LUBRICANTS AND SPECIAL FLUIDS
TRIBOLOGY SERIES Advisory Editor: D. Dowson Editorial Board W. J. Bartz (Germany) R. Bassani (Italy) B. Briscoe (Gt. Britain) H. Czichos (Germany) K. Friedrich (Germany) N. Gane (Australia)
W. A. Glaeser (U.S.A.) M. Godet (France) H. E. Hintermann (Switzerland) K. C. Ludema (U.S.A.) T. Sakurai (Japan) W. 0. Winer (U.S.A.)
VOl. 1 Tribology - A Systems Approach to the Science and Technology of Friction, Lubrication and Wear (Czichos) VOl. 2 Impact Wear of Materials (Engel) VOl. 3 Tribology of Natural and Artificial Joints (Dumbleton) VOl. 4 Tribology of Thin Layers (Iliuc) VOl. 5 Surface Effects in Adhesion, Friction, Wear, and Lubrication (Buckley) Vol. 6 Friction and Wear of Polymers (Bartenev and Lavrentev) VOl. 7 Microscopic Aspects of Adhesion and Lubrication (Georges, Editor) VOl. 8 Industrial Tribology - The Practical Aspects of Friction, Lubrication and Wear (Jones and Scott, Editors) VOl. 9 Mechanics and Chemistry in Lubrication (Dorinson and Ludema) VOl. 10 Microstructure and Wear of Materials (Zum Gahr) VOl. 11 Fluid Film Lubrication - Osborne Reynolds Centenary (Dowson, Taylor, Godet and Berthe, Editors) VOl. 12 Interface Dynamics (Dowson, Taylor, Godet and Berthe, Editors) Vol. 13 Tribology of Miniature Systems (Rymuza) Vol. 14 Tribological Design of Machine Elements (Dowson, Taylor, Godet and Berthe, Editors) Vol. 15 Encyclopedia of Tribology (Kajdas, Harvey and Wilusz) Vol. 16 Tribology of Plastic Materials (Yamaguchi) Vol. 17 Mechanics of Coatings (Dowson, Taylor and Godet, Editors) VOl. 18 Vehicle Tribology (Dowson, Taylor and Godet, Editors) Vol. 19 Rheology and Elastohydrodynamic Lubrication (Jacobson) VOl. 20 Materials of Tribology (Glaeser) VOl. 21 Wear Particles: From the Cradle to the Grave (Godet, Dowson and Taylor, Editors) VOl. 22 Hydrostatic Lubrication (Bassani and Piccigallo) Vol. 23 Lubricants and Special Fluids (Stepina and Veself)
Tribology series, 23
LUBRICANTS AND SPECIAL FLUIDS Vaclav StGpina Research Institute of Fuels and Lubricants, Prague, Czecho-Slovakia (retired)
Vaclav Vesely Slovak Technical University, Bratislava, Czecho-Slovakia (retired)
ELSEVIER Amsterdam London New York Tokyo 1992
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Distribution of this book is handled by the following publishers: exclusive sales rights in the East European countries, Democratic Republic of Vietnam, Mongolian People's Republic, People's Republic of Korea, People's Republic of China, Republic of Kuba ALFA Publishers, Hurbanovo nim. 3. 815 89 Bratislava, CSFR in all remaining areas Elsevier Science Publishers Sara Burgerhartstraat 25 P. 0.Box 21 1, 1000 AE Amsterdam, The Netherlands
ISBN 0-444-98674-X 0 V. Stspina and V. Veselj, 1992
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, mechanical, photocopying, recording, or otherwise, without the prior written permission of the copyright owner
Printed in Czecho-Slovakia
CONTENTS
,
Foreword to the English Language Edition Acknowledgements
1
The Definition and Classification of Lubricants ............................................................... References .............................................................................................................................
1 7
2
The General Properties of Lubricants ...............................................................................
9
2.1 2.1.1 2. I .2 2.1.2.1 2.1.2.2 2. I .2.3
The Functional Properties of Lubricants .....
.............................................................
9 9 12 17 22
2.1.2.4 2.1.3 2.1.3. I 2.1.3.2 2.1.3.3 2.1.4 2. I .5 2.1.6 2.1.6. I 2.1.6.2 2. I .6.3 2.1.6.4 2.2 2.2. I 2.2.2 2.2.3 2.2.4 2.3 2.3. I 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.2.4 2.3.3 2.4 2.4.1
............................................................. .................................................................................... Viscosity - Temperature Relationships................................................................................. Viscosity - Pressure Relationships........................................................................................ The Relationship between Viscosity, Viscosity - Temperature Effects and Chemical
............................................................. 32 Relationships between Viscosities of Oils, Viscosity Indexes and Practical Applications . 35 Miscellaneous Rheological Properties .................................................................................. 37 Pseudo-Plastic Lubricants with Structural Viscosity ........................................................... 38 Quasi-Plastic Lubricants .................................................................................... Dilatant Lubricants ... .................................................................... 48 The Compressibility of Liquid Lubricants ................................................... Thermal Conductivity and Specific Heat ....................................................... Electrical Properties of Lubricants ............................................................... Electrical Conductivity ............... ..................................................................... 56 Electrical Strength .............................................. ............................................. 57 Dielectric Losses ....................................................................................................... 57 ......................................................................................... 59 Regions for the Applications of Lubricants .................60 Cloud-Point and Pour-Point ................................................................................................. 60 ............62 Vaporisation, Ignition and Explosion ................................................ Drop-Point and Threshold Strength Value of Lubricant Greases ..... The Low-Temperature Properties of Greases .................................... Service Life of Lubricants .................................................................................... Resistance to Oxidation ........................................................................................ 72 The Effect of Energy Ab ........................................................................................ 80 Thermal Stability ........................................................ The Effect of Light .......................................................................... The Effect of High-Energy Radiation .................................................................................. 86 ........................................... 90 The Effects of Electric Discharges and Electric Fields Resistance to Chemicals ............................................... Surface Properties ................................................................................................................ 92 Foam Formation in Oils ..................................................................................................... 101 V
2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.5 2.6
Atomisation of Oil Emulsions . ........................................................................................... The Solvent Power The Detergent and Rust and Corrosion Protection by Oils The Physiological Properties of Lubricants .......................................................................
3
Types of Lubricants and their Compositions
3.1 3.2 3.2.1 3.2.1. I 3.2.1.2 3.2.1.2.1 3.2.1.2.2 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.2.2.4 3.2.2.5 3.2.2.6 3.2.2.7 3.2.2.8 3.2.2.9 3.2.2.10 3.2.2.1 I 3.2.3 3.3 3.3.1 3.3.1.1 3.3.1.2 3.3.1.3 3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5 3.5
Gaseous Lubricants ............................................................................................................ 125 Liquid Lubricants ............................................................................................................... 126 Mineral Oils ..... Crude Oil Comp The Effects of Processing Techniques ............................................................................... 132 Separation Processes .......................................................................................................... 132 Refining Process ......................................... Synthetic Oils ... ........ Polyalkenes (Polyo .................................................................................................. 149 Aromatics and Cycloaliphatics .......................................................................................... 152 Fluorinated and Chlorinated Lubricants ............................................................................ 154 157 Polyalkylene Glycols and Polyalkyl Ethers ....................................................................... Polyphenyl Ethers and Polyphenyl Sulphides .... .................................... 161 Organic Esters .................................... 162 Phosphoric Acid Esters ...................................................................................................... 175 Aryl and Alkyl Esters of Silicic Acid ................................................................................ 178 ................... I80 Polysiloxanes . ............................................. 184 The Behaviour Miscellaneous Synthetic Oils ............................................................................................. 186 Inorganic Liquid Lubricants and Melts ............................................................................. 191 Lubricating Greases ........................................................................................................... 192 .................................. Composition of Lubricating Greases ..... Oil Components of Greases ..... .................................. Thickeners .......................................................................................................................... 204 Additives for Greases ......................................................................................................... 226 Solid Lubricants ................................................ Inorganic Lubricants ........ ................. Organic Compounds .......................................................................................................... 239 Soft Metals and Alloys ...................................................................................................... 242 Friction Reducing or Sliding Lacquers .............................................................................. 243 Self-Lubricating Materials ..................................................................................... Biodegradable Lubricants ...................................................................................... ................................................. ..................................... 247
4
Additives
4.1 4.1 .I 4.1.2 4.1.3 4.1.3.1 4.1.3.2
Antioxidants (Oxidation Inhibitors) ................................................................................... Radical Acceptor-Type Low Temperature Antioxidants .................................................. Metal Deactivators ............................................................................................................. Peroxide Decomposer Antioxidants .................................................................................. Sulphur-ContainingDecomposers ..................................................................................... Sulphur- and Nitrogen-ContainingDecomposers ..............................................................
VI
................................................................
...........................................................................................................................
103
125
255 258 258 268 271 272 273
4.1.3.3 4.1.3.4 4.1.3.5 4.1.3.6 4.2 4.2.1 4.2.2 4.2.2.1 4.2.2.2 4.2.2.3 4.2.2.4 4.2.3 4.2.3.1 4.2.3.2 4.2.3.3 4.2.3.4 4.2.3.5 4.2.4 4.2.5 4.2.6 4.3 4.3. I 4.3.2 4.4 4.4.1 4.4.2 4.4.3 4.5 4.5.1 4.5.2 4.6 4.7 4.8 4.9 4.9. I 4.9.2 4.10
Phosphorous-Containing Decomposers ...... Decomposers Containing both Sulphur and Ashless Peroxide Decomposers Containing Sulphur. Phosphorus and Nitrogen .............287 Future Developments ......................................................................................................... 288 Detergents and Dispersants ................................................................................................ 289 Significance of Detergent-Dispersant Additives - Mechanisms of their Action ...............289 Detergents .......................................................................................................................... 301 Alkarylsulphonates ............................................................................................................. 303 Alkylphenolates (Alkylphenolsulphides) ........... .............................. 308 Alkenyl Phosphonates and Thiophosphonates .... .............................. 310 Carboxytates ....................................................... .............................. 312 Dispersants ......................................................................................................................... 315 Succinimides and Bis-succinimides ................................................................................... 316 Miscellaneous Polyalkenepolyamine Derivatives ........... Esters of Alkenylsuccinic Acids ............. Nitrogen-Containing Copolymers .................................. Miscellaneous Dispersants ................................................................................................. 325 Combining Antioxidants sants ....................................................... 326 Packaged Additives ........ ................................................................ 326 Effects of Antioxidant an t Additives on Oil Quality Parameters .. 331 Synergism between Antioxidant and Detergent-Dispersant Additives ............................. 332 Antioxidant Synergism .... Detergent-Dispersant Syn .............................................................................. 338 Rust. Corrosion and Fatig Rust Inhibitors .................................................................................................................... 338 Corrosion Inhibitors and Metal Passivators ....................................................................... 340 ................................................... 342 Anti-fatigue Additives ............................................... Modifiers of Viscosity and Viscosity-Temperature Characteristics .................................. 343 Dispersant VI Improvers .................................................................................................... 368 Polymers as Anti-wear Additives ...................................................................................... 371 Pour-Point Depressants ...................... ..................................................................... 372 Anti-foams ............................................................................. Emulsifiers and Demulsifiers ................................................. ..................................... 377 Extreme Pressure, Anti-seizure, Lubricity and Anti-wear Additives ................................ 381 Extreme Pressure (EP) Additives ...................................................................................... 383 Lubricity Additives (Friction Modifiers) ........... ..................................................... 391 Miscellaneous Additives .. ...................................................................... 393 References .......................................................................................................................... 399
5
The Classification and Applications of Liquid Lubricants ..........................................
408
5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.1.5 5.1.6 5.1.7 5.1.8 5.1.9 5.2 5.2.1
.................................................................. Internal Combustion Engine Oils ....... Oils for Four-Stroke Internal Combust nes ........................................................... Oils for Two-Stroke Gasoline Engines .............................................................................. Oils for Rotary Gasoline Engines ...................................................................................... Oils for Marine Diesel Engines .......................................................................................... Railroad Diesel Oils ........................................................................................................... Oils for Dual-Fuel and Miscellaneous Internal Combustion Engines ............................... Oils for Running-in Engines .............................................................................................. ................................................................... Upper Cylinder Lubricants ................. Perspectives ......................................... ...................................................................... Aircraft Oils ....................................................................................................................... Oils for Lubricating Aircraft Piston Engines .....................................................................
409 409 470 473 474 477 478 479 480 481 482 482
VII
5.2.2 Oils for Gas Turbine Aircraft Engines .............................................................................. ........ .......................................... 5.2.2.1 5.2.2.2 Aircraft Hydraulic Oils ...................................................................................................... Performance Test for Aircraft Oils .................................................................................... 5.2.3 Compressor Oils ................................................................................................................. 5.3 Oils for Air and Gas Compressors ..................................................................................... 5.3.1 Oils for Refrigerating Compressors ................................................................................... 5.3.2 Lubricants for Vacuum Pumps (High-Vacuum Lubricants) ............................................. 5.3.3 Air-Tool and Rock-Drill ................................................................................... 5.3.4 ........................................................................................... Steam Engine Oils ........ 5.4 5.5 Turbine Oils .................. ................................................................................... 5.5.1 Steam Turbine Oils ............................................................................................................ 5.5.2 Water Turbine Oils .. .................................................................................................... Oils for Stationary Ga urbines ....................................................................................... 5.5.3 5.6 Bearing or Machine OiIs .................................................................................................... Spindle Oils ........................................................................................................................ 5.6.1 5.6.2 Electric Motor Oils ............................................................................................................ .............................................................................................................. 5.6.3 I1 Bearings ........................................................................................... 5.6.4 Bearing Oils for the Textile Industry ................................................................................. 5.6.5 Gear Oils ............................................................................................................................ 5.7 5.7.1 Types of Gear Oil .............................................................................................................. 5.7.1.1 Automotive Gear Oils ........................................................................................................ 5.7.1.2 Aircraft Gear Oils .............................................................................................................. 5.7.1.3 Multi-purpose Tractor Gear Oils ....................................................................................... 5.7.1.4 Industrial Gear Oils ............................................................................................................ 5.7.1.5 Chain Transmission Oils .................................................................................................... 5.7.1.6 Friction Clutch and Transmission Oils .............................................................................. 5.7.I .7 Traverse Screw and Rack Oils ........................................................................................... ........................................................... 5.7.1.8 Oils for Steel Wire Cables ................................. Hydraulic Oils (Hydraulic Fluids) ..................................................................................... 5.8 Hydrostatic Power Transmission Fluids ............................................................................ 5.8.1 5.8.1.1 Brake Fluids ....................................................................................................................... Hydrodynamic Transmission Oils or Fluids ...................................................................... 5.8.2 Metal-working Lubricants ................................................................................................. 5.9 Cutting Oils or Fluids ........................................................................................................ 5.9.1 Lubricants in Chipless Metal-forming ............................................................................... 5.9.2 5.9.2.1 Sheet-metal Rolling Lubricants ......................................................................................... 5.9.2.2 Lubricants for Drawing Wire, Bar and Tube ..................................................................... 5.9.2.3 Lubricants for Pressing Processes ...................................................................................... 5.9.2.4 Lubricants for Forging ....................................................................................................... Special Fluids ..................................................................................................................... 5.10 5.10.1 Insulating Oils .................................................................................................................... ............................................................................................ 5.10.1.1 Transformer Oils ....... ............................................................................................ 5.10.1.2 Oils for Contact-break ............................................................................................ 5.10.1.3 Capacitor Oils ........... 5.10.1.4 Cable Oils ........................................................................................................................... 5.10.2 White (Vaseline) Oils ........................................................................................................ 5.10.2.1 Medicinal White Oils ...... .............................................................................................. 5.10.2.2 Technical White Oils .......................................................................................... 5.10.3 Heat Treatment Oils for Metals ......................................................................................... 5.10.3.1 Quench Oils ....................................................................................................................... 5.10.3.2 Oils for Tempering (Stress Relieving) ...............................................................................
VIII
484 484 487 490 493 495 503 511 512 513 515 515 518 518 519 527 527 528 528 529 529 535 535 546 546 549 559 559 560 560 560 562 581 587 599 600 614 617 620 621 622 623 623 623 627 627 628 631 631 634 634 639
Heat Transfer Fluids . Hydrocarbon Oils ...... Synthetic Oils ........................ Low-flammability Heat-transfer Fluids ... .......................................... 5.10.5 Preventive Oils and Petrolatums .................................................. ........................ 5.10.6 Miscellaneous Special Fluids ............................................................................................. ...................................................... 5.10.6.1 Damper and Shock-absorber Oils ....................................... 5.10.6.2 Mould Oils and Release Agents .. 5.10.6.3 Air Filter Oils ..................................................................................................................... 5.10.6.4 Anti-seizure Compounds ................................................................................................... 5.10.6.5 Bolt-blackening Oils ............. ...................................................... 5.10.6.6 Rust-release or "Penetrating" 5.10.6.7 Textile Fibre Lubricating Oils ........................................................................................... 5.10.6.8 Turbine Washing Oils ........................................................................................................ 5.10.6.9 Special Agricultural Oils ................................................................................................... 5.10.6.10 Process Oils and Extenders for Rubbers and Plastics ....................................................... References ..........................................................................................................................
655 656 656 657 657
6
Types and Applications of Lubricating Greases ...........................................................
664
6.1 6.1.1 6. I .2 6.1.3 6.1.4 6.1.5 6.1.6 6.2 6.3 6.3.1 6.3.1.1 6.3.1.2 6.3.1.3 6.3.1.4 6.3.1.5 6.3.1.6 6.3.2 6.3.3 6.3.3.1 6.3.3.2 6.3.3.3 6.3.3.4 6.3.3.5
Classification of Greases according to Machine Parts Lubricated .................................... 668 668 Lubricating Greases for Anti-friction Bearings ................................................................. 670 Greases for Anti-friction Bearings in Railroad Vehicles .................................................. .................................................................. 670 Greases for Plain Bearings.......... .................................................................. 671 Greases for Gears ....................... .................................................................. 676 Greases for Sliding Guideways .. ....... ..................... ................676 Miscellaneous Grease-like Materials ... 677 Multi-purpose Greases ....................................................................................................... 678 Classifications of Greases by Types of Machine .............................................................. 679 Automotive Greases .......................................................................................................... Greases for Front-wheel Bearings ..................................................................................... 679 681 Greases for Drive-shaft Joints ............................................................................................ 681 Greases for Periodic Chassis Lubrication .......................................................................... Sealed for Life Chassis Greases ........................................................................................ 682 683 Multi-purpose Automotive Greases ................................................................................... Greases for Miscellaneous Automotive Assemblies ......................................................... 684 ................................................................. 685 Aircraft Greases ......................................... Miscellaneous Lubricating Greases ........... ................................................................. 688 688 Steel Rolling-mill Lubricants ............................................................................................. 688 Instrument Greases ............................................................................................................. Textile Machine Greases ............................................................................................ Greases for Mechanical Handling Equipment .................................... Miscellaneous Special Greases ................................................................................... ............................................................................................................ ~ 9 1 697 Subject Index .....................................................................................................................
5.10.4 5.10.4. I 5.10.4.2 5.10.4.3
642 643 651 651 654 655 655
IX
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FOREWORD TO THE ENGLISH LANGUAGE EDITION
All books reflect the culture and society in which their authors live and work and technical books are no exception. Much of what has been written on lubrication and lubricants stems from activity in the United States of America. The immensely successful industrial developments in that country owe much to collaboration in the area of lubricants and fuels which took place through such agencies as the American Society for the Testing of Materials (ASTM) and the Society of Automotive Engineers (SAE). In many ways, this collaboration was unique. The technical literature on lubricants and special fluids has a distinctly American flavour - and a military one. The motor industry, the steel industry and the development of efficient, largescale agriculture have many of their roots in Detroit and Pittsburgh. US Military standards, and civilian specifications since derived from them, have dominated the world market for fuels, lubricants, brake fluids and the whole range of functional fluids. Much of the technical literature in the area of practical tribology is one of the most obvious outcomes. Since the 1960s, the European Societies have made their own contribution, indeed, many of the more recent developments - particularly of smaller cars and small diesel engines - have had European origins, so that a reverse flow of technology has been added to that of science from the universities and institutes. This book joins those written from a distinctly European perspective. Dr. Stgpina and Professor Vesely first published their work as a text-book in the Czech language in 1980, under the title “Maziva a Specia‘lni Oleje - Zdklady tribotechniky” (Lubricantsand Special Oils - TribotechnologicalFoundations). They wrote this book, among other reasons, to provide a comprehensive source of fact and opinion for those working in both the academic and industrial fields over a wide area of technical activity in what could then be described as the somewhat closed atmosphere of Eastern Europe. Much of the relevant literature was published in English and other languages, so that it was not readily accessible to many of their colleagues, linguistically and also in terms of ready availability of the original sources. They set out to examine, in detail, technology published from all sources and incorporated with it their own findings from many years of involvement in the Institute of Fuels and Lubricants of Benzina in Prague and the Slovak Technical University in Bratislava. Their activities encompassed, not only technical work in Czechoslovakia, but also work in an advisory capacity in other Comecon countries, including the USSR. Their own professional interests included the establishment of a sound technical basis for the development and selection of lubricants and other specialised functional fluids in Czechoslovakia.
XI
Since the book was published in Czech, their own technical activities have continued through a time when access to technical literature has improved, and when many exciting new developments have taken place over the many fields in which tribology is applied. European specifications had by 1980 already supplemented, and sometimes replaced, the American originals. This process has continued apace, and .developmentsin the USA, such as those aimed at smaller, more fuel efficient and environmentally acceptable engines, have flourished. The authors have sought, in this English language edition, to update the text where necessary but to develop and retain some of its original themes. They have expanded the title to cover some developments of functional fluid technology outside that which would normally be contained in the term “oils”. In a fast-moving field such as this, much of any text is inevitably out of date before it has been printed. Some obsolete specifications are deliberately included. The authors would claim, however, that there are few cases where such earlier work is ever completely obsolete; this is an evolving technology and each new development builds on what went before. It is valuable to review the earlier work, although prudent to check what has happened more recently. One of the challenges in writing any text which covers even a significant part of this subject is the organisation of the text. The authors start with a general overview, including some of the basic science, in Chapter 1. This introduces, in particular, some of the language and definitions which will be used later, and some of the literature. In Chapter 2, the emphasis is on the more detailed science and the way this is called upon to explain and systematise the many functions which these fluids must perform. In Chapter 3, they examine the chemistry of the base fluids in relation to their various applications. The chemistry of additives, on which many of the premium properties of modern lubricants rests, is presented and discussed in Chapter 4. Chapters 5 and 6 are a synthesis of all the ideas brought out in the earlier chapters of the book, in which they are seen to form part of the basis of design of practical, working commodities, from sophisticated lubricants for high-output gasoline engines, to more humble products such as protectives of wire ropes. It has been my privilege to have had the opportunity to assist the authors in preparing this text. This task has illuminated, for me, many corners of a subject which has been one of my own professional concerns for several years. During the work, +have gained an appreciation, not only of the erudition of the authors in compiling a formidable synthesis of the work of many - including themselves - in this important field, but also of the efforts of the first translator, Robert Wiesner.
W J. Fox
XI1
ACKNOWLEDGEMENTS
We are indebted to R. Wiesner for translating the original Czech text of Maziva a Specihlni Oleje (Z6klady tribotechniky) and our up-dated script into English; Bill Fox undertook the task of converting the original translation into the present form of this book. In particular, we would like to acknowledge the technical contributions, including advice on recent developments in the manufacture of additives for lubricants and oil testing, of John Crawford and Gareth Hughes and the support of Bill Smith and Heinz Laher in bringing the work to English-speaking readers. In compiling a text of this scope, we have drawn heavily on discussion and debate over many years with our colleagues in the Institute of Fuels and Lubricants of Benzina in Prague and the Slovak Technical University in Bratislava, and with many workers in tribology at national and international conferences. We offer them and the many authors mentioned in the bibliographies our thanks for their teaching and stimulation.
V . Veselj V . StEpina Prague, 1992
XI11
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CHAPTER ONE THE DEFINITION AND CLASSIFICATION OF LUBRICANTS
This short, first Chapter of the book is intended to provide an introduction to the subject and some of its language and to delineate its scope. The first Bibibliography at the end of this Chapter includes some of the outstanding original texts on the subject; the number of languages in which they are written demonstrates the internationality of this most practical of the applied sciences. In later Chapters, we will examine some of the facets of the subject in more detail and enquire into the fundamental physical and chemical background necessary for a more complete understanding of it, then place it in an engineering and commercial context.
Tribology - which includes the study of lubricants - deals with the behaviour of surfaces in contact, in mutual motion or incipient mutual motion. The principal object of this inter- and multi-disciplinary science is the study of friction, wear and lubrication, which involves physics and mechanics (tribophysics) as well as chemistry (tribochemistry), metallurgy and materials science in general, and even, on occasion, geology (tribogeology) and biotechnology (biotribology) (1-27). When two solid surfaces, or a solid surface and a fluid - a gas, a liquid or fluidised particles - interact, relative motion of the surfaces creates a force resistant to the motion - friction - which causes wear. Friction itself, as a rule, is undesirable because it results in disproportionate energy consumption. Wear is also normally undesirable, as it results in the loss of both material and energy, although wear is an essential part of machining processes. Friction occurs at the friction (tribological) unit, which can comprise two, three or four elements (two friction surfaces, two friction surfaces plus an intermediate layer such as a lubricant, or these together with the environment of the unit, such as air, inert gas, high vacuum, radiation, etc.). These elements can affect each other: they can interact at different levels - mechanical, thermal and chemical. Changes in a friction element itself can occur, such as the interaction between the surface and the interior mass, and these may be accompanied by changes in the mechanical and chemical properties of the body. The properties of a lubricant may also change with time, independently of what may be occurring in the other elements. Friction may be classified as: - internal (taking place within a bulk solid or a fluid), - external (occurring at phase boundaries), - dry (relating to contact between solid surfaces), (relating to contact between a solid surface and a fluid), -fluid - static ( resistance to the onset of motion), 1
- dynamic
(relating to materials in motion). Each of these divisions may be sub-classified into sliding and rolling friction. The resistance to motion (tangential frictional force) between two bodies in their mutual contact zone is defined by the following simple relationships: Sliding friction may be expressed as Ft = p L s Fthus , p s = Ft/ F
(1.1)
where Ftis the frictional force ( N ), F is the normal load on the friction surface ( N ) and ps is the coefficient of sliding friction. Rolling friction may be expressed as Mt = Ftr = pVF,so that p, = Ft IF = MtIF
where Mtis the friction moment ( Nm ), pv is the coefficient of rolling friction (m) and r is the radius of curvature of the body (m). Equations (1.1) and (1.2) state that for sliding friction, the friction rate index, i.e.,the coefficient of friction p, is defined as the ratio of the frictional force to the normal load on the frictional surface; for rolling friction, it is the ratio of the friction moment to the normal load. For identical materials, the coefficients of rolling friction are smaller than those of sliding friction. Sliding friction coefficients, ps,are in the region 0.2 - 0.8 and higher, whilst rolling coefficients, p,,,are only 0.001 - 0.01, When solid surfaces come directly into contact, adhesive, abrasive and vibratory wear (fretting corrosion) may occur, also wear due to fatigue or plastic deformation (creep). The contact of a solid and a liquid can produce wear from corrosion, erosion and cavitation, whilst contact between a solid and a gas or vapour can cause corrosive or erosive wear. Lubrication reduces both friction and wear. The lubricant is designed to act to prevent direct contact between surfaces in relative mutual motion, and thus reduce both the frictional force between these surfaces and wear. However, in addition to this primary function, the lubricant is also required to remove contaminants from the friction surfaces and to protect the metal surfaces from corrosion. In particular cases, the lubricant must act as electrical insulant, perform as a medium for the transfer of force and as behave as a shock absorber. Modem lubricants - most often the product of petroleum chemistry - may be classified by the following criteria: In terms of physical state, they may be divided into liquid (the most important), plastic, gaseous and solid lubricants. In terms of chemical composition, several significantly different chemical types of liquid lubricants are used. They may be broadly classified into hydrocarbons, including those derived from crude oil (the prevalent type) and synthetic hydrocarbons (polyolefins or alkyl aromatics) and non-hydrocarbon lubricants. The latter may also be “natural” or “synthetic”. Aliphatic oils from natural sources were, 2
historically, the earliest lubricants to be used, but their importance has diminished considerably. On the other hand, the importance of synthetic oils is growing, particularly oils used for special purposes, for example, esters of organic acids and alcohols, and esters of phosphoric and silicic acids. The important ether lubricants consist of polyglycols in which the free hydroxyls are esterified or etherified, and polyphenylethers. Halogenated oils, particularly fluorinated derivatives, are also used. Other important synthetic oils include the polysiloxanes (“silicone oils”). Water, aqueous solutions, emulsions and melts - in which compounds of triboactive elements are also used - can be grouped with the non-hydrocarbon synthetics. Gaseous lubricants are used in a limited area of application, although their importance is growing. They may classified into oxygenated and oxygen-free lubricants. The common process fluids (steam, turbine flue gas) have recently been added to this category. Plastic lubricants are colloidal systems based on a solution or dispersion of a thickener, which may be (but is not necessarily) a soap in a viscous medium. Solid lubricants may be either inorganic or organic and, structurally, lamellar and amorphous. Their function is to reduce dry friction. Classification by Rheology distinguishes between Newtonian and non-Newtonian lubricants; the former type includes most natural and synthetic oils, as well as water and many solutions, whilst the latter comprises pseudoplastic and quasiplastic lubricants. Examples of pseudoplastic lubricants are solutions of polymers in oils or water emulsions. Lubricant greases are important quasiplastic lubricants. In terms of Mode oflubrication, Newtonian and pseudoplastic lubricants can act as hydrodynamic and hydrostatic lubricants and, under certain conditions, as boundary and mixed lubricants. The same applies to the quasiplastic lubricants, with the exception of the hydrostatic mode of lubrication. The prefix “rheo” is sometimes used instead of “hydro”. Gaseous lubricants can operate in gas-lubricated bearings as aerostatic or aerodynamic lubricants. Solid lubricants act as dry lubricants. Friction in the gap between the friction elements may be dry or fluid. In dry friction (see& 1.1, I), both friction surfaces are in direct contact. In fluid friction, the surfaces are separated by a viscous liquid and the modes of lubrication may be defined by the dimensionless viscosity parameter qw‘p, where q is the viscosity (or its related rheological magnitude in Pas), o is the sliding velocity expressed as a frequency and P is the pressure per unit area of surface. As this parameter increases, the regime changes from dry friction ( q d P = O ) to boundary friction (ZI)- with the thickness of the fluid film approaching zero, then to the mixed friction regime (ZZI) - where the thickness of the fluid film approximates to the roughness profile of the surface (R) - and finally to the fluid friction regime (IV), where the fluid film is thicker than the highest asperity of the surface. The elastohydrodynamic regime lies at the transition from mixed to fluid lubrication. This regime is characteristic of highly-loaded, non-uniform, curved surfaces. Surfaces operating in the dry friction regime can remain unlubricated or be lubricated with a solid lubricant. Fluid, quasiplastic and gaseous lubricants can operate in the other regimes. In all regimes, the friction coefficient p depends on load, geometry of the surface and surface
roughness, as well as on sliding velocity and the environment in which the system exists. Figure 1.1 is based on a modification of Stribeck's diagram (39). All aspects of the properties, preparation and use of lubrication are treated in the following chapters. This section deals with the lubricant and the solid surface, and in this respect, lubricants can be sub-divided into inert and reactive.
"kr
Fig. 1 . 1 Modified Stribecks diagram
The hydrocarbons - alkanes, cycloalkanes - are relatively inert. They are adsorbed on to the bare metal surface by physisorption, i.e., according to the cil.+,itinii: M'
+ CH,R-CH,R
'
[(M)(CH2R-CH2Rf>]'
(1.3)
where M' is a metal with an unpaired electron. If oxygen is absent, and at lower temperatures, no major changes occur and wear is small. At higher temperatures, hydrocarbon is desorbed, accompanied by decomposition of the hydrocarbon and the adsorption complex to give free radical products: [(M)(CHlR-CH,R')'
[M-CH,R]
+ R'CH,'
(1.4)
and, with further scission of the -C-C- and -C-H bonds, into unsaturated hydrocarbons (alkenes), cycloalkanes, aromatic compounds, alkynes and, when the surface is very active, carbon. These are all bonded to the surface and may react with it, e.g., to form surface carbides (58). In the presence of oxygen, oxidation of the free radical intermediates in the lubricant and on the surface occurs, and the oxidation products cause deterioration of the lubricant, corrode the metal surface and cause wear. Typical examples of this type of lubricant include highly-refined oils, such as white oils and some oils derived from severe hydrocracking of paraffinic base-stocks. Such oils have low solvent power, even for their own oxidation products, and the products of decomposition can separate out in the form of hard deposits on, e.g., bearing surfaces. Polyaromatic oils are more reactive; they adsorb more readily on to the bearing surface. If saturated hydrocarbons are also present, they are displaced from the surface by the aromatic oils because of the lower solvent power of the saturated hydrocarbons. As a result of this, the viscosity at the surface is higher than in the
4
bulk oil (the polyaromatics are highly viscous). In the absence of oxygen, and at lower temperatures, the polyaromatics are chemically bonded to the fresh metal surface: M’
+ CHR=CHR’ =*
[M-CHR-CHR’]’
(1 3 )
The tendency of polyaromatics to adsorb on the surface is reinforced by the planar structure and residual valence bonds typical of polynuclear aromatics. Contrarywise, adsorption is impeded by steric hindrance, e.g., by numerous and highly-branched substituents. The presence of double bonds in aromatic compounds means they can donate electrons to an acceptor surface or accept electrons from a surface of donor character. In the first case, aromatic cation radicals may be formed, and in the second, anion radicals:
+
/
[M-CHR-CHR’]’
M- + CHR-~HR’
(1.6.1)
-\
‘*
M+ + CHR-~HR’.A ,k.
[(M+)(CHR-dHR’)]
(1.6.2)
In the absence of oxygen, as in case (1.6.l), the metal is supersaturated by the electrons and thus protected against corrosion and wear. In case (1.6.2),the metal enters into the ion complex, which is soluble in the excess of polyaromatics and the surface is depleted even in the absence of oxygen. In the presence of oxygen, and at higher temperatures, the hydrocarbon oxidises, but more slowly than in the case of alkanes and cycloalkanes, because polyaromatics are more stable than alkyl or cycloalkyl radicals. The generation of corrosive agents is thus slower, and the surface is coated with soft deposits (resins). Appeldoorn and his co-workers (29, 30) were the first to explain the apparent contradictions in the behaviour of the saturated and the polyaromatic oils - low wear in the absence of oxygen and higher wear in the presence of oxygen in the case of saturated oils, but higher wear in the absence of oxygen and lower wear in the presence of oxygen in the case of polyaromatics. Their ideas have been further developed here to emphasise the importance of the composition of lubricant surfaces. The friction surface can behave as a miniature chemical reactor, particularly if bare metal is exposed. The products of the chemical conversion can be identified as having increased surface adsorptive capacity - as in the case of saturated or polyaromatic hydrocarbons and carbonaceous or oxygenated substances - but many processes are unclear and have not yet been sufficiently investigated. The progress of these various processes often adversely affects the stability both of the lubricant (e.g., towards oxidation) and the surface (e.g., towards corrosion, wear or embrittlement caused by hydrogen generated by dehydrogenation). Tribologically reactive lubricants (either mineral oil with additives or synthetic 5
with or without additives) are selected when improved wear protection or friction reduction is desirable. The additives concerned are either those conferring improved lubricity - the capability to lubricate - or those which prevent seizure under “extreme pressure” (EP) conditions. The former are particularly designed to operate under moderate conditions involving boundary or mixed friction, whilst the latter - the EP additives - are effective under very severe conditions of temperature and pressure in elastohydrodynamic lubrication and in metal-forming processes. The dominant mechanism is the chemical reaction of the additive with the surface. Phenomena are encountered in the behaviour of additive-containing lubricants which are somewhat difficult to explain. For example, aliphatic acids, long known as anti-wear additives (8,31-33),are more effective in cetane (CI6H,, n-alkane) than in squalane (C,,H,, iso-alkane). This may be caused by bonding between the additive and the oil, or by stronger adsorption of the heavier hydrocarbon on to the surface. The fact that the effects of acids is lower if the oil is more aromatic may be explained in terms of mutual competition for occupation of the surface between the additive and the oil. Similar competition for surface sites may also exist among some additives themselves, such as between zinc dialkyldithiophosphates(anti-wear and anti-seizure effects) and aliphatic acids at higher concentrations, or, similarly, surfactant additives such as sulphonate detergents (58) and amino additives at particular concentrations in oil (35). Synergistic effects have also been observed (36). This phenomenon of mutual competition for surface sites is general among polar additives. Similar behaviour should also be expected in synthetic oils containing additives, and in the so-called partially synthetic oils, e.g., mixtures of mineral and synthetic lubricating oils with additives. Additives to improve performance are now a significant component of most modern lubricants and special fluids, that is to say, added organic and inorganic substances of natural and synthetic origin provide the lubricant or special fluid with many of the beneficial properties it should have. Whilst the first use of lubricants occurred in antiquity, real technical development of lubrication was not recorded until the latter part of the last century and the development of modem lubricant types did not start until the second decade of this century, associated with the development of synthetic oils and additives. This development accelerated particularly during the second World War and in the post-war years. Technical development also stimulated scientific activity involving studies of the relationships between lubricant properties on the one hand and friction elements and conditions prevailing in the tribological (friction) unit on the other. The first pioneer work was that of Petrov (37), who investigated the relationships between frictional force, oil viscosity, shaft speed and the thickness of the oil film in sliding bearings. Reynolds (38)worked out the elements of hydrodynamic theory of lubrication, still valid even now. The general properties of lubricated surfaces in terms of oil viscosity, speed and load in a bearing was investigated by Stribeck (39).Hardy (40) started work on boundary lubrication, and his work encouraged the use of additives in oil. The years following the second World War were characterised by the change6
over to lighter-weight machines operating under higher loads, speeds and temperatures. This period is also associated with the development of the theory of elastohydrodynamic lubrication (EHD), involving, amongst others, Grubin (41), Dowson (42) and Hamrock (43). Tribotechnology (44-57) deals with the application of tribological principles. In technical practice, the lubricant constitutes an essential and unavoidable structural element affecting the rate of energy loss and the wear of parts of machines and, consequently, the operating capacity and predicted operational life of the machine and associated equipment. One principle must always be borne in mind : when highquality lubricants are used, a high-quality machine may operate well, reliably and economically. However, unless the machine is properly designed, manufactured with precision and well maintained, a lubricant of the same high quality will not be able to assert its qualities. The same standards must be imposed on both the choice of composition of the lubricant and the materials of construction and design and fabrication of the machine.
Chapter 1 - References 1. AMONTONS, A. G.: Histoire de 1’Acad. Roy. des Sci. avec les MCmoires de Math. et Physique, 206, 1699. 2. DE LA HIRE,Ph.: Histoire de 1’Acad. Roy. des Sci., 104, 1732. J. A,: Proceed Roy. Inst. of Great Britain, 13, 1892,387. 3. EWING, 4. GOMBEL, L.: Jahrbuch der Schifbautechn. Gesellsch., 18, 1917,236. 5. TOMLINSON, G. A.: Phil. Mag., 7, 1929,905, Proceed. J. Mech. Engng., 141, 1939,2056. 6. HOLM,R.:Wiss. Veroffentl. Siemens Werke, 17, 1938, 38. 7. ERNST, H. - MERCHANT, H. E.: Proceed. Special Summer Conf., Friction and Surface Finish, Cambridge, Mass., MIT Press, 76, 1940. 8. BOWDEN, F. P. - TABOR. D.: The Friction and Lubrication of Solids. Oxford Clarendon Press, Vol. 1, 1950, Vol. 2, 1964. 9. BURWELL, J. T. - STRONG, C. D.: Proceed. Roy. Soc.( London), A 212, 1952,470; J. Appl. Phys., 23, 1952, 18. 10. ARCHARD, J. F.: J. Appl. Physics, 24, 1953, 981; Proc. Roy. SOC.(London), A 236, 1956, 397. 11. CHRUSCOV, M. M.: Conf. on Lubrication and Wear. London, Oct. 1957, Paper 46. , S.: Razvitije nauki o trenii, Izd. AN USSR 1956. 12. KRAGELSKIJ, 1. V. - ~ E D R O V V. 13. CAMERON, A,: Principles of Lubrication. Longmans 1966. 14. JOST,H. P.: Committee on Lubrication (Tribology). Report, London, H.M.S.O. 1966. 1. V.: Trenije i iznos. Moscow, MaSinostrojenije 1968. 15. KRAGELSKIJ, 16. RABINOWICZ, E.: Friction and Wear of Materials. New York, J. Wiley and Sons 1965. 17. NEALE, M. J.: Tribology Handbook. London, Butterworth 1971. 18. QUINN, T. F. J.: The Application of Modem Physical Techniques to Tribology, London, Butterworth 1973. 19. BOWDEN, F. P. - TABOR D.: Friction - an Introduction to Tribology, London, Heinemann 1973. 20. LING,F. F.: Surface Mechanics. New York, J. Wiley and Sons 1973. J. (Ed.): Principles of Tribology. London,.McMillan 1971. 21. HOLLING, 22. ENGEL, P. A.: Impact Wear of Material, Amsterdam, Elsevier 1976. 23. CUCHOS, H.: Tribology. A System Approach to the Science and Technology of Friction. Amsterdam, Elsevier 1978.
7
24. IUUC,J.: Tribology of Thin Layers. Amsterdam Elsevier 1980. 25. BUCKLEY, D. H.: Surface Effects in Adhesion, Friction, Wear and Lubrication. Amsterdam, Elsevier 1981. 26. JONES,M. H. - S c o n , D.: Industrial Tribology. Amsterdam, Elsevier 1983. 27. S T ~ P I NV. A ,- VESELY,V.: Maziva v tribologii (Lubricants in Tribology). Bratislava, Veda 1985. 28. DERIAGIN, B. V. et al.: Wear, 1, 1958,277. 29. APPELDOORN, J. K. - TAO,F. F.: Wear, 12, 1968, 117. 30. GOLDBLATT, I. L.: Ind. Eng. Chem., 10, 1971, 270. 31. BOWDEN, F. P. et al.: Nature, 156, 1945,97. 32. LEVINE, 0. - ZISMAN, W. A.: J. Phys. Chem., 61, 1957, 1068. 33. FEIN,R. S.: ASLE Trans., 8, 1965,59. 34. CHRUSCOV, M. M. - MATVEJEVSKI, R. M.: Vestnik malinostrojenija, 34,1954, 12. 35. ROUNDS, F. D.: ASLE Trans., 24, 1981,431. V. et al.: Ropa a Uhlie, 24, 1982, 10. 36. STEPINA, N.: Engineering J., St. Petersburg, 71, 1883,228, 337, 436, 535; Summary: Ostwalds 37. PETROV, Klassiker der Wissenschaften, Nr. 218, Leipzig 1927. 38. REYNOLDS,0.: Phil. Trans. of the Roy. SOC.of London, A 166,1875,935; A 177,1886, 156; Summary: Ostwalds Klassiker der Wissenschaften, Nr. 21 8, Leipzig 1927. 39. STRIBECK, R.: VDI Z., 46, 1902, 1341, 1432, 1463. 40. HARDY, W. B. - DOUBLEDAY, J: Proc. Roy. SOC.(London), A 160, 1922,550. 41. GRUBIN, A. N.: Trudy C. N. I. I. T. Mas., Vol. 30, Moscow 1949. 42. DOWSON,D. D. - HIGGINSON, G. R.: Elastohydrodynamic Lubrication - The Fundamentals of Roller and Gear Lubrication. London, Pergamon Press 1960. 43. HAMROCK, B. J. - DOWSON, D. D.: J. Lubric. Technol., 98, 1976,223 ;99,1977, 15; 100, 1978, 236; 101, 1979,92. 44. SOMMERFELD, A.: Z. Math. Phys., 50, 1904.97. 45. MARTIN, H. M.: Engineering (London), 119, 1916, 11. 46. GO~TNER, G. H.: Einfuhrung in die Schmiertechnik. Diisseldorf, K. Marklein Verlag 1966. 47. HERSEY, M. D.: Theory and Application in Lubrication. New York, J. Wiley and Sons 1966. 48. KosTEcKu, B. J.: Trenije, smazka i iznos v masinach. Kiev, Technika 1970. 49. THIESSE, P. A. - MEYER, K. - HEINICKE, G.: Grundlagen der Tribochemie. Berlin, Akademie Verlag 1967. 50. O'CONNOR, J. J.: Standard Handbook of Lubrication Engineering, New York, McGraw-Hill 1968. 51. SAFR,E.: Technika mazani (Technique of Lubrication). Praha, SNTL 1970. 52. MOORE,D. F.: Principles and Applications of Tribology. Oxford, Pergamon Press 1975. 53. VOCEL, M. - DUFEK,V. et al.: Tieni a opotfebenl strojnlch soutisti (Friction and Wear of Machine Parts). Praha, SNTL 1976. 54. EMINOV, E. A. et al.: SpravoEnik po primeneniju i normam raschoda smazdnych materialov. Vol. 1 and 2, Moscow, Chimija 1977. 55. BRENDEL, H. et al.: Wissenspeicher Tribotechnik. Leipzig, VEB Fachbuchverlag 1978. 56. KRAGELSKIJ, 1. V. et al.: Trenije, iznagivanije i smazka. SpravoEnik, Vol. 2, Moscow, MaSinostrojenije 1979. 57. ST~PINA, V. - VESELY,V.: Maziva a speciilnl oleje (Lubricants and Special Oils). Bratislava, Veda 1980. 58. FEIN,R. S.: In: Interdisciplinary Approach to the Lubrication of the Concentrated Contacts. Ed. Ku. P.M., Washington, NASA, 489, 1970.
8
CHAPTER TWO THE GENERAL PROPERTIES OF LUBRICANTS
A lubricant must have properties appropriate to the functions it is meant to perform. These properties will not suffice by themselves if it does not have the ability to keep them for a long enough time under the effects of the various factors in the environment in which it is intended to operate. These factors include, a priori, the presence of oxygen, temperature and pressure and, last but not least, water, light, radiation and electrical field, all under the possible catalytic co-effect of metals and dust. Also, the chemical environment - acids and bases etc. - may be detrimental. A lubricant must be able to withstand all these effects to the maximum extent. On the other hand, the lubricant itself must be, as far as is possible, “unaggressive” and biologically inactive. It must have low volatility and, in some cases, its tendency to spontaneous ignition must be low. Mechanical and colloidal stability can be extremely important, particularly in the case of lubricating greases. The functional properties of particular lubricant types in different matter states can vary widely, as can their resistance to most or all of these effects. These properties clearly depend on chemical composition and the type, composition and concentration of fortifying additives.
2.1 THE FUNCTIONAL PROPERTIES OF LUBRICANTS Density, viscosity and other rheological properties of lubricants, and electrical, thermal and other, more superficial characteristics affect functionality, but to different extents.
2.1.1 Density Density is not, of itself, an important functional property of lubricants, except in the case of fluids for fluidmechanical coupling devices and transformer oils. Its principal importance for liquid lubricants lies in the characterisation of composition, for the inter-conversion of mass and volume and for the computation of kinematic viscosity from dynamic viscosity. Density is defined as the mass (m)per unit volume ( v ) of a substance at a given temperature ( t ) , usually 20 “C for liquids and 0 “C at a pressure of 10-1MPa (760 Torr) for gases -
pt = m/v (kg.m-3)
(2.1)
A number of standard methods are specified for the determination of the density of oils. CSN 65 6199 offers 3 methods for density determination, a densitometer, the Mohr-Westfield balance and a pyknometer. ASTM D-287 and D-1298 describe methods for density (“gravity”) determination with the so-called API hydrometer (analogous to the densitometers). IP 160 is equivalent to ASTM D-1298.
9
ASTM D-941, D- 1481 and IP 189 details the measurementof density in Lipkin’s bi-capillary pyknometer. DIN 51-7575 covers the methods specified in ASTM D-941, D-1298 and D-1481.Bingham’s pyknometer (ASTM D-1480) is used for density determination of highly-viscous oils and greases produced to ASTM D-1480. IP 190uses a similar closed-capillary pyknometer. IP 59/method D specifiesthe Mohr-Westfield balance. The density of quasi- and pseudo-plastic lubricants is less subject to scrutiny than that of fluid lubricants. Density determination of such lubricants is, however, useful for identifying the dosages and establishing the presence of fillers and the like (SEB 181-30 and ASTM D-1480/62 are suitable only for liquid melts, since the pyknometer specified cannot be filled with a non-liquid substance).
“Relative Density” (note: in the text that follows, the symbols p and d are used to designate density and relative density, respectively, at specified temperatures) is sometimes used; it is the ratio between the densities of the test substance and a comparison substance at temperatures tl and f2: d:: = Plt,’P2tz
US literature uses the term “gravity” quoted in API (American Petroleum Institute) degrees. The relationship between degrees API and relative density at 15.6 “C (60 O F ) , d::: , is given by:
“API o - oil, w
=u - 131.5 d::%
(2.3)
- water Degrees API increases as density increases. The density of substances of similar composition is an additive property. Generally, density increases as the molecule increases in size, and, in the case of hydrocarbons, from alkanes to cycloalkanes to aromatics. Asphaltic compounds have the highest density. The density of a lubricant fluid can provide indications of its composition and nature.However, to be any more precise, other physical properties must be considered, e.g., viscosity, viscosity-gravity constant (see page 17), boiling-point (see characterisation factor), relative molecular mass, etc. (I) - see also n-d-M analysis. The density of lubricants, predominantly hydrocarbons, varies between about 860 and 980 kg.m” (33 and 13 “API). For the same viscosity, paraffinic oils have the lowest density and aromatic oils the highest. At the same density, synthetic lubricants - except for organosilicates - have higher density - often over 1,OOO kg.m-3. The density of liquid lubricants changes with temperature and pressure. It decreases linearly with temperature, the rate of decrease depending on chemical composition. However, ab initio calculation of absolute values has proved impossible, as fluid density is governed by intermolecular forces and anisotropy, and none of these effects has been sufficiently investigated as yet. Because of this, an empirical equation has been used: where apand pp are coefficients of thermal expansion. 10
In technical practice, only the correlation coefficient a is used. For hydrocarbon lubricants, its value is 0.65 for the density range 83 1 to 9!0 and 0.60 for the density range 95 1 to 1,000 kg.m-3. Since the decrease in density with increasing temperature itself decreases with increasing pressure, these coefficients are only applicable to pt at atmospheric pressure. By contrast, density increases non-linearly with pressure, according to: PJW
= p l / o , = p 2 / q= . . .
(2.5.1)
where o is the “expansion factor” of Watson and Gamer, which changes with temperature and pressure as shown in the chart (2). The density/pressure relationship is a function of the compressibility of the oil. Since this decreases with increasing pressure, the pp isotherms in the chart are concave, i.e., degressive with respect to the tangent to the p-axis. The following empirical rule was derived for mineral oils for temperature T K and P MPa : p TIP = po(l+ CIT + C2T2)[1+C3~.51~g(P-P,)]-’ (2.5.2) where P,, po, C,,C2 and C3are constants having values:
C,= -1.1 . 10-3 C2 = 4.10-7 C3 = -(6.0 - 0.0031p1).10~3 p1 = p a t 15.6 OC and 0.1 MPa P, = 110 Pa po = 0.727 + 0.000346 . pI2 po can also be derived by incorporating the known values of p, P and Tin equation 2.5.1. This relationship is virtually independent of whether the oil is paraffinic, cycloparaffinic or aromatic (135). Wright’s empirical equation (136) is simpler: 1%” PT1P = A + B.log,
POT
(2.5.3)
where por is the density at temperature T and atmospheric pressure, A and B are quantities dependent on pressure and temperature but independent of oil composition - the author plotted these in a chart which enables pT,? to be calculated, and also, when pOTisknown, the module of elastic compression (see Chapter 2.14). Another relationship is also available (137): PT1P= POT
(1 + aP4b + P )
(2.5.4)
where pOT= pTolPo [1 - ap(T - To)]. apis the coefficient of thermal expansion, pOTis the density at T+To / P = 0, and a and b are constants. From pT,Pit is possible to derive the “coefficient of isothermal compressibility” (see Chapter 2.14). 11
The simplified equation:
p = po[l
+ 0.6P/(1+1.7P)]
(2.5.5)
is less accurate.
2.1.2 Viscosity Viscosity is one of the most important properties of a fluid lubricant, which determines the fluid friction involved in lubrication, the load-carrying capacity of the lubricant film, its resistance to the initiation of relative movement of moving parts and the sealing capacity, pumpability and heat transfer properties of the lubricant. It is a measure of the internal friction taking place in a fluid - the mutual resistance to relative motion of the fluid molecules.
Dynamic Viscosity According to Newton’s law of friction (3),in fluids undergoing laminar flow, the shear stress zin the plane parallel to the direction of flow is directly proportional to the velocity gradient dvldz, i.e., to the shear-rate D fig. 2.1).
z = q.dv/dz = qD (Pa)
(2.6.1)
where z is the shear stress (Pa) per unit area in the plane x, y, v is the velocity (m.s-’) in direction x , z is the distance (m) from the parallel plane x , y, dvldz = D is the velocity gradient or shear-rate (s-l), q is the dynamic viscosity (or viscosity) - the coefficient of internal friction of the fluid (in Pa.s or kg.s-’.m-’). Fluids which conform to this relationship in laminar flow are termed “Newtonian Fluids”.
pqd:,-lf./
d2
,‘
- _ - - _; _ I
---,<
-T
I’
X
Fig. 2.1. Graphic representation of Newton’s law of friciton in a fluid
In the SI system, the unit of dynamic viscosity is the Pascal Second (Pa.s), (i.e., the tangential stress produced by a velocity gradient of 1 s-’ crosswise to the flow). In the CGS system previously used, the Poise (after Poiseuille) (P dyne.s.cm“) is equivalent to lo-*Pa.s and CP to mPa.s. In the English literature, the Reyn (after Reynolds) is sometimes used. 1 Reyn, which is expressed in the dimensions lb.s.inch‘l, equals 1.45.1(1-~Pa.s.The reciprocal of viscosity, q-l, is “fluidity”. Viscosity is, in fact, the analogue in shear terms of Hook’s module, G, applied to an elastic body and defined by the relationship:
z = G.dv/dz 12
(Pa)
(2.6.2)
Dynamic viscosity must obviously be determined under laminar flow conditions. Capillary viscometers are based on the Poiseuille formula:
q = 7nA A P W v = ?AP/81u
(Pa.s)
(2.7.1)
where v is the volume (m3) flowing in time t (s), AP is the pressure gradient (Pa) over length 1 (m) of a tube of radius r (m), and u is the linear velocity of flow (m.s-l). A constant pressure gradient must be applied throughout the measurement. Capillary viscorneters are suitable for the majority of low shear stress and low shear gradients (CSN 65-6248, 7163-63, and DIN 5 1-561 and 51-569). Viscometers are, however, also available for viscosity measurements at up to 1O'O P a s at high shear stresses (30 to 1.2.105 Pa), temperatures up to 230 "C and pressures as high as 700 MPa (138). Stokes' law is used to calculate viscosity from measurements with falling-ball viscometers: q = 2?g(p2-p1)/9u (kg.s-'.m-') (2.7.2) where r is the radius of the falling-ball (rn), g is the acceleration due to gravity (m.s-2), p1and p2 are the densities of the ball and the fluid, respectively (kg.m") and u is the uniform velocity of the ball (m.s-l). Hoppler's viscometer is an example of this type of instrument (DIN 51-015).This viscorneter is suitable for fluids of very high viscosity. Pressure viscometers suitable for measuring viscosity at different pressures and temperatures are mostly of the falling-ball type. Rotary viscometers depend on the measurement of fluid resistance to the rotation
e
c
3
4
'
1
2
Fig. 2.2. Scheme of rotary viscometer 1 - rotor, 2 - fluid film, 3 - inlet of heating medium, 4 - outlet of heating medium, 5 - thermometer
of coaxial cylinders (fig. 2.2). Couette's equation is used in this case: = M ( r 2 - r 1 ) 2 / ~ . 4 d r 1 2(Pa.s) r~
(2.7.3)
where M is the torque (N.m), w is the angular velocity (s-l), 1 is the length of the cylinder (m), and ( r 2 - r l )is the clearance between the cylinders (m). 13
Couette and VolaroviE viscometers are suitable for opaque fluids, a wide range of viscosities and high values of r and D. Cone-and-plate viscometers are suitable for shear-rates up to lo3 s-l (139). In this case, viscosity can be calculated from the equation:
q = 3MtgqY2do (Pa.s)
(2.7.4)
where r is the radius of the cone, 0 is the angle between the cone and the plate, the other symbols having the same meaning as in he previous example. Ultrasonic viscometers are designed for measuring shear stress which changes periodically in strength and direction (140). These viscometers are especially suitable for determining Newtonian relaxation times in Newtonian and nonNewtonian lubricants.
Kinematic Viscosity Dynamic viscosity cannot be directly determined with viscometers based on flow under gravitational force. The property relevant to measurements under these conditions is called kinematic viscosity, which is linked to dynamic viscosity by the equation: v = q/p (m2.s-') (2.8.1) I
Fig. 2.3. Capillary viscometer
-
-
a ,b timing marks, B timing bulb
The earlier unit for kinematic viscosity was the Stoke (St, [104.m2.s-']). The viscosity of liquid lubricants, particularly those derived from crude petroleum, was expressed in cSt. Whilst this unit is still widely used, m2S1with the same numerical value is increasingly employed. Kinematic viscosity is usually determined by measuring the time t for a given amount of oil to flow at a constant rate through a capillary held at constant temperature. The viscometer is calibrated with a fluid of known viscosity @g. 2.3). 14
The following equation applies:
v=kt
(2.8.2)
where k is the viscometer constant derived by measuring the flow-rate of the calibration fluid. Ubbelohde, Ostwald-Fenske,PinkeviE, Vogel and Cannon-Fenske(CSN 65-6216.65-6248. GOST 625852, DIN 5 1-33, 51-561.5 1-562, IP 71 and ASTM D-445) viscometers are the most widely used. Normal test temperatures are 10 to 15OOC. Occasionally, very low temperatures - down to -55°C - are used (CSN 65-6236, GOST 1929-51 and DIN 61-569). The IS0 3104 method is being introduced for petroleum oils.
“Conventional” Viscosities Older viscosity measuring methods and symbols remain in use in some countries. For example, Engler’s viscometer and Engler degrees ( O E ) are used in Central and Eastern Europe (CSN 65-6217, DIN 51-560), Saybolt’s or Redwood’s viscometer with “seconds” and “SSU” in the USA (ASTM D-88), RI in the U.K. (IP 70). The deficiency of such “conventional” viscosity standards is the impossibility of defining them in terms of more fundamental physical units. They are, obviously, only comparative expressions. Approximate conversions into the corresponding kinematic viscosity can be made with empirical formulae, tables and charts. Their use should be discouraged because these units can convey entirely the wrong picture of internal friction. Thus, an oil of viscosity 76 m 2 S 1 (cst) at 20 “C has an Engler viscosity 10 times that of water, whereas its kinematic viscosity is 76 times higher! The following conversion factors can be used to convert Anglo-Saxon units into mz.s-’ (the figures are approximate): n = 0.226 x n = 0.226 x n = 0.260 x n = 0.247 x
SSU - 195 x SSU-’ for 32 < SSU < 100 SSU - 135 x SSU‘’ for SSU > I 0 0 RI -179 RP’ for 34 < RI < 100 RI - 56 x RI-’ for RI > 100
But kinematic viscosity itself, of course, cannot be directly used as an expression describing internal friction unless the substances being compared are of the same density. For example, air of density 1.1 kg.m” and oil of density 900 kg.m’3 have an identical kinematic viscosity of 14 rn2.s-l at 50 OC, whilst their dynamic viscosities are 1 5 . 4 ~ 1 0 -and ~ 12 MPa.s respectively. Viscosity-density ratio is therefore becoming a more commonly used term for kinematic viscosity, the latter being used chiefly in hydraulic engineering. Relative Viscosiry is the ratio of the viscosity (77) of a fluid to the viscosity of a standard fluid (qo),e.g., water, at a conventional temperature (e.g., 20 “C):
77 = 77rel
770
(2.9.1)
Einstein’s relationship is applicable to the relative viscosities of solutions or dispersions of (spherical) colloidal particles in a viscous fluid: 15
q,e1= 1+ k$
(2.9.2)
where @ is the volume ratio of a dissolved or suspended substance to the total volume, and k is a coefficient with a value of about 2.5. Specific Viscosity is a related quantity. It can be defined by the equation:
(2.10) Specific viscosity represents the “contribution” of a solute to the viscosity of the solution, q, where the viscosity of the solution is q,,. It is applicable, for example, to solutions of polymers in oil. Reduced Viscosiry is a similar concept, defined by: qred
= qspec lc (m3.kg-’)
(2.11)
which allows for the viscosity effect of the solute. The limiting value of the reduced viscosity, when C (the concentration of dissolved substance) and D (the shear rate) approach zero is the “lnrrinsic Viscosity” [q]= l-I.qspec/C which is a characteristic of the hydrodynamic volume of the polymer when it is in a coiled state in the solution.
The Viscosity of Blends In contrast to density, viscosity is not an additive property. The kinematic viscosity of a mixture of gases can be calculated from their reciprocals as follows: = nl/vl + n 2 h 2 + . . . n/vi 1lvblend
(2.12)
where n is the volume fraction of components 1, 2 . . . i in the blend. The Walther viscosity function can be used for hydrocarbon oils (see below):
w = log (log v + c)
(2.13)
and the viscosity of the blends can be calculated from:
Wblend = nl W,
+ n2W2 + . . . niWi
(2.14)
where n , . . . ni are the volume fractions of the components of the blend. Some “viscograms” (e.g., that of Ubbelohde) can be used to derive these values graphically.
A more general relationship defining the dynamic viscosity of a blend of similar liquids was derived by Kendall and Monroe:
(2.15) where x, . . . xi are the mole fractions of the blend components.
16
Viscosity and Density Both these properties are associated with the chemical composition of a liquid. The viscosity of mineral oil lubricants of the same molecular weight increases from alkanes to aromatics. This was the reason for deriving the “viscosity-density constant” (VDC) or “viscosity-gravity constant “ (VGC), which is a parameter independent of the hydrocarbon cornposition in the oil. It approaches 0.800 for paraffinic oils and exceeds 0.900 for aromatics (4).According to PinkeviE, it can be derived from: VGC =
d,,
+ 0.0925 - 0.776 log . log( IOv,,,,
- 4) 1.082 - 0.72 log . log( lov,,., - 4)
ASTM D-250 describes the method for determining VGC at 37.8 “C (100 O F ) for oils of viscosity above 40 SSU.
2.1.2.1 Viscosity-Temperature Relationships The viscosity of a gas or a liquid changes with temperature. The dynamic viscosity of gases increases with temperature, independently of density and pressure, provided they follow the ideal gas equation P V = RT. Their kinematic viscosity changes, by definition, with density and is indirectly proportional to pressure. By contrast, both dynamic and kinematic viscosities of liquids decrease with temperature. This results from the fact that molecules in the liquid state tend to cluster at lower temperatures. As the temperature rises, these clusters disperse and the free volume they occupy grows (the difference between the total volume of the liquid and the aggregate molecular volume increases). The relationship between viscosity and temperature of gases is given by Sutherland’s equation:
(2.16) where T is the temperature in K and B and C are constants. When q is known at temperature T I ,then: 172/171
= ( T , / T ~ ) ~+”W WT~ ~+ C?
(2.17)
Values for q and C for some gases are listed in Table 2.2. Derivations of the relationships between the viscosity of liquids and temperature (and pressure) have been based on various theories of liquid structure (241,242): Cellular rheory - each molecule is placed in a “cell” surrounded by its nearest neighbours, Caviry theory - cavities or “holes” are supposed to exist in the lattice structure of liquids, reminiscent of holes in solids, Tunnel theory - molecules are arranged unidirectionally in parallel “tunnels”, together with a significant body of liquid structure theory based on the model of
17
vacant molecular sites set in motion by molecules moving in leaps as a result of the formation of fresh vacant sites. Each leap requires activation energy sufficient to overcome an energy barrier. All these theories, whose development is still incomplete (I43),are based on the theory of free volume of the liquid. Flow starts if sufficient free volume (Vf) is provided around the flow unit (the molecule or molecular cluster) which itself has volume V*, so that the flow unit is capable of moving about in the free volume V,. Table 2.1. Viscosity of Gases Type of Gas
Viscosity (10' Pas)
Air Hydrogen Nitrogen Oxygen Water vapour Methane Ethylene Ethane
171 88 176 204 127 108 99
92
Temperature ("C)
0 21 23 23 100 17 15 100
Constant ("C) 120 80 111 124 650 226 226 700
The relationship between viscosity and these parameters at temperature T and pressure P is: qTlP, = Aqe (V"vf)" (2.18.1) The index n has been allocated values of 1 , 2 and others by different authors. Some semi-empirical viscosity-temperaturerelationships agree with this theory. Generally, the relationship between viscosity and temperature of non-polar liquids follows Andrade's equation: qL = AeBtT (2.18.2) or modified equation: (2.18.3) log (qL/qoL) = B (1/T - l/To) where B is the slope of the log q/T plot and To is the intercept of this line with the x-axis, corresponding to the temperature at which qL= 1 Pas. The constant B relates to the activation energy of viscous flow, W , by the J.Kg-P). Values relationship B = W,,/K, where K is the Boltzman constant (1.31. for this and To have been determined for n-alkanes and found to be dependent on carbon number. For other organic compounds, correction factors are needed related to the presence of aromatic nuclei and functional groups. Such derivations, many of which are cited in the literature, can also be used for computation of viscositytemperature relationshipsof non-hydrocarbon synthetic lubricants, provided they do not involve associated or highly viscous blends. In these cases, more accurate equations are available involving three constants, e.g., that of Vogel(Z25): 18
(2.19) in which 0is the temperature at which the free volume of the liquid is zero and its viscosity infinite. Walther’s equation (5) is of practical importance for relating the viscosity of mineral oils to temperature:
W = log. log ( V + C ) = A - B . log T
(2.20)
where A, B and C are constants (C = 0.6 for higher and 0.8 for lower viscosities, A and B vary with the oil). Viscosity thus varies linearly with temperature with the coordinates log T and log . log(v + C). The constant B is the slope of the straight line. This is usually symbolised m and can be calculated from:
m=
Wl -w2 logT2 - l0gT1
(2.21)
Values of B vary among mineral oils between 2.0 and 4.4. It can be used for calculating the viscosity of liquids over a wide range of temperatures (about -25 to 160 “C) provided two viscosities at different temperatures are known. Umstatter’s equation (6)is more accurate and is suitable for liquids of viscosity 1-l0 m2.s-’, ar sh lnv= A - B 1nT
(2.22)
Walther’s equation and the slope rn provide good models of mineral oils and most synthetic fluids, except for highly associated substances, but fail to deal with light and heavy oils simultaneously. m can be calculated from known kinematic viscosities at two or more temperatures with Walther’s viscosity-temperature equation. The value of m can also be determined graphically using Ubbelohde’s viscosity-temperature chart (ASTM D-341, DIN 5 1-563). The viscosity-temperature coefficient is a simple parameter: kV = 17r,/vt, (f2
’
fl)
(2.23)
This can be used to characterise synthetic oils. It increases in an homologous series with increasing molecular size. In technical practice, various other relationships are used to characterise kinematic viscosity - temperature variations. Of these, the most widely used is Viscosity Index (Vl), established by Dean and Davis (7). The viscosity-temperature variation of the given oil is compared with, on the one hand, an oil from a Pennsylvanian crude which has a relatively small change in viscosity with temperature (VI = 100) and on the other with an oil from a Mexican Gulf crude with a large change (VI = 0). The following equation is then used: VI=100.-
L-u L-u =loo.L-H D
(2.24)
19
where L is the viscosity (mm2.s-l) at 100 O F (37.78 "C)of an oil with VI = 0 of which the viscosity at 210 O F (98.89 "C)is the same as that of the test oil at the same temperature, U is the viscosity at 100 O F of the test oil, H is the viscosity at 100 O F of an oil with VI = 100 of which the viscosity at 210 OF is the same as that of the test oil at the same temperature, and D = (L -H). Values for L, H and D for the measured kinematic viscosity of the test oil at 210 O F can be found from tables. If the viscosity of the test oil at 210 OF exceeds 75 m m 2 s 1 ,values for L and D can be calculated as follows: L = 1.01523Y2+12.15449Y- 155.61 D = 0,8236 Y2 + 0.5015Y- 53.03
(2.25) (2.26)
where Y is the viscosity of the oil in mm2.s-I at 210 OF.
As thus defined, VZ has a number of serious deficiencies: it has been established primarily for medium-heavy oils and it is not suitable for characterising all oils. At the same viscosity coefficient (v lv f l < f 2 ) , lighter '1 '2' oils have a higher VI; it is not an additive property; it is compiled from viscosities at two ends of a defined scale and so fails to provide information needed at higher and lower temperatures. the scale fails at VI around 140 and above, since two types of oil of identical VI and identical viscosity at 100 O F can have significantly different viscosities at 210 O F (fig. 2.4.) (8).
5
10
20
50
100
m m
1aJJ
mm?+'AT 37.8OC
Fig. 2.4. Relationship between viscosities of oils of differing V1 at 37.8 and 98.9 OC
20
The method of VI determination described above is therefore not suitable for oils of very high VI, e.g., modem engine oils, hydraulic oils and some synthetic lubricants. For such oils with VI over 100, the "extended Vl" (VIE) has been proposed. Several methods are available for overcoming these problems. For example, Blott and Verver (8)replaced viscosities in equation (2.24) by their logarithms to obtain a "viscosity modulus". This idea has been taken up. Following the acceptance of the SI units system, the International Standards Organisation (ISO) prepared a draft standard to establish viscosity index from viscosities measured at 40 "C and 100 "C (to harmonise with the new oil classifications based on viscosity at 40 "C, I S 0 3448). This draft, international standard ISODIS 29022 - Identification of Petroleum Products from Kinematic Viscosities - does not differ in essence from the original method based on viscosities at 100 "F and 2 10 O F . However, a new symbol of VI to cover the ranges up to and over 100 has been established. The determination of kinematic viscosity is based on the viscosity of distilled water, which is 1.0038 mm2.s-I at 20 "C. Two methods are given for determination of VZ from viscosities at 40 and 100 "C: -Method A for VI 0 -100 - Method B for VI > 100 Method A
If the kinematic viscosity of the oil at 100 "C is less than or equal to 70 mm2.s-', the Land D values can be obtained from a table. If it is higher, these values are calculated from: L=0.835313Y2+ 14.6731Y-216.246 D = 0.666904Y2+ 2.8238Y - 1 19.298
(2.27) (2.28)
where L is the viscosity in mm2.s-l at 40°C of a mineral oil of VI = 0 which has the same viscosity at 100 "C as the test oil, Y is the viscosity in mm2.s1 at 100 "C of the test oil, H i s the viscosity in mm2.s-' at 40 "Cof a mineral oil of VI= 100, which has the same viscosity at 100 "C as the test oil. As before, D = (L - H). Viscosity index is then calculated from:
L-u
V l = -. 100
D
(2.29)
where U is the viscosity in mm2.s-l at 40 "C of the mineral oil under test. Method B
If the kinematic viscosity at 100 "C of the test oil is less than or equal to 70 rnm2s1, the corresponding H value is obtained from a table. If the viscosity is higher, the value of H is calculated by:
21
H = 0.168409Y2 + 11.8493Y - 96.9478
(2.30)
The VI equation is the same as in Method A. Calculation methods for VI of oils are contained in CSN 65-6218, ASTM D-2270, IP 226 and DIN 51-563.
2.1.2.2 Viscosity-Pressure Relationships The dynamic viscosity of ideal gases does not change with pressure; kinematic viscosity is inversely proportional to pressure. The dynamic viscosity of real gases, however, increases with pressure. At the critical conditions (TK, PK),the critical viscosity qK of gaseous hydrocarbons of molar mass m is defined as:
qK = 7.7.1 0-7.M'/2P2/3 / TK1l6 (Pa.s)
(2.31)
The reduced viscosity qR of hydrocarbon gases at other temperatures and pressures can be derived from the critical viscosity qK, according to reference (9) and the viscosity can then be calculated from: (2.32) The viscosity of liquid lubricating oils and greases also increases with pressure (see fig. 2.6). An exception to this generalisation is water, in which the increase occurs after an initial decrease. The extent of change depends on the chemical composition of the liquid, as in the case of changes with temperature discussed above. For example, the viscosity of mineral oils at 1,000 MPa increases lo4 to lo5 times, but that of water only 2 times. The increase in viscosity with alcohols is low, but becomes greater as molecular weight increases. Various approximations have been formulated to help define viscosity-pressure variation. One of the most commonly used is due to Barus (162):
qp = qo exp aP (Pas)
(2.33)
where P is the pressure in Pa. qo is the dynamic viscosity in Pas at atmospheric pressure, a is a coefficient, the magnitude of which varies in oils between 1.4 and over 5.0. Pa-' (10)(for example, in paraffinic mineral oils it is 1.6 - 2.6. Higher values are observed for oils of higher viscosity. For cycloalkanes, it is 1.95 2.6 and for cycloalkylaromatics it is 2.3 - 5.1. Among the synthetic oils, it is 1.54 for diesters, 2.05 for polyesters, 5.4 and 3.09 for polyphenylethers and 3.59 for polybutenes.) In technical practice, simpler models are used which ignore the nature and composition of the liquid. The viscosity-pressure effect is less than the viscositytemperature effect - an increase in pressure of 2.5 MPa causes the same increase in the viscosity of liquids as a drop in temperature of 1°C. Some studies of the correlation between composition and molecular size and configuration and pressure-viscosity changes at different pressures have been carried out using a high-pressure capillary viscometer (11)and have indicated that:
22
in contrast to the absolute value of viscosity determined at atmospheric pressure (lo-’ MPa), a is independent of chain-length and size of the molecule; a decreases with increasing internal mobility of the molecule, e.g., when oxygen bridges (ether bonds) are present, and increases when certain functional groups are introduced (e.g., C1- or -OH); the number and position of side-chains, e.g., methyl groups, strongly affects the magnitude of a.Whereas a for iso-hexane (one side-chain) is very little less than that for n-hexane, differences in a for isomers with 2 side-chains are substantially greater. In iso-hydrocarbons with 3 or more side-chains, differences in a coefficients increase still further. The pressure coefficient a is thus dependent on the degree of branching of hydrocarbons. This can be defined by:
v = R,~IR: - I
(2.34)
where Rn2 and R; are the mean values of the squares of the imaginary radii of unbranched and branched hydrocarbons (12); a increases with the number of phenyl groups in the molecule: in compounds with a large number, viscosity increases more steeply - even exponentially - with pressure, i.e., a is not constant but its magnitude increases with pressure; hydrogenation of planar phenyl groups to puckered cycloalkyl groups increases the pressure coefficient a. Viscosity studies in pressure viscometers on oils with chemical composition differences, particularly in respect of aromatic and cycloalkyl content, have given results which depart from Barus’ model. The departure increased with pressure, showing that the pressure coefficient a is not a constant but changes with pressure (fig. 2.5).
P
Fig. 2.5. Differences between measured and computed viscosities according to the Barus relation of oils of different chemical composition
A plot of log q vs. P for different viscosities shows that the isotherms of most mineral and synthetic oils have a greater or lesser degree of concavity, i.e., change regressively with curvature with respect to the P-axis fig. 2.5 Z). By contrast, oils with a high aromatic content and synthetic oils, e.g., polymethylphenyl siloxanes and polyphenyl ethers, show convexity, i.e., curvature changes progressively with
23
respect to the log q axis (fig. 2.5 14.On the other hand, graphs plotted according to Barus' model are linear (fig. 2.5 IZZ) (177). Barus' model overestimates the effects of pressure for most oils, but underestimates it for some. There are even oils for which the log q-P isotherm is S-shaped, with regressive curvature in the low pressure region and progressive curvature in the high pressure region (178). The difference between measured and calculated viscosities cannot be minimised unless the Barus model is extended by a second pressure coefficient p which describes the curvature of the isotherm, as follows: 77p = qo exP(ap + PP2)
(2.35.1)
where a is the pressure coefficient at the start of the isotherm (Pa-'), p is the pressure coefficient for the region of isotherm curvature (Pa-2). The p-coefficient is negative in oils whose log q vs. P isotherms are regressive and positive in oils with progressive isotherms. The values of this latter coefficient vary between 1.0 and 4.0.10-17 (Pa-2)at 40 "C. Fig. 2.6 (11)illustrates the differences in the q-P relationship in mineral and synthetic oils. Because of the wide variation in composition of both mineral and synthetic oils, the magnitude of the pressure coefficient acannot be predicted. Also, a increases with increasing temperature, so that the temperature effect complicates the issue even further. a is thus best determined, in the pressure viscometer. log ll
t=2SoC
7.0
6.2 5.4
4.6 3.8
3.0
22
14
0 40 80 1201M)m
MI% Fig. 2.6. Differences in q - P relationship of oils of different origin
A, B, C - mineral oils of different nature, D - ester dl based on sebacic acid, E - polyether oil, F - silicone oil
Notwithstanding these problems, the following principles can be deduced and applied: - the pressure coefficient a within a single group of oils is higher if the viscosity determined at atmospheric pressure is high, - a close parallel exists between viscosity-pressure change and viscositytemperature change within a closely-limited group of oils; the greater the viscosity change with temperature, the greater its change with pressure, - the magnitude of the pressure coefficient a usually increases with increasing aromatic and cycloalkanes in the oil, 24
- the dependence of viscosity on pressure increases with increasing temperature (viscosity decreases with temperature more rapidly; the dependence of viscosity on pressure as pressure rises is greater in oils of low VZ measured at atmospheric pressure; these differences increase as pressure rises). The above general conclusions emphasise the inaccuracy of the Barus model the coefficient a is far from constant. Cameron also offers a model with 2 coefficients: (2.35.2) 17 = 170 (1 + PO" (126) Models similar to that of Barus involving the coefficient a have also been used for kinematic viscosity. Pywell (135). Kouzel (263), Roelands (164), Fresco (165), Kim (166), So and Klause (267) and others, have proposed other attempted correlations between viscosity and pressure. To allow for the decrease in viscosity with temperature, Roelands (264) defined a pressure/temperature relation as:
77 = exp (In qo + 2.76) (1+-)'
2
( T + 135 )O-' To+ 135
- 2.76
(2.35.1)
where qo is the viscosity in mPa.s at T = To and P = 0, P is the pressure in Wa, z is the viscosity-pressure index, and So is the viscosity-temperature index. A plot of the index z against viscosity and density is shown in fig. 2.7.
Fig. 2.7. Dependence of the viscosity - pressure index z on the density and viscosity
25
It will be observed that the dependence of viscosity on temperature becomes more marked as pressure increases. For instance, the ratio q ( d q ( 3 0 changes with pressure as follows (172): P (MPa) 0.1 100 500 1000 5000
30
T(OC) 60
100
1 1 1 1 1
0.222 0.140 0.043 0.014 4.9. 10-5
0.056 0.025 0.0025 0.00029 6.5 . 10-9
So and Klause (267) suggested and tested the equation which displays the smallest deviations between experimental and calculated values for the pressure coefficient a (see Table 2.2):
aK = 1.216 + 4.413 (logVo) 3*0627 + 2.848.104m~*'903(lOgVo) 1.5976-
- 3.999 (logvo) 3.0975pO IlfJ9
(2.36)
where aK is the pressure coefficient of kinematic viscosity in Pa.10-8, vo is the kinematic viscosity (mm2.s-') at the base temperature used and atmospheric pressure, mo is a viscosity/temperature parameter based on atmospheric viscosity at 38.8 OC and 98.9 "C, i.e., on the ASTM slope divided by 2, p is the density ( g . ~ m -at ~ )the base temperature and atmospheric pressure. This equation provides a pressure coefficient of viscosity between 0 and 135 "C and at pressures up to 20 Pa for additive-free oils, oils containing polymeric additives, pure hydrocarbons and non-hydrocarbon oils. The parameters selected define the relationship between viscosity and pressure and the flow unit (according to Eyring's theory of viscosity) as well as the density of molecular clusters and the resilience of the molecules. The equation takes account of enmeshing of the molecules and the tlow of molecular segments in conformity with the theory of "holes" in the liquid. It also defines the connection between polymeric and nonpolymeric liquids with respect viscosity, temperature and pressure. Viscosity-pressure coefficients have been proved to be different under static and dynamic conditions (145). Both coefficients are lower under dynamic conditions than under static conditions. a diminishes with increasing peripheral viscosity, time, increasing load and pressure. other irregularities are experienced in oil blends. The presence - even at minimal concentration - of low viscosity oil can reduce the pressure coefficient by as much as the value corresponding to the material added, and in polyester-oil and polyether oil blends the value observed for the blend was lower than the values corresponding to those of each component in isolation (168).
26
Table 2.2. Pressure Coefficient of Kinematic Viscosity of some Oils at 37.8 "C (167) Oil identification Mineral, alkanic
Mineral, cyclanic Mineral, cyclanicaromatic
Viscosity (mm2.s-1)
Density
m0
aK (Pa-'. 10-8)
(kg.rn-3)
4.844 9.902 99.33 8.811 78.69
815.9 841.9 871.0 864.3 876.5
4.100 4.000 3.550 4.200 3.810
1.672 1.933 2.567 1.971 2.716
145.9 51 1.1
916.0 932.0
4.235 4.235
3.836 5.041
4.067 10.38 7.665 8.181 8.048 9.790
818.8 847.2 848.0 872.8 888.2 931.6
4.145 3.890 4.010 4.020 4.200 3.660
1.610 I .975 2.064 2.059 2.329 2.348
12.56 88.64 124.4 115.7
901.5 957.5 837.4 828.5
3.505 2.980 3.670 3.640
1.537 2.049 3.587 3.587
1293
909.0
1.830
2.283
1917 1984 29.0
898.0 865.0 997 (20 "C)
1.400 1.805
2.306 2.861 0.95
928.0 886.0
2.230 2.045
1.983 2.325
Mineral, aromatic CdC,/C, by n-d-M analysis
C2+
C2,+
67/22/11 6612 1I13 52J4810 52/27/21 3 1/69/0 3 1134135 Diester, di-2-ethylhexyl sebacate Polyester Polybutene Hydrogenated polybutene Mineral. cyclanic-with light polymethacrylate -with heavy p l y methacrylate -with polybutene Polyester (168) Diester - with light polymethacrylate -with plybutene Polyphenylether bis(hydroxyphenoxy)benzene (Z68) Hydrocahons 9-n-octylheptadecane 9-(2-phenylethyl)heptadecane 2-(2-~yclohexylethyl)heptadecane 1,l-diphenyltetradecane 2-n-buty l-n-hexylnaphthalene 2-(Ar)-n-butyl-3(Ar)n-hexyl-tetralin
2-n-butyl-3-n-hexyl-decalin
226.8 224.7
380
1207 (20 "C)
5.4
8.931
790.5
3.830
1.614
9.383
844.1
3.805
1.756
14.73 18.62
82 1.6 906.9
3.860 3.805
2.003 2.025
12.49
920.9
4.405
2.000
14.83 15.28
896.7 863.9
4.205 4.425
2.115 2.379
27
A general relationship exists between the viscosity-pressure coefficient, the viscosity-temperature coefficient A(T), the compressibility of a polymeric fluid ( 6 ) and the coefficient of thermal expansion a(p): (2.37)
However, its applicability is limited by a lack of information (146). The relationship between 77 and P(T)has mostly dealt with, so far, for Newtonian liquid oils. However, such oils may, in certain regions of temperature, exhibit strongly non-Newtonian behaviour (e.g., highly paraffinic or waxy oils at temperatures close to their congealing point or “pour-point”). Solid paraffin waxes can be separated from oil by the application of pressure. The consequence of this is a sharp increase in viscosity. At temperatures close to the pour-point, a relatively low pressure change is sufficient to convert a liquid oil into a solid (see Jig. 2.8) (179).
0
50
100
150
200
250 MPa
Fig. 2.8. Dependence of the viscosity of paraffinic oil on the temperature and pressure
Polymer-thickened oils are also non-Newtonian fluids. These polymers may be viscosity modifiers and viscosity index improvers (e.g., in multi-grade engine oils, gear oils and high VZ hydraulic oils). Knowledge of qoT- !(T) relationships for such oils has practical importance. Comparison of the changes in viscosity with pressure (and temperature) of such thickened oils, compared with oils made from raffinate alone have been published (I75).The oils tested were commercially-available oils, prepared by thickening to almost identical viscosity at 100 “C, a raffinate (a so-called high-pressure hydrogenate - oil A in Table 2.3) with: polyisobutene (PIB) - case 1, polymethacrylate (PMA) - case 2, styrene-butadiene copolymer (SBC) - case 3, styrene-isoprene copolymer (SIC) - case 4, ethylene- propylene copolymer (OCP) - case 5. 28
The q-p(n vs. P relationship was examined up to 250 MPa and temperatures 40 to 80 OC. Results for these oiVpolymer blends are shown in Table 2.3. The q vs. P(T)characteristics of non-thickened, high-pressure hydrogenated oil are shown for comparison. This oil was produced from a paraffinic crude at a pressure of 15 MPa and about 400 OC over a Ni-W/Al,O,.SiO, catalyst, and formed the base oil for oils 1-5 listed in Table 2.3. Also included are results from a Duosol raffinate (Oil B) and a blend (Oil C) of Oils A and B to approximately the same viscosity as Oils 1-5 in Table 2.3. Results for tests on these oils are given in Table 2.4. The q vs. P(n characteristics of the polymer-free raffinate oil blends A,B, and C confirmed the expectation that coefficients increase with the aromatic content of the oil. The effect of a higher aromatic content in oils B and C was not off-set by the substantially higher content of cycloalkanes in oil A. This demonstrates that the effect of aromatics on the sensitivity of an oil to pressure is substantially greater than the effect of cycloalkanes. Because viscosity is a measure of the degree of mutually attractive intermolecular forces or internal friction, i.e., resistance to movement among themselves - the rate of movement being determined being determined by the distance between the molecules and their mutual alignment - it follows that the increase of oil viscosity with pressure is a function of density, and hence of compressibility of the oil. Since compressibility decreases with pressure, pressure coefficients also decrease with pressure. Of the raffinate oils, the highest decrease occurred with oil C, from which it can be inferred that the isotherm log q vs. P is affected not only by the oil's aromatic content, but also by other factors, probably the cycloalkane content. It has additionally been confirmed that oils with an adverse viscosity-temperature relationship have higher pressure coefficients. The non-Newtonian, polymer-containing oils 1-5 mostly have lower pressure coefficients a than oil C, which has an almost identical viscosity at 100 OC.The pressure coefficients of some oils with polymers are even lower than those of oil A of substantially lower viscosity. It may be inferred from this that the presence of polymers in oil adversely affects the q-P relationship. Knowledge of the viscosity-pressure behaviour of oils of various compositions is essential in technical practice in a number of applications, such as gear lubrication, hydraulic systems, heavily-loaded anti-friction bearings (operating up to 700 MPa) and, generally, everywhere that the elastohydrodynamic regime is expected to be involved. The importance of viscosity-pressure effects is demonstrated by the Dowson-Higgins equation (176), for the thickness of the lubricating film in EHD lubrication:
h0 -- 2.65
a0.54 q00.7 y0.7 ~ 1 0 . 0 3,E0.43(~Nn)-0.13(m)
(2.37.2)
where a is the pressure coefficient of viscosity (m2.N-' = Pa-'), qo is the dynamic viscosity at atmospheric pressure of oil entering the lubricating wedge (N.s.m-2 = Pas), E' is the mean elasticity modulus of the friction surfaces (Pa), 29
-
Table 2.3. q P(T)Relationships in Oils Containing Different Polymers Oil identification
1
2
3
4
5
90.9 9.1
94.0
99.04
98.15
99.33
Composition (% wt.) Oil A PIB (1) PMA(2) SBC (3) SIC (4)
6.0 0.96
1.85 0.67
(5)
Properties: Density (kg.mS3)at 40 "C at 80 "C Dynamic viscosity (mPa.s) at 40 "C at 80 "C Kinematic viscosity (rnm2.s-') at 100 "C Viscosity Index Pour-point ("C) Viscosity (mPa.s) at different pressures (MPa) and 1 temperatures ("C)
40
Pressures 0.1
72.17 207.5 542.5 151.7 2792
50
100 160 200 'IS0
80
853.7 829.4
854.0 829.6
851.8 827.3
853.2 828.7
851.3 826.7
72.17 16.16
58.82 15.45
64.99 14.92
62.79 15.81
67.52 15.34
12.01 150 -16
12.0 182 -42
11.13 150 -14
11.52 160 -16
11.47 143 -16
2
40
80
16.16 58.82 15.45 37.89 165.7 35.1 82.36 427.2 73.1 189.2 1184 156.9 310.1 2176 243.2
Oil identification 3 Temperatures 40 80 40
4
80
5
40
80
64.99 14.92 62.79 15.81 67.52 15.34 183.7 32.3 178.7 36.71 87.8 34.93 417.5 66.2 45.8 78.3 480.4 73.79 184.8 146.1 1233 175.1 132.7 162.9 250.6 236.9 2193 280.4
at pressure (MPa) 0.01 200
al.lO'E(Pa-l) %.lO-E(Pa-l) p.lO-l7(Pa-2)
4.47 9.0 2.21 1.83 -1.89
3.81 8.95 1.78 1.48 -1.15
2.16 1.80 -1.77
4.37 10.48 1.73 1.37 -1.76
2.16 1.83 -1.68
3.97 7.82 1.66 1.38 -1.15
2.20 1.78 -2.10
4.40 1.77 1.44 -1.63
2.13 1.79 -1.67
1.73 1.41 -1.58
al characterises viscosity rise with pressure at the beginning of the isotherm; 3 at the end (at final pressure). Notes: (1) oil concentrate containing about 50% (wt.) PIB of MW = 3.10.104; (2) oil concentrate containing about 45% (wt.) PMA of M W = 9.16; (3) 100%copolymer MW = 1.16; (4) 100%copolymer MW = 9.104; (5) 100%copolymer MW = 8.104.
30
-
Table 2.4. 17 P(n Relationships in Raffinates Raffinate Oil Identification
Hydrocracked oil (A)
Duosol Raffinate (B)
Blend (C) = (A +B)
100
33 67
15 72 13
15 37 48
15 48 37
8 4 1
38 9 1
29 7 1
at 40 "C at 80 "C
85 1.2 826.6
883.3 859.6
871.8 847.7
Dynamic viscosity (mPa.s) at 40 "C at 80 "C Kinematic viscosity (mm2.s-') Viscosity Index Pour-point ("C)
34.2 8.28 6.05 110 -16
185.1 27.62 16.95 87 -10
94.6 17.42 12.05 94 -1 1
Constituent Ratio (96 wt.) Hydrocracked oil Duosol raffinate
100
Hydrocarbons: composition (96 wt.) Alkanes Cyclanes Aromatics of which: mono-aromatics di-aromatics tri- & tetra-aromatics Properties: Density ( k g d )
Viscosities (mPa.s) at high pressures (MPa) and temperatures ("C) 40 Pressures 0.1 50 100 160 200
Temperatures 80
40
80
40
80
34.2 8.28 98.15 19.19 259.6 41.12 748.6 92.55 1421 149.7
185.1 626.7 1933 6607 13920
27.62 70.92 169.1 435.1 770.1
96.46 303.1 862.3 2652 5145
17.12 42.8 96.73 233.4 394.7
qm at pressure (MPa.s) -
0.1
4.13
6.70
5.54
'Is0
200
9.49
18.08
13.04
2.19 1.86 -1.63
1.76 1.45 -1.57
2.53 2.18 -1.86
1.96 1.66 -1.48
2.39 1.99 -1.99
1.87 1.56 -1.54
31
rE is the mean radius (m), and FN,, is the load on the friction surfaces per unit contact length.
Viscosity at Phase Boundaries The magnitude of viscosity at phase boundaries is still the subject of much discussion. Arguments based on molecular theory lead to the conclusion that the viscosity of a liquid is higher the closer the phase boundary is approached. In addition, viscosity at curved surfaces is higher than at flat surfaces and higher at concave surfaces than at convex surfaces. The following equation has been used:
(2.38) where qE is the viscosity at the boundary, q is the viscosity inside the liquid, v is the surface tension, M is the relative molecular area, r is the radius of curvature of the contact surface or its asperities, p is the density of the liquid, CF is a property related to the composition of the phase in contact, T is the absolute temperature, and k is a constant. All effects which increase qE also increase the load-carrying capacity of the lubricant film. Phenomena occurring at the phase boundary can be affected by the adsorption of foreign substances, e.g., additives. The effective viscosity at the phase boundaries can then grow significantly.
2.1.2.3 The Relationship between Viscosity, Viscosity-Temperature Effects and Chemical Composition The chemical composition of liquid lubricants has substantial effects on their viscosity and viscosity-temperature behaviour. The viscosity of oils varies with the nature and structure of the compounds they contain and with their molecular weight and boiling-points. Generally, viscosity in an homologous series increases with molecular size and boiling-point. Among hydrocarbons, the lowest viscosities are found in straight chain (normal) and branched chain (iso) alkanes. For the same carbon number, the viscosity of isoalkanes decreases as the length of the main chain diminishes, in comparison to those of n-alkanes, whilst the viscosity of isoalkanes increases with increased branching (especially at the end of the chain). The viscosity of cycloalkanes and aromatics increases, for a given molecular weight, with the number of rings. Cycloalkanes with 2 or more rings in the molecule have higher viscosities than mononuclear aromatics, while cyclohexanes are more viscous than cyclopentanes of the same carbon number and with similar structure
32
(Table 2.5). As the number of rings in the molecule increases, the difference in viscosity widens between polynuclear aromatics, polycycloalkylaromatics and polycycloalkanes, the viscosities of the polynuclear aromatics being the highest. Complexity of ring structure, the insertion and number of lateral substituents (and their degree of branching) augment the increase in viscosity of cyclic hydrocarbons. On the other hand, viscosity drops with unsaturation of the substituents. Similar principles also apply to synthetic oils. Polypropylene glycols, for instance, have higher viscosities than polyethylene glycols, because of the branching introduced by the pendant methyl groups. Phthalate esters have higher viscosities than straight-chain sebacates of the same alcohols, even at lower molecular weight. Viscosity decreases when methylene groups in hydrocarbons are replaced by ether or ester groups. By contrast, substitution by a carbonyl group or a sulphur bridge raises the viscosity. The carbonyl group has a similar effect to branching (23). Viscosity can be further increased by replacing hydrogen in a molecule with a halogen. Compounds possessing a dipole moment, or capable of forming hydrogen bonds, have, at lower temperatures, higher viscosities than non-polar substances. For example, esters of trimethylolethane with fatty acids have higher viscosities the more non-esterified hydroxyl groups they contain (24). Non-esterified and non-etherified glycols have higher viscosities than their analogues where hydroxyl is replaced by hydrogen (23).This is the result of molecular association which makes the effective molecule larger than the assumed molecule. Generally, viscosity increases as free volume decreases, i.e., the more molecules get in each other's way in a given volume of liquid and the higher are the physical or chemical forces of attraction between them. Viscosity would be infinite if free volume were equivalent to zero. The change in viscosity with temperature of a liquid lubricant is affected by the nature and structure of its constituent compounds. Generally, all effects which increase oil viscosity also increase the change in viscosity with temperature.
Table 2.5. Viscosities, Viscosity Coeffkients, Viscosity Indexes and Pour-Points of C,, Hydrocarbons Viscosity (mm2.s-1) at Viscosity Coefficient VIPourpoint 98.9OC 37.8"C for 37.8198.9"C ("C) Pentacosane 9-n-oct ylheptadecane 9-(2-ethylphenyl)heptadecane 9-(2-ethylcyclohexyl)-heptadecane 9-(2-propylcyclohexyI)-heptadecane 1,5-diphenyl-3-(2-ethylphenyl)pentane 1,5-dicyclohexyl-3-(2-ethylcyclohex y I)-pentane I ,7-dicyclopentyl-3-(propylcyclopentyl)-heptane
3.00 2.49 2.53 3.29 2.91
10.4 8.93 9.38 14.72 1 1.53
3.46 3.6 3.71 4.55 3.97
163 116 108 115
53.3 -13.8 -26.7 -30.8 -20.5
3.83
25.7
6.71
-15
-32
10.13
170
16.8
-6
-41.2
4.64
25.68
5.55
107
-23.7
101
33
Among hydrocarbons, the n-alkanes have a flat viscosity-temperature curve (high VI). Iso-alkanes have lower Vl’s than n-alkanes of the same carbon number. These differences increase with increasing branching of the chain and asymmetry of the alkyl substituents. The coefficient of viscosity and VI of cyclic hydrocarbons are lower than those of alkanes and the differences increase with increasing ratio of cyclically-bonded carbons to alkane carbons in the molecule; the lowest Vl’s are those of the polycyclic hydrocarbons with low-carbon number alkyl chains. Remarkable exceptions among the bicycloalkanes are the bicyclopentane derivatives (Table 2.5). The dependence of viscosity on temperature increases with the number of rings in hydrocarbons (both alkylaromatics and alkylcyclohexanes); condensed polynuclear aromatics have lower VI than polycycloalkanes.Viscosity increases and VI decreases with hetero-atom content (O,N,S). The VI of oils produced from the same crude source decreases with increasing density and viscosity of the oil fractions. Similarly, the viscosity-temperature curves of synthetic oils of differing composition vary. Generally, compounds characterised by a very low change in viscosity with temperature have linear structures, i.e., a low ratio of molecular “thickness” to molecular length, and low resistance to free rotation around the -C-C- bond; they pucker and coil when cool and extend when heated as they have no chemical groupings which would initiate molecular association (13). Polysiloxanes exhibit relatively small changes in viscosity with temperature. The reason for this is the free rotation of atoms around the large silicon atoms and the presence of numerous oxygen atoms in ether bonds. The polymethylsiloxane molecule resembles, at low temperatures, a compressed spring. The “spring” extends as the temperature rises and the free volume of the liquid diminishes more slowly than if the molecule had remained compressed. The extended molecules are attracted to each other by the expanded exposed surface and by the dipole moment of the Si-0-Si- groups; all this increases viscosity and flattens the viscosity-temperature curve. Polysiloxanes with other alkyl groups or with phenyl as substituents are less elastic and their viscosity-temperature behaviour is less advantageous (15). Changes in viscosity with temperature increase in the sequence: Silicate esters Polymethylsiloxanes Polyethylene glycols esterified or etherified on both hydroxyls Mixed polyethylene and polypropylene glycols Esters of dicarboxylic acids and monohydric alcohols and complex esters of monocarboxylic acids and neopentyl alcohols n-Alkanes Iso-alkanes Alkylcyclopentanes Condensed alkylcycloalkanes Paraffinic mineral oils Cycloalkanic mineral oils Poly nuclear aromatics 34
Halogenated hydrocarbons have the steepest viscosity-temperaturecurves. The large halogen atom hinders free rotation around the -C-C- bond and reduces the elasticity of the molecule. The curve steepens increasingly from fluorine to iodine. Although perfluorinated compounds contain small F atoms, the molecules are not elastic, because fluorine - as an electron-acceptor - reduces the strength of the -C-C- bond. Also, Van der Waal's intermolecular forces are weak and thermal expansion and free volume changes are large. The viscosity-temperature curve is therefore steep. Weak Van der Waal's forces in fluorinated compounds also explain their low boilingpoint, high volatility and low surface tension (13, 16). Similarly, chemical composition of oil affects the magnitude of deviations which result from the fact that VZ is not an additive function. In some anomalous cases, the VZ of a blend of oils can be higher than that of the individual components. 2.1.2.4 Relationships between Viscosities of Oils, Viscosity Indexes and Practical Applications
Fluid viscosity, and hence lubricant viscosity, is an important factor in fluid mechanics. The very nature of fluid flow in pipes of specified size and at specified velocity is determined by fluid viscosity. The characteristics of flow in pipes are determined by the Reynolds Number (Re), defined by the equation: (2.39) where v is the fluid velocity in m.s-', d is the internal diameter of the pipe in m, v is the kinematic viscosity in m2.s-'. At Re c 2040, flow is laminar; at Re > 2040, it is turbulent, and in the region 2040 c Re c 2040 its nature is uncertain. Viscosity also affects resistance to flow (pressure drop in the pipe) during pumping of a lubricant fluid or oil. This resistance increases and the amount of oil pumped decreases with increasing viscosity. Given the length of the pipe, its internal diameter needs to be calculated from the lubricant viscosity and the effective operating temperature in order to ensure an adequate supply of lubricant. The viscosity of a lubricant is one of its most important properties in tribomechanics. A sufficiently high viscosity is imperative to establish conditions for fluid friction and therefore affects wear of the components of a machine. In many cases, the minimum or optimum value of viscosity can be calculated from equations (or derived from charts) that take into account the load-carrying capacity of the lubricant film. In some cases, lubricant viscosity is selected by approximations and guide-lines which allow safety margins for deviations from standard operating conditions. In the first category of approach to the problem, a suitable oil is typically chosen according to its viscosity. The flow required to achieve adequate lubrication of a plain bearing under specified operating conditions (pressure and peripheral velocity) can then be calculated.
35
The minimum quantity of lubricant required to achieve full-flow fluid lubrication depends on the size of the bearing, the clearance, the operating conditions and the viscosity of the lubricant (In.This minimum lubricant demand is calculated from the so-called minimum thickness of the lubricant layer or film (ho) for which the following equation applies:
(2.40) where v is the peripheral velocity in m.s-’, P is the pressure required in Pa, h is the dynamic viscosity in Pa.s, k’ is a coefficient depending on the relative length of the bearing Z/d,
y is the bearing clearance = where D is the bearing diameter and d is d
the diameter of the journal. The thinnest lubricating film should not exceed 0.25 x (D- d)in order to prevent internal friction losses within the lubricant. Last but not least, it is important, in order to maintain a fluid friction regime, to balance the outflow of the lubricant into the appropriate part of the lubricating film with the outflow from the bearing. The pressure at the point of delivery required to achieve this state is defined by the classic Reynold’s equation : (2.41)
where P is the average pressure of the oil film (Pa), x is the axial distance between the inlet and the outlet of the oil (m), u is the relative velocity of the sliding surfaces (m.s-l), h is the average thickness of the oil film at atmospheric pressure (m), h, is the average thickness of the oil film at pressure P (m). The oil viscosity which is required to meet design criteria can be calculated in a similar way for the lubrication of slide-ways and gears, or can be determined from charts. The viscosity of the lubricant has to match the operating conditions of the mechanical component or machine which is to be lubricated. The external temperature must therefore be included in the calculation. Different types of oil are therefore available with different viscosities suitable for specific fields of application (bearings, engines, gears. etc.) and these viscosity ranges are significant factors in quality standards for lubricating oils. The classification of oils by viscosity has been advanced to a high level particularly in the automotive sector (engine and gear oils). Viscosity-temperature relationships are vital for liquid lubricants exposed to extreme temperatures. Such lubricants must have the highest VZ so as to exhibit the least possible viscosity at low temperatures (to facilitate start-up of the mechanical components) and a high enough viscosity (including viscosity reserve) at the operating temperature of the machine to provide hydrodynamic lubrication.
36
Automotive engine oils are typical in having these sorts of requirements. Similarly, the viscosity-pressure-temperature relationships of liquid lubricants exposed in operation to high pressures, e.g., gear and hydraulic oils, need to be well defined.
2.1.3 Miscellaneous Rheological Properties’ If the viscosity of a liquid is not constant at a given temperature and pressure but depends on shear rate, its viscosity is termed “apparent viscosity” and the liquid is a non-Newtonian fluid. The fluid properties of such a substance cannot be derived from the laws of conventional hydrodynamics, but must be measured using a variable shear-rate viscometer. The proportionality coefficient of a substance is referred to as its apparent viscosity, (77’) (18, 19, 20). Several types of nonNewtonian fluid are classified by the nature of the function z = A D ) (fig. 2.9).
Fig. 2.9. Fluid flow curves u - Newtonian fluid, b - pseudo-plastic fluid (of structural viscosity), c - ideally plastic substance (Bingham),
d - quasi-plastic substance (Bingham-Casson), e - dilatant substance, f -dilatant substances with a limit of fluidity (a rare category)
The apparent viscosity of a non-Newtonian substance is dependent on the shear stress. However, the majority of these materials are characterised by changes in viscosity with the period of time for which the shear stress has operated. This phenomenon is termed rheopexy or thixotropy, respectively, according to whether the viscosity increases or decreases with time. If a substance can regenerate, at rest after its structure has been destroyed by flow, this phenomenon is referred to as reversible thixotropy. A typical example of this is the reversible conversion of a gel into a sol (and vice versa); this conversion is accompanied by major changes in viscosity. Higher and lower degrees of thixotropy can also be identified, which depend on the time required for the substance to regenerate at rest after its structure has been modified by flow. The substance may not always fully regain its original structure. Thixotropy is mostly met in lubricating greases. Decomposition and regeneration of their structures can occur in two ways: I “Rheology” is the study ofthe flow properties and behaviour of substances. especially those existing in the broad range between the liquid and the solid states of matter (18.19.138,147.182,183).Bondi (147) summarises the rheology of lubricating oils and greases up to 1960, Hutron up to 1972 and Briant et al. (182) up to 1989.
37
- a change in the forces among the particles of thickener, - changes in the size and shape of the thickener particle. Rheopexy is a comparatively rare phenomenon among lubricants. 2.1.3.1 Pseudo-Plastic Lubricants with Structural Viscosity Pseudo-plastic liquid lubricants contain substances which form an ordered structure in the lubricant in the equilibrium state. Such substance include, for example, paraffin wax crystals near the actual waxy pour-point of oils, asphaltenes in residual oils and polymeric additives in oil. This structure does not change provided the system is at rest. If a small change in shear stress or shear rate occur, no change in the structure need take place and the viscosity remains unchanged. At this point, the lubricant is in the so-called first Newtonian region. With increasing stress or shear rate, the structure gradually breaks up and the viscosity drops increasingly. Finally, the structure is fully broken down and the viscosity stabilises. The lubricant is then in the so-called Second Newtonian region (19,20). This loss in viscosity is temporary; it re-increases when the shear-rate decreases. In the transition region, viscosity changes according Oswald’s equation:
q ’ = -Zn o r qr =- r DC D
(2.42)
where q’ is the apparent viscosity, n is the constant of plastic flow (n >1, at n < 1 the phenomenon is dilatancy), c is the complex flow index (Ih), z is the applied shear stress, and D is the shear-rate. In the case of polymer-thickened oils, the second Newtonian region can be observed at shear rates generally above lo3 s-*.In this region, the originally coiled and tangled polymer molecules are extended in the direction of flow, which reduces their thickening effect. This temporary viscosity loss can be expressed as a percentage of the original viscosity of the thickened oil. It is higher when greater forces have to be overcome to extend the molecule, when the viscosity of the original oil is higher when the concentration of thickener in the oil is higher, when the temperature is lower and when the pressure is higher (high pressures may be involved causing substantial viscosity increase). Polymer effects are also involved; the greater the viscosity loss, the lower is the shear stability of the polymer (134). Emulsions and foam are also pseudo-plastic, as a result of a flattening of the dispersed spherical particles in the direction of flow. Temporary viscosity loss can also arise, indirectly, by another mechanism. At high load (2) and high speed of the mechanism (where D equals or even exceeds lo6 s-*)heat is evolved. This heat evolution (Q) per unit volume of liquid per unit time can be calculated from the equation: Q = zD = qD2 = qT
38
o ( P a s ’ = J.m-3.s-1)
(2.43)
Because of this effect, the temperature of the lubricant film rises and its viscosity decreases. Pseudo-plastic lubricants containing high molecular weight substances are of practical importance. They are significant in the context of lubrication in two ways: 4.0 3.5
g 3.0 20 15 1.0 0.5 0
2 4 6 8 d Z 4 6 8 d 2 4 68105 S-’
Fig. 2.10. Relationship between the apparent viscosity of structurally viscous oil and the shear rate and temperature
The shape of the viscosity-shear rate curve changes with pressure, as shown in fig. 2.10 (21) (pressure acts in the opposite direction to temperature). From these curves, a linear decline in viscosity with increasing shear rate may be observed in certain sections of the curve, in the case of a logarithmic representation of the shear rate as in fig. 2.11.
Fig. 2.1 1. Simplified progress of dynamic viscosity in correlation to shear rate in logarithmic scale The function [ q’(D)) can then be derived, according to Bartz and Schultz (22)by the followingempirical equations: For median values of D (qzc q’(D) c q,),
q’(D) = qz + (Aqp - rn!logD+)
(2.44.1)
qID) = qz + AVD
(2.44.2)
For very low values of D, For very high values of D, (2.44.3) tllD) = 1 ‘, where qz is the dynamic viscosity of the base oil, Aqp is the increment in viscosity caused by the structural viscosity contribution of the added substance,
39
rn‘is the slope of the linear portion of the viscosity curve (in a logarithmic plot),
D , where Do = 1 s-I, the reference shear rate. D+ = DO To permit mathematical modelling of the lubrication of, for example, plain bearings, with structurally viscous oils, a new Reynold’s equation has to be set up on the basis of the above relationships. The situation is complicated by the fact that the shear rate has a different value at each point of the bearing gap, not only in the direction of the periphery of the bearing (in the sense of shaft rotation, but also in a longitudinal or axial direction. The reason for this is the lateral outflow of the oil. Viscosity, in the mathematical model, must therefore be treated as a vector. The merits of structurally viscous lubricants in the lubrication of plain bearings are evident especially when high and low speeds of rotation alternate.This type of oil helps to achieve a more uniform pressure distribution in a peripheral direction and, in consequence, slightly lower maximum pressures and lower friction increase during an increase in speed than with a Newtonian oil (23).
Another significant rheological characteristic is so-called “visco-elasticity”. The shear stress effect can straighten and rearrange macro-molecules which at rest are curled and tangled. During this process, these molecules acquire a certain amount of “elastic energy”. As soon as the shear stress ceases, this energy is recovered in the form of retraction of the molecules into the rest state. This cancels a portion of the shear stress imparted to the oil - a phenomenon referred to as “recovered shear stress” (24). This elastic energy is particularly evident when rapid, short-term changes in shear stress occur, as in the engagement of gears or in the lubrication of non-stationary plain bearings. In such cases, a significant proportion of elastic energy may accumulate in the lubricant (in comparison to distributed energy) and this energy can manifest itself in the form of,an increased resistance to fluid flow. The advantage of this is that these oils are less easily expelled out of the contact zone of the surfaces in relative motion than Newtonian oils. The constant for the decay of these phenomena is the relaxation time from the following equation: x = xoe-ar (2.44.4) where xo is the state at the onset of the phenomenon, x is the state after time t, and u is the decay constant - the higher the constant, the more rapid the decay phenomenon. A characteristic relaxation time can be chosen in the form of llu, i.e., the time period in which x drops to l/e (0.37) of the original value. Tension (z) caused by deformation (B) is produced in a solid, elastic body of modulus of elasticity E: z = EB (2.44.5) Expressed as a rate equation: dDB dz -=E.(2.44.6) dt dt Equation 2.44.4can be regarded as an expression of the elastic behaviour of a body.
40
However, the elastic behaviour of real bodies is not perfectly elastic, since deformation lags tension, so that in the simple case, according Maxwell: (2.44.7)
where t A is the relaxation time constant. The right-hand side of equation 2.44.7 expresses the decay of tension generated by plastic deformation of the body. This relationship also applies to visco-elastic substances, e.g., some liquids or polymers, which can behave as elastic bodies if the force acting on the substance achieves a maximum and than decays sufficiently fast that the molecule cannot revert and no “creep-strain” occurs (e.g., in ultrasonic viscometers (140)). Relaxation times can thus be determined by measuring the absorption of ultrasonic waves of known frequency acting on the substance (148, 149).
The following relaxation times at 20 O C have been recorded: to 10-7sin mineral oils of molecular weight (MW) 250-600, 10-9to 10% for MW 500- 1,500, 10-6 to 10% for visco-elastic oils, lo-’ to 10’s for lubricating greases. Increasing pressure by 1,000-fold can increase the relaxation time 50 - 100-fold. A temperature increase of 50 K can reduce the time by 1/25 or more. Increasing polymer concentration and molecular weight increases the time. The relaxation time also changes with a change in frequency . The “relaxation frequency spectrum” of Newtonian oils is highly susceptible to the presence of impurities and additives, but it is less affected by the chemical composition of these oils themselves. Volume deformations in oil are a consequence of relaxation and are essential in oils exposed to marked rises in hydrostatic pressure, e.g., in hydraulic devices. Volume deformation can be calculated from the rate of absorption of ultrasonic energy acting on the molecule. The periodicity of load on the lubricant in high-speed gears and bearings may be roughly coincident with relaxation time, if oils used are of the visco-elastic type. As a result, the lubricant is not expelled from the contact zone (e.g.,during engagement of gears (25)) and the load-carrying capacity of the lubricant and the gear-train may be increased. The elastic energy accumulated in a structurally viscous oil may be a source of force acting in a vertical direction. Such forces were observed and measured first by Weissenberg (26) - the so-called Weissenberg effect. Elasticity can cause a tensile stress in the fluid acting in a direction parallel to the shear plane. Since the flow in a bearing is circular, the tensile stress also acts in a circular direction. This results in a centripetal pressure, which itself generates an inwards-acting vertical force. This force tends to separate the friction surfaces (fig. 2.12). Axial bearings designed on this principle can be loaded without the need for any hydrostatic or hydrodynamic pressure and practical tests have confirmed this theory (27). 41
Another, related phenomenon concerning the lubrication of plain bearings, still under discussion, can be attributed to structural viscosity. A Newtonian oil in service is expelled laterally from the bearing gap and sucked back only in the region under pressure. In contrast, a structurally viscous oil could be recycled back into the region of no pressure in the bearing, so that the rate of flow-out would reduce and the loadcarrying capacity would increase. Howel :r, these theories have not been confirmed experimentally. la
II a
Ib
II b
111 a
111 b
Fig. 2.12. Demonstration of Weissenberg’s phenomenon I-immobile bar in the rotating vessel, 11 - sliding disc in the rotating vessel, Ill - free rotating vessel, a - Newtonian oil, b - pseudoplastic oil
2.1.3.2 Quasi-Plastic Lubricants Plastic substances (“Bingham’s simple bodies”) behave, up to a point of critical shear stress (zo = shear stress threshold value), as solid substances, deforming elastically according to Hook’s law. Beyond this value, they behave like viscous fluids and dD changes linearly. These relationships are defined by Bingham’s equation, which characterises “Bingham’s flow”:
z-
To = qpD
(z>
To)
(2.45)
where qp is the so-called “plastic viscosity”, a constant. Quasi-plastic substances (“Bingham’s complex bodies”) behave similarly, with the difference that the function z/D is exponential above the shear stress threshold value at first, and Newtonian flow becomes evident only at high shear rates. The De Wael-Bingham equation applies:
(z-TO) = 77’0 (z> TO) or Casson’s equation:
1 (z-1/2 q’ = -
zo112)2 D This equation has been modified by Czarny and Moes (169)to: q’ = 1 (T l/n - zol/n)n D
(2.46) (2.47.1)
(2.47.2)
where n is a variable dependent on the type and consistency of the lubricant and on the type and temperature of the thickener. It can be calculated from known values of z and D and generally n > 1. Lubricating greases are examples of Bingham’s complex bodies. The 42
characteristics of their fluid behaviour are described, for example, by Forster’s equation (28): (2-
= ( n - qinf)
(2.48)
where n is the constant of plastic flow and qinfis the viscosity at infinite shear. Other equations provide definitions of the dependence of viscosity on shear rate of lubricating greases. Sisko, for example, states that:
77 = a + bSi
(2.49.1)
where S , (= 4 Q / d ) is the nominal shear rate at the pipe wall, r is the radius of the pipe, Q is the quantity of lubricant moving in unit time and a, b and n are constants. Czarny and Moes (169) have closely examined the wall effect which is a characteristic of lubricating greases, which form a layer on contact surfaces - where they meet a metal or other substance - which has different rheological properties from the rest of the grease present. The thickness of this layer varies with the nature of the grease, and the shear stress threshold value on the wall can be many times lower than that in the bulk lubricant mass. If z is the distance from the wall, d the imaginary parameter at which the threshold value would be zero and s the distance at which zo attains the threshold value throughout the entire available volume, the following equation applies: (2.49.2)
zo = zo(vol) when z = 0. The following values have been observed for lithium greases:
1 2 3
2 2.4 3
9.5 5.0 3.0
1.5 0.6 0.6
0.55 1 .o 0.2
260 543 1367
38 90 200
The following equation has been established for flow in the volume of the liquid for the pressure gradient, AP,per unit length, 1 A P 2
(2.49.3)
At the wall: 43
where Q is the flow in m3.s1. A significant consequence of the wall effect is that resistance to flow is reduced in very small orifices. In central piping manifolds, a substantial proportion of the lubricant - sometimes all of it - flows within a “wall” regime. These drops in viscosity of a lubricating grease are a consequence of a change in its internal structure caused by shear stress; kinetic energy is consumed in the process of achieving interaction between the thickeners and the continuous liquid phase (solvation effect) to deform the structural geometry of the dispersed particles and separation of the particles bonded together by electrostatic and Van der Waals forces of attraction (30). If the shear rate continues to increase, particle alignment becomes sufficiently pronounced as to give rise to optical anisotropy (manifested by double refraction) (31).To study the rheological properties of greases, it is necessary to define not only the shear stress or shear rate but also the length of time during which it persists. Such a relationship is illustrated in the 3-dimensional chart infig. 2.13 (32). Herschel and Buckley have found that viscometric values for lubricating greases fit a rheological model containing 3 characteristic parameters: z = zy+ q.)Dn
(2.50.1)
where z is the shear stress, D is the shear rate, and z is the yield stress (threshold value). Y If n is less than 1 (greases thinned by shear rate), the viscosity at D -> infinity would be equivalent to zero; on the other hand, if n is greater than 1 (greases thickened by shear rates), the viscosity at D ->infinity would be infinite. These two solutions are physically unacceptable. It appears that at very high shear rates or shear stresses, the fibres will be aligned in the direction of flow or the grease structure may even disintegrate. In either case, the end-result will be something like the base oil, or a suspension of soap in oil. So, at extremely high shear rates, grease viscosity tends towards that of the base oil. Experiments have shown, that where “lubricant starvation” is avoided, greases have formed films thicker than or at least as thick as those formed by the base oil. Mathematically, this means that in order to describe performance up to these limits, a third term is necessary in the Herschel-Buckley model:
r = zY + q.)D(”+nd/D
(2.50.2)
where nb is the base oil viscosity. Viscosity of a grease is the most significant property for its application as a lubricant. It affects the capability of the lubricant to penetrate the site to be lubricated, the ease of start-up of the lubricated mechanism, energy losses etc. The
44
viscosity of the lubricant at its lowest operating temperature should not exceed 1,500 - 2,000 P a s at a shear rate D of 10 s-I. The value of D also affects the nature of flow. With a low D, flow in piping is plug-flow; at higher D, the velocity is distributed parabolically from the wall to the centre line of the pipe. The viscosities of greases depends on the origin and properties of both the dispersed and continuous phases. Low viscosity oils are suitable for making greases to operate at very low temperatures. Good base-oil viscosity-temperature properties also improve this property in a grease made from the oil; the set-point or pour-point of the oil, however, do not substantially affect the viscosity of the grease. Composition and concentration of the thickeners have considerable influence.The viscosity of the grease increases with increased dispersancy of the thickener and its concentration in the lubricant.
0
1
2
3
4
5
Fig. 2.13. Relation between the viscosity (q),shear rate (0) and shearing time (t) of lubricating grease The apparent viscosities of greases can be determined in a rotary viscometer of the co-axial cylinder or cone-and-plate types (55). Standardised methods are described in GOST 7163-63, CSN 65-6332 and ASTM D-1092/62. In these, a specimen of the greases is forced through a capillary by a piston linked to a hydraulic system, The apparent dynamic viscosity can be calculated from the speed and force exerted in the system using Poiseuille’s equation (2.7.1). Measurement of viscosity and yield-point of greases with a plasto-viscometer is specified in GOST 9127-59. The nature of dynamic viscosity determined in this way provides information on the pumpability of a grease, i.e.. its susceptibility to being forced through the pipes in the lubricant manifold. However, these standard methods have the disadvantage that the capillaries are too short to allow changes in the
45
grease to develop (changes which do occur in long pipes). The National Lubricating Grease Institute (NLGI) has dealt with this deficiency by suggesting various modifications to the method, e.g., ASTM D- 1092-62. Better results were obtained with a testing device developed in Germany. The main functional component of this apparatus is a 3m long, 7mm diameter, accurately-machinedmetal capillary (the device is described in Stahl-Eisen Betriebsblatter Nr. 181 306.61). In addition, the German method, incorporated in DIN 5 1-80, includes a device attributed to Kesternick for determining the exit pressure of the lubricant. The test lubricant is used to fill a nozzle, adjusted to the required temperature and forced out under gas pressure. The pressure necessary to expel the lubricant at test temperature is measured.
In characterising the flow properties of greases by apparent viscosity, shear rate and the duration of application of shear stress must also be defined; however, in technical practice, a much simplified criterion - “consistency” - is normally used. This term has a variety of different definitions. For example, the Society of Rheology (33) defines consistency as that property of a substance which offers resistance to permanent changes in its form. NLGI (34) defines consistency in a similar way, indicating that it is an attribute of plasticity, whereas viscosity is an attribute of fluidity. ASTM defined consistency in 1958 as the resistance of a nonNewtonian body to change in form. All these, and many other definitions, are essentially unsatisfactory because they fail to allocate dimensions to consistency. Consistency is usually determined by measuring the depth of penetration in 10-1mm of a standard metal cone into the surface of the lubricant at 25 OC. The test may be carried out on the original lubricant or on a sample which has been “worked by a certain number of strokes (e.g., 60, 10,OOO, 100,000) in a grease-working machine. Methods of determining grease penetration appear in CSN 65-6307, GOST 5346-50, ASTM D-217-65, IP 50 A N D IP167 (for very soft lubricants) and DIN 51-804, Blatt 1.
Greases can be classified by the rate of penetration into several classes of consistency, from OOO to 6, as shown in Table 2.6. Table 2.6. NLGI Consistency of Greases Penetration at 25 “C (Io-‘mm) 445-475 400-430 355-385 3 10-340 265-295 220-250 175-205 130-160 85-1 I5
Consistency (degree)
Consistency (description)
OOO
almost fluid extremely soft very soft soft medium soft medium medium tough tough very tough (blocks)
00 0 1
2 3 4 5 6
Whilst consistency can be expressed in terms of a simple and conventional test such as penetration, it is, however, necessary to know how temperature and mechanical stress can change consistency. The principal value of determination of 46
consistency by penetration is for inspection and shipping of greases. Penetration can indicate, to some extent, pumpability and flow characteristics of the lubricant in pipes (35)and it provides some information about the likely behaviour of the lubricant in a bearing (36). However, it is also necessary to appreciate that the shearing conditions under which the penetration value was obtained can differ in shear rate and particularly in duration (even after working the lubricant for various times) from the actual conditions which the lubricant will be exposed in pipework or bearings. Unfortunately, even other laboratory methods which attempt to simulate lubricant flow or apparent viscosity at different temperatures, fail to attach sufficient importance to the relationship between shear rate and time. This is especially troublesome at low temperatures. The structure of the thickener (e.g., a soap) may undergo phase changes with increasing temperature; soap solvation may occur as viscosity decreases. Hence, consistency may change with temperature. It may decrease or increase or it may remain constant. All this depends on the origin and composition of the thickener. The properties of most lubricant greases do not change after heating to their melting-points and re-cooling. However, the consistency and shear stress threshold value of’some lubricants rise after a heatingkooling cycle. The reasons for these variations are as yet unknown. In order to measure the mechanical stability of a grease, its penetration after working is determined, i.e., after the lubricant has been exposed to a small shear stress for a specified period of time . As a result of this working, some lubricants become soft (display thixotropy) or hard (and are characterised as rheopectic), accompanied by changes in shear stress threshold value and viscosity. A characteristic indication of the degree of colloidal stability of lubricant greases is “syneresis”. This is the physical phenomenon resulting from the agglomeration of particles in the “solid” phase of the lubricant, either spontaneously or as the result of forces of attraction operating over a period of time, or because of load or temperature. The result of this is the expulsion of the liquid, continuous phase from the colloidal system of the lubricant. The mechanical stability of a lubricant depends on the composition of the thickener, its origin and concentration and the presence of polar substances. It usually increases with thickener concentration. The composition of the thickener usually has a considerable effect on the mechanical stability of the lubricant. Standard methods for assessing the mechanical stability of lubricant greases are based on measuring penetration before and after working. These methods are detailed in CSN 65-6329, ASTM D-217-65, IP 50/64 and DIN 51-804. However, dynamic mechanical tests are more significant. In recent years, some methods have been developed based on the use of electron microscopy studies of lubricant greases after exposure to mechanical stress.
The colloidal stability of lubricant greases depends on a large number of factors. It improves with increased concentration and dispersancy of the thickener dispersed as a solid phase and with increased viscosity of the liquid, continuous phase; the size of the thickener crystallites has a considerable effect. This aspect of stability
47
(syneresis) can be measured by various standard methods which are supposed to simulate the behaviour of the lubricant under storage conditions. The method detailed in GOST 7142-74 (also accepted as CSN 65-6331) most closely approaches operating conditions. The lubricant sample is pressed into a flat metal disk equipped with a weighted piston. The lubricant is subjected to a pressure of 1 0 ' MPa at 25°C for 30 minutes; the loss of mass of lubricant is then determined. Methods for determining the separability of oil from greases are included in the following standards: GOST 2633-48, ASTM D-1742-64, FTMS 321.2, IP121/63, GOST 7142-74. These methods differ in the design of the devices, operating temperature and in the magnitude of the pressure applied to the lubricant. In addition to these static tests, faster methods are available based on centrifugation of the grease sample.
Some further, specific ideas need to be taken into consideration regarding the use of quasi-plastic substances as lubricants. The theory that the viscosity of plastic lubricants in a plain bearing gap is different in peripheral (tangential) and longitudinal (axial) directions applies to greases and is even more relevant than for the structurally viscous oils. The development of hydrodynamic pressure in the direction of the bearing periphery is more uniform than is the case with structurally viscous oils. Therefore, a grease provides the same load-caving capacity at even lower pressures. The existence of a threshold value of shear stress is also beneficial. Because of this factor, the grease has to be forced by external pressure into the bearing gap before it starts to flow. Part of the lubricant is, however, expelled by the action of the hydrodynamic pressure in the bearing. Since the shear stress outside the bearing quickly drops virtually to zero, the lubricant ceases to flow and constitutes a seal, preventing contaminants from entering the bearing and hindering further leakage of the lubricant. The apparent viscosity of quasi-plastic substances is affected - in greater measure than that of the structurally viscous fluids - by the magnitude and duration of shear stress. It is affected, also, by the shear stress applied in the grease gun and by transfer through piping to the site of lubrication. Air bubbles and oil separation can change the rheological properties of the lubricant (37).It is hence very difficult (as it is with structurally viscous oils) to define the effective viscosity of the lubricant at the lubrication site. The main problem with theoretical considerations in this area arises from the fact that the flow properties of a grease may be considerably affected by its viscoelasticity and that the relaxation time of greases are relatively large (0.01 to 10 s) (38).
2.1.3.3 Dilatant Lubricants Dilatancy in lubricants is a rather unusual and little-studied phenomenon. In dilatant lubricants, apparent viscosity increases with shear rate. The reason for dilatancy is probably the breakdown of coarse particles to finer ones with a larger surface area, producing a higher tendency towards solvation. Immobilisation of a major
48
proportion of the oil and an increase in viscosity occur. Some dispersed lubricants, such as those containing polymer particles, are dilatant. In most cases, dilatancy is not a preferred property of lubricants. The growth of viscosity with increasing shear rate, i.e., with increasing tangential speed or revolutions, is undesirable. The exception is the category of pseudo-plastic lubricants, i n which viscosity is permanently lost under these conditions as a result of depolymerisation of polymeric additives. In such cases, it could be desirable to incorporate additives which would impart dilatancy and compensate for the loss of viscosity. Such systems or suitable combinations have not so far been described.
2.1.4 The Compressibility of Liquid Lubricants Compressibility has become increasingly important with the development of hydraulic systems. To design a hydraulic system, it is essential to know how the fluid used for the application of force will change in use: efforts are made to employ fluids of the lowest possible compressibility in order to maximise the efficiency of servo-systems. The maximum acceptable compressibility is specified for certain areas of application, e.g., aerospace. The more compressible or elastic a fluid is, the less effective is the response of the hydraulic control system or servo-mechanism and the more difficult it is to ensure uniform flow of the fluid when it is pumped under high pressure (117). Compressibility can affect lubrication properties - an incompressible lubricant cannot be expelled from the lubricant surface and is more suitable for preventing metal-to-metal contact under conditions approaching boundary friction. In general, fluids with a high modulus of elasticity of compression (low compressibility) are most in demand; the exception is for shock absorber fluids. The relationship between fluid volume and the pressure to which fluids are exposed is non-linear, as illustrated in fig. 2.14.
A
Fig. 2.14. Effect of pressure on oil volume - heavy solvent raffinate (M95) at 20 "C, A' - at 200 "C. B - light solvent raffinate (VI 95) at 20 OC,B' - at 200 OC, C - gas oil at 20 "C,D - wide fraction of heavy gasoline at 20 OC
49
This relationship can be expressed as adiabatic or isothermal compressibility. The volume change under adiabatic compression is very rapid and the heat of compression cannot be removed during the period of compression. The following equation applies to adiabatic compression:
-1 dV y= - - (m2.N-') Vo dP
(2.5 1.1)
The reciprocal l/yis the modulus of volumetric elasticity (also represented by K). For isothermal compression: (2.5 1.2)
The ratio between the compressibility coefficients is the same as the ratio between the specific heats: (2.52)
Values of the ratios for lubricating oils are 1.12 - 1.13 (39).The coefficients of compressibility are also in the same ratio as those of the isobaric coefficient of thermal expansion a to the isochoric coefficient of expansion P, which defines the rise in pressure at constant volume for a temperature change of 1 O C . The following equation can be derived:
a o=-
(2.53)
PP
In technical practice, simpler equation are used, such as:
v, = PVO(P - Po)
(2.54.1)
or AV = VP (PI - Po) where Vo is the fluid volume at pressure Po, V, is the fluid volume at pressure P I , and is a coefficient whose value, [4-9.10-5] depends on the nature of the oil. For empirical estimation, oil volume drops about 0.7% for a pressure rise of 10 MPa. Compressibility can be similarly computed using the method of Watson and Gamson (2). The following equations have also been derived (146): - between compressibility and surface energy, yE 0 = k, ~ 3 1 2
(2.54.2)
- between modulus of elasticity of compression and the molecular parachor P K = k2(P/V,,,0,)6 50
(2.54.3)
where k , and k, are constants, Vmol is the molecular volume. Since the parachor is an additive quantity, K can be calculated from tabulated values of group contributions to parachor. The relationship between compressibility and pressure coefficient has already been mentioned (see above, page 28). Compressibility is further related to elastic properties of isotropic substances: G=K(-
3 2
1-2v -1 l+v
and E = 3K(1 - 2v) = 2G(1 + v)
(2.54.4) (2.54.5)
where G is the elastic modulus in shear, E is the elastic modulus in tension, and v is Poisson’s constant - its value changes from 0.5 (for liquids) to 0.1 - 0.2 (for purely elastic bodies). Values for polymers average 0.35 for glassy polymers, 0.4 for semi-crystalline polymers and 0.5 for elastomers. The relationship between K and cohesive energy Ecoh is very important : (2.54.6) where Ecoh is the “density of cohesive energy”. The compressibility of liquid lubricants depends on their chemical composition, on the size and flexibility of their molecules and on inter-molecular forces. It increases with temperature and decreases with pressure. It decreases with molecular weight in an homologous series of hydrocarbons. For example, the approximate adiabatic modulus of elasticity of compression (in MPa.103) at 38 “C and atmospheric pressure is: mineral oils - 1.4 carboxylic acid diesters - 2.1 silicate esters - 1.4 polyphenylethers - 1.7 phosphate esters - 2.75 silicones - 1.0 to 1.4 Aromatic oils are less compressible than alkanes. The smaller the oil molecule, the more compressible is the oil1. Examples of percentage volume reduction with rising pressure at a specified temperature for some petroleum distillate products are shown in Table 2.7 and elasticity constants in Table 2.8.
For more detailed information, see Report on Pressure Viscosity (4040).
51
Table 2.7. Compressibilities of Petroleum Products (in Terms of Percentage Volume Reduction) (44) Product
Temperature (“C)
71 Octane Broad petroleum fraction Gas oil Light oil distillate Heavy oil distillate Refined oil, VI 95
20 20 20 20 20 20 200
5.1 3.9 3.4 2.9 6.5 7.7
Pressure (MPa) 142 213 8.45 6.7 5.8 5.1
-
-
10.0 8.8 7.7 6.8 12.5 14.4
284
355
-
17.5 14.5 12.2 10.9 9.7 16.5 18.1
12.8 10.7 9.4 8.4
-
Table 2.8. Elasticity Values of some Materials (103 MPa) (146) Substance
V
E
G
K
0.5 0.5
0 0
0 0
2.0 1.33
0.38 0.33 0.3 0.45
3.2 4.15 2.35 1.o
1.2 1.55 0.85 0.35
100 60
77 24.5
3.0 4.1 3.3 3.3 4.1 2.4 2.0 4.0 4.0 39 37
15 90 70 120 220 2000 1000 1000
5.3 35 26
Liquids: Water Organic liquids Polymers: Polystyrene (amorphous) Polymethacrylate(amorphous) Polyamide 6/6 (plycrystalline) HD Polyethylene (semi-crystalline) Polyvinylchloride (amorphous) Polycarbonate (amorphous) Polytrifluorochloroethylene(semi-crystalline) Polytetrafluorethylene(semi-crystalline) Polyformaldehyde (crystalline) Minerals: Quartz Glass Metals: Mercury Lead Cast iron Aluminium Copper Steel (soft) Electrodes: a-alumina Sic Graphite
0.07 0.23 0.5 0.45 0.27 0.33 0.35 0.28
25
44.5
66 lo00
500 500
36 66 70 134 166 667 333 333
Compressibility can be determined either directly, by measuring the volume of a given amount of fluid at different pressures, or indirectly by measuring the speed of sound in the fluid according to the equation (44: y= l/pV2 (m2.N-’)
where p is the density of the fluid (kg.rn”) and V is the speed of sound (m.s-l).
52
(2.55)
Measurement should be made at low frequencies, e.g., 500 Hz, to avoid distortion of the results by structural relaxation phenomena (150). Since the measurement of liquid compressibility is somewhat difficult, methods - which are all subject to certain errors - are available for the calculation of fluid compressibility from the other, more readily measurable, physical variables such as viscosity and density (42, 43).
2.1.5 Thermal Conductivity and Specific Heat The capability of a substance to conduct heat is expressed as its specific thermal conductivity A (W.m-'. K-I) and is defined by the equation: AQ = sAt.(e, -e2)ii
(2.56)
where AQ is the quantity of heat (W.s), At is the time (s), S is the surface area in contact (m2), and (8, - 02)Ais the temperature gradient (K.m-l). Liquid lubricants normally have a very low specific thermal conductivity. This property increases with increasing density and decreases with increasing temperature. The relationship between specific thermal conductivity, temperature and density for mineral oils is illustrated in fig.2.15. Esters are somewhat more conductive than hydrocarbon oils.
7 0.15
63
Y
ci
E 0.14
'
OJ3
800 850
Q12
900 950 1000 1050
0.11
010
0.09
0
50
100
150
200
250°C
Fig. 2.15. Relation between thermal conductivity, temperature and density of mineral oils
Gases also have low specific thermal conductivities. For example, gases used as lubricants have A values at 20 (100) O C as follows (all in W.m-'. K): Air Nitrogen Helium
co2
0.0256 0.0255 0.151 0.0159
(0.0310) (0.0306) (0.1710) (0.0235)
Solid lubricants (graphite, metal sulphides and tellurides, soft alloys etc.) have thermal conductivities several orders of magnitude higher (e.g., graphite 12-150 W.m-I.K, depending on temperature and purity).
53
Table 2.9. Specific Heats (kJ.kg-'.K-') of Mineral Oils Density at 15 "C (kg.tr3) 800 850 900 950 lo00
0°C 1.905 1.842 1.779 1.717 1.633
Specific Heat at 20°C 50°C
70°C
100°C
1.989 1.926 1.863 1.800 1.717
2.198 2.135 2.052 1.989 1.905
2.324 2.261 2.177 2.114 2.031
2.114 2.052 1.968 1.905 1.821
Table 2.10. Group Contributions to Molar Specific Heat at 25 "C (kJ.m~l-~.K")(161)
-CH3 -CH,-CH< >C< =CH, -CH,- in cyclopentanes -CH2- in cyclohexanes =CH- in aromatics =C< in aromatics -0-S-
-F
-c1 -OH -SH -NH, =NH >N-
-co-coo-COOH -CONH-
(s) = solid
54
(1) = liquid
30.85 25.30 15.55 6.15 22.6 13.85 17.95 15.4 8.5 16.8 24.0 (21.3) 27.0 16.9 46.7 20.9 14.2 17.05 23.0 (4.0) (50.2) (37.6-54.4)
36.8 30.4 20.9 7.35 21.75 26.35 26.35 22.15 12.10 35.5 44.75 (20.9) (39.7) 44.75 52.25 31.8 (43.9) 52.7 (54.8) 98.7 (89.9)
85.5
122.95
78.6
112.9
64.9
92.85
Cragee's equation relates the effective specific heat of liquid hydrocarbons at t "C to their relative density (diz:z)1/2(compared to water):
ct=
4.187
(df::)
(0.403
+ 0.000801t)
(kJ.kg-'.K-*)
(2.57)
li2
The specific heat of liquid hydrocarbon is thus indirectly proportional to density and directly proportional to temperature. Pressure dependence is small, and can be ignored for technical purposes. The specific heat of mineral oils varies between 1.7 and 3.3 kJ.kg-'.K-', depending on density and temperature. The above equation is accurate to *4% between 0 and 400 "C for hydrocarbons and hydrocarbon oils with densities 720 - 960 kg.m-3, except for aromatic hydrocarbons and cracked products whose specific heats are a little lower. The specific heats of mineral oils of different densities and at different temperatures are shown in Table 2.9. The specific heats of diesters, phosphates and silicates are similar to those of hydrocarbon oils. These values increase with temperature. Silicones and fluorinated lubricants have slightly lower specific heats and hence may require more cooling capacity. The thermal conductivity and specific heat of lubricants are important for the removal of heat from operating equipment, particularly, for example, from aircraft turbines. The size and weight of radiators and other heat exchangers is determined by these properties. In the case of gases and hydrocarbon vapours, specific heats at constant volume (C,) must be distinguished from those at constant pressure (C,). Their specific heats also change with temperature and density, but they are more dependent on pressure than those of liquids -the specific heats of gases increase with pressure. The specific heats of gaseous lubricants are about half those of liquid lubricants, e.g., that for air at 100 "C and 2MPa is 1.034, nitrogen 1.059; for C 0 2 at 20 "C and the same pressure, it is 1.072 k.J.kg-'.K-'. Solid lubricants also have lower specific heats than liquids. For example, the specific heat of graphite is about 1.0 and that for sulphur is about 0.75 kJ.kg-'.K-'. Table 2.11. Specific Heats of Polymers at 25 "C (161) Polymer Polyethylene (s) Polyisobutene (I) Polyacetal (s) Polyethylene oxide (I) Polyvinylchloride (s) Polypropylene oxide (I) Polyamide 6 (s) Polyamide 6/6 (s) Polytetrafluorethylene(s) Polychlortrifluorethylene (s)
M.rnol-' .El
kT.kg-l.K-*
46
1.68 1.96 1.42
111.2
42.7 90.3 60.0 110.8 164 328
2.05 1.05 1.92 1.46 1.46
96.8
0.97
104.6
0.92
(s) = solid (I) = liquid
55
The specific heats of organic compounds can be calculated with fair accuracy from the molar contributions of the groups in the molecules, as shown in Table 2.10. When the specific heats of polymers are calculated, the molecular weight of the monomer as well as the crystallinity of the polymer, and whether it is solid (s) and liquid (l), have to be allowed for. Table 2. I I specifies the calculated values of some tribologically significant polymers.
2.1.6 Electrical Properties’ofLubricants 2.1.6.1 Electrical Conductivity Electrical conductivity, G, is defined by the equation:
G =-= E
- (m-2.kg-’.s3.A2) R
(2.58)
where I is the current, E is the voltage, and R is the resistance. The unit of conductivity is the Siemens (S), which represents the conductivity of a conductor of resistance 1 ohm equivalent to the “mho” or reciprocal ohm, which appears widely in the older text-books. Clean, dry, additive-free hydrocarbon oils are poor conductors. Their conductivity rises, however, with the concentration of substances capable of dissociating into electrolytic ions, or of molecules into radicals and “electromorphous ions”. These ions migrate in an electrical field and enable electric current to pass through the oil. The increase in conduction in oil with electrical potential difference is exponential rather than ohmic (linear). There is a similar exponential increase with temperature, because the ions move more easily through a less viscous oil. The following empirical relationship applies:
(a),
G = G, e-EdkT
(2.59)
where Go and k are constants for the material and EG is the activation energy for the process of conduction. Time is also an important variable. At constant potential difference, conductivity decreases with time because ions are accumulated at the electrodes, discharged and stabilised at a constant value, so that the formation and discharge of ions reaches an equilibrium. The electrical conductivity of dry, contaminant-free, well-refined and additivefree mineral oils at equilibrium is about S.m-’. By comparison, a 0.5% aqueous solution of NaCl has conductivity about 1 S.m-’. This emphasises the importance of thorough drying and the removal of substances liable to dissociate and, especially, the removal of all contaminants, gases and volatile materials in cases where the oil is required to act as an electrical insulator. Higher conductivity results from the presence of acids produced by aging of oils, so resistance to oxidation is essential 56
for electrical insulating oils. Polar additives increase the conductivity of oils by several orders of magnitude.
2.1.6.2 Electrical Strength Electrical strength is defined as the highest voltage an insulator can withstand without a discharge occurring. It is commonly defined as kV per unit distance. Pure petroleum products normally have a high and roughly equal electrical strength. This property is very little affected by chemical composition and temperature, unless a second phase forms in the oil. Heterogeneous contaminants such as dispersed water, solid contaminants, corrosion products and sludge from oxidation can all have a major influence. Electrical strength can be measured by determining the break-down voltage, Ep, in a homogeneous electrical field between two electrodes of specific shape and size at a predetermined distance from each other, h, by increasing the voltage in a prescribed manner. The following equation holds: E = E,,.h-' (kV.cm-')
(2.60)
Break-down voltage, when it is reached, is manifested by a spark flash between the electrodes. In CSN 34-6632, the electrodes are stipulated to be circular and 20 mm in diameter, positioned 3 m m apart. In GOST 982-56, they are disk-shaped, 25 mm diameter and 3 mm apart. The test apparatus according to ASTM D-877 and IP 120 has similar electrodes. The German VDE standards specify spherical [disks] 2.5 mm apart. Swiss SED regulations require 12.5 mm spheres 5 mm apart. The quantity of oil in the spark chambers varies from 350 to 750 cm3.
Electrical strength is principally important for electrical insulating oils, e.g., transformer and cable oils.
2.1.6.3 Dielectric Losses A dielectric behaves like a capacitor and electric current can pass if an A.C. electric field is applied across it. The ideal dielectric involves no loss in energy, and the current vector leads the voltage vector by 90°. Real dielectrics produce losses, and the angle between the voltage and current vectors, @, is less than 90". The complementary angle, 6, between @ and 90°,is the "loss angle" and represents the magnitude of dielectric loss, P, i.e., the loss of energy expressed as the ratio of wattless power to the power input according to the equation:
P=EZcos@=EIsin6=VZtan6(W)
(2.61)
or tan .IOO = 100 tan 6 (%) (2.62) KI Dielectric loss is normally expressed as t a d , or the loss factor, 100tan6, in terms of percentage loss of power. The loss grows with increasing conductivity (hence also with increasing electrical field and temperature) and polarity of the dielectric is a highly sensitive criterion of the cleanliness of fresh oils and their degree of aging. Low-viscosity oils have lower losses, because the oscillation of molecules or
K=
57
particles in a thin environment requires less energy. Likewise, very high-viscosity oils exhibit low losses, because the oscillation is retarded by the viscous environment. Losses grow initially with increasing frequency, but stabilise as soon as the'particles are no longer able to keep pace with rapid changes in the field (Table 2.12). Table 2.12. Dielectric Losses at 50Hz and 100 "C Loss Factor (%)
Oil distillate Oil raffinate, 24.6% CA Chromatographic fraction from this raffinate % of this fraction %CA 6 0 20 11 24 16.6 18 25.3 6 37.6 Turbine Oil Laboratory-aged turbine oil Colophony-thickened turbine oil Laboratory-aged, colophony-thickened cable oil Polyethylene-thickened cable oil Laboratory-aged, polyethylene-thickened cable oil
30 - 45 0.42
0.02 0.05 0.19 1.54 11.3 0.5
1.7 - 13.0 0.05 - 0.7 1.5 - 2.0 0.2 0.3
If the heat generated by electrical losses exceeds the loss of heat dissipated to the environment around the unit, breakdown may occur and the dielectric may be destroyed. Dielectric loss is an essential criterion for insulating oils, particularly cable oils. Dielectric losses -tan 6 - can be determined with the Schering bridge, which comprises 4 arms. One arm is connected to a capacitor containing the test dielectric, C,, and one arm to a comparison capacitor C,, which should be loss-free. High-voltage A.C. supply is connected to the junctions of the two m s . The other two arms contain capacitors and resistors to balance the bridge. At balance: C, = C,,.R4/R3
tan 6, = wC,.R, = IOC, where C, is the capacity of the capacitor containing the test dielectric oil in pF, C,, is the capacity of the comparison capacitor in pF, R, is the value of the resistor in the arm R3 (SZ), R, is the value of the resistor R4, usually 318.4Q2, wis the angular frequency 2@(s-'), and C, is the capacity of the capacitative decade.
58
(2.63)
2.1.6.4 Permittivity
The effect of the application of electrical fields to dielectrics is to bring about polarisation of the particles; charges present in the dielectric shift mutually and a dielectric flow occurs. The dielectric permeability of a substance, permittivity, is called its dielectric constant. It can be used to measure the molar polarisation of a substance, P, by means of the following equation by Claudius and Masotti:
P=
(E-
1)
M
(&+
2)
p‘
(2.65)
where E is the permittivity, M is the molecular weight, and p is the density. In non-polar substances:
(2.66) where P, is the electron polarisation caused by shifts in the centres of gravity of the negative electrons and the positive atomic nuclei in the electrical field of the light beam, RLL is the molecular refraction according to Lorentz and Lorenz, a quantity theoretically independent of temperature, and n is the refractive index. If E is determined at lower frequencies, P, is constant and E, = n2. In polar substances, electron polarisation is augmented by polarisation due to the alignment of permanent dipoles, PD. If the minute atomic polarisation, PA,caused by the mutual shift of atomic nuclei because of electron shift is neglected, the following applies: P = PE + P , (2.67.1) and E,, > n2
(2.67.2)
The difference ( E - n2) is an approximate measure of the polarisation of the molecules. As temperature and frequency rise, the alignment of dipoles breaks down, P , falls to zero and E,, decreases and approaches n2. These relationships can be used to calculate the dipole moment:
pp = 0.0127.10~’*{(P -RLL)T}’”
(2.68)
The value of the dielectric constant of electro-insulatingoils at 20 O C and 50 Hz is usually 2.1 to 2.3 and that of chlorinated biphenyls 2.5 to 6.4 (depending on composition). Values ranging from 2 (PTFEi) to 4 (polyamide 616)have been found for polymers. Permittivity can be determined, together with tan 6,using a Schering bridge (45).
59
2.2 CRITERIA FOR DEFINING TEMPERATURE REGIONS FOR THE APPLICATIONS OF LUBRICANTS In the preceding section, we have examined the relationships between the changes in a number of physical properties of lubricants which have functional significance. In this section, some specific, temperature-related, physical properties are discussed which the lubricant must possess in order to fulfil the functional requirements. In particular, we shall evaluate the limiting temperature conditions for particular needs, and the additional problems which may arise. The boundaries of operating temperature are associated with phase changes in liquid and plastic lubricants. These phase changes can be followed by several conventional methods - the determination of cloud-point, pour-point, flash- and fire-points in liquid lubricants, and in lubricating greases, droppoint and threshold strength value. It is possible to draw conclusions from such information which can give some ideas, albeit only approximate, about the expected functional properties of the lubricant within the temperature region which is contemplated for its use.
2.2.1 Cloud-Point and Pour-Point When a liquid lubricant is cooled, it does not usually pass abruptly from the liquid to the solid state. The process normally occurs gradually, in two stages. In paraffinic, wax-containing mineral oils, crystals of predominantly paraffinic (n-alkanoic) hydrocarbons first start to precipitate at a certain temperature - the cloud-point. In synthetic oils - e.g., organic ester types - this cloud-point may be an indication of congelation of some components of the blend. In all oils which contain dissolved water, clouding may be associated with the separation of water or ice. On further cooling, the precipitation of paraffinic crystals continues and the paraffin crystal lattice strengthens to such a point that motion of the fluid components is completely inhibited. This is the “true” or “paraffinic” pour-point. In wax-free oils, or oils with a very low wax content, fluidity decreases mainly because of the rise in viscosity with reduction in temperature; at a certain temperature, the viscosity becomes so high (over lo6 mm2.s-’) that the oil simply ceases to flow entirely. This is the “viscosity” or “false” pour-point. “Crystallisation-point” is a more suitable term for the “waxy” substance and “vitrification-point” for non-crystalline materials. The pour- or cry stallisation-point depends on the composition of the oil; it increases with size, symmetry, polarisability and polarity of the molecule. Methods for measuring cloud- and pour-points are practically identical. Hot oil is chilled in a prescribed manner in a test-vessel of specified dimensions. The temperature at which the first wax crystals appear at the bottom of the vessel is the cloud-point (ASTM D-2500, IP 219, DIN 51-583, for transparent oils of cloud-point c49 “C). According to CSN 65-6572, GOST 1533-42 and DIN 51-583, the pour-point is the temperature at which with any further chilling the oil stops flowing. By contrast, according to ASTM D-97, IP 15 and DIN 51-597, pour-point is the lowest temperature at which the oil still flows. The pour-point of hydrocarbon waxes, crystalline and micro-crystallineparaffins - ceresine, petrolatum and vaseline - is determined by observing the bulb of a rotating thermometer. It is defined as the lowest temperature at which the chilled product solidifies (CSN 65-7012, GOST 4255-48, Zhukov’s method, ASTM D-938, IP 76 and DIN 51556).
60
Mineral oils have pour-points in the region -45 - 60 “C and higher. n-Alkanes have high pour-points, because of their molecular symmetry. Asymmetrical isoalkanes have lower and symmetrical higher pour-points than n-alkanes of the same carbon number. Similar rules also apply to cycloalkanes and aromatics; the nonsubstituted and symmetrical hydrocarbons have higher pour-points than substituted and asymmetrical hydrocarbons. The polarisable aromatics solidify at higher temperatures than cycloalkanes. A double bond increases molecular polarity, but at the same time, can cause asymmetry. Generally, pour-point decreases with predominantly asymmetrical molecules and increases with polarity. Ethers usually have lower pour-points than hydrocarbons of the same carbon number in the main chain, particularly when they are asymmetrical (13). This is mainly observed in polyalkylene glycols and particularly in polypropylene glycols, where branching furtherlowers pour-point, as compared with polyethylene glycols. Diester oils, in which every carbonyl group acts as a locus of branching, have low pour-points. These can be further reduced by introducing branched alcohols (e.g., 2-ethylhexanol or 3,5,5-trimethylhexanol) or a slightly branched acid (e.g., 2methyladipic or trimethyladipic acids) into the molecule, or, in the case of polyol esters, esterification with a blend of acids. Trimethylolpropane esters pour at lower temperatures than trimethylolethane esters or pentaerythritol esters (48). Even in these cases, the asymmetry effects a further lowering of pour-point. Polysiloxanes (silicones) generally have very low pour-points. Fluorocarbons usually pour at a higher temperature than their parent hydrocarbons, however, nearly always below -20 “C. Fluidity of wax-free lubricants at lower temperatures is generally better if the viscosity-temperature curve for an oil is flat, and at lower viscosity grades. The requirement for a sufficiently low pour-point for aircraft and instrument oils can only be satisfactorily met by synthetic oils. The most commonly used are ester and polysiloxane oils, which have both low pour-points and low viscosity-temperature coefficients (very high VZ). There is no sharply-definedpour-point for some liquid lubricants. Chilling merely increases their viscosity until the “false” pour-point is reached. Deep chilling below the true pour-point or crystallisation-point can also result in a gradual increase in low-temperature viscosity over time. Because of this, such a procedure may be prescribed for some conditions lubricants must meet at very low temperatures. For example, MIL-L-7808 for aircraft hydraulic oils specifies that the viscosity of a lubricant should not change by more than 6% at -65 O F (-18 “C) when stored 3 hours 35 minutes, and after 72 hours’ exposure at this temperature, the viscosity should not rise from the initial maximum of 13,000 mm2.s-’ to more than 17,000 mm2.s-l (49). Clearly, these conditions are rather intricate and difficult to satisfy. In mineral oils, the “true” pour-point was formerly regarded as an adequate criterion for the use of oils at low temperatures. More recently, the functional qualities of lubricants in this temperature region have been regarded as viscosityrate functions, which are, as mentioned earlier, of a non-Newtonian type;pour-point is merely considered as the temperature limit up to which the lubricant can be treated 61
as a liquid. However, the limiting temperature for pumpability of an oil is not necessarily coincident with its pour-point, but is more usually higher and can be defined with some exactness only from the viscosity, which should not exceed about 5 Pas. Pour-point is not always conclusive in terms of pumpability. It is essential to define the point at which the rate of chilling of the oil in a laboratory test matches that experienced under field conditions. This rate of chilling affects the pour-point and the viscosity of the oil at a given temperature; at a more rapid chill-rate, higher pour-points can be achieved as well as higher oil viscosities, due to the formation of a large number of minute crystals. On the other hand, slow chilling creates conditions for slower growth af a smaller number of large crystals in the viscous solution. This illustrates that it is essential to preserve the same relationship between rate of chilling and time in any test.
2.2.2 Vaporisation, Ignition and Explosion This section deals with boiling-point, vapour-pressure, volatility and distillation limits, together with flash- and fire-points, explosive limits and auto-ignition. Liquid lubricants are characterised by high boiling-points, low vapour-pressures, low volatility and high flash- and fire-points. In mineral oils, boiling-points are not sharply-defined, since they usually comprise a wide range of hydrocarbon distilling at temperatures equivalent to 350 - 510 "C reduced to atmospheric pressure. In practice, hydrocarbons are unable to withstand such temperatures without decomposition and they are distilled at sub-atmospheric pressure and much lower temperatures.
Boiling-Point For the same carbon number, or roughly the same molecular size, the highest boilingpoints are those of polynuclear aromatics, followed by dinuclear aromatics. The boiling-points of mononuclear aromatics, dicycloalkanes, monocycloalkanes and n-alkanes are very similar; the iso-alkanes have the lowest boiling-points. Oils of higher viscosity index have higher boiling-points at a given viscosity (fis. 2.16).This is, however, only true for oils comprising very narrow distillation cuts, with 10 - 90% boiling within a maximum range of 10 "C. Synthetic oils, which are often individual compounds rather than complex mixtures, are better characterised by boiling-points than by distillation curves. At a given viscosity, boiling-point decreases in the series (51): linear ethers and esters linear polymethyl siloxanes aromatic ethers polyphenyl siloxanes chlorinated hydrocarbons fluorinated hydrocarbons
62
The relationship between boiling-point Tv and vapour-pressurep may be defined both thermodynamically and empirically. Antoine's equation is applicable over a relatively wide range: (2.69)
where the constants A, B and C are related to the composition of the substances. Constants for many substances are given in thermodynamic tables. They have been determined with fair precision for alkanic mineral oils.
316
360 404 469 493 538 BOILING POINT IOCI AT 100 kPa
Fig. 2.16. Relationship between viscosities and boiling points in oils of different viscosity indexes (51)
Boiling-point can be one of the characteristicsfor classifyingpetroleum fractions. For example, the characterisation factor: (2.70)
(Tv,sis the mean molar boiling-point of the petroleum fraction in K) is additive on a mass basis and has roughly the following values: 12-13 for alkanic, 11-12 for cycloalkanic, 10-11 for aromatic and cycloalkanic-aromatic and less than 10 for aromatic and heterocyclic petroleum fractions. Volatility The vapour-pressure at temperature up to 50 "C, even of light liquid lubricants, is very low, so volatility at these temperatures can be neglected. Vapour pressure rises, however, with increasing temperature, depending on the concentration of light fractions. The volatility of petroleum oils can be reduced by specifying very narrow boiling fractions. For example, the vapour pressure at 180 OC of oils of viscosity 32 mrn2.s-l at 38 "C is about 200 Pa in a conventional oil, about 50 Pa in a 20 "C distillation cut and about 2.5 Pa in a 10 "C cut. 63
The volatility of lubricating oils is an essential selection factor with important consequences in the context of oil loss and safety in use and handling. Knowledge of oil volatility must be involved in design calculations. In many machines, the lubricant operates in surface films exposed to high temperatures, such as gear teeth, cams, engine bearings, etc. In such cases, volatile losses can cause failure of the lubrication system, due to lack of lubricant or decrease in its ability to lubricate. Oil volatility studies (in an instrument prescribed in MIL-L-7808 for aircraft turbine oils) have shown that volatilisation of engine oil from a surface is dependent on the area of the surface and its temperature, but indepeqdent of oil thickening with polymeric viscosity modifiers (52). Noack's method is usually employed for measuring the volatility of petroleum and ester oils and engine oils in particular (DIN 51 -581). The oil sample is heated for 1 hour at 250 "C and a metered stream of air passed over its surface. Air is then exhausted, along with vapours, at a vacuum of about 0.2 Pa. Evaporation loss is determined by weighing.
Other methods for determination of volatility are described in CSN 65-6245 (heating the oil at specified temperatures between 100 "C and 250 "C for a specified time under an air stream over the oil sample), GOST 9566-74, ASTM D-972-59 (temperatures between 99 and 149 "C for 1 hour), IP 183 and MIL-L-7808 for aircraft hydraulic oils. Methods for determining the volatility of lubricating greases are given in CSN 65-6245 (temperature 100 to 150 "C, time 22 hours) and ASTM D-2595 (93 to 3 16 "C, 22 hours). Empirical relationships between evaporative loss, measured with Noack's apparatus (DIN 5 1-58I), and temperature, time, vapour pressure and external pressure have been published by Rumpf (50).
Flash- and Fire-Points Flush-point can be used as a rough criterion to evaluate the temperature at which an oil is likely to present a fire hazard, the danger that it may mix with air to generate an explosive mixture, and, to a certain degree, as a guide to the magnitude of volatile losses. As defined, it is the temperature at which vapours accumulate in an open or closed testing vessel in a quantity sufficient to ignite when a flame is applied. At a still higher temperature, thefire-point , vapours are generated at a sufficient rate to sustain a fire once it has been ignited. As a rough guide, flash-point is related to Engler distillation volume range by the equation: Flash-point ("C) = 0.64t - 62
(2.71)
where I is the arithmetic mean of the temperatures on the distillation curve at the zero point and those at which 5% and 10% of the oil distils over.
Flash-point is associated with the chemical and structural composition of the oil. At equal viscosity, polycyclic oils have the lowest and paraffins the highest flash-
64
points (polycyclic oils have, for a given viscosity, smaller molecules and higher volatility). Polycyclic oils also have the lowest explosion limits at equal viscosity. Flash-point is very sensitive to the presence of light volatile fractions in the oil. Very low flash-point and a small difference between flash-point and fire-point indicate the presence of light, volatile fractions and the possibility of high evaporation losses. The presence of large amounts of light fractions in an oil is also indicated by the difference between open and closed cup flash-points. A large difference indicated the presence of low-boiling constituents. Methods for measuring flash- and fire-points in an open cup are described in CSN 65-6212, ASTM D-92, IP 36, DIN 51-376 (Cleveland method), CSN 65-6244, DIN 51-584 (Marcusson's method) and GOST 1369-42 and 4333-48 (Brenken's method). The Pensky-Marten methods for measuring flashpoints in a closed vessel are described in CSN 65-6191, GOST 6356-52, ASTM D-93 and IP 34. "Closed-cup'' flash-points are usually 10-30% lower than "Open-cup"; the difference can relate to structure, composition and the concentration of silicone anti-foam additives.
Flammability Flash-point lies about 3 - 5 "C above the temperature at which the concentration of vapours in air corresponds to the lower flammable or explosive limit of the oil vapour/air mixture. At the other end of the concentration range, the upper flammable or explosive limit marks the vapour concentration above which the mixture will not burn. The connection between flash- and fire-points has been used as basis for the classification of liquids in respect of the fire hazard they present in handling, storage and shipping. CSN 65-0201 (which is essentially the same as other national standards) classifies flammable liquids or mixed solutions of them into 3 hazard classes: Class 1 : flammable liquids of flash-points up to 21 "C, Class 2 : flammable liquids of flash-point above 21 and up to 65 "C, Class 3 : flammable liquids of flash-point above 65 and up to 125 "C. Flammable liquids with flash-points over 125 "C are considered unclassified. Flash-points for Classes 1 and 2 can be measured with the Abel-Pensky apparatus, Class 3 with Pensky-Martin. The flammability of lubricants becomes important when they are used in an environment where they can easily catch fire, or where the prevention of fire is essential. Liquid lubricants are generally classified into 3 categories of nonflammability:
-
fluids which are non-flammable in contact with liquid oxygen (e.g., perhalogenated hydrocarbons and esters), - fluids which are non-flammable in contact with air (e.g., halogenated hydrocarbons containing more than 60% halogen), 65
-
fluids of limited flammability (e.g., water/glycol mixtures, oiYwater emulsions, phosphate esters, halogenated hydrocarbons of low halogen content). There is as yet no definition for the formal classification of fluids by nonflammability.
Auto-ignition Igniting a mixture of air and the vapours of a flammable substance by means of an external source of ignition (between the upper and lower explosive limits) occurs more readily the greater the energy of the ignition source. However, at a certain temperature appertaining to each flammable substance, spontaneous ignition may occur on contact with air or other oxidant without any intervention by an external source of combustion energy. This is the auto-ignition temperature. Oxidation chainreactions at this temperature achieve such velocities that the heat generated cannot be abstracted fast enough and the temperature rises to that needed for flame formation. This temperature is not a constant for a substance, but depends mostly on external effects and the operating conditions under which the temperature is measured. These external effects include, in particular, the overall pressure and the oxygen partial pressure; generally, self-ignition temperature decreases as these pressures rise. At low pressures, as temperature rises, flame formation may occur, followed by dying out of the flame, then further flame and burning of the lubricant. At higher pressures, the flame does not die out. These interesting effects are important when oils are used as fuels in incineration plants (61). Time is also an important variable - the possibility of auto-ignitionincreases with extended time at a given temperature. Positive and negative inorganic and organic catalysts also have a considerable effect. Positive catalysts - initiators - include some metals and their oxides, NO,, aldehydes, peroxides and some organic compounds containing carbon-nitrogen double and triple bonds, such as nitriles, i.e., substances which enhance the formation of radicals. Negative catalysts comprise some organic substances, such as aromatic amines, bromine compounds, some metallo-organic and inorganic substances, such as metal carbonyls, some metal oxides (e.g., Sb,O,, Sb,O,) and, quite generally, compounds which can absorb or enhance the nonradical-forming decay of radicals. The walls of the vessel in which the flammable substance is confined can act as catalyst. Whilst the extended wall surface has a negative effect due to heat abstraction, the materials forming the walls can provide surfaces with varying degrees of catalytic effect. Even inactive surfaces can significantly affect autoignition temperature, e.g., cotton-waste soaked in lubricating oil can spontaneously inflame even at ambient temperatures, as can inert lagging soaked by oil leaking from heated vessels. The auto-ignition temperature of flammable liquids, and hence that of most liquid lubricants, depends on their chemical composition. In general, auto-ignition temperature of n-alkanes decreases with increasing carbon number. Various
66
relationships have been derived for hydrocarbons (53,54) which are somewhat contradictory. However, it does appear to hold good that the tendency to autoignition increases with decreasing stability of transient radical species. Hence, aromatics with short alkyl substituents are more stable than those with long alkyls; aromatics than similar cycloalkanes, polyalkenes than monoalkenes and isoalkanes with numerous quaternary and tertiary carbons than alkanes, and so on. These principles also apply to synthetic liquid lubricants. The data in Table 2.13 should be treated with caution, however, because the numbers may change as experimental techniques develop. Auto-ignition temperature also provides an important index of the potential fire hazard during the handling of flammable substances, e.g., when they come into contact with hot bodies during heating processes, etc. Flammable substances (“combustibles”) are therefore classified in some countries into categories based on their auto-ignition temperatures.
Table 2.13. Auto-ignition Temperatures of Some Pure Hydrocarbons, Compounds and Commercial Products Hydrocarbon or product
Hexane Nonane Hexadecane 2,3,3,4-tetramethyl pentane p-butylbenzene tert-butylbenzene Decahydronaphthalene Tetrahydronaphthalene 1-methylnaphthalene Diphenyl oxide Hexachlorodiphenyl oxide Perfluorodimethylcyclohexane Tricresyl phosphate Tetra-aryl silicate Gasoline (20% aromatics) Kerosene Engine oil (SAE 10) Engine oil (SAE 50)
Auto-ignition Ignition temperature delay* (“C) (minutes) 26 1 234 230 431 348 477 272 423 543 646 628 65 1 600 577 482 249 382 410
0.5 1.1 0.4 0.I 1.2 0.3 0.1
0.4 0.2 0.01 0.1 0.1
0.2 1.1 0.02 0.1
* The term “ignition delay” or “induction time” is used to represent the time period between the injection of the specimen of the substance tested and the onset of ignition.
67
For example, in Germany, inflammables are classified, in accordance with the VDE 0165 standard, into 5 classes by auto-ignition temperature: Class GI G (33 G4
G5
Auto-ignition Temperature ("C) >450 300-450 200-300 135-200 100-135
Auto-ignition temperature is determined experimentally, according to GOST 13920-68 or DIN 51 794 (equivalent to ASTM D-2155) by measurement of the lowest temperature of a heated vessel containing air at lo-' MPa pressure, at which a flash appears in the vessel after the oil under test is injected.
2.2.3 Drop-Point and Threshold Strength Value of Lubricant Greases The droppoint is that temperature at which, under the other conditions of a standard apparatus, the first drop separates from the grease. At this point, either the lubricant as whole begins to be converted into a liquid phase, or, as a result of the collapse of its internal structure, a portion of the liquid-phase separates as a liquid (oil) phase. Drop-point is defined as the temperature at which a sample of grease heated in the prescribed test apparatus at a specific rate becomes sufficientlyliquid for a drop liquid to fall under gravity. The methods and apparatus used for the measurement of drop-point of greases are described in CSN 65-6305, GOST 6793-53, ASTM D-566-64 and D-2265, IP 132/65 and DIN 51-801 Blatt. In technical practice, drop point is regarded as a rough - and perhaps problematical - indicator of the upper temperature limit for operational use of some types of grease.
According to generally accepted guide-lines, calcium and sodium hydrocarbontype greases can operate up to temperatures within 20 "C below their drop-points. However, this is not applicable to greases which contain high-drop-pointsoaps, such as lithium and barium soaps. For example, lithium greases with a drop-point of about 190 "C have an operational limit of 120 "C. For these greases, and for greases containing inorganic thickeners (which have no drop-point), the upper limit of operating temperature must be defined from other quality parameters, such as threshold strength value, volatility, oxidation stability etc. Drop-point data are suitable for deducing the type of soap from which a lubricating grease has been manufactured. The higher the melting-point of the soap, the more saturated the fat from which the soap was made and the higher the drop-point of the grease, as shown in Table 2.14. In the USSR,the strength limit is regarded as a better criterion for greases than the drop-point, which is rather doubtful for determination of the point at which the lubricant stops being plastic and starts acting as a liquid, i.e., the upper temperature limit of the operating capability of the grease. The strength limit expressei the
68
critical load at which the grease begins to become liquid, i.e., at which it reaches the “threshold strength value”. Table 2.14. Relationship between the Drop-point of Lubricating Greases and the Degree of Saturation of the Fatty Substances Saponified (54) Fatty substance Linseed oil Cottonseed oil Beef tallow Linseed oil Cottonseed oil Beef tallow
Iodine No. (gUlOOg)
Soap tYPe (metal)
Drop-point (“C)
176 110 41 176 110 47
Na Na Na Ca Ca Ca
103 125 148 71 89 95
The strength limit of most greases lies between 0.05 and 2 kPa (0.5 and 20 g.cm2) within the range 20 “C and 120 “C. According to experience in the USSR, the lowest strength limit of a grease at its highest operating temperature should be 1 - 2. lo2 Pa and at ambient temperature 3.102 - 1.5. Pa. The method and apparatus for determining strength limits are described in GOST 7143-54.The method is based on the determination of the pressure at which movement of the grease occurs at a given temperature in the plastometer capillary.
The strength limit of a grease changes with temperature and shear rate. Strength limit decreases with increasing temperature in most lubricants; exceptions in this regard are greases compounded with silica gel, calcium complexes and indanthrene thickeners, as well as some others where the phenomenon is reversed. A similar correlation exists between strength limit and shear rate. Strength limit decreases with increasing shear rate, to a specific point at which it reaches a value called the liquid limit. The strength limits of lubricating greases depend on their chemical composition, structure and method of manufacture. The size of the dispersed particles and their origin and concentration are most significant. The strength limit of the lubricant increases with the thickening power of the thickener. For example, the addition of 12-15% of lithium soap, 20% of sodium soap or 8-10% of silica gel is necessary to achieve a 1 kPa strength limit at 20 “C in a grease (55). In lubricating greases containing inorganic, complex and other thickeners, the chemical composition of the liquid phase can have a considerable effect. For example, silica gel greases made from mineral oils have strength limits several times greater than the same greases made from silicone oils. The limit is also affected by the method o€ manufacture, especially the pattern of chilling and the final homogenisation of the lubricants and soaps.
69
It is interesting to note that the strength limit of greases does not affect the startability of machines lubricated with such materials (30).This i s because in startup, a shear rate is required between the parts high enough to exceed a critical value of deformation, the threshold limiting value of shear. However, the strength limit significantly affects the ability of the lubricant to be pumped out of its container; high strength limit greases are difficult to transport to the suction piping of the pump.
2.2.4 The Low-Temperature Properties of Greases A number of methods is available for measuring the apparent viscosity of lubricating oils. These simulate field conditions as closely as possible. The behaviour of lubricants at low temperatures, i.e., at the lower boundary of their usability, can be established with reasonable precision by measuring their cloud-points and pourpoints. The problem is, however, much more difficult with greases. The study of low temperature properties is chiefly important in connection with pumpability (transport through piping) of the lubricant, increases in bearing torque and the energy required to operate pumping units at low temperatures.
The difficulty arises from the fact that investigation of a grease at low temperatures comprises identifying the intricate correlations between temperature, shear stress and time, which all affect the fluidity of the lubricant. Efforts to establish generally applicable rheological principles for greases have not yet been successful. Attempts have therefore been made to develop methods which can simulate, with fair accuracy, the behaviour of the grease under low-temperature field conditions. Such methods must allow shear rate variation over the range to 5. lo2 and even as high as lo3 s-' and variation of the period of application of shear stress, whilst maintaining adequate reproducibility of shear stress and stabilisation of temperature. The method devised by de Limon of Shell enables shear rates to be produced in the rheometer which closely match those in the field. This method has, for example, been used to establish the relationship between the composition of a lithium grease, made from soaps based on 12-hydroxystearic acid and paraffinic oils, and pumpability at temperatures below -20 "C and shear rates from loo to lo2s-l (56). During these tests, the viscosity and pour-point of the oil was changed, together with the soap concentration in the lubricant. This work showed that at a given shear rate, viscosity, viscosity index, pour-point and soap concentration all decisively affect the pumpability of the grease. The lower the viscosity of the oil component, the better the pumpability of the resultant grease (57-60). Since the oil viscosity must be sufficiently high at the service temperature of the lubricant to provide an elastohydrodynamic lubricating film, the viscosity index of the oil becomes important. The higher the VZ,the better are the flow properties of the grease at low temperatures. However, the thickening power of one commonly-used thickener, lithium 12hydroxystearate, diminishes in high VZ paraffinic oils and this must be compensated by increasing the concentration of thickener. This then adversely affects the
70
rheological properties of the lubricant at low temperatures; at constant shear rate, the apparent dynamic viscosity of the grease increases linearly with increasing soap content. Similarly, the high pour-point of a paraffinic oil has an adverse effect, particularly if it is higher than the temperature at which the lubricant is to be pumped. Methyl methacrylate pour-point depressants provide no solution to this problem, indeed, on the contrary, since these compounds themselves act as thickeners. This example demonstrates that cyclo-paraffinic oils are to be preferred for the manufacture of lubricating greases. Also, the adverse effect of the increased viscosity of cyclo-paraffinic oils on the low-temperature properties of greases is substantially lower than that of paraffinic oils. A number of factors affects the fluidity of greases at low temperature which can conflict with the entire range of desirable lubricant properties. Correlations between the composition of the grease, the properties of its constituents and apparent viscosity under different shear rates in the lower temperature region can only be determined empirically; while the differences between the results of laboratory tests and the effects of field conditions can be significantly influenced by the choice of test and test conditions. All the methods suggested up to now have proved unsatisfactory in some respects, including determinations o f apparent viscosity by ASTM D-1092-62, rotational torque in a roller bearing by ASTM D-1478-63, IP 186/64, CSN 65-6327 AND FTMS 334.2, low-temperature penetration, pour-point by Bosch’s method, forces needed to drive the lubricant boundary from a pipe by the method of Knoll (Siemens), the amount of lubricant driven through defined pipe-sections in a rheometer, by the method of de Limon (Shell), viscosity by the “viscosity balance” according to EMPA F 10 108, apparent viscosity at variable shear rates and after variable shear periods with the Couette or coneand -plate system, according to DIN 51 805 and SEBl8l 306-61. A detailed evaluation of these tests has been published by H.Gravert (32).
2.3 SERVICE LIFE OF LUBRICANTS A lubricant can be exposed in service to a variety of conditions which can change its composition, and, in consequence, its functional properties. As the lubricant ages, its desirable properties are gradually curtailed to the point that it must be replaced. The severity of these conditions and the capability of the lubricant to withstand them determines the service life of the lubricant. The various conditions which affect lubricants can be classified as follows: - the effect of oxygen in conjunction with heat and other sources of energy (light, radiation, electric discharge, electric field) and/or pressure, catalysts and water, - the effect of acids and bases, - the effects of mechanical contaminants from an external source or as a result of the operation of the lubricated mechanism, e.g., wear debris, metal swarf. 71
These effects do not operate separately, but often occur in a complex way and not always homogeneously. In comparison with universal problems of the actions of oxygen, heat, catalysts, water, pressure and light these other effects may only arise under particular, specialised service conditions. Service life expectancy is the resistance of the lubricant to these external conditions.
2.3.1 Resistance to Oxidation The ageing of liquid lubricants and greases resulting from the reaction of their components with oxygen is a common phenomenon. The evaluation of oxidation stability is therefore one of the elementary tests of lubricants and special oils. Laboratory tests consist in exposing the oil to more severe conditions to achieve rapid aging, although the same qualitative course of the oxidation reactions needs to be followed as is characteristic of the slower aging process in actual service. Test conditions therefore vary with the type of lubricant tested or according to the different conditions to which it will exposed in the field. Tests employed under both static and dynamic conditions differ according to the oxidant gas (air or pure oxygen), the manner of its contact with the oil (contact of
Table 2.15. Summary of some Standard Tests for the Determination of the Oxidation Stabilitv of Oil Standard
Intended for oil type:
CSN 65-6235
Mineral oils
CSN 65-6224
COST 18136-72 COST 11257-65
72
Characteristic conditions of Test
Oxidation by excess air or oxygen in the presence of catalyst (Cu). Time, oil temperature, volume and type of oxidising gas selected according to quantity of test oil. Change in viscosity, neutralisation number and oil carbonisation residue evaluated. Additive-free mineral oils 140 "C, oxygen stream, 12 hours; oxidation value is the sum of the weights of resins, asphalt and coke (Resin = substances soluble in ethanolic NaOH solution from which it is separated by addition of HCI; resin is soluble in chloroform. Asphalt = substances precipitated by light petroleum hydrocarbons, soluble in benzendethanol mixture. Coke = substances insoluble in oil, light hydrocarbons and hot ethanoybenzene mixture).
Lubricating oils Turbine oils
Equivalent to CSN 65-6235. Oxidation under static conditions, based on convection produced by temperature difference between internal and external parts of the instrument; oil temperature 120 OC, 50 hours, Cu catalyst. Acid value, amount of sediment and free acid content evaluated.
Table 2.15. contd. Standard
Intended for oil type:
COST 14297-69
Transformer oils
COST 11063-77
Characteristic conditions of Test
120 "C, 50 mumin. air stream, 14 hours, Cu-Fe catalyst, light products washed with water to determine free acid-content; if these are being produced, the test is terminated; if not, the changes in acid value, oil colour and water concentration are estimated. Engine oils with additives Test in a tilted rotary machine at 200 "C for 50 hours. Viscosity increase and production of petroleum ether-insoluble sediment evaluated.
Additve-free oils P-48/57 (British Air Ministry PAM] Test)
200 "C, 15 yh oxygen in 3 x 6-hour periods. Criteria viscosity change and Ramsbottom carbonisation residue after 15-30 hour test.
IP-56/57
Transformer oils
IP- 1 14/56 T
Turbine oils
150 "C, Cu-catalysed, 90 hours air stream. Criteria amount of sludge (substances precipitated by nheptane) and acidity change. 110 "C, Cu-catalyst, 90 hours air stream. Criteria oil acidity change.
Turbine oils with inhibitors ASTM D-943 IP- 157156 DIN 51-587 ASTM D-2272 Turbine Oils IP-229 (Turbine Oil Stability Test ("TOST")**
ASTM D-2893
Gear oils
DIN 51-352
Oils with and without additives Mineral oils
DIN 5 1-554/ BI. I ("Baader Test")
DIN 51-554B1.2 ("Baader Test)
Electroinsulating oils
DIN 51-554B1.3 ("Baader test")
Turbine oils
95 "C, oxygen and catalyst (Fe, Cu). Criteria time taken for acidity value and oil colour to change.* Rapid test in rotary bomb containing oxygen at 0.63 MPa and 150°C; water and Cu-catalyst present. Criteria - time for absorption of predetermined volume of oxygen measured by pressure decrease in bomb. 95 "C, dry air, 312 hours. Criteria - increase of viscosity and precipitation number by ASTM D-91 (ml of precipitate produced by mixing 10 ml oil with 90 ml 50/130 "C boiling range petroleum ether). 200 "C, air stream, in 2 x 6-hour periods. Criteria increase in Conradson carbonisation residue. 1 10°C. air-stream in 2 x 6-hour periods, intermittent immersion of Cu spiral as catalyst, 30 ml oil, 70 hours. Criteria - saponification number, sludge formaton, evaluation of some standards as in B1.2 or 3. 28 days for basic test, 3 days for routine test at 90 "C.Criteria - change in saponification number, loss factor. Criteria - change in saponification number after 3-days' ageing.
* In high-quality turbine oils, this may take several thousand hours
tl
Simulated field service (SFS) test is similar.
73
the gas with the hot oil surface, introduction of the gas into the oil or forcing the gas through the oil), the gas pressure (ranging from atmospheric to about 0.6 MPa.), and the temperature of the test oil (the oxidation stability of bearing, hydraulic, turbine, electro-insulating,etc., oils is tested at lower temperatures, in the range 95 - 150 "C). The tests can also involve the use of a catalyst (particularly copper and iron in sheets, spirals or in solution, e.g., as the metal naphthenate) and variations are stipulated in the duration of test conditions (from 12 to several thousand hours) and the size of the test sample (from 2g or less to 1 litre or more). Table 2.15 details the main characteristics of the standard methods for measuring the oxidative stability of oils. This review shows, in addition, the considerable differences in the variables selected for evaluating the test results. Methods based on other principles measure the induction period and the rate of oxidation from the amount of oxygen absorbed at the test temperature within a given time period under static and dynamic conditions. The extent and rapidity of occurrence of the changes caused by oxidation depend on the chemical composition of the lubricants, the presence of substances accelerating or retarding the oxidising reactions, etc., but chiefly on temperature, the constant, common factor governing oxidations, both at initiation and during subsequent progression (62). The most suitable terms in which the applicability of lubricants for use at higher temperatures can be expressed are thus thermo-oxidising reactions or thermo-oxidative stability. The oxidation of oils is a chain reaction of free radicals with degenerate branching and proceeds essentially in 3 stages: initiation, propagation and termination. The initiation phase consists in the decomposition of substances RH, where R is an alkyl, alkaryl or cycloalkyl group, into radicals, which can occur with some difficulty by the action of external energy (mainly thermal) according to the equation:
RJi-R*+H'
(2.72)
or more easily as:
RH + 0,2RH + 0,-
+ HOO' 2R' + H202 R'
(2.73.1) (2.73.2)
During the initiation phase, no visible changes occur in the oil. This is the socalled induction period. The second, propagation phase, during which the reaction chain is maintained or extended, begins after a sufficient quantity of radicals has been produced during the induction period. In this phase, hydroperoxides are both formed:
R'
+ 0, -
ROO'+ RH and decomposed:
74
ROO'
(2.74.1)
- ROOH + R'
(2.74.2)
ROOH
- RO’ + HO.
(2.75.1)
and at a higher peroxide concentration: 2ROOH
- RO’+ROO’+H,O
(2.75.2)
In the termination phase, the chain reactions terminate by disappearance of the reactive radicals, e.g., by recombination or coupling: R’
+ R’ - R-R
- R-0-0-R 2 ROO’ - R04R - ROOR + 0, R’+ ROO’
(2.76.1) (2.76.2) (2.76.3)
and by other reactions which produce non-radical products. All the reactions depicted above can be accelerated by the effects of energy (light and heat), metallic catalysts, etc., or retarded by inhibitors, which suppress the formation of radicals or neutralise those which have been produced. Kinetics of the reactions show that the velocity and ease of oxidation are higher the easier radicals are formed and peroxy-radicals produced from them react with un-reacted oil components, and the slower the peroxy radicals themselves decompose. In the case of mineral oils, the formation of radicals in the initiation phase is easiest with, e.g., alkylaromatic hydrocarbons and substances with a labile sulphur bond, followed by, in decreasing order of reactivity, hydrocarbons with tertiary, secondary, primary and quaternary carbons. The ease of initiation of oxidation of hydrocarbons of the general formula RH also follows this sequence. In subsequent phases of the oxidation, the order of stability is exactly opposite; the slowest reactions are those of peroxy-radicals with the least energy of formation, so that oils with the longest induction periods oxidise with the highest velocity in their later phases (63). The resistance of an oil to oxidation can be deduced from its composition according to these principles. However, it should be noted that the pattern of oxidation for blends is different from that for their component substances. There is no general rule of additivity which applies to blends. Some hydrocarbons, e.g., the alkanes, cycloalkanes and some mononuclear aromatics exhibit positive and negative autocatalytic activity. Autocatalysis occurs when natural inhibitors are absent or when they have been depleted during the course of the oxidation. Autoretardation occurs when “natural” inhibitors, e.g., phenolic compounds, are produced as a product of the oxidation. The oxidation stability of individual compounds increases in the following sequence: alkane polymers/mononuclear aromaticdalkanes and cycloalkanes/dinuclear aromatics and trinuclear aromatics. The reactivity of all hydrocarbons increases with extension of the alkyl chain and that of alkylaromaticsdecreases with the introduction of a quaternary carbon in the alpha position into the aromatic nucleus (67-70). 75
In the case of mineral oils and greases derived from them, interpretation of the effects of structure on oxidation is rendered intricate by the presence of a wide variety of different hydrocarbons and other compounds. The character and composition of such oils changes with the source of the crude oils and their subsequent processing to lubricants (64-66). Therefore, investigation of the oxidation stability of hydrocarbon types apparently of homogeneous composition, e.g., monoalkanes, etc., derived from crude oils of differing provenance, may give rise to different results. Temperature is of great importance in oxidation studies. At the lower temperature range (up to about 150 “C), the pattern of oxidation reactions and hence oxidative stability may differ substantially from that at higher temperatures (63, 66, 67). In the general case, however, it is supposed that the primary products of the oxidation are peroxy radicals, and that these undergo further changes to produce, e.g., dialkyl peroxide radicals:
+ ROO’-
R’
ROOR
(2.77)
Primary peroxy radicals react to produce primary alcohols and aldehydes: 2RCH200’
- RCHO + RCH20H + 0,
(2.78)
secondary peroxy radicals to give secondary alcohols and ketones: 2RCHR’OO’
- RR’CO + RR’CHOH
(2.79)
and tertiary peroxy radicals to give dialkyl peroxides: 2RR’R”OO’
- RR’R”CO-OCRR’R‘’+
0,
(2.80)
The peroxy radicals can themselves isomerise and cleave to produce ketones plus hydroxyl, which can then form other radicals and water: RR’CHOO’
- RR’C’-OOH - RR’CO + ‘OH
(2.81)
The transition products can themselves be further oxidised, e.g., aldehydes via peroxy-acids to carboxylic acids, etc. Similarly, alkoxy-radicals,RO’, can give primary, secondary or tertiary alcohols, aldehydes and ketones and further oxidation products form these compounds. Hydroperoxides can decompose to alcohols and water, ketones and alcohol or phenol, and they can also oxidise the original substrate and the transitional oxidation products, e.g., in the case of sulphur compounds:
-S--+ ROOH -SO- + ROOH -SO,- + ROOH -SO,H Oxidation of the transitional oxidation products proceeds further according to the same general mechanism, leading ultimately to the most stable end-products, acids, keto-acids, lactones, estolides (complex lactones), and, finally, oil resins, asphaltenes, carbenes and carboides (coke). The pathway from hydrocarbon to condensation product is longer in the case of saturated oils than aromatics, and the oxidised oils are characterised above all by
76
their acidity. In contrast, in the case of aromatic oils, condensation proceeds more rapidly, and the oxidation is characterised by thickening and sludge formation. This process can be illustrated as follows: Saturated oils peroxides acids
\
h ydroxy acids
keto-acids, lactones, estolides asphaltogenous acids resins asphaltenes carbenes carboides
1
aldehydes, phenols peroxides Aromatic oils
Temperature, catalysts, e.g., metals like Cu/Fe, or initiators - substances which readily produce radicals - accelerate these processes, whilst inhibitors such as radical scavengers, peroxide decomposers and metal deactivators, retard them. The more important products of oxidation have the following properties (63): Peroxides initiate other chain reactions; oxidations, polymerisations and, at higher temperatures, cleavage. They contain an acid hydrogen and can oxidise and corrode metals, resulting oxides and salts dissolving in oil and catalysing further oxidation. They can be detected in oils during the early stages of oxidation, but their concentration is drastically reduced as oxidation proceeds to its later stages. Alcohols, ethers and ketones have no detrimental properties in themselves, on the contrary, they can improve the “lubricity” or “oiliness” of oils, but they can be readily converted into other, detrimental oxidation products. Aldehydes are unstable to oxidation and initiate other chain reactions. They are readily converted into undesirable peroxy-acids and carboxylic acids. They undergo condensation reactions, alone and in combination with aromatics and phenols. Products of these condensations increase the viscosity of used oils and can eventually precipitate as sludge. Low molecular weight carboxylic acids promote corrosion. They attack bearing metals to form insoluble salts which deposit in the oil system and can interfere with the normal function of the oil. Whilst higher molecular weight carboxylic acids are less corrosive, they produce oil-soluble metal salts which may catalyse further oxidation. In the presence of water - another oxidation product - they act as emulsifying agents. Keto-acidshave similar properties. Hydroxy-acids are derived from carboxylic acids by more severe oxidation. They are insoluble in oil and eventually settle in hydrodynamically stationary sites as lacquers.
