Lubricant AdditivesChemistry and ApplicationsSecond EditionCRC_59645_FM.indd i 11/12/2008 7:53:38 PMCHEMICAL INDUSTRIESA Series of Reference Books and TextbooksFounding EditorHEINZ HEINEMANNBerkeley, CaliforniaSeries EditorJAMES G. SPEIGHTUniversity of Trinidad and TobagoO'Meara Campus, Trinidad1. Fluid Catalytic Cracking with Zeolite Catalysts, Paul B. Venuto and E. Thomas Habib, Jr.2. Ethylene: Keystone to the Petrochemical Industry, Ludwig Kniel, Olaf Winter,and Karl Stork3. The Chemistry and Technology of Petroleum, James G. Speight4. The Desulfurization of Heavy Oils and Residua, James G. Speight5. Catalysis of Organic Reactions, edited by William R. Moser6. Acetylene-Based Chemicals from Coal and Other Natural Resources,Robert J. Tedeschi7. Chemically Resistant Masonry, Walter Lee Sheppard, Jr.8. Compressors and Expanders: Selection and Application for the ProcessIndustry, Heinz P. Bloch, Joseph A. Cameron, Frank M. Danowski, Jr., Ralph James, Jr., Judson S. Swearingen, and Marilyn E. Weightman9. Metering Pumps: Selection and Application, James P. Poynton10. Hydrocarbons from Methanol, Clarence D. Chang11. Form Flotation: Theory and Applications, Ann N. Clarke and David J. Wilson12. The Chemistry and Technology of Coal, James G. Speight13. Pneumatic and Hydraulic Conveying of Solids, O. A. Williams14. Catalyst Manufacture: Laboratory and Commercial Preparations,Alvin B. Stiles15. Characterization of Heterogeneous Catalysts, edited by Francis Delannay16. BASIC Programs for Chemical Engineering Design, James H. Weber17. Catalyst Poisoning, L. Louis Hegedus and Robert W. McCabe18. Catalysis of Organic Reactions, edited by John R. Kosak19. Adsorption Technology: A Step-by-Step Approach to Process Evaluation and Application, edited by Frank L. Slejko20. Deactivation and Poisoning of Catalysts, edited by Jacques Oudar and Henry WiseCRC_59645_FM.indd ii 11/12/2008 7:53:38 PM21. 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. Somorjai22. Catalysis of Organic Reactions, edited by Robert L. Augustine23. Modern Control Techniques for the Processing Industries, T. H. Tsai, J. W. Lane, and C. S. Lin24. Temperature-Programmed Reduction for Solid Materials Characterization, Alan Jones and Brian McNichol25. Catalytic Cracking: Catalysts, Chemistry, and Kinetics,Bohdan W. Wojciechowski and Avelino Corma26. Chemical Reaction and Reactor Engineering, edited by J. J. Carberry and A. Varma27. Filtration: Principles and Practices: Second Edition, edited by Michael J. Matteson and Clyde Orr28. Corrosion Mechanisms, edited by Florian Mansfeld29. Catalysis and Surface Properties of Liquid Metals and Alloys, Yoshisada Ogino30. Catalyst Deactivation, edited by Eugene E. Petersen and Alexis T. Bell31. Hydrogen Effects in Catalysis: Fundamentals and Practical Applications, edited by Zoltn Pal and P. G. Menon32. Flow Management for Engineers and Scientists, Nicholas P. Cheremisinoff and Paul N. Cheremisinoff33. Catalysis of Organic Reactions, edited by Paul N. Rylander, Harold Greenfield,and Robert L. Augustine34. Powder and Bulk Solids Handling Processes: Instrumentation and Control,Koichi Iinoya, Hiroaki Masuda, and Kinnosuke Watanabe35. Reverse Osmosis Technology: Applications for High-Purity-Water Production,edited by Bipin S. Parekh36. Shape Selective Catalysis in Industrial Applications, N. Y. Chen, William E. Garwood, and Frank G. Dwyer37. Alpha Olefins Applications Handbook, edited by George R. Lappin and Joseph L. Sauer38. Process Modeling and Control in Chemical Industries, edited by Kaddour Najim39. Clathrate Hydrates of Natural Gases, E. Dendy Sloan, Jr.40. Catalysis of Organic Reactions, edited by Dale W. Blackburn41. Fuel Science and Technology Handbook, edited by James G. Speight42. Octane-Enhancing Zeolitic FCC Catalysts, Julius Scherzer43. Oxygen in Catalysis, Adam Bielanski and Jerzy Haber44. The Chemistry and Technology of Petroleum: Second Edition, Revised and Expanded, James G. Speight45. Industrial Drying Equipment: Selection and Application, C. M. vant Land46. Novel Production Methods for Ethylene, Light Hydrocarbons, and Aromatics,edited by Lyle F. Albright, Billy L. Crynes, and Siegfried Nowak47. Catalysis of Organic Reactions, edited by William E. Pascoe48. Synthetic Lubricants and High-Performance Functional Fluids, edited byRonald L. Shubkin49. Acetic Acid and Its Derivatives, edited by Victor H. Agreda and Joseph R. Zoeller50. Properties and Applications of Perovskite-Type Oxides, edited by L. G. Tejucaand J. L. G. Fierro51. Computer-Aided Design of Catalysts, edited by E. Robert Becker and Carmo J. PereiraCRC_59645_FM.indd iii 11/12/2008 7:53:39 PM52. Models for Thermodynamic and Phase Equilibria Calculations, edited byStanley I. Sandler53. Catalysis of Organic Reactions, edited by John R. Kosak and Thomas A. Johnson54. Composition and Analysis of Heavy Petroleum Fractions, Klaus H. Altgelt and Mieczyslaw M. Boduszynski55. NMR Techniques in Catalysis, edited by Alexis T. Bell and Alexander Pines56. Upgrading Petroleum Residues and Heavy Oils, Murray R. Gray57. Methanol Production and Use, edited by Wu-Hsun Cheng and Harold H. Kung58. Catalytic Hydroprocessing of Petroleum and Distillates, edited by Michael C. Oballah and Stuart S. Shih59. The Chemistry and Technology of Coal: Second Edition, Revised and Expanded, James G. Speight60. 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 Jos M. Parera62. Catalysis of Organic Reactions, edited by Mike G. Scaros and Michael L. Prunier63. Catalyst Manufacture, Alvin B. Stiles and Theodore A. Koch64. Handbook of Grignard Reagents, edited by Gary S. Silverman and Philip E. Rakita65. Shape Selective Catalysis in Industrial Applications: Second Edition, Revised and Expanded, N. Y. Chen, William E. Garwood, and Francis G. Dwyer66. Hydrocracking Science and Technology, Julius Scherzer and A. J. Gruia67. Hydrotreating Technology for Pollution Control: Catalysts, Catalysis, and Processes, edited by Mario L. Occelli and Russell Chianelli68. Catalysis of Organic Reactions, edited by Russell E. Malz, Jr.69. Synthesis of Porous Materials: Zeolites, Clays, and Nanostructures, edited byMario L. Occelli and Henri Kessler70. Methane and Its Derivatives, Sunggyu Lee71. Structured Catalysts and Reactors, edited by Andrzej Cybulski and Jacob A. Moulijn72. Industrial Gases in Petrochemical Processing, Harold Gunardson73. 