Several thermal stability tests are described in Federal Specifications.35-37 The test should allow for decomposition of a significant portion of the test sample and provide an analysis
Trang 2Table 8 SURFACE TENSION OF SEVERAL BASE OILS
Surface tension
Water 72 (× 10–3 )
Mineral oils 30—35 (× 10–3) Esters 30—35 (× 10–3) Methylsilicone 20—22 (× 10–3) Fluorochloro compounds 15—18 (× 10–3) spinning drop apparatus.34Relationships in this unit are expressed in the following equation
where ρ1 = density of the heavy phase, ρ2 = density of the light phase, d = drop width,
θ = rotational speed, and K = constant characteristic of the test unit
With this unit interfacial tensions down to 10–4to 10–5dyn/cm (10–7to 10–8N/m) can be measured The detergents and dispersants in many automotive lubricants are so effective at reducing interfacial tension that used crankcase oils contaminanted with 10 to 15% water form a stable emulsion which defies separation by techniques which do not involve distil-lation These general techniques can be used to suspend graphite in motor oil, disperse calcium carbonate in over-based diesel lubes, or prepare a 95% water invert emulsion Both surface tension and interfacial tension are altered by additives and by lubricant degradation
THERMAL STABILITY
Thermal stability is the resistance of the lubricant to either molecular breakdown or rearrangement at elevated temperatures in the absence of oxygen Stability in an ordinary air environment (oxidation stability) is covered in the next section
One method of measuring thermal stability involves the isoteniscope, a closed vessel with
a manometer for measuring the rate of pressure increase at a specified heating rate Thermal gravimetric and differential thermal analyses can also be used to evaluate thermal stability Several thermal stability tests are described in Federal Specifications.35-37 The test should allow for decomposition of a significant portion of the test sample and provide an analysis
of the liquid and solid decomposition products as well as the gases formed.37 Fluids such as mineral oils with a substantial percentage of C–C single bonds as the most vulnerable point for breakdown exhibit a thermal stability of about 650 to 700°F (343 to 371°C) Synthetic hydrocarbons prepared by a polymerization or aligomerization process and then hydrogenated involve the same basic structures as mineral oils, but exhibit a thermal stability of 50°F(28°C) or more below that of a mineral oil In thermal breakdown, a mineral oil produces more moles of methane than of ethane and ethylene That is, the molar quantities
of the thermal decomposition product tend to decrease continuously with increasing molecular weight A synthetic hydrocarbon made by polymerization will produce a significant quantity
of the monomer from which it was made as a telltale fingerprint
Molecules containing only aromatic linkages or aromatic linkages with methyl groups as side chains show a thermal threshold of the order of 850 to 900°F (454 to 482°C) Polyphenyl ethers, chlorinated biphenyls, and condensed ring aromatic hydrocarbons fall in this category With organic acid esters the functional group is the weak link in the molecule, and thermal stabilities range from 500 to 600°F (260 to 316°F) The presence of metals such as iron in
246 CRC Handbook of Lubrication
Trang 3Table 9 SPECTRAL DATA OBTAINED FOR VARIOUS LUBRICANTS
Wavelength coefficient Wavelength coefficient
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
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 system35 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 breakbreak-down
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
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.
Trang 4To 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.
Trang 5Table 10 INTERNATIONAL ORGANIZATION FOR STANDARDIZATION (ISO)
VISCOSITY CLASSIFICATION SYSTEM FOR INDUSTRIAL FLUID LUBRICANTS
Viscosity grade ranges (cSt at 40°C) ISO viscosity
original base oil was oxidized The large molecules separated by GPC are found to be rich
in metal corrosion products These large molecular size products appear to be condensation polymers with a characteristic beta keto conjugated unsaturation (–C=C–C–) which can be found in oxidation products from dibasic acid esters, polyol esters, monoesters, and mineral oils These fluids all show oxidation products with the same general UV absorption patterns
as shown in Table 9 In Figure 8 the rates of oxidation for the same polyol ester show that
a copper catalyst has an inhibiting effect, while lead and iron accelerate the primary oxidation rate
The effect of temperatures on stable life of lubricants is illustrated in Figure 9 This extrapolation system relating log of life to temperature provides a design guideline for the limiting bulk lubricant temperatures in a system
LUBRICATION SPECIFICATIONS
Several widely used specifications include SAE engine oil grades, SAE gear lubrication grades, ASTM/International Organization for Standardization (ISO) grades for industrial
Note: The viscosity grade numbers for
the ISO System are identical to those shown for the ANSI/ASTM system (ASTM D 2422, ISO 3448
— 1975).