77
Estolides are lactone-type condensation products of several hydroxy-acids. They are similarly insoluble in oil and deposit as resins and lacquers. Phenols can retard or inhibit oxidation, but can also condense to form resins, which are detrimental. Sulphoxides are believed to act as anti-oxidants. SuZphones are neutral substances. Sulphonic acids are highly detrimental, promoting corrosion, oxidation and condensation processes, as well as forming sludges in oil themselves. Generally, the relationships between the composition and oxidative stability of the main, homogeneous, viscous oil types (derived by absorptive refining of a sulphur-containing crude petroleum oil) can be classified in the following way (63): Alkylcycloalkanes - if virtually sulphur- and nitrogen-free - are very unstable. Mononuclear aromatics which contain sulphur compounds or groups are the most stable components of oils. However, when they are de-sulphurised (e.g., by hydrogenation) or derived from sulphur-free crude, they are unstable in the same way as alkylcycloalkanes. Polynuclear aromatics can contain even more sulphur compounds, but are less stable than the corresponding mononuclear aromatics. They are, however, more stable than the alkylcycloalkanes. Their stability decreases with an increase in the number of rings. Desulphurisation (e.g., hydrogenation) improves their stability. The addition of sulphur-free aromatic components to sulphur-free alkylcycloalkanes improves the stability of the resulting oil blend. The efficacy of the aromatics as anti-oxidants increases with the number of benzene rings, and with the concentration of sulphur compounds. Whilst the maximum stability, expressed as time required for the uptake of a given amount of oxygen, can be achieved by adding about 20% of sulphur-containing mononuclear aromatics to an alkylcycloalkane, the same effect can be achieved with about 2% of dinuclear aromatics and less than 0.5% of trinuclear aromatics: the improvement in stability increases substantially with the transition from mono- to polynuclear aromatics. However, a surplus of aromatics can decrease the oxidation stability of an oil blend. In the case of hydrocarbon oils derived from crude petroleum, the following general conclusions may be drawn: To achieve the maximum stability against deterioration with time, oils must contain an optimum amount of aromatics. Oils with a lower concentration are termed, in manufacturing practice, “over-refined”, whilst oils with higher concentrations are referred to as “under-refined”. This manufacturing experience leads to the well-known concept of “optimum aromaticity” (151). The position of this optimum depends, however, on other factors: the types of aromatics present and the presence of naturally-occurring sulphur and nitrogen compounds. It shifts to lower values when passing from mono- to polynuclear aromatics and with increasing concentrations of sulphur and nitrogen compounds. At higher temperatures, the optimum shifts in the direction of higher aromaticity. It is further influenced by the presence of synthetic anti-oxidants - which shift it towards lower values -, by the working environment (vacuum, inert gas, air or oxygen), the ratio of the oil surface exposed to the environment to the total volume
78
of oil or to the surface of the oil-container, the presence of metals, metal oxides or metal compounds which may act as catalysts, inhibitors, or neutral agents. From all the discussion above, it appears that polynuclear aromatic sulphur compounds are efficient, naturally-occurring oxidation inhibitors. They behave as radical traps and also peroxide decomposers, i.e., they act both as low- and hightemperature anti-oxidants. The draw-back of these natural inhibitors is their tendency to impair oxidation stability and to form condensation products if they are present at too high a concentration, which must therefore be closely monitored. The behaviour of synthetic oils is less complicated, provided their make-up is clearly defined and their composition simple. Polyethylenes oxidise, forming acid products soluble in oil and a small amount of resin. Polyisobutylenes depolymerise at higher temperatures quite readily to form volatile products and leave behind practically no solid residues. The same is true for polyglycols, which may, however, decompose at temperatures above 230 OC into aldehydes, which have the undesirable properties mentioned earlier. Polymethylsiloxanes have very good oxidation stability up to 200 OC. Above this, they decompose into volatile products, particularly formaldehyde and formic acid, and deposit deleterious cyclic gels. The phenylmethylsilicones can withstand temperatures up to 250 "C, exceptionally to 300 "C if phenyls predominate over methyls - the more phenyls they contain, the more stable they become. Diester oils are of low stability, similar to that of non-aromatic mineral oils, but they are receptive to oxidation inhibitors. Their oxidation products are water-soluble. Esters of highest stability are derived from primary alcohols, those of lowest stability from tertiary alcohols. Methyl groups bonded to tertiary carbons can considerably enhance oxidation stability, in either the alcoholic or the acidic part of the molecule. Polyphenyl ether lubricants are very stable, but at the temperatures at which they are used (about 300 "C), they are subject to oxidative condensation reactions. This process can be suppressed with oxidation inhibitors. The fluorinated lubricants have the highest stability to oxidation. Fluorinated esters, derived from aromatic acids and fluorinated alcohols, maintain a high oxidation stability up to about 320 "C. Fluorocarbons can withstand heat and oxidants such as liquid oxygen, 90% hydrogen peroxide and fuming nitric acid, so they have found application in rocketry. The same is true, to a lesser degree, for the polychlorotrifluorethylenes. Chlorinated aliphatic and, to a lesser extent, aromatic hydrocarbons are subject to changes associated with the elimination of hydrogen chloride under oxidation conditions and at higher temperatures. The fatty oils, acylglycerides of higher fatty acids, have very poor oxidation stabilities and diminish the stability of the mineral oils to which they are sometimes added to improve lubricity. The same also applies to greases, especially those manufactured from soaps of unsaturated fatty compounds. The thermooxidation reactions of greases containing metal soap thickeners can be catalysed by the metal. The free acids produced by such reactions not only, as in the case of liquid lubricants, promote corrosion by the grease, but can also be a factor causing breakdown of the mechanical stability of the lubricants.
79
The oxidation stability of greases can be determined by measuring the decrease in oxygen pressure in a bomb containing a grease sample. the test is carried out under various specified sets of conditions. According to CSN 65- 6318, for example, the initial pressure in the bomb is 8.10-' MPa, lubricant temperature 100 "C and the test lasts for 100 or 200 hours. Similar methods and machines to the CSN standard are described in ASTM D-942-60, IP 142/65 and DIN 5 1 808. The Russian method GOST 573476 is based on the determination of the amount of organic acid produced during the oxidation of the grease sample by atmospheric oxygen. According to the Abel-Koppen method used in the GDR, the test lubricant is exposed to oxidation by air whilst it is simultaneouslysubjected to mechanical-dynamicand thermal stress (130). Methods described in GOST 5734-76 and ASTM D-1402 standards can be used to ascertain the catalytic effect of copper on the oxidation stability of the grease.
Some solid lubricants are very sensitive indeed to oxidative effects; for example, some soft metals or metal alloys oxidise readily and this, being linked to changes in their functional properties caused by the formation or decomposition of corrosion products, is manifested even at ambient temperatures. Graphite is susceptible to severe oxidation in pure oxygen within the temperature range 550 to 600 "C, molybdenum disulphide at about 300 "C. A conjoint factor in these cases is surface area; as surface areas increase, these temperatures decrease - graphite with a surface area of 500 m2.g" can be pyrophoric, even at ambient temperature.
2.3.2 The Effect of Energy Absorption The energy absorbed by a lubricant (i.e., the energy supplied by heat, light and radiation) can directly promote changes in its chemical composition, with consequential reduction in stability. The quantity of energy absorbed by one mole of a substance can be calculated by Planck's equation:
E = h v = Nhcll = 11.9711(J)
(2.82)
where h is Planck's universal constant (Js-'), v is the frequency of the radiation (s-l), 1 is the wave-length (m), c is the velocity of light (m.s1), and N is the Avogadro number. The action of energy is referred to as thermolysis (or pyrolysis at high temperatures) in the case of heat energy, photolysis for light and radiolysis for ionising radiation. Regardless of the type of energy, the first effect on the lubricant molecule is the endothermic scission of bonds to form free radicals: R-CH2-CH2-R'
- R-CH,-R'-CH,
(2.82.1)
This effect of radiant energy can be preceded, however, by excitation and ionisation of the molecule: R-CH2-CH2-R'
80
- (R-CH,-CH2-R')+ - R-6H-R'-dH2
+e
(2.82.2)
The ionised molecule may also undergo polymerisation or dehydrogenation:
+ H2
R’-CH(R.CH-CH,R’)-CH-R
(R-CH2-CH2-R’)+ + e
+ H$
R-CH=CH-R’
(2.82.3) H2 The free radicals may recombine, condense with molecules appearing earlier in the sequence or, at higher temperatures, decompose into lighter products including hydrogen. 2R-CH2
R-dH2
- R-CH2-CH2-R’
- R-CH2-CH2-R /
R-CH,-CH-R’
I
(2.82.4)
+
CH2-R
\ R-CH, + R-CH2iH-R’ R-dH2
(2.82.5)
(etc.)
- lighter hydrocarbons
Condensations predominate at lower temperatures, decompositions at high temperatures. The more stable free radicals, e.g., benzyl aromatics, Ar-CK-R, undergo condensations, whilst less stable radicals, such as primary and secondary Table 2.16. Approximate Bond Energy in Organic Substances (kJ)
c-x
C-C
in alkenes: R-CH=CH-CH, R-CHSH-CHZ-CH,
330-350 320 310
c-0 C-O C S
295 625 260-300
4-H -0-H
-N-H
355-380 375-410 over350
380
C-CI C-F
280
Si-0
445
445
s-s
215
P-S R-CH2-H (R)2CH-H
400 370
N-S
230(est.) 2lO(est.)
(R),C-H R-CO-H
350 300-350
in aromatics: k-Ar
k-CH3 k-CH2-CH2-Ar
in aldehydes: in multiple bonds: C=C C=C
2-X
over 380 380 330 300-305
435 510
81
alkyls, tend to decompose. The tendency to decomposition grows with size of the molecule. The presence of oxygen promotes these processes; reaction products are then oxygenated and undergo further reactions. In scissions, the first bonds to break are those of lowest energy, namely, 4-C- bonds in preference to 4 - H ; in heterogeneous compounds -S-S-, -O-O-, C-S- bonds in preference to -S-H. However, the nature of the organic grouping to which these bonds are attached is also important (72) (Table 2.16). Light products reduce viscosity and increase volatility; heavy products form undesirable sediments in lubrication sites and may also cause other problems. The effects of energy absorption on lubricating greases may, additionally, be to disturb the forces of attraction and change the internal structure and properties. The lubricants first become soft, then - as a rule - hard and insoluble; they may even decompose into separate liquid and solid phases. The metals in the soap thickeners in lubricating greases may generate dangerous radionuclides on exposure to neutrons.
2.3.2.1 Thermal Stability
Every lubricant is exposed in the course of its application to heat energy. The result of the action of heat is a function of the operating temperature (lower in vucuo), time and pressure. Decomposition of mineral oils can be observed at ambient pressure at temperatures as low as 150 O C , particularly if the oils contain compounds of low bond energy, e.g., non-aromatic sulphur bonds. Readily perceptible decomposition occurs at temperatures from 250 to 300 "C,particularly if the heat exposure is prolonged. Themal stability, i.e., the temperature region in which thermal decompostion of oil and additives occurs, can be determined by thermoanalytical methods, or methods such as mass spectrometry, gas chromatography etc. including thermal analysis. The thermal stability of electrically conductive additives can be followed by methods based on the direct measurement of resistance/conductance of liquid dielectrics. The most common thennoanalytical methods are differential thermal analysis, DTA, and differential calorimetry, DSC, together with thermogravimetry and thennodilatometry. DTA and DSC methods follow the exothermic and endothermic reactions occurring in a steadily-heated sample. In a graphical correlation of heat input and temperature of the sample under test and a comparison standard, sample decomposition appears as a discontinuity,often with a characteristic shape. Thermogravimetricmethods follow the correlation between weight changes in the heated sample and its temperature, using a special thermal balance; thermodilatometricmethods involve the determination of volume changes. The method of measuring the thermal stability of conducting additives consists in plotting the variation of conductance with temperature of the additive under test, e.g., ZnDDTP in oil; the temperatureof the maximum change in electrical conductance corresponds to the temperature of maximum change of the additive.
82
Investigation of oils which was restricted to thermal stability would be of limited value. An oil in a machine is exposed not only to heat, but also to oxygen, the influence of catalytic metals, contaminants, etc. Tests in which all these effects are involved are more indicative of performance. The best tests are those carried out under field conditions, e.g., testing of engine oils in model test engines. However, such tests are expensive and time-consuming, and efforts have been made to develop short-term, laboratory screening tests, particularly for oils which operate at high temperatures, such as engine oils. These methods are based on the evaluation and measurement of the amount and nature of deposits formed by the test oils - the oil is sprayed for a given time on to a hot metal plate or made to flow down this plate for part of its circulation in the system. In addition to deposit assessment, other changes in the quality of the oil can be established, e.g., viscosity, carbonisation residue, acidity and/or basicity, etc.. The detergency (cleaning power) and dispersancy (ability to cause the suspension of solids) of the oil, which affects the formation of deposits on the plate, can also be assessed in these tests. The most common screening test based on the spraying of oil on to a hot plate at a selected constant temperature by means of a rotating distributing device is the so-called Panel Coker Test (76). The test was originally developed for the evaluation of aircraft engine oils at a constant temperature of 315 "C. It was later used for all types of engins oils. Different laboratories have made many modifications to the design of the test machine, the test conditions and the test procedure itself. For instance, the shape of the plate can vary, different methods are used to maintain constant volume of the oil, the duration of the test varies (most tests last 4 to 8 hours), as does the temperature of the plate (as much as 390 "C). The oil may be cooled or not cooled, air may be blown across the oil or not, the test may proceed without interruption or with short breaks for oil drainage, etc.
Thermooxidative stability of the oil is usually determined from the amount of deposition on the plate, the colour and nature of the deposits and from changes in the quality of the oil (mostly viscosity, carbonisation residue and acidityhasicity). The rate of build-up of deposits whilst the temperature is being raised gives a certain amount of guidance on the maximum temperature range of applicability of the oil. That temperature at which rapid deposition occurs may indicate the upper temperature limit at which the oil can be used, e.g., the piston crown in an internal combustion engine. A well-known version of the original PCT is the tester developed by the French company Antar, modified by the Institut Franqais du Pktrole (77). Another laboratory method based on oil flowing over a plate is Wolfs plate test (79). In this procedure, the test oil is supplied to the plate by a pump and after running over the plate is collected and recirculated. The plate temperature is usually 250 "C, the oil circulates at 50 cm3h-' twice over 6 hours with a 16 hour break. The weight of deposits on the plate is measured, and the appearance of the plate, and changes in the viscosity of the oil and its carbonaisation residue are determined. It is also worth mentioning MacKee and Fritze's apparatus (78). based on Stager and Kinzler's method (79), and the Thin Oil Oxidation apparatus also used for testing aircraft synthetic oils (80). Accumulated knowledge gained from a large number of these tests enables a reasonable comparison to be drawn between results obtained by these methods with those from standardised test engines, e.g., the Petter AVl engine, in relation to deposits in the piston grooves. One of the older methods for evaluating thermooxidative stability of oils is Papok's high temperature stability test (GOST 4953-49, CSN 65-6226), in which the behaviour of oil in a thin layer at 250 - 350 "C is examined. In this, the time taken to convert the oil between two metal surfaces into a lacquer film of
83
unit strength is the criterion measured. COST 5737-53 evaluates the volatility, extent of lacquer formation and the operational characteristicsof the oil in relation to time of heating at a constant temperature or to temperature for a fixed time.
The thermal stability of mineral oils depends on the size of the molecule and the composition of the oil. As the molecule grows in size, the thermal stability of high viscosity oils should decrease, but this expectation may be complicated by the fact that, at the same time, there is not a consisteiit increase in the cycloaromatic content in the heavy fraction, which leads to improved thermal stability. The constitution of the cycloaromatic compounds themselves is also important. Thus, Boguslavskaja and Velikovskij (73),from examination of hydrocarbon types isolated from residual oils from Emba crude, arrived at the following conclusions: - aromatic hydrocarbons with a small number of rings are of higher thermal stability than the cycloalkanes derived from the same oils, - thermal stability of cycloalkanes decreases with increasing number of rings, - the highest stability is possessed by aromatics with the molecular formula CnH2n-12 (naphthalenes); their thermal stability falls abruptly at lower hydrogen contents. Similarly, the thermal stability of synthetic oils also depends on their composition. Generally, it is higher than that of mineral oils. Phosphoric acid esters are the exception - they can only withstand 100-150 "C maximum, depending on their aromaticity. The highest stability is normally ascribed to the fluorinated hydrocarbons (boiling-point above their decomposition temperature) and fluorinated esters, at over 300 "C, and the methylphenyl silicones in which phenyl predominates, at up to 360 "C, and the polyphenylethers, up to 400 "C, and, for a short time, even above 500 "C. The decomposition temperatures of polymers are lower than those of their parent monomers, and lower than the temperature which could be predicted from their characteristic bond structure. Some polymers degrade at temperatures below 180 "C. The thermal stability of polymers is an extremely important property when they are used in multi-grade engine oils. The best thermal stability, in the absence of oxygen, is found in ethylene - propylene copolymers, followed by the polyisobutylenes, polymethacrylates with a nitrogen in the molecule and nitrogen-free polymethacrylates. This sequence almost reverses in oil solution in the presence of oxygen (74). The presence of aromatic groups in a polymer increases its stability both in the absence and the presence of oxygen. It has so far not been established if and to what extent polymers influence the optimum aromaticity of lubricating oils. The thermal stability of lubricant greases is chiefly influenced by the nature and origin of the thickeners - the soap cation and the type and composition of the fatty substances used for the manufacture of the soap - and also the composition of the liquid phase. Drop-point is generally regarded as the limit of thermal stability for a grease. However, this is not valid if thermal stability is regarded as the ability of the lubricant to maintain its structure and basic functional properties under the prolonged effect of heat. The thermal stability limit and hence the temperature limit in service
84
of the lubricant is sometimes higher. For instance, at 130-140 O C , some sodium greases pass through a transitional sub-phase manifested as a “false” drop-point, but retain their functional properties up to 150 -160 “C (81). Some greases form at higher temperatures a tough, elastic mass which ceases to adhere to the metal surface, so that the bearing runs dry. This phenomenon is commonly - but inexactly - termed “gelling”. The tendency of a grease to “gelling” can be established, among a number of dynamic mechanical tests, by using Weflescheid’s apparatus. In this, the lubricant sample is heated for one or more days at a specified temperature, typically 80 “ C ,in a hollow metal cylinder with a perforated bottom through which any separated oil trickles. The sample is then pushed out of the lubricant cylinder with a wooden piston and placed in the measuring device proper. This consists of a longitudinally bisected cylindrical mould with a tilting top-section connected with a pointer on a millimeter scale which indicates the angle of opening or closing of the two halves of the mould. If the pointer returns elastically more than 2 mm after the lubricant cylinder has been compressed 10 mm and released, the lubricant is judged to have “gelled”. Apart from dynamic-mechanical tests, only qualitative methods are used for determining the thermal stability of greases. For example, according to FTMS 2503.1, a grease sample is spread on two steel plates. The plates are pressed together and exposed to a specified temperature for seven days. Qualitative changes in the lubricant are then observed. IP 180 specifies a stability test for calcium greases.
High thermal stability can be achieved in greases manufactured from synthetic oils (mainly silicone types) and thermostable, non-soap thickeners. Extremely high thermal stability greases can be achieved with metallic gallium, which is liquid over the range 25 to 2,000 “C, thickened with a special kind of bentonite. The thermal stability of inorganic solid lubricants, such as sulphides and selenides of some metals and graphite, is very high, particularly in vacua Organic solid lubricants are significantly less heat-resistant.
2.3.2.2 The Effect of Light Equation (2.90) and Table 2.17 indicate that the quantity of radiant energy per light quantum absorbed increases with reducing wave-length. For example, visible yellow light cannot impart sufficient energy to break a -C-C- bond, but ultra-violet can. Table 2.17. Wave Length of Radiant Energy per Absorbed Light Quantum Type of Radiation X-rays Ultra-violet Visible violet Visible yellow Visible red Infra-red
Wave length (llm)
Absorbed Energy (kJ/mol)
20 200 400 600 800 to 4. I O5
6000 600 300 200 150
85
The primary photo-chemical effect is, once more, splitting of the lubricant molecules into free radicals. This is followed by all the radical reactions which have already been cited, especially the condensations. The first sign of this effect, even with normal visible light, is the gradual darkening of the oil. This is followed by precipitation of deposits as products of condensations, polymerisations and oxidations. Ultra-violet radiation enhances and accelerates these effects. In contrast to oxidation reactions, which are characterised by reaction of peroxy radicals, ROO', the usual action of light and of energy in general is to initiate reactions predominantly of R' radicals.
2.3.2.3 The Effect of High-Energy Radiation The effect of radiation on the service life of lubricants assumes importance in establishments in which nuclear energy is handled (atomic power plants) and generally where the lubricated machinery comes into contact with ionising radiation. This problem becomes more important as nuclear energy and space flight programmes develop. Data about the admissible dosages of lubricants with ionising radiation in nuclear installations are presented in a number of publications (8287,96). The following radiation dosages can be encountered in nuclear power plants (by individual lubricated machines or their components): W.kg-' Turbines Control devices Devices for introduction of fuel elements Primary pumps and blowers Valves Slide & gate valves
3.2.10-6 3.2.10-8to 3.2.10-2 3.2.10-8to 3.2.10-2 3.2.10-7 to 6.4.10-I 3.2.10-7 to 3.2.10-2 3.2.10" to 6.4.10.'
radyear
(0 - 104) (102- 108) (102 - 108) (103 - 2.109) (103 - 108) (107- 2.109)
Some studies also provide the results of investigation of the effect of inert atmospheres (mainly helium and carbon dioxide) on the problems of lubrication in gas-cooled nuclear reactors (88-90). These gases must not contain impurities originating in the lubricant. Lubricants exposed to y-radiation do not become radioactive and can be handled as conventional lubricants. Neutrons can make the lubricant radioactive, particularly if the lubricant contains a heavy metal or some other readily activated element, such as chlorine, sodium, phosphorus or sulphur. The lubricant can obviously be rendered useless if it is contaminated by a radioactive substance. Radioactive lubricants are difficult to handle; handling is costly and has to be carried out observing strict safety precautions. Storage and disposal of such lubricants also presents problems. The usual procedure is to leave them in a separate
86
place for many years until the radionuclides present fully decompose. Incineration in special devices with special filters is costly and causes problems in the handling of the fuel residues. Salt mines are used for storing radioactive wastes; the lubricants are confined in a strong “matrix”, i.e., mixed with a suitable material such as asphalt which is capable of binding the radioactive lubricant and which is itself resistant, to a certain extent, to radiation, so that there is no decomposition over a substantial period of time. Conventional liquid and plastic lubricants (greases) deteriorate on exposure to radiation. As compared to the effects of oxidation, the used oils have a relatively low neutralisation number and substantially more severe cleavage of -C-H bonds compared with thermolysis and photolysis. The oils release gases, chiefly hydrogen and methane and become dark, their viscosity increases, deposits are formed and, eventually, they become tough gels. Greases lose their consistency, and, in a subsequent phase of deterioration, are converted into coke-like solids. It has been found that alterations to the lubricant brought about by radiation are consistent for all four types of radiation (a, p, yand neutron radiation), but the most severe effects are those of yradiation and neutrons with energies above 2.5.10-13 J (1.6 MeV). Dosages of such radiation can be determined by calorimetry and activation analysis in the case of slow thermal neutrons, although the effects of the neutrons themselves on the lubricants may be negligible. CSN 01-1308 classifies neutrons as follows: Slow neutrons:
cold thermal resonance Medium-energy neutrons Fast neutrons High energy neutrons Very high energy neutrons
3.10.10-22J 3.10.10-22- 8.1OZoJ 9.10T20- 1.6.10-16J 1.6.10-16J 8.10-14- 1.6.10-12J 1.6.10’2 - 8.10-l2J 8.10-12J (=50 M eV)
The particular elements in the lubricant absorb different amounts of radiation energy. Table 2.18, below, details the values observed in the atomic pile, plotted against neutron type and unit area (96). Values given in Table 2.18 can be used to estimate energy absorption for compounds of the four elements listed. Calculations show that absorption in an homologous series is, with cycloalkanes, independent of molecular size; in alkanes absorption decreases with increasing molecular size, whilst in unsaturated (e.g., aromatic) hydrocarbons it increases, and in hydrocarbons with a given carbon number it increases with hydrogen content. Absorption decreases with the introduction of oxygen atoms into the hydrocarbon molecule and even more markedly with the introduction of sulphur atoms. On the basis that the absorption by cycloalkanes is rated as loo%, absorption by n-alkanes CI0H2, is equivalent to 106%, that of naphthalenes ClOH8.62% and that of naphthalenes C5,Hg8 93%. Compared again with the corresponding cycloalkanes, for compounds containing 20 carbon atoms, the absorption by the respective naphthalene is 81%, that of the terphenyl75%, diphenyl oxide 71% and diphenyl sulphide 69%. 87
The effect of yradiation alone under the same conditions results in the following absorptions: refined petroleum oils 9.2.105 J.kg-' (loo%), aromatic extracts from selective solvent refining 7.3.105 J.kg-' (79%) and for greases, the same order of magnitude, lo6 J.kg-'. In addition to the rate of energy absorption, the so-called "G-factor", the number of molecules converted by the absorption of a given quantity of energy, e.g., 1.6.lO-I7 J (100eV), is also an important parameter. Table 2.29 shows the G-factors for different hydrocarbon types and reactions. Aromatic hydrocarbons have the lowest G-factor, as well as the lowest absorption. More profound changes start to occur in oils once the dosage exceeds 1MJ.kg-'. Typical effects on hydrocarbon oils are the formation of gases (hydrogen, methane), increase in viscosity, decrease in flash-point, deterioration of oxidation stability and resistance to rusting, darkening and the formation of sludge. Anti-wear properties do not appear to undergo any substantial changes. An alkanic oil, after absorption of lo4 J.kg-', released about 20 cm3.kg-l of a gas containing 90% hydrogen. With increasing dose up to lo6 J.kg-I, the amount of gas increased; on increasing the dose above this it dropped to about 6 crn3.kg-l of oil. The unsaturated hydrocarbon content of the gas decreased at the same time and at lo7 J.kg-' no hydrocarbons were found in the gas, because of polymerisations occurring in the oil. The most sensitive hydrocarbons to radiation are the polyalkenes. Saturated oils are more resistant, whilst the most resistant are aromatic and cycloalkyl-aromatic oils (91).At a yradiation dose of 5.106 J.kg-'i in the presence of air and at 98.9OC, thickening decreased linearly when the aromatic content was increased from ca. 5 to 20%. It is advantageous to increase the aromatic content by addition of partially hydrogenated aromatic compounds with short alkyl substituents. Long alkyls are not suitable. The same is true for polynuclear aromatics with polyalkene or methylene bridges, particularly those with weak bonds. Some sulphurous substances show similar effects to the aromatics. An increase in sulphur content from 0.2 to 2% causes a reduction in the viscosity increase from 65 to 45%. However, too high a sulphur content is disadvantageous in the presence Table 2.18. Energy Absorption (J.kge1.lOl2per Neutron per cm2.104) Flux of Fast Neutrons Flux of Radiation Flux of Thermal Neutrons Carbon Hydrogen (CH2) Oxygen Sulphur
0.48 41.2 6.25
*- Less than 0.15.10-'2J.kg' 88
%
Y
%
17 90 70
2.37
83 10 30
4.7
2.7
Total
%
**-
%
-
2.85
-
45.9
8.95 2.88 3.36
100 100 100 100
Table 2.19. G-factors for Radiochemical Reactions of Hydrocarbons of Different Types (93) Reaction type Saturated Polymerisation Isomerisation Hydrogen cleavage Methane formation Decomposition of irradiated materials
G-factors for hydrocarbons Unsaturated
10.1 about 1 2-6 0.06 - 1 4-9
10-’10,000 6 - 14 1 0.1 - 0.4
6 - 2.000
Aromatic
10.05 I1 0.04 - 0.4 0.001 -0.08 11
of neutrons, which convert the sulphur into radioactive 32P with a half-life of 1.3 . lo6 s. Disulphides of the dibenzyl disulphide type are in one class of effective aromatic sulphur compounds; they also improve the anti-wear properties of the oils. Polyphenylethers are among synthetic oils which are substantially more resistant to radiation effects than mineral oils; small changes in viscosity and other properties only start to be observed when the radiation dose exceeds 10 MJ.kg-’. These oils are, however, rather expensive and they are too viscous to be suitable for some types of application. Polyphenylsilicones, biphenyls and terphenyls are also, generally, superior in this respect to mineral oils, but their lubricating power is usually lower. Polyglycols and phosphoric acid aryl esters are equivalent to mineral oils, whilst carboxylic diesters, silicon esters and polyalkylsilicones are significantly less resistant - see Table 2.20 (95,96,88). Additives can, to some extent, affect the resistance of lubricant to the effects of radiation. Two types of additives exist, in this context: energy absorbers and radical scavengers. Energy absorbers convert the radiation energy into less detrimental thermal energy. They are characterised by the presence of conjugated double bonds in a polynuclear aromatic condensed system (hydrocarbons, pigments) which can transform the energy of the incident radiation into resonance energy. Their function is to suppress the decomposition of excited molecule-ions into olefins. Free radical scavengers suppress radical polymerisation and consequential viscosity increase and, to a lesser extent, the production of olefins and hydrogen. The principal representatives of this type are the aromatic disulphides and some organic diselenides, the latter being less common (92). Both types of additive are so-called “anti-rads”. The choice of metal-containing additives is restricted by the possible formation of radioactive isotopes and the necessity to avoid damage caused to the reactor in the event of accidental leakage of the lubricant. In particular, metals with a high neutron-absorption capacity, e.g., cadmium, mercury, must be eliminated. Polymeric oil additives are subject to extensive depolymerisation as a consequence of irradiation if they are non-aromatic or only slightly aromatic.
89
Table 2.20. Resistance of Oils to Radiation (97) (Dosages in MJ.kg" to Achieve ca. 25% Change in Oil Viscosity or Acidity) Polypheny lethers Polyphenyl siloxanes Al kylaromatics Mineral oils Polyglycols
40 20 10 1 1
Phosphoric acid arylesters Alkyl diesters Silicic acid esters Polyalkyl siloxanes Polyolefins
1
0.5 0.5 0.05 0.05
Aromatic polymers depolymerise to a substantially lesser extent, so that it is possible to obtain an oil whose viscosity changes very little on exposure to radiation by combining such a polymer with an aromatic oil. Conditions such as these are also encountered by lubricating greases, which are expected to be more efficient than liquid lubricants in places of high radiation density, in the fuel cell charging mechanisms in the cores of nuclear piles. Here, they may be exposed to temperatures as high as 200 "C in contact with C 0 2 under pressures as high as 1 MPa. However, the composition of greases means that they tend to be less resistant to radiolysis than lubricating oils. The metallic soaps of fatty acids in the CI6 to C,, range, or the corresponding hydroxy acids, are subject to decomposition even at low radiation doses, with resultant softening of the lubricant. However, this softening can, in many cases, be compensated by increasing viscosity of the oil. In the higher radiation intensity region (lo6 - lo8 J.Kg-'; lo8 - 1O'O rad), drastically high increases in oil viscosity cause toughening of the grease. the grease can grow significantly in volume because of the formation of gases during radiolysis above 1O6 J.kg-', iT they cannot continuously escape. Therefore, metallic soap-based greases cannot be used where high intensity ionising radiation occurs (above lo6 J.kg-'); greases with soap-free thickeners, such as bentonites, silica gels, phthalocyanines, indanthrenes, octadecylterephthalamates, etc., should be used instead. Moreover, synthetic oils are generally the preferred oil component to petroleum mineral oils. Inorganic solid lubricants, such as graphite, MoS,, soft metals and alloys, etc., are very resistant to radiation, although the crystal lattice in graphite can be damaged in a neutron flux. These lubricants are therefore suitable for direct application, or as additives to liquid lubricants and greases for the lubrication of devices exposed to extremely high radiation doses. Gaseous lubricants are also suitable for this type of application. In direct contrast to this, some solid organic lubricants (particularly those based on substituted ethylene and propylene polymers), have very low resistances to radiation.
2.3.2.4 The Effects of Electric Discharges and Electric Fields Lubricants used in electrical devices, particularly electroinsulating oils,are often subject to the temporary effects of electric discharges and the permanent effects of electric fields. 90
The effect of electric discharges is similar to that of heat. Cleavage, dehydrogenation, polymerisation and condensation reactions of all the hydrocarbon groups may occur, although not all to the same degree. Gaseous products, mainly hydrogen and, to a lesser extent, C2-C4 hydrocarbons, are produced during these reactions. Philippovich (99) reports a gas composition of 50.4- 65.7% hydrogen, 3.0 - 3.7% methane, 23.3 - 25.0% C2-C4 hydrocarbons, 2.4 - 1.0% oxygen, 4.9 4.0% nitrogen and 15.0 -% oil-soluble decomposition products. The reactions observed can be explained by impact-ionisation of the hydrocarbon molecules, the primary reaction being cleavage of the molecule into hydrocarbon and hydrogen radicals and ions. This is followed by radical or ionic reactions, depending on the potential, frequency, pressure and other conditions (98). Silent electrical discharge acting on petroleum or fatty oils or their blends at moderately elevated temperatures, reduced pressure and in the presence of hydrogen produces the so-called “Voltols” or “Electrions”, viscous oils with flat viscositytemperature curves and good oxidation stability and lubricity. For example, Electrion R, prepared from a mixture of petroleum and fatty oils, has density of 910 kg.m-3, viscosity 2,500 mm2.s-’ at 37.8 “C and 260 mm2.s-*at 98.9 OC and saponification number 90 mg KOH g-’ can mix in any ratio with petroleum, animal and vegetable oils (but not with castor oil). 10% of Electrion R in bright stock reduces the static friction coefficient of the latter from 0.16 to 0.09 at 25 “C and increases the VZ from zero to 75 or from 75 to 127. A 25% solution in toluene can maintain 0.5% soot in suspension for years, whereas in its absence the soot settles out in minutes. Electrion also improves the oxidation stability of oils. When lubricity and detergency are important, Electrion can be added to mineral base oils at up to 2.5% mass. 2.5 to 10% can be added to engine oils and 15% to metal-machining and forming oils (152). The effect of the action on oils of a high voltage electric field in releasing gases, chiefly hydrogen and some hydrocarbons, reduces the dielectric strength of the oil and rakes dielectric losses, whilst the gases accumulate in cables at high points and can cause rupture and spoilage of the cable. Wax is also caused to separate from the oil. This phenomenon is referred to as the“gas strength” of the oil; it is most probably initiated by radical reactions of the gas phase distributed in the oil (100). Aromatics have the highest capacity to absorb and retain these gases and so hinder their escape. Saturated oils have the greatest sensitivity to these effects, whilst that of cycloalkanes is substantially lower. The presence of certain additives, e.g., quinones, nitroaromatics and sulphur has a beneficial effect.
2.3.3 Resistance to Chemicals During service, lubricants can come into contact with chemicals and react with them to produce products which reduce the functional capabilities of the lubricant or even cause damage to the lubricated surfaces. So the service life of the lubricant can also be determined by its resistance to chemical attack. Superior resistance to substances
91
such as water, hydrocarbon vapour, solvents, formaldehyde, acids, alkalis, polymers and many others may be required, particularly in the chemical industry. Special lubricants are produced for such duties. Solid lubricants, especially inorganics, have the highest resistance to chemicals. The most resistant liquid lubricants are fluorinated oils. With lubricating greases, resistance is particularly dependent on the composition of the thickener. The resistance of lubricating greases to the effects of liquid fuels can be determinedby FTMS 5414.1; a mixture of the grease and the fuel in a specified ratio is agitated for a specified time then centrifuged. The amount of grease leached into the liquid phase is determined by evaporation of the fuel. The resistance of greases to solvent attack may be measured in the same way.
2.4 SURFACE PROPERTIES The molecules in a body attract each other by cohesive forces. The resultant force is zero. However, at an interface, the cohesive forces acting on both of its sides are unequal. Their resultant is directed towards the matter which has higher cohesive force. Thus, a surface is energetically unsaturated and a surJuce tension, forms on it. In a narrower sense, surface tension acts on the liquid (or melt) to vapour, vacuum or gas interface, yL,or on the solid to vapour, vacuum or gas interface, 'ys. Znterfaciul surface tension, yLl,L2, or ysl,s2 acts on two liquid or solid interfaces, and interfuciul suguce confucttension, yslL, on solid-to-liquidinterfaces. Surface tension is related to surface length and its dimensions are N.rn-". Work is required to expand a surface by an amount AS. In the case of a reversible isothermal process, the following equation is valid: AW =AS.y (J)
(2.83.1)
This work, related to unit surface, is termed thefree surface energy, 0,its dimensions are J.mm2and its numerical value is y Heat also participates in the expansion of surface. The overall surface energy includes components of both work and heat, which must be supplied to the surface in order to achieve unit expansion of the surface (by 1 m2). The following equation then applies:
CO= 0 -T.dafdT
(2.83.2)
The other entropy component involved in this relationship, the so-called latent heat of suguce formation, q, can be determined by measuring y at different temperatures. Its values are about two or more orders of magnitude lower than the values of 0.The reduction or elimination of surface is accompanied by the release of energy. All three types of tension influence the behaviour and properties of liquid lubricants. Surface tension effects are responsible for undesirable foaming of oils, as well 92
as in intentional atomisation of the lubricant, for example in oil-mist lubrication. Interfacial tension is important in the formation of oil-water emulsions, which may desirable and intentional, as in the production of lubricating and cooling emulsions for metal-working, or undesirable, e.g., in aged oils. Contact tension is associated with the spreading ability of oils, their capability to wet metal surfaces and to form protective films which can provide protection against water and corrosive effects. The surface tension of liquids can be measured by the method of capillary rise, weighing drops (stalagmometric measurement), drawing a ring from the interface according to the method of Lecomte-du Nouy, measuring the pressure of gas (H2, N,, CO,) in bubbles, measuring the length of waves produced on the surface by a source of oscillations of known frequency, measuring the frequency of vibrating drops etc. (254). The surface energy of solid surfaces is difficult to measure; this determination can only be made indirectly and is inaccurate. The particles (e.g., metal atoms) move slowly on the surface, surfaces are uneven and the surface energy is unbalanced; it changes with the area of crystals and is higher on asperities and edges than on plane surfaces. The surface has other defects, so that tension cannot reach equilibrium, i.e., it cannot reach a state of lowest energy. If the free surface energy is defined as: AU = LwfS$
(2.84.1)
where AHf is the heat of fusion and S$ is the surface per atom on the interface. the value is not very different from the surface energy of the melt at the metal melting-point (the difference is typically about 20%). Suitable methods for mensuration include determining the contact angle on the solid-liquid interface, the flow velocity of a monocrystalline wire near the melting-point, the relaxation of metallic surfaces, damping of sinusoidal oscillation on the surface of monocrystals, crystal cleavage energy etc. (255).
The surface tension and free surface energy of inorganic and organic liquids, hydrocarbons and polymers is low, that of molten metal about two orders of magnitude higher and that of solid metals higher still - see Table 2.21. All such values are drastically reduced by adsorbing to surface saturation oxygen, steam, sulphur and contaminants in general. The surface tension of liquids decreases with temperature according to the following empirical relationship: (2.84.2)
where Tr is the reduced temperature T/Tc. At the critical temperature Tc the surface tension is zero. An indirect method of calculating the surface tension of liquids or organic polymers of known composition is available. Plausible results can be obtained using the following equation: y= ( P/Vm)4
(2.84.3) 93
where P is the parachor and V, the molar volume. Both values can be derived from the tabulated incremental group or atom contributions for the substance under examination (146). Vm represents the molar volume in the liquid or solid state according whether the substance is solid or liquid. This method is particularly useful for calculating the yvalues for polymers. The interfacial surface tension of liquids which are mutually partially soluble can be estimated by the equation: (2.84.4)
yi,L, = yi, - YLz
where yi and y are the surface tensions of their saturated solutions. 1 Lz Estimation of ys and ySL tensions for solid polymers is based on the value of the contact angle at the phase boundary and on the following equations (156): Y S L = YS + yi - 2H(Ys?j)1’3
(2.84.5) (2.84.6)
(2.84.7) (2.84.8)
Equation 2.84.7 can be used to calculate the surface tension of a solid, ys, if the angle Ois known. Equation 2.84.5 provides for the calculation of the contact tension,
Table 2.21. Surface Energies of Some Substances (J.mm2) Y Fluids: argon n-hexane benzene methylene iodide lubricating oils asphalt glycerol water silicones polyisobutene RMM 410 RMM 2840 diethyl adipate n-perfluoro-pentane diethyl octafluoroadipate
94
T(“0
13.2 18.4 28.85 50.8 32-35.5 30.0 63.4 24.0 24.0
-188 20 20 20 30 100 20 20 20
24.0 24.0 28.0 9.9 22.7
28.4 33.4
(Table 2.2 1. contd.)
Y Melts: potassium lead antimony copper silver iron steels cadmium aluminium gallium mercury B2°3
LiF2 BaF2 PbO NaCl NaB02 Na2B407
Nal Solids: low density polyethylene polyvinyl chloride polyvinyl fluoride polyvinylidene fluoride polyhexafluorpropylene polytrifluorethylene plytetrafluorethylene polytrifluorochloroethylene polyethylene terephthalate polymethyl methacrylate polyamide 6 and 6/6 polypropylene pol yisobutene cis-polybutadiene polystyrene pol yacrylonitrile polyvinyl acetate pol yformaldehyde paraffin wax cellulose graphite - in very high vacuum Mica - in very high vacuum
1 19.0 442.0 383.0 182.0 923.0 1700.0 950-1250 608 900.0 735.0 476.1 79.5 450 280.0 182.0 580.0 114.0 194.0 212.0 86.0
31-33.2 39-42.0 28-36.7 25-30.3 16-21 22-24.8 18.5-28.0 27-31 41-49 39-42 40-54 31-32 27-33 32.0 33-43 44-60 34-40 31-36 45 200 73-123 1750 300 10250
T(OC) 350 350 635 900
995 1530 370 700 30 20 900
-196 -196 900 2050 803 1016 1000 706 20 20 20 20 25 20 20 20 20 20 20 20 20 20 20 20 20 20 25 20 25 25 25 25
95
ysL when ys is known. Equation 2.84.8 can be used to calculate the contact angle 8 at the liquid-solid interface. Use of the auxilliary index # makes it possible to derive general relationships for interfacial or contact tensions of the two phases of substances 1 and 2, which can be either solid or liquid. The following then applies (157): (2.84.9)
Y1,2 = Iq +Y2 -2fi(Y1Y2)1/2
# frequently has a value of 1, in which case: y1,2 = (y1 lI2
- y2192
(2.84.10)
This equation only applies to contacts in which only spreading forces are acting. Where hydrogen bonds are involved, the following generally applies: (2.84.11) y= y d + yh where 9 is the tension due to dispersant forces and yh is that due to hydrogen bonding. Equation 2.84..10 then becomes: Y1,2 = Yl + r2 - W,d.Y,d)’” - 2(Y,h . r 2h) 112 (2.84.12) or: Yl,2
= [(r,d)’”
- (“/2d)”212 + [(Y:)’”
- ( 7 2h) ID12
(2.84.13)
If one, non-polar phase, with & = y and a second phase with hydrogen bonds are selected, the rd and yh values may be determined for the second phase from equations 2.84.10 and 2.84.12. From measurement of the contact angles of two liquids on a solid substance, rd and yh values for solids may be calculated from the equation:
Table 2.22. Some Values of y and y n-alkanes water polyethylene polyvinyl chloride p l y tetrafluorethylene polyamide 6/6 graphite
96
(mN.M1)
Yd
Yh
18-22 22.1 33.2 40.0 18.6 40.8 123.0
0 50.1 0 1.5 0.5 6.2
Surface Tension and Cohesive Energy When a surface is formed, work is performed against the cohesive forces which bind together the molecules in the bulk substance. Two new surfaces arise, each possessing a free surface energy, 0.The work which must be done on the system in order to form a surface of unit energy, given by: A w k = 2 0 (J.m-2)
(2.84.15)
is the free cohesion energy. This energy is liberated when the surfaces rejoin to return to the bulk condition. Equation 2.84.15 is valid for normal and sub-cooled fluids and for isotropic non-crystalline substances. In the case of substances of lower molecular weight, molar cohesive energy is involved, which is closely associated with molar heat of evaporation: Ekoh
= AHv - RT (J.mo1-’)
(2.84.16)
with the solubility parameter: (2.84.17)
and with surface energy:
d = k (0/V1/3)1/~ (J.m-3)1/2 = pa112
(2.84.18)
For heavier fluids and polymers, molar cohesive energy can be determined indirectly, e.g., from swelling or dissolving polymers in fluids of known cohesive energy or from the additive function (158): (2.84.19)
Group contributions to cohesive energy or F-function are tabulated in Table 2.23 (146).
Table 2.23. Group Contribution to Cohesive Energy (kJ.mol-l at 293 K) Group CH,= CH3CH= C(CH&< C6H5
C6H,= C=CH CH=CH
Contribution
Group
Contribution
1.o 2.3
-0-
coo-
3.2
0.1 3.0
CONCHCNF CI Br
14.5
6.8 6.0
2.0
1.5 6.9
1.2 3.1 3.7
2.7
Values of cohesive energy for some polymers are given in Table 2.24.
97
Table 2.24. Cohesive Energy of Polymers (kJ.mo1-') Polymer
Cohesive Energy
polytetrafluorethylene <polyethylene< polyformaldehydecpolymethylsilicone
6-12
pol ytrifluorochlorethylenecpolypropylene< polyisobutenecpolybutadiene
12-16
polyvinyl chloridecpolymethacrylate< pol ymethylmethacrlate<polyacrylonitrile
16-40
pol ybu tylacrylate<polybutylmethacrylatec polyethylene terephthalatecpolyamide 6 & 6/6
over 40
Data in this table support the following conclusions relevant to tribology: the presence of the phenyl group and, contrary to general belief, of numerous methyl substituents, oxygen, sulphur and nitrogen groups in organic compounds augments cohesive energy. This is an important guide for the selection of additives where high surface energy is required in addition to other properties; good self-lubricating capability can be expected in polymers of low cohesive energy (low cohesive energy implies a low unity or strength of the surface, which can indicate the probability of self-lubricating properties); liquid polymers of high cohesive energy are expected to have good lubricating properties - providing they are soluble in oil and adsorb on the surface; solid polymers are good as structural materials, e.g., as bearing liners.
Surface Tension and Adhesive Phenomena Similarly, work must be done to separate two bodies, 1 and 2, which are in contact on a common surface, because two new surfaces are formed, with energy a,, and a,,, and the surface ceases to exist. Work needed per unit area is the free adhesion energy: AWa = asl+ a,, -
(J.m-2)
(2.84.20)
The overall energy also includes a heat component, that heat which is absorbed or released when the surface is changed; equation 2.84.21 must be extended by the appropriate entropic component -T.dok/dT. If AWa has a positive value, work is necessary to separate the surfaces; if its value is negative, the surfaces separate spontaneously. For example, surfaces S, and S, separate spontaneously with respect to the liquid, provided the following obtains: (2.84.2 1)
98
In the tribological context, these relationships are useful for predicting the behaviour of materials of polymeric structure. Thus, in a system comprising: polypropylene ( y t 22.5; $ 37.6) 1 I methacrylatdvinyl choride copolymer (yd 38.9; 14.7) s2 2 alkaryl sulphonate solution (yf 29.0; y k 8.6; yL 37.2), substitution in equation 2.84.13 and 2.84.20 and formulation of the results in energy units gives:
rsh
oSl,,,= 3.4 a,,,
= 0.9 a ,
= 1.6 rnJ.m-, and AW, = -0.9 rnJ.m-,.
This system separates spontaneously.
Adhesive Phenomena on Wetted Solid Surfaces When a liquid, L, is placed on a solid surface, S,, a drop-shaped configuration is usually formed. In an energetic equilibrium, the following equation is valid when the wetting angle 8 < 90":
as, = asl,
,,, and for 8 > 90": a
+ aL.cose = a,, + oL.cos(n - B)
(2.84.22) (2.84.23)
i.e., in both cases: aL.cose = oSl- o~~~
(2.84.24)
By substitution in equation 2.84.21, the so-called "degree of wetting" is obtained: AW, = oL(1 + COSB)
(2.84.25)
For 8 = 180°, cos @ = -1 and AW, = 0 - the surface is not wetted. For 8 = 90", cos @ = 0 and AW, = C T , , ~ - conditions for the onset of wetting. At 8 = 0", cos @ = 1 and AW, = 20,.~ - the surface is fully wetted and the work released is equivalent to the free adhesive energy. In the presence of a liquid L between two surfaces S, and S,, the free adhesion energy decreases. While in the absence of the liquid equation 2.84.21 applies, in its presence the analogous relationship holds:
= OSI,L + 0 ~ 2 - , O~S ~ , S ~ and the difference in the free adhesion energies is: 'Wa,L
AWa AWa,L = as1 + as2 - OSlL - 0 ~ 2 ~
(2.84.26)
(2.84.27)
Combining equations 2.84.25 with 2.84.28 gives: A W -~ AWa,L = oL(cosel + COS~,)
(2.84.28)
99
If cos8 is substituted by the expression in equation 2.84.19, the following results: (2.84.29) In the case of metallic surfaces, where cos 8, = cos 8, = 1, the free adhesion energy decreases by 2aL, according to equation 2.84.28. In a metal-to-polymer friction pair, cos 8, = 1, cos 8, is, according to equation 2.84.18, equivalent to 2@(ys11yL)-1, @isin most cases equal to one and the free adhesion energy decreases by 2(ys2.y#2. Adhesive energy decreases in the presence of a liquid more rapidly the higher is the surface tension of both the solid and liquid surfaces (159).
Adsorption at Phase Interfaces Surfaces which are not saturated tend to adsorb foreign substances. The process is slow, equilibrium being reached within hours in the case of liquids, where it can be monitored by noting the changes of yLwith time. On solid surfaces, e.g., metals, it is even slower, and can take several years. Notwithstanding this, considerable changes in surface composition may occur in a short time. In an alloy of copper with 1% aluminium in the bulk solid, the aluminium concentration may be quintupled on the surface (160). Phenomena occurring on the interface between a liquid and a second phase are affected by the addition of substances which are soluble - at least colloidally - in the liquid. The cohesive forces in the liquid are reinforced by adhesive forces between the liquid molecules and the added substance. Nothing changes in the liquid or on the surface when these forces are equal. When they are unequal, the equilibrium stabilises according to the Gibbs adsorption isotherm: c dY (kmol.m-2) r=- (2.85) RT dc where Tis the concentration of the added substance at the surface, c is its concentration in the interior bulk, dyis the change in surface tension for a change of concentration dc of the added substance. If dy increases with the the increase of concentration dc, r acquires a negative value and the added substances passes from the surface to the interior bulk of the liquid. This is referred to as negative adsorption. Negative adsorption is typically found, for example, in the case of electrolytes in water. If, on the other hand, dydecreases with an increase in dc, r h a s a positive value which increases until the surface is more or less saturated with a monomolecular layer of the added substance. This is called positive adsorption. Substances which accumulate on the surface in this way have surface or capillary activity and are called su$actants. Surfactants are characterised by the presence of non-polar hydrophobic and polar hydrophyllic constituents. The hydrophobic constituent, e.g., a long alkyl chain, has an affinity to oil or to a non-polar phase, whilst the hydrophyllic constituent, e.g., 100
oxygen- or nitrogen-bearing groups, has an affinity to water or a polar phase. In respect of ion-formation or ionogenity, surfactants may be anion-active (anionic), cation-active (cationic) and non-ionogenic, or amphipatic - anionic in cationic media and vice versa. Representative anionic surfactants include substances of the types (RCOO-),Xn+, (R-SO;),Xn+ and (ROSO;),Xn+, where R is a long-chain alkyl. Monovalent X cations, e.g., Na+, K+,NH,+ - but not Li+, increase the solubility of the surfactant in water, multivalent cations, e.g., Ca2+,Mg2+,Ba2+, increase solubility in oil. The cations may be metallic (ash-forming) or organic (ashless). Examples of cationic surjactunts include the salts of nitrogen bases, ammonium salts (R-NR',+)Y-, pyridinium salts etc.; R is again a long-chain alkyl, whilst R' is a short alkyl and Yan anion such as halide. Nun-ionic surjuctunts include, for example, the reaction products of ethylene or propylene oxides or polyethylene amines with fatty acids, alcohols, aminies, e.g., the following type: R-Z-( CH2-CH2-O-),-CH2-CH2OH and R-Z-(CH2-CH,-NH),-CH,-CH2-NH2 where Z is e.g., -0-, -CO-O-, -CO-NH-. Ampholytic substances, also known as Zwitter ions, carry both positive and negative charges, e.g., some betaines:
N+(R R'R )-R '-COOThe length of the alkyl chain R affects the functionality of these types of products. Hydrotropic agents, with carbon number not exceeding c8,give rise to true solutions in water and substantially increase the water-solubility of substances which themselves have only limited solubility. c8-Cl2 alkyls form colloidal solutions in water, confer wetting properties on water and may have a demulsifying effect. c1+18 alkyls increase the detersive power of water and oil solutions - they are examples of detergents, and, in water, suponutes or soaps. C,, alkyls impart emulsifying properties to surfactants. Anionic substances have been the most important type for lubricants but the importance of non-ionic surface active agents is growing.
2.4.1 Foam Formation in Oils Foam formation in oils is an undesirable phenomenon having an adverse effect on their functional properties, for example, by reducing the strength of the lubricating layer, increasing the tendency to ageing, decreasing oil viscosity, density and thermal conductivity, diminishing the performance of the oil pump and consequently the rate of oil circulation, increasing oil losses, and many others. A foam is essentially a dispersed system, in which water, gas or vapour are finely dispersed in the dispersive liquid environment in the form of small bubbles separated from one another by fine films of increased viscosity or strength. The foaming 101
tendency of an oil and foam stability depend on the cleanliness of the oil (uncontaminated, homogeneous oils do not foam), its surface tension, viscosity and temperature and pressure. However, the ease and speed of foam formation also depend on the conditions under which the oil operates, such as changes in velocity of flow, pressure changes, the amount of air absorbed by the oil and the amount of gas or vapour entrained into the oil circulation system. A variety of methods is available for evaluatingthe foaming tendencies of oils. Some of these methods are established standards. All are based on a common principle - air is driven for a specified period of time through the oil which is heated to a specified temperature. The relative volume of foam is measured immediately after the end of this process and again after a specified period at rest. According to standard method CSN 65-6238, the volume of foam formed on the surface of the oil at 25 and 95 "C is measured after passage of finely dispersed air at a specified rate and again after 10 minutes at rest. ASTM D-892 and IP 146 specify an oil temperature of 24 "C, passage of air for 5 minutes and a rest period of 10 minutes. The test is repeated with another sample at 93.5 "C and, after the foam has collapsed, at 24 "C. Some quality standards for turbine and hydraulic oils prescribe tests of the ability of the oil to separate air, e.g., by Impiger's method as in DIN 51-381: air is blown at a test temperature of, e.g., 25.5 or 75 "C, through a nozzle under pressure into the test oil. After a dispersion of air in oil has been formed, the rate of air-release from the oil is measured by following the change of oil density with time. In sulphuric acid-refined oils, soap residues in inadequately washed oils may cause increased foaming and even emulsification. For this reason, some types of oil, particularly turbine and hydraulic oils plus some others, manufactured by refining techniques such as this, are subjected to the lye test. According to CSN 65-6227, the oil sample is boiled with sodium hydroxide, hydrochloric acid is added to the lye extract and clarity or turbidity is examined by observing the legibility of letters attached to the wall of a test tube of specified dimensions. GOST 6473-53 contains a similar method (ZOZ).
The foaming tendency of oils and foam stability are linked to the change in the surface tension of the oil in relation to gas (air) or vapour. So surface-active substances which dissolve at least partially to form colloidal solutions in the oil (organic acids, resins, asphaltenes, some lubricant additives, etc.) aggravate the tendency of the oil to foam and increase foam stability. Foaming tendency decreases with the degree of refining of mineral oils and increases with ageing of the oils, with the concentration of some additives and, generally, with the presence of certain heterogeneous substances ( I 02). The influence of oil viscosity on foaming is not satisfactorily established. Some researchers believe (103) that foaming increases with oil viscosity, others maintain the opposite view (204). According to KiEkin (103, the foaming tendency of oils changes in different viscosity ranges; within the range 4.1 to 128 mm2.s-* the tendency to form foams decreases with viscosity, between 128 and 325 mrn2.s1 it increases, whilst at even higher viscosities it decreases again. The position regarding foam stability is different. Foam stability increases with increasing viscosity (105). In the case of temperature effects, it is necessary to distinguish between relationships governing foams formed by gases and those formed by vapours. Whilst foaming in vapour increases with temperature up to a certain point, after which it decreases, gas foaming decreases with rising temperature, probably due to falling oil viscosity (99).Foam stability also decreases with increasing temperature and, at 102
a temperature of about 95 "C, the foam stability of different, uncontaminated mineral oils remains at about the same level (205). A rise in pressure caused by compression of vapour or gas may bring about a decrease in the amount of foam which, however, increases again as soon as the pressure is released. There are still many obscure points in the relationship between foaming and foam stability of various substances. Even the correlation with the value of surface tension is not self-evident. Unexpected and apparently inexplicable cases of oil foaming may be met in practice.
2.4.2 Atomisation of Oil Lubrication systems exist which make use of liquid lubricants in the form of an oil mist. The object is usually to atomise the oil into particles of such a size that the maximum oil is delivered to the point of lubrication with the minimum mist scatter into the surrounding area, so as to prevent or at least minimise the sometimes objectionable effects of the aerosol. The surface tension of the oil versus air can be decreased, and the size of the oil particles in the air can be reduced by adding surfactants, e.g., fatty acid esters like oleic acid. Oil particles reach the friction surface faster and more easily when they are tiny. On the other hand, particles which are too small with a lesser tendency to coalesce can be more readily scattered into the surroundings, causing higher lubricant losses and greater physiological hazards for operators, e.g., in the lubrication of pneumatic machinery and metal-working and the like. According to Hartman (206), the amount of scatter diminishes with increasing oil viscosity and the concentration of certain polymers in the oil. With increasing concentration of the right polymer, the particle size distribution in the aerosol and their ability to coalesce are improved. The mechanism of this effect has not yet been elucidated, but the general opinion is that it is connected with the viscoelasticity of the oil and the magnitude of electric charges. The most commonly used polymer is polybutene; atactic polypropylene has also been suggested for this purpose (153).
2.4.3 Emulsions Lubricating oils supplied in the form of emulsions in water are useful in cases where a high cooling capacity is needed as well good lubrication efficiency. Emulsions are suitable for metal-working, sheet rolling processes and the like, where high frictional heat release occurs and adequate and rapid heat abstraction is desirable to prevent undesirable metallurgical changes in the materials being processed. Another large sphere of application for oil emulsions are hydraulic systems in which the working fluid must be fire-resistant, particularly where the use of high fire-point materials is specified, as in mines. On the other hand, situations are encountered in technical practice where oil emulsions form, but where they are undesirable. They can cause deterioration in the 103
quality of the lubricating oil and mechanical malfunction. This is a particular problem with oils which contain surface-active substances, such as engine oils formulated from mineral oils containing detergenddispersant additives, mineral oils compounded with fatty oils, and used oils; such oils must be handled, stored and used accordingly. The stability of an oil-in-water emulsion can be expressed in the form of a demulsification number or index. The demulsification number, according to CSN 65-6230, DIN 51 589 and IP 19, is the time in seconds in which an emulsion, formed by driving water vapour through the oil, separates under test conditions into water and oil. Methods for measuring demulsification are contained in, for example, ASTM D-1401and D-2711: oil and distilled water in specified ratio are stirred for a specified period of time at a specified temperature; after a prescribed time at rest the amounts of oil and water separated from the resultant emulsion are measured. DIN 51 589 prescribes a test of the capability of steam turbine oils to separate water after contact with steam. This involves the formation of an artificial emulsion by driving water vapour through the test oil and measuring the time taken for the water to separate from the emulsion.
An oil-in-water emulsion is a colloidal dispersion system. The size of the droplets in milky emulsions is about 2 pm and in transparent emulsions up to 0.05 pm. A certain amount of work is required to disperse an oil in water to form an emulsion, e.g., intensive stirring. The overall surface and internal energy of the system grows with the number of small particles formed. The magnitude of the interfacial surface tension between oil and water governs the amount of energy needed for the surface to grow, i.e., to form the emulsion. The lower the interfacial surface tension of the two phases the easier it is for the emulsion to form. The function of an emulsifier is to reduce the interfacial surface tension. The formation and stability of oil emulsions are promoted by adding surfactants (emulsifiers) which reduce the surface tension of water or increase the surface tension of oil, i.e., reduce the interfacial tension of the water and oil system. The thermodynamic stability of the system, i.e., the tendency of the dispersed phase to retain its surface, depends on the ability of the emulsifier to reduce the interfacial tension. This thermodynamic stability is defined by the amount of work resulting from the difference between the surface tensions between the two constituents of the emulsion. For example, if we consider that yfor water is about 72 mN.m-' and y for oil is about 30 mN.m-' at 20 OC,it is not surprising that a considerable amount of work is necessary for the emulsifier to form the emulsion. The magnitude of the interfacial tension in an oil and water system can be determined, for example, by the ring method already described and specified in ASTM D-971. This involves measuring the force required to lift a platinum ring perpendicularly (normal to the liquid surface) from the surface of the liquid of higher surface tension. The measured force is corrected by an empirical factor, the value of which depends on the force applied, the densities of the oil and water and the size of the ring.
Substances which reduce the surface tension of water include surfactants (emulsifiers) with an alkyl chain preferably above C20:They can only form colloidal solutions in water, and thus separate in the form of thin skins at the interface of the two phases. They orient themselves with their hydrophyllic groups towards the water 104
phase and their hydrophobic groups towards the oil phase. In this way, a film on the interface of the two phases is formed. The stability of the emulsion depends on the stability of this film, which is stronger if the emulsifier molecule is larger. On the other hand, surfactants with shorter hydrocarbon chains cannot form sufficiently stable emulsions; they may even cause it to collapse, because they accumulate on the phase interface and disturb the strength of the film formed by the emulsifier. Examples of water-soluble surfactants include, in addition to the above, surfactants of alkali metal hydrosilicates, partially degraded proteins, starch, dextrins, etc. Examples of substances which increase the surface tension of oil include polar compounds which are practically water-insoluble, but which are partially soluble in oil, such as oil-soluble phenolates, synthetic sulphonates and petroleum sulphonates of alkaline earth and heavy metals, petroleum resins, asphaltenes, carbenes, polar impurities from oil-refining processes and substances produced by the ageing of oils. The tendency of oils to form emulsions may also be increased by other substances, provided they are able to form sufficiently strong films on the phase interface. Finely dispersed solids may have this effect. Many of the emulsions arising under field conditions are formed in this very way, for example, the finely dispersed soot particles originating from combustion products (especially in indirect injection diesel engines), dust etc., are responsible for the formation of emulsions and sludges in crankcase oils. Other factors affect emulsion stability. It increases with increasing viscosity of the dispersive (continuous) phase, reduction in the density difference between the phases and with the particle size of the dispersed phase, as indicated by the equation derived from Stokes' Law: 1
-=K U
'-
r% ( P I
(s.m-1)
(2.86)
P2)
where u is the velocity of phase separation, or llu is the fodemulsion stability, q is the dynamic viscosity of the external phase in kg.s.m-', r is the radius of the particle of the dispersed phase in m, p1 is the density of the dispersed phase in kg.m", p2 is the density of the external phase in kg.me3, g is the acceleration due to gravity m.s2, and K is a constant. Emulsion stability also increases with increasing pH in the alkaline zone, and by mechanical movement at an optimum intensity. Emulsion stability is, however, disturbed by: - the renewal of clean boundary surfaces by fresh particles coming from the interior of the fluid mass where they form clusters, - mechanical motion away from the intensity optimum, - electrical effects (passage of a.c. or d.c. current, mixing with an emulsion of opposite charge, addition of multivalent ions at a sufficient concentration), 105
-
temperature increase and consequential change of density and viscosity of the dispersed phase (unless the solubility of the components promoting emulsion formation increases as temperature rises), - change in pH, especially towards the acid region, - filtration, removing from the emulsion the finest particles which promote emulsion formation and stability. Two types of emulsion may occur in an oil-water system, oil-in-water or waterin-oil. According to Bankroft’s rule, an oil in water emulsion is formed when the emulsifier is soluble in water, and a water in oil emulsion is formed when the emulsifier is soluble in oil. The volume ratio of the phases is also important. There is a tendency to form oil in water emulsions when water is present in excess, and a tendency to form water in oil emulsions when there is excess oil. Reversal of the emulsion from one type to the other may be brought about by changing the type of emulsifier or changing the volume ratio. The emulsion may be broken either by using these procedures or by adding a surfactant of shorter hydrophobic chain length.
2.4.4 The Solvent Power of Liquid Lubricants In the process of dissolving a substance, it is necessary to overcome cohesion between its molecules by solvation, association with the molecules of the solvent. A principle once enunciated for mutual solubility of liquids - somewhat muddied by alchemical ideas - was that “like dissolves like”, meaning that a similarity in physical and chemical properties was involved. More accurate criteria are now offered by thermodynamics. In many instances, solubility remains surprisinglyunpredictable, particularly the solubility of simple, crystalline solids in liquids such as water, and most solubility data are inevitably based on empirical results, even though post-hoc explanation is often advanced (183).
One rough criterion is the difference in surface tension.; the greater this difference, the greater is interfacial tension and the lower the mutual solubility (96, 115). Another quantitative criterion may be the similarity of the attractive forces in the liquids, Van der Waals’ intermolecular forces (orientation, induction and dispersion forces) and hydrogen bonds between strongly electronegative elements and protons (71, 104). According to the nature of these types of forces present, liquids may be distinguished as being polar and non-polar. The greater the difference between two liquids in terms of polarity and polarisability (expressed, for instance, as their dielectric constants - see page 59) and of the relative size of their molecules, the poorer is their mutual solubility. Since the dielectric constant of polar substances decreases with rising temperature and that of non-polar substances remains constant, the miscibility of mutually insoluble substances may increase with temperature until a single phase is formed at an upper critical temperature of solution. 106
Mineral oils are classed as liquids of low polarity, which decreases still further with increasing concentration of alkane hydrocarbons and with decreasing viscosity. These lubricants are, therefore, readily soluble in non-polar solvents, e.g., light hydrocarbons, carbon tetrachloride, or in polar fluids of low polarity, e.g., chloroform, diethyl ether, and, by contrast, relatively immiscible with highly polar substances, such as lower alcohols, ketones and - in particular - water. The more paraffinic and the less viscous oils have lower solvent power and more readily precipitate the products of ageing, resins and asphaltenes, and they are less tolerant to admixture with polar synthetic oils, additives and polar polymers of which the molecular weight is several orders of magnitude greater than their own. The mutual tolerance of different oil components can be a very important property. It can be improved in paraffinic oils by adding suitable, less polar hydrocarbon and non-hydrocarbon oils. Miscibility (transition into a single liquid phase) of liquid lubricants with other organic substances can bring about changes in their other functional properties. Good miscibility of oils with other liquids over a broad range of temperatures may be an important condition for their use in specific equipment, for example, for the lubrication of Freon-type refrigerating compressors. Because of environmental problems, certain Freons are unacceptable as refrigerant fluids in many areas. Replacement of Freons is itself complicated by solubility of the replacements in mineral oils. For this and other reasons, synthetic oils such as diesters or polyglycols may be preferred as refrigerator lubricants.
On the other hand, good solvent power of lubricants can be deleterious; for example, liquid lubricants can strip the protective paint off machines, damage sealing materials and dissolve the plasticisers in plastic packaging causing embrittlement. The solvent properties of oils and their influence on sealing materials can be, to some extent, estimated from the magnitude of the aniline point. This point is that temperature at which a defined, hot, homogeneous mixture of oil, aniline and n-heptane separates into separate phases. Methods for the determination of aniline points of oils and solvents are specified in CSN 65-6180 and the equivalent ASTM D-611, IP 2 and DIN 51 787. Aromatic and, generally polar oils are characterised by lower temperature aniline points and paraffinic and generally less polar oils by higher aniline points. Oils with lower aniline point temperatures have, in general, better solvent power and have a detrimental effect on sealing materials, causing swelling of rubber jointing and packing. Paraffinic oils, on the other hand, cause their shrinkage. Oils are more mutually soluble and miscible if the difference between their aniline points is closer.
Solvent power may be a very important property in some applications of lubricating oils. Separability of water from oil is a related property. separability of oils increases as their polarity decreases. The miscibility (compatibility) of lubricating greases is of even greater importance in their application, but its measurement is more diffkult than that of liquid lubricants. The simplest method is to estimate compatibility from the droppoint and penetration after the lubricants have been kneaded together. 107
The solubility parameter mentioned earlier provides a useful basis for the calculation of solvent power (170). For pure non-cyclic hydrocarbons and highly refined oils the following equation may be used: d, = 7.26 + 0.01203~(MPa)ln
(2.87.1)
j to 886 kg.m-3. A rather less accurate This equation is applicable for values of ~ 1 . up correlation can be used for heavier hydrocarbons and all non-hydrocarbon oils:
d, = 8.63 nD + 0.96
(MPa)"*
(2.87.2)
where nD is the refractive index. Some d, values for mineral and synthetic oils are quoted in Table 2.25. Table 2.25. Solubility Parameters of Some Oils and Gases (271) d, (MPa)'" Mineral Oils: density 880 k g m 3 at 15 "C density 980 kg.m3 at 15 "C Synthetic Oils: di-2-ethylhexyl adipate di-2-ethylhexyl sebacate pentaerythritol caprylate diphenoxydiphenyl ether diphenoxytriphenyl ether polychlorotrifluoroethylene dimethylsilicone methylphenylsilicone tri-(2-ethylhexyl) phosphate tricresyl phosphate Gases: helium hydrogen nitrogen air carbon monoxide oxygen argon methane carbon dioxide
d, (MPa)'I2
17.95 21.71 18.05 17.94 18.95 23.24* 23.67* 15.55 14.20 15.14 18.24 18.82 3.35 5.52 6.04 6.69 7.47 7.75 7.77 9.10** 14.81***
* Not suitable at temperatures above 90 "C. 8.
Minimum solubility of methane in oil occurs at 150 "C.
*** Not suitable in synthetic oils; solubility of CO, in mineral oils decreases drastically with pressure, whereas those of hydrogen, oxygen and air increase.
As a general rule, liquids whose solubility parameters differ by less than 3 are miscible in all proportions.
108
The solubility of gases in oils can be calculated from the following equation (2 71):
In a = [0.0345(d1- d2)2- 2.66](1 - 273/T) - 0.0303d1- 0.0241(17.6- d2)2-5.731
(2.87.3)
where a is Ostwald's coefficient - the ratio of the volume of the dissolved gas to the volume of the liquid at the temperature and pressure of the test (or the ratio of the gas concentration in the liquid to that above it). Essentially, it is independent of pressure. d, and d2 - see Table 2.25 - are solublity parameters at 298 K, T the test temperature in K, the numerical constants are derived from some generally accepted thermodynamic values. The dependence of solubility of a gas in a mineral oil on density can be determined using ASTM D2779. Equation 2.87.3, however, gives more precise results and is also applicable to non-hydrocarbon oils. This relationshipis very important in tribology, since it helps in the determination of the relationship between the behaviour of gases dissolved in oil and their concentration.
It is well known that a small amount of oxygen in oil is needed to form a protective film on steels; a lack of oxygen leads to adhesive wear but a surplus causes corrosive wear. Dissolved oxygen affects the ageing of oils with lower oxidation induction periods. Air released from solution in oil in sites of reduced pressure may erode the protective liquid boundary layer and cause seizure; it may affect cavitation wear favourably or adversely. In aircraft hydraulic and sealing oils, release of gas with decreasing pressure may cause the escape of oil from tanks as foam; the released gas may promote the formation of vapour pockets in the suction lines of pumps and cause an undesirable increase in oil foam compressibility. As an example, we may calculate the amount of air released from 5 litres of aircraft oil (pentaerythritolcaprylate) at 200 "Cwhen the aircraft climbs to a height of 10 km, where the pressure falls to 27.6% of ground pressure (271). Equation 2.87.3 indicates In a = -1.494, a = 0.224. 5 1 of oil will contain 1 . 1 2 1 of air at ground level; at a height of 1 0 km, the quantity of dissolved air drops to 1 . 1 2 x 0.276 = 0.309 1. The quantity of air released will be 0.8 11 1 at ground pressure, or 0.811 x .276 = 2.95 1 at a height of 1 0 km. Gases behave quite differently under super-critical conditions. Their solvent power grows markedly above their critical pressure and at their critical temperature. This property can be used for the separation of substances of differing molecular sizes and polarities, Thus, ethane under supercritical conditions (P > 7.29 MPa, T > 304 K) preferentially dissolves non-polar substances, whilst carbon tetrachloride (P > 7.29 MPa, T > 304 K) dissolves polar substances. Dissolved substances can be recovered by returning the solvent to the sub-critical state. This principle is applied in process technology. 109
2.4.5 The Detergent and Dispersant Properties of Oils In order that machine parts function reliably and wear is minimised, it is essential that their friction surfaces be free of deposits and contaminants which can cause seizure. Deposits may arise from thermooxidations of the lubricant, and solid particulate impurities which may enter the lubricant, originating from a polluted environment, products of combustion and wear of the friction surfaces. A lubricant is required to keep these contaminants in suspension and inhibit their deposition, as well as to keep the functional parts of the machine clean and to disperse the dirt which is released throughout its bulk. Clearly, therefore, the lubricant must have both detergent and dispersant (DD) properties. The detergent effect of the oil is based on the same principle as that of using a soap to release dirt from dirty surfaces: it depends on the power of the oil to wet the solid surface and, consequently, on the magnitude of contact tension on the liquidto-solid interface. The wetting effects improves, and so does the detergent power of the oil, if the contact tension becomes lower. The dispersant power of the oil is controlled by principles which are generally applicable to the formation and stabilisation of suspensions of solid substances in liquids. Like emulsions, suspensions are dispersed systems of which the stability depends - in addition to other factors - on the magnitude of the contact tension in the interface between the dispersed solid particles and the oil which envelopes them; the lower the contact tension, the more stable is the entire dispersive system. Thus, systems which have a lower contact tension will have both higher detergent and dispersant power. Accordingly, detergent and dispersant properties of oils improve with increasing concentration of high-polarity components, namely, from alkanes through cycloalkanes and aromatics to high-polarity synthetic oils, and from additive-free oils to oils containing surfactant additives. Differences may, however, exist between related oils. It is well known, for example, that the detergent and dispersant power of diamylanthracene is higher then that of mono-, di- and polynaphthalene. This power increases with the number of rings in the molecule, although the length of any alkyl substituents also exerts an influence (105). Differences in DD properties have also been found among solventrefined and reclaimed engine oils which have different concentrationsof aromatics, particularly di- and poly-nuclear aromatics (108). Other factors enhancing the stability of suspensions of solids in oils are the same as those indicated above in the section in which emulsions were discussed: small differences in the densities of both phases, small dimensions of dispersed particles, high viscosity of the continuous phase and motion occurring at an optimum magnitude. Higher density oils form suspensions of higher stability. Increasing temperature, which causes a fall in viscosity, leads to a decrease in the stability of the system. At a defined, critical temperature, the suspension may flocculate, i.e., coagulate, and a solid phase be separated from the liquid. This critical temperature is dependent on the stability of the system, and in particular on the efficiency and concentration 110
of additives which increase the disperant power of the oil and the stability of the dispersion. Detergenddispersantpower is a vital property, particularly in engine oils, but also in many other oil types. The only way to evaluate this property in engine oils is suitable engine tests, because - among other factors - high temperature effects must be taken into consideration. High temperatures not only influence the types and physical and chemical properties of the products formed in the oil but also the functional power of the detergenddispersantadditives. On the other hand, simulation tests may be sufficient for other types of oil. Simulation tests may, however, be employed as screening tests for engine oils and detergenddispersantadditives. They are based on observation of the stability of suitable artificial suspensionsby various techniques: sedimentation, filtration , centrifugation and microscopic and chromatographic examination. It is very important that the substance selected for the preparation of the suspension resembles the solids formed in service as closely as possible. Many methods are available for use as screening tests to evaluate the detergenvdispersant properties of oils and additives. These methods differ in the operating principles, the selection of the dispersed solid, the technique of preparing the emulsion and that of following its stability, or the efficiency of the detergenVdispersant additive. In the Wood-River rest, asphalt dissolved in benzene is added to the oil. After distilling off the benzene, the mixture is filtered and the amount of asphalt passing the filter is the criterion of the detergenVdispersantpower of the oil. Sedimentation tests consist in adding soot to the oil, along with detergenVdispersant additive, and letting the mixture settle. DD power is indicated by the ratio of the the time it takes for a defined amount of soot to settle to the sedimentation time of the same amount of soot in oil without the DD additives. In the preparation of suspensions for sedimentation tests used for assessing the DD properties of metal-working fluids, pigments are used whose nature resemble or coincides with the wear debris produced during the machining process. These include, for example, graphite dust, iron oxides and grey cast iron or steel wear debris. Cenfrrfigation methods use soot, which is added to the test oil in a special gasoline cut. After the soot has been centrifuged off, the light-transmission of the turbid oil solution is compared with that of a soot-free control. In onefiltration or chromatographicmethod, soot dispersed in the oil is filtered in a glass tube through sand layers separated by filter papers. Dispersive power is evaluated from the number of filter papers blackened by the passage of soot. The essential deficiency of the above methods is that they omit the effects of operating temperature. for this reason, tests on hot plates are used for the evaluation of DD qualities of engine oils and the DD additives used in them. The drop and microscopic methods are relatively quick and simple. Either method can be used for evaluating both the DD power of an oil at different temperatures and the residual detergent level in oils after differing times in field service at elevated temperatures, in fresh oil, after an artificial suspension has been formed and in used oils after natural suspensions of solid have formed by thermooxidation reactions. Drop tests consist in forming a drop of the test oil on a sheet of specified filtration paper. After a time, the oil and the “carbon” particles (in used oil) diffuse through the pores of the paper filter and form a characteristic spot. This spot consists of zones, from which the DD properties of the oil, the extent of ageing of the oil and the presence of oxidised heavy fuel components in the oil may be identified. There are four zones in the spots: - a central zone with a light or dark coloured centre,
111
- a dark halo around the central zone, connected to it, - a diffused zone, - a peripheral transparent zone formed by soaking of the oil. The DD properties of the oil are characterised by the diffused zone. Only oils containing suspended particles in sizes up to 0.5 km form this diffusion zone. Reduction in size, or disappearance of this zone indicatesthat the suspension has flocculated and the DD power of the oil has decreased.The more intense the yellow colouration of the peripheral transparent zone, the deeper has been oil oxidation or the greater the concentration of heavy fuel fractions in the oil. The Mirsubishi merhod uses a complexing agent for identification of alkalinity or acidity in the drop test. High alkalinity is indicated by a broad blue zone around the dark central portion, low alkalinity by a narrow zone. Neutral or acidic oil leaves a light yellow or yellow-green ring around the central portion. The colour reaction appears about one minute after the drop has fallen on the portion of the paper with a spreading agent. The colours and the breadth of the particular zones stabilise within 15 minutes, then the colours either fade or merge. In the microscopic method, a drop is formed on a hot plate at a specified temperature. The formation or rate of flocculation of solid particles in oil suspension is examined as a function of temperature with a microscope. A microscopic method described by Bowden (132) evaluates the DD power of an oil as the value of the so-called agglomeration temperatures, I -1V.These are the temperatures at which 3.6, 15 and finally all particles of impurities in the oil coalesce. The higher these numbers the greater the efficiency of the detergentldispersanttested. Results by this method are claimed to correlate well with the results of engine tests. Of all properties of oils for which “screening” tests have been devised (with the possible exception of wear), detergent-dispersantproperties are perhaps those for which it is the most difficult to devise a reliable sequence of test methods. Full-scale performance tests in engines themselves lack reproducibility, and the difficulties of correlating the results from one type of engine with those from another are wellknown. The multiplicity of effects which results in the eventual result - an estimate of the ability of an oil to keep an engine clean - is such that it is impossible to predict engine behaviour from the results of screening tests with any reliability. False results -both positive and negative indications - are frequently obtained.Tests such as those described above are valuable in comparing chemically and physically similar lubricants with each other and in providing a basis for searching for new and improved lubricants. Their advantages lie in the possibility of controlling the conditions of the tests; however, their comparatively low cost and good reproducibility unfortunately cannot be taken to justify their use as a substitute for full-scale engine testing, which is extremely expensive.
2.4.6 Rust and Corrosion Protection by Oils Some oils are used in the lubrication systems of steam and water turbines and for the lubrication of machines installed in a moist environment. Water may then penetrate into the oil. In such cases, the oil must not only be able to wet the surfaces to be lubricated (this being part of its function as a lubricant) but also to be able to provide protection to the metallic surfaces against rusting. The same applies to lubricating greases coming into contact with water (e.g., lubrication of steering joints and water pumps in motor vehicles, lubrication of bearings of mining, road and agricultural machines, etc. The protection afforded by turbine oils is tested by ASTM D-54T. The rust protection capability (stability of the protective layer) of lubricating greases is tested, according to CSN 65-656319 by visually evaluating the extent of corrosion of a plate coated with the lubricant and then exposed to moist air. According to CSN 65-6324, the stability of the protective lubricant layer is tested by exposing the lubricant layer, which is spread out over the surface of a steel roller, to a specified temperature for a
112
specified time. The lubricant remaining on the roller is then weighed. The rust-protective properties of greases are also tested by GOST 4699-53. The tendency of a grease to separate from a lubricated surface, and hence its ability to protect, is specified by GOST 6037-51. GOST 6953-53 describes a method for evaluating the stability of the protective layer on a metallic surface. In ASTM D-1743-64, the lubricant is applied to an anti-friction (taper-roller) bearing and the bearing is immersed in distilled water for two weeks. A particular feature of this method is that the construction of the bearing provides wedge-shaped crevices which tend to express the lubricant; these sites are particularly prone to rusting, and a repeating pattern of discolouration of the metal surface of the bearing shell can be readily observed with inferior lubricants.The most common test is the EMCOR TEST, detailed in DIN 5 1 802 and IP 220; this evaluates not only the anti-corrosive properties of greases, but also the tendency of these lubricants to emulsify in the presence of water. ASTM D-1264 and IP 215 methods test the resistance of greases to water.
Rust protective substances, whether oils, greases or so-called double-function oils, must offer outstanding qualities; they must provide reliable lubrication while the machine is running, plus rust protection when it is shut down. The protective qualities of oils are associated with surface phenomena existing at the metal-water-oil system. These qualities - as well as the oiliness of the lubricant - improve when the molecule contains one or more polar groups. The groups promote the formation of a compact, firmly adhering hydrophobic layer on the metal surface even when water is present. The suitability of substances which reduce the contact angle of a water drop on the oil surface has been proved. For the oil itself to have a highly polar or, at least, some aromatic character is desirable, but not quite sufficient, since the oil is also usually expected to have high stability. Therefore, high-quality oils and greases designed for the lubrication of machines under particular conditions are formulated with special additives which ensure that the lubricants have the required wetting and protective properties. Oils intended to provide prolonged surface protection are also carefully formulated with additives. Clearly, lubricants which are intended to provide protection against an aggressive environment or rusting must not themselves be corrosive. Corrosion by lubricants may be caused by free organic or inorganic acids, and acid components in general, or by additives if these release at elevated temperatures substances such as hydrogen sulphide or hydrogen chloride which can form corrosive acids in the presence of water. The concentration of free acidic constituents in both fresh and oxidised (used) oils can be identified by the acid or neutralisation number or total acid number (TAN), expressed in terms of the number of milligrams of KOH required to neutralise unit mass. The methods employed, based on a titration against a colour indicator or a potentiometric titration of the lubricant dissolved in toluene/ethanol or toluene/isopropanol mixture are included in many standards: CSN 65-6070 - titration in alcohol-toluene against a colour indicator, CSN 65-6233 - potentiometric titration, GOST 5985-59 - titration against a colour indicator, ASTM D-974 and IP 139 - titration in isopropanol-toluene mixture against a colour indicator, ASTM D-664 and IP 177 - potentiometric titration, IP 1 and IP 182 - determination of neutralisation number and inorganic acidity, after extraction of inorganic acids with hot water, by titration with a colour indicator, DIN 51-809 - determination of acid number of greases after hot solution in alcohol-benzene mixture by titration against a colour indicator,
113
DIN 51-812 - by potentiometric titration, and DIN 51-558 - determination of total acidity of oil and water-soluble acids. Similar methods can be used for the determination of the saponificationnumber, which may be used to measure the amount of hydrolysable ester present. The concentration of strong (inorganic) acids in oil is expressed as Strong Acid Number ( S A N ) and determined by potassium hydroxide titration in the pH range below 4 by ASTM D-664. Many methods, some standardised, are available for testing the corrosivity of oils and greases. For example, PinkeviE's corrosivity test is described in GOST 5162-49; in this, the test plates are periodically dipped at 140 "C in the test oil 15-16 times per minute. After 50 hours the change in weight and the corrosive effect on the surface of the plate are evaluated. CSN 65-6321 specifies corrosion tests of lubricating greases on copper and steel at ambient temperature; CSN 65-6309 the same tests at 100 and 50 "C, CSN 65-6322 and GOST 29170-78 accelerated corrosion tests for greases and soft waxes at 100 "C. In all these tests, a panel of the material under test is immersed in the test lubricant at a specified temperature for a specified time and changes to the surface of the material assessed. There are many quality standards for lubricants and special oils which specify the copper strip test; a polished copper strip is immersed in a specified quantity of the test oil which is heated at its field service temperature. After a specified time interval, the changed appearance of the copper strip is compared with a standard scale, e.g., ASTM Copper Strip Corrosion Standard. This method is the subject of ASTM D130 and equivalents, CSN 65-6183, IP 154 and DIN 51-759. The Volkswagen (VW) test - a corrosion test on steel and copper at 100 "C for 48 hours - is also widely known. Corrosion tests of lubricating greases are described in IP 112 and DIN 51811. Visual assessment methods for determining the concentration of sulphur-containing constituents which are corrosive to copper in electroinsulatingoils are the subjects of CSN 65-0147 and ASTM D-1275, whilst DIN 51-353 deals with testing silver-corrosion by sulphur compounds in the same type of oil. ASTM D-1662, IP 155 and DIN 52-364 specify methods for the determination of sulphur corrosion of copper by cutting oils. The corrosivity of metal-workingemulsions is usually evaluated by Herbert's test (CSN 65-6256, IP 125). This is a very severe test: steel chips are scattered on a finely ground cast-iron plate and the emulsion poured on to the chips. After 24 hours, the chips are removed and the surface of the cast-iron plate is examined. CSN 65-6251, for testing the corrosiveness of metal-working emulsions is less sophisticated and less severe. However, electrochemical methods are more suitable for studying corrosion processes, the effects of corrosion inhibitors and for evaluating the extent of corrosion of oil emulsions, oils and oil-in-water solutions for metal-working. For example, a potentiometric polarisation method is available based on the fact that the more easily electrons separate from the crystal lattice of the metal, the more electropositive is the metal compared to the surrounding electrolyte. Under suitable test conditions, the magnitude of the current which indicates the rate of metal dissolution, and hence the extent of the corrosion process, can be measured. A large number of both static and dynamic test methods has been developed as screening tests for evaluating the corrosive effect of engine oils on bearing metals (lead, cadmium, silver, copperflead and other alloys), resulting from the generation of acidic oxidation products, e.g., the Sohio Polyveriform Test (109). the Shell Existent and Potential Corrosivity Test (110). the Sunbury (BP) Beaker Corrosion Test (ZZZ), the MacCoul Corrosion Test ( Z Z t ) , the Indiana Stirring Test (ZZJ), the Underwood Corrosion Machine (114,etc. Corrosion may occur both above and below the level of the oil surface. This phenomenon can be observed in closed gear-trains filled with oils containing additives with reactive sulphur and chlorine atoms. Swiss S N V 81052 represents a suitable method for the evaluation of the corrosivity of oils containing anti-wear additives above and below the oil level. Two steel plates are placed for 72 hours at 120 "C in the hot oil in a test-tube fitted with a reflux condenser, then dried, weighed and suspended for 168 h in a dessicator over distilled water at 20 "C. After being dried, they are then re-weighed. Weight loss and the extent of corrosion of the plates are the criteria evaluated. In the Deutsche Bahn method, 20 g of the test oil is placed for 48 hours at 140 "C in a 250 cm3 flask fitted with a reflux condenser. A copper strip lies on the bottom of the flask and a steel sheet is suspended
114
under the reflux condenser. Oil from the test is dissolved in 250 cm3 of n-heptane and left in the dark and in the absence of air for 24 hours. After filtration and stripping of the n-heptane, the amount of sludge is measured. The amount of substances insoluble in n-heptane should not exceed 0.1%,no substances insoluble in chloroform should be present, and the sheet should remain intact.
2.5 THE PHYSIOLOGICAL PROPERTIES OF LUBRICANTS Hydrocarbons and other organic compounds and - of particular concern to us here - lubricants can be harmful to plant, animals and humans. The extent of the harm they may cause depends on the structure and size of the molecules of these substances. Lubricants and products from physical and chenrical transformations of lubricants may affect human and animal organisms by causing skin complaints (dermatoses), by irritating respiratory and other organs and they may be mutagenic and carcinogenic. The most frequent and widespread harmful effects are dermatoses. The effects of hydrocarbons and organic compounds in general on the skin has not yet been fully investigated, but there is a generally-held belief that the cause of skin diseases may be (115,116): - removal of the slightly acidic lipoid layer from the skin, - penetration by hydrocarbons and other compounds into the skin affecting its metabolism, - weakening the skin’s resistance to repeated contact so that constant irritation is caused leading to skin damage. Low molecular weight volatile hydrocarbons and other compounds are supposed to penetrate the protective layer of the skin but evaporate quickly, so that the contact period may be short. High-molecular weight substances, on the other hand, penetrate the skin less easily, but once there they remain longer and affect skin reactions and changes. However, every type of hydrocarbon and other compounds differs in the severity of these effects. The biological effects of substances are assessed either by tests on experimental animals or on human volunteers (and by careful scrutiny of the results of unfortunate accidents). Several criteria are used for evaluation: the number of applications required for the development of distinct skin reactions; the severity of the disorder suffered (hyperaemia, diffusive erythrema, edematous infiltration, edema, dermatitis, necrosis); duration of the disorder and the time for it to heal; consequential and ephemeral changes to the skin (pigmentation and hyperceratosis, fissures, scars of differing severity). Skin tests with model hydrocarbons may be used to characterise the acute biological agressiveness of particular hydrocarbon groups and sulphur-compounds as follows (118,121):n-alkanes over C,, c isoalkanes c monocyclic cycloalkanes c alkenes and alkanes up to C,, , polycyclic alkanes c aromatics. Among sulphur compounds, slight biological aggressivity is observed in dilute solutions of dialkyl disulphides, approximately at the same level as higher alkanes. Sulphides with 115
shorter akyls and dialkyldisulphides containing secondary carbon prove more irritant. The conditions existing in hydrocarbon blends including liquid lubricants are considerably more complicated and individual hydrocarbons and other compounds, as well as their constituent groups, may affect each other as synergists or antagonists. The aggressive biological effect of liquid lubricants on the skin should not be under-estimated. Anyone exposed to prolonged contact over a period of time should protect themselves by frequently cleaning or replacing their clothing and underclothes, wearing gloves and aprons to prevent oil-soaked overalls, avoiding unnecessary contact with lubricants and solvents, by washing exposed skin with warm water and mild detergents before eating, drinking, smoking or using the lavatory and using suitable barrier and reconditioning creams to avoid dermatitis (185). Liquid lubricants applied as mists or being converted during service (e.g., in machining processes) into aerosols, may enter the operator’s respiratory tract and provoke irritant effects. The nervous system may also be affected. It is not certain whether they enhance the genesis of respiratory cancer, but they may be associated with a slight increase in cancer of the gastrointestinal tract. Lubricants containing volatile ingredients may provoke pneumonia. Those additives derived from physiologically-active elements (halogens, sulphur, nitrogen, phosphorus) are more suspect and may be dangerous. It is believed that if a threshold limit value of 5 mg oil per m3 is not exceeded, most people may be exposed for 8 hours daily without adverse effects on health. Mutagenicity and carcinogenicity present hidden dangers from oils and the products of incomplete combustion of fuels. The latent period from first exposure to the occurrence of cancer may be long, e.g., for skin cancer over 20 years and for scrota1 cancer 10 to 40 years. Tests of liquid lubricants and lubricating greases for personal protection are compulsory in most countries, e.g., in Czechoslovakia, lubricants are classified into three groups and measures for protection against health hazards appropriateto each group are stipulated (119,124). Since 1984, every new product has had to be submitted for obligatory testing, comprising the determination of LD50 value, of peroral application to rats, LD50 value for primary skin irritation, LD50 value for primary eye irritation, 90 days’ toxicity on rats and tests of mutagenisity with one test for genes, one for chromosomes and one for detection of damage and modifications to DNA (187). Mutagenicity may be determined with the modified Ames’ test (188)and dermal carcinogenicity by skin tests (186).
Some hydrocarbons are themselves carcinogenic, e.g., benz-a-pyrene, cholanthrene and some poly-nuclear aromatics with more than four rings. Carcinogenicity increases with the number of aromatic nuclei. Poly -condensed nitrogenous and sulphur compounds may display similar effects. Substitution of the aromatic nucleus by methyl may aggravate the carcinogenicity of aromatics, while increasing the length of the substituent or the presence of other alkyls may reduce the hazard. These correlations have not yet been explored in detail. Braukman (123) provides a summary of the carcinogenicity of many aromatic hydrocarbons, Burghardtova (189) that of several lubricating oils. 116
Oils probably do not affect lower organisms in water, unless they prevent oxygen from dissolving in it or reduce its concentration, although they may affect the taste of fish meat (Z27).However, lower organisms can adversely affect mineral oils. They can infect oils and bring about its decomposition in the presence of dissolved oxygen or dispersed water. Bacteria predominate at pH above 7, yeasts at lower pH. Decomposition is brought about by enzymes produced by the microorganisms. The primary decomposition is aerobic, a later type anaerobic. Long chains degrade, methyl groups producing carboxyl, peroxides accumulating in aromatic components and sulphurous substances producing hydrogen sulphide. Oils become acidic, corrosive and smelly; they become hygienically unacceptable. Emulsion-type fluids are particularly susceptible to these problems, which can be suppressed by the use of bactericidal additives (see Chapters 4.10 and 5.9.1j. The effects of oils on plants, which is particularly important where oils are used in plant protection, depends on their physical properties, especially viscosity (which affects penetration of the oils through pores, causing them to become clogged and hindering transpiration), contact tension (adhesion of the oil to the leaves) and volatility (affects the duration of the effect). The severity of the combined chemicalhiological effect of oils on plants may cause temporary effects in acute or local cases (burning of leaves by light saturated or aromatic hydrocarbons or products of oil ageing) as well as chronic changes ( the plant may defoliate permanently due to the effect of heavy oil fractions). Toxicity towards vegetation increases in the sequence: alkanes, cycloalkanes, aromatics, phenols and mercaptans. It increases as molecular weight increases up to 350 then stops (102). Among the synthetic oils, the polymethyl siloxanes have the lowest activity. They are regarded as non-toxic. Polypropylene glycols are of low physiological activity. Non-aromatic ester oils can be as harmful as mineral oils, but are no worse. Polyphenyl ethers which contain aromatic nuclei may be more harmful. Of the phosphate esters, tricresyl phosphate and esters with a high (over 1%) concentration of 0-cresol are particularly toxic. Chlorinated aromatic hydrocarbons (biphenyls and poly-phenyls in general) are toxic. Polychlorinated biphenyls (PCB’s) are not only toxic but they also degrade very slowly in the environment. They are in the course of being banned in all countries. Data about fluorinated derivatives are not entirely unanimous; some of these substances are physiologically inactive, some very harmful, e.g., fluorinated fatty acid derivatives with an even number of carbon atoms and the products of decomposition of polytetrafluorethylene, polybifluorchlorethylene and similar compounds. Another danger arises from metal-containing lubricants such as greases when these are exposed to ionising radiation, particularly neutrons; this is discussed in detail elsewhere (see page 90). Little has been published about the physiological properties of additives. Most detergenddispersant additives - alkarylsulphonates, alkylphenolates and alkylsalicylates of Ca and Mg are comparatively non-toxic, together with commercially produced packaged additives for engine oils. The value of oral LD50 for many of 117
these is more than 10 mVkg of mammalian body weight, and through the skin over a 24 hour period more than 2 mVkg. Phenolic antioxidants are of higher toxicity; LD50 can be 1 to 5 gkg, except for selected products used up to now in the food industry. Zinc dialkyldithiophosphates also have lower LDSO’s, 1.5 to 2 mVkg (173). Regulations in many countries, including the EEC and the USA, now require disclosure of the composition of additive blends and the registration of new products, in order to obtain a license permitting their use in lubricants. Prudent technical practice is to consider these factors in conjunction with the use to which products are likely to be put and to set up safe working codes accordingly. However, in many cases it is found that the major health hazard resides in the base oil as much as in the additives; the rules of good hygiene mentioned earlier should always be followed, even if the numerical toxicity index values appear low. Some substances are no longer used on grounds of toxicity or that of their degradation products. These include additives containing barium and lead, chlorinated naphthalenes once used in cutting oils, combinations of nitrites and amines formerly used in emulsions, mercaptobenzthiazoles (antioxidants) and chlorinated solvents, such as trichlorethylene, once used as diluent in oils for open gears.
2.6 THE EFFECTS OF LUBRICANTS ON SEALING MATERIALS Compatibility with the materials used to compound sealing devices, such as packings and O-rings, is essential if lubricants are to be reliable in field service. Lubricants can often adversely affect such materials, causing swelling, shrinkage and hardening. The result of this is a loss in sealing efficiency and consequential loss of the lubricant by leakage. Two factors correlate with effects on sealing materials: chemical composition and viscosity of the oil. Chemically, aromatics concentration is of primary importance. The effects of these factors on elastomers of several types are shown in Table 2.26 (180). Table 2.26. Effects of Oil on Some Elastomer Types Effect on sealing efficiency of (elastomer type) Factor
Fluorinated Acrylic rubber rubber
Nitrile rubber
Silicone rubber
Oil viscosity drop at constant aromatics content
very low
low
high
very high
Increased aromatics content at constant viscosity
very low
high
veryhigh
low
Synthetic oils may affect the chemical structures of elastomers in various ways. Most synthetic oils cause swelling or disintegration by their solvent effect on the 118
elastomers, e.g., diester oils, whilst polyolefin oils cause shrinkage, as do low aromatic petroleum raffinates such as those made by hydrocracking of distillates highly refined with SO,. Some additives may also have an adverse effect; for example, embrittlement of some types of fluoro-elastomers can be caused by additives containing amine groups (190). The following sealing materials are generally used for lubricants (temperature ranges quoted for use are approximate): Nitrile rubbers (acrylonitrile-styrene-butadiene)for mineral oils, polyalphaolefins, diester oils, polyglycol oils. Used from -25 to 100 OC.Accounts for about 80% of all sealing materials. Polydiene rubbers (natural rubbers, polybutadiene, polyisoprene) for vegetable oils and glycol ether brake fluids. Used from -30 to 130 "C. Poor resistance to mineral oils. Polyolefn rubbers (Butyl rubbers - polyisobutene-polybutadiene,EP rubbers polyethylene-propylene, EPDM rubbers - polyethylene-propylene-dienemixtures) for phosphate esters, glycol ethers, vegetable oils. From about -10 to 120 "C. Poor resistance to mineral oils. Chlorinated rubbers (polychloroprene, neoprene) for phosphate esters from -40 to 200 "C Fluorinated rubbers (perchloropropylene-vinylidenefluoride and analogues Viton rubbers) for mineral oil and phosphate esters, from -40 to 200 "C. Silicone rubbers (methyl-vinylsiloxanes, phenyl-methyl-vinylsiloxanes) for diesters, polyesters and chlorinated methylphenylsiloxanesfrom -60 to 150 "C.Poor abrasion resistance. Polyurethane rubbers (polytolyldiisocyanate-polyglycol-basedand analogues) for mineral oils from -40 to 70°C. Non-aggressive sealant, but poor moisture and heat resistance. Designed for special purposes. Acrylic rubbers (polyacrylates and copolymers) for mineral oils from -20 to 150 "C. Poor hydrolytic stability. Polysulphide rubbers (a,wchloroalkanes -Na polysulphide thiokols) resistant to hydrocarbons and chemicals from -40 to 90 "C. Otherwise poor properties. Chlorosulphonate rubbers (polyethylene sulphodichorides, Hypalox) resistant to chemicals, oils and ageing, from -40 to 100 "C.
119
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Chapter 2 References 1. WATERMAN, H. I. - BOELHOUWER, C. - CORNELISSEN, J: Correlation Between Physical Constants
and Chemical Structure. Amsterdam, Elsevier 1958. B. W. : Ind. Eng. Chem., 35, 1943,388. 2. WATSON, K. M. - GAMSON, J: Philosophiz naturalis principia mathematika. Lib 11. Kap. 2, London 1687. 3. NEWTON, 4. HILL,J. B. - COATS,H. B. : Ind. Eng. Chem., 20, 1928,641. 5 . WALTHER, C. : Oel u. Kohle, 1,1933,71. 6. UMSTATTER, H. : Kolloid Z, 90,1940, 172. 7. DEAN,E. W. - DAWIS,G. H. B. : Chem. metall. Eng., 36, 1929,618. C. G. : J. Inst. Petrol., 38, 1952, 193. 8. BLOTT,J. F. T. - VERVER, 0. A. : Nat. Petr. News, Techn., 4 , 1944, 10. 9. WATSON, K. M. - NYEHARA, 10. VOLAROVIC, M. P. : Trenije i iznos v maginach. 11. Moscow, AN USSR, 1940,281. 11. Kuss, E. : Mineraloltechnik, 18, 1973,7 - 8. P. : Z. Phys. Chem., N. F., 68,1969,205, 12. Kuss, E. - POLLMANN, W. M. : Ind. Eng. Chem., 42,1950,2462. 13. MURPHY, C. M. - ZISMAN, R. G.: Ind. Eng. Chem., 41, 1949,933. 14. THORPE, L. G. - LARSEN, 15. MCGRECOR, R. R. : Ind. Eng. Chem., 46, 1954,2323. A. V. - CADY, G. H. : Ind. Eng. Chem., 39, 1947,367. 16. GROSSE, 17. SAFR,E. : Technika mazinl (Technique of Lubrication). Praha, SNTL 1970. 18. REINER, M. : Rheologie in elementarer Darstellung. Miinchen, C. Hauser Verlag 1968. G. W. : A Survey of General and Applied Rheology. New York, Pitman 19. SCOTT-BLAIR, Publishing Cop. 1949. 20. GREEN, H. : Industrial Rheology and Rheological Structures. New York, J. Wiley and Sons 1949. 21. BARIZ,W. J. : VDI Forschungsheft, 1968,530. G. R. : Erdol-Erdgas Z., 85, 1969,516. 22. BARTZ,W. J. - SCHULIZE, H. : Erdol u. Kohle, 17, 1964,919. 23. BOSMA, R. - NAYLOR, H. H. et al. : 5th World Petroleum Congress, Sect. VI, Paper 19, 289 - 305, 24. HOROWITZ, New York 1959. 25. CAMERON, A. :J . Inst. Petrol., 49, 1954, 191. 26. WEISSENBERG, K. : Abnormal Substances and Abnormal Phenomena of Flow. New York. NorthHolland Publishing Co. 1949. 27. KIRSCHKE, K. : Proc. Int. Congr. Rheology, 3, 1963,401. 28. FORSTER, E. 0. : NLGI Spokesman, 24, 1961,462. A. W. : Ind. Eng. Chem., 50, 1958, 1789. 29. SISKO, 30. SINICYN, V. V. : Podbor i primenenije plastitnych smazok. Moscow. Chimija 1969. G. V. : Doklady AN USSR, 71,1950,505. 31. VINOGRADOV, 32. GRAVERT, H. : Mineraloltechnik, 13, 11, 1968,7. 33. BINGHAM, E. C. : J. Rheol., 1, 1930,507. 34. ORR,A. S. : NLGI Spokesman, 25, 1961, 160. 35. SKOCLUND, R. D. : NLGI Spokesman, 24, 1960,47. 36. MILNE,H. : Kolloid Z., 129, 1954,96. 37. DOTERWEICH, E. : Untersuchungen des Schmiervorganges an fettgeschmierten Gleitlagern (Dissertation) Darmstadt-Technische Hochschule 1965. 38. FORSTER, E. 0. : Lubric. Eng., 16, 1960, 523. A. : J. Inst. Petrol., 31, 1945,421. 39. CAMERON, 40. ”Pressure Viscosity Report’’ Vol. 1 and 2, ASME 1953. J. : ASTM Bull., 235, 1959,51. 41. PEELER, R. L. - GREEN, 42. Dow, R. B. - FINK,C. E. : J. Appl. Phys., 11, 1940, 353. 43. HARTUNC, H. A. : Trans. ASME, 78, 1956,941.
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44. O’CONNOR, J. J. : Standard Handbook of Lubrication Engineering. New York, McGraw-Hill 1968. 45. TICHY,VI. et al. : Transformstorovy’olej (Transformer Oil). Bratislava, SVTL 1962. M. R. : ASTM Bull., 215,6, 1956, 87-94. 46. KLAUS, E. E. - FENSKE, 47. KLAUS, E. et al. : I & EC Product Research and Development, 2, 1963, 332. 48. ZORN,H. : Erdol u. Kohle, 8, 1955, 414. 49. GUNDERSON, R. C. - HART,A. W. : Synthetic Lubricants. New York, Reinhold Publ. Co. 1962. 50. RUMPF, K. : Erdol u. Kohle, 27, 1974, 306. 51. KLAUS, E. E. - TEWKSBURY, E. J. : Hydrocarbon Proc., Dec. 1974,67. 52. KLAUS,E. E. : Proceedings USA Aerospace Fluids and Lubricants Conference. San Antonio (Texas) 1963. 53. WIEZEWICH, N. et al. : Ind Eng. Chem., 27, 1935, 152. 54. JACKSON, J. L. : Ind. Eng. Chem., 43, 1951, 2869. 55. BRUSS, H.: Schmiertechnik, 4, 1957, No. 3. 56. SCHMIDT, H. : Mineraloltechnik, 18, 1973, 10. 57. BUEHLER, F. A. - RAICH,H. : NLGl Spokesman, 31, 1967,49. 58. KLAMANN, D. et al. : Erdol u. Kohle, 20, 1967, 219. G. H. : Schmiertechnik, 11, 1964, 143. 59. GOTTNER, - MICHIO:Bull. Jap. Petrol. Inst., 10, 5, 1968,42. 60. HOSHINO A. R. : Sci. Petroleum, 4, 1938, 2947. 61. UBBELOHDE, 62. JESTER, W. A. - KLAUS, E. E. : Nuclear Applications, 3, 1967,375. 63. VESELY,V: Ropa a Uhlie, 11, 1969, 297. 64. REVENDA, J. : Ropa a Uhlie, 5, 1963, 2. 65. SmazoEnyje matenaly, prisadki i parafiny. Moscow, VNIINP 1958. 66. ST~PINA, V. : Ropa a Uhlie, 13, 1971,6; 14, 1972.4. 67. SCOTT,G. : Atmospheric Oxidation and Antioxidants. Amsterdam, Elsevier 1967. 68. CERNO~UKOV, N. 1. - KREJN,L. E. : Okisljajemost minerahych masel. Moscow, G l T I 1955. R. A. : Ind. Eng. Chem., 35, 1943,581. 69. LARSEN, R. G.- ARMFIELD, 70. ZUIDEMA, W. H. : The Performance of Lubricating Oils. New York, Reinhold Publishing Co. 1952. 71. EWELL, R. H. et al. : Ind. Eng. Chem., 36, 1944, 871. 72. SEMJONOV, N. N. : Osnovnyje problemy chimiteskoj kinetiki. Moscow, AN USSR 1959. A. C. : Neft. Choz., 3, 1947, 59. 73. BOGUSLAVSKAJA, M. C. - VELIKOVSKIJ, 74. ARLIE,J. P. - DENIS,J. : Colloques Internationaux du CNRS, No. 233, Brest, 1974. et,al. : Chimija mineralnych masel. Moscow, G’ITI 1951. 75. ~ E R N O ~ U N. K O1. V 76. ROUNDS,F. G. : SAE Preprint 255 Nov. 1957. A. : IngCnieurs de l’Automobile, 1960, 82. 77. MACNAB, M. - DE GOLDLIN, S.A. - F R ~A., R. : Anal. Chem., 21, 1949,568. 78. MACKEE, H. : Schweizer Archiv fur angewandte wirtschaftliche Technik. Aug. 79. STAGER, H. - KUNZLER, 1942, No. 8. 80. WILLIAMS, A. L. - OBERRIGHT, N. : ASME, Oct. 1959, 59. 8 1. LIEBL,X:Plasticke mazivi (Lubricating Greases). Bratislava, Prka 1970. E. R. : Lubr. Eng., 13, 1957, 199. 82. HAUSMAN, R. F. - BOOSER, 83. FREEWING, J. J. - SCARLEIT,N. A. : 4th World Petroleum Congress. Roma 1959. 84. MARLES, A. J. : Scientific Lubrication, 12, 1958,33. 85. WILLS,J. G. : Nuclear Power Plant Technology. New York-London, J. Wiley & Sons 1967. 86. BARNETT, R. S . : NLGl Spokesman, 7, 1961.92. 87. VAILE,P. E. B. : Roc. Inst. Mech. Engng., 176, 2, 1962,27. 88. FRICKER, H. W. : Inst. Mech. Engng. Lubrication and Wear Convent. 1963, Paper 30.46. 89. HELFRICH, F. : Schmiertechnik, 11, 1964, 339. A. - GREENE, D. F. : ASLE Trans., 4, 1961, 87. 90. BEERBOWER, S. et al. : J. Phys. Chem., 62, 1958,20. 91. GORDON, 92. CARROLL, J. G. - BOLT,R. 0. : ASLE Trans., 2, 1959, I .
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RODENBUSH, H. - PROKOP, W. R. : Mineraloltechnik, 19, 1974, 1. RODENBUSH, H. : Erdol u. Kohle, 19, 1966, 583. J. G. - CALISCH, S. R. : Lubr. Engng., 13,1957,388. CARROLL, HOLLINGHURST, R. : J. Inst. Petrol., 52, 1966, 9. POLISHUK, A. T. : 41st Ann. NLGI Meeting, Houston 1973. VANIERDE SAINTAUNEY et al. : Chimie et Ind., 29, 1933, 1011. PHILIPPOVICH, A. : Chemisch-physikalische Grundlagen der Verwendung von Erdol und seinen Produkten. Wien, Springer-Verlag 1960. 100. KRWN, J. E. - KULAKOVA,R. V: Neftjanyje izoljacionnyje masla. Moscow GEI 1957. 101. RYBAK, B.,M. : Analyz nefti i nefteproduktov. Moscow, GTTI 1962. 102. VESELY,V. : ChCmia a technol6gia ropy I. (Chemistry and Technology of Petroleum I.). Bratislava, SVTL - SNTL 1963. 103. STEINBACH, H. L. : Erdol u. Kohle, 12, 1959, 397. 104. KOHLE,H. : Z. VDI, 95, 1953,761. 105. KICKIN,N. 1. : Chim. Technol. Topl. Masel, 4, 1966. 106. HARTMANN, L. M. : Lubr. Eng., 28, 1, 1972, 21 107. BALDWIN, R. R. - DANIEL,S. G. : J. Inst. Petrol., 39, 1953, 105. 108. SACHANOV, A. N. - KAGAN, G. M. : ChimiE. sostav neftej i neftjanych produktov. Moscow, GTTI 1931. E. C. : Petr. Proc., 9, 1952, 1274. 109. HUGHES,M. A. - HUGHES, 110. LARSEN, R. G. et al. : Ind. Eng. Chem., 17, 1945, 19. 111. MATTHEWS, F. W. H. : J. Inst. Petrol., 6, 1949, 436. 112. MACCOULL, N. et al. : SAE Journal, Transactions, 50, 1942, 338. 113. LAMB,G. G. et al. : Ind. Eng. Chem., 13,1941, 317. 114. UNDERWOOD, A. F. : SAE Journal, Transactions, 43,1938, 385. 115. SACHPARANOV, M. I. : Vvedenije v molekuljarnuju teoriju rastvorov. Moscow, GITTL 1956. 116. ZUIDEMA, M. H. : Ekspluatacionnyje svojstva smazoEnych masel. (Translated into Russian) Moscow, G’ITI 1957. 117. GODDARD, D. R. : Lubrication and Lubricants. Ed. E. R. Braithwaite. Amsterdam, Elsevier 1967. 118. SEHRENS, H. T. : Berufsdermatosen, 4, 1956, 201. 1 19. VANA, V. - LISKOVA,B. : Pracov. Lek., 10, 1958,364. 120. ANDRADE, E. N. da C. : Nature, 125, 1930,309,352. 121. CWKA,M. et al. : Ropa a Uhlie, 11, 1969,629.Idem. Berufsdermatosen, 18, 1970,281. 122. VANVELZEN,D. et at. : Ind. Eng. Chem., Fundamentals, 11, 1970, 20. 123. BRAUKMANN, F. : Erdol u. Kohle, 6, 1953, 804. 124. CWKA,M. et al. : Z. ges. Hyg., 1965, 9. 125. VOGEL,H. : Z. Physik, 22, 1921,645. 126. CAMERON, A. : J. Inst. Petrol., 48, 1962, 147. 127. NELSON, W. L. : Oil Gas Journal, 57,49, 1959, 72. 128. CWKA,M. - STEPINA, V. : Ropa a Uhlie, 10, 1968, 193. 129. BIKBULATOV, N. T. : Chim. seraorg. sojedinenij 11. Ufa, AN USSR 1959. 130. ABEL,W. - KOPPEN,J. : Schmierungstechnik, 3,1972, 302. 131. VESELY,V. - UDE,G. : Chem. Techn., 17, 1965,549. 132. BOWDEN, J. N. et al. : Preprints ACS, Div. Petrol. Chem., Sept. 1961. 133. NLGI Spokesman, 19, 1956.30; 20, 1958,69; 28, 1964,289. 134. RUMPF,K. K. : Viskositat und Fliessvermogen von Mehrbereichs-Motorolen. Esslingen, Techn. Akademie 1978, Bericht No. 2. 135. PYWELL,R. P. In: The Rheology of Lubricants. Ed. T. C. Davenport, Barking, Appl. Science Publ. 1973. 136. WRIGHT,W. A. : ASLE Trans., 10, 1967,349. 137. DOWSON, D. et al. : J. Mech. Eng. Sci., 4/2, 1962, 121. 138. HUTTON, J. F. In: The Rheology of Lubricants. Ed. T. C. Davenport, Barking, Appl. Science Publ. 1973. 93. 94. 95. 96. 97. 98. 99.
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139. WALTERS, K. In: The Rheology of Lubricants. Ed. T. C. Davenport, Barking, Appl. Science Publ. 1973. 140. WISLICKI, B. : Eurotrieb ‘81,111 A, 245, Warsaw 1981. 141. EYRING, H. - JHON,M. S. : Significant Liquid Structure. New York, J. Wiley & Sons 1969. 142. EYRING, H. - JHON,M. S. : The Structure of Liquids. In: Interdisciplinary Approach to the Lubrication of Concentrated Contacts. Ed. Ku, P. M., Washington, NASA 1970. J. K. : The Structure of Liquids. In: Interdisciplinary Approach to the Lubrication 143. APPELDOORN, of Concentrated Contacts. Ed. Ku. P. M., Washington, NASA 1970. 144. ROELANDS, C. J. A. et al. : ASME, Lubr. Div. Symp., 1962. S. : Intertribo ‘81,II, 145, Vysokc? Tatry, CSSR 1981. 145. JANCZAK, K. J. - HOFMAN, D. W. : Properties of Polymers. Amsterdam, Elsevier 1972. 146. VAN KREVELEN, 147. BONDI,A. In: Rheology, Theory and Applications. Vol. 111, Ed. Eirich T. R., Academic Press 1960. 148. FORSTER, E. K. : ErdoI u. Kohle, 13, 1960,478. 149. TASKOPRULU, N. S. et al. : J. Acoust. SOC.Am., 33, 1961,278. 150. MATHESON, A. J. : Molecular Acoustic. New York, J. Wiley & Sons 1971. H. : Ind. Eng. Chem., 34,1942,927. 151. VON FUCHS,H. H. - DIAMOND, 152. Company bibliography Electrion, S. A., Gent. Belgium. 153. UK Patent 852991. A. J. : Adsorption and Surface Energetics. In: Interdisciplinary Approach to the 154. HALTNER, Lubrication of Concentrated Contacts. Ed. Ku P. M., Washington, NASA 1970. 155. Contact Angle, Wettability and Adhesion. Advances in Chem. Series, 43, ACS, 1964. 156. GIRIFALCO, L. A. - GOOD,R. J. : J. Phys. Chem., 61, 1957,904,62,1958, 1418; 64, 1960, 561. 157. OWENS, D. K. - WENDT,P. C. : J. Appl. Polym. Sci., 13, 1969, 174. 158. SMALL, P. A. : J. Appl. Chem., 3, 1953,7. Z. : Intertribo ‘81, I, 187, Vysokc? Tatry, CSSR 1981. 159. RYMUZA, 160. BUCKLEY, D. H. : Discussions on reference (15.4). 161. VAN KREVELEN, D. W. - HOWZER,P. J. : hoperties of Polymers - Correlation with Chemical Structure. Amsterdam, Elsevier 1972. 162. BARUS, C. : Amer. J. of Science, 45, 1893, 87. 163. KOUZEL, B. : Hydrocarb. Process, 44/3,1965, 120. 164. ROELANDS, J. C. A. : Thesis. T. U. Delft, 1966. 165. FRESCO, G. P. : MS Thesis. Pennsylv. State University 1%2. 166. KIM, H. W. : MS Thesis. Pennsylv. State University 1970. E. F. : ASLE Trans., 23, 1980,409. 167. SOA,B. Y.C. - KLAUSE, C. J. : ASLE Trans., 24, 1981,542. 168. SPIKES,A. - HAMMOND, 169. CZARNY, R. - Moos, H. : Eurotrib ‘81, Warsaw, 1981,111,68. 170. HILDEBRAND, J. H. et al. : Regular and Related Solutions. New York, Van Nostrand Reinhold Co. 1970. A. : ASLE Trans., 23,1980,335. 171. BEERBOWER, 172. TENNAPEL,W. E. : Tribologia e Lubrificazione, 15, 1980, 127. 173. CLARK, D. G. : Erdo1 u. Kohle, 31, 1978, 584. Jr. : ASLETrans., 26, 1, 1983, I . 174. LOWWEB.SARGENT, 175. STEPINA, V. et al. : Intertribo ‘84, VysokC Tatry, CSSR 1984; Tribologie & Schmierungs technik, 14,2, 1987, 113. G. R. : ElastohydrodynamicLubrication - The Fundamentals of 176. DOWSON, D. D. - HIGGINSON, Roller and Gear Lubrication. London, Pergamon Press 1960. 177. Kuss, E. : Materialpriifung, 2, 1960, 189 - 197. 178. BRADBURY, D. et al. : ASME Trans., 1951,667. 179. DGMK - Forschungsbericht 198, 1982. A. A. et al. : Mineraloltechnik,25, 1, 1980. 180. REGLITZKY, 181. POLISHUK, A. T. : NLGI Spokesman, 4, 1985.32. 182. DAVENPORT, T. C. : The Rheology of Lubricants. London, Appl. Science Publ. 1973.
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183. ULBRECHT, J. - UITSCHKER, F. : Chemicke infenfrstvi ne-newtonskych kapalin (Chemical Engineering of nowNewtonian Liquids). Prague, Academia 1973. 184. HOTIER,J. et al. : ACS Reprints, Petrol Div., 32, No.2, 1987,496. 185. EYRES, A.Q.: In : Industrial Tribology. Jones, M.H.- Scott, D. (Ed.): Industrial Tribology. Amsterdam, Elsevier 1983. 186. ROY,T. A. et al. : Fund. and Appl. Toxicology, 10, 1988,466. N. : Proc. 33rd. Conf. on Petroleum. Bratislava, December 1988. 187. HAJDAKOVA, G.R. et al. : Cell. Biol. Toxicol. 2, 1986.63. 188. BLACKBURN, K. : Roc. 33rd. Conf. on Petroleum. Bratislava, December 1988. 189. BURGHARDTOVA, 190. STREIT,S. - SONSE,S.: Kautschuk, Gummi, Kunstoffe, 38,1985,471
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CHAPTER THREE TYPES OF LUBRICANTS AND THEIR COMPOSITIONS
Substances in all states of matter can be used as lubricants - gases, liquids, plastic and solid materials, pure organic or inorganic compounds, their homogeneous and heterogeneous mixtures, solutions, melts, alloys, emulsions and suspensions - providing they reduce friction and wear between materials in contact in mutual, relative motion or intended motion.
3.1 GASEOUS LUBRICANTS Gases are used as lubricants in gas-lubricated bearings (14, 222) which operate in the aerodynamic (at high speeds -10,000 to 600,000 r.p.m.) or aerostatic regimes at temperatures from 15 to 800 "C or more, where liquid lubricants would freeze or decompose, in hostile environments (e.g., radiation, etc.), where there is danger of contamination and other difficult or unusual situations. The advantages of gas lubrication include the low viscosities of gases, which increase with temperature (see Table 2.1), low pressure loss, low coefficients of friction, small frictional heat generation, absence of local hot-spots, 'the omnipresence of the lubricants on the surfaces and the absence of sealing, clogging and contamination problems. The drawbacks are the lower load-carrying capacity of gas-bearings, limited to 35-70 kPa in the aerodynamic regime and to l00kPa in the aerostatic regime (externally pressurised bearings), the tendency to instability and turbulence, highfrequency vibrations and the danger of direct contact which may lead to failure. Gas lubrication requires the highest precision in manufacture and assembly of gas bearings, leak-proof mechanisms and smooth surfaces, with asperities below 0.35 pm or even less. In addition to conventional steel-to steel, steel- or stainless steelto graphitised bronze, chromium-to-cobalt friction pairs, less usual pairs are often used, such as vanadium-to-Al2O3 (223) or S i c and other materials with ionimplantation of MoS, (224). Graphite coated with CdO(8-10 pm)-to-Ag using a Na silicate binder has proved satisfactory up to 430 "C and sputtered (1 pm) Cr,O, up to 650 "C for bearings and detonation-coated(60 - 90 pm) Ni-Cr on Cr203 for both bearing and shaft. Air is suitable up to 650 "C, CO, to 650 "C, He and N, to 1,OQO"C and more, hydrogen (higher cooling capacity, but flammable) or methane for gas turbines, working fluids like steam for steam turbines, and fuel or exhaust gases for internal 125
combustion turbines. Reactive compounds such as CF2Br2and SF6 are sometimes added to the gas in closed circuit systems to improve its performance functions in critical and transition regimes (212). Use of gas lubrication is increasing in high-speed equipment: precision optical instruments, dental drilling, measuring instruments, electronic computers, precision grinding spindles, and in the pharmaceutical, chemical, food, textile and nuclear industries where contamination must be avoided and the loads are low. Externally pressurised gas bearing working in the aerostatic region can carry higher loads, are less critical on clearances and tolerances and can be used even at lower speeds.
3.2 LIQUID LUBRICANTS The majority of lubricants are liquids. They may be classified according to their origin into two groups: mineral oils derived from petroleum (the majority) and synthetic oils. Most liquid lubricants contain additives to improve their natural properties, or impart new properties to them, so that they have qualities suitable for their applications and their operability may be prolonged. Liquid lubricants are used either as such or as water emulsions, where efficient cooling is required as well as lubrication duty. Liquid lubricants also form the principal component of plastic lubricants, more often referred to as lubricating greases.
3.2.1 Mineral Oils Mineral lubricating oils having a variety of physical and chemical properties are now in use. To obtain oils of the required properties, selection of the right crude petroleum source has to be combined with the right manufacturing processes and the use of the right additives in optimum proportions and dosages.
3.2.1.1 Crude Oil Composition Effects Crude oils may be characterised according to their overall composition or state of aggregation, their fractional composition and the properties of their constituents. Under the heading of state ofaggregation, crude oils contain gaseous, liquid and solid constituents, The concentration of gases (alkanes from methane to butanes) is negligible. Liquids are the principal constituents. Solids referred to as waxes accompany lubricating oil cuts, and these are predominantly saturated. Macrocrystalline or paraffin waxes originate from lighter cuts. Their main constituents are isoalkanes, cycloalkanes and some aromatics, all with long alkyl chains up to C8@ The microcrystalline or ceresine waxes (ceresines for short) originate from heavier cuts. The “amorphous” (actually microcrystalline) waxes (referred to as petrolatums in the USA and vaselines in several countries) originate from some asphalt-free or refined distillation residues. 126
Petroleum distillation residuum represents the transition state between liquid and solid. The residua may be pseudoplastic or quasi-plastic. They are termed “long residuum” if they originate from atmospheric distillation (also mazoute in Eastern Europe) and asphalt (in the USA) or bitumen (in Western Europe) if they come from a vacuum distillation process. In terms of fractional composition, crudes are classified into light, heavy and asphaltic crudes. By convention, light crudes contain more than 50% “white products” - petrol (gasoline in the USA), kerosene and gasoil boiling up to 360 “C, heavy crudes more than 50% viscous oils and asphaltic crudes more than 50% asphalt. Intermediate crudes contain less than 50% light, heavy and asphaltic constituents. Asphaltic crudes are often rich in sulphur compounds (more than 3% as S), but the other crudes, especially the heavier ones, may also be sulphurous. In terms of chemical composition, crudes may, by an analogous convention, be classified into alkanic (or paraffinic) crudes with more than 50% alkanes, cycloalkanic or cyclic (or naphthenic) crudes with more than 50% cycloalkanes, aromatic (rare) with more than 50% aromatics in the distillate and mixed crudes with less than 50% alkanes, cycloalkanes and aromatics. Alkanic crudes are mostly light, wax-beiring and of low sulphur content. Cycloalkanic crudes are often heavy, waxfree or low wax and may contain sulphur compounds. Crudes are classified according to wax content, into paraffinic, semi-paraffinicand non-paraffinic, at more than 2%, 0.5 - 2% and less than 0.5% wax concentration (the term “paraffinic” should not - but often is - confused with “alkanic”!). Crudes are classified in terms of sulphur content into high, medium and low sulphur - above 2%, 0.5 - 2% and less than 0.5% S respectively in the aggregate crude. The fractional and chemical composition of crudes can be conveniently represented in triangular diagrams (5). Frost and ObrjadEikov (I) distinguish high-temperature and low-temperature crudes. High-temperature crudes - supposedly formed at temperatures above about 300 “C - are usually lighter, they are rather alkanic and their fraction rich in nalkanes. Cyclopentanes predominate over cyclohexanes in light cuts. In the lubricating oil fractions, they contain macrocrystalline waxes. Low-temperature crudes, supposedly formed below 150 “C, are usually heavier, cycloalkanic, isoalkanes predominate over n-alkanes and cyclohexanes over cyclopentanes in the lighter cuts. The lubricating oil cuts may be low-wax or contain microcrystalline waxes. The character of the crude oil is important, but not decisive, in the manufacture of lubricating oils. Modem processes allow the production of lubricants and special oils with specifically required properties from almost any crude oil source, especially with the use of additives. The problem is rather one of economics. Waxfree cycloalkanic crudes can be processed to produce lubricants of low pour-point without the need for an expensive de-waxing process. However, these crudes are becoming rare. Crudes with a high sulphur concentration require deep refining, which appreciably increases the cost of manufacture. Earlier refining methods did not allow fundamental changes in lubricant composition - they just removed the most objectionable constituents. Modern processes enable the composition to be altered 127
to match product needs by removing some of the less desirable constituents, e.g., by solvent refining, or by transforming them into desirable compounds, e.g., by hydrogenation.
The Chemical Composition of Petroleum Mineral Oil Fractions Oil fractions mainly contain hydrocarbons, a huge number of species of widely different varieties. From experimental data (2 5), their properties depend on the number of carbon atoms in the average molecule, i.e., on the size of the molecules expressed as relative molecular mass (RMM - 250 or more), on the distillation range and on the hydrocarbon groups present. These are represented by straight-chain and branched alkanes, alkylcycloalkanes, alkylaromatics and alkylcycloaromatics. Compounds of sulphur, oxygen and nitrogen are also present, some of them as organometallic porphyrins (V, Ni) and oil resins. Straight-chain alkanes (n-alkanes, n-paraffins) have, at the same carbon number, the lowest viscosity (below 15 mm2s-l at 40 "C), the highest viscosity index (VZ about 200), the lowest evaporation loss and the highest flash-point. Their pour-point is high (40 OC or more) and in high concentration they are undesirable. They are removed from lubricating oil fractions by de-waxing or catalytic cleavage and isomerisation. Branched alkanes (isoalkanes, iso-paraffins) differ from the n-alkanes by having alkyl substituents on the main chain. A typical n-alkane may be represented as:
-
CH,-(CH,),-CH,
n > 24, RMM > 360
and iso-alkanes by:
n > 24, RMM > 360 If the alkyls are short and not very numerous, there is not a big difference between the properties of n- and isoalkanes. Pour-points are high, and the oil must be dewaxed. If the alkyls are long, more numerous and branched, iso-alkanes have a low pour-point, a high VI, low volatility and evaporation loss and a high flash-point. Such isoalkanes are present i n classical mineral oils in small amounts, but in unconventional oils produced, e.g., by hydrocracking, they may predominate. They are also manufactured synthetically (see polyolefins, PAO). Alkylcycloalkanes(alkylcyclanes,naphthenes) molecules contain one or more (up to six) cyclopentane or cyclohexane rings, either separated or directly linked, or mostly condensed with alkyl substituents. A typical molecule may be represented by the structural formula
128
One alkyl is often longer with six or more carbon atoms, while the other alkyls are mostly methyl and less frequently ethyl and isopropyl. These hydrocarbons (and the alkylaromatics)control the viscosity of mineral oils. They are rarely present at less than 30% and their concentration may be as high as 70%.At the same carbon number, they have higher viscosity, lower VI, lower flash-point and higher evaporation loss than the alkanes or isoalkanes, but their pour-point is generally low. The carbon number and the length and degree of branching of alkyls have the same influence on their properties as in the case of the isoalkanes. There is insufficient evidence to be able to classify oils as being predominantly cyclopentanic or cyclohexanic. One remarkable cycloalkane is adamantane (see page 187), discovered first in a Czechoslovakian cycloalkanic crude. Later, alkyl homologues of adamantane and polyadamantanes (diamantane, etc.) were found as minor constituents in other crudes (52, 62). Alkyfuromutics are characterised by one or more aromatic nucleus, separated (as in alkyl biphenyls) or - mostly - condensed, often with cycloalkane rings in the
Alkyl substituents have the same effect on the properties of alkyl aromatics as on isoalkanes and alkylcycloalkanes. However, aromatic nuclei can have an even greater effect than cycloalkanic rings on the viscosity of lubricating oil cuts and at the same time decrease their VI; with up to two nuclei less than the cycloakanes, but with more than two nuclei substantially more. Polynuclear aromatics with more rings and numerous short alkyls not only impair the rheological properties of oils, but also affect their oxidation stability - they enhance the formation of oxidised resins leading to sludge and solid deposits. Therefore, they are commonly removed from most lubricating oils by solvent refining or by hydrogenation into hydrocarbons with fewer nuclei, more cycloalkanic rings and even more, longer alkyls, e.g., into alkylbenzenes. Alkylbenzenes substituted with long alkyls have high VI, and may have a low pour-point and are regarded as high-grade oils. Their concentration in virgin oil cuts is rather limited and increases in hydrogenated stocks. Some of them are produced by synthesis (see page 153). Besides these alkylbenzenes, cyclohexylbenzene and tetralin-type mononuclear aromatics are also present. Dinuclear aromatics are classed as alkylated and cycloalkylated naphthalenes and biphenyls, tri-nuclear aromatics with the analogous phenanthrenes, whilst anthracenes seem only to be present in minor concentrations. With mass spectrometry, the presence of other aromatics can be detected: acenaphthenes, acenaphthylenes, fluorenes, chrysenes, pyrenes, etc. Most hydrocarbons in lubricating oil cuts are hybrid types; they contain, simultaneously, aromatic nuclei, cycloalkanic rings and alkyl substituents. It is, 129
therefore, difficult to classify them into individual hydrocarbon groups. For this reason, statistical methods of analysis, introduced by the Waterman Dutch school and based on correlations between the physical properties and chemical constitution of hydrocarbons are still extensively used in spite of the progress made in chromatographic and spectral methods (6, 7,9,251).One of the most useful is the n-d-M method, which offers useful information on the relative ratio of carbon bonded in alkanes (paraffins, Cp), cycloalkanes (naphthenes, CN), and aromatics (C,) and the average number of cycloalkanic (RN), aromatic (RA) and total rings (RT) from the density, refractive index and relative molecular mass. Different variants are available involving spectroscopic methods and addressing departures from conventional oil compositions, e.g., a higher concentration of aromatics or sulphur compounds. The non-hydrocarbon constituents comprise oxygen, sulphur and nitrogen compounds and high-molecular weight petroleum pitches - oil resins, asphaltenes, carbeaes, carboids and asphaltogenic acids and anhydrides. They are present, except for carbenes and carboids, in all fractions of virgin crude oils and predominate in heavy oil cuts and residues. Oxygen compounds mostly comprise naphthenic acids, carboxy derivatives of cyclopentyl, cyclohexyl and polycycloalkyl hydrocarbons. They confer acidity on fractions starting from kerosene. Their highest concentration frequently appears in light lubricating oil cuts. They are not desirable in lubricating oils, but they find application in petrochemistry and in the production of lubricating greases. Besides these acids, some neutral oxygen compounds are present. Their constitution and influence on lubricant properties is not well known. Some of them are classified as benzofurans. Sulphur compounds exist in concentrations from one hundredth to more than eight percent in terms of sulphur. In virgin crudes, they are present as free sulphur, mercaptans, alkyl-, cycloalkyl-, aryl- and mixed sulphides, and as heterocyclic compounds, alkylated thiophanes and thiophenes of the general formula:
The concentration of these compounds increases from light to heavy distillation cuts and residuals, where they may predominate over hydrocarbons. For example, the trinuclear aromatic oil cuts of Romashkino crude with 5.5% S contains 15% dialkylsulphides, alkylcycloalkylsulphidesand cycloalkylthiophanes, almost 60% thiophenes (mostly benzo-, dibenzo-, acenaphtheno- and fluorothiophenes) and only 25% of hydrocarbons (7). All sulphur compounds can improve the load-carrying capacity (lubricity, oiliness) of lubricants. Acyclic and alicyclic compounds are thermo-labile, aromatic compounds are thermo-stable. Both can be oxidised by peroxides to sulphoxides and sulphones and act as peroxide decomposers., i.e., natural anti-oxidants (4,9).The 130
final products of oxidation, sulphonic acids, are, however, strongly acidic and corrosive. Too high a concentration of sulphur compounds impairs resistance to ageing. Therefore, their concentration in oils is reduced by refining processes, particularly by hydrogenation, which enables the sulphur to be recovered as a product. Nitrogenous substances are contained in crude oils at lower concentrations (from hundredths to 1.5% by weight). They are present mainly in asphaltic and sulphurous crudes. Their concentration increases with the size of the molecules, accumulating chiefly in the high-molecular weight aromatic fractions, oil resins and asphaltenes. Their chemical nature is not well known as yet; they can be classified into bases, neutral and acid substances, according to their reactions with perchloric acid in acetic acid (strong bases), acetic anhydride (weak bases) or with quaternary ammonium bases (pyrrole) (8). Bases identified include alkylated pyridines, quinolines, acridines and hydrogenated carbazoles. Acid and neutral substances include pyrrolic and acidic nitrogen, nitriles, phenazines, carbazoles and, frequently in the heavier petroleum components, organo-metallicporphyrin derivatives, containing vanadium and nickel. Nitrogen compounds are regarded as deleterious components, since they can cause deterioration in stability towards light and oxidation and also impair the activity of catalysts in some processing treatments. However, some may possibly possess anti-oxidant properties, e.g., in the presence of metals. Petroleum resins (also called malthenes) are highly viscous, even plastic substances of density about one, with a relative molecular weight ranging from 280 to 1200 or higher. They appear in all crude oil fractions and particularly in heavy oil cuts and distillation residua - vacuum asphalt, where they may predominate. Their molecules comprise polycondensed cycloalkanes and aromatics, with short alkyl or alkylene chains, plus oxygen, sulphur or nitrogen atoms in the rings or bridges. Their general formula is : CnH2n-mOpSPs
n 5100, m I60,p 5 4 , r
l
1.5, s I1
Oil resins are soluble in heavier hydrocarbons, lubricating oil cuts, polar solvents and sparingly soluble in light alkanes, e.g., propane. They are readily adsorbed on to polar adsorbents and react with sulphuric acid to yield acid-soluble products. They are not resistant to heat and oxidation, and give rise to sticky substances and eventually asphaltene as the final product. Therefore, their concentration in lubricating oils is reduced by oil-treatment processes, such as de-asphalting, acid or solvent refining or hydrogenation. Asphaltenes are involatile, powdery constituents of distillation residues melting with decomposition and condensation over 100 "Cto carbenes and carboids. In oils they form colloidal solutions in which micelles agglomerate to form polynuclear aromatic clusters. From these solutions, asphaltenes precipitate by the action of light alkanes from ethane to isooctane. They are believed to be condensation products of oil resins with a similar chemical composition, but with molecules at least twice as large and richer in oxygen, sulphur and nitrogen. They are, with resins, the main constituents of asphalts, and in lubricants they are undesirable as the main pre131
cursors of carbon deposits. Therefore, they are removed by distillation and refining processes (mainly propane deasphalting). Carbenes and carboids are the ultimate condensation products of oil resins and asphaltenes. The carbenes are soluble in carbon disulphide, the carboids insoluble in any solvent. Asphaltogenic acids and anhydrides are considered to be the highest homologues or analogues of naphthenic acids. They accumulate in residues, from which they can be extracted by ethanol. Their concentration does not exceed 1%.
3.2.1.2 The Effects of Processing Techniques The manufacture of lubricating oils from crude oils includes separation, refining and de-waxing processes. Raw lubricating oil cuts are obtained by separation processes. Refining processes are applied to the cuts to remove from them any unsuitable components, or to transform these into ones which are wanted. De-waxing separates from the raffinates (or crude cuts) solid hydrocarbons, in order to obtain oils with a low pour-point and good rheological properties at low temperatures. The last stage in oil manufacture is blending together of base stocks and doping with additives (10, 253,258).
3.2.1.2.1 Separation Processes
Distillation In the first stage of distillation, the crude oil is subjected to atmospheric distillation, where petrol (gasoline), kerosene and gas oil cuts are obtained (depending on crude source, a primary flash-distillation stage may be used to concentrate the lightest hydrocarbon constituents, up to about C, which are then used as feedstocks for chemical manufacture and for converhon into other hydrocarbons for gasoline, etc.). The viscous oil cuts are obtained from the “long residuum” (atmospheric residue, mazoute) by vacuum distillation. A high vacuum (25 Wa or less) is essential to permit distillation at temperatures at which thermal decomposition of the stock is minimised. Usually, the temperature does not exceed 410 “C; however, even this temperature may be too high for some crude oils, especially alkanic and sulphurous crudes. Vacuum distillation is normally used to produce 3 or 4 lubricating oil distillates and vacuum asphalt as distillation residue. Lubricating oil distillates differ from one another by distillation range, average relative molecular weight, viscosity, flash- and pour-points, etc. The main criteria used are viscosity and flash-point. Wide or narrow distillation cuts can be obtained from vacuum distillation; narrow cuts are generally more desirable. The distillation range of a cut should not exceed 50 - 60 “C. At any given viscosity, wide cuts contain both light and heavy oil fractions. The light fractions increase volatility, decrease flash-point, increase the difference between closed cup and open cup flash-point and very often cause a 132
deterioration in thermooxidative stability. On the other hand, they may improve VZ, which does not mean, necessarily, that they would flatten the viscosity-temperature curve. The heavy fractions in the cut aggravate darkening, decrease VZ and increase coking, but can improve thermooxidative stability. Wide cuts are harder to process than narrow cuts. During the refining process, the light portions tend to be overrefined and the heavier portions under-refined. This is particularly true for refining with selective solvents (so-called light-heavy selectivity). A similar irregularity occurs with deep hydrogenation. In de-waxing processes, wide cuts impair wax crystallisation, which again hampers the achievement of low pour-points and good yields of both oil and wax. Oils of all viscosity ranges, corresponding, for example to the I S 0 international viscosity classification (Table 3.1) can be prepared from refined and de-waxed narrow cuts. Oils of higher quality can be made by blending close-neighbouring narrow fractions than wide fractions.
Table 3.1. IS0 Viscosity Classification of Industrial Oils I S 0 Viscosity Grade
VG2 VG3 VG5 VG7 VGlO VG15 VG22 VG32 VG46 VG68 VGIM) VG150 VG220 VG320 VG460 VG680 VGlOOO VG 1500
Mean Viscosity (mm2.s-*)at 40 O C 2.2 3.2 4.6 6.8 10 15 22 32 46 68 100 150 220 320 460 680 1000 1500
LimitingValues of Kinematic Viscosity (mm2.s-') at 40 O C 1.98 - 2.42 2.88 - 3.52 4.14 - 5.06 6.12 - 7.48 9 - 11 13.5 - 16.5 19.8 - 24.2 28.8 - 35.2 41.4 - 50.6 61.2 - 14.8 90- 110 135 - 165 198 - 242 228 - 352 414 - 506 612 - 748 900- 1100 1350 - 1650
Deasphalting of Vacuum Distillation Residues Vacuum distillation residues - the asphalts - contain three main constituents: highly viscous hydrocarbon oils, oil resins and asphaltenes. The oils can be separated from the asphaltenes and partially from the oil resins by distillation in high vacuum. A more suitable method is, however, deasphalting with propane. The hydrocarbon oils and the low-polarity resins dissolve in a large excess of propane. The asphaltenes 133
and higher polarity resins remain undissolved as a heavy phase. Selectivity can be controlled by adjusting the propane: asphalt ratio (as high as 1O:l) and the temperature (up to 80 "C). n-Butane, which is less selective, can also be used. After the propane has evaporated from each of the two phases, the heaviest oils (propane deasphaltisates or brightstocks) and propane asphalt are isolated. The process amounts to a selective extraction by a non-polar solvent. Industrial exploitation has started of extractive distillation of residues by gases in a critical state. It makes use of the earlier observation that some fluids possess, in their super-critical states, the transport and diffusing properties of gases together with the solvent properties of liquids. This is true of non-polar fluids, e.g., methane to butane, as well as polar ones, such as toluene, carbon dioxide and ammonia (252, 255,259). Carbon dioxide dissolves, at 7.3 MPa and 31 "C the non-asphaltic parts of distillation residues and liberates them again when the pressure is lowered and the temperature is raised. This procedure (sometimes termed destraction) is also suitable for extracting the non-asphaltic components from tars and from coal hydrogenation residues, for separating oil from used oils or from heavy crude oil deposits or asphalt sands (199). 3.2.1.2.2 Refining Processes
Refining substantially changes the chemical composition of lubricating oil distillates and can thus control the properties of raffinates (10, 250). Its primary effect is to reduce the concentration of the most polar constituents - oxygen, nitrogen and sulphur compounds, poly-nuclear aromatics and - to the extent that they are present - olefinic hydrocarbons, the products of local over-heating. The object of refining processes is to improve colour, VZ, long-term stability against ageing in storage and at operating temperatures, the reduction of coking, etc. Unless refining is accompanied by profound chemical transformations (e.g., as in hydrogenation), distillation ranges and flash-point do not change significantly. Pour-point may rise somewhat if the paraffin wax crystallisation inhibitors (some resins) are removed, or if transformation into n-alkanes or long-chain alkyl substituents occurs. However, it may decrease in the event of isomerisation or cleavage of n-alkanes or n-alkyls takes place. In contrast to distillation, both density and viscosity decrease. Amenability to treatment with additives improves. Colour can be an indication of the depth of refining, but - as the people of experience say - colour does not lubricate! GOST 2667-52 specifies a colorimetric method for oil colour determination; ASTM D-1500-64, IP 196/66 and DIN 51 578 specify visual methods. The colour of the oil in a test-tube of prescribed dimensions is compared with a scale of coloured glass strips. The standards are numbered and the test result is defined as the ASTM colour and the number of the matching glass strip. Accuracy is 0.5 unit up to 8. Oils of colour exceeding 8 are diluted with kerosene in a specified ratio. The IP 17 method, which employs a Lovibond Comparator (originally used principally for measuring the d o u r of beer), is less frequently used: the colour of the oil is compared with the colour formed by combination of red, yellow and blue standard glass strips with numerical graduations, e.g., 2 red, 3 yellow, 1 blue,
134
etc. The ASTM D-156method is also suitable for determining the colour of “white” oils :the oil sample is compared with coloured glass standards numbered +30 to -16 . The lighter the colour, the higher the number. Another criterion is the amount of residue produced on “carbonisation”. This type of test consists in determining the percentage of solid formed by the thermal decomposition of the oil in the absence of air under specified conditions. Two methods are used in practice: the Conradson Carbon Test (CCT), used more in Europe, and the Ramsbottom Test, used in the USA. The CCT method is described in ASTM D-189 and the identical CSN 65 6210,GOST 5987-51and DIN 51 551. The carbonisation residue is determined by the thermal decomposition of a 10 g oil sample in a porcelain crucible under prescribed heating conditions. The Ramsbottom Test is specified in ASTM D-524and IP 14.The carbonisation residue is determined by heating, vaporising and decomposing the oil sample in a furnace at 550 ”C.The CSN 65 621 1 carbonisation test is identical to the CCT, except that a 2 g sample is used in a smaller porcelain crucible. The assessment of other quality criteria (VI, oxidation stability, etc.) which relate to refining is described elsewhere in this book.
The main types of refining processes now employed are refining with chemical agents, selective solvents, adsorbents and hydrogen (hydrogenation).
Refining with Chemical Agents (I&) This is the earliest refining process. The main agents used are sulphuric acid and sometimes oleum or sulphur trioxide, or, more rarely, other agents such as anhydrous aluminium trichloride (104.Refining with acid agents is followed by neutralisation, usually with calcium hydroxide or caustic alkali. Sulphuric acid causes polymerisation, condensation and, at elevated temperatures and concentrations, sulphonation and oxidation. It also partly acts as a selective solvent. Saturated and mono-nuclear aromatic hydrocarbons are comparatively little attacked. The unsaturated components, e.g., thermally-changed lubricating oils, produce acid esters and polymers, which pass into the acid layer. Poly-nuclear aromatics tend to undergo condensations and sulphonations; the products of these transformations also pass into the acid layer. The same applies to oil resins and nitrogenous compounds, oxygen and sulphur compounds. They partly dissolve in the acid, are partially converted into ionic species, condense or are sulphonated. The non-aromatic naphthenic acids dissolve in the acid, whilst the aromatic ones can, additionally, be sulphonated. Nitrogenous bases are neutralised and pass into the acid. However, the selectivity of these processes is rather poor. The process produces two layers: a less polar raffinate and a layer of acid sludge (acid resins). The layers are separated by settling, centrifugation or electrostatic processes. The raffinate is extracted with caustic alkali and washed with water. After drying, it may be further post-refined with bleaching agents to make the final raffinate. This is a product showing good quality parameters, although the VZ is not much higher than that of the distillate. The acid sludge is a troublesome waste. It is hard to handle, corrosive and undergoes further condensation and oxidation reactions connected with the formation of asphaltenes, carbenes and carboids, releasing irritating sulphur dioxide. Disposal procedures are still not satisfactory, which is one 135
of the reasons why no new plants are being erected. Existing plants, as long as they continue in operation, are used for the production of light bearing, turbine, transformer and similar lubricants from wax-free or de-waxed distillates.
Reclaiming Used Oils Refining with sulphuric acid is also used for the reclaiming used engine and some industrial oils (22). Reclaimed oils may achieve composition and attributes similar to the original oils, however, their refining is somewhat troublesome. Most of the problems stem from the products of physical and chemical ageing and by contaminants and additives present. One of the procedures worth mentioning is the Czechoslovak process which involves the transport of these contaminants into a water-glass solution, whereby the contaminants are adsorbed on the colloidal silicic acid produced by hydrolysis in the water layer (22).Waste oils may also be decarbonised and dewatered with surfactants, e.g., oxyethylated phenol, and after settling, subjected to conventional treatment (23).However, even for the treatment of waste oils, sulphuric acid processing is being replaced by other processes, such as solvent treatment, hydrogenation or combinations of these (257). Modem reclaiming techniques based on combined distillation, deasphalting, solvent refining and, chiefly, hydro-refining processes are capable of producing reclaimed oils which match the quality of virgin raffinates. The quality of a reclaimed oil used in engine lubricants is shown in Table 3.2. Table 3.2. Quality Specification for a Re-refined Used Engine Oil Parameter
ASTM Method
Flash-point Insoluble Substances Sulphated Ash Glycols Sulphur
D-92 D-893 D-874 D-2982 D-2552 D- 129
Phosphorus Chlorine Water Neutralisation No.
-
Aniline Point Viscosity Index Trace Metals
Limits as in virgin raffinate 0.01% by weight max. 0.01% by weight max. absent 0.25% by weight max.
50 p.p.m. max. 100 p.p.m. max. 0.01% max. D-664 0.15 TAN max., strong bases, 0. strong acids, 0. D-611 93 "C min. D-2270 90 min. X-ray fluorescence, 200 p.p.m. max. atomic absorption, or emission spectroscopy
The Manufacture of White Oils Medicinal oils must not contain aromatics. Fuming sulphuric acid and, more recently, sulphur trioxide are used for their manufacture, as well as for the 136
manufacture of technical white oils. After pre-treatment with acid and removal of the acid sludge, or after a pre-treatment with hydrogen, the aromatics and resins are sulphonated by repeated dosages of oleum or SO,. The sulphonic acids formed pass partly to the oil and partly to the acid layers. The oil layer is extracted with an alcoholic solution of caustic alkali. The recovered “mahogany” sulphonates are oilsoluble and’are valuable raw materials for the manufacture of detergent additives for oils and emulsifiers. The “green” sulphonates extractable from the water layer are inferior, but cheap wetting agents. Again, in the manufacture of white oils, hydrogenation is displacing acid-refining if sulphonates are not required (22).
Solvent Retining The main criterion of solvent power is similarity of chemical composition or of molecular mass. Solvent refining consists in the separation of wax-free oil cuts into two layers: one rich in less-polar components, poor in solvent, the raffinate layer, the other rich in polar components and solvent, the extract layer. After flashing off the solvent, the raffinate comprises the high-grade part of the original oil, characterised by high VZ (up to 100, but rarely more), light colour and high stability against ageing, whilst the extract is that part with undesirable attributes (with few exceptions) - dark colour, sticky consistency, high density (even over 1,OOO kg m”), high viscosity, low or even negative VZ, low resistance to ageing and a tendency to form carbonisation residues. Solvents are selected according to their selectivity and solvent power. Selectivity is that property which distinguishes between wanted and unwanted components and provides the means to separate them; solvent power defines the quantity of solvent necessary to transfer the more soluble components into the extract. Both properties change with increasing temperature: selectivity decreases, solvent power increases (with a few exceptions, e.g., propane in the deasphalting process). At the critical temperature, one phase is formed and no separation occurs. Thus, by changing the temperature and the solvent to oil ratio, the desired depth of refining can be achieved. The most common selective solvents are furfural (Ff), suitable for all oil stocks (even gas oils and brightstocks), and phenol (Ph), more used for waxy oils than waxfree, and for more heavy than light cuts. More recently, N-methylpyrrolidone (NMP) has also been used. It has good selectivity (Ff > NMP > Ph), solvent power (NMP > Ff > Ph), stability (NMP > Ph > Ff), biodegradability (NMP > Ph > Ff) and lower toxicity. This all leads to lower investment and operating costs, so that many furfural and especially phenol plants are being converted to NMP (201). Other solvents are known (e.g., wet tricresol, SO,, benzene), but they have only found limited application or are now regarded as out-dated.
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Solvent Precipitation In selective precipitation processes, non-polar solvents precipitate the most polar asphaltenes and oil resins in oil distillates and distillation residues, in oils so deasphalted wax components (as in some de-waxing processes), and polar oxidation products and additives from used oils. The less polar or light constituents are transported into the solvent. Conventional precipitants are alkanes, chiefly liquid propane. The lighter ethane precipitates too much of the oil with the asphalt; nbutane to hexanes, on the other hand, precipitate selectively only the asphaltenes and the most condensed resins of highest softening point. The effects of both polar and non-polar solvents may be combined in one unit operation. Thus, in the Duosol process for distillation residues, asphaltenes and resins are precipitated by propane and the propane simultaneously extracted with a phenol-cresol mixture.
Adsorption Refining Processes Adsorption refining is based on the adsorption of polar substances on the surface of a polar adsorbent, for example, natural or synthetic aluminosilicate, silica gel, alumina or bauxite. These processes nowadays are mainly used for reclaiming used oils after acid or solvent refining to remove refining agent and other impurities and colour. Although it is possible, in adsorptive refining, to separate the desirable from the undesirable oil components better than with solvent refining, and to obtain higher yields of raffinate, it is rarely employed as the main refining process (15). Adsorptive refining with high surface area graphite may become the exception. Here, the n-alkanes (oil waxes) and polynuclear aromatics are primarily adsorbed. Oils of substantially higher VZ and, particularly, low viscosity at low temperatures are recovered (210). Adsorptive refining is classified into contact and percolative processes. In contact processes, the oil is mixed with a minor proportion of the finely divided adsorbent (1 - 3%) at below 100 OC in the cold-contacting process, before separation of the spent adsorbent. This variant is used for cleaning light industrial oils (turbine oils, electroinsulating oils, etc.) as well as reclaiming these oils. The hot contacting process proceeds at temperatures up to 340 OC, up to the flash-point of the oil but never above its decomposition temperature, with the object of achieving more thorough recleaning of engine oils, compressor oils and, generally, heavy oils and brightstocks. Percolation processes consist in filtering the oil at lower temperatures through a stationary bed of relatively coarse-grained adsorbent and recovering the first eluates, which become gradually darker as the percolation proceeds. The later eluates are returned to the oil-stock feed-tank. Adsorptive refining improves oil colour, brightness and oxidation stability, and removes water and contaminants which might be corrosive. A high-quality oil should not contain water and physical contaminants. CSN 65 6231 and GOST 1541-74 specify a qualitative test for water present in oil. The sample is heated in an oil bath to 150 "C;
138
if water is present, the oil sputters and foams. Quantitative distillation methods are specified by CSN 65 6062,GOST 1594-69,ASTM D-95,IP 74 and DIN 51582.The oil is diluted with a solvent, e.g., a petroleum spirit, plus oleic acid to suppress foaming, or xylene, and distilled. The solvent vapour plus entrained water vapour is condensed and accumulated in a gauge-glass. The volume of water is measured after phase separation. Determination of water by potentiometric titration with Karl Fischer reagent is specified in ASTM D-1744and DIN 51-777. Physical contaminants in oils are determined by CSN 65 6219,GOST 6370-59and DIN 51592.Solid particles are filtered off from a solution of the oil in a suitable solvent, such as hot xylene-ethanol mixture or benzene.
Adsorption processes used to be common as the final treatment before oilblending to required quality. Problems of disposal of spent adsorbent have contributed to their replacement by hydrogenation processes.
Hydrogenation (IOe) Unlike acid refining and solvent refining, hydrogenation produces various depths of conversion of undesirable components into lighter, low-viscosity and heavier, high viscosity hydrocarbons. No troublesome wastes are produced. Three types of hydrogenation process may be distinguished: hydrofinishing, hydrofining and high-pressure hydrogenation or mild hydro-cracking. All three types proceed at higher temperatures, from 250 to 420 "C and at pressures from 2 to 20 MPa, both parameters higher and lower in exceptional cases due to the use of catalysts, which are usually transition metals and their compounds (oxides, sulphides) on a more or less acid substrate, in a hydrogen environment and with the consumption of hydrogen. In the hydrofinishing processes, the last traces of impurities remaining in the oil after acid or solvent refining are removed. In hydrofining, some deeper transformations occur, including partial saturation of aromatic nuclei and partial hydrogenation of 0, S and N-containing components. In the HP-hydrogenation, at the upper temperature and pressure limits - virtually a mild hydrocracking process - deep transformations occur, almost complete hydrogenation and hydrogenolysis reactions, isomerisations and cracking reactions.
Hydrofinishing and Hydrofining vpical operating conditions for these processes are temperatures of 250 to 370 "C, pressures from 2 to 7 MPa, liquid-hourly space velocities (LHSV's) 0.5 to 3 h-l, hydrogen-to-feed ratios 300 to 800 vol. H2/vol. feed , and hydrogen consumption 5 to 80 vol./vol. of feed, the lower values for hydrofinishing. Typical catalysts are cobalt molybdates or more active nickel and nickel-cobalt molybdates on a slightly acidic alumina (e.g., 10 to 15% MOO,, 3 to 5% COO,4 to 5% NiO) for hydrofining and less active catalysts, e.g., for hydrofinishing. Yields of raffinate are up to 98% in hydrofinishing and less in hydrofining. The light by-products ,areflashed off within the unit.
139
In hydrofinishing, no substantial chemical transformations take place. In hydrofining, substantial reduction of oxygen compounds and of 20 to 70% of sulphur compounds (in terms of sulphur content) occurs - hence the synonymousdesignation hydrodesulphurisation. Nitrogen compounds are troublesome, heavy compounds being converted into light compounds (17) which contribute to accelerated deactivation of acid centres in the catalysts. Resin concentration is reduced, as is carbonisation residue (up to 50%), colour is improved, viscosity slightly reduced and VZ slightly increased. Stability to ageing and additive response are increased. In this respect, medium pressure hydrogenation is a mild refining process which normally follows, but may precede, solvent refining of oil distillates.
High-PressureHydrogenation - Hydrocracking of Oils The importance of HP-hydrogenation as the main treatment of lube oil distillates or deasphalted distillation residues is increasing. It enables oils to be obtained with a high VI or even extremely high VZ (XHVI - e.g., up to 150 from hydro-cracked slack waxes). This process was probably first introduced under field conditions in Czechoslovakia in 1961; about 40% of lube oils are now products of HPhydrogenation. Typical operating conditions are temperatures 370 - 420 OC, pressures 10 - 30 MPa, LHSV 0.5 - 1.5 h-', hydrogen-to-feed ratio 900 - 2,000 vol./vol. and consumption of hydrogen 80 - 250 vol./vol. Even at the same VZ, the composition of hydrocracked oils differs significantly from that of solvent raffinates, as shown by Table 3.3 (21, 200). Catalysts used are more active than those in hydrofining, being mostly sulphides of transition metals, like MoS2, WS,, MoS2.NiS, and, in the case of pre-hydrogenated stocks in a second stage of hydrogenation, including nickel and noble metals, Ni, Pd, Pt, all on y or q-alumina or natural or synthetic aluminosilicates, both amorphous and crystalline, e.g., molecular sieves ( H(A1.SiO4),OH] (22,23).Pore dimensions and the acidity of the catalysts are carefully balanced so as to impede rapid deactivation and to guide the transformation in the desired direction. The metallic component enhances hydrogenation, leading to cycloalkanes and alkylated and cycloalkylated benzenes from 0, S and N constituents, resins, polynuclear aromatics and to a certain extent polycycloalkanes (261). The noble metals enhance the hydrogenolytic opening of cycloalkane rings to branched-chain alkyls. The acidic component facilitates cleavage, producing hydrocarbons with lower viscosities and the isomerisation of cyclohexanes to cyclopentanes and to branched, saturated hydrocarbons with lower pour-points. It also enhances the hydrocracking of substances with electron-donor properties (containing 0, S , N and double bonds) because of their stronger adsorption on to electron-acceptor centres. However, this increases the risk of catalyst deactivation by carbonaceous products (so that pore dimensions must be optimised (22)). Hydrogenation is thermodynamically a low temperature process, but its kinetics require higher temperatures. This contradiction must be compensated by higher pressures. Active catalysts permit temperatures and pressures to be decreased. Under 140
Table 3.3. Comparison of Duo-Sol Solvent Extraction and Hydrocracking of Romashkino-Mukhanovo Crude (21) Duo-Sol Solvent Extraction Viscosity (mm2.s-') at 50 "C VI Pour-point ( "C)
Hydrocracking
31.31 90 -12
35.13 91 -10
20.2
21.1
5.6 11.3 28.5 45.4
12.7 16.4 31.6 60.7
8.2 5.6 3.6 2.2 4.2
5.5 4.1 3.2 0.8 2.2
2.9 26.7
2.0 17.8
Benzothiophenes Dibenzothiophenes Total thiophenes
3.8 2.7 6.5
0.0
Resins
1.2
0.4
Chemical Composition by MassSpectrometry, % weight Alkanes
Monocycoalkanes Dicycloalkanes Higher Alkanes Total Cycloalkanes Alkylbenzenes Indanes + Tetralins Dic yclanobenzenes Naphthalenes + Tricyclanobenzenes Acenaphthenes + Biphenyls Fluorenes + Acenaphthalenes * Dicyclanonaphthalenes Total Aromatics
0.0 0.0
* Relative data.
otherwise identical conditions, an increase in temperature increases hydrogen consumption and affects the composition of the hydrogenate (22). Reducing the pressure, under otherwise identical conditions, cause hydrogen consumption to decrease, prolongs the operating cycle, but adversely affects the hydrogenation of aromatic hydrocarbons and heteroatomic components, so that the residual concentration of heavy aromatics and resins in the product is higher. So to achieve products of the same VZ with less active catalysts, the temperature must be raised. Hydrogenation changes the composition of all homogeneous oil groups (24) see Table 3.4 and 3.5.The viscosities, densities, sulphur content (%S) and aromatic content (% C A , RA) are all reduced, whilst VZ, hydrogen content, concentration of cycloalkanic compounds and rings (% C N , RN) are increased. The true (paraffinic) pour-point of oils are increased, whilst the false (viscosity) pour-points are decreased. This suggests that it may be advantageous to subject to HP-hydrogenation those stocks which are roughly unified as a group in terms of their constituents, and use processes which simplify the composition of the feed, e.g., hydrofining, or to proceed in more than one stage. 141
Table 3.4. Changes in the Composition and Properties of an Oil and its Chromatographic Constituents - before and after Hydrocracking (24) oil
original
before
after
Yield (% vol.) Density at 20 "C (kg.m-3) Viscosity at 50 "C (mm2.s-')
100 919 53.4
100 879 24.3
VI %H %S
58 12.4 1.18 -18
91 13.0 0.063 4.5
20.3 20.7 59.0 2.58 1.08 1.58
5.1 36.4 58.5 2.37 0.21 2.16
Pour-point ( "C) Composition by Type (ndhQ* %CA %CN %CP
Rc RA RN
* Diammatics and Polyammatics according to Hazelwood
a lkanocyc1anes before after
38.8 856 22.8
71.9 856 20.0
107 121 14.1 14.0 0.0015 0.033 -9 6.2
0 29.4 70.6 1.86 0 1.80
0 30.7 69.3 1.82 0 1.82
monoaromatics before after 26.8 899 31.7 68 12.63 0.67 -17 15.0 22.6 62.4 2.39 0.80 1.59
16.2 898 22.4 68 12.93 0.063 17.5 155.1 25.2 59.7 2.52 0.77 1.75
diaromatics before after
polyaromatics before after
17.7 995 357.5
6.45 987 71.0
9.7 1049 736
50 11.2 0.472 -13.5
-415 9.81 3.13 12
-122 10.8 2.89 8
40.9 18.1 41.0 4.04 2.68 1.36
33.3 29.4 37.3 3.53 1.68 1.85
44.8 34.5 30.7 5.20 2.58 2.62
0
-
Table 3.5. Changes in Viiosity, Viscosity Index and Chemical Composition at Increased Hydrogenation Severity (21) Hydrocracked Oils from Vacuum Distillate Viscosity Index
91.2
98
Viscosity (mm2.s.') at 50 oc Chemical Composition by Mass-Spectrometry (% weight)* Alkanes
35.13
28.15
21.1
22.4
Monocycloalkanes Dicycloalkanes Higher cyclanes
12.7 16.4 31.6
Cyclanes Alkylbenzenes Indanes + tetralins Dicyclanobenzenes Naphthalenes + tricyclanobenzenes Acenaphthenes + diphenyls Fluorenes + acenaphthalenes + dicyclanonaphthalenes Aromatics Resins Relative data.
115
Hydrocracked Oils from Propane Deasphaltate 123.5
83.5
121
127
22.94
19.9
88.27
35.92
29.63
21.6 19.1 16.0
28.7 26.7 19.6 12.3
45.1 19.6 10.2 6.0
15.1 17.4 9.5 23.1
37.5 21.7 18.0 7.6
32.8 29.0 18.4 8.3
60.7
56.7
58.6
35.8
50.0
47.3
55.7
5.5 4.1 3.2 0.8 2.2
9.0 4.7 3.3 1.o 0.8
6.4 2.5 1.4 0.4 0.7
7.0 5.4 2.3 0.8 1.5
17.2 7.5 5.1 1.7 1.2
9.3 2.5 1.2 0.4 0.6
4.9 3.8 0.9 0.4 0.6
2.0 17.8
1.8 20.6
0.8 12.2
1.5 18.5
1.8 34.5
0.7 14.7
0.1 10.7
0.4
0.3
0.5
0.6
0.4
0.5
0.8
HP-hydrogenation (i.e., mild hydrocracking) enables high-grade and high- VZ oils to be produced economically. Oils of this quality cannot be obtained by solvent refining, and even if they could, yields would be very low. The superiority of HPhydrogenation consists in the possibility of obtaining oils with Vl's up to 130 (exceptionally even higher) from almost any type of oil in high yields, in better utilisation of by-products (high-quality motor fuels and light lube oil cuts), good response to additives, lower volatility at the same viscosity and lower biological activity. Less convenient characteristics are poorer stability to light, higher corrosivity after oxidation and lower solvent power for polar substances, e.g., for oxidation products and additives. The overall concentration and type of aromatic compounds, together with the concentration of sulphur and nitrogen compounds, are decisive factors in the properties in service of HP-hydrogenates, particularly in respect of thermooxidative stability. This stability grows up to a certain critical concentration of aromatics, then when this is exceeded it becomes much worse (169, 202). The relative molecular mass of the aromatics, the number of nuclei present and the concentration of sulphur compounds are also very important. The heavier the aromatics and the greater the concentration of sulphur compounds, the lower is h e critical concentration (170, 171). Hydrogenates containing less than 5% of aromatics have a rather poor oxidation stability. In comparison with selective raffinates, hydrogenates are especially sensitive above 210 - 220 "C.This manifests itself by a tendency towards more serious carbon deposition in the first piston grooves with engine oils formulated from hydrogenates. This inconvenience may be overcome by optimising the structure and content of additives (anti-oxidants, detergents and dispersants) or using doped blends of selected raffinates and HP-hydrogenates. Oils made from HP-hydrogenates usually have inferior anti-wear properties. This is due to the absence of polar components. Suitable additives enable this disadvantage to be overcome (172). A further drawback associated with HP-hydrogenates is low solvent power for additives, particularly for some polymers, and for the polar products of ageing. This also is due to the low concentration of aromatic components, which is also responsible for a high aniline point of HP-hydrogenates and for shrinkage of rubber seals and packings (aromatic content which is too high causes swelling). Pale-coloured sludges are formed in hydrocracked oils by the action of light. The constituents which are responsible for this effect have not yet been identified, but compounds with a reactive tertiary carbon, traces of olefins originating from dehydrogenation, or some hydroaromatic or polyaromatic compound are suspected. Light stability can be improved by removing or converting these components, e.g., by low-temperaturehydrogen after-treating in a fine-finishing converter, by catalytic conversion on molecular sieves in the absence or presence of added alkenes (220) (which also improves resistance to ageing), by further reducing the concentration of heavy aromatics, e.g., by clay treatment, and by using suitable anti-oxidants, because the natural anti-oxidants (some resins, some poly-nuclear aromatics) have to a large extent been removed. 144
The preferred feedstocks for HP-hydrogenation include heavy oil distillates, deasphalted vacuum distillation residues and, more recently, distillation residues from high-severity cracking to make motor fuels. Slack waxes may even be hydrocracked to XHVI oils. Specific processes have been developed (22). Some have been commissioned or are being promoted in the USA (Chevron - 261),France and Germany (the IFP process - 262), the Soviet Union (263),East Germany (264) and in Czechoslovakia (the MVOL process - 265). Yields up to 80% of viscous products may be attained. Relatively low viscosities limit their applications, although viscosities up to about 12 mm-2s-1 at 100 "C can be achieved by hydrogenating brightstocks. So a wide variety covering almost 98% of the lube oil range can be produced, with the exception of heavy compressor and cylinder oils :light distillates of lower VZ and low pour-point for oils of IS0 VG 2 to 15 grades, medium distillates or hydrogenates for bearing, turbine, hydraulic, transformer, white and similar oils and heavier hydrogenates emanating from hydrogenated brightstocks or hydrogenates of higher viscosities from oil distillates as components of engine oils improved with suitable additives or with predominantly di-nuclear aromatic oil components or simply with heavier solvent raffinates. These hydrogenates must be freed from light components, which affect the thermooxidation stability of the product. As by-products, saturated and virtually sulphur-free motor fuels and refined waxes with improved filtrability are obtained. HP-hydrogenates are serious competitors of synthetic polyalphaolefin (PAO) oils, with the advantage of lower price.
De-Waxing Processes Solvent dewaxing is the dominant process in the recovery of oils of low pourpoint and good rheological properties at low temperatures from waxy distillates and raffinates. The oil is diluted with the solvent, chilled to low temperature and the separated wax crystals filtered off (107). Suitable solvents include propane, particularly for residual oils such as propane deasphaltates, ethane, particularly for producing oils of very low pour-point, or socalled solvent/anti-solvents mixtures. The anti-solvent precipitates the wax and promotes the formation of wax crystals which are easily filtered and washed; the solvent dissolves the oil which is occluded in the filter cake. Examples of antisolvent/solvent pairs are methylethyl ketone (2-butanone)-toluene and 1,2-dichlorethane-dichlormethane.Precipitating and dissolving effects are combined in methylisobutyl ketone (4-methylpentan-2-one). Light oils may also be de-waxed by processes based on other principles (108): adsorption of n-alkanes by molecular sieves, formation of urea adducts with alkanes and degradation of paraffins by fermentation. These processes are of limited importance. Oils of extremely low pour-points and low viscosities at low temperatures (for bearing, hydraulic, transformer and other oils) are still mostly manufactured by refining wax-free cycloalkanic oil cuts. Oils with similar properties may also be 145
obtained by de-waxing waxy cuts at very low temperature. Since the latter method is expensive, and wax-free oils are scarce, processes based on combinations of conventional de-waxing and the further decrease of oil pour-points and viscosities at low temperatures by adding pour-point depressants have been developed. The advantage of such a procedure is that oils produced in this way have lower viscosities at low temperatures than oils made from cycloakanic crudes and that they retain relatively high Vl's and the required pour-points. These low pour-point oils, however, have limited applications. They cannot be used in service where the presence of precipitated wax would be a nuisance, e.g., for the lubrication of refrigerating compressors, particularly those of the Freon type. Catalytic de-waxing processes have been developed recently (254,265).Low oil pour-points are achieved by isomerisation and hydrocracking of constituents with long alkyl-chains at about 360 "C and pressures up to 10 MPa, in the presence of hydrogen and hydrogenation catalysts (Pt, Pd, Ni, etc.) on acidic molecular sieves with pores which allow the adsorption of straight-chain hydrocarbons, e.g., mordenites in the H-form. Light oil-cuts, e.g., from HP-hydrogenation, are suitable feed-stocks, and the product - obtained in 70-80% yields - has pour-points down to -40 "C and lower. The by-products are gases and light liquid hydrocarbons.
Lubricant Base Stocks The processes described earlier are used for the production of base oils for several different types of lubricating oils. These oils are nowadays regarded as being sorted into conventional types, and essentially classified under three group headings, each group being capable of further improvement with additives: A. High viscosity index raffinates, designed mainly for high quality engine oils and some special types of industrial oils (Table 3.6). B. Medium viscosity index raffinates, used either alone or in blends with the above oils for high quality industrial oils and greases (Table 3.7). C. Low viscosity index raffinates, suitable for some types of industrial oils, where the viscosity-temperature characteristics need not be considered and where thermooxidative stability is not decisive (Table 3.8).
3.2.2 Synthetic Oils Although modem types of mineral oils reinforced by additives are capable of meeting a broad range of lubricating and other requirements, their properties are in some cases significantly surpassed by those of synthetic oils, for example, where very low or very high temperatures prevail, or under conditions where a wide operating temperature range or an unconventional environment, e.g., self-ignition hazard or exposure to ionising radiation, impose unusual requirements (30,31,32). Notwithstanding their limited availability and high price, there are fields where synthetic oils find application and are economically viable. Some types of synthetic oils are suitable for use in admixture with mineral oils - the so-called "semi146
Table 3.6. High Viiosity Index Raffinates Solvent Neueals.
Parameter Viscosity at 100oc(mm2.s-') Viscosity Index (min.) Rash-point (P.M., "C,min.) (O.C., "C, min.) Fire-point ( oc. min.) Pow-point ( "C. ma.) CCT (% wt, max.) Colour (ASTM. max.) Colour stabiitytt (48 h.100 "C, max.) Acidity (mg KOWg, max.)
Ash (%wt, ma.) Clouding by separated parafFm
after 3 months at 20-25 "C
Brightstockst
100
150
200
350
4.14.6 100
5.2-5.6 97
6.3-6.7 97
9.2-9.6 95
10.0-10.5 95
12.3-13.5 95
215 240 265 -12
220 245 270 -9 0.1 2.5
230 250 275 -9 0.1 3.0
230 250 275 -9 0.1 3.0
im
200
230
230
245
245 -15 0.03
-17 0.02 1.5
0.05
400
500
300 -9 0.15 3.5
2.0
1.o 0.05
1.0
1.o
1.o
0.05
0.05
0.05
0.01
0.01
0.01
0.01
0.05 0.01
1.o 0.05 0.01
0.01
-
-
-
-
-
-
-
1.o
t Conventional term for raffvlates from deasphaltedpetroleum distillate residuum. tt Deterioration of oil colour at 100 "C aArr 48 hours exposure to lighr
P 4
240 260
1.5
Conventionalterm for fully-refined oils of low acidity; the numbers represent average values of viscosity in SSU at 37.8 OC.
L
600
14.2-15.0 95
1.o 0.05
l00BS 20.0-21.2 95 255 280 320 -9
0.6 6 1.o 0.05 0.01
15OBS 31.5-32.5 95 265 295 320 -9 0.8 6 1.o 0.05 0.01
nohazeproduced
Table 3.7. Selective Ramnates of Medium Viscosity Index Parameter
very light
Viscosity (mm2.s-I) at 40 "C Viscosity Index (min.) Flash-point (P.M., "C, min.) (O.C., "C, min.) Fire-point ( "C, min.) Pour-point ( "C, max.) Colour (ASTM, max.) Colour Stability (48 h,100 "C, max.) CCT (96 wt., max.) Acidity (mg KOWg, max.) Ash (S wt., max.)
light
8-9 50 150 165 180 - 40
Raffinate medium
13- 15 45 150 165 180 - 40 1.o 1.o 0.03 0.05
I .o 1.o
0.02 0.05 0.0 1
0.01
32 - 37 45 185 205 225 - 35 1.5 1.o 0.05 0.05 0.01
heavy 130 - 145 40 220 230 250 20 2.0 1.o 0.05 0.05 0.01
-
Table 3.8. Low Viscosity Index Ramnates Parameter Viscosity (mm2.s-1) at-40 "C at100 "C Viscosity Index Flash-point (P.M., "C, min.) (O.C., "C, min.) Fire-point ( "C, min.) Pour-point ( "C, max.) Colour (ASTM, max.) Colour Stability (48 h,100 "C, max.) Acidity (mg KOWg, max.) Ash (8 wt., max.)
I
-
18 23 0 155 175 195 -35 2
I1
I11
105- 115
-
0
-
185 215 235 -20 3.5
IV
V
16- 18 0
18 - 20 0
33 - 37
205 230 255 -7 6
205 235 265 -7 6
255
0
270 300 0
dark
1.o
1.o
1.o
1.o
-
0.1
0.1 0.01
0.1
0.1
0.01
0.01
0. t 0.05
0.01
synthetic" oils - as well as additives for mineral oils. Synthetic oils now in use are generally chemical compounds of defined composition with defined functional groups, and may be broadly classified into: - alkene polymers, - alkylaromatics, - chlorinated hydrocarbons, - fluorinated hydrocarbons, - polyalkylene glycols and polyalkylene ethers, - alkyl esters of monocarboxylic aliphatic acids, - alkyl esters of dicarboxylic aliphatic acids, - alkyl and aryl esters of oxy-acids of phosphorus, - alkyl and aryl esters of silicic acids,
- polsiloxanes, and - oils of miscellaneous chemical chemical families. The majority of synthetic oils can be improved by additives (234 - 236).
3.2.2.1 Polyalkenes (Polyolefins) The alkene polymers were among the earliest synthetic oils and they are becoming more significant again. They are produced by polymerisation of lower alkenes, C, to C,,, in the presence of Lewis acids (e.g., Al,Cl,, BF, and their complexes) or Ziegler catalysts (e.g., A1(C2H5)&lx and promoters) in the liquid phase, at lower temperatures around 100 OC, both in the presence and the absence of hydrogen (48). At the lower end of the polymerisation temperature range, the reaction is slower and the catalyst must be more active, but the product viscosity is higher. The pressure must be high enough to maintain the charge in the liquid state. Inert solvents, such as n-alkanes or low-viscosity oligomers, are therefore used. As the reaction time is usually long, batch-wise polymerisations are used. The alkenes must have a terminal double bond. If they are long-chain, the chain should be unbranched, or branched only at a site remote from the double bond, in order to produce viscous, high VZ oils. The viscosity index of the product depends on the structure of the monomer; the shorter and more branched the monomer, the lower the VI. Isobutylene is, however, a notable exception to this. The residual double bond is a significant group in these hydrocarbon synthetic oils. Its presence imparts to the oils properties of adhesion to surfaces, improves their lubricating ability, but adversely affects their oxidative stability. Polyalkenes are light in colour, without fluorescence, and contain no aromatics or only traces, but they may contain some cycloalkanes. They are susceptible to deterioration with age, but respond well to antioxidants, e.g., phenyl - 1-naphthylamines or phenothiazines. Hydrogenation improves their stability to ageing. If exposed to temperature stress, polyalkenes do not coke but depolymerise. They bum, when ignited, with minute soot formation. They blend with mineral oils and also tolerate other oils, provided these are of low polarity. Those of highest viscosity do not blend with polar solvents, e.g., ketones and acetates. Polybutene oils are of great interest.They are produced from butene fractions (blends of C, alkenes, e.g., from catalytic or steam cracking) in which isobutene predominates. In the absence of isobutene, oils similar to polypropylenes are produced, with a slightly higher VZ (249). In the presence of isobutene, oils of viscosity ranging from 4 rnm2.s-l at 40 "C to over 4,000 mm2.s-l,VZ 70 to 120 and more and pour-point -50 OC to 0 "C result, depending on the isobutene content and the polymerisation temperature. Oils of the highest viscosity vitrify. According to the polymerisation used, they have the attributes of viscous oils, oil thickeners, adhesives and elastomers. The oil products have excellent electrical properties, high electrical resistance and low loss factor. Their adhesive power - which increases with molecular size - is important in some applications. Polybutenes can be blended, at normal temperatures, with hydrocarbon solvents, chlorinated hydrocarbons, ethers 149
and esters; they do not dissolve in polar solvents (water, alcohols, ketones and acids). They are compatible with most of the hydrocarbon polymers and other natural and synthetic polymers and condensation products, like rubber, resins, asphalts, etc. They can be emulsified with all types of emulsifiers. vpical properties of industrial polybutenes of various molecular sizes are shown in Table 3.9. Polybutenes are used as components of lubricating oils, where they contribute to the improvement of viscosity-temperature characteristics, shear stability, anti-wear properties and the reduction of carbonisation residue. They are used in a pure state for lubricating high pressure ethylene compressors and the last stage of air compressors, where they reduce explosion hazard, for bearings in furnaces and chain conveyors, in aluminium cold-rolling and steel hot rolling, extrusion and drawing processes, where they leave no residue or staining and need not be removed prior to annealing. They can be graphitised for applications at higher temperatures. They are used as components for oils for gas-filled cables, as liquid dielectrics and impregnating oils for electro-insulatingpapers. They find further application in oils for two-stroke gasoline engines, special oils (e.g., textile oils), as additives for increasing adhesive properties and for the reduction of spatter. Another very wide field of application is the manufacture of special sealant, adhesive and filler materials. The high-molecular weight polybutenes are thickeners and VZ improvers, the highest synthetic rubbers. One propeaty of polybutenes which can be of particular value in fuel additive compositions is their ability to crack under combustion zone conditions into low molecular weight hydrocarbons. As a constituentof gasoline additive compositionsin fuel-injectedengines, evaporation of the gasolinecreates a concentrated film of detergent additives in polybutene. This is claimed to act so as to lubricate the valve-stem and to retain deposits in suspension, conveying them into the combustion chamber, where the light hydrocarbons produced by cracking of the polybutene augment the combustion qualities of the gasoline.
Polyalphaolefins Polyalphaolefin oils (PAO) are manufactured in the presence of Lewis acids, e.g., BF3 or A12C16, by oligomerisation of l-alkenes in the range C, - C,,, followed by hydrogenation of the double bond. They are almost pure isoalkanes, with 24 or more carbon atoms in long alkyl chains (216 -219),e.g., CH,-[CH2],-CH, I
CH3-[CH&-CH(CH3)-CH2-CH \
('3#62)
These PA0 oils are characterised by high thermal stability, low viscosities even at low temperatures, low pour-points (down to -75 "C), high VZ (up to 195), high flash-points and low volatility. They are hydrolytically stable, non-corrosive and non-toxic, compatible with mineral oils and responsive to additives. Because they 150
Table 3.9. Properties of Commercial Polybutene Oils of Varying Molecular Size Oil Mean Relative Molecular Mass 'Molecular Mass Distribution Density at 15 "C (l~g.rn-~) Viscosity (mm2.s-1) at 40 "C at 100°C . Viscosity Index Flash-point (O.C., "C, min.) Fire-point ( "C. min.) Pour-point ( "C. min.) Volatility (%weightloss a i k 10 h at 98.9 "C)
A
B
C
D
E
F
G
H
I
J
K
225
320 1503000 837
420
460
610
660
750
920 20015oooO 889
1290 30025000 898
2060 300-
2300
-
810 3.9-6.2
18-24
-
-
-
69 132 154 -50
102 108
8.8
-
850
-
857
106- 112 210-227
-
-
90 150 163
95 155 166 -35
-40
6.1
5.6
-
863
-
872
-
881
40000 904
-
906
-
48-59 97 163 171 -27 5.1
74- 79 109- 125 196- 233 627-675 2903-3231 3871-4161 100 104 109 117 122 122 166 166 193 227 243 243 307 307 185 193 233 273 -15 -15 -7 2 . 18 18 3.0
1.5
1.45
1.0
0.32
0.25
are saturated, their aniline points are high and therefore enhance shrinkage of sealant materials (216 - 219,231). A comparison between the properties of oligomers of 1-decene and 1-octene is given in Table 3.10, and the properties of 1-decene oligomers are described in Table 3.11. Table 3.10. Comparison of the Properties of Liquid Oligomers of 1-Decene and 1-Octene Oligomers of Viscosity (mm*.s-') at 40 "C at 100 "C Viscosity Index Pour-point ( "C) Flash-point ( "C) Volatility (Noack test,% wt.)
Dimer
1-Decene
Trimer Tetramer Dimer
1-0ctene Trimer Tetramer Pentamer
4.95 1.68
14.72 30.79 2.80 3.49 5.66 1.19 125 116 -69 -57 <70 216 242 120 14 3.5 95
9.95 2.84 97 -66 168 38
-
-72 160 80
22.74 4.42 103 -57 208 9
31.04 5.58 119 -51
250 7
Table 3.11. 'Mica1 Properties of 1-Decene Oligomers Viscosity (mm2.s-1) at 100 "C at 40 "C at -40 "C(mPa.s) Pour-point ( "C) Flash-point (O.C., "C) Fire-point ( "C) Volatility (Noack test,% wt.) Density (kg.mV3) Bromine Index
1.75 -
-68 160 -
50
4.0 17.7 2690 -66 215 248 15 820 50
6.0 10.0 8.0 32.1 48.0 64.5 21300 42000 9000 -50 -57 -60 244 256 260 284 290 270 2.5 2.0 7.5 829 50 50 50
20.0 160 -50
270 300 1.5 50
Polyalphaolefins are used both as fully synthetic oils and as blends with mineral or ester oils. They are recommended especially as automotive gear lubricants, automotive hydraulic and transmission fluids, aviation gas turbine lubricants, high temperature gear and bearing lubricants, ammonia and R 22 refrigeration compressor lubricants, long-life air compressor oils, diesel locomotive and marine engine lubricants for severe service, wide temperature range greases and fully synthetic or semi-synthetic engine oils. In comparison with competing hydrocracked oils, no trace of wax crystals is formed at very low temperatures.