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 OConnor75. Catalysis of Organic Reactions, edited by Frank E. Herkes76. The Chemistry and Technology of Petroleum: Third Edition, Revised and Expanded, James G. Speight77. Synthetic Lubricants and High-Performance Functional Fluids: Second Edition,Revised and Expanded, Leslie R. Rudnick and Ronald L. Shubkin78. The Desulfurization of Heavy Oils and Residua, Second Edition, Revisedand Expanded, James G. Speight79. Reaction Kinetics and Reactor Design: Second Edition, Revised and Expanded,John B. Butt80. Regulatory Chemicals Handbook, Jennifer M. Spero, Bella Devito, and Louis Theodore81. Applied Parameter Estimation for Chemical Engineers, Peter Englezos and Nicolas Kalogerakis82. Catalysis of Organic Reactions, edited by Michael E. Ford83. The Chemical Process Industries Infrastructure: Function and Economics,James R. Couper, O. Thomas Beasley, and W. Roy Penney84. Transport Phenomena Fundamentals, Joel L. PlawskyCRC_59645_FM.indd iv 11/12/2008 7:53:39 PM85. Petroleum Refining Processes, James G. Speight and Baki zm86. Health, Safety, and Accident Management in the Chemical Process Industries,Ann Marie Flynn and Louis Theodore87. Plantwide Dynamic Simulators in Chemical Processing and Control,William L. Luyben88. Chemical Reactor Design, Peter Harriott89. Catalysis of Organic Reactions, edited by Dennis G. Morrell90. Lubricant Additives: Chemistry and Applications, edited by Leslie R. Rudnick91. Handbook of Fluidization and Fluid-Particle Systems, edited by Wen-Ching Yang92. Conservation Equations and Modeling of Chemical and Biochemical Processes,Said S. E. H. Elnashaie and Parag Garhyan93. Batch Fermentation: Modeling, Monitoring, and Control, Ali inar, Glnur Birol,Satish J. Parulekar, and Cenk ndey94. Industrial Solvents Handbook, Second Edition, Nicholas P. Cheremisinoff95. Petroleum and Gas Field Processing, H. K. Abdel-Aal, Mohamed Aggour, and M. Fahim96. Chemical Process Engineering: Design and Economics, Harry Silla97. Process Engineering Economics, James R. Couper98. Re-Engineering the Chemical Processing Plant: Process Intensification,edited by Andrzej Stankiewicz and Jacob A. Moulijn99. Thermodynamic Cycles: Computer-Aided Design and Optimization, Chih Wu100. Catalytic Naphtha Reforming: Second Edition, Revised and Expanded,edited by George T. Antos and Abdullah M. Aitani101. Handbook of MTBE and Other Gasoline Oxygenates, edited by S. Halim Hamidand Mohammad Ashraf Ali102. Industrial Chemical Cresols and Downstream Derivatives,Asim Kumar Mukhopadhyay103. Polymer Processing Instabilities: Control and Understanding, edited by Savvas Hatzikiriakos and Kalman B. Migler104. Catalysis of Organic Reactions, John Sowa105. Gasification Technologies: A Primer for Engineers and Scientists, edited byJohn Rezaiyan and Nicholas P. Cheremisinoff106. Batch Processes, edited by Ekaterini Korovessi and Andreas A. Linninger107. Introduction to Process Control, Jose A. Romagnoli and Ahmet Palazoglu108. Metal Oxides: Chemistry and Applications, edited by J. L. G. Fierro109. Molecular Modeling in Heavy Hydrocarbon Conversions, Michael T. Klein,Ralph J. Bertolacini, Linda J. Broadbelt, Ankush Kumar and Gang Hou110. Structured Catalysts and Reactors, Second Edition, edited by Andrzej Cybulskiand Jacob A. Moulijn111. Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology,edited by Leslie R. Rudnick112. Alcoholic Fuels, edited by Shelley Minteer113. Bubbles, Drops, and Particles in Non-Newtonian Fluids, Second Edition,R. P. Chhabra114. The Chemistry and Technology of Petroleum, Fourth Edition,James G. Speight115. Catalysis of Organic Reactions, edited by Stephen R. Schmidt116. Process Chemistry of Lubricant Base Stocks, Thomas R. Lynch117. Hydroprocessing of Heavy Oils and Residua, edited by James G. Speight and Jorge Ancheyta118. Chemical Process Performance Evaluation, Ali Cinar, Ahmet Palazoglu, and Ferhan KayihanCRC_59645_FM.indd v 11/12/2008 7:53:39 PM119. Clathrate Hydrates of Natural Gases, Third Edition, E. Dendy Sloan and Carolyn Koh120. Interfacial Properties of Petroleum Products, Lilianna Z. Pillon121. Process Chemistry of Petroleum Macromolecules, Irwin A. Wiehe122. The Scientist or Engineer as an Expert Witness, James G. Speight123. Catalysis of Organic Reactions, edited by Michael L. Prunier124. Lubricant Additives: Chemistry and Applications, Second Edition, edited byLeslie R. RudnickCRC_59645_FM.indd vi 11/12/2008 7:53:39 PMEdited byLeslie R. RudnickDesigned Materials GroupWilmington, Delaware, U.S.A.Lubricant AdditivesChemistry and ApplicationsSecond EditionCRC Press is an imprint of theTaylor & Francis Group, an informa businessBoca Raton London New YorkCRC_59645_FM.indd vii 11/12/2008 7:53:39 PMCRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa businessNo claim to original U.S. Government worksPrinted in the United States of America on acid-free paper10 9 8 7 6 5 4 3 2 1International 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 valid-ity 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 uti-lized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopy-ing, 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 orga-nizations 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 DataLubricant 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 2008621.89--dc22 2008034106Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.comand the CRC Press Web site athttp://www.crcpress.comCRC_59645_FM.indd viii 11/12/2008 7:53:40 PMixContentsPreface ........................................................................................................................................... xiiiContributors ..................................................................................................................................... xvPART 1 Deposit Control AdditivesChapter 1 Antioxidants ................................................................................................................. 3Jun Dong and Cyril A. MigdalChapter 2 Zinc Dithiophosphates ............................................................................................... 