Trang 6Table 11 TYPICAL MILITARY SPECIFICATIONS FOR HYDRAULIC FLUIDS AND LUBRICANTS
Specification designation
cSt viscosit at
Note: MIL-H-27601 — Hydraulic fluid, petroleum base, high temperature, flight vehicle, MIL-H-83282 — Hydraulic fluid, fire resistant synthetic hydrocarbon base, aircraft, MIL-L-6387 — Lubricating oil, synthetic base, MIL-L-7808 —
Lu-bricating oil, gas turbine, aircraft, and MIL-L-23699 — LuLu-bricating oil, aircraft turbine engine, synthetic base.
Trang 7Table 12 PHYSICAL PROPERTIES OF SEVERAL FLUIDS
Table 13 PROPERTIES OF TYPICAL SAE GRADE LUBRICANTS
fluid 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
Note: For automotive oil specifications, sec “Automobile Engines” and subsequent chapters in
Volume I.
Trang 8military specifications, with respect to oxidation stability, thermal behavior, and wear characteristics
General specifications for a fluid type do not imply that all 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 provided in the chapter on “Lubricant Properties and Test Methods” in Volume I
NOMENCLATURE
_
B = Isothermal secant bulk modulus
Bs = Adiabatic bulk modulus
Br = Isothermal tangent bulk modulus
ΔE = Energy of activation
h = Planck’s constant
N = Rotational speed N
_
= Avagadro’s No
n = Power law index
nD20 = Refractive index
V
_
= Molecular volume
VI = Viscosity index
α = Viscosity-pressure coefficient
η = Viscosity in centipoise
v = Viscosity in centistokes
ρ = Fluid density
α = Surface tension; interfacial tension
ω = Angular velocity
252 CRC Handbook of Lubrication
Trang 91 Fredrickson, A G., Principles and Applications of Rheolagy, Prentice-Hall, Englewood Cliffs, N.J., 1964,
118.
2 Fenske, M R., Klaus, E E., and Dannenbrink, R, W., The comparison of viscosity-shear data obtained
with the Kingsbury tapered plug viscometer and the PRL high shear capillary viscometer Special Tech.
Publ No 111, Symposium on Methods of Measuring Viscosity at High Rates of Shear, Tech Publ 111,
American Society for Testing and Materials, Philadelphia, Pa., 1950, 45.
3 Gerrard, J E., Steidler, F E., and Appeldoorn, J K., Viscous healing in capillaries, Ind Eng Chem.
Found., 4, 332, 1965; 5, 260, 1966.
4 Ewell, R H and Eyring, H J., Chem Phys., 5, 726, 1937.
5 Fresco, G P., Klaus, E E., and Tewksburg, 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, 1953.
8 Klaus, E E., Johnson, R H., and Fresco, G P., Development of a precision capillary-type pressure
viscometer, ASLE Trans., 9, 113, 1966.
9 Kim, H W., Viscosity-Pressure Studies of Polymer Solutions, Ph.D thesis, Pennslyvania State University,
University Park, Pa., 1970.
10 So, B Y C and Klaus, E E., Viscosity-pressure correlation of liquids, ASLE Trans., 23, 409, 1980.
11 Jones, W R., Johnson, R L., Sanborn, D M., and Winer, W O., Viscosity-pressure measurements
for several lubricants to 5.5 × 10 8 N/m 2 (8 × 10 4psi), and 149°C (300°F) Trans ASLE, 18, 249, 1975.
12 Novak, J and Winer, W O., Some measurements of high pressure lubricant rheology, J Lubr Technol.
Trans ASME, 90, 580, 1968.
13 Jakobsen, J., Sanborn, D M., and Winer, W O., Pressure-viscosity characteristics of a series of
siloxanes, J Lubr Technol., Trans ASME, 96, 410, 1974.
