polymeric VI improvers. In addition, some synthetics are polymeric in nature, e.g., silicones, polyglycol ethers, polyesters, polyperfluoro ethers, etc. The presence of a polymer in the lubricant raises the possibility of two viscosity-related effects. First, under high shear rates and streamlined flow, the viscosity of the solution may be reduced reversibly. In the case of turbulent flow and extremely high shear rates, the polymer can be mechanically degraded and the viscosity of the solution lowered permanently. Characteristics of flow in the lubricant system determine the severity of mechanical degradation and, thereby, limits the size and effectiveness of polymeric additives that can be used. Shear rates in lubricant applications range from low values to the order of 10 6 reciprocal seconds in various common lubricating systems. Another important area is the shear rate or shear stress for cold starting. In an operating automotive engine, oils of the order of 3 to 15 cP(0.003 to 0.015 Pa . sec) are subjected to 5 × 10 5 /sec shear rate. At cold starting, oils of 3000 to 50,000 cP(3 to 50 Pa . sec) are subjected to shear rates of the order of 10 3 to 10 4 / sec. Atypical polymer solution gives a characteristic behavior as shown in Figure 2. Viscosity remains constant up to a critical shear rate after which the viscosity falls linearly to a stable or second Newtonian zone. The greater this slope, (n – 1), the higher the molecular weight of the polymer in these “power law” fluids. Relative polymer size can be judged by the power law index ηin the Ostwald de Waele equation η = K γ n−1 where K is a constant typical of the polymer system. Aplot of percent temporary viscosity loss vs. log of the shear rate also provides a straight line which, when extrapolated to lower shear rates, predicts the shear rate at which non-Newtonian behavior begins. With polymer molecular weights limited by mechanical viscosity loss, the second Newtonian zone appears to be greater than 10 6 /sec. Non-Newtonian lubricants may provide some advantages in a journal bearing. Studies have shown that a non-Newtonian lubricant can maintain the film thickness predicted by the low shear viscosity and show as much as a 40% friction reduction. 22 Non-Newtonian lubricants also tend to give lower than predicted friction in EHD bearings, possibly by partial starvation of the EHD contact. While polymer-containing lubricants lower friction and improve gas mileage in automotive engines, the specific mechanism responsible is not well defined. PolymerDegradation Polymers undergo permanent size reduction under the turbulence and cavitation involved in the valving system in pumps, relief valves in hydraulic systems, and all types of EHD and boundary lubrication. 23 Three types of test devices to evaluate mechanical breakdown in polymer solutions are (1) a pump system with an orifice or needle valve in the discharge line to create a pressure drop and severe cavitation, (2) an ultrasonic oscillator, and (3) a roller bearing rig to provide severe mechanical degradation with an EHD contact. Alarge number of cycles are required to achieve a final breakdown value. For a given pressure drop across an orifice, viscosity reduction will approach an asymptote after 5000 to 10,000 c. The breakdown is also a function of severity, but is surprisingly independent of the char- acteristics of all but the most severe unit in a system. Recent studies have shown that the amount of mechanical degradation of a given polymer is a function of the initial molecular weight and either the pressure drop across an orifice type loading device or Hertzian pressure in EHD contacts, as illustrated in Figure 3. 24 The roller bearing data were determined in a tester comprising two loaded tapered roller bearings running at 3500 rpm with 15 mᐉ of lubricant. 