3.2.2.2 Aromatics and Cycloaliphatics Viscous oils of the general formulae: 152
(where R is a long alkyl chain, R’ is a short one) can be obtained by the alkylation of lighter aromatics by heavier alkylating agents or of heavier aromatics with lighter alkylating agents in the presence of Friedel-Crafts catalysts. Light aromatics in this context are benzene and its homologues and heavy aromatics naphthalene, biphenyl, terphenyls, aromatic extracts of kerosene and lighter viscous oils. Light alkylating agents include ethylene, propylene and heavy agents their oligomers and linear I-alkenes with 4 or more carbon atoms and linear alkylchlorides, e.g., dodecy lchloride. Dialkylbenzenes are representative of Type I., and butyl- to hexylnaphthalenes and isopropylterphenyls of Type 11. The longer the alkyl chains, the higher is viscosity, VI and pour-point, and the lower the resistance to ageing and radiation, and solvent power. Branched chains and molecular asymmetry reduce pour-point. Viscosity increases with the number of nuclei in the molecule, whilst VZ reduces, pour-point rises and resistance to heat and radiation improves. Mononuclear aromatics have low solvent power, but a better response to additives than di- and trinuclear aromatics. Dialkylbenzenes with straight C, - C,, alkyl chains have lower viscosities, 3 to 4 mm2.s-l at 100 “C,high VZ (about 100) and pour-points of -50 OC or less. As compared with PAO’s, they have lower Vl’s, higher pour-points, poorer volatilities, but better solvent power and seal compatibilities. They are used as base oils for engine and gear oils, hydraulic fluids and greases for arctic climates. Alkylbenzenes with lower Vl’s and pour-points are used either by themselves or in blends with cycloalkanic mineral oils as compressor oils for refrigeration at very low temperatures. These oils have better miscibility with FCH refrigerants and a lower critical solution temperature. Their poor lubricity is improved by cycloalkanic mineral oils. Alkylnaphthalenes have some potential as heat-resistant fluids, but their Vl’s are very low (-66 for hexyl and +53 for dihexylnaphthalene).During WW 11, they found limited use as substitutes for cycloalkanic mineral oils. Alkylated polyphenyls, e.g., isopropylterphenyls, are used as heat and radiationresistant fluids. Cycloaliphatics At the present time, cycloaliphatics are represented commercially by perhydrogenated dimers of a-methylstyrene, e.g., 153
-
I
CH3 CH CH3 I CH3 CH3
Oils and greases compounded from these synthetic base fluids are increasingly being used as high traction transmission fluids in traction drives (the basic principle of a tractioq drive is the capability to transmit power from one smoothly turning element to another, without intermediate gears, chains, belts, etc.). The power output of a traction drive is directly proportional to the traction coefficient of the lubricant. The superiority of cycloaliphatic-based lubricants in this duty is shown in the following comparisons (all at 49.5 kW, 28 "C, 980 Mpa) : Base Fluid Class
Traction coefficient
Cycloaliphatics Silicones Phosphate esters Polyalphaolefins Polyglycols Diesters Polyols Naphthenic mineral oils
0.095 0.075 0.060 0.050 0.045 0.040 0.035
-
0.058 0.065
Cycloaliphatic-based lubes become shear-resistant semi-solids at the high momentary contact pressure in adjustable speed traction drives. Thus, they provide high torque with increased power transmission capability. After leaving the contact zone, they revert to low viscosity oils (246).
3.2.2.3 Fluorinated and Chlorinated Lubricants The earliest fluorinated lubricants were the polytrifluorochloroethylenes
(PTCFE),[-CF2-CFCl-],, of 500 to about 1,OOO relative molecular weight in the liquid state (Table 3.12). They are characterised by relative chemical inertness to most chemicals and metals, as well as by high stability to oxidation, low flammability, high density, excellent electrical properties and outstandinglubricating ability. Their deficiencies are a comparatively high volatility typical of whollyfluorinated compounds, steep viscosity-temperature curves and high toxicity, particularly from the carbonyl fluoride produced at temperatures exceeding 260 OC. PTFCE's protect metals from corrosion, but react vigorously with aluminium, and attack some organic materials, e.g., amines in rocket fuels. To avoid the risk of explosion, they should be prevented from coming into contact with aluminium components - e.g., compressor pistons and cylinders - in lubrication service. They are good lubricants in the mixed-friction regime. 154
Table 3.12. Properties of Polytrifluorochlorethylenes Relative Molecular Weight
Density (kg.m-3)
Boiling-pint
(mm2.s-*) (mPa.s)
(mm2.s-')
ns
1865 1925
5 100
10 190
2 1s
880
194s
500
970
950
1953 19u1
750
1460
1150 1235
1937.
-
-
45 75 320 620
560
At 71 O C .
(mPas)
Pour-pint
("(3
at71 "C
at 37.8 "C
at 130 Pa (1 Tom)
4
10
30 80
-
124 625 1200
Surface-tension at 25 O C (mN.m-')
-59 -15
23 23
38 71
4.5 10 29.5
23 23
93
35
-
Polytrifluorochloroethylenes are suitable for lubricating air and oxygen compressors, all devices for manufacturing and storing compressed oxygen and air, compressors, vacuum pumps, and all parts of equipment, including hydraulictransmission and other systems and valves, where contact with acid gases, corrosive and reactive chemicals cannot be avoided. A particularly useful application is in the rolling of thin titanium sheet and for lubricating titanium surfaces. Their good lubricity, in combination with these other properties, is highly desirable in gyroscopes and rocketry. They are used as oil components in lubricating greases intended for a temperature range between -40 and +200 O C , in which case the heavier polymers can be used as thickeners. Perjluorinuted hydrocarbons are particularly suitable for militaIy lubricant and hydraulic fluid applications, being non-flammable and difficult to oxidise and able to withstand high temperatures. Their deficiencies are high volatility and pour-point and low lubricity. Fluorinated esters do not suffer these drawbacks to the same extent. The incorporation of the ester group increases boiling-point and decreases pour-point. Esters of fluorinated acids or alcohols are less suitable than esters of nonfluorinated acids and fluorinated alcohols, which are more resistant to hydrolysis, are more stable to oxidation between 250 and 350 O C and do not decompose below about 320 O C . The terminal group in perfluorinated alcohols must be -CH20H rather than -CF20H, as the latter introduces instability and a tendency to split off hydrogen fluoride. As compared with their non-fluorinated analogues, the esters of dibasic acids and fluoro-alcohols have higher viscosity, rather higher volatility, rather poor viscosity-temperature curves and higher pour-points. However, they have better oxidation stability and lubricating power (see Table 3.13). Table 3.13. Comparison of Fluorinated and Unfluorinated Esters Viscosity (mm2.s-'1 at 98.9 OC 1,6-hexandiol-bis-$otanoate* 1,6-hexan-diol dioctanoate bis-(@'-butyl)sebacate* Dibutyl sebacate
2.87 2.62 1.84 2.11
at 37.8 "C 12.0 8.42 6.63 6.1 1
rn Constant Crystallising in the Walther point equation ( "3( 0.92 4 0.70 8 0.86 2 0.7 1 15.5
* Fluorinated derivatives sometimes bear the following symbols (34): 4, fatty acid F(CF,),COOH I#',
fatty alcohol F(CF,)CH,OH
y. fatty acid H(CF,),COOH
W: fatty alcohol H(CF,),CH,OH
They have found application in heavy-duty gas turbines operating with an oilsump temperature of 200 O C , in submarines as hydraulic fluids and lubricants (being heavier than water, they leave no oil slick at the surface), in the lubrication of electric motors and generators containing silicone insulation and operating at elevated temperatures, as high dielectric constant fluids for capacitors and as useful components of lubricant greases designed for high temperature operation. In the process of switching from conventional lubricants to fluoroesters, the lubricating and hydraulic systems must be rinsed thoroughly to get rid of the old 156
lubricants and their oxidation products. Fluoroesters are compatible with aliphatic esters and diesters, phosphates and polyglycols, but not with hydrocarbon lubricants, polyphenylethers, silicones and silicates. Perfluoropolyalkylethers (PFAE) of the general formula CF 1 [- CF - CFZ- 0 -In (n = 2 - 35) are fluids with a low chemical affinity, very good viscosity-temperature characteristics and, low pour-points, outstanding dielectric properties and good radiation stability. Their viscosity increases with growing size of molecules from 10 to 1,OOO rnm2.s-l at 20 "C, density from 1,850 to 1,920 kg.m", VI from 50 to 140. Pour-points are from -20 to -95 "C and surface tensions are low, 18 to 2 1 mN.m-'. They are non-flammable and non-toxic. Their radiation and thermal stabilities in contact with air are good, up to 310 "C in the absence of metals and up to 260 "C in their presence, particularly iron and titanium alloys. Their oxidation stability can be increased up to 340 "C by suitable additives, such as arylamines, benzimidazoles, selenium compounds and triarylphosphines (C,H,),P and their oxides. PFAE's are insoluble in most solvents except fluorinated solvents, and are resistant to hydrolysis, motor and rocket fuels, strong acids and bases, halogens, liquid oxygen and oxidants, but not to A12Cl,, BF3 and C0F2. They tolerate conventional elastomers and plastics, but attack fluorinated sealants. In machining processes, they attack aluminium, magnesium and their alloys. The vapours so produced contain hexafluorpropylene (one of the starting monomers), acetyl fluoride and carbonyl fluoride, and are toxic. PFAE's are very good lubricants in the elastohydrodynamic and boundary regimes, suitable for corrosive environments and severe conditions, such as in plants for producing and handling oxygen and chemicals, in nuclear plants and as hydraulic fluids, lubricants and dielectrics in vacuum and electrical engineering devices and in spacecraft (266 - 268). Chlorinatedfluidswere developed for applicationsin which limited flammability is required. The most commonly used were the polychlorinated biphenyls (PCB). Their non-flammability increases with increasing C1 content, but at the same time their toxicity and resistance to degradation in the environment increases also. Their use was strictly limited to closed systems, and recently their production was stopped because of problems with waste disposal.
3.2.2.4 Polyalkylene Glycols and Polyalkyl Ethers Polyalkylene glycols and their ethers, manufactured from ethylene oxide and propylene oxide, have the general formula:
R I R O - [ - C% - CH - 0 -Ix-
R
where R is H or methyl, R and R" are H, alkyl or acyl.
157
and have every third carbon in their main chain replaced by oxygen, and, instead of terminal methyls, a free or substituted hydroxyl. The oxygen confers on the products affinity to most surfaces, in other words, excellent lubricating power and hydrophylic behaviour. Products of this type in which R is hydrogen or methyl and R' and R' hydrogen, alkyl or acyl, are listed below: I :polyalkylene glycols (R = R' = R" = H) 11:polypropylene glycols (R =CH,, R = R '= H) 111:polyethylene propylene glycols (copolymers I and 11) IV :polyalkylene glycol diethers (R = H or CH,, R' = R' = alkyl) V :polyalkylene glycol monoethers (R = H or CH,, R' = H, R' = alkyl) VI :polyalkylene glycol diesters (R = H or CH,, R = R' = acyl) VII :polyalkylene glycol monoesters (R = H or CH,, R' = H, R = acyl) VIII :polyalkylene glycol etheresters (R = H or CH,, R' = alkyl, R' = acyl) Polyethylene glycols (Iare ) liquids or, if x is large, water-soluble waxes. Polyglycols (11) are liquids or waxes, insoluble in water from MW 900 upwards, but hydrophylic. The copolymers (III) are soluble in water if the ratio of ethylene to propylene is 3: 1 ( the solution is clear up to 80 "C and turbid at higher temperatures; the turbidity disappears after chilling) and insoluble in hydrocarbons; at 1:1 they are still waterand alcohol-soluble but insoluble in hydrocarbons and at 0: 1 they are insoluble in water and with limited solubility in hydrocarbons. Polyglycols IV, VI and VIII are water-insoluble oils. Polyglycols V and VII are surfactant liquids or waxes with lubricant or anti-static properties. Similar colourless oily or waxy derivatives, poorly soluble in water, are produced by polymerisation of tetrahydrofurane on Lewis acid catalysts (BF,, A12C1,, etc.): HO-[CH2-CH2-CH2-CH2-0],-H
Ethylene oxide may be used as a co-monomer. Polytetrahydrofuranes were used in the Second World War for lubricating torpedoes because, having densities exceeding 1,000 kg.m"? they left no oil traces on the water surface. Copolymers with ethylene oxide are oils thermally stable up to 240 "C. The properties of these products vary with the ratio of tetrahydrofurane to ethylene oxide. For example, at 2:1, the viscosity of the product is about 100 mm2.s-l at 100 "C, VZ is about 140, density 1.024 g.cm", flash-point above 260 "C, pour-point below -25 "C and carbonisation number (CCT) 0.2%. The toxicity of tetrahydrofurane polymers is very low: the LD,, value is more than 10 g/kg. They do not affect the skin and mucous membranes. The viscosity of polypropylene polymers varies with the chain length over a wide range, for example between 10 and 20,000 mm2.s-l at 40 "C. Polyglycols with end hydrogen atoms and identical molecular size have substantially higher viscosity than their partially or fully etherified counterparts. The reason for this is the formation 158
of hydrogen bonds. The viscosity-temperature curve of the polyglycols is flat and further flattens after substitution of the end-hydrogen atoms. Viscosity indexes vary between 100 and 200, mostly exceeding 150. Polyglycols have high flash-points, between 200 and 260 O C , and low volatilities. At elevated temperatures and in an oxidising environment they depolymerise. Depolymerisation can be controlled with antioxidants. The flash-points of polyglycols is associated more with the oxidation stability of the lubricant than with its volatility; they may be raised by over 50% with simultaneous suppression of polymer degradation by the addition of suitable antioxidants. These polymers have "false" pour-points. They stop flowing and vitrify at a viscosity of 100,000 to 150,000 mrn2.s-l. At the same viscosity at 100 "C, copoly ethylene/propylene glycols congeal at lower temperatures than polypropylene glycols. Polyglycol ethers and diethers have still lower pour-points. Uninhibited polyglycols have relatively low oxidation stabilities at elevated temperatures. They are, however, responsive to antioxidants and the stability of inhibited polyglycols can exceed the stability of inhibited mineral oils. In the absence of oxygen and traces of alkali, polyglycols remain stable up to 260 O C . At 300 O C , they decompose violently to produce aldehydes and acids, but no solid residues are formed. The radiation stability of polyglycols is roughly the same as that of mineral oils. The advantage of polyglycols is that thermal and oxidation degradation with time proceeds by depolymerisation, with the formation of volatile and less-viscous prodycts. Because of this attribute, they are suitable as vehicles for the solid lubricants, such as graphite, MoS2, used for high temperature bearings. Other advantages, namely, good lubricity and coolant properties, make polyglycols suitable for forging, pressing and metal-machining operations under heavy-duty conditions, as well as for heavily-loaded bearings and gears; they can be used also as hydraulic lubricants. In some applications, they surpass all other lubricants, sometimes even those containing anti-wear additives. The positive attributes of polyglycols can be further improved with additives. In comparison with mineral oils containing high-pressure additives and Table 3.14. Friction Coefficients of Polyglycol Oil (Mobil Glygoyle 30) Compared with Mineral Oils Containing Extreme Pressure Additives And Polyalphaolefin Oil at 20 "C, Tested on Different Friction Pairs Friction Pair:
Polygl ycol Oil
Steel aeainst:-
Sn-white metal (WM 80) Sn-white metal (V 738) Sn-white metal (V 840) Pb-white metal (Thermit) Leaded Bronze (Tego 11) Mean Value
0.101 0.106
0.121 0.1 16 0.130 0.115
Mineral Oil with E.P. Additive, containing:
Polyalphaolefin Oil
Pb
SIP
0.189 0.184 0.232
0.10
1.126
1.144
0.174
0.186
0.180
0.171
0.163 0.190
0.176
0.193 0.166 0.1 19
0.167
0.155 159
polyalphaolefins of approximately the same viscosities, the very good lubricity of polyglycols is evident from the low friction coefficients at 20 "C determined for different friction pairs with the Tanner apparatus (Table 3.24) Polyglycols have not been found to be toxic and their biological degradability is limited. They do not affectplastic or elastic materials, nor promote rubber swelling, and are therefore widely used in rubber plastics and paper processing, for example in calendering, where temperatures are as high as 260 "C,and for lubricating rubber packings, seals, etc. They do not promote corrosion of metals, but they provide no protection against rusting. They have affinity to water and absorb it (even compounds with blocked end-hydroxyl groups). For this reason, anti-rust agents must be added whenever this is important. Polyglycols are chiefly used as brake fluids and coolants. These fluids are expected to have good oxidation stability, satisfactory lubricity to metals and rubber, compatibility with water, low pour-points, high boiling-points and flat viscositytemperature curves. Polyglycols meet these requirements. They are also capable of application as hydraulic fluids for wide temperature ranges, from -40 "C. The satisfactory oxidation stability and other properties of polyglycols even after ageing make them suitable for lubricating compressors (but not high-pressure air compressors) and vacuum pumps. In cases where removal of entrained lubricant mist is required, water-soluble glycols can be used; the glycols can be recovered by waterwashing. Polyglycols have also been proposed for lubrication of internal combustion engines (35).The flat temperature-viscosity curves, satisfactory lubricity and clean oxidation products make them advantageous for this purpose. There is, however, a risk of oxidative depolymerisation to aldehydes capable of forming acid and condensation products leading to undesirable lacquers. Polyglycols are suitable for two-stroke engines, where the lubricant is mixed with the fuel, since they help to reduce substantially the lubricant: fuel ratio and produce fewer harmful substances in the exhaust gas, but they are rather expensive. Polyglycols are suitable components of both soap-containing and soap-free lubricating greases, designed for high-temperature service and containing solid lubricants. Water solutions based on ethylene or ethylene-propylene oxides, which contain, essentially, water, glycol, polyglycols and additives, are used as low-flammability lubricants of hydraulic fluids, for example in mining machinery, where low flammability is vital. One problem not yet satisfactorily resolved is the poor lubricity of these solutions in the mixed and rolling friction regimes. Substantial differences may be involved between the polarity, volatility, viscosity and adhesion of the particular components, i.e., water, glycol and high-molecular weight polyether oil. The problem may be alleviated by means of additives, e.g., phosphine oxides. Another problem is protection against corrosion. Metals must be protected, not only in the liquid phase, but also in the vapour phase above it. Some salts - borax, phosphates, molybdates, tungstates and fluorosilicates - are effective in the liquid phase, while ammonium 160
salts of weak acids with high vapour pressures, e.g., nitrites or benzoates of monoor diethylamine and diisopropylamine, provide protection in the vapour phase. For example, a hydraulic fluid intended for pressure casting, hydraulic presses and aircraft engineering consists of 55% glycol and 45% water. Its viscositytemperature properties are improved with a copolymer of 75% ethylene oxide and 25% propylene oxide, plus 1% of dimethylammonium laurate to improve its antiwear properties, 1% diisopropylammonium nitrite and 2% salicylateammine to inhibit corrosion and 2% mercaptobenzthiazole as its sodium salt to protect copper components (36). Since they do not stain and can be readily washed off, water-soluble polyglcols are used for lubricating and smoothing textile fibres, metal-machining and in the preparation of lubricating greases which are soluble in water and insoluble in hydrocarbons, which do not affect rubber and burn without leaving solid residues. Polyglycols of Type I11 blended with water are used as quenching liquids. Glycerol, which is related to glycol, lost its significance as a lubricant because of its ready oxidation (due to the presence of secondary alcohol groups) and poor thermal stability; it dehydrates at 200 "C and above to give acrolein, an unsaturated aldehyde which polytnerises rapidly. It has been used to a limited extent for the lubrication of food-processing machines (it is non-toxic)., pumps for hydrocarbon gases (it does not dissolve hydrocarbon gases), ground-glass joints (it can be easily washed off with water) and rubber pumps and miniature gears (it does not affect plastics). It is also used for the manufacture of oil-insoluble lubricating greases and graphite and molybdenum disulphide lubricating pastes.
3.2.2.5 Polyphenyl Ethers and Polyphenyl Sulphides The polyphenyl ethers are the aromatic analogues of the polyglycol ethers, and they have the general formula:
where R, R' and R" are H or alkyl. According to the number of p-, m-,or 0substituted phenyl (P) and ether oxygens (E) present, an abbreviated formula may be used, e.g., pm4P3E for the ether:
In the analogous polyphenyl sulphides, the oxygen atoms are replaced by sulphur. The presence of benzene rings in these compounds increases thermal and oxidation stability; however, the viscosity-temperature characteristics are affected adversely (VZbelow 0) and pour-point are elevated. Flash-points exceed 200 "C and 161
are highest for p-derivatives, all ether constituents being equivalent. Alkyl substituents scarcely improve the negative low temperature properties of these compounds. Asymmetric ethers of satisfactory pour-points may be obtained by exchanging the bond in the ortho and para position for that in the meta position, or by using eutectic mixtures. Polyphenyl ethers and polyphenyl sulphides are intended for service at high temperatures. They have good thermal (up to 390 - 450 “C)and oxidation (as high as 230 - 320 “C) stabilities, with high auto-ignition temperatures (exceeding 500 “C), which makes them suitable for use as hydraulic oils for lubrication of high vacuum diffusion pumps, of lubrication of electrical contacts when noble metals are used and of equipment exposed to high levels of radiation (37,38).Their lubricity and wear resistance in the mixed friction regime and at medium to high temperatures is equivalent to that of mineral oils, but better in polyphenyl sulphides.
3.2.2.6 Organic Esters Monoesters The monoesters of monobasic carboxylic acids and monovalent alcohols are not widely used as synthetic lubricants, even though some of them have high Vl’s and all have good lubricity. They also have high volatilities, high pour-points and low oxidation stabilities. Their pour-points can be reduced by combining branched acids and alcohols, but at the cost of VI. Their properties are summarised in Table 3.15. Sperm oil was at one time more important. Its principal constituents are saturated (about 35%) and unsaturated (about 60%) esters of higher fatty acids (Cl0 - C20) and higher alcohols (C,2 - C2$. Its low pour-point components are used as lubricants for precision mechanisms, often in blends with diester oils. Sulphurised, it is a component of oils for EHD, in the mixed and boundary regimes. It is a biodegradable and raw material for fatty alcohols. Sperm oil is now rare and costly. International conservation pressures to protect whales from extinction have led to the cessation of use of sperm oil as a lubricant. Several polymeric substitutes of equivalent functionality are now available for general use as raw materials for the manufacture of lubricatingoil additives, whilst other, less problematicalnaturally-occurringmaterials are now cultivated for more specialised applications. One equivalent substitute is the jojoba oil recovered from Simmondis Californica shrubs which gmw wild in the sub-tropical zones of America and are now cultivated in the USA, Mexico and Israel. This has a similar composition and similar applications to sperm oil.
Diesters Although there has been research and development in the USA on the synthesis and uses of aliphatic diesters of dibasic carboxylic acids of the general formula:
R0.CO.R” .CO.OR’ where R’ is an alkyl C, - C,, and R” an alkyl C, - C,, or phenylene group, the 162
Table 3.15. Properties of Some Commercially Available Monoesters* Starting Materials Carboxylic Alcohol Acid 2ethylhexanoic
heptanoic
Carbon Number 15
-
2-ethylhexanol
16
1.05
iso-octadecanol
26
3.32
isononanol(oxo)
16
1.3
3.3
isotridecanol(ox0)
20
1.88
5.82
3.17
11.69
2.84 3.34
10.12
-26
fatty acids 2ethylhexanol 25 isomdecanol 26 * Manufactured by: Nyco, BASF and Technochemie (39).
8
Low-temp. viscosity (mm2.s-') 571-40"C 132 I -50 OC 220 I -56 "C 951-40OC 2.61 244 I -50 OC 420 I -56 "C 18.45
n-heptanol
isodecanoic n-hexadecanol
c.
Viscosity (mm2.s-I) at 98.9 "C 37.8 "C
-
-
-
-
-
136
-
23
-56 <-95
-
>125
77 -
-75
168
-
115
-3
-
18
147 -
-1 1
-
-5
-
28.5 21
651-40"C 1380/ -70 "C 56.1 1-17.8 "C 113 322 1-40 OC 1500 I -54 "C
-
Pourpoint ( "C)
Flashpoint ( "C) 128
VI
Volatility (Noack) (90) -
!z
Table 3.16. Properties of Some Commercially Available Diester Oils* Starting Materials Dicarboxylic Alcohol Acid
Carbon Number
Fumaric Giutaric 3-tertbutyladipic Adipic
30 -29 26 22
9.8 1 3.56 3.1 3.57 2.35 12.38
22 22 -24 24 24 26
2.74 2.83 3.24 3.38 3.45 3.6
21 23 25 25 25
2.21 2.55 2.9 2.84 3.15
isotridecanol decanolldodecanol 2-ethylhexanol n-octanol I n-decanol 2-ethylhexanol
isooctanol 2,2,4-trimethylpentanol 30/70(wt.) n-octanoll ndecanol 3.5.5-trimethylhexanol iso-nonanol isodecanol 2,2,4-mmethyladipic n-hexanol n-heptanol n-octanol 2-ethylhexanol iso-octanol n-hex yl-2-hydrox y ethyl ether n-nonanot iso-nonanol ndecanol isodecanol ndodecanol n-tridecanol
25 27 27 29 29 33 35
Viscosity (mm2.s-I) at 98.9 "C 37.8 "C
Low-temp. viscosity (mm2.s-')
81.84 14680 I -17.8 "C 211.1 1-17.8 "C 13.67 14.34 14.87 8.17 18.22 108 I-17.8 "C 807 1-40 "C 9.8 990/-40°c 10.22 11.43 13.0 1755 1-40 "C 13.43 3200 I -40 "C 15.01 50001-40°c 7.25 8.92 10.79 11.96 13.08
VI
Pourpoint ( "C)
108 163 76 135 117
-38 -39 -55 -58 ~-5W-68
Flashpoint ( "C)
267 244 217 215 182 I 202
-
Volatility (Noack)
(a) 3.0 8.4
-
36 I 52
138 141 177 152 137 137
-79 -57 21 -37 <-70 <-70
245
41 49 9.9 39.5 31 145
129 132 160 1-17.8 "C 135 89 324 I -17.8 "C 114
-8 1 -79 -77 -65 -59
191 210 226 21 1 216
61 34 18.6 35 26
23 1 235 226 1220 239 242 246 248
24 9.6 141 19 7.5 9.6 4.8 4.1
-
11.35 141 3.03 3.31 12.78 147 4.011 4.03 21.97 118.88 113 I 124 3.7 1 15.05 246 1-17.8 "C 151 20.18 776 1-17.8 "C 115 4.1 1 4.62 19.86 371 1-17.8 "C 170 24.33 196 5.60
-64
-72 -49 1-50 -57 -54 -22 -13
193
-
Table 3.16. contd. iso-tridecanol n-tetradecanol iso-octanol 2ethylhexanol
35 37 25 25
6.82 5.90 3.34 2.96
isotridecanol n-octanol iso-octanol 2ethylhexanol 3,5,5-trimethylhexanol Dodecanoic 2ethylhexanol (decaneiso-octanol dicarboxylic) iso-nonanol iso-decanol
30 26 26 26
6.5 3.68 3.79 3.29
28 28 28 30 32
4.66 3.86 4.30 5.28 5.58
19.3 15.22 17.34 23.19 25.69
2-ethylhexanol 2ethylhexanol 2ethylhexanol 2-ethylhexanol
24 24 24 24
4.36 4.7 1 5.05 3.56
30.15 32.6 30.6 20.18
2-ethylhexanol
24
3.57
16.7
-
2-ethylhexanol 43/57(wt.) n-octanol / ndecanol iso-nonanol
24
4.06
16.6
26.3 26
4.24 4.8
18.09 23.0
Azelaic
Sebacic
Phthalic iso-phthalic terephthalic hexahydrophthalic hexahydroisophthalic hexahydroterphthalic
3695 / -17.8 "C 1300/ -17.8 "C 184 I-17.8 "C 1150/ -40 "C 356 133OCCS/ -17.8 "C 14.84 12.46 183 / -17.8 "C 42.43 24.97 12.7 11.0
* Manufactured by Amoco, BASF, Geigy, Hiils, Nyco. Technochemie and Veba (39).
124 209 154 139
-39 -2 -65 -73
258 25 1 -
-
4.0 3.5 19 20
150 191 167 153
-55 20 -5 1 -62
252 22 1
4.5 13 11
183 262 I-17.8 "C 168 178 184 5401nPa.d 161 .17.8 "C(CCS) 20 47 100 35
-42 -66 -27 -25 -46
249
13 8.8 8.0 6.0 3.5
-43 -46 -48 -50
223 224 227 222
16.3
104
-54
22 1
-
-
165
-51
227
-
-
158 145
-6 -25
248 218
6.2 12
-
266
-
-
-
widest development and practical applications in the twentieth century were accomplished in Germany during the Second World War (40). The development of diester oils in all industrially-developed countries was associated with aviation technology (228,229). Unlike hydrocarbons of the same chain length, esters oils have lower pour-points and volatilities; the viscosity of the diester group increases with the alcohol chain length when the same acid is used (40,4I).However, if the aggregate number of carbon atoms is considered, diester properties fluctuate according to the acid or the alcohol used. Among esters made from straight-chain dicarboxylic acids, those prepared from branched alcohols substituted in the p position (e.g., 2-ethylhexanol) have the lowest viscosities. Esters of branched dicarboxylic acids have lower viscosities than esters of straight-chain acids, if the molecules have the same size. Generally, esters prepared from iso-alcohols (except for alcohols substituted in the pposition) have higher viscosities than diesters of the same carbon number made from n-alcohols. The viscosities of diesters increase substantially at the same carbon number if cyclic alcohols are employed. Examples are shown in Table 3.16. As in the case of hydrocarbons, the viscosity-temperature properties of ester oils depend on the branching of the chains. The diesters of straight-chain aliphatic dicarboxylic acids and straight-chain or branched alcohols always have higher VI than alkane-based mineral oils. Straight-chain esters have the highest Vl’s. VI decreases with increasing branching of the molecule. The drawback of the straightchain esters is their high pour-points, which decrease with branching of the molecule. The optimum compounds of this type for lubricants are hence esters in which chains are only moderately branched, and it is desirable for methyl groups to be adjacent to the ester bond. Generally, the VZ of esters decreases with increasing viscosity, as is the case with mineral oils. The oxidation stability of esters is not high; it is comparable with that of alkanebased mineral oils. Esters decompose at higher temperatures, with the formation of alkenes which react more rapidly with oxygen. Oxidation stability of esters depends mainly on the stability of the alcohols used in their manufacture. Tertiary and, especially, quarternary carbons in the pposition increase the oxidation stability of both alcohols and acids. Substituted alcohols which have no phydrogen are particularly suitable for imparting high stability. Oxidation stability decreases with the length of the substituent chains. Esters of secondary alcohols have lower stability than those of primary alcohols. Esters containing 5- or 6-membered rings show good oxidation stability, however, most changes in the configuration of the ester molecule which increase oxidation stability also reduce VI. Manufacturers and users of compressors and aircraft turbines and manufacturers of oils have developed a number of tests for evaluating the stability to oxidation of diester oils which simulate as closely as possible field conditions (other properties are also tested, including: auto-ignition temperature, volatile losses, storage stability, tendency to pitting, thermal stability, corrosion protection, etc.). Tests employed are: thin layer oxidation test (Mobil), differential thermal analysis, differential scanning calorimetry, the Pneurop oxidation test, Alco and Erdco tests and others. 166
The thin layer oil oxidation test used for aircraft turbine oils consists in dropping the oil heated to 200 "C on to a rotating aluminium disk maintained at 300 to 360 OC. The disk rotates at 2,500 rpm and centrifugal force causes the thin film of oil to move to the edge, where it accumulates and is returned into circulation. Filtered air is blown over the surface of the disk. The appearance of the disk is evaluated after 85 minutes. Whilst mineral oils form coke-like deposits on the disk, diesters leave no trace. The reason for this is that the esters are good solvents for the products of ageing and wash them off the surface. Diester lubricants are sensitive to the presence of metals, especially copper, and are therefore always treated with anti-oxidants and usually also with metal deactivators. Anti-oxidants based on C- or N-alkylated phenylnaphthylamines and phenothiazines have given good results in this respect. Ester oils behave relatively well at low temperatures. Their behaviour improves with molecular branching and with the introduction of oxygen atoms into the molecule, which also raises the viscosity of the oil. High viscosity esters, however, behave as non-Newtonian fluids. The pour-points of esters depend on the composition of the molecule, i.e., on the nature and length of the chain of the acid and the alcohols used in its preparation. Esters made from acids of longer, straight chains generally have higher pour-points. The pour-point of the iso-octyl ester of adipic acid is -79 "C, azelaic acid -65 "C, sebacic acid -5 1 "C and didecanoic acid (CI2) -27 "C. If a straight-chain dicarboxylic acid is used as start-point for ester manufacture, the alcohol used for esterification of the C, and lower acids should preferably be branched in the pposition. This is the only way of obtaining low pour-point esters, and can be noted from the pour-points of various diesters detailed in Table 3.20. As noted earlier, ester oils have higher thermal stability than mineral oils. According to current interpretations, their decomposition proceeds at temperatures above 240 "C via cyclic intermediates (42): (R)&
I
CHZ-
H R ' I I I
0-co
I
I-
(R)2C = CH2 + RCOOH
(3.1)
If the P-carbon atom in the alcohol group is fully substituted, radical decomposition occurs, which yields the same products, but requires more energy and higher temperatures. The thermal stability of diesters may be improved by substituting the hydrogen on the pcarbon with methyl or fluorine. Esters from secondary alcohols have lower thermal stability then those from straight-chain primary alcohols, and start to decompose at 190 "C. Esters from primary alcohols are stable up to 260 - 3 15 "C. At high temperatures, esters leave no coke-like deposits and do not form lacquers, because they do not contain resins. Increased branching in the /%positionreduces their tendency to hydrolysis (43). 167
A tendency to hydrolyse with reversion to their raw materials is a serious deficiency of diester oils. Hydrolysis may not occur at elevated temperatures, since the water evaporates. Diesters from hexahydrophthalic acid have greater resistance to hydrolysis (43). Among the esters of branched acids, those containing dicarboxylic acids branched in the a-position have the higher resistance. Hydrolytic stability of esters of nonbranched acids grows with the length of the acid chain. These differences are evident from the values of TAN (total acid number) of various diesters in pre- and post hydrolysis conditions (Table 3.17). These reactions are inhibited if the carbon atom adjacent to the carboxyl group is blocked by an alkyl group (44). One of the essential tests to which diester oils are subjected is hydrolyltic stability (storage stability). According to the Rolls-royce method, a 250 cm3 sample is agitated with 25 cm3 of water at 90 “C for a period of 192 h. After this time, the neutralisation number of the oil should not exceed 1.5 mg K0H.g-’ of oil.
Table 3.17. Post-hydrolysis Changes in TAN of Different Diesters Determined by the VEBA Method* Starting Materials
TAN
VI
(mg K O W )
Alcohol
Acid
preposthydrolysis
preposthydrolysis
Volatility (Noack %wt.) preposthydrolysis
0.12 0.32 0.62 43.3 0.6 56.0 RC4 0.05 17.3 iC10 55.0 11 53 0.2 155 43 nC*/nC, 0.2 1.1 148 150 13 13 * This method is very severe: 3 parts of water and 2 parts of ester are stirred for 20 hours at 180 “Cand 1MPa. The changes in neutralisation number (TAN),viscosity, VI and volatility by the Noack method are determined.
nC8/nC10 nC8/nC10 nC8/nClo
Hexahydrophthalic Hexahydroterephthalic Hexahydroisophthalic Nonadecanoic carboxylic Adipic Trimethyladipic
A deficiency of diester oils is their solvent power for most polymers, particularly rubber and polyethylene. Their particular advantages are high shear stability, wear control in the mixed friction region and low foaming tendencies. The volatility of most diester oils is lower than those of mineral oils of the same viscosities, but substantially higher than those of PAO’s, particularly at elevated temperatures. Their volatility decreases with increasing viscosity. However, molecular structure also plays an important part. For example, esters containing cyclic alcohols, e.g., esters of dicarboxylic acids and 3,3,5-trimethylcyclohexanol, have relatively high volatility. Chemical composition and, probably, the concentration of volatile “ester impurities” of low boiling-point are very important factors in determining the load carrying (anti-wear) properties of esters, whereas viscosity is, in this respect, the comparable factor with mineral oils ( Table 3.18). The reason for this different behaviour may lie in the somewhat lower rate of
168
Table 3.18. Anti-wearPropertiesof Some Dieters from FZG Tests (169) in the Niemann Test Gear-box _____
~~
Diester Composition
~~
~~
Volatility
Viscosity
(Noack) (8)
at 98.9 "C
2,2,4-trimethyladipidoctanol Azelaidisooctanol Hexahydroterephthalicl2ethylhexanol Sebacidiso-octanol Adipidtridecanol Dodecanoidiso-octanol Dodecanoidiso-nonanol
13 17 19 14 6 11 5
VI Min. Load carrying capacity (mg)
(mrn2.s-')
3.3 3.1 4.1 3.4 5.7 3.9 5.9
150 156 148 165 142 170 163
6 7 7 8 8 8 8
Wear (mg) 13 14 18 13 16 17 18
Wear at next highest load (mg) 192 26 1 306 220 27 1
60 117
viscosity increase with pressure in ester oils as compared with mineral oils. This phenomenon is exploited in the application of ester oils in high pressure hydraulic systems operating at low temperatures. The relatively good lubricity and load-carrying capacity of the lubricating film are not only attributable to the polar groups, but also, probably, to partial hydrolysis when the primary acid - which reacts chemically with the metal surface - is released and provides a boundary layer with good load-carrying capacity. For some special applications, both the lubricating ability and the thermal stability of diester oils can be improved if hydrogen atoms in the alcohol are replaced by fluorine; however, this occurs at the expense of impairment of the viscosity-temperature characteristics of the oil. The oils most generally used (Table 3.16) are made principally from aliphatic dicarboxylic acids - adipic, sebacic, azeleic and dodecanoic(di) acids and primary alcohols in the C, - C,, range, the typical di-2-ethylhexyl sebacate (I) being chiefly used in aircraft (45, 46):
0 0 It II CH3-(CH2)3CH-CH,-O-C -(CH2)8 -C -O-CH,-CH-(CH,)-CH, 72H5
The development of diester oils in the years between 1948 and 1955 was due to the demand for lubricants for modern aeroplanes. Most of the military specifications come from that period. Later, they became accepted by the civil aviation industry. Diesters of the di-Zethylhexyl sebacate type were found suitable for jet engines. Propellor-driven aircraft, however, required oils of higher viscosities, which was achieved by addition of polyol-based complex esters. Ester oils and lubricating greases made from them find application, outside the aircraft industry, as lubricants for machine parts exposed to electric power, fine mechanisms, precision instruments, textile machinery, high pressure hydrogen compfessors (where traces of sulphur act as catalyst poisons) and as components of part-synthetic engine and gear oils, where they improve VZ without impairing shear stability, reduce viscosity and lower pour-point.
Complex Esters Complex esters are made in two principal configurations: (a)monoalcohol-diacid-diol-diacid-monoalcohol, (b)monoacid-diol-diacid-diol-monoacid. Primary c6 - C, alcohols (from aldolisations or 0x0-syntheses), polyethylene glycols, c6 - C,, diacids (as in diesters) and C5- C9 monoacids are the principal starting materials . By comparison with diesters, viscosities are higher (7 - 11 at 100 "C), Vl's similar (140 - 150), pour-points lower (-50 "C and less ) and flashpoints similar at 220 - 270 "C. Complex esters are also used as components of aviation and engine lubricants.
170
Polyesters The esters of monocarboxylic acids and polyols are products of great importance. They are made from aliphatic acids in the C, - C, range (e.g., valeric, caprylic, pelargonic), their blends and methylated analogues, iso-acids (e.g., 2methylpentanoic), neo-acids with a quaternary carbon atom (e.g., 2,2-dimethylpentanoic,5,5-dimethylhexanoic, 4,4,4-trimethylhexanoic)and, more rarely, benzoic acids. The polyols, mostly neo-alkylpolyols, include trimethylolpropane(I), pentaerythritol (11), neopentylglycol (111), trimethylolethane (IV), dipentaerythritol (V) and polytrimethylolpropanes (VI) (n = 1 - 4) (232).
CH2OH
CH2OH
I
I
(I) CH3 - CH2 - C - CH2OH
(11) HO - CH2C - CH2OH
I
I
CH2OH
CH2OH
CH2OH
CH2OH
I
(m
I
CH3 - C - CH3
I
I
CH2OH
CH2OH
CH2OH (V)
CH3 - C - CH2OH
(IV)
CH2OH
I
I
HOH$ - C - CH2 - 0 - CH2 - C - CH2OH
I
I
CH2OH
CH2OH
CH2OH
CH2OH
I
(VI)
CH3 - CH, - C - CH2 - 0
H
I CHZOH
CH2OH
n
Diols of polyglycol (VII) (n > 4) are also possible:
(W) HO
t
CH 2 - CH - 0
H
hj. 171
c.
Table 3.19. Properties of Some Commercially-Available Diol-Ester Oils
4 N
Diol
Starting Materials Carboxylic Acid
neo-pentylglycol iso-decanoic acid ethylcaproic + iso-tridecanoic n-octanoic +decanoic 16-hexanediol 2ethylhexanoic n-nonanoic iso-decanoic 2-ethylhexanoic + isotridecanoic dipropyleneglycol n-nonanoic pelargonic 1,44imethylolcyclohexane iso-nonanoic n-octanoic + n-decanoic iso-tridecanoic himethylhexme diol n-octanoic 2-ethylhexanoic iso-octanoic iso-nonanoic n-nonanoic n-decanoic n-undecanoic ndodecanoic n-tetradecanoic
Carbon Number
98.9 "C
Viscosity (mmZ.s-') at -40 "C 37.8 "C
Pourpoint ( "C)
Flashpoint ( "C)
Volatility (Noack)
80
-6 1
-
39.4
66 144 138 I -17.8 "C 111 180 119 119
-56 -63 -72 -8 <-70 <-70
-
28 32.8 37.7 8.6 17.3 32.8
VI
(a)
25
3.21
15.05
26 22 24 26 27
4.12 2.5 1 2.39 3.01 3.79 3.79
23.93 8.47 8.52 10.20 17.41 17.41
24 24
2.8 2.7
10.14 9.8
138 130
-57 -59
-
-
%3
210
-
21
6.47
41.57
-
117
-48
321
8.8
-
4.54 9.65
19.78 83.45
163 102
-17 -43
248
6.0 3.0
3.m 3.02 3.55 4.56 3.5 4.05 4.57 5.09 6.18
10.5 13.35 15.58 23.05 13.532 16.5 19.41 22.3 28.48
1010 180 394.6 I -17.8 "C 86 407.4 1-17.8 "C 120 834 1-17.8 "C 124 1860 I-17.8 "C 166 165 172 180 189
-60 -7 1 -65 -59 -63
214 209 225 230 230 237 247 255 266
-
34
25
25 25 27 27 29 31 33 37
15300 56OOO
-
-
-
-40
-30 -16 5
213
-
242 -
213
-
27.4 26.2 20.0 9.4 9.2 6.9 5 .O
3.6 2.2
Table 3.19. contd. trimethylhexane diol n-nonanoic + oleic naphthenic 1.12-dodecane 2-ethylhexanoic diol iso-nonanoic
28 30
5.52 7.92 3.74 5.58
23.79 64.09 15.68 25.25
7815 1-17.8 "C 3200 -
196 -97 142 182
-33 -34 C-70 -38
259 26 1 -
3.2 4.2 12.3 7.4
Table 3.20. Properties of Some Commercially-Available Trimethylolpropane Trio1 Ester Oils* Triol
Starting Materials Carboxylic Acid
trimethylolpropanen-heptanoic n-cctanoic n-nonanoic n-decanoic
Carbon Number 27 30 33 36
98.9 "C 3.5 4.06 4.25 5.3
Viscosity (mm2.s-') at 37.8 "C -40 "C 15.0 18.3 19.3 26.15
2350 2436 3552
-
VI 125 135 140 153
Pourpoint ( "C)
Flashpoint ( "C)
-60 -57 -51
220 235 235
8
-
Volatility (Noack) (%)
8.0 4.9 3.5 2.1
Manufactured by Geigy and Nyco (39).
Table 3.2 1. Properties of Some Commercially-Available Pentaerythritol Tetra-Esters* Triol
Starting Materials Carboxylic Acid
Pentaerythritol
e
4
w
Carbon
Number
98.9 "C
Viscosity (mm2.s-1) at 37.8 "C -40 "C
n-heptanoic
33
5.1
26.0
n-octanoic n-nonanoic n-decanoic iso-decanoic iso-stearic
37 41 45 45 77
5.49 6.47 7.02 10.3 17.9
28.6 1 35.97
Manufacturedby Geigy and Nyco (39).
-
99.0 150
618 I-17.8 "C 7200
-
13600 1-17.8 "C 1747J -17.8 "C
Pourpoint ( "C)
Flashpoint ( "C)
140
-65
250
3.9
144 146
-4 -1 -30 -40 -18
-
2.4 1.7 1.5
VI
93 140
250 300
Volatility (Noack) (%)
-
1.2
Examples of some esters are described in Tables 3.19 to 3.21. Esters derived from pentaerythritol have higher viscosities than the trimethylolpropane analogues. Their viscosity indexes are also higher. Viscosity and viscosity index increase with increasing length of the alkyl chains, but pour-point increases at the same time. Branched acids give rise to esters of higher viscosity and, usually, lower pour-point and lower viscosity indexes than the straight-chain acids. Oxidation stability is increased by the presence of neo-acids. The viscosities of polyol esters prepared from branched chain acids are higher than the viscosities of polyol esters made from n-acids of the same carbon number. Some of the high viscosity polyol esters behave as nowNewtonian fluids at low temperatures.Measured and extrapolated viscosities at -40 "C of pentaerythritol (PE) and trimethylhexandiol (TMH) esters are compared in Table 3.22. Whilst the measured viscosity of the PE derivative is a little lower than its extrapolated viscosity, the measured viscosity of the TMH ester is twice its extrapolated viscosity (measured at 11 "C below the pour-point). Detailed knowledge of differences in low temperature viscosities is particularly important in aeronautical engineering. Table 3.22. Viscosities of "wo Different Polyol Esters at Low Temperatures (268) Properties Viscosity (mm2.s-')
at 98.9 "C at -40"C (measured in a capillary) (extrapolated) Viscosity index Pour-point ( "C)
Pentaerythritol ester with blend of C,-C,, acids
Trimethyl hexandiol ester with iso-nonanoicacid
5.0
4.56
1570 9200 126 -70
14165 6800
123 -5 1
Esters of neoalkylpolyols and monocarboxylic acids are thermally stable in the presence of antioxidants up to about 3 15 "C. The volatility of most polyol esters is fairly low, pentaerythitol esters and some trimethylolpropane esters being particularly involatile. As in most other types of esters, volatility mostly varies inversely with viscosity. However, some esters of branched diols, such as 1,4-dimethylolcyclohexaneand trimethylhexanediols, are exceptions in that they have rather higher volatility than would be expected from their viscosities. The pour-points of branched acid esters of straight-chain diols or polyols are lower than the pour-points of the esters of the same diols and straight-chain acids. For example, the pour-point of the 1,lZdodecanediol diester of iso-nonanoic acid is -38 "C, with 2-ethylhexanoic acid -70 "C; that of the pentaerythritol tetraester with n-decanoic acid +30 "C, with iso-decanoic acid -40 "C and with iso-stearic acid -18 "C. Low pour-points of the substituted polyol esters can be achieved by esterification with a mixture of acids of odd and even total carbon number (C,,C, and C8,c1,, respectively), or by esterification of a blend of straight and branched 174
chain C6,C, acids. In the former case, the esters so produced will have higher volatility and corrosivity, in the latter poorer VZ. At the same viscosity, the anti-wear properties of polyol esters are similar to those of diester oils and, to a large extent, mineral oils. The polyol esters, however, have better friction properties in respect of stick-slip phenomena. They ate receptive to treatment with anti-wear additives. Diol esters of mono-carboxylic acids have considerably higher resistance to hydrolysis than the corresponding dicarboxylic acid esters. Neo-alkylpolyol esters are suitable for a wide temperature range; they have relatively low viscosities at low temperatures, low pour-points, very high viscosity indexes and low volatility. They are commonly inhibited with amine anti-oxidants, and have very good oxidation stabilities. They have higher viscosities than diester oils. Some are used as lubricants for aircraft gas turbines (oil temperature 220 "C;viscosity 24,000 mm2.s-l at -55 "C) and industrial gas turbines. They are also used as heat-transfer fluids in machines operating at very high temperatures and high speeds, the cold-rolling of metals, as components in lubricating greases intended for use at high temperatures and additives for mineral and synthetic oils, where they improve low temperature properties and reduce attack on elastomers. They improve the lubricity of silicone oils. Acylgly cerols Oily acylglycerols (fatty oils) are lubricants of low oxidative stability. However, they are biodegradable and have good properties under conditions of mixed or boundary lubrication. They are therefore added - in non-sulphurised and sulphurised forms, e.g., sulphurised rapeseed oil - to mineral oils and oils to be voltolised. Rapeseed oil is suitable if oxidative stability is unimportant. Castor oil is important, to a limited extent, in applications where immiscibility with mineral oils is demanded (sometimes for the lubrication of alcohol-powered racing engines). The -OHgroup in ricinoleic acid provides the immiscibility. Very effective anti-wear additives can be made by converting ricinoleic acid into its esters.
3.2.2.7 Phosphoric Acid Esters In the generic formula for phosphoric acid esters: 0
II
RIO - P - OR2
I OR3
R,, R, and R3 represent organic radicals, which may be identical, as in trialkyl and triaryl phosphates, or different, as in alkaryl phosphates. Phosphates in which only one hydrogen is substituted by an organic radical, have no lubricant attributes 175
in themselves, but may be used as corrosion or rust inhibitors, as well as for lubricant and other additive applications. Although triaryl and trialkyl esters of phosphoric acid have been known for a long time as plasticisers, particularly for PVC,they have only recently been used as lubricants, or as components of lubricants which improve their lubricating properties. The properties (viscosity, viscosity index and pour-point) of phosphate esters differ widely according to their chemical composition (Table 3.23). Tables 3.23. Properties of some Phosphoric Acid Esters (47) Viscosity (mm2.s-') at 37.8 "C 98.9 O C tricresyl phosphate (typical commercial product) n-octyldicresyl phosphate di-n-octylcresyl phosphate tri-n-octyl phosphate tri-(2-ethylhexyl)phosphate diphenylcresyl phosphate 2-ethylhexyldiphenyl phosphate
4.37 3.15 2.63 2.56 2.23 3.25 2.45
VI
Pourpoint ( "C)
35.11 15.30 9.99 8.48 7.9 17.53 10.01
ioa
-1 1 -5 1 -60
148 94 28 65
-35 -53 -34 -60
0 61
Phosphate esters have excellent lubricating properties even under boundary friction conditions. They react with metallic surfaces to form, for example, phosphides and phosphates (233).They can therefore be used as anti-wear additives and corrosion inhibitors for lubricants. Their essential merit as lubricants is their low flammability and combustibility, although these properties vary with chemical composition and molecular size. Unlike most organic compounds, their resistance to flammability grows with decreasing viscosity and relative molecular weight, since the phosphorus: organic ratio in the molecule increases; this is demonstrated in Table 3.24. Table 3.24. Correlation between the Auto-ignition Temperature and Viscosity of Phosphate Esters Viscosity at 40 "C (mm2.s.') 17.3 28.2 40.3 68.1
Auto-ignition temperature ( OC)by ASTM ~ - 2 8 6 650 605
590 585
Phosphate lubricants are stable and better than petroleum lubricants under oxidising conditions. On the other hand, their thermal stability is only satisfactory in the medium-temperature range. 176
Phosphoric acid triaryl esters are stable in a neutral environment, and the addition of anti-oxidants can improve their oxidation stabilities. The stability of the alkaryl esters depends on the position of the alkyl substituent in the benzene ring. Differential thermal analysis shows that tris-o-isopropylphenyl phosphate is more stable than the p-isomer. Oxidation of phosphate esters proceeds via a auto-catalytic reaction of the P-0-R bond in which diarylphosphoric acid and high molecular weight products are formed. A rise in acidity and viscosity may therefore indicate the onset of oxidation. Phosphate lubricants decompose at high temperatures and are therefore unsuitable, particularly in the presence of metals or acids; triaryl phosphates may be exposed indefinitely to temperatures up to 175 "C and alkaryl phosphates up to 125 "C.The triakyl phosphates are more sensitive to heat, but their thermal stability may be improved by replacing the hydrogen atom in the Pposition by other substituents, e.g., methyl or halogen. Hydrolytic stability is also greatly influenced by structure, improving with increasing size of the molecule. The alkaryl phosphates have lower stabilities towards hydrolysis than the trialkyl phosphates. Substitution of the hydrogen atom in trialkyl phosphates in the Pposition relative to the ester oxygen with other substituents improves hydrolytic stability (47).Generally, their hydrolytic stability is average or rather low and must be tested; nevertheless, it is sufficient for most practical applications. In contrast to the esters of carboxylic acids, hydrolysis of phosphate esters does not proceed as far as liberation of the original acid and alcohol. The hydrolysis of triaryl phosphates proceeds with decreasing reaction velocity in the sequence tertiary>secondary>primary esters, and stops at the stage of formation of the diary1 ester. The composition of the ester groups has a significant effect on hydrolytic stability. Triphenyl phosphate and diphenylcresyl phosphate have substantially lower stabilities than tris-o-isopropylphenyl phosphate and trixylenyl phosphate. The lower viscosity esters, because of their structure, have lower hydrolytic stabilities. The general rules discussed in earlier sections apply to the correlations between viscosity and viscosity-temperature properties, on the one hand, and chemical composition of the organic portion of the molecule on the other. As a rule, phosphates have similar viscosity-temperature properties to those of average petroleum lubricants. The same applies to volatility, when lubricants of the same viscosity are compared. Phosphates are good solvents. This is advantageous when they are mixed with other lubricants and for compatibility with additives, but it can be disadvantageous in relation to contact with plastics, elastomers and paints, e.g., polyethylenes, polyperfluorethylenes, polyamides and butyl and silicone rubbers. Fluorocarbons are the only suitable materials for seals and glands. Some phosphates are toxic, particularly those tricresyl phosphates which have a high content of o-cresol. Others are efficient bactericides and some are acceptable as plasticisers for food packings. 177
Toxicity in this context concerns the effect of decomposition products on the central nervous system. 0-Cresyl phosphate stands out in this respect, but neurotoxicity has been found recently in other phosphates. It is therefore important to use the latest information on toxicity relating to phosphates. The radiation stability of phosphates is unsatisfactory. Phosphate esters have found application in non-flammable lubricants and hydraulic fluids. alone, or blended with chlorinated aryl esters, they meet all the civil and military requirements for resistance to auto-ignition and non-flammability. Air-freighters use branched trialkyl phosphates with the required low-temperature properties to fill hydraulic equipment. In industry, for example in steam turbine governors, welding machines, metal forges and rolling mills, the cheaper blends of chlorinated biphenyls or diphenyl oxides with triaryl phosphates were preferred. These blends are good, safe lubricants for air compressors, but they are at present unacceptable because of the toxicity of the chlorinated aryls, particularly biphenyls, and disposal problems. Phosphate esters are suitable in hydraulic devices not only for the transmission of force, but also for the protection against wear of pumps and valves. This is demonstrated, for example, by comparison of the results of Vickers pump tests of phosphate ester and mineral oil of the same viscosity (Table 3.25). Table 3.25. Comparison of Tests of Phosphate Ester and Mineral Oils of the Same Viscosities in the Vickers Pump Test, VlMC, at 14 Mpa, after 250 Hours Phosphate Ester ISONG46 5
Index Loss of ring weight (mg) Loss of lamella weight (mg) Total loss (mg)
5 10
Mineral Oil ISONG46 20 to 120 10 to 30 30 to 150
3.2.2.8 Aryl and Alkyl Esters of Silicic Acid The esters of ortho-silicic acid (ortho-silicates - I) and its dimer - hexaalkoxydisiloxanes and hexa-aryloxydisiloxanes diortho-silicates - 11) are relative newcomers to tribotechnology. OR
I RO-Si-OR
I
OR (1)
where R is alkyl, aryl or alkaryl. 178
OR
OR
I
I
RO-Si-0-Si-OR
I
I
OR
OR (11)
These substances are characterised by good viscosity and viscosity-temperature properties. Tetra-alkyl silicates have very high viscosity indexes and those of the disiloxanes of the same viscosity are a little higher. Viscosity increases and the viscosity-temperature curve becomes steeper with the introduction of branched alkyls or of aryls. Tetraalkyl silicates and hexaalkoxydisiloxanes have excellent low temperature properties. There alkyl derivatives congeal at low temperatures, which is not necessarily a deficiency if they are designed for high temperature use. The deficiency of silicon esters is their tendency to hydrolyse. The tendency to hydrolysis increases in the sequence: carboxylic acid esters/ phosphates/ silicates/ titanates. Tetraethyl silicates can be decomposed by water at normal temperature. Resistance to hydrolysis grows with increasing length and branching of the alkyl chain. Ortho-silicates with C, or longer alkyls can be kept in open vessels. Tetraaryl silicates, on the other hand, must be protected against atmospheric moisture. Disiloxanes are normally more stable under hydrolytic conditions. The presence of two alkyls - two in silicates and four in disiloxanes - is beneficial. Alkaryl silicates have higher stability than pure alkyl or aryl silicates. The first products of hydrolysis are gels and the final products is colloidal silicic acid. Resistance to hydrolysis can be increased by the use of additives, e.g., phenyl- 1-naphthylamine. Silicon esters have high boiling points and negligible volatility; their thermal stability is, as a rule, excellent and somewhat exceeds that of the organic esters. Di2-ethylhexyl sebacate decomposes at 274 "C, whereas tetra-2-ethylhexyl silicate decomposes - under similar conditions - at 337 "C and tetraphenyl silicate at 425 "C. Alkyl silicates decompose to alkenes and silicic acid, aryl silicates to yield phenols and silicic acid. In their oxidation stability, silicates are comparable with hydrocarbon lubricants. They are, however, very amenable to anti-oxidant treatment. The first products of oxidation are peroxides, then acids, which promote hydrolysis. Thus, the resistance of alkyl silicates to hydrolysis is improved by anti-oxidants. Silicone esters have average lubricating properties. They are satisfactory for the hydrodynamic regime, poorer under boundary conditions. However, their lubricity can be improved by the introduction of polar groups (Cl, F, N, o-alkyl, sec-alkyl) into the R substituent, or by blending with other lubricants (e.g., polyglycols or esters), or additives. Silicates and their decomposition products are not corrosive, but neither do they provide anti-corrosion properties. However, the latter may be imparted to them by the use of suitable additives. Advantages of silicates include their low solvent power towards plastics and sealants, but after prolonged contact at elevated temperatures they may cause hardening. Additives to control rubber swelling are also available. Generally, silicates may be improved by additives which increase their viscosity index (polysiloxanes), lubricity and oxidation stability and suppressing their tendency to promote rust or foam formation. The properties of some silicon esters are illustrated in Table 3.26. 179
Table 3.26. Properties of Some Silicic Acid Esters P20
(kg.m") Orthosilicates: tetra-n-butyI tetra-(2-ethyl-butyl)tetra-(2-ethyl-hexyl)tetra-(3,3,5-trimethylhexyl)tetra-(isotridecy1)Disiloxanes: hexa-(2-ethyl-hexyl)-*
Viscosity (mm2.s-1) at ( "C) -40 98.9 37.8
896 902 894 879 875
1.04 1.67 2.36 3.41 6.6
932
11.27
1.85 12 656 84.04 6.83 260 11.40 695 36.0 21460 35.32
2296
point
Flashpoint
("C)
("C)
78
175 161 135
-127 <-65 <-65 -76 -52
219 262
-
<-70
202
PourVI
238
-
-
-
*Commercial product, Omnite High Temperature Fluid 8200, improved with polysiloxane with oxidation and corrosion inhibitors.
Silicate esters are often used as heat transfer fluids at high temperatures, as coolants for electronic devices, hydraulic fluids for wide temperature ranges (from -50 to 200 "C), particularly in aeronautics and rocketry, and as lubricants for automatic air-borne weapons for the same temperature range. They are also used in the preparation of lithium and other lubricating greases and pastes operating over a wide temperature range or at very low temperatures, e.g., hexa-alkoxydisiloxanes with colloidal calcium carbonate. The use of silicon esters is still developing. Polysilicate clusters (III) are at the next stage of development: and analogues (III)
R'-Si-[-O-Si-(OR)3]3
Some of them have VZ 290 to 320, viscosity 7 to 11 mrn2.s-l at 100 OC, 500 to 1,500 mrn2.s-*at -40 "C, pour-point below -70 "C and negligible volatility. They are resistant to hydrolysis, compatible with most fluid lubricants except the aryl phosphates and polypropylene glycols and their lubricity is good. they are non-toxic and they do not imtate the skin. The areas in which they are employed are similar to those of silicates (225). 3.2.2.9 Polysiloxanes
Polymethyl and polyrnethylphenyl siloxanes (I) are known in technical practice as silicones. Their development began in 1904, but their industrial manufacture did not start until as late as the Second World War in the USA and Germany. R R R
(I)
'f
R-Si
I
:t I
0-Si
0-Si-R
R is alkyl (most frequently methyl) or aryl (most frequently phenyl) groups or mixtures of them. 180
The most important examples of the silicone oils are the polymethyl siloxanes. Their relative molecular weights are from 200 to over 140,000;their viscosity ranges from 1 to over 700,000 m m 2 s 1 at 25 "C. They have the best viscosity-temperature properties of any fluid lubricants. Viscosity and the slope of the viscositytemperature curve increase gradually if the methyls are substituted by ethyls, phenyls and fluorinated alkyls and if polyglycol chains are incorporated into them. However, even highly phenylated silicones have much better viscosity-temperature properties than most lubricants. The polymethyl silicones are Newtonian fluids at viscosity up to 1 ,OOO mrn2.s-l at 20 "C and shear rates up to 1 ,OOO s-l, non-Newtonian fluids and structurally viscous at higher viscosities. Under prolonged shear stress, silicone polymers partially depolymerise. This depolymerisation is, however, considerably less than that of mineral oils thickened with polymeric additives. Unlike other organic lubricants, virtually all silicones have very low volatility, several times lower than those of diester oils. Their flash-points are generally above 315 "C. Silicones have very low pour-points; the introduction of asymmetry enables them to become even lower. Their flat viscosity-temperature curve, low volatility and low pour-points make silicones suitable for use over a very wide temperature range. Another most advantageous property of the silicones is their good thermal stability. Polymethyl silicones start to decompose at about 315 "C,undergoing partial isomerisation (which increases with temperature) of those chains with 8 to 14 members. The weak point is the poor stability of the Si-0 bond, which enhances the formation of cyclic polymers with higher volatilities. The introduction of phenyl improves the thermal stability to 370 "C and more. Excessive temperatures reduce the viscosities of polymethylated silicones and increases that of polymethylphenylated silicones in which there is a significant phenyl content. Decomposition products are not usually corrosive if halogen is absent from the molecule. Polymethyl siloxanes have up to 200 "Cbetter oxidation stability than any other organic lubricants. As oxidation begins to be perceptible at higher temperatures, volatile oxidation products appear, namely, formaldehyde, formic acid, water and carbon dioxide, together with cross-linked polymers and finally gels, which considerably increase viscosity. Sludge does not usually form. Oxidation stability increases with increasing pheny1:methyl ratio. Polymethyl siloxanes can withstand an oxidising atmosphere up to 300 "C for a short time. The oxidation follows a chain reaction sequence for which the mechanism on the page 182 has been proposed. This mechanism is consistent with that suggested for hydrocarbon mineral oils. The stability of silicones can be improved by special antioxidants based on aromatic amines or compounds of iron, copper, nickel, cobalt and cerium. It is worth noting that the stability of silicone-ester blends is poorer than that of each constituent separately.
181
I - Si - CH3
- Ai - dH2 + H 0 2 ,
+ 02
I
I
I - Si- CH2CO0 ,
- s i - dH2 + 0,
1
I I
- Ai-
- Si - CH200
I - Si - CH200 + - Si - CH3
I
0 - CH20,
I
I
I
-- SiII
- CH200H
+
I - Si - CH200H
-
I - Si- CH20+ H 6 ,
I - Si - CH20
-
I -Si*+ CH20
I
I
I
Si- dH2 ,
I
I
I
Radiation converts silicones into gels via Si* and SiCH2*radicals and methane, hydrogen and volatile hydrocarbons. No acid products are formed. Radiation stability is improved by increasing the pheny1:methyl ratio. Silicones are not vulnerable to hydrolysis below 200 "C. At higher temperatures, the viscosity of the very viscous polymethylated and polyphenylated silicones decreases. According to present knowledge, the silicones are non-toxic; they are chemically inert and water-repellent. The low viscosity polymethyl siloxanes have low solvent ability for dissolving elastomers, plastics and paints. At elevated temperatures, plasticisers may occur and the shrinkage and hardening of rubber. This necessitates careful selection of sealing materials, particularly because the surface tension of silicones is very low (16 - 21 mN.m-l; for comparison, the surface tension of oil is 30 - 35 and water 72). As viscosity increases above 10mm.s-*at 20 OC solvent power diminishes; the introduction of phenyls has the opposite effect - surface tension increases in both cases. Generally, the low solvent power of the viscous silicones has the effect that they do not blend (or only blend to a limited extent) with mineral, diester and polyglycol lubricants and only reluctantly accept additive treatment. Table 3.27 lists the properties of some silicones. Polymethyl silicones are used for lubricating electric motors, speedometers, precision machines and instruments, as fluids for shock absorbers, for impregnation of self-lubricated bronze bearings, etc. They are widely used for lubricating plastic and rubber components in refrigerators, pumps, pneumatic systems, tape recorders, film cameras, etc. 182
Table 3.27. Properties of Some Silicones Viscosity (mm2.s-l) at 20 "C Polymethyl-siloxanes
5.0
100 500 1000 3 m
0.55 0.57 0.59 0.60 0.62 0.62 0.61
-65 -65 -55 -55 -50 -50 -44
135 163 274 316 316 316 >316
918 940 955 971 972 973 973
19.7 20.1 20.8 20.9 21.1 21.2 21.5
100 500 100 500 63
0.62 0.65 0.76 0.84 0.69
-73 -73 -51 -21 <-70
274 274 299 301 288
990 990 1020 1100 1040
24.1 24.4 28.5 -
10.0
50.0
Polymethyl-phenyl siloxanes 4: Me = 0.35
4: Me = 0.30 qj: Me = 0.75 Polychlorphenyl siloxane '
v337.8-v98,9 Pour- Flash- p20 Surface v37.8 point point &g.m-3) tension ("C) ("C) at 20 "C (mN.m'')
@ = phenyl, Me = methyl
Polymethyl siloxanes improve the viscosity index of high-temperature silicate hydraulic fluids. They suppress foaming and increase the flash-point of mineral oils. Polymethylphenyl silicones with a pheny1:methyl ratio of about 0.2 are suitable for service at -55 "C and retain their stability at 230 OC. They may be used as lubricants for internal combustion engines and as components of lubricating greases. Lubricants with a pheny1:methyl ratio of 0.35 are used for anti-friction bearings operating permanently at elevated temperatures, for precision devices (watches, electric shavers, etc.) and as the basis of lubricating greases designed for elevated temperatures. Silicones with a pheny1:methyl ratio of 0.75 are suitable for even higher temperature use, as lubricants for furnace doors, anti-seizure lubricants for screws and guide-pins, as heat transfer and fire-resistant fluids and other high temperature service. At a pheny1:methyl ratio of about 1.3, both the temperature and oxidation stabilities increase, and the lubricants are relatively radiation-resistant. Such lubricants can be used alone or as constituents of lubricating greases for temperatures exceeding 3 15 "C. Chlorphenyl silicones have improved lubricity and are used as lubricants for clock mechanisms and for thread cutting. Silicones may be used as blends with esters for lubricants of improved lubricity. Blends with diesters are applicable at low temperatures, and those with neopentyl esters at higher temperatures, for example as special compressors oils, hydraulic fluids, instrument oils, and constituents of lubricating greases. Silicones are good lubricants where rolling friction is involved, but relatively poor in cases of sliding friction - particularly in steel-to-steelpairs and in the region of mixed friction. One factor may be the spherical shape of the molecule, which prevents sufficient adhesion of the polar groups to the friction surfaces. Another is the chemical inertness of silicones and also the very low surface tension, which 183
causes excessive spreading on metal surfaces (one of the disadvantages of silicones which impedes the formation of adequately thick films). Improvement in the lubricating and other properties has been achieved in recent years by introducing other elements into the polysiloxane molecule, particularly halogens into the nucleus (the efficiency increases in the order F>Cl>Br) and aluminium, titanium or tin atoms into the chain (49). For example, fluorinated silicones combine the best properties of fluorinated hydrocarbons and silicones. They have ,very good viscosity-temperature characteristics in a wide temperature range, low pour-points (below -65 "C), good lubricity and oxidation stability (above 200 "C). They readily hydrolyse, but may be stabilised to some extent by a tertiary butoxy- group. They are good high-temperature lubricants and hydraulic fluids (50, 51). Silicone oils can be readily utilised for the lubrication of other friction couples, such as bronze or brass on aluminium, copper or zinc under conditions of very low loading of the friction surfaces (52). Some methyl-alkyl silicones, particularly those with long side-chains (1,500 to 10,OOO relative molecular weight), are very good lubricants for aluminium and, with suitable additives, also for stainless steel and titanium up to a temperature of about 200 "C (53).Life expectancy above 200 "C can be multiplied 30 to 500 times by the incorporation of cerium soaps and chelatetype addditives. Silicone oils are excellent lubricants for plastic components. The drawback of silicone oils and greases is their ability to act, even in trace amounts, as separators of lubricants of different origins, paints, varnishes and even enamels. Therefore, surfaces which have been lubricated with silicone must be thoroughly cleaned when the lubricant is changed, and the use of silicone lubricants in lacquer shops should be avoided. The properties which may govern the use of the main types of synthetic and mineral oils are summarised in Table 3.28.
3.2.2.10 The Behaviour of Elastomers in Synthetic Oils Synthetic oils can affect elastomers (sealant materials) chemically and physically, resulting in changes in the properties - and eventually their sealing capability. Chemical changes occur if the oil attacks the polymer chains chemically, especially at elevated temperatures. The elastomer may lose its elasticity and become brittle and resinous. Physical changes of the elastomer can result from two mechanisms absorption of the oil by the elastomer or extraction of soluble constituents, such as plasticisers (both effects usually occur simultaneously). If absorption predominates, the result is swelling; if extraction, shrinkage occurs. Different elastomer types have different resistances to the effects of the various sorts of synthetic oils, at different temperatures. The general rule for selecting suitable oiYelastomer combinations, expressed elsewhere in this book in another context, is that "like dissolves like". Consequently, non-polar polymers (e.g., plastics, thermoplastic elastomers and non-vulcanised rubbers) dissolve in non-polar oils and polar, linear polymers in polar oils. Elastomers, having a network structure, do not dissolve, but swell excessively in chemically-related fluids. 184
Table 3.28. Properties Important for Selecting Base Oils Mineral Oil
PA0
Maximum Temp. ( "C) - in absence of oxygen 200 200 -in presence of oxygen 150 150 Minimum Temp. ( "C) due to viscosity increase 0 to -50 -50 Oto 140 1401 190 Typical VI 150/200 220 Flash-point ( "C) 5 5 Fire resistance 4 4 Antiwear properties 3 2 Lubricating properties Oxidation stability at high temp. (with antioxidant) 4 2 4 2 Waterseparability , 3 2 Airseparability 1 Rust protection (withinhibitors) 1 1 Miscibility with mineral oils 2 1 Compatibility with paints 2 4 Compatibility with elastomers 1 2 Additive solubility 1 1 Hydrolytic stability 4 1 Resistance to evaporative loss 3 1 Toxicity 1 3-10 Cost relative to mineral oils,
Dialkyl Diesters Polyols Poly- Phosphates Silicates Silicones Polyphenylbenzenes glycols Methyl- Methyl- Chlor- Ethers phenyl phenyl 260 200
250 210
300 240
260 200
120 120
300 200
220 100
320 250
305 230
450 320
-50 100 200 5 4 3
-35 145 230 4 4 3
-65 140 205
-20 160 180 3 1 1
-50 0 200 1 2 2
-50 160 200 2 4-5 4-5
-50 200 310 2 5 4-5
-30 175 290 2 5 4-5
-65 195 270
0 -60 275 2 4 4
3 3 3 1 1 2 2 1 1 3 3 2-3
3 2 2 4 3 5 4 3 4 1 2 5
1 2 2 4 4 5 4 3 4 1 2 10
3 5
4 2 2 4 5 5 4 3 4 3 4 10
3
3 5 5 3 5 1 1 5 3 3 1 25
2 5 5 3 5 2 2
3 5 5 3 5 2 2
Scale: 1 = excellent, 2 = very good, 3 = good. 4 = fair, 5 = poor.
4
4 2
4
3 5 3 3 4
2 3 3 5
4 4 2 2 5 5 2 2 10
1 4 3
5
5
3 3 1 50
4 3 2
60
..
1
. .. .
2
These rough guide-lines are insufficient for the practical selection of suitable elastomer/oil combinations. There are examples of swelling of certain elastomers in chemically dissimilar oils. Such oils can, for example, cause some swelling of plasticiser-free elastomers and shrinkage of plasticised rubbers. Therefore, the results of prolonged immersion tests are essential for practical applications, in addition to a knowledge of the relationships outlined above. A summary of experience in matching synthetic oils to sealant elastomers is presented in,Table 3.29. Table 3.29. Guide-lines for the Selection of Sealant Elastomers for mpical Synthetic Oils (Nagdi, K., Parker-Pradifa GmBH, Germany) Synthetic oil type 100
p l y alphaolefins alkylated polyaromatics polyalkylene glycols organic esters phosphate esters silicone oils
polypeffluoroalkyl ethers polyphenyl ethers chlorinated hydrocarbons
Elastomer types to be considered at ( "C): 120 150 200
CR* low NBR* NBR
CSM* ECO* ECO
NBR CR NBR
NR*, SBR* CR*,NBR*
EPDM high NBR EPDM high NBR EPDM FPM CSM* ECO*
EPDM EPDM FPM
FPM FPM FPM
EPDM
AEM* high NBR* high NBR ACM EPDM high NBR EPDM high NBR FPM
FPM MVQ, FPM
EPDM * high NBR*, AEM * FPM FPM FPM
FPM FPM
-
(FPM)** (FPM)** -
* Plasticiser-free compounds to avoid shrinkage.
** Long-term tests are necessary.
ACM = polyacrylate rubbers AEM = ethylene-acrylic rubbers CR = chlorprene rubbers CSM = chlorosulphonated polyethylene rubbers ECO = epichlorhydrin rubbers EPDM = ethylene-propylene-dienerubbers FPM = fluoro- rubbers NBR = acrylonilrile-butadiene-nitrilerubbers SBR = styrene-butadiene rubbers VMQ = vinyl-methyl silicone rubbers
3.2.2.11. Miscellaneous Synthetic Oils The development of synthetic oils is still going on and new compounds are still being discovered. The extent to which they will find application can only be anticipated with difficulty as yet.
186
Oils based on adamantune have generated considerable interest. This compound - tricyclo-3,3,1 ,23.7)decane, discovered by Landa and MachdEek (56) in Czechoslovakian cyclanic crude oil, is one of the most stable hydrocarbons, so this attribute can be expected to be found in its derivatives. Ester oils based on 1adamantylcarboxylic acid (I) (57) and 1,3-bis(carboxymethyl) adamantane (11) (60, 61) have, contrary to analogues with the same number of carbon atoms, high viscosity, low pour-point, good thermal stability but unacceptably low oxidation stability (due to the presence of reactive tertiary carbon atoms): CH2COOR
I
COOR a
C
H
2
C
O
O
R
Esters derived from 1,3-dihydroxy-5.7-dimethyladamantane (III) (58,59)do not have this drawback, although their other properties are similar. Viscosity, as well as viscosity index, can be increased by extending the alkyl chain to the range of over 100. Also, alkylated and arylated adamantanes (IV, V) and the esters derived from them (VI) have been investigated (62). C4H9
CH3 I
R - COO
OCO - R
187
Another group of adamantane derivatives available as lubricants are the highly fluorinatedalkyl adamantanes (190), which are liquid over a wide temperature range and have pour-points mostly below -30 "C and - due to the presence of the adamantane nucleus and the high level of fluorination - extremely high stability. In other studies, a series of esters has been prepared both by esterification of primary alcohol adamantyl methanol with aliphatic monocarboxylic acids, and by esterification of aliphatic polyols with alkyl adamantane carboxylic acids (Z91,230).Thebasic properties of the esters tested are shown in Table 3.30. All ester oils prepared from adamantyl methanols have kinematic viscosities of 3.0 m m 2 d at 100 OC and very good rheological properties at low temperatures.Viscosity index grows with the linear part of the molecule (e.g., dimethyladamantylmethyl laurate has VZ of 125 and the myristate 136). Even with the same length of alkyl chain, VZ decreases when the molecular cross section is increased by the ethyl substituent on the adamantane rings (for example, the VZ of ethyldimethyladamantylmethyl laurate is only 84). The neo-pentyl-alkyladamantyl carboxylates have very low VZ,but very good thermal stabilities, although the other adamantanyl ester oils are also relatively very stable, as the 1-adamantylmethanol forms similar sterically hindered esters as the neo-pentyl-type alcohols. The thermal stability of these compounds depends neither on the degree of substitution on to the adamantane nucleus, nor on the nature of the ester bond. Their oxidation stabilities are relatively good, and comparable with those of conventional ester oils. The mixed esters have substantially lower pour-points than the individual adamantanyl esters. The pour-point rises as the linear chain in the molecule is extended. The volatility of the individual adamantyl esters is very low, that of the mixed esters higher. However, the volatility of the mixed esters is still comparable with that of conventional ester oils and substantially lower than that of mineral oils of similar viscosities. The neo-pentylalkyladamantylcarboxylates have high volatilities due to their low relative molecular weight. The lubricity of the adamantyl ester oils is similar to or better than that of conventional ester oils (e.g., the wear index measured in the four-ball apparatus fluctuates in adamantyl ester oils between 0.5 1 an 0.53 mm scar diameter, compared with that of 2-ethylhexyl sebacate of 0.95 mm). Adamantyl dicarboxylate esters (e.g., bis-3,5-dimethyl-l-adamantyl adipate) are difficult to manufacture, although the process has been patented. Moreover, some esters, whilst initially liquid, solidify after prolonged storage (melting-point50 "C). Some new types of ester oils include halogen oligomers of unsaturated esters, e.g., rnethacrylutes. They are characterised by high viscosity and VZ,low volatility, fair thermal and oxidation stabilities (55) and very good lubricity. Diesters of &phenols have been proposed as heavy-duty aviation lubes (269). Oily derivatives of borazones, pyrazines, triazines, tetra-substituted ureas, tetraalkyl silanes have been reported. 188
Table 3.30. Properties of Some Adamantane Ester Oils Viscosity kinematic at
(IlldS-')
5OoC (3,5-dimethyl - 1-adamantyl-methyl) 14.6 myristate (3,5-dimethyl-1 -adamantyl-methyl) laurate 11.9 (3ethyl-5,7-dimethyl-adamantyl-methyl) laurate 15.9 (alkyladamanty1-methyl) laurate (mixture of esters) 14.3 (1-neopentyl- alkyl- adamantyl) carboxylate (ester mixture) 10.1 bis-(2ethylhexyl) sebacate (for comparison)
8.9
100°C
Brookfield
dynamic (Mas) -17.8"C
VI
Flashpoint ("C)
POWpoint ("C)
Thermal Volatility Lubricity stability (Noack) (+ball wear (% decomp. (% wt.) scar diameter at 400 "C for mm) lh 6h
4.49
-
-
136
222
-10
3
32
12.2
0.52
3.83
240
-
125
214
-25
5
33
20.0
0.5 1
4.32
-
-
84
202
-23
6
29
-
0.53
4.18
680
620
I07
-
-47
7
36
24.8
-
2.9
890
740
3
-
-52
2
21
99.8
-
-
-
134
214
-56
100
100
24.0
0.95
3.23
Tetrualkyl silunes, CH,-Si-R,R',_, (n =3) with 25 to 35 carbon atoms in the alkyls have, for example - for the same viscosity measured at 100 "C - viscositytemperature curves of similar slope (VZ 120 - 155) and similar pour-points (-6 to -42 "C) as the organic silicates, but are resistant to hydrolysis and have flash-points above 270 "C. They are thermally stable up to 370 "C and their oxidation stability (in the presence of anti-oxidants, e.g., sterically hindered phenols or arylamines)and anti-wear properties (in the presence of organic phosphates) are excellent (213). They can be used under the same conditions as organic silicates and also in wet environments. These fluids doped with antioxidants and anti-wear additives are suitable for use in space-craft (237). They are not suitable for use as the oil component of lubricating greases. Examples of triuzine-bused lubricants include the condensation products of cyanuric acid and chlorinated polyglycol ethers of the following type:
/N\
CH,O - [ - CH2- CH2- 0 -In- CH2 -CH2 -0 -C
C - O-CH2- CH2- [ - OCH2-CH2-In- OCH,
II
I
N
N
\C'
I
0 CH2- CH2 - [ - 0 -CH2 -CH,-]"-
OCH3
If n =3, the fluid has a viscosity of 7.18 mm2.s-l at 98.9 "C, flash-point of 200 "C and pour-point of -43 "C. It is hard to ignite, although it contains no phosphate or halogen (214). The per-fluorinated analogue is a fire-resistant lubricant and heattransfer fluid for high-temperature applications (237). 2,3-dialkyl-substituted4-pyrazines with long alkyls are recommended as aircraft oils for heavy-duty reactive (e.g., jet) engines (215), e.g., 2-@-nonylphenoxy)-3methylpyrazine. Ferrocene and urea derivatives are also of interest. Ferrocene (bis-(cyclopentadienyl)iron(JI) possesses remarkable thermal stability, up to about 460 "C; substituted derivatives have been proposed as the basis for hightemperature lubricants. Liquid alkylated ferrocenes of good tribological properties have been obtained. However, their thermal decomposition temperatures are 100 "C lower than that of ferrocene itself, and oxidation leads to a sharp increase in viscosity (287). Alkylferrocenes have also been proposed as anti-wear and anti-seizure lubricants and additives (275). Tetrusubstituted ureus with aromatic substituents are stable up to 370 "C. Some alkyl - or alkaryl - derivatives are liquid at room temperature, however, they do not possess the high stability of the parent compounds. The lubricating properties of the urea derivatives which have been investigated are unsatisfactory because of negative VZ's and only moderate boundary lubrication properties.
190
In general, the significance of synthetic oils is growing, although the demand for them still remains a fraction of that for mineral oils.
3.2.3 Inorganic Liquid Lubricants and Melts The cheapest of all liquid lubricants is water (Table 3.31). Table 3.3 1. Lubricant Properties of Water Density at 20 "C Specific heat at 20 "C Thermal conductivity at 20 "C Surface tension at 20 "C Compressibility between 0 and 10 MPa between 90 and 100 MPa Viscosity (mPa.s) at pressure (MPa) 0.1 at 20 "C 1.002 50 "C 0.546 100 "C 0.282 200 "C Gas solubility (cm3 in 1 dm3water) Oxygenat 0°C 20 "C Nitrogen at 0 "C CO, at 0 "C SO, at 0 "C
(kg.m-3) (kJ.kg-'. K-I) (W.rn-'. K-I) (mN.m-*)
998 4.180 0.6075 72.75
(rn2.N')
0.483.10-9 0.375.10-9
5 10 1.001 1.OOO 0.547 0.549 0.287 0.293 0.139 0.141
20 lo00 0.998 2 0.552 0.301 0.145 49 31 23 171 40
The advantages of water are its very high specific heat and thermal conductivity and its total resistance to oxidation and ignition; it has virtually no effect on polymeric structural materials (with the exception of swelling of certain polyamide polymers by water-absorption). However, water also has many deficiencies. Its viscosity is low, and the change of viscosity with temperature is somewhat greater than that of alkanic oils. This fact is of little significance in practice, because of the rather narrow temperature range over which water can be used in service at normal operating pressures, i.e., 0 to 100 O C . The change is viscosity with pressure is very modest: at 1,OOO MPa viscosity roughly doubles. Water can fairly readily dissolve atmospheric gases, oxygen rather more than nitrogen, and carbon and sulphur dioxides, when it becomes corrosive. Pure water lacks lubricity there is the added risk of erosion and cavitation wear. Water has long been used, alone or in the form of solutions and emulsions, for metal-working (see chapter 5.9.1). Its significance as both labricant and coolant has grown with increasing application of engineering plastics and ceramics (e.g., AhO3) in friction pairs (270). Its viscosity can be increased by the addition of polyhydric alcohols (glycols, pentaerythritol, some sugars), alkali carboxylates (e.g., solutions of lactates, at a 191
concentration as high as 60%)and water-soluble polymers (polyglycols, polyvinyl alcohols, cellulose derivatives); these solutes allow the freezing-point limit to be crossed. Corrosivity can be suppressed by keeping it slightly alkaline and adding anti-corrosive salts, e.g., borates, phosphates, vanadates (IV), tungstates and fluorotungstates,or water-soluble rust inhibitors. These compounds not only inhibit corrosion but also enable it to withstand EHD or boundary friction conditions. For example, spraying phosphate solution on small-radius rails helped reduce stick-slip and, consequently, squealing of trams (204). The use of nitrites and nitrates is prohibited because of health hazards, whilst that of alkanolamines and borated derivatives is growing. Sulphuric acid can be used for the lubrication of the cylinders of chlorine compressors. The sulphuric acid concentration should exceed 93% (it is usually 98%) and the operating temperature must be below 90 "C;failure to meet these conditions leads to severe corrosion. Sodium glass melts (Na-Ca, Na-Ca-Al, Na-B) are suitable for chipless metalworking (205).Their softening-pointis between 700 and 750 "C. Viscosity at the softening-point is about 3 MPa.s, and the viscosity-temperature curve is steep. A problem with glass melts is their adhesion to the surface of the work-piece or tool, and the necessity for and difficulty of removing such deposits by sand-blasting or with hydrofluoric acid. Eutectic phosphatehorate melts are not prone to the same problems (206). For example, the blend of 90% KPO, and 10% Na,B,O, softens at 300 "C and with 10% K,B,O, at 450 "C. The corrosivity of the phosphates diminishes with the addition of heavy metals. The viscosity-temperature curve is comparatively flat, so that these eutectics are applicable over a wide temperature range. They are applied either as powders or aqueous solutions. Low-friction coefficient layers are supposed to form on the surface. In contrast to glasses, lubricant residues can be readily removed with water or acid solutions.
3.3 LUBRICATING GREASES Lubricating greases or plastic lubricants are colloidal systems (dispersions), mostly gels, rarely sols, in which the dispersive (continuous)phase is formed of lubricating oil and the dispersed phase (thickener) is an anisotropic solid which penetrates the liquid phase so that the gel produced acquires properties characteristic of the plastic (solid) state. From a macroscopic aspect, the lubricant may have a consistency resembling butter, or it may be fibrous, spongy or, rarely, grainy. There is a number of monographs describing lubricating greases (63, 68, 69, 75, 98). From a microscopic point of view, plastic lubricants are mostly two-phase systems with an internal structure formed by thickener crystallites (64). The internal structure and size of the crystallites characterise the particular types of lubricating grease as well as their consistency (figs. 3.1.1 to 3.1.19). The consistency of the lubricant is more uniform when the ratio of fibre length to diameter is greater. 192
I93
194
Fig. 3.1. Electromicrophotographsshowing the internal structure of thickener crystallites in lubricating greases made from
1 - Ca-soaps of fatty acids. 2 - Ca-soaps of synthetic acids, 3 - Ca-stearate. 4 - Ca-oleate, 5 - Ca-soap of 25 % synthetic acids and 75 % oleic acid, 6 - Ca-grease with alkalinity 0.04 % NaOH, 7 - Ca-grease with alkalinity 0.18 % NaOH, 8 - neutral Ca-grease. 9 - Ca-grease with 2.2 % oleic acid acidity, 10 - Ca-complex soap from stearic and acetic acids. 1 1 - Ca-complex soap from synthetic acids (C,,,-C20), 12 - Ca-complex soap from 12-hydroxystearic and acetic acids, 13 - Na-soap from fatty acids, 14 - Li-soap from stearic acid, 15 - Li-soap from oleic acid, 16 - Li-soap from 1Zhydroxystearic acid, 17 - Ba-soap of fatty acids, 18 - bentonite thickener. 19 - silica gel thickener
195
The crystallites are swollen by the action of the oil phase, and the liquid phase is usually a saturated solution of the solid phase, provided that the latter is at least partially soluble in oil. Interaction of the phases is indispensable; if it is absent, the result is a pasty lubricant, whilst undesirable interaction yields colloidally instable lubricants (67). It is the colloidal nature of plastic lubricants which differentiates them from liquid lubricants. Gel-base lubricants are, above all else, non-Newtonian, pseudo-plastic or quasi-plastic substances. Their internal structure can be destroyed at high shear stress and repeated strain, and above the threshold value of the shear rate viscosity decreases. In thixotropic lubricants, the structure is restored when the shear rate and viscosity approach the initial values. In rheopectic lubricants, a stronger and more stable structure is formed, and threshold shear rate and viscosity increase above the initial values. Lubricants in which rheological properties differ little between the modified and unmodified states are referred to as mechanically stable lubricants. Rheological properties also change with temperature. These changes may be both reversible and irreversible. Lubricants of which the structure is not destroyed by temperature increase and then restored by chilling, or only destroyed at high temperatures, are referred to as thermally stable lubricants. The structure of the lubricant can deteriorate with time during which the lubricant is at rest. Separation (synerisis) of the oil component occurs and, eventually, the macroscopically homogeneous colloid -1yogel - can decompose into a viscous component and a colloid - xerogel - insoluble in this viscous component (67).Such lubricants are colloidally unstable. Mixtures of different types of greases can be colloidally unstable, even though each grease by itself can be colloidally stable. Therefore, mixing - even casual - of different commercial products should be avoided. Changes in rheological and colloidal properties may be associated with the oxidative stability of the lubricant. Changes in the dispersing and dispersed phases may occur due to the absorption of oxygen and oxidation reactions. In a lubricant which is not stable towards oxidation, this may cause undesirable changes in properties leading ultimately to complete destruction of the lubricant.
3.3.1 Composition of Lubricating Greases The principal components of lubricating greases are lubricating oil and thickener. Many lubricants contain in addition, fillers and additives. These components, along with manufacturing conditions, can have decisive effects on the properties of the lubricants. ASTM D-128-64descibes an overall systematic analysis of greases. Chromatography on silica gel or an ion exchanger may also be employed for separation of the individual components of the grease (164). The lubricant may be broken down into its components by dialysis, according to DIN 5280, or with sodium hydroxide (265),or by extraction with trichlorotrifluoroethane in the Soxhlet aapparatus (166).
Infra-red spectroscopy is suitable for identification of the base oil, thickeners and additives; it can
196
also be employed for the quantitative determination of water (167). GOST 521 1-50 specifies methods for the determination of soap, mineral oil and acid content, and COST 52-50 the determination of water-soluble soaps. ASTM D- 128-64 describes the spectroscopic analysis and conventional analytical methods for determination of Na, Ca, Li, Ba and other metals. The lead content can be determined electrochemically on a platinum electrode as PbO, by ASTM D- 1262. Total chlorine content can be determined by ASTM D-808-63, provided that other halogens are absent. The lubricant sample is burned in a bomb with oxygen with excess added sodium hydroxide solution, the product is dissolved and chlorine is determined as AgCI. Chloride ion and chlorine that can be converted into chloride ion by the action of dilute HNO, can be determined according to DIN 51800. Since chloride ion and ionogenic chlorine may promote corrosion in anti-friction bearings, their concentration in lubricants needs to be monitored. Free bases and organic acids may be determined by CSN 65 631 1, GOST 6707-76, ASTM D-12864 and IP 37/66 or IP 274. Both free and bonded glycerols can be determined by ASTM D-128-64. Solid additives added intentionally to the lubricant can be dctermined by CSN 65-6310 visually in a 0.5 mm layer of lubricant or glass. According to GOST 6479-73, the lubricant sample is decomposed with 10% HCI and dissolved in petroleum ether and ethanol-benzene mixture; the weight of the undissolved residue is then determined. The concentration of graphite and MoS, can be determined by DIN 51813; the lubricant is dissolved in a mixture of benzene and glacial acetic acid and the residue is washed, dried, weighed and analysed to determine the MoS2 content. All solid particles, whether they are lubricants, fillers or special additives are regarded - according to CSN 65-6312 as “physical impurities”; CSN 65-6304 specifies visual evaluation of them between two glass paltes; at the same time, the grooves, if any, formed by rotation of the plates (concentration of “seizing impurities”) are observed (CSN 65-6304 prescribes a brass template for forming a 0.5 mm lubricant layer). A similar method, in ASTM D-1404-64, stipulates the use of a plexiglass plate. A very small amount of physical impurities can be determined by forcing 0.5 to 1 kg of sample through a finemesh screen; this is followed by dispersing the residue accumulated on the sieve in a solvent blend and filtering the suspension so produced through a filtration crucible. This method permits the determination of particle size by selection of the mesh size of the sieve, but it fails to differentiate between lubricating and other additives and physical impurities. GOST 92770-59 determines the size and concentration of particles by means of an automatic particlecounting microscope of 60 x magnification. Similar methods are provided by FTMS-3005.2 and IP 134. A method which determines impurities in lubricants by torque in a miniature ball-bearing is also available (168). CSN 65-6317 and COST 1036-75 describe methods for the determination of impurities insoluble in ethanol-benzene mixture and boiling water. CSN 65-63 16 determines physical impurities insoluble in a 10% HCI-petroleum ether mixture. It differs from GOST 6479-73 in that it does not stipulate the dissolution of the residue remaining on the filter in boiling distilled water. Both methods fail to differentiate the insoluble lubricating additives, graphite, MoS2 and the like, from physical impurities insoluble in the product concerned or water. n-Hexane insolubles can be determined by ASTM D-128-64; this method can also be used to measure insoluble physical impurities and solid additives, as well as the concentration of soap, inorganic fillers and asphaltenes. The IP 5 standard contains a method for the determination of the ash-content of lubricating greases. The concentration of water in the grease can be measured in the same way as that in oil by CSN 656306, GOST 1044-41, DIN 51-682, ASTM D-95-62 and 1P 74/64.
-
3.3.1.1 Oil Components of Greases The properties of the oils which constitute lubricating greases affect a number of essential properties of the latter. Greases mostly contain mineral oils, rarely synthetic 197
oils. Mineral oils are significantly cheaper and, mostly, readily available. Some of them are also suitable for the manufacture of lubricants of exceptional properties. Lubricating greases based on mineral oils have proved to be efficient lubricants for space craft engines and instruments. The problems of extreme temperatures and vacuum proved to be much less troublesome than had been anticipated, provided oils of suitable quality were used and the components (bearings, gear-boxes, etc.) lubricated with the greases were perfectly sealed (70).Mineral oils have better radiation resistance than the majority of synthetic oils.
Mineral Oils The relationships between the composition and properties of the mineral oil and the lubricating grease it forms are complex and inadequately known; in addition, they are complicated by the presence of the thickener. The effects of the constituent groups in the oil and its viscosity-temperature properties on the properties of the grease depend on the type of thickener used. Greases containing soap thickeners made from cycloalkanic oils of lower VZ are softer, but have higher colloidal stabilities, than lubricants made from high VZ alkanic oils, which are harder but applicable over a wider temperature range because of their flatter viscoscity-temperature curves. Soaps can disperse more easily and form stronger bonds with cycloalkanic oils. It is therefore customary in the trade to disperse the soap in a small amount of such oil and to add the alkanic oil later. Cationic interactions are also important. Thus, lithium lubricants can be made from high VZ oils, whereas low VZ oils are more suitable for aluminium lubricants (containing aluminium stearate) and for greases of a softer consistency. Lithium lubricants are very sensitive to changes in chemical composition. For example, those made from severely hydrogenated oils have lower shear stress limiting values and viscosities. Calcium and sodium lubricants are not so sensitive in this respect, but even here, oils of not too high a VZ are to be preferred. The strength properties of Table 3.32. Properties of Lipophylic Bentonite Gels Made from Mineral Oil and Hydrocarbon Fractions *(72) Apparent viscosity (17) (Pas) at 20 "C and D(s-') 12
62
1428
Strength limit (.r) (Pa) at 0°C 50°C
Oil separability (96)
Bearing Oil? 1380 300 23 24 9 Fractions: 2400 470 30 46 43 alkanes-cycloalkanes monoaromatics 2570 500 32 30 25 diaromatics 2000 410 25 15 1.4 triaromatics 550 160 15.8 12 1.1 polyaromatics 830 210 18.5 13.6 2.5 * Bentonite lubricants containing 16% thickener. t Mineral oil of viscosity 50.6 mm'.s-' at 50 O C and 6 rnrn2.s1 at 100 "C, pour-point -10 "C.
198
1.4 2.5 2.2 1.5 1.2 1.9
the complex calcium greases are not affected by the concentration of heavy aromatics in the oil (71).The opposite rules prevail for lubricants made with nonsoap bentonite thickeners. As Table 3.32 illustrates, the viscosity, limiting shear strength and colloidal stability of these plastic lubricants decreases with aromatic content, particularly multi-nuclear species. Polar constituents (polycyclic aromatics and resins) enhance the efficiency of thickeners and the colloidal - and sometimes thermal - stability of the calcium and sodium lubricants. Thus, stearate and oleate lubricants made from distillates may be more stable than those made from raffinates (73).Lubricants made from vacuum distillates or even from extracts usually have high strength, viscosity and colloidal stability. However, polar constituents adversely affect the quality of aluminium lubricants and may even inhibit the formation of gels (68).The presence of resins in the oil in complex calcium lubricants reduces their shear stress threshold value, resistance to degradation and the thickening effect of soaps (71). Lubricants of high oxidation and thermal stabilities should be prepared from alkanic raffinates of high oxidation stability, which also confers a good response to oxidation inhibitors. This is particularly true for lubricants containing soap thickeners, since the service life of lubricants operating at elevated temperatures is directly related to their oxidation stability, and metallic soaps catalyse the oxidation of oils. However, greases made from highly-refined oils may tend to harden at high temperatures, and the viscosities of such lubricants may decrease with the depth of the oil refining process. By contrast, with a highly de-waxed oil, the viscosity and shear strength of the lubricant is higher and the strength-temperature characteristics better (74). From the stand-point of applications, the viscosity of the oil should be approximately equivalent to that which would be selected if the oil alone were to be employed for lubrication (75).However, viscosity affects both the structure and the stability of greases. For example, in some lubricants containing simple soaps, soap crystallisation and the formation of clusters is suppressed with increasing oil viscosity, which causes shorter and thicker soap fibres to be formed and produces softer lubricants. The use of oils of too high viscosity may cause the formation of gritty, granular lubricants of low stability, and it can happen that no grease is formed at all. In contrast, it has been observed that the colloidal, physical and thermal stabilities of complex calcium and silica gel lubricants depend less on the viscosity and more on the composition of the oil (76,79). The viscosity and viscosity index of the oil component also affects the rheological properties of the lubricant. Its pumpability at low temperatures depends on both viscosity and viscosity index of the oil, although the lubricant structure and the nature of the dispersed phase are co-active. Lubricants containing oils of lower viscosity and higher viscosity index are preferred for low-temperaturelubricants, in order to prevent solid parafins separated from the oil at low temperatures from affecting the primary structure of the plastic lubricant. According to Sinicyn (69).the importance of low pour-point of the oil is often over-estimated. The suitability of an oil for the manufacture of a low-temperature 199
grease is not governed only by pour-point. Sometimes, a very small amount of solid paraffin makes the oil congeal at a temperature at which the grease made from this oil operates without any trouble. This is attributable to the effect of the thickener, which surpasses the effect of the paraffin. The magnitude of such phenomena depend on the type and purity of the thickener and on the presence of free acids and bases. Therefore, viscosity may be more influential than pour-point in determining the operational suitability of a plastic lubricant at a specific low temperature. Also, the magnitude of the torque on a bearing lubricated with a grease depends on the viscosity of the oil component of the grease. The viscosity of the oil component considerably affects the colloidal stability of the grease. Lubricants containing less viscous oils usually tend more towards separability of the oil, even when these oils contain higher amounts of thickener. Among all the properties of greases mentioned, the viscosity of the oil employed predominates over the influence of its chemical nature and composition; this principally applies to lithium, sodium and some other hydrocarbon lubricants (78). By contrast, it does not apply to complex calcium and silica gel lubricants (79). Lubricants which contain less viscous, mainly alkanic oils are more transparent. Oil viscosity has only a slight effect on shear strength limit, penetration, mechanical stability and some other properties of greases. The constitution of oil greases, relative molecular weight and flash-point affect evaporative losses of a lubricating grease and the temperature limit of its applicability in hot environments. The mineral oils currently used for lubricant grease manufacture are chiefly medium to high VI raffinates, rarely distillates and extracts of low VI. Oil viscosity varies over a wide range, however, values of 15 - 60 mm2.s-l at 40 "C are prevalent. The higher viscosity oils are mostly used for high-temperature lubricants with superior anti-corrosive properties and higher load-carrying capacity and for lubricants operating under high vacuum conditions.
Synthetic Oils Because of their relatively high prices, synthetic oils are mainly used for the manufacture of special-purpose lubricating greases. Ester and silicone oils are the most often used, plus, less frequently, polyalkene and polyfluoralkene polyesters, and polyphenyl ethers, especially for radiation-resistant lubricants. The particular properties of the synthetic oils are reflected in the lubricating greases made from them. In the synthetic oil used for special bentonite or lithium greases, if the hydrogen atom is substituted by deuterium (239), products of higher thermooxidative stability are obtained because of the substantially greater bondstrength of C-D compared with C-H (202,240).
200
Ester-based Greases Lithium greases containing adipates, azealates and sebacates of c6-Cl 2 aliphatic alcohols (principally 2-ethylhexanol and iso-octanol) have good lubricating ability, excellent low-temperature properties, low corrosive tendencies, high oxidation stability and low volatility. Their service temperature range is wide, ranging from -50 "C (sometimes -70 " C ) and 100 "C and more. Higher temperature limits depend on the nature of the dispersed phase. The disadvantage of ester-based lubricants is a tendency to attack natural and synthetic polydiene rubbers?but this adverse effect on rubbers is reduced if isostearic acid is used in the preparation of the esters (80). The physical properties of some of these lubricants are shown in Table 3.33. Table 3.33. Properties of Greases Made from Lithium 1Zhydroxy Stearate and Ester Base Oils Ester Base Oil Penetration (IO-lrnrn) Penetration after 10.000 strokes, % changes Drop-pint ( "C) Evaporative Loss (%, 22 h, 177 "C) Oil Separation (%, 30 h, 100 "C) Rubber swelling (a) A = di-2-ethylhexyl azealate
C = pentaerythritol isostearate
A
B
C
D
222 +I7 192 10.3 0 78
314 0 192 9.0 0 75
248 +8 197 5.2 0 6
21 1 +37 197 10.5 0 17
B = di-2-ethylhexyl sebacate D = hexadecyl isostearate
Silicone-based Lubricating Greases Some outstanding, but also negative, properties of silicone-based lubricating greases result from the properties of the silicone base oils they contain (81).In contrast to lubricants based on mineral oils, silicone-based lubricants are characterised by relatively small changes in penetration over a wide temperature range, which enables them to be used at substantially low and high temperatures (fig. 3.2). Among other merits of silicone-based lubricants are exceptionally high thermal and oxidation stabilities, which enables them to be used for a relatively long period of time in applications in which they are exposed to elevated thermal stress (fig. 3.3). The low surface tension of silicones causes any traces of them on a surface to appear as a separate layer. Because of this, surfaces must be carefully cleaned when switching from a silicone lubricant to one of another kind. For the same reason, silicone greases must not be used in lacquers and paint shops. Another deficiency of these lubricants is their low resistance to pressure. Silicone lubricants are hence used only in plain bearings exposed to low-pressure loads, e.g., with friction-pairs steel-to-bronze, brass, aluminium or cadmium. They are also suitable for plastic and elastic bearings and for friction pairs combined of these materials and metal.
20 1
Table 3.34. The Properties of Some Silicone Base Oils and Greases Prepared from Them (82) A ~~~~
B
C
D
~
Oil Properties*: 100 75 900 300 Viscosity (mm2.s-') at 25 "C 74 55 600 190 37 "C 25 17 170 30 98.9 "C 300 300 200 215 Viscosity Index -74 -68 -57 -49 Pour-point ( "C) 260 287 210 294 Flash-point ( "C) 18 27 15 46 Volatile Loss (%) after 7 days, 232 "C) 27.4 25.5 22.7 Surface Tension (mN.m-') -74 to 230 -68 to 205 -57 to 177 -49 to 230 Operating Temperature Range ( "C) Grease Properties?: +20 +10 +50 +19 Penetration after 1 6 strokes, (% change) Drop-point ( "C) 210 207 >305 >305 Oxidation Stability (in bomb pressure 10 10 14 20 drop in kPa after 500h at 98.9 "C) Oil Separation (%) (after 30 h at 177 "C) 4 3 0.1 4 Volatile Loss (%) (after 30 h at 177 "C) 5 4 6 2 Operating Temperature Range ( "C) -75 to 180 -70 to 175 -50 to 175 -45 to 235 *Silicone Oil Type A - phenylmethylB - chlorphenylmethylC - alkylmethylD - fluorphenylmethyl-
tThickener Type. A - Li stearate
B - Li stearate C - Bentonite D - polytetrafluorethylene
I
-60 -40 -20
0
20
40 60 80
100 120 UO 160
OC
Fig. 3.2. Comparison of penetration curves of Li-based lubricating greases manufactured from A - mineral oil and B - silicone oil
202
Table 3.35. Advantages and Disadvantages of Silicone-based Lubricants Type of silicone base oil
Advantages
Disadvantages
Fluorinated siloxanes
Resistant to solvent and chemical attack, usable from -45 to 230 "C, high loadcarrying capacity. Usable from -85 to 220 "C
High price, spreads on surfaces, cannot be coloured.
Phenylmethyl siloxanes
Dimethyl siloxanes
Usable from -55 to 235 "C
Alkylmethyl siloxanes
Usable from -80 to 200 "C, miscible with lubricants containing mineral oils, can be coloured, usable for lubricating diferent metal friction pairs (steel-Al. steel-brass)
Low load-canying capacity. Spreads on surfaces, cannot be coloured. Immiscible with lubricants containing mineral oils, spreads on surfaces, cannot be coloured. Usable from 175 to 200 "C for short time. Spreads on surfaces.
770.
700-
g 600800w 400a 3 300200100-
Fig. 3.3. Comparison of oxidation stability of Li-based lubricating greases manufactured from A - mineral oil and B - silicone oil (according to ASTM D 942 or DIN 51808)
The silicones of dimethyl siloxane, phenylmethyl siloxane, akylacetyl siloxane and chloro- or fluorophenylmethyl siloxane types are the oil components used for these purposes. The preferred thickeners are primarily the lithium-based soaps (stearates and 12-hydroxy stearates), polyaryl ureas, polytetrafluorethylene, bentonites and carbon black. The characteristicproperties of these oils and of greases made from them are shown in Table 3.34. The advantages and disadvantages of lubricants made from different silicone oils are listed in Table 3.35. 203
Polyalphaolefin (PAO) Lubricating Greases Lithium-base lubricants prepared from polyalphaolefins may be employed in a very wide temperature range, from -55 to 180 "C. Lubricants suitable for -70 to 240 "C(e.g., those based on I-decene oligomers) are also available. These lubricants have additional advantages, namely, good oxidation stability, flat viscositytemperature curves, low volatility, good lubricity (particularly at higher loads) and respond well to additives. Table 3.36 illustrates the properties of these lubricants. Polyalphaolefins have been used to only a limited extent so far, but considerable development is expected (83). Table 3.36. Properties of Lithium-based Greases based on Polyalphaolefins Penetration (worked) (IO-lrnm) Penetration after 100,OOO strokes (10'rnm) % of penetration change Drop-point ( "C) Oxidation stability (pressure drop kPa after 500 h) Oil separation (% weight) (after 30 h at 177 "C) Evaporative loss (% weight) (after 22 h at 177 "C) Operating temperature range ( "C)
315
325 3 >260 "C 300 4 7
-65 to 235
Perfluoropolyether (PFAE) Greases The perfluoropolyether lubricating greases are prepared from non-soap thickeners, like bentonite, zinc oxide, boron nitride, phthalocyanines and, chiefly, polymers of tetrafluorethylene and fluorinated ethylene propylenes. Many of them are not particularly efficient. For example, 20 to 38% dispersed phase in the PFAE is required for the preparation of a grease of consistency 2. The properties of some perfluoralkylether oils and greases made from them (thickened with polytetrafluorethylene) are shown in Table 3.37. Polyphenyl ethers have so far found little application as oil components for lubricating greases. They may, however, be used in the future if a method can be found of mixing them with other components to improve their poor low-temperature properties. Some of their notable properties, particularly their radiation resistance, could stimulate such a project (86).
3.3.1.2 Thickeners A thickener is a substance which converts a fluid into a plastic state. The concentration and the properties of the thickener substantially affect the structure and properties of the plastic lubricant - the lubricating grease. The consistency and the colloidal stability of a lubricating grease grow with increasing thickener concentration. The properties of the thickener and the required consistency grade determine the thickener concentration, which varies mostly 204
Table 3.37. Properties of Some PFA Ether Oils and Greases Made from Them Oil Average relative molecular weight Viscosity (mm2.s-') at 37.8 "C at 98.9 "C Pour-pint ( "C)
2000 18 3.3 -58
3600 85 10.3 -42
5800 270 26 -35
7800 495 43 -29
Lubricating Grease Thickener content (F'TFE) (% weight) 17.0 15.5 14.7 14.1 Penetration (after working) (10-'mm) 285 283 282 275 Penetration (after 105 strokes) (10'mm) 346 315 312 315 21 11 11 15 Change of penetration (%) Roll stability test (ASTM D-1831) 18 16 27 (%change after 6 h) 37 230 230 230 230 Drop-pint ( "C) 4 4 3 3 Oil separation (% weight after 30 h at 100 "C) 65 9 2 2 Volatile loss (% weight after 22 h at 205 "C) -55 to 150 -40 to 235 -35 to 290 -30 to 290 Operating temperature range ( "C)
between 7 and 30%. As much as 50% of thickener may be employed, but only in lubricants thickened with hydrocarbons and some organic and inorganic substances with low thickening power. Ash content also grows, however (with the obvious exception of lubricants thickened with hydrocarbon or other special, ash-free thickeners). As in the case of oils, "oxide" or "sulphated' ash is normally determined. The combustion residue of a specified amount of the grease, to which ammonium nitrate has been added, is calcined to constant weight at 775 "C; the result is referred to as oxide ash. Sulphated ash is determined by calcining the combustion residue at 600 "C and again adding ammonium nitrate. After cooling, sulphuric acid is added to ensure acid reaction and the substance calcined at 775 "C to constant weight (CSN 65-6308, ASTM D-482, IP 5 and DIN 5 1-803).
Thickeners used can be classified into several groups according to the scheme shown in Table 3.38. Among lubricating greases manufactured at present, soap-based types are the most important. More than 90% are simple or mixed soaps. Lubricants with complex soaps are, for the time being, in a minority, however they are becoming increasingly popular, particularly calcium and aluminium complex soaps. Among the simple soaps, the proportion of lithium soaps has recently increased, whilst the market share of calcium and sodium soaps has decreased. The proportion of the remaining soaps and lubricants with soap-free thickeners is relatively low (these are mostly used for lubricants for limitqd, speciality applications). For the present, the greatest attention is paid to lithium soaps, complex calcium soaps, organophilic bentonites and polyphenyl ureas.
205
Table 3.38. Summary of Thickeners used in Lubricating Greases
I
I
I
Simple
Mixed
sodium lithium calcium barium strontium lead zinc
sodium-potassium sodium sodium-lithium lithium sodium-calcium calcium sodium-barium barium sodium-lead calcium-lead sodium-aluminium aluminium lithiumcalcium(strontium) lithium-aluminium calcium-lead sodium-lithium-calcium lithium-aluminium-zinc
I
**
I
Complex
I
I
InOTniC
bentonites silica gels carbon black asbestos boron nitride zinc oxide miscellaneous
Organic!(ash-free)* polyalkenes lithium soap (polyethylene, complex Al soap polypropylene, +bentonite poly-1-butene, lithium (sodium) poly-4-butene soap + polyurea poly4methylpolyurea + 1-pentene terephthalamate propylenepolyurea + piperylene inorganic copolymer thickener polytetrafluorethylene miscellaneous tetrafluorethylenecombinations hexafluorpropy lene copolymer cellulose derivatives organic pigments (phthalocyanines, terephthalamates, indanthrenes) polyamides polyimides polyureas (alkyl, acyl, alkacyl ureas)
Organic thickeners also include paraffins, ceresins and waxes used for preparation of hydrocarbon lubricating greases.
.* A new type of soap-based thickener is thixotropic over-based calcium alkaryl sulphonate (containing clystalline calcium carbonate (250).
+
Soap Thickeners Soap thickeners are manufactured by saponification of technical animal and vegetable fatty acid glycerols, or by neutralisation of the higher fatty acids. The animal products used are mostly tallow, lard oil and fish oil and the vegetable products olive, peanut and, to a lesser extent, sunflower, soya-bean, rape-seed and cotton oils. The most important are the C,, and C,, fatty acids, i.e., saturated stearic, palmitic and, to some extent, myristic and arachidic acids and the unsaturated oleic acid and the acids produced from so-called semi-drying oils. Acids with conjugated double bonds are unsuitable. Synthetic fatty acids originating from the oxidation of hard paraffin wax are also used, as well as hydroxy-acids, such as 10- and 12hydroxystearic acids. Of lesser importance are naphthenic, sulphonaphthenic or resin acids and the technical waxes, lanolin (from sheep wool), beeswax, carnauba and montan waxes. Raw waxes may be used, but the physically or chemically modified types, such as distilled, recrystallised, hydrogenated, partially saponified or reesterified montan waxes are more suitable. Lower organic acids, including saturated (e.g., acetic), unsaturated (e.g., acrylic), hydroxy (e.g., tartaric), aromatic (e.g., benzoic) and inorganic acids (e.g., phosphoric and even hydrochloric (91)),are used for the manufacture of complex soaps. Fatty acids are better raw materials than their acyl glycerides, as their saponification proceeds more easily and is more complete, the yield is higher and the soap which results is of more uniform quality. The preparation of soaps from acyl glycerides or acids is, as a rule, the first stage in the manufacture of greases. Ready-made soaps (commercial products) are, however, also used, because the manufacturing process and equipment needed are less sophisticated, less heat is required and batches are more uniform, although the process is more expensive where raw materials are in limited supply and there is limited possibilities for combination and substitution. Simple soaps can be obtained by saponification or neutralisation of the above acids or acyl glycerides by the relevant hydroxide. A mixture of hydroxides is used for the production of mixed soaps. The soap so produced normally disperses readily in oil to form the lubricant. Exceptions here are the aluminium soaps, which are manufactured in two stages. A water-soluble soap (e.g., sodium soap) is produced in the first stage, then in a second stage an aqueous solution of aluminium sulphate is used to precipitate the aluminium soap, according to the following reaction:
6 C,,H,,COONa
+ Al,(S04)3.18H,0
- 2 A1(C,,H3,C00),
+ 3 Na2S04+ 18 H20
The precipitated soap is washed to remove contaminants and dried. Only the dried soap is suitable as a thickener. The term “complex soap” describes metal salts with multiple anions, in which, as a rule, the long anion of a fatty acid and the short anion of some other acid (e.g., acetic acid) or a hydroxyl anion (92) is bound to the same polyvalent metal. Complex soap types include both soaps with one cation (e.g., Ca, Ba, Sr, Mg, A1 and Pb) and those with several cations (e.g., Cr-Pb). Complex soaps of the bivalent metals are
207
normally made from a mixture of higher fatty acids (e.g., stearic acid) and acetic acid: CH, - (CH,),6
- COO
\
CH3COO /"
where M is the bivalent metal. The aluminium complex soaps contain a higher fatty acid, usually combined with benzoic acid: CH, - (CH,),6 - COO
\
They can be prepared by the reaction of a mixture of the fatty acid and benzoic acid and the aluminium isopropylate or its trimer tris-oxyaluminium tri-isopropoxide or tri-isobutoxide: R
I
0 I / *I\
0
I
0
I
RO - A1
A1 - OR
'0' where R is isopropyl or isobutyl. Other types are complex sodium soaps (e.g., a complex of sodium salt of a higher fatty acid and sodium acetate), and lithium soaps (complexes of lithium stearate and various lithium salts). Lubricating greases made from complex soaps have some advantages over lubricants made with simple soaps, e.g., higher drop-points. Some are reversible, i.e., when melted then cooled down again, they regain their original structure. They have better structural stability under shear stress and during processing, better oxidation stability and they withstand the action of water. Complex sodium lubricants, for example, have better water resistance than simple sodium lubricants. These advantages predicate the use of complex lubricants for applications as multi-purpose greases. Complex barium lubricants thus became the first multipurpose greases for the automobile and for many applications in industry. The disadvantage of complex lubricant greases is their limited mutual compatibility, even incompatibility with each other or with lubricants containing simple soaps. For example, the complex sodium lubricant made from furoic acid (furan-Zcarboxylic 208
acid) forms a liquid product when mixed with calcium or lithium lubricants (89). The higher price of complex lubricants is a limiting factor on their more extensive use. In greases, simple, mixed and complex soaps form a fibrous structure, in which the fibres (fibrils) are probably associated by dipoles (e.g.,simple soaps of monovalent and bivalent metals) or by the formation of coordination compounds (e.g.,simple aluminium and complex soaps), as, for example, in the case of aluminium (9491):
where R, = R2 (e.g., C,,H,,) in simple soaps, or R, is an alkyl group (e.g., C1,H3,) and R, is a short alkyl or aryl group (e.g., C,H,) in complex soaps.
The properties of lubricating greases are affected by both the cation and the anion of the soap, but usually predominantly by the cation. Greases are therefore classified by cation type.
Cation Effects - greases based on simple soaps Lubricating greases containing simple soaps have the following characteristics, classified by cation type: Sodium greases have a fibrous or grainy texture. This texture is due to the large size of the fibres of the dispersed soap, which has a lower thickening power than that of calcium or lithium soaps. Sodium lubricants are hydrophilic and they absorb water, but they decompose after prolonged contact with water. Water has a deleterious effect on the mechanical properties if a sodium grease, reducing its strength limit and drop-point and aggravates iron corrosion. However, if the moisture content of the system is low, sodium lubricants can, by absorbing the moisture, protect against corrosion. In some applications, e.g., in the lubrication of textile machinery, the solubility of sodium lubricants in water can be advantageous, since soiled fabrics can be cleaned by warm water. Sodium lubricants can be exposed for a longer time to higher temperatures, even above their drop-points; they liquefy, but regenerate on re-cooling. The drop-points of most sodium greases vary between 150 and 190 "C, and the working temperature ranges between -20 and 110 (130) "C, depending on the lubricant composition. Typical properties of brick (block) and fibrous sodium lubricants are illustrated in Tables 3.39 and 3.40 (68).
209
Table 3.39. Brick (Block) Sodium Greases Penetration (unworked) (lO-1mm)
35140
45155
40160
20125
30140
Drop-point ( "C) 135 190 120 210 205 Na - soaps (% weight) 25 *20 35 42 40 Water (max.,% weight) 0.5 trace 2.0 trace 8 Mineral oil viscosity (mm2.s-'l "C) 20-25198.9 65-70137.8 43-48137.8 38-40D8.9 38-40198.9 *Also contains 3% calcium soap.
Table 3.40. Fibrous Sodium Greases NLGI Grade
0
1
2
3
4
5
6
Penetration (worked) (1O-lrnrn)
3351385 3101340 2651295 2201250 1751205 1301260 651115 8-10 11-14 14-17 18-22 27-41 35-40 155 157 160 163 166 170 0.4 0.5 0.6 0.8 1.0 1.4 0.3 0.4 0.5 0.5 0.6 0.7 0.2 0.3 . 0.3 0.4 0.4 0.5
4-6 Na soap (% weight) Drop-point ( "C) 150 Water (% weight, max.) 0.3 Free alkali (8 weight, rnax.) 0.2 Free fatty acids (% weight, max.) 0.1 Mineral Oil viscosity at 38.8 "C (mm2.s-1) ASTM colour (max.)
-
65 70 3
Potassium greases are of soft consistency and emulsify readily with water. Lithium greases are of smooth, butter-like consistency. Unlike those described above, they are resistant to water. Another virtue is their stability at high temperatures (drop-point between 180 and 200 "C),which makes them virtually allpurpose greases, suitable for temperatures ranging between -60OC and 120-140 "C (depending on composition).Lithium greases also have high mechanical stabilities and the ability to retain their structures even under very high strains. The composition of lithium greases has a considerable effect on their properties. The drop-point of lithium lubricants decreases - due to higher solubility of the soap with increasing length of the alkyl chain in the fatty acid and with its degree of unsaturation, and is lower in cycloalkanic than in alkanic oils (Table 3.41). As the length of the fatty acid alkyl increases, bleeding of the oil due to syneresis decreases. This decrease is more pronounced in cycloalkanic oils. Lithium greases are expensive, high-quality lubricants and are therefore manufactured from high-quality saturated fatty components (Table 3.42),most frequently from stearic or 12-hydroxystearic acids or their mixtures. Calcium greases are available in different consistencies depending on soap content, ranging from briquettes to semi-liquid lubricants. They are smooth, have a fairly good resistance to water and average mechanical stability. Their good colloidal stability is conditioned by the water content (0.5 to 5% depending on soap type), which is probably bound as a complex. Since calcium greases decompose when they lose water, their upper limit of applicability is determined by the temperature at 210
Table 3.41. Correlation between Fatty Acid Chain Length and Drop-point of Li-greases Containing 12% of Li-soap (91) Fatty Acids
Drop-point ( "C) Cvcloalkanic Oils Alkanic Oils
Saturated: C8
-*
-*
201
-* -*
-*
ClO c12 '14 '16
and c22 Unsaturated: Ricinoleic acid Oleic acid *Li-soap incompatible with oil. c20
-*
202 188 183
190 181
153 182
153 181
Table 3.42. Correlations between Fatty Raw Material mpes and Properties of Lithium Greases (92) Fatty substance stability Tallow Hydrogenated tallow Stearic acid Hydrogenated fish oil Hydrogenated castor oil (12-hydroxy-stearic acid) 1 = best, 4 =worst ratings
Chemical Mechanical stability stability
Droppoint
Water- Compatibility resistance with oil
4
2
3
1 1
I
1
3 3 3
1
4 1 1 1
1
1
4
3
1
1
2 3 3
2
which water - present as a structure-stabiliser - evaporates. These lubricants become soft when heated and decompose completely and irreversibly at temperatures above their drop-points. They do not protect against corrosion as they do not absorb free water if it is present. The drop-points of simple Ca-greases are relatively low, mostly 60 to 110 "C. The composition of the soap used is, however, also important in this respect. Calubricants, for example, containing the soaps of sulphonic acids can have drop-points as high as 150 "C. Similarly, lubricants containing Ca-soaps of hydroxy-stearic acid can have drop-points as high as 145 "C and, since they are virtually water-free, they can be used up to 120 "C. As compared with sodium or potassium lubricants, calcium lubricants have lower apparent viscosities if they contain the same oil. They are therefore more pumpable at the same consistency. Calcium lubricants are less thixotropic than other plastic lubricants except for sodium lubricants. If they are exposed to shear stress and then left at rest, they tend - on prolonged storage towards hardening (rheopexy). 21 1
Barium soups are comparatively rarely used, on toxicity grounds. They have very high drop-points (about 200 "C) and can therefore be used at relatively high temperatures (up to 170 "C). They have high water resistance, good mechanical stability and the ability to retain their initial plasticity and structure even after repeated heating and cooling. Their deficiencies include a tendency to soften or to change their structure, poor low-temperature properties and the toxicity of barium. Since Ba-soap has a lower thickening power, lubricants of the desired consistency are produced by adding more soap and therefore at higher cost than in the case of lubricants with other metal soaps. It has been suggested that barium lubricants with high drop-points can only be made from complex barium soaps of fatty acids (93,94). Lead and zinc greases are soft, because of the low thickening power of lead and zinc soaps. The special attribute of zinc lubricants is their high water-resistance; lead lubricants form lubricant films with high load-carrying capacities, superior shear resistance and low rolling friction. Although lead lubricants are insoluble in water and resistant to the formation of water emulsions, they tend to separation of oil and soap in a moist environment. Aluminium greases are semi-fluid and ductile, because of the low thickening power of aluminium soaps. This property of Al-soaps is associated with the process by which they crystallise in oil; unlike other types of soaps, Al-soaps crystallise as small, symmetrically-shaped particles. They form no fibres, needles, ribbons or the like. Aluminium lubricants show very good resistance to both hot and cold water. Lubricating films have high load-carrying capacity and adhere well to metal surfaces. Adhesion and anti-corrosion abilities increase with the viscosity of the base oil used. On the other hand, aluminium lubricants are mechanically unstable and their shear stability and stability to working depends on the the type of Al-soap used. The structure of these lubricants changes considerably when they are heated and rapidly cooled, which causes them to be converted into rubbery, cohesive substances, Pas
121 o c
Fig. 3.4. Relationship between apparent viscosity and temperature of the lubricating greases A - aluminium-. B - sodium- and C - calcium greases (95)
212
with a drop-point of about 120 "C. They can operate up to a temperature of 80 "C. As shown in f i g . 3.4, aluminium soaps have unusual graphs for the relationship between apparent viscosity and temperature. Unlike lubricants with other cations, the pumpability and rheological properties of Al-lubricants at low temperature depend more on the nature of the soap than on the viscosity and pour-point of the base oil (96).
Cation effects - greases from mixed Soaps Mixed soups can be made either from simple soaps of different cations or from blends of simple and complex soaps of different cations. The components must be compatible with each other. A liquid blend can result, for example, if a sodium grease containing soap formed from a blend of low and high molecular weight acids is mixed with a calcium or lithium grease. A lubricant blend containing mixed sodium-barium soaps of fatty acid and the sodium lubricant formed from castor oil soap is liquid. The attributes of those simple and complex soaps which form mixed soaps find application in plastic lubricants containing mixed soaps. Sodium soaps are usually mixed with calcium or aluminium soaps. The resulting lubricants retain the high drop-points, good anti-corrosion abilities and good shear stabilities of sodium lubricants, whilst their water-resistance is higher and their structure is smoother. The ratio of sodium to calcium or aluminium soaps in such lubricants is usually 4-5: 1. Sodium-calcium greases are more common than sodium-aluminium greases (68,98). Sodium-calcium greases are characterised by a smoother structure and a higher shear strength. Some of them are insoluble in gasoline (e.g., a blend of 32% hard sodium grease, 52% soft potassium grease, 4% graphite and 12% glycerol). Sodiumlithium greases with a small amount of calcium soaps provide low torques at normal temperature (99). The resistance of sodium lubricants to water can be substantially improved by the addition of 0.5 to 1% of magnesium, cadmium or zinc 12-hydroxy-stearate (68). Sodium-lead lubricants are smooth and form lubricant films with higher loadcarrying capacities, and sodium-barium lubricants have higher thermal stabilities. Calcium soaps are generally blended with lead soaps, particularly with lead naphthenate, to increase their resistance to pressure. The addition of the barium soap to the calcium soap extends the operating temperature range of calcium lubricants and reduces their tendency to decompose at elevated temperatures (above 120 "C). The properties of lithium lubricants can be further improved by combining them with other soaps. For example, the combination of lithium with calcium or strontium soaps in the ratio of 3-6:1 improves the structural stabilities of lubricants under shear stress (100). The addition of potassium soap has a similar effect and a smooth structure is achieved directly without homogenisation (101). Aluminium soaps reduce the tendency to water-separation of lithium lubricants (102). Zinc soaps in combined lithium-aluminium lubricants improve their oxidation stability (103). 213
Anion Effects The properties of greases made from mixed, simple or complex soaps also change with changes in their anions, both with respect to chain-length and shape, and to the unsaturated bond content. As the alkyl group in a soap from saturated fatty acids becomes shorter, the solubility of the soap in oil decreases, the drop-point increases and the lubricant becomes harder (higher consistency), but its colloidal stability usually diminishes. By contrast, as the length of the alkyl chain increases, the dispersivity and, consequently, the thickening power of the soap increases, the size of its fibres decreases and, at lower soap concentrations, higher viscosity, strength limit and colloidal stability of the lubricant are achieved (110). Soaps of stearic acid have the greatest thickening power. The most favourable balance of plastic lubricant properties can be achieved if a blend of fatty acids of differing alkyl chain lengths is employed. The fractional composition of the fatty acids can be extended to an optimum which favourably affects the rheological and other properties of lubricants. However, these properties can deteriorate if the optimum is exceeded (110). In the manufacture of calcium greases from synthetic fatty acids from paraffin wax oxidation, the most favourable properties are achieved if the soap contains C to C20acids and no acid over C, (112). '9 The addition of 4 -10% oxidised petroleum hydrocarbons rich In aromatics (aniline point below 75 "C) into the fatty substances used for the manufacture of the soap can avoid hardening of sodium greases during storage (113). Naphthenic acids are not suitable alone for the manufacture of greases; they give lubricants of low stability. If added to fatty acids in small amounts (0.5 to l%), they can, however, improve some properties, e.g., the adhesion capacity of aluminium lubricants, the colloidal and mechanical stabilities of sodium and lithium lubricants, etc. (91, 115).
The higher hydroxycarboxylic acids deserve a closer look, especially 12hydroxy-stearic acid. It is mainly suitable for the manufacture of lithium greases characterised by high mechanical, colloidal and thermal stabilities. Lubricants (e.g., lithium greases) with higher colloidal stabilities, lesser tendency to hardening and higher water-resistance can be obtained by using a blend of 35-40%hydroxy acids (e.g., 12-hydroxy-stearic acid) and 65-60%unsaturated fatty acids (116). Petroleum sulphonic acids are only used to a limited extent for the manufacture of soaps for plastic lubricants; however, they have favourable effects on the structure of the lubricants (127,118).
The degree of unsaturation of the acid anions has a very significant effect on the structure and properties of lubricating greases (119,123). Soaps from unsaturated acids dissolve better in oil, so that the lubricants produced have better mechanical, shear and colloidal stabilities and higher lubricant film strength (124).However, they have a deleterious effect on water-sensitivity and, particularly, on oxidation stability and they reduce the drop-point of the lubricants. Generally, fatty raw materials of iodine number above 35 are regarded as less suitable and those above 70 unsuitable for the manufacture of plastic lubricants. In particular, the presence of conjugated double bonds impairs oxidation stability. 214
The fatty acids from animal fats and vegetable oils always contain a certain amount of unsaturation acids. By contrast, synthetic fatty acids from oxidation of solid alkanes or paraffin waxes are fully saturated. The calcium greases made from these synthetic acids are worse in service than those made from natural fatty acids; their rheological properties at low temperature are less satisfactory, their shear strength threshold value, viscosity-temperature and viscosity-shear relationships are worse, their colloidal stability is diminished and their thixotropy is higher. These deficiencies can be avoided in calcium-based lubricants if they are blended with unsaturated fatty acids; in addition, the thickening efficiencies of the soaps improves with increasing unsaturated acid content. For example, a blend of 60-75% unsaturated acids (such as oleic acid) and 25-40% synthetic fatty acids is necessary to obtain high-quality calcium greases. The unsaturated acids in such blends increase the thickening effect of the soap and the saturated acids favourably affects the thermal and chemical stabilities of both soap and lubricant. The stability of the lubricant is, however, impaired if the proportion of unsaturated acids exceeds 75%.
Unsaturation adversely affects the properties of lubricants containing hydroxylated fatty acids. Complex calcium lubricants prepared from castor oil and 12-hydroxy-stearic acid (Iodine Number 6.9 - 33g 12/100g)have good properties in service and their mechanical stability exceeds that of lubricants made from soaps of stearic or synthetic fatty acids. However, the presence of unsaturated acids (ricinoleic or oleic acids) in technical 12-hydroxy-stearic acid reduces the thickening power of the soaps. The strength limit, viscosity, drop-point and colloidal and oxidation stability of lithium lubricants made from these soaps decreases with unsaturation.
Complex-soap Greases The manufacture of greases based on complex soaps depends the interaction of anions of long-chain fatty acids and anions of short-chain organic acids and inorganic acids - and even hydroxyls - to produce coordination compounds. The most important are calcium, calcium-lead, lithium, barium and aluminium lubricating greases; sodium lubricants are less important. As compared to simple calcium lubricants, calcium lubricants based on complexes, mostly with acetates, have many positive attributes - higher drop-point, high water-resistance, applicability at elevated temperatures, the ability to retain their structure (consistency) at elevated temperatures and good anti-wear properties (104). Anti-wear properties depend to a considerable extent, however, on the ratio of acetic and fatty acids in the soap complex; they improve with increasing acetic acid content in the complex up to a ratio of 3 - 4:1. The ratio of low- and highmolecular weight acids in the complex soap, plus the soap content of the lubricant, also affects their properties in service, such as strength limit and, to a lesser extent, their rheological properties and the colloidal stabilities of the lubricants. The thickening power of the soap increases with increasing molecular ratio of low- to high-molecular weight acids, but mechanical stability decreases and the thixotropy of the lubricant increases. The chainlength of the high molecular weight acid is also a factor. In a complex calcium lubricant with useful, balanced characteristics, the ratio of acetic to stearic or palmitic acid is about 3 1 and the concentration of the stearate soap is about 9 - lo%, palmitic 12 - 13%(ZOS).
215
However, the effect of the ratio of 12-hydroxy-stearic acid to acetic acid on lubricant properties is substantially less. This ratio affects, in some measure, the strength limit, but it does not affect the lubricity, colloidal or mechanical stabilities or the resistance of the lubricant to temperature and water (106). In contrast to lubricants made from stearate-acetate complexes, the consistency of lubricants based on hydroxy-stearate complexes changes very little with minor changes in alkalinity or acidity (108). The main deficiency of complex calcium lubricants is that they harden at elevated temperatures and tend - particularly the higher consistency types - to rheopexy after prolonged storage and use. They are colloidally unstable when exposed to pressure, and their pumpability is worse at lower temperatures. Also, their mechanical stability is worse and at higher shear stress they soften and liquefy relatively rapidly, particularly the softer types of 0 and 1 consistency grades. Because of these deficiencies, these lubricants are not considered by some users to be suitable or multi-purpose lubricants (107). Mixed complex calcium lubricants, such as CaAl, Na-Ca-Al and (mainly) Ca-Pb, of which the properties are somewhat better, are also used, however, they too tend to harden during service and storage. This effect may be alleviated with anti-oxidants (109). Complex barium lubricants vary in structural properties; the structure is butterlike if the hydroxyl anion is present in the complex-builder but changes to fibrous if the lower organic acid forms a complexing anion. These products are mainly fibrous, with drop-points between 190 and 250 "C. Conventional barium lubricants are comparatively mechanically stable, water-resistant and provide corrosionprotection. Because of a high thickener content, their pumpability at low temperature is poor. This deficiency can be overcome by selection of a suitable oil component (107). Complex aluminium lubricants are mostly prepared from hydroxy-stearic acid and other high molecular weight acids, also benzoic and phthalic acid (91).They have some good multi-purpose characteristics, such as high drop-points (about 250 "C), excellent water-resistance and good thermal, colloidal and shear stabilities. Because of the properties of the dispersed phase, their pumpability at low temperature is relatively good. They are highly pressure-resistant, which can be further enhanced by additives, to which they are very responsive (107). Some of the properties of complex sodium lubricants are better than those of the simple sodium lubricants. They are prepared from the complex soaps of high- and low-molecular weight organic acids (e.g., stearic, benzoic, acetic and tartaric acids). They have high drop-points (above 200 "C) and better water-resistance than lubricants made from simple soaps. Sodium soaps made from terephthalic acid have high water-resitance and mechanical stability. Complex lithium lubricants of a conventional composition - stearic acid-acetic acid-mineral oil - have not found any special application in the market, even though their high water- and temperature-resistanceis good and their working range is large, from -30 "C to 150 OC. However, more recently-developed greases based on Li salts of 12-hydroxy-stearic acid-boric acid-salicylic acid (271) or hydroxy-stearic acidazelaic acid (272) represent a break-through in multi-purpose and multi-grade 216
greases; they have high drop-points (250 to 300 "C), very good shear stabilities assessed, for example, by the low difference between penetration after 100,000and 60 double strokes -, water- and high temperature-resistance and a working range from -50 to 220 "C, especially when based on high-grade (hydrocracked) mineral oils or synthetic oils (PAO, stable esters). They have been termed "third-generation greases" (273).Some can be used at temperatures as high as 250 O C (69). The properties of some complex greases are illustrated in Table 3.43. Table 3.43. Properties of Some Complex Calcium, Aluminium and Lithium Greases Manufactured in the USA Complex Lubricating Grease Ca Al Li Penetration at 25 "C (10-'mm) - after 60 strokes 325 - % change after lo5 strokes -26 260 Drop-point ( "C) Soap (% weight) 35.8 Working temperature range ( "C) -18 to 177 Roll stability test (Change in stability after 100 h) -39 Anti-corrosion test pass Oil separation (aweight after 30 h at 100 "C) 3.0 Evaporation loss (% weight after 22 h at 98.9 "C) Oxidation stability (pressure drop Wa) - after 100 h 21 - after 500 h 70 Static hardening (change in penetration of unworked lubricant) - after 28 days at 25 "C -54 at 100 "C -54 at 121 "C -68
270 -1 260 7 -32 to 177
275 16 260 10 -54 to 177
56 fail 2.4 -
18 pass 3.4 0.1
44
28 49
154
-11.5 -5.5 8.5
-
-
Miscellaneous Effects in Soap-based Lubricants Glycerol and primary alcohols remain in soaps if the latter are prepared from acyl glycerides or waxes. Glycerol stabilises, to some extent, the structure, and improves the lubricity of the lubricants produced, but its adverse effects dominate; it impairs the water- and temperature-resistance of complex calcium lubricants if its concentration in the soap exceeds 2% (125). Generally, it causes the oxidative and mechanical stabilities of the lubricants to deteriorate and it reduces the thickening power of the soaps. Below a maximum limit of 1 % in the grease, it does not significantly affect its properties. Primary monoalcohols, which have a higher oxidation stability, are better stabilisers of lubricant structure. Water modifies the structure of lubricants containing hydrated soaps. The 217
relationship between the amount of water in soap and the properties of plastic lubricants made from them has not yet been fully established. There is a generally-held view that the water content which is regarded as optimum varies according to the type of soap from which the lubricant is prepared. It is different in natural and synthetic acids, and if free glycerol or mono-alcohols are present. For example, 2% is regarded as the minimum water content, in calcium lubricants prepared from soaps of natural fats and fatty acid, in order to produce a stable lubricant (75). Changing the water content from 2 to 5% in lubricants prepared from soaps of synthetic fatty acids does not affect the bulk mechanical properties and structure of the lubricant (Z26). A low water content (less than 1%) is often adequate to stabilise the structure of sodium lubricants.
The structure, the rheological properties and the mechanical and colloidal stabilities of lubricating greases depend in large measure on the concentration of free acids and buses. Even small amounts of acid or base can alter the shape and size of the particles in the dispersed phase and, consequently, the essential characteristics of the plastic lubricants. Relationships of this nature differ between lubricants prepared from acids of different compositions (127,128). For example, the structure of a neutral calcium lubricant prepared from simple soaps of stearic and oleic acids in a ratio of 1:3 and containing 0.7 to 1.1% water is characterised by long, twisted fibres. an increase in the free acid content up to about 0.5% does not affect the shape of the crystallites. However, if the acid content is higher, particularly above 1.2%, the nature of the crystallites changes rapidly; thick, flat platelets are formed and the twisted shapes of the fibres disappear progressively. At an acid content above 2.2%, large aggregates of dispersed phase with dispersed fragments of soap crystallites appear. This change of structure results in a reduction in strength limit, viscosity and colloidal stability of the soap. On the other hand, at the transition into the alkaline region, the thickening effect of the soap grows, up to about 0.12% NaOH; the strength limit and viscosity of the lubricant increase and oil-separability decreases.The structureof the dispersed phase is provided by dual shapes of particle: ribbons and twisted fibres, which have a strong thickening effect. By contrast, in lithium lubricants prepared from the soaps of 12-hydroxy-stearic acid, neutral lubricants exhibit maximal strength limit and viscosity and minimal oil-separability.
These changes in the morphological properties of the soap fibres, which constitute the structure of the lubricant, resulting from acidity or alkalinity, are accompanied by changes in the rheological and other charactistics of the lubricant. The effects of free bases are closely associated with the formation of hydroxyl groups in the soap molecule. These groups may be a part of the soap anion if, for example, hydroxy acids are present, and they may originate form water which is present, or from an excess of free metal hydroxide. High molecular weight polymers can also be used to modify the structure and improve the properties of some plastic lubricants. Polyalkylene glycols (0.1 to 1.O%) of relative molecular weight 400 - 7,000and polyacrylates or polymethacrylates (6%) can be used to improve the thixotropic properties of lithium lubricants. Polybutenes (1%) of molecular weight about 12,000 or styrene-isobutene copolymers can be added to diester oiVlithium lubricants to improve their stability at elevated temperatures.
218
The structure and properties of soap-based greases depend significantly on the process conditions under which they are manufactured, namely, the temperature at which the soap is dispersed in the oil, the final temperature of this final manufacturing stage, the method of formation of the complex, the degree of agitation during the mixing process, the rate at which the liquid is cooled, the nature of the final treatment of the lubricant (intensity of milling and homogenisation) etc. These conditions are not only specific for particular lubricant types, but they must be tailored to the selected soap raw materials and the nature and properties of the continuous phase (i.e., the oil phase in which the soap is dispersed). The optimisation of these conditions depends in considerable measure on experience, since the results of laboratory experiments and those achieved in pilot plant cannot always be exactly reproduced in full-scale manufacture.
Soap-free Thickeners Soap-free thickeners comprise various inorganic and organic oleophilic substances which, unlike soaps, do not generate fibrous structures in lubricating greases (131). The structure of bentonite and silica gel lubricants contain clusters of platelets, among which the continuous phase is enclosed. The structure of urea-based lubricants is formed of fine, needle-shaped crystals reminiscent of lithium stearate fibres. The most important inorganic thickeners are the hydrophobic bentonites. Other examples include zinc oxide, boron nitride, asbestos, carbon black, carbon fibres and silica gels. Greases containing these thickeners do not need as high manufacturing process temperatures as soap-based lubricants. They can be prepared by mixing the thickener into oil at ambient temperature. A suitable dispersant solvent, such as methanol or acetone (about 0.5 to 3%) can be added to facilitate thorough wetting and dispersing of the thickener after the gel has formed, the solvent is flashed off. Lubricants consisting of concentrated solid lubricants (graphite, MoS2, WS,. etc.) in oil are termed lubricating pastes.
Bentonite Greases These are thickened with hydrophobic bentonite, natural montmorillonite type aluminosilicatein which the mono-valent metal cation (Na) has been substituted by an oleophilic quaternary ammonium cation, e.g., methyl-dioctadecylamine type (132). The thickening power of the bentonite depends on the type and size of the active surface of the montmorillonite and on the chemical composition of the oleophilic reagent. The strength limit of the lubricant is increased by increased bentonite concentration (133, 134). Generally, the lubricant contains 5 - 8% by weight of thickener in mineral oil, PAO, or silicone oil. Bentonite lubricants have many positive and even excellent properties (274). They are suitable as universal (multi-purpose)automotive and industrial lubricants.
219
They are available suitable for lubrication at high temperatures, for use under high load conditions, in radiation environments, etc. Depending on the thermal stability of the hydrophobic bentonite and the oil component, the upper working temperature limit can be as high as 150-230 "C (and even 300 "C with polyethers). Bentonite lubricants are characterised by high shear stress threshold values (which decrease somewhat with temperature), fair colloidal stabilities, satisfactory thermal, viscosity and shear stabilities and water-resistance. On the other hand, their deficiencies include low mechanical stability, poor anti-corrosion effects, incompatibility with other greases, poor response to additives and unsatisfactory anti-wear properties.
Silica Gel Greases Silica (silicon oxide) rendered hydrophobic by blocking the surface silanols (Si-OH) with alcohols (e.g., n-butanol, long-chain alcohols), di-isocyanates, adsorbed polymers (e.g., polybutadiene-styrene, polysiloxanes, polyglycols) or tensides (surfactants) such as N-cetylpyridinium cations is used as the thickener in silica gel lubricants (135,Z36,137,139).The thickening power of silica gels depends on their active surface area (usually 50 - 380 m2.g-l or more). However, it changes with type of oil. It is highest in the hydrocarbon oils and lowest in the ester oils. Oil type can also affect changes in strength limits with temperature. As in other types of lubricants, the most drastic fall in strength limit occurs when the oil component is non-polar (e.g., highly-refined mineral oils, PA0 or polysiloxane oils). However, the strength limit and consistency of lubricants prepared from polar oil components increases over the temperature range 20 - 250 "C. The viscosity characteristics of silica gel lubricants are similar to those of lubricants containing soap thickeners. Their viscosity decreases with increasing shear-rate, but their viscosity-temperature characteristics remain good. Silica gel lubricants are resistant to water and have good mechanical stability. When exposed to intense deformation, their properties change less than those of most other lubricants. The change in their thixotropic properties over time is low. The most significant attributes of silica gel lubricants are their high thermal stability - working temperatures as high as 150 "C with mineral oils and 250 - 300 "C with silicones -,their resistance to ionising radiation, fair oxidation stability and resistance to aggressive chemical environments. The magnitude of these attributes is largely dependent on the resistance of the reagent used to render the gels hydrophobic. Additives are available which increase this resistance. For example, polysiloxanes containing chlorine confer high resistance to nitric acid attack on silica gel lubricants (140). These typical merit factors make silica gel lubricants suitable for use in the lubrication of atomic reactor components and in rocketry (especially parts in contact with the oxidisers) and in hot environments (hot bearings, gears and valves) and in special lubricants for the steel industry. Deficiencies of silica gel lubricants include their poor corrosion protection properties; however, these can be improved with suitable additives. 220
Greases made from Special Thickeners Organic soap-free thickeners are now used mainly for the manufacture of special greases. They may be classified into aromatic and non-aromatic derivatives. The development of aromatic thickeners began in the 1950's, with the developments in rocketry and nuclear energy, which utilised their properties which made them suitable for service at high temperature and radiation environments and exposure to powerful oxidisers, etc. The most important example of aromatic thickeners are pigments, particularly copper phthalocyanine (I) (241), indanthrene (11) (143) and metal terephthalamates (111) (242): 0 II
CIOH37 - NH - CO
0
COOM
These pigments have low thickening power and their concentration in the lubricant needs to be high (20 - 50%). Polysiloxanes and polyphenyl ethers, which have the highest thermal stability of all oils (but lower than those of pigments alone, which have melting-points as high as 500 "C), are mostly used as the continuous (dispersing) phase. Pigment-based lubricants are hence very stable and the upper working temperatures are up to 350 "C. Since the pigments themselves contain functional groups which can inhibit oxidation and have polyaromatic conjugated systems with anti-radiation properties, pigment-based lubricants have - even without antioxidants - high oxidation stabilities and radiation resistance. They resemble silica gel lubricants in that their strength limit and consistency increase with increasing temperature up to 100-150 "C. They have flat viscosityshear curves (there is a small decrease in viscosity with increasing shear-rate), fair mechanical and colloidal stabilities and satisfactory anti-wear properties - the 22 1
pigments themselves are good solid lubricants. They have very high densities (e.g., the density of copper phthalocyanine is 1620 kg.cm-,), so that pigment-based lubricants are heavier than water. A further characteristicis their fine-grain structure and colour; indanthrene lubricants are deep blue, whilst phthalocyanines produce violet lubricants. The addition of pigments to other thickeners (e.g., bentonite or lithium or other simple and complex soaps) increases the thermal and oxidation stabilities of the resulting lubricants. Some 1,3,5-triazinederivatives have a similar effect. Polyurea greases appeared on the market at the same time as those based on pigments (144-146).Polyarylureas are the thickeners normally used, but polyalkyl and polyalkaryl ureas are also used; however, these have lower thickening power and oxidation stability (147, 148). Polyureas can be made by the reaction of diisocyanates with diamines and monoamines "in situ", i.e., directly in the oil phase, as follows: R, - NH2 t OC = N - R2- N = CO t H2N - R, - NH2 t OC = N - R2 - N = CO t H2N -R4
(in situ)
t
R, - NH - CO - NH -R2 - NH - CO - MI - RJNH- CO - NH - CO - NH -R,NH -CO - NH - R4
u x
2
L
v
x
(two - dimensional tetra - urea)
X'
---- NH-CO-N
I
(t diisocyanate in excess)
X"
,.,---
I
co I
R,
(formation of biuret bridges)
I
NH I
co X"
---- NI-CO-NH ---- x
(threedimensional polyurea)
where R,, R, are long-chain alkyls, R, is arylene and R, alkene chains. The thickening power and other properties of the resulting lubricant depend on the R groups, on the size of the network built by the biuret bridges, on the particular combination of aliphatic and aromatic groups and on the presence of heteroatoms in the hydrocarbon chains interspersed in the carbamide groups. Polyurea greases have high drop-points, good heat, mechanical and colloidal stabilities even under shear stress, they are resistant to oxidation (being ashless and
222
metal-free) and to water and they have good anti-wear properties (because of the presence of oxygen and amine nitrogen). Suitable oil components include mineral and synthetic oils, e.g., PAO, silicones, polyphenylethers and fluorinated oils for high temperature use. They are compatible with other lubricating greases and respond well to additive treatment. Polyurea greases made with mineral oils are satisfactory in the range -35 to I50 "C and with synthetic oils from -70 to 200 OC. They are used for the lubrication in critical situations of heavily-loaded medium-speed and high-speed antifriction bearings in mines, quarries, cement works, steel mills and paper mills, and in belt, road, rail and air transport equipment. As multi-purpose greases, they compete with lithium-based greases based on hydroxystearic acids (274). The composition and properties of polyurea greases are shown in Table 3.44. Examples of newer types of highly efficient urea thickeners include benzimidazobenzisoquinoline ureas of the following type:
where R is a c,6+,2 alkyl or triazine-urea compounds (274) of the type:
where R groups are c,6-c,, alkyls. These are suitable for the preparation of greases stable at high temperatures (Table 3.45). Another more recent type of product comprises the polyurethane complex lubricants. In this case, the thickener is a blend of polyurea and calcium acetate; this thickener improves the anti-wear capability of the lubricant. 4-6 weight % of polyurea and 12-15 weight % of Ca acetate are required to prepare a grease to NLGI No.2 consistency grade. This saves half the more expensive polyurea by substituting the cheaper Ca acetate, whilst retaining a high drop-point and anti-wear capability. Lubricants compounded with blends of polyurea, Ca acetate and Ca carbonate have high resistance to softening at low shear-rates and to hardening at high shear rates; with suitable oils, they can be low-temperature lubricants (238).
223
Table 3.44. Composition and Characteristics of Polyurethane Lubricant (151) Composition:
Polytetraurea content (% weight) Petroleum oil Oxidation and corrosion inhibitor
Properties:
Penetration (lo-' mm) Drop-point ( "C) Oil separated after 1 week at 40 OC (DIN 51 817) (%weight) Pressure drop in oxidiser vessel (DIN 5 1 808) ( H a ) (a) after 100 h at 98.9 OC (b) after 400 h at 98.9 "C
10
283 232
1.2 15
35
The lubricant passed the SKF test (DIN 51 806), A & B.
Table 3.45. Composition and Characteristics of Semi-liquid Polyalkylene Greases (ZSZ) Composition:
Polyethylene/poiypropylenecontent (% weight)
6
Petroleum oil Oxidation and corrosion inhibitor and anti-wear additive Properties:
Penetration (10-lmm) Penetration after 105 strokes (I0"mm) Four-ball test (weld load, N) Timken load stage (N) FZG test (run A), damage load stage Working temperature range ("C)
437 452 373013930 180 >I2 -10 to 120
Non-Aromatic Thickeners In addition to the well-known paraffins and ceresines, polyalkylenes and their polyfluorinated analogues are used. Lubricants thickened with polyethylene or ethylene-propylene copolymers of molecular weight 5,000 to 50,000 are useful for lubricating open gears, ground joints in vacuum equipment and the like. An acceptable lubricant of good colloidal stability can be prepared from high-density (950 k g . ~ m - ~linear ) polyethylene (152). Blends of such polyethylene with polyalkenes are even more suitable. Although lubricants thickened with polyalkenes display good colloidal and chemical stability, they have a low capacity for adhesion to metallic surfaces. For this reason, polyalkenes are usually combined with other thickeners ( 1-3% by weight of lubricant). Addition of 1% of HD-polyethylene into lithium and complex calcium lubricants reduces their friction coefficients. High temperature lubricants can be prepared by adding graphite or MoS,. 224
Greases prepared by thickening mineral or synthetic oils with polytetrafluorethylenes (PTFE) or polytetrafluorchloroethylenes (PTFCE) have good thermal stability - up to 230 "C, good lubricity and high load-carrying capacity at low torque. Known exceptions are friction pairs comprising soft metals (Al, Mg) and steel under high loads, where these lubricants fail to provide satisfactory seizure protection. They are also unsuitable for lubricating the surfaces of copper-containing alloys, since the lubricants rapidly darken and promote corrosion, particularly in the presence of water.
These lubricants withstand strong oxidisers and other chemically active substances but they have some elementary drawbacks; in particular, they have poor viscosity-temperature characteristics - they are only applicable over a relatively narrow temperature range (up to 100 "C), low resistance to water and low mechanical stability. They are unstable towards ammonia and amines (PTCFE) and hence are unsuitable for lubricating rocket components, since amines are sometimes used as components of rocket fuel. New types of lubricants produced by the combination of perfluortrialkylamines as the continuous phase and PTFE as the dispersed phase not only withstand strong oxidisers but also hydrocarbon fuels containing amines (e.g., asymmetric dimethyl hydrazine) and are, therefore, suitable for the lubrication of rocket components (154).
Lubricants containing halogenated polyalkenes (PTFE, PTFCE) are not themselves toxic and hazardous to living organisms; however, when heated at 300500 "C they decompose to yield vapours of very toxic compounds, for example peffluorisobutylene, ten times as toxic as phosgene (155). Other types of ashless thickeners, such as ethyl- and hydroxyethyl derivatives of cellulose, polyamides and polyimides have not achieved any significance in application, because of their poor thickening capacity and since greases made from some of them fail to achieve the quality of other types of ashless lubricants. The so-called hydrocarbon greases usually contain, as thickeners, petroleum oil residues, solid paraffin, ceresine or "natural" waxes (beeswax, carnauba wax, lanolin). Hydrocarbon lubricants comprising the materials referred to in technical practice as vaselines or petrolatum stocks are not usually regarded as lubricating greases. However, since the solid hydrocarbon or wax crystals in them form basic structures similar to soap fibres with oil molecules bound on to them, the colloidal system is, in fact, similar to those of greases.
These lubricants are characterised by low drop-points (35 to 60 "C). In exceptional cases, high drop-points, e.g., 75 "C, can be achieved if very hard ceresines or waxes are employed for thickening the oil. These lubricants cannot compete, in this respect, with other grease types and are not suitable as regular lubricants. However, because of their good resistance to water, they find broad application as preventative agents and can serve, thanks to their good chemical 225
stability, as protective coatings against the effects of chemically aggressive substances. They are also suitable for impregnation of ropes, for sealants in the chemical industry and similar applications.
Mixed Thickeners Mixed thickeners - combinations of soap-based and soap-free - are also used in grease compounding. These special lubricants can exhibit all the advantages of the lubricants produced with all the thickeners present in the mixture. All types of thickeners (e.g., bentonites, lithium and aluminium soaps, polyureas with sodium terephthalamate or lithium or sodium-based soaps or with inorganic thickeners, particularly bentonite and silica gels) are used in practice. Examples of lubricants of very high thermal stability include those containing complex aluminium soap and bentonites. These are suitable for lubricating automobile wheel hubs with highly efficient disc brakes which develop much heat (156). Obviously, only mutually compatible thickener mixtures are admissible.
3.3.1.3 Additives for Greases The performance properties of greases are normally augmented with additives. These are classified, according to their solubility in the oil component, into solid additives or fillers and soluble additives or improvers. The fillers are mostly inorganic substances which are unable to form a grease structure (Table 3.46). They can be classified according to chemical composition as follows (examples in parentheses): - carbonaceous substances (graphite, carbon black), - silicates (mica, talc), - metal powders (Al, Cu, Pb, Zn), - metal oxides (A1203,MgO, PbO, Pb,O,, TiO,, ZnO), - metal carbonates (CoCO,, PbCO,), - metal sulphides (Sb,S,, Sb,S,, PbS, ZnS+BaS04 (lithopon), MoS,, WS,), - metal sulphates (BaSO,, PbSO,). The chief function of these fillers - which actually perform as solid lubricants is to improve the lubricity and anti-wear properties of the grease, to increase the anti-shock characteristics, to improve the resistance to aggressive chemical agents and similar properties. Some fillers, e.g., soft metals and metal sulphides, act on rough metal surfaces as smoothing or lapping agents by filling the gaps between surface asperities. Greases containing fillers also have better sealing properties. Suitable additives or improvers include various compounds from among those covered later in Chapter 4.The main types are oxidation and corrosion inhibitors, anti-wear additives, additives which increase the load-carrying capacity of the lubricant film and, in lesser measure, additives which improve water-repellency and increase adhesion to metal surfaces, metal deactivators, anti-rads, anti-foams and colour stabilisers. Viscosity-temperature behaviour improvers and pour-point
226
Table 3.46. Commonlv-used Fillers Filler Type. Graphite: Colloidal Flakes Powder MoS2 ZnO
Usual concentration (% weight)
Grease Type
Applications
0.01 - 1.0 2 - 10 2 - 10 0.5 - 18 2 - 10
arbitrary calcium-base calcium-base arbitrary arbitrary
improvement of anti-wear capacity and high temperature use
aluminium-base calcium-base
vehicle chassis lubricants thread lubricants
calcium-base calcium -base
thread lubricants
Lead Powder
1-20 1 -2 0
Pb304 Zinc Powder
10 - 20 10 - 40
depressants have little significance as their effect is neutralised by adsorption on to the thickener. The use of additives is subject to some peculiarities, which must be respected. The choice of an additive or compound producing the required effect is to a large extent dependent on the composition of the grease. For example, the efficiency of an additive in lubricants incorporating soap-free inorganic thickeners, particularly bentonites, can be considerably reduced by the adsorptive effects of the thickener. Improving one property can adversely affect other attributes of the grease or even alter its structure. For example, some anti-wear additives and lubricity improvers, as well as some antioxidants, can reduce the shear strength limit, depress consistency, reduce drop-point, increase syneresis and impart corrosivity to the lubricant. Bentonite and analogous lubricants are particularly prone to this effect. Lead-base soaps affect the consistency and corrosivity of bentonite lubricants. Such undesirable phenomena can be largely suppressed by mixing the additives into the lubricant at as low a temperature as is feasible. Monovalent phenol antioxidants lengthen the fibres in sodium-base greases; bivalent analogues have the opposite effect; resorcinol shortens the fibres and hydroquinone destroys the structure; isopropyloxydiphenylamine in lithium greases incorporating silicone oils inhibits oxidation and gellation of the lubricant at elevated temperatures.
Some types of grease need no additives because some thickeners themselves act as improving agents. It is well known, for example, that some pigments are excellent antioxidants and calcium soaps have anti-wear properties, and lead soaps improve the strength of the lubricant film. As might be expected, out of all types of additives, only some, dependent on the type of solid and liquid phases present, can be used in greases.
Oxidation Inhibitors Doses vary between 0.1 and 1 .O%; most are efficient up to 150 "C but decompose above this. Special antioxidants are available for service at high temperatures. For 227
example, 9-hydroxy-9,lO-boroxarophenathrene-basedcompounds stable up to 290 OC are used in bentonite and pigment lubricants; also, some multi-functional dialkyl dithiocarbamates (e.g., of Zn, Cd, Pb), phenothiazine and selenium and tellurium compounds (e.g., dilauryl selenide or telluride) are heat-stable. In lubricants of neutral or alkaline nature, aromatic amines are mostly used, for example, alkylated diphenylamines, phenyl-1-naphthylamine, phenothiazine (chiefly in calcium, sodium, lithium and even in barium and strontium lubricants). In lubricants containing free fatty acids, phenolic antioxidants, such as di-tert-butylp-cresol and analogues, alkyl- or phenylphenols (e.g., 4-tert-butyl-2-phenylphenol) are preferred. Some of the low-temperature antioxidants also perform as anti-rads.
Metal Deactivators and Passivators Several polar substances act as corrosion inhibitors, like ethylene diamines, sodium and calcium alkylsulphonates, sodium benzoate, sodium nitrite, zinc and lead naphthenates, alkylphenol sulphides and sugar and fatty acid esters, in concentrations of 0.5 to 2% weight. Amphipatic polar compounds used as rust preventatives also have anti-wear properties. However, the molecular structure optimal for rust prevention is not optimal for wear protection (129). Corrosivity of greases is tested by CSN 65 6309 for 48 hours with copper or steel plates at 50 or 100 "C. CSN 65 632 I , 65 6322 and GOST 5757 67 are analogous, the duration of the test and the material used for the plates varying in the different standard methods. According to DIN 5171 1, ASTM D-1261 and IP 122/56, the corrosivity of greases is tested on copper strips; in R M S 5304 a ball of the grease is placed on a copper strip and the changes apparent after 24 hours are evaluated. However, the EMCOR test is mostly used (DIN 51 802).
EP Additives Substances available for improving the load-carrying capacity of the lubricating film include those which contain, as active elements, sulphur, phosphorus, chlorine and lead. Sulphurised alkenes, terpenes, dibenzyl disulphide, chlorinated paraffins have been used in doses ranging from 2 to 15% weight (see Chapter 4.10). However, one type of compound is unlikely to be satisfactory in every type of grease (Table 3.47). For example, impairment of the load-carrying capacity of lithium greases in which different types of EP additives during storage has been observed when no stabiliser has been present (Table 3.48). The load-carrying capacity of the lubricant film (anti-wear properties) of the grease can be characterised by dynamic-mechanical tests, notably with the Timken tester or the Four-ball machine. A special Gear Wear Test, FTMS 791a-335 can be used to test the load-carrying capacity of greases intended for the lubrication of gears. The operating portion of the tester is a spiral gear; the driving pinion is of brass, the idle gear of steel. The gear couple is rotated and the rotation transmitted to a winch provided with a weight. In the first 6,000 working phases, the winch applies a lifting force of 22 N, in the next 3,000 phases the weight descends with a force of 44.5 N and causes the gear to rotate in the reverse direction. The extent of wear can be determined by weighing the gear pair before and after the test.
228
Table 3.47. Different Concentrations of EP Additives in Greases* Made with Different Thickeners (130) Thickener
EP additive concentration (% weight) for additives incorporating different active elements needed to achieve the same load-carrying capacity of the lubricating film (16-18 kg with the Timken Tester) sulphur-chlorine
Ca soap Li soap
lead soap 2.5 2.5
5 10
-
2
Bentonite
*In all cases, the greases contained the same oil of viscosity 14-20 mm2.s-'/98.9 "C.
Table 3.48. Effect of EP Additive on the Load-carrying Capcity of Lithium Grease Lubricating Film Active elements in additive
s, P
s, CI, P S, CI
+ stabiliser
Concentration of additive (aweight) 10 10 10
Load-carrying capacity (N) in the Timken Tester initial
after 1 month
180
135 112 180
180 180
Water Repellants Although water-repellancy is a normal property of the components of lubricating greases, it can be increased by the addition of 0.5 to 3% by weight of carboxylic copolymer of molecular weight 10,000to 20,000 in which the carboxylic group is partially saponified. Alkaline or alkaline earth salts of polyamyl, octyl, dodecyl and hexadecylacrylates, among others, are suitable for this purpose.
Additives for Improving Adhesion to Metal Surfaces The most common additives of this type are polyisobutenes with molecular weight over 100,000 (e.g., in lithium lubricants) and latex (e.g., in aluminium lubricants). They are added to the lubricant just before it is filled in the containers, since they lose their efficiency when heated and mechanically processed. The method for testing the adhesiveness of greases to metallic surfaces, the so-called DB-Hafttest, was developed by Deutsche Bundesbahn (Deutsche Bundesbahn, September, Annex.7,1961-TL 958103). The test consists in determining the force needed to separate two plates with a layer of the test lubricant between them. In the NLGl test (114). the lubricant is spread in specified thickness on a horizontal disk, which is then caused to rotate. The weight of lubricant left on the disk after a specified period of time determines the degree of adhesion. This method is less suitable for discriminating between the adhesive capacity of various types of lubricating greases due to the effect of cohesive forces and the relationship between centrifugal force and lubricant density.
229
Colour Stabilisers Compounds similar to those used in lubricating oils are employed for stabilising the colour of lubricating greases against the effects of light and heat. These include, in particular, aliphatic amines, such as dibutyl, triamyl and cyclohexylamine and substituted hydroquinone, at concentrations of 0.01-0.1% weight. A stabilising effect is also shown by polysiloxanes at 0.001-0.05%weight, and these are recommended for use in petroleum waxes. Dicyclohexylaminoethyl acetate and other types of dicyclohexylamine compounds (0.03-0.05%weight) and some Schiff's bases (0.010.1% weight) are also recommended.
3.4 SOLID LUBRICANTS Increased use of solid lubricants is associated with the developments in the design and operation of machines under conditions of extreme pressure, temperature, radiation and other effects where no organic lubricant would be able to retain stability, liquid lubrication would be impossible, semi-dry friction is prevalent and dry friction is imminent (289, 290). The areas of application of solid lubricants extends from space to ground technologies. Solid lubricants can lubricate the steering surfaces of space craft, bearings and gears of precision mechanisms, door hinges and locks, slide windows grooves and drawers in cars, household applications, etc., including in the form of aerosols. Some solid lubricants can be applied outside their normal sphere, for example in oils as friction modifiers and in greases as thickeners and additives. By the term solid lubricant is meant a direct means of providing a thin film of solid substance between friction surfaces which is capable of reducing friction and wear. Such requirements can only be met by those lubricating films which have low shear strength, are soft, have high adhesion to the surfaces and which resist pressure, are able to regenerate their integrity of surface coverage if it is broken (self-heal break) and which do not contain abrasive contaminants. Such properties can be derived from the correlation of friction coefficient p in the zone of boundary lubrication:
azs+(1 - a)zm P=
D
(3.3)
'm
where a is the relative area of metal-to metal contact, zs is the shear stress of the metal-to-metal contact junction, zm is the shear strength of the lubricating film, and P , is the pressure developed on the contact surface. This equation is applicable to solid lubricant films between the friction pair surfaces. This film is so thin that the pressure Pm corresponds to the degree of effective contact of the friction couple. If the film is uninterrupted, then a = 0, p = zm/Pm,zmis low and p is minimal. 230
If the solid lubricant layer is pierced by the asperities of the friction surfaces, the friction coefficient p increases with a and attains the value given by the ratio zJPm at a = 1, i.e., after the solid lubricant film has been removed. Equation 3.4 below gives the value of wear for moderately-loaded, unlubricated metal surfaces of similar hardness: K.P v= -
Pm
(3.4)
where V is the volume of wear particles per unit shear distance, K is a constant, and P is the total pressure. This equation can be applied as a general guide to the durability of lubricating films. The following attributes are required for solid lubricants for them to be capable of forming an effective lubricating film: strong tendency to adhere to the surface to be lubricated, low zm value (low shear strength), thermal stability, often in a range where liquid lubricants are inapplicable (300 to 1,100 "C), high melting-point; above their melting-point, solid lubricants lose all or most of their capacity to reduce friction, small particle size; the smaller the particles, the easier it is for them to form a stable dispersion of solid matter in solvent and the more efficient is the lubricant film - size needs also to be adapted to the height of the surface asperities, high thermal conductivity; at low thermal conductivities of solid substances - as in the case of plastics - local melting of the lubricating film or shear surface occurs, leading to the loss of capability to reduce friction. The thermal conductivity of non-metallic substances can thus be usefully increased by the addition of metal powder or fibre, so that bearing materials of long service life, which do not seize even when non-conventional lubricants are applied, can be obtained, high anti-corrosive ability, optimal electrical conductivity; high conductivity is required if the solid substance is expected to reduce wear of electric contacts, but low conductivity is needed for the lubrication of the contact surfaces of insulators, low density; more stable dispersions of substances of lower density can be achieved where solid lubricants are to be applied in the form of a dispersion in a solution, colour; colourless lubricants must be used in those cases where colouration of materials is to be avoided, e.g., in food, textiles and metal-forming industries. Types of solid lubricants can be classified according to their chemical composition into: 23 1
Inorganic Lubricants
-
with a laminar structure: sulphides or more rarely selenides or tellurides (chalcogenides) of Mo, W, Ti, Zr, Nb, Ta, graphite and fluorinated graphite, halogen compounds of Cd, Pb, Hg, some borates, boronitride. The laminar structure is not a sufficient prerequisite for solid lubricants. A necessary property is adhesion to the surface; e.g., mica does not lubricate, and boron nitride only at high temperature, - without a laminar structure: Sb,S,, Bi,S,, B,O, (with a chain structure), SbSbS,, AsAsO,, PbO, CdO, CaF,, BaF,, LiF, CaO, phosphates of Zn, Fe, Mn and others similar.
Organic Lubricants
- polymers, polytetrafluorethylene (PTFE) and others, - polyaromatics: some anthraquinone dyestuffs, phthalocynanins, - fats, waxes and their derivatives. Soft Metals
- Pb, Sn, In, Ga, Cd, Ag, Au and their alloys and others. Solid lubricants are applied as powders, friction-reducing lacquers, liquid dispersions and suspensions in volatile solvents or oils containing as much as 35 % weight and pastes with as much as 70% weight of solid lubricants, as components of dry bearing materials, of lubricating greases, as aerosols and enamels for very high temperatures.
3.4.1 Inorganic Lubricants The most important inorganic lubricants have a laminar structure with molecules arranged in parallel layers of high transverse strength but minute coherence in the longitudinal direction, which makes it possible to displace them with minimum frictional resistance. Molybdenum disulphide is probably the most commonly used solid lubricant. Molybdenum disulphide (MoS,), the more important properties of which are detailed in Table 3.49, is a black-brown coloured, soft, non-poisonous powder of very high chemical stability, although its resistance to hot concentrated acids and oxygen above 370 "C is lower; however, the product of its oxidation, MOO,, has lubricating properties up to about 650 "C and does not cause seizure. The lubricating power of molybdenum disulphide is not lost provided there remains some unoxidised molybdenum disulphide on the surface. X-ray diffraction studies of oxidation rate indicate that, as might be expected, the smaller is the particle size the higher is the oxidation rate (241).
232
Molybdenum disulphide, in lubricant form, has a hexagonal crystal structure similar to that of graphite. The distance between adjacent sulphur layers is greater than the thickness of the layers themselves. This accounts for the ready shear and good lubricating properties of MoS2. Very pure natural MoS2 is more suitable for lubrication than the synthetic variety. According to the UK specification BS 2819, the MoS2 content must be above 99% and the remainder must not include any contaminants which cause seizure, such as SiO,, and no acidic residues must be present.
The very low friction coefficients of MoS2 are remarkable; they vary, at low load, around 0.1 and can decrease with increasing load down to values approaching 0.05 cfig, 3.5).In field applicatons, they vary around 0.06, decreasing in the presence of air and increasing shear velocity. JJ 0.14
032 010
OD8 a06 0.04
a02 2al
400
800
1200
1600
2000 2400 LOAD ldaN 1
Fig. 3.5. Dependence of the friction coefficient of MoS2 on the load Because of the crystal structure of molybdenum disulphide and graphite, both solids rearrange rapidly when rubbed on surfaces. The basal planes are essentially parallel to the substrate surface, which facilitates shear and, accordingly, leads to low friction. Both friction and wear with these solids are affected by orientation. With the basal plane normal to the interface, the rate of wear is high. When the basal plane is parallel to the interface, the wear-rate decreases.
Unlike graphite, the lubricating characteristics of molybdenum disulphide do not depend on the presence of adsorbents. In fact, MoS, lubricates better in the absence of adsorbed films. The presence of adsorbates such as water vapour increases the friction coefficient obtained with MoS2. Wear is also observed to increase with an increase in the concentration of water in the system. If MoS, is employed as a solid lubricant additive (friction modifier), it suppresses the effects of polar additives copresent in the oil (193). The strong adhesion of MoS, to solid surfaces is not only due to its laminar structure, but also to chemisorption of the active sulphur atoms. The same is true for all chalcogenides and for Se and Te atoms. This may explain the increased lubricating power of MoS, in combination with ZnS. However, the presence of Mo atoms is also beneficial; thus, molybdenum silicides and oil-soluble molybdenum compounds (ammonium molybdates, phosphomolybdates, dithiophosphates) also have some outstanding lubricant properties. 233
Tungsten disulphide, WS,, is similar to molybdenum disulphide. It is more expensive, but on the other hand it forms non-coloured sliding surfaces and it is more resistant to oxidation. Its electrical conductance is better than that of other chalcogenides. It is, therefore, preferred for lubricating electrical contacts (183). Selenides of Mo and W and selenides of Zr, Nb and Ta are also suitable as solid lubricants. A more recent development is antimony thioantimonate SbSbS, (211), which has been stated to be an excellent solid additive to lithium and bentonite greases made with silicone oils, which in heavy-duty applications (e.g., lubrication of stainless and tool steels) substantially surpasses MoS,. The As analogue, ASASS,, is similar. A disadvantage of MoS, is its inability to prevent corrosion and continuing wear due to corrosion in service. SbO, can be added to alleviate these drawbacks. Graphite (see Table 3.49 for characteristics) is a black, soft, non-poisonous powder with a metallic lustre; it is chemically stable, although it has less resistance to hot, concentrated inorganic acids. Its thermooxidation stability and lubricity are similar to those of MoS,. It oxidises at temperatures above 550 "C to yield carbon dioxide, leaving no residue, and above 650 "C pure graphite powder is susceptible to auto-ignition. In vucuo, it evaporates rapidly, loses lubricity and can even be abrasive. Natural flaked graphite, synthetic colloidal graphite in oil or water, constructional electrocarbon and electrographite, graphite saturated with resins (e.g., PTFE), strengthened with silica, containing intercalated compounds such as chalcogenides, carbon fibres and graphite fibres, all find wide application in tribology (188,283,291).
Ob
.
0.3 -
0.20.1 -
AMBIENT PRESSURE. TORR
Fig. 3.6. Friction coefficients of graphite and MoS, in different environments 1 - graphite, 2 - MoS,
Friction coefficients for graphite are approximately double those for MoS,. Unlike molybdenum disulphide, the adhesion of graphite is only provided by physical adhesive forces and a thin layer of adsorbed water vapour or other polar contaminant is required for effective adhesion. Therefore, in vucuo, where polar substances are absent, graphite has poor lubricating characteristics and a high 234
coefficient of friction, substantially higher than that of MoS2 (see f i g . 3.6). Its lubricant film load capacity is lower than that of MoS2. In graphite, which has an identical hexagonal structure to MoS2, each carbon atom is bonded to four others. Therefore, the C-C bond within the lamellae is relatively strong, but weakened by the presence of contaminants,such as water vapour, oxides or hydrocarbons (278).Since graphite requires the presence of a polar substance in order to adhere to a metal surface, it shows good lubricity at normal temperatures due to the adsorbed water and above 500 “C, when adhesion to the surface is improved by metal oxides (as, for example, in metal-forming operations), but not at temperatures between 100 and 500 O C nor in high vacuum (e.g., in outer space), where it evaporates and water and oxygen are desorbed from the surface. This drawback can be neutralised by incorporating suitable contaminants or intentionally “soiling” the surface (279).A favourable factor is the considerable ability of graphite to form intercalates with many compounds (280, Z82).
The lubricating capacity of graphite can be improved by a running-in procedure, in which the crystals become optimally oriented. Table 3.49. Basic Characteristicsof Graphite and MoS, Characteristics
Graphite
Density (kg.m3) Required purity, min, (a) Hardness (Moshe) Melting-point ( “C) Thermal conductivity (W.m-’. K-l) Electrical conductivity Nature of adhesion to metal surfaces
2400 99 0.5 - 1.0 3540 48* good
4800 99 1.0 - 1.5 -1180 2 - 3.5* semi-conductor
adhesion+watervapour
adhesion, chemisorption
0.1 - 0.2 -180 to 450 up to 800
0.04 - 0.08 -180 to 420 up to 650 -
-
-
Typical friction coefficient, for dry steel Thermal stability in air ( “C) Thermal limit in the absence of air Lubricity in vacuo Thermal limit in vacuo Resistance to ionising radiation Chemical stability
-
almost no change unstable towards hot concentrated sulphuric and nitric acids
Decomposition products Hardness Typical friction coefficent of MOO, to steel Toxicity Particle size distribution in suspensions (pn) 50% 90%
MoS~
-
1100
fully resistant unstable towards hot concentrated inorganic acids and strong oxidising agents, chlorine and phenol Mo03,S02 (Mo, S) 0.15 - 2*
-
- 0.15 - 0.3* non-toxic
non-toxic
<0.2
<0.3 <0.8
4.6
* estimate
235
In some applications, the electrical conductance of graphite can be an advantage; it is higher than that of MoS, (the electrical resistance of graphite is only 4.10-2 Ohm.cm-'.) The characteristics of MoS2 and graphite coincide in some applications and they can be substituted one for the other. Graphite, being cheaper, is then preferred. Blends of MoS2 and graphite or MoS,, graphite and aluminium silicates (e.g., 20% MoS,, 40% graphite and 40% bentonite) have been shown to be advantageous. The MoS2-graphite blends have lower friction coefficients than those computed from their arithmetic ratio, and lower than those of each component of the mixture. MoS2graphite blends also show very distinct synergy with respect to the load-carrying capacity of the lubricating film. A blend of 3% weight of MoS, and 3% weight of graphite imparted to a grease the same load-carrying capacity as about 10%weight of MoS2 alone (175). Synergism is also evident from the observation that an MoS,graphite blend allows electric charge to leak away better t'lan either component separately (207). The application is still less widespread of the more recently developed solid lubricants, carbon polyfluoride and potassium borate; they can both be very good high-pressure additives and friction modifiers in both liquid lubricants and greases (159, 176). The characteristics of carbon polyfluorides (fluoro-graphites), (CF,),, are to a great extent dependent on the fluorine content of the products, as shown in Table 3.50. The colour of the (CF,), powder differs according to the fluorine content: thus, carbon polyfluorides with a minor fluorine content (x = 0.27 to 0.9, i.e., 27 to 58 % by weight F)are black or grey, whilst those with x > 1.O are fine white powders. The cheaper fluorides with x < 0.9 can be employed as solid lubricants. The very expensive fluorides with x > I .O are used chiefly for the manufacture of dry cells, (CFJJLi, of high energy density and in electronics, where their high electrical resistance is advantageous.
The fluoro-graphites are highly resistant even to very strong acids (sulphuric, nitric and hydrochloric acids, aqua regia). However, according to recent investigations (242), the fluoro-graphite acts in the presence of some organic compounds (Lewis bases, nucleophiles) as an oxidiser, decomposing them into carbon and fluoro-compounds. The fluoro-graphites do not react with hydrogen up to 400 "C. In the presence of fluorine at elevated temperatures, they burn. They are hydrophobic and insoluble in organic solvents (acetone, ether, benzene and alcohol). Fluoro-graphites of low fluorine content (x = 0.25 to 0.33). when kept in closed vessels, liberate hydrogen fluoride, which can be washed out of the product with a 1:l mixture of methanol and water.
The rheological properties of (CF,), results from its laminar structure, thermal stability and chemical inertness. According to one reference (243), the friction coefficients of compounds with x = 0.8 to 0.9 are lower than those of graphite and comparable with those of MoS2. They decrease with increasing load and shear rate. The anti-wear and anti-seizure (anti-stick-slip) properties of these compounds are 236
almost coincident with those of MoS2. In blends with graphite, synergistic phenomena are evident; the lubricating properties of (CF,),, MoS2 and graphite have been compared (277). Table 3.50. Typical Characteristics of (CF,), Powder Characteristics Specific density (kg.dm") Specific surface (m2.g-1) at x = 1.0
1.14 1.198 1.23 Melting-point it does not melt Surface characteristics Thermal stability (decomposition temperature ( "C) at x = 1.0 1.1
1.2 Electrical resistance (ohm . cm-I) at x = 0.4 0.49 0.55 0.85 1.1 Dielectric constant 2377 at 7000 MHz Toxicity virtually non-toxic
Typical data
2.68 116 130 156 160 hydrophobicflyophobic surface tension 6 m Nm-I
380450 580 640 1.599 1.93 246 9.104 1.10~
(LDSo>9kg.kg-') Chemical reactivity
strongly inert to acids
Compatibility with materials
and solvents excellent compatibility with all conventional materials, (metals, elastomers, plastics, pigments, etc.)
Fluoro-graphites have very good capabilities in protecting metal surfaces from corrosion. Iron and non-ferrous metal surfaces coated with (CFX),,do not corrode under the conditions of standard tests, whereas the surfaces of similar materials coated with MoS2 oxidised rapidly (245).The good anti-corrosion properties of the fluoro-graphites are also manifest in application as lubricating additives in oils. Table 3.51 illustrates the positive effects on the change in weight of metal specimen after the oxidation-corrosion test on GL5 gear oils. The test method used was that of the French Air Force, AIR 165 (72 h at 150 "C). Fluoro-graphites have found tribological applications mainly in the lubrication of aircraft engine bearing components (247). There are also available self-lubricating bearing materials based on synthetic elastomers and (CF,), (248). 237
Table 3.51. Effect of (CF,), in GL5 Gear Oils on Weight Changes in Metals (AIR 1651 Test) Oil Gear oil (MIL -L-2105C) Gear oil + 0.5% wt .(CF,),
Weight changes in metal specimens (mg) Copper Aluminium Lead +2.6 +0.08
-196 -178
+2.4 4.05
Other halogenated solid lubricants include cadmium iodate CdIO,, silver iodide AgI, and calcium and barium fluorides. Cadmium iodide has very good lubricating properties, but has a tendency to corrosion. The same is true for silver iodide. Calcium and barium fluorides find their application in high temperature lubricants, particularly in friction-reducing lacquers and enamels. Boron compounds are relative newcomers. Boron oxide, B,O,, is suitable for temperatures from 400 to 650 "C. At higher temperatures, friction coefficient and viscosity both decrease (1.5 at 500 "C and 0.1 at 650 "C and lo5 Pa.s at 400 "C and 20 P a s at 600 "C, respectively). Borax, N%B40,, has similar applications to those of potassium borate, and also in high temperature glasses. Potassium borate (KB305.H20) is used commercially as a 35% dispersion in oil. Table 3.52 illustrates its elemental composition and properties. Table 3.52. Qpical Elemental Analysis and Properties of OLOA 6750 Commercial Product of about 35% of Potassium Borate in Oil Composition Element concentration: Calcium Nitrogen Boron Potassium
-
% weight
0.06 0.17 7.60 8.90
Properties Density (kg.m") Viscosity (rnm2.s-'/100 OC)
1.140 12.8
P ssium borate can also be used as a lubricating additive in gear, engine, machining and other lubricating oils and greases, at a concentration from 1 to 11% by weight. It may also be combined with zinc dialkyldithiophosphates (ZnDDP). According to Adam and Godfrey (179, the EP-films set up by potassium borate and ZnDDP comprise two layers: the frst layer contains mainly potassium, boron and oxygen, the second zinc, sulphur and oxygen.
The positive characteristics of potassium borate are its good thermal stability (up to 400 "C), high load-carrying capacity, the ability to reduce friction coefficient and to form virtually colourless lubricating films. The demerit of potassium borate is its 238
partial solubility in water. In systems where the lubricant comes into contact with water or an environment containing water repeatedly or for long periods, it decomposes. Boron nitride, BN, is a high temperature solid lubricant with laminar structure effective from 500 to 900 "C, with friction coefficients decreasing as the temperature rises, from 0.5 - 0.7 at 400 "C to approximately 0.4 at 900 "C. Oxides, hydroxides and phosphates of alkaline earth metals and zinc are collectively known as white lubricants. They are used in oil suspensions and dispersions, pastes and greases for lubricating instruments and automotive equipment, and in metal cold-working fluids. They protect against corrosion, do not colour workpieces and are easily removed. They have also proved useful in molybdenum disulphide pastes. Lead oxide is suitable for temperatures between 430 and 650 "C. It is also used with lead silicate up to 675 "C or with CaF, for the preparation of enamel frits baked on to friction surfaces. These enamels adhere strongly to metals and have outstanding lubrication properties up to 1,000 "C (284). Sliding layers are increasingly produced directly on materials to be used as friction surface pairs. This is done by mechanical, thermal, chemical or combination procedures, e.g., by annealing, hardening, tempering, rolling and similar treatments. Surfaces are coated with non-metallic (C, N, 0, B), metallic (Cu, Au, Sn, Pb, Co, Ni) and derived compounds (MoS, etc.), polymers (PTFE, polyamides, elastomers) and ceramic layers (carbides, nitrides, borides of Cr, Mo, W,Ti, A1,0,, TiAl and others) by sintering, weld-formation, thermochemically or electrochemically, by ion implantation, ion plating, splittering, from flame, arc, plasma or explosion sputtering (242, 277-280). Some friction problems can only be solved by pre-coated layers, e.g., the lubrication of hot hydrostatic bearings of automotive gas turbines (650 "C) (280) or fittings and mountings in power equipment (282) (Table 3.53). Phosphatising is the most important chemical treatment. Iron, steel zinc or cadmium surfaces are treated in baths containing dihydrogenophosphates of zinc, manganese and/or iron, acid fluorides and borates or electrochemically in caustic solutions of phosphates and chlorates. More recently, processes combining surface pre-treatment and lubrication into one operation are making progress. Phosphates, borates, oxalates etc., with an organic substituent can be used for such procedures, e.g., the reaction products of P,O, with amines or alkanolamines with balanced acidity, alkalinity or of neutral reaction (282), are particularly suitable for metalworking operations.
3.4.2 Organic Compounds Soaps, fatty acids, fats such as tallow and waxes are among the oldest solid lubricants. Stearates of sodium, calcium or magnesium (often with added organic phosphates) enable low friction coefficients to be attained in wire-drawing processes. Their application has, however, temperature and pressure limits. 239
N
P 0
Table 3.53. Chemical and Thermal Stability of Friction-reducing Lacquers from Plastics (160) Type of plastic matrix Thermal stability ( "C) Acids Bases short-term long-term weak strong weak strong 100 80 polyviny lchloride polystyrene 80 <65 polymethacrylate 100 430 polyamide 140
1 1
I
1
3
1 1 1 1 2
2 1 1 1
3 2 3 2
2 1 1 1
I 3
2 3
3
3 = unstable
I
Resistance (at 20 "C) to Hydrocarbons C, C, C, halog.
3 1
3 2
3 3 1 3
3 1 1 2
1 1
I
3 1 1 3
3 3 1 3
1 1 1
1 1 1
2 2 1
3 3 1
1
1
I
3
1 1 1 1
3
2
1 1 3 1 2
2 1 1 1
3 1
3 3 3 1 1
1 2 2 1
1
3 3
3 3 3 3 2
1 3 1 1 1
3 3
1 1
ket.
3 3 3 1 2
1 2 1 1 1
2
Oils petr. animal alc.
1
Chemicals est. H,O(g)*
3 3 2 3
Fluorinated polyethylenes (PTFE, Teflon) and the copolymers of tetrafluoroethylene and perfluoropropylene have the lowest friction coefficients of all solid lubricants, good resistance to oxygen and all chemicals (except liquid sodium) and good performance in vacuum at low temperatures. Their operating temperature range is from -270 to about 205 O C and exceptionally 260 OC.They have, however, poor resistance to pressure and ionising radiation. Their decomposition products are toxic. Substances of slightly higher friction coefficients, but harder and easier to form, can be made by chlorinating polytetrafluoroethylenes. Polytetrafluoroethylene has a laminar structure, but loses it at higher shear velocities (over 1.5 m.s-*) (187). Its friction coefficients are low (0.01 - 0.04)over a wide temperature range.
