Lubricant Additives: Chemistry and Applications

Lubricant Additives Chemistry and Applications Second Edition

CHEMICAL INDUSTRIES A Series of Reference Books and Textbooks

Founding Editor HEINZ HEINEMANN Berkeley, California

Series Editor JAMES G. SPEIGHT University of Trinidad and Tobago O'Meara Campus, Trinidad

1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Fluid Catalytic Cracking with Zeolite Catalysts, Paul B. Venuto and E. Thomas Habib, Jr. Ethylene: Keystone to the Petrochemical Industry, Ludwig Kniel, Olaf Winter, and Karl Stork The Chemistry and Technology of Petroleum, James G. Speight The Desulfurization of Heavy Oils and Residua, James G. Speight Catalysis of Organic Reactions, edited by William R. Moser Acetylene-Based Chemicals from Coal and Other Natural Resources, Robert J. Tedeschi Chemically Resistant Masonry, Walter Lee Sheppard, Jr. Compressors and Expanders: Selection and Application for the Process Industry, Heinz P. Bloch, Joseph A. Cameron, Frank M. Danowski, Jr., Ralph James, Jr., Judson S. Swearingen, and Marilyn E. Weightman Metering Pumps: Selection and Application, James P. Poynton Hydrocarbons from Methanol, Clarence D. Chang Form Flotation: Theory and Applications, Ann N. Clarke and David J. Wilson The Chemistry and Technology of Coal, James G. Speight Pneumatic and Hydraulic Conveying of Solids, O. A. Williams Catalyst Manufacture: Laboratory and Commercial Preparations, Alvin B. Stiles Characterization of Heterogeneous Catalysts, edited by Francis Delannay BASIC Programs for Chemical Engineering Design, James H. Weber Catalyst Poisoning, L. Louis Hegedus and Robert W. McCabe Catalysis of Organic Reactions, edited by John R. Kosak Adsorption Technology: A Step-by-Step Approach to Process Evaluation and Application, edited by Frank L. Slejko Deactivation and Poisoning of Catalysts, edited by Jacques Oudar and Henry Wise

21. Catalysis and Surface Science: Developments in Chemicals from Methanol, Hydrotreating of Hydrocarbons, Catalyst Preparation, Monomers and Polymers, Photocatalysis and Photovoltaics, edited by Heinz Heinemann and Gabor A. Somorjai 22. Catalysis of Organic Reactions, edited by Robert L. Augustine 23. Modern Control Techniques for the Processing Industries, T. H. Tsai, J. W. Lane, and C. S. Lin 24. Temperature-Programmed Reduction for Solid Materials Characterization, Alan Jones and Brian McNichol 25. Catalytic Cracking: Catalysts, Chemistry, and Kinetics, Bohdan W. Wojciechowski and Avelino Corma 26. Chemical Reaction and Reactor Engineering, edited by J. J. Carberry and A. Varma 27. Filtration: Principles and Practices: Second Edition, edited by Michael J. Matteson and Clyde Orr 28. Corrosion Mechanisms, edited by Florian Mansfeld 29. Catalysis and Surface Properties of Liquid Metals and Alloys, Yoshisada Ogino 30. Catalyst Deactivation, edited by Eugene E. Petersen and Alexis T. Bell 31. Hydrogen Effects in Catalysis: Fundamentals and Practical Applications, edited by Zoltán Paál and P. G. Menon 32. Flow Management for Engineers and Scientists, Nicholas P. Cheremisinoff and Paul N. Cheremisinoff 33. Catalysis of Organic Reactions, edited by Paul N. Rylander, Harold Greenfield, and Robert L. Augustine 34. Powder and Bulk Solids Handling Processes: Instrumentation and Control, Koichi Iinoya, Hiroaki Masuda, and Kinnosuke Watanabe 35. Reverse Osmosis Technology: Applications for High-Purity-Water Production, edited by Bipin S. Parekh 36. Shape Selective Catalysis in Industrial Applications, N. Y. Chen, William E. Garwood, and Frank G. Dwyer 37. Alpha Olefins Applications Handbook, edited by George R. Lappin and Joseph L. Sauer 38. Process Modeling and Control in Chemical Industries, edited by Kaddour Najim 39. Clathrate Hydrates of Natural Gases, E. Dendy Sloan, Jr. 40. Catalysis of Organic Reactions, edited by Dale W. Blackburn 41. Fuel Science and Technology Handbook, edited by James G. Speight 42. Octane-Enhancing Zeolitic FCC Catalysts, Julius Scherzer 43. Oxygen in Catalysis, Adam Bielanski and Jerzy Haber 44. The Chemistry and Technology of Petroleum: Second Edition, Revised and Expanded, James G. Speight 45. Industrial Drying Equipment: Selection and Application, C. M. van’t Land 46. Novel Production Methods for Ethylene, Light Hydrocarbons, and Aromatics, edited by Lyle F. Albright, Billy L. Crynes, and Siegfried Nowak 47. Catalysis of Organic Reactions, edited by William E. Pascoe 48. Synthetic Lubricants and High-Performance Functional Fluids, edited by Ronald L. Shubkin 49. Acetic Acid and Its Derivatives, edited by Victor H. Agreda and Joseph R. Zoeller 50. Properties and Applications of Perovskite-Type Oxides, edited by L. G. Tejuca and J. L. G. Fierro 51. Computer-Aided Design of Catalysts, edited by E. Robert Becker and Carmo J. Pereira

52. Models for Thermodynamic and Phase Equilibria Calculations, edited by Stanley I. Sandler 53. Catalysis of Organic Reactions, edited by John R. Kosak and Thomas A. Johnson 54. Composition and Analysis of Heavy Petroleum Fractions, Klaus H. Altgelt and Mieczyslaw M. Boduszynski 55. NMR Techniques in Catalysis, edited by Alexis T. Bell and Alexander Pines 56. Upgrading Petroleum Residues and Heavy Oils, Murray R. Gray 57. Methanol Production and Use, edited by Wu-Hsun Cheng and Harold H. Kung 58. Catalytic Hydroprocessing of Petroleum and Distillates, edited by Michael C. Oballah and Stuart S. Shih 59. The Chemistry and Technology of Coal: Second Edition, Revised and Expanded, James G. Speight 60. Lubricant Base Oil and Wax Processing, Avilino Sequeira, Jr. 61. Catalytic Naphtha Reforming: Science and Technology, edited by George J. Antos, Abdullah M. Aitani, and José M. Parera 62. Catalysis of Organic Reactions, edited by Mike G. Scaros and Michael L. Prunier 63. Catalyst Manufacture, Alvin B. Stiles and Theodore A. Koch 64. Handbook of Grignard Reagents, edited by Gary S. Silverman and Philip E. Rakita 65. Shape Selective Catalysis in Industrial Applications: Second Edition, Revised and Expanded, N. Y. Chen, William E. Garwood, and Francis G. Dwyer 66. Hydrocracking Science and Technology, Julius Scherzer and A. J. Gruia 67. Hydrotreating Technology for Pollution Control: Catalysts, Catalysis, and Processes, edited by Mario L. Occelli and Russell Chianelli 68. Catalysis of Organic Reactions, edited by Russell E. Malz, Jr. 69. Synthesis of Porous Materials: Zeolites, Clays, and Nanostructures, edited by Mario L. Occelli and Henri Kessler 70. Methane and Its Derivatives, Sunggyu Lee 71. Structured Catalysts and Reactors, edited by Andrzej Cybulski and Jacob A. Moulijn 72. Industrial Gases in Petrochemical Processing, Harold Gunardson 73. Clathrate Hydrates of Natural Gases: Second Edition, Revised and Expanded, E. Dendy Sloan, Jr. 74. Fluid Cracking Catalysts, edited by Mario L. Occelli and Paul O’Connor 75. Catalysis of Organic Reactions, edited by Frank E. Herkes 76. The Chemistry and Technology of Petroleum: Third Edition, Revised and Expanded, James G. Speight 77. Synthetic Lubricants and High-Performance Functional Fluids: Second Edition, Revised and Expanded, Leslie R. Rudnick and Ronald L. Shubkin 78. The Desulfurization of Heavy Oils and Residua, Second Edition, Revised and Expanded, James G. Speight 79. Reaction Kinetics and Reactor Design: Second Edition, Revised and Expanded, John B. Butt 80. Regulatory Chemicals Handbook, Jennifer M. Spero, Bella Devito, and Louis Theodore 81. Applied Parameter Estimation for Chemical Engineers, Peter Englezos and Nicolas Kalogerakis 82. Catalysis of Organic Reactions, edited by Michael E. Ford 83. The Chemical Process Industries Infrastructure: Function and Economics, James R. Couper, O. Thomas Beasley, and W. Roy Penney 84. Transport Phenomena Fundamentals, Joel L. Plawsky

85. Petroleum Refining Processes, James G. Speight and Baki Özüm 86. Health, Safety, and Accident Management in the Chemical Process Industries, Ann Marie Flynn and Louis Theodore 87. Plantwide Dynamic Simulators in Chemical Processing and Control, William L. Luyben 88. Chemical Reactor Design, Peter Harriott 89. Catalysis of Organic Reactions, edited by Dennis G. Morrell 90. Lubricant Additives: Chemistry and Applications, edited by Leslie R. Rudnick 91. Handbook of Fluidization and Fluid-Particle Systems, edited by Wen-Ching Yang 92. Conservation Equations and Modeling of Chemical and Biochemical Processes, Said S. E. H. Elnashaie and Parag Garhyan 93. Batch Fermentation: Modeling, Monitoring, and Control, Ali Çinar, Gülnur Birol, Satish J. Parulekar, and Cenk Ündey 94. Industrial Solvents Handbook, Second Edition, Nicholas P. Cheremisinoff 95. Petroleum and Gas Field Processing, H. K. Abdel-Aal, Mohamed Aggour, and M. Fahim 96. Chemical Process Engineering: Design and Economics, Harry Silla 97. Process Engineering Economics, James R. Couper 98. Re-Engineering the Chemical Processing Plant: Process Intensification, edited by Andrzej Stankiewicz and Jacob A. Moulijn 99. Thermodynamic Cycles: Computer-Aided Design and Optimization, Chih Wu 100. Catalytic Naphtha Reforming: Second Edition, Revised and Expanded, edited by George T. Antos and Abdullah M. Aitani 101. Handbook of MTBE and Other Gasoline Oxygenates, edited by S. Halim Hamid and Mohammad Ashraf Ali 102. Industrial Chemical Cresols and Downstream Derivatives, Asim Kumar Mukhopadhyay 103. Polymer Processing Instabilities: Control and Understanding, edited by Savvas Hatzikiriakos and Kalman B. Migler 104. Catalysis of Organic Reactions, John Sowa 105. Gasification Technologies: A Primer for Engineers and Scientists, edited by John Rezaiyan and Nicholas P. Cheremisinoff 106. Batch Processes, edited by Ekaterini Korovessi and Andreas A. Linninger 107. Introduction to Process Control, Jose A. Romagnoli and Ahmet Palazoglu 108. Metal Oxides: Chemistry and Applications, edited by J. L. G. Fierro 109. Molecular Modeling in Heavy Hydrocarbon Conversions, Michael T. Klein, Ralph J. Bertolacini, Linda J. Broadbelt, Ankush Kumar and Gang Hou 110. Structured Catalysts and Reactors, Second Edition, edited by Andrzej Cybulski and Jacob A. Moulijn 111. Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology, edited by Leslie R. Rudnick 112. Alcoholic Fuels, edited by Shelley Minteer 113. Bubbles, Drops, and Particles in Non-Newtonian Fluids, Second Edition, R. P. Chhabra 114. The Chemistry and Technology of Petroleum, Fourth Edition, James G. Speight 115. Catalysis of Organic Reactions, edited by Stephen R. Schmidt 116. Process Chemistry of Lubricant Base Stocks, Thomas R. Lynch 117. Hydroprocessing of Heavy Oils and Residua, edited by James G. Speight and Jorge Ancheyta 118. Chemical Process Performance Evaluation, Ali Cinar, Ahmet Palazoglu, and Ferhan Kayihan

119. Clathrate Hydrates of Natural Gases, Third Edition, E. Dendy Sloan and Carolyn Koh 120. Interfacial Properties of Petroleum Products, Lilianna Z. Pillon 121. Process Chemistry of Petroleum Macromolecules, Irwin A. Wiehe 122. The Scientist or Engineer as an Expert Witness, James G. Speight 123. Catalysis of Organic Reactions, edited by Michael L. Prunier 124. Lubricant Additives: Chemistry and Applications, Second Edition, edited by Leslie R. Rudnick

Lubricant Additives Chemistry and Applications Second Edition

Edited by

Leslie R. Rudnick Designed Materials Group Wilmington, Delaware, U.S.A.

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-5964-9 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Lubricant additives: chemistry and applications / editor, Leslie R. Rudnick. -- 2nd ed. p. cm. -- (Chemical industries ; 124) Includes bibliographical references and index. ISBN 978-1-4200-5964-9 (alk. paper) 1. Lubrication and lubricants--Additives. I. Rudnick, Leslie R., 1947- II. Title. III. Series. TJ1077.L815 2008 621.8’9--dc22 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

2008034106

Contents Preface ........................................................................................................................................... xiii Contributors ..................................................................................................................................... xv

PART 1 Chapter 1

Deposit Control Additives Antioxidants .................................................................................................................3 Jun Dong and Cyril A. Migdal

Chapter 2

Zinc Dithiophosphates ............................................................................................... 51 Randolf A. McDonald

Chapter 3

Ashless Phosphorus–Containing Lubricating Oil Additives ..................................... 63 W. David Phillips

Chapter 4

Detergents................................................................................................................. 123 Syed Q. A. Rizvi

Chapter 5

Dispersants ............................................................................................................... 143 Syed Q. A. Rizvi

PART 2 Chapter 6

Film-Forming Additives Selection and Application of Solid Lubricants as Friction Modifiers ...................... 173 Gino Mariani

Chapter 7

Organic Friction Modifiers....................................................................................... 195 Dick Kenbeck and Thomas F. Bunemann

PART 3 Antiwear Additives and Extreme-Pressure Additives Chapter 8

Ashless Antiwear and Extreme-Pressure Additives................................................. 213 Liehpao Oscar Farng

Chapter 9

Contents

Sulfur Carriers ......................................................................................................... 251 Thomas Rossrucker and Achim Fessenbecker

PART 4

Viscosity Control Additives

Chapter 10 Olefin Copolymer Viscosity Modifiers .................................................................... 283 Michael J. Covitch Chapter 11 Polymethacrylate Viscosity Modifiers and Pour Point Depressants ........................ 315 Bernard G. Kinker Chapter 12 Pour Point Depressants............................................................................................. 339 Joan Souchik

PART 5 Miscellaneous Additives Chapter 13 Tackifiers and Antimisting Additives ...................................................................... 357 Victor J. Levin, Robert J. Stepan, and Arkady I. Leonov Chapter 14 Seal Swell Additives................................................................................................. 377 Ronald E. Zielinski and Christa M. A. Chilson Chapter 15 Antimicrobial Additives for Metalworking Lubricants ........................................... 383 William R. Schwingel and Alan C. Eachus Chapter 16 Surfactants in Lubrication ........................................................................................ 399 Girma Biresaw Chapter 17 Corrosion Inhibitors and Rust Preventatives ............................................................ 421 Michael T. Costello Chapter 18 Additives for Bioderived and Biodegradable Lubricants ......................................... 445 Mark Miller

PART 6 Applications Chapter 19 Additives for Crankcase Lubricant Applications ..................................................... 457 Ewa A. Bardasz and Gordon D. Lamb

Contents

Chapter 20 Additives for Industrial Lubricant Applications ...................................................... 493 Leslie R. Rudnick Chapter 21 Formulation Components for Incidental Food-Contact Lubricants ......................... 511 Saurabh Lawate Chapter 22 Lubricants for the Disk Drive Industry .................................................................... 523 Tom E. Karis Chapter 23 Additives for Grease Applications ........................................................................... 585 Robert Silverstein and Leslie R. Rudnick

PART 7 Trends Chapter 24 Long-Term Trends in Industrial Lubricant Additives ..............................................609 Fay Linn Lee and John W. Harris Chapter 25 Long-Term Additive Trends in Aerospace Applications .......................................... 637 Carl E. Snyder, Lois J. Gschwender, and Shashi K. Sharma Chapter 26 Eco Requirements for Lubricant Additives .............................................................. 647 Tassilo Habereder, Danielle Moore, and Matthias Lang

PART 8 Methods and Resources Chapter 27 Testing Methods for Additive/Lubricant Performance ............................................669 Leslie R. Rudnick Chapter 28 Lubricant Industry–Related Terms and Acronyms .................................................. 685 Leslie R. Rudnick Chapter 29 Internet Resources for Additive/Lubricant Industry ................................................ 707 Leslie R. Rudnick Index .............................................................................................................................................. 761

Preface Lubricant additives continue to be developed to provide improved properties and performance to modern lubricants. Environmental issues and applications that require lubricants to operate under severe conditions will cause an increase in the use of synthetics. Owing to performance and maintenance reasons, many applications that have historically relied on petroleum-derived lubricants are shifting to synthetic lubricant-based products. Cost issues, on the contrary, tend to shift the market toward group II and III base oils where hydrocarbons can be used. Shifts to renewable and biodegradable fluids are also needed, and this will require a greater need for new effective additives to meet the challenges of formulating for various applications. There are several indications that the lubricant additive industry will grow and change. Legislation is driving changes to fuel composition and lubricant components, and therefore, future lubricant developments will be constrained compared to what has been done in the past. Registration, Evaluation, Authorisation and Restriction of Chemicals (REACh) in the European Union (EU) is placing constraints on the incentive to develop new molecules that will serve as additives. The cost of introduction of new proprietary materials will be the burden of the company that develops the new material. For many common additives that are produced by several manufacturers, they will share costs to generate any needed data on the toxicology or biodegradability of the materials. Continued progress toward new engine oil requirements will require oils to provide improved fuel economy and to have additive chemistry that does not degrade emission system components. This will require a new test to evaluate the volatility of phosphorus in engine oils and to improve the oil properties in terms of protecting the engine. Future developments and requirements will undoubtedly require new, more severe testing protocols. The market for lubricant additives is expected to grow. China and India, for example, represent highly populated markets that are expected to see growth in infrastructure, and therefore a growth in industrial equipment and number of vehicles. Many U.S. and EU companies continue to develop a presence in Pacific and Southeast Asia through either new manufacturing in that region or sales and distribution offices. More advanced technologies will require application of new types of lubricants, containing new additive chemistries required for exploration of space and oceans. Since these remote locations and extremes of environment require low maintenance, they will place new demands on lubricant properties and performance. This book would not have developed the way it has without the invaluable help and encouragement of many of my colleagues. I want to thank all of the authors of the chapters contained herein for responding to the deadlines. There is always a balance between job responsibilities and publishing projects like this one. My heartfelt thanks to each of you. It is your contributions that have created this resource for our industry. I especially want to thank Barbara Glunn, at Taylor & Francis Group, with whom I have worked earlier on Synthetics, Mineral Oils and Bio-Based Lubricants, for her support to this project from its early stages through its completion. I also want to thank Kari Budyk, project coordinator, who has been invaluable in every way in the progress of this project and has been a tremendous asset to me as an editor and helpful to the many contributors of this book. I also want to thank Jennifer Derima, Jennifer Smith, and the team at Macmillan Publishing Solutions for their efforts, patience, and understanding during the time I have been working on this book. I also thank Paula, Eric, and Rachel for all of their support during this project. Les Rudnick xiii

Contributors Ewa A. Bardasz The Lubrizol Corporation Wickliffe, Ohio

Tassilo Habereder Ciba Specialty Chemicals Inc. Basel, Switzerland

Girma Biresaw Cereal Products and Food Science Research Unit NCAUR-MWA-ARS-USDA Peoria, Illinois

John W. Harris Shell Global Solutions Houston, Texas

Thomas F. Bunemann Uniqema Gouda, The Netherlands Christa M. A. Chilson PolyMod Technologies Inc Fort Wayne, Indiana

Tom E. Karis Hitachi Global Storage Technologies San Jose Research Center San Jose, California Dick Kenbeck Uniqema Gouda, The Netherlands

Michael T. Costello Chemtura Corporation Middlebury, Connecticut

Bernard G. Kinker Degussa, RohMax Oil Additives LP Kintnersville, Pennsylvania

Michael J. Covitch The Lubrizol Corporation Wicklife, Ohio

Gordon D. Lamb Lubrizol International Laboratories Belper, Derby, United Kingdom

Jun Dong Chemtura Corporation Middlebury, Connecticut

Matthias Lang Process & Lubricant Additives Ciba Specialty Chemicals Basel, Switzerland

Alan C. Eachus Independent Consultant Villa Park, Illinois

Saurabh Lawate The Lubrizol Corporation Wickliffe, Ohio

Liehpao Oscar Farng ExxonMobil Research and Engineering Company Annandale, New Jersey

Fay Linn Lee Shell Lubricants Houston, Texas

Achim Fessenbecker Rhein Chemie Rheinau GmbH Mannheim, Germany

Arkady I. Leonov University of Akron Akron, Ohio

Lois J. Gschwender AFRL/RXBT Wright-Patterson Air Force Base, Ohio

Victor J. Levin Functional Products Inc. Macedonia, Ohio xv

xvi

Contributors

Gino Mariani Acheson Colloids Company Port Huron, Michigan

Leslie R. Rudnick Designed Materials Group Wilmington, Delaware

Randolf A. McDonald Functional Products Inc. Cleveland, Ohio

William R. Schwingel MASCO Corporation R&D Taylor, Michigan

Cyril A. Migdal Chemtura Corporation Middlebury, Connecticut

Shashi K. Sharma AFRL/RXBT Wright-Patterson Air Force Base, Ohio

Mark Miller Terrasolve Technologies Eastlake, Ohio Danielle Moore Ciba Specialty Chemicals plc Process & Lubricant Additives Macclesfield, Cheshire, United Kingdom

Robert Silverstein Orelube Corporation Bellport, New York Carl E. Snyder AFRL/RXBT Wright-Patterson Air Force Base, Ohio

W. David Phillips W. David Phillips and Associates Stockport, Cheshire, United Kingdom

Joan Souchik Evonik RohMax USA, Inc. Horsham, Pennsylvania

Syed Q. A. Rizvi Santovac Fluids St. Charles, Missouri

Robert J. Stepan Functional Products Inc. Macedonia, Ohio

Thomas Rossrucker Rhein Chemie Rheinau GmbH Mannheim, Germany

Ronald E. Zeilinski PolyMod Technologies Inc. Fort Wayne, Indiana

Part 1 Deposit Control Additives

Antioxidants Jun Dong and Cyril A. Migdal

CONTENTS 1.1 1.2 1.3 1.4 1.5 1.6

Introduction ...............................................................................................................................4 Sulfur Compounds ....................................................................................................................5 Sulfur–Nitrogen Compounds ....................................................................................................6 Phosphorus Compounds............................................................................................................7 Sulfur–Phosphorus Compounds ............................................................................................... 8 Amine and Phenol Derivatives ............................................................................................... 10 1.6.1 Amine Derivatives ....................................................................................................... 10 1.6.2 Phenol Derivatives ....................................................................................................... 13 1.6.3 Amine and Phenol-Bearing Compounds..................................................................... 13 1.6.4 Multifunctional Amine and Phenol Derivatives ......................................................... 13 1.7 Copper Antioxidants ............................................................................................................... 16 1.8 Boron Antioxidants ................................................................................................................. 17 1.9 Miscellaneous Organometallic Antioxidants ......................................................................... 18 1.10 Mechanisms of Hydrocarbon Oxidation and Antioxidant Action .......................................... 18 1.10.1 Autoxidation of Lubricating Oil ................................................................................ 19 1.10.1.1 Initiation ..................................................................................................... 19 1.10.1.2 Chain Propagation ...................................................................................... 19 1.10.1.3 Chain Branching ........................................................................................ 19 1.10.1.4 Chain Termination .....................................................................................20 1.10.2 Metal-Catalyzed Lubricant Degradation ...................................................................20 1.10.2.1 Metal Catalysis ........................................................................................... 21 1.10.3 High-Temperature Lubricant Degradation................................................................. 21 1.10.4 Effect of Base Stock Composition on Oxidative Stability ......................................... 21 1.10.5 Oxidation Inhibition................................................................................................... 23 1.10.6 Mechanisms of Primary Antioxidants.......................................................................24 1.10.6.1 Hindered Phenolics ....................................................................................24 1.10.6.2 Aromatic Amines .......................................................................................26 1.10.7 Mechanisms of Secondary Antioxidants ...................................................................28 1.10.7.1 Organosulfur Compounds ..........................................................................28 1.10.7.2 Organophosphorus Compounds .................................................................28 1.10.8 Antioxidant Synergism .............................................................................................. 29 1.11 Oxidation Bench Tests ............................................................................................................ 30 1.11.1 Thin-Film Oxidation Test .......................................................................................... 31 1.11.1.1 Pressurized Differential Scanning Calorimetry ........................................ 31 1.11.1.2 Thermal-Oxidation Engine Oil Simulation Test (ASTM D 6335; D 7097) ............................................................................ 31 1.11.1.3 Thin-Film Oxidation Uptake Test (ASTM D 4742) ................................... 33

Lubricant Additives: Chemistry and Applications

1.11.2

Bulk Oil Oxidation Test ............................................................................................. 33 1.11.2.1 Turbine Oil Stability Test (ASTM D 943, D 4310)..................................... 33 1.11.2.2 IP 48 Method ..............................................................................................34 1.11.2.3 IP 280/CIGRE ............................................................................................34 1.11.3 Oxygen Update Test ...................................................................................................34 1.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272) ........................34 1.12 Experimental Observations .....................................................................................................34 1.13 Antioxidant Performance with Base Stock Selection ............................................................. 37 1.14 Future Requirements ............................................................................................................... 38 1.15 Commercial Antioxidants ....................................................................................................... 39 1.16 Commercial Metal Deactivators ............................................................................................. 41 References ........................................................................................................................................ 41

1.1

INTRODUCTION

Well before the mechanism of hydrocarbon oxidation was thoroughly investigated, researchers had come to understand that some oils provided greater resistance to oxidation than others. The difference was eventually identified as naturally occurring antioxidants, which varied depending on crude source or refining techniques. Some of these natural antioxidants were found to contain sulfur- or nitrogen-bearing functional groups. Therefore, it is not surprising that, certain additives that are used to impart special properties to the oil, such as sulfur-bearing chemicals, were found to provide additional antioxidant stability. The discovery of sulfurized additives providing oxidation stability was followed by the identification of similar properties with phenols, which led to the development of sulfurized phenols. Next, certain amines and metal salts of phosphorus- or sulfur-containing acids were identified as imparting oxidation stability. By now numerous antioxidants for lubricating oils have been patented and described in the literature. Today, nearly all lubricants contain at least one antioxidant for stabilization and other performance-enhancing purposes. Since oxidation has been identified as the primary cause of oil degradation, it is the most important aspect for lubricants that the oxidation stability be maximized. Oxidation produces harmful species, which eventually compromises the designated functionalities of a lubricant, shortens its service life, and to a more extreme extent, damages the machinery it lubricates. The oxidation is initiated upon exposure of hydrocarbons to oxygen and heat and can be greatly accelerated by transitional metals such as copper, iron, nickel, and so on. when present. The internal combustion engine is an excellent chemical reactor for catalyzing the process of oxidation with heat and engine metal parts acting as effective oxidation catalysts. Thus, in-service engine oils are probably more susceptible to oxidation than most other lubricant applications. For the prevention of lubricant oxidation, antioxidants are the key additive that protects the lubricant from oxidative degradation, allowing the fluid to meet the demanding requirements for use in engines and industrial applications. Several effective antioxidant classes have been developed over the years and have seen use in engine oils, automatic transmission fluids, gear oils, turbine oils, compressor oils, greases, hydraulic fluids, and metal working fluids. The main classes include oil-soluble organic and organometallic antioxidants of the following types: 1. 2. 3. 4. 5. 6. 7.

Sulfur compounds Sulfur–nitrogen compounds Phosphorus compounds Sulfur–phosphorus compounds Aromatic amine compounds Hindered phenolic (HP) compounds Organo–copper compounds

Antioxidants

8. Boron compounds 9. Other organometallic compounds

1.2

SULFUR COMPOUNDS

The initial concepts of using antioxidants to inhibit oil oxidation date back to the 1800s. One of the earliest inventions described in the literature [1] is the heating of a mineral oil with elemental sulfur to produce a nonoxidizing oil. However, the major drawback to this approach is the high corrosivity of the sulfurized oil toward copper. Aliphatic sulfide with a combined antioxidant and corrosion inhibition characteristics was developed by sulfurizing sperm oil [2]. Additives with similar functionalities could also be obtained from sulfurizing terpenes and polybutene [3–5]. Paraffin wax has also been employed to prepare sulfur compounds [6–9]. Theoretical structures of several sulfur compounds are illustrated in Figure 1.1. Actual compounds can be chemically complex in nature. Aromatic sulfides represent another class of sulfur additives used as oxidation and corrosion inhibitors. Examples of simple sulfides are dibenzyl sulfide and dixylyl disulfide. More complex compounds of a similar type are the alkyl phenol sulfides [10–15]. Alkyl phenols, such as mono- or di-butyl, -amyl, or -octyl phenol, have been reacted with sulfur mono- or dichloride to form either mono- or disulfides. As shown in Figure 1.1, the aromatic sulfides such as benzyl sulfide have the sulfur attached to carbon atoms in the alkyl side groups, whereas the alkyl phenol sulfides have the sulfur attached to carbon atoms in the aromatic rings. In general, the alkyl phenol sulfide chemistry appears to have superior antioxidant properties in many types of lubricants. Mono- and dialkyldiphenyl sulfides obtained by reacting diphenyl sulfide with C10 –C18 alpha-olefins in the presence of aluminum chloride have been demonstrated to be powerful antioxidants for high-temperature lubricants especially those utilizing synthetic base stocks such as hydrogenated poly-alpha-olefins, diesters, and polyol esters [15]. The hydroxyl groups of the alkyl phenol sulfides may also be treated

O CH3 CH2 CH2 S CH2

C C

CH3

(CH2)x

CH CH2

CH3

(CH2)x

CH2

(CH2)x

CH3

(CH2)x

CH2

(CH2)x

CH3

CH2

O CH3

Sulfurized ester

Sulfurized dipentene CH3

CH3

(CH2)x

CH3

(CH2)x

CH3

CH2

Dibenzyl sulfide Sulfurized olefin OH R

(S)x

R HO

Dialkylphenol sulfide

FIGURE 1.1

Examples of sulfur-bearing antioxidants.

Lubricant Additives: Chemistry and Applications

with metals to form oil-soluble metal phenates. These metal phenates play the dual role of detergent and antioxidant. Multifunctional antioxidant and extreme pressure (EP) additives with heterocyclic structures were prepared by sulfurizing norbornene, 5-vinylnorbornene dicyclopentadiene, or methyl cyclopentadiene dimer [16]. Heterocyclic compounds such as n-alkyl 2-thiazoline disulfide in combination with zinc dialkyldithiophosphate (ZDDP) exhibited excellent antioxidant performance in laboratory engine tests [17]. Heterocyclic sulfur- and oxygen-containing compositions derived from mercaptobenzthiazole and beta-thiodialkanol have been found to be excellent antioxidants in automatic transmission fluids [18]. Novel antioxidant and antiwear additives based on dihydrobenzothiophenes have been prepared through condensation of low-cost arylthiols and carbonyl compounds in a one-step high-yield process [19].

1.3 SULFUR–NITROGEN COMPOUNDS The dithiocarbamates were first introduced in the early 1940s as fungicides and pesticides [20]. Their potential use as antioxidants for lubricants was not realized until the mid-1960s [21], and since then, there have been continuous interests in this type of chemistry for lubricant applications [22]. Today, dithiocarbamates represent a main class of sulfur–nitrogen-bearing compounds being used as antioxidants, antiwear, and anticorrosion additives for lubricants. Depending on the type of adduct to the dithiocarbamate core, ashless and metal-containing dithiocarbamate derivatives can be formed. Typical examples of ashless materials are methylene bis(dialkyldithiocarbamate) and dithiocarbamate esters with general structures being illustrated in Figure 1.2. Both are synergistic with alkylated diphenylamine (ADPA) and organomolybdenum compounds in high-temperature deposit control [23]. In particular, methylene bis(dialkyldithiocar bamate) in combination with primary antioxidants such as arylamines or HPs and triazole derivatives is known to provide synergistic action in stabilizing mineral oils and synthetic lubricating oils [24–26]. This material has been used to improve antioxidation characteristics of internal combustion engine oils containing low levels (<0.1 wt%) of phosphorus [27]. In another effort to reduce phosphorus content in aviation gas turbine lubricants, methylene-bridged bis(dialkyl) or bis(alkylar yldithiocarbamate) was used as high-temperature antioxidant and antiwear agent to replace tricresyl phosphates that are of a concern to produce neurotoxic ortho-cresol isomers in trimethylolpropane triester base oil under high-temperature service conditions [28]. It has been known that metal dithiocarbamates such as zinc, copper, lead, antimony, bismuth, and molybdenum dithiocarbamates (MoDTCs) possess desirable lubricating characteristics including antiwear and antioxidant properties. The associated metal ions affect the antioxidancy of the additives. Within the group, MoDTCs are of greater interest particularly for engine crankcase lubricants. Certain molybdenum additives posses good oxidation resistance and acceptable corrosion characteristics, when prepared by reacting water, an acidic molybdenum compound, a basic nitrogen complex, and a sulfur source [29,30]. Oil-soluble trinuclear MoDTCs prepared by reacting

O S S

S R

C N

Bis(disubstituted dithiocarbamate)

FIGURE 1.2

Ashless dithiocarbamates for lubricants.

N R

C S

C C

R C O

Dithiocarbamate ester

Antioxidants

ammonium polythiomolybdate with appropriate tetralkylthiuram disulfides were found to be superior to dinuclear molybdenum compounds in terms of providing lubricants antioxidant, antiwear, and friction-reducing properties [31]. When combined with an appropriate aromatic amine, MoDTCs can exhibit synergistic antioxidant effects in oxidation tests [32]. As a result, molybdenum dialkyldithiocarbamates (C7–24) and ADPAs are claimed broadly for lubricating oils [33]. More restrictive are claims for molybdenum dialkyldithiocarbamates (C8–23 and C3–18) and ADPAs in lubricating oils that contain <3 wt% of aromatic content and <50 ppm of sulfur and nitrogen [34]. Molybdenum dialkyldithiocarbamates and HP antioxidants are jointly claimed for lubricating oils that contain 45 wt% or more one or two ring naphthenes and <50 ppm sulfur and nitrogen [35]. MoDTC was used to top-treat engine oils formulated with group I base stocks (>300 ppm S) and an additive package designed for group II base stocks. The oils passed the sequence IIIF oxidation test, in which the oils would otherwise fail without the molybdenum top-treatment [36]. Further demonstrated is a combination of ADPAs, sulfurized olefin, or HP and oil-soluble molybdenum compounds including MoDTC. The mixture is highly effective in stabilizing lubricants, especially those formulated with highly saturated, low-sulfur base oils [37]. Thiadiazole derivatives, particularly the monomers and dimers, represent another class of sulfur- and nitrogen-bearing multifunctional additives with antioxidant potency. For example, the monomeric 2-alkylesterthio-5-mercapto-1,3,4-thiadiazole has been reported to increase oxidative stability of engine oils under thin-film oxidation conditions by using thin-film oxygen uptake test (TFOUT) [38]. Lithium 12-hydroxystearate grease containing 2,5-dithiobis(1,3,4-thiadiazole2-thiol), a dimer, exhibited superior oxidative stability in the American Society for Testing and Materials (ASTM) D 942 pressure bomb oxidation test [39]. When used in conjunction with ADPA and organomolybdenum compound, the thiadiazole derivative improved the thermal-oxidation engine oil simulation test (TEOST) deposition (ASTM D 7097) characteristic of an engine oil from the control oil containing sulfurized isobutylene instead [40]. In addition to providing antioxidant benefit, the thiadiazole derivatives have been widely used as ashless antiwear and EP additives. Some of them can also provide corrosion inhibition and metal deactivation properties to nonferrous metals such as copper. Phenothiazines are also well-known sulfur- and nitrogen-bearing antioxidants and have been used to stabilize aviation fluids. Recent advances have lead to N-substituted thio alkyl phenothiazines, having improved antioxidant activities and oil solubility [41] as well as N-aminopropylphenothiazine that can be used for further derivatization of the N-amino group [42]. For example, alkyl phenothiazines together with aromatic amines can be attached to olefin copolymers to result in a multifunctional antioxidant, antiwear agent, and Viscosity index (VI) improver for lubricants [43]. Diamine sulfides, including diamine polysulfides, can also provide effective oxidation control when used in conjunction with oil-soluble copper. In demonstration, dimorpholine disulfide and di(dimethyl morpholine) disulfide were compared to primary alkyl ZDDP and found to be superior in controlling oil viscosity increase of engine crankcase lubricants at elevated temperatures [44].

1.4 PHOSPHORUS COMPOUNDS The good performance of phosphorus as an oxidation inhibitor in oils was identified early on in lubrication science. The use of elemental phosphorus to reduce sludge formation in oils has been described [45]. However, elemental phosphorus, like elemental sulfur, may have corrosive side effects to many nonferrous metals and alloys, so it is rarely incorporated in oils in this form, rather oil-soluble organic compounds of phosphorus are preferred. Naturally occurring phosphorus compounds such as lecithin have been utilized as antioxidants and many patents have been issued on these materials for single use or in combination with other additives [46–49]. Lecithin is a phosphatide that has been produced commercially as a by-product from the processing of crude soybean oil. The antioxidant property of synthetic neutral and acid phosphite esters has been known for sometime. Alkyl and aryl phosphites such as tributyl phosphite and triphenyl phosphite are efficient

Lubricant Additives: Chemistry and Applications

TABLE 1.1 Applications of Organophosphites as Antioxidants for Lubricants Applications Compressor oils

Automotive and industrial lubricants Automotive and industrial lubricants

Hydraulic fluids, steam turbine oils, compressor oils, and heat-transfer oil Steam turbine oils, gas turbine oils Hydraulic fluids, Automatic transmission fluids

Phosphites

Supplementary Antioxidants

References

Trinonylphenyl phosphite, tributyl phosphite, tridecylphosphite, triphenylphosphite, trioctylphosphite, dilaurylphosphite Triaryl phosphites, trialkyl phosphites, alkyl aryl phosphites, acid dialkyl phosphites Triphenyl phosphite, diisodecyl pentaerythritol diphosphite, tri-isodecyl phosphite, dilauryl phosphite Steric hindered tributyl phosphite, bis(butylphenyl pentaerythritol) diphosphite Triphenyl phosphite, trialkylsubstituted phenyl phosphite

Secondary aminic and hindered phenolic

(3,5-Di-t-butyl)4-hydroxybenzyl isocyanurate

Alkylated diphenylamine, phenyl-naphthylamine

Trialkyl phosphites

Secondary aminic and hindered phenolic including bis-phenol

antioxidants in some petroleum base oils, and many patents have been issued on such compositions [50,51]. Table 1.1 summarizes the patenting activities of the past three decades on the stabilization of various lubricants with organophosphites. For optimum antioxidant performance, phosphites are customarily blended with aminic or HP antioxidants that can lead to synergistic effect. For better hydrolytic stability, tri-substituted phosphites with sterically hindered structures such as tris-(2,4di-tert-butylphenyl) phosphite and those based on pentaerythritol as described in the U.S. Patent 5,124,057 [52] are preferred. The aluminum, calcium, or barium salts of alkyl phosphoric acids are another type of phosphorus compound that displays antioxidant properties [53,54].

1.5

SULFUR–PHOSPHORUS COMPOUNDS

The identification of sulfur and phosphorus compounds as powerful antioxidants for protection of hydrocarbons has led to the development of oil-soluble antioxidants, having both elements in one molecule. Numerous patents have been issued on such compositions, and a considerable number have been used commercially [60–67]. In fact, antioxidants containing both sulfur and phosphorus are usually more effective and efficient in a wider variety of base stocks than those containing only phosphorus or sulfur. Many commercial oils have employed one kind or other of these sulfur–phosphorus-type additives. One widely used class of sulfur–phosphorus additive is the metal dialkyldithiophosphates, which are typically prepared by the reaction of phosphorus pentasulfide with alcohols to form dithio-phosphoric acids, followed by neutralization of the acids with an appropriate metal compound. Many types of alcohols such as the aliphatic, cyclic [62], and phenolic derivatives have been used, and those of relatively high molecular weight (such as lauryl, octyl, cyclohexyl, methyl cyclohexyl alcohols, and amyl [65] or butyl phenols) are preferred to give sufficient thermal stability to the final products while rendering sufficient solubility in oils. For the second-step reaction, zinc,

Antioxidants

9 S 2 ROH + P2S5

2 RO

SH + H2S

RO S 2 RO

S SH + ZnO

Zn + H2O

RO 2

FIGURE 1.3

Synthesis of ZDDP.

barium, molybdenum, or calcium oxides are usually chosen. For more than 60 years, zinc salts of dialkylthiophosphoric acids (ZDDP) have been one of the most cost-effective antioxidants and therefore have been included as a key component in many oxidation inhibitor packages for engine oils and transmission fluids. In addition, ZDDPs show good antiwear properties, especially in the valve train area owing to the formation of sulfide and phosphate films through corrosive reactions on metal surfaces. These films can also provide protection against corrosive attack from the organic acids formed during the oxidation process. The salts of C4/C5 dialkyldithiophosphoric acid are the most common, but a broad range of other alkyl and aryl derivatives have been developed to meet special needs, for instance, protection at higher temperatures. The reaction scheme of making ZDDP is shown in Figure 1.3. A number of patents describe modifications to the first step of the reactions shown in Figure 1.3; by conducting preliminary condensation reaction of phosphorus pentasulfide with unsaturated organic compounds such as terpenes, polybutenes, wax olefins, fatty acids, fatty esters, sperm oil, and so on to form high-molecular-weight intermediate products [68–89]. During these reactions, hydrogen sulfide is liberated, and the intermediates are usually acidic. The mechanism of the P2S5 reaction with olefins in these cases may be one of substitution (replacement of reactive hydrogen atoms) as well as of addition. In preparing the final additives, these acidic intermediates were neutralized by the treatment with alkaline earth oxides or hydroxides to form metal salts. The calcium, barium, or potassium salts are the most preferred products. Some additives may also display detergency characteristics. The concept of conducting preliminary condensation reactions provides a facile route to the synthesis of a wide variety of products from the reaction of phosphorus pentasulfide and an unsaturated organic moiety. Several of these, particularly the terpene and polybutene reaction products, have been used extensively in commercial applications. To reduce the staining effect of ZDDP on metal parts (especially copper), addition of alkyl or aryl phosphites during the synthesis has been attempted [90]. For example, triphenyl phosphite is added to the dialkyldithiophosphoric acid and heated at 110ºC for an hour before the addition of zinc oxide. In another patent, a novel dithiophosphate with improved oxidation stability is described [91]. An acid is reacted with a glycol, to give a monoester having a hydroxyl group, which is then reacted with P2S5 to give the dialkyl dithiophosphoric acid. Zinc oxide is subsequently added to give the novel dithiophosphates. To improve solubility, the salts can be made of lower dialkyl dithiophosphates by utilizing both primary and secondary alcohols, including butyl alcohols in the process [92]. Mixed metal salts of dialkyl dithiophosphoric acids and carboxylic acids are claimed to have higher thermal stability [93]. Many descriptions have recently appeared of organomolybdenum phosphorodithioate complexes that impart excellent oxidation stability to lubricants. In certain circ*mstances, oil-soluble molybdenum compounds are preferred additives owing to their multifunctional characteristics such as antiwear, EP, antioxidant, antipitting, and antifriction properties. For instance, several molybdenum dialkylphosphorodithioate complexes with varying alkyl chain length of amyl, octyl, 2-ethylhexyl, and isodecyl were reported to exhibit appreciable antioxidation, antiwear,

Lubricant Additives: Chemistry and Applications

and antifriction properties [94]. Novel trinuclear molybdenum dialkyldithiophosphates prepared by reacting an ammonium polythiomolybdate and an appropriate bis(alkyldithiophosphoric) acid possess excellent antioxidant as well as antiwear and friction-reducing properties [31]. Some molybdenum compounds have been used commercially in engine oils and metal working fluids as well as in various industrial and automotive lubricating oils, greases, and specialties [95]. The combination of ZDDP with a molybdenum-containing adduct, prepared by reacting a phosphosulfurized polyisoalkylene or alpha olefin with a molybdenum salt, has been described [96]. In this case, the molybdenum adduct alone gave poor performance in oxidation tests, but the mixture with ZDDP provided good oxidation stability. Novel organomolybdenum complexes prepared with vegetable oil have been identified as synergist with ADPAs and ZDDPs in lubricating oils [97]. Owing to increasing concerns on the use of metal dithiophosphates that are related to toxicity, waste disposal, filter clogging, pollution, etc., there have been extensive research activities on the use of ashless technologies for both industrial and automotive applications. A number of ashless compounds based on derivatives of dialkylphorphorodithioic acids had been reported as multifunctional additives. Upon reacting diisoamylphosphorodithioic acid with various primary and secondary amines, eight alkylamino phosphorodithioates with varying chain length from C5 to C18 were obtained and found to possess excellent antiwear and antioxidant properties as compared to ZDDP [98]. Alkylamino phosphorodithioates obtained from reacting heptylated or octylated or nonylated phosphorodithioic acids with ethylene diamine, morpholine, or tert-alkyl (C12–C14) amines have been demonstrated to impart similar antioxidant and antiwear efficacy and superior hydrolytic stability over ZDDP [99]. Phosphorodithioate ester derivatives containing a HP moiety are also known to have antioxidant potency. This type of chemistry can be obtained by reacting metal salts of phosphorodithioic acids with HP halides [100] or with HP aldehydes [101]. Substituting the phenol aldehydes with hindered cyclic aldehydes, in which the carbon atom attached to the carbonyl carbon contains no hydrogen atoms, may also result in products having excellent antioxidant and thermal stability characteristics [102].

1.6 AMINE AND PHENOL DERIVATIVES Oil-soluble organic amines and phenol derivatives such as pyrogallol, gallic acid, dibutylresorcinol, hydroquinone, diphenylamine, phenyl-alpha-naphthylamine, and beta-naphthol are early examples of antioxidants used in turbine oils and lubricating greases [103,104]. In engine oils, these types of compounds showed only limited effectiveness. Other amines and phenol derivatives such as tetramethyldiaminodiphenylmethane and alizarin were used to some degree, rarely alone, but more often in combination with other types of antioxidants. For example, a mixture of a complex amine with a phosphorus pentasulfidepolybutene reaction product has been reported [105]. Another reported mixture is a complex phenol derivative such as alizarin in combination with an alkyl phenol sulfide and a detergent additive [106]. As technology advances, numerous amine and phenol antioxidants have been invented, and many of them have become the most widely used antioxidants in the lubricant industry.

1.6.1

AMINE DERIVATIVES

ADPAs are one of the most important classes of amine antioxidants being used today. Owing to their higher reactivity over the unsubstituted diphenylamine, ADPAs have been workhorse antioxidants for engine oils and various industrial lubricants for more than two decades. Figures 1.3 and 1.4 illustrate the typical synthesis routes of some commonly used ADPAs. The reactions start with benzene, which is first converted into nitrobenzene [107], followed by a high-temperature reduction to aniline [108]. Under very high-temperature (400–500°C) and high-pressure (50–150 psi) conditions, aniline can undergo a catalytic vapor-phase conversion to form diphenylamine [109].

Antioxidants

NO2 + H2 Catalyst

+ HNO3 Benzene

Nitrobenzene

NH2

Catalyst

Aniline

N Diphenylamine

H N

C8 olefin

C4, C8

FIGURE 1.4

C9 olefin C9

C4 + C8 olefin C4, C8

C8 olefin + styrene C8, St

H N

C8, St

Synthesis routes of ADPA antioxidants.

To make ADPAs, diphenylamine is reacted with an appropriate alkylating agent such as alcohol, alkyl halide, aliphatic carbonyl compound, or an olefin. The olefins are preferred for economic reason. The most commonly used are isobutylene (C4), diisobutylene (C8), nonenes (C9), styrene, and propylene tetramer (C12). Depending on the acidic catalyst, olefin, and other reaction conditions, for instance, the temperature, the degree of alkylation will vary from mono- to di-alkylation. Mono-ADPA is generally more effective than the corresponding disubstituted on a weight basis because additional alkylation substantially reduces the number of moles of diphenylamine per weight unit. However, in practice, obtaining monosubstituted diphenylamine in relatively pure format is difficult because as soon as the diphenylamine is monoalkylated, it quickly proceeds to dialkylation. Attempt in the preparation of high content of mono-ADPAs has led to the use of novel clay catalyst with greater selectivity in alkylation reactions and C6 –C18 linear olefins to produce high levels (at least 50 wt%) of mono-ADPAs with lower levels of dialkyl diphenylamines and undesirable unsubstituted diphenylamine [110]. Alkyl groups of six or more carbon of mono-ADPA tend to render the material lower yellow color and higher resistance to discoloration [111]. It was found that monosubstituted diphenylamines more readily oligomerize under various conditions to produce higher-molecular weight, linear oligomers. Oligomers with 2–10 degrees of polymerization are desirable antioxidants especially for high-temperature applications. Disubstituted and polysubstituted diphenylamines, however, are more restricted from forming oligomers higher than dimers. Oligomeric versions of monosubstituted diphenylamine prepared from reacting diphenylmine with C4 –C16 olefins have been described for use in ester lubricants [112]. The products are claimed to be more effective than simple diphenylamines for extremely high-temperature applications. hom*o-oligomers of alkylated (C4 –C8) diphenylamines, styryenated diphenylamines,

Lubricant Additives: Chemistry and Applications

or cross-oligomers of the ADPAs with substituted N-phenyl-α(β)-naphthylamine (PNA) are claimed to possess superior antioxidant efficacy in synthetic ester lubricants for high-temperature applications [113]. Oligomeric products derived from thermal and chemical condensation of ADPA and alkylated PNA in the presence of aldehyde can provide high performance and nonsludging attributes, as evident in the rotating pressure vessel oxidation test (RPVOT, ASTM D 2272) and the ASTM D 4310 sludging tendency test designed for turbine oils [114]. There appears to be a great number of patenting activities on the process of using isobutylene derivatives as alkylating agents. Under certain mole ratio range, diphenylamine can be reacted with diisobutylene at a temperature of 160°C or higher to facilitate chain scission of diisobutylene [115]. In the presence of an acid clay catalyst, the resulting product has <25% of 4,4′-dioctyl diphenylamine, which yields a liquid at room temperatures. In another process that involves twostep reactions [116], a light-colored, liquid product is obtained by first reacting diphenylamine with diisobutene, followed by reaction with a second olefin, preferably isobutene. Specific mole ratio, reaction temperature, and reaction duration are critical to obtain the desired ADPAs. To obtain higher levels (>50 wt%) of monosubstituted diphenylamine content in the final product, diisobutylene is allowed to react at a lower temperature range of 105–157°C in the presence of a clay catalyst. By carefully controlling mole ratio of the reactants together with reaction duration, the process, as disclosed, selectively results in a higher proportion of mono-ADPA and a lower proportion of unsubstituted diphenylamine and disubstituted or polysubstituted diphenylamines [90,117]. U.S. Patent 6,355,839 [118] discloses a one-step process using highly reactive polyisobutylene oligomers having an average molecular weight of ~160 to 280 and at least 25% of 2-methylvinylidene isomers as the alkylating agents to make ADPAs and other types of alkylated diarylamine. The resulting products are liquid at ambient temperatures. Several antioxidant patents based on alkylation of benzotriazole compounds have been issued. One particular benefit of using this class of antioxidant over the ADPAs is their additional activity in the reduction of copper corrosion. Examples are N-t-alkylated benzotriazoles obtained by reacting a benzotriazole with an olefin such as diisobutylene [119], and the reaction products of a benzotriazole with an alkyl vinyl ether or a vinyl ester of a carboxylic acid such as vinyl acetate [120]. Antioxidant and antiwear properties were reported for benzotriazole adducts of an amine phosphate [121] or an organophosphorodithioate [122]. The former type also exhibited rust prevention characteristics in the ASTM D 665 corrosion test. Aromatic diamines are a broad group of aminic antioxidants suitable for lubricants. 3,5diethyltoluenediamines with the amino moieties being located on the 2,4 and 2,6 positions relative to the methyl group have been claimed to be effective in the prevention of oil viscosity increase and acid buildup [123]. The additives are relatively noncorrosive to copper and lead bearings and are compatible with seals at high temperatures and pressures. Substituted benzylamines or substituted 1-amino-1,2,3,4-tetrahydronaphthalene is particularly useful for synthetic lubricants such as polyalphaolefins (PAOs) or polyol esters. Oils bearing these additives demonstrate very low metal corrosion, low viscosity increase, and low sludge buildup [124]. N,N′-diphenyl-p-phenylenediamines in which the phenyl groups may be substituted with methyl, ethyl, or methoxy have been claimed as effective antioxidants [125]. A broader range of substituted p-phenylenediamines has been claimed for crankcase lubricating oils for use in environments where iron-catalyzed oxidation reactions can take place [126]. 2,3-Dihydroperimidines that are prepared from the condensation of 1,8-diaminonaphthalenes with ketones or aldehydes show good oxidation inhibition in the RPVOT (ASTM D 2272). Synergistic behavior of the amines was also observed when an appropriate phenolic antioxidant is present [127]. Oils containing N,N′disubstituted-2,4-diaminodiphenyl ethers and imines of the same ethers have shown low viscosity increase, low acid buildup, and reduced metal corrosion in bench tests [128,129]. The reaction product of a hydrocarbyl succinic anhydride and 5-amino-triazole demonstrated antioxidant efficacy in a railway diesel oil composition [130].

Antioxidants

1.6.2

PHENOL DERIVATIVES

Phenols, especially the sterically hindered phenols are another class of antioxidants being extensively used in industrial and automotive lubricating oils and greases. Based on the chemical structure, phenols may be customarily categorized into simple phenols such as 2,6-di-tert-4-methylphenol (also known as BHT) and complex phenols that are typically in polymeric forms having molecular weights of 1000 or higher. The structures, important physical properties, and typical applications of some commonly used HPs are given in Table 1.2. Similar to the alkyl phenol sulfides discussed earlier, the combinations of HPs and sulfur chemistry have been widely reported. For example, the reaction products of simple phenols such as the 2,6-di-tert-butylphenol listed in Table 1.2 with selected thioalkenes have shown effectiveness in the prevention of acid buildup and oil viscosity increase, without causing lead corrosion [131]. Another patent describes a process for preparing hydrocarbylthio-HPs by reacting substituted phenols with hydrocarbyl disulfides using an aluminum phenoxide catalyst [132]. Using a 4,4′-methylene bis(2,6di-tert-butylphenol) as reference, the thiophenols were found to be superior in bulk oil oxidation tests and bench corrosion test on bearings. High oligomeric phenolic antioxidants in the form of hindered and sulfur bridged have been developed [133]. These compounds have lower volatility, better thermal stability, and improved seal compatibility and corrosion properties. In general, sulfurbridged HPs are more effective than the conventional phenolics under high-temperature oxidation conditions and are considered particularly suitable for the lubricants formulated with highly refined base stocks [134]. Figure 1.5 shows structures of some commercial sulfur-bridged HPs that have found use in various lubricant formulations. Thioalkene-bridged hemi-HPs prepared from catalytic reaction of HP with thioalkene have also been reported to be active in the stabilization of mineral oils and synthetic oils [135].

1.6.3

AMINE AND PHENOL-BEARING COMPOUNDS

Given the high popularity and effectiveness of amine and phenol derivatives as lubricant antioxidants, the combination of amine and phenolic moieties in one molecule represents a logic approach to enhance performance. In a prior art [136], fusing amine with a long carbon chain 3,5di-tert-butyl-4-hydroxyphenalkyl group that separates the phenol group from the amino nitrogen leads to novel products with lower volatility, better thermal stability, and higher solubility in oils. Nelson and Rudnick [137] reacted an ethyoxylated alkyl phenol with an alkyl arylamine in the presence of an aldehyde. The resulting product had improved antioxidant potency owing to a synergistic action between the phenolic moiety and the amine, and also showed enhanced solubility in oils owing to the presence of alkylated aromatic moiety in the molecule. Phenolic imidazolines have been prepared from polyaminophenols and carbonyl compounds [138]. In addition to providing antioxidant activity, the products also have corrosion inhibition and metal deactivation properties owing to the cyclic imidazoline moiety. Multifunctional additives containing sulfur, nitrogen, and phenolic moieties in one molecule have been reported. In this instance, mercaptobenzothiazoles or thiadiazoles are Mannich reacted with HP antioxidants to yield oil-soluble compounds with antioxidant and antiwear properties [139]. More complex product having similar functionalities was obtained by reacting a sulfur-containing HP ester with an ADPA [140].

1.6.4

MULTIFUNCTIONAL AMINE AND PHENOL DERIVATIVES

The industry-wide trend in the reduction of phosphorus and sulfur, in particular, ZDDP in finished lubricants has led to increasing activities in the development of novel multifunctional additives that have combined properties of antioxidancy, antiwear, and to some extent dispersancy, while having low-to no-sulfur and phosphorus contents. It has been shown that products obtained from

(3,5-Di-tert-butyl-4hydroxyhydrocinnamate)methane

Octadecyl 3-(3′,5′-di-tert-butyl-4′hydroxyphenyl)propionate

2,6-Di-tert-butyl-4-methylphenol (BHT)

2,6-Di-tert-butyl-phenol

Phenols

Structure

OC18H37

OCH2

50–55

110–125

36–37

Melting Point (°C)

0.1–0.5

Solubility (in mineral oils)

TABLE 1.2 Structure, Physical Properties, and Typical Applications of Commercial Hindered Phenols for Lubricants

Industrial oils, eco-friendly oils

Industrial oils, food grade lubricants, greases

Industrial oils, power transmission fluids, food grade lubricants, greases

Industrial oils, power transmission fluids, greases, fuels

Applications

14 Lubricant Additives: Chemistry and Applications

2-Propenoic acid, 3-[3,5-bis(1,1dimethylethyl)-4-hydroxyphenyl]-1,6hexanediyl ester

2,2′-Methylene bis(4-methyl-6-tert-butylphenol)

3,5-Di-tert-butyl-4-hydrocinnamic acid, C13–C15 alkyl ester

4,4′-Methylene bis(2,6-di-tert-butylphenol)

3,5-Di-tert-butyl-4-hydroxyhydrocinnamic acid, C7–C9 alkyl ester

CH2

O C7 − C9

C6H12

OC13 − C15

105

128

154

Liquid at 25°C

2–5

Industrial oils, greases, food grade lubricants

Engine oils, industrial oils, greases

Engine oils, industrial oils, food grade lubricants, greases

Engine oils, power transmission fluids, industrial oils

Antioxidants 15

Lubricant Additives: Chemistry and Applications O

OH S

HO 2

FIGURE 1.5

CH3

Examples of commercial sulfur-bridged phenolic antioxidants.

reacting alkyl or alkenyl succinic acid anhydride with an appropriate amine may impart such multifunctionalities. Product made by reacting a polyalkenylsuccinic acid or anhydride first with an aromatic secondary amine, then with an alkanol amine, was found to provide appreciable antioxidancy, dispersancy, and anticorrosion effects to engine oils as tested in a Caterpillar engine test [141]. A more recent U.S. Patent literature [142] discloses materials made from the reaction of alkyl or alkenyl succinic acid derivative with a diamino naphthyl compound for use as antioxidant, antiwear, and soot dispersing agents for lubricating oils. By fusing a HP moiety to an alkenyl succinimide domain, a novel dispersant having antioxidant property was obtained [143]. The product improved the performance of engine oils in the sequence VG, an industry recognized sludge test to evaluate the ability of a lubricant in preventing the formation of sludge and varnish deposits in a fired engine. U.S. Patent 5,075,383 [144] describes novel antioxidant–dispersant additives obtained by reacting amino-aromatic polyamine compound, including aromatic secondary amines, with ethylene–propylene copolymer grafted with maleic anhydride. Engine oils containing the additives displayed improved performance characteristics in laboratory oxidation and sludge dispersancy tests, as well as in the sequence VE and the MWM-B engine tests.

1.7 COPPER ANTIOXIDANTS The ability of copper compounds to function as oxidation inhibitors has been of interest to the lubricant industry for years. Copper is usually considered to be an oxidation promoter, and its presence is of a concern in lubricants such as power transmission oils, where fluid contact with copper-containing bearings and sintered bronze clutch plates takes place [145]. It has been suggested that copper corrosion products, originating from surface attack of copper metal, are generally catalysts that accelerate the rate of oxidation [146], whereas oil-soluble copper salts are antioxidants [147]. To maximize the full antioxidant strength of a copper compound, the initial concentration needs to be maintained at an optimum range, normally from 100 to 200 ppm [145,147]. Below this range, the antioxidant effect of the copper compounds will not be fully realized, whereas above the range, interference with antiwear additives may occur, leading to pronounced increase in wear on high-stress contact points [148]. Examples of oil-soluble copper antioxidants developed in early years were a group of copper– sulfur complexes, obtained by sulfurizing certain types of unsaturated hydrocarbons in the presence of copper [149–151]. A more recent patent describes lubricant compositions that are stabilized with a zinc hydrocarbyl dithiophosphate (ZDDP) and 60–200 ppm of copper derived from oil-soluble copper compounds such as copper dihydrocarbyldithiophosphate or copper dithiocarbamates [148]. Oxidation data are given for fully formulated engine oils containing the ZDDP and various supplemental antioxidants including amines, phenolics, a second ZDDP, and copper salts. Only the blends with copper salts passed the oxidation test. With the other additives, the viscosity increase was excessive. Organo-copper compounds including copper naphthenates, oleates, stearates, and polyisobutylene succinic anhydrides have been reported to be synergistic with multiring aromatic compounds in controlling high-temperature deposit formation in synthetic base stocks [147].

Antioxidants

More complex compounds obtained from further reactions of copper salts have also been reported to be effective antioxidants in various lubrication applications. For example, copper carboxylate or copper thiocyanate was reacted with a mono-oxazoline, bis-oxazoline, or lactone oxazoline dispersant to form coordination complexes, wherein the nitrogen contained in the oxazoline moiety is the ligand that complexes with copper. The resulting products exhibit improved varnish control and oxidation inhibition capabilties [152]. Reaction products of a copper salt (acetate, carbonate, or hydroxide) with a substituted succinic anhydride derivative containing at least one free carboxylic acid group are effective high-temperature antioxidants and friction modifiers. When incorporated in an engine oil formulation, the oil passed rust, oxidation, and bearing corrosion engine tests [153]. In another patent [154], a HP carboxylic acid was used as the coupling reagent. The resulting copper compounds are reported to be effective in the controls of high-temperature sludge formation and oil viscosity increase when used alone or in synergistic mixtures with a conventional aminic or phenolic antioxidant.

1.8 BORON ANTIOXIDANTS The search for more eco-friendly additives to replace ZDDP has led to renewed interest in boron esters owing to their ability to improve antioxidation, antiwear, and antifriction properties of lubricants when used alone or in combination with other additives. The complex tribological behavior of boron compounds in formulated lubricants depends on their particular chemical structures and the interactions between boron and other active elements such as sulfur, phosphorus, nitrogen, or their combinations when present [155,156]. A number of boron–oxygen-bearing compounds have been reported to be effective oxidation inhibitors in terms of prevention of oil viscosity increase and acid formation at elevated temperature (163°C) [157–161]. Representatives are boron epoxides (especially 1,2-epoxyhexadecane) [157], borated single and mixed alkanediols [158], mixed hydroquinone-hydroxyester borates [159], phenol esters of hindered phenyl borates [160], and reaction products of boric acid with the condensates of phenols with aromatic or aliphatic aldehydes [161]. Borate esters with nitrogen are known for their antioxidant activity and improved antiwear properties probably due to the formation of additional boron nitride film on rubbing surface [162]. Borated adducts of alkyl diamines with long-chain hydrocarbylene alkoxides and low-molecularweight carboxylic acids have been reported to have antifriction properties and high inhibition ability especially at elevated temperatures [163]. Appreciable oxidation inhibition effect has also been reported for borate esters of hydrocarbyl imidazolines [164], borates of mixed ethyoxyamines and ethoxyamides [165], and borates of etherdiamines [166]. Synergistic antioxidant effect of borate esters with ADPAs or with zinc dithiophosphates has been established. When tested at 180°C in a PAO using a pressurized differential scanning calorimetry (PDSC), strong synergistic antioxidant action was observed between borate esters and a dioctyl diphenylamine at a 1:1 (w/w) blending ratio [167]. Similar effect was observed in the mixtures of borate esters and a ZDDP [155]. The synergism with ZDDP is of practical importance as it allows reduced phosphorus level in a finished lubricant without sacrifice of oxidative stability. The catalytic effect of boron in enhancing antioxidant performance has led to the development of phenolic-phosphorodithioate borates, obtained from coborating HP and alkyl phosphorodithioatederived alcohol. The borates were found to possess exceptional antioxidant and antiwear properties. Both the HP moiety and the phosphorodithioate alcohol moiety were believed to provide the basis for the synergy each of which are subsequently enhanced by the integral boron coupling moiety [168]. Despite many tribological and antioxidation benefits that borate esters can offer, large use of the chemistry for lubricant applications has not taken place. One serious drawback with most borate esters has been their high susceptibility to hydrolysis, a process that liberates oil-insoluble and

Lubricant Additives: Chemistry and Applications

abrasive boric acid. Following attempts have been made to address the issue with varying degrees of success: 1. Incorporation of HP moiety to sterically inhibit the boron–oxygen bonds from hydrolytic attack. Commonly used HPs are 2,6-dialkyl phenols [169], 2,2′-thiobis(alkylphenols) and thiobis(alkylnaphthols) [170]. 2. Incorporation of amines that have nonbonding pairs of electrons. The amines coordinate with the electron-deficient boron atom, thus preventing hydrolysis. U.S. Patents 4,975,211 [171] and 5,061,390 [172] disclose the stabilization of borated alkyl catechol against hydrolysis by complexing with diethylamine. Significant improvement in hydrolytic stability was reported for borate esters incorporated with a N,N′-dialkylamino-ethyl moiety [156]. It was hypothesized that the formation of a stable five-member ring structure in molecules involving coordination of nitrogen with boron substantially inhibited the hydrolytic attack from water. 3. Use of certain hydrocarbon diols or tertiary amine diols to react with boric acid to form stable five-member ring structures [173].

1.9 MISCELLANEOUS ORGANOMETALLIC ANTIOXIDANTS More recently, a number of oil-soluble organometallic compounds, for example, organic acid salts, amine salts, oxygenates, phenates and sulfonates of titanium, zirconium, and manganese have been claimed to be effective stabilizers for lubricants [174,175]. Some of the compounds are essentially devoid of sulfur and phosphorus, therefore, suitable for modern automotive engine oils where lower contents of the two elements are desired. In one example [174], lubricating oils having 25 to ~100 ppm of titanium derived from titanium (IV) isopropoxide exhibited excellent oxidative stability in the high-temperature (280°C) Komatsu hot tube test and ASTM D 6618 test evaluate engine oils for ring sticking, ring and cylinder wear, and the accumulation of piston deposits in a four-stroke cycle diesel engine. In another example [175], titanium (IV) isopropoxide was used to react with neodecanoic acid, glycerol mono-oleate, or polyisobutenyl bis-succinimide to form respective titanated compounds. These compounds, when top-treated in a SAE 5W30 engine oil to result in 50 to ~800 ppm of titanium in oil, improved the deposit control capability of the oil as tested by using the TEOST (ASTM D 7097). Similar antioxidant effect was observed for neodecanoates of zirconium and manganese in the same oil. Oil-soluble or dispersible tungsten compounds, more specifically, amine tungstates and tungsten dithiocarbamates, have been attempted as antioxidants for lubricants and found to be synergistic with secondary diarylamine and alkylated phenothiazines. The mixtures, when added to an engine crankcase lubricant to result in ~20 to 1000 ppm of tungsten, were highly effective in controlling oil oxidation and deposit formation [176]. Sulfur-free molybdenum salts such as molybdenum carboxylates have been attempted as antioxidants and found to be synergistic with ADPAs in lubricating oils [177,178]. The synergistic mixtures improved oxidation stability of crankcase lubricants while providing additional friction modification characteristics.

1.10

MECHANISMS OF HYDROCARBON OXIDATION AND ANTIOXIDANT ACTION

It is now understood that oxidation of hydrocarbon-based lubricants undergoes autoxidation, a process that leads to the formation of acids and oil thickening. To a more severe extent, oil-insoluble sludge and varnish may be formed, causing poor lubrication, reduced fuel economy, and increased wear.

Antioxidants

Antioxidants are essential additives incorporated in lubricant formulations to delay the onset of autoxidation and minimize its impact. The mechanisms of lubricant degradation and its stabilization by antioxidants are discussed in the following sections.

1.10.1 AUTOXIDATION OF LUBRICATING OIL The well-documented autoxidation mechanism involves a free-radical chain reaction [179–181]. It consists of four distinct reaction steps: chain initiation, propagation, branching, and termination. 1.10.1.1

Initiation 2 R ⫺ H  → R i ⫹ HOO i

(1.1)

R ⫺ R Energy → R i ⫹ R i

(1.2)

The initiation step is characterized as the formation of free alkyl radicals (R•) from the breakdown of hydrocarbon bonds by hydrogen abstraction and dissociation of carbon–carbon bonds. These reactions take place when hydrocarbons are exposed to oxygen and energy in the form of heat, UV light, or mechanical shear stress [182]. The ease of hom*olytic cleavage of an R–H bond follows this order, as determined by the C–H bond strength and the stability of the resulting radical [183]: phenyl < primary < secondary < tertiary < allylic < benzylic. Thus, hydrocarbons containing tertiary hydrogen or hydrogen in an alpha position to a carbon–carbon double bond or aromatic ring are most susceptible to oxidation. The reaction rate of chain initiation is generally slow under ambient conditions but can be greatly accelerated with temperature and the presence of catalytic transitioning metal ions (copper, iron, nickel, vanadium, manganese, cobalt, etc.). 1.10.1.2 Chain Propagation R i ⫹ O 2 → ROO i

(1.3)

ROO i ⫹ RH → ROOH ⫹ R i

(1.4)

The first propagation step involves an alkyl radical reacting irreversibly with oxygen to form an alkyl peroxy radical (ROO•). This reaction is extremely fast, and the specific rate is dependent on the radical’s substituents [179]. Once formed, the peroxy radical can randomly abstract hydrogen from another hydrocarbon molecule to form hydroperoxide (ROOH) and a new alkyl radical (R•). Based on this mechanism, each time a free alkyl radial is formed, a large number of hydrocarbon molecules may be oxidized to hydroperoxides. 1.10.1.3 1.10.1.3.1

Chain Branching Radical Formation ROOH → RO i ⫹ HO i

(1.5)

RO i ⫹ RH → ROH ⫹ R i

(1.6)

HO i ⫹ RH → H 2O ⫹ R i

(1.7)

Lubricant Additives: Chemistry and Applications

1.10.1.3.2 Aldehyde or Ketone Formation RR⬘HCO i → RCHO ⫹ R⬘ i

(1.8)

RR⬘R ⬙CO → RR⬘CO ⫹ R ⬙ i

(1.9)

The chain-branching steps begin with the cleavage of hydroperoxide into an alkoxy radical (RO•) and a hydroxy radical (HO •). This reaction has high activation energy and is only significant at temperatures >150°C. Catalytic metal ions accelerate the process. The resulting radicals will undergo a number of possible reactions: (a) the alkoxyl radical abstracts hydrogen from a hydrocarbon to form a molecule of alcohol and a new alkyl radical according to reaction 1.6, (b) the hydroxyl radical follows the pathway of reaction 1.7 to abstract hydrogen from a hydrocarbon molecule to form water and a new alkyl radical, (c) a secondary alkoxyl radical (RR′HCO•) may decompose through reaction pathway 1.8 to form an aldehyde, and (d) a tertiary alkoxy radical (RR′R″CO•) may decompose to form a ketone (reaction 1.9). The chain-branching reaction is a very important step to the subsequent oxidation state of the oil as not only will a large number of alkyl radicals be formed that expedites the oxidation process, but also the lower-molecular-weight aldehydes and ketones generated will immediately affect the physical properties of the lubricant by decreasing oil viscosity and increasing oil volatility and polarity. Under high-temperature oxidation conditions, the aldehydes and ketones can undergo further reactions to form acids and high-molecular-weight species that thicken the oil and contribute to the formation of sludge and varnish deposits. Detailed mechanisms will be discussed in Section 1.10.3. 1.10.1.4

Chain Termination R i ⫹ R⬘ i → R ⫺ R⬘

(1.10)

R i ⫹ R⬘OO i → ROOR⬘

(1.11)

As oxidation proceeds, oil viscosity will increase due to the formation of high-molecular-weight hydrocarbons. When oil viscosity has reached a level that diffusion of oxygen in oil is significantly limited, chain termination reactions will dominate. As indicated by reactions 1.10 and 1.11, two alkyl radicals can combine to form a hydrocarbon molecule. Alternatively, an alkyl radical can combine with an alkyl peroxy radical to form a peroxide. This peroxide, however, is not stable and can easily breakdown to generate more alkyl peroxy radicals. During the chain-termination processes, formation of carbonyl compounds and alcohols may also take place on the peroxy radicals that contain an extractable α-hydrogen atom: RR⬘CHOO i ← → RR⬘CHOOOOHCR⬘R ⫺O

(1.12)

2  → RR⬘C ⫽ O ⫹ RR⬘CH ⫺ OH

1.10.2 METAL-CATALYZED LUBRICANT DEGRADATION Metal ions are able to catalyze the initiation step as well as the hydroperoxide decomposition in the chain-branching step [184] through a redox mechanism illustrated in the following section. The required activation energy is lowered for this mechanism, and thus, the initiation and propagation steps can commence at much lower temperatures.

Antioxidants

1.10.2.1 Metal Catalysis 1.10.2.1.1

Initiation Step M(n⫹1)⫹ ⫹ RH → M n⫹ ⫹ H⫹ ⫹ R i

(1.13)

M n⫹ ⫹ O2 → M(n⫹1)⫹ ⫹ O⫺2

(1.14)

M(n⫹1) ⫹ ROOH → M n⫹ ⫹ H⫹ ⫹ ROO i

(1.15)

M n⫹ ⫹ ROOH → M(n⫹1)⫹ ⫹ HO⫺ ⫹ RO i

(1.16)

1.10.2.1.2 Propagation Step

1.10.3 HIGH-TEMPERATURE LUBRICANT DEGRADATION The preceding discussion provides the basis for the autoxidation stage of lubricant degradation under both low and high-temperature conditions. The end result of low-temperature oxidation is the formation of peroxides, alcohols, aldehydes, ketones, and water [185,186]. Under high-temperature oxidation conditions (>120°C), breakdown of peroxides including hydroperoxides becomes predominant, and the resulting carbonyl compounds (e.g., reactions 1.8 and 1.9) will fi rst be oxidized to carboxylic acids as shown in Figure 1.6. As an immediate result, the oil acidity will increase. As oxidation proceeds, acid or base-catalyzed Aldol reactions take place. The reaction mechanism is illustrated in Figure 1.7 [187]. Initially, α,β-unsaturated aldehydes or ketones are formed, and further reaction of these species leads to high-molecular-weight products. These products contribute to oil viscosity increase and eventually can combine with each other to form oil-insoluble polymeric products that manifest as sludge in a bulk oil oxidation environment or as varnish deposits on hot metal surface. Oil viscosity increase and deposit formation have been identified to be the principal oil-related factors to engine damages [188].

1.10.4 EFFECT OF BASE STOCK COMPOSITION ON OXIDATIVE STABILITY Mineral base stocks used to formulate lubricants are hydrocarbons that are originated from crude oils and essentially contain mixtures of n-paraffins along with isoparaffins, cycloparaffins (also called naphthenes), and aromatics having about 15 or more carbon atoms [189]. In addition, small amounts of sulfur-, nitrogen-, and oxygen-containing species may be present depending on the refinery techniques employed. In the American Petroleum Institute (API) base oil classification system, mineral oils largely fall into the groups I, II, III, and V, with some distinctions shown in Table 1.3 in terms of saturates, sulfur contents, and viscosity index. Group I base oils still dominate the base oil market, accounting for more than 50% of global capacity. Groups II and III base stocks

O R

ROO• −ROOH

C•

O 2R

O O2

• OO RH• −R

O OOH

OH + O2

FIGURE 1.6 High-temperature (>120°C) lubricant degradation leading to the formation of carboxylic acids.

Lubricant Additives: Chemistry and Applications O

(CH2)n

CH + H3C

(CH2)nCH

Acid or base

OR′

O R

CHCOR' + "R

(CH2)nCH

CHCOR′ + H2O

O (CH2)p

COR"' −H2O O

O R

Sludge precursors "R

C(CH2)n−1CH

(CH2)p

CHCOR′

COR"'

FIGURE 1.7 High-temperature (>120°C) lubricant degradation leading to the formation of high-molecularweight hydrocarbons.

TABLE 1.3 API Base Oil Categories API Category

Percent Saturates

Percent Sulfur

Viscosity Index

Group I Group II Group III Group IV Group V

≤90 ≥0.03 ≥80 and ≤120 ≥90 ≤0.03 ≥80 and ≤120 ≥90 ≤0.03 ≥120 PAOs Includes all other base oils not included in the first four groups

are on the horizon, and their use is expected to grow in large scale in the coming future, especially after the completion of nearly a dozen new group II/III oil refinery plants worldwide [190]. It has been widely recognized that base oil composition, for example, linear and branched hydrocarbons, saturates, unsaturates, monoaromatics, polyaromatics, together with traces of nitrogen-, sulfur-, and oxygen-containing heterocycles, etc., plays an important role in the oxidative stability of the oil. There have been quite extensive research activities attempting to establish correlations between base stock composition and oxidative stability [191–195]. However, owing to the large variations in the origin of the oil samples, the test methods, test conditions, and the performance criteria employed, the conclusions are not always consistent and in some cases contradictory to each other. In general, it has been agreed that saturated hydrocarbons are more stable than the unsaturated toward oxidation. Of the different saturated hydrocarbons found in mineral oils, paraffins are more stable than cycloparaffins. Aromatic compounds, due to their complex and large variation in the chemical makeup, play a more profound role. Monocyclic aromatics are relatively stable and resistant to oxidation, whereas bi and polycyclic aromatics are unstable and susceptible to oxidation [196]. Alkylated aromatics oxidize more readily due to

Antioxidants

the presence of highly reactive benzylic hydrogen atoms. Kramer et al. [193] demonstrated that the oxidative rate of a hydrocracked 500N base oil doubled when the aromatic content increased from 1 to 8.5 wt%. Naturally occurring sulfur compounds are known antioxidants for the inhibition of the early stage of oil oxidation. Laboratory experiments have shown that mineral oils containing as little as 0.03% of sulfur had good resistance to oxidation at 165°C over sulfur-free white oils and PAOs [145]. In hydrocracked oils that are essentially low in aromatics, better oxidative stability was found with elevated sulfur concentration (>80 ppm) versus a level at 20 ppm or lower [192]. It has been proposed that sulfur compounds act as antioxidants by generating strong acids that catalyze the decomposition of peroxides through a nonradical route or by promoting the acid-catalyzed rearrangement of arylalkyl hydroperoxides to form phenols that are antioxidants [145,179]. Contrary to sulfur, nitrogen-bearing compounds, especially the heterocyclic components (also called “basic nitrogen”), accelerate oil oxidation even at relatively low concentrations [197]. In highly refi ned groups II and III base stocks that are essentially devoid of heteroatom-containing molecules, aromatic and sulfur contents are considered as the main factors which influence the base oil oxidative stability [192,193]. It has been shown that oxidative stability of a given base stock can be enhanced when the combinations and concentrations of base stock sulfur and aromatics are optimized [194].

1.10.5 OXIDATION INHIBITION The proceeding mechanistic discussion makes clear several possible counter measures to control lubricant oxidation. Blocking the energy source is one path. However, this is only effective for lubricants used in low shear and temperature situations. A more practical approach for most lubricant applications is the trapping of catalytic impurities and the destruction of alkyl radicals, alkyl peroxy radicals, and hydroperoxides. This can be achieved through the use of a metal deactivator and an appropriate antioxidant with radical scavenging or peroxide decomposing functionality, respectively. The radical scavengers are known as primary antioxidants. They function by donating hydrogen atoms to terminate alkoxy and alkyl peroxy radicals, thus interrupting the radical chain mechanism of the auto-oxidation process. The basis for a compound to become a successful antioxidant is that peroxy and alkoxyl radicals abstract hydrogen from the compound much more readily than they do from hydrocarbons [198]. After hydrogen abstraction, the antioxidant becomes a stable radical, the alkyl radical becomes a hydrocarbon, and the alkyl peroxy radical becomes an alkyl hydroperoxide. HPs and aromatic amines are two main classes of primary antioxidants for lubricants. The peroxide decomposers are also called secondary antioxidants [180]. They function by reducing alkyl hydroperoxides in the radical chain to nonradical, less-reactive alcohols. Organosulfur and organophosphorus compounds and those containing both elements, such as ZDDPs, are well-known secondary antioxidants. Since transitional metals are present in most lubrication system, metal deactivators are usually added to lubricants to suppress the catalytic activities of the metals. Based on the functioning mechanisms, metal deactivators for petroleum products can be classified into two major types: chelators [180] and surface passivators [199]. The surface passivators act by attaching to metal surface to form a protective layer, thereby preventing metal–hydrocarbon interaction. They can also minimize corrosive attack of metal surface by physically restricting access of the corrosive species to the metal surface. The chelators, however, function in bulk of the lubricant by trapping metal ions to form an inactive or much less-active complex. With either mechanism, metal deactivators can effectively slow the oxidation process catalyzed by those transitional metals, which in turn lends metal deactivators an antioxidant effect. Table 1.4 lists examples of metal deactivators that are commonly found in lubricant formulations.

Lubricant Additives: Chemistry and Applications

TABLE 1.4 Metal Deactivators for Lubricants Surface Passivators

Basic Structure N

Triazole derivative

CH2NR2

N N N Benzotriazole

N H S

2-Mercaptobenzothiazole

SH N N R1

N N

Tolyltriazole derivative

CH2N

R3 R4

Chelators CH3

H C

CH2

H N

N,N′-disalicylidene-1,2-diaminopropane OH

1.10.6 MECHANISMS OF PRIMARY ANTIOXIDANTS 1.10.6.1

Hindered Phenolics

A representative example of HP antioxidant is 3,5-di-t-butyl-4-hydroxytoluene (2,6-di-t-butyl-4methylphenol), also known as BHT. Figure 1.8 compares the reaction of an alkyl radical with BHT versus oxygen. The reaction rate constant (k2) of alkyl radical with oxygen to form alkyl peroxy radicals is much greater than that (k1) of alkyl radical with BHT [179]. Hence with an ample supply of oxygen, the probability of BHT to react with alkyl radicals is low. As oxidation proceeds with more alkyl radicals being converted to alkyl peroxy radicals, BHT starts to react by donating a hydrogen atom to the peroxy radical as shown in Figure 1.9. In this reaction, the peroxy radical is reduced to hydroperoxide, whereas the BHT is converted into a phenoxy radical that is stabilized through steric hindrance and resonance structures. The steric hindrance provided by the two butyl moieties on the ortho positions effectively prevent the phenoxy radical from attacking other hydrocarbons. The cyclohexadienone radical resonance structure can further combine with a second alkyl peroxy radical to form the alkyl peroxide, which is stable at temperatures <120°C [200]. Without resonance transformation, an alternative reaction pathway for phenoxy radicals is to combine with

Antioxidants

25 O•

OH R• +

k1 + RH

R• + O2

FIGURE 1.8

ROO •

k2 >> k1

Reactivity of BHT with alkyl radical.

O• + ROO •

+ ROOH

CH3

O + ROO •

H3C

FIGURE 1.9

OOR

H3C

•

Hydrogen donation and peroxy radical trapping mechanisms of BHT.

O•

+ CH3

FIGURE 1.10

+ CH3

CH3

CH2

Termination reaction of phenoxy radicals.

each other into a chain termination step. As depicted in Figure 1.10, a hydrogen atom is donated from one phenoxy radical to the other, thus regenerating one BHT molecule and one methylene cyclohexadienone. Under higher-temperature oxidation conditions, the cyclohexadienone alkyl peroxide previously formed is no longer stable. As illustrated in Figure 1.11, it will decompose to form an alkoxy radical, an alkyl radical, and a 2,6-di-t-butyl-1,4-benzoquinone. As can be expected, the generation of new radicals will deteriorate the overall effectiveness of the BHT under high-temperature oxidation conditions.

Lubricant Additives: Chemistry and Applications O

O Heat

CH3

+ RO • + • CH3

OOR O

FIGURE 1.11

Decomposition of cyclohexadienone alkyl peroxide at higher temperatures.

H R

FIGURE 1.12

RO•, ROO•

• N

R + ROH, ROOH

ADPA as a hydrogen donor.

• N

ROO •

O• R

R + RO •

N ROO •

O− R

N +

−ROR

O− R

N +

OOR

ROO • O• R

−RO •

O + O

ROO

FIGURE 1.13

Low-temperature function mechanism of ADPA.

1.10.6.2 Aromatic Amines A particularly effective class of aromatic amines useful as primary antioxidants is the ADPAs. The reaction of the antioxidants begins with hydrogen atom abstraction by alkyl peroxy radical and alkoxy radical, the mechanism of which is illustrated in Figure 1.12. Owing to the rapid reaction of alkoxy radicals with oxygen, the resulting alkyl peroxy radical is present at higher concentration, and its reaction with ADPA predominates. The aminyl radical formed can undergo a number of possible reaction pathways depending on temperature, degree of oxidation (relative concentration of peroxy radicals versus alkyl radicals), and the chemical nature of the ADPA [201]. Figure 1.13 shows the low-temperature (<120°C) oxidation inhibition mechanism [179,202], starting with aminyl radical attacking a second alkyl peroxy radical to form a nitroxyl radical and alkoxy radical. The nitroxyl radical is stabilized through three possible resonance structures [182,202], as illustrated in Figure 1.14. Next, a third alkyl peroxy radical reacts with the nitroxyl radical to form a nitroxyl–peroxide complex, which can further eliminate an ether molecule, forming a nitroxyl cyclohexadienone. Following reactions involve a fourth alkyl peroxy radical being added to the nitroxyl cyclohexadienone to form nitroxyl cyclohexadienone peroxide, followed by a dissociation reaction to form 1,4-benzoquinone and an alkylated nitrosobenzene. Therefore, on

Antioxidants

27 O• R

O− R

N •

O− R

FIGURE 1.14

R •

Resonance structures of nitroxyl radical.

• N

H • R C CR

R C CR R

R ROO •

Radicals

O•

H R

OH R

O R C R′

RR′IIC•

CHRR′ O R

FIGURE 1.15

ROOH

ROO •

High-temperature (>120°C) function mechanism of ADPAs.

an equal mole basis, in theory, ADPAs can quench twice as many alkyl peroxy radicals as a mono HP under low-temperature conditions. At high temperatures (>120°C), the nitroxyl radical intermediate can undergo one of two possible reaction pathways by either reacting with a secondary or a tertiary alkyl radical to form an N-sec-alkoxy diphenylamine [179,186,203] or an N-hydroxyl diphenylamine intermediate, respectively. These mechanisms are illustrated in Figure 1.15. In the former case, the resulting alkoxy amine intermediate can thermally rearrange to form a ketone and regenerate the starting ADPA. In the latter case, nitroxyl radical is regenerated upon reaction of the hydroxyl diphenylamine intermediate with an alkyl peroxy radical. Thus, at high temperatures, one molecule of ADPA can catalytically scavenge a large number of radicals before the nitroxyl

Lubricant Additives: Chemistry and Applications

radical is destroyed. It has been reported that such regeneration process can provide ADPAs with a stoichiometric efficiency of more than 12 radicals per molecule [186].

1.10.7 MECHANISMS OF SECONDARY ANTIOXIDANTS 1.10.7.1 Organosulfur Compounds Organosulfur compounds function as hydroperoxide decomposers through the formation of oxidation and decomposition products. The mechanism is illustrated in Figure 1.16 for an alkyl sulfide. The antioxidant action starts with the reduction of an alkyl hydroperoxides to a less reactive alcohol, with the sulfide being oxidized to a sulfoxide intermediate. A preferred mechanism for the subsequent reaction of sulfoxide intermediate is the intramolecular beta-hydrogen elimination, leading to the formation of a sulfenic acid (RSOH), which can further react with hydroperoxides to form sulfur-oxy acids. At elevated temperatures, sulfininc acid (RSO2H) may decompose to form sulfur dioxide (SO2), which is a particularly powerful Lewis acid for hydroperoxide decomposition through the formations of active sulfur trioxide and sulfuric acid. Previous work has shown that one equivalent of SO2 was able to catalytically decompose up to 20,000 equivalents of cumene hydroperoxide [204]. Further enhancing the antioxidancy of sulfur compounds is that, under certain conditions, the intermediate sulfur-oxy acids (RSOxH) can scavenge alkyl peroxy radicals, thus giving the sulfur compound primary antioxidant characteristics: • RSO xH ROO  → RSO x • ⫹ ROOH

1.10.7.2 Organophosphorus Compounds Phosphites are a main class of organophosphorus compounds being used as secondary antioxidants. Phosphites decompose hydroperoxides and peroxy radicals following the reaction

R2 R

ROOH −ROH

H2C

RSOH + H2C=CR1R2

O H

ROOH

−ROH

RSO2H ROOH −ROH

RSO3H

RH + SO2 ROOH

−ROH

ROSO3H H2O

Decompose more hydroperoxides

H2SO4 ROOH ROOR R2C=O

FIGURE 1.16

Antioxidation mechanism of alkyl sulfide.

Antioxidants

R′′O •

P(OR′)2 + R

O•

P(OR′)2

R′′OO •

FIGURE 1.17

R′′O

O R′′O

P(OR′)2 + R

O•

Alkoxy and peroxy scavenging mechanisms of phenyl phosphite.

mechanisms. In these reactions, phosphite is oxidized to corresponding phosphate, with the hydroperoxide and the peroxy radical being reduced to less-reactive alcohol and alkoxy radical, respectively. (RO)3 P ⫹ R⬘OOH → (RO)3 P ⫽ O ⫹ R⬘OH

(1.17)

(RO)3 P ⫹ R⬘OO • → (RO)3P ⫽ O ⫹ R⬘O •

(1.18)

Phosphites with certain substituted phenoxy groups may also behave as peroxy and alkoxy radical scavengers as shown in Figure 1.17. The resulting phenoxy radicals can further eliminate peroxy radicals upon resonance transformation to cyclohyxadienone radical as discussed earlier. Owing to the steric hindrance provided by the alkyl groups on the ortho-positions, these phosphites tend to be more stable against hydrolysis and are preferred for use in moist lubrication environment.

1.10.8 ANTIOXIDANT SYNERGISM* Antioxidant synergism describes the effect or response of a combined use of two or more antioxidants being greater than that of any individual antioxidant. Synergistic antioxidant systems offer practical solutions to problems where using a single antioxidant is inadequate to provide satisfactory results, or where the treatment level has to be limited due to economic or environmental reasons. Three types of synergy have been proposed for lubricant antioxidants [205]: (a) hom*osynergism, (b) heterosynergism, and (c) autosynergism. hom*osynergism occurs when two antioxidants acting by the same mechanism interact, generally in a single-electron-transfer cascade. A classic example is an ADPA in combination with a HP antioxidant. ADPA is initially more reactive than HP in scavenging alkyl peroxy radicals. As illustrated in Figure 1.18, the amine is first converted to an aminyl radical, which is relatively less stable and will accept a hydrogen atom from the HP to regenerate the alkylated amine [179,182]. In consequence, the HP is converted to a phenoxy radical. The driving force for this regeneration cycle to occur is the higher reactivity of the ADPA compared to the HP and the greater stability of the phenoxy radical relative to the aminyl radical [201]. After the HP is consumed, the aromatic amine antioxidant begins to deplete. By regenerating the more reactive amine, the overall effectiveness of the system is enhanced, and the useful antioxidant lifetime is extended. * With permission from Dong, J. and C.A. Migdal, Synergestic Antioxidant Systems for Lubricants. 12th Asia Fuels and Lubes Conference Proceedings, Hong Kong, 2006.

Lubricant Additives: Chemistry and Applications RO • , ROO •

ROH, ROOH

H R

FIGURE 1.18

• N

O•

Mechanism of synergism between ADPA and hindered phenol.

Heterosynergism occurs when antioxidants act by a different mechanism and hence complement each other. This type of synergy usually takes place when a primary antioxidant and a secondary antioxidant are present in one lubricant system. The primary antioxidant scavenges radicals, whereas the secondary antioxidant decomposes hydroperoxides by reducing them to more stable alcohols. Through these reactions, chain propagation and branching reactions are significantly slowed or inhibited. A representative example of a heterosynergism is an aminic antioxidant in combination with a ZDDP. Autosynergism is a third type of synergistic response that results from two different antioxidant functions in the same molecule. Usually, antioxidants having functional groups that provide radical scavenging and hydroperoxide decomposing functions exhibit autosynergy. Examples of this type of antioxidants are sulfurized phenols and phenothiazines.

1.11 OXIDATION BENCH TESTS Oxidative degradation of lubricants can be classified into two main reactions: bulk oil oxidation and thin-film oxidation. Bulk oil oxidation usually takes place in a larger oil body at a slower rate. The exposure to air (oxygen) is regulated by the surface contact kinetics, and the gas diffusion is limited. The oxidation leads to increases in oil acidity, oil thickening, and, to a more severe extent, oil-insoluble polymers that may manifest as sludge when mixed with unburned/oxidized fuel components, water, and other solids. Thin-film oxidation describes a more rapid reaction in which a small amount of oil, usually in the form of a thin-film coating on metal surface, is exposed to elevated temperatures and air (oxygen). Under these conditions, hydrocarbons decompose much more quickly and the polar oil oxidation products formed at the oil–metal interface can rapidly build up on the metal surface, leading to the formation of lacquer or deposits. Over the years, many oxidation bench tests have been developed and proven to be valuable tools for lubricant formulators, particularly in the screening of new antioxidants and the development of new formulations. Most bench tests attempt to simulate the operating conditions of more expensive engine and field tests, when taking into consideration the oxidation mechanisms described earlier. In addition, a third mechanism based on oxygen uptake in a closed system has been employed in some bench tests, such as the RPVOT [206].

Antioxidants

Owing to the limitation of laboratory setup, a single bench test cannot address all oxidation aspects of a real world scenario. The large variation in test conditions, particularly test temperature, use of catalyst, performance parameter, oxidation mechanism, and targeted oxidation stage, etc., makes it rather difficult or even impractical to correlate one test with another. It is therefore a common practice to run multiple tests at a time when characterizing a lubricant formulation and its additives. This section selectively reviews oxidation bench tests more closely related to the characterization of antioxidants. These tests have been standardized by some of the international standardization organizations such as ASTM and the Co-ordinating European Council (CEC), etc., and are more widely used in the industry. It is important to note that there are a number of custom-tailored test methods designed for specific needs that have been proven to be advantageous in certain circ*mstances. The value of these tests should not be underestimated.

1.11.1 THIN-FILM OXIDATION TEST 1.11.1.1 Pressurized Differential Scanning Calorimetry Differential scanning calorimetry (DSC), including PDSC, is an emerging thermal technique for rapid and accurate determination of thermal-oxidative stability of base oils and performance of antioxidants. PDSC has been a more sought after technique for two main reasons. First, high pressure elevates boiling points, thus effectively reducing experimental errors caused by volatilization losses of additives and light fractions of base oil; second, it increases the saturation of the reacting gases in sample, allowing the use of lower test temperature or shorter test time at the same temperature [207]. PDSC experiments can be run in an isothermal mode to measure oxidation induction time (OIT) corresponding to the onset of oil oxidation or in a programmed temperature mode to measure the onset temperature of oxidation. The temperature technique has been utilized to study depositforming tendency of five engine oils, and the results obtained were consistent with their engine test ranking [208]. The OIT technique, however, is more commonly used for its simplicity and speed. Its early use can be traced back to the 1980s when Hsu et al. [209] tested a number of engine oils and found the induction periods of the samples to be indicative of the sequence IIID viscosity break points. Soluble metals consisting of lead, iron, copper, manganese, and tin together with a synthetic oxidized fuel were included as catalysts to promote oil oxidation. The CEC L-85 and the ASTM D 6186 [210,211] are two standard methods that are based on OIT technique. Key test conditions of the methods are listed in Table 1.5. The CEC L-85 test method was originally developed for European Association des Constructeures Europeens de l’Automobile (ACEA) E5 specification for heavy-duty diesel oils and has been incorporated in the current E7 specification. The test is capable of differentiating between different quality base oils, additives, indicating antioxidant synergies and correlating with some bulk oil oxidation tests [212]. With appropriate modifications to the standard methods, PDSC has been successfully utilized in the characterization of various lubricants in addition to automotive engine oils. These include, but not limited to, base oils [213,214], greases [215], turbine oils [214], gear oils [216], synthetic ester lubricants [217], and biodegradable oils [218,219]. Using PDSC to study the kinetics of base oil oxidation [220] and antioxidant structure–performance relationship [221] has also been reported. 1.11.1.2 Thermal-Oxidation Engine Oil Simulation Test (ASTM D 6335; D 7097) The TEOST was originally developed to assess the high-temperature deposit-forming characteristic of API SF quality engine oils under turbocharger operating conditions [222]. The original test conditions were specified as the 33C protocol and subsequently standardized in the ASTM D 6335 method [223]. In this test, oil containing ferric naphthenate is in contact with nitrous oxide and moist air and is cyclically pumped to flow past a tared depositor rod. The rod is resistively heated through 12 temperature cycles, each going from 200 to 480°C for 9.5 min. After the heating cycle

Test Designation

D 6186

CEC L-85

D 6335

D 7097

D 4742

D 943, D 4310

IP48

IP 280

D 2272

Test

PDSC

TEOST 33C

TEOST MHT

TFOUT

TOST

IP48

IP 280/CIGRE

RPVOT

O2 uptake

Bulk

O2 uptake, thin film

Thin film

Bulk

Thin film

Oxidation Regime

TABLE 1.5 Conditions of Oxidation Test Methods

160

120

200

160

285

100, 200–480

210

130, 155, 180, 210

Temperature (°C)

Air

N2O, moist air Dry air

Air

Gas

90 psig

1 L/h

15 L/h

3.0 L/h

90 psig

10mL/m

100 psi, no flow 3.6 mL/min

500 psi, 100 mL/m

Gas Flow or Initial Pressure

Cu, H2O

Cu, Fe naphthenates

None

Oil-soluble Fe, Pb, Sn Fuel, naphthenates of Fe, Pb, Cu, Mg, and Sn, H2O Fe, Cu, H2O

Fe naphthenate

None

Catalyst

50 mL

25 g

40 mL

300 mL

1.5 g

8.4 g

116 mL

3.0 mg

Sample Size

∆P = 25 psi

164 h

6h×2

∆TAN = 2.0 1000 h

Sharp pressure drop

Occurrence of oxidation exotherm 120 min maximum 12 Programmed cycles 24 h

EOT

TAN, sludge, metal weight loss Viscosity, carbon residue Volatile acids, oil acidity, sludge OIT

Deposits, volatile OIT

Deposits

OIT

Parameter Measured

32 Lubricant Additives: Chemistry and Applications

Antioxidants

is complete, deposit formed on the depositor rod is determined by differential weighting. The 33C protocol was found capable of discriminating engine oils with known ability in resisting deposit formation in critical areas of engines [222]. The successful use of high-temperature deposition test to characterize engine oils has led to the development of a TEOST mid-high temperature (MHT) protocol, a simplified procedure for the assessment of oil deposition tendency in the piston ring belt and under-crown areas of fired engines [224]. Thin-film oxidation condition was thought to be predominant in these areas, and accordingly, the depositor assembly was revised to allow the oil flows down the rod in a slow and even manner to obtain a desired thin film. To better reflect the thermal-oxidative conditions of the engine zone of interest, a continuous depositor temperature of 285°C together with modified catalyst package and dry air is employed. The test runs for 24 h, and afterward, the amount of deposits formed on the tared depositor is gravimetrically determined [225]. Since introduction, the TEOST MHT has been incorporated in the International Lubricant Standardization and Approval Committee (ILSAC) gas fuel (GF)-3 and GF-4 engine oil specifications with an upper limit of 45 and 35 mg, respectively. Aside from being a thermal-oxidation test, TEOST can also be used to characterize neutral and overbased detergents of automotive engine oils [226]. 1.11.1.3

Thin-Film Oxidation Uptake Test (ASTM D 4742)

The TFOUT method was originally developed under the U.S. Congress mandate to monitor batchto-batch variations in the oxidative stability of re-refined lubricating base stocks [227]. The test stresses a small amount of oil to 160°C in a high-pressure reactor pressurized with oxygen along with a metal catalyst package, a fuel catalyst, and water to partially simulate the high-temperature oxidation conditions in automotive combustion engines [228]. Better oxidative stability of oil corresponds to a longer time it takes to observe a sharp drop in oxygen pressure. TFOUT can be carried out in a RPVOT apparatus upon proper modification to the sampling accessories. Based on the results obtained from testing a limited number of reference engine oils, qualitative correlation between TFOUT and the sequence IIID engine dynamometer test has been established [229]. Since being adopted as an ASTM standard method, there has been a wider utilization of the TFOUT to screen lubricants, base stocks, and additive components before sequence III engine testing [227].

1.11.2 BULK OIL OXIDATION TEST 1.11.2.1

Turbine Oil Stability Test (ASTM D 943, D 4310)

The turbine oil stability test (TOST) has been widely used in the industry to assess the oxidative stability of inhibited steam turbine oils under long-term service conditions. It can be used on other types of industrial lubricants such as hydraulic fluids and circulating oils and in particular on those that are prone to water contamination in service. The test runs at relatively low temperature (95°C) to represent the thermal-oxidative conditions of real steam turbine applications. Two versions of the TOST, namely, ASTM D 943 and D 4310 [230,231], have been developed. Both the methods share some common test conditions including test apparatus, catalysts, sample size, temperature profile, and gas, with minor differences in the test duration and target oxidation parameters to be monitored. The ASTM D 943 measures oxidation lifetime, which is the number of hours required for the test oil to reach an acid number of 2.0 mgKOH/g or above. The ASTM D4310 determines the sludging and corrosion tendencies of the test oil by gravimetrically measuring oil-insoluble products after 1000 h of thermal and oxidative stresses. The total amount of copper in oil, water, and sludge phases is also determined. A modified TOST method that operates at higher temperature (120°C) and in the absence of water has been proposed [232]. The procedure requires RPVOT as a monitoring tool and is specifically suitable for the determination of sludging tendencies of long-life steam and gas turbine oils formulated with the more stable groups II and III base stocks and high-performance aminic

Lubricant Additives: Chemistry and Applications

antioxidants. The “dry” TOST method is a potential alternative to the original methods that have found to be less discriminatory on such high-performance turbine oils. 1.11.2.2 IP 48 Method The Institute of Petroleum (IP) 48 is a high-temperature bulk oil oxidation test originally designed for the characterization of base oils [233]. The test stresses a 40 ml of oil sample in a glass tube at 200°C, along with air bubbling at 15 L/h, for two 6 h periods with a 15–30 h standby period in between. Oil viscosity increase and the formation of carbon residue are determined after the oxidation. The test is considered unsuitable for additive-type oils (other than those containing ashless additives) or those which form solid products or evaporate more than 10% by volume during the test. However, successful assessment of engine oils using a modified IP 48 method with four 6 h cycles has been reported [233,234]. 1.11.2.3

IP 280/CIGRE

The IP 280, also known as the CIGRE test, was designed to assess the oxidative stability of inhibited mineral turbine oils, targeting formations of volatile acid products (through water absorption), sludge, and increase of oil acidity [235]. The IP 280 and the TOST D 943 are similar to each other in terms of the oxidation regime employed. However, their test conditions are different, and it is a common practice to conduct both tests because in some internal turbine oil specifications, the limits for both the tests are stipulated. The IP 280 test was found to be more suitable for discriminating performance of additive packages, whereas the D943 is more suitable for comparative evaluation of base oils derived from different crude source and processing techniques [236].

1.11.3 OXYGEN UPDATE TEST 1.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272) The RPVOT, originally known as the rotating bomb oxidation test (RBOT), was designed to monitor the oxidative stability of new and in-service turbine oils having the same composition. It can also be used to characterize other types of industrial lubricants, for example, hydraulic fluids and circulating oils. The test utilizes a steel pressure vessel where sample oil is initially pressurized to 90 psi with oxygen and thermally stressed to 150°C in the presence of water and copper coil catalyst until a pressure drop of 25 psi is observed [237]. The test temperature was chosen to promote measurable oil breakdown in a relatively short time. However, such temperature causes a lack of representation to most steam turbines that operate below 100°C and to the combustion turbines that operate at much higher temperatures [238]. Owing to its sensitivity to specific additive chemistries, RPVOT finds limited use in comparing differently formulated oils. In addition, the test is more suitable for the determination of remaining useful life of in-service turbine oils rather than the qualification of new oils. Attempting to correlate RPVOT to the lengthy TOST D 943 on steam turbine oils has been successful, suggesting that the results from RPVOT may be used to estimate the relative lifetime of turbine oils in the TOST D943 [239].

1.12

EXPERIMENTAL OBSERVATIONS

The following two experiments demonstrate (a) performance behaviors of aminic antioxidant versus HP that is in agreement with the mechanisms discussed earlier, and (b) how proper selection and combinations of antioxidants can lead to synergy that further enhances performance. In the first experiment, two turbine oils, each formulated with a base oil selected from an API group I or group IV base stock, a standard additive package of metal deactivator and corrosion inhibitor, and 0.8 wt% of antioxidants of interest were tested by using the TOST D 943 lifetime method. The aminic antioxidant was an ADPA containing a mixture of butylated and octylated diphenylamines. The HP was a C7–C9 branched alkyl ester of 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid. As can be seen from Figure 1.19, in either oil, the HP significantly outperformed the ADPA by

Antioxidants

2.0

HP, GI

HP + ADPA, GI

HP, GIV

TAN (mgKOH/g)

2.5

ADPA, GIV

ADPA, GI

3.0

1.5

HP + ADPA, GIV

1.0

0.5

8064

7552

7048

6544

6040

5536

5032

4528

4024

3520

3016

2512

2008

1504

1000

500

0.0

Time (h)

FIGURE 1.19

TOST results of turbine oils containing group I or IV base oil and 0.8 wt% of antioxidant.

800 700

RPVOT OIT (min)

600 500 400 300 200 100 0 HP

FIGURE 1.20

HP + ADPA

ADPA

RPVOT results of turbine oil containing a group I base oil and 0.5 wt% of antioxidant.

providing longer protection time against oxidation. Mixtures of the ADPA and the HP at 0.4 wt% each in the oils provided even stronger protection, leading to an extended lifetime of ~5000 h for the group I turbine oil and well over 8000 h for the group IV turbine oil. Thus, under the low-temperature test conditions, the HP was superior to the ADPA. Proper mixing of the two additives produced synergy, in this case a hom*osynergism that led to the maximum protection. In the second experiment, a turbine oil formulated with an API group I base stock, a metal deactivator, a corrosion inhibitor, and a 0.5 wt% of the same antioxidants as before was tested by using the RPVOT (ASTM D 2272). The results are graphically presented in Figure 1.20. At the higher

Lubricant Additives: Chemistry and Applications 140

120

Deposits (mg)

100

0 Baseline

ADPA

ADPA + HP

FIGURE 1.21 TEOST results of a prototype PCMO containing a group II base stock and a total of 1.0 wt% of antioxidant.

test temperature (150°C), the OIT of the blend containing ADPA was ~600 min, while the ADPA was depleted. The HP protected the oil for ~300 min, indicating that the HP is only half as effective as the ADPA under the same test conditions. A mixture of ADPA and HP with 0.25 wt% of each additive present provided a protection for over 700 min. Therefore, in contrast to the TOST results, under high-temperature conditions, the ADPA was superior to the HP. Similar to what was observed in the TOST, a synergistic mixture of the two additives provided the maximum protection. The superiority of ADPA over HP and the benefit of antioxidant synergy for maximum oxidation protection have been further demonstrated in a GF-4 prototype passenger car motor oil (PCMO). The oil contained an API group II base oil, a low level (0.05 wt%) of phosphorus derived from ZDDP, and a number of other additives (detergents, dispersant, viscosity index improvers, pour point depressant, etc.) that are commonly found in engine oil formulations. The ADPA, HP, and their mixture were tested at 1.0 wt% in the oil on a TEOST MHT apparatus, using the ASTM D 7097 standard procedure. The results are presented in Figure 1.21. The baseline blend, which contained all other additives except the antioxidant, produced a fairly high level (130 mg) of deposits. With the addition of the HP, the deposit was substantially reduced to ~80 mg, with the ADPA, down to ~55 mg. By properly mixing the two antioxidants while keeping the total level constantly at 1.0 wt%, the deposit was further reduced to ~40 mg. The TEOST results confirm the superior performance of ADPA and further demonstrate the benefit of antioxidant synergy for high-temperature oxidation conditions. The antioxidant mechanisms discussed earlier well explain the experimental results and can serve as a foundation to guide lubricant formulators in the selection of correct antioxidant(s) for a particular end use. To obtain a successful formulation, other factors such as cost performance, volatility, color, solubility, odor, physical form, toxicity, and compatibility with other additives also need be taken into consideration. From a performance standpoint, HPs are excellent primary antioxidants for their stoichiometric reactions with free radicals under lower-temperature conditions. In contrast, ADPAs are excellent primary antioxidants for high-temperature conditions owing to their catalytic radical scavenging actions. The hom*osynergism facilitated between the

Antioxidants

ADPA and the HP is powerful in the inhibition of different stages of oil oxidation as demonstrated. It is, however, important to note that the generation and the magnitude of an antioxidant synergy are dependent on the formulation, base oil, and test method used. The ADPA/HP synergy appears robust as it was successfully reproduced in two oil formulations and tests that vastly differ from each other in terms of base oil makeup, additive type and complexity, test conditions, and oxidation regimes. In fact, this type of synergy has been used in a wide range of lubricants. In a more recent development, a methylene-bridged HP was utilized and found to be synergistic with ADPA in low-phosphorus engine oils [240]. Several instances of other types of synergy have been demonstrated and discussed in greater depth elsewhere. These include, but are not limited to, synergy between sulfur-bearing HP and ADPA antioxidants for hydro-treated base stocks [134,241], synergy between aminic antioxidants [242], and synergy between primary antioxidants and oragnophosphites [57].

1.13 ANTIOXIDANT PERFORMANCE WITH BASE STOCK SELECTION Driven by escalating environmental and performance requirements, the lubricant industry is rapidly changing for the better with the advances of additive and base oil technologies. One notable change from a formulation point of view is that the conventional solvent-extracted base oils (group I) are gradually being replaced by high-quality, high-performance groups II and III base stocks made from hydrotreated (hydrocracked), hydrotreating, and hydrocatalytic dewaxing processes. These processes provide oils with low sulfur, high degree of saturation, and viscosity index (Table 1.3). Lubricants formulated with these base stocks generally have improved performance characteristics such as superior oxidative stability, lower volatility, improved low-temperature properties, longer drain intervals, and improved fuel economy. Because of these benefits, the API group III base oils are becoming a serious challenge to synthetic PAOs for top-tier oil formulations. Many efforts have been made to understand the relationship between the base oil composition and the response to added antioxidants. Such knowledge is extremely important for lubricant formulators when comes to the selection of an appropriate antioxidant system for a given oil. Figure 1.22 shows the RPVOT test results of four base oils with and without the presence of an antioxidant. Each oil

RPVOT OIT (min)

1800 1600

No AO HP

1400

ADPA

1200 1000 800 600 400 200 0 API group I

FIGURE 1.22

API group II

API group III

RPVOT results of HP and ADPA in API groups I–IV base oils.

