Lubricants and Special Fluids Tribology Series, Manuais, Projetos, Pesquisas de Engenharia Mecânica

Baixe Lubricants and Special Fluids Tribology Series e outras Manuais, Projetos, Pesquisas em PDF para Engenharia Mecânica, somente na Docsity! TRIBOLOGY SERIES, 23 LUBRICANTS AND SPECIAL FLUIDS V. STÉPINA and : V. VESELY ELSEVIER LUBRICANTS AND SPECIAL FLUIDS 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 ............................................................... 1 References ............................................................................................................................. 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 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 The Functional Properties of Lubricants ..... ............................................................. 9 ............................................................. 9 .................................................................................... 12 Viscosity - Temperature Relationships ................................................................................. 17 Viscosity - Pressure Relationships ........................................................................................ 22 The Relationship between Viscosity, Viscosity - Temperature Effects and Chemical ............................................................. 32 Relationships between Viscosities of Oils, Viscosity Indexes and Practical Applications . 35 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 ............................................................... Miscellaneous Rheological Properties .................................................................................. 37 Electrical Conductivity ............... ..................................................................... 56 Electrical Strength .............................................. ............................................. 57 Regions for the Applications of Lubricants ................. 60 Dielectric Losses ....................................................................................................... 57 ......................................................................................... 59 Cloud-Point and Pour-Point ................................................................................................. 60 ............ 62 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 Resistance to Chemicals ............................................... Surface Properties ................................................................................................................ 92 Foam Formation in Oils ..................................................................................................... 101 Vaporisation, Ignition and Explosion ................................................ The Effects of Electric Discharges and Electric Fields V 2.4.2 Atomisation of Oil 2.4.3 Emulsions . ........................................................................................... 103 2.4.4 The Solvent Power 2.4.5 The Detergent and 2.4.6 2.5 2.6 Rust and Corrosion Protection by Oils The Physiological Properties of Lubricants ....................................................................... 3 Types of Lubricants and their Compositions ................................................................ 125 3.1 Gaseous Lubricants ............................................................................................................ 125 3.2 Liquid Lubricants ............................................................................................................... 126 3.2.1 Mineral Oils ..... 3.2.1 . I Crude Oil Comp 3.2.1.2 The Effects of Processing Techniques ............................................................................... 132 3.2.1.2.1 Separation Processes .......................................................................................................... 132 3.2.1.2.2 Refining Process ......................................... 3.2.2 Synthetic Oils ... ........ 3.2.2.1 Polyalkenes (Pol yo .................................................................................................. 149 3.2.2.2 3.2.2.3 3.2.2.4 Aromatics and Cycloaliphatics .......................................................................................... 152 Fluorinated and Chlorinated Lubricants ............................................................................ 154 Polyalkylene Glycols and Polyalkyl Ethers ....................................................................... 157 3.2.2.5 Polyphenyl Ethers and Polyphenyl Sulphides .... .................................... 161 3.2.2.6 Organic Esters .................................... 162 3.2.2.7 Phosphoric Acid Esters ...................................................................................................... 175 3.2.2.8 Aryl and Alkyl Esters of Silicic Acid ................................................................................ 178 3.2.2.10 The Behaviour ............................................. 184 3.2.2.1 I Miscellaneous Synthetic Oils ............................................................................................. 