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 thi