API group IV

Lubricant Additives: Chemistry and Applications

represents an API group from I to IV. The HP and the ADPA are the same as before. Clearly, without the protection of antioxidant, all oils performed equally poor. A 0.5 wt% of the HP antioxidant gave modest levels of protection that marginally increase from API group I to IV. When the base oils were treated with the same level of the ADPA, a drastic performance boost is seen across the board. The performance responses of the highly refined groups II, III, and the group IV to the added ADPA appear to be particularly strong. The superior antioxidant response of the groups II and III base oils over the conventional group I base oils may be attributed to the removal of aromatic hydrocarbons and polar constituents and the large presence of saturated hydrocarbons in the oils [243,244]. One school of thought hypothesized that oxygen-, sulfur-, and nitrogen-containing polar species may exist in the form of micellar aggregations in base oil. When an antioxidant is added, some of the natural polar molecules may interact physically and chemically with the additives, leading to a reduction in additive effectiveness in some circ*mstances. In those highly refined base oils where the natural compounds are essentially low or absent, the added antioxidant is able to exert its maximum effectiveness [245]. ZDDP, another important class of antioxidant/antiwear agent, has been studied by others, and the results indicated that its antioxidant performance is dependent on the base oil aromatics, alkylsubstituted aromatics, average chain length of hydrocarbons, and the relative presence of normal paraffins and isoparaffins [196]. In group I base oils, ZDDP gave good responses to highly saturated hydrocarbons characterized with normal paraffins having shorter chain length. Isoparaffins were found to decrease the antioxidant activity of ZDDP due to the steric hindrance of the side chains, which restricts the additive molecules from interacting with the hydrocarbons. In oils with higher monoaromatic hydrocarbons, ZDDP tends to perform better, which was believed to be related to improved solvency.

1.14 FUTURE REQUIREMENTS The need for antioxidants in future lubricants will continue and the demands for both quality and quantity will likely to increase to meet new environment and performance requirements. Although such trend is inevitably to take place in the entire lubricant industry, three specific areas may see more rapid and dynamic advancements: (a) modern engine oils, (b) biodegradable lubricants, and (c) engine oils that operate on biofuels. New engine designs and engine oil formulations are being frequently rolled out. The primary driving force is environmental in nature: the requirements for less oil consumption, better fuel economy, extended drain intervals, and lower emissions (particulates, hydrocarbons, CO, and NOx). To meet these requirements, new automotive engines are designed lighter and smaller but are required to operate under more severe conditions for higher output and speeds, which lead to higher engine and oil temperatures. It has been well established that every 10ºC of temperature increase will approximately double the rate of oil oxidation. Therefore, to maintain satisfactory service lifetime in a more severe service environment, increasing the level of antioxidant and using those suitable for high-temperature conditions, such as aminic antioxidants, are expected. Modern catalytic converters are highly effective in reducing the emissions. However, they are vulnerable to the deterioration effects of sulfur, phosphorus, and ash derived from engine oils and fuels. Accordingly, initiatives have been made to reduce the ZDDP content in engine oils. The current ILSAC GF-4 specification has limited the sulfur and phosphorus levels to a maximum of 0.7 and 0.08 wt%, respectively, and these numbers are likely to be even lower for future engine oils. With the ZDDP being reduced, it is expected that the uses of ashless, primary antioxidants as well as secondary antioxidants (as a substitute for ZDDP) will increase. To meet the increasing performance requirements set for modern engine oils, high-quality groups II and III base stocks are emerging to replace the conventional solvent-refined group I oils. The hydrotreating and isodewaxing processes that are used to make these oils significantly lower the unsaturated hydrocarbons and polyaromatics, which contribute to poor oxidative stability.

Antioxidants

However, the naturally occurring sulfur species that can function as antioxidants have also been largely removed. Previous discussion has clearly demonstrated that the superior oxidative stability of these oils can only be realized when an appropriate synthetic antioxidant is used. Therefore, as lubricant formulators increase the use of hydrotreated and synthetic base stocks, the requirements for antioxidants in lubricants are expected to rise. The environmental and toxicity issues of petroleum-based oils as well as their rising cost related to a global shortage have led to renewed interest in the use of vegetable oils, such as soybean oil, canola oil, sunflower oil, and coconut oil as lubricants and industrial fluids. The industry has already seen vegetable oils being utilized to make environmentally friendly automotive engine oils, two-cycle engine oils, hydraulic fluids, total loss lubricants, and marine lubricants. Vegetable oils generally possess some excellent lubrication properties, for example, good inherent lubricity, low volatility, high viscosity index, excellent solvency for lubricant additives, and easy miscibility with other fluids. However, vegetable oils are known for their poor oxidative stability as compared to mineral oils. Research has found that typical soybean oil formed polymers at a rate an order of magnitude faster than mineral oils [246]. To overcome this drawback, it is expected that vegetable oil–based lubricants will need more antioxidants to meet the performance requirements set for mineral oil–based lubricants. Owing to the unique hydrocarbon composition of vegetable oils, antioxidant response in these new fluids will be different from mineral oils. Recent work [218] has indicated that ADPAs that have proven track of high-temperature performance in mineral base oils are not as effective in stabilizing soybean oil at elevated temperatures (e.g., 170°C). In addition, environmentally friendly lubricants need to use additives that satisfy biodegradation and bioaccumulation standards. Antioxidants suitable for mineral oils may become problematic for use in vegetable oil–based lubricants. As such, the development of new classes of biodegradable antioxidants may be needed. The increasing use of biofuels such as biodiesel methyl esters derived from oil seeds, animal fats, and reclaimed cooking oils, represents a new challenge to the stabilization of engine oils that operate on such fuels. Recent work [247] has discovered that biodiesels of various vegetable sources (canola, soybean, palm oils, coconut, etc.) promote the oxidation of in-service engine oils even at a low dilution level, primarily shortening inhibited period, and leading to rapid oil viscosity increase. Further aggravating the situation is the high and narrow boiling points of biodiesels, which make them more persistent than mineral diesels after entering the crankcase. For an engine oil to perform satisfactorily with these new fuels, antioxidant level has to be maintained at an effective level to counteract the strong degradation impact from the biofuels. Overall, future lubricants will favor antioxidants having high-performance, cost-effective, ashless, multifunctional, and environmentally-friendly attributes. The ultimate driving force is environmental in nature while taking into account the emerging base oil technologies and performance specifications.

1.15 COMMERCIAL ANTIOXIDANTS Product Ethanox®

310 Ethanox 323 Ethanox 376 Ethanox 4701 Ethanox 4702 Ethanox 4703 Ethanox 4716 Ethanox 4733 Ethanox 4735 Ethanox 4755 Ethanox 4872J

Company Albemarle Albemarle Albemarle Albemarle Albemarle Albemarle Albemarle Albemarle Albemarle Albemarle Albemarle

Chemistry Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane Nonylphenol disulfur oligomer 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C18 alkyl ester 2,6-Di-t-butyl phenol 4,4′-Methylene bis(2,6-di-t-butyl phenol) 2,6-Di-t-butyl-alpha-dimethylamino-p-cresol 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C7–C9 alkyl ester Mixture of mono-, di-, and tri-t-butyl phenols Mixture of t-butyl phenols Boron containing derivatives of Ethanox 4702 Multiring t-butyl phenol, 53% active (Continued )

Lubricant Additives: Chemistry and Applications

Continued Product

Company

Chemistry

Ethanox 4827J Ethanox 4777

Albemarle Albemarle

Multiring t-butyl phenol, 30% active Alkylated diphenylamine

Additin® 7010 Additin 7130 Additin 7110 Additin 7120 Additin 7115 Additin 7135

Rhein Chemie Rhein Chemie Rhein Chemie Rhein Chemie Rhein Chemie Rhein Chemie

Oligomerized 1,2-dihydro-4-trimethylquinoline Phenyl-alpha-napthylamine 2,6-Di-t-butyl-p-cresol 2,6-Di-t-butyl phenol Phenol derivative sterically hindered Styrenated diphenylamine

Naugalube® 15 Naugalube 16 Naugalube 18 Naugalube 22 Naugalube 32 Naugalube 37 Naugalube 38 Naugalube 531 Naugalube 438 Naugalube 438L Naugalube 635 Naugalube 640 Naugalube 680 Naugalube AMS Naugard® PANA Naugalube APAN Naugalube TMQ Naugalube 403 Naugalube TPP

Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura Chemtura

2,2′-Thiodiethylene bis(3,5-di-t-butyl-4-hydroxyphenyl)propionate 4,4′-Thiobis(2-t-butyl-5-methyl phenol) 2,2-Thiobis(4-methyl-6-t-butyl phenol) Mixture of t-butyl phenol Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C18 alkyl ester 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C13–15 alkyl ester 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C7–C9 alkyl ester Dioctyl diphenylamine Dinonyl diphenylamine Styrenated diphenylamine Butylated-, octylated-diphenylamine Octylated-, styrenated-diphenylamine Alpha-methystyrenated DPA Phenyl-alpha-napthylamine Alkylated PANA Oligomerized 1,2-dihydro-4-trimethylquinoline N,N′-di-sec-butyl-p-phenylenediamine Triphenyl phosphite

Irganox® L 01 Irganox L 06 Irganox L 57 Irganox L 67 Irganox L 101 Irganox L 107 Irganox L 109 Irganox L 115 Irganox L 118 Irganox L 135 Irganox E 201 Irgaphos® 168

Ciba Ciba Ciba Ciba Ciba Ciba Ciba Ciba Ciba Ciba Ciba Ciba

Dioctyl diphenylamine Octylated PANA Butylated, octylated diphenylamine Dinonyl diphenylamine Tetrakismethylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C18 alkyl ester Hindered bis-phenol 2,2′-Thiodiethylene bis(3,5-di-t-butyl-4-hydroxyphenyl)propionate High MW liquid hindered phenolic with thioether 3,5-Di-t-butyl-4-hydroxy-hydrocinnamic acid, C7–C9 alkyl ester Liquid di-alpha-tocopherol (vitamin E) Tri-(di-t-butylphenyl) phosphite

Vanlube® AZ Vanlube EZ

RT Vanderbilt RT Vanderbilt

Vanlube NA Vanlube PCX Vanlube RD Vanlube SL Vanlube SS

RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt

Zinc diamyldithiocarbamate in oil Zinc diamyldithiocarbamate and diamyl ammonium Diamyldithiocarbamate Nonylated, ethylated diphenylamine 2,6-Di-t-butyl-p-cresol Oligomerized 1,2-dihydro-4-trimethylquinoline Octylated, styrenated diphenylamine Octylated diphenylamine (Continued )

Antioxidants

Continued Product

Company

Vanlube 81 Vanlube 7723 Vanlube 869 Vanlube 8610 Vanlube 887 Vanlube 887E Vanlube 9317 Vanlube 961 Vanlube 996E

RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt

Chemistry Dioctyl diphenylamine Methylene bis(dibutyldithiocarbamate) Zinc dithiocarbamate/sulfurized olefin blend Antimony dithiocarbamate/sulfurized olefin blend Tolutriazole compound in oil Tolutriazole compound in ester Organic amine in synthetic ester Butylated, octylated diphenylamine Methylene bis(di-butyl-dithiocarbamate) and tolutriazole derivative

1.16 COMMERCIAL METAL DEACTIVATORS Product Ethanox®

Company

Chemistry

Albemarle

N,N-disalicylidene-1,2-diaminopropane

Irgamet® 30 Irgamet 39 Irgamet 42 Irgamet BTZ Irgamet TTZ

Ciba Ciba Ciba Ciba Ciba

Triazole derivative Tolutriazole derivative Water-soluble tolutriazole derivative Benzotriazole Tolutrizole

Cuvan® 303 Cuvan 484 Cuvan 826 NACAP® ROKON® Vanchem® NATD Vanlube 601 Vanlube 601E Vanlube 704

RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt RT Vanderbilt

N,N-bis(2-ehtylhexyl)-ar-methyl-1H-benzotriazole-1-methanamine 2,5-Dimercapto-1,3,4-thiadiazole derivative 2,5-Dimercapto-1,3,4-thiadiazole derivative Sodium 2-mercaptobenzothiazole, 50% active 2-Mercaptobenzothiazole Disodium, 2,5-dimercaptothiadiazole, 30% active Heterocyclic sulfur–nitrogen compound Heterocyclic sulfur–nitrogen compound Proprietary blend

4705

REFERENCES 1. Baird, J. Great Britain Patent 1516 (1872). 2. Haas, F. Lubricant. U.S. Patent 2,162,398 (June 13, 1939, Archer-Daniels-Midland Company). 3. Kobbe, W.H. Sulphur-containing oil composition and method of making the same. U.S. Patent 1,844,400 (February 9, 1932). 4. Palmer, R.C. and P.O. Powers. Sulphurized terpene oil and process of preparing the same. U.S. Patent 1,926,687 (September 12, 1933, Newport Industries, Inc.). 5. Knowles, E.C., F.C. McCoy, and J.A. Patterson. Lubricating oil and method of lubricating. U.S. Patent 2,417,305 (March 11, 1947, The Texas Company). 6. Lincoln, B.H., W.L. Steiner, and G.D. Byrkit. Sulphur containing lubricant. U.S. Patent 2,218,132 (October 15, 1940, Continental Oil Company). 7. Lincoln, B.H., W.L. Steiner, and G.D. Byrkit. Sulphur containing lubricant. U.S. Patent 2,313,248 (March 9, 1943, The Lubri-Zol Development Corporation). 8. Lincoln, B.H., G.D. Byrkit, and W.L. Steiner. Method for the synthesis of sulphur-bearing derivatives of high molecular weight. U.S. Patent 2,348,080 (May 2, 1944, Continental Oil Company). 9. Farrington, B.B., V.M. Kostainsek, and G.H. Denison Jr. Compounded lubricant. U.S. Patent 2,346,156 (April 11, 1944, Standard Oil Company of California). 10. Mikeska, L.A. and E. Lieber. Preparation of phenol sulfides. U.S. Patent 2,139,321 (December 6, 1938, Standard Oil Development Company).

Lubricant Additives: Chemistry and Applications

11. Mikeska, L.A. and C.A. Cohen. Mineral oil stabilizing agent and composition containing same. U.S. Patent 2,139,766 (December 13, 1938, Standard Oil Development Company). 12. Mikeska, L.A. and E. Lieber. Stabilized lubricanting composition. U.S. Patent 2,174,248 (September 26, 1939, Standard Oil Development Company). 13. Mikeska, L.A. and E. Lieber. Polymerization and condensation products. U.S. Patent 2,239,534 (April 22, 1941, Standard Oil Development Company). 14. Richardson, R.W. Oxidation inhibitor. U.S. Patent 2,259,861 (October 21, 1941, Standard Oil Development Company). 15. Hu, S.M., C.L. Gao, J.J. Tang, J.C. Zhang, and H.F. Liang. Properties of mono- and dialkyldiphenyl sulfides for high temperature lubricants and their molecular structures. Acta Petrolei Sinica (Shiyou Xuebao), S1, 118–130, 1997. 16. Askew, H.F., G.J.J. Jayne, and J.S. Elliott. Lubricant compositions. U.S. Patent 3,882,031 (May 6, 1975, Edwin Cooper & Company Ltd.). 17. Spence, J.R. Lubricating compositions containing normal-alkyl substituted 2-thiazoline disulfide antioxidants. U.S. Patent 4,485,022 (November 27, 1984, Phillips Petroleum Company). 18. Salomon, M.F. Antioxidant compositions. U.S. Patent 4,764,299 (August 16, 1988, The Lubrizol Corporation). 19. Oumar-Mahamat, H., A.G. Horodysky, and A. Jeng. Dihydrobenzothiophenes as antioxidant and antiwear additives for lubricating oils. U.S. Patent 5,514,289 (May 7, 1996, Mobil Oil Corporation). 20. Hester, W.F. Fungicidal composition. U.S. Patent 2,317,765 (April 27, 1943. Rohm & Haas Company). 21. Denton, W.M. and S.A.M. Thompson. Screening compounds for antioxidant activity in motor oil. Inst. Petrol. Rev. 20(230), 46–54, 1966. 22. Holubec, A.M. Lubricant compositions. U.S. Patent 3,876,550 (April 8, 1975, The Lubrizol Corporation). 23. Karol, T.J., S.G. Donnelly, and R.J. Hiza. Improved antioxidant additive compositions and lubricating compositions containing the same. PCT 03/027215 A2 (2003, R.T. Vanderbilt Company Inc.). 24. Chesluk, R.P., J.D. Askew Jr., and C.C. Henderson. Oxidation inhibited lubricating oil. U.S. Patent 4,125,479 (November 14, 1978, Texaco Inc.). 25. Yao, J.B. The application of ashless thiocarbamate as lubricant antioxidation and extreme pressure additive. Lubricating Oil, 20(6), 41–44, 2005. 26. Doe, L.A. Antioxidant synergistis for lubricating compositions. U.S. Patent 4,880,551 (November 14, 1989, R.T. Vanderbilt Company Inc.). 27. Nakazato, M., J. Magarifuchi, A. Mochizuki, and H. Tanabe. Low phosphorus engine oil composition and additive compositions. U.S. Patent 6,351,428 (March 11, 2003, Chevron Oronite Company LLC). 28. Khorramian, B.A. Phosphorus-free and ashless oil for aircraft and turbo engine application. U.S. Patent 5,726,135 (March 10, 1998). 29. deVries, L. and J.M. King. Process of preparing molybdenum complexes, the complexes so-produced and lubricants containing same. U.S. Patent 4,263,152 (April 21, 1981, Chevron Research Company). 30. deVries, L. and J.M. King. Process of preparing molybdenum complexes, the complexes so-produced and lubricants containing same. U.S. Patent 4,265,773 (May 5, 1981, Chevron Research Company). 31. Stiefel, E.I., J.M. McConnachie, D.P. Leta, M.A. Francisco, C.L. Coyle, P.J. Guzi, and C.F. Pictroski. Trinuclear molybdenum multifunctional additive for lubricating oils. U.S. Patent 6,232,276 B1 (May 15, 2001, Infineum USA L.P.). 32. deVries, L. and J.M. King. Antioxidant combinations of molybdenum complexes and aromatic amine compounds. U.S. Patent 4,370,246 (January 25, 1983, Chevron Research Company). 33. Shaub, H. Mixed antioxidant composition. European Patent 719313 B1 (August 6, 1997, Exxon Chemical Patents Inc.). 34. Arai, K. and H. Tomizawa. Lubricating oil composition. U.S. Patent 5,605,880 (February 25, 1997, Exxon Chemical Patents Inc.). 35. Tomizawa, H. Lubricating oil composition for internal combustion engines. U.S. Patent 5,688,748 (November 18, 1997, Tonen Corporation). 36. Kelly, J.C. Engine lubricant using molybdenum dithiocarbamate as an antioxidant top treatment in high sulfur base stocks. IP.com Journal, 1(6), 22, 2001. 37. Gatto, V.J. Antioxidant system for lubrication base oils. U.S. Patent 5,840,672 (November 24, 1998, Ethyl Corporation). 38. Yao, J.B. Recent development of antiwear and extreme pressure-resistant additives for lubricating oils and greases. Lubricating Oil, 21(3), 29–37, 2006. 39. Hoffman, D.M., J.J. Feher, and H.H. Farmer. Lubricating compositions containing 5,5′-dithiobis(1,3,4thiadiazole-2-thiol). U.S. Patent 4,517,103 (May 14, 1985, R.T. Vanderbilt Company Inc.).

Antioxidants

40. Karol, T.J., S.G. Donnelly, and R.J. Hiza. Antioxidant additive compositions and lubricating compositions containing the same. U.S. Patent 6,806,241 (October 19, 2004, R.T. Vanderbilt Company Inc.). 41. Salomon, M.F. N-Substituted thio alkyl phenothiazines. U.S. Patent 5,034,019 (July 23, 1991, The Lubrizol Corporation). 42. Germanaud, L., P. Azorin, and P. Turello. Antioxidant nitrogen-containing additives for lubricating oils. France Patent 2,639,956 (June 8, 1990, Elf France). 43. Kapuscinski, M.M. and R.T. Biggs. Dispersant and antioxidant VI improver based on olefin copolymers containing phenothiazine and aromatic amine groups. U.S. Patent 5,942,0471 (August 24, 1999, Ethyl Corporation). 44. Colclough, T. Lubricating compositions. PCT Intl. Appl. WO 9525781 A1 (September 28, 1995, Exxon Chemical Ltd.). 45. Brown, A.L. Treatment of hydrocarbon oils. U.S. Patent 1,234,862 (July 31, 1917, Westinghouse and Electric Manufacturing Company). 46. Ashburn, H.V. and W.G. Alsop. Lubricating oil. U.S. Patent 2,221,162 (November 12, 1940, The Texas Company). 47. Hall, F.W. and C.G. Towne. Method of lubrication. U.S. Patent 2,257,601 (September 30, 1941, The Texas Company). 48. Musher, S. Lubricating oil and the method of making the same. U.S. Patent 2,223,941 (December 3, 1940, The Musher Foundation). 49. Loane, C.M. and J.W. Gaynor. Lubricant. U.S. Patent 2,322,859 (June 29, 1943, Standard Oil Company). 50. Moran, R.C., W.L. Evers, and E.W. Fuller. Petroleum product and method of making same. U.S. Patent 2,058,343 (October 20, 1936, Socony-Vacuum Oil Company Inc.). 51. Moran, R.C. and A.P. Kozacik. Mineral oil composition. U.S. Patent 2,151,300 (March 21, 1939, SoconyVacuum Oil Company Inc.). 52. Cohen, S.C. Synergistic antioxidant system for severely hydrocracked lubricating oils. U.S. Patent 5,124,057 (June 23, 1992, Petro-Canada Inc.). 53. Farrington, B.B. and J.O. Clayton. Compounded mineral oil. U.S. Patent 2,228,658 (January 14, 1941, Standard Oil Company of California). 54. Farrington, B.B., J.O. Clayton, and J.T. Rutherford. Compounded mineral oil. U.S. Patent 2,228,659 (January 14, 1941, Standard Oil Company of California). 55. Meyers, D. Method of lubricating compression cylinders used in the manufacture of high-pressure polyethylene. U.S. Patent 6,172,014 B1 (January 9, 2001, Pennzoil-Quaker State). 56. Holt, A. and G. Mulqueen. Stabilizing compositions for lubricating oils. U.S. Patent Appl. 2003/0171227 (September 11, 2003, Great Lakes Chemicals). 57. Dong, J. and C.A. Migdal. Stabilized lubricant compositions. U.S. Patent Appl. 206/0069000 A1 (March 30, 2006. Crompton Corporation). 58. Durr, A.M. and R.A. Krenowicz. Turbine oil compositions. U.S. Patent 3,923,672 (December 2, 1975, Continental Oil Company). 59. Messina, N.V. and D.R. Senior. Stabilized fluids. U.S. Patent 3,556,999 (January 19, 1971, Rohm and Haas Company). 60. Rutherford, J.T. and R.J. Miller. Compounded hydrocarbon oil. U.S. Patent 2,252,984 (August 19, 1941, Standard Oil Company of California). 61. Rutherford, J.T. and R.J. Miller. Compounded oil. U.S. Patent 2,252,985 (August 19, 1941, Standard Oil Company of California). 62. Asseff, P.A. Lubricant. U.S. Patent 2,261,047 (October 28, 1941, The Lubri-Zol Corporation). 63. Cook, E.W. and W.D. Thomas Jr. Crankcase lubricant and chemical compound therefore. U.S. Patent 2,342,572 (February 22, 1944, American Cyanamid Company). 64. Cook, E.W. and W.D. Thomas Jr. Crankcase lubricant and chemical compound therefore. U.S. Patent 2,344,392 (March 14, 1944, American Cyanamid Company). 65. Cook, E.W. and W.D. Thomas Jr. Lubricating oil composition. U.S. Patent 2,344,393 (March 14, 1944, American Cyanamid Company). 66. Cook, E.W. and W.D. Thomas Jr. Lubricating composition. U.S. Patent 2,358,305 (September 19, 1944, American Cyanamid Company). 67. Cook, E.W. and W.D. Thomas Jr. Lubricating compositions. U.S. Patent 2,368,000 (January 23, 1945, American Cyanamid Company). 68. Davis, L.L., B.H. Lincoln, and G.D. Byrkit. Method of synthesizing sulphur-bearing high molecular weight hydrocarbons. U.S. Patent 2,278,719 (April 7, 1942, Continental Oil Company).

Lubricant Additives: Chemistry and Applications

69. Kelso, C.D. Phosphorus sulphide-hydrocarbon reaction product. U.S. Patent 2,315,529 (April 6, 1943, Standard Oil Company). 70. Loane, C.M. and J.W. Gaynor. Lubricant. U.S. Patent 2,316,078 (April 6, 1943, Standard Oil Company). 71. White, C.N. Lubricant. U.S. Patent 2,316,091 (April 6, 1943, Standard Oil Company). 72. May, R.L. Reaction products of aliphatic alcohols and terepene-phosphorus sulphide. U.S. Patent 2,356,073 (August 15, 1944, Sinclair Refining Company). 73. May, R.L. Zinc salt of the reaction products of aliphatic alcohols and terepene-phosphorus sulphide. U.S. Patent 2,356,074 (August 15, 1944, Sinclair Refining Company). 74. Musselman, J.M. and H.P. Lankelma. Preparation of compounds for lubricants, etc. U.S. Patent 2,357,346 (September 5, 1944, The Standard Oil Company). 75. Berger, H.G., T.T. Noland, and E.W. Fuller. Mineral oil composition. U.S. Patent 2,373,094 (April 10, 1945, Socony-Vacuum Oil Company). 76. Mixon, L.W. Interface modifier. U.S. Patent 2,375,315 (May 8, 1945, Standard Oil Company). 77. Mixon, L.W. and C.M. Loane. Lubricant. U.S. Patent 2,377,955 (June 12, 1945, Standard Oil Company). 78. May, R.L. Reaction products of alkylated phenols and terepene-phosphorus sulphide. U.S. Patent 2,379,312 (June 26, 1945, Sinclair Refining Company). 79. Noland, T.T. Mineral oil composition. U.S. Patent 2,379,453 (July 3, 1945, Socony-Vacuum Oil Company). 80. Hughes, E.C. and W.E. Scovill. Mineral oil beneficiation. U.S. Patent 2,381,907 (August 14, 1945, The Standard Oil Company). 81. May, R.L. Lubricant. U.S. Patent 2,392,252 (January 1, 1946, Sinclair Refining Company). 82. May, R.L. Lubricant. U.S. Patent 2,392,253 (January 1, 1946, Sinclair Refining Company). 83. Musselman, J.M. and H.P. Lankelma. Lubricants. U.S. Patent 2,396,719 (March 19, 1946, The Standard Oil Company). 84. Bartleson, J.D. Additives for lubricants. U.S. Patent 2,403,894 (July 9, 1946, The Standard Oil Company). 85. May, R.L. Lubricating oil. U.S. Patent 2,409,877 (October 22, 1946, Sinclair Refining Company). 86. Lincoln, B.H. and G.D. Byrkit. Lubricating oil. U.S. Patent 2,415,296 (February 4, 1947). 87. Berger, H.G. and E.W. Fuller. Mineral oil composition. U.S. Patent 2,416,281 (February 25, 1947, Socony-Vacuum Oil Company). 88. Rogers, T.H., R.W. Watson, and J.W. Starrett. Lubricant. U.S. Patent 2,422,585 (June 17, 1947, Standard Oil Company). 89. Fuller, E.W. and H.S. Angel. Mineral oil composition. U.S. Patent 2,455,668 (December 7, 1948, SoconyVacuum Oil Company). 90. Clason, D.L. and C.W. Schroeck. Phosphite treatment of phosphorus acid salts and compositions produced thereby. U.S. Patent 4,263,150 (April 21, 1981, The Lubrizol Corporation). 91. Rivier, G. Novel metallic dithiophosphates and their use as additives for lubricating oils. U.S. Patent 4,288,335 (September 8, 1981, Orogil). 92. Schroeck, C.W. Metal salts of lower dialkylphosphorodithioic acids. U.S. Patent 4,466,895 (August 21, 1984, The Lubrizol Corporation). 93. Clason, D.L. and C.W. Schroeck. Mixed metal salts and lubricants and functional fluids containing them. U.S. Patent 4,308,154 (December 29, 1981, The Lubrizol Corporation). 94. Sarin, R., D.K. Tuli, A.V. Sureshbabu, A.K. Misra, M.M. Rai, and A.K. Bhatnagar. Molybdenum dialky lphosphorodithioates: synthesis and performance evaluation as multifunctional additives for lubricants. Tribology International, 27(6), 379–86, 1994. 95. Lubricant additives. Technical Bulletin 506. (R.T. Vanderbilt Company Inc.). 96. Levine, S.A., R.C. Schlicht, H. Chafetz, and J.R. Whiteman. Molybdenum derivatives and lubricants containing same. U.S. Patent 4,428,848 (January 31, 1984, Texaco Inc.). 97. Nalesnik, T.E. and C.A. Migdal. Oil-soluble molybdenum multifunctional friction modifier additives for lubricant compositions. U.S. Patent 6,103,674 (August 15, 2000, Uniroyal Chemical Company Inc.). 98. Sarin, R., D.K. Tuli, V. Martin, M.M. Rai, and A.K. Bhatnagar. Development of N, P and S-containing multifunctional additives for lubricants. Lubrication Engineering, 53(5), 21–27, 1997. 99. Ripple, D.E. Zinc-free farm tractor fluid. PCT 2007005423 A2 (January 11, 2007, The Lubrizol Corporation).

Antioxidants

100. Schadenberg, H. Dithiophosphate ester derivatives and their use for stabilizing organic material. British Patent 1,506,917 (September 30, 1975), Shell International Research. 101. Davis, R.H., A. Okorodudu, and M. Sedlak. Lubricant compositions containing a dithiophosphoric acid ester-aldehyde reaction product. European Patent Appl. 00090506 A2 (March 3, 1983, Mobil Oil Corporation). 102. Braid, M. Lubricating oils or fuels containing adducts of phosphorodithioate esters. U.S. Patent 3,644,206 (February 22, 1972, Mobil Oil Corporation). 103. Rogers, T.H. Refined viscous hydrocarbon oil. U.S. Patent 1,774,845 (September 2, 1930, Standard Oil Company). 104. Rogers, T.H. Refined viscous hydrocarbon oil. U.S. Patent 1,793,134 (February 17, 1931, Standard Oil Company). 105. Roberts, E.N. Lubricant. U.S. Patent 2,409,799 (October 22, 1946, Standard Oil Company). 106. Jenkins, V.N. Diesel engine lubricating oil. U.S. Patent 2,366,191 (January 2, 1945, Union Oil Company of California). 107. Dubois, L.O. and R.N. Gartside. Continuous manufacture of nitrobenzene. U.S. Patent 2,773,911 (December 11, 1956, E.I. du Pont de Nemours and Company). 108. Karkalits, O.C., Jr., C.M. Vanderwaart, and F.H. Megson. New catalyst for reducing nitrobenzene and the process of reducing nitrobenzene thereover. U.S. Patent 2,891,094 (June 16, 1959, American Cyanamid Co.). 109. Addis, G.I. Vapor phase process for the manufacture of diphenylamine. U.S. Patent 3,118,944 (January 21, 1964, American Cyanamid Co.). 110. Lai, J.T. and D.S. Filla. Process for production of liquid alkylated diphenylamine antioxidant. U.S. Patent 5,750,787 (May 12, 1998, The B.F. Goodrich Company). 111. Popoff, I.C., P.G. Haines, and C.E. Inman. Alkylation of diphenylamine. U.S. Patent 2,943,112 (June 28, 1960, Pennsalt Chemicals Corporation) 112. Lai, J.T. Synthetic lubricant antioxidant from monosubstituted diphenylamines. U.S. Patent 5,489,711 (February 6, 1996, The B.F. Goodrich Company). 113. Lai, T.J. Lubricant composition. U.S. Patent 6,426,324 (July 30, 2002, Noveon IP Holdings Corp. and BP Exploration & Oil Inc.). 114. Andress, H.J., Jr. and R.H. Davis. Arylamine-aldehyde lubricant antioxidants. European Patent Appl. 0083871 A2 (July 20, 1983, Mobil Oil Corporation). 115. Franklin, J. Liquid antioxidant produced by alkylating diphenylamine with a molar excess of diisobutylene. U.S. Patent 4,824,601 (April 25, 1989, Ciba-Geigy Corporation). 116. Lai, J.T. Method of manufacturing alkylated diphenylamine compositions and products thereof. U.S. Patent 6,204,412 (March 12, 2001, The B.F. Goodrich Company). 117. Lai, J.T. and D.S. Filla. Liquid alkylated diphenylamine antioxidant. U.S. Patent 5,672,752 (September 30, 1997, B.F. Goodrich Company). 118. Onopchenko, A. Alkylation of diphenylamine with polyisobutylene oligomers. U.S. Patent 6,355,839 (March 12, 2002, Chevron U.S.A. Inc.). 119. Braid, M. Lubricant compositions containing N-tertiary alkyl benzotriazoles. U.S. Patent 4,519,928 (May 28, 1985, Mobil Oil Corporation). 120. Shim, J. Lubricant compositions containing antioxidant mixtures comprising substituted thiazoles and substituted thiadiazole compounds. U.S. Patent 4,260,501 (April 7, 1981, Mobil Oil Corporation). 121. Shim, J. Multifunctional additives. U.S. Patent 4,511,481 (April 16, 1985, Mobil Oil Corporation). 122. Shim, J. Triazole-dithiophosphate reaction product and lubricant compositions containing same. U.S. Patent 4,456,539 (June 26, 1984, Mobil Oil Corporation). 123. Wright, W.E. Antioxidant diamine. U.S. Patent 4,456,541 (June 26, 1984, Ethyl Corporation). 124. Bandlish, B.K., F.C. Loveless, and W. Nudenberg. Amino compounds and use of amino compounds as antioxidants in lubricating oils. European Patent 022281 B1 (September 14, 1983, Uniroyal Inc.). 125. Muller, R. and W. Hartmann. N,N′-Diphenyl-p-phenylenediamines, method for their production and their use as stabilizers for organic materials. European Patent Appl. 072575 A1 (February 23, 1983, Chemische Werke Lowi GmbH). 126. Colclough, T. Lubricating oil components and additives for use therein. U.S. Patent 5,232,614 (August 2, 1993, Exxon Chemical Patents Inc.). 127. Malherbe, R.F. 2,3-Dihydroperimidines as antioxidants for lubricants. U.S. Patent 4,389,321 (June 21, 1983, Ciba-Geigy Corporation).

Lubricant Additives: Chemistry and Applications

128. Roberts, J.T. N,N′-Disubstituted 2,4′-diaminodiphenyl ethers as antioxidants. U.S. Patent 4,309,294 (January 5, 1982, UOP Inc.). 129. Roberts, J.T. Imines of 2,4-diaminodiphenyl ethers as antioxidants for lubricating oils and greases. U.S. Patent 4,378,298 (March 29, 1983, UOP Inc.). 130. Sung, R.L. and B.H. Zoleski. Diesel lubricant composition containing 5-amino-triazole-succinic anhydride reaction product. U.S. Patent 4,256,595 (March 17, 1981, Texaco Inc.). 131. Braid, M. Phenolic antioxidants and lubricants containing same. U.S. Patent 4,551,259 (November 5, 1985, Mobil Oil Corporation). 132. McKinnie, B.G. and P.F. Ranken. (Hydrocarbylthio)phenols and their preparation. U.S. Patent 4,533,753 (August 6, 1985, Ethyl Corporation). 133. Gatto, V.J. and A. Kadkhodayan. Sulfurized phenolic antioxidant composition method of preparing same and petroleum products containing same. U.S. Patent 6,001,786 (December 14, 1999, Ethyl Corporation). 134. Dong, J. and C.A. Migdal. Synergistic antioxidant systems for lubricants. 12th Asia Fuels and Lubes Conference Proceedings. Hong Kong, 2006. 135. Braid, M. Phenolic antioxidants and lubricants containing the same. U.S. Patent 4,551,259 (November 5, 1985, Mobil Oil Co.). 136. Gatto, V.J. Hydroxyphenyl-substituted amine antioxidants. U.S. Patent 5,292,956 (March 8, 1994, Ethyl Corporation). 137. Nelson, L.A. and L.R. Rudnick. Mannich type compounds as antioxidants. U.S. Patent 5,338,469 (August 16, 1994, Mobil Oil Corporation). 138. Oumar-Mahamat, H. and A.G. Horodysky. Phenolic imidazoline antioxidants. U.S. Patent 5,846,917 (December 8, 1998, Mobil Oil Corporation). 139. Camenzind, H., A. Dratva, and P. Hanggi. Ash-free and phosphorus-free antioxidants and antiwear additives for lubricants. European Patent Appl. 894,793 (February 3, 1999, Ciba Specialty Chemicals Holding Inc.). 140. Hsu, S.Y. and A.G. Horodysky. Sulfur-containing ester derivative of arylamines and hindered phenols as multifunctional antiwear and antioxidant additives for lubricants. U.S. Patent 5,304,314 (April 19, 1994, Mobil Oil Corporation). 141. Andress, H.J. and H. Ashjian. Products of reaction involving alkenylsuccinic anhydrides with aminoalcohols and aromatic secondary amines and lubricants containing same. U.S. Patent 4,522,736 (June 11, 1985, Mobil Oil Corporation). 142. Nelson, K.D. and E.A. Chiverton. Fused aromatic amine based wear and oxidation inhibitors for lubricants. U.S. Patent Application 2006/02237 A1 (October 5, 2006, Chevron Texaco Corporation). 143. Loper, J.T. Dispersant reaction product with antioxidant capability. U.S. Patent Appl. 2006/0128571 (June 15, 2006, Afton Corporation). 144. Migdal, C.A., T.E. Nalesnik, and C.S. Liu. Dispersant and antioxidant additive and lubricating oil composition containing same. U.S. Patent 5,075,383 (December 24, 1991, Texaco Inc.). 145. Colclough, T. Lubricating oil oxidation and stabilization, in Atmospheric Oxidation and Antioxidants. G. Scott ed., Elsevier Science Publishers B.V., Amsterdam, 1993. 146. Klaus, E.E., J.L. Duda, and J.C. Wang. Study of copper salts as high-temperature oxidation inhibitors. Tribology Transactions, 35(2), 316–324, 1992. 147. Holt, D.G.L. Multiring aromatics for enhanced deposit control. European Patent Appl. 0709447 (May 1, 1996, Exxon Research and Engineering Company). 148. Colclough, T., F.A. Gibson, and J.F. Marsh. Lubricating oil compositions containing ashless dispersant, zinc dihydrocarbyldithiophosphate, metal detergent and a copper compound. U.S. Patent 4,867,890 (September 19, 1989). 149. Downing, F.B. and H.M. Fitch. Lubricant. U.S. Patent 2,343,756 (March 7, 1944, E.I. du Pont de Nemours & Company). 150. Fox, A.L. Solution of Copper mercaptides from terpenes. U.S. Patent 2,349,820 (May 30, 1944, E.I. du Pont de Nemours & Company). 151. Downing, F.B. and H.M. Fitch. Lubricating oil. U.S. Patent 2,356,661 (August 22, 1944, E.I. du Pont de Nemours & Company). 152. Gurierrez, A., D.W. Brownawell, and S.J. Brois. Copper complexes of oxazolines and lactone oxazolines as lubricating oil additives. U.S. Patent 4,486,326 (December 4, 1984, Exxon Research & Engineering Co.). 153. Hopkins, T.R. Copper salts of succinic anhydride derivatives. U.S. Patent 4,552,677 (November 12, 1985, The Lubrizol Corporation).

Antioxidants

154. Farng, L.O. and A.G. Horodysky. Copper salts of hindered phenolic carboxylates and lubricants and fuel containing same. U.S. Patent 4,828,733 (May 9, 1989, Mobil Oil Corporation). 155. Stanulov, K., H.N. Harbara, and G. Cholakov. Antioxidation properties of boron-containing lubricant additives and their mixtures with Zn dialkyldithiophosphates. Oxidation Communications, 22(3), 374–386, 1999. 156. Yao, J.B. and J.X. Dong. Improvement of hydrolytic stability of borate esters used as lubricant additives. Lubrication Engineering, 51(6), 475–479, 1995. 157. Horodysky, A.G. Borated epoxides and lubricants containing same. U.S. Patent 4,410,438 (October 18, 1983, Mobil Oil Corporation). 158. Horodysky, A.G. Borated hydroxyl-containing compositions and lubricants containing same. U.S. Patent 4,788,340 (November 29, 1998, Mobil Oil Corporation). 159. Farng, L.O. and A.G. Horodysky. Mixed hydroquinone-hydroxyester borates as antioxidants. U.S.Patent 4,828,740 (May 9, 1989, Mobil Oil Corporation). 160. Braid, M. Phenol-hindered phenol borates and lubricant compositions containing same. U.S. Patent 4,530,770 (July 23, 1985, Mobil Oil Corporation). 161. Koch, F.W. Boron-containing compositions and lubricants containing them. U.S. Patent 5,240,624 (August 31, 1993, The Lubrizol Corporation). 162. Yao, J.B., W.L. Wang, S.Q. Chen, J.Z. Sun, and J.X. Dong. Borate esters used as lubricant additives. Lubrication Science, 14–4, 415–423, 2002. 163. Horodysky, A.G. Borated adducts of diamines and alkoxides, as multifunctional lubricant additives, and compositions thereof. U.S. Patent 4,549,975 (October 29, 1985, Mobil Oil Corporation). 164. Horodysky, A.G. and J.M. Kaminski. Friction reducing additives and compositions thereof. U.S. Patent 4,298,486 (November 3, 1981, Mobil Oil Corporation). 165. Horodysky, A.G. and J.M. Kaminski. Friction reducing additives and compositions thereof. U.S. Patent 4,478,732 (October 23, 1984, Mobil Oil Corporation). 166. Horodsky, A.G. and R.S. Herd. Etherdiamine borates and lubricants containing same. U.S. Patent 4,537,692 (August 27, 1985, Mobil Oil Corporation). 167. Yao, J.B. and P. Ma. Interaction of organic borate ester containing nitrogen with other lubricant additives. Lubricating Oil (Runhuayou), 21(2), 32–47, 2006. 168. Farng, L.O. and A.G. Horodysky. Lubricant composition containing phenolic/phosphorodithioate borates as multifunctional additives. U.S. Patent 4,956,105 (September 11, 1990, Mobil Oil Corporation). 169. Hinkamp, J.B., J.D. Bartleson, and G.E. Irish. Boron esters and process of preparing same. U.S. Patent 3,356,707 (December 5, 1967, Ethyl Corporation). 170. Braid, M. Borate esters and lubricant compositions containing such esters. U.S. Patent 4,547,302 (October 15, 1985, Mobil Oil Corporation). 171. Small, V.R., Jr., T.V. Liston, and A. Onopchenko. Diethylamine complexes of borated alkyl catechols and lubricating oil compositions containing the same. U.S. Patent 4,975,211 (December 4, 1990, Chevron Research Company). 172. Small, V.R., Jr., T.V. Liston, and A. Onopchenko. Diethylamine complexes of borated alkyl catechols and lubricating oil compositions containing the same. U.S. Patent 5,061,390 (October 29, 1991, Chevron Research Company). 173. Wright, W.E. and B.T. Davis. Haze-free boronated antioxidant. U.S. Patent 4,927,553 (May 22, 1990, Ethyl Corporation). 174. Brown, J.R., P.E. Adams, V.A. Carrick, B.R. Dohner, W.D. Abraham, J.S. Vilardo, R.M. Lange, and P.E. Mosier. Titanium compounds and complexes as additives in lubricants. U.S. Patent Appl. 2006/0217271 (September 28, 2006, The Lubrizol Corporation). 175. Esche, C.K., Jr. Additives and lubricant formulations for improved antioxidant properties. U.S. Patent Appl. 2006/0205615 (September 14, 2006, New Market Services Corporation). 176. Ravichandran, R., F. Abi-Karam, A. Yermolenka, M. Hourani, and I. Roehrs. Amine tungstates and lubricant compositions. WO 2007/009022 A2 (January 18, 2007, King Industries Inc.). 177. Gatto, V.J. and M.T. Devlin. Lubricant containing molybdenum compound and secondary diarylamine. U.S. Patent 5,650,381 (July 22, 1997, Ethyl Corporation). 178. Gatto, V.J. and M.T. Devlin. Lubricating composition. Great Britain Patent Application 2,307,245 (May 21, 1997, Ethyl Corporation). 179. Rasberger, M. Oxidative degradation and stabilisation of mineral oil based lubricants, in Chemistry and Technology of Lubricants. R.M. Motier and S.T. Orszulik, eds., Blackie Academic & Professional, London, UK, 98–143, 1997.

Lubricant Additives: Chemistry and Applications

180. Paolino, P.R. Antioxidants, in Thermoplastic Polymer Additives. J.T. Lutz Jr., ed., Marcel Dekker, Inc., New York, 1–35, 1989. 181. Reyes-Gavilan, J.L. and P. Odorisio. NLGI Spokeman, 64(11), 22–33, 2001. 182. Pospisil, J. Aromatic and heterocyclic amines in polymer stabilization. Advances in Polymer Science, 124, 87–190, 1995. 183. Lowry, T.H. and K.S. Richardson. Mechanism and theory, in Organic chemistry. Harper and Row Publishers, New York, 472, 1976. 184. Colclough, T. Role of additives and transition metals in lubricating oil oxidation. Industrial & Engineering Chemistry Research, 26, 1888–1895, 1987. 185. Maleville, X., D. Faure, A. Legros, and J.C. Hipeaux. Oxidation of mineral base oils of petroleum origin: The relationship between chemical composition, thickening, and composition of degradation. Lubrication Science, 9, 3–60, 1996. 186. Jensen, R.K., S. Korcek, L.R. Mahoney, and M. Zinbo. Liquid–phase autoxidation of organic compounds at elevated temperatures. 1. The stirred flow reactor technique and analysis of primary products from n-hexadecane autoxidation at 120–180°C. Journal of the American Chemical Society, 101, 7574–7584, 1979. 187. March, J. The aldol condensation, in Advanced Organic Chemistry, 3rd edn., John Wiley & Sons, Inc., New York, pp. 829–834, 1985. 188. Hamblin, P.C. and P. Robrbach. Piston deposit control using metal-free additives. Lubrication Science, 14(1), 3–22, 2001. 189. Sequeira, A., Jr. Crude oils, base oils, and petroleum wax, in Lubricant Base Oil and Wax Processing. Marcel Dekker, Inc., New York, 1994. 190. Demarco, N. Global supply: links and kinks. Lubes’N’Greases, April, 15–18, 2007. 191. Murray, D.W., C.T. Clarke, G.A. MacAlpine, and P.G. Wright. The effect of base stock composition on lubricant performance, SAE Technical Paper 821236. 192. Cerny, J., M. Pospisil, and G. Sebor. Composition and oxidative stability of hydrocracked base oils and comparison with a PAO. Journal of Synthetic Lubrication, 18(3), 199–213, 2001. 193. Kramer, D.C., J.N. Ziemer, M.T. Cheng, C.E. Fry, R.N. Reynolds, B.K. Lok, M.L. Sztenderowicz, and R.R. Krug. Influence of group II & III base oil composition on V.I. and oxidative stability. Presented at the 66th NLGI Annual Meeting, Tucson, AZ, October, 1999. 194. Igarashi, J., T. Yoshida, and H. Watanabe. Concept of optimal aromaticity in base oil oxidative stability revisited. Symposium on Worldwide Perspectives on the Manufacture, Characterization and Application of Lubricant Base Oils: 213th Annual Meeting Preprint, American Chemical Society, San Francisco, CA, April 13–17, 1997. 195. Wang, H.D. and X.L. Hu. The study advance on effects of molecular structure of group II, III base oils on the oxidation stability. Lubricating Oil, 20(2), 10–14, 2005. 196. Adhvaryu, A., S.Z. Erhan, and I.D. Singh. The effect of molecular composition on the oxidative behavior of group I base oils in the presence of an antioxidant additive. Lubrication Science, 14(2), 119–129, 2002. 197. Yoshida, T., J. Igarashi, H. Watanabe, A.J. Stipanovic, C.Y. Thiel, and G.P. Firmstone. The impact of basic nitrogen compounds on the oxidative and thermal stability of base oils in automotive and industrial applications. SAE Paper 981405, 1998. 198. MacFaul, P.A., K.U. Ingold, and J. Lusztyk. Kinetic solvent effects on hydrogen atom abstraction from phenol, aniline, and diphenylamine. The importance of hydrogen bonding on their radical-trapping (antioxidant) activities. Journal of the Organic Chemistry, 61, 1316–1321, 1996. 199. Hamblin, P.C., D. Chasan, and U. Kristen. A review: ashless antioxidants, copper deactivators and corrosion inhibitors. Their use in lubricating oils, in 5th International Colloquium on Additives for Operational Fluids, J Bart zed., Technische Akademie Esslingen, 1986. 200. Boozer, C.E., G.S. Hammond, C.E. Hamilton and J.N. Sen. Air oxidation of hydrocarbons II. The stoichiometry and fate of inhibitors in benezene and chlorobenezene. Journal of the American Chemical Society, 77, 3233–3237, 1955. 201. Gatto, V.J., W.E. Moehle, T.W. Cobb, and E.R. Schneller. Oxidation fundamentals and its application to turbine oil testing. Presented at the ASTM Symposium on Oxidation and Testing of Turbine Oils, December 5, 2005, Norfolk, VA. 202. Berger, H., T.A.B. Bolsman, and D.M. Brower. Catalytic inhibition of hydrocarbons autoxidation by secondary amines and nitroxides. In Developments in Polymer Stabilisation, 6. G. Scott, ed., Elsevier Applied Science Publishers, London, 1–27, 1983.

Antioxidants

203. Jensen, R.K., S. Korcek, M. Zinbo, and J.L. Gerlock. Regeneration of amines in catalytic inhibition oxidation. Journal of Organic Chemistry, 60, 5396–5400, 1995. 204. Bridgewater, A.J. and M.D. Sexton. Mechanism of antioxidant action: reactions of alkyl and aryl sulphides with hydroperoxides. Journal of the Chemical Society, Perkin Trans. II, 530, 1978. 205. Scott, G. Synergism and antagonism, in Atmospheric Oxidation and Antioxidants. vol. 2, G. Scott, ed., Elsevier Science Publishers B.V., Amsterdam, 431–457, 1993. 206. Hsu, S.M. Review of laboratory bench tests in assessing the performance of automotive crankcase oils. Lubrication Engineering, 37(12), 722–731, 1981. 207. Sharma, B.K. and A.J. Stipanovic. Development of a new oxidation stability test method for lubricating oils using high-pressure differential scanning calorimetry. Thermochimica Acta, 402, 1–18, 2003. 208. Zhang, Y., P. Pei, J.M. Perez, and S.M. Hsu. A new method to evaluate deposit-forming tendency of liquid lubricants by differential scanning calorimetry. Lubrication Engineering, 48(3), 189–195, 1992. 209. Hsu, S.M., A.L. Cummings, and D.B. Clark. Evaluation of automotive crankcase lubricants by differential scanning calorimetry. SAE Technical Paper 821252, 1982. 210. CEC L-85-T-99. Hot surface oxidation — pressure differential scanning calorimeter (PDSC). 211. ASTM Standard D 6186-98. Standard test method for oxidation induction time of lubricating oils by pressure differential scanning calorimetry (PDSC). 212. Adamczewska, J.Z. and C. Love. Oxidative stability of lubricants measured by PDSC CEC L-85-T-99 test procedure. Journal of Thermal Analysis and Calorimetry, 80, 753–759, 2005. 213. Adhvaryu, A., S.Z. Erhan, S.K. Sahoo, and I.D. Singh. Thermo-oxidative stability studies on some new generation API group II and III base oils. Fuel, 81(6), 785–791, 2002. 214. Migdal, C.A. The influence of hindered phenolic and aromatic amine antioxidants on the stability of base oils. 213th ACS National Meeting Preprint, San Francisco, April 13–17, 1997. 215. Rohrbach, P., P.C. Hamblin, and M. Ribeaud. Benefits of antioxidants in lubricants and greases assessed by pressurized differential scanning calorimetry. Tribotest Journal, 11(3), 233–246, 2005. 216. Jain, M.R., R. Sawant, R.D.A. Paulmer, D. Ganguli, and G. Vasudev. Evaluation of thermo-oxidative characteristics of gear oils by different techniques: effect of antioxidant chemistry. Thermochimica Acta, 435(2), 172–175, 2005. 217. Nakanishi, H., K. Onodera, K. Inoue, Y. Yamada, and M Hirata. Oxidative stability of synthetic lubricants. Lubrication Engineering, 53(5), 29–37, 1997. 218. Sharma, B.K., J.M. Perez, and S.Z. Erhan. Soybean oil-based lubricants: a search for synergistic antioxidants. Energy & Fuels, 21, 2408–2414, 2007. 219. Cheenkachorn, K., J.M. Perez, and W.A. Lloyd. Use of pressurized differential scanning calorimetry (PDSC) to evaluate effectiveness of additives in vegetable oil lubricants. ICE (American Society of Mechanical Engineers), 40, 197–206, 2003. 220. Gamlin, C.D., N.K. Dutta, N.R. Choudhury, D. Kehoe, and J. Matisons. Evaluation of kinetic parameters of thermal and oxidative decomposition of base oils by conventional, isothermal and modulated TGA and pressure DSC. Thermochimica Acta, 392–393, 357–369, 2002. 221. Gatto, V.J., H.Y. Elnagar, W.E. Moehle, and E.R. Schneller. Redesigning alkylated diphenylamine antioxidants for modern lubricants. Lubrication Science, 19(1), 25–40, 2007. 222. Florkowski, D.W. and T.W. Selby. The development of a thermo-oxidation engine oil simulation test (TEOST), SAE Technical Paper 932837, 1993. 223. ASTM Standard D 6335-03b. Standard test method for determination of high temperature deposits by thermal-oxidation engine oil simulation test. 224. Selby, T.W. and D.W. Florkowski. The development of the TEOST protocol MHT bench test of engine oil piston deposit tendency. Presented at 12th Esslingen Colloquium, January 11–13, 2000, Esslingen, Germany. 225. ASTM Standard D 7097-05. Standard test method for determination of moderately high temperature piston deposits by thermal-oxidation engine oil simulation test — TEOST MHT. 226. Anonymous. Correlation of TEOST performance with molar soap concentration for optimal deposit performance. Research Disclosure, 409, 531, 1998. 227. Sun, J.X., P.T. Pei, Z.S. HU, and S.M. Hsu. A modified thin-film oxygen update test (TFOUT) for lubricant oxidative stability study. Lubrication Engineering, May, 12–19, 1998. 228. ASTM Standard D 4702-02a, Standard test method for oxidation stability of gasoline automotive engine oils by thin-film oxygen uptake (TFOUT).

Lubricant Additives: Chemistry and Applications

229. Ku, C.S. and S.M. Hsu. A thin-film oxygen uptake test for the evaluation of automotive crankcase lubricants. Lubrication Engineering, 40(2), 75–83, 1984. 230. ASTM Standard D 943-04a, Standard test method for oxidation characteristics of inhibited mineral oils. 231. ASTM Standard D 4310-03. Standard test method for determination of the sludging and corrosion tendencies of inhibited mineral oils. 232. Yano, A., S. Watanabe, Y. Miyazaki, M. Tsuchiya, and Y. Yamamoto. Study on sludge formation during the oxidation process of turbine oils. Tribology Transactions, 47, 111–122, 2004. 233. Cerny, J., D. Landtova, and G. Sebor. Development of a new laboratory oxidation test for engine oils. Petroleum and Coal, 44(1–2), 48–50, 2002. 234. Cerny, J., Z. Strnad, and G. Sebor. Composition and oxidation stability of SAWE 15W-40 engine oils. Tribology International, 34(2), 127–134, 2001. 235. IP 280/89. Determination of oxidation stability of inhibited mineral turbine oils, in standard methods for analysis and testing of petroleum and related products. The Institute of Petroleum, 1994. 236. Jayaprakash, K.C., S.P. Srivastava, K.S. Anand, and K. Goel. Oxidation stability of steam turbine oils and laboratory methods of evaluation. Lubrication Engineering, 49(2), 89–95, 1984. 237. ASTM Standard D 2272-02. Standard test method for oxidation stability of steam turbine oils by rotating pressure vessel. 238. Swift, S.T., K.D. Butler, and W. Dewald. Turbine oil quality and field application requirements, in Turbine Lubrication in the 21st Century, ASTM STP 1407, W.R. Herguth and T.M. Warne, eds., American Society for Testing and Materials, West Conshohocken, PA, 2001. 239. Mookken, R.T., D. Saxena, B. Basu, S. Satapathy, S.P. Srivastava, and A.K. Bhatnagar. Dependence of oxidation stability of steam turbine oil on base oil composition. Lubrication Engineering, 53(10), 19–24, 1997. 240. Gatto, V.J. and W.E. Moehle. Lubricating oil composition with reduced phosphorus levels. U.S. Patent Application 2006/0223724 A1 (October 5, 2006, Albemarle Corporation). 241. Gatto, V.J. and M.A. Grina. Effects of base oil type, oxidation test conditions and phenolic antioxidant structure on the detection and magnitude of hindered phenol/diphenylamine synergism. Lubrication Engineering, 55(1), 11–20, 1999. 242. Dong, J. and C.A. Migdal. Lubricant compositions stabilized with multiple antioxidants. U.S. Patent Application 2006/0128574 A1 (June 15, 2006, Crompton Corporation). 243. Niu, Q.S., H. Chui, and L.P. Yang. Effects of lube base oil composition on the lubricant oxidation. Shiyou Xuebao Shiyou Jiagong, 2(2), 61, 1986. 244. Adhvaryu, A., Y.K. Sharma, and I.D. Singh. Studies on the oxidative behavior of base oils and their chromatographic fractions. Fuel, 78, 1293, 1999. 245. Hsu, S.M., C.S. Ku, and R.S. Lin. Relationship between lubricating basestock composition and the effects of additives on oxidation stability. SAE Technical Paper 821237, 1982. 246. Adhvaryu, A., S.Z. Erhan, Z.S. Liu, and J.M. Perez. Oxidation kinetic studies of oils derived from unmodified and genetically modified vegetables using pressurized differential scanning calorimetry and nuclear magnetic resonance spectroscopy. Thermochim Acta, 364, 87–97, 2000. 247. Stunenburg, F., A. Boffa, R. van den Bulk, K. Narasaki, M. Cooper, and G. Parsons. Impact of biodiesel use on the lubrication of diesel engines. Presented at the 13th Annual Fuels and Lubes Asia Conference. Bangkok, Thailand, March 7–9, 2007.

Zinc Dithiophosphates Randolf A. McDonald

CONTENTS 2.1 Introduction ............................................................................................................................. 51 2.2 Synthesis and Manufacture ..................................................................................................... 51 2.3 Chemical and Physical Nature ................................................................................................ 52 2.4 Thermal and Hydrolytic Stability ........................................................................................... 53 2.5 Oxidation Inhibition ............................................................................................................... 56 2.6 Antiwear and Extreme-Pressure Film Formation................................................................... 58 2.7 Applications ............................................................................................................................ 59 References ........................................................................................................................................ 61

2.1

INTRODUCTION

Zinc dialkyldithiophosphates (ZDDPs) have been used for more than 50 years in the lubricant industry as low-cost, multifunctional additives in engine oils, transmission fluids, hydraulic fluids, gear oils, greases, and other lubricant applications. The power of this particular compound is in its ability to simultaneously function as an excellent antiwear agent, a mild extreme-pressure (EP) agent, and an effective oxidation and corrosion inhibitor, all at a very low cost in comparison with the alternate chemistries available in the market. This is why it is still manufactured on a large scale by companies such as the ExxonMobil Corporation, Chevron Corporation, Ethyl Corporation, Lubrizol Corporation, and others. To date, as much as 300 million lb of ZDDP is still manufactured annually in the industrialized West.

2.2 SYNTHESIS AND MANUFACTURE ZDDP was first patented on December 5, 1944, by Herbert C. Freuler of the Union Oil Company of California in Los Angeles [1]. The multifunctionality of ZDDP was immediately noticed as Freuler indicated a noticeable increase in both the oxidation and the corrosion resistance of the lubricants tested with the novel compound at a 0.1–1.0% treatment level. The initial synthesis Freuler carried out involved the reaction of 4 mol of the intermediate dialkyldithiophosphate acid and 1 mol of hydrogen sulfide S →

4 ROH + P2S5

2(RO)2 P SH + H2S

(2.1)

followed by neutralization of the acid with 1 mol of zinc oxide S

S 2(RO)2 P SH + ZnO

→

2(RO)2 P SZnS P(OR)2 + H2O

(2.2)

Lubricant Additives: Chemistry and Applications

This synthetic route is still used today in the manufacturing of ZDDP. The P2S5, a flammable solid produced from the high-temperature reaction between elemental sulfur and phosphorus, is provided to the ZDDP manufacture in sealed aluminum bins containing 500–7200 lb P2S5. The P2S5 is hoppered into the reactor containing alcohol under a blanket of nitrogen. This is due to the ignitability of both the alcohol and the P2S5 when exposed to air. The hydrogen sulfide by-product, a highly toxic gas, is either converted to sodium sulfide solution in a caustic scrubber or thermally oxidized to sulfur dioxide. The heat of reaction and rate of hydrogen sulfide evolution are controlled by the addition rate of the P2S3 as well as the flow rate of the cooling water. The acid is then neutralized by zinc oxide; the reaction temperature is controlled by the addition rate of reaction depending on whether it is an acid to oxide or oxide to acid, addition scheme. Enough zinc oxide is used to neutralize the acid to a pH range, which will give a product suitably stable to thermal degradation and hydrogen sulfide evolution. The water formed from the reaction and the residual alcohol is vacuumdistilled. Any unreacted zinc oxide is then filtered, requiring a filtration system capable of removing particles as small as 0.1–0.8 μm. A larger molar excess of zinc oxide is often necessary to obtain the pH required for stability. The various manufactures have done much work to reduce the amount of zinc oxide used to obtain product stability (such as the addition of low-molecular-weight alcohols or carboxylic acids to lower the amount of residual sediment in the product before filtration) [2]. The filtered liquid product, with or without additional petroleum oil, is then provided to the customer in drums or in bulk.

2.3 CHEMICAL AND PHYSICAL NATURE ZDDP is an organometallic compound having four sulfur atoms coordinated to the zinc atom, which is in a tetrahedral, sp3 hybridized state. A Raman spectrum of ZDDP shows a strong P–S symmetric stretching band near 540 cm–1 and the absence of a strong Raman band near 660 cm–1, indicating a symmetrical sulfur–zinc coordination arrangement as in the following structure S

P RO

(2.3)

versus structure 2.4 RO

(2.4) OR

often given in the literature. The strong IR band at 600 cm–1 pointing to P=S stretching would be more consistent with PS2 antisymmetrical stretching in light of the Raman spectrum [3]. The neutral ZDDP molecules as represented in structure 3.1 actually exist as monomer, dimer, trimer, or oligomer depending on the state of the ZDDP, crystalline or liquid, the concentration of ZDDP in solvent, and the presence of additional compounds. The proposed structure for a tetramer in the case of a neutral zinc diisobutyldithiophosphate in hexane as determined by dynamic light scattering is shown in the following structure [4]: OR

RO S

RO P RO

S Zn

S S

Zn S

P OR

P RO

S P

(2.5)

P S

Zinc Dithiophosphates

Under overbased condition, when the ratio of dialkyldithiophosphate acid to zinc oxide is less than 2:1, a basic zinc salt  S  (RO) PS 2   

   Zn4O  6

(2.6)

will be synthesized along with the neutral salt. The basic salt is a tetrahedron of zinc atoms surrounding a central oxygen atom with (RO)2PS2 ligands along each edge of the tetrahedron. Crystallographic analysis of pure basic zinc salts has established the near equivalency of P–S–Zn bonds. Raman spectra have also shown symmetrical P–S stretching, supporting a symmetrical sulfur–zinc coordination arrangement for the basic ZDDPs [3]. In the presence of water, as one would encounter during commercial ZDDP manufacture, the basic zinc salt will be in equilibrium with the basic zinc double salt as seen in the following reaction:  S  (RO)2PS  

  + H2O 2  Zn4O  6 R = alkyl, phenyl, or alkylphenyl

S    (RO)2PS  

   Zn2OH  3

(2.7)

The stoichiometric excess of zinc oxide used in commercial ZDDP manufacture gives rise to a mixture of basic zinc salt (or zinc double salt) and neutral salt, the ratio depending on the amount of excess zinc oxide used and the molecular weight of the alkyl groups involved, where short alkyl groups tend to promote the formation  S    (RO) PS Zn4O 2  

 S    3 (RO) PS Zn + ZnO 2   2 R = alkyl, phenyl, or alkylphenyl

(2.8)

but performance differences seen between the two salts with respect to wear would imply that a more complex situation may exist [5]. As reported in the literature, basic ZDDP salts spontaneously decompose in solution into neutral complexes and zinc oxide when the temperature is increased [6]. Pure ZDDPs, with alkyl groups of four carbons or less, are solid at ambient temperatures (with the exception of sec-butyl, which is a semisolid at room temperature) and tend to have limited or no solubility in petroleum base stocks. ZDDPs with aryl or alkyl groups with more than five carbons are liquid at ambient temperature. To utilize the less-expensive and more readily available low-molecular-weight alcohols and yet to produce oil-soluble products, commercial manufactures use mixtures of high- (i.e., more than four carbons) and low-molecular-weight alcohols to obtain a statistical distribution of products favoring lesser amounts of pure low-molecular-weight ZDDPs. Other methods have also been developed to increase the amount of lower-molecular-weight alcohols in ZDDPs. These include the addition of ammonium carboxylates to inhibit precipitation [7] and the use of alkyl succinimides as solubilizing-complexing agents [8].

2.4 THERMAL AND HYDROLYTIC STABILITY The study of the thermal degradation of ZDDP is important in that much of the tribological characteristics of ZDDP can be explained by the effects of its decomposition products. The thermal decomposition of ZDDP in mineral oil has been found to be extremely complex. ZDDP in oil, upon heating to degradation, will give off volatile compounds such as olefin, alkyl disulfide,

Lubricant Additives: Chemistry and Applications

and alkyl mercaptan. A white precipitate will also form, which has been determined to be a lowsulfur-containing zinc pyrophosphate. The oil phase will contain varying amounts of S,S,S-trialkyltetrathiophosphate, O,S,S-trialkyltrithiophosphate, and O,O,S-trialkyldithiophosphate depending on the alkyl chain and the extent of degradation. The decomposition products of ZDDPs made from secondary alkyl alcohols, straight-chain primary alkyl alcohols, and branched primary alkyl alcohols appear similar in content but differ in proportions. This implies a similar mechanism for both primary and secondary ZDDP decomposition. O-alkyl thiphosphate esters are powerful alkylating agents. The P–O–R group is susceptible to nucleophilic attack, thus producing an alkylated nucleophile and thiophosphate anion. The incoming nucleophile initiates the reaction by an attack on the alpha carbon. This shows a kinetic dependence on alkyl structure S P

S R + Nu−

O− + Nu−R

(2.9)

Steric hindrance to the approach of the nucleophile will play a large rate-controlling factor here. The only nucleophile initially present is the dithiophosphate itself. The decomposition is initiated by one dithiophosphate anion attacking another, possibly on the same zinc atom: O−

OR 2 S

S−

OR S−

+ S

(2.10)

The resulting di-anion then attacks the triester, producing O,S-dialkyldithiophosphate anion O− S

(2.11)

resulting in the migration of an alkyl group from oxygen to sulfur. This anion then, in a route analogous to the dialkyldithiophosphate anion, reacts with itself in a nucleophilic attack to effect another alkyl transfer from oxygen to sulfur producing O,S,S-trialkyldithiophosphate OR RS

(2.12)

The net effect of the above reactions is a double alkyl migration from oxygen to sulfur OR −S

P OR

O S

(2.13)

O−

The major gases associated with ZDDP decomposition are dialkylsulfide (RSR), alkyl mercaptan (RSH), and olefin. The relative amounts of each of these gases depend on whether the alkyl group in the ZDDP is primary, branched primary, or secondary [9]. In the presence of mercaptide anion (RS –)

Zinc Dithiophosphates

from the intermediate zinc mercaptide (Zn[RS2]), O,S,S-trialkyldithiophosphate will react with mercaptide to produce alkyl mercaptan and results in the following structure: O− RS

(2.14)

The nucleophilic phosphoryl oxygen (P=O) will then attack another phosphorus atom to produce a P–O–P bond as in the following reaction: P

SR + O

P−

+ −SR

(2.15)

A mercaptide anion subsequently cleaves the P–O–P bond at the original P–O site, giving rise to a net exchange of one atom of oxygen for one atom of sulfur between the two phosphorus atoms: P

+ −SR

O− +

(2.16)

This gives rise to a net reaction for conversion of Structure 2.14 to S,S,S-trialkyltetrathiophosphate, dialkylsulfide and S-alkylthiophosphate di-anion as shown in the following reaction: O

O 3 RS

P O−

SR + −SR

2 RS

P O−

S O− + R2S + RS

(2.17)

The dialkylsulfide and S,S,S-trialkyltetrathiophosphate decomposition products are soluble in oil. The S-alkylthiophosphate decomposition product can also react with itself by way of a phosphoryl nucleophilic attack and elimination of mercaptide anion as in Reaction 2.15. This process will continue until a zinc pyro- and polypyrophosphate molecule with low sulfur content is formed. The chain will continue to extend until the product precipitates out of solution. The decomposition of primary alkyl ZDDPs can be accurately described as discussed earlier. ZDDPs made from branched primary alcohols will decompose in a similar fashion, although at a much slower rate. This can be explained by the fact that the alpha carbon of the branched primary alkyl group, being more sterically hindered than the unbranched primary alkyl group, will be less susceptible to nucleophilic attack, as described in Reaction 2.9. The increased steric hindrance from beta carbon branching will also decrease the amount of successful mercaptide anion attack on the branched alkyl P–O–R bond, resulting in less dialkylsulfide formation and a higher yield of mercaptan, an olefin by-product (through a competing protonation or elimination reaction with mercaptide anion). Lengthening the alkyl chain will have a much less pronounced effect on thermal stability than branching at the beta carbon due to the greater steric hindrance derived from the latter. The decomposition of secondary alkyl ZDDPs, although similar to primary decomposition, shows that olefin formation is much more pronounced. The increase in elimination over nucleophilic substitution in secondary ZDDPs over primary ZDDPs is easily explained by the fact that elimination is accelerated by increasing the alkyl substitution around the double bond formed. Thus, secondary alkyl groups will favor a thermal decomposition into olefins and phosphate acids at the expense of the sulfur–oxygen interchange noted earlier. In a similar but much more pronounced way, tertiary ZDDP decomposition will be dominated by facile production of olefin through elimination. This occurs at even moderate temperatures, making their use in commercial applications prohibitive.

Lubricant Additives: Chemistry and Applications

Aryl ZDDPs, due to the stability of the aromatic ring, are not susceptible to nucleophilic attack. Thus, the initial thermal decomposition reaction described in Reaction 2.9 cannot occur. Also the formation of olefin from an acid-catalyzed elimination reaction cannot occur. Aryl ZDDPs are, therefore, very thermally stable. A rating of various ZDDPs in terms of thermal stability would, therefore, be aryl > branched primary alkyl > primary alkyl > secondary > tertiary. The varying amounts of decomposition products that depend on the heat history and the alkyl or aryl chain involved will directly control the amount of EP and wear protection the ZDDP will provide in a given circ*mstance [10]. Hydrolysis of ZDDP begins with cleavage of the carbon–oxygen bond of the thiophosphate ester, with the hydroxide anion displacing the thiophosphate-anion-leaving group. The stability of the intermediate alkyl cation determines the ease with which this cleavage takes place. The secondary alkyl cation is more stable and more easily formed than the primary alkyl cation; therefore, hydrolysis of secondary ZDDP occurs more easily than hydrolysis of primary ZDDP. For the case of an aryl ZDDP, the carbon–oxygen bond cannot be broken, and the site of hydrolytic attack is the phosphorus–oxygen bond with the displacement of phenoxide anion with hydroxide anion. The order of hydrolytic stability is, therefore, primary > secondary > aryl.

2.5 OXIDATION INHIBITION Base oils used in lubricants degrade by an autocatalytic reaction known as auto-oxidation. The initial stages of oxidation are characterized by a slow, metal-catalyzed reaction with oxygen to form an alkyl-free radical and a hydroperoxy-free radical as seen in the following reaction: +

RH ⫹ O2 M→ R* ⫹ HOO*

(2.18)

This reaction is propagated by the reaction of the alkyl-free radical with oxygen to form an alkylperoxy radical. This radical further reacts with the base oil hydrocarbon to form alkyl hydroperoxide and another alkyl radical as seen in the following reaction: R * ⫹ O 2  → ROO* RH → ROOH ⫹ R*

(2.19)

This initial sequence is followed by chain branching and termination reactions forming highmolecular-weight oxidation products [11]. The antioxidant functionality of ZDDP is ascribed to its affinity for peroxy radicals and hydroperoxides in a complex pattern of interaction. The initial oxidation step of ZDDP by hydroperoxide is the rapid reaction involving the oxidative formation of the basic ZDDP salt as seen in the following reaction: S    Zn + R′OOH 4 (RO)2PS     2



 S    (RO) PS  Zn4O + R′OH 2    6

 S    +  (RO)2PS     2

(2.20)

In this reaction, 1 mol of alkyl hydroperoxide converts 4 mol of neutral ZDDP to 1 mol of basic ZDDP and 2 mol of the dialkyldithiophosphoryl radical (which subsequently reacts to produce the disulfide) [12]. The rate of hydroperoxide decomposition slows during an induction period during which the basic zinc thermally breaks down into the neutral ZDDP and zinc oxide [6]. This is followed by the neutral ZDDP further reacting with hydroperoxide to produce more dialkyldithiophosphoryl disulfide and more basic ZDDP. When the concentration of the basic ZDDP becomes low enough,

Zinc Dithiophosphates

a final rapid neutral salt-induced decomposition of the hydroperoxide will occur in which the dialkyldithiophosphoryl radical will not react with itself to form the disulfide but will react with hydroperoxide to form the dialkyldithiophosphoric acid as seen in the following reaction [13]: S

(RO)2PS• + R′OOH

(2.21)

(RO)2PSH + R′OO•

The dialkyldithiophosphoric acid then rapidly reacts with alkyl hydroperoxide, producing oxidation products that are inactive in oxidation chain reactions. The simplest reaction scheme for the reduction of the hydroperoxide is seen in the following reaction: S

 S    (RO) PS  + R′OH + H2O 2    2

2 (RO)PSH + R′OOH

(2.22)

Oxidation products include the disulfide mentioned earlier, the analogous mono- and trisulfides, and compounds of the form (RO)n(RS)3–n P=S and (RO)n(RS)3–n P=O [3]. These products show little activity as either oxidation inhibitors or antiwear agents. The literature also reveals an ionic process that will produce more dialkyl-dithiophosphoric acid as seen in the following reaction: S

Zn + R′OO•

(RO)PS

R′OO− +

S + (RO)2PS Zn+ (RO)2PS•

(2.23)

followed by Reaction 2.21 [14]. At low concentrations of ZDDP, hydrolysis of the ZDDP to the zinc basic double salt and dialkyldithiophosphoric acid becomes viable. At temperatures >125°C, the dialkyldithiophosphoryl disulfide decomposes into the dialkyldithiophosphoryl radicals, which further react with hydroperoxide to produce more dialkyldithiophosphoric acid [6]. Thus, many pathways are available to form the active dialkyldithiophosphoric acid. The neutral ZDDP also reacts with alkyl peroxy radicals. This is an electron-transfer mechanism that involves the stabilization of a peroxy intermediate. An attack by a second peroxy radical leads to the intramolecular dimerization of the resulting dithiophosphate radical forming the inactive dialkyldithiophosphoryl disulfide as seen in the following reaction: RO S P RO S

S OR P S OR

RO2• R OO • RO S P RO S

R OO

S•• P• RO S

S OR P S OR

RO2• RO RO

S P S

S OR P S OR

2 RO2− + Zn2+

(2.24)

Lubricant Additives: Chemistry and Applications

The zinc metal atom provides an easy route for heterolysis of the radical intermediate; thus, the disulfide, by itself, has little antioxidant functionality [15]. ZDDP acts as an oxidation inhibitor not only by trapping the alkyl radicals, thus slowing the chain reaction mechanism, but also by destroying alkyl hyperoxides and inhibiting the formulation of alkyl radicals. Empirical determination of the relative antioxidant capability of the three main classes of ZDDP shows secondary ZDDP > primary > aryl ZDDP. The relative performance of each ZDDP type may correlate with the stabilization of the dialkyl(aryl)dithiophosphoryl radical and its subsequent reactivity with alkyl hydroperoxide to produce the catalyzing acid. Commercial ZDPs are a mixture of both neutral and basic salts. It has recently been determined that neutral and basic ZDDPs give essentially equivalent performance with respect to antioxidant behavior. This can be explained by the equilibrium shown in Reaction 2.8. At elevated temperatures, as would occur in an oxidation test, the basic ZDDP is converted into the neutral ZDDP. As the temperature is lowered, the equilibrium shifts back toward the formation of the basic ZDDP, indicating that the concentration of basic ZDDP as a function of temperature. The solvent used and the presence of other additives also play a role in this equilibrium. Thus, the exact composition of neutral versus basic salts at any time in an actual formulation is a complex function of many variables.

2.6 ANTIWEAR AND EXTREME-PRESSURE FILM FORMATION ZDDPs operate mainly as antiwear agents but exhibit mild EP characteristics. As an antiwear agent, ZDDP operates under mixed lubrication conditions with a thin oil film separating the metal parts. Surface asperities, however, intermittently penetrate the liquid film, giving rise to metal-onmetal contact. The ZDDP reacts with these asperities to reduce the contact. Likewise, when the load is high enough to collapse the oil film, the ZDDP reacts with the entire metal surface to prevent welding and to reduce wear. A great deal of study has been done to determine the nature of this protective film and the mechanism of deposition, where the thermal degradation products of the ZDDP are the active antiwear agents. The antiwear film thickness and composition are directly related to temperature and the extent of surface rubbing. Initially, ZDDP is reversibly absorbed onto the metal surface at low temperatures. As the temperature increases, catalytic decomposition of ZDDP to dialkyldithiophosphoryl disulfide occurs, with the disulfide absorbed onto the metal surface. From here, the thermal degradation products (as described in Section 2.3) are formed with increasing temperature and pressure until a film is formed on the surface [16]. The thickness and composition of this film have been studied using many different analytical techniques, but no analysis gives a concise description of the film size and composition for the various kinds of metal-to-metal contact found in industrial and automotive lubrication regimes. In general, the antiwear/EP ZDDP film can be said to be composed of various layers of ZDDP degradation products. Some of these degradation products are reacted with the metal making up the lubricated surface. The composition of the layers is temperature-dependent. The first process that takes place is the reaction of sulfur (from the ZDDP thermal degradation products) with the exposed metal leading to the formation of a thin iron sulfide layer [17]. Next, phosphate reacts to produce an amorphous layer of short-chain ortho- and metaphosphates with minor sulfur incorporation. The phosphate chains become longer toward the surface, with the minimum chain length approaching 20 phosphate units. Some studies have indicated that this region is best described as a phosphate “glass” region in which zinc and iron cations act to stabilize the glass structure. At the outermost region of the antiwear film, the phosphate chains contain more and more organic ligands, eventually giving way to a region composed of organic ZDDP decomposition products and undegraded ZDDP itself. The thickness of the film has been analyzed to be as small as 20 nm using ultra thin film interferometry and as large as 1 μm using electrical capacitance [18–21].

Zinc Dithiophosphates

Recent work has concluded that, although the rate of film formation is directly proportional to temperature, a stronger correlation exists between film formation and the extent of metal-tometal rubbing as quantified by the actual distance that the metal slides during a given test period. The film reaches a maximum thickness at which point a steady state between formation and removal exists, the rate of formation being more temperature-dependent than the rate of removal. It was also found that the ZDDP reaction film has a “solid-like” nature (as opposed to be a highly viscous liquid) due to the lack of reduction of film thickness observed with time on a static test ball [22]. Another mechanism of wear found to be inhibited by ZDDP is wear produced from the reaction of alkyl hydroperoxides with metal surfaces. It was found that the wear rate of automobile engine cam lobes is directly proportional to alkyl hydroperoxide concentration. The mechanism proposes the direct attack of hydroperoxide (generally through fuel combustion and oil oxidation) on fresh metal, causing the oxidation of an iron atom from a neutral charge state to Fe+3 by reaction with 3 mol of alkyl hydroperoxide as described in the following reactions: 2 ROOH ⫹ Fe  → 2 RO∗ ⫹ 2 OH⫺ ⫹ Fe⫹2

(2.25)

ROOH ⫹ Fe⫹2  → RO∗ ⫹ OH⫺ ⫹ Fe⫹3

(2.26)

The ZDDP and its thermal degradation products neutralize the effect of the hydroperoxides by the mechanism described in Reactions 2.20 through 2.23 in Section 2.5. It was also shown that peroxy and alkoxy radicals were far less aggressive toward metal surfaces than hydroperoxides, indicating that free-radical scavengers such as hindered phenols would be ineffective in controlling this kind of engine wear. This may explain why the antiwear performance of ZDDP is directly related to its antioxidation performance in the order of secondary ZDDP > primary ZDDP > aryl ZDDP rather than correlating with the order of thermal stability (aryl > primary > secondary) [23]. A recent study has been conducted to investigate the difference in wear performance between neutral and basic ZDDPs in the sequence VE engine test. The neutral ZDDP performed better in value train wear protection than the basic ZDDP. The basic salt actually failed the sequence VE engine test, indicating that using commercial ZDDPs with lower basic salt content may be preferred when limited to 0.1% maximum phosphorus content (as mandated by the International Lubricant Standardization and Approval Committee [ILSAC] GF-3 motor oil specification). It was suggested that the increased wear protection by neutral ZDDP could be explained by the superior adsorption of the oligomeric structure of the neutral salt, leading to the formation of longer polyphosphate chains relative to the basic salt [5].

2.7

APPLICATIONS

ZDDPs are used in engine oils as antiwear and antioxidant agents. Primary and secondary ZDDPs are both used in engine oil formulations, but it has been determined that secondary ZDDPs perform better in cam lobe wear protection than primary ZDDPs. Secondary ZDDPs are generally used when increased EP activity is required (i.e., during run-in to protect heavily loaded contacts such as valve trains). ZDDPs are generally used in combination with detergents and dispersants (alkaline earth sulfonate or phenate salts, polyalkenyl succine amides or Mannich-type dispersants), viscosity index improvers, additional organic antioxidants (hindered phenols, alkyl diphenyl amines), and pour point depressants. A typical lubricant additive package for engine oils can run in high at 25% in treatment level. The ILSAC has designated its GF-3 engine oil specification to include a maximum limit of 0.1% phosphorus to minimize the engine oil’s negative impact on the emissions catalyst. For the GF-4 specification, the limit in phosphorus was reduced even further. As a result

Lubricant Additives: Chemistry and Applications

of the minimum phosphorus requirement, the treatment level for ZDDP in organic oils is limited to ∼0.5 to 1.5%, depending on the alkyl chain length used. The new challenge to motor oil formulators is in passing the required ILSAC tests while keeping the ZDDP level low. Yamaguchi et al. have shown that the antioxidant effect of ZDDP is significantly enhanced in API group II base stocks with as much as 50% increase noted for a basic ZDDP. An increase in antioxidancy was also noted when using ZDDPs in polyol ester [24]. Several studies have also shown that ZDDP oxidation by-products are in effective antiwear agents. The use of these base-stock effects to extend the oxidation life of the ZDDP may be a suitable method for the formulator to reduce the level of ZDDP needed to accommodate the GF-3 limits. The synergistic effect between organic molybdenum compounds and ZDDP in wear reduction is currently being studied as a means of lowering phosphorus content in engine oils. In U.S. patent 5,736,491, molybdenum carboxylate is used with ZDDP to give a synergistic reduction in friction coefficient by as much as 30%, thus allowing a reduction in the total phosphorus content and an improvement in fuel economy [25]. The patent literature has sited other organic molybdenum compounds such as molybdenum dithiocarbamates (MoDTC) and dialkyldithiophosphates (MoDTP) as being useful, synergistic secondary antiwear agents [26]. ZDDPs are also used in hydraulic fluids as antiwear agents and antioxidants. The treatment level for ZDDP in hydraulic fluids is lower than that used for engine oils, typically running between 0.2 and 0.7% by weight. They are used in combustion with detergents, dispersants, additional organic antioxidants, viscosity index improvers, pour point depressants, corrosion inhibitors, defoarmers, and demulsifiers for a total treatment level of between 0.5 and 1.25% [27]. Primary ZDDPs are preferred over secondary ZDDPs due to their better thermal and hydrolytic stability. One problem faced by hydraulic fluid formulators is the need for a fluid that will service both high-pressure rotary vane pumps and axial piston pumps, preferably out of the same sump. High-pressure vane pumps require a hydraulic fluid with antiwear properties and oxidative stability commonly achieved through the use of ZDDPs. High-pressure piston pumps need only rust and oxidation protection and do not require ZDDPs. ZDDPs can cause catastrophic failure to axial piston systems by adversely affecting the sliding steel–copper alloy interfaces. The patent literature has several examples of formulators trying to overcome this problem with the use of additional wear-moderating chemistries such as sulfurized olefins, polyol esters or borates of them, fatty acid imidazolines, aliphatic amines, and polyamines. Another problem faced by hydraulic fluid formulators is the interaction of ZDDPs with overbased alkaline earth detergent salts (as well as the interaction of carboxylic acid and alkenyl succinic anhydride rust preventatives with these detergents) in the presence of water to give filter-clogging by-products. Formulators have tried to overcome this problem of poor “wet” filterability by using nonreactive rust inhibitors (i.e., alkenyl succinimides) and improving the hydrolytic stability of ZDDP antiwear agent [28]. ZDDPs are used in EP applications such as gear oils, greases, and metalworking fluids. Secondary ZDDPs are preferred due to their thermal instability resulting in quick film formation under high loads. In automotive gear oils, ZDDPs are used at 1.5–4% in combination with EP agents (such as sulfurized olefins), corrosion inhibitors, foam inhibitors, demulsifiers, and detergents. Total multifunctional additive package treatment levels for automotive gear lubricant additives are from 5 to 12% by weight. Industrial gear oil formulators have generally gone to ashless systems using sulfur–phosphorus-based EP antiwear chemistries at total additive package treatment levels of 1.5–3%. In general, the recent focus in gear oil technology improvement has centered on increased thermal stability and EP properties. ZDDPs are used in greases in chemical systems that closely resemble gear oil formulations. Many gear oil lubricant additives are used in EP greases. In general, the ZDDP treatment level for greases is in the same range as that used for gear oils. ZDDP, usually secondary or a mixture of secondary and primary, is used in combination with sulfurized olefins, corrosion inhibitors, ashless antioxidants, and additional friction modifiers. A recent advancement in grease technology is the use of ZDDP/sulfurized olefin synergy to replace antimony and lead in high-EP grease

Zinc Dithiophosphates

formulations. This has generally been limited to the European market, having been pioneered in Germany. ZDDPs, in combination with sulfurized olefins, are also used to replace chlorinated paraffins in medium- to heavy-duty metalworking fluids. This is due to the possible carcinogenicity of the low-molecular-weight analogs of chlorinated paraffin. European formulators, and to a certain extent Japanese formulators, use ZDDPs in this way. The use of ZDDPs in metalworking fluids in the United States is limited due to environmental concerns. The U.S. Environmental Protection Agency classifies them as marine pollutants. In conclusion, after 50 years, ZDDPs still enjoy a wide variety of uses in the lubrication industry, with production volumes remaining at high levels. The majority of ZDDP production is used in automobile engine oil. The impact of the GF-2 and GF-3 phosphorus-level specification of 0.1%, however, was reduction of ZDDP production in the past 10 years. The Ford Motor Company is currently evaluating engine oils with 0–0.6% phosphorus levels in fleet tests in preparation for the looming GF-4 standard in 2004, which will require engine oils to have minimal impact on emission system deterioration. This could further negatively impact ZDDP production. The need to understand clearly how ZDDPs function in terms of wear and oxidation protection is reinforced by the need to develop satisfactory phosphorus-free alternatives to ZDDP. The development of such chemistries, within the economic and functional limits that ZDDPs impose, will be a daunting task for future researchers. Until that time, the elimination of ZDDPs from various industrial lubricants will mandate either higher costs or less performance.

REFERENCES 1. Freuler, H.C. Modified lubricating oil. U.S. Patent 2,364,284 (December 5, 1944, Union OIL Co. of California). 2. Adams, D.R. Manufacture of dihydrocarbyl dithiophosphats. U.S. Patent 5,672,294 (May 6, 1997, Exxon Chemical Patents, Inc.). 3. Paddy, J.L. et al. Zinc dialkyldithiophosphate oxidation by cumene hydroperoxide: kinetic studies by Raman and 31P NMR spectroscopy. Trib Trans 33(1):15–20, 1990. 4. Yamaguchi, E.S. et al. Dynamic light scattering studies of neutral diisobutyl zinc dithiophosphate. Trib Trans 40(2):330–337, 1997. 5. Yamaguchi, E.S. The relative wear performance of neutral and basic zinc dithiophosphates in engines. Trib Trans 42(1):90–94, 1999. 6. Bridgewater, A.J., J.R. Dever, M.D. Sexton. Mechanisms of antioxidant action, part 2. Reactions of zinc bis(O,O′-dialkyl(aryl)phosphorodithioates) and related compounds with hydroperoxides. J Chem Soc Perkin II:1006–1016, 1980. 7. Buckley, T.F. Methods for preventing the precipitation of mixed zinc dialkyldithiophosphates which contain high percentages of a lower alkyl group. U.S. Patent 4,577,037 (March 18, 1986, Chevron Research Co.). 8. Yamaguchi, E.S. Oil soluble metal (lower) dialklyl dithiophosphate succinimide complex and lubricating oil composition containing same. U.S. Patent 4,306,984 (December 22, 1981, Chevron Research Co.). 9. Luther, H., E. Baumgarten, K. Ul-Islam. Investigations by gas chromatography into the thermal decomposition of zinc dibutyldithiophosphates. Erdol und Kohle 26(9):501, 1973. 10. Coy, R.C., R.B. Jones. The chemistry of the thermal degradation of zinc dialkyldithiophosphate additives. ASLE Trans 24(1):91–97, 1979. 11. Rasberger, M. Oxidative degradation and stabilization of mineral based lubricants, in R.M. Moritier and S.T. Orszulik, eds., Chemistry and Technology of Lubricants, 2nd ed. London: Blackie Academic and Professional, 1997, pp. 82–123. 12. Rossi, E., L. Imperoto. Chim Ind (Milan) 53:838–840, 1971. 13. Sexton, M.D., J Chem Soc Perkin Trans II:1771–1776, 1984. 14. Howard, S.A., S.B. Tong. Can J Chem 58:92–95, 1980. 15. Burn, A.J. The mechanism of the antioxidant action of zinc dialkyl dithiophosphates. Tetrahedron 22:2153–2161, 1966.

Lubricant Additives: Chemistry and Applications

16. Bovington, C.H., Darcre, B. The adsorption and reaction of decomposition products of zinc dialkyldithiophosphate on steel. ASLE Trans 27:252–258, 1984. 17. Bell, J.C., K.M. Delargy. The composition and structure of model zinc dialkyldithiophosphate antiwear films, in M. Kozna, ed., Proceedings 6th International Congress on Tribology Eurotrib ’93, Budapest, 2:328–332, 1993. 18. Willermet, P.A., R.O. Carter, E.N. Boulos, Lubricant-derived tribochemical films—An infra-red spectroscopic study. Trib Intl 25:371–380, 1992. 19. Fuller, M. et al. Chemical characterization of tribochemical and thermal films generated from neutral and basic ZDDPs using x-ray absorption spectroscopy. Trib Intl 30:305–315, 1997. 20. Allison-Greiner, A.F., J.A. Greenwood, A. Cameron. Thickness measurements and mechanical properties of reaction films formed by zinc dialkyldithiophosphate during running. Proceedings of IMechE International Conference on Tribology—Friction, Lubrication and Wear 50 Years on, London, IMechE, 1:565–569, 1987. 21. Tripaldi, G., A. Vettor, H.A. Spikes. Friction behavior of ZDDP films in the mixed boundary/EHD regime. SAE Tech. paper 962036, 1996. 22. Taylor, L., A. Dratva, H.A. Spikes. Friction and wear behavior of zinc dialkyldithiophosphate additive. 43(3):469–479, 2000. 23. Habeeb, J.J., W.H. Stover. The role of hydroperoxides in engine wear and the effect of zinc dialkyldithiophosphates. ASLE Trans 30(4):419–426, 1987. 24. Yamaguchi, E.S. et al. The relative oxidation inhibition performance of some neutral and basic zinc dithiophosphate salts. S.T.L.E. Preprint No. 99-AM-24, pp. 1–7, 1989. 25. Patel, J.A. Method of improving the fuel economy characteristics of a lubricant by friction reduction and compositions useful therein. U.S. Patent 5,736,491 (April 7, 1998, Texaco, Inc.). 26. Naitoh, Y. Engine oil composition. U.S. Patent 6,063,741 (May 16, 2000, Japan Energy Corporation). 27. Brown, S.H. Hydraulic system using an improved antiwear hydraulic fluid. U.S. Patent 5,849,675 (December 15, 1998. Chevron Chemical Co.). 28. Ryan, H.T. Hydraulic fluids. U.S. Patent 5,767,045 (June 16, 1998, Ethyl Petroleum Additives, Ltd.).

Ashless Phosphorus– Containing Lubricating Oil Additives W. David Phillips

CONTENTS 3.1 Introduction and Scope ...........................................................................................................64 3.2 Historical Background ............................................................................................................ 65 3.3 Manufacture of Phosphorus-Containing Lubricating Oil Additives ...................................... 68 3.3.1 Neutral Alkyl and Aryl Phosphate Esters .................................................................... 68 3.3.1.1 Natural Phosphates.......................................................................................... 69 3.3.1.2 Synthetic Phosphates from Isopropylphenols ................................................. 70 3.3.1.3 Synthetic Phosphates from Tertiarybutylphenols ........................................... 70 3.3.2 Acid Phosphate Esters .................................................................................................. 71 3.3.2.1 Alkyl and Aryl Acid Phosphates (Non-ethoxylated) ...................................... 71 3.3.2.2 Alkyl- and Alkylarylpolyethleneoxy Acid Phosphates................................... 72 3.3.3 Amine Salts of Acid Phosphates and of Polyethyleneoxy Acid Phosphates ................ 72 3.3.4 Neutral Phosphite Esters............................................................................................... 73 3.3.5 Alkyl and Aryl Acid Phosphites ................................................................................... 73 3.3.6 Dialkyl Alkyl Phosphonates ......................................................................................... 74 3.4 The Function of Lubricity Additives....................................................................................... 74 3.4.1 The Basic Mechanism of Lubrication and Wear and the Influence of Additives ......... 75 3.5 Investigations into the Mechanism and Activity of Phosphorus-Containing Additives ......... 79 3.5.1 Early Investigations into Antiwear and Extreme Additives .........................................80 3.5.2 Neutral Alkyl and Aryl Phosphates..............................................................................80 3.5.2.1 Historical Background ....................................................................................80 3.5.2.2 Recent Technical Developments ..................................................................... 86 3.5.2.3 Recent Commercial Developments ................................................................. 89 3.5.3 Alkyl and Aryl Acid Phosphates .................................................................................. 91 3.5.3.1 Non-ethoxylated .............................................................................................. 91 3.5.3.2 Alkyl and Alkarylpolyethleneoxy Acid Phosphates .......................................92 3.5.3.3 Amine Salts of Acid Phosphates ..................................................................... 95 3.5.4 Neutral Alkyl and Aryl Phosphites .............................................................................. 98 3.5.4.1 Use as Antiwear/Extreme-Pressure Additives ................................................ 98 3.5.4.2 Use as Antioxidants for Lubricating Oils .......................................................99 3.5.5 Alkyl and Aryl Acid Phosphites ................................................................................. 100 3.5.5.1 Amine Salts of Acid Phosphites.................................................................... 102 3.5.6 Phosphonate and Phosphinate Esters.......................................................................... 103 3.5.7 A Summary of the Proposed Mechanism for Antiwear and Extreme-Pressure Activity of Phosphorus-Based Additives .................................................................... 104 3.6 Market Size and Commercial Availability............................................................................ 105 63

Lubricant Additives: Chemistry and Applications

3.7 Toxicity and Ecotoxicity ....................................................................................................... 108 3.8 The Future for Ashless Phosphorus-Based Lubricating Oil Additives................................. 110 3.9 Lubricating Oil Formulations (General) ............................................................................... 111 3.10 Hydraulic Oils ....................................................................................................................... 111 3.11 Automotive Engine Oils ........................................................................................................ 111 3.12 Fuels ...................................................................................................................................... 113 3.13 Conclusions ........................................................................................................................... 113 References ...................................................................................................................................... 114 Appendix A: Early Patent Literature on Phosphorus-Containing Compounds ........................... 118 Neutral Phosphates................................................................................................................ 118 Neutral Phosphites ................................................................................................................ 118 As Antioxidants .................................................................................................................... 118 Acid Phosphates/Phosphites ................................................................................................. 118 Phosphonates ......................................................................................................................... 119 Alkyl- and Arylpolyethyleneoxy-Phosphorus Compounds .................................................. 119 Amine Salts ........................................................................................................................... 119 Physical Mixtures of Phosphorus and Sulfur and Chlorine Compounds ............................. 119 Miscellaneous Phosphorus Compounds ............................................................................... 120 Appendix B: Additional Literature and Patent References on the Mechanism and Performance of Phosphorus-Containing Additives .............................................................. 120 Neutral Phosphates................................................................................................................ 120 Neutral Phosphites ................................................................................................................ 121 Alkoxylated Phosphates ........................................................................................................ 121 Amine Salts ........................................................................................................................... 121 Dialkyl Alkyl Phosphonates ................................................................................................. 121 Mixtures of Phosphorus and Sulfur Compounds.................................................................. 122 General References ............................................................................................................... 122

3.1 INTRODUCTION AND SCOPE In any discussion of phosphorus-containing lubricating oil additives, the products that probably come most rapidly to mind are the zinc dialkyldithiophosphates (ZDDPs)—multifunctional additives that have been widely used in both automotive and industrial oils for many years. However, a wide variety of ashless phosphorus-containing additives are used in the lubricating oil industry. As with the metal-containing dithiophosphates, they have been in use over a long period and, despite considerable research into alternative chemistries, the basic structures introduced in the 1930s are still used today. In contrast, the technology of most other additive types and that of the base stocks themselves have steadily developed over this time. Many different types of phosphorus-containing molecules have been examined as additives for lubricating oils, with most attention given to their potential as antiwear (AW) and extreme-pressure (EP) additives. Consequently, the patent literature contains a host of references to different structures displaying this characteristic. However, regardless of composition, all the additives used in this application serve the same and specific function of bringing phosphorus into contact with the metal surface, where it can be adsorbed and, under certain conditions, react. The resulting surface film improves the lubrication properties of both mineral and synthetic oils. This chapter discusses the use of chemicals that contain only phosphorus to improve the performance characteristics of oils, specifically neutral and acid phosphates, phosphites and phosphonates, and the amine salts of the acids (see Figures 3.1 and 3.2 for an outline of the main classes and their structures). These are the principal types of phosphorus compounds in current commercial use, but other types have also been examined and claimed in the patent literature, for example, phosphoramidates. There are also ashless compounds in which sulfur or chlorine has been incorporated

Ashless Phosphorus-Containing Lubricating Oil Additives

Phosphate esters Alkyl or aryl phosphates

Neutral phosphates

Acid phosphates

Non-ethoxylated

Ethoxyalkyloxy-

Amine salts

Phosphites and phosphonates Alkyl or aryl phosphites/phosphonates

Neutral phosphites/phosphonates

Acid phosphites/phosphonates

Neutral amine salts

FIGURE 3.1 The main classes of phosphorus-containing additives for lubricating oils.

into the molecule as, for example, in thiophosphates and chlorinated phosphates. These are, however, outside the scope of this discussion, but the performance of mixtures of compounds separately containing phosphorus and sulfur or chlorine will be mentioned. In addition to examining the impact of ashless phosphorus compounds on lubrication performance, this chapter also looks at their performance as antioxidants, rust inhibitors, and metal passivators. Additionally, their polar nature makes them good solvents and assists the solution of other additives in nonpolar base stocks. The versatility displayed by phosphorus-containing additives is such that usage of these products continues to grow nearly half a century after their introduction, and they find application in the latest technological developments.

3.2 HISTORICAL BACKGROUND Until the 1920s, additive-free mineral oils met the majority of industry’s lubrication requirements. In the applications where their performance was unsatisfactory, an increase in viscosity or the sulfur content of the oils then available usually provided adequate lubrication. For very severe applications, the oil would be blended with animal or vegetable oils—for example, tallow or rapeseed oils were used for steam engine cylinder lubrication. Fish oils were used in the early locomotive axle boxes, while castor oil reduced friction in worm gear drives and flowers of sulfur were added to cutting oils. However, when hypoid gears were introduced, they quickly revealed the limited lubrication of oils then available. This resulted in the development of additives such as sulfurized lard oil and lead naphthenate. These were followed by sulfurized sperm oil, an additive that eventually became widely used in both industrial and automotive applications. The earliest type of an organic phosphorus chemical to find use as a lubricating oil additive is thought to have been a neutral triaryl phosphate, specifically tricresyl phosphate (TCP). This material was originally synthesized in about 1854 [1] although trialkyl phosphates were synthesized slightly earlier, in about 1849 [2]. Commercial production of TCP began in about 1919, when

Lubricant Additives: Chemistry and Applications R

O RO

O O

R Trialkyl phosphate R

O RO

Triaryl phosphate

O O

Alkyl monoacid phosphate

Aryl diacid phosphate

Trialkyl phosphite

Triaryl phosphite O

Dialkyl phosphite

Dialkyl alkyl phosphonate

O RO

O OH.H2NR

OR Amine phosphate

FIGURE 3.2

R Alkyl dialkyl phosphinate

The structures of some common phosphorus-containing lubricating oil additives.

this product was introduced as a plasticizer for cellulose nitrate, but it was not until the 1930s that patents began to appear claiming improved lubrication when TCP was blended with mineral oil. In 1936, this use was claimed in gear oils [3], but a detailed investigation into their behavior as AW additives was not published until 1940 [4,5], by which time TCP was already said to be in widespread use. During World War II, extensive research into phosphorus-containing additives took place in Germany [6,7]. This research was facilitated by the recent availability of test equipment for assessing wear and load-carrying behavior, for example, the four-ball machine [8]. The results of the research concluded that for high load-carrying (later known as EP) performance, the molecule must contain the following: • A phosphorus atom • Another active group, for example, Cl– or OH– (for attachment to the metal surface) • At least one aryl or alkyl group (phosphoric acid was not thought to be active)

Ashless Phosphorus-Containing Lubricating Oil Additives

Subsequent to these studies, the market adopted chlorine-containing phosphates such as tris(2-chloroethyl) phosphate, but they were later replaced in most applications by other EP additives as chlorine tended to produce corrosion. The 1940s and 1950s saw significant development activity in the oil industry involving TCP, and patents appeared claiming the use of this AW/EP additive in general industrial oils [9], rolling oils [10], cutting oils [11], greases [12], rock drill lubricants [13], and aviation gas turbine lubricants [14,15]. Some military specifications, for example, on hydraulic oils (NATO codes H515/520/576), were published, which initially called for the use of this additive. However, in the late 1960s, the difficulty of obtaining good-quality feedstocks for the manufacture of natural phosphates based on cresol and xylenol, together with the concern regarding the neurotoxicity of TCP [16,17], led to the reformulation of many products with the less toxic synthetic triaryl phosphates based on alkylated phenols. TCP is still used today in aviation applications, but the quality of the phosphate in terms of its purity and freedom from the o-cresol isomers that were mainly responsible for the neurotoxicity behavior, has significantly improved in the past 10–20 years. In addition to its use as an oil additive, TCP was also used for a period in the 1960s as an ignition control additive for motor gasoline to avoid preignition arising from the deposition of lead salts. These were formed by the interaction of the lead tetraethyl antiknock additive and the alkyl halide scavenger [18–21]. Alkyl phosphates were claimed for this application in 1970 [22]. As a result of their polar nature, neutral triaryl phosphates have also been claimed as corrosion inhibitors for hydrocarbons [23,24], but they are unlikely to be promoted for this application today in view of the availability of more active species, such as the acid phosphates. The use of trialkyl phosphates as AW and EP additives has been much less extensively evaluated. Although a flurry of patent activity took place in the late 1920s and 1930s covering methods for their manufacture [25–34], there was little interest in their use as lubricating oil additives for further 20 years. This was probably a result of the focus, in the interim period, on chlorinated derivatives. It was not until the late 1950s that tributyl phosphate (TBP) was disclosed in blends with isopropyl oleate [35] for use in gear oils and claimed in blends with chlorinated aromatics. In 1967, a patent appeared claiming the use of alkyl phosphates or the amine salts of alkyl acid phosphates in a water-based lubricating composition [36]. In addition to alkyl phosphates, various other types of phosphorus-containing compounds have been evaluated as AW/EP additives. Patents on acid phosphates claiming their use as EP additives for oil appeared in 1935 and 1936 [37–39], whereas the first publication with detailed information on the use of ethoxylated alkyl or aryl phosphate oil additives (in metalworking applications) appeared in 1964 [40]. Patents on the use of these products in mineral oils [41] and in synthetic esters [42] appeared later. Alkoxylated acid phosphates were also found to have good rust inhibition properties [43], a feature that was additionally observed for the alkyl (or aryl) acid phosphates [44,45]. Neutral amine salts of alkyl acid phosphates were claimed in 1964 [43] and, in 1969, in admixture with neutral phosphates [46]. These are, however, just a few examples of the patent estate covering these product groups. The other main phosphorus-containing products to be discussed are the phosphites. The basic chemistry of alkyl and aryl phosphites, like that of the phosphate esters, was also uncovered in the nineteenth century. In a similar fashion, their utilization as oil additives was not exploited until much later. Patents appeared in 1940 on the use of mixed aryl phosphites as oil antioxidants [47] and on their activity as AW/EP additives at least as early as 1943 [48]. Isomeric with the acid phosphites are phosphonates (Figure 3.2). Dialkyl alkyl phosphonates were claimed as lubricants in 1952 and 1953 [49,50] but not until about 1971 as friction modifiers and EP additives [51,52]. The preceding summary focused on the use of phosphorus compounds alone. In reality, they are widely used in admixtures with sulfur-containing materials to provide good lubrication over a wider range of performance requirements. Examples of some of the combinations patented are given in Appendix A.

3.3 3.3.1

Lubricant Additives: Chemistry and Applications

MANUFACTURE OF PHOSPHORUS-CONTAINING LUBRICATING OIL ADDITIVES NEUTRAL ALKYL AND ARYL PHOSPHATE ESTERS

Although phosphate esters can be regarded as salts of orthophosphoric acid, they are currently not produced from this raw material because the yields are relatively low (∼70% for triaryl phosphates). Instead, phosphorus oxychloride (POCl3) is reacted with either an alcohol (ROH), a phenol (ArOH), or an alkoxide (RONa) as indicated in the following reactions: 3ROH ⫹ POCl3 → (RO)3PO ⫹ 3HCl trialkyl phosphate

3ArOH ⫹ POCl3 → (ArO)3PO ⫹ 3HCl triaryl phosphate

3RONa ⫹ POCl3 → (RO)3PO ⫹ 3NaCl trialkyl phosphate

(3.1)

(3.2)

(3.3)

Reactions 3.1 through 3.3 pass through intermediate steps as shown in the following reaction: ROH ⫹ POCl3 → ROPOCl 2 ⫹ HCl ROH  →(RO)2 POCl ⫹ HCl ROH  →(RO)3PO ⫹ HCl

(3.4)

The intermediate products are called phosphorochloridates, and, if desired, it is possible to obtain a mixture rich in a particular intermediate by changing the ratio of reactants. In 2001, a two-stage process for the production of a tertiarybutylphenyl phosphate with low levels of triphenyl phosphate (TPP) was described [53]. In this process, the POCl3 is first reacted with sufficient tertiarybutylphenol (C4H9ArOH) to produce mainly the diphosphorochloridate. Phenol is then added to the reaction mixture to produce predominantly the mono-tertiarybutylphenyl diphenyl phosphate (reaction 3.5). This product was said to give unusually low air entrainment values and was therefore mainly of interest as a hydraulic fluid base stock. C4H 9 ArOH + POCl3  → C4H 9 ArOPOCl2 PhOH → C4 H9 ArOPO(OPh)2 + 2HCl

(3.5)

The production of mixed products can be achieved by using different alcohols or by an alcohol and alkoxide (reaction 3.6). These materials are not in significant use as AW/EP additives. ROH ⫹ POCl3 → (RO)2 POCl ArONa → (RO)2 PO(OAr) ⫹ NaCl mixed alkyl aryl phosphate

(3.6)

Trialkyl (or alkoxyalkyl) phosphates can be produced by either reaction 3.1 or 3.3, although in reaction 3.1, unless catalyzed, a considerably excess alcohol is required to drive the reaction to completion. The hydrogen chloride (HCl) by-product is removed as rapidly as possible—usually by vacuum or water washing while the reaction temperature is controlled to minimize the thermal degradation of the phosphate. In the alkoxide route (reaction 3.3), the chlorine precipitates as sodium chloride (NaCl), somewhat simplifying the purification treatment. After a water wash to remove the NaCl, purification consists of a distillation step to remove excess alcohol, an alkaline wash, and a final

Ashless Phosphorus-Containing Lubricating Oil Additives

distillation to remove water [54]. With the alkoxide method, any residual chloride can be removed by water washing followed by a final distillation under vacuum. Despite early research into the alkyl phosphates, a rigorous investigation into the preparation of the lower alkyl derivatives and their properties did not take place until 1930 [55]. In contrast to the large range of aryl phosphates available, the range of neutral alkyl phosphates is currently limited to tri-n-butyl phosphate and tri-iso-butyl phosphate, trioctyl phosphate, and tributoxyethyl phosphate. Other ether phosphates [56] have been claimed in the past but, as far as is known, are not currently manufactured. Although the neutral trialkyl phosphates have been available for sometime, they have not been widely used as additives for mineral oil. Those products in commercial production are used principally as components of aircraft hydraulic fluids, in turbine oils, rolling oils, or as solvents in industrial processes. However, interest in these materials as AW additives for applications where the release of phenols from the degradation of the phosphate is to be avoided currently exists. They also offer advantages as alternatives to the acid phosphates, alkoxylated acid phosphates, and their salts in metalworking applications, where there are concerns over instability in hard water and foam production in use (Canter, N., Private Communication, August 2001). Triaryl phosphates, which are the most widely used of all ashless phosphorus-based AW additives, are currently manufactured almost exclusively by reaction 3.2. Phosphorus oxychloride is added to the reaction mass containing excess of phenol in the presence of a small amount of catalyst, typically aluminum chloride or magnesium chloride, before heating slowly. The hydrogen chloride is removed as it is formed under vacuum, followed by absorption in water. On completion of the reaction, the product is distilled to remove most of the excess phenol(s), the catalyst residue, and traces of polyphosphates. Finally, the product may be steam-stripped to remove volatiles including residual phenol(s) and is dried under vacuum. The raw material for the manufacture of triaryl phosphates was originally obtained from the destructive distillation of coal. This process yields coal tar, which is a complex mixture of phenol and alkyl phenols including cresols (methylphenols) and xylenols (dimethylphenols). Distillation of this mixture (sometimes known as cresylic acids) produces feedstocks rich in cresols and xylenols, which are then converted into the neutral phosphate. An early patent on the production of triaryl phosphates from tar acids was issued in 1932 [26]. Unfortunately, in the 1960s, as the number of coal tar distillers declined due to the move from coal to natural gas as a fuel, it became progressively more difficult to obtain cresols and xylenols from this source. As a consequence, the phosphate manufacturers turned their attention to the use of phenol, which was alkylated with propylene or butylene. The resultant mixtures of alkylated phenols were then converted into phosphates [57,58]. To distinguish phosphates from these two sources of raw materials, the cresol and xylenol-based products became known as natural phosphates and the phosphates from alkylated phenols as synthetic phosphates. This distinction is no longer valid today as synthetic cresol and xylenol are now available and used in phosphate manufacture. However, the nomenclature remains a simple way of distinguishing between the cresol/xylenol-based products and the newer products based on phenol. As the physical and chemical properties of each product type are slightly different, customer selection may depend on the application. For example, if the requirement is for a product that requires good oxidation stability, then the choice would be a tertiarybutylphenyl phosphate (TBPP), but a xylyl phosphate would be selected if the product required the best hydrolytic stability. 3.3.1.1

Natural Phosphates

The main products available in this category are TCP and trixylyl phosphate (TXP) (Figure 3.3). These products, based on cresols and xylenols, are complex mixtures of isomeric materials [59]. However, the variation in phosphate isomer distribution, which arises from changes to the feedstock

Lubricant Additives: Chemistry and Applications CH3

CH3

O O

CH3

O CH3

CH3

O P

O CH3

CH3 CH3 Tricresyl phosphate

Trixylyl phosphate

FIGURE 3.3 The structures of tricresyl and trixylyl phosphate.