Other plastics are applied to a lesser extent, such as polyamides (e.g., polyamides with bronze powder, with Ag+MoS2 and similar fillers are suitable for lubrication at temperatures as low as -235 "C),polyimides, polyurethanes and polyacetals or polyparaphenylenes (with properties placing them between graphite and polyimides). Other solid lubricants or additives for solid lubricants worth mentioning include the polybenzthiazol derivatives (I) synthesised from mixed toluidines and sulphur
Further condensation with 4-aminophthalimide and ZnO yields lubricants which, in combination with MoS2 and Sb2S3, are suitable for supersonic aircraft (16Z). The thermally-stable phthalocyanines (page 22 1), metal-free or with copper, have properties superior to those of graphite but inferior to molybdenum disulphide. Metals are attached by chelate bonds. In addition to polyamides, some anthraquinone pigments and the mechanically strong and thermally stable aromatic polyesters and polyamides, polyimides and analogues find applications in bearing and gear materials and in packings and valve seats (178).For instance, the self-lubricating polyhydroxybenzoate (11) is suitable for coatings on metal surfaces and the polybenzenetetracarboxyimide (111) for high-pressure lacquers (188): r
1
24 1
3.4.3 Soft Metals and Alloys Soft metals, such as lead, silver, gold, tin, indium and cadmium with a low shear strength or friction coefficient are used as soft platings on harder metals. Frequently, they are components of other solid lubricants, increasing their resistance to high temperatures and improving their behaviour in emergency conditions. Solid lubricants capable of withstanding the extraordinary tasks of lubricating the instruments and other equipment in spacecraft, for example, include combinations of gold and graphite, or silver with both MoS2 and Teflon (FTFE). Friction coefficients of these metals are higher than those for MoS2, but they are useful in those applications in which anti-corrosion protection is vital, in addition to lubricity. For instance, in a steelindium combination, the layers must be of optimum thickness; layers which are too thin can cause distortion of the steel, whilst if they are too thick, scratching can result (278).
Other soft metals, such as mercury, gallium, sodium-potassium and caesiumpotassium alloys, etc., are inapplicable in many applications, in spite of their good lubricating and rheological characteristics, principally because of their ready oxidisability even at normal temperatures. In recent years, ternary or quaternary alloys of bismuth, indium, lead, tin, cadmium and also of gallium, titanium and zinc have been extensively studied. These alloys have very low melting-points (70 to 105 "C), which is particularly significant for equipment in nuclear reactors, rockets and space-craft. Their major merit is their exellent lubricity within the range of 100 to 250 "C (so-called prolonged zone of moderate wear), good cooling capacity and the fact that their viscosity is virtually invariant - they behave as Newtonian liquids. On the other hand, their great disadvantage, which limits their wider application for now, is their oxidisability at normal temperatures, as a result of which oxide coatings or dust form, and, above all, their corrosive effects on structural materials resulting from inter-metallic reactions leading to new brittle compounds. The following equation applies for the friction coefficient 01)of the friction system:
where
zs is the shear strength of the coating layer, and Py is the yield point of the base material.
For systems of identical materials, rs and P, change in a parallel manner. p can be reduced by a convenient combination of coating material with a low rs and a base material with a given P,. It is essential to find the optimum coating thickness. If the thickness is too great, P, becomes also related to the properties of the coating and the reduction in pcannot be achieved. If, on the other hand, the thickness is too small, the properties of the base material will predominate and p will fail to decrease; in addition, the durability of the coating will be reduced by abrasion. A thin coating must also meet other criteria: good adhesion to the base and mutual compability.These properties are, unfortunately, mutually contradictory. For instance, if the surfaces of the friction pair are identical, (e.g., steel-to-steel), the coating material (A) would adhere to both friction surfaces equally, which would cause seizure. The remedy consists in choosing a second coating (B) which is compatible with both of the materials - steel and coating (A). The result is then a (steel-to-B-to-A-to-steel) system.
242
If, however, B diffuses into A, the result is again seizure. In this case, a further coating which can be soluble in both A and B but not in steel must be selected. In this way, a series of solid solutions is produced, from B to C to A to steel, in which it is necessary that the dilatation coefficient increases towards the surface and there is a gradation of mechanical properties in a reverse direction from the surface. An example of such a system is the steel-to-nickel-to-gold-to-silvercombination, in which the coatings are applied electrolytically and the silver layer (5 p) is the thickest. Combinations of this type are suitable as construction materials for bearings (276).Mixed coatings of high ductility as a bond and a metal resistant to high temperature as the high temperature lubricant (e.g., a gold-to-molybdenum combination) are under investigation (189).
Table 3.54 summarises commonly used high-temperature lubricants (162) and materials for bearings.
3.4.4 Friction-Reducing or Sliding Lacquers Friction-reducing lacquers are lubricants applied to surfaces subjected to low or high temperatures, high loads and low sliding speeds in continuous, intermittent or cyclic service. Their components are solid lubricants embedded in a matrix binder, the whole dispersed in an organic solvent or water. Compared with the more common industrial lacquers, they contain more solid constituents and less binder. They are applied by painting, immersion or spraying and form a wear-resistant film on the surface after drying in air or baking at 100 to 250 "C. The surface must be well-cleaned and prepared, e.g., by phosphatising, and have an optimum roughness Ra (centre line average height) of 5-10 pm with a phosphate layer of 3pm and a lacquer layer after baking of 10pm. Their composition depends on the duties for which they are designed. The solid lubricants are polytetrafluoroethylene (up to 200-250 "C), MoS2, other chalcogenides and graphite (up to 650 "C), PbO-Pb silicates (up to 675 "C), molybdates and tungstates of Na, K, Pb (up to 700 "C),BN (from 400 to 650 "C), silver (from 500 to 975 "C), fluorides of Ca, Ba, Li, also as eutectics (from 500 to 675 "C, exceptionally to 1,350 "C), and compositions are available for exceptional environments, e.g., liquid sodium or hydrogen. The matrix materials for air-drying lacquers for low temperature applications (down to -270 ") include cellulose derivatives, alkyd, acrylic, urethane and polyvinylbutyral resins. For higher temperatures, suitable matrices can be made with aminoplastic and phenoplastic resins, aromatic polyamides (aramides) and polyimides, epoxy or silicone resins, mostly applied by baking. The aromatic character of the matrix is particularly important the higher the temperature to which the lubricant film is exposed. For the highest temperatures, inorganic binders based on phosphates (Al), fluorides (Ca, Ba) and silicates (alkali metals, Pb) are suitable. Additives which improve heat dissipation and other properties such as strength, adhesivity and resistance to corrosion, can also be used, e.g., metal powders. Friction-reducing lacquers are applied in automotive and electrical engineering (e.g., for transmission shafts, aluminium engine cylinders, magnetic armatures of contact starters, and on titanium or austenitic screws) and in space technology (284, 293,294).
243
E
Table 3.54. High Temperature Solid Lubricants Summarised Type of solid lubricant
Highest operating temperature in service ("C)
MgOIMolFe
500
CdO/graphite Ni/BaF2/CaF2
500 535
Na silicatdgaphite 535 Metal sulphides, selenides and tellurides of Group IVb, Vb, VIb and VIII elements 575 MoSdgraphiteJNa silicate/PbS/B203 650 NilCr blends with eutectic mixtures of fluorides 650 Fluorochlorinated hydrocarbons 650 PbOlPb silicate 675 Ca fluoride %75 under certain conditions Moo3, pb molybdate, K molybdate, 700 Na tungstate, B203, Ag CaF2 + LiF blends
GraphitelB203/silverhalides GraphitdCKl, Carbides and borides
(in hydrogen) 8 15 (in air) 615 (in liquid sodium) 535 815 1,000 suitable up to 1.400 tested up to 2,000
Notes Self-lubricating bearing material used in gas turbines. Typical composition: 65% Mo powder, 30% Fe powder, 5% MgO. Soft CdO acts as a binder. Bearing material. The fluorides are held in pores in the nickel, which is the base material. Abrasion rate of this mixed lubricant decreases as temperature increases. Na silicate acts as a binder. Vacuum lubricants Base: nickel alloys Pressure-produced NUCr blends impregnated with 40% eutectic mixture of fluorides. Lubricating coatings formed by chemical reactions with bearing surfaces. Sprayed as water paste on bearing surfaces and baked at 995 "C. Chemically stable in hot oxidising and reducing atmosphere; low vapour pressure, low solubility in water, ready cleavage of crystals. All these substances protect bearings at high temperatures from serious abrasion and display low friction coefficients. Unsuitable at low temperatures. E.g., optimum temperature for silver is 315 to 975 "C. Sprayed as water paste on C r N alloys and baked under hydrogen cover. Good thermal and chemical stability, lubricity constant from 25 "C to specified temperature. Silver halides are efficient high temperature lubricants but highly abrasive at normal temperatures. Friction coefficient decreases with increasing temperature. Friction coefficients of Nb,Ta, B, Ti and Zr carbides decrease with temperature starting from normal temperature; dramatically increasing values found, however, in the range 800-1,400 "C.
3.4.5 Self-Lubricating Materials Two kinds of self-lubricating materials are summarised here: plastics and porous metallic bearings impregnated with lubricating substances. Representative selflubricating plastics include polyamides, aromatic polyimides, polyesters and their parent compounds, polyacetals and polycarbonates, either used as such and shaped to the desired form (e.g., of bearings) or poured as monomer and polymerised in a mould (e.g., polyamide 6 from anionic polymerisation). These plastics may be reinforced with glass, metal or ceramic fibres or best with carbon and graphite fibres which themselves have lubricating properties; they can be improved with solid lubricants of types previously discussed, PTFE being the best. Porous bearings consist of sintered bronze or iron powders with interconnecting pores. The porosity and the impregnated lubricant must obviously be adapted to the design duty: high porosity and high lubricant content are required for high-speed and light-load applications whilst low porosity materials may be more suitable for cyclic, reciprocating, low-speed and intermittent operation. These bearings are characterised by having high graphite contents and the use of lubricants containing anti-wear and anti-seizure additives (286). Self-lubricating bearings for high temperature applications can be based on sintered metals, e.g., Ga-In, Co-Ag, Ni, Ni-Cr, Ta (288). A Ni-Cr bearing impregnated with a eutectic mixture of 38% weight CaF, and 62% BaF, in vacuum is proposed for operation at a temperature range from 540 to 820 "C (289). All these materials have critical mechanical properties and require special machining procedures.
3.5. BIODEGRADABLE LUBRICANTS The concern to respect the ecological requirements has led to the introduction of biodegradable lubricants. Their use became imperative especially in mechanisms operating in areas of underground fresh water reservoirs, surface water basins, swimming pools, water works, biological sewage treatment plants, water ways, swimming excavators, dredgers, in agriculture, forestry and systems with high oil leaking rate. The biodegradation of lubricants proceeds by enzymatic action of microorganisms. The final products of degradation are CO,, H,O and the biomass of decayed microorganisms, in the case of anaerobic degradation even CH,. The mechanism of aerobic degradation is better known than that of the anaerobic one. Several methods of assessing the aerobic degradation have been proposed. In Europe the CEC-L-30-T-82 has been adopted. It consists of an intensive aeration of 0.05 ml tested oil in the form of a 15 % CCl, solution in 150 ml of water containing nutritive inorganic salts (ammonium phosphates and other cations and anions), yeast extract and 1 ml inoculum, i.e., 1 ml outlet-water from a biological sewage treating plant. After 3 weeks the subsisting 245
oil is extracted with CCl, and its content determined by IR spectroscopy at the 3930 cm-l band. Several alternative methods have been developed (297) based, e.g., on the formation of water, CO,, release of energy, multiplication or decay of microorganisms, formation of biomass, gravimetric determination of the amount of remaining oil (lubricant). In the widespread closed bottle test the oiloxygen content in water saturated with oxygen and containing the tested oil is determined volumetrically, i.e., the total oxygen demand of the oil (TOD), the chemical oxygen demand (COD) at the start and the biological oxygen demand after x days of exposition (BOD,, as a rule BOD,), all in 02/mg oil. Actually they are not universally accepted tests for assessing the soil alteration and anaerobic degradation.
The accumulated knowledge allows to formulate the following rules: Straight chain hydrocarbons and straight alkyls are readily degraded. The process starts at the end of the chain, its mechanism and the enzymes involved are well known. Paraffin waxes are degrading this way. Branching of the chain, i.e., the presence of tertialy and quaternary carbons slows down the process. Polyalphaolefins (PAO) are markedly resistant. Double bonds have a similar effect, e.g., in polypropylene and polybutylene oils, where however branching is the primordial cause. Cyclic and aromatic hydrocarbons are degraded first in the alkyl chain where the cited rules are valid. The nucleus in benzene is transformed via diols (pyrocatechol), in naphthalene via 1,2-dihydro-1,2-dihydroxynaphthaleneto dicarboxylic acids and CO, and H,O. Similarly cycloalkanes (alkylated cyclohexane or dekahydronaphtalene) are rather fairly degraded. By contrast polycyclic and polyaromatic hydrocarbons with condensed rings, such as present in mineral oils, are resistant, even the dearomatised white oils( !), as well as synthetic biphenyl and terphenyl lubricants. Didodecylbenzenes are biodegradable, provided dodecyls are not branched. Natural straight-chain fatty products, alcohols, acids, their esters as, e.g., monoesters in sperm oil, triacylglycerols in fatty oils belong to the fully degradable lubricants. Analogous esters based on modified fatty acids (e.g., isostearic acids) and neoalkylpolyols (e.g., trimethylolpropane, pentaerythritol) belong to high degradable oils. Fully degradable are also the polyoxyethylene glycols, not so the polyoxyprophylene glycols, where the branching in the chain and water insolubility impede their degradation. Generally, the synthetic ester oils of carboxylic, phosphoric or silicic acids are fairly, even well, degraded, provided they respect the expressed rules. Esters based, e.g., on trimethyladipic acid, phtalic or trimellitic acid, on oxoalcohols prepared from dimers and trimers of propylene and butylenes are appreciably resistant. Polyphenyl ethers are very resistant. Halogenated oils are unacceptable. Lubricating greases are biodegradable if their components, the thickener and the oil, are biodegradable. Na, K, Li, Ca soaps meet this requirement. Ba and Pb soaps are inadmissible. Vegetal fatty oils, some ester oils and polyoxyethylene glycols are appropriated as oil phases.
246
Addirives may affect the biodegradability. Antioxidants suitable for foods are acceptable in oils. Biocides must be avoided. In unclear situations inspiration may be found at producers or users of polymers, pesticides and surfactants. A new criterion of ecological acceptability, the behaviour towards water, has been introduced. In a DIN tentative specification 4 classes of lubricants are distinguished: lubricants not impairing the quality of water (class 0), slightly impairing (class I), impairing (class 2), heavily impairing (class 3). This criterion is embracing the toxicity of the lubricant to man, animal and vegetal land and aquatic life as defined in existing legal prescriptions. In the Table 3.55 some typical lubricants are compared from the ecological standpoint.
Table 3.55. Biodegradability and Behaviour towards Water of Some Lubricants Biodegradability (%)
Rape-seed oil Polyoxyethylene glycols Polyox ypropylene glycols Polyoxyethylene-propyleneglycols Aliphatic esters Aromatic esters Polyalphaolefins (PAO), white oils Mineral oils Emulsions O/W
99 99 15
15 - 55 75 - 100 45 - 80 4 5 4 5
<15*
Behaviour towards water (class) 0 0 2 1-2 0- 1 1-2 1-2
2 3
* in the presence of biocides; in their absence >15
Biodegradable lubricants find application as hydraulic fluids, metal working oils and emulsions, quenching fluids, slide-way oils, compressor lubricants, gear oils and greases, internal combustion engine oils (especially for two-stroke motorcycles, outboard motors, chain-saws, incl. chain lubrication), rubber and textile lubricants. The area of their use is extending.
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-
25 1
198. U.S. Patent 4,061,566. K. : Erdol u. Kohle, 34, 1981,296. 199. EISENBACH, W. - NIEMANN, 200. BILLON, A. et al. : 10th World Petroleum Congress PD. 1911. Bucharest 1979. 201. SEQUEIRA, A. et al. : Hydrocarbon Processing, W 9 , 1979, 155. 202. VESELY,V. : Ropa a Uhlie, 11, 1969,297. 203. Chem. Ind., 32, 1980,736; 33, 1981, 3. H. : Tribology. Amsterdam, Elsevier 1978. 204. CZICHOS, J. : Lubr. Eng., 11, 1955,389. 205. SEJOURNET, J. - DELACROIX, 206. GRAUE, G. : Schmiertechnik, 9, 1962,245. 207. LODES,A. : Unpubtished Comments. Slovak Inst. of Chemical Tech. Bratislava. J. L. : Accounts Chem. Res., 11, 1978,296. 208. KAMARCHIK, P. Jr. - MARGRAVE, F. : Wear, SO (2), 1978,343. 209. YAMAMOTO, Y. - HIRANO, 210. GROSZEK, A. J. : ASLE Trans., 24, 1981,490. Y. : ASLE Trans., 24, 1981,497. 21 1. KING,J. P. - ASMEROM, R. L. : Technical Notes, 197, Washington, NASA 1959. 212. BUCKLEY, D. H. - JOHNSON, 213. TAMBORSKI, C. et al. : Preprints ACS, Petrol. Div. 27 (2). 1982, 314. 214. GAVIN,D. F. et al. : Preprints ACS, Petrol. Div. 27 (Z), 1982,349. 215. DOLLE,R. E. : Ind. Eng. Chem., Prod. Res. Dev., 6, 1967, 177. 216. CAMPEN, M. et al. : Hydrocarbon Process., 61 (2). 1982,75. L. F. : Preprints ACS, Petrol. Div. 27 (2), 1982, 349. 217. NELSON, N. T. - HECKELSBERG, 218. SHUBKIN, R. L. et al. : Ind. Eng. Chem., Prod. Res. Dev., 19, 1980, 15. 219. BRENNAN, J. A. : Ind. Eng. Chem., Prod. Res. Dev., 19, 1980,2. 220. YAN,T. Y. et al. : Ind. Eng. Chem., Prod. Res. Dev., 17, 1978, 366. 221. BARTZ,W. J. (Ed.) : Intern. Jahrbuch d. Tribologie, Grafenau-Hannover, Exp. Vincenta Verl. 1982. 222. SWNBERG, S. A. et al. : Podsipniki skolzenija s gazovoj smazkoj. Moscow, MaSinostrojenije 1979. S. V. : In : Intern. Jahrbuch d. Tribologie, Grafenau-Hannover, Exp. VincentaVerl. 223. PINJEGIN, 1982,525. 224. BHUSHAN, B. : In : Intern. Jahrbuch d. Tribologie, Grafenau-Hannover, Exp. VincentaVerl. 1982, 575. 225. Scorn, R. N. et al. : Preprints ACS, Petrol. Div., 24,2, 1979, 818. 226. SCHMID, W. A. : 4th Intl. Colloquium. Synthetic Lubricants & Operational Fluids. Esslingen 1984. 227. MILLER,J. W. : ibid. 228. SZYDYWAR, J. : ibid. 229. WILDERSOHN. M. : ibid. 230. PODEHRADSKA, J. et al. : ibid. 231. DAIN,C. J. - JAMES,B. D. : ibid. 232. WENZEL, B. : ibid. 233. PYTKO,S. et al. : ibid. H. : ibid. 234. RUMPF,Z. - SCHINDLBAUER, K. : ibid. 235. KRISTEN, U. - MULLER, 236. JENSEN, R. K. et al. : ibid. 237. SNYDER, C. E. : ASLE Trans., 1971,237; 1982,299. 238. VARTANIAN, P. F. : NLGI Spokesman, 6, 1985,98. 239. ATKINSON, J. G. et al. : Can. J. of Chem., 45, 1957, 151 1. 240. STALLINGS, L. et al. : NLGI Spokesman, 9, 1984, 184. 241. BUCKLEY, D. H. : Surface Effects in Adhesion, Friction, Wear and Lubrication. AmsterdamOxford-New York, Elsevier 1981. K. : Meeting on Chemistry of Fluorine. Meznf Louka, Czechoslovak Society for 242. KLOUDA, Science & Technology 1984. 243. STEPMA,V. - CHVOJKA, M. : Unpublished Works. Res. Inst. Fuels Lubr., Praha 1985.
252
244. FUSARO, R. L. - SLINEY, H. E. : NASA Technical Note D - 5097, 1969. 245. Pamphlets from Pennwalt, Tulsa, Oklahoma (USA). 246. CAMPEN, M. : Techn. Meeting of Mechanical Engineers. Stockholm 1980. 247. Ind. Lubr. Tribol., 5/6, 1985, 116. 248. Ind. Lubr. Tribol., 9/10, 1985, 194. 249. STEPINA, V. et al. : Ropa a Uhlie, 30,6, 1988, 373. 250. MUIR,R. J. : NLGI Spokesman, 7, 1988, 140. 251. HALAS . et al. : Analysis of Complex Hydrocarbon Mixtures. Amsterdam, Elsevier 1981. 252. HOTIER,G. et al. : Preprints ACS, Petrol. Div., 132, 2, 496. 253. HOBSON, G. D. : Modern Petroleum Technology. Chichester, J Wiley 1984. 254. BUSHWELL, J. et al. : Proceedings 10th World Petroleum Congress. 1979, 10. 255. PUCCI,A. et aI. : Les Fluides Supercritiques. M. Pernut (Ed.), Pont-A-Mousson 1987. 256. Hydrocarbon Processing, 6Y9, 1982, 101. 257. Proceedings Int. Conf. Waste Oil Recovery and Reuses. Washington DC.,copyright by Am. Petrol. Re-Refiners, 1978. 258. RIEDIGER,B. : Die Verarbeitung des Erdoles. Berlin, Springer 1971. 259. GEARHARDT, J. A. - GARWIN, L. : Hydrocarbon Process., 58’1, 1979, 12. 260. TREBICKY, VI. : Thesis. Prague, Inst. of Chem. Technology 1988. 261. ROSS, W. J. et al. : Hydrocarbon Processing, 57/5, 1978, 112. 262. IFP Technology for the Refining of Lube Base Oils, White Oils, Waxes. Ref. 29,676, 1982. 263. AGAFONOV, A. V. et al. : Sbornik trudov VNIINP. 18. Moscow, 1978.2. 264. BAn, R. : Ropa a Uhlie, 24, 1982,471. 265. NOVAK,V. - VYBfHAL, J. : Ropa a Uhlie, 24, 1982,471. 266. STANESTI, D. et al. : Makromol. Chemie, 102, 1967, 115; Chim. e Ind. (Milano), 50, 1968, 206; Wear, 18, 1971, 85. 267. SNYDER, C. E. Jr. et al. : Lubr. Eng., 35, 1979,451. 268. JONES,W. P. Jr. - SNYDER, C. E. Jr. : ASLE Trans., 23, 1980, 253; In: 221,395. 269. CHAO,T. S . et al. : Preprints ACS, Petrol. Div., 24/2, 1979, 836; Ind. Engng. Chem., Prod. Res. Dev., 22, 1983, 357. 270. TAMBORSKI, C. et al. : Ind. Eng. Chem., Prod. Res. Dev., 22, 1983, 357. 271. U.S. Patents 3,758,407( 1973); 3,791,973 ( 1974) (Esso Res. Eng. Co.). 272. U.S. Patent 3,809,650 ( 1974) (Mobil Oil Co.). 273. CAMPBELL, 1. D. et al. : NLGI Spokesman, 40, 1976, 193. 274. SCHMID, G. : Intern. Jahrbuch d. Tribologie. Grafenau-Hannover, Exp. Vincenta Verl. 1982,421. 275. KAJDAS, Cz. - DOMINIAK, M. : Schmierungstechnik, 16, 1985,205. 276. ROEMER, E. : Intern. Jahrbuch d. Tribologie. Grafenau-Hannover, Exp. Vincenta Verl. 1982,546. 277. JAMES,D. H. et al. : Wear, 34,1975, 373. 278. FROMIN, I. 1. : Elektrodugovaja naplavka. Moscow, Masgiz 1961. 279. HARTLEY, N. E. W. et al. : Friction and Wear of Implanted Metals. London, Plenum Press 1974; Wear, 34, 1975, 373. P. : Intertribo ‘84. Vysok6 Tatry, CSSR, Czechoslov. Soc. for Science 280. BLASKOVIC, P. - BALLA, and Technology 1984. 281. Tribotechnika v teorii a praxi (Tribotechnology in Theory and Practice). Plzefi, Czechoslov. Soc. for Science and Technology 1984. 282. SMRKOVSKY, E. : ibid., 140. 283. KRAL,E. : ibid., 126. 284. ASSMANN, J. : ibid., 134. 285. WILSON,R. W. - SHORE, E. B. : In : Industrial Tribology. Jones, M. H. - Scott, D. (Ed.). 286. CS Patents 130 296 and 143 610 (1966) (VeselS;, V. et al.). 287. KLAMANN, D. : Lubricants and Related Products. Weinheim, Verl. Chemie 1984. 288. TSUYA,Y.et al. : Lubr. Eng., 31, 1975,402. 289. LIPP,L. C. : Lubr. Eng., 32, 1976, 574. 290. BARTZ,W. J. (Ed.) : Festschmierstoffe i. d. Praxis, Grafenau. Wurt, Expert Verl. 1978.
253
291. DEVINEY, M. L. - OGRADY,T. H.(Ed.) : Petroleum Derived Carbons. Washington DC.,h e r . Chem. Soc. 1976. 292. KOZAK,P. - RABL, V. et al. : 32nd and 33rd Annual Conferences on Petroleum. Bratislava 1987 and 1988. 293. LAPPLE, W. : In: Intern. Jahrbuch d. Tribologie.Grafenau-Hannover, Exp. Vincenta Verl. 1982, 445 294. GANSHEIMER, J. : Intern. Jahrbuch d. Tribologie. Grafenau-Hannover, Exp. Vincenta Verl. 1982, 455. 295. U.S.Pat. 210434 and 210435 (1988). 296. SHEN,Q. S. et al. : 7th Internationales Kolloquium. Esslingen 1990. 297. V O L ~M. , : Schmiemngstechnik, 22, 1991, 177.
254
CHAPTER FOUR ADDITIVES
It should be apparent from the first three paragraphs of this book that many of the properties which all but the simplest lubricants need in order to fulfil their functions in the design and operation of machines can only be obtained - in absolute terms or economically - by using chemical additives. In this chapter, we examine the nature, composition and properties of many different types and species of addititive in all the classes of lubricants and special fluids already surveyed. Much of this examination relates not only to the effect of the indidual additives themselves, but also the product of the interaction between additives and lubricant bases, between additives and between all of these and the surfaces to be lubricated.
Additives can be defined as substances which improve the performance of lubricants, either by imparting new properties to a base oil or grease, or by enhancing properties already present. The performance of lubricating oils is classified according the effects of base oil and additives respectively is summarised in Table 4.1 below. Table 4.1. Influence of Base Oil Stock and Additives on the Properties of
Lubricating Oils Effects dominated by base oil choice
Effects common to base oil and additives
Effects dominated by additives
Density Evaporation rate Flash- and fire-points Compressibility
Viscosity Viscosity-temperature characteristics Pour-point Low temperature rheology Resistance to foaming Thermal stability Oxidation stability Carbonisation residue Miscibility and solvent power Colour Physiological effects Odour
Detergent-dispersanteffects Emulsibility and demulsibility Friction properties (lubricity) Anti-wear properties Anti-corrosion and anti-rust properties Adhesion to metal surfaces Alkalinity Ash content
Specific heat Thermal conductivity Electrical conductivity Resistance to radiation
The need for improvement in the performance of lubricants by the use of additives has been brought about largely by the increased technical specifications 255
of machines - more severe conditions of extended operation, attempts to prolong the service life of machines and decrease the consumption of lubricants and the adoption of new and more sophisticated processes. Additives offer very important improvement possibilities and are indispensable in the production of modem lubricants. The use of additives, in the modem sense, began in the 1930’s and enormous growth has been seen since in both their production rates and the scope of their applications. Annual world-wide consumption of additives for lubricants and special oils is estimated at more than 2.5 million tonnes (including, in 1990, 0.2 million te zinc dialkyldithiophosphates, 0.8 million te metallic detergents, 0.5 million te ashless dispersants and 0.5 million te VI improvers). The major portion is used in the production of engine oils. For example, in 1990, the US additive consumption could be broken down into 64% used for blending engine oils (37% for multi-grade automotive engine oils, 20% for monograde engine oils and 7% for other engine oils) and the remaining 36% for blending industrial oils and greases (13% for hydraulic and automatic transmission fluids, 8% for cutting and metal-forming fluids, 2% for gear oils, 13% for other types of oils and greases (396). The increased dosage of additives in different lubricants (especially in engine oils) has led to a much higher increase in the consumption of additives than that of finished lubricants. The production of additives has become an important and innovative branch of the speciality chemical industry.
Usage of most lubricating oil and grease additives may be divided into two groups according to their function: (1) additives which improve the physical properties of the lubricant, (2) chemically-acting additives, whose effects can usually be measured by some efficiency characteristic of the lubricant. However, the boundary between these two groups is not clearly defined. The type of additive determines the importance and extent of its application. Hence, additives may be classified as primary and special. Primary additives are commonly employed in the most frequently used groups of lubricating oils, e.g., automotive oils. Special additives impart specific properties to certain types of lubricants. The largest quantity and number of additives are used for the improvement of petroleum-based oils in the manufacture of finished engine oils. Many lubricant additives exhibit multifunctional properties and, for instance, some typical antioxidants simultaneously act as corrosion and rust inhibitors, whilst also acting as metal deactivators and anti-wear agents. Many viscosity index improvers also act as pour-point depressants and some increase the adhesivity of the oil to metal surfaces. Some film-strength improvers also improve lubricity. Both synergistic and antagonist effects can occur when a number of different additives is used together. This must be taken into account when selecting both the appropriate compounds for a particular application and the correct dosage to achieve the overall desired effect. Manufacture of additive components themselves is an important area of chemical technology, but selection of the best balance of component species and relative dosages for each application is an equally demanding and expensive field of study. The identification of additives present in oils and, especially. quantitative determination of individual components can be a very difficult task, in both multi-component concentrates and in finished oils.
256
Dialysis, together with suitable spectroscopic and chromatographic techniques, can be recommended as a convenient analytical tool (182,192,319). In dialysis, similar in principle to ultra-filtration, smaller molecules diffuse relatively rapidly through the pores of a membrane, such as a rubber diaphragm, from a solution of the multi-component additive dissolved in petroleum ether or n-heptane, whilst larger molecules dialyse very slowly, if at all. Base oil, alkyl- and arylphosphates, esters of fatty acids, alkylphenols, alkyl borates, dialkyldithiophosphates and metal dithiocarbamates have small and easily dialysed molecules. Molecules of metal phenolates and sulphonates dialyse slowly. Molecules of basic metal phenol sulphides, sulphonates and salicylates, barium phosphonates, polyisobutenes and polymethacrylates do not dialyse at all. Sometimes, only the lower molecular weight portions of additive mixtures can dialyse, e.g., in polymethacrylates and succinimides. The individual components of the dialysate and the residue can be determined by IRspectroscopy, NMR or other methods, advantageously in combination (320). Some types of oil additives can be identified by the amount of the characteristic chemical elements. For example, zinc indicates the likely presence of antioxidants of the zinc dialkyldithiophosphate (ZDDP) type; higher amounts of nitrogen the presence of a succinimide-type ashless dispersant or other compounds with amino or amido groups. Calcium, barium and magnesium indicate the presence of ashforming detergents; both chlorine and lead are typical for high-pressure anti-wear agents, and silicon for anti-foams. Elemental analysis can thus reveal some of the characteristic elements typical in individual additive types and may prove useful in the identification of additives in the treated oil. Both emission and atomic absorption spectrometry are convenient methods for the determination of the metal content of lubricants, and have to a large extent supplanted in routine use the more traditional gravimetric, volumetric, chelatometric, colorimetric and photometric methods which are specified in many standard specifications.
In practical applications, additives must be fully soluble in the oil, with a high resistance to cloud and sediment (deposit) formation, even after long-term storage conditions and after temperature fluctuations. Some additives are mutually incompatible and, when they are used together, cloud and sediment can be formed. The same can happen when oils are mixed which contain incompatible additives. Compounded oils must therefore be tested not only for storage stability but also for blending with other finished oils and storage. The solvent properties of the base oil into which additives are compounded can have important effects. High viscosity index, paraffinic and hydrocracked stocks present particular blending problems. It is normal practice in the industry to test additives by blending into selected, “difficult” base stocks, as well as for miscibility with other blends which may be encountered in the application when lubricant reservoirs are replenished from other sources of supply. Storage stability can be tested either in accelerated tests under artificial conditions (the oil is heated to 120-150 “C, then cooled to the ambient temperature of storage and the amount of sediment measured by centrifugation), or under long-term storage conditions (the oil is, typically, stored at ambient temperature for 60 days, without mixing and without exposure to light. The homogeneity of the oil and the amount of sediment are then determined.). Additive compatibility in the case of oil-mixing is typically tested by mixing test oils in various ratios, from 1:l to 9:1,and testing the resultant mixtures by similar methods to those used for testing storage stability.
It is particularly important during additive manufacture, handling and storage to prevent quality deterioration - by over-heating, contact with moisture, mixing with incompatible additives - and to protect operators from possible harmful physiological effects ( I 72). 257
4.1 ANTIOXIDANTS (OXIDATION INHIBITORS) Two types of antioxidants are used, either separately or in combination, to improve the oxidation stability (or resistance to oxidation) of finished lubricants: (1) radical acceptors or scavengers, (2) peroxide decomposers. Antioxidants are sometimes combined with metal deactivators or passivators (1-3, 136). Current antioxidants are less effective or even ineffective in unrefined or under-refined oils. Such oils are less responsive to the normal types of antioxidants due to their high aromatic content. Some oil constituents, especially resinous materials derived from the crude oil, may make antioxidants ineffective.
Oils for turbine, hydraulic, electro-insulating and similar, relatively low temperature applications, are mostly blended with radical acceptors and metal deactivators, peroxide decomposers being used to a lesser extent. Low temperature oxidation, i.e., at temperatures up to about 150 "C,is involved under these operating conditions. In engine and other oils which work at high temperatures, peroxide decomposers are predominantly used, sometimes in combination with metal passivators and, to a lesser extent, radical acceptors. Radical acceptors also improve the storage stability of such oils. Radical acceptors, used in combination with peroxide decomposers,may markedly improve the overall oxidation stability of oils. Oils exposed to strong radiation may be blended with anti-rads.
4.1.1 Radical Acceptor-Qpe Low Temperature Antioxidants Common low temperature antioxidants which are based on radical acceptors or scavengers, InH, interrupt the oxidation chain reaction both by entering into the propagation phase and by hydrogen transfer to R* and RO,* radicals (I):
- ROOH + In*
ROO* + InH
- RH + In*
R* + InH
(4.1) (4.2)
Depending on the strength of the InH bond and the character of the In* radical, other reactions can occur. For instance: initiation: InH + 0,
- In* + HO,
(4.3)
RH + In*
- R* + InH
(4.4)
- In - In
(4.5)
propagation:
terminations: 2 In* 25 8
2 In* + ROO*
In* + R*
- InOOR
(4.6)
- InR
(4.7)
During termination reactions, inactivation of the radicals produced takes place; the higher the reaction rate and the less reactive is the In* radical, the more effective is the inhibitor and the slower the oxidation of the oil. By contrast, a higher reactivity of the In* radical enhances chain transfer reactions and the oxidation proceeds according to reaction (4.4)despite the presence of the inhibitor, albeit at a lower rate. Initiation by reaction (4.3) is also possible at high inhibitor concentrations (2,3,136).Because of the dependence of all the individual reaction rate constants on temperature and oil composition, the effects of inhibitors varies with oil type. In extreme cases, the inhibitor may even exhibit oxidising properties. Reactions in oils are even more complex because of the presence of “natural” inhibitors, already present in the virgin oil or formed during its oxidation. Moreover, the interaction of various oil constituents or their oxidation products with inhibitors and their oxidation products (I)may make the situation even less clear. The most frequently used low temperature antioxidants of the free radicalacceptor type are electron-donor compounds with particular reference to their reactions with peroxy radicals ROO.. As examples, phenols, aromatic amines and compounds which form phenol and amines by their decomposition may be cited. Phenols may either contain one ring with one or more functional groups, or a multiplicity of rings condensed or linked by bridges. Electron-acceptor compounds are less common as inhibitors. The alkylperoxy radicals do not readily lose their electrons and add to double bonds, except where these are activated. For example, in some polyaromatic hydrocarbons such as alkylnaphthalenes, anthracene or phenanthrene (4, 6), the following reactions are believed to occur:
+ROO*-
&@ ‘ H
HOO
H
Disproportionation products
>
‘OOR 102
@ ROO
H
In reality, the action of these aromatics is to slow down oxidation, rather than inhibit it.
259
Phenols Phenols alone do not act as antioxidant. Even mono-alkyl phenols show little activity (5, 136). The most important compounds, in this context, are sterically-hindered phenolic rings of the type: OH
R3
in which R,, R,, R, represent alkyls. These compounds react with ROOand ROO. radicals and eventually form quinones, via relatively stable semiquinone radicals (6-9): OH 0. 0
+ ROO.
(4.9)
(-ROOH)
I
I
R3
R3
InH + ROO*
ROO
R,
- InH-OOR -In-OOR + ROOH +ROO*
OH
(4.10)
OH
- R+ooR
R*+Ro;*
I
R3
R3
R3
InH + ROO.
- n-complex - In*+ ROOH + ROO*
R3
0
In* + ROO.
260
ROO
- InOOR
R3
(4.12)
(4.13) R3 (4.14)
(4.15)
2 In*
- In -In
(4.16)
The following alkylphenols are examples of antioxidants of this type:
tBU
tBu
Me
tBu
Me
where Me represents methyl and tBu tert-Butyl. In hydrocarbon medium, the effectiveness of these phenols increases from (I) to (V) (10) and is enhanced by the presence of at least one tert-butyl group in the ortho position with respect to the hydroxyl (10, 11). Both di-tert-butyl-4-methylphenol (111) and 2,4-dimethyl-6-tert-butylphenol(V) are commonly used. Methyl groups in positions 2 and 4 and tert-butyl groups in positions 2 and 6 stabilise the transition radicals by inductive, hyperconjugative and steric effects, increase the solubility of phenols in hydrocarbons, decrease their solubility in water and protect them from the direct influence of oxygen. The presence of the bulky tertiary alkyl group in position 4 is undesirable, as it hinders the reactivity of the ROO* radical in this position (7, 10). There is an optimum steric hindrance of the hydroxyl group; too intensive stenc hindrance depresses the course of the reaction, whilst weak steric hindrance enhances chain-transfer reactions. Substituents, such as alkyl, aryl and alkaryl, which increase the electron-donor character of the inhibitor, enhance its activity when present in the para or ortho positions. Electron-acceptor substituents (-Cl, -NO,, -COOH, etc.) have a negative effect on inhibition (12). 2,6-ditertbutyl-4-methylphenolis the most frequently used monohydroxy phenol used in mineral oils. It acts efficiently in both refined and highly-refined oils. Its efficiency is strongly depressed in poorly-refined oils. However, in contrast to some other antioxidants, it does not enhance the formation of sludges in the course of ageing ( I ) . Its disadvantage is its high volatility, which limits its usage at higher 26 1
temperatures. This phenol can also be used in synthetic oils and lubricating greases, provided they are not basic. The same holds true for other phenols of a similar type. 2- and 3-tert-butyl-4-methoxyphenolis used as a special inhibitor for oils or waxes which come into contact with food products. 2,6-di-tert-butyl-4-methylphenol is also believed to be non-toxic.
Poly-hydroxy single ring phenols, such as pyrocatechol, hydroquinone and pyrogallol and their derivatives, have not found application in mineral oils, but they can be used as inhibitors in some synthetic oils such as polyglycols. The same is true for hydroxylated, condensed aromatics, e.g., polyalkylated naphthols and dihydroxylated anthraquinones, which are used in ester oils. Bis-phenols with inter-ring bridges ortho or para to the hydroxyls are very important. 2,2’-methylene-bis-phenol,4,4’-methylene-bis-phenol, thio-bis-phenols and dithio-bis-phenols can be cited as examples. In these compounds, one alkyl in the ortho or para positions with respect to the hydroxyl should be tert-butyl. The activity of bis-phenols, which are obviously analogues of hindered monohydroxy phenols, is higher than would result from the doubled hydroxyls. The molar concentrations of bis-phenols required to achieve the same effect is therefore less than half that for mono-hydroxylic phenols. Another advantage is their much lower volatility. However, they have a greater tendency to form oxidation sludges, which increases in the order methylene-bis-phenolscthiobisphenolscdithiobisphenols(I). Examples of this type of inhibitor are 2,2’-methylene-bis-(4-methyl-6-tertbutylphenol) and 4,4’-methylene-bis-(2-methyl-6-tert-butylphenol), the latter being normally a little less effective. Both phenols are typical radical scavengers, unable to decompose hydroperoxides. They actually inhibit peroxide decomposition, as evidenced by studies with cumene hydroperoxide as model compound. On the other hand, analogous 2,2’-or 4,4’-thio- and dithio-bis-phenols impart both radical acceptor and peroxide decomposer properties and are, therefore, even more effective than methylene-bis-phenols (I).Their tendency to form sludges can be suppressed by other additives. It has been found that, in combination with dialkyldithiophosphates, these ashless bis-phenols effectively suppress thickening of engine oils, acid formation, bearing corrosion and the formation of insoluble materials in heavily-loaded passenger car gasoline engines (13). It is well-known that the replacement of sulphur by selenium can be advantageous in some antioxidants (38).In the case of bis-phenols, however, this substitution did not prove to be useful ( I ) .
Amines Primary, secondary and tertiary aromatic amines and diamines are effective antioxidants. The effectiveness of aliphatic amines is doubtful; they are used in lubricants only as colour stabilisers. Several views exist on the mechanism of amine action. In the case of primary and secondary amines, hydrogen transfer is assumed to be the first step, followed by the formation of comparatively stable iminoxyl radicals (13,15):
262
- R’N*R” + ROOH R’N*R” + ROO* - ROO+ R’R”N.0
ROO* + R’NHR”
(4.17) (4.18)
The R’N*R” radicals react further with peroxy radicals to form an end-product of the hydroxylamine type. Although this mechanism cannot be applied to tertiary amines, the possibility of hydrogen splitting from the ortho position with respect to nitrogen cannot be excluded. Theories which assume the existence of ionic transition states or ionic complexes formed by the electron shift have been postulated (14): ROO*+fi(-RO6+*6(,
or [ R O 6 * 6 C ]
(4.19)
or, alternatively, the following complex:
This complex further reacts with another peroxy radical to form molecular compounds (I6,17). Generally, the antioxidant properties of amines are enhanced by three factors: (i) effective delocalisation of the unpaired electron formally placedat the nitrogen atom, (ii) high electron density at the nitrogen atom to facilitate electron transfer to the radical, (iii) sufficient steric hindrance of the nitrogen to suppress chain transfer by the arylamine radical. In both mineral and synthetic lubricants, secondary amines are the most often used. Diphenylamine and its C-alkylated or N-alkylated, arylated or alkarylated derivatives, such as 4.4’-dioctyldiphenylamine,phenyl- 1-naphthylamine, phenyl-2naphthylamine, are the most important amines from this group. Both phenylnaphthylamines are very effective low temperature antioxidants, especially in mineral, non-aromatic oils and oils with optimum aromaticity. They act as radical acceptors and moderate peroxide decomposers. Their primary decomposition products act as inhibitors in the later period of ageing by oxidation. The inferior colour-stability and tendency to enhance the formation of insoluble products ( I ) is one disadvantage of these antioxidants. The established carcinogenicity of 2-naphthylamine makes the application of phenyl-2-naphthylamine dubious, in spite of its higher antioxidant efficiency as compared with pheny 1- 1-napthy lamine. Phenyl- 1-naphthy lamine and alky lated diphenylamines can even be used for the stabilisation of slightly basic synthetic oils and lubricating greases. For example, a mixture of 50 parts of phenyl-1-napthylamine, 25 parts of diphenyl-pphenylenediamine and 25 parts of p-diisopropyldiphenylamineis recommended as a universal inhibitor for lubricating greases.
263
Tertiary amines, such as N,N’-tetramethyldiaminodiphenylmethane(4,4’methylene-bis-[N,N-dimethylaniline])can be used in lubricating oils: Me
.Me
This amine is recommended for use in industrial oils. It is noteworthy that it belongs to a small group of antioxidants which, when used at a high enough concentration, is able to increase the stability of oils with a higher than optimum aromatics content. However, this inhibitor adds colour to oils, and like all antioxidants which do this, it enhances the formation of oil-insoluble oxidation products. Furthermore, its primary oxidation products seem to have only sIight antioxidant effects in mineral oil lubricants. Among the cyclic amines, 2,2,4-trimethyl- 1,2-dihydroquinoline, substituted in the 6- position, and its polymers are recommended for mineral, ester and polyglycol oils and greases: R
This inhibitor also possesses anti-ozone properties, important in plastics and elastomers. At the time of writing, its use is allowed in products which come into contact with food. In addition to phenols and amines, aminophenolic inhibitors are used. One example is 2,6-di-tert-butyl-4-N-dimethylaminomethylphenol: OH
This compound is used in industrial oils, e.g., turbine oil, in diester lubricants and also in lubricating greases. In some respects, it is similar to N,N’-tetramethyldiaminodiphenylmethane, already mentioned. At a suitable concentration, it can improve the properties of oils with higher aromatic contents, but its efficiency is lower. As in the case of 2,6-di-tert-butyl-4-methylphenol, it impedes the formation of insoluble oxidation products. However, it is very volatile, and its effectiveness, especially in non-aromatic oils and oils with optimum aromaticity, is lower. Contrary to expectations, a lower tendency, in comparison with other antioxidants, to form acid oxidation products has not been observed (I). Another aminophenol, commonly used in the USSR, is 4-hydroxydiphenylamine: 264
This has proved especially useful in oils of lower viscosity, e.g., in transformer oils. Many other compounds have been proposed as low temperature antioxidants of the radical acceptor type. Some of them have been used for special purposes, but they have not achieved broad acceptance (270). The following general conclusions can be postulated for the use of synthetic low temperature antioxidants in mineral oil lubricants: Synthetic antioxidants increase the oxidation stability, especially, of non-aromatic oils and oils of optimum aromaticity. The optimum composition of oils with respect to aromaticity shifts towards oils of lower aromaticity if both the inhibitor concentration and the severity of oxidation increase. Consequently, non-aromatic oils are more responsive to inhibitors during the early stages of oxidation, i.e., in the early stages of the life of the oil, whereas oils of optimum aromaticity, adequately blended with additives, are more responsive to inhibitors when the oil is more fully oxidised, i.e., in the later stages of its working life. Oils of high aromaticity are exceptional, in that they can only be blended with a few special inhibitors; inhibitors which can be widely and generally applied are not yet available for highly aromatic oils. When the more common inhibitors are used in these oils, they do not improve oxidation stability and can even cause it to deteriorate. Generally, it is necessary to avoid over-treating with inhibitors, whether they are “natural” or “synthetic” (1). General recommendations on similar lines regarding the choice of inhibitor for synthetic oils cannot yet be formulated. The choice of inhibitor for both polyester oils, based on polyols, and diester oils, based on mono-alcohols or polyglycols, depends on the temperature conditions under which the oil is to be used. Stericallyhindered phenols are recommended for temperatures below 125 “C, amines, selenides and phenothiazines for temperatures up to 150 and 175 OC (28).Recently, amines such as phenyl-1-naphthtylamine at a concentration of 1-2%, in combination with C- or N-substituted diphenylamines have been applied at temperatures up to 200 “C. For higher temperatures (up to 260 “C), polyalkylated naphthols, polyhydroxylated biphenyls and hydroxylated benzophenones (19) have been suggested. C- and N-alkylated phenothiazines, phenothiazine carboxylic acids (20) and some silicon compounds, such as 5-ethyl-10,lO’-diphenylazasilazene(I) ( 2 4 , and boron derivatives, such as lO-hydroxy-9,1O-boroxazophenthrene, also exhibit the required properties (11) (22).
265
These are frequently used in combination with both phenyl- 1-naphthylamine and a suitable metal deactivator (23).Alternatively, some metallic substances showing a synergistic effect, such as organo-tin compounds, e.g., (III) and (IV) (24) and alkali metal (Li,Na, K, Rb,Cs) salts of carboxylic acids (25), can be used together with compounds (I) and (11) above. Rl\
/R2
R3/ sn\R4
(111)
iF [ Sn
x-
[1:
2
- Sn / R1
(IV)
Phosphate ester oils are usually very resistant to oxidation, so antioxidants are not necessary. This is even more valid for polymers of trifluorochloroethylene. On the other hand, the diesters of fluorinated alcohols and dicarboxylic acids are somewhat less stable, and, moreover, almost insensitive, even antagonistic to common antioxidants. Both phenyl- 1-naphthylamine and phenothiazines have proved useful in polyglycol ester oils, as have some phenols, e.g., dihydroxyphenols (hydroquinone) and dibutyl-4,4’-diphenylolpropane,and some amines, such as substituted pphenylenediamines (N,N’-di-(2-naphthyl)-p-phenylenediamine,and in polyglycols, trimethyl- 1,2-dihydroxyquinolineand its polymers. Antioxidants developed for polyphenylether oils are very interesting in that they also inhibit the oxidation of aromatic compounds. Useful compounds include some organometallics, especially transition metal (Ti, Mn, Fe, Co, Ni) acetylacetonates. Similar compounds can be recommended for aromatic ester oils (26). The inhibition of silicone oils against oxidation is very difficult. The solubility of compounds of a different nature from the silicone is limited and special techniques sometimes need to be used. Silicone oils show good resistance to oxidation, but tend to become gelatinous at higher temperatures. Suitable inhibitors such as polyaromatics (e.g.,1.1 ’-dinaphthyl-1,Zbenzanthrene,fluoroanthrene)are effective up to a maximum temperature of 285 “C (27).Some organometallic compounds are especially recommended - ferrocene (cyclopentadienylmanganesetricarbonyl)(28). tin dibutyl laurate (29) and cerium (30) and selenium (dodecylselenide)derivatives. It is interesting to note that polymethylsilicones themselves retard the oxidation of mineral oils under static conditions. This is probably a surface phenomenon; the thin superficial layer of silicone hinders the transport of oxygen from the surface into the bulk oil (31). In contrast to silicone oils, the oxidation stability of silicon acid esters is relatively low. Nevertheless, they are very responsive to antioxidant treatment. The most effective antioxidants in this case are aromatic amines (especially phenyl-lnaphthylamine). Because peroxide formation is suppressed, hydrolytic stability increases at the same time as oxidation stability: the formation of peroxides and their 266
decomposition products, particularly water, is effectively controlled.
The Susceptibility to Oxidation Inhibition of Oil Lubricants During the oxidation of an oil, the sequence of reactions 2.82a and 2.82b which lead to non-branching oxidation chains must be stopped. An inhibitor interferes with the oxidation by reaction 4.1 (page 258) when a hydroperoxide is formed which is stable under the conditions of the oxidation. In reaction 4.2 which is atypical - the radical R* reacts very rapidly with oxygen - reactions 2.82a and 4.6 - with the formation of unreactive products. These reactions can be summarised in the general scheme: InH + nROO* = stable, unreactive products
(4.20a)
The stoichiometric coefficient n represents the number of ROO. radicals reacting with one inhibitor molecule. Inhibitors of the radical-acceptor type can be natural, synthetic or formed during oil oxidation. Very often, a mixture cif inhibitors is present, together with other additives, fresh or aged, in laboratory, static or road tests. The total amount of inhibitor present can be determined from the oxygen uptake of the oil, measured over time under suitable conditions (tig. 4.14. When effective antioxidants are present in the oil, its properties (viscosity, acidity, deposit formation) should not change during the tests (fig. 4.1b). (abs 1 I021
h
nllnl
b
a
Fig. 4.1. a) Determination of the induction period T~and T~ b) Relations between the induction capacity and the changes characterising the oil ageing (e.g., of viscosity, acidity, deposits) in dependence on time, on length of path (idealized model) - the broken line after the addition of antioxidant The “titration” of antioxidants by peroxy radicals is based on these principles (384). The rates of oxygen absorption by the oil under investigation and cyclohexene (Cy-H) are determined in an inert solvent (hexadecane) and the reaction is initiated by the decomposition of azobisisobutyronitrile (ANNA). The titration is carried out at a temperature such that the decomposition of primary hydroperoxides is negligible. 1 cm3 of Cy-H plus 9 cm3 of n-hexadecane is placed in a suitable reactor (39Z).0.5 cm3 of antioxidant InH solution in n-hexadecane (2-20.104 mol) is added at 60 “C. In a separate test, an equivalent amount of the oil under investigation in hexadecane plus 0.5 crn3 of AN-NA solution in chlorobenzene (0.03 0.1 mol) is used. The rate of oxygen absorption is measured until all the antioxidant is consumed. The following reactions occur: AN-NA = 2Aa + N2
(4.20b)
267
A. + 02=AOO* A 0 0 0 + CyH = AOOH + Cy*
(4.20~) (4.2Od)
The cyclohexenyl radical is further converted by the following reaction sequence:
cy. + 0, = cyoo. CyOO.
+ CyH = CyOOH + C y
(4.20e) (4.200
in the presence of the inhibitor, reaction 4.20f is suppressed: CyOO* + InH = CyOOH + In* CyOO. otherwise a chain reaction would ensure. Generally,
+ In*= CyOO-In
InH + nCyOO* = non-reactive products
(4.2Oi)
The oxygen absorption rate is plotted and the induction periods determined graphically for strong (z,) and total (3 inhibitor content. The length of the induction period is proportional to the inhibitor concentration and inversely proportional to the initiator concentration.The antioxidantconcentration can be calculated as follows: z R. [In HJ = (m~l.dm-~) (4.20j)
n
where Ri is the the rate of radical formation, for azobisisobutyronitrile Ri = 4.6.10-8 dm3.s-' at 60 OC. For a given antioxidant at a known concentration, equation (4.20j) enables comparison to be made of the effectiveness of different antioxidants in the same medium or the same antioxidant in different media. The stoichiometric coefficient n can be determined by means of equation (4.20j) only if the antioxidant is not substantially destroyed during the induction period. For hindered phenols n = 2, for bis-phenols n = 4, for zinc dialkyldithiophosphates which also act as radical acceptors n = 1 to 4, typically 3. From the value of the stoichiometriccoefficient, the mechanism of antioxidant action can be deduced. When there is insufficient information on the nature of the antioxidant, the total inhibition capacity of the oil can be determined by equation (4.20k): 7 R. n[ln HJ = 2 (mol.dm3) (4.20k) [OI where [O]is the concentration of oil in the test solution. The expression n[ln HJ applies to all antioxidants of the radical acceptor type. Metal deactivators and peroxides are not included in this definition. Over a period of time, determination of the value of this expression can be used to follow the gradual consumption of antioxidants, to monitor synergistic and antagonistic interactions between antioxidants and other additives and to study the influence of the conditions under which the oil has operated (e.g., temperature). In addition, the conditions can be determined under which it is necessary to recondition or rejuvenate the oil by addition of fresh antioxidant, or estimation of the oil drain interval (385,389).
4.1.2 Metal Deactivators The presence of metals and their compounds, even trace amounts present as impurities, is a very important factor in the ageing of lubricants. The catalytic action of metals and especially transition metals and their compounds depends on several factors which are still not understood in detail - metal type, nature of the oxidised substrate, oxidation products which are able to coordinate their bonding to metal,
268
etc. All these factors influence the redox potential of the metal compound - probably the most important variable - and the solubility of the metal compound and can be involved in steric hindrance (32). Metals and their compounds can influence the oxidation in the initiation, propagation and termination phases. The following processes can occur in the initiation phase: - reaction of metal compounds with the substance, e.g., RH + M"+
- R* + H+ + M("-') - R* + M"+
(4.21)
- reaction of metal compounds with hydroperoxides, e.g., ROOH + M("-')+
- RO* + HO- + Mn+
(4.22)
HOa + M("-1)+
(4.23)
HO- + M"+
- in summary: ROOH
- RO* + HO.
(4.24)
- or at higher peroxide concentrations: ROOH + Mn+
- ROO* + H+ + M("-')+
- ROO+ HO- + M"+ 2ROOH - RO* + ROO* + H20
ROOH + M("-')+
- in summary,
(4.25) (4.26)
(4.27)
- activation of the oxygen molecule:
02+
M"+/
M("+')+ 00,
+ RH
/
R* + HOO* + M"+
(4.28)
Mn++ 00 In the propagation phase, the activation of oxygen by metals can occur, leading to:
- ROO* ROO* + RH - ROOH + R* R * + 0,
(4.29) (4.30)
In the termination phase, radicals can be converted into anions by the action of metal compounds: ROO* + M@-')+
- ROO- + M"+
(4.31)
and metal compounds can accelerate the conversion of peroxy radicals into nonradical products: 2R00* (+ M"+)
- molecules (+M("-')+)
(4.32)
Note that the metals act as antioxidants in reactions (4.3 1) and (4.32). 269
Oxidation can be influenced by metals without any changes in their valency; zinc compounds, for example, interact in this way. The following reactions are believed to occur:
- ROOMX + HX
ROOH + MX,
- RO* + MXO* MXO* + ROOH - MXOH + ROO* ROOMX
MXOH + HX or, in summary: 2ROOH
(4.33) (4.34) (4.35)
- MX, + H,O
(4.36)
- ROO*+ ROa + H,O
(4.37)
Copper, iron and lead are the metals which most often come into contact with lubricants and their power to influence oxidation in lubricants normally descends in this order (34). However, this order itself may change, depending on the reaction conditions. It is different for homogeneous and heterogeneous catalytic action and depends on temperature. For instance, below 150 "C, copper is more active than iron, whereas above 180 OC, iron becomes more active than copper (36). The metal-catalysed oxidation of lubricants is greatly influenced by electrondonor substances. They can act as oxidation promoters, if their complexes with metals are unstable, or as metal deactivators if these complexes are stable. Moreover, they act as antioxidants if they irreversibly transfer an electron to the metal, or decompose the peroxides. Finally, if the electron-donor group is sterically-hindered, they are ineffective as additives (35). The prevention of oxidative ageing enhanced by metal ions can be achieved with the so-called metal deactivators. The most effective are nitrogen compounds, which convert the ions into catalytically inactive chelates. Metal passivators are substances which reduce the solubility of metals by interaction with its surface. Some examples of Schiff's bases very important as metal deactivators, include N,N' -bis-salicylideny1- 1,2-diaminopropane: H I CH=N-CH-CH,N=C
q
OH
I
CH3
D
HO
and the analogous, but less soluble, diaminoethane (BSED) derivative, anthranilic acid, some acid amides (oxamides), thiadiazoles, imidazoles, pyrazoles (mercaptobenzimidazole), mercaptobenzthiazole, mercaptobenz-imidazoline,etc. Metal deactivators are commonly combined with radical acceptors in lower temperature applications, at lower concentrations (5 - 30 p.p.m.). Synergisticeffects can frequently be observed in these systems. Some metal deactivators are multi-
270
functional; for instance, anthranilic acid acts as radical scavenger, metal deactivator and anti-corrosive agent (37). The use of metal deactivators can, however, bring some problems. Their effect depends on the type of metal, their concentration, the oil environment in which they operate and other, often unclear factors. An example of this variety of behaviour is provided by the low temperature oxidation of tetralin (used as a model cycloalkane aromatic), initiated by azobisisobutyronitrile.Oxygen absorption was found to be hindered by BSED in the presence of Cu,Zn, Ni and Co acetonylacetonates, but BSED was less effective in the presence of Fe acetonylacetonateor caprylate and Cu stearate. Under similar conditions, anthranilic acid was effective in the presence of both elemental copper and its salts, as well as the acetonylacetonates of Ni and Zn. However, it was almost ineffective in the presence of Fe and Co acetonylacetonates and Fe caprylates. N-phenylanthranilicacid, acting simultaneously as a diphenylamine antioxidant, was effective in the presence of Co, Ni and Zn acetonylacetonatesand Cu stearate, but was practically ineffective in the presence of iron salts, Cu acetonylacetonate and elementary copper. The effect of the additive is expressed either by the reduction of oxygen absorption without change in the induction period, or by a marked prolongation of the induction period after which rapid oxygen absorption may follow (35).
The deactivator must therefore be selected with much care, because it is unusual to know the form in which metal is likely to be present in the lubricant. Certain chelates, such as the Co-chelate of BSED, activate the oxygen and can act as oxidants. They are recommended, for example, as catalysts for the oxidative “sweetening” of gasolines (39).
4.1.3 Peroxide Decomposer Antioxidants ROO. radicals are produced and disappear very quickly in oils at high temperatures, so that phenols or amines have insufficient time to intervene. Thus, another type of additive must be used if the oxidation proceeds beyond this stage. Oxidation retarders or high-temperature antioxidants, also called peroxide decomposers, are effective inhibitors. They act by decomposing hydroperoxides and peroxides immediately they have been formed during the propagation phase of the oxidation chain reaction, in such a way that the products of decomposition are molecular compounds rather than radicals. The propagation stage is thus eliminated and the reaction terminates in stable products after initiation. A typical termination reaction is the conversion of aromatic hydroperoxides to molecular compounds, by either (a) radical or (b) cationic mechanisms. Phenols formed in this way have antioxidant effects: -
R-oH
+ ’*
(4.38.1)
+ 9”
(4.38.2)
R-Co-R‘ Common types of peroxide decomposer inhibitor include sulphur and phosporus compounds, with both elements often simultaneously present in the same molecule. 27 1
As a rule, peroxide decomposers are themselves or produce substances of acidic character, acting by a cationic mechanism. Anionic mechanisms, resulting from basic decomposers, are less commonly encountered.
R\ R\ CHOOCHOOH + H O ' R/ R/ or, in the case of tertiary amines, ROOH + R',N
+ H20
mR'+ R/
CO
- ROH + R;NO
HO-
(4.39)
(4.40)
The majority of these compounds simultaneously exhibit both anti-corrosion and anti-wear properties. However, some sulphur compounds, such as sulphides with mixed alkyls, cycloalkyls and cycloalkql substituents, certain mercaptans and tiobis-propionic acid esters, may increase the corrosiveness of the oil. This corrosivity is caused by the gradual oxidation of the sulphur compound to sulphoxide (noncorrosive), sulphone (non-corrosive) and sulphonic acid (corrosive). Whereas the conversion to sulphoxide and sulphone is a desirable reaction because simultaneous reduction of the peroxide occurs (e.g., of ROOH to ROH), severe oxidation is dangerous. Sulphoxides are also peroxide decomposers, but are sometimes less effective than sulphides.
4.1.3.1 Sulphur-containing Decomposers Certain simple aromatic sulphides and disulphides, such as benzyl sulphide and dibenzyldisulphide, are peroxide decomposers, although they are more often employed as anti-wear additives. Sulphur-containing alkylphenols and aromatic amines, which also act as radical acceptors, are more efficient sulphide antioxidants. Examples of alkylphenol sulphides include: OH OH
where R and R' are C1-CI2alkyls or cycloalkyls, R" - alkyl, cycloalkyl, oxyalkyl, arylalkyl (alkyl C1-CI2), R"' - alkyl C,-C,,, n=1-2. 2,2'-dithio-bis-(3-methyl-4,6-di tertbutylphenol: (CH3)2C OH
(CH&C 212
CH3
CH3
C(CH3)2
and 4,4’-thio-bis-(2-methy1-6-tertbutylphenol) (1 ). Aromatic thiols and their derivatives are known to be corrosion and oxidation inhibitors and also anti-seizure additives: SH
SH
SH
SH
Q R/&)y+ RO
SH
R
where (a) are .I-alkylphenols, R = alkyl C, - C,,, (b) are 2,5-dialkylphenols, R = alkyl C, - C,, (c) are 2,5-dialkyloxythiophenols,R = alkyl C, - C,, (d) is dithiohydroquinone. The sulphur compounds contained in base oils also have antioxidant action and are often referred to as “natural” antioxidants.
Selenium compounds, such as dialkyl and diarylselenides,e.g., dilaurylselenide, are even more effective. Some of them are effective up to 270 “C,others react with peroxides in the same way as sulphur decomposers by forming selenoxides, from which selenides are regenerated at higher temperatures (4 1). The disadvantages of selenides are their corrosivity and toxicity. 4.1.3.2 Sulphur- and Nitrogen-containing Decomposers
Phenothiazines and dithiocarbamates are the most important compounds in this category which contain both sulphur and nitrogen. Phenothiazines tend to form oxidation sludges in oils and hence find their applications mainly in the form of C- or N-alkylated derivatives of longer alkyls, such as the following:
where R groups are C,,
- C,, alkyls, and
R
273
where R,R’ are alkyls, aryls and cycloalkyls, etc.. R:’ R”’ the same or hydrogen. Metal dialkyldithiocarbamates(I), the products of the reactions of organic mines with CS, and metal hydroxide (most frequently zinc), are efficient oxidation and corrosion inhibitors, e.g., for silver bearings (404) and good metal deactivators. Some have anti-wear properties. According to Sanin (84), they are more efficient than the zinc dialkyldithiophosphates(ZDDP) and their thermal stability is 50-60 “C higher. They can be used in both oils and greases, but have not, so far, found as broad application as the ZDDP. They are, however, important as synergists for
ZDDP:
r
1
(1) where R are alkyls (identical or different) or heterocyclic radicals and M is Zn, Cd, Pb, Ni, Ba, Mg or other metals, like Co, As, Bi, Sb, Se and Mo. Compounds of this type which have been commercialised as additives include cadmium diamyldithiocarbamate (withdrawn because of cadmium toxicity), lead and zinc diamyldithiocarbamates.The zinc salt has been used as a complete or partial replacement of ZDDP in engine oils, for improvement of “factory fill” engine oils (engine, gear-box and final-drivehousings) and as oxidation inhibitors in lubricating greases for operation at elevated temperatures. Lead diamyldithiocarbamate is a good multifunctional antioxidant, with anti-wear, anti-seizure and anti-corrosion properties; it is also valuable as a friction modifier. Antimony dialkyldithiocarbamate has similar properties. Having a low ash content, it is suitable for two-stroke oils. The molybdenum analogue has been claimed to improve the anti-wear properties of lubricating greases twice to four times as much as MoS2 (399). Other sulphur- and nitrogen-containing oxidation inhibitors with anti-wear and anti-corrosion properties include derivatives of 2J-dimercapto- 1,3,4-thiodiazoles:
N-N
I I
R - (S)x - C C - (S),, - R ‘S’ These are predominantly used in gear oils, but also in turbine and hydraulic oils and fluids for automatic transmissions. Incorporated in emulsion fluids, they prevent welding of metal surfaces during metal-working processes at high pressures and act as rust inhibitors and biocides (42). 4.1.3.3 Phosphorus-containing Decomposers Some compounds of phosphorus (111), for instance trialkylphopshites, 274
triarylphosphites and trialkarylphosphites, can decompose peroxides. The peroxides are reduced, whilst the phosphites are converted into phosphates:
/ \ OR
O = P - O ROR
(R = butyl or longer alkyl or aryl group). Phosphites show very marked anti-corrosion properties, but they are insufficiently stable in the presence of moisture. They are used in plastics more often than in oils.
4.1.3.4 Decomposers Containing both Sulphur and Phosphorus Dithiophosphates The most widely-used peroxide decomposer inhibitors contain both sulphur and phosphorus. Sulphur provides the antioxidant properties whilst the anti-corrosion effect is supplied by the phosphorus. Certain simple sulphur-containing alkylphosphonates belong to this group of decomposers: OR
,
RS - CH,- P’
I \ OR
0
(R is alkyl C,, - C,,, R is hydrogen or C,
- C,,
alkyl)
Complex esters of phosphoric acid and bis-(alkylpheno1)-disuphides or polysulphides are other examples (43):
However, the most widely used types are the 0.0’-dialkyl-, diaryl- or alkylaryldithiophosphates (DDP) of metals, of which zinc is the most frequently encountered (perhaps more properly referred to as phosphorodithioates, but the term di-(alkylor aryl) dithiophosphates - abbreviated to DDP - is now almost universally used except by purists and will be used throughout here). They are produced by the reaction of alcohols (ROH), phenols (ArOH), alkylphenols (RArOH) or mixtures of these with phosphorus pentasulphide: 4 ROH + P2S5
- 2(RO)2P(S)SH + H2S
(4.4 1)
275
The acid produced by this reaction is then neutralised by a metal or its oxide or hydroxide. Another route is via the double-decomposition of alkali dithiophosphates by heavy-metal salts. If a metal oxide is used for neutralisation, a mixture of normal and basic salts is formed. Both salts are good anti-oxidants; of the two, only the basic salt is insoluble in methanol. Small amounts of other esters are formed in the reaction of PzS, with alcohols: OH I S=P-OR I SH
SH I S=P-SR I SR
OR OR I I S=P-s-P=S I I OR OR
OH SR OR OR I I I I S=P-OR S=P-OR S=P-s-s-P=S I I I I OR OR OR OR The extent of the side reactions by which these products are formed depends on the purity of the P,S, and alcohols and on the temperature of the neutralisation step. The presence of these by-products reduces the efficiency of the MDDP, they are less thermally stable, some are corrosive and have an unpleasant smell. The melting-point of the P,S, should be close to 138 "C, P-contents 28.2-28.5% by weight, Scontent to 100%. with the lowest attainable organics content. Alcohols used should be water-free. To prevent the acid decomposing during the neutralisation,the temperature should not exceed 82 "C during this stage in the process.
The following structures have been attributed to metal DDP (62):
M is a divalent metal and R can be alkyl, alkaryl or aryl. In solution, MDDP molecules combine together in associated form (57, 268). Complexes can be formed from MDDP and some compounds, such as phenol (46). amines (47). pyridine and picolines (48). 1- 10-phenanthroline (45) and other nitrogen-containing bases (49). Complexes are also formed with peroxy compounds.
Zinc is the most frequently used metal in DDP, but other metals can be used. Dithiophosphates of the alkali metals are water-soluble, promote the formation of emulsions and are corrosive. This last property excludes them from practical use.
276
Alkaline earth metals (Ca, Ba) impart detergency and dispersancy. Barium and nickel DDP also act as anti-corrosion agents. Nickel DDP improves the stability of hydrocracked oils against light. The iron derivatives do not have anti-oxidant properties. Lead DDP increases the strength of the lubricating film. Antimony and bismuth DDP's are effective high-pressure additives in industrial gear oils and the oil-components of those lubricating greases where the lubricant film must have a very high load-carrying capacity. They also have good anti-wear properties, but their thermal stabilities are lower than those of ZDDP's. Molybdenum dithiophosphates are used as friction modifiers. Salts of some organic amines, such as guanidine, are effective antioxidants (84). Thermal Stability of DDP There is a close connection between the antioxidant and anti-wear efficiency and the thermal stability of MDDP. Conclusions from investigation of the relationship between the thermal stability and composition of DDP can be summarised as follows: impurities in DDP decrease thermal stability (64), thermal stability decreases in the order arybn-alkybisoalkybbenzyl (40, 60), generally, thermal stability increases with increasing length of constituent alkyl groups, a similar effect is brought about by the presence of a quaternary carbon atom in the /%position in the alkyl group, the type of cation also affects thermal stability (66). in alkyl derivatives, thermal stability decreases in the order Zn>Ni>Ba (280), in aryl derivatives, thermal stability decreases in the order Zn>Ni>Cd>NH4 (281), decomposition temperatures established by differential thermal analysis (DTA) varied in their dependence on experimental conditions in the range 135 - 190 "C for alkyl (85) and around 250 "C for aryl compounds (fig. 4.2), thermal stability is also influenced by other factors, such as the chemical nature of the solvent, or the oil environment; thus, the di-n-butylamine derivative remains almost unchanged after 5 hours at 175 "C in decalin, but decomposes partially in a white oil and totally in engine oil. The following decomposition products have been identified in different media: O,O', S - tri-n-butyldithiophosphatein vaseline oil, O,O', S - tri-n-butyltetrathiophosphate in engine oil, S, S', S" - tri-n-butyltetrathiophosphate in the absence of solvent (54, 55, 282). The final zinc- and phosphorus-containing product of thermal decomposition is a metathiophosphate (84)or a polymeric, glassy, water-soluble residue to which has been attributed the ability to form load-bearing films on metal surfaces (64). However, the zinc compounds may remain oil-soluble without precipitation and formation of solid deposits after oxidation of use in an engine (72),despite then higher molecular weight of the decomposition products. In addition, a variety of 277
gaseous products after thermal decomposition have been identified by various authors: at lower temperatures di-n-butylsulphide (55), sulphane (hydrogen sulphide) and alkenes (84, 283), di-n-butylsulphide, butyl mercaptans; mercaptan and sulphane (60), alkene and mercaptans, and at higher temperatures, sulphane, sulphides and disulphides (64).
I
I
,
OTA 1
3
L
mg
TG
0 100
200 300 400 500 600
700 800
900 o0l-
Fig. 4.2. Derivatogram of ZnDDP with varying alkyls 1 - isobutyl-isoamyl, 2 - dihexyl, 3 - diisooctyl, 4 - di-n-cetyl
There has been much less investigation of the decomposition products of zinc diaryldithiophosphates.Decomposition occurs at a much higher temperature and the mechanism differs from that of the dialkyl compounds. The residue is oil-soluble and has a much lower load-carrying capacity than its analogue for alkylated derivatives (65). Fig. 4.2.I illustrates the changes in ZDDP concentration in an experimental lubricant formulation with time during an engine test. This also shows the appearance of the decomposition products and the disulphide form of the dialkyldithiophosphates.
-
TIME INTO TEST
Fig. 4.2.1. Changes in phosphorous chemistry of an experimental engine oil in a test engine as monitored by P-31 N M R (418)
278
The accurate determination of decomposition temperature is difficult and rather imprecise. Some deviations between results described by various authors for substances of the same chemical composition can be accounted for by various factors, all of which have important consequences for the storage and use of these additives. The results obtained depend on the rate of heating, heat input, amount of the sample, the concentration of MDDP in solution in the sample tested, the detailed arrangement of the apparatus, etc. The decomposition of MDDP starts with evolution of gas at about 70 "C. At this temperature, the degree of decomposition can be observed as weight loss after 24 hours of thermal exposure. Storage and handling temperature of concentrated solutions of MDDP should not exceed 50-55 "C. When heating MDDP, the skin temperature of the container and heating surfaces should not exceed 90 O C and only warm water or a similar heat exchange fluid should be used (not steam). Dilute hydrocarbon solutions of MDDP, such as finished oil blends containing around 3% of additive concentrate, are less heat sensitive (especially in properly balanced formulations with other additives) and can therefore be exposed to higher sustained temperatures (e.g., in engines), but the decomposition-temperaturdcomposition relationship is an important factor in the selection of MDDP additives for particular applications.
Antioxidant Effectiveness The antioxidant properties of dithiophosphates depend on the cation, the nature of their alkyl or aryl groups, the oil environment and the temperature conditions to which they are exposed. Zinc is the preferred cation. Nickel, barium and calcium are effective up to 170 "C (280).However, their effectiveness decreases at higher temperatures, and eventually a pro-oxidation effect can be observed (50). The following conclusions appear to hold for the alkyl and aryl derivatives (69):a short alkyl chain is more effective than a longer one (unfortunately, the opposite conclusion also appears in the literature (84);a branched alkyl chain is more effective than a straight chain with the same carbon number (in this respect, propyl and isopropyl alkyls show anomalous behaviour - n-Pr is more effective than iso-Pr); alkylaryl and diary1 dithiophosphates are less effective than dialkyl dithiophosphates synthesised from both primary and secondary alcohols. It is generally accepted that the thermal stability of dithiophosphates is the main factor which governs their effectiveness as antioxidants - the lower their stability, the higher their efectiveness and consequently the lower the concentration of DDP required to achieve the same effect in oil (68). The operating temperature in an engine for an oil blended with DDP should not be far above its decomposition temperature, otherwise excessive amounts of deposits are liable to form in those parts of the mechanism which are at a higher temperature. In an operating engine, the lubricant meets its maximum temperature at the piston crown and in the piston-ring zone, with a temperature gradient down the piston defined by the mode of operation and the thermal design of the piston. This must be particularly taken into account when dialkyldithiophosphates are used (71). Heavy inorganic residues and carbonaceous deposits are formed at temperatures which greatly exceed the decomposition temperatures of dialkyldithiophosphates. Both types of residue are insoluble in oil and enhance the formation of deposits on the upper parts of the piston. The decomposition temperatures of alkyl-aryl- and
279
diaryldithiophosphates are, in contrast, higher and therefore closer to the operating temperature in this part of the engine (about 250 "C). Furthermore, the final decomposition products are semi-solid, soluble in oil and do not form deposits. This difference can be especially significant in the formulation of oils for high-output diesel engines (see below). Another advantage of diaryldithiophosphates is that they form oil-soluble hydrolysis products, whereas dialky ldithiophosphates hydrolyse in the presence of water or steam to a white, oil-insoluble product, which participates in deposit formation. These factors must be balanced against the lower anti-oxidant and anti-wear properties of the diaryldithiophosphates. Resistance to hydrolysis among dithiophosphates decreases in the order shorter alkyls (secondary> primary) >longer alkyls>alkyl-aryl. The differing thermal stability of ZDDP thus very strongly affects piston cleanliness of engines operating at high temperatures. This has been demonstrated i n Petter AVB engine test (Table 4.2) and in Caterpillar 1-G engine tests. Although this engine is now obsolete as the basis of a test procedure (the level of severity being much less than those of engines in many vehicles), these results clearly demonstrate the effects of this aspect of additive chemistry. Table 4.2. Results of Petter AVB Tests of API CD-level Oils Containing Different ZDDP Additive content
Oil A
Oil B
Oil C
ZDDP (0.1% weight Zn in oil)
Zn dialkyldithiophosphate C,+Ca iso-a1kyls;low thermal stability 8% weight
Zn dialkyldithiophosphate Ca+C,, n-alkyls; higher thermal stability 8% weight
Thermally stable Zn alkylaryldithiophosphate
6.518.314.8 8.316.2
6.519.4110 9.719.6
7.0
7.8
7.2l9.619.4 9.919.1 9.0
70
80
90
Detergen t-dispersant additive (0.5% weight 300 TBN Ca-sulphonate, 2.5% 250 TBN Ca phenolate, 5.0% succinimide
8% weight
Merit rating of piston (10 max.) Piston grooves Piston lands Piston rings (average) Total rating
The higher thermal stability of zinc alkylaryl- and diaryldithiophosphatesis the reason why they are preferred for use in oils for heavy-duty engines, despite their inferior antioxidant and anti-wear properties. Additionally, they have the advantage of some detergent-dispersantcontribution,the magnitude of which depends on the size of the molecule (these properties can also be observed in some DDP with long alkyls). The first engine oils which contained zinc dialkyldithiophosphates and met the MIL-C specification have been satisfactorily used in both diesel and gasoline engines for many years. However, oils containing zinc diaryldithiophosphates have been used more often since the introduction of the Caterpillar engine
280
clutch test. They were shown to be effective in diesel engines, but exhibited somewhat worse anti-wear properties in gasoline engines. A compromise can be found with zinc alkylaryldithiophosphates.
The antioxidant properties of DDP also depend on the composition of the oil in which they are used. The presence of aromatic and cycloalkanoaromatic hydrocarbons in oils enhances these properties (285). However, even here the general rule is valid, that an optimum concentration exists for both natural and synthetic antioxidants; above the optimum, the antioxidant effect starts to decrease (1, 36). Many opinions have been published on the mechanism of the antioxidant action of ZDDP (including 44,50, 51, Z37,287). This property has been investigated by following the decomposition of cumene hydroperoxide, tetralin hydro-peroxide and di-tert-butyl hydroxide in the presence of ZDDP. Some authors presume that the active form of the antioxidant is the product of reaction of ZDDP with a peroxy compound. According to Bums (53),decomposition of a peroxide in the presence of ZDDP proceeds in three stages at different rates: the first step is rapid, the second slow and the third and final step is again rapid. According to this mechanism, bis(dialkyloxythiophosphory1)-disulphide is formed as first intermediate: RO\
4s
Sq, / O R +Roo. RO, 4 SOOR Sq, / O R
P RO1p\ S - Z n - S "OR
0
P RO/'S-Zn-S
J
SOOR Sq, / OR +Roo. RO\/ P - P / p\ RO S - Z n - S "OR RO/ 'S-Zn-S RO\
P "OR
\
/
(4.42)
OR
(4.43)
/'\ OR
+ 2R00- + Zn2+ Rib1 et al. (54, SS), in similar terms, described the three-stage decomposition of cumene hydroperoxide in excess zinc di-n-butyldithiophosphate.In the first step, a complex of the hydroperoxide with the ZDDP is rapidly formed. Slow decomposition of the complex, accompanied by the precipitation of zinc compounds, occurs in the second step. In the third, DDP decomposition products cause rapid decomposition of further quantities of hydroperoxide which have been added. Bums et al. (50, 51,52, 103) assumed that there was hydroperoxide interaction with the sulphur atom in the dithiophosphate molecule, whilst Rhbl concluded that a peroxide-zinc bond had been formed in the complex. This conclusion is based on the observation that complexes are not formed from dithiophosphate decomposition products, which are effective antioxidants. Rhbl found the following phosphorus containing decomposition products, among others: 0,O:S-tri-n-butyldithiophosphate, 0 , O :S-tri-n-butyl-thiophosphateand 0,O:O ', S-tetra-nbutyldithiophosphate (286). The same products were formed from bis(dibutoxythiophosphory1)-disulphide. This implies that this disulphide is itself a 28 1
transient product in the decomposition chain. Decomposition products containing thionic sulphur can decompose peroxides very rapidly, whilst sulphur-free decomposition products decompose peroxides only very slowly. The activity of thiocontaining products is intermediate between these extremes. The following phosphorus-free decomposition products were formed: di-n-butyl-sulphide, disulphide, sulphoxide and sulphones, the first three of which are well-known peroxide decomposers. Sanin and co-workers (84) reached similar conclusions. According to them, both thionic and thiolic sulphur act in depressing the rate of oxidation, but in different stages in the reaction sequences; thiolic sulphur acts in the early phase whilst thionic sulphur influences more advanced stages. The reaction mechanism of MDDP interaction with hydroperoxides is radical in the opening phase, changing to ionic in the advanced phase. The simultaneouspresence of cumyl alcohol, a-methyl styrene, phenol and acetone in the products of ZDDP interaction with cumene hydroperoxide suggests that radical and ionic mechanisms participate in the decomposition of peroxides. However, at lower temperatures, the radical scavenging function of ZDDP cannot be excluded from consideration (36,54,288,320).
Neutral zinc salts seem to act as radical acceptors. The same property can be attributed to DDP acids which are formed in the thermal decomposition of ZDDP. However, basic salts are inactive in this respect, but become active after interaction with decomposition products. Another product of thermal oxidation, bis(dialkyldithiophosphory1)-disulphide,is not active as a radical scavenger (385). From this, it is possible to conclude that neutral ZDDP’s are active antioxidantseven during the early stages of operation of an oil, whereas this property is lacking in the basic salts. There is less information available about the mechanism of action of diaryldithiophosphates. It is known that no complex is formed from zinc diphenyldithiophosphate and cumene hydroperoxide (54).
Anti-wear Properties The presence of sulphur or phosphorus in MDDP imparts to these additives pronounced anti-wear and sometimes even extreme pressure characteristics. However, these attributes are also influenced by the metal, alkyl and aryl groups, and by thermal stability. The terms “anti-wear’’ and “extreme pressure (EP)” properties are conventionally used to describe additive action in several levels of severity of conditions, lubrication regimes and potential wear in those cases where the lubricated surfaces are not uniformly separated by a lubricant film. In this text, “extreme pressure” action is used to describe that sector of this range of situations in which the original lubricant film has virtually ceased to exist in the contact zone; catastrophic failure of the mechanism is prevented by the local establishmentof low-shear layers of high thermal stability between the surfaces. Surface-to surface contact has occurred and with it significant surface damage, but the process has been arrested. The term “anti-wear agent” covers behaviour over a wide range of circumstances, including those in
282
which a partial film of the original lubricant is retained. Anti-wear agents can thus act by enhancing adhesion of the lubricant to the lubricated surfaces and by wholly or partly replacing it. The differences between the various regimes may be manifest principally in terms of a difference in the local temperature in the contact zone (the EP region denoting a much higher temperature), but the magnitude of these temperature boundaries varies considerably among lubricated systems. The practical result of moving from one regime to another is often apparent as a change of friction coefficient - the EP region being characterised by p much higher than the lower end of the anti-wear additives’ sphere of influence. “Friction modifier” effects can result from influence on both these phenomena.
Ability to reduce wear by MDDP decreases in the following order of cations: Cd>Zn>NilFe>Ag>Pb>Sb; Sn>Bi. Extreme pressure properties follow the following sequence: BiZAg, Pb>Se, Sn> Cd, Fe, Ni>Zn (289) and for the organic substituents: sec-alkybp-alkyl>alkaryl>aryl (69,290). Investigation on the four-ball apparatus (289) of the load-carrying capacity of films formed by di-4methyl-2-pentyl-dithiophosphatesshowed that whereas zinc was the most effective under mild load, it was the least effective under high load. However, when used in conjunction with cadmium, its anti-seizure properties in a valve-train system gave better results than when nickel was used (289). This emphasises that choice of dialkyldithiophosphates must be guided by consideration of the actual regime which is dominant. This is further supported by the observation that some zinc dialkyldithiophosphatesin engine oil reduce wear - but increase friction coefficient (370,375).
The best anti-wear properties among zinc dithiophosphates have proved to be obtained with mixed secondary and primary C, - c6 alcohols, or even better, C, - C , - the former may cause pitting corrosion (to an extent dependent on the type of material lubricated). Individual short-chain alcohols can be ruled out; they produce crystalline ZDDP of limited solubility, separating from oil on storage. MDDP may not be soluble in oil if the alkyl chain has fewer than five carbon atoms. If a mixture of alcohols is employed, one of them may have fewer than five carbon atoms and the resultant additive remain oil-soluble.
ZDDP prepared from mixed C, - c6 secondary alcohols have given good results in the FZG test (11th to 12th load stages can be achieved in a normal run) and in four-ball machine tests. Such additives are incorporated mostly into gear, hydraulic and cutting oils to provide anti-wear properties.They were also used in engine oils for higher anti-wear properties if these were especially desirable, whereas ZDDP prepared from primary alcohols were used primarily in engine oils because of their better thermal stability. ZDDP with primary and secondary alkyls with a carbon number average of 8 are regarded as universal for oils in gasoline and lightly-loaded diesels. Zinc dialkyldithiophosphates of long-chain primary alcohols (Cs - C,o) and, particularly, mixtures of zinc dialkyl- and diaryldithiophosphatesare used in engine oils to meet higher specifications, relating to supercharged engines. The high anti-wear efficiency of the ZDDP’s and their ability to increase the strength of the lubricating film are also used in other applications in the field. For instance, reduction of scuffing in engine valve-trains can be achieved by increased 283
dosage of ZDDP of lower thermal stability, although this is chiefly a problem of design and metallurgy. Some of the more recent studies of the influence of additives on the anti-wear efficiency of oils have concentrated mainly on the mechanism and kinetic effect of the metal (especially zinc) ZDDP and their decomposition products. These efforts have been based on the theory accepted hitherto that the thermal decomposition of the dithiophosphatesoccurs at temperatutes prevailing on the friction pair surface and hence that their decomposition products from a protectivd coat either by direct reaction with the materials of the friction surface or by “in situ” polymerisation on the friction surface. Research and field experience have shown that the decomposition kinetics and chemical compositiotl of the decomposition products depend to a great extent on the types and compositions of other additives simultaneouslypresent, as well as on the composition of the base oils, the presence of oxygen in the environment and other factors and interactions among the decomposition products of the MDDP and other additives present, and that there is a considerable amount of mutual influence of all these effects (321‘-325,382).
I
0.7-,
-..-..-..- ..-..-..-.. -..-..
0.5a3 -
0.l. 1
80
120
1$0
l77
& OC
$3
w
9i 120 MINUTES
Fig. 4.3. Decomposition kinetics of ZnDDP in the presence of differing DD-additives 1 - ZnDDP-1.2 (1.08)% wt in base oil. 2 - ZnDDP-1.2 (1.08)% wt + Ca-suplhonate (TBN 295)-6.0 (1.6)% wt in base oil, 3 - ZnDDP-1.2 (1.08)% wt + Ca-natural sulphonate (TBN 5)-6.0 (2.0)%wt in base oil, 4 - ZnDDP-1.2 (1.08)% wt + Ca-phenolsulphide(TBN 250)-6.0 (3.66)Wwt in base oil, 5 - ZnDDP-1.2 (1.08)% wt + bissuccinimide (TBN 25)-6.0 (3.0)%wt in base oil
The decomposition of three ZDDP‘s was investigated (326,383)by examining the extinction of the P-0-C region in their infra-red spectra as a function of temperature and time in base oils from different sources. Kinetics of the decomposition of ZDDP was affected by all the above factors, and different detergent-dispersant (DD) additives - which are significant synergists of the ZDDP anti-wear effect affected anti-wear properties in different ways (fig. 4.3).Essentially,those DD-additives which accelerate and intensify the thermal decomposition of ZDDP have a significant influence. The greatest effect is that of multi-component DD-additives. However, the concentration of particular constituents plays a part, as does the type of polymer modifier, if present. It is worth noting that oils containing ZDDP lose their antiwear properties after their antioxidant effect has been exhausted (386).
The anti-weareffect of ZDDP’s also depends on the character of the base oil. For example, they have only a very smalE effect in polypropylene oils; in solvent raffinates and hydrocracked oils their effect increases with increasing aromatic content.
284
It may be speculated that this interaction is influenced by the solvent properties of the base oil. The EP activity of thiophosphates has been shown by several workers to involve the deposition of various solid products of thermal decomposition on the surface of the metal. The polarity of the oil, acting as solvent, may influence the course of the decomposition reactions and the rate at which deposition occurs.
Effects of DDP on the Friction Properties of Oils ZDDP’s reduce the friction coefficient of oils. At the same concentration of ZDDP in oil, the magnitude of this effect depends on: - the chemical structure of the ZDDP, and hence the value of its decomposition temperature; ZDDP with shorter and mainly secondary alkyls are therefore more effective than ZDDP with longer alkyls or alkylaryls (419), - the electrochemical reactivity of the ZDDP with the friction surface; the effect of the ZDDP on the friction coefficient of the oil increases with increase in reactivity (420), - the temperature of the oil and the contact pressure; friction coefficients of oils decrease as a result of ZDDP action more as temperature and contact pressure increase.
Anti-corrosion Properties The pronounced, favourable effect of ZDDP of lower thermal stability is demonstrated, for example, in the observed reduction of wear of Pb-Cu or Pb-Cd bearing bushes. Bearing metals containing lead can be attacked, especially, by peroxides and organic acids. The action of peroxide on lead produces lead oxides, which with organic acids give lead salts. These catalyse the further oxidation of lead. ZDDP in behaving as an antioxidant suppresses the formation of peroxides and organic acids; its decomposition products react with lead to form complex inorganic products, which contain sulphur and phosphorus and form a protective film on the bearing surface. This film reduces contact of the acidic, corrosive compounds with the surface of the metal. Other compounds containing sulphur and phosphorus, such as phospho-sulphurised unsaturated hydrocarbons (terpenes, alkenes) may react with lead in a similar fashion to form a protective coating.
However, mixtures of oils containing ZDDP with oils containing other anti-wear additives containing sulphur and phosphorus sometimes cause increased corrosivity, particularly towards copper, It is, therefore, advisable to use an adequate corrosion inhibitor at the same time as the anti-wear additive. The mechanism of formation of this protective coating has not been satisfactorily explained, as yet. According to prevalent opinion, it is produced by an adsorption process and by the exchange of atoms between the bearing material and the decomposition products of ZDDP.Measurement of the phosphorus concentration in the surface film with radioactive 32Phas shown that the phosphorus concentration is almost 100 times as high under mechanical load than without the load (29Z). The rate of growth of the film is proportional to the friction produced at the point of contact (292). The surface film contains, in addition to phosphorus, sulphur and zinc (70).
285
ZDDP is not suitable for engines with bearing metals containing silver because it causes corrosion; it can also promote the corrosion of bearings made from older types of phosphor-bronze.
Zinc Dithiophosphates in Engine Oils ZDDP's have so far been unsurpassed as additives for engine oils. Their merit is their multi-functionality - they act as antioxidant, anti-wear, passivator and anticorrosion additives. They are synergistic with other types of antioxidants, such as alkylphenols, metal deactivatorsand zinc dialkyldithiocarbamates and they improve the efficiency of some detergent-dispersant additives. Zinc, phosphorus and sulphur are now being introduced into all types of lubricating oils for 4-stroke engines, except oils for engines with silver bearing bushes (73).They cannot be used above a low maximum concentration (0.4 - 0.5%) in 2-stroke engine oils in which the oil is mixed with gasoline. In this case, the ZDDP, if used at higher concentrations,acts as a source of deposits on the spark plugs, causing failure and reducing service life. Oils containing low ZDDP concentrations may thicken when exposed to severe conditions, when the oil in the crankcase heats up to about 150 O C . The reason for this is the accumulation of high-temperature sludges, as well as the evaporation of light components. An increase in the concentration of ZDDP may only suppress the formation of sludge, and it is advantageousto use, simultaneously,other antioxidants such as akylphenols (ZO6). The evaporation of light components can only be avoided by a judicious choice of base oil. The concentration of ZDDP in engine oils depends on the required zinc content. This varies, as a rule, between 0.04 and 0.1% by weight, but it can be as high as 0.15% when, for example, improved anti-wear properties or lower thickening rate are desirable (Table 4.3). The modem type of gasoline engine oil, produced since 1972, usually contain 0.10 to 0.15% zinc; in diesel engine oils, it is recommended not to exceed 0.12% zinc. Table 4.3. Effects of Increased Zinc Content in Oil on Results in the 4-hour Sequence IIID Test in a Gasoline Engine Zn concentration in oil (% weight) 0.08 0.12 0.15
SE specification limit
1180
124
32
375 max.
Piston varnish rating
8.6
9.5
9.7
9.1 min.
Oil sludge rating
6.8
8.1
9.8
9.2 min.
Viscosity increase (%)
The properties of ZDDP are affected by the metal: phosphorus ratio. The metal should be in a slight stoichiometric excess and the ZDDP reaction at most only slightly acidic. Currently-available products have Zn:P ratio of 105-1lo%,pH 4.5286
5.0 - this value should not fall below 4.0.The thermal stability of ZDDP improves with increasing Zn:P ratio. The presence of excess alcohols and their oxidation products is undesirable. It has been alleged that the concentration of zinc on the roads exceeds, in some areas, the threshold value of toxicity (74).The maximum admissible concentration of zinc in air is, at present, 5 mg.m3. This value is far from being reached even at a consumption of 1 litre of oil per 1,600km.However, zinc on the road can originate from other sources, such as tyre wear. Zinc dithiophosphates are slightly toxic. They are effective fungicides (265).The LD50 of commercial products is about 2.5g per kg of the living organism. A number of ZDDP’s has been found to be toxicologically active as skin and eye irritants.
The Determination of Zinc, Phosphorus and Sulphur in DDP’s and in Oils Dialkyldithiophosphate additive concentrates usually contain up to 9% by weight Zn, 8.5% P and 17% S; the alkylaryl- and dialkyl- compounds contain up to 3.6% Zn, 3.3% P and 7% S. The zinc content of both ZDDP and oil can be determined by IP 117/47; the sample is incinerated and Zn determined gravimetrically as zinc hydroxyquinolate. ASTM D-1549 and GOST 14330-69 specify a polarographic method of determination of zinc in oil and additives (containing up to 2% Zn and in the absence of Cd). Measurement by atomic absorption spectroscopy (e.g., by ASTM D-4628-86) is currently routinely used and the use of isotopic dilution analysis is envisaged (310). Phosphorus in additives and oils can be determined by ASTM D-1091 and IP 148/72, IP 149 and IP245 (photometric methods for determination of P at 0.002-2% weight). The sample is mineralised with H,SO, and the phosphorus transformed into orthophosphoric anions with HNO, and HCIO, or H,O,. According to the photometric method, ammonium vanadate and molybdenate solutions are added to form a yellow complex, which is measured photometrically at a wave-length of 420-470 nm. According to the titration method, phosphate anions are precipitated as quinoline phosphomolybdate from boiling HCI by sodium hydroxide, the excess of which is titrated with HCI. The photometric method is also described in CSN 65 6253. The standard emission spectroscopic method for P, Zn, Ba and Ca is specified in 1P 187. The total sulphur content in oils and additives can be determined by CSN 65 6228, GOST 1437-47. ASTM D-1551, IP 63 and DIN 51786, in which the sample is burned in an air-jet in a quartz tube. The sulphur oxides produced are absorbed in 3% H,O, and converted into H,SO,, which is determined by a chelatometrical method, titration with NaOH or gravimetrically as barium sulphate. ASTM D-1552 is more intricate. The sample is burned in an oxygen flame by the Wickbold method (DIN 51409 and IP 243). and absorption of SO, in H,O, gives H,SO,, which is then determined as sulphate by titration with barium perchlorate, or by nephalometric, gravimetric or photometric methods. According to KiriEenko, sulphur can be determined by hydrogenation, followed by coulometric determination of H,S (123).
4.1.3.5 Ashless Peroxide Decomposers Containing Sulphur, Phosphorus and Nitrogen Where metal-containing antioxidants cannot be used, ashless antioxidants containing phosphorus, nitrogen and thionic sulphur can be employed (79,for example:
287
/OR S=P-OR \ SNR'R"
/OR S=P-OR \ SCH,NHR'
/ NHR S=P-NHR \NHR
/(C18H37)
[
S = P - OC6H5 NHRR' or
S = P -FS H3 ' S I
NHRR
where R is octyl, - CO - (CH2)8- CONH,, R',R" - hydrogen or methyl). These additives are stable up to comparatively high temperatures (about 300 "C) and suitable for high specification engine oils. Some effectively suppress oil thickening whilst avoiding contamination of exhaust gas catalysts. Inexpensive antioxidants with good anti-wear properties can be derived from Sderivatives of 0,O'-dialkyldithiophosphoricacids. As stated earlier, some of them are products of decomposition of ZDDP. The S-alkyl, S-alkoxymethyl-, Salkylthiomethyl- and S-hydroxybenzyl- derivatives can be cited as examples (7679). The S-carbamoyldithiophosphates,reaction products of mono- and diisocyanates with dialkyldithiophosphoricacid, are also good antioxidant additives (as effective as ZDDP) and have good anti-wear properties (better than ZDDP) (80): R' - NCO + HS - P(S)(OR)L-
R'
- NH - CO - S - P(S)(OR)z
(4.42)
where R' is an alkyl with 12 or more C atoms. These derivatives of the comparatively unstable S-carbamoyl group have lower decomposition temperatures than ZDDP; their deficiency is their tendency to revert, during storage, into gels. They fail to protect metals from corrosion and need to be combined with a corrosion inhibitor.
4.1.3.6 Future Developments Prospects for future development of oils containing ZDDP and S- and P-containing ashless antioxidantscan to some extent be jeopardised by the introduction of exhaust gas post-combustion catalysts. The deleterious elements are phosphorus, which acts as a poison for catalysts based on noble metals (Pt, Pd) and sulphur, which poisons the NO,-reduction catalysts. Unless sufficiently resistant catalysts are developed, or some other way found of separation of the harmful elements from the exhaust gases, 288
massive departures from present additive systems and the adoption of phosphorusand sulphur-free additives may be the consequence. This holds true not only for antioxidants, but also for other types of engine-oil additives (59, 68, 73). The question of the biodegradabilityof ZDDP and other phophorus-containingadditives, and their effects on the environment in waste oil-disposal, may bring about future changes in the range of additives in large-scale use.
4.2 DETERGENTS AND DISPERSANTS Detergents and dispersants (DD) are essential additives, particularly for engine oils. Their chief function is to avoid or suppress the formation of deposits on the hot surfaces of internal combustion engines - particularly on pistons - and disperse the corrosive cold sludges into the oil in those parts of the engine operating at relatively low temperatures.They are,in addition, required to neutralise the acidic components contained in the oil and acid substances entering the oil, and thus reduce corrosive wear of the metal surfaces and protect ferrous surfaces from rusting. They can increase the thermal stability of the oil and affect the process of its oxidation. They have also made considerable progress in gasoline formulations, where they keep the inlet manifold from the fuel reservoir to the inlet valves clean, and diesel oils, where they improve clean combustion.
4.2.1 Significance of Detergent-Dispersant Additives Mechanisms of their Action In addition to efficient oil filtration, the detergent-dispersant ability of the oil is a major determinant of the extent and rate of the deposition on the working parts of the machine of substances formed in or entrained into the oil. In the case of an internal combustion engine, these parts may especially include the piston-ring grooves or on the piston surface; insoluble substances produced during incomplete fuel combustion can form deposits and varnishes, which may obstruct the operation of vital parts of the engine, impede heat flow, etc. In essence, two types of deposits can be formed: deposits caused by heat (the thermooxidation products of fuel and oil which, because of flocculation promoted by heat, deposit on the hot parts of the engine) and - cold sludges caused by phenomena taking place at lower temperatures (carbon particles depositing by co-action of water on colder parts of the engine). Organic acids condensing on the hot parts of the engine are responsible for the formation of varnishes and lacquers. The rate of deposit formation can also be affected to a considerable extent by mechanical contaminants (dust) entering the oil through poor filtration and to abrasion of the metallic friction surfaces. Deposits and varnishes are, together with wear of the engine by solid particles, the decisive factors in ensuring good operation, output and reliability of the engine.
-
289
Since there are differences between diesel and gasoline engines in terms of fuel composition, the thermal regimes in the engine and operating conditions, the conditions giving rise to the formation and the nature of insoluble substances in the oil, and for the formation of deposits and varnishes differ also. Anderson (81) illustrates the problem in the charts depicted below:
Diesel Engines fuel
incomplcte combustion and diffusion products of ignition
combustion H,O + SO, + NOx
pre-ignition products
$-
H2S0, + oxidation products
low-molecular weight oxidised,polymers
soot
+
1. t
varnishes
Gasoline Engines PISTON VARNISH
t
fuel
oil
blow -by
+ heat + NO + 0,
oxidised gaseous
oxidation products
products (droplets)
DEPOSITS
piston ring-zone
I
t
carbon deposits
d
varnish
inorganic b sats
water SLUDGES
These charts illustrate that one of the significant promoters of deposit formation in diesel engines is the sulphuric acid produced via sulphur oxides from the sulphur-compounds present in diesel fuel and oil. Nitrogen oxides, originating form nitrogenous substances present in fuel and lubricant and. predominantly, from the direct combination of nitrogen and oxygen at high engine temperatures,promote the production of sulphuric acid and also contribute themselves to oxidative transformations in the
290
lubricant. These transformations,together with other reactions between sulphuric acid and oil, cause the formation of insoluble products which adhere strongly to the hot metal surfaces. The amount of deposit grows with increasing fuel sulphur content, with increased engine temperature and the rate of formation of nitrogen oxides. The acidic products from burnt fuel may condense on the relatively cool cylinder walls and on the piston surfaces, and they can catalyse the polymerisation of substances produced by thermooxidation reactions to sludge and varnish deposits. If deposition of solids (which besides carbon can contain sulphate crystals, additive residues, wear debris and oxidised fuel and lubricant) becomes excessive, the piston grooves can become full, which, together with varnishes, immobilises them; in consequence, combustion products may penetrate past the piston at a greater rate into the sump, or the oil may penetrate into the combustion chamber. This again causes formation of deposits, increases oil consumption and decreases output and, eventually, inevitable stoppage of the engine. Varnish deposits on the piston also impede heat transfer, which can cause an increase in piston temperature and decreases the service life of the oil. Varnish in spark-ignition engines causes an increase in the octane requirement of the engine. Burgess and colleagues (83).in studying the oxidation products of oil in gasoline engines, found that the action of oxygen on oil first produces low-molecular weight liquid products (oxidised monomers) of low oil-solubility in oil (so-called red oil). When these monomers are exposed to constant heating, highmolecular weight, oil-insoluble products result. When these products come into contact with the piston surface, they harden and form varnishes. According to prevalent opinion, 90% of the deposits on the piston come from the oil and only 10% from the fuel. The formation of deposits in the gasoline engine depends on the type and operating conditions. Essentially, there is a difference between the oxidation of the oil in the low temperature and high temperature in the engine. At low temperatures,e.g., during stop-go running of an insufficiently warmedup engine, the main sources of low-temperature deposits (cold sludges) are the products of incomplete fuel combustion - partially cracked and oxidised hydrocarbons, olefins, dienes, aldehydes, acids, phenolics etc., produced in the relatively cool flame region near the relatively cold walls of the combustion chamber. The chemical composition of low temperature deposits has not been sufficiently investigated. A part of the product of incomplete fuel combustion penetrates as blow-by into the crankcase. Here, the products condense and form oil-insoluble droplets. These droplets either return to the piston surface with the oil, directly or through positive crankcase ventilation. Because they are more polar than oil, they are more readily bonded by adhesive forces to the hot metal surface and reactions between their active components occur. The liquid is first transformed into an adhesive, half-solid substance, which further hardens and forms varnish deposits. In addition, these droplets react with water and lead compounds derived from anti-knocks, if present, to form semi-solid resin deposits - cold sludges. Nitrogen oxides take part in the formation of deposits and cold sludges (81).This can be demonstrated by analysis of the carbonaceous products in which the nitrogen is substantially higher than in the fuel or the lubricant. Although the conditions affecting the formation of NO, and their concentration is known in, for instance, diesel engine exhaust gases, their influence on the formation of deposits is considerably less understood. It is supposed that they are a positive factor, although considerably less significant, than air or sulphur oxides.
Suppression of the lay-down of deposits and varnishes from fuel oxidation products, and the neutralisation of substances which enhance the formation of these oxidation products, can be achieved by: - neutralisation of acidic substances and suppression of polymerisation of oxidised polymers, - solubilisation, i.e., transformation of liquid but poorly-soluble or insoluble substances such as asphaltenes into soluble form, - peptisation of solid particles, e.g., carboides and soot, by converting them into
29 1
stable suspensions in oil,
- preventing the particles from depositing on the metal surface in relatively nonturbulent sites. DD additives provide chemical neutralisation of acids, mainly strong acids, and prevent polymerisation of oxidised monomers, as long as the additive retains an alkaline reserve, until its alkalinity is exhausted. During this neutralisation process, excess metal hydroxide or carbonate - which constitutes the alkaline reserve of the detergent, is converted into metal sulphate. Thus, the higher the sulphur content of the fuel, and the more the tendency for acid products from the oxidation of fuel and oil to accutnulate in the oil, the higher the alkalinity of the additive to be used needs to be. From the commentary earlier, it is apparent that oils for lubricating diesel engines generally need to be compounded with DD additives of higher alkaline reserve than those for gasoline engines; as the sulphur content of the diesel fuel and the operating temperature to which the oil is exposed increase, the alkaline reserve of the DD additive should also increase. High performance, high compression ratio gasoline engines used for high-load highway operation for prolonged periods on leaded gasoline also require oils with basic additives. In these cases, chemical neutralisation of inorganic acids, HCl and HBr, originating from anti-knock additives, and organic acids is necessary. Over recent years, developments have been aimed at reduce diesel engine piston temperatures for a given power output, e.g., by improved cooling through piston design, and at the use of low-sulphur diesel fuel. On the other hand, the specific output and long-term loading of gasoline engines has been increased, without designing to reduce oil temperature and other potential sources and promoters of oil oxidation (e.g., by introducing positive crankcase ventilation, increasing the aromatic and/or anti-knock agent content of the gasoline). These factors, in respect of which the requirements of diesel and gasoline engines have tended to move closer, determine the alkaline reserve necessary in DD additives, and have led, amongst other needs, to the development of “mixed-fleet’’ oils for lubricating both gasoline and diesel engines.
The total base number (TBN) and total acid number (TAN) of “over-based” additives in oils is associated with the concentration and composition of MDDPtype antioxidants, detergents and dispersants (TAN measures the extent to which the additive reacts with strong base, whilst TBN is the amount of strong acid; additives may and do have, simultaneously, both high TAN and TBN values). MDDP’s have very high acidity. Metal dialkyldithiophosphates with mainly shorter alkyls have higher acidities than alkylaryl- and diaryldithiophosphates. The following TAN’S have been measured, by ASTM D-664, in commercial ZDDP concentrates: 146 mg KOHlg for ZDDP with C,-C, primary alkyls, 124 mg KOWg in those with C , alkyls and 70 mg KOWg for alkylaryl compounds.
Among detergents and dispersants, metal alkarylsulphonates have relatively high acidities, even when their total alkalinity is also high. The TAN depends on the method of manufacture (extent of neutralisation of sulpho-acids or residual SO,) and may exceed 30 mg KOH/g, particularly in low-base sulphonates, in which the
292
TAN can even be higher than the TBN. By contrast, basic metal alkylphenates and, even more, succinimide dispersants, have very low acidities (the alkalinity of succinimides or Mannich products results from amine groups, chiefly secondary amines, which have higher alkalinities than primary amines; for this reason, bissuccinimides have higher alkalinities than mono-succinimides). Acidity i n succinimides results from the presence of unreacted alkenylmaleic anhydrides. Metal thiophosphonates have relatively low acidities (e.g., commercial Ba thiophosphonate with TBN 65 mg KOWg has TAN 5.5 mg KOWg). The TAN of metal salicylates is a little higher (e.g., commercial Ca akylsalicylate with TBN 142 mg KOWg has TAN 12 mg KOWg). High oil acidity, caused by the acidity of some additives as well as by organic acids, when it cannot be neutralised by strong bases, is obviously undesirable in that it causes corrosion, promotes fatigue wear of friction surfaces and accelerates oil ageing (some oxidation reactions of hydrocarbons and their derivatives proceed more rapidly in an acid environment). For these reasons, very low-base petroleum sulphonates (TBN <30) with a high TAN can be an undesirable component of DD packages for engine oils. To prolong the interval between oil changes, the difference between TBN and TAN of the fresh oil should be as high as possible. The level of alkaline reserve in the oils for gasoline engines is limited by the ash content of the oil. This increases with the alkaline reserve of the additive. Gasoline engines are more sensitive to oil ash content than diesel engines. Similarly, conventionally-lubricated two-stroke gasoline engines, in which the oil is mixed with the gasoline, are sensitive to ash content, particularly water-cooled engines. Low-ash oils are mandatory for rotary gasoline engines. Fouling of the combustion chamber causes the conditions for combustion and the quality of the exhaust gases to deteriorate and leads to deposit formation on the exhaust valves; this impairs their operation and leads to a drop in output. Ash, acting as thermal insulator, causes the thermal regime in the engine to advance, leading to pre-ignition. When the combustion chamber is fouled with deposits derived from ash, better antiknock properties (higher octane rating) of the gasoline are required. The base oil itself induces a higher octane requirement as its heavy component (brightstock) concentration increases. However, metal DD additives, the main ash precursors in oil, have a far more significant influence. The type of metal is also a contributor to this problem. For example. tests of mono-grade SAE 30 oils with and without additives in a Fiat 125B engine fueled with lead-free gasoline showed an octane requirement increases after 216 hours as follows (87):
Base oil (free of ZDDP and metal detergents) Oil containing ashless dispersant Oil containing ZDDP Oil containing ZDDP + Mg-containing detergent (0.63% ash) Oil containing ZDDP + Ca-containing detergent (0.68%) ash Oil containing ZDDP + Ba-containing detergent (0.82% ash)
3-4 5-6 6 6-7 7-8 8
Other authors (88)confirmed this effect with SAE low140 SE oils of differing ash content on octane requirement measurements in a Ford Cortina 1600 engine fueled with lead-free and leaded fuels (see Table 4.4).
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Table 4.4. Influence of Ash Content in Oil on Octane Requirement Increase Ash content of oil (96 weight)
leaded
1.29 1.15 0.27 0.02
Gasoline lead-free
8.1 8 .O 5.0 2.8
5.6 5.3 3.9
As a result of the polymeric VI improvers they contain, multi-grade oils have less influence on octane requirement than mono-grade oils. Polyolefins have proved to have the best performance in this respect (89). Other authors (90.92) have also reported on the effects of ash content of lubricating oils on engine octane requirement. The tendency to knock can, in these cases, be explained by low thermal conductivity of the deposits in the combustion chamber and on the piston crown, which causes a general increase in engine temperature and also local over-heating. These hot-spots cause irregular ignition (pre-ignition). The occurrence and extent of this phenomenon depends on the composition of the ash deposits and/or the type of metal they contain. For example, lead-containing deposits originating from the gasoline were shown to have a lesser effect than barium-containing deposits produced from the oil. This is associated with the decomposition temperature of the deposits. Table 4.5 illustrates the decomposition temperature of various compounds which occur in combustion chamber deposits.
Table 4.5. Melting-points and Decomposition Temperatures of Compounds Found in Combustion Chamber Deposits in Gasoline Engines ( "C) Cation
Anion Sulphate Carbonate Chloride Bromide Nitrate Oxides
Lead
Magnesium
Calcium
Barium
1170 (d)315-400 50 1 513 (d)470 888
(d)l124 (d)350 708 700
1193-1450 900 772 730
2800
2580
1580 (d)1360 963 847 592 1923
(d) indicates decomposition
Combustion chamber temperatures vary between 1,000 and 1,300 "C, so all lead compounds melt or decompose, whereas barium sulphate and carbonate are unchanged (lead is first deposited as carbonate, from which oxides are gradually formed; these are then converted into chlorides,bromides and sulphates). The melting-point and decomposition temperature clearly suggest that magnesium and calcium salts should give rise to less trouble than barium salts. A higher content of calcium and magnesium in detergents is more acceptable, in this respect, than barium. The apparent importance of lubricant-derived deposits on octane requirement increase in 4-stroke engines has diminished somewhat in response to a downwards trend in oil consumption, itself driven by emission control regulations. For much of its life, the engine of a modem passenger car consumes virtually no oil between drains. The opportunity for ash-forming additive residues from the lubricant to penetrate the combustion zone has therefore been much reduced.
Fewer problems with ash occur in diesel oils when high alkaline reserve oils are used. Some problems have been noted and linked to ash deposition on exhaust valves 294
and turbo-compressor blades when API CD oils have been used in highlysupercharged, medium-speed diesel engines (92).Ash also constitutes a considerable proportion (around 20% by weight) of deposits in the first piston groove; up to 58% ash has been identified in crown land deposits in Cummins NTC 400 tests (421). Diesel oil development efforts have included attempts to reduce ash levels. For example, Daimler-Benz allows 1.8-2.0% maximum by weight of ash (depending on lubricant performance level) in oils approved for lubricating their supercharged diesel engines; Detroit Diesel permits a maximum of 1%. The reason for this is the very close tolerance between the piston grooves and rings, which are liable to be fouled by deposits derived from ash in the oil.
Modern mixed DD additives offer two ways of resolving these problems: combination of ash-containing metal detergents and ash-free dispersants (chiefly succinimides) and the combination of different metals in ash-containing metallic detergents. For instance, in formerly used combinations of calcium and barium, calcium generally produced less sulphate-ash than barium, but the bulk of the calcium ash is higher than that of the barium ash. Calcium salts adhere more strongly to metal surfaces than barium salts, so that barium salts can be expelled with the exhaust gases and more readily leave the engine. Magnesium salts produce the least amount of ash. Basic metallic detergents specifically neutralise inorganic and organic acids in oil and inhibit the polymerisation of oxidised monomers. On the other hand, ashfree dispersants, being of low basicity, cannot neutralise strong acids. Their beneficial effect is due to another mechanism. The mechanism of the solubilising and peptising effects of DD additives is associated with the structure of the DD additive and with the properties of its components. Oils containing DD additives are, in fact, colloidal solutions in which the molecules of the additives form micelles of up to 10 to 500 molecules. The size of the micelles is dependent on the concentration of the additive in oil and on the temperature. Polar compounds, such as water, alcohols, etc., increase the size of the micelles. Excess basicity in over-based detergents is solubilised in the detergent micelles. The ratio of number of molecules to number of ions is around 30; this ratio varies with temperature, concentration of the additive and the concentration of polar components in the oil. The presence of ions or electrically-charged particles is a significant factor in the stabilisation of contaminants in used oils. Different DD additives present the same essential chemical structural features, comprising polar components connected to long, oleophilic hydrocarbon chains. The polar component enables the molecule to become bound to polar targets (e.g., contaminant particles) whilst the non-polar oleophile imparts solubility in oil and prevents the coagulation and sedimentation of the particles. Solubilisation occurs when the polar components of the molecule become bound to polar intermediate products of oil and fuel oxidation, which otherwise tend to agglomeration and 295
flocculation form the oil as the temperature increases. Peptisation by the DD additive takes place during the process of binding to solid particles, products of thermooxidation reactions of fuel and oil, dust particles, metallic wear debris, etc.. The oil-DD additive system involves a number of forces besides the generalised dispersive forces (82): induction forces generated by the interaction of the permanent dipoles of the additives and the induced dipoles in their surroundings; orientation forces generated by hydrogen bonds and the transfer of charge from electron-donor molecules (transformation of electron-donor-acceptor complexes) and, sometimes, chemical bonds. Dispersive forces operate particularly between additive and oil. Interactions occur among additive molecules, in which all these forces are involved except chemical bonding, and give rise to the formation of micelles. Similar forces act between the micelles and their surroundings: polar, oil-insoluble liquids (oxidation products of hydroxy acids, sulphuric acid salts) are solubilised. The solubilised products may enter into the interior of the micelle or its shell, or be adsorbed on to its surface. Thus, solubilisation may be inter-micellar, micellar or super-micellar, depending on the strength of the interaction and the nature of the substances involved. Interaction may occur between micelles, insoluble oxidation products and contaminants (“carbon”). The resultant effects are the adsorption of the micelles on to the surface of the contaminant particles, establishment of electrical double-layers (the so-called electrostatic barrier), mutual repulsion of particles of like charge and peptisation of the particles. A similar barrier system may be formed by adsorption of micelles on to the metal surface. Such barriers prevent settlement of the contaminant and the formation of varnish on the metal surface; interactions with the metal can also have anti-corrosion and anti-friction and anti-wear effects. Dispersive forces do not change with temperature; the chemical forces become stronger, whilst the remaining interactions become weaker. The weakening effect dominates overall. Temperature increase has, in general, a deleterious effect which requires the use of higher quality additives. Some highly polar contaminants in the additives or the environment in which they operate, such as hydrophilic substances (e.g., low molecular weight sulphonates) and, especially, water, have adverse effects. They dissolve if they are present in small amounts. At higher concentrations, they are preferentially adsorbed on to surfaces and repel the additives from them. The processes taking place in these systems are varied and intricate. Many deficiencies can be overcome by using additives in combination, such as metal detergents with ashless dispersants. Such combinations also minimise the risk of a sudden collapse of the system, followed by separation of sludges from oil and settlement of these sludges on the metal surface (sludge dumping).
Solubilising and peptising effects of DD additives are associated with the magnitude of the charge on and the size of the micelles. Additives forming large micelles of small charge (e.g., succinimides) usually have solubilising effects, whilst additives which form small micelles of high charge (such as polymeric DD additives, metal sulphonates and alkylsalicylates) mostly have peptising effects. The latter types, unlike the former, are adsorbed on the surface of the metal and on the sooty particles, forming electrostatic barriers and preventing the adsorption of other contaminants on the metals and suppressing the agglomeration of insoluble particles. The effectiveness of detergent-dispersant additives depends on their type, the size of their non-polar moieties, and on their constitution and basicity. That of ashless additives depends principally on the nature and amount of their active organic groups. The concentration of the additive is also important. An optimum concentration exists at which maximum efficiency can be achieved. Electrical phenomena are important, but they are not the sole criteria. The nature and presence of polar compounds in the oil are very important; they generally reduce the effect 296
of DD additives by becoming adsorbed on particles in suspension and reducing the detergent surface, solubilising additive micelles and changing the colloidal properties of the system. During service, both oil and additive undergo chemical changes, the amount of additive in oil diminishes and the oil loses its detergentdispersant capacity. Test of solubilising effects on oil-insoluble hydroxy- and keto-acids have shown that ashless dispersants have solubilisation effects 10 times greater than those of metal detergents (84,94).On the other hand, metal detergents are excellent peptisers of solid particles of diameter 2-1 5 nm dispersed in oil, forming a layer 3-5 nm thick which is physically adsorbed on the surface of the particles. The peptising power of sulphonates and phosphonates is drastically suppressed when water is present. The reason for this may be the partial removal of the film from the particle surface by hydration. Thus, the effective dispersive capacity of sulphonates and phosphonates is lower in a moist environment, e.g., in an engine crankcase, than in water-free oil. This explains why the efficiency of these additives is substantially lower under cold-running conditions or stop-go operation when the amount of condensed watcr in the oil increases. Polymeric dispersant VI improvers peptise 2-100 nm particles with a 2-3 nm thick film by means of hydrogen bonds between the dispersant group in the polymer and the polar sites on the particle surface. This film forms a barrier between the particles and prevents them coagulating. Unlike in the case of the metallic detergents, water does not reduce the peptising effects of polymeric dispersants. These dispersants are also preferable to metallic detergents, i n that they peptise solid particles i n the critical size region of 1-5 nm, which is typical for soot (3-5 nm) produced during the partial combustion of fuel (95).
Succinimides not only possess solubilising but also peptising effects. They peptise particles between 2 and 1.5 nm by forming a 10 nm thick surface film. This is formed with 2-50 nm particles by means of hydrogen bonds and with larger particles (up to 1.5 nm) by salt formation or physical adsorption (96).
It is evident from the different solubilising and peptising abilities of different DD additive species that it is desirable to fortify the oil with a combination of detergent and dispersants of different types. These must also be chosen according to the engine type and field conditions for which the oil is intended, and the additive dosage must be adjusted so as to match their concentration to the oxidation products and contaminants which are expected.
Anticorrosion Ability (Control of Corrosive Wear) The acid products discussed in the previous section are corrosive; they can become a primary source of corrosive wear of engine components, especially the metal surfaces of pistons and piston rings, cylinder liners, valves, valve- lifters and bearings, because of direct chemical reaction between the acids and the metals. The effect of oxidation products on copper-lead bearing bushes is not, however, direct and wear is not always of the corrosive type. According to Dennison ( 9 9 ,the first phase of attack is the conversion of the lead into oxides by the action of peroxides produced by oil oxidation; this oxide is then atacked by the acids. A lead-free composition rich in copper is of low strength and is prone to pitting corrosion. So although bearing corrosion can also be caused by their metallurgy, the extent of their corrosive wear is usually affected by the amount and nature of the acidic substances produced during the oxidation of
297
the fuel and the oil. It is therefore necessary to identify the sources of and conditions for the occurrence of corrosion phenomena in both diesel and gasoline engines.
The main sources of corrosion and corrosive wear in diesel engines, particularly of the piston and its rings, are acids of sulphur, if the fuel is sulphurous and the temperature of the metal surfaces lies below the dew-point of the combustion products. These conditions are especially characteristic of the design and operation of slow-speed diesels in marine service. On the other hand, the main agents of corrosion in gasoline engines run on leaded fuels are the halogen hydride acids from organic chlorides and bromides used as lead-scavengers in lead-based anti-knock compositions. Chlorine, in particular, is highly corrosive. Liquid acid oxidation products also participate in corrosion in both types of engine. The degree of corrosion is considerably influenced by engine temperature - as temperature decreases, corrosive wear increases. During engine operation at low temperatures, acid-containing blow-by of fuel combustion products causes cylinder and piston-ring corrosion, if the temperature lies below its dew-point. Gasoline engines are more sensitive than the smaller diesels in this respect.
The most effective way of reducing corrosive wear of engine components is the neutralisation of the acids (i.e., organic acids and sulphuric acid) by the alkaline reserve of the DD additives in the oil. It has been established (99) that these basic detergents reduce corrosive wear substantially up to oil pH 6 in gasoline engines and 4.5 in diesel engines. It has been clearly demonstrated that the extent of corrosive wear decreases with increasing TBN of the oil. This is evident fromfig 4.4, which illustrates the correlation between wear of the first piston-ring of a stationary diesel engine running on diesel fuels of varying sulphur content and alkaline reserve of the lubricants used. Tests by Cook (ZOO) and Bolnes (ZOZ) showed that there was no difference between sulphonates, calcium phosphonate and barium phosphonate in this respect. However, the contrary opinion appears to prevail, that higher efficiency is obtainable by the use of package detergents, mainly basic sulphonates. Ashless detergents are also beneficial. This synergistic effect of ashless detergents can be explained by
$5. a-
3
4-
2a 3 z 2-
e
P ’-0-
r
0.1
. .
I
I
.
I
0.2 0.3 0.4 0.5 0.6 0.7
s
08
-
09 % S
Fig. 4.4. Correlation between the piston rings wear (measured by radioactive method) of a stationary diesel engine, lubricating oils of varying alkalinous reserve (TBN) and diesel fuels with increasing sulphur content
298
so-called physical dispersion, in which the ashless dispersants enclose the sulphuric and organic acids in comparatively strongly-bound micelles, so that the acids cannot act freely. Tests made with Caterpillar L-1 engines showed that ashless dispersants are effective up to oil pH 1-1.5. Ashless dispersants are believed to be effective up to oil pH 4-4.5.
Neutralisation of the acidic products from the products of combustion and thermooxidation products gradually exhausts the alkaline reserve of the oil. The process of exhaustion depends on the quantity of acid produced, the nature of operation of the engine, the oil composition and other factors. After some time, the level of oil alkalinity becomes stabilised. However, the alkalinity of the fresh oil must be high enough so as not to drop into the acid region throughout the entire operating life of the oil.
Anti-rust Protection Favourable conditions for the rusting of steel arise whenever the engine temperature drops below the dew-point of water when the engine is stationary. Basic sulphonates and phenolates are effectiveanti-rust agents because they neutralise the acids which, in the presence of water, promote rusting. However, neutral sulphonate has better ability to become adsorbed on the metal surfaces and thus prevent them coming into contact with water directly (102). The best results have been obtained with detergents containing a combination of metals. Zinc and lead sulphonateshave also given good results. Anti-rust boosters comprising alkylphenolethoxyethanol(I) or copolymers of ethylene-propylene oxide (11) types are sometimes incorporated into mixed packages containing alkaline metallic detergents:
0 - CH2CH2(OCH2CH2)nOH
R (R is usually iso-octyl and n = 4 -9)
HO fCH2CH2 .+CH
I CH3
- CH2 - 0 -j+CH,CH,O
+OH
(11)
(x is about 4 and y about 55)
If these products are used alone, they do not act as rust inhibitors. However, combination with basic detergents causes interactions and this and the products of the interactions neutralise acid contaminants in oil very effectively. Dosages used should be such that the concentration in oil is 0.1 to 0.5% by weight.
Thermal Stability Improvement The correlation between the amount of deposits and oil temperature can be studied using a modified Panel Coker Test (PCT). In this test, the temperature is gradually 299
raised and the quantity of deposit determined at each temperature level after 4 hours of exposure at that temperature level until a temperature is reached at which the oil breaks down. This break-down is manifested by an abnormal increase in deposit formation. Tests have confirmed that the thermal stability of oil increase with increasing DD additive concentration and increasing oil TBN (103). Tests run in parallel between modified PCT and diesel engines have established that the thermal stability of oil containing DD additives gradually decreases with time in service (104).This also shows that thermal stability is related to DD additive concentration; as these additives and their alkaline reserves become exhausted, the thermal stability of the oil decreases.
Effects on the Rate of Oil Oxidation Tests made in model engines such as the Petter W1 have established that the oxidation stability of the oil increases if suitable DD additives are present in addition to antioxidants. Comparison of laboratory oxidation tests together with tests made in model engines has shown that the oxidation stability of oils dosed with ZDDP additives at conventional levels is increased with increasing dosages of calcium petroleum sulphonate and with its alkaline reserve (99).This leads to the conclusion that conventional DD additives with a high alkaline reserve actually improve the antioxidant properties of oils in general, because unlike phenolates, salicylates and phosphonates, petrosulphonates do not themselves behave as antioxidants (106) on the contrary, low-base petroleum sulphonates, in the absence of other additives, act as pro-oxidants and only exhibit antioxidant behaviour at higher concentrations and higher alkalinity (320). There are various interpretations of the mechanism of this effect of DD additives. Sechter et al. (105) attribute it to the solvent power of the detergent-dispersants for the oxidation products and their incorporation into DD micelles, thus rendering them inactive. Phenolates, phosphonates and salicylates (unlike sulphonates), are also believed to produce - after reaction with acidic components in the oil substances which themselves act as antioxidants.
Behaviour towards Water All detergent-dispersant additives are amphipatic, oil-soluble surface-active compounds containing both hydrophobic and hydrophilic groups, able to reduce the inter-facial tension in the oil-water interface. Contact of oil concentrates of DD additives leads to hydrotropy and the formation of emulsions, the stability of which depends on the interfacial surface tension. Differences in polarity result in differences in the effects of particular compounds on the decrease in interfacial surface tension and on emulsion stability. The most effective, in both respects, are those compounds which contain hydrophilic groups of high polarity, such as S0,X (sulphonates), -NH-(e.g., succinimides)and -COOX(carboxylates).
300
This is the reason why DD additives and oils containing them must be protected from contact with water. The DD additive itself does not, in fact, lose its efficiency when it is transferred into the emulsion phase, however, contact of the oil containing the additives causes partial dissolution of the additive in water, which again causes a decrease in the concentration of the DD additive in the oil. The solubility in water of different compounds used as DD additives varies. Hence, resistance of DD additives to water is one of the tests in use. This test consists in determining the decrease in sulphated ash content of the oil after it has been washed with water.
4.2.2 Detergents Detergents mostly used at present comprise calcium, magnesium and, to a much lesser extent, barium salts of petroleum-based and synthetic alkarylsulphonic acids, alkylphenols, and alkenylphosphonic,thiophosphonic and alkylsalicylic acids (oncepopular barium salts have now almost disappeared from additive treatments, on toxicity and other grounds). It has been suggested by Crawford that mixtures of different metals may exhibit synergistic effects and so provide a performance improvement. Additives intended for other purposes may exhibit detergent effects if they have long alkyl chains. For example, zinc dialklydithiophosphates with alkyls around C, have marked detergent properties which can even surpass the performance of ZDDP’s with shorter alkyls in combination with detergents. Thus, at the same molal concentration, the following numerical ratings have been observed in Petter W1 tests (141): base oil + 2.2% weight ZDDP (Cz0) base oil + 0.8% weight ZDDP (C,) base oil + 0.5% weight ZDDP (C,) + 0.6% weight 10 TBN petroleum sulphonate base oil + 0.5% weight ZDDP (C,) + 0.6% weight 10 TBN synthetic Ca sulphonate
26.1 18.4 23.2 20.0
Detergents can either be neutral or basic (super-basic, hyperbasic), i.e., containing super-stoichiometric amounts of metals. The latter are more frequently termed overbased detergents, or detergents with an alkaline reserve. The alkaline reserve or degree of overbasing is expressed by the TBN; this number represents the total alkalinity expressed as amount of KOH per gram. ASTM D-2896 and DIN 51 596-Blatt 1 specify the potentiometric method for determination of total alkalinity. The oil sample is dissolved in a water-free mixture of chlorobenzene and glacial acetic acid and tiuated potentiometrically with a solution of perchloric acid in acetic acid. The number of mg of KOH equivalent to the perchloric acid consumed per gram of sample is the TBN. A coulometric method is also available (123). ASTM D-4739-87 (ASTM D-664 was formerly used) specifiesTBN determinationby potentiometric titration of the oil sample dissolved in a mixture of toluene and isopropanol with an alcoholic solution of hydrochloric acid. Compared with ASTM D-2896, it gives lower values for TBN, since it involves the neutralisation of strong acids with weak bases (bases are stronger in D-2896, since the medium is
30 1
glacial acetic acid); also, the reproducibility of the results is poorer as the transition-point is less distinct, which can be especially important in the case of used oils.
Overbased detergents containing as much as 30 times the stoichiometric amount of metals have been developed over recent years. They can be prepared by heating a mixture of an acid or neutral substrate with a large excess of metal base in solution in a solvent in the presence of a promoter, followed by filtration. Carbonation of the reaction mixture with CO, before filtration is often used to increase the incorporation of metal in the form of carbonate, which is colloidally dispersed andor bound in complexes in the filtered basic detergent (110-114,260).The promoters can be C, - C5 alcohols (mostly methanol) because of their high efficiency and low price, other alcoholates, polyglycols, phenols, mercaptans, ketones, nitro-compounds, carboxylic acids, phosphorus thioacids, oximes and, generally, organic compounds at least 0.0005% soluble in water at 50 "C, with an ionisation constant above about 1.10-lo in the presence of water at 25 "C and a pH at least 7 in saturated water solution, also at 25 "C. The basic detergent shows good solubility when the carbonate is produced in amorphous form; crystalline carbonates (except magnesium carbonate) diminish the solubility of the detergent in oil. The detergent over-basingprocess occurs in two stages: hydration of MO to M(OH), and carbonation with C 0 2 in a water-free environment. Ammonia is used in the hydration process to neutralise acidic components (e.g., sulphonic acid in the manufacture of overbased sulphonates). The solvents employed for the neutralisation and carbonation include volatile hydrocarbons of boiling-point below 150 "C, e.g., hexane, heptane, light gasoline, benzene and xylenes, which are vaporised by the heat developed during the exothermic neutralisation and carbonation reactions. The colloidal dispersion of over-based salts may contain both hydrated and non-hydrated carbonates. These differ in respect of their stability and their ability to form colloidal dispersions. For instance, an overbased magnesium detergent was found to contain the following salts: MgCO, . Mg(OH), . 3H20, MgCO,. MgO. 5H2Q, MgCO3.3H20, MgCO3.5H2O.
A detergent can be characterised by a number of parameters: relative molecular mass, constitution of its organic portion, length of the alkyl chains, total base number (TBN), ratio of metal in the alkaline reserve to metal in the neutral molecule ("salt to soap ratio"), type of metal and, in some cases, sulphur and phosphorus content.
Table 4.6. Limiting Values of Some Currently-used Detergents Sulphonates Relative molecular mass
Phenolates Whenolsubhides)
Salicylates
375-700
160-600
(sulphonic acid)
(alkylphenol)
250- 1,OOO (alkylsalicylic acid)
TBN (mg KOWg) 0-500 Ratio of metal in alkaline reserve to metal in neutral compound 1-30 Type of metal Ca,Mg,Ba Neutral compound content (56 weight) 10-45
50-400
64-345
0.8-10 Ca,Mg,Ba
1-10 Ca,Mg
30-50
10-45
302
Increased detergent alkalinity reduces deposits and wear, giving improved oil consumption figures and longer engine life. Selection of metallic detergent components is critical in securing anti-wear protection and lowest oil consumption. Limiting values of these variables in some currently-used types of detergents are shown in Table 4.6. These variables are not, however, sufficient for assessing the effectiveness of a detergent in oil, which can only be determined by field tests.
4.2.2.1 Alkarylsulphonates
Petroleum sulphonates or, often, synthetic sulphonates are used. Either can be neutral or overbased, calcium or magnesium or sometimes barium alkaryl-sulphonates soluble in oil. They can provide the essential, thermally stable components of detergent additives in all types of oil. The composition of neutral (also referred to as “normal”) sulphonates can be represented by the general formula: (RAr.SO,),M where M is Ca, Mg or Ba in stoichiometric ratio to the acidic group, and RAr is an alkylaromatic group of petroleum or synthetic origin. The alkalinity of basic and overbased sulphonates is predominantly produced by a dispersion of carbonates of the metal concerned in a neutral sulphonate. This has been confirmed by electron microscopy studies (107). The general formula of overbased sulphonates is thus: (RAr.SO,),M.nM’CO, In basic sulphonates, 3 to 15 metal atoms in the alkaline components are associated with one in the neutral component. M and M’ need not be identical; for example M can be Ca and M’ Ba or Mg. The excess metal in basic sulphonates can also be bound to hydroxyl; some authors (108,209)quote the following formula: (RArS0,)MOH Kuliev (2) suggests the following formula for the two forms: (RArSO,),M.MO.M(OH),,
(RArS03),M.M0.MC0,
The most frequently used formula for basic sulphonates is:
RR Ar - SO, -0
0 II M- 0 - C - 0 --+M-.OH
+
Overbased sulphonates can be prepared by a great variety of processes. The most common comprises neutralising a sulphonic acid with excess calcium oxide or calcium oxide in the presence of a promoter, typically methanol, then carbonating the mixture to produce a colloidal dispersion of what is term a calcium carbonated overbased sulphonate.
303
The dispersed calcium carbonate is amorphous, with a particle size from 20 to 150 Angstroms. It is solubilised in a mineral oil camier by the calcium sulphonate, giving bright, clear dispersions. These products are characterised by being relatively non-viscous and having no thixotropic properties. When an overbased calcium sulphonate is treated with an active hydrogen compound such as water, alcohol or a lower carboxylic acid, the amorphous calcium carbonate can be converted into the crystalline form. The predominantly calcium carbonate may exist in three modifications: calcite, vaterite and aragonite. Crystalline calcium carbonate has much larger particles, from 150 up to as much as 500 Angstroms. X-ray diffraction data have confirmed that these calcite particles consist of thin, wafer-like platelets, with a pronounced tendency to orient parallel to the calcium sulphonate in mineral oil. A pronounced change in rheological properties on modification of the calcium carbonate into calcite is attributed to a high degree of association between these parallel platelets (403).Because of this, thixotropic overbased calcium sulphonates with crystalline calcium carbonate have higher viscosities, are more resistant to mechanical break-down, possess good extreme pressure properties and provide better corrosion protection.
Mineral-oil based sulphonates, so-called petroleum sulphonates, were the first detergents used for engine oils during World War 11. They are valuable by-products of white-oil production or can be deliberately produced by sulphonating suitable oil distillates. The following structure can be used as a model for the hydrocarbon portion (215):
-
CH19H39
Ca
According to Danvik et al. (280). the chemical composition of mineral oil sulphonates is strongly affected by the choice of sulphonating conditions.
Sulphonate-type detergents used today in lubricating and other oils (e.g, in diesel fuel, heating and quenching oils) are predominantly calcium salts of relatively low alkalinity (TBN up to about 25), exceptionally Ba salts of TBN up to about 70, together with over-based calcium and magnesium petroleum sulphonates of TBN as high as 400, with TBN 500 rare but available. Supply problems with petroleum sulphonates, their comparatively high price and the manufacture of synthetic alkarylsulphonates for laundry detergents have encouraged petroleum sulphonates to be complemented by synthetic sulphonates. These are predominantly alkylbenzene sulphonates with at least 20 carbon atoms in the alkyl chain (e.g., polydodecylbenzene sulphonate), manufactured either intentionally by alkylating benzene, naphthalene or their alkyl derivatives with propene or isobutene 1-alkeno-oligomers, or extracted as by-products from the manufacture of laundry detergents (261).Their chemical structure can be illustrated by the formula: 304
where M is Ca, Mg, Ba, R are C,, to C3, alkyls, x > 1. Synthetic sulphonate detergents are manufactured and used chiefly as calcium salts at TBN up to around 400, and to a much lesser extent, barium salts up to TBN 80 and magnesium salts at TBN up to about 500. The magnesium salts are more expensive, sensitive to water, with which they can form gels when stored. They have. however, very good anti-rust properties. They are therefore particularly suitable for engines with hydraulic valve-lifters, which are prone to rust failure, which causes valve-knocking. Another advantage is their relatively low ash content, which is important in avoiding pre-ignition.
Examples of commercial products are shown in Table 4.7. Barium content in oils and additives is usually determined by 1P 1 lot743 the sulphated ash is dissolved i n perchloric acid and the Ba is determined gravimetrically as sulphate. Metals which form insoluble sulphates such as lead must not be present. After Ba has separated, Ca and Al may be determined. IP 1 1 1/74 specifies a particular method for determining Ca in the presence of P, Fe, Al, Ba and Mg. Oil is incinerated, the ash dissolved in HCI and the Ca precipitated as oxalate, which can then be determined by titration with KMnO,. ASTM D-811 specifies a process for separating Ba,Ca, Mg, Zn, Sn. Si and Al in both fresh and used oils in the presence of S, P and Cl; the sulphate ash is dissolved in HCl and the individual metals determined by the wet method. Methods for analysing the composition of sulphonates by liquid chromatography are specified in ASTM D-2548 (for Na sulphonates) and ASTM D-2894 (for Ca and Ba sulphonates). For the determination of Na sulphonates, the sample is dissolved in chloroform and adsorbed on silica gel. Oil is eluted with chloroform and sulphonate with alcohol; the amounts present are determined gravimetrically. For the determination of Ca and Ba sulphonates, the sample is dissolved in diethyl ether and the sulphonates converted with HCI into sulphonic acid. The latter is converted into sodium sulphonate. The further analysis proceeds according to ASTM D-2548. ASTM D-855 specifies the detailed analysis of sulphates. Factors for the conversion of metal concentration in oil into sulphated ash are: Metal
Conversion factor
Ca Mg Ba Zn
3.4 4.95 1.7 1.24
The ash value is obviously dependent on the metal (Ba sulphonate has the highest, Ca sulphonate lower and Mg sulphonate the lowest ash content) and on the alkaline reserve. The relative molecular weight for good solubility of metal sulphonates used as detergents and anti-corrosion additives for lubricating oils should be between about 900 and 1100. Detergent capability and solubility both improve as it increases. It is
305
Table 4.7. Composition of Commercial Petroleum and Synthetic Sulphonates Used as Components in Engine Oil Detergent Additives* Properties and Compositions
Density at 15°C (kg.m-)) (approx.) Content (%weight) of: Ba
Petroleum sulphonates calcium calcium neutral slightly basict
calcium neutral medium basic
Synthetic sulphonates barium highly neutral medium basic basic
magnesium highly basic
1100
1 loo
930
940
950
lo00
1 100
-
-
-
-
11-13 (16.2) 1.8-2.0 28-32
Mg Ca S Active sulphonate (%)
1.4-2.1 1.7-2.4
I .5-2.4 1.6-2.4 35-47
1.7-1.9 2.7-3.0
TBN (mg KOWg)
max.10
10-30
max. 10
Sulphated ash (% weight)
4.8-7.2
5.1-8.5
5.8-6.5
2.84.0 2.7-3.0 42-47
20-60 280-310 (400) 9.5-13.6
13.U.2(55)
900 5.5-6.1
2.6-2.9
max.10 9.4-10.4
-
12-15 2.6-2.9 4247
1.8-2.0 min. 29
50-80
to 500
20.4-25.5
7-10
-
45-50
35-509b solutions of metal sulphonates.
Commercial, medium-base (TBN 10-12) products are also available derived from petroleum, also over-based (TBN 300-400). Their preparation is substantially more difficult.
also desirable that the dispersion of molecular weights in the product is as low as possible. Detergent power, naturally, depends on the effective concentration of active sulphonate. Sulphonates which have low inorganic salt and sediment content (below 0.1% by weight) as a result of purification with low molecular weight alcohols such as isopropanol are more effective as detergents. The anti-corrosion and anti-rust properties of barium and magnesium salts are better than those of calcium salts. This property, inherent in neutral sulphonates, can be impaired by high metal chloride content, e.g., CaC1, present as residue from the conversion of sodium into calcium salts. The chlorine content of petroleum sulphonates should not exceed 0.03% by weight. According to Anand and co-workers (272,272), the most effective detergent-dispersantsare Ca-Ba petroleum sulphonates prepared from sodium petroleum sulphonates with an average molecular weight of 460-482 (this may be achieved either by fractionation, e.g., according to Fische et al. (273). or by mixing petroleum sulphonates of different ranges of molecular weight obtained from different oil fractions). The chemical nature of the oil fraction used as starting material for sulphonation has a significant effect on the dispersive power of the finished sulphonate. The best detergent-dispersant efficiency in engine oils is that of Ca-Ba petroleum sulphonate produced from oil fractions with a CN:CAratio of 4 - 5 . The petroleum sulphonate molecule should contain only one aromatic ring and the alkanic side-chain should have at least 13 carbon atoms. Tests by Bowden’s microscopic method have shown that the detergent-dispersanteffect of oils do not increase linearly with the concentration of Ca-Ba petroleum sulphonate, but that there are optimum concentrations at which maximum efficiency is achieved (efficiency varies with gradually increasing sulphonate concentration in oil, up an optimum). The efficiency variation of petroleum sulphonates with good detergent-dispersant ability is minimal and such petroleum sulphonates possess good efficiency, even when the optimum concentration is exceeded, whereas the efficiency of medium- or low-quality sulphonates deteriorates rapidly with further increases of concentration in oil.
The desirable cleansing, dispersant and neutralising power of metal synthetic alkarylsulphonates depends mainly on: - the length and configuration of the alkyl chains, - the ratio of polar to non-polar parts of the molecule, - basicity. Solubility in oil and detergent-dispersant action of the sulphonate increases with length of the alkyls. This is illustrated in Table 4.8: The detergent efficiency of these sulphonates decreases with the number of substituents on the alkyl chain. Also, di- and polyphenyl derivatives cause deterioration in the beneficial effects of monophenyl derivatives. These phenomena are the consequence of adverse changes in the polar and non-polar portions of the molecule (85).Basicity is obviously the determinant of the neutralising power of the alkaryl sulphonate. Neutralising capacity improves with alkaline reserve, but the detergent-dispersant efficiency decreases, because of the decrease in active sulphonate concentration. These factors must be taken into account in formulating oils with sulphonates. 307
Table 4.8. Petter AV1 Engine Test Results with Oils Treated with Barium Monoalkarylsulphonatesof Various Alkyl Chain Lengths Alkyls
Relative molecular weight'
Average number of C atoms
c10-c14
167
11.8
c15-c22 c22-c33 c24-c35
280 360 390
19.8 26.0 27.7
c,,-c,* (n-dkyl) CZ6-C2,(polyisobutenyl)
518 510
-
Total test merit rating points
Not measured - sulphonate insoluble in oil 86 91.7 93 91 80
* Relative molecular weight of sulpho-acids.
Metal sulphonates usually have good thermal stabilities. Therefore, relatively high temperatures can be used in handling them, namely, up to 80 OC bulk temperature and up to 120 "C skin temperature. Contamination with water must be avoided in order to prevent emulsion formation. The biological activity and human skin effects are similar to those of mineral oils, and the same regulations are applicable. However, recent work has raised questions of possible skin-sensitisation potential with certain sulphonates. \
4.2.2.2 Alkylphenolates (Alkylphenolsulphides) The first detergents of this type (also referred to as phenates, sulphurised phenates and sulphurised alkylphenates) were first used during World War I1 in the form of calcium and barium tert-octylphenolsulphide and tert-amylphenolsulphide (116),the condensation products of calcium tert-amylphenolate with formaldehyde (117)and calcium and barium phenolates with long alkyl substituents (118).The presence of a labile sulphur atom in the alkylphenolsuphide enhances its antioxidant properties and imparts anti-wear properties. The detergent action depends on the size and configuration of the alkyl group. The patent literature quotes a number of alkylphenolate and alkylphenol-sulphide type detergent additives. Some alkylphenolsulphides have, in addition, anti-foam effects. The superiority of the phenolate detergents resides in their ability to alleviate oxidation and even corrosion. Therefore, they enable the amount of additives in oil to be reduced. In lubricants in which MDDP are not permitted, they perform the full function of antioxidant. The high neutralising power of magnesium overbased phenolates can makes them preferred additives in lubricants for marine diesel engines requiring high alkalinity, although their use in this application has been suspected of causing cylinder wear. All detergents decrease bearing weight loss, but, on an equal metal and base molal content, phenates are slightly better than salicylates and sulphonates. Some advantages of phenates stem from the fact that they are derived from relatively weak acids (phenols and C02). On reaction with strong acids, 308
sulphurised alkylphenols are liberated, which can be expected to provide antioxidant and anti-wear characteristics. Phenolates have good high-temperature properties and are therefore suitable as detergent components in oils for diesel engines and highly thermally-loaded gasoline engines. Phenolates with longer alkyl groups improve the solubility of polymeric viscosity modifiers in oil. This is particularly important for high VZ hydrocracked and polyolefin oils, in which the solubility of polymeric modifiers is poorer. The synergistic effect of phenolates and sulphonates is also of considerable importance. Calcium, barium and magnesium alkylphenolates are produced and used either as hydroxylated salts: OMOH
OMOH
OMOH
or, with alkaline reserve, in the form of metal carbonates: M -0 - CO- 0 - M 0I
0I
I
!-o-Co-o-M
I 0
where M is Ca, Ba or Mg, x is 1 - 5 , R is a straight-chain or branched C,-C,, alkyl, typically nonyl or decyl. The alkaline reserve in the calcium phenolates available now is as high as 400 TBN, but in barium phenolates below 120, because of the high ash content in high TBN Ba products. The preparation and production of basic alkylphenolates is simpler than that of sulphonates and phosphonate, in that the phenol itself (without promoter) enhances the production of the excess metal oxide in the colloidal form (119). Promoters are used in commercial processes; these include methanol, methoxyethanol and ethylene glycol. The presence of petroleum or synthetic sulphonate has a beneficial effect. This interaction between phenolates and sulphonates can be utilised in the preparation of package overbased DD additives with alkylphenolates and sulphonates in suitable ratios. This is advantageous particularly in the case of those petroleum sulphonates where the alkaline reserve is difficult to produce or to introduce into finished oils without problems of oil-solubility.
Examples of commercial products are illustrated in Table 4.9. Alkylphenolates can be handled safely at temperatures up to 120 "C (skin temperature up to 180 "C). Calcium-containing products have a similar effect on the human organism as mineral oils. Bariumcontaining products are toxic and must be handled more carefully; this toxicity has led to the virtual removal of barium-containing additives from commercial use.
309
Recently (1988-9), high TBN sulphurised calcium alkylphenolates have appeared on the market, with TBN around 400 mg KOWg (ADX410from Adibis). These represent a departure from the phenate materials hitherto available. During manufacture, long-chain naturally-occurring acids are introduced into the phenate molecule. The viscometric properties of these high TBN products are unusual in that in the concentrate form they have viscosity index over 150, compared about 70 for conventional phenates at 250 TBN. The incorporation of long-chain acids also imparts significant friction modification properties, again unusual in phenates. Normal treatment levels in automotive lubricant formulations reduces the friction coefficient from levels of 0.13 to 0.085.
Table 4.9. Properties and Composition of Commercial Alkylphenolates and Phenol Sulphides Used as Components of Detergent Additives in Engine Oils* ROpertY
Calcium type medium high basicity
neutral
1100
1000
Barium type medium overbased basicity 1000
1100
4.5 8.5-10 3.5-5.5 2.6-2.8 2.8-3.5 85-120 250-400 10max. 70-80 9.9-16 31.6-47.6 7.7-1 1.9 14.5-17 * All are solutions in suitable diluent oil raftinate; actives content around 50%weight.
Density at 15 "C (kgm3) Composition (% weight) Ca Ba S TBN (mg KOWg) Sulphated ash (% weight)
1100 2.9-4.7
9.3-14.0
-
-
-
11-13 3-4 95-120 19.5-25.5
4.2.2.3 Alkenyl Phosphonates and Thiophosphonates These DD additives have almost entirely lost their market position because of their inferior thermal stability, their high ash content as barium salts, their high cost of manufacture and the general effort to reduce the phosphorus content of oils for gasoline engines fitted with catalytic converters. Since they were largely useful as barium salts, the toxicity of barium has also tended to discourage their use. They were used for the first time during World War II, when it was found that metal salts of the alkenylphosphonic acids produced by the reaction of phosphorus pentasulphide with liquid polyalkenes of 500-2000 relative molecular weight, mainly polybutenes (99,I22),have highly detergent-dispersant effects on high- and low-temperature sludges in lubricating oils. One of the first commercial products was the potassium salt prepared by the reaction product of PzSs with liquid polyisobutene of viscosity about 120 mm2.s-' at 38 "C (220). Later commercial products were salts of alkaline earth metals, particularly barium salts (122).
The action of steam or a prolonged hydrolysis prior to or during the neutralisation stage can cause substitution of part of the sulphur in the thiophosphonate group, PS(SM),, by oxygen, so that a mixture of thiophosphonate and phosphonate is
3 10
produced. For simplicity, the former commercial products are sometimes called phosphonates (although they contain sulphur); they were given the general formula:
x x II/
\
M.M(OH),
R P
or
MCO,
\ / X
where R is a long alkenyl radical with minimum relative molecular weight 500, X is oxygen or sulphur, M is a bivalent metal. Calcium salts are difficult to prepare and only soluble in oil to a limited extent. In packaged detergent engine oil formulations, now obsolete, medium-base barium (thio)phosphonates of TBN up to about 120 were used, of the following type:
X X It II R-P-S-P-R I
I
where R is any alkenyl C,,-C,, and X is S or 0. The commercial products usually had 12.0% by weight barium, 0.8-1.2% phosphorus, 1-3% sulphur and up to 20% sulphated ash. TBN varied around 70 and the additive concentrate usually contained about 50% ac'tive component. Combination products of the following type were more frequently used: /O\ R e O - PI
P-O-@R I
"\ /"
M.M(OH),
or
MCO,
where R is an alkyl C,-C,,, M is Ca. Phosphonates of higher alkaline reserve may also be prepared, but such a reserve in the barium compounds would be associated with an extremely high ash content. Similar promoters or solvents as in the production of sulphonates are employed for other over-based products, including phenols, phenolsulphides (124,alcohols (125) and acids, such as formic acid (226).
Although now largely obsolete and replaced by sulphonate/phenate combinations on grounds of toxicity and ash content, the phosphonates and thiophosphonates were 31 1
highly efficient detergent-dipersants suitable for improving piston cleanliness. They were particularly suitable for oils intended for lubricating gasoline engines, less suitable for diesel oils, because they have lower stability at higher temperatures. Phosphonates could sometimes be used in such cases as DD additive components, mainly phosphonates of higher Ba/P ratio (above about 5/1), because they contributed efficiently to the reduction of carbon deposits in the piston grooves. They were, to some extent, able to prevent the formation of "cold sludge", particularly thiophosphonates with low Ba/P ratios. The relative molecular weight, the BaP ratio, the method of preparation and the number of unsaturated linkages in the alkenyl chain determine the efficiency of thiophosphonates. According to Karl1 et al. (262),the efficiency of Ba thiophosphonates grows at the same molal concentration up to a relative molecular weight of 1,250;according to MAFKI staff (263),Ba thiophosphonates alkylated with polyisobutene of relative molecular weight 850 to 2,300 have similar efficiencies. Because of the neutralisation effect, the efficiency of thiophosphonates increases with increasing metal content up to a Ba/P atomic ratio of 1.5/1. Phosphonic acids can be prepared from polyalkene and P,S, at 200 to 250 "C. The barium salt is then obtained by neutralisation with barium hydroxide, after steam treatment at 150-200 "C and filtration. According to the MAFKI work (263),a better product was obtained if the thiophosphonic acids were purified by sedimentation only, without steam treatment, provided sediment content does not exceed 1.5 to 2% weight (the barium content of the sediment is substantially higher, and exists largely in the phosphonate form, which reduces the efficiency of the additive). High efficiency barium thiophosphonates cannot be made from alkenes with more than one double bond in the chain - dienes reduce the effectiveness of the additive.
Alkenylthiophosphonates are the most thermally sensitive of all the ashcontaining detergent-dispersants. They should not be heated above 60 "C and only liquid heat-transfer media should be used for warming them. Thiophosphonates are .toxic and ingestion or long-term inhalation should be avoided. 4.2.2.4 Carboxylates
The first detergents used were the soaps of high molecular weight carboxylic acids, such as calcium dichlorostearate: [CH, - (CH2)7- CH- CH I I c1 Cl or calcium chorophenylstearate:
- (CH,),
0 II - C - 01Ca
0 II [CH, - (CH2)7 - CH - CH - (CH2)7 - CI
I
01 Ca
Cl Although these substances had good detergent capacity, they were abandoned because of they promoted oxidation and caused corrosion of copper-lead bearings.
312
Calcium naphthenate with a high alkaline reserve is still used occasionally, e.g., in diesel cylinder lubricants in cross-head marine engines operating on a total oilloss system:
r
0
1
II (CH2)n- C - 0 Ca.CaCO3
IR L
J2
Alkylsalicylates form a separate group of general formula:
where M is a bivalent metal, usually Ca, but also Mg, and R is a c,-C,o alkyl. The salicylate esters originally used, such as zinc diisopropyl salicylate, were later substituted by calcium salicylates with longer chains, such as octyl, attached to the aromatic nucleus. Basic calcium and magnesium salicylates are now used. Typical products, made by Shell, are characterised as follows: Shell Code SAP001 SAP002 SAP005 SAP007 SAP008*
Metal type Ca Ca Ca Mg Mg
%w/w 6.0 2.3 10.0 7.4 6.0
TBN (mgKOWg)
165
64 280 345 280
* Also contains 2.9 weight boron. Magnesium alkylsalicylates are mainly used in marine engine oils, because of their good anti-corrosion properties. They help achieve high alkalinity values at low ash-levels, but they tend to have lower thermal stability then the calcium salts. Alkylsalicylates are prepared by carboxylation of water-free metal alkylphenolates with carbon dioxide at elevated pressure (227). Preferred starting materials are rnonoalkylphenolates substituted, preferably, in the para-position;2,6-dialkylphenolsand 2,4,6-trialkylphenolsare not suitable, in that they fail to carboxylate, or only carboxylate at low yield at high tempratures and pressures (228).The alkaline reserve is usually in the form of carbonate. Alkylsalicylates of exceptional purity can be made by carboxylation in wiped-film reactors (387).
313
Treatment with carbon dioxide (carbonation) of a mixture of salicylate and added metal hydroxide in the presence of a promoter, e.g., an alcohol, is a general procedure for the preparation of overbased detergents. Alternatively. a colloidal complex of a metal carbonate can be prepared in siru and then combined with the acid or its salt to give the required excess basicity. This chemical route is followed in the case of magnesium phenates, where the former reaction path is less satisfactory. In both cases, the degree of basicity can be expressed as metal ratio or "basicity index", which is the ratio of the metal present to the metal contained in the neutral salt.
An advantage of the alkylsalicylates is their high thermal stability, which makes them particularly suitable for oils used in lubricating heavy-duty diesel engines. Overbased alkylsalicylates have good anti-corrosion properties, but they have the disadvantage of incompatibility with some mineral and synthetic sulphonates unless other DD additives are present. Magnesium sulphonates are less sensitive in this respect, but have the disadvantage of lower thermal stabilities. Salicylates also have a significant ability to reduce friction. The basic metal salts of sulphurised alkylsalicylic acids are also good detergents (294). Alkyl esters, e.g., ethylhexyl esters, of alkylsalicylates,act as antioxidants. Because of their antioxidant and detergent actions, alkylsalicylatesmay be used alone, however, they are usually combined with ZDDP to improve anti-wear properties and with succinimides to prevent the formation of low-temperature deposits, to improve piston cleanliness and to reduce the ash content of the oil. Like phenolates, they show synergistic effects with alkaryl sulphonates. Combinations with phenolates can be extremely effective.
With regard to handling temperatures and effects on the human organism, the same guide-lines apply as for alkylphenolates. According to Grjaznov et al. (295),neither basic Ca alkylsalicylate nor the low-base Ba alkylsalicylate decomposed when heated at 180 "C for 2 hours in a nitrogen atmosphere. After heating at 300 "C, Ca salicylate retained some of its properties by forming a CaCO, dispersion, in spite of decomposition; on the other hand, Ba alkylsalicylatelost its detergent and anti-corrosion properties. Sodium alkylsalicylate decomposed at 180 "C.
The composition of the commonly-used ash-containing detergents, sulphonates, phenolates and/or salicylates, has tended to stabilise somewhat in recent years, except for the continued pressure towards ever-higher overbasing, in some cases involving novel process chemistry. A comparison of constitution and properties, and effects in gasoline and diesel engines, is given in Table 4.10. There has also been comparatively little development evident from the patent literature in this area. Among newer compounds may be noted: - basic calcium polyisobutyl sulphonates by the reaction of PIB (r.m.w. around 950) with chlorosulphonic acid (327), - sulphurised basic magnesium alkylphenolate with TBN around 250 mg/KOWg (320), - basic metal polyarylaminosulphate and polyarylaminophenolsulphides (329), - very high TBN (>400) overbased sulphurised calcium alkylphenolate.
314
Table 4.10. Comparison of Ash-containing Detergents in Engine Oils Effect Diesel engine oils -deposit reduction above piston rings -deposit reduction below piston rings -increase in TBN per 8 ash -TBN retention Gasoline engine oils -prevention of oil-thickening (MS IIID) -rust prevention (MS IID) -reduction in varnish (MS VD) Both diesel and gasoline engines -increase in TBN per Q ash -prevention of CuPb type bearing corrosion -effect on Friction coefficient
Sulphonates Phenolates Salicylates
3 3
2 2 1
1 2 1
2 3 3
1 1 2
1 2 1
1
1 =best 2 = medium
3 =worst Note: Calcium alkylsalicylates have a beneficial effect on piston cleanliness in diesel engines, particularly in engines with high temperatures in the first ring groove (e.g., Caterpillar 1G-2) and on the reduction of carbon in that groove. Similar effects can also be achieved with mixtures of overbased calcium alkylphenolate and low-base calcium sulphonate in a suitable ratio. By contrast. basic calcium phenolate by itself is the least effective in this respect.
4.2.3 Dispersants The term dispersant is used in this context to denote ash-free additives capable, among many other useful functions, of dispersing cold sludge-forming substances in the crankcases of gasoline engines. The tendency to form low temperature sludges and, consequently, to increase viscosity is also displayed by oils in smaller diesel engines. Special methods for testing the resistance of these oils to the formation of cold sludges have been developed (e.g.,formerly, the Perkins Diesel Oil Thickening Test and Daimler Benz OM 616 test, a replacement test now under consideration by CCMC, with the OM 602A as the likely candidate). The sludge formed in this type of engine is different from that formed in gasoline engines, in that it arises as a result of temperature distribution in the flame-front in the diesel combustion chamber and the volatility of diesel fuel. In small diesels, the surface area of metal exposed per unit volume of combustion space is relatively high, leading to soot being formed in the cooler parts of the flame. Soot build-up in the crankcase oil of engines of this type can be quite rapid and can cause it to become thixotropic, leading to engine failure as a result of inadequate oil-flow. Whilst cold sludges in gasoline engine also contain sooty material, the starting materials from which the soot is made are different and the overall chemistry of the deposits is also different. However, the phenomena in both types of engine are amenable to dispersant treatment.
Dispersants are, moreover, capable of stabilising the products of thermooxidation, peptising solid contaminants and preventing them from settling; they are synergistic with detergents and are satisfactorily stable at elevated temperatures. For these reasons, they are conventionally used as components in DD additive packages. Modem high-quality engine oils usually contain 4 to 8% by weight of ashless dispersant. The first compound with proven ability to suppress significantly the
315
formation of cold sludges in gasoline engines was the copolymer of lauryl methacrylate with diethylaminoethyl methacrylate (130) (see also under VI improvers). A number of dispersants suitable as components of DD additives has been developed since. The mechanism of one aspect of DD action - maintaining contaminants in a peptised state - can be deduced from consideration of the probable nature of the peptised particles. In the case of ashless dispersants such as succinimides, relatively small (0-500 A) particles may be produced, by forming a thick, non-polar film around the contaminant (which is polar), thus preventing coagulation. Polymeric dispersants achieve a similar effect, producing larger particles (0-1.000 A). The structure and shape of the polymer chain may exert an important influence. This action of ashless dispersant can also be supposed to operate with metallic dispersants in the case of the formation of small particles (0-200 A). However, metallic dispersants can also form large particles (5,000-15,000 A) on which a surface charge prevents coagulation by electro-static repulsion.
4.2.3.1 Succinimides and Bis-succinimides Succinimides and bis-succinimines are the reaction products of alkenylsuccinic anhydrides and polyethylene polyamines (131): R - C H = C H - C H -CO I CH, - CO’
‘
N - (CH,), - (NHCH, - CH,), - (CH& - NH,
(mono) succinimide R - C H = C H - C H -CO, I CH, - CO
,\
N (CH,),
- (NHCH,
- CH,), - (CH,), - N
/CO - CH - CH = CH - R I - CH,
’ ‘CO
bis - succinimide where R is usually a polybutenyl fragment of relative molecular weight 800 to 2,500 (mostly 800 to 1,200) and x = 2 to 5. The alkenylsuccinic anhydrides are made by the reaction of a polyalkene with maleic anhydride. The distribution of molecular weights in the alkenyl should be as namow as possible, in order to reduce its resistance to cleavage, which causes the formation of products of only limited solubility in oil or partial solubility in water. The alkenyl anhydride must therefore be tested prior to use in reactions with polyethylenepolyamines for the concentration of water-soluble constituents (with alkyls up to about Cl0) and oil-soluble constituents (with alkyls above C,d. Copolymers of polyisobutene (PIB) with styrene or PIB monomers with functional groups, or other polyalkenes, e.g., poly-n-butenes, copolymers of ethylene-propylene, propylene-isobutene, etc., have been used in place of polyisobutene. Since the reaction of polyalkene with maleic anhydride is never fully complete, some residual polyalkene remains which is virtually impossible to extract in a practical industrial process; this polyalkene therefore becomes part of the resulting product and affects its viscosity. This is one reason why the viscosities of commercial products varies over such a wide range.
316
The addition reaction of maleic anhydride with PIB is normally carried out with excess maleic anhydride in the initial charge by simply heating the starting materials together. At the end of the reaction, the excess maleic anhydride is removed by distillation from the crude polyisobutenyl succinic anhydride. The addition proceeds vigorously at first, but slows after a substantial proportion of the PIB has been converted. The conversion of PIB in the later stages of the reaction can be considerably increased by the addition of haloalkanes or halogens. Chlorinated PIB reacts much more readily than the original polymer. This method allows polyisobutenyl succinic anhydride to be produced with significantly less unconverted residual PIB, thus enabling a more concentrated succinimide product to be made, which has lower viscosity (especially at low temperatures) than products containing a higher concentration of unreacted PIB. PIB succinimides are used as the principal dispersant component in many lubricating oil formulations for a variety of reasons. Prominent among the advantageous factors are high thermal stability, superior dispersant activity, flexibility in modification of the molecule to meet particular performance requirements, ready availability and moderate cost of raw materials and ease of handling. Perhaps the biggest disadvantages in the longer-term are oil-thickening at low temperatures and the presence of chlorine (normally 50 - 150 p.p.m.). The degree of low temperature thickening is largely a function of the structure of the molecule and its restricted rotation due to the methyl groups. If large amounts of dispersant are needed in the final oil to meet engine test standards, this can emphasise the problem. Chlorine content in PIB succinimides is a function of the method of manufacture. Some can result from catalyst residues present in PIB, but it can also be present as a result of the method of manufacture of the polyisobutenyl succinic anhydride intermediate. Depending on the routes chosen, it can vary between 50 and 1,500 p.p.m. Chlorine in lubricating oils is an emotive issue because there it can lead to the possibility of emission of dioxins in vehicle exhausts. At least one PIB manufacturer has offered chlorineand bromine-free PIB, which may well be the preferred raw material in the future. The manufacture of mono- and bis-succinimides depends on the accurate determination of the acidity of the polyalkylenyl anhydride which forms the non-polar part of the molecule. This determines the conditions for the reaction of the anhydride with the polyalkylene polyamine, especially the molar ratio of the two starting materials. Smaller inaccuracies in this determination can lead to differences in the ratio of mono- and bis-succinimides in the final product. Technical monosuccinimides always contain some bis-succinimide and vice versa. The manufacture of packaged succinimides containing different amounts of mono- and bis-succinimides is based on this principle. In addition, mono-succinimides can form amide complexes, similar to bis-succinimides, of the following type: R - C H = C H - C H -CO I
CH2 - CO'
'
N - (CH2)2- (NHCH, - CHz),- (CH,),- NH
\
CO- CH - CH = CH - R
CO - CH2
by proton-transfer to the tertiary nitrogen in one succinimide ring from a primary amine nitrogen in the mono-succinimide, causing ring-opening and condensation. The condensation is reversible and the monosuccinimide is probably re-formed at higher temperatures (150-180 "C). It is possible that this sort of mechanism is responsible for the observed variation in the viscosity of mono-succinimide additive concentrates after storage at intermediate temperatures (50-90 "C). It seems unlikely that this condensation affects the properties of the dispersant in finished oil, although this possibility cannot be entirely ruled out.
Mono-succinimideshave excellent dispersant action on low temperature sludges in oil and the products of incomplete gasoline combustion, as well as improving the cleanliness of the entire piston surface in gasoline engines. Their effect on the high 317
molecular weight products of diesel fuel combustion is less marked, in that they do not sufficiently prevent the formation of deposits in the first ring groove, although they do materially contribute to the reduction of varnish deposition on the lower (skirt) portion of diesel pistons. These products of diesel operation are better dealt with by bis-succinimides, which have higher molecular weight. The bis- compounds also efficiently disperse gasoline combustion by-products, provided a certain amount of the basic nitrogen is present as mono-succinimide. Since the nitrogen content in bis-succinimides is significantly lower than in mono-succinimides, nitrogen is usually present in higher concentration in gasoline engine oils than in diesel engine oils. Compared with bis-succinimides. mono-succinimides are more aggressive towards sealing materials of the Viton (@ Dupont) and copper-lead type bearing materials. Bis-succinimides have higher thermal and oxidation stabilities than monosuccinimides.Moreover, the cleavage of mono-succinimidesgives rise to potentially insoluble products, which must be dispersed by the remaining succinimide. On the other hand, the cleavage of bis-succinimides produces two oil-soluble, potentially beneficial succinimides. For this reason, it is good practice to formulate with a combination of both succinimide types in a suitable ratio. The use of such combinations is essential for oils formulated to lubricate both gasoline and diesel engines, such as oils to meet MIL-L-46152 E, class API SG/CD, which must pass both MS Sequence VE (gasoline test) and Caterpillar 1-G 2 (high-output diesel test). Generally, the thermal stability of succinimides is high. The -NH, groups cleave at temperatures up to 250 to 350 “C; polyisobutene is stable up to about 310 “C. the decomposition of succinimides accelerates above 250 OC if the oil also contains ZDDP above a critical ratio because of reactions with -NH, and -NH- groups.
The nitrogen content of commercial succinimide concentrates (about 60-70% actives in oil) varies between 1.4 and 2.1% by weight, depending on the lengths of the polyalkenyl and polyethyleneamine chains. The nitrogen content of bissuccinimides is in the range 1.1 to 1.4%. Nitrogen can be determined by ter-Meulen hydrogenation, Kjeldhahl or Dumas volumetric methods (252) (ASTM D-3228).
Succinimides are only slightly basic, and their chemical neutralising power is therefore lower than that of overbased detergents. They are, however, able to neutralise acidic products by so-called “physical neutralisation”. The dispersant power and oil-solubility of succinimides is affected mainly by the length of the polyalkenyl and polyethyleneamine chains. The presence of other compounds, like ZDDP, or higher polyamines of the type:
is also important. 318
The reaction products of pol yalkylenepolyamineswith a mixture of alkenylsuccinic anhydrides and aliphatic mono-carboxylic acids also have good dispersant properties and increase the thermal stability of ZDDP’s.
These types of succinimides have no anti-wear properties. They must therefore be used together with wear- and corrosion-inhibitors. The most frequently used inhibitors are ZDDP, n-alkenylsuccinates, calcium bis-phenolates and metal dialkyldithiocarbamates. Mutli-functional succinimides are also available with anticorrosion and anti-wear properties; their molecular structure contains functional groups of phosphorus, sulphur, boron and similar atoms. Combinations of succinimides (and primary amines in general) with ZDDP are very sensitive to concentration in relation to their anti-wear properties. For a given ZDDP concentration, a critical amine concentration exists above which the anti-wear efficiency of the ZDDP is drastically reduced. On the other hand, it increases synergistically up to this point. ZDDP’s with long alkyl chains are more sensitive to this phenomenon; the aryl dithiophosphates are insensitive (381). Succinimidesby themselves have a corrosive effect on copper and its alloys because of the formation of chelate bonds between the copper and the amide and amine nitrogens in the succinimide molecule. This reaction can be inhibited by adding a small amount of acid, e.g., succinic acid or terephthalic acid, or by incorporating boron in the succinimide molecule.
The presence of boron or sulphur improves both the dispersant efficiency and anti-wear properties of succinimides. The boron content, expressed by the mass ratio to nitrogen, can vary between 0.1 and 5.5:l. Succinimides containing boron can be prepared by reaction with various boron compounds, e.g., boron trioxide or boric acid and its esters and halogenides (277, 421). The patent literature describes of other types of succinimide dispersants (270). Interesting examples include those prepared by the reaction of alkenylsuccinic anhydrides with condensation products of urea with polyalkylenepolyamine(278) of the type: H2N-(ANH),-CO-(NH-A),-NH, and the reaction products of piperidine derivatives (296) of the general formula - CHR
I
c -z II 0 where n is 2 to 200 and Z is a linkage of the type:
t”
,CH2CHz\
-0-N
‘CH,CH2 / N - M - o -
319
,
/ CHZCH2\
\ CH2CH2/ C H - D - C H
\ CHzCH2/ N - M - o -
CHZCHZ\
0-A-N
where A,M,D are bivalent saturated aliphatic hydrocarbon radicals with 2 - 10 carbon atoms, and R is a univalent alkenyl radical of 30 to 200 carbon atoms. Similar examples include the reaction products of alkenylsuccinic anhydrides with derivatives of 1(aminoethyl) piperazine, 1,4-bis-(aminopropyl)piperazineand dipropylenetriamine. The halogen-assisted reaction, mentioned earlier, of polyalkenes with maleic anhydride, can produce multiple substitution on the polyalkene chain. This presents the possibility of increasing the number of polar groups in the molecule and an increase in dispersant effect.
4.2.3.2 Miscellaneous Polyalkenepolyamine Derivatives Dispersant additives with pronounced antioxidant properties can be made by Mannich reactions from long-chain alkylphenols (most often from polyisobutenes or polypropylenes of molecular weight about 1,OOO), polyethylenepolyamines and formaldehyde (279):
CH2 - NH - (CH,CH,NH-),
- CH2CH2 - NH2
R OH
OH CH, - NH - (CH2- CH2- NH), - CH2 - CH2
R Their effectiveness depends on the average molecular weight of the akyls and on the mutual ratio of the string materials. The thermal stability of these Mannich base products is lower than those of the succinimides. Other dispersants of the general formula: OH CHZ- NH - (A -NH),-
X II A - NH - C -NH
- A - (A - NH),- NH,
R-& OH
X II CH, -NH - (A - NH),- A - NH - C
OH
- NH - A - (A - NH),- NH- CH,
can be prepared from Mannich bases by condensation with urea and thiourea (A is -CH2-CH2-and x is oxygen or sulphur).
320
These additives, like succinimides, can be improved by introduction of boron into the molecule, which also gives them antioxidant properties. Polyethylenepolyamines and carboxylic, sulphonic or organophosphoric acids can be used for preparation of amidic dispersants, e.g., of the following type: R-CO-NH-(CH2-CH2-NH-).&H2-CH2-NH2, where R is an alkyl up to C35. These products are thermally stable up to about 260 "C and, because of the high polarity of the mnolecule, have very good dispersive power. The ester of isostearic acid and tetraethylenepentamine is a well-proven dispersant for use in oils for watercooled two-stroke gasoline engines. Although these products are thermally stable, the upper limit is below that required for air-cooled two-stroke engine oils, such as for chain-saw and similar duties. In these cases, the even higher stability and detergent qualities of ash-containing additives are required. As mentioned earlier, additives which contain amine groups, especially mono-succinimides, can damage fluoro-elastomers, making them brittle. Fluorocarbons are used for sealings in automobile engines because of their very good thermal stability (up to 200 "C) and resistance to oxidation. One such product, Viton A (8Dupont), is a vinylidene fluoridehexafluoropropylene copolymer: - ( - CF - CF2),- ( - CH2 - CF2- )YI CF3
and analogous to ter- and quadripolymers of vinylfluoride, tetrafluorethylene,hexafluor-propylene and the oxygencontaining, hydrogen-free copolymer: - ( - CF - CF,), - [- CF2 - CF(OCF3) -IyI CF,
Some engine manufacturers have developed their own bench tests to evaluate the compatibility of fluoro-elastomer sealing materials with lubricating oils. The conditions of these monitoring tests and admissable borderline values are shown in Table 4.11. It has become evident that the results of test conducted on fresh oil may lead to wrong conclusions, since oils which have been in use in the engine for longer periods of time have exhibited substantially better results (398). The possible effects of additives on cross-linking must not be overlooked (405).
Table 4.11. Test Conditions for Compatibility of Lubricating Oils with Fluoro-elastomerSeal Materials - Permitted Limits Engine manufacturer
Daimler Benz & MAN
ope1 Volkswagen Peugeot Citroen Caterpillar*
Test conditions Seal performance parameters Temperature Duration ElongationTensile Volume Hardness strength (GM60256) ( "C) (h) ..................(% change) .................
150
I68 150 72-1000
150 150 177
96 168 96
f5 E3
-45 +10 -40
-40 4 0 -10
-
f25
-
-
-40 -30
-15
-5
f5
-
-
f5
E3
-
*Test of effect of lubricating oil on clutch plates faced with glass fibre and fluoro-elastomer.
32 1
It has been observed that base-oils of different Vl's have different effects on fluoro- elastomer packings. Base-oil VI changes of 70 to 96 can cause up to 40% differences in the elongation of fluoroelastomers. Compounds containing active sulphur have adverse effects on nitrile-rubber packings. On the other hand, nitrogen-free succinate esters, which in some respects are excellent dispersants, show good compatibility with fluoro-elastomer packings.
4.2.3.3 Esters of Alkenylsuccinic Acids Succinate esters, containing neither nitrogen nor basicity as TBN, are used commercially as ashless dispersants, and have the following general fomulse:
R - CH = CH - CH - CO - C2H4OH I CH2COOH R - CH = CH - CH - CO - C2H4OH I CH2 - CO - 0 - C2H4OH R - CH = CH- CH -CO - C2H40 - C2H4 - 0 -CO - CH -CH = CH - R I I CH2COOH HO - CO - CH2 To form these compounds, polyisobutenylsuccinic anhydride is condensed with a polyol to give the succinate ester. An excess of hydroxyl over acid is generally used, so that the succinate ester contains free, unreacted hydroxyl groups.
The alcohols can contain 1-40 carbon atoms and many different subtituents, such as chlorine, bromine, phenyl, alkoxy groups and ester groups (e.g., ethylene glycol monooleate). The most usual are polyvalent alcohols containing at least three hydroxyls, of which a portion is esterified with c8 - C,, carboxylic acids (e.g., sorbitol monooleate, sorbitol distearate, glycerol monooleate or monostearate and erythritol didocanoate) as well as unsaturated alcohols, aminoalcohols, phenols, mono- to tri-alkylphenols with alkyl groups (of relative molecular weight as high as 1,000). 2-chlorophenol, resorcinol, pyrocatechol, bis-alkylphenols with methylene, sulphide and polysulphide bridges and a- and pnaphthols. R is a polyalkene or alkene copolymer containing at least 50 carbon atoms. In order to have sufficient stability and for the ester to be soluble in oil, the R group must contain at least 80% of aliphatic a-mono-alkenes and the proportion of double bonds must not exceed 5% of all the -C-C- bonds. In addition to the polybutene currently employed, copolymers containg, e.g., 95% isobutene and 5% styrene, or 98% isobutene, I% piperylene and I % chloroprene, or 80% ethylene and 20% propylene may be used. The relative molecular weight may be 700 to 5,000. Polymers of relative molecular weight 10,000 to 1 million may be used if VI improver properties are required. The polyalkyl chain may also contain hetero-atoms or other groups, such as chlorine, bromine, sulphur, keto- or nitrogroups, but the amount should not exceed a limit of about 10%.at which the unsaturated nature of the alkene would be lost.
322
The most recent compounds of this type to find wide application are the esters of polyalkylene succinic acids and pentaerythritol:
R - CH = CH - CH - CO - 0 - CH,C(CH,OH), - CH2O I CH2 - CO - 0 - CH,C(CH,OH),
- CH2O
They have good thermal and oxidation stabilities, poorer abilities to disperse products of the incomplete combustion of engine fuel but a fair ability to disperse cold sludges. They are therefore suitable as ashless dispersants for gasoline and diesel engine oils, as well as for “mixed fleet” oils, in combination with bissuccinimide an a ratio of about 60:40. They can also be used for partially and fully synthetic oils. Other esters which may be used include those which result from the reaction of alkylsuccinic acid anhydrides with epoxides or mixtures of epoxide with water (e.g., with ethylene, propylene, butylene, styrene, cyclohexene, 1,2-0ctene oxides and butadiene monoxide). The effects in gasoline and diesel oils of the main types of ashless dispersants are compared in Table 4.12. Table 4.12. Comparative Data on the Performance of the Main Types of Ashless Dispersant in Gasoline and Diesel Oils Function monosuccinimide Gasoline engines varnish sludge cold sludge Diesel engines deposits above piston rings deposits below piston rings soot dispersion Gasoline/diesel CuPb bearings fluoro-elastomer compatibility 1 = best
2 = medium
1 1 2 2 3 1
3 3
Dispersant Type bispentaerythritol/ succinimide alkenylsuccinic acid ester
2 2 3
3 3 1
1
2 2
3 1 3
2
1
2
1
3 =worst
4.2.3.4 Nitrogen-containing Copolymers In most of these products, one comonomer molecule provides oil-solubility, whilst the other contains the functional, basic nitrogen group. These products combine VI improver and dispersant properties and are further discussed in a later section (4.5). They may be represented by a general formula (133):
323
- CH- CH2- CH- CH2- CH - CH2- CH - CH2 - CH - CH2 I L I
I
L I
R
I L I B
R
I
I
L I
L I
R
R
where R is an oleophilic group, e.g.,- CnH2n+,,
B is a basic nitrogen group, e.g., -(CH2),-NR2 with n > 2
L is a group providing a bond between R or B and the polym-r ch in: -0-
0 II
0 II
c - , - c- NH -, - 0 -
The most commonly encountered commercial products are copolymers of: dodecylmethacrylate (90%)and n-diethylaminoethylmethacrylate(1 0%)(134): CH3 I
CH-CI
c=o I
OC 12H25
CH3 I CH2-CI
c=o
I CH2 I
CH2 I
N /\
n
dodecylmethacrylate (90%)and vinylpyridine or N-vinylpyrrolidone(10%):
-.CH
324
cHf
dodecylfumarate (95%) and N-dimethylaminoethylmethacry late (5%):
c=o
I
I
CH - CH
c=o I -
12’25
Jm
A separate category of polymeric products which have powerful dispersant and VI improver properties is now widely used, comprising “functionalised” olefin
copolymers (OCP) and ethylene-propylene-dienecopolymer mixtures (EPDM). Materials derived from polyolefins represent, by a large margin, the cheapest VI improvers. Since these substances contain at least one double bond per molecule, dispersant VI improvers can be produced from such polymers by thermal reaction with maleic anhydride followed by reaction with polyalkylene polyamines. The reaction with maleic anhydride can also be carried out under high-shear conditions to generate reactive molecular fragments. Several commercial dispersant VI improvers, prepared by this and other routes, are now avaliable. In terms of formulating technology, the dispersant polymers enable solutions to be found to particular problems associated with blending multi-grade oils to meet high performance specifications, particularly for gasoline engine oils. Monomeric dispersants are viscous and have a pronounced thickening effect on lubricating oils. At the high dosages required to meet modem engine-cleanliness requirements, monomeric dispersants thicken the oil so much that, when combined with the dose of polymeric W improver needed to achieve the required high temperature (100 “C) viscosity of the lubricant grade, low temperature viscosity limits may be exceeded. Polymeric dispersants, having both VI improver and dispersant properties, can be used to achieve the required viscosity limits by effecting a reduction in the dosage of monomeric ashless dispersant needed to meet the engine performance stipulations.
4.2.3.5 Miscellaneous Dispersants
Included under this category are, for example, polyethyleneglycol dibenzoate, polyvinyl polystearate and dialkylnonyl borate. Over-based sulphonates and medium-TBN Ba phosphonates can, at high concentrations, reduce the formation of sludges produced during “cold” operation of engines (Z29). Their use in this way is, however, expensive and raises problems associated with high ash content.
Unlike ash-containing metallic detergents, the development of ashless dispersants is still relatively active (330). In addition to esters of polyalkenylsuccinic acid with polyamines or polyvalent alcohols, Mannich compounds and carboxylated amides, new types of compounds have appeared, including biscarbamides (331), polyaminocycloalkanes (332),substituted pyrimidine and triazine compounds (333), 325
polyesters containing carboxypyrolidone (334, reaction products of chlorinated paraffins (336, esters of alkylsalicylic acids (e.g., with pentaerythritol) (337) and polyalkylene amides of alkylsalicylic acids (338).
4.2.4 Combining Antioxidants with Detergent-Dispersants Modem heavy-duty (HD) engine oils are generally formulated with mixed detergentdispersant additives together with antioxidants such as ZDDP and alkylphenols or aromatic amines, and frequently with added rust inhibitors, demulsifiers and antifoams. The DD components are mostly combinations of two or three types of detergents, for example, high- and low-base calcium (exceptionally magnesium) sulphonates plus overbased or medium-based calcium phenolate or calcium (less frequently magnesium) salicylate and, almost always, ashless dispersant, usually succinimide. Different packaged additives are produced for different specifications, comprising oils mainly for lubricating gasoline or diesel engines, particularly for high piston temperature operation, and also for water- or air-cooled two-stroke gasoline engines (i.e., operating over different temperature ranges). The metal used in detergents is usually calcium, less frequently magnesium, and, very rarely, barium. The value of the alkaline reserve (TBN) in particular detergent components is chosen in relation to the desired effect and also in respect of the required ash content and finished oil TBN. All components contribute alkalinity proportional to their TBN and concentration in the package. The different ash content and TBN of engine oils differ is illustrated in Table 4.13. These data show the correlation between severity of duty, alkalinity and ash content; alkalinity and ash increase with increasing severity. Overbased oils, required for lubricatinglarge stationarydiesel engines fed with heavy sulphur fuels (such as crosshead marine engines), can contain magnesium, in order to achieve lower ash contents at very high TBN, as compared with calcium.
Table 4.13. Ash Limits and TBN in Engine Oils Oil specification
Ash content (% weight max.)
All season oils for gasoline engines MIL-L-46152E Diesel Oils: MIL-L-21WE Medium-basicity oils High-basicity oils Low-speed engine oils CCMC D-5
TBN (mgKOWg)
1.o
9-10
1.5
10-12 up to 30 up to 70 about 40 about 15
3.5 8.5 4.7 up to 2.0
4.2.5 Packaged Additives The need to combine different kinds of additives in engine oils, as mentioned above, led to the development of so-called packaged additives. The components in these packages of additives differ in type and dosage, according to the nature and quality 326
of the base oil, the desired quality and performance of the finished engine oils and the operating conditions of particular engines. Polymer viscosity modifiers and pourpoint depressants may, exceptionally, be incorporated into the packages. The use of package additives simplifies and improves handling, storage and quality control of additives, makes dosage more precise and reduces losses. An interesting approach to package additives is the use of “universal”, “cascade” or “booster” packages. This is a means of rationalising production of a series of oils for different performance levels by the use of combinations of additives designed to enhance particular performance characteristics of oils.
The following section discusses the dosages and characteristics of the mostfrequently used antioxidants and detergent-dispersants in package additives.