186 3.2.3 Inorganic Liquid Lubricants and Melts ............................................................................. 191 3.3 Lubricating Greases ........................................................................................................... 192 3.2.2.9 Polysiloxanes . ................... I80 3.3.1 Composition of Lubricating Greases ..... .................................. 3.3.1.1 Oil Components of Greases ..... .................................. 3.3.1.2 Thickeners .......................................................................................................................... 204 3.3.1.3 Additives for Greases ......................................................................................................... 226 3.4 Solid Lubricants ................................................ 3.4.2 Organic Compounds .......................................................................................................... 239 3.4.3 Soft Metals and Alloys ...................................................................................................... 242 3.4.4 Friction Reducing or Sliding Lacquers .............................................................................. 243 3.4.5 Self-Lubricating Materials ..................................................................................... 3.5 Biodegradable Lubricants ...................................................................................... 3.4.1 Inorganic Lubricants ........ ................. ................................................. ..................................... 247 4 Additives ........................................................................................................................... 255 4.1 Antioxidants (Oxidation Inhibitors) ................................................................................... 258 4.1 . I Radical Acceptor-Type Low Temperature Antioxidants .................................................. 258 4.1.2 Metal Deactivators ............................................................................................................. 268 4.1.3 Peroxide Decomposer Antioxidants .................................................................................. 271 4.1.3.1 Sulphur-Containing Decomposers ..................................................................................... 272 4.1.3.2 Sulphur- and Nitrogen-Containing Decomposers .............................................................. 273 VI 5.10.4 Heat Transfer Fluids . 5.10.4. I Hydrocarbon Oils ...... 5.10.4.2 Synthetic Oils ........................ 5.10.4.3 Low-flammability Heat-transfer Fluids ... .......................................... 642 5.10.5 Preventive Oils and Petrolatums .................................................. ........................ 643 5.10.6 Miscellaneous Special Fluids ............................................................................................. 651 5.10.6.1 Damper and Shock-absorber Oils ...................................................... 651 5.10.6.2 Mould Oils and Release Agents .. ....................................... 5.10.6.3 Air Filter Oils ..................................................................................................................... 654 5.10.6.4 Anti-seizure Compounds ................................................................................................... 655 5.10.6.6 Rust-release or "Penetrating" 5.10.6.7 Textile Fibre Lubricating Oils ........................................................................................... 655 5.10.6.8 Turbine Washing Oils ........................................................................................................ 656 5.10.6.9 Special Agricultural Oils ................................................................................................... 656 5.10.6.10 Process Oils and Extenders for Rubbers and Plastics ....................................................... 657 References .......................................................................................................................... 657 5.10.6.5 Bolt-blackening Oils ............. ...................................................... 655 6 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 Types and Applications of Lubricating Greases ........................................................... 664 Classification of Greases according to Machine Parts Lubricated .................................... 668 Lubricating Greases for Anti-friction Bearings ................................................................. 668 Greases for Anti-friction Bearings in Railroad Vehicles .................................................. 670 Greases for Plain Bearings .......... .................................................................. 670 Greases for Gears ....................... .................................................................. 671 Greases for Sliding Guideways .. .................................................................. 676 Miscellaneous Grease-like Materials ... ....... ..................... ................ 676 Multi-purpose Greases ....................................................................................................... 