23 The mechanical breakdown appeared to be stepwise with an estimated nine successive molecular scissions for a 5000 nm molecule to the stable size of 5 to 8 nm. Polar polymers exhibit a lower initial rate of breakdown than do nonpolar polymers, indicating some reduction in mixing rate near the bearing surface. 236 CRC Handbook of Lubrication 227-254 4/10/06 2:07 PM Page 236 Copyright © 1983 CRC Press LLC are the Cleveland Open Cup flash and fire points (ASTM D92). The flash and fire points of a well-distilled petroleum fraction should differ by about 10°F (5.5°C)/100°F (55.5°C) of fire point. Thus, a flash point of 400°F (204°C) and a fire point of 440°F (227°C) would be expected of a typical lubricating oil fraction. A larger spread would indicate a relatively poor separation by distillation. As a rule of thumb, the fire point of a typical mineral oil fraction is approximately equal to the 20% boiling point at 10 mmHg (1.33 kPa) pressure. A careful measure of the boiling points for a typical mineral oil neutral fraction by tem- perature-programed gas chromatography indicates that a boiling range from the 5 to 95% boiling points of 150 to 170°C (302 to 338°F) is typical. Base oils for most industrial and automotive lubricants exhibit this range. For a variety of synthetic compounds or narrow boiling range (30°C) mineral oil fractions, viscosity-boiling point properties are correlated with viscosity-temperature properties in Figure 4. Evaporation losses from a relatively thin film evaporation test give another useful measure of volatility. The boiling point or boiling range of lubricant fractions or components can be measured using temperature programed gas chromatography (ASTM D2887) for boiling points up to 1000°F (538°C). One convenient method of converting the boiling point to vapor pressure or going from a vacuum fractionation to normal boiling points is the vapor pressure chart 238 CRC Handbook of Lubrication FIGURE 4. Viscosity-volatility relationship. 227-254 4/10/06 2:07 PM Page 238 Copyright © 1983 CRC Press LLC for hydrocarbons shown in Figure 5. 25 This relationship was generated using hydrocarbons and esters of organic acids as single compounds. The figure can also be used to convert a 10 or 50% normal boiling point to reduced boiling points or vapor pressures. A good method of predicting vapor pressures of lubricants down to 10 –6 mmHg (1.33 × 10 –4 Pa) is given by Beerbower and Zudkevitch. 26 Vapor pressure of a typical mineral oil lubricant is influenced strongly by its more volatile components. Thus, in a lubricant with a 150°C (302°F) boiling range, the 5 to 20% boiling points are the most important in establishing a vapor pressure for the system. In addition to oil consumption, evaporation, and safety (flammability), volatility plays a role in boundary lubrication. There is evidence 27,28 that lubricants with high volatilities cause higher wear in systems than do lubricants with matched viscosities and fluid types of lower volatility levels. DENSITY Specific gravity is defined as the ratio of the weight of a given volume of product at 60°F (15.6°C) to the weight of an equal volume of water at the same temperature. The petroleum industry has modified the Baume scale to provide an API gravity defined by the equation: (12) A high API gravity value matches a low specific gravity and vice versa. Tables are available for conversion of density or gravity measurements at any temperature between 0°F ( – 17.8°C) Volume II 239 FIGURE 5. Vapor pressure chart for hydrocarbons. 227-254 4/10/06 2:07 PM Page 239 Copyright © 1983 CRC Press LLC Table 4 COEFFICIENT OF EXPANSION FOR MINERAL OIL LUBRICANTS ESTIMATED FROM ASTM TABLES and 500°F (260°C) to the standard conditions of 60°F (15.6°C) (ASTM D1250). Density change with temperature (coefficient of thermal expansion) is more sensitive to the boiling point of the hydrocarbon fraction than to its density, although both independent variable are necessary to correlate the data properly. 