Zinc Dialkyldithiophosphates (ZDDP) Zinc dialkyldithiophosphates are a regular component of package additives intended for oils for the lubrication of gasoline and naturally-aspirated and lightlysupercharged diesel engines. The ZDDP content of the package usually varies between 10 and 15%, so that the zinc or phosphorus contents in oil are 0.04 to 0.15% weight according to the antioxidant and anti-wear effects required for the oil. The ZDDP content also depends on the presence of alkylphenolate or alkylsalicylate in the package, because both have antioxidant properties. Because of their relatively high decomposition temperatures, ZDDP’s of longchain alkyls or, better still, alkyl-aryl or diaryldithiophosphates are preferred in package additives intended for use in heavily-loaded diesel engines. However, ZDDP with good anti-wear properties (dialkyldithiophosphates of C,-C, alkyls) must also be present in amounts equivalent to 0.3 - 0.5 weight % in the finished oil. In oils containing dispersants with NH- groups, the ZDDP dosage must be very carefully adjusted. The zinc content must not exceed the equivalent of amino- or amido- groups, since this would cause excessive formation of varnishes on the pistons if the first piston ring temperature exceeds 220 OC (297). On the other hand, free -NH2 and =NH groups, in the absence of or with low concentrations of ZDDP, form chelate bonds with copper and thus cause corrosion of copperflead bearing materials. The ratio of ZDDP to succinimide or, in general, to any dispersant which contains amino- and/or amido- groups must be so adjusted so that all the -NH, and =NH groups can form complexes with ZDDP’s; theoretically, one ZDDP molecule should correspond to one -NH2 or =NH group. Phosphorus-based ZDDP is the principal source of phosphorus in an engine oil. This is very important in connection with catalysts used for the reduction of harmful exhaust emissions. One of the main sources of catalyst poison are oils with a high ZDDP content. Over the years, a number of workers have demonstrated the adverse effect on catalyst metals of lubricant-derived phosphorus, using combustor rigs and engine dynamometers. One poisoning mechanism is the formation of an impervious glaze of zinc pyrophosphate on the catalyst surface.
3 27
Catalyst deterioration is highest on exposure to exhaust gases containing unburnt oils which contain ZDDP's while operating at catalyst temperature below about 480 "C. When both phosphorus and lead are present, plugging of catalyst pores by lead phosphate has also been mentioned as a means of further action by phosphorus. Much of the earlier work on phosphorus contamination of catalysts was done using base stocks and ZDDP alone. More recent studies on fully-formulated lubricants have indicated that the behaviour of ZDDP is influenced by the metallic additives used as detergents and that the adverse effect of the ZDDP on catalyst life is much reduced in practice. However, metallic additives themselves have a deleterious effect on catalysts as a result of the front face becoming blocked by ash. Lubricant-derived phosphorus has also been found to have adverse effects on the oxygen sensor in the system. The poisoning mechanism here appears to cause an increase in sensor response time, so that the engine is more likely to operate outside its aidfuel control window. High ash content oils or high oil consumption can also cause sensor-blocking.
Catalyst poisoning can be reduced in four main ways: - engine oil consumption should be minimised. In particular, oil access to the exhaust system should be reduced by improving the effectiveness of exhaust valve stems and guides, - to avoid zinc phosphate formation, the catalyst should operate at higher temperatures, above about 540 OC, in a warmed-up engine, perhaps by moving the catalytic converter nearer to the exhaust gas manifold, - use of catalysts which are more resistant to phosphorus, - cutting down the ZDDP in the lubricant so as to minimise phosphorus content; supplementary anti-weadantioxidant additives may be necessary. Workers in this field have found that adequate oxidation and wear control, as measured in the Sequence IIlE test can be achieved by the use of additional antioxidant supplements to compensate for the reduced phosphorus level. Caterpillar 1H-2 and 1G-2 diesel tests as well as the CRC L-38 bearing corrosion test do not appear to be affected by lower phosphorus levels. In Japan, SG/CD quality engine oils have been available for some years with phosphorus levels as low as 0.08% weight. No in-service problems have been reported with these oils. Some engine manufacturers (e.g., Volkswagen) require a minimum phosphorus content in engine oils of 0.08% weight in the interests of engine durability.
Low-temperature Antioxidants The antioxidant effect of a package can be enhanced by the presence of a lowtemperature antioxidant of the radical-scavenger type together with the ZDDP. Package additives in engine oils contain antioxidants of the hindered phenol type or aromatic amines. Their concentration in the package should correspond to about 0.30.5% weight in the finished oil.
Sulphonates Calcium sulphonates of 5 to 300 TBN (mg KOWg) are mostly used. Magnesium sulphonates of TBN up to 400 have proved useful mainly in gasoline engine oils, where they are efficient rust-inhibitors and agents for the reduction of wear in valve328
train components. However, they aggravate bore-polishing when used in diesel oils. There is virtually no difference between the detergent effects of calcium and magnesium sulphonates. Mixed fleet oils can therefore contain both calcium and magnesium salts. Low-base sulphonate of TBN up to about 25 can be incorporated particularly when anti-rust characteristics need to be enhanced; they have also proved useful in oils for high-performance diesel engines, especially in combination with high-TBN phenolates. Overbased sulphonates are more often used in gasoline engine oils, but their concentration in oil should not exceed 1.5% weight, because they can promote more severe deposit formation in the piston grooves. Phenolates (Phenol Sulphides) Calcium salts (70 to 400 TBN) are much more often used than magnesium salts. The latter are mostly found in oils for marine engines. Barium salts of 70 to 120 TBN formerly found limited application in two-stroke gasoline engines. The concentration of phenolates in package additives is about 20 to 60 % weight. The higher concentration is recommended in oils for highly thermally-loaded engines, where they sometimes perform the function of antioxidants. Higher dosage of package additives with a high proportion of phenolates can to some extent improve the performance of base oils of poor oxidation stability. Salicylates Much the same comments apply as for the phenolates. They are competitive with the more widely-used phenolates largely because of their strongly positive effects on engine cleanliness in the 1 G-2 diesel test and on friction reduction. Again, combinations of salicylates and phenolates together with sulphonates and succinimides can be used to produce very effective DD packaged additives for oils to meet all performance specifications. Phosphonates (Thiophosphonates) These must now be regarded as obsolete in packaged DD additives. They were regarded as especially suitable for gasoline engine oils, in which they contributed significantly to piston cleanliness. Packages typically contained up to 40% by weight of phosphonates, usually with basic sulphonates and succinimides. Ashless Dispersants Succinimides predominate in this category, with bis-succinimides in diesel oils and mono-succinimides in gasoline engine oils. Except for dispersant VI improvers, other ashless dispersant types are found much less often. Package additives usually contain 30-60% of succinimide concentrate, depending on the duty required (gasoline engine oils, especially “all-season oils”, containing the higher proportion). 329
Gasoline engine oils to meet API SF performance standards usually contain 4-5% by weight succinimide in the finished oil, whilst SG performance demands 7-8% succinimide or equivalent. Typical total dosages of detergent-dispersant additives in engine oils for different performance specifications are illustrated in Table 4.14. Multigrade oils usually contain 20-30% more DD additive than mono-grades in order to meet the same performance standards. Table 4.14. Concentration of Detergent-DispersantAdditives in Engine Oils for Different Specifications API classification SA SB SC SD SE SF,SG CA CB
cc CD, CD+, CE,CF-2
Other classifications
DD-additive content (% weight in finished oil)
Ford M,C lOlA Ford M,C IOIB, GM 6041M Ford M,C IOIC, GM 6136M, MIL-L-46152A MIL-L-46152B MIL-L-2 l04A, DEF-2 1OlC (without ashless dispersant) Supplement 1, DEF-2IOID MIL-L-2104B Caterpillar Series 3, MIL-L-45199B. MIL-L- 2104C, D &E
0.5-1.0 2.5-4.0 4.0-6.0 6.0-9.5 up to 12.0 1.5-2.2 2.2-4.5 4.5-8.0 UP to 20.0
The total amount of DD additive needed to meet the required performance level depends on the synergistic effects, if any, of the components selected, the quality of the base oil and its response to additives and the type and concentration of other additives present, among other, less important factors.
As mentioned earlier, the composition of DD additives used for two-stroke gasoline engine oils with conventional lubrication systems differs from those of DD additives in oils for four-stroke engines, and depends on the type of engine and its thermal load. Oils dosed with ash-containing dispersant-detergents (mainly highly thermally-stable low- and high-base calcium sulphonates and (formerly) mediumto high-base barium phenolates (barium ash being more readily carried out of the engine) or (currently) calcium phenolates (calcium producing less ash and, consequently, causing less spark plug fouling) are more suitable for lubricating low cubic capacity, air-cooled engines (e.g., motor-cycle, chain-saw and mower engines) which operate at a first ring temperature around 260 "C. The alkaline-reserve of these ash-containing detergents is present as metal hydroxide in preference to metal carbonate. For water-cooled engines with a lower temperature on the first piston ring, such as sports boats, ashless dispersants are preferred, which can be succinimides, high molecular weight carboxylate condensation products with polyethylenepolyamines. These oils are also suitable for lubricating two-stroke 330
automobile gasoline engines. The DD additive content of two-stoke engine oils is usually in the range 3 to 8% by weight. Package additives normally also contain antifoams (e.g., polysiloxanes) of a suitable composition and molecular weight to suppress foaming in the finished oil.
Diluent Oils Package additives are supplied and used as solutions in oil to facilitate handling. The diluent oil content of packages usually varies between 10 and 50%, depending on the composition of the package and the additive components. Preferably, the diluent oil should be moderately highly-refined and should not affect perceptibly the composition and physical properties of the base oil in which the package is mixed. Since the additive components are frequently viscous and may be at the limit of their solubility, the diluent oil must have good solvent characteristics, low viscosity, reasonably high VI and low volatility. A 100 Solvent Neutral is a common choice for this purpose, preferably of narrow boiling range and high naphthenics content. Detergent packages which contain both ZDDP and overbased detergents are very sensitive to water. Even a low water content (0.4% weight), at temperatures above about 60 O C , can cause hydrolysis of the ZDDP and reaction of the hydrolysis products with the alkaline reserve of the detergents, giving calciumcontaining compounds of limited oil-solubility. These compounds contribute to cloud-formation in the oil and can lead to deposit-formation which is difficult to remove by filtration: it causes deterioration in the antioxidant and anti-wear properties of the additive. The diluent oil specification therefore normally includes restrictions on water content; care must be exercised in handling and storing the diluent oil to avoid contamination with water. Depending on the crude oil source of the diluent oil, it may have an appreciable sulphur content (0.05 - 1.0% by weight). This sulphur may be present in a form which has significant antioxidant effect and this may be synergistic with antioxidants deliberately added in the package. Package additives made from different diluent oils may therefore exhibit different performance properties in finished oil.
4.2.6 Effects of Antioxidant and Detergent-Dispersant Additives on Oil Quality Parameters The presence of oxidation inhibitors and DD additive packages in finished oil not only affects its thermooxidation stability and detergent-dispersant behaviour, but also other properties. Colour: package additives darken oils; the effect of metallic detergents, particularly phenolates and salicylates, can be very marked. Kscosiry: package additives increase the viscosity of the finished oil. This varies with the nature of the package, as the viscosities of individual additive concentrate components also vary considerably, as do their concentrations. The viscosities of zinc dialkyldithiophosphates are about 10 mm*.s-l at 100 "C, whereas the viscosities of zinc diaryldithiophosphates can be twice these values. The viscosity of ashcontaining detergents depends on the nature of the organic moiety, on the type and concentration of the metal and on the TBN. Whereas the viscosity of calcium petroleum sulphonate of TBN up to 10 is usually about
33 1
20 mm2.s-' at 100 O C , it increases to about three times this value at TBN 20-25. Synthetic calcium sulphonates have relatively low viscosities - 300 TBN calcium sulphonates 30-50 mm*.s-' at 100 "C. The commonly-used succinimide ashless dispersants can have very high viscosities (150 - 800 mm2.s-* at 100 "C)- see also note earlier (page xxxx) on dispersantlVI improver formulation effects. Viscosities of commercial package additives are normally between 40 and 150 mm2.s" at 100 "C. The contribution by the additive package to the viscosity of the finished oil can be a significant constraint on formulation of the additive package, especially in low-viscosity multi-grade oils.
Pour-point: the pour-point of the compounded finished oil is principally affected by the pour-point of the diluent oil, rather than any effects of the additive materials themselves, unless the package contains polymeric pour-point depressant or viscosity modifier. In some packages, a significant amount of polymer may be present not only as pour-point depressant (usually at a concentration equivalent to about 0.5% by weight of the finished oil, but also in the form of dispersantlV1 improver (see page 325). Blending practice varies; in many instances, the VI improver is blended separately from the rest of the package.
Flush-point: the flash-point of the finished oil is also mainly affected (in relation to the contribution of the additive package) by that of the diluent oil; currently, it usually varies around 200 "C and hence is close to that of the base oil. Both flashand pour-points are important factors involved in the choice of diluent oil for additive manufacture (it should be noted that the additive concentrates themselves contain up to about 50% diluent oil in order to be handlable during manufacture. The diluent oil content of the finished oil is, therefore, the sum of that in the components and any further quantities used to adjust the viscosity and composition of the package additive). Ash-content: the principal source is obviously ash-containing additives. This depends on TBN and the atomic weight of the metals used, which vary in the order Zn<Mg
4.3 SYNERGISM BETWEEN ANTIOXIDANT AND DETERGENT-DISPERSANT ADDITIVES Every type of additive used in lubricating oils is represented by a series of chemical compounds of similar effects. If these compounds are present in the oil alone, mainly as functional additives, then their individual effects can be measured unequivocally, 332
and will be found to depend on the composition of the additive species, its concentration and purity and the nature of the oil, which determines the susceptibility of the oil to additive treatment. However, when packaged additives are employed, synergistic and antagonistic additive effects may be involved (135). In the case of synergism, the effects of two or more substances present simultaneously exceeds the expected sum of the effects of the individuals applied separately, or exceeds the effect of the most active substances applied at the same concentration. In the case of antagonism, the opposite effects occur; the individuals appear to “interfere” with each other’s actions. A practical consequence of this can be that a substance which is difficult or expensive to obtain can be replaced by a cheaper synergistic combination which has a similar or better effect. Synergism cannot be exactly identified and proved unless it can be expressed quantitatively. The physical or physico-chemical effects may not be sufficient. Only some types of additives, especially antioxidants, show a clear chemical effect; others, such as detergent-dispersants,show physical effects - their quantitative evaluation is difticult to prove and is inaccurate. Therefore, only a few additive types can be termed “true” synergists (or antagonists), whereas other are quasi-synergistic, presenting effects similar to synergism.
The terms “homosynergism” and “heterosynergism” are used to distinguish different types of action (136).In homosynergism each component acts according to the same mechanism; for example, in the case of radical scavenger antioxidants, one phenol may deliver a missing hydrogen to another, or in the case of peroxide decomposers, more effective decomposers may arise by reaction with the substrate. In heterosynergism, each of the substances present may act by a different mechanism, for example, one accelerates termination of radical chains by acting as radical acceptor, whilst the other retards propagation by acting as peroxide decomposer. The result, in each case, is reduction of the production and accumulation of radicals.
4.3.1 Antioxidant Synergism The efficiency of 2,6-di-tert-butyl-4-methylphenolcan be enhanced by a homosynergistic effect by admixture with simpler phenols which are unsubstituted in the ortho- position. Maximum synergistic effect of the phenol mixture (such that the phenols being complementary in the ortho- position) is achieved if one phenol has at least one ortho- tert-alkyl group while this position is vacant in the other position. A synergistic effect also exists between alkylphenols and dialkyl phosphites (269) or alkylphenols and aromatic amines, and between aromatic secondary amines (e.g., phenyl-1-naphthylamine) and aromatic diamines (e.g., diaminonaphthalenes), basic carboxylates (and fluorinated analogues), enolates and partially saponified tetraminoacetic acid esters, polyhydrobenzophenones and diarylthioureas (388). The interaction between hindered and un-hindered phenols in the presence of ROO. has been examined by e.s.r. spectroscopy (390).In a mixture of both types of phenol, the concentration of radicals
333
arising from unhindered phenols increases. Evidently, the unsaturated phenol delivers hydrogen to phenoxy radical from the hindered phenol, so that its other reaction is suppressed and the initial, efficient antioxidantis regenerated. Earlier experience of synegism in hinderdunhindered phenol mixtures (389) is c o n f i e d by this finding.
The efficiency of phenolic antioxidants can also be enhanced by the heterosynergism of ZDDP. For example, a stronger antioxidant effect (identified by laboratory oxidation tests) in highly-refined oils can be achieved with a combined dose of 2,6-ditert-butyl-4-methylphenoland ZDDP than with the alkylphenol alone. The efficacy of ZDDP can also be increased by the heterosynergistic effect of some alkylphenols, such as di-tert-butyl-bis-phenol (137). The heterosynergism between ZDDP and mine-type antioxidants is much weaker. This combination has, however, proved useful in improving the performance of oils of low aromatic content made by hydrocracking (103). Mixtures of ZDDP with alkylphenols substituted in the ortho- position, particularly bis-phenols in socalled "ashless antioxidants", can not only reduce the ash content of engine oils, but also provide an economic solution to the problem of oil-thickening caused by thermooxidation stress, for instance in gasoline engines in high performance passenger cars operating over prolonged periods of time (106). This type of performance can be simulated in laboratory engines (see Table 4.15). Table 4.15. Engine Oil Viscosity Increase in the Petter W1-ET 325 Engine Test as a Result of Different Antioxidant Treatments (140) S A E 20W-50 oil, to former Ford M,C lOlB specification Viscosity increase at 37.8 "C
(a)
Antioxidant comprising:
2.55 % (weight) Zn dinonylphenyl-dithiophosphate 1.O % Zn dinonylphenyldithiophosphate+ 1.O % alkylphenol 1.O% alkylphenol Inhibitor-free oil (test terminated after 41 hours because of stuck piston ring) Crankcase sump oil temperature Coolant temperature Test duration 25g oil replenished after Total oil charge
925 208 407 2700
163 "C 177 "C 48 hours 16 & 36 hours 90Og)
The combination of ZDDP and N,N'-disalicylidenethylene diamine with conventional DD additives is also synergistic. Quite low doses (about 0.1%) of this typical metal deactivator can distinctly improve the result in engine tests on treated oils (139). Engine oils often exhibit interesting synergism of ZDDP and detergentdispersants; the results are frequently surprising and difficult to explain (Z38). 334
Forbes and Wood (142) reported the favourable effect of ZDDP on enhancing the dispersing and solubilising power of succinimides; this effect has also been observed by Russian and other researchers (84, 264, 265); it is, however, accompanied by a slight diminution in the ability of succinimide to stabilise the dispersion of solid matter in oil. The rules which govern the synergism of succinimides and antioxidants have not been satisfactorily explained. The influence of MDDP on the micellar structure of the colloidal solution of succinimides in oil, or the interaction between succinimides may be involved. The reason for the reduction in the stabilising power of succinimides in the presence of MDDP may be the growth of the charge on its micelles (electrical conductivity grows if succinimides are mixed with MDDP), which could result in the decrease in stabilising power (266). The decrease in adsorptive capacity may be caused otherwise.
Synergistic effects between added antioxidants and “natural” antioxidants (usually sulphur compounds) in the base oil may also be encountered, depending on the crude source of the lubricant. Synergy may amplify the effect of base oil sulphur content variation, requiring the adjustment of addtive treatment to allow for variations caused by the use of very low-sulphur crudes, e.g., some North Sea crudes.
4.3.2 Detergent-Dispersant Synergism The effects of antioxidants can be identified by laboratory methods (e.g., length of induction period, growth of acidity in the oil, fall in oxygen pressure) but it is considerably more difficult to establish detergent-dispersant effects and the results are always less objective. Laboratory methods available typically measure the ability of the DD additive to disperse a solid, such as soot or TiO,, or the tendency to form deposits on a hot-plate. It is doubtful if such methods have a sufficiently valid chemical basis on which to base conclusions regarding synergism. It is therefore necessary and reasonable to rely on the evaluation of the detergent-dispersant ability of additives from engine test results. In the case of engine oils, their effects can be quantified by numerical rating of piston deposits. For many years, detergents were used individually. Later, with the development of more heavily thermally-loaded engines, package DD additives or detergents with metal combinations, proved more useful. Both types of additives act to produce an increase in efficiency, and in both cases phenomena similar to synergy appear. It is, in fact, necessary to take advantage of this effect in formulating oils for heavily thermally-loaded engines (138,143). The advantages of the package additive approach to DD formulation can be demonstrated in many ways. A single basic calcium sulphonate mixed with ZDDP cannot satisfy requirements more severe than MIL-L-2 104A/Supplement 1 specification, whereas with a number of detergent types combined into a package with ZDDP and other additives, almost any desired performance level may be achieved by gradually increasing the dosage. Other examples (see Table 4.16) can be used to demonstrate how the total additive content of the oil to reach a given performance level may be reduced if different detergents and dispersants are
335
suitably combined. Metal alkylphenolates can produce a remarkable synergistic effect in combination with both sulphonates and succinimides. For example, all the present specification limits can be satisfied with overbased phenolate or salicylate plus overbased calcium sulphonate, ashless dispersant, ZDDP and other additives in minor amounts by gradually increasing the dosage and optimising the base oil composition. It is also worth mentioning the distinctly favourable effect of overbased metallic detergents on the low- and medium- temperature dispersancy properties of succinimide-containing oils, whether or not they also contain ZDDP. This effect grows with increasing TBN of the detergents. By contrast, neutral and low-base sulphonates have negative effects (144). To meet modem gasoline engine oil specifications in wide-range multi-grade oils in which ash and zinc contents is controlled to low limits, a disproportionate amount of dispersant may be required. The polymeric viscosity modifier itself may introduce problems similar to base oil effects, particularly the ethylene-propylene copolymers (OCP) which provide cost-effective control of viscosity-temperature characteristics. This has led to attempts to introduce limited VI improver characteristics into the dispersant molecule (as well as dispersancy into the Vl improver) to reduce the amount of hydrocarbon VI improver required.
Efforts continue to develop tests which provide information on the physical and physico-chemical basis of DD additive action in oil solution; laboratory test so far available are unreliable and engine tests are expensive. The objective is to provide, above all, simple, inexpensive, but reliable methods. Although basic DD additives have neutralising capabilities, their effect is mostly of a physical or physico-chemical nature. DD additives are surface-active substances which carry positive or negative charges and which have electron donor and electron-acceptor properties. These affect the interaction between additive and oil, and among additive molecules, among micelles formed from additives and solvents, among micelles themselves and between micelles and the surfaces of the structural material or insoluble contaminants (82). Interesting studies of DD additive solutions in oils have been carried out using methods which have proved useful elsewhere in studies of similar substances in a water environment. These studies involve electrical phenomena, such as determination of electrokinetic potential (potential difference between interface of solid phase with adsorbed ions and the interior of the solution); the type, size and density of particle charges; structure and composition of micelles and the thickness of diffusion double layers; mobility of micelles or solvated ions and variation in electrical
Table 4.16. Composition of Detergent-DispersantPackage Additives to Satisfy Engine Oil Specifications at 0.05% Zn Component ratios (% weight)
Alternative 1
2
ZDDP Ba thiophosphonate Ca phenolate (100 TBN) Ca sulphonate (IOOTBN) Succinimide
13 87 -
8 weight of additive package required to meet DEF 2101D
5.4
4.8
3.7
3.7
336
3
4
14
18
60 26
37
-
18 40 12
30
30
15
conductivity of oil solutions with time, voltage, concentration and combination of additive species (145157,249).The results are then compared with those from other tests: detergent capability (e.g., by COST 10734-64, where the oil solution is oxidised at 250 "C in the presence of a standard substance and the rate of oil filtration and degree of filter-fouling are evaluated), polarisation and polarisability of the additives, solubilisation of substances insoluble in oil (e.g., pigments, soot) and the results of engine tests. Although the results gained in water or polar environments are not comparable with those in non-polar oils of varied composition and dissociation of additives into ions several orders of magnitude lower, some remarkable results have been obtained (146,148-150,249). It has been established that potential changes with temperature, the temperature gradient up to 100 O C can be both positive (e.g., that of petroleum sulphonates) and negative (e.g., that of alkylphenolates and phosphonates). Differences in potential and the magnitude of micellar charge increase with temperature. The temperature thus significantly affects the surface properties of the additive (145). This effect is positive at the beginning and negative at the end, when desorption processes prevail on the surface, The conductivity of oil solutions is low but measurable; measurements are reproducible - conductance grows with increasing additive concentration. The relationship between conductance and time indicates that some particles possess surface charges and can discharge themselves rapidly on electrodes (e.g., petroleum sulphonates and succinimide micelles), whereas other particles possess internal charges (e.g., solvated ions) and discharge themselves slowly, generating non-homogeneous electric fields (e.g., alkylphenolates, phosphonates (145,249)).This demonstrates that the charged particles are attributable to the additives, which generate an electrostatic barrier layer on the surfaces of the metal or the solid particles (e.g., soot). When substances possess their own detergent effect they are characterised by small micelles with large charges; when the additives have a solubilising effect, the micelles are large with small charges (150,152,249). The conductivity of oil solutions of additives usually decreases with increasing applied voltage. However, with some additives, the decrease is small (e.g., petroleum sulphonates) or zero (succinimides), and with some discontinuous (alkylphenolates) or eventually stabilising at a boundary value (phopshonates) (249).These changes may be linked to the rate of dissociation of the additive to ions in the oil environment. The dissociation is most rapid in succinimides and petroleum sulphonates, slower in phenolates and phosphonates. The former type have a predominantly solubilising effect, whilst the latter have a barrier effect. It is possible to provide some logical basis for the use of additives in combination in terms of these different mechanisms of solubilisation and barrier-like effects, which may be synergistic or antagonistic. The phenomena have been followed by measuring the conductance of mixed additive solutions. In some cases, the conductance is additive, in others there may be evidence of slight antagonism, e.g., in the case of combination of phenolates and phosphonates. The combination of sulphonates with phenolates or phosphonates can be shown to be distinctly synergistic (145,146,249).The presence of ZDDP, in terms of conductance, is generally synergistic. The degree of interaction is, moreover, influenced by the ratio of concentration of the additives. It is, however, remarkable that dithiophosphates, phenolates and phosphonates actually reduce the otherwise excellent solubilising power of petroleum sulphonates and succinimides (249). The relationships between electrokinetic properties and the results of engine tests have also been studied. Results to date have not provided any clear conclusions. Some authors have found a correlation (146,155).some a certain amount and some none at all (154).The reasons for these discrepancies appear to be that the test conditions vary, that lubricant and colloidal systems change during the test and that the research has not been sufficiently intensive or systematic. Nevertheless, the study of electrokinetic processes has contributed to our knowledge of these systems and has the potential to enable the best use to be made of the surface properties of DD additives and, in particular, their interactions.
331
4.4 RUST, CORROSION AND FATIGUE INHIBITORS
4.4.1 Rust Inhibitors Additive-free mineral and synthetic oils and greases do not usually provide sufficient protection of the surfaces of ferrous metals from the effects of water or moisture, atmospheric oxygen and acidic contaminants which may - particularly if the machine is at rest, cause corrosion, i.e., rust-formation. Rust-inhibitors improve the protective capacity of lubricants in this specific respect and are incorporated into special rustpreventive oils. The main task of these oils is to prevent rusting of machine components and materials. However, these inhibitors are also incorporated into many types of lubricating oils, such as turbine, hydraulic, gear and instrument oils, as well as into most high-quality lubricating greases, especially those with lithium and synthetic bases, and into the so-called dual-purpose or double function oils. These dual-purpose oils, for example for automotive engines and gears (in which case they are frequently known as “factory-fill” oils) act as preventatives during stationary storage of the equipment and also as conventional lubricants when the automobile is set in motion. By this means, costly stripping of the preventive oil is avoided. Sheet metal rolling-oils also have a double function, acting as lubricants during the rolling process and as preventatives during transport and storage of the sheets. These oils need not be stripped off the sheets before further processing.
Suitable compounds for use as rust inhibitors include organic compounds, readily soluble in mineral and synthetic oils, which are amphipatic, resistant to hydrolysis and have good adhesion to metal surfaces. The mechanism of their protective action consists in adsorption of the polar group on to the steel surface, so that the hydrocarbon portion of the molecule forms a hydrophobic, water-repellent film. This protective film can be strong enough to withstand the effects of organic and inorganic acids and protect the surface from galvanic corrosion. Rust inhibitors currently used include low molecular weight alkylsuccinimides (258)and alkenylsuccinic acid esters (162)and alkylenesuccinic anhydrides (mainly for turbine and hydraulic oils). A typical product is made by the reaction of alkenylsuccinic acid with propylene oxide (400): R - CH = CH - CH - COOH I CH2COO - CH - CH2 - OH I
(R = CI,H21)
CH,
- derivatives of alkylthioacetic acids (259): R-S-CH,COOH
-
(where R is a long alkyl)
N-acyl derivatives of sarcosine: R-CO-N(CH,)-CH,-COOH
and of similar fatty mines;
- substituted imidazohnes (260) (chiefly for preventive oils): 338
/N R-C (X can be (CH, residue);
- CH, - NY),Y,
CH I 'NX - CH2 -
n = 0 to 7, Y is a mono- or di-carboxylic acid
-
amine or alkanolamine salts of dialkylphosphates (161),which also have antiwear properties;
-
non-ionic esters of fatty acids and polyhydroxy alcohols (e.g., penta-erythritol, sorbitol), used chiefly in preventive oils;
- 4-alkyl-phenoxycarboxylicacids, e.g., 4-(nony1phenoxy)-acetic acid: R
0 (CH,), - COOH
- amides or sulphamides of the type: R-CO-NH-(CH,),-COOH Ar-S02-NH-(CH,),-C00H
(Na) (R is a longer alkyl) (Na)
(Ar is an aryl group, n = 1-5)
- petroleum and synthetic alkaryl sulphonates soluble in oil, including, for example, sodium and calcium salts (both neutral and basic), lead, and zinc sulphonates and ethylenediamine sulphonates. The addition of 0.2% by weight of sodium petroleum sulphonate reduces the surface tension of water from 76 to 42 mN.m-', and 0.1% weight reduces the interfacial tension between mineral oil and water from 51 to 2.1 mN.m-'. These petroleum sulphonates are prepared from petroleum sulphonic acids of relative molecular weight 400 to 550. The oil-soluble sodium petroleum sulphonates are also used as emulsifiers, water-repelling agents, wetting agents and stabilisers for the structure of lubricating greases, and contain around 60% by weight of water, up to 0.5% of inorganic salts, 14-164 ash (as Na2S04) and 16-19%of SO,. The synthetic sulphonates can be prepared from dinonylnaphthalene sulphonic acid, e.g., its ethylenediamine salt: r
1
or, more frequently, by neutralisation of sulphonated alkylbenzenes of which the alkyl is a polypropylene of 40-520 relative molecular weight. Synthetic sulphonates are regarded as being more efficient than petroleum sulphonates. Each of the above examples has its own specific properties and applications (163). Ammonium sulphonate is a good rust inhibitor for automatic transmission fluids and flushing oils and for greases, as well as a corrosion inhibitor against HBr and HCI for gasoline engine oils. Sodium sulphonate is suitable for cutting oils, lithium greases, aluminium complex greases and greases based on
339
diester synthetic oils. In water-free media, it is also an effective inhibitor of galvanic corrosion, able to solubilise water produced. In aqueous media, it is displaced from the surface. It is inefficient in light hydrocarbons (403). Neutral calcium sulphonates are used as rust inhibitors in preventive, cutting and metal-working oils which contain chlorinated paraffins, rolling-mill and rinse oils and in lithium greases. They are characterised by high thermal stability, and can also act as inhibitors of galvanic corrosion. Mediumbase calcium sulphonates (40 - 50 TBN) are virtually all-purpose rust inhibitors suitable for all types of mineral and synthetic oils and greases, high chlorinated paraffin cutting-oils, oils and greases with a high load-carrying capacity, MoS2-containing lithium greases and other, similar preparations. They can neutralise organic and inorganic acids. Lead sulphonate can partly or completely replace lead soaps in oils with a high load-carrying film, and is thus a suitable rust inhibitor for gear oils. It also acts as a de-watering agent and is therefore a convenient additive for lubricants for Morgoil type bearings and for hydraulic oils. Zinc sulphonate is useful in hydraulic oils because of its demulsifier properties, thermal stability above 190 "C and its capacity to reduce sludge formation in oils containing ZDDP. It is also suitable for greases, particularly those based on bentonite, which are softened by other sulphonates. Ethylenediamine sulphonate is particularly useful as a rust inhibitor in lithium and calcium-based greases prepared from 12-hydroxystearic acid, aluminium complex greases and emulsion oils for metalworking processes, because of its good emulsifying properties in hard water. The concentration of these rust inhibitors in both oils and greases varies between 0.5 and 2% by weight; greases usually have the higher concentration. Sarcosine and imidazoline derivatives are of considerable importance. They are used in preventive, turbine, hydraulic (both mineral and synthetic) fluids and cutting oil emulsions, non-flammable water/glycol fluids, greases (particularly bentonite and MoS2-containing types) and even in light fuels, both alone and in mixtures with other types of rust inhibitors, in concentrations from 0.01 to 2% by weight. In engine oils, they improve the capacity to neutralise acids (e.g., HBr and HCI produced by the decomposition of alkyl halides present in lead anti-knocks). Substituted imidazolines dissolved in light hydrocarbons are used for de-watering metal surfaces to speed up drying. Lauryl- and oleyl-sarcosines and their sodium and/or ammonium salts and esters and their reaction products with alkylamines are also used. Imidazolines and sarcosines display marked synergism. The advantage of the imidazolines is their high thermal stability. They are compatible with many other types of additives. However, they are very sensitive to oxidation. They form quaternary ammonium compounds with alkyl halides, and react with acids to form salts. Their salts with low molecular weight acids, e.g., acetic, hydrochloric and phosphoric acids, are oil-soluble. Their salts with higher fatty and naphthenic acids and alkyl- and alkaryl-sulphonic acids are insoluble in water but soluble in oil.
Rust protection is also essential for some water solutions and oil emulsions in water. Water-soluble inorganic salts, such as alkali metal or ammonium borates, metaphosphates, vanadates, molybdates and tungstates have good anti-rust properties. They have even proved to be satisfactory under elasto-hydrodynamic and mixed lubrication conditions. Nitrates and fluorosilicates also provide corrosion protection. Among organic compounds, water-soluble benzoates and acetylenic compounds such as propargyl alcohol and butynediol possess outstanding anti-rust and anti-corrosion properties.
4.4.2 Corrosion Inhibitors and Metal Passivators Corrosion inhibitors are substances which provide protection to both ferrous and non-ferrous metal components - mostly bearings - against attack by acidic contaminants contained in or produced by oils and greases. They also act as metal
340
passivators, inhibiting the catalytic effect of metals in oil oxidation by forming surface layers on the metal and impeding the emission of metal ions into the oil. The mechanism of this corrosion inhibitor effect consists in chemical reaction of the corrosion inhibitor with the metal surfaces. This reaction produces a protective film which is resistant to corrosion agents (264,165).The film must firmly adhere to the metal surface - it must be adhere sufficiently firmly to prevent other surfaceactive oil components, such as detergents and dispersants and acidic substances, from destroying it. The formation of a protective film is an intricate process whose nature, speed and thickness depend on the composition of the additive and the metal type and their interaction. For example, sulphurcontaining additives are believed to promote three typical reactions with metals: formation of mercaptide or thio-acid salts, formation of metal sulphide and metal-additive complexes (166,167).The reactions of metals with polysulphides, particularly those with a high sulphur content, causes the formation of M,S, metal sulphides. The protective film is destroyed with an increasing concentration of acidic substances. However, if sufficient inhibitor is present, it may re-form. Destruction and re-establishment of the film proceed simultaneously; the destructive process does not become prevalent until the inhibitor is exhausted. This process is accelerated by temperature (168).
The majority of compounds used as corrosion inhibitors are peroxide decomposer-type antioxidants, compounds containing sulphur, phosphorus or both. The earliest corrosion inhibitors for engine oils were organic phosphites. They were mostly mixtures of mono-, di- and tri-organophosphites, made by the reaction of alcohols or hydroxy esters, e.g., methyl lactate or trimethyl citrate, with PCI, (269,170),with the general formula (RO),P(OH)3-n, R being an organic radical and n =I, 2 or 3. These phosphites were widely used in premium-types of engine oils in the late 1930’s and the early 1940’s often in combination with film-strength improvers such as methyl chlorostearate or hexachloro-diphenyl ether (271).By the mid-1 940’s, the majority of these inhibitors had been replaced by new substances which contained sulphur or phosphorus or both, which became prototypes for the products now in use. These may be classified as follows: dialkyl or diaryldithiophosphates of metals, chiefly zinc and nickel; metal diorganodithiocarbamates, chiefly zinc; sulphurised terpenes or higher (C,2) alkenes produced by heating elemental sulphur with the terpene and washing the crude product with caustic soda or alkaline sodium sulphide to remove dissolved, corrosive sulphur (273); phosphosulphurised terpenes - produced by heating terpenes with P2S, (2 74); thiazoles (I) and triazines (11):
which act as chelating agents, forming, with copper and silver, a complex insoluble in water and many organic substances. This complex produces a transparent film on the metal surface. These compounds are, therefore, used as special corrosion
34 1
inhibitors for copper and silver alloys and are suitable for turbine (including aircraft turbine) oils, hydraulic fluids, electro-insulating oils, cutting and rolling-mill oils and special lubricating greases. The concentration in oil used is very low, from 0.01 to 0.05 by weight. They act during oil oxidation as passivators of copper and its alloys in oils which come into contact with copper. They are also used as anti-stain inhibitors in gear oils, preventing changes in the colour of copper resulting from the presence of corrosive sulphur. Substituted triazines (339),the reaction products of benzotriazine and alkylsulphides (34.9, also act as oxidation and corrosion inhibitors.
4.4.3 Anti-fatigue Additives The primary cause of metal fatigue appears to be subsurface and surface cracks in the solid friction surface. These cracks are first generated by repeated cyclic stressing of the surface which leads in its first stage to strengthening of the material by work-hardening but then to the loss of elasticity and cracking. The formation of cracks is aggravated by defects in the material, irregularities on the surface, the presence of a second, foreign phase (oxides, carbides, sulphides, brittle impurities) on and under the surface, defective treatments leading to residual stress and unequal distribution of surface energy, discontinuities in contact geometry and operational factors such as elastohydrodynamic friction, tangential forces without shearing, rolling with shearing, elastic deflection of the bearing, oscillations, stick-slip effects etc. (407).As soon as they form, the cracks tend to form a branching network, and the surface disintegrates with flaking and pitting (372,408).The formation of cracks can be reduced by plating both friction surfaces with very thin protective layers (e.g., O.lpm Cd on steel). Lubricants have an undoubted effect on wear by fatigue. Oil penetrates into the cracks and enlarges them by hydrostatic pressure (408). It appears that higher viscosity oils apparently impede fatigue by forming thicker films on the surface and penetrating more slowly into the cracks. However, the chemical constitution of the oil can be more important in the fatigue-failure mechanism than its viscosity (408). Reactive oils such as esters are more problematical than the less reactive mineral oils. Oils which tend to release hydrogen by dehydrogenation are worse than oils with a lesser tendency to dehydrogenate. Acid products of oil deterioration encourage fatigue failure. The same may hold for some additives. For instance, some EP additives reduce the life expectancy of gears (420). Atmospheric oxygen does not seem to influence fatigue, but dissolved oxygen seems to be active. One of the most deleterious substances is dissolved or dispersed water, present as an impurity or produced as a product of oil deterioration (372).Another hypothesis suggests the decomposition of water to atomic oxygen and hydrogen on fresh surfaces of the cracks. The oxygen then acts corrosively and the hydrogen causes hydrogen embrittlement and corrosion (242). The relative stability of additive species, particularly ZDDP, can influence fatigue failure of metal surfaces in severe contact conditions. Pitting of cam-followers and cam-noses has been observed as an
342
oil-related problem when low-viscosity multi-grade (SAE 1 OW-30) oils were used in over-stressed valvetrains in prototype gasoline engines. The problem was more severe with relatively thermally unstable, lower alkyl-substituted ZDDP than with the more stable higher branched-chain alkyl, or diaryl, additives. However, the less stable additives gave significantly better protection against scuffing wear of the flanks of the cams. This problem was largely overcome metallurgically but the case illustrates the possibility of additive interactions with metal surfaces leading to fatigue failure in severe EHD conditions.
In summary, all corrosive substances which may penetrate into the cracks and all factors which promote corrosion aggravate fatigue phenomena and all factors which suppress corrosion can be beneficial, e.g., non-corrosive antioxidants and inhibitors, non-corrosive thickening agents (e.g., polyolefins), and non-corrosive solid lubricants which smooth the friction surface and products with similar properties. Special attention must be paid to water. The life-expectancy, L, of a material depends on the concentration of water ( c ) in p.p.m. according to the empirical relationship: (4.43) If, for example, the durability is 1 at 100 p.p.m., then at 25 p.p.m. it is 2.6 and at 400 p.p.m. it is 0.52 (422). The principal function of anti-fatigue additives is to suppress the effect of water. They include: compounds which neutralise protons, e.g., isopropylaminoethanol (423,373); compounds which produce a hydrophobic film on the surface, e.g., higher fatty alcohols (such as n-octadecanol or the salts of octadecanoic acid and organic amines); compounds which bind free water which is present; for example, organic amine salts of carboxylic acids such as succinates prevent water adsorption on the metal surface and its diffusion into the extremities of the cracks (374); compounds which produce compact surface films, e.g., alkyl-phosphonates with organic cations (375). Particular combinations of the above types can be devised so as to produce synergism.
4.5 MODIFIERS OF VISCOSITY AND VISCOSITYTEMPERATURE CHARACTERISTICS Additives of this type affect the rheological properties of the oil, increasing its viscosity at higher temperatures without degrading its other properties, mainly lowtemperature flow and pumpability and thermal and chemical stability, and without interfering with the effects of other additives. They are mainly used in engine, hydraulic and gear oils, but are also applied in other lubricants. They are mostly linear, non-crystalline or atactic polymers and copolymers, with a variety of chemical compositions, in the mean relative molecular weight range from 5,000 to 2 million. 343
Unlike pure substances of lower molecular weight, polymers contain macromolecules of varying relative molecular weight. They are, therefore, characterised by mean molecular weights, of which the most significgt is number average molecular weight, M,, and mass average molecular weight, M,. Relevant equations are:
where Niis the number of molecules in the system and M iis their relative molecular weight. For example, if a system contains 5 molecules each of 20,000,40,000,60,000,80,000 and 1OO.OOO molecular weight (g.mol-I) respectively, then:
The k, value can be determined osmometrically, cryoscopically or ebullioscopicdly and the Gw value from light diffusion or the rate of sedimentation in polymer solutions (302). The precursors of the synthetic polymers were animal oils, blown vegetable oils, condensed oils and voltolised oils. The viscosity of these oils can be substantially increased by blowing with air at 70-120 "C. The viscosity of mineral oils can be increased by compounding them with blown animal and vegetable oils, such as rape-seed, cotton, castor and sperm oils, which also improve their lubricity, but at the cost of oxidation stability. A considerable increase in viscosity can be achieved by voltolisation.
The oldest viscosity and viscosity index improvers are the polybutenes, above all the polyisobutenes (I), which are commercial products (known as,e.g., Paratone, Exanol, Oppanol and Hyvis) (I75) and polyalkylstyrenes (11) (e.g., Santodex), used as early as before World War 11.
where R is an alkyl c&, radical. These additives are still in use. Atactic polypropylene with iw around 30,000, fully separated from isotactic constituents and stereoblocks can also be used. It has similar properties to polyisobutene, but lower thickening power at identical Mw.
Additives of the above type have been improved on considerably in respect of some of their properties by other polymer types, including: 344
- copolymers of methacrylate esters (polymethacrylates or PMA) (111) (Z76), marketed as Acryloid, Garbacryl, Plexol and Viscoplex:
{CH2
-73ry ;# 1 COORI
CH2 - COOR2
(111)
with i w o f about 6.0.104to 1.0.106; - similar polyacrylates (PA) (IV) (177,Z78)(e.g.,Glissoviscal)
{CH2
-
CH2 - COOR2
(IV)
COORl
n m where n = 100 - 3,000, R, and R, are C, - C,, aliphatic radicals (identical or different), average C, to CI2; - ethylene-propylene copolymers (EPC or OCP) (V)
-
styrene-ethylene-propylenecopolymers (Va)
of different mole ratios of ethylene to propylene, ranging from 1.5 to 0.43:1 (311) and with Gwaround 8.10. The standard ethylene content in these copolymers is 50 to 70%; if it reaches 808, the copolymer becomes partially crystalline and the stability of its solutions in oil is affected.
345
- copolymers of ethylene, propylene and a diene (EPDM) (VI)
where x,y and z can be 1 - 100.
-
The content of non-conjugated dienes is 5 10%. The dienes most frequently used are trans-1,C hexadiene, 5-ethylidene-2-norborneneand dicyclopentadiene.
-
hydrogenated block co-polymers of styrene and butadiene or isoprene (VII) with M,around 1. lo5, containing about 50-70% of styrene
-ri
-
The high polymer block of the hydrogenated polydiene olcfin chain is terminated by two lower polymer polystyrene blocks. This structure allows a super-molecular arrangement, which decisively affects the properties of block polymers.
- copolymers of alkyl fumarates and vinyl acetates (VIII) (279)
and mixtures between these and other polymers. The order of monomer units in the polymers can be of the block type, as shown, or random. Each of these products is a mixture of polymers of different molecular size. The distribution of the relative molecular weights differs in different types of polymeric viscosity modifiers, as shown in fig.4.5.
Sn
The difference between and k, characterises the extent of the distribution or the heterogeneity of the relative molecular weights, which can be defined by the equation: (4.44)
346
0
3
5 7 9 BISSUCCINIMIDE,%wt
Fig. 4.5. Correlation between bissuccinimite content (% wt) in oil and merit ratings at Petter AVB engine test The patent literature indicates a series of other compounds suitable as viscosity and viscosity index modifiers for lubricating oils, for example (298): - ethylene copolymers with C, to C,, a-olefins containing 60 to 80 mole % ethylene (313), - ethylene, propylene and 1,4-Fexadiene terpolymers containing 25 to 5% ethylene (314, - cis-l,4-polybutadiene with M, 7.5.104to 2.106 (3Z5), - among adamantyl polymers, poly-3,5-dimethyl-l-adamantylacrylate with G, 80,000to 285,000; this should be a more effective VI improver than conventional polymethacrylates (180).
Recent developments in VI improvers have taken place in the following directions: - developments of polyolefin-types. These include, for example, hydrogenated polybutadienes of G, about 20,000: r
cH2-i;21 1
(CH,
- CH, - CH2 - CH,), - (CH, - CH = CH - CH,),
-
Their properties are similar to those of polybutene (high thermal and mechanical stabilities and good thickening power, especially at lower temperatures); - copolymers of olefins with alkyl methacrylates. The aim here is to combine the advantages and neutralise the deficiencies of the respective homopolymers. Such products include alkyl methacrylate/ethylene-propylene copolymers (341). styrene/alkyl methacrylate block copolymers (342)containing 550% of styrene and as much -as 95% - of lauryl methacrylate, with a M , of 10,OOO to 50,000 and a ratio of M , / M , of less than 2, and similar products; - new polymeric components acting as both dispersant and VI improvers, including, for example, tetramer mixtures of three alkyl methacrylates and an N,N'-dialkylaminoalkylmethacrylamide(343), epoxidised terpolymers of ethylene, C3-C8 a-olefins and unconjugated dienes, oxidised copolymers of acrylonitrile, reaction products of polyolefins (e.g., atactic polypropylene) and polyethylene polyamines, terpolymers of ethylene, propylene and Nvinylimidazolines or cyclic imides, etc.;
347
-
polymers with multi-functional groups, such as sulphonic-copolymers of allyl derivatives, e.g., allyl alcohol, allyl acetate, allyl esters of c12+8 carboxylic acids and I-alkenes, e.g., 1-hexadecene. These compounds can act as viscosity and VI modifiers, pour-point depressants, dispersants, EP additives and rust and oxidation inhibitors. Other compounds have also become available, which combine the properties of viscosity and VI modifiers and synthetic oils, such as: - adducts of amines (e.g., methylbenzylamine) with phosphoric acid esters of the
OH I
R-O-P-OR II 0 (R is hydrogen or an alkyl up to C,,, R is hydrogen or an alkyl up to C7),operating as viscosity and VI modifiers, anti-wear and anti-corrosion inhibitors; - polyoxyalkyleneglycol diethers (345) of the following general formula:
CH3 [RO-(CH2dH
72%
CH,CH -O++
CH3 CH, (!XI -&H-o&CH,
(R is C, to Ctl alkyl and a+b+c = 5 to 100). Mixed ViscositylVImodifiers now in use include, for example, polymethacrylates with polyolefins, styrene-diene copolymers with poly -isobutenes (or with ethylenepropylene copolymers)and others. The aim is to achieve more complex end-results, including easier handling and economy. The advantages of ester polymers, particularly polymethacrylates, include a strongly beneficial effect on the viscosity-temperature curve, adaptability to use in a variety of oils and compatibility with DD additives; their main demerit (besides a somewhat high cost) is their tendency to form deposits in thermally loaded diesel engines. The advantage of the ethylene-propylene and hydrogenated styrenediene polymers is their high thickening effect at low doses and hence their cost-effectiveness as VI improvers. They have good mechanical shear stability and form less carbon deposit at elevated temperatures; they are, therefore, especially recommended for use in multigrade oils for supercharged diesel engines. Their disadvantages include a lesser effect than PMA on V1, a higher viscosity increase, mainly at low temperatures and low shear rates and, in particular, their inability to reduce pour-point.
All these polymers are normally handled as concentrates in raffinate oil solutions of 2 - 30 mm2.s-l average viscosity at 50 "C. The polymer concentration in commercial products depends on the solubility of the polymer in oil. Commercial polymethacrylates contain 30-808 active polymer. alkene copolymersup to about 18% and styrene-butadiene copolymers only 5 to 7%. The viscosity of the diluent oil is also important. Styrene copolymers are hence mostly supplied in the solid state for direct dilution in the oil they are intended to modify at elevated temperatures (about 130 - 150 "C).
348
These polymers increase both oil viscosity and viscosity index; some also act as pour-point depressants, antioxidants and, in some cases, anti-wear agents (326).
Thickening Effects of Polymers When a polymer is added to an oil solvent, its macromolecules assume the shape of random clusters in varying degrees of development depending on the nature of the oil and the temperature of the solution and on the structure and rigidity of the polymer chain. The less rigid polymers (according to Kuhn, those having shorter chain segments), being more flexible, tend to form thicker clusters. Polymer rigidity increases with increasing structural branching from polyolefinsthrough polystyrenes to polyalkylmethacrylates,as illustrated in Table 4.17. Fig.4.6 shows the cluster structures of the most important examples of polymeric viscosity modifiers. PMA and ethylene-propylene copolymer (OCP) clusters have roughly the same size, however OCP clusters are formed by longer and more flexible chains, whereas PMA clusters are formed by thicker and more rigid chains. The central polyolefin block in hydrogenated polystyrene-diene copolymers (SDC) is more or less the same as that of OCP. However, unlike OCP, SCD does not exist in oil solution as an isolated individual form but in an associated form around the insoluble terminal blocks, so that a much more developed structure is formed.
The viscosity increase at a given temperature and shear stress of an oil due to the presence of a polymer can be expresses by the specific viscosity, qSFF= e, or by the reduced viscosity, Vred = e.c-l,or, most accurately, by the intrinsic viscosity [ q ] . This is a fundamental parameter characteristic of the polymer/solvent pair at a given temperature and shear rate ( D ) in macromolecular chemistry. The thickening power of polymers is connected with the nature of the oiVpolymer interaction and depends on the polymer type and the composition of the base stock. Table 4.17. Lengths of Chain Segments of Selected Polymers, According to Kuhn Polymer Structure Length (A) Polyethylene
-CH,-CH,-
14.5
Polypropylene
-CHZ-CHI CH3
15.2
Poly-(n-pentene-1)
-CHZ-CH-
20.4
I
C3H7 Polystyrene
Polyalkyl methacrylate
- CH,
- CH -
21.6
7H3
-30
349
t
a
b
W(M'
C
Fig. 4.6. The distribution of the relative molecular weights of different types of polymeric viscosity modifiers a - PMA, b - OCP, c - HSBCP
In the oiVpolymer system, two main interactions operate (181);polymer with polymer, Ipp,leading to the formation of the structure in the oil, and polymer with oil, IP. leading to the solution of the polymer in the oil. If no polymer/oil interaction occurs, i.e., if the oil is a poor polymer solvent, the polymer chain tends to form intermolecular aggregates, coiled clusters, as the most likely conformation of the macromolecules, and avoid contact with the oil. In polymer/oil interaction when the oil is a good polymer solvent, the chains interact with the oil and straighten due to solvation, so that the clusters expand, their density decreases and the viscosity of the system increases, according to the following equation: (4.45) this is the so-called which can be derived from the Einstein expression for relative viscosity q/qo.pes,rq!; equivalent density of the cluster under limiting conditions, i.e., the density of the equivalent spheres which would - with respect to solution viscosity increase - be present, as well as the shape of the dissolved macromolecule, of which the shape cannot be determined. The oiVpolymer interaction depends on the nature of the oil and the structure of the polymer and can be expressed as the hydrodynamicJolume [q]. M,where [q] is the intrinsic viscosity and M the average of the relative molecular weight M,. For example, the different dissolving powers of oils of various compositions for PMA, expressed as hydrodynamic volume, are illustrated in Table 4.18 (347).
Table 4.18. Solution Properties of PMA in Different Oils Oil type Cycloalkanic oil Highly aromatic oil Synthetic (ester) oil Cycloalkanic + 6% (weight) synthetic oil
[q].M.
(cm3.mol-') 4.3 5.8 7.0
6.1
Fundamental differences exist between polymeric viscosity modifiers, in respect of the relationships between polymer thickening power (related to its cluster bulk) and temperature. Fig.4.7 illustrates the
350
differences in the cluster bulks of different oils (expressed as the hydrodynamic bulk in relation to temperature within the range 10-160 OC (347). Whereas the bulk of PMA increases within this range, those of olefin (OCP) and styrene-isoprene copolymers (SIC) decreases. The shapes of the curves of reduced viscosities of different viscosity modifiers, as shown infig. 4.8, is in agreement with this (348). The thickening power of PMA mostly increases within this temperature range, whereas that of OCP, SIC and PIB sytematically decrease.
C
Fig. 4.7. The cluster structures of different types of polymeric viscosity modifiers u - PMA, b - OCP, c - HSBCP
12 10 8 6
4
Fig. 4.8. The differences in the dimensions of the hydrodynamic bulks of different types of polymers in relation with different temperatures a OCP, b - HSBCP, c - PMA
-
These differences have significant practical consequences on the viscosity at low and high temperatures, for example, of multigrade engine oils made from different viscosity modifiers, as shown infig. 4.9.
The operating temperature range involved can be identified from the ratio of by the index Q specific, reduced or intrinsic viscosities at two temperatures defined by the relationship: (4.46)
351
qred
3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6
1.4 1.2 1.0 0.8 0.6 0 .4 0.4
0.3 0.2 0.1
0
- 0.4
I :
I
I
1 I
-30
-10
0
20
40
60
80 100 TEMPERATURE ( O C )
Fig. 4.9. Characters of curves of reduced viscosities of oils of different origin with different viscosity modifiers 1 - PMA, 2 - HSICP, 3 - HSBCP, 4 - OCP,5 - PIB
_____
in cycloalkanicraffinate (3.8 m * . ~ - ~ / l O "C) o in paraffinicraffinate (6.5 m 2 . s ~ 1 / 1 0"C) 0
At e>l, the solution process is endothermic and the solvent power within the temperature range considered is poor, but it improves with increasing temperature. The polymer has low solvent power, but increases the VI; ester polymers such as PMA provide an example. At Q
352
Fig. 4.10. Comparison of the thickening power of OCP and PMA at a) 98.9 "C and b) -17.8 "C (CCS)
Fig. 4.1 I . Comparison of the thickening power of HSBCP and PMA at a) 98.9 "C and b) -17.8 "C (CCS)
Fig. 4.12. Comparison of the thickening power of PIB and PMA at a) 98.9 "C and b) -17.8 "C (CCS) The compatibility of polymer with oil can be tested by centrifuging the solution for 1 hour at 0 "C at 25,000 G. The solution is then stored for one month at -20 "C. The following observations can result: - the solution remains homogeneous, - the solution converts to a gel at -20 "C, - the polymer separates from the solution at -20 "C, - the polymer separates from the solution at 0 OC or even at normal temperature. Incompatibility can be observed when the oil and the polymer we. very dissimilar (250).
353
Table 4.19. Effects of Different Polymers and Brightstock in the Same Base Oil (133) Viscosity
Viscosity increase Specific viscosity Q VI at ("C) 37.8 98.9 37.8 98.9 37.8 98.9 37.8l98.9 35.15 5.41 96 45.9 7.96 10.75 2.55 0.305 0.379 1.239 130 50.05 7.47 14.9 2.06 0.424 0.381 0.900 119 56.99 7.47 21.84 2.06 0.622 0.381 0.613 101
Oil type
(mm2.s-') at ( "C)
Base oil +1.13% PMA 4.84% PIB +lo% Brightstock
Table 4.20. Thickening Power of Various Polymers over the Temperature Range -17.8 to 98.9 "C Polymer type*
Thickening effect of 10%weight of polymer in SAE 10 Engine Oil qmd at 98.9 "C
PMA-1 PMA-2 OCP PIB SBC
18.3 15.5
8.9 9.0 8.6
q,d at -17.8 "C 2.41 5.2 4.8 5.2 9.65
Q98.91-17.8 7.6 2.38 1.85 1.73 0.89
.
PMA = polymethacrylate OCP = ethylene-propylene copolymer PIB = polyisobutene SBC = styrene-butadiene copolymer
The Influence of Base Oil Base oil affects solvent power and compatibility in the oiVpolymer system. The Q values in the same oil differ according to the polymer type and vice versa. Polymer type being the same, Q can be affected by the chemical composition (alkanicity, aromaticity) or viscosity of the oil; a more distinct effect is normally achieved in oils of higher alkanicity or lower viscosity. Highly polar oils from cycloalkanic or aromatic types are more susceptible to polymethacrylate modifiers, whereas alkanic oils of lower polarity (particularly hydrocracked oils or PAO) are receptive to polyolefin types. This suggests that the thickening power of PMA is higher in the cyclic oils and that of polyolefins in alkanic oils (401). Currently available commercial oils are fairly compatible with most commercially available polymers. Exceptions are hydrocarbon oils of very high VI (above 130) and low aromatic content, in which the highly polar polymers may separate, e.g., PMA with average side-chain carbon number below 10 and C4 alkyls and shorter below 30% (276).Compatibility may also be expected to be low in the systems comprising hydrocarbon-free (e.g., ester) oils and non-polar polymers (e.g., some polyalkenes).
354
Influence of Polymer Composition The thickening power of polymethacrylates, polyacrylates and polyfumarates is influenced by the length and configuration of the R substituent; it increases with the length of the alkyl chains and is greater with branched than with straight-chain alkyls of the same carbon number. Ester polymers with short alkyls have a greater influence on VZ increase, since their Q is higher and their solubility in oil at lower temperatures is lower (Table 4.21). To ensure solubility in and compatibility with oils, the average size of alkyls in ester polymers should not be shorter than C4. This is particularly applicable to hydrogenates. Thus, those ester polymers containing isoalkyls which are more soluble in oil are better VI improvers for hydrogenates. The differing effect of alkyls of different chain-lengths and configurations is used in the selection of alcohols for ester polymers of the required thickening power and/or effect on oil VZ. Hydrocarbon polymers exist as crystalline and amorphous types. Only the amorphous linear-chain polymers which are soluble in both mineral and synthetic oils under conventional operational conditions are used as viscosity and VZ modifiers. They all have Q
PMA PMA PMA PIB OCP
Mw.10-3Dispersion Alkyls Index C, C,,, M,JMn (%) 4.5 52 40 8.7 500 22 4.5 34 8.8 500 100 0 4.8 13.8 530 3 125 2.7 94
c,
Q
qred(110cm3.g-')at ( "C) 0
37.8
0.238 0.219 0.300 0.622 1.12
0.380 0.310 0.360 0.548 1.01
98.9
0.470 0.410 0.390 0.500 0.85
150
0.430 1.97 0.450 1.87 0.410 1.30 0.455 0.80 0.74 0.76
*PMA = polymethacrylate EPC = ethylene-propylene copolymer PIB = polyisobutene SBC = styrene-butadiene copolymer
Influence of Relative Molecular Weight Relative molecular weight has a decisive effect on the thickening power of polymers. This can be generally expressed by the Mark-Houwink equation: 355
[ql =K[,].Ma
(4.47)
where Kiql and a are constants depending on the solvent, polymer and temperature. As a rule, a is greater than 0.5 (it is generally 0.65 to 0.75). Attempts have been made to establish experimentallythe relationship between K and a and properties such as polymer type, rigidity of the chain, size and disposition of blocks in block polymers, affinity between solvent and monomer, etc. (377,378).
At the same concentration of polymer in oil, the effect on VZ increases with increasing relative molecular weight. For example, 1% of polymer of higher relative molecular weight (30,000 - 50,000) or 5-10% of the same polymer of lower relative molecular weight (5,000 - l0,OOO) produce the same increase in VZ. Reduced viscosity and the index Q increase with increasing relative molecular weight at any temperature between 0 and 150 “C. The difference in relative molecular weight can, to some extent, explain the lower effect of brightstock (of 600 to 6,000 relative molecular weight) on oil VI increase compare with the synthetic polymers, the relative molecular weight of which are usually several orders of molecular weight higher.
Effects of Polymer Concentration in the Oil Viscosity increases with increasing polymer concentration. The effect of concentration c on reduced viscosity can be expressed by the empirical Huggins equation: (4.48)
where kH is a constant. Viscosity increase is proportional to concentration at lower polymer concentrations and to as much as the fifth power of the polymer concentration at higher concentrations. This change in the relationship can be explained by “entangling” of the polymer chains at higher concentrations. Viscosity index also increases with concentration in systems with Q>l, but the progression of oil thickening increment with increasing polymer concentration differs from that of viscosity index increment. Whilst thickening increases constantly with increasing amount of polymer, the maximum VZ increase is achieved at an optimum concentration. Below this point, VZ increases with the amount of polymer incorporated but the VZ increment per unit polymer decreases; above it, VZ changes little or may even decrease. This phenomenon must be taken into account when multigrade engine oils of different viscosity classifications are being formulated. The decrease of the effect of VZ increase at increasing polymer concentration has, moreover, economic consequences which must be considered, since polymers are relatively costly and the achievement of high VI can be expensive. It is important to select an oil with the maximum response to polymer treatment and never to exceed the optimum concentration for the particular polymer used.
356
Effects of Temperature The thickening power of a polymer is strongly affected by the Flory temperature 0 when 0 = 1 (299). Reduced viscosity decreases above and increases below this temperature. Changes are greatest close to the temperature 0. Every polymer type has a different 0; for alkene polymers, it is below 20 "C, for styrene-diene copolymers slightly higher and for polymethacrylates the highest, above 129 "C. Changes in intrinsic viscosity [ q ]with temperature are highest in oils of lower solvent power for polymers, and with polymers with short side-chains (e.g., polymethacrylates). In oils of good solvent power for polymers e.g., polyalkenes, the dependence of [ q] on temperature is slightly lower. Within the temperature range 20 - 120 OC, the thickening power of polyolefins is higher than that of polymethacrylates, although it decreases with increasing temperature (reduced viscosity decreases). The thickening power of polyolefins within this temperature range decreases in the order OCP > SIC > SBC > PIB. Thickening power is distinctly higher in the lower temperature ranges (-10 to 20 "C) and decreases in the order OCP > PIB > SBC > SIC (401). An increase in intrinsic viscosity with temperature can be observed in some systems where Q > 1; after the maximum has been exceeded, intrinsic viscosity decreases. This phenomenon has not yet been satisfactorily explained. Some authors suggest the existence of a second temperature in the higher temperature region (the Prigozhin temperature), or the existence of interactions between the main and side-chains of the polymer which are affected by temperature. Decreased thickening power at higher temperature can have adverse effects on, for example, the piston-ring region in internal combustion engines.
Effects of Pressure The pressure-viscosity relationship in oils thickened with polymers was discussed in Chapter 2.1.2.2. Thickening power increases with increasing pressure. However, the relationship between the viscosity coefficient a and the thickening effect of polymers has not yet been sufficiently investigated (379, 380).
Viscosity Changes with Shear Stress Oils containing polymeric additives lose their initial Newtonian nature and become pseudoplastic with, in the ideal case, viscosity reversible with changing shear stress and shear rate, D.Polymers of different types show different behaviour, in this respect, in oil, which is illustrated infig.4.13, which shows the changes in apparent viscosities of oils thickened with various polymers (274). The viscosities of these oils at different temperatures are listed in Table 4.22.
357
Table 4.22. Viscosities at Different Temperatures of Multigrade Engine Oils (Oil Viscosity around 17 mrn2.s-') Containing Different Polvmeric VZ Modifiers Polymer *
PMA I PMA I1 OCP SBC PIB I PIB I1
Kinematic viscosity (mm2.s-') at ("c) 180
150
98.9
5.86 5.23 4.46 4.35 4.38 4.7
8.1 7.79 6.96 6.55 6.6 6.99
17.06 16.97 16.88 16.84 17.26 17.18
Dynamic viscosity (mPa.s) at ("C) 37.8
VI
Pour-point
("(3
-17.8 (CCS)
84.7 101.4 130.0 157.7 163.8 135.1
1.650 2.870 1.875 2.250 12.700 6.180
231 193 151 124 123 149
-38 -34 -14 -14 -14 -14
* PMA = polymethacrylate PIB = polyisobutene SBC = styrene-butadiene copolymer
OCP = olefin copolymer
BASE STOCK
10
lo2
lo3
lo4
1 0 ~ 2
Fig. 4.13. Dependence of apparent viscosities of oils containing different polymers on the shear rate Knowledge of the dependence (decrease) of viscosity on shear rate is particularly important for polymeric VI modifiers for use in oils exposed to high shear stress, such as engine, hydraulic and gear oils, since the viscosity at high shear may fall below the critical limit for increased wear (about 5 mm2.s-') or below the break-down limit (about 2 mm2.s-').
Polymers of different chemical composition, configuration and molecular weight yield oils of different sensitivity to shear stress, and they therefore exhibit different temporary viscosity loss and different degrees of thixotropy on being returned to the rest state. Temporary viscosity loss may be expressed as a temporary shear stability index (TSSI): TSSI =
d' .1 "d.1
358
- 'd,h - 'd.0
. loo
(4.49)
is the dynamic viscosity of the fully-formulated oil measured at low shear-rate (mPa.s), vd,h the same measured at a high shear-rate, vd,o the dynamic viscosity of the oil formulated without the VI improver, all in the same units.
vd,l
The temporary decrease in the viscosity of an oil which contains polymeric additives under increasing shear stress is the result of two factors acting simultaneously: the orientation of the polymer molecules and heating up of the oil by internal friction. Molecular orientation, which is influenced by the chemical nature of the polymers, is the dominant factor at low shear rates, while at high shear rates it is accompanied by temperature rise in the oil which becomes the dominant influence. Tests with straight mineral oil have shown that the oil temperature increase is minimal at low shear rates - about 2 "C at 1 .2.104s-', but increases with increasing shear rate, reaching about 17 O C at 6.6. 105.s-*.This relationship is probably dependent on the nature of the oil. The result of these effects is that SAE 50 engine oil, for example, behaves as an SAE 30 oil around the piston rings and SAE 20 in the main engine bearings (187).
However, the magnitude of the temporary shear loss in viscosity in oils thickened with different polymers depends not only on the shear rate but also on the temperature at which the oil viscosity is measured at a given shear rate. These differences are shown in Table 4.23. The decrease in the apparent viscosity of an oil thickened with polyethylene-propylene and, especially, polystyrene-diene, at elevated temperature and at a given shear rate is substantially lower than that of an oil thickened with polymethacrylate. This is an important factor in field service, when it is necessary to sustain a sufficiently high engine oil viscosity in sites of high shear rate and high temperature, for example in the piston ring zone.
Mechanical Stability (Shear Stability) Temporary viscosity loss in oils containing polymeric VZ improvers must be distinguished from the permanent viscosity loss caused either by partial mechanical depolymerisation attributable to the effects of shear stress (the subject of this section) or by the thermooxidation of the polymer at elevated temperatures in the presence of oxygen (see fig 4.14). The former is the result of mechanical resistance to shear and the latter of resistance to oxidation and the effect of heat, i.e., the thermooxidative stability of the polymer in the oil. 'II ORIGIN OIL
' I 1 ORIGIN OIL
D 0
D
b
Fig. 4.14. Difference between a) temporary and b) permanent loss of oil viscosity containing a polymeric VI modifier
359
Table 4.23. Temporary Viscosity Loss of Oils Containing Different Polymeric VZ Improvers and Dependence on Shear Rate and Oil Temperature Polymer Type Oil viscosity (mm2.s-') at 37.8 "C 98.9 "C 130 "C @Pas) at -17.8 "C Oil temperature ( "C)
PMA
OCP
SBC
74.97 14.11 8.67
106.77 15.44 8.86
1 10.06 16.14 8.80
1.650
Shear rate
1.950
1.,900
Temporary viscosity loss (%)
(~-').105
70
2.99 1.01 0.4
38.1 29.8 24.7
34.1 26.9 18.4
37.8 25.9 19.3
100
3.44 I .44 0.5 1
36.6 20.3 9.4
32.0 22.7 16.0
22.5 11.4 1.9
I30
3.61 1.24 0.22
28.5 26.2 20.4
27.8 27.8 19.9
14.8 11.4 11.4
I50
3.75 1.08 0.67
30.8 18.2 21.4
26.8 15.4 18.7
7.1 3.8 3.8
Mechanical stability is affected by lubricant stress, which is a function of shear stress and shear rate, by the time over which the stress is acting, by the frequency of changes in tension, by the resistance to flow, which is dependent on temperature and viscosity of the base oil and concentration and average molecular weight of the polymer (257) and, especially, by the type of polymer. All effects which tend to increase the viscosity of the additive-containing oil also tend to diminish mechanical stability. Under comparable conditions, polymers which contain an aromatic nucleus exhibit higher stabilities than copolymers of alkenes or aliphatic esters, such as methacrylates. Other effects are involved which may reverse this order. Molecular size is of primary importance; the larger the molecule, the lower is its mechanical stability. For example, if polymethacrylate with an average molecular 100,OOO to weight M, up to 100,000 degrades& about 5%, then that of 200,000 degrades by 10%and that of M , over 400,000 by more than 20%. At equal M,, polyalkenes (e.g.,ethylene-propylenecopolymers) are less stable than PMA (250,253). This may be explained in terms of the better solubility of polyalkenes in oil, which results in larger particles more susceptible to stress. At equal length of the main chain and equal thickening power, all polyalkenes are, however, of lower and more stable than PMA. influenced _ by the dispersion of polymer Mechanical stability of a polymer is also _ sizes, which can be expressed by the ratio M , / M,,the dispersion index. A small
kw
Mw
3 60
ratio indicates a narrow distribution of molecular sizes and a high mechanical stability. Styrene-diene copolymers have a low dispersion index, between 1 and 2. Ethylene-propylene copolymers and polymethacrylates mostly have high dispersion indices, up to 10. The larger molecules degrade first, so that the dispersion becomes narrower and the G,+,/ M , ratio smaller (318).Mechanical stability decreases with increasing length of alkyl chains in the polymers. The better solubility of the lower ester polymers is evident here also. Cross-linking reduces mechanical stability. Base oil effects also play a part in these phenomena; as a rule, mechanical stability diminishes in good solvents (Q < 1). It should be noted that those factors which lessen mechanical stability usually improve thickening power of the polymer. The choice of a suitable polymer is hence a compromise between these two properties. The mechanical stability of polymers or oil thickened with polymers can be expressed by the shear stability index (SSZ): SSI =
v1
- v2
.loo
(4.50)
"1 - v o
in terms of the viscosities - usually expressed in mm2.s-I - where: v1 - oils containing polymer at the start of the test, v2 - the same at the end of the test, vo - base oil viscosity. The lower the SSI, the more stable is the product towards mechanical shear. The mechanical stability of polymer-containing oils can be established by standard laboratory tests, of which the most commonly used is the Bosch injector test. In this, the oil is repeatedly driven through a narrow orifice under standard conditions (CEC-L-14A-78, DIN 51-382). In the magnetostriction test, the oil is exposed to high-frequency vibrations for 5 to 30 minutes (ASTM D-2603-70 (1988)). There are also other tests available, such as the Orbahn injector., the Nieman gear test rig (Z89), the power steering pump test and others. Results from the different tests are not interchangeable (e.g., polyalkenes are more stable in a sonic test than in an injector) and do not correlate with engine tests. However, they offer useful information on the mechanical stability of oils and demonstrate that polymer molecules become smaller with continuing mechanical stress until they stabilise at a certain point, e.g., within 30 cycles in the Bosch injector, or 5 minutes in the oscillator. The results of tests on engine oils containing different polymers (349) show that there is good agreement between results obtained from the Orbahn injector and the Nieman gear-box (in the FZG test). There is no correlation between the results of the Peugeot 204, and those from the Orbahn injector and Nieman gear-box tests. The results differ with every polymer. The main reason for this may be thermooxidation, which is involved in the Peugeot engine test but not in the rig tests. (The Peugeot 204 test is now obsolete and does not appear in the European specifications, because no cars are now manufactured with the integral gear-box which formed the basis of this test. However, these observations have been retained in the text to illustrate the effect of an environment in which the oil is simultaneously exposed to mechanical shear and oxidative deterioration.)
Thermal and Thermooxidation Stability Unlike mechanical stress, thermal or thermooxidation stress affects all polymer molecules and the viscosity loss has no lower limit.
36 1
It would be expected that polymethacrylates would have higher oxidation stabilities than ethylenepropylene copolymers; the molecule contains no tertiary carbon, no carbon adjacent to an aromatic nucleus and no carbon in the a-position to a double bond, as in the case of the other polymers mentioned. It is well known that these carbons bring about the lowest resistance to the effects of oxygen. However, the effects of structural factors on the oxidation stabilities of polymers in, for example, engine operation has not been satisfactorily explained entirely. Engine and other oils also contain antioxidants with different synergistic and antagonistic effects, which strongly affect the resistance to degradation of these polymers (303). Thermal stability alone can be followed thermogravimetrically (Table 4.24) at a given rate of temperature increase (e.g., 2 "Clminute) and in an inert or oxygen atmosphere. Studies of the behaviour of polymethacrylates in nitrogen have shown that the thermal resistance of polymethacrylates in nitrogen increases with molecular size, so long as their alkyls are identical. The effects of side chains are small, and polymethacrylates prepared by anionic synthesis have lower dispersion indices, higher purities and better stabilities. The presence of nitrogen in the molecule increases the thermal stability of polymethacrylates. Some polyalkenes have higher stabilities (250,254,257)(Tables 4.26, 4.27) than the polymethacrylates. The reason for these differences have not been explained: there may be differences in manufacture, in the effect of base oil, additives and by failure, in comparing the properties of polymers, to select the optimum type of polymer for the conditions of the application. It can, therefore, be desirable to verify and place in context much of the data contained in both manufacturers' and scientific papers.
The thermooxidation and mechanical stabilities of oils can be tested under laboratory or field conditions in full-scale engines (188). The results may differ considerably from those obtained in laboratory rigs, although similar relationships between polymer compositions and thermooxidation stability may be observed; the thermooxidation stability of a given type of polymer decreases with increasing molecular size, and the degree of branching, cross-linking, dispersion and length of the polymer side-chains. However, as shown in Table 4.28, the presence and type of antioxidants is also important. Suitable standard test engines for this work include CRC L-38 (10 hour test), Caterpillar 1432 and 1-H2 (see page 447) and the now obsolete Peugeot 204. The engine which was used in the Peugeot 204 (CEC L-25-A-78) test was a 1.1 litre 4-cylinder OHC gasoline engine in which the transmission, final drive gears and the engine were all lubricated with the same oil. The test was operated for SO hours at an engine speed of 4,100 r.p.m. and an oil temperature of 1 15 "C. Mechanical and thermooxidation stability were evaluated from differences in viscosity after removal of fuel contaminants before and after the test.
Polymer-containing oils are road-tested by driving a minimum distance of 800 km; this distance is regarded as sufficient to demonstrate the extent of polymer degradation. The greatest viscosity and VI losses are, however, observed after a minimum distance of 1,800 km. Road tests can, unfortunately, provide results which rank polymers in different orders, depending on the polymer type and the effects of the type and design of engine and transmission, and, of course, the operating conditions which produce the stress on the lubricant. Gel permeation chromatography tests on oil samples from "hot tests" in four-cylinder engines operating with sump temperatures in the region have enabled distribution curves to be obtained which show the break-down of the largest molecules in polymethacrylates (fig. 4.154 and a broadening of the distribution in ethylene-propylene (fig. 4.1%) and ) (186). styrene-butadiene (fig. 4 . 1 5 ~copolymers
362
Table 4.24. Thermal Stability of Polymers by ThermogravimetricAnalysis Side-chain composition (mol %) c, M, ,104 M, I M,
Polymer Type
cl
Polymethacrylate (commercial products)
Polymethacrylate (anionic preparation) Polymethacrylate Polyisobutene Ethylene-propylene copolymer
clZ
26
33
-
100
‘14
19
Decompositiontemperature (“C) for 2% 10% weight loss
‘18
‘16
16
6
10.5
55 155 286 1130
1.8 2.4 3.2 9.0
245 244 260 276
270 272 288 308
-
10
588
1.56
90 94
2.5 2.7
305 300-320 332 400
332 340-350 370 432
Table 4.25. Comparison of Typical Additive Polymers Polymer Type
v
spec. at (“C )
SSI
Q 98.91 -17.8”C at polymer concentration (%)
at O 149 PMA-1
PMR-2
w m w
Styrene-diene Styrene-diene+PIB PIB Polyalkene
1.05 2.6
-
-17.8 0.3 0.7
-
5 6.6 2.0 3.5 1.5 1 2.5
10 5.0 2.1 4.7 1.7 0.95 2.7
15
20
4.3 4.1 2.3 2.75 6.2 >7 1.8 2.05 0.9 0.89 3.1 3.6
Thennooxidation stability (160 “C, 5 1 air), viscosity at 98.9% change (%) after
(%I
150 42
26 0 2 0 14
C
98.9
Viscosity at 98.9% loss by DIN 51-382
at
O C
24h
72h
0
18
35
-
-
0 0
-12
-
0
-3
22 22 15 11 18
89 91 75 66 82
37.8 -17.8
35 29 24 12 25 8 1 0 0 0 16 18 40
TGA % loss
-23 -2.5
0
-
-
0
-3
-6
300
400
(%)
42.2 17.1 3.3 7.7 -
15.7
1 ,,,,_,A, ' .' .. -... lo5 lo6
,.._..."
lo3
10'
*
MW
b
C
Fig. 4.15. Envelope curves of: a - PMA, b - OCP, c - HSBCP contained in engine oil subjected to the "hot test"
Table 4.26. Viscosity Changes in Oxygen and Nitrogen Atmospheres of Oils Containing Different VZ Improvers (Z85) Polymer Type Polymer concentration Oil viscosity Viscosity loss (a) (% by weight)
polymethacrylate dispersant p l y methacrylate ethylene-propylenecopolymer styrene-butadienecopolymer un-thickened oil
2.0 2.0 2.0 2.0
-
(mm2.s-') at in nitrogen* in oxygen* 98.9 "C (24h) (48h) (72h) (72h) 10.73 3.5 3.3 3.7 2.9 11.16 8.0 9.3 9.7 7.6 25.37 20.5 36.5 44.0 67.5 16.1 21.6 25.7 34.8 12.54 6.19
* 5 1 /hournitmgen or oxygen. In standard Caterpillar diesel engine tests, styrene copolymers have proved superior to polyrnethacrylates(255 and others). In general. polyrnethacrylateshave poorer thermlioxidationstabilities than polyalkene and polystyrene copolymers in thermally highly-stressed, particularly in supercharged diesel engines, They lose their solubility in oil, forming varnishes which adversely affect engine cleanlinessalthough, dispersant polymethacrylates can be exceptions. However, good engine cleanliness results (particularly in piston ratings) cannot be achieved unless - even with hydrocarbon polymers - the composition of the dispersant-detergent additives and their concentration in oil matches the VI improver type. Tests in automotive engines with integral gear-boxes, when these were current, were very severe. The oil was subjected to much more severe conditions than in engines with separate gear-boxes and final drive units. Viscosity losses in oils containing polymethacrylates and polyalkenes can be reliably forecast from FZG tests in the Niemann gear-box (189,190).Oils are also subjected to higher stress in automobiles with rear-mounted, aircooled engines.
364
Table 4.27. Tests on SAE 10W/50 VI Improver-containing Oils in the NSU Prinz Engine (Hot Test) Comparison with DIN 51-383 (185)
-
Copolymer Type -
Engine Test Duration (hours) 0 5 15 25
styrene-butadiene average viscosity molecular loss weight* (%) 98,000 94,000 76,000 55,000
30.0
ethylene-propylene
p l y methacrylate
average viscosity molecular loss weight* (%) 90,000 85,000 60,000 50,000
average viscosity molecular loss weight* (%)
-
12.0
240,000 240,000 225,000 200,000
30.5
~~
DIN 51-383 (30 cycles) 5.7 - 18.8 * Measured by gel permeation chromatography on the oil after the prescribed test period.
-
9.5
Table 4.28. Oxidation Stability of Polymeric VI Improvers in the Presence of Antioxidants (Results from Tests at 150 "C with FdCdPb Catalysts)(276) Polymer Type
M.103
Induction period (minutes)
Oxygen Oil viscosity absorption change (mm2.s-') ( m o l 0 9 ) after (96) at 37.8 OC after 420 minutes 420 minutes
(a) in the presence of phenolic antioxidant: polymer-free oil N-free PMA
4.0% 4.4% 6.0% N-containing PMA 4% 5% Anionic PMA 4% PIB 2.4% OCP 1.5%
270 155 392
-
90 94
294 265 215 >420 >420 340 340 285
0.9
+35 (+ insoluble substances) 0.71 +37 1 +34 +52 1.1 0.20 -7 -1 0.12 0.6 +25 0.6 -6 0.8 -24
.o
(b) in the presence of ZDDP: polymer-free oil N-free PMA
4% 4.4% 6% N-containing PMA 4% 5% Anionic PMA 4% PIB 2.4% OCP 1.5%
155 392
-
90 94
344 463 405 355 306 32 1 392 360 360
0.24 0.13 0.13 0.25 0.62 0.46 0.2 0.26 0.2
+5 4 +1 +9 -1 +15 +8 +5.5 -15
365
Thermooxidation effects not only cause the breakdown of polymer molecules but also their oxidation, so that both oxidation and condensation processes occur. These usually cause viscosity increase as the lubricant ages. In this respect, a certain amount of degradation of the polymer may have a beneficial effect by acting to counteract the oil-thickening process. These are, however, complex circumstances, which depend on the nature of the base oil, the type and concentration of additives, the type and operating regime of the engine etc. For this reason, different results are obtained in laboratory engine and road tests, which may obscure the basic mechanisms of the processes involved. Thermooxidative instability of VZ improvers at higher concentrations can become a major cause of deterioration in engine oil performance, frequently manifest in the form of piston fouling. Such problems with multi-grade oils with high polymer concentrations must be countered by increased dosage of detergent-dispersant additives; polymers of high stability towards shear and thermooxidation are obviously preferred on both technical and economic grounds. Considerable practical effort has been devoted to the development and manufacture of multi-grade oils for both gasoline and diesel engines which show small viscosity and VI decrease throughout their working lives (so-called “stay-in-grade” oils). Mixed fleet oils for fleets involving both gasoline and diesel engines must incorporate stable VI improvers. These highly stable VI improvers are also important in the formulation of shear-stable oils for all-season hydraulic fluids, gear and other types of oils.
Low-temperature Properties An important factor in the evaluation of viscosity modifiers and VZimprovers is their effects on the changes in oil viscosity at low temperature and variable shear-rates. All polymeric modifiers increase oil viscosity at low temperatures. At low shear stress, the differences in the thickening power of particular polymers at low temperatures may be very large. However, these values tend to level off with increasing shear stress because of the different susceptibilities of different polymers to shear stress, manifest as differences in the temporary viscosity decrease. These differences are illustrated in Table 4.29. The thickening power of every polymer at low temperature depends on its molecular weight and the molecular weight distribution curve. It also depends , in copolymers, on the concentration ratios of the component monomers, and in those polymers which themselves have no pour-point depressant effect, on the efficiency of any added pour-point depressant. Table 4.30. provides a comparison of the effects on base oil viscosity of commercial polymethacrylates and ethylene-propylene copolymers with different shear stabilities, within a defined low temperature range, as measured at the shear rate of the cold cranking simulator (CCS). It shows that changes in the specific viscosities of mixtures in the same polymer type in the same temperature range are relatively small. It is interesting to note that changes among ethylene-propylene copolymer (OCP) types are negative, so that the viscosities of oils thickened with polymer at these temperatures and shear rates are lower than the base oil viscosities. 366
Table 4.29. Viscosities of Oils Thickened with Commercial Polymeric Additives of Different Shear Stabilities (246) Product Type PMA PMA PMA OCP OCP PMNOCP PMNOCP PMAPIB
Concentration
in oil (% weight)
Polymer SSI
8.0 9.9 10.1 14.0 13.5 11.0 10.8 11.2
Viscosities (mPa.s) at -28.9 "C Brookfield (low shear)* CCS (high shear)t
45 24 15 22 12 23 23 30 ~~
15,480 23,000 17,100 876,000 93,000 818,000 43,000 24,400 ~~~
~
~~~
~
13,500 13,250 13,250
not measurable 12,750
not measurable 13,250 14,250 ~
~~
* The Brookfield viscometer measures the viscosity at shear rates up to 1.5 s-l by ASTM 2983-87. It comprises a cylinder or disc suspended in a ruby thrust bearing; the cylinder or disc rotates, powered by a synchronous electric motor through a beryllium-copper spring, in the liquid under test. The deviation of the spring is read off on a dial. In order to measure the viscosity, the reading must be multiplied by a simple calibration constant.
t The CCS (cold cranking simulator) measures viscosity at a shear rate of about 104 to 16 s-' by ASTM D-2602-86 and DIN 5 1-377; a universal constant-voltage electric motor powers the rotor, which is close-mounted in the stator of the measuring unit. A small amount of oil is charged into the annulus between the rotor and the stator and adjusted to the specified temperature. At constant motor input, the rotor velocity is a function of the oil viscosity, which can be deduced from a calibration curve plotted from measured viscosities of calibrating oils.
Table 4.30. Effect of Polymethacrylatesand Ethylene-PropyleneCopolymers of Different SSI on Oil Viscosity at Low Temperatures by CCS (246) Oil A B
Polymer SSI % weight Viscosity (CCS) (mPa.s) at ( "C) Viscosity change at ( "C)
OCP OCP c OCP D PMA E PMA F PMA Base oils for: A-C D-F -
29 25 19 23 17 1
12.6 13.0 14.0 9.0 9.0 16.6
98.9
-17.8 -12.2
-6.7
-1
15.07 14.91 15.05 14.94 15.12 15.46
2675 2350 2075 1950 2075 2225
1560 1220 1000 1030 1090 1220
985 800 660 700 750 820
645 535 440 470 500 555
- 1060 - 860
690 570
445 385
-17.8 -12.2 -6.7 0.18 0.03 0.09 0.26 0.35 0.15
0.47 0.15 -0.06 0.20 0.27 0.42
0.34 0.16 -0.04 0.23 0.32 0.44
-1 0.45 0.20 -0.01 0.22 0.30 0.44
This phenomenon is due either to exceptional properties of the polymer, or to the effect of the oil contained in the commercial polymer concentrate. The specific viscosities of oils thickened with OCP's are usually lower than those of oils thickened with PMA's, because of the greater influence of shear rate on the apparent viscosity decrease of the OCP or the oil thickened with it. The thickening power of PMA in the low temperature region increases with increasing shear strength of the PMA whereas the reverse applies in the case of OCP, so that there also appear to be differences in this aspect of behaviour among polymer types. Differences in the thickening power of polymeric VI improvers of different types at low temperatures and at high and low shear rates are illustrated in Table 4.31. 367
Table 4.3 1. Low Temperature Viscosities at High (CCS) and Low (Brookfield) Shear Rates of Engine Oils Containing Different Polymers (183) Additive type:
Styrene-isoprene copolymer
PMA-1
PMA-2
OCP
SAE viscosity grade
Viscosity
1OWl50 1ow140 1OWl40 1OWl40 1OWl40 (mm*.s-'/98.9 "c) 21.7 15.3 14.1 14.5 17.3 2250 1410 1450 1730 (mPa.s/-17.8"C)(CCS) 2550 (mPa.s)(Brookfield)at "C -1 1 3360 2340 1680 1460 2550 -18 7890 5430 3890 3840 5400 -22 171000 124000 7300 6400 46500 -26 333000 251000 127000 394000 1235000 121oooO 893000 192oooO ..not measurable.. -31
In selecting polymers for use as viscosity and VZ modifiers, other properties or potential effects of the polymers on the behaviour and properties of the oil, must also be considered. This includes effects on the demulsibility of the oil, pour-point effects (many alkene and akene copolymers fail in this respect) and compatibility with other additives.
4.5.1 Dispersant VZ Improvers Tri-functional polymethacrylates prepared by the copolymerisation of certain polar monomers - particularly N-containing compounds - form a special group, together with bi-functional olefin copolymers and olefin-diene copolymers, which also contain polar groups (introduced either by copolymerisation with a functional comonomer or by post-copolymerisationreaction of the copolymer). PMA-type VI improvers with both pour-point depressant and dispersant effects include the following types:
1
CH,
CH2
-
I
c=o
I 0 - (-CH2)2
0 - (-CH,),CH,
(Older type, X is - OH, -N
C2H5 2' H5
368
-r3
or-N
/
-X
C H 2 - C H 2\
\CH2 - CH2
0)
I
CH,
-
Other nitrogenous comonomers such as vinylpyridine are N-vinylmorpholine are also suitable. Polyolefin types of dispersant VZ improvers tend to lack pour-point depressant characteristics to the same degree as PMA's and may require the addition of additional lower molecular weight PMA's to correct this deficiency. These compounds include styrene and ester copolymers: CH -CH I I
c = oc = o n
OR
OR
c=
c
\ /
m
N I
The motive for the development of these additives, apart from a general effort to achieve multi-functionality, was to reduce the adverse effects of high concentrations of polymethacrylate VI improvers on engine cleanliness. This had led to the need for a higher dosage of DD additives in multi-grade engine oils, with economic penalties and, as mentioned earlier, formulation problems resulting from the high viscosity contribution from both DD additive and VZ improver; this made the achievement of low viscosity multi-grades difficult without recourse to expensive, close-cut base oils or volatility problems with normal base oils. The earlier versions of dispersant VZ improvers, mostly nitrogenous PMA types, had the additional capability - which was a prime reason for their introduction - of dispersing cold sludge in gasoline engine oils. They also provided some dispersancy in oils used for the lubrication of normally-aspirated diesel engines. Later developments aimed at improving the dispersant power of these additives at higher operating temperatures. The shear strength of dispersant copolymers varies. Their thickening power is generally lower than that of non-dispersant types. The ability of these copolymers to disperse cold sludges in gasoline engine oils is beneficial in the formulation of DD packages, in that they reduce the ashless
369
dispersant concentration required in order to achieve better engine cleanliness; this is shown by the values in Table 4.32, in which results of Sequence VC tests are compared. Table 4.32. Dispersant Polymethacrylates - Effects on the Reduction of Ashless Dispersant Concentration in SAE 1OW/40Engine Oils (191) Composition of oil (% weight): Base oil (mixture of selected raffinates) Detergent and antioxidant Ashless dispersant Polymethacrylate: - non-dispersant type I - dispersant type I - non-dispersant type I1 - dispersant type I1 Oil properties: viscosity (mm2.s-') at 98.9 "C viscosity (CCS)(mPa.s.) at -17.8 "C active elements concentration Ca P Zn Sequence VC engine test results (192 hours) sludge rating* varnish rating* piston-crown varnish rating* carbon groove fill (%) oil screen clogging (%)
A
B
83.5 4.8 2.5
84.75 4.8 0.75
C
D
84.27 4.8 3.13
95.92 4.8 1.08
9.2 9.7 7.8 8.2 14.75 2170
14.81 2150
15.08 2240
14.4 2150
0.23 0.11 0.12
0.23 0.1 1 0.12
0.23 0.11 0.12
0.23 0.11 0.12
7.1 6.4 7.0 2.7
9.6 8.0 8.1 0
0
0
8.6 6.2 6.9 2.0 0
9.6 7.3 7.2 1.5 0
* Merit rating on a scale of 0 - 10.
In effect, about 4% of the dispersant polymeric VI improver was able to replace about 1% of ashless dispersant. The contrast between the effects of dispersant and non-dispersant VZ improvers on piston cleanliness in supercharged diesel engines is evident from the Caterpillar 1 G test results shown in Table 4.33. These clearly indicate that the dispersant types are more effective in these engines than the non-dispersant types. Table 4.33. Results of Caterpillar 1 G Tests of SAE 20W/40Oils Containing Dispersant and Non-dispersant VZ Improvers (292) VI improver type* Concentration Sulphated in oil (%)
Ash (Wwt.)
Test Duration (hours)
Groove Fill Land Deposits (8 fill) (% area) 1 2 3 4 1 2 3
1.8 1.5 480 61 0 1.8 1.5 120 48 23 Polymers with the same shear strength, 9% viscosity loss by DIN 51382.
Dispersant Nondispersant
370
0 17
0 3
0 66
0 98
0 94
In the selection of polymethacrylate VZ improvers, particularly dispersant types, base oil properties and the presence and concentration of other viscosity modifiers must be taken into account. Dispersant polymethacrylates can produce cloudiness in hydrocracked oils at lower temperatures, which fails to disappear after the oil has been heated. These products are not always miscible with polybutenes.
4.5.2 Polymers as Anti-wear Additives Special anti-wear additives are normally incorporated into oils to reduce wear. Conventional anti-wear additives include sulphur- and phosphorus-containing compounds, but polymers are also becoming available (316). As early as 1961, it was found that frictional losses in internal combustion engines were lower with thickened oils than with non-thickened oils of the same viscosity (184,185).These lower frictional losses are accounted for not only by the temporary loss of viscosity at higher shear stress and shear rate, but also by the visco-elastic properties of oils containing polymers (258).This effect finds application in bearings and other components exposed to shock loads. A significant proportion of the stress is absorbed by elastic deformation of the lubricant film and so, as was established by Harnoy (304). The load-carrying capacity of the film is independent of viscosity. On the other hand, in the static stress condition, the viscous properties of the lubricant dominate. This is applicable, for example, to the crankshaft bearings in engines, of which the wear-rate is dependent on the oil viscosity at a given temperature and shear rate, and hence also on the mechanical stability of the polymers (305,317). The anti-wear properties of polymers are affected by the molecular size, concentration and chemical composition of the polymers and the base oil (295).At the same concentration, polymers of lower relative molecular weight are more effective; in the case of polymethacrylates, a polymer of M , 42,000 is more effective than that with 2, 75,000 - 350,000. Wear decreases with increasing polymer concentration. Less wear occurs with SAE 10W/30 multi-grade oils than with unthickened SAE 30 oils; a 10W/30 oil with 6% polymer is better than the same oil with 1.7% polymer and a lOW/50 oil is better than a 10W/30, because it contains more polymer. Polymers which form a tougher structure in oil - the measure of which is the unit chain-length segment - are more effective, according to Kuhn (251). The toughness of the chain is the result of interactions in the oil/polymer system. In mineral oils of the same conventional viscosity classification, such as 10W/50, polymethacrylates are more effective than styrene-diene or ethylene-propylene copolymers. Dispersant polymethacrylates, when used at higher concentrations,.are very effective. Diester oils further increase the effectiveness of polymethacrylates, but further study is required to produce more accurate correlations.
37 1
4.6 POUR-POINT DEPRESSANTS Pour-point depressants are added in order to overcome the effects of residual solid hydrocarbons (waxes, paraffins and ceresines) which have not been separated from the oil in the de-waxing process and so reduce the fluidity limit (true pour-point) of the oil. Pour-point depressants do not prevent the cry stallisation of residual paraffins and ceresines, but do prevent them forming interlocking networks and separating from the oil in the form of felt-like lattice. The mechanism of the pour-point depressant effect can be explained either by the adsorption of a thin film of the pour-point depressant on to the surface of the nascent crystals of paraffin and ceresine (polyalkyl naphthalene acts in this way) or by co-crystallisation with them (as in the case of polymethacrylates, polyacrylates, polyacrylamides, alkene copolymers). This prevents the formation of undesirable, extended structures of needles and platelets (293, 294). Pour-point depressants prevent paraffin crystals from forming networks and ceresine crystals from swelling. This mechanism also explains their lack of effect in cycloalkanic oils (in which the solid phase does not form), their limited effect in high pour-point oils and their marked effect in partly de-waxed oils (containing ceresine microcrystals). Pour-point depressants do not affect cloud-point to a notable degree, so that a difference of more than about 10 "C between cloud-point and pour-point indicates the presence of a pour-point depressant.
The oldest known pour-point depressants are the condensation products of chlorinated paraffins with naphthalene (Paraflow) or phenol (195, 296):
(where R is a long-chain alkyl and R' is hydrogen or a short alkyl). These pour-point depressants were used in the early 1930's. They are now used to a limited extent for decreasing the pour-point of oils, but more often to improve the filtrability of paraffins in solvent de-waxing processes. They enhance the free formation of compact crystals moving in the oil.
Tetra-alkylphenyl and bis-(tetra-alkylphenyl) phthalates are similar, older types of pour-point depressants:
/R
aco0 coo
/R
(R is a long alkyl, examples include Santopour and Xylopour). The most frequently used oil pour-point depressants are the polymethacrylates (Acryloid, Plexol, Viscoplex, Garbacryl, etc.) and polyalkylacrylates (Glissoviscal) of higher alcohols (197, 298):
(where n = 2,000 to 8,000, R is a C, to C,, alkyl, and R’ is H or -CH,). Their advantage is their multi-functional effect, in that they simultaneously act as both pour-point depressant and viscosity index improvers. A number of polymers and copolymers have similar properties, for example polyacrylamides (Z99), vinyl carboxylate and dialkyl fumarate copolymers (200 - 202), polyalkylstyrenes (203), some alkene polymers and copolymers (204) and maleic acid polyesters (3Z6). Pour-point depressants have relatively low mean molecular weights, ranging between 500 and 10,000. More-recently developed pour-point depressants for oil include: - alpha-olefin copolymers (60 - 95 mol % 1-hexene and 5 - 4 mol 8 1-octadecene) (350). - p-alkylbenzylchloride polymers (e.g.,p-dodecylbenzylchloride)or its copolymers with styrene, propylene or 1 -hexene (35Z), - ethylene and vinyl ester copolymers of saturated C, to C, carboxylic acids (353).
The reduction of the pour-point of an oil depends on the composition and properties of the oil and on the pour-point depressant type, its constitution, relative molecular weight and concentration in oil. Pour-point depressants have little or no effect on non-refined oils which contain polyaromatic hydrocarbons and resins; also these compounds have themselves some pour-point depressant effect, they act as antagonists towards synthetic pour-point depressants. In strongly aromatic oils with higher pour-points, depressants have only a slight effect, however the effect is more pronounced in lower viscosity oils. The use of pour-point depressants in very high viscosity oils is totally without effect, since when these oils are cooled, they are immobilised because of the considerable increase in viscosity which occurs and on which the pour-point depressant has no influence; the result is a “viscosity” or “false” pour-point. Generally, pour-point depressants have no effect on engine oils in the viscosity classifications above SAE 30. Different types of pour-point depressants influence pour-point in different ways. However, in all cases, there is an optimum concentration in oil. If this optimum is exceeded, the pour-point increases again. In the region of the optimum, the pourpoint reduction with increasing dosage decreases. The optimum concentration can only be established empirically. Within a polymer group, the pour-point depressant capability depends on the composition of the monomers and their concentration in oil (fig. 4.16). The distribution of molecules by size has more influence than molecular weight; to be
373
efficient, the substances should be as homogeneous as possible and no molecules should be present which are too large or too small.
--- WLYMETHACRYLATE WLYACRYLATE
- 40 I
0.1
0.2
0.3
DEPRESSANT To w t
Fig. 4.16. Effect of the C , , alkyl polyacrylate and polymetacrylate on the pour points of oil
The nature of the side-substituent is also of importance in the cases of polymethacrylates and polyacrylates; n-akyls are currently preferred. These alkyls should be long enough to ensure good solubility of the polymers in oil and their length also affects the efficiency as pour-point depressants (133).If good efficiency is to achieved, the choice of optimum length can be very important. However, the efficiency is still higher if alkyls of different lengths but whose mean carbon number corresponds to the optimum are employed. This factor is important in commercial manufacturing practice. The use of some types of pour-point depressants, particularly the older types, in insufficiently dewaxed or bright-stock-containing oils can cause a considerable increase in pour-point when such oils are stored for an extended period of time at temperatures close to their cloud-points.The same effect may occur if alternate solution and precipitation of paraffins takes place. This phenomenon is termed pourpoint reversal. The pour-point may increase in this way by 20-30 "C. This behaviour has not been completely explained. It has been suggested that heating the oil causes the desorption of the pour-point depressant and that the solid hydrocarbons, becoming exposed, redissolve in oil; when the oil cools down again, the pour-point depressant fails to re-adsorb on to the hydrocarbon crystals. The magnitude of this phenomenon varies, and it depends on the composition and concentration of the solid phase in oil, the concentration and nature of the pour-point depressant and on the pattern of the temperature changes. It may also be provoked by the presence of some detergents, which can form a gel in the oil. It is interesting to note that it has never been observed in multi-grade oils which do not contain brightstock and in which polymetacrylates are used as VI improvers and pour-point depressants. It may be concluded from this that pour-point reversal is connected with the concentration of ceresine, which are mainly present in brightstock, and with the type of pour-point depressant which is used.
The interaction of oil, paraffin, polymer and pour-point depressant leads, at low temperatures, to a gel of differing degrees of robustness. Such gels contribute to an 374
increase in viscosity and plasticity of the lubricant, and so-called gel-viscosityoccurs (306). The gel structure breaks down and the gel viscosity decreases with increasing shear rate D; the structure partially regenerates as D decreases, as a result of pseudoplasticity and thixotropy. Gel viscosity can be calculated from the area delineated by the hysteresis curve representing the change of viscosity with change and extent of the shear stress variation within which the viscosity has been followed. Its share in low temperature viscosity with a given base oil and paraffin composition is strongly affected by the composition of the polymer and the pour-point depressant. Antioxidant and detergent-dispersant additives have a lesser influence (307). Some modem hydrocarbon copolymer V l improvers, particularly ethylene-propylene copolymers, can exhibit problems of gel formation at low temperatures which have in some situations led to engine failures. These have been observed when automobiles have been allowed to stand in severe winter weather conditions and, especially, when the oil in the crankcase has been subjected to repeated temperature cycle below and above about -15 "C. Gelling of the lubricant has occurred to a sufficient extent to seriously impede circulation of the oil by the oil pump through the filter screen and the relatively narrow oil-ways. The use of supplementary electrical power from an auxilliary system (when the automobile's own battery is unable, because of the low temperature) to turn the engine causes the engine to turn virtually unlubricated and damage to the moving components has resulted. In response to this problem, the engine builders have insisted on the inclusion of pumpability tests in the June, 1989 up-date of SAE 5300.
4.7 ANTI-FOAMS Anti-foam additives became important with the extensive use of detergent-dispersant additives, which enhance the formation and stability of foam. The presence of antifoams is therefore highly desirable in engine oils, especially those containing DD additives in high dosages. Foams are particularly liable to be generated in engines with well-cooled crankcase and rocker-box covers; it is well known that lower or higher susceptibility to foaming is associated with the design of the oil circulation systems in engines.
Anti-foam additives are also essential for other oil types used in circulation systems wherever extensive contact with air is involved, such as turbine, hydraulic and automatic transmission oils and fluids. Considerable foaming may also occur in gear trains; anti-foam additives are also indicated for gear-oils. Many factors are involved in oil foaming and, while empirical rules exist, they do not provide a sufficient explanation for this undesirable effect. Foaming may be caused by changes in viscosity and surface tension in oils and hence by changes in oil temperature, and by contamination of the oil by surfaceactive substances (see Chapter 2.4.1 for details).
It is necessary to distinguish between surface foaming and the formation of emulsions of air in oil. Surface foaming can be limited or eliminated by the addition of anti-foam agents. The earliest known were calcium and lithium soaps, lanolin, alkylsulphates,
375
potassium oleate and other compounds, which are no longer in use. Polysiloxanes, mainly polymethylsiloxanes and polyvinylsiloxanes and other compounds of these types can now be regarded as universal anti-foams. They are capable not only of preventing the formation of foam but also of breaking existing foam (205 -210). To be efficient, an anti-foam must be insoluble in oil, have a lower surface tension than the oil and be very finely dispersed in it. In order to be oil-insoluble, the viscosity of silicone oils (polydimethylsiloxane) must exceed 80 mm2.s-*at 40 "C. Their viscosity increases with relative molecular weight. Silicone oils of viscosity 10,000to 15,000 mm2.s-I are generally even more effective. The surface tension of silicone oils in these viscosity ranges is as low as 20.9 to 21.5 mN.m-', as compared with mineral oil at 28 to 35 mN.m-I. In order to achieve maximum effect and stability during prolonged storage of the oil, the size of the dispersed silicone oil particles should not exceed 1 pm. This requirement can be met by first dispersing the silicone oil in a low-boiling aromatic liquid and then adding it to the oil. The dosage level of silicones in oil is important. The practical dosage used is 35 p.p.m. in engine oils, 1-10 p.p.m. in hydraulic oils and 15-20 p.p.m. in automatic transmission fluids. If the polysiloxane dose is too low, the effect achieved may be opposite to that desired, because true solutions may result in which the same foaming tendency increase may occur as is observed with other soluble contaminants. On the other hand, excessive dosage with polysiloxane may result in a slight clouding effect due to the limited solubility in oil of the polysiloxane. The mechanism of the action of these additives is unclear. They may reduce the strength of the surface films separating the gaseous phase from the liquid phase, which causes the gas bubbles to burst. These additives must, therefore, have only limited solubility in oil and a lower surface tension.
Polysiloxanes are effective in acidic, neutral and slightly basic environments, however their effectiveness diminishes sharply in the presenece of strong base. They are highly stable thermally and have a beneficial effect on other oil properties, reducing the saturation vapour pressure and thus the volatility. When oil containing polysiloxane is oxidised, formation of resins and acids decreases (36),the oxidation induction period is extended, varnish formation is reduced and, to a lesser extent, the thermal stability and detergent capability of the oil is enhanced (209). In addition to polysiloxanes, other organo-silicon compounds have an anti-foam effect, for example CH2=CH-SiR,(OC,H,),, where (x+y) = 3. Also available on the market are some alkyl acrylate homo- and copolymers, such as 2-ethylhexyl acrylate copolymers with ethyl acrylate, which are recommended for use in gear oils at 100300 p.p.m.. Other products specified in the patent literature include calcium and potassium oleates, sodium alkylsulphonates, various fluorinated compounds, salts of alkylalkylenedithiophosphates, trialkylmonothiophosphates,esters of sulphated ricinoleic acid and mixtures of polyethylene-glycol ethers with polyethylene sulphides. It seems possible that fluorinated compounds may become of major importance.
376
Anti-foam additives do not suppress the formation of emulsions of air in oil. On the contrary, decreasing foaming at the oil surface may retard the desorption of air from oil, and this factor must be considered in the choice of an anti-foam for a particular system. This is especially true for oils such as hydraulic fluids in which absorbed air may be a problem. For instance, some fluorinated compounds and polyether siloxanes - in contrast to polymethylsiloxanes - when used so as to exert significant anti-foam effect exhibit a substantially lower tendency to prevent the desorption of air from oil (402).
4.8 EMULSIFIERS AND DEMULSIFIERS Emulsifiers (ionic and non-ionic surfactants or tensides) reduce the interfacial surface tension at the water-oil phase boundaries to about 10 mN.m-l and form at these sites thin, continuous, elastic films which prevent the agglomeration of the droplets. A water-emulsifier-oil interface replaces that of water-oil. In order to form a stable emulsion, it is necessary for the continuous (external) phase containing the dissolved emulsifier to envelop completely the dispersed (internal) phase droplets with emulsifier to prevent agglomeration (220, 221). In determining the nature of the emulsion, the decisive factors are the ratio of water to oil and the hydrophiliclipophilic nature of the emulsifier. An excess of water and hydrophilic emulsifiers produce oil-in-water emulsions, whilst excess oil and oleophilic or lipophilic emulsifiers result in water-in-oil emulsions (fig.4.27).
-- - ----_ TYPE WIO
Fig. 4.17. Schematic representation of the “oil in water” and “water in oil” types of emulsions 1 - emulsifier molecule Some hydrophilic emulsifiers must be soluble in oil, so that so-called emulsion oils can be made. These emulsion oils form the concentrate used for the preparation of oil-in-water emulsions. This solubilisation of hydrophilic emulsifiers which are normally insoluble in oil is brought about by the use of auxilliary solvents, e.g., alcohols and glycols.
377
Hydrophilic emulsifiers are used as components of metal-working and rollingmill emulsions and, mostly, of the so-called non-flammable oils. Lipophilic emulsifiers are used as components of oils which are capable of emulsifying water so as to overcome the harmful effects of moisture. Rust inhibitors and some detergent additives have lipophilic emulsifier properties. Therefore, oils which contain additives such as these must be prevented from coming into contact with water, which would otherwise cause emulsification. The size of the particles dispersed in the emulsion depends on the emulsifier ratio. The proportionality of the soap content to the total surface can be determined in emulsions with a very low concentration of soap-type emulsifiers. Further reduction of particle size can be achieved by increasing the emulsifier content from a threshold value; excess emulsifier molecules form colloidal micelles (353).Emulsions can be classified by droplet diameter (in nm) into very coarse (25-lo), coarse (10-1),coarse-colloidal(10.1 I), medium-colloidal (0.1-0.01), fine-colloidal (0.01-0.001) and transparent micro-emulsions (less than 0.001). The coarser the droplets, the more rapidly they settle. By Stokes’ Law, the settling rate of 0.1pm diameter drops at a viscosity of 1 mPa.s is 4.7.10-3c d d a y , and at a viscosity of 1 P a s it is 4.7.10-6c d d a y . In a coarse emulsion with 25 nm diameter droplets, the rates are 294 and 0.294 respectively, in the same units.
In order to evaluate the nature of an emulsifier, the HLB system (hydrophiliclipophilic balance) has been established. Surface-active substances can be classified in this way by the HLB from 0 to 40 (210)(see fig. 4.18). This system is, however, only applicable to non-ionic emulsifiers. The HLB values of surface-active agents can be estimated roughly, e.g., from their composition, the properties of the tenside-water mixture, comparison of the properties of the emulsion and by paper, gas and liquid chromatography. For example, with non-ionic surface active substances, increased content of polyethylene glycol groups and free hydroxyls, higher neutralisation number of the fatty acid and a low ester saponification number increase the HLB value (267). HLB values can also be determined indirectly by titrating with water a mixture of 4% benzene and 96% dioxane with 1% emulsifier until the mixture becomes cloudly. The amount of water added (the water value of the emulsion) is proportional to the HLB value. Each homologous series has an inherent proportionality constant.
Substances of low HLB value are more lipophilic and those of high HLB value more hydrophilic. The nature of any given emulsion of a specified type depends on the hydrophilic-lipophilicbalance of the emulsifier. The most commonly used watersoluble emulsifiers are sodium, potassium and ammonium soaps of higher fatty, naphthenic, alkylarylsulphonic and sulphonaphthenic acids. The most widely used oil-soluble emulsifiers are calcium, barium and magnesium salts of the same acids and phenols. The emulsion type can also be changed by substitution of the cation in the external phase - that in which the emulsifier is dissolved. The two types act as antagonist in their emulsifying effect and admixture can cause the emulsion system to break down. Some solid substances may form oil-in-water emulsions. For instance, hydrophilic SiO, can form oil-in-water emulsions, whilst oleophilic soot forms water-in-oil emulsions. However, no emulsion can form at a certain Si02-to-soot ratio, since the effects of these substances neutralise each other.
378
Emulsifiers may be classified in terms of ionogenicity into ionic and non-ionic types. In ionic emulsifiers, the emulsifier molecule dissociates into a capillarily active anion or cation. In the first case, this is referred to as an anion-active emulsifier and in the second to a cation-active emulsifier. Non-ionic emulsifiers do not dissociate. In anionic or anion-active emulsifiers, the hydrophobic anion tends to pass or effectively passes, during the ionisation process, into water solution in the oil phase, whereas the cation remains in the water phase. Examples of these emulsifiers are the sodium, potassium and ammonium salts of fatty acids (I): CH,-(CH2),-COO- Na'
(K', NH,')
(1)
(n > 16, most frequently stearic or unsaturated oleic acid),
or of naphthenic acids: CH3 I CH
/ \ H2C
CH - (CH2), - COO- Na'
I H2C -CH, I
(K', NH;)
or amine soaps, e.g., di- and triethanolamine salts of fatty acids (111): CH,(CH,),-COO-
[NH(CH2CH,OH),]'
(111)
Sodium and amine salts of sulphated fatty oils or fatty alcohols and alkyl- and alkylarylsulphonic acids are also very widely used. The common deficiency of sodium and ammonium soaps of fatty acids is their tendency to form soaps which are insoluble in water with calcium and magnesium ions. The solid soaps can precipitate to form scums and deposits which coat machine surfaces, eventually causing abrasive wear, and block filter screens and oil-ways. This deficiency can be limited or even eliminated by adding complexing agents, such as ethylenediaminotetracetic (EDTA) salts (247). On the other hand, naphthenic acid soaps are almost insensitive to electrolytes, but promote oil foaming. The difference between the sodium and ammonium soaps of fatty acids used as emulsifiers consists in the stability of the emulsions formed towards pH effects; the sodium soaps form more stable emulsions at higher pH than the ammonium soaps. The effectiveness of sulphonates is influenced by ions which make water hard to a much lesser extent than is the case with fatty acid soaps. These drawbacks can be practically eliminated by the use of a suitable combination with non-ionic emulsifiers, of which a further advantage is the protection of ferrous surfaces against corrosion.
Cation-active or cationic emulsifiers contain hydrophobic cations which tend to pass into the oil phase during the ionisation process. Examples of such substances include quaternary ammonium, pyridinium and similar salts containi'ng a long alkyl in the cation residue [(CH,)3N-(CH,),-CH,]+ Hal-@ > 11) (N) , the so-called
379
inverse soaps. Examples include cetylpyridinium bromide and chloride (V), cetyltrimethylammonium bromide or chloride and some nitrogen derivatives of fatty acids (e.g., imidazolines):
Quaternary salts form stable emulsions in the neutral or acid regions. In non-ionic emulsifiers, the hydrophobic and hydrophilic moieties are mutually bound in a polar, undissociated molecule. Examples include ethoxylated substances with active hydrogens: RXf CH,CH2-0 % H (VI), where R is a long alkyl or alkaryl, X is -0-, -COO-, -CONH-, etc., and n > 6, (for example ethoxylated higher fatty alcohols, alkylphenols, sugars, amines, acids and their amides). Whereas the long polyethylene glycol residue is hydrophilic, the similar polypropylene glycol chain is hydrophobic. Use of non-ionic emulsifiers has substantially increased. Their advantage is their ability to form stable and non-corrosive emulsions at pH close to or higher than 7 even at a low concentration in oil (up to 3%); they are less sensitive to water hardness, only mildly irritant to the skin and not as readily susceptible to bacterial decomposition as other types of emulsifiers. Because of these qualities, their increasing use is unsurprising. A summary of commercially available emulsifiers is given in Table 4.34. These emulsifiers differ in their solubility in organic substances - propylene glycol, isopropanol, perchlorethylene, xylene and the others. The presence of such substances may therefore affect their emulsifying capablities.
Table 4.34. Some Qpes of Hydrophilic and Oleophilic Non-ionic Emulsifiers Hydrophilic emulsifiers
HLB value
Oleophilic emulsifiers
polyoxyethylene esters of stearic acid
16.9 - 17.9
mono- and diacyl glycerides of comestible fats*
2.8 - 3.5
polyoxyethylene ethers
14.5 - 15.4
sorbitol and fatty acid esters (lauric, palmitic, stearic, oleic)
1.8 - 8.6
polyoxyethylene esters of acylglycerols
1 1 - 18.1
polyoxyethylene -alkaryl esterst
13 - 13.3
polyoxyethylene esters of sorbitol and oleic acid 9.2 - 10.2
Insoluble in water at higher temperatures (75 "C). t Insoluble in mineral oil at normal temperatures (25 "C).
380
HLB value
Demulsifiers Unwanted emulsions may be produced when oil comes into contact with water, for instance in manufacturing (e.g., refining with acid or alkali) or in applications (e.g., turbine oils). These emulsions have to be broken and demulsifiers may be used for this purpose. Both emulsifiers and demulsifiers are surface-active substances. The difference between them resides in their constitution, in the length and configuration of their alkyl substituents. In emulsifiers these tend to be long and straight-chain, in demulsifiers shorter and often branched. In this respect, they are similar to wetting agents. Emulsifiers form a relatively tough and strong film on the phase boundary of the droplets, whilst demulsifiers destroy this film and replace with a thin film which does not impede the coalescence of the droplets. Typical examples are anionic alkali metal and ammonium salts of alkarylsulphonic acids, e.g., didocylbenzene- or dinonylnaphthalenesulphonatesfor oil-in-water emulsions and the calcium analogues for water-in-oil emulsions. Antimony salts have also been reported. Lead naphthenates act as anti-emulsion agents. Ampholytic reaction products of polyalkyleneamines and dicarboxlic acids (415) or of polyisocyanates with polyamines and sulphuric acid (416) have been proposed. A11 these products must be used at very low concentrations. A particular problem arises in engine oils which contain detergent-dispersant additives, especially those in which high dosages of ashless dispersants, e.g., succinimide, are used. The dispersants can act as effective non-ionic emulsifiers and in those parts of the engine in which water is present in liquid form can give rise to water-in-oil emulsions, which resemble butter in appearance, frequently referred to as emulsion sludge or cold sludges. The problem can be aggravated by certain types of engine design, in which blow-by gases from the combustion chamber (gas, containing water resulting from fuel oxidation, which passes the sealing piston-rings in the cylinder bores) are vented via cooler parts of the engine, such as the rocker-box, in order to minimise oil-losses to atmosphere. This problem can be largely overcome by engine designs in which the blow-by gases are recirculated into the fuel-air intake system, but it can be alleviated by incorporation of demulsifiers into the engine oil.
4.9 EXTREME PRESSURE, ANTI-SEIZURE, LUBRICITY AND ANTI-WEAR ADDITIVES Extreme pressure additives include compounds which react chemically with the material of the friction pair, so that new compounds are formed which create a lubricating film firmly bound to the surface and resistant to rupture. This film increases the effectiveness of the lubricant and prevents direct metal-to-metal contact, welding and seizure of the friction surfaces. The objective is not necessarily the reduction of the friction coefficient. Lubricity or “oiliness” additives are compounds which, by physical adsorption and chemisorption of their polar molecules on to the surfaces of the friction pair form boundary films of higher load-carrying capacity and lower friction coefficients than the unfortified oil.
381
Extreme pressure additives often act as anti-seizure agents, at high surface temperatures. A sudden increase in load causing rupture of the lubricant film produces momentary temperature flashes and the surface becomes very hot locally. Under such conditions, the additive decomposes and forms a reactive, predominantly inorganic layer (especially chloride, sulphide, phosphide, phosphite or low-melting eutectics). This layer is resistant to micro-welding and wear is arrested. Simultaneously,the temperature flashes are suppressed, the hot-spot temperature falls below the decomposition point of the additives and reactive layer-formation ceases. The process is repeated if the layer is broken. This self-controlling mechanism is only set in operation when the local temperature rises above the decomposition temperature of the additive. Lubricity additives, which are mostly organic compounds, are affective only up to the temperatures at which they desorb from the surfaces. These temperatures are lower in the case of physisorption and higher for chemisorption; as a general rule, they do not exceed 150 "C. The transition between the two types of additive may be continuous if chemisorption is translated into chemical reaction as temperature rises, so that anti-wear action becomes anti-seizure protection.
Both groups of additive contribute to the reduction of galling and abrasion and, consequently, to the reduction of wear of the friction surfaces in shearing and rolling motion, particularly under mixed friction, extreme pressure and shear.
Fig. 4.18. Illustration of dependence of friction coefficients on the temperature
Fig.4.18 (356)illustrates the variation of friction coefficient p with temperature. The behaviour of the base oil is represented by curve I. The inevitably weak bonds between the non-polar oil and the surface weaken with increasing temperature and the friction coefficient increases. The friction coefficient remains low until the softening point (T,) of, e.g., the soap produced from the additive, e.g., a fatty acid, is reached (curve II). The extreme pressure additive (curve 111) strongly affects the friction coefficient after a reaction temperature (T,) is exceeded; at this point, the extreme pressure additive starts to react with the surface. A hypothetical combination of lubricity and extreme pressure additives should behave according to curve IV. 382
Fig.4.29 (359) illustrates the behaviour of additives in terms of wear in relation to normal load FN expressed as the Archard coefficient K (m3.N-’.m“). Dry friction causes slight abrasion up to a critical point FNcrit. After this point has been passed, abrasion increases drastically and also affects sub-surface layers (curve a). With additive-free oil (curve b), abrasion reduces to lower values down to a certain loadpoint. If an anti-wear additive is added, the coefficient of abrasion reduces by AK‘ (curve c). The extreme pressure additive allows the load to be increased by a value AFN (curve d).
I
I
I
FN
Fig. 4.19. Illustration of the correlation between the behaviour of friction modifiers, extreme-pressure additives and the normal load There is no unified nomenclature in the technical literature for the description and classification of these additives. Soviet literature uses the terms “prisadki antifrikcionnyje” (anti-friction additives), “protivoiznosnyje” (extreme pressure, anti-seizure). The terms oiliness agents, friction modifiers, film strength agents, anti-wear agents, extreme pressure (EP) agents, anti-seizure agents and anti-welding agents can all be encountered. One reason for this is that the substances employed for any particular purpose often act in a number of ways; however, some are more effective as lubricant film strength improvers, some as friction modifiers, and so on.
4.9.1 Extreme Pressure (EP) Additives EP additives are compounds with one or more reactive sites in the molecule which contain atoms or group which, when certain temperatures are reached, react with the metal surfaces of the friction pair. Since these active groups principally contain -C1, -S-, Pc and/or =Pc, reaction with the metal surface produces chloride, sulphide, phosphide or phosphate layers, which prevent the metal surfaces from welding or seizing. The local temperature, which accelerates the reaction between the metal surface and the active atoms of the EP additive, increases with increasing load on the friction surfaces by increasing the surface pressure. The temperature of the friction surfaces drops as soon as a sufficiently thick layer of reaction products is formed. At this point, further decomposition is suspended as long as the protective layer is able to prevent the temperature increase or local over-heating, unless rupture 383
of the protective layer due to further pressure rises, etc. occurs. The surface decomposition temperatures of EP additives differ; for example, those of chlorinecontaining additives are about 300 OC whilst those of sulphur-containing additives are about 600 to 800 “C. The combination of different EP additives with different active elements together with lubricity additives enables reduced wear of the friction surfaces over a wide pressure and temperature range to be achieved (213). Substances capable of increasing the strength of lubricant films or establishing a firm lubricating layer between the surfaces of the friction pair can be used as additives for lubricants exposed to boundary or mixed friction. In this situation, lubricants without such additives can expose the surfaces to severe wear, extensive development of frictional heat and surface damage. This can result in short operating life, impaired functioning and possible breakdown of the machine parts. Typical locations of such problems include the surfaces of gear teeth, so these types of additives are above all found in gear oils. However, they are also found in other oils, for example hydraulic fluids, oils used for lubricating pneumatic tools and in oils used for metal-working, such as cutting, drawing and rolling-mill oils. Many types of compounds have been used or recommended for use as EP additives for liquid lubricants and greases, including compounds of sulphur, phosphorus, chlorine, bromine, fluorine, iodine, boron, lead and many others. Iodine-based aromatic complexes are suitable for lubricating “un-lubricatable” metals such as titanium and some stainless steels. Titanium-to-titanium pairs seize and stainless steel pairs are hard to lubricate. A constantly regenerating layer of metal di-iodides (Ti12, FeI,) with a lamellar structure is supposed to be formed. The iodine-organic complex can be used alone or in oil solution (214-216).
Not all compounds with these active elements are suitable for lubricant additive use. Factors to be considered include, not only anti-wear activity, but other properties, such as solubility, volatility, thermal and oxidation stability, miscibility with the base oil involved, compatibility with other additives (synergism and antagonism occur), reactivity with water, effects on different metals and packing and sealing materials, activity over a prescribed temperature range, physiological activity - and, of course, price. The thermal stability of the additive is particularly important for gear oils. The decomposition temperature must be high enough to prevent the additive decomposing at the bulk oil temperature in the gear system and the decomposition products must not be corrosive to the metal surfaces at these temperatures. On the other hand, the rate of decomposition and reaction with the metal must be high enough to form a chemical layer immediately with increasing load on the bare surface under conditions of boundary friction between the surfaces of the teeth. Commercial products in use contain one or more active elements or comprise combinations of compounds containing various active elements. These compounds are usually more effective and their action more complex because particular active elements become active at different temperatures and thus complement one another over the whole operating temperature range. 3 84
Since the phosphorus-, chlorine- and sulphur-based compounds used do not react until higher temperatures are reached, they may be combined advantageously with additives which react at lower temperatures (up to about 180 "C), such as fatty acids or lead soaps. The temperature range of the package additive can be extended in this way and friction coefficients reduced; synergistic phenomena identifiable by radiotracer techniques - can be utilised to the maximum extent. The compounds used in such combinations also complement each other in respect of shear strength and reduction of friction coefficient. Shear strength increases in the order P
CI>S. This sequence is, however, also dependent on temperature.
In some cases, the effective compound cannot be formed until the active elements in the mixture of compounds reaches certain temperature and pressure conditions. For instance, chlorine- and sulphur-based additives are chemically more active. However, metal chloride films with a melting-point of about 680 O C , suitable for lubricating steel surfaces at temperatures over 800 "C, are formed more rapidly in combination with sulphur compounds as well as chlorine (227). Combinations of lead soaps and sulphur compounds form active lead sulphide under high pressure, which is sufficiently plastic under pressure at 700 'C and which has, under these conditions, a low shear strength. In the absence of lead, sulphur compounds on their own produce iron sulphide, which melts at around 1180 "C.
It is very important to match the reactivity of the additive to the application. The requirements of gear oils - when the contact time is longer and the compound must be less aggressive - are very different in this respect from those of metalworking fluids, where the contact time can be very short and reactivity must be higher. Among gear and metal-working operations themselves, there are marked differences in the operating environment. In general, EP additives should only be reactive enough to ensure that chemical wear (corrosion) does not exceed mechanical wear. With respect to chemical composition, EP additives currently available may be classified as follows: - compounds containing one active element, such as chlorine or other halogens, phosphorus, sulphur, selenium, tellurium, lead or boron; - compounds containing two or more active elements, such as chlorine and phosphorus, chlorine, phosphorus and sulphur, or sulphur, chlorine, lead or molybdenum. The effect of chlorine-based compounds depends on the lability of chlorine in the compound and on the degree of chlorination (201). A certain lability of the -C-C1 bond is essential for the effectiveness of the additives. Thus, chlorinated paraffins containing about 40% chlorine in aliphatic bonds have found wider application than, for example, chlorinated naphthalenes with relatively strong aromatic bonds (disregarding the higher animal toxicity of the chlorinated naphthalenes). The -CCl, group is highly effective in this type of EP additive. According to Daney (220) compounds containing phosphorus and -CCl, groups, like tris(trichlorethy1)-phosphate or tris-(trichloro-ten-butyl)phosphate exhibit excellent ability to reduce friction under high loads. In addition to these compounds, many chlorine-containing substances have been proposed and patented, such as trichloroni tropropanol, chlorodibenzyldisulphide,dichlorodiphenyltrichloro-ethaneand
385
chlorinated kerosene, as being suitable for use as EP additives, but not all have found any wide application. The human toxicity of many of them is an important factor. Chlorinated substances become effective on ferrous surfaces at temperatures around 200 "C and above, giving rise to active surface layers of iron chlorides.These substances are virtually without effect under low loads (365).Their effectiveness increases with increasing load, probably due to tribological activation of the surface. The additive itself undergoes chemical change during its action, for example, chloroform is converted into hexachlorethane (220).
Chlorine-based compounds are still used in exceptional cases as EP additives, mainly in cutting and metal-working oils, for examples for machining titanium and its alloys. They are less-used in gear-oils, because of their relatively high susceptibility to hydrolysis and low thermal stability, together with their tendency to from inactive decomposition products and corrosive hydrogen chlorides in the presence of water. Classic chlorinated paraffins have been recommended, e.g., in cutting oils, with 300-400TBN Ca (Mg) sulphonate in combination with ZDDP or other S-P compounds and natural or synthetic esters. However, the use of chlorinebased additives has declined on ecological and health grounds, because burning of chlorparaffins in the presence of aromatic oils can give rise to potentially carcinogenic and mutagenic compounds, including the formation of toxic furans and dioxins. Methods for the determination of chlorine in oils and additives depend on burning the sample and determining the hydrogen chloride produced gravimetrically or by titration. According to CSN 65 6234, combustion products are absorbed in a 3% H202 solution and HCI is determined either by titration with mercury (11) nitrate or gravimetrically as AgCI. ASTM D-808-87 and DIN 51-577 Method A specify methods for the determining chlorine in oils, additives and greases involving oxidation of the sample in a compressed oxygen bomb. The hydrogen chloride evolved is absorbed in Na2C0, solution and measured gravimetrically as AgCI. DIN 51-577 involves burning the sample according to the GroteKreckeler method in a stream of air and absorbing the combustion products in 10% and 5% ammonia solution. Chloride ion is determined gravimetrically or volumetrically (the method cannot be used in the presence of other halogens, with the exception of fluorine). Chlorine in oils, greases and additives can be determined in the absence of other halogens by ASTM D-1317-89 and IP 118;the sample is dissolved in low-boiling hydrocarbons and refluxed with sodium metal and n-butanol. The sodium chloride produced is determined by titration with AgNO,.
Other compounds containing halogens have been tested alone or in mixtures with sulphur compounds (222). Iodine-, bromine- and fluorine-based compounds are very effective but rather expensive. They are, therefore, only normally considered for special oils; for example, fluorine compounds have been used in special silicone gear oils. Phosphorus-based compounds used are mostly alkyl and aryl phosphites and phosphates. Phosphites are more effective than phosphates and the aliphatic esters are better than aromatic esters (220). According to other authors (222,223), tertiary phosphites are weak EP additives and the effects of alkyl phosphites decreases with the size of the alkyls. Phosphorus compounds are preferred as EP additives to chlorine- and sulphur-based compounds at low velocities. In order to increases adhesion of phosphorus compounds to metal surfaces, other active groups such as hydroxyl or chlorine and at least one aryl or alkyl group must be incorporated.
386
Compounds containing phosphorus alone are not used much except in cases of low torqueAow speed applications. The most commonly encountered additives contain phosphorus in combination with other active elements. Some triaryl phosphates, such as triphenyl, tricresyl and trixylyl phosphates and mixtures are marketed. It is not entirely clear how triaryl phosphates function. At one time, it was supposed that they formed metal phosphides, analogous to sulphides and chlorides, of lower melting-points and shear strengths to those of the metals comprising the friction surfaces (220,224). Later, it was suggested that they form metal phosphates on the friction pair surfaces and that the EP effect of triaryl phosphates was connected with their tendency to hydrolysis (225). Thus, the presence of minor amounts of water and oxygen is necessary to bring about hydrolysis, eventually to phosphoric acid. The function of the neutral phosphates is to provide solubility in oiI(336).
Although triaryl phosphates are not as effective as many sulphur and chlorine compounds, they are still used, because they do not corrode metals and they substantially reduce wear in machines operated for along time under low load. They are also effective anti-corrosion agents and suitable as anti-wear agents in all types of circulating oils in greases, gas-turbine synthetic oils, compressor oils and metalworking fluids. In the last example, the polishing effect of triaryl phosphates is used; they also provide a smooth surface no machined materials which does not tend to rust. The concentration of triaryl phosphates used varies between 0.5 and 2% depending on the type of lubricant. Tricresyl and trixylyl phosphates are also recommeded as rubber swelling agents. They are incorporated into alkanic oils which come into contact with packings and seals and would otherwise cause shrinkage of the rubber and consequent leakage. The phosphate concentration in this application is about 2% by weight. Liquid trialkyl phosphates are also used as non-flammable components in aircraft hydraulic fluids, ashless hydraulic oils and as liquid components for special lubricating greases.
Sulphur-basedcompounds are much more widely used, as EP additives in metalworking fluids and gear and other oils. The most widely-used substances are the disulphides, e.g., dibenzyl disulphide, butylphenyl disulphide, polysulphides, sulphurised sperm oil (now almost totally replaced for conservation reasons, e.g., by jojoba oil), sulphurised terpenes and alkene polymers, alkylxanthates, dialkyldithiocarbamates and others. The compounds usually contain a reactive sulphur, able to form iron sulphides on iron and steel. These sulphides form a film of their own which withstands high loads, although the friction coefficient, which is higher than that of a chloride or a phosphate film (2Z8), remains comparatively high (at around 0.5) and should be complemented by another film, such as a chloride film. Tests of abrasion particles collected from iron surfaces have shown that they contain more iron oxide than iron sulphide, although the sulphides are, in fact, the protective agents (367). The more effective the additive, the more sulphide is formed relative to oxide. For example, 1 1 to 14% S and 15% 0 is formed from diphenyl disulphide and 8 to 18% S and 8% 0 from di-tert-butyl disulphide. This is probably connected with the strength of the -C-S bond, which decreases in disulphides in the order diphenybdisec-butybdi-tert-butybdibenzyl, whereas the effectiveness decreases in the same order (368). Mercaptans and sulphides can provide an effective protective layer for lighter loads.
387
Sulphur-based compounds are regarded as more effective EP additives for highspeed applications than chlorine- or phosphorus-containing compounds (226). This can be explained in terms of the autocatalytic reaction between sulphur and iron and by the dependence of this reaction on temperature. Some authors have asserted that sulphides and disulphides are effective additives and mercaptans ineffective (227). Other claim that disulphides and polysulphides are effective, whilst monosulphides fail to act as EP additives. These controversies could reflect differences in the conditions of test used for different compounds, as well as the type of sulphur compound. Selenium- and tellurium-based compounds act in the same way as sulphur compounds. They are, however, expensive and toxic and therefore not easily used. Like sulphur compounds, some of them have anti-oxidant properties and also act as anti-rads.
Combined chlorine-phosphorus based compounds are very little used. Di-(2-chlorethyl)vinyl phosphonate (228), the bis-(p-chlorphenyl) ester of phosphorous acid, trichloromethanechlorophosphorousacid and their salts have been proposed as additives for gear oils.
Chlorine-sulphur bused compounds and mixtures of them can be effective EP additives, chiefly in gear oils intended for heavily-loaded hypoid gears and cutting fluids. In this combination, one active element complements the other over the range of operating conditions. The metal chlorides form more rapidly in the presence of sulphur and the “anti-seizure’, film comprises either a mixed layer of iron chlorides and sulphides or a complex of both. Substances containing these two active elements with useful EP properties include chlorobenzyl disulphide, sulphochlorinated sperm oil and its substitutes, pinene and mineral oil, and chlorobenzylalkyl xanthogenates. Phosphorus-sulphur compounds are at present the most widely used EP additives, particularly for automotive gear oils, as well as for industrial gear oil, hydraulic oils and oils used for moderate severity metal-working applications. They are, however, also suitable for other applications. Automotive gear oils now incorporate S / P compounds as their only EP additives. They meet the requirements imposed by increasing speed and load of the gear trains of modem automobiles, as well as at the high speeds of passenger cars and the high torques of trucks (230,231).They also possess other virtues; they are not corrosive in the presence of water, indeed they perform as corrosion inhibitors, they improve the thermooxidation stability of oils, suppress - in the presence of suitable friction modifiers - vibration in limited-slip differentials and they do not adversely affect packing and sealing materials, in contrast to chlorinated additives (232). Sulphurphosphorus based EP additives, having good oxidation stability and quite good thermal stability and demulsifier properties, are employed in industrial oils, where their lubricant film load-carrying capacity matches that of conventional EP additives of the sulphurised sperm oiVlead naphthenate type. They are virtually the only antiwear and antioxidant additive in hydraulic oils. Sulphur-phosphorus based EP additives used now are mainly of two types: metal dialkyldithiophosphates (mainly zinc and/or antimony and tin - especially for compounds containing shorter alkyls) and similar compounds of molybdenum or
388
tungsten, such as:
(Molyvan L manufactured by R. T. Vanderbilt Co.) and polyalkenes with a bridge containing sulphur and phosphorus, e.g., products prepared from polybutene and P2S5:
H3C - C?H3 I
CH3
_...qn CH3
s
s
CH3
II
I
YH3 II ‘s s‘P-CH=C-CH, CHZ-C=CH-P’ ’
I
C-CH,
C-CH,
CH3
CH3
I
The ratio of sulphur to phosphorus differs among these additives, but the sulphur content is usually many times larger. Chlorine-phosphorus-sulphur based compounds and their mixtures are highly efficient EP additives, suitable for lubricating heavily-loaded gears and oils for the more demanding metal-working operations. Compounds made from all three active elements include, for example, condensation products of chlorinated kerosene, chlorinated paraffins and the salt of dialkyldithio-phosphoric acid containg 33.7% C1,0.99% P and 2.2% S (233). Lead soaps have been effectively used as EP additives for many years and include the first additives to oils used for lubricating hypoid gears. They are characterised by the ability to suppress wear in gears where both sliding and rolling action occurs (e.g., hypoid and worm gears) and behave as steel corrosion inhibitors in the presence of water (acting as de-watering agents). In the presence of a compound containing active sulphur, they form sulphurised lead soaps; these form anti-seizure films effective at high temperatures and extend the range of phosphorus-chlorinesulphur additives from 90 to 180 “C. The most commonly-used soaps are lead naphthenates, since they are cheap and readily soluble in oils, particularly cycloalkanic and dark oils, less so in high VZ oils. Commercial concentrates of lead naphthenates in oil contain 20-30% lead. Lead oleates, 12-hydroxystearatesand similar compounds are also used, but some authors claim that lead naphthenates are better EP additives than other lead soaps, especially in the presence of water, because they suppress the formation of oil-inwater emulsions (234). However, they impair the oxidation stability of oils. Lead soaps are not usually used as sole EP additives in gear oils, but are complemented by other additives containing active sulphur or phosphorus. EP additives comprising Pb soaps and sulphur and chlorine-based compounds are recommended as special additives for oils intended for lubricating hypoid axles in trucks operating under high torque/low speed conditions. The relative merits and 389
demerits of lead soaps mixed with sulphurised fatty oils and sulphur- and phosphorus compounds are illustrated in Table 4.35. Table 4.35. Comparison of Gear Oils to MIL-L-2105B Specification with Various EP Additives EP additive type:
Lead naphthenatd sulphurised fatty oil
EP properties: 4-ball tests: wear index 59-74 3188-5395 weld load (N) Timken: OK load (N) 333 Falex: No. of teeth (fewer teeth, less wear) 21
SdphW-phoSphomsbased additive
50-80 2452-3924 333 10
Thermal stability (50 h at 162.8 "C): Viscosity increase (%) Insolubles content (%) Benzene-insolubles (%)
220 9.5 3.5
9.7 0.08 0.05
Corrosion stability (ASTM 130-56): 6 h at 98.9 "C 3 h a t 121.1 "C
Ib 2c
lb 2c
Foaming test (ASTM 982-46) Sequence I - tendency - stability Sequence I1 - tendency - stability Sequence I11 - tendency - stability
10 0 50 0 0 0
10 0 10 0 0 0
Demulsification (4001111 water, 200g oil, stirred for 10 min. at 51.7 "C) - quantity of water separated (cm3) after: 10 min. 200 315 Ih 220 360 24 h 245 385 Lead content in oils and additives can be determined by CSN 65 237 and IP 120; copper and iron can be measured at the same time. The sample is fully oxidised with H,SO, + HNO, + H,O,, and the filtered PbSO, is boiled with Na,C03 solution in order to separate other sulphates. Pb is precipitated from the solution with H,S, PbS is oxidised to sulphate and measured gravimetrically. Iron is separated from the filtrate as hydroxide and determined colorimetrically with thioglycollic acid or o-phenanthroline; copper is also determined colorimetrically as yellow diethyldithiocarbamate. In lubricating greases, lead is determined by ASTM D-1262-81: the sample is oxidised with the same mixture of sulphuric and nitric acids and hydrogen peroxide. The solution is mixed with dilute sulphuric acid and alkali acetate, the precipitate filtered and boiled with aqueous ammonia. After acidification with nitric acid, the lead is determined by electrolysis as lead peroxide. These elements are now more conveniently measured by atomic emission spectroscopy (ASTM D4951-89).
In addition to the compounds containing the main active elements typical of EP additives, other compounds are also used, such as organic borates (chlorophenyl390
boric acid dibutyl ester) which, like ZDDP, are also effective antioxidants, and various complexes containing halogenides of two different metals, such as silver, copper, tin, manganese and cobalt, and arsenates or antimonates (235). EP additives may also incorporate other compounds such as tartaric acid, boric acid, lecitin, alkylsuccinic acids, nitro-compounds, epoxidised aliphatics, phenylisocyanates and many more which are, by themselves, able to increase the strength of the lubricating film and also bring about other beneficial effects. Polymers, such as fluorinated polymers, polyamides, polyvinyl chloride, high density polyethylene and polyisobutenes, can be used as EP additives in some special cases, for example in lubricating greases. They not only increase the strength of the lubricating film but they also increase the adhesion and resistance to washout with water of the lubricants and increase their drop-points (236). The loadcarrying and antiwear capacities of both liquid and plastic lubricants can be increased by combining oil-soluble EP additives with dispersions of solid lubricants, particularly graphite, MoS2 and poly-p-phenylene (see below) at a concentration of r 1 5-2596 (4Z4).
However, the effect of particular solid lubricants varies in this respect and differs with concentration and composition of the oil-soluble additive, i.e., with the different type and concentration of the active elements. It is not manifest in combinations with EP additives containing active sulphur and chlorine (237), and the operating conditions must be taken into account in applying these additives. They cannot be employed in oils which must be clear and bright, as the addition of even a small amount of graphite or MoS2 makes the oil dark; the possibilities must always be taken into account of the partial separation of the solid lubricant from the oil when it is undisturbed or partially caught on filters. Possible synergism and antagonism must be allowed for when mixtures of additives of different effects are used. For instance, it is well known that some detergent and dispersant additives neutralise the effect of extreme pressure additives, when the adhesion of the detergent or dispersant to the metal surface is stronger than that of the EP additive. Such interactions must be monitored experimentally.
4.9.2 Lubricity Additives (Friction Modifiers) These mostly comprise compounds with long, straight hydrocarbon chains (about 10 carbons or more) - which provide oil-solubility - and an end polar group which has sufficient adsorption and/or chemisorption capacity towards the friction surface, and which reduces the friction coefficient of the lubricant. The mildest additives of this type are C,, - C,, alcohols; long-chain carboxylic and hydroxycarboxylic acids and their esters, higher amides and imides, phosphites, phosphates, phosphonates, the important dithiophosphates (Mo), thiocarbamates and dithiocarbamates (Sb) (2Z9), and other derivatives, including those of boron. Sperm oil used to be a
39 1
frequent component in lubricating oil additives, but its scarcity and conservation pressures stimulated the development of substitutes, e.g., esters prepared by the selective hydrogenation of soya bean and linseed oils, the hydrogenolysis of the resulting fatty acids to the fatty alcohols and esterification of the acids and alcohols obtained to long-chain esters with unsaturated bonds in the acid group (296). The oil derived from the seed of the jojoba is very similar in composition to sperm oil. Some authors mention a beneficial effect of cholesterol type liquid crystals on the lubricity of oils (394). Each of the compounds mentioned exerts a specific influence on the change in static or dynamic friction coefficient. In a package additive, the so-called friction modifier is able to change the friction profile of the lubricant - the curve in which is plotted the change in friction coefficient from nil shear rate (static friction) to a certain value of shear rate (kinetic or dynamic friction) - and to suppress the “stickslip” phenomenon at low shear rates, which can be achieved by careful blending. Modern transmission systems, such as automatic transmissions, limited slip axles, wet brakes and many slide-ways (e.g., in machine tools) require a lubricant with specific properties to secure a specific relationship between friction coefficient and shear rate. This relationship - the ratio between static and dynamic friction coefficients -can vary even in mechanisms which are apparently identical but with small design differences; friction coefficients are affected by a number of factors such as pressure, shear rate, type of materials and surface finish in the friction couple, temperature and products of reaction of the components of the lubricant (238,239,246).The wrong ratio may adversely affect the performance and operation of the mechanism. A high static coefficient combined with a low dynamic coefficient may cause difficulty in gear movement and a distinct squawking noise in automatic transmissions; on the other hand, too low a static coefficient may cause sliding and delayed gear engagement, and inadequate transmission of power. Limited-slip axles require oils which combine EP additives and additives which provide a sufficiently high static friction coefficient to prevent the gears from vibrating. It is not easy to meet these conflicting requirements, especially since antagonistic effects between sulphur- and phosphorus-based EP additives and some friction modifiers have been observed (258,259).The wrong relationship between friction coefficient and shear rate can cause stick-slip motion in slide-ways and adversely affect machining accuracy in machine tools.
SLIDING VELOCITY
Fig. 4.20. Effect of different types of lubricating additives on the change in static and dynamic friction coefficients of the base stock A - additive-free, B - with 1 % glycolate, C - with 1 % oleic acid, D - with 1 % dioleylphosphite
392
Neither mineral nor synthetic base oils provide the right friction profile for many such applications and friction modifier additives must be added in the right combination to achieve the correct ratio between static and dynamic friction coefficients. Fig. 4.20 illustrates how particular additives affect the ratio of these coefficients in different ways. Some compounds which are suitable for use as lubricity additives and components of package friction modifiers are used for special purposes. For example, aminodithiophosphates are used as vibration suppressors in oils designed for lubricating limited-slip axles (24Z). and N-acylsarcosine derivatives (24Z), sulphonated fatty acids and their esters (242, combinations of organo-phosphoric and fatty acids (243), esters of dimerised fatty acids (244) etc. in oils to suppress squawking in automatic transmissions. Lubricity additives are also important in automotive gear and engine oils. Decreasing friction coefficient decreases the resistance to motion between the friction surfaces and reduces fuel consumption. Ash-containing friction modifiers are also used including dispersions of MoS2 or organo-metallic compounds forming active sulphides in situ, as are ashless compounds containing active elements or groups, which do not affect the colour of the oil and do not deposit dark coatings on machine parts, such as primary akylamines of the laurylamine long-chain type (381). other oil-soluble amines, esters of long-chain carboxylic and hydroxycarboxylic acids, oxazolines, imidazolines and, especially, phosphorus and boron derivatives (393).
4.10 MISCELLANEOUS ADDITIVES Special additives are encountered which are designed for very well-defined duties, or which are only used for some specially-designed lubricants. Disinfectants are used to prevent the growth of bacteria and moulds in oil emulsions used for machining processes and in non-flammable hydraulic emulsions. The oil-in-water emulsions used for metal-working repeatedly and rapidly undergo undesirable changes, including decomposition, pH decrease, increased corrosivity, oil separation, colour change, generation of offensive odours and increased biological activity towards the human skin. These phenomena may be caused by microorganisms, such as pseudomonas oleovorans, p . aeruginosa, escherichia coli, micrococcus aureus, candida albicans and aspergillus niger, which often occur in the water used to make the emulsion but may also enter the emulsion by contamination during operation.
Appropriate disinfectants (used at dosages up to about 0.2% by weight) include a variety of phenols, chlorinated compounds, organic nitrogen-bases (e.g., alkylated hexahydrotriazines), biocidally-active zinc dialkyldithiophosphates and atkylphenyltributylstannates (279).The most effective treatments for metal-working emulsions and coolants are claimed to be condensed aldehydes and heterocyclics
393
containing sulphur, nitrogen or both (354). Preferred phenols, on grounds of economy, toxicity and environmental impact are phenol, cresols and xylenols, preferably containing C, - C, alkyls. Polychlorinated phenols are banned on ecological grounds. The mechanism of the antimicrobial effect of phenols consists in two reactions of phenols within the cell of the organism: halting the oxidation-phosphorylationprocess and coagulation of the cell protein at higher concentrations. The advantage of phenols is the rapid destruction of bacteria; their disadvantage is their ready solubility in water (in systems operating at a very high water content they must be used as their alkali metal salts), their loss of activity in the presence of non-ionic emulsifiers and the reaction of some phenols (those unsubstituted in the ortho- and para-positions) with formaldehyde, resulting in inactive products. Objections from hygienists are expected.
The most effective aldehyde is formaldehyde as formalin, a 30-40% solution in water. However, because of the problems of using free formaldehyde (volatility, corrosivity of its oxidation products and more recently identified carcinogenicity), condensed forms have been developed, O-formals and N-formals. The oxygen formals can be produced by the reaction of alcohols and glycols (e.g., 1,2-propylene glycol (I)) or benzyl alcohol (11) with formaldehyde to produce semi-formals - full formals are more stable chemically and consequently have little or no effect. H I H,C - C - CH2OH I 0 - CHZOH
@
CH2 - OCH20H
(1)
Nitrogen-formals are produced by the reaction of formaldehyde with amines and amides. The substances relevant here are various substituted hexahydrotriazines, condensation products of formaldehyde with primary alkyl amines, hydroxyalkyl amines and alkoxyalkyl amines, e.g.,
/cHfN - CH2 - CH2 - OR
RO - CH2CH2 - N I H2C
\
/
I CH,
N I CH2 - CH2 - OR
(hexahydro- 1,3,5-trialkoxyethy1-1,3,5-triazine). In contrast to formaldehyde, they are less volatile, longer-acting, anti-corrosive and harmless to the skin. They are effective at concentrations of 400 -1,500 p.p.m. (163).
Compounds which are more efficient and quicker acting have been developed, including methylene-bis-oxazoline (I), methylene-bis-oxazine (11) and aminal (III), condensation products of 2- and 3-hydroxyalkylamines with excess formaldehyde:
394
CH? -
L
-
0 - CH2
CH-0
/CH,-CH2\ \
CH, - CH2
/O CH2 - CH,
(111)
In aqueous solution, these compounds are less alkaline than hexahydrotriazines, and hence less harmful to the skin. The third group of aldehyde disinfectants are the reaction products of formaldehyde with urea and/or chlorinated urea, such as dimethylol urea (IV) and N-methylolchloroacetamide(V): HOCH2-NH-CO-NH-CH,OH
(IV)
ClCH,-CO-NH-CH*OH
(V)
The latter is the more effective; however, it has the disadvantage that it hydrolyses in water to produce corrosive hydrochloric acid. It must therefore be used in conjunction with a suitable corrosion inhibitor. The effects of aldehyde-type disinfectants derives from their reaction with carboxylic. amino, hydroxyl and sulph-hydryl groups in cell wall proteins. This reaction destroys the osmotic balance which kills the microorganism. Formaldehyde-based biocides have little effect on moulds or yeasts, but this effect can be increased by combining them with a small amount of phenolic biocides. However, the pH of the mixture must not exceed 9 - 9.5 (397).
A further group of effective disinfectants is compounds containing sulphur and nitrogen, nitrogen, phosphorus and metal dithiocarbamate and dithiophosphate (the metal may be zinc, manganese or sodium). Effective substances containing sulphur and nitrogen, which can also contain halogen, are the substituted derivatives of
395
and 1,Zbenzthiazole
0
where R is H, alkyl or aryl. Some newer, very effective biocides include organic boron compounds. Biocides must meet the following requirements (309):disinfecting effect on a wide range of microorganisms, stability under field conditions, absence of odour, zero human toxicity in the concentration employed, no irritating effect on the skin and respiratory system and no adverse effects on machine components, machined material and paints used on the machinery. The use of mercury-based compounds (phenyl mercuric chloride, mercury phenyl borates etc.) formerly applied in conjunction with phenols - has ceased because of health hazards. In any case, mercury-based disinfectants are unsuitable for emulsions used for machining aluminium and its alloys because these react with mercury and the work-pieces can be damaged.
Biocides have to be changed from time to time, because the microorganisms become resistant to one type continuously applied. Inevitably, biocides are harmful to the human organism, and contaminate waste water after the emulsions in use have decomposed. For this reason, efforts have been made to find alternative methods of bacterial control, including ionisation and pasteurisation of emulsions, the use of bactericidal ion exchangers (358,359)and the oligodynamic effect of metallic silver. The principal advantage of ion exchangers is that the ion exchanger particles anchor the reactive, ionic bactericidal compound and the concentration of this compound in solution is negligible. As the bacterial suspension flows through the active layer of the fixed-bed exchanger, the cells come into contact with the particle surface and the cells absorb a small amount of the bactericidal compound and become inactive. Bactericidal ion exchangers can be prepared by impregnating a cation-active exchanger of 0.3 - 0.6 nm diameter particles with an alkaline aqueous solution of bis-tributyl zinc oxide (360):
Some chlorinated hydrocarbons are recommended for suppressing the formation of deposits in two-stroke engine spark plugs (245). Tackiness impmvers can be added to lubricating oils and greases to improve adhesion to the lubricated surface and reduce the separability of oils from greases. Important groups of compounds used for this purpose comprise aluminium soaps of high molecular weight fatty acids and high molecular weight polyalkenes, mainly polybutenes. They are particularly used in lubricants which can stain the materials being processed by the lubricated machinery, e.g., lubricants for textile machinery. 396
However, they can also reduce splash-out of greases from bearings and leakage of oils from gear-boxes. Quench oils,particularly those used for warm quench baths, incorporate special additives to increase the quenching-rate, increase the oxidation stability of the oil and prevent the formation of deposits on the hardened work-pieces. These preparations include mixtures of ashless and ash-containing antioxidant and detergent-dispersant additives containing calcium, phosphorus, sulphur and nitrogen.
Additives Enhancing Swelling of Packings and Sealants Elastomers, for example butadiene-acrylonitrile copolymers, are often used for sealing rings and packings for automatic transmissions and gear-boxes. The sealing materials must be compatible with the oil used and they must not shrink or soften. Slight swelling is desirable in order to achieve a tight seal. If the oil itself fails to produce such mild swelling, a suitable aromatic additive - usually an aryl phosphate - will be incorporated in the oil.
Additives Producing Low-melting Eutectics Organo-metallicadditives have been recommended recently for liquid lubricants and greases which are supposed to form eutectics with the metal asperities on the sliding surfaces. This reduces the time for running-in, decreases friction, wear and temperature and reduces the dependence of lubrication on viscosity. The so-called third-generation products aim to replace conventional lubricants with EP additives and solid lubricants in certain situations (194).
Tribopolymers These products are now attracting increased attention. They are additives formed in situ, either directly in oils by polymerisation, or on the tribochemically activated surfaces from monomers present in the oil (361).Tribopolymers may, for instance, be products of zinc dialkyldithiophosphate transformations (362).According to Sor et al. (363), ZDDP forms a boundary film of a colloidal nature on the friction surfaces and, simultaneously, the metal undergoes a chemical transformation. These films may be associated with the aromatic components of the mineral oil and their oxidation products, as well as micelles of other functional additives present in the oil. Close attention has been paid to tripolymers in the USSR (364). Some tribopolymers are claimed to surpass the anti-wear properties of MoS2 and also protect surfaces containing lead and copper from corrosion.
397
DY= Lubricants are sometimes dyed to enable the different types of liquid lubricants and greases to be distinguished. Oil-soluble and water-insoluble azo-dyes are used, including:
-
a typical red azo-dye ( 1-(2,4-dimethyl-benzoazo)-2-hydroxy-naphthy11-azo)azobenzene HO ,C*3
N%
- a typical brown-red monoazo-dye (1-(2,4-dimethyl-benzazo)-2-hydroxynaphthalene)
Dosage of these materials in oil seldom exceeds 0.05% by weight. The appearance of the oil may also be improved by the addition of fluorescent organic substances. Some solid lubricants also used as additives are coloured, e.g., the indanthrenes (blue) and the phthalocyanines (blue and green). Several other special additive types are worth mentoning: - anti-ruds - which suppress the destructive effects of radiation on lubricants, - de-watering agents - e.g., alkylarylsulphonates of Pb and Zn, - unti-creep agents - soaps of fatty acids, fluorinated polyacylates (477), - anti-static agents for textile oils - cationic surfactants, - water-repelling agents - e.g., (for greases) metallic soaps (Na,Ca) of C, - C, polymethacry lic acids.
398
Chapter 4 - References 1 . VESELY’, V. : Ropa a Uhlie, 11, 1969, 297. 2. KUUJEV, A. M. : Chimija i technologija prisadok k maslam i toplivam. Moscow, Izd. Chimija 1972. J. : Antioxidanty. Raha, Academia 1968. 3. POSPISIL, 4. DUNN,J. R. et al. : J. Chem. Soc., 1954, 580. 5. BEAN,X. : Rev. GCn. Caoutch., 31, 1954,49. 6. SPACHT, R. B. et al. : Ind. Eng. Chem., 44, 1962, 102. INGO GOLD, K. N. : J. Inst. Petrol., 45, 1959,244; J. Phys. Chem., 64,1960, 1636. %COOK,J. et al. : J. Amer. Chem. SOC.,78, 1956, 4159. ~ . D E M S OE.VT. , - EMANUEL, N. M. : Usp. Chim., 27, 1958,365. lO.Sco?r, G. : Chem. and Ind., 1963,271. 1 ~.ROSENWALD, R. - HOADSON, I. : Ind. Eng. Chem., 42, 1950, 162. 12. BOGDANOV, G. N. - BOLDIN, A. A. : Neftechimija, 3, 1963, 594. 13. THOMAS, J. P. : J. Amer. Chem. SOC.,82, 1960, 595. 14. MYERS,H. : Reprints ACS, Div. Petrol. Chem., 13, 2, 1968, B61. 15.MAHONEY, L. R. : Angew. Chem., 8, 1969,547. LOO BOOZER, C. E. - HAMMOND, G. S. : J. Amer. Chem. SOC.,76, 1954,3861. 17.LEVIN, P.J . - MICHAJLOV, V. V. : Usp. Chim., 39, 1970, 1687. 18.ZISMAN, W. A. : Ind. Eng. Chem., 45, 1963, 1406. 19. U.S. Patents 3,326,802, 3,493,570. 20. STRANGE, H. D. et al. : Ind. Eng. Chem., Prod. Res. Dev., 6, 1967, 33. 21. U.S. Patent 3,287,269. 22. BRIDCER, R. F. et al. : Ind. Eng. Chem., Prod. Res. Dev., 5, 1966, 226. 23. ELLIOT, J. S. - EDVARDS, E. D. : Synthetic Lubricants for Gas Turbines. IP Reprint 1960. 24. HRONEC, M. et al. : J. Oxidn. Commun., 8, 1985,51; Ropa a Uhlie, 28, 1986, 303. 25. CHAO,T. S . et al. : Preprint ACS, Petrol Div., 27 (2), 1982, 362; Ind. Eng. Chem., Prod. Res. Dev., 23, 1984,21 26. U.S. Patents 3,399,139, 3,413,223, 3,422,014. 27. ACTON,E. M. et al. : J. Chem. Eng. Data, 6, 1961, 64. 28. SCHIEFER, H. M. et al. : 137 ACS Nat. Meeting 1960. 29. KOEIZOVA,R. 1. et al. : Chim. Technol. Topl. Masel, 11 (1 I), 1966,52. 30. Hydrocarbon Processing - Petrol. Refiner, 40 ( I I), 1961,41. M. S. : Chim. Technol. Topl. Masel, 8 (6), 1963,52. 31. BLAGOVIDOV,I. F. - BOROVAJA, 32. SHELDON, R. A. - KOCHI,J. K. : Oxid. Comb. Revs., 5, 1973, 135. 33. OCHIDI, E. : Tetrahedron, 20, 1964, 1819. 34. CHAKRAVARTY, N. K. : J. Inst. Petrol., 49. 1963, 353. 35. HRONEC, M. - VESELY,V. : Ropa a Uhlie, 15, 1973, 169; 16, 1974,353,437. 36. VESELY,V. et al. : Niektort probltmy vq’skumu a spracovania ropy (Some Problems of Crude Oil). Bratislava 1978, 11. 37. CS Patent 95903, (1959) (Vesely, V. et a].). 38. ZORN,H. : ErdoI u. Kohle, 3, 1950, 171. 39. LOOMIS, R. J. : Petrol Refiner, 41 (2). 1962, 139. 40. LARSON, R. : Sci. Lubr., 10 (8),1954, 12. 41. DENISON, G. H. - CONDIT, P. C. : Ind. Eng. Chem., 41, 1949,945. 42. U.S. Patents 2,719,125, 2,983,716, 2,719,126. 43. KUUIEV,A. M. et al. : Azerb. Neft. Choz., 7, 1962, 23. 44, SCANLEY, C. S. - LARSON, R. : SAE Meeting. Tulsa (Oklahoma), Nov. 1958. 45. FERNANDO, Q. - SHIRO, M. : J. Chem. SOC., 1971,350. 46. IVANOV, S. K. - KORSALYKOV, Ch. : Erdol u. Kohle, 24,1917,516.
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379.SIRINOV. Ch. D. et al. : Izv. VUZ Neft. i Gaz, 8/10, 1965.75, 12/2, 1%9, 53. W. D. : J. Lubr. Technol., Trans. ASME, Series F, 90, 1968,580. 380.NOvAK. J. D. - WINER. F. G. : ASLE Trans., 24,198 1,431. 38 1 .ROUNDS, 382.KAPSA. Ph. et al. : Trans. ASME, 103, 10, 1981,486. 383.STEPINA. v . et a]. : Ropa a Uhlie, 24, 10, 1982, 596. 384.MAHONEY, L. R. et al. : Ind. Eng. Chem., Prod. Res. Dev., 17, 1978,250. 385.KORCEK. S. et al. : SAE Paper 7380955 (1978); 810014 (1981). 386.WILLERMET, P. A. et a]. : 1978 Annual ASLE Meeting, Paper No. 78-AM-18-1. 387.T~At,A. et al. : CS Patents 193 886,193 890, 193 892; Ropa a Uhlie, 22,1980,297. J. W. : Preprint ACS, Petrol. Div., 10 (2), 1965, 127; Ind. Eng. 388.DAv1s.T. G. - THOMPSON, Chem., Prod. Res. Dev., 5, 1966,76. M. A. : J. Amer. Chem. SOC.,89,1967,5619. 389.MAHONEY, L. R. - DAROOGE, 390.TKAt, A. et al. : 10th Annual Meeting of French-Czechoslovak Cooperation on Oxidative Ageing and Burning of Polymers. CS Society for Science and Technology, HlubokP, Oct. 19-23, 1976. 391.MAHONEY. L. R. : Angew. Chem., Intern., Ed., 8, 1969,547. 392.MAHONEY, L. R. et al. : Prepflnts ACS, Petrol. Div., 24 (3), 1979,802. 393.HORODYSKY, A. G. et al. : Preprints ACS, Petrol. Div., 27 (2), 1982,589. 394.GRIBAJLQ A. P. eta]. : cH?TM, 7, 1983, 18. 395.SIEBER. I. : Schmierungstechnik 10, 1983,299-306. 396.US International Trade Commission “Synthetic Organic Chemicals, US Production and Sales”. 397.PAULUS, W. : ErdoI u. Kohle, 37, 1984, 303. 398.KILLER, C. : Mineraloltechnik, 3, 1984.3. 399.ZAJMOVSKNA, T. A. et d.: Chim. Technol. Top]. Masel, (4), 1984, 38. U. : Katalysatoren, Tenside und Mineraloladditive. Stuttgart, G. 4W.FALBE, J. - HASSERODT, Thieme Verl. 1978. ~ O ~ . T R E B I C K YV. , et al. : Ropa a Uhlie, 29, 1987, 160. k.- RICHMAN,U. : Schmierungstechnik, 16, 1985, 51 ~O~.WEBER, 403.Mur~,R. J. : NLGI Spokesman, 7,1988, 140. M. W. : SAE Reprints, 1 IOB, 1960. ~W.BOWMAN, L. D. - SAVAGE, STREI RE IT, G. - DUNSE, S. : Kautschuk and Gummi, Kunststoffe, 38, 1985,471. 406.KENNEDY, P. J. : ASLE Trans., 23, 1980.370. 407.LITmAN. W. E. : In : Approach to Lubrication of Concentrated Contacts. Ku, P. M.,(Ed.) Washington, NASA 1970,309. 408.S~.S. P. : Wear, 25, 1973, 111; et al. : Wear, 32, 1975. 33. 409.APPELWORN,J. R. : In : Approach to Lubrication of Concentrated Contacts. Ku, P. M., (Ed.) Washington, NASA 1970, 309. M. F. : SAE Paper 790 329, 1976. 410.KF.PPLER, R. K. - JOHNSON, V. : Ropa a Uhlie, 24, 1982,99. 41 ~.VESELY, 412.Chem. Week, 27 (6). 1979. J. : Ropa a Uhlie, 20,1978,520. 413.KLUCKS. P. - KEKENABY, 414.GAYLE, J. : J. Appl. Polymer Sci., 22, 1978, 1971. 415.U.S. Patent 4,151,173. 416.U.S. Patent 4,016,131. 417.AHLBORN. G. H. : In : Intern. Jahrbuch d. Tribologie 1982. Bartz, W. J., (Ed.), GrafenauHanover, Expert-Vincent2 Verl. 1982. 418.Lubrizol Newsline, Vol. 3, No. 1, (1985). ~ ~ ~ . V A L DJ.AetUal. F , : Ropa a Uhlie, 31,584 (1989). 4 2 0 . w ~S~. S~. .et a]. : Tribology Trans., 31, 381, (1988). 421.Lubrizol Newsline, Vol. 6, 3 (1988); U.S. Patent 1,565,627.
407
THE CLASSIFICATION AND APPLICATIONS OF LIQUID LUBRICANTS
Oils of both mineral and synthetic origin, mainly employed as lubricants but also used for other purposes, are termed liquid lubricants and are manufactured as such for commercial use. They are classified,according to their area of application, into lubricatingoils and special fluids. The annual worldwide consumption of lubricants 39.725 million tonnes in 1990 (incl. 1.2 mil. t by bunkering), composed of 36.134 mil. t virgin base oils, 1.569 mil. t %-refinedoils, 0.547 mil. t synthetic oils, 0.01 mil. t vegetable oils, 0.343 mil. t water and 1.122 mil. t additives (effective substances only = 2.356 mil. t with diluting oil content - commercial products). It is stated to by growing at a rate of 0.5 - 1.0 % annually in last years. (Lubricants included 45.5 % of engine oils, 7.9 % of automative gear oils, A W and greases and 46.6 % of industrial lubricants.) (276). In this Chapter, the different requirements of the machinery in which lubricants and special fluids are applied are examined in detail and related to those compositional and functional properties of the products which underlie the various classification systems used in commercial and technical practice.
As a result of technical developments over a considerable period of time, many types of lubricating oils and special fluids now exist; these technical needs are expressed in various commercial criteria used for oil classification. The type of equipment, and its component parts, for which the oil is intended define the basis of the oil classification; the operating conditions of the lubricated part provide other important criteria needed to define the characteristics required in the oil. The main oil classifications in terms of equipment are into engine oils, turbine oil, steam turbine oils etc. Oils for various parts of the equipment include bearing oils, transmission oils, hydraulic oils etc. Another classification system deals with the special requirements of certain industrial sectors. Thus, for example, textile oils, electrical insulating oils, quenching oils and oils for application in radioactive environments are specially manufactured and classified. I S 0 introduced in 1983 a set of classifications, prefixed L for lubricating oils and special fluids with the following designations according to application (fuels are similarly designated by the prefix F, solvents and chemical feed-stocks S, bitumens B and paraffin waxes W): L - A - oils for once-through lubrication L - B - release agents L - C - gear oils L - D - Compressor oils L - E - oils for internal combustion engines 408
L-G L-H L-ML-P L-R L-Q L-T L-u L-v L-x L-z -
slide-way lubricants hydraulic oils metal-machining oils oils for pneumatic tools temporary anti-corrosion agents heat transfer fluids turbine oils heat treatment oils insulating oils lubricating greases steam cylinder oils
Oil properties are evaluated by laboratory tests (Table 5.1), in standard engines and other machines and by field tests. Table 5.1. Common Laboratory Tests of Oils Oil Type Property
Engine
X Oxidation stability Thermal stability X X Shear stability Detergency Dispersancy Varnish formation General corrosion Rusting X Copper corrosion Effect on sealing materials Friction coefficient EP properties Wear Foaming tendency X Miscibilitykompatibility with other oils x
Gear
Hydraulic Industrial
Special
X
X
X
X
X
-
X
-
X
-
(XI
X
X
X
X
X
-
-
X
X
X
X
X
X
X
X
X
5.1 INTERNAL COMBUSTION ENGINE OILS Oils for automotive and stationary internal combustion engines (both gasoline and diesel) need to fulfil many functions and must meet many - sometimes conflicting - requirements. They form the most complex type of oil encountered in practice.
5.1.1 Oils for Four-Stroke Internal Combustion Engines The following properties are required from lubricating oils for four-stroke, reciprocating, internal combustion engines: 409
1. Lubricating power (i.e., sufficient load-carrying capacity of the lubricating film) under hydrodynamic, elastohydrodynamic, mixed and boundary conditions up to temperatures which can exceed 250 "C. 2. Adequate rheological properties (fluidity) at low temperatures, often well below 0 "C. 3. Sufficiently high viscosity over the whole range of operating conditions to seal the gaps between the piston rings and cylinder walls. 4.Sufficient thermal and oxidation stability and resistance to the effects of products of incomplete fuel combustion for the longest possible time, in order to prevent the formation of significant amounts of undesirable carbon and other deposits which limit the operation of functional parts of the engine. 5. Protection of the engine against the formation of corrosive, acidic sludges during cold-running of an insufficiently warmed-up engine. 6. Ability to conduct and dispose of the heat produced by the engine in the cooling surfaces and the walls of the crankcase and cylinder bore. 7. Ability to neutralise the corrosive products of fuel combustion. 8. Ability to prevent the accumulation and harmful effects of thermooxidation products formed by the reaction of fuel with oil and impurities on the surfaces in contact with oil. 9. Low volatility and low oil consumption during operation. 10. No adverse effect on common sealing materials, including when the oil is hot; it should not smell and it should not be physiologically aggressive. 11. Protection of metal and other surfaces during prolonged shut-down periods, immediately preceded by running, possibly under adverse conditions. The required composition and properties of oils vary according to the type of engine, its design, construction, power output and operating characteristics, and the conditions under which it is used. The most severe demands on internal combustion engine oils are those encountered in road vehicles and tractors, i.e., automotive engine oils. The need for light and compact engines leads to high specific engine output in relation to both intake volume and engine mass, resulting in a huge heat release when relatively large quantities of fuel are burned in a cylinder of a given volume. The reject heat must be removed so that the operating temperature does not exceed an acceptable value. Heat flow via the piston rings to the cylinder walls creates significant temperature gradients in the oil layer on the surfaces of both the piston and the cylinder. The oil must be capable, also, of removing very large amounts of heat from the piston gudgeon pin and the big-end bearing. The heat transferred into the oil causes the oil temperature in the crankcase to rise and a proportionally large increase in the oil temperature at the piston and the bearings. The need to keep the engine mass low and to minimise cubic capacity leads to a decrease in the oil volume in the engine. This small amount of oil has to be able to absorb heat from the moving parts of the engine so it has to operate at a higher specific circulation rate (i. e., a higher rate of oil-pumpoutput relative to crankcaseoil volume). The oil in the crankcaseremains in continuous,rapid motion and any solid impurities,formed by abrasion of the friction surfaces and sucked in by the intake air, cannot settle. Thus, the demands for perfect functioning of both the oil and the air and oil filtration systems increase. The amount of heat to be removed by the oil may be so high that the heat exchange area in the crankcase and the temperature gradients are insufficient, and an oil-cooler becomes necessary. This makes it even more difficult to prevent used oil deposits from settling on the metal surfaces in the engine. These deposits, have low heat
410
transfer coefficients, diminish the cooling efficiency of the oil and limit or even nullify the functions of the piston rings and other parts of the engine. The low mass of the engine and its correspondingly low thermal capacity bring about rapid temperature changes, dependent on its temporary running conditions. The cooling system of an engine is designed to cope with the unwanted heat from a fully-loaded engine at the maximum temperature of the ambient air. Under partial load conditions, the only thing the thermostat can do is to shut off the cooling system. If the heat output of the engine is insufficient to cover heat losses other than those produced by the cooling system, the engine operating temperature decreases rapidly, especially at low ambient air temperatures. Under these conditions, the temperature of the circulating oil decreases and the rate of evaporation of water produced by fuel combustion falls. The condensed water combines with the blow-by gases passing between the piston and the cylinder into the crankcase, leading to the formation of acid, corrosive and bulky sludges containing up to 40% of water, which tend to clog the scraper rings and the oil grooves on the piston. Moreover, they form deposits on the walls of the crankcase and clog the sump of the oil-pump and the oil filter. These deposits are the so-called cold sludges and are usually formed during stop-go running, especially in cold weather. A vehicle which is shut-down for a long period cools down to the ambient temperature and, at the same time, the viscosity of the engine oil reaches the same temperature. When the cold engine is started, the oil - at a higher viscosity - offers a higher resistance to flow through the oil passages and is therefore returned to the sump through the oil pressure relief valves. The engine runs with insufficient oil on the friction surfaces until the oil temperature rises by absorbing combustion heat and that produced by shearing forces in the oil pump. Up to 70% of engine wear originates in this period of poor engine lubrication. Naturally, this percentage increases when the oil temperature is low and the time taken for the oil to become sufficiently fluid is prolonged. Although all these factors which affect the characteristics of the oil apply to all types of internal combustion engines used in road vehicles and tractors (except two-stroke gasoline engines with conventional lubrication systems), there are certain differences between the different types of engine and their construction.
The following classification of engines is generally used: - gasoline (spark ignition) and diesel (compression ignition) engines, - four-stroke gasoline engines, water- and air-cooled, - four-stroke diesel engines, supercharged and naturally-aspirated, - two-stroke gasoline and diesel engines. The different effects on the oil and the variations in the demands in its properties arise, above all, in the varying heat and pressure regimes of different engines. In four-stroke internal combustion gasoline engines, higher heat-loading can occur in air-cooled and supercharged types. The most decisive factors, however, are the value of the mean effective pressure on the piston (b.m.e.p.) and the r.p.m. of the crankshaft. These are both related to the specific output of the engine, its heat regime and consequently the temperature of the oil. During sustained highway driving, temperatures in high-output engines with high compression ratios can reach 150 "C in the oil-pump area, 170-180 "C in the the connecting-rod bearings and almost 250 "C in the top piston ring groove. Modem practice in engine manufacture has led to even higher oil sump temperatures, up to around 160 "C (7).This presents a series of design and operational problems connected with oil-foaming, attack by the oil on seal and bearing materials, high oil consumption, a greater tendency tb form deposits in piston ring grooves, a greater rate of oil oxidation and more severe demands on the load-canying capacity of the oil film. All this complicates the choice of appropriate base stocks and additives, especially
41 1
polymeric viscosity modifiers.These developments have led to the development of new high temperature engine tests, of which the Volkswagen Digifant is the most likely candidate among the European engines for selection by CCMC, supplementing the ASTM Sequence IIIE on a GM engine, already included in the CCMC specifications;the Toyota 1G-FE test has been proposed among Japanese engines.
The heat and pressure regimes of diesel engines are more severe than those of gasoline engines. This results from the higher pressures of both compression and combustion, the high temperature of the intake air and a higher combustion temperature. Such high temperatures are very significant in supercharged engines with high-pressure intake air; even with cooled pistons, the first ring groove temperature can reach 250 OC.In engines with uncooled pistons, the first ring groove temperature approaches 300 "C.Similarly, high heat-loading of the oil occurs in twostroke diesels, which generally have higher specific outputs than four-stroke designs. Another disadvantage of two-stroke diesels without valve-gear lies in the design of the ports of the cylinder wall, which hinder the formation of a continuous, homogeneous oil film. Some oil penetrates into the exhaust passage, especially during idle and low-speed running; thermal decomposition of the oil occurs on the hot walls, leading to soot formation, reducing the flow of gases through the exhaust casing, causing both a decrease in engine output and an increase in operating temperature. Moreover, ash in gasoline engines and the heat regime and temperature in diesels depends on engine r.p.m., the load on the engine, the magnitude of oilflow and the efficiency of cooling. In the main oil passage, the temperature of the lubricating oil can increase, especially with increasing r.p.m., almost independently of engine-loading. If an oil-cooler is not in use, the temperature of the cooling-water has only a modest effect on oil temperature. Running in low gear at high engine r.p.m. causes a temperature increase in the main oil passage. Fortunately, this temperature increase is almost proportional to the ambient temperature, simplifying the choice of the viscosity classification of oil suitable for various seasons of the year and different climatic conditions (8). However, modem practice in many vehicles is to specify one oil grade for year-round use, and it is often only in specialised fleets and in response to extreme climatic differences that this degree of freedom can be exercised. This sensitivity to changes in ambient temperature is much less for diesel engines than gasoline. The sensitivity of diesels to temperature changes caused by stop-go running is less marked and their demands on the low temperature rheological properties of oils are less severe. However, the demands placed on the thermal and oxidation stability of these oils and their ability to neutralise acidic combustion products (especially in engines operated on high-sulphur fuels) and to inhibit the deposition of thermooxidation products formed by the reactions on functionalengine surfaces of fuel components with the oil are much more severe. The composition and properties of oils for diesel engines in both rail vehicles and large stationary engines operating on diesel fuel or heavier fuels (such as industrial and marine engines) must fulfil similar requirements. The need for special viscosity-temperature characteristicsand overall sensitivity to other factors are less 412
critical with these engines, because they mostly run at constant speed and temperature. However, the neutralising power and detergency of these oils must be relatively high, especially in engines operated on heavy fuels. Oil change after a certain time period or distance travelled is dictated by the following major factors: - changes in both physical and chemical properties of the oil occuring during its use, - the accumulation of soot and sludge, - contamination by water and other impurities, - separation of non-combustible residues from the fuel and other sources. The actual drain intervals depend not only on the quality of the oil, but also on the running conditions, climatic conditions, and other factors. The drain interval recommended by the engine manufacturer is based on the outcome of practical tests, including extended field test programmes, with the type of oil in question. Over the past few years, users of diesel engines with direct fuel injection have sought to extend the oil drain interval to 20,000 km or more. This is more easily possible in engines with a large sump volume (i.e., a lower oil circulation factor), equipped with cooled pistons, improved oil and air filtration, good sealing of the piston rings and, generally, in engines in a good mechanical condition (in contrast to this, drain intervals in indirect injection engines should generally not exceed 5,000 km, because of the considerable oil-thickening which occurs due to the accumulation of carbonaceous material produced by imperfect fuel combustion. The content of sooty material in the oil should not exceed 6% by weight). There is thus a constant pressure to extend oil-drain intervals and to reduce specific oil consumption. Former and present recommendations are summarised below, on the basis of API CD oils. This extension, and the effects of very long-distance travel, dictate that oil charges must be suitable for a wide range of climatic conditions. The former reluctance of diesel engine builders and users to specify multi-grade oils has been overcome, with added complications for diesel engine lubricating oil formulators. Engine manufacturer Detroit-Diesel 2-stroke 4-stroke Cummins Caterpillar Mack API CD specification oils.
Oil consumption (I per 1,000 km) former presenthture 0.98 0.59 0.49 0.65 0.3
0.49 0.59 0.33 0.33 0.3
Oil-drain interval (h)* former present/future 32,000 12,900 27,400 12,900 40,000
24,000 12,900 27,400 12,900 80,000
More careful and frequent oil inspections are necessary when reduced oil consumption and possible extended drain intervals are adopted. Various simplified, rapid methods of quality assessment are. used. Greater care must also be devoted to both oil and air filtration.
413
Changing or cleaning the filters needs to be carried out at regular intervals to ensure reliable operation. The efficiency of the full-flow oil filter for the separation of impurity particles in the size range 5-10 pm is very important. In one practical example, improved filter efficiency (impurities in the size range 6-9 pm were separated, compared with the original limit of 15 pm minimum), a 30-60% decrease in the impurity content of the oil resulted and the oil drain interval could be doubled
(230). Eastern European producers recommend for Skoda (Czechoslovakia) and Lada (USSR) gasoline engines oil-drains (with API CC-SF oils) of 10.000 km,and 20,000 km for API SE-CD oils in Tatra and Liaz supercharged diesels. European manufacturers of diesel engines for commercial vehicles are striving to extend oil drain intervals, despite pressures for improved durability and the need to meet emissions legislation, impelled by the financial need to minimise vehicle down-time. Before recommending this type of service, the engine manufacturer must be confident that the lubricant will give satisfactory performance in extended drain service. Long-life oils must not only comply with standard specifications (API, MIL-L- and manufacturers' own specifications),they must pass longduration tests under field conditions in full-scale engines. In these, the oil is replaced at prescribed, lengthy intervals and the oil tested in a prescribed sequence. For example, API CD oils meeting the Volvo VDS specification can be used for 30,000 km between oil-drains on long-distance service. These oils must be tested in 300,000 km field service tests in 3 trucks fitted with Volvo TD 120 series engines with oil-drains at 50,000 km intervals, the condition of the oil being monitored throughout the test and deposits and wear are assessed at the end. The fresh oil may be SAE 10W30or 15W40(other viscosities, including SAE 5W30 can be accepted by prior agreement with Volvo) and comply with the API CD specification. The Ford Tornado borepolish test is used, with a limit on bore polish determined by comparison with valid tests on high and low performance reference oils. The fuel sulphur content is controlled at 0.746 (max.) and the fuel consumption is measured. At each 50,000 km drain, the used oil must meet the following limits: - viscosity at 100 "C - 9.0 mm2.s-' min. for SAE 10W30, - 12.0 mm2.s-l min.for SAE 15W40, - TBN (ASTM D-2896) - 50% of fresh-oil TBN, - wear-metals and additive elements are measured. At the end of the test, bore polish must be less than 300 cm2 max., 100 cm2 max.per cylinder; valvetrain and valve-guide wear, bearings, ring-wear, piston deposits and general cleanliness of the engine are assessed.
It has been speculated that oil-drains up to 100,000 km will be reached during the 1990's with this type of engine and service. In this respect, the manufacturers of commercial diesel engines have apparently been more ready to recommend extended oil-drain intervals than gasoline engine builders.
Viscosity Characteristics of the Oil The main contribution - 60-70%- of the entire friction load of an engine is accounted for by the contact between piston rings and cylinder liner, the remainder being due to the bearings and the valve mechanism. In order to achieve minimum friction in the piston area, the oil film must be 2-5 pm thick; this depends on the viscosity of the oil. 414
The choice of the oil is, above all, governed by the viscosity requirements of the engine and its dependence on viscosity-temperature effects. For high engine temperatures, oils of higher viscosity must be used. The satisfactory lubrication of crankshaft bearings depends mainly on oil viscosity. For operation with single-grade oils, 4 - 5 m m 2 x 1 is traditionally held to be the minimum, critical value for viscosity in big-end bearings, lubrication failure occurring at 2 mm2.s-' (22). These critical values are, however, rather empirical and arbitrary, since the creation of the conditions for hydrodynamic lubrication depends not only on the viscosity in the oil film, but also on the geometry of the bearings, r.p.m. and the specific loading of the oil film. The semi-qualitative relationships mentioned can be expressed in a more quantitative way in the dimensionless Sommerfeld's number, So, or its reciprocal value, the bearing number S;.' Notwithstanding these considerations, an oil viscosity of 4 - 5 mm2.s-*at 100 "C can be accepted as a practical, minimum value for engines with a sump temperature not exceeding 100 "C.Thus, an SAE 20 oil having a minimum viscosity at 100 "C equal to 5.6 m m 2 s 1 can be used in engines with an oil sump temperature below about 120 "C at the oil-pump outlet, since the viscosity in the big-end bearing, which is 20 - 30 "C hotter, should not fall below the limiting value of 4 mm2.s-I. As the temperature increases, an oil with a higher viscosity classification, such as SAE 50, should be used. Increased wear of damage to bearing may not only be caused by insufficient lubrication due to low oil viscosity. It may be the result of corrosion of the yellow metal material in the bearing by acid products formed by the oxidation of the fuel-oil combination, the interaction of the bearing metals with metallic and other constituentsfrom additives,cavitation by bubbles from low-boiling fractions if these are present at high concentrations in the oil, electrolytic effects due to the differences in electrochemical potential of alloy metals in the bearing system and abrasion by hard particles of wear metals and soot.
The viscosity of nowNewtonian multigrade engine oils containing polymeric viscosity index improvers depends not only on temperature but also on the shear rate; the effect of this on viscosity decrease depends on both temperature and pressure (129). As the temperature decreases, the rate of viscosity decrease diminishes at a given shear rate. The decrease in viscosity of a multigrade oil at a given temperature, pressure and shear stress depends on the shear stability of the polymer, i.e., on its resistance to temporary loss in viscosity. The conditions for the determination of the minimum critical viscosity for lubrication of the main engine bearings are, therefore, more complex with multigrade oils than single grades. According to references (130) and (259), the critical values can be established approximately by measuring viscosity at a temperature of 155 "C and a shear rate of 5.105 s-l. Under these conditions, the critical minimum viscosity is 2.6 - 2.8 mm2.s1. Usually, a certain viscosity reserve or safety margin is retained. Its magnitude, for a given type of engine, depends on the operating temperature of the engine, the clearances in moving parts, the sealing of the bearings, the size of the gap between the piston rings and the cylinder wall and on the efficiency of oil cooling, which in turn depends on the oil circulation factor. In addition, the pattern of performance of the engine, whether it will be operated mostly at full load or partial load, must be allowed for. It must also be remembered that increased viscosity of the lubricant leads to a decrease in mechanical efficiency of the engine and an increase in fuel consumption. 415
The effect of frictional losses in the engine on both mechanical efficiency and fuel consumption is significant. In mixed cityhighway operation in a car with a spark ignition engine, out of 100% of the energy available from combustion of the fuel, 33% is rejected in the heat of the exhaust gases, about 29% in the cooling water and only about 38% remains as so-called power output. However, 6% is consumed for overcoming friction in the engine and only 25% provides the so-called brake horse power. 2.5% is needed to drive the auxiliary units, such as the alternator, water-pump, fan servo and steering booster, 1.5% for overcoming the friction in the gears and about 1.5% is lost overcoming friction in the rear axle unit; about 4% is dissipated in idle running and 3.5% in the application of brakes. After all this, only about 12% of the total energy input of the fuel is available to drive the wheels. Out of this, about 6% each is necessary to overcome tyre friction and air resistance. 1
j
0
I
2
I I
lo?
