Handbook of Lubrication part 7 pot

The boiling point or boiling range of lubricant fractions or components can be measuredusing temperature programed gas chromatography ASTM D2887 for boiling points up to1000°F 538°C.. A

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 thelubricant raises the possibility of two viscosity-related effects First, under high shear ratesand 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 degradedand the viscosity of the solution lowered permanently Characteristics of flow in the lubricantsystem determine the severity of mechanical degradation and, thereby, limits the size andeffectiveness of polymeric additives that can be used

Shear rates in lubricant applications range from low values to the order of 106reciprocalseconds in various common lubricating systems Another important area is the shear rate orshear 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 × 105/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 103to 104/sec

A typical polymer solution gives a characteristic behavior as shown in Figure 2 Viscosityremains 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 thepower law index η in the Ostwald de Waele equation η = K γn−1 where K is a constanttypical of the polymer system A plot of percent temporary viscosity loss vs log of theshear rate also provides a straight line which, when extrapolated to lower shear rates, predictsthe shear rate at which non-Newtonian behavior begins With polymer molecular weightslimited by mechanical viscosity loss, the second Newtonian zone appears to be greater than

106/sec

Non-Newtonian lubricants may provide some advantages in a journal bearing Studieshave shown that a non-Newtonian lubricant can maintain the film thickness predicted bythe low shear viscosity and show as much as a 40% friction reduction.22 Non-Newtonianlubricants also tend to give lower than predicted friction in EHD bearings, possibly by partialstarvation of the EHD contact While polymer-containing lubricants lower friction andimprove gas mileage in automotive engines, the specific mechanism responsible is not welldefined

Polymer Degradation

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 EHDand 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 dischargeline to create a pressure drop and severe cavitation, (2) an ultrasonic oscillator, and (3) aroller bearing rig to provide severe mechanical degradation with an EHD contact A largenumber of cycles are required to achieve a final breakdown value For a given pressure dropacross 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 orificetype loading device or Hertzian pressure in EHD contacts, as illustrated in Figure 3.24 Theroller bearing data were determined in a tester comprising two loaded tapered roller bearingsrunning at 3500 rpm with 15 m of lubricant.23 The mechanical breakdown appeared to bestepwise with an estimated nine successive molecular scissions for a 5000 nm molecule tothe 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

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) offire point Thus, a flash point of 400°F (204°C) and a fire point of 440°F (227°C) would beexpected of a typical lubricating oil fraction A larger spread would indicate a relativelypoor separation by distillation As a rule of thumb, the fire point of a typical mineral oilfraction 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 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 andautomotive lubricants exhibit this range

tem-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 inFigure 4 Evaporation losses from a relatively thin film evaporation test give another usefulmeasure of volatility

The boiling point or boiling range of lubricant fractions or components can be measuredusing temperature programed gas chromatography (ASTM D2887) for boiling points up to1000°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

FIGURE 4 Viscosity-volatility relationship.

for hydrocarbons shown in Figure 5.25This relationship was generated using hydrocarbonsand 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 volatilecomponents Thus, in a lubricant with a 150°C (302°F) boiling range, the 5 to 20% boilingpoints are the most important in establishing a vapor pressure for the system

In addition to oil consumption, evaporation, and safety (flammability), volatility plays arole in boundary lubrication There is evidence27,28that lubricants with high volatilities causehigher wear in systems than do lubricants with matched viscosities and fluid types of lowervolatility levels

DENSITYSpecific 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 petroleumindustry has modified the Baume scale to provide an API gravity defined by the equation:

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 boilingpoint of the hydrocarbon fraction than to its density, although both independent variableare necessary to correlate the data properly.29 For mineral oil lubricants an engineeringapproximation for the coefficient of expansion is summarized in Table 4, The chapter

“Lubricant Properties and Test Methods” in Volume I gives typical densities of commerciallubricants

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 amongparaffinic, naphthenic, and aromatic structures in the lubricant base oil (ASTM D3238).Lubricant compressibility is usually expressed as bulk modulus which is defined by theequation:

BULK MODULUSBulk modulus expresses the resistance of a fluid to compression (reciprocal of compress-ibility) This property, which varies with pressure, temperature, and molecular structure, issignificant in (1) hydraulic and servosystem efficiencies and response time, (2) resonanceand water-hammer effects in pressurized-fluid systems, (3) explanation of viscosity-pressureproperties in hydrodynamic and EHD lubricants, and (4) in thermodynamic considerations

of liquids

Two general methods used to measure bulk modulus are (1) pressure-volume-temperaturedetermination of density or density change directly, and (2) velocity of sound in a liquid atthe desired temperature and pressure The former method provides isothermal secant bulkmodulus or average values over a pressure range Tangent bulk modulus or bulk modulusfor a specific pressure is obtained by differentiation from the secant data Velocity of soundmeasurements provide adiabatic tangent bulk modulus values

Klaus and O’Brien30 measured the isothermal secant bulk modulus for a variety of bricants over the range of 0 to 10,000 psi (0.101 to 69 MPa) For engineering accuracy,

lu-the isolu-thermal secant bulk modulus, B

_, can be converted to an isothermal tangent bulkmodulus B, in accordance with the relationship:

Wright31proposed a useful method for predicting isothermal secant bulk modulus valuesfor mineral oils based on Figures 6 and 7 Figure 6 shows the relationship between B

_andtemperature 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 Theserelationships work well for mineral oil base stocks and formulated lubricants, organic acidesters, synthetic hydrocarbons, and phenyl ethers Both silicones and perfluoropolyethersshow 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 showstwo deleterious effects on system response First, any entrained air dissolves upon raisingthe pressure, causing a greater volume reduction than the compressibility of the originalfluid Secondly, the gas-saturated fluid is somewhat more compressible than the same fluidwith only air saturation at atmospheric pressure Air saturation at atmospheric pressure doesnot measurably change B

_over that of a degassed fluid

GAS SOLUBILITYSolubility of gases in lubricants is a physical property that in turn affects related lubricantproperties 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 againreduced to that in the reservoir or sump, the gas comes out of solution to produce foam orjust entrained gas bubbles The dissolved oxygen, in the case of air, can also react with thelubricant 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 gaschromatograph (GC) with a precolumn of solid adsorbent to remove the liquid which containsthe 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 gassolubility 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., CO2,gas solubility decreases with increasing temperature At fluid temperatures where the vaporpressure of the liquid is 60 mmHg (8 kPa) or above, gas solubility falls below levels predictedfrom lower temperatures At the normal boiling point, gas solubility drops to zero Smallamounts of volatile products in the lubricant can have the same effect as a more volatilebase oil and result in reduced gas solubility With gas mixtures, solubility of individualgases follows the partial pressure of the gas in the mixture

Volume II 241

Table 5 GAS SOLUBILITY PARAMETERS

a L applied only to petroleum liquids of 0.85 kg/dm 3 density,

d, at 15°C To correct the other densities, Lc = 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 isthe equilibrium volume of gas dissolved in a unit volume of oil This coefficient can beused 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 conditionsencountered in lubrication systems The weight solubility of air at 2 atm is then double thesolubility at 1 atm for a given temperature Liquid solubility parameter, S1, is approximately18.0 for diesters commonly used in aircraft fluids, 18.5 to 19.0 for higher esters, 18.41 formethyl 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 gasesdissolve rapidly The resulting fluid can show a dramatic reduction in viscosity Typicalviscosity 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 ENTRAINMENTTendency to foam generally increases with increasing fluid molecular size, increasingviscosity, or decreasing temperature Foaming is caused by the escape of insoluble gases orthe physical mixing of excess gas with the fluid The best way to minimize foam is withmechanical design The chemical approach to reducing foaming is the use of a siliconeadditive that tends to lower surface tension at gas-liquid interfaces