51Randolf A. McDonaldChapter 3 Ashless PhosphorusContaining Lubricating Oil Additives ..................................... 63W. David PhillipsChapter 4 Detergents ................................................................................................................. 123Syed Q. A. RizviChapter 5 Dispersants ............................................................................................................... 143Syed Q. A. RizviPART 2 Film-Forming AdditivesChapter 6 Selection and Application of Solid Lubricants as Friction Modifers ...................... 173Gino MarianiChapter 7 Organic Friction Modifers ....................................................................................... 195Dick Kenbeck and Thomas F. BunemannPART 3 Antiwear Additives and Extreme-Pressure AdditivesChapter 8 Ashless Antiwear and Extreme-Pressure Additives ................................................. 213Liehpao Oscar FarngCRC_59645_FM.indd ix 11/12/2008 7:53:40 PMx ContentsChapter 9 Sulfur Carriers ......................................................................................................... 251Thomas Rossrucker and Achim FessenbeckerPART 4 Viscosity Control AdditivesChapter 10 Olefn Copolymer Viscosity Modifers .................................................................... 283Michael J. CovitchChapter 11 Polymethacrylate Viscosity Modifers and Pour Point Depressants ........................ 315Bernard G. KinkerChapter 12 Pour Point Depressants............................................................................................. 339Joan SouchikPART 5 Miscellaneous AdditivesChapter 13 Tackifers and Antimisting Additives ...................................................................... 357Victor J. Levin, Robert J. Stepan, and Arkady I. LeonovChapter 14 Seal Swell Additives ................................................................................................. 377Ronald E. Zielinski and Christa M. A. ChilsonChapter 15 Antimicrobial Additives for Metalworking Lubricants ........................................... 383William R. Schwingel and Alan C. EachusChapter 16 Surfactants in Lubrication ........................................................................................ 399Girma BiresawChapter 17 Corrosion Inhibitors and Rust Preventatives ............................................................ 421Michael T. CostelloChapter 18 Additives for Bioderived and Biodegradable Lubricants ......................................... 445Mark MillerPART 6 ApplicationsChapter 19 Additives for Crankcase Lubricant Applications ..................................................... 457Ewa A. Bardasz and Gordon D. LambCRC_59645_FM.indd x 11/12/2008 7:53:40 PMContents xiChapter 20 Additives for Industrial Lubricant Applications ...................................................... 493Leslie R. RudnickChapter 21 Formulation Components for Incidental Food-Contact Lubricants ......................... 511Saurabh LawateChapter 22 Lubricants for the Disk Drive Industry .................................................................... 523Tom E. KarisChapter 23 Additives for Grease Applications ........................................................................... 585Robert Silverstein and Leslie R. RudnickPART 7 TrendsChapter 24 Long-Term Trends in Industrial Lubricant Additives .............................................. 609Fay Linn Lee and John W. HarrisChapter 25 Long-Term Additive Trends in Aerospace Applications .......................................... 637Carl E. Snyder, Lois J. Gschwender, and Shashi K. SharmaChapter 26 Eco Requirements for Lubricant Additives .............................................................. 647Tassilo Habereder, Danielle Moore, and Matthias LangPART 8 Methods and ResourcesChapter 27 Testing Methods for Additive/Lubricant Performance ............................................ 669Leslie R. RudnickChapter 28 Lubricant IndustryRelated Terms and Acronyms .................................................. 685Leslie R. RudnickChapter 29 Internet Resources for Additive/Lubricant Industry ................................................ 707Leslie R. RudnickIndex .............................................................................................................................................. 761CRC_59645_FM.indd xi 11/12/2008 7:53:40 PMCRC_59645_FM.indd xii 11/12/2008 7:53:40 PMxiiiPrefaceLubricant 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 fuids 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 Pacifc and Southeast Asia through either new manufacturing in that region or sales and distribution offces.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 encour-agement 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 RudnickCRC_59645_FM.indd xiii 11/12/2008 7:53:40 PMCRC_59645_FM.indd xiv 11/12/2008 7:53:40 PMxvContributorsEwa A. BardaszThe Lubrizol CorporationWickliffe, OhioGirma BiresawCereal Products and Food Science Research UnitNCAUR-MWA-ARS-USDAPeoria, IllinoisThomas F. BunemannUniqemaGouda, The NetherlandsChrista M. A. ChilsonPolyMod Technologies IncFort Wayne, IndianaMichael T. CostelloChemtura CorporationMiddlebury, ConnecticutMichael J. CovitchThe Lubrizol CorporationWicklife, OhioJun DongChemtura CorporationMiddlebury, ConnecticutAlan C. EachusIndependent ConsultantVilla Park, IllinoisLiehpao Oscar FarngExxonMobil Research and Engineering CompanyAnnandale, New JerseyAchim FessenbeckerRhein Chemie Rheinau GmbHMannheim, GermanyLois J. GschwenderAFRL/RXBTWright-Patterson Air Force Base, OhioTassilo HaberederCiba Specialty Chemicals Inc.Basel, SwitzerlandJohn W. HarrisShell Global SolutionsHouston, TexasTom E. KarisHitachi Global Storage TechnologiesSan Jose Research CenterSan Jose, CaliforniaDick KenbeckUniqemaGouda, The NetherlandsBernard G. KinkerDegussa, RohMax Oil Additives LPKintnersville, PennsylvaniaGordon D. LambLubrizol International LaboratoriesBelper, Derby, United KingdomMatthias LangProcess & Lubricant AdditivesCiba Specialty ChemicalsBasel, SwitzerlandSaurabh LawateThe Lubrizol CorporationWickliffe, OhioFay Linn LeeShell LubricantsHouston, TexasArkady I. LeonovUniversity of AkronAkron, OhioVictor J. LevinFunctional Products Inc.Macedonia, OhioCRC_59645_FM.indd xv 11/12/2008 7:53:40 PMxvi