3
c 5 6
100
150
250
200
3
OC
0 CONTEMPORANEOUS OUTLOOK
Fig. 5.1. Contemporaneous outlook heat load of various engine parts
1 -1st piston groove, 2 - piston undercrown, 3 -journal bearings (outlet), 4 - valve train, 5 - oil sump, 6 - coolant
z
W
20 w
a
097-
1195,
,
,
,
,
, ,
,
,
,
,
4 6 8 10 12 14 16 l 8 20 22 24 VISCOSITY AT 99.9 oClmm?slJ
Fig. 5.2. Dependence of fuel consumption on the viscosity (viscosity class SAE) of engine oil measured by high shear rate (Abscisses mark the 95 % limits of reliability)
Frictional losses in engines increase with oil viscosity. For example, in a multi-cylinder engine, mechanical efficiency decreases about 5% for each SAE grade of viscosity increase. It appears from published results of tests carried out on various types of engine with different cylinder capacity and different types of oil (both single and monograde, mineral and synthetic), under different conditions (highway and city driving) that 0.7 - 1.0% fuel economy can be achieved by dropping one SAE viscosity grade. Similar results are obtainable from theoretical calculations. Excessively high oil viscosity is therefore undesirable economically.Obviously, frictional losses also depend on the operating temperature of the oil and on its low temperature fluidity (263).
416
Other publications by other authors based on the results of highway tests also confirm the advantage of multigrade oils in the context of frictional loss and fuel consumption. For example, Bame and Coone (173) and Lawrence (218) showed that the use of multigrade oil can produce a gain in fuel economy of about 5 - 10% in comparison with a monograde oil of the same viscosity at 100 “C. Other authors quote somewhat lower values (5 - 8%).These results should, however, be treated with some caution, as they depend not only on the test methods used to obtain them, but also on other properties of the oils used, such as oxidation stability, volatility, etc. Moreover, the mechanical and thermooxidative stability of polymeric additives can be very important. During high-load running of the engine, the inevitable energy losses due to friction occur in places where the oil is subjected to high shear, namely, between the piston rings and the cylinder bore and in the main crankshaft bearings. Under these conditions, oils thickened with polymers exhibit temporary viscosity loss. Viscosity loss can also be permanent, when the polymer is destroyed, and this depends on its resistance to mechanical stress. Viscosity drop results in decreased fuel consumption. The magnitude of the differences in fuel consumption, when using either single or multigrade oils, depends on the “true” oil viscosity at the given temperature and shear stress. This can also be seen from the data obtained in tests with an engine running at both constant speed and load (146) (fig. 5.1 and 5.2). The use of an oil of lower viscosity or a multigrade oil gives a better fuel economy gain when the engine is insufficiently warmed up, e.g., in winter and/or stop-go driving, than in sustained, e.g., highway driving, where the operating temperature of the oil is maintained nearer its optimum value. However, the relationship between relative fuel consumption generally depends on the engine type and the extent of use of non-Newtonian and Newtonian oils. Friction modifiers can also decrease friction and fuel consumption. Low-viscosity multigrade oils, e.g., SAE 5W20 or 10W30, containing friction modifiers are sometimes referred to as “fuel economy oils”.
In most cases, oil consumption decreases as viscosity increases; oils of higher viscosity are generally less volatile and penetrate to a lesser extent between the cylinder walls and the piston rings into the combustion chamber. The consumption of monograde oils varies linearly with viscosity measured at 100 “C; a similar relationship obtains for multigrade oils if viscosity values near piston ring z 1.M LL
r
g
I50
1m
a97
4
6
0
10 12 14 16 18 20 22 VISCOSITY AT 9 ~ 1 . 9 0 lmm?il ~ AND SHEAR RATE lo’s-’
I
Fig. 5.3.Dependence of fuel consumption on the viscosity (viscosity class SAE) of engine oil measured by high shear rate (Abscises mark the 95 96 limits of reliability).
417
temperature are used (229). For example, j g . 5.3 illustrates the oil consumption relative to oil viscosity, using the now obsolete Caterpillar OL-2 engine test (147). The Caterpillar OL-2 engine test for measuring oil consumption was carried out in a six-cylinder Caterpillar 1693 TA engine operating on diesel fuel containing 0.4% sulphur. The test duration was 50 h and it was run for 750 cycles in the following phase pattern: Phase
r.p.m. (min-')
Time (s) 30 30 30 90
1. 2. 3.
4.
1
Engine load (kW)
,ooo
0 0 317
2,385 2,100
maximum
1,500
Oil consumption was measured after 50 h.
Fig. 5.4 relates oil consumption with the extrapolated kinematic viscosity of mono- and multigrade oils at 280 "C in the OL-2 test. The results demonstrate not only the lower consumption as viscosity increases, but the lower consumption of multigrade oils in general. 4.0 SAE 30
W
r
015W140
K
2OW140
1D 1
1.2
1.4 1.5 1.6 EXTRAPOLATED VISCOSITIES AT 280 O C
1.3
I rnrnfj0 Fig. 5.4. Differences in the consumption of engine oils with different viscosities at 280 "C
However, it should be noted that abnormally high wear or even engine seizure can occur even at high oil viscosities if the oil is diluted with too much fuel. In gasoline engines, temporary fuel dilution of the oil occurs after starting the engine, especially at low ambient temperatures, when the choke is in operation for a long time; similarly, fuel dilution takes place during extended periods of idling. This type of dilution does not cause permanent deterioration of the lubricant, as the gasoline evaporates when the engine is warmed up to operating temperature. The most significant failures are caused by dilution of the fuel with gasoline fractions containing residual amounts of boiling-point above about 220 "C. These fractions are not completely burned and migrate to the crankcase with the blow-by, where they cannot evaporate even at the operating temperature. A more frequent cause of failure in diesel engines is dilution of the oil by fuel due to malfunctioning of one or more injector nozzles. Once the nozzle misbehaves for any reason and the correct spray pattern
418
is changed, fuel droplets are formed, or the fuel may even dribble directly from the nozzle. Also, if deposits on the nozzle deform the spray pattern cone, the liquid passes into the oil and will not evaporate. The consequent fall in oil viscosity may lead to melting of plain bearing alloys and in engines with rollerbearings failure and run-out of the piston pin. If the nozzle is so badly obstructed that the fuel is not distributed at all and emerges from the nozzle in the form of large drops, the lubricant is washed off the walls of the cylinder and piston seizure may result. Engines with plain-bearing are more susceptible to run-out failure (120).
While the lubricating oil should have a sufficiently high viscosity at higher engine operating temperatures, it is desirable that its viscosity is as low (or its fluidity is as high) as possible at lower ambient temperatures. Viscosity (or fluidity) of the oil under high shear conditions in the engine influences the rate of formation of the lubricating film, its ability to reduce friction and wear and the magnitude of the internal resistance which affects cold starting (23,24). The starting of an engine is influenced by the viscosity of the oil under a combination of low temperature and high shear rate conditions, whilst the rate of formation of the lubricant film depends on the pumpability of the oil, which relates to its viscosity at low temperatures and low shear rates - the higher the viscosity of the oil, the lower its rate of flow to the surfaces to be lubricated. As well as viscosity, the pour-point of the oil and the type and concentration of viscosity modifier used affect its pumpability. Moreover, the design of the engine, the type of filter and whether it is new or used, the type and location of the by-pass system and the output of the oil-pump are all very important. The Rocker Arm Oiling Test (RAOT) is used to measure the pumpability of oils. In this test, the time in seconds for the oil to reach the valve gear mechanism - the valve rocker arms is measured after starting the engine. Other devices have also been used for this purpose. For example, that jointly developed by Rohm & Haas and the Renault company gives results in good agreement with viscosity values from the Brookfeld viscometer, mini-rotary viscometer or vacuum capillary.
-
The critical moment in engine starting is just after the starter motor cuts out, when the engine itself must overcome the resistance from internal friction of the oil molecules and the friction between the lubricant and the frictional surfaces of the moving parts of the engine (15).When an engine is cold, its ease of starting depends on its type, design and mechanical condition, and on the rheological properties and composition of the oil. Startability is worse for engines with higher compression ratios, so that diesel engines always have worse starting characteristics than gasoline engines. Diesel engines are, therefore, often equipped with heating plugs for preheating the oil. In engines with automatic transmissions the problems of starting are even more complicated. The engine must also overcome the resistance of the torque converter. As a result, different types and designs of engine require different minimum r.p.m. to start successfully. This depends not only on the capacity of the engine source, but is also significantly influenced by the frictional resistance, which is directly related to oil viscosity. Test with engines of different types and designs have shown that the oil viscosity limit for easy starting for an engine in good mechanical condition and with adequate battery capacity is 2.5 to 5.0 Pas. 419
Multigrade oils of the same upper viscosity limit as monograde oils give easier starting, because their viscosity at low temperature is lower. Differences in this respect between monograde and multigrade oils are illustrated in fig. 5.5. bT
zz
55200-
o=
z
SAE low130
'\
150-
MIN. STARTING REVOLUTIONS
0
100
-5
*5
0
AMBENT [OIL) TEMPERATURE . O C
-lo
Fig. 5.5. Differences in the influence of engine oils of different viscosity class on the starting-ease of engine at low temperatures
As the temperature decreases, the minimum r.p.m for a successful start increases. With an SAE 30 oil, starting is possible at around 0 OC and higher. For an SAE 10W30 oil, the lowest temperature for successful starting is about 7 "C lower. In considering the relationship between oil viscosity and engine startability at low temperatures, fuel dilution and oil thickening by ageing should be taken into account. Fig. 5.6 shows the influence of fuel dilution on oil viscosity at 0 O F (-17.8 "C) and -20 O F (-28.9 "C), and its effects on r.p.m. in a gasoline engine. The effect of pentaneinsoluble products of oil ageing on viscosity are shown in fig. 5.7 (Id). mi6
p0.s
-
-
$2 2 0 >
$
0
)
z 0
-=------ --
0-f
v
\
-17.8 QC
I
5
I
- B w
- 60
10 15 DILUTION (Yo Val)
Fig. 5.6. Influence of oil dilution by fuel on the oil viscosity and r.p.m. of a gasoline engine at different temperatures
Ageing of the oil may result in it becoming gelatinous when the sump temperature reaches 150 "C or more (26). The effect of both soluble and insoluble products of oil ageing, products of incomplete fuel combustion and of "cold sludges" formed in the oil during low temperature engine operation is also significant. As the concentration of these materials in the oil increases, viscosity increases and low temperature starting is 420
impaired. However, a warm engine may be hard to start, too. This may not only be the result of vapour locks in the fuel piping system in a gasoline engine, it can be the result of complete or partial loss of the lubricant film between the piston rings and the cylinder wall. 50
o
2
4 6 a N) 129/owt INSOLUBLES IN PENTANE
Fig. 5.7. Effect of the ageing products insoluble in n-pentane on the viscosity of oil in gasoline engine
Low viscosity at low temperature and high viscosity at high temperature are needed for engines used for varying loads and ambient temperature variation effects, typical duties of most vehicle engines. In these engines, oil viscosity must change as little as possible, i.e., they must have the highest possible VI. Oils for use in large stationary engines which operate predominantly under constant load and temperature conditions do not have to meet such exacting requirements. The choice of oils by viscosity for specific engine types was facilitated by the introduction, in I9 11, of the classification system by the US Society of Automotive Engineers (SAE).The latest (June 1989)version of the S A E Viscosity Classification of Automotive Engine Oils is detailed in Table 5.2: Table 5.2. Viscosity Classification of Engine Oils SAE J 300 June 89 SAE Viscosity grade
Max.Viscosity(cP = mPa.s) at temperature ( "C ) Cranking (1) Pumpability (2)
ow
3250 at -30 3500 at -25 3500 at -20 3500 at -15 4500 at -10 6OOO at -5
5w 1ow
15W 20w 25W
30000 at -35 30000 at -30 30000 at -25 3oooO at -20 3oooO at -15 3oooO at -10
-
20 30 40 50 60
-
Kinematic Viscosity (cSt = mm2.s-') at 100 "C min. max. (3,4) -
3.8 3.8 4.1 5.6 5.6 9.3 5.6 9.3 12.5 16.3 21.9
-
<9.3 <12.5 <16.3 ~21.9 <26.1
(1) See Appendix A of SAE June 89.
(2) ASTM D-4684 (also Appendix B and text. Section 1.1) - note that this method does not allow any detectable
yield stress. (3) ASTM D-445. (4) Some engine manufacturers also recommend limits on viscosity measured at 150 "C and 106 s.'.
42 1
The former June 1986 SAE 5300 classification was inadequate for modem technical practice, because oil stress had substantially increased and the range of physical conditions for which the viscometric properties of oils must be specified had widened. Furthermore, polymeric VI improver dose rates have substantially increased,imparting non-Newtonian characteristicsto these oils and requiring more detailed viscometric information in order to specify the operating ability of the oil in the engine. Current viscosity classifications of engine oils suffer the following deficiences: Viscosity is specified at 100 "C, which is an unsuitable temperature for modem engines. Oils thickened with polymers are non-Newtonian and their viscosity depends on the shear rate. This factor is only allowed for in the current specifications in so far as it relates to startability with the oils in question. Viscosities are only measured at higher temperatures under low-shear conditions; this fails to reflect the effective viscosities of the oils because the high shear rate, which is vital for evaluating the relationship between oil viscosity and engine wear, has not been allowed for (17,18,221). It was therefore proposed that subsequent specifications should incorporate viscosity measurement at high temperature (HT)and high shear (HS), so as to bring viscosity measurement conditions more into line with those prevailing in an operating engine, but this has not yet been adopted in the SAE specification. Specifications which include minimum viscosities under HT/HS conditions have been established in Europe: Source of specification Test method Volkswagen Brunswick British Leyland Not specified Ford To be established CCMC CEC L-36-T-84 * To be deferred to a higher CCMC specification.
Temperature ( "C)
I50 150 150
I50
Shear-rate Viscosity (sec-')xl06 (mPa.s)(min.) 1 .o 1 .O 1 .o 1.o
3.5 3.2* 3.0
3.5
CEC Working Group CEC CL-23 has been considering a number of possible techniques, including the Ravenfield viscometer and the tapered bearing simulator. These are both rotational viscometers (the Brunswick is a high pressure capillary). The Ravensfield HTA-IS Rotary Viscometer was adopted for CEC-L-36-T-84.
The SAE classification distinguishes between single-season, monograde oils, which must be changed according to the season of the year, and all-season or multigrade oils. During cold weather, when the engine is unlikely to overheat and the cooling system is virtually redundant, oils which are fluid at low temperature are preferred and a viscosity of about 5 mm2.s-l is quite suitable. SAE 5W is suitable for arctic climates and SAE 1OW for milder regions. During the warmer seasons, a reasonable viscosity grade is SAE 30 and SAE 40 or even 50 preferable in hot climates. The use of oils falling into only one SAE viscosity grade is now largely limited to engines for constant operating conditions, such as stationary, marine and rail-road engines, and also for engines in road vehicles used for very long-mileage duties, where the oil has to be changed frequently anyway, such as trucks and passenger service cars - taxis, company car fleets, delivery vans, etc. Whilst multigrade oils were formerly used chiefly for gasoline engines in cars, they have become standard in diesel engined cars increasingly used in trucks. These modem multigrade oils have very high viscosity indexes, so that they display acceptably low viscosity in sub-zero temperatures and yet still have a sufficiently high viscosity reserve in the 422
summer. Economical oil-drains are possible after a recommended distance, regardless of climatic conditions, season or altitude. S A E 10W/40 oils predominate at present in gasoline engines, but 5W/30 oils are increasingly in demand, particularly with the availability of the partially or wholly synthetic oils, as well as wholly synthetic 5W/20's, which - as compared with 10W/40 or 15W/40 mineral oils - save fuel and facilitate starting. 15W/40 oils are broadly applied in diesel engines, particularly for the Central European climate. The latest generation of multigrade engine oil developments is based on S A E OW/X, using mixtures of very high VZ hydrocarbon oils with PAO, ester 6r polyalkylene glycol synthetics. The Caterpillar Tractor Company, for example, recommends for their engines the following oil viscosity grades for different temperature zones: 1ow
-20 to +I0 -20 to 4 0 -15 to +50 0 to +40 +5 to +50
15W30 20W40 30 40
Caterpillar issues separate, detailed recommendations for operating below -20 "C (form SEBU5898 - Operation and Maintenance Manual for Cold Weather Recommendations). The former Ford ESE-M2C 153B specification, which includes oils at performance level SF for gasoline engines for both trucks and passenger cars, recommended for various ambient temperatures oils with SAE viscosity grades according to the diagram in jig. 5.8. AMBIENT TEMPERATURES, OC
-23
-12
-7
0
10
16
32
Fig. 5.8. Recommendation of oil viscosity grades with respect to the out-of-door temperatures according to Ford ESE-M2C 153 B specification
The advantage of multigrade oils in diesels are not so explicit as they are in gasoline engines. High performance multigrade oils are, of course, required for 423
multi-fuel engines, in military service and in mixed fleets. Diesel engine manufacturers were slow to recommend multigrades. The reason for this reluctance included, for example, the increase deposit-forming tendency on the pistons of engines operating at high temperatures and lubricated with oils containing a high concentration of polymeric VI improvers of conventional polymetacrylate types. This reluctance has, however, receded with the introduction of new types of polymers (29,20), such as nitrogen-containing polymers, mixtures of polyalkenes and styrene,-dienecopolymers of higher shear stability and higher oxidation stability. However, there are problems associated with increased wear of valve gear (cams, lifters, etc.) and pitting corrosion in diesel engines caused by multigrade oils; these problems can, however, be suppressed by the use of suitable materials (232). The differences between monograde and multigrade oils with respect to viscosity and viscosity index are illustrated infig. 5.9.
mm2i11 150
T
50
100
~~
I
130 OC
Fig. 5.9. Differences between the monograde and multigrade oils with respect to viscosities and viscosity indices
Multigrade oils cover two to five SAE viscosity grades, so that the designations 5W20, 10W30,10W40,15W40,15W50,20W30,20W40 and 20W50 are used. The viscosities and viscosity indexes of oils corresponding to these viscosity class ranges have limited boundaries; the minimum viscosity indexes are specified in ASTM D2270-86. High quality multigrade engine oils should retain their viscosity class even under long-term stress, e.g., in highway operation. Such stay-in-grade oils can be made from mineral base oils and VZ improvers of very good mechanical and thermooxidation stability. 424
Synthetic Engine Oils Intensive efforts to reduce the consumption of fuels has enhanced interest in the use of some synthetic oils in engines. They are used either alone or in blends with mineral oils (243, 244, 245). Suitable synthetic oils (particularly polyalphaolefins and esters of monocarboxylic acids and polyols, esters of alcohols and dicarboxylic acids, dialkylbenzenes and polyglycols) have low viscosities at low temperatures and high viscosity indexes and, at comparable viscosities of the mineral oils, their volatility is substantially lower. Supplies of mineral oil raffinates of viscosities below 6.0 mm2.s1at 100 "C (150 Solvent Neutral) is strictly limited and oils of viscosities below 5 mm2.s-l at 100 OC (100 Solvent Neutral) are unsuitable as major constituents of engine oils because their volatility is too high. Compared with mineral oils, synthetics have higher thermal stability and can therefore solve many lubricating problems in engines which are heavily loaded thermally. Synthetic base oils do not require so high a percentage of polymeric VI modifiers; this contributes to improved cleanliness of heavily loaded engine components. Polyalphaolefins (PAO) - hydrogenated oligomers of 1-octene to dodecene, with viscosities usually between 4 and 6 mm2.s-* at 100 "C are used alone, but more usually blended with 15-3096 of esters for the manufacture of fully synthetic oils or blended with mineral raffinates (10 to 40% PAO) to make partially synthetic engine oils at SAE 5 to lOW/20-50. They have very good rheological properties at low temperatures and low volatility. The low volatility of PA0 keeps down oil consumption and the formation of lacquer on the pistons, and slows down the increase of oil viscosity during use in the engine. The PA0 contributes to reduced formation of carbon in the piston grooves at high engine temperatures. Fully synthetic oils prepared from blends of PAO's with esters have very good properties (148).Oils of this type containing about 30% ester have also proved suitable for lubricating small indirect injection diesel engines (149),where the oil must be frequently drained because of thickening with soot. Synthetic oils have a higher capability to disperse soot than mineral oils, but a lesser ability to protect bearings from wear; they also adversely affect nitrile rubber sealants. "Ester engine oils" comprise esters of dicarboxylic acids (chiefly 2,2,4trimethyladipic, hexahydroisophthalic, decan-dicarboxylic and heptan-dicarboxylic acids) and C, to c26 mono-alcohols (21)(Type l), and those of neoalkylpolyols and monoacids (Type 2) of viscosity 3.5 - 4 mm2.s-l at 100 OC, which achieve a suitable compromise between desirably low volatility and low viscosity. They are frequently constituents in fully synthetic oils in blends with PA0 or of partially synthetic oils in blends with petroleum raffinates, at a concentration of 1530%. Polyol ester oils have especially high thermal stabilities. During differential thermal analysis, partially synthetic oils of Type 1 display inflections between 385 and 392 "C, higher stability oils with Type 2 esters between 455 and 460 "C. There are no substantial differences between oils of these two types in respect of other properties, such as lubricity, effect on sealing materials or resistance to hydrolysis (21).
425
The weak point of ester oils is their sensitivity to water, since they are less resistant to hydrolysis, and their deleterious effect on elastomers. Their ease of hydrolysis causes partial decomposition of the esters into acid and alcohol, and increased corrosion of engine components during shut-down or during operation of a cold engine in city traffic and a cold climate. The adverse effect on elastomers may cause swelling of rubber and erosion of sealants, leading to oil loss by leakage. Ester oils are less responsive to antioxidants such as ZDDP, but ashless antioxidants such as alkylphenols, arylamines and phenothiazines improve their oxidation stability. The absence of ZDDP, however, may cause deterioration of their anti-wear and anti-corrosion properties; these drawbacks need to be compensated by the use of other additives (23). Synthetic and semi-syntheticengine oils made with polyalphaolefins and esters can be improved with detergent-dispersant additives similar to those used for petroleum-based engine oils. Polyol esters are less responsive to some detergents, e.g., sulphonates; the lower ability of PA0 to dissolve some detergent-dispersant packages requires the selection of suitable types. For example, calcium petroleum sulphonates must have a higher molecular weight, about 1200. Suitable ashless dispersants can be prepared by esterifying succinic acid with pentaerythritol. Their oxidation stability can be improved with a combination of ZDDP and a thermally more stable ashless antioxidant, e.g., alkyl-bis-phenols or aromatic amines; this also enables a low viscosity increase of the oil during service in the engine to be achieved.
Polyglycols and dialkylbenzenes are rarely used for the preparation of synthetic engine oils. The merits of the polyglycol oils are their good rheological and detergent properties and the fact that they do not leave sludge after decomposition. Their demerit is their poor compatibility with mineral oil and additives. Dialkylbenzenes, having very good rheological properties at low temperatures, are useful in applications in arctic regions (143).
Lubricity (Load-carrying Capacity) of Lubricant Films in Engines The most severe wear occurs in those componentsof the engine where the conditions are not suitable for the development of hydrodynamic (or elasto-hydrodynamic) friction and mixed or boundary friction prevails. This is particularly descriptive of the piston rings and cylinder surfaces where, in addition to the effects of corrosion promoters in the form of acidic constituents in the blow-by gases and the oil itself, solid impurities (carbon, etc.) on the surfaces and problems of accuracy of design and manufacture, severe scratching, scoring and scuffing may occur because the film is insufficiently thick, or because the load is unevenly distributed on the piston or cylinder surfaces. Aggravated wear (scuffing) occurs mainly during the running-in process, when asperities on the friction surfaces may come into direct metal-to -metal contact and may weld. Such scuffing wear can be controlled by treating the piston ring and cylinder with molybdenum disulphide. Severe. wear of the piston
426
rings and cylinder skirts may also be caused by the corrosive effects of fuel combustion products. These substances are a particular cause of wear of the top rings.
Severe scuffing takes place mainly at both the “dead-centres”of the piston stroke, where the sliding speeds are low or nil, particularly at top dead centre where the temperature and pressure of the gas is highest. Wear also occurs on valve lifters, cams and, in the case of higher compression ratios and oil temperatures, on gear surfaces, because of the high pressure and also because of the difficulty of retaining enough oil on the lubricated surfaces. In such cases, control of scuffing depends critically on the lubricity of the oil, the concentration and nature of the polar substances contained in it and the load-carrying capacity of the oil film,which can be enhanced by sufficiently high viscosity and the presence of polymeric additives, the viscosity of which reversibly increases in sites of lowest shear and, in addition, confer viscoelastic properties on the oil (16,144). Methods are available for measuring oil-film thickness in the piston ring zone and there is enough knowledge of the effects of ring profile and position on the formation of a lubricating film of adequate load-canying capacity. This is based on studies of the mechanism of the hydrodynamic phenomena on the contact points of the piston rings and the cylinder surface. Film-thicknessmeasurements have shown that the effective film-thickness is less than the theoretical and that piston rings operate in a condition of partial oil-starvation (24-26).In connection with work stimulated by efforts to overcome the harmful effects of exhaust gases, it was shown that the lead-free gasoline aggravated cam and lifter wear, with conventional materials of construction, in some car prototypes made in the late 1970’s (27). Modern spark-ignition and compression-ignition engines contain valve gears of various designs. Earlier designs favoured the OHV (over-head valve) system, where the cylinder head valves are operated by push-rods and rockers from the camshaft which is mounted in the crankcase (fig. 5.10a).In the OHC (over-head camshaft) system favoured in many high-output modem designs, the valves in the cylinder head are operated by a camshaft also mounted in the cylinder head through rockers and levers or cams which directly operate the valves.
I
/
1
a
3
b
Fig. 5.10. Systems of actuating gear: a) OHV, b) OHC
1 - camshaft,2 - rocker follower, 3 - valve stem,4 - valve spring, 5 - valve lifter, 6 - valve, 7 - control rod
427
The valve-gear zone is the most critical element in the engine design with respect to wear (both scuffing and pitting corrosion). It significantly affects the functional life of the engine, fuel consumption, performance and the magnitude of dangerous emissions. There are more serious problems associated with the lubrication of OHC systems in which the rocker comes into direct contact with the cam. This not only causes high shear, but also experiences changes in direction relative to the motion of the friction surfaces. This has a deleterious effect on the conditions for (elasto-) hydrodynamic friction, so that boundary friction prevails. High fatigue wear may, however, also occur on the surfaces of the valve lifters in an OHV system, particularly if high Hertzian pressures exist, if the cam profiles are unsuitable and if the combination of materials and oil consumption are adverse. Such phenomena can be observed in large diesel engines with heavy valve-gear components and very strong valve springs, especially in the case of exhaust valve lifters, where the contact with the valves produces a higher tension, since not only the force of the valve spring but also that of the exhaust back-pressure must be overcome. Also, the different exhaust valve cam profile is less favourable for the lubrication of the contact surface with a lifter with too great a sliding friction component (150). The theory of elastohydrodynamic lubrication provides an explanation of the formation of a lubricating film between the surfaces of the cam and the valve lifters. The thickness of the film is determined by variables in the Dowson-Higginsequation for the thinnest lubricating film in EHD contact, namely, the materials of the friction pair, the geometry of the friction surfaces, the mean speed of the surfaces, the specific tension of the contact zone and the oil composition. Allowance must also be made for the fact that the contact surfaces of the valve gear operate under non-stationary conditions, at varying temperatures, speed and load and that changes in the profile of one or both components determines the rate of shear, which may be unfavourable for the formation of a hydrodynamic film. According to (151),the thickness of the lubricating film on the intake valve cam periphery varies during rotation of the cam. The film is thinnest in the section between the vertex and a point about 42" from the vertex. The film becomes thinner with change of temperature (as a result of decreasing oil viscosity) and may become, in this critical section, so thin (0.1 - 0.5 pm) that mixed or boundary friction predominates. The thickness of the lubricating film of oil depends, at the contact point, on the viscosity-temperature and viscosity-pressure coefficients. The determination of these parameters is extremely difficult and so the minimum admissible viscosity for a given cam-to-valve lifter combination at the coolant temperature ( e g , 100 "C) determined empirically is used in engineering practice. According to (152). the lowest critical viscosity of engine oils for such friction pairs is 4 -5 mm2.s-' at 100 "C. However, criteria determined in this simple way can involve great inaccuracies and differences in wear. Extensive wear and damage to the valve lifters due to the lubricating oil film being too thin - and so failing to cover the surface asperities - cannot always be prevented, despite increased antiwear activity of the oil achieved by increasing the concentration of a suitable ZDDP in the oil. According to (153).heavy wear of diesel engine valve gear may also be caused by soot contamination in the oil due to the adsorption of ZDDP decomposition products on the surface of these contaminants. Wear rate also depends on the material of the friction pairs. The following materials are reported by Scott (251) for camshafts: - hardenable grey cast iron, - water-quenched high-carbon or oilquenched alloy steels of high carbon content in a throughhardened condition, - carburised steels with selective flame or induction hardening of the surfaces, - (for cams) Cr- and Mo-containing surface-hardened irons, and for tappets: - through-hardened high carbon, Cr or Mo types of carburised low alloy steels, - grey hardenable cast iron contaning Cr, Mo and Ni, - chilled cast iron. Surface finish treatments, oxide coatings, phosphatisation, salt-bath nitridingand stress relieving can be beneficial. Wear of friction pairs made from heterogeneous materials of different metallurgical compositions is lower. The combination of hardened cast iron and chilled cast iron, or cast iron and steel, is more resistant
428
than steel and steel. Chilled cast iron is very resistant to scuffing but susceptible to pitting corrosion. The reverse is the case with cast iron, but the resistance of cast iron to scuffing is greater than that of steel (the different susceptibility to fatigue wear of cast iron is caused by differing content and distribution of graphite). The austenite content of steel is very important because it is susceptible to wear. Thus, the concentration of elements which promote the formation of austenite in steel (Ni, Mn, C and N) should be reduced and that of elements which suppress it (Cr, W, V, Si) increased. The profile of the curve and the manner of achieving contact should be such as to minimise sliding friction, but this tribologically desirable condition cannot always be met. As the lift of the intake valve should be rapid and high enough to achieve adequate cylinder fill and the lift of the exhaust valves long enough to achieve complete discharge of the cylinders, both profiles affect engine performance.
One of the sources of lubricity in oil is brightstock, so that its presence is desirable, particularly in heavy-duty oils. However, brightstock increases carbon deposits in the piston grooves. ZDDP, which is normally now present in engine oils, reduces scuffing (82). High concentrations of ZDDP’s containing secondary and short-chain alkyls (C, to c6) and of low decomposition temperature contributes to the control of excessive wear of cams and lifters, but does not prevent pitting corrosion of chilled cast iron. The relationship between zinc concentration and the antiwear properties of oils is non-linear, especially in relation to the reduction of cam and lifter wear. In formulating oils to increase antiwear protection, the composition of the ZDDP is not the only important factor; the fractional make-up, chemical composition and viscosity of the base stock, the concentration and composition of other additives, the presence of special antiwear agents, the surface finish and the sliding speed between the friction surfaces are also of considerable importance (222,254,255 and others). The antiwear properties of the oil are related to the formation of protective films on the friction surfaces and the reactions between additives and the decomposition products of additives with polar contaminants and with the materials of the friction surfaces. Since so many factors are involved, the mechanism and kinetics of these phenomena are to a large extent unknown. Contrary results are often obtained, particularly when different devices are used and different configurationsof friction contact, loading rates, sliding speed and shear rates, and, as a result, different temperatures on the contact surfaces. One type of test device was used in attempts to achieve a true simulation of the processes occurring in the valve-train system under field conditions is exemplified by the cam and tappet rig introduced by the CEC in 1970 and the CEC test method (CEC L-15-T-74) in 1974 (the design of this was based on the MIRA Cam and Follower rig introduced in 1961, originally intended for the evaluation of materials for cam and lifter surfaces (90,91, IS@). The test was intended to investigate the effects of engine oils on scuffing (load to cause seizure) and fatigue wear (the number of cycles or the time for the onset of pitting corrosion under certain conditions). The design of the rig makes possible both OHV and OHC valve gear arrangementsand the replacementof the materials of friction couples. However, this procedure is no longer used. Similar devices were developed in the UVMV in Prague (257) and in NAMI in Moscow. Some automobile builders and oil manufacturers developed their own material and oil testing devices, e.g., Fiat (257) and Simca (259). Such single-purpose test devices have both advantages and disadvantages. Their advantage is that they provide the possibility of varying the relationships between load, speed, cam and lifter geometry, oil composition and so on. Their main disadvantage is the impossibility of identifying the influences at work in actual field operation, e.g., contamination of the oil with combustion products, engine vibration, etc. Therefore, full-scale engine tests in dynamometer rigs are always more meaningful and decisive in the final stages of oil development. However, useful studies have been carried out with “motored engines, such as Renault and Toyota, in which, after a period of normal operation, the engine is driven by an electric motor under precisely controlled conditions in order to evaluate individual wear parameters. Standard engine tests have been developed by the American engine builders and have been adopted
429
in oil specifications (e.g., MS-Sequence 11 and IV). Since the design, performance and materials of construction of European and Japanese engines differ from American practice, engine builders in these countries have introduced their own tests. Some have appeared from time to time in the European specifications for engine oils (e.g., Volvo B20, Mercedes-Benz OM 616, Daimler-Benz M102 E HKL and EPA-AMA, Volkswagen 5106 and Peugeot TU-3).The CEC -L-38-T-87 (valve-train wear) method, which uses the Peugeot TU-3 has been adopted for CCMC.
Compatibility with Sealants
As the concentration of additives, especially ashless dispersants increases, and the use of synthetic oils becomes more widespread in oils for modem engines, the compatibility of the oil with elastomers must be closely watched. Special tests have been developed, such as VW 3334, CEC L-39-T-87, is also available, but it is not as rigorous as the VW method. Oil Volatility Oils must retain low volatility even under thermally severe conditions in order to minimise oil consumption and to preserve their desirable properties. Volatile loss due to high temperature occurs on the piston surfaces and in the combustion chamber. This factor is absent in supercharged diesels, probably because the intake system is at or above atmospheric pressure. The viscosity of an oil increases with losses of light constituents by evaporation. Evaporative thickening may surpass the effects of products of oxidation at high temperatures (28). Oil volatility determines the amount of oil vapour produced in the crankcase; in order to minimise atmospheric pollution, this vapour is conveyed to the combustion chamber nowadays (Positive Crankcase Ventilation - PCV). High oil losses may also have other causes, such as foaming, which causes oil leakage from the sump, and fouling and immobilisation of the piston rings as the oil ages. Mechanical defects in the engine, design factors and severe field conditions may also cause the leakage of oil into the combustion chamber through the gap between the piston rings and the cylinder wall (the result of ring wear, cylinder distortion, crossoscillation of the rings, over-pressure in the crankcase, etc.). Increased leakage of oil around the valve guidein gasoline engines can be the result of the sub-atmosphericpressure in the intake system. Excessive top-land deposits cause bore-polishing and high oil consumption. Top-land design is critical for proper oil control. Improved oil consumption figures with cut-back top-lands are due to quicker seating of the top ring and a more favourable pressure distribution among the rings, together with minimum contact between carbonaceous deposits and the cylinder wall (262).
Oil volatility relates to the nature, the fractional composition and the viscosity of the base oil, the concentration of light components, the presence of additives and the working conditions of the engine. It generally decreases with increasing viscosity, but the magnitude of viscous drag and consequently fuel consumption increase at the same time. Multigrade oils are preferable in this respect, provided they satisfy one essential condition. Because of the thickening effect of the polymer at low temperatures, they must be prepared from low viscosity base oils. These must 430
have very narrow distillation ranges to minimise volatility, as well as high viscosity indexes. Engine builders not only specify among other oil parameter requirements maximum volatility (e.g., VW 500.00accepts 13% weight volatile loss in the Noack test - 1 hour at 250 "C - and VW 501.00 20% max. for SAE 5W oils and 15% max. for 10W oils, VW 505 13% max.) but also use special oil consumption tests, such as Cummins NTC-400 in the USA and D-B OM 102E in Europe. In terms of composition, alkanic, ester and other synthetic components, i.e., those of high VI, have advantages in respect of volatility at the same viscosity, as do very narrow oil fractions. Volatility decreases significantly if the first 20% of oil distils above 390 "C at atmospheric pressure. It is therefore recommended to measure the yield of components distilling between 360 and 400 "C, or the temperature at which the first 20 to 40% of oil distils (29).The fractional composition of the oil may also be conveniently determined by gas chromatography (31).Volatility is also indicated by flash-point; the minimum is 193 - 204 "C (30) and, for higher-rated engines, at least 220 "C. A linear correlation enables oil consumption to be predicted from volatility, based on the viscosity at 98.9 "C, the temperature at which the first 20%of oil distils and the VI of the oil (81).However, other factors are involved and the correlation between oil consumption and volatility is not linear (22.29).
Volatility is, as already noted, also influenced by the presence of polymers. Under
. comparable conditions, the consumption of thickened multigrade oils is lower than
that of monograde oils of the same viscosity at 100 "C.This is not only due to higher viscosity at high temperatures because the VI is higher, but also to the elasticity of the lubricant and lower spattering accompanying rapid changes in shear stress, particularly under the severe conditions of a heavily loaded engine. The higher the polymer concentration, the lower the oil consumption even at the same apparent volatility (28).The polymer must, however, possess good mechanical and oxidation stability at high temepratures; if not, the consumption of a multigrade oil can be higher than that of a monograde (16). The working regime of the engine, especially the temperature and speed of movement of the pistons, are, not surprisingly, the factors which most affect volatile losses and oil consumption. In a simplified analysis, as viscosity decreases, the oil can be imagined to leak between the piston rings and valve guides into the combustion chamber at an increasing rate, if the piston speed is ignored. As the piston speed is increased, the quantity of oil conveyed on the piston and cylinder surfaces by the intensive pumping action of the rings during the piston strokes also increases. This causes some oil to spatter into the combustion chamber, causing losses. However, at a certain piston speed and load, the thickness of the lubricant film reaches a limiting value, at which the amount of oil between the piston rings and the cylinder wall starts to become substantially lower, spattering into the combustion chamber decreases and oil losses, even at low viscosity, are lower. The effects of polymers and operating conditions have been well demonstrated in tests with gasoline engines with dry sumps, which enables more accurate measurementsof oil consumption to be made (15).
43 1
SAE 40 and IOW/40 oils (the multigrade oil containing a mechanically "clean" and stable polymer) were compared. Road tests showed that the consumption of both SAE 40 and 10W/40 oils increased proportionately to increasing speed, however the consumption of the SAE 40 oil was higher. These differences can be explained in terms of the viscosities of the oils at temperatures around I50 - 200 "C on the piston surfaces. Whereas the viscosities of both oils were the same at 100 "C, the extrapolated viscosity of the SAE 40 oil at 200 "C was 2.8 mm2.s-l and that of the 10W/40 oil was 3.4 mm2.s-'. It was, however, surprising that the consumption of an oil at SAE 10 with viscosity 1.3 mm2.s-l increased at vehicle speeds up to 80 - 100 km.h-' and - as might be expected -rapidly fell again, relatively, to the consumption level of the SAE 10W/40 oil when the speed was further increased to 110 k m . h l . Logically, there is the danger of a complete collapse of the oil film followed by boundary friction under such extreme conditions. However, it follows from this that anomalous situations may occur and oil consumption cannot be predicted from the oil viscosity at 100 "C, but at higher temperatures,more nearly approaching those prevailing on the piston.
Performance Characteristics The oil comes into contact with oxygen from the air in the crankcase and during circulation in the engine. The larger the contact surface, the greater the intensity of this contact. For example, it is high in those places where oil splashes; it increases with the thoroughness of mechanical mixing (e.g., in the oil gear-pump), with the speed of rotation of the engine components and with the oxygen partial pressure in those places where the fuel or combustion mixture contacts the oil. The effect of oxygen on the oil is also enhanced by increasing temperature and is therefore related to the design, performance and operation of the engine. Air and blow-by entrain into the oil moisture, fuel combustion products (sulphur and lead oxides, carbonaceous residues, acidic combustion products of anti-knock additives if any), reactive components of partially burnt fuel, nitrogen oxides and dust. In addition, the finely dispersed metal particles produced by abrasive wear of the friction surfaces gradually accumulate in the oil. Increased penetration of unburnt fuel into the crankcase can be manifested by an increased amount of lead in the oil sludge and in deposits in the oil ring groove and the piston skirt. This phenomenon, called "grey-paint" or "lead-paint", appears to occur more often in engine tests than under field conditions, although it is a genuine service problem.
Oil ages by the effects of temperature and oxygen and by the catalytic effect of some contaminants, particularly metal particles produced by scuffing. Acidic and condensation products are generated. They produce, along with the dirt entrained by air and blow-by, deposits and varnishes on the frictional surfaces of the engine and they are the cause of increased wear of those surfaces, corrosion of metal components (particularly bearings) and piston rings and high oil consumption. The rate of ageing determines the functional life of the oil, i.e., the time during which it is capable of performing its basic function and can be left in the engine without dangers of excessive Occurrence of the phenomena listed above. Oil ages rapidly in diesel engines, particularly supercharged types with high piston temperatures, despite the fact that substantial reduction in piston temperature 432
is achieved by cooling the pistons with oil splashed from the lower section of the engine. Oil also ages very rapidly in high compression ratio gasoline engines with inadequate oil tanks and oil coolers. Air filtration, especially for engines operating in a dusty environment, is essential. The dust level in quarries and on fields, etc., may be as high as 1 g.m-3,on bitumen road-ways of the g m 3 and on dusty roads lo-’ to order of g.mV3.Filters are available with efficiencies approaching 1 OO%, e.g.. filters with special paper inserts which are star-shaped to enlarge the surface, two-stage mechanical filters (e.g.. Donaldson filters) and combined oil-bath/insert filters.
It is well known that pistons cannot be made perfectly tight. Even good compression rings allow to pass 0.2 to 1% of the combustion product volume into the crankcase and, again, some oil leaks between the rings into the combustion chamber, where it is burned incompletely and products of its partial combustion contaminate the oil. Oil contamination increases with deterioration in the physical condition of the engine, which is associated with poor functioning of the compression rings and increased clearance between the piston and the cylinder wall. Oil ageing is also enhanced by suction of gases from the oil sump back into the engine, particularly if oil- foam is entrained through the intake system into the cylinder, where incomplete combustion occurs, producing chemically unstable products and carbon, and constituting another source of corrosion and deposition. Intake valves can be fouled. The tightly enclosed forced ventilation system of the crankcase is therefore connected to atmosphere via a valve, which allow subatmospheric pressure in the system to develop to a certain point, and an over-rated trap for entrained drops is provided on the sump inlet port. Free ventilation of the crankcase allows as much as 25% of the light products of the thermooxidation products of fuel and oil to escape into the atmosphere, causing pollution. The reduction and even complete prevention of fuel escape can be achieved by forced ventilaton of the crankcase system (PCV); the gases are exhausted via a hose equipped with a regulating valve into the filtered air intake manifold, whilst the oil filling socket is hermetically sealed. The oil is thus readily purged of acidic and unsaturated combustion product fractions, which are partly responsible for “cold sludge” in the oil. On the other hand, some new problems have arisen with corrosion, particularly in the region of the valve gear, and oil consumption has increased.
Some other arrangements also have a deleterious effect on the oil. For instance, engine modifications aimed at reducing the adverse effects of exhaust gases, such as “lean-burn” combustion conditions, lower compression ratios and retarded ignition result in poorer utilisation of the energy of the gasoline and a greater waste heat output, which can raise the oil temperature 8 - 11 “C. Lean-burn combustion produces nitrogen oxides at a higher rate in the combustion products; these oxides leak along with other gases past the piston into the crankcase and enhance sludge production in the oil. This is a particular in engines operating at high temperatures and pressures. NO, compounds play an important r61e in engine oil degradation. They contribute to sludge and varnish formation, oil oxidation and thickening. Mechanisms of NO, effects have not been fully explored.
433
It is generally accepted that they react with unsaturated fuel-derived compounds in blow-by to form deposit-precursors and interact with additives and base oil components. NO, disproportionatesinto nitric oxide, NO, and nitrogen dioxide, NO, (dinitrogen tetroxide, N,O,). At high temperatures and low oxygen concentrations, NO predominates, whilst at lower temperatures and higher oxygen concentrations NO2 increases. The NO, content of blow-by may be 8 - 500 p.p.m. (including 1 -160 p.p.m. NO,). Literature references and preliminary experiments suggest that NO, is the active component which contributes to engine oil degradation.
These effects may become particularly troublesome in those designs in which the problem of reducing the deleterious effects of exhaust gases is overcome by returning it to the combustion chamber with the injected fuel, making contact of the oil with nitrogen oxides and unburned residues more intimate. Excessive amounts of diesel fuel in the oil, caused by poor operation of the injector nozzles, leakage of the injection pump lubricated with engine oil from the sump, etc., may have serious adverse effects on the cleanliness of diesel engines. For example, 166 hour tests in a Skoda ML 634 diesel engine of clean SAE 30 engine oil at API “CB” specification and the same oil containing 5% by weight of diesel fuel demonstrated that the fuel in oil seriously impaired maintenance of the cleanliness of the piston set (227), albeit in a very lightly additive-treated oil. Excessive amounts of diesel fuel in the oil may necessitate premature oil drain. Oil viscosity and viscosity index can have significant effects on engine cleanliness. It has been observed in engine tests that oils with the same concentration and composition of DD additives but higher effective viscosity yielded poorer results. This may be attributable to the lower mobility and thus poorer cooling efficiency of the higher viscosity oil, as well as the formation of a thicker oil film with poorer heat-transfer properties. Consequently,engine temperatures rise and the oil is exposed to more severe operating conditions. The piston temperature may increase by as much as 20 “C. This may also explain why multigrade oils, particularly those covering a wide viscosity range require a higher dosage or compositional changes of additives to achieve the same performance level, and why different concentrations of additives must be used in monograde oils of different viscosity classes and different VZ. The depth of refining of the base stock from which engine oil is prepared is also of considerable importance. Insufficiently refined, or over-refined, oil adversely affects the performance of engine oils and this deficiency cannot always be overcome by increasing the concentration of antioxidantsand detergent-dispersants. The interactive effects of oils and engines operated on lead-free gasoline, now widespread on ecological grounds, have not been entirely assessed. Some workers have observed that there is no adverse effect in respect of wear, sludge formation, varnish and rust, but there is increased scuffing of valve seats (30). Others have noted increased pressure, higher octane requirement and increased wear (35);other again lesser wear of rings and liners (38).The opinion has been expressed that the reduction of lead in gasoline and the change in associated engine design changes may bring about the reduction in deposits and decrease in rusting (from the absence of lead-scavengers in the gasoline) but an increase in cold sludge (36, whilst it has been suggested (37) that the absence of lead adversely affects engine cleanliness. One of the main reasons for this diversity of observation and opinion may be differences in the aromatic contents of the gasolines used.
434
The reduction of lead in gasoline gave rise to the observation,first in Germany in 1982 and the United Kingdom (1984) and later in the USA, Japan and other European countries, of the formation of “black sludge” on the rocker cover, the valve deck, the oil-pump screen and in the sump of gasoline engine. CCMC defines this as: “A soft black sludge formed in the flow areas of the engine which is transformed after a number of oil changes into a hard lacquer which, through the effect of highly detergent oils, flakes off and is carried to the sump, where it blocks the oil-pump intake screen”. It has been found that this phenomenon is connected with increased concentrations of olefins and oxygenated products (e.g., methanol and co-solvents, which promote the formation of N204 and also precipitate oil-insolubles if excessive dilution takes place) in gasoline, but also with changes in engine technology, high oil loading and engine oil technology. Lean-burn engines with an air: fuel ratio >17, used to reduce fuel consumption, produce more N,O,. This promotes the formation of sludge precursors. Lead oxides produced by the combustion of leaded gasoline act as radical traps which limit the nitration of fuel olefins. In their absence, compositions have been developed involving the use of hindered phenols in the oil as radical traps to overcome this problem with lead-free gasoline. The development of combinations of ZDDP and hindered phenols has been stimulated by the change to lead-free gasoline. Crankcase ventilation controls the amount of blow-by gases (containing sludge precursors) which are condensed in the engine. Fouled air systems, such as those used in Ford engines, are very sensitive to sludging. Long drain intervals (e.g., 15,000 km in Germany) - introduced in response to pressure to reduce environmental contamination with waste oil as well as for economic reasons - result in increased concentrations of oil-insolubles. Oils with lower solvent capacity towards oil additives and insolubles may be more sensitive to the accumulation of insoluble. Succinate ester dispersants, the least aggressive towards Viton seals, are the least active in dealing with sludge. The consequences of this situation were the development of the MS-Sequence 11-Eand V-E tests (the basis of the API “SG’classification), because API “ S F oils were unsatisfactory, the modification of the drain interval recommendation by Ford USA (l0,OOO km, 6 months max.), and the development of the Daimler-Benz M-102E sludge test and inclusion of it in Daimler-Benz 226.1 and 228.1, and the Volkswagen 501.01 and 521.07 specifications in Europe. The first modification of the Daimler-Benz M102Etest consists of 150 cycles of one hour each. These cycles, in aggregate, are equivalent to 10,OOO km of service. The operating conditions for each cycle are: seven stages of different duration (I to 27 minutes), speed (650 to 5,500 r.p.m.), different cooling fluid temperatures (40 to 100 “C) and different sump oil temperatures (50 to 135 “C).
The most severe service conditions for an engine are those in which it heats up excessively and those in which it remains cold. Excessive heating of the engine and the oil charge in highway vehicles occurs when travelling at high speed for a long time on the highway at full engine load (e.g., with a trailer). Over-heating of the oil also occurs with engines equipped with automatic transmissions (by 5 to 8 “C),with inadequate oil charges and oil-circulationfactors, with wrongly-mountedcrankcases and as a result of excessively long intervals between oil drains (39,40).Conditions such as these result in premature ageing of the oil and its constituents, accompanied by the formation of deposits and varnish on the pistons, sticking of the piston rings, wear of components and pre-ignition and an increase in the octane requirement of the engine. By contrast, inadequately-heated oil (e.g., during warm-up, stop-go running and in winter) lead to condensation of water in the engine and penetration into the crankcase oil of unburned or partially-burned fuel, oxygenated products and soot. Acidic substances and cold sludges form from the combination of these products with water, which clog oil-ways and piston grooves, increase oil viscosity, etc. (Table 5.3). 435
Table 5.3. Typical Composition of Cold Sludge (%)
Oil Water Lead (present as PbBr2) Soot and carbon Oxidised hydrocarbons Iron, silicon and the like
Gasoline engines (leaded fuel)
Diesel engines
30 - 50 1-8 10-20 20 - 25 10- 15 1-2
40 - 45 1-5 30 - 4 0 15-20 1-2
Oil is not only adversely affected by the products of incomplete fuel combustion; complete combustion produces sulphur oxides from the fuel, nitrogen oxides from air, lead and hydrogen chloride and hydrogen bromide (from lead scavengers) in leaded gasoline, and products from the interaction of all or some of these, e.g., corrosive acids, with water. The adverse effect of acidic fuel combustion products on engine cleanliness and component wear increases with thermal load on the engine. This effect can be suppressed by the basicity reserve of the lubricating oil in use, expressed as total base number, TJ3N. The minimum TBN requirement increases with increasing engine performance characteristics and some engine builders stipulate their own TBN specifications for oils used in their engines. For example, Caterpillar determined the minimum TBN of API-CD engine oils to be used for lubricating their diesel engines, and related the sulphur content of the fuel in use (from 0.5 to 1.5% weight sulphur in fuel) to the minimum TBN of the used oil, so as to provide an index which indicates the need to replace the engine oil (fig. 5.11).
0.5 l:o 1.5 SULPHUR IN FUEL l%wt)
Fig. 5.1 1. Minimal TBN-values (according to ASTM D 2896) of original and used engine oils API CD in dependence of the sulphur content in fuel for the Caterpillar engine a - for original oil, b - for used oil Caterpillar also required that the engine oil be drained and replaced if the wear rate expressed as iron and chromium content of the oil exceeds an acceptable level, regardless of oil TBN. Oil basicity can be achieved by a combination of additives, of which some may be more efficient than others in reducing corrosive wear. Total wear is not, of course, solely due to corrosion and may not be proportional Lo TBN.
436
High oil basicity cannot, however, guarantee that the weak acids produced by the thermal oxidation of oil and additives or their hydrolysis will be neutralised. These weak acids can cause corrosive wear even if the TBN is high.Therefore, not only the TBN, but also the total acidity (TAN) of the oil must be monitored. Oil basicity decreases as a function of the production of the concentration of acidic products produced during operation. The extent and rate of decreases vary with the type and physical condition of the engine, the operating variables, the composition of the fuel, etc. Sulphur content of diesel fuels, in particular, is especially significant. Fig.5.12 illustrates the effect of sulphur in diesel fuel on the decrease in engine oil basicity measured during the operation of a two-stroke compression-ignition engine operated at 900 r.p.m., with 230 mm cylinder bore and lubricated with an oil of TBN 7.5 mg KOWg (160). When diesel fuel of 1% weight sulphur was used, the basicity dropped to 1.0 TBN after 500 hours; wear and deposits increases appreciably with a further drop in TBN.
P 8 I 0 7 Y 6
$ 5 - 4 z 3 + 2 1 0
200 400 600 800 1000 1200 1400 1600 WORK TIME OF ENGINE Ih 1
Fig. 5.12. Dependence of TBN change on the content of sulphur in diesel fuel and the time of oil use in the engine A - 1 .O % wt, B - 0.75 9% wt, C - 0.4 % wt sulphur in the fuel
km
Fig. 5.13. Relation between the corrosion wear and TAN-value of the engine oil
According to one reference (Z6I),the decrease in oil basicity during heating and oxidation is independent of the base oil used, but results from decomposition of the basic additives, their reaction with other additives, their neutralisation by acidic oxidation products and retention of particles which contribute to the alkaline reserve by the oil filter. The total acidity (TAN) and corrosive wear (expressed as amount of iron, or, if CuPb bearing shells are used, of copper in the oil) increases proportionately with 437
decrease in the alkaline reserve of the oil (expressed as TBN). An abrupt, exponential increase in corrosive wear occurs in that region where TAN exceeds TBN (see fig. 5.13). The quantity of foreign matter penetrating into the engine is related to the composition of the fuel, particularly its concentration of heavy fractions, sulphur and lead, and to the efficiency of combustion. The combustion of fuel is always to some extent imperfect (e.g., as a result of changes in operating conditions,or maloperation of some part of the engine) under field service conditions, particularly during idling or braking. Imperfect combustion in diesels occurs during overload, when the amount of fuel injected increases and the injection period lengthens. This delays ignition and causes imperfect combustion. Also, poor condition and operation of injector nozzles (e.g., due to clogging of the injector holes), sticking of the needle in the nozzle, maladjustment of the fuel injection timing, very high compression and a high concentration of heavy fractions in the fuel may all contribute to imperfect combustion, manifested by smoking of the engine exhaust. Accumulation of soot, which occurs mainly in high-speed, small displacement diesels, causes excessive oil-thickening. If the soot concentration exceeds about 12% by weight, the oil becomes thixotropic and is only mobile at a certain sliding speed. Soot also impairs the efficiency of antiwear additives (e.g., by adsorption on to its own surface) and may itself cause wear in the piston area. Soot with a higher oxygen content causes less wear. The functional life of an oil is thus dependent to an appreciable extent on factors external to the oil itself. The oil-drain intervals may relate not only on the distance travelled by the vehicle, hours on the engine or the amount of fuel consumed, but also on the prevalent service of the engine. The API recommendation introduced in 1972 was based on these factors. This recommendation specified the factors which dictate the reduction of oil-drain intervals, such as city stop-go running, particularly in winter, insufficient filtration of oil in by-pass systems, frequent towing of trailers and sustained highspeed, highway driving. Should any of these factors, which are deleterious to oil quality, be present, it appears desirable to reduce the recommended oil-drain period for a given engine by 30%;should two or more arise, the drain interval must be reduced to about 50%.
The performance characteristics of engine oils cover their ability to withstand adverse effects such as these in engines of various types and various designs, operated under varied conditions, with different types of fuels and required to neutralise the effects of fuel combustion products and, as ageing advances, the effects of contaminants. Performance characteristics are determined by the composition and properties of the base oil and by the composition and concentration of additives, especially antioxidants and detergent-dispersants, in the oil. Engine oils can thus be classified by their per$ormance Characteristics. The first classification of engine oils by their properties and the prevailing operating conditions was introduced by the API in 1947. There were three classes of engine oils: Regular oils - pure mineral oils, or those containing only pour-point depressants or viscosity modifiers, suitable for light-duty gasoline and diesel engines, Premium oils - containing oxidation and corrosion inhibitors but no DD additives, suitable for medium-duty gasoline and diesel engines, Heavy-duty (HD)oils - containing, in addition, detergent-dispersantadditives, and suitable for heavyduty engines.
At present, a classification produced in collaboration with the ASTM and the SAE is in operation. A similar classification - GOST 17479-72 - was proposed in 438
1971 for the Comecon countries. The methodology of both classifications and comparisons between them are detailed in Table 5.4 (see also (145)). S A E J 183 specifies the bench tests required and the minimum standards to be achieved for the classification of engine oils (Table 5.5); the basic operating parameters of the selected engine tests are detailed in Table 5.6, In addition to the API (ASTM-SAE) stipulations for the classification of oils according to their minimum properties, more detailed performance classifications have been in use in the USA since 194 1 for commercial and military specifications. The military specifications, carrying the prefix MIL-, normally require performance standards in gasoline and diesel engines over a wide range of operating conditions (Table 5.7). Commercial specifications normally only cover one type of engine. Thus for CD oils, Caterpillar Tractor Co. have at various times established their own engine tests for oils to be used in their engines, namely: Caterpillar OL-1 - 250 hours, oil drain after 125 hours, multi-cylinder diesel engine. Caterpillar OL-2 - evaluation of oil quality from its consumption, Caterpillar OL-3 - similar to the OL-I, in a supercharged diesel engine with direct fuel injection and 2.54 mm piston-crown clearance, Carerpillar OL-4 - similar to OL-2, in the same engine as the OL-3, but with 0.75 mm pistoncrown clearance. Caterpillar I-K (current) - single-cylinder test to evaluate 1991-type US diesel lubricants. The OL- series tests were developed and run by Caterpillar, although they were also available in other laboratories. Development of the I-K test was initially carried out by Caterpillar, but the oil, motor and additive industries finalised development of the procedure under the auspices of the ASTM. Mack have similar tests (Mack T-1 to T-7).This test was stated to be the ”cornerstone of the proposed API CF4 specification”. It will probably be incorporated into the new CF and CF-2 specifications, along with Detroit diesel’s 6V92TA test. A new parameter, WD-K, will be used to assess piston deposits: the amount of top land heavy carbon will be measured, plus brake specific oil consumption (GMIKW-HR) as an additional parameter.
Table 5.4. Approximate Equivalence of APUSAE J 183 and GOST 17479-72 Engine Oil Classifications API Recommendation for oil usdother class specifications complied with
GOST class
SA* For older engines operating under such mild conditions that the protection afforded by compounded oils is not required;only additives pour-point depressants and viscosity modifiers.
Straight For spark ignition engines with no oils high quality oil requirements.
SB* For older engines operating under mild conditions; minimum additive dose of antioxidant, anticorrosion and antiwear agents only. (Oils in categories SA, SB should nor be used in any engine unless specifically recommended by the manufacturer.)
A
Recommendation for oil use
For spark-ignition engines operating under mild thermal load.
439
(Table 5.4 contd.) API Recommendation for oil uselother class soecifications comolied with
GOST Recommendation for oil use class
~~
BI SC* For spark-ignition engines of passenger cars and some trucks, 1964-67 models, operating under conditions specified by engine manufacturers; oils contain additives protecting against high- and lowtemperature deposits,corrosion and wear (tvoical swcification Ford M2C 10111967).
For spark ignition engines operating under medium and slightly increased load, enhancing formation of engine deposits and bearing corrosion.
SD* For spark-ignition engines of VI passenger cars and trucks, 1968-7I models, operating under conditions specified by engine manufacturers; oils contain additives similar to SC oils at a higher level of effectiveness (typical specification Ford M2C 101B and GM 604 I , 1968).
For spark-ignition engines having higher oil-quality requirements; for diesel engines requiring oils resistant to formation of deposits and with antiwear properties.
GI SE* For passenger car spark-ignition engines of 1971 and some 1971-1979 models operating under conditions specified by the engine manufacturer; oils contain additives providing protection against high-temperature deposits and low-temperature sludges, corrosion, rust and wear (typical specification Ford M2C, GM 6136M, MIL-L-46152A (1972); oils meet MIL-L-46152 and the CC specification). SF* For spark-ignition engines of passenger cars of 1980 and later models and lightduty trucks requiring oils of higher thermooxidation stability and superior antiwear properties. SG For spark-ignition engines of passenger cars, vans and light trucks of 1988 and later models operating to manufacturers’ recommended maintenance procedures (mainly using lead-free gasoline). Oils developed for this service provide improved control of engine deposits (including “black sludge”), oil oxidation and engine wear compared with oils of earlier classifications, plus rust and corrosion protection. Includes performance properties of oils meeting CC specification; can also be used where API SF, SE, SF1CC or SWCC are recommended.
For spark - ignition engines operating under adverse conditions; for diesel engines operating under higher load and more adverse conditions
440
-
-
(Table 5.4 contd.) API Recommendation for oil uselother class specifications complied with
GOST Recommendation for oil use class
CA* For spark-ignition and naturallyBZ aspirated diesel engines on low-sulphur fuel, operating under mild conditions; oils contain additives which reduce hightemperature deposits and corrosion of bearings (meets specification MIL-L-2104A).
For medium-stressed diesel engines
CB* For spark-ignition and normallyaspirated diesels on low-sulphur content fuel requiring better protection against wear and deposits: oils contain additives suppressing high-temperature deposits, bearing corrosion (oils comply with Supplement 1 and DEF-2101D).
VZ
As above
GZ CC* For moderately supercharged diesel engines for trucks and tractors operating under medium- and heavy-duty conditions and for some spark-ignition heavy-duty engines; oils contain additives suppressing high-temperature deposits, bearing corrosion and rusting and low-temperature sludge in spark-ignition engines (meet reauirements of MIL-L-2104B & 46152).
Asabove
CD For heavy-duty and high r.p.m. supercharged engines on fuels of widely variable quality; oils contain additives suppressing efficiently high-temperature deposits and bearing corrosion (comply with Caterpillar Tractor Co. specification Series 3, MIL-L-45 199B & 2104C: oils complying with --MIL-L-2104C meet requirements of SC). CD lIFor 2-stroke diesel engines; meet all requirements of API CD.
D(E)
For diesel engines whose operation and fuel supply enhance formation of deposits, corrosion and wear, or which operate under extremely adverse conditions.
-
CE For turbocharged or supercharged heavy-duty diesels manufactured since 1983 and operated under both low-speedhigh-load and high-speedhigh-load conditions. Oils designed for this service may also be used when previous API engine service categories for diesel engines are recommended. CF-4Adopted 1990, describes oils for highspeed, 4-stroke diesels. Replaces CE. Meets CE requirements and provides improved control of oil consumption and piston deposits. Particularly suitable for on-highway, heavy-duty truck applications.
-
-
* Indicates obsolete test techniques.
441
Table 5.5. API Classification 5-183 - Engine Test Requirements API Engine test class
Primary performance criteria
SA
None
None
SB
L-4* CRC L-38 Sequence IV*
Bearing weight loss, mg max.
500 500
Cam scuffing Lifter scuff rating, max.
None 2
SC
SD
442
CRCL-38 Bearing weight loss, mg max. Sequence IIA*/IlIA* Cam & lifter scuffing Avg. cam + lifter, in./mm max. Avg. rust rating, min. Avg. sludge rating,min. Avg. varnish rating, min. Sequence IV* Cam scuffing Lifter scuff rating, max. Sequence V* Total engine sludge rating, min. Avg.piston-skirt varnish rating, min. Total engine varnish rating, min. A v g h t a k e valve tip wear, in./mm, max. Ring sticking Oil ring blockage, % max. Oil screen plugging, % max. L-1*/0.95% S fuel Groove l(top), carbon fill, % vol. max. Groove 2 and below Sequence IIB*/IIIB* Cam & lifter scuffing Avg. cam + lifter wear, in./mm max. Avg. rust rating, min. Avg. sludge rating, min. Avg. varnish rating, min. Cam scuffing Sequence IV* Lifter scuff rating, max. Sequence VB* Total engine sludge rating, min. Avg. piston skirt varnish rating, min. Total engine varnish rating, min. Avg. intake valve tip wear, in.(mm), max. Oil ring clogging, % max. Oil screen plugging, % max. CRC L-38 Bearing wt. loss, mg, max. L-l* (0.95% min. Groove No. l(top) carbon fill, S fuel) or Cat. 1-H* vol.% max. Groove No. 2 lacquer coverage, % area, max. Groove No. 2 and below Land No. 3 and below Falcon* Avg. engine rust rating, min.
50
None 0.0025/0.064 8.2 9.5 9.7 None 2 40 7.0
35 0.0020/0.051 None 20 20 25 Essentially clean None 0.0030/0.076 8.8 9.6 9.6 None 1 42.5 8.0 37.5 0.00 1S(0.038) 5 5
40 L-1 25
1-H 30
50 Essentially clean - Essentially clean 9.0
(Table 5.5 contd.) API Engine test class SE CRCL-38 Sequence IIB*, IIC* or IID Sequence IIIC* or IIID
Primary performance criteria Bearing wt. loss, mg m a .
40 IIB 8.9 None
Average engine rust rating Number of stuck lifters
Viscosity increase at 100°F (37.8OC) and 40 h, % max. Viscosity increase at 40°C and 40 h, % max. Avg. engine ratings at 64 test h Sludge rating, min. (CRC manual No. 12) Piston skirtvarnish rating, min. (CRC manual No. 9) Oil ring land deposit rating, min. (CRC manual No. 9) Ring sticking Lifter sticking Scuffing and wear at 64 test h, Cam and lifter scuffing Cam+lifter wear, in. (mm), avg. . max.
Sequence VC* or VD Avg. engine sludge rating, min. (CRC manual No. 12) Avg. piston skirt rating, min. (CRC manual No. 9) Avg. engine varnish rating, min. Oil screen clogging, % max. Oil ring clogging, % max. Compression ring sticking Cam wear, in (mm)
SF
Sequence IID Sequence IIID
Avg. engine rust rating, min. Number of stuck lifters Viscosity increase at 40°C at 64 test h, % max. Avg. sludge rating Avg. piston varnish rating(CRC manual No. 9) Avg. oil ring land deposits rating (CRC manual No. 9) Lifter sticking Ring sticking Scuffing and wear Cam and lifter scuffing Cam+lifter wear, in. (mm),avg. (max.) (avg.)
IIC 8.4 None IIIC
IID 8.5 None IIID
400
400 375
9.2
9.2
9.3
9.1
6.0 None .None
4.0 None None
None None O.OOlO(O.025) 0.0040(0.102) 0.0020(0.051) 0.0080(0.203) vc VD 8.7 9.2 7.9 8.0
6.4 6.3 5 10.0 5 10.0 None None - Rate & report 8.5 None 375 9.2 9.2 4.8
None None None None '0.004q0.102) 0.0080(0.203)
443
(Table 5.5 contd.) API Engine test class Sequence VD
CRC L-38 SG
CRCL-38 Sequence IID Sequence IllE
Sequence VE
Caterpillar 1-H2
CA
Avg. engine sludge rating, min. (CRC manual No. 12) Avg. piston skirt varnish rating, min. (CRC manual No. 9) Oil ring clogging, % max. Oil screen clogging, % max. Compression ring sticking Cam wear, in. (mm),avg., max. max. Bearing wt. loss, mg, max. Bearing wt. loss, mg, max. Avg. engine rust rating, min. Number of stuck lifters Piston skirt varnish rating, min. Avg. engine sludge rating, min. Ring land deposits rating, min. Number of stuck lifters Number of stuck rings Scuffed cams and lifters Viscosity increase at 40 'C, 64 test h, % max. Cam+lifter wear, in. (mm),avg., max. max. Oil consumption, max. Avg. engine sludge rating, min. Rocker arm cover sludge rating, min. Avg. piston skirt varnish rating, min. Avg. engine varnish rating, min. Oil ring clogging, % max. Oil screen clogging, % max. Compression ring sticking Cam wear, in.(mm), avg, max. max. Top groove fill, vol.% max. Weighted total demerits, max. Ring side clearance loss, in.(mm), max.
L-4* or CRC L-38
L-l*(0.35%S min. fuel) CB
Primary performance criteria
Bearing wt. loss, mg, max. Piston skirt varnish rating, min. Groove No.l(top) carbon fill, vol. % max. Groove No. 2 and below
L-4* or CRC L-38 Bearing wt. loss, mg, max. Piston skirt varnish rating, min. L- 1*(0.95%S Groove No. 1 (top) carbon fill, min. fuel) ~ 0 1 . 8max. Groove No. 2 and below
9.4 6.6 10.0 7.5 None 0.001O(0.025) 0.0025(0.064) 40 40 8.5 None 8.9 9.2 3.5 None None None 375 0.0012(0.030) 0.0025(0.064) 4.88 9.0 7.0 6.5 5.0 15.0 20.0 None 0.005(0.013) 0.0015(0.038) 45 140 O.OOO5(0.0 13) L-4 L-38 120-135 50 9.0 9.0 25 Essentially clean 120-135 50 9.0 9.0 30 Essentiallv clean
(Table 5.5 contd.) API Engine test class
Primary performance criteria
CC
Bearing wt. loss, mg, max. Piston skirt varnish rating, min.
CRCL-38 LTD* or modified LTD*
Sequence IIA*, IIB*, IIC* or IID Cat. I-H* or 1-H2
Piston skirt varnish rating, min. Total engine varnish rating, min. Total engine sludge rating, min. Oil ring plugging, % max. Oil screen clogging, % max. IIA Avg. engine rust rating, min. 8.2
35 25 25
50 9.0 modified LTD 7.5 42 42 10 10
IIB 8.2 1-H
IIC 7.6 1-H2
LTD 7.5
Groove No. 1 (top carbon fill, 30 vol. %, max. Groove No. 2 lacquer, % area, max. 50 Land No. 3 and below Essentially clean Weinhted - total demerits, max. Ring side clearance loss,. in.(mm), max CD
CRCL-38 Cat. I-D* Cat.1-G* or 1-G2
Bearing wt. loss, mg, max. Piston skirt varnish rating, min. Groove No.I(top) carbon fill, vol.%, max. Groove No.2 and below 1-G Groove No. 1(top) carbon fill, 60 vol.% max. Land No.2, carbon and lacquer coverage, % area, max. 50 Groove No.2, carbon and lacquer coverage, Iarea, max. 50 Land No.2 and below Essentially clean Weighted total demerits, max. Ring side clearance loss, in.(mm), max.
CDII This oil must meet the engine test requirements for API CD and, in addition, the limits of the Detroit-diesel 6V-53T test: Piston area Weighted total demerits, avg., max. Hot stuck rings Face distress, No.3&3 rings, demerits, max. Liner and Head area Liner distress, avg. % area Valve distress
IID 7.7
45
-
140 0.0005(0.0l3)
50 9.0 75 Essentially clean 1-G2
80 80
-
300 0.0005(0.013)
400 None
13.0 12.0 None
445
(Table 5.5 contd.) API Engine test class CE
Primary performance criteria
CRCL-36 Bearing wt. loss, mg, max. Caterpillar 1-G2 As for CD C u m i n s NTC-400 Oil consumption Crown land deposits, 8 covered with heavy carbon, avg., max. Piston deposits, third ring land, total CRC demerits for all 6 pistons, max. 40 Camshaft roller follower pin wear, mm, max. Mack T-6 Merit rating, min. Avg. rate of viscosity increase during Mack T-7 last 50 h, mm*.s-' at 100"Chour, max. 0.040
50
< reference oil 25
0.05 1
90
Indicates obsolete. tests.
-
Table 5.6. Basic Parameters of Engine Tests APUSAE 5-183Classification of
Engine Oils CRC-L-38
Purpose: Engine used: Fuel: Test operating conditions: Duration,h Speed, r.p.m. Fuel flow, Ib/h Air/fuel ratio Spark advance, BTDC Crankcase vacuum, in.
To evaluate the oxidation, copperflead bearing corrosion and depositforming tendencies of a crankcase lubricating oil. Labeco CLR laboratory test engine, singlecylinder 3.8x3.75" spark ignition. Iso-octane + 3 ml TEL motor mix per US gallon. 40 plus 4.5 3150 25 4.75 f 0.25 14.0 1 f 0.5 35 f 1 2 f 0.2
*
run-in. Coolant outlet temp., 0F 200f2 Coolant inletloutlet A temp., "F 10 f 2 Gallery oil temp.,"F 290 f 2* Oil pressure, p.s.i. 40f2 Crankcase blow-by 30f 1
* 275 f 2°F for SAE 10 only. Procedure reference: US Fed.STD 791-3405 MS Sequence llD Purpose:
To evaluate low-temperature rusting and corrosion characteristics of an oil. Engine used 1977 Oldsmobile V-8,5.7 litre, 8.5:l compression ratio. Fuel: GMR 995 gasoline. Phase I1 Phase I11 Test operating conditions: Phase I Duration, h 28 2 2 Speed, r.p.m. 1400f20 1500f20 3600f20 Load, BHP 25f2 25f2 100f2 Oil inlet temp., "F 120 f 2 120f2 260f2 llOf 1 Coolant outlet temp., "F 120f 1 200f2 Procedure reference: ASTM STP 3 15
446
(Table 5.6 contd.) IIID
MS Sequence
Purpose: Engine used:
IIIE
To evaluate the high-speed, high-temperature oxidation wear and deposit-forming tendencies of motor oils for gasoline service. 1977 Oldsmobile V-8.5.7 litre, Buick V-6, 3.8 litre 8.5: 1 compression ratio GMR 995 gasoline
Fuel: Operating conditions: Engine is run for 64 h at constant speed and load, with a shutdown every 8 h for oil sampling and levelling. 3000 f 20 3000 f 20 Speed, r.p.m. Performance, kW 74.6 50.6 Coolant temp., "C 118 118 Oil temp.,"C 149 149 Air inlet temp., "C 27 27 Air inlet humidity, g.kg'' 11.4 11.4 Aidfuel ratio 16.5: 1 1651 V-D
MS Sequence
Purpose: Engine used: Fuel:
V-E
To evaluate sludge, wear and varnish performance of an oil under high-, medium- and low-temperature conditions. Ford Pinto 2.3 litre Phillips J. unleaded gasoline; in V-D, test fuel enters engine via carburettor, in V-E via direct injection.
Operating conditions: Duration, h in repeated cycles Speed, r.p.m. Performance, kW Coolant outlet temp., "C Oil temp., "C Air inlet temp., "C Air inlet humidity, g.kg-' Water temp., "C EGR
192 1 (120 min.) 2500 f 25 25 57 79(68) 27 11.4 57 on
288 2 (75 min.) 2500 f 25 25 68 86(99) 27 11.4
69 on
3 (45 min.) 750 f 25 0.7 49 49(46) 29 11.4 49 off
Procedure reference: ASTM STP 315 Caterpillar
Purpose:
Engine used: Engine arrangement: Fuel:
1-H2
1-G2
To evaluate the effect of an oil on ring-sticking, wear and accumulation of deposits under high-speed, medium supercharged (1-H2) or supercharged (14 2 ) conditions. Single-cylinder Caterpillar diesel 5.125x6.5" (130.2 mmx165.lmm), piston type 1Y510. 1Y73 1Y73 supercharged diesel fuel (0.45%sulphur)
447
(Table 5.6 contd.) Operating conditions: Speed, r.p.m. Load, BTU/min. Exhaust temp., "C Compression ratio: Oil inlet temp., "C Water outlet temp., "C Air inlet temp., "C Air inlet pressure, in. Hg (abs.) Procedure references: US Fed.STD ASTM
1800 f 10 4950 f 50 566 f 10 16.4 f 0.5 82 f 2 71 f 2 77f2 40f3
1800 f 10 5850 f 50 593 It 10 16.4 f 0.5 96 f 2 88 f 2 119f2 53 f 0.3
79 1-346 509
791 -341 509A
Cummins NTC-400
Purpose: Engine used: Fuel: Test procedure:
T o evaluate deposit formation and oil consumption control in a highly-loaded turbocharged and after-cooled diesel engine. Cummins 6-cylinder turbocharged and after-cooled direct injection diesel engine of 855 cu.in. capacity, rated at 400 BHP at 2100 r.p.m.. FTM 791-346 diesel fuel (0.4%sulphur) A production 400 HP engine is modified by restricting water flow to oil cooler and after cooler in order to achieve the conditions listed below. The engine is run for 200 h at 5% above normal maximum fuel rate and oil consumption monitored by weighing and external oil tank which feeds a constant level system. Average oil consumption rate in the 20-40 h period is compared with that in the 180-200 h period.
Operating conditions: Time, h Speed, r.p.m. Fuel flow, Ibh Oil temp. after cooler, "C Air inlet temp., "C
200 2100 151 121 143
Mack T-6
Purpose: Engine used: Fuel: Operating conditions: Duration
To evaluate the ability of an oil to control piston deposits, ring wear and oxidation in severe service. Mack ETAZ 673 6cylinder, 4-stroke, after-cooled turbocharged diesel, 672 cu.in. (keystone rings). diesel fuel (0.1 to 0.3%sulphur).
600 h in 50 x 12 h cycles: Stage 1 Time, h 4 Speed, r.p.m. 1400 Load, ft.lb. 990 Fuel flow, Iblsec. 39.3 Manifold pressure, in. Hg 29 Oil temp., "C 113 Coolant temp., "C 88 Air inlet temp., "C 29.4
Procedure reference: Mack 5GT49
448
Stage 2 4 1800 918 32.7 41 113 88 29.4
Stage 3 4 2100 816 29.3 46 113 88 29.4
(Table 5.6 contd.) Mack T-7 Purpose:
To evaluate the viscosity increase characteristics of an oil under lowspeed, full-load conditions in a 4-stroke, turbocharged and aftercooled diesel engine. Engine used: Mack EM 6-285 6-cylinder, 4-stroke, turbocharged and inter-cooled diesel engine of 672 cu.in. capacity and rated at 283 BHP at 2100 r.p.m. Fuel: diesel fuel (0.5% max. sulphur), cetane index 40 min. Test procedure: Engines are replaced or re-built every 2000 h and conditioned by flushing and break-in procedures. Individual oil evaluations are made by running engine for a maximum of 150 h at full load. Viscosity increase rate between 100 and 150 h is measured and referenced by poor and good reference oils. he-test flush Test Operating conditions: Break-in 2 150 0.5 0.5 Time, h 2100 1200 Speed, r.p.m. 1200 2100 700 Load, klb. 1060 700 1080 Fuel flow, Ib./sec. 40 0.6 Manifold pressure, in. Hg 30 f 5.6 113 Oil temp., “C max. 85 zt 2 Coolant temp., “C 29.4 zt 2 Air inlet temp., “C
*
Procedure reference: Mack 5 GT 57
Table 5.7. US Military Engine Oil Specifications - Laboratory Engine Test Requirements Engine test 2 104E (Diesel) CRC L-38 Bearing weight loss, mg max. Piston varnish, min.
50
Sequence IID Avg. rust, min. Lifter sticking Sequence IIIE Visc. at 40 “C, increase at 64 h, %,max. Avg. rating at 64 h: Piston varnish, min. Oil-ring land deposits, min. Sludge, min. Ring sticking Lifter sticking Scuffing & wear at 64 h: Cam or lifter scuffing Carn+lifter wear, inches: Average Maximum
MIL-L46152D 21260D 46167B (Gasoline) (“Factory fill”) (“Arctic”) 40 9.0
50
9.0 none
50
none
375 8.9 3.5 9.2 none none none
none
none
none
0.0025 0.0070
0.0012 0.0025
0.0025 0.0070
0,0025 0.0070
449
(Table 5.7 contd.) Engine test 2104E (Diesel)
MIL-L46152D 212601) 46167B (Gasoline) (“Factory fill”) (“Arctic”)
Sequence V-E Avg. engine sludge, min. Rocker cover sludge, min. Avg. engine varnish, min. Piston skirt varnish, min. Oil-ring clogging, %, max. Oil-screen clogging, %, max. Compression-ring sticking (hot) Cam+lifter wear: Average, pm, max. Maximum, pm
8.5 6.5 4.2 6.0 15.0 23.0 none
6.5
8.5 6.5 4.2 6.0
20.0 none
8.5 6.5 4.2 6.0 15.0 23.0 none
203 457
I27 381
I96 441
203 457
Caterpillar I -H2 Top groove fill, %, max. Weighted total dements, max.
-
45 140
Caterpillar 1-G2 Top groove fill, %, max. Weighted total dements, max.
80 300
80 300
80 300
Detroit Diesel Piston area: Avg. total deposits, max. 400 Hot stuck rings none Avg. ring face distress, max.: Fire ring report Nos. 2 & 3 compression 13.0 Liner and head area: Avg. liner scuffing,% area, max. 12.0 Valve distress none report Port plugging, $6, max. Allison C-3 (Seal) Total immersion (Buna M): Ot0+5 Volume changes, % Of5 Hardness changes, points Dip cycle (polyacrylate): Volume changes, % o t o 10 Hardness changes, points oto+5 Dip cycle (silicone) Ot0+5 Volume changes, % Hardness changes, points oto-10 Allison C-3 (Time. torque) Slip time at 5500 cycles, s., max. 0.85
9.0 7.0 5.0
15.0
15.0
23.0 none
400 none
- 13.0
report
12.0 none report
-
-
0 to +5 Of5
-
0 to 10 0 to +5
-
0 to +5 0 to -10 0.85
Allison C-3 (Anti-wear) Degree of grinding pattern remaining Scuffing. scoring or chatter marks
none
Caterpillar TO-2 Stopping time increase.%. rnax. 350
350
450
360
The special Caterpillar TO-2 test was intended for testing the suitability of oils in reduction gears under load, used in construction machinery. This test identifies differences in the friction characteristics of oils in “wet” (oil-immersed)clutches with bronze-steel friction pairs. The clutch consisting of bronze discs and steel separator plates is cyclically loaded in a system specially designed by Caterpillar, similar to an engine-powered SAE 2 machine. After a certain number of cycles, the motor drive is shut down at the same instant as pressure is exerted on the clutch. The time interval during which the machine stops by the action of the clutch friction is measured; the change in time interval to stop is a measure of the friction characteristic. This test has been replaced by the TO-4. A similar test, the Detroit Diesel Allison C-3 test is also made in the SAE 2 machine, but graphitesteel friction is employed. This is specified in “Detroit Diesel Allison Type C-3” for gear fluids. The change in torque to stop is measured. The main characteristics of both tests and limiting values are detailed in Table 5.8.
Table 5.8. Basic Characteristics and Limit Values of Caterpillar TO-2 and Detroit Diesel Allison C-3 Tests ~
~~
~~
~~
~
Test
Friction materials
Test duration
Limit values
TO-2
Bronzdsteel
15000 cycles (one cycle = 15 sec.)
C-3
Graphiwsteel
5500 cycles
Stopping time increase 15% max. Avg. wear, mm max. bronze 0.254 steel 0.102 Torque 101.7 Nm min. Stopping time 0.85 s. max. Torque difference between 1550 and 5500 cycles 40.7 Nm max.
Other engine builder tests by Detroit Diesel, Ford, General Motors and Cummins include the Ford Tornado test (Table 5.9). This is intended for testing polishing of the cylinder bore, accompanied by oil consumption, blow-by and scuffing of the piston rings and cylinder barrel surfaces. This phenomenon may be observed mostly in high performance engines under load during operation at maximum performance when oils of insufficient performance levels and low basicity are employed. Fuel quality is also involved; it has been found that high-sulphur (1%) diesel fuels significantly increase bore polish and piston-ring weight loss. It has been generally accepted that it is caused by abrasion of the friction
-
Table 5.9. Ford Tornado Engine Test Basic Parameters Purpose: Evaluation of the ability of the lubricant to prevent bore polishing.
Engine: Ford Tornado 6-cylinder, 5.6 litre turbocharged engine. Fuel: 1.O% sulphur diesel fuel. Test Conditions: 101 - 200 hours 0 - 100 hours 200 hours Duration 2400 Speed (r.p.m.) 123 150 Power (bhp) Oil temperature “C 80 100 s! Coolant temperature “C 25 30 Fuel flow, kg/h Procedure Reference: CEC L-27-T-79
45 1
surfaces, mainly the piston crown and the top land (261). by hard particles, such as carbon and other thermooxidation and decomposition products of oil and additives (sulphates, carbonates,phosphates and metal oxides) under certain thermal and physical conditions.
The American specifications have had a considerable influence outside the USA, not only because they were the first but also because of the international nature of the automobile, oil and additive businesses and the roots of these businesses in large American corporations. Nevertheless, quite separate test methods and specifications have also been developed in Western Europe. Examples include the British military DEF specifications (1951), D.C.E.A. in France (1952), BT-PO in Belgium and TL, in Germany. European spark ignition engines have usually been smaller than the American types and frequently have to operate at higher temperatures, on the upper limits of output, so that they are more sensitive to pre-ignition and require oils with better detergent-dispersant properties; on the other hand, they are less prone to rusting in service. The requirements for oils for truck diesel engines also differ between the USA and Europe, although good detergent properties at high temperatures are demanded in both areas. Antioxidation, anti-scuffing and anti-wear properties are emphasised in Europe, and good dispersant for stop-go running and anti-rust properties in the USA (242). Some requirements of European diesel engines cannot be met at all by oils which nevertheless meet the requirements of the American specifications (81, 89,240).
For these reasons, European standard engine tests have been developed (some of these are now obsolete): Petter W 1 (oxidation stability, anti-corrosion), Petter AV 1 and AVB (thermal stability and detergent-dispersant properties), Petter AV 1 (ORE-7) (detergentdispersant and anti-wear properties in the presence of ethylene glycol in the oil), MWM-A and B (thermal stability, detergent-dispersant and anti-wear properties), Fiat 124 AC and 132 (influence on harmful preignition), Fiat 600D (dispersant effects at low temperatures), Ford Cortina Sequence I (anti-rust and corrosion), Sequence I1 (detergent-dispersant and anti-wear properties under severe high-temperature, high-speed conditions), Sequence I11 (suspension of insolubles, blocking of filter screens and oil filters under low and medium temperature conditions), Ford Kent (detergent-dispersant properties), Daimler-Benz OM 352A (prevention of deposit formation, wear and bore polish), OM 6 16 (prevention of wear, ring-stick and sludge accumulation under coldlwarm, medium speed and hot operating conditions), M-102E (“black-sludge” test), M- 102E HKL (cam and tappet wear), 452
Peugeot 204 (shear stability), TU-3 (cam and tappet wear), Poyaud 6 SPZir (prevention of ring-stick and deposits), Volvo B20 (camshaft failure), TD 96B (prevention of ring-stick, wear and deposits under high temperature condtions), TD 120A (bore polish), Volkswagen 1302 hot test (prevention of wear, ring-stick and accumulation of deposits), 1431 (prevention of ring-stick and accumulation of deposits), 5106 (cam and tappet wear), 3334 (effects on sealing materials). Oil specifications developed by European engine manufacturers include: British Leyland (now Rover Group) - OL-01, 22 OL-02, 22 OL-06, 22 OL-07, Daimler Benz - sheet 226.0, 226.1, 227.0, 227.1, 228.1, MAN - 269,270, 271, Perkins Engine, Poyaud, Scania and Volvo specifications, Volkswagen - 500.00, 501.01, 505.00, 521.07, 728, 797. Engine tests developed and introduced in the USSR are summarised in Table 5.10. As example, the principal features of the VW 500.00 specification for fuel-efficient SAE 5W/X and l0Wlx multigrade oils (June, 1988) are: - minimum performance level CCMC G3/D2; - sulphated ash less than 1.5% weight mandatory for gasoline oils. Oils that meet the VW 500.00 specification but exceed 1.5% ash may only be used in naturally-aspirated diesel engines; - phosphorus level 0.08% weight minimum. For oils with 0.08-0.10% it may be necessary to carry out additional functional or field tests; - high-temperaturehigh-shear viscosity 3.5 mPa.s minimum, at 150 "C, 106.s-'; - evaporation loss (Noack test) 13.0% maximum; - piston cleanliness/ring-stick in the I .6 litre, naturally aspirated VW diesel engine according to VW 1435; piston cleanliness (DIN 51361 rating): 68 minimum ring-stick: none; - high-temperature deposits and wear in the VW 1302 test (air-cooled, 4-cylinder, horizontallyopposed engine) according to PV 9800 in-house procedure. Average piston rating requirement minimum is 70 (IP rating procedure). Requirements for wear, viscosity increase and oil consumption unchanged from earlier specifications; - M 102E sludge test according to the test procedure being developed within CEC Project Group PL-37.
As has been mentioned earlier, a unified European engine oil performance classification was developed by the ComitC des Constructeurs d' Automobiles du March6 Commun (CCMC). The new CCMC Oil Sequences/SpecificationsClasses 453
R P Table 5.10. Summary of Engine Tests Used in the USSR according to GOST 17479-72 (123) Oil group Detergentdispersant properties A Bl B2 "I "2
GI G2
D
E
NAMI-I-high temp.( 1) NAMI-I-high-temp. UIM-fj-NATI(2) or SMD- 14(4) NAMI-I-high-temp. UIM-6-NATI or SMD-14 NAMI-I-high-temp. UIM4N-NATI (3) & IM-I ( 5 ) or JAMZ-Z38NL3(6) IM-I or JAMZ-238NB DK-2[7)
Performance properties of oils Antioxidant properties
Anti-corrosion properties
Petter W1 or IKM-I(8) same same same same same same same same
Petter W1 same same Petter W 1 or JAZ-204 Petter W 1 or JAZ-204 Petter W 1 Petter W1 or JAZ-204 same same -
-
Resistance to cold-sludge formation
Anti-wear properties
NAMI-I low-temp. same UIM-6-NATI or SMD-14 same same UIM-6-NATI or SMD-14 same same same IM-1,UIMd-NATI or JAMZ-238NB same same IM-I or JAMZ-238NB DK-2
(1) NAMI-I (GOST20991-75) - 12% (15 x 8 h periods) on a single-cylinder, 4-stroke gasoline engine; each period consists of 2 h cold-mode operation at 1100 r.p.rn. (oil temperature 40 "C, water temperature 30 O C , inlet air temperature 50 "C), 6 h hot-mode operation at 3.000 r.p.m. (oil temperature 120 "C, water temperature 90 "C, air temperature 50 "C). (2) UIM-6-NATI (COST11637-65)- 120 h test (12 x 10 h periods) on a single-cylinder. 4-stroke. non-supercharged diesel engine at 1,500 r.p.rn. at 95 "C oil temperature and 115 "C coolant temperature. (3) IJIM-6N-NATI(COST1265847) - 120 h test (12 x 10 h periods) on a single-cylinder. 4-stroke supercharged diesel engine of 1,500 r.p.m. at 95 "C oil temperature, 115 "C coolant temperature and 85 "C inlet air temperature. (4) SMD-14 (type method) - 960 h test on a kylinder. 4-stroke, supercharged diesel engine at 1700 r.p.m. at 90 "C oil and water temperatures, oil change after 480 h (in tests with fuel containing 0.5% max.S) or after 240 h (with fuel containing 1.0%max.S). (5) IM-I (GOST 20303-74) - % h test on a singlecylinder. 4-stroke, supercharged diesel engine at 1535 r.p.m. at 105 "C oil temperature and 130 "C coolant temperature. (6) JAM-238NB (type method) - 960 h test on an S-cylinder, 4-stroke. supercharged diesel engine at 85-95 "C oil and water temperature at the engine outlet; fuel contains 1% S;D p u p oil change after 480 h and G, after 240 h. (7) DK-2 (GOST20992-75) - 36 h test on a 2-stroke free-piston compressor with free piston at 15 MPa pressure in fourth stage with warm water outlet (32 "C); fuel contains 0.9 to 1.0% S. ( 8 ) IKM-I (GOST 20457-75) - 40 h test on air-cooled, single-cylinder gasoline engine at 1500 r.p.m. at 200 O C cylinder and 120 "C oil temperatures; 0.5kg oil in sump without change. (9) JAMZ-204 (GOST20302-74) - 125 h test on a 4cylinder. 2-stroke diesel engine at 1800 r.p.m. at 130 O C oil temperature and 85 to 90 "C coolant temperature; fuel contains 1.0%S.
G4 and G5 (gasoline), D4, D5 and PD2 (diesel), valid from 1.1.90, were published on the occasion of the CEC Symposium (19-21.4.89) in Paris. Details of these specifications are given in Tables 5.11 and 5.12. The existing G1 and G2 specifications were withdrawn at the same meeting; existing G2, G3, D2 and PDl specifications were valid to end- 1989. The latest specifications (April, 1991) are detailed in Tables 5.12.1 and 5.12.2. The basic test parameters are listed in Table 5.13. The new specifications became necessary because of - extended oil drain intervals, - increased oil temperatures, - fuel quality changes, - changed engine operating conditions, which caused lubricant requirements to become more severe. The following additional tests are included in the new specifications: for gasoline engine oils - valve-train scuffing wear (PSA-engine, TU-3), - formation of black sludge (DB M102E), - sequence HIE and VE with modified limits compared to the MI and MIL- specifications, - low-speed oil-thickening test (Mack T-7); this for diesel engine oils test will probably be replaced by a European test, - bore-polishing (OM 364A), - elastomer seal compatibility, for all engine oils - foaming tendency. These tests are generally devised by independent scientific and technical institutes and the specifications by engine, oil and additive manufacturers or consumers (including the military), acting in collaboration. The coordinating organisation in the USA is the Coordinating Research Council (CRC) and in Europe the Conseil EuropCen de Coordination (CEC); both share their experiences and there is effort to achieve world-wide unification of the evaluation of engine oils (145). Modem engine oils correspond to performance classes SF to SG/CC to CD+ by API (in Europe) or SF to SG/CD to CE (in the USA) (high performance lead-free gasoline engines require "SG' oils). These oils can also be used as mixed fleet oils and for turbocharged gasoline and diesel engines, where the temperature in the sump can exceed 150 "C and reach 230 to 250 "C and, briefly, even 300 "C in the top ring groove. The Motor Vehicle Manufacturers Association (MVMA) in the USA and the Japanese Automobile Manufacturers Association (JAMA), through an organisation called the International Lubricant Standardisation and Approval Committee (ILSAC), jointly developed and tentatively approved a specification for gasoline-fuelled passenger car engine oils. This ILSAC specification comprises five parts. The first section on viscosity uses the SAE viscosity classification (SAE J300). The second encompasses the API SG performance requirements. The third contains specifications for bench test performanceparameters - HT/HS viscosity at 150 "C and 106s-' (ASTM D-4683, or CEC L-36T-84, min. 2.9 mPa.s), volatility (either by distillation range - ASTM D-2887 - 17% max.distilled at 371 "C,
455
Table 5.11. CCMC European Oil Sequence for Service-fill Oils for Gasoline Engines Classes Gl,G2,G3 (June, 1984) Properties
Tests
Ratings
Units
Values
Test method
G1
Low-temp. Seq. VD sludge
Avg. engine sludge, min.* Avg. piston skirt varnish, min.* Avg. engine varnish, min.* Cam wear, avg.(max.) Oil screen clogging, max. Oil ring clogging, max. Compression ring sticking
Bearing corrosion
CRC-L-38 Bearing weight loss, max. or Petter W-1 Bearing weight loss, max.
High-temp. Seq. IIID Visc.increase at 40 "C, max. oxidation Avg. piston skirt varnish, min.* Avg. oil ring land deposits, min.* Avg. engine sludge, min.* Ring sticking Lifter sticking Cam or lifter scuffing Cam and lifter wear avg.(max.)
Pm % %
G2 G3 approximately equivalent to (API SE) (API SF) (API SF -low visc.) 9.2 9.4 9.4 ASTM 315H Part II(4) 6.7 6.7 6.4 6.3 6.7 6.6 2x64) 25(64) 10 7.5 7.5 10 10 10 none none none
mg
40
40
40
mg
25
25
25
%
375@40h 9.2 4.8 9.2 none none none 102(254)
375@64h 9.2 4.8 9.2 none none none 102(203)
200@64h Pt.II(4)
9.1 4.0
Pm
9.2 none none none 102(203)
ASTM-STP 509A Pt.IV or CEC-L-02-A-78 ASTM 315H
(Table 5.11 contd.) High-temp. Ford Kent Cold ring sticking deposits or Piston skirt varnish, min.* Oil thickening Oil consumption Cortina Cold ring sticking, min.* Oil thickening Oil consumption Re-ignition FIAT 132 Hours without pre-ignition Wear
D-BOM616 Cam-wear: avg.(max.) (One outlier allowed for calculation of the average) Cylinder wear: avg.(max.)
Rust Shear stability
Seq.IID Bosch injector
Visc.at 100 "C after 30 cycies based on S A E 5300 Sept.1980, without multilabelling.
loss
Max. weight loss after l h at 250 "C
Oil seal compatibility
To be determined
Viscosity
High shearhigh-temp.
Oil consumption
To be determined
Evaporative Noack
R4
Avg. engine rust, min.* Lifter sticking
* Rated on a scale 0 to 10,where 10 = clean.
none 7
9.8
none 7 rate and report rate and report
none 7
CEC-L-29-T-81
9.8
9.8
CEC-L-03-A-70
rate and report rate and report
h
ao
ao
ao
Pm
30(60)
30(60)
30(60)
P
CEC-L-i 7 - ~ - 7 a
lO(24) 8.5
8.5
8.5
none
none
10W30>9 XW4b12 XW5b14
5W30>9.3 10W30>9.3 5W40>12 10W40>12
10wx:20 15WX:15 20wx:15
ASTM 3 15H Part I(4) CEC-L-14-A-78
DIN 51 581 15
3.5 mPa.s at 150 "C and 106s-'
CEC-CL-23
$ Table 5.12. CCMC European Oil Sequence for Service-fill Oils for Diesel Engines Classes D1, D2, D3 and PDl (June, 1984) 00
Values Properties Tests
Ratings
Low-temp. Seq.VD sludge (to be
replaced by D-B OM616 Pt.A for D3 and PD1 Bearing corrosion
D1 (naturally aspirated light duty operation)
D2 D3 PD 1 (naturally (naturally (passenger car aspirated aspirated diesels naturally and and aspirated and turbocharged turbocharged turbocharged) heavy duty extra heavy operations duty operations
Avg. engine sludge, min.*
9.2
8.7
8.7
8.7
Avg. piston skirt varnish, min.* Avg. engine varnish, min.*
6.4 6.3
6.0 5.9
6.0 5.9
6.0 5.9
% %
10 10 none
10.0 10 none
10.0 10.0 none
10.0 10.0 none
mg
40
40
40
40
mg
25
25
25
25
%
375@40h 9.1 4.0 9.2 none none none 102(254)
-
Oil screen clogging, max. Oil ring clogging, max. Compression ring sticking
CRC-L-38 Bearing weight loss, max. or Petter W-1 Bearing weight loss, max.
High-temp. Seq.IIID oxidation
Units
Test method
Visc.increase at 40 OC, max. Avg. piston skirt varnish, min.* Avg. oil ring land deposits, min.* Avg. engine sludge, rnin.* Ring sticking Lifter sticking Cam or lifter scuffing Cam and lifter wear avg. (max.)
Wn
ASTM 3 15H Part II(4)
ASTM-STF' 509A Pt.IV or CEC-L-02-A-78 ASTM 315H Pt.II(4)
4.8
4.8
102(254)
102(254)
102(254)
(Table 5.12 contd.) High-temp. Cat.lH2 or MWM-B
or Cat.lG2 Bore-polish D-B OM 352A
Top groove fill, m a . Weighted total demerits, max. Ring sticking Varnish & carbon deposits, min.
45
140 none 55
none 65
CEC L-12-A-76
80 300
Top groove fill, max. Weighted total demerits
ASTM STP 509A
Part I <2 58
Bore-polish Piston cleanliness, min.*
Ring sticking V W 1431 Ring sticking & piston rating Piston cleanliness, min.* Rust
Seq.IID
Avg. engine rust, min.*
8.1 10W30>9 XW40>12
Wear
30(60)
Visc.at 100°C after 30 cycles based on S A E J300 Sept.1980, without multilabelling. DB OM616 Cam-wear: avg.(max.) (One outlier allowed for calculation of the average) Cylinder wear: avg.(max.)
loss
Max. weight loss after 1h at 250 "C
Oil seal compatibility To be determined
* Rated on a scale 0 to 10, where 10 =clean. ** Not applicable to monogades. \o
8.1
Shear Bosch stability** injector
EvaporativeNoack
~JI
8.5
***Including monogrades.
CEC-CL-29DB (Sept.83) none 58
CEC L-35-T-84
8.1
ASTM 3 15H Part 1(4) CEC-L-14-A-78
CEC-L-17-A-78
lO(24) 1owx:20
A1 others 15***
DIN 51 581
Table 5.12.1. CCMC European Oil Sequence for Service-fill Oils for Gasoline Engines - Classes G-4, G-J(April,1991) Requirements
Properties
Test method
Unit of measure
1 Laboratory Tests 1.1 Viscosity Characteristics according to SAE J300 June 1987
1.2 Shear Stability Viscosity at 100 "C after CEC L-14-A-78 (Bosch mm2.s-1 injector) 30 cycles
1.3 High Shear-rate, High Temperature Viscosity CEC L-36-T-84 mPa.s q = l 5 0 "C, [email protected]) 1.4 Evaporative Loss (Noack) Weight loss after lh at 250 "C
1.5 Oil-Elastomer Seal Compatibility Max. variation of characteristics after immersion for 7 days in fresh oil without pre-ageing. Hardness DIDC Tensile strength Elongation rupture Volume variation 1.6 Foaming Tendency Tendency-Stability
%
G-5
only SAE grades lOWX 15WX 20wx
only SAE grades 5WX lOWX
XW30>9 Stay in XW40> 12 grade XW50>14 after shear test. >3.5
>3.5
1owxc15 15WXc15 >13 20WX45
CJX L-39-T-87
Elastomer Type RE1 RE2 RE3 RE4 (fluoro-) (ACM) (silicone) WBR) -51+5 -2510 -51+5 points 01+5 % -5010 -151+10-301+10 -2010 % -6010 -35/+10 -201+10 -5010 -51+5 01+30 -51+5 % 01+5
ASTM D 892 Sequence I(24"C): without Option A Sequence II(94"C): Sequence III(24"C):
or
460
G-4
CEC L-40-T-87
2 Engine Tests High Temperature Tests 2.1 .Corrosion Bearing weight loss, max. L-38 Petter W1
Values
Bearing weight loss, max.
ASTM STP 509A mg or CEC L-02-A-78 mg
10-nil 50-nil 10-nil
40
40
25
25
(Table 5.12.1 contd.) 2.2 High Temperature Oxidation (ME, Visc.increase at 4OoC,max. Buick)
Piston skirt varnish, min. Ring land varnish, min. Sludge, min. Ring sticking Lifter sticking Cam & lifter wear, avg. max.
2.3 High Temperature Deposits Ring sticking Piston skirt varnish, min. Oil thickening Oil consumption
Low and High Temperature Sludge and Wear 2.4 Low Temperature Sludge WE, Engine sludge, avg.min. Ford) Piston skirt varnish, min. Avg. engine varnish, min. Oil ring clogging Comp. ring sticking Oil screen clogging Avg. cam wear, max. Max. cam wear, max.
ASTM Research %
300
300
Report D-2:1225 merit
pm
8.9 3.5 9.2 none none <30
pm
<60
8.9 3.5 9.2 none none <30 <60
Present Cortina test merit Test procedure inadequate for today’s under engines. CEC asked to development develop immediately adequate procedure. In meantime, tests under 2.6.
ASTMResearch merit Report D-2: 1126 merit merit
Avg. engine sludge, min. Ring sticking Piston deposit Cam wear Follower wear
2.7 Pre-ignition Test (Fiat 132) Hours without preignition, min. 2.8 Rust Test (Oldsmobile IID)
9 6.5 5 el5 none <20 I30 380
pm pm merit
el5 <20 7.5
4 5 <20 7.5
% %
2.5 Valve Train Scuffing Wear CECL-38-T-87 (TU-3) Cam wear avg. Max. wear (100H) Pad merit (avg. of 8 pad, min.) 2.6 Black Sludge (M 102E)
pm pm
9 6.5 5 el5 none <20 130 380
CEC L-41-T-88
merit merit merit pm
pm
>RL 140 >RL140 >RL139 >RL140 >RL139 >RL140 >RL139 >RL140 >RL139
CEC L-344-82
ASTM IID STP 315 H (Part I)
h
80
80
merit
8.5
8.5
(This sequence defines the minimum performance level of a product for presentation to CCMC members. Performance parameters other than those covered by the following tests or more stringent limits may be indicated by individual members companies.)
46 1
Table 5.12.2. CCMC European Oil Sequence for Service-fill Oils for Diesel Engines Classes PD-2, D-4,D-5 (April, 1991)
-
Requirements
Properties
Test method Unit of measure
Values G-5 PD-2 G-4 (Passenger (Industrial cars) vehicles)
1 Laboratory Tests 1.1 Viscosity According to SAE J300 June 1987
XW30 XW40 XW50 XW30
20W20 30 40 XW30 XW40 XW50
20W20 30 40 XW40 XW50
1.2 Shear Stability (Bosch Viscosity at 100 OC after injector) 30 cycles
CEC L-14-A-78 mm2.sV1 XW3019 XW3029 XW40212 XW40212 XW502 14 no requirements for single grades. 1.3 High Shear-rate, High Temperature Viscosity CEC L-36-T-84 (T=150°C, [email protected]') 23.5 mPa.s 23.5 (23.3 for 20W20 & 30) 1.4 Evaporative Loss (Noack) Weight loss after l h CEC L-40-T-87 at 250 "C % lOWXSl5 -213 for all others 1.5 Oil-Elastomer Seal Compatibility Max. variation of characteristics after immersion for 7 days in fresh oil without pre-ageing. Hardness DIDC Tensile strength Elongation rupture Volume variation 1.6 Foaming Tendency Tendency-Stability
462
CEC L-39-T-87 RE1 (fluoro-) 01+5 points 9% -5010 76 %
a010 01+5
ASTM D 892 without Option A
Elastomer Type RE2 RE3 RE4 (ACM) (silicone)(NBR) -51+5
-2510
-151+10 -35/+10
-301+10 -2010 -201+10 -5010 01+30 -51+5
-51+5
-51+5
Sequence I(24"C): 10-nil Sequence II(94"C): 50-nil Sequence III(24OC): 10-nil
(Table 5.12.2 contd.) 2 Engine Tests
High Temperature Tests 2.1 Ring Sticking and Piston cleanliness (VW 1.6 TC Ring sticking
2.2 Bore Polishing and Piston cleanliness (OM-364A) Bore Polishing Piston cleanliness
CEC L-35-T-84 merit >RL148
CEC L-42-T-89 %
516 124
merit
12 238
Low and Medium Temperature Sludge and Wear 2.3 Low Speed Oil Thickening Test* (Mack T-7) Viscosity increase ASTM RRD2-1220
csth 2.4 Low Temperature Oil Thickening (OM-602A PL-38) 2.5 Rust (IIW
Avg. engine rust, min.
2.6 Wear** (OM-616)
to be developed
-<0.04---
-limit to be determined-
ASTM 315H Part 1 merit
8.5
8.1
8.1
10 20
10 20 10 24
10 20
CEC L-17-A-78 Cam wear: avg. m a .
prn pm
Cylinder wear: avg. max.
Pm pm
5 12
5
12
* A suitable European test should be developed. ** Interim.
463
-
Table 5.13. Basic Parameters of Engine Tests CCMC Classification of Eneine Oils Petter W-1
To measure the effect of an oil on corrosion of copperflead bearings, high-temperatureoxidation stability and accumulation of deposits. Single-cylinder Petter W-1 spark-ignition laboratory engine, 82.5 x 85mm. Engine used: Leaded gasoline meeting AF 21 14 specification. Fuel: Operating conditions: 146*4 Duration 36 h + 4 h run-in Water inlet temp., "C Speed, r.p.m. 15oof15 Water outlet temp., "C 15ofl 138fl* Load, BHP 3.3 approx. Oil inlet temp.,"C Fuel flow, mu,. 113M.5 Hot-spot temp.,"C 20035 12.0: 1fo.1 Aidfuel ratio Manifold depression, in. Hg 233.2 * 130 "C for SAE 10 grade only. Purpose:
Ford Kent
To determine the effect of an oil on ring sticking and piston deposits. Purpose: Engine used: Ford Kent 1600GT 4-cylinder, 4-stroke gasoline engine, Design No. 2265E. Fuel: AF 21 14. Operating conditions: 100 Duration, h Speed, r.p.m. 4ooM50 Load Full Nominal power, kw 48.5 Coolant outlet temp.,"C 11w2 42f2 Coolant A, "C Oil gallery temp.,"(= 1m2 Air inlet temp., "C 4232 Fuel consumption, litresh 19.3fo.3 Procedure reference: CEC L-29-T-81 Ford Cortina
Purpose:
To evaluate ring sticking, deposit-forming and wear characteristics of an oil under severe high-temperature, high-speed conditions. Engine used: Ford Cortina 4-cylinder 120E spark ignition engine, 15OOcc capacity, compression ratio 8.3:1 Fuel: Regular grade gasoline meeting AF 21 14. Operating conditions: 5h run-in plus lOOh on-test Duration: 35oof50 Speed, r.p.m. 28.5-29.0 Fuel consumption, pintdh 9of2 Lube oil temp. Exhaust back pressure, in.Hg, 2.0 max. Full, approx. 48 Load, BHP 8232 Coolant outlet temp.,"C m 5 Oil pressure, p.s.i. 6fl Coolant A across engine, "C
464
(Table 5.13 contd.) Every 10 h, measure timing to give detonation. Procedure references: Ford UK requirement CEC L-03-A-70 IP 246 Fiat 132
To evaluate the effects of lubricating oils on the occurrence of destructive pre-ignition. Fiat 132 4-cylinder, 4-stroke gasoline engine, Design No. 503.309. Engine used: CEC reference fuel RF 82-A-81 plus 1% of test oil. Fuel: Operating conditions: Duration 100 h minimum (comprising repeats of a 3-mode cycle) Phase 2 Phase 3 Phase 1 Cycle time, min. 90 20 5 5OOOk50 Speed, r.p.m. 25oof50 4ooof50 Power, kW 10.1 30.1 56.8 Fuel consumption, I/h 6 . M .1 Coolant outlet temp., "C 8 W 85f2 9oi2 Coolant A, "C 3fl 4f2 Exhaust back pressure, m m Hg 9of10 Spark advance, " BTDC 23f2 28fl 32f1 Aidfuel ratio 13.5:1-14.5:1 Oil temp.,"C 9333 125f5 Procedure reference: CEC-L-34-T-82
Purpose:
Daimler-Benz OM 616 (Kombi)
To determine the effect of an oil on wear, ring sticking and accumulation of deposits under coldlwarm (PartA), medium speed (Part B) and hot (Part C) operating conditions. 4-cylinder, 4-stroke Daimler-Benz pre-combustion chamber diesel engine, Engine used: 91mm bore x 92.4mm stroke, capacity 2404 cc, compression ratio 21:l. 0.4% S automotive diesel fuel. Fuel: Operating conditions: 10h run-in. Part B Part c Duration Part A 2x60 h periods of 135 s. 50 h 36 h run/90s. stop Speed, r.p.m. 45oof25/0 325of25 45m25 Fuel consumption, 351-360 cmhin. 351-36010 267-274 463930 43E2.5 46k2.5 Load, kW 9OfW36f2 9032 m 2 Water outlet temp.,"C 6.5f1.5 9+3 Water temp. A, "C 9f3/4.5fl.5 Engine oil temp., "C 1l o w 9839 128f2 35w35f5 35f5 Intake air temp., OC Oil and filter changed after each period. Purpose:
(Table 5.13 contd.) Procedure reference: Daimler-Benz Test Programme 616-201/0ct 78 CEC-L-17-A-78
MWM-B To evaluate the effect of an oil on ring sticking, wear and accumulation of deposits under high-temperature conditions. M W M KD 12E single-cylinder diesel engine, 95mm x 120mm stroke, 85Occ. Engine used: capacity, compression ratio 22: 1. 1% S gas oil to specification RF-9 1-A-8 1. Fuel: Operating conditions: 50 plus 10 for run-in* Duration, h Speed, r.p.m. 450 Power, kW 51 Coolant inlet temp., “C 85-90 Coolant outlet temp., “C 90-95 Oil sump temp., “C 125-140 750 Exhaust temp., “C, max. Boost pressure, bar 0.70 Exhaust back pressure, p.s.i. 2-3 Fuel consumption, Vh 20.0 Procedure reference: CEC-35T-84.
Purpose:
Bosch Injector Rig To evaluate the shear stability of lubricating oils containing polymers. Apparatus consists of a motor-driven Bosch injector pump with a spray nozzle, atomisation chamber, containers, etc. Operating conditions: Test oil is circulated through the apparatus with the injector nozzle set to 175 bar (175 kp/cm2) for 30 cycles. Oil viscosity is measured before and after shearing. Procedure reference: CEC-L-14-T-84. Purpose: Apparatus used:
Daimler-Ben2 OM 352A To determine the effect of an oil on piston deposits, wear and bore- polishing in a highly-rated, turbocharged diesel engine. 4-stroke, direct injection diesel engine with turbocharger, Mercedes Benz Engine used: OM 352A Type 353.975. Fuel: To meet DIN 51601. Operating conditions: Duration - 300 h (comprising 3 repeats of two 50 h phases, one steady-state, one cyclic). Phase 1 Phase 2 lo00 idle 2800 Speed.r.p.m. 2800 I800 Durati0n.h 1x20 1/2x20 1/2x20 1/2x20 50 85-90 > Water outlet temp.,”(= c Oil temp.,”C < >120- 12 Exhaust temp.,”(= < 65C > Boost pressure, mbar < 950- 1 1 5 ’ L > Purpose:
466
(Table 5.13 contd.) Oil pressure, mbar, max. c Blow-by, Vmin.,max. c
-3 > " 3 1
>
VW 1431 Purpose:
To determine the effect of an oil on ring sticking and piston deposits in a turbocharged passenger car diesel engine. Engine used: VW 068 ATL 1.6 litre, 4-cylinder, turbocharged, indirect injection diesel engine. Fuel: Not specified. Operating conditions: Duration 7 h run-in + 50 h on-test. Speed, r.p.m. 2200+25 100 Coolant inlet temp.,"C Coolant outlet temp.,"C 1lof2 Fuel consumption, g/h 3 1OOf40 Sump oil temp.,"C I1M2 Air intake temp.,"C 3ok5 Procedure references:
CEC-L-2A-76 DIN 51361(Part 4)
* Run-in conducted on test oil for factory fill approval or in comparison with reference oil for service-fill approval. exceptionally 20% max. for SAE 5W30, or, as evaporative loss by DIN 51581 - 20% max. after 1 hour at 250 "C, or, similarly, 25% max. for SAE 5W30), foaming tendency - ASTM D-892 (Seq. 1 max. 10/0 ml, Seq. I1 max. 50/0 ml, Seq.I11 max. 10/0 ml), filtrability - GM 9099P (max. 50% flow reduction), flash-point - ASTM D-93 (min. 190 "C) or ASTM D-92 (min. 205 "C), shear stability (L-38 test, 10 hour stripped viscosity - must remain in original SAE grade), physical appearance and odour (shall be clear and bright with no objectionable odour), homogenicity and miscibility ( R M 791B, method 3470) - shall remain homogeneous and, when mixed with SAE reference oils, shall remain miscible). The fourth section contains requirements which are not a formal part of the specification but may be used in certain regions of the world; fuel efficiency - ASTM Seq. VI test (improvement of 2.7% min.), catalyst compatibility (phosphorus content 0.12 weight 6 max.),low-temperature viscosity (cranking viscosity 3,500 mPa.s at -25 "C, pumpability 30 P as at -30 "C max.)
Problems may be encountered in the lubrication of engines fuelled with alcohols (methanol,ethanol). The acidic products of incomplete combustion of such alcohols are very aggressive in the presence of condensed water and if they pass as blow-by into the sump oil they can cause severe corrosion. In addition, such alcohols, being highly polar, can react with additives and hinder or weaken their effect (164,165). Problems may also arise (mentioned earlier) in the lubrication of small, indirect injection diesel engines, the oil charge of which can quickly become contaminated with oil-insolubles (soot); this causes rapid viscosity increase and the liquid oil is transformed into a gel (94).The oil-drain interval, even with very high-quality oils, must be short. Additives which provide protection against gelling are sought (266).
467
The concentration of additives in oils, especially detergent-dispersants,inevitably grows with increasingly stringent performance needs placed on the engine oils. Better types are sought and the requirements for appropriate additive combinations grows. Short alkyl ZDDP’s (C, to C6) with high antiwear activitiescontributeto the reduction of the frictional load; however, they also have quite low thermal stabilities.They are therefore used only as a part of the ZDDP dosage (at up to about 0.3% in finished oil). ZDDP’s from C, to C, alkyls with balanced antioxidant and anti-wear properties and average thermal stabilities are particularly suitable for gasoline engine and mixed fleet oils. ZDDP’s from C, to C,, alkyls, alkylaryldithiophosphates and diaryldithiophosphates,having very high thermal stabilities, are the typical antioxidants for oils used in thermally heavily loaded, mainly diesel engines. Only those oils in which these latter ZDDP predominate are appropriate for friction clutches, since they meet the friction characteristic requirements of the Caterpillar TO-2 test. Ashless antioxidants (alkylbis-phenols and/or aromatic amines), together with ZDDP, are not employed as regularly, although they are effective ZDDP synergists at low concentrations in oil (up to 0.5%). reducing the amount of deposits in piston grooves and corrosion on the pistons. Metal-based detergents used in modem additive combinations are predominantly calcium and magnesium sulphonates, phenolates and alkylsalicylates. The concentration of calcium or magnesium sulphonate should not exceed about I .5% weight in the finished oil; above that figure, deposits in the top ring-groove may increase and the friction coefficient of the oil rises (alkylsalicylates, on the other hand, decrease the friction coefficient). Ashless dispersants, mainly succinimides in balanced mono:bis ratios, are present in higher concentrations than the metallic detergents in modem oils which are mainly intended for gasoline engines (for example, API “SF’ oils contain 4 to 5 % weight, API “SG’as much as 8% weight of succinimides). They act as efficient synergists with the metallic detergents. The concentration of detergent-dispersantadditives and suitable combinations of them may depend to a considerable extent on the concentration and type of polymeric viscosity modifier (VI)in multigrade engine oils. All polymers decompose progressively with the thermal stress on hot engine surfaces and their decomposition products aggravate the formation of deposits and varnishes. The chemical nature of the polymer determines its thermal stability and affects to some extent the rate of decomposition and the nature of the decomposition products. Usually, the higher the concentration of polymer, the more engine fouling occurs. For this reason, the concentration of antioxidant and detergent-dispersant additives in multigrade oils is higher than those in mono-grade oils. This is particularly true for oils with a higher concentration of light fractions. They evaporate on the hot surface of the piston, crack and the oxidised and polymerised products of thermal cleavage form deposits in the piston grooves and varnishes on the top lands.
Antifoam Properties of Oils Surface-activeDD additives in oils increase their tendency to foam. This can be very troublesome, as persistent foaming causes oil loss through breather ports, impairs the performance of the oil pump and, consequently, the supply of oil into the distribution system. Internal foaming adversely affects the physical properties of the oil, reducing its density, viscosity and thermal conductivity and increasing its tendency to age. This can be remedied by the addition of anti-foam agents, mainly the polysiloxane types. 468
Emulsion Formation A thick layer of soft, creamy, white or grey-brown deposit, firmly adhering to metal surfaces and resistant to detergents, sometimes forms, mainly on the internal surface of the rocker box cover and the oil-fill plug. This is a persistent water-in-oil emulsion containing about 80% water by volume. It forms rapidly on relatively cool engine surfaces as a result of emulsification of oil coming into contact with moisture or water vapour condensing on the cooler metallic surfaces of the engine. Its formation is, therefore, related to the temperature and moisture content of the air and the operational mode of the engine; short duration running with a cold engine or prolonged running with the engine surfaces thoroughly cooled with cold air creates the pre-conditions for this type of emulsion formation. Engine design affects this phenomenon (41). Positive crankcase ventilation (PCV)reduces emulsion formation. On the other hand, favourable conditions for emulsion formation exist in those engines in which a very low pressure difference arises between the air intake into the crankcase or valve-gear compartment and the air offtake. The same applies for engines in which some parts (particularly the rocker box cover) remain cold during engine operation.
Some polymers, especially dispersant types, and nitrogen-containing ashless dispersants are potent promoters of emulsion formation; organo-metallic detergentdispersants and ZDDP produce less-persistent emulsions. These emulsions have no serious adverse effects on the engine and disappear partially or completely with a change in engine temperature. At carefully-controlled concentration, proprietary emulsion-breaking additives can be used in combination with dispersants.
Effects of Glycol and Water Water and antifreeze coolant chemicals are undesirable contaminants of engine oils, as they impair the functioning of additives and promote excessive formation of sludge, deposits, emulsions, corrosion of metal surfaces and plugging and deterioration of the filtration materials in the oil filters. At a concentration in oil above 0.2 - 0.4%, water or glycol coolant has a detrimental effect on ZDDP-type antioxidants and detergent-dispersants as a result of hydrolysis. This leads to the formation of ineffective, oil-insoluble compounds, the dispersive forces existing between the DD additive and the oil are broken, the additive separates from the oil, thick emulsions, gels and solids form and the dispersive power of the DD additive decreases (262). This in turn impairs the action of the oil filter, clogging the filter cartridge pores with deposits, gels and precipitated additives or their oil-insoluble metal derivatives, and the back-pressure across the filter increases. The problems multiply as the oil-insolubles coagulate and the particles produced by incomplete fuel combustion (soot) and the thermooxidation reactions of the oil become larger. A specific phenomenon caused by the interaction between glycol and oil thermooxidation products is the generation of sticky, sweettasting deposits, which can be washed off with water and alcohol, on the surfaces
469
of pistons, bearings, lifters, etc. When the engine cools down, these deposits can clog it to the extent that it fails to re-start. The severity of this phenomenon varies, depending on the composition of the oil, the rate of ageing, the concentration of water and glycol and the mode of service of the engine. The composition and the resistance to water (which varies according to the particular type of ZDDP and detergent-dispersants present) have a considerable effect. These problems become more severe as the extent of ageing increases and with increased TAN. The concentrations of water and glycol vary in response to engine temperature changes. The trouble especially arises during operation of a cold, insufficiently warmed-up engine which has been shut down for some time, particularly in a cold climate, since the interaction reaction proceed during the shut down period. The water and glycol both evaporate as they are heated up to engine operating temperature. Glycols evaporate from oil quite readily. During engine tests, when glycol was continuously fed to the sump, two thirds was blown out by gas at an oil temperature of 88 "C. Laboratory tests also showed that oil containing 4% glycol only retained 10%of its glycol content when heated for 8 hours at 93 "C; no trace of the glycol remained in the oil at 121 "C. Oil filters are able to separate glycol from oil, particularly high efficiency by-pass filters. This separation is facilitated if the glycols react with additives to produce bulky gels, which can be readily trapped by the filter. Obviously, these remedial processes cause additive depletion and filter loading, which must be corrected. The Petter AVUORE-7 engine test evaluates the resistance of engine oils to glycol; the effects of the oil on pistons and ring wear and deposits in the presence of glycol is measured. The test uses a singlecylinder Petter AV1 engine operated on 1 % sulphur diesel fuel. During the test, a mixture of 80%distilled water and 20% ethylene glycol is added at the start of every hour's running unloaded (10 cm3dropwise for 15 min., 100 cm3 total) in ten dosing operations. The test duration is 145 hours (3 x 45 hours loaded and 2 x 5 hours unloaded) at 1500 r.p.m, 85 (45) "C oil temperature, 108 (35) "C coolant (outlet) temperature, 500 "C (max.) exhaust gas temperature.
5.1.2 Oils for Two-Stroke Gasoline Engines Two-stroke gasoline engines used for powering four- and two-wheeled road vehicles, small boats, mowers, etc., can be classified in the context of lubrication under two groups: Engines with conventional lubrication systems in which the lubricating oil is conveyed to the points of lubrication dissolved in the fuel; the oil is mixed with the fuel in the fuel tank and the fue1:oil ratio is constant. Engines with separate lubrication systems in which the oil is mixed with fuel in the carburettor, or is supplied to the lubricating points separately; the fue1:oil mixture ratio changes with machine r.p.m. The basic principle is the same for both systems - the dispersed oil accumulates in the working compartments of the engine and, after having fulfilled its lubricating function, it is burned along with the fuel. Since engines with conventional lubrication
470
systems have different designs, operate under different conditions and, to some extent, require oils of different compositions from other lubricants, oils for this system comprise a separate group among lubricating oils. Initially, high viscosity oils were recommended, because the oil was blended in the engine with the gasoline. However, experience and systematic tests showed that the value of high oil viscosity had been overestimated. Oil viscosity which was too low might cause partial seizure of the pistons and damage to the bearings, but if the viscosity was too high it led to an unacceptable increase in deposits and bridging of the spark-plug gaps, without improving the reliability of lubrication. Optimum values were found over quite a wide range, between 50 and 110 mm2.s1 at 50 "C, so that oils in S A E 30 to 50 grades were judged suitable. The beneficial effects of flat viscosity-temperature characteristics (high VZ), which had formerly been regarded as unimportant, proved to be very significant. The oscillating and sometimes high temperatures of two-stroke engines particularly air-cooled models emphasise the apparent need for oils of good lubricity was legitimate, as it suppressed partial or complete piston seizure. For heavilyloaded engines, lubricity has the highest priority and must always be regarded as a more important property than any others when additives are selected. Equally important, the oil concentration must be minimised, in order to minimise smoking and environmental contamination. Two-stroke engines for four-wheeled vehicles with a cylinder displacement over 150 cm3 can be satisfactorily lubricated during normal operation with a fuel mixture containing2%oil containing suitable additives. The fuel mixture for low-volume, heavily-loaded engines such as two-wheeled vehicles, boats and machines like saws and mowers must contain more oil (4 - 5%). as the amount of oil supplied to the cylinder at one stroke relates to the intake volume, whereas the oil requirement relates to the area of the surface to be lubricated. Volume increases with the cube of the bore whilst surface with the square, so small volumes engines are less well provided with lubricant and the oi1:fuel ratio must be increased. Also, the smaller engines must operate at higher temperatures.
If the oil and/or fuel leaves excessive amounts of residues, a tendency to the formation of conductive bridges across spark-plug gaps is unavoidable. Bridging can be reduced by the use of ashless additives with amine groups, or a small amount of magnesium sulphonate can be added. Residues and deposits in the exhaust ports reduce their diameter and roughen their surface finish; this reduces the flow-velocity of the exhaust gases and impairs the quality of cylinder scavenging and charging, with consequent detriment to engine performance. The formation and properties of these deposits is dependent more on the nature of the oil than on any other factor. In contrast to earlier suggestions that a low carbonresidue oil was required, it has become apparent that it is more important for the carbon residue to be brittle and low density, so that it can be detached and removed more easily. The same applies to the ash from ash-containing DD additives. The detergent-dispersant additives now contained in all two-stroke oils help reduce deposits, particularly those formed on the piston, especially those in the hottest top portion, and on the electrodes of the spark-plugs (105). 47 1
The crankshaft roller bearings must be properly lubricated. Lubrication is provided by the oil remaining after the gasoline has evaporated from the gasoline/oil mixture passing into the crankcase. The rate of gasoline evaporation depends on the instantaneous temperature existing in the crankcase. The temperature of an adequately-loaded engine is high enough to ensure that the oil, after the gasoline has evaporated, can form a lubricant film with sufficient load-carrying capacity on the surface of the balls or rollers and in the race-ways of the roller bearings. This film is continuously renewed during operation of the engine and thus provides satisfactory lubrication. However, troubles may arise if the engine is insufficiently loaded and its operating temperature is too low, particularly after it has been shut down. At low temperature, less gasoline evaporates from the mixture and the lubricating film of oil diluted with gasoline may not provide adequate lubrication if its viscosity or lubricity is too low. When the engine stops, the oil, diluted with gasoline, runs down the metal surfaces and fails to protect them from corrosion by moisture sucked into the engine with air and blow-by of combustion products past the piston rings, or from acids arising from the hydrolysis of halogenated lead scavengers. Two-stroke engine oils are therefore also dosed with corrosion inhibitors; this anti-corrosion function is also performed by some detergents, e.g., sulphonates. Two-stroke engines with separate lubricating systems are lubricated with highquality automotive engine oils, either monograde SAE 30-40 or multigrade SAE 1OW30 types. Two-stroke engines with conventional lubricating systems are lubricated with special, additive-treated oils. The base-stock should be a high-quality raffinate, containing 10-30% of brightstock to provide sufficient lubricity and yielding a light carbonaceous residue which can be readily carried away. However, exhaust smoke becomes more troublesome with increasing brightstock content. Synthetic oils, such as polybutene-based types, are also useful; they have good adhesive properties, suppress smoke and keep the engine clean, and they are more environmentally acceptable. Some less thermally loaded engines, mainly those in automotive use, need only 1% oil mixtures for adequate lubrication and corrosionprotection. This is valid also for rape-seed oil and ashless esters of modified natural fatty acids, e.g., isostearic acid (a derivate of oleic acid) and polyols, such as trimethylol-propane. Their merits are biodegradability and excellent lubricity. Modern oils containing detergent-dispersant additives, predominantly calcium sulphonates and calcium phenolates, with sufficient TBN, which prevent excessive ash-formation in the engine after the oil has burned by forming a brittle and easilyremovable ash. Suitable sulphonates are calcium sulphonates of low relative molecular weight. According to the thermal regime of the engine, succinimides may be added, or succinimides alone or with other ashless additives may be used. Calcium sulphonate also acts as a corrosion inhibitor. Other additives, such as special anti-wear additives often containing chlorine, which helps carry away lead from the combustion chamber, may also be present. Oils intended for the lubrication for a short time of extremely highly-loaded racing engines require (unless they are lubricated with vegetable oils, e.g., castor oil, as was common with dirt-track 472
machines), require a high dose of a lubricity additive, such as esters of high molecular weight fatty acids. Additives which form conductive bridges and deposits on spark-plug electrodes when they are burnt (e.g., phosphorus-containing antioxidants) are inadmissable. Top-class oils must unconditionally pass performance class test TC. Very highperformance engines, operating at over 10,000 r.p.m., require oils meeting still higher criteria; only a few types of special, mostly synthetic oils achieve these standards. CEC and ASTM Committee D-2, Tech. B Section VI have attempted to systematise two-stroke engine oil testing to satisfy the known requirements of various engines in field service . Each designation is applicable for a specific enginetype, e.g., TA, TB and TC, etc. TD is a BIA-quality oil to be used for large, water-cooled outboard engines and those air-cooled engines specified or recommended by the engine manufacturers (168). TA covers higher ash-content oils (typically containing metal sulphonates, phosphates and salicylates) intended for lubricating small, air-cooled engines in mopeds, lawn-mowers, small generators and pumps. The Yamaha CE5OS engine is used for testing. TI3 covers oils for motor scooters, small ( ~ 2 5cm3) 0 motor cycles and chain-saws operating on higher fueVoil ratios. The Vespa 125TS engine is used for testing power-loss, tightening and pre-ignition. TC relates to oils for lean fueUoil ratio chainsaws, high performance motor cycles and snow-mobiles. The Yamaha 350M-2 engine is used to test for piston deposits and ring-sticking, and the Yamaha CE5OS for tightening and pre-ignition. TD covers outboard motor oils. NMMA TC-WS-18-82 and OMC 9OTLCOS are used to test for piston-scuffing, engine deposits, ring-sticking and pre-ignition. Rust protection, miscibility and filtration tests are also applied.
5.1.3 Oils for Rotary Gasoline Engines The future of the rotary (Wankel) engine remains uncertain. Although problems of materials and sealing have been largely overcome, fuel consumption and air pollution are still troublesome. The likely world market share by Wankel-type engines was forecast to be between 1% (realistic) and 27% (optimistic) by the 1990’s (Z-5, 9, ZO, 27), but this growth has failed to materialise, in spite of its use by the Japanese company Mazda. Oils for rotary engines were still under development at the time of writing (53,54). The smaller, air-cooled engines are lubricated with a mixture of fuel and oil. The oils must burn cleanly and protect the engke from corrosion and wear. Monograde SAE 30 and 40 oils, oils for conventionil two-stroke gasoline engines or ashless oils for low-capacity, two-stroke boat engines are suitable for rotary engines. A higher dosage of anti-wear additives is, however, needed. Larger, watercooled engines are lubricated jointly from the oil and fuel tanks, the interior surfaces with fueUoil mixture and the bearings and other engine parts with oil alone. 473
In addition to the requirements listed earlier, oils must show good starting properties at low temperatures, adequate dispersancy,good thermooxidaton stability, provide sealing between rotor and trochoid (corresponding to piston and cylinder in a conventional engine) and provide heat transfer. The ash content should be low, to suppress pre-ignition (to which these engines have a tendency), to minimise progressive increase in octane requirement, to protect spark-plugs, maintain sealant properties (preventing excessive wear of trochoid vertices) and to prevent exhaust port fouling., Carbonaceous deposits lead to similar problems which may be suppressed by the use of antioxidants and detergents. Rotary engine oils need to be more thermally stable than oils for conventional engines, because the oil transports about 7% of the heat (about half the heat absorbed by the cooling water, where the corresponding value is about one eighth). The oil tank temperature may be as high as 150 "C. Oils meeting DEF 2101-D specification for stability and MIL-L-46152 or MILL-2104B for detergency were described as being suitable. Cold sludge forms to a lesser extent in the Wankel engine as there is less contact of the oil with blow-by gases. API SC and SD levels are satisfactory. Metallurgical changes have overcome problems of scuffing, but oils must have as good anti-wear properties as those for conventional engines and must meet the tenth seizure-load stage in the FZG test by DIN 5 1 354: this remains the best available criterion. Low viscosity - 1800 to 4500 mPa.s (CCS) at -18 "C - is needed from the standpoint of heat transfer, friction losses and startability in winter, and high viscosity - 9 to 12 mm2.s-I at 100 "C - for anti-wear properties and high performance conditions. Rusting and corrosion problems encountered mainly in stop-go running can be overcome with SC and SD oils. High shear-stability is not a problem; that portion of the oil which is exposed to shear stress (about 80%) mixes with the fuel and is burnt. Oils showing a 25% viscosity loss in the Bosch injector test (DIN 5 1 382) are suitable. Multigrade SAE 10W30 to 10W40 and, preferably, SAE 20W30 (which gives greater latitude to the manufacturer) oils are quite suitable for lubricating watercooled rotary engines. 500- 1500 RMW polybutenes or esters of natural fatty acids are desirable base-oil components; they bum cleanly, have good adhesiveness and allow the fue1:oil ratio, which is usually 1:20 to 1:25, to be reduced. Development has been aimed at achieving joint lubrication of engine and gears. If this is successful, it may be necessary to adopt a hydraulic fluid specification such as DEXRONII(seepage 596).
5.1.4 Oils for Marine Diesel Engines Four-stroke and two-stroke trunk-piston diesel engines of output up to about 7 MW and two-stroke cross-head engines providing over about 7 MW are used for propelling river- and sea-going craft. The former can be suitably lubricated with oils similar to automotive lubricants, as well as with synthetic types (144, of various viscosity grades (SAE 1OW to 50) 474
and performance characteristics (API CB to CD classes) according to type and performance (load) of the engine, with an alkaline reserve related to the sulphurcontent of the fuel. Special specifications have been introduced for marine engine oils; for example, MIL-L-9000F (ships) in 1965 and MIL-L-9000G (ships) in 1970 in the USA, STM 7250 in France, DG Ships/6926 and DF STAN 91 -22 in the UK, 3-GP-903a in Canada, BN-PO-178A in Belgium and TL 9150-0031 in Germany. According to these specifications, oils must meet basic quality requirements and, additionally, those of some special laboratory tests, like low foaming tendency, homogeneity, compatibility with other oils, ability to protect bearings from sea-water corrosion and pass simulator engine tests. For example, the MIL-L-9000F and G specifiactions cover oils for lubricating ship and submarine diesel engines operating under normal performance and thermal conditions and powered by fuel corresponding to MIL-F-16884 specification (0.95 - 1.05 weight % sulphur); the oils must pass the Caterpillar 1G-2 test (FTMS 791a, method 341.4) and GMC-71 (ITMS 791a. method 339.3) (Table 5.14). Oils complying with this specification belong to API classes CC or CD.
Table 5.14. Characteristics of the GMC-71 (US Navy)Engine Test Parameters evaluated
Engine Fuel Operating conditions: Test duration
Performance characteristics (piston deposits, corrosion, wear) of the oil operating under severe conditions with high-sulphur fuel, the oil being contaminated with sea-water. CiM-71 3-cylinder, 2-stroke diesel 1% sulphur diesel fuel
330 hours total (15 cycles of 20 hour runs and 4 hour shut-downs) Engine speed (r.p.m.) 18oof10 Fuel temperature ( "C) 49 121 Sump oil temperature ( "C) 93 Coolant temperature ( "C) Inlet air temperature ( "C) 30 150 cm3 sea-water is added to the oil charge at the start of every cycle
To meet the requirements of the STM 7250 specification, oils must pass the following: foaming test (max. foam bulk after ASTM D-892-581 test, after 10 minutes rest 300 cm3 at 24 "C, 25 cm3 at 93 "C and 300 cm3 at 24 "C after heating), homogeneity test (two methods are used: 100 cm3 of oil is placed in a tube at -32"C for 24 hours the oil heated to room temperature must be clear; or the oil is centrifuged in two 100 cm3 tubes for 4 hours at 38 "C at an acceleration of lo00 g; not more than 0.01% vol. of deposits may separate), test of Compatibility with other oils (50 cm3 of test oil and 50 cm3 of reference oil are agitated for 15 minutes in a 100 cm3 tube then centrifuged for 30 minures at 38 "C and lo00 g - not more than 0.01 % vol. of deposits may separate), engine tests: 120 hour test in a Petter AVl engine by the CEUAT method, 300 hour test in the GMC 3.71 engine by the FlWS 791/344 method under non-standard conditions (80 "C offtake water temperature) and 300 hour compatibility test in the GMC 2.171 engine (a mixture of identical portions of reference and test oils is tested).
475
Under the DG Shipd6926 specification, OMD I13 oil used for lubricating all ship-board diesel engines must have a viscosity of 11.9 - 13.5 mm2.s-I at 98.9 "C, 3000 mm2.s-' max. at 0 "C, closed cup flash-point 182 "C min., pour-point -17.8 "C max., foam persistency 25 cm3 at 93 "C and 300 cm3 at 24 "C and good water separability. It must pass the following engine tests: 480 hour Caterpillar L-1 (modified IP 124-60),240 hour Caterpillar 1-G, Petter W-1(as for DEF 2101C). Petter AVl and Rootes TS-3 (300 hour for marine regulations) (Table 5.15).
Table 5.15. Characteristics of the Rootes TS-3 Engine Test Parameters evaluated
Engine Fuel Operating conditions: Test duration Engine speed Inlet oil temperature ( "C) Coolant temperature ( "C) inlet outlet
Performance characteristics of the oil; exhaust valve port plugging and piston ring wear are particularly evaluated. 3-cylinder, 2-stroke Rootes TS-3 diesel 1% sulphur diesel fuel 300 hours without oil change 2m10 53 about 61 66
The two-stroke cross-head engines used for propelling very large vessels employ separate lubrication systems for the cylinders (cylinder oils or MDCL's) and the crankshaft bearings (system oils). These low-speed engines are less sensitive to fuel quality and can therefore be powered with heavy fuels of high sulphur content. These fuels contain appreciable amounts of solids, such as resins and coke and catalyst fines resulting from refinery processing. Such solids can present special problems of abrasive wear. The bearings of these engines can be lubricated and the pistons cooled with conventional engine oils of suitable viscosity (SAE 20 and 30), e.g., of API CB quality. However, high-stability oils with viscosities in the region of 16-23 mm2.s1 at 100 OC and a high alkaline reserve to neutralise acidic fuel combustion products are needed to lubricate the cylinders (by circualtion). The oil is distributed on to the periphery of the cylinder and is spread by the movement of the piston rings; some of it is burnt and the rest is circulated via a centrifuge and an oil tank. The alkaline reserve of these oils is around TBN 60-70 and is achieved by high dosages of overbased calcium or magnesium phenolates, sulphonates or naphthenates. If low-sulphur fuel is employed, over-based oils can cause excessive wear. This phenomenon has not yet been explained.
Also, these oils must be compatible with sea-water, display adequate demulsification properties, wet the metal surfaces and be non-corrosive. Simulation engine tests are used for identifying the performance characteristics of these oils: Petter
AV1,Petter MK-11, Caterpillar I-G and Bolnes engines (100hour test in a 2-cylinder engine at 55 K W and 430 r.p.m. at coolant temperature 60 "C, or 300 hour test in a 3-cylinder supercharged engine at 190
476
KW and 514 r.p.m. at a sump temperature of 65 "C and the same coolant temperature (108, 109). However, the maintenance and replacement costs of large marine engines are so high that extensive fullscale testing in the field is essential when new oil formulations are introduced for approval by engine builders. A number of techniques for ship-board testing of lubricants, especially anti-wear characteristics of MDCL's, have been developed. These include procedures whereby the gradual change in dimensions of a tapered insert in the piston ring is monitored electrically; this gives a read-out related to ring-wear which can be obtained whilst the engine is running under fully realistic conditions.
Compared with automotive applications, marine practice is characterised by a relatively small number of very high-value operating units and very high operating costs. The development of new and improved lubricant types is therefore accompanied by particularly close collaboration between engine builders, operators and the manufacturers of oils and additives. The tendency to develop standard tests and specifications for oils for use in a large variety of engines is much less than in the land-based automotive area.
5.1.5 Railroad Diesel Oils Railroad diesels are normally lubricated with S A E 30 and 40 oils similar to API CC and CD automotive oils. The CD oils are intended for heavy-duty, supercharged diesel engines operating at higher temperatures and in engines with extended intervals between oil and filter changes and periodic overhauls. These conditions obviously require higher-quality oils. Design improvements are constantly being made (e.g., piston-cooling, filtration improvements, reduction in the oil circulation factor) which moderate these demanding operating conditions for the oils. Oil development tends towards the use of oils containing a combination of ashless dispersants which suppress the formation of cold sludges and, consequently, excessive viscosity increase during periods of cold running. Fully synthetic oils are promising in this respect. Resistance to cold-sludge formation was tested in the Perkins 4-109 diesel engine, at 18 kW and 2500 r.p.m., 70 "C oil and coolant temperatures, without oil filtration, over a 120 hour period (Perkins Diesel Oil Thickening Test). Oil quality is evaluated from engine cleanliness, the generation of oil-insolubles and viscosity-increase (120).
Modem engines use silver-based bearing shells, which are more resistant to critical lubrication conditions, so diesel engine oils for these engines do not usually contain ZDDP, resistance to oxidation being provided by increased concentrations of alkylphenolates or alkylsalicylates. Relevant properties of railroad diesel oils are tested in the same way as those of automotive oils. Some specifications, such as those of the American railroad companies and French and Belgian Railways, require special tests, for example: - silver corrosion test, e.g., EMD 201-47; in addition, the maximum zinc content of the oil (lo p.p.m.) is specified, - silver disc friction test, - lead corrosion test (e.g., by the Alco method),
477
-
copper corrosion test (by ASTM 0-130-55). oxidation test, NYC (Wocot),which also identifies copper, lead, silver and iron corrosion, thermal stability test (Chrysler heat test at 149 OC, beaker gel test - 7-day test at 176.7"C), glycol resistance test; 145 hour test in Petter AVl by ORE 7 method (see also page 470). Each engine builder company has its own preferences based on field experience and only proven quality in extended road testing can earn product approval (Table 5.16).
Table 5.16. Railroad Engine Builders' Requirements Engine builder
SAE grade
Sulphated Ash
Zinc content
API class
Road test
10-20 10-20
-
-
30
-
1.5
0.05% min.
40
-
1.5
0.05% min.
15Wl40
-
1.8
0.05% min.
1 0-20 7-13
-
-
-
-
-
3 locos,l year 3 locos, 100,OOO miles SWCC required SWCD SWCC required SWCD SWCC required SWCD CD CD CD required
10 min. 10 min.
-
-
CD CD
GM-EMD 40 40 GE MTU
TBN
(ASTM D28%)%weight (max)
Alco 40 Bombardier 40 Sulzer 40 SEMTPielstick 40 40 SACM
10 p.p.m max. -
required required
5.1.6 Oils for Dual-Fuel and Miscellaneous Internal Combustion Engines Dual-fuel compression-ignition engines using diesel oil and natural gas, or sparkignition gas engines using natural gas, liquefied hydrocarbons or waste gas (e.g., from methane fermentation of organic waste) require low-ash oils for lubrication which minimise the formation of deposits on the piston-crown, in the combustion chamber and on the spark-plugs (6). Spark-ignition gas engines are very sensitive to ash in oils. However, some natural gas and liquefied hydrocarbon engines using low-ash monograde oils have exhibited severe exhaust-valve wear and it has been suggested by some authors that ash in oil has some anti-wear properties. Waste-gas has a non-standard or cyclical composition and hence it causes variation in the fue1:air ratio and, having a higher than usual concentration of sulphurous and nitrogenous compounds, accelerates the ageing of the oil. The oil composition must be adjusted accordingly. The formulation of oils for these duties can be unusually intricate. Examples of the composition of gas engine oils are shown in Table 5.27. WiIs (246) has provided a comparison of the mineral and synthetic oils which are used for the lubrication of gas engines.
478
Table 5.17. Typical Composition of Gas Engine Lubricating Oils EngindOil type SAE grade Component 250 Solvent Neutral 600 Solvent Neutral 150 Brightstock Rust inhibitor Antifoam ZDDP antioxidant Ashless antioxidant Detergent-dispersant Ashless dispersant
2-strokellow ash 30 31.0
68.0 0.05 0.005 1.oo -
4-strokdmedium ash 40 8 weight
dual fueVashless 40
-
86.0 10.0
84.5 9.5
-
-
.oo
1
-
3.0
-
-
0.5 5.5
5.1.7 Oils for Running-in Engines The surfaces of pistons, piston-rings and cylinder liners of all new engines have major or minor micro- and macro-geometric faults. Microgeometric faults are surface asperities, i.e., differences between the measured and the effective surface. Macrogeometric faults are the differences between the effective mean profile and its theoretical contour.
Target engine performance with minimum oil consumption and minimum blowby into the sump is achievable by running-in the engine, i.e., by bringing about mutual adaptation (mating) of the friction surfaces without changing their rated dimensions.
-
Running-in can be seen as comprising two sequential stages: microgeometric running-in, when the asperities in mutual contact grind off, so that the surfaces actually in contact extend and the local stress in the areas of effective contact decreases; high local loads develop local hot-spots which can promote micro-welding of mutually-opposed asperities; macrogeometric running-in, when opposed profiles become mated by gradual wear of the surfaces, so that, for example, mutual adaptation of piston-rings and cylinder liners enables sufficient tightness to be achieved to prevent leakage of oil into the combustion chamber, or of combustion products into the oil sump.
It is important to try to achieve the best possible mating of the engine components as quickly as possible, since the engine must not be heavily-loaded during the running-in period; excessive loading can cause imperfect adaptation of the friction surfaces and, eventually, to seizure due to excessive heat release. The running-in process can be accelerated and better run-in may be achieved by either of the following methods:
479
-
introduction ofabrasives into rhe engine intake system: by this rather drastic method, fine silicatebased “polishing” powder is introduced into the intake manifold; the powder enhances the grinding-off of the surface roughness. The principal deficiency of this method for multi-cylinder engines is uneven distribution of the abrasive into the individual cylinders. Silicate particles are liable to get into the oil and unwanted wear of other engine parts, such as bearings, or increase piston deposits. - aditives in thefuel: about 2% weight of these agents is added to the fuel. The combustion heat decomposes them into fine abrasive aluminium, chromium, nickel, etc. oxide-based substances,which help polish the unevenness existing between the cylinder wall and the piston rings. Since these particles are very fine and become further crushed between the cylinder wall and the piston rings, they have no detrimental effect on other portions of the engine if carried there by the oil. These additives cease to be effective as soon as tightness of the piston ring is achieved. - additives in rhe oil: high-pressureand lubricity agents and solid lubricants, e.g., MoS? These additives diminish the danger of seizure, but instead of reducing the time for run-in, they extend it. Artificial anti-seizure treatment of the friction surfaces by phosphatising, sulphatising and copper coating of the piston rings has a similar effect. Suitable organo-metallicadditives, which form deformable eutectics with metals in areas of roughness, have been recommended.
Special oils with wear-accelerating agents are in many ways the best means of optimising running-in. These agents are mainly carboxylic acid esters which react with the metal at high temperatures and generate metal salts soluble in the oil. They appreciably improve the running-in properties of oils which contain detergentdispersant additives normally used for lubricating the engines. The reaction products have no abrasive effects and can be removed readily from the engine when the oil is changed. Polymers (polyisobutene, etc. (144)) are also suitable as running-in additives. Oils for running-in engines should have the lowest possible viscosity consistent with satisfactory bearing lubrication and effective heat removal. SAE 20 - 30 oils (according to engine type) are the most suitable. For effective running-in, an adequately high pressure must be applied behind the piston rings. This requires a fairly rapid increase in engine loading, which can be advantageous in that the engine can be operated quite normally within a very short time, without special limitations. As soon as run-in is complete, additives cease to be effective, as the pressure in the contact area drops substantially after the asperities have disappeared and smooth surfaces have been created. Nevertheless, a true and effective running-in mode of operation must be established, to avoid excessive wear on the piston skirt and cylinder liner surfaces, particularly on the recessed side of the piston.
5.1.8 Upper Cylinder Lubricants When the motor is started, especially after an extended shut-down period and for some time afterwards, the top portions of the cylinders and some portions of the valve gear are insufficiently lubricated by the normal lubrication system “from below”. Until the engine heats up to at least 80 “C,fairly intensive cylinder wear occurs, accompanied by the corrosive effect of the acidic products of combustion. Test have shown that the lubrication of these areas of the engine under these adverse conditions can be substantially improved by adding a small amount (0.25 - 1 .O 96 vol.) of oil to the fuel, or injecting during start-up or shortly afterwards a small 480
amount of oil into the intake manifold (214,so that when the mixture is compressed, the cylinder walls become covered with an oil-mist. Upper cylinder lubrication enables oil to reach valve gear components more rapidly, particularly the intake valves, where it acts both as lubricant and as solvent of any resin form fuel oxidation which has settled on the intake valves. Upper cylinder lubrication has been mostly used in spark-ignition engines, since diesel fuel, especially its heavier fractions, have a certain lubricant quality themselves. Well-refined mineral oils with low viscosities (around 30 - 35 mm2.s-' at 20 "C) are applicable as upper cylinder lubricants. Upper cylinder lubricants have mostly lost their original significance with the advent of package detergent additives for automotive gasolines, which also contain a raffinate fraction with a viscosity around 10 mrn2.s1 at 100 "C which are also designed to reduce deposits on the intake valve stems.
5.1.9 Perspectives World energy and raw-material problems are very much reflected in the automotive industry. Efforts have been made to cut substantially the fuel consumption of small passenger cars to below about 4 litres per 100 km,with the target of achieving consumptions as low as 2.7 litres per 100 km. Tribological means can be used to contribute to this effort. For instance, total mechanical losses by friction and leakage in present vehicles of progressive designs represent 25 to 50%, losses in the pistonto-cylinder assemblages about 12% of the brake horse-power of the units. It is estimated that these losses could be reduced by about one quarter. The continuously variable transmission (CVT) can achieve optimum effectiveness of the vehicle in each gear and an improvement in fuel consumption of up to 25 to 30%. Utilising braking energy by coupling the gears with the flywheel can bring about a 50 to 80% reduction. For large diesel engines, a so-called adiabatic, un-cooled engine with insulated cylinder walls and a system of waste-heat recycle in a couple gas turbine has been under development. The temperature on the cylinder walls is believed to be as high as 540 "C.It is evident that completely new tribological approaches must be considered in such projects (228,229). Other reductions can also be made from meticulous tribological analyses of friction, wear and lubrication conditions. Oils of better thermal and oxidation stabilities and lower viscosities will be applied under conditions of pure hydrodynamic lubrication. Suitable synthetic oils will be increasingly used. Under elastohydrodynamic, mixed and boundary lubrication conditions, lubricants with efficient friction modifiers or high-pressure additives will be employed. A significant issue in the correct application of tribological principles in designing friction elements and operating conditions will be a reduction in material losses by wear, lower repair and servicing demands, higher reliability and higher operability of vehicles and other machines powered by internal combustion engines.
481
5.2 AIRCRAFT OILS For the propulsion of aircraft, gas turbine engines are obviously now completely dominant. Both military and commercial aircraft powered by gas turbines can reach high, including supersonic, speeds at high altitudes, and transport heavy loads. The development of turbo-jet and turbo-prop aircraft continues to accelerate. On the other hand, the development of aircraft piston engines has slowed down. These engines are mostly used in smaller, sport and agricultural aircraft and in helicopters. For these aircraft, piston engines will retain their importance for many years to come. Very high and quite specific demands are placed on lubricants for aircraft engines. These demands differ in many ways from those imposed on oils for lubricating other types of internal combustion engines used in other applications. Especially high demands are placed on oils for jet aircraft.
5.2.1 Oils for Lubricating Aircraft Piston Engines The lubricating oil in aircraft piston engines is exposed to much higher thermal stress than that in ground vehicles. It is exposed to a very wide range of temperatures. The oil film in the piston ring zone is at 250 - 300 O C and the oil charge 80 - 125 O C , so that a large proportion forms a mist saturated with air. The oil on the lubricated surfaces is exposed to high pressures; the load on the main bearings is 10 -20 MPa and on the connecting rod as high as 25 MPa. In addition, aircraft engine bearings typically run at very high r.p.m. The sliding-speed of the basic components is of the order of 6-15 m.s-', the speed of the connectingrod shaft exceeds 2.500 r.p.m., and that of the compressors blade vanes is as high as 30,000 r.p.m.
The thermal and pressure load on the oil obviously varies according to the performance and operating regime of the engine. Whereas large engines with output over 735 kW operate predominantly at constant speed, usually lower than the maximum speed (which is reached during take-off and sharp climb), small engines, for example those of sporting aeroplanes, operate in an unstable regime where full performance of the engine predominates. These severe changes have adverse effects on the oil. The performance requirements of aircraft piston engine oils may be deduced, in broad outline, from the above statements. They must have high thermal and oxidation stabilities and the ability to disperse the products of oil ageing and fuel combustion, their viscosities must be not too dependent on temperature (to make take-off with a cold engine possible) and they must have low volatility and a low tendency to foaming. Aircraft piston engine oils are prepared from high-quality raffinates in a narrow fractional range. The old types of oil were dosed with ash-producing additives (e.g., under British specification DERD 2472B12) but they were not widely applied. The newer types are treated with ashless additives, particularly antioxidants and 482
dispersants and may also contain anti-wear agents. British specification DERD 245012, US specification MIL-L-2285 1B-Amendment No. 1 and Canadian specifications 3-GP 315,320 and 321 are applicable to these oils. Additive-free oils are used to a lesser extent. These include oils to Soviet specification GOST 2174376, British specifications DERD 2472A, 2472B/0 (Grade 1100) and 2472C, US specification MIL-L-60824D (Grade 1065 and 11OO), Canadian specifications 3-GP60a, 80a and 120a and French specifications AIR 3560D-80 and 3560-DE-100. The particular types of oil differ mainly in their viscosities; the standard types have viscosities form 12 to 25 mm2.s-l at 100 “C, VI from 95 to 125 and pour-point from -12 to -30 “C. The flash-point of additive-free oils ranges from 230 to 290 “C. At one time, most oils for the lubrication of aircraft piston engines were monogrades. Multigrade oils have now significantly appeared in this application also. The sale of such an oil was started in 1980 by Shell (Aeroshell Oil W 15W/50 Multigrade). A reduction of 50% in oil consumption was achieved with this oil in Pratt & Whitney R-I830 aircraft engines (210). The SAE J 300 classification now includes an SAE 60 grade for use in aircraft engines. The majority of specificationsdo not indicate oil density; in practice,this varies between 875 and 895 kg.m” at 15 “C. Soviet GOST 9230-60 and the specifications of the other countries stipulate viscosity index limits. GOST specifies the ratio of the kinematic viscosities, PVK 50/100, and the temperatureviscosity coefficient TKV (GOST 3153-51). GOST specifies the flash-points in open and closed cups; the US and British specificationsdefine flash-point in Cleveland open cup and the German specifications use Marcusson. Carbonisation residue is determined in Soviet, US and German specifications by Conradson and in British specificationsby Ramsbottom. Oxidation ash is prescribed in additive-free oils, sulphated ash in additive treated oils. Pour-point is defined in the Soviet and German specifications by DIN 5 1 538, in the American and British specificationsby ASTM D-97. The MIL specificationsstipulate pour-points after thinning with xylene and gasoline. Oil-contamination is defined in the Soviet specificationsas the concentration of physical contaminants (GOST 6370-59), in the US specificationsas the Contamination Test (FTMS791-3006), with the result expressed as oil contamination with particles of a size over 74 pm and as precipitation number in terms of the amount of substances separated from the oil after thinning with gasoline and centrifuging. Neutralisation number is determined in the additive-free oils by a conventional method, in additivetreated oils and varying TBN by ASTM D-974. GOST and other specifications limit the sulphur content; the methods employed vary. The Soviet specificationsprescribe the Pinkevitch corrosion test (GOST 5 162-49);other specificationsdefine copper corrosion by ASTM D- 130. The specifications limit the foaming tendency of the oil and prescribe determinations at two different temperatures, e.g., by ASTM D-892. ASTM D-1500 and DIN 51 578 are used to determine oil colour; GOST 9230-60 prescribes colour (Oil MS-20s) which is determined by GOST 2667-52. Soviet specificationsrequire the evaluation of thermooxidation stability by the Papok method (GOST 495349). other specifications stipulate engine tests. Storage life is only defined in MIL-L-22851; it is expressed as the amount of deposits produced during prolonged storage of the sample. Solvent content from selective refining is only prescribed in Soviet specifications GOST 6350-56 and 1057-67. The “Work Factor” is defined in the US and British specifications and evaluated by a Work Factor Test (FTMS 791-3451.1); W K 0.85 max. is permitted for 1065- and 1100-type oils.
483
5.2.2 Oils for Gas Turbine Aircraft Engines Lubricating oils are those intended for lubricating the main shaft bearings and cooling the engine and, in turbo-prop engines, for lubricating the propeller-gear; hydraulic oils are designed for the automatic, control and servo-systems. Oils in turbine-driven aircraft are exposed to high temperatures, which increase proportionately with airspeed and power, dependent on the aircraft design. For example, the temperature of the circulating oil in sub-sonic aircraft varies between 70 and 1 15 "C (the temperature in the tail-bearing in the turbine is as high as 180 "C) and in supersonic aircraft operating at Mach 1 - 2, between 135 and 190 "C (in the hot bearings as high as 200 "C), at Mach 3 between 200 and 260 "C (bearing temperatures up to 315 "C) and at Mach 4 between 315 and 370 "C (bearing temperatures up to 425 "C). The oil is heated by the heat generated by the work of the engine and by the aerodynamic heat generated by flight. The heating of the aircraft is related to the airspeed, height of flying and the material from which the aircraft is made. Steel-titanium airframes temperatures vary at Mach 2.2 between 130 and 370 "C Airframe heat is transferred to the lubrication and hydraulic systems. Cooling is hindered by the high temperature of the intake air, which increases - due to compression in proportion to airspeed (for example, the temperature at 1,200 km.h-' is about 40 "C,at 2,400 km.h-' as high as 200 "C (43). The oil is also exposed to high prtssures and because the oil filters are small (7 - 30 likes), high circulation factors are experienced (1,500 to 2,500 1.h-I). According to the type and size of the engine, the main engine shaft is operated at between 5,000 and 20,000 r.p.m.. The mechanical loading on the front bearing is 500 MPa, the middle 300 MPa and the rear bearing 450 MPa. The reduction gears of turboprop engines are loaded up to 1,500 MPa.
-
5.2.2.1 Lubricating Oils
The working conditions sketched in above require the oil to have high thermal and oxidation stabilities, between 200 and 250 "C, low viscosity at low temperatures, extremely low pour-point, sufficiently high viscosity at working temperature (3 - 5 mm2.s-I at 100 "C for turbo-jet engines and 5-8 mm2.s-*at I00 "C for turboprop engines), which must all remain constant even under conditions of high mechanical stress, plus a low foaming-tendency, and high flash- and self-ignition points; the oil must be neither corrosive nor be liable to damage sealing materials. Mineral oils fail to meet these requirements fully and gas turbines have therefore been lubricated with synthetics since the advent of their use. Development of these oils intensified after World War II, particularly in the USA, Britain and the USSR and is still in progress (254). The first generation of synthetic oils includes the types corresponding to MILL-7808J, DERD 2487 and GOST 13076-67 specifications. Different esters of aliphatic dicarboxylic acids and C,-C alcohols, particularly diisooctyl sebacates and azelate (made from 2-ethylhexanol) containing oxidation inhibitors such as Calkylphenothiazines, anti-wear agents (e.g., tricresyl phosphate), anti-foam agents and, eventually, several others, are used to comply with these specifications. These oils can be used for temperatures up to 175 "C and, for a short time, up to
484
240 "C in aircraft engines exposed to high thermal stress, at speeds of Mach 2 and higher, unless the operating temperatures exceed these values. At higher temperatures, these oils decompose. They have good viscosity-temperature characteristics, pour-points below -60 "C, they are resistant to hydrolysis, they have low volatility and good lubricity and they are non-toxic. Their deficiency is their low viscosity (2-5 mm2.s-l at 100 "C),their aggressiveness to rubber and surface coatings and their tendency to penetrate seals. Oils with higher thermal stabilities were developed in the early 1960's, however, their low temperature properties were inconvenient. These second-generation oils comply with Soviet specification MRTU 38-1- 157-65 and with MIL-L-9236B (now cancelled), MIL-L-9236D, MIL-L-23699B and DERD 2497/1 (for turboprop engines). These oils were initially polyglycol types, but esters of neoalkyl polyols later dominated. Pentaerythritol and trimethylolpropane esters with c 6 to C12 mono-carboxylic acids are the most important. They contain amine-type oxidation inhibitors, corrosion inhibitors, anti-wear agents and polymeric VI improvers. Compared with the first-generation oils, they have higher thermal stability (by 50 "C or more), adequate lubricity, lower volatility and they are less toxic. Because of their lower volatility, higher flash- and self-ignition points, these oils are more reliable lubricants for supersonic aircraft, where the fire hazard is greater than in sub-sonic aircraft. The third generation oils, still under development, are required to operate at temperatures of 260 "C and above an airspeed over Mach 3. They correspond to the MIL-L-27502 specification (see Table 5.18). They can be prepared from polyalkylarylsiloxanes, halogenated hydrocarbons, polyphenyl-ethers, etc. (45). Table 5.18. Specifications for Jet Aircraft Engine Lubricants MIL-L- specification 9236
9236A
9236B
23699B
27502
204 ocmin. 98.9 O C 10.0 37.8 O C 3.5 -40 "C -54 "C 13,000
3.0 (3.0) (3.5) -
1 .o -
-
1.o
5.6 13,000 -
-
288 -59
260 -59
5 -
5 -
6 218 -59 passes 15 12-35
-
-
56
Oil properties Viscosity (mm2.s-I) at
Viscosity stability at -54 O C , increase after 3 hours (% max.) Flash-point ( "C, min.) Pour-point ( "C, max.) Foaming tendency test Volatility (% max.) Rubber swelling (%) Lubricity, Ryder machine, compared with reference oil (% min.) Thermooxidation stability test in bearing Miscibility test with other oils 100 hour engine test
-
-
21 8 -54
6 260 -49
20 12-35
12-35
414
100
-
Pass
-
10
passes passes -
-
-
485
Table 5.19. US Classification of Aircraft Oils with Good Oxidation Stability according to Engine Temperature ( "C) Category Engine bearings Engine oil charge Oil in bearing inlet
1
1'I2
2
2'J2
3
3 'I2
4
260 171 149
260 204 177
260 227 204
288 260 232
316 288 260
343 316 288
371 399 316
= esters of dicarboxylic acids containing old-type. antioxidants. 11/, = esters of azelaic acid containing more effective antioxidants. 2 = neopentyl esters (5 mrn2.s-l at 100 "C). 2I/* to 4 = not available. 1
Table 5.20. Properties of Synthetic Aircraft Oils Density at 15 "C ( k g ~ n - ~ ) Viscosity (mm2.s-1) at 200 "C at 98.9 "C at 37.8 "C at -40 "C Pour-point ("C) Fire-point ("C) Self-ignition temperature ( "C) Acidity (TAN) (mg K0H.g') Lead corrosion, 1 hour at 162.5 "C ( m g . ~ m - ~ ) Volatile loss (% weight) Deposits (% weight) Change in viscosity at 37.8 "C (a) Acidity increase (mg K0H.g-1) Rubber swelling "XNIOS" (%) Lubricant film load-carrying capacity (IAE machine, specification DERD 2487); critical load with oil at 100 "C at 2000 r.p.m. at 6000 r.p.m. Volatility after 6.5 h at 204 "C (% weight m a . ) Water content at 20 "C (saturated liquid)(% weight max.) at 37.8 "C Specific heat (kJ.kg-'.K-') Thermal stability after 24 h at 279.6 "C, % change in viscosity at 37.8 "C Foaming tendency (cm3.s-') at 23.9 "C at 93.3 "C at 23.9 "C after 93.3 "C Surface tension (mN.m-')
940 2.25 7.60 35.9 12,500 -58 260 400 0.1 -0.3 0 0.07 -3.6 4.43 15
294+35 171f31 19 0.04 I .9 -4 5/60 25/25 10/60 30
These compounds have high thermooxidation stabilities (up to 400 "C) but poorer low-temperature properties. Third-generation oils are at the limit beyond which "conventional" lubricantscan no longer be used. Supersonic aircraft operating at above Mach 5 need lubricants capable of operating at temperatures as high as 600 "C. Lubricants envisaged for this include liquid metals (Cs, K, Na), metal solutions, silicon dioxide and boron
486
trioxide, graphite and MoS2, Cu and Fe phthalocyanines and gaseous lubricants not harmful to the ozone layer, e.g., CF2Br2, CF3Br, CF2CI2, SF6. In the USA, classifications have been established for oils with good oxidation stability in terms of engine temperature (Table 5.19). In this index, which provides interesting and useful perspectives, the oils are classified into four headings.
Examples of oils with higher load-carrying capacities corresponding to the British DERD 2487 specification and French AIR 3517 are detailed in Table 5.20. Oil must be compatible with the following elastomers: nitrile, Viton, silicone, PTFEC. The quality specificationsof aircraft turbine oils do not contain all the laboratory tests which can be employed for these oils. Some tests are only used for development, checking the effects of metallurgical changes, etc. (Tuble 5.21). Nevertheless, these tests are still applicable for testing aircraft turbine oils of relevant quality specifications, such as GOST 6457-66, 11552-65, 102328-68, 1081764, MIL-L-6081C. 25336, DERD 2490, 2479/0, etc. for mineral oils, and MRTU-38-1-164-65and 38-1-158-65, MIL-L-7808,25502,9236 and DERD 2497 for synthetic oils. Oils corresponding to these specifications are regarded as high-quality aircraft turbine oils.
Table 5.2 1 . Some of the More Important Methods for Evaluating Oxidation, OxidatiodCorrosionand Thermooxidation Stabilities of Aircraft I’urbine Oils Type of test Oxidation-corrosion (Sunbury Beaker Tests) Oxidation-corrosionstability Oxidation-corrosion stability, Pratt Whitney Co. USA Oxidation-corrosion stability, Allison Co. USA Thermooxidation stability, Rolls-Royce Co. Thermooxidation stability, USSR Thermooxidation stability, Deutsche Versuchsanstalt fur Luft- und Raumfahrt Thermooxidation stability Thermooxidation stability WPAB (Wright Patterson Airforce Base, Dayton Ohio, USA
Prescribed standard DERD 2487 FTMS 791a-5308 PWA 521B FMS-35H RR 1001 GOST 98 1-47
Oil specification DERD 2487 & 2497 MIL-L-6081 & 23699 -
-
DVL-Bericht Nr.287 FTMS 791-2504 MIL-L-2105 -
-
5.2.2.2 Aircraft Hydraulic Oils
High fire-point synthetic oils are usually employed for the control and power systems of turbine aircraft (249, 250). In the USSR, polysiloxane fluids, mainly blends with esters of organic acids or fluorinated silicones, are used. In the USA, these applications are mostly served with higher esters of phosphoric acid, especially in the commercial aircraft, or higher esters of silicic acid, mainly in supersonic gas turbine aircraft. Polyphenylethers are used for extremely high temperatures, for example, polyphenylethers together with chlorosiloxanes are used in the Tu 144, Concorde and Boeing 2707. 487
Synthetic aircraft hydraulic oils are covered, in the USA, by MIL-L-8446 B, 7808 A, 6387 A, 6085 A and 7083, in the USSR by GOST 13032-67 and 13004-67,the most commonly-used fluid 7-503-3 being covered by MRTU 38-1-195-66.The most important requirements for these fluids are high thermal stability and nonflammability. An example of a synthetic aircraft hydraulic oil complying with the MIL-L-6085 A specification and containing oxidation and corrosion inhibitors and anti-wear agents is described in Table 5.22. Table 5.22. Properties of Synthetic Aircraft Hydraulic Oils of Very Low Pour-Point and Low Volatility ~
~
~~
Colour, NPA Density at 20 "C (kg.m-3) max. Acid number (TAN)(mg K0H.g-I) max. Viscosity (mm2.s-') at 54 "C at -54 'C Flash-point ( "C) Fire-point ( "C) Pour-point ( "C) max. Stability (72 h at -54 "C) Volatility (22h at 100 "C) (% weight) max. Oxidation stability (168h at 121 "C): change in viscosity at 54 "C (%) max. change of TAN (mg KOHg'), max. deposits (mg. IOOml~'),max. Copper and steel corrosion test (10 days at 27 "C and 50% relative humidity Protection in moist environment (100 h, 49 "C, 100% relative humidity) Sedimentation index Rubber swelling "H'(%) Accelerated storage stability test (168 h at 110 lead corrosion ( m g . ~ m -max. ~) Foaming tendency (cm3.s-I) at 23.9 "C, max. at 93.3 "C, max. at 23.9 - 93.3 "C. max.
5 917 - 927 0.5 8 - 8.6 12,000 210 - 235 255 -57 Separation, gelatinisation or crystallisation do not occur. 1 5
0.5 10
No corrosion or pitting No corrosion or pitting 0 12 - 35 10
10 251180 100l300
Oil must not attack copper, steel, cadmium, aluminium, magnesium alloys or silver.
The US specifications for petroleum-based hydraulic oils are AAF 3580 D, MILH-5606 A and MIL-H-6083 B Type 1, the British specifications are DTD 585 and 5540, and the French AIR 3520 A. The properties of mineral oils meeting MIL-H5606 A, DTD 585 and AIR 3520 A are shown in Table 5.23.
488
Table 5.23. Properties of Mineral Oil-based Aircraft Hydraulic Oils with Oxidation and Corrosion Inhibitors, VZ Modifiers and Anti-wear Additives* Density at 20 "C (kg.m-3) Acid number (TAN)(mg K0H.g-I) max. Viscosity (mm2.s-') at 37.8 "C at -40 OC at -54 "C Flash-point (open-cup)( "C) Pour-point ( "C) Content of solid particles: number of particles in 100 cm3 of oil retained by filter identified by method specified in AIR 1653/A (pm, max.) 5 - 15 15 - 25 25 - 50 5 0 - 100 gravimetrically in 100 cm3 of filtered oil as specified in AIR 1653/A (mg, max.) Water content (p.p.m.) max. Foaming tendency (cm3 foam after 5 minutes blowing at 23.9 "C) (rnax. volume of foam after 10minutes storage, cm? max.) Volatility at 64.5 "C Copper corrosion (after 72 h at 135 "C); colour of copper sheet, max.) Oxidation and corrosion resistance (168 h at 135 "C) - magnesium and aluminium alloys, cadmium-plated steel and steel corrosion (mg.cm'2) - copper corrosion (mg.cm-2) - change of colour of copper sheet - change of viscosity at 37.8 "C (%) - change of acid number (mg K0H.g-') - insoluble substances Rubber swelling, "L"type (168 h at 69.8 "C)(%) Compatibility with other liquids used for hydraulic transmissions Stability at low temperatures (72 h at -54 "C) Shear stability by AIR 1653 A Stability after 12 months' storage at ambient temperature
860 0.20 1 4 - 15 450 - 500 200 - 2500 95 - 105 -60 to -70
16,000 2850 506 90
I .2 100 65
.
complete collapse pass 2e
0.2 max. 0.4 max. 3b max. -5 to 20 0.2 max. 0 19 - 28 pass pass pass pass
* This oil is recommended, for example, for use. in the following fixed-wing aircraft: Super Mystbre-B2, Vaucour (ATAR IOlE & G),Mirage 111 (ATAR 9B & C), Mirage IV (ATAR 9K). Mirage F1, Mirage 5, Mirage G8.Jaguar, Br6guet Atlantic, Transall, Starfighter F104, Fouga CM 170R (Turbomeca Marbon? VI), Morane MS 760 Paris 11 (Turboma Marbod VI), Morane MS 760 Paris IA (Turbomeca Marbon? VI), and the following helicopters: Alouette 11 (Turbomeca Astajou), Alouette 111 (Turbomeca Artouste IIIB), Superfrelon (Turbomeca Turmo IIIc), SA 330, SA 315, Puma,Gazelle.
The purity of hydraulic oils with regard to the size of solid particles contained in them must also be monitored. Regulations on this aspect are increasingly more strict. MIL-L-560 H, for example, sets
489
the following limits on particle size distribution (number of particles per 100 cm3 of oil): 5 - 1 5 p m 2500 lo00 16-25pm 26-50pm
51-100pm over100pm
250 25 0
5.2.3 Performance Tests for Aircraft Oils Performance-related properties of aircraft engine oils are examined in laboratory tests and in both standard and full-scale engine tests. Laboratory Tests Thermooxidation stability of aircraft engine oils is tested in the USSR by GOST 985-56. The oil is oxidised in the presence of metals (Cu or Fe). The test conditions can be varied. When the oil is oxidised in the engine, it is present as a thin film, whereas in this test the entire bulk of the oil is subjected to the oxidation process. For this reason, more thorough methods have been developed, for example those of Shmelev, Papok and Zuseva. Oil stability is assessed from the amount of deposits produced on a hot plate and from changes occurring in the oxidised oil. Volatility is measured from the loss in the quantity of oil. Other tests, in the PZZ apparatus over 2 hours, measure thermooxidation stability, volatility and corrosion properties of the oil.
US and British methods for measuring oxidation-corrosion stabilities are listed in Table 5.21. The Soviet GOST 5 162-49 specification prescribes aluminium alloy, steel and copper corrosion tests, and other specifications in other countries copper and silver (FTMS 791a - 5305) and the SOD lead test (FTMS 791a - 5321 and ASTM D-2955). The Panel Coker Test (FTMS791-3462) meausres the tendency to form varnish; the test conditions attempt to simulate the actual operating conditions of the aircraft oil, i.e., 100 r.p.m., temperatures of 330/357/371 O C , 2.4 g.min-' oil-injection, test duration 8 hours. Volatility and fractional stability are measured by vacuum distillation or, indirectly, by methods such as the Papok method in the USSR (GOST 5737-53 or 10306-75)and ASTM D-972-56 in other countries. The Soviet specificationsdo not requirefoaming tendency measurement, whereas ASTM D-892 and IP 146/565 methods are used in the USA and the UK. FTMS 791-3604 seal-swell rest is carried out over protracted periods and at high temperatures; the test is made in a sealed vessel. Storage stability (mainly a test of the tendency to separate additives) is determined by a test at elevated temperatures over 45 days. Test conditions are specified in MIL-L-7808 F. Resistance to mechanical stress (shear stability) is tested in a fuel pump/nozzle system through which the oil is circulated under specified conditions; the criterion measured is viscosity, which must not change from the initial viscosity by more than *2%.
490
Lubricity (lubricant-filmcarrying capacity) is evaluated in various test machines; the most-commonly used is the Ryder Gear-Erdco Universal Test of the PrattWhitney Co., the test conditions being specified in ASTM D-1947-64 and FTMS 791a-6508 (46). The test device is a gear-box, in which turbo-jet gears are used as test pieces; 270 cm3.min-' of test oil is injected on to the gears which turn at 10,OOO r.p.m.. The intake oil temperature is 74 O C and the load is increased every 10 minutes. the result is expressed as the critical load at which scuffing and scoring appear on one third of the surface of 65% of the teeth. Oils which withstand a critical load of 2,950 N are deemed suitable for field service.
In the UK, aircraft oils, particularly those intended for turboprop engines, are tested, additionally, in the IAE Gear Machine (IP 166/65) at 110 "C.The load is increased until partial scuffing occurs, as manifested by a decrease in speed. GOST 9490-75 specifies tests in the four-ball machine. The BSE-Olympus test is designed to measure load-carryingcapacity under conditions prevailing in supersonic aircraft equipment; the test machine is the Olympus gear-box with working temperatures as high as 180 "C and bearing temperatures of 260 to 3 16 "C (47). Other tests are applied to aircraft turbine oils in which the oils are subjected to high temperatures, high mechanical loads and high speeds (r.p.m.), such as the bearing Head test (FTMS 791a-3410) in the USA and the Bristol-Siddley BSE 1 Test in the UK. Test machines for these procedures are virtually identical. They comprise a heated plain bearing powered by an electric motor (the same motor used in the Ryder Gear Machine). The methods, however, differ. In the American test, the test conditions (inlet and outlet temperature) are related to the oil type; the temperatures vary between 149 and 204 "C and 17 1 and 227 "C,respectively, the test lasts 100 hours, with a speed of 10,OOO r.p.m., load 2,220 N, bearing temperature 260 OC, oil flow 600 crn3.min-l and air-rate 10 I.min-'. In both methods, the viscosity increase, neutralisation number and extent of deposition are evaluated.
Formation of varnish and carbon is tested in the Rolls-Royce reduction gear-box at 1,700 kW (the test duration 150 hours at 1,500 r.p.m., intake oil temperature 115 "C) and in a similar Tyne reduction gear from the same company.
Aircraft Oil Engine Tests As in the case of automotive engine oils, the performance properties of aircraft engine oils are tested in performance-simulating engines and other equipment. Fullscale engines are used for extended tests. Piston engine and turbine oils are tested separately. Automotive oil test engines can be used, mainly for testing piston engine aircraft oils; the tests are conducted under different conditions. The tests which have been used include: Oil corrosion and varnish-formation tendencies were tested in the 6-cylinder Chevrolet engi'ne and oxidation stability by the L-38method in the single-cylinder L a b engine. More usual is the high-
49 1
temperature Wisconsin Pausson Engine Test, which is more severe (no piston-ring sticking in 60 hours). Continental Motors prescribed, in their MHS 24 specification, testing the antioxidant properties in the Lauson engine. The single-cylinderLauson engine used normally for testing automotive oils is subjected, in the CMC test, to higher thermal stress. The coolant temperature is adjusted to 218 "C. The first signs of piston-ring sticking should not appear before 20 hours and engine cleanliness should score at least 6 on a scale of 10. Ethyl Corporation use the CLR engine (the same as the Labeco engine) for testing ashless oils; oxidation stability at high temperatures, detergent-dispersant ability and dispersant stability in lowtemperature operation are evaluated (MIL-L-22851 and DERD 2450 specifications). The procedure involves two tests: 1. Oxidatioflhermal-Stability Test - CLR Engine Test, according to ITMS 3407 of 1965; the engine runs 40 hours at 3,100 r.p.m., 0.85 air excess coefficient, 190.6 "C mean oil temperature on the piston, 107.2 "C outlet coolant temperature, criteria for lubricant oxidation and thermal stability being the extent of fouling of the piston and other lubricated components, change of oil viscosity and neutralisation number, n-pentane-insolublecontent, ash-content and the amount of carbonaceous residue. 2. Low-temperature Dispersancy and Detergency - CLR Engine Test according to FTMS 791a-347.I; the CLR-LTD engine is run at 1,800r.p.m., mixture ratio 15.0 : I , intake temperature 79.5 "C and coolant inletloutlet temperatures 46/52 O C respectively; the engine is dismantled once every 20 hours and the fouling-rate evaluated on the CRC Deposit Rating Scale; oil samples are tested for viscosity by FTMS 791-3132.3 and oil-dilution by ASTM D-322-63.
Soviet standards do not prescribe engine tests. Engine oils must correspond to samples which have passed tests conducted in aircraft engines by the state testing institutes. Tests are, however, carried out in engine simulators. The NAMI-1 test is made in a single-cylinder gasoline engine, in which engine oils are classified under both hot and cold operating conditions; the test is similar to the CLR tests. Aircraft turbine engine oils are tested in simulators which closely imitate service conditions of aircraft turbines; the evaluation of thermal and mechanical loading capabilities of the oil over a wide range is particularly emphasised. The simulator usually comprises an oil tank, coolers, heat exchangers, pumps, filters, superheaters, de-foamers and test-bearings, together with instruments for monitoring temperatures, pressures, torque, etc. The operating conditions can be adjusted, in most cases, automatically;the oil is evaluated by changes in oil properties, deposition and corrosion rates, changes in the properties of sealant materials and continuous recording of load and torque.
The Wright Air Development Centre in the USA developed the deposition number test, which has become the FTMS 791a-5003 standard contained in American quality specifications for aircraft turbine engine oils. Tests made in full-scale aircraft piston engines are similar to prolonged automotive engine tests and the test results are evaluated from the post-test condition of the engine, by the performance, contamination rate of particular components, the wear-rate of functional surfaces, etc., and by physical and chemical changes occurring in the oil, e.g., changes in viscosity and acid number, as well as saponification rate, carbonaceous residues, benzene- and hexane-insolubles and fuel dilution. The principle throughout is that the regime of the bench-engine test must approach actual conditions in flight. Bearing and oil temperatures, oil foaming,, oil 492
consumption, engine performance and other indices are monitored throughout the test; post-test conditions are evaluated, including wear-rate, corrosion and cleanliness (deposits on filters, in bearings, etc.) and the anti-wear properties of the oil, as well as the anti-wear properties of the oil during and after the test. MIL-L-5009 specifies a 100 hour test in the 5-57-19 or 5-57-29 engines; the test includes twenty 5-hour cycles. The test can be classified, according to the test results obtained, into one of grades 0 to 4; the 4th grade can be achieved by an oil‘which is manifestly better than a specimen reference oil. Engine builders also have their own tests.
5.3 COMPRESSOR OILS This category of oils comprises fluids used for lubricating the internal and external components of machines designed for compressing, conveying and expanding gases. In terms of design, these machines can be classified into piston and rotary compressors; by the intake and delivery pressures into high-pressure compressors, blowers (which handle large volumes of gases at relatively low delivery pressures) and vacuum pumps (which evacuate gases from closed compartments). In pneumatic machinery, compressed gas expands to atmospheric pressure and does work. Piston machines are classified into horizontal and vertical: rotary machines into pocket, vane and centrifugal turbo-compressors. They may be single-stage (with a maximum compression ratio up to about 10) and multi-stage machines. The gases processed may be classified according to their effect on lubricating oils into inert, hydrocarbon, chemically active and gaseous coolants (Table 5.24). Table 5.24. Classification of Gases by their Effects on Lubricating Oils in Compressors Inert gases
Hydrocarbon gases
Chemically-active gases
Gaseous coolants
Nitrogen Hydrogen Synthesis gas (CO+H,) Carbon dioxide Helium
Natural gas Refinery Gases Hydrogen chloride Towns gas Saturated & unsaturated C, to C, hydrocarbons
Oxygen Chlorine Carbon dioxide Hydrogen sulphide Sulphur dioxide
Ammonia Sulphur dioxide Freons Methylene chloride
There are no substantial differences, with respect to their lubrication requirements, among these various machines. Therefore, oils used for lubricating them are grouped together under the heading of “compressor oils” and classified by the types of compressor for which they are intended (such as air and gas compressor oils, oil for coolant compressors, oils for blowers and pneumatic machines). PNEUROP classifies compressors into the following types:
493
1 Positive displacement compressors (with interrupted flow) 1.1 Oscillatory compressors 1.1.1 Crankshaft compressors 1.1.1.1 Piston compressors (fig. 5 . 1 4 ~ ) 1.1.1.2 Diaphragm compressors 1.1.2 Crankshaft-less compressors 1.1.2.1Free-piston compressors 1.1.2.2 Electric vibrators 1.2 Rotary compressors 1.2.1 Single-shaft compressors 1.2.1.1 Sliding-vane compressors (fig. 5.14b) 1.2.1.2 Fluid-piston compressors (fig. 5 . 1 4 ~ ) 1.2.2 Multi-shaft compressors 1.2.2.1 Screw compressors (fig. 5.144 1.2.2.2 Straight-wing compressors (fig. 5.14e) 2 Turbo-compressors (with continuous flow) 2.1 Radial compressors (fig. 5.14’ 2.2 Axial compressors (fig. 5.14g)
f
d
Fig. 5.14. Outline of different types of compressors
a - piston,
494
-
b - sliding vane, c - fluid-piston, d - screw, e straight-wing,f- radial hukcompressor, g - axial turbocompressor
5.3.1 Oils for Air and Gas Compressors The basic task of the oil in air and gas compressors is to lubricate the cylinder, glands, big-end bearings, cross-head pin, guideways and position pins. The lubrication requirements of the different components may differ. Horizontal compressors use separate lubricating systems with oils of differing properties. Such an arrangement is not possible in vertical compressors and they therefore use a single oil type, which provides lubrication, as required, for particular components. These factors are basic for the selection of oils, their viscosities and chemical compositions. The main criteria for the oil are the operating temperature and pressure and the purity of the gas handled. Temperature is related to the compression ratio, the physical constants of the gas (Poison constant k) and the rate of heat abstraction during compression. Some compression heat is removed by the air, water or oil cooling system. Gas temperature (in K) are therefore lower than under adiabatic conditions, where the relation T,/T2 = @I/p2)kapplies to ideal gases. The heat capacity of the relatively cool intake gas may reduce the temperature in single-stage compressors. Temperatures in multistage compressors are reduced to some extent, but still remain relatively high unless inter-stage heat exchange is employed (Table 5.25). Table 5.25. Approximate Maximum Air Temperatures ( "C)as a Function of Compression Pressure Delivery pressure (MPa) 0.4 0.7
1.o 1.4 2.0
Single-stage compressor 177
260 300
Double-stage compressor with inter-cooler 100 120 140 155
177
Temperatures are roughly the same for the compression of oxygen, nitrogen and carbon monoxide and a little lower for hydrocarbon gases, carbon dioxide and ammonia.