Air entrainment is similar to the problems of foam In hydraulic systems, air entrainmentcan result in response problems, while in gear systems air entrainment can result in reducedheat transfer and higher operating temperatures Antifoam additives are not necessarilyhelpful; several commercial additives are available to improve air entrainment characteristics

THERMAL PROPERTIESThermal properties of lubricants are involved in considering heat transfer, temperature

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 boilingtemperature decreases with the increasing pressure (and increasing boiling point) A typicalmineral oil, e.g., ISO grade 68, exhibits the following values

Latent heat Pressure of vaporization

It appears that alteration of the electrical conductivity of the base oil is primarily due to theionic nature of many additives, impurities such as water and chlorides, or oxidative orthermal degradation of the base stock Electrical conductance of the unused lubricant isconsidered critical primarily in such applications as electrical equipment and in some aircraftand 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 isthe manifestation of this surface free energy at a gas-liquid interface, while interfacial tensionexists 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 awayfrom the surface is measured

Volume II 245

Table 9 SPECTRAL DATA OBTAINED FOR VARIOUS LUBRICANTS

Polymers used as VI improvers tend to have thermal stability thresholds that are lowerthan 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 belowthose 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 activethe 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 importantchemical property Unlike thermal stability, oxidation stability can be altered significantly.Additives control oxidation by attacking the hydroperoxides formed in the initial oxidationstep or by breaking the chain reaction mechanism Aromatic amines, hindered phenols, andalkyl 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 surfacesand soluble metal salts from catalyzing the condensation polymerization reactions of oxidizedproducts to produce sludge and varnish

A number of bulk oxidation tests are described in the ASTM (D2272, D1313) and FederalTest Method Standards No 791, Method No 5308 These tests are good for measuringstable life or the effectiveness of oxidation inhibitors Oxygen diffusion limits the value ofthese tests in correlations with many actual lubrication systems

The first step in oxidation of hydrocarbons is formation of a peroxide at the most vulnerablecarbon-hydrogen bonds This initiates a free radical chain mechanism which propagatesformation of hydroperoxides Further oxidation leads to other oxygen-containing moleculessuch as aldehydes, ketones, alcohols, acids, and esters A similar peroxide path of oxidationhas 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 length, but extinction coefficient not reported, MLO 7558 — paraffinic white oil, MLO 7828 — naphthenic white oil, and MLO 7219 — partially hydro- genated aromatic stock.

wave-To monitor the oxidation process, a microoxidation test has been developed along withanalytical procedures based on gel permeation chromatography (GPC) and atomic absorptionspectroscopy (AAS).39 In these tests, oxidations were carried out until 50% or more of the

Table 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 condensationpolymers with a characteristic beta keto conjugated unsaturation (–C=C–C–) which can befound in oxidation products from dibasic acid esters, polyol esters, monoesters, and mineraloils 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 oxidationrate

The effect of temperatures on stable life of lubricants is illustrated in Figure 9 Thisextrapolation system relating log of life to temperature provides a design guideline for thelimiting bulk lubricant temperatures in a system

LUBRICATION SPECIFICATIONSSeveral widely used specifications include SAE engine oil grades, SAE gear lubricationgrades, 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).

Table 11 TYPICAL MILITARY SPECIFICATIONS FOR HYDRAULIC FLUIDS AND LUBRICANTS

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 — Lubricating oil, aircraft turbine engine, synthetic base.

Table 12 PHYSICAL PROPERTIES OF SEVERAL FLUIDS

Table 13 PROPERTIES OF TYPICAL SAE GRADE LUBRICANTS

fluid lubricants, and military specifications Examples of these standards and classificationsare shown in Tables 10 and 11 and in pertinent chapters of Volume I These specificationsdefine 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.

military specifications, with respect to oxidation stability, thermal behavior, and wearcharacteristics.

General specifications for a fluid type do not imply that all fluids meeting the requirementsare of equal quality Relative quality must be determined by the ultimate user in his particularapplication 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 gradelubricants are shown in Table 13 Characteristics of a variety of commercial lubricants arealso 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