29 For mineral oil lubricants an engineering approximation for the coefficient of expansion is summarized in Table 4, The chapter “Lubricant Properties and Test Methods” in Volume I gives typical densities of commercial lubricants. A similar straight line relationship exists between temperature and density over the range of 0 to 500°F ( – 17.8 to 260°C) for high-boiling synthetic lubricants. In addition to its usual engineering applications, density often offers a simple way of identifying specific lubricants. In petroleum and hydrocarbon-based lubricants, gravity can aid in distinguishing among paraffinic, naphthenic, and aromatic structures in the lubricant base oil (ASTM D3238). Lubricant compressibility is usually expressed as bulk modulus which is defined by the equation: (13) where B _ = isothermal secant bulk modulus, psi (Pa), P = pressure of measurement, psi (Pa), P o = atmospheric pressure, 0 psi (101.3 kPa), V o = relative volume at P o , and V = relative volume at P. BULK MODULUS Bulk modulus expresses the resistance of a fluid to compression (reciprocal of compress- ibility). This property, which varies with pressure, temperature, and molecular structure, is significant in (1) hydraulic and servosystem efficiencies and response time, (2) resonance and water-hammer effects in pressurized-fluid systems, (3) explanation of viscosity-pressure properties in hydrodynamic and EHD lubricants, and (4) in thermodynamic considerations of liquids. Two general methods used to measure bulk modulus are (1) pressure-volume-temperature determination of density or density change directly, and (2) velocity of sound in a liquid at the desired temperature and pressure. The former method provides isothermal secant bulk modulus or average values over a pressure range. Tangent bulk modulus or bulk modulus for a specific pressure is obtained by differentiation from the secant data. Velocity of sound measurements provide adiabatic tangent bulk modulus values. Klaus and O’Brien 30 measured the isothermal secant bulk modulus for a variety of lu- bricants over the range of 0 to 10,000 psi (0.101 to 69 MPa). For engineering accuracy, 240 CRC Handbook of Lubrication 227-254 4/10/06 2:07 PM Page 240 Copyright © 1983 CRC Press LLC the isothermal secant bulk modulus, B _ , can be converted to an isothermal tangent bulk modulus. B, in accordance with the relationship: tan Bp ≅secant B _ 2p (14) Iso thermal and adiabatic tangent bulk modulus are related by the equation: (15) where B s = adiabatic tangent bulk modulus, B r = isothermal tangent bulk modulus, γ= ratio of bulk moduli or specific heats, C p = specific heat at constant pressure, and C v = specific heat at constant volume. Wright 31 proposed a useful method for predicting isothermal secant bulk modulus values for mineral oils based on Figures 6 and 7. Figure 6 shows the relationship between B _ and temperature at 20,000 psi (138 MPa) as a function of fluid density at atmospheric pressure. Figure 7 shows a relationship between isothermal secant bulk modulus and pressure. These relationships work well for mineral oil base stocks and formulated lubricants, organic acid esters, synthetic hydrocarbons, and phenyl ethers. Both silicones and perfluoropolyethers show a relatively low bulk modulus (high compressibility) based on a density correlation. Bulk modulus is a physical property of the base fluid which cannot be changed significantly by additives. Entrained air (or other gas) in a hydraulic system being pumped at high pressure shows two deleterious effects on system response. First, any entrained air dissolves upon raising the pressure, causing a greater volume reduction than the compressibility of the original fluid. Secondly, the gas-saturated fluid is somewhat more compressible than the same fluid with only air saturation at atmospheric pressure. Air saturation at atmospheric pressure does not measurably change B _ over that of a degassed fluid. GAS SOLUBILITY Solubility of gases in lubricants is a physical property that in turn affects related lubricant properties such as viscosity, foaming, bulk modulus, cavitation, heat