ContributorsGino MarianiAcheson Colloids CompanyPort Huron, MichiganRandolf A. McDonaldFunctional Products Inc.Cleveland, OhioCyril A. MigdalChemtura CorporationMiddlebury, ConnecticutMark MillerTerrasolve TechnologiesEastlake, OhioDanielle MooreCiba Specialty Chemicals plcProcess & Lubricant AdditivesMacclesfeld, Cheshire, United KingdomW. David PhillipsW. David Phillips and AssociatesStockport, Cheshire, United KingdomSyed Q. A. RizviSantovac FluidsSt. Charles, MissouriThomas RossruckerRhein Chemie Rheinau GmbHMannheim, GermanyLeslie R. RudnickDesigned Materials GroupWilmington, DelawareWilliam R. SchwingelMASCO Corporation R&DTaylor, MichiganShashi K. SharmaAFRL/RXBTWright-Patterson Air Force Base, OhioRobert SilversteinOrelube CorporationBellport, New YorkCarl E. SnyderAFRL/RXBTWright-Patterson Air Force Base, OhioJoan SouchikEvonikRohMax USA, Inc.Horsham, PennsylvaniaRobert J. StepanFunctional Products Inc.Macedonia, OhioRonald E. ZeilinskiPolyMod Technologies Inc.Fort Wayne, IndianaCRC_59645_FM.indd xvi 11/12/2008 7:53:41 PMPart 1Deposit Control AdditivesCRC_59645_S001.indd 1 10/20/2008 9:14:04 AMCRC_59645_S001.indd 2 10/20/2008 9:14:04 AM31AntioxidantsJun Dong and Cyril A. MigdalCONTENTS1.1 Introduction ............................................................................................................................... 41.2 Sulfur Compounds .................................................................................................................... 51.3 SulfurNitrogen Compounds .................................................................................................... 61.4 Phosphorus Compounds ............................................................................................................ 71.5 SulfurPhosphorus Compounds ............................................................................................... 81.6 Amine and Phenol Derivatives ............................................................................................... 101.6.1 Amine Derivatives ....................................................................................................... 101.6.2 Phenol Derivatives ....................................................................................................... 131.6.3 Amine and Phenol-Bearing Compounds ..................................................................... 131.6.4 Multifunctional Amine and Phenol Derivatives ......................................................... 131.7 Copper Antioxidants ............................................................................................................... 161.8 Boron Antioxidants ................................................................................................................. 171.9 Miscellaneous Organometallic Antioxidants ......................................................................... 181.10 Mechanisms of Hydrocarbon Oxidation and Antioxidant Action .......................................... 181.10.1 Autoxidation of Lubricating Oil ................................................................................ 191.10.1.1 Initiation ..................................................................................................... 191.10.1.2 Chain Propagation ...................................................................................... 191.10.1.3 Chain Branching ........................................................................................ 191.10.1.4 Chain Termination ..................................................................................... 201.10.2 Metal-Catalyzed Lubricant Degradation ................................................................... 201.10.2.1 Metal Catalysis ........................................................................................... 211.10.3 High-Temperature Lubricant Degradation ................................................................. 211.10.4 Effect of Base Stock Composition on Oxidative Stability ......................................... 211.10.5 Oxidation Inhibition ................................................................................................... 231.10.6 Mechanisms of Primary Antioxidants ....................................................................... 241.10.6.1 Hindered Phenolics .................................................................................... 241.10.6.2 Aromatic Amines ....................................................................................... 261.10.7 Mechanisms of Secondary Antioxidants ................................................................... 281.10.7.1 Organosulfur Compounds .......................................................................... 281.10.7.2 Organophosphorus Compounds ................................................................. 281.10.8 Antioxidant Synergism .............................................................................................. 291.11 Oxidation Bench Tests ............................................................................................................ 301.11.1 Thin-Film Oxidation Test .......................................................................................... 311.11.1.1 Pressurized Differential Scanning Calorimetry ........................................ 311.11.1.2 Thermal-Oxidation Engine Oil Simulation Test (ASTM D 6335; D 7097) ............................................................................ 311.11.1.3 Thin-Film Oxidation Uptake Test (ASTM D 4742) ................................... 33CRC_59645_Ch001.indd 3 12/4/2008 3:33:15 PM4 Lubricant Additives: Chemistry and Applications1.11.2 Bulk Oil Oxidation Test ............................................................................................. 331.11.2.1 Turbine Oil Stability Test (ASTM D 943, D 4310)..................................... 331.11.2.2 IP 48 Method .............................................................................................. 341.11.2.3 IP 280/CIGRE ............................................................................................ 341.11.3 Oxygen Update Test ................................................................................................... 