14 Appledoorn, J K., Okrent, E H., and Philippoff, W., Viscosity and elasticity at high pressures and
high shear rates, Proc Am Pet Inst., 42(3), 1962.
15 Foord, C A., Wedeven, L D., Westlake, F J., and Cameron, A., Optical elastohydrodynamics, Proc.
Inst Mech Eng., 184, 487, 1969/1970.
16 Nagaraj, H S., Sanborn, D M., and Winer, W O., Surface temperature measurements in rolling and
sliding EHD contacts, ASLE Trans., 22, 277, 1979.
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, 1978.
18 API, Technical Data Book — Petroleum Refining, 3rd ed., American Petroleum Institute, Washington,
D.C., 1977.
19 Johnston, W G., A method to calculate the pressure-viscosity coefficient from bulk properties of lubricants,
ASLE Trans., 24, 232, 1981.
20 Alsaad, M., Bair, S., Sanborn, D M., and Winer, W O., Glass transitions in lubricants: its relation
to EHD lubrication, J Lubr Technol Trans ASME, 100, 404, 1978.
21 Bair, S and Winer, W O., Shear strength measurements of lubricants at high pressure, J Lubr Technol.,
Trans ASME, 101, 251, 1979.
22 Dubois, G B., Ocvirk, F W., and Wehe, R L., Natl Advisory Committee for Aeronautics, Contract
No NAw6197, Prog Rep 9 (revised), August 1953.
23 Klaus, E E and Duda, J L., Effect of Cavitation on Fluid Stability in Polymer-Thickened Fluids and
Lubricants, Sp Publ 394, U.S National Bureau of Standards Washington, D.C., 1974, 88.
24 Bhatia, R., Mechanical Shear Stability and Blending Efficiency of Polymers in Lubricant Formulations,
M.S thesis, Pennsylvania State University, University Park, Pa., 1978.
25 Myers, H S., Jr., Volatility Characteristics of High-Boiling Hydrocarbons, Ph.D thesis, Pennsylvania
State University, University Park, Pa., 1952.
26 Beerbower, A and Zudkevitch, D., Predicting the evaporation behavior of lubricants in the space
en-vironment, ACS Meet 8, C-99, Div Pet Chem., American Chemical Society, Los Angeles, April 1963, preprint.
27 Klaus, E E and Bieber, H E., Effects of some physical and chemical 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.,
1970, chap 12.
29 Maxwell, J B., Data Book on Hydrocarbons, D Van Nostrand, New York, 1950.
Trang 1030 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, 1967.
32 Wilkinson, E L., Jr., Measurement and Prediction of Gas Solubilities in Liquids M.S thesis, Pennslyvania
State University, University Park, Pa., 1971.
33 Beerbower, A., Estimating the solubility of gases in petroleum and synthetic lubricants, ASLE Trans., 23,
335, 1980.
34 Cayias, J L., Wade, W H., and Schecter, R S., The measurement of low interfacial tension via the
spinning drop techniques, Adsorption at Interfaces, ACS Symp Ser No 8, American Chemical Society,
Washington, D.C., 1975.
35 Military Specification, MIL-L-23699B, Lubricating Oil, Aircraft Turbine Engine, Synthetic Base, U.S Department of Defense, Washington, D.C., 1969.
36 Federal Test Method Standards No 791, Lubricants, Liquid Fuel, and Related Products; Methods of Testing, U.S Bureau of Standards, Washington, D.C., 1974.
37 Military Specification MIL-H-27601A (USAF), Hydraulic Fluid, Petroleum Base, High Temperature, Flight Vehicle, U.S Department of Defense, Washington, D.C., 1966.
38 Klaus, E E., Tweksbury, E J., Jolie, R M., Lloyd, W A., and Manning, R E., Effect of Some
High Energy Sources on Polymer-Thickened Lubricants Spec Tech Publ No 382, American Society for
Testing and Materials, Philadelphia, Pa., 1965, 45.
39 Lockwood, F E and Klaus, E E., Ester oxidation under simulated boundary lubrication conditions,
ASLE Trans., 24, 276, 1981.
254 CRC Handbook of Lubrication