transfer, oxidation, and boundary lubrication. In many cases, gas is entrained at low pressures and then dissolved in the high-pressure portion of lubrication and hydraulic systems. As the pressure is again reduced to that in the reservoir or sump, the gas comes out of solution to produce foam or just entrained gas bubbles. The dissolved oxygen, in the case of air, can also react with the lubricant as the temperature in bearings or hot portions of the system reaches the threshold of the oxidation reaction. Gas solubility can be measured with precision at temperatures up to 260°C in a gas chromatograph (GC) with a precolumn of solid adsorbent to remove the liquid which contains the gas. 32 The experimental data can be plotted as a straight line of log gas dissolved vs. 1/temperature K. As the molecular weight of the gas increases, the rate of increase in gas solubility with temperature rise drops off. At a molecular weight of about 32 (oxygen), change in gas solubility with temperature is small. At higher molecular weights, e.g., CO 2 , gas solubility decreases with increasing temperature. At fluid temperatures where the vapor pressure of the liquid is 60 mmHg (8 kPa) or above, gas solubility falls below levels predicted from lower temperatures. At the normal boiling point, gas solubility drops to zero. Small amounts of volatile products in the lubricant can have the same effect as a more volatile base oil and result in reduced gas solubility. With gas mixtures, solubility of individual gases follows the partial pressure of the gas in the mixture. Volume II 241 227-254 4/10/06 2:07 PM Page 241 Copyright © 1983 CRC Press LLC Table 5 GAS SOLUBILITYPARAMETERS OstwaldSolubility coefficient LparameterS 2 for Gasat 0°C a Equation 16, MPa 1/2 Helium0.0123.35 Neon0.0183.87 Hydrogen0.0405.52 Nitrogen0.0696.04 Air0.0986.69 Carbon monoxide0.127.47 Oxygen0.167.75 Argon0.187.77 Methane0.319.10 Krypton0.6010.34 Carbon dioxide1.4514.81 Ammonia1.7 Ethylene 2.0 Xenon3.3 Hydrogen sulfide5.0 a Lapplied only to petroleum liquids of 0.85 kg/dm 3 density, d, at 15°C. To correct the other densities, L c = 7.70L(0.980–d) (see ASTM D2779 for details). Gas parameters to use in this equation are given on Table 5. The Ostwald coefficient is the equilibrium volume of gas dissolved in a unit volume of oil. This coefficient can be used directly for many engineering approximations below 5 atm pressure and 373 K (100°C). Solubility of air is, for instance, about 9.8% by volume in petroleum oils under conditions encountered in lubrication systems. The weight solubility of air at 2 atm is then double the solubility at 1 atm for a given temperature. Liquid solubility parameter, S 1 , is approximately 18.0 for diesters commonly used in aircraft fluids, 18.5 to 19.0 for higher esters, 18.41 for methyl phenyl silicone, 15.14 for dimethyl silicone, 18.29 for tri-2-ethylhexyl phosphate, and 18.82 for tricresyl phosphate. 33 In cases where thin films of lubricant are exposed to gases at high pressures, the gases dissolve rapidly. The resulting fluid can show a dramatic reduction in viscosity. Typical viscosity effects are shown on Table 6. 18 In general, the effectiveness of dissolved nitrogen in reducing viscosity negates the normal augmenting effect of pressure on viscosity. FOAMING AND AIR ENTRAINMENT Tendency to foam generally increases with increasing fluid molecular size, increasing viscosity, or decreasing temperature. Foaming is caused by the escape of insoluble gases or the physical mixing of excess gas with the fluid. The best way to minimize foam is with mechanical design. The chemical approach to reducing foaming is the use of a silicone additive that tends to lower surface tension at gas-liquid interfaces. Air entrainment is similar to the problems of foam. In hydraulic systems, air entrainment can result in response problems, while in gear systems air entrainment can result in reduced heat transfer and higher operating temperatures. Antifoam additives are not