677 Classifications of Greases by Types of Machine .............................................................. 678 Automotive Greases .......................................................................................................... 679 Greases for Front-wheel Bearings ..................................................................................... 679 Greases for Drive-shaft Joints ............................................................................................ 681 Greases for Periodic Chassis Lubrication .......................................................................... 681 Sealed for Life Chassis Greases ........................................................................................ 682 Multi-purpose Automotive Greases ................................................................................... 683 Greases for Miscellaneous Automotive Assemblies ......................................................... 684 Aircraft Greases ......................................... ................................................................. 685 Miscellaneous Lubricating Greases ........... ................................................................. 688 Steel Rolling-mill Lubricants ............................................................................................. 688 Instrument Greases ............................................................................................................. 688 Textile Machine Greases ............................................................................................ Greases for Mechanical Handling Equipment .................................... Miscellaneous Special Greases ................................................................................... ............................................................................................................ ~ 9 1 Subject Index ..................................................................................................................... 697 IX This Page Intentionally Left Blank 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, large- scale 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 ” (Lubricants and Special Oils - Tribotechnological Foundations). 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 This Page Intentionally Left Blank 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), -fluid (relating to contact between a solid surface and a 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 Sliding friction may be expressed as mutual contact zone is defined by the following simple relationships: Ft =pLsF, thus p s = Ft/ F (1.1) where Ft is 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 = Mt IF where Mt is 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 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 : (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-CHR’]’ + / M- + CHR-~HR’ -\ [M-CHR-CHR’]’ ‘* M+ + CHR-~HR’ .A (1.6.1) ,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 change- 6 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 high- quality 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. 3. EWING, J. A,: Proceed Roy. Inst. of Great Britain, 13, 1892,387. 4. GOMBEL, .: 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. 12. KRAGELSKIJ, 1. V. - ~ E D R O V , V. S.: Razvitije nauki o trenii, Izd. AN USSR 1956. 13. CAMERON, A : Principles of Lubrication. Longmans 1966. 14. JOST, H. P.: Committee on Lubrication (Tribology). Report, London, H.M.S.O. 1966. 15. KRAGELSKIJ, 1. V.: Trenije i iznos. Moscow, MaSinostrojenije 1968. 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. 21. HOLLING, J. (Ed.): Principles of Tribology. London,.McMillan 1971. 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 ASTM D-941, D- 1481 and IP 189 details the measurement of 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 190 uses a similar closed-capillary pyknometer. IP 59/method D specifies the 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 t l 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 =u - 131.5 (2.3) d::% o - oil, w - 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 ap and 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 : (2.5.2) p TIP = po(l+ CIT + C2T2)[1+C3~.51~g(P-P,)]-’ where P,, po, C,, C2 and C3 are 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) 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 pOTis known, the module of elastic compression (see Chapter 2.14). where por is the density at temperature T and atmospheric pressure, Another relationship is also available (137): PT1P = POT (1 + aP4b + P ) (2.5.4) where pOT = pTolPo [1 - ap(T - To)]. ap is the coefficient of thermal expansion, pOT is the density at T+To / P = 0, and a and b are constants. From pT,P it is possible to derive the “coefficient of isothermal compressibility” (see Chapter 2.14). 