341.11.3.1 Rotating Pressure Vessel Oxidation Test (ASTM D 2272) ........................ 341.12 Experimental Observations ..................................................................................................... 341.13 Antioxidant Performance with Base Stock Selection ............................................................. 371.14 Future Requirements ............................................................................................................... 381.15 Commercial Antioxidants ....................................................................................................... 391.16 Commercial Metal Deactivators ............................................................................................. 41References ........................................................................................................................................ 41 1.1 INTRODUCTIONWell 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 differ-ence was eventually identifed as naturally occurring antioxidants, which varied depending on crude source or refning 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 identifcation 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 identifed 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 identifed 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 functiona-lities 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 preven-tion of lubricant oxidation, antioxidants are the key additive that protects the lubricant from oxida-tive degradation, allowing the fuid 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 fuids, gear oils, turbine oils, compressor oils, greases, hydraulic fuids, and metal working fuids. The main classes include oil-soluble organic and organometallic antioxidants of the following types: 1. Sulfur compounds 2. Sulfurnitrogen compounds 3. Phosphorus compounds 4. Sulfurphosphorus compounds 5. Aromatic amine compounds 6. Hindered phenolic (HP) compounds 7. Organocopper compoundsCRC_59645_Ch001.indd 4 12/4/2008 3:33:16 PMAntioxidants 5 8. Boron compounds 9. Other organometallic compounds1.2 SULFUR COMPOUNDSThe 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 sulfde with a combined antioxidant and corrosion inhibition characteristics was developed by sulfurizing sperm oil [2]. Additives with similar func-tionalities could also be obtained from sulfurizing terpenes and polybutene [35]. Paraffn wax has also been employed to prepare sulfur compounds [69]. Theoretical structures of several sulfur compounds are illustrated in Figure 1.1. Actual compounds can be chemically complex in nature.Aromatic sulfdes represent another class of sulfur additives used as oxidation and corrosion inhibitors. Examples of simple sulfdes are dibenzyl sulfde and dixylyl disulfde. More complex compounds of a similar type are the alkyl phenol sulfdes [1015]. 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 disulfdes. As shown in Figure 1.1, the aromatic sulfdes such as benzyl sulfde have the sulfur attached to carbon atoms in the alkyl side groups, whereas the alkyl phenol sulfdes have the sulfur attached to carbon atoms in the aromatic rings. In general, the alkyl phenol sulfde chemistry appears to have superior antioxidant properties in many types of lubricants. Mono- and dialkyl-diphenyl sulfdes obtained by reacting diphenyl sulfde with C10C18 alpha-olefns 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-olefns, diesters, and polyol esters [15]. The hydroxyl groups of the alkyl phenol sulfdes may also be treated CH2CH2CCH2CHCCH3SCCH3CH2SSulfurized dipenteneSulfurized esterCH3(CH2)x CH CH (CH2)xSCH3CH3(CH2)x CH CH (CH2)xCH3Sulfurized olefinCH3(CH2)x COO (CH2)xCH3CH3(CH2)x C CH2(CH2)xCH3OCH CH (CH2)xSCH CH (CH2)xCH2S CH2Dibenzyl sulfideOHRHOR (S)xDialkylphenol sulfideSOSCH2FIGURE 1.1 Examples of sulfur-bearing antioxidants.CRC_59645_Ch001.indd 5 12/4/2008 3:33:17 PM6 Lubricant Additives: Chemistry and Applicationswith 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 cyclo-pentadiene dimer [16]. Heterocyclic compounds such as n-alkyl 2-thiazoline disulfde in combi-nation 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 auto-matic transmission fuids [18]. Novel antioxidant and antiwear additives based on dihydrobenzothio-phenes have been prepared through condensation of low-cost arylthiols and carbonyl compounds in a one-step high-yield process [19].1.3 SULFURNITROGEN COMPOUNDSThe dithiocarbamates were frst 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 sulfurnitrogen-bearing compounds being used as antioxi-dants, 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(dialkyldithiocarbamate) in combination with primary antioxidants such as arylamines or HPs and triazole deriva-tives is known to provide synergistic action in stabilizing mineral oils and synthetic lubricating oils [2426]. This material has been used to improve antioxidation characteristics of internal combus-tion engine oils containing low levels (5Industrial oils, eco-friendly oilsCRC_59645_Ch001.indd 14 12/4/2008 3:33:18 PMAntioxidants 153,5-Di-tert-butyl-4-hydroxyhydrocinnamic acid, C7C9 alkyl esterO C7C9OHOLiquid at 25C>5Engine oils, power transmission f uids, industrial oils3,5-Di-tert-butyl-4-hydrocinnamic acid, C13C15 alkyl esterHOOC13 C15OLiquid at 25C>5Engine oils, power transmission f uids, industrial oils4,4-Methylene bis(2,6-di-tert-butylphenol)CH2OHHO154NAEngine oils, industrial oils, food grade lubricants, greases2,2-Methylene bis(4-methyl-6-tert-butylphenol)OHCH2OH12825Engine oils, industrial oils, greases2-Propenoic acid, 3-[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-1,6-hexanediyl esterCHCHC OOHO2C6H12105150C. 