necessarily helpful; several commercial additives are available to improve air entrainment characteristics. THERMAL PROPERTIES Thermal properties of lubricants are involved in considering heat transfer, temperature Volume II 243 227-254 4/10/06 2:07 PM Page 243 Copyright © 1983 CRC Press LLC conventional mineral oil base lubricants have talent heats of vaporization between 60 and 90 Btu/lb (140 to 209 kJ/kg) at atmospheric pressure. The heat of vaporization at the boiling temperature decreases with the increasing pressure (and increasing boiling point). Atypical mineral oil, e.g., ISO grade 68, exhibits the following values. Latent heat Pressureof vaporization atmkPaBtu/lbkJ/kg 0.0131.3100233.0 1.0101.375174.0 10.01013.0511.6 By comparison, heat of vaporization of water is 969.7 Btu/lb (2255 kJ/kg) at atmospheric pressure. Much less heat is required for evaporation of oil compared with water under conditions where these fluids exhibit the same vapor pressure. Electrical Conductivity Electrical conductivity can generally be related to relative amounts of ions and ion-forming materials in the lubricant. In water base lubricants, the suppression of electrical conductivity is desirable since metallic corrosion is related to electrical activity. Metal corrosion products also tend to add to further degradation of the lubricant. Electrical conductivity of well-refined and dry mineral oil and most synthetic lubricant base stocks is extremely small, in the 10 –14 mho/cm 2 range. In some hydraulic control systems, streaming potential or zeta potential has been related to corrosion in servo valves. It appears that alteration of the electrical conductivity of the base oil is primarily due to the ionic nature of many additives, impurities such as water and chlorides, or oxidative or thermal degradation of the base stock. Electrical conductance of the unused lubricant is considered critical primarily in such applications as electrical equipment and in some aircraft and industrial control systems where streaming currents have caused damage. Surface Tension Surface and interfacial tension are related to free energy at a surface. Surface tension is the manifestation of this surface free energy at a gas-liquid interface, while interfacial tension exists at an interface between two immiscible liquids (ASTM D971). Surface tension can be measured by the du Nouy ring method. In this procedure a platinum wire ring is placed in contact with the clean surface of the liquid and the force F required to pull the ring away from the surface is measured. F = 4πrσ(17) where F = force in dyn/cm 2 (N/m 2 ), r = radius of the platinum ring, and σ = surface tension in dyn/cm (N/m). The surface tension of several base oils is shown in Table 8. Surface tension for the finished lubricant is surprisingly sensitive to additives. For example, less than 0.1 wt% of a siticonc in a mineral oil will reduce the surface tension to essentially that of the silicone. The use of additives to lower surface tension does improve surface wetting of the bearing. Interfacial tension between two immiscible liquids is approximately the difference between the surface tensions of the two liquids. Additives that create stable emulsions and micro- emulsions are capable of reducing the interfacial tension between the two phases to very low values approaching zero. The preferred method of measuring low interfacial tensions Volume II 245 227-254 4/10/06 2:07 PM Page 245 Copyright © 1983 CRC Press LLC Table 9 SPECTRAL DATA OBTAINED FOR VARIOUS LUBRICANTS Extinction Extinction Wavelength coefficient Wavelength coefficient Lubricant λ (nm) ( ᐉ/g-cm) λ (nm) (ᐉ/g-cm) DEHS (orig.) ~280 NA ~220 NA DEHS (HMW) 277 7.19 219 13.69 MLO 7558 (HMW) 278 11.65 225 17.62 MLO 7219 (HMW) 275 48.47 223 73.18 MLO 7828 (HMW) 277 14.00 226 17.14 TMPTH (HMW) ~280 A ~220 A TDP (HMW) ~280 A ~220 A a nitrogen atmosphere tend to push the thermal stability limit of the common dibasic acid esters and polyol esters toward the low end of this range. An all-glass system 35 produces a thermal stability advantage for the polyol esters that is probably not reflected in use in a lubrication system. Methyl esters have thermal stability levels about the same as those of mineral oil. Polymers used as VI improvers tend to have thermal stability thresholds that are lower than smaller molecules of the same general structure. Polymethacrylates show thermal break- down at 450°F (232°C) and polybutenes at 550°F (288°C). In both cases, thermal breakdown is distinctly different from mechanical degradation. 