11 The simplified equation: is less accurate. p = po[l + 0.6P/(1+1.7P)] (2.5.5) 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”. -_ - -__ -T ;I pq-fl,:d,:/. - ---,< d2 ,‘ 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 (Pa) (2.6.2) 12 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 6258- 52, 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 (OE) 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 n = 0.226 x SSU - 195 x SSU-’ for 32 < SSU < 100 n = 0.226 x SSU - 135 x SSU‘’ for SSU > I 0 0 n = 0.260 x RI -179 RP’ for 34 < RI < 100 n = 0.247 x 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 15 .4~10-~ 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): are approximate): 77 = - 77 rel 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-’) (2.1 1 ) 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: (2.12) 1lvblend = nl/vl + n2h2 + . . . n/vi 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: d,, + 0.0925 - 0.776 log . log( IOv,,,, - 4) VGC = 1.082 - 0.72 log . log( lov,,., - 4) ASTM D-250 describes the method for determining VGC at 37.8 “C (100 OF) 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. 172/171 = ( T , / T ~ ) ~ ” W ~ + T ~ + C? When q is known at temperature T I , then: (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 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< f2 ) , lighter 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). '1 '2' 5 10 20 50 100 m m 1aJJ mm?+'AT 37.8 OC 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, IS0 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 "C of 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 where U is the viscosity in mm2.s-l at 40 "C of the mineral oil under test. Method B (2.29) 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) (2.3 1) 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): (2.33) qp = qo exp aP (Pas) 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 viscosity- temperature 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 - 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: 17 = 170 (1 + PO" (126) (2.35.2) 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+-)' ( T + 135 )-'O - 2.76 (2.35.1) 2 To+ 135 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 changes with marked as pressure increases. For instance, the ratio q(dq(30 pressure as follows (172): P (MPa) T(OC) 30 60 100 0.1 1 0.222 0.056 100 1 0.140 0.025 500 1 0.043 0.0025 1000 1 0.014 0.00029 5000 1 4.9. 10-5 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 non- polymeric 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 Viscosity Density m0 aK (mm2.s-1) (kg.rn-3) (Pa-'. 10-8) Mineral, alkanic Mineral, cyclanic Mineral, cyclanic- aromatic Mineral, aromatic CdC,/C, by n-d-M analysis 67/22/11 6612 1 I1 3 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 - -with polybutene methacry late Polyester (168) Diester - with light polymethacrylate -with plybutene Polyphenylether bis- (hydroxyphenoxy)- benzene (Z68) Hydrocahons C2+ 9-n-octylheptadecane 9-(2-phenylethyl)- 2-(2-~yclohexylethyl)- heptadecane heptadecane C2,+ 1,l-diphenyltetradecane naphthalene n-hexyl-tetralin 2-n-buty l-n-hexyl- 2-(Ar)-n-butyl-3(Ar)- 2-n-butyl-3-n-hexyl-decalin 4.844 9.902 99.33 8.811 78.69 145.9 51 1.1 4.067 10.38 7.665 8.181 8.048 9.790 12.56 88.64 124.4 115.7 1293 1917 1984 29.0 226.8 224.7 3 80 8.93 1 9.383 14.73 18.62 12.49 14.83 15.28 815.9 4.100 841.9 4.000 871.0 3.550 864.3 4.200 876.5 3.810 916.0 4.235 932.0 4.235 818.8 4.145 847.2 3.890 848.0 4.010 872.8 4.020 888.2 4.200 931.6 3.660 901.5 3.505 957.5 2.980 837.4 3.670 828.5 3.640 909.0 1.830 898.0 1.400 865.0 1.805 997 (20 "C) 928.0 2.230 886.0 2.045 1207 (20 "C) 790.5 3.830 844.1 3.805 82 1.6 3.860 906.9 3.805 920.9 4.405 896.7 4.205 863.9 4.425 1.672 1.933 2.567 1.97 1 2.716 3.836 5.041 1.610 I .975 2.064 2.059 2.329 2.348 1.537 2.049 3.587 3.587 2.283 2.306 2.861 0.95 1.983 2.325 5.4 1.614 1.756 2.003 2.025 2.000 2.115 2.379 27 Table 2.3. q - P(T) Relationships in Oils Containing Different Polymers Oil identification 1 2 3 4 5 Composition (% wt.) Oil A 90.9 94.0 99.04 98.15 99.33 PIB (1) 9.1 PMA(2) 6.0 SBC (3) 0.96 SIC (4) 1.85 (5) 0.67 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) 853.7 854.0 851.8 853.2 851.3 829.4 829.6 827.3 828.7 826.7 72.17 58.82 64.99 62.79 67.52 16.16 15.45 14.92 15.81 15.34 12.01 12.0 11.13 11.52 11.47 150 182 150 160 143 -16 -42 -14 -16 -16 Viscosity (mPa.s) at different pressures (MPa) and Oil identification temperatures ("C) 1 2 3 4 5 Temperatures Pressures 40 80 40 80 40 80 40 80 40 80 0.1 72.17 16.16 58.82 15.45 64.99 14.92 62.79 15.81 67.52 15.34 50 207.5 37.89 165.7 35.1 183.7 32.3 178.7 36.71 87.8 34.93 100 542.5 82.36 427.2 73.1 417.5 66.2 45.8 78.3 480.4 73.79 160 151.7 189.2 1184 156.9 184.8 146.1 1233 175.1 132.7 162.9 200 2792 310.1 2176 243.2 250.6 236.9 2193 280.4 at pressure (MPa) - 'IS0 0.01 4.47 3.81 4.37 3.97 4.40 200 9.0 8.95 10.48 7.82 al.lO'E(Pa-l) 2.21 1.78 2.16 1.73 2.16 1.66 2.20 1.77 2.13 1.73 %.lO-E(Pa-l) 1.83 1.48 1.80 1.37 1.83 1.38 1.78 1.44 1.79 1.41 p.lO-l7 (Pa-2) -1.89 -1.15 -1.77 -1.76 -1.68 -1.15 -2.10 -1.63 -1.67 -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 MW = 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 Duosol Blend (C) oil (A) Raffinate (B) = (A +B) Constituent Ratio (96 wt.) Hydrocracked oil Duosol raffinate 100 33 100 67 Hydrocarbons: composition (96 wt.) Alkanes 15 15 15 Cyclanes 72 37 48 Aromatics 13 48 37 of which: mono-aromatics 8 38 29 di-aromatics 4 9 7 tri- & tetra-aromatics 1 1 1 Properties: Density ( k g d ) at 40 "C 85 1.2 883.3 871.8 at 80 "C 826.6 859.6 847.7 Dynamic viscosity (mPa.s) at 40 "C 34.2 185.1 94.6 at 80 "C 8.28 27.62 17.42 Kinematic viscosity (mm2.s-') 6.05 16.95 12.05 Viscosity Index 110 87 94 Pour-point ("C) -16 -10 -1 1 Viscosities (mPa.s) at high pressures (MPa) and temperatures ("C) Temperatures Pressures 40 80 40 80 40 80 0.1 50 100 1 60 200 34.2 8.28 185.1 27.62 96.46 17.12 98.15 19.19 626.7 70.92 303.1 42.8 259.6 41.12 1933 169.1 862.3 96.73 748.6 92.55 6607 435.1 2652 233.4 1421 149.7 13920 770.1 5145 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 1.76 2.53 1.96 2.39 1.87 1.86 1.45 2.18 1.66 1.99 1.56 -1.63 -1.57 -1.86 -1.48 -1.99 -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 Halogenated hydrocarbons have the steepest viscosity-temperature curves. 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 boiling- point, 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 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 : d (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 non- Newtonian 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 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 Tension (z) caused by deformation (B) is produced in a solid, elastic body of z = EB (2.44.5) period in which x drops to l/e (0.37) of the original value. modulus of elasticity E: Expressed as a rate equation: dz dDB - = E . - dt dt (2.44.6) Equation 2.44.4 can 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 10-7s in mineral oils of molecular weight (MW) 250-600, 1 0-9 to 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 load- carrying capacity would increase. Howel experimentally. la I b II a :r, these theories have not been confirmed 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 1 D (z> TO) zo112)2 or Casson’s equation: q’ = - (z-1/2 - (2.46) (2.47.1) This equation has been modified by Czarny and Moes (169) to: (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 q’ = - 1 (T l/n - zol/n)n D 42 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 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-machined metal 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. D- 1092-62. 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 non- Newtonian 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 10-1 mm 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 Consistency (Io-‘mm) (degree) Consistency (description) 445-475 400-430 355-385 3 10-340 265-295 220-250 175-205 130-160 85-1 I5 OOO almost fluid 00 extremely soft 0 very soft 1 soft 2 medium soft 3 medium 4 medium tough 5 tough 6 very tough (blocks) 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 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= - PP (2.53) In technical practice, simpler equation are used, such as: v, = PVO(P - Po) or AV = VP (PI - Po) (2.54.1) 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. values of group contributions to parachor. been mentioned (see above, page 28). Compressibility is further related to elastic properties of isotropic substances: Since the parachor is an additive quantity, K can be calculated from tabulated The relationship between compressibility and pressure coefficient has already 3 1-2v 2 l + v G = K ( - 1 (2.54.4) and E = 3K(1 - 2v) = 2G(1 + v) (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 Pressure (“C) (MPa) 71 142 213 284 355 Octane 20 - 8.45 - 17.5 Broad petroleum fraction 20 5.1 6.7 10.0 12.8 14.5 Gas oil 20 3.9 5.8 8.8 10.7 12.2 Light oil distillate 20 3.4 5.1 7.7 9.4 10.9 Heavy oil distillate 20 2.9 - 6.8 8.4 9.7 Refined oil, VI 95 20 6.5 - 12.5 - 16.5 200 7.7 - 14.4 - 18.1 Table 2.8. Elasticity Values of some Materials (103 MPa) (146) Substance V E G K Liquids: Polymers: Minerals: Metals: Electrodes: Water Organic liquids 0.5 0.5 Polystyrene (amorphous) 0.38 Polymethacrylate (amorphous) 0.33 Polyamide 6/6 (plycrystalline) 0.3 HD Polyethylene (semi-crystalline) 0.45 Polyvinylchloride (amorphous) Polycarbonate (amorphous) Polytrifluorochloroethylene (semi-crystalline) - Polytetrafluorethylene (semi-crystalline) Polyformaldehyde (crystalline) Quartz 0.07 Glass 0.23 Mercury Lead Cast iron Aluminium Copper Steel (soft) a-alumina S ic Graphite 0.5 0.45 0.27 0.33 0.35 0.28 0 0 3.2 4.15 2.35 1 .o 100 60 15 90 70 120 220 2000 1000 1000 0 0 1.2 1.55 0.85 0.35 77 24.5 5.3 35 26 44.5 66 lo00 500 500 2.0 1.33 3.0 4.1 3.3 3.3 4.1 2.4 2.0 4.0 4.0 39 37 25 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-’) (2.55) where p is the density of the fluid (kg.rn”) and V is the speed of sound (m.s-l). 