Temperatures determine the required oil viscosity. At the maximum temperature, the oil must retain a high enough viscosity to prevent metal-to metal contact and to enable the oil to fulfil its sealant function. However, if it is too high, it is unable to wet the entire friction surface adequately and it will retain solid contaminants and transport them to the less-exposed valve or piston-ring grooves. Oils of higher VZ have viscosities which are adequate at working temperatures and low enough for starting. Oils must have good thermal stability (e.g., as evidenced by a low carbonisation residue), because compressors, especially single-stage types, experience high temperatures. In air compressors, where the oil comes into contact with air in the form of a thin film generated between piston and cylinder, the oil must have 495
sufficient oxidation stability under hot conditions, as well as detergent capacity. This is less important for horizontal compressors with short-term lubrication, in which fresh oil is continuously fed via cylinder and valves into a separator, but is very important for vertical compressors, with lubricant entering the cylinders from the crankcase, in the same way as in internal combustion engines, and with a continuous circulation system with filtration, i.e., long-term lubrication. For air compressor oils, PNEUROP specify an oxidation test at 200 "C in the presence of an Fe,O, catalyst, in a 4 x 6 hour test. Conradson carbon residue is determined before and after the test. The difference in the CCT results represents degree of thermlioxidation stability. In addition, volatility and viscosity are measured before and after the test.
Elevated pressure tends to expel the oil from between the piston-rings and the cylinder wall and to press the rings against the wall, creating the risk of boundary friction. For this reason, the viscosity must be sufficiently high and, in addition, good anti-wear properties of the oil are highly desirable. The intake air transports dust into the compressor. With dust levels, prevalent in industrial zones at 11 g.m-3, a compressor may inspire as much as 1 kg of dust in a month. This can result in solid deposits in which the inorganic content exceeds the organic. Obviously, efficient filtration can overcome this. Moisture is another problem of compressor operation with implications for oil quality. The higher the temperature of the gas being handled, the higher its moisture content. Moisture may condense in the compressor at elevated pressure and, when it leaves the compressor, cause corrosion in the intermediate and final coolers, the compressed air manifold and reservoirs. Corrosion in the compressor itself may appear to be minimal, but the moving piston scrapes visible corrosion damage off and the naked surface is repeatedly exposed to corrosion. The result is increased scuffing, abrasion and wear. The compressor oil is therefore required to adsorb itself on to the surfaces and expel water from them. Where there is a large amount of water present, the oil should form with it a lubricating emulsion which adheres to the metal. In any case, intake air must be dried. Conventional air compressor oils were made from naphthenic oils which, as a result of thermooxidative decomposition, produce loose carbon which does not cohere; this carbon is supposed to be carried away by the air. Critics of this approach maintain that loose carbon, being in intimate contact with air, can incandesce; the incandescent particles carried out of the compressor can cause explosions in the delivery piping. However, no explosion hazard should result if the compressor is properly serviced and overhauled and not over-lubricated. The oil used should, however, have a high flash-point (low volatility), leave the cylinder in liquid form and be readily retained by the separator. Explosion is most likely to be caused by the vapours of the lighter fractions, which can be ignited by hot deposits of hot drops of the heavier fractions. Compressor oils must, therefore, always be composed of closely related, very narrow fractions. Blending of light and heavy fractions to achieve the desired viscosity is inadmissable. The use of brightstocks in any compressor application is therefore very doubtful.
496
The work of some Soviet authors (235) offers very instructive ideas of the working conditions of oils in air compressors. Their suggestions concern the problems of dealing with elevated temperatures caused by the increase of the level of compressor performance; oil temperature has an upper limit at which the oil is unable to fulfil all lubricant functions. The remedy frequently adopted is to use high-viscosity residual oils; unfortunately, these promote the formation of carbon deposits on valves and in the delivery piping. Tests were carried out in two-stage compressors with a 230 "C output air temperature and 1.2 MPa pressure. The test was continued non-stop for 72 hours, during which time the air, heated during its passage through the compressor and containing oil mist, was run through 3 centrifugal separators and vented, via a reduction valve, to atmosphere. The separated oil was led into specially-cooled collectors. These tests led to the following conclusions: An efficient oil fractionation process proceeds in the compressor, at temperatures which are low compared with the boiling-points of the hydrocarbons contained in the oil at 1.2 MPa pressure. The heaviest oil fractions, of high viscosity, and containing 1.5 to 45 by weight of carbonaceous residue separated close to the compressor. The amount of oil which settled in the first collector depended on the hcavy fraction content of the oil; about 30% from distillate oils, 40% from mixed oils and 50 to 70% from residual oils appeared in the first collector. Oils made from a narrow fractional cut delivered a lesser proportion into the first and third collector and more into the second collector. The fractionation process was accompanied, on the delivery side, by vigorous oxidation of the finely dispersed oils. All oil fractions passing the separator had a substantially higher acidity than the original oils. The oxidised oil settled as a mist and formed a thin film which moved on the pipe wall ten times slower than the air stream. Under these conditions, further oxidation and transformation of the oxidised products into hard deposits occurred. Piston-ring wear was negligible and was little dependent on oil viscosity. Distillate oils gave rise to higher wear. Low-viscosity oils gave minimum loss of compressor performance (compared with 100% residual oil, they achieved 87.7% with distillate oil and 89.9% with mixed oils).
The presence of some additives in the oil can contribute substantially to the reduction of deposition, for example, antioxidants and detergent-dispersants of the same type as those used in ehgine oils. Among the DD additives, ashless dispersants are preferred, because ash-based additives also contribute to deposit formation. Polymeric viscosity modifiers are undesirable as they promote the formation of carbon at high temperatures. The temperature differences between air and gas compressors necessitates the availability of oils from a wide range of viscosities. They vary, for both horizontal and vertical compressors, between 4 and 39 mm2.s-l at 100 O C with VZ of 90 minimum, flash-point 200 to 270 "C, pour-point -20 to -35 "C and 0.1 to 0.3% weight carbon residue. DIN 5 1 506 (1975), which complies with the European Committee of Compressor, Vacuum Pump and Pneumatic Machine Builders (PNEUROP), classifies oils for the lubrication of piston and rotary compressors into several groups. The quality requirements for these oils are detailed in Table 5.26. IS0 6521 classifies piston and air compressor oils into three groups: ISO-L-D AA - oils for low thermal load, ISO-L-D AB - oils for medium thermal load, ISO-L-D AC - oils for high thermal load. The quality requirements for these oils are shown in Table 5.27 and 5.29. 497
s 00
Table 5.26. Quality Parameters of Compressor Oils to DIN 51506 Designation'
V -B5
+vc5
22
46
100
& VB-L6 150 220
3.8
5.6 to 9.4
9.4
12.6
16.3
to
to
to
12.6
16.3
22.0
22.0
5.6
205 -3
210
225
255
175 -9
450
22
46
above
3.8
5.6 to 9.4
& VC-L6+ 100 150
22
46
100
150
I
DIN test No.
9.4
12.6
3.8
12.6
51561
to
to
to
to
12.6
16.3
5.6
5.6 to 9.4
9.4
to
12.6
16.3
205 -3
210
175 -9
VD-~6
Viscosity at 100 "C mm2.s-1
to 5.6
Flash-point (O.C.) "C minimum Powpoint2 ocmax. Ash
175 -9
195
0
to
195
195
205 -3
51562
210 51376 51597
0.02% weight maximum oxide ash for VB & VC groups; supplier should report sulphate ash for VB-L.5 1575
VC-L & VD-L PUPS Water extract reaction Neutralisation number (total acid content) Asphaltenes content
neutral
51558,
Part 1
0.15 mg KOH g" for VB & VC groups; to be reported by supplier for VB-L, VC-L & VD-L groups
5 1595
For Groups VB & V C below limits of possible quantitative determination For Groups VB-L & VC-L: not specified
51595
Water content (% weight m a . )
0.1
not specified 51532
Cff after ageing in air jet (% weight max.) 2.0 CCT after ageing in air jet and in the presence of iron oxide (% weight max.)
2.5
not specified
2.0
1.5
not specified
not specified
2.5
3.0
5132 Sheet l4
51352 Sheet 2
CCT of residue after distilling off 80%by volume by DIN 513563, % weight max.
not specified
0.3
0.75
0.3
0.75
51551
1. Designation numbers correspond to IS0 classification. 2. Low pour-point oils must be employed if they are to be exposed to low temperatures. 3. This parameter cannot be adhered to for VC-L group oils. 4. The method is suitable for base oils and for oils containing additives (with antioxidants and ashcontaining oils DD additives) from which not more than 15% by volume evaporates during the test.Test procedure: air is passed twice every 6 hours through the oil at 200 "C and Conradson carbon residue determined. 5.For mobile air compressors with final compression temperatures of 140 "C (VB) and 220 'C (VC); additive-free oils. &For air compressors with compressed air storage tank or piping system with final temperature 140 'C (VB-L: with additives), 160 "C (VC-L: with additives) and 220 "C (VD-L: with additives for mobile compressors).
P \Q
Table 5.27. Quality Parameters of Piston Air Compressor Oils to IS0 6521 ISO-L-D AA
Group Viscosity class (VG)
32
46
I
68 100 150 I
ISO-L-D AB 32
46
IS0 test method
68 100 150
Quality parameters: Kinematic viscosity (+lo%) 32 46 68 100 150 32 46 68 100 150 3104 mmz.s-' at 40 "C at 100 "C -typical values must be supplied by the oil manufacturerPour-point ( "C) -9 -9 3016 Copper corrosion 1 (3 h, 100 "C) max. 1 2160 Demulsification 3 ml at Iminute. at DP6614 30 30 30 30 30 54 54 54 82 82 temperature "C Rusting after 24 h no rusting7120 Seq.A Oxidation stability: Oil properties after ageing in air .jet at 200 "C: Evaporative loss 70 weight max. I 15 15 1 15 DP 6617 Pt.1 CCT increase 70weight. max. 1.5 1.5 1.5 2.5 1.5 - after ageing in air jet at 200 "C in presence of iron oxide: Evaporative loss % weight max. 20 20 20 20 20 CCT increase % weight rnax. 2.5 2.5 2.5 2.5 2.5 Properties of residue from evaporation: CCT, % weight max. 0.3 0.3 0.3 0.3 0.3 DP6616 DP 6615 IS0 3104 Ratio of kinematic viscosity to fresh oil viscosity at 40 "C, max. 5 5 5 5 5
Oils for rotary compressors (wing and screw compressors) are also classified into three groups: ISO-L-D AG - oils for low thermal load, suitable for compressors with delivery side temperature I 100 "C; ISO-L-D AH - oils for medium thermal load, suitable for compressors with delivery side temperatures 100 to 110 "C; ISO-L-D AJ - oils for high thermal load, suitable for compressors with delivery side air temperature exceeding 110 "C. The quality requirements for rotary screw air compressor oils to I S 0 6521 are listed in Table 5.28 and 5.29.
500
Table 5.28. Quality Parameters of Rotary Screw Air Compressor Oils to I S 0 6521 ISO-L-D AH
Group Viscosityclass(VG)
I
ISO-L-D AG
IS0 test method
15 22 32 46 68 100 I 15 22 32 46 68 100
Quality parameters: Kinematic viscosity (&lo%) mm2.s-' at 40 "C 15 Pour-point ( "C) Viscosity index min. Copper corrosion (3h, 100 "C) max. Demulsification max.3 ml emulsion after minutes 30 at temperature "C 54 Oxidation stability: Foaming at 24 "C ml, rnax. Foam stability (ml) max.
22 32 46 68 100 -9 80 Ib
15 22 32 46 68 100 -9 80 lb
30 30 30 30 30 30 30 30 30 30 54 54 54 54 82 54 54 54 54 54 will be determined 300 0
300 0
3104 3016 2909 2160
30 82
DP6614
DP 6247
Air compressors for pressures up to 15 MPa use oils of viscosity up to 12 mm2.s-', up to 30 MPa 12-16 mm2.s-I and over 30 MPa 16-20 mrn2.s-l, all at 100 "C. Viscosities exceeding 16 mm2.s-I are used for higher compression ratios, for inadequately-cooled compressors (delivery temperatures over 140 "C)and for highfriction compressors (without spacer rings). Oils must be well-refined and stable. Horizontal compressors can use straight oils (without additives); oils for vertical compressors may contain antioxidants, detergents, rust inhibitors and anti-wear additives. Under critical operating conditions, e.g., at the extremely high final temperatures which can occur in single-stage compressors, mineral oils may release excessive amounts of carbon. Diesters, polyol esters of long chain fatty acids and polyalphaolefins have been introduced to meet these needs. Wits (247) has prepared a summary of synthetic compressor oils in use and Jayce and Jones (248) have reported their development work (see Table 5.29). The PNEUROP test has shown that these oils produce very little coke-like residue (0.19 or 0.24 by Conradson). The nature of the gas being transported by the compressor must obviously be taken into account when selecting compressor oils. Znerf gases have, with the exception of air, very little influence on changes in properties (i.e., ageing) of the lubricating oil. Hydrocarbon gases condensed on the walls of the cooled cylinders tend to wash off the oil film. Such cases require mineral oils of high viscosity or oils containing additives which enhance their adhesivity to metal surfaces, or watersoluble polyglycol oils with oxidation inhibitors (48) or polyether oils, which only dissolve the hydrocarbons to a limited extent. 50 1
Table 5.29. Specification of Synthetic Air Compressor Oils to ISO-L-D AC and/or D AJ Quality parameters Viscosity grade (VG) Kinematic viscosity(flO%) (mm2s-' at 40 "C) Viscosity index
Oxidation stability Pour-point "C
Self-ignition "C Emulsification at 54.4 "C Corrosion Copper corrosion (3h at 100 "C, max.) Foaming at 24 "C
Limits
Suitable synthetic oils (see coding below)
Test method
32,46,68,100,150
all
IS0 3448
32,46,68,100,150 all I S 0 3104 IS0 2909 60 to 165 12,3 200 4 not required 5 will be determined 4-30 1,2,3 IS0 DP 3016 <-50 4 not required 5 >350 1,2,3,4 ASTM D-2155 to be supplied by oil manufacturer--IS0 DP 6614 1 2/45 IS0 PR 7120 no rustingnot required 3
Ib 0 after 10 minutes (not required for ISO-L-D AC)
all all
IS0 2160 IS0 DP 6247
Synthetic oil types: 1 = organic esters 2 = polyalphaolefins 3 = phosphate esters 4 = siloxanes 5 = polyalkylene glycols
In addition to the hydrocarbon gases, carbon oxides and argon are significantly soluble in oils. The viscosity of oil saturated at 25 "C with carbon dioxide drops to about one third of the original value and with argon to about two thirds (212). Again, thicker oils or synthetics must be used in such cases. The lubrication of "pure" nitrogen (nitrogen containing 2 p.p.m. oxygen) compressors is worth mentioning. Compressors lubricated with oil through which nitrogen bubbles run seized after 300 hours operation. There were no lubrication problems when the lubricant was contacted with air (222). The failure to form a protective layer in the former case may have been the cause.
Fatty oils (hoof and claw, tallow, lard oil) can be used as blending agents to achieve increased adhesivity, at 2-5%. However, they have low oxidation stabilities and are not suitable in compressors with closed lubrication systems. Contamination of the gas with oil mist can be suppressed by the addition of up to 1 % polymer into the oil (Z44). Mineral oils, and hydrocarbon oils in general, are of little use or quite unsuitable for lubricating compressors used for transporting chemically active gases. Contact between lubricating oils and oxygen at higher pressures and temperatures can result in violent oxidation and explosion; oil vapours mixed with compressed oxygen is explosive. Oxygen compressor cylinders can be lubricated either with difficult to ignite synthetic oils or with distilled water, which may contain 5-7% glycerol to improve its lubricity. These compressors are usually fitted with special carbon piston-rings so that the cylinder requires no fluid lubrication. 502
Compressors for handling other chemically active gases mostly use nonhydrocarbon lubricants. The oil must be suited to the nature of the gas handled, e.g., polyglycol fluids for hydrocarbon gases or vapours. Compressors for chlorine are lubricated with 98% sulphuric acid or with halogenated hydrocarbons; those for sulphur dioxide are either un-lubricated (since sulphur dioxide itself has lubricant properties) or lubricated with white oils, which can also be suitable for some other chemically active gases. White oils are also used to lubricate ethylene compressors. Some rotary machines, such as vane blowers (e.g., Rootes blowers) do not require internal lubrication. This also applies to some screw compressors and centrifugal or axial turbo-compressors. On the other hand, vane compressors operating at 3304,000r.p.m., compression ratios 4 or more and temperatures up to 280 "C are as demanding in terms of lubrication as piston compressors. The function of the oil is to lubricate, seal and cool, in a continuous lubrication system. The viscosity of the oil should be the same or a little higher than that for piston compressors working under similar conditions; it should be at least 10 mm2.s-l, as centrifugal force can create the risk of rupturing the oil film. The oil must have good thermooxidation stability to prevent sliding vanes becoming stuck in their grooves; the oil is usually doped with antioxidant and detergent additives. Dust and moisture are a nuisance, as with piston compressors. External elements of the system, such as bearings, pins and guideways, are not particularly demanding in lubrication terms in all types of compressors. Standard bearing oils can be used, unless the compressor oil (as in the case of vertical compressors) is used, or the same oil is used as that for the propulsion machinery (such as a turbine).
5.3.2 Oils for Refrigerating Compressors Refrigeration is a closed thermal-exchange cycle. The refrigerant, in the form of a vapour, is compressed, causing its temperature to rise. The vapour enters a condenser, where it gives up its heat and condenses. The liquid so produced passes through a reduction valve (the expansion system) where its pressure drops and into the evaporator, where it evaporates, acquiring the necessary latent heat of evaporation from the refrigerated medium which cools. The basis of this cycle is shown infig. 5.15. The refrigerant vapour compressor is the central component of the whole refrigerating system and its reliable operation is clearly essential to achieve the required refrigerating effect. Oil lubricates the compressor pistons, bearings and valves and helps the sealing rings perform their function. The oil can be exposed to high temperatures in contact with refrigerant, oxygen and water. Halogenated hydrocarbons which are used as refrigerants accelerate the ageing of the oil. Leakage in the low-pressure section and the evaporator - if the pressure here is sub-atmospheric (e.g., if the evaporating temperature is below -23 "C for R 11, below -30 "C for R 12 or below -41 "C for R22, or below -35"C for ammonia)
503
- may cause ingress of air and moisture into the system and impair the stability of the oil. 7
Fig. 5.15. Outline of binary cooling system 1 - container of fluid coolant, 2 - expansion valve, 3 - evaporator, 4 - cooling chamber, 5 - compressor,
6 - separator of oil, 7 - condensing vessel, a - coolant in steam-state, b - coolant in liquid state, c - cooling agent
The following substances have been used as refrigerants: Ammonia - its chemical and physical interactions with mineral oil are negligible. Ammonia and oil mixtures are, however, explosive. Damaging emulsions may be produced and corrosion occurs in the presence of water and the products of oil ageing. Carbon dioxide is only slightly soluble in mineral oils, and it does not affect their properties. It absorbs moisture and is corrosive when moist. Sulphur dioxide when liquefied dissolves aromatic and heterocyclic fractions in mineral oils; these fractions must therefore be excluded from the oils; moist SO, corrodes and access of moisture must therefore be avoided. Fluorinated hydrocarbons - (sold under commercial names including Freons, Ledons, Frigenes, Arctones, Genetrones and Isotrones) are still used; difluorochloromethane- CF2C12(Freon 12 or R12) are the most important; other refrigerantsused include fluorotrichloromethane,CFCI, (Freon 1 1 or R1 I), difluorochloromethane, CHF,CI (Freon 22 or R22). trifluorochloromethane, CF,CI (Freon 13 or R13), trifluorotrichloroethaneC,F3CI, (Freon 1 13), tetrafluorodichloro-ethane,C,F,C,, Freon 1 14) and some brominated Freons. Freons 11 and 12 are fully soluble in all mineral and some synthetic oils, other are partially soluble. Solubility may also be affected by the composition of some mineral oils; alkanic oils are less soluble - their mutual solubility increases with increasing cycloalkane concentration and, particularly, with aromatic content (52)(fis. 5.16). Freon 12 reduces oil viscosity at 38 "C, at a concentration as low as as 5% weight, by almost one half. The higher the oil viscosity, the more Freon it absorbs. For this reason, Freon compressors use oils of higher viscosity than other operating under the same conditions. Freons, particularly Freon 22, severely reduce the surface tension of oils. It has also been suggested that piston-seizure on start-up can be explained by reduced surface tension of the oil film (50).Some authors have stated that Freons act as anti-wear additives,increasing the lubricant film strength (49). The use of Freons has become a major cause for concern. Well-founded evidence exists that they destroy the protective ozone-layer in the stratosphere.Freons R12, R11 and R113 are especially harmful and their use prohibited in many temtories. Replacements for these materials as refrigerants are not readily available; Freon 124, C,H,F,, has been proposed, but its properties need further investigation and manufacturing methods also need more development work. In spite of the diminished use and importance of certain Freons, data and mechanisms of action of these compounds has been retained in this text; the authors believe that the technical problems and solutions which accompany the use of Freons will be faced by users of replacements, and hope that this account will facilitate the necessary development work.
504
Chiorinnred hydrocarbons (ethylene chloride, methyl chloride and methylene chloride) are currently used in household refrigerators. The presence of moisture can cause partial hydrolysis producing corrosive hydrogen chloride; these refrigerators are therefore usually fitted with dehydrators.Chlorinated hydrocarbons blend with mineral oils, but the resultant viscosity drop is acceptable: it is much less than in the case of Freons.
20
-30 -40
\
oi02o3oaom600 CONTENT OF OIL (%wt
I
Fig. 5.16. Solubility limits for refrigerant blends of R 22 and cycloalkanic mineral oil of varying aromatic content Refrigerants operate in a closed, high-temperature (compressor, oil separator), high-pressure (compressor to condenser) and low-pressureflow-temperature (evaporator/compressor intake) system connected by the expansion flap. Some lubricant is entrained by the refrigerant and some is separated in the separator; however, some also proceeds as far as the cold section of the circuit. Oil-refrigerant interaction therefore occurs in both hot and cold zones.
Oil and refrigerant are most exposed to stress in the thin film on the hot piston and cylinder walls, on the pressure valve and in the entire pressure branch up to the condenser, unless they are retained within this section by the separator. The oil temperature in heavy-duty compressors can reach 250 "C in this zone. The viscosity of the oil must therefore be high enough to give the oil the required lubricating power at these temperatures and in the oil-refrigerant mixture, provided the latter is soluble in oil. The oil must also have adequate thermal and thermooxidation stability, as it contacts air under reduced pressure and must not produce sludge or deposits, which might impede closure of the delivery valves, cause seizure of the piston rings, bring about refrigerant blow-through, impair heat transfer in the cooler and enhance copper corrosion, etc. The thermal stability of oils mixed with refrigerants (halogenated hydrocarbons or SO,) must be tested for the effects of both temperature and refrigerant (and/or its decomposition products). If both effects act together, they promote significant chemical changes in the oil. The stability of the refrigerant must also be tested. The thermal stability of the oil and Freon refrigerants and the resistance of the oil to these refrigerants is usually determined in the Philipp test, corresponding to DIN 51 593. The test device comprises a sealed U-tube vg. 5.174. One branch of the tube holds the test oil, which is maintained at a temperature of 250 "C, the other the refrigerant, which is maintained at 40 "C. After 96 hours, the chemical changes occurring in the oil are evaluated from the rate at which the oil darkens, or from the magnitude of visible reaction products. A simple device illustrated in fig. 5.17b is intended for testing
505
the catalytic effect of metals (iron, copper) and other materials (insulants, paints, etc.) on the oil and the refrigerant. The oil-refrigerant mixture is held at 175 "C in the presence of iron and copper strips, or of wire sealed into a glass tube. The chemical changes in the oil-refrigerant mixture are evaluated from the degree of corrosion of iron and copper and the amount of sludge produced.
a
b
Fig. 5.17. Devices for testing of thermal stability
1 - oil (1 ml), 2 - coolant (1 ml), 3 - oil (0.5 ml) + coolant (0.5 ml), 4 - steel strip - copper spiral
CSN 65 6246 specifies a similar test: a test-tube filled with oil and refrigerant is sealed and heated in a prescribed manner. Decomposition, if any, of the refrigerant is checked every 24 hours; refrigerant decomposition is manifested by silvery drops which settle on the tube walls or by etching of the glass in the case of fluoro-refrigerants, or yellow spots with sulphur dioxide. Such tests are of no value for oils used in compressors with ammonia or hydrocarbon-gas refrigerants. However, it is important to establish the thermbxidation stability of these oils, since this can help in the estimation of their functional life and the risks of faults arising in the system as a result of sludge, deposits and emulsions. Emulsions can arise from the surface-activity of amides, produced by the reaction of organic acids with ammonia. The thermiioxidation stability of these oils may be determined by the same method used for the determination of the thermbxidative stability of air-compressor oils, i.e., from the carbon residue increase, or by the methods used for the determination of the thermooxidative stability of other types of lubricating oils. Oils operating in the cold section of the circuit must have low enough pour-points and exhibit good fluidity at low temperatures. The pour-points should be at least 10 "C lower than the lowest oil temperature in the cold section of the circuit. Low temperature fluidity can be determined by DIN 51 568: the oil is placed in a U-tube and attemperated. The time taken for the oil meniscus to move between two gauge lines is measured; the temperature at which the oil flows at 10 mrn.s-l is determined.
Oil fluidity is essential for proper functioning of the evaporator. In a dry-evaporator system (formed by a pipe coil), the refrigerant fully evaporates and any oil dissolved in it is released. Oil which is fluid enough can return to the compressor even if the temperature is very low; thus, the oil does not fill the evaporator and impair its performance. In the wet (flooded) evaporators used for oil-immiscible refrigerants such as ammonia, the refrigerant evaporates and the vapour is evacuated from the flooded vessel, at the bottom of which is the oil and in the upper part the liquefied refrigerant. An oil of adequate fluidity can be drained - unless it is emulsified - from the lowest point in the evaporator. Oilhefrigerant (ammonia) emulsions impair the performanceof the evaporator.
506
The fluidity of oiVFreon mixtures at low temperatures improves with increasing concentration of Freon in the mixture. Notwithstanding this, the hydrocarbon composition of the oil is also relevant; higher aromatic hydrocarbon contents have a beneficial effect. Semi-synthetic oils, including mineral and alkylbenzene-based oils have high fluidities. Also, the oil is required not to precipitate paraffin waxes at temperatures higher than the temprature of the refrigerant in the evaporator, to avoid these paraffins flocculating, coating the walls of the evaporator and impairing the operation of the controls, particularly in the expansion system. Most troubles arise in systems in which the expansion section is formed by long, thin capillary tubes, which may block with precipitated paraffins. Paraffin crystals grow as a result of excessively slow temperatures decrease. These paraffin crystals may block capillaries. The risk of problems of this type, due to crystal separation and growth, is less for larger refrigerators fitted with expansion valves; the relatively large temperature drop occurs within a relatively short time. However, a significant quantity of fine paraffin crystals may form; these may not produce a direct risk, but they may settle on pipe walls and reduce the free diameter of the piping. Paraffins are precipitated at higher temperatures from mixtures of oils and halogenated hydrocarbons, which act as paraffin anti-solvents or precipitating agents, than from the oil alone. Paraffins are less soluble in the halohydrocarbon mixture than in the oil. By contrast, with oil mixtures with Freons, which act as cosolvents, paraffin precipitation occurs at temperatures substantially lower than the temperature of the cloud-point of the oil itself. Knowledge of the temperature at which precipitation of the oillR-coolant mixture is therefore essential. For refrigerators with capillaries in the expansion section, this temperature should be lower than the evaporation temperature of the refrigerant (e.g., -29 "C for Freon 12). Alkylbenzenes, polyalkenes and cyclanic mineral oils have a lower tendency to separate solids under low temperature conditions. In alkanic mineral oils, the paraffin precipitation point depends on the depth of de-waxing treatment. The temperature at which paraffin is precipitated from an oivrefrigerant mixture (the paraffin flocculation point) can be determined, for example, by DIN 51 351: the refrigerant plus 10% added oil is slowly cooled in a pressure tube until, at a certain temperature, the mixture clouds or flakes of wax start to separate. The amount of oil (i.e.. the precipitated paraffin wax) insoluble at low temperature in Freon 12 can be determined by DIN 51 590 or CSN 65 6243. Under British Standard BS 2626, no more than 0.3%of paraffin wax must separate from the oiVFreon 12 mixture at -40 "C.
The temperature at which solid substances separate from the oivrefrigerant mixtures also depends on the concentration of oil in the mixture. The higher the concentration, the higher the temperature limit. In addition to the properties detailed above, low foaming tendency is also desirable in refrigerant compressor oils. Foaming can lead to insufficient oil being supplied to the points of lubrication. Foaming tendency is primarily related to the concentration of refrigerant in the oil. During compressor start-up, leakage of oil 507
foam from the compressor may occur as a result of the pressure drop and, consequently, the refrigerant may separate from the oil. This phenomenon is highly undesirable. Oils for ammonia refrigerating compressors can be tested for foaming tendency by driving ammonia through the oil. Oils must be moisture-free, since any ice formed impairs the operation of expansion flaps and aggravates corrosion, emulsion, sludge and foam. Moisture in refrigerant compressor oil can be determined by CSN 65 6241 (also applicable to transformer and capacitor oils). The oil is heated at 60 "C, inert gas expels water from it and this water is quantitatively trapped by phosphorus pentoxide.
Mutual solubility of oil and refrigerant is of the utmost importance. Systems operating with refrigerants insoluble in oil are advantageous from the point of view of the lubricant function of the oil, since the oil viscosity is unchanged. Troublefree operation of the refrigerator can not really be assured in the absence of knowledge of changes occurring as a result of miscibility of oil and refrigerant. For example, in a dry evaporator, the fluidity of the oil-rich phase formed by the oil is much poorer than that of the original mixture from which this phase was formed. In a wet evaporator and its receiver, the light phase may be selectively washed off if the solubility limit is exceeded; this can lead to performance loss and equipment failure. If the mixture in the crankcase can separate into two phases, care must be taken to ensure that the oil-pump does not abstract the oil-lean phase, in order to avoid insufficient lubrication (TubZe 5.30). Table 5.30. Composition, Boiling-point and Miscibility of Major Refrigerant Types with Mineral Oils and Alkylaromatics-Mineral Oil Mixtures ~
Refrigerant type *
Chemical formula
Boiling-point Miscibility with O C cyclanic mineral mineral oil oil alkylbenzene (1 :1)
R11 24 CC13F Yes Yes -30 R12 CC12F2 Yes Yes no no R13 -8 1 CCIF, limited(< -35"C) limited (< -70 "C) R13B1 -58 CBrF3 no no -128 R14 CF4 limited(< -10 "C) limited (< -38 "C) CHClF2 -4 1 R22 no no R23 -82 cHF3 RJ 13 48 Yes Yes C2F3C13 4 R114 limited limited CClF$XlF2 -39 Rl15 no no C2F4C12 R2URl15t R502 no -46 limited (< - I9 "C) -89 R 13lR23t no no R503 -33 Ammonia no no NH2 * The symbols for the fluorinated refrigerants correspond to the American designations of Freons (the last number indicates the number of fluorine atoms, the previous number that of hydrogen plus one. the previous that of carbon plus one); R indicates refrigerant. t Azeotropic mixture. ~~
508
Reactivity of the mixture of oils and chlorinated hydrocarbons with the friction surfaces provides a lubricating film of higher load-carrying capacity, which clearly benefits lubrication. By contrast, fluorine-based refrigerants substantially reduce the surface tension of the oil and can decrease the lubricity of the oil to the point at which serious scuffing of the lubricated components occurs (e.g., the piston and cylinder of the refrigerating compressor during start-up). The extent by which Freons reduce the surface tension of the oil varies; for example, R22 causes a larger drop than R12.Surface tension is the most significant property of mixtures of compressor oils with Freons. Mineral oils of higher polarity and higher aromatics content and alkylbenzene synthetics have higher surface tensions. DIN 8978 specifies a test for the measurement of wear with refrigerating compressor oils.
For lubricating refrigerating compressors, mineral oils are mostly used, together some synthetics. The most suitable mineral oils are, typically, cycloalkanic types with C, above 38% and C, 7- 15%; Freon-type refrigerants readily dissolve in them and they have low pour- and paraffin wax flocculation points and good fluidity at low temperatures. The most important quality criterion is viscosity, which varies between 10 and 55 mm2.s-l at 50 "C, and pour-point, which varies between -30 and -40 "C. Suitable oils may also be derived by deep refining of alkanic oils; these oils have C, 33% maximum and C, 4 to 15%. The viscosity of the lubricandrefrigerant, as already noted, is very important in determining the lubricating ability of the oil. Oil-soluble refrigerants vary in their effects on oil viscosity reduction. This applies not only to hydrocarbon refrigerants but also Freon types; the type of Freon can be important - R22>R12>Rll.Viscosity decreases with increasing refrigerant concentration in the mixture; at the same refrigerant concentration, cycloalkanic oils exhibit a higher decrease than alkanic oils. Freon and ammonia refrigerating compressors mostly use cycloalkanic and alkanic lubricating oils. These oils are not suitable, however, for lubricating SO, compressors - among mineral oils, only the white oils are suitable. White oils are not suitable for lubricating Freon compressors because these oils do not have sufficient oxidation stability and do not mix with Freon-type refrigerants. Suitable additives for refrigerator lubricants include metal deactivators, oxidation inhibitors (when making a selection, detrimental effects on oil-refrigerant stability must be considered), corrosion inhibitors (for corrosive refrigerants such as moist SO,, CO, and chlorinated hydrocarbons), overbased additives (in cases where decomposition of the refrigerant may cause excessive oil acidity) and silicones (to reduce oil foaming in the crankcase) (54). Freon-type refrigerants have pronounced EP characteristics and improve the antiseizure properties of lubricants. This behaviour is attributed to chlorine produced on the rubbing surface form the reaction between the metal surface and chlorine in the refrigerant molecule. Fluorine, on the other hand, does not take part in the reaction. 509
Organic phosphonates improve lubricity in the refrigerant environment. At elevated temperatures, organic phosphates also show excellent effects. Phosphonates and phosphites improve thermal stability of the lubricant in the presence of refrigerants but phosphates lower it (268). Additives for refrigerating compressor oils have not found much favour in field service, however, because in spite of their beneficial effects, they can be troublesome in other parts of the refrigerator system, making choices very difficult. DIN 5 1 503 applies to oils for lubricating refrigerating compressors with refrigerant evaporation temperatures exceeding -30 "C. Oils are classified into Group A (oils for NH3 and CO, refrigerant compressors) and Group B (oils for compressors using fluorinated hydrocarbons, such as R11, R21, R22, R40 and R114 as refrigerants). Quality parameters for these oils are detailed in Table 5.31. Table 5.3 1. Quality Parameters for Refrigerating Compressor Oils to DIN 51503 Quality parameters
Oil group A
DIN test B
Appearance clear " 61 Flash-point ( "C) Neutralisation No. -0.08 (without mineral acids and bases)(mg K0H.g-I max.) Saponification No. (mg K0H.g-' max.) A . 2 . 0 1 Ash (% weight max.) A -25 Fluidity in U-tube at "C -30 and below (for refrigerating compressors with evaporation temperature -20 to -30 "C). -20 and below for refrigerating compressors with evaporation temperatures above -20 "C). No settled water to be present in 10 1 Water content of oil as supplied sample of oil as delivered in bulk. No settled water evident in oils delivered in drums. Not more than 30 mg water per 1 kg in oils delivered in small, waterproof packages. Resistance to Freon R12 (hours, minimum) 96 Freon R12 insolubles (% weight max.) 0.05 Kinematic viscosity (mm2.s-1, min.) 33/20 "C 76/20 "C 10/50 "C 17/50 "C for refrigerating compressor oils for evaporation temperatures -20 to -30 "C. and 53/20 "C 14/50 "C for refrigerating compressor oils for evaporation temperatures above -20 "C.
510
51584 5 1538 51559 51575 51568
51552 or 51777 B1.1
51593 51590 51561 51562
Semi-synthetic oils have become more popular and more widely used. These include mixtures of cycloalkanic oils containing 30 - 60% alkylbenzenes with viscosities 20-25 mm2.s-I at 50 "C. Tests on alkylaromatics alone revealed good dissolving power with Freons but unsatisfactory lubricant capability. The addition of anti-wear agents worsened their miscibility with refrigerants (55).
Alkylaromatics have fairly high viscosities and, consequently, semi-syntheticoils prepared from them cannot have the low viscosity desired. Efforts have been made to lower their viscosities by adding polyalkenes (56).Fully-synthetic poly-burenebased oils,with good thermal stability, high VI and good lubricating capacity have also been used. Polyalphaolefins are especially recommended for ammonia compressors at evaporation temperatures below -70 "C. These oils are not suitable for Freon compressors because of poor oiVFreon solubility. Neither polyethylene nor polypropylene-based oils have proved very successful. Synthetic, water-soluble, branched polypropylene glycols give good results because of their lubricity, low pour-point, low-solubility in compressors which use hydrocarbons such as propane and propylene as refrigerants. Polypropylene glycols may be improved with an oxidation inhibitor, such as phenyl- 1 -naphthylamine (48). Polyphenylether oils (e.g.. Polyran M 15 refrigerant made by Bayer) perform well, e.g., in open systems using R22 as refrigerant. Silicic acid polybutyl ester (Fluisil S55K made by Bayer) is well-suited to Freonrefrigerant compressors. This oil exhibits good fluidity at low temperatures (pour-point -85 "C)and is miscible in higher concentrationswith R13. Its deficiency is that it is rather sensitive to water; hence it is only applicable for the lubrication of hermetically-sealed refrigerating compressors. A long-standing problem with refrigerating machines is the precipitation of copper from oil on the hot, rotary components of the compressor. This separation is aggravated by a high concentration of resins resulting from ageing of the oil, as well as by instability of the refrigerant, moisture in the system, contaminants and acidic substances in the oil (e.g.. HCI) and by high temperatures. The copper precipitated may cause machine failure.
5.3.3 Lubricants for Vacuum Pumps (High-vacuum Lubricants) The quality requirements for compressor oils also apply to oils for piston and oilseal rotary vacuum pumps. I S 0 6521 classifies vacuum pump oils into six groups: Oils for reciprocating piston vacuum pumps, rotary boost-lubricated and rotary oil-flooded (vane and screw) vacuum pumps: ISO-L-D VA - for low-vacuum (10 to 1 mbar) and non-aggressive gases, ISO-L-D VB - for low-vacuum and acidic gases, Oils for rotary, rotary-wing and rotary piston vacuum pumps: mbar) and non-aggressive ISO-L-D VC - for medium vacuum (1 to gases, ISO-L-D VD - for medium and acidic gases, 51 1
to mbar) and non-aggressive gases, ISO-L-D VE - for high vacuum ( ISO-L-D VF - for high vacuum and acidic gases. Diffusion pump oils must contain as little light fractions as possible. The degree of vacuum attained depends, inter alia, on the amount of these fractions evaporated from the oil into the vacuum space. The vacuum must thus relate to the vapour pressure of the saturated vacuum oil vapour; it is usually in the region of 1.3.10-1 to 1.3.lo4 Pa ( to Torr) at 25 "C. Special, molecularly-distilled oils with viscosity 35-70 mm2.s-l at 50 OC, viscosity index about 90, flash-point over 210 "C and pour-point below -10 "C are usually used. One of the well-known commercial products is Apiezon. Oils generally suitable for operation at high vacuum (i.e., 1.3.10-4to 1.3.10-5Pa Torr) and at temperatures up to 100-120 "C can be prepared by to molecular-distillation of wax-free or de-waxed raffinates. These oils should contain anti-wear additives able to react with the surface, since no surface-coating of metal oxides which could moderate boundary or mixed friction can be generated in the vacuum. It is advantageous if the surfaces contain oxide or sulphide occlusions. Suitable surfaces, comprising a hard carrier on a soft substrate, include Cu-Pb, NiAg, Ni-Sn and Ni-Si (220).For pressures of l .3.10-* Pa Tom) and lower, only self-lubricating materials, solid lubricants and soft metal surfaces (see Chapter 3.4) can be used. Some graphite-lead blends in oleophilic form have good lubricating properties in both open applications and in vacuum (227).
5.3.4 Air-Tool and Rock-Drill Oils Compressed air is used for powering pneumatic machines used in mines, construction, quarrying, etc.(pneumatic hammers, picks, drills, rammers, etc.). The tool is driven either by a rotary motor or by a piston oscillating in a cylinder into which compressed air is fed via valve or slide gear alternately to each side of the piston. Air fed to the machine expands adiabatically when it leaves and cools. This increases its relative humidity; when the dew-point is reached, water condenses and, if the temperature of the air stream is low enough, immediately freezes. Isothermal expansion, which occurs during expansion in the working cylinder, causes, by contrast, a decrease in relative humidity; air dries and prevents the formation of ice, which otherwise would impair the operation of the piston. This pattern of behaviour dictates some of the properties which lubricating oils should possess. The viscosity required of the oil must correspond to the load, temperature and speed of the components to be lubricated; it should, moreover, facilitate the formation of a fine mist of oil to be sprayed through a nozzle into an air-jet. Lower viscosities facilitate atomisation. Low viscosity is particularly required for pipe-mounted lubricators, in which the only means of oilsupply is in the form of a fine mist. On the other hand, if the lubricator is mounted on the machine itself, oils of higher viscosity are more advantageous, because larger oil drops settle more readily on to the friction surfaces; drops which are too fine can easily be entrained into the environment and cannot properly fulfil their lubrication function.
512
The temperature of the air in the pipework running from the air reservoir to the machine determines the maximum temperature which may be reached in the pneumatic machine. This temperature usually varies between 20 and 65 "C and depends on the length of piping. The temperature in the machine itself is no higher, despite the effect of frictional work, because the air-flush is very vigorous and the expansion of the air reduces the temperature of the machine. From these considerations, oils in use have viscosities between 3 and 25 mm2,s'' at 50 "C; these viscosities enable the oil to adapt to the conditions prevailing. The minimum temperatures in the working cylinder may be very troublesome, particularly at certain points in the air intake and outlet. The temperature here may be -20 "C or even lower. The pour-points of the oils used therefore vary between -20 and -40 "C. High oxidation stability is essential because of the intimate contact between the finely atomised oil and air. Oils are doped with additives which increase adhesion to metal surfaces and the strength of the lubricant film, since it is exposed to high temperatures and impact loads. They may also contain polymeric additives (e.g., polymethacrylates, polybutenes, etc.) which can reduce the escape of oil mist into the environment. They usually contain a rust-inhibitor to protect the metal surfaces from attack by water; the rust inhibitor produces an emulsion with water which covers the lubricated surface. An agent which forms a water-repellent coating on the surface may also be used.
5.4 STEAM ENGINE OILS Unlike internal combustion engine and compressor cylinders, steam engine cylinders are insulated to prevent heat loss and may be provided with a steam jacket, intended to maintain a high working-temperature on the inside surfaces. The working medium is wet, saturated or superheated steam at elevated pressure. Wet steam contains over 3% water. Saturated steam is converted into wet steam when it expands and superheated steam can turn into wet steam during low- and oscillatory-loading of the engine. High working temperatures require oils with very high viscosities. Adequate viscosity is also needed to assist sealing of the piston and to transfer pressure between the piston-rings and the cylinder wall. Higher the steam temperature requires higher viscosities. Viscosity is normally specified at 100 OC,but viscosity at working temperature is more important and must be higher at higher VI. High viscosity oil is not suitable for high-speed engines, particularly if the temperature of the wet steam is low; the oil cannot properly wet the surfaces, being washed off and collecting in hotter sites where, under continuous action of their high temperatures, it forms carbonaceous residues, which can clog and immobilise pistonrings and form deposits on valves. Stuck piston-rings and passing valves give rise to blow-by, loss of capacity and bore wear. The oil must therefore have good thermal stability, which can be indicated by a low carbonisation number. The oil must also have low volatility and high flash-point. Oil evaporation is, 513
on the one hand, diminished by the overall high pressure in the cylinder, but it is enhanced, on the other, by the low partial pressure of the thin layer of oil on the cylinder surface in the presence of excess water from the steam. Lubricity of the oil and/or its adhesivity to the metal surfaces is essential. The inner components of the engine (piston, piston-rod, slide-valve) undergo rectilinear and reciprocating motion; movement stops at dead centre, where no lubricant wedge can be formed to provide liquid friction, and, as a result, mixed friction occurs at these points. In addition, water from the steam tends to wash off the oil film and corrode the bare metal surfaces. Oil adhesivity can be enhanced by the addition of suitable fatty oil, like hoof, tallow or lard oils. More fatty oil is needed at lower temperature and pressure of the intake steam and the more the operation of the machine fluctuates. Oil distillates or vacuum distillation residues with low bitumen content are also suitable for this purpose. Oils for superheated steam are usually raffinate types and fatty oils must not be added. 10-12% of fatty oils are suitable for very wet steam, 5-10% for saturated steam and 3% for moderately superheated steam. The addition of fatty oils is avoided for those engines in which the condensate returns to the boiler and must be oil-free. Solid lubricants (e.g., 0.2%of colloidal graphite or MoS,) may be added instead of fatty oils. Some examples of cylinder oils are described in Table 5.32. Table 5.32. Qpes and Basic Properties of Oils used for Lubricating Steam Engine Internals Raftinate Viscosity (mm2.s-1) at 100 "C (approx.) Flash-point ( "C) min. Pour-point ( "C) max. Viscosity index, min. Carbonisation residue (weight %) max. Applicable to use with steam up to ( "C)
30 280
Distillate
30 240 10
-
40 285 10 -
50 305 10
1.5
3.0
3.0
3.0
2330
250
300
>300
60 330
5 95
40 295 5 95
50 310 5 95
5 95
1.0
1.0
1.5
260
300
330
-
Bearing oils with viscosities corresponding to the working temperature of the component lubricated can be used for lubricating those external engine components (main and big-end bearings, cross-head guideways and pins, connecting-rod guideways, valve-gears and governor drive-pins) which are exposed to the surrounding temperature plus radiant heat and heat conducted by the engine components from the steam cylinder.
5 14
5.5 TURBINE OILS Oils used for lubricating the bearings and gears of steam turbines and for abstracting heat from the steam turbine gear-bearings, water-turbine supporting, guide and suspension bearings and the moving components of gas turbines (except aircraft turbines) are referred to as turbine oils.
5.5.1 Steam 'hrbine Oils In a steam turbine, the oil must lubricate the bearings, clutches, gears and governing system, abstract heat conveyed by the shaft from tne turbine interior, seal gas-cooled turbo-generators, protect the surfaces from rust, corrosion and deposits and transmit pilot pulses. The oil must be fully reliable in all these functions. The lubrication system is of the closed or continuous type. The functional life of the oil must be as long as 20,000 operational hours; oil change intervals of 60,000 hours are not exceptional and a requirement is made that the oil has to have as long an operational life as that of the turbine. Steam enters the turbine at up to 600 "C or more. Bearing temperatures may be as high as 75 OC;this temperature is precisely monitored by built-in thermocouples and a temperature rise of 10 "C activates a danger signal. The cooler keeps the bearing temperature below 70 "C (which must not be exceeded). Because of the high rotational speeds (small turbines run at up to 30,000 r.p.m.), lubrication is hydrodynamic except at shut-down and start-up. The shaft is lifted hydrostatically for start-up and during running down to avoid mixed and boundary friction. The high temperatures create a fire risk. Relatively small oil charges and a high circulation factor (about 8) are therefore used. Oil distribution systems are laid out so as to avoid high temperature zones. Hydrogen-cooled sets have gas-vents in the main oil tank. Under normal operating conditions, the oil dissolves 6.5% by volume of hydrogen and 10% of air. This gas is explosive and must be evacuated from the system. Gears reduce the high speeds at which turbines must operate for best efficiency down to lower speeds for coupled machinery. These gears are fitted with either separate or integrated lubrication systems. Oils for integrated lubrication systems must have a somewhat lower viscosity. To compromise between the requirement of lower viscosity for the bearings and higher viscosity for the gears, low viscosity is compensated by additives. The governor system is lubricated by the main oil stream. This system is very sensitive, and must not fail. These comments imply that there are exceptionally demanding oil viscosity requirements. Oil of viscosity 25-55 mm2.s-' at 50 "C and VI over 90 meets all requirements. Lower viscosity facilitates heat removal, whilst high viscosity strengthens the lubricant film. These oils must have very good oxidation stability. They are made from a wellrefined base-stock and protected by a low-temperature antioxidant, which should 515
not darken or form deposits. 2,6-ditert-butyl-4-methylphenolis suitable for this purpose. The oxidation stability of the oil can also be improved by supressing temperatures in the oil circuit, limiting contact with air and by systematic removal of solid contaminants (which include metallic contaminants arising from galling) from the oil-tank bottom by means of an in-line centrifuge, filter or magnetic separator. Copper, brass and bronze components are tin-plated. No galvanised pipes are used, since both copper and zinc aggravate oxidation. The oil usually contains an anti-rust agent. Rust-protection is particularly important for stand-by machines. However, the anti-rust agent must not promote corrosion of non-ferrous metals. Examples include high molecular weight alkenyl succinic acids, their esters and anhydrides. CSN 65 6249, ASTM D-665, IP 135 and DIN 51 587 specify rusting tests for turbine oils: a steel bar whose surface has been prepared in a standardised manner is exposed to the action of 300 c d of oil and 30 cm3 of distilled water with vigorous stirring at 60 "C. Visual examination is carried out after 24 hours. Simulated sea-water may be used instead of distilled water in this test.
Besides rusting, other types of corrosion may occur. These can be caused by eddy currents in the shaft and frame, as well as by defective insulation, electrostatic charges, electrolytic cells formed between dissimilar metals in the oil circuit, etc. The nature of the oil has no direct influence on these types of corrosion. The oil must not form emulsions with water. Emulsions increase the bulk and viscosity of the oil, promote deposits and may cause the failure of the whole lubrication system. Emulsion formation is promoted by polar substances, inherent and introduced contaminants in the oil and micro-organisms originating in air and water. The oil must therefore be well-refined and have a high oxidation stability, and all contaminants must be systematically removed. The centrifuged oil must contain less than 0.2% of water. Foaming is also undesirable. Foam impairs heat transfer in the oil cooler, promotes oxidation as a result of increased surface area between oil and air and interferes with the functioning of the governor system. The oil therefore contains anti-foam agents such as silicones. However, over-dosing can impede the release of gas bubbles and actually increase foaming. A de-foaming tank may be incorporated into the oil circuit which seals hydrogen-cooled generators. Foaming may be suppressed by other design arrangements. The quality parameters for turbine oils in Europe and the USA are detailed in Table 5.33. Oils for lubricating both bearings and gears should contain EP additives. The selection of such additives is very difficult; they are normally surface-active substances, but they must not promote emulsification, foaming and corrosion of metals, or impair the oxidation stability of the oil. Also, they must be highly stable. Some esters of acids containing phosphorus and sulphur and some of their zinc salts meet these criteria. 5 16
Table 5.33. European and US Industrial Turbine Oil Specifications Performance test
British Standard BS 489
Non-EP oils DIN 51515
80 Viscosity index min. Flash-point "C min. 168 -6 Pour-point "C max. Neutralisation no. mg K0H.g' max 0.2 Air releaselDIN 5 1381, ASTM D-3427 5- 10' Foam tendency/stability, mi 450/0 at 24 "C at 93 "C 50/02 24 "C after 93 "C 45010 Demulsibilitylsteam, max. 300 /ASTM D-1401 max. Oxidation stability/ ASTM D-943, TAN to /IP 280 TOP max. I .o /Sludge max. 0.4 RBOT 1P 2272. min. Copper corrosion/ ASTM D- 130/100 "C13 h, max. 2 Rust/ASTM D-665A B Pass Load-carrying/FZG A 20/8.3/90 Fail -
EP oils General Brown Boveri Electric HTGD GEK 32568A 901 I7 90 160-215 I 215 185 -6 -12 -6 Report Report 5-6l 5 300 -
2 at 2000 -
1o/o
20/0 1o/o -
2 at 20003
450110 50/10
450/10 300 1800 2 at 2000
-
1.8
450
0.4 -
1
2 Pass -
Pass -
2 Pass -
-
-
6-7l
( 1 ) Depending on viscosity grade. (2) For IS0 VG 68 and 100. requirements are tendency/stability at 24 "C 450/40, at 93 "C 100/10, at 24 "C after 93 "C 450/40. (3) Additionally for oxidation lest FTMS 5308.6, viscosity change -5 to +20%,TAN increase 3.0 max.
British Admiralty specifications OEP-69 and OEP-90 and American MIL-L-17331B stipulate the requirements for these oils.
In steam turbines in which the oil is heavily contaminated with water, Pseudomonas microorganisms, particularly I! aeruginosa, may develop increased biological activity. This may have detrimental effects on steam turbine lubricating systems resulting in the formation of deposits in the oil and fouling the tanks with slime, which firmly adheres to the tank walls. In these cases, a biocide should be added to the oil (236-238). Air pollution with sulphur dioxide and nitrogen oxides, even in trace amounts, impairs the colour of turbine oil, reduces the effective concentration of additives and shortens the functional life of the oil. The additive concentration can decrease in about 2,000 hours operation to 60-80% of its original value. Steel corrosion is evident after about 6,000 hours. On the other hand, there is no evidence of similar effects with hydrogen sulphide and hydrocarbons. If the drain intervals of oils which may come into contact with these gases is to be extended, air filtration must 517
be fitted or the oil system rearranged to avoid the oil contacting the contaminated environment (258). The oil in the main tank must be topped up whenever necessary. No more than 10% of the original oil charge should be added at any one time. Fresh oil tends to foam and may also cause precipitation of the products of ageing from the original oil. Since some additives, such as antioxidants and anti-rusts, may be exhausted during operation of the turbine, make-up oil may contain increased dosages, which must, however, be checked by a specialist. The oil must be drained if its acid number reaches 1.5 mg K0H.g-' (0.5 has also been quoted), or the saponification value reaches 4 mg K0H.g-'. The viscosity should not increase by more than 25%, the flash-point should not fall below 180 "C and the interfacial tension between oil and water should not fall below 14 - 17 mN.m-'. Some turbines operate at steam temperatures as high as 650 "C and their governor systems must work at high temperatures with absolute reliability - this also applies to lubrication. Petroleum-based oils are unable to meet these needs and, therefore, synthetic oils with high self-ignition temperature (to prevent fire) and good thermoooxidation stability and lubricity must be employed instead. Suitable oils which meet these requirements include triaryl phosphates, such as trixylenyl phosphate. A product of low toxicity and low foaming tendency can be prepared by reacting POCI, with a narrow xylene cut boiling between 212 and 222 "C (234).Its self-ignition temperature is about 739 O C and its toxicity (LD,, = 24 g.kg-') is about ten times lower than that of tricresyl phosphate (2.6 g.kg-') and twice as low as the trixylenyl phosphate prepared from a wide xylene cut boiling between 205 and 235 "C (12 g.kg-').
5.5.2 Water Turbine Oils Water turbines use the same oils for their circulating lubricant system and for the lubrication of the supporting and guide bearings, so the oils must meet the same requirements. These oils must have very low pour-points. Higher viscosity oils are used if lubrication of the thrust bearings and gears is integral and if the operating conditions demand it. Suspension bearings, which must absorb a high axial thrust, also need higher viscosities, as do small turbines fitted with suspension bearings, mostly ring-type, and oil-bath lubrication. These also need additives to increase oil adhesion (fatty oils are commonly used) to ensure liquid friction in otherwise unfavourable conditions.
5.5.3 Oils for Stationary Gas "urbines Stationary industrial gas turbines use similar oils to steam turbines, working conditions being similar. The oil does not come into contact with condensed water but the outlet temperatures are higher (600-700 "C) than the temperatures of the inlet steam, and the rotational speed of gas turbines is higher. Oils containing EP 518
additives are also suitable for lubricating the gears. However, if the gear-box is highly stressed, a separate lubrication system with oil at higher viscosity is preferable. Gas turbines for railroad locomotives and ships require oils with higher oxidation and thermal stabilities, since small charges of high circulation-factor oils are used in order to reduce the weight of the assemblage. These oils must therefore contain more oxidation inhibitor or be drained more frequently. Aircraft-type stationary turbines are characterised by a high power: weight ratio, compactness and relatively low purchase cost, They are, however, less efficient and less economical to run, so that they are not suitable for sustained operation. They are suitable for auxiliary or stand-by tasks, such as in power plants to assist when consumption peaks. Unlike the earlier types of turbines, they are fitted with antifriction bearings and can therefore run at full power within seconds of start-up. Oil circulation is high and the oil mixes thoroughly with air in the bearings, which increases its rate of evaporation and ageing. Because of these factors, the same synthetic oils which are used for airborne turbines are recommended. Oils for the lubrication of all types of turbine must be of the highest quality. They are usually available at a range of viscosities enabling the optimum viscosity to be selected for the relevant operating conditions. The methodology of oil cleaning, drain and quality-monitoring tend to be the same for all types of turbine; they must be carefully chosen and well looked after.
5.6 BEARING OR MACHINE OILS The terms “bearing” or “machine” oils reflects the original task of these types of oil, when short-term lubrication was prevalent and only unsophisticated long-term lubrication systems existed (such as ring and chain lubrication). With the development of more sophisticated long-term lubrication systems, bearing oils have taken over the task of lubricating other machine elements, such as machine tool gears and slideways, and have even been used as pressure-transfer fluids in moderatelydemanding hydraulic systems. The properties required of these oils are in many cases influenced by the requirements of these other elements, so the term “bearing oils” scarcely reflects their present function; there is no single, explicit definition which reflects the exact area of application of these oils. All these developments have created a need to produce and employ a wide variety of bearing oils which differ in their composition, properties and application. Bearing oils can be classified into those intended for once-through, short-term lubrication systems (general purpose oils), for example for drop-feed lubricants and wick-oilers, and those for long-term, circulatory lubrication systems (high-quality or mark oils). They differ mainly in oxidation stability, achieved by deeper refining of the distillates, and by the use of low-temperature antioxidants. High-quality oils have high viscosity indexes, brighter colour, lower resin content, higher anilinepoints, etc. 519
The fundamental property of bearing (machine) oils is their viscosity. They are available in a wide assortment of types of differing viscosities, (between 2 and 60 mm2.s-l at 50 "C) since the oil must be selected for a particular size, peripheral velocity, load etc. of the bearing. Plain bearings (radial and axial bearings with hydrodynamic and hydrostatic lubrication systems), self-lubricated bearings of porous materials and plain bearings with wick-feed lubrication systems (bearings with boundary lubrication) are lubricated exclusively with oil (as opposed to grease). Oil is also advantageous for anti-friction bearings with a high sliding speed (providing low friction loss) or for working at high temperatures and when other machine elements are simultaneously lubricated with oil. Plain bearing with hydrodynamic lubrication systems (fig. 5.18) can be subdivided into statically-loaded bearings (with constant load in respect of rate, direction and sense) and dynamically-loaded bearings, with variable, fluctuating or impact loading (e.g., bearings in piston engines).
OIL INLET
Fig. 5.18. Important magnitudes and distribution of oil loading in the plain bearing with hydrodynamic lubrication system FN - total loading, R - radius of bearing, r - radius of journal. e - excentricity of journal, h, - width of the lubricating layer A knowledge of effective oil viscosity is essential to be able to calculate the effective boundaryloading (FmaX) or boundary speed (nmax)of the bearing and the minimum thickness of the lubricating oil film (ho). i.e., the boundary values of these variables required for the bearing to be able to work in the hydrodynamic lubrication mode at the effectiveconditions of temperature, pressure and shear stress, as defined in the following equations (57):
(N)
(5.1)
where F is the total loading in N, C , is a coefficient characterising the geometric shape, the surface roughness of the sliding surfaces and the accuracy of manufacture and assembly (values between 1 and 2). q is the dynamic viscosity of the oil in Pas, D is the diameter of the bearing in m, 1 is the width of the bearing in m, n is the rotational speed of the bearing in r.p.m.,
520
($.
l ) is the effective working area of the bearing in m2.
The hydrodynamic theory of lubrication provides a basis for understanding of the effects of these variables in the above equations on the functional properties of the plain bearing, in various mathematical solutions. These use dimensionless expressions to advantage, such as the dimensionless criterion of viscosity: P (5.3) 170
in which P is the mean specific pressure of the bearing in Pa, o is the angular velocity of the journal = ?-!L in s-', or the Summerfeld number (So) or its 30 inverse (Si')and the friction number p/v, where p is the coefficient of sliding friction and
so = P 9 lqw
(5.4)
R-r
where w is the relative free play in the bearing play = V where R is the ideal radius of the bearing in m, r is the ideal radius of the journal in m, v is the peripheral speed in m.s-l. The minimum height h, of the oil level necessary to ensure the hydrodynamic regime is given by the relation
where k' is a coefficient dependent on the realtive length Nd. Bearings are similar geometrically and hydrodynamically if their dimensionless variables are iden tical. Rules for computation of plain bearings and bushes are specified in various national standards, e.g., CSN 02 3090, and many bearing design charts are based on these correlations. Table 5.34 illustrates the correlation between oil viscosity and bearing loading (96).
Table 5.34. Dependence of Viscosity on Load and Peripheral Velocity Specific pressure in bearing low to0.5 to 1
Peripheral velocity (nu-') medium high to2 to3 to4 to5
very high to 10 to15
Viscosity (mm2.s-I) at 50 O C 0.5 I .o 1.5 2.0 2.5 3.0 4.0 5.0 6.0 8.0 10.0 12.0 15.0 20.0 25.0
30 65 85 -
15 30 45 60 75
-
-
85 -
-
-
-
-
-
-
8
15 20 30
35 45 60 75 85 -
-
6 10 15 20 25 30 40 50 60
75
loo 135 -
-
5
8 12 15 17.5 25 30 35
4 5 7.5 8 10.5
12.5 15
20
2.8 4
5 7.5 8 8 12 14
45
25
17
60 75 85
35 40 50 60 80
25 30
115 150 -
110
35 45
55 75
2.8 2.8 4
6 7.5 7.5 8 10.5 -
52 1
Should the peripheral (shear) velocity of the bearing be too low and the load on the bearing too high, conditions do not exist for the generation of a sufficiently thick hydrodynamic layer of lubricant with sufficient load-bearing capacity. Such a layer must then be generated by increasing the pressure of the oil supply to the bearing. Bearings with hydrostatic lubrication are based on this principle (Fg. 5.19).
IFNri&-
UNDER PRESSURE
7
Fig. 5.19. Scheme of bearing with hydrostatic lubrication system
For lubricating medium or large anti-friction bearings, the viscosity of the oil should match the working temperature of the bearing; the higher the temperature, the higher should be the oil viscosity. As a rough guide, 12 m m 2 s 1 is about right for average working temperatures in a well-designed bearing. More exact values are shown in Table 5.35. Table 5.35. Kinematic Viscosity at 50 "C of Bearing Oils for Lubricating Anti-friction Bearings at Various Operating Temperatures Operating temperature ( "C) 30 40 50
60 70
80 90 100
Kinematic viscosity of oil at 50 O C (mm2.s-')
7.0 8.5 12.0 17.0 25.0
35.0 50.0 70.0
Spherical-roller thrust bearings operating at low r.p.m. are lubricated with oil of viscosity around 400 mm2.s-*at 50 "C. For operation at high r.p.m., the correlations given above relating working temperature to oil viscosity apply. If these thrust bearings work under normal operating conditions, the viscosity of the oil can be 522
deduced from the velocity factor D, (i.e., the bearing bore diameter mm (D) multiplied by the r.p.m (n)), as shown in Table 5.36. Higher oil viscosity should be proportionally related to the bearing loading. Table 5.36. Relationship between Oil Viscosity in Anti-friction Bearings and Bearing Velocity Factor Dn 1,OOo 10,OOo
I00,000 200,000
Kinematic viscosity at 50 "C (mm2.s-') 150-300 60-150 30-60
20-50
Pour-points are related to the climatic conditions under which the oils work and to whether the machines work indoors or outside. Oils made from non-paraffinic or de-waxed paraffinic distillates and/or containing pour-point depressants may form a separate series of low pour-point oils. Opinion varies on whether machine-tool gears require bearing oils of higher lubricity. Some oil manufacturers produce oils containing lubricity additives (e.g., tallow and lard oils). This practice varies however, since machine-tool gears are readily designed so as to deal the temperature and pressure between the teeth with oils of natural lubricity, lubricating film capacity and corresponding viscosity. ISO-3498- 1979 (E) classifies machine-tool lubricants into six groups according to their lubricated elements and the working conditions to which the oil viscosity, lubricating grease penetration and composition must correspond (see Table 5.37).
Bearing oils available can be classified into three main groups: - general purpose oils of moderate viscosity index (about 60% of alkanic oils and 40% cycloalkanic oils) with average oxidation stability. These are suitable for single-pass lubrication systems, oil-baths exposed to moderate thermal stress and circulation systems with a short functional life. Wax-free mineral oils have relatively low pour-points, low viscosity oils from -40 "C and maximum viscosity oils up to -15 "C. In application, the oils are classified by their viscosities, which lie between 2 to 25 mrn2.s-l at 50 "C; - additive-free oils - predominantly de-waxed selective raffinates or hydroraffinates from alkanic distillates with good oxidation stability and high VZ (above 80-90). They are intended for long-life lubrication systems with low circulation factors and working temperatures up to 60 "C. They are suitable for light-duty gears, particularly those in machine tools and for filling those hydrostatic systems which are not very demanding on the oxidation stability of the oil. Because of their versatility in use, their viscosities range from 1.5 to 60 mm2.s-*at 50 "C. Pour-points are usually between 0 "C and -10 "C; - oils containing antioxidant additives (often labelled as machine oils; the label should indicate that the oils are suitable for long-duration lubrication of bearings
523
VI h)
Table 5.37. Classification of Metal-workingLubricants I S 0 3498-1979(E)
P
Group Site
L-A
complete systems
L-C
closed transmissions
L-F
Special properties of the lubricant
ISO-L* & viscosity class
Mode of application or other use
refined mineral oils
L-AN 6g2
general lubrication of lightly-loaded parts
mildly loaded
refined mineral oils of good oxidation stability (including with antioxidants)
L-CB 32 L-CB 68 LCB 150
pressure, dip and oil-mist lubrication of closed gears and headstock, feed-box and slide (etc.) bearings under moderate load; CB 32 and 68 are also suitable for flood lubrication of mechanically-controlled clutches
heavily loaded
the same, with load resistance
L-CC 150 L C C 320
pressure and dip lubrication of all types of heavily loaded gears (except hypoid) and bearings up to 70 "C; suitable also for manual or central lubrication of feed- and slide-guideways
spindles and bearings
refined mineral oils of very good oxidation stability, anti-corrosion and anti-wear properties
L-FD 2 L-FD 5 L-FD 10 L-FD 22
pressure, dip and oil-mist lubrication of plain and antifriction bearings; suitable where low viscosity is required, e.g., fine hydraulic mechanisms, electro-mechanical clutches, air ducts and hydrostatic bearings
spindles, bearings
refined mineral oils of very good oxidation stability and anticorrosion properties
L-FC 2 L-FC 10 L-FC 22
pressure, dip and oil-mist lubrication of plain and antifriction bearings; suitable for lubricating systems (including couplings) requiring the absence of anti-wear agents
Application
spindles,bearings and attached couplings
Detail
L-G
slideways
L-H
hydraulic systems
refined mineral oils of good lubricity and adhesivity, preventing “stick-slip’’
L-G 32 L-G 68 L-G 150 L-G 220
lubrication of plain bearings, particularly suitable for low shear-rates and minimising or preventing stick-slip motion and for use in moderately loaded, continuously operating worm gears
refined mineral oils of good oxidation stability and anti-corrosion properties
L-HL 15 L-HL 32 L-HL 46 L-HL 68
hydraulic system filling, lubrication of plain and antifriction bearings and gears, except hypoidal types
refined mineral oils of good oxidation stability anticorrosion and antiwear properties
L-HM 15 L-HM 32 L-HM 46 L-HM 68
filling hydraulic systems which control heavily loaded components; HM 32, HL 32, HM 68 and HL 68 may replace CB 32 and CB 68
L-HG 32 L-HG 68
suitable for lubricating separate slideways if oil of this viscosity is required
L-XM l 3 L-XM 2 L-XM 3
grease XM 1 is suitable for central lubrication systems; XM 2 and XM 3 are better for manual lubrication; equipment manufacturer should specify lubricant for first fill so that compatible lubricants can be later used
~~~
hydraulic and slideway refined mineral oils of systems HM type, able to tlrevent stick-slip L-X
wheregrease is required
multi-purpose grease
greases with good anti-oxidant and anticorrosion properties
~~
( I ) “L”is the IS0 identifier for “lubricants, industrial oils and similar products”. (2) The numbers which follow describe the kinematic viscosity of the lubricant in m2.s-’at 40 “C (IS0 3448). (3) The NLGI penetration values corresponding to the IS0 grease classifications are: IS0 XM 1 XM 2 XM 3
Penetration after working (m-’) 3 10 to 340 265 to 295 220 to 250
Table 5.37.1. Cincinnati Milacron Specification for Special Slideway Oils Specification
P-47 (medium-heavy way oil)
Description:
(ASTM method) bperties: API gravity at 60 OF (D-257) Visc. SUS at 100 OF (D-445) Colour (D-1500) Flash-point (o.c.) (D-92) Fire-point (o.c.) (D-92) Neut.No.* (mg KOWg oil) (D664)
Results after heat test: Evaporative loss (%) Precipitate or sludge Condition of steel rod Condition of copper rod
P-50
Compounded medium-heavy oil for machine-tool slideways. A non-corrosive additive to be used to provide anti-stick-slip characteristics. A tackiness additive is used to provide required adhesive properties.
P-53
(heavy way oil)
(combination way and hydraulic oil)
Compounded heavy oil for machine-tool slideways. A non-corrosive additive to be used to provide anti-stick-slip characteristics. A tackiness additive is used to provide required adhesive properties.
Good quality liquid oil for use in a central system for lubricating hydraulic pump units and slideways. This lubricant should contain a noncorrosive additive to provide anti-stick-slip characteristics.
18 to 27 284 to 346 8 max.
900 to 1100
8 max.
8 max.
330 O F min. 360 O F min.
350 OF min. 410 OF min.
315 OF min. 355 OF min.
1.7 max.
1.7 max.
0.6 max.
1.0 max.
none satisfactory light brown stain
18 to 27
1.0 max. none satisfactory tight brown stain
20 to 30 135 to 165
1.0 max. none satisfactory slight discolouration
* This value can be subject to negotiation with individual suppliers. Srick-dip test - all oils must pass the MMC Co. stick-slip test, i.e.. the ratio between the coefficientsof static and dynamic friction must not exceed 0.85. Hen?test procedure - place polished pieces of copper and steel rods (0.25 in. dia, >3.0in. long) in a 100 cc. Griffin beaker containing a weighed amount of oil sample. about 35 to 40 g. Put beaker and metal samples in an electric drying oven for 24 hours, maintaining a temperature of 210 to 220 "F. This test is made to determine evaporative loss, precipitate or sludge formationand effects of the oil on steel and copper. Field tesfs - the lubricant must have all the general qualities required to ensure satisfactory performance as a machine-tool slideway lubricant when it is applied at quarterly intervals under conditions consistent with good machine-tool practice. ASTM tests - the latest ASTM Standards will be followed for all tests except for stick-slip and heat tests.
and other elements in the machine set). They should be resistant to the formation of sludge, varnish and other oxidation products. They coincide with the second group in terms of viscosity classification.
5.6.1 Spindle Oils The lowest viscosity bearing oils are referred to as spindle oils. These oils are intended for the lubrication of machine tool spindles. These spindles are assembled very precisely to ensure a perfect circular motion without any vibration which would leave marks on the surface of the work-piece. The plain bearings used have extremely small clearances. Multiple-wedge bearings are used when extreme accuracy of the assembly is required. The tight bearing clearances allow very fine oil films to be produced, often at a very high shear-rate. Thus, the oils used must have low viscosities, namely, about 5-10 mm2.s-l at 50 "C,for the most accurate mountings and 15-25 mm2.s-* for less demanding mountings. Since these oils operate in long-life circulation systems, they must be sufficiently stable to oxidation and this requires additive treatment. They are also normally doped with anti-rust additives, since the low-viscosity oils run off the surface during shut-down periods and do not provide adequate protection against rusting, produced by humid air. The natural lubricating power of these low-viscosity oils may not be sufficient and they can be doped with lubricity additives. Anti-foam agents may be required. Spindles may also be mounted in anti-friction bearings provided with taper-rings for the accurate adjustment of clearance and/or some pre-tension. These bearings are highly demanding in terms of lubrication and are mostly not connected to the oil circulation system. Well-refined oils with viscosities from 12 to 25 mm2.s-l and with anti-rust additives are usually employed in drop-feed lubrication.
5.6.2 Electric Motor Oils Insulant oils are commonly used for electrical equipment. These oils must not come into contact with conductive surfaces such as brushes, commutators, slip-rings and contacts. The ability of oils to creep and penetrate must not damage insulators or lead to puncturing. Excess lubricant must therefore be avoided. Lubricating greases are used for small motors fitted with anti-friction bearings. Some motors have self-lubricated porous bearings. These bearings require oils of excellent oxidation stability in the presence of metals and good sliding and adhesive properties. Motors fitted with plain bearing rotors are lubricated with oils of viscosity matching the working temperature and loading, of high oxidation stability and good anti-corrosion properties, which requires additive treatment. Oils such as these are also suitable for lubricating bearings in centrifugal pumps, fans, workheads, gear-boxes and feed-boxes in machine tools and crank mechanisms in small and medium-sized machines with circulatory lubrication systems, and as hydraulic fluids in machine tools and presses. 527
5.6.3 Slide-way Oils These oils are intended to provide a sufficiently strong lubricating layer between the slide and the slideway in machine tools. The slide providing the drive for some of the machine elements travels on the slide-way and imposes a perpendicular load on it. Unless the rate of movement is very slow and the perpendicular load is low, there are no particular requirements for the lubricant. However, the carriages of some machine tools move extremely slowly, often at less than 100 mm.min-l. The viscosity/pressure-wedge effect cannot occur at such a velocity and the conditions for the formation of a liquid layer do not exist unless hydrostatic lubrication is used (constant feed of oil under pressure between the friction surfaces). The kinematic friction coefficient increases sharply - depending on the material of the friction pair - and approaches the static coefficient. This leads to resistance to motion and elastic stress on the driving set. The spring-effect of the driving set sets up stick-slip oscillations, which impair the accuracy of machining. This problem can be eliminated by using special oils which have high oxidation stability and contain special lubricity additives which give the oil favourable frictional properties, and an additive which reduces the spreadability and increases the adhesiveness of the oil. Oils of viscosities between 3 and 15 mm2.s1 at 50 O C are used, viscosity being chosen according to load. Universal oils suitable for machine tool slideways, gears and hydraulic systems are also available; some can also be used as cutting oils. The desired properties can be achieved by a balanced combination of additives, including antioxidant, corrosion (rust) inhibitor, EP additive, friction modifier and anti-foam agent. The Cincinnati Milacron specification for a special table-way oil is given in Table 5.37.1.
5.6.4 Oils for Rolling-mill Bearings Rolling-mill oils are expected to produce accurate, precisely reproducible and reliable work under conditions of high load and severe impacts, which places high demands on the functioning of bearings and hence their lubrication. The bearings, which are often designed as original parts (e.g., Morgoil bearings by Morgan Construction Co.) are characterised by accurate dimensions and very tight tolerances. The oil with which they are lubricated must never fail; it must reduce friction, take away huge amounts of heat to keep the bearing at a low temperature (35-40 "C), protect the bearing from contaminants produced by rusting, corrosion and seizure; it must not have foaming or emulsification tendencies and it must separate readily from any water with which it comes into contact. Its viscosity, determined by the speed and load, varies from 70 to 320 rnm2.s-*;its VZis high (over 80 - usually 90-95), its pour-point is below -7 OC and its acid number below 0.1 mg KOWg. These oils may contain oxidation and rust inhibitors plus VZ improvers and additives enhancing separation from water. Lubrication is circulatory and sustained. A contaminant separator (particularly for scale removal), operating continuously or intermittently, is incorporated into the circuit.
528
ASTM D-892-89 specifies the oil foaming tendency test to be used (the foam must disappear immediately) and a modified ASTM D-1401 or D-2711-86 is used for demulsification. According to ASTM D-1401, a mixture of 40 cm3 of oil and 40 cm3 of water is stirred for 5 minutes at 54.4 'C. The two phases must separate into layers of 40 cm3 of oil and 37cm3 of water within 20 minutes at 82.2 "C; oils of viscosity exceeding 340 mm2.s-' at 50 "C should separate within 40 minutes at 50 "C.The Hershel test (FSSK 320.32), which was formerly used, has been dropped.
Some oil manufacturers specify, in their catalogues, special groups of oils which meet these requirements; other manufacturers tailor their oils to meet the requirements of the bearing manufacturers.
5.6.5 Bearing Oils for the Textile Industry Lubrication methods in the textile industries are matched to the equipment rather than to the type of fabrics manufactured in it. Oil viscosity relates to speed and load. Low-viscosity spindle oils are used for machines operating at speeds up to 15,000 r.p.m. and high-viscosity oils for the bearings of drying machines. Textile oils should not stain fabrics. They must, therefore, be thoroughly refined, light-coloured or colourless, of good oxidation stability (a small quantity of oil is often exposed to long term contact with moist air). They must not produce coloured oxidation products and must protect the machine, particularly during shut-down, or in situations where the fibres are soaked with water emulsions, from rusting (which also stains). They must form strong lubricating films. Low pour-point oils are required for machines which are started in cold rooms. Textile machine lubricants contain antioxidant and anti-rust agents, lubricity improvers (for spindles), EP additives (for looms), spatter-reducing polymers (e.g., polyisobutenes), creep depressants (fatty acid soaps) and additives to facilitate oilwashing (surfactants, such as sulphonates, monooleyl glycerols, etc.). These agents must not leave any colour stains on the fabric. To minimise spattering, machines must not be over-lubricated. Polypropylene oils have shown very good capabilities in these applications. Sintered porous bronze bearings are suitable for low loads. They can be impregnated with oil (as much as 25% by volume) of good oxidation stability and containing lubricity improving agents (95).These bearings may also be coated with polytetrafluorethylene (PTFE),which is partly transferred on to the shaft.
5.7 GEAR OILS Gear oils are defined here as oils which are primarily intended for the lubrication of gear-trains, in which the gears are permanently in mesh and transmit torque from one shaft to another. These oils are expected to minimise the wear of teeth and prevent damage, to lubricate the gear bearings, to remove frictional heat, protect teeth and bearings from corrosion and rusting, both during operation and shut-down, 529
to minimise noise levels and vibration of the gearing, to damp impact between the teeth and to wash off dirt. Gear damage may be classified into that caused by adhesion and wear, abrasion, fatigue, plastic flow and fracture. The lubricant can be responsible for, mainly, adhesive and abrasive wear and surface scratching. Adhesive wear is generally caused by breakdown of the lubricant film if its load-carrying capacity is insufficient and the surfaces are heavily loaded, making possible direct metal-to-metal contact with increasing temperature and, in consequence, welding of surface asperities and weld-fracture. This is manifested by accelerated metal loss or scratching on the tooth surfaces, either on the drive side or the reverse (beginning at the trailing edge) and transfer of the wear debris from one tooth to the other. If this wear debris, and any other hard contaminants contained in the oil, are larger than the thickness of the lubricant film (about 0.6 pm), it can abrade and scratch the tooth surfaces in the direction of shear. These scratches are usually short and do not reach the top of the teeth. A large number of parallel scratches in the direction of shear on the addendum and dedendum of the driving gear is evidence that the gear is operating in a zone of insufficient lubrication; increased temperature may cause surface bum-off. Adhesive wear can be avoided, or at least reduced, by using a lubricant with sufficient load-carrying capacity provided by oil viscosity and anti-wear additives. Abrasive wear can be avoided by effective filtration and sealing against the infiltration of hard contaminants into the lubricant. Increased oil viscosity can diminish plastic deformation of the surfaces of teeth and fatigue wear. However, plastic distortion is actually caused by excessive loading of the surfaces, exceeding the elastic limit and it can be avoided in the design of the gears. Plastic flow of the surface layers is spread in the direction of shear and may change the profile of the teeth. Fatigue of tooth surfaces is manifested by pitting and is frequently the cause of gear distress. Initial pitting - which may also be transient - usually occurs on teeth with unhardened surfaces. It may be spread randomly over the tooth sides or concentrated around the pitch line in the vicinity of the dedendurn. Further straining of the material creates permanent deep, wide pits; this usually occurs at high r.p.m. and low speeds, hence it mostly occurs in the gearing of heavy-duty trucks. Oil viscosity and composition (type of EP or lubricity additives) may enhance or moderate the onset and progress of pitting. Tooth fracture is always caused by severe shock loads, material defects or fatigue, design faults, etc. and it is not associated with the lubricant.
Shearing speed at the tooth, in the first place, then peripheral speed of the gear, type of transmission, tooth shape and modulus, loading between the teeth, material strength, temperature, type of lubrication and ambient conditions all influence the selection of the lubricant and its properties. This is mostly true for lubricating oils and, to some extent, also true for lubricating greases (where the lubricant is required to stay in place and not leak). The theory of gear lubrication has still not been sufficiently explored. The differences in opinion are caused by the differing requirements for the composition and properties of the oils. The basic problem is the nature of the contact involved in meshing. In plain spur gears, mutual rolling occurs in the vicinity of the pitch circle, whilst shear may occur on more remote surfaces. The amount of shear is greater in the more intricate gear design, particularly hypoid gears (fig.5.20). In addition to variable shear rate in the direction of rolling, an element of constant cross shear is involved and the short (theoretically point) contact of the teeth is also associated with very high specific loads. Tooth shape, however, does not promote the formation of the hydrodynamic layer, and zones of mixed and boundary lubrication are believed to exist. More recently, the view has tended to be accepted that an elasto-hydrodynamic layer, which remains intact and continuous in the tooth-mesh for a short time, forms and prevents metal-to-metal contact. This lubricating layer must, however, be thick enough to overcome the effects of surface roughness. The formation of such a layer depends on a number of factors, and temperature and other non-static factors
530
must also be considered. Shear also has a considerable effect. It can cause the temperatures to rise, both of the friction surface and of the lubricant, causing the lubricant to be expelled from the contact zone, thus reducing the thickness and the load-canying capacity of the lubricating film. Tooth geometry, particularly those elements which determine the maximum shear speed, is hence of considerable importance. It is remarkable that the load which causes scuffing decreases with increasing shear speed, but increases slightly again after the lowest point has been passed. This behaviour is attributed to the s (218). visco-elastic properties of the oil, which can be exhibited over very short periods, less than
8[ SPUR GEA
I
I
,‘HYPOID EAR
I I
CIRLLL PITCH
-- -
BASIC CIRCLE
I
I
-----
\
”
\ -L---
----
SLIDING VELOCITY
TO HEEL Fig. 5.20. Outline demonstration of sliding velocity in contact of teeth in gears Oil temperature in the contact zone rises with increasing load and peripheral velocity (173). Also, the effective width of the contact and the pressure in the lubricating film increase with increasing load. However, the load varies locally in particular parts of the tooth surface, so that the pressure in the lubricating film and the oil temperature fluctuate. The tribological configuration of the contact zone and the various mutual relationships are thus rather intricate. Nevertheless, all this implies that oil viscosity is highly important and that alkanic and those cycloalkanic oils with a lower dependence of viscosity on temperature and higher thermal conductivity are advantageous. However, the benefits of viscositypressure coefficient grow with increasing aromatic and cyclic structures. Because of the intricacy of the tribological configuration of gears, there are many obscurities in the application of hydrodynamic friction theory to the lubrication of gears and some views appear rather contradictory.Thus, according to Ibrahim and Cameron (59).theoretical assumptionsabout the minimum thickness of the lubricating film apply only at low velocities in gear-trains, whereas at high velocities, despite a sufficient thickness of the lubricating film, a critical situation may be attained where the lubricating capacity of the lubricant may fail and surface wear increase. According to Wellaner and Holloway (208). the critical specific thickness, A, of the film covering the surface roughness must be. taken into account; this width is considerably smaller in gears than in the rolling elements of bearings; it is also related to velocity in the zone of the pitch circle. From experiments, a curve relating the specific thickness of the lubricating film 1 and the velocity of the pitch circle has been plotted (fig. 5.21) (209). From this curve, wear should not occur at low velocity if 1 is less than 0.I . To avoid damage to the tooth surface, 1 must grow with increasing velocity up to 2.0. However, oils with EP additives must be employed for 1 below 1 .O. Leach and Kelly (60)maintain that a limiting temperature is the critical variable; wear increases above this limit. This also explains the fact that EP and lubricity additives reduce wear; their chemical effect (reaction between active atoms in the additives and metal surfaces) or adsorption (chemisorption)of the polar constituents of the molecule on to the metal surfaces reduces the temperature generated between the friction surfaces and the intensity of local overheating. and consequently keeps the temperature fall to below the limiting temperature.
53 1
Some authors (215) attach the highest importance to the maximum flash-temperaturereached in the friction couples, others (216) to the permanent temperature of the tooth sides. The flash-temperature is higher when the friction temperature, load per unit length, difference between driving wheel and pinion velocities, reduced modulus of elasticity and reduced radius, thermal conductivity and specific heat of the gear cluster are higher. The maximum tolerable load with an additive-freeoil can be determined from the highest theoretical maximum flash-temperature(217). The permanent temperature of the tooth sides is approximately equivalent to the lubricant temperature in the gear cluster during idle running, to which the product of the load per unit length, peripheral velocity of the pitch circle and a constant of the proportional effect of the gear is added.
33zz
55 -1
::
T 2I-
5 tE- 0.5 LL
w u
ko2_J
i3 0.l-
k a 0
7-
a2
a5
1
2
5
100 zoo 500 VELOCITY IN PITCH CIRCLE (M.cM’)
10
20
50
Fig. 5.21. Relationship between the critical specific thickness of the lubricating film k in contact of teeth in gears and the velocity of the pitch circle This is in agreement with experience with oils containing various EP additives (i.e., various active elements),of which the reaction with metal surfaces is also related to temperature and of which the effect changes with varying operating conditions of the gears. Gear oils containing phosphorus (or sulphur) alone, e.g., phosphite, phosphate or thiophosphate, are suitable for high torquellow speed operation, as in truck gears (oils containing 0.1% weight and even less phosphorus can pass the CRC-L 37 test). Oils working under high spedshock-load conditions require S/P additives, in which the sulphur must be highly active (this activity increases in the order disulphides
Fe
L 2 R-S-S-R(orR-S-H.R-S-R)
R-S-Fe
- FeS
(5.6)
The rate of this reaction depends on the sulphur bond energy. Reactive compounds containing chlorine produce lubricating films of high carrying-capacity. even at low temperatures, but the chlorine films do not retain their load-carrying capabilities at temperatures as high as sulphur compounds. The possibility of mixed or boundary friction occurring despite apparently thick lubricant films justifies the use of solid additives in gear oils, since they also have an anti-wear effect (176.177).
532
The choice of suitable viscosity of oils for field service is based on experience or on tables and charts which specify the relationship between oil viscosity and peripheral velocity, or between gear diameter and r.p.m., and the material, ambient temperature and operating conditions (66). English-language literature recommends the following formula for the computation of the approximate viscosity of oil to be used for lubricating spur and spiral gears operating at high peripheral speeds (above 1,200 r.p.m.), on the basis of elasto-hydrodynamiclubrication theory and field experience: 58,000 V =
Vd
(5.7)
( p )06 P+l
where v is the kinematic viscosity at 37.8 "C in mm2 s 1 , V is the peripheral speed in ftmin-I, d is the pinion diameter in inches, p is the gear ratio. Lechner (229) compares the effects of factors which enable the load on the gears to be increased without seizure occurring: Factor
Load multiplier
1 Lubricant effects 1 (base-line) I . 1 Mineral oil 1.2 Same plus additive 5 1.5 1.3 Doubling viscosity at 50 "C(minera1 oil) 1.4 Same plus additive 5.75 2.5 I .5 Synthetic oil, half friction coefficient of mineral oil 2 Design effects 6 2.1 Change in gear geometry I .5 to 2 2.2 Design of teeth
3 Effects of materials 3. I I :10 diminution of surface roughness 3.2 Increase in material hardness 3.3 Surface phosphatising 3.4 Surface nitriding 4 Working conditions 4.1 Shear rate changes (general purpose oils) 4.2 Same highly-refined oils 4.3 20 "C reduction in oil temperature
-
2 4
I .4 I .8
2.5 8 I .8
The following properties are essential for gear oils, in varying degrees: - adhesion to the metal surface, which holds the lubricant on to the surface despite centrifugal force. Adhesiveness is especially important for open gears. It is related to the concentration of polar compounds and of substances with large molecules and high melting-points, such as asphaltenes, resins and various polymers. With the exception of some polymers, these substances are, however, undesirable, since their liquid friction is high and their cooling effect low; - capability to produce a lubricating film of sufficient carrying capacity between the metal surfaces which is resistant to excessive load at high pressure. This 533
property is imparted to the oil by viscosity and, to some extent, by the composition and concentration of the EP additives used and the type and concentration of the active element; The capacity of the oil film to withstand pressure without scuffing increases with oil viscosity within the range 2,000-20,000pinion r.p.m. (6Z).
-
lubricity. This reduces friction coefficient, imparts the required frictional characteristics to the lubricant and increases its resistance to the effects of shear, which operate to expel it. Lubricity is imparted to the oil by polar constituents. However, the required friction characteristics can be achieved only by special additives in suitable ratios. Control of friction characteristics is particularly important for those gear oils which are used simultaneously as hydraulic system fill and for special oils used to lubricate limited slip differentials; - good viscosity-temperature characteristics, i.e., adequately high viscosity and good fluidity at the lowest operating temperatures (high VI);
The oil alone - without VI improver - should have a high VI, since depolymerisation of polymers occurs at high shear stress, with the loss of the thickening effect of polymers. However, some highmolecular weight petroleum resins and also some polymers improve the VI of the oil and retain high shear stability. Polyglycol ester and polyalphaolefin oils are suitable for some specific applications because of their excellent viscosity-temperature curves. Their adhesiveness and lubricity are very good, even in the absence of additives.
-
high thermooxidative stability, which suppresses the formation of corrosion products, viscosity increase and accumulation of hard, abrasive, carbonaceous debris. Most oils are doped with antioxidants; demulsification, enabling water to be separated and preventing the functioning of the oil being impaired by emulsion formation;
The tendency to form emulsions increases with increasing viscosity and the concentration of polar substances. Gear oils, particularly those used for open gears, should therefore contain additives which function as water-repellents (e.g., lead or zinc sulphonates).
- high shear strength under sudden load increase; - low foaming tendency, since foam causes oil leakage from the gearbox, reduces the load-carrying capacity of the oil film and impairs lubricating power; Venting and foaming increase with increasing peripheral speed and some EP additives aggravate this tendency. Gear oils should therefore contain anti-foam agents.
-
oils should not cause corrosion of the metal surfaces, indeed they must provide protection from the effects of corrosive substances during both operation and shut-down;
The oxidation and hydrolysis products of some. EP additives may promote m i o n of steel, particularly that of copper-basealloys both below and above the oil level. Thermally stable EP additives which arc difficult to hydrolyse. or those which do not react with water to form corrosive substances,
534
are therefore preferred. The use of chlorine-based additives is limited by the temperature at which hydrogen chloride separates, since this generates with water a highly corrosive acid. Corrosion- and rustpreventatives are frequently used.
-
oils should not damage sealing materials, must be compatible with other oils and must readily dissolve additives. These requirements are not always met.
Oxidation products and some additives may impair the function of packings and accelerate ageing. Chlorine-based compounds, which promote swelling of rubber packings, are an example of this. Some EP additives only dissolve with difficulty in high VI, alkanic oils. If a cycloalkanic oil, in which EP additives dissolve readily, is mixed with a high VI oil, the EP additives may separate to some extent. Also, mixing oils containing lead-soaps with oils containing active-sulphur compounds may produce a precipitate of only slightly-soluble lead sulphide.
Gear oils are not normally required to have detergent properties. However, this property is important for oils used for lubricating control mechanisms or as hydraulic system fill, since any deposits contained in these oils may impair their function.
5.7.1 Types of Gear Oil Gear oils are usually classified by application, into automotive, aircraft, multipurpose tractor and industrial gear oils. Industrial gear oils may include oils used for lubricating chain transmissions, friction clutches and transmissions, power screws and wire ropes. The differences in the compositions and properties of these oils result from this classification and the conditions under which the oil must do its job, as well as the tooth sliding speed, loading, temperature, method of lubrication, ambient conditions, etc. 5.7.1.1 Automotive Gear Oils
Automotive gear oils must meet higher standards than industrial gear oils since relatively small gears are required to transmit high power at high tooth pressure, under conditions of frequent shock-loading ahd vibrations produced by the engine, unevenness of the roadway and the activity of the driver. Oils of higher viscosity and hence higher film load-carrying capacity cannot be employed, because of high peripheral speed and because operational ability at low temperature must be ensured. Oils with EP additives are therefore most suited to the job. Automotive gear oils are intended chiefly for lubricating the driving (axle) gears (final drive gear) and hand-control gearboxes. Depending on the type of gears in both gear assemblies, one or two oil types may be used. The final drive (differential) configurations are intricate, since there are four basic gear types and each requires a different oil (fig. 5.22): (a) spur (spiral) gear; (b) bevel gear; (c) worm and (d) hypoid gears. Vehicles with cross-mounted engines usually have spur (spiral) gears mounted on the driving axle. The gearbox and the engine sometimes share the same lubrication system. These gears have a relatively low shear rate and tooth load and 535
can, under normal load conditions, be lubricated with a suitable engine oil, since such oils contain ZDDP which adequately protects the teeth from wear. Oils with an average load-carrying capacity are fully suitable for such gears even under high load conditions.
Fig. 5.22. Basic gear types
a - spur (spiral), 6 - bevel, c - worm. d - hypoid gears
In the design of European cars being built in the 1990’s. the integral gear-bodengine system has mostly been abandoned, with the exception of the British Rover Group Mini, originally the first massproduced car to use it. Most cars with cross-mounted engines now have separate lubrication systems for engine and gear-box. When the engine and gear-box share the same lubrication system, the effects on polymers in the oil of simultaneous mechanical and thermooxidative shear may, as mentioned in Chapter 4, be to cause permanent reduction in oil viscosity.
Spiral bevel gears are the most widely used type for driving axles, mainly for trucks, buses, tractors and even the older types of passenger cars. These gears have low sliding speeds and short tooth slip and only a small portion of the tooth profile participates in the sliding contact. Lubricants for these gears require higher resistance to pressure than spur gear lubricants, but oils of average load-carrying capacity are
536
mostly adequate. Under moderate conditions, engine oils containing ZDDP can be used; if other circumstances permit, higher viscosity oils are preferable. Worm gears are used almost exclusively for the driving axles of heavy-duty motor vehicles (trucks), but they have been to a large extent replaced by spiral bevel, and especially hypoid gears. The disadvantage of worm gears is their relatively heavy weight in comparison with their performance and a high sliding speed at the contact point, although the pressure on the teeth are relatively low. Their mechanical efficiency, particularly under heavy-duty working conditions (e.g., when towing a trailer) and at elevated ambient temperatures, is low. Under some conditions, they have relatively short working lives because of wear or pitting of the bronze worm shaft. Pitting is also promoted by the use of oil with insufficient oxidation stability. Oil which contains chlorine and active sulphur compounds can also accelerate wear.
Worm gears, especially if they are heavily loaded, need an oil of as high a viscosity as possible (provided it is sufficiently fluid at low temperature to facilitate starting), with adequate oxidation stability and high load-carrying capacity. The same types of oil are used as those suitable for moderately loaded hypoid gears. They must not, however, contain additives based on chlorine or active sulphur. Polyglycol oils with antioxidant and anti-rust additives also meet these requirements. Hypoid gears are a combination of worm and spiral bevel gears, where the axis of one member is off-set with respect to that of the other. The frictional configuration here is quite intricate, since the teeth roll over each other at high slip rates in the direction of rolling and at the same time considerable side-slip occurs. Pressure between the teeth are several times higher than those of spur and worm gears and the conditions for the establishment of hydrodynamic lubrication are less favourable than those for worm gears. Most modern passenger cars with both front and rear wheel drives use this type of gearing. They are less common in front engine-front wheel drives and rear engine-rear wheel drives, where bevel gears are preferred. The advantages of hypoid gears are low noise, vibration-free operation and long functional life, since more rigid, large diameter pinions and larger bearings can be used.
Oils for lubricating hypoid gears have to meet the highest standards of lubricating film load-carrying capacity and anti-wear properties. Limited-slip difleerentiuls are involved in a special type of operation. On starting or at high vehicle loading, they transfer the majority of the driving force to the wheel providing the highest thrust via a limited-slip, multi-plate clutch, whilst operating as a conventional differential gear during normal conditions. These gears must be lubricated with oils which possess the additional frictional characteristics required to achieve vibration- and squeak-free slippage of the coupled clutch. These vibrations are caused by stick-slip occurring on the clutch plates due to adverse friction characteristicsof the lubricant (62).This phenomenon occurs when the friction coefficient increases with decreasing clutch sliding speed, and is dependent on the friction characteristics of the clutch. It has been
537
found that some oils which meet the requirements of MIL-L-2105and 2105-B specifications are able to lubricate these clutches without causing squeaking. This must obviously be associated with the composition of the EP additives, which also exhibit differences in this respect, EP additives which have some friction modifier properties also contribute to a decrease in the temperature of the oil in the gearbox during vehicle travel.
The lubrication of manual gearboxes is less complex than that of the final drive units. Since spur or spiral gears are used, sliding speeds on the mesh side of the wheels are low and tooth-loading is relatively low; the smaller gears present an exception, but they only operate for short periods of time. Conventional gearboxes do not use heavily -loaded taper-roller bearings, being usually arranged so that sufficiently large bearing can be used. All these factors enable the lubrication of these gears to be carried out with a wide variety of oils and impose only moderate demands on the carrying-capacity of the lubricant film.In many cases, oils with suitable viscosity containing ZDDP as a mild EP additive are quite adequate. However, gear oils with medium load-carrying capacities are preferred. The above discussion illustrates the existence of considerable differences in the requirements for automotive gear oils. This has led to the development of specifications which facilitate the choice of oils for various applications, which classify oils by viscosity, performance and type of application. As in the case of engine oils, gear oils are classified by SAE viscosity (Table 5.38). In the Eastern European countries, a unified viscosity classification was established in 1974 for automotive gear oils (Table 5.39). Table 5.38. Viscosity Classification of Automotive Gear Oils by SAE J 306 March 85 SAE viscosity grade
Maximum temperature ( "C) Viscosity at 100 "C (mm2.s-') Maximum for viscosity of 150 Pas* Minimum
-55
-
-40 -26
-
4.1 4.1 7.0 -12 11.0 90 13.5 140 24.0 41.0 250 * Measured according to ASTM D-2983-87 in the Brookfield viscometer (63).
70W 75w 80W 85W
-
24.0 41.0
-
Table 5.39. Unified viscosity Classification of Automotive Gear Oils in COMECON Countries Viscosity class
9 18 34
Kinematic viscosity (mm2.s-') at -18 "C 100 "C Min. Max. Min. Max. 3250.
-
21700
-
4.0 14.0 24.0
Equivalent SAE J 306 Viscosity grade
14.0 24.0t 42.0
Not prescribed if viscosity at 100 ' C min. is at least 6.5 d . d . Not prescribed if viscosity at -18 "C max. is at least 160,OOO mm*.s-' (extrapolated).
538
80W 90 140
viscosity classification is extremely important in relation to the type and working conditions of the gear system. The maximum viscosity at low temperatures determines the startability of the gears during the winter season, which is determined by the passive resistance of the gears to which the oil contributes a greater share if its viscosity is high. Viscosity is even more important at temperatures which the oil may reach during vehicle operation; it determines the carrying-capacity of the lubricant film and thus the wear-rate or damage which occurs to the teeth and bearings, as well as the amount of noise (if the viscosity is too low) and energy losses (if the viscosity is too high). Oils with higher viscosity, and hence higher load-canying capacity, are preferable, because of the higher pressure which exists between the teeth in mesh and the fact that higher viscosity oils can provide better conditions for elasto-hydrodynamiclubrication. However, teeth entrain larger amounts of oil than that which remains in the lubricant f i l m and this oil surplus must be expelled through the gap between the teeth. This causes an intensive kneading action of the oil which requires more energy the higher the oil viscosity. This energy consumption reduces the mechanical efficiency of the gears and increases the temperature of the oil. These adverse phenomena are more marked at higher gear speeds. Since the time during which the teeth are in contact diminishes with increasing speed, oil viscosity can be reduced so that formerly conventional oils with high viscosities are now used only in large gears in industrial machinery.
When considering the maximum temperature of the oil in the gearbox, the stable temperature must be distinguished from the temperature gradients which exist during prolonged, continuous vehicle travel. Dependent on a number of factors, including the arrangement of the gears, the peripheral speed of the gear wheels, tooth loading, quantity of oil in the gearbox, the degree of cooling of the oil, the distance between the gears and the hot surfaces of the engine, the method of lubrication, etc., the oil temperature in the gears (mainly the final gears) of modern engines varies between 75 and 150 "C, and that in sports vehicles and those equipped with catalytic converters from 160 to 165 "C.It is well known that a substantial part of the heat which causes the oil in the gearbox to heat up is derived from friction in the gear bearings; the rate of this heat-flux depends on the arrangement of the bearings, the manufacturing tolerances and the load. Measurement carried out on the oil in various vehicles during prolonged, continuous operation have shown that the temperature of the oil may be much higher; the peak temperature may reach 180 to 220 OC (64). This temperature
OC
I
Fig. 5.23. Progress of the oil charge temperatures in the driving axle hypoid gears of a passenger car
539
increases rapidly during the first 10-20 km, reaches a maximum after 80-150 km and then decreases; the decrease is rapid at first, then stabilises up to about 800 km (fig. 5.23) The values quoted above do not fully apply to normal operation of automotive vehicles, but have been identified in cars driving constantly near their maximum speed and fresh gears have been used in each test. The ambient temperatures must also be considered, since these can affect the test temperature by 320 OC. Friction modifiers can decrease the peak operating temperatures by as much as 35 "C and the stable temperature by 15 "C; the amount of temperature reduction depends, however, on the concentration of friction modifier in the oil.
Since oils must have adequate viscosity at both low and high temperatures, their viscosity index must be sufficiently high to achieve this. Multigrade automotive gear oils in SAE viscosity grades 75W80,75W90,80W90,80W140 and 85W140 can be quoted as examples. Multigrade automotivegear oils have more recently attracted interest in the context of fuel economy. According to one reference (Z23),fuel economy with heavily-loaded heavy-duty truck gears improves by 1.2-2.8% if an SAE SOW140 multigrade gear oil is used. A synthetic 75W90 oil improved fuel economy in city travel by 2.2% and in highway travel by 1.6% in comparison with an SOW90 oil (143). Suitable friction modifiers in gear oils can improve gear efficiency and decrease fuel consumption. The effect of friction modifiers increases with increasing temperature since oil viscosity decreases and the proportion of mixed friction increases. The oil temperature in the gearbox (final drive housing) first increases and then becomes stable for the whole time of operation of the vehicle, whilst the efficiency of the gears increases with increasing oil temperature in the presence of friction modifier. Under severe conditions of service, where low viscosity gear lubricants formulated with conventional sulphuro-phosphorus EP additives are thought to be questionable in terms of durability, an EP additive system based on a dispersion of potassium triborate has been shown to provide superior bearing and gear protection. The use of this borate EP additive system in low viscosity lubricant formulations provides, at the same time, the potential for improved fuel efficiency (260).
Multigrade gear oils, particularly those in the lower viscosity grade 75W, can be suitably prepared from part-synthetic oils, e.g., from mixtures of mineral oils with polyalphaolefins or from fully synthetic oils. In comparison with petroleum-based oils, these lubricants have low viscosities at low temperatures and relatively high viscosities at high temperatures; they provide a lubricant film with satisfactory loadcarrying capacity and good rheological properties, which allows friction forces to be reduced, i.e., gear efficiency to be increased, and fuel consumption to be cut (251, 252,253). Since synthetic components have high Vl's, partially or fully synthetic gear oils need little or no added polymeric viscosity and VI modifier, allowing high shear stabilities to be achieved (however, very highly shear stable polymers, such as polyisobutene, can be used). On the other hand, oils with a higher polyolefin content need a higher EP additive dosage to achieve the required performance; the reasons for this do not appear to be clear. The US Army and the analogous British War Office specifications classify oils by performance parameters. The API classification categorises oils by application under typical operating conditions. 540
Performance parameters of gear oils are largely influenced by the concentration of EP and anti-wear additives and by the concentration and interaction of their active elements. Combinations of active elements have both advantages and disadvantages and have a considerable bearing on the suitability of an oil for particular types of gears and working conditions. Combinations of lead soaps and compounds based on active sulphur suppress tooth surface scuffing, particularly of un-phosphatised gears. They can give oils good performance characteristics at high vehicle speeds. They are, however, disadvantageous at high torque and their high chemical activity is evident from high wear of the bearings and gears. Oils containing such additives in combination are especially suitable for first-fill for passenger car gearboxes, for run-in during the first 1,500-2,000 km. They do not comply with MIL-L-2105 and 2105B specifications and may, in some cases, have a detrimental effect on seals. Combinations of lead soaps and sulphur- and chlorine-based compounds have proved generally satisfactory for conditions of high torque at low speeds, but they are relatively ineffective in passenger cars at high speeds. Oils with these additives are generally used as factory- or first fill for heavy-duty vehicle gears. They have good anti-rust properties and comply with MIL-L-2105 in this respect. On the other hand, they do not comply with MIL-L-2105B tests for high speeds and torques. They may damage conventional seals and cause gearbox leakage. Sulphur-, chlorine- and phosphorus-based combinations have been used, because of their high chemical activity, mainly for hypoid bearings during the early stages of their development. Some combinations are effective at high speeds, some at high torques. Many of the original oils which met the requirements of VVL-761 and MIL-L-2105 contained this type of additive combination. Oils with a combination of sulphur- and chlorine-based compounds are less effective than oils containing the former combinations under high torque conditions and they do not comply with the majority of military specifications. Most gear oils manufactured between 1952 and 1%2 which met the MIL-L-2105 and 2 105B specificationscontained 5-9.576 of EP additives comprising S/P/C1-based compounds. They have been replaced more recently by oils containing combinations of sulphur- and phosphorus-based compounds, which provide better thermal stability, high storage stability and better anti-corrosive properties in the presence of water. MIL-L-2105 oils contain about 3.5% weight and MIL-L-2105B oils about 6.5% weight of such additives. The composition and properties of the base oil also dictates the efficiency and dosage of EP additives.The thennooxidative stability of the gear oil also depends on the quality of the base stock. The base oil should be a highquality, selective raffinate. High aromatics content impairs the effect of EP additives. Better results are obtained with narrow, homogeneous cuts than with oils which contain several fractions of differing viscosities. The EP additive concentration required to achieve the same performance level in a gear oil decreases with increasing viscosity of the oil. Some polymeric VI improvers of low shear stability have a deleterious effect; detergent-dispersant additives do not generally affect the performance of gear oils.
Table 5.40. Quality Parameters for Automotive Gear Oils Meeting MIL-L 2105 Parameters 75 Viscosity (SUSat 100 "F) 150-190 (31.9 - 40.7) (mm2.s-' at 37.8 "C) (SUSat 210 "F) (mm2.s-' at 98.9 "C) Viscosity index min. 85 Flash-point ( "C) min. 149 -45.5 Channel point ( "C) max. Copper corrosion l b max. (1 hat 121 "C - ASTM D-130) Antirust protection (24 h - ASTM D-665A) Pass
Viscosity grade 80
90
55-65 (8.9-1 1.7) 85 163 -45.5
80-90 (1 1.5-18.0) 85 163 -17.8
l b max. Pass
l b max. Pass
-
54 1
The first US Federal performance specification test was WL-761 in 1942. This was replaced in 1945 by the military specification 2- 105B and later by MIL-L-2105. Table 5.40 lists the requirements gear oils must meet for this specification. In addition, oils must pass simulation tests in a final axle drive gear test according to CRC-L-22 (corrosion test with water), CRC-L-19 (test with high peripheral speed and low driving shaft torque) and CRC L-20 (high torque-low speed); it must also undergo a storage stability (fluidity) test, a test of foaming tendency (ASTM D-892) and a test for compatibility with other oils. The channel point temperature is standardised in Fl?vlS 791a-3456.The oil is atemperated at 45 f2"C and cooled at a prescribed rate, and the fluidity (channel point) of the oil is measured. A channel is made with an inclined bar on a steel plate and the time taken for the channel to be filled is recorded. The channel point is the limiting temperature at which channel filling takes less than 10 seconds. TheCRC L-19 test is made on a Chevrolet rear axle in four run-in cycles (16 -64-16 km.h*)and 10 test cycles (%-128-96 km.h-') in which the throttle is alternatively fully open and fully closed. Scuffing on the tooth mesh side is evaluated. The CRC L-20test is made on a Dodge truck rear axle at 62 r.p.m. for run-in for 20 minutes and at 62 r.p.m. for a test run for 30 minutes; during run-in the oil is not cooled and during the test run it is cooled at 93-121 "C. Deposits, corrosion, rusting, wear and tooth surface scratching rate are evaluated.
Since gear oils for passenger cars from the 1950's onwards required high performance characteristics, the MIL-L-2105B specification, introduced in 1953, prohibited the use of residual dark oils. Residual dark oils caused excessive oil thickening and deposit formation, resulting in bearing damage, and scuffing and scratching of the gear teeth. High quality selective raffinates must be used for the preparation of these gear oils. All these measures do not, however, solve scuffing and scratching problems, although the solution of these problems is, in fact, the task of EP additives. MIL-L-2105B specifies the following requirements for gear oils: - the quality parameters must be the same as those of MIL-L-2105 (Table 5.40), except that the viscosity classification is different (see Table 5.4Z) and the copper corrosion test is carried out at the same temperature but over a period of 3 hours; Table 5.41. Viscosity Classification of Automotive Gear Oils according to MIL-L-2105B Viscosity (mm2.s-1) at "C
98.9 -17.7 (w.)
-
80 8.8-1 I .6
10,850
Viscosity grade 90
16.8-19.2 65,200
140 25.7-34.3
-
the oil must pass the stability test, the fluidity test, foaming tendency test and the test of compatibility with other oils; the oil (120 cm3) must pass the TOST oxidation test according to FIUS 7912504, conducted with a spur gearbox at 1725r.p.m of.thedriving and 2450 r.p.m.
542
of the driven shafts, at 162 "C in the presence of a copper catalyst and in a 1.1 I 1itre.h" air stream. The test lasts 50 hours. After the test, the change in viscosity and the concentration of n-pentane- and benzene-insolubles are measured. The oil viscosity increment must not exceed 100%. the concentration of n-pentane insolubles 3% weight and benzene-insolubles 2% weight; - the oil must pass a performance test in simulated gearboxes by the following methods (97): CRC L-37 test at alternately low and high r.p.m. and high and low torque; CRC L-42 test at high r.p.m. and shock load; CRC L-33 water corrosion test (7days). The CRC L-37 test (ITMS 791b-6506 of 1%9) was originally carried out on the Chrysler rear hypoid axle and later on the Dana L-37D axle. The test comprises two runs - during the fust, the oil is exposed to operation at 149 "C for 100 minutes at high speed (440 r.p.m.) and low torque (1064 Nm); the second run involves operation at 135 "C for 24 hours at low speed (80 r.p.m.) and high torque (4710 Nm). Deposits, corrosion, rust and tooth surface damage are evaluated after each run. The CRC L-42( R M S 791b-6507.1 of lW9) test is based on the 1956 Chevrolet V-8.The test identifies the ability of the oil to prevent tooth surface scratching and includes a series of runs with fully open and closed throttle, simulating operation at high speed and shock load. The bearing noise level is also evaluated. High-speed run conditions
Number of acceleration and deceleration cycles Starting speed Maximum speed Mean load on teeth mesh side, min. Mean load on teeth relieved side, min. Shock-loadrun Conditions Number of acceleration and deceleration cycles Starting speed Maximum speed Mean load on teeth mesh side, min. Mean load on teeth relieved side, min.
-5 - 550 r.p.m. - 1,100 r.p.m. - 14,950 N.m-' - 12,910 N.m-' - 10 - 550 r.p.m.
- 650 r.p.m.
- 31,200 Nm-' - 22,400 N d
Noise level test
Slow acceleration up to 850 r.p.m. and slow deceleration down to 700 r.p.m. are repeated three times. After stopping, the oil is left to cool down to 93 "C. The CRC L-33 ( F I U S 791b-5326.1) seven day test is based on the Dana axle with 2.3 litre oil charge; 28 cm3 of distilled water is added to the oil. Run-in is carried out without load at 2,500 r.p.m. and oil temperature 82.2 "C for four hours. The final drive housing is allowed to stand for 162 hours at 51.7 "C (oil temperature) and the extent and nature of corrosion evaluated.
MIL-L-2105C specification was submitted in draft in 1970. Oils meeting this specification must pass the following tests: - CRC L-37, L-42 (FI'MS 791a-6506.1 and 6507.1) tests with the same acceptance limits as those specified for MIL-L-2105B, - CRC L-33, L-42 (FTMS791a-5236.1) seven-day water corrosion test with the same acceptance levels as those specified in MIL-L-2105B. 543
modified thermooxidation test (TOST) by the Canadian Army procedure, in which post-oxidation viscosity of the filtered oil is measured (97);the viscosity increase should not exceed 50% and the concentration of substances insoluble in n-pentane should not exceed 3% weight and in benzene 2% weight (ASTM D893h storage stability test by FTMS 79 1-3440, the oil must have a satisfactory viscosity at low temperature (determined in the Brookfield viscometer), the oil must pass the FTMS 791-3440 test for compatibility with other oils. MIL-L-2105C oils are available in various viscosity classes, listed in Table 5.42 with their other properties: Table 5.42. Quality Parameters for Automotive Gear Oils Meeting
MIL-L-2105c Viscosity grade Properties
75w
8OWBO
85Wl140
Viscosity (mm2.s-I) at 100 oc min. 4.1 Temperature ("C max.) for max. -40 Viscosity (Brookfield) of 150 Pa.s -45 Channel-point ( "C), max. Flash-point (o.c., "C),min. 150 2c Copper corrosion (ASTM D-130,3 h,121 "C) Foaming tendency (ASTM D-892): 20 Sequence I & 11 (cm4, max. 50 Sequence 111 (cm3), max.
13.5-24.0
24.0-41.0
-26 -35 165 2c
-12 -20
Note: replaces previous specification MIL-L-
10 324A
180
2c
20
20
50
50
2105B
2105B
In 1987, MIL-L-2105C was replaced with MIL-L-2105D as the primary US military gear oil specification. The physical properties of the lubricants and the performance test requirements are identical to those of MIL-L-2 105C. The new specification allows the use of re-refined oils, but requires that approved lubricants and lubricant components fully comply with US health, safety and environmental requirements. It introduces the L-60 test and leaves out the L-37 test. European test requirements include those made on the FZ and IAE (98)machines and the British IP-MIRA high-speed shock test (99). The IP-MIRA high-speed shock test was made on Vauxhall hypoid rear axles with prescribed, alternating acceleration and deceleration. Wear and overall conditions of the gears are evaluated after the test.
The British CS 2798 specification of 1954 was replaced in 1957 by CS 3000A and laster by CS 3000B. These specifications are similar to US MIL-L-2105 and 2105B. The 1969 API classification divided oils into six classes, MI-GL-1 to API-GL544
6, according to performance characteristics and application. The API classification is described in Table 5.43. Table 5.43.API Classification of Automotive Gear Oils API class GL-1
Characteristics
Other relevant specifications’
Oils for lubricating automotive bevel gear-transmissions with hclical teeth. worm gears, some gears and transmissions with manual gear-shift operating under low loads and at low speeds and low shear rates, capable of being lubricated with pure mineral oils incorporating oxidation and rust inhibitors, de-foaming agents and pour-point depressants; do not contain bl’ additives 01- friction modifiers.
_ _ ~ ~ _ ._ .
(;I,-.!
Oil,. foi Ilihricating automotive end worm gear transmissions ~ and shear operating uriilcr such loads and ai m A temperatures rates that GL-I oils provide inadequate lubrication; do not contain EP agents or friction modifiers.
GL-3
Oils for luhricating gear transmissions with manual gear-shift and bevel end transmissions with helical teeth, operating at low speed and under low load. The bad-carrying capacity of the lubricant film exceeds that of GL-I and GL-2 oils but these oils fail to meet the requirements of GL-4; contain about 2.7% EP additives.
GL-4
Oils for lubricating transmissions of passenger cars and other motor vehicles operating at high speed and low torque; oils should provide as good as or better anti-score protection than CRC reference oil RGO-105 and comply with the tests specified in ASTM Research Report No. RR25:D2 of May, 1965 (March 1967) and amendment of October 1968**; contain about 3.5 weight EP additives.
Oils of this class should meet the requirements of specification MIL-L-2 105
GL-5
Oils for lubricating mainly hypoid gears in passenger cars and other vehicles operating at high speed and under impact load; high speed and low torque, low speed and high torque; oils must provide as good as or better anti-score protection than CRC reference oil RGO-I 10 and comply with test specified in ASTM Research Report No. RR25:D2**; contain about 6.5% weight EP additives***.
Oils of this class should comply with MIL-L-2105B
GL-6
Oils for lubricating hypoid gears with specific axial off-set (above 2.0 off-set, approx, 25% of gear diameter) of passenger cars and other motor vehicles operating under conditions of high speed and performance. Oils should provide anti-score protection as good as or better than L-lo00 reference oil and comply with the test specified in ASTM Research Report No. RR25:D2**; contain as much as 10% weight of additives.
Oils of this class comply with Ford ESW-M2C specification and “Ford Controlled Temperature 500 Drag Start test”.
Not included in the API specification - relates to experience and technical practice. Complete title is “Laboratory Performance Test API-GL-4. GL-5and GL-6 Automotive Gear Lubricants.” **t Oils for use in limited slip differentials must incorporate a suitable friction modifier. It
545
In 1988 API, working closely with S A E and ASTM, proposed two new gear lubricant categories, PG-1 and PG-2. These new performance classes had been requested by the builders of heavy-duty equipment. PG-1 is an interim designation for lubricants needed for heavy-duty manual transmissions for trucks and buses. PG2 designates high performance lubricants for truck and bus axles. In both lubricant categories, the emphasis is on high temperature performance and specific, quantitative oil-seal compatibility. PG-2 will include requirements for high temperature (150 “C)stability and component cleanliness, oil-seal compatibility and surface fatigue, in addition to all the current GL-5 requirements. PG-1focusses only on high temperature cleanliness, oil-seal compatibility and compatibility with copper alloys. API GL-5 is not necessarily a basis for PG-1performance. When test development of the PG-1 and PG-2 categories is complete and approved, they will be upgraded to a full API GL designation. Motor vehicle builders have issued specifications of the properties required for oils used for lubricating limited-slip differentials employed in their vehicles. These are summarised in Table 5.44 (61). 5.7.1.2 Aircraft Gear Oils
Gears and their bearing employed in aircraft (helicopter gears, propeller pitch gears, turbo-jet reducer gears) are mostly subject to high loads and they require special oils as lubricants. The properties of these oils are laid down in military specifications which have also been adopted for civilian use. Originally, mineral oils were used for aircraft gear oils. These have been gradually supplanted by synthetics, which have better viscosity-temperature characteristics, lower volatility, etc., and are used in aircraft as multi-purpose oils. Both military and engine builders’ specifications exist. For example, Pratt and Whitney in the USA and Rolls-Royce in the UK issue their ,own specifications for aircraft oils. 5.7.13 Multi-purpose Tractor Gear Oils
Significant changes took place in the development of tractor gear oils in the 1960’s. These changes were stimulated by two factors, namely, efforts to reduce the number of oil types in use and developments in the design of tractors and associated equipment. The result was the introduction of two types of multi-purpose (universal) tractor oils: - “tractor universal oil (TOU)” and “super tractor universal oil (STOU)” for the lubrication of engine and gears and for cooling the “wet” brakes, - universal oil or hydraulic/transmission fluid for lubricating the gears and also suitable for use as fluid for the hydraulic systems. The first TOU oils corresponded to MIL-L-2 lO4B but had a higher ZDDP content (0.08 - 0.1% weight Zn) which improved the anti-wear properties of the oils. They were not, however, suitable for the recently-introduced wet brakes, because their friction characteristics were not adequate, so they were unable to prevent stick-slip 546
Table 5.44. Specifications of Oils Used for Lubricating Limited-slip Differentials Specification Properties
Ford M2C42 Ford M3C34A (SAE 80) (SAE 90)
Viscosity SUS at -17.8 "C, max. 100,Ooo (mm2.s-') (22,600) at 37.8 "C, max. (mm2.s-1) 90-100 at 98.9 "C 55-65 (mm2.s-1) (8.9-1 1.7) (18.25-20.65) 90 90 Viscosity index, min. -23 Pour-point. "C, max. -35 Flash-point "C, min. 177 171 204 Fire-point "C, min. 193 Concentration of actives (46 weight min.) 2.0 Lead as PbO 2.0 1.2 1.2 Sulphur - total - active Chlorine 1.25 1.25 Phosphorus 14.5 Saponified substances 14.5 Fatty substances 0.2 Water content, % max. 0.2 Tests: 80(350) Timken OK load, (Ib (N)), min. 75(205) 5 5 Wear (mg), max. Copper corrosion 3 h at 22 "C, max. I I lb Ib 3 h at 98.9 OC, max. Moisture corrosion test* 0 0 Oxidation test** 10 Evaporative loss (%), max. 10 Viscosity increase at 98.9 "C 10 ( % max.) 10 Foaming tendency 25 Foam vol. at 93.3 OC (cm3), max 25 0 Foam stability (46) 0
Studebaker (M.S.907) -
1,400 (316) 85 min. ( 17.45) 90 -23 177 2.5 0.03-0.045 0.2 0.2
50(220)
-
-
4 10
-
* Ford Laboratory test Method BJ5. ** Ford method - 200g of oil is heated at 149 "C for 100 h without air stream; Studebaker method - 200g oil heated at 85-93 "C for 125 h.
and vibration of brakes in operation. These oils were therefore additionally treated with friction modifiers and labelled STOU. Manufacturers' quality specifications for STOU are detailed in Table 5.45. The principle of wet brakes is similar to that of disk brakes in passenger cars. The braking effect is achieved by the application of pressure to rotating disks. Wet brakes are equipped with several disks, which are wetted with oil. The oil is required to abstract heat. The brakes are mounted on the final drive housing on each wheel.
547
Table 5.45. Tractor Manufacturers' Specifications and Tests for STOU Universal Tractor Oils Tractor manufactureslspecifications
Propertieflests John Deere J20A
Ford M2C86A
International Massey-Ferguson Harvester M2C34A B-6 M-1135 M-1138
Viscosity at 98.9 OC
sus (mrn2.s-1)
56min. 61-65 (8.20 (10.6-11.7)
Viscosity at -17.8 "C (Brookfield) mPa.s, max. 4000 Viscosity index, min. Flash-point "C, ox., min. 193 Pour-point "C max. -28 Copper corrosion: 3 h at 98.9 "C, max. 3 h at 121 "C, max. 3 h at 149 "C, max. 1 24 h at 70 "C, max. Foaming tendency (ASTM D-892) 25/0 Sequence I (cm3), max. Sequence I1 (cm3), max. 50/0 Sequence 111 (cm3), max. 25/0 Corrosion test (Falex pin) Oxidation test(l00 h at149 "C): evaporative loss (%) max. 5 vischcrease at 37.0" C max. 12 visc.increase at 98.9 OC max. 6 deposits 0 Seal test (70 h, 100 "C): change in volume 0-+5 change in hardness 0 Seal test RDR 008 (168 h a t 121 "C) Timken OK load (Ib) rnin. 15 (N) (66) Timken abrasion test (6 h at 401b (176N) weight loss (mg) max. 1.o Falex test (Ib), min.
58min. 47min. 60-65 (9.7) (6.5) (10.3-1 1.7)
55-58 (8.9-9.7)
-20
196 -37
9000 95 -15
4000 120 -28
1 lb
1 -
la 1 -
Ib -
100/0
20/0
100/0 I 00/0 pass
100/0
100/0 100/0
100/0 100/0
-
50-10 50-10 50-10 -
100/0 -
100/0 -
-
0
-
15
0
-
-
5
5
+5 -
0
-
-
-
-
-
-
pass
pass -
-
1750
-
-
-
-
-
9000 95 218 -15
9000 100 205
Ib 1 -
-
1750
20/0
-
-
(kN) (8.7) (8.7) 4-ball machine (1 h, 65 "C, 1500 r.p.m., 40 kP (392N)) scar dia, (mm), max. 0.4 ASTM mean Hem load, kP(N), min. 38(375) load wear index, (kP(N))rnin. - 89(875) weld load (kP(N)), rnin. - 158(1550) The oil must additionally undergo a wet brake test, tests in a hydrodynamic clutch and in gear trains and a miscibility (compatibility) test at 1:l ratio.
Most of these oils are cycloalkanic and SAE 20W30. 10-12% of package additive makes them suitable for lubricating engines to MIL-L-2104C or 46152 performance levels and provides adequate load-carrying properties and suitable friction characteristics. These properties are achieved with high shear strength viscosity index improvers and pour-point depressants. Universal hydraulic/transmission fluid is suitable for lubricating gears and for filling the hydraulic systems with which modern tractors are mostly equipped. The hydraulic system mainly comprises clutches, torque converters, steering gear boosters, controls for trailer implements, etc.
These fluids are also suitable for cooling wet brakes. They are prepared according to manufacturer’s specifications for STOU oils; some of these specifications are illustrated in Table 5.46. Fluids contain packaged additives at a dosage of about 9% by weight; this imparts suitable friction characteristics, high load-carrying capacity and anti-wear properties, oxidation stability, anti-corrosive and anti-rust properties, water-repellency and low foaming tendency; a high shear-strength VZ improver provides improved rheological properties at lower temperatures.
5.7.1.4 Industrial Gear Oils These category covers oils suitable for lubricating all gears (except those of motor vehicles) including spur, bevel and spiral bevel gears of varying sizes and performance levels, at varying loads and including both open and closed gears with different lubrication systems. Lubrication of these gears is accomplished with a wide variety of oils, of different composition, viscosity, additive content and properties, often used for lubricating other components. These oils are thus often listed among other categories of oil, such as bearing, compressor and dark oils. Open gears are not very demanding on oil quality, since they are mostly robust and operate at low r.p.m. and built with larger tolerances as compared with enclosed gears. Friction in open gears is usually boundary type and a lubricating wedge cannot be formed. With viscosity corresponding to velocity and load, the oil must adhere well to the teeth, must not run off despite elevated temperature, pressure and shock load, and it must not absorb water or be washed off easily by rain. Dark oils, e.g., atmospheric or vacuum distillation residues of cyclic crudes, are suitable types for this purpose. These oils are characterised by a natural adhesiveness of the lubricating film, since they contain polar oil resins and asphaltenes, but they have low oxidation stabilities and a poor capacity to expel water. Better quality open-gear oils may contain additives to improve their spreadability (e.g., latex or lead soaps). The concentration of these substances may be high enough to give the oil the consistency of a soft grease. Solid agents, such as graphite or MoS,, may be added to improve the load-carrying capacity of the lubricant film. Lubricants which form a dry, non-sticky surface, to which dirt does not readily adhere, are preferred for open gears working in a moist and dusty environment. 549
Table 5.46. nactor Manufacturers' Specifications and Tests for Hydraulidhansmission Oils Propertiernests
Tractor manufacturedspecifications AllisChalmers PF 821
Viscosity at 98.9 "C SUS,min. 55 (mm2.s-l), min. (8.9) after 30 min. ultrasound shear stress, SUS min. at 37.8 "C 250 (mm2.s-1). min. (56) Viscosity (Brookfield) mPa.s, at -17.8 "C max. at -27.8 "C, max. Viscosity index, min. Flash-point "C, o.c., min. 193 Pour-point "C max. -28 Metal content (% wt.),min. Ca P Zn Copper corrosion: 2 h at 100 "C, max. 3 h at 100 "C, max. lb 3 h at 149 "C, max. 24 h at 70 "C, max. Foaming tendency (ASTM D-892) Sequence I (cm3), max. 2510 Sequence I1 (cm3), max. 2510 Sequence I11 (cm3), max. 2510 Foaming tendency (with 1% water) QIO Sequence I (cm4, max. 2510 Sequence I1 (cm3), max. Sequence 111 (cm3), max. OIO Corrosion test (Falex pin) Oxidation test (100h at149 "C): 5 evaporative loss (%) max. viscincrease at 37.0 "C max. 12 vischcrease at 98.9 "C max. 6 deposits 0
J.1.Case 143
John Deere Ford
Int. MasseyHarvester Ferguson J 14B M2C134A B-6 M-1138
54 (8.6)
54-58 (8.6-9.7)
58 (9.7)
47 (6.5)
55-58 (8.9-9.7)
255 (57)
-
-
-
-
-
-
-
3100
4000
9000
-
2600 14000 95-115 196 -37
4000
-
0.38 0.30 0.1
-
-
-
188 -35
193 -28
100 205 -32
-
-
-
-
-
-
-
-
120
-28
-
-
-
lb
-
-
-
lb
-
-
lb
-
-
2510 2510
2010 10010 2010
50- 10 50- 10 50- 10
-
2510
2510 2510 2510
-
-
-
-
-
-
50-10 50-10 %lo
10010 1OO/O 10010
pass
-
-
-
5
-
-
-
-
-
-
-
12 6 0
5 -
-
15
-
-
1
-
-
-
Other tests applied are similar to those in Table 5.45; in addition, the oil must comply with field tests in wet brakes, hydraulic clutch and gears.
Examples include ozokerite or asphalt-based lubricants, often diluted with volatile solvents which allow them to be sprayed on as aerosols. Modern open gears
operating indoors are often lubricated with soft grease applied to the tooth surface by periodic spraying. Enclosed gears are more demanding on the oil but only a few types require special oils with effective additives to improve the load-carrying ability of the film. Their viscosity corresponds to the operating conditions (peripheral speed, magnitude and type of load, temperature) and to the materials of the gears (hard, structural metals require higher viscosity oils because they can carry higher loads). Other properties may be necessary according to the application, such as good oxidation stability, low foaming tendency, water separability, etc. US Steel, AGMA and others set out detailed limiting values for the quality parameters for open and closed gear oils (see Table 5.47). Table 5.47. Equipment Manufacturers' Specifications of Oils for Industrial Transmissions Manufachlres/specifications
Propertiesflests
US Steel 224 Timken (ASTM D-2782) OK load (lb., min.)
60
222
AGMA
David Cincinnatti Ford Brown Milacron Motor Co. (5EP) P-59(C-320) M-2C142C (6EP)
250.04(5EP) 250.03(6EP)
-
45
-
1.1
-
-
7.5 max. 75max.
-
-
3max. 3max.
-
1
2
-
pass pass
clean
NR
pass NR
10
10
-
45'
4-Ball EP test (ASTM D-2783)
Weld load (kg min.) 250 Seizure load (kg min.) N R Loadlwear index (kg minJ45 Wear test (ASTM D-2266) (54 "C, 1800 r.p.m., 20kg.l h) Scar dia. (mm max.) 0.35 FZG N8.3l90l20 1I Pass load stage min. Coefficient of friction p on David Brown 6 inch CRS Disc machine, ratio candidate p: reference p, max. Dynamic demulsibility at 180°F top Iwater bottom 96 Corrosion (David Brown standard test) - bronze - steel Copper-strip ASTM D- 130) la 3 h a t 100 "C, max. Rust (ASTM D-665,IP-135) A - distilled water B - synthetic sea water Oxidation stability (ASTM D-2893) % visc. increasel95 "Cmax. -
55 1
(Table 5.47 contd.) Propertieflests
Manufacturedspecifications
US Steel 224
222
Oxidation (modified ASTM D-2893) (121C/312 M101 airlh) % visc. increase, max. 6 Demulsibility (ASTM D-271 I , modified for 90 ml water) % water in oil, max. NR total free water, ml min. 80 emulsion, ml max. 1 Pour-point (ASTM D-97) oil visc. up to 130 SUS at 210"F, "F max. +15 +20 Foam tendency/stability (ASTM D-892,IP-146) at 24 "C, mum1 at 93 "C, mVml Air release (DIN 51381,lP-313) at 90 "C, minutes, max. Thermal stability test (CM procedure B) % visc. increase (ASTM D-2161) sludge condition of steel rod condition of copper rod l)
-
-
AGMA 250.04(5EP) 250.03(6EP)
David Cincinnatti Ford Brown Milacron Motor Co. (5EP) P-59(C-320) M-2C142C
(6EW
-
+10
0
-
- 5 max. none nodepositd discolouration -CM colour 2 max. -
45 Ib Timken OK Load or 9 Pass Load Stage FZG.
*) Test targets vary with viscosity. 3, Ford test method BJ1-2.
Viscosity is a decisive factor in choosing an oil. Given the peripheral speed, gearwheel diameter and r.p.m., ambient temperature and working conditions, the recommended viscosity at 50 "C can be found from tables and charts (66). For gears operating with high loads on the tooth surfaces, low r.p.m. and at elevated ambient temperatures, lubricated either manually or by immersion, and top worm gears, higher viscosity oils are required. These oils reduce the noise level and wear of tooth surfaces under conditions of high thermal and mechanical loading. However, they cause the tooth temperature to rise because of high internal friction and poor ability to abstract heat, and they increase the energy losses and hence the efficiency of the gear system. Gears operating at high r.p.m. at low temperatures, and gears lubricated with circulation systems, require lower viscosity oils, which also ensure rapid starting at low temperatures and reduce energy consumption. Such oils provide better heat removal and penetrate more readily into lubrication sites. 552
Because of the importance of oil viscosity for the choice of gear oils, the American Gear Manufacturers’ Association (AGMA) has adopted a viscosity classification for gear and circulation oils. Oils employed for enclosed gears are classified by viscosity values in SSU according to AGMA numbers (see Table 5.48). Table 5.48. AGMA Specifications for Oils for Closed Gears AGMA No.
Viscosity (SSU) at ( “C) 37.8 98.9
Approximate equivalent mm2.s-*at 50 OC IS0 classification (VG) ~~~~
I 2 3 4
23.5-90 100-125 160-220 220-270 5 80-105 250-400 6 105-1 25 400-5 10 7 comp 125-150 5 10-740 8 150-190 740-1000 8 comp 150-190 740-1000 8A comp 190-250 1000-1500 Note: the “comp” oils contain 3-101neutral tallow or any suitable animal oil.
180-240 280-360 490-700 700-1000 -
46 68 100
150 220 320 460 680 680 1000
Oils for gears operating under wide temperature range conditions with frequent temperature fluctuations must have higher viscosity indexes, lower pour-points and adequate low-temperature fluidities. Pure mineral oils are usually satisfactory for the lubrication of most industrial gears. However, some gears with intricate configurations which do not favour the generation of liquid friction, such as hypoid, spiral and worm gears, or those subjected to high specific load (e.g., small gears for heavy-duty performance), need oils compounded with animal or vegetable oils or effective EP agents. Industrial gear oils used to be mainly treated, to improve their EP and anti-wear properties, with lead compounds (e.g., naphthenates) and chlorinated hydrocarbons (e.g., chlorinated solid paraffins) in combination with sulphurised aliphatics, especially sperm oil but also other animal or vegetable oils, e.g., rape-seed oil. These combinations had both merits and demerits. For example, lead soap and chlorinated hydrocarbons improved EP properties and sulphurised aliphatics improved anti-wear properties. However, lead soaps and sulphurised aliphatics impaired oxidation stability, chlorinated hydrocarbons tended to hydrolyse to produce highly corrosive hydrochloric acid. The scarcity of sulphurised sperm oil and objections to its use and that of lead compounds - on conservation and environmental grounds have led to restrictions. Sulphur- and phosphorus-based additives, similar to those used for automotive gear oils, have been more recently developed. Examples include various combinations of ZDDP compounds, alkenyl thiophosphates, sulphurised hydrocarbons (mainly polybutenes) and sulphurised animal fats. Anticorrosion agents are also usually added. Although the EP properties of oils containing S-P-based additives are not as good as those based on lead or chlorine compounds (the best results in the Timken apparatus have been given by oils containing lead, in the four-ball tester those containing chlorine), anti-wear properties are excellent, particularly in relation to phosphorus content, which can be demonstrated on the FZG test. In this test, oils containing ZDDP and sulphurisedhydrocarbons gave very good results. These oils also have better thermooxidation stability and friction characteristics. Anti-rust properties are satisfactory. Results of copper and steel
553
corrosion tests are somewhat less satisfactory because of the higher active sulphur concentration, particularly after pre-oxidation of the oil. High-quality industrial gear oils contain about 3% weight of EP and anti-wear S-P type and anti-corrosion additives. If sulphurised animal fats are used alone, the concentration of these additives may be as high as 10% weight.
Table 5.49 illustrates the differences in the load-carrying capacities of oils containing EP additives of various compositions, in terms of MPa (Hertzian pressure). A summary of oils suitable for open and closed industrial gears under various working conditions is shown in Table 5.50. Table 5.49. Load-carrying Capacities of Gear Oils Oil type Pure mineral oils
Maximum load-carrying capacity (MPa)
800
1400 Oils with medium-power EP additives, e.g., S and P compounds, sulphonated fatty substances. Oils with very efficient EP additives, 1900 e.g., chlorinated hydrocarbons, leadbased soaps of fatty or naphthenic acids.
Factors which augment loadcarrying capacity higher oil viscosity, higher concentration of natural polar substances in the oil higher content of EP additives higher oil viscosity higher content of EP additives higher oil viscosity
Test of Anti-wear Properties of Gear Oils Several test machines which simulate gears have been developed in order to determine the anti-wear properties of gear oils. Among the better-known are the FZG, IAE gear machine and Ryder gear machines. (FZG stands for Forschungstelle fu'r Zahnruder und Getriebebau - Research Institure for Gears and Gear Manufacture - (Technische Hochschule, Munchen), and IAE for Institution of Automobile Engineers.) The FZG test method is described in DIN 51-354 and CEC L-07-T-70 specifications. The Nieman test gearbox which is used is depicted in fig. 5.24 and is essentially an enclosed power circuit formed by two gear sets (driving and test gears) mounted on the ends of two parallel shafts, one torsion and one stationary shaft, which is interrupted by an arresting coupling. The torsion stress on this coupling can be adjusted by means of a lever-gear, fitted with a weight. The test gear housing contains a heating coil and a thermostat to maintain the specified test temperature. Increased shear is produced by off-setting the tooth profiles; this increases the liability to scuffing of the tooth sides. The tooth sides therefore do not require as high a load for testing EP additive-containing oils as other test machines (67).According to the load-carrying capacity of the oil tested, either wide gears (20 mm) or narrow (10 mm) gears working at lower (8.3 m.s-') or higher (16.6 m.s-l) gear speed. The working temperature of the oil is normally maintained at 90 O C , but higher temperatures are also possible. Anti-seizure capacity is characterised by the weight loss of the gears under particular loads. The scale is divided into 12 loading levels, classified by the increase in the perpendicular force per unit tooth width F i b . This test identifies the level of loading - the damage load stage - at which wear of the test gears has risen markedly as compared with the previous load stage and traces of scoring covering at least 20% of the total tooth side surface can be seen with the naked eye.
554
The modified FZG test (fig. 5.24) is used for testing automotive gear oils; in this, the magnitude and rate of temperature increase are monitored for a specific time, e.g., 120 minutes. Slower and smaller temperature increases indicate superior lubricity of the test oil.
I
1
l2
5
Fig. 5.24. Sketch of the FZG Testing Machine
1 - test gear set, 2 - torque measuring coupling, 3 - bearing, 4 torque arresting coupling, 5 - electric motor, ~
6 - transmission gear set, 7 - suspension, 8 - weight
IP-166/65 specifies the use of the IAE gear machine test Vg. 5.25). This machine comprises one torsion and one stationary shaft. Spur gear pairs are mounted on the shaft ends. One gear pair is coupled with the driving set and has wide teeth, the other pair of narrow teeth is the test set. The torsion shaft is interrupted by a coupling, which can alter the torsion pre-stress and hence the load on the test gears. The off-set of the test gear profile varies in the ranges common to gearings, the mesh angle being relatively large (68).The criterion for limiting load is the formation of scratches or seizure on the tooth addendum or dedendum, as a result of four measurements.
1
5
Fig. 5.25. Sketch of the IAE Gear Machine
1 - test gears, 2 - transmission gears, 3 - coupling, 4 - be11transmission,5 - electric motor
The Ryder test is particularly intended for evaluating the lubricatingcharacteristics of aircraft turbine oils.
FZG, IAE and Ryder test results cannot be mutually compared, because of a lack of research results. It is generally maintained that the results of the FZG test can be used to predict the performance of spur and bevel gears in field service, but it is uncertain whether they apply to hypoid gear performance (69).Tests conducted in full-scale hypoid gear housing rigs are more reliable for automotive gear oils. Some of these tests have become standard, such as FTMS-79/9-6506 and FTMS-79la6507.
555
VI a\
Table 5.50. Simplified Scheme for Selection of Oils for Lubricating Industrial Gears Peripheral velocity
Specific pressure
oil type
Open spur & bevel gedmanual or oil-bath
very low (
medium to high a)light duty b)heavyduty
dark oils* without EP additives
Closed spur, screw and bevel gears/ circulation or splash-lubrication
high (>15 m.s-')
low to medium
highquality selective dinates without EP 14-17 (bearing & compressor oils)
Type of gearhethod of lubrication
medium (5-15 m.s-1) low (<5 m.s-') high medium low
Closed hypoid & highly-loaded spiral-bevel gedcirculation or splash lubrication
additives
high
high (>1800 r.p.m.) medium to medium (900-1800 r.p.m.) high low (up to 900 r.p.m.)
gear oils, with medium effect EP addtives
Characteristicproperties of oil viscosity at 50 "C film-load oxidation Uld.S*' capacity stability 35-40 190-230
medium to high
-
8-1 1 low
good
24-32 7-9 11-19 19-32
gear oils with highly 4-24 (SAE 80-90) efficient FP 4-42(SAE 80-140) additives 14-42(SAE 90-140)
other
medium to high VI, low pour-points, goodwaterseparation, low foamingtendency
medium to high medium good
VI, low pourpoints, good water-separation low foamingtendency
high VI, low high
good
pour-points, good water separation,low foamingtendency
Closed worm gear /splash or oil-bath lubrication of
high (>1800 r.p.m.)
low to
medium
low-velocity gears
high
high-quality 8-1 1 selective raffinates without EP additives (bearing & compressor oils)
low
gear oils 7-9 with medium effect EP additives
medium
highquality 21-25 medium low to medium (900-1800 r.p.m.) selective raffinates without EP additives (bearing or compressor oils) high gear oils 11-18 low medium gear oils 32-48 with medium (up to 900 r.p.m.) to high to very effective EP additives
Opem worm gears/ manual or oil-bath lubrication
low
medium to high
bearing or 32-40 dark oils (for cast gears) without EP additives
good
mediumto high VI,low pour-points, good waterseparation, low foaming same
low
high
good good
same
medium
-
-
*So-cdled “dark oils” are oils prepared directly from residual vacuum distillates, or by mixing vacuum distillates with bitumen. The spread of the distillation residue, or the ratio of distillate to asphaltic bitumen determines the viscosity of the oil. The advantage of these oils is their high natural adhesivity and higher lubricant film loadcarrying capacity. resulting from the high concentration of polar substances (asphaltenes and resins), which, however, causes poor water separability. particularly in higher viscosity oils. Because of their composition, they have low oxidation stability, low viscosity indexes. high acid numbers and large carbonisation residue values. Their flash-points depend on the viscosity of the distillate used and the concentration of asphaltic bitumen; pour-pointsdepend on the the nature of the petroleum crude from which the dark oil has been produced.
Other Tests of the Properties of Gear Oils In addition to the tests mentioned in the gear oil specifications, a number of tests is available for evaluating the performance properties of gear oils. The three friction tests described below are unlike the gear tests described above in that no attempt is made in them to imitate the design and operation of real gear systems. Instead, particular aspects of friction and lubrication are isolated and studied under more or less precisely-definedconditions. In one sense, they are designed to provide information rather than to demonstrate success or failure in meeting a performance standard, although the results are frequently described as "pass" or "fail". The relationship of laboratory test results to performance in field service, or "correlation", is one of the more contentious subjects in practical tribology. Full-scale tests are not only expensive,they are often not very reproducible. The complex conditions prevailing in any practical machine are difficult to define precisely and to reproduce; expense is multiplied by multiple replication to obtain a valid result. Tests like those described below are more reproducible and much cheaper to run. However, the relationship of the results with those of field service is often questionable. The Falex test (ASTM D-2670) is used to measure the EP and anti-wear properties of a lubricant or grease. A rotating pin 0.25 inch diameter is clamped between two V-blocks to which a load is applied. Wear at a given load is determined by measuring the width of the contact areas or the weight loss of the pin and the blocks. In the Four-bull test (CSN 65-6254, ASTM D-2783, DIN 51 -530, GOST 9490-60), a steel ball under load rotates on three stationary balls clamped in a small cup which also holds the test oil. The EP performance of the oil is assessed by increasing the load until welding occurs; anti-wear performance can be investigated by measuring the diameter of wear scars on the balls at the point of contact (Shell method). The Timken load test (ASTM D-2509) is used to examine boundary lubrication under heavy-load operation. The apparatus is designed to bring a flat, rectangular test block into line contact with a rotating cylinder. The load (normally in Ibs.) until scoring occurs is measured and the appearance of the test block is recorded. Thennooxidation stability - US Steel S-200 is identical to ASTM D-2893, however the extent of sludge formation is measured as well as viscosity increase. The Beaker Oven test lasts 72 hours at 135 "C; sludge formation in the oil and deposits on a steel plate are evaluated. In the US Steel Corp. procedure, 10 litres of dry air is blown over 312 hours through a 300 cm3 sample of the oil in the tube, which is dipped in an oil-bath at 95 "C. Increase in the viscosity of the oil at 98.9 "C and the amount of insolubles are measured. The Panel Coker test is used to evaluate thermal stability at 260 "C for 30 minutes. In Czechoslovakia, gear oil thermooxidation stability is tested at 120 "C or 130 "C for 72 hours with an air-jet at 5 1.h-I. The amount of deposits on a catalytic copper plate is evaluated. After the oxidation test, copper and steel corrosivity are tested at 100 "C for 3 hours or at 20 "C for 24 hours. Copper and Sreel Corrosion Tests - in addition to the current ASTM D- 130 test, the copper oxidation test (COCT) is also used. This test is for 17 hours at 135 "C; BT-10 is for 100 hours at 121.1 "C In Czechoslovakia, the comsivity of gear oils towards copper and steel is tested at 100 "C for 3 hours and at 20 "C for 48 hours; an accelerated corrosion test is available (100 "U3hours) in CSN 41-220. LimiredSlip Properties - these can be evaluated by a "large wheel/ small wheel" road test (GM-UTSR-4A-1-4). The vehicle travels with a constant difference between the speed of revolution of the wheels so that friction surface scuffing and vibration are produced; the distance travelled up to the onset of vibration is measured. Performance Tesring - some automobile builders use their own tests for the evaluation of gear oils for capabilities beyond those of the more widely applicable specifications. Examples include the Mack Heavy-duty Power-divider Test, in which 98,000 Ib (432 kN) shock-load is produced by a sudden drop in r.p.m. If a suitable lubricant is used, there is no cracking sound and minimum wear. The European High Speed Endurance Test, made with a special vehicle, measures the temperature increase in the differential and tooth scoring is evaluated after 384 km travel. In the USA, the Ford Controlled Temperature 500 Drag Start Test and Ford Trailer Towing Test are used to measure the increase in oil temperature in the differential.
558
5.7.1.5 Chain Transmission Oils Special oils are not required for the lubrication of link and sprocket chain transmissions. In the majority of cases, dark oils of high viscosity and good natural adhesivity are suitable for open or part-encapsulated transmissions. Encapsulated transmissions can be lubricated with compressor oils. Oils containing medium effect EP additives must be employed for heavy-duty transmissions. Chain-drive conversion units can be used to test these oils. The unit comprises a hydraulicallygoverned, infinitely variable-speed chain transmission with plain friction plates, which are surrounded by a special chain section. The test oil is sprayed from inside the chain transmission.
As in the case of industrial gearings, the viscosity of the oil must be chosen to
match the chain speed and the ambient temperature; at higher chain speeds, the viscosity required is reduced (see Table 5.51). Table 5.5 1. Oil Viscosities for Lubricating Chain Transmissions Chain velocity (m.s-') up to 2.5 2.5 to I 7 to 10 above 10
Recommended viscosity (mm2.s-') at 50 O C for ambient temperature ( "C)
<-I0 30-40 20-30 15-20 15-20
-10 to +20 60-80 40-60 30-40 20-30
20 to 40
A0
120-160
above 160 90-120 60-90
60-90 40-60 30-40
40-60
Manual lubrication is used for plain, open chain transmissions and pressure lubrication or oil-bath (or oil-spray for encapsulated transmissions) for large sprocket chains.
5.7.1.6 Friction Clutch and Transmission Oils Multi-plate clutches and transmission controls must be lubricated to achieve engagement which is free from stick-slip and to avoiding heating and wear of the steel plates. Friction clutches and transmissions are therefore lubricated, in spite of the fact that the lubricant reduces the friction coefficient and the efficiency of the friction coupling. The oils used have low viscosities (up to about 50 mm2.s-l at 50 "C) and good oxidation stability to avoid sticky deposits, which would be detrimental to operation; for the same reason, the oils cannot contain EP additives. In contrast, infinitely-variable-speedchain gear-boxes need oils with good antiwear capabilities to prevent excessive wear of the chain and plate and consequential loss of transmission efficiency. Since an oil-bath lubrication system is employed, the oil can be subject to considerable thermal stress and so must have good oxidation stability. Durable, pure oils (e.g., compressor oils) of higher viscosities (60-1 10 mm2.s-l at 50 "C, depending on ambient temperature) can be used in a higher temperature 559
environment; lower viscosity oils with appropriate EP additives are suitable at lower temperatures.
5.7.1.7 Traverse Screw and Rack Oils The conditions for the generation of hydrodynamic friction are very adverse in these manually-lubricated, low-speedhigh-load machines, so that they need oils with a superior load-carrying capacity, such as dark oils with a minimum viscosity of 40 mm2.s-l at 50 "C.
5.7.1.8 Oils for Steel Wire Cables Steel wire cables for hauling and carrying are subjected to high stress and must therefore be protected from wear and corrosion. The lubricant must perform its protective and lubricating function both at the cable surface and inside its core. It must be able to penetrate the capillaries to the core and there develop its preventative and lubricating power. It must also have good elasticity over the whole range of operating temperatures to which it may be exposed. Highly adhesive lubricants are mostly used for steel wire cables. Examples include dark oils or viscous semi-solid lubricants prepared from deasphaltised distillation residues and containing additives such as latex which make the oil sticky enough to adhere to the surface of the cable and prevent it running down at elevated ambient temperatures. Cables for indoor and outdoor work are lubricated in summer with high-viscosity oils (above 150 mm2.s1 150 "C) and in winter with lowerviscosity oils (around 30 mm2.s-'/50 "C) and very low pour-points (below -30 "C). These lubricants may be tested in the Winkler and Pietsche Tester (70), in which wear of two steel wires lubricated with the test lubricant is assessed. Two steel test wires, wound 360" with respect to each other, are clamped between two pairs of rotary plates. A gear crank imparts opposing reciprocating motion to the first pair of plates. The second pair of plates transmits the motion to the test wires by means of a rocker. This allows passive, opposed reciprocating movement of this pair of plates. This harmonic interaction imparts to the test wires a periodic, axial, reciprocating motion, so that the two wires rub against one another. The rubbing action is similar to that between two steel wire strands in a cable. The test lubricant is continuously fed to the friction surfaces. The wear rate of the wires is measured by determining the loss in weight. Friction coefficients can be measured with a tensometer spring and an oscilloscope..
5.8 HYDRAULIC OILS (HYDRAULIC FLUIDS) In hydraulic transmissions or drives, the basic principles of hydraulics are used to transmit and control power, in order to multiply pressure or where accurate and reliable control is required. Hydraulic power transmission can be favoured over mechanical transmission on the grounds of structural compactness, infinitely variable speed control, simple running reversal, applicability to rotary and straight560
Table 5.52. ISO/DIS 6743/4 Classification of Hydraulic Fluids Type of Application hydraulic transmission Hydrostatic
Composition and properties
Additive-free mineral oils (DIN 51517, Pt.1). Mineral oils with oxidation & rust inhibitors (DIN 51504, Pt.1) HL oils with EP additive (DIN 5 1504Jt.2)
Symbol
HH HL HM
HL oils with VI improver (higher Vr) HR HM oils with VI improver (higher Vr) HV Synthetic fluids of normal flammability
Note
HS
For hydraulic systems with loaded components For construction and marine equipment Special properties
Common HM oil with anti-stick-slip additives lubrication of hydraulics and slideways
HG
Environment Oil-in-water emulsions or aqueous requiring non- solutions. flammable liquids Aqueous solutions of polymers (e.g., polyalkylene glycols) Water-in-oil emulsions
HFAE Typically less than 20% flammables content
Polymer-in-water solutions Fire-resistant, non-aqueous fluids, further divided into: synthetic, water-free liquids consisting of phosphate or silicate esters Synthetic, water-free liquids consisting of chlorinated hydrocarbons Synthetic, water-free liquids consisting of a mixture of HFDR and HFDS Synthetic, water-free fluids of other types Hydrostatic Automatic transmissions Couplings and convertors
For machines with common lubrication of hydraulic system and slideways, where slide damage at low velocities must be minimised
HFAS Typically more than 80% water HFB Less than 25% flammables HFC Typically less than 80% water HFD HFDR
Both HFDS and H m T liquids must be handled HFDT withdue precautions because of adverse HFDU environmental and human toxicity effects
HFDS
HA HN
561
line motion with ready inter-conversion, remote control, simple servicing, etc. Hydraulic power transmissions have found a very broad range of applications in machinery. The fluids used must be able to perform as lubricants, to dissipate the heat generated and to protect the internals of the machine against corrosion. The properties required of it depend on the design, performance and working conditions of the hydraulic system, hence on the method of power transmission - whether the type of transmission is hydrostatic or hydrodynamic. The I S 0 classification of hydraulic fluids is described in Table 5.52.
5.8.1 Hydrostatic Power lkansmission Fluids Hydrostatic power transmissions are characterised by low kinetic energy of the fluid, which is virtually constant throughout the system, and by high potential energy. The hydrostatic pressure is relatively high and can be varied simultaneously throughout the system. Hydrostatic power transmission is based on Pascal's principle of 1650 pressure of a fluid enclosed in a vessel increases and diminishes uniformly in all directions, regardless of the shape of the vessel, and acts equally everywhere on its walls fig.5.26); equation 5.15 applies.
Fig. 5.26. Scheme of the principle of hydrostatic power transmission The ratio of the forces F, and F2 opposed to two pistons is proportional to the ratio of their surface areas $,and S2 and indirectly proportional to the distances they travel I , and 12:
-_F L S , = F2
S2
12
4
(5.8)
Thus, a small force F, applied to small piston of surface area S, with a long stroke I , can produce a large force F2 on the working cylinder S2 with a short stroke 12. All hydrostatic hydraulic systems comprise at least six components (Fg. 5.23, namely, the hydraulic fluid reservoir, a pressure generator (hydraulic pump), pressure and directional flow control element, a means for converting the pressure into mechanical motion (hydraulic motor), fluid transfer piping and hydraulic fluid. The systems are complemented by fluid treatment devices, such as filters and coolers. The system may be fitted with gear (external or internal gearing), plate or piston (axial or radial) hydraulic pumps. External, spiral, spur or herringbone gear pumps can develop pressures up to 22 MPa; internal gear pumps can produce pressures in the system up to 30 MPa. sliding-vanepumps (widely used in mobile and industrial hydraulic systems) up to about 18 MPa and axial piston pumps up to 50-60 MPa. Fluid circulation can be of the open type (fluid returns to the reservoir after every cycle and cools down) or enclosed (fluid circulatesbetween the hydraulic pump and the hydraulic motor and the reservoir makes up the liquid deficit - this results in higher mechanical and thermal stress on the fluid). The fluid flows from the pump to the motor and a control element, or reversal of the rotation of the pump, changes
562
the direction of flow if required. A relatively low frequency of reversal of the direction of flow is typical of such systems. More recently, broad application has appeared for alternating fluid flow systems, which are unlike the conventional, unidirectional flow hydraulic systems in that the hydraulic pump is coupled with the hydraulic motor by means of one or several enclosed and mutually-separated pipes in which the fluid oscillates. This design causes much greater dynamic and thermal stresses on the fluid. The fluid is subjected to the highest stress in the pumps, control devices and operating machinery, since the highest pressure and velocity changes occur in these locations. For example, in hydraulic presses the pressure can be up to 50 MPa, in construction and earth-moving machines up to 25-32 MPa, in machine tools up to 2.5-6 MPa and in control devices 0.5-0.6 MPa. These machines therefore require fluids with highest standards of performance and cleanliness.
1
LOW PRESSURE FLUID
HIGH PRESSURE FLUID
6
Fig. 5.27. Outline of the hydraulic system with 4-way direction of flow (pressure) of fluid
I - fank, 2 - deaerating, 3 - deaerating valve, 4 - hydraulic motor, 5 - hydraulic generator, 6 - 4-ways valve
The most elementary criterion of the fluid is its viscosity. The lubricating and sealing ability of a fluid improve with increasing viscosity, but high viscosity slows down the response of the system and increases internal friction, pressure gradient, performance losses and working temperature and necessitates the use of larger pumps. Cavitation effects generated on the suction side of the pump may be caused by inappropriate fluid viscosity. Fluids which are too viscous may cause voids in the piping, so that the pump has insufficient suction capacity, the oil flow becomes restricted and pump wear increases. In choosing fluid viscosity, the requirements of the components of the hydraulic system, which may differ in design, must be taken into account as well as the basic factors such as speed, load and temperature. Working temperatures may vary over quite a wide range, especially with outdoor systems, so that changes in viscosity with temperature must be allowed for. The fluid must have adequate low-temperature fluidity (pumpability) to facilitate start-up of the pump and its viscosity at the highest operating temperature must not fall below the minimum for proper lubrication. Typical viscosity characteristics of different types of hydraulic fluid are shown in Table 5.53. The viscosity indexes of hydraulic fluids are mostly high. The VI of generalpurpose mineral oils is around 100, whilst that of improved and synthetic oils is as high as 160-170. VI improvers used must be highly shear-stable so that oils containing them have the lowest possible permanent viscosity loss. The magnitude of temporary viscosity loss must also be taken into account since the oil in sliding-
563
Table 5.53. 'Qpical Viscosity Characteristics of the Most Common Hydraulic Fluids - Swedish Specifications Viscosity (mm2.s-') at 40 OC Pour-point ( "C) max. Max. permitted oil temperature ( "C) to maintain min. viscosity of 10 mm2.s-l' Lowest temperature ( "C) at which pump start is possible (max. oil viscosity of 1500mm2.s-1)** Min. oil viscosity (mm2.s-'/98.9"C) after 50 cycles in Bosch injector (DIN 51-382) Max. oil viscosity decrease after Bosch injector test (%) Max. viscosity (mm2.s-') at: -17.8 "C -28.9 "C
SH 15
SH32
SH46
SH68
15 -39
32 -39
46 -39
68 -24
50
75
70
85
-30
-25
-20
-5
3.2
6.0
5.5
7.5
20 1500
20 3500
25 1500 -
25 4000 -
The viscosity of the oil after the Bosch injector test (by DIN 51-382) should be 10 mm2.s-]min.. ** The lowest possible oil temperature at which the pump starts may depend on the design of the suction side.
Table 5.54. Poclain Specification for Hydraulic Oils Prooertiedtests Kinematic viscosity (mm2.s-')
at 40 "C at 100 "C
VI Kinematic viscosity after shear stress, min.
Method
Poclain specification
NF T 60-100
46-50 8.3 min. 150 min. 11/85 "C 10185 "C 7
NF T 60-136 DIN 51-382 POC Q-40899-20 NF E 48-614 ASTM D-2619
Air release at 50 "C (minutes),max. Hydrolytic stability copper weight loss (mg.cm3),max. water acidity (mg KOHg'), max. Foaming tendency/stability, max.NF T 60-129 at 24 "C at 93 "C at 24 "C after 93 "C Flash-point (o.c., "C), min. NF T 60-1 18 Pour-point ( "C), max. NF T 60-105 Compatibility with elastomers (vol.% change),max. NF E 48-610 Copper corrosion (3 h at 100 "C), max. NF M 07-15 ASTM D-665 method A Rust protection 4-ball test (40 kgll Id17 mm.s-') NF E 48-617 FZG test (A/20/8.3/90) DIN 51354 Aniline point ( "C), min. NF M 07-021 Water separation (minutes), max. NFT 60-125 Concentration (96 weight) of Zinc Phosphorus Aciditv index. max. NF T 60-1 12
564
0.5
6 30/0 3010 3010 180 -36 12 lb
no rust scar dia. 0.6 max. load stage 11 pass 95 20 0.06-0.08 0.05-0.07 1.4-1.6
vane pumps is subject to high shear-stress in the by-pass valves and on the sliding vanes. Oils for hydraulic systems for outdoor service, where wide temperature differences are met must also meet high standards in this respect (e.g., in hydraulic systems for construction machinery). This is illustrated in Table 5.54, which lists the requirements of the Poclain specification for oils for hydraulic systems. VI improvers must always be carefully selected. Those which have adequate shear stability for motor oils may be inadequate for hydraulic fluids, where the required standards of permanent shear stability may be much higher (134).
Because of the high pressure in hydraulic systems, viscosity should not be very dependent on pressure, particularly at low temperature when viscosity may substantially increase; a high VZ is obviously desirable for this reason also. The viscosities of oils which have high viscosities at atmospheric pressure show a greater tendency to increase with increasing pressure. For instance, the viscosity of an oil with an atmospheric pressure viscosity of 140 rnm2.s'l increases 3-fold under a pressure of 35 MPa, whereas an oil with atmospheric viscosity of 33 rnrn2.s-' increased only two-fold. At higher oil temperatures, the change in viscosity with increasing pressure is less fig. 5.28). 400 7a 350N'
E
300250200 150 -
loo50 -
la
50
100 MPo
Fig. 5.28. Change of viscosity of a typical mineral hydraulic fluid with pressure at different temperatures
The choice of fluid viscosity is particularly dependent on the type and design of the pump and the motor. The viscosity ranges required for various pump types are indicated in Tuble 5.55. This shows that the viscosity classes of hydraulic fluids are wide and should generally be between 7 and 80 mm2.s-l at 50 "C. The lowest viscosity should never be lower than that indicated, because there is a tendency to use extremely fine tolerances at increasing pressure in hydraulic systems. Table 5.55. Viscosities Recommended for Hydraulic Oils for Various Pump Qpes (mm2.s-1) Pump type
Sliding-vane Piston Gear or screw
Maximum recommended viscosity at lowest operating temperature
Minimum recommended viscosity at highest operating temperature
800-1500 500-2500 1200-2500
10-15 4-15
5-10
565
Pump makers usually define the limits for viscosities of fluids for starting and full operation of their pumps, working under different outdoor temperature conditions. Sperry Vickers Co. prescribe the following viscosities for fluids and oils for their mobile hydraulic systems: - Engine oils - SAE 5W, 5W20 and 5W30 at outdoor temperatures of -23 to 54 "C (5W or 5W20 in the Arctic) -18 to 83 "C SAE 1OW ................................................................... SAE 10W30 ............................................................... -18 to 99 "C SAE 20W20 ............................................................... 10 to 99 "C - Hydraulic fluids containing anti-wear addtives IS0 22 at outdoor temperatures of -21 to 60 "C IS0 32 ............................................ -15 to 77 "C IS0 46 ............................................... -9 to 88 "C IS0 68 ............................................... -1 to 68 "C Special polyalphaolefin or ester synthetic oils or their mixtures are recommended for Arctic use. Abex Denison Co. prescribe the following viscosity limits (mm2.s-') for their pumps:
Piston Pumps Maximum viscosity at cold start Maximum viscosity in full operation Optimum viscosity at working temperature Maximum viscosity at full performance
1618 162 30 10
Sliding-vane Pumps
1618 108 30 10
Because of the high pressures involved, low viscosities do not always ensure full lubrication and the suppression of wear on the friction surfaces. Therefore, hydraulic fluids contain anti-wear additives, mostly sulphur and phosphorus compounds such as ZDDP, which also suppress both oxidation and corrosion, and, as rule, lubricity additives. These additives should help achieve trouble-free operation of all components, particularly the control system items. The lubricity additive prevents stick-slip in machine tools, where guideways operate at a low feed speed and are lubricated with the hydraulic system fluid. In addition to ZDDP in conventional zinc-containing hydraulic fluids (with either high - 0.07% weight min. - or low - 0.03 to 0.045% weight max. zinc content in the fluid), ashless anti-wear addtives, such as arylphosphates (e.g., tricresyl phosphate ot triphenylphosphothionate)may be used (178).They are used at concentrationsof 0.5 to 1% by weight in oil. However, a suitable ashless antioxidant, usually an alkylphenol type (around 0.5% weight) and a rust inhibitor (up to 0.1% weight) must also be added. The cleanliness of systems with long-life fluid-fill is safeguarded with detergentdispersant additives; this is particularly applicable to the hydraulic systems in mobile machinery, but also to some stationary machines such as machine tools and generally to machines working in a moist environment. The detergent-dispersant disperses and renders harmless both oxidation products and water in the fluid. The fine dispersion generates a stable emulsion which gives the oils a milky, cloudy appearance. Hydraulic fluids containing DD additives are labelled H-LPD fluids. For rapid and precise functioning of a hydraulic system, it is essential that the power fluid be practically incompressible, although slight compressibility dampens 566
pressure surges caused by switching and thus provides smoother operation. Compressibility depends, especially, on the content of dissolved air and other gases, which increase it. The fluid itself is less important in this respect. Compressibility increases with increasing pressure. At pressures up to about 30 MPa, fluid bulk decreases by about 0.5% for every 7 MPa; at pressures up to 40 MPa by about 3% and at 100 MPa by about 5.2%. These values apply over the temperature range 10 to loo "C. Fluid compressibility is also related to the change in density with increasing pressure. It is estimated that fluid density increases by 0.01 for a pressure rise of 14 MPa (fig. 5.29).
Fig. 5.29. Change of water and mineral oil density with the change in pressure
Air separability also depends on pressure. High-quality mineral oil dissolves 810% by volume of air at 20 OC and atmospheric pressure. Air solubility and fluid compressibility increase with increasing viscosity and increasing pressure @g. 5.30).
1
2
3
4
5
PRESSURE (MPo 1
Fig. 5.30. Dependence. of air solubility in hydraulic oil on the pressure and temperature of oil Air enters the hydraulic fluid mainly at the initial filling, on draining and re-filling and during repairs to the system and replacement of piping. This produces air pockets in the cylinder, motor and upper pipework. This air is compressible and dissolves in the fluid if the pressure increases. During operation, the fluid comes into contact with air in the fluid reservoir. Air may also be sucked into the system through leakage in the reservoir.
567
Increasing air content of the fluid may have adverse effects on the performance and function of the pump, causing a rise in its noise level and vibration. Contact of the fluid with air must be minimised by design of the system. This particularly applies to the pump suction side and the intake point for the returning oil below the fluid level in the reservoir. Air separability can be determined by ASTM D-3427or DIN 51381. The solubility of air in oil can be expressed as:
vv = V Q a p
(5.9)
where Vv is the volume of dissolved air at saturation (m3), Vo is the volume of oil, a is Bunsen's coefficient (0.08-0.1 between 0 and 100 "C for hydraulic fluids), p is the working pressure of the fluid (Pa). The volume of dissolved air can be calculated by DIN 5 1381 from the relationship: L=
(Po-PJ * 100 Po - PL
(5.10)
where L is the concentration of the dispersed air (air bubbles) in the fluid (%), pa is the density of the bubble-free fluid (kg.~n-~), px is the density of the fluid containing the dispersed air(kg.~n-~), pL is the density of air at test temperat~re(kg.m-~).
With decreasing pressure, air is released from the fluid and causes instability of servo-systems, inaccurate operation of the mechanisms, decrease in power transmission, high noise levels and erosion by cavitation when the bubbles burst. Generally, cavitation produced by bursting of vapour bubbles is distinguished from that caused by air bubbles. However, the vapour and air bubbles are produced simultaneously and when the pressure drops, the resulting bubbles contain a mixture of fluid vapour and air. Pressure drop in the fluid may occur in the pump suction piping, behind throttle valves, behind sharp bends in piping, etc. It is generally held that air separates more readily from less viscous fluids. This is only true if all the air bubbles are of the same size. Small bubbles may be produced in the pump in a fluid of lower viscosity. These bubbles then rise more slowly to the surface of the fluid in the reservoir.
Foaming is commonly experienced and this causes all the properties mentioned to deteriorate. Foaming tendency can be suppressed with an anti-foam agent, which most hydraulic fluids contain. These agents can sometimes reduce separation of air from oil. Slow separation of air and consequential fluid foaming may arise for other reasons, such as admixture with other fluids (oil), physical contaminants, residual traces of materials used for cleaning the system, etc. Inadequate design of the circulation system can have adverse effects, as can too high a fluid circulation factor, inadequate fluid return to the reservoir so that the fluid does not have sufficient time to settle, and streaming of fluid in the reservoir, so that the pump mostly sucks in freshly-returned fluid and only part of the fluid charge is working. The compression of a fluid containing air bubbles may generate adiabatic heating, to temperatures as high as 700 "C,and increase its pressure by about 7 MPa. Despite the fact that only minute bubbles heat up like this and the rest of the fluid 568
bulk remains unheated, the immediate surroundings of the bubbles cannot withstand this level of temperature and accelerated ageing commences, the fluid becoming dark and emitting a characteristic scorching odour. This odour and the darkening of the fluid are the consequence of the spontaneous ignition of the fluid-air mixture in the air bubbles due to a violent increase in the temperature of the air in the bubbles (the so-called Lorentz effect).
This is also one of the reasons why the fluid should contain an antioxidant additive. Another reason is to prolong the service-life of the fluid, which works under significantly high temperature and pressure conditions, in mixture with air and in the presence of catalytic metals and moisture. The fluid must not give rise to appreciable amounts of soluble or insoluble products of ageing, which would increase its viscosity, clog the filters, piping and narrow passages and promote corrosion, emulsification and foaming. The condition of the fluid must therefore be closely monitored and drained whenever the acid number - the symptom of sludge formation - rises appreciably. The oxidation stability of hydraulic fluids can be determined by ASTM D-943 or DIN 51-587. In the GIGRE test, IP-280, the oil is oxidised in air-jet in the presence of copper and steel for 163 hours. The increase in acidity and the amount of sludge and volatile acids are determined. The CERL test, IP-331, made under the same conditions,is used to assess the oxidation and corrosion resistance of fresh hydraulic fluids which contain antioxidants,anti-corrosionand anti-wear additives. In addition to the determination of volatile and soluble products, deposits and insoluble corrosion products, corrosivity of the fluid may be gauged by weighing the two metal test pieces after the test. The thermal stability of hydraulic fluids at elevated temperatures, particularly those containing ZDDP, must also be monitored (thermal decompositionof ZDDP which contain alkyl radicals, used as anti-wear additives, yields fluid-insolubleproducts which, together with the products of decomposition of the fluid, can clog the functional parts of the system). For this purpose, the Cincinnati Milacron Thermal Stability Test is suitable. 250 ml of the test fluid is exposed in the presence of copper and steel to a temperature of 135 "C for 168 hours. Deposits (25 mg max.) and copper loss (10 mg max.) are measured, together with the development of acidity and viscosity increase.
Vupour pressure at elevated temperature is an important property of hydraulic fluids. High vapour pressure may cause cavitation and so increase erosive wear. High vapour pressure hydraulic fluids (e.g., oil emulsion in water and water solutions of glycols) can therefore only be used at lower temperatures.
Volumetric expansion with increasing temperature can be a significant factor for hydraulic systems with large charges of fluid. At atmospheric pressure, the volume expands by about 0.7% for a temperature increase of 10 OC (a temperature increase of 60 "C increases the volume of a 200 1 charge of circulating fluid by 8 I). The coefficient of expansion decreases with increasing temperature. Fig. 5.32 depicts the relationship between hydraulic fluid volume, temperature and pressure. In some hydraulic systems, penetration by water cannot be avoided. Moisture accelerates ageing of the fluid, enhances corrosion and promotes the formation of
569
emulsions, which often take the form of thick sludge, impedes fluid flow, blocks control elements and causes them to malfunction. Fluids must, therefore, be capable of separating from water (which can be achieved with suitable additives, mainly polymethacrylates). The fluid circuit must incorporate water separators, in addition to filters for separating physical contaminants (dust, wear debris). However, a small water concentration cannot be avoided in any hydraulic system. Since mineral oilbased fluids are usually highly-refined, they have insufficient natural rust-inhibitory properties (particularly at low viscosities) so they are usually dosed with anti-rust agents. Many of these react with zinc and this cannot be used (e.g., in the form of electroplated surfaces) as a material of contstruction for hydraulic systems.
110
1 I
0
1 0 . l Mrn
50
100 150 TEMPERATURE PC)
Fig. 5.31. Correlation between the increase of hydraulic fluid volume and temperature and pressure
Water dissolves only slightly in mineral oil (about 0.02% of water dissolves in oil at 50 "C and atmospheric pressure). It tends to separate in circulating oil as droplets, then settle in the reservoir if undisturbed to form a water-in-oil emulsion. The water content of non-emulsifiable fluids should not exceed 0.2% by weight, since water promotes wear, particularly in bearings (pitting) and, in steel-bronze couples, rusting and oil ageing, associated with sludge formation and oil foaming. In addition, emulsions can alter the viscosity of the fluid and impair its filtrability, in a similar way to sludges. Water may also cause the control elements to freeze. If contaminationof the system with water cannot be avoided, oils containing detergentdispersant additives must be used. Oils containing ZDDP are particularly vulnerable to the effects of water. Some ZDDP hydrolyse with the production of substances which have a strongly corrosive effect on copper and its alloys (copper and copper alloys are widely used for axial piston pumps). Also, hydrolysed ZDDP loses its effect as an anti-wear additive for steel friction surfaces (279,180). ASTM D-2619 specifies a test for the hydrolytic stability of hydraulic fluids. Water plus I% by volume of oil are shaken for 5 minutes; the mixture is poured into a gauge cell and left to stand for 168
570
hours at 21-27 “C. The amount of emulsion and free water, the clarity of the water and the amount of sediment are estimated.
Severe requirements are laid down for the content and size of physical contaminants, which may increase wear of the friction surfaces. Concentration of these materials, determined with a suitable diaphragm filter, should not exceed 0.008% by weight. The maximum permissible size of the particles is related to the smallest clearances in the orifices through which the fluid must flow. This size is controlled by the fineness of the filter used for cleaning the fluid entering the system. Particle ( B in f i g 5.32) substantially smaller than the critical size of the orifice ( E ) cannot cause a sudden breakdown of the system. However, they may gradually cause abrasion of the valve and slide valve edges and other parts. Particles larger than the critical size ( E ) can block the inlet to the orifices and put the filter element out of action. Particles with sizes (C, D)equal to the critical size pose the biggest wear risk; they may bring about sudden breakdown and wear of the system. 100 cm3 of fresh hydraulic fluid may contain around 16 solid particles up to 10 pm and several hundreds or thousands of particles up to 100 pm.
Fig. 5.32. Dependence between the size of particles and the smallest cleafence of the openings
Since trouble-free operation and service life of the system depend on the cleanliness of the hydraulic fluid, the builders of hydraulic systems and standard specifications define the limiting values for the amount and sizes of particulate contaminants. DIN 51524 and 51525 require 0.05% weight maximum, i.e., about 45 mg per 100 cm3, solid substances in fresh hydraulic fluids. Some specifications also define the maximum permissible concentration of particles of particular sizes. U S NASA 1638 classifies hydraulic fluids by their degree of purity into 12 classes (see Table 5.56). SAE 749 into 7 classes, British commercial organisations into 9 and Comecon into 19. “Pure” and “super-pure” hydraulic fluids and oils are also available. The requirements for fluid purity are similar to those given in MIL-H5606A (page 488) for aircraft hydraulic fluids. An I S 0 purity code is under preparation. The purity of fluids is defined by the number of particles up to 5 pm (evidence of the amount of sludge) and up to 15 pn (evidence of potential for increased wear). The advantages claimed for this code are its universality, the classification of particles into the two most important size ranges, the unification of all ranges of contaminant concentrations and the identification of degree of contamination with only two numbers (139,140). Automatic counters can be used (e.g., HIAC Pacific Scientific Inst. USA) for counting the number of particles in a fluid volume and measuring their size distribution; the counter can be used for both fresh
57 1
and used fluids. Morphological analyses of the particles can be made with an automatic shape analyser (e.g.,Omnicon Fas 11). The ferrographicmethod, i.e., analysis of the particles in a strong magnetic field, in which they are classified into ferromagnetic particles and others, has also been recommended.
Table 5.56. NASA 1638 Classification of Hydraulic Oils by Contaminant Content Class
00 0 1 2 3 4 5 6
7 8 9 10 11 12
Number of particles Particle size range (pm)
5-15
15-25
25-50
50-100
>lo0
125 250 500 1000 2000 4000 8000 16ooo 32000 64000 128000 256000 512000 1024000
22 44 89 178 356 712 1425 2850 5700 11400 22800 45600 91200 182400
4 8 16 32 63 126 253
1 2 3 6 11 22 45 90 180 360 720 1440 2880 5760
0 0 1 1 2 4 8 16 32 64 128
506
1012 3025 4050 8100 16200 32400
256
512 1024
Compatibility with the materials from which seals, hoses and filter cartridges is another essential property of hydraulic fluids. Fluids must not alter the volume and hardness of these elastomers when they are in contact. The quality specifications for hydraulic fluids normally indicate limiting values for changes produced in this way. Above all, hydraulic fluids must not cause seals to shrink, as this would cause leakage of the fluid and loss of efficiency of the system. The magnitude of the effects of the oil on these materials depends on the composition of the oil and its additives, the viscosity of the oil and the composition and properties of the elastomeric materials. Low-viscosity cycloalkanic and aromatic oils cause swelling of of elastomers, whilst alkanic oils cause shrinkage and sulphurbased additives may cause hardening. The adverse effects of alkanic oils can be reduced, to some extent, by swelling agents, e.g., triarylphosphates. The interactions between oil and elastomers is aggravated at increased temperature. The composition of most hydraulic oils may be divided into mineral and synthetic oils, water, water solutions and emulsions. Mineral hydraulic oils are prepared from high-quality raffinates of good oxidation stability and high VZ.The choice of a suitable oil should be made according to the working conditions in the system, which varies in the amount of power transmitted, the design of the system, temperatures, etc. For instance, high-quality, additive-free bearing oils (low pour-point oils in a low-temperature environment), or, better still, long-service oils (e.g., turbine oils) containing rust and oxidation 572
inhibitors, can be used in hydraulic presses and machine tools, etc., where the temperatures of the power fluid do not exceed 60 "C and a short service life (around 3,000 hours) of the fluid is expected. These oils may also be used for systems in which the temperature exceeds 60 "C, such as governing devices, hydraulic motors, etc. Special hydraulic oils containing the above additives are needed for highperformance hydraulic systems under heavy thermal and pressure-stress, incorporating slide-valve manifolds and high-speed, enclosed oil circulation, where long service-life of the oil charge is expected (over 10,OOO working hours). The various quality standards reflect this differentiation of hydraulic oils. Oil-insoluble oil oxidation (ageing) products form sludges and deposits, foul the pump vanes and clog filters and piping. Soluble ageing products separate partially in low-temperaturezones and can foul or even block hydraulic fluid coolers. Contamination of hydraulic fluid may also be caused by the penetration into it of other oils, if these oils are used for the lubrication of other components (e.g., slideways and gears in machine tools, etc.). Such oils may contain additives which may cause corrosion of parts of the hydraulic system.
DIN 51-524 classifies hydraulic mineral oils into groups H, H-L and H-LP. L signifies oils containing antioxidants and anti-corrosion agents and P those containing anti-wear additives. H oils are intended for systems in which hydrostatic power and moderate pressure predominate. HL oils are designed mainly for moderate-temperature hydrostatic systems where frequent drain-intervals would be unavoidable with an H oil or for systems where water-corrosion may develop. H and H-L oils cannot be used for hydrodynamic power transmission unless they meet specified requirements, neither are they suitable for automatic transmissions for cars, power-assisted steering or hydraulic brakes, for which special engine oils are specified by the builders. H-LP oils are applicable mainly in hydrostatic power systems with high thermal loads and corrosion hazard which require oils with increased anti-wear capabilities to decrease wear of the pump or the friction surfaces in the hydraulic motor. The same guidelines as for these oils apply to oils used in hydraulic power transmissions, powerassisted steering and brakes.
The French classification AFNOR NFE 48603 includes performance classes HH, HL, HM,HR and HV and eight viscosity classes (10-150 rnm2.s-l at 40 "C). This classification of hydraulic mineral oils, and the recommendations of RP 75H and the ComitC EuropCen des Transmissions OlCohydrauliques et Pneumatiques (CETOP), roughly correspond to IS0 TC 28/SC 4 (see Table 5.57). The anti-wear properties of hydraulic oils can be evaluated by a 250 hour test in a Vickers 104-C or 105-C sliding-vane rotary pump in a closed circuit through which the test oil is circulated. The oil viscosity is maintained at 13 mm2.s-' (it must not drop below 10 mm2.s-') and at 14 MPa pressure. This test is used in Europe according to Vickers XO-02 technical data. Ring wear must not exceed 200 mg and vane wear 50 mg with an oil of good anti-wear capabilities (with pure mineral oil, the pump stops within 10 hours because of excessive wear. With oils containing low concentrations of anti-wear agent, loss of ring and vane material should not exceed 350 mg and 150 mg, respectively). ASTM D-2882-90 and lP-281/725 also prescribe tests in the same Vickers pumps. Similar tests are carried out in the Abex-Denison T5D-042 sliding-vane pump (2.400 r.p.m., 16 MPa, 7 1.1 ' C in the first phase and 98.9 "C in a second phase) and in an Abex-Denison 46 piston pump (2,400 r.p.m., 36 MPa, 71.1 "C f m t phase, 98.9 "C second phase).
573
-
Table 5.57. EORC 28/SC4 Quality Parameters of Hydraulic Oils
P
Quality parameter
Units
GROUP
Test method
HH 10
Kinematic viscosity at 40 "C
mm2.sec1 ISOlDIS 3104 from 9 to 11
Dynamic viscosity
mPas
Viscosity index (min.)
15
22
32
HL 100
150
10
15
13.5 19.6 28.8 41.4 61.2 90 16.5 24.2 35.2 50.6 74.8 110
135 165
9 11
13.5 19.6 28.8 41.4 61.2 90 16.5 24.2 35.2 50.6 74.8 110
46
68
22
32
46
68
100 150 135 165
DIN 53015 E & ASTM D-2893 Isor2909
95
Density
kg.~n-~ ISOlDIS 3675
Pour-point (min.)
"C
IS03016 -15
Flash-point (min.)
OC
IS02909 140 165
95
90
90
suppliers to report -15
-15
-12
-12
-9
-9
-6
180
-15
-15
140
165
-15
-12
-12
-9
-9
-6
180
Water content (max.)% weight ISOlDIS 3733 & ASTM D-1744 (Karl-Fischer)
Waterseparation (demulsification)
cm3
0.1 40 - 37 - 3 to 30 min.
ASTM D-1401
Foaming at 24 "C. cm3 max. ASTM D-892 93 "C & 24 "C after 93 "C)
Deanation (max. minutes time at given temo.) O C
Tendency 300, Stability 10
2
DIN 51381
25
NeutralisationNo.
mgKOWg ASTM D-974
Comercorrosion
(max.) max.
IS0 2160
I_ 25
2
2
50
50
10
50
10
50 5
indication of reaction
0.05 15
@ 75
_____
Anti-rust action Oxidation stability
ASTM D-665* mgKOWg ASTM D-943 after lo00 h
Max. acid number increase 2 mg KOWg after lo00 h
_____
Compatibility with elastomers Anti-wear Vickers 4-ball
~
% change in hardness
mg
mm
Shear stability
~
to be supplied
ASTM D-28882 ASTM D-2266 DIN 51-382
GROUP
HM Kinematic viscosity at 40 "C
mm2.s-'
HV
10
15
100
150
10
15
9
13.5 19.6 28.8 41.4 61.2 90 16.5 24.2 35.2 50.6 74.8 110
135 165
9
13.5 19.6 28.8 41.4 61.2 90 16.5 24.2 35.2 50.6 74.8 110
22
32
46
68
from to rnPa.s
Viscosity index (min.)
11
DIN 53015 E & ASTM D-2893
46
68
100
11
~
at -20 "C (or at -10 "C or -5 "C) at -10 "C 05 -5 "C 95
ISW2909
Density
kg.mS3
ISO/DIS 3675
Pour-point (min.)
"C
IS0 3016
-18
-18
Flash-point (min.)
"C
IS0 2909
140
165
Water content (mu.) % weight ISO/DIS 3733 8c ASTM D- I744 (Karl-Fischer) m 4 m
32
ISO/DIS 3104
~
Dynamic viscosity
22
130
90
90
suppliers to report -15
-15
-12
-12
-9
-9
180
0.1
-42
-39
140
165
-36
-35
-30
27
-24 180
VI
2
(Table 5.57 contd.) Quality parameter
Units
Test method
GROUP
HH 10
cm3
Foaming at 24 "C,
cm3 m a . ASTM D-892
Deaeration ( m a . time at given temp.)
minutes "C
Neutralisation No.
mgKOHlg ASTM D-974 (mu.)
Copper corrosion
max.
Anti-rust action
DIN 51381
32
46
HL
68
-5 - 5 _ 5 _ 5
! o @ g g
25
50
25
50
mg mm
ASTM D-28882 ASTM D-2266
15
22
32
50
50
75
75
indication of reaction Ib
ASTM D-665*
96 change in hardness
10
150
to be supplied
no trace of rust
mgKOWg ASTM D943 after lo00 h
Anti-wear Vickers 4-ball
100
40 - 37 - 3 to 30 min.
IS0 2160
Compatibility with elastomers
Shear stability
22
ASTM D-1401
Water separation (demulsification)
Oxidation stability
15
Max. acid number increase 2 mg KOWg after lo00 h to be. supplied
0
DIN 51-382
*Method A (fresh water) or method B (seawater) for specific applicahons.
be. supplied after defining test method
46
68
100
150
FZG tests can be instructive, despite the fact that correlation between Vickers and FZG tests have not been sufficiently established. HL-P hydraulic oils should satisfy at least load-stage 9 in the FZG, for use in a wide-gear (20 mm) gear-box at 8.3 r.p.m. and oil temperature 90 O C (good oils achieve passes at load-stages 10-12). In four-ball machine screeningtestsby ASTM D-2266 at loads of 294 or 392N (30 or 40 klb), duration I hour, scuffing scar diameter is assessed (there are various modification of load and test duration). This test is, however, only suitable for hydraulic oils containing ZDDP additives.
Limiting values of quality parameters of high-quality hydraulic oils meeting the requirements of hydraulic system manufacturers are detailed in Table 5.58. Fire-resistant hydraulic oils are suitable for the following temperatures (see also Table 5.52): HFAE - (oil-in water emulsions) over the range 5 to 55 O C , HFB - (water-in-oil emulsions) from 5 to 60 OC, HFAS - (water emulsions of polymers) from -20 to 60 O C , HFDR - (water-freesynthetic fluids - polyglycols, phosphoric acid esters, silicic acid esters, chlorinated hydrocarbons and their mixtures) from -20 to 150 "C. Water itself suffers several deficiencies. It lacks lubricity and does not protect against corrosion. Despite this, it is used in large systems which cannot be made leak-proof, such as forging presses incorporating plunger pumps, valve manifolds and leather glands, and for systems presenting a fire hazard (like spray presses) or in environments where fire hazard must be avoided, such as in mines. Generally, it is used where leaking fluids may contact hot surfaces or naked flames.