38 Additives used for lubrication improvement tend to have thermal stability limits below those of base oils. Zinc dialkyldithiophosphates used to improve boundary lubrication prop- erties show thermal degradation at 400 to 500°F (204 to 260°C). Generally, the more active the EP additive, the lower the thermal stability threshold. OXIDATION STABILITY Stability of a lubricant in the presence of air or oxygen is commonly its most important chemical property. Unlike thermal stability, oxidation stability can be altered significantly. Additives control oxidation by attacking the hydroperoxides formed in the initial oxidation step or by breaking the chain reaction mechanism. Aromatic amines, hindered phenols, and alkyl sulfides are compounds that provide oxidation protection by one of these mechanisms. A third type of oxidation control involves metal deactivators that can keep metal surfaces and soluble metal salts from catalyzing the condensation polymerization reactions of oxidized products to produce sludge and varnish. A number of bulk oxidation tests are described in the ASTM (D2272, D1313) and Federal Test Method Standards No. 791, Method No. 5308. These tests are good for measuring stable life or the effectiveness of oxidation inhibitors. Oxygen diffusion limits the value of these tests in correlations with many actual lubrication systems. The first step in oxidation of hydrocarbons is formation of a peroxide at the most vulnerable carbon-hydrogen bonds. This initiates a free radical chain mechanism which propagates formation of hydroperoxides. Further oxidation leads to other oxygen-containing molecules such as aldehydes, ketones, alcohols, acids, and esters. A similar peroxide path of oxidation has been shown for dibasic acid esters and polyol esters. Volume II 247 Note: DEHS — di-2-ethylhexyl sebacate, HMW — high molecular weight oxidation product, NA — no absorption at this wavelength, A — absorbs at this wave- length, but extinction coefficient not reported, MLO 7558 — paraffinic white oil, MLO 7828 — naphthenic white oil, and MLO 7219 — partially hydro- genated aromatic stock. 227-254 4/10/06 2:07 PM Page 247 Copyright © 1983 CRC Press LLC To monitor the oxidation process, a microoxidation test has been developed along with analytical procedures based on gel permeation chromatography (GPC) and atomic absorption spectroscopy (AAS). 39 In these tests, oxidations were carried out until 50% or more of the 248 CRC Handbook of Lubrication FIGURE 8. Oxidation of trimethylolpropane triheptanoate at 498 K. FIGURE 9. Oxidation stability as a function of temperature. 227-254 4/10/06 2:07 PM Page 248 Copyright © 1983 CRC Press LLC [...]... (130°F) 37. 8°C (100°F) –40°C (–40°F) –54°C (–65°F) Viscosity index COC flash point (°C) Pour point (°C) Total acid no MIL-H- 276 01 2: 07 PM Properties CRC Handbook of Lubrication Specification designation 4/10/06 250 Table 11 TYPICAL MILITARY SPECIFICATIONS FOR HYDRAULIC FLUIDS AND LUBRICANTS 2 27- 254 4/10/06 2: 07 PM Page 251 Volume II 251 Table 12 PHYSICAL PROPERTIES OF SEVERAL FLUIDS Table 13 PROPERTIES OF. .. Turbine Engine, Synthetic Base, U.S Department of Defense, Washington, D.C., 1969 36 Federal Test Method Standards No 79 1, Lubricants, Liquid Fuel, and Related Products; Methods of Testing, U.S Bureau of Standards, Washington, D.C., 1 974 37 Military Specification MIL-H- 276 01A (USAF), Hydraulic Fluid, Petroleum