52 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): 4.187 ct = (0.403 + 0.000801t) (kJ.kg-'.K-*) (df ::) li2 (2.57) 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 C02 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 M.rnol-' .El kT.kg-l.K-* 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) (s) = solid (I) = liquid 46 11 1.2 42.7 90.3 60.0 110.8 164 328 104.6 96.8 1.68 1.96 1.42 2.05 1.05 1.92 1.46 1.46 0.97 0.92 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’of Lubricants 2.1.6.1 Electrical Conductivity Electrical conductivity, G, is defined by the equation: G =-= - (m- 2.kg-’.s3.A2) E 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 (a), 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: 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 additive- free 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 mm 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 K = Dielectric loss is normally expressed as t a d , or the loss factor, 100 tan6, 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 57 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, drop- point 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-crystalline paraffins - 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 iso- alkanes have lower and symmetrical higher pour-points than n-alkanes of the same carbon number. Similar rules also apply to cycloalkanes and aromatics; the non- substituted 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., 2- methyladipic 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-defined pour-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 viscosity- rate 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 boiling- points 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 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 flash- points 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 non- flammability: - 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 non- flammability. 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 chain- reactions 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-ignition increases 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 non- radical-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 auto- ignition 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 auto- ignition 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 Auto-ignition Ignition temperature delay* (“C) (minutes) Hexane 26 1 0.5 Nonane 234 1.1 Hexadecane 230 2,3,3,4-tetramethyl pentane 431 0.4 p-butylbenzene 348 0. I tert-butylbenzene 477 1.2 Tetrahydronaphthalene 423 0.1 Diphenyl oxide 646 0.2 Hexachlorodiphenyl oxide 628 0.01 Perfluorodimethylcyclohexane 65 1 0.1 Tricresyl phosphate 600 Tetra-aryl silicate 577 0.1 Gasoline (20% aromatics) 482 0.2 Kerosene 249 1.1 Engine oil (SAE 10) 382 0.02 Engine oil (SAE 50) 410 0.1 Decahydronaphthalene 272 0.3 1-methylnaphthalene 543 0.4 * 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 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 start- up, 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 pour- points. 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 lo2 s-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 12- hydroxystearate, 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, 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 cone- and -plate system, according to DIN 51 805 and SEBl8l 306-61. IP 186/64, CSN 65-6327 AND FTMS 334.2, 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: Characteristic conditions of Test CSN 65-6235 Mineral oils CSN 65-6224 Additive-free mineral oils 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. 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). COST 18136-72 Lubricating oils Equivalent to CSN 65-6235. COST 11257-65 Turbine oils 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. 72 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 (2.76.1) R’+ ROO’ - R-0-0-R (2.76.2) 2 ROO’ - R04R - ROOR + 0, (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/di- nuclear 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: R’ + ROO’- 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: + ROOH + ROOH + ROOH -S-- -SO- -SO,- -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 ydrox y acids keto-acids, lactones, estolides asphaltogenous acids \ resins asphaltenes carbenes carboides aldehydes, phenols 1 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-acids have 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 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 5734- 76 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 simultaneously subjected to mechanical-dynamic and 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' - (R-CH,-CH2-R')+ + e (2.82.2) - R-6H-R'-dH2 80 The ionised molecule may also undergo polymerisation or dehydrogenation: R’-CH(R.CH-CH,R’)-CH-R + H2 (R-CH2-CH2-R’)+ + e R-CH=CH-R’ + H$ (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-CH2-CH2-R (2.82.4) R-CH,-CH-R’ + I CH2-R R-dH2 - R-CH2-CH2-R’ / \ R-CH, + R-CH2iH-R’ (etc.) R-dH2 - lighter hydrocarbons (2.82.5) 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-C c-x 2-X in alkenes: R-CH=CH-CH, R-CHSH-CHZ-CH, in aromatics: k - A r k - C H 3 k-CH2-CH2-Ar in aldehydes: in multiple bonds: C=C C=C 330-350 c-0 320 C-O 310 C S 380 C-CI C-F over 380 R-CH2-H 380 (R)2CH-H 330 300-305 435 (R),C-H 510 R-CO-H 295 4 - H 355-380 625 -0-H 375-410 260-300 -N-H over350 280 Si-0 445 445 s-s 215 P-S 230(est.) 400 N-S 2lO(est.) 370 350 300-350 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. Thermogravimetric methods follow the correlation between weight changes in the heated sample and its temperature, using a special thermal balance; thermodilatometric methods 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 temperature of the maximum change in electrical conductance corresponds to the temperature of maximum change of the additive. 82