CH3 C

CH2

H Propylene

Phenol

ortho-

OH +

meta-

para-

Isomers of isopropyl phenol OH

CH3 C

CH2

OH +

CH3 Butylene

Phenol

meta-

para-

Isomers of tertiarybutyl phenol

FIGURE 3.4

Process for the production of feedstocks used in the manufacture of synthetic phosphates.

composition, has little impact on AW properties. Of greater importance are the actual phosphorus content and the level of impurities present, particularly those that are acidic. In the past, the tri-o-cresyl phosphate content was a source of much concern in view of the high neurotoxicity of the material (see Section 3.19). However, the feedstock that is most widely used today in the production of TCP, is predominantly a mixture of m- and p-cresol, and o-cresol levels are extremely low. 3.3.1.2

Synthetic Phosphates from Isopropylphenols

In this case, phenol is alkylated with propylene to produce a mixture of isomers of isopropylated phenol (Figure 3.4). Depending on the reaction conditions and the degree of alkylation, it is possible to produce a range of isopropylphenyl phosphates (IPPPs) with viscosities varying from ISO VG 22 to VG 100. In seeking an alternative product to TCP (an ISO 32 viscosity-grade fluid), the products with the closest phosphorus contents and viscosities (IPPP/22 and IPPP/32) are most widely used. 3.3.1.3 Synthetic Phosphates from Tertiarybutylphenols In a similar fashion to the manufacture of isopropyphenyl phosphates, it is possible to produce a range of phosphates from butylated phenols prepared by the reaction of isobutylene with phenol (Figure 3.4). The tertiarybutyl substituent is larger in size than the isopropyl substituent, and this

Ashless Phosphorus-Containing Lubricating Oil Additives

TABLE 3.1 Phosphorus Contents and Viscosity Levels for Neutral Trialkyl and Triaryl Phosphate AW Additives Phosphate Ester TiBP TOP TBEP TCP TXP IPPP/22 IPPP/32 TBPP/22 TBPP/32 TBPP/100

Phosphorus Content (%)

Typical Viscosity at 40°C (cSt)

11.7 7.8 7.1 8.3 7.8 8.3 8.0 8.5 8.1 7.1

2.9 7.9 6.7 25 43 22 32 24 33 95

reduces the overall level of alkylation in the molecule necessary to achieve the same viscosity, resulting in the presence of more unsubstituted phenyl groups. Again, the TBPPs from this range, which are used as AW additives in mineral oil, are those closest in phosphorus content and viscosity to TCP, that is, TBPP/22 and TBPP/32 (Table 3.1).

3.3.2 3.3.2.1

ACID PHOSPHATE ESTERS Alkyl and Aryl Acid Phosphates (Non-ethoxylated)

The manufacture of acid phosphates, particularly alkyl acid phosphates, is also based on technology that has its roots in the nineteenth century but was commercialized only during the past 50 years. The process involves the reaction of phosphorus pentoxide with an alcohol in the absence of water (reaction 3.7). P2O 5 ⫹ 3 ROH → ROP (OH)2 ⫹ (RO)2POH monoalkyl acid phosphate

(3.7)

dialkyl acid phosphate

The ratio of monophosphate to diphosphate is usually 40–50% monophosphate and 50–60% diphosphate, with very small amounts of phosphoric acid (⩽1%). Small amounts of neutral ester may also be produced. Products commercially available are made from C5, C7–C9 alcohols, mixtures of C10–12 and alcohols, and C18 alcohols. Monoaryl and diaryl acid phosphates (also known as monoaryl and diaryl hydrogen phosphates) are by-products in the manufacture of triaryl phosphates and may be produced by stopping the reaction before completion and hydrolyzing the intermediate phosphorochloridates (reaction 3.8). −

POCl3 ⫹ ArOH → ArOPOCl 2 ⫹ (ArO)2 POCl OH  → ArOPO(OH)2 ⫹ (ArO)2 PO ⭈ OH monoaryl diacid phosphate

diaryl monoacid phosphate

(3.8)

Di(alkyl)aryl monoacid phosphates, soluble in mineral oil, are reported to be produced by reacting phosphorus oxychloride with an alkylated phenol in the presence of base (reaction 3.9) or by the reaction of monoarylphosphorodichloridate with an alkylated phenol (reaction 3.10). The reactions

Lubricant Additives: Chemistry and Applications

are carried out at temperatures of about 60–90°C using lower than equivalent quantities of phenol. The former process gives predominantly the monoacid phosphate with a small amount of neutral phosphate ester. The latter produces a somewhat greater amount of neutral mixed phosphate ester but mainly the mixed monoacid phosphate [60]. Examples of commercially available lubricating oil additives of this chemistry are amylphenyl- and octylphenyl acid phosphates. ⫺

POCl3 ⫹ RC6H 4OH OH  →(RC6H 4O)3 PO ⫹ (RC6H 4O)2 PO ⭈ OH ⫺

RC6 H 4 OPOCl2 ⫹ ArOH ⌷⌯  → RC6 H 4 O(ArO)2 PO ⫹ RC6 H 4 OArOPO ⭈ OH

(3.9)

(3.10)

Other processes reported for the production of mixtures of mono-2-ethylhexyl and di-2-ethylhexyl acid phosphates include the chlorination of bis-(2-ethylhexyl) phosphonate followed by hydrolysis (reaction 3.11) or the hydrolysis of the tris-(2-ethylhexyl) phosphate. ⫺

⫺

(C8H17O)2 POH Cl  →(C8H17 )2 PO ⭈ Cl OH  →(C8H17O)2 PO ⭈ OH

(3.11)

The alkyl acid phosphates are quite widely used as AW/EP additives in metalworking lubricant applications and as corrosion inhibitors for circulatory oils. 3.3.2.2

Alkyl- and Alkylarylpolyethleneoxy Acid Phosphates

A range of polyethyleneoxy acid phosphate esters was introduced for metalworking lubricant applications in the early 1960s. These products, which consisted of both the free acids and their barium salts, were manufactured by reacting an ethoxylated alcohol with phosphorus pentoxide (reaction 3.12). The properties of the resulting acid phosphate mix can vary significantly depending on the chain length of the alcohol and the number of units of ethoxylation. For example, products that are only soluble in either oil or water can be produced as well as compounds that are soluble (or dispersible) in both media. P O

2 5 ROH ⫹ (C2H 4O)nH → RO(C2H 4O)nH  → [RO(C2H 4O)n ]2 PO ⭈ OH ⫹ RO(C2H 4O)n PO(OH)2

(3.12)

3.3.3 AMINE SALTS OF ACID PHOSPHATES AND OF POLYETHYLENEOXY ACID PHOSPHATES Although the acidic products are very active AW/EP additives, their acidity can lead to precipitation problems in hard water and potential interaction with other additives. To minimize such adverse effects, the acids are sometimes used as their neutral amine (or metal) salts. The salts are produced by reacting an equivalent weight of the base with that of the acid (reaction 3.13). The choice of base will depend on whether oil or water solubility is required. The use of short-chain amines will normally result in water-soluble additives, whereas using, for example, tertiaryalkyl primary amines with a chain length of C11–14 will tend to produce oil-soluble derivatives. The chain length of the acid phosphate also influences the solubility. The selection of the appropriate mixture of amine and phosphate for a given application is largely a compromise because the most active mixtures may also produce disadvantageous side effects, for example, on foaming and air release properties. The fact that a neutral salt is used also does not prevent the product from titrating as an acid and from forming a different salt in the presence of the stronger base. RNH 2 ⫹ (R1O)1⫺2 PO(OH)2⫺1 → (R1O)1⫺2 PO(OH ⭈ H 2 NR)2⫺1

(3.13)

Ashless Phosphorus-Containing Lubricating Oil Additives

where R is an alkyl group, typically C8 −C22. It is also possible to use secondary and tertiary amines, R2NH and R3N in the production of these salts.

3.3.4

NEUTRAL PHOSPHITE ESTERS

As with phosphate esters, it is possible to produce both neutral and acid phosphite esters. The neutral triaryl phosphites are produced by reacting phosphorus trichloride with a phenol or substituted phenol (reaction 3.14). PCl3 ⫹ 3ArOH → (ArO)3P ⫹ 3HCl triaryl phosphite

(3.14)

This is also a stepwise process occurring through the production of the monoaryl and diaryl hydrogen phosphite intermediates. The production of neutral trialkyl phosphites using PCl3 requires the addition of a tertiary amine base to neutralize the acid formed (reaction 3.15). Unless the HCl is removed quickly, it can cause the process to reverse with the production of an alkyl halide and the dialkyl acid phosphite (reaction 3.16). PCl3 ⫹ 3ROH ⫹ 3R 3N → P(OR)3 ⫹ 3R 3NHCl trialkyl phosphite

P(OR)3 ⫹ HCl → RCl ⫹ (RO)2POH dialkyl acid phosphite

(3.15)

(3.16)

The use of mixtures of different alcohols or different phenols can result in the production of mixed alkyl or aryl phosphites. Mixed alkylaryl phosphites can be produced by reacting triaryl phosphite with alcohols to give a mixture of aryldialkyl- and alkyldiaryl phosphites (reaction 3.17). A commercial example of such a product promoted as an oil additive is decyldiphenyl phosphite. (ArO)3P ⫹ ROH → (ArO)2 POR ⫹ ArOH ROH  → ArOP(OR)2 ⫹ 2ArOH alkyldiaryl phosphite

(3.17)

aryldialkyl phosphite

Because of their widespread use in the plastics industry as stabilizers for polyvinylchloride, etc., many different neutral phosphites are commercially available. These range from C2 to C18 alkyl (normally saturated) and from C1–9 alkaryl. In lubricant applications, the most common products are those with alkyl chains of C8 − C18 and C10 −C15 alkaryl, for example, trioctyl phosphite and tris(2,4-ditertiarybutylphenyl) phosphite. The last type is increasingly important in view of its better hydrolytic stability.

3.3.5

ALKYL AND ARYL ACID PHOSPHITES

Alkyl and aryl acid phosphites are manufactured by reacting together phosphorus trichloride, an alcohol (or phenol), and water (reaction 3.18): PCl 3 ⫹ 2ROH ⫹ H 2O → (RO)2 POH ⫹ 3HCl

(3.18)

dialkyl acid phosphite

Mixtures of alcohols, as indicated earlier, may also be used to produce di-mixedalkyl phosphites. The commercially available dialkyl acid phosphites vary from C1 to C18 with use as oil additives falling mainly in the range of C8 −C18.

Lubricant Additives: Chemistry and Applications

Little mention is made in the literature of the use of aryl acid phosphites, and there is no known oil industry using ethoxylated neutral or acid phosphites. Phosphites are, however, generally unsuitable for applications where water contamination is likely in view of their hydrolytic instability, and ethoxylation, certainly in respect of water-soluble products, would not offer any obvious advantage.

3.3.6

DIALKYL ALKYL PHOSPHONATES

Although these products are isomeric with the dialkyl phosphites (Figure 3.2), they are a distinct class of materials with different properties. They are claimed as friction modifiers as well as AW/EP additives and are prepared by the Arbusov rearrangement in which a trialkyl phosphite is heated with an alkyl halide, for example, an iodide (reaction 3.19): P(OR)3 ⫹ R ′I → R ′PO(OR)2 ⫹ RI

(3.19)

dialkyl alkyl phosphonate

Commercially available materials range from the dimethyl methyl derivative to products based on dodecyl phosphite, although the higher-molecular-weight products are likely to be of greatest interest for oil applications. Polyethyleneoxy phosphonates, produced by the reaction of diphosphites with epoxides, have been claimed as friction modifiers [61], whereas diaryl hydrogen phosphonates, such as diphenyl phosphonate, are produced by hydrolysis of the corresponding phosphite with water.

3.4 THE FUNCTION OF LUBRICITY ADDITIVES The earliest additives used for improving lubrication performance were known as oiliness additives and film strength additives. While these descriptions are no longer used, others are now employed. The current terminology together with typical examples of the chemistries employed is shown in Table 3.2.

TABLE 3.2 Different types of additives used to improve lubrication performance Additive Description

Performance

Mechanism

Friction modifier

Reduces friction under near-boundary lubrication conditions

Physical adsorption of polar materials on metal surfaces

Antiwear additive (usually with mild EP properties)

Reduces wear at low to medium loads

Extreme-pressure additive, also known as: -film strength additive, -load-carrying additive -antiscuffing additive

Increases the load at which scuffing, scoring, or seizure occurs

Reacts chemically with the metal surface to from a layer (normally a metal soap) that reduces frictional wear at low-medium temperature and loads Reacts chemically with the metal surface to form a layer, e.g. as a metal halide or sulfide which reduces frictional wear at high temperatures/loads

Typical Chemistries Long-chain fatty acids and esters, sulfurized fatty acids, molybdenum compounds, long-chain phosphites, and phosphonates Neutral organic phosphates and phosphites, zinc di-alkyldithiophosphates

Sulfurized or chlorinated hydrocarbons, acidic phosphorus-containing materials, and mixtures thereof; some metal soaps, e.g. of lead, antimony, and molybdenum

Ashless Phosphorus-Containing Lubricating Oil Additives

TABLE 3.3 A General Classiﬁcation of Chemicals as Friction Modiﬁers, AW, and EP Additives Additive Natural oils and fats Long-chain fatty acids, amines, and alcohols Organo-molybdenum compounds Synthetic esters Organo-sulfur compounds ZDDP Phosphorus compounds Sulfur compounds Chlorine compounds

Friction Modiﬁer

AW Additive

EP Additive

1 1 1 2 2 3 3 4 5

4 4 2 3 2 1 1 3 4

5 5 4 4 3 3 3 1 1

Note: The lower the number, the better the rating.

Table 3.3 [62] offers a generalized classification of the different chemical types of additives used to improve lubrication performance, but, depending on the structure of the additive, some variation in the performance can be expected. In reality, the distinction between AW and EP additives is not clear-cut. AW additives may have mild EP properties, whereas EP additives can have moderate AW performance, and both produce coatings on the metal surface. In fact, EP additives have been described as additives that reduce or prevent severe wear [63]. However, as seen from the Table 3.3, EP additives are unlikely to function satisfactorily as friction modifiers, and vice versa.

3.4.1

THE BASIC MECHANISM OF LUBRICATION AND WEAR AND THE INFLUENCE OF ADDITIVES

An understanding of the basic mechanism of lubrication is useful to appreciate the way in which additives behave and their relative performance. The following is therefore a somewhat simplified explanation of a complex process. Lubrication can be described as the ability of oil (or another liquid) to minimize the wear and scuffing of surfaces in relative motion. It is a function of the properties of the lubricant (e.g., viscosity), the applied load, the relative movement of the surfaces (e.g., sliding speeds), temperature, surface roughness, and the nature of the surface film (hardness and reactivity, etc.). All surfaces are rough. Even those that appear smooth to the naked eye, when examined microscopically, consist of a series of peaks and troughs. The simplest situation arises when the lubricating film is thick enough to completely separate the two surfaces so that metal-to-metal contact does not occur (Figure 3.5). Such a situation could arise at low loads or with highly viscous liquids, and the lubricating characteristics depend on the properties of the lubricant as the load is fully supported by the lubricant. This condition is known as hydrodynamic or full-film lubrication. As the load increases, the lubricating film becomes thinner and eventually reaches a condition where the thickness is similar to the combined height of the asperities on the mating surfaces. At this stage, metal contact commences, and as the asperities collide, they are thought to weld momentarily (causing friction) before shearing with loss of metal (wear) (Figure 3.5). The wear particles then abrade the surface and adversely affect friction, with the resulting damage depending on the hardness of the particle and the surface it contacts. This condition is known as mixed-film lubrication as it is a mixture of full-film lubrication and boundary lubrication with the trend toward the latter with increasing load. As the film thins still further, the load is increasingly supported by the metal surface and friction rises rapidly. When eventually a film that is only a few molecules thick separates the surfaces,

Lubricant Additives: Chemistry and Applications

Metal Full-film (hydrodynamic) lubrication Oil

Metal

Mixed-film lubrication

Oil

Metal

FIGURE 3.5 A diagrammatic representation of full-film (hydrodynamic) and mixed-film lubrication.

the roughness, composition, and melting point of the surfaces strongly influence the resulting friction. At this stage, viscosity plays little or no part in the frictional behavior. This stage is known as boundary lubrication and is characterized by high frictional values that now change little with further increases in load or sliding speed. The wear process that takes place under boundary conditions is perhaps the most complicated of those involved in lubrication in that it involves four different types of wear: corrosive, fatigue, ploughing, and adhesive. Corrosive wear occurs when the metal surfaces react with their environment to form a boundary film, whereas fatigue wear is the process of the fracture of asperities from repeated high stress. Micropitting is an example of this form of wear, which is the subject of considerable investigation today. Micropitting is the result of plastic deformation of the surface that eventually causes the fracture of the asperity, leaving a small pit in the surface. Ploughing wear arises when a sharp particle is forced along the surface, leaving a groove behind, whereas adhesive wear is the tendency of very clean surfaces to adhere to each other. However, this action requires the generation of fresh surfaces during the wear process, perhaps by plastic deformation. It is now thought that this mechanism is much less prevalent than was earlier believed [64]. The relationship between friction, viscosity, load, and sliding speeds can be represented graphically for a bearing by what is known as a Stribeck curve. This is shown in Figure 3.6 [65], where the frictional coefficient is plotted against the dimensionless expression ZN/P, where Z represents the fluid viscosity, N the sliding speed, and P the load. Friction is reduced as the value of ZN/P is lowered until a minimum is reached. For a bearing, this minimum value is ∼0.002 for an ideal hydrodynamic condition. At this point, metal contact begins, and friction rises and continues to do so with increasing contact. In the mixed friction zone, the friction value lies in the region of 0.02–0.10. Eventually, when the film is very thin, friction becomes independent of viscosity, speed, and load and can reach a value of 0.25. By experiment it was established that • Continually increasing the load reduced the ZN/P value, assuming that speed and the viscosity remained constant. The same results can be obtained by reducing either the speed or viscosity, or both, provided the unit load remains constant or is increased.

Ashless Phosphorus-Containing Lubricating Oil Additives

Regions of lubrication

0.001−0.002 Minimum fluid friction

Mixed

Boundary lubrication

Friction

Boundary friction 0.15−0.25 Hydrodynamic lubrication (Fluid film)

Viscosity × speed load

(ZN /P )

FIGURE 3.6 Relationship between coefficient of friction and ZN/P.

• Friction varied directly with viscosity; it was proportional to velocity at lower speeds but varied inversely with velocity at higher speeds [65]. As the surfaces move closer together, the lubricant is squeezed out from between them. Some additives, when adsorbed onto the surface, display a molecular orientation perpendicular to the surface that reduces the level of contact and hence lowers the friction. Such products are known as friction modifiers. Those additives effective in reducing wear and (usually) friction in the mixed friction zone are called antiwear additives, whereas products effective in reducing wear (and increasing seizure loads) in the boundary lubrication process are known as extreme-pressure additives. However, due to the importance of temperature in the lubrication process, it has been pointed out in the past that the latter should, perhaps, be better described as extreme-temperature additives. The temperature at which an additive reacts physically or chemically with the metal or metal oxide surface significantly affects its activity. Each AW/EP additive type has a range of temperature over which it is active (Figure 3.7) [66]. The lowest temperature in the range would normally be the temperature at which physical adsorption takes place. This can occur at ambient or at higher temperatures depending on the polarity of the additive and the impact on surface energy. The greater the reduction in surface energy, the stronger will be the absorption of the surface film and the greater will be the likelihood that the additive remains in place for a chemical reaction with the surface. Additives that are only weakly bound to the surface may desorb as the temperature rises and cease to function further in the wear-reducing process. As the temperature increases so does the surface reactivity. Fatty acids and esters react at fairly low temperatures to produce metal soaps followed by chlorine-containing compounds (to form chlorides), phosphorus (as phosphates, polyphosphates, and/or phosphides), and, finally, sulfur, which reacts at very high temperatures to form metal sulfides [66]. Chlorine-based additives can be film-forming even at ambient temperatures, but as the temperature rises they become aggressive and, with the release of HCl, can cause significant corrosion. Although the FeCl2 film has a fairly well-defined melting point at 670°C, the optimum operating temperature is much lower. Klamann [67] indicates that the efficiency of metal chlorides starts to

Lubricant Additives: Chemistry and Applications 0.5 Mineral base oil Friction coefficient

0.4 P 0.3 Cl

0.2 Fatty acids 0.1 Cl + P + S + fatty acids

0 0

100

200

300

400

500

600

700

800

1000

1100

Temperature (°C)

FIGURE 3.7 Effect of temperature on EP additive activity. (From Mandakovic, R., J. Syn. Lubr., 16(1), 13–26, 1999. With permission.)

TABLE 3.4 Corrosion Films Formed on Sliding Iron Surfaces Lubricant Type

Nature

Dry or hydrocarbon

Fe FeO Fe3O4 Fe2O3 FeCl2 FeS

Chlorine Sulfur

Friction Coefﬁcient (Dry)

Melting Point (°C)

1.0 0.3 0.5 0.6 0.1 0.5

1535 1420 1538 1565 670 1193

Source: Fundamentals of Wear, Lubrication, 12(6), 61–72, 1957. Permission from Chevron.

drop above 300°C and that the friction coefficient at 400°C is already a multiple of the optimum value. However, the dry friction coefficient of the chloride film is substantially lower than that for iron sulfide (Table 3.4) [68]. The relatively low friction associated with this film is probably one reason why chlorinated products are so effective as EP additives. Phosphorus, by comparison, does not react until at higher temperatures and then at slower rates. However, the upper temperature limit of ~550°C in an air environment is thought to be a result of the oxidation of the carbon in the film rather than the degradation of a metal soap (Forster, N.H., Private Communication, July 2007). The soaps, phosphates/phosphides, chlorides, and sulfides formed on the metal surface were originally considered to produce a lower melting and less-shear stable film than that of the metal/metal oxide. This film would cause a smoothing of the metal surface that was then able to support a higher unit loading. This is now thought to be an oversimplified explanation as research has found the EP films to be considerably different to those postulated and without the expected lower shear stability [69]. What it certainly does not consider are additional “subprocesses” of removal of the film by mechanical wear and its possible regeneration in situ by further action of the AW/EP additive (Figure 3.8). Since surface temperature is largely dependent on load, additives that might be effective at high loads may be completely ineffective at low loads (and vice versa). Under such circ*mstances, therefore, significant wear could occur before the load-carrying properties of the EP additive come into play. To minimize this effect, additives are often used in combination, resulting in extending the temperature (and load) range over which they are active.

Ashless Phosphorus-Containing Lubricating Oil Additives

Physical adsorption Desorption Chemical reaction Regeneration of film

Increase in temperature

Mechanical wear

Destruction of film

FIGURE 3.8 Basic processes involved in the mechanism of action of lubricity additives.

−7

Additive (none) 0.4% Organic phosphate 0.4% Fatty acid 0.4% Organic phosphate and 0.4% fatty acid

−8

−9

−10

−11

100

1000

10000

FIGURE 3.9 Effect of fatty acid and phosphate ester on wear rate. (Fundamentals of Wear, Lubrication, 12(6), 61–72, 1957. Permission from Chevron.)

Although single AW/EP additives can be used to meet application and specification requirements, combinations of additives can produce both synergistic and antagonistic effects. The use of mixtures of phosphorus- and chlorine- or sulfur-containing compounds, to extend the temperature range over which a lubricating film is available, has already been mentioned. Another example of synergism was reported by Beeck et al. [5], who described the effect of combinations of TCP and long-chain fatty acids. It was suggested that the use of such mixtures in some way improved the packing of the film on the surface and therefore helped to reduce metal contact. Figure 3.9 [68] in fact shows that combinations of phosphate and fatty acid can result in lower wear rates than either component. Such synergy is useful in that it reduces additive costs and the possibility that the additives might have an adverse effect on product stability, etc. An example of additive antagonism is given in Section 3.12.

3.5

INVESTIGATIONS INTO THE MECHANISM AND ACTIVITY OF PHOSPHORUS-CONTAINING ADDITIVES

Many papers have been written about the way in which TCP and other phosphorus-containing compounds work as AW/EP additives. As might be expected, researchers have had differences

Lubricant Additives: Chemistry and Applications

of opinion. These have probably arisen as a result of the different test conditions found in the wide variety of test equipment developed for measuring wear. For example, different test specimen geometries, surface finish, sliding speeds, and the use of additives with different levels of purity have meant that the data have not been strictly comparable. After a brief review of the early development of AW/EP additives, a number of papers exploring the mechanism of action of different phosphorus-based additives are summarized in this section. It is not inclusive, and the results of many other workers could have been mentioned. An additional selection of papers on the topic is therefore given in Appendix B. Some papers evaluate several classes of product (e.g., phosphates, phosphites, phosphonates, etc.); these may be located in sections other than that on neutral phosphates if information on these other structures is limited.

3.5.1

EARLY INVESTIGATIONS INTO ANTIWEAR AND EXTREME ADDITIVES

Some of the earliest experiments into the effects of different lubricants on friction were carried out by Hardy in 1919 [70], who noted the superior performance of castor oil and oleic acid. He found that good lubricating properties were closely related to the ability of substances to lower surface energy. A series of papers from Hardy and Doubleday followed in 1922–1923 examining the activity of lubricants under boundary conditions. In 1920, Wells and Southcombe [71] discovered that the addition of a small amount of a long-chain fatty acid significantly reduced the static coefficient of friction of mineral oil. Bragg postulated in 1925 [72] that long-chain molecules with a polar terminal group were attached to the surface by adsorption of the polar group and that the long hydrocarbon chains were orientated perpendicular to the surface. He also suggested that the formation of films on both the moving surfaces assisted lubrication by sliding over one another, with their long chains being “flattened” as the distance between the surfaces was reduced. However, in 1936, Clark and Sterrett [73] showed that the lubricating film could be up to 200 molecules in thickness but that only the first layer would have the strength to withstand the shearing stresses produced under sliding conditions. They also found that certain ring structures (e.g., trichlorophenol) that were active as “film strength” additives also showed molecular orientation, in this case, parallel to the metal surface, and attributed the good load-carrying performance to the ability of the layers to slide over one another. Orientation was not the only factor involved, as compounds with a similar orientation could show a wide difference in performance. The mechanism and influence of additives on boundary lubrication were first investigated and reported by Beeck et al. [4]. They found that friction was reasonably constant with sliding velocity up to a critical velocity, beyond which there was a significant reduction. Additives were found to reduce the friction at low speeds relative to the base oil alone and also had a significant, but variable, effect on the reduction at different critical velocities. Low critical velocities were found for compounds that were strongly adsorbed and that showed orientation of the surface film. It was recognized that the adsorbed layer is thinner than the roughness of even the best machined surfaces and that high temperatures (or loads) at points of contact would cause decomposition of the molecules with the formation of a high-melting corrosion product and an increase in friction. If the surfaces were highly polished, then sliding could take place without destruction of the surface film. It was concluded that most of the friction-reducing compounds, principally, the long-chain fatty acids, were not able to produce a highly polished surface and therefore were not effective AW additives.

3.5.2

NEUTRAL ALKYL AND ARYL PHOSPHATES

3.5.2.1 Historical Background One additive examined by Beeck et al. [5] that was able to reduce both friction and wear was TCP, a product that was, at that time, beginning to find widespread commercial use as an AW additive. The authors proposed that TCP acted by a corrosive action, preferentially reacting with the high

Ashless Phosphorus-Containing Lubricating Oil Additives

spot on the surface, where the surface temperatures are highest (from metal contact). It was thought that in the reaction, the phosphate ester formed a lower melting phosphide (or possibly an iron/iron phosphide eutectic) that flowed over the surface and caused a smoothing or chemical polishing effect. They also observed that there appeared to be an optimum level of addition of the TCP (1.5%), a conclusion later confirmed by other workers in the field. Beeck et al. claimed in these papers that their research had produced a better understanding of the AW mechanism and enabled more precise distinctions to be drawn between the different types of additives; more specifically, that A wear prevention agent reduces pressure and temperature through better distribution of the load over the apparent surface. If the resulting minimum pressure is still too high for the maintenance of a stable film, metal to metal contact will take place in spite of the high polish. Since in this case the surface of actual contact is relatively very large, seizure and breakdown will follow very rapidly …

The intervention of the war years encouraged German researchers to prepare and evaluate a number of phosphorus compounds as EP/AW additives, principally phosphinic acid derivatives and also acid phosphates [6,7], while other workers [74] continued to investigate the behavior of TCP. The performance of the latter in white oil was examined, and it was suggested that the additive reacted with steel to form a thin, solid, nonconducting film that prevented seizure by shearing in preference to metal-to-metal contacts. The improved behavior of blends of TCP with fatty acids was explained as being due to the improved adsorption of the fatty acid onto the surface of the chemically formed film. In 1950, an extensive evaluation of different neutral alkyl and aryl phosphates and phosphites, in some cases containing chlorine and sulfur, was undertaken [75]. The results of this investigation showed that the action of sulfur and chlorine on the surface is to form a sulfide and a chloride film, respectively. In the presence of phosphorus, mixed films of phosphide/sulfide or phosphide/chloride were formed. The presence of phosphide was established chemically by the liberation of phosphine in the presence of hydrochloric acid. Although the concept of phosphide film formation was challenged at this time [76,77], it remained as the generally held theory until the mid-1960s when several papers appeared with contradictory data. Godfrey [78] pointed out that the experiments that had indicated the presence of phosphide had all been static, high-temperature investigations, and none had identified phosphide on a sliding surface lubricated with TCP. He experimented with the lubrication of steel-on-steel surfaces by TCP followed by an examination of the metal surface. This revealed the presence of white crystalline material, which was shown by electron diffraction measurements to be predominantly a mixture of ferric phosphate, FePO4, and its dihydrate, FePO4.2H2O. Phosphides, if present, were in extremely small quantities. Furthermore, a paste made from the dihydrate showed similar frictional characteristics to TCP, whereas a paste from iron phosphide showed no significant reduction in friction. Tests also suggested the importance of air to the performance of TCP as tests carried out under nitrogen revealed substantially increased wear. Pure TCP was evaluated and, unlike commercial material, showed no significant friction-reducing properties. The presence and role of impurities in the activity of commercial TCP was the subject of investigations using radioactive P32 [79]. Results suggested that the phosphorus-containing polar impurities—not the neutral TCP—were adsorbed onto the metal surface. The P32 found in the wear scar appeared to the chemically bound—not physically adsorbed, but the latter process seemed to be the way that the phosphorus was initially made available on the surface. The authors indicated that the impurities resembled acid phosphates (rather than phosphoric acid, which Godfrey had assumed) and carried out wear tests comparing the neutral ester with both an acid phosphate (dilauryl acid phosphate) and hydrolyzed TCP. They found that lower concentrations of these compounds generally gave equivalent performance to the neutral ester. Of interest was the observation that, although TCP showed no wear minimum in the reported tests (cf. the results given by

Lubricant Additives: Chemistry and Applications

TABLE 3.5 Effects of Various Additives on the Adsorption of P32 Additive Concentration (wt%) 0.5% TCP alone +2% Barium sulfonate A +2% Barium sulfonate B +0.1% Rust inhibitor +0.5% Diisopropyl acid phosphite +0.1% Dilauryl acid phosphate +5.5% Acryloid dispersant +7.9% Polymeric thickener +0.7% Sulfur–chlorine EP additive +0.5% Thiophosphate +0.5% 2,2′-Methylene-bis(2-methyl, 4-tertiarybutyl phenol) +0.5% Sulfurized terpene

Activity (Counts/min) 280 0 80 16 25 24 82 78 120 150 250 290

Source: Klaus, E.E., Bieber, H.E., ASLE Preprint 64-LC-2, 1964. With permission.

Beeck et al. [5]), the data on acid phosphate, acid phosphite, and phosphoric acid did display such minima. The work using radioactive P32 also allowed a study of the competition between TCP and different types of additives for the metal surface. This was determined by measuring the residual surface radioactivity after wear tests. Table 3.5 shows the effect of various types of additives on the adsorption of P32 from TCP. The lower the number of counts, the greater the interaction between the additive and TCP. Radiochemical analysis was also the technique used to investigate the deposition of phosphorus on steel surfaces in engine tests [80]. In this study, the effect of different types of aryl phosphates (TPP, TCP, and TXP) on case-hardened tappets was examined. The results suggested that the efficacy of these additives is correlated directly with their hydrolytic stability, that is, their ability to produce acid phosphates as degradation products. This was confirmed by tests on a series of other phosphates (largely alkyldiaryl phosphates), which showed good correlation between antiscuffing performance and hydrolytic stability (Table 3.6). Examination of the tappet surface revealed the presence of aryl acid phosphates on the surface and the absence of phosphides. Adsorption studies of the neutral aryl and acid phosphates on steel surfaces indicated that, although the film of neutral ester could be more easily removed, the adsorption of the acid phosphate was irreversible, suggesting salt formation. These studies led the authors to conclude that the mechanism involved initial adsorption of the phosphate on the metal surface followed by hydrolytic decomposition to give an acid phosphate. This reacted with the surface to give iron organophosphates, which then decomposed further to give iron phosphates. The importance of impurities in determining the level of activity of TCP was confirmed in yet another paper [81]. The composition of impurities in commercial grades of TCP was determined using thin-layer chromatography and analysis by neutron activation. Acidic impurities, probably the monocresyl and dicresyl acid phosphate (and also small amounts [2 × 10−4%] of phosphoric acid), were found at 0.1–0.2%, that is, at levels that had previously been shown to produce a significant reduction in wear when added to mineral oil. Other impurities ranged from 0.2 to 0.8%. This latter category was assumed to contain chlorophosphates based on the amount of chloride ion produced.

Ashless Phosphorus-Containing Lubricating Oil Additives

TABLE 3.6 Correlation between the Antiscufﬁng Performance and Ease of Hydrolysis (Acid Formation) of Organic Phosphates Time to Scufﬁng (min)b at a Spring Load

Additive (0.08% wt Added P)

Relative Ease of Hydrolysisa

305 lb

340 lb

Benzyldiphenyl phosphate Allyldiphenyl phosphate Ethyldiphenyl phosphate Octyldiphenyl phosphate Triphenyl phosphate Tritolyl phosphate 2-Ethylhexyldiphenyl phosphate None

100 100 80 50 50 30 5 —

>30 >30 28 15 15 8 2–3 2–3

9 Not tested Not tested 6 5 Not tested Not tested Not tested

Note: Camshaft, Ford Consul (cams phosphated); Tappet, Ford Consul (non-phosphated); Camshaft speed, 1500 rpm (equivalent engine speed 3000 rpm); Base oil, SAE 10W/30 oil without EP additive. a Because of the wide range of hydrolytic stability of these compounds, it was not possible to compare the stabilities of all these compounds in the same acid medium. Consequently, an arbitrary scale was drawn up with benzyldiphenyl phosphate assigned a value of 100. b Mean of several tests. Source: Barcroft, F.T., Daniel, S.G., ASME J. Basic Eng., 64-Lub-22, 1964. With permission.

The authors commented that the TCP used for the investigation was the best grade available, but even this material contained up to 25% polar impurities. It was thought typical of the TCP used in the wear studies to date and reported in the literature. Wear tests on TCP, acid phosphates, and phosphites in a super-refined mineral oil and a synthetic ester (di-3-methylbutyl adipate) indicated that relatively small amounts (0.01%) of additive can produce a significant wear reduction in mineral oils and that the acidic materials were more active. However, in the polar base stock, where there is competition for the surface, the amount of TCP required to provide a similar reduction in wear is substantially greater. The effectiveness of the alkyl acid phosphates is not significantly reduced in the synthetic ester, suggesting that their polarity (and hence adsorption) is greater than that of the neutral phosphate, the synthetic ester, and its impurities (Table 3.7). The authors concluded that the activity of TCP was due to the acidic impurities and that the neutral ester acted as a reservoir for the formation of these impurities during the life of the lubricant. Until about 1969, the theory regarding the production of a phosphate film on the steel surface seemed to be widely accepted. Reports then appeared suggested that the situation was more complicated. One paper [82] examined and compared the corrosivity toward steel, the load-carrying capacity, and the AW performance of several phosphorus compounds. Using the hot-wire technique at 500°C [83] followed by an x-ray analysis of the surface films that were produced, the reactivity (or corrosivity) was studied. Perhaps, not surprisingly, the neutral phosphate and phosphite evaluated showed relatively little reactivity with the steel, whereas the acid phosphate and phosphite produced substantially more corrosion. The anomaly was the behavior of a neutral alkyl trithiophosphite, which showed a very high reactivity but low load-carrying ability, suggesting a different mode of breakdown. Analysis of the films formed confirmed the major presence of basic iron phosphate (or principally iron sulfide in the case of the thiophosphite), but small amounts of iron phosphide were

Lubricant Additives: Chemistry and Applications

TABLE 3.7 Effect of Concentration on the AW Properties of Phosphorus-Containing Additives in a Synthetic Ester Average Wear Scar Diameter (mm) Additive

Concentration (wt%)

1 kg

10 kg

40 kg

— 1.0 3.0 0.5 0.1 1.0 0.01 0.05 1.0 0.02 0.05 0.15 0.001 0.01 1.0

0.39 0.38 0.40 0.23 0.57 0.17 0.21 0.19 0.17 — 0.16 — 0.41 0.16 0.38

0.71 0.71 0.64 0.25 0.74 0.25 0.41 0.28 0.28 0.72 0.25 0.33 0.69 0.37 0.60

0.91 0.97 0.97 0.78 — 0.46 0.84 0.43 0.42 — — — 0.90 0.50 0.78

None TCP

Hydrolyzed TCP Dilauryl acid phosphate

Diisopropyl acid phosphite

Phosphoric acid

Note: ASTM D 4172. Four-ball wear test conditions: test time, 1 h; test temperature, 167°C; test speed, 620 rpm. Source: Bieber, H.E., Klaus, E.E., Tewkesbury, E.J., ASLE Preprint 67-LC-9, 1967. With permission.

also found in all the x-ray analyses of the degradation products. Evaluation of the load-carrying capacity of the additives was found to vary directly with corrosivity except for the alkyl trithiophosphite. The authors surmised from this that the load-carrying capacity of phosphorus-containing additives was not only due to the reactivity of the films but also due to the properties of the film that was formed. The relationship between wear and reactivity also varied directly for several compounds, but in the case of the neutral phosphite and the alkyl trithiophosphite, there was no correlation. This was attributed to the different composition of the film in these cases. In fact, the authors proposed that the main reaction product of the phosphite could be iron phosphide. They suggested that the load-carrying capacity of the films formed by EP additives fell in the following order: phosphide > phosphate > sulfide > chloride whereas the order of AW properties was sulfide > phosphate > phosphide The first of these sequences is, of course, different to the order in EP activity predicted from the stability of the films formed on the metal surface and from the general perception that phosphorus is less active than either chlorine or sulfur. Similarly, for AW performance, phosphorus is normally regarded as more active than sulfur. A paper by Goldblatt and Appeldoorn [84] cast doubt on the theory that the activity of TCP was due to the generation of acidic impurities. In this study, the activity of TCP in different atmospheres and in different hydrocarbon base stocks was examined. The resulting data showed that TCP

Ashless Phosphorus-Containing Lubricating Oil Additives

was much more effective in a low viscosity white (paraffinic) oil than in an aromatic base stock. Aromatics are good AW agents and compete with the TCP for the surface. Under these conditions, either the iron phosphate reaction products are less stable or perhaps a thinner and less complete layer is produced and is worn away, leading to an increase in corrosive wear. Surprisingly, the AW performance of the mixed aliphatic/aromatic base stock was better than either of the components and was not improved by addition of TCP. The behavior of TCP in different atmospheres focused on the effect of moisture in a wet-air atmosphere and also under dry argon, that is, in the absence of oxygen and moisture. No significant differences were found in the results indicated previously for the different hydrocarbon base stocks. However, in a further series of tests comparing the behavior under both wet and dry air and wet and dry argon in an ISO 32 grade white oil, TCP was shown to have a slight AW effect. The exception was in wet air, when it increased wear but also generally showed higher scuffing loads than when used in dry argon. In a naphthenic oil of similar viscosity, the use of wet air (or wet argon) again resulted in increased wear and exhibited higher scuffing loads. This behavior was also observed with other phosphates and phosphites. The authors suggested that in dry air the TCP film forms very rapidly and metallic contact quickly falls. In dry argon the same thing happens, only at a slower rate. In wet air the film is not as strong, and metallic contract remains high, whereas in the case of wet argon, it does not form at all. “Thus the formation of a protective film is enhanced by oxygen but hindered by the presence of moisture.” The observation [78] that air was necessary for the action of TCP did not consider that moisture was present in the air and could have been responsible for the improvement in wear performance. The previous theory indicating it was necessary for the TCP to hydrolyze to form acid phosphates before it became active was also challenged. Wear tests on standard and very low acid TCP in dry argon showed no significant difference in activity. It was concluded that TCP was reacting directly with the surface without first hydrolyzing to acid phosphate and without being preferentially adsorbed at the metal surface. In 1972, Forbes et al. [85] summarized the current thinking on the action of TCP, which indicated that TCP was an effective AW additive at high concentrations independent of the base oil, but at low concentrations was adversely affected by the presence of aromatics. The acidic degradation products have similar properties but show better performance at low concentrations. It was felt that TCP adsorbed onto the metal surface decomposed to give acid phosphates that reacted with the surface to give metal organophosphates. The results of further investigations into the effects of oxygen and temperature on the frictional performance of TCP on M-50 steel were published in 1983 [86]. The critical temperature at which friction is reduced as surface temperature rises was measured under different conditions and was found to be 265°C in dry air (<100 ppm water) when full-flow lubrication is used; 225°C under conditions of limited lubrication and 215°C under nitrogen, also with limited lubrication. Analysis of the surface indicated that TCP had reacted chemically at these temperatures, causing a substantial increase in the amount of phosphate deposited (phosphide was not observed). Oxygen was said to be necessary for this reaction, but the suggestion that prior hydrolysis of the phosphate was required could not be substantiated. The debate regarding the formation of iron phosphate or phosphide as reaction products in the wear mechanism rumbled on into the late 1970s and early 1980s. In 1978, Yamamoto and Hirano [87] carried out scuffing tests on several aryl and alkyl phosphates. The aryl phosphates showed better scuffing resistance, and it was suggested that the alkyl phosphates reacted with the steel surface, forming a film of iron phosphate under mild lubricating conditions, but that the aryl phosphates reacted only slightly until conditions became more severe with the formation of iron phosphide. The implication was that the phosphide (formed as a result of a reaction between the phosphate and the metal surface) acted as a good EP additive but that the iron phosphate had only AW activity. Surface roughness measurements showed a polishing action for the aryl phosphates (particularly for TCP) but not, under these conditions, for the alkyl phosphates.

Lubricant Additives: Chemistry and Applications

The concept of corrosive wear and of phosphates as chemical polishing agents as expressed by Beek et al. [5] was examined by Furey in 1963 [88]. In his work, surfaces of different roughness were prepared and friction measurements were made when in contact with a solvent refined oil under different applied loads. In tests on an additive-free oil (unfortunately, no information was available on the sulfur or aromatic content), it was found that friction, in addition to being load-dependent, was low for highly polished surfaces and rose with increasing roughness up to a roughness of ∼10 µin. At about this roughness, the percentage metal contact was also found to be at its maximum but decreased thereafter. The explanation given for this was that with increasing roughness, the distances between the peaks and troughs increase but the peaks become flatter. The flatter the peak, the better the loadcarrying capacity, whereas the deeper troughs allow for a greater reserve of oil available locally for lubrication and cooling. When several AW/EP additives were evaluated in the oil, it was found that, although there was a reduction in surface roughness, it was less than that found by the oil alone. Furthermore, at low loads, TCP was able to reduce metal contact significantly but had no effect on surface roughness. At moderate to high loads, although the metal contact was reduced, the surface roughness was increased. The author concluded that TCP was not acting through a polishing action. In 1981, Gauthier et al. [89] looked again at the wear process and film formation. They categorized the process into three wear phases: an initial, very rapid phase followed by a medium wear rate, and finally a slow wear phase. In the rapid wear phase, a brown film was formed that, on analysis, was found to be a mixture of ferrous oxide and phosphate. A blue film, which is formed as the wear rate slows (and the surface becomes smoother), contained no iron and was described as a polymeric acid phosphate. (No mention was made of the “white crystalline film” Godfrey reported.) When both films were removed and the roughness of the underlying surface was measured, it was found that the surface below the brown film was very smooth. The surface under the blue film was much rougher and ∼1000 Å thicker. The authors suggested that the smooth surface was the result of polishing arising from corrosive wear. They concluded that in the fi rst phase of wear, a corrosive wear process is involved because of the presence of ferrous phosphate on the surface. When a “critical value” for the surface coverage by the phosphate was achieved, the organic phosphoric acids produced by the decomposition of TCP polymerized to form a polyphosphate. As a result, in the last two wear phases, “the wear of metal is almost completely replaced by the wear of the additive.” In this way, the disparate observations of TCP behavior (polishing versus increased surface roughness) could be related and combined. The presence of polyphosphate was also noted by Placek and Shankwalkar [90] when investigating the films produced on bearing surfaces by pretreatment with phosphate esters. Tests were carried out on 100% phosphates and also on their 10% solutions in mineral oil, the latter condition because the combination had been reported to provide better wear protection than the individual components alone, apparently by the formation of a “friction polymer” [91,92]. Phosphates chosen for the work included both aryl and alkyl types. Analyses of the films formed by immersion in the phosphates at 250°C revealed the presence of a high level of carbon together with iron phosphate/polyphosphate and a small amount of phosphide. At 300°C, the hydrocarbon had all but disappeared and no phosphide was detected. The films formed by the mineral oil solutions were mainly hydrocarbon-based, but the film formed by the alkyl phosphate was unique in that it contained needlelike fibers. The effect of pretreatment on wear found under four-ball test conditions is indicated in Table 3.8. The bearings treated with the mineral oil solutions displayed at least as good wear reduction as those treated with the 100% phosphate. 3.5.2.2

Recent Technical Developments

In 1996, Yansheng et al. [93] reported on the effect of TCP on the wear performance of sulfurized, oxy-nitrided, and nitrided surfaces. A synergistic effect on nitrided and oxy-nitrided surfaces was found, resulting in significant increases in load-bearing capacity while reducing friction and wear, but no improvement was seen on sulfurized surfaces. A recent application in this brief survey relates to the use of aryl phosphates as vapor-phase lubricants. Although not strictly an additive application, this development has been the focus of

Ashless Phosphorus-Containing Lubricating Oil Additives

TABLE 3.8 Friction and Wear Reduction from Bearing Surface Pretreatment by Phosphate Esters Bearing Preparation

Average Scar Diameter (mm)

Improvement (%)

Maximum Torque (gf m)

Improvement (%)

1.00 0.72 0.75 0.81 0.72 0.72 0.64

– 28 25 19 28 28 36

46.1 18.4 18.4 18.4 18.4 15.0 19.6

– 60 60 60 60 68 58

Untreated reference TCP IPPP TOF 10% TCP in mineral oil 10% IPPP in mineral oil 10% TOF in mineral oil

Note: ASTM D 4172–88. Four-ball wear test conditions: test time, 60 min; test temperature, 75°C; test load, 40 kgf; test speed, 600 rpm. IPPP = isopropylphenyl phosphate; TCP = tricresyl phosphate; TOF = tris (2ethylhexyl) phosphate. All wear tests performed in 100 solvent neutral paraffinic mineral oil. Source: Placek, D.G., Shankwalkar, S.G., WEAR, 173(1-2), 1994. Permission from Elsevier.

Wear scar (mm)

0.8

0.6

0.4

Hertz line

0.2

0 0

0.2

0.4

0.6

0.8

1.20

TCP (mol %)

FIGURE 3.10 Four-ball wear values at 370°C with vapor lubrication as a function of tricresyl phosphate (TCP) vapor concentration. (From Klaus, E.E., Jeng, G.S., Duda, J.L., Lubr. Eng. 45(11), 717–723, 1989. With permission.)

most recent analytical studies into the mode of action of these additives; therefore, the conclusions represent the current thinking. Aryl phosphates were chosen for this application because of their oxidation stability and good boundary lubrication performance at high temperatures. The initial studies took place with TCP [94] and involved examination of the films formed on tool steel balls and on iron, stainless steel, copper, nickel, tungsten, and quartz wire specimens. (TCP vapor had previously been shown to form tenacious films on graphite, tungsten, and aluminum at temperatures above its thermal decomposition point [95].) Wear tests on tool steel with vapor at 370°C showed low levels of wear even at 0.1 mol% concentration (Figure 3.10). An optimum concentration was reached at ∼0.5 mol%. Reaction with the metals indicated above is displayed in Figure 3.11, which shows that deposition on iron and copper is relatively fast but slow for quartz, nickel, and tungsten.

Lubricant Additives: Chemistry and Applications Copper Iron Stainless steel Nickel Quartz Tungsten

Deposit weight (mg/cm2)

2100 1800 1500 1200 900 600 300 0 0

15 Time (min)

FIGURE 3.11 Deposition on various substrates with 1.55% TCP in a nitrogen stream at 700°C. (From Klaus, E.E., Jeng, G.S., Duda, J.L., Lubr. Eng., 45(11), 717–723, 1989. With permission.)

Rates of formation are, of course, temperature-dependent, but films are produced up to at least 800°C. Increases in temperature and TCP concentration caused an increase in deposit formation. The use of TCP vapor to lubricate high-speed bearings made from M50 steel at 350°C was examined by Graham et al. in 1992 [96] with excellent results. In fact, the wear area was smoother than the unused surface. Surprisingly, similar results were found when lubricating silicon nitride surface without prior activation. Here, the results were clouded by the transfer of copper to the test specimens, and it was thought that activation could have occurred by reaction of TCP with copper components of the vapor delivery system, which was then deposited onto the ceramic surface. Analysis of the film formed by TCP on a ceramic surface was also investigated by Hanyaloglu and Graham [97]. In this case, the ceramic was activated by a film (∼20 atoms thick) of iron oxide. The presence of TCP at 0.5% in nitrogen or air at 500°C gave a friction coefficient of 0.07 and produced a polymer containing mainly carbon, oxygen, and a small amount of phosphorus with a molecular weight range of 6,000–60,000 g/mol. A combination of vapor and mist lubrication has also been evaluated in the lubrication of gas turbine bearings [98]. The data indicated that organophosphates worked well with ferrous metal due to the rapid formation of a predominantly iron phosphate film. This was followed by the development of a pyrophosphate-based film over the iron phosphate. As long as iron was present, the organophosphates worked well, but continued production of the phosphate/pyrophosphate film reduced access to iron and eventually led to surface failure. Morales and Handschuh [99] reported a solution to this problem in which the phosphate contained a small quantity of ferric acetylacetonate. Evaluation of this solution in comparison with the pure phosphate showed that the iron salt enabled a phosphate film to be successfully deposited onto an aluminum surface, which the pure phosphate is unable to do. (Neutral phosphates are known not to wet the surface of aluminum.) Vapor/mist lubrication of a gearbox using pure phosphate was compared with the performance of the phosphate containing the iron salt; a significant improvement in scuffing performance was noted. This was enhanced when the mist was directed onto the gear teeth immediately before contact. Evaluation of the surface film on the gear teeth revealed no phosphorus when the pure phosphate was tested but showed the presence of “fair amounts” of both iron and phosphorus when using the soluble iron salt. A recent study of the mechanism of film formation by aryl phosphates [100] involved examining the reaction of phosphates with metal in the form of foil or powder and also with various metal oxides in different oxidation states. The tests were carried out in both oxygen-rich and oxygendepleted environments and they revealed that the reactivity of both the commercial grade of TCP

Ashless Phosphorus-Containing Lubricating Oil Additives

and the pure isomers increased with steel and other metals with increasing oxidation state of the metal/oxide. In comparison with little or no degradation in the absence of metal/metal oxides, limited degradation took place in the presence of metal, but almost complete breakdown of the phosphate occurred (at the same temperature—in the range 440–475°C) in the presence of Fe2O3 and Fe3O4. The isomeric forms of TCP also displayed different levels of reactivity with tris-orthocresyl phosphate (TOCP), more active than the meta and para isomers. The authors indicated that these relativities are consistent with the oxide’s free energy of formation; those oxides with the highest free energy of formation show the lowest level of activity, and vice versa. Different types of steel surface also displayed different levels of reactivity, with 316C stainless steel being the least active. Surface analysis of the steel specimens used indicated that, depending on whether the metal surface was oxygen-rich or poor, different mechanisms of degradation predominate. When excess oxygen was present, the film produced was a polyphosphate with good lubricating properties, whereas a surface with only a thin oxide coating produced iron phosphate, which has poor lubrication properties. No phosphide was found in the surface coating, but an iron/amorphous carbon layer, possibly rich in fused aromatics, arising from the degradation of the aromatic part of the phosphate was found when using the TBPP, but not when TCP was examined. Since these aromatics have a planar structure, they may assist with lubrication by allowing the surface to move more easily over one another. However, it is likely that the end result is a composite of the behavior of the polyphosphate and the carbonaceous film, if formed. Indeed the author suggests that the polyphosphate may be acting as a “binder” for the carbon, and it is the latter that is providing the lubrication. The proposed mechanism for the formation of the polyphosphate film was thought to involve the cleavage of the C– O bond on one of the pendant groups as the phosphate attaches itself to the surface (presumably through the –P=O function), eliminating a cresyl radical. This is followed by the elimination of another cresyl radical as the second C–O bond breaks, and an Fe– O bond is formed. In this way a “lattice of cross-linked PO3 is formed with the Fe surface.” Wear of the film is not a problem as it appears to be self-healing due to diffusion of Fe ions through the polyphosphate layer to the surface where reaction with phosphate continues. There was no suggestion that hydrolysis of the phosphate is involved. 3.5.2.3 Recent Commercial Developments Although the majority of phosphates used as AW/EP additives are relatively low-viscosity products, interest has been expressed in materials of high molecular weight for aerospace applications, where low volatility is important; for example, high-temperature lubricants for aero-derivative gas turbines and greases for space vehicles. Three products have become commercially available and have been evaluated: an ISO 100 tertiarybutylphenyl phosphate with low TPP content, resorcinol tetraphenyl bisphosphate (Figure 3.12), and isopropylidene di-p-phenylenetetraphenyl bisphosphate (Figure 3.12). The hydrolytic stability of the resorcinol diphenyl phosphate is relatively poor, but this would not be of major concern for aerospace applications, for example, in greases. However, this material has been claimed as an AW additive for fuels and lubricants [101], whereas the TBPP has been incorporated into an aerospace grease formulation [102]. As part of an assessment of the high-molecular-weight additives for use in high-temperature aviation gas turbine oils, they were compared under co*king, four-ball wear, and oxidation test conditions. The results are given in Table 3.9. Although the AW performance of the butylphenyl phosphate is not as good as that of TCP, the reduced impact on deposit formation and magnesium corrosion performance has made it the most promising candidate. Although much of the recent focus of activity has been on aryl phosphates, there have also been developments with alkyl phosphates. TBP, for example, is now used as an EP additive for EP steam and gas turbine oils used when the turbine is driving a reduction gear (Ertelt, R. Private Communication, September 2001). About 1.5% of the additive is used to increase the FZG gear test performance (DIN 51354) from a load stage failure of about 6–8 to 10–11. Again, the neutral nature of the molecule is of advantage in minimizing interaction with other components of the formulation.

Lubricant Additives: Chemistry and Applications O

O O

O P

Resorcinol tetraphenyl bisphosphate

O O

O P

Isopropylidene di-p-phenylene tetraphenyl bisphosphate

FIGURE 3.12

Structures of high-molecular-weight phosphate esters.

TABLE 3.9 The Effect of High-Molecular-Weight AW Additives on the co*king, Wear and Magnesium Corrosivity of Ester-Based Gas Turbine Oil Formulations AW Additive Blank—no additive TCP Tris-C9–C10 alkylphenyl phosphorodithioate TBPP Resorcinol tetraphenyl phosphate

Deposit Formationa (mg)

Wearb (mm)

89 98 103

0.655 0.40 0.505

High High Pass

94 Not determined

0.54 0.425

Pass Fail

Magnesium Corrosivityc

Note: Additives used at 1% addition in the ester base. a

b c

Fluid held at 300°C for 3 h: method described in paper by Gschwender et al., Lubrication Engineering, pp. 20–25, May 2000. ASTM D 4172-88. Four-ball wear test for 1 h at 40 kg, 600 rpm, and 75°C. 20 mL sample held for 48 h at 232°C with 1 1/h, air flow.

Source: Gschwender, L., Private Communication, August 2001. With permission.

An additional application where interest has been expressed in alkyl phosphates is metalworking. Owing to a desire on environmental grounds to move away from chlorine, mixtures of neutral phosphates and sulfur-containing additives have been promoted as alternatives [103–106]. As concerns exist about the possible release of phenolic materials into the environment, the alkyl phosphates are, perhaps, best suited for this application and are able to provide similar or better performance to the chlorparaffins when used together with sulfur carriers. Table 3.10 summarizes the drill life and other AW/EP performance in a neat oil for both neutral isopropylphenyl phosphate and neutral alkyl phosphate in combination with a sulfurized olefin when compared with a chlorparaffin. In an extension to this work, drill life test data were obtained on tri-isobutyl and tributoxyethyl phosphate in comparison with a commercially available acid phosphate (oleyl acid phosphate). Each phosphate was evaluated at the same phosphorus level in the presence of a sulfur carrier (a 4:1 mixture of a sulfurized fatty acid ester with 26% total sulfur and a dialkyl polysulfide with 40% total sulfur content), and all additives were dissolved in a neat paraffinic mineral oil of

Ashless Phosphorus-Containing Lubricating Oil Additives

TABLE 3.10 A Comparison of the AW/EP Performance of a Chlorparafﬁn and an Alkyl or Aryl Phosphate/Active Sulfur Combination in a Simple Oil-Based Cutting Fluid Formulation Formulation and Test Data ISO 22 paraffinic oil Tri-isopropylphenyl phosphate Trialkyl phosphate Active sulfur compound (40% S) Chlorinated paraffin (40% Cl) Four-ball wear test (ASTM D 4172)-mm Four-ball EP properties (ASTM D 2783) Weld load (kgf) Seizure load (kgf) Load wear index Pin and V-block wear (ASTM D 3233) Failure load (lb) Drill Life test Holes drilled to failure (EN24T mild steel at 1200 rpm/0.13 mm/min feed rate)

92 — — — 8 0.65

95.7 — — 4.3 — —

96.9 — 0.6 2.5 — —

— 1.0 — 2.5 — 0.43

400 80 51

— — —

620 80 104

>3100

—

2726

140

100

280

200

TABLE 3.11 Results of Drill Life Tests on Alkyl and Alkoxyalkyl Phosphates in the Presence of Sulfur Carriers Formulation (w/w) and Test Data

Sulfur carrier Tri-isobutyl phosphate Tributoxyethyl phosphate Oleyl acid phosphate Neat oil Holes drilled to failure

5.2 — — — 94.8 84

5.2 4.17 — — 90.63 432

5.2 — 6.4

5.2 — — 10.0 84.8 18

88.4 >500

ISO VG 22. The test was carried out on an automatic drilling machine, drilling holes of 18 mm depth in a 40 mm thick disk of stainless steel type 304 with a feed rate of 0.13 mm/rev and at 1200 rpm. The test was concluded when either the drill broke or showed excessive wear. The results in Table 3.11 [107] show a significant improvement for the butoxyethyl phosphate over the isobutyl phosphate, whereas the oleyl acid phosphate showed little activity. The reason for the poor behavior of the acid phosphate is not known. Also, in the field of metalworking, phosphates have been claimed as components of hot forging compositions [108,109].

3.5.3 3.5.3.1

ALKYL AND ARYL ACID PHOSPHATES Non-ethoxylated

Although the range in commercial use is limited, acid phosphates are important components of metalworking oils—frequently in combination with chlorparaffins. However, because of

Lubricant Additives: Chemistry and Applications

environmental concerns associated with the use of chlorinated hydrocarbons, their possible replacement by mixtures of phosphorus and sulfur compounds has been investigated [66]. Mixtures of monophosphoric and diphosphoric acid esters were compared with a dithiophosphate acid amide in macroemulsions using a variety of EP tests. Performance in drilling and tapping tests (which are regarded as the conditions most closely simulating cutting performance) indicated that the dithiophosphate amide gave the best performance, whereas the monoacid and diacid phosphates produced levels of performance similar to or better than that of the chlorparaffin alone. Traditionally, the acid phosphates in commercial use have high acid numbers (200–300 mg KOH/g). As a consequence, in addition to their use as AW/EP additives, they are used as corrosion inhibitors [110], and certain structures are promoted as copper passivators [110]. A recent development has been the availability of aryl phosphate-based products that have a relatively low level of acidity (typically 10–15 mg KOH/g) while offering a combination of good AW/EP performance with rust prevention and oxidation inhibition. The multifunctionality of this product type offers opportunities for the simplification of additive packages and use in a wide range of hydraulic and circulatory oils, metalworking, and gear applications, whereas the lower level of acidity reduces the potential for additive interaction and the promotion of foaming, etc. Increased activity in alkyl acid phosphates has been reported in the patent literature. This arises from the use of long-chain alcohols (C16−C18) to produce an acid phosphate ester mix with a high monoacid content (preferably greater than 80:20% monoacid–diacid ratio) [111]. With this acid distribution, it has been possible to achieve lower wear than for the conventional ethoxylated alkyl phosphates with a monoacid to diacid ratio of 60:40%. 3.5.3.2

Alkyl and Alkarylpolyethleneoxy Acid Phosphates

Polyethyleneoxy acid phosphates are a potentially very large class of compounds. Not only are variations possible in the type of alcohol or phenol chosen but also in the type of alkoxylation (although ethylene oxide [EO] is invariably used) and the EO content. Products of this process were originally claimed to be more active than the non-ethoxylated variety, but the latest advances in the latter types [111] suggest this may no longer be the case. Depending on the choice of raw materials, the fi nished product may be oil- or water-soluble or water-dispersible. Alkyl and (alk)arylpolyethyleneoxy acid phosphate esters acids containing <55% EO were found to be oil-soluble; products with an EO content of more than 60% were water-soluble as the free acids and their amine salts, whereas products with 40–60% of EO were both oil- and water-soluble or water-dispersible [40]. The free acids are used in oil applications, whereas amine (usually triethanolamine) or metal salts of the acids are used in aqueous applications. The alcohols and phenols initially selected for evaluation were lauryl and oleyl alcohols and nonyl, dinonyl, and dodecyl phenol. Other raw materials used today include C8 − C10 alcohols, 2-ethyl hexanol, tridecanol, cetyl-oleyl mixed alcohols, and phenol. The products are nonionic surfactants with excellent wetting and emulsification properties, and certain types do not support bacterial growth. They are also good corrosion inhibitors—an important factor for their use in metalworking applications. The higher EO-content products tend to produce a heavy and stable foam, and materials containing ∼45% EO are therefore preferred for metalworking applications [40]. The effect of the alcohol or phenol and the impact of EO content on the wear behavior in a naphthenic oil can be seen in Figures 3.13 and 3.14, respectively [112]. The performance of the product based on oleyl alcohol is interesting in that it does not appear to change with EO content, yet is simultaneously capable of producing materials that vary from oil- to water-soluble. However, the four-ball or pin and v-block tests, although widely used as screening tests for the metal-working application, are not considered to be capable of predicting the performance under cutting conditions. This is confirmed in the paper given in 1995 by Werner et al. [113], which compares the performance of different ethoxylated acid phosphates under various test

Ashless Phosphorus-Containing Lubricating Oil Additives (a)

0.7 0.6 0.5 0.4 0.3 0.2 0.1

ra l)

ic )

(n e

e at

lp ho s

oi l( 10 0

in o

l he

ur yl

lp ny

yl od

al co

ol en ph

lc o ec

le y D

Test conditions: 40 kg, 100 rpm, 60 min, 121°C Four-ball wear scar diameter (mm)

(b)

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

co al

Ba s

oi l

yl ur La

ol ph yl

on in D

od D

ol en ph yl

yl le O

)

Test conditions: 100 kg, 100 rpm, 60 min, 121°C Four-ball wear scar diameter (mm)

FIGURE 3.13 Effect of hydrophobe on wear properties—four-ball wear scar diameter. Acid phosphate esters based on nonionics containing 23–25% ethylene oxide.

Lubricant Additives: Chemistry and Applications (a)

1 Oleyl alcohol 40 kg 0.8

Dinonyl phenol 40 kg

0.6

0.4

0.2

0 0

Ethylene oxide (wt%) Test conditions: 40 kg, 100 rpm, 121°C, 60 min Four-ball wear scar diameter (mm)

(b) 1 Oleyl alcohol 100 kg 0.8

Dinonyl phenol 100 kg

0.6 0.4 0.2 0 0

Ethylene oxide (wt%)

Test conditions: 100 kg, 100 rpm, 121°C, 60 min Four-ball wear scar diameter (mm)

FIGURE 3.14 Effect of ethylene oxide content on wear properties 1% of additive in 100 SUS (100°F) naphthenic base oil.

conditions. Of greater relevance than conventional four-ball or pin and v-block tests are actual cutting or tapping torque tests. The results given in Table 3.12 show that (1) products with a hydrophilic–lipophilic balance (HLB) value of 11–12 give the best results and (2) in general, the further the value deviates from this, the worse the results become. Unfortunately, no studies appear to have

Ashless Phosphorus-Containing Lubricating Oil Additives

TABLE 3.12 Phosphate Ester Surfactant Ranking on Steel in a Water-Based System as a Function of Structure

Phosphate EO Units

Alcohol

Pin-on-Vee Block

Rankings of Sample Four-Ball Wear

Tapping Torque

Total of Rankings

Overall Rating

HLB Number

C9–16

5.5

C18

Nonylphenol

C13

C12

C8–10

11.5

C12

C13

11.5

C12

Nonylphenol

Phenol

15.4

Dinonylphenol

Nonylphenol

9.5

C13 butanediol

4 6

13 15

10 15

15 13

38 43

14 15

9.7 —

been made on the nature of surface film deposited on the metal, but the adsorption mechanism indicated previously is probably still valid. 3.5.3.3

Amine Salts of Acid Phosphates

One amine phosphate that appeared in the patent literature as early as 1934 as a corrosion inhibitor for aqueous systems (and is still occasionally used) is triethanolamine phosphate [114]. Formed by the neutralization of phosphoric acid with triethanolamine, this product was widely used as a corrosion inhibitor for automotive antifreeze formulations for many years [115]. In 1970, Forbes and Silver [116] reported on their work investigating the effect of chemical structure on the load-carrying properties of different phosphorus compounds. In this case, the structures under review were di-n-butylphosphoramidates, amine salts of di-n-butyl phosphate, and derivatives of dialkylphosphinic and alkylphosphonic acids. The results indicated that the phosphoramidates were more effective load-carrying additives than the neutral phosphates, TBP and TCP, but less active than the amine salts of di-n-butyl phosphate. The evaluation of the series of dialkylphosphinic and alkylphosphonic acid esters indicated that the AW performance related directly to the strength of the acid from which they had been produced (Figure 3.15), suggesting that adsorption through the polarity of the ester group was an important step in the process. In addition to the work carried out in hydrocarbon base stocks, some testing was also performed in a synthetic ester. This fluid enabled a comparison to be made of tetra-alkylammonium salts of dibutylphosphate (otherwise insoluble in mineral oil), which displayed the best AW/EP properties of all the amine phosphates tested (Table 3.13). The authors suggested that this was probably due to the stability of the ions.

Lubricant Additives: Chemistry and Applications 1: n -butyl di-n-octylphosphinate 2: Di-n -butyl hexylphosphonate 3: Di-n -butyl phenylphosphonate 4: Tri-n -butyl phosphate 5: Diethyl benzyl phosphonate 6: Diethyl o -nitrophenylphosphonate

3.5 x1

x2 2.5

pKA of acid

3.0

2.0

x5 x3 x4 x

1.5 6

0.3

0.4

0.5

0.6

0.65

Four-ball wear scar diameter (mm)

FIGURE 3.15 Effect of acid strength on AW performance. (From Forbes, E.S., Silver, H.B., J. Inst. Pet. 56(548), 90–98, 1970. With permission.)

TABLE 3.13 Four-Ball Test Results of Various Amine Dibutyl Phosphates in Diisooctyl Sebacate EP Test (BuO)2 PO2 NR1 R2 R3 R4 nC4H9NH2 nC6H11NH2 PhNH2 [nC4H9]NH2 [nC4H9]3NH [nC4H9]4N [CH3]2 [nC8H17]2N None TCP

AW Test

MHL (kg)

WL (kg)

ISL (kg)

30 min

WSD (mm) after 45 min

60 min

40.5 40.0 43.6 34.8 37.8 54.4 56.1 18.6 19.8

130 130 140 140 150 150 165 120 110

125 115 120 100 110 150 165 55 60

0.38 0.37 0.37 0.26 0.37 0.25 0.30 0.57 0.31

0.38 0.38 0.38 0.27 0.38 0.25 0.35 0.61 0.33

0.36 0.39 0.39 0.33 0.42 0.26 0.40 0.64 0.35

Note: % additive = 4 milliatoms of P/100 g of fluid. (MHL, Mean Hertz Load; WL, Weld Load; ISL, Incipient Seizure Load; WSD, Wear Scar Diameter.) Source: Forbes, E.S., Silver, H.B., J. Inst. Pet., 56(548), 90–98, 1970. With permission.

A further study of the mechanism of amine phosphates by Forbes and Upsdell appeared in 1973 [117]. Adsorption/reaction studies of dibutyl and di-2-ethylhexyl phosphates with either noctylamine or cyclohexylamine and iron powder showed that both the amine and the acid phosphate were adsorbed onto the metal surface and that the rate and extent of their adsorption/desorption varied with chemical structure. The higher the solubility of the iron–phosphate complex formed,

FZG load stage failure

Ashless Phosphorus-Containing Lubricating Oil Additives

>12

Amine phosphate

10 8

TPPT 6 4 0

0.5

1.0

1.5

2.0

Amine phosphate (%)

Additive concentration (%) 1.5 >12 1.0 12 load stage 0.5

0 0

0.5

1.0

1.5

TPPT (%)

FIGURE 3.16 FZG performance of an amine phosphate and TPPT separately and in mixtures in an ISO VG 32 polyalphaolefin. (From Kristen, U., Additive für Schmierstoffe, Expert Verlag, Renningen-Malmshaim, German, 1994. With permission.)

the greater was the likelihood of desorption. Furthermore, good AW performance depended on high phosphate and amine adsorption and retention of the phosphate moiety on the surface. The conversion of a dialkyl acid phosphite to an amine phosphate and the use of the mixed product as a multifunctional AW/EP additive, antioxidant, and corrosion inhibitor with improved metal passivation properties were claimed in 1997 [118]. An amine salt and TCP were studied as AW agents for different synthetic esters by Weller and Perez [119] and compared with a sulfurized hydrocarbon. The neutral ester (TCP) generally showed an increasing amount of wear up to 1% addition before reducing at higher levels. The amine salt, however, rapidly reduced wear to very low levels. Friction coefficients were also consistently lower with the amine salt. Kristen [120] reported the effect of additive interaction between amine phosphates and a phosphorothionate. The additives were evaluated under FZG gear test conditions (DIN 51354). The results showed that the additives respond differently in nonpolar and polar base stocks, specifically a polyalphaolefin and a synthetic ester (Figures 3.16 and 3.17). In the synthetic hydrocarbon base, a level of 0.75% amine phosphate and 0.25% phosphorothionate (or perhaps 0.5% of each) provided a borderline FZG 12 load stage pass/fail. In comparison, 0.75% of amine phosphate ester and 1% of phosphorothionate were required to achieve the same level of performance in the ester. Monitoring the response of additive combinations reveals not only the most cost-effective mixtures but also any antagonisms between additives, as were found here in the ester base at higher additive levels. Such information is invaluable to formulators when trying to meet specification requirements and ensuring that the performance level is consistently above the minimum limit. Amine salts, for example, triethanolamine salts of alkyl and arylpolyethyleneoxy acid phosphates, are widely used in metalworking applications. Some of these products are not only commercially available but are also produced in situ when the pH of the product is adjusted by the addition of base to ensure the product is alkaline in use. This is to avoid corrosion and minimize skin irritation.

Lubricant Additives: Chemistry and Applications FZG load stage failure

98 >12

Amine phosphate

TPPT

8 6 4 0

0.5

1.0

1.5

2.0

Amine phosphate (%)

Additive concentration (%) 1.5 12 load stage 1.0 11

0.5 10 0 0

0.5

1.0

1.5

TPPT (%)

FIGURE 3.17 FZG performance of an amine phosphate and TPPT separately and in mixtures in an ISO VG 22 pentaerythritol ester. (From Kristen, U., Additive für Schmierstoffe, Expert Verlag, 1994. With permission.)

3.5.4 3.5.4.1

NEUTRAL ALKYL AND ARYL PHOSPHITES Use as Antiwear/Extreme-Pressure Additives

The earliest known reference to the evaluation of phosphites as AW/EP additive is in a 1950 paper by Davey [75]. As a result of these investigations, which also included a comparison with phosphates and the effect of incorporating chlorine into the phosphate/phosphite molecule, it was found that • Phosphites have superior EP properties to the phosphates, and long alkyl chains are more effective than aryl groups. • Evaluation of TBP and TXP revealed similar optimum concentrations of between 1 and 2% as were found in the previous study with TCP [5]. • Polar compounds such as acids or esters improve the lubricating (AW) properties of phosphites and phosphates by being strongly adsorbed on to the surface. • The incorporation of chlorine or sulfur into the molecule (or the addition of small amounts of free sulfur) improves the EP properties. Chlorine is more effective when part of an alkyl residue, and when sulfur is added to a P/Cl compound (e.g., a chlorinated phosphite), the EP properties are further improved. Following the study by Davey, a number of patents appeared claiming the use of phosphites in lubricant applications [121–124], but it was not until 1960 that a further detailed study of the behavior of phosphites, this time by Sanin et al. [125], was published. The study emphasized the correlation between structure and activity, and the short-chain derivatives were found to be the most active. In 1993, Ohmuri and Kawamura [126] carried out fundamental studies into the mechanism of action of phosphite EP additives. They found that initial adsorption rates of phosphorus-containing

Ashless Phosphorus-Containing Lubricating Oil Additives

esters depended largely on the existence of – OH and –P=O bonds in the structure. The extent of adsorption was influenced by the hydrolytic stability of the esters, and this process was found to occur through reaction with water adsorbed onto the iron surface. Adsorption of the phosphites varied depending on the degree of esterification; triesters were adsorbed after being decomposed hydrolytically to monoesters, whereas diesters were adsorbed without hydrolysis. Phosphite esters eventually hydrolyzed to inorganic acid regardless of the degree of esterification, followed by its adsorption and conversion to the iron salt. It was suggested the adsorbing and hydrolyzing properties of the esters depended on the arrangement of the molecules physisorbed onto the surface. Evaluation of a range of alkyl phosphites as EP additives in gear oils was reported by Riga and Rock Pistillo [127]. The most effective products were those with short chains, particularly dibutyl phosphite, which resulted in a wear layer of >1000 Å and the formation of both iron phosphate and phosphide. Other phosphites formed only traces of phosphide, and as the chain length increased, the resulting film became thinner and contained less phosphorus, possibly due to steric hindrance. Long-chain (C12) alkyl phosphites have also been claimed as AW additives for aluminum rolling oil [128] and, in fact, are still used in metalworking applications. In view of the work carried out on the use of phosphates as vapor-phase lubricants, an investigation into the effect of phosphites on the frictional properties of ceramic-on-ceramic and ceramicon-metal surfaces was carried out in 1997 [129]. The phosphites (and other additives evaluated) had no effect on ceramic-on-ceramic friction; in fact, short-chain phosphites significantly increased friction. When several types of metal were slid against oxide ceramics, the alkyl phosphites were found to lower the friction for each metal except copper. Apparently, the reaction products between copper and the phosphite had adhesive properties and increased friction. The decomposition of trimethylphosphite on a nickel surface was also studied to obtain insight into the initial steps in the decomposition of phosphates when used as vapor-phase lubricants [130]. The main breakdown path is the cleavage of the –P–O– bond to yield the methoxy species, which then degrades to CO and H2 or reacts with the nickel surface. Following heating to 700°K, the surface loses adsorbed species other than phosphorus, which is seen as a simple way for the controlled deposition of phosphorus onto a metal surface. 3.5.4.2

Use as Antioxidants for Lubricating Oils

In addition to their use as AW/EP additives, neutral (and acid) phosphite esters have long been used as antioxidants or stabilizer for hydrocarbons. They were originally introduced as stabilizers for rubber and thermoplastics. Trisnonylphenyl phosphite, for example, was first used to stabilize styrene-butadiene rubber in the early 1940s; this was shortly followed by patents claiming phosphites as antioxidants for lubricants [48,122,123,131,132]. Phosphites function as decomposers of hydroperoxide, peroxy, and alkoxy radicals (reactions 3.20 through 3.22) rather than eliminating the hydrocarbyl-free radicals formed in the chain initiation process. They also stabilize lubricants against photodegradation [133]. R1OOH ⫹ (RO)3P → (RO)3P ⫽ O ⫹ R1OH hydroperoxide

R1OO• ⫹ (RO)3P → R1O• ⫹ (RO)3P ⫽ O alkylperoxy radical

R1O• ⫹ alkoxy radical

(RO)3P → R1OP(RO)2 ⫹ RO•

(3.20)

(3.21)

(3.22)

100

Lubricant Additives: Chemistry and Applications

O P

Tris-(2,4-ditert butylphenyl) phosphite

O O

P O

Bis-(2,4-ditert butylphenyl) pentaerythritol diphosphite

FIGURE 3.18

Structures of some commonly available hindered phosphites.

This behavior as secondary antioxidants by destroying the hydroperoxides, etc., formed in the chain propagation process results in their use in synergistic combination with those antioxidant types that are active as radical scavengers in the initiation process, for example, the hindered phenols and aromatic amines [134–138]. Phosphites are useful additives because of their multifunctionality. However, although they are still used as antioxidants in hydrocarbon oils, their relatively poor hydrolytic stability and the formation of acidic compounds that could affect the surface active properties of the oil have prompted the introduction of “hindered” phosphites with better hydrolytic stability: for example, tris-(2,4-ditertiarybutylphenyl) phosphite or tris-(3-hydroxy-4,6-ditertiarybutylphenyl) phosphite, and, where solubility permits, cyclic phosphites, for example, based on pentaerythritol such as bis(2,4-ditertiarybutylphenyl) pentaerythritol diphosphite (Figure 3.18). These types are claimed as stabilizers or costabilizers for lubricating oils [139–142]. Table 3.14 [141] illustrates the significant improvement in oxidation stability shown by such blends. In common with most other types of phosphorus-containing products, neutral (and acid) phosphites have also been claimed as corrosion inhibitors [143,144].

3.5.5

ALKYL AND ARYL ACID PHOSPHITES

As might be predicted from the behavior of the other types of phosphorus-containing additives, the acid phosphites have good AW/EP properties; the nonylphenyl acid phosphite is particularly effective [145,146]. When used in aviation gas turbine lubricants, the acid phosphites were sometimes formulated in combination with neutral phosphates (TCP); blends of the two products showed synergy even when the amount of the phosphite was very low [147]. The acid phosphites are also claimed to be corrosion inhibitors [144] and antioxidants [47,149,150]. The effects of structure on the AW and load-carrying properties of dialkyl phosphites was studied by Forbes and Battersby [151] in a liquid paraffin. AW performance was best with longchain compounds (Figure 3.19), whereas the short-chain (highest phosphorus content) derivatives displayed the best load-carrying performance. Scuffing behavior, however, appeared to reach a minimum at about a C8 carbon chain length (Figure 3.20). This parallels the behavior of the neutral phosphites. Adsorption studies also showed that the phosphorus content of the solution was depleted in the same order of the load-carrying performance, namely, the most active products showed the highest loss from solution. The presence of water increased the uptake of phosphorus from solution. Comparison of the performance

Ashless Phosphorus-Containing Lubricating Oil Additives

101

TABLE 3.14 Antioxidant Synergism between Hindered Aryl Phosphites and a Hindered Phenol Oxidation Stability Base Stock 1

Antioxidant

% Viscosity Change

Total Acid Number Increase

Hindered phenol (0.5%) Hindered phosphite A (0.5%) Hindered phenol (0.1%) + phosphite A (0.4%) Hindered phenol (0.17%) + phosphite A (0.33%) Hindered phenol (0.5%) Hindered phosphite B (0.5%) Hindered phenol (0.1%) + phosphite B (0.4%) Hindered phenol (0.17%) + phosphite B (0.33%)

357 438 8.7 9.4 712 452 8.1 8.7

11.5 12.2 0.01 0.06 14.2 10.6 0.05 0.03

Note: Hindered phenol is tetrakis-(methylene-3,5-ditert-butyl-4-hydroxy hydrocinnamate) methane; Phosphite A is tri-(2,4-ditert-butylphenyl) phosphite; Phosphite B is bis-(2,4-ditert-butylphenyl) pentaerythritol diphosphite. Test conditions: IP 48 (modified), 200°C for 24 h, air at 15 1/h in an ISO VG 32 mineral oil. Source: U.S. Patent 4,652,385, Petro-Canada Inc., 1987.

0.7

Ethyl Compounds blended at 4 mmol/100g liquid paraffin

Wear scar diameter (mm)

n-Butyl 0.6

0.5

0.4

2-Ethylhexyl Lauryl

0.3

Cyclohexyl

Stearyl

0.2 2

8 10 12 14 16 Carbon chain length

FIGURE 3.19 Effect of chain length on the four-ball AW performance of dialkyl phosphites. (From Forbes, E.S., Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.)

of the phosphites against the corresponding acid phosphate revealed that the phosphites had better load-carrying but inferior AW behavior (see Table 3.15). The authors suggest that the activity of the phosphites is due to an initial hydrolysis to produce the following intermediate either in solution or on the metal surface:

O P

This reacts with the iron surface to give an iron salt that was thought to be responsible for the AW properties of the product. Under much more extreme conditions as are found with scuffing,

102

Lubricant Additives: Chemistry and Applications Ethyl 220

Compounds blended at 4 mmol/100 g liquid paraffin

210

Initial seizure load (kg)

200 190 180 170 160

n -butyl

150 Stearyl

140 130

Lauryl 2-Ethylhexyl

120 110

Cyclohexyl 100 2

6 8 10 12 Carbon chain length

FIGURE 3.20 Effect of chain length on the initial seizure loads of dialkyl phosphites. (From Forbes, E.S., Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.)

TABLE 3.15 Comparison of the Load-Carrying Properties of Dialkyl Phosphates and Dialkyl Phosphites at 4 mmol/100 g Base Oil

Diethyl phosphite Diethyl phosphate Dibutyl phosphite Dibutyl phosphate Di-2-ethylhexyl phosphite Di-2-ethylhexyl phosphate Dilauryl phosphite Dilauryl phosphate

Initial Seizure Load (kg)

Wear Scar Diameter (60 min) mm

225 160 155 85 125 80 130 80

0.70 0.43 0.64 0.42 0.36 0.29 0.32 0.35

Source: Forbes, E.S., Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.

the aforementioned salt was thought to decompose further to give the phosphorus-rich layer of the following type: O H P O O The authors postulated the load-carrying mechanism of phosphites given in Figure 3.21. 3.5.5.1

Amine Salts of Acid Phosphites

In 1975, Barber [152] investigated the four-ball test performance of several long-chain amine salts of short-chain acid phosphites, which were found to be very active. Unfortunately, he did not investigate

Ashless Phosphorus-Containing Lubricating Oil Additives RO

103

O P

Adsorption RO

Hydrolysis in solution RO

O P

RO Fe (a) Hydrolysis (b) Reaction

O P

H RO

Reaction with iron surface

P O

H Antiwear region Fe Hydrolysis

O P

H Fe

H P O

O Antiscuff region Fe

FIGURE 3.21 Mechanism of load-carrying action of dialkyl phosphites. (From Forbes, E.S., Battersby, J., ASLE Trans., 17(4), 263–270, 1974. With permission.)

the effect of increasing the chain length of the phosphite while reducing the length of the amine. Most of the paper concerns the behavior of a wide range of phosphonate esters (see Section 3.16).

3.5.6

PHOSPHONATE AND PHOSPHINATE ESTERS

A large group of phosphonate esters was prepared by Barber in 1975 [152] and evaluated using the fourball machine. Although short-chain esters were more effective in preventing scuffing, the most effective products were those containing chlorine. However, even at high levels of chlorine, the performance was still inferior to the amine phosphite reaction products reported earlier. In comparison with TCP, incipient seizure loads were generally higher, but the weld loads were broadly similar. Unfortunately, there were no direct comparative data under wear test conditions. A limited number of phosphinate esters were evaluated and found (also by four-ball tests) to give similar performance to the phosphonate esters. The activity of a range of phosphonates was studied by Sanin et al. [153], who concluded that their effectiveness depended on their structure and the friction regime, but esters containing no chlorine had “no effect at either low or high load.” A further study, by the same authors, of the mechanism of activity of phosphonates again suggested the reaction of decomposition products with iron and the formation of a protective layer. Under severe conditions, this layer is removed, resulting in a sudden increase in friction followed by seizure or welding. Studies of the reaction of a dibutyltrichlorophosphonate (Cl3CPO(OBu)2) indicated that a reaction took place at 405–408K to give chlorobutane and an iron-containing polymer. At 413K, this polymer decomposed to give FeCl3, which gave additional protection. Phosphonate (and pyrophosphonate) esters, as their metal or amine salts, have appeared in the patent literature over many years as AW/EP additives. Amine salts of dinonylphosphonate are, for example, claimed in aircraft gas turbine lubricants [154], and dimethyltetradecyl phosphonate has

104

Lubricant Additives: Chemistry and Applications S

PIB + P2S5

H2O

PIB

(3.23)

OH where PIB = polyisobutylene S PIB

2(CH3

CH2)

CH O

PIB

CH3 O

(OCH2

OH)2

(3.24)

FIGURE 3.22 An example of the preparation of a phosphorus-based detergent. (From Colyer, C.C., Gergel, W.C., Chemistry and Technology of Lubricants, VCH Publishers, New York, 1992. With permission.)

been used in water-based formulations with good pump wear characteristics [155,156]. One of the most recent applications has been in refrigeration compressor oils (e.g., for automobiles) that are compatible with the more ecologically acceptable refrigerants. The reason for their selection in this application has probably been their good hydrolytic stability in view of the need for a long fluid life [157,158]. Other automotive industry applications for these products include use as friction modifiers, for example, in automatic transmission fluids [159], or possibly as detergents in engine oils [160–162] to keep insoluble combustion and oil oxidation products dispersed in the oil. An alternative method for incorporating phosphorus into dispersant is exemplified in Ref. 160. This method involves reacting P2S5 with a sulfurized hydrocarbon, such as sulfurized polyisobutylene, at high temperatures to form a thiophosphorus acid (see reaction 3.23, Figure 3.22). This intermediate is then reacted with propylene oxide to form the hydroxypropyl esters of the phosphorus acid (see reaction 3.24, Figure 3.22). Aminoethane phosphonate copolymers have also been claimed to provide dispersancy, corrosion protection, and pour point depression [163]. Among other applications mentioned in the literature for these products or their salts in lubricating oils are the extrusion, cold rolling, and cold forging of aluminum [164], offering improved rust inhibition [165] and antioxidant performance [166].

3.5.7

A SUMMARY OF THE PROPOSED MECHANISM FOR ANTIWEAR AND EXTREME-PRESSURE ACTIVITY OF PHOSPHORUS-BASED ADDITIVES

In attempting to produce an explanation for the activity of phosphorus-containing additives, it is not easy, as explained earlier, to compare the results of the preceding investigations because conditions vary from one investigation to another. No report evaluates all the different types of additives with the same (high) level of purity under identical test conditions. However, it is possible to draw together some of the more consistent “threads” running through the many papers. One parameter highlighted in past reports (and confirmed by recent observations) is that the presence of oxygen on the metal surface appears to be important for the activity of neutral aryl phosphates. This could perhaps be one of the major reasons why TCP is sometimes found to be inactive. The composition of the film formed on the surface is not yet completely defined, but current work points toward the formation of a self-regenerating polyphosphate layer in which amorphous carbon may be providing the lubrication benefits. The mechanism of formation of the polyphosphate layer and the role, for example, of moisture is not yet clear but appears to be a stepwise process as follows: • The adsorption of the material onto the surface (occurring through the –P=O and –P– OH bonds in the molecule). • Either the hydrolysis of a –P–OR bond to form –P– OH (probably arising from water on the surface but may also occur in solution) with the formation of acid phosphates/phosphites

Ashless Phosphorus-Containing Lubricating Oil Additives Amine phosphites Amine phosphates Acid phosphites Acid phosphates Neutral phosphites Neutral phosphates Neutral phosphonates

Improvement in EP properties

Improvement in AW properties

105

Impact on stability, etc.

FIGURE 3.23 An approximate ranking of the effect of structure on the AW, EP, and stability properties of the base stock.

or, in the case of neutral phosphates, the cleavage of the C–O bond to release an aryl radical and a residual –P– O• radical. • Either reaction of the –P– OH or –P– (OH)2 with the metal surface to form an iron salt, possibly followed by further hydrolysis to release the remaining hydrocarbon moieties and reaction of the new –P– OH groups with the surface to form polyphosphate, or the reaction of the residual –P– O• radical with the iron surface to form a succession of Fe–O–P– bonds leading to the formation of polyphosphate. • Products that contain –P– C bonds (e.g., the phosphonates and particularly the phosphinates) are less likely to operate by a mechanism involving hydrolysis, and the stability of the P– C bond might be expected to prevent or delay the formation of the phosphorus-rich surface layer with an adverse effect on EP properties. However, the same stability could result in better friction-modification properties. The fact that phosphinates and phosphonates are active as AW/EP additives suggests that the –P=O bond is also involved in the surface adsorption process, but that either the nature of the surface film may be different or a polyphosphate film is produced as a result of the scission of a –P– C bond. • The formation of amine salts results in an increase in activity, possibly as a result of the stability of the ion and improved adsorption on the metal surface. The mechanism of formation of phosphide, which is reported in many instances, has not yet been clarified but might possibly involve the amorphous carbon that then acts as a reducing agent on the phosphate/phosphite layer as it forms on the surface. These conclusions lead, as a broad generalization, to the order in activity and impact on surface chemistry/stability as shown in Figure 3.23. The preceding comments are, however, a simplification of the situation. Depending on the length of the alkyl or alkaryl chain, if the iron salts that are formed are soluble in the oil they may desorb from the surface, leading to poor AW/EP performance. Interaction with other surface-active materials will inevitably influence the performance of AW/EP additives, whereas depletion in use due to oxidation, etc., will also affect performance.

3.6

MARKET SIZE AND COMMERCIAL AVAILABILITY

Information on the market size for ashless phosphorus-containing AW/EP additives is limited. An approximate total market of ∼10,000 tpa is broken down, as given in Table 3.16. The data exclude the use of phosphites as antioxidants in oil applications, which is separately estimated to be between 100 and 200 tpa. The wide use of phosphorus-containing AW/EP additives is due, in addition to their good lubricity performance, to the following features of value to formulators: • Ashless • Low odor, color, and volatility • Low acidity/noncorrosive (applies to the neutral esters only)

106

Lubricant Additives: Chemistry and Applications

TABLE 3.16 A Breakdown of the Market for Ashless PhosphorusContaining AW/EP Additives Product

Market Size (t)

Alkyl phosphates Aryl phosphates Acid phosphates, ethoxylated alkyl and aryl phosphates, and amine salts of acid phosphates Phosphites, acid phosphites, dialkyl alkyl phosphonates, and amine salts of acidic products

100 6000 3000 1000

TABLE 3.17 Typical Physical Properties of the Most Widely Used Grades of Phosphorus-Based AW Additives Property Viscosity at 40°C Viscosity at 0°C Specific gravity Pour point Acid number Water content Phosphorus level Flash point

• • • •

Unit cSt cSt 20/20°C °C mg KOH/g % % °C

TiBP

TCP

IPP/32

TBPP/32

2.9 10.0 0.965 <–90 0.06 0.01 11.7 155

25.0 1000 1.140 –28 0.05 0.06 8.3 240

32.3 990 1.153 –27 0.05 0.05 8.0 245

33 1500 1.170 –26 0.06 0.05 8.1 255

Low toxicity Biodegradable (many but not all products) Compatible with most other types of additives (particularly the neutral esters) Soluble in a wide range of base stocks, both mineral oil and synthetic, and able to assist the solvency of other additives

Although the physical properties of phosphorus-containing additives are not critical, the values for the most widely used types of phosphate ester AW additives are given in Table 3.17, with TiBP as an example of an alkyl phosphate, TCP as a natural phosphate, and ISO 32 grades of both types of synthetic ester. The major suppliers of phosphorus-containing lubricant additives are listed in Table 3.18, and their current oil industry applications are summarized in Table 3.19. Undoubtedly, the most important applications for the neutral aryl phosphates are hydraulic, turbine, and general circulatory oils, whereas almost the entire market for the ethoxylated alkyl and aryl acid phosphates is to be found in metalworking. The acid phosphates, acid phosphites, and amine salts of these acidic materials are used in a mixture of metalworking, gear oils, hydraulic oils, etc., as indicated in Table 3.19. The selection of an AW/EP additive depends on the specific requirements for the application; for example, whether both AW and EP performance is needed and what levels are required. When this has been ascertained, secondary considerations may be the level of stability required (oxidative or hydrolytic), potential interaction with other components of the formulation, and the effect on surface-active properties such as foaming. Table 3.20 attempts to identify the additives that should

Asahi Denka Chemtura Ciba Spec. Chem. Croda Daihachi Dover Chemicals Johoku Chemicals Krishna Lanxess Libra Chemicals Rhein Chemie Rhodia Sumitomo Supresta United Phosphorus Vanderbilt

Producer

× × ×

× ×

× × × ×

Aryl

Alkyl

Neutral Phosphates

Nonethoxylated

Ethoxylated

Alkyl/Aryl Acid Phosphates

× ×

Amine Phosphates

× ×

× × ×

× ×

× × ×

Aryl

Neutral Phosphites Alkyl

TABLE 3.18 Principal Suppliers of Ashless Phosphorus-Containing Lubricating Oil Additives

Acid Phosphites

Alkyl Phosphonates

Ashless Phosphorus-Containing Lubricating Oil Additives 107

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TABLE 3.19 Principal Applications for Ashless Phosphorus-Containing AW/EP Additives Application Automotive ATF Gear oil Power steering Shock absorber Electric motor Industrial Hydraulic oils Gear oil Turbine oils Compressor oils Gas oil Universal tractor Metalworking Grease Way oils Circulating oil Vegetable oil Aircraft Piston engine Turbine engine Grease

Triaryl Phosphate

Trialkyl Phosphate

Amine Phosphate

Acid Phosphates

Alkyl/Aryl Phosphites

be given prime consideration when taking these secondary requirements into account. Products that demonstrate better AW than EP activity, and vice versa, are shown. However, the boundary between AW and EP performance is not clear-cut and much depends on the application requirements.

3.7 TOXICITY AND ECOTOXICITY It was mentioned earlier in this chapter that concern had been expressed in the past regarding the toxicity of phosphorus-containing products, particularly TCP. Today, with increasing focus on the environmental behavior of chemicals, their ecotoxicity is also under scrutiny. As a result, detailed investigations into both the toxicity and the ecotoxicity have been carried out on alkyl and aryl phosphate esters. The results are summarized in recent publications [59,167], and most are available in the safety data sheets associated with different product types. The data demonstrate a relatively low (but variable) order of toxicity and ecotoxicity. No significant risks in handling are anticipated, provided the manufacturer’s guidance, which is essentially the same as for mineral oils, is followed. The concerns over TCP arose as a result of the o-cresol content in the feedstock as tri-orthocresyl phosphate (TOCP) was found to be a significant neurotoxin. Initially, the level of o-cresol in the feedstock was high (up to ∼25%) and significant amounts of TOCP were present in the finished product. Although the initial focus was on TOCP, it was later acknowledged that any isomer containing the o-cresyl moiety was neurotoxic (e.g., mono-o-cresyldiphenyl phosphate was said to be 10 times more neurotoxic than TOCP [168]). For these reasons, the o-cresol content of the feedstock used in the manufacture of TCP has been progressively reduced over time. In recent years, production has moved to the use of 99% minimum m- and p-cresol. Levels of o-cresol in the feedstock are now frequently <0.05%, and the TOCP content can be as low as parts per billion. Mackerer et al. [169]

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TABLE 3.20 Guidance on the Selection of AW and EP Additives Required Characteristic

Good AW Performance

Non-phenolic additive

Neutral alkyl phosphates; dialkyl alkyl phosphonates

Good hydrolytic stability

TXP, dialkyl alkyl phosphonates

Good oxidation stability

Neutral tertbutylphenyl phosphates Neutral phosphates, dialkyl alkyl phosphonates Neutral tertbutylphenyl phosphates Neutral IPPPs

Low foaming/air release

Good toxicity performance Good ecotoxicity performance Multifunctionality, for example, rust inhibition, antioxidant

Good EP Performance

Combination of AW and EP Performance

Acid alkyl phosphates; acid alkyl phosphonates and their salts; neutral and acid alkyl phosphites Acid alkyl phosphonates and their salts Hindered aryl phosphites Neutral phosphites

Mixtures of neutral and acid phosphates, etc.

—

— —

Acid IPPPs

—

— Neutral and acid phosphites, acid phosphates

— Acid IPPPs

estimate that the toxicity of the TCP now available commercially is ∼400 times less than that of material available in the 1940s and 1950s, and a recent evaluation of the organophosphorus-induced delayed neurotoxicity (OPIDN) of a commercial aviation gas turbine oil containing TCP was negative [170]. However, in view of past concerns, the use of TCP is now largely restricted to aviation gas turbine oils. Most general industrial applications that require an aryl phosphate AW additive now use the isopropylphenyl or, to a lesser extent, the tertiarybutylphenyl variants. In standard tests, neither of these types display OPIDN from acute oral ingestion. There are, however, some differences in the toxicity and the ecotoxicity behavior between the different aryl phosphates. For example, the reproductive toxicity of the synthetic aryl phosphates, together with TXP, was recently studied in rats (according to Organisation for Economic Co-operation and Development [OECD] method 422). Both the isopropylphenyl phosphate and the TXP showed adverse effects at moderate to high dose levels, but these were reversible when exposure ceased. The TBPP (produced according to reaction 3.2) did not display any adverse effects. Differences are also seen in ecotoxicity behavior. Owing to the high TPP content in the lowerviscosity grades of the synthetic phosphates (particularly ISO VG 22 and 32), these products have the worst ecotoxicity behavior. The tertiarybutylphenyl phosphates normally have a higher TPP content than the corresponding grade of IPPP and therefore, of the synthetic phosphates, possess relatively worse ecotoxicity. By comparison, the IPPPs generally show satisfactory behavior in these tests. One ISO VG 46 isopropylphenyl phosphate-based AW/EP additive has, for example, been approved by the German Environment Agency (Umweltbundesamt) for use in rapidly biodegradable hydraulic fluids, products that are eligible for the “Blue Angel” environmental award [171]. Another difference between the product types is displayed in biodegradability tests. The tests were carried out according to OECD method 301F (Manometric Respirometry). In this test, biodegradation is measured as the net oxygen uptake over that occurring in blank tests containing only

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TABLE 3.21 OECD 301F Biodegradability Data on Different Types of Aryl Phosphates Product (ISO VG 46 Base Stocks) TXP IPPP TBPP

% Biodegradability After 10 Days

28 Days

68 Days

5 18 25

29 47 62

70 65 72

inoculated medium. The extent of biodegradation is calculated from the mass of test material added to the test vessels and its theoretical oxygen demand for complete biodegradation. The test was carried out in triplicate on the ISO VG 46 grades of different types of aryl phosphates manufactured according to reaction 3.2. The results are summarized in Table 3.21. The results are initially in the order of their hydrolytic stability, but it is interesting that TXP, after a slow start, eventually reaches the same level as the synthetic fluids and might have exceeded them had the test been extended. In view of these data, the tertiarybutylatedphenyl phosphate would be regarded as readily biodegradable (Pw1), whereas the TXP and IPPP would be classified as inherently biodegradable (Pw2). Despite of relatively benign ecotoxicity of the higher viscosity grades of aryl phosphates, all these products are classified as marine pollutants because of the UN Marine Pollutant Classification. However, because they are used at low concentrations, they are unlikely to contribute significantly to the finished product’s ecotoxicity. The toxicity of other phosphorus-containing compounds is less well documented. Drake and Calamari [172] indicate that dialkyl alkyl phosphonates generally have a low level of acute toxicity, which decreases with increasing chain length, apparently a general observation for these classes of compounds. As with alkyl phosphates, certain short-chain products can be skin irritants. No clues were found to their environmental behavior, but in view of the absence of phenolics and improved hydrolytic stability, it might be surmised that fish toxicity could be good but biodegradability would be inferior to that of the phosphates. Neutral phosphites, particularly the alkyl phosphites, would be expected to have good toxicity and biodegradability behavior, but their ease of hydrolysis, which is the factor assisting the biodegradation, would probably result in poor aquatic toxicity. The future of the nonylphenyl phosphites is uncertain; the U.S. National Toxicology Program currently lists nonylphenol as an estrogen mimic and also as a thyroid disruptor. The acid phosphates, acid phosphites, and their salts, particularly amine salts, are likely to be classified as irritants and, due to their ease of hydrolysis, may again be toxic to aquatic organisms. In all cases, it is essential that reference be made to the health and safety information provided by the manufacturer.

3.8

THE FUTURE FOR ASHLESS PHOSPHORUS-BASED LUBRICATING OIL ADDITIVES

Although ashless phosphorus-containing additives are used in many industrial applications, there are certain market segments where they have not, to date, been successful. These are principally in automotive engine oils where the use of ZDDP dominates due to a combination of price and multifunctionality and in gear oils where sulfur continues to be the EP additive of choice. However, the use of chlorine as an EP additive, particularly in metalworking applications, is in decline for environmental reasons and is expected to be slowly substituted by P/S combinations. The use of sulfur alone in applications requiring high EP performance may also move to P/S mixtures to reduce the total sulfur level and the ability to more readily “tailor” the balance of AW and EP performance

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to the application. The potential in other market segments including those traditionally using ZDDP is discussed in greater detail in the following:

3.9 LUBRICATING OIL FORMULATIONS (GENERAL) The current trend toward the use of groups II and III mineral oil base stocks for general industrial applications, with improved antioxidant response but inferior lubricity as a result of the removal of aromatics and sulfur compounds, could encourage the wider use of phosphorus. The lack of competition for the surface, which has previously been shown for TCP in stocks containing naphthenics and aromatics, should result in the increased activity of phosphorus compounds. Their use may also be beneficial due to their ability to aid the dissolution of additives that might otherwise have limited solubility. In Europe, legislation (Directive 2000/769/EC) implemented in 2004 requires a substantial reduction in sulfur dioxide (SO2) emissions from the combustion of waste materials including waste oil. This may result in a move to lower sulfur levels in lubricating oils (including metalworking oils) and a possible replacement by phosphorus to restore the level of AW/EP performance.

3.10 HYDRAULIC OILS In recent years, there has been a move toward the use of ashless hydraulic oils. This is mainly for two reasons. First, as a result of the sensitivity of ZDDPs toward moisture and the resulting deposition of zinc oxide/sulfide. This deposit can adversely affect the filterability of the oil and reduce oxidation stability. Second, there is increasing concern regarding the environmental behavior of heavy metals. Regulatory controls, however, are likely to extend further to cover metals such as zinc, as in the Great Lakes Initiative between the United States and Canada. As the zinc cannot be easily removed from waste at the effluent plant, there has been a focus on the reduction in use levels. Concern has also been expressed in certain countries regarding the smell of sulfur arising from the degradation of the ZDDP when the hydraulic oil is used, for example, in elevators (Dixon, R., Shell Global Solutions, Private Communication, November 2007).

3.11 AUTOMOTIVE ENGINE OILS Vehicle emissions legislation (e.g., in the United States, Europe, and Japan) now exists to control and substantially reduce the levels of particulates, hydrocarbons, carbon monoxide, and oxides of nitrogen in the engine exhaust. The engine manufacturers have met these requirements by a variety of design changes that impact the composition of oils and fuels in the following ways: • The introduction of catalytic converters to oxidize the hydrocarbon and carbon monoxide components to carbon dioxide and water, and reduce the nitric oxide (NO) to nitrogen, has been very successful in reducing emissions. When they operate at their normal operating temperature and optimum level of efficiency, they are almost 100% efficient and most of the remaining emissions occur in the time before the catalyst reaches “light-off” temperature. Many studies into reducing this period to achieve yet lower emissions have been conducted. Although much success has been achieved, further progress may be hindered by the formation of a deactivating film on the catalyst surface by the phosphorus from the ZDDP antioxidant and AW/EP additive. As a consequence, there is pressure to reduce the phosphorus content of engine oils to minimize catalyst fouling. Currently, oil specifications such as ILSAC GF-4 and ACEA Cx limit the phosphorus content of both diesel and gasoline engine oils to 0.05–0.09% with the actual level being linked to the amount of catalyst used and the expected service interval. Further reductions below 0.05% are being considered, but there is a concern that such a low level could adversely affect the durability of certain engine parts, for example, the valve train and timing chain, as reducing the ZDDP content also reduces the wear protection. However, a recent study [173] suggests that

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the behavior of phosphorus compounds in wear and catalyst tests varies according to the way in which phosphorus is incorporated into the molecule. Further work reports that it is possible to achieve improvements in catalyst protection (and fuel economy) by reducing the ZDDP content and then adding a metal-free phosphorus-containing AW additive [174]. In an attempt to increase fuel economy, the so-called fuel-efficient lubricants are being developed. These are usually lower-viscosity products (since energy losses decrease with viscosity), sometimes complemented by the use of friction modifiers. However, low-viscosity oils may cause increased wear of some engine components, and the necessity for improving the wear protection is being studied. The ILSAC GF-5 specification, for example, will necessitate the use of some form of friction modifier to guarantee the required level of economy. Currently, molybdenum compounds or long-chain esters are under evaluation, but other approaches (e.g., the use of functionalized viscosity modifiers) are also being studied as the ability of these additives to deliver reduced friction over long periods is uncertain (Mainwaring, R., Shell Global Solutions, U.K.). ZDDP is linked to increased friction and therefore reduced ZDDP levels may also be required. To reduce the particulate (soot) levels in exhaust gas, the diesel engines in many passenger cars and trucks need to use particulate filters. These filters can also remove the ash-containing residue produced from metallic fuel and lubricant additives, and if they are not occasionally cleaned, they will block causing a deterioration in engine performance. The engine builders, however, are trying to preserve or even extend service intervals and consequently are interested in reducing the ash content of the oil. Although ZDDP is not the only source of metals in the oil, a reduction in zinc content will follow automatically from any reduction in the phosphorus content (as long as ZDDP remains in formulations) and will therefore help to reduce engine oil ash content (Mainwaring, R., Shell Global Solutions, U.K.). One of the techniques used to remove the soot from the particulate filters (and thereby maintain an acceptable engine back pressure) has been to oxidize the deposited carbonaceous material by nitrogen dioxide (NO2). This is obtained from the exhaust gas by catalytically oxidizing the NO component. The oxidation of the soot to carbon dioxide effectively removes carbonaceous filter deposits, and the NO2 is such a powerful oxidant that it enables the process to be carried out at a relatively low temperature (∼250°C). Unfortunately, the catalyst used to oxidize the NO preferentially oxidizes any SO2 in the exhaust, thereby reducing the efficiency of the NO conversion. Additionally, the sulfur trioxide (SO3) formed passes through the trap in the gas phase and is converted there into sulfuric acid by the water in the exhaust. The sulfuric acid (monitored as “sulfates”) contributes, as droplets, to the overall level of particulate emissions and is clearly undesirable if the exhaust gas is inhaled. Any reduction in sulfur content by lowering ZDDP levels in engine oils also reduces the phosphorus content arising from this additive. Supplemental phosphorus may therefore be needed. Increased emphasis on fuel economy led some manufacturers to introduce direct injection stratified charge gasoline engines. Conventional catalysts cannot remove oxides of nitrogen (NOx) in these “lean-burn” engines, and, as a result, NOx storage catalysts have been developed in which the oxides are stored as nitrates by reaction with barium sulfate contained in the catalyst coating. When the barium-containing sites become saturated, the engine switches to stoichiometric or slightly rich operation at which temperature the nitrates break down and release the NOx, thus promoting its reduction through the conventional route of reaction with hydrocarbons and carbon monoxide. Unfortunately, barium sites react preferentially with any sulfur oxides present, reducing their ability to “store” NOx. As a consequence, there is again pressure to reduce the fuel sulfur content. However, these levels are already being lowered (see “Fuels” below), and at such levels, the engine oil begins to be a significant

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contributor to exhaust “sulfur” content.A debate has therefore arisen regarding the future level of lubricant sulfur, and diesel engine manufacturers have already expressed interest in lubricants with a sulfur level as low as 0.2%— considerably below the current value of ∼1% (Mainwaring, R. Shell Global Solutions, Private Communication, January 2008). In 1999, the European Union (EU) issued emission requirements for heavy-duty diesels that anticipated significant reductions in NOx, CO, unburnt hydrocarbons, and particulates over the period from 2001 to 2008. The greatest challenge was to lower NOx while at the same time reducing particulates as measures to correct the former normally resulted in an increase in the latter. In addition, in reducing NOx, the efficiency of the diesel engine would be impaired, and the result would be an increase in fuel consumption and CO2 emissions. However, a technique called selective catalytic reduction (SCR) has now been developed and adopted by several engine manufacturers in the EU [175]. This involves injecting an aqueous solution of urea (CO(NH2)2) into the exhaust stream where it degrades to carbon dioxide and ammonia (NH3). The NH3 then reduces the NOx to nitrogen (N2) and water on a tungsten/vanadium catalyst. It is obviously important to avoid NH3 being released into the atmosphere, and another catalyst is required to oxidize any residual NH3 while avoiding oxidation of the nitrogen. To date, SCR (low NOx but high particulates) has generally been favored over diesel particulate filters (high NOx but low particulates) for reducing emissions by many EU manufacturers. It allows engines to operate more efficiently—indeed sufficiently so as to more efficiently and more than offsets the cost of the urea. However, although it is an effective technique, there are concerns about its size, weight, guaranteed availability throughout Europe, and its efficacy at the lower temperatures encountered in the exhaust of light-duty applications. These problems have so far prevented its application to passenger cars. The effects on pollution if urea is not used in a system designed for its use and how the solution would be made available to the ordinary motorist are currently the subject of further investigation and debate.