Base, High Temperature, Flight Vehicle, U.S Department of Defense, Washington, D.C., 1966 38... fluids meeting the requirements are of equal quality Relative quality must be determined by the ultimate user in his particular application A summary of some properties for several classes of fluids with potential use in the formulation of lubricants is shown in Table 12 Properties of some typical SAE grade lubricants are shown in Table 13 Characteristics of a variety of commercial lubricants are also... lubricants, and military specifications Examples of these standards and classifications are shown in Tables 10 and 11 and in pertinent chapters of Volume I These specifications define the lubricants in terms of physical properties and in some cases, particularly the Copyright © 1983 CRC Press LLC 2 27- 254 4/10/06 2: 07 PM 252 Page 252 CRC Handbook of Lubrication military specifications, with respect... Inst Mech Eng., 184, 4 87, 1969/1 970 16 Nagaraj, H S., Sanborn, D M., and Winer, W O., Surface temperature measurements in rolling and sliding EHD contacts, ASLE Trans., 22, 277 , 1 979 17 Nagaraj, H S., Sanborn, D M., and Winer, W O., Direct surface temperature measurements by infrared radiation in EHD, and the correlation of the Blok flash temperature theory, Wear, 49, 43, 1 978 18 API, Technical Data... properties of lubricants on boundary lubrication, ASLE Trans., 7, 1, 1964 28 Fein, R S., Chemistry in concentrated-conjunction lubrication, in An Interdisciplinary Approach to the Lubrication of Concentrated Contacts, National Aeronautics and Space Administration, Washington, D.C., 1 970 , chap 12 29 Maxwell, J B., Data Book on Hydrocarbons, D Van Nostrand, New York, 1950 Copyright © 1983 CRC Press LLC 2 27- 254... 254 2: 07 PM Page 254 CRC Handbook of Lubrication 30 Klaus, E E and O’Brien, J A., Precision measurement and prediction of bulk-modulus values for fluids and lubricants, J Basic Eng., ASME Trans., 86 (D-3), 469, 1964 31 Wright, W A., Prediction of bulk moduli and pressure-volume-temperature data for petroleum oils, ASLE Trans., 10, 349, 19 67 32 Wilkinson, E L., Jr., Measurement and Prediction of Gas... E J., Measurement and prediction of viscosity-pressure characteristics of liquids, J Lubr Tech., Trans ASME, 91, 454, 1969 6 Kuss, E., The Viscosities of 50 Lubricating Oils Under Pressures up to 2000 Atmospheres, Rep No 17 on Sponsored Res., (Germany), Department of Scientific and Industrial Research, London, 1951 7 ASME, Pressure-Viscosity Report, American Society of Mechanical Engineers, New York,... systems and pressure grease cups Copyright © 1983 CRC Press LLC 255-268 4/11/06 266 11:58 AM Page 266 CRC Handbook of Lubrication viscosity of the grease can be very high Once motion begins and the rate of shear is increased, the apparent viscosity of grease approaches, but never reaches, the viscosity of the fluid component In industrial applications, the apparent viscosity is useful in predicting: 1 2... lubricants, indicates areas of application in Table 1, discusses their particular advantages and limitations, and shows how they can be used in the four main ways which are outlined in Table 2 Reviews of solid lubricants are also given in References 1 to 5 Discussions of the nature and influence of surface films, boundary lubrication, and wear mechanisms are covered in earlier handbook chapters Many factors . (orig.) ~280 NA ~220 NA DEHS (HMW) 277 7. 19 219 13.69 MLO 75 58 (HMW) 278 11.65 225 17. 62 MLO 72 19 (HMW) 275 48. 47 223 73 .18 MLO 78 28 (HMW) 277 14.00 226 17. 14 TMPTH (HMW) ~280 A ~220 A TDP (HMW). MPa 1/2 Helium0.0123.35 Neon0.0183. 87 Hydrogen0.0405.52 Nitrogen0.0696.04 Air0.0986.69 Carbon monoxide0.1 27. 47 Oxygen0.1 67. 75 Argon0.1 87. 77 Methane0.319.10 Krypton0.6010.34 Carbon dioxide1.4514.81 Ammonia1 .7 Ethylene. more of the 248 CRC Handbook of Lubrication FIGURE 8. Oxidation of trimethylolpropane triheptanoate at 498 K. FIGURE 9. Oxidation stability as a function of temperature. 2 27- 254 4/10/06 2: 07 PM