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 fol-lows 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 (RRHCO) may decompose through reac-tion pathway 1.8 to form an aldehyde, and (d) a tertiary alkoxy radical (RRRCO) 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 vola-tility 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 R R R i i (1.10) R R OO ROOR i i (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 signifcantly 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 pro-cesses, formation of carbonyl compounds and alcohols may also take place on the peroxy radicals that contain an extractable -hydrogen atom: RR CHOO RR CHOOOOHCR RRR C O RR CH OHO2 i (1.12)1.10.2 METAL-CATALYZED LUBRICANT DEGRADATIONMetal 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.CRC_59645_Ch001.indd 20 12/4/2008 3:33:20 PMAntioxidants 211.10.2.1 Metal Catalysis1.10.2.1.1 Initiation Step M RH M H R(n 1) n i (1.13) M O M On2(n 1)2 (1.14)1.10.2.1.2 Propagation Step M ROOH M H ROO(n n 1) i (1.15) M ROOH M HO ROn (n 1)i (1.16)1.10.3 HIGH-TEMPERATURE LUBRICANT DEGRADATIONThe 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 (>120C), breakdown of peroxides including hydroperoxides becomes pre-dominant, and the resulting carbonyl compounds (e.g., reactions 1.8 and 1.9) will frst 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, ,-unsaturated aldehydes or ketones are formed, and fur-ther 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 identifed to be the principal oil-related factors to engine damages [188].1.10.4 EFFECT OF BASE STOCK COMPOSITION ON OXIDATIVE STABILITYMineral base stocks used to formulate lubricants are hydrocarbons that are originated from crude oils and essentially contain mixtures of n-paraffns along with isoparaffns, cycloparaffns (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 refnery techniques employed. In the American Petroleum Institute (API) base oil classifcation 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 OC ROC ROCOOROC OOHOC OH + O2ROOHROO O2 RHR2 RH 2 RFIGURE 1.6 High-temperature (>120C) lubricant degradation leading to the formation of carboxylic acids.CRC_59645_Ch001.indd 21 12/4/2008 3:33:20 PM22 Lubricant Additives: Chemistry and Applicationsare 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 refnery 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 oxida-tive stability of the oil. There have been quite extensive research activities attempting to establish correlations between base stock composition and oxidative stability [191195]. 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, paraffns are more stable than cycloparaffns. 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 FIGURE 1.7 High-temperature (>120C) lubricant degradation leading to the formation of high-molecular-weight hydrocarbons.OC (CH2)nR CHO OC OR H3COC (CH2)nCH R CHCOR + H2OOOC (CH2)p"R COR"'O OC(CH2)nCHR CHCOR'OOC C(CH2)n1CH R CHCOROC+(CH2)p"RAcidor baseCOR"'H2OOSludge precursors+TABLE 1.3API Base Oil CategoriesAPI Category Percent Saturates Percent Sulfur Viscosity IndexGroup I 90 0.03 80 and 120Group II 90 0.03 80 and 120Group III 90 0.03 120Group IV PAOsGroup V Includes all other base oils not included in the frst four groupsCRC_59645_Ch001.indd 22 12/4/2008 3:33:20 PMAntioxidants 23the 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 inhi-bition 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 165C over sulfur-free white oils and PAOs [145]. In hydrocracked oils that are essentially low in aromatics, better oxi-dative 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 pro-moting the acid-catalyzed rearrangement of arylalkyl hydroperoxides to form phenols that are antioxidants [145,179]. Contrary to sulfur, nitrogen-bearing compounds, especially the hetero-cyclic components (also called basic nitrogen), accelerate oil oxidation even at relatively low concentrations [197]. In highly refned 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 infuence the base oil oxidative stability [192,193]. It has been shown that oxida-tive 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 INHIBITIONThe 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 lubri-cant 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 deactiva-tor 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. Organo-sulfur 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 mech-anisms, metal deactivators for petroleum products can be classifed 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 metalhydrocarbon interaction. They can also minimize cor-rosive 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 deacti-vators an antioxidant effect. Table 1.4 lists examples of metal deactivators that are commonly found in lubricant formulations.CRC_59645_Ch001.indd 23 12/4/2008 3:33:20 PM24 Lubricant Additives: Chemistry and ApplicationsTABLE 1.4Metal Deactivators for LubricantsSurface Passivators Basic StructureTriazole derivativeNNNCH2NR2BenzotriazoleNNNH2-MercaptobenzothiazoleNSSHTolyltriazole derivative NNNR1R3R4CH2NChelatorsN,N-disalicylidene-1,2-diaminopropane HOOHCHCH2CHCH3NHN C1.10.6 MECHANISMS OF PRIMARY ANTIOXIDANTS1.10.6.1 Hindered PhenolicsA representative example of HP antioxidant is 3,5-di-t-butyl-4-hydroxytoluene (2,6-di-t-butyl-4-methylphenol), 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 hydro-carbons. The cyclohexadienone radical resonance structure can further combine with a second alkyl peroxy radical to form the alkyl peroxide, which is stable at temperatures > k1R +OHOk1k2ROO+ RHFIGURE 1.8 Reactivity of BHT with alkyl radical.OHCH3OCH3+ ROOH+ ROO+ ROOOH3C OOROH3C FIGURE 1.9 Hydrogen donation and peroxy radical trapping mechanisms of BHT.+OCH3OHCH3+OCH2CH3OFIGURE 1.10 Termination reaction of phenoxy radicals.CRC_59645_Ch001.indd 25 12/4/2008 3:33:21 PM26 Lubricant Additives: Chemistry and Applications1.10.6.2 Aromatic AminesA 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 radi-cal 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 (120C), the nitroxyl radical inter-mediate 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 FIGURE 1.14 Resonance structures of nitroxyl radical.NORNORRNOR RRFIGURE 1.15 High-temperature (>120C) function mechanism of ADPAs.ROOHROONHRNORNORCHRRNOHRRRCRROORRCRHN RRadicalsOC RRRRIICRRRCCRR CRC_59645_Ch001.indd 27 12/4/2008 3:33:22 PM28 Lubricant Additives: Chemistry and Applicationsradical is destroyed. It has been reported that such regeneration process can provide ADPAs with a stoichiometric effciency of more than 12 radicals per molecule [186].1.10.7 MECHANISMS OF SECONDARY ANTIOXIDANTS1.10.7.1 Organosulfur CompoundsOrganosulfur compounds function as hydroperoxide decomposers through the formation of oxi-dation and decomposition products. The mechanism is illustrated in Figure 1.16 for an alkyl sul-fde. The antioxidant action starts with the reduction of an alkyl hydroperoxides to a less reactive alcohol, with the sulfde 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, sulfninc 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 giv-ing the sulfur compound primary antioxidant characteristics: RSO H RSO ROOHxROOx 1.10.7.2 Organophosphorus CompoundsPhosphites are a main class of organophosphorus compounds being used as secondary anti-oxidants. Phosphites decompose hydroperoxides and peroxy radicals following the reaction R RROOHROHC R1R2H2CHOR RSOH + H2C=CR1R2ROOH ROHRSO2HROOH ROHRSO3HRH + SO2ROOH ROHROSO3HH2OH2SO4ROOHROORR2C=ODecompose more hydroperoxidesS SFIGURE 1.16 Antioxidation mechanism of alkyl sulfde.CRC_59645_Ch001.indd 28 12/4/2008 3:33:22 PMAntioxidants 29mechanisms. 