3.12

FUELS

• As a result of the concern regarding the direct and indirect impacts of fuel sulfur on engine emissions, there is pressure to reduce the sulfur content. In the EU, a limit of 50 ppm maximum sulfur in diesel fuel was introduced in 2005, with a further reduction to 10 ppm in 2008 for gasoline engines and in 2009 for diesel engines. At such low levels, it may be necessary to restore the lubricity of the fuel by additives, and incorporating a small amount of phosphorus has been shown to be effective [176]. • Considerable concern currently surrounds carbon dioxide emission and its connection with global warming. Within the EU, the auto producers have reached a voluntary agreement with the commission to achieve a carbon dioxide emission target of 140 g/km by 2008 with a further reduction to 130 g/km by 2012. This will encourage a move toward more fuelefficient vehicles, particularly those that are smaller and lighter, and perhaps also to thinner lubricants requiring even better AW protection. In the United States, the Senate has ruled that carbon dioxide emission from trucks are a pollutant rather than a “by-product” of combustion. This is expected to promote reductions in carbon dioxide levels and hence encourage the use of lower viscosity, more fuel-efficient diesel engine oils (Mainwaring, R., Shell Global Solutions, Private Communication, January 2008).

3.13 CONCLUSIONS Ashless phosphorus-containing additives are available in a wide range of structures and performance. Although most are used as AW and EP additives for industrial oils, they can also function as antioxidants, rust inhibitors, metal passivators, and detergents. In some cases, the multifunctionality

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can be found within the same molecule. Their advantageous physical properties—for example, low color and odor and good solubility for other additives—make them attractive components for additive packages. However, although the future looks bright in industrial oil applications in view of current pressure on sulfur and chlorine (mainly as a result of environmental concerns), the potential in automotive engine oil remains uncertain due to the current downward pressure on phosphorus. In fuels, the question is whether the reduced level of sulfur will require replacement by other AW additives and, if so, whether phosphorus can be incorporated without adversely affecting other properties (e.g., catalyst efficiency).

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Williamson, S. Ann. 92: 316, 1854. Vogeli, F. Ann. 69: 190, 1849. British Patent 446, 547, The Atlantic Refining Co., 1936. Beeck, O., J.W. Givens, A.E. Smith. On the mechanism of boundary lubrication-I. The action of long chain polar compounds. Proc Roy Soc A 177: 90–102, 1940. Beeck, O., J.W. Givens, E.C. Williams. On the mechanism of boundary lubrication-II. Wear prevention by addition agents. Proc Roy Soc A 177: 103–118, 1940. Tingle, E.D. Fundamental work on friction, lubrication and wear in Germany during the war years. J Inst Pet 34: 743–774, 1948. West, H.L. Major developments in synthetic lubricants and additives in Germany. J Inst Pet 34: 774– 820, 1948. Boerlage, G.D., H. Blok. Four-ball top for testing the boundary lubricating properties of oils under high mean pressures. Engineering, 1, July 1937. U.S. Patent 2,391,311, C. C. Wakefield, 1945. U.S. Patent 2,391, 631, E. I. du Pont de Nemours, 1945 U.S. Patent 2,470,405, Standard Oil Development Co., 1949. U.S. Patent 2,663,691, The Texas Co., 1953. U.S. Patent 2,734,868, The Texas Co., 1956. U.S. Patent 2,612,515, Standard Oil Development Co., 1952. British Patent 797,166, Esso Research and Engineering Co., 1958. Morgan, J.P., T.C. Tullos. The jake walk blues. Ann Int Med 85: 804–808, 1976. Johnson, M.K. Organophosphorus esters causing delayed neurotoxic effect: Mechanism of action and structure/activity studies. Arch Toxicol 34: 259, 1975. British Patent 683,405, Shell Refining and Marketing Co., 1952. Greenshields, R.J. Oil industry finds fuel additives can help in controlling pre-ignition. Oil Gas J 52(8): 71–72, 1953. Jeffrey, R.E. et al. Improved fuel with phosphorus additives. Petrol Refiner 33(8): 92–96, 1954. Burnham, H.D. The role of tritolyl phosphate in gasoline for the control of ignition and combustion problems. Am Chem Soc Petrol Div Symp 36: 39–50, 1955. U.S. Patent 3,510,281, Texaco Inc., 1970. U.S. Patent 2,215,956, E. I. du Point de Nemours, 1940. U.S. Patent 2,237,632, Sinclair Refining Co., 1941. French Patent 681,1770, ICI, 1929. U.S. Patent 1,869,312, Combustion Utilities Corp., 1932. Russian Patent 47,690, R. L. Globus, S. F. Monakhov, 1936. U.S. Patent 2,071,323, Dow Chemical Co., 1937. British Patent 486,760, Celluloid Corp., 1938. U.S. Patent 2,117,290, Dow Chemical Co., 1938. Shlyakhtenko, A.I., P.P. Lebedev, R. Mandel. Novosti Tekhniki, 20: 42, 1938. Russian Patent, 52,398, A. I. Shlyakhtenko, P. P. Lebedev, 1938. Canadian Patent 379,529, Celluloid Corp., 1939. U.S. Patent 2,358,133, Dow Chemical Co., 1944. British Patent 872,899, Esso Research and Engineering Co., 1961. French Patent Addn. 89,648, Establissem*nts Kuhlmann, 1967. British Patent 424,380, N. V. de Bataafsche Petroleum Maatschappij, 1935.

Ashless Phosphorus-Containing Lubricating Oil Additives

115

38. U.S. Patent 2,005,619, E. I. du Pont de Nemours, 1935. 39. French Patent 797,449, E. I. du Pont de Nemours, 1936. 40. Beiswanger, J.P.G., W. Katzenstein, F. Krupin. Phosphate ester acids as load-carrying additives and rust inhibitors for metalworking fluids. ASLE Trans 7(4): 398–405, 1964. 41. U.S. Patent 3,547,820, GAF Corp., 1970. 42. U.S. Patent 3,567,636, GAF Corp., 1971. 43. British Patent 1,002,718, Shell International Research Maatschappij, 1965. 44. U.S. Patent 2,381,127, Texas Co., 1945. 45. U.S. Patent 2,642,722, Tide Water Associated Oil Co., 1953. 46. British Patent 1,266, 214, Esso Research and Engineering Co., 1972. 47. U.S. Patent 2,236,140, Atlantic Refining Co., 1944. 48. U.S. Patent 2,325,076, Atlantic Refining Co., 1944. 49. Dutch Patent 69,357, N. V. Bataafsche Petroleum Maatschappij, 1952. 50. U.S. Patent 2,653,161, Shell Development Co. 1953. 51. British Patent 1,247,541, Mobil Oil Corp., 1971. 52. U.S. Patent 3,600,470, Swift & Co. 1971. 53. U.S. Patent 6,242,631, Akzo Nobel, 2001. 54. Phillips, W.D., D.G. Placek, M.P. Marino. Neutral phosphate esters. In Synthetics, Mineral Oils and Bio-based Lubricants-Chemistry and Technology, ed. L.R. Rudnick. Boca Raton, FL: Taylor & Francis, 2006. 55. Evans, D.P., W.C. Davies, W.J. Jones. The lower trialkyl phosphates. J Chem Soc Pt I: 1310–1313, 1930. 56. U.S. Patent 2,723,237, Texas Co., 1955. 57. British Patent 1,165,700, Bush, Boake Allen Ltd., 1965. 58. British Patent J. R. Geigy, 1,146,173, 1966. 59. Phillips, W.D. Phosphate ester hydraulic fluids. In Handbook of Hydraulic Fluid Technology, ed. G.E. Totten. New York: Marcel Dekker, 2000. 60. U.S. Patent 5,779,774, K. J. L. Paciorek, S. R. Masuda, 1998. 61. Tang, J., Q. Cang, X. Zong. Preparation and evaluation of phosphorus friction modifiers used in automatic transmission fluids. Shiyou Lianzhi Yu Huagong 31(3): 17–20, 2000. 62. Anonymous. Product review—lubricant additives. Ind Lubr Trib 49(1): 15–30, 1997. 63. Lansdown, A.R. Extreme pressure additives. In Chemistry and Technology of Lubricants, eds. R. M. Mortier and S. T. Orszulik. New York: VCH Publishers Inc., 1992. 64. Anonymous. Boundary lubrication. Lubrication, 57: 1, 1971. 65. Gunther, R.C. Lubrication. Bailey Bros. and Swinfen Ltd., Pennsylvania: Chilton Books, 1971. 66. Mandakovic, R. Assessment of EP additives for water miscible metalworking fluids. J Syn Lubr 16(1): 13–26, 1999. 67. Klamann, D. Lubricants and Related Products. Verlag Chemie, Germany: Weinheim, 1984. 68. Anonymous. Fundamentals of wear. Lubrication 12(6): 61–72, 1957. 69. Anonymous. Lubrication fundamentals. Lubrication, 59(Oct–Dec): 77–88, 1973. 70. Hardy, W.B. Note on the static friction and lubricating properties of certain chemical substances. Phil Mag 38: 32–49, 1919. 71. Wells, H.M., J.E. Southcombe. Petrol World 17: 460, 1920. 72. Bragg, W.H. The investigation of the properties of thin films by means of X-rays, Nature 115: 226, 1925. 73. Clark, G.L., R.R. Sterrett. X-ray diffraction studies of lubricants. Ind Eng Chem 28(11): 1318–1322, 1936. 74. Thorpe, R.E., R.G. Larsen. Antiseizure properties of boundary lubricants. Ind Eng Chem 41: 938–943, 1949. 75. Davey, W. Extreme pressure lubricants—phosphorus compounds as additives. Ind Eng Chem 42(9): 1841–1847, 1950. 76. Larsen, R.G., G.L. Perry. Chemical aspects of wear and friction. In Mechanical Wear, ed. J.T. Burwell. American Society for Metals, Ohio, USA, 1950. 77. Bita, O., I. Dinca. Behaviour of phosphorus additives. Rev Mecan Appl 8: 441–442, 1963. 78. Godfrey, D. The lubrication mechanism of tricresyl phosphate on steel. ASLE Preprint 64-LC-1, 1964. 79. Klaus, E.E., H.E. Bieber. Effects of P impurities on the behaviour of tricreslyl phosphate as an antiwear additive. ASLE Preprint 64-LC-2, 1964. 80. Barcroft, F.T., S.G. Daniel. The action of neutral organic phosphates as EP additives. ASME J Basic Eng 64-Lub-22, 1964.

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81. Bieber, H.E., E.E. Klaus, E.J. Tewkesbury. A study of tricresyl phosphate as an additive for boundary lubrication. ASLE Preprint 67-LC-9, 1967. 82. Sakurai, T., K. Sato. Chemical reactivity and load carrying capacity of lubricating oils containing organic phosphorus compounds. ASLE Preprint 69-LC-18, 1969. 83. Barcroft, F.T. A technique for investigating reactions between EP additives and metal surfaces at high temperature. Wear 3: 440–453, 1960. 84. Goldblatt, I.L., J.K. Appeldoorn. The antiwear behavior of TCP in different atmospheres and different base stocks. ASLE Preprint 69-LC-17, 1969. 85. Forbes, E.S., N.T. Upsdell, J. Battersby. Current thoughts on the mechanism of action of tricresyl phosphate as a load-carrying additive. Proc Trib Conv 1: 7–13, 1972. 86. Faut, O.D., D.R. Wheeler. On the mechanism of lubrication by tricresylphosphate (TCP)—the coefficient of friction as a function of temperature for TCP on M-50 steel. ASLE Trans 26(3): 344–350, 1983. 87. Yamamoto, Y., F. Hirano. Scuffing resistance of phosphate esters. Wear 50: 343–348, 1978. 88. Furey, M.J. Surface roughness effects on metallic contact and friction. ASLE Trans 6: 49–59, 1963. 89. Gauthier, A., H. Montes, J.M. Georges. Boundary lubrication with tricresylphosphate (TCP). Importance of corrosive wear. ASLE Preprint 81-LC-6A-3, 1981. 90. Placek, D.G., S.G. Shankwalkar. Phosphate ester surface treatment for reduced wear and corrosion protection. Wear 173: 207–217, 1994. 91. Klaus, E.E., J.M. Perez. Comparative evaluation of several hydraulic fluids in operational equipment. SAE Paper No. 831680, 1983. 92. Klaus, E.E., J.L. Duda, K.K. Chao. A study of wear chemistry using a microsample fourball wear test STLE. Trib Trans 34(3): 426–432, 1991. 93. Yansheng, Ma., J. Liu, Y. Wu, Z. Gu. The synergistic effects of tricresyl phosphate oil additive with chemico-thermal treatment of steel surfaces. Lubr Sci 9–1: 85–95, 1996. 94. Klaus, E.E., G.S. Jeng, J.L. Duda. A study of tricresyl phosphate as a vapor-delivered lubricant. Lubr Eng 45(11): 717–723, 1989. 95. Cho, L., E.E. Klaus. Oxidative degradation of phosphate esters. ASLE Trans 24(1): 119–124, 1981. 96. Graham, E.E., A. Nesarikar, N.H. Forster. Vapor-phase lubrication of high-temperature bearings. Lubr Eng 49(9): 713–718, 1993. 97. Hanyaloglu, B., E.E. Graham. Vapor phase lubrication of ceramics. Lubr Eng 50(10): 814–820, 1994. 98. Van Treuren, K.W. et al. Investigation of vapor-phase lubrication in a gas turbine engine. ASME J Eng Gas Turbines Power 120(2): 257–262, 1998. 99. Morales, W., R.F. Handschuh. A preliminary study on the vapor/mist phase lubrication of a spur gearbox. Lubr Eng 56(9): 14–19, 2000. 100. Saba, C.S., N.H. Forster. Reactions of aromatic phosphate esters with metals and their oxides. Trib Lett 12(2): 135–146, 2002. 101. European Patent 0521628, Ethyl Petroleum Additives. 1992. 102. Didziulis, S.R., R. Bauer. Volatility and performance studies of phosphate ester boundary additives with a synthetic hydrocarbon. Aerospace Report TR 95-(5935)-6. 103. Japanese Patent 05001837, Toyota Central Res. & Dev. Lab, 1988. 104. Japanese Patent 02018496, New Japan Chemical Co., 1990. 105. Japanese Patent 02300295, Toyota Jidosha & Yushiro Co., 1991. 106. U.S. Patent 6,204,277, PABU Services, 2001. 107. European Patent Appl., EP1618173, Great Lakes Chemical Corp., 2004. 108. U.S. Patent 5,584,201, Cleveland State University, 1986. 109. Soviet Union Patent 810,767, Berdyansk Exptl. Pet. Plant, 1981. 110. Metal Passivators, Newsletter No 10, ADD APT AG, June 2000. 111. Werner, J.J., R.L. Reierson, J.-L. Joye. European Patent Appl. WO 00/37591, 2000. 112. Katzenstein, W. Phosphate ester acids as load-carrying additives and rust inhibitors for metalworking fluids. Proc 11th Int Trib Coll Esslingen, Germany, 1741–1754, 1998. 113. Werner, J.J., M. Dahanayake, D. Lukjantschenko. Relationship of structure to performance properties of phosphate-ester surfactants in metal working fluids. STLE Annual Conference, Chicago, 1995. 114. U.S. Patent 1,936,533, E. I. Du Point de Nemours, 1933. 115. British Standard 3150, Corrosion-inhibited antifreeze for water-cooled engines, Type A. British Standard Institution, 1959. 116. Forbes, E.S., H.B. Silver. The effect of chemical structure on the load-carrying properties of organophosphorus compounds. J Inst Pet 56(548): 90–98, 1970.

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117. Forbes, E.S., N.T. Upsdell. Phosphorus load-carrying additives: Adsorption/reaction studies of amine phosphates and their load-carrying mechanism. Paper C-293/73, 277–298. 1st Eur Trib Cong, London 1973. 118. Farng, L.O., W.F. Olszewski. U.S. Patent 5,681,798, 1997. 119. Weller, D., J. Perez. A study of the effect of chemical structure on friction and wear, Part 1—Synthetic ester fluids. Lubr Eng 56(11): 39–44, 2000. 120. Kristen, U. Aschefreie extreme-pressure-und verschleiss-schutz-additive. In Additive für Schmierstoffe, ed. W.J. Bartz. Expert Verlag, Germany: Renningen-Malmsheim, 1994. 121. U.S. Patent 2,722,517, Esso Research & Engineering Co., 1955. 122. U.S. Patent 2,763,617, Shell Development Co., 1957. 123. U.S. Patent 2,820,766, C. C. Wakefield & Co., 1958. 124. U.S. Patent 2,971,912, Castrol Ltd., 1961. 125. Sanin, P.I. et al. Wear 3: 200, 1960. 126. Ohmuri, T., M. Kawamura. Fundamental studies on lubricating oil additives. In Adsorption and Reaction Mechanism of Phophorus-Type Additive on Iron Surface, eds. Toyota Chuo Kenkyusho R. and D. Rebyu. 28(1): 25–33, Japan: Toyota, 1993. 127. Riga, A., W. Rock Pistillo. Surface and solution properties of organophosphorus chemical in wear tests. NTAS Ann Conf Therm Anal App 28: 530–535, 2000. 128. French Patent 1,435,890, Albright & Wilson Ltd., 1996. 129. Ren, D., G. Zhou, A.J. Gellman. The decomposition mechanism of trimethylphosphite on Ni (III). Surf Sci 475(1–3): 61–72, 2001. 130. British Patent 682,441, Anglamol Ltd., 1952. 131. U.S. Patent 2,764,603, Socony Vacuum Oil Co. 1954. 132. Rasberger, M. Oxidative degradation and stabilisation of mineral oil-based lubricants. In Chemistry and Technology of Lubricants, eds. R. M. Mortier and S. I. Orszulik. New York: VCH Publishers, 1992. 133. European Patent 0049133, Sumitomo Chemicals., 1982. 134. Japanese Patent 63156899, Sumiko Junkatsu-Zai., 1988. 135. Japanese Patent 2888302, Tonen Corp., 1991. 136. Japanese Patent 05331476, Tonen Corp., 1994. 137. Japanese Patent 06200277, Tonen Corp., 1994. 138. U.S. Patent 4,656,302, Koppers Co. Inc., 1987. 139. European Patent Appl. 475560, Petro-Canada Inc., 1992. 140. U.S. Patent 4,652,385, Petro-Canada Inc., 1987. 141. U.S. Patent 5,124,057, Petro-Canada Inc., 1993. 142. U.S. Patent 3,329,742, Mobil Oil Corp., 1967. 143. U.S. Patent 3,351,554, Mobil Oil Corp., 1967. 144. U.S. Patent 3,321,401, British Petroleum Co., 1967. 145. U.S. Patent 3,201,348, Standard Oil Co., 1965. 146. Messina, V., D.R. Senior. South African Patent 6707230. 147. U.S. Patent 3,115,463, Ethyl Corp., 1963. 148. Forbes, E.S., J. Battersby. The effect of chemical structure on the load-carrying and adsorption properties of dialkyl phosphites. ASLE Trans 17(4): 263–270, 1974. 149. Barber, R.I. The preparation of some phosphorus compounds and their comparison as load-carrying additives by the four-ball machine. ASLE Preprint 75-LC-2D-1, 1975. 150. Sanin, P.I. et al. Antiwear additives of the phosphonate type. Neftekhimiya 14(2): 317–322, 1974. 151. U.S. Patent 3,553,131, Mobil Oil Corp., 1971. 152. U.S. Patent 4,246,125, Ethyl Corp., 1981. 153. U.S. Patent 4,260,499, Ethyl Corp.,1981. 154. European Patent Appl. 510633, Sakai Chemicals, 1992. 155. Japanese Patent 05302093, Tonen Corp., 1993. 156. U.S. Patent 4,225,449, Ethyl Corp., 1980. 157. Colyer, C.C., W.C. Gergel. Detergents/dispersants. In Chemistry and Technology of Lubricants, eds. R. M. Mortier and S. T. Orszulik. New York: VCH Publishers, 1992. 158. Japanese Patent 62215697, Toyota Res. and Devt., 1987. 159. Czech Patent 246897, Kekenak. 1988. 160. U.S. Patent 3,268,450, Sims, Bauer and Preuss, 1966. 161. Japanese Patent 8126997, Showa Aluminium Co., 1981. 162. U.S. Patent 4,123,369, Continental Oil Co., 1978.

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163. U.S. Patent 3,658,706, Ethyl Corp., 1972. 164. Goode, M.J., W.D. Phillips. Triaryl phosphate ester hydraulic fluids-A reassessment of their toxicity and environmental behaviour. SAE Paper 982004, 1998. 165. Henschler, D., H.H. Bayer. Toxicological studies of triphenyl phosphate, trixylenyl phosphate and triaryl phosphates from mixtures of hom*ologous phenols. Arch Exp Pathol Pharmakol 233: 512–517, 1958. 166. Mackerer, C., M.L. Barth, A.J. Krueger. A comparison of neurotoxic effects and potential risks from oral administration or ingestion of tricresyl phosphate and jet engine oil containing tricresyl phosphate. J Toxicol Environ Health, Part A 57(5): 293–328, 1999. 167. Daughtrey, W., R. Biles, B. Jortner, M. Ehrlich. Delayed neurotoxicity in chickens: 90 day study with Mobil Jet Oil 254. The Toxicologist, 90, Abstract 1467, 2006. 168. Durad® 310M, Great Lakes Chemical Corp., May 2002. 169. Drake, G.L., Jr., T.A. Calamari. Industrial uses of phosphonates. In Role of Phosphonates in Living Systems, ed. R. Hildebrand. Boca Raton, FL: CRC Press, 1983. 170. Roby, S.H., J.A. Supp. Effects of ashless antiwear agents on valve train wear and sludge formation in gasoline engine testing. Lubr Eng 53(11): 17–22, 1977. 171. Devlin, M.T., R. Sheets, J. Loper, G. Guinther, K. Thompson, J. Guevremont, T.-C. Jao. Effect of ashless phosphorus antiwear compounds on passenger car emissions and fuel efficiency. Additives 2007 Conf., London, April 2007 172. Trautwein, W.-P. AdBlue as a Reducing Agent for the Decrease of NOx Emissions from Diesel Engines of Commercial Vehicles, Research Report 616-1, DGMK, Hamburg, 2003. 173. U.S. Patent 5,630,852, FMC Corp., 1997. 174. Crosby, G.W., E.W. Brennan. Am Chem Soc Div Petrol Chem, Preprints 3(4A): 171, 1958. 175. Demizu, K., H. Ishigaki, M. Kawamoto. The effect of trialkylphosphites and other oil additives on the boundary friction of oxide ceramics against themselves and other metals. Trib Intl 30(9): 651–657, 1997. 176. German Patent 1,271,874, Hüls Chemicals. 1968. 177. German Patent 1,286,675, Hüls Chemicals. 1969.

APPENDIX A:

EARLY PATENT LITERATURE ON PHOSPHORUSCONTAINING COMPOUNDS

NEUTRAL PHOSPHATES U.S. Patent 2,723,237, Texas Oil Co.

NEUTRAL PHOSPHITES As AW/EP Additives British Patent 1,052,751, British Petroleum (chlorethyl phosphite and a chlorparaffin) British Patent 1,164,565, Mobil Oil Corp. (alkyl or alkenyl phosphite and a fatty acid ester) British Patent 1,224,060, Esso Research and Engineering Co. U.S. Patent 2,325,076, Atlantic Refining Co. U.S. Patent 2,758,091, Shell Development Co. (haloalkyl or haloalkarylphosphites) U.S. Patent 3,318,810, Gulf Research & Development Co. (phosphites and molybdenum compounds)

As Antioxidants U.S. Patent 2,326,140, Atlantic Refining Co. U.S. Patent 2,796,400, C.C. Wakefield & Co.

ACID PHOSPHATES/PHOSPHITES British Patent 1,105,965, British Petroleum Co Ltd. (acid hydrocarbyl phosphite and phosphates or thiophosphates or phosphoramidates)

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British Patent 1,153,161, Nippon Oil Co. U.S. Patent 2,005,619, E. I. du Pont de Nemours U.S. Patent 2,642,722, Tide Water Oil Co. French Patent 797,449, E.I. du Pont de Nemours

PHOSPHONATES British Patent 823,008, Esso Research and Engineering Co. (dicarboxylic acid and either a haloalkane phosphonate, a haloalkyl phosphate or phosphite and optionally a neutral alkyl or aryl phosphate) British Patent 884,697, Shell Research Ltd. (dialkenyl phosphonates) British Patent 899,101, British Petroleum Co. (amino phosphonates) British Patent 993,741, Rohm and Haas Co. (aminoalkane phosphonates) British Patent 1,083,313, British Petroleum Co. (amino phosphonates) British Patent 1,247,541, Mobil Oil Corp. (Dialkyl-n-alkylphosphonate or alkylammonium salts of dialkylphosphonates) U.S. Patent 2,996,452, US Sec of Army (di-(2-ethylhexyl) lauroxyethyl phosphonate) U.S. Patent 3,329,742, Mobil Oil Corp. (diaryl phosphonates) U.S. Patent 3,600,470, Swift & Co. (hydroxy or alkoxy phosphonates and their amine salts) U.S. Patent 3,696,036, Mobil Oil Corp. (tetraoctyl-(dimethylamino) methylene diphosphonate) U.S. Patent 3,702,824, Texaco Inc. (hydroxyalkylalkane phosphonate)

ALKYL- AND ARYLPOLYETHYLENEOXY-PHOSPHORUS COMPOUNDS U.S. Patent 2,372,244, Standard Oil Dev. Co.

AMINE SALTS British Patent 705,308, Bataafsche Petroleum Maatschappij (substituted monobasic phosphonic acid and amine salts thereof) British Patent 978,354, Shell International Research (alkali metal-amine salt of a halohydrocarbyl phosphonic acid) British Patent 1,002,718, Shell International Research (alkylamine salt of diaryl acid phosphate) British Patent 1,199,015, British Petroleum Co. Ltd. (quaternary ammonium salts of dialkyl phosphates) British Patent 1,230,045, Esso Research and Engineering Co. (quaternary ammonium salts of alkyl phosphonic and phosphonic acids) British Patent 1,266,214, Esso Research and Engineering Co. (neutral phosphate and a neutral alkylamine hydrocarbyl phosphate) British Patent 1,302,894, Castrol Ltd. (tertiary amine phosphonates) British Patent 1,331,647, Esso Research and Engineering Co. (quaternary ammonium phosphonates) U.S. Patent 1,936,533, E. I. du Pont de Nemours. (triethanolamine salts) U.S. Patent 3,553,131, Mobil Oil Corp. (tertiary amine phosphonate salts) U.S. Patent 3,668,237, Universal Oil Products Co. (tertiary amine salts of polycarboxylic acid esters of bis(hydroxyalkyl)-phosphinic acid)

PHYSICAL MIXTURES OF PHOSPHORUS AND SULFUR AND CHLORINE COMPOUNDS British Patent 706,566, Bataafsche Petroleum Maatschappij (a phosphorus compound, e.g., a trialkyl phosphate, a glycidyl either and a disulphide) British Patent 797,166, Esso Research and Engineering Co. (TCP and a metal soap of a sulphonic acid) British Patent 841,788, C.C. Wakefield & Co. Ltd. (chlorinated hydrocarbon, a disulphide, and a dialkyl phosphite) British Patent 967,760, The Distillers Co. Ltd. (disulphides, chlorinated wax, and a haloalkyl ester of an oxyacid of phosphorus) British Patent 872,899, Esso Research and Engineering Co. (trialkyl phosphates and chlorinated benzene) British Patent 1,222,320, Mobil Oil Corp. (diorganophosphonate and a sulphurized hydrocarbon or sulphurized fat)

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British Patent 1,287,647, Stauffer Chemical Co. (phosphonates or halogenated alkylphosphates, sulphurised oleic acid, and sebacic acid) British Patent 1,133,692, Shell International (TCP and triphenylphosphorothionate) British Patent 1,162,443, Mobil Oil Corp. (neutral or acid, alkyl or alkenyl phosphite, and a sulphurized polyisobutylene, triisobutylene, or a sulphurized dipentene) U.S. Patent 2,494,332, Standard Oil Dev. Co. (thiophosphates and TCP) U.S. Patent 2,498,628, Standard Oil Dev. Co. (sulfurized/phosphorized fatty material and TCP or tricresyl phosphite) U.S. Patent 3,583,915, Mobil Oil Corp. (di(organo)phosphonate, and an organic sulphur compound selected from sulphurized oils and fats, a sulphurized monoolefi n or an alkyl polysulphide)

MISCELLANEOUS PHOSPHORUS COMPOUNDS British Patent 1,035,984, Shell Research Ltd. (diaryl chloralkyl phosphate or thiophosphate) British Patent 1,193,631, Albright & Wilson Ltd. (hydroxyalkyl disphosphonic acid/alkylene oxide reaction products) British Patent 1,252,790, Shell International Research (pyrophosphonic and pyrophosphinic acids and their amine salts) U.S. Patent 3,243,370, Monsanto Co. (phosphinylhydrocarbyloxy phosphorus esters) U.S. Patent 3,318,811, Shell Oil Co. (diacid diphosphate ester) U.S. Patent 3,640,857, Dow Chemical Co. (tetrahaloethyl phosphates)

APPENDIX B: ADDITIONAL LITERATURE AND PATENT REFERENCES ON THE MECHANISM AND PERFORMANCE OF PHOSPHORUS-CONTAINING ADDITIVES NEUTRAL PHOSPHATES Garaud, Y., M.D. Tran. Photoelectron spectroscopy investigation of tricresyl phosphate anti-wear action. Analusis 9(5): 231–235, 1981. Ghose, H.M., J. Ferrante, F.C. Honecy. The effect of tricresyl phosphate as an additive on the wear of iron. NASA Tech. Memo, NASA-TM-100103, E-2883, NASI. 15: 100103. Han, D.H., M. Masuko. Comparison of antiwear additive response among several base oils of different polarities. Tribol Trans 42(4): 902–906, 1999. Han, D.H., M. Masuko. Elucidation of the antiwear performance of several organic phosphates used with different polyol esters base oils from the aspect of interaction between the additive and the base oil. Tribol Trans 41(4): 600–604, 1998. Kawamura, M., K. Fujito. Organic sulfur and phosphorus compounds as extreme pressure additives. Wear 72(1): 45–53, 1981. Koch, B., E. Jantzen, V. Buck. Properties and mechanism of action of organism phosphoric esters as antiwear additives in aviation. Proc 5th Int Tribol Coll Esslingen, Ger Vol. 1, 3/11/1–3/11/12, 1986. Morimoto, T. Effect of phosphate on the wear of silicon nitride sliding against bearing steel. Wear 169(2): 127–134, 1993. Perez, J.M. et al. Characterization of tricresyl phosphate lubricating films. Tribol Trans 33(1): 131–139, 1990. Riga, A., J. Cahoon, W.R. Pistillo. Organophosphorus chemistry structure and performance relationships in FZG gear tests. Tribol Lett 9(3,4): 219–225, 2001. Riga, A., W.R. Pistillo. Surface and solution properties of organophosphorus chemicals and performance relationships in wear tests. Proc 27th NATAS Ann Conf Therm Anal Appl 708–713, 1999. Ren, D., A.J. Gelman. Reaction mechanisms in organophosphate vapor-phase lubrication of metal surfaces. Tribol Int 34(5): 353–365, 2001. Weber, K., E. Eberhardt, G. Keil. Influence of the chemical structure of phosphoric EP-additives on its effectiveness. Schmierungstechnik 3(12): 372–377, 1972. Wiegand, H., E. Broszeit. Mechanism of additive action. Model investigations with tricresyl phosphate. Wear 21(2): 289–302, 1972.

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Yamamoto, Y., F. Hirano. Effect of different phosphate esters on frictional characteristics. Tribol Int 13(4): 165–169, 1980. Yamamoto, Y., F. Hirano. The effect of the addition of phosphate esters to paraffinic base oils on their lubricating performance under sliding conditions. Wear 78(3): 285–296, 1982. Yanshang, Ma., et al. The effect of oxy-nitrided steel surface on improving the lubricating performance of tricresyl phosphate. Wear 210(1–2): 287–290, 1997.

NEUTRAL PHOSPHITES Barabanova, G.V. et al. Effect of phosphoric, thiophosphoric and phosphorus acid neutral ester-type additives on the lubricating capacity of C5-C9 synthetic fatty acid pentaerythritol ester. Pererabotke Nefti 17: 57–61, 1976. Orudzheva, I.M. et al. Synthesis and study of some aryl phosphites. 3rd Tekh Konf Neftekhim 3: 411–415, 1974. Sanin, P.I. et al. Tr Inst Nefti Akad Nauk SSSR, 14: 98, 1960. Sanin, P.I., A.V. Ul’yanova. Prisadki Maslam Toplivam, Trudy Naucha. Tekhn Soveshch 189, 1960. Wan, Y., Q. Xue. Effect of phosphorus-containing additives on the wear of aluminum in the lubricated aluminumon-steel contact. Tribol Lett 2(1): 37–45, 1996. U.S. Patent 3,115,463/4. U.S. Patent 3,652,411. U.S. Patent 4,374,219.

ALKOXYLATED PHOSPHATES Jia, X., X. Zhang. Antiwear property of water-soluble compound phosphate ester. Runhua Yu Mifeng 4 (25–25): 67, 1999.Wang, R. Study on water-soluble EP additives. Runhua Yu Mifeng 3(17–18): 51, 1994. Zhang, X., X. Jia. Water soluble phosphate esters. Hebei Ligong Xueyuan Xuebao 21(2): 58–61, 1999.

AMINE SALTS Forbes, E.S. et al. The effect of chemical structure on the load carrying properties of amine phosphates. Wear 18: 269, 1971. Shi, H. Development trend of phosphorus-nitrogen-type extreme-pressure antiwear agents. Gaoqiao Shi 12(3): 37–41, 1997.

DIALKYL ALKYL PHOSPHONATES Cann, P.M.E., G.J. Johnston, H.A. Spikes. The formation of thick films by phosphorus-based antiwear additives. Proc Inst Mech Eng, I. Mech. E. Conf 543–554, 1987. Dickert, J.J., C.N. Rowe. Novel lubrication properties of gold O, O-dialkylphosphorodithioates and metal organophosphonates. ASLE Trans 20(2): 143–151, 1977. Gadirov, A.A., A.K. Kyazin-Zade. Diphenyl esters of alpha-aminophosphonic acids as antioxidant and antiwear additives. Khim Tekhnol Topl Masel 3: 23–24, 1990. Lashkhi, V.L. et al. Antiwear action of organophosphorus and chloroorganophosphorus compounds in lubricating oils. Khim Tekhnol Topl Masel 2: 47–50, 1975. Lashkhi, V.L. et al. IR spectroscopic study of phosphonic acid ester-antiwear additives for oil. Khim Tekhnol Topl Masel 5: 59–61, 1977. Lozovoi, Y., P.I. Sanin. Mechanism of action of phosphorus acid ester-type extreme pressure additives. IZV Khim 19(1): 49–57, 1986. Tang, J., Q. Cang, X. Zong. Preparation of phosphorus friction modifiers used in automatic transmissions fluids. Shiyou Lianzhi Yu Huagong 31(3): 17–20, 2000. Wan, G.T.Y. The performance of one organic phosphonate additive in rolling contact fatigue. Wear 155(2): 381–387. Xiong, R.-G., W. Hong, J.-L. Zuo, C.-M. Liu, X.-Z. You, J.-X. Dong, Z. Pei. Antiwear and extreme pressure action of a copper (II) complex with alkyl phosphonic monoalkyl ester. J Tribol 118(3): 676–680, 1996.

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MIXTURES OF PHOSPHORUS AND SULFUR COMPOUNDS Kawamura, M. et al. Interaction between sulfur type and phosphorus type EP additives and its effect on lubricating performance. Junkatsu 30(9): 665–670. Kubo, K., Y. Shimakawa, M. Kibukawa. Study on the load-carrying mechanism of sulfur-phosphorus type lubricants. Proc JSLE Int Tribol Conf 3: 661–666, 1985. PCT Int Appl. WO 2002053687A2 (Shell Int Res). Qiao, Y., B. Xu, S. Ma, X. Fang, Q. Xue. Study in synergistic effect mechanism of some extreme pressure and antiwear additives in lubricating oil. Shiyou Xuebao, Shiyou Jiagong 13(3): 33–39, 1997. Qiao, Y., X. Fang, H. Dang. Synergistic effect mechanism of the combination system of two typical additives containing sulfur and phosphorus in lubricating oil. Mocaxue Xuebao 15(1): 29–38, 1995. Xia, H. A study of the antiwear behaviour of S-P type gear oil additives in four-ball and Falex machines. Wear 112(3–4): 335–361, 1986.

GENERAL REFERENCES Hartley, R.J., A.G. Papay. Function of additives. Antiwear and extreme pressure additives. Toraiborojisuto 40(4): 326–331, 1995. Palacios, J.M. The performance of some antiwear additives and interference with other additives. Lubr Sci 4(3): 201–209, 1992.

Detergents Syed Q. A. Rizvi

CONTENTS 4.1 Introduction ........................................................................................................................... 123 4.2 Detergent Types .................................................................................................................... 125 4.3 Detergent Parameters ............................................................................................................ 125 4.4 Detergent Substrates.............................................................................................................. 127 4.5 Synthesis of Neutral and Basic Detergents ........................................................................... 130 4.6 Testing ................................................................................................................................... 135 References ...................................................................................................................................... 139

4.1 INTRODUCTION Modern equipment must be lubricated to prolong its lifetime. A lubricant* performs a number of critical functions. These include lubrication, cooling, cleaning and suspending, and protecting metal surfaces against corrosive damage [1]. Lubricant comprises a base fluid and an additive package. The primary function of the base fluid is to lubricate and act as a carrier of additives. The function of additives is either to enhance an already-existing property of the base fluid or to add a new property. The examples of already-existing properties include viscosity, viscosity index, pour point, and oxidation resistance. The examples of new properties include cleaning and suspending ability, antiwear performance, and corrosion control. The extent of the desirability of various properties differs from lubricant to lubricant and largely depends on the conditions of use. Automotive use, for example, requires lubricants with good oxidation resistance, suitable low- and high-temperature viscosities, high-viscosity index (i.e., minimum loss in viscosity with an increase in temperature), and good cleaning and suspending ability. Conversely, the use as nonautomotive lubricants, such as industrial and metalworking lubricants, emphasizes oxidation resistance, antiwear performance, corrosion control, and cooling ability. One of the most critical properties of the automotive lubricants, especially engine oils, is their ability to suspend undesirable products from thermal and oxidative degradation of the lubricant. Such products form when the by-products of fuel combustion, such as hydroperoxides and free radicals, go past piston rings into the lubricant and, being reactive species, initiate lubricant oxidation. The resulting oxidation products are thermally labile and decompose to highly polar materials with a tendency to separate from the bulk lubricant and form surface deposits and clog small openings. The former will lead to malfunctioning of the closely fitted surfaces, such as those between pistons and cylinder walls, and the latter will impair oil flow to parts needing lubrication. The separation tendency of these products relates to their high polar to nonpoplar ratio [2], which makes them less soluble in largely nonpolar base oil. A lubricant with high-oxidation resistance, due to the quality of the base fluid or the presence of a good oxidation inhibitor additive package, will slow down the formation of these undesirables.

* The terms “lubricant” and “oil” are interchangeable and are different from the terms “base oil” and “base fluid.” Lubricant and oil imply base oil or a base fluid plus additives.

123

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Oxidation inhibitors, detergents, and dispersants make up the general class of additives called stabilizers and deposit control agents. These additives are designed to control deposit formation, either by inhibiting the oxidative breakdown of the lubricant or by suspending the harmful products already formed in the bulk lubricant. Oxidation inhibitors intercept the oxidation mechanism, and dispersants and detergents perform the suspending part [3,4]. Detergents are the topic of this chapter, and dispersants are the topic of the subsequent chapter. Detergents are metal salts of organic acids that frequently contain associated excess base, usually in the form of carbonate. Dispersants are metal-free and are of higher molecular weights than detergents. The two types of additives work in conjunction with one another. The fi nal products of combustion and lubricant decomposition include organic and inorganic acids, aldehydes, ketones, and other oxygenated materials [4,5]. The acids have the propensity to attack metal surfaces and cause corrosive wear. Detergents, especially basic detergents, contain reserve base that will neutralize the acids to form salts. Although this decreases the corrosive tendency of the acids, the solubility of the salts in the bulk lubricant is still low. The organic portion of the detergent, commonly called soap, has the ability to associate with the salts to keep them suspended in the bulk lubricant. However, in this regard, detergents are not as effective as dispersants because of their lower molecular weight. The soap in detergents and dispersants also has the ability to suspend nonacidic oxygenated products such as alcohols, aldehydes, and resinous oxygenates [4]. The mechanism by which this occurs is depicted in Figure 4.1. Dispersants and detergents together make up the bulk, ∼45 to 50%, of the total volume of the lubricant additives manufactured. This is a consequence of their major use in engine oils, transmission fluids, and tractor hydraulic fluids; all of which are high-volume lubricants [6]. As mentioned earlier, detergents neutralize oxidation-derived acids as well as help suspend polar oxidation products in the bulk lubricant. Because of this, these additives control rust, corrosion, and resinous buildup in the engine. Like most additives, detergents contain a surface-active polar functionality and an oleophilic hydrocarbon group, with an appropriate number of carbon atoms to ensure good oil solubility [2]. Sulfonate, phenate, and carboxylate [7] are the common polar groups present in detergent molecules. However, additives containing salicylate and thiophosphonate functional groups are also sometimes used.

Oil Oil

Oil

Polar oxidation product

Oil

Oil Oil Oil

Oil

FIGURE 4.1 Oil suspension of polar oxidation products.

Detergents

125 (RSO3)a M·X Mb CO3·y M(OH)c

(RPhO)a M ·X Mb CO3·y M(OH)c

Basic sulfonate

Basic phenate

(RCOO)a M·X Mb CO3·y M(OH)c Basic carboxylate a and c = 1 and b = 2, if the metal M is monovalent; a and c = 2 and b =1, if the metal M is divalent

FIGURE 4.2 General formulas for detergents. (Adapted from Rizvi, S.Q.A., Additives and additive chemistry., ASTM Manual on Fuels and Lubricants.)

4.2 DETERGENT TYPES Detergents are the metal salts of organic acids. The acids normally used to synthesize these compounds include arylsulfonic acids such as alkylbenzenesulfonic acids and alkylnaphthalenesulfonic acids [8–11]; alkylphenols [12–16]; carboxylic acids such as fatty carboxylic acids, naphthenic acids, and petroleum oxidates [17–20]; and alkenylphosphonic and alkenylthiophosphonic acids [21–23]. Sometimes, a mixture of different types of acids is also employed [24]. The reaction of these acids with inorganic bases, such as metal oxides, metal hydroxides, and metal carbonates, results in the formation of salts [7]. The quantity of the metal used may be equal to (stoichiometric amount) or in excess of the exact amount necessary to completely neutralize the acid functionality. The presence of metal in stoichiometric amount results in the formation of the neutral salt, often referred to as a neutral detergent or soap. If the metal is present in excess, the detergents are called basic, overbased, or superbased [7,25]. It is important to note that basic detergents appear as clear hom*ogeneous fluids, the same as neutral detergents, because the excess metal is present in a colloidal form [26]. The general formulas for metal sulfonates, metal phenates, and metal carboxylates are presented in Figure 4.2. The excess base in basic detergents may be present as metal hydroxide, metal carbonate, or both. For neutral detergents, x and y in the formulas in Figure 4.2 are zero. For low overbased detergents, such as those with a base number of about 50 or less, x may be zero and y may be a low number, or both x and y may be low numbers. This implies that slightly overbased detergents are either carbonate-free or contain a mixture of both the hydroxide and the carbonate. Highly overbased detergents invariably have a large amount of carbonate as the reserve base. That is, in their case, y is low and x is very high. In some cases, x can be as high as 20, or more. In summary, the excess base per equivalent of acid in metal hydroxide–containing detergents is generally lower than that in metal carbonate–containing detergents.

4.3

DETERGENT PARAMETERS

Detergents are described chemically in terms of their metal ratio, percent sulfated ash, degree of overbasing or conversion, soap content, and total base number (TBN) [7]. The metal ratio is defined as the total equivalents of metal per equivalent of acid. The percent sulfated ash is the ash produced when the detergent is treated with sulfuric acid and burned. All organic material in the detergent burns, leaving behind the metal sulfate ash. Sulfate ash results from the reaction of metal compound with sulfuric acid either directly, as with metal hydroxide and metal carbonate, or through the oxidative degradation of the metal sulfonate. Detergents are not the only additives that result in sulfate ash. Other metal-containing additives in the lubricant also contribute toward it. Such additives include metal carboxylates and metal dialkyldithiophosphates such as zinc dialkyldithiophosphate. The former compounds are sometimes used as friction modifiers and corrosion inhibitors, and the latter compounds are commonly used as oxidation inhibitors and antiwear agents. Because the metal compounds can lead to the formation of the inorganic material

126

Lubricant Additives: Chemistry and Applications

(ash) on combustion, a formulator must know the metal content of a formulation to offset any problem that might occur. This is because the lubricant travels past piston rings into areas that experience flame and high temperatures, such as the top land and the groove behind the top ring; it burns to produce ash. Ash is undesired because it is believed to initiate deposit formation. Sulfated ash is one of the methods used to assess the metal content of a lubricant, and the methods to determine this are described in the ASTM Standards D 482 and D 874 [27]. The degree of overbasing is the number of equivalents of the metal base per equivalent of the acid substrate. This is usually expressed as conversion, which indicates the amount of inorganic material relative to that of organic material. Conversion is expressed as the number of equivalents of base per equivalent of acid times 100 [7]. The soap content is the amount of neutral salt as a percent of detergent composition. The TBN of the detergent reflects its ability to neutralize acids. For basic sulfonate and phosphonate detergents, only the overbased portion of the detergent, that is, the carbonate and the hydroxide (see Figure 4.2), possesses this capability. The neutral metal sulfonates and phosphonates, that is, soaps, lack this ability. However, for basic carboxylates, salicylates, and phenates, soaps also possess the acid-neutralizing ability. This is because, unlike sulfonates and phosphonates that are strong acid–strong base salts, metal carboxylates, metal salicylates, and metal phenates are strong base–weak acid salts. This makes them Lewis bases, hence the acid-neutralizing ability. Let us try to calculate the detergent parameters for a detergent of a hypothetical molecular formula (RSO3)vCaw(CO3)x(OH)y.* In this formula, v, w, x, and y denote the number of sulfonate groups, the number of calcium atoms, the number of carbonate groups, and the number of hydroxyl groups, respectively. The metal ratio, the total equivalents of metal per equivalent of acid, for such a detergent equals to 2w/v. The coefficient 2 signifies the divalent nature of calcium. For metals such as sodium and potassium, which are monovalent, the ratio equals to w/v. The degree of overbasing or conversion, which is metal ratio times 100, is (w × 100)/v for monovalent metals and (2w × 100)/v for divalent metals. Neutral detergents, or soaps, have a conversion of 100 because the ratio of equivalents of base to the equivalents of acid is 1. Soap content for such a detergent can be calculated using the following equation: Percent soap ⫽

formula weight [(RSO3 )2 Ca] ⫻ 100 effective formula weight

(4.1)

The effective formula weight is the weight of all the atoms that make up the formula (RSO3)v Caw(CO3)x(OH)y plus the diluent, if present. The diluent can be the incidental alkylate that does not get sulfonated or the diluent oil that is intentionally added. If one must add oil, most of it is added to reactants at the beginning of the reaction, especially during the manufacture of basic detergents. The presence of diluent is believed to facilitate micelle formation, thereby making the process more efficient. Adding oil after the reaction is not as effective. The TBN indicates a detergent’s ability to neutralize acids. In additives and formulated lubricants, the TBN is expressed as mg KOH/g of additive [27]. The method to determine base numbers is described in the ASTM Standard D 974 [28]. For sulfonate and phosphonate detergents, it can be calculated by using the number of equivalents of excess metal after salting the acid, that is (2w – v), according to Equation 4.2. TBN (mg KOH/g) ⫽

(2w ⫺ v) ⫻ 56 ,100 effective formula weight

* The correct formula for such a detergent is (RSO3)Ca.xCaCO3.yCa(OH)2.

(4.2)

Detergents

127

To calculate the base number of monovalent metal-derived sulfonates, one must use only (w – v) in Equation 4.2. For divalent metal-derived carboxylate, salicylate, and phenate detergents, Equation 4.3 is to be used. TBN (mg KOH/g) ⫽

(2w) ⫻ 56 ,100 effective formula weight

(4.3)

For monovalent metal salt of this type, the numerator will be w × 56,100. As mentioned earlier, the percent sulfated ash is the quantity of solid metal sulfate that results when the detergent is treated with sulfuric acid and the mixture ignited. Theoretical sulfated ash for divalent and monovalent metals can be calculated using the following equations. Equation 4.4 is for divalent metals, and Equation 4.5 is for monovalent metals. Percent sulfated ash ⫽

w ⫻ molecular weight of M 2SO 4 ⫻ 100 atomic weight of metal M ⫻ effective formula weight

(4.4)

Percent sulfated ash ⫽

0.5w ⫻ molecular weight of M2SO 4 ⫻ 100 atomic weight of metal M ⫻ effective formula weight

(4.5)

4.4 DETERGENT SUBSTRATES Various organic acids are used to synthesize detergents. These include alkylaromatic sulfonic acids, alkylphenols, alkylsalicylic acids, fatty carboxylic acids, and alkenylphosphonic acids. Alkylaromatic sulfonic acids, such as alkylbenzenesulfonic acids and alkylnaphthalenesulfonic acids, are made by reacting the respective alkylbenzenes and alkylnaphthalenes with a sulfonating agent. The alkylaromatic starting materials are made through alkylation of aromatics—benzene and naphthalene [29–33]. To make synthetic sulfonates, benzene or naphthalene is first alkylated and then sulfonated. The alkylating agent is either an alkyl halide or an olefin. The olefins can be α-olefins, internal olefins, or olefin oligomers such as polypropylene and polyisobutylene. An acid catalyst is usually required. One may choose from many acids. These include mineral acids such as sulfuric acid and phosphoric acid, Lewis acids such as aluminum chloride and boron trifluoride, organic acids such as methanesulfonic acid, and mixtures thereof [32,33]. Some of the inorganic acids such as sulfuric acid are also available on a solid support, such as Fuller’s earth or silica. Unlike other catalysts that require neutralization at the end of the reaction, these catalysts just require filtration to remove them. Zeolites and related mixed-metal oxides also enjoy the same advantage as the solid alkylation catalysts [30,31]. Another class of catalysts, exemplified by Amberlysts®, is aromatic polymer-derived sulfonic acids [31,34]. Although they have the advantages of being insoluble, hence easier to remove, and of multiple use, they have the disadvantage of being expensive. It is important to note that not all catalysts have equal effectiveness in all alkylations. Alkylaromatic sulfonic acids are derived either from the sulfonation of alkylaromatics, such as alkylbenzenes and alkylnaphthalenes, or from petroleum refining. The alkylbenzenes and alkylnaphthalenes are converted into respective sulfonic acids by reacting them with a sulfonating agent. The acids thus obtained are called synthetic sulfonic acids. Alkylbenzenesulfonic acids are also available from petroleum refining. These are referred to as natural sulfonic acids. Detergents made from synthetic sulfonic acids are called synthetic sulfonates and those made from natural sulfonic acids are called natural or petroleum sulfonates. The steps involved in making synthetic sulfonic acids are shown in Figure 4.3. The degree of branching in alkylbenzenes and alkylnaphthalenes, commonly called the alkylate, increases as we go from α-olefins to internal olefins to olefin oligomers. More branching of the alkylate implies

128

Lubricant Additives: Chemistry and Applications R + Olefin or alkyl halide

Acid

Benzene

Alkylbenzene

Acid

+ Olefin

R Naphthalene

R Alkylnaphthalene

SO3H

+ SO3 or oleum R

Alkylbenzene

Alkylbenzenesulfonic acid

FIGURE 4.3 Synthesis of sulfonic acid substrates. R R

Monoalkyl

Dialkyl

Trialkyl

Sulfonatable

Not sulfonatable if R group is highly branched

Not sulfonatable

Alkylbenzene constituents

FIGURE 4.4

Alkylbenzene structures.

somewhat less-efficient sulfonation but better oil solubility of the final sulfonate detergent. The common reagents used to sulfonate alkylaromatics are sulfur trioxide, fuming sulfuric acid or oleum, and chlorosulfonic acid [35]. Oleum is 15–30% sulfur trioxide dissolved in concentrated sulfuric acid. In general, the alkylate is dissolved in a hydrocarbon solvent, such as hexane or heptane, and reacted with the sulfonating reagent. One obtains a mixture of a monosulfonic acid and disulfonic or higher sulfonic acids. The latter must be removed because of the high polarity of their metal salts, hence potentially lower oil solubility. This can be easily achieved by water washing. The disulfonic and higher sulfonic acids are also undesired because, when reacted with polyvalent metals, they have the tendency to make polymeric salts that are usually of low lubricant solubility as well. Not all components of the alkylate are sulfonatable. In the case of alkylbenzenes, the species that do not sulfonate easily include polyalkylated benzenes, such as trialkylbenzene, or highly branched dialkylbenzenes. Their sulfonation difficulty is primarily a consequence of the steric crowding of the sulfonatable positions. Monoalkylbenzenes do not suffer from this drawback and hence sulfonate easily. In general, the sulfonation of branched alkylbenzenes is slower than linear alkylbenzenes. In the case of alkylbenzenes, Figure 4.4 shows structures that are sulfonatable and those that are not.

Detergents

129

With naphthalene, however, steric factors are not as important because of its bicyclic nature. The alkyl groups are likely to be attached to different aryl rings, except for very highly alkylated naphthalenes. Commercial NA-SUL® products are based on alkylnaphthalene chemistry. During petroleum refining, crude mineral oil is washed with a sulfonating agent such as sulfur trioxide or oleum [36]. Crude mineral oil contains reactive unsaturated compounds containing multiple bonds and alkylaromatics. These react with sulfur trioxide to form sulfonic acids. This is a desirable step because oils containing unsaturates and aromatics have a greater susceptibility toward oxidative breakdown, which could lead to the formation of increased deposits. If this occurs, it is likely to lead to equipment malfunction [5,23,37–42]. An analogous process is used to manufacture medicinal-quality white oil from petroleum. In the subsequent reaction, the sulfonic acid fraction is reacted with sodium hydroxide to convert the acids into sodium salts. These salts are washed with water to extract green acid soaps, which are used in many consumer products. The residual waterinsoluble material is then extracted with alcohol. This results in the isolation of mahogany acid soaps, which are useful in making detergent additives. The process is summarized in Figure 4.5. Alkylphenols are made in a manner analogous to alkylbenzenes, that is, by alkylating phenol with an olefin in the presence of an acid catalyst. The preferred catalysts are sulfuric acid, aluminum chloride, and boron trifluoride [39–42]. The alkylphenols can be either converted directly into their neutral or basic salts or further reacted with sulfur or sulfur dichloride to form sulfur-bridged alkylphenols and with formaldehyde to form methylene-bridged alkylphenols. This is shown in Figure 4.6. Alkylsalicylic acids are prepared from alkylphenols by reacting the alkali metal, especially potassium, phenates with carbon dioxide. The reaction is known as the Kolbe–Schmitt reaction [43]. Like the natural sulfonate process, this process yields alkali metal salts. These must be either neutralized with a mineral acid to free acids to use them to make detergents or reacted directly with a metal halide, such as calcium chloride or magnesium chloride, to make the calcium or magnesium soaps [44]. Alkenylphosphonic and alkenylthiophosphonic acid detergents are only rarely used. The acids are prepared by reacting polyisobutylene of varying molecular weights with phosphorus pentasulfide and the subsequent hydrolysis of the resulting adduct [45,46]. The adduct is believed to result from an ene-type addition of phosphorus pentasulfide to polyolefin. This type of addition does not result in the loss of the double bond, but it shifts the double bond down the carbon chain. Unless steric factors hinder the reaction, at least theoretically, the ene product can react with another molecule of phosphorus pentasulfide. This process can extend further. The adduct is hydrolyzed by the use of steam. One obtains a complex mixture of acids that include fully hydrolyzed (sulfur-free) Mineral oil + SO3

Sulfonated unsaturated + NaOH and aromatic compounds

Water extraction

Green acid soaps

Mahogany acid soaps

Alcohol extraction

Natural detergent substrates Largely paraffinic mineral oil

FIGURE 4.5 Isolation of natural sodium sulfonates.

130

Lubricant Additives: Chemistry and Applications OH

OH +

Acid

Olefin

R Alkylphenol

Phenol OH

R OH

Methylene-coupled alkylphenol

CH2O

R = alkyl group, typically C6−C12 X = 1−3 R Alkylphenol

S or SCI2 OH

OH (S)x

Sulfur-coupled alkylphenol or phenol sulfide

FIGURE 4.6 Synthesis of alkylphenol substrates.

alkenylphosphonic acids and partially hydrolyzed (contain residual sulfur) alkenylthiophosphonic acids. The reactions to synthesize alkylsalicylic acids and alkenylphosphonic and thiophosphonic acids are shown in Figure 4.7. Recently, the development of basic detergents that are not derived from organic acids has been reported [47,48]. Substrates that can be overbased include organic amines and polyamines, ethers, and organic sulfides (sulfurized olefins). Only alkali metal–derived overbased materials have been reported.

4.5 SYNTHESIS OF NEUTRAL AND BASIC DETERGENTS To make detergents, the organic acids are reacted with a metal base, such as a metal oxide or a metal hydroxide. In general, the reaction between the organic acid and the inorganic base is not good because of poor contact between the two reactants. A number of compounds, called promoters, are used to facilitate salt formation and the subsequent carbonation or a related reaction. Common promoters include ammonium hydroxide; low-molecular-weight carboxylic acids, such as formic acid and acetic acid; low-molecular-weight alkylphenols; and other polar compounds, such as nitroalkanes and imidazolines. A comprehensive list of such agents is provided elsewhere [49]. Most of these reagents are used in combination with water, except for high-temperature overbasing reactions where water will not stay in. In such cases, alcohols, such as 2-ethylhexanol or iso-octyl alcohol, and alkylphenols that have high boiling points are used. When water is present as part of the promoter system, it either is added or results from the neutralization reaction. The promoters are surfactants, that is, they contain a hydrophilic moiety, such as a hydroxyl group or a carboxylic acid functionality and a reasonably sized alkyl group to impart a somewhat hydrophobic character to the molecule.

Detergents

131 OH

OH COOK

COOH HCI

KOH CO2 R Alkylphenol R

R Potassium alkylsalicylate

KCI

R Alkylsalicylic acid R

R + P2S5 Phosphorus pentasulfide R

P2S5

R R

Polyisobutylene  P2S5 adduct

∆

H2O

R R

S P

OH OH

R R R1 Polyisobutenylphosphonic acid

FIGURE 4.7

Synthesis of alkylsalicylic acids and alkenylphosphonic acids.

Not all promoters are effective for all overbasing reactions, and one has to experiment to select the right promoter system. For low-temperature overbasing (≤100°C), alcohol–water mixtures are commonly used; for high-temperature overbasing (≥100°C), low-molecular-weight alkylphenols are used. The structure of the final detergent from the two processes is believed to be different and hence the performance in certain tests. The role of promoters in the overbasing reaction is not well understood. One explanation regarding their role is based on their preferential reaction with the base to form an alkoxide or a phenoxide. This species then transfers the metal to the substrate, thereby facilitating salt formation and overbasing. The other explanation is based on their acting as a surfactant and a wetting agent. This improves contact between the base and the substrate, thereby assisting the reaction to occur. The second explanation is definitely more plausible than the first. However, in high-temperature overbasing reactions, usually carried out under anhydrous conditions, the first explanation may have merit. Although a number of metals can be used to make neutral salts (soaps), only a fewer metals have the ability to result in oil-soluble basic detergents. The common metals that can be used for this purpose include lithium, sodium, and potassium in group I and magnesium, calcium, strontium, and barium in group II of the periodic table. Aluminum is the only overbasable metal in group III. Overbased salts of transition metals such as zinc, copper, cadmium, molybdenum, copper, manganese, cobalt, nickel, and iron, from sulfonic acids, alkylphenols, and naphthenic acids are also reported in the patent literature [19,50,51]. The ability to overbase relates to a metal’s base strength: the higher the basic character, the easier it is to overbase. For group I metals, where basic character increases from lithium to sodium to potassium, it is easier to overbase potassium than lithium. In group II metals, the basic character increases from magnesium to calcium to strontium to barium; hence, it is easier to overbase barium than magnesium.

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Lubricant Additives: Chemistry and Applications

Various detergents derived from metal anions other than hydroxide and carbonate are reported in the patent literature. The anions include sulfites, sulfates, thiosulfates, borates, and phosphates [32,49,52–55]. These detergents are obtained either from the carbonate detergent by displacing the carbonate anion with the alternative anion or by using the anion precursor during overbasing. For example, one can obtain metal sulfite overbased detergent either by blowing sulfur dioxide during overbasing or by displacing carbon dioxide in a carbonate detergent with sulfur dioxide. The resulting metal sulfite detergent can be oxidized to a sulfate detergent by using an oxygen source, such as oxygen gas or peroxide, or to a thiosulfate detergent by reacting it with elemental sulfur [49,52,53]. Borate and phosphate overbased compositions can be made using boric acid or phosphoric acid during the reaction [54,55]. Common commercial detergents are derived from calcium, magnesium, sodium, and barium. The metals are listed in order of preference. As mentioned, neutral detergents are made by reacting the acid substrate with a stoichiometric amount of the metal base, and overbased detergents are made by reacting the substrate with an excess amount of base in the presence of carbon dioxide. To make calcium and magnesium salts from natural sulfonic acids and alkylsalicylic acids, one must convert commercially available alkali metal (sodium and potassium) salts (see Figures 4.5 and 4.7) into free acids by reacting them with a mineral acid and then reacting the acids with magnesium oxide or calcium hydroxide. Alternatively, alkali metal salts can be converted directly into magnesium and calcium salts through a double-decomposition reaction with a metal halide, as shown in Figure 4.8. To make the natural sulfonate detergent, one must react the mahogany acid soap with a metal halide such as calcium chloride. The reaction converts the sodium sulfonate soap into calcium sulfonate, which can be overbased if desired. Because of the extensive branching, petroleumderived sulfonates have better oil solubility than synthetic sulfonates of similar molecular weight. Figure 4.9 presents the idealized structures of neutral detergents. To make overbased detergents, one can use either a two-step process or a one-step process. Generally, the one-step process is preferred over the two-step process. In the two-step process, the neutral salt or the soap is made first, which is subsequently overbased. In the one-step process, the excess metal base is charged to the reaction; once the neutral salt formation is complete, carbon dioxide blowing (carbonation) of the reaction is initiated. When carbon dioxide uptake stops, the reaction is considered complete and it is worked up to isolate the product. Figure 4.10 summarizes the two processes. For making overbased natural sulfonates and alkylsalicylates, one can double-decompose alkali metal salts in situ by reacting with a metal halide and overbasing. The alkali metal halide by-product need not be removed until the overbasing is complete. It comes out during the final filtration, which is employed to remove any unreacted excess base and other particulate materials.

SO3Na

SO3 + CaCl2

2 R

Ca + 2NaCl R

OH CO2K

CO2 + CaCl2

FIGURE 4.8 Double-decomposition reaction.

Ca + R

2KCl

Detergents

133 SO3)x M

SO3)x M

O)x M

Metal salt of alkylbenzenesulfonic acid Metal salt of alkylnapthalenesulfonic acid

Metal salt of alkylphenol

OH M or

O)x M

R O

X = 1 or 2 Y = S or CH2 M = Na, Mg, Ca

Metal salt of alkylsalicylic acid M M

M O

Y or

R R R Sulfur and methylene-bridged phenates

O R

O M

Metal phosphonate

Metal thiophosphonate

Metal thiopyrophosphonate

FIGURE 4.9 Idealized structures of neutral salts (soaps). Two-step process: Neutral salt

Substrate + metal oxide or hydroxide Mx O

M(OH)y

Base

Base Stoichiometric amount

CO2 or no CO2

Basic or overbased salt

One-step process: Substrate + metal oxide or hydroxide MxO

M(OH)y

CO2

Basic or overbased salt

or no CO2

Excess base Basic salts = neutral salts · Mx O, M(OH)y, or Mx CO3 M = Na, Mg, Ca, or Ba x = 1 and y = 2 for Mg, Ca, and Ba (divalent metals) x = 2 and y = 1 for Na (monovalent metal)

FIGURE 4.10

Processes to make basic detergents.

134

Lubricant Additives: Chemistry and Applications

As mentioned, common metals that can be used to make neutral or basic detergents include sodium, potassium, magnesium, calcium, and barium. Calcium and magnesium find most extensive use as lubricant additives, with a preference for calcium due to its lower cost. The use of bariumderived detergents is being curbed due to concerns for barium’s toxicity. Technically, one can use metal oxides, hydroxides, and carbonates to manufacture neutral (nonoverbased) detergents; for nonoverbased detergents, oxides and hydroxides are the preferred bases. Sodium hydroxide, calcium hydroxide, and barium hydroxide are often used for sodium, calcium, and barium detergents. For magnesium detergents, however, magnesium oxide is the preferred base. During the synthesis of calcium detergents, overbasing is usually stopped before all the metal base is converted into calcium carbonate. As a result, the excess base is present as a mixture of calcium hydroxide and calcium carbonate. The calcium carbonate predominates because, if the reaction is overblown with carbon dioxide, the amorphous calcium carbonate, which is desired, is converted into crystalline calcium carbonate. Of low solubility in the overbased system, crystalline calcium carbonate falls out of solution, and one obtains an oil-insoluble gel-like product. Although such products are of little use as lubricant additives, they are useful as rheology control agents in coatings. The challenge is to make them on a consistent basis. Lubrizol supplies such products derived from alkylbenzenesulfonic acids as its Ircogel® product line. Gelled carboxylates and solid calcium micellar complexes have also been reported in the patent literature [56–58]. Basic detergents contain reserve base, which is entrained into the detergent in a colloidal form. The base, such as the carbonate, is believed to be encapsulated by soap molecules. In this arrangement, the polar head group (sulfonate, phenate, or carboxylate) of the soap associates with the carbonate, and the hydrocarbon portion of the soap associates with the oil (see Figure 4.11). The base neutralizes acids that result from oxidation of the fuel and the lubricant and from the oxidation and thermal decomposition of thermally labile additives. Some detergents are marketed as neutral or nonoverbased. However, most of them have a small amount of reserve base present. In other words, they are overbased to some degree. This implies that no effort was made to overbase them, and their reserve base is due to the presence of the unreacted base used to make them. For example, commercially available neutral sulfonates have a TBN of 30 or less, and the base is commonly present as a hydroxide such as calcium hydroxide. Conversely, basic or overbased detergents have a much higher base number, that is, they typically have a TBN of 200–500, and the base is commonly present as a metal carbonate. Calcium-based phenate detergents are easier to make than magnesium-based detergents because alkylphenols are weak acids, and their reaction with magnesium oxide, a weak base, is not facile. To make the neutral salt, one must react the alkylphenol with a strong base such as

O S O O O

CaCO3

S O O Sulfonate head group Hydrocarbon group Neutral calcium sulfonate

FIGURE 4.11

Micelle structure of detergents.

Basic sulfonate inverse micelle structure; size: 100 −150Å

Detergents

135

magnesium alkoxide. This reagent can be prepared by reacting magnesium metal with an excess of highly reactive alcohol such as methanol. However, this method is hazardous because of the hydrogen gas by-product and costly because of the price of the magnesium metal. Once the neutral salt or soap formation occurs, the excess alcohol is exchanged for an inert solvent, such as toluene or mineral oil, before overbasing. Alternatively, one can use a high-temperature overbasing procedure using a low-molecular-weight alkylphenol as a promoter [59]. In the case of methylene or sulfur-bridged phenols that are more acidic than regular alkylphenols, the reactivity toward magnesium oxide is not a problem. And these compounds form neutral and overbased magnesium salts without difficulty. Neutral and basic calcium phenates from alkylphenols, bridged or unbridged, are easy to make because calcium hydroxide, being a stronger base, reacts with them readily. Other acids, that is, alkylsalicylic acids, fatty carboxylic acids, and alkenylphosphonic acids, react with calcium and magnesium bases without any problem. The synthetic sequences used to make common types of neutral and carbonate overbased detergents are outlined in Figures 4.12 through 4.17. Sulfonate, salicylate, and carboxylate detergents are commercially available as calcium and magnesium salts, and phosphonates are available as calcium salts. Some specialty sulfonates, for example, NA-SUL BSB®, are also available as barium salts. Phenate detergents are commonly available as calcium salts, and phosphonate detergents are available as both calcium and barium salts. Basic calcium sulfonates make up ~65% of the total detergent market, followed by phenates at ~31%.

4.6

TESTING

Detergents are used in engine lubricant formulations to perform two key functions. One is to neutralize the acidic by-products of lubricant oxidation and thermal decomposition, and the other is to suspend neutral but highly polar-oxygenated species in the bulk lubricant.

SO3H Base (stoichiometric amount) R Alkylbenzenesulfonic acid O

S O M O S O

R SO3Na

Divalent metal salt of alkylbenzenesulfonic acid (soap)

Metal halide

R Excess base

Natural sodium sulfonate

CO2

x MCO3

O · S O M O S

O O

R Overbased or basic sulfonate M = Ca, Mg, Ba

FIGURE 4.12

Synthesis of neutral and basic metal sulfonates.

136

Lubricant Additives: Chemistry and Applications OH O

Base Stoichiometric amount

R Divalent metal salt of alkylphenol (soap)

Alkylphenol

Excess base

CO2

x MCO3

R Overbased or basic phenate M = Ca, Mg

FIGURE 4.13

Synthesis of neutral and basic metal phenates.

M OH

OH Y

R Sulfur- or methylene-bridged alkylphenol

O Y

Base Stoichiometric amount

R R Divalent metal salt of sulfur- or methylene-bridged alkylphenol (soap)

Excess base Y = S, CH2 M = Ca, Mg

CO2

x MCO3

M O

O Y

R R Overbased or basic bridged phenate

FIGURE 4.14 Synthesis of neutral and basic bridged metal phenates.

All components of a lubricant—base oil, additives, and viscosity modifier—oxidize because of their organic nature. Oxidation of the API group I base oils is more facile than that of the group II and group III base oils, primarily because of the presence of aromatic and sometimes olefinic components. These compounds oxidize to form hydroperoxides and radicals [6]. These species are highly reactive and start the oxidation chain reaction. The result is the oxidative breakdown of all components of the lubricant to highly oxygenated polar species, which are of low lubricant solubility. Because of this, these materials, both acidic and neutral, tend to separate on surfaces, thereby

Detergents

137 OH

OH COOK

COOH

HCI

Potassium alkylsalicylate

Alkylsalicylic acid Base Stoichiometric amount

Metal halide

O M O

CO2

O M O

Excess base

Basic or overbased metal salicylate

Neutral metal salicylate (soap)

M = Ca or Mg

FIGURE 4.15

n MCO3

KCI

Synthesis of neutral and basic metal salicylates.

R R

S P

OH OH

Base Stoichiometric amount

O M O P HO OH

R R

R1 Polyisobutenylphosphonic acid

R R

R R1

S P

O M O

R R1

R = H or alkyl M = Ca, Ba

X = O or S

Divalent metal salt of polyisobutenylphosphonic acid (soap)

Excess base CO2

O M O P HO OH

R R1

x MCO3

R R R1

Overbased or basic alkenylphosphonate

FIGURE 4.16

Synthesis of neutral and basic phosphonates.

138

Lubricant Additives: Chemistry and Applications O C

Oleic acid Base

Stoichiometric amount O C

Divalent metal oleate (soap)

Excess base

M 2

CO2 O C

M · X MCO3

O 2

Overbased or basic carboxylate M = Ca, Mg

FIGURE 4.17

Synthesis of neutral and basic fatty carboxylates.

impairing the proper functioning of the various equipment parts. The acidic materials such as sulfur oxides, and organic acids resulting from lubricant oxidation can attack metal surfaces and cause corrosion. Sulfur oxides and sulfuric acid result from the combustion of fuel sulfur or the oxidation of sulfur-containing additives, such as sulfurized olefins or zinc dialkyldithiophosphates. The function of the detergent in this case is to neutralize these acids, thus causing corrosion. The reserve base in detergents primarily performs this function. The soap portion keeps oil-insoluble polar products suspended in oil. Phenates, sulfurized phenates, and salicylates can also act as oxidation inhibitors because of the presence of the phenol functionality. Phenols are well known for their oxidation-inhibiting action [5]. Detergents are also effective corrosion inhibitors, especially basic detergents [25,26], because they not only neutralize corrosive acidic products but also form surface films that isolate metal surfaces from corrosive agents [6]. The carbonate portion in the detergent performs acid neutralization, and the soap portion forms the protective surface film. NA-SUL 729® and NA-SUL CA-50® are examples of commercial corrosion inhibitors belonging to this class of additives. The tests that evaluate rust and corrosion are described elsewhere [3,6]. Detergents derived from fatty carboxylic acids are good friction modifiers, primarily because of the linear structure of their soaps [6]. Detergents find primary use in engine oils, which are responsible for more than 75% of the total consumption. The detergent level in marine diesel engine oils is the highest because marine engines use high-sulfur fuel, which leads to highly acidic combustion products such as sulfuric acid. The lubricants for these engines therefore require the base reserve of highly overbased detergents. A variety of proprietary and industry-established tests are used to determine a detergent’s effectiveness in lubricants. For gasoline engine oils to be used in North America, these include the CRC L-38, TEOST (Thermo-Oxidation Engine Oil Simulation Test [60]), ASTM Sequence IIIE/IIIF, and ASTM Sequence VE/VG tests. For European gasoline engines, in addition to performance in the ASTM Sequence IIIE/IIIF and VE/VG Engine tests, performance in Peugeot TU3M HighTemperature Test and MB M111 Black Sludge Test is also required. These tests are part of the ACEA 2002 Standard. The standard also includes a sulfated ash limit that directly affects the amount of detergent used in formulations since it is the primary contributor to sulfated ash. The efficacy of diesel engine oils for North American use is evaluated by the use of both the single-cylinder and the multicylinder engine tests. Single-cylinder tests include CRC L-38 and

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139

Caterpillar 1K, 1M-PC, 1N, 1P, and IR engine tests. CRC L-38’s viscosity requirement, an imprecise measure of a detergent’s effectiveness, is a part of the API CG-4 standard. Caterpillar 1K test is a part of the API CF-4, API CH-4, and API CI-4 standards. Caterpillar 1M-PC is a part of the API CF, CF-2, and the U.S. Military’s MIL-PRF-2104G specifications. Caterpillar 1N is a part of the API CG-4, API CI-4, and the U.S. Military’s MIL-PRF-2104G specifications. Caterpillar 1P is a part of the API CH-4 specification, and caterpillar 1R is a part of the API CI-4 specification. The pass ratings for these tests require meeting the overall appearance of the cylinder and its parts, which are expressed in terms of weighted demerits, percent top groove fill, ring side clearance loss, top land heavy carbon, oil consumption, and stuck rings. All these parameters are related to deposits on the piston and its parts. The multicylinder tests that determine the effectiveness of a detergent include Detroit Diesel 6V92TA engine test (a part of the API CF-2 and MIL-PRF-2104G specifications) and Mack M11 engine test (a part of the API CH-4 and CI-4 specifications). These two tests evaluate a detergent’s ability to prevent deposit-related port plugging and engine sludge, respectively. For European use, oils must pass the requirements of tests such as VW1.6L TC diesel, XUD11BTE, VWDI, MB OM 364 LA, and MB OM 441 LA. These tests evaluate ring sticking, piston cleanliness, viscosity increase, and filter plugging. It is important to note that technically most of these tests evaluate the combined effectiveness of the detergent and the dispersant. The performance of the individual additive is hard to unravel. Detergents find additional use in automatic transmission fluids and tractor hydraulic fluids. In this case, the primary function of these additives is not to neutralize acids or to minimize deposit formation, but to alter the frictional properties of these fluids. This is critical if the fluids are to perform effectively as driveline lubricants.

REFERENCES 1. Sieloff, F.X., J.L. Musser. What does the engine designer need to know about engine oils? Presented at the Detroit Section of the Society of Automotive Engineers, March 16, 1982. 2. Schilling, A. Motor Oils and Engine Lubrication. London: Scientific Publications, 1968. 3. Rizvi, S.Q.A. Additives: chemistry and testing. In E.R. Boozer, ed Tribology Data Handbook—An Excellent Friction, Lubrication, and Wear Resource. Boca Raton, FL: CRC Press, 1997, pp. 117–137. 4. Rizvi, S.Q.A. Lubricant additives and their functions. In S.D. Henry, ed. Metals Handbook, 10th Ed. 1992, pp. 98–112. 5. Ingold, K.U. Inhibition of autoxidation of organic substances in liquid phase. Chemical Reviews 61:563– 589, 1961. 6. Rizvi, S.Q.A. Additives and additive chemistry. ASTM Manual on Fuels and Lubricants. 7. Gergel, W.C. Lubricant additive chemistry. International Symposium on Technical Organic Additives and Environment, Interlaken, Switzerland, May 24–25, 1984. 8. Rolfes, A.J., S.E. Jaynes. Process for making overbased calcium sulfonate detergents using calcium oxide and a less than stoichiometric amount of water. U.S. Patent 6,015,778, 1/18/2000. 9. Moulin, D., J.A. Cleverley, C.H. Bovington. Magnesium low rate number sulphonates. U.S. Patent 5,922,655, 7/13/99. 10. Sabol, A.R. Method of preparing overbased barium sulfonates. U.S. Patent 3,959,164, 5/25/76. 11. Hunt, M.W. Overbased alkali metal sulfonates. U.S. Patent 4,867,891, 9/19/89. 12. Robson, R., B. Swinney, R.D. Tack. Metal phenates. U.S. Patent 4,221,673, 9/9/80. 13. Brannen, C.G., M.W. Hunt. Preparation of overbased magnesium phenates. U.S. Patent 4,435,301, 3/6/84. 14. Jao, T.C., C.K. Esche, E.D. Black, R.H. Jenkins Jr., Process for the preparation of sulfurized overbased phenate detergents. U.S. Patent 4,973,411, 11/27/90. 15. Liston, T.V. Methods for preparing group II metal overbased sulfurized alkylphenols. U.S. Patent 4,971,710, 11/20/90. 16. Burnop, V.C.E. Production of overbased metal phenates. U.S. Patent 4,104,180, 8/1/78. 17. Shiga, M., K. Hirano, M. Matsush*ta. Method of preparing overbased lubricating oil additives. U.S. Patent 4,057,504, 11/8/77. 18. Stuart, F.A., W.A. Tyson Jr., Recovery of overbased alkaline earth metal additives from centrifugates. U.S. Patent 4,910,334, 3/20/90.

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19. Ali, W.R. Process for preparing overbased naphthenic micronutrient compositions. U.S. Patent 4,243,676, 1/6/81. 20. Slama, F.J. Process for overbased petroleum oxidate. U.S. Patent 5,013,463, 5/7/91. 21. Van Kruchten, M.G.A., R.R. Van Well. Prepartion of basic salt. U.S. Patent 4,810,398, 3/7/89. 22. Weamer, G.L. Additives—A way to quality in motor oils. Petroleum Refiner 38:215–219, 1959. 23. Popkin, A.H. Metal salts of organic acids of phosphorus. U.S. Patent 2,785,128, 3/12/57. 24. Cease, V.J., G.R. Kirk. Prepartion of overbased magnesium sulfonates. U.S. Patent 4,148,740, 4/10/79. 25. King, L.E. Basic alkali metal sulfonate dispersions, process for their preparation, and lubricants containing same. U.S. Patent 5,037,565, 8/6/91. 26. Koch, P., A. Di Serio. Compounds useful as detergent additives for lubricants and lubricating compositions. U.S. Patent 5,021,174, 6/4/91. 27. ASTM Standards D 482 and D 874, Section 5, Petroleum Products, Lubricants, and Fossil Fuels. 1999 Annual Book of ASTM Standards. American Society of Testing and Materials. 28. ASTM Standard D 974, Section 5, Petroleum Products, Lubricants, and Fossil Fuels. 1993 Annual Book of ASTM Standards. American Society of Testing and Materials. 29. Price, C.C. The Alkylation of Aromatic Compounds by the Friedel–Crafts Method. Organic Reactions, Vol III. New York: Wiley, 1946, pp. 1–82. 30. Kovach, S.M. Alkylation of aromatics with olefins in the presence of an alumina catalyst. U.S. Patent 4,219,690, 8/26/80. 31. Knifton, J.F., P.R. Anantaneni, M. Stockton. Process and system for alkylation of aromatic compounds. U.S. Patent 5,770,782, 6/23/98. 32. Olah, G.A. Friedel–Crafts and Related Reactions. Vol II. New York: Interscience-Wiley Publishers, 1964. 33. Kirkland, E.V. Aromatic alkylation. U.S. Patent 2,754,341, 7/10/56. 34. Dwyer, F.G., Q.N. Lee. Aromatics alkylation process. U.S. Patent 5,191,135, 3/2/93. 35. Suter, C.M., A.W. Weston. Direct sulfonation of aromatic hydrocarbons and their halogen derivatives. Organic Reactions, Chapter 4, Vol III. New York: Wiley, 1946, pp. 141–197. 36. Bosniack, D.S., P.F. Korbach. Conversion of sulfonic acids into a hydrocarbon oil of superior oxidation stability. U.S. Patent 4,188,500, 5/12/80. (See “Background of Invention” portion of the patent.) 37. Kreuz, K.L. Gasoline engine chemistry as applied to lubricant problems. Lubrication 55:53–64, 1969. 38. Kreuz, K.L. Diesel engine chemistry as applied to lubricant problems. Lubrication 56:77–88, 1970. 39. Kirk–Othmer Encyclopedia of Chemical Technology, Vol. I. Alkylation of phenols. 1963, pp. 894–895. 40. Kirk–Othmer Encyclopedia of Chemical Technology, Vol. II. Ion exchange, New York: Interscience Publishers, 1967, pp. 871–899. 41. Merger, F., G. Nestler. Manufacture of alkylphenol compounds. U.S. Patent 4,202,199, 5/13/80. 42. Kolp, C.J. Methods for preparing alkylated hydroxyaromatics. U.S. Patent 5,663,457, 9/2/97. 43. March, J. Carboxylation with carbon dioxide, Kolbe–Schmitt reaction. In Advanced Organic Chemistry, Reactions, Mechanisms, and Structure, 4th Ed. New York: Wiley-Interscience Publication, 1992, Chapter 11, Section 1–20, pp. 546–547. 44. Van Kruchten, E.M.G.A., G.W.J. Heimerikx. Additives for lubricating oils and processes for producing them. U.S. Patent 5,089,158, 2/18/92. 45. Schallenberg, E.E., R.G. Lacoste. Ethylenediamine salts of thiophosphonic acids. U.S. Patent 3,185,728, 5/25/65. 46. Brois, S.J. Olefin-thionophosphine sulfide reaction products, their derivatives, and use thereof as oil and fuel additives. U.S. Patent 4,042,523, 8/16/77. 47. Huang, N.Z. Overbased products using non-ionic substrates—Chemistry and properties. Symposium on Recent Advances in the Chemistry of Lubricant Additives, Division of Petroleum Chemistry, National Meeting of the American Chemical Society, New Orleans, LA, August 22–26, 1999. 48. Huang, N.Z. Non-conventional overbased materials. U.S. Patent 5,556,569, 9/17/96. 49. Asseff, P.A., T.W. Mastin, A. Rhodes. Metal complexes and methods of making same. U.S. Patent 2,777,874, 1/15/57. 50. Hunt, M.W., S. Kennedy. Metal-containing lubricant compositions. U.S. Patent 4,767,551, 8/30/88. 51. Steckel, T.F. Process for overbasing via metal borate formation. U.S. Patent 6,090,757, 7/18/2000. 52. Cahoon, J.M., J.L. Karn, N.Z. Haung, J.P. Roski. Sulfurized overbased compositions. U.S. Patent 5,484,542, 1/16/96.

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53. Asseff, P.A., T.W. Mastin, A. Rhodes. Methods of preparation of superbased salts. U.S. Patent 2,695,910, 11/30/54. 54. Steckel, T.F. Process for overbasing via metal borate formation. U.S. Patent 5,064,545, 11/12/91. 55. Bleeker, J.J., M. Booth, M.G.F.M. van Grieken, W.J. Krijnen, G.D. van Wijngaarden. Borated basic metal salt and lubricating oil composition. U.S. Patent 4,539,126, 9/3/85. 56. Jaynes, S.E., W.R. Sweet. Overbased carboxylate gels, U.S. Patent 5,919,741, 7/6/99. 57. Vinci, J.N., W.R. Sweet. Mixed carboxylate overbased gels. U.S. Patent 5,401,424, 3/28/95. 58. McMillen, R.L. Calcium containing micellar complexes. U.S. Patent 3,766,067, 10/16/73. 59. Nichols, W.P., J.L. Karn. Magnesium overbasing process. U.S. Patent 5,173,203, 12/22/92. 60. Florkowski, D.W., T.W. Selby. The development of a thermo-oxidative engine oil simulation test (TEOST). SAE Technical Paper 932837. Fuels and Lubricants Meeting and Exposition, Philadelphia, PA, October 18–21, 1993.

Dispersants Syed Q. A. Rizvi

CONTENTS 5.1 5.2 5.3 5.4 5.5 5.6

Introduction ........................................................................................................................... 143 Nature of Deposits and Mode of Their Formation ............................................................... 144 Deposit Control by Dispersants ............................................................................................ 147 Desirable Dispersant Properties............................................................................................ 147 Dispersant Structure ............................................................................................................. 147 Dispersant Synthesis ............................................................................................................. 148 5.6.1 The Hydrocarbon Group ........................................................................................... 149 5.6.2 The Connecting Group .............................................................................................. 151 5.6.3 The Polar Moiety ....................................................................................................... 152 5.7 Dispersant Properties ............................................................................................................ 157 5.7.1 Dispersancy ............................................................................................................... 157 5.7.2 Thermal and Oxidative Stability ............................................................................... 160 5.7.3 Viscosity Characteristics ........................................................................................... 161 5.7.4 Seal Performance....................................................................................................... 162 5.8 Performance Testing ............................................................................................................. 163 References ...................................................................................................................................... 166

5.1 INTRODUCTION Lubricants are composed of a base fluid and additives. The base fluid can be mineral, synthetic, or biological in origin. In terms of use, petroleum-derived (mineral) base fluids top the list, followed by synthetic fluids. Base oils of biological origin, that is, vegetable and animal oils, have not gained much popularity except in environmentally compatible lubricants. This is because of the inherent drawbacks these base oils have pertaining to their oxidation stability and low-temperature properties. Additives are added to the base fluid either to enhance an already-existing property, such as viscosity, of a base oil or to impart a new property, such as detergency, lacking in the base oil. The lubricants are designed to perform a number of functions, including lubrication, cooling, protection against corrosion, and keeping the equipment components clean by suspending ordinarily insoluble contaminants in the bulk lubricant [1]. Although for automotive applications all functions are important, suspending the insoluble contaminants and keeping the surfaces clean are the most critical. As mentioned in Chapter 4 on “detergents,” this is achieved by the combined action of the detergents and the dispersants present in the lubricant. Dispersants differ from detergents in three significant ways: 1. Dispersants are metal-free, but detergents contain metals, such as magnesium, calcium, and sometimes barium [2]. This means that on combustion detergents will lead to ash formation and dispersants will not. 2. Dispersants have little or no acid-neutralizing ability, but detergents do. This is because dispersants have either no basicity, as is the case in ester dispersants, or low basicity, as 143

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is the case in imide/amide dispersants. The basicity of the imide/amide dispersants is due to the presence of the amine functionality. Amines are weak bases and therefore possess minimal acid-neutralizing ability. Conversely, detergents, especially basic detergents, contain reserve metal bases as metal hydroxides and metal carbonates. These are strong bases, with the ability to neutralize combustion and oxidation-derived inorganic acids, such as sulfuric and nitric acids, and oxidation-derived organic acids. 3. Dispersants are much higher in molecular weight, approximately 4–15 times higher, than the organic portion (soap) of the detergent. Because of this, dispersants are more effective in fulfilling the suspending and cleaning functions than detergents. As mentioned in Chapter 4, dispersants, detergents, and oxidation inhibitors make up the general class of additives called stabilizers and deposit control agents. The goal of oxidation inhibitors is to minimize the formation of deposit precursors, such as hydroperoxides and radicals [3,4]. This is because these species are reactive, and they attack the hydrocarbon base oil and additives, which make up the lubricant, to form sludge, resin, varnish, and hard deposits. The goal of the dispersant and the soap portion of the detergent is to keep these entities suspended in the bulk lubricant. This not only results in deposit control but also minimizes particulate-related abrasive wear and viscosity increase. When the lubricant in the equipment is changed, the deposit precursors and the depositforming species are removed with the used oil. The dispersants suspend deposit precursors in oil in various ways. These comprise the following: • Including the undesirable polar species into micelles. • Associating with colloidal particles, thereby preventing them from agglomerating and falling out of solution. • Suspending aggregates in the bulk lubricant, if they are formed. • Modifying soot particles so as to prevent their aggregation. The aggregation will lead to oil thickening, a typical problem in heavy-duty diesel engine oils [5,6]. • Lowering the surface/interfacial energy of the polar species to prevent their adherence to metal surfaces.

5.2 NATURE OF DEPOSITS AND MODE OF THEIR FORMATION A number of undesirable materials result from the oxidative degradation of various components of the lubricant. These are base oil, additives, and the polymeric viscosity modifier, if present. In engine oils, the starting point for the degradation is fuel combustion, which gives rise to hydroperoxides and free radicals [7]. The compounds in the fuel that are most likely to form peroxides, hydroperoxides, and radicals include highly branched aliphatics, unstaurates such as olefins, and aromatics such as alkylbenzenes. All these are present in both gasoline and diesel fuels. American Society for Testing and Materials (ASTM) test methods D 4420 and D 5186 are used to determine the aromatic content of gasoline and diesel fuels, respectively [8]. The fuel degradation products (peroxides, hydroperoxides, and radicals) go past the piston rings into the lubricant as blowby and, because they are highly energetic, attack largely the hydrocarbon lubricant. Again, the highly branched aliphatic, unsaturated, and aromatic structures are among those that are highly susceptible. ASTM Standard D 5292 is commonly used to determine the aromatic content of the base oil [8]. The reaction between the contents of the blowby and these compounds results in the formation of the lubricant-derived peroxides and hydroperoxides that either oxidatively or thermally decompose to form aldehydes, ketones, and carboxylic acids [3,4,9]. Acids can also result from the high-temperature reaction of nitrogen and oxygen, both of which are present in the air–fuel mixture; the oxidation of the fuel sulfur; and the oxidation, hydrolysis, or thermal decomposition of additives such as zinc dialkyldithiophosphates. The reaction between nitrogen and oxygen to form NOx is more prevalent in diesel

Dispersants

145

engines and gasoline engines that are subjected to severe service, such as long-distance driving for extended periods. The NOx formation initiates when the temperature reaches 137°C [10,11]. Zinc dialkyldithiophosphates are commonly used as oxidation inhibitors in engine oils [12,13]. All these acids are neutralized by basic detergents to form inorganic metal salts and metal carboxylates. These compounds are of low hydrocarbon solubility and are likely to fall out of solution. The aldehydes and ketones undergo aldol-type condensation in the presence of bases or acids to form oligomeric or polymeric compounds. These can further oxidize to highly oxygenated hydrocarbons, commonly referred to as oxygenates. The oxygenates are usually of sticky consistency, and the term resin is often used to describe them [14]. Resin is either the basic component in or the precursor to all types of deposits. Common types of deposits include varnish, lacquer, carbon, and sludge [15,16]. Varnish, lacquer, and carbon occur when resin separates on hot surfaces and dehydrates or polymerizes to make tenacious films. The quantity and the nature of deposits depend on the proximity of the engine parts to the combustion chamber. The parts closer to the combustion chamber, such as exhaust valve head and stem that experience approximate temperatures of 630–730°C [17,18], will develop carbon deposits. The same is true of the combustion chamber wall, piston crown, top land, and top groove, which are exposed to approximate temperatures of 200–300°C. Carbon deposits are more common in diesel engines than in gasoline engines and result from the burning of the liquid lubricating oil and the high-boiling fractions of the fuel that adhere to hot surfaces [19]. As we move away from these regions to the low-temperature regions, such as the piston skirt, the deposits are not heavy and form only a thin film. For diesel engine pistons, this type of deposit is referred to as lacquer; for gasoline engine pistons, this type of deposit is called varnish. The difference between lacquer and varnish is that lacquer is lubricant-derived and varnish is largely fuel-derived. In addition, the two differ in their solubility characteristics. That is, lacquer is watersoluble and varnish is acetone-soluble [15]. Lacquer usually occurs on piston skirts, on cylinder walls, and in the combustion chamber, whereas varnish occurs on valve lifters, piston rings, piston skirts, valve covers, and positive crankcase ventilation (PCV) valves. The coolest parts of the engine, such as rocker arm covers, oil screen, and oil pan, that are exposed to temperatures of ≤200°C experience sludge deposits. Sludge can be watery or hard in consistency, depending on the severity of service. If the service is extremely mild and of short duration, as in the case of stop-and-go gasoline engine operation, the sludge is likely to be watery or mayonnaiselike [15]. This type of sludge is called low-temperature sludge, which occurs when the ambient temperature is <95°C. The high-temperature sludge is more common in diesel engines and gasoline engines with long, continuous operation. This type of sludge occurs when the ambient temperature is >120°C and is hard in consistency. In the former case, the engine does not get hot enough to expel combustion water, which stays mixed with oil, imparting sludge, a mayonnaiselike appearance. In the latter case, however, the ambient temperature is high enough to expel water, thereby resulting in hard sludge. Sludge is common in areas that experience low oil flow, such as crankcase bottoms and rocker boxes. Another component of the combustion effluent that must be considered is soot. Soot not only contributes toward some types of deposits such as carbon and sludge, but it also leads to a viscosity increase. These factors can cause poor lubricant circulation and lubricating film formation, both of which will result in wear and catastrophic failure. Soot is particulate in nature and results from the incomplete combustion of the fuel and of the lubricating oil from the crankcase that might enter the combustion chamber by traveling past the piston rings [20]. Fuel-derived soot is a chronic problem in the case of diesel engines because diesel fuel contains high-boiling components that do not burn easily. In addition, diesel engine combustion is largely heterogeneous, with poor air–fuel mixing, hence poor combustion [20]. Soot is made of hydrocarbon fragments with some of the hydrogen atoms removed. The particles are charged and hence have the tendency to form aggregates. When aggregates occur on surfaces, such as those of the combustion chamber, soot deposits result. These deposits are soft and flaky in texture. If these occur in oil, lubricant experiences an increase in viscosity. A soot-related viscosity increase usually requires the presence of polar materials in oil that

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have the ability to associate with soot. These can be additives or polar lubricant oxidation and degradation products. Carbon deposits are lower in carbon content than soot and, in most cases, contain oily material and ash. This makes knowledge of the ash-forming tendency of a lubricant important to a formulator. This concern was addressed in Chapter 4. When soot associates with resin, one gets either resin-coated soot particles or soot-coated resin particles [16]. The first type of particles results when resin is in excess, and the second type results when soot is in excess. The amount of soot in resin determines the color of the deposits: the higher the soot, the darker the deposits. Sludge results when resin, soot, oil, and water mix [9]. Deposit formation in gasoline engines is initiated by NOx and oxidation-derived hydroperoxides that react with hydrocarbons in the fuel and the lubricant to form organic nitrates and oxygenates [14,21]. Being thermally unstable, these species decompose and polymerize to form deposits. The deposits typically include resin, varnish, and low-temperature sludge. In diesel engines, however, soot is an important component of the deposits, which include lacquer, carbon deposits, and hightemperature sludge [16]. Typically, carbon deposits are of high metal content, which is mainly due to the presence of detergent additives in the lubricant [22,23]. Detailed mechanism of deposit formation in engines is described elsewhere [24,25]. The mechanism is based on the premise that both the lubricant and the fuel contribute toward deposit formation. The role of the blowby, NOx, and high-temperature oxidative and thermal degradation of the lubricant, described earlier, are substantiated [24]. The importance of oxygenated precursors—their decomposition, condensation, and polymerization to form deposits—is also supported. The deposit precursors consist of approximately 15–50 carbon atoms and contain multiple hydroxy and carboxy functional groups. Because of the polyfunctionality, these molecules have the ability to thermally polymerize to high-molecular-weight products [14,16]. As mentioned earlier, soot associates with polar oxidation products in oil to cause a viscosity increase. Viscosity increase can also occur in gasoline engine oils that have little or no soot. This happens when the oxygen content of the precursors is low and the resulting polymer is of low molecular weight and of good oil solubility [14]. This phenomenon is commonly referred to as oil thickening [6]. Conversely, if the oxygen content of the precursors is high, the polymerization results in the formation of high-molecular-weight products of low lubricant solubility. Such products constitute resin, which is of low oil solubility and separates on surfaces. If the surfaces are hot, subsequent dehydration and polymerization lead to the formation of varnish, lacquer, and carbon deposits. It is important to note that deposits are a consequence of lubricant oxidation that accelerates once the oxidation inhibitor package in the lubricant is exhausted. Three other internal combustion engine problems—oil consumption, ring sticking, and corrosion and wear—are also related to lubricant degradation. Oil consumption is a measure of how much lubricant travels past piston rings into the combustion chamber and burns. A certain minimum amount of the lubricant is necessary in the vicinity of the piston rings to lubricate cylinder walls and cylinder liners and hence facilitate piston movement and minimize scuffing. However, if too much lubricant ends up in the combustion chamber, serious emission problems will result. Modern piston designs, such as articulated pistons and pistons with low crevice volume, allow just enough lubricant to minimize scuffing, but without adversely contributing to emissions [26,27]. Other parameters that affect oil consumption include the integrity of pistons and cylinders and the viscosity, volatility, and sealing characteristics of the lubricant. Pistons with stuck rings and out-of-square grooves and cylinders with increased wear will result in a poor seal between the crankcase and the combustion chamber [15]. As a consequence, a larger amount of blowby will enter the crankcase and increase the rate of lubricant breakdown. This will complicate the situation further. Ring sticking occurs when sticky deposits form in the grooves behind the piston rings. This is a serious problem because it not only results in a poor seal but also leads to poor heat transfer from the cylinder to the wall. If not controlled, this will result in nonuniform thermal expansion of the pistons, loss of compression, and ultimately the failure of the engine [15]. The wear of pistons and the cylinders is undesired for the same reasons. Wear of engine parts is either corrosive or abrasive. Corrosive wear arises from the attack of fuel sulfur-derived products, such as sulfur oxides or sulfuric acid, or the acidic by-products of

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147

lubricant oxidation and degradation, such as carboxylic and sulfonic acids. Fuel sulfur–derived piston ring wear and cylinder wear are serious problems in large, slow-speed marine diesel engines that use a high-sulfur fuel. Corrosive wear is controlled by the use of lubricants with a base reserve, that is, those containing a large quantity of basic detergents. This was discussed in Chapter 4. Abrasive wear results from the presence of the particulate matter, such as large soot particles, in the lubricant. Dispersants are crucial to the control of soot-related wear.

5.3 DEPOSIT CONTROL BY DISPERSANTS Fuel and lubricant oxidation and degradation products, such as soot, resin, varnish, lacquer, and carbon, are of low lubricant (hydrocarbon) solubility, with a propensity to separate on surfaces. The separation tendency of these materials is a consequence of their particle size. Small particles are more likely to stay in oil than large particles. Therefore, resin and soot particles, which are the two essential components of all deposit-forming species, must grow in size through agglomeration before separation. Growth occurs either because of dipolar interactions, as is the case in resin molecules, or because of adsorbed polar impurities such as water and oxygen, as is the case in soot particles. Alternatively, soot particles are caught in the sticky resin. Dispersants interfere in agglomeration by associating with individual resin and soot particles. The particles with associated dispersant molecules are unable to coalesce because of either steric factors or electrostatic factors [28]. Dispersants consist of a polar group, usually oxygen- or nitrogen-based, and a large nonpolar group. The polar group associates with the polar particles, and the nonpolar group keeps such particles suspended in the bulk lubricant [16]. Neutral detergents, or soaps, operate by an analogous mechanism.

5.4 DESIRABLE DISPERSANT PROPERTIES Dispersing soot, deposit precursors, and deposits is clearly the primary function of a dispersant. Dispersants, in addition, need other properties to perform effectively. These include thermal and oxidative stability and good low-temperature properties. If a dispersant has poor thermal stability, it will break down, thereby losing its ability to associate with and suspend potentially harmful products. Poor oxidative stability translates into the dispersant molecule contributing itself toward deposit formation. Good low-temperature properties of a lubricant are desired for many reasons: ease of cold cranking, good lubricant circulation, and fuel economy. Base oil suppliers have developed a number of ways to achieve these properties. The methods they use include isomerization of the base stock hydrocarbons through hydrocracking and the use of special synthetic oils as additives. Since dispersant is one of the major components of the engine oil formulations, its presence can adversely affect these properties, which must be preserved.

5.5 DISPERSANT STRUCTURE A dispersant molecule consists of three distinct structural features: a hydrocarbon group, a polar group, and a connecting group or a link (see Figure 5.1). The hydrocarbon group is polymeric Connecting group Hydrocarbon group Nitrogen- or oxygenderived functionality Polar moiety

FIGURE 5.1

Graphic representation of a dispersant molecule.

148

Lubricant Additives: Chemistry and Applications

in nature, and depending on its molecular weight, dispersants can be classified into polymeric dispersants and dispersant polymers. Polymeric dispersants are of lower molecular weight than dispersant polymers. The molecular weight of polymeric dispersants ranges between 3,000 and 7,000 as compared to dispersant polymers, which have a molecular weight of 25,000 and higher. Although various olefins, such as polyisobutylene, polypropylene, polyalphaolefins, and mixtures thereof, can be used to make polymeric dispersants, the polyisobutylene-derived dispersants are the most common. The number average molecular weight (Mn) of polyisobutylene ranges between 500 and 3000, with an Mn of 1000–2000 being typical [29]. In addition to Mn, other polyisobutylene parameters, such as molecular weight distribution and the length and degree of branching, are also important in determining the overall effectiveness of a dispersant. Substances obtained through a polymerization reaction, especially those made by using an acid catalyst or a free-radical initiator, often contain molecules of different sizes. Molecular weight distribution, or polydispersity index, is commonly used to assess the heterogeneity in molecular size. Polydispersity index is the ratio of weight average molecular weight (Mw) and Mn, or Mw/Mn [30–32]. These molecular weights are determined by subjecting the polymer to gel permeation chromatography (GPC). The method separates molecules based on size [33]. The larger molecules come out first, followed by the next size. When the molecules are of the same size, Mw/Mn equals 1 and the polymer is called a monodisperse polymer. The polymers with an index >1 are called polydisperse polymers. For most applications, monodispersity is desired. Polyisobutylene, derived from acid-catalyzed polymerization reaction, typically has a polydispersity index between 2 and 3. This will impact many of the dispersant properties described below. Dispersant polymers, also called dispersant viscosity modifiers (DVMs) and dispersant viscosity index improvers (DVIIs), are derived from hydrocarbon polymers of molecular weights between 25,000 and 500,000. Polymer substrates used to make DVMs include high-molecular-weight olefin copolymers (OCPs), such as ethylene–propylene copolymers (EPRs), ethylene–propylene–diene copolymers (EPDMs), polymethacrylates (PMAs), styrene–diene rubbers (SDRs) of both linear and star configurations, and styrene–ester polymers (SEs). The polar group is usually nitrogen- or oxygen-derived. Nitrogen-based groups are derived from amines and are usually basic in character. Oxygen-based groups are alcohol-derived and are neutral. The amines commonly used to synthesize dispersants are polyalkylene polyamines such as diethylenetriamine and triethylenetetramine. In the case of DVMs or dispersant polymers, the polar group is introduced by direct grafting, copolymerization, or by introducing a reactable functionality. The compounds used for this purpose include monomers such as 2- or 4-vinylpyridine, N-vinylpyrrolidinone, and N,N-dialkylaminoalkyl acrylate and unsaturated anhydrides and acids such as maleic anhydride, acrylic acid, and glyoxylic acid. The details of these reactions are described in Section 5.6, which deals with the dispersant synthesis. Amine-derived dispersants are called nitrogen or amine dispersants, and those that are alcohol-derived are called oxygen or ester dispersants [28]. Oxygen-derived phosphonate ester dispersants were popular at one time, but their use in engine oils is now restrained because of the phosphorus limit. Phosphorus limit pertains to its tendency to poison noble metal catalysts used in catalytic converters. Formulators prefer to take advantage of the phosphorus limit by using zinc dialkyldithiophosphates, which are excellent oxidation inhibitors and antiwear agents. In the case of amine dispersants, it is customary to leave some of the amino groups unreacted to impart basicity to the dispersant. The reasons for this are described in Section 5.7.

5.6

DISPERSANT SYNTHESIS

Since it is not easy to attach the polar group directly to the hydrocarbon group, except in the case of olefins that are used to make DVMs, the need for a connecting group or a link arises. Although many such groups can be used, the two common ones are phenol and succinic anhydride. Olefin, such as polyisobutylene, is reacted either with phenol to form an alkylphenol or with maleic anhydride

Dispersants

149

to form an alkenylsuccinic anhydride. The polar functionality is then introduced by reacting these substrates with appropriate reagents.

5.6.1

THE HYDROCARBON GROUP

Polyisobutylene is the most common source of the hydrocarbon group in polymeric dispersants. It is manufactured through acid-catalyzed polymerization of isobutylene [34,35]. Figure 5.2 depicts the mechanism of its formation. In Figure 5.2, polyisobutylene is shown as a terminal olefin, whereas in reality it is a mixture of various isomers. Those that predominate include geminally disubstituted (vinylidene), trisubstituted, and tetrasubstituted olefins. Figure 5.3 shows their structure and the possible mechanism of their formation. Polyisobutylenes of structures I and II result from the loss of a proton from carbon 1 and carbon 3 of the intermediate of structure V. Polyisobutylenes of structures III and IV result from the rearrangement of the initially formed carbocation, as shown in Figure 5.3. The reactivity of these olefins toward phenol and maleic anhydride varies. In general, the more substituted the olefin, the lower the reactivity, which is a consequence of the steric factors. Similarly, the larger the size of the polyisobutyl pendant group, that is, the higher the molecular weight, the lower the reactivity. This is due to the dilution effect, which results from low olefin-to-hydrocarbon ratio. As mentioned earlier, polyisobutylene is the most commonly used olefin. One of the reasons for its preference is its extensive branching. This makes the derived dispersants to possess excellent oil solubility, in both nonassociated and associated forms. However, if the hydrocarbon chain in the dispersant is too small, its lubricant solubility greatly suffers. Because of this, the low-molecularweight components in polyisobutylene are not desired. This is despite their higher reactivity. These must be removed, which is carried out through distillation. Alternatively, one can minimize the formation of these components by decreasing the amount of the catalyst during polymerization and by lowering the polymerization reaction temperature. A new class of dispersants derived from ethylene/α-OCP with an Mn of 300–20,000 has also been reported, primarily by the Exxon scientists [36,37]. Such dispersants are claimed to have superior low- and high-temperature viscometrics than those of the polyisobutylene-derived materials. As mentioned earlier, dispersant polymers are derived from EPRs, styrene–butadiene copolymers, polyacrylates, PMAs, and styrene esters. The ethylene–propylene rubbers are synthesized by Ziegler–Natta catalysis [38]. The styrene–butadiene rubbers are synthesized through anionic polymerization [38]. Polyacrylates and PMAs are synthesized through polymerization of the monomers using free-radical initiators [38]. Styrene esters are made by reacting styrene–maleic

H+

H3C

CH2

H3C

CH3

H3C

CH3 CH3 +

CH3

CH3 Isobutylene

CH3

H2C

CH3

R = polyisobutyl

H3C R

CH3 CH3 CH2

H3C − H+

FIGURE 5.2 Acid-catalyzed polymerization of isobutylene.

CH3 CH3 + CH 3

150

Lubricant Additives: Chemistry and Applications CH3

H CH2

CH3 CH 3

H3 C

Trisubstituted olefin

4 R

CH3 CH3

Trisubstituted olefin

H 1 2 C H + H

R = polyisobutyl

Carbocation intermediate V

IV H H R

CH3 CH3

H3C CH3 CH3 1

Tetrasubstituted olefin

H3C

III

CH3 CH3

H3C

Terminal olefin (vinylidene)

CH3

1 CH3

2 + H3C CH3 CH 3 1 V

C3 to C2 hydride transfer

H 4

R H3C

3 +

H3C

C2 to C3 methide transfer

1 CH3 2

4 R

CH CH3 H 1 3

H3C CH3

4 R

1 CH3

CH3

CH3 1

H CH3 H

R C3 to C4 hydride transfer

4 + CH3

VII

C4 to C3 methide Not shown transfer by arrow CH3 +

1 2 CH3 +

C3 proton loss

CH3

1 CH3

R CH3 1

VIII

H3C

H 2 CH3 CH3 1 3

Trisubstituted olefin III

C2 proton loss

CH3 4 R

1 CH3

Tetrasubstituted olefin

2 H CH3

CH3 1

FIGURE 5.3 Polyisobutylene structures and the mode of their formation.

anhydride copolymer or styrene–maleic anhydride–alkyl acrylate terpolymer with alcohols, usually in the presence of a protic acid, such as sulfuric or methanesulfonic acid, catalyst. Since complete esterification of the anhydride is hard to achieve, the neutralization of the residual carboxylic acid anhydride is carried out by alternative means [38–40].

Dispersants

5.6.2

151

THE CONNECTING GROUP

As mentioned in Section 5.5, succinimide, phenol, and phosphonate are the common connecting groups used to make dispersants. Of these, succinimide and phenol are the most prevalent [2]. Succinimide group results when a cyclic carboxylic acid anhydride is reacted with a primary amino group. Alkenylsuccinic anhydride is the precursor for introducing the succinimide connecting group in dispersants. Alkenylsuccinic anhydride is synthesized by reacting an olefin, such as polyisobutylene, with maleic anhydride [2]. This is shown in Figure 5.4. The reaction is carried out either thermally [29,41,42] or in the presence of chlorine [43]. The thermal process involves heating the two reactants together usually >200°C [29,41,42], whereas the chlorine-mediated reaction with a mixture is carried out by introducing chlorine to react containing polyisobutylene and maleic anhydride [43–48]. Depending on the manner in which chlorine is added, the procedure is either one-step or two-step [44]. If chlorine is first reacted with polyisobutylene before adding maleic anhydride, the procedure is considered two-step. If chlorine is added to a mixture of polyisobutylene and maleic anhydride, it is a one-step procedure. The one-step procedure is generally preferred. The chlorine-mediated process has several advantages, which include having a low reaction temperature, having a faster reaction rate, and working well with internalized or highly substituted olefins. The low reaction temperature minimizes the chances of thermal breakdown of polyisobutylene and saves energy. The major drawback of the chlorine process is that the resulting dispersants contain residual chlorine as organic chlorides. Their presence in the environment is becoming a concern because they can lead to the formation of carcinogenic dioxins. A number of strategies are reported in the literature to decrease the chlorine content in dispersants [49–54]. The thermal process does not suffer from the presence of chlorine, although it is less energy-efficient and requires the use of predominantly a terminal olefin, that is, the polyisobutylene of high vinylidene content. The mechanism by which the two processes proceed is also different [46,47,50–52]. The thermal process is postulated to occur through an ene reaction. The chlorine-mediated reaction is postulated to proceed through a Diels–Alder reaction. The mechanism of the diene formation is shown in Figure 5.5. Chlorine first reacts with polyisobutylene 1 to form allylic chloride II. By the loss of the chloride radical, this yields the intermediate III, which through C4 to C3 methyl radical transfer is converted into the intermediate IV. A C3 to C4 hydrogen shift in the intermediate results in the formation of the radical V. This radical can lose hydrogen either from C4 to yield the diene VI or from C5 to result in the diene VII. The resulting dienes then react with maleic anhydride through a 4 + 2 addition reaction, commonly called a Diels–Alder reaction [55], to form alkenyltetrahydrophthalic anhydrides [50,52]. These reactions are shown in Figure 5.6. These anhydrides can be converted into phthalic anhydrides through dehydrogenation by using sulfur [50–52]. These compounds can then be transformed into dispersants by reacting with polyamines and polyhydric alcohols [51,52]. During the thermal reaction of polyisobutylene with maleic anhydride, that is, the ene reaction, the vinylidene double bond moves down the chain to the next carbon. Since thermal reaction requires a terminal olefin, further reaction of the new olefin with another mole of maleic anhydride will not occur if the double bond internalizes, and

O O

O +

Heat Polyisobutylene

Polyisobutenyl

or CI2 O Maleic anhydride

FIGURE 5.4

Alkenylsuccinic anhydride formation.