Table 5.58. Limiting Values of Quality Parameters - High Quality Hydraulic Oil Quality parameter
Limiting value
Viscosity index, min. Neutralisation number (mg KOWg), min. TOST test (ASTM D-943) - time (h) to reach neutralisation number 2 mg KOWg, min. - properties of oil after 1000 h neutralisation number (mg KOWg), max. total sludge (mg), max. total Cu (mg).max. total Fe (mg),max. Thermal stability: Cincinnatti Milacron Heat 'Test (168 h at 135 "C) copper corrosion (ASTM D-130) (3 h at 120 "C), max. Anti-rust (ASTM D-665) Hydrolytic stability (ASTM D-2619) (Cu weight loss, m g . ~ m - ~max. ), -total water acidity (mg KOWg), max. Demulsification (ASTM D-1401), time to reach 40-37-3 cm3 emulsion, (minutes), max. Anti-wear (a) FZG test (A20/8.3/90/1), load stage for pass, min. (b) Vickers test, ring and vane weight loss after 24 h at 14 MPa, (mgh max. Filtrability by Denison test (s), max.
90 0.5 1500 1 .o
200 50 50
pass Ib pass 30 4.0 30 12
100 600
577
Oil-in-water emulsions (up to 20% oil) are less flammable than oils and have better lubricity and can be less corrosive than water. Stable water-in-oil emulsions (up to 60% oil) have better lubricity still and are still of low flammability (self-ignition temperatures about 430 "C). They are the most used non-flammable, water-based fluids. The lubricating components in these compositions are low-viscosity oils (1 5-30 m m 2 s 1at 50 "C), but the emulsions have much higher viscosities and are nonNewtonian. Their viscosity decreases with increasing shear stress. The stability of the emulsion depends on the emulsifier. Oil-soluble sulphonates, carboxylates or phenolates with C , , and longer alkyls are used. The water droplets should be uniform in size, about 2 pm diameter. These emulsions can be improved by the addition of sulphur-based anti-wear additives (e.g., dibenzyl disulphide, dithiophosphates, dithiocarbamates, alkylated xanthates) or chlorinated anti-wear additives (chloroparaffins, etc.). The oil component normally contains a lowtemperature antioxidant. Corrosion inhibitor (derivatives of amine carboxylic acids) is added to either water or oil. Because of their superior lubricity, emulsions can be used for hydraulic systems incorporating sliding-vane and gear pumps, slide-valve manifolds and hard or polychloroprene glands.. They have good viscositytemperature characteristicsand they are relatively cheap. Their deficiency is a lower sealing capacity and a lack of operational stability and capacity to expel solid contaminants (because of their relatively high density). The anti-wear characteristics of emulsions can be evaluated with the Four-ball or Falex testers.
Few problems are experienced with polymer-water emulsions, in which solutions of higher polyethylene or polypropylene glycols are used. They can contain oxidation and corrosion inhibitors as well as anti-wear agents. They are of limited flammability, their self-ignition temperature being about 650 "C. Their viscosity varies with water content, decreasing as water concentration goes down. Their lowtemperature fluidity and viscosity-temperature properties are good. Their effect on elastomers is relatively small but they are harmful to paints. Zinc and cadmium promote corrosion in the presence of water. Stainless or nickel- and tin-plated metals are better in these situations. General-purpose elastomers, particularly polychloroprene, are convenient sealant materials; impregnated cork and leather are unsuitable.
Water solutions of polyglycol oils with a minimum water content of 35% by volume are used as non-flammablepressure fluids (HFC) for mining equipment over the temperature range -20 to 60 "C. Two types of polyglycol oil are available commercially (Table 5.59). Type A is fairly compatible with mineral oils and limited miscibility with water (10-25% without phase-separation). The solubility of polyglycol in mineral oil or water depends on the oxygen content in the molecule - the lower the oxygen content, the better the oil-solubility and the lower the water-solubility.5 p e B has fairly good 578
compatibility with water and limited solubility in mineral oil (it can absorb 0-200% of water without phase-separation). The solubility of polyglycol in water is determined by the capacity of the oxygen to hydrogen-bond water molecules via ether and hydroxyl end-groups. Water-insolubleproducts result if the oxygen in the hydrogen bond is screened by a methyl or an ethyl group.
Table 5.59. Comparison of H-LP w p e Polyglycol Synthetic and Mineral Hydraulic Oils Properties
Synthetic oils Type B Type A 150-170 160-175
Viscosity index Shear stability (% viscosity loss) 5-15 Operational temperature range ( "C)120 to -40 -55 to -65 Pour-pint ( "C) Air separation (DIN 51-381), minutes at 50 "C 0-3 Anti-wear (Vickers test by ASTM D-2882,250 h, 50 "C. 1450 r.p.m., 14 MPa) wear in mg of vanes 0-2.6 rings 0-12
Mineral oils
120 to -40 -55 to -65
95-120 5-20 90 to -30* -20 to -50*
2-5
5-10*
1.0-2.8 5-12
4.0 (average) 18
0-10
Depending on viscosity of oil.
Fire-resistant esters of phosphoric acid (aryl phosphates) or silicic acid are mostly used in aircraft hydraulic systems. Since they are very expensive, they are rarely found in industrial hydraulic systems. They are almost exclusively used for environments with a high fire risk, their self-ignition temperature being about 650 "C. npical commercial products for use in industrial hydraulic systems based on phosphoric acid esters have viscosity around 4.5 mPa.s at 50 OC and 1.8 mPa.s at 100 "C, pour-point below -20 "C and density about 1.3 at 15 "C, which ensures good lubricating properties. These ester oils usually contain Vf modifiers, rust inhibitors and anti-foams, even though they dissolve additives with difficulty. They are harmful to most sealants and paints; butyl rubber is a suitable sealant material. Chlorinated aromatic hydrocarbons, mainly tri- and tetrachlorbiphenyls containing 40-5096 chlorine have some useful properties, however, they are aggressive to all elastomers, toxic, non-biodegradabel and environmentally unacceptable. Polychlorbiphenyls(PCB) are carcinogens (0.1 p.p.m. at 42% chlorine, as low as 0.05 p.p.m. at 54% chlorine). In addition, they interfere with the formation of haemoglobin in the liver. Highly toxic dioxins and polychlordibenzofurans result from chemical and/or thermochemical transformation of PCB at 600 - 700 "C in the presence of oxygen. To destroy these materials incineration at 1,200 - 1,300 "C in a surplus of oxygen is used. The requirements of ecology have led to the introduction of a new range of hydraulic oils labelled according to a tentative supplement of DIN 51 524 H-TG(vegetal oils), H-E(synthetic esters), H-PG (polyoxyethylene glycols) and H-X (others). Their quality must satisfy parameters of highquality
579
hydraulic oils including the biodegradability (90-100% according to CEC-1-33-T-82) and nondegradation of water quality. In general, non-flammable or low-flammability materials can be tested according to the Bureau of Mines regulations. The Association of Hydraulic Equipment Manufacturers (AHEM) specify standard tests for fire resistance of hydraulic fluids in “Fire-resistant Fluidsfor Fluid Power Systems”. The requirements for properties and testing of non-flammable hydraulic fluids for mining equipment in European Community countries were set out in the Luxembourg Report of 15:11:74. CETOP has adopted these requirements and methods. The tests prescribed are divided into compulsory and optional (those intended for special applications, e.g., in mining equipment (124): Compulsory tests - self-ignition temperature (ASTM D-2155-66, DIN 51-794), - flammability test for high-pressure spray (FTMS 791a-6052), - flammability test for fluids in contact with hot metal surfaces (FTMS 791a-6053, CETOP RP 65 H (draft)), - effect of fluid volatility on fluid flammability (FI’MS 701a-352, CETOP RP 64 H (draft)). Optional tests - fluid flammability test for high-temperature spray with screening (5th Luxembourg Report, Section II1.3.1), - fluid flammability test for high-pressure spray in the shape of a hollow cone (ibid, Section VI.6.15), - fluid flammability test in CFR diesel engine with variable compression ratio* (ASTM D61361T, CETOP RP 56 H (draft)), - fluid flammability test in a coaldust environment (5th Luxembourg Report, Section VI.6.16), - wick test (ibid., idem).
The development of the composition and qualities of hydrostatic power transmission oils needs to be considered against the following probable scenario: It can be expected that the working pressures in hydraulic power generators and motors will increase to 40-50 MPa. Such pressures cause the viscosity of the oil to increase and oils of lower viscosities at atmospheric pressure must then be used.This in turn necessitates lower oil volatility to reduce the risk of cavitation caused by oil vapour and dissolved air. The anti-wear requirements of oils will increase with increasing pressure. Oil temperatures are expected to increase up to about 100 “C and pump speeds up to 2,500 r.p.m. This will emphasise the need for oils with high VI (IS0 “HV” group), containing highly mechanically stable VI improvers. Tank charges will be smaller, less than 2-3 times the volume of oil in circulation in the system. Consequently, the oil circulation factor will be greater. At the same time, oil drain intervals will be extended. All this will require oils with higher oxidation stabilities. Cleaner oils, containing particles no bigger than 5 pm will be stipulated.
* The US Navy Bureau of Ships (125) defines as the criterion of resistance of a fluid to ignition that compression ratio at which the fluids ignites; the higher the compression ratio, the higher the resistance to ignition of the fuel (e.g., tricresyl phosphate ignites at a compression ratio of 50:1) The Bundesanstalt f& Materialpriifung developed a bomb test for a similar purpose (126).
580
5.8.1.1 Brake Fluids
Automobiles are now almost exclusively fitted with hydraulically-operated brakes, essentially a hydrostatic system in which the fluid transmitting the power developed by the action of a pedal generates pressure in a master brake cylinder. The master cylinder forces fluid uniformly through the pipework to the wheel brake cylinders, where the pistons press the shoes or blocks against the drums or disks and thus brake the vehicle. Vacuum boosters, which use the suction power of the engine, are used on heavy vehicles in which the power applied to the brake pedal would not achieve the development of the required pressure.
Reliable functioning of the hydraulic system and the braking effect are to a large extent dependent on the properties of the brake fluid. The fluid must operate reliably over a wide range of temperatures, from -50 to 200 OC and more. It is expected to have low viscosity (high fluidity) at low temperatures. High- and low-viscosity fluids can transmit hydrostatic pressure to the same effect but the braking operation consists in transmitting power, i.e., the product of force and piston length, and it is essential that the piston is able to return smoothly and rapidly to the starting position; this is easier if the fluid viscosity is low.
The fluid must have a sufficiently high boiling-point or it must be non-flammable, even when it contains dissolved water, which reduces its boiling-point. During braking, kinetic energy is converted into thermal energy by friction between the braking surfaces. A considerable amount of heat is thus released over a very short period of time. Although this heat is conducted to the rest of the vehicle and much of it is removed by the slip-stream air, the braking mechanism itself heats up considerably. When braking is frequent or prolonged, as in mountain driving and violent braking at high speed, drums or disks can become red-hot. The brake cylinders, and hence the brake fluid, heat up and the brake fluid temperature may reach 150-2Ott "C. Temperatures usually vary around 100 "C; the temperature of the fluid in disk brakes is usually higher.
If the fluid boiling-point is lower than the fluid temperature reached during braking, vapour-locks form in the brake cylinders after the pressure is released, the compressibility of the fluid increases and the brakes fail to operate. Water cannot be completely excluded from the brake fluid, either during operation or filling. After one or two years' operation, the fluid may contain as much as 2.5% moisture. Water reduces the service life of the brake fluid. The fluid should be capable of absorbing water, but this water reduces the boiling-point of the fluid. The size of this reduction depends on the chemical composition of the fluid. The higher the initial, ("dry") boiling-point of a fluid which is miscible with water, the sharper is the rate of decrease, down to a water content of 2%. The rate of decrease with increasing water content after this point to the "wet" boiling-point is almost the same for all fluids of this type. In fluids which contained 2% by volume of water, the boiling-point was lowered by 140 "C in fluids of original ("'dry")boilingpoint of about 300 "C. whereas in fluids of which the dry boiling-point was 210 "C, the observed
581
boiling-point decrease was only 50 "C. The wet boiling-point of both fluids was 160 "C when they both contained 2% by volume of water; in both cases, this fell to 130 "C at 5% water content.
In this respect, highly hygroscopic brake fluids are better than non-hygroscopic, even though their density increases with increasing water content. They absorb more water, but their boiling-points decrease less than in the case of the non-hygroscopic fluids. Moreover, they are more temperature-stable. The increase of viscosity at low temperatures is relatively small - for example, 1% water by volume causes the viscosity at -40 O C to increase to about 30 mm2.s-l and their pour-points are virtually unchanged. In contrast, free (undissolved) water in non-hygroscopic fluids freezes at low temperature, which can cause system failure. However, even with hygroscopic fluids, different chemical compositions and hence different boiling-points result in differences in the rate of lowering of boiling-point with increasing watercontent. Brake fluids must be sufficiently thermally-stable, and they should not decompose so as to evolve gases over the operating temperature range. The fluid must not break down into its components, become cloudy or release deposits at very low temperatures. These properties must persist, even when dissolved water is present in the fluid. For these reasons, the compatibilityof the fluid with water must always be assessed. The appearance of the fluid and its cleanliness must be checked, if it contains 3.5 % water by volume and it has been exposed to low temperatures for some time. Also, the rate of passage of air bubbles must be measured to determine the ease of air separation.
Separation of deposits must not occur, even after prolonged operation at high temperatures;deposits can clog narrow passage-ways in the manifold and even cause the brake system to fail. Brake fluids must therefore possess adequate oxidation stability, sufficient to maintain the pH of the fluid-water mixture in the alkaline region (7- 11.5) and suppress corrosion of the metal surfaces. Pitting-corrosion or roughening of the metal surfaces must be avoided at all costs. Only a very small change in the colour of non-ferrous metal surface is admissable. One of the most valuable properties of a brake fluid is inertness to the metals which it may encounter and it must have minimal effect on the rubber components of brake systems. Brake fluids should cause slight swelling of sealant materials (mostly made now from styrene-butadiene rubbers) in order to provide an effective seal. But excessive swelling can generate increased friction and hence deterioration of the collar and restriction of the passage of fluid between the packing and the walls of the cylinder, which can result in inadequate lubrication of the cylinder slide-ways. In no case must the fluid cause shrinkage and hardening of the collar. Since brake fluids also act as lubricants, they must possess good lubricity and high load-carrying capacity in the lubricant film so as to minimise wear of the cylinder and the friction surfaces of the piston. Conventional hydraulic brakes are designed to have short friction paths, low speeds and high pressures to be applied to the friction surfaces, leading to high pressures in the fluid. At full braking, the pressure can be as high as 15 MPa. 582
All these factors necessitate a high degree of lubricating properties in the brake fluid. Low foaming tendency, rust-preventing properties and miscibility with other brake fluid type are also needed. The SAE has specified the quality standards for brake fluids. The former SAE J 70 classified brake fluids by minimum boiling-point into 3 categories: 150 "C (Rl), 110 O C (R2) and 190 "C (R3).Other specificationswere issued later, including S A E J 70b,J 7Oc and the latest, J 1703 Jan. 80. Federal Motor Vehicle Safety Standard (FMVSS)1 16, with SAE J 1703 Jan. 80, now applies to topquality brake fluids for high performance automobiles (Table 5.60). Table 5.60. Brake Fluid Specifications FMVSS 116 SAE DOT 3* DOT 4 DOT 5'1 1703Jan.80
J 1702f (arCtiC)
Viscosity (mm2.s-') at 4 0 "C, max. 1500 1800 900 1800 1500 1.5 1.5 1.5 3.5 at 100 "C, min. 205 320 Boiling-point ( "C), min. 260 190 150 Thermal stability: change in boiling-point "C, max., 3 3 3 per "C at boiling-point over 225 "C M.05 "C M.05 "C M.05 O C Boiling-point of fluid after water140 155 180 retention test ( "C) pH of fluid-water/alcohol mixture 7.0-1 1.5 7.0-1 1.5 7.0-11.5 Low temperature stability: compatibility with water (6 h at -60"C): transparent, no deposiappearance of fluid time for rise of air bubbles (s), max. 35 35 35 (144 h a t 40 "C): appearance of fluid transparent, no deposits time for rise of air bubbles (s), max. 10 10 10 (further 24 h at 60 "C): transparent, no deposiappearance of fluid 0.05 0.05 deposits at time of testing (96 vol.), max. 0.05 deposits on delivery (96 vol.), max. 0.15 oxidation stability; aluminium weight loss (mg.cm2). m u . 0.05 0.05 0.05 -0 maces of attack or roughening appearance of metal discolouration slight rubbery residues minute compatibility with standard SAE RM or other admissible fluids: transparent, no deposits or layering (after 12 h at 4 0 "C) transparent,no deposits or layering (after another 24 h at 60 "C) deposits at time of testing (96 vol.), max. 0.05 0.05 0.05 volatility (%I weight), max. 80 80 80 appearance of residue after evaporation transparent,no deposits after 5 6 effect on SBR rubber*+ (120 h at 70 OC): change in collar dia. (mm) 0.15- 1.4 0.15- 1.4 0.15-1.4 -
583
(Table 5.60 contd.) FMVSS 116 SAE DOT 3* DOT 4 DOT 5*J 1703 Jan.80
J 1703f (arctic)
decrease in hardness (IRHD)***
10
10
10
-
(72 h at 120 "C): decrease in hardness (IRHD),max.
15
15
15
-
corrosion test (120h at 100 "C), ave. weight loss (mg.cm-'), max.: tin-plated (white) metal steel aluminium alloy cast iron brass copper appearance of metal appearance of fluid deposits (% vol.), max.
0.2 0.2 0.1 0.2 0.4 0.4
0. I
7-11.5 fluid pH appearance of rubber decrease in hardness of rubber 15 (IRHD), max. increase in dia. of rubber collar I .4 (mm), max. test with original components of drum brakes number of strokes at 120 "C after 16,000 strokes at 20 "C 85000
0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.2 0.4 0.4 0.4 0.4 no roughening or pitting very slight colour change 0.1 0.1 7-11.5 7-11.5 no signs of decomposition
-
-
15
15
-
1.4
I .4
-
85000
85000
* Department of Transportation. ** Synthetic styrene-butadienerubber. tll International Rubber Hardness Degrees
Some of the tests which are used for the evaluation of brake fluids are described below: Water-retention (lowering of boiling-point with water content) SAE RM 1 standard reference fluid is used to assess the hygroscopicity of the fluid. lWcm3 of the reference fluid containing 0.5% water is placed in a vessel and the same quantity of the test fluid in another and both vessels placed in a dessicator at standard humidity (450 g ammonium sulphate crushed in water at 23 "C) and left until the water content of the reference fluid reaches 3.5% (usually 48 h). The water content and boiling-point of the test fluid are then determined. Low-temperature stability - compatibility with water 100 cm3 of the fluid is mixed with 3.5 cm3 of water and left to stand for a specified period of time in a glass centrifuge tube. The tube is inverted and the time taken for the air bubble to reach the top of the liquid is recorded. PH 50 cm3 of the test fluid is mixed with the same quantity of a mixture at pH 7 comprising 80% alcohol and 20% water. pH is determined using a glass electrode to ASTM D-1121-67. Volarilify Four 25 cm3 samples of the test fluid are weighed into four Petri dishes to an accuracy of 0.lg. The dishes are placed in an oven at 100 "C and left for 24 h at this temperature, then cooled to room temperature, re-weighed and heated again for 24 h at 100 "C. If, after this time, the weight loss is less than 60%, the test is terminated and the volatile loss recorded. If the weight loss exceeds 60%. the test is repeated until the weight loss in all dishes is less than 0.2 g, or until the maximum loss after 7 days is reached. After completion of the test and the weight losses in each dish has
5 84
been measured, the residues are combined, stood at -5 "C for 60 minutes, and the fluidity is observed: after 5 seconds, the fluid must flow for at least 5 minutes. Thermal stabilify The test fluid is placed in a flask which is fitted with a burette and a thermometer and heated in a liquid bath. The temperature at which gas first appears in the burette is noted as the critical temperature. Thermal stability variation is indicated by the temperature at which the same amount of gas is evolved. Field conditions are best simulated in the PeugeotRenault method. 50 cm3 of the fluid is heated in a steel vessel, to which is welded pressure piping, and which is fitted with a thermocouple. The vessel is pressurised to 3 MPa at intervals; the time taken to pressurise the vessel is noted. As soon as gas starts to develop in the liquid, its compressibility increases and the time taken to achieve 3 MPa also increases.
Currently-available brake fluids fall into the two ctaegories mentioned earlier of hygroscopic and non-hygroscopicfluids.
The hygroscopic group includes glycol ether of various chemical compositions and boiling-points - between 190 and 300 "C - and polyglycols, which increase the lubricity of the fluid. These fluids contain antioxidants, corrosion inhibitors and, sometimes, anti-wear agents. More recently, boric acid esters (I)/glycol ether mixtures, containing 30% or more borate, have appeared. These esters are characterised by high boiling-point, good lubricity and anti-corrosive action. They hydrolyse in the presence of dissolved water, yielding the corresponding dialkyl hydrogen diborates (11)and alcohol, both of which have high boiling-points and good solubility in the fluid.
R CH3 -[- 0 - CH - CH2-13-0-B I
R
/
I 0 -[-CH - CH20-1, - CH,
' 0 -[-CH - CH20 -1, I
- CH3
H2O
(1)
R (5.1 I )
R I
/ 0 -[-CH - CH2O -13 - CH3 HOB
'
+ CH, -[-
0 - CH, - CH2-I3-OH
(n)
0 -[-CH - CH20 -I3 - CH, I
R
(R is H or CH,)
Among representative brake fluids containing borate esters are included mixtures of monoalkoxytrialkylene glycol, diethylene glycol monoalkyl ethers and borate esters (Z8Z), condensation products of glycol borates and mixtures of borate esters of polyalkylene glycols (Z82), diesters of dicarboxylic acids (e.g., adipic acid) and mono- or polyglycol ethers.
585
Silicates of glycol ethers are other relative newly developed products. These hydrolyse according to:
R I
X-Si-R-OR I
-
R
R I
X-Si-OH+ROH I R
(5.12)
where X is a hydrocarbon radical, R is an alkyl, and R' is a glycol ether group. These compounds absorb water -J a lesser exter than the analogous fluids based on boron (e.g., DOT 4). Their dry boiling-point is high, e.g., 320 "C, their wet boiling-point after 20 hours is 300 "C and after 120hours 260 "C (the corresponding values for DOT 4 being 235 and 145 "C,respectively. Their viscosity-temperature curves are flat and their low-temperature viscosity is low. They are interchangeable with glycol ethers and glycol ether borates (183). For a summary see (211)and (211~). The non-hygroscopic group includes mineral and synthetic oils (e.g., PAO) and silicones. These comply with the standards, except for the boiling-points of the fluids in the presence of 3.5% water. They are not compatible with current brake fluids and they attack SBR; nitrile, chlorobutadiene (neoprene), or fluorinated (e.g., Viton) rubbers or polyurethanes must be used. One characteristic of silicone brake fluids is that they are not significantly hygroscopic; this has both advantages and disadvantages. During the time taken for a polyglycol to absorb 3% of water, a silicone fluid only absorbs 300 to 800 p.p.m. Consequently, their boiling-points - which are high (above 250 "C) are not depressed by moist air. However, they do not prevent low-temperature freezing of free water - this may be overcome by inclusion of an adsorbent drier in the system. They have very high viscosity indexes and hence relatively low viscosity at low temperatures. They are good dielectrics, chemically inert, non-corrosive, non-toxic, biologically non-aggressive and they have good thermal and mechanical stabilities. They are compatible with current brake fluids, sealants and paints. these make them good fluids for "sealed for life" systems, despite the fact that they are relatively Table 5.61. Basic Properties of Dow-Corning Brake Fluid Q2-1141 Rope* at 100 oc at 25 "C at -40 "C Boiling-point ( "C) of fresh fluid after exposure to moisture Corrosion of iron, aluminium and copper (mg.mr2) Operating temperature range ( "C)
Kinematic viscosity (mm2.s-1)
5 86
97 Typical values
7 20 1 20 260 260 0.002-0.000 -40 to 288
compressible and need more force in the operation of the pedals. The properties of Dow-Corning's 42-1141 brake fluid are shown in Table 5.61. The properties of the main component of the brake fluid, the glycol ether, depend on the number and type of molecules of the alkylene oxide (ethylene or propylene oxide) used in its preparation. The larger the hydroxyalkylated chain, the higher is the boiling-point of the polyglycol ether, the poorer is its lowtemperature stability and the higher its low-temperature viscosity. The effect on rubber also depends on the chemical composition of the polyglycol and glycol ether used for the preparation of the brake fluid. The hydroxyl and ether groups cause the rubber to shrink and the alkyf and mainly aryl groups enhance swelling. The longer the hydrocarbon chains, the more the glycol ether tends to promote swelling.
The older types of brake fluid, essentially mixtures of castor oil and alcohols (mostly butanol), are now obsolete. Their main deficiencies were low boiling-points (around I10 "C), immiscibility with water and inability to interchange with glycol ether-based fluids due to their significantly lower boiling-points and different effects on rubber. If the price of current brake fluids is indexed at 100, the corresponding indexes for borate-based fluids are 110-120, silicone fluids 300-400, mineral oil fluids down to 50 and synthetic hydrocarbons, such as PAO, 150-250.
5.8.2 Hydrodynamic Transmission Oils or Fluids Fluid or hydrodynamic drives are characterised by low, almost constant pressure and high, variable kinetic energy (speed) of the power fluid. They are suitable where high starting torque and more rapid and flexible acceleration are required, particularly in road and rail vehicles. The use of automatic, hydrodynamic tranmissions in passenger cars has increased markedly. Their development began in the USA shortly before World War II. By 1%9,90% of all passenger cars built in the USA used automatic transmission. This development came later in Europe, in about 1971. As few as 15%of all manufactured cars were then equipped with automatic gear-boxes. By 1975, however, the figure risen to 40% (71).
Automatic transmission enables a soft start, automatic, stepless adjustment of r.p.m. to load, without shocks and torsional vibrations. All this increases the mechanical effect of the engine and prolong the effective life of the equipment. However, fuel consumption is rather higher. Two basic hydrodynamic components are involved: hydraulic clutch and hydraulic torque converter (fig. 5.33). The fluid with which both the clutch and the converter are filled - now identified with the symbol ATF (automatic transmission fluid) - serves as power transmission medium, hydraulic medium for the regulating mechanism, lubricant for the gears, clutch disks and bearings, and as friction modifier and heat transfer fluid. This multiplicity of functions, which involves a much higher load on the fluid than in a hydrostatic system (less fluid in an enclosed circuit, higher speed, higher heat input, cooling difficulties, atomisation and hence more intimate contact with air), is highly demanding on the properties of semi-automatic and automatic transmission fluids. 587
This is particularly true for passenger cars, but also for trucks, buses, tractors and railway vehicles.
Fig. 5.33. Sketches of hydraulic clutch (a) and hydraulic torque converter (b) 1 - pump, 2 - turbine, 3 - oil flow
Hunstad et al. (72)define ATF as combining the properties of - transmission oils which provide a lubricating film of medium load-bearing capacity and serve to protect spiral and bevel gears subjected to high loads, - non-foaming liquids suitable for torque converters and clutches, - hydraulic fluids which enable the control system to operate within the range -35 to 150 "C (modern ATF from -40to about 180 "C), - lubricants for wet clutches and gears producing smooth, noiseless and slip-free starting, - lubricants with enough oxidation stability to withstand the effect of atmospheric oxygen for over 40,000km at high temperatures in the presence of metal catalysts without forming sludges and varnishes which may harm control valves, - lubricants which provide corrosion and rust protection, which are not themselves detrimental to many metals, rubber sealants, glands, adhesives, clutch linings or strip pads which integrate these devices. Hydrodynamic transmission fluids are thus among the most complex of lubricants, with a combination of premium properties of base oil and additives; Table 5.62 illustrates this. In addition, acceptable levels of colour and odour, low toxicity and high flash- and fire-points are sometimes emphasised (73). The requirements for hydraulic transmission fluids vary from manufacturer to manufacturer and this is reflected in the specifications and the tests used.
Selection of Viscosity, Viscosity Index and Base Oil Every specification prescribes viscosity limits in the low- and high-temperature ranges. The oil must both retain sufficient fluidity at very low temperatures be pumped and lubricate the shift gears and sufficient viscosity at high temperatures that it can form a lubricating film and seal the hydraulic system. 588
Table 5.62. Approaches Used for Achieving the Required Properties of Automatic Tkansmission Fluids ATF ~romttyreauired Low pour-point and viscosity suitable for the entire operating temperature range. Correct friction characteristics over the operating temperature range. Resistance to oxidation and thermal stability. Ability to prevent deposit and varnish formation. Anti-wear properties and load-canying capacity of the lubricant film. Anti-corrosion and anti-rust properties. Compatibility with metallic and other components of the transmissions. Absence of deleterious effects on sealant materials.
Approach used Choice of suitable base oil, use of V1 improver, viscosity modifier and pour-point depressant. Use of friction modifiers; this is often referred to as the most important characteristic property of ATF. Choice of suitable base oil and antioxidant. Use of detergent-dispersants,choice of base-oil. Use of EP and anti-wear additives. Use of corrosion and rust inhibitor. Choice of all types of additives and antifoam agents. Choice of base-oil and additives.
Operation of the gear-box is directly related to good low-temperature viscosity, which - facilitates starting; the engine and a directly-coupled converter generate resistance, - provides trouble-free performance of the gear-shift hydraulic manifold; too high a viscosity restricts the supply of fluid from the pump to the manifold and may cause clutch slippage or even burning, - allows accurate gear-shifting; too viscous a fluid impairs pressure adjustment in the hydraulic system and gear-shifting becomes too "soft".
All this leads to oils being selected with very high viscosity indexes (150 minimum). Beacause the very high shear stress imposed on the oil throughout the system, the VI improvers used must have high shear stabilities.Dispersant VZ improvers have been advocated, but sometimes give poorer results than non-dispersant additive VZ improver types due to the effects of other additives (73). The selection of highly-refined, de-waxed base-oil with VZ around 100 or more and pour-point below -15 "C is the result of efforts to obtain as flat a viscositytemperature curve as possible with the lowest possible concentration of polymeric VI improver, to minimise viscosity loss and VZ during shear stress. Alkanic oils have better oxidation stabilities than cycloakanic oils; on the other hand, cycloalkanic oils have lower pour-points and lower low-temperature viscosities. Also, cycloalkanic oils promote swelling of sealant materials, whereas alkanic oils promote shrinkage. It is therefore advantageous to combine alkanic and cycloalkanic constituents in the base-oil in a ratio of about 4: 1.
589
Friction Characteristics Different designs of hydrodynamic systems, with different materials of construction, different friction materials and different gear-shift systems qualities result in varying requirements for friction characteristics, i.e., the relationship between the static and dynamic friction coefficients of the power transmission fluid. Thus, General Motors' hydrodynamic transmission systems cannot use fluids with high static (adhesive) friction coefficients; such fluids cause squeaking during gear-shifts and a sudden torque at the end of the shift-operation may manifest itself by jolting or shaking which makes the clutch and the entire mechanism vibrate (74) (this phenomenon arises mainly at low speeds in systems with less rigid clutch shafts). Moreover, fluids giving low static friction coefficients reduce the speed of deceleration, so that a more gentle gear-shift is achieved. However, this coefficient must be high enough to enable the gear-box members to be disengaged. On the other hand, Ford gear-boxes with disk clutches give the best results with fluids of high static friction coefficients. The relationship between friction coefficient and shear velocities at different temperatures with Ford M 2C 33F (curve A) and GM Dexron (curve B) fluids is illustrated in fig. 5.34. c1
mi
" " 0o
PO 400 600 800 1OOOrniri' REVOLUTIONS
Fig. 5.34. Comparison of friction characteristics of the Ford M2C 33 F fluid A - at 20 O C , A'
- at 121 "C;Dexron fluid B - at 20 "C. E - at 121 "C
General Motors have developed two methods and various apparatus designs for examining ATF characteristics. The older, Low-velocity Friction Apparatus (LVFA) comprises a steel, ring-shaped plate which is pressed against, and causes to rotate, the test disk to which a lining is attached. The pressure applied between the plates ("holddown pressure") is adjustable and the torque transmitted, from which the friction coefficient may be calculated, is measured tensometrically. The disadvantage of this apparatus is that it is impossible to correlate the relative r.p.m. and working conditions of the steel plate and disk in the apparatus with the field r.p.m. and conditions of the gearbox. This problem led to the development of the SAE?No.2 apparatus, which simulates field conditions in gear-boxes better. The apparatus uses the same clutch as is used in the actual gear-box. It is powered by an engine from start up to 3,000 r.p.m.; after this is reached, the engine is driven to provide different speeds.The hold-down pressure of the disk can be varied according to the energy absorbed by the clutch; this energy can be adjusted by means of a flywheel.
The friction characteristics which are required can be achieved with suitable lubricant additives. However, synergistic and antagonistic additive interactions can 590
also occur. Figures 5.35 to 5.38 illustrate the different friction characteristics at 93.5 "C and 0.85 MPa of oils containing various additives (71).
P
0.15 0.10 4050
3
6
9
12 15m.miri'
SLIDING VELOCITY
Fig. 5.35. Effect of ZnDDP with long (A) and short (B) alkys with the same phosphorus content on the mineral oil friction coefficient )1
035
0
3
6
9
12
15m.mirl'
SLIDING VELOCITY
Fig. 5.36. Effect of Ca-sulphonate with TBN 200 (A) and 300 (B) on the mineral oil friction coefficient
I.' 025
015 0.10
OLE SLIDING VELOCITY
Fig. 5.37. Effect of polymetacrylate (A) and polyisobutylene (B) on the mineral oil friction coefficient
a
0
3
m
6
9
12
1
15 m.mid
SLIDING VELOCITY
Fig. 5.38. Effect of ashless dispersant (A) and (B) on the mineral oil friction coefficient
59 1
The differing effects on the magnitude and rate of change of the friction coefficient of the base-oil is related to the activity of the particular compound on the metal surface. ATF additives may be classified (71) into: inactive additives (e.g., polymetacrylates, polyisobutenes, some ashless antioxidants, metal deactivators and corrosion inhibitors); these have no effect on the friction characteristics of the base-oil; low-activity additives (e,g., sulphurised fatty oils and some EP additives); these reduce the static friction coefficient only at high concentrations in the oil; moderate-activity additives (e.g., detergents, ash-containing anti-oxidants, antiwear additives, other corrosion inhibitors and metal deactivators); these reduce the friction coefficient at elevated temperatures and low shear velocities; medium-activity additives (other types of detergents, metal deactivators and corrosion inhibitors); their friction curves show an almost constant friction coefficient regardless of shear velocity. Friction decreases at elevated temperatures and low shear velocities; high-activity additives (e.g., some antioxidants, anti-wear additives and corrosion inhibitors); these are effective even at low concentrations and their effects are dominant over those of other compounds with milder effects; friction coefficient decreases with decreasing shear velocity. However, the selection of suitable additives is still largely empirical. The effects of lubricity additives is gradually exhausted as they decompose and react with the oxidation products of the oil. Friction coefficient thus changes over time, depending on the shear velocity fig. 5.39). The friction coefficients of ATF also changes with temperature (64).
0
6
12 18 24 30 rn.rnin-' SLIDING VELOCITY
Fig. 5.39. Change of ATF friction characteristic in relation to duration of operation A -after 16,OOOkrn. B-after 32,OOOh, C - at start
Oxidation and Thermal Stability Modem automatic gear-boxes are usually fitted with control systems which include many valves with close clearances and with power-servo mechanisms. Even very small amounts of deposits can impair the precise operation of these components and the correct operation of the whole hydrodynamic transmission. If the power fluid is 592
too thick, or the metal surfaces are corroded by acids in the fluid, operation may similarly deteriorate. Resins and carbon are generated by thermal and oxidative stress. They settle on the friction surfaces, clog their pores and produce a glassy layer on the lining; the friction coefficient rapidly falls and the dwell-time in gear-shifting is prolonged. Intimate contact with air which occurs at high fluid velocities in the presence of metals or other catalysts, high temperatures of the oil charge (as high as 180 "C)in the converter during acceleration, when a part of the energy is absorbed by the fluid, and contact between the fluid and hot surfaces (e.g., clutch disks, which reach 320 "C)provide the severe operating conditions to which an ATF is subjected. All these effects promote thermooxidation reactions in the fluid, so that the base-oil must have good oxidation stability characteristics and be dosed with effective antioxidants and/or suitable detergent-dispersant additives. GM-type fluids contain thermally-unstable friction modifiers and therefore require higher doses of effective .antioxidant and DD additives. Ford-type fluids, having higher thermal and oxidation stabilities, do not need these additives to the same extent. Various simulators are available for assessing the oxidation stability and functional life of ATF fluids. GM fluids are tested i n the Powerglide gear-box under conditions prescribed in the ATF specification. The Dexron fluid test lasts 300 hours at an air-rate of 30 cm3.min"; for new-generation fluids, the airrate is 60 or 90 cm3.min-l at 160 "C. After the test is complete, the gear-box is dis-assembled and all components are evaluated for varnish and deposits (which must be absent). The Mercomatic test is similar to the Powerglide; it is operated for 300 hours, 100 cm3.min-' (200 cm3.min-' for M2C 33G fluids) and at 163 "C. In the AT 12 test, a gear-box powered by an internal combustion engine is used. The gears are engaged at low r.p.m. and the test lasts 225 hours ( this is expected to be extended to 400 hours); the gear-shift dwell time should not exceed 0.8 seconds. After the test, the gear-box is dismantled and wear of the functional surfaces and the condition of the clutch lining are assessed.
The AT 13 test is similar, except that the gears are engaged at high r.p.m.. The test duration is 250 hours and the gear-shift dwell should not exceed 0.95 seconds.
Anti-wear Properties Because the conditions in some functional parts where shock-loading at low speed occurs are not conducive to the generation of hydrodynamic friction, and because of the low fluid viscosities, ATF must be dosed with EP additives.
Foaming The torque converter and pumps keep the fluid constantly in motion with a churning action. The fluid is thus intimately mixed with air and there is a risk of foaming. The detrimental effects of foam have been discussed earlier. The ATF must be capable of separating readily from absorbed air and exhibit low foaming tendencies.
593
Foam can impair the efficiency of the converter, cause functional failures in the hydraulic manifold and cavitation in the pump, aggravate the ageing of the fluid, impede proper lubrication of the gears by breaking the oil film with air in the fluid and cause loss of fluid from the system.
To avoid foaming, ATF's are dosed with anti-foam agents. The fluid must, additionally, be resistant to the absorption of air. If it is not, and significant amounts of air are absorbed, the anti-foam agent prevents separation of this air, which remains in the fluid enclosed in microscopic bubbles. The compressibility of the fluid increases and this causes defective performance in the hydraulic manifold.
Anti-corrosion Properties Moisture, air and aggressive products of oxidation promote corrosion of the metal surfaces, which causes inaccurate operation of the valves and control systems. The power fluid must, therefore, not be corrosive itself and must provide protection against corrosion.
Cleaning Properties Detergent-dispersantadditives in the power fluid help keep the precision mechanism in hydraulic tranmissions and their control systems perfectly clean.
Compatibility with Sealant Materials Seals in automatic transmission systems mostly use nitrile rubbers (butadienestyrene-acrylonitrile),which can be used at temperatures up to 120 "C(nitrile rubber Buna N), polyacrylate up to 175 "C and silicone rubbers up to 150 "C; use of vinylidene fluoride copolymers (Viton A) has become widespread. F"E(Teflon) is suitable for temperatures up to 260 O C . Each of these sealant materials has, however, certain deficiencies. Nitrile rubber ages and hardens during operation, particularly during operation at elevated temperatures (high gear-box temperatures preclude use of this material). Silicone rubbers are highly resistant to attack by oil, but are sensitive to damage while mounted and prone to swelling. Polyacrylates do not tend to swell, but they have poor low-temperature properties. Fluorinated rubbers have high thermal stabilities and do not swell, but they are easily damaged during assembly and they are very expensive. Some additives promote the swelling of rubber. For instance, ZDDP's decompose at elevated temperatures to produce sulphides, which vulcanise rubber and increase its hardness. Friction modifiers have a similar effect; their molecules penetrate the structural lattice and cause the rubber to swell.
Some base-oils and additives cause the sealant material to deteriorate which leads to fluid leakage. It is desirable for the fluid to cause moderate swelling of the packing and create tighter seals. For this reason, alkanic base-oils are dosed with swelling agents. 594
Numerous, severe and often contradictory requirements for performance properties of hydraulic transmission fluids have obliged manufacturers to develop specifications and tests for fluids for particular types of transmission. ATF specificationsare available for railway vehicles, e.g., those by Voith, types R,S, and T, or by Svenska Rotor Maskine (SRM) for road vehicles, especially passenger cars. There are now three types of transmission fluids in use. DEXRON-I1 fluids are designed primarily for General Motors transmissions, MERCON for post- 1981 Ford transmissions and Type F fluids (meeting Ford’s M2C 33F specification for pre-1978 and some pre- 1981 Ford transmissions. Most suppliers offer a single ATF approved against the DEXRON-I1 and MERCON specifications. The main difference between the three types lies in their frictional characteristics. Mis-application can cause unacceptable gear change performance (“shift-feel”) and, ultimately, damage to the transmission. All are essentially light (5W) mineral oils with good low temperature fluidity and contain antioxidants, anti-foam agents, viscosity improvers, friction modifiers, anti-wear additives and seal-swell modifiers. They are dyed red for identification. A detailed quality specification is listed in Table 5.63. The more compact European cars are supposed to be more highly stressed than American cars in respect of their ATF requirements, but no European specifications had been issued at the time of writing. Some tests are under consideration, for example the FZG anti-wear test (DIN 5 1 354), the GFC low temperature test in the Brookfield viscosmeter (IP 267 170), the DKA frictional characteristic test, etc. (200-102),which may provide a basis for specifications. Further growth in the demand for ATF may be expected. It will be enhanced by the increase in the number of compact cars in the USA,growing use of caravans for tourism, the introduction of exhaust gas clean-up devices, new designs, metallurgy, sealant materials, etc. Rationalisation of conflicting frictional characteristics is still at an early stage. It may be advantageous to introduce a fluid with frictional characteristics similar to those of base oils, so that such a fluid would not require treatment to produce the right static friction coefficient and service life would be longer (103). The patent literature discloses base oils derived from partially-hydrogenatedalpha-methyl styrene dimer (187). I -hexene and I -octadecene coplymers (188) and alkyl phthalates (189).
New additives for hydrodynamic transmission oils also continues. Two relatively new types of friction modifiers include zinc, calcium or alkylamine salts of alkenylphosphonicacids (190),the reaction products of alkylphosphitesand C,&, epoxides (190, Nalkylmercapto-succinimides(192) and N-(hydroxyalky1)-alkenyI-succinic acid (193). Rubber-swelling agents reported include polyfunctional nitriles (194) and substituted sulpholanes (195). Hydroxyalkyl thioethers (196) are among newer antioxidants and anti-wear additives.
Ranney (297) has prepared a detailed summary of patents issued for hydraulic oils in the USA. 595
ul
Table 5.63. Automatic lkansmission Fluid Specifications
\o
ch
colour/rating miscibility odour toxicity viscosity at 100 "C (mm2.s-')
DEXRON-11'
MERCON~
Tv~F e
red recommended no separation or colour change
6.0-8 .O/red no separation or colour change not objectionable non-toxic
7.0Ired no separation not objectionablenot objectionable non-toxic
6.803
6.8 5.5
acceptable fresh fluid used fluid
5.5
7.0/7.S3 5.5
BrooWield viscosity (mPa.s) -18 "C,max
-23.3 "C, max. -40 "C. max.
pour-point ( "C), max. flash-point ( "C). min. fire-point ( "C), min. copper strip rust & corrosion anti-foam properties
not required 160 175 no blackening with flaking no rust (incl. humidity cabinet test) no foam at 95 "C 10 mm max. foam height 23 sec. to break at 135 'C
1700 1700 4000 5oooo not required 177 not required 1b max. no rust seq.1, ml, max. 100/0 seq.2, ml, max.100/0 seq.3, ml, max. 100/0 seq.4, ml, max. 100/0
5
m 5oooo
40O C 175 190
l b max. no rust seq.1, ml, max.100/0 seq.2, ml, max.100/0 seq.3, ml, max. 100/0 seq.4, ml, max. 100/0
sealant compatibility % volume change
durometer, pts. %change in elongation bend test
BUNA N +1 to +5 0 to -5
+I t o 4 -5 to +5
+1 to +5 + I to +5 -60 max. no cracks
% volume change
durometer, pts. % volume change durometer, pts. reversion SAE No. 2 Friction
oxidation test
POLYACRYLATE 0 to +I0 0 to +5 SILICONE 0 to +5 0 to -10
+3 to +8 -5 to +5
not required not required not required not required
no reversion HEFCAD - satisfactory operation for 100 h; dynamic torque between 115 and 175 Nm; static minus dynamic equals 14 Nm max.; clutch engagement between 0.45 and 0.75 s; good plate condition.
4OOO cycle friction durability
THOT - clean transmission; minimal cooler corrosion; TAN increase c 7;viscosity at 23.3 "C 6000 mPa.s max.;differential IR c 0.8; viscosity 5.5 mm2.s-' at 100 "C during test.
M O T - equal to or better than reference at 155 "C, 300 h; 1.O% max.pentane insolubles at 200 h 5.0 max.TAN change at 250 h; 50 cm-' max.diff. IR at 250 h; 50% max. viscosity increase at 40 "C at 250 h; 3b max.Cu strip at 50h and 300 h; no varnish A1 strip at 300 h.;no sludge at 300 h.
"Six-pack" static coefficient SD-1177 plates; mid-point dynamic at 100 cycles 0.167 min.; torque (between 5 and 4000 cycles) dynamic coefficient at 100 120-150 Nm; static break-away at cycles to fit within prescribed 4.37 r.p.m. (between 200 and 4OOO envelope. cycles) 90-130 Nm; engagement time (between 5 and 4ooo cycles) 0.8 to 1.0 s; 5 cycle low speed friction coefficient peak (dynamic torque) 155 Nm; static torque to mid-point dynamic torque ratio (at 200 cycles) 0.90 to 1.OO ABOT equal to or better than reference in aluminium beaker test at 135 "C, 175 h; 1.0% max. pentane insolubles; 4.5 max. TAN increase; 45 cm-' max. diff. IR.
(Table 5.63 contd.) VI \o
00
cycling test
wear test
vacuum diaphragm compatibility ride-test (shift-feel)
DEXRON-11' TCHT-acceptableoperation and parts at 20,000 cycles; 1-2 shift -time between 0.35 and 0.7 s; no cooler corrosion; TAN increase c6;diff.IRc0.8; viscosity at 100 "C during test 5.5 mm2.s-1 min..
MERCON'
Type F
TCHT - same as DEXRON-I1 except no cooler corrosion requirement.
Vehicle cycling- satisfactory operation and parts for 8000 cycles at 1 1 W "C; viscosity at 100 oc 5.5 mm2.s-1 min.; 1700 mPa.s max. at -18 "C.
power-steering pump wear equal to or better than reference; no excessive wear, scuffing or scoring on cam-ring.
vane-pump(ASTM D-2882 at 80 "C, 6.9 MPa) weight loss 15mg max.
4-ball - 0.45mm max. scar dia.
equal to reference
Ford Taurus with AXOD shift-feel after 250 miles and equal to reference; friction materials - no distress.
1.2- DEXRON-U and MERCON are registered trademarks of General Motor Corp. and Ford Motor Co.. respectively; 3 - recommended minimum for compliance with used oil requirement.
zero fails Max. 1 step removed from factory fill
5.9 METAL-WORKING LUBRICANTS Many different processes for machining and forming metals are involved in mechanical engineering and metallurgical technology. The lubricants used in these processes for cooling as well as lubrication are indispensable; the composition and properties of the lubricants dictate energy losses, surface finish of the work-piece, service life of the tools and many other characteristics of these processes. Metal-working processes may be divided into two categories: - metal-cutting processes in which the required shape is achieved with an acceptable finish by removing excess material from the work-piece in the form of chips, filings, etc., e.g., turning, milling, drilling, boring, planing, sawing and grinding), - metal-forming processes in which the shape of the work-piece is changed without loss of material (chipless working, e.g., rolling, drawing, pressing and forging). The tribology of metal-working directly influences friction at the tool/work-piece interface, tool wear and the surface finish of the the material machined. It thus contributesto energy and material consumption and eventual quality of the work-piece (199 - 201,231,232,239).The following factors must, therefore, be considered, which bear on the tribology of all metal-working processes: - the type of motion transmitted (cutting, drawing, etc.); - the working conditions (load, interfacial pressure, initial and frictional heat load, duration of the operation, flow of material, rate of deformation); - the structure of the system - properties of and relationship between the tribological elements work-piedtool (composition, geometry, dimensions, ductility, hardness, surface roughness, previous history), lubricant (type, chemical composition, viscosity and viscosity change with temperature. pressure and shear rate) and the environment (configuration, moisture present): - relationships between the process inputs and outputs (power demand and performance, tooUworkpiece friction and frictional heat, wear and surface finish of the work-piece). Tribological factors often determine the serviceability of the tool in metal-removing processes. For example, reduction in tool wear by using better materials, new coatings, better lubricants and the way in which the lubricants are fed to the tool produces energy savings, which can be quite high. It has been shown that increases in cutting speed over normally recommended values can appreciably reduce friction, improve surface texture and improve tool efficiency. The energy saving in metal-cutting has been estimated, rather modestly, at 1.25 to 2.5% (202). The situation in chipless metal-working is similar. Frictional losses can be 35% lower when oils of the correct viscosity are used in cold-rolling; energy consumption can be reduced by 15% if roller bearings are used in steel-strip rolling mills. The American metallurgical industry saves annually some 750,000 to 800,000 tons coal-equivalent (29 MJ.kg-') in cold- and hot-rolling processes. Additional savings may be achieved if warm-rolling replaces hot-rolling (203,204).
Different requirements exist for lubricant composition and properties; there are different working methods and every process has its own specific characteristics. Lubricants are therefore sub-divided into those suitable for cutting (chip-removing) and forming (chipless working) processes for metallic materials. Metal-working and heat-treatment are usually followed by wet cleaning. All water must be removed to avoid staining and corrosion of the surface. Ar-drying processes can be len@hy or even impracticable, particularly for intricately-shaped work-pieces. De-watering processes are therefore popular. Liquids for
599
this duty may be solvents, such as gasoline, alkylaryl polyglycol ethers, and chlorinated hydrocarbons, containing surfactants which promote rapid wetting of the metal with the solvent and complete dewatering of the surface without emulsification of the solvent. Wetted work-pieces must be protected against corrosion for an appreciable time so that the use of anti-corrosion agents is indispensable.
5.9.1 Cutting Oils or Fluids The cutting process consists in mutual motion of the tool and the work-piece which results in the separation of part of the material in the form of a chip. The process itself proceeds in two stages; in the first, a layer of material is cut off by the tool, and in the second, the chip separates from the site where the tool bites. The start of the process is characterised by plastic deformation of the material; deformative creep of a thin layer of the material occurs. In the later stage, the material is cut off and the chip moves towards the face of the tool. The departing chip rubs against the face of the tool and can damage the tool blade by friction on the face land. Both internal friction (by creep of material within the chip) and external friction (between the chip and the tool face) occur. Friction occurring on the tool face causes fresh plastic deformation, which results in further deformation of the chip. Cutting fluid fed into the cut is expected, as a result of its cooling effect, to influence the whole process of chip formation, i.e., to reduce the mechanical and thermal load applied to the cutting edge, to reduce both internal and external friction and thus to reduce tool wear and prolong its life in service (135). The cooling effect of the fluid may both improve and impair the cutting process. The work needed to achieve plastic deformation of the material is converted into heat. The temperature of the material depends on its type, the cutting speed and the cross-section of the chip; it can vary from 200 to 500 "C. This value is very important in relation to the strength of the material. The strength of carbon steel grows up to about 300 OC (the brittleness temperature) then decreases. The temperature from which the material cools down is thus important. If the temperature exceeds 300 OC, the strength of the material increases when it is cooled; however, resistance to chip formation and the work involved in deformation also increase. If the temperature is below 300 O C , both material strength and deformation work decrease. In addition to the cooling and lubricating effects of the lubricant, the Rehbinder effect (205) is an important feature of chip-producing machining. Rehbinder and his team found that the hardness of steel wetted with a suitable liquid decreases, making machining of the steel easier, independently of the lubricating and cooling effect of the fluid. Derjagin and Kusakov (206) have offered a possible explanation of this phenomenon arising out of work on the beneficial effect of water in mica-splitting. According to these workers, water drains into the cracks, saturating the crystal lattices with the energy which is so released and thus preventing attraction between the split surfaces. This suggestion has been corroborated by a number of observations. Carbon tetrachloride has a similar effect in the machining of metals (207).
The lubricating effect of fluids is manifested by the formation of a load-carrying boundary film in the fine surface cracks created during the plastic deformation of 600
the metal. This facilitates deformative creep of the material during formation of the chip and thus reduces the work of deformation. The cooling and lubricating effect of the cutting fluid is also beneficial in the rubbing of the chip over the tool and the rubbing of the tool-flank over the cut surface. Wear of the tool occurs here on its contact surfaces with the chip and the cut surface, which shorten the life of the tool. Local wear on the cut contact sites results from abrasion by hard chips and the cut surface on the tool-flank, from the abrasive effect of the materials coming into contact and from abrasion of the surface microlayer resulting from its enhanced plasticity. The abrasive effect itself is due to the intrusion of the hard part of the chip or the cut surface into the tool face and the edge of the surface. Adhesive wear results from welding of the chip material in contact with the tool face, and the cut surface in contact with the tool flank surface. Welding is promoted by the fact that two fresh, clean metal surfaces contact each other under high pressure and temperature conditions. Tool edge wear then occurs by breaking and separation of the welded local junctions between the edge, chip and material of the work-piece. The degree of wear increases with increasing temperature at the contact site and increased pressure. The difference in hardness of the materials in contact zones at the working temperature is also involved. The temperature of the edge-chip contact can be as high as 200-1200 OC and the specific pressure as high as 103-104MPa, depending on the cutting speed, the crosssection of the chip and the material being cut. Adhesive wear occurs at 850-900 O C . Diffusion causes mutual penetration of the elements of the two metals which are pressed together and heated at the “diffusion temperature”; diffusion can thus only arise under exceptional conditions, e.g., when sintered carbide-tipped tools are used for machining. When sintered carbide contacts the work-piece material at high temperatures, the elements in the carbide - W, Ti, Cr or Co - intrude into the work-piece material and the work-piece elements - Fe, Mn and Si - into the surface layer of the sintered carbide. Consequently, the surface layer of the edge is dis-aggregated in the contact zones and smaller particles of the disaggregated layer may be removed by the chip and the cut surface.
The magnitude of the abrasive effect depends on the cooling efficiency of the cutting fluid, which may be positive or negative, and this depends on how the ratio of hardness of the tool and the work-piece material changes at the working temperature. Cutting fluids affect adhesive wear by reducing the temperature of the contact zone through preventing direct metal-to-metal, chip-to-tool face and workpiece-to-tool flank contacts and by reducing the coefficient of friction. This desirable lubricating effect of the cutting fluid is achievable through the formation of a strong boundary film between the surfaces in contact, strong enough to be retained even at high contact pressures. The film must be sufficiently resistant to high temperatures at high cutting speeds, when the temperature in the contact zone may exceed 400 OC. Cutting fluids containing EP additive can create such lubricating films with adequate load-carrying capacities. However, the composition and concentration of the EP additives must be matched to the pressure and temperature prevailing at the contact zone. The lubricating effect of the EP additive cannot appear unless the chemicallycreated boundary layer is softer than the material of the edge at the temperature prevailing in the contact. Some boundary films, such as oxides, which have adequate load-carrying capacity, do not meet this requirement. They may reduce the degree of adhesion, and hence adhesive wear, but they increase abrasive wear. 601
Metallic soaps of fatty acids can create boundary films up to 200 "C, but lose their effect at their melting points. Moreover, they are not sufficiently resistant to pressure. Only oils containining EP additives are suitable for high temperatures and pressures. Suitable additives for cutting fluids include polar vegetable and animal fat glycerides, which form boundary films by adsorption on to the metal surfaces, and certain organic substances containing, mainly, sulphur, phosphorus, chlorine or iodine, which produce sulphides, phosphides, phosphates and chlorides or iodides. Nitrogen compounds are ineffective as EP additives by themselves, but can act as synergists with the other compounds listed. The boundary films so produced show lower friction coefficients, because they have lower shear strengths. Their thermal stability varies and is limited by their temperatures of transition into other physical states. fig. 5.40 depicts the temperature ranges within which these EP additives are effective and the friction coefficients of the boundary films which are produced (269). p 0.5
0.4 0.3
a2 0.1 0 0
-
100 200 300 400 500 600 700 800 900 1m1100 TEMPERATURE ,"C EP-HOLE
Fig. 5.40. Temperature-rangesof the effectivity of various EP-additives Chlorides generate lower coefficients of friction and lower shear strengths than sulphides. Chlorinebased compounds are therefore more suitable for low temperatures, low cutting speeds and small chip cross sections. However, the use of chlorine compounds as EP additives has more recently been considerably restricted or prohibitedon environmentalgrounds, which has produced significant problems. Inability to use them initially created a mne in the temperature range 120 to 180 "C in which the friction surfaces were not separated by an efficient boundary film (the "EP hole" in&. 5.40). These compounds have been replaced by special synergistic sulphur-, phosphorus- and complex ester-based compounds, suitable for most cutting fluids for machining operations. However, replacement is more difficult for special metal-cutting and forming operations, such as broaching, cutting of thin metals and cold drawing. Fluids for these applications have previously contained 50% and more of chlorinated paraffin waxes and this problem has not been satisfactorily resolved.
Diffusion wear occurs at high temperatures and increases with temperature. The cooling effect is the main factor which restricts or suppresses it. The lubricating effect of the fluid at excessively high temperatures can be suspect because of thermal instability of the boundary layer. Other factors influencing wear in cutting operations include the built-up edge cfig. 5.41). This prolongs tool-life, but makes the work-piece surface rather rough.
602
The built-up edge reduces edge wear by protecting the edge from direct contact with the material layer which is cut, separating it from the highest temperature sites and itself taking over the function of the edge. Since its rake angle and thus cutting angle are smaller, the chip-severing process proceeds more easily, less force being required, and the edge is not subjected to high mechanical and thermal stress.
a
b
FORMATION IRATE I OF BUILT-UP EDBE
Fig. 5.41. Effect of built-up edge rate on tool wear and roughness of machined surface Fig. 5.41a. Outline demonstration of built-up edge TI - constant portion of the built-up edge, 'I2 - variable portion of the built-up edge, 1 - workpiece, 2 - chip, 3 - built-up edge, 4 - blunting, 5 - tool
Since the built-up edge forms in the temperature range 100-600 "C, but most effectively at around 300 OC (referring to carbon steel), variation in the cooling power of cutting fluids can influence its formation considerably. Only a fluid which possesses properties and composition which match the working conditions of pressure and temperature at the tool-edge/work-piece contact can fulfil its main tasks, which are to lubricate, cool, assist in the removal of the chip in the proportion required, prolong tool life and control the surface finish of the work-piece, all with minimum power consumption and with no negative effects in other respects. The working conditions themselves depend on the severity of the machining process (the type of machining operation), the cutting speed, the thickness of the chip and the properties and composition of the work-piece material. Machining operations can be graded in order of increasing severity as; centreless grinding, surface grinding, sawing, milling, shallow-hole boring, turning, slotting, shaping, planing, boring, turret-late turning, hobbing, shape-grinding, deep-hole drilling, reaming, teeth-milling, male and female thread-cutting, internal and external broaching. The selection of machining fluids for these duties should be made according to the following guidelines: machining operations in the early part of the series require cooling and lubricating fluids, those in the latter part fluids with a strong lubricating effect. The higher the cutting speed and the thicker the chip, the higher the pressure and temperature in the cutting area and the more severe the duty of the fluid. The relationship between the material of the work-piece and the quality and composition of the cutting fluid is based on these principles: in the case of steels,
603
higher lubricating and cooling performance is needed for higher strength steels. Emulsions or pure mineral oils are suitable for medium-strength carbon steels; oil emulsions with EP additives are required for higher strength steels. Alloy steels, with increasing strength, require oil emulsions, fatty oils, mineral oils compounded with fatty oils, cutting oils with a high EP additive concentration or concentrates of EP additives. To machine steel with sintered carbide-tipped tools at high cutting-speeds requires salt solutions or very dilute oil in water emulsions. Dry machining is generally only used for cast iron. If very clean cuts and dust reduction in the environment are required in the case of grinding or cutting with sintered carbide-tipped tools, water solutions of salts will serve. Oil emulsions are convenient for high-precision cutting and produce a good surface finish, for instance in reaming and thread-cutting and the machining of malleable cast iron. Cutting oils containing active sulphur cannot be used for copper and its alloys. These oils colour and darken the surfaces. Sulphur-containing additives are not generally recommended. Oil emulsions, compounded oils and cutting oils with sulphur-free EP additives are suitable. Dry-machining or emulsion-machining (e.g., for internal combustion engine pistons) or the use of compounded oils (for machining tough alloys) are used for aluminium alloys. Magnesium alloys are easy to machine and the oils do not need to form films with high load-carrying capacity. Emulsions cannot be used, since magnesium alloys decompose water into hydrogen and oxygen, generating a fire hazard. Pure mineral or compounded oils are suitable; the high adhesivity of compounded oils ensures that the work-piece and chip surfaces are thoroughly coated and hence protected from contact with oxygen in the air. Their drawback is the formation of sticky deposits of magnesium soaps. Titanium and its alloys are among the most difficult metals to machine. Machining is more difficult than that of nickel and chromium steels. Titanium and its alloys have a great tendency to adhere and weld to any surface in which they are in sliding contact. To machine them requires high concentrations of effective EP additives, especially chlorine-based compounds. In Table 5.64, a simplified scheme is presented for classifying the type of metalworking process and the type of metal being worked. Cutting fluids can be classified by their cooling or lubricating abilities into water solutions of salts, oil emulsions, pure mineral oils, mineral oils compounded with fatty oils, mineral oils or synthetic oils with EP additives, concentrates of EP additives and solid lubricants. In evaluating them, not only their cooling and lubricant properties but also properties such as detergency and protective properties, foaming tendency, effects on health and safety, behaviour towards protective paints on machinery and their stability in operation must be considered. Adequate detergency and dispersancy of the fluid is required for operations which produce very small chips and filings, especially for grinding, where the material is removed by abrasion. If fluids with low detergent-dispersant effect are used, abrasion debris may firmly adhere to the working surfaces of the grinding wheels, 604
which impairs the finish of the work-piece, aggravates wear and shortens the working life of the wheel. The coolants used in these situations must therefore contain detergents, which must also control corrosion, for example ethanolamine salts of alkyl- or arylsulphamidoacetic acid, at about CI6. Soft water, containing inorganic and organic compounds which act as anticorrosion, wetting, lubricity improver, bactericidal and anti-foam agents can be used when a high cooling effect is required (compared with mineral and fatty oils, the specific heat of water is three times higher and it has a much higher latent heat of evaporation and thermal conductivity and lower viscosity). Good lubricating and anti-corrosion properties and low foaming tendency in the water phase are provided by alkali or ammonium salts of alkyl- or arylsulpharnidocarboxylicacids of the general formula: X-SO,-N(methy1. alky1)-(CH,),COHY where X is a C,, to C, alkyl or an aryl (e.g., phenyl) with an alkyl, alkyloxyl or halogen nuclear substituent and n = 1- 5. Derivatives of N-methyl-a-aminoacetic acid, N-methyl4aminobutyric acid, 5-aminovaleric acid or 6-aminocaproic acid have also been suggested (222). Some of these compounds also have excellent properties as emulsifiers for coolant emulsions. Their lubricating properties can be improved by high molecular weight polyethylene-propylene glycols, which may be OH-group substituted by higher fatty alcohols, amines, acids or amides. Ions such as B,O?-, PO:., MOO:- and WOi- confer excellent anti-corrosion properties to water and the ability to deal with EHD or mixed friction regimes; they also have bactericidal properties. Condensation products of P,O, with triethanolaminehave also been suggested in this connection (270).
Oil-in-water emulsions are used for the same purpose. Their oil content should correspond to the lubricating ability required and accordingly varies between 1 and 20%. The lubricating effect and wetting ability, and its stability and anti-corrosion properties, increase with increasing oil content. Emulsions are made by dispersing mineral oil, preferably cycloalkanic types, containing emulsifier or emulsion oil in softened water. Emulsion made from very soft water tend to foam whilst water which is too hard gives restricted emulsion stability. The best level of hardness is 5-8" German water hardness (for conversion to English, French or American values see DIN 53 910, Pt.1). Emulsions made with nonionic emulsifiers usually have higher stabilities, even with harder water. The most common anionic emulsifiers are sodium salts of fatty and naphthenic acids and sulphurised alcohols and acids; nonionic types include esters of higher fatty acids and polyalkylene glycols. The lubricating effect and anticorrosive properties of emulsions depend, among other factors, on the properties of the emulsifier. Oils commonly used for the preparation of cutting emulsions include petroleum distillates with viscosities normally between 15 and 20 rnm2.s1 at 50 "C. Emulsifiers with HLB (hydrophiliCnipophilicbalance see Chapter 4, between 5 and 5.5 are suitable. Emulsifiers with HLB 7 and above form oil-in-water emulsions.
Micro-emulsions, a relatively recent development, comprise water, oil and an emulsifier of adequate HLB or, preferably, a blend of two non-ionic emulsifiers, one of which promotes the formation of a water-in-oil emulsion and the other promotes oil-in-water emulsion, with a balanced HLB. The resultant product is a clear, homogeneous fluid which offers better control of the work, because the operator can see both tool and work through the stream of emulsion.
Table 5.64. A Simplified Scheme for Classification of Cutting Fluids according to the Metal-working Process and the Material Being Worked
Ul
Machining operation
Metal Ferrous metals
common turning form turning, automtic lathe turning drilling deep drilling -g
common milling, slotting, shaping & planing gear milling, slotting. Bhnping & planing sawing common thread cutting thnxd cutting, automatic lathe
low€ steels
high€ steels
alloy steels
E-A(3-5) CF0,SCFO
E(3-5) E-EP(5). CR0,CMO E(3-5)
E-EP(5-10) SCFO, EP-MO E(3-5)
E-A(3-5)
E-EP(5-10). E-A(5-10) EP-MO SCF0,SCMO. EP-MO, SCFO EP-MO SCFO EP-C,EP-MO, E-EP(5-lo), EP-CBP-MO, SCFO SCFO SCFO E(3-5) E(3-5) EA(3-5)
EP-MO.
SCFO.
EP-MO,
SCFO
EP-MO E(3-5) F,MFO,
SCFO
CFO
CF0.F EP-MO,
E(3-5) SCFO,
CFOP EP-MO. SCFO
SCFO.CM0. EP-MO
g-
EP-CJP-MO
CFO,
common grinding
E-A(2-3). VSS EP-MO SCFO
grinding of - thrrads -toothflanks -shapes
Non-ferrous metals
EA(3-5) SCFO,
cast iron
copper
copper alloys light A1 alloys
E(2-3) E(3-5)
~(2-5) E-A(3-5)
E(2-3)
E(2-3)
E(3-5)
E(3-5) MO E(3-5). MO
EA(3-5). MFO EA(3-5) MFO E(2-3)
MF0,CFO
M0,MFO
E(3-5) E(2-3) E(3-5). MO
FNFO, CFO E(3-5)
E(2-3) M0,MFO
E(3-5) F,MFO
MO
CM0,CFO
E(3-5). MO WS-E, ~(2-3)
M0,MFO
FNFO CFO E(2-3)
E(2-3)NO E(3-5)
MFO
SCFO
EP-CBP-MO SCF0,EP-MO MO E(2-3). WS-E(2-5). WS-E E-EP SCFO EP-MO EP-MO SCFO E-EP(3-5) EP-MO
E(2-5)MO E-A(3-5) MO E(3-5) M0,MFO MFO E-A(3-5) E-A(3-5) MFO E(2-3)NO
E(2-3)
CMO.CF0 CF0,CMO
EA(3-5). MFO MFO, E-A(3-5) E(2-3). MO.CMO E-A(3-5) CFO E-A(3-5)
high-speed grinding shape grinding solid material grinding lapping. honing, super-finishing
working in machining centres
EP-MO SCFO
SCFO EP-MO
EP-MO SCFO
CFO.CMO
CMO
CFO.CM0
WS-E,
CMO
SCFO.CM0, E-EP(5- 10)
CM0,SCFO E-EP(5-10)
EP-MO SCFO
E-A(3-5)
E-A(3-5) MFO
ws-s
E-A(3-5)
codes used; WS-E: water solutions of electrolytes ws-s; w a t a solutions of synthetics &%): oil-in-water emulsions, with oil concentration in % E-A(%):&-in-water emulsions activated with surfactants, with oil concentration in % E-EP(%): oil-in-water emulsions with EP additives, with oil concentration in % M O mineral oils ("sowcutting fluids) F: vegetable or animal substanm ("semi-hard" cutting oils) MFO mineral oils compounded with fatty substances ("soft to semi-hard" cutting oils) CMO.CFO: mineral or compounded fatty oils, chlorinated or containing chlorine-based additives ("semi-hard" cutting oils)* SCM0,SCFO mineral or fatty oils containing chlorine and sulphur+ EP-MOmineral oils with a high concentration of combined C1-, S-or P-and fatty acid EP additives ("hard" cuaing oils)* EPC. EP additive concenmtes used in mixture with MO in varying ratios,depending on the suength of the material and the nature of the machining o p t i o n . *Chlorine oils are becoming excluded from cutting oils on ecological and health grounds.
Long-life emulsions must contain either a biocidal agent or an emulsifier which prevents the growth of microorganisms. When pronounced cooling, penetration and cutting effects are required, activated emulsions prepared from mineral oils containing EP additives are used. Generally, emulsions must be stable, have a maximum pH of 9.0, if possible be translucent or transparent to allow both the tool and the work-piece to be inspected, must not corrode metal surfaces - indeed protect them from corrosion - or damage the protective paint on the machine, they must have low foaming tendency and no physiological effect on the skin or internal organs, they must resist microbial decomposition and they must not become smelly even after prolonged use. The resistance of emulsions to anaerobic bacteria - which promote decomposition and odour - can be increased with bactericidal additives or by post-purification pasteurisation of the circulating emulsion. Filtration through silver has also been proposed. Suitable corrosion inhibitors for emulsion oils also include organic amines, such as alkanolamines and their derivatives, which also increase basicity and can act to some extent as emulsifiers. Sodium nitrite (NaNO,), formerly used as an anticorrosion agent, is now prohibited. Nitrosoamines, the reaction products of sodium nitrite and some organic amines, are dangerous - according to one reference (298) and many other authors, 80% of nitrosoamines are carcinogenic. This effect, however, varies. Diethyl nitrosoamine is much more dangerous than diethanol nitrosoamine and the reaction which gives rise to nitrosoamines follows a different pattern in acidic and alkaline media. This factor needs to be borne in mind when cutting emulsions containing corrosion inhibitors are selected and handled. Fresh oils and fluids are usually slightly basic, but they may become acidic as they age, due to the action of anaerobic bacteria. If the vapours are ingested by breathing, their pH changes in the acidic environment of the stomach. Emulsions in colloidal form, with an enormously extended surface area of the dispersed constituent, are vulnerable to the rapid growth of microorganisms. Once an emulsion is infected, the microorganisms multiply extremely rapidly, which can lead to complete decomposition of the emulsion. Gram-negative bacteria are most prevalent in infected cutting oil emulsions. In the later stages, when the oxygen content in the system has diminished, anaerobic bacteria, such as clostridium and desulphovibrinium genera can multiply. However, fungi and yeast can also occur, particularly at low pH; these multiply more slowly than the bacteria. Test methods for microbial infections are available (e.g.,
264).
Emulsions are particularly useful where heat abstraction is more important than lubricating effect. However, emulsions for demanding machining operations must be very carefully selected. Whilst they have excellent cooling properties (an oil-inwater emulsion containing 10% of oil has five times the coolant effect as a mineral oil with viscosity 16 mm2.s-'), their lubricating effect is considerably less than that of oils. The increase in boiling-point with pressure (e.g.. to 262 "C at 5 MPa) is also very significant. During operations in which it comes into contact with a very hot
608
tool edge, despite the cooling effect, the emulsion can be exposed to very high pressure (e.g., in deep-hole drilling). The vapour bubbles which form disappear at the effluent point very rapidly, but allow the influx of much cooler fluid into the angle of the tool rake. This sudden cooling shock - particularly on sintered carbide tips - causes cracks, peeling and cratering. The stability of metal-working emulsions can be determined by CSN 65 6250 and ASTM D-1479, which are almost identical. Oil-in-water emulsion of the specified oil concentration is mixed and allowed to stand at room temperature for a specified temperature and the amount of oil which separates is mcasured. It is, however, essential to cany out the test at various temperatures and with water of various degrees of hardness.
pH is very important for fluids which contain water. Ferrous metals tend to rust rapidly in an acidic medium, whereas non-ferrous metals corrode in a basic environment. When the alkalinity is too high (over about pH 9.3, paint on the machine and the rubber seals are damaged and dermatoses and catarrhal problems of the respiratory tract occur. The development of microorganisms slows with increasing pH and stops when the pH exceeds 9.5. A pH of 7.4 to 8.8 is recommended for ferrous metals, and pH 7 (moderately alkaline) for light metals, copper and copper alloys. Emulsion in the system may sometimes become contaminated with oil, such as hydraulic oil, as a result of leakage from another hydraulic circuit. This oil can change the water-in-oil ratio in the whole system. Used or degraded emulsions can be destroyed by acidification with hydrochloric or sulphuric acids, which change the emulsion pH to 3-4 and induce separation into water and oil phases (some polymers and demulsifiers can help break the emulsion). The separated water must then be cleaned and the oil either reclaimed or used as fuel. Straight mineral oils of relatively low viscosity (15-35 m m 2 s 1 at 50 "C)and suitable low-temperature fluidity are useful for operations in which the lubricating effect is more important than the coolant effect, for example, on low-cutting-speed, automatic lathes, for machining low-carbon steels, some non-ferrous metals and light alloys. For some operations, such as machining aluminium alloys, honing and lapping, oils with even lower viscosities (around 5 mm2.s-l at 50 "C) are used to achieve a cleaning effect and better heat transfer. The use of kerosene, which is volatile and harmful to health and which constitutes a fire hazard, must be avoided.
Unlike emulsions, mineral oils are unsuitable for high-speed operations with carbide-tippedtools. On the other hand, they provide better corrosion-protectionfor the metal surfaces and are not susceptible to microbial decomposition. They are compatible with lubricating oils and can therefore be used for multi-purpose oils for metal-working, machine lubrication and hydraulic systems. Synthetic oils have not, so far, been much used in metal-working. Polyol glycols and their mixtures with 3040% water can be considered for this purpose. They
609
exhibit good lubricity and protective qualities, they are difficult to ignite and more stable in service than mineral oils (255,256). Polyethylene glycols have a low level of toxicity, cause no skin irritation and waste fluids are biodegradable. The coolant effect and lubricating power of mineral oils can be improved with fatty oils, which improve their wetting power and affinity for metal surfaces. These mixtures are referred to as Compounded mineral oils; they may contain animal and vegetable oils, e.g., sperm, hoof and horn, tallow, lard, rape seed and castor oils or fatty acids, especially oleic acid. In some operations, fatty oils (preferably rape-seed oil) are used alone for both cooling and cutting. Fatty and compounded oils form boundary films with low friction coefficients as a result of physical adsorption of the polar group on to the metal and chemisorption, resulting in metal soaps. The deficiency of these boundary films is their limited thermal stability and low load-carrying capacity, so that fatty and compounded oils are suitable only for low-speed operations (up to about 30 r.p.m.), for example, working alloy steels, provided that low surface roughness is also required, and, preferably, for working non-ferrous metals. Both fatty and compounded oils have relatively low stability to oxidation and their acidity increases with time giving metal soaps and resins. The oiis become thick and foul the functional parts of the machine as a result of their adhesivity. The loadcarrying capacity of boundary films increases appreciably when fatty substances (acids or their glycerides) which contain sulphur - the so-called sulphurisedfutty oils - are added. These substances improve mineral cutting oils, which must possess balanced lubricating, anti-wear and wetting properties and be usable over a wider range of machining operations and materials to be machined, in terms of the tool-work interface temperature. Another useful additive combination is that of a fatty substance and a chlorinated organic compound (e.g., a chlorinated paraffh wax); the fatty oil or fatty acid acts as lubricating and wetting additive and the chlorinated compound as EP agent. Oils compounded in this way can be used for working copper and copper alloys as they do not cause darkening of the work surface. Multi-purpose oils can be made in this way, which can be used not only as machining oils but also as hydraulic fluids and slideway oils. Chlorinated fatty substances which have a similar effect to their sulphurised equivalents provide lubricating, anti-wear and wetting properties and are also suitable for copper-based materials. Iron chloride is only thermally stable up to a temperature of 50 "C and at 200 "C is formed much more slowly than the iron sulphide which forms when additives containing active sulphur are used. Oils treated with chlorinated fatty substances are therefore used for low-cutting-speed operations or where the surface temperature must be kept low to prevent thermal transformations and hardening of the material, as is the case with stainless and heatresistant steels. The most effective cutting oils available which are usable over a wide temperature range, high pressures and high cutting-speeds,are those containing both fatty substances and chlorine and active sulphur compoumfs. Examples include sulphurised or chlorinated fatty substances with chlorinated or sulphurised organics, 610
of chlorinated fatty substances with elementary sulphur, and fatty substances, chlorinated hydrocarbons and elementary sulphur. Oils like this have been used mainly for the working of tough alloy steels. Again, the health and safety characteristicsof chlorinated compositions are frequently suspect and relevant safety data must be sought, as with all applications of lubricants. The load-carrying capacity and anti-wear properties of the boundary film can be adjusted by varying the concentration of the EP additive in the oil and selection compounds of varying chlorine or sulphur activity, which depends on the bond strength of chlorine and sulphur in the compound. High EP activity compounds are desirable in high-performance cutting fluids, but if the activity is too high, the oil can cause corrosion of the work-piece surface. Correct selection of the compounds used is very important, especially if EP additive concentrates are used. These concentrates are mixtures of EP additives dissolved at high concentration in mineral oil. They are diluted with low-viscosity mineral oil for the particular severity of the application. EP additive concentratesproduce a boundary film with very high loadcarryingcapacity,even at high temperatures.However, their wetting and cooling properties may be poor. In use, there may be problems with the formation of the built-up edge, which may severely affect tool-life and the surface finish of the work.
EP additive concentrates are recommended for working ferrous metals when the tool cuts a minimum chip and the cutting speed is between 2 and 20 r.p.m. (external and internal broaching, thread cutting, reaming, slotting, shaping and hobbing). Polymeric additives are also used in cutting fluids. At high local temperatures (600-900 "C), they gradually dehydrogenate and are converted into a graphitic structure which has an anti-wear effect. At the same time, the nascent hydrogen penetrates the metal, reduces its cohesion and creates a reducing atmosphere around the metal. These phenomena have been observed when polymethacrylates have been used (233). Solid lubricants (mainly graphite and MoS2)can only be used in metal-working fluids as dispersions in mineral oils. They create boundary films with very high loadcanying capacity, however it is very difficult to make dispersions of them which will remain stable for a sufficient length of time. Moreover, these dispersions have only very restricted applications because of their very low cooling capacity. For these reasons, solid lubricants have not been much used in metal-working. Special cutting fluids may contain, in addition to lubricating and extreme pressure anti-wear additives, additives to improve adhesion (high molecular weight polymers and aluminium soaps), odorants (turpentine oils and some esters), scavengers for free hydrogen chloride (e.g.. some phenol epoxides), biocides (e.g., triazines) and other additives.
611
Performance Testing of Cutting Fluids There are no unified, universally accepted quality specifications or machine builders’ specifications,as there are for other types of oil. However, some principles for the evaluating the properties of cutting fluids exist. Essentially, three stages of test are used. Laboratory tests of the chemical properties of cutting fluids comprise, in addition to the more elementary quality indexes, measurement of chemical and physical stability,corrosive effects on ferrous and non-ferrous metals, pH of the water extract, foaming tendency, surface tension, effects on elastomers and machine paint, safety in respect of fire hazard, general hygiene and aggressivity to the skin, and, for some types of cutting fluids (especially emulsions), evaluation of microbial and microbiological degradation properties. Mechanical tests of lubricity (load-carrying capacity of the lubricant film) are made in simulation machines, such as the Fourball tester, the Timken and the Almen-Wieland apparatus, where wear is assessed at constant and variable (increasing) load and for various periods of time. It should be noted that no reliable correlation between the values determined by the various laboratory tests and performance in service of cutting fluids has been found. Tests of cutting fluids have either been dropped, or used only for information. Satisfactory results from laboratory tests can be used to justify proceeding to the next stage of testing, in full-scale machines under simulated operating conditions. In these full-scale tests, the principal functional properties of the cutting fluid are assessed, including the effects of the fluid on the durability of the tool edge and the surface finish of the work, the ability of the fluid to abstract heat from the cutting zone and its cleaning effect. There are many non-standard methods used to examine the effects of fluids on the cutting process. The use of these various methods for particular machining processes is virtually impossible because of the enormous variability of cutting conditions, which are influenced by the different machining operations and materials worked. However, some conditions are identical, or at least similar, namely chip thickness and cutting speed, and these determine the amount of heat generated in the cut. It is therefore possible to select one or two machining operations as typical of a certain range of cutting speed and chip thickness. The Research Institute of Machine Tools and Machining Processes (WOSO) in Prague uses, for the evaluation of the properties of cutting fluids, a set of properties relating to particular machining operations: - operations where a thick chip is removed and the cutting speed is low (e.g., broaching, reaming and tapping); - operations where a smaller chip is removed at a higher cutting speed (e.g., turning); - operations where small particles are removed and the cutting speed is high (e.g., grinding). Reaming has been selected as a suitable process for the evaluation of fluids of the first type. This is based on the effect the fluid has on the formation of a builtup edge, which is the dominant factor in the sliding zone in machining at low cutting 612
speeds. Tool flank wear decreases to a certain point with increase in the built-up edge, so that the surface roughness remains satisfactory. If both the constant and the variable parts of the the built-up edge grow, tool-wear increases and the surface finish deteriorates, since the variable part is constantly renewed and the hardened particles are carried away by the chip and the workpiece and grind off the tool flank. Cutting fluids containing active additives are able to influence the formation of the built-up edge. The less the ability of the fluid to have this effect, the greater the tendency for the reamed hole to be wider than the actual diameter of the reamer. The method assesses statistically the deviation of hole diameter from reamer diameter and the relationship between the mean hole size and the number of holes reamed, at different hole sizes. Changes in roughness as a function of the number of holes reamed, average roughness and its deviations are evaluated at the same time by the same statistical method. A complementary method is available for determining the effect of fluids on the durability of tools intended for cutting at small chip thickness. This is based on the torque on taps. The difference between two fluids is measured as the difference in torques. Fluids intended for cutting with a thicker chip and higher speed can be evaluated on lathes with ferrous metals. The criterion of fluid quality is the durability of the tool. The relationship between tool durability and cutting speed can be determined from Taylor’s exponential expression:
T.Vm = C
(5.13)
where T is the tool durability in minutes, v is the cutting speed in r.p.m., m is the exponent and C is a constant. In a logarithmic plot, this equation gives a straight line; this can, however, break in some elevated temperature regions as a result of the formation of craters, or at lower speeds because of the considerable formation of built-up edges. T values at least three cutting speeds are required to plot a Taylor chart; tan a,which determines the slope of the line, or the exponent m, can be calculated from the plot. The value of rn depends on the quality of the fluid, but it can change according to the metal being machined. The properties of the cutting fluid can thus be characterised by its influence on the exponent m,or, if m values are identical, by its influence on the shift of the entire straight line (i.e., on the magnitude of the constant C). Surface roughness can be determined at the same time from the turning operation. Values for grinding fluids can be determined by measuring the ground surfaces of various steels. These steels may undergo various heat treatment processes. They vary in hardness and different wheels are used for grinding. These methods give some information, but not enough to distinguish in detail between the qualities of different fluids, and particularly emulsions. The VUOSO Research Institute in Prague, already mentioned, has developed a method for the evaluation of cutting fluids for this process which enables the 613
qualities of different fluids to be assessed rather more readily. The method uses a modified full-scale machine and special cutting conditions. The results are then processed by computer. The criterion determining fluid quality for the grinding process is derived from the specific energy, expressed by the equation: (5.14) where F; is the tangential cutting force corresponding to lmm2 active surface area of the grinding wheel (N.mm-'), vs is the peripheral speed of the grinding wheel (r.p.m.), z' is the quantity of material removed per mm of active width of the grinding wheel (mm3/mm.s), h is the equivalent depth of cut (mm) - (the change from rough to finish eg gnnding is at he = 0.1.1Oq3). e' is essentially &e energy requirement to grind lmm3 of material, in J.mm-3. The effect of the grinding fluid can be evaluated from grinding wheel wear, using the grinding ratio: G = vM/vk
(5.15)
where VM is the volume of material ground off (mm3) and V, is the volume loss of the grinding wheel (mm3). The efficiency of the fluid can be determined from the relationship: (5.16) where e' is the specific energy. This method is more objective than that mentioned earlier, in that it is used to assess the effects of the fluid not only from the specific energy requirement, but also from the rate of wheel wear. These tests are followed by a third stage of testing, essentially full-scale field trials under actual operating conditions. In these, the fluid is tested for its anticorrosive properties towards both the machine and the work-piece, its stability, foaming tendency, effects on elastomers and paints, its effects on operator health, including effects on skin and respiratory tract, general hygienic properties, its tendency to foul grinding wheels and the sedimentation rate of grinding dust (used for emulsions). The disposal of waste fluid and associated environmental and economic problems are also evaluated.
5.9.2 Lubricants in Chipless Metal-forming Chipless metal-forming includes hot and cold drawing and rolling of sheet metal, wire and profile section drawing, deep drawing of sheet metal, forging, cutting and stamping (271).There are two main types of forming process, hot and cold, but due 614
to the enormous work expended in forming, which is mostly converted into heat, the material being handled heats up in the cold process so much that the two processes become mixed. The metallurgical difference between hot and cold forming is related to the crystallisationtemperature of the metal (changes in the crystallinity of the system). The advantage of hot forming is lower resistance to deformation because of reduced inter-atomic forces; the material does not harden. On the other hand, the cooling period prolongs the process. Hot forming is applicable only to material within a certain temperature range, of which the upper limit must be below the temperatureat which some of the metallic constituents begin to melt (in the inter-crystalline layer) and the lower limit above the transition temperature. For example, this range is 1315-750 "C for steel, 1200-1100 "C for nickel, 450-35 "C for aluminium and 150-110 "C for zinc alloys. Initial heating can be avoided by cold-forming, but deformation can only proceed so far, determined by the degree of hardening. Forming beyond this, in a second stage, is impractical without annealing. The selection of hot and cold forming depends on the type of material and the properties which need to be achieved in the forming process.
Whereas much material is lost in chip-type metal-working (sometimes as much as go%), the forming process involves no loss of material. Wherever possible, therefore, forming tends to displace chip-type metal-working. Some forming processes are illustrated diagrammatically in fig. 5.42, and the basic parameters of the forming process listed in Table 5.65. Table 5.65. Basic Parameters in Metal-forming Processes Process
Parameters temperature
pressure MPa
velocity m.s-1
drawing
up to 20
0.0005 (bars and pipes) up to 1 (thicker wires) 15 to 30 (thin wires)
cold-drawing - ambient plus heating by friction to 200-300, heat-drawing 400-600; hot-drawing about 800 (for Mo,W
pressing
up to 80
0.02 to 0.05
ambient or medium temperatures, 500-800 (2000 for Ti) normal or elevated temperature (400-1250 under hot conditions)
OC
rolling
20 to 150
5 to 25 (cold rolling) 0.1 to 0.5 (hot rolling)
forging
30 to 130
0.02 to 0.3 (hydraulic forging) 100 to 200 (cold process) up to 10 (power hammer forging) 500-600(hot process) up to 1200
extrusion 100 to 200
0.05 (hot blast extrusion) 0.5 to 5
normal or high frequency (lo00 to 1200). 600 for nonferrous metals (hot process)
The lubricant is an important factor in these processes. Most forming processes are not practical without a suitable lubricant. The composition, properties and method 615
of application of the lubricant decide the service life of the tool, the surface properties of the materials formed and the size of the forming work required (i.e., energy consumption) - the properties of the lubricant must match up to these requirements. The summary below has, doubtless, general validity, but each of the properties mentioned is more or less important, depending on the particular application, and each must be optimised for the relevant forming process.
b
a
/2
1 1
3 C
d /3
e Fig. 5.42. Schemes of the principles of various forming processes a - rolling, b - drawing, c - pressing, d - forging, e - extrusion A typical example illustrates this point. Unlike in the general case of tribotechnology - that the lubricant should minimise friction - some forming processes need a friction coefficient such that friction force and energy consumption are reduced, but sufficient friction to prevent slipping and achieve engagement of the cylinder and ensure the right thickness, shape and surface qualities of the material formed remains. The following equation can be used to calculate the optimum friction coefficient:
(5.17) where Ah is the required thinning of the material and D is the cylinder diameter.
A good lubricant must be able to produce films with sufficient load-carrying capacity and optimal friction coefficient, it must not be corrosive or cause stains on the metal surface or develop offensive odours, it must be physiologically harmless and it must be thermally stable in order to retain its properties at the temperatures generated during the forming operation.Liquid lubricants must wet the metal 616
surfaces and remove heat. Because of the wide range of properties required for the different forming operations, both liquid and solid lubricants, as well as greases, are used. Examples of suitable liquid lubricants include mineral, synthetic, animal, and vegetable oils (or fatty acids) with and without additives, or with solid lubricants as additives (where the lubricating function is emphasised) and emulsions - particularly water-in-oil and aqueous solutions of sodium and potassium soaps (where the cooling function is emphasised). Solid lubricants are used where high load-carrying capacity is required and the abstraction of heat is not critical. The composition and properties of the lubricant must match the tasks involved in forming as well as the conditions to which it is exposed in service. In this respect, lubricants can be classified according to the forming process used.
5.9.2.1 Sheet-metal Rolling Lubricants
In the rolling process, the material passes between two rollers between which it is squeezed to the required cross section. Not only does the shape of the material change, but also its internal structure and mechanical properties. The definitive deformation of the material is the deformation of its crystals. Plastic deformation occurs by the application of an external force (pressure) by the cylinders in excess of its deformation strength. However, the process of plastic deformation does not continue without constraint. As a consequence of the intercrystalline action of the force, and a certain amount of elastic deformation, some residual stress remains in the material after deformation. This stress is manifest by an increase in the work required for deformation - work-hardening. This hardness disappears with a change in structure if the material is heated above its transition temperature and new crystals are created. Hardening does not occur in hot-rolling when the material is heated above its recrystallisation temperature; cold-rolling must always be followed by annealing, i.e., the relief of residual stress. Prior to the hotrolling process, the material - in the form of ingot or bloom - must be heated to the forming temperature. Cold-rolling is used for applications where the material is in the form of, e.g., sheets and strips, which cannot be hot-rolled because they are too thin and would cool too quickly, or where a better surface finish and higher strength are required. A special type of hot-rolling process is used for the manufacture of seamless tube. As stated earlier, the lubricant must match the type of rolling process and the material being rolled. In the hot-rolling of steel, where the temperature exceeds 1,000 OC, lubricants are required with high cooling effect and able to wash off scale, which rapidly forms.The only lubricants suitable for this purpose are water solutions of soaps and oil-in-water emulsions. Oils alone decompose at these high temperatures and leave undesirable residues on the surfaces of the steel. Emulsions are also used for aluminium rolling, in which, although temperatures are substantially lower (up to 500 "C), oil still cannot be employed, for the same reasons. 617
Other non-ferrous metals, such as copper and its alloys, are hot-rolled with water or emulsion, or even in the absence of any liquid lubricant. In the selection of an emulsion, its stability, concentration (oil content in the emulsion),its lubricating power and its effect on the quality of the rolled metal must all be considered. Emulsion stability and concentration, which considerably influence the extent of lubricating effect, are closely associated. Weaker and more stable oil-in-water emulsions have a higher cooling and lower lubricating effect, richer and less stable (metastable) emulsions show lower cooling and higher lubricating effects. Emulsion type, stability and concentration are related to the material rolled and the rolling conditions (degree of reduction of sheet thickness, rolling speed, etc.), the surface quality required and the cooling and lubricating effects needed. The tendency to form undesirable residues and contamination of the rolled surfaces grows with increasing concentration of oil in the emulsion. As the emulsion comes into contact with the hot surface of the rollers and the rolled metal, the water evaporates rapidly leaving behind a thin film of oil, which is subjected to further thermal decomposition. Thus, the amount and type of deposits left on the surface of the sheet also depends on the chemical and fractional composition of the oil and its viscosity; cycloalkanic oils, free from heavy aromatics, without a very high distillation end-point and of low viscosity are therefore preferred. The emulsifiers used for the preparation of emulsions may also have adverse effect. The hot-rolling of strip and'de-scalingof the surfaces is followed by cold-rolling, to achieve the thickness required and to improve surface finish and mechanical strength. Cold-rolling lubricants are expected to remove heat generated by the work of deformation and by the friction produced between the rollers and the work-piece, and to reduce frictional resistance to a minimum consistent with sustaining the rolling process. Lubricant selection is based on the rolling-speed, the material being rolled, the target thickness of sheet and surface finish and the type of rolling mill. The essential criterion, however, is whether heat removal or lubricating power - and hence surface finish - is the dominant requirement. For heat removal, i.e., at high speed, oil emulsions are used; for lubrication, mineral oils, usually treated with additives or compounded with vegetable oils. Stable oil-in-water emulsions are mostly used. However, metastable emulsions are used in exceptional cases, for example in rolling white metal sheets. White metal sheets are rolled to a very low final thickness (0.2 mm or less). Stable emulsionsproduce a dull, matte surface, which affects subsequent electroplating processes, e.g., with thin layers of zinc or chromium.
Emulsions must have sufficient load-carrying capacity (resistance to pressure) to protect the metal surface from damage, but they must not themselves attack the surface so as to promote corrosion. They must not leave residues on the surface of the sheet after annealing, they must have some ability to neutralise acids which could intrude from pickling, they must be mistant to microorganisms and show low foaming tendency and suitable stability. 618
Emulsions are made from emulsion oil, which contains mineral oil, emulsifier, solvent and - if the lubricating properties and load-carrying ability of the lubricant are important - additives. The chemical and fractional composition of the oil are important factors which can restrict the formation of residues on the sheet surface after stress-relieving or after hot-rolling. The emulsifier can affect the anti-rust properties of the emulsion; anionic emulsifiers are better in this respect than nonionics, although some may leave ash after annealing, which fouls the surface. The stability of sheet-metal rolling emulsions can be measured by ASTM D-3342-90.
Cycloalkanic oils containing additives are used for cold-rolling steel at low speeds (below 250 m.min-'), when heat removal is more important than lubrication. Synthetic oils, e.g., polypropylene types, are becoming popular. The rolling oil may, in some instances, be used for lubricating the bearings of the back-up rollers. Oils of lower viscosity (around 3 mm2.s-lat 50 "C) are preferred if clean post-annealing surfaces are required for the sheet. However, oil of such a low viscosity does not have enough load-carrying capacity and must contain additives. These additives must not spoil the post-annealing cleanliness of the sheet surface. Suitable additives include fatty alcohols or sheep wool fat. Some typical EP additives are unsuitable for this purpose. Palm oil is useful for rolling very thin sheets (less than 0. lmm)where lubricating power is important. Palm oil has no real equivalent substitute in cold-rolling, although isopropyl oleate has been proposed and used. Aluminium cold rolling requires either pure mineral oil of low viscosity (5-10mm2.s-l at 50 "C) but adequately high flash-point and additives (usually fatty oils or alcohols). The same oils are suitable for cold-rolling copper and brass, however, 3-596emulsions are also suitable. Bronze requires mineral oils, of similar viscosity, compounded with fatty oils. Additive-treated oil is suitable for titanium. The lubricating power of cold-rolling oils can be assessed by preliminary, simulation tests. The cup test is used for judging the suitability of sheet-metal for deepdrawing. The rolled sheet is pressed into cups of identical diameters. After each pressing, a successively larger diameter blank is used. The depth of the cup increases until a critical deformation is reached and the bottom of the cup breaks. The lubricant properties are assessed from the pressing force applied. since the lubricant has only a small effect on the magnitude of the deformation force @-lo%), the influence of high quality lubricants is not evident until heavy deformations at the limit of the strength of the material occur. Wiegand and Moss's method (78)is quite widely used. The test metal, in the form of a strip, is drawn between two jaws and the relationship between the friction coefficient and the lubricant is measured from the jaw pressure and the reduction in thickness of the specimen. The method has been modified in the Mechanical Engineering and Textile Technology university in Liberec (Czechoslovakia) so that the friction conditions in the test better simulate field conditions. Lowa surface pressures are used and the friction surfaces are substantially larger. In the Iron Metallurgy Research Institute in Prague, experimental two-high rolling mills are used, in which the thickness of the rolled test strip with a layer of lubricant is measured befon? and after the test. Lubricant prope-rtiesare assessed from the change in thickness.
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5.9.2.2 Lubricants for Drawing Wire, Bar and 'hbe
This process consists in cold-drawing wires, bars and tubes through dies with the same diameters as the required product cross-section. The final products must be precisely shaped and have a high surface finish. The so-called wet-drawing process, which delivers an end-product with accurate cross-section and high surface finish, demands careful selection of the lubricant to reduce friction and provide cooling. The dry-drawing method is used for light-duty operations - small cross-section reduction and low material strength, and for thick wires for which a high surface finish is not required. Powdered soap mixed with graphite or other solid lubricants are used. The types and properties of lubricant must relate to the drawing process and the material being handled and its pre-treatment. Pre-treatment of the material can include phosphatising, lime leaching, borax pickling, silicate treating (steel) and oxalate coating (stainless steel). Lubricating ability is emphasised at high load and low speed, viscosity at high speed. The better the lubricant adheres to the surface, the harder it is to remove, so graphite, MoS2 or EP additives are avoided if possible
(104. Hard metals, such as nickel and chromium steels, Ti, Zr, V, Nb, U, Ta, Mo and W, are difficult to draw. The lubricant exclusively used for making superfine lamp filaments with diamond dies is a water emulsion of colloidal graphite and 10%H,SO, (96).
The selection of lubricants for tube-drawing is also dependent on the materials and their pre-treatment. Soaps, fats - alone or with 20-30% of fillers such as lithopone - and concentrated oil-in-water emulsions (1 530%) are suitable for steel, chlorinated waxes for lime-leached stainless steel, graphitised soap powder for pickled stainless steel, aqueous soap solution or "activated" emulsions (with compounded oils) for phosphatised steel and compounded oils. concentrated emulsions, vegetable oils such as rape-seed oil, animal fats such as tallow and other fatty substances or soft lubricating greases are suitable for copper, aluminium and their alloys. Bar-drawing is less demanding on lubricants. The bar-stock, pre-treated by pickling, leaching or burning, is drawn through a die flooded with a liquid lubricant, usually a compounded mineral oil of viscosity 60-150 mm2.s-l at 50 OC (depending on the drawing conditions). Emulsions or powdered soaps may also be used for steel bars and fatty oils for non-ferrous metals. Sheep wool fat is suitable for aluminium and rape-seed for copper and its alloys. In wet wire drawing, concentrated metastable emulsions of good lubricity are usually employed. The property of metastability may be emphasised by using activated emulsion lubricants containing oil compounded with fatty substances or EP additives. Combined lubricants are sometimes used, for example, a liquid lubricant such as mineral oil in the circulating system and a grease in the lubricating system ahead of the tool. A high viscosity mineral oil (about 250 mm2.s1at 50 "C) mixed with fatty oil and soaps is recommended for rough drawing aluminium alloy 620
wires. Oil with lower viscosity (50-150 mm2.s-*at 50 "C) is suitable for medium drawing and 20 mm2.s-l at 50 "C for fine drawing operations. Oil-in-water emulsions are used for copper wire drawing. Mineral or vegetable oils are being replaced by synthetic ester oils and anionic or non-ionic emulsifiers are replacing soaps. Anti-foam and antioxidant additives are also used. The finer the drawing operation, the lower is the concentration of lubricating oil in the emulsion, about 12-18% for rough drawing, 5- 10% for medium drawing, 3-7% for fine drawing, 2-5% for very fine drawing and 1-2% for hair drawing (136). There are few simulators available for testing drawing lubricants. One is the Gumminski & Willis tester (80).The test involves static deformation,in which a sheet metal strip is subjected for a few seconds to high pressure. After the pressure has been relieved, the change in thickness of the sheet, which relates to lubricant properties, is measured. The Press-Fit Test is specified as standard in ASTM TVL-1-2. A steel roller is pressed at 15 mm.min-' for about 3 minutes into a hardened socket of diameter 0.025 mm smaller than that of the roller. The force needed to press in the rollers is characteristic of the lubricant. Stick-slip and friction coefficient are also measured. Selection of suitable lubricants for wire-drawing is difficult, as the drawing operation itself, the nature of the material and its pre-treatment are all involved. The Kellerman and Turlach (79) simulation tester can provide useful information. In this test, a wire drawn through a groove in a block is pressed from the bottom side against the periphery of a disk. As the pressure of the wire against the disk is increased, the wire stops sliding at a certain point and reverses the direction of torque on the disk, so that the latter turns in the direction of the motion of the drawn wire. The properties of the lubricant can be gauged from the pressure reached at the point of equilibrium of the two torques.
5.9.2.3 Lubricants for Pressing Processes
Pressing is taken to mean the processing of metal by pressure resulting in permanent change of shape and appreciable change in thickness of the original metal stock. Pressing includes a number of forming processes, in a narrow sense, like drawing, extrusion and stamping and in a broader sense blanking, cutting-out, bending and punching. Drawing and extrusion processes are the most demanding on the lubricant. Both die and work are normally lubricated. The lubricant is expected to reduce the frictional force in the drawing or extrusion operation, to make the material creep more easily and to avoid cold welds, improve the durability of the tool and ensure that tolerances of the work are met. The lubricant influences the residual stresses in the material, i.e., the ratio of the external to internal friction in the drawn layers. The influence of the lubricant increases with increasing ratio of work surface in contact with the tool to overall surface. Frictional forces can act both positively and negatively on the drawing process. They are desirable if they allow the required deformation. If they impede changes in shape and increase resistance to it, they must be reduced by a lubricant. Because of the very high pressures involved, mixed or boundary friction predominates. The lubricant must match the type of material k i n g drawn, its strength and the degree of drawing. Pure, cycloalkanic mineral oil of medium viscosity (around 50 mm2.s1 at 50 "C) or lower viscosity (around 20 mm2.s-' at
62 1
50 "C) with EP additives, e.g., chlorinated paraffins, or added solid lubricants, e.g., graphite, MoS2, talc, ZnO, PbO, vegetable oils, e.g., rape-seed oil, alone or sulphurised, animal oils and fats (tallow, fish oil), general-purpose oil emulsions or those activated with EP additives, water solutions of sodium or potassium soaps and dry soap powders are all suitable for this purpose in different situations. Solid lubricants - graphite, MoS2 - on their own are suitable for hot-pressing, deep drawing and drawing alloy steels of high strength and low ductility. Glass dust is useful for severe operations like pressing steel, high-temperature nickel alloys, titanium, zirconium, molybdenum and tungsten up to 2,000 "C (104). Non-conductive solid lubricants and lubricants with nonconductive fillers are unacceptable in electrowelding of sheets. Multi-purpose lubricants intended for deep drawing of sheet metal (e.g., automobile body sheets) must not only possess good lubricating power but also prevent rust and be easily removable from the surface of the sheet. These requirements can be met by medium-viscosity, refined, cycloalkanic mineral oils with good surface wetting properties, containing a lubricity additive and corrosion inhibitors.
5.9.2.4 Lubricants for Forging Forging is a hot-forming process. The required metal shape is achieved by deformation with anvil, die-hammer, power-hammer or press-forging processes. Among the forging processes (bending, punching, etc.), die-forging is the most demanding in terms of lubricant selection. The lubricant must protect the die from wear, cool it, reduce friction between the die and the work of die-forging, separate the die surface from the work surface, prevent sticking of scale in the die and improve the creep of the forged material. Lubricants with excellent lubricity and high load-carrying capacity can achieve the required standards. Oils by themselves are unsuitable as lubricants for forging operations with tool temperatures of 189320 "C, because they are insufficiently resistant to pressure, too volatile and tend to decompose at high temperatures. They can, however, be quite suitable as carriers of the lubricants mentioned earlier, unless these are used by themselves. Soaps, e.g., of aluminium or fatty substances, e.g., tallow, sheep wool fat, olein) can be added; these substances improve lubricity and stabilise the dispersion of solid lubricants in the oil. Water or volatile solvents, salt solutions, e.g., of sodium chloride, potassium nitrate, may contain water-soluble sodium or potassium soaps and can act as the dispersing phase for some solid lubricants at lower temperatures. Cold forging of soft metals or low-strength steels requires particularly good lubrication, to promote metal creep and prevent sticking of the metal to the tool. Mineral oils, especially compounded oils, with EP additives are advantageous, as solid lubricants are difficult to remove from the forging surface. soap solutions are suitable for some metals, including copper.
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5.10 SPECIAL FLUIDS "Special fluids" is used here to describe "oily" fluids of which the primary function is not lubrication, such as insulating oils, vaseline or white oils, heat-treatment oils, preventive oils and heat-transfer fluids. Other special fluid types include mould oils, bolt-blackening oils, textile fibre oils, turbine-washing oils, shock-absorber fluids, etc., which are specially designed for very restricted applications.
5.10.1 Insulating Oils This term covers oils used in electrical engineering to provide insulation and heat transfer and dissipation in transformers and contactors (transformer oils), for impregnating paper dielectrics in capacitors (capacitor oils) and for impregnating cable paper insulation or cable insulants (cable oils). 5.10.1.1 Transformer Oils
The main function of the oil in a transformer are to dissipate heat and to act as insulant (128). Transformer internals are cooled by convection and spontaneous circulation of the oil. Circulation results from the variation in density of the oil, which is heated in the hot portion of the transformer and cooled as it comes into contact with the outer wall and cooling elements in the transformer. The higher the oil temperature difference,the more vigorous is its circulation; the rate of circulation decreases with increasing internal friction in the fluid, implying that transformer oils must have as low a viscosity as possible. However, viscosity is restricted by the minimum acceptable flash-point, which is 145 "C (open-cup) or 135 "C (PenskyMartens). This temperature must allow for a considerable safety margin, because of the oil temperature in the transformer (as much as 35 "C higher than the ambient temperature in aircooled transformers). Worldwide standards are unified in this respect and specify this temperature, restricting the application of lower-viscosity oils and, consequently,oils with a high cooling effect. However, these oils have a higher volatility when vacuum-dried and present a higher fire hazard. Modem methods of oil-treatment in transformers necessitate preparation of the oils from n m w distillation cuts, which offer both low viscosity and low volatility. The flash-point of the oil must be monitored periodically during its service life. A drop in flash-point of 15 "Cindicates a serious deficiency, which may be accompanied by evolution of gases and pressure increase in the transformer. The transformer stops automatically if this happens. Since the gases are flammable, this pressure must not be vented unless the lines carrying the gas are earthed and un-protected lamps and other sources of ignition removed. When the insulation is damaged, odourless gases can be evolved. If sparking occurs below the level of the oil, hydrogen, acetylene and hydrocarbon gases are formed. Gas composition monitoring with gas chromatography (224) provides very valuable information on disturbances which may occur in transformers (225).
The viscosities of transformer oils are usually in the range 30 to 35 mm2.s-l at 20 "C and do not exceed 40 rnm2.s-l at 20 "C.The density of a transformer oil must be below 890 kg.m-3at 20 "C.Both these features are essential for winter operation
623
of the transformer; ice which may develop from moisture in the oil is lighter than water and must be prevented from floating in the oil (which in turn becomes denser when cold), as this ice may cause a short circuit. The low-temperature properties of the oil are characterised either by pour-point or by the maximum permitted viscosity at a specified low temperature; usually, both of these criteria are applied. These requirements follow from the need to ensure that the oil remains sufficiently fluid for trouble-free operation of transformers exposed to low ambient temperatures outdoors (including when the transformer is only intermittently under load). The values demanded depend on the climatic zone of application; in the European climate, the pour-point should be below -40 "C and the viscosity below 3,800 mm2.s-l at -30 "C. Insulation resistance depends on the degree of refining and the purity of the oil. The highest insulation resistance is that of pure, water-free hydrocarbons containing no sulphur or oxygen compounds. Such products also do not damage copper and non-ferrous metals in the transformer. The utmost care must be exercised in ensuring the absence of moisture and the purity of the transformer oil during production, packing, transportation, filling and operation. Perfectly dry oil is hygroscopic and can take up or absorb moisture very quickly when exposed to humid air, even for a short time. Even minute traces of water severely reduce the insulating effect of the oil. BSI tests show, that 100p.p.m. of water can reduce the electric strength of transformer oils from 90 to 75 kV. Moisture can also aggravate corrosion in the compartment above the oil level. It may be absorbed in the oil during operation, for example by breathing of the transformer. The vents are therefore usually fitted with a silica-gel, or molecular sieve dessicant. Oil moisture must be monitored periodically or once every six months during operation, Oil which sputters when heated in dry test-tube is moist and must be dried.
Important transformer oil purity criteria include acid number (limited to 0.05 mg KOWg maximum), saponification number (0.1 mg KOWg maximum), negative copper corrosion test and an electrical strength in dry oil of 205 kV.cm'I minimum at 105 "C and 95 kV.cm-' minimum in the freshlydelivered oil. Various apparatustypes used give different values for electrical strength or breakdown voltage. Most standard specificationsrequire the oil to be dried before testing in a prescribed manner. The requirement for electrical strength to be specified on delivery, i.e., before drying, which appears in many quality specifications, does not Seem to be very sensible. This property changes rapidly during handling, transportation and storage of the oil, and its significance is doubtful, since the oil must be dried before refilling the transformer anyway. Some standards do not actually specify any electrical strength value at all, others prescribe periodic monitoring once per year under normal operating conditions and every six months under exceptional circumstances (in dusty or moist environments, or if air circulation or heat transmission are poor). If the electrical strength drops below a certain value, the reason for this must be found.
The loss factor (or phase retardation) , tan 6 at 90 O C is determined on the oil before drying; it must not exceed 0.005, and serves both as an indicator of the severity of refining and of the purity of the oil.
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Functional life and oxidation stability of the oil are economically important, not only in relation to the value of the oil in the transformer but also in relation to the damage which can be caused by metal corrosion (Cu, Fe) and damage to insulation which can be caused by acidic products, deposits in the pipework, impaired heat transfer and losses from oil draining, transformer shut-down and the laborious job of cleaning the internals of the transformer. The oil in the transformer ages at relatively low temperatures. It is thus difficult to find reliable laboratory tests from which drain intervals can be predicted. Tests can only be accelerated by increasing temperature over a limited range, otherwise the thermooxidationprocesses occuring in the oil may change not only quantitatively but qualitatively. There are a number of tests, differing in test conditions and methods of evaluation. Usually, air or oxygen is introduced into the oil, which is heated at 100- 120 OC in the presence of a catalyst, usually copper or sometimes iron; after a fairly long period, 100-200 hours, the condition of the oil is evaluated for acidity and sludge content. In CSN 6235, the oil is oxidised at 100 "C by introducing 3 1 .h-I of oxygen. The acid number after 90 hours must not exceed 0.35 mg KOWg and the sludge content 0.5% weight.
The oxidation stability of the oil can be improved with a low-temperature oxidation inhibitor, e.g., 2,6-ditert-butyl-4-methyl phenol and a metal deactivator and/or passivator. Oils containing such agents must be declared to be inhibited, as some manufacturers and users have remained sceptical about the benefits of inhibited oils. Transformer oils containing oxidation inhibitors may be changed once every 40 years, whereas uninhibited oils need to be changed every 20-30 years. The need to change the oil is indicated by the acidity of the oil, which is tested once per year, up to a neutralisation number of 0.5 mg KOWg. Values of 0.5-1.0 mg KOWg indicate risk and the oil must then be tested more frequently. The oil must be changed if a value of 1.0 mg KOWg is reached, or if the rise in acidity is steeper after the 0.5 mg KOWg value has been reached earlier. The oil is changed if water-soluble acids appear, if n-pentane-insoluble sludges start to develop or if the interfacial tension of the oil decreases below the critical value of 40 mN.m-'.
When ambient temperatures are high and the temperature of the oil reaches its limit, the space above the oil is filled with an inert gas. The stability of transformer oils may be impaired by silent electrical discharges. Discharges occurring at the oiUgas interface cause gas evolution, mainly hydrogen and light hydrocarbons, and the generation of unsaturation in the oil. Gas bubbles reduce the effective thickness of the insulating medium and increase the risk of breakdown, fire and explosion. Unsaturated compounds reduce the oxidation stability of the oil. The gas stability of the oil can be tested by a method developed by the International Electric Commission (IEC). This uses the effect of a 50 kV mm-I, 50Hz electrical field over 18 h on the oil in a gas-tight annular glass capacitor, with a gap of 2 mm and an inner steel electrode. The capacitor is two
625
thirds filled with the oil saturated with the test gas and blanketed with hydrogen or nitrogen. The criterion used is the release or absorption of gas per unit time.
Most national standards specify the minimum quality of transformer oils required for operational safety. Table 5.66 shows the quality standards in force in Czechoslovakia, the USSR, Germany and the USA. The quality of commercial transformer oils mostly exceeds these levels.
Table 5.66. National Standards for 'hnsformer Oil Quality Quality parameter
CSFR USSR GOST BTS KO-100 (CSN (inhibited) 982-68 65 6845) (PND%W~I)
Germany USA DIN ASTM 57370 111040-69(2)
density (kg.m-3at 20 "C), max. 900 900 895 840-910 viscosity (mm2.s-l), max.at -30 "C 3800 2200 1500 1800 0 "C 69 20 "C 40 32 25 37.8 "C 12 40 "C 14 50 "C 9.0 pour-point ( "C), max. -40 -45 -45 -40 135 130 flash-point (P-M)( "C), min. 135 140 (o.c.)( "C), min 146 oxidation stability (3) (4) (IEc444) (ASTMD-2440) 9% sludge, max. 0.05 0.05 0.01 0.06 0.7 acidity (mgKOWg). max 0.35 0.35 0.10 0.30 2.6 acid number (mgKOWg), max. 0.05 0.05 0.02 0.03 0.05 sap. number (mg KOWg), max. 0.1 0.60 lmax. lye test (CSN 65 6227) 1 max. corrosive sulphur test (CSN 65 6247) (GOST 859-66) (ASTM D-1275) negative negative negative negative negative water content (p.p.m.), max. 35 ash (46 weight), max. 0.008 0.005 electrical strength (kV .cm-'), min. as supplied 95 22 30 after drying at 105 "C for 30 minutes 205 46 50 loss factor (tan 6)at 90 "US0 Hz as supplied 0.005 0.003 0.01Y70°C 0.005 (GOST6581-66) after pre-treatment 0.18 interfacial tension (mN . m-l).'),min. 40 colour, max. 25 1 (GOST2667-52) (1) Inhibited with 2,6-ditert-butyl-4-m1hylphenol. (2) The requirements of ASTM specifications for inhibited oils am identical, except for ASTM D-2112. stability after 195 h, is applicable to inhibited oils. (3) 100 "U96hourSl3 1itre.s O2per hour. (4) 130 Oc/90 hoursn litres 0, per hour.
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oxidation test:
Cycloalkanic oils with excellent low-temperature properties and a low concentration of light aromatics (which act as gas strength improvers) are preferred, but rather scarce. Alkanic oils have therefore been introduced. These are severely de-waxed or, alternatively, moderately de-waxed and dosed with suitable pour-point depressants. Antioxidants and light aromatics are also added. Polyalphaolefins or dialkylbenzenes are used in special cases, alone or blended with mineral oils. Chlorinated biphenyls were used as non-flammable or fire-resistant oils. However, they are toxic, non-biodegradable, and prone to the evolution of hydrogen chloride, being subject to silent electrical discharges. Therefore, they are no longer used, being substituted where fire-resistance is needed by silicones and other special oils (238).
5.10.1.2 Oils for Contact-breakers
I
The main function of contact-breaker or switch-gear oils is to quench the electric arc generated between contact-breaker conductors at the moment of disconnection of the current, to act as insulators during the short time of disconnection and to cool and dissipate the products generate from the arc. This requirement is best met by transformer type oils with low viscosities (including at low temperatures),low pourpoint and higher aromatics content, with higher oxidation stability. They must not contain or produce corrosive sulphur, even at the cost of some oxidative stability. The decomposition products of the arc produce gases and soot. The latter reduces the electric strength of the oil and deposits in the pipe-work. The oil must therefore be filtered from time to time and the pipes coated with soot-repellent paint.
5.10.1.3 Capacitor Oils In a capacitor, the function of the oil is to soak the paper dielectric and fill the compartment between the electrode foil bundle and the casing of the capacitor. Because of the relatively thin dielectric layer, the load on a capacitor oil is greater than that on transformer oils or oil in high-voltage cables. Capacitor oils thus have to possess low volatility, high resistivity, low loss factor, high gas and overall stability in a strong electric field (as in the case of contactbreaker insulating oils), high oxidation and thermal stability to avoid thermal decomposition and the generation of light gases, especially hydrogen. Transformer oil types refined to give a low loss factor are suitable. The quality parameters of such capacitor oils for use in the USSR are shown in Table 5.67. Permitivity is a significant property, expressed by the dielectric constant (DC). High permitivity allows volumetric capacity to be increased, which brings about savings in material and, in the case of capacitors with paper dielectrics, an increase in the voltage gradient resulting from better distribution of the electric stress between the paper and the impregnating agent. Permitivity depends on the constitution of the product. Polychlorobiphenyls (PCB) were formerly used for this reason (222).These products, with high DC’s (up to 11) cannot be used now, on safety and environmental grounds. They have been replaced by castor oil (“autoregenerative”DC 5.3), diester 627
Table 5.67. USSR Quality Standards for Capacitor Oils Quality parameter viscosity (mm2.s-1) at 20 oc at 50 "C acid number (mg KOWg), max. lye test ash (% weight), max. ( "C),min. flash-point (P-M) pour-point ( "C),max. water-soluble acids and bases mechanical contaminants transparency at +5 "C specific volume electrical conductance (S.m-') at 20 "C, min. at 100 "C, min. electrical strength at 50 Hz (kV.cm-I), min. dielectric strength at 20 OC at 1000 Hz at 50 Hz loss factor (tan 8)at 100 "C at lo00 Hz,max. at 50 Hz,max.
COST 5775-68 37-45 9-12
0.02 1 0.0015 135 -45
absent absent transparent 1.10'6 1.104 200 2.1-2.3 2.1-2.3 0.002 0.005
oils (DC 5.2), silicones (DC 2.2-2.4) and their fluorinated derivatives (DC 5.2). Synthetic aromatic oils with two (rarely more) benzene nuclei, condensed aromatics such as alkylnaphthalenes and alkyl (isopropyl) biphenyls, or alkyl-bridged aromatics like partially-hydrogenatedalkylstyrene dimers are quite extensively used.
5.10.1.4 Cable Oils The properties of cable oils are related to the type of cable. In conventional highvoltage cable (up to 70 kV), the paper insulant used is impregnated with a very high viscosity oil, further thickened with a natural resin (colophony) or a polymer such as polyisobutene. The insulant is normally enclosed in a lead sheath. Under operational load, the insulant alternately expands and contracts. The lead sheath cannot follow these movements and remains permanently expanded. The pressure in the insulant drops, and significantly sub-atmospheric pressure can arise, which causes the formation of gas and vacuum bubbles (vacuoles), in which ionisation occurs, which a resultant risk of electrical breakdown. Therefore, insulant in very high voltage cables (above 70 kV) is maintained under permanent pressure (up to 2 MPa) by inert gas (nitrogen) or oil, to prevent the generation of vacuoles. The pressure medium may be separated from the insulant by a lead or other sheath, or it can be in direct contact with the insulant in a hollow cable core or in the sheath channels. Transformer oil type, low-viscosity oils are preferred where the lower and medium pressure zones are separated and medium-viscosity oils for direct contact
628
and higher pressure. The higher the voltage, the lower should be the viscosity of the oil in order to control heating of the cables through ohmic losses in the conductor and dielectric losses in the insulant. The requirement for good mobility is not necessary in the case of gas-filled cables and high-quality impregnating oils thickened with polyisobutene (as much as 30%) of relative molecular weight 1,5OO-2,ooO, or with the so-called non-migrating insulators for rugged terrain, ceresine (up to 50%) or colophony (around 5%). Regardless of viscosity, all cable oils must possess certain properties in common, such as low dielectric losses (low tan 6, low loss factor), low permitivity (high dielectric constant),high resistivity, electric and gas strength and oxidation stability. Well-refined oils have low tan 6. Colophony does not affect dielectric loss up to about 80 "C,but increases it at higher temperatures. Polyisobutene is preferable on these grounds. The dielectric constant is a simple criterion of the polarity and polarisability of the oil. The higher the dielectric constant, the higher the dielectric losses. Water, dust, miscellaneousphysical contaminants and refining agent residues all increase conductivity. Oils with low tan 6 also have good electric strength. Gas strength influences the formation of vacuoles. It depends on the composition of the oil. Alkanic oils have low gas strength, cycloalkanic oils are more resistant. The thickeners mentioned above improve gas strength of viscous oils. Low and medium viscosity oils can be improved by blending with 1520% of high-quality, well-refined aromatic extracts, particularly those which can be converted by hydrogenation into oils containing cyclohexane rings (106). Besides aromatic extracts, synthetic C,-C,, dialkyl benzenes are used for blending (106). Good gas strength is more important in cable oils than in transformer oils. Gas strength may also be tested in a hydrogen environment at a 4 kV.mm-' voltage gradient for two hours, or deduced from UV absorption.
Oxidation stability is an important property. Unstable oils, during oxidation, produce soluble and insoluble substances which damage electrical properties and, above all, aggravate dielectric losses. Despite the fact that the oil in the cable does not come into contact with air, oxidation stability effects appear as a result of residual oxygen arising from the impregnation process. The impregnation process takes place at 120-130 O C , with restricted access by air, and can last as long as 48 hours. Oxidation usually occurs at 100-120 "C in the presence of copper. A sign of oxidation instability is a change in tan 6 and the appearance of the catalyst after oxidation . Viscosity index is another essential attribute. The VI of the viscous oils for impregnating paper should be as low as possible, since the oil should be thin during impregnation but thick during use, to avoid it running down the cable. Colophony further decreases the VZ of the oil. On the other hand, the oil used as the pressure medium in hollow cables must have a high VI in order to remain mobile in spite of temperature fluctuations. Pour-points should be low. Viscous impregnatingagents should not solidify when 629
the cable is being laid and low-viscosity oils should remain mobile even at low temperatures. Both low- and high-viscosity oils are preferable made from cycloalkanic crudes. Low-viscosity, severely de-waxed alkanic oils may be moderately improved by adding light aromatic extracts. Heavier, wax-free extracts are also suitable as highly viscous paper impregnating agents; they reduce the VI and prevent separation of the colophony from solution. Their compatibility with polyisobutenes should be tested. Colophony must be pure and distilled. It tends to crystallise, but this tendency can be suppressed by the extract itself (107). The use of colophony is becoming obsolete.
The quality specifications of pressure cable oils are detailed in Table 5.68 and examples of high-viscosity oils and impregnating agents are illustrated in Table 5.69. Table 5.68. U.S. Quality Standards for Cable Oils Quality parameter
density (kg.m") at 15.6 "C viscosity (mm2.s-') at 37.8 "C at 98.9 "C flash-point (o.c.)( "C), min. pour-point ("C), min. total sulphur (%), max. corrosive sulphur (ASTM D-1275) acid number (mg KOWg), max. colour (ASTM D-1500). max. loss factor(tan 6) at 100 "C and 50 Hz,max. electrical resistivity (0hm.m.') at 100 "C, min. electric strength (kV) by ASTM D-1934 -without catalyst), min. oxidation stability by ASTM D-1934 -without catalyst acidity (mg KOWg), max. colour (ASTM D-1500), max. loss factor (tan 6)at 100 "C and 50 Hz,max. electrical resistivity (0hm.m-l) at 100 "C, min. water content (p.p.m.) specific optical dispersion at 25 "C
630
ASTM D-1818 Oils for low pressure cable systems
ASTM D-1819 Oils for high pressure cable systems
ASTM D-2297 Oils for capacitors and cable accessories
890-905 21 .l-22.4 2.6-4.2 149 -40 0.25 negative 0.014 1
9 17-930 162-173 9.7-11.1 193 -21 0.35 negative 2
922-934 432-561 20.4-22.5 235 -5 0.35 negative 0.04 2.5
0.002
0.006
0.005
500.109
25.1O9
10.109
30
30
30
0.02 2
0.1
4
0.06 5
0.007
0.028
0.02
20.109 45 113.104
5.109 45 110.104
1.109 35 1 13.104
0.05
Table 5.69. Properties of High-viscosity Cable Oils and Colophony Impregnants Oils lighter heavier
927 density ( k g . ~ n - ~ ) at 20 "C viscosity (mm2.s-') at 60 OC 100 at 100°C 17 flash-point (ox.)( "C) 220 pour-point ( "C) -10 0 loss factor(tan6)at 100 "C, 50 Hz,initial 0.001 after ageing specific resistance (ohm.cm-1.1012)at 100 "C initial after ageing -
Impregnants lighter heavier
932 220 34 240 5 0.001
320 33 210 8
-
0.5 1
0.3 0.75
-
7 3.5
10 5
960
960 530 530 222
5.10.2 White (Vaseline) Oils White oils are essentially neutral mixtures of pure liquid hydrocarbons, predominantly alkanes and cycloalkanes, non-aromatic or low in aromatics and free from heterocyclics. They are made by deep acid-refining or hydrogenation of waxfree, usually cycloalkanic oils. They are classified by their composition, properties and applications into medicinal and technical oils, e.g., in the USA (by the Food and Drugs Administration -FDA) into the categories: 21 CFR 172.878 (for processing of foods which must contain not more than 0.1-0.5% of oil), 21 178.3620a (permitted to come into direct contact with foods), 178.3620b (technical white oils).
5.10.2.1 Medicinal White Oils Medicinal white oils, Puruflnum Liquidum, must conform with the pharmacopoeia of the relevant country. They are used in pharmaceutical products as carriers and solvents for solid, semi-solid, plastic and liquid pharmaceuticals, ointments, separation agents in tablets and capsules and in other medicaments, e.g., laxatives. Developing of specifications must obviously be strictly followed. In the cosmetic industry, they are used in the manufacture of creams, lotions and many special formulations. In foodstuffs, they can be found in use everywhere that a lubricant comes into contact with food, e.g., in greasing baking-pans, the prevention of sticking of doughs, in the manufacture of dried albumin, doughs and paste and as ingredients in chewing gums (to which special requirements apply). In all these industries, machinery is lubricated mainly with these oils if it contacts the product. Medicinal white oils are high purity products. They must be transparent and nonfluorescent, including in UV light, with restricted absorbance at 220-280 nm. They must be colourless, tasteless and odourless. They must be insoluble in water and alcohol, soluble in ether and chloroform. They must be free from readily
63 1
carbonisable residues and of reducing substances. They must not contain paraffin wax, sulphur compounds, chlorides and sulphates. They must not be detrimental to health and must not be skin-irritant. The pharmacopoeia in different countries agree on essential requirements but differ on details. Thus, the US specification specifies density of 840-905 ,UK 870890 and Germany 830-870 kg.m". Viscosities should be 38.1 mm2.s1 minimum at 37.8 OC in the USA, 64 mm2.s1 minimum at 37.8 O C in the UK and 100 mPa.s at 20 "C minimum (for paruffinurn subliquidurn) in Germany. Only specified antioxidants are allowed in specified concentrations, e.g., tocopherol, 2-tert-butyl4-methoxy phenol, at e.g., 10 p.p.m.. Medicinal oils for chewing gum must satisfy very special requirements; oils of very low viscosities were used in the preparation of nasal sprays, but this was abandoned when it was demonstrated that prolonged inhalation of the aerosol could give rise to lipoid pneumonia.
Medicinal oils used as lubricants have wide viscosity ranges, from about 6 to 43 mm2.s-l minimum at 50 OC,medium viscosity indexes (50-75) and mostly low or very low pour-points. Some quality specifications for medicinal white oils are detailed in Table 5.70. Table 5.70. National Specificationsfor White Medicinal Oils CSFR USSR USA CSN 65 7310 GOST 3164-52 USP XVIII 1970 NF 1970
density (kg.~n-~) at 20 "C viscosity(mm2.s.') at 20 "C at 37.8 "C at 50 "C alkalinitylacidity purity tests: solid alkanes sulphur compounds
850-905
875-890
847-907
820-882
65 min.
-
-
38.1 min.
37 max. 0
0
28-36 0
-
0 stipulates maximum opalescence at 0 "C
complies with
absent
CS No.3 pharmaopeoia
test with sodium plumbate negative, some colouration in acidic layer allowed.
carbonisable substances
chlorides sulphates fluorescent substances
complies with pass CS No.3 pharmaopeoia
reducing substances skin irritation (CSN 65 7310, Pt.5) pass UV absorbance of dimethyl sulphoxide extract at 260-350 mm -
632
-
0.1 max.
-
5.10.2.2 Technical White Oils White and semi-white oils are used for lubrication as well as for other purposes. They are used for lubricating and processing in the textile industry (lubrication with self-acting lubricators with these oils is compulsory; insufficiently-refinedoils may cause scrota1 cancer), in compressors for the manufacture of polyolefins and where staining of products and a hazard to health contact with the lubricant poses. Other applications include the manufacture of cosmetic products (where, however, they tend to be displaced by medicinal oils), preventive coatings for eggs, textile fibre processing (silk and synthetics) and as extenders and plasticisers for rubber and plastics. Special applications include dielectric fluids for metal-working by sparkerosion and as absorbents in gas filters. Technical white oils may also include the compounded oils which are used for coatings in the manufacture of of aluminium crown caps for milk bottles, etc. These applications do not require as strict physiological control as for medicinal oils, but they must be colourless, their colour must remain stable to heat and light and, for some applications, e.g., lubrication of textile fibres, oxidation stability is required. Their versatility in application calls for a wide range of viscosities, from 4 to 22 mm2.s1at 50 "C. Viscosity index requirements are the same as those for medicinal oils. They are not biodegradable. Quality parameters for technical white oils are illustrated in Table 5.71. Table 5.7 1. National Quality Specifications for Technical White Oils
csm Light Heavy cosmetic oil (TPD 42/953-61)
UK Spindle oil (1)
density (kg.m3) at 20 "C 870 880 viscosity(mm2.s-') 22-30 30-49 at 20 "C at 60 "C 16-21 flash-point (o.c.)( "C),min. 130 140 -30 -25 pour-point ( "C), max. 0 0.03 acid number (mgKOWg), max. 0.01 0.003 ash (% weight), max. colow (IP 17B) max. 0.5 UV absorbance (3) 280-289 nm 290-299 nm 300-329 nm 330-350 nm (1) Under S1 1953 amendment no. 1545 the oil must be deep-refined with sulphuric acid. (2) Produced according to Federal Register 2, Dec.1964,29. 16079, Section 121.2589. (3) Of dimethyl sulphoxide extract.
USA Technical white oil (2) -
-
1.75 4.0
3.3 2.3 0.8
633
5.10.3 Heat Treatment Oils for Metals The term heat treatment is used here to describe the controlled chilling or heating of metals in the solid state to change their properties. Oils are mainly used for quenching, heating or steel-tempering (stress-relief). 5.10.3.1 Quench Oils
Quenching is the most common method of heat-treating steel. This process enables steel of a specific composition to acquire the appropriate properties for specific purposes. The steel is heated to a temperature over 750 "C, at which it has the austenitic structure, and then chilled, so that the austenitic structure of the gamma iron is changed into other structures, depending on the rate of chilling. Generally, the more rapid the chilling the harder the steel fig. 5.43).
STABLE AUSTENITE
9c a0
#
300
f
200
F
100 -0
1
hENi!lTE*PERUfi PERLITE 1
1s
lmin
lh TIME
Fig. 5.43. Effect of chilling speed on the metalographic structure of steel (S stands for steel) Slow chilling converts austenite into soft perlite, more rapid chilling gives rise to the harder bainite and very rapid chilling hard and not very tough martensite. The critical rate of chilling at which the austenite is converted into martensite alone depends on the composition of the steel.
Chilling proceeds in three stages, the progress of which determines the final structure and properties of the treated metal. In the first phase, the surface of the work-piece is surrounded by the vapour of the violently-heated chilling liquid. Chilling occurs by radiation through this vapour envelope, of which the conductivity is low; the effect in this phase is also low. The duration of this phase is prolonged if low-boiling liquids are used. In the second phase, the work directly contacts the liquid. No continuous vapour envelope develops and the liquid vigorously boils on the surface of the work. Residual vapour in the form of bubbles penetrates into the bulk of he liquid and is carried away by convection. This is the most rapid phase of chilling. In the final phase, chilling occurs by convection and conduction, which
634
depend on the viscosity and thermal conductivity of the quench liquid. This is the slowest phase. Three methods of quenching are used - continuous or “cold” quenching, thermal quenching and isothermal quenching. Cold quenching, with which a high degree of hardness of carbon steel can be achieved, requires rapid chilling and uses water or 510% sodium carbonate or calcium chloride solution as chilling agent. The salts present break the vapour layer in the first phase by crystallising on the work surface, bursting with the heat and churning up the vapour layer, thus acceleratingthe first phase of the chilling process. Mineral oils are used for most alloy steels and air alone for high-grade alloy steels. The process results in the generation of a very hard steel, but it has low toughness and suffers from high internal stress, which can cause distortion and cracks in the case of very large or intricately-shaped work-pieces. This risk is considerably greater when water is used for chilling, as the chilling effect is much greater than when oil is used. Oil quenching, where the chilling proceeds more slowly, yields steel with lower levels of internal stress and less distortion. Additivefree oils may, however, cause problems in optimising through hardenability and can therefore yield softer structures. “Through-hardenability”is the ability of the steel to acquire the required depth of hardness. Unlike hardenability, i.e., the ability of the steel to acquire through quenching a certain degree of hardness, which depends on the carbon content of the steel, through-hardenability is mostly influenced by the presence of alloying additives and is closely connected with the shapes and positions of the curves in the phasediagram chart of austenite conversion in continuous chilling.
W
850 800
OIL TEMPERATURE 180 OC
-
BASE OIL
a P O 600-
6 0
2 500400300200 100
I
5
10
15 20 25 3orcc TIME SINCE THE BEGINNINO OF CHILUNG
Fig. 5.44. Differences in chilling curves of additive-free oil and oils with different amount on additives A - v a p w blanket, B - nucleate boiling. C - convection
The through-hardenability of steel can be improved with special agents which are added either into the steel (boron) or into the quenching oil. ‘These additives improve the chilling effect of the oil and the rate of chilling, particularly at a
635
temperature of around 500 "C, where perlite is generated as well as martensite. The chilling curve of an additive-treated oil differs from the curve of an additive-free oil and lies nearer the chilling curve of water (fig. 5.44) Additive-containing quenching oils may be used to achieve the required hardness of thinner cross-sections of low-alloy steels and even plain carbon steel. A high-performance quenching oil should have the following characteristics: - accelerated cooling-rate to give maximum steel hardening response and permit the use of cheaper steels, - minimum deposition on the steel surface and minimum sludge formation, - minimum viscosity increase to reduce drag-out loss of oil on the surface of the components, - minimum acid formation to prevent staining of bright steel components, - minimum tendency to produce cracking and distortion. This performance must be maintained throughout the service life of the oil. The very severe treatment that quenching oil receives can cause the properties of the oil to change dramatically during service. Additives which improve the rate of chilling of quenching oils are polar compounds with high affinity to the treated steel surface. This affinity helps break the vapour envelope in the first phase of chilling and thus shortens it, making the wetting effect of the oil in the second phase more effective, consequently changing progress along the chilling curve. Vegetable oils (e.g., rape-seed oil) or animal oils (e.g., whale oil) have higher affinities than mineral oils, but their deficiency is instability to oxidation and the production of rubbery substances, sludges and soaps which foul the work surface, spoiling the chilling effect and reduce the service life of the oil. For this reason, detergent-dispersants, such as calcium sulphonate and phenolate and succinimides and their mixtures are nowadays regarded as superior quenching additives; they are added in combination with oxidation inhibitors to prolong the service life of the oil, and anti-foams. With a suitable emulsifier, the oil may be converted into an emulsion after the quenching process and washed off. These oils can contain 2 4 % by volume of detergent-dispersants; the amount depends on the type of quenching process - at higher quenching-bath temperatures, the detergent-dispersant concentration must be higher. The base oil, or the additive-free oil, should have adequate viscosity, high flashpoint, high initial boiling-point and short range of boiling-points after this, i.e., a narrow fractional composition, and a long service life. The quenching efficiency of the oil may depend on its thermal conductivity, latent and specific heats, but there are not marked differences in these properties and they need not be further considered here. Higher chilling rates and reduced losses by oil being carried out with the work-piece can be achieved with low-viscosity oils, but at a possible cost in terms of volatility and flash-point - important in the elimination of fire risk. Wellrefined, alkanic oils with viscosities of 10-15 rnm2.s-l at 50 "C, flash-points of 160-180 "C, initial boiling-points of 300-350 "C and boiling range about 100 OC are preferred. 636
Synthetic quenchants, e.g., polyethers, can also be used. In comparison with mineral oils, the first quench phase is shorter, they possess higher oxidation stabilities and they offer a lower degree of fire hazard. Modern quench fluids are non-flammable aqueous solutions of polyalkylene glycols. These are fully soluble in water up to 80 “C,but separate from water solution at higher temperatures (e.g., Breox Quenchant offered by British Petroleum Co. see Table 5.72). They produce on the surface of work dipped into the quenching bath a polymer film, which often prevents the occurrence of a chilling shock at high temperatures, which can cause cracks and stresses in the treated metal. At lower temperatures, the polymer enters into solution and accelerates the chilling process. Table 5.72. Composition and Properties of Breox Quenchant Products Breox A appearance
composition
Breox B
semi-thick, honey-like, colourless to slightly yellow odourless liquids; readily miscible in water and common organic solvents ethoxylated and propoxylated n-butanol in molar ratio of 4:1
properties: relative molecular weight (dry matter) 2090 viscosity (mPa.s) at 20 “C 975 refractive index n2E 1.4122 alkalinity (mg KOWg) 26.3 water content (% weight) 50.3 ethoxy-group content (% weight) 47.5
ethoxylated and propoxylated n-decanol in molar ratio of 9:l
2440 1255 1.4137 24.6 48.1 48.2
A second group of products includes polymers, e.g., polyacrylamide, which generate a protective layer around the work and thus stabilise the vapour envelope and settle on the work at the end of the process, reducing the rate of chilling in the lower temperature phase. The rate of chilling can be controlled by the polymer film which depends on the concentration of polymer in the chilling bath, the bath temperature and the extent of mixing of the tank. These non-flammable water solutions of polymers are useful for cast iron, steel and aluminium alloy. Some colloids, e.g., sulphite waste liquors from cellulose manufacture, have a similar effect to these polymers in the second group.They suffer the deficiency that the water solution deteriorates through microbial action. In thermal quenching, also referred to as “marquenching”, “martempering” or “step-quenching”, the work is dipped into the oil-bath which is held at 120-200 O C . This type of quenching is suitable mainly for thin-walled, intricately-shaped work, where the high internal stress and distortion normally developed during simple quenching must be avoided. The basis of the process consists in prolongation of the heat-soak period just above the austenitic transition temperature. The work is dipped
637
into the warm bath and held there sufficiently long for the temperature throughout the whole section to reach that of the bath; it is then chilled by ambient air. A structure is formed in the steel which comprises bainite, martensite and residual austenite; this steel is very hard and quite tough. In this quenching process, the oil is exposed to high temperature for long periods. It must therefore have a high initial boiling-point and flash-point (at least 10-20 "C above the bath temperature) and high thermooxidation stability. Well-refined oils with viscosity 15-30mm2.s-l at 100 "Cand flash-point 230 "Cminimum, containing packaged additives corresponding to the composition mentioned earlier. These increase the rate of queching, oxidation stability and the detergent-dispersant effect of the oil. In isothermal quenching ("austempering"), the work is heated at the austenitic zone temperature is allowed to soak in the oil-bath, but in this case at 200-240 "C to complete the transition from the austenitic structure into the hard, tough, bainitic structure. Isothermal quenching at about 400 O C is particularly suitable for small work, like oil-quenched carbon steel wire, (the so-called patenting) , for which a strength above 1-1.2.103 MPa is usually required. Oils with a high flash-point are generally required as these oils are subjected to higher thermal load. NUCLEATE SOILING COOLING RATE T/SEC
850
700
600
500 u)o 300 200 P R O E TEMPERATURE OC
Fig. 5.45. The record of speed. of the sample chilling in laboratory quenching speed test (Wolfson steel probe test) The quenching ability of fluids can be assessed by a laboratory quenching rate test, in which the metal sample equipped with a thermocouple is heated at over 800 "C and then dipped into the test fluid. The rate of chilling of the sample is then recorded; the record shows three phases (fig. 5.45). Three tests are based on this principle: the Wolfson test which uses a steel specimen, the nickel-ball test and the Silver Ball Quenchometer. High-quality thermal quenching oils and fluids should reach a quench-rate of about 80 "C.s-l in the boiling zone during the Wolfson test. In the silver specimen test, the chill-rate is substantially higher because the thermal conductivity of silver is higher; the chill-rate with high-quality fluids can be as high as about 400 "C.s-'. In the nickel-ball test , the progress of temperature change is recorded as the ball heated at 850 "C is chilled to 354 "C (the Curie point) at which nonmagnetic nickel is converted into the magnetic form. This method is less informative than those mentioned earlier.
638
Oil ageing tests are also important. The oil is repeatedly subjected to cyclic thermal shocks under oxidation conditions. The change in chill-rate before and after the test, deposits on the heating elements, the amount of sludge in the oil, changes in oil viscosity and the acid number of the oil are all assessed.
5.10.3.2 Oils for Tempering (Stress Relieving) Internal stresses developed during rapid quenching may be detrimental in subsequent processing of the steel. To eliminate internal stress, the oil is gradually heated to a temperature of about 300 "C (low tempering), so that martensite is converted into less hard tempered martensite, or to 500-650 "C, when cementite particles in the steel grow, so that its hardness decreases and its toughness improves. This heating is followed by soaking at pre-determined temperatures for specific periods. After this, the steel is rapidly chilled by dipping in water or oil; this prevents the development of temper-brittleness (which occurs in nickel-chromium and chromium-manganese steels) as a result of separation of carbides, phosphides or nitrides. Oils can be used for both heating and chilling. Heating (in low tempering) requires oils of good thermal stability, sufficiently high initial boiling-points, the highest attainable flash-points, etc. Chilling requires refined oils of lower viscosities and better thermal conductivity; oils may contain oxidation inhibitors to prolong the service life of the oil-bath. Bearing oils of viscosities around 30 mrn2.s-l at 50 "C are normally used for this purpose.
5.10.4 Heat Transfer Fluids The disadvantages of direct heating and high-pressure steam heating have stimulated the development and use of indirect heating by means of heat-transfer fluids or melts (heat-transfer media). Direct heating requires a self-contained heat source for every unit. It is difficult to maintain a constant temperature of the materials being heated or the required temperature pattern. There are risks of local over-heating of the material, fire or explosion if the materials are flammable and come into contact with the naked flame. Indirect heating with steam also has drawbacks; it requires expensive, high-pressure installations and feed-water treatment. Long distribution lines involve considerable thermal losses form condensation, leakage, etc., and many problems arise if heat has to be supplied for several processes, each of which requires different working temperatures. The process may become uneconomic.
Heating with heat-transfer fluids allows the use of non-pressurised heating systems, rapid and accurate temperature control and ready transitions from heating to cooling. The operation is safe and several processes may be incorporated in the same heating circuit even though they each operate at different temperatures. The heating unit may be separate from the heat-generator if space is restricted. Energy saving may also be possible. The usual heat-transfer fluids are hydrocarbon oils, mostly mineral oils, and synthetic oils - mostly non-hydrocarbon oils.
639
Water may be used as a heat-transfer fluid; pressurised water is used for temperatures between its freezing point and its pressure-elevated boiling-points; aqueous solutions of ethylene glycol are favoured for tracing lines which contain products which may solidify. Melts of some metallic salts are also used, e.g., mixtures of sodium nitrite and potassium nitrate (up to 550 "C - with a fire hazard), barium chloride (up to 1,300"C), metal melts, e.g., sodiudpotassium (up to about 750 "C) or sodium alone (up to 900 "C) (both carrying fire and explosion hazards, especially in contact with water), carbon dioxide and, exceptionally,mercury vapour (which carries health and recovery problems). Some of these heat-transfer systems are to be found in very special, demanding situations, such as nuclear power plants, where other safety considerations dictate unusual approaches to heat-transfer .
5.10.4.1 Hydrocarbon Oils
The use of heat-transfer oils is confined to the temperature range between their pourpoints and boiling-points. In practice, the range of use is narrower than this, for several reasons. Starting up a cold system with liquids with inadequate fluidity, or a high pourpoint, is prolonged by the time required to reach a viscosity sufficiently low for the pump to provide adequate circulation in the system (i.e., less than 300 mrn2.s-l at the pump intake). This means that wax-free cycloalkanic or aromatic oils are preferred. On the other hand, the bulk temperature of the oil should always be at least 20 "C lower than the initial boiling-point and at least 40 "C lower than the cracking temperature, which is about 360 "C for alkanic oils and 380-390 "C for aromatic oils; it is higher for light than for heavy oils. To reduce the tendency to crack (which promotes the formation of undesirable decomposition and heavy products in the oil, impairs water separability, increases foaming tendency and coking of the walls, reduces heat-transfer and increases fire risk by bursting of blocked tubes), no hot spots must occur on the inner surfaces of the tubes in the heat generator. The viscosity of the fluid at the bulk operating temperature should be about 5 mm2x1. At the same viscosities at 50 "C - between 15 and 35 rnm2.s1 - a viscosity of 5 mm2.s-' is reached at lower temperatures with oils of lower VZ, i.e., with cycloalkanic and aromatic oils rather than alkanic oils. This implies that alkanic oils should have a lower viscosity at 50 "C than aromatic or alicyclic oils; the lower viscosity of alkanic oils also ensures that they will have higher stabilities at operating temperatures. Any oil used must consist of close-boiling cuts and its closed cup flash-point must be sufficiently high. Aromatic oils have better thermooxidation stability than inhibited alkanic oils. In both types of oil, the use of suitable antioxidants and DD additives may be beneficial. However, oxidation stability is less important in enclosed systems. Apart from dissolved air, the oil seldom comes into direct contact with oxygen; air can only penetrate into the enclosed oil circuit through the cold expansion vessel which is installed to compensate for volume changes and this vessel is often blanketed with inert gas.
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Low viscosity, low pour-point oils are used for outdoor systems where considerable ambient temperature variation occurs, e.g., in road construction. Mineral oils of appropriate composition, viscosity and flash-point are used for open heating systems up to 250 "C and in enclosed systems up to 320 "C. Aromatic oils or their hydrogenates are used at elevated temperatures, e.g., hydrogenated terphenyls (e.g., Therminols from Monsanto) and heavy cuts of akylbenzenes, byproducts from surfactant manufacture (e.g., Dubotherm from Slovchemia). Cycloakanic and, especially, alkanic oils with a low pour-point (e.g., Humblethem from Exxon) are suitable for use at lower temperatures. In handling aromatic oils, it is important to avoid blending them with alkanic oils. Such admixture can result in precipitation followed by sedimentation of the heavier components. These products can impair and even stop oil circulation and affect heattransfer as a result of coke build-up in exchanger tubes. Water must be excluded from the system. It increases energy consumption, causes foaming, produces interruption to circulation and causes other problems. Oils with good waterseparability are required. The decrease in density with increasing temperature must be allowed for in the design of the system. The volume increase is greater for aromatic oils than alkanics. Vessel volume should be designed with a safety margin of 1.5 for a 1,000 litre fluid fill, and 1.3 for larger units. 5.10.4.2 Synthetic Oils
Synthetic heat-transfer liquids can be classified into flammable (e.g., biphenylldiphenyl oxide, polyether and polyphenyl ether) fluids and those of low flammability (e.g., halogenated aromatics and organosilicates). BiphenyVdiphenyl oxide is used as a eutectic mixture of biphenyl (26.5%) and diphenyl oxide (73.5%), known as Dowtherm. It has the following characteristics: boiling-point at atmospheric pressure pour-point viscosity (mm2.s-1) at 20 "C at 400 "C vapour pressure(kPa) at 250 "C at 300 "C at 350 "C at 375 "C flash-point working temperature range without pressuri sing in the vapour regime
256M.5 "C 12.3 "C 5.5 0.17 100
252 569 810 128 "C 20-255 "C up to 388 "C
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Thermo-chemical stability remains unchanged at temperatures up to 400 "C; a small amount of high-boiling material forms above 400 "C, which remains in solution and does not affect heat transfer. These products are not corrosive. BiphenyVdiphenyl oxide eutectic has the following advantages:
- relatively low demands on the circulating pump because of low viscosity over the entire working temperature range; - good heat-exchange characteristics (high transmission of heat from condensing vapour, the condensing temperature remaining constant over the whole exchanger surface); - applicable in both liquid and vapour states for forced and natural circulation; - possibility of exact adjustment of temperature by controlling vapour pressure; - volume reduction on solidification, so that the valve system is unlikely to be damaged; Disadvantages include fire risks from the low flash-point, a tendency to creep which requires very tight valves and pumps to be used in order to avoid leakage and a relatively high pour-point, which prolongs the period from cold start to full operation. Polyether or polyglycol oils can be used at low temperatures. They have the following characteristics: viscosity (mm2.s-')
at - 18 "C at 38 "C at 100 "C
viscosity index pour-point flash-point
9000 100 18 145 -40 "C 230 "C
With added low-temperature antioxidant \;.g., phenyl- a - nap..thylamine), they can be used as heat-transfer media for temperatures between the pour-point and 250 "C. Polyphenylether oils have high thermal stability because of their aromatic character. Some of them can be used at temperatures up to 400 "C. Qpical terphenylether oils for heat-transfer duty up to about 340 "C have the following characteristics: pour-point -27 "C flash-point 168-180 "C self-ignition temperature approx. 360 "C
5.10.4.3 Low-flammability Heat-transfer Fluids These liquids have become important and widely applied. Generally, the rate of flame spread and flammability decreases in compounds with fluorine, chlorine, bromine, nitrogen and silicon elements or P=O groups. Many compounds with these
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elements or groups having high flash-points have been made and patented, but most of them have not been widely used and some pose a serious health hazard. Polychlorbiphenyls (PCB) were formerly widely used. They have outstanding fire-resistant characteristics and do not form explosive mixtures with air, despite their low flash-point (200 "C). But they are very toxic and non-biodegradable. They must be disposed of by burning in special incinerators or blast furnaces at temperatures exceeding 1,200 "C. If mis-handled, they liberate noxious products, including hydrogen chloride, which must be neutralised, and chlorinated benzofurans and benzodioxins, among them the most potent poison known, 2,3,7,8tetrachlorobenzodioxin. The production of PCB has been discontinued and the disposal of used products must be conducted under strict control with stringent precautions. Organosilicates, e.g., tetraoctyl, dioctyldicresyl and tetracresyl silicates, are partial substitutes for PCB. They are non-flammable and non-toxic, their selfignition temperatures are over 600 "C (higher for aryl-products) and their working temperature range extends from extremely low temperatures up to 180 "C - for some derivatives even more. They must, however, be protected from ingress of moisture to avoid hydrolysis, which results in the formation of insoluble silica products.
5.10.5 Preventive Oils and Petrolatums These substances are intended for the temporary protection of metal surfaces from atmospheric corrosion. This corrosion is primarily electrochemical, in which ionic oxidation and reduction reactions occur on two different places (anode and cathode) and the resulting reaction products separate out in a third place (protection from chemical corrosion, where both oxidation and reduction reactions on the same site occur, is only of limited relevance to temporary corrosion preventives). The main factors affecting atmospheric corrosion are moisture and oxygen and corrosive gases, vapours and salts. In the electrochemical rusting of iron, for example, the moisture forms an electrochemical cell. The oxygen present releases hydroxyl ion (OH-) on the cathode, which reacts with ions on the anode to form metal hydroxide: Fe 1
-0,+2e 2 &-+H20 Fe2++ 2HOalso, Fe2++&-
-
-
-
Fe2++2e
(5.18)
02-
(5.19)
2HO-
(5.20)
Fe(OH),
(5.21)
FeO
(5.22)
In addition to oxygen corrosion, acidic corrosion may occur if gases are present which form acids with water (CO,, SO,, C12). The lower the pH, the more extensive is this corrosion. Hydrogen sulphide promotes sulphidic corrosion iron (moist hydrogen sulphide) and on non-ferrous metals (both moist and
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dry hydrogen sulphide). Salts (e.g..sodium chloride on sites near the sea, or from finger-prints)participate in electrochemical reactions mainly as strong electrolytes.
The preventive reduces or suppresses surface conductance by creating between the metal and the water a permanent protective, eletrically non-conducting film which adheres strongly to the metal surface and prevents contact between gases, including oxygen, and electrolytes and the metal surface. The durability of the preventive depends on its resistance to the effects of oxygen and acidic gases. Its electric conductivity depends on the concentration of substances which can dissociate, its impermeability to oxygen and other gases on its viscosity and consistency, its adhesivity to the metal surface and water-repellency or its ability to bond water in the form of an emulsion and surface tension effects. The selection of a preventive and expectations of its ability to provide the required film thickness must firstly be based on the environment in which protection to the metal has to be provided, i.e., the degree of aggressiveness of the atmosphere. This aggressiveness is determined by the climate, moisture, proximity to sources of corrosive gases, distance from the sea and the conditions of storage (enclosed room, open shed, outdoors, etc.). Another criterion is the period of time for which protection needs to be provided. This may be anything from a few days to several years. It is important to establish whether the surface to be coated will be clean, moist, rough, etc., and whether the preventive coating must be stripped off before further treatment of the metal takes place (in this context, the ease of stripping the preventive coating is important) and whether the preventive needs to serve as lubricant, or only to be compatible with other lubricants which will be applied separately. The number and diversity of requirements for metal corrosion preventives has resulted in a large number of these products being available (Table 5.73). As can be seen from this summary, preventive oils, petrolatums, greases and asphalt bitumens form a significant part of the metal corrosion-preventive market. They are cheap and extensively used, but their service life is shorter and they are less effective than the other substances. The higher-quality products contain corrosion inhibitors which are by themselves able to produce on the metal surface a sufficiently stable, thin and water-insoluble preventive film and additives which improve adhesion to the metal surface. Examples of anti-corrosion additives include sheep wool fat, oxidised petrolatums, oil-soluble surfactants like stoichiometric and over-based petroleum sulphonates and synthetic sulphonates of calcium, magnesium and zinc, nitrated hydrocarbons and organic substances containing an acetylenic bond. De-watering additives reduce the surface tension towards metal; they can be cationic surfactants (imidazolines) or organic amine or ammonium organic phosphates. They must, however, be compatible with the anti-corrosion additives. Examples of additives which improve adhesion include polybutenes and latex. Low-temperature antioxidants and modifiers of rheological properties may also be present.
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Table 5.73. Summary of Types of Temporary Metal Corrosion Preventives Protection by oil film formation
Straight mineral oils Mineral oils containing corrosion inhibitors Solvent-diluted, inhibited mineral oils Emulsion oils Synthetic oils containing corrosion inhibitors
Protection by thixotropic oil films
Mineral oils containing corrosion inhibitors and thixotropic agents (usable in cold conditions) Mineral oils containing corrosion inhibitors and thixotropic agents (usable in hot conditions) Mineral oils containing corrosion inhibitors, thixotropk agents and solvents
Protection by soft, sticky films
Petrolatums or hydrocarbon polymers without additives Petrolatums or hydrocarbon polymers with corrosion inhibitors Wool fat with corrosion inhibitor and solvent Soft sodium lubricating greases
Protection by non-sticky wax films
Hydrocarbon waxes Natural and synthetic resins Metal soaps and asphalts containing solvents Wax emulsions (water dispersions)
Protection by hard varnishes
Transparent varnishes; cumarondindene resins, alkyds and natural resins containing softeners and solvents Drying oils
Protection by strippable, elastic foils
Substances which form protective layers at 185-195 "C (after the work has been dipped): cellulose acetobutyrate, ethyl cellulose Stripping foils: polyolefins or PVC + solvent
Protection by vapour
Organic and inorganic corrosion inhibitors with low vapour pressures, e.g., nitrites or benzoates of cyclohexylamine
Special agents containing water- and moisture-displacing additives are also available for removing finger-prints, which contain corrosive sweat salts.
For ease of cold-handling, some preventives, particularly those of higher viscosity, are thinned with volatile diluents (mostly heavy gasoline); oil-in-water emulsions (e.g., at 1 5 ratio) are designed for short-term use (up to 3 months) in the protection of objects indoors. The viscosity of preventive oils varies from about 20 mm2.s-I at 50 "C to 50 20 mrn2.s-l at 1cK) OC.The prrVcrlti\c Lilrii ia thicker at higher viscosity and the protection afforded lasts longer.
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Water-soluble preventives can be classified into 4 main groups: substances forming oil-in-water emulsions with water; their composition is similar to those of metal-working emulsion oils. These emulsions are stable up to 80-90 "C.After the object to be coated has been dipped into the emulsion and the water has evaporated, a protective film remains on the surface. The film can be readily washed off the surface. These emulsions are therefore only useful for short-term protection of steel, particularly for phosphatised surfaces (e.g., components kept in work-in-progress stores), when the corrosion-protective requirements are not very severe; - substances which can be emulsified with water at high temperatures (over 60 "C); after the object has been dipped into the emulsion at 70-80 "C and the water has evaporated, it remains coated with a protective film which is not emulsifiable with water and thus provides extended protection; - substances which form metastable emulsions with water and, unlike the previous class, be applied at ambient temperatures. After the component has been coated by dipping or spraying, the surface remains coated with a protective film: the water separates without evaporation.This surface dries rapidly without any heattreatment. Metastability makes the film difficult to emulsify with water so that it provides better and longer-lasting protection (the emulsion-concentration is usually 5-30%, depending on the protection or thickness of film which is required); - emulsions made from waxes, which can be emulsified by special methods only and containing 50-80% water. After application to the surface and evaporation of the water, a hard, firmly-adhering film giving long-term protection remains. In addition to conventional protective oils, special double-function oils can be used, especially for post-rolling protection of sheets.They protect the sheets from corrosion during transport and storage and also act as lubricants for deep-drawing forming processes. Oils used for internal protection of engines and gears need not be removed before commissioning of the equipment and act, for some time (mainly during the run-in period) as lubricants. Such oils must contain, in addition to anticorrosive additives, antioxidant, detergent-dispersant, lubricity and anti-wear agents, which must all be compatible with the corrosion inhibitor which is present. Preventive petrolatums provide long-term protection of metal surfaces from atmospheric corrosion. Their main constituent is petrolatum, the others comprising petroleum products, such as cylinder oil and asphalt bitumen, animal fat (mainly lanoline) and waxes. Lanoline has inferior oxidation stability and has now been replaced by more effective corrosion inhibitors and additives which improve adhesion to metal surfaces as well as de-watering agents, such as aluminium stearate.
-
Tests for Protective Ability and Durability The development of test methods for preventives has been stimulated by the need for their functions. They must maintain their functional characteristics in the application, exhibit sufficient preventive capacity, with some reserve, throughout the
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time they are applied, be easily applied, adhere well enough to the surface so as not to drop or run down the surface after application, not to be aggressive towards the surface themselves and to be stable in storage. Preventives which are not removed from the protected surface before equipment is commissioned must be readily miscible and compatible with the lubricant employed in its operation. Preventives must retain their functional properties under specific conditions: for example, preventives used for treatment between operations of metallurgical materials, blanks and components which must withstand extended storage, local and long-distance sea and land transportation, extended storage after shipping, transport under different climatic conditions, erection, etc. Test methods are often very similar. Attempts have therefore been made to restrict the present large number of test methods to a small number of unanimously agreed, precisely defined and internationally standardised tests. They may be categorised as follows: - tests in air-conditioned chambers, - tests under field conditons, - special tests. Air-conditioned chambers create artificial, intensively corrosive, but typical climatic conditions for different metals, which may or may not be preventive-coated. Ambient temperatures are adjusted and changed; water vapour condensation repeatedly applied; the concentration of active gaseous components of the atmosphere artificially increased (particularly that of CO,); maritime, tropical or arctic conditons simulated; W radiation increased; rain simulated, etc. Climatic chamber tests for petroleum-based preventives are specified in Czechoslovakia in CSN 03 8205 (Regulations for Temporary Protection of Metal Goods from Atmospheric Effects during Storage and Transportation) and CSN 03 813 1 (Condensation Chamber Test). Degreased, dry steel plates with ground surfaces are weighed and dipped into the preventive,removed and suspended in the condensation chamber, where they are exposed for 16 days to condensing water vapour in the absence of SO, (for preventive oils) or in the presence of SO,. Weight loss due to corrosion is measured; protective ability is expressed in terms of g.m-2 of the test plates in comparison with maximum limits.
In DIN 51-359, which is similar, 875 1 . h-' of air is constantly supplied at 100% relative humidity and 50 OC and the changes occurring on the surface of the plates evaluated after a test period. DIN 50-017 specifies a standard condensation chamber; the steel plates are suspended from a horizontally rotating carousel frame and tested under non-cyclic condensation conditions. In ASTM D-17 48-62, three steel plates are dipped into a liquid or molten preventive and then exposed for 16 hours to water vapour at 49fl "Cin a 2.4 m3.h-' air stream. Not more than three rust stains of diameter not exceeding lmm may appear on the surfaces after 30 days, except that corrosion stains within a distance of 3mm from the edges do not count. US FTMS-79la-5319 uses ball-bearing specimens instead of test plates. The radiation test in an air-conditioned chamber assesses the resistance of the materials to sunlight and radiant heat. 647
CSN 03 8825 describes a cyclic test with 24 hour cycles, which include 4 hour exposure to moist heat (4W2 "C and 93f3% relative humidity) and 20 hour exposure to light and heat radiation in the ultra-violet, visible and infra-red spectral regions. Relative humidity during the test must not exceed 50% and the test lasts 28-42-52 days (for materials to be exposed to direct sunlight) or 7-14-21 days (for materials which are expected to be exposed to dispersed sunlight). In FTMS 791a-6151, the test plates are exposed to radiation from an arc lamp, with intens,ity corresponding roughly to June sunlight at noon; the chamber is airconditioned in 2 hour cycles. During each cycle, the plates are exposed to radiation without shower, then for 18 minutes with a shower of water at 15 "C at 62.8 "C maximum. After 300 hours, not more than three corrosion stains of diameter not exceeding lmm may appear on the surface of the plates. Tests in an air-conditioned chamber with fog produced by atomising NaCl solution are intended to simulate atmospheric conditions near the sea. In CSN 03 8 132, plates coated with the preventive are exposed to a constant relative humidity of 90-100% at 20f2 "C and, once every 24 hours, to salty fog. The duration of the test depends on the intended application of the preventive. In DIN 51 358, three plates are coated with the test preventive and exposed to salty fog (20% NaCl in distilled water) at 35 "C for seven days. Not more than three corrosion stains of diameter not exceeding lmm are allowed on the plate. FTMS 791a-4001 specifies a similar procedure. The fog-chamber test uses atomised distilled water instead of NaCl. The tropical chamber test is similar to the humid heat cyclic test (e.g., CSN 03 8823), except that higher temperatures are used (42-48 "C). The surface of the test sample is sometimes inoculated with mould. Long-term field tests prove the actual protective ability and durability of test preventives if adequate test conditions are used. In CSN 03 8110, climatic effects and atmospheric corrosion under field conditions are defined by: - the test environment (situation for exposure, location and specification of the climatic conditons at the site), - additional stress imposed, simulating stress existing under field conditons, - season of the year when exposure takes place, and duration (e.g., April to May, September to October, one or several years). Plates coated with preventive may be mounted on stands and exposed to the outdoor environment, located under a roof (for protection from direct sunlight and precipitation) located in test booths equipped with blinds or shutter walls or in enclosed rooms (workshops, stores, etc.). In addition to assessing protective ability and durability, preventives may be subjected to special tests. The military specifications MIL-C-I 6173 and MIL-C11796 contain details of tests for corrosive effects on metals and alloys: The test metal plates are placed in a glass vessel and immersed in the preventive; the plates must not touch each other. The vessel is sealed and placed in a dessicator where it is kept at 54.4 "C for 7 days (MIL-C-16174) or at 82 "C for 2 weeks (MIL-C-I 17%). Maximum weight loss of the plates is specified; the plates must not exhibit pitting corrosion or colour change.
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MIL-C-2 1260A specifies a sea-water or salt-air resistance test for preventive: The steel plates are coated with the preventive and placed for 20 hours in artificial sea-water (solution of 25 g NaCI, 11 g MgCI2.6H2O, 1.2 g CaCI2 and 4 g Na2S0, in one litre of distilled water). Three small corrosion stains on a plate are allowed.
DIN 51 358 is similar. MIL-C-5545A includes a test for ability to neutralise hydrogen bromide: Steel plates are dipped alternately for 1 second into an aqueous solution of 0.1% HBr and into the test preventive oil 12 times per minute. The corrosive effect after 4 hours' standing in a warm room. No corrosion should form outside a 3 mm margin from the edge.
DIN 51 357 is similar. MIL-C- 1 1796B specifies low-temperature adhesion: The test plate is dip-coated with the preventive and placed in a dust-free room to dry for 24 hours. One hour after the drying process has started, 4 longitudinal and 4 perpendicular scratches 2.5 cm long in the form of a cross are made in the preventive coat at 2.5 cm.s.' and at -17.8 "C or -40 "C, using a special engraving tool. If the coating peels off more than 0.8 mm from the scratches, adhesion is regarded as unacceptable.
MIL-C-6529C specifies resistance to high and low temperatures: 25 cm3of preventive oil is placed in a 19 mm diameter, 15 cm long vessel for 24 hours at 96 "C and for 15 hours at -17.8 "C, then 10 days at room temperature. Changes in the oil, including deposits, are assessed.
MIL-C-16173 specifies a test of rate of drying: A plate is dip-coated with the preventive, dried in a vertical position (4 hours for grade 1 and 24 hours for grade 2) and then tested to determine whether it is possible to touch the surface without damaging it.
MIL-C- 16173C specifies miscibility with lubricating oils: 5 cm3 of preventive oil and 95 cm3 of lubricating oil are mixed and agitated. The mixture is allowed to stand at 77 "C for 15 minutes and the degree of any separation is measured.
MIL-C- 16173 specifies the ability of the preventive to be spray-coated; this should be manifest at temperatures down to +4 "C. The preventive oil at +4 "C should form a continuous film when sprayed with a gun on to a glass plate.
Tests for ease of removal from the work-piece after a period of storage are described in some specifications:
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Different types of test apparatus are available; all use preventive-soaked wicks sliding against the work in a reciprocating motion at a frequency of 40 cycles per minute. No preventive film should be perceptible on the rubbed surface after a specified number of cycles.
The effect of the preventive on dessicants with indicators is tested if the work is to be transported or stored in rooms or packages where dessicants are present. The preventive must not impair the performance of the dessicant or its indicator. In MlL-C-65229C, 1.5 1 .hl of air is driven for 3 hours at 71 "C through the preventive oil under test. The air passes into a U-tube filled with the dessicant (e.g., silica gel) with a colour indicator. The dessicant is then left overnight in a chamber at 100% relative humidity. The dessicant should change colour as a result of the effect of the moisture absorbed; if not, the preventive is unacceptable.
MIL-C-6529C specifies a test of storage life for preventive oils: 4.5 1 of preventive oil is stored for 12 months in a wide-necked, dark-glass bottle. changes in the oil and the extent of deposition are examined and corrosion tests carried out.
Non-standard Tests The contact-angle test ("Randwinkeltest") is based on the fact that a relationship exists between the preventive properties of an oil and the angle produced between the edge of a drop of water and the surface of a horizontal metal surface coated with the oil film. If the angle is more obtuse (e.g., 70-85 O ) . the drop is well-rounded and the duration of the protective ability of the oil is short. Flatter drops (with a more acute angle) indicate greater protective ability. For example, it stands at about 55' in the condensation chamber 100 hours and at 30-35" 200 hours; oils which contain sulphonates have this effect - the angle can be measured with a microgoniometer. In the Shell dynamic test, water or electrolyte (e.g., NaCl) runs down an inclined plate coated with preventive. Occurrence and extent of corrosion on the plate are evaluated. A similar test is used to characterise the occurrence and extent of corrosion; hot factory-supply water rains on to an inclined test plate coated with the preventive from a height of 60 cm. In static tests, metal plates coated with preventive are placed in a perforated desiccator plate and distilled water, sometimes containing electrolyte, is dropped on to the preventive coating. Drops should neither run down nor unite. The degree and severity of corrosion under the drops and the size of the drops are measured. Creep tests measure the ability of the preventive to maintain a minimum preventive coating on the metal surface. The weight of preventive which remains on 1 cm2 of surface after the preventive has run down it at a specified temperature is determined. The wash-off tendency of preventive films is tested by exposing the plates to a water-shower. The dried plates are weighed before and after the test. Two tests are available to identify the ability of preventives to displace water:
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(a) Horizontal Water Displacement (HWD) test: a steel plate is half-immersed in a 3% solution of NaCl in water, then left to drain and placed in a horizontal position and a drop of the test preventive is then placed on the centre of the wet portion. After one minute in this position and 2 minutes suspended, the degree of water displacement is measured. The extent of displacement, i.e. of drying of the relevant portion of the plate is then verified by climate-chamber tests. (b)Vertical Water Displacement (VWD) test: the plate is wetted with a 3% salt solution and left to drain and then dipped for 2 minutes into the test preventive, then suspended in a vertical position. After one hour in this position, it is left for 72 hours in a desiccator at 92% relative humidity and ambient temperature. No corrosion should occur. The importance of preventives is evidenced by the large number of tests which exist. This is also emphasised by losses through corrosion. For instance, losses due to corrosion in the UK were at one time estimated at 3.5%of Gross National Product. The situation in other countries is similar.
5.10.6 Miscellaneous Special Fluids A large number of special fluids, usually intended for a very limited range of applications,can be found in lists of commercial products. These oils also have some special properties and compositional characteristics, particularly in respect of types and amounts of additives. 5.10.6.1 Damper and Shack-absorberOils
A damping force against the oscillatory motion of an unsprung and spring-mounted mass, for example in a road vehicle, can be brought about by throttling the movement of oil produced by the motion of a piston in a shock absorber cylinder. This throttling action can be provided by a spring-loaded plate or valve which can, to some extent, control the damping force. Modem motor vehicles frequently have telescopic, double-action shock absorbers, the design of which resulted from the introduction of coiled springs as spring elements. Self-contained wheel suspension on vertical struts (e.g., MacPherson types) have also been widely used. The vertical struts act simultaneously as vertical steering pivots and shock-absorber piston rods.
Damper or shock-absorber dash-pots are usually filled with low-viscosity, cycloalkanic mineral oils, in the range 20-100 mm2.s-' at 20 "C with low pourpoints. The viscosity of the oil must match the design of the shock-absorber. The oil must have a sufficiently high viscosity index to be able to reach the required minimum viscosity at elevated working temperatures, which through internal friction may be as high as 130 "C and sufficient low-temperature fluidity for the environment.
65 1
Inadequate low-temperature fluidity makes the action of the shock-absorber hard during cold starting. The resistance of the oil flowing through the valves generates a large amount of heat, so that the oil heats up rapidly and the shock-absorber action soon settles.
The oil viscosity at the lowest conceivable working temperature should not exceed 250 to 4,000 rnm2.s-l, depending on the design. To satisfy this requirement, the oil must have a very high VZ (as high as 170 and, exceptionally, 300). Such high Vl’s can only be achieved in mineral oils by the addition of polymeric VZ improvers. However, it is necessary to use polymers with very high mechanical stabilities; the oil must have a high shear stability to maintain efficient performance of the shockabsorber over time and temperature. For these reasons, the natural properties of the oil must include good viscosity-temperature properties. Long service life of the oil can be achieved by deep refining of mineral oils or by changing to selected synthetic oils with very high Vl’s and low pour-points. Damper or shock-absorber oils contain antioxidants and anti-rust agents, as the intrusion of moist air into the oil cannot be eliminated. For the same reason, these oils also contain antifoams, as the oil is subjected to vigorous mixing with the air and moisture which has been absorbed, and to pressure changes. Formation of oil foam is manifested by the same sort of hard shock reaction behaviour in the shockabsorber as results from shortage of oil. Lubricity and anti-wear properties, especially with mineral oils, must be provided by additives; these additives must be highly efficient because of the elevated pressures and deformations to which the oil is subjected. The anti-wear additives prevent seizure of the piston and the cylinder; this particularly applies to MacPherson systems. The lubricity additive or friction modifier can suppress low-frequency shock-absorber noise, caused by the piston rubbing against the gland. The noise level depends on the material used (e.g., a chromium steel-Viton combination is noisy). High-frequency noise results from foam; the composition of the oil has relatively insignicant effect. The design of the shock-absorber is the decisive factor - there is no noise so long as the oil remains under pressure (213).
5.10.6.2 Mould Oils and Release Agents Release agents (also termed abherents, slip-aids, anti-blocking agents) act under static conditions - under dynamic conditions they act as lubricants. Their r6le is to prevent or reduce adhesion of material to itself or to another material (275). The field of use includes moulding, casting and, generally, transfer of materials in the construction industry, metallurgy, glass casting, rubber, plastic and paper processing, the food industry (e.g., ice-making) and packaging. Release agents must meet two conflicting needs: to bond strongly to one surface, while providing complete release from the other. Polarity and surface charge are involved. Further, these agents must possess wetting ability, low surface tension with at least one of the materials and appropriate volatility, stability and solubilisation 652
power with respect to the material in question. They must not interfere with subsequent processing and they must be easily removed. Typical release agents include mineral and silicone oils (in dilute solutions or emulsions), hydrocarbon or natural and synthetic ester waxes, polymers (e.g., polyolefin trays in most refrigerators - these may also be incorporated in the material being processed), fatty substances (alcohols, acids, stearates) water-soluble polyvinyl alcohols and polyethylene glycols (easily removable), together with solid powders (e.g., graphite for high temperatures, talcum powder for low temperatures) and solid films (e.g., PTFE) for lower temperatures.
Mould Oils in Concrete Processing Processing of concrete, either on- or off-site (e.g., the manufacture of moulded goods such as thick-walled pipe by rotary-moulding), requires shuttering or moulds. Concrete tends to adhere to their walls, so moulds are coated with lubricants to prevent sticking and improve the surface of the concrete product. A good quality mould oil should not be absorbed by the concrete, spoil its surface, stain the concrete wall or change its colour. The lubricant must protect metal moulds from corrosion and adhere well to the mould surface. The following types of mould oils are used: - pure, emulsifier-free mineral oils; these provide uniform colouring, but promote the development of bubbles; - mineral oils with emulsifiers; they provide uniform colour, do not promote bubble formation, but have a moderate effect on the concrete setting process; - water-in-oil emulsions; these provide unifrom colouring, suppress the formation of bubbles, provide the best-quality surface and are generally regarded as being among the best moulding lubricants; - synthetic polypropylene oils (e.g., Propyloil by Slovnaft Co.), alone or in emulsions, particularly suitable for heated and vibrated concrete work, complex profiles and plastic-concrete combination panels. Different viscosity oils can be used. Oils with viscosity 2-4 mrn2.s-l at 20 "C containing olein can be added to ceramic mixtures before moulding. Oils of appreciably higher viscosities (45-65 mm2.s-l at 50 "C) containing adhesionpromoting additives (resins, polybutenes, atactic polypropylene) are suitable for lubricating moulds and as additives to concrete for rotary-moulding of largediameter, thick-walled pipe. Various "rule-of-thumb" tests are available for adhesiveness. A simple laboratory test is based on the principle of stretched oil fibre length. A 6.5 mm diameter glass rod, flattened at one end, is immersed 15 mm under the surface of the test oil in a beaker at 20 O C and lifted at 6 cm.s-'. The distance between the oil level and the rod when the fibre breaks is read to an accuracy of 5 mm on a rule placed at the side of the beaker. The longer the fibre which is lifted, the better the oil or additive adheres. Another simple test is the anvil test. Some oil is put on the anvil and struck with a hammer. A piece of paper surrounds the anvil at a certain distance; the spatter pattern of the oil on the paper indicates how well it adheres.
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Mould oils are used in the later stages of clay brick and tile fabrication, in which they prevent bricks and tiles sticking to one another and to the metal moulds. The oil must be effective throughout the drying process and allow the bricks or tiles to separate when the process is complete. The oil must therefore be non-volatile, must thoroughly wet the surfaces and must also be able to absorb moisture to some extent. Mineral oils with viscosities 45-60 mm2.s1 at 50 "C containing wetting agents can be used.
Mould Oils in Metal Casting Mould oils or powders are also used in the continuous casting of steel, aluminium and copper (272). Their r61e is to prevent sticking of the solidified bloom in the mould and facilitate sliding the bloom out of the mould. Lime, silicic acid, alumina and fluxes such as fluorspar and alkaline earth oxides. Mineral oils with viscosity 30-60 mm2.s1at 50 OC,compounded oils or semi-syntheticester oils are examples of liquid lubricants used. These may also contain solid lubricants (molybdenum dioxide, graphite) as additives. The oil must yield a sufficient quantity of carbonaceous matter at casting temperature, the vapours produced must not be toxic or smelly, they must not cause eye irritation or respiratory problems and they must not cause too much smoke to be developed.
5.10.6.3 Air Filter Oils Mineral oils which have the ability to bind dust and other physical dirt entrained by air are used in air filters. These oils should not be volatile or resinify in the air stream, they must be highly resistant to oxidation and able to soak the filter cartridges (this attribute is achieved with additives)and they must not drain off the cartridges (unless in a special design incorporating bottom oil-filling). In some cases, ease of emulsification is required so that they are able to bind moisture in the filtered air. They must not be smelly and they must be resistant to attack by microorganisms. They must be easily removed from the filter cartridge when it is cleaned. Oils of medium viscosity (15-20 mm2.sm1at 50 "C), of high oxidation stability and containing bactericides are used for filters to clean air at moderate temperatures. White or viscin oils are used for special purposes, particularly in the food, pharmaceutical and medical products industries. Viscin oil is obtained by extracting the bark of white mistletoe (viscum album) with ether; this oil has good fluidity at low temperatures and a natural bactericidal action. Emulsifiable oils with viscosities 35-40 mm2.s-I at 50 "C are used for filtration of moist air. They are not normally expected to have any bactericidal effect. Filter cartridge oils for diesel locomotive engines must meet severe requirements. They should have a gel structure to adhere well to the surface of the cartridge but present a clean surface to dusty air; they must remain stable through temperature changes, be unaffected by bacteria, non-corrosive and able to help the cartridge withstand bad weather. 654
5.10.6.4 Anti-seizure Compounds Lubricants are normally used for separating two surfaces in relative sliding motion. However, it is sometimes necessary to separate surfaces which are temporarily in contact at rest, under high pressure and temperature, but required to make the transition into motion. Anti-seizure compounds are used based on solid lubricants, such as graphite or molybdenum disulphide, in a suitable oil substrate, petrolatum or lubricating grease. The solid lubricant must be selected to withstand strongly oxidising conditions. Thin films of poly-tetrafluoroethylene (PTFE)are often used for similar purposes, in the form of tape or spray-on coatings.
5.10.6.5 Bolt-blackening Oils Oils which form non-corrosive, fine emulsions with water are used for blackening hot steel products, such as nuts and bolts. The treatment should improve their rustresistance. The water evaporates and the oil bums off.
5.10.6.6 Rust-release or “Penetrating” Oils Light mineral oils of viscosity 2-5 mm2x1at 20 “C and flash-point (PM)over 60 “C containing additives such as sodium petroleum sulphonate are suitable for releasing rusted bolts and other components. Light silicone oils are also suitable. These oils are often mixed with light hydrocarbon and other solvents to speed penetration and facilitate application, which can include by spray-can.
5.10.6.7 Textile Fibre Lubricating Oils Lubricants are used in the textile industries not only for lubricating machinery but also to lubricate the fibres. Oil which can be removed by washing, or emulsions containing up to 50% oil in water, are mostly involved. These lubricants can be used throughout the process, from the first operation to final weaving. The function of the lubricant is to reduce the rate of dust formation (particularlyin processing rough cotton), to replace natural fat (wool), to reduce static electricity generated between the fibres and between fibres and equipment (especially with synthetic fibres), to prevent fibres breaking and to reduce friction and wear throughout the process. The process is lengthy and often interrupted; the lubricant can remain on the fibre for a long time and is sometimes even left on the finished goods. The oil must therefore have adequate, relevant characteristics: fastness to light, good oxidation stability (it must not form deposits which are difficult to wash off), it must be physiologically harmless (this property is increasingly emphasised) and readily removable. Well-refined, very light mineral oils are suitable, at viscosities around 5 mrn2.s1 at 50 OC. In some cases, technical or medicinal white oils or polypropylene oils must be used; water soluble polyethylene glycols are suitable for polyamides and polyesters yams and fabrics. The properties of these oils can be 655
improved by non-staining, harmless additives, such as antioxidants (e.g., 2,6-ditertbutyl-4-methyl phenol), anti-static additives (e.g., cationic surfactants) and water-soluble detergents and emulsifiers (because of the incompatibility of anionic and cationic tensides, non-ionic compounds such as esters of fatty acids and polyglycols or ethers of alkylphenols or fatty alcohols and polyglycols are preferred to the more usual mineral or synthetic sulphonates). Emulsions must meet the same requirements and, in addition, they must contain anti-corrosion additives, since they must not corrode themselves and also provide corrosion protection. These emulsions must be stable under all conditions of application and like the oils, be readily removable by washing. Oils and emulsions of similar compositions also find similar applications in leather processing. Emulsion stability should be tested in both hard and soft water. The oil, under standard conditons, is introduced into water with vigorous agitation then allowed to stand. After 24 hours, water or oil must not have separated, no more than a thin layer of emulsion should be visible floating on the surface. It is also possible to introduce water into oil; after inversion of the emulsion type has occurred, more water is rapidly added. Emulsion stability is then tested as before. Removability by washing is tested under standardised conditions on cotton and silk patches, which are first de-oiled then impregnated with the test oil, washed and the residual oil finally extracted with cyclohexane or light hydrocarbon solvent. The oil extracted is weighed. The amount of oil in the finished goods can also be measured spectroscopically in ultra-violet light.
5.10.6.8 I'urbine Washing Oils Two types of oil are used for washing turbines. One has low viscosity and contains detergent additives to dissolve sludge, strip deposits which have settled on the internal surfaces during the cleaning process and keep the cleaned surfaces free from corrosion. The washing oil must be miscible with the turbine oil. After this first type of turbine washing oil has been drained, the system is filled and washed with the second type. This oil has the same properties as the turbine oil specified for the machine and is also drained. The method of washing is laid down in detail and must be followed carefully. Cleanliness is imperative both in washing and operation.
5.10.6.9 Special Agricultural Oils Oils are used in agriculture either alone or as pesticide solvents. Refined oils, from which aromatics and unsaturated hydrocarbons have been removed, are not very toxic towards vegetation. White oils, emulsified with water, are suitable. Emulsions containing4-8% of oil, which contains non-sulphonatablecomponents (determined with 37N H2S04)and has viscosity 30-100 mm2.s-l at 20 "C,can be used for winter treatment of out-of-season plants. Summer treatment requires emulsions containing 1-2% of oil of lower viscosity, e.g., 3-7 rnm2.s-l at 20 OC,containing a higher concentration of non-sulphonatable components, 90% for lower and 99% for higher temperatures. Oils which are too viscous block the pores of the plants. High656
aromatic, sulphur-based and unsaturated oils possess fungicidal properties, but they are phytotoxic and some of them are carcinogenic (273).
5.10.6.10 Process Oils and Extenders for Rubbers and Plastics The so-called process oils facilitate mechanical processing of mixtures in the plastics industry. In the rubber industry, they function more over as extenders, which may replace a part of the monomers without significantly affecting the properties of the final rubber mixture (274). Aroinatic oils (above 30% CA, 60-70% aromatics by chromatography, 2040% CN) improve and accelerate the process and improve the tensile strength of diene, chloroprene, acrylonitrile and ethylene-propylene-diene (EPD) rubbers, sulphochlorinated polyethylene elastomers (e.g., Hypalon) and polyindene and other petroleum resins. Dark oils may contaminate the product. Cyclanic oils (30-50%C,, 1-28 CA,20% aromatics by chromatography) function as universal process oils for plastics and elastomers. They can be used for both natural and synthetic rubbers (styrene-butadiene, polybutene, polyisoprene, butyl and EPD rubbers) and, provided their aromatic content is at the lower limit specified, also for ethylene-propylene (EP) rubbers. The rubbers gain in colour stability, light stability, resistance to mechanical depolymerisation and low-temperature properties. These oils do not foul the products. Alkanic oils (2-5% CA, less than 30% C,, less than 10% aromatics by chromatography) are compatible with low diene content rubbers, such as butyl, EPD and EP rubbers. They improve low-temperature properties in diene rubbers, however, they sweat out and can contaminate the product. Polyethylene glycols as water solutions and polypropylene glycols in emulsions or pure find application as anti-stick agents for uncured rubber, mould release agents, machining lubricants for hard rubber, tyre lubricants, mandrel lubricants for rubber hose and lubricants for rubber products.
Chapter 5 - References 1. RODGERS, T. W. et al. : SAE Meeting, Paper 720 467, 1972. 2. BEHLING, R. D. - WEISE,E. : SAE Meeting, Paper 730 048, 1973. G . et al. : In : Performance Testing of Lubricants for Automotive Engines and 3. SOUILLARD, Transmissions. Ed. C. F. McCue et al., Essex, Applied Science Publ. 1974.313. 4. EBERAN-EBERHORST, C. G . A. - WEISE,E. : ibid., 333. 5. BRANDOUE, B. Du JEU, J. : Proc. Instn. Mech. Engrs., 188, 1974, 14. 6.CAVERHILL, J. R. : ASME, Paper 71-DGP-10, 1971. 7. HOLLINGHURST, R. : In : Performance Testing of Lubricants for Automotive Engines and Transmissions. Ed. McCue, C. F., et al., Essex, Applied Science Publ. 1974, 447. 8.BROSINSKY, H. : Mineraloltechnik. 15, 3, 1970. 9. FABIAN, R. J. : Automotive Engng., 81.5, 1973,28. 10. W