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) P R OOH O R OH3 (RO) P3 (1.17) (RO) P3 R OO (RO) P O R O3 (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 antioxi-dants 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) homosynergism, (b) heterosynergism, and (c) autosynergism.Homosynergism 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 frst 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 effective-ness 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.O R P(OR)2RORO P(OR)2 + RRO P(OR)2O+ RROOOOFIGURE 1.17 Alkoxy and peroxy scavenging mechanisms of phenyl phosphite.CRC_59645_Ch001.indd 29 12/4/2008 3:33:22 PM30 Lubricant Additives: Chemistry and ApplicationsHeterosynergism occurs when antioxidants act by a different mechanism and hence comple-ment each other. This type of synergy usually takes place when a primary antioxidant and a sec-ondary 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 signifcantly 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 TESTSOxidative degradation of lubricants can be classifed into two main reactions: bulk oil oxidation and thin-flm 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 com-ponents, water, and other solids. Thin-flm oxidation describes a more rapid reaction in which a small amount of oil, usually in the form of a thin-flm coating on metal surface, is exposed to ele-vated temperatures and air (oxygen). Under these conditions, hydrocarbons decompose much more quickly and the polar oil oxidation products formed at the oilmetal 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 feld 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].NHR RN RRO , ROO ROH, ROOHOROHRRFIGURE 1.18 Mechanism of synergism between ADPA and hindered phenol.CRC_59645_Ch001.indd 30 12/4/2008 3:33:22 PMAntioxidants 31Owing 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 tempera-ture, use of catalyst, performance parameter, oxidation mechanism, and targeted oxidation stage, etc., makes it rather diffcult 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 specifc needs that have been proven to be advantageous in certain circumstances. The value of these tests should not be underestimated.1.11.1 THIN-FILM OXIDATION TEST1.11.1.1 Pressurized Differential Scanning CalorimetryDifferential 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 deposit-forming tendency of fve 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 lAutomobile (ACEA) E5 specifcation for heavy-duty diesel oils and has been incorporated in the current E7 specifcation. 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 modifcations 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 lubri-cants [217], and biodegradable oils [218,219]. Using PDSC to study the kinetics of base oil oxidation [220] and antioxidant structureperformance 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 characteris-tic of API SF quality engine oils under turbocharger operating conditions [222]. The original test conditions were specifed 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 fow past a tared depositor rod. The rod is resistively heated through 12 temperature cycles, each going from 200 to 480C for 9.5 min. After the heating cycle CRC_59645_Ch001.indd 31 12/4/2008 3:33:22 PM32 Lubricant Additives: Chemistry and ApplicationsTABLE 1.5Conditions of Oxidation Test MethodsTestTest DesignationOxidation RegimeTemperature(C)GasGas Flow or Initial PressureCatalystSample SizeEOTParameter MeasuredPDSCD 6186Thin f lm130, 155, 180, 210O2500 psi, 100 mL/mNone3.0 mgOccurrence of oxidation exothermOITPDSCCEC L-85Thin f lm210Air100 psi, no f owNone3.0 mg120 min maximumOITTEOST 33CD 6335Bulk 100, 200480N2O, moist air3.6 mL/minFe naphthenate116 mL12 Programmed cyclesDepositsTEOST MHTD 7097Thin f lm285Dry air10mL/mOil-soluble Fe, Pb, Sn 8.4 g24 hDeposits, volatileTFOUTD 4742O2 uptake, thin f lm160O290 psigFuel, naphthenates of Fe, Pb, Cu, Mg, and Sn, H2O1.5 gSharp pressure dropOITTOSTD 943, D 4310Bulk 95O23.0 L/hFe, Cu, H2O300 mLTAN = 2.0 1000 hTAN, sludge, metal weight lossIP48IP48Bulk200Air15 L/hNone40 mL6 h 2Viscosity, carbon residueIP 280/CIGREIP 280Bulk120O21 L/hCu, Fe naphthenates25 g164 hVolatile acids, oil acidity, sludgeRPVOTD 2272O2 uptake160O290 psigCu, H2O50 mLP = 25 psiOITCRC_59645_Ch001.indd 32 12/4/2008 3:33:22 PMAntioxidants 33is 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 simplifed procedure for the assessment of oil deposition tendency in the piston ring belt and under-crown areas of fred engines [224]. Thin-flm oxidation condition was thought to be predominant in these areas, and accordingly, the depositor assembly was revised to allow the oil fows down the rod in a slow and even manner to obtain a desired thin flm. To better refect the thermal-oxidative conditions of the engine zone of interest, a continuous depositor temperature of 285C together with modifed 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 specifcations 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 batch-to-batch variations in the oxidative stability of re-refned lubricating base stocks [227]. The test stresses a small amount of oil to 160C 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 cor-responds 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 modifcation 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 TEST1.11.2.1 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 fuids and circulating oils and in particular on those that are prone to water contamination in service. The test runs at relatively low temperature (95C) 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 profle, and gas, with minor differences in the test duration and target oxidation parameters to be moni-tored. 