Polyisobutenylsuccinic anhydride

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Lubricant Additives: Chemistry and Applications

H3C R

H3C CH3 CH3

CH3CH3 + 4 3 2 CH 2 1

CI2

4 3 2 CI

−CI•

H3C CH3 CH3 R

CH3 1

3 •2

CH2 1

H III

• CH3 transfer CH3 CH3 R

CH3

CH2 1

H3C H CH3

−H• (From C4)

R 4 •3 2 CH 2 • 1 H transfer H C5 H H

CH3 CH3 4 •3 2 CH2 H CH 1 3

−H• (From C5) H3C H R

CH3

4 3 2 CH2

CH2 1

VII

FIGURE 5.5 Mechanism of chlorine-assisted diene formation.

the reaction will stop at this stage. This is shown in reaction 5.3 of Figure 5.6. If the new double bond is external, the reaction with another molecule of maleic anhydride is possible [45]. This is shown in reaction 5.4. For dispersants, polyisobutylphenol is the alkylphenol of choice. It is synthesized by reacting polyisobutylenes with phenol in the presence of an acid catalyst [56–58]. Lewis acid catalysts, such as aluminum chloride and boron trifluoride, are often employed. Boron trifluoride is preferred over aluminum chloride because the reaction can be carried out at low temperatures, which minimizes acid-mediated breakdown of polyisobutylene [58]. This is desired because dispersants derived from low-molecular-weight phenols are not very effective. Other catalysts, such as sulfuric acid, methanesulfonic acid, and porous acid catalysts of Amberlyst® type, can also be used to make alkylphenols [59,60]. Polyisobutylene also reacts with phosphorus pentasulfide through an ene reaction, as described in Chapter 4. The resulting adduct is hydrolyzed by the use of steam to alkenylphosphonic and alkenylthiophosphonic acids [2,3]. The methods to synthesize alkylphenols and alkenylphosphonic acids are shown in Figure 5.7. A new carboxylate moiety derived from glyoxylic acid to make dispersants has been reported in the literature [61–65]. However, at present, no commercial products appear to be based on this chemistry.

5.6.3

THE POLAR MOIETY

The two common polar moieties in dispersants are based on polyamines and polyhydric alcohols. The structures of common amines and alcohols used to make dispersants are shown in Figure 5.8. The polyamines are manufactured from ethylene through chlorination, followed by the reaction with ammonia [66]. The reaction scheme is given in Figure 5.9. As shown, polyamines

Dispersants

153

Diels−Alder reaction

CH2

H3C

O H H3C

O H

CH2

H3C

(5.1)

CH2

H3C

H3C H CH3

C R

CH3

(5.2)

O H3C

H3C

R Ene reaction

H3C CH3 H

CH2

H H

R O H3C

H C

CH2 H

(5.3)

O H

O H3C

H H3C H3C

H3C CH3

H3C H3C

(5.4)

H H

FIGURE 5.6 Mechanism of alkenylsuccinic anhydride formation.

contain piperazines as a by-product. Examining the structures of various amines, one can see that they contain primary, secondary, and tertiary amino groups. Each type of amino group has different reactivity toward alkenylsuccinic anhydride. The primary amino group reacts with the anhydride to form a cyclic imide, the secondary amino group reacts with the anhydride to form an amide/carboxylic acid, and the tertiary amino group does not react with the anhydride at all [67]. However, it can make a salt if a free carboxylic acid functionality is present in the molecule, as is the case in amide/carboxylic acid. These reactions are shown in Figure 5.10. New highmolecular-weight amines derived from phosphoric acid–catalyzed condensation of polyhydroxy compounds, such as pentaerythritol, and polyalkylene polyamines, such as triethylenetetramine, are known [68]. These amines are claimed to form high total base number (TBN) dispersants with

154

Lubricant Additives: Chemistry and Applications OH

Polyisobutylene

Acid

Phenol

R Polyisobutylphenol +

Polyisobutylene

P2S5

Adduct

Phosphorus pentasulfide

H2O

S Polyisobutenyl

O OH

polyisobutenyl

Polyisobutenylthiophosphonic and polyisobutenylphosphonic acids

FIGURE 5.7 Synthesis of alkylphenols and alkenylphosphonic acids.

low free-amine content and better engine test performance than dispersants made from conventional polyamines. Imide and ester dispersants are made by reacting polyamines and polyhydric alcohols with alkenylsuccinic anhydrides. The reaction typically requires a reaction temperature between 130 and 200°C to remove the resulting water and complete the reaction [44]. As mentioned earlier, imide dispersants are made by the use of polyalkylene polyamines, such as diethylenetriamine and triethylenetetramine. Many polyhydric alcohols can be used to make ester dispersants. These include trimethylolpropane, tris(hydroxymethyl)aminoethane, and pentaerythritol. When one uses tris(hydroxymethyl)aminoethane as the alcohol, one can obtain an ester dispersant with basicity. The reactions to make succinimide and succinate dispersants are depicted in Figure 5.11. The alkylphenol-derived dispersants are made by reacting an alkylphenol, such as polyisobutylphenol, with formaldehyde and a polyamine [58,69]. The result is the formation of 2-aminomethyl4-polyisobutylphenol. The reaction of ammonia or an amine, formaldehyde, and a compound with active hydrogen(s), such as a phenol, is called the Mannich reaction [70,71]. Hence, such dispersants are called Mannich dispersants. For making phosphonate dispersants, the common method is to react the free acid with an olefin epoxide, such as propylene oxide or butylene oxide, or an amine [2,72,73]. These reactions are shown in Figure 5.12. Salts derived from the direct reaction of amine and metal bases with olefin-phosphorus pentasulfide adduct are also known [74,75]. It is important to note that structures in figures are idealized structures. The actual structures will depend on the substrate (alkylphenol and alkenylsuccinic anhydride)-to-reactant (formaldehyde and polyamines) ratio. Because of the polyfunctionality of the succinic anhydride group and of the amines and polyhydric alcohols, various dispersants can be made by altering the anhydride-to-amine or anhydride-to-alcohol ratios. These dispersants differ not only in their molecular weight but also in their properties. Polyfunctionality of the two reactants leads to dispersants, which have molecular weights that are three to seven times higher than expected if the two reactants were monofunctional.

Dispersants

155

Diamines 1° NH2

H2N

CH3 N H3C 3°

Ethylenediamine

1° NH2 NH2 Primary amino group (1°)

N,N-dimethylaminopropylamine

Triamines 2° H N

H2N

1° NH2

Diethylenetriamine

1° H2N

3° N

2° NH

Secondary amino group (2°)

Tertiary amino group (3°)

Aminoethylpiperazine

Tetramines

H2N

H N

1° NH2

2° N H

Triethylenetetramine

H2N

2° H N

3° N

2° HN

1° NH2

Aminoethylaminoethylpiperazine 1° NH2

3° N

Bis(aminoethyl) piperazine Alcohols

CH3

NH2

Pentaerythritol

Trimethylolpropane Tris(hydroxymethyl)propane

Tris(hydroxymethyl)aminoethane

FIGURE 5.8 Amines and alcohols used to synthesize dispersants.

The methods to make DVMs are shown in Figures 5.13 through 5.15. These are synthesized by • Grafting or reacting of a dispersancy-imparting monomer on an already-formed polymer, as in the case of EPRs and SDRs [76–84]. • Including such a monomer during the polymerization process, as in the case of polyacrylates and PMAs [85]. • Introducing a reactive functional group in the polymer that can be reacted with a reagent to impart dispersancy, as in the case of styrene–maleic anhydride copolymers [40,86–93]. Although most of the examples in Figures 5.13 through 5.15 pertain to the introduction of the basic nitrogen-containing moieties, neutral DVMs are also known in the literature. These are made by using nonbasic reactants, such as N-vinylpyrrolidinone, alcohols, or polyether-derived methacrylate ester [79,94,95]. Recently, dispersant viscosity–improving additives with built-in oxidation

156

Lubricant Additives: Chemistry and Applications

H2C=CH2 + CI2

CICH2CH2CI

Ethylene

NH3

CICH2CH2NH2 Chloroethylamine

Ethylenedichloride

NH3 NH2CH2CH2NHCH2CH2NH2

CICH2CH2NH2

NH2CH2CH2NH2 Ethylenediamine

Diethylenetriamine CICH2CH2CI

NH2CH2CH2N

Aminoethylpiperazine

FIGURE 5.9 Manufacture of polyamines.

(a) Primary amine O

O Polyisobutenyl

RNH2

Polyisobutenyl

Imide

O (b) Secondary amine

O O

Polyisobutenyl

R2NH

NR2 Amide

O (c) Tertiary amine

O O

O NR2

Polyisobutenyl

NR2

Polyisobutenyl

+ R3N

NR2 − + O NHR3

Polyisobutenyl

Salt O

FIGURE 5.10 Amine–anhydride reaction products. (Based on Harrison, J.J., Ruhe, R., Jr., William, R., U.S. Patent 5,625,004, April 29, 1997.)

inhibiting and antiwear moieties have been reported in the patent literature [77,96,97]. Dispersant polymers containing oxidation-inhibiting moieties are commercially available from Texaco Chemical Company now part of Ethyl Petroleum Additives Company. As the examples show, grafting usually allows the introduction of the connecting group in the dispersant polymers at the same time as the polar moiety.

Dispersants

157

N O PIB O

H N H2N

O N

Polyalkylenepolyamine

Polyisobutenylsuccinic anhydride

H2C H2 C

PIB N

CH2

NH C H2

O Polyisobutenylsuccinimide O R R′

PIB O

R′ +

HOH2C

PIB

CH2 O O

CH2OH

CH2

R O

Polyisobutenylsuccinic anhydride

Polyhydric alcohol

PIB = Polyisobutenyl

FIGURE 5.11

CH2

C R′

CH2 R

Polyisobutenylsuccinate ester

Synthesis of imide and ester dispersants.

5.7 DISPERSANT PROPERTIES A dispersant consists of a hydrocarbon chain, a connecting group, and a polar functionality. Although each structural feature imparts unique properties to a dispersant, the dispersant’s overall performance depends on all the three. The overall performance is assessed in terms of its dispersancy, thermal and oxidative stability, viscosity characteristics, and seal performance. These criteria primarily relate to engine oils, where dispersants find major use.

5.7.1

DISPERSANCY

As mentioned, dispersancy pertains to a dispersant’s ability to suspend by-products of combustion, such as soot, and lubricant degradation, such as resin, varnish, lacquer, and carbon deposits. The overall performance of a dispersant depends on all the three of its structural features: the hydrocarbon chain, the connecting group, and the polar moiety. The molecular weight of the hydrocarbon group in a dispersant determines its ability to associate with undesirable polar species and suspend them in the bulk lubricant. For dispersants that have the same connecting group and the polar moiety, the lower the molecular weight, the higher the ability to associate with polar materials and the lower the ability to suspend them. Because of the trade-off between the two properties, the hydrocarbon chain must have the correct size and branching. The size affects a dispersant’s affinity toward polar materials, and branching affects its solubility, both before association and after association with the species, a dispersant is designed to suspend in oil. Experience has demonstrated that hydrocarbon groups containing 70–200 carbon atoms and extensive branching, as in the case of polyisobutylenes, are extremely suitable to design dispersants with good dispersancy. The hydrocarbon chains of larger size, even if the branching is similar, lead to dispersants with low affinity toward polar materials. That is why dispersant polymers possess lower dispersancy than polymeric dispersants. However, since dispersant polymers have additional attributes, such as good thickening efficiency and

158

Lubricant Additives: Chemistry and Applications OH + CH2O

H N

H2N

R Polyalkylenepolyamine Polyisobutylphenol

OH H2 C

N H

CH2

N H

NH H2 C

N H

R Polyaminomethylpolyisobutylphenols

CH3

S S PIB

PIB

+ O

OCH2CH

Propylene oxide Polyisobutenylthiophosphonic acid

CH3 Bis-hydroxypropyl polyisobutenylthiophosphonate

FIGURE 5.12

Synthesis of Mannich and phosphonate dispersants.

in some cases good thermal and oxidative stability, their use is advantageous. They usually replace additives, called viscosity modifiers, in the package. Since they impart some dispersancy because of their structure, the amount of polymeric dispersant in engine oil formulations is somewhat decreased [79,98]. Both the connecting group and the polar moiety are important to the dispersancy of the dispersant molecule. They must be considered together since both contribute toward polarity. In Mannich dispersants, the phenol functional group, and in imide and ester dispersants, succinimide, succinate, and phosphonate functional groups are also polar, the same as the amine and the alcohol-derived portion of the molecule. The polarity is a consequence of the electronegativity difference between carbon, oxygen, nitrogen, and phosphorus atoms. The greater the electronegativity difference, the stronger the polarity. This implies that groups that contain phosphorus–oxygen bonds are more polar than those containing carbon–oxygen bonds, carbon–nitrogen bonds, and carbon–phosphorus bonds. The electronegativity difference for such bonds is 1.4, 1.0, 0.5, and 0.4, respectively [99]. However, since dispersants have many bonds with various combinations of atoms, the overall polarity in a dispersant and its ability to associate with polar materials are not easy to predict.

Dispersants

159

CH3 CH

CH2

CH3

CH2 n

Ethylene − propylene copolymer

4-Vinylpyridine

Radical initiator

CH3

CH3 CH2

CH2

n −1

N Dispersant olefin copolymer (DOCP)

FIGURE 5.13

Dispersant viscosity modifier synthesis through grafting.

CH3

CH3 n H2C

COOR

CH2

CH3

COOR

CH3

Dimethylaminoethyl methacrylate

Radical initiator

OCH2CH2N

Alkyl methacrylate

CH2

CH3 C

CH2

COOR CH2

C CH3

n−x O

CH3

C OCH2CH2N

CH3 Polyacrylate-Type Dispersant Viscosity Modifier

FIGURE 5.14

Dispersant viscosity modifier synthesis through copolymerization.

160

Lubricant Additives: Chemistry and Applications H C

H C

CH2

C O

Styrene−maleic anhydride polymer

H C

ROH O

H2N n

CH2

R′

H C

O n

N R

R′

Styrene ester−based dispersant viscosity modifier

FIGURE 5.15

Dispersant viscosity modifier synthesis through chemical reaction.

Because some of the materials with which the dispersant associates are acidic, such as carboxylic acids derived from lubricant oxidation, the presence of an amine nitrogen is an advantage because of its basic character. Therefore, in certain gasoline engine tests, nitrogen dispersants are superior to ester dispersants. Ester dispersants are usually superior in diesel engine tests because of their higher thermo-oxidative stability. Mannich dispersants are good low-temperature dispersants; hence, they are typically used in gasoline engine oils. As mentioned earlier, commercial polyisobutylenes have a molecular weight distribution. This will lead to dispersant structures of varying size, hence molecular weight. An optimum ratio between the molecular weight of the hydrocarbon chain and that of the polar functionality (polar/ nonpolar ratio) is a prerequisite for good dispersancy. If a dispersant composition has an excessive amount of components with short hydrocarbon chains, that is, of low molecular weight, its associating ability increases, but its oil solubility suffers. This is likely to deteriorate its dispersancy, especially after associating with polar impurities. Such structures in dispersants are, therefore, undesired. Their formation can be minimized by using polyolefins of low polydispersity index, controlling the formation of low-molecular-weight components, removing such components through distillation [100], or postreacting with another reagent, preferably of the hydrocarbon type. Polyolefins of low polydispersity index (≤2) are available from BP and Exxon Chemical Company. Controlling the formation of low-molecular-weight components is exemplified by the use of boron trifluoride catalyst for making alkylphenols instead of aluminum chloride, which tends to fragment polyisobutylene. Removing the lower-molecular-weight components, although not easy, is possible at the precursor stage, which is before reacting with the alcohol or the amine. A number of reagents can be used for the postreaction [101]. Hydrocarbon posttreatment agents include polyepoxides [102], polycarboxylic acid [103], alkylbenzenesulfonic acids [104], and alkenylnitriles [105]. Whenever postreacted dispersants are used in engine oils, improved dispersancy, viscosity index credit, improved fluorocarbon elastomer compatibility, hydrolytic stability, and shear stability are often claimed.

5.7.2

THERMAL AND OXIDATIVE STABILITY

All the three components of the dispersant structure determine its thermal and oxidative stability, the same as dispersancy. The hydrocarbon group can oxidize in the same manner as the lubricant hydrocarbons to form oxidation products that can contribute toward deposit-forming species [4,9]. (This is described in Section 5.2.) Although the rate of oxidation is quite slow for largely paraffinic hydrocarbon groups, such as polyisobutyl group, it is quite high for those that contain multiple bonds, such as polyisobutenyl, and the benzylic groups. The benzylic functional group is present in styrene

Dispersants

161

butadiene and styrene ester–derived dispersant polymers. Purely paraffinic hydrocarbon groups that contain tertiary hydrogen atoms, such as EPRs, oxidize at a faster rate than those that contain only primary and secondary hydrogen atoms. Styrene isoprene–derived materials contain both benzylic and tertiary hydrogen atoms. This implies that highly branched alkyl groups, such as polyisobutyl and polyisobutenyl, have a higher susceptibility toward oxidation than linear or unbranched alkyl groups. Dispersant polymers with built-in oxidation-inhibiting moieties are known in the literature [77,78,96]. The polar moiety in an amine-derived dispersant is also likely to oxidize at a faster rate than the oxygen-derived moiety because of the facile formation of the amine oxide functional group on oxidation. Such groups are known to thermally undergo β-elimination [40], called the cope reaction, to form an olefin. This can oxidize at a faster rate as well as lead to deposit-forming polymeric products. From a thermal stability perspective, the hydrocarbon group in the case of high-molecularweight dispersant polymers, such as those derived from OCPs, is more likely to break down (unzip) than that derived from the low-molecular-weight polymers. Dispersants based on 1000–2000 molecular weight polyisobutylenes are relatively stable, except at very high temperatures that are experienced in some engine parts, such as near the top of the piston [17,18]. Thermal breakdown of the oxidized amine polar group is mentioned in the previous paragraph. The chemical reactivity of certain dispersants toward water and other reactive chemicals present in the lubricant formulation is an additional concern. The most likely reaction site is the connecting group. The common connecting groups are amide and imide in amine-derived dispersants and ester in alcohol-derived dispersants. All three can hydrolyze in the presence of water [106], but at different rates. Esters are easier to hydrolyze than amides and imides. The hydrolysis is facilitated by the presence of bases and acids. Basic detergents are the source of the metal carbonate and metal hydroxide bases, which at high temperatures catalyze the hydrolysis reaction. Additives, such as zinc dialkyldithiophosphates, are a source of strong acids that result when these additives hydrolyze, thermally decompose, or oxidize. The fate of the ester-, amide-, and imide-type dispersant polymers, such as those derived from polyacrylates, PMAs, and styrene ester substrates, is the same. Some OCP-derived dispersant polymers, such as those obtained by grafting of monomers 2- or 4-vinylpyridine and 1-vinyl-2-pyrrolidinone [76,80], do not suffer from this problem since they do not contain easily hydrolyzable groups. Reactivity toward other chemicals present in the formulation is again prevalent in the case of ester-derived dispersants. Reaction with metal-containing additives, such as detergents and zinc dialkyldithiophosphates, can occur after hydrolysis to form metal salts. This can destroy the polymeric structure of a dispersant and hence its effectiveness. Some formulations contain amines or their salts as corrosion inhibitors or friction modifiers. Depending on the molecular weight and the ambient temperature, these can displace the polyol or sometimes the polyamine, thereby altering the dispersant structure, hence its properties.

5.7.3

VISCOSITY CHARACTERISTICS

The amount of dispersant in automotive engine oils typically ranges between 3 and 7% by weight [79], making it the highest among additives. In addition, dispersant is the highest molecular-weight component except the viscosity improver [107]. Both of these factors can alter some physical properties, such as viscosity, of the lubricant. A boost in the viscosity of a lubricant at high temperatures is desired, but at low temperatures it is a disadvantage. At high temperatures, the lubricant loses some of its viscosity [108], hence its film-forming ability, resulting in poor lubrication. Maintaining good high-temperature viscosity of a lubricant is therefore imperative to minimize wear damage. This is usually achieved by the use of polymeric viscosity modifiers [3,109]. Some dispersants, especially those that are based on high-molecular-weight polyolefins and have been oversuccinated partly fulfill this need [44]. Therefore, the amount of polymeric viscosity modifier necessary to achieve specific high-temperature viscosity is reduced. Unfortunately, dispersants that provide a viscosity advantage lead to a viscosity increase at low temperatures as well. The low-temperature viscosity requirements for engine oils have two components: cranking viscosity and pumping

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viscosity [110]. Cranking viscosity is an indication of how easily the engine will turn over in extremely cold weather conditions. Pumping viscosity is the ability of the lubricant to be pumped to reach various parts of the engine. For cold weather operation, low to moderate cranking and pumping viscosities are highly desirable. Although pumping viscosity and the pour point can be lowered by the use of additives, called pour point depressants [3,13], lowering cranking viscosity is not easy. In the case of base oils, this is usually achieved by blending carefully selected base stocks. An ideal polymeric dispersant must provide high-temperature viscosity advantage without adversely affecting the cold-cranking viscosity of the lubricant. Dispersant polymers have the same requirement. Good high-temperature viscosity to cranking viscosity ratio in polymeric dispersants can be achieved by • Carefully balancing the type and the molecular weight of the hydrocarbon chain [111] • Choosing the optimum olefin to maleic anhydride molar ratio [112] • Selecting the type and the amount of the polyamine used In dispersant polymers this can be achieved by selecting (1) a polymer of correct molecular weight and branching and (2) a suitable pendant group. Dispersant polymers derived from mediummolecular-weight, highly branched structures, and ester-type pendant groups are best suited for use as additives. Examples include polyacrylate, PMA, and styrene ester–derived dispersants. These additives not only act as viscosity modifiers and dispersants but also act as pour point depressants, thereby improving the low-temperature properties of the lubricant. A number of patents pertaining to dispersants with balanced high-temperature viscosity and low-temperature properties are reported in the patent literature [113–117]. A Mannich (alkylphenol) dispersant, derived from ethylene/1-butene polymers of Mn 1500–7500, has been claimed to possess improved dispersancy and pour point [113]. Another patent claiming the synthesis of a dispersant with superior dispersancy and pour point depressing properties has also been issued [114]. The dispersant is based on the reaction of maleic anhydride/lauryl methacrylate/stearyl methacrylate terpolymer with dimethylaminopropylamine, and a Mannich base was obtained by reacting N-aminoethylpiperazine, paraformaldehyde, and 2,6-di-t-butyl phenol. A number of patents describe the use of ethylene/α-olefin/diene interpolymers to make dispersants [115–117]. These dispersants are claimed to possess excellent high- and low-temperature viscosities, as defined by VR´/VR. Here VR´ pertains to the dispersant and VR pertains to the precursor, such as alkylphenol or alkenylsuccinic anhydride. VR´ is the ratio of the –20°C cold-cranking simulator (CCS) viscosity (cP) of a 2% solution of dispersant in a reference oil to the 100°C kinematic viscosity (cSt) of the dispersant. VR is the ratio of the –20°C CCS viscosity (cP) of a 2% solution of precursor in the reference oil to the 100°C kinematic viscosity (cSt) of the precursor. The values of 2.0–3.9 for VR and VR´ and of <1.11 for VR´/VR are considered suitable for balanced low- and high-temperature viscosities.

5.7.4

SEAL PERFORMANCE

Seals in automotive equipment are used for many purposes, the most prominent of which are to have easy access to malfunctioning parts to perform repair and to minimize contamination and loss of lubricant. Various polymeric materials are used to make seals. These include fluoroelastomers, nitrile rubber, polyacrylates, and polysiloxanes (silicones). Maintaining the integrity of seals is critical; otherwise, the lubricant will be lost, and wear damage and equipment failure will occur. The seals fail in a number of ways. They can shrink, elongate, or become brittle and thus deteriorate. The damage to elastomer seals is assessed by examining volume, hardness, tensile strength change, and the tendency to elongate and rupture [118]. Two primary mechanisms by which seal damage can occur include abrasion due to particulate matter in the lubricant and the attack of various lubricant components on the seals. The lubricant-related damage can occur when some of its components

Dispersants

163

diffuse into the seals. This will either cause a change in the seal’s hardness, thereby leading to swelling and or elongation, or extract the plasticizer, an agent used to impart flexibility and strength to polymeric materials. Abrasive damage is not common since most equipment has an installed lubricant filtration system. The lubricant-related damage, however, is of primary interest to us. The lubricant is a blend of base stocks and an additive package. Certain base stocks, such as those of high aromatics content or those that are of the ester type, have the tendency to extract the plasticizer because of their high polarity. Additives, however, have the ability to diffuse into the seal material and alter its properties as well as remove the plasticizer. Among additives, dispersants are the most implicated in causing seal damage, especially to fluoroelastomer (Viton®) seals. Although in many cases seal failure can be corrected by the use of additives, called the seal-swell agents, it is wise to eliminate such damage by prevention. Elastomer compatibility requirements are a part of the current United States, Association des Contsructeurs Européens de l’Automobile (ACEA), and Japanese standards for engine oils and worldwide automotive transmission and tractor hydraulic fluid specifications [119]. Damage to seals is prevalent in the case of nitrogen dispersants. In general, the higher the nitrogen content, the higher the seal problems [118]. Rationally, these problems occur due to the presence of lowmolecular-weight molecules in the dispersant. These include free amine either as such or in a labile form, such as an alkylammonium salt, or low-molecular-weight succinimides and succinamides. Because of their high polarity and smaller size, these molecules are more likely to diffuse into the seal material and alter its physical and mechanical properties [120]. It is believed that in the case of Viton seals, the loss of fluoride ions is responsible for seal deterioration. Removal of the free amine and of low-molecular-weight succinimides will improve seal performance. Alternatively, one can posttreat dispersants with reagents, such as boric acid and epoxides, which will either make such species innocuous or hinder their diffusion into the seal material. Many chemical treatments of dispersants, covered in Section 5.7.1, claim to improve seal performance of dispersants and crankcase lubricants that use them. These reagents react with seal-damaging amines and lowmolecular-weight succinimides to make them harmless. Strategies other than those listed earlier are also reported in the patent literature [121–125].

5.8 PERFORMANCE TESTING Engine oils account for almost 80% of the automatic transmission dispersant use. Other applications that use these additives include automatic transmission fluids, gear lubricants, hydraulic fluids, and refinery processes as antifoulants. Dispersants of relatively lower molecular weight are also used in fuels to control injector and combustion chamber deposits [126,127]. Such dispersants usually contain a polyether functionality [128]. Succinimide and succinate ester–type polymeric dispersants are used in gasoline and diesel engine oils, but the use of alkylphenol-derived dispersants, that is, of the Mannich type, is limited to gasoline engine oils. Dispersant polymers derived from ethylene–propylene rubbers, styrene–diene copolymers, and PMAs are also used in both gasoline and diesel engine oils. As mentioned earlier, dispersant polymers lack sufficient dispersancy to be used alone and hence are used in combination with polymeric dispersants. The PMA and styrene ester–derived dispersant polymers are used in automatic transmission fluids, in power-steering fluids, and, to a limited extent, in gear oils. Additive manufacturers use various laboratory screen tests and engine tests to evaluate a dispersant’s effectiveness. Many of the screen tests are proprietary, but all are developed around evaluating performance in terms of a dispersant’s ability to disperse lamp black or used engine oil sludge. The laboratory engine tests are industry-required tests and include both gasoline engine and diesel engine tests. These are listed in International Lubricant Standardization and Approval Committee (ILSAC), American Petroleum Institute (API), ACEA 2002, Japanese Automobile Standards Organization (JASO), and Bureau of Indian Standards (BIS) standards. It is important

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to note that the U.S. military and original equipment manufacturers (OEMs) have their own performance requirements, which are over and above those of the API. Although the details of various tests are available in these standards and elsewhere [119], the important engine tests that evaluate a dispersant’s performance are listed in Tables 5.1 through 5.4.

TABLE 5.1 U.S. Gasoline Engine Tests Engine Test

Engine Type

CRC L-38

CLR single-cylinder engine

ASTM sequence IIIE ASTM sequence IIIF ASTM sequence VE ASTM sequence VG TEOST High-temperature deposit test

1987 Buick V6 engine 1996 Buick V6 engine Ford Dual-Plug head four-cylinder engine Ford V8 engine Bench test Bench test

Evaluation Criteria Bearing corrosion, sludge, varnish, oil oxidation, and viscosity change Sludge, varnish, wear, and viscosity change Sludge, varnish, wear, and viscosity change Sludge, varnish, and wear Sludge, varnish, and wear Thermal and oxidative stability High-temperature deposits

TABLE 5.2 U.S. Diesel Engine Tests Engine Test

Engine Type

Evaluation Criteria

Caterpillar 1K Caterpillar 1M-PC Caterpillar 1N Caterpillar 1P Mack T-6

Caterpillar single-cylinder engine Caterpillar single-cylinder engine Caterpillar single-cylinder engine Caterpillar single-cylinder engine Multicylinder engine

Mack T-7 Mack T-8 Mack T-9

Multicylinder engine Multicylinder engine Multicylinder engine

Piston deposits and oil consumption Piston deposits and oil consumption Piston deposits and oil consumption Piston deposits and oil consumption Piston deposits, wear, oil consumption, and oil thickening Oil thickening Oil thickening Soot thickening

TABLE 5.3 European Gasoline Engine Tests Engine Test

Engine Type

ASTM sequence IIIE

Six-cylinder engine

ASTM sequence VE Peugeot TU-3M high temperature

Four-cylinder engine Four-cylinder single-point injection engine Four-cylinder multipoint injection engine Four-cylinder carbureted engine

M-B M111 black sludge VW 1302 VW T-4

Four-cylinder multipoint injection engine

Evaluation Criteria High-temperature oxidation (sludge, varnish, wear, and viscosity increase) Low-temperature sludge, varnish, and wear Piston deposits, ring sticking, viscosity increase Engine sludge and cam wear Piston deposits, varnish, wear, and oil consumption Extended drain capability

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165

TABLE 5.4 Current European Diesel Engine Tests Engine Test

Engine Type

VW 1.6TC diesel intercooler

Four-cylinder engine

VW D1

Four-cylinder direct-injection engine

Peugeot XUD11ATE Peugeot XUD11BTE M-B OM 602A M-B OM 364A/LA

Four-cylinder indirect-injection engine Four-cylinder indirect-injection engine Five-cylinder indirect-injection engine Four-cylinder direct-injection engine

M-B OM 441LA

Six-cylinder direct-injection engine

MAN 5305

Single-cylinder engine

Mack T-8

Multicylinder engine

Evaluation Criteria Piston deposits, varnish, and ring sticking Piston deposits, viscosity increase, and ring sticking Piston deposits and viscosity increase Piston deposits and viscosity increase Engine wear and cleanliness Bore polishing, piston deposits, varnish, sludge, wear, and oil consumption Piston deposits, bore polishing, wear, oil consumption, valve train condition, and turbo deposits Piston deposits, bore polishing, and oil consumption Soot-related oil thickening

As mentioned earlier, soot-related viscosity increase and deposit-related factors are the primary criteria for evaluating a dispersant’s performance. Moreover, as commented in Chapter 4, neutral detergents (soaps) also help control deposits such as varnish, lacquer, sludge, and carbon. Therefore, besides the control of soot-related viscosity increase, which is the sole domain of dispersants, deposit control is the result of a joint performance of the detergent and the dispersant. However, in this regard, the dispersant plays a more prominent role. Besides engine oils, transmission fluids are the primary users of dispersants. Certain parts of the transmission see very high temperatures, which lead to extensive lubricant oxidation. The oxidation products, such as sludge and varnish, appear on parts; for instance, clutch housing, clutch piston, control valve body, and oil screen components. This can impair the functioning of these parts. A turbohydramatic oxidation test (THOT) is used to determine a transmission fluid’s oxidative stability. Polymeric dispersants are useful in controlling sludge buildup [129]. When friction modification of the transmission fluid is the goal, either dispersants or their precursors, such as alkenylsuccinic acids or anhydrides, are used in combination with metal sulfonates [130–134]. In many such formulations, the borated dispersant and the borated detergent (metal sulfonate) are used. Dispersants are used in gear oils to improve their properties also. Gear oils usually contain thermally labile extreme-pressure additives. Their decomposition by-products are highly polar, and dispersants are used to contain them to avoid corrosion and deposit formation [135,136]. Polymeric dispersants are used in hydraulic fluids to overcome wet filtration (Association Française de Normalisation [AFNOR]) problems, which is often required for HF-0-type fluids [137]. Filtration problems occur due to the interaction of water with metal sulfonate detergent and zinc dialkyldithiophosphate that are used as additives in hydraulic fluid formulations. Fouling is a common problem in many processes, including refinery processes. Fouling refers to the deposition of various inorganic and organic materials, such as salt, dirt, and asphaltenes, on heattransfer surfaces and other processing equipment. This results in poor heat transfer, among other problems. Antifoulants are chemicals used in refi nery operations to overcome fouling. Detergents and dispersants are often used for this purpose [138–140].

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REFERENCES 1. Sieloff, F.X., J.L. Musser. What does the engine designer need to know about engine oils? Presented to Detroit Section of the Society of Automotive Engineers, March 16, 1982. 2. Colyer, C.C., W.C. Gergel. Detergents/dispersants. In R.M. Mortier, S.T. Orszulik, eds. Chemistry and Technology of Lubricants. New York: CH Publishers, Inc., 1992, pp. 62–82. 3. Rizvi, S.Q.A. Lubricant additives and their functions. In S.D. Henry, ed. American Society of Metals Handbook, 10th edition, 1992, Vol. 18, pp. 8–112. 4. Ingold, K.U. Inhibition of autoxidation of organic substances in liquid phase. Chemical Reviews 61: 563–589, 1961. 5. Kornbrekke, R.E., et al. Understanding soot-mediated oil thickening—Part 6: Base oil effects. SAE Technical Paper 982,665, Society of Automotive Engineers, October 1, 1998. Also see parts 1–5 by E. Bardasz et al., SAE Papers 952,527 (October 1995), 961,915 (October 1, 1996), 971,692 (May 1, 1997), 976,193 (May 1, 1997), and 972,952 (October 1, 1997). 6. Covitch, M.J., B.K. Humphrey, D.E. Ripple. Oil thickening in the Mack T-7 engine test—fuel effects and the influence of lubricant additives on soot aggregation. Presented at SAE Fuels and Lubricants Meeting, Tulsa, OK, October 23, 1985. 7. Obert, E.F. Internal Combustion Engines and Air Pollution. New York: Intext Educational Publishing, 1968. 8. Petroleum products, lubricants, and fossil fuels. In Annual Book of ASTM Standards. Philadelphia, PA: American Society of Testing and Materials, 1998. 9. Cochrac, J., S.Q.A. Rizvi. Oxidation and oxidation inhibitors. ASTM Manual on Fuels and Lubricants, to be published in 2003. 10. Gas and expansion turbines. In D.M. Considine, ed. Van Nostrand’s Scientific Encyclopedia, 5th edition, New York: Van Nostrand Reinhold, 1976, pp. 1138–1148. 11. Zeldovich, Y.B., P.Y. Sadovnikov, D.A. Frank-Kamenetskii. Oxidation of Nitrogen in Combustion. Moscow-Leningrad: Academy of Sciences, U.S.S.R., 1947. 12. Ford, J.F. Lubricating oil additives—a chemist’s eye view. Journal of the Institute of Petroleum 54: 188–210, 1968. 13. Rizvi, S.Q.A. Additives: Chemistry and testing. In Tribology Data Handbook—an Excellent Friction, Lubrication, and Wear Resource. Boca Raton, FL: CRC Press, 1997, pp. 117–137. 14. Kreuz, K.L. Gasoline engine chemistry as applied to lubricant problems. Lubrication 55: 53–64, 1969. 15. Bouman, C.A. Properties of Lubricating Oils and Engine Deposits. London: MacMillan and Co., 1950, pp. 69–92. 16. Kreuz, K.L. Diesel engine chemistry as applied to lubricant problems. Lubrication 56: 77–88, 1970. 17. Chamberlin, W.B., J.D. Saunders. Automobile engines. In R.E. Booser, ed. CRC Handbook of Lubrication, Vol. I, Theory and Practice of Tribology: Applications and Maintenance. Boca Raton, FL: CRC Press, 1983, pp. 3–44. 18. Obert, E.F. Basic engine types and their operation. In Internal Combustion Engines and Air Pollution. New York: Intext Educational Publishing, 1968, pp. 1–25. 19. Schilling, A. Antioxidant and anticorrosive additives. In Motor Oils and Engine Lubrication. London: Scientific Publications, 1968, Section 2.8, p. 2.61. 20. Patterson, D.J., N.A. Henein. Emissions from Combustion Engines and Their Control. Ann Arbor, MI: Ann Arbor Science Publishers, 1972. 21. Lachowicz, D.R., K.L. Kreuz. Peroxynitrates. The unstable products of olefin nitration with dinitrogen tetroxide in the presence of oxygen. A new route to α-nitroketones. Journal of the Organic Chemistry 32: 3885–3888, 1967. 22. Covitch, M.J., R.T. Graf, D.T. Gundic. Microstructure of carbonaceous diesel engine piston deposits. Lubricant Engineering 44: 128, 1988. 23. Covitch, M.J., J.P. Richardson, R.T. Graf. Structural aspects of European and American diesel engine piston deposits. Lubrication Science 2: 231–251, 1990. 24. Nahamuck, W.M., C.W. Hyndman, S.A. Cryvoff. Development of the PV-2 engine deposit and wear test. An ASTM Task Force Progress Report, SAE Publication 872,123. Presented at International Fuels and Lubricants Meeting and Exposition. Toronto, Canada, November 2–5, 1987. 25. Rasberger, M. Oxidative degradation and stabilization of mineral oil based lubricants. In R.M. Mortier, S.T. Orszulik, eds. Chemistry and Technology of Lubricants. New York: VCH Publishers, Inc., 1992, pp. 83–123.

Dispersants

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26. Oliver, C.R., R.M. Reuter, J.C. Sendra. Fuel efficient gasoline-engine oils. Lubrication 67:1–12, 1981. 27. Stone, R. Introduction to Internal Combustion Engines, Society of Automotive Engineers, 1993. 28. Rizvi. S.Q.A. Additives and additive chemistry. ASTM Manual on Fuels and Lubricants, to be published in 2003. 29. Stuart, F.A., R.G. Anderson, A.Y. Drummond. Lubricating-oil compositions containing alkenyl succinimides of tetraethylene pentamine. U.S. Patent 3,361,673 (January 2, 1968). 30. Cooper, A.R. Molecular weight determination. In J.I. Kroschwitz, ed. Concise Encyclopedia of Polymer Science and Engineering. New York: Wiley Interscience, 1990, pp. 638–639. 31. Ravve, A. Molecular weights of polymers. In Organic Chemistry of Macromolecules. New York: Marcel Dekker, 1967, pp. 39–54. 32. Deanin, R.D. Polymer Structure, Properties, and Applications. New York: Cahner Books, 1972, p. 53. 33. Randall, J.C. Microstructure. In J.I. Kroschwitz, ed. Concise Encyclopedia of Polymer Science and Engineering. New York: Wiley Interscience, 1990, p. 625. 34. Fotheringham, J.D. Polybutenes. In L.R. Rudnick, R.L. Shubkin, eds. Synthetic Lubricants and HighPerformance Functional Fluids, 2nd edition, New York: Marcel Dekker, 1999. 35. Randles, S.J. et al. Synthetic base fluids. In R.M. Mortier, S.T. Orszulik, eds. Chemistry and Technology of Lubricants. New York: VCH Publishers, 1992, pp. 32–61. 36. Gutierrez, A., R.A. Kleist, W.R. Song, A. Rossi, H.W. Turner, H.C. Welborn, R.D. Lundberg. Ethylene alpha-olefin polymer substituted mono- and dicarboxylic acid dispersant additives. U.S. Patent 5,435,926 (July 25, 1995). 37. Gutierrez, A., W.R. Song, R.D. Lundberg, R.A. Kleist. Novel ethylene alpha-olefin copolymer substituted mannich base lubricant dispersant additives. U.S. Patent 5,017,299 (May 21, 1991). 38. Stambaugh, R.L. Viscosity index improvers and thickeners. In R.M. Mortier, S.T. Orszulik, eds. Chemistry and Technology of Lubricants. New York: VCH Publishers, 1992. 39. Bryant, C.P., H.M. Gerdes. Nitrogen-containing esters and lubricants containing them. U.S. Patent 4,604,221 (August 5, 1986). 40. Shanklin, J.R., Jr., N.C. Mathur. Lubricating oil additives. U.S. Patent 6,071,862 (June 6, 2000). 41. Morris, J.R., R. Roach. Lubricating oils containing metal derivatives. U.S. Patent 2,628,942 (February 17, 1953). 42. Sparks, W.J., D.W. Young, Roselle, J.D. Garber. Modified olefin–diolefin resin. U.S. Patent 2,634,256 (April 7, 1953). 43. Le Suer, W.M., G.R. Norman. Reaction product of high molecular weight succinic acids and succinic anhydrides with an ethylene polyamine. U.S. Patent 3,172,893 (March 9, 1965). 44. Meinhardt, N.A., K.E. Davis. Novel carboxylic acid acylating agents, derivatives thereof, concentrate and lubricant compositions containing the same, and processes for their preparation. U.S. Patent 4,234,435 (November 18, 1980). 45. Rense, R.J. Lubricant. U.S. Patent 3,215,707 (November 2, 1965). 46. Weill, J., B. Sillion. Reaction of chlorinated polyisobutene with maleic anhydride: Mechanism of catalysis by dichloromaleic anhydride. Revue de I’Institut Francais du Petrole 40(1): 77–89, 1985. 47. J. Weill. Ph. D. dissertation, 1982. 48. Weill, J., J. Garapon, B. Sillion. Process for manufacturing anhydrides of alkenyl dicarboxylic acids. U.S. Patent 4,433,157 (February 21, 1984). 49. Baumanis, C.K., M.M. Maynard, A.C. Clark, M.R. Sivik, C.P. Kovall, D.L. Westfall. Treatment of organic compounds to reduce chlorine level. U.S. Patent 5,708,097 (January 13, 1998). 50. Pudelski, J.K., M.R. Sivik, K.F. Wollenberg, R. Yodice, J. Rutter, J.G. Dietz. Low chlorine polyalkylene substituted carboxylic acylating agent compositions and compounds derived thereform. U.S. Patent 5,885,944 (March 23, 1999). 51. Pudelski, J.K., C.J. Kolp, J.G. Dietz, C.K. Baumanis, S.L. Bartley, J.D. Burrington. Low chlorine content composition for use in lubricants and fuels. U.S. Patent 6,077,909 (June 20, 2000). 52. Wollenberg, K.F., J.K. Pudelski. Preparation, NMR characterization and lubricant additive application of novel polyisobutenyl phthalic anhydrides. Symposium on Recent Advances in the Chemistry of Lubricant Additives. Paper presented before the Division of Petroleum Chemistry, Inc., 218th National Meeting of the American Chemical Society, New Orleans, LA, August 22–26, 1999. 53. Harrison, J.J., R. Ruhe, Jr., R. William. One-step process for the preparation of alkenyl succinic anhydride. U.S. Patent 5,319,030 (June 7, 1994). 54. Harrison, J.J., R. Ruhe, Jr., R. William. Two-step thermal process for the preparation of alkenylsuccinic anhydride. U.S. Patent 5,625,004 (April 29, 1997).

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55. Morrison, R.T., R.N. Boyd. The Diels–Alder reaction. In Organic Chemistry, 3rd edition, Boston, MA: Allyn and Bacon, 1976, Section 27.8, pp. 876–878. 56. Alkylation of phenols. In Kirk-Othmer Encyclopedia of Chemical Technology,. New York: Interscience Publishers, 1963, Vol. 1, pp. 894–895. 57. Ion exchange. In Kirk-Othmer Encyclopedia of Chemical Technology,. New York: Interscience Publishers, 1967, Vol. 2, pp. 871–899. 58. McAtee, J.R. Aromatic Mannich compound-containing composition and process for making same. U.S. Patent 6,179,885 (January 30, 2001). 59. Merger, F., G. Nestler. Manufacture of alkylphenol compounds. U.S. Patent 4,202,199 (May 13, 1980). 60. Kolp, C.J. Methods for preparing alkylated hydroxyaromatics. U.S. Patent 5,663,457 (September 2, 1997). 61. Adams, P.E., M.R. Baker, J.G. Dietz. Hydroxy-group containing acylated nitrogen compounds useful as additives for lubricating oil and fuel compositions. U.S. Patent 5,696,067 (December 9, 1997). 62. Pudelski, J.K. Mixed carboxylic compositions and derivatives and use as lubricating oil and fuel. U.S. Patent 6,030,929 (February 29, 2000). 63. Baker, M.R., J.G. Dietz, R. Yodice. Substituted carboxylic acylating agent compositions and derivatives thereof for use in lubricants and fuels. U.S. Patent 5,912,213 (June 15, 1999). 64. Baker, M.R. Acylated nitrogen compounds useful as additives for lubricating oil and fuel compositions. U.S. Patent 5,856,279 (January 5, 1999). 65. Baker, M.R., K.M. Hull, D.L. Westfall. Process for preparing condensation product of hydroxy-substituted aromatic compounds and glyoxylic reactants. U.S. Patent 6,001,781 (December 14, 1999). 66. Ethylene amines. In Kirk-Othmer Encyclopedia of Chemical Technology, 2nd edition. New York: Interscience Publishers, 1965, Vol. 7, pp. 22–37. 67. Morrison, R.T., R.N. Boyd. Amines II. Reactions. Organic Chemistry, 3rd edition. Boston, MA: Allyn and Bacon, 1976, pp. 745–748. 68. Steckel, T.F. High molecular weight nitrogen-containing condensates and fuels and lubricants containing same. U.S. Patent 5,053,152 (October 1, 1991). 69. Pindar, J.F., J.M. Cohen, C.P. Bryant. Dispersants and process for their preparation. U.S. Patent 3,980,569 (September 14, 1976). 70. Harmon, J., F.M. Meigs. Artificial resins and method of making. U.S. Patent 2,098,869 (November 9, 1937). 71. March, J. Aminoalkylation and amidoalkylation. In Advanced Organic Chemistry, Reactions, Mechanisms, and Structure, 4th edition. New York: Wiley-Interscience, 1992, pp. 550–551. 72. Schallenberg, E.E., R.G. Lacoste. Ethylenediamine salts of thiphosphonic acids. U.S. Patent 3,185,728 (May 25, 1965). 73. Schlicht, R.C. Friction reducing agents for lubricants. U.S. Patent 3,702,824 (November 14, 1972). 74. Brois, S.J. Olefin-thionophosphine sulfide reaction products, their derivatives, and use thereof as oil and fuel additives. U.S. Patent 4,042,523 (August 16, 1977). 75. Brois, S.J. Olefin-thionophosphine sulfide reaction products, their derivatives, and use thereof as oil and fuel additives. U.S. Patent 4,100,187 (July 11, 1978). 76. Kapusciniski, M.M., B.J. Kaufman, C.S. Liu. Oil containing dispersant VII olefin copolymer. U.S. Patent 4,715,975 (December 29, 1987). 77. Kapuscinski, M.M., R.E. Jones. Dispersant-antioxidant multifunction viscosity index improver. U.S. Patent 4,699,723 (October 13, 1987). 78. Kapuscinski, M.M., T.E. Nalesnik, R.T. Biggs, H. Chafetz, C.S. Liu. Dispersant anti-oxidant VI improver and lubricating oil composition containing same. U.S. Patent 4,948,524 (August 14, 1994). 79. Goldblatt, I., M. McHenry, K. Henderson, D. Carlisle, N. Ainscough, M. Brown, R. Tittel. Lubricant for use in diesel engines. U.S. Patent 6,187,721 (February 13, 2001). 80. Lange, R.M., C.V. Luciani. Graft copolymers and lubricants containing such as dispersant-viscosity improvers. U.S. Patent 5,298,565 (March 29, 1994). 81. Sutherland, R.J. Dispersant viscosity index improvers. U.S. Patent 6,083,888 (July 4, 2000). 82. Stambaugh, R.L., R.D. Bakule. Lubricating oils and fuels containing graft copolymers. U.S. Patent 3,506,574 (April 14, 1970). 83. Trepka, W.J. Viscosity index improvers with dispersant properties prepared by reaction of lithiated hydrogenated copolymers with 4-substituted aminopyridines. U.S. Patent 4,402,843 (September 6, 1983). 84. Trepka, W.J. Viscosity index improvers with dispersant properties prepared by reaction of lithiated hydrogenated copolymers with substituted aminolactams. U.S. Patent 4,402,844 (September 6, 1983).

Dispersants

169

85. Seebauer, J.G., C.P. Bryant. Viscosity improvers for lubricating oil composition. U.S. Patent 6,124,249 (September 26, 2000). 86. Adams, P.E., R.M. Lange, R. Yodice, M.R. Baker, J.G. Dietz. Intermediates useful for preparing dispersant-viscosity improvers for lubricating oils. U.S. Patent 6,117,941 (September 12, 2000). 87. Lange, R.M., C.V. Luciani. Dispersant-viscosity improves for lubricating oil composition. U.S. Patent 5,512,192 (April 30, 1996). 88. Lange, R.M. Dispersant-viscosity improvers for lubricating oil compositions. U.S. Patent 5,540,851 (July 30, 1996). 89. Hayashi, K., T.R. Hopkins, C.R. Scharf. Graft copolymers from solvent-free reactions and dispersant derivatives thereof. U.S. Patent 5,429,758 (July 4, 1995). 90. Nalesnik, T.E. Novel VI improver, dispersant, and anti-oxidant additive and lubricating oil composition containing same. U.S. Patent 4,863,623 (September 5, 1989). 91. Mishra, M.K., I.D. Rubin. Functionalized graft co-polymer as a viscosity index improver, dispersant, and anti-oxidant additive and lubricating oil composition containing same. U.S. Patent 5,409,623 (April 25, 1995). 92. Kapuscinski, M.K., C.S. Liu, L.D. Grina, R.E. Jones. Lubricating oil containing dispersant viscosity index improver. U.S. Patent 5,520,829 (May 28, 1996). 93. Sutherland, R.J. Process for making dispersant viscosity index improvers. U.S. Patent 5,486,563 (January 23, 1996). 94. Bryant, C.P., B.A. Grisso, R. Cantiani. Dispersant-viscosity improvers for lubricating oil compositions. U.S. Patent 5,969,068 (October 19, 1999). 95. Kiovsky¸ T.E. Star-shaped dispersant viscosity index improver. U.S. Patent 4,077,893 (March 7, 1978). 96. Patil, A.O. Multifunctional viscosity index improver-dispersant antioxidant. U.S. Patent 5,439,607 (August 8, 1995). 97. Baranski, J.R., C.A. Migdal. Lubricants containing ashless antiwear-dispersant additive having viscosity index improver credit. U.S. Patent 5,698,5000 (December 16, 1997). 98. Sutherland, R.J., R.B. Rhodes. Dispersant viscosity index improvers. U.S. Patent 5,360,564 (November 1, 1994). 99. Brady, J.E., G.E. Humiston. Chemical bonding: General concepts—polar molecules and electronegativity. In General Chemistry: Principles and Structure, 2nd edition. New York: Wiley, 1978, pp. 114–117. 100. Diana, W.B., J.V. Cusumano, K.R. Gorda, J. Emert, W.B. Eckstrom, D.C. Dankworth, J.E. Stanat, J.P. Stokes. Dispersant additives and process. U.S. Patents 5,804,667 (September 8, 1998) and 5,936,041 (August 10, 1999). 101. Degonia, D.J., P.G. Griffin. Ashless dispersants formed from substituted acylating agents and their production and use. U.S. Patent 5,241,003 (August 31, 1993). 102. Emert, J., R.D. Lundberg, A. Gutierrez. Oil soluble dispersant additives useful in oleaginous compositions. U.S. Patent 5,026,495 (June 25, 1991). 103. Sung, R.L., B.J. Kaufman, K.J. Thomas. Middle distillate containing storage stability additive. U.S. Patent 4,948,386 (August 14, 1990). 104. Ratner, H., R.F. Bergstrom. Non-ash containing lubricant oil composition. U.S. Patent 3,189,544 (June 15, 1965). 105. Norman, G.R., W.M. Le Suer. Reaction products of hydrocarbon-substituted succinic acid-producing compound, an amine, and an alkenyl cyanide. U.S. Patents 3,278,550 (October 11, 1966) and 3,366,569 (June 30, 1968). 106. Morrison, R.T., R.N. Boyd. Hydrolysis of amides, pp. 671–672; Alkaline and acidic hydrolysis of esters, pp. 677–681. In Organic Chemistry, 3rd edition. Boston, MA: Allyn and Bacon, 1976. 107. Baczek, S.K., W.B. Chamberlin. Petroleum additives. In Encyclopedia of Polymer Science and Engineering, 2nd edition. New York: Wiley, 1998, Vol. 11, p. 22. 108. Klamann, D. Viscosity–temperature (VT) function. In Lubricants and Related Products. Weinheim, Germany: Verlag Chemie, 1984, pp. 7–12. 109. Schilling, A. Viscosity index improvers. In Motor Oils and Engine Lubrication. London: Scientific Publications, 1968, pp. 2.28–2.43. 110. Engine oil viscosity classification. SAE J300–Revised December 1999, SAE Standard, Society of Automotive Engineers. 111. Adams, D.R., P. Brice. Multigrade lubricating compositions containing no viscosity modifier. U.S. Patent 5,965,497 (October 12, 1999).

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112. Emert, J., R.D. Lundberg. High functionality low molecular weight oil soluble dispersant additives useful in oleaginous compositions. U.S. Patent 5,788,722 (August 4, 1998). 113. Emert, J., A. Rossi, S. Rea, J.W. Frederick, M.W. Kim. Polymers derived from ethylene and 1-butene for use in the preparation of lubricant dispersant additives. U.S. Patent 6,030,930 (February 29, 2000). 114. Hart, W.P., C.S. Liu. Lubricating oil containing dispersant VII and pour depressant. U.S. Patent 4,668,412 (May 26, 1987). 115. Song, W.R., A. Rossi, H.W. Turner, H.C. Welborn, R.D. Lundberg, A. Gutierrez, R.A. Kleist. Ethylene alpha-olefin/diene interpolymer-substituted carboxylic acid dispersant additives. U.S. Patents 5,759,967 (June 2, 1998) and 5,681,799 (October 28, 1997). 116. Song, W.R., A. Rossi, H.W. Turner, H.C. Welborn, R.D. Lundberg. Ethylene alpha-olefin polymer substituted mono- and dicarboxylic acid dispersant additives. U.S. Patent 5,433,757 (July 18, 1995). 117. Song, W.R., R.D. Lundberg, A. Gutierrez, R.A. Kleist. Borated ethylene alpha-olefin copolymer substituted Mannich base lubricant dispersant additives. U.S. Patent 5,382,698 (January 17, 1995). 118. Harrison, J.J., W.A. Ruhe, Jr. Polyalkylene polysuccinimides and post-treated derivatives thereof. U.S. Patent 6,146,431 (November 14, 2000). 119. Ready reference for lubricant and fuel performance. Lubrizol Publication. Available at http://www. lubrizol.com. 120. Stachew, C.F., W.D. Abraham, J.A. Supp, J.R. Shanklin, G.D. Lamb. Engine oil having dithiocarbamate and aldehyde/epoxide for improved seal performance, sludge and deposit performance. U.S. Patent 6,121,211 (September 9, 2000). 121. Viton seal compatible dispersant and lubricating oil composition containing same. U.S. Patent 5,188,745 (February 23, 1993). 122. Nalesnik, T.E., C.M. Cusano. Dibasic acid lubricating oil dispersant and viton seal additives. U.S. Patent 4,663,064 (May 5, 1987). 123. Nalesnik, T.E. Lubricating oil dispersant and viton seal additives. U.S. Patent 4,636,332 (January 13, 1987). 124. Scott, R.M., R.W. Shaw. Dispersant additives. U.S. Patent 6,127,322 (October 3, 2000). 125. Fenoglio, D.J., P.R. Vettel, D.W. Eggerding. Method for preparing engine seal compatible dispersant for lubricating oils comprising reacting hydrocarbyl substituted dicarboxylic compound with aminoguanidine or basic salt thereof. U.S. Patent 5,080,815 (January 14, 1992). 126. Cunningham, L.J., D.P. Hollrah, A.M. Kulinowski. Compositions for control of induction system deposits. U.S. Patent 5,679,116 (October 21, 1997). 127. Ashjian, H., M.P. Miller, D-M. Shen, M.M. Wu. Deposit control additives and fuel compositions containing the same. U.S. Patent 5,334,228 (August 2, 1994). 128. Mulard, P., Y. Labruyere, A. Forestiere, R. Bregent. Additive formulation of fuels incorporating ester function products and a detergent-dispersant. U.S. Patent 5,433,755 (July 18, 1995). 129. Gear and transmission lubricant compositions of improved sludge-dispersibility, fluids comprising the same. U.S. Patent 5,665,685 (September 9, 1997). 130. Otani, H., R.J. Hartley. Automatic transmission fluids and additives thereof. U.S. Patent 5,441,656 (August 15, 1995). 131. O’Halloran, R. Hydraulic automatic transmission fluid with superior friction performance. U.S. Patent 4,253,977 (March 3, 1981). 132. Ichihashi, T., H. Igarashi, J. Deshimaru, T. Ikeda. Lubricating oil composition for automatic transmission. U.S. Patent 5,972,854 (October 26, 1999). 133. Kitanaka, M. Automatic transmission fluid composition. U.S. Patent (September 28, 1999). 134. Srinivasan, S., D.W. Smith, J.P. Sunne. Automatic transmission fluids having enhanced performance capabilities. U.S. Patent 5,972,851 (October 26, 1999). 135. Conary, G.S., R.J. Hartley. Gear oil additive concentrates and lubricants containing them, U.S. Patent 6,096,691 (August 1, 2000). 136. Lubricating oil composition for high-speed gears. U.S. Patent 5,756,429 (May 26, 1998 Ichihashi, Toshihiko). 137. Ryan, H.T. Hydraulic fluids. U.S. Patent 5,767,045 (June 16, 1988). 138. Forester, D.R. Use of dispersant additives as process antifoulants. U.S. Patent 5,368,777 (November 29, 1994). 139. Forester, D.R. Multifunctional antifoulant compositions. U.S. Patent 4,927,561 (May 22, 1990). 140. Forester, D.R. Multifunctional antifoulant compositions and methods of use thereof. U.S. Patent 4,775,458 (October 4, 1988).

Part 2 Film-Forming Additives

Selection and Application of Solid Lubricants as Friction Modiﬁers Gino Mariani

CONTENTS 6.1 Introduction ........................................................................................................................... 173 6.2 Solid Lubricant Properties .................................................................................................... 175 6.2.1 Graphite ..................................................................................................................... 175 6.2.1.1 Sources of Graphite..................................................................................... 177 6.2.1.2 Lubrication .................................................................................................. 178 6.2.2 Molybdenum Disulfide.............................................................................................. 179 6.2.3 Boron Nitride ............................................................................................................ 180 6.2.4 Polytetrafluoroethylene ............................................................................................. 181 6.3 Preparation for Lubricant Application .................................................................................. 182 6.4 Applications .......................................................................................................................... 185 6.4.1 Wear Protection and General Lubrication................................................................. 185 6.4.2 Lubrication for Plastic Deformation of Metals ......................................................... 189 References ...................................................................................................................................... 194

6.1

INTRODUCTION

Solid lubricants are considered to be any solid material that reduces friction and mechanical interactions between surfaces in relative motion against the action of a load. Solid lubricants offer alternatives to the lubricant formulator for situations where traditional liquid additives fall short on performance. An example is a high-temperature lubrication condition in which oxidation and decomposition of the liquid lubricant will certainly occur, resulting in lubrication failure. Another example is for situations that generate high loads and contact stresses on bearing points of mating surfaces, producing a squeeze-out of the liquid lubricant and a resulting lubricant starvation (see Figure 6.1). Solid lubricants, used as a dry film or as an additive in a liquid, provide enhanced lubrication for many different types of applications. Typical hot-temperature applications include oven chain lubrication and metal deformation processes such as hot forging. Solid lubricants are also helpful for ambient-temperature applications such as drawing and stamping of sheet metal or bar stock. Solid lubricants are effectively used in antiseize compounds and threading compounds, which provide a sealing function and a friction reduction effect for threaded pipe assembly [1]. Applications involving low sliding speeds and high contact loads, such as for gear lubrication, also benefit from solid lubricants. The solid lubricant effectively provides the required wear protection and loadbearing performance necessary from gear oil, especially capable when used with lower-viscosity base oils.

173

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Lubricant Additives: Chemistry and Applications Spot welds

Liquid lubricant is squeezed out No welds

Solid lubricant stays in there

FIGURE 6.1

Contact stresses on bearing points of mating surfaces cause a squeeze-out.

Solid lubricants also assist applications where the sliding surfaces are of a rough texture or surface topography. Under this circ*mstance, the solid lubricant is more capable than liquid lubricants for covering the surface asperity of the mating surfaces. A typical application is a reciprocating motion that requires lubrication to minimize wear. Another application for solid lubricants is for cases where chemically active lubricant additives have not been found for a particular surface, such as polymers or ceramics. In this case, a solid lubricant would function to provide the necessary protection to the mating surfaces, which would normally occur due to the reaction of a liquid component with the surface [2]. Graphite and molybdenum disulfide (MoS2) are the predominant materials used as solid lubricants. These pigments are effective load-bearing lubricant additives due to their lamellar structure. Because of the solid and crystalline nature of these pigments, graphite and MoS2 exhibit favorable tolerance to high-temperature and oxidizing atmosphere environments, whereas liquid lubricants typically will not survive. This characteristic makes graphite and molybdenum disulfide lubricants necessary for processes involving extreme temperatures or extreme contact pressures. Other compounds that are useful solid lubricants include boron nitride, polytetrafluoroethylene (PTFE), talc, calcium fluoride, cerium fluoride, and tungsten disulfide. Any one of these compounds may be more suitable than graphite or MoS2 for specific applications. Boron nitride and PTFE are discussed along with graphite and molybdenum disulfide in this chapter. What are the basic requirements for an effective solid lubricant? Five properties must be met in a favorable way [3]. 1. Yield strength. This refers to the force required to break through the lubricant or deform its film. There should be high yield strength to forces applied perpendicular to the lubricant. This will provide the required boundary lubrication and protection to loads between the mating surfaces. Low yield strength of the film should be present in the direction of sliding to provide reduced coefficient of friction. This dependency on directional application of forces is considered an anisotropic property. 2. Adhesion to substrate. The lubricant must be formulated in a manner that maintains the lubricant film on the substrate for a sufficient period necessary for the lubrication requirements. The force of adhesion should exceed that of the sheer forces applied to the film. Any premature adhesion failure will result in a nonprotective condition between the two sliding surfaces that require lubrication. 3. Cohesion. Individual particles in the film of solid lubricant should be capable of building a layer thick enough to protect the high asperities of the surface and to provide a “reservoir” of lubricant for replenishment during consumption of the solid film (see Figures 6.2 and 6.3). 4. Orientation. The particles used must be oriented in a manner that parallels the flow of the stress forces and provide the maximum opportunity for a reduction in the coefficient of friction. For this to occur, it is necessary for the dimensions of the particles to be greatest in the direction of low shear.

Solid Lubricants as Friction Modiﬁers

FIGURE 6.2

Surface asperities.

FIGURE 6.3

Burnished lubricant.

175

5. Plastic flow. The lubricant should not undergo plastic deformation when loads are applied directly perpendicular to the direction of motion. The solid should be able to withstand the intimate contact between the mating surfaces so that a continuous film of lubrication is maintained. This chapter attempts to guide the formulator toward making successful choices in solid lubricants. It briefly summarizes the physical and chemical properties of the solid lubricant and discusses the merits of each type of major lubricant as well as the recommended application. The information will assist in understanding the chemistry of the lubricant and its general mechanism of lubrication.

6.2 SOLID LUBRICANT PROPERTIES 6.2.1

GRAPHITE

Graphite is most effective for applications involving high-temperature and high load-carrying situations. These capabilities make graphite the solid lubricant of choice for forging processes. Solid lubricants such as MoS2 will oxidize too rapidly to be of any value at the typical hot-forging temperature range of 760–1200°C, although MoS2 has a greater lubrication capability than graphite.

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Why is graphite such a good lubricant? The answer lies in the platelet, lamellar structure of the graphite crystallite (see Figure 6.4). Graphite is structurally composed of planes of polycyclic carbon atoms that are hexagonal in orientation. Short bond lengths between each carbon atom within the plane are the result of strong covalent bonds (see Figure 6.4). Weaker van der Waals forces hold together a number of planes to create the lattice structure. The d-spacing bond distance of carbon atoms between planes is longer and, therefore, weaker than the bond distance between carbon atoms within the planes. As a force is applied perpendicular to the crystallite, a strong resistance is applied against the force. This high yield strength provides the load-carrying capacity for the lubricant. Concurrent with the force applied perpendicular to the substrate is a sliding force applied parallel to the direction of sliding. The weak bond between the planes allows for easy shearing of the planes in the direction of the force. This creates a cleaving of the planes and results in friction reduction. The lamellar motion of graphite cleavage can be illustrated by the concept of a hand applying a force on a deck of playing cards as shown in Figure 6.5. Forces applied perpendicular to the deck are resisted by the stack’s thickness 1.415 Å

A d −Spacing 3.354 Å B

FIGURE 6.4

Structure of graphite.

FIGURE 6.5

Representation of lamellar lubrication.

Solid Lubricants as Friction Modiﬁers

177

TABLE 6.1 Coefﬁcients of Friction Provided by Graphite Films Test Method

Graphite Film

Three-ball slider Bowden–Leben machine

Unlubricated Metal

0.09–0.12 0.07–0.10

Mineral Oil on Metal

0.16–0.18 0.40

0.15–0.17 0.17–0.22

TABLE 6.2 Natural Graphite Amorphous % Carbon % Sulfur % SiO2 % Ash Mesh

–85.0 –0.30 6.0–7.0 10–15 −325

Crystalline Flake 1 90–95 0.15–0.20 0.20–0.30 7–10 −325

Crystalline Flake 2 96–98 0.10–0.70 0.05–0.2 2.0–3.0 −325

and yield strength. Yet, a far easier force is required to rupture the stack when the force is applied parallel along the face of the deck, resulting in the shearing of the cards. The effects of the lamellar structure of graphite can be observed when sliding conditions are applied onto metal surfaces. Coefficient of friction data can be generated by various bench test methods for measuring the lubricity of sliding conditions. In comparison to unlubricated or oil-lubricated metal surfaces, graphite provides excellent lubricity [4]. This is summarized in Table 6.1. 6.2.1.1

Sources of Graphite

There are many types and sources of graphite. These sources influence the properties of the graphite, which affects the performance of the end product that uses graphite. Graphite is characterized by two main groupings: natural and synthetic. Natural graphite is derived from mining operations worldwide. The ore is processed to recover the usable graphite. Varying quality of the graphite will be evident from the ore quality and the postmining processing of the ore. High-purity natural graphite will normally be highly lubricating and resistant to oxidation. This is due to the high degree of crystal structure and graphitization usually associated with naturally derived graphite. Natural graphite of lesser quality is also available. A lower total carbon content and a lower degree of graphitization characterize the lesser quality. The end product is graphite that is more amorphous in nature, with a higher content of ash components, which are mostly oxides of silicon and iron. Lubrication functionality decreases as crystallinity and graphitization decrease. Lubrication functionality also decreases as total ash content of the graphite increases. Commercially available natural graphite is provided in a variety of grades. The suitability of the grades depends on the intended application and economic constraints. Table 6.2 characterizes examples of commercially available natural graphite. Selecting the type of natural graphite to use is based on the degree of lubrication required for the application, the particle size of the graphite necessary for the application, and the economic constraint. For situations where the lubrication demand is severe, a high-carbon crystalline flake or crystalline vein graphite is desired. The high degree of crystallinity and graphitization provides superior lubrication. A more economical alternative is the lower-carbon-content flake graphite.

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TABLE 6.3 Synthetic Graphite Typical Values

% Carbon % Sulfur % SiO2 % Ash

Primary

Secondary

99.9 Trace 0.02 0.1

99.9 0.01 0.05 0.1

For most situations, these types of graphite perform adequately in lubricating conditions that do not require the purity and lubricity of higher-quality crystalline graphite. For occasions where only minor lubricity is needed and perhaps a more thermally insulating coating is required, amorphous graphite would be chosen. Amorphous graphite is also the least expensive of the commercially available natural graphite grades. Combining amorphous and crystalline graphite can also be done to modify the amount of lubrication to suit the requirements of the application. Synthetic graphite is an alternative source for lubricating graphite. Synthetic graphite is characterized as primary or secondary grade (see Table 6.3). Primary grade is derived synthetically from production within an electric furnace, utilizing calcined petroleum co*ke as well as very high temperatures and pressures to produce the graphite. The result is usually a product of high purity and can approach the quality of natural graphite flake in terms of percent graphitization and lubrication capability. Secondary synthetic graphite is derived from primary graphite that has been used for the fabrication of electrodes. This type of graphite is usually less lubricating than natural or primary grades of graphite because of its lesser degree of crystallinity and graphitization and the presence of binding agents and surface oxides that do not contribute to lubrication. Secondary synthetic graphite is perfectly capable of lubricating effectively for many applications that can afford a lesser degree of lubricity. The chief benefit in using secondary synthetic graphite is the cost, with the secondary graphite costing significantly less than primary-grade synthetic graphite or high-purity natural graphite. 6.2.1.2 Lubrication Appropriate-quality graphite is able to meet the five criteria for an effective solid lubricant. Graphite possesses the necessary yield strength for successful lubrication. It is able to adhere sufficiently to metal surfaces due to its affinity to metal and its packing within and above the microstructure of the surface. Graphite has a burnishing capacity desirable for lubrication mechanisms that require a “memory” effect. Proper orientation of graphite particles is achieved by the natural tendency for the graphite crystal to orient itself parallel to the substrate and in the direction of lowest shear. The anisotropic characteristic of graphite lends itself well to its lubricating capability and friction reduction property. The planar orientation of the graphite particles on the substrate takes advantage of the anisotropic property. Proper orientation allows the lamellar functionality of graphite where easy shear is achieved along the crystal plane when sliding forces are put along the length of the particles. The high yield strength in graphite is maintained in the direction perpendicular to the direction of shear force, providing for the load-carrying capability. Graphite is best suited for lubrication in a regular atmosphere. Water vapor is a necessary component for graphite lubrication. The role that adsorbed water vapor plays in the lubricating properties of graphite has been studied [5]. It is theorized that water vapor helps to reduce the surface energy of the graphite crystallite. The adsorption of a water monolayer onto the planar surface of

Solid Lubricants as Friction Modiﬁers

179

the graphite likely reduces the bonding energy between the hexagonal planes of the graphite to a level that is lower than the adhesion energy between a substrate and the graphite crystal. This allows for lamellar displacement of the graphite crystals when shear forces are applied to the graphite film. The result is a reduction of friction and corresponding lubrication. Because water vapor is a requirement for lubrication, graphite is usually not effective as a lubricant in a vacuum atmosphere. The lubricating ability of graphite as a function of temperature is very good. Graphite is able to withstand continuous temperatures of up to 450°C in an oxidizing atmosphere and still provide effective lubrication. The oxidation stability of graphite depends on the quality of the graphite, the particle size, and the presence of any contaminants that might accelerate the oxidation. Graphite will also function at much higher temperatures on an intermittent basis. Peak oxidation temperatures are typically near 675°C. For these instances, modifying the composition of the graphite mixture may be necessary as a way to control its rate of oxidation. The thermal conductivity of graphite is generally low. For example, primary-grade synthetic graphite has a conductivity of ~1.3 W/mK at 40°C. Amorphous graphite is even less conducting and is sometimes considered for providing some degree of thermal insulation for specific applications.

6.2.2

MOLYBDENUM DISULFIDE

Molybdenum disulfide is the second significant solid lubricant widely used in industry. It has been used since the early nineteenth century for lubrication applications. MoS2, also known as molybdenite, is a mined material found in thin veins within granite. Lubricating-grade MoS2 is highly refined by various methods to achieve a purity suitable for lubricants [6]. This purity usually exceeds 98%. MoS2 is commercially available in a variety of particle size ranges. Table 6.4 lists basic properties for molybdenum disulfide. The low friction of MoS2 is an intrinsic property related to its crystal structure, whereas graphite requires the adsorption of water to behave as an effective lubricant. Molybdenum disulfide achieves its lubricating ability with a mechanism similar to graphite. Just like graphite, MoS2 has a hexagonal crystal lattice structure. Sandwiches of planar hexagonal Mo atoms are interspersed between two layers of sulfur atoms. Similar to graphite, the bond strength between the hexagonal planes between the sulfur atoms are weak van der Waal-type bonds when compared to the strong covalent bond between molybdenum and sulfur atoms within the hexagonal crystal. Orientation of the MoS2 crystallites is important if effective friction reduction is to be achieved. MoS2 has anisotropic properties that are comparable to graphite. When a force is applied parallel along the hexagonal planes, the weak bond strengths between the planes allow for easy shearing of the crystal, resulting in a lamellar

TABLE 6.4 Characteristics of Hexagonal Molybdenum Disulﬁde Property Bulk hardness Coefficient of friction Color Electrical conductivity Luster Melting point Molecular weight Service temperature Specific gravity Thermal conductivity

Value 1.0–1.5 Ʊ 0.10–0.15 Blue-gray to black Semiconductor Metallic >1800°C 160.08 Up to 700°F 4.80–5.0 0.13 W/mK at 40°C

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Lubricant Additives: Chemistry and Applications

mechanism of lubrication. At the same time, the crystal structure and strong interplanar bond forces of MoS2 allow for high load carrying against forces applied perpendicular to the plane of the crystal. This is necessary for the prevention of metal on metal contact for high-load applications such as gearbox lubrication. MoS2 scores well in the other criteria for an effective solid lubricant. It forms a strong cohesive film, that is smoother than the surface of the substrate on which it is bonded. MoS2 film has sufficiently high adhesion to most metal substrates, which it successfully burnishes onto the wearing surfaces, thus minimizing metal wear and prolonging friction reduction. This characteristic is an exception, however, with titanium and aluminum substrates due to the presence of an oxide layer on the metal surface, which tends to reduce the tenacity of the MoS2 film. The lubrication performance of MoS2 often exceeds that of graphite. It is most effective for high load-carrying lubrication when temperatures are <400°C. Another advantage of MoS2 is that it lubricates in dry, vacuum-type environments, whereas graphite does not. This is due to the intrinsic lubrication property of MoS2. On the contrary, the lubricating ability of MoS2 deteriorates in the presence of moisture because of oxidation of MoS2 to MoO3. The temperature limitation of MoS2 is due to similar decomposition issues of the material as that experienced with moisture. As MoS2 continues to oxidize, MoO3 content increases, which induces abrasive behavior and increases coefficient of friction for the surfaces to be lubricated. The effectiveness of MoS2 improves as contact forces increase on the lubricated surface. Burnished surfaces exhibit coefficient of friction reduction as a function of increasing contact forces [7]. In contrast, graphite does not necessarily exhibit this behavior. The frictional property of MoS2 systems has been reported to be generally better than graphite in many instances, up to the service temperature limitations for the lubricant. The particle size and film thickness of MoS2 will affect lubrication. Generally, the particle size should be matched to the surface roughness of the substrate and the type of lubrication process considered. Too large a particle distribution may result in excessive wear and film reduction as mechanical abrasion is experienced. Too fine a particle size may result in accelerated oxidation in normal atmospheres as the high surface area of the particles promotes the rate of oxidation.

6.2.3

BORON NITRIDE

Boron nitride is a ceramic lubricant with interesting and unique properties. Its use as a solid lubricant is typically for niche applications when performance expectations render graphite or molybdenum disulfide unacceptable. The most interesting lubricant feature of boron nitride is its high-temperature resistance. Boron nitride has a service temperature of 1200°C in an oxidizing atmosphere, which makes it desirable for applications that require lubrication at very high service temperatures. Graphite and molybdenum disulfide cannot approach such higher service temperatures and still remain intact. Boron nitride also has a high thermal conductivity property, making it an excellent choice for lubricant applications that require rapid heat removal. A reaction process generates boron nitride. Boric oxide and urea are reacted at temperatures from 800 to 2000°C to create the ceramic material. Two chemical structures are available: cubic and hexagonal boron nitride. As one might expect, the hexagonal boron nitride is the lubricating version. Cubic boron nitride is a very hard substance used as an abrasive and cutting tool component. Cubic boron nitride does not have any lubrication value. The hexagonal version of boron nitride is analogous to graphite and molybdenum disulfide. The structure consists of hexagonal rings of boron and nitrogen, which are connected to each other, forming a stack of planar hexagonal rings. As with graphite, boron nitride exhibits a platelet structure. The bond strength within the rings is strong. The planes are stacked and held together by weaker bond forces. Similar to graphite and molybdenum disulfide, this allows for easy shearing of the planes when a force is applied parallel to the plane. The ease of shear provides the expected friction reduction and resulting lubrication. Concurrently, the high bond strength between boron

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181

TABLE 6.5 Hexagonal Boron Nitride Property Coefficient of friction Color Crystal structure Density Dielectric constant Dielectric strength Molecular weight Service temperature Thermal conductivity Size (grades)

Value 0.2–0.7 White Hexagonal 2.2–2.3 g/cm3 4.0–4.2 ∼35 kV/mm 24.83 1200°C (Oxidizing atmosphere) ∼55 W/mK 1–10 µm

and nitrogen within the hexagonal rings provides the high load-carrying capability that is necessary to maintain metal–metal separation of the substrates. Similar to MoS2, boron nitride has intrinsic lubrication properties. Boron nitride effectively lubricates in a dry as well as a wet atmosphere. It is very resistant to oxidation, more so than either graphite or MoS2, and maintains its lubricating properties up to its service temperature limit. Commercial grades are available in a variety of purities and particle sizes. These varieties influence the degree of lubrication provided by boron nitride since particle size affects the degree of adhesion to substrate, burnishing ability, and particle orientation within a substrate. Impurities such as boric oxide content need to be considered with respect to the lubrication capability of boron nitride powder since this will influence the ability of the powder to reduce the coefficient of friction for an application. The variation in grades will also influence the thermal conductivity properties and ease of suspension in a liquid carrier. Table 6.5 summarizes typical properties for hexagonal boron nitride.

6.2.4

POLYTETRAFLUOROETHYLENE

Polytetrafluoro-ethylene (PTFE) has been in use as a lubricant since the early 1940s. Structurally, the polymer is a repeating chain of substituted ethylene with four fluorine atoms on each ethylene unit: ⫺ (CF2 ᎏ CF2 )n ⫺

(6.1)

Contrary to the other lubricants discussed, PTFE does not have a layered lattice structure. The lubrication properties are at least partially the result of its high softening point. As frictional heat begins to increase from sliding contact, the polymer maintains its durability and is able to lubricate. Various grades are produced and applied to specific applications as a result of the properties imparted by the grade. For example, molecular weight and particle size are two characteristics that can alter the performance of the polymer as a lubricant. The critical characteristic of PTFE—the one it is widely known for [8]—is the outstandingly low coefficient of friction imparted by the molecule. PTFE has one of the smallest coefficients of static and dynamic friction than any other solid lubricant. Values as low as 0.04 for sliding conditions have been reported for various combinations of PTFE films on substrates [9]. The lowfriction property is attributed to the smooth molecular profile of the polymer chains, which orient in a manner that facilitates easy sliding and slip. It is postulated that the PTFE polymer results in rod-shaped macromolecules, which can slip along each other, similar to lamellar structures.

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TABLE 6.6 Typical Physical Properties of PTFE Property Coefficient of friction (ASTM D1894) Dielectric constant Hardness Melting point Service temperature Specific gravity

Value 0.04–0.1 2.1–2.4 50–60 Shore D 327°C Up to 260°C 2.15–2.20

Its chemical inertness makes it useful in cryogenic to moderate operating temperatures and in a variety of atmospheres and environments. Operating temperatures are limited to ∼260°C due to the decomposition of the polymer. One consideration in using PTFE is the cold weld property of the material. This could eliminate its use for some applications where extreme pressure is encountered. Such pressure may result in the destruction of the polymer particle and in the lubrication failure, as the PTFE congeals and fails to remain intact on the rubbing surface. PTFE finds many uses in bonded film lubrication at ambient temperature. These applications include fasteners, threading compounds, and chain lubrication and engine oil treatments. PTFE is widely used as an additive in lubricating greases and oils, for both industrial and consumer applications (see Table 6.6 for basic properties). Although difficult to accomplish due to the low surface energy of PTFE, colloidal dispersions of PTFE in oil or water can be produced. This is useful for applications requiring the stable suspension of PTFE particles in the lubricating medium such as for crankcase oil or hydraulic oil. The nature and feedstock of the PTFE influence the ability to create a stable, unflocculated dispersion, which is necessary for effective lubrication.

6.3 PREPARATION FOR LUBRICANT APPLICATION For a lubricant to be effective, the solid has to be applied in a manner that provides an effective interface between the mating substrates that require wear protection or lubrication. Dry-powder lubrication can be used, but it is limited in its scope of application. In other words, the dry powder can be sprinkled onto the load-bearing substrate. By a combination of the rubbing action from sliding and the natural adhesion properties of the solid lubricant, some measure of attachment to the substrate will occur by burnishing to provide lubricating protection [10]. MoS2 seems to function particularly well from this manner of application, as it has an effective burnishing capability. The use of free powder has limitations. The films tend to have a short duration of service since adhesion is usually insufficient to provide any longevity for a continuous application. The use of dry powder also makes it difficult in many circ*mstances to accurately apply the lubricant to the place intended, with the possible exception of tumbling metal billets for achieving a coating over phosphated substrates. This can be overcome by the use of bonded films. Bonded films will provide a strong adhesion to the substrate requiring protection. It also allows for a more controlled rate of film wear, which depends on the properties of the bonding agent and the film thickness of the bonded film. Bonded films can be achieved by a number of ways, all by use of secondary additives that promote a durable and longerlasting film. The intended application will dictate the appropriate type of bonding agent. For applications of continual service, resin and polymer bonding agents are typically used. These include phenolic resins, acrylics, celluloses, epoxies, polyimides, and silicones. Some of the binders such as epoxies are

Solid Lubricants as Friction Modiﬁers

183

curable at room temperatures. Others such as the phenolic resins require elevated-temperature curing. Service temperature may be the limiting consideration for the chosen bonding agent. To overcome service temperature limitations, alternative type bonding agents are also widely used. Most typical are inorganic salts such as alkali silicates, borates, and phosphates. These types of salts overcome temperature limitations of organic bonding agents, transferring the burden of temperature consideration to the solid lubricant. Conversely, the use of inorganic salts as bonding agents typically does not provide for a coating life that is as durable as an organic bonded coating. This usually limits the application to those requiring constant replenishment of the lubricant. To facilitate the application of the solid lubricant, dispersion in a liquid is most commonly used. The liquid can be a solvent, oil, synthetic oil, or water. Suspension within a liquid allows for the easy and precise application of the solid lubricant to the intended areas that require protection. Compared to dry-powder application, film control is easily achieved through spray, dip, or flow methods onto the substrate. Environmental cleanliness is also improved since the solid particles are entrapped within a liquid matrix, preventing the airborne dispersion of the particles. For applications in which the solid lubricant is a secondary additive in a liquid, proper suspension is critical for achieving effective lubrication. A consideration for liquid suspensions is that the shelf life of the lubricant is limited. Because the particles require suspension within a liquid carrier, eventual sedimentation of the solid lubricant will occur. This necessitates proper mixing procedures for the handling of the suspension to provide for consistent lubricant performance within the stated shelf life of the material. Adjustment to formulations with respect to dispersion and viscosity controls will influence the time it takes for the suspension to destabilize. The quality of the suspension will also determine how easily the settled pigment is redispersed with mild agitation (see Figure 6.6). To create the suspension, the solid lubricant particles require treatment of the particle surface to make it amenable to suspension within the carrier liquid. This is similar to paint, where the colorant is chemically treated to provide the required dispersion characteristics and form what is considered a colloidal suspension (see Figure 6.7). This treatment is necessary to maximize the available particles for lubrication and provide the degree of dispersion stability required for the job. Without such treatment, particle agglomeration and rapid sedimentation will occur. This would negatively influence the application of the lubricant onto the substrate in a manner that creates an inferior and ineffective film. Wetting agents and suspending agents such as polymeric salts, starches, and polyacrylics are used to treat the surface of the solid lubricant to render it capable of suspension within the liquid carrier. When creating the dispersion, the particle size distribution of the solid lubricant has to be considered. Small, submicron particles are easier to suspend and retain physical stability than large, coarse particles. To this end, milling action on the solid lubricant is usually necessary to alter the size distribution to the desired range of sizes (see Figure 6.8).

FIGURE 6.6

Particle sedimentation.

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Lubricant Additives: Chemistry and Applications

FIGURE 6.7

Colloidal dispersion.

%CHANNEL 20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0 0.100

0.0 1.000

10.00

100.0

1000

Size (µm)

FIGURE 6.8

Particle size distribution of colloidal graphite suspension.

Fine sized particles are not necessarily the best distribution for a particular lubricating application (see Figure 6.9). Some consideration is required for the most beneficial particle size to match up with the surface roughness and nature of the application. This consideration could run contrary to what is the best particle size for dispersion stability. Therefore, some degree of compromise may be necessary to achieve a balance of dispersion stability and lubrication performance.

Solid Lubricants as Friction Modiﬁers

185

%CHANNEL 20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0

0.0 1.000

0.100

10.00

100.0

1000

Size (µm)

FIGURE 6.9

Coarse graphite particle size distribution.

Some type of substrate preparation for the load-bearing surface may be required to facilitate the application of the solid lubricant. This is usually necessary for metal deformation processes so that the film thickness, film uniformity, and durability of the applied lubricant on a billet will be robust enough to lubricate. Typical treatments of the surface include phosphating, peening, and shot blasting, which are especially useful for powder tumbling applications. With water-based dispersions, heating the substrate to some elevated temperature is often necessary to activate the bonding agents. Substrate heating serves a dual purpose: it facilitates the evaporation of the water carrier, and it also initiates the physical/chemical bonding of the film onto the substrate.

6.4

APPLICATIONS

Two major lubrication applications are considered here: metal wear protection lubrication and lubrication for plastic deformation of metal. The former concerns applications such as constant sliding or reciprocating motion, for example, gear, chain, or journal lubrication. The latter concerns applications where metal is under plastic flow, such as metal-forming or metal-cutting applications.

6.4.1

WEAR PROTECTION AND GENERAL LUBRICATION

Wear protection and general lubrication applications are meant to include processes requiring hydrodynamic lubrication, elastohydrodynamic lubrication, and boundary lubrication. Examples of such applications include chain lubrication, gear lubrication, and engine oil treatments. In essence, any application where repetitive sliding or rolling contact occurs between two surfaces can be considered under the umbrella of wear protection lubrication. The intention is for the lubricant to reduce the coefficient of friction and protect against wear (see Figure 6.10). The benefits include savings in power consumption and service life of the component and efficiency gains due to the increased uptime resulting from proper lubrication. Solid lubricants are useful and required for applications and conditions when conventional liquid lubricants are inadequate. These conditions include the following: 1. High operating temperatures that eliminate or reduce the functionality of the liquid lubricant 2. Contact pressure of sufficient magnitude that breaches the integrity of the liquid lubricant

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Lubricant Additives: Chemistry and Applications

5 Kg

1 Kg

FIGURE 6.10

Lubrication of sliding surfaces—friction reduction.

TABLE 6.7 Worm Gear Dynamometer Tests Performance Parameters Output Torque = 113 N m Description AGMA #8 gear oil AGMA #8 gear oil + 1% colloidal MoS2 dispersion AGMA #7 gear oil AGMA #7 gear oil + 1% colloidal MoS2 dispersion Synthetic PAG #2 oil Synthetic PAG #2 oil + 1% MoS2 dispersion

Mean Input Torque (N m)

Percent Efﬁciency

Mean Oil Sump Temperature (°C)

6.02 5.92

62.6 63.6

92.1 95.5

6.05 5.89

62.3 64.0

93.6 93.4

6.09 5.79

61.8 65.1

108.8 88.4

Source: Pacholke, P.J., Marshek, K.M., Improved worm gear performance with colloidal molybdenum disulfide containing lubricants, ASLE paper presented at the 41st Annual Meeting in Toronto, Ontario, Canada, May 12–15, 1986.

3. 4. 5. 6. 7.

Performance enhancement that extends the capability of the conventional liquid lubricant Performance enhancement that extends the service life of the conventional liquid lubricant Applications that undergo a “start/stop” routine Applications that require low sliding speed but heavy bearing load Applications that require “fool-proofing” for potential catastrophic lubrication failures that result from lubricant starvation

For successful incorporation of a solid lubricant as a secondary additive into liquid lubricants, a well-formulated colloidal dispersion is required. As an example, consider a case study where gear oil performance is enhanced above that of a conventional liquid lubricant by use of colloidal solids. The addition of 1% colloidal molybdenum disulfide to AGMA No. 7 and AGMA No. 8 gear oils reduced the break-in times and steady-state operating temperatures of low-viscosity synthetic oils as compared to nonfortified gear oils [11]. Table 6.7 summarizes a comparison of the performance of various blended gear oils to the measured output criteria as tested on a worm gear dynamometer. Another example concerns the potential lubrication improvement from solid lubricants for friction-modified engine oils. Because of the burnishing property that solid lubricants such as colloidal graphite or colloidal MoS2 would have on metal surfaces, friction reduction in engine and

Solid Lubricants as Friction Modiﬁers

187

axle components might be expected. Along with friction reduction, there should be a corresponding increase in fuel efficiency for motor vehicles. Various studies seem to support that conclusion. One report claims that in fleet trials conducted according to EPA 55/45 fuel economy testing with reference motor oils fortified with either MoS2 or graphite, both in a colloidal dispersion, the fuel economy was improved by 4.5% [12]. In another fuel economy study using a fleet of taxicabs, the use of 2% colloidal graphite or colloidal MoS2 in low-viscosity-formulated engine oils and rear axle lubricants improved the fuel economy by 2.5% [13]. The friction-reducing influence of colloidal graphite in oil is illustrated in one study by a dynamometer evaluation conducted on a 2.3 L engine [14]. The study indicates that graphite properly dispersed in an appropriate liquid lubricant will considerably reduce friction with the subsequent benefit of fuel economy savings. Solid lubricants are also applied as bonded films for certain applications. For example, applications requiring a permanent or semipermanent lubricating film would require a bonded film. Bonded coatings are commonly formulated with MoS2 or PTFE. One example would be for selflubricating composites that require high-temperature stability, such as for what may be needed for engine piston ring protection [15]. Other examples that benefit from a bonded lubricant include fasteners, chains, and reciprocating mechanisms that require a persistent lubricating film. For these applications, PTFE stands out due to its low coefficient of friction. This is summarized in Table 6.8 by comparative coefficient of friction data for PTFE, graphite, and MoS2, which are bonded onto cold-rolled steel substrates. In assessing the lubrication potential for dispersed solid lubricants, some type of bench testing is utilized to characterize the apparent lubrication performance of the material. The most typical lubrication tests are Shell 4-Ball Wear method, Shell 4-Ball EP method, Falex Pin–Vee method, Plint Reciprocating method, Incline Plane method, and FZG Gear Lubrication method. In many cases, custom lubrication tests are developed for the specific application to be considered. When conducting bench testing for lubricant performance, correlation is best achieved when the mode of contact and conditions of the application are closely replicated by the bench test. The configuration of the contact points for the application is matched with a similar mode of contact for the bench test. For an illustration of laboratory lubrication assessments, see Table 6.9 [16] to compare the empirical performance of the four solid lubricants dispersed in an oil carrier. The lubricants were tested according to two common methods of lubrication evaluation. In this example, the dispersion of MoS2 and PTFE provides effective load bearing, wear resistance, and coefficient of friction reduction when evaluated by a point-to-point contact (4-ball) and line-to-point contact (Falex Pin–Vee). Interpretation of any bench test result must be done carefully to ensure the validity of extrapolating the test performance to the actual application. What criteria should be considered for an application when selecting the preferred or optimal solid lubricant? First, consider the service temperature for the application. This dictates which solid

TABLE 6.8 Coefﬁcient of Friction for Bonded ﬁlms Coefﬁcient of Frictiona MoS2 Graphite PTFE a

0.23 0.15 0.07

Evaluated at room temperature, ASTM D4918.

Source: Watari, K., Huang, H.J., Turiyama, M., Osuka, A., Yamamoto, O., U.S. Patent 5,985,802, 11/16/99.

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Lubricant Additives: Chemistry and Applications

TABLE 6.9 Bench Lubrication Test Results Four-Ball Lubrication Test Wear ASTM D-4172

Base oil With 1% colloidal graphite With 1% colloidal MoS2 With 1% colloidal PTFE With 1% colloidal BN

Extreme Pressure ASTM D-2783

Falex Lubrication Test Wear ASTM D-2670

EP ASTM D-3233

Coefﬁcient of Friction

20 kg mm

40 kg mm

Weld (kg)

Load Wear Index (kg)

Teeth

lb to Failure

Calculated

0.678 0.695

1.060 0.855

126 160

17.20 18.7

Fail 78

875 1000

0.159 0.132

0.680

0.805

200

24.3

4375

0.077

0.50

0.84

200

29.04

4500+

0.0568

0.37

0.72

126

19.9

Fail

500

0.1602

Source: Acheson colloids test data.

lubricant can be used. For example, MoS2 generally has a higher load-carrying capability than graphite. Yet, at service temperatures above 400°C, MoS2 degrades and loses its lubricating capacity. MoS2 is, therefore, eliminated from consideration if the service temperature is above 400°C. The second consideration is environment. Atmospheric restrictions will eliminate the use of certain solid lubricants. For example, a vacuum environment will eliminate the use of graphite. As mentioned previously, graphite requires adsorption of water molecules to its surface to function as an effective lubricant. MoS2, on the contrary, as well as PTFE and boron nitride have intrinsic lubrication properties and do not require water molecules on their surface to provide friction reduction value. The third criterion is the nature of the lubricant; either a liquid fortified with solid lubricant additives or a bonded solid lubricant film. Some pigments are easier to disperse in liquid than others. For example, graphite and MoS2 are comparatively easier to disperse in liquids than PTFE and boron nitride. This is mostly due to particle size-reducing capability, surface energy, and surface chemistry of the solid lubricant. The particle size of the pigment has an influence on lubrication performance. The size of the particulate and the size distribution of the particles should be optimized for the application (see Figure 6.11). For example, larger particles tend to give better performance for applications that are slow in speed or oscillating in nature. Large particles also tend to give better performance on substrates where the surface roughness is relatively coarse. A finer particle size tends to provide superior results for applications with constant motion and high speeds. Finer particles tend to function better where the surface roughness is relatively fine. Although not always predictable, the influence of particle size needs to be considered not only for dispersion requirements but also for the intended use application. The fourth criterion involves cost-effectiveness of the lubricant. When the application conditions are met with two or more solid lubricants, cost will dictate the choice. Generally, graphite will be the least expensive. High-purity graphite is more expensive than lower-purity natural graphite or secondary synthetic graphite, which are more expensive than low-quality graphite. Molybdenum disulfide will be next, followed by PTFE and boron nitride as the more expensive solid lubricants. Cost-effectiveness for any of the solid lubricants will be influenced by the quality of the lubricant

Solid Lubricants as Friction Modiﬁers

FIGURE 6.11

189

Orientation of solid lubricant particles in the direction of motion.

TABLE 6.10 Solid Lubricant Selection Comparison and Rating Criteria Normal atmosphere Vacuum atmosphere Ambient temperature Continuous service temperature to 260°C in air Continuous service temperature to 400°C in air Continuous service temperature to 450°C in air Burnishing capability Hydrolytic stability Thermal conductivity Load-carrying lubrication Friction reduction Dispersability Color Relative cost

Graphite

MoS2

PTFE

Boron Nitride

1 3 1 1

1 1 1 1

N/A

1 1 2 2 2 1 Black 1

1 2 3 1 2 1 Gray 2

3 1 3 1 1 3 White 2

2 1 1 2 3 2 White 3

Note: 1 = best, 2 = good, 3 = ok

and formulation that utilizes the lubricant. The effectiveness of the final formulation may prove that a costlier solid lubricant is more cost-effective in use. Table 6.10 attempts to rate the effectiveness of the solid lubricants for various criteria of application.

6.4.2

LUBRICATION FOR PLASTIC DEFORMATION OF METALS

Lubrication requirements for assisting metal deformation operations such as forging and metal drawing are far more demanding than those for wear lubrication. The metal movement process creates very fast metal flow and rapid new surface generation. This creates a demand for a lubricant to flow with the metal, remain adhered to the surface, maintain sufficient film cohesion to “meter” out the lubricant with the advancing metal, and interact rapidly with the newly formed metal surface. Metal-forming operations are inherently high-load and high-stress processes, which put a significant demand on protective lubrication. Most applications are conducted at an elevated temperature region. Under this circ*mstance, conventional liquid lubricants fail to withstand the stresses for the application. Solid lubricants

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Lubricant Additives: Chemistry and Applications

are most appropriate for such applications because of their ability to withstand the operating temperatures, orient and adhere to the substrate surface, provide the coefficient of friction reduction necessary to promote metal flow, and provide the required load-carrying properties to prevent metal-on-metal contact. Indeed, most applications that involve plastic deformation of metal will utilize solid lubricants as either the primary or the secondary lubricant within a formulation. What application criteria are used for determining the necessity for a solid lubricant? Severity of metal movement is the most significant factor. In cases where it is judged that metal movement would be considered extreme, solid lubricants will most likely be required. Application examples include forward, backward, and extreme lateral extrusion of metals. For example, forging of spindles, constant velocity (CV) joints, crankshafts, and hubs would fall in this category. For these and similar cases, liquid lubricant technology falls short of providing the necessary lubrication, coefficient of friction reduction, and die wear protection. Once it has been determined that a solid lubricant is necessary, the temperature criteria need to be determined. Metalworking applications done at ambient temperature can utilize MoS2 as the solid lubricant. MoS2 has the best lubrication properties among the four lubricants discussed. In fact, for applications such as cold forging, MoS2 is the preferred lubricant because of its ability to handle the very high load and stress applied onto the part being deformed. In some cases, application of the MoS2 is by dry-powder tumbling of the billets. Usually the billets are phosphated before applying powder to anchor the MoS2 onto the surface and within the structure of the phosphate coating. The phosphate coating acts as an anchor for the powder and allows the lubricant to advance with the metal deformation. Table 6.11 compares forging performance for bare versus coated steel. Lubrication is improved as press tonnage falls and spike height of the forged billet increases. Dry-powder tumbling is an effective application method for some cases. Other situations will require a more detailed and accurate depositing of MoS2 film onto the substrate. This requires the use of a dispersed MoS2 to provide a controlled coating thickness and particle size distribution considered appropriate for the job. There may be instances where MoS2 is not desirable—for example, environmental concerns or housekeeping issues. In these instances, PTFE or boron nitride would be appropriate. The white color of the pigments alleviates concerns regarding cleanliness of using graphite and molybdenum disulfide. Situations that require a reduction in emissions and material reactivity would favor boron nitride since PTFE will decompose at typical warm and hot forging temperatures. Both would effectively lubricate, with perhaps boron nitride faring better than PTFE for applications with significant metal flow. PTFE can, however, stand out as a lubricant for cold metal-forming operations involving sheet stock and bar stock. The low coefficient of friction imparted by PTFE will provide the necessary lubrication to assist metal flow in a manner far better than boron nitride and much cleaner than graphite or molybdenum disulfide. All the solid lubricants would be appropriate for bonded-film applications for metal deformation processes. Bonded films are desirable for sheet metal applications where coil or blank metal is

TABLE 6.11 Cold-Forging Lubrication Sample Bare steel Bare steel + zinc phosphate Bare steel + zinc phosphate + MoS2 Source: Acheson Colloids test data.

Press Tonnage

Spike Height (mm)

80.2 79.6 78.4

10.67 11.11 11.46

Solid Lubricants as Friction Modiﬁers

191

prepared with a dry-film lubricant. When developing bonded-film lubricants, consider the formulation of effective binders and bonding agents so that the solid lubricant can function as intended. For metalworking applications at elevated temperatures, the operating temperature will determine which solid lubricant can be used. All the solid lubricants mentioned would be suitable for temperatures up to 260°C. Above that temperature, PTFE will be eliminated from consideration due to its decomposition. MoS2 will be suitable for applications up to 400°C in an oxidizing environment. Above that temperature, decomposition of MoS2 will occur. Both graphite and boron nitride will lubricate effectively above an operating temperature of 400°C. Graphite is the predominant lubricant used for plastic deformation at elevated temperatures. The use of graphite is common and preferred for what is considered warm- and hot-forging situations. The forging process is considered warm forging when billet temperatures are up to 950°C. The process is considered hot forging when billet temperatures exceed 950°C. In both cases, oxidation of graphite will occur. But the rate of oxidation depends on temperature and is regulated by the formulation and characteristics of graphite. Graphite quality, contaminants, crystallite size, and particle size will influence the rate of oxidation. The components of the finished formulation also play a role in controlling the oxidation rate of graphite, allowing it to survive for an appropriate length of time necessary for lubricating the process. The type and quality of graphite play an important role in performance. Its consideration is the first step in a selection process. The first choice is to choose between natural and synthetic graphites. Often the choice is dictated by the degree of graphite quality suitable for the application. For instances where average lubrication is required, natural graphite of lesser quality can be used. More demanding lubrication will require the use of high-purity synthetic or natural graphite. Selection of the particle size of graphite will vary depending on the intentions for the job. Particle size should be matched to the type of metal movement expected from the process, the surface roughness of the die and part, and the degree of stability required for the formulated lubricant. If a large particle distribution is desired, then concern about physical stability of the lubricant must be addressed. Rapid settling and hard packing of graphite could occur due to the large particle size if countermeasures are not taken. This would create handling costs and product inconsistency for the end user. For most circ*mstances, high-quality graphite should be used so as to minimize performance inconsistency. The quality and characteristic of graphite can affect the lubricating performance. Table 6.12 illustrates a lubricity comparison of standard formulations produced with different graphites. In this example, the application is warm forging of steel. Actual forging of a steel billet generates lubrication data where the spike height is determined using preset forging press parameters (see Figure 6.12). A greater spike height and lower coefficient of friction suggest better lubrication from the coating. Once the type of graphite to be used is selected, then the cost of the powder needs to be considered versus the benefit derived from its use. In general, high-purity natural or primary synthetic graphite will be costlier than secondary synthetic graphite. However, the performance benefit of using the higher-cost material may justify its selection for the application. Benefits normally associated with

TABLE 6.12 Graphite Inﬂuence on Forging Lubrication (800°C Forging Temperature) Graphite A B C

Spike Height (mm)

Coefﬁcient of Friction

1.5 1.3 1.1

0.05 0.08 0.10

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Lubricant Additives: Chemistry and Applications

FIGURE 6.12

Deformed billet and spike.

TABLE 6.13 Lubrication Comparison of Forging Lubricants (800°C Forging Temperature) Lubricant Graphite A Graphite B Nongraphite lubricant

Spike Height (mm) 1.5 1.3 0.7

Coefﬁcient of Friction 0.05 0.08 0.15

the higher-cost materials are consistency, lubricating performance, and reduced oxidation rates of graphite. The chosen graphite should be of a specific particle size distribution to derive certain benefits in performance. These benefits include the ease of dispersing graphite into a liquid carrier, the stability of graphite within the concentrated product, the application and film formation of the product onto the workpiece, and the optimized lubrication for the deformation process. Forging processes normally require a temporary bond of the lubricant onto the workpiece and tool. This is achieved by the use of the type of bonding agents mentioned previously in this chapter. The use of dry powder or simple liquid–powder mixes will not perform adequately because of the poor adhesion onto the substrate. To illustrate the value of graphite for hot-temperature metalworking applications, consider the example cited in Table 6.13. A comparison is made between two formulated graphite products and a nongraphite product tested under the same procedures of warm forging. In this example, the degree of spike height and coefficient of friction generated by the forging process are determined. The lower spike height and higher coefficient of friction for the nongraphite lubricant are indications of reduced lubrication capability in comparison to the graphite-containing materials. In certain instances, graphite is not desirable due to either the operating temperature or concern about housekeeping and cleanliness. Hexagonal boron nitride is a capable alternative to graphite for these conditions. It is considered the “white graphite” due to its lamellar structure. It has a reasonably low coefficient of friction that approaches and sometime exceeds that of graphite. It is able to withstand operating temperatures up to 1200°C in oxidizing environments. This makes boron nitride an effective material for high-alloy isothermal forging, where extremely high temperatures

Solid Lubricants as Friction Modiﬁers

193

and long contact times are encountered. A profile of oxidation characteristics provides a comparison of oxidation stability between boron nitride and graphite (see Figure 6.13). The ability for boron nitride to remain intact at a very high temperature makes it ideal for applications that require a long residency time for lubricant coating. 108

0.2 Residue: 107.2% (12.72 mg)

Weight (%)

0.0 −7.517% (−0.8918 mg)

104

− 0.2 102

− 0.4

0.3150% (0.03737 mg)

100

98 0

200

Derivation weight (%°C)

106

400

600

Temperature (°C) 110

800

−0.6 1000

Universal V2.3C TA Instruments 1.2

691.17°C

100 1.0

0.8

Weight (%)

70 60

0.6

50 0.4

40 30

Derivation weight (%°C)

0.2

20 10

0.0

0 0.8139%

−10 0

200

400

600

Temperature (°C)

800

−0.2 1000

General V4.0D DuPont 2100

FIGURE 6.13 Comparison of peak oxidation temperatures of boron nitride and graphite. (From Acheson Colloids test data.)

194

Lubricant Additives: Chemistry and Applications

Another advantage of using boron nitride is the heat conductivity property of the material. For applications that would require rapid heat dissipation, boron nitride serves quite well and is superior to graphite in that regard. Thermal conductivity values of boron nitride powder will vary depending on its quality. But, boron nitride in any of its grades is invariably more thermally conductive than graphite or MoS2. Applications such as high-performance cutting oils are claimed to deliver benefits of enhanced lubrication and heat withdrawal when finely dispersed submicron particles of boron nitride are incorporated into the fluid [17].

REFERENCES 1. Jacobs, N.L. U.S. Patent 5,180,509. 1/19/93. 2. Ludema, K.C. Friction, Wear, Lubrication, A Textbook in Tribology. Boca Raton, FL: CRC Press, 1996, p. 123. 3. Acheson Colloids company, J. Brian Peace Lecture. 4. Clauss, F.J. Solid Lubricants and Self-Lubricating Solids. New York: Academic Press, 1972, p. 45. 5. Savage, R.H. Graphite lubrication. J Appl Phys 19:1, 1948. 6. Barry, H.F. Factors relating to the performance of MoS2 as a lubricant. J Am Soc Lubr Eng 33(9):475–480, 1977. 7. Kohli, A.K., B. Prakash. Contact pressure dependency in frictional behavior of burnished molybdenum disulphide coatings. Tribology Trans 44(1), 2001. 8. Du Pont Teflon® Fluoroadditives brochure. 9. Bowden, F.P., D. Tabor. The Friction and Lubrication of Solids. New York: Oxford University Press, 1986, p. 165. 10. Kaur, R.G., C.F. Higgs, H. Hesmat. Pin-on-disc tests of pelletized molybdenum disulfide. Tribology Trans 44:79–87, 2001. 11. Pacholke, P.J., K.M. Marshek. Improved worm gear performance with colloidal molybdenum disulfide containing lubricants. ASLE paper presented at the 41st Annual Meeting in Toronto, Ontario, Canada, May 12–15, 1986. 12. Haviland, M.L., M.C. Goodwin. Fuel economy improvements with friction-modified engine oils in Environmental Protection Agency and road tests. Society of Automotive Engineers Technical Paper 790,945, Oct. 1979. 13. Haviland, M.L., J.L. Linden. Taxicab fuel economy and engine and rear axle durability with low viscosity and friction modified lubricants. Society of Automotive Engineers Technical Paper 821,227 Oct. 1982. 14. Broman, V.E. et al. Testing of friction modified crankcase oils for improved fuel economy. Society of Automotive Engineers Technical Paper 780,597, June 1978. 15. Peters, J.A. U.S. Patent, 5,702,769. 12/30/97. 16. Acheson Colloids test data. 17. Watari, K., H.J. Huang, M. Turiyama, A. Osuka, O. Yamamoto. U.S. Patent 5,985,802. 11/16/99. 18. ZYP Coatings technical data sheet, Boron Nitride Powders for Research and Industry. 19. Booser, R.E. Theory and Practice of Tribology, Vol. II. Theory and Design. Boca Raton, FL: CRC Press, 1983, p. 276.

Organic Friction Modiﬁers Dick Kenbeck and Thomas F. Bunemann

CONTENTS 7.1 7.2

Introduction ........................................................................................................................... 195 Friction and Lubrication Regimes ........................................................................................ 196 7.2.1 Friction Reduction through the Lubricant ................................................................. 197 7.3 Friction Modifiers versus Antiwear/Extreme-Pressure Additives ........................................ 199 7.4 Chemistry of Organic Friction Modifiers .............................................................................200 7.4.1 Friction Modifier Mechanisms ..................................................................................200 7.4.1.1 Formation of Reacted Layers ......................................................................200 7.4.1.2 Formation of Absorbed Layers ................................................................... 201 7.4.1.3 Formation of In Situ Polymers ....................................................................202 7.5 Chemistry of Other Friction Modifiers .................................................................................202 7.5.1 Metallo-Organic Compounds ....................................................................................202 7.5.2 Mechanical Types ......................................................................................................203 7.6 Factors Influencing Friction-Reduction Properties ...............................................................203 7.7 Friction Modifiers: Current Practice .....................................................................................203 7.8 Friction Modifier Performance .............................................................................................204 7.8.1 Stribeck Curve Determinations .................................................................................205 7.8.2 Friction as a Function of Temperature ......................................................................207 7.9 Consequences of New Engine Oil Specifications and Outlook ............................................208 7.10 Bench Tests to Investigate Friction-Reducing Compounds ..................................................209 References ......................................................................................................................................209

7.1 INTRODUCTION Friction modifiers (FMs) or friction reducers have been applied for several years. Originally, the application was for limited slip gear oils, automatic transmission fluids, slideway lubricants, and multipurpose tractor fluids. Such products made use of friction modification to meet requirements for smooth transition from static to dynamic condition as well as reduced noise, frictional heat, and startup torque. Since fuel economy became an international issue, initially to reduce crude oil consumption, FMs have been introduced into automotive crankcase lubricants, as well, to improve fuel efficiency through the lubricant. In the United States, additional pressure is imposed on original equipment manufacturers (OEMs) by the corporate average fuel economy (CAFE) regulation. Following the introduction of vehicle exhaust emission regulations in various regions around the world, emphasis on friction reduction further increased. This can be well understood if it is realized that 20–25% of the energy generated in an engine by burning fuel is lost through friction [1]. The biggest part is lost by friction on the piston liner/piston ring interface and a smaller part by bearing and valve train friction. It is predicted that in future engines the contribution of the piston group to engine friction will increase up to 50% [2].

195

196

Lubricant Additives: Chemistry and Applications

Reduction of fuel consumption and emissions can be achieved through [3] engine design changes and modifications, such as • • • • • •

Application of roller followers Use of coatings Surface modifications Material selection Fuel quality The engine lubricant

All these aspects are looked at and applied in the automotive industry. This chapter concentrates on the engine lubricant. The need to measure fuel savings has led to the development of American Petroleum Institute (API) test sequences such as VI and VIA in the United States. Sequence VIB will be used for International Lubricant Standards Approval Committee GF-3. In Europe, a fuel economy test has been developed by Conseil Européen de Co-ordination pour le Dévelopments des Essais de Performance des Lubrifiants et der Combustible pour Moteurs (CEC) (test number CEC L-54-T-96) for the Association des Constructeurs Européens d’automobiles A1 and B1 specifications using the DBM 111 engine. Both tests require that the candidate lubricant shows decreased fuel consumption relative to reference oil.

7.2 FRICTION AND LUBRICATION REGIMES Friction is defined as the resistance a body meets while moving over another body in respect of transmitting motion. The friction coefficient is defined as FW Fn

(7.1)

where F W is the frictional force and Fn the normal force or load. For a lubricated surface, the coefficient of friction is determined by the lubrication regime. In simple terms, the following three lubricant regimes can be distinguished: 1. Elasto-hydrodynamic lubrication (EHL) regime characterized by a (relatively) thick lubricant film [4]. The mating surfaces are far enough from one another to prevent metal-to-metal contact. The load on the system is completely carried by the lubricant film, and the viscosity of the lubricant determines the friction coefficient. Viscosity depends on temperature and pressure/viscosity coefficient. 2. Boundary lubrication (BL) regime characterized by a thin lubricant film [5]. Under high loads, high temperature, or with low viscosity oils, most of the lubricating film is squeezed out between the metal surfaces, and metal-to-metal contact occurs. The load is entirely carried by the metal asperities. A thin layer of absorbed or otherwise deposited molecules is necessary to prevent the two surfaces and their asperities from plowing into one another. 3. Mixed lubrication (ML) regime characterized by a lubricant film of intermediate thickness [6]. The two metal surfaces have come closer compared to hydrodynamic lubrication, and metal-to-metal contact occasionally occurs. The load is carried by both the lubricant and the asperities. These regimes are related to the friction coefficient f by a lubricant parameter defined as su su or F p

(7.2)

Organic Friction Modiﬁers

197

Boundary lubrication

Friction coefficient f

Mixed lubrication

Elasto-hydrodynamic lubrication EHL Lubricant parameter (viscosity × speed/load)

FIGURE 7.1

Stribeck curve at high contact pressure.

Boundary lubrication

Friction coefficient f

Mixed lubrication

Elasto-hydrodynamic lubrication EHL Lubricant parameter (viscosity × speed/load)

FIGURE 7.2

Stribeck curve at low contact pressure.

where s = system speed u = lubricant dynamic viscosity F = load (Fn) p = contact pressure The so-called Stribeck curve gives the relationship between f and these lubricant parameters. The shape of the Stribeck curve and the transitions from BL to ML and ML to EHL depend on a number of parameters such as material roughness (microgeometry), contact pressure, and lubricant viscosity. High contact pressure such as that present at point contacts leads to a different Stribeck curve as at line contact (lower contact pressure) (see Figures 7.1 and 7.2).

7.2.1

FRICTION REDUCTION THROUGH THE LUBRICANT

Engine friction originates from several components, that operate at different conditions of load, speed, and temperature. Hence, these components may experience various combinations of EHL, ML, and BL during engine operation. For each of these regimes, a number of factors govern engine friction.

198

Lubricant Additives: Chemistry and Applications

Basically, two options to reduce friction and improve fuel efficiency come forward [7,8]. 1. Use of low-viscosity engine oils (SAE 0W/5W-20/30) when fluid lubrication (EHL) is the governing factor [9–11]. Fluid lubrication is especially prevalent in the bearings. The gradual reduction of engine oil viscosity over the years has already brought significant fuel savings (see Figure 7.3). In the preceding case, oil selection is crucial. In terms of frictional characteristics, one must emphasize low kinematic viscosity, high viscosity index, low “high-temperature, high-shear” (HTHS) viscosity, and a low-pressure/viscosity coefficient [12,13]. However, it has to be realized that other base fluid properties, such as volatility and thermal/oxidation stability, must not be ignored. 2. Addition of friction-reducing agents when BL and ML are the governing factors [14]. These are prevalent in the valve train and the piston group. In the preceding case, additive system design is the crucial factor. One must emphasize selecting proper FMs and controlling additive–additive and additive–base fluid interactions. To assess possible fuel economy improvements in the engine sequences prescribed, an overview of the lubrication regimes existing in various test engines is provided. The data those used for current and previous ILSAC specifications are given in Table 7.1.

Relative fuel savings (%)

5W-30

10W-40

4 15W-40

15W-50

2 20W-50 0 6

14 8 10 12 High shear viscosity at 100°C, 4.105 s-1 (mm/s)

FIGURE 7.3 Relationship between SAE viscosity grades and fuel savings based on fleet car trials.

TABLE 7.1 Lubrication Regimes in API Sequences VI and VIA API Sequence VI (%) Boundary lubrication Mixed lubrication Elasto-hydrodynamic lubrication

37 15 48

API Sequence VIA (%) 24 4 72

Organic Friction Modiﬁers

199

TABLE 7.2 Lubrication Regimes in the DB M111E Engine Frictional Loss (%) Valve train Piston assembly Bearings

25 40 35

Main Lubrication Regime Boundary lubrication Mixed lubrication Elasto-hydrodynamic lubrication

In Sequence VIA, which is prescribed for ILSAC GF-2, EHL is dominating, leading to a substantial effect of engine oil viscosity on fuel economy. Effects of FMs will be small due to the low presence of BL and ML conditions, which is due to the application of roller followers. Hence, the Sequence VIA test engine is often indicated as “a very expensive viscometer.” This characterization of the Sequence VIA engine will be addressed by Sequence VIB, to be used for ILSAC GF-3, for which an engine, a bucket tappet sliding valve train will be used, leading to an increase of the BL and ML regimes [3]. In Europe, the M111 engine is used for the CEC L-54-T-96 fuel economy test, which is prescribed in the ACEA A1 and B1 engine oil specifications. Similar data as aforementioned are not available to the authors, but data given in a Shell paper [15] indicate the frictional loss occurring in this engine, which can be translated to lubrication regimes (Table 7.2). On the basis of the relatively high amount of frictional loss in the valve train and piston assembly, the M111 engine should be sensitive to FMs. This is due to the use of four valves per cylinder to improve combustion efficiency and so to obtain more power from a given amount of fuel. However, compared to other engine designs, the frictional loss in the M111E valve train will be higher. Provided that the higher-power output obtained from the four-valve assembly is significantly higher than that is lost by higher valve train friction, this approach is favorable with regard to fuel economy.

7.3 FRICTION MODIFIERS VERSUS ANTIWEAR/ EXTREME-PRESSURE ADDITIVES A point of debate is often about the difference between FMs and antiwear/extreme-pressure (AW/EP) additives, especially when it is about FMs active at BL conditions. For a good understanding, this should be clarified; therefore, this section deals with the principal difference between these two additive categories [16]. AW/EP additives are types of compounds that provide good BL. Such materials have the capacity to build strong BL layers under severe load conditions. Hence, AW/EP additives protect closely approaching metal surfaces from asperities damaging the opposite surface. On the contrary, most AW additives have little friction-modifying properties. The crucial differences between AW/EP and FM films are their mechanical properties. AW/EP films are semiplastic deposits that are difficult to shear off. Thus, under shearing conditions, their coefficient of friction is generally moderate to high. Conversely, FM lubricant films are built up of orderly and closely packed arrays of multimolecular layers, loosely adhering to one another and with the polar head anchored on the metal surface. The outer layers of the film can be easily sheared off, allowing for a low coefficient of friction.

200

Lubricant Additives: Chemistry and Applications

TABLE 7.3 Lubrication Modes versus Friction Coefﬁcient Lubrication Mode

Friction Coefﬁcient

Comparison

0.5–7 0.12–0.18 0.06–0.08 0.001–0.01

Dragging an irregular rock over rocky ground Dragging a flat stone over a flat rock Ice skating Hydroplaning

Nonlubricated surface AW/EP films Friction-modified films EHL

TABLE 7.4 FM Type and Mode of Action Mode of Action/Type of FM Formation of reacted layers Formation of absorbed layers Formation of polymers Mechanical types

Products Saturated fatty acids, phosphoric and thiophosphoric acids, sulfur-containing fatty acids Long-chain carboxylic acids, esters, ethers, amines, amides, imides Partial complex esters, methacrylates, unsaturated fatty acids, sulfurized olefins Organic polymers

The difference between the two types of films and other lubrication modes is best illustrated by the data presented in Table 7.3.

7.4 CHEMISTRY OF ORGANIC FRICTION MODIFIERS Organic FMs are generally long, slim molecules with a straight hydrocarbon chain consisting of at least 10 carbon atoms and a polar group at one end. The polar group is one of the governing factors in the effectiveness of the molecule as an FM. Chemically, organic FMs can be found within the following categories [16]: • • • •

Carboxylic acids or their derivatives, for example, stearic acid and partial esters Amides, imides, amines, and their derivatives, for example, oleylamide Phosphoric or phosphonic acid derivatives Organic polymers, for example, methacrylates

Another classification can be given by mode of action and FM type (Table 7.4). Owing to the different mode of actions, the mechanism of friction reduction varies for each category. The next section further deals with details about their mode of action, and another section deals with the current chemistry used as well as specific products.

7.4.1

FRICTION MODIFIER MECHANISMS

7.4.1.1

Formation of Reacted Layers

Similar to AW additives, protective layers are formed by chemical reaction of the additive with the metal surface. However, the principal difference is that the reaction has to occur under the relatively

Organic Friction Modiﬁers

Van der Waals forces

Long, nonpolar chains

Polar heads

Van der Waals forces

Dipole−dipole interactions Adhesive hydrogen bonding

Oxidized and hydroxylated metal surface

FIGURE 7.4

Organic FMs—formation of adsorbed layers.

201 Metal surface //////////////////// H H H H H = Polar head T T T T T T = Hydrocarbon tail TTTTTT HHHHH HHHHH TTTTTT OIL OIL OIL OIL OIL OIL OIL OIL OIL TTTTTT HHHHH HHHHH TTTTTT TTTTTT HHHHH //////////////////// Metal surface

FIGURE 7.5 Multilayer matrix of FM molecules.

mild conditions (temperature and load) of the ML regime. These conditions require a fairly high level of chemical activity as reflected by the phosphorus and sulfur chemistry applied. An exception to this is stearic acid. Theoretically, the friction-reducing effect of stearic acid should decrease with increasing temperature due to desorption of the molecule from the metal surface. However, stearic acid experimentally shows a remarkable drop of friction with increasing temperature, which can only be explained by the formation of chemically reacted protective layers. 7.4.1.2

Formation of Absorbed Layers

The formation of absorbed layers occurs due to the polar nature of the molecules. FMs dissolved in oil are attracted to metal surfaces by strong absorption forces, which can be as high as 13 kcal/mol. The polar head is anchored to the metal surface, and the hydrocarbon tail is left solubilized in the oil, perpendicular to the metal surface (see Figure 7.4). Next the following steps occur: 1. Other FM molecules have their polar heads attracted to one another by hydrogen bonding and Debye orientation forces, resulting in dimer clusters. Forces are ∼15 kcal/mol. 2. Van der Waals forces cause the molecules to align themselves such that they form multimolecular clusters that are parallel to one another. 3. The orienting field of the absorbed layer induces further clusters to position themselves with their methyl groups stacking onto the methyl groups of the tails of the absorbed monolayer [17,18]. As a result, all molecules line up, straight, perpendicular to the metal surface, leading to a multilayer matrix of FM molecules (see Figure 7.5). The FM layers are difficult to compress but very easy to shear at the hydrocarbon tail interfaces, explaining the friction-reducing properties of FMs. Owing to the strong orienting forces, mentioned earlier, sheared-off layers are quite easily rebuilt to their original state. The thickness and effectiveness of the absorbed FM films depend on several parameters, four of which are explained here. 1. Polar group. Polarity itself is not necessarily sufficient for adsorption; the polar group must also have hydrogen-bonding capability. Molecules with highly polar functional groups that are not capable of forming hydrogen bonds, such as nitroparaffins, do not adsorb.

202

Lubricant Additives: Chemistry and Applications

Hence, these do not function as friction-reducing additives. However, polarity plays a major role among the various lateral surface interactions through strong electrostatic dipole–dipole interactions. These may be either repulsive or attractive, depending on the orientation of the adsorbed dipoles with respect to the surface [19]. 2. Chain length. Longer chains increase thickness of the absorbed film, and the interactions between the hydrocarbon chains increase as well [18]. 3. Molecular configuration. Slim molecules allow for closer packing as well as increased interaction between adjacent chains, leading to stronger films. Therefore, straight chains may be preferred. 4. Temperature. Temperature influences FM film thickness and tenacity. Adsorption of frictionreducing compounds to the metal surface does occur at relatively low temperatures. AW additives form protective layers by chemical reactions for which higher temperatures are needed. If the temperature is too high, enough energy might be provided to desorb the frictionreducing molecules from the metal surface. 7.4.1.3

Formation of In Situ Polymers

The formation of low-friction-type polymer films can be considered a special case. Instead of the usual solid films, fluid films are formed under influence of contact temperature (flash temperature) and load. Another difference is that the polymers are developed at the interface between metal asperities without reacting with the metal surface. The requirements of such polymers are 1. Polymers must have relatively low reactivity. Polymerization must be generated by frictional energy. 2. The polymers formed must be mechanically and thermally stable and should not be soluble in the lubricant. 3. The polymers must develop a strong bond to the metal surface either by absorption or by chemical bonding. 4. The formation and regeneration of films must be fast to prevent competitive adsorption by other additives. Examples of polymer-forming FMs are • Partial complex esters, for example, a sebacic acid/ethylene glycol partial ester methacrylates • Oleic acid (olein), which may be explained through thermal polymerization (formation of dimers and higher oligomers)

7.5 CHEMISTRY OF OTHER FRICTION MODIFIERS Within this group, the following categories can be distinguished by chemical type: 1. Metallo-organic compounds 2. Oil-insoluble materials Classification by type appears in Table 7.5.

7.5.1

METALLO-ORGANIC COMPOUNDS

Molybdenum dithiophosphate, molybdenum dithiocarbamate, and molybdenum dithiolate as well as copper-oleate, copper-salicylate, and copper-dialkyldithiophosphate are examples of frictionreducing metallo-organic compounds.

Organic Friction Modiﬁers

203

TABLE 7.5 Classiﬁcation of Other FMs Types of FMs Metallo-organic compounds Mechanical types

Products Molybdenum and copper compounds Molybdenum disulfide, graphite, teflon (PTFE)

The mechanisms of operation of this class of products are not fully understood, but the following hypotheses are presented: • • • •

Diffusion of molybdenum into the asperities Formation of polymer-type films In situ formation of molybdenum disulfide (most accepted hypothesis) Selective transfer of metal (copper) leading to the formation of thin, easy-to-shear metal films

7.5.2

MECHANICAL TYPES

In this group, the classical types such as graphite and molybdenum disulfide as well as some more recent FMs such as teflon (polytetrafluoroethylene, PTFE), polyamides, fluoridized graphite, and borates can be found. The friction-reducing mechanisms can be explained by • The stratified structure and formation of easy-to-shear layers • The formation of elastic or plastic layers on the metal surface

7.6 FACTORS INFLUENCING FRICTION-REDUCTION PROPERTIES This section lists the main factors that impact friction-reducing properties. 1. Competing additives. Other polar additives with affinity to metal surfaces such as AW/EP and anticorrosion additives as well as detergents and dispersants may compete with FMs. This emphasizes that lubricant formulations have to be balanced carefully to achieve optimal performance. 2. Contaminants. Short-chain acids, which are formed by oxidative degradation of the lubricants, may compete at the metal surface, resulting in a loss of friction-modifying properties. 3. Metallurgy. The type of steel alloy used will affect the adsorption of FMs. 4. Concentration. Increase of FM concentration results in an increase of friction reduction up to a point above which improvements are marginal. Generally, the friction-reducing effect is most (cost-)effective at concentrations of ∼0.25 to 1% for organic FMs and 0.05–0.07% for molybdenum dithiocarbamates.

7.7 FRICTION MODIFIERS: CURRENT PRACTICE The most frequently used organic FMs include 1. Long-chain fatty amides, specifically oleylamide (Figure 7.6). This is a reaction product of olein (main component oleic acid, a straight-chain unsaturated C18 carboxylic acid) and ammonia (NH3).

204

Lubricant Additives: Chemistry and Applications Oleylamide

Glycerol mono-oleate (GMO) NH2 OH O OH O O

FIGURE 7.6

Organic FMs—structural drawings.

S N

Mo S

Zn (dtp)2

Mo(dtc)(dtp)

Zn (dtp)(dtc)

Mo(dtp)2

Zn(dtc)2

R C

Mo(dtc)2

N R

FIGURE 7.7 Molybdenum dithiocarbamate— structural drawing.

FIGURE 7.8 Molybdenum dithiocarbamate— exchange of functional groups.

2. Partial esters, specifically glycerol mono-oleate (GMO) (Figure 7.6). GMO is a reaction product of glycerin (natural alcohol with three hydroxyl groups) and olein (as mentioned previously). Investigations have shown that the alpha version (terminal hydroxyl groups esterified) is the active component rather than the beta one (middle hydroxyl group esterified). Special production techniques are required to manufacture high-alpha-containing products. The mode of action of both product groups is based on the formation of adsorbed layers that can easily be sheared off, leading to reduced friction. It is expected that further research will result in new and improved types to cope with more severe requirements with regard to friction retention over time. Within the group of metallo-organic compounds, molybdenum dithiocarbamate [Mo(dtc)2] seems almost exclusively to be used to obtain friction reduction (Figure 7.7). Research [7,20] has shown that the friction-reducing activity of Mo(dtc)2 is based on the exchange of functional groups with zinc dialkyldithiophosphates [Zn(dtp)2] (Figure 7.8). It was found that oxidation affects these exchange reactions significantly and that the most effective friction reduction is achieved at the later stages of oxidation when the concentrations of the single-exchange product [Mo(dtc)(dtp)] and the double-exchange product [Mo(dtp)2] are high. When both products are nearly consumed by oxidation, friction reduction ceases.

7.8 FRICTION MODIFIER PERFORMANCE Literature suggests that FMs act both in the BL and ML regimes [7,16,21]. Their mode of action should depend on FM chemistry and prevailing engine conditions. It is further suggested that organic FMs are most active in the mixed regime, whereas metallic types are predominantly active

Organic Friction Modiﬁers

205

Friction coefficient f

Boundary lubrication

Mixed lubrication

Elasto-hydrodynamic lubrication Contact pressure (wear) High

Low

Lubricant parameter (viscosity × speed / load)

FIGURE 7.9

Influence of wear/contact pressure on ML/EHL transition.

in the BL regime. Recent investigations by the authors, carried out with a pin-on-ring tribometer, showed that it is likely that organic FMs act predominantly in the BL regime as well. Tests were carried out with CEC reference oil RL 179/2, which is applied in the CEC L-54-T-96 fuel economy test. RL 179/2 is a formulated 5W/30 engine oil that does not contain any FM and that has a proven fuel economy benefit CEC round-robin tests. Frictional behavior was investigated by establishing stabilized Stribeck curves. By determining these, both boundary and mixed friction can be investigated. Stabilized Stribeck curves are obtained by measuring the coefficient of friction over a speed range from ∼0.0025 to 2 m/s at appropriate steps. A number of runs are carried out until two consecutive runs give a good match. Usually, after four runs, the curve has stabilized, meaning that process roughness has stabilized to a large extent. Performance criteria in considering the results are frictional level in the BL and ML regimes in combination with specimen wear. The reason for looking at wear is that this parameter corresponds with contact pressure, which in turn influences the ML/EHL transition. The relationship between wear and contact pressure is given by the expression Fn = p A

(7.3)

where Fn = normal force (load) A = wear scar p = contact pressure Consequently, a high-wear scar leads to a lower contact pressure, and a lower contact pressure does shift the ML/EHL transition in the Stribeck curve to the left (see Figure 7.9).

7.8.1

STRIBECK CURVE DETERMINATIONS

Stabilized Stribeck curves have been determined with a pin-on-ring tribometer at which the ring was a 100Cr6 stainless steel ring with a 730 mm diameter. These rings are high-quality materials used in standard bearings and therefore easily available. The pin used was a cylinder from the same material with an 8 mm diameter, also used in bearings. To get proper line contact, the cylinders have been provided with flexible ends to allow full alignment with the ring.

206

Lubricant Additives: Chemistry and Applications

Ring roughness, Ra, was ∼0.15 µm, although the cylinder was very smooth. Hence, the roughness of the ring determined the shape of the Stribeck curve, specifically the BL/ML transition. The load (normal force Fn = 100 N) was chosen such that heat development in the contact zone was negligible, so that the viscosity was constant. Hence, it was possible to determine the Stribeck curve only as a function of speed. The temperature chosen was 40˚C. The following graph (Figure 7.10) shows comparative data for RL 179/2 as well as this oil with addition of 0.5% GMO and addition of 0.5% of organic FMs A and B. (A and B are products with both free and esterified hydroxyl groups.) All the FMs studied here show a significant reduction of the friction coefficient in the BL regime. Organic FMs A and B show a reduction in the mixed regime as well. On first sight this looks favorable. The next graph (Figure 7.11) shows the wear, taken at similar sliding distances. 0.16

Friction coefficient f (−)

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.010

0.100

1.000

10.000

Contact speed v (m/s) RL 179/2 (5W/30)

FIGURE 7.10

+ 0.5% GMO

+ 0.5% OFM A

+ 0.5% OFM B

Stribeck curves of CEC RL 179/2 plus organic FMs. 9.000 8.000

Wear scar (mm2)

7.000 6.000 5.000 4.000 3.000 2.000 1.000 0.000

Wear area RL179/2

FIGURE 7.11

+ 0.5% GMO

+ 0.5% OFM A

Wear scars of CEC RL 179/2 plus organic FMs.

+ 0.5% OFM B

Organic Friction Modiﬁers

207

The wear of the oil containing A and B is twice as high as those of the reference oil and that oil with addition of 0.5% GMO. Consequently, the contact pressure p is twice as low as those of the others and is what makes the ML/EHL transition shift to the left. Thus it seems that organic FMs are predominantly active in the BL regime and that the shifts observed in the mixed regime are likely to be caused by other phenomena that must not be ignored.

7.8.2

FRICTION AS A FUNCTION OF TEMPERATURE

Another aspect of FM performance is friction as a function of temperature. Temperature plays an important role with regard to adsorption/desorption phenomena as for the formation of adsorbed layers as well as regarding those of reacted layers. The graph in Figure 7.12 shows the frictional behavior of some organic FMs as a function of temperature, using the pin-on-ring tribometer as before with the same specimens and configuration. Again, CEC reference oil RL 179/2 was used, and the speed chosen (0.03 m/s) assured operation well within the BL regime. All the organic FMs studied show a significant friction reduction over the temperature range tested. GMO and oleylamide perform best, and the optimum adsorption seems to be obtained at ∼70°C. At higher temperatures, desorption may start to occur as well as some kind of competition with other surface-active additives, leading to a higher coefficient of friction. Oleylamide, however, continues to show high friction-reducing properties at elevated temperatures. Figure 7.13 shows a comparison between organic FM GMO and metallic-type FM molybdenum dithiocarbamate. Two sources of the latter were used at a concentration equivalent to 0.07% molybdenum. GMO and the molybdenum dithiocarbamates show a marked performance difference. Although GMO is active over a wide temperature range, the molybdenum dithiocarbamates start to reduce friction at temperatures of 120°C and above only. This has to be considered as an induction period that can be explained by the necessary exchange of ligands between molybdenum dithiocarbamate and zinc dialkyldithiophosphate (see Section 7.7). Once molybdenum dithiocarbamate has “lighted off,” a fast drop of friction is noticed. At the end of the test cycle 140°C, the system has not stabilized and the friction coefficient might decrease further.

0.14

Friction coefficient f (−)

0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 40

RL 179/2

FIGURE 7.12

90 80 100 Temperature (°C)

+ 0.5% GMO

+ 0.5% OFM A

110

120

130

+ 0.5% Oleylamide

Friction coefficient versus temperature—CEC RL 179/2 plus organic FMs.

140

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Lubricant Additives: Chemistry and Applications 0.14

Friction coefficient f (−)

0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 40

RL 179/2

FIGURE 7.13

80 90 100 Temperature (°C) + 0.5% GMO

110

+ 0.07% Mo (A)

120

130

140

+ 0.07% Mo (B)

Friction coefficient versus temperature—CEC RL 179/2 plus molybdenum dithiocarbamates.

The difference in friction-modifying characteristics between organic FMs and molybdenum dithiocarbamate suggests that it might be beneficial to use a combination of these materials.

7.9

CONSEQUENCES OF NEW ENGINE OIL SPECIFICATIONS AND OUTLOOK

Although initial fuel economy requirements were focused on fresh oil only, new engine oil specifications will address fuel economy longevity as well. A good example is Sequence VIB, which has been developed for the ILSAC GF-3 specification. Sequence VIB includes aging stages of 16 and 80 h to determine fuel economy as well as fuel economy longevity. These aging stages are equivalent to 4000–6000 mi of mileage accumulation required before the EPA metro/high-highway fuel economy test. That test is used in determining CAFE. To obtain engine oil formations that are optimized with regard to fuel economy longevity, high requirements are demanded for base oil selection and additive system design [3,7,22]. These requirements are • To minimize the increase of viscosity thereby maintaining a low electrohydrodynamic friction coefficient • To maintain low boundary/mixed friction A minimum increase of viscosity can be obtained by base fluid selection (in terms of volatility, oxidation stability, and antioxidant susceptibility) and selection of antioxidants and their treat level. The market is already anticipating requirements by increasing the production capacity of groups II (HIVI) and III (VHVI) base fluids and by increased interest in groups IV (PAOs) and V (a.o. esters) base fluids. To achieve low friction under BL and ML conditions, the use of effective friction-reducing additives is needed. To maintain low boundary and mixed friction over time, it is necessary to prevent consumption of these additives by processes such as oxidation and thermal breakdown. Therefore, selecting suitable antioxidant systems for molybdenum compounds and organic FMs and developing organic FMs with highest thermal/oxidative stability will be key for high fuel economy longevity and a successful application in engine oil formulations.

Organic Friction Modiﬁers

209

Further studies on the mechanisms of FM action, for example, through molecular modeling techniques, could also speed up the development of optimized additives and additive systems. Apart from frictional properties, other important tribological parameters such as wear rate and surfacemetal geometry should be investigated as well. In most papers studied, this seems to be ignored, although all three parameters should be considered in relation to one another.

7.10

BENCH TESTS TO INVESTIGATE FRICTION-REDUCING COMPOUNDS

Several bench tests can be thought of to investigate the frictional properties of base fluids and formulated products. In recent literature [8,23,24], the following test equipment has been used: 1. The high-frequency reciprocating rig (HFRR) to measure boundary friction. Although originally developed to measure diesel fuel lubricity, the equipment can be successfully applied to measure lubricant properties as well. Frequency Stroke length Load Ambient temperature

10–200 Hz 20–2000 µm 0–1000 g 200°C

A 6 mm diameter ball is the upper specimen and a 3 mm thick smooth disk with a 10 mm diameter is the lower specimen. HFRR specifications are the test conditions that the authors applied to screen FMs; these include a 40 Hz frequency, a stroke of 1000 µm, and a 400 g load. 2. A mini traction machine (MTM) to measure mixed and (E)HD friction, for example, by the determination of Stribeck curves. The MTM rig is capable of measuring at either constant or varying slide/roll ratios if required. Speed range Slide/roll ratio Load Ambient temperature

Up to 5 m/s 0–200% (Full rolling to full sliding) 0–75 N 150°C

Standard specimens are a 19.05 mm diameter ball as upper specimen and a 50 mm diameter disk as lower specimen. Both are manufactured from AISI 52100 bearing steel. The standard disk is smooth, which allows measurement of mixed-film and full-film friction. Alternatively, rough disks are available for measurements in the BL regime. MTM specifications are the test conditions that the authors applied to test FMs; these comprise a speed range of 0.001–4 m/s, a 30 N load, and a 200% slide/roll ratio. 3. An optical rig provided with a disk coated with a spacer layer to measure EHD film thickness. Such a rig enables film-thickness measurements down to <5 nm with a precision between 1 and 2 nm. Some other literature refers to the low-velocity friction apparatus (LVFA). Alternative reciprocating rigs may be suitable as well.

REFERENCES 1. Wilk, M.A., W.D. Abraham, B.R. Dohner. An investigation into the effect of zinc dithiophoshpate on ASTM sequence VIA fuel economy. SAE Paper 961,914, 1996. 2. Houben, M. Friction analysis of modern gasoline engines and new test methods to determine lubricant effects, 10th International Colloquium, Esslingen, 1996.

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3. Korcek, S. Fuel efficiency of engine oils—current issues. 53rd Annual STLE Meeting, Detroit, 1998. 4. LaFountain, A., G.J. Johnston, H.A. Spikes. Elastohydrodynamic friction behavior of polyalphaolefin blends. Tribololgy Series 34:465–475, 1998. 5. Spikes, H.A. Boundary lubrication and boundary films. Tribology Series 25:331–346, 1993. 6. Spikes, H.A. Mixed lubrication—an overview. Lubrication Science 9(3):221–253, 1997. 7. Korcek, S. et al. Retention of fuel efficiency of engine oils. 11th International Colloquium, Esslingen, 1998. 8. Sorab, J., S. Korcek, C. Bovington. Friction reduction in lubricated components through engine oil formulation. SAE Paper 982,640, 1998. 9. Goodwin, M.C., M.L. Haviland. Fuel economy improvements in EPA and road tests with evine oil and rear axle lubricant viscosity reduction. SAE Paper 780,596, 1978. 10. Waddey, W.E. et al. Improved fuel economy via engine oils. SAE Paper 780,599, 1978. 11. Clevenger, J.E., D.C. Carlson, W.M. Keiser. The effects of engine oil viscosity and composition on fuel efficiency. SAE Paper 841,389, 1984. 12. Dobson, G.R., W.C. Pike. Predicting viscosity related performance of engine oils. Erd-1 und KohleErdgas 36(5):218–224, 1982. 13. Battersby, J., J.E. Hillier. The prediction of lubricant-related fuel economy characteristics of gasoline engines by laboratory bench tests. Proceedings of International Colloquium, Technische Akademie Esslingen, 1986. 14. Griffiths, D.W., D.J. Smith. The importance of friction modifiers in the formulation of fuel efficient engine oils. SAE Paper 852,112, 1985. 15. Taylor, R.I. Engine friction lubricant sensitivities: A comparison of modern diesel and gasoline engines. 11th International Colloquim, Esslingen, 1998. 16. Crawford, J., A. Psaila. In R.M. Mortier and S.T. Orszulik, eds. Chemistry and Technology of Lubricants, Miscellaneous Additives. London, 1992, pp. 160–165. 17. Allen, C.M., E. Drauglis. Boundary lubrication: Monolayer or multilayer. Wear 14:363–384, 1969. 18. Akhmatov, A.S. Molecular physics of boundary lubrication. Gos. Izd. Frz.-Mat. Lit., Moscow, p. 297, 1969. 19. Beltzer, M., S. Jahanmir. Effect of additive molecular structure on friction, Lubrication Science 1–1: 3–26, 1998. 20. Arai, K. et al. Lubricant technology to enhance the durability of low friction performance of gasoline engine oils. SAE Paper 952,533, 1995. 21. Christakudis, D. Friction modifiers and their testing, additives for lubricants. Kontakt Stud 433:134–162, 1994. 22. Effects of aging on fuel efficient engine oils. Automotive Engineering (Feb):1996. 23. Moore, A.J. Fuel efficiency screening tests of automotive engine oils. SAE Paper 932,689, 1993. 24. Bovington, C., H.A. Spikes. Prediction of the influence of lubricant formulations on fuel economy from laboratory bench tests. Proceedings of International Tribology Conference, Yokahama, 1995.

Part 3 Antiwear Additives and Extreme-Pressure Additives

Ashless Antiwear and Extreme-Pressure Additives Liehpao Oscar Farng

CONTENTS 8.1 Introduction ........................................................................................................................... 214 8.2 Chemistry, Properties, and Performance (Classified by Elements) ...................................... 215 8.2.1 Sulfur Additives ........................................................................................................ 215 8.2.1.1 Sulfurized Olefins ....................................................................................... 216 8.2.1.2 Sulfurized Esters and Sulfurized Oils ........................................................ 219 8.2.1.3 Other Sulfur Additives ................................................................................ 220 8.2.2 Phosphorus Additives................................................................................................ 220 8.2.2.1 Phosphate Esters ......................................................................................... 221 8.2.2.2 Phosphites ................................................................................................... 222 8.2.2.3 Dialkyl Alkyl Phosphonates .......................................................................224 8.2.2.4 Acid Phosphates ......................................................................................... 225 8.2.3 Sulfur–Phosphorus Additives ................................................................................... 225 8.2.3.1 Ashless Dithiophosphates ........................................................................... 225 8.2.3.2 Ashless Phosphorothioates and Thiophosphates ........................................ 226 8.2.4 Sulfur–Nitrogen Additives ........................................................................................ 228 8.2.4.1 Dithiocarbamates ........................................................................................ 228 8.2.4.2 Dimercaptothiadiazole and Mercaptobenzothiazole Additives.................. 229 8.2.4.3 Other Sulfur–Nitrogen Additives ............................................................... 230 8.2.5 Phosphorus–Nitrogen Additives ............................................................................... 231 8.2.5.1 Amine Phosphates ...................................................................................... 231 8.2.5.2 Amine Thiophosphates and Dithiophosphates ........................................... 232 8.2.5.3 Other Phosphorus–Nitrogen Additives....................................................... 232 8.2.6 Nitrogen Additives .................................................................................................... 232 8.2.7 Additives with Multiple Elements ............................................................................. 234 8.2.8 Halogen Additives ..................................................................................................... 235 8.2.9 Nontraditional Antiwear/Extreme-Pressure Additives ............................................. 236 8.3 Manufacture, Marketing, and Economics............................................................................. 237 8.4 Evaluation Equipment/Specification ..................................................................................... 238 8.4.1 Lubricant Specifications ............................................................................................ 238 8.4.2 Additive Specifications.............................................................................................. 239 8.4.3 Test Methods and Equipment .................................................................................... 239 8.5 Outlook ................................................................................................................................. 243 Acknowledgment ........................................................................................................................... 245 References ......................................................................................................................................246

213

214

8.1

Lubricant Additives: Chemistry and Applications

INTRODUCTION

To optimize the balance between low wear and low friction, machine designers specify a lubricant with a viscosity sufficient to generate hydrodynamic or elastohydrodynamic oil films that separate the machine’s interacting surfaces, but not too high to induce excessive viscous drag loss. In practice, the various contact types in a machine, the incidence of operating conditions beyond the design range, and the pressure to improve efficiency by reducing oil viscosity conspire to reduce oil film thickness below the optimum. The high spots, or asperities, on the interacting surfaces then start to interact with one another, initially through micro-elastohydrodynamic lubrication (EHL) films, and at the end through direct surface contact, resulting in increased friction and the likelihood of surface damage. Antiwear and extreme-pressure (EP) additives are added to lubricating oils to decrease wear and prevent seizure under such conditions. A common way to demonstrate the viscosity optimization is shown in Figure 8.1; this is known as a Stribeck curve. The curve is a composite of a boundary friction curve and a viscous friction curve that decreases as viscosity and, therefore, film thickness increase and that increases as viscosity and speed increase, respectively. A good operating target is represented slightly to the right of the minimum in the curve. Improving the surface finish of contacting surfaces can move the minimum in the curve to a lower viscosity range, saving energy but increasing the cost of components. The hardening or coating of surfaces can increase their durability under increased levels of contact with lower viscosity, but again at an increase in component cost. Notwithstanding these component manufacturing improvements, the need for antiwear and EP additives will continue, but the nature of their chemistry is likely to change due to environmental constraints, component material developments, and the continuing increase in severity of machine operating conditions. The distinction between antiwear and EP additives is not clear-cut. Some are classed as antiwear in one application and EP in another, and some have both antiwear and EP properties. To add to the confusion, EP additives come in mild and strong flavors, and some are only effective in low-speed, high-load situations and others only in high-speed, high-temperature applications. Generally, antiwear additives are designed to deposit surface films under normal operating conditions and thereby reduce the rate of continuous, moderate wear, whereas EP additives are expected to react rapidly with a surface under severe distress and prevent more catastrophic modes of failure such as scuffi ng (scoring), galling, and seizure. Recently, it has been suggested that EP additives be renamed as antiscuffi ng additives, since there is no pressure

Boundary friction 0.15 Mixed lubrication Coefficient of friction

Hydrodynamic regime

Boundary lubrication

Fluid film

0.001−0.002 Minimum fluid friction Z = viscosity of oil N = speed of sliding P = pressure between surfaces (load)

FIGURE 8.1

Regions of lubrication—Stribeck curve.

ZN P

Ashless Antiwear and Extreme-Pressure Additives

215

distinction between them and antiwear additives; only an expectation of a performance boost under severe conditions. EP/antiscuffing additives tend to be very reactive, and some can have adverse effects on oxidative stability of oils, can be corrosive to nonferrous materials, and can reduce the fatigue life of bearing and gear surfaces. They should only be used when severe distress is a distinct possibility. Antiwear additives function in various ways. Some deposit multilayer films thick enough to supplement marginal hydrodynamic films and prevent asperity contact altogether. Some develop easily replenishable monolayer films that reduce the local shear stress between contacting asperities and are preferentially removed in place of surface material. Others bond chemically with the surface and slowly modify surface asperity geometry by controlled surface material removal until conditions conducive to hydrodynamic film generation reappear. EP additives are designed to prevent metal–metal adhesion or welding when the degree of surface contact is such that the natural protective oxide films are removed and other surface-active species in the oil are not reactive enough to deposit a protective film. This is most likely to occur under conditions of high-speed, high-load, or high-temperature operation. EP additives function by reacting with the metal surface to form a metal compound such as iron sulfide. They act in a manner similar to that of antiwear additives, but their rate of reaction with the metal surface and therefore the rate of EP film formation are higher and the film itself is tougher. Some EP additives prevent scoring and seizure at high speed and under shock loads; others prevent ridging and rippling in high-torque, low-speed operations. In both cases, EP additives and surface metal are consumed, and a smoother surface is created with an improved chance of hydrodynamic action, resulting in less local distress and lower friction. In the absence of such additives, heavy wear and distress well beyond the scale of surface asperities would occur, accompanied by very high friction. A wide variety of antiwear and EP additives are commercially available, and many other chemicals with antiwear and EP functionality have been reported in the literature and in patents. To be commercially viable, additives must be adequately soluble in lubricant formulations and reasonable in cost, must neither overly reduce the lubricant’s oxidative stability nor increase the corrosivity of metals contacted by the lubricant [1–3]. Lead naphthenates were extensively used early in the industry’s history, but environmental concerns have led to their virtual disappearance. Similarly, chlorinecontaining additives are in decline. Zinc dialkyldithiophosphates (ZDDPs or ZnDTPs) are the best known and most widely used antiwear additives in engine oils, transmission fluids, and hydraulic oils. However, the concern for phosphorus poisoning of automotive catalysts and for zinc as an environmental contaminant has resulted in a pressure to find metal- and phosphorus-free replacements for both automotive and industrial applications. This has resulted in a move toward ashless antiwear and EP additives, and this chapter covers these additives in terms of their chemistry, properties, and performance characteristics, applications, marketing, sales, and outlook.

8.2 8.2.1

CHEMISTRY, PROPERTIES, AND PERFORMANCE (CLASSIFIED BY ELEMENTS) SULFUR ADDITIVES

Sulfur-containing additives are used to provide protection against high pressure, metal-to-metal contacts in boundary lubrication. The magnitude of the EP activity is a function of the sulfur content of the additive; high-sulfur-content additives are usually more effective EP agents than are lowsulfur-content additives. The sulfur content of the additive must be balanced against requirements for thermal stability and noncorrosiveness toward copper-containing alloys. The additive’s composition and structure represent a chemical compromise between conflicting performance requirements. In general, any compound that can break down under an energy-input stress, such as heat, and allow for a free sulfur valence to combine with iron would do well as an antiwear and EP additive. Sulfur additives are probably the earliest known, widely used EP compounds in lubricants.

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Lubricant Additives: Chemistry and Applications

Sulfurization by addition of sulfur compounds [elemental sulfur, hydrogen sulfide, and mercaptans] to unsaturated compounds has been known to the chemical industry for years [4–8]. The two most common classes of additives are called sulfurized olefins [9,10] and sulfurized fatty acid esters [11], because they are produced from reactions of olefins and naturally occurring or synthetic fatty acid esters with sulfur compounds. In the absence of initiators, the addition to simple olefins is by an electrophilic mechanism; and Markovnikov’s rule is followed. However, this reaction is usually very slow and often cannot be done or requires very severe conditions unless an acid catalyst is used. In the presence of free radical initiators, H2S and mercaptans add to double and triple bonds by a free radical mechanism, and the orientation is anti-Markovnikov. By any mechanism, the initial product of addition of H2S to a double bond is a mercaptan, which is capable of adding to a second molecule of olefin, so that sulfides are often produced (reaction 8.1):

C C

H2S

C C HC

(8.1)

8.2.1.1 Sulfurized Oleﬁns Sulfurized olefins are prepared by treating an olefin with a sulfur source under proper reaction conditions. The more the sulfur used, the higher is the sulfur content. Suitable olefins preferably include terminal olefins and internal olefins, mono-olefins and polyolefins. However, to provide adequate oil solubility, the olefin should provide a carbon chain of at least four carbon atoms. Accordingly, suitable alpha olefins are butenes, pentenes, hexenes, and preferably higher alpha olefins such as octenes, nonenes, and decenes. Isobutylene is a very unique olefin that not only exhibits very high reactivity toward sulfur reagents (high conversion rate) but also can produce sulfurized products having very good stability and lubricant compatibility. Therefore, sulfurized isobutylene (SIB) has been by far the most cost-effective, widely used EP additive in lubricants. 8.2.1.1.1 Chemistry and Manufacture Initially, sulfurized olefins were synthesized through a two-step chloride process, and often, the products were referred to as “conventional sulfurized olefins.” Sulfur monochloride and sulfur dichloride were used in the first step to produce chlorinated adducts, and then the adducts were treated with an alkali metal sulfide in the presence of free sulfur in an alcohol–water solvent, followed by further treatment with an inorganic base (reactions 8.2 and 8.3) [12]. The final product is a light yellow–colored fluid with oligomeric monosulfides and disulfides as the main compositions, as typified by reactions 8.2 and 8.3. CH3 CH2 + S2Cl2

Cl-C(CH3)2-CH2-S-S-CH2-C(CH3)2-Cl

CH3

(8.2)

Major adduct

NaSH SIB + 2 NaCl

Cl-C(CH3)2-CH2-S-S-CH2-C(CH3)2-Cl Na2S

(8.3) SIB =

S-C(CH3)2-CH2-S-S-CH2-C(CH3)2-S n

Ashless Antiwear and Extreme-Pressure Additives

217

The manufacture of conventional sulfurized olefins involves sulfur monochloride, and the final product contains some residual chlorine. The process also generates aqueous waste with halogenand sulfur-containing by-products that must be disposed of. Chlorine in lubricants and other materials is increasingly becoming an environmental concern because chlorinated dioxins can be formed when chlorine-containing materials are incinerated. Chlorinated waxes have been eliminated from many lubricants for this reason. Residual chlorine content is also becoming a major concern in many areas of the world. Germany currently has a 50 ppm maximum limit on the chlorine content of automotive gear oils. This requirement is a problem for automotive gear oil suppliers as well as additive suppliers if their technology is based on conventional sulfurized olefins, since the residual chlorine content is a consequence of the chemistry required to manufacture conventional sulfurized olefins. By fine-tuning the manufacturing process, the chlorine content of conventional sulfurized olefins may be reduced from a typical 1500 ppm to <500 ppm. However, manufacturing changes to reduce the residual chlorine content will probably slow the production process, require additional capital investments, and possibly generate more aqueous waste. In the late 1970s, the high-pressure sulfurized isobutylene (HPSIB) process was developed to replace the conventional, low-pressure chlorine process. HPSIBs are usually mixtures of di-tert-butyl trisulfides, tetrasulfides, and higher-order polysulfides [13–16]. Some HPSIBs contain oligomeric polysulfides of poorly defined composition or other materials such as 4-methyl-1, 2-dithiole-3-thione (Structure A, [8,17], and reaction 8.4). The higher-order polysulfides generally favor EP activity at the expense of oxidative stability and copper corrosivity compared to the monosulfides and disulfides of conventional sulfurized olefins. In the absence of other reagents, the straight reaction of elemental sulfur and isobutylene results in a dark-colored liquid that contains a significant amount of dithiolethiones (thiocarbonates). 4-Methyl-1, 2-dithiole-3-thione is a pseudoaromatic heterocyclic compound. Owing to its rigid ring structure, dithiolethiones can be easily precipitated as yellowish solids that cause severe staining problems. Therefore, dithiolethione is often not a desirable side product in SIB. CH3 CH2 CH3

Dithiolethione Polysulfides Oligomeric polysulfides

Sulfur High pressure

(8.4)

S CH3

S S

STRUCTURE A

In the presence of various catalysts (or basic materials), such as aqueous ammonia, alkali metal sulfides, or metal dithiocarbamates, amounts of dithiolethiones (Structure A) and oligomeric polysulfides can be reduced, and low-molecular-weight polysulfides (X = 2 to 6 in Structure B) are the predominant products [18]. (CH3)3C

S X

C(CH3)3

STRUCTURE B

The use of hydrogen sulfide in the high-pressure sulfurized olefin process can ease the reaction complexity and also yield high-quality, low-molecular-weight polysulfides. The compositions of products prepared from this process usually have good clarity, low odor, light color, and high EP activity. Hydrogen sulfide is a very foul smelling and toxic gas. It leads to collapse, coma, and death as result of respiratory failure within a few seconds after one or two inhalations. Liquefied hydrogen sulfide has a high vapor pressure that requires additional, adequate protective equipment. There are considerable risks associated with its routine use on an industrial scale, but hydrogen

218

Lubricant Additives: Chemistry and Applications

sulfide is a low-cost, commodity chemical, which can often offset the additional costs for safe use. High-pressure sulfurized olefins can also be prepared with reagents that generate hydrogen sulfide within the reactor during the course of the reaction. Direct handling of hydrogen sulfide is thus avoided, but there can be processing penalties, usually in the area of aqueous waste handling. Performance wise, high-pressure sulfurized olefins could replace conventional sulfurized olefins in suitable applications. A decision to manufacture high-pressure sulfurized olefins by one process or another will require a careful assessment of acceptable risks versus economic requirements. Other olefins or mixed olefins are also used in the preparation of various sulfurized olefins. Among these, di-tert-nonyl and di-dodecyl trisulfides and penta-sulfides are very popular additives. Diisobutylene (2,4,4-trimethyl-1-pentene) is also used extensively to make higher-viscosity sulfurized products. In addition, sulfurized hydrocarbons such as sulfurized terpene, sulfurized dicyclopentadiene, or sulfurized dipentene olefin, and sulfurized wax are also widely used due to low raw material costs. 8.2.1.1.2 Applications and Performance Characteristics Sulfurized olefins played a key role in establishing superior ashless sulfur/phosphorus (S/P) additive systems for lubricating automotive and industrial EP gear oils in the late 1960s [19–21]. The early EP gear oil additives were clearly dominated by chlorine, zinc, and lead, which had difficulty in adequately protecting heavy-duty equipment. On the contrary, the S/P gear oil additive technology, based on ashless and chlorine-free components, possesses very good thermal-oxidative stability and rust inhibition (CRC L-33 and ASTM D665B); therefore, this is a significant performance improvement over the metal- and chlorine-based technologies. Sulfurized olefins function mainly through thermal decomposition mechanisms. Sulfur prevents contact between interacting ferrous metal surfaces through the formation of an intermediate film of iron sulfide. By doing this, sulfur usually decreases the wear rate but accelerates the smoothing of the surfaces. This smoothing actually helps reduce the wear rate. Furthermore, a higher percent of active sulfur in a molecule increases the chances of reaction with the metal surface and favors EP (antiseizure) more than antiwear properties. Thus, SIB is mainly a strong antiscuffing additive, with outstanding scuffing protection properties (e.g., CRC L-42 performance). Table 8.1 shows coefficients of friction and dimensions with respect to metal surface, oil molecules, and sulfide layers. It can be seen that the friction coefficients of the sulfide layers are about half of those for metal-to-metal surfaces. The sulfide layers retard the welding of the moving metal surfaces, but do not prevent wearing. Particles of iron sulfide are constantly sloughed off from the metal surface. This wear can be determined by an analysis of the lubricating oils (residual iron content), and subsequent sludge formation can be controlled by the use of dispersants. Besides heavy-duty gear oil applications [22], sulfurized olefins have also found usefulness in other lubricant areas, such as metal processing oils, greases, marine oils, and tractor transmission oils.

TABLE 8.1 Typical Surface Characteristics Surface Steel:steel FeS:FeS Copper:copper CuS:CuS Material Size of oil molecules Size of sulfide layers Surfaces with superfinish

Coefﬁcient of Friction 0.78 0.39 1.21 0.74 Dimension (Å) 50 3000 1000

Ashless Antiwear and Extreme-Pressure Additives

219

8.2.1.2 Sulfurized Esters and Sulfurized Oils The oldest widely used sulfur-based additive that is still found in commercial lubricants is sulfurized lard oil (SLO), a sulfurized animal triglyceride. In 1939, H. G. Smith made one of the most important discoveries in the history of lubricant additives. He found that sulfurized sperm whale oil (SSWO) was more soluble in paraffinic base oils, even at low temperatures, and had a much higher thermal stability than SLO. Thus, over 60 years ago, he recognized that the improved stability of sulfurized sperm oil resulted from its monoester structure, compared with the triester structure of SLO. With long-chain monoesters, the sulfur has little potential to form bridges between the molecules, as it does when triglycerides are being sulfurized. SSWO is an excellent boundary lubricant and is highly resistant to gumming, resin formation, or viscosity increase, when subjected to high temperature and high pressure [23]. Unfortunately, from a lubricant cost-performance viewpoint, this additive is no longer available due to restrictions on the use of sperm whale oil. Sulfurized jojoba oil, an expensive alternative, is available; it is also a mixture of long-chain alcohol fatty acid compounds. All these sulfurized fats or esters are usually manufactured to contain 10–15% sulfur and are often good antiwear and mild EP agents. 8.2.1.2.1 Chemistry and Manufacture Sperm oil is a waxy mixture of esters of fatty alcohols and fatty acids with a small amount of triglycerides. After the separation of solid waxes by filtration or centrifuge, a liquid wax remains, consisting mainly of an ester of oleic alcohol and oleic acid. Such a structure could not be better for sulfurizing purposes. Similar to sulfurized olefins, sulfurized esters can be made by either direct sulfurization with elemental sulfur or sulfurization with hydrogen sulfide under superatmospheric pressures. Nowadays, they are mainly made from vegetable oils having one or more double bonds. Sulfurized esters are made from unsaturated fatty acids such as oleic acid, and esterified with an alcohol such as methanol. Frequently the sulfurization of fats is made in the presence of an olefin, preferably of long chain, and the resulting commercial product is a mixture of the two types. Equation 8.5 shows a typical example of sulfurization of methyl oleate with elemental sulfur. When sulfurized with hydrogen sulfide, products usually possess light color and lower odor.

CH3(CH2)7-CH

CH-(CH2)7COOCH3

S CH3(CH2)7-CH=CH-(CH2)7COOCH3

CH3(CH2)7-CH--CH-(CH2)7COOCH3

S S

CH3(CH2)7-CH--CH-(CH2)7COOCH3

(8.5)

Heat CH3(CH2)7-CH-CH2-(CH2)7COOCH3

CH3(CH2)7-CH-CH2-(CH2)7COOCH3 + other products

x = 1, 2

8.2.1.2.2 Properties, Performance Characteristics, and Applications The load-carrying property of sulfurized oils is directly linked to the amount of active sulfur in the additive. Percent of active sulfur (which is believed to provide EP activity) and total sulfur can be determined by proper analytical methods, and the difference is the percent of inactive sulfur.

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Lubricant Additives: Chemistry and Applications

The more the active sulfur present, the higher the load-carrying property. However, there is also a direct correlation between active sulfur and copper corrosivity—the more the active sulfur, the poorer the copper corrosion protection. More active sulfur can also lead to cleanliness and stability challenges. Therefore, the ultimate product properties for a specific lubricant product will dictate which sulfurized products to use. Although the sulfur content may not be as high as in many sulfurized olefins, sulfurized esters are attractive for their exceptionally good frictional properties in many applications. This is because combining sulfur with fat in a lubricant additive provides a synergistic effect. In this instance, the fat provides reduced friction, and sulfur provides wear and EP protection. Of all the elements, sulfur probably gives the best synergistic results in combination with other components and organic compounds. As to EP characteristics, sulfurized esters have a surface activity conferred by a small amount of their normal free fatty acids. These are polar species that tend to be absorbed in layers of molecular dimensions at the metal interface. The interposition of such films is effective in preventing metal seizure under conditions of EP or under conditions tending to displace the lubricating film between the bearing surfaces. Here, film strength and EP phenomena are often used synonymously. Film strength implies that metal-to-metal contact and welding are prevented as a result of the film formation (or replenishment) by the chemical reaction of the metal and an EP additive. Also, fatty oils and sulfurized fatty oils because of their affinity for metal surfaces are less easily displaced from metal surfaces by water than are mineral oils. The ferrophilic ester groups improve the EP properties. Depending on the molecular structure and its polarity, the surface activities vary. Since the surface activity or polarity of the substances used for sulfurization plays an equally decisive role in lubricating action, it should be taken into serious consideration when one formulates a product for a specific application. Comparing sulfurized triglycerides (e.g., SLO) with sulfurized monoesters (e.g., SSWO), the EP properties of the triglycerides are better. Two factors may be responsible for this phenomenon: (1) as the triester structure is more ferrophilic, hydrogen bridging may occur; (2) as triglycerides decompose at high temperatures to form acrolein moieties during the lubrication process, the polymerized acrolein film can add strength to the sulfide film and improve the EP characteristics. However, this EP activity of triglycerides has limited value due to their poor stability and oil solubility. Stability tests at elevated temperatures show faster and heavier sludging for SLO than for SSWO. Therefore, a proper balance of all properties is an essential part of product formulations. Sulfurized fats or esters are used extensively in lubricants such as metalworking fluids, tractortransmission fluids, and greases. 8.2.1.3

Other Sulfur Additives

Elemental sulfur provides good EP properties; however, it leads to corrosion. It dissolves in mineral oils up to certain levels depending on the type of base oils. Low polarity paraffinic/naphthenic type group II and III base oils usually have very limited solubility of elemental sulfur. Sulfurized aromatics such as dibenzyl disulfide, butylphenol disulfide, diphenyl disulfide, or tetramethyldibenzyl disulfide generally containing less-active sulfur improve the EP characteristics of lubricants only moderately; they are therefore used predominantly in combinations with other sulfur or phosphorus-containing EP additives [24,25]. Other sulfur carriers such as sulfurized nonylphenol, dialkyl thiodipropionates (S[CH2CH2C(=O)OR]2), derivatives of thioglycolic acid esters (HS–CH2C(=O)OR), derivatives of thiosalicylic acid, and trithians are also available [26]. However, materials with low sulfur content are usually less active as antiwear/EP additives, but more effective as antioxidants.

8.2.2

PHOSPHORUS ADDITIVES

Phosphorus-containing additives are used to provide protection against moderate to high pressure, metal-to-metal contacts in boundary lubrication and EHL. Unlike sulfur additives, where their EP activity must be balanced against performance requirements for thermal stability and mild

Ashless Antiwear and Extreme-Pressure Additives

221

corrosivity toward copper-containing alloys, phosphorus additives usually possess very good corrosivity control. Owing to totally different mechanisms involved in surface film formation rates and film strengths, phosphorus additives cannot replace sulfur additives in many applications and vice versa. Typically, phosphorus additives are extremely effective in applications with slow sliding speeds and high surface roughness. 8.2.2.1

Phosphate Esters

Phosphate esters have been produced commercially since the 1920s and have gained importance as lubricant additives, plasticizers, and synthetic base fluids for compressor and hydraulic oils. They are esters of alcohols and phenols with a general formula O=P(OR)3, where R represents alkyl, aryl, alkylaryl, or very often, a mixture of alkyl and aryl components. The physical and chemical properties of phosphate esters can be varied considerably depending on the choice of substituents, and these can be selected to give optimum performance for a given application. Phosphate esters are particularly used in applications that benefit from their high-temperature stability and excellent fire-resistance properties in addition to their adequate antiwear properties [27]. 8.2.2.1.1 Chemistry and Manufacture Phosphate esters are produced by reaction of phosphoryl chloride with alcohols or phenols as shown in reaction 8.6. 3ROH + POCl3 ⇒ O=P(OR)3 + HCl

(8.6)

Early production of phosphate esters was based on the so-called crude cresylic acid fraction or tar acid derived by distillation of coal tar residues. This feedstock is a complex mixture of cresols, xylenols, and other heavy materials and includes significant quantities of ortho-cresol. The presence of high concentrations of ortho-cresol results in an ester that has been associated with neurotoxic effects, and this has led to the use of controlled coal tar fractions, in which the content of ortho-cresol and other ortho-n-alkylphenols is greatly reduced. Phosphate esters using coal tar fractions are generally referred to as natural as opposed to synthetic, where high-purity raw materials are used. The vast majority of modern phosphate esters are synthetic, using materials derived from petrochemical sources. For example, t-butylated phenols are produced from phenols by reaction with butylene. The reaction of alcohol or phenol with phosphoryl chloride yields the crude product, which is generally washed, distilled, dried, and decolorized to yield the finished product. Low-molecularweight trialkyl esters are water-soluble, requiring the use of nonaqueous techniques. When mixed alkylaryl esters are produced, the reactant phenol and alcohol are added separately. The reaction is conducted in a stepwise process and the reaction temperature is kept as low as possible to avoid transesterification reactions from taking place. The most commonly used phosphate esters for antiwear performance features are tricresyl phosphates (TCP), trixylenyl phosphates (TXP), and tributylphenyl phosphates (TBP). 8.2.2.1.2 Physical and Chemical Properties The physical properties of phosphate esters vary considerably according to the mix and type of organic substituents, the molecular weights, and structural symmetry, all proving to be particularly significant. Consequently, phosphate esters range from low-viscosity, water-soluble liquids to insoluble high-melting solids. As mentioned previously, the use of phosphate esters as synthetic base fluids arises mostly from their excellent fire resistance and superior lubricity, but is limited due to their hydrolytic and thermal stability, low-temperature properties, and viscosity index. Although phosphate esters are widely used as antiwear additives for lubricants, the concerns about hydrolytic stability, thermal stability, and of course, satisfactory antiwear properties are equally important. In that sense, triaryl phosphates are dominant over trialkyl phosphates, because their hydrolytic–thermal stability is much better.

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The thermal stability of triaryl phosphates is considerably superior to that of the trialkyl esters, which degrade thermally by a mechanism analogous to that of the carboxylic esters (reaction 8.7). R

C R

P CH2

C OR

HO CH2

(8.7)

+ O

With respect to hydrolytic stability, aryl phosphate esters are superior to the alkyl esters. Increasing chain length and degree of branching of the alkyl group leads to considerable improvement in hydrolytic stability. However, the more the substituent is sterically hindered, the more difficult it is to prepare the ester. Alkylaryl phosphates tend to be more susceptible to hydrolysis than the triaryl or trialkyl esters. The low-temperature properties of phosphate esters containing one or more alkyl substituents tend to be reasonably good. Many triaryl phosphates are fairly high-melting point solids, but an acceptable pour point can be achieved by using a mixture of aryl components. Coal tar fractions, used to make natural phosphate esters, are already complex mixtures and give esters with satisfactory pour points. Phosphate esters are very good solvents and are extremely aggressive toward paints and a wide range of plastics and rubbers. When selecting suitable gasket and seal materials for use with these esters, careful consideration is required. The solvency power of phosphate esters can be advantageous in that it makes them compatible with most other common additives and enables them to be used as carriers for other less-soluble additives to generate additive slurry. 8.2.2.2

Phosphites

Phosphites are the main organophosphorus compounds used to control oxidative degradation of lubricants. They eliminate hydroperoxides and peroxy and alkoxy radicals, retard the darkening of lubricants over time, and also limit photodegradation. In addition to their important role as antioxidants, phosphites are also found to be useful antiwear additives. Dialkyl hydrogen phosphites and diaryl hydrogen phosphites are neutral esters of phosphorus acid. These materials have two rapid equilibrating forms: the keto form, (RO)2P(=O)H, and the acid form, (RO)2P–O–H. Physical measurements indicate that they exist substantially in the keto form, associated in dimeric or trimeric groupings by hydrogen bonding. Trialkyl phosphites and triaryl phosphites are neutral trivalent phosphorus esters. These materials are clear, mobile liquids with characteristic odors. 8.2.2.2.1 Chemistry and Manufacture Phosphites are produced by reaction of phosphorus trichloride with alcohols or phenols as given by 3ROH + PCl3 + 3NH3 ⇒ P(OR)3 + 3NH4Cl

(8.8)

When mixed alkylaryl phosphites are produced, the reactants phenol and alcohol are added separately with the reaction temperature being controlled carefully. High-molecular-weight phosphites can be produced from transesterification reaction of either alcohols or phenols with trimethyl phosphite under catalytic (acidic) conditions. P(OCH3)3 + 3ROH ⇒ P(OR)3 + 3CH3OH

(8.9)

With acid catalyzed hydrolysis, dialkyl or diaryl hydrogen phosphites can be produced from trialkyl or triaryl phosphites as shown in reactions 8.10 through 8.12.

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223

P(OR)3 + H2O ⇒ (RO)2P(=O)H + ROH

(8.10)

P(OR)3 + HCl ⇒ (RO)2P(=O)H + RCl

(8.11)

2P(OR)3 + HP(=O)(OH)2 ⇒ 3(RO)2P(=O)H

(8.12)

By carrying out the preceding reactions in the presence of hydrogen chloride acceptors such as pyridine, the isolation of mono, di, and trialkyl phosphites is feasible. However, with alcohols of normal reactivity, the product is often mainly dialkyl hydrogen phosphite. This can be made in up to 85% yield, by adding PCl3 to a mixture of methanol and a higher alcohol at low temperature. The methyl and hydrogen chlorides are then removed by heating under reduced pressure on a steambath. PCl3 + 2ROH + CH3OH ⇒ (RO)2P(=O)H + CH3Cl + 2HCl

(8.13)

The commonly used phosphites available in the marketplace are dimethyl hydrogen phosphite, diethyl hydrogen phosphite, diisopropyl hydrogen phosphite, dibutyl hydrogen phosphite, bis(2ethylhexyl) hydrogen phosphite, dilauryl hydrogen phosphite, bis(tridecyl) hydrogen phosphite, dioleyl hydrogen phosphite, trisnonylphenyl phosphite, triphenyl phosphite, triisopropyl phosphite, tributyl phosphite, triisooctyl phosphite, tris(2-ethylhexyl) phosphite, trilauryl phosphite, triisodecyl phosphite, diphenylisodecyl phosphite, diphenylisooctyl phosphite, phenyldiisodecyl phosphite, ethylhexyl diphenyl phosphite, and diisodecyl pentaerythritol diphosphite. 8.2.2.2.2 Chemical and Physical Properties Phosphites tend to hydrolyze when exposed to humidity in the air or moisture in the lubricant. The extent of hydrolysis depends on the moisture content of the ambient atmosphere, the temperature, and the duration of exposure. Generally, liquid phosphites are more stable than solids because of the reduced surface area available for moisture pickup. But hydrolysis can be minimized if proper precautions, such as dry nitrogen atmosphere, cool storage, and use of tight seals, are observed. The lower dialkyl hydrogen phosphites hydrolyze in both acidic and alkaline solutions to monoalkyl esters and phosphorus acid. Rates of hydrolysis normally decrease with increasing molecular weight. The lower esters of trialkyl phosphites are rapidly hydrolyzed by acids; however, they are relatively stable in neutral or alkaline solutions. In general, the hydrolytic stability of the trialkyl phosphites increases with molecular weight. Since the dialkyl hydrogen phosphites are predominately in the keto form, they are somewhat resistant to oxidation and do not complex with cuprous halides. Both of these reactions are characteristic of trivalent organic phosphorus compounds [28–30]. These esters are relatively resistant to reaction with oxygen and sulfur, but react quite readily with chlorine and bromine giving the corresponding dialkyl phosphorohalidates ((RO)2P(=O)X where X = Cl or Br) [27]. The hydrogen atom of the dialkyl hydrogen phosphites is replaceable by alkali but is not acidic in the usual sense. The alkali salts are readily obtainable by reaction of ester with metals. In contrast with the parent compound, these salts readily add sulfur to form the corresponding phosphorothioates. Sodium salts of phosphites can be reacted with alkyl chlorides to produce alkyl phosphonates. These salts react with halophosphites to produce pyrophosphites and with chlorine or bromine to yield the corresponding hypophosphates. Dialkyl hydrogen phosphites add readily to ketones, aldehydes, olefins, and anhydrides, and these reactions are catalyzed by bases and free radicals. This type of reaction provides an excellent method for preparing phosphonates. Sulfur reacts readily with trialkyl or triaryl phosphites to form corresponding trialkyl or triaryl phosphorothioates, which are also very useful antiwear additives. The reaction of trialkyl phosphites with halogens is an excellent method for preparing dialkyl phosphorohalidates. Acyl halides and most polyfunctional primary aliphatic halides can be used. Triisopropyl phosphite provides a unique means for preparation of unsymmetrical phosphonates and diphosphonates because the by-product isopropyl halide reacts very slowly and thereby does not compete with the primary reaction.

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Lubricant Additives: Chemistry and Applications

8.2.2.2.3 Applications and Performance Characteristics Dialkyl (or diaryl) hydrogen phosphites, besides being excellent antiwear agents, are considered the most potent form of phosphorus, suited to high-torque, low-speed operations. This is the area where antiwear processes are taken to the extreme and is one of the most important sections of the EP performance spectrum. Sulfur can be quite incapable of giving protection under such conditions. Only a phosphorus source, if active enough and in sufficient concentration, can help here. Conversely, phosphorus components are of little use in high-speed and shock operations where sulfur components can be excellent. Dialkyl or diaryl phosphites are also potent antioxidants. With dialkyl phosphites, it has been reported that oxidation produces a phosphate anion, which tends to act as a bridging ligand to form an oligomeric iron (III) complex, that is, an iron oxide complex resembling Structure C. O R

P OH

O O

STRUCTURE C

However, there is also a weak, high-viscosity, nonsolid film that increases the overall thickness of the total film at high speeds [24,31]. Dialkyl phosphites are widely used in gear oils, automatic transmission fluids (ATF), and many other applications. Spiro bicyclodiphosphites are also reported to be used in continuously variable transmission fluids [32] (Structure D). O R

P O

STRUCTURE D

8.2.2.3

Dialkyl Alkyl Phosphonates

Dialkyl alkyl phosphonates [R–P(=O)(OR)2] are stable organic phosphorus compounds that are miscible with ether, alcohol, and most organic solvents. Besides being used as additives in solvents and low-temperature hydraulic fluids, they can also be used in heavy metal extraction, solvent separation, and as preignition additives to gasoline, antifoam agents, plasticizers, and stabilizers. Dialkyl alkyl phosphonates are prepared from either dialkyl hydrogen phosphites or trialkyl phosphites as described in reactions (Michaelis–Arbuzov reaction). (RO)3P + R′X ⇒ (RO)2P(=O)R′ + RX

(8.14)

(RO)2P(=O)H + R′OH + CCl4 ⇒ (RO)2P(=O)R′ + H2O

(8.15)

(RO)2P(=O)H + NaOH ⇒ (RO)2P∙O∙Na + R′X ⇒ (RO)2P(=O)R′ + NaX

(8.16)

In principle, the thermal isomerization of all phosphites to phosphonates can be carried out. The stability of these compounds varies greatly; however, depending on the nature of the R group, other products may be formed during heating. For R = methyl, complete conversion occurs at 200°C in 18 h, but for R = butyl, the compound is stable at 223°C. It is thought by some that isomerization of phosphites may be possible only if traces of phosphonate are already present as an impurity [33]. (RO)3P + Heat ⇒ (RO)2P(=O)R

(8.17)

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225

8.2.2.4 Acid Phosphates Acid phosphates are also potent additives, useful in similar areas of antiwear and EP to the dialkyl phosphites. Orthophosphoric (monophosphoric) acid (H3PO4), the simplest oxyacid of phosphorus, can be made by reacting phosphorus pentoxide with water. It is widely used in fertilizer manufacture. Orthophosphoric acid has only one strongly ionizing hydrogen atom and dissociates according to the following reaction H3PO4 ⇔ H+ + H2PO4– ⇔ H+ + HPO4 2– ⇔ H+ + PO4 3–

(8.18)

Since the first dissociation constant, K1 (7.1 × 10−3), is much larger than the second (K 2 = 6.3 × 10−8), very little of the H2PO4 produced in the first equilibrium goes on to dissociate according to the second equilibrium. Even less dissociates according to the third equilibrium since the third constant K3 is very small (K3 = 4.4 × 10−13). The acid gives rise to three series of salts containing these ions, for example, NaH2PO4, Na2HPO4, and Na3PO4. 8.2.2.4.1 Chemistry and Manufacture Alkyl (aryl) acid phosphates are made from alcohol (phenol) and phosphorus pentoxide. Generally, a mixture of monoalkyl (aryl) and dialkyl (aryl) phosphates is produced. 3ROH + P2O5 ⇒(RO)2P(=O)OH + (RO)P(=O)(OH)2

(8.19)

Pure monoalkyl or dialkyl (aryl) phosphates can be synthesized through different reaction routes as follows: ROH + POCl3 ⇒ ROP(=O)Cl2 ⇒ (Hydrolysis) ⇒ (RO)P(=O)(OH)2

(8.20)

(RO)2P(=O)H + Cl2 ⇒ (RO)2P(=O)Cl ⇒ (Hydrolysis) ⇒ (RO)2P(=O)(OH)

(8.21)

8.2.2.4.2 Properties, Performance Characteristics, and Applications Phosphoric acids tend to hydrolyze further when exposed to humidity. The extent of hydrolysis depends on the moisture content of the ambient atmosphere and the duration of exposure. Wherever possible, phosphoric acids should be handled in a dry nitrogen atmosphere to prevent hydrolysis. Therefore, for applications where incidental moisture contact is inevitable, acid phosphates are not recommended. Acid phosphates are used as rust inhibitors and antiwear additives. However, they are not as widely used as their amine-neutralized derivatives, for example, amine phosphates.

8.2.3

SULFUR–PHOSPHORUS ADDITIVES

Sulfur–phosphorus additives are used to provide protection against moderate to high pressure, metal-to-metal contacts in boundary lubrication, and EHL. Metallic sulfur–phosphorus additives, such as zinc dithiophosphates (ZnDTPs), are the most important antiwear/EP components used in engine oils. Ashless sulfur–phosphorus additives are used less extensively, and the most commonly available S/P additives in the marketplace are based on chemistries of dithiophosphates, thiophosphates, and phosphorothioates. Other important applications of S/P compounds are in matches, insecticides, flotation agents, and vulcanization accelerators. 8.2.3.1 Ashless Dithiophosphates Numerous patents were issued on the use of phosphorodithioic acid esters in lubricating oils in the early days. U.S. Patent 2,528,732 describes alkyl esters of phosphorodithioic acid. U.S. Patent 2,665,295

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Lubricant Additives: Chemistry and Applications

describes the S-terpene ester, whereas U.S. Patent 2,976,308 describes an anti-Markovnikov addition of phosphorodithioic acid ester to various olefins, both aromatic and aliphatic. Amine dithiophosphates and other novel dithiophosphate esters are reported in the literature [34–38]. Coupling with vinyl pyrrolidinone, acrolein or alkylene oxides (to make hydroxyl derivatives) are also known [39–41]. 8.2.3.1.1 Chemistry and Manufacture Similar to metallic dithiophosphates, ashless dithiophosphates are also based on phosphorus pentasulfide (P2S5) chemistry. They can be prepared from the same precursor of ZnDTP, dithiophosphoric acid (reaction 8.22) through the reaction of alcohol (or alkylphenol) and P2S5. 4ROH + P2S5 ⇒ 2(RO)2P(=S)SH + H2S

(8.22)

The dithiophosphoric acids are further reacted with an organic substrate to generate ashless derivatives. Typical organic substrates are compounds such as olefins, dienes, unsaturated esters (acrylates, methacrylates, vinyl esters, etc.), unsaturated acids, and ethers. The efficiency and stability of the ashless dithiophosphates very much depends on components used in their manufacture and the reaction conditions. The most common ashless dithiophosphate used in the marketplace is a dithiophosphate ester made from ethyl acrylate and o,o-diisopropyl dithiophosphoric acid as described in the following: [C3H7–O–]2–P(=S)S–CH2–CH2–C(=O)O–C2H5 Treatment of terpenes, polyisobutylene (PIB), or polypropylene (PP) with phosphorus pentasulfide and hydrolysis give thiophosphonic acids [R–P(=S)(OH)2 where R = PIB, terpenes, or PP). They can be further reacted with propylene oxide or amines to reduce acidity. However, this type of additive belongs to the same class of chemicals called ashless dispersants. Hence, they can be dual functional dispersants with improved antiwear/EP properties. 8.2.3.1.2 Applications and Performance Characteristics Unlike ZnDTP, ashless dithiophosphates are usually not as versatile, and therefore cannot be considered as multifunctional additives. Although ashless dithiophosphates have fairly good antiwear and EP properties, their anticorrosion properties are not as good as ZnDTP. This is closely related to the stability and decomposition mechanisms of ashless dithiophosphates. Relatively weak corrosion protection also limits their application at high concentrations in engine oils as well as some industrial oils. Ashless dithiophosphates can be useful in metalworking fluids, automotive transmission fluids (ATF), gear oils, greases, and non-zinc hydraulic fluids [42,43]. 8.2.3.2

Ashless Phosphorothioates and Thiophosphates

Numerous esters of the phosphorothioic acids are known. In salts and esters of these oxygen/sulfur (O–S) acids, there may be a preferred location of the multiple bonds, but in general, this is not well known. Thus in principle, there are two series of possible acids, each of which might give rise to salts and esters as described in the following: HO

O H3PO3S

Phosphorothionic acid

Phosphorothiolic acid

Phosphorothioic acid

(Thionophosphoric)

(Thiolophosphoric)

(Thiophosphoric)

Ashless Antiwear and Extreme-Pressure Additives HO

O H3PO2S2

227

Phosphorothiolothionic acid Phosphorodithiolic acid

Phosphorodithioic acid

(Thiolothionophosphoric)

(Dithiophosphoric)

(Dithiolophosphoric)

The “thionic” acids contain the group P=S, whereas the “thiolic” acids contain the group P–SH. The term “thioic” is often used when the molecular form is unknown or when specification is not desired. One form of these acids is usually more stable than the other, and it may not be possible to prepare both esters as, for example, the isomers of phosphorothioic acid. RO

RO P

P RO

In the case of some esters, the thiolo form is the most stable, but the phenyl ester exists 80% in thiono, (PhO)2P(=S)OH, and 20% in thiolo, (PhO)2P(=O)SH forms. The equilibrium of these compounds is liable to be dependent on the nature of the R groups, the solvent used, and even the concentration. Intermolecular hydrogen bonding may be expected to play a part in such equilibrium [33]. 8.2.3.2.1 Chemistry and Manufacture The creation of a compound with a phosphorus–sulfur linkage can often be carried out simply by heating the appropriate phosphorus compound with sulfur [44]. Likewise, the replacement of oxygen by sulfur in compounds containing P–O linkages can also be achieved simply by heating them with P2S5. Inorganic phosphorothioates (thiophosphates) are usually prepared from sulfur-containing phosphorus compounds. They are produced during the hydrolytic breakdown of phosphorus sulfides and are often themselves unstable in water. They hydrolyze to the corresponding oxy compounds with the evolution of H2S. Phosphorus–sulfur compounds are often thermally less stable than their oxy analogues. A few examples are listed as follows: P4S10 + 12NaOH ⇒ 2Na3PO2S2 + 2Na3PS3O + 6H2O

(8.23)

(BuO)2P(=O)SH + RI ⇒ (BuO)2P(=O)SR + HI

(8.24)

(PhO)3P + S ⇒ (PhO)3P=S

(8.25)

(PhO)3P + PSCl3 ⇒ (PhO)3P=S + PCl3

(8.26)

Hydrolysis of phosphorothioate esters results in a progressive loss of sulfur as hydrogen sulfide (H2S) and its replacement by oxygen. (RO)3P=S + H2O ⇒ (RO)3P=O + H2S

(8.27)

8.2.3.2.2 Applications and Performance Characteristics It has been known for many years that sulfur compounds form a film of iron sulfide, and phosphorus compounds form iron phosphate, on the mating metal surfaces. Generally, the films formed from sulfur sources such as SIB are expected to contain FeS, FeSO 4, as well as organic fragments from

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Lubricant Additives: Chemistry and Applications

the additive decomposition. With phosphorus sources, such as dialkyl phosphites, films containing FePO4, FePO3, as well as organic fragments are expected. When both sulfur and phosphorus are present, both elements contribute to the nature of the film, and which one predominates depends on the S/P ratio, the decomposition mechanisms, and the operating conditions, for example, high speed and shock or high torque/low speed. Ashless phosphorothioates are widely used as replacements for metallic dithiophosphates in many lubricant applications where metal is less desirable [43,44]. Phosphorothioates are often present (generated in situ) in lubricant formulations when both sulfur and phosphorus additives are used. Aryl phosphorothioates provide good thermal stability and good antiwear/EP properties as evidenced by their strong FZG performance.

8.2.4

SULFUR–NITROGEN ADDITIVES

Sulfur and nitrogen-containing additives are used to provide protection against moderate to high pressure, metal-to-metal contacts in boundary lubrication, and EHL. Both open chain and heterocyclic compounds have attracted a considerable amount of research effort to explore their potential as antiwear and EP additives. Among open chain additives, dithiocarbamates are the most widely used. Other additives, such as organic sulfonic acid ammonium salts [45], and alkyl amine salts of thiocyanic acid [46] are reported in the literature, but are of relatively low commercial value. Nitrogen and sulfur-containing heterocyclic compounds, such as 2,5-dimercapto-1,3,4-thiadiazole (DMTD, Structure E), 2-mercapto-1,3-benzothiazole (MBT, Structure F), and their derivatives, have been used for many years as antioxidants, corrosion inhibitors, and metal passivators; generally at relatively low concentrations. N

S C

STRUCTURE E

8.2.4.1

STRUCTURE F

Dithiocarbamates

The dithiocarbamates, the half amides of dithiocarbonic acid, were discovered as a class of chemical compounds early in the history of organosulfur chemistry [47,48]. The strong metal-binding properties of the dithiocarbamates were recognized early, by virtue of the insolubility of metal salts and the capacity of molecules to form chelate complexes. Other than applications in lubricant areas, dithiocarbamates have been used in the field of rubber chemistry as vulcanization accelerators and antiozonants. 8.2.4.1.1 Chemistry and Manufacture Organic dithiocarbamates can be made by a one-step reaction of dialkylamine, carbon disulfide, and an organic substrate. The organic substrate is preferably an olefin, diene, epoxide, or any other unsaturated compounds as exemplified in the literature [49,50]. Organic dithiocarbamates can also be made through a two-step reaction involving ammonium or metal dithiocarbamate salts and organic halides [51]. In the case of their ammonium salts, N-substituted dithiocarbamic acids, RNHC(=S)SH or R2NC(=S)SH, are formed by reaction of carbon disulfide with a primary or secondary amine in alcoholic or aqueous solution before they are further reacted with ammonia. To conserve the more valuable amine, it is a common practice to use an alkali metal hydroxide to form the salt. RNH2 + CS2 + NaOH ⇒ RNHC(=S)S–Na + H2O

(8.28)

The dithiocarbamic acid can be precipitated from an aqueous solution of dithiocarbamate by adding strong mineral acid. The acids are quite unstable but can be held below 5°C for a short time.

Ashless Antiwear and Extreme-Pressure Additives

229

The most common additive, methylene bis-dibutyl dithiocarbamate, is prepared from sodium dibutyl dithiocarbamate and methylene chloride. 2(C4H9)2NC(=S)S–Na + CH2Cl2 ⇒ [(C4H9)2NC(=S)S]2CH2 + 2NaCl

(8.29)

8.2.4.1.2 Applications and Performance Characteristics Unlike metallic dithiocarbamates that have been widely used in lubricants, ashless dithiocarbamates have only been gaining more attention recently. Relatively high cost is certainly a major factor in limiting wider use. The success of metallic dithiocarbamates also overshadows their ashless counterpart. Certain metallic dithiocarbamates, such as molybdenum dithiocarbamates, offer exceptionally good frictional properties that cannot be matched by their ashless analogues also. However, ashless dithiocarbamates have been found to be versatile, multifunctional additives in a few areas. They can be effective antiwear/EP additives as well as good antioxidants and metal deactivators [52–55], (Structures Ga and Gb). They tend to generate less sludge or deposits than mostly metallic additives and they are very compatible with various base oils. S

S R′

R′ N

N R

R OH

STRUCTURE Ga S R′ R′′

N R

STRUCTURE Gb

8.2.4.2

Dimercaptothiadiazole and Mercaptobenzothiazole Additives

Additives derived from DMTD and 2-mercaptobenzo-thiazole (MBT) are well documented in the literature. Owing to strong ring stability (partial aromaticity and resonance delocalization), balanced sulfur–nitrogen distributions, and reactive mercaptan groups, both heterocyclic compounds can be versatile core molecules to make many useful additives with many beneficial characteristics, such as improved thermal/oxidative stability and reduced corrosivity. Unfortunately, some potentially good reactions are hampered by the limited solubility of DMTD and MBT in common petrochemical solvents. Therefore, a suitable sample preparation procedure is very critical to help achieve desirable antiwear/EP additives. 8.2.4.2.1 Chemistry and Manufacture Many differing organic reactions can be applied to functionalize the mercaptan groups of DMTD and MBT. Oxidative coupling reactions involving other alkyl mercaptans can bring in additional sulfur for EP performance and additional alkyl chains for improved solubility [56]. (Addition reactions with organic compounds containing activated double bonds can link DMTD or MBT heterocyclic core molecules with long chain esters, ketones, ethers, amides, and acids together [57–60]). Likewise, ring opening with epoxides to generate alcohol derivatives is also known [61]. Direct amine salts formation and linking alkyl amines through Mannich base condensation are also extensively studied [62–64]. A number of examples are listed in reactions 8.30 through 8.33, where TD is the abbreviation for the thiadiazole moiety and BT is for the benzothiazole moiety.

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Lubricant Additives: Chemistry and Applications

Oxidative coupling DMTD + 2RSH + 2H2O2 ⇒ RS–S–(TD)–S–SR + 4H2O

(8.30)

Mercapto alkylation and Mannich alkylation DMTD + 2CH2=O + 2RSH ⇒ RS–CH2–S–(TD)–S–CH2–SR + 2H2O

(8.31)

MBT + CH2=O + RNH2 ⇒ (BT)–S–CH2–NHR + H2O

(8.32)

Amine salt formation DMTD + 2RNH2 ⇒ RNH3–S–(TD)–S–NH3R

(8.33)

8.2.4.2.2 Applications and Performance Characteristics MBT is a light yellow powder with limited solubility in hydrocarbons. It is more soluble in aromatic solvent (~1.5% in toluene), polar solvents, and highly aromatic oils. MBT is used as a copper corrosion inhibitor in fuels as well as a corrosion inhibitor/deactivator in numerous industrial lubricants such as heavy-duty cutting and metalworking fluids, hydraulic oils, and lubricating greases. DMTD is also a light yellow powder with very limited solubility in hydrocarbons. It is considered a versatile chemical intermediate suitable for making various oil-soluble derivatives. Both MBT and DMTD derivatives are widely used as copper passivators and nonferrous metal corrosion inhibitors. Some proprietary load-carrying additives are substituted MBT and DMTD compounds that are used in various applications either as a component or as a part of additive packages with a specific purpose [65,66]. In the absence of any phosphorus moiety in MBT and DMTD, their oil-soluble derivatives are suitable for replacing zinc dithiophosphates in some lubricant applications. For example, a commercial, high-density, powder-like MBT and DMTD derivatives is used as a dual functional antioxidant/EP agent in greases. 8.2.4.3 Other Sulfur–Nitrogen Additives In addition to DMTD, MBT, and dithiocarbamate additives, there are other sulfur–nitrogen-containing additives available in the marketplace or reported in the literature. Among these, phenothiazine derivatives (Structure H, PTZ), substituted thiourea additives (Structure I, TU), thionoimidazolidine derivatives (Structure J, TIDZ), thiadiazolidine and oxadiazole (ODZ) derivatives (Structures K and L), thiuram monosulfides, thiuram disulfides, and benzoxazoles are of particular interest because they are all sulfur- and nitrogen-rich molecules [67–73]. Thiuram disulfides, chemically similar to dithiocarbamates, can be used in the rubber industry as vulcanizers. 2-Alkyldithio-benzoxazoles also offer good frictional properties in addition to strong antiwear/EP properties [74]. H N

S R′

STRUCTURE H, PTZ

R2 S

STRUCTURE I, TU

Ashless Antiwear and Extreme-Pressure Additives

231

H R1

R2 O

H O R5

O R5

STRUCTURE J, TIDZ R2 H N

N S

N SH

X R3

STRUCTURE K, TDZL

8.2.5

STRUCTURE L, ODZ

PHOSPHORUS–NITROGEN ADDITIVES

Phosphorus–nitrogen additives are used to provide protection against moderate to high pressure, metal-to-metal contacts in boundary lubrication, and EHL. Ashless phosphorus–nitrogen additives are used as dual functional antiwear/antirust additives extensively, and those that are most commonly available in the marketplace are based on chemistries of amine dithiophosphates, amine thiophosphates, amine phosphates, and phosphoramides. 8.2.5.1

Amine Phosphates

Amine phosphates are by far the most important phosphorus–nitrogen-containing additives used in lubricants. In fact, they are multifunctional additives possessing very good antirust properties as well as antiwear/EP properties. 8.2.5.1.1 Chemistry and Manufacture Amine phosphates are produced by treating acid phosphates with alkyl or aryl amines. Under various conditions, neutral, overbased, and underbased amine phosphates can be synthesized. If mixed monoand dialkyl acid phosphates are used as starting materials, mixed mono and dialkyl amine phosphates are produced. The final additives usually possess high total acid number (TAN) and high total base number (TBN), although reaction adducts are considered fairly neutral. It is known that a complete neutralization of both phosphoric acid groups in monoalkyl acid phosphates with amines cannot be easily achieved, and therefore, under normal conditions, a partially neutralized amine phosphate is formed. (RO)2P(=O)(OH) + R′NH2 ⇒ (RO)2P(=O)O∙NH3R′

(8.34)

(RO)P(=O)(OH)2 + R′NH2 ⇒ (RO)P(=O)(OH)O∙NH3R′

(8.35)

8.2.5.1.2 Applications and Performance Characteristics Amine phosphates are extensively used in industrial oils, greases, and automotive gear oils. They offer very good rust protection as demonstrated in various bench rust tests (ASTM D665 and CRC L-33). They also show very good antiwear/EP characteristics (Four-Ball Wear and Four-Ball

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EP, FZG, Timken, and CRC L-37). Since amine phosphates are very polar species, they interact strongly with other additive components, making their performance very dependent on the formulation. Hence, extra attention is needed when amine phosphates are used. 8.2.5.2

Amine Thiophosphates and Dithiophosphates

Amine thiophosphates and amine dithiophosphates can be found in engine oils and industrial oils where zinc dithiophosphates and other nitrogen-containing additives are used, either as decomposition products or as in situ-produced products. They are critical to the lubricant performance because of their high activity toward metal surfaces. 8.2.5.2.1 Chemistry and Manufacture Amine thiophosphates are produced by reacting thiophosphoric acid with alkyl or aryl amines [75]. Likewise, amine dithiophosphates are synthesized from dithiophosphoric acid and amines. (RO)2P(=S)SH + H2NR′ ⇒ (RO)2P(=S)S∙H3NR′

(8.36)

(RO)2P(=O)SH + H2NR′ ⇒ (RO)2P(=O)S∙H3NR′ + (RO)2P(=S)O∙H3NR′

(8.37)

8.2.5.2.2 Applications and Performance Characteristics Amine thiophosphates and dithiophosphates are also multifunctional additives providing good rust inhibition and antiwear properties. Owing to their high activity and low stability, amine thiophosphates and dithiophosphates are not as extensively used as either amine phosphates or metallic dithiophosphates. A detailed study of their antiwear mechanisms suggested that a tribofragmentation process is involved [76,77]. Relatively poor corrosion control is one area of concern that needs attention. With proper formulation adjustments, it is quite feasible to overcome certain intrinsic weaknesses and apply both chemistries to various lubricant products. 8.2.5.3

Other Phosphorus–Nitrogen Additives

There are many other phosphorus–nitrogen-containing ashless antiwear additives reported in the literature. Some are proprietary technologies, and their commercial status is unknown. Organophosphorus derivatives of benzotriazole (BZT) are a group of additives based on triazole and dialkyl or dialkylphenyl phosphorochloridate chemistry [78]. Arylamines and dialkyl phosphites can be coupled through a Mannich condensation reaction to form unique phosphonates that are used as multifunctional antioxidant and antiwear additives [79]. Bisphosphoramides are also reported [80].

8.2.6

NITROGEN ADDITIVES

Nitrogen-containing additives are used to provide rust inhibition and cleanliness features in various lubricant applications. For example, nitrogen-containing ashless dispersants are a key component for engine oils, and alkoxylated amine compounds are used in lubricating greases to provide corrosion inhibition [81]. Furthermore, arylamines are widely used as antioxidants due to their ability to terminate radical chain propagation and decompose peroxides. Very few nitrogen additives alone are considered effective antiwear/EP additives, and their performance is either very specific to industrial applications or fairly dependent on product formulations. However, when used in combination with other sulfur, phosphorus, or boron additives, nitrogen-containing additives can be very effective supplements to enhance antiwear/EP performance. 8.2.6.1 Chemistry, Manufacture, and Performance Several novel chemistries are available in the literature for nitrogen-only antiwear additives. Among these, dicyano compounds were tested and they exhibited very good Four-Ball Wear activities [82]. Polyimide-amine salts of styrene–maleic anhydride copolymers are also reported as antiwear additives; however, high additive concentrations (5–10%) are needed [83]. Alkoxylated amines (Structure M) and mixtures of fatty acid, fatty acid amide, imide or ester derived from substituted

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TABLE 8.2 SAE 5W-20 Prototype Motor Oil Formulation Component

Formulation A (wt%)

Solvent neutral 100 Solvent neutral 150 Succinimide dispersant Overbased calcium phenate detergent Neutral calcium sulfonate detergent Rust inhibitor Antioxidant Pour point depressant OCP VI improver Antiwear additivea a

22.8 60 7.5 2 0.5 0.1 0.5 0.1 5.5 1

In the case of no antiwear additive present in the formulation, solvent neutral 100 is put in its place at 1.0 wt%.

TABLE 8.3 Four-Ball Wear Results Compound

Formulation

No antiwear additive 1.0 wt% ZDDP 0.5 wt% ZDDP 5-Heptadecenyl-1,3,4-oxadiazole 5-Heptyl-1,3,4-oxadiazole 5-Heptadecenyl-2,2-dimethyl-1,3,4-Oxadiazole 5-Heptadecenyl-2-furfuryl-1,3,4-Oxadiazole a

Wear Scar Diameter (mm) 0.73 (0.74)a 0.50 (0.51) 0.70 (0.67) 0.38 (0.38) 0.54 (0.56) 0.7 0.38 (0.39)

A A A A A A A

Duplicated runs.

succinic acid or anhydride have been identified to be good fuel lubricity additives [84] (Structure M). Alkyl hydrazide additives possessing two adjacent nitrogen atoms have also been claimed to exhibit good antiwear properties [85] (Structure N). Products of nitrogen heterocycles, such as ODZ (Tables 8.2 and 8.3 for performance evaluations), BZT, tolyltriazole (TTZ), alkyl succinhydrazide (SHDZ), and borated hydroxypyridine (BHPD) (Structures O, P, Q, R, S, respectively), with pendant alkylates, amines or carboxylic acids have been found to be effective antiwear additives in both lubricants and fuels [86–92]. Although triazoles are costly chemicals, they have unique geometric structures that contribute to high surface film–forming efficiency. RO(C4H8O)nCH2CH2CH2NH2 (M) STRUCTURE M R4

N R1

N H

STRUCTURE N (AHDZ)

R3 R1

H N N

STRUCTURE O (ODZ)

STRUCTURE P (BZT)

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Lubricant Additives: Chemistry and Applications R2 O R3 N

H N

H3C

STRUCTURE Q (TTZ)

STRUCTURE R (SHDZ)

OR4 OR5

STRUCTURE S (BHPD)

Both BZT and TTZ derivatives are also effective copper deactivators at low concentrations. Therefore, these types of additives indeed have dual functions. They find applications in industrial oils, greases, and fuels. Table 8.2 lists a prototype engine oil formulation used for the evaluation of ODZ additives where various ODZs can be blended at 1 wt% in place of the same amount of light base oil. Table 8.3 lists the Four-Ball Wear performance data where a series of ODZs were evaluated against 0.5 wt% and 1 wt% ZDDP. As demonstrated, those ODZ additives exhibited fairly good antiwear properties in this bench test [86].

8.2.7

ADDITIVES WITH MULTIPLE ELEMENTS

Complex additives with multiple elements can be derived from various S/P, sulfur/nitrogen, phosphorus/nitrogen, and many other traditional additive building blocks. As a result, molecules with more than four, five, six, or even more elements are created (S/P/N/B in addition to C/H/O). Derivatization frequently adds a degree of complexity, yet provides a chance of achieving better synergisms among all critical elements that can not only satisfy the performance needs but also help neutralize any potentially added costs associated with the new chemistry under development. Many examples are available in the literature as well as in the marketplace, such as amine salts of dithiophosphates and thiophosphates (Section 8.2.5.2); borated derivatives of dithiophosphates [41], dithiocarbamates [50], and dimercapto-thiadiazole [93]; urethane derivatives of dithiophosphates [94] (Structure T); and reaction adducts of dialkyl phosphites, sulfur, and acylated amines [95]. O C

S RO

R S

CH3 O

O NH

C O

OR′

R S

OR′

STRUCTURE T

Several complex additives have different chemistries involved with the same element in a single molecule to attain strong synergisms. As exemplified in the following case, where both phosphite chemistry and phosphate chemistry are incorporated into the same molecule, greater antiwear performance can be achieved ([96]; reaction 8.38).

Ashless Antiwear and Extreme-Pressure Additives O

235 O

P2O5

R′′-CH=O RO

CHR′′

OH Amines

OR′

O OR RO

CHR′′

P(O)(O)2

(H3N

OR′

CHR′′

OR′

CHR′′

R′′′)2

+ RO

P(O)-O

H3N

R′′′

OR′

(8.38) The synergistic antiwear performance of the aforementioned complex phosphorus additives are illustrated in Chart 8.1. Nine different analogues were synthesized and tested at 1 wt% in base oils using three different conditions in the Four-Ball Wear test. As demonstrated, they all exhibit exceptionally good antiwear properties.

8.2.8

HALOGEN ADDITIVES

Chlorine was one of the earliest antiwear and EP elements used in the lubricant industry. Chlorine-containing additives are still used in cutting oils and related metalworking lubricants, in combination with sulfur additives. Iodine was mentioned in aluminum-processing lubricants for wear control. Fluorine, in perfluorinated compounds, is well known to reduce wear and especially friction.

WSD Four-Ball Wear Test (WSD in millimeters) 2.5 Test 1−93C, 40 kg, 1800 rpm, 30 min

Test 2−93C, 60 kg, 1500 rpm, 30 min 1.5

Test 3−135C, 60 kg, 100 rpm, 30 min

1 0.5 0 BS

BS +

1% (A) 1% (B) 1% (C) 1% (D) 1% (E) 1% (F) 1% (G) 1% (H) 1% (I) All tested additives (A−J) were derived from dibutyl phosphite, butyraldehyde, and a selected amine at various molar ratios. A and B: using Primene JMT at different ratios, C and D: using Adogen 183 at different ratios, E,G, and I: using Duomeen O at different ratios, F and H: using bis 2-EH amine at different ratios.

CHART 8.1

Complex phosphorus additives. (BS, base stock; WSD, wear scar diameter.)

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Lubricant Additives: Chemistry and Applications

The chlorine compounds act and function in that they coat the metal surface with a metal chloride film under the influence of high pressure at point of lubrication and in the presence of traces of moisture. FeCl2 melts at 672°C and has low shear strength when compare with steel. The effect of chlorine compounds depends on the reactivity of the chlorine atom, temperature, and concentration. Hydrogen chloride formed in the presence of larger quantities of moisture can cause severe corrosion of the metal surfaces. As the corrosion hazards increase along with the EP properties with increasing reactivity of the chlorine atoms, a compromise must be found in the development of chlorine-containing additives. Chlorinated paraffins such as trichlorocetane represent a group of important EP additives used in the past. They can significantly increase the load stages in the FZG test with increasing concentration. The chain length has practically very little influence on the EP effect; on the contrary, the load-carrying capacity increases with increasing degree of chlorination. In practice, chlorinated paraffins with ~40 to 70 wt% chlorine are used; however, they are sensitive to moisture and light and can easily evolve hydrogen chloride [97]. Compounds such as phenoxy-propylene oxide, amines, or basic sulfonates neutralize hydrogen chloride and thus act as stabilizers. Good results are also obtained with chlorinated fatty acids and their derivatives; particularly those with trichloromethyl groups in the end position, since the additives with CCl3 groups are particularly effective. Owing to their high stability, chlorinated aromatics have less favorable EP properties than the chlorinated aliphatics. Alkylaromatics with chlorinated side chains improve the load-carrying capacity much more than those chlorinated in the ring; the efficiency increases with the number of carbon atoms in the side chain. Chlorinated fatty oils and esters as well as chlorinated terpenes and amines have also been patented as EP additives. Sulfur–chlorine additives were found to be satisfactory for gear lubrication in passenger cars in the mid-1930s. Apparently, this type of additive could satisfy the high-speed and moderate-load operation of passenger cars used in that time period. When sulfur and chlorine are combined in the organic molecule, sulfur somewhat reduces the corrosive tendency of chlorine; on the contrary, the EP properties of the combined moieties are improved in comparison with the individual compounds. Chlorinated alkyl sulfides, sulfurized chloronaphthalenes, chlorinated alkyl thiocarbonates, bis-(pchlorobenzyl) disulfide, tetrachlorodiphenyl sulfide, and trichloroacrolein mercaptals [Cl2C=CCl– CH(SR′)–SR″, where R′ and R″ are alkyl or aryl] must be mentioned in this class. Reaction products of olefins and unsaturated fatty acid esters with sulfur chlorides contain highly reactive β-chlorosulfides, which due to their reactive chlorine and sulfur atoms give very good EP agents, yet show more or less strong corrosive tendencies. However, severe wear was frequently encountered in truck axles where performance under high-torque, low-speed conditions is of greater importance. Later on, the presence of chlorine, although a good EP agent, was found to be detrimental to lubricant thermal stability. Hence, for the past 30 years, chlorine has not been used in gear oils. Chlorinated trioleyl phosphate, condensation products of chlorinated fatty oils with alkali salts of dithiophosphoric acid diesters, and reaction products of glycols with PCl3 are examples of chlorine–phosphorus additives used in earlier years. The most serious drawback for chlorine antiwear and EP additives is in the environmental area. Legislation around the industrial world limits the chlorine content of many lubricants to parts per million. Therefore, except for the cutting oil industry, which is also under pressure to change, chlorine additives are not considered a viable option for modern lubricants.

8.2.9

NONTRADITIONAL ANTIWEAR/EXTREME-PRESSURE ADDITIVES

Traditional sulfur, phosphorus, and halogen-related compounds are considered to be the dominant antiwear/EP additives in the marketplace. However, as environmental concerns escalate, the future trends will favor products that diminish potential hazard and disposal problems. Recent clean fuel

Ashless Antiwear and Extreme-Pressure Additives

237

activities are driving sulfur levels toward 10–50 wt ppm ranges. Subsequently, the petroleum industry is favoring lower sulfur lubricants since sulfur is also known to poison the catalytic system used for NOX reduction. Therefore, the use and development of nontraditional antiwear additives is becoming more valuable. A number of nonsulfur, nonphosphorus ashless antiwear additive technologies have been reported in the literature [98–102]. Among these, high hydroxyl esters (HHE), dimer acids, hydroxyamine esters, acid anhydrides, cyclic amides, and boron derivatives are recognized as leading technologies. Graphite and polytetrafluoroethylene (PTFE) possess excellent friction reduction properties and indirectly contribute some antiwear/EP characteristics. However, both materials need to be dispersed in the oil as they have very limited lubricant solubility, which hampers their usefulness. Organic borates are considered as effective friction modifiers, antioxidants, and cleanliness agents. Recent studies indicate that some borates can be good antiwear additives. Potassium borates have been used in gear oils for years, but these types of metallic borates are outside the scope of this chapter. Esters are known to possess good lubricity properties. The properties can be further improved to offer antiwear characteristics through proper functionalization. Several companies have marketable products in this area.

8.3

MANUFACTURE, MARKETING, AND ECONOMICS

All major additive suppliers produce ashless antiwear and EP additives that are available as components and packages. Following is a list of major producers (arranged in alphabetical order). • • • • • • • • • • • • • • • • • • • •

Afton Corporation Akzo Nobel Atofina Chemicals (former Elf Atochem NA and Pennwalt Corporation) BASF Chemtura (former Great Lakes Chemical’s Durad Division) Ciba Specialty Chemicals Chevron Corporation (Oronite Division) Clariant Dover Chemical (former Keil Chemical Division, Ferro) Dow Chemical (former Angus Division) Elco Corporation (Detrex) FMC Hampshire Chemical Corporation (former Evans Chemetics) ICI America (Uniqema) Infineum International Limited Lubrizol Corporation Polartech Rhein Chemie Rhodia (former Albright & Wilson) Zeneca

Ashless antiwear and EP additives are supplied in various chemistries, including single and multiple blends formulated to maximize performance and minimize adverse effects (e.g., dropout and corrosion). Product designations vary by chemical class and concentration. Many of them are formulated into additive packages according to applications, such as passenger car engine oils, heavy diesel engine oils, automotive transmission oils, automotive gear oils, hydraulic fluids, and others. Since the product offering information can be supplier-specific, it is recommended to contact the suppliers directly or go to their corresponding Web site for further information.

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Lubricant Additives: Chemistry and Applications

There has been some consolidation in the additive business, but the market has not changed much as a result. Following are the major changes by year. 1992 1996 1997 1999

1999 2001 2003 2004

2005 2007

Ethyl acquired Amoco Petroleum Additives (U.S.) and Nippon Cooper (Japan) Ethyl acquired Texaco Additives Company Lubrizol bought Gateway Additives (Spartanburg, South Carolina) Infineum, the new petroleum additives enterprise, a joint venture between Exxon Chemical, Shell International Chemicals Ltd., and Shell Chemical Company, unveiled its new corporate identity and became fully operational on January 1, 1999 (the largest merge of additive companies in history) Crompton completed a merger with Witco Texaco Oil merged into Chevron Oil whereas Chevron Chemical Oronite Division was kept intact Dover Chemical acquired the Keil Chemical petroleum additives business from Ferro Corporation Ethyl Corporation transformed into NewMarket Corporation, the parent company of Afton Chemical Corporation and Ethyl Corporation to maximize the potential of its operating divisions—petroleum additives and tetraethyl lead fuel additive business Chemtura was formed by the merger of Crompton and Great Lakes Chemical Corporation Chemtura bulked up its specialty lubricants business with the assets of Kaufman Holdings Corporation, parent company of Anderol and Hatco

Since most lubricant additives are produced through batch processes, consolidation can lead to improved operations and reduced costs (e.g., reducing plant idle time with better chemical manufacturing management systems). There are still many manufacturing facilities using equipment and procedures that are 30–40 years old. Hence, any investments in automation and continuous processing for a plant will be a competitive advantage. However, the business is so cost-competitive that most suppliers have difficulties in justifying major capital expenditures.

8.4 EVALUATION EQUIPMENT/SPECIFICATION 8.4.1

LUBRICANT SPECIFICATIONS

Lubricant components and formulated products are manufactured as per the rigid specifications in petroleum refineries and lubricant blending plants, and must also meet detailed commercial, industrial, and military specifications. As an example, the U.S. Military has rigid specifications for automotive lubricants, although the automotive manufacturers have similarly rigid but not necessarily the same specifications to assure quality and consistency of lubricant manufacture. In addition, there are performance specifications that must be met from such original equipment manufacturers (OEMs) as farm machinery and other off-highway automotive equipment. These specifications are designed to enable the user to select appropriate lubricants and to be assured of adequate performance over a specified service life. The industry is, for the most part, adequately self-regulating with minimal government input concerning performance specifications. The most elaborate system for developing and upgrading lubricant and fuel specifications is for automotive lubricants. The American Society of Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and the American Petroleum Institute (API) all have defined roles in determining specifications for products such as passenger car motor oils and heavy-duty motor oils. These three organizations, working together in the United States, are known as the Tripartite. Extending internationally, the International Lubricant Standardization and Approval Committee (ILSAC) is also active in all phases of engine lubricant category development.

Ashless Antiwear and Extreme-Pressure Additives

239

In other product categories, lubricant and additive suppliers, OEMs, and industry trade associations work together to determine performance requirements and product specifications. In addition to the three industry organizations mentioned earlier, the National Lubricating Grease Institute (NLGI), the National Marine Manufacturers Association (NMMA), the American Gear Manufacturers Association (AGMA), the Society of Tribologists and Lubrication Engineers (STLE), and other groups, associations, and key equipment builders can influence lubricant specifications. In addition to meeting all military and industrial specifications, many leading lubricant marketers and finished lubricant suppliers develop their own internal specifications to be used for new product launching, competitive product analysis, and future product development. Proprietary fieldtesting is an integral part of the overall new lubricant product development processes and is often the most critical step to assure technical success and customer satisfaction for new products.

8.4.2

ADDITIVE SPECIFICATIONS

Specifications for antiwear/EP additives focus primarily on application, base oil compatibility, and quantification of elemental constituents. In addition, specifications typically identify specific and critical performance standards for applications. Common specifications for antiwear/EP additives are shown in Table 8.4. In addition to typical specifications as reported in the Certificate of Analysis (C of A) from additive suppliers, individual lubricant marketers often prefer to conduct their own internal additive specifications, such as infrared analysis and key performance testing.

8.4.3

TEST METHODS AND EQUIPMENT

In the United States, a number of bench and advanced tests were developed and approved by ASTM, and these tests have gained widespread reception throughout the industry. However, there are also a few selected lab-bench and advanced tests that were developed and approved only by specific OEMs, but represent certain critical and desirable performance features (Figures 8.2 and 8.3). This chapter is not intended to cover all evaluation tests in detail, but rather to illustrate a few representative tests to highlight the key assessment criteria. 1. Four-Ball Wear and EP Test. This tester was developed to evaluate the antiwear, EP, and antiweld properties of lubricants. It is a simple bench test machine designed to measure the protection a lubricant provides under conditions of high unit pressures and various sliding velocities. The Four-Ball Wear tester consists of four 1.5 in. diameter steel balls arranged in the form of an equilateral tetrahedron. The three lower balls are held immovably in a TABLE 8.4 Typical Speciﬁcations for Antiwear and EP Additives Chemical Class Amine phosphates Methylene bis-dialkyl dithiocarbamate Sulfurized lard, esters, fatty acids Triphenyl phosphorothioate Chlorinated paraffins, fatty acids

Property

Performance Test

Percent of nitrogen, phosphorus, and TAN/TBN Percent of sulfur, nitrogen, and residual chlorine, amine Percent of total sulfur Percent of active sulfur Percent of sulfur, phosphorus, and melting point Percent of chlorine Acid value

Four-Ball Wear, Four-Ball EP, FZG, rust/oxidation test Four-Ball EP, FZG, Falex EP, oxidation/corrosion test Four-Ball Wear, Four-Ball EP, stick-slip, Cu corrosion Four-Ball EP, FZG, Falex EP, oxidation/corrosion test Four-Ball Wear, Falex EP, Timken, Cu corrosion

240

Lubricant Additives: Chemistry and Applications Boundary Machine ways heavily loaded gears

Elastohydrodynamic Gears rolling element bearings

Hydrodynamic Journal bearings

Friction

Viscosity × speed / load Controlling lubricant factors

1. Viscosity Viscosity and viscosity index 2. Pressure-viscosity coefficient

Active antiwear and EP additives

FIGURE 8.2

Lubrication regimes.