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 prod-ucts after 1000 h of thermal and oxidative stresses. The total amount of copper in oil, water, and sludge phases is also determined.A modifed TOST method that operates at higher temperature (120C) and in the absence of water has been proposed [232]. The procedure requires RPVOT as a monitoring tool and is spe-cifcally 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 CRC_59645_Ch001.indd 33 12/4/2008 3:33:23 PM34 Lubricant Additives: Chemistry and Applicationsantioxidants. 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 MethodThe 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 200C, along with air bubbling at 15 L/h, for two 6 h periods with a 1530 h standby period in between. Oil vis-cosity increase and the formation of carbon residue are determined after the oxidation. The test is con-sidered 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 assess-ment of engine oils using a modifed IP 48 method with four 6 h cycles has been reported [233,234].1.11.2.3 IP 280/CIGREThe IP 280, also known as the CIGRE test, was designed to assess the oxidative stability of inhib-ited 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 specifcations, 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 TEST1.11.3.1 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 fuids 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 150C 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 100C and to the combustion turbines that operate at much higher temperatures [238]. Owing to its sensitivity to specifc additive chemistries, RPVOT fnds limited use in comparing differ-ently formulated oils. In addition, the test is more suitable for the determination of remaining useful life of in-service turbine oils rather than the qualifcation 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 OBSERVATIONSThe following two experiments demonstrate (a) performance behaviors of aminic antioxidant ver-sus 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 frst 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 inhibi-tor, 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 C7C9 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 signifcantly outperformed the ADPA by CRC_59645_Ch001.indd 34 12/4/2008 3:33:23 PMAntioxidants 35providing 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-tempera-ture test conditions, the HP was superior to the ADPA. Proper mixing of the two additives produced synergy, in this case a homosynergism that led to the maximum protection.In the second experiment, a turbine oil formulated with an API group I base stock, a metal deac-tivator, 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 500100015042008251230163520402445285032553660406544704875528064Time (h)TAN (mgKOH/g)HP, GIADPA, GIHP + ADPA, GIHP + ADPA, GIVADPA, GIVHP, GIV3.02.52.01.51.00.50.0FIGURE 1.19 TOST results of turbine oils containing group I or IV base oil and 0.8 wt% of antioxidant.0100200300400500600700800HP HP + ADPA ADPARPVOT OIT (min)FIGURE 1.20 RPVOT results of turbine oil containing a group I base oil and 0.5 wt% of antioxidant.CRC_59645_Ch001.indd 35 12/4/2008 3:33:23 PM36 Lubricant Additives: Chemistry and Applicationstest temperature (150C), 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 beneft of antioxidant synergy for maximum oxi-dation 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 con-tained 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 confrm the superior per-formance of ADPA and further demonstrate the beneft 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 con-ditions. In contrast, ADPAs are excellent primary antioxidants for high-temperature conditions owing to their catalytic radical scavenging actions. The homosynergism facilitated between the FIGURE 1.21 TEOST results of a prototype PCMO containing a group II base stock and a total of 1.0 wt% of antioxidant.020406080100120140Baseline HP ADPA ADPA + HPDeposits (mg)CRC_59645_Ch001.indd 36 12/4/2008 3:33:23 PMAntioxidants 37ADPA and the HP is powerful in the inhibition of different stages of oil oxidation as demon-strated. 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 dif-fer 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 SELECTIONDriven 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 benefts, 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 FIGURE 1.22 RPVOT results of HP and ADPA in API groups IIV base oils.020040060080010001200140016001800API group I API group II API group III API group IVRPVOT OIT (min)No AOHPADPACRC_59645_Ch001.indd 37 12/4/2008 3:33:23 PM38 Lubricant Additives: Chemistry and Applications represents an API group from I to IV. The HP and the ADPA are the same as before. Clearly, with-out 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 refned 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 aggre-gations 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 circumstances. In those highly refned 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, alkyl-substituted aromatics, average chain length of hydrocarbons, and the relative presence of normal paraffns and isoparaffns [196]. In group I base oils, ZDDP gave good responses to highly saturated hydrocarbons characterized with normal paraffns having shorter chain length. Isoparaffns 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 REQUIREMENTSThe 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 specifc 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 econ-omy, 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 10C 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 specifcation 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-refned group I oils. The hydrotreating and isodewaxing processes that are used to make these oils signifcantly lower the unsaturated hydrocarbons and polyaromatics, which contribute to poor oxidative stability. CRC_59645_Ch001.indd 38 12/4/2008 3:33:24 PMAntioxidants 39 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