• Availability • Novel chemistries are available (S/P/N/B) • Protecting film can be formed from low-shear, nonmetallic species • Process development, registration, and commercialization plan pending on market demand and timing • Applicability • Bench tests only serve as indicators • Balance of performance is key • Selection • Identifying critical tests first • Seeking combinations of additives • Using QSAR analysis

Four-Ball Wear/EP HFRR & Plint Multispecimen Optimol SRV Pin-on-V-block

FIGURE 8.3

Seq IVA and IIIG JAMA chain wear FZG and FE8 M11 EGR 35VQ25 pump OM 602/TU 3M L37/L42 Engine and industrial oils • Reduced exhaust emissions • Improved stability • High temperature oxidation/nitration • Oxidation resistance/hydrolytic stability • Improved compatibility and cleanliness • Improved surface fatigue-wear protection • Pitting and micropitting • Antiscuffing • Lo w friction • Fuel economy and energy conservation • Improved durability • Extended drain • Fill for life

Ashless antiwear additives: availability, applicability, selection, and future needs.

clamping pot, while the fourth ball is made to rotate against them. Test lubricant is added in the test pot, covering the contact area of the test balls. During a test, wear scars are formed on the surfaces of the three stationary balls. The diameter of the scars depends on the load, speed, temperature, duration of run, and type of lubricant. The Four-Ball EP tester runs at a fixed speed of 1770 ± 60 rpm and has no provision for lubricant temperature control. A microscope is used to measure the wear scars. Two of the standard tests run on the Four-Ball machine are Mean-Hertz Load and Load-Wear Index. ASTM D 2596 covers the detailed calculation procedure of Load-Wear Index for greases and D-2783

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for oils. These procedures involve the running of a series of 10 s tests over a range of increasing loads until welding occurs. From the scar measurements, the mean load (loadwear index) is calculated and it serves as an indicator of the load-carrying properties of the oil being tested. FZG Four-Square Gear Test Rig. The FZG test equipment consists of two gear sets, arranged in a four-square configuration, driven by an electric motor. The test gear set is run in the test fluid, while increasing load stages (from 1 to 13) until failure. Each load stage is run for a 15 min period at a fixed speed. Two methods are used for determining the damage load stage. The visual rating method defines the damage load stage as the stage at which more than 20% of the load-carrying flank area of the pinion is damaged by scratches or scuffing. The weight loss method defines the damage load stage as the stage at which the combined weight loss of the drive wheel and pinion exceeds the average of the weight changes in the previous load stages by more than 10 mg. The test is used in developing industrial gear lubricants, ATFs, and hydraulic fluids to meet various manufacturers’ specifications. Falex EP/Wear Tester. The Falex test machine provides a rapid method of measuring the load-carrying capacity and the wear properties of lubricants. The test consists of rotating a test pin between two loaded journals (V-blocks) immersed in the lubricant sample. There are two common tests run in this machine: one is an EP test (subjecting a test lubricant to increasing loads until a failure occurs) and the other is a wear test (subjecting a lubricant to a constant load for a definite period of time while measuring the wear pattern). Timken EP Test. This test provides a rapid method of measuring abrasion resistance and the load-carrying capacity of lubricants. A number of lubricant specifications require Timken “OK” loads above certain minimum values. The mode of operation consists of rotating a Timken tapered roller bearing cup against a stationary, hardened steel block. Fixed weights force the block into contact with the rotating cup through a lever system. The OK load is the highest load the cup and block can carry without scoring during a 10 min run. Timken abrasion tests are run under fixed loads for extended time periods, and the weight loss of the cup and block are a measure of the abrasion resistance of the lubricant. L-37 High Torque Test. The CRC L-37 test operates under low-speed, high-torque conditions. It evaluates the load-carrying ability, wear stability, and corrosion characteristics of gear lubricants. The test differential is a Dana Model unit driven by a Chevrolet truck engine and four-speed transmission. A complete, new axle assembly is used for each test after a careful examination of gear tooth and bearing tolerance. After break-in at reduced load and high speed, the test continues for 24 h under low-speed (80 axle rpm) and hightorque conditions. L-42 High Speed Shock Test. The CRC L-42 test is established to evaluate the antiscore performance of EP additives in gear lubricants under high-speed, shock load conditions. The test axle is a Dana Model unit driven by a Chevrolet engine through a four-speed truck transmission. The procedure requires five accelerations in fourth gear with inertia loading and 10 accelerations in third gear with dynamometer loading. The lubricant evaluation is based on the amount of scoring, and test results are expressed as percent tooth contact area scored. fa*g FE-8 Test. fa*g developed this test frame to be a flexible tribological system to conduct tests over a wide range of operating conditions with different test bearings. Shortduration standardized tests have been developed for different applications. fa*g also uses longer-term testing (e.g., fatigue) for comprehensive evaluations. The FE-8 gear oil test was developed specifically to evaluate the effectiveness of antiwear additives. The test runs under heavy load and low speed that forces the bearing to operate under boundary lubrication conditions.

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Cylindrical roller/thrust loaded 7.5 rpm 114 kN Variable 80 h

Other tests including Optimol SRV, Cameron-Plint, high-frequency reciprocating rig (HFRR), Falex multi-specimen, Vickers vane pump, Vickers 35-VQ-25 pump, and Denison high-pressure pump tests are also used widely in evaluation of various lubricants and greases. Appropriate field tests are also arranged in proprietary test sites to ensure good product quality and equipment compatibility/ friendliness before the introduction of a new product into the marketplace. On the engine oil side, the ILSAC is active in all phases of passenger car category development, and the SAE is the technical society for those with interest in transportation. Within the SAE is a Fuels and Lubricants Division/Engine Oil Technical Committee (TC-1) that serves as a forum for open discussion of technical issues related to current and future engine lubrication needs and standard development. With the introduction of GF-3 in 2001, the industry moved to a completely new set of engine tests for validation of passenger car engine oil performance. Although some new tests replaced previous tests, which were running out of parts, others provided a means to measure performance in new areas. The current category is GF-4, which superseded GF-3 in the summer of 2004. Among the GF-4 tests, the most critical engine tests related to antiwear/EP performance are the Sequence IVA and the Sequence IIIG. The Sequence IVA is an ASTM designation of a test previously referred to as the KA24E, originally developed by the Japan Automotive Manufacturers Association. It is included to replace the wear component of the Sequence VE. The Sequence IVA is designed to evaluate an oil’s ability to prevent cam lobe wear in slider valve train design engines operated at low temperature, short trip, and “stop and go” conditions (low-speed/low-temperature operation). Following is a list of the test conditions and specifications: Engine Engine speed Engine torque Oil temperature Cycle duration Test length 7-Point cam lobe wear

Nissan 2.4 L inline 4 cylinder 800 and 1500 rpm cycles 25 N m 50–60°C 50 min low speed/10 min high speed 100 h 120 µm maximum

The Sequence IIIG is a replacement for the Sequence IIIF and uses a current production version of the GM 3800 Series II V-6 engine. Special camshaft and lifter metallurgy and surface finishing are used to increase wear. The Sequence IIIG procedure is designed to evaluate the oil resistance to oxidation and wear in high-speed and high-temperature vehicle operation. The test conditions and specifications are summarized as follows: Engine Engine speed Engine load Valve spring load Oil temperature Coolant temperature Test length Average cam and lifter wear

GM 3800 Series II V-6 (231 CID) 3600 rpm 250 N m 205 lb 150°C 115°C 100 h 60 µm maximum

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The next ILSAC category is ILSAC GF-5, which is targeted to be introduced around mid-2010. The ILSAC/Oil Committee has decided that the Sequence IIIG and Sequence IVA tests will be retained to ensure that acceptable wear protection is achieved in the upcoming ILSAC GF-5 category. In the heavy-duty diesel engine oil area there are a number of industry standard engine tests that measure the wear performance. These tests required to meet both industry and engine manufacturer requirements such as API CJ-4 and various specifications from Caterpillar, Cummins, Detroit Diesel, Mack, and Volvo. The key wear tests assess the ability of an oil to control valve train or ring and liner wear under severe operating conditions, which include high-load duty cycles, use of exhaust gas recirculation, and high levels of soot contamination. The API CJ-4 category requires three tests that include valve train wear as a pass/fail parameter. The Roller Follower Wear Test (ASTM D5966) is run in a 6.5 liter V-8 GM diesel engine; it was initially developed for the older API CG-4 category, which was developed for the introduction of low sulfur (500 ppm maximum) fuel. However, this test has remained as a requirement in all subsequent specifications. At the end of this 50 h test, the used oil soot level is typically 3.5 to 4.0%. The level of wear on the stationary pin in the hydraulic cam followers is measured. The Cummins ISB test (ASTM procedure in progress) was introduced as an industry requirement for API CJ-4. This 350 h test runs in a 5.9 liter in-line 6 cylinder engine running of ultra low sulfur (15 ppm maximum) diesel fuel. The first 100 h are run at steady-state conditions to generate 3.25% soot in the oil. The final 250 h are run under cyclic conditions to stress cam and tappet wear, which are the primary pass/fail criteria. The third diesel engine test that measure valve train wear is the Cummins ISM test (ASTM procedure in progress). The Cummins ISM is the third in a series of Cummins heavyduty wear tests developed for API and engine builder diesel oil specifications. Similar to previous Cummins M11 HST (ASTM D6838) and Cummins M11 EGR (ASTM D6975) tests, the Cummins ISM alternates between 50 h soot generation and 50 h wear stages. This test runs for 200 h using 500 ppm sulfur diesel fuel. The used oil typically contains 6 to 7% soot, and the key pass/fail wear parameters are focused on the crossheads (bridges for the inlet and exhaust valves) and the adjusting screw for the fuel injectors. The Mack T-12 test (ASTM D7422) measures ring and liner wear under severe operation using 15 ppm sulfur fuel. This 300 h test runs with a very high EGR rate for the first 100 h to generate 4.3% soot. During the final 200 h, the engine runs over-fueled at peak torque conditions to create a very severe environment for top ring weight loss and liner wear at the point of top ring reversal, which are the key wear parameters for this test.

8.5

OUTLOOK

The additives business has experienced an economic upturn in recent years, primarily due to the imbalance between demand and supply as a result of tight feedstock availability and increased demand in the Far East region. The basic chemicals used to produce additives are subject to short supply as new and large capacity has not been effectively added to the manufacturing side for several years. A number of natural disasters such as the hurricane Katrina certainly made the situation even worse. The additive suppliers have successfully passed the raw material costs to their customers resulting in escalated unit pricing and improved profitability. The increased volume demand has been neutralized by several factors, such as longer drain lubricants and the reduction of ash additives. Despite the push for new engine oils meeting more stringent requirements, a major rationalization is occurring because of the ability to use additives longer and the recycling of products in the industry. Consequently, the total additive volume demand is growing slowly. Ashless antiwear/ EP additives are no different from other additives in terms of market demand. Antiwear additives are a mature function class, and business opportunities in the next few years will be modest. The dominant position of zinc dithiophosphates in engine oils is gradually diminishing, but is not expected to be in jeopardy in the near term. Therefore, a total switch to ashless antiwear additives in engine oils is not likely to occur very soon, but minor changes are in progress.

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The impetus for significantly improved lubricant additives is found on a number of fronts. Governmental and regulatory requirements continue to challenge the industry for improved products with lower toxicity. New engine developments, such as increased use of diamond-like carbon (DLC)– coated engine parts and ceramic components for wear resistance and higher contact temperatures, are on the horizon and present opportunities for antiwear additives that can function at very high operating temperatures. Space technology and other advanced transportation needs present new challenges to the industry. And, of course, there will always be a need for low product costs and ease of production. Four particular developments may have a major impact on the lubricant industry in the near term: (1) a move toward low-sulfur hydroprocessed (groups II and III), sulfur-free gas-to-liquid (GTL) and synthetic (groups IV and V) base stocks; (2) the imminent trend toward lower ash, sulfur, and phosphorus in engine oils; (3) a desire to reduce or eliminate chlorine in lubricants, particularly in metalworking fluids; and (4) a move to eliminate heavy metals and achieve low ash or even ashfree in both engine and industrial oils. To meet the growing needs for better thermal/oxidative stability and better viscometrics, synthetic base stocks such as polyalpha olefins, together with hydrotreated petroleum base stocks and GTLs are continuing to expand in all lubricant sectors. These types of materials have no aromatic hydrocarbons or greatly reduced amounts of aromatic hydrocarbons, which are potentially problematic for additive solvency; as a result of removing these solubilizing aromatics, the additives tend to precipitate out of the oil. This is particularly true for surface-active, polar components such as antiwear additives. Therefore, greater compatibility with nonconventional base stocks (groups II–IV) will be an essential requirement for all ashless antiwear/EP additives. Meanwhile, there are noticeable synergies identified among certain ashless antiwear/EP additives and nonconventional base stocks in a number of lubricant applications. Therefore, the choice of proper ashless additives will be vitally important. Because of the large number of automobiles equipped with catalytic converters that are sensitive to phosphorus derived from zinc dithiophosphates in the crankcase oil (possible reduction of catalytic efficiency), strong needs exist for engine oils with lower phosphorus content. Initially, ILSAC GF-4 aimed to reduce phosphorus levels to as low as 0.05% (about one-half of the former GF-3 level), but settled on a maximum phosphorus level of 0.08% instead (a 20% reduction). In addition to phosphorus limits, GF-4 oils also offered improved oxidation stability (including nitration control), hightemperature wear discrimination, high-temperature deposits control and used oil pumpability [103]. As vehicle emission regulations become more challenging, increasing restrictions are likely to be placed on other lubricant elements besides phosphorus that can impact emission control systems. Sulfur and metals are also under scrutiny as sulfur is suspected as a poison of DeNOx catalysts, and ash (from metals) may plug after-treatment particulate traps. Modern engine oils rely heavily on ZDDP to provide antiwear, antioxidation, and anticorrosion protection. Since ZDDP is rich in phosphorus, sulfur, and zinc, it becomes an obvious target for emission control. In fact, at former use level, ZDDP was almost solely accountable for more than two-thirds of sulfur and all the phosphorus and zinc present in engine oils, excluding sulfur from base oils. Oils with ZDDP at former levels could make it difficult for OEMs to optimize (for cost and life) their exhaust after-treatment systems. Therefore, the future trend will likely be toward further reduced ZDDP in engine oils providing that the performance integrity can be maintained through the use of alternate additives [104]. To satisfy performance requirements in terms of oxidation control and deposit levels, more antioxidants could be added to the engine oil formulation. These ashless antioxidants (hindered phenols and arylamines) may effectively compensate for the loss of oxidation protection due to the reduction in ZDDP concentrations. However, since ZDDP is such a cost-effective additive and is the sole antiwear component used in many engine oils, a reduction in ZDDP treat levels may not provide the needed wear protection. Recently, engine builders are requiring even greater antiwear protection, and more demanding test protocols are being put in place to ensure that lubricants can meet these more stringent specifications. Therefore, there is a strong need for advanced ashless antiwear systems to replace or supplement ZDDP to satisfy emission regulations while ensuring high levels of wear protection [105,106].

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Not all phosphorus-containing additives behave the same in engine oils. Furthermore, even within the same ZDDP family, not all ZDDP respond the same to after-treatment devices as evidenced by their relative volatility performance. Data indicated that volatilized phosphorus showed very low statistical dependence on either oil volatility or phosphorus concentration in the fresh oil. Rather the data seemed to indicate that the chemistries of the phosphorus-containing additives and their formulation with other additives were the controlling cause of phosphorus volatility and, by extension, emission level. Selby’s Phosphorus Emission Index (PEI) and Sulfur Emission Index (SEI) shed some insights into the volatility impact on emission issues and better S/P volatility control than the current ZDDP that is highly desired for future ashless antiwear additives [107–109]. Chlorine in lubricants and other materials is becoming an increasing environmental concern. Legislation around the industrial world limits the chlorine content of many lubricant products to 50 ppm or less. The Montreal Protocol mandated a gradual phase-out of the use of chlorinecontaining refrigerants, such as hydrochlorofluorocarbon (HCFC) and chlorofluorocarbon (CFC), and replacement with alternative hydrofluorocarbons (HFC). Increased wear occurred in the refrigeration compressor when HFC refrigerants were substituted for CFC, and the cause of this increased wear was believed to be inferior antiwear capability of the alternative HFC refrigerant as the environmental gas, compared to that for CFC [110]. This offers some opportunities for the development of new ashless antiwear additives for refrigeration compressor oils. The cutting oil industry is facing similar ecological pressures, and future changes to reduce or eliminate chlorine are expected. The most significant opportunity is perhaps driven by human health and waste disposal issues concerning the use of chlorinated paraffins. Chlorinated paraffins are used extensively as EP additives in metalworking fluids. The National Toxicology Program (NTP) listed chlorinated paraffins, derived from C12 feedstocks and chlorinated at 60%, as a suspect carcinogen. Although few metalworking fluids are formulated with this class of chlorinated paraffin, the image of chlorinated paraffins in general has suffered due to uncertainties about future NTP reclassification of all such additives. Gear additives are another area of concern. Because of the problems associated with chlorine additives, their use in gear oils has been greatly reduced. However, a number of processes for making gear additives utilize chlorine or chlorine-containing reagents at some point in the reaction sequence. Small amounts of chlorine still remain in the final product. The complete removal of chlorine is therefore expected to become an important priority, but will be difficult to attain in the near future. Finally, the use of metallic antiwear/EP additives is diminishing due to the influence of environmental concerns. Heavy metals are considered pollutants, and their presence is no longer welcomed in the environment. Given equal performance and costs, ashless antiwear additives will be preferred for many future lubricants. In the future, the lubricant additive business will continue to grow and will need more ashless antiwear/EP additives [111]. Possible new markets include biodegradable lubricants, biodiesel fuel–friendly lubricants, advanced transportation lubricants, robotics, ceramics, and space technology lubricants. Traditional markets in engine oils, ATFs, marine, aviation, gear, hydraulic, circulating oils, metalworking, and other industrial lubricants are also expanding. Healthy growth for nonconventional base oils (groups II–V) is expected in many of these areas. Clearly advanced ashless antiwear additives with environment-friendly features, excellent stability, and unique performance properties, especially for nonconventional base oils, will be the additives of choice for increasingly demanding lubricant applications.

ACKNOWLEDGMENT I thank Pat Dedert and Elvin Hoel for assistance in the literature search and also many of my colleagues, especially Dr. Douglas Deckman, Dr. David Blain, Dr. Steven Kennedy, Dr. Andy Horodysky, and Dr. Andy Jackson, for their valuable comments.

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Ranney, M.W., Lubricant Additives, Chemical Technology Review, No. 2, 1973. Ranney, M.W., Synthetic Oils and Additives for Lubricants—Advances Since 1977. Ranney, M.W., Synthetic Oils and Additives for Lubricants—Advances Since 1979. March, J., Advanced Organic Chemistry, p. 703, Second Edition, McGraw-Hill Book Company, 1977. Pozey, J.S. et al., Reactions of Sulfur with Organic Compounds, Edited by J.S. Pizey, Consultants Bureau, New York, A Division of Plenum Publishing Corporation, 1987. Reid, E.E., Organic Chemistry of Bivalent Sulfur, Volume I–V, 1958–1963, Chemical Publishing Co., Inc., 1963. N. Kharasch, Organic Sulfur Compounds, Volume 1, Symposium Publications Division, Pergamon Press Inc., Chapters 8, 10 and 20, 1961. Landis, P.S., The Chemistry of 1,2-dithiole-3-thiones, Chemical Reviews, 65, 237, 1965. Jones, S.O. and Reid, E.E., The Addition of Sulfur, Hydrogen Sulfide and Mercaptans to unsaturated hydrocarbons, The Journal of the American Chemical Society, 60, 2452, 1938. Louthan, R.P., Preparation of Mercaptans and Thioether Compounds, US Patent No. 3,221,056, 1965; US Patent Nos. 3,419,614 (1968); 4,194,980 (1980) and 4,240,958 (1980). US Patent Nos. 2,012,446 (1935) and 3,953,347 (1976). Myers, H., Lubricating Compositions Containing Polysulfurized Olefin, US Patent No. 3,471,404, 1969; US Patent Nos. 3,703,504 and 3,703,505 (1972). Davis, K.E., Sulfurized Compositions, US Patent Nos. 4,119,549, 1978 and 4,191,659, 1980. Davis, K.E. and Holden, T.F., Sulfurized Compositions, US Patent Nos. 4,119,550, 1978 and 4,344,854, 1982. Dibiase, S.A., Hydrogen Sulfide Stabilized Oil-soluble Sulfurized Organic Compositions, US Patent No. 4,690,767, 1987. Horodysky, A.G. and Law, D.A., Additive for Lubricants and Hydrocarbon Fuels Comprising Reaction Products of Olefins, Sulfur, Hydrogen Sulfide and Nitrogen Containing Polymeric Compounds, US Patent No. 4,661,274, 1987. Horodysky, A.G. and Law, D.A., Sulfurized Olefins as Antiwear Additives and Compositions thereof, US Patent No. 4,654,156, 1987; US Patent No. 2,995,569 (1961). Johnson, D.E. et al., Sulfurized Olefin Extreme Pressure/Antiwear Additives and Compositions thereof, US Patent No. 5,135,670, 1992; US Patent Nos. 2,999,813 (1961); 2,947,695 (1960) and 2,394,536 (1946). Papay, A.G., Lubrication Engineering, 32(5), 229–234, 1975. Korosec, P.S. et al., NLGI Spokesman, 47(1), 1983. Macpherson, I. et al., NLGI Spokesman, 60(1), 1996. Buitrago, J.A., Gear Oil Having Low Copper Corrosion properties, EP Patent Application 1 471 133 A2, 2004. Rohr, O., NLGI Spokesman, 58(5), 1994. Papay, A.G., Lubrication Science, 10(3), 1998. Mortier, R.M. and Orszulik, S.T., Chemistry & Technology of Lubricants, 1992. Habeeb, J.J. and Haigh, H.M., Premium Wear Resistant Lubricant Containing Non-Ionic Ashless Antiwear additives, US Patent Application 2006/0135376 A1, 2006. Samuel, D. and Silver, B.L., The Journal of the American Chemical Society, 3582, 1963. Smith, T.D., The Journal of the American Chemical Society, 1122, 1962. Venezky, et.al., The Journal of the American Chemical Society, 78, 1664, 1956. Orloff, H.D., The Journal of the American Chemical Society, 80, 727–734, 1958. Lacey, I.N., Macpherson, P.B., and Spikes, H.A., Thick Antiwear Films in EHD Contacts, Part 2: Chemical Nature of the Deposited Film, ASLE Preprint, 1985. Ishikawa, M. and Watts, R.F., Continuously Variable Transmission Fluid, US Patent Application 2005/0250656 A1, 2005. Corbridge, D.E.C., Phosphorus: An Outline of its Chemistry, Biochemistry and Technology, pp. 213, 249, 401, 1985. Lange, R.M., Norbornyl Dimer Ester and Polyester Additives for Lubricants and Fuels, US Patent No. 4,707,301, 1987. Pollak, K., Amine Derivative of Dithiophosphoric Acid Compounds, US Patent No. 3,637,499, 1972. Shaub, H., Amine Salt of Dialkyldithiophosphate, US Patent No. 4,101,427, 1978. Michaelis, K.P. and Wirth, H.O., Di- or trithiophosphoric Acid Diesters, US Patent No. 4,244,827, 1981.

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247

38. Horodysky, A.G. and Gemmill, R.M., Phosphosulfurized Hydrocarbyl Oxazoline Compounds, US Patent No. 4,255,271, 1981. 39. Farng, L.O. et al., Phosphorodithioate-derived Pyrrolidinone Adducts as Multifunctional Antiwear/ Antioxidant Additives, US Patent No. 5,437,694, 1995. 40. Ripple, D.E., Phosphorus Acid Compounds-Acrolein-Ketone Reaction Products, US Patent No. 4,081,387, 1978. 41. Farng, L.O. et al., Lubricant Additive Comprising Mixed Hydroxyester or Diol/Phosphorodithioatederived Borates, US Patent No. 4,784,780, 1988. 42. Farng, L.O. et al., Lubricant Additives Derived from Alkoxylated Diorgano Phosphorodithioates and Isocyanates to Form Urethane Adducts, US Patent No. 5,282,988, 1994. 43. Le Sausse, C. and Palotai, S., Ashless Additives Formulations Suitable for Hydraulic Oil Applications, US Patent Application 2005/0096236 A1, 2005. 44. Cardis, A.B., Reaction Products of Dialkyl and Trialkyl Phosphites with Elemental Sulfur, Organic Compositions Containing Same, and Their Use in Lubricant Compositions, US Patent No. 4,717,491, 1988. 45. Bosniack, D.S., US Patent No. 4,079,012, 1978. 46. Nebzydoski, J.W. et al., US Patent No. 3,952,059, 1976. 47. Debus, H., Uber die Verbindungen der Sulfocarbaminsaure. Ann Chem (Liebigs), 73, 26, 1850. 48. Thorn, G.D. and Ludwig, R.A., The Dithiocarbamates and Related Compounds, Elsevier Publishing Company, 1962. 49. Lam, W.Y., US Patent No. 4,836,942, 1989. 50. Cardis, A.B. et al., Borated Dihydrocarbyl Dithiocarbamate Lubricant Additives and Composition thereof, US Patent No. 5,370,806, 1994. 51. Farng, L.O. et al., Dithiocarbamate-derived Ethers as Multifunctional Additives, US Patent No. 5,514,189, 1995. 52. Gatto, V.J., Dithiocarbamtes Containing Alkylthio and Hydroxy Substituents, EP Patent Application No. EP 1306370 B1, 2003. 53. Cardis, A.B. and Ardito, S.A., Biodegradable Non-Toxic Gear Oil, US Patent Application No. 2003/0125218 A1, 2003. 54. Daegling, S., Use of a Noise-reducing Grease Composition, EP Patent Application No. 1188814 A1, 2002. 55. Cartwright, S.J., Ashless Lubricating Oil Composition with Long Life, CA Patent Application No. CA2465734 A1, 2004. 56. Little, R.Q., US Patent No. 3,087,932, 1963. 57. Gemmill, R.M. et al., US Patent No. 4,584,114, 1986. 58. Davis, R.H. et al., Antiwear/Antioxidant Additives Based on Dimercaptothiadiazole derivatives of Acrylates and Methacrylates Polymers and Amine Reaction Products thereof, US Patent No. 5,188,746, 1993. 59. Karol, T.J. Maleic Derivatives of 2,5-Dimercapto-1,3,4-thiadiazoles and Lubricating Compositions Containing Same, US Patent No. 5,055,584, 1991. 60. Fields, E.K. US Patent No. 2,799,652, 1957. 61. Davis, R.H. et al., Dimercaptothiadiazole-derived, Organic Esters, Amides and Amine Salts as Multifunctional Antioxidant/Antiwear Additives, US Patent No. 4,908, 144, 1990. 62. Vogel, P.W., US Patent No. 3,759,830, 1973. 63. Fields, E.K. et al., US Patent No. 2,703,784 and 2,703,785, 1955. 64. Hsu, S-Y. et al., Quaternary Ammonium Salt Derived Thiadiazoles as Multifunctional Antioxidant and Antiwear Additives, US Patent No. 5,217,502 and 5,194,167, 1993. 65. Srinivasan, S. et al., Automatic transmission fluid comprises major amount of base oil and minor amount of additives comprising dihydrocarbyl - thiadiazole, sulfurized fat and ester and metal containing detergent, US Patent Application 780998 20010209 and EP Patent Application 1231256 20020814. 66. Srinivasan, S. et al., Automatic Transmission Fluid, for Automatic Transmission Equipment Platforms, Comprises Major Amount of Base Oil and Minor Amount of Additives Comprising Ashless Dialkyl Thiadiazole and Amine Antioxidants, US Patent Application 800017 20010305 (March 5, 2001) and EP Patent Application 1239021 20020911 (September 11, 2001). 67. Vann, W.D. et al., Lubricant Containing a Synergistic Composition of Rust Inhibitors, Antiwear Agents, and a Phenothiazine Antioxidants, US Patent No. 7,176,168 B2, 2007. 68. Esche, C.K., Gatto, V.J., and Lam, W.Y., Effective Antioxidant Combination for Oxidation and Deposit Control in Crankcase Lubricants, US Patent No. 6,599,865 B1, 2003. 69. Nalesnik, T.E. and Barrows, F.H., Substituted Linear Thiourea Additives for Lubricants, US Patent No. 6,187,726 B2, 2001.

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70. Mukkamala, R., Thioimidazolidine Derivatives as Oil-Soluble Additives for Lubricating Oils, EP Patent Application No. EP 1,229,023 B1, 2003 and EP 1,361,217 B1, 2005. 71. Nalesnik, T.E., Oxadiazole Additives for Lubricants, US Patent No. 6,551,966 B2, 2003. 72. Nalesnik, T.E., Thiadiazolidine Additives for Lubricants, US Patent No. 6,559,107 B2, 2003. 73. Camenzind, H. and Nesvadba, P., Lubricant Composition Comprising an Allophanate Extreme-pressure, Anti-wear Additive, US Patent No. 5,084,195, 1992 and 5,300,243, 1994. 74. Zhang et al., A Study of 2-Alkyldithio-benzoxazoles as Novel Additives, Tribology Letters, 7, 173–177, 1999. 75. Polishuk, A.T., and Farmer, H.H., NLGI Spokesman, 43, 200, 1979. 76. Schumacher, R. et al., Improvement of Lubrication Breakdown Behavior of Isogeometrical Phosphorus Compounds by Antioxidants, Wear, 146, 25–35, 1991. 77. Schumacher, R. et al., Tribofragmentation and Antiwear Behavior of Isogeometric Phosphorus Compounds, Tribology International, 30(3), 199, 1997. 78. Okorodudu, A.O.M., Organophosphorus Derivatives of Benzotriazole, US Patent No. 3,986,967, 1976. 79. Farng, L.O. and Horodysky, A.G., Phenylenediamine-derived Phosphonates as Multifunctional Additives for Lubricants, US Patent No. 5,171,465, 1992. 80. Hotten, B.W., Bisphosphoramides, US Patent No. 3,968,157, 1976. 81. Andrew, D.L. and Moore, G.G., Lubricating Grease Composition with Increased Corrosion Inhibition, EP-903398, 1999. 82. Cier, R.J. and Bridger, R.F., US Patent No. 4,025,446, 1977. 83. Pratt, R.J., US Patent No. 3,941,808, 1976. 84. Daly, D.T., Adams, P.E., and Jackson, M.M., US Patent No. 6,224,642 B1, 2001. 85. Nalesnik, T.E., Alkyl Hydrazide Additives for Lubricants, US Patent No. 6,667,282 B2, 2003. 86. Nalesnik, T.E., 1,3,4-Oxadiazole Additives for Lubricants, US Patent No. 6,566,311 B1, 2003. 87. Avery, N.L. et al., Friction Modifiers and Antiwear Additives for Fuels and Lubricants, US Patent No. 5,538,653, 1996. 88. Farng, L.O. et al., Triazole-maleate Adducts as Metal Passivators and Antiwear Additives, US Patent No. 5,578,556, 1996. 89. Farng, L.O. et al., Fuel Composition Comprising Triazole-derived Acid-esters or Ester-amide-amine Salts as Antiwear Additives, US Patent No. 5,516,341, 1996. 90. Nalesnik, T.E., Alkyl-Succinhydrazide Additives for Lubricants, US Patent No. 6,706,671 B2, 2004. 91. Nalesnik, T.E. and Barrows, F., Tri-glycerinate Vegetable Oil-Succinihydrazide Additives for Lubricants, US Patent No. 6,559,106 B1, 2003. 92. Levine, J.A. and Wu, S., Borate Ester Lubricant Additives, US Patent Application 2004/0235681, 2004. 93. Farng, L.O. et al., Mixed Alcohol/Dimercaptothiadiazole-Derived Hydroxy Borates as Antioxidant/ Antiwear Multifunctional Additives, US Patent No. 5,137,649, 1992. 94. Farng, L.O. et al., Lubricant Additives, US Patent No. 5,288,988, 1994. 95. Watts, R.F. et al., Power Transmission Fluids with Improved Extreme Pressure Lubrication Characteristics and Oxidation Resistance, US Patent No. 6,534,451 B1, 2003. 96. Farng, L.O. et al., Load-Carrying Additives based on Organo-Phosphites and Amine Phosphates, US Patent No. 5,681,798, 1997. 97. Anon, The Future of Chlorine in Metalworking Fluids, Lubrication Engineering, 35(5), 266–271, 1979. 98. Furey, M.J. and Kajdas, C., Wear Reducing Compositions and Methods for Their Use, US Patent No. 5,880072, 1999. 99. Baranski, J.R. and Migdal, C.A., Phenolic Borates and Lubricants Containing Same, US Patent No. 5,698,499, 1997. 100. Roby, S.H. and Ruelas, S.G., Engine Oil Compositions, US Patent Application 2005/0070450 A1, 2005. 101. Williamson, W.F and Rhodes, B., Non-Phosphorus, Non-Metallic Anti-wear Compound and Friction Modifier, International Patent Application WO 00/42134, 2000. 102. Yoon, B.A. et al., Borated-Epoxidized Polybutenes as Low Ash Anti-wear additives for Lubricants, US Patent Application Serial No. 10/951356, EP Patent Application No. EP1699909 A1 and PCT/ US2004/042149, 2004. 103. Tan, I., Lubricant Additives—Treats and Opportunities, Lubricants World, July/August, 16–19, 2003.

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104. Farng, L.O. et al., Ashless Anti-wear additives for Future Engine Oils, 14th International Colloquium Tribology, Stuttgart, Germany, 1547–1553, January 13–15, 2004. 105. Korcek, S. Jensen, R.K., and Johnson, M.D., Assessment of Useful Life of Current Long Drain and Future Low Phosphorus Engine Oils, Proceedings of second World Tribology Congress—Scientific Achievements—Industrial Applications—Future Challenges, 259–262, Vienna, Austria, 2001. 106. Korcek, S. Jensen, R.K., and Johnson, M.D., Engine Oil Performance Requirements and Reformulation for Future Engines and Systems, SAE Paper #961146, 1996. 107. Selby, T.W., Development and Significance of the Phosphorus Emission Index of Engine Oils, 13th International Colloquium Tribology—Lubricants, Materials and Lubrication, Technische Akademie Esslingen, Germany, January 15–17, 2002. 108. Selby, T.W., Fee, D.C., and Bosch, R.J., Analysis of The Volatiles Generated During The Selby-Noack Test by 31P NMR Spectroscopy, Elemental Analysis Symposium, ASTM D02 Meeting, Tampa, FL, December 2004. 109. Selby, T.W., Fee, D.C., and Bosch, R.J., Phosphorus Additive Chemistry and Its Effects on The Phosphorus Volatility of Engine Oils, Elemental Analysis Symposium, ASTM D02 Meeting, Tampa, FL, December 2004. 110. Mizuhara, K. et al., The Friction and Wear Behavior in Controlled Alternative Refrigerant Atmosphere, Tribology Transactions, 37, 1, 120, 1994. 111. Farng, L.O. and Deckman. D.E., Novel Anti-Wear Additives for Future Lubricants, Additives 2007 Conference: Applications for Future Transport, London, U.K., April 17–19, 2007.

Sulfur Carriers Thomas Rossrucker and Achim Fessenbecker

CONTENTS 9.1 9.2

Introduction ........................................................................................................................... 252 History................................................................................................................................... 253 9.2.1 First Synthesis and Application (1890–1918) ............................................................ 253 9.2.2 First Application in Metalworking Oils (1920–1930) ...............................................254 9.2.3 Sulfurized Compounds for Gear Oils and Other Lubricants (1930–1945) ...............254 9.2.4 Scientific Research on Chemistry and Application (1930–1949) .............................. 255 9.2.5 Summary of the Past 50 Years .................................................................................. 256 9.3 Chemistry .............................................................................................................................. 257 9.3.1 Chemical Structure of Sulfur Carriers ...................................................................... 257 9.3.1.1 Sulfurized Isobutene ................................................................................... 258 9.3.1.2 Active-Type Sulfurized Olefins................................................................... 258 9.3.1.3 Inactive Sulfurized α-Olefins ...................................................................... 258 9.3.1.4 Sulfurized Synthetic Esters (Light Color) .................................................. 258 9.3.1.5 Sulfurized Fatty Oil (Black Color) ............................................................. 259 9.3.1.6 Sulfurized Fatty Oil/Olefin Mixture (Light Color)..................................... 259 9.3.2 Current Commercial Production Processes .............................................................. 259 9.3.2.1 General Aspects .......................................................................................... 259 9.3.2.2 Black Sulfurization .....................................................................................260 9.3.2.3 High-Pressure H2S Reaction ....................................................................... 261 9.3.2.4 Mercaptan Route ......................................................................................... 262 9.3.3 Other Synthetic Routes .............................................................................................. 262 9.3.3.1 Sulfurchlorination Route............................................................................. 262 9.3.3.2 Alkylhalogenide/NaSx ................................................................................ 263 9.3.4 Raw Materials............................................................................................................264 9.4 Properties and Performance Characteristics.........................................................................264 9.4.1 Chemical Properties ..................................................................................................264 9.4.1.1 Effect of Additive Structure on Performance .............................................264 9.4.2 Physical Properties .................................................................................................... 267 9.4.2.1 Effect of Additive Structure on Properties ................................................. 267 9.5 Comparative Performance Data in Pertinent Application Areas .......................................... 269 9.5.1 Metalworking ............................................................................................................ 269 9.5.1.1 Cutting/Forming ......................................................................................... 269 9.5.1.2 Contribution of Sulfur Carriers to Metalworking ....................................... 269 9.5.1.3 Replacement of Sulfur Flowers ................................................................... 269 9.5.1.4 Copper Corrosion ........................................................................................ 270 9.5.1.5 Substitutes for Chlorinated Paraffins .......................................................... 270 9.5.1.6 Substitute for Heavy Metals ........................................................................ 270 9.5.1.7 Carbon Residue Reducing in Rolling Oils .................................................. 270 9.5.1.8 Water Miscible Metalworking Products ..................................................... 270 251

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9.5.2 Grease ........................................................................................................................ 271 9.5.3 Industrial Oils ............................................................................................................ 271 9.5.3.1 Industrial Gear Oils .................................................................................... 272 9.5.3.2 Slideway Oils .............................................................................................. 272 9.5.3.3 Hydraulic Fluids .......................................................................................... 272 9.5.3.4 Multifunctional Lubricants ......................................................................... 272 9.5.3.5 Agricultural Applications ........................................................................... 272 9.5.3.6 Automotive Applications ............................................................................ 272 9.5.4 Synergies/Compatibility with Other Additives ......................................................... 273 9.5.4.1 Zinc Dialkyldithiophosphates ..................................................................... 273 9.5.4.2 Basic Alkali Metal Salts ............................................................................. 274 9.5.4.3 Antioxidants ................................................................................................ 274 9.5.4.4 Esters/Triglycerides .................................................................................... 274 9.5.5 Cost-Effectiveness ..................................................................................................... 275 9.6 Manufacture and Marketing Economics............................................................................... 275 9.6.1 Manufacturers............................................................................................................ 275 9.6.2 Marketers ................................................................................................................... 276 9.6.3 Economics ................................................................................................................. 276 9.6.4 Government Regulations ........................................................................................... 276 9.6.4.1 Competitive Pressures ................................................................................. 276 9.6.4.2 Product Differentiation ............................................................................... 276 9.7 Outlook.................................................................................................................................. 276 9.7.1 Crankcase/Automotive Applications ......................................................................... 276 9.7.2 Industrial Applications .............................................................................................. 277 9.8 Trends .................................................................................................................................... 277 9.8.1 Current Equipment/Specification .............................................................................. 277 9.8.1.1 Types of Equipment .................................................................................... 277 9.8.1.2 Additives in Use .......................................................................................... 277 9.8.1.3 Deficiencies in Current Additives ............................................................... 277 9.9 Medium-Term Trends............................................................................................................ 278 9.9.1 Metalworking ............................................................................................................ 278 9.9.2 Industrial Oils ............................................................................................................ 278 References ...................................................................................................................................... 278

9.1 INTRODUCTION In the lubricant industry, a great variety of sulfur-containing additives are known and in use today. We list only a few of the most common types: • • • • • •

Sulfur carriers (sulfurized olefins, esters, and fatty oils) Sulfur/phosphorus derivatives (dithiophosphates, thiophosphonates, thiophosphites, etc.) Thiocarboxylic acid derivatives (dithiocarbamates, xanthogenates, etc.) Heterocyclic sulfur (mercaptobenzothiazoles, thiadiazoles, etc.) Sulfonates (Na-, Ca-salts of alkylbenzenesulfonic acids, nonylnaphthalenesulfonates, etc.) Others (sulfated fatty oils/Turkish red oils, sulfurchlorinated fatty oils, sulfur-linked phenols and phenates)

This long list gives a good impression of the versatility and importance of sulfur chemistry in lubricants. But as versatile their chemistry is, the range of applications of sulfur-containing lubricant additives is just as versatile. In this chapter, we try to review major aspects of a group of additives commonly known as sulfur carriers. This is a generic name that has been accepted in the marketplace

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and used to summarize a group of additives that provide extreme-pressure (EP) and antiwear (AW) properties and are used in gear oils, metalworking fluids, greases, and engine oils. The vast majority of them are sulfurized fats, esters, and olefins. To distinguish them from other sulfur-containing products and avoid misunderstandings, a suitable definition of sulfur carriers is the following: Sulfur carriers are a class of organic compounds that contain sulfur in its oxidation state 0 or –1, where the sulfur atom is bound either to a hydrocarbon or to another sulfur atom • That does not contain other hetero atoms except oxygen • Produced by adding sulfur to all kinds of unsaturated, double-bond-containing compounds such as olefins, natural esters, and acrylates or by substitution reaction with reactive organic halides and alike Lubricant additives fitting this definition are the main focus of this chapter. Owing to the overwhelming versatility of sulfur chemistry, other sulfur-containing product groups cannot be discussed in depth but are mentioned in the context where appropriate. Although this group of additives has been used in the lubricant industry for more than eight decades, sulfur carriers are not at all an endangered species. In fact, we are still seeing increasing usage today. This is partly due to continuous ongoing R&D work done in this area, which brings about innovation and product improvement. Also, many chemical aspects and applications are waiting to be discovered. Furthermore, sulfur carriers are essential additives for the solution of upcoming lubricant market requirements such as chlorinated paraffin substitution; heavy metal replacement; and health, safety, and environmental issues. Therefore, we expect to see substantial future growth of light-colored, low-odor, and odor-free sulfur carriers.

9.2 HISTORY As we look back on more than 100 years of sulfurized compounds, the authors had to rely on literature sources for the time before 1950s. During the literature studies, it turned out that one of the most fruitful sources for the time period before 1950 is the review articles of Helen Sellei [1,2] published in 1949. Much of what follows is based on their content, but we have tried to reinterpret the information with today’s background knowledge.

9.2.1

FIRST SYNTHESIS AND APPLICATION (1890–1918)

Sulfurized fatty oils have been commercially produced for more than 100 years. Long before they were used as additives in lubricants, they had become important additives for the rubber industry. The addition of 4–8% sulfur to an unsaturated natural oil such as rapeseed oil at high temperatures (120–180°C) gives a flexible, gummy polymer called factis. Sulfur undergoes an addition reaction to the double bonds of the natural oil and builds up a three-dimensional structure of sulfur bridges between the triglyceride molecules. This is comparable to the vulcanization process of latex, which results in rubber. In the late nineteenth century, rubber was an expensive natural raw material, and with the rapid industrialization in general and the growing automobile industry in particular, rubber tires were needed in increasing amounts. It soon turned out that factis also provided special, positive properties to rubber goods during the vulcanization process. This was the starting point of smaller chemical factories producing additives for the rubber industry. In 1889, Carl Benz submitted the patent for the world’s first automobile in Mannheim, Germany. In the same year and city, Rhein Chemie Rheinau GmbH was founded and started to produce sulfurized natural oils. Germany had seen a special national aspect to the industrial history of sulfurized fats and rubber before 1914. Because Germany had very few colonies, all rubber had to be imported. During the national tensions in the first decade of the twentieth century and subsequent trade boycotts, the search for alternatives

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had been strongly pushed, leading to the development of synthetic rubber (Buna). Subsequently, because it was cheap and based on locally available raw materials, factis had found increasing use as rubber substitute and rubber diluent.

9.2.2

FIRST APPLICATION IN METALWORKING OILS (1920–1930)

In the very early days of modern lubrication, it had become known that sulfur is an important element to improve frictional properties and prevent seizure under high loads. Free sulfur and sulfurcontaining heterocyclic molecules are known as part of natural crude oil (thiophenes, thioethers, etc.). In early refining technology, they were not removed effectively, especially from the higherviscosity oils that were typically used for gear oils, which had up to 3–4% sulfur. This natural sulfur contributed to mild EP performance (antiwelding). After the positive effects of sulfur for lubricant oil formulations were recognized, the next step was to physically dissolve sulfur flower into the lubricant oil at elevated temperatures. This sulfur, however, is very reactive and corrosive against copper and its alloys. Also, sulfur flower has a limited solubility in mineral oil, which limits its maximum dosage and final EP performance achievable. Sulfurized esters were first used in metalworking. For heavy-duty operations with a high degree of boundary lubrication conditions, it was realized that the addition of oil-soluble sulfur compounds had a tremendous effect on the performance. The first milestone literature that reports this effect was published in 1918 by the E.F. Houghton Corporation [3] for cutting oils. It is claimed that a mixture of lard oil, mineral oil, and wool fat treated with sulfur flower at elevated temperatures results in a sulfurized product that increases the performance of cutting oils enormously. In particular, the tool life is extended, and smoking of the coolant is reduced greatly due to friction and temperature reduction. These observations are still valid today and may be considered the starting point of the application of sulfur carriers as additives for lubricants. In comparison with the solid, rubberlike material factis, which has been commercialized for several decades, Houghton Corporation’s breakthrough was to produce a liquid fatty material that was soluble in mineral base oil at any ratio. It overcomes the solubility limits of sulfur flower and allows the adjustment of the EP performance level according to treat rate. They achieved this simply by using nonreactive mineral oil and wool wax as chain-breaking agents and diluent to control the polymerization reaction of lard oil to keep it liquid. From thereon, the use of sulfurized oils has become quite common in metalworking.

9.2.3

SULFURIZED COMPOUNDS FOR GEAR OILS AND OTHER LUBRICANTS (1930–1945)

Some years later, the idea of improving load-carrying capacity under high-pressure and high-temperature conditions had been picked up by automotive lubricant researchers and applied to oils for the newly constructed hypoid gear boxes. With the advent of hypoid gears in automotive applications in the 1920s and 1930s, wear and seizure under high-load conditions became a major technical problem that lubricant companies needed to solve. Most of the development work had been done within these lubricant companies, and the new technology had not been published in detail. However, the number of patents on sulfur compounds for lubricants developed very rapidly throughout the 1930s and 1940s. 1936 1940 1941 1946

First patent review on EP lubricants [4] Patent review 1938–1939 [5] General publication on lubricating additives including an extensive patent bibliography [6] Review article on sulfurization of unsaturated compounds [7]

This clearly indicates that the ideas that had been invented and first applied by the metalworking people also worked for gears. Combinations of sulfurized products with lead soaps and lubricity

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esters were the first high EP performance technology in gear oils. Many years later, Musgrave [8] stated in an article on hypoid gear oils that it was just by chance that the synergistic effect of sulfur with lead soaps had been discovered in the early 1930s. An interesting historical dimension was added to the EP gear oil development during World War II. Most of the EP gear oil development occurred in the United States due to the great importance of automotive industry in the 1920s and 1930s. German gear oil technology was not as advanced as that of United States. Eyewitnesses report of frequent gear box failures of German tanks and heavy equipment during the attack against Russia. The reason was that in autumn Russian roads were turning into mud and the heavy vehicles were operated most of the time at maximum power. The only mildly additized gear oils were not just good enough to prevent scoring and welding.

9.2.4

SCIENTIFIC RESEARCH ON CHEMISTRY AND APPLICATION (1930–1949)

Between 1930 and 1950, the important basics of sulfur carrier technology had been developed. Patents from this period include most of today’s raw materials and reaction pathways. Raw materials used were animal oils [3], vegetable oils/organic acids [9], pine oils [10,11], whale oil (sperm oil), acrylates, olefins [12], alcohols [13], synthetic esters [14], and salycilates [15]. Even thiocarbonates [16] and xanthogenates [17] were synthesized and used as organic, oil-soluble sulfur-containing EP additives. Reaction pathways mentioned in patents mentioned earlier include • • • •

Sulfur flower reaction with and without H2S, aminic, and other suitable catalysts Sulfur chlorination with S2Cl2 [18] Organic halides with alkalipolysulfides Mercaptan route [19]

Important product properties that are still part of today’s development work were also mentioned in that period: • • • • •

Stability of sulfurized products, for example, diisobutene [20] Active and inactive sulfur compounds Corrosive and noncorrosive compounds Light- and dark-colored derivatives High- and low-odor products

Parallel to new chemistry, the development of test machines for tribological research progressed quickly along with publications on mechanistic studies of additives. In 1931, at an API meeting, Mougay and Almen [21] presented the first chemical interpretation for the load-carrying capacity of sulfur-containing EP additives and their synergy with lead soaps. They attributed the performance to the formation of a separating film between the frictional partners—a theory generally accepted today in tribological science. In 1939, this film-forming theory of sulfur compounds was proven using the four-ball tester [22]. In 1938, Schallbock et al. published [23] standard-setting results on investigations in the field of metalworking. Empirical correlations were found among cutting speed, temperature, and tool life that are still valid. In 1946, synergistic effects of chlorinated additives with sulfur additives were explained based on a chemical reaction theory [24] under the aspect of newest generation hypoid gear formulations. Phosphorus additives (tricresylphophate [25], zinc dialkyldithiophosphates [ZnDTP]), primary antioxidants (AOs) (phenyl-α-naphthylamine, butylated hydroxytoluene [BHT]), and detergent/dispersants [26] (salycilates) also joined the world of lubricant additives during this period and have been used since in combination with sulfur carriers. Most of the development work at that time had been done in a deductive way in a trial-and-error approach. Theoretical explanations, tribological, and chemical modeling always trailed behind (looking back from today’s point of view, it is quite astonishing that not so much has changed in 70 years).

256

9.2.5

Lubricant Additives: Chemistry and Applications

SUMMARY OF THE PAST 50 YEARS

With the fundamentals of sulfur carriers being explored so early in lubricant additive history, the literature from 1950 until today concentrates around improvements in production procedures, combinations and synergies with other additives, improvement in product qualities, and search for special applications. The use of sulfur carriers has been extended from metalworking and automotive engine to industrial oils and greases. Ashless hydraulic oils may contain sulfurized EP additives for special applications. Now this product group is used throughout the lubricant oil industry. Tribology has been defined as a particular field of scientific research, and several basic models of additive response have been worked out. A review article [27] in 1970 summarizes the state of the art at that time, including many literature references. Until the 1950s, sulfur carriers were mostly made by the lubricant manufacturers. However, increasing environmental awareness, growing market, and the need for more specialized products brought about change. As the sulfurization process involves deep chemical knowledge and production know-how and includes extremely high safety risks, specialty chemical companies became active in this area. It is expected that the few lubricant companies that still produce a small quantity of black sulfurized fats in-house may discontinue sooner or later. Since the 1950s, the sulfur carrier market was split into two segments: automotive and industrial. In automotive gear oil applications, sulfurized isobutene (SIB) soon became the standard EP product because it is high in sulfur content but low in corrosivity. The typical, rather strong smell of SIB is no real problem in this field of application because gear boxes are totally closed systems. In any open lube system, this EP technology is not acceptable. The big oil companies had their petrochemical subsidiaries (Mobil Chemical, BP Chemical, Shell Chemical, Exxon Chemical, Chevron Chemical, etc.) and added additive manufacturing as the market grew including SIB production units. So SIB production has always been the target for those companies with focus on automotive additives and lubricants. Over the decades, the SIBs have gone through changes in chlorine level due to environmental requirements [28]. Starting at 2–3% in the early days, today high qualities no longer contain chlorine because of a chlorine-free, high-pressure H2S production processes. Also, the amount of active sulfur in SIBs has been reduced to improve the long-term abrasive wear of gear oil formulations in bearings and to meet today’s fill-for-life requirements. But in principle, in automotive applications, the same sulfur chemistry is in use today as it was some 60 years ago. The other big field of application of sulfurized EP additives is industrial lubrication. Traditionally this area is less regulated and restricted by Original Equipment Manufacturer (OEM) approvals, general specifications, and standards—it is particularly true for the metalworking market. Here much more differentiated, problem-solving additives have been and still are in use. This environment has generated a greater variety of smaller volume sulfur carriers that address strongly differentiated technical requirements of metalworking processes and grease applications. Subsequently smaller, more specialized chemical companies entered the lube additive business. The first products were dark in color, but as early as 1962, Rhein Chemie commercialized its first light-colored, lowodor sulfurized synthetic ester based on chlorine-free production technology (see Section 9.3.2). A big milestone in the history of sulfur carriers has been the international banning of sperm oil (whale oil) in 1971. Up to this year, sperm oil and lard oil (pig fat) have been the dominant fatty raw materials for sulfur carriers. The sperm-oil-based products in particular showed excellent solubility and lubricity in addition to their sulfur-related EP properties. The extensive research activities of this period resulted in various patents [29,30]. The new raw materials turned out to be vegetable oils in combination with either synthetic esters or olefins. Another aspect that strongly influenced the sulfur carrier market has been the change of refinery technology for base oils. The driving force behind was the necessity to improve environmental as well as health and safety aspects of the major refinery products: fuels. These requirements led to drastic reduction of aromatic components and sulfur content in fuels and subsequently of base oils. From a lubricant point of view, the reduction of aromaticity of the base oils had strong negative

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impact on their solvency for additives and thus triggered intensive adjustment work on the additive producer’s side including sulfur carrier manufacturers. The reduction in sulfur content however has contributed substantially to the market growth of sulfurized additives. The reduction of naturally occurring sulfur in base oils through the desulfurization units now needs to be balanced for specific applications through the addition of synthetic, oil-soluble sulfur components to keep EP/AW properties as well as AO performance. This trend started in the 1970s but is getting stronger today with the increase in the availability of XHVI base oils/groups II and III as well as completely synthetic basestocks (polyalphaolefins [PAOs]). In the late 1970s and early 1980s, a new class of sulfur carrier has been introduced into the industrial lubrication market: dialkylpolysulfides. They are based on C8, C9, or C12 olefins and contain up to 40% sulfur in a very reactive form. They can be looked at as liquid, oil-soluble sulfur flower. The starting point for the development of these additives was the requirement of many lubricant-blending companies for an alternative to sulfurization of base oils with sulfur flower. It is a very time-consuming step and may generate toxic gas (hydrogen sulfide, H2S) and sulfur dropout during application. Sulfur flower can be dissolved in mineral oil just above its melting point of 115°C in concentrations of typically 0.4–0.6% and is used if appropriate in heavy-duty metalworking applications or running-in gear oils (see Section 9.5.1.3). The solution that has been offered from additive manufacturers has been the new class of organic polysulfides of light color and rather low odor. Diisobutenepentasulfide and tert nonyl- and dodecylpentasulfide have been introduced as easy to blend liquids and substitutes for sulfur flower. Today, these active type pentasulfides have become the most important and wide-spread class of sulfur carriers on the industrial oil side. In 1985, it was found that sulfur carriers, preferably polysulfide types, show a strong synergistic EP/AW behavior when combined with high total base number (TBN) ASTM-D4739 sodium and calcium sulfonates [31]. This has become known as the PEP technology (passive EP) in neat oil metalworking. In the beginning, it was hoped that this combination would be a general and simple solution to upcoming chlorinated paraffin replacement issue that started in Western Europe and Scandinavia in the mid to late 1980s. But as it turns out today, the PEP technology can only partially match the universal properties of chlorinated paraffin formulations especially under low-speed/ high-pressure operation conditions (for more details, see Section 9.5.4.2). From the late 1980s to early 1990s, a totally new aspect of sulfurized esters and fats has gained substantial ground—the toxicology and ecotoxicology of these chemicals. Workers’ safety, environmental compatibility, biodegradability, and similar requirements need to be addressed in industrial more than in automotive lubrication, because workers in machine shops often cannot avoid constant direct contact with the lubricant. The fact that the use of natural, renewable raw materials and optimized production procedures may give low-toxic and biodegradable sulfur carriers refreshed the interest of development chemists in these special, environmentally safe but classic additives. The twenty-first century’s central question of additive and lubricant R&D departments is how to further optimize energy efficiency and reduce friction. Again, sulfur carriers play a role. Spanning from engine oils to wind turbine gear boxes, formulators take advantage of their multipurpose character.

9.3 CHEMISTRY 9.3.1

CHEMICAL STRUCTURE OF SULFUR CARRIERS

For the majority of sulfur carriers, discrete structures are very hard to sketch for several reasons: The raw materials are very often mixtures of isomers: in the case of olefins, for example, diisobutene, there are five main isomers; tetrapropylene shows some 35 components in the gas chromatogram (GC). Natural fatty oils have a distribution with mono-, double- and triple unsaturated acids with unsaponificable matter.

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Depending on the temperature, inter- or intramolecular bonding of sulfur occurs preferably. The catalyst directs the addition of sulfur in a certain way (Markovnikov, etc.). It is a fact that sulfur carriers are technical products based on technical raw materials. In the following, the most typical structures of sulfur carriers based on different, contemporary raw materials are shown. Taking the rather complex reaction pathways of a sulfurization reaction into account, they necessarily are simplified model structures. 9.3.1.1 Sulfurized Isobutene This is the standard EP additive for gear oils with typical sulfur contents in the range of 40–50% (Figure 9.1). 9.3.1.2

Active-Type Sulfurized Oleﬁns

These are the polysulfide types of sulfur carriers that have been introduced as substitute for sulfurization of base oil and are widely used today in metalworking applications (Figure 9.2). 9.3.1.3

Inactive Sulfurized α-Oleﬁns

These are used in noncorrosive lubricant applications ranging from metalworking, greases to even engine oil applications (Figure 9.3). 9.3.1.4

Sulfurized Synthetic Esters (Light Color)

These are widely used in metalworking and grease applications. Depending on the type of synthetic ester chosen, special properties such as low temperature stability/fluidity and low viscosity may be achieved (Figure 9.4).

FIGURE 9.1

Sulfurized isobutene.

S 0−4 S

S 0−4

FIGURE 9.2

Sulfurized diisobutene, sulfurized tetrapropylene.

FIGURE 9.3

Inactive sulfurized α-olefins.

Sulfur Carriers

259 O S

O S

n O

FIGURE 9.4

Sulfurized synthetic esters (light color).

S R2

R1 S S

Chromophoric group

S O

S R1

S R2

FIGURE 9.5

Sulfurized fatty oil (black color).

9.3.1.5 Sulfurized Fatty Oil (Black Color) See Section 9.3.2.2 (Figure 9.5). 9.3.1.6

Sulfurized Fatty Oil/Oleﬁn Mixture (Light Color)

This special group of sulfur carriers is outstanding in properties as they combine the positive effects of sulfurized olefins (e.g., hydrolytic stability, high sulfur content) with the excellent lubricity and film-forming properties of sulfurized fatty oils (Figure 9.6).

9.3.2

CURRENT COMMERCIAL PRODUCTION PROCESSES

9.3.2.1 General Aspects In any current sulfurization process, the raw material is based on unsaturated compounds (olefins or unsaturated esters). All reactions are addition reactions to olefinic double bonds, and textbook chemistry applies to this kind of reaction (catalytic conditions, mechanisms, addition patterns— Markovnikov–anti-Markovnikov orientation). Today’s large-scale production technology in general avoids any halogen-containing reaction steps because there are low limits (maximum 30 ppm chlorine) in the final lubricants that may not be exceeded, for example, in automotive gear oils. Also, expensive removal/workup steps, for example, by washing, can be avoided if halogens are not used.

260

Lubricant Additives: Chemistry and Applications

O S S

O O

FIGURE 9.6

Sulfurized fatty oil/olefin mixture (light color).

Any sulfurization process involves either the possible formation or the actual use of hydrogensulfide (H2S), an extremely toxic and corrosive gas. The smell of H2S is generally that of rotten eggs, which is offensive to the human nose. In earlier times, the H2S released during the sulfurization reaction (see Section 9.3.2.2) was just vented through a chimney or, if at all, scarcely absorbed in alkaline scrubbers. Today, this procedure is no longer tolerated almost worldwide, and smaller factories that have not yet done so need to invest into expensive safety equipment. This is another reason why the smaller lubricant oil companies that still do a little bit of sulfurization are considering stopping. The processes are either pure batch or semicontinuous processes. For industrial applications, the volumes and varieties would not completely justify continuous productions. 9.3.2.2 Black Sulfurization This is the simplest and oldest of all production technologies for sulfur carriers. The first patented sulfur carrier was done this way. The manufacturing equipment needs to withstand pressure above 1–2 bars (it may even be pressureless). Raw materials may be olefins as well as natural or synthetic esters with a certain degree of unsaturation. The other reactant introduced into the olefin-containing reaction vessel is sulfur flower. The mixture is heated above the melting point of sulfur. An uncatalyzed reaction starts to become exothermic above 150–160°C, with the evolution of substantial amounts of H2S. Catalyzed reactions start just above the melting point of sulfur, in the range of 120–125°C. Typical catalysts are organic amines, metal oxides, and acids. Mechanistic studies of this reaction have been reported [32] and are very complex. At temperatures between 120 and 160°C, intermolecular reactions are preferred. At 160–190°C, intramolecular reactions may be detected. The first reaction step in this so-called black sulfurization process is the ring opening of the sulfur flower (S8-ring structure) and the subsequent oxidative attack of the sulfur on the vinylic protons (Equation 9.1). This quite uncontrolled reaction ends in the release of H2S and the formation of vinylic mercaptans, vinylic thioethers, vinylic alkyl- and dialkylpolysulfanes, vinylic thioketones, and even sulfur-containing heterocycles (thiophenes, etc.). R ⫺ HC ⫽ CH ⫺ CH 2 ⫺ R ⫹ Sx → R ⫺ HC ⫽ CH ⫺ CH(SR ) ⫺ R ⫹ HS( x⫺1) ⫹ H 2S

(9.1)

Sulfur Carriers

261

A great part of the in situ generated H2S does not leave the reaction mixture but is directly adsorbed by the double bonds, thus producing saturated alkylmercaptans (Equation 9.2a). They react further, oxidatively, with sulfur flower to generate alkyl- and dialkylsulfides with the release of more H2S (Equation 9.2b). R ⫺ HC ⫽ CH ⫺ R ⫹ H 2S → R ⫺ H 2 C ⫺ CH(SH) ⫺ R Olefin

(9.2a)

Alkylmercaptan intermediate

2R ⫺ H 2 C ⫺ CH(SH) ⫺ R ⫹ Sx → R ⫺ H 2 C ⫺ CHR ⫺ S ⫺ (Sx⫺1 ) ⫺ S ⫺ CHR ⫺ CH 2 ⫺ R ⫹ H 2S Alkylmercaptan

Dialkylpolysulfide

(9.2b) The final product consists of a full range of organic sulfur derivatives. Some of them are still unsaturated, with isomerized double bonds and conjugated, chromophoric (color-deepening) sulfur compounds such as thioketones and thiophenes, which cause the product to be dark black in color and rather smelly. From an application point of view, these products exhibit EP/AW performance, but because of their remaining double bonds, they have the following negative characteristics: • They will continue to polymerize during use and even under normal storage conditions. • They are easily oxidizable and form residues on fresh metal surfaces/discoloration. • They will cause a TAN increase within a short time in circulation systems and cause short oil drain intervals. • They will even generate H2S/mercaptan during high-temperature usage in lubricant systems (see Sections 9.4.2.1.7 and 9.4.2.1.2). So today’s main use of these black sulfurized products are total loss lubricants in which long-term stability and bad smell are not an issue. It is the cheapest way of making sulfurized additives. 9.3.2.3

High-Pressure H2S Reaction

High-quality sulfur carriers, which have improved properties compared to the black materials, are produced today using high-pressure/high-temperature equipment. The handling of toxic H2S under high-pressure conditions requires sophisticated handling techniques and safety measures. Furthermore, H2S is an expensive gas. All these aspects contribute to significantly higher production costs compared to the simple black sulfurization. In this process, the olefins, sulfur, and H2S are added to a high-pressure-resistant reactor and heated to 120–170°C. The reaction is also catalyzed by amines, metal oxides, acids, etc. For lowboiling olefins such as isobutene, the pressure may go up as high as 50–60 bar. For higher-boiling olefins such as diisobutene, typical pressure is in the range of 2–15 bar. The presence of H2S as reducing agent and strong nucleophile makes a total difference to the black sulfurization process. The oxidative attack of sulfur on the vinylic carbon–hydrogen (C–H) bond is effectively suppressed. The side reaction of the black sulfurization process becomes the main reaction here: the addition of H2S to the double bonds to form mercaptans (Equation 9.2a) which then quickly react with sulfur in a redox reaction to form dialkyldi-, tri-, tetra-, and polysulfides and release 1 mol equivalent of H2S (Equation 9.2b). This procedure gives much more controlled reaction conditions and fi nally fewer side products. The most important effect of this reaction pathway is the fact that the double bonds are gone after the reaction and no conjugated systems with chromophore (color-deepening) properties can be formed. The sulfur carriers produced by this way are much more oxidatively stable, and they are of light color. This one-step process is an advantage in terms of total production time and turnover.

262

9.3.2.4

Lubricant Additives: Chemistry and Applications

Mercaptan Route

Few producers synthesize sulfur carriers in a two-step process. 1. In the first step, H2S is added to olefins under the catalytic action of Lewis acids. If strong activators such as BF3 are used, the reaction takes place at as low as –20°C. Another procedure works at 60–90°C. The resulting alkylmercaptans are distilled from the reaction mixture and isolated as intermediates (Equation 9.3). The nonreacted olefins are circulated back to the reaction vessel. R ⫺ HC ⫽ CH ⫺ R ⫹ H 2 S → R ⫺ H2 C ⫺ CH(SH) ⫺ R Olefin

Alkylmercaptan

(9.3)

2. The mercaptans are oxidized either with hydrogen peroxide (H2O2) (Equation 9.4a) to the dialkyldisulfides 2R ⫺ H 2C ⫺ CH(SH) ⫺ R ⫹ H 2O2 → R ⫺ H 2C ⫺ CHR ⫺ S ⫺ S ⫺ CHR ⫺ CH 2 ⫺ R Alkylmercaptan

Dialkyldisulfide

(9.4)

or by stoichiometric amounts of sulfur to trisulfides (Equation 9.5) and polysulfides (Equation 9.6) 2R ⫺ H 2 C ⫺ CH(SH) ⫺ R ⫹ 2S → R ⫺ H 2 C ⫺ CHR ⫺ S ⫺ S ⫺ S ⫺ CHR ⫺ CH 2 ⫺ R ⫹ H 2S Alkylmercaptan

(9.5)

Dialkylpolysulfide

2 R ⫺ H 2 C ⫺ CH(SH) ⫺ R ⫹ Sx → R ⫺ H 2 C ⫺ CHR ⫺ S ⫺ (Sx⫺1 ) ⫺ S ⫺ CHR ⫺ CH 2 ⫺ R ⫹ H 2S Alkylmercaptan

Dialkylpolysulfide

(9.6) This may be summarized in the reaction shown in Figure 9.7. This process is mainly applied to olefin-based sulfur carriers based on tri- and tetrapropylene as starting material because the resulting tertiary dodecylmercaptan may be used chemical intermediate and in other applications such as rubber processing as.

+ 2 H2S

BF3 2 −20°C

Tetrapropene

+2S − H2S

Tertiary dodecyltrisulfide

FIGURE 9.7

9.3.3

Summary of two-step process.

OTHER SYNTHETIC ROUTES

9.3.3.1 Sulfurchlorination Route Sulfur carriers can be synthesized in a two-step process using disulfur dichloride and sodium sulfide solution (Figure 9.8). It had been widely used because it is a controlled way of adding discrete S2-bridges to double bonds with little side reactions occurring.

Sulfur Carriers

263 Cl 2

+ S2Cl2

S Cl

FIGURE 9.8

Sulfurchlorination reaction.

S S

Cl S 2

+ 2 Na2S

− 4 NaCl

Cl S

n Sulfurized isobutene

FIGURE 9.9

Dechlorination reaction.

Cl 2

−2 NaCl

0−5

S 0−5

Arylhalogenide (or Alkylhalogenide)

FIGURE 9.10

Sodium polysulfide in water

Diarylpolysulfide (or dialkylpolysulfide)

Dechlorination of arylhalogenides.

Step 1. Addition of disulfur dichloride to double bonds. In case of fatty oil being used as olefin source, the resulting product is a sulfurchlorinated fatty oil useful as chlorine and sulfur-containing EP additives in metalworking. From a technical point of view, their biggest problem is the split-off of chlorine and subsequent severe corrosion problems that are difficult to control. From today’s point of view, the presence of chlorine is not favorable anymore because of environmental concerns. Step 2. Subsequent treatment of the sulfurchlorinated products with sodium sulfide (NaS2) solution in water. It is a substitution reaction of sulfur versus chlorine (Figure 9.9). Intermolecular linkage as well as ring closure may occur. The water-soluble sodium chloride is washed out. 9.3.3.2

Alkylhalogenide/NaSx

This process is closely related to the first step reaction discussed in Section 9.3.3.1. Starting materials may be alkyl- or arylhalogenides. As shown in Figure 9.10, it is possible to substitute halogens with sulfur using alkali sulfides. If Na2S is used, monosulfides are generated. In case alkali polysulfide is applied, alkyl- or arylpolysulfides are the resulting derivatives. This route has not found commercial interest as raw material costs are too high compared to other synthetic methods.

264

9.3.4

Lubricant Additives: Chemistry and Applications

RAW MATERIALS

In principle, any single- or multi-double-bond-containing molecule may be sulfurized. Therefore, the list of olefinic raw materials is long. The list of sulfur-containing materials is rather short. It is mainly sulfur flower (S8), hydrogen sulfide gas, some S2Cl2, and some alkali polysulfide (e.g., NaSx). On the olefin side, patent literature reports of the following: • Vegetable oils (soybean, canola, rapeseed, cottonseed, rice peel, sunflower, palm, tall oil, terpenes, etc.) • Animal fats and oils (fish oils, lard oil, tallow oil, sperm oil, etc.) • Fatty acids • Synthetic esters • Olefins (isobutene, diisobutene, triisobutene, tripropylene, tertapropylene, α-olefins, n-olefins, cyclohexene, styrene, polyisobutene, etc.) • Acrylates, methacrylates • Succinic acid derivatives, and more The choice of commercially applied raw material is certainly limited to those compounds that have a reasonable price level and give certain performance benefits. Sulfur carriers based on low-boiling olefins (e.g., C4-types) are limited to closed lubricating systems due to the volatility of the decomposition products and associated offensive smell. For water-based lubricant oil systems, sulfurized fatty acids that can be easily emulsified and active types of olefins that cannot be hydrolyzed are preferred. In oil applications, one can find the full range of products.

9.4 PROPERTIES AND PERFORMANCE CHARACTERISTICS 9.4.1 9.4.1.1

CHEMICAL PROPERTIES Effect of Additive Structure on Performance

9.4.1.1.1 Raw Materials The additive structure is mainly influenced by the choice of the raw materials and the sulfurization method. A general overview of the performance properties of sulfurized products based on different raw materials is shown in Table 9.1. 9.4.1.1.2 Inﬂuence of Raw Materials on Extreme Pressure and Antiwear The raw material determines the polarity and, therefore, the affinity of the product to a metal surface [33]. With increasing polarity, an increasing EP performance can be observed. Straight sulfurized

TABLE 9.1 Performance Properties of Sulfurized Products Ester

Extreme pressure Antiwear Reactivity Cu corrosion Antioxidant Lubricity

Triglyceride

Inactive

Active

Fair Good Low Low Good Fair

Good Low High High Low Fair

Inactive

Active

Good Very good Low Low Good Very high

Very good Low High High Poor Very high

Oleﬁns Inactive Low Good Low Low Good Poor

Active Fair Poor Very high High Poor Poor

Sulfur Carriers

265

olefins are nonpolar and show a relative poor affinity for metal surfaces (see Section 9.4.2.1.4). As the polarity increases from olefin < ester < triglyceride, the EP performance increases in the same order. This behavior is demonstrated in a simple four-ball EP test. Chart 9.1 shows the four-ball weld load (DIN 51350 Part 2) of sulfurized additives over the sulfur level in oil. The products with a high polarity (C and D) show considerably higher EP loads than the nonpolar additives (A and B). The content of active sulfur is only of minor importance on the EP performance, but the polarity and chemical structure play a major role. 9.4.1.1.3 Activity Active sulfur is the amount of sulfur available for a reaction at a certain temperature. A common method for its determination is ASTM D-1662 [34]. The amount of active sulfur is determined by reacting copper powder with the sulfurized product for 1 h at 149°C. Depending on the raw materials and on the sulfurization method, the active sulfur content can vary greatly. The activity is a function of the temperature. Chart 9.2 shows typical active sulfur contents of sulfurized products based on different chemistry and sulfurization methods. The activity depends mainly on the sulfur chain in the molecule. Mono- and disulfides are not aggressive against yellow metals. Pentasulfides are highly reactive and, therefore, suitable for heavy-duty machining of steel The long-term inhibition of these products against yellow metals is hardly possible. Long-chain sulfur bridges in polysulfides (A) are thermally less stable than short sulfur bridges, where sulfur is linked to the carbon atom of the raw material. For this reason, the reaction with the metal surface is possible at relatively low temperatures. Mono and disulfides show only a medium activity, because sulfur will be released only at higher temperatures [35]. The active sulfur at a given temperature is an indication of the ability of the product to provide sufficient reactive sulfur to form metal sulfides. Published work on the mechanism of the influence of

7500 A

7000

Four-ball weld load (N)

6500

6000 5500

5000

4500 4000 3500 3000 2500 2000 1500 1000 0

0.5

1.5

2.5

3.5

4.5

5.5

% Sulfur in oil Type

Total Sulfur

Olefin

Ester

Triglyceride

CHART 9.1

Active Sulfur

Activity (%)

0.5

Influence of raw materials on EP performance.

6.5

266

Lubricant Additives: Chemistry and Applications

90 A

B Active sulfur (% of total sulfur).

70 C 60 D 50

40 30 20 10 0 50

110

130

150

Temperature (°C)

CHART 9.2

Active Sulfur at 149°C

Type

Total Sulfur

Olefin

Triglyceride

0.5

Triglyceride

10.5

Olefin/ Triglyceride

4.5

Active sulfur of various sulfurized products.

organosulfur compounds on the load-carrying properties of lubricating oils indicates that this is due to their ability to form sulfide films that are more easily sheared than the metallic junctions under EP conditions [36]. Therefore, active sulfur has a significant influence on the AW performance. Higher sulfur activity results in faster formation of the metal sulfide and higher wear. This performance is visualized in Chart 9.3. The chart shows the four-ball wear scar (DIN 51350 Part 3) of sulfurized products with various activities. 9.4.1.1.4 Copper Corrosion ASTM D-130 [37] is a common method to determine the copper corrosion of additives. This copper corrosion does not necessarily reflect the activity of a sulfurized product, because very often yellow metal deactivators are used to mask the active sulfur. The degree of copper corrosion depends on the amount of active sulfur and the presence of yellow metal deactivators. Inactive sulfurized products will show a long-term inactivity toward yellow metals, whereas active sulfur, masked with yellow metal deactivators, will react with the yellow metal as soon as the deactivator is consumed/reacted. Therefore, the only statement that can be made is that a product will not stain copper under the given test parameters. This method is not suitable to determine the activity that is of major relevance for the performance of a sulfurized product (see Section 9.4.1.1.3).

Sulfur Carriers

267

Four-ball wear scar (mm)

1.6

A B C D

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

Type

A B C D

CHART 9.3

Hydrocarbon Hydrocarbon Triglyceride Triglyceride

0.5

1.5 2 2.5 % Sulfur in oil

Total Sulfur 40 20 10 18

Active Sulfur 36 5 0.5 10.5

3.5

Activity (ASTM D-1662)(%) 90 25 5 58

Influence of activity on AW performance.

9.4.1.1.5 Antioxidant Sulfurized products with low active sulfur content are suitable to improve the AO behavior of lubricants. This is particularly important if hydrocracked, almost sulfur-free base fluids are used. During the synthesis of these oils, the natural sulfur (mainly heterocycles, inactive) is removed. The reintroduction of inactive sulfur carriers improves the oxidation stability, especially in combination with other secondary AOs. 9.4.1.1.6 Lubricity Lubricity can be described as friction reduction under low-pressure conditions. Under these conditions, physical adsorbed lubricating films are effective (see Section 9.4.2.1.4). Inactive sulfurized triglycerides are widely used to improve the lubricity of a lubricant. In general, the lubricity of sulfur carriers increases with the polarity. Sulfurized olefin (no lubricity) < sulfurized ester (medium lubricity) < sulfurized triglyceride (high lubricity). Special products with enhanced lubricity are based on synergistic raw material blends such as triglyceride/long-chain alcohol, triglyceride/fatty acid, and triglyceride/olefin. 9.4.1.1.7 Color The color of sulfurized compounds is mainly influenced by the production method and virtue of the raw materials. Light color is not only a matter of cosmetics but also a quality feature. Light-colored products manufactured with high-pressure hydrogen sulfide processes or by mercaptan oxidation do not have remaining unsaturated double bonds, and therefore, they show better oxidation stability in general.

9.4.2 9.4.2.1

PHYSICAL PROPERTIES Effect of Additive Structure on Properties

9.4.2.1.1 Raw Materials The selection of the raw materials and the production process determine the chemical structure of the compound. The physical properties of a sulfurized product are dependent on the chemical structure. An overview is given in Table 9.2.

268

Lubricant Additives: Chemistry and Applications

TABLE 9.2 Physical Properties of Sulfurized Products Ester Polymerization Solubility Polarity Viscosity Biodegradability

9.4.2.1.2

Low Good Moderate Low Good

Triglyceride High Fair–good High High Excellent

Oleﬁn Very low Very good Low Very low Poor

Polymerization

During the sulfurization process, the molecules of the raw materials are linked through sulfur. Depending on the structure of the raw material, two or more raw material molecules will be linked. Triglycerides such as lard oil and soybean oil do polymerize and form solid, rubberlike products, if the polymerization is not controlled through chain terminators such as esters or olefins, containing only one double bond. Olefins with only one double bond do not polymerize. Two molecules are linked by a sulfur chain where length depends on the production process. Esters behave in a similar way but due to varying amounts of multiple unsaturated compounds in natural esters, some polymerization takes place. Dark sulfurized products not only show less oxidation stability compared with light-colored, completely saturated compounds but will also resume polymerization after the production process is finished. 9.4.2.1.3 Solubility The solubility is mainly a function of the polarity of the product. As the polarity increases from olefin < ester < triglyceride, the solubility decreases. Polarity as well as the grade of polymerization determines the solubility. In general, sulfurized olefins have excellent solubility in solvents and all mineral oils. Depending on the sulfurization method, esters can exhibit good solubility even in group II and group III base oils if their polymerization grade is controlled during production. Sulfurized triglycerides are, in general, limited in their solubility due to their high polarity. But even more, the grade of polymerization plays a predominant role. A controlled reaction/polymerization can lead to light-colored products that will be soluble in paraffinic base oils, whereas uncontrolled polymerization will lead to dark-colored products, soluble only in oils with higher polarity and aromatic content such as, for example, naphthenic base oils. 9.4.2.1.4 Polarity Polarity determines the adhesion of a sulfurized product to the metal surface. The polarity depends on the raw materials used for the sulfurization. The organic portion of the molecule is responsible for the polarity and the affinity of the sulfurized product to the metal surface [35]. As the polarity increases from sulfurized hydrocarbon < sulfurized ester < sulfurized triglyceride, the affinity (physical adsorption) to metal surfaces also increases. Therefore, sulfurized products based on triglycerides, fatty acids, or alcohols provide superior lubricity compared with sulfur carriers based on less polar esters or nonpolar olefins. 9.4.2.1.5 Viscosity Viscosity of a sulfurized product depends on the type of raw material used for the sulfurization and polymerization grade. A higher degree of polymerization (molecular weight) results in higher viscosity. The raw materials determine the viscosity index (VI) of a sulfur carrier. Short-chain sulfurized olefins show low VIs, whereas sulfurized triglycerides have VI above 200.

Sulfur Carriers

269

9.4.2.1.6 Biodegradability Depending on the raw materials and on the sulfurization process, sulfurized products cover the whole range from nonbiodegradable to readily biodegradable. Besides the raw material, the production technology plays a predominant role. Catalysts used, impurities in the raw materials, and side components formed during the synthesis have a strong influence on the biodegradability. Therefore, biodegradability cannot be predicted but has to be tested for every single product. Biodegradable sulfur carriers are available for various applications [38]. 9.4.2.1.7 Stability Storage stability is obtained by total reaction of the double bonds in the sulfur carrier and in eliminating H2S and mercaptans. Especially mercaptans, but also H2S, are left over from the sulfurization process. If H2S or mercaptans are not removed completely, they will evaporate under severe conditions in the final application or even under unfavorable storage conditions. Mercaptans can react with the polysulfanes and thereby release H2S. Depending on the raw materials and the type of sulfurization process, some sulfurized products continue to polymerize during storage. Especially triglycerides, sulfurized with flower of sulfur under atmospheric pressure, show a steady and sometimes very strong polymerization during storage.

9.5

COMPARATIVE PERFORMANCE DATA IN PERTINENT APPLICATION AREAS

9.5.1 9.5.1.1

METALWORKING Cutting/Forming

In principle, we have to deal in all cutting processes with abrasive wear (i.e., cutting) and adhesive wear (i.e., build up edges). Depending on the particular process, the machining parameters, one of these wear types, play a dominant role. At low machining speeds, like in most of the forming operations, adhesive wear (cold welding), the formation of build up edges, and wear on the flank of the cutting edge are very often the limiting factors for tool life. At high machine speeds and increasing contact temperatures, the abrasive wear determines the tool life. The reactivity of additives depends on temperature and pressure. Field and laboratory tests showed that different types of sulfur carriers (same sulfur content but varying raw materials or production processes) lead to significantly different results in a metalworking operation [39]. 9.5.1.2

Contribution of Sulfur Carriers to Metalworking

Sulfurized products can be designed to meet technical and ecological requirements in metalworking processes. They are used successfully for more than 80 years to avoid abrasive and adhesive wear and enhance lubricity. In cutting operations, their main function is to support the cut and to prevent wear of the tool, whereas in forming processes, sulfurized products should form a pressurestable lubricant film and prevent adhesive wear. 9.5.1.3

Replacement of Sulfur Flowers

In the past, it was very common to dissolve sulfur flowers in metalworking fluids to obtain a high reactivity and good EP properties. This procedure is very cost-intensive because it has to be done under controlled temperature conditions below the melting point of sulfur and has several disadvantages such as limited solubility (maximum 0.8% S), limited stability (sulfur dropout), and high corrosivity toward yellow metals. Also, there is a risk of H2S generation, a highly toxic gas well known because of its rotten-egg odor. Today, this process is widely substituted by using sulfurized products. If just reactivity is required, sulfurized olefins with high total and active sulfur content are

270

Lubricant Additives: Chemistry and Applications

used, although it is possible to adjust almost any activity/lubricity ratio while using the combination of appropriate sulfurized products. 9.5.1.4

Copper Corrosion

Depending on the process and on the metals machined, corrosion control toward yellow metals can be a requirement. If inactivity (no staining) toward yellow metals is required, it is important to use either absolute inactive sulfurized products or medium-active sulfur carriers in combination with yellow metal deactivators. Active sulfurized products can be inhibited short term but will, in the long term, turn active again. 9.5.1.5 Substitutes for Chlorinated Parafﬁns The driving forces for the replacement of chlorinated paraffins are mainly ecological and toxicological reasons. Users and waste oil disposal facilities have additional concerns over the corrosivity of the chlorinated paraffin decomposition products, primarily hydrochloric acid. Chlorinated paraffins work because of their ability to form a highly persistent lubricating film even at low temperatures or moderate pressure. At high-temperature/high-pressure conditions, they decompose, and the formed hydrogen chloride forms metal chloride with the metals involved in the process [39]. Chlorinated paraffins can be substituted with sulfurized products. Depending on the main function of the chlorinated compound in the particular process used, lubricity, or activity, suitable sulfurized products are available, which can function as alternatives. The lubricity performance is mainly covered by highly polar, inactive sulfur compounds (see Section 9.4.1.1.6), whereas the activity will be covered by reactive sulfurized olefins or mixed sulfurized olefins/triglycerides. 9.5.1.6

Substitute for Heavy Metals

Heavy metals, particularly antimony, molybdenum, and zinc compounds are used as EP and AW additives in severe metalworking processes. Sulfur carriers have proven to be suitable substitutes, particularly when used with synergistic compounds such as polymer esters, phosphates, phosphites, dialkyldithiophosphates, and sulfonates. 9.5.1.7

Carbon Residue Reducing in Rolling Oils

Sulfur carriers are used in cold rolling of steel to prevent carbon residues build up on the surface of the metal sheets during the annealing process. Carbon residues are generated by oxidation/polymerization of additives used in rolling oils. Therefore, typical rolling oils contain sulfur-based antisnakey edge and carbon-reducing additives [40]. Clean burning of the lubricant is important to obtain a clean metal surface that can be evenly coated in subsequent process steps. Typical products used for this application are sulfurized olefins with low to medium activity or inactive sulfurized triglycerides. 9.5.1.8

Water Miscible Metalworking Products

Sulfurized products are used in water-miscible metalworking systems to provide EP performance and, depending on the type of sulfur carrier, lubricity. By far the biggest applications are soluble oils or emulsions. Standard sulfurized products are not water-soluble. Surfactants must be used to keep the sulfur carrier in the emulsion. Compared to applications in non-water-based systems, the water-based systems require hydrolytically stable products that can react at relative low temperature. Therefore, active sulfurized olefins, preferably pentasulfides, are widely used for this application. Sulfurized esters and triglycerides are also used, especially if additional lubricity is required. Specialty sulfur carriers are reaction products of sulfurized fatty acids and sulfurized olefins. These sulfur carriers have good emulsifying properties with relatively high hydrolytic stability and activity.

Sulfur Carriers

271

Straight sulfurized fatty acids such as sulfurized oleic acid, are used in semisynthetic metalworking fluids. The sulfurized fatty acid will be reacted with alkaline compounds such as amines or potassium hydroxide to form a soap. This soap is water-dispersible and needs much less emulsifiers than a sulfurized olefin. However, hard water stability can become a problem with this type of sulfur carrier.

9.5.2

GREASE

High demands on load-carrying capacity of machine parts require the use of EP and AW additives to avoid material loss and the destruction of the surfaces of the friction partners. Older technology still uses typical gear oil sulfur carriers based on short-chain olefins such as isobutene. These sulfur carriers provide a high sulfur content, but their distinct, strong odor prohibits their use in open lubricating systems. As it is almost impossible to mask the activity of sulfurized products in greases by using yellow metal deactivators or sulfur scavengers, inactive sulfurized products are widely used as EP additives in greases. Especially if the grease is designed for a wide application range, it is imperative that truly inactive sulfur carriers are used, because yellow metals are widely present as friction partners (e.g., brass cages in bearings). In addition, there are increasing demands on high-temperature stability for various grease types. This also calls for inactive, oxidation-stable sulfurized products. Typical sulfur carriers for greases are shown in Table 9.3. Sulfurized products are also used to substitute heavy-metal-containing compounds, which are traditionally used as EP additives in greases. Besides their excellent performance, these heavy-metalcontaining compounds show some weak points. Antimony and bismuth compounds are known to have some weakness regarding copper corrosion, and lead compounds are toxic. In the meantime, many of these products have been replaced by special sulfurized products either as a direct replacement or in combination with synergistic compounds such as zinc dialkyldithiophosphates, phosphate esters, or overbased sulfonates [41]. Sulfurized products are also used in greases for constant velocity joints (CVJ) [42]. They are very efficient in combination with molybdenum compounds (e.g., molybdenum dithiocarbamate [MoDTC], molybdenum dithiophosphate [MoDTP], Mo-organic salts) as a sulfur source to support the formation of lubricating, active molybdenum disulfide in the friction zone. There is an increasing demand for EP greases for environmentally sensitive applications such as railroad wheel flange lubrication, railroad switches, and agricultural equipment such as tractors or cotton picker spindles. Some sulfurized products are biodegradable and show excellent ecological data [39]. Therefore, these products are used rather than heavy-metal-containing compounds to enhance EP and AW properties in such applications.

9.5.3

INDUSTRIAL OILS

An increasing variety of industrial fluids use sulfurized products as EP and AW additives.

TABLE 9.3 Typical Sulfurized Products for Greases Type

Total Sulfur

Active Sulfur

Features

8–12 13–15

0.5–3 4–7

Olefin

10–15

Triglyceride/olefin

Ester

9–11

1–3

Mainly inactive, limited EP performance Mainly active, hard to mask Cu corrosion long term, good EP High EP performance, very distinct odor, only for encapsulated systems Mainly inactive, high EP performance Mainly inactive, limited EP performance, excellent low-temperature pumpability

Triglyceride Triglyceride

272

9.5.3.1

Lubricant Additives: Chemistry and Applications

Industrial Gear Oils

Typical sulfur carriers for this application are short-chain sulfurized olefins. Sulfurized Isobutene (SIB) or Sulfurized Diisobutene are widely used as EP additive in industrial gear oils. SIB is used for some decades as the EP additive of almost all industrial gear oil packages. The high sulfur content, combined with a relative low active sulfur level, is ideally suited to match the requirements. Unfortunately, these products have a very distinct odor and, depending on the manufacturing process, can contain chlorine compounds. Newer developments are based on sulfurized olefins with a longer chain length. Specialty products with additional demands on lubricity are based on sulfurized triglycerides or mixtures of sulfurized olefins and triglycerides. 9.5.3.2 Slideway Oils Slideway oils are a special type of gear oils with very good anti-stick–slip properties. Besides the austere requirements on coefficient of friction, there are also demands on compatibility and demulsibility with metalworking emulsions. Inactive sulfurized triglycerides are suitable to reduce the coefficient of friction. Unfortunately, most of these products are easy to emulsify and will, therefore, not meet the requirements on demulsibility without extensive formulation work. Modern slideway oils are based on demulsifying sulfurized olefin/triglyceride-based products that have the advantages of low coefficient of friction, good demulsibility, and high EP loads. 9.5.3.3 Hydraulic Fluids It is possible to use inactive sulfur carriers in hydraulic systems with only moderate requirements on thermal stability. Typical products are sulfurized olefins and triglycerides or mixtures thereof. 9.5.3.4 Multifunctional Lubricants Multifunctional lubricants cover more than just one lubrication application. There are increasing demands for this lubricant type, especially in metalworking shops. As one lubricant will be used for different applications with sometimes very different requirements, it is important that multifunctional additives are used. Depending on the overall performance requirements, sulfur carriers are used as EP, AW, or lubricity additive (see Tables 9.1 and 9.2). Multifunctional lubricants are almost always a compromise in their formulation. For example, metalworking machines with combined gear oil/process oil sump require a fine-tuned additive, especially on the EP side. Sulfur carriers with a medium activity, additionally passivated with sulfur scavengers, are widely used for this application. 9.5.3.5

Agricultural Applications

Lubricants in agricultural applications can be spilled on soil because of either the machine design or leaks in hydraulic and gear systems. Therefore, there are increasing requirements on environmentally compatible or less harmless lubricants. Sulfurized products are ideally suited for this type of applications. They can be designed to meet performance and ecological requirements (e.g., biodegradability). A wide range of lubricants exists for outdoor equipment based on vegetable oils (e.g., soybean, canola, rapeseed, and sunflower oil). Sulfur carriers for these applications are mainly based on vegetable oils and synthesized in strictly controlled manufacturing processes. Typical sulfur carriers for agricultural applications are shown in Table 9.4. 9.5.3.6

Automotive Applications

It is disclosed in U.S. Patent Nos. 4,394,276 and 4,394,277 that various sulfur-containing alkane diols may be formulated with lubricating oils to effectively reduce fuel consumption in an internal combustion engine. Sulfurized products in general and inactive sulfurized, oxidatively stable

Sulfur Carriers

273

TABLE 9.4 Sulfur Carriers for Agricultural Applications Sulfur Carrier Type

Application

Ester, inactive Triglyceride, inactive Triglyceride, medium-active Olefin, inactive Triglyceride/olefin, inactive

Gear greases (NLGI class 000), cotton picker spindle lubricants Gear lubricants, hydraulic fluids, greases for bearing lubrication, chassis lubricants Gear lubricants, chain saw, and bar saw lubricants Gear greases (NLGI class 000), cotton picker spindle lubricants Gear lubricants, hydraulic fluids, greases for bearing lubrication, chassis lubricants

TABLE 9.5 Synergistic Effect of ZnDTP on Copper Corrosion (ASTM DISO) Type Triglyceride Ester Triglyceride a

Total Sulfur

Active Sulfur

Treatment Level (%)

18 17 15

10.5 8.5 5

5 5 5

Cu Corrosion 3 h at 100°C

Cu Corrosion (+1.5% ZnDTPa) 3 h at 100°C

4c 3b 3a

3b 1b 1b

Thermally stabilized zinc dialkyldithiophosphate based on 2-ethylhexyl alcohol.

olefins in particular are known to reduce friction efficiently in engines. They provide not only AW and antifriction but also antioxidation properties. However, they cannot substitute the multifunctional zincdialkyldithiophosphates in this application. Besides crankcase applications, the use of sulfurized products in automotive gear lubricants is far more important. Since the middle of the twentieth century, almost every gear lubricant for automotive applications has been formulated with SIB. The advantages of SIB are its high sulfur content, oxidation stability, and low corrosivity, but the very distinct odor and the low lubricity are its disadvantages. Sulfurized esters and triglycerides are used in special transmission fluids to adjust the stick–slip properties. These sulfur carriers are also used in other lubricants in the automotive area such as wheel bearing or CVJ grease.

9.5.4

SYNERGIES/COMPATIBILITY WITH OTHER ADDITIVES

Sulfurized products are compatible with most of the additives used in lubricants. Only strong acids and bases must be avoided in combination with sulfur carriers. 9.5.4.1

Zinc Dialkyldithiophosphates

ZnDTPs are used in combination with sulfurized products in various applications. Besides their primary functions as AW and AOs, there is a well-known synergistic effect in regard to stabilization and improvement of copper corrosion of sulfur carriers. This behavior is demonstrated in the ASTM D-130 copper corrosion test (Table 9.5). In addition, ZnDTP can have a very positive effect on the odor of sulfur carriers.

274

9.5.4.2

Lubricant Additives: Chemistry and Applications

Basic Alkali Metal Salts

Sulfurized products show a very strong synergistic effect in combination with basic alkali metal salts [43], often referred to as overbased sulfonates or carboxylates. Particularly, active sulfur in combination with overbased calcium or sodium sulfonates exhibits advantageous performance with regard to improved load-carrying and AW properties. These additive combinations are used in lubricants for severe metalworking operations. It is disclosed in International Patent WO 87/06256 [41] that the load-bearing characteristics of a grease composition and gear lubricant may be unexpectedly improved by formulating these compositions with an additive mixture comprising overbased salts of alkaline earth metals or alkali metals and at least one sulfurized organic compound. From today’s point of view, the overbased products/ sulfur combination has its advantages in some stainless steel cutting and forming operations. But the high alkalinity of such formulations shows big compatibility problems when in contact with lubricity esters and other types of acidic additives. Alkaline washing baths get used up quickly and need much more frequent changes as calcium soaps built up. Welding without cleaning the metal surface is also impossible as the high TBN sulfonates are generating high amounts of oxide ash. 9.5.4.3 Antioxidants Inactive sulfur carriers show a synergism with aminic AOs. This effect is very distinct in low or even sulfur-free base fluids. Active sulfurized products do not show this synergy. On the contrary, the active types deteriorate the oxidation stability. Table 9.6 demonstrates the oxidation stability (ASTM D-2270, RPVOT Test) of active and inactive sulfur carriers based on the same raw materials. The inactive product improves the oxidation stability (250 min) twice as much as the active type (120 min). In combination with the aminic AOs, the synergistic effect is obvious. Although the inactive sulfur carrier improves the AO properties of the aminic AO, the active type has a detrimental effect and reduces the oxidation stability. 9.5.4.4 Esters/Triglycerides Esters are used either as base fluids or as additives. It is important to coordinate ester type and sulfur chemistry to achieve optimum performance. Unsaturated esters show strong synergistic effects with active sulfur, whereas inactive sulfur shows distinct synergies with saturated esters. The performance of sulfur carriers in saturated esters is similar to their performance in mineral oil. These synergies are widely used in the formulation of lubricants. Combinations of active sulfur and unsaturated esters or triglycerides (mainly vegetable or animal oils such as canola, rapeseed, tall, sunflower oil, and esters thereof) are very common in all types of metalworking fluids, in oils, as well as in water-based systems. The combination of these products shows better EP and AW properties than the single components. This performance is illustrated in the four-ball test (see Table 9.7).

TABLE 9.6 Synergisitc Effect of ZnDTP on Copper Corrosion

Base oil 0.2% Aminic AO (alkylated diphenylamine)

Base Oil Hydrocracked, Dewaxed, No Sulfur (min)

1.0% Inactive Oleﬁn, 20% S, 5% active S (min)

1.0% Active Oleﬁn, 39% S, 30% active S (min)

250

120

400

540

135

Sulfur Carriers

275

TABLE 9.7 Esters, Synergistic Effect on EP and AW Sulfurized Oleﬁn 40% S, 36% active 1.5% — 1.5%

TMP Oleate — 5.0% 3.5%

Four-Ball Weld Load DIN 51350 Part 2 (N) 2800 800 3200

Four-Ball Wear Scar DIN 51350 Part 3 (mm) 0.8 0.6 0.55

Other applications for the combination of active sulfur and unsaturated ester are heavy-duty gear oils in agricultural applications and environmentally-friendly chain saw and bar saw lubricants (see Section 9.5.3.5). If Cu corrosion presents a problem, the use of medium active sulfurized products in combination with unsaturated esters is of advantage. Some of these sulfur carriers are active enough to create the synergistic effects, but their Cu corrosion can be controlled with suitable yellow metal inhibitors. Besides the improvement of EP and AW performance, inactive sulfur carriers can boost the AO properties. Saturated esters are used where good oxidation stability is required. The performance of sulfurized products in saturated esters is comparable to the performance in mineral oil. Inactive sulfurized olefins and sulfurized triglycerides, and mixtures thereof, are typically used as additives in lubricants based on saturated esters.

9.5.5

COST-EFFECTIVENESS

Sulfurized products cover the whole range from relative cheap commodity to high-price specialty. Depending on the type and treatment level, sulfurized products can be a major cost factor in lubricants. However, the use of sulfur carriers enables us to run processes and to overcome lubrication problems that cannot be solved in a cost-efficient way with other additives. For example, it is possible to increase machine speeds and thus productivity while using appropriate sulfur carriers instead of esters or chlorinated paraffins. Depending on the type of sulfur carrier, other commonly used additives in a formulation, for example, esters or yellow metal deactivators, can be saved. In comparison with heavymetal- or chlorine-containing lubricants, the disposal costs for the used lubricant can be much lower. Besides direct cost savings, respectively, cost efficiency due to higher productivity and lower disposal costs, there are secondary cost factors. In comparison with some traditionally used EP additives such as chlorinated paraffins (HCl-formation > rust), overbased sulfonates (difficult degreasability, incompatibility with other additives), or heavy-metal-containing additives (residue formation), sulfurized products show, in general, lower cost-effective side effects.

9.6 MANUFACTURE AND MARKETING ECONOMICS 9.6.1

MANUFACTURERS

Afton, United States Arkema, France DAI Nippon Inc., Japan DOG-Chemie, Germany Dover Chemical Corp., United States Elco, United States Harrison Manufacturing Company, Australia Hornett, United Kingdom

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Lubricant Additives: Chemistry and Applications

Lubrizol, United States Miracema Nuodex, Brasil PCAS, France Rhein Chemie, Germany Additionally, there are some lubricant manufacturers who still sulfurize dark-colored products for their own use, and some local sulfurization plants also sulfurize commodity-type products.

9.6.2

MARKETERS

In general, the manufacturers are also marketing the products. Some local manufacturers buy sulfurized products, blend them with esters, mineral oil, etc., and sell them under their own brand name. Sulfur carriers are intermediate and not consumer products.

9.6.3

ECONOMICS

Market prices vary depending on the raw materials, the production process, and the performance level of the products. Low-quality, dark sulfurized fats with distinct odor and limited stability sell for less than U.S.D 1.6/kg. High-performance, top-quality, low-odor, light-color products achieve prices of more than U.S.D 4.0/kg. Not only the raw materials, but much more the production process, determine the price for a sulfurized product. For example, the sulfurization of a typical gear oil sulfur carrier with di-sulfur-di-chloride and the necessary subsequent washing steps are more costly than the sulfurization of a fat with flower of sulfur.

9.6.4

GOVERNMENT REGULATIONS

9.6.4.1

Competitive Pressures

There are no government regulations concerning the use of sulfurized products, but depending on the location of the manufacturing plant, very stringent regulations and conditions concerning emission standards can apply. Therefore, there is a competitive distortion in production between more and less environmentally aware countries. Production technology and in particular low-emission production are key cost factors. 9.6.4.2

Product Differentiation

Apart from some commodities, there is a clear product differentiation mainly derived from quality and performance. A first criterion for differentiation is color, followed by sulfur content, raw materials, and odor. A classification is hardly possible because many of the products are tailor-made either to cover a specific performance profile or to meet specifications in various applications. A simple categorization by sulfur content or raw material bases would be to coarse and would not take performance into account. Even products based on the same raw materials but manufactured with a different process can be completely different in performance. Modern, light-colored, high-performance products with low odor are sulfurized using H2S or mercaptans. Even the appearance distinguishes these products from conventionally sulfurized darkcolored, smelly products.

9.7 9.7.1

OUTLOOK CRANKCASE/AUTOMOTIVE APPLICATIONS

Steady demands on the reduction of phosphorus levels in motor oils as well as requirements for increased fuel efficiency, that is, friction reduction will open new opportunities for sulfur chemistry. Sulfurized products are already used in this type of application (see Section 9.5.3.6).

Sulfur Carriers

9.7.2

277

INDUSTRIAL APPLICATIONS

Multifunctional and multipurpose lubricants are on the wish list of many end users. The development for sulfurized products that can be used in these types of lubricants is in full progress, and the products have already been commercialized. Mainly light-colored products based on mixed, well-balanced raw materials to ensure a broad performance range are used for multipurpose applications. Replacement of heavy metals and chlorinated paraffins in almost all industrial lubricants is also an ongoing project that is widely found in the lubricants industry. Sulfur carriers are playing a predominant role as substitutes for these products. Increasing demands for environmentally more acceptable lubricants has led many formulators into the development of lubricants based on natural triglycerides such as canola oil, soybean oil, tall oil, or esters. Biodegradable sulfur carriers are used as EP and AW additives as well as secondary AOs in these applications.

9.8

TRENDS

9.8.1

CURRENT EQUIPMENT/SPECIFICATION

Sulfurized products are single components and not complete performance packages such as hydraulic or crankcase packages. Therefore, sulfur carriers are used in the whole variety of lubricants rather than in a specific equipment. Also, no national or international specification standards exist for these products. The manufacturer sets the specification in agreement with the user. 9.8.1.1

Types of Equipment

As already mentioned, the biggest use of sulfurized products (excluding SIB) is in industrial applications. Metalworking and grease applications followed by industrial gear oils are formulated with sulfur carriers. A lot of old equipment is still in use. Many mid-size and small companies have not modernized their metalworking machines for more than three decades. This older, robust equipment is very often running at low machining speeds and nonoptimized machining parameters. Modern machining equipment requires thermally stable fluids, based on highly refined or synthetic base fluids. Improved solubility in nonpolar oils and thermal stability of sulfurized products gain importance. 9.8.1.2

Additives in Use

Today’s additive usage depends very much on regional technological requirements and local legislation. In countries with low, old or standard technology and little environmental concerns, additives such as chlorinated paraffins or heavy metals are used for the formulation of lubricants, often in combination with sulfurized products. In countries where legislation has put some pressure onto the formulators and users of lubricants (higher disposal costs for chlorine-containing lubricants, limits on heavy metals in waste water, etc.), sulfurized compounds play an even more important role. They are the main EP additives, very often combined with sulfonates, salicilates, phosphoric acid esters, dialkyldithiophosphates, or carboxylic esters to complement AW and lubricity performance. 9.8.1.3

Deﬁciencies in Current Additives

All available sulfur carriers are limited in their thermal stability. This is a desired feature, because reactive sulfur will only be released when the molecule breaks down. However, there are applications running at a high temperature, where a fast decomposition of the EP product is not desired. Corrosion toward yellow metals is another deficiency of sulfurized compounds. In high-temperature applications, the active sulfur will react with copper to form copper sulfide.

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Lubricant Additives: Chemistry and Applications

9.9 MEDIUM-TERM TRENDS In general, there is a trend toward higher economy and ecologically and toxicologically safe lubricants.

9.9.1 METALWORKING Increased lubricant temperatures are a consequence of higher machine speeds, completely encapsulated machines, and reduced process steps. In the future, the thermal stability of metalworking lubricants and their toxicological safety will be on the focus. Owing to integrated applications (e.g., one lubricant for process and machine lubrication), the additives need to cover wide temperature ranges. The trend to replace multiple cutting steps with forming operations exists. Therefore, the type of additives will also change. Minimum amount lubrication requires new lubricant concepts in regard of performance and marketing. Maintenance of lubricants will further be reduced. Again this trend calls for increased stability of additives. Sulfurized products for metalworking application will need improved thermal stability in combination with good solubility in high paraffinic or even synthetic base fluids. Ecological and toxicological safety will be the basic requirements. Improved lubricity and excellent compatibility with process materials such as cleaners and paints will be essential for the formulation of modern lubricants for deformation processes (e.g., cold forging and deep drawing).

9.9.2 INDUSTRIAL OILS Synthetic fluids such as PAOs, polyalkyleneglycols (PAGs), extra high VI mineral oils (XHVI), or synthetic esters are being used in increasing volumes for the formulation of high-performance industrial lubricants. Smaller lubricant sumps, reduced sizes of components, and increased performance will place high demands on the lubricants. Especially in mobile equipment (e.g., excavator and lawn mower), ecologically and toxicologically harmless lubricants will become a demand. Reduced maintenance and longer lubricant change intervals require high lubricant stability. Improved thermal stability, low copper corrosion, and excellent solubility in synthetic fluids are demands on sulfurized products for the new generation of industrial lubricants.

REFERENCES 1. Sellei, H., Sulfurized extreme-pressure lubricants and cutting oils, Part 1, Petroleum Processing, 4, 1003–1008, 1949. 2. Sellei, H., Sulfurized extreme-pressure lubricants and cutting oils, Part 2, Petroleum Processing, 4, 1116–1120, 1949. 3. Base for metal-cutting compounds and process of preparing the same, George W. Pressell, Houghton & Co. PA, US 1,367,428/GB 129132 (1921). 4. Byers, J.T., Patents show trend in extreme pressure lube technology, National Petroleum News, 28, 79, 1936. 5. van Voorhis, M.G., 200 lubricant additive patents issued in 1938 and 1939, National Petroleum News, 32, R-66, 1940. 6. Miller, F.L., W.C. Winning, and J.F. Kunc, Use of additives in automotive lubrication, Refiner and Natl. Gas Manuf. 20(2), 53, 1941. 7. Westlake, H.E., Jr., The sulfurization of unsaturated compounds, Chemical Reviews 39(2), 219, 1946. 8. Musgrave, F.F., The development and lubrication of the automotive hypoid gear, Journal of the Institution of Petroleum 32(265), 32, 1946. 9. Lubricating compound and process of making the same, Leonard A. Churchill, US 1,974,299 (1934). 10. Method of sulphurizing pine oil and data thereof. M.C. Edwards, J. Heights, J.V. Congdon US 2,012,446 (1935).

Sulfur Carriers

279

11. Method of sulphurizing terpenes, abiethyl compounds, etc. J.W. Borglin, Hercules Powder Company, Del. US 2,111,882 (1938). 12. Pure compounds as extreme pressure lubricants. E.W. Adams, G.M. McNulty, Standard Oil Company, IL, US 2,110,281 (1938). 13. Production of mercaptanes, K. Baur, IG Farbenindustry AG, US 2,116,182 (1938). 14. Sulphurized Oils, B.H. Lincoln, W.L. Steiner, Continental Oil, OK, US 2,113,810 (1938). 15. Lubricating Oil, E.A. Evans, C.C. Wakefield & Co Ltd., US 2,164,393 (1939). 16. Extreme Pressure Lubricating Composition, B.B. Farrington, R.L. Humphreys, Standard Oil Company, CA, US 2,020,021 (1935). 17. Lubricant composition, E.W. Adams, G.M. McNulty, Standard Oil Company, IL, US 2,206,245 (1940). 18. Lubricating Oil, J.F. Werder, US 1,971,243 (1934). 19. Lubricant containing organic sulphides, L.A. Mikeska, F.L. Miller, Standard Oil Development Company, US 2,205,858 (1940). 20. Compound lubricating oil, C. Winning, Westfield, D.T. Rogers, Standard Oil Development Company, US 2,422,275 (1947). 21. Mougay, H.C., and G.O. Almen, Extreme pressure lubricants, Journal of the Institution of Petroleum, 12, 76, 1931. 22. Baxter, J.P. et. al., Extreme pressure lubricant tests with pretreated test species, Journal of the Institution of Petroleum, 25(194), 761, 1939. 23. Schallbock et al., Vorträge der Hauptversammlung der deutschen Gesellschaft für Metallkunde, VDI Verlag, pp. 34–38, 1938. 24. Prutton, C.F. et.al., Mechanism of action of organic chlorine and sulfur compounds in E.P. lubrication, Journal of the Institution of Petroleum, 32(266), 90. 25. Lubricant, Herschel, G. Smith, Wallingford, Gulf Oil Corporation, PA, US 2,179,067 (1939). 26. Lubricant, E.W. Cook, W.D. Thomas, American Cyanamid Company, NY, US 2,311,931 (1943). 27. Forbes, E.S., Load carrying capacity of organo-sulfur copmpounds—a review, Wear, Lausanne, 15, 341, 1970. 28. Process for producing sulfurized olefins, A.G. Horodysky, Mobil Oil Corporation, US 3,703,504 (1972). 29. A sulphurised mixture of compounds and a process for ist production, Ingo Kreutzer, Rhein Chemie Rheinau GmbH, GB 1371949 (1972). 30. Cross-sulfurized olefins and fatty monoesters in lubricating oils, B.W. Hotten, Chevron Research Company, CA, US 4,053,427 (1977). 31. Metal working using lubricants containing basic alkali metal salts, J.N. Vinci, The Lubrizol Corporation, OH, US 4,505,830 (1985). 32. Prof. Hugo, Rhein Chemie Rheinau GmbH internal studies, Freie Universität Berlin, 1980. 33. Korff, J., Additive fuer Kuehlschmierstoffe, Additive fuer Schmierstoffe, Expert Verlag, Renningon Germany, 1994. 34. ASTM D 1662, Standard Test Method for Active Sulfur in Cutting Oils. 35. Fessenbecker, A., Th. Rossrucker, and E. Broser. Rhein Chemie Rheinau GmbH—performance and ecology, two inseparable aspects of additives for modern metalworking fluids, TAE, International Colloquium, 1992. 36. Allum, K.G. and J.F. Ford. The influence of chemical structure on the load carrying properties of certain organo-sulfur compounds, Journal of the Institute of Petroleum, 51(497), 53–59, 1965. 37. ASTM D-130, Detection of Copper Corrosion from Petroleum Products by the Copper Strip Tarnish Test (2000). 38. Roehrs, I. and T. Rossrucker, Performance and ecology—two aspects for modern greases. NLGI Annual Meeting, 1994. 39. Rossrucker, T. and A. Fessenbecker, Performance and mechanism of metalworking additives: new results from practical focused studies, Rhein Chemie Rheinau GmbH, STLE Annual Meeting, 1999. 40. Deodhar, J., Steel rolling oils—cold rolling lubrication, the metalworking fluid business, The College of Petroleum Studies, December, 1989. 41. Vinci, J.N., Grease and gear lubricant compositions comprising at least one metal containig composition and at least one sulfurized organic compound, The Lubrizol Corporation, OH, International Publication No. WO 87/06256, 1987. 42. G. Fish. Greases, GKN Technology Limited, WO 94/11470, EP 0668 900 B1, 1994. 43. Vinci, J.N. Metal working using lubricants containing basic alkali metal salts. The Lubrizol Corporation, OH, US 4,505,830 (1985).

Part 4 Viscosity Control Additives

Oleﬁn Copolymer Viscosity Modiﬁers Michael J. Covitch

CONTENTS 10.1 Introduction ......................................................................................................................... 283 10.2 Classes of Olefin Copolymers .............................................................................................284 10.3 Chemistry ............................................................................................................................284 10.3.1 Synthesis by Ziegler–Natta Polymerization ...........................................................284 10.3.2 Synthesis by Metallocene Polymerization ............................................................. 286 10.3.3 Functionalization Chemistry ................................................................................. 286 10.4 Manufacturing Processes .................................................................................................... 287 10.4.1 Solution Process ..................................................................................................... 288 10.4.2 Suspension Process ................................................................................................ 289 10.4.3 Postpolymerization Processes................................................................................ 289 10.4.4 Making the OCP Liquid Concentrate ....................................................................290 10.5 Properties and Performance Characteristics.......................................................................290 10.5.1 Effect of Ethylene/Propylene Ratio on Physical Properties of the Solid ...............290 10.5.2 Effect of Copolymer Composition on Rheological Properties in Solution ........... 292 10.5.2.1 Low-Temperature Rheology .................................................................. 292 10.5.2.2 High-Temperature Rheology ................................................................. 295 10.5.3 Effect of Diene on Thermal/Oxidative Stability ................................................... 299 10.5.4 Comparative Rheological Performance in Engine Oils ........................................300 10.5.4.1 Comparative Study of OCP Viscosity Modifiers in a Fixed SAE 5W-30 Engine Oil Formulation .....................................................300 10.5.4.2 Comparative Study of 37 SSI OCP Viscosity Modifiers in an SAE 15W-40 Engine Oil Formulation .................................................. 301 10.5.5 Interaction with Pour Point Depressants ...............................................................302 10.5.6 Field Performance Data ......................................................................................... 305 10.6 Manufacturers, Marketers, and Other Issues ......................................................................306 10.6.1 EP/EPDM Manufacturers ......................................................................................306 10.6.2 Olefin Copolymer VM Marketers ..........................................................................307 10.6.3 Read Across Guidelines .........................................................................................308 10.6.4 Safety and Health................................................................................................... 310 References ...................................................................................................................................... 310

10.1 INTRODUCTION Olefin copolymer (OCP) viscosity modifiers are oil-soluble copolymers comprising ethylene and propylene and may contain a third monomer, a nonconjugated diene, as well. By virtue of their high thickening efficiency and relatively low cost, they enjoy a dominant share of the engine oil viscosity modifier market [1]. First introduced as a lubricant additive by Exxon in the late 1960s, the chemical 283

284

Lubricant Additives: Chemistry and Applications

and physical properties of OCPs continue to evolve to achieve improvements in low-temperature rheology, thickening efficiency, and bulk handling characteristics. Several excellent reviews of OCP viscosity modifiers have been published [1–3]. This chapter serves as an update and current compilation of information relating to the chemistry, properties, and performance characteristics of this important class of lubricant additives.

10.2 CLASSES OF OLEFIN COPOLYMERS There are many ways to classify OCP viscosity modifiers. From a user’s perspective, OCPs are marketed as either solids or liquid concentrates. The physical state of the solids depends on several factors, primarily on the ethylene/propylene (E/P) mass ratio. When E/P is in the 45/55–55/45 range, the material is amorphous and cold flows at room temperature. Thus, OCPs of this composition are most commonly sold as bales, packaged in rigid boxes to maintain bale shape. When E/P is higher than 60/40, the copolymer becomes semicrystalline in nature and does not cold flow under ambient conditions. Thus, both bales and pellets can be produced. Liquid concentrates of OCP in mineral oil contain enough rubber to raise the kinematic viscosity (KV) to 500–1500 cSt (mPa s) range at 100°C. A typical viscosity/concentration relationship is shown in Figure 10.1. From the preceding discussion, OCPs can also be classified according to crystallinity, which is measured by x-ray diffraction or differential scanning calorimetry. The influence of crystallinity on rheological performance will be discussed in Section 10.5. Shear stability is another parameter by which OCP viscosity modifiers are categorized. The higher the molecular weight of a polymer, the more prone it is to mechanical degradation when elongational forces are imposed by the fluid flow field. This subject is dealt with in detail in Section 10.5.2.2.3. Finally, chemical functional groups can be grafted to the OCP backbone, providing added dispersancy, antioxidant activity, and low-temperature viscosity enhancement. A number of chemical routes for functionalizing OCPs are described in Section 10.3.3.

10.3 CHEMISTRY 10.3.1 SYNTHESIS BY ZIEGLER–NATTA POLYMERIZATION Although methods for synthesizing high-molecular-weight polymers of ethylene were commercialized in the 1930s (the Imperial Chemical Industries (ICI) PLC, currently a division of AkzoNobel high-pressure process), the polymers contained a significant number of short- and long-chain

Viscosity (cSt) at 100°C

4000

3000

2000

1000

0 0

OCP concentration, mass %

FIGURE 10.1 Kinematic viscosity of 50 permanent shear stability index (PSSI) amorphous OCP dissolved in 100N mineral oil. (Minick, J., A. Moet, A. Hiltner, E. Baer and S.P. Chum, J. Appl. Poly. Sci., 58, 1371–1384, 1995. Reprinted with permission of John Wiley & Sons, Inc.)

Oleﬁn Copolymer Viscosity Modiﬁers

285

R Cl Cl - Mt Cl Cl At crystal surface

R Cl RÕ Mb

Cl - Mt Cl Cl

Base metal complex (soluble specie)

FIGURE 10.2 Active center in Zieger–Natta catalysts. Mt = transition metal (such as Ti); Mb = base metal (such as Al). (Adapted from Boor, J., Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York, 1979.)

branches that limited the ability to produce high-density polyethylene. The first commercially viable synthesis of linear polyethylene at low monomer pressure was pioneered by Ziegler in 1953, and the stereoregular polymerization of α-olefins was demonstrated by Natta the following year [4]. The secret to their success was the discovery of catalysts (called Ziegler or Zieger–Natta catalysts), which are molecular complexes between halides and other derivatives of group IV–VIII transition metals (Ti, V, Co, Zr, and Hf) and alkyls of group I–III base metals. A typical catalyst of this type comprises an aluminum alkyl and a titanium or vanadium halide having the general structure shown in Figure 10.2. Electron donors, such as organic amines, esters, phosphines, and ketones, may be used to enhance reaction kinetics. Finally, molecular weight control is often aided by the use of chain transfer agents such as molecular hydrogen or zinc alkyls [5], which are effective in terminating chain growth without poisoning the active metal center. Ziegler–Natta polymerization is probably the best-known example of insertion, coordination, stereoregular, or stereospecific polymerization. This nomenclature has been adopted to describe the mechanism(s) by which olefin monomers insert into the growing polymer chain, as directed by both steric and electronic features of the coordination catalyst. A commonly accepted chain propagation mechanism involves monomer insertion at the transition metal–carbon bond [4]. The main purpose of the base metal alkyl is to alkylate the transition metal salt, thus stabilizing it against decomposition. As pointed out by Boor [4], Ziegler–Natta catalysts may be modified to produce copolymers with varying degrees of randomness or, from a different perspective, blockiness of one or both comonomers. Owing to the higher reactivity ratio of ethylene to that of propylene [4–7], the formation of long runs of ethylene is more favored than long sequences of propylene. This is substantiated by 13C NMR spectroscopy [8–13]. Ziegler–Natta catalysts are available in two forms—heterogeneous and hom*ogeneous. Heterogeneous catalysts are insoluble in the reaction medium and are suspended in a fluidized-bed configuration. Reaction takes place at the exposed faces of the metal complex surface. Since each crystal plane has a slightly different atomic arrangement, each will produce slightly different polymer chains in terms of statistical monomer insertion and molecular weight distribution. Thus, they are often called multisite catalysts. hom*ogeneous Ziegler–Natta catalysts are soluble in the reaction solvent and, therefore, function more efficiently since all molecules serve as potential reaction sites. Since the catalyst is not restrained in a crystalline matrix, it tends to be more “single-site” in nature than heterogeneous catalysts. Polymers made by hom*ogeneous polymerization generally are more uniform in microstructure and molecular weight distribution and, therefore, are favored for use as viscosity modifiers [1,4]. Nonconjugated dienes are often used in the manufacture of ethylene–propylene copolymers (known as EPDMs-Ethylene Propylene Diene Monomer) to provide a site for cross-linking (in nonlubricant applications) or to reduce the tackiness of the rubber for ease of manufacture and handling. Certain dienes promote long-chain branching [2,5,14,15] that, in turn, increases the modulus in the rubber plateau region. The terpolymer is then easier to handle as it is dried and packaged [1]. A disadvantage of long-chain branching is that it reduces the lubricating oil thickening efficiency relative to a simple copolymer of similar molecular weight and copolymer composition, although low levels of vinyl norbornene or norbornadiene are claimed [15] to improve cold flow without loss in thickening efficiency or shear stability.

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FIGURE 10.3 Chemical structure of generalized bridged bis-cyclopentadienyl metallocene catalyst. M is a group IVB transition metal; X is a halogen or alkyl radical. (Rubin, I.D. and A. Sen, J. Appl. Poly. Sci., 40, 523–530, 1990. Reprinted with permission of John Wiley & Sons, Inc.)

10.3.2

SYNTHESIS BY METALLOCENE POLYMERIZATION

The desire to achieve higher levels of control over stereoregularity, composition, and molecular weight distribution led to the development of activated metallocene catalysts. Although known to Zieger and Natta, the technology was rediscovered by Kaminsky and Sinn in 1980 and further developed by workers such as Brintzinger, Chien, Jordan, and others [16–20]. Metallocene catalysts consist of compounds of transition metals (usually group IVB: Ti, Zr, and Hf) with one or two cyclopentadienyl rings attached to the metal. The most common activator is methylaluminoxane (MAO). A large number of variants have been reported, but the highest levels of stereospecificity have been achieved with bridged, substituted bis-cyclopentadienyl metallocenes (Figure 10.3) [21]. One of the major advantages of metallocenes over Ziegler–Natta catalysts is the ability to incorporate higher α-olefins and other monomers into the ethylene chain. The fi rst commercial use of metallocene single-site catalysts to manufacture EPDM elastomers was DuPont Dow Elastomers’ Plaquemine, LA facility, which began operation in 1996 using Dow’s Insite® constrained geometry catalyst [22,23]. The catalyst is described as “monocyclopentadienyl Group 4 complex with a covalently attached donor ligand … requiring activation by strong Lewis acid systems [such as] MAO…” Several advantages of this technology over conventional Ziegler–Natta processes were reported. Since the catalyst is highly efficient, less is needed; therefore, the process does not require a metal removal or de-ashing step. In addition, the copolymers produced by this chemistry are reported to have narrow molecular weight distributions for good thickening efficiency and shear stability as well as good control over copolymer microstructure. Metallocene-catalyzed polyolefins also differ from Ziegler– Natta polymers in that the former contains a predominance of unsaturated ethylidene end groups [24].

10.3.3 FUNCTIONALIZATION CHEMISTRY Traditionally, OCPs are added to lubricating oil to reduce the degree to which viscosity decreases with temperature, that is, to function solely as a rheology control agent. Other lubricant additives—such as ashless succinimide dispersants, a variety of antioxidants, detergents, antiwear agents, foam inhibitors, friction modifiers, and anticorrosion chemicals—provide other important functions (dispersing contaminants, keeping engines clean, maintaining piston ring performance, preventing wear, etc.). It has been recognized for many years that it is possible to combine some of these performance and rheology control features on the same molecule. Some report that “both dispersant and antioxidant functionality may exhibit more potent activity when attached to the polymer backbone than in their monomeric form” [25]. Three hybrids have been commercialized, although many more have been disclosed in the patent literature.

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They include dispersant OCPs (DOCPs), dispersant antioxidant OCPs (DAOCPs), and grafted OCPs (gOCPs). The addition of antiwear functionality has also been reported [25,26]. Although many grafting reactions have been described in the literature, two general classes have received the most attention. Free radical grafting of nitrogen-containing monomers or alkylmethacrylates onto the OCP molecule is one class. Nitrogen-containing monomers such as vinyl pyridines, vinylpyrrolidinones, and vinylimidazoles are often cited in the patent literature [28–33]. Free radical grafting with phenothiazine is claimed [28–33] to provide antioxidant functionality as well. The grafting reaction may be conducted with the OCP molecule dissolved in mineral oil or another suitable solvent [77,86,130,131]; alternately, solvent-free processes have been disclosed [28–33] in which the reaction is conducted in an extruder. Mixtures of alkylmethacrylate monomers, which are typical of those found in poly(alkyl methacrylate) (PMA) viscosity modifiers, may be grafted to OCPs [34] to provide improved lowtemperature properties. Adding nitrogen-containing monomers to the alkylmethacrylate mixture provides dispersancy characteristics as well. A common side reaction is hom*opolymerization, which can be minimized by process optimization. hom*opolymers of nitrogen-containing monomers are usually not very soluble in mineral oils and often lead to hazy products and can attack fluoroelastomer seals. hom*opolymers of alkylmethacrylates are fully soluble in oil, however. Thus, optimizing the grafting process is much more critical when working with nitrogen-containing monomers. A second class of grafting reactions involves two steps [25,35–47]. In the first step, maleic anhydride or a similar diacyl compound is grafted onto the OCP chain, assisted by free radical initiators, oxygen, and heat [54,77,86,130,131]. In the second step, amines and alcohols are contacted with the anhydride intermediate to create imide, amide, or ester bonds. In many respects, this chemistry is very similar to that used to create ashless succinimide dispersants. An advantage of this approach over free radical monomer grafting is that hom*opolymerization is avoided. The patent literature describes a related functionalization process in which free primary or secondary nitrogens of highly basic succinimide dispersants may be used to couple preformed dispersants to the maleic anhydride–grafted OCP molecule [49–52]. Amine derivatives of thiadiazole, phenolic [25], and amino-aromatic polyamine [53,54] compounds have been reacted with maleic anhydride–grafted OCP to provide enhanced antioxidant character to the additive [25]. Although maleic anhydride is the most common chemical “hook” for attaching functional groups to OCP polymers, a number of other approaches have been reported [55–61]. Further elaboration is beyond the scope of this chapter. Another approach for attaching functionality to the OCP chain is through the nonconjugated diene in the terpolymer [26,62]. For example, 2-mercapto-1,3,4-thiadiazole is attached to the ethylidenenorbornene site on an EPDM polymer through addition of the thio group across the ethylidene double bond. The thiadiazole group is claimed to provide antiwear, antifatigue characteristics to lubricants containing the grafted OCP.

10.4

MANUFACTURING PROCESSES

Two polymerization processes have been used for the manufacture of E/P copolymer viscosity modifiers: solution and slurry. In the solution process, the gaseous monomers are added under pressure to an organic solvent such as hexane, and the polymer stays in solution as it forms. By contrast, the slurry or suspension process utilizes a solvent such as liquid propylene in which the resultant copolymer is not soluble. It is reported [63] that removing the catalyst residue from the polymer is more difficult in the slurry process, although some contend [27] that the levels of catalyst are so low that catalyst removal is not necessary. Ethylene–propylene rubber was reported [64] to have been successfully manufactured in a fluidized-bed gas-phase reactor. However, the use of fluidization aids such as carbon black is necessary to process low-molecular-weight grades that are typical of lubricating oil viscosity modifiers. Thus, the gas-phase process is not appropriate for manufacturing OCP viscosity modifiers.

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SOLUTION PROCESS

The most common method for manufacturing OCP viscosity modifiers is the solution process as described in Figure 10.4 [65]. It is made up of four sections—polymerization, polymer isolation, distillation, and packaging. In the polymerization section, monomers, an organic solvent such as hexane, and a soluble catalyst are introduced into a continuously stirred polymerization reactor. During polymerization, the polymer remains in solution and causes the bulk viscosity of the reaction medium to increase. To maintain good agitation, monomer diffusion, and thermal control, polymer concentration in the polymerization reactor is typically limited to 5–6 wt% [27]. Up to five reactors arranged in series have been reported in the literature [66]. The effluent from the last reactor is contacted with an aqueous shortstop solution to terminate polymerization and wash away the catalyst, although this step is often omitted when using metallocene catalysts due to their high reactivity and, therefore, low concentration [22]. While the copolymer is still in solution, extender oils or antioxidants can be added. In the isolation section, three techniques have been described in the literature. In the most common method (shown in Figure 10.4), the polymer is flocculated with steam, and the solvent and unreacted monomers are recovered, purified, and recycled. The aqueous polymer slurry is mechanically dewatered, granulated, and air-dried. A second nonaqueous method for isolating the polymer has been described in which the polymer is concentrated in a series of solvent removal steps [67,68]. The final step may be conducted in a devolatilizing extruder. A third technique does not isolate the polymer as a solid; rather, it mixes the polymer solution into mineral oil and distills off the solvent, producing a finished liquid OCP product [2]. Another type of solution polymerization process that has received a great deal of attention has been Exxon’s tubular reactor technology. Its purpose is to generate a polymer with long blocks differing in monomer composition for improved performance as a viscosity-improving polymer [3,69–71]. A schematic of this process is shown in Figure 10.5. Monomers and solvent are premixed

Polymerization

Dewatering Water

Termonomer AO Catalyst

Shortstop

Extender oil

Ethylene Propylene

Waste water

Solvent

Expeller Off gas Steam Expander Solvent Fresh solvent High-boils Distillation

Baling wrapping

FIGURE 10.4. Solution process for manufacturing EPDM. (Adapted from Hydrocarbon Processing, 164, November 1981.)

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Solvent Catalyst

Cocatalyst

Solvent/monomers

To collection

Tubular reactor Solvent/ monomers Temperaturecontrolled catalyst Mixing premixing zone device

Solvent/ monomers

Polymerization zone

FIGURE 10.5 Tubular reactor process for preparation of multiblock ethylene–propylene copolymers. (U.S. Patent 4874820, 1989; U.S. Patent 4804794, 1989; U.S. Patent 5798420, 1998.)

with a highly active Ziegler–Natta polymerization catalyst and metered into a plug flow reactor under conditions that minimize chain transfer and termination reactions. Ethylene or propylene is injected into the tube at different points to adjust the local monomer concentration and, thereby, the monomer composition along the growing polymer chain. In comparing the rheological properties of different A-B-A type block compositions, Ver Strate and Struglinski reported [12] that “chains with high ethylene section in the center of the chain … associate at low temperature with little intermolecular connectivity.” When the high ethylene (crystallizable) segments are at the ends, polymer networks can form at low temperatures, which can impart a gelatinous texture to the solution.

10.4.2

SUSPENSION PROCESS

Ethylene and a nonconjugated diene, if desired, are contacted with liquid propylene, which acts both as a monomer and a reaction medium [27,72]. In the presence of a suitable catalyst, polymerization takes place rapidly, producing a suspension of copolymer granules that are insoluble in the reaction medium. The heat liberated during the polymerization reaction is dissipated by propylene evaporation, thus providing a convenient mechanism for temperature control. In addition, since the polymer is not soluble in the reaction medium, viscosity remains low. Thus, relative to typical solution processes, the polymer concentration in a suspension reactor can be five to six times higher. Upon exiting the polymerization reactor, the polymer suspension is contacted with steam to strip off unreacted propylene that is then recycled. According to Corbelli and Milani [72], the Dutral® process does not include a catalyst washing step. The copolymer product, in aqueous suspension, is dewatered, dried, and packaged in a similar fashion to polymer made by the solution process.

10.4.3

POSTPOLYMERIZATION PROCESSES

There are two main types of packaging processes in practice today [64]. In one, the isolated polymer is mechanically compressed into rectangular bales. The bales are often wrapped in a polyolefin packaging film to prevent the bales from adhering to one another during storage and foreign matter from sticking to the tacky rubber surface. Typical types of polyolefins films include poly(ethyleneco-vinyl acetate), low-density polyethylene, and ethylene/α-Olefin Copolymers (OCP). Another method for packaging solid OCP rubber is to extrude the polymer, pass the melt stream into a water-cooled pelletizer, and dry the final product. The pellets may be packaged in bags or boxes or may be compressed into rectangular bales.

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The mechanical properties of the rubber often dictate what type of isolation and packaging processes are the most appropriate. Amorphous Ethylene propylene (EP) copolymers are often too sticky to successfully traverse the conventional flocculation/drying/baling process. One way to modify these compositions to improve their handling characteristics is by introducing long-chain branching [5,48,73] through the use of low concentrations of nonconjugated dienes or other branching agents. For nonfunctionalized OCPs, this is the main reason that some commercial viscosity modifiers contain dienes [2]. Copolymer compositions higher in ethylene content (>60 wt% [5]) are often semicrystalline and may be amenable to packaging in pellet form. In some cases, the pellets may contain an anticaking or antiblocking agent to prevent agglomeration. Another type of manufacturing process has been used to manufacture low-molecular-weight OCP viscosity modifiers that are difficult to isolate and package in conventional equipment. A higher-molecular-weight feedstock of the appropriate composition may be fed into a masticating extruder or Banbury mixer to break down the polymer chain to lower-molecular-weight fragments using a combination of heat and mechanical energy [74–81]. Several patents describe the use of oxygen [82–85] or free radical initiators [86,87] to enable this process.

10.4.4 MAKING THE OCP LIQUID CONCENTRATE After the solid OCP viscosity modifier has been manufactured, it must be dissolved in oil before it can be efficiently blended with base stocks and other additives. The first stage entails feeding the rubber bale into a mechanical grinder [2] and then conveying the polymer crumb into a high-quality diluent oil that is heated to 100–130°C with good agitation. The rubber slowly dissolves, raising the viscosity of the oil as shown in Figure 10.1. Certain high-intensity hom*ogenizers can also be used in which the entire rubber bale is fed directly into a highly turbulent diluent oil tank at high temperature; this bypasses the pregrinding step. When the solid polymer is supplied in pellet form, the rubber can be fed directly into hot oil, or, if it is slightly agglomerated, it may first be passed through a low-energy mechanical grinder.

10.5 PROPERTIES AND PERFORMANCE CHARACTERISTICS 10.5.1

EFFECT OF ETHYLENE/PROPYLENE RATIO ON PHYSICAL PROPERTIES OF THE SOLID

The comonomer composition of E/P copolymers has a profound influence on the physical properties of the rubber. These properties, in turn, dictate the type of containers in which the product can be stored and how it is handled during distribution and use. 13 C NMR has been used extensively to characterize the sequence distribution of EP copolymers [88–93]. As the ratio of E/P increases, the fraction of ethylene–ethylene (EE) sequences (dyads) rises, as demonstrated by the data in Figure 10.6. Concurrently, the total fraction of E/P dyads decreases (forward and reverse propylene insertion are designated as p and p*, respectively). Thus, the average length of contiguous ethylene increases with ethylene content. Above ∼60 wt% ethylene, these sequences become long enough to crystallize, as measured by differential scanning calorimetry (Figure 10.7) or x-ray diffraction. When the degree of crystallinity exceeds ∼25%, EP copolymers become unsuitable as viscosity modifiers due to limited solubility in most mineral oils. As the propylene content approaches zero, the copolymer takes on the physical characteristics of high-density polyethylene, which, due to its inertness to oil, is used as the packaging material of choice for engine oils and other automotive fluids. Microstructural investigations of metallocene ethylene/α-OCPs by Minick et al. [94] concluded that the relatively short ethylene sequences of low-crystallinity (<25%) samples are capable of crystallizing into fringed micelle or short-bundled structures (Figure 10.8). Higher-order morphologies such as lamellae or spherulites are not observed. Therefore, the physical properties of semicrystalline OCPs fall in between those of polyethylene and amorphous EP rubber.

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0.8 fEp+fEp*

Dyad mole fraction

0.7

fEE

0.6

0.5

0.4

0.3

0.2 45

Ethylene content (wt%)

FIGURE 10.6 The effect of ethylene content (wt% as measured by NMR) on ethylene–ethylene (EE) and ethylene–propylene (EP) dyad concentration.

DSC heat of fusion (J/g)

0 50

%Ethylene

FIGURE 10.7 The effect of ethylene content (wt%) on crystallinity as measured by differential scanning calorimetry (DSC) for a range of experimental EPDM copolymers.

Polyethylene is a rigid, high-modulus solid at room temperature. Amorphous E/P rubber is a relatively soft material under ambient conditions, which, cold, flows and exhibits a tacky feel. The degree of tack is inversely proportional to its molecular weight and can be reduced by the incorporation of long-chain branching. Solid bales of this type of rubber are easily compressed and further densify during storage. Semicrystalline OCPs hold their shape during storage but are slightly tacky to the touch. Higher compression pressures, longer compression times, and higher finishing temperatures are required to successfully produce dense bales. Typical physical properties of E/P copolymers are summarized in Table 10.1.

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# SCB/1000 C Crystallinity (%) Density (g/cc)

0.86

0.87

0.88

0.89

Type I

0.90

0.91

Type II

0.92

0.96

Type III

Type IV

Fringed micelles No lamellae

Type II Fringed micelles Lamellae

Type III No fringed micelles Lamellae

Type IV No fringed micelles Lamellae

No spherulites

Spherulites

FIGURE 10.8 Schematic illustration of solid-state morphologies of four types of poly(ethylene-co-octene) copolymers. # SCB/1000 C is defined as the number of side chain branches per 1000 backbone carbon atoms. (Minick, J., Moet, A., Hiltner, A., Baer, E., and Chum, S.P., J. Appl. Poly. Sci., 58, 1371–1384, 1995. Reprinted with permission of John Wiley & Sons, Inc.)

TABLE 10.1 Typical Physical Properties of Ethylene–Propylene Copolymers Property

Typical Value

Density (kg/m3) Heat capacity (cal/g °C) Thermal conductivity (cal/cm s °C) Thermal diffusivity, (cm2/s) Thermal coefficient of linear expansion (per°C)

860 0.52 8.5 × 10−4 9.2 × 10−4 2.2 × 10−4

Source: Adapted from Corbelli, L., Dev. Rubber Tech., 2, 87–129, 1981.

10.5.2 10.5.2.1

EFFECT OF COPOLYMER COMPOSITION ON RHEOLOGICAL PROPERTIES IN SOLUTION Low-Temperature Rheology

Rubin et al. [95–101] and others [102] measured the intrinsic viscosity [η] of EP copolymers as a function of temperature in various solvents (Figure 10.9). Intrinsic viscosity, a measure of polymer coil size in dilute solution, is fairly insensitive to temperature for noncrystalline OCPs. Semicrystalline copolymers undergo a precipitous drop in [η] as temperature drops below ∼10°C. In this region, the polymer begins to crystallize, forming intermolecular associations that effectively cause the molecule to shrink in on itself, yet remain sufficiently solvated to remain suspended in oil solution. The viscosity of a dilute polymer solution often follows the Huggins equation [103] sp c ⫽ [] ⫹ k ′ [] c 2

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2.50 2.25 2.00

Intrinsic viscosity (dL/g)

EPA1 1.75

EPA2

1.50

EPB1 EPC3

1.25 1.00 0.75 0.50 0.25 0.00 −20

−10

Temperature (°C)

FIGURE 10.9 Intrinsic viscosities of EP copolymers in SNO-100 base oil at various temperatures. EPC3, EPB1, EPA2, and EPA1 have 73, 61, 50, and 50 wt% ethylene, respectively. Differences in intrinsic viscosity above 20°C are attributable to differences in molecular weight. (Rubin, I.D. and Sen, A., J. Appl. Poly. Sci., 40, 523–530, 1990. Reprinted with permission of John Wiley & Sons, Inc.)

where c = polymer concentration (g/dL) ηsp = specific viscosity (η – η 0)/η 0 η = solution viscosity ηo = solvent viscosity k′ = constant Thus, for a specific lubricant composition, c and ηo are fixed, and the temperature dependence of the solution viscosity is directly related to that of the intrinsic viscosity. Low-temperature viscosity is an important rheological feature of automotive lubricants. For the vehicle to start in cold weather, the lubricant viscosity in the bearings should be below a critical value as determined by low-temperature engine startability experiments [7] and defined within SAE J300 [104] for all “W” grades. The cold cranking simulator (CCS) test, ASTM D5293, is a high-shear rate rheometer operating at fixed subambient temperatures, designed to simulate oil flow in automotive bearings during start-up. After the engine starts, the lubricant must also be able to freely flow into the oil pump and throughout the internal oil distribution channels of the engine. This is the other half of the low-temperature viscosity specification for motor oils [104]. The mini-rotary viscometer (MRV), ASTM D4684, is a low-shear rate rheometer designed to simulate pumpability characteristics of a multigrade oil in a vehicle that was sitting idle for about 2 days in cold weather. SAE J300 also contains upper viscosity limits for MRV viscosity and yield stress for all W grades. Thus, the mechanism of intermolecular crystallization, which leads to molecular size contraction in solution, affords high ethylene OCP viscosity modifiers the opportunity to contribute less to

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Safety Kleen

Mobil Paulsboro

Sun HPO LTOCP1

Chevron RLOP

LTOCP3 Exxon

OCP1

Sun Tulsa 2000

2500 3000 3500 CCS viscosity (mPa s) at −25°C

4000

FIGURE 10.10 Cold cranking simulator (CCS) viscosity for six SAE 5W-30 lubricant formulations, each blended with one of three viscosity modifiers: LTOCP1, LTOCP3, or OCP1. Within each base oil type, the ratio of high and low-viscosity base oils was kept constant.

Safety Kleen Mobil Paulsboro Sun HPO

Yield stress failures

Chevron RLOP

LTOCP1 LTOCP3

Exxon

OCP1 Sun Tulsa 0

10,000

20,000

30,000

40,000

50,000

60,000

MRV TP-1 (mPa s) at −35°C

FIGURE 10.11 Mini-rotary viscosity results for six SAE 5W-30 lubricant formulations, each blended with one of three viscosity modifiers: LTOCP1, LTOCP3, or OCP1. Within each base oil type, the ratio of high and low-viscosity base oils and the type and concentration of pour point depressant were held constant.

viscosity at low temperatures than noncrystalline or amorphous copolymers of similar molecular weight. For this reason, the class of EP copolymers having ethylene content greater than ∼ 60 wt% are often called low-temperature OCPs or LTOCPs. A rheological comparison of two LTOCPs and one conventional OCP viscosity modifier in several SAE 5W-30 oil formulations is seen in Figures 10.10 and 10.11. These data illustrate the low-temperature rheological benefits of LTOCPs. A number of workers have cautioned, however, that the long ethylene sequences of LTOCPs are similar in structure to paraffin wax and can interact with waxy base oil components at low temperatures. In many cases, they require specially designed pour point depressants (PPDs) to function properly in certain base stocks. Thus, LTOCP viscosity modifiers have been implicated in problems such as MRV failures in comingled fresh oils [105] and used passenger car lubricants [106,107].

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10.5.2.2 High-Temperature Rheology Copolymer composition has less influence on high-temperature rheological behavior than it has at low temperatures, partly because lightly crystalline OCPs have melting points well below 100°C [97]. Since both high-temperature KV and high-temperature high-shear rate viscosity (HTHS) are used to classify multigrade engine oils [104], it is important to understand how copolymer composition and molecular weight influence these key parameters. 10.5.2.2.1 Kinematic Viscosity For both economic and performance reasons, it is desirable to limit the amount of polymer needed to achieve a given set of rheological targets. Therefore, it is important to quantify the effects of molecular weight, molecular weight distribution, and branching on thickening efficiency. Thickening efficiency has been defined in many ways, but the most common definitions are (1) the amount of polymer necessary to increase the KV of a reference oil to a certain value or (2) the KV or specific viscosity (see Section 10.5.2.1) of a given polymer concentration in a reference oil. For polymers of equal molecular weight, thickening efficiency increases with ethylene content and is highest for copolymers with narrow molecular weight distributions [1,2]. In Figure 10.12, a plot of intrinsic viscosity versus weight average molecular weight (M) demonstrates the familiar Mark–Houwink power law relationship:

[] ⫽ K ′ Ma where K′ and a are constants. Assuming a single value for the power law constant a = 0.74, Crespi et al. [5] published a table of K′ values as a function of ethylene content, which is reproduced in Table 10.2. This clearly shows that thickening efficiency can be improved by increasing ethylene concentration. This is further illustrated by plotting data from Kapuscinski et al. [98] in Figure 10.13. The 80 mol% ethylene copolymer requires less polymer to achieve a target viscosity than a 60 mol% copolymer; therefore the former has a higher thickening efficiency than the latter.

Intrinsic viscosity (dL /g) Decalin 135°C

1.0

0.1

0.01 103

104 105 Weight average molecular weight

106

FIGURE 10.12 Intrinsic viscosity as a function of weight average molecular weight for EP copolymers of narrow polydispersity (Mw / Mn ∼ 2). (Spiess, G.T., Johnston, J.E., and VerStrate, G., Addit. Schmierst. Arbeitsfluessigkeiten, Int. Kolloq., 5th, 2, 8.10-1, Tech. Akad. Esslingen, 1986. Reproduced with permission of ExxonMobil Chemical Company.)

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TABLE 10.2 Mark–Houwink K′ Constants for E/P Copolymers Containing Different Mole Percentages of Ethylene (a = 0.74) Mol% ethylene

K′ × 104

Mol% ethylene

K′ × 104

2.020 2.115 2.205 2.295 2.390 2.485 2.585 2.690 2.795 2.910

55 60 65 70 75 80 85 90 95 —

3.020 3.140 3.260 3.385 3.515 3.645 3.790 3.940 4.240 —

5 10 15 20 25 30 35 40 45 50

Source: Adapted from Crespi, G., Valvassori, A., and Flisi, U., Stereo Rubbers, W.M. Saltman, ed., Wiley-Interscience, New York, NY, 365–431, 1977.

Polymer Concentration (wt%)

1. 5

60 mol% ethylene

80 mol% ethylene

0. 5

0 0

100

200

300

400

500

Mw × 10−3

FIGURE 10.13 Polymer concentration needed to raise the kinematic viscosity of a 130N base oil to 11.5 cSt. (Data from Kapuscinski M.M., Sen, A., and Rubin, I.D., Soc. Automot. Eng. Tech. Paper Ser. No. 892152, 1989.)

Long-chain branching has a directionally detrimental effect on thickening efficiency for polymers of equal molecular weight. This is not surprising, since the average chain end-to-end distance of a random coil in solution is controlled, in large part, by the length of the main chain. Branching essentially shortens the chain and lowers its hydrodynamic radius. For example, Table 10.3 contains thickening efficiency data for two sets of noncrystalline OCP viscosity modifiers, one linear and the other containing 2% branching agent, each set differing only in molecular weight. 10.5.2.2.2 High-Temperature, High-Shear Rate Viscosity Since concentric journal bearings operate in the hydrodynamic or elastohydrodynamic lubrication regimes, oil film thickness is a critical factor influencing wear [108,109]. For this reason, SAE J300 specifies a minimum HTHS viscosity for each viscosity grade [104]. HTHS viscosity is measured at very high shear rates and temperatures (106 s–1 and 150°C, respectively), which is similar to the

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TABLE 10.3 Thickening Efﬁciency of Linear and Branched OCP Viscosity Modiﬁers Mw

Linear

Branched

230,000 180,000

13.50 11.17

12.03 10.87

Note: Thickening efficiency is defined as the kinematic viscosity (at 100°C) of a 1.0 wt% polymer solution in 6.05 cSt mineral oil.

TABLE 10.4 Rheological Comparison of Lubricants Containing OCP Viscosity Modiﬁers Differing in Molecular Weight Viscosity Modiﬁer OCP1 OCP2 OCP3

Weight Average Molecular Weight

PSSI

Capillary Viscosity (cP) at 150°C

HTHS (cP) at 150°C

% TVL

160,000 80,000 50,000

45 30 22

5.33 5.33 5.33

3.43 3.77 3.88

36 29 27

Source: Adapted from Spiess, G.T., Johnston, J.E., and VerStrate, G., Addit. Schmierst. Arbeitsfluessigkeiten, Int. Kolloq., 5th, 2, 8.10-1, Tech. Akad. Esslingen, 1986.

flow environment in an operating crankshaft bearing at steady state. At these rates of deformation, most high-molecular-weight polymers will align with the flow field [110], and a temporary reduction in viscosity is measured. The difference between low-shear rate viscosity and HTHS at 150°C is termed temporary viscosity loss (TVL) or percent temporary viscosity loss (relative to the low-shear rate KV). As is true for most polymers [110], TVL is proportional to molecular weight [1] as seen in Table 10.4. For polymers of equal weight average molecular weight, those with narrow molecular weight distributions undergo less TVL than those with broad Mw / Mn values [1]. HTHS viscosity can be adjusted by increasing the viscosity of the base oil or by increasing the viscosity modifier concentration, as shown in Figure 10.14. Since the formulation also has to meet KV and CCS viscosity limits, there is often only limited flexibility to adjust HTHS viscosity within the bounds of a given set of base oils and additives. 10.5.2.2.3 Permanent Shear Stability The tendency of an OCP molecule to undergo chain scission when subjected to mechanical forces is dictated by its molecular weight, molecular weight distribution, ethylene content, and degree of long-chain branching. Mechanical forces that break polymer chains into lower-molecular-weight fragments are elongational in nature, causing the molecule to stretch until it can no longer bear the load. This loss in polymer chain length leads to a permanent degradation of lubricant viscosity at all temperatures. In contrast to temporary shear loss, permanent viscosity loss (PVL) represents an irreversible degradation of the lubricant and must be taken into account when designing engine oil for commercial use. PVL is similar to TVL, except that the viscosity loss is measured by KV before and after shear. Permanent shear stability is more commonly defined by the permanent shear stability index (PSSI) or simply SSI, according to ASTM D6022 as follows: PSSI ⫽ SSI ⫽ 100 ⫻ ( V0 ⫺ VS ) ( V0 ⫺ Vb )

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Lubricant Additives: Chemistry and Applications 5

HTHS Viscosity (cP)

4.75

4.5

4.25 * 4

* * *

3.75

36.5

20.0

11.2 % 600N Oil

0.0

4.8

49.2

5.8

3.5

6.8 7.8 8.8

ier

dif

sit

is %V

o ym

FIGURE 10.14 The effects of base oil composition and viscosity modifier concentration on HTHS viscosity. SAE 15W-40 engine oil consisting of European API Group I base oils (150N + 600N), an ACEA A3-98/B3-98 quality performance additive, an oil diluted amorphous OCP viscosity modifier, and a pour point depressant. Bars marked with an asterisk comply with ASTM D445 and D5293 limits for SAE 15W-40 oils. 60

Shear stability index

50 40 30 20 10 0 5.0

5.1

5.2 5.3 5.4 Log weight average molecular weight

5.5

5.6

FIGURE 10.15 Relationship between weight average molecular weight and SSI (ASTM D3945) for a series of OCP viscosity modifiers.

where Vo = viscosity of unsheared oil Vs = viscosity of sheared oil Vb = viscosity of the base fluid (without polymer) SSI represents the fraction of viscosity contributed by the viscosity modifier that is lost during shear. SSI is proportional to log10 molecular weight (Mw), as shown in Figure 10.15. Commercial OCP viscosity modifiers have SSI values in the range of 23–55.

Absolute viscosity (cP)

Oleﬁn Copolymer Viscosity Modiﬁers

299

150°C (a)

(b)

Low (KV)

Shear rate

High (HTHS)

FIGURE 10.16 Viscosity loss trapezoid, per Selby: (a) Fresh oil viscosities and (b) oil viscosities after permanent shear. (Selby, T.W., Soc. Automot. Eng. Tech. Paper Ser. No. 932836, 1993.)

Although ASTM D6022 provides a definition for SSI, it is important to recognize that the only component that is responsible for viscosity loss during shear is the high-molecular-weight polymer. If the additive for which SSI is calculated happens to be a concentrated polymer solution in oil, according to the strict definition of ASTM D6022, the composition of the base fluid does not include the viscosity modifier (VM) diluent oil. Since the diluent oil viscosity is usually lower than the base blend viscosity for most viscosity grades, V b is higher than it would be if the VM diluent oil viscosity was factored into V b. For example, take an SAE 15W-40 engine oil formulated with a liquid OCP concentrate containing 10 wt% polymer in a 5.1 cSt mineral oil. V0 and Vs are 15.2 and 12.8 cSt, respectively. The base blend viscosity (when the VM component is a liquid) is 9.4 cSt. When the VM component is defined as the solid polymer, V b is 9.15 cSt. Calculated shear stability index values are 41.4 and 39.7, respectively. Thus, the numerical value of SSI is dependent on the definition of the polymeric additive in question. The concept of “stay-in-grade” is generally used to refer to a lubricating oil, when tested in vehicles or laboratory shearing devices, which maintains its KV within the limits of its original SAE viscosity grade. The problem with viscosity measurements of engine drain oils is that many factors other than permanent polymer shear influence viscosity—such as fuel dilution, oxidation, and soot accumulation. Therefore, it is customary to measure PVL after shear in a laboratory rig, the most common being the Kurt Orbahn test, ASTM D6278. Several reviews of methods for determining the shear stability of polymer-containing lubricating oils have been published [111–113]. Selby devised a pictorial scheme for mapping the effects of shear rate and PVL on hightemperature viscosity, the viscosity loss trapezoid (VLT) [114], shown in Figure 10.16. The corners of the trapezoid are defined by viscosity data, and the points are connected by straight lines. Note that the straight lines do not imply that there is a linear relationship between viscosity and shear rate. The VLT is a convenient graphical representation of the temporary and permanent shear loss characteristics of polymer-containing oils. Molecular weight degradation causes a permanent loss in both KV and HTHS, but the magnitude of the former is always larger than the latter. The shape of the VLT is the characteristic of polymer chemistry and molecular weight. It is experimentally observed [115] that the Kurt Orbahn shear test breaks molecules above a threshold molecular size; molecules smaller than the threshold value are resistant to degradation. Selby [114] uses this observation to derive certain qualitative conclusions of the polymer molecular weight distribution from the shape of the VLT.

10.5.3

EFFECT OF DIENE ON THERMAL/OXIDATIVE STABILITY

There has been little solid scientific data published in the literature to compare the relative thermal/ oxidative stability of oil solutions containing E/P copolymers versus EPDM terpolymers. Marsden [2]

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states that “introduction of a termonomer … can … detrimentally affect shear and oxidation stability, dependent on the monomer,” but he offers no data. Others [5,27] cite high-temperature aging experiments on solid rubber specimens, which demonstrate that EP copolymers are more stable (in terms of tensile properties) than EPDM terpolymers of similar E/P ratio. Copolymers containing higher levels of ethylene are claimed to have better thermal/oxidative stability than more propylene-rich copolymers, presumably due to the lower concentration of oxidatively labile tertiary protons contributed by the propylene monomer. High thermal stresses are sufficient to promote hydrogen abstraction by a free radical mechanism. The relative susceptibility of protons to hydrogen abstraction follows the classical order tertiary > secondary > primary. In the presence of oxygen, peroxy radicals are formed, which can accelerate the degradation process. Despite these suggestions that diene-containing E/P copolymers may be less thermally stabile than EP copolymers, the author is not aware of any definitive studies that have shown that EPDM viscosity modifiers are more likely to degrade in service than EP copolymers. Indeed, engine oils formulated with both types have been on the market for years.

10.5.4

COMPARATIVE RHEOLOGICAL PERFORMANCE IN ENGINE OILS

The most influential factors governing the rheological performance of OCPs in engine oils are molecular weight and monomer composition. The effects of molecular weight and molecular weight distribution were discussed in Section 10.5.2.3, and the influence of E/P ratio on lowtemperature rheology was covered in Section 10.5.2.1. In this section, two comparative rheological studies are presented to further illustrate the links between OCP structure and rheological performance. 10.5.4.1

Comparative Study of OCP Viscosity Modiﬁers in a Fixed SAE 5W-30 Engine Oil Formulation

There are two ways to compare the relative performance of several viscosity modifiers. One is to choose a fixed engine oil formulation where the base oil composition and additive concentrations are held constant, and the VM level is adjusted to achieve a certain 100°C KV target. The other is to adjust both base oil composition and VM concentration to achieve predetermined KV and CCS viscosity targets. Section 10.5.4.1 offers an example of the first approach and Section 10.5.4.2 illustrates the second approach. Four OCP viscosity modifiers were blended into an SAE 5W-30 engine oil composition consisting of a 95/5 w/w blend of Canadian 100N/250N mineral base stocks, an ILSAC GF-2 quality performance additive, and a polyalkylmethacrylate PPD. The viscosity modifiers are described in Table 10.5. OCP1 and OCP2 are high SSI polymers differing in both E/P ratio and diene content. OCP3 and OCP4 are progressively more shear stable and have essentially 0% crystallinity. Rheological data are summarized in Table 10.6. Comparing OCP1 and OCP2, the former is a more efficient thickener because it contains no long-chain branching (no diene monomer) and it has

TABLE 10.5 Properties of OCP Viscosity Modiﬁers Used in Table 10.6 Viscosity Modiﬁer Code OCP1 OCP2 OCP3 OCP4

Shear Stability Index (ASTM D6278)

Copolymer Type

55 50 37 25

EP, LTOCP EPDM, amorphous EP, amorphous EP, amorphous

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TABLE 10.6 Rheological Properties of SAE 5W-30 Engine Oils Containing Different OCP Viscosity Modiﬁers Polymer content (wt%) Kinematic viscosity (cSt) at 100°C Viscosity index CCS viscosity (cP) at −25°C MRV viscosity (cP) at −30°C MRV viscosity (cP) at −35°C HTHS (cP)

OCP1

OCP2

OCP3

OCP4

0.58 10.17 156 3080 13,900 40,100 2.95

0.71 10.09 160 3280 26,500 Yield stress 2.88

0.73 9.99 160 3510 26,700 Yield stress 2.96

1.05 10.10 158 3760 28,100 Yield stress 3.07

TABLE 10.7 OCP Viscosity Modiﬁers Used in Rheological Study in Table 10.8 Viscosity Modiﬁer Code OCP7 OCP3 OCP8 OCP9

Copolymer Type EPDM, amorphous EP, amorphous EPDM, LTOCP EP, LTOCP

a higher ethylene content (see Figure 10.13). The latter property also manifests itself in lower CCS viscosity. The MRV viscosity is also lower for OCP1 relative to the other amorphous OCPs, but this is highly dependent on the particular PPD that was chosen for this study. The fact that most of the oils displayed yield stress failures at –35°C shows that the PPD was not optimized for this particular set of components. As SSI decreases from OCP1 to OCP4, the polymer concentration needed to reach a KV target of 10 cSt increases. In other words, polymer thickening efficiency is proportional to shear stability index. Among the noncrystalline OCPs, increasing polymer level causes the CCS viscosity to increase. Since all oils were formulated with the same base oil composition, high-temperature HTHS is relatively constant, independent of OCP type. OCP4, the lowest molecular weight polymer, should have the lowest degree of TVL, and it indeed has the highest HTHS viscosity of the group. 10.5.4.2

Comparative Study of 37 SSI OCP Viscosity Modiﬁers in an SAE 15W-40 Engine Oil Formulation

In this example, the base oil ratio and polymer concentration were adjusted to achieve the following targets: KV = 15.0 cSt and CCS = 3000 cP at –15°C. The base stocks were American Petroleum Institute (API) Group I North American mineral oils, the additive package was of API CH-4 quality level, and the PPD was a styrene ester type that was optimized for these base oils. All viscosity modifiers (see Table 10.7) were nominally 37 SSI according to ASTM D6278. Rheological results are summarized in Table 10.8. OCP3 is the same polymer as in Table 10.5. Although OCP8 and OCP9 are semicrystalline LTOCPs, they represent different manufacturing technologies, broadly described in Figures 10.4 and 10.5, respectively. Incidentally, OCP1 in Table 10.5 was also manufactured by the tubular reactor technology described in Figure 10.5.

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Lubricant Additives: Chemistry and Applications

TABLE 10.8 Rheological Properties of SAE 15W-40 Engine Oils Containing Different OCP Viscosity Modiﬁers Polymer content (wt%) 150N base oil percentage 600N base oil percentage Kinematic viscosity (cSt) at 100°C Viscosity index CCS viscosity (cP) at −15°C MRV viscosity (cP) at −20°C MRV viscosity (cP) at −25°C HTHS (cP)

OCP7

OCP3

OCP8

OCP9

0.95 76 24 15.04 141 3,080 10,000 20,500 4.17

0.85 76 24 14.97 140 3,040 9,900 18,600 4.38

0.85 70 30 15.25 140 3,070 8,800 18,300 4.25

0.64 70 30 15.12 135 3,010 Solid Solid 4.42

The rheological data in Table 10.8 further illustrate several features of LTOCPs mentioned earlier. Their inherently lower CCS viscosity contributions permit the greater use of higher-viscosity base oils, which can be beneficial in meeting volatility requirements. The low-temperature MRV performance of OCP9 was far inferior to that of the other copolymers, indicating that the PPD chosen for this particular study was not optimized for OCP9 in these base stocks. Another polyalkymethacrylate PPD was found to bring the MRV viscosity of the OCP9 formulation down to 7,900 and 18,000 cP at –20 and –25°C, respectively. More about interaction with PPDs is discussed in Section 10.5.5. Again, the higher thickening efficiency of EP copolymers versus EPDMs of similar molecular weight (shear stability) is clearly demonstrated in Table 10.8. Another feature worth noting is that increasing base oil viscosity can nudge HTHS viscosity upward (compare OCP7 with OCP8 or OCP3 with OCP9).

10.5.5 INTERACTION WITH POUR POINT DEPRESSANTS Although base oil and VM play a role in determining low-temperature oil pumpability, the PPD provides the primary control in this area. SAE J300 [104] specifies the MRV test (ASTM D4684) as the sole guardian of pumpability protection, although it acknowledges that other tests may also be useful in the development of lubricants from new components. The Scanning Brookfield test (ASTM D5133) and Pour Point (ASTM D5873), although not required within SAE J300, are often contained in other standards established by original equipment manufacturers, oil marketers, and governmental agencies and, therefore, must also be considered in the development of modern engine oils. Advances in base oil technology have led, in recent years, to a wide range of mineral and synthetic lubricant base stocks [116], classified as API Group I, II, III, IV, and V stocks. The API system classifies oils according to viscosity index (VI), saturates content, and sulfur level. Group I mineral oils are defined as having <90% saturates, VI >80 and more than 0.03% sulfur. Groups II and III oils have <0.03% sulfur and >90% saturates, but they differ mainly in VI. Group II oils have VI >80, whereas Group III stocks have VI values in excess of 120. Formulating these conventional and highly refined oils to meet all the rheological requirements of SAE J300 is not always straightforward. An important aspect of base oil technology that is not embodied within the API Group numbering scheme is the type of de-waxing process or processes employed. It is well known [117–123] that the low-temperature oil pumpability performance of engine oils is often impeded by nucleation and growth of wax crystals, which can coalesce and restrict the flow of oil at low temperatures. The type and amount of wax that forms dictates the

Oleﬁn Copolymer Viscosity Modiﬁers

303

type and concentration of PPD that will be effective in keeping wax crystals small so that they do not form network structures and lead to high viscosity and yield stress. Both the feedstocks and dewaxing steps used in the manufacture of a given base oil determine wax composition and, in turn, PPD response. Certain types of viscosity modifiers can interact with base oils and PPDs at low temperatures and can lead to excessively high MRV viscosities in some situations (see, for example, OCP9 in Table 10.8). Formulating with amorphous OCPs has not, in the author’s experience, posed many difficulties. Conversely, LTOCPs possess longer ethylene sequences that have the potential to interact with wax crystals at low temperatures. Several reports in the literature [66,76] suggest that high-ethylene OCPs can, under certain circ*mstances, interact negatively on MRV viscosity and yield stress and may be more sensitive to the type of PPDs that will be effective in some formulating systems. A previously unpublished study of low-temperature interactions among base oils, OCP viscosity modifiers, and PPDs was carried out in the author’s laboratory using components listed in Tables 10.9 through 10.11. Fully formulated SAE 5W-30 and 15W-40 engine oils were blended using performance additive packages DI-1 (at 11 wt%) and DI-2 (at 13 wt%); API SJ quality and CH-4 quality, respectively; and all combinations of base oil, VM, and PPD.

TABLE 10.9 Base Oils Used in Low-Temperature Viscosity Modiﬁer/Pour Point Depressant Interaction Study Base Oil Code (API Group) B1-L (I) B1-M (I) B1-H (I) B2-L (I) B2-M (I) B2-H (I) B3-L (II) B3-M (II) B3-H (II) B4-L (III) B4-H (III)

Saturates (wt%)

Kinematic Viscosity (cSt) at 100°C

Sulfur (wt%)

Viscosity Index

CCS Viscosity (cP) at –25°C

73.6 71.8 61.8 75.2 75.0 72.3 100 100 100 100 100

3.88 5.15 12.10 4.18 4.91 12.73 4.20 5.49 10.72 4.50 6.49

0.276 0.553 0.381 0.193 0.544 0.412 0.006 0.011 0.016 0.007 0.006

104 102 97 105 106 99 100 117 98 123 131

1170 4060 — 1510 3060 — 1570 2430 — 1120 2710

TABLE 10.10 Viscosity Modiﬁers Used In Low-Temperature Viscosity Modiﬁer/Pour Point Depressant Interaction Study VM Code

OCP Type

SSI (ASTM D6278)

VM-1 VM-2 VM-3

Amorphous LTOCP LTOCP

37 35 37

TABLE 10.11 Pour Point Depressants Used In Low-Temperature Viscosity Modiﬁer/Pour Point Depressant Interaction Study Pour Point Depressant Code PPD-1 PPD-2 PPD-3 PPD-4

Chemistry Poly(styrene-maleate ester) Poly(alkylmethacrylate) Poly(styrene-maleate ester) Poly(alkylmethacrylate)

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Lubricant Additives: Chemistry and Applications

Figures 10.17 through 10.20 summarize VM/PPD effects on MRV viscosity for each base oil type. In these graphs, the letter Y adjacent to a vertical bar denotes a yield stress failure. For the API Group I base oil B1 (SAE 5W-30, Figure 10.17), only PPD-3 is effective with all three viscosity modifiers. Both VM-1 and VM-3 suffer yield stress failures with at least one PPD. In the 15W-40 formulation, VM-3 exhibits yield stress behavior with all four PPDs, even in one case in which the MRV viscosity is quite low (PPD-1). In the author’s experience, it is quite unusual to observe yield stress failures in the MRV test when viscosity is below ∼40,000 cP. Y Y

80,000 Y 60,000 40,000 20,000

VM-3 VM-2 VM-1

0 PPD-1 PPD-2 PPD-3

FIGURE 10.17

MRV viscosity (cP) at −25°C

MRV viscosity (cP) at −35°C

60,000 Y 40,000 20,000 VM-3 VM-2 VM-1

0 PPD-1

PPD-4

PPD-2

PPD-3

40,000 Y

20,000 0

VM-3 VM-2 VM-1

MRV viscosity (cP) at −25°C

60,000

80,000 60,000 40,000 20,000

VM-3 VM-2 VM-1

0 PPD-1 PPD-2

PPD-3 PPD-4

Rheological results for oil B2. SAE 5W-30 (left) and SAE 15W-40 (right).

80,000 60,000

40,000 20,000

0 PPD-1 PPD-2

PPD-3

PPD-4

VM-3 VM-2 VM-1

MRV viscosity (cP) at −25°C

MRV viscosity (cP) at −35°C

PPD-1 PPD-2 PPD-3 PPD-4

MRV viscosity (cP) at −35°C

PPD-4

Rheological results for oil B1. SAE 5W-30 (left) and SAE 15W-40 (right).

80,000

FIGURE 10.19

80,000

FIGURE 10.18

80,000 60,000 Y 40,000 20,000

VM-3 VM-2 VM-1

0 PPD-1 PPD-2 PPD-3 PPD-4

Rheological results for oil B3. SAE 5W-30 (left) and SAE 15W-40 (right).

Oleﬁn Copolymer Viscosity Modiﬁers

305

MRV viscosity (cP) at -35°C

80,000 Y

60,000

Y Y

40,000 20,000 VM-3 VM-2 VM-1

0 PPD-1

PPD-2 PPD-3

PPD-4

FIGURE 10.20 Rheological results for oil B4. SAE 5W-30.

TABLE 10.12 Number of MRV Failures due to Yield Stress in LowTemperature Viscosity Modiﬁer/Pour Point Depressant Interaction Study VM VM-1 VM-2 VM-3 Total

PPD-1

PPD-2

1 2 3

6 6

PPD-3

PPD-4

Total

3 1 2 6

5 1 13 19

3 4

The other API group I blended oils, B2 (Figure 10.18), respond to PPDs in a similar manner to B1, but only in the SAE 5W-30 formulation. In the 15W-40 case, VM-3 is the only VM to experience yield stress failure, but only in the presence of PPD-2; all other combinations demonstrate acceptable pumpability performance. Figure 10.19 describes the MRV map of B3, the API Group II oil. Overall, low-temperature viscosities are all quite low for all VM/PPD combinations, although VM-3 again experiences one yield stress failure in each viscosity grade. VM-1 fails the yield stress criterion once. Finally, the API Group III SAE 5W-30 formulation (Figure 10.20) was the most difficult to pour depress. All four oils blended with VM-3 were yield stress failures, and each of the other two VMs showed significant yield stresses for one PPD each. In summary, the number of MRV failures due to yield stress is given in Table 10.12. Clearly, one of the LTOCP viscosity modifiers, VM-3, is substantially more sensitive to PPDs than the other two polymers. It is especially incompatible with PPD-2. The other LTOCP in this study, VM-2, and the amorphous VM-1 were found to be far more tolerant of PPD type. Similar to the discussion in Section 10.5.4.2, one of the major differences between VM-3 and VM-2 is that they were manufactured by different technologies, broadly described in Figures 10.5 and 10.4, respectively.

10.5.6

FIELD PERFORMANCE DATA

Multigrade lubricants containing EP viscosity modifiers have been tested in passenger car and heavy-duty truck engines for over three decades, but very few studies devoted to VM effects on engine cleanliness and wear have been published. It is generally believed that adding a polymer to the engine lubricant will have a detrimental effect on engine varnish, sludge, and piston ring-pack

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TABLE 10.13 120 h Caterpillar 1H2 Piston Deposit Ratings of SAE 10W-40 Oils Formulated with N-Vinyl PyrollidoneGrafted OCPs Sample

Nitrogen (wt%)

TGF

WTD

0 0.3 0.5 0.7 0.26 0.28

– 46 39 47 28 11

>800 244 173 149 156 139

OCP MFOCP5 MFOCP6 MFOCP7 MFOCP6a MFOCP2a

Note: TGF, top groove fill rating; WTD, weighted total (piston) demerits rating. a OCP grafted with maleic anhydride and subsequently reacted with amines. Source: Adapted from Spiess, G.T., Johnston, J.E., and VerStrate, G., Addit. Schmierst. Arbeitsfluessigkeiten, Int. Kolloq., 5th, 2, 8.10-1, Tech. Akad. Esslingen, 1986.

deposits [1,2], but the performance additive can be formulated to compensate for these effects. Kleiser et al. [124] ran a taxicab fleet test designed to compare the performance of a nonfunctionalized OCP viscosity modifier with a highly dispersant-functionalized OCP (HDOCP) as well as to test other oil formulation effects. They were surprised to find that SAE 5W-30 oil containing a higher concentration of non-DOCP showed statistically better engine deposit control when compared to a similar SAE 15W-40 oil with lower polymer content. They also observed significant improvements in sludge and varnish ratings attributed to the use of HDOCP. Others have also reported that DOCPs can be beneficial in preventing buildup of deposits in laboratory engines such as the Sequence VE [125], VD [1], and Caterpillar 1H2/1G2 [1] tests. These authors found that engine oils containing certain DOCPs need less ashless dispersant to achieve an acceptable level of engine cleanliness than NOCPs. The actual level of deposit prevention is highly influenced by the functionalization chemistry as well as the number of substituents per 1000 backbone carbon atoms, as shown in Table 10.13 (MFOCP = Multi-functional OCP).

10.6 10.6.1

MANUFACTURERS, MARKETERS, AND OTHER ISSUES EP/EPDM MANUFACTURERS

EP copolymers and EPDM terpolymers are manufactured by a number of companies around the globe. Table 10.14 [126] contains a listing of those with production capacity >30,000 metric tons/ year (£65 million per annum). Not all are necessarily supplying rubber into the viscosity modifier market. The vast majority of the capacity goes into other applications such as automotive (sealing systems, radiator hoses, injection molded parts), construction (window gaskets, roofing/sheeting, cable insulation, cable filler), and plastics modification. Various grades are often classified by melt viscosity, EP ratio, diene type and content, and physical form and filler type and level (carbon black, pigments, or extender oils). Melt viscosity is measured by two main techniques—Mooney viscosity (ASTM D1646 or ISO 289) and melt index (ASTM D1238 or ISO 1133-1991, also called melt-mass flow rate). Mooney viscosity is directly proportional to molecular weight, whereas melt index is inversely proportional to molecular weight.

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307

TABLE 10.14 Manufacturers with Production Capacity Greater than 30,000 Metric Tons/Year Manufacturing Location(s)

Company Dow Chemical

DSM Elastomers

ExxonMobil Chemical

Plaquemine, Louisiana Seadrift, Texas Geleen, The Netherlands; Triunfo, Brazil Baton Rouge, Louisiana, United States; Notre Dame de Gravenchon, France; Kumbo Polychem, South Korea (JV) Yokkaichi and Kashima, Japan

Capacity (Metric tons/Year) 230,000

185,000

Technology Metallocene, solution and gas-phase processes Ziegler–Natta, solution process

Trade Name

Comments

Nordel IP

EPDM

Nordel MG Keltan

EP and EPDM

272,500

Ziegler–Natta and metallocene, solution process

Vistalon

EP and EPDM

87,500

Ziegler–Natta, solution process

Esprene

EPDM

Orange, Texas Marl, Germany

110,000

Buna EP T Buna EP G

EP and EPDM

93,000 120,000a

Royalene, Trilene Mitsui EPT

EP and EPDM

Mitsui

Geismar, Louisiana, United States Chiba, Japan

Polimeri Europa

Ferrara, Italy

85,000

Dutral

EP and EPDM

Sumitomo

Japan

45,000

Ziegler–Natta, suspension and solution processes Ziegler–Natta, solution process Ziegler–Natta and metallocene, solution process Ziegler–Natta, suspension process Ziegler–Natta, solution process

Esprene

EPDM

JSR Corporation (Japan Synthetic Rubber) Lanxess

Lion Copolymer

EP and EPDM

Includes 75,000 metric tons/year metallocene plant to begin operation in 2007.

10.6.2

OLEFIN COPOLYMER VM MARKETERS

Companies which provide EP copolymers and EPDM terpolymers to the viscosity modifier market are listed in Table 10.15. A wide variety of products, varying in shear stability and level of crystallinity, are available in both solid and liquid forms. Functionalized polymers that provide added dispersancy and antioxidancy are available from several suppliers. The reader is advised to update this information periodically, since each company’s product lines change over time. Mergers and acquisitions have also contributed to significant flux in the OCP market. For example, the Paratone® product line was originally developed and marketed by the Paramins Division of Exxon Chemical Company. When Exxon and Shell combined their lubricant additives businesses to form Infineum in 1998, the Paratone business was sold to Oronite, the lubricant additives division of Chevron Chemical Company. Ethyl’s purchase of Amoco and Texaco OCP product technology in the 1990s resulted in rebranding of Texaco’s TLA-XXXX products to Ethyl’s Hitec® product line. Ethyl Additives changed its name to Afton Chemical Company in 2004. Dupont originally marketed EPDM—manufactured at its Freeport, Texas, facility—into the viscosity modifier market under the Ortholeum® trademark until it was sold to Octel in 1995. Thereafter, DuPont adopted

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Lubricant Additives: Chemistry and Applications

TABLE 10.15 Marketers of E/P Copolymers and EPDM Terpolymers as Engine Lubricating Oil Viscosity Modiﬁers Company

Headquarters

Trade Name

Product Classes

Afton Chevron Oronite

Richmond, Virginia Richmond, California

Hitec 5700 series Paratone

NDOCP, DOCP, DAOCP NDOCP, DOCP

Dow Chemical Infineum

Midland, Michigan Linden, New Jersey

Nordel IP Infineum V8000 series

NDOCP NDOCP

Lubrizol

Wickliffe, Ohio

Lubrizol 7000 series

NDOCP, DOCP

RohMax (Degussa AG)

Darmstadt, Germany

Viscoplex

NDOCP, mixed PMA/OCP

Product Form Liquid concentrates Pellet, bales, and liquid concentrates Bales and pellets Pellets and liquid concentrates Bales and liquid concentrates Liquid concentrates

Note: DAOCP = olefin copolymer with dispersant and antioxidant functionality; DOCP = dispersant-functionalized olefin copolymer; NDOCP = nonfunctionalized olefin copolymer; PMA = poly(alkyl methacrylate).

the NDR brand name. DuPont and Dow Chemical Company formed a 50/50 joint venture in 1995, merging their elastomer businesses. Shortly following the successful start-up of their metallocene plant in Plaquemine, Louisiana, 2 years later, the Freeport facility was closed. Dow Chemical Company acquired control of the EPDM product line, marketed under the Nordel® IP trade name, in 2005. Bayer transferred its EPDM business to a new company named LANXESS in 2004. Although primarily a poly(alkylmethacrylate) company, Rohm GmbH of Darmstadt, Germany, developed several OCP-based viscosity modifiers under the Viscoplex® trade name, currently owned and marketed by RohMax Additives, a Degussa company. In mid-2007, Crompton sold its EPDM business to Lion Copolymer.

10.6.3

READ ACROSS GUIDELINES

Various lubrication industry associations have published highly detailed guidelines [127–129] for defining conditions under which certain additive and base oil changes to a fully or partially qualified engine oil formulation may be permitted without requiring complete engine testing data to support the changes. The purpose of these standards is to minimize test costs while ensuring that commercial engine oils meet the performance requirements established by industry standards, certification systems, and original equipment manufacturers (OEMs). From a viscosity modifier perspective, changes are often driven by one or more of the following needs: 1. To optimize viscometrics within a given viscosity grade 2. To improve the shear stability of the formulation 3. To interchange one polymer for another (cost, security of supply, customer choice) There are similarities and differences among codes of practice adopted by the American Chemistry Council (ACC) and the two European agencies, ATC (Technical Committee of Petroleum Additive Manufacturers of Europe) and ATIEL (Technical Association of the European Lubricants Industry). All permit minor changes in VM concentration (no more than 15% relative on a mass basis) to accomplish the first need mentioned earlier. The European codes explicitly allow the interchange of one VM for another (if both are from the same supplier) if the VM supplier deems them to be “equivalent and interchangeable.” VMs from different suppliers, or those from the same supplier that are not judged to be “equivalent,” must undergo a rigorous engine testing program such as that outlined in Table 10.16.

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TABLE 10.16 Engine Test Requirements for Interchanging Viscosity Modiﬁers within the ATIEL Code of Practice Performance Category

NDVM to NDVM (1, 2, 3, 4)

DVM to DVM or NDVM to DVM (1, 2, 3, 4)

Gasoline

TU572 (8) M111 or VG (9) M111FE

Light-duty diesel

VWICTD (9) VW DI M111FE

OM602A VWICTD (9) XUD11 (12) VW DI M111FE

Gasoline/light-duty diesel

TU572 (8) M111 or VG (9) VWICTD (9) VW DI M111FE

TU572 (8) M111 or VG (9) OM602A VWICTD (9) XUD11 (12) VW DI M111FE

Gasoline/light-duty diesel with after treatment devices

TU572 (8) M111 or VG (9) VW DI M111FE

TU572 (8) M111 or VG (9) OM602A XUD11 (12) VW DI M111FE

Heavy-duty diesel

OM364LA (10) Mack T8 (6) Mack T8E (7) OM 441 LA M11 (5) (11)

OM364LA (10) OM602A Mack T8 Mack T8E OM 441 LA M11 (5) (11)

Note: 1. Full testing required for any other viscosity modifier interchange (VMI) not listed above. 2. Physical mixes of non-dispersant viscosity modifier (NDVM) and DVM are treated as DVM. 3. Only the tests included in the ACEA sequence for which read across is required have to be run. 4. Where alternative tests are listed, for example, “M111 or VG,” the alternative test cannot be run to document read across if a failing result has already been obtained on the other test. 5. Not required if the new oil formulation has the same or greater HTHS value compared with the original tested formulation. 6. The Mack T8 requirement is waived, if the replacement NDVM has the same or poorer shear stability as determined on candidate test oil formulated to similar viscosity as the original NDVM tested formulation and measured by CEC-14-A93 (ASTM D 6278-98), and if the requirements of note 7 are also met. 7. The T8 or T8E requirement is waived, if the replacement NDVM is within the same chemical type as the tested NDVM (“chemical type” means chemical family such as, but not limited to, styrene ester, polymethacrylate, styrene butadiene, styrene isoprene, polyisoprene, olefin copolymer, and polyisobutylene). 8. TU3MH results can be used in support of A2-96 issue 3 provided the results are passing for A2-96 issue 2. 9. The VWICTD may be replaced by the VWDI (with B4 performance limit). 10. The OM364LA requirement may be met by a passing OM441LA at E5 level. 11. The M11EGR test may be used in place of the M11. 12. The XUD 11 test can be replaced by the DV4 test. Source: Code of Practice for Developing Engine Oils Meeting the Requirements of the ACEA Oil Sequences, ATIEL, Technical Association of the European Lubricants Industry, Issue No. 13, Appendix C, Brussels, Belgium, November 28, 2005.

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TABLE 10.17 CAS Index of EP and EPDM Copolymers EP/EPDM

CAS Index

EP EPDM (ENB termonomer) EPDM (DCPD termonomer) EPDM (1,4-hexadiene termonomer)

9010-79-1 25038-36-2 25034-71-3 25038-37-3

The ACC guidelines impose two levels of data needed to support viscosity modifier interchange. Level 1 support is defined as analytical and rheological test data. Level 2 support includes both level 1 data and full-length valid ASTM engine tests, intended to demonstrate that the proposed VM interchange presents no harm in terms of lubricant performance. There are three categories of engine tests that can be used to satisfy the ACC level 2 criterion: (1) statistically designed engine test matrices, (2) complete programs, or (3) partial data sets from the same technology family. This broad definition of additive interchange testing is more open for interpretation than the ATIEL guidelines as represented in Table 10.16. Minor formulation modifications needing only level 1 data do not permit changes in VM type, defined as polymers of a “specific molecular architecture with a specific shear stability characterized by a specific trade name, stock or code number.” When a change in shear stability is required, level 2 support is sufficient for polymers of the same chemical family (e.g., OCPs) and from the same manufacturer. Otherwise, a full engine testing program is needed. The ACC guidelines also specify that if a dispersant viscosity modifier (DVM) is used in a core multigrade formulation, the additional dispersant needed to read across to a monograde or other multigrades with lower VM concentration requires a Sequence VG test and level 2 support in other tests.

10.6.4

SAFETY AND HEALTH

E/P copolymers as well as EPDMs are classified as “nonhazardous” substances by OSHA (1910.1200) and the European Economic Community (EEC). They are generally considered to be not acutely toxic, similar to other high-molecular-weight polymers. Material that is heated to the molten state can emit fumes, which can be harmful and irritating to the eyes, skin, mucous membranes, and respiratory tract, especially copolymers containing nonconjugated diene termonomers. Proper ventilation and respiratory protection are recommended when handling EP and EPDM copolymers under these conditions. Appropriate personal protective equipment is also advised to guard against thermal burns. EP/EPDM grades are indexed by the Chemical Abstract Service in Table 10.17.

REFERENCES 1. Spiess, G.T., J.E. Johnston and G. VerStrate, “Ethylene Propylene Copolymers as Lube Oil Viscosity Modifiers,” Addit. Schmierst. Arbeitsfluessigkeiten, Int. Kolloq., 5th, 2, 8.10-1, W.J. Bartz, ed., Tech. Akad. Esslingen, Ostfildern, Fed. Rep. Germany, 1986. 2. Marsden, K., “Literature Review of OCP Viscosity Modifiers,” Lubr. Sci., 1(3), 265, 1989. 3. Ver Strate, G. and M.J. Struglinski, “Polymers as Lubricating-Oil Viscosity Modifiers,” in Polymers as Rheology Modifiers, D.N. Schulz and J.E. Glass, eds., ACS Symp. Ser., 462, 256, Am. Chem. Soc., Washington, DC, 1991. 4. Boor, J., Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York, 1979. 5. Crespi, G., A. Valvassori and U. Flisi, “Olefin Copolymers,” in Stereo Rubbers, W.M. Saltman, ed., Wiley-Interscience, New York, NY, pp. 365–431, 1977. 6. Brandrup, J. and E.H. Immergut, eds., Polymer Handbook, 2nd Ed., Wiley , New York, p. II-193, 1975. 7. May, C.J., E. De Paz, F.N. Gixshick, K.O. Henderson, R.B. Rhodes, S. Iseregounis, and L. Ying, “Cold Starting and Pumpability Studies in Modern Engines,” ASTM Res. Report RR-DO2-1442, 1999.

Oleﬁn Copolymer Viscosity Modiﬁers

311

8. Wilkes, C.E., C.J. Carmen and R.A. Harrington, “Monomer Sequence Distribution in EthylenePropylene Terpolymers Measured by 13C Nuclear Magnetic Resonance,” J. Poly. Sci.: Symp. No. 43, 237, 1973. 9. Carmen, C.J. and K.C. Baranwal, “Molecular Structure of Elastomers Determined with Carbon-13 NMR,” Rubber Chem. Tech., 48, 705, 1975. 10. Carmen, C.J., R.A. Harrington and C.E. Wilkes, “Monomer Sequence Distribution in Ethylene– Propylene Rubber Measured by 13C NMR. 3. Use of Reaction Probability Model,” Macromolecules, 10(3), 536, 1977. 11. Randall, J.C., Polymer Sequence Determinations: Carbon-13 NMR Method, Academic Press, New York, p. 53, 1977. 12. Carmen, C.J., “Carbon-13 NMR High-Resolution Characterization of Elastomer Systems,” in Carbon-13 NMR in Polymer Science, ACS Symp. Ser., W.M. Pasikan, ed. 103, 97, 1978. 13. Kapur, G.S., A.S. Sarpal, S.K. Mazumdar, S.K. Jain, S.P. Srivastava and A.K. Bhatnager, “StructurePerformance Relationships of Viscosity Index Improvers: I. Microstructural Determination of Olefin Copolymers by NMR Spectroscopy,” Lubr. Sci., 8-1, 49, 1995. 14. U.S. Patent 4666619, 1987. 15. U.S. Patent 4156767, 1979. 16. Olabisi, O. and M. Atiqullah, “Group 4 Metallocenes: Supported and Unsupported,” J.M.S.—Rev. Macromol. Chem. Phys., C37(3), 519, 1997. 17. Soares, J.B.P. and A.E. Hamielec, “Metallocene/Aluminoxane Catalysts for Olefin Polymerization. A Review,” Poly. Reaction Engr., 3(2), 131, 1995. 18. Gupta, V.K., S. Satish and I.S. Bhardwaj, “Metallocene Complexes of Group 4 Elements in the Polymerization of Monoolefins,” J.M.S.—Rev. Macromol. Chem. Phys., C34(3), 439, 1994. 19. Hackmann, M. and B. Rieger, “Metallocenes: Versatile Tools for Catalytic Polymerization Reactions and Enantioselective Organic Transformations,” Cat. Tech, 79(December), 1997. 20. Chien, J.C.W. and D. He, “Olefin Copolymerization with Metallocene Catalysts. I. Comparison of Catalysts,” J. Poly. Sci. Part A: Poly. Chem., 29, 1585, 1991. 21. Montagna, A.A., A.H. Dekmezian and R.M. Burkhart, “The Evolution of Single-Site Catalysts,” Chemtech, 26(December), 1997. 22. McGirk, R.H., M.M. Hughes and L.C. Salazar, “Evaluation of Polyolefin Elastomers Produced by Constrained Geometry Catalyst Chemistry as Viscosity Modifiers for Engine Oil,” Soc. Automot. Eng. Tech. Paper Ser. No. 971696, 1997. 23. Rotman, D., “Dupont Dow Dobuts Metallocene EPOM.” “Synthetic Rubber,” Chem. Week, 15,(May 14), 1997. 24. U.S. Patent 5151204, 1992. 25. Mishar, M.K. and R.G. Saxton, “Polymer Additives for Engine Oils,” Chemtech, 35(April), 1995, pp. 35–41. 26. U.S. Patent 5698500, 1997. 27. Corbelli, L., “Ethylene-Propylene Rubbers,” Dev. Rubber Tech., 2, 87–129, 1981. 28. European Patent 199453, 1986. 29. U.S. Patent 4092255, 1978. 30. U.S. Patent 4699723, 1987. 31. U.S. Patent 4816172, 1989. 32. U.S. Patent 5814586, 1998. 33. U.S. Patent 5874389, 1999. 34. Pennewib, H. and C. Auschra, “The Contribution of New Dispersant Mixed Polymers to the Economy of Engine Oils,” Lubr. Sci., 8(2), 179–197, 1996. 35. U.S. Patent 4089794, 1978. 36. U.S. Patent 4160739, 1979. 37. U.S. Patent 4171273, 1979. 38. U.S. Patent 4219432, 1980. 39. U.S. Patent 4320019, 1982. 40. U.S. Patent 4489194, 1985. 41. U.S. Patent 4749505, 1988. 42. U.S. Patent 5210146, 1992. 43. U.S. patent 5252238, 1993. 44. U.S. Patent 5290461, 1994. 45. U.S. Patent 5401427, 1995.

312

Lubricant Additives: Chemistry and Applications

46. U.S. Patent 5427702, 1995. 47. U.S. Patent 5534171, 1996. 48. Garbassi, F., “Long Chain Branching: An Open Question for Polymer Characterization?” Polymer News, 19, 340–346, 1994. 49. U.S. Patent 4517104, 1985. 50. U.S. Patent 4632769, 1986. 51. U.S. Patent 5540851, 1998. 52. U.S. Patent 5811378, 2000. 53. European Patent 338672, 1989. 54. U.S. Patent 5075383, 1991. 55. U.S. Patent 4500440, 1985. 56. U.S. Patent 5035819, 1991. 57. European Patent 470698, 1992. 58. European Patent 461774, 1991. 59. U.S. Patent 5021177, 1991. 60. U.S. Patent 4790948, 1988. 61. European Patent 284234, 1988. 62. U.S. Patent 6117941, 2000. 63. Ondrey, G. and T. Kamiya “Synthetic Rubber is Resilient,” Chem. Eng., 105(12), 30, 1998. 64. Italiaander, E.T., “The Gas-Phase Process—A New Era in EPR Polymerization and Processing Technology,” KGK Kautschuk Gummi Kunststoffe, Jahrgang, 48(October), 742–748, 1995. 65. Hüts, B., “Ethylene-Propylene Rubber,” Hydrocarbon Processing, p. 164, November 1981. 66. “EPM/EPDM,” Gosei Gomu, 85, 1–9, 1980. 67. Darribère, C., F.A. Streiff and J.E. Juvet, “Static Devolatilization Plants,” DECHEMA Monographs, 134, 689–704, 1998. 68. U.S. Patent 3726843, 1973. 69. U.S. Patent 4874820, 1989. 70. U.S. Patent 4804794, 1989. 71. U.S. Patent 5798420, 1998. 72. Corbelli, L. and F. Milani, “Recenti Sviluppi Nella Produzione Delle Gomme Sintetiche EtileneProilene,” L-Industria Della Gomma, 29(5), 28–31, 53–55, 1985. 73. Young, H.W and S.D. Brignac, “The Effect of Long Chain Branching on EPDM Properties,” Proceedings of Special Symp. Advanced Polyolefin Tech., 54th Southwest Reg. Meeting, American Chemical Society, Baton Rouge, LA, 1998. 74. British Patent 1372381, 1974. 75. Canadian Patent 991792, 1976. 76. U.S. Patent 4464493, 1984. 77. U.S. Patent 4749505, 1988. 78. U.S. Patent 5290461, 1994. 79. U.S. Patent 5391617, 1995. 80. U.S. Patent 5401427, 1995. 81. U.S. Patent 5837773, 1998. 82. U.S. Patent 3316177, 1967. 83. U.S. Patent 3326804, 1967. 84. U.S. Patent 4743391, 1988. 85. U.S. Patent 5006608, 1991. 86. U.S. Patent 4743391, 1988. 87. U.S. Patent 5006608, 1991. 88. Carman, C.J. and K.C. Baranwal, “Molecular Structure of Elastomers Determined with Carbon-13 NMR,” Rubber Chem. Technol., 48, 705–718, 1975. 89. Carman, C.J., R.A. Harrington and C.E. Wilkes, “Monomer Sequence Distribution in EthylenePropylene Rubber Measured by 13C NMR. 3. Use of Reaction Probability Model,” Macromolecules, 10(3), 536–544, 1977. 90. Carman, C.J., “Carbon-13 NMR High-Resolution Characterization of Elastomer Systems,” Carbon-13 NMR in Polymer Science, ACS Symp. Ser., 103, 97–121, 1978. 91. Wilkes, C.E., C.J. Carman and R.A. Harrington, “Monomer Sequence Distribution in Ethylene– Propylene Terpolymers Measured by 13C Nuclear Magnetic Resonance,” J. Poly. Sci.: Symp. No. 43, 237–250, 1973.

Oleﬁn Copolymer Viscosity Modiﬁers

313

92. Di Martino, S. and M. Kelchtermans, “Determination of the Composition of Ethylene-PropyleneRubbers Using 13C-NMR Spectroscopy,” J. Appl. Poly. Sci., 56, 1781–1787, 1995. 93. Randall, J.C., Polymer Sequence Determinations: Carbon-13 NMR Method, pp. 53–138, Academic Press, New York, 1977. 94. Minick, J., A. Moet, A. Hiltner, E. Baer and S.P. Chum, “Crystallization of Very Low Density Copolymers of Ethylene with α-Olefins,” J. Appl. Poly. Sci., 58, 1371–1384, 1995. 95. Rubin, I.D. and M.M. Kapuscinski, “Viscosities of Ethylene-Propylene-Diene Terpolymer Blends in Oil,” J. Appl. Poly. Sci., 49, 111, 1993. 96. Rubin, I.D., “Polymers as Lubricant Viscosity Modifiers,” Poly. Preprints, Am. Chem. Soc. Div. Poly. Chem., 32(2), 84, 1991. 97. Rubin, I.D., A.J. Stipanovic and A. Sen, “Effect of OCP Structure on Viscosity in Oil,” Soc. Automot. Eng. Tech. Paper Ser. No. 902092, 1990. 98. Kapuscinski M.M., A. Sen and I.D. Rubin, “Solution Viscosity Studies on OCP VI Improvers in Oils,” Soc. Automot. Eng. Tech. Paper Ser. No. 892152, 1989. 99. Kucks, M.J., H.D. Ou-Yang and I.D. Rubin, “Ethylene-Propylene Copolymer Aggregation in Selective Hydrocarbon Solvents,” Macromolecules, 26, 3846, 1993. 100. Rubin, I.D. and A. Sen, “Solution Viscosities of Ethylene-Propylene Copolymers in Oils,” J. Appl. Poly. Sci., 40, 523–530, 1990. 101. Sen, A. and I.D. Rubin, “Molecular Structures and Solution Viscosities of Ethylene-Propylene Copolymers,” Macromolecules, 23, 2519, 1990. 102. LaRiviere, D., A.A. Asfour, A. Hage and J.Z. Gao, “Viscometric Properties of Viscosity Index Improvers in Lubricant Base Oil over a Wide Temperature Range. Part I: Group II Base Oil,” Lubr. Sci., 12(2), 133–143, 2000. 103. Huggins, M.L., “The viscosity or dilute solutions of Long-chain molecules. IV. Dependence on concentration” J. Am. Chem. Soc., 64, 2716, 1942. 104. “Engine Oil Viscosity Classification,” Soc. Automot. Eng. Surface Vehicle Standard No. J300, Rev. December 1999. 105. Rhodes, R.B., “Low-Temperature Compatibility of Engine Lubricants and the Risk of Engine Pumpability Failure,” Soc. Automot. Eng. Tech. Paper Ser. No. 932831, 1993. 106. Papke, B.L., M.A. Dahlstrom, C.T. Mansfield, J.C. Dinklage and D.J. Rao, “Deterioration in Used Oil Low Temperature Pumpability Properites,” Soc. Automot. Eng. Tech. Paper Ser. No. 2000-01-2942, 2000. 107. Matko, M.A. and D.W. Florkowski, “Low Temperature Rheological Properties of Aged Crankcase Oils,” Soc. Automot. Eng. Tech. Paper Ser. No. 200-01-2943, 2000. 108. Alexander, D.L., S.A. Cryvoff, J.M. Demko, A.K. Deysarkar, T.W. Beta, R.I. Rhodes, M.F. Smith, Jr., R.L. Stambaugh, J.A. Spearot and M.A. Vickare, “High-Temperature, High-Shear (HTHS) Oil Viscosity: Measurement and Relationship to Engine Operation,” J.A. Spearot, ed., ASTM Special Tech. Pub., STP 1068, 1989. 109. Bates, T.W., “Oil Rheology and Journal Bearing Performance: A Review,” Lubr. Sci., 2(2) 157–176, 1990. 110. Wardle, R.W.M., R.C. Coy, P.M. Cann and H.A. Spikes, “An ‘In Lubro’ Study of Viscosity Index Improvers in End Contacts,” Lubr. Sci., 3(1), 45–62, 1990. 111. Alexander, D.L. and S.W. Rein, “Relationship ‘between Engine Oil Bench Shear Stability Tests,” Soc. Automot. Eng. Tech. Paper Ser. No. 872047, 1987. 112. Bartz, W.J., “Influence of Viscosity Index Improver, Molecular Weight, and Base Oil on Thickening, Shear Stabilit, and Evaporation Losses of Multigrade Oils,” Lubr. Sci., 12(3), 215–237, 2000. 113. Laukotka, E.M., “Shear Stability Test for Polymer Containing Lubricating Fluids—Comparison of Test Methods and Test Results,” Third International Symposium—The Perform. Evolution of Automotive Fuels and Lubricants, Co-ordinating European Council Paper No. 3LT, Paris, April 19–21, 1987. 114. Selby, T.W., “The Viscsosity Loss Trapezoid – Part 2: Determining General Features of VI Improver Molecular Weight Distribution by Parameters of the VLT,” Soc. Automot. Eng. Tech. Paper Ser. No. 932836, 1993. 115. Covitch, M.J., “How Polymer Architecture Affects Permanent Viscosity Loss of Multigrade Lubricants,” Soc. Automot. Eng. Tech. Paper Ser. No. 982638, 1998. 116. Kramer, D.C., J.N. Ziemer, M.T. Cheng, C.E. Fry, R.N.Reynolds, B.K. Lok, M.L. Sztenderowicz and R.R. Krug, “Influence of Group II & III Base Oil Composition on VI and Oxidation Stability,” National Lubricating Grease Inst. Publ. No. 9907, Proceedings from 66th NLGI Annual Meeting, Tucson, AZ, October 25, 1999. 117. Rossi, A., “Lube Basestock Manufacturing Technology and Engine Oil Pumpability,” Soc. Automot. Eng. Tech. Paper Ser. No. 940098, 1994.

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118. Rossi, A., “Refinery/Additive Technologies and Low Temperature Pumpability,” Soc. Automot. Eng. Tech. Paper Ser. No. 881665, 1988. 119. Mac Alpine, G.A. and C.J. May, “Compositional Effects on the Low Temperature Pumpability of Engine Oils,” Soc. Automot. Eng. Tech. Paper Ser. No. 870404, 1987. 120. Reddy, S.R. and M.L. McMillan, “Understanding the Effectiveness of Diesel Fuel Flow Improvers,” Soc. Automot. Eng. Tech. Paper Ser. No., 1981. 121. Xiong, C-X, “The Structure and Activity of Polyalphaolefins as Pour-Point Depressants,” Lubr. Eng., 49(3), 196–200, 1993. 122. Rubin, I.D., M.K. Mishra and R.D. Pugliese, “Pouir Point and Flow Improvement in Lubes: The Interaction of Waxes and Methacrylate Polymers,” Soc. Automot. Eng. Tech. Paper Ser. No. 912409, 1991. 123. Webber, R.M., “Low Temperature Rheology of Lubricating Mineral Oils: Effects of Cooling Rate and Wax Crystallization on Flow Properties of Base Oils,” J. Rheol., 43(4), 911–931, 1999. 124. Kleiser, W.M, H.M Walker and J.A. Rutherford, “Determination of Lubricating Oil Additive Effects in Taxicab Service,” Soc. Automot. Eng. Tech. Paper Ser. No. 912386, 1991. 125. Carroll, D.R. and R. Robson, “Engine Dynamometer Evaluation of Oil Formulation Factors for Improved Field Sludge Protection,” Soc. Automot. Engr. Tech. Paper Ser. No. 872124, 1987. 126. R.J. Chang and M. Yoneyama, “CEH Marketing Research Report, Ethylene-Propylene Elastomers,” 525.2600, Chemical Economics Handbook—SRI Consulting, Mento Park, California, January 2006. 127. Petroleum Additives Product Approval Code of Practice, American Chemistry Council, Appendix I, Arlington, Virginia, April 2005. 128. ATC Code of Practice, Technical Committee of Petroleum Additive Manufacturers in Europe, Section H, Brussels, Belgium, October 2006. 129 Code of Practice for Developing Engine Oils Meeting the Requirements of the ACEA Oil Sequences, ATIEL, Technical Association of the European Lubricants Industry, Issue No. 13, Appendix C, Brussels, Belgium, November 28, 2005. 130. U.S. Patent 4169063, 1979. 131. U.S. Patent 4132661, 1979.

Polymethacrylate Viscosity Modiﬁers and Pour Point Depressants Bernard G. Kinker

CONTENTS 11.1

Historical Development....................................................................................................... 315 11.1.1 First Synthesis ........................................................................................................ 315 11.1.2 First Application .................................................................................................... 316 11.1.3 First Manufacture and Large-Scale Application ................................................... 316 11.1.4 Development of Other Applications ...................................................................... 317 11.2 Chemistry ............................................................................................................................ 317 11.2.1 General Product Structure ..................................................................................... 317 11.2.2 Monomer Chemistry .............................................................................................. 318 11.2.3 Traditional Polymer Chemistry ............................................................................. 319 11.2.4 Patent Review......................................................................................................... 321 11.3 Properties and Performance Characteristics....................................................................... 323 11.3.1 Chemical Properties............................................................................................... 323 11.3.1.1 Hydrolysis ............................................................................................... 323 11.3.1.2 Thermal Reactions: Unzipping and Ester Pyrolysis ............................... 323 11.3.1.3 Oxidative Scissioning ............................................................................. 324 11.3.1.4 Mechanical Shearing and Free Radical Generation ............................... 325 11.3.2 Physical Properties................................................................................................. 325 11.3.2.1 Pour Point Depressants ........................................................................... 325 11.3.2.2 Viscosity Index Improvers ...................................................................... 327 11.4 Manufacturers, Marketers, and Economics ........................................................................ 332 11.4.1 Manufacturers and Marketers ................................................................................ 332 11.4.2 Economics and Cost-Effectiveness ........................................................................ 333 11.4.3 Other Incentives ..................................................................................................... 334 11.5 Outlook and Trends ............................................................................................................. 334 11.5.1 Current and Near-Term Outlook ............................................................................ 334 11.5.2 Long-Term Outlook................................................................................................ 335 References ...................................................................................................................................... 336

11.1

HISTORICAL DEVELOPMENT

11.1.1 FIRST SYNTHESIS The first synthesis of a polymethacrylate (PMA) intended for potential use in the field of lubricant additives took place in the mid-1930s. The original work was conducted under the supervision of Herman Bruson, who was in the employ of the Rohm and Haas Company (a parent of RohMax), and 315

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it was conducted in Rohm and Haas’ Philadelphia Research Laboratories. Bruson was exploring the synthesis and possible applications of longer alkyl side chain methacrylates [1]. He had proposed poly lauryl methacrylate as a product that might serve as a potential thickener or viscosity index improver (VII) for mineral oils. The result of the work was the 1937 issuance of two U.S. patents, for “Composition of matter and process” [2] and for “Process for preparation of esters and products” [3].

11.1.2

FIRST APPLICATION

Bruson’s invention did indeed thicken mineral oils, and it was effective in increasing viscosity at higher temperatures more so than at lower, colder temperatures. Since this behavior influences the viscosity–temperature properties or VI of a fluid, these materials eventually became known as viscosity index improvers (VIIs). Although PMAs successfully thickened oils, there were other competitive thickeners of that time, which increased the viscosity of mineral oils; these were based on poly isobutylene and alkylated polystyrene. The commercial success of PMA was not at all assured. The driving force behind PMA eventually eclipsing the other commercial thickeners of the era was PMA’s value as a VII rather than as a simple thickener of oils. In other words, PMAs have the ability to contribute relatively little viscosity at colder temperatures such as those that might be encountered at equipment start-up, but have a much higher contribution to viscosity at hotter temperatures at which equipment tends to operate. This desirable behavior enabled oil formulators to prepare multigrade oils that could meet a broader range of operating-temperature requirements. The positive enhancement of VI ensured the future success of PMAs.

11.1.3

FIRST MANUFACTURE AND LARGE-SCALE APPLICATION

The commercial development of PMAs as VIIs lagged until the beginning of World War II, when the U.S. government board rediscovered Bruson’s VII invention. The board was charged with searching the scientific literature for useful inventions that might aid the war effort. When considering potential utility, they hypothesized about a PMA VII providing more uniform viscosity properties over a very broad range of temperatures, particularly in aircraft hydraulic fluids. The fluids of that era were judged to be deficient particularly in fighter aircraft because of the exaggerated temperature/time cycles experienced. On the ground, the fluids could experience hightemperature ambient conditions and engine waste heat; and then after a rapid climb to high, very cold altitudes, the fluid might experience temperatures below −40°C. After successful trials of the multigrade aircraft hydraulic fluid concept, Rohm and Haas, in cooperation with the National Research Defense Committee, rapidly proceeded to commercialize PMA VIIs and delivered the first product, Acryloid HF, in 1942. These multigraded hydraulic fluids were quickly adopted by the U.S. Army Air Corps and were followed by other multigraded hydraulic fluids and lubricants in ground vehicles that incorporated VIIs. After the war, Rohm and Haas introduced PMA VIIs to general industrial and automotive applications. Early passenger car engine oil VIIs were first introduced to the market in 1946. The adoption of “all season” oils in the commercial market was greatly influenced by two events. First, the automotive manufacturers’ viscosity specification introduction of the new designation “W” (for winter grades); and then by Van Horne’s publication [4] pointing to the possibility of making and marketing cross-graded oils such as the now well-known “10W-30” as well as other cross-grades. By the early 1950s, use of multigrade passenger car oils became widespread in the consumer market. Methacrylates played a major role in enabling the formulation of that era’s multigrade engine oils. The use of PMA VIIs has since been extended to gear oils, transmission fluids, and a broad array of industrial and mobile hydraulic fluids in addition to the early usage in aircraft hydraulic fluids.

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11.1.4

317

DEVELOPMENT OF OTHER APPLICATIONS

Another important application area for PMA chemistry is in the field of pour point depressants (PPDs). When a methacrylate polymer includes at least some longer alkyl side chains, relatively similar to the chain length of waxes normally present in mineral oil, it can interact with growing wax crystals at sufficiently low temperatures. Wax-like side chains can be incorporated into a growing wax crystal and disrupt its growth. The net effect is to prevent congealing of wax in the oil at the temperature where it would have occurred in the absence of a PPD. Early PMA PPDs were used first by the military and later by civilian industry when Rohm and Haas offered such products to the industrial and automotive markets in 1946. Although PMAs were not the first materials used as PPDs (alkylated naphthalenes were), PMAs are probably the predominant products in this particular application now. Another use of wax-interactive PMAs is as refinery dewaxing aids. The process of dewaxing is carried out primarily to remove wax from paraffinic raffinates to lower the pour point of the resulting lube oil base stocks. PMA dewaxing aids are extremely interactive with waxes found in raffinates and thus function as nucleation agents to seed wax crystallization and promote the growth of relatively large crystals. The larger crystals are more easily filtered from the remaining liquid so that lube oil throughputs and yields are improved, whereas pour points are lowered by virtue of lower wax concentrations. Incorporating monomers more polar than alkyl methacrylates into a PMA provides products useful as ashless dispersants or dispersant VIIs. The polar monomer typically contains nitrogen and oxygen (other than the oxygen present in the ester group), and its inclusion in sufficient concentration creates hydrophilic zones along the otherwise oleophilic polymer chain. The resulting dispersant PMAs (d-PMAs) are useful in lubricants since they can suspend in solution what might otherwise be harmful materials ranging from highly oxidized small molecules to soot particles. PMAs have also been used in a number of other petroleum-based applications albeit in relatively minor volumes. An abbreviated list would include asphalt modifiers, grease thickeners, demulsifiers, emulsifiers, antifoamants, and crude oil flow improvers. PMAs have been present in lubricants for about 65 years now, and their longevity stems from the flexibility of PMA chemistry in terms of composition and process. Evolution of the original lauryl methacrylate composition to include various alkyl methacrylates and nonmethacrylates has brought additional functionality and an expanded list of applications. Process chemistry has also evolved such that it can produce polymers of almost any desired molecular weights (shear stability) or allow the synthesis of complex polymer architectures. The evolution of efficient processes for controlled radical polymerization in the 1990s has led to the development of taper and block copolymers and has permitted the development of products with narrower molecular weight distribution.

11.2 11.2.1

CHEMISTRY GENERAL PRODUCT STRUCTURE

Typically, a methacrylate VII is a linear polymer constructed from three classes (three distinct lengths) of hydrocarbon side chains. These would be short, intermediate, and long-chain lengths. A more extensive discussion is given in Section 11.3, but an abbreviated description is given here to better understand the synthesis and chemistry of methacrylate monomers and polymers. The first class is short-chain alkyl methacrylate of one to seven carbons in length. The inclusion of such short-chain materials influences polymer coil size particularly at colder temperatures and thus influences the viscosity index of the polymer in oil solutions. The intermediate class contains 8–13 carbons, and these serve to give the polymer its solubility in hydrocarbon solutions. The longchain class contains 14 or more carbons and is included to interact with wax during its crystallization and thus provide pour point depressing properties.

318

Lubricant Additives: Chemistry and Applications CH3 CH3

(

H2C

)x H

H2C

C C

FIGURE 11.1 Generalized structure of polyalkylmethacrylate.

FIGURE 11.2 Alkyl methacrylate monomer.

The structure of PMAs used as PPDs differs from that of a VII by virtue of normally containing only two of these sets of components. These are the long-chain, wax crystallization interactive materials, and intermediate chain lengths. The selected monomers are mixed together in a specific ratio to provide an overall balance of the aforementioned properties. This mixture is then polymerized to provide a copolymer structure in which R represents different alkyl groups and x indicates various degrees of polymerization. The simplified structure is shown in Figure 11.1.

11.2.2

MONOMER CHEMISTRY

Before discussing lubricant additives based on PMA, it is necessary to give an introduction to the chemistry of their parent monomers. The basic structure of a methacrylate monomer is shown in Figure 11.2. The four salient features of this vinyl compound are as follow: 1. The carbon–carbon double bond that is the reactive site in addition polymerization reactions. 2. The ester functionality adjacent to the double bond, which polarizes and thus activates the double bond in polymerization reactions. 3. The pendent side chain attached to the ester (designated as R). These chains may range from an all-hydrocarbon chain to a more complex structure containing heteroatoms. A significant portion of the beneficial properties of PMAs is derived from the pendant side chain. 4. The pendant methyl group adjacent to the double bond, which serves to shield the ester group from chemical attack, particularly as it relates to hydrolytic stability. As mentioned earlier, various methacrylate monomers, differing by length of the pendant side chains, are normally used to construct PMA additives. The synthesis chemistry of these monomers falls into two categories: shorter chains with four or fewer carbons and longer chains with five or more carbons. The commercial processes used to prepare each type are quite different. The short-chain monomers are often mass produced because of their usefulness in applications other than lubricant additives. For instance, methyl methacrylate is produced in large volumes and used primarily in the production of Plexiglas® acrylic plastic sheet and as a component in emulsion paints and adhesives. It is also used in PMA lubricant additives, but the volumes in this application pale in comparison with its use in other product areas. Methyl methacrylate is generally produced by either of two synthetic routes. The more prevalent starts with acetone, then proceeds through its conversion to acetone cyanohydrin, followed by its hydrolysis and esterification. The other route is oxidation of butylenes followed by subsequent hydrolysis and esterification. Recently, additional processes for methyl methacrylate have been introduced based on propylene and propyne chemistry, but their use remains comparatively small.

Polymethacrylate Viscosity Modiﬁers and Pour Point Depressants CH3 ROH + H2C

CH3 H2C

C C

H2C

CH3

H2C C

OCH3

FIGURE 11.4

Direct esterification of methacrylic acid and alcohol.

CH3 ROH

H2O

FIGURE 11.3

319

CH3OH

Transesterification of methyl methacrylate and alcohol.

The long-chain monomers are typically but not exclusively used in lubricant additives and can be produced by either of two commercial processes. The first is direct esterification of an appropriate alcohol with methacrylic acid. This well-known reaction is often used as a laboratory model of chemical strategies used to efficiently drive a reaction to high yield. These strategies involve a catalyst, usually an acid; an excess of one reagent to shift the equilibrium to product; and removal of at least one of the products, typically water of esterification, again to shift the equilibrium. The relevant chemical equation is given in Figure 11.3. A second commercial route to longer-side-chain methacrylate monomers is transesterification of methyl methacrylate with an appropriate alcohol. The reaction employs a basic compound or a Lewis base as a catalyst. The equilibrium is shifted to product by use of an excess of methyl methacrylate and by removal of a reaction product, that is, methanol (if methyl methacrylate is used as a reactant). Figure 11.4 shows the reaction equation.

11.2.3

TRADITIONAL POLYMER CHEMISTRY

A combination of alkyl methacrylate monomers chosen for a given product is mixed together in specific ratios and then polymerized by a solution, free radical–initiated addition polymerization process that produces a random copolymer. The reaction follows the classic pathways and techniques of addition polymerization to produce commercial materials [5]. Commercial polymers are currently synthesized through the use of free radical initiators. The initiator may be from either oxygen or nitrogen-based families of thermally unstable compounds that decompose to yield two free radicals. The oxygen-based initiators, that is, peroxides, hydroperoxides, peresters, or other compounds containing an oxygen–oxygen covalent bond, thermally decompose through hom*olytic cleavage to form two oxygen-centered free radicals. Nitrogen-based initiators also thermally decompose to form two free radicals, but these materials quickly evolve a mole of nitrogen gas and thus form carbon-centered radicals. In any event, the free radicals attack the less hindered, relatively positive side of the methacrylate vinyl double bond. These two reactions are the classic initiation and propagation steps of free radical addition polymerization and are shown in Figures 11.5 and 11.6. The reaction temperature is chosen in concert with the initiator’s half-life and may range from 60 to 140°C. Generally, a temperature–initiator combination would be selected to provide an economic, facile conversion of monomer to polymer and avoid potential side reactions. Other temperature-dependent factors are taken into consideration. Chief among these might be a need to maintain a reasonable viscosity of the polymer in the reactor as it is being synthesized. Obviously,

320

Lubricant Additives: Chemistry and Applications CH3 CH3 H2 C

C C

H2C

FIGURE 11.5

Free radical initiation of methacrylate polymerization. CH3

CH3 CH3 I

H2C

+ X

H2C

H2 C

)x H O

Monomer addition—propagation step. CH3

CH3 RSH + R′

(

H2C

C C

R′

(

H2 C

C C

FIGURE 11.7

(

FIGURE 11.6

)x H

+ RS

Termination by chain transfer.

the temperature can be utilized (as well as solvent) to maintain viscosity at a level appropriate for the mechanical agitation and pumping systems within a production unit. Excessive temperatures must be avoided to avoid the ceiling temperature of the polymerization, which is the temperature where the depolymerization reaction commences (see Section 11.3.1.2). Normally, a mixture of alkyl methacrylate monomers is used to produce a random copolymer. No special reaction techniques are needed to avoid composition drift over the course of the reaction since reactivity ratios of alkyl methacrylates are quite similar [6]. The most important concern during a synthesis reaction is to provide polymer at a given molecular weight so as to produce commercial product of suitable shear stability. As normal for vinyl addition polymerizations, methacrylates can undergo the usual termination reactions: combination, disproportionation, and chain transfer. Chain transfer agents (CTA), often mercaptans, are the most commonly chosen strategy to control molecular weight. Selection of the type and amount of CTA must be done carefully and with an understanding that many other factors influence molecular weight. Numerous factors can impact the degree of polymerization: initiator concentration, radical flux, solvent concentration, and opportunistic chain transfer with compounds other than the CTA. An undesirable opportunistic chain transfer possibility is hydrogen abstraction at random sites along the polymer chain leading to branched polymers that are less efficient thickeners than strictly linear chains. The mercaptan chain transfer reaction is shown in Figure 11.7. In addition to chain transfer, the other usual termination reactions of chain combination or disproportionation can occur with methacrylates. Commercial products cover a broad range of polymer molecular weights ranging from ∼20,000 to ∼750,000 Da. Molecular weight is carefully controlled and targeted to produce products that achieve suitable shear stability for a given application.

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321

Higher-molecular-weight PMAs are rather difficult to handle as neat polymers; hence, it is necessary in almost all commercial cases to use a solvent to reduce viscosity to levels consistent with reasonable handling properties. Additionally, it is important to maintain reasonable viscosity during the polymerization reaction (although it always increases as monomer to polymer conversion increases), so that sufficient agitation can be maintained. Thus, solvents are almost invariably employed. An appropriate solvent would (1) be nonreactive, (2) be nonvolatile (at least at the reaction temperature), (3) avoid chain transfer reactions, and (4) be consistent with the intended application of the resulting product. It turns out that mineral oil meets the aforementioned criteria reasonably well, so that a solvent choice can be made from higher-quality, lower-viscosity-grade mineral oils. Nonreactivity demands relatively higher saturate contents; hence, better quality American Petroleum Institute (API) Group I (or higher API group) mineral oil can be used. Choice of solvent viscosity primarily depends on the end application; choices range from very-low-viscosity oils of 35 SUS to light neutrals typically up to 150N. Alternatively, one can use a nonreactive but volatile solvent when mineral oil might interfere with a sensitive polymerization and then do a solvent exchange into a more suitable carrier oil. The amount of solvent added to commercial PMA VIIs is sufficient to reduce viscosity to levels consistent with reasonable handling or container pump out properties. This amount is dictated by polymer molecular weight, as this also heavily influences product viscosity. Generally, a higher-molecular-weight polymer requires more solvent. Commercial products may thus contain polymer concentrations over a very broad range of ∼30 to 80 wt%. d-PMAs were fi rst described by Catlin in a 1956 patent [7]. The patent claims the incorporation of diethylaminoethyl methacrylate as a way of enhancing the dispersancy of VIIs and thus providing improved deposit performance in engine tests of that era. The original dispersant methacrylate polymers utilized monomers that copolymerized readily with alkyl methacrylates and did not require different polymerization chemistry. Beyond these original random polymerizations, grafting is also an important synthetic route to incorporate desirable polar monomers onto methacrylate polymers. Stambaugh [8] identified grafting of N-vinylpyrrolidinone onto a PMA substrate as a route to improved dispersancy of VIIs. Another approach is to graft both N-vinylpyrrolidinone and N-vinyl imidazole [9]. An obvious benefit of grafting is an ability to incorporate polar monomers that do not readily copolymerize with methacrylates due to significant differences in reactivity ratios. Grafting reactions are carried out after achieving high conversion of the alkyl methacrylates to polymer. Bauer [10] identified an alternate synthetic route to incorporate dispersant functionality by providing reactive sites in the base polymer and then carry out a postpolymerization reaction. For example, maleic anhydride copolymerized into or grafted onto the polymer backbone can be reacted with compounds containing desirable chemical functionality such as amines. This strategy is a route to incorporate compounds that are otherwise not susceptible to addition polymerization because they lack a reactive double bond. This discussion characterizes most of the chemistry used to prepare the great majority of commercial PMA products. Additional chemical strategies as well as some novel processes and polymer blend strategies are reviewed in the literature and in the patent section below.

11.2.4

PATENT REVIEW

A review of pertinent literature and patents shows PMAs to have been the subject of numerous investigations over the course of years. A huge body of PMA patent literature exists, and a large subset of it is related to lubricant additives. A summary from the additive-related patents suggests five major areas of investigation, which can be categorized as variation of polymer composition; incorporation of functionality to enhance properties other than rheology, that is, dispersancy; improved processes to improve economics or enhance a performance property; polymer blends to provide unique properties; and finally, polymer architecture. A brief discussion of only a few of the more important patents within the five categories ensues.

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Lubricant Additives: Chemistry and Applications

Since the first PMA patents [2.3], there has been a continuing search for composition modifications to methacrylate polymers to improve some aspects of rheological performance. As expected, much of the earlier work explored uses of various alkyl methacrylate monomers and examined the ratios of one to another; this work is part of the well-established art. But, even more modern patent literature includes teachings about PMA compositions. For instance, highly polar PMA compositions made with high concentrations of short-chain alkyl methacrylates are useful in polar synthetic fluids such as phosphate esters to impart rheological advantages [11]. There are numerous examples of incorporating nonmethacrylates into polymers; a good example is the use of styrene [12] as a comonomer to impart improved shear stability. However, styrenic monomers have different reactivity ratios than methacrylates, and the usual processes lead to relatively low conversions of styrene. This can be overcome by a process utilizing additional amounts of methacrylate monomers near the end of the process to drive the styrene to high conversion [13]. Incorporation of functional monomers to make dispersant versions of PMA has been discussed in the preceding section on chemistry. Despite the well-known nature of d-PMA, it remains an area of active research as exemplified in Ref. 14, which describes a dispersant for modern diesel engine oil soot. Although nitrogen-based dispersants are the focus of much research, oxygen-based dispersants such as hydroxyethyl methacrylate [15] and ether-containing methacrylates [16] have also been claimed. In addition to incorporating dispersant functionality, significant efforts to incorporate other types of chemical functionality such as antioxidant moieties [17] have been made. Novel processes have been developed to improve either economics or product properties. Tight control of molecular-weight distribution and degree of polymerization can be achieved through constant feedback of conversion information to a computer control system that adjusts monomer and initiator feeds as well as temperature [18]. Coordinated polymerizations are useful in preparing alternating copolymers of methacrylates with other vinyl monomers [19]. A process has been described to prepare continuously variable compositions, which can obviate the need to physically blend polymers [20]. Polymer blends of PMA and olefin copolymer (OCP) VIIs provide properties intermediate to the individual products with OCP imparting efficient thickening (and economics) and PMA imparting good low-temperature rheology. However, a physical mixture of the two VIIs in concentrated form is incompatible. This problem is overcome by using a compatibilizer, actually a graft polymer of PMA to OCP, to make an ∼70% PMA and ∼30% OCP mixture compatible [21]. Very-high-polymer content products can be prepared by emulsifying the mixture, so that the PMA phase is continuous in a slightly polar solvent, whereas the normally very viscous OCP phase is in micelles [22,23]. Blends of PMAs can provide synergistic thickening and pour depressing properties [24]. PMA polymer architecture is being very actively investigated today. Preparations of PMA blocks, stars, combs, and narrow MWD polymers are all the subject of relatively recent patents or patent applications. For instance, the newer polymerization technique of controlled radical polymerization (CRP), specifically atom transfer radical polymerization (ATRP), has been used to prepare PMAs of very narrow molecular weight distribution to improve the thickening efficiency/shear stability balance of the resulting product [25]. Similarly, a CRP nitroxide-mediated polymerization (NMP) has been described [26] as providing products with similar improved properties. The ATRP technique has been used to prepare PMAs with functional (polar) monomers in blocks to enhance physical attraction to metal surfaces and thus improve frictional properties under low-speed conditions [27,28]. Star-shaped PMAs made through CRP processes including reversible addition–fragmentation chain transfer (RAFT) polymerization, NMP, and ATRP have been described as providing improved solution properties [29]. Star shapes and other polymer architectures through various CRP processes are described [30,31] as having enhanced thickening efficiency/shear stability and VI contribution relative to the more traditional linear polymers. Another new polymer architecture of interest is comb polymers with polyolefin and PMA elements as described in Ref. 32; the same enhancement of properties as mentioned previously applies to these structures.

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11.3 PROPERTIES AND PERFORMANCE CHARACTERISTICS 11.3.1

CHEMICAL PROPERTIES

PMAs are rather stable materials and do not normally undergo chemical reactions under moderate to even relatively severe conditions. The chemical design of any VII or PPD clearly entails avoiding reactive sites in their structure to provide as high a degree of stability as possible in the harsh environments to which lubricants are exposed. It is expected that these PMA addi

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