Lubricating oil is used to reduce friction and wear between bearing metallic surfaces that are moving with respect to each other by separating the metallic surfaces with a film of the oil. Lubricating oil is distinguished from other fractions of crude oil by a high (>400°C/>750°F) boiling point
In the early days of petroleum refining, kerosene was the major product, followed by paraffin wax wanted for the manufacture of candles. Lubricating oils were at first by-products of paraffin wax manufacture.The preferred lubricants in the 1860s were lard oil, sperm oil, and tallow, but as the trend to heavier industry increased, the demand for mineral lubricating oils increased, and after the 1890s petroleum displaced animal and vegetable oils as the source of lubricants for most purposes.
TEST METHODS
The number of tests applied for product character and quality varies with the complexity of the product and the nature of the application. The more important tests, such as viscosity, flash point, and color are usually performed on every batch. Other tests may be on a statistical (or as-needed) basis dependent on data that can be presented in the graphical form of a fingerprint that is specific for the blend of components and additives in a particular formulation. Comparison of the fingerprint with a known standard can be used as a check on the composition.
- Acidity and Alkalinity
Unused and used petroleum products may contain acidic constituents that are present as additives or as degradation products formed during service, such as oxidation products (ASTM D-5770). The relative amount of these materials can be determined by titrating with bases.The acid number is used as a guide in the quality control of lubricating oil formulations. It is also sometimes used as a measure of lubricant degradation in service. Any condemning limits must be empirically established.
Thus the acid number is a measure of this amount of acidic substance in the oil, always under the conditions of the test. Because a variety of oxidation products contribute to the acid number and the organic acids vary widely in corrosion properties, the test cannot be used to predict corrosiveness of an oil under service conditions.
The acid number is the quantity of base, expressed in milligrams of potassium hydroxide per gram of sample, that is required to titrate a sample in this solvent to a green/green-brown end point with pnaphtholbenzein indicator solution (ASTM D-974, IP 139). However, many higher-molecular-weight oil products (dark-colored oils) that cannot be analyzed for acidity because of obscurity of the color-indicator end point can be analyzed by an alternate test method (ASTM D-664). The quality of the mineral oil products renders them suitable for determination of the acid number.
In a manner akin to the acid number, the base number (often referred to as the neutralization number) is a measure of the basic constituents in the oil under the conditions of the test.The base number is used as a guide in the quality control of oil formulation and is also used as a measure of oil degradation in service.
The neutralization number, expressed as the base number, is a measure of this amount of basic substance in the oil, always under the conditions of the test.The neutralization number is used as a guide in the quality control of lubricating oil formulations. It is also sometimes used as a measure of lubricant degradation in service; however, any condemning limits must be empirically established.
Samples of oil drawn from the crankcase can be tested to assess the reserve of alkalinity remaining by determining the total base number of the oil (ASTM D-664, ASTM D-2896, ASTM D-4739, IP 177, IP 276). Essentially, these are titration methods in which, because of the nature of the used oil, an electrometric instead of a color end point is used. The reserve alkalinity neutralizes the acids formed during combustion. This protects the engine components from corrosion. However, the different base number methods may give different results for the same sample.
Lubricating oil often contains additives that react with alkali to form metal soaps, and the saponification number expresses the amount of base that will react with 1 g of the sample when heated in a specific manner. In the test method (ASTM D-94, IP 136), a known weight of the sample is dissolved in methyl ethyl ketone or a mixture of suitable solvents and the mixture is heated with a known amount of standard alcoholic potassium hydroxide for between 30 and 90 min at 80°C (176°F). The excess alkali is titrated with standard hydrochloric acid, and the saponification number is calculated. The results obtained indicate the effect of extraneous materials in addition to the saponifiable material present.
- Ash
The ash formed by the combustion of lubricating oil (ASTM D-482,ASTM D-2415, IP 4) is, as defined for other products, the inorganic residue, free from carbonaceous matter, remaining after ignition in air of the residual fuel oil at fairly high temperatures. The ash content is not directly equated to mineral content but can be converted to mineral matter content by the use of appropriate formulae.
- Asphaltene Content (Insoluble Constituents)
The asphaltene fraction (ASTM D-2006, ASTM D-2007, ASTM D-3279, ASTM D-4124, ASTM D-6560, IP 143) is the highest-molecular-weight most complex fraction in petroleum. Insofar as the asphaltene content gives
an indication of the amount of coke that can be expected during exposure to thermal conditions .there is little need for the application of the test to lubricating mineral oil. Use of the oil under stressful conditions where heat is generated may introduce the need to determine the amount of insoluble constituents precipitated by the addition of a low-boiling hydrocarbon liquid to mineral oil.
Pentane-insoluble constituents can be determined by membrane filtration (ASTM D-4055). In this method, a sample of oil is mixed with pentane in a volumetric flask, and the oil solution is filtered through a 0.8-mm membrane filter. The flask, funnel, and filter are washed with pentane to completely transfer any particulates onto the filter, after which the filter (with particulates) is dried and weighed to give the pentane-insoluble constituents as a percentage by weight of the sample.
The precipitation number is often equated to the asphaltene content, but there are several issues that remain obvious in its rejection for this purpose.
For example, the method to determine the precipitation number (ASTM D-91) advocates the use of naphtha for use with black oil or lubricating oil and the amount of insoluble material (as a % v/v of the sample) is the precipitating number. In the test, 10 ml of sample is mixed with 90 ml of ASTM precipitation naphtha (which may or may nor have a constant chemical composition) in a graduated centrifuge cone and centrifuged for 10 min at 600–700 rpm.The volume of material on the bottom of the centrifuge cone is noted until repeat centrifugation gives a value within 0.1 ml (the precipitation number). Obviously, this can be substantially different from the asphaltene content.
On the other hand, if the lubricating oil has been subjected to excessive heat, it might be wise to consider application of the test method for determining the toluene-insoluble constituents of tar and pitch (ASTM D-4072, ASTM D-4312). In these methods, a sample is digested at 95°C (203°F) for 25 min and then extracted with hot toluene in an alundum thimble. The extraction time is 18 h (ASTM D-4072) or 3 h (ASTM D-4312). The insoluble matter is dried and weighed. Combustion will then show whether the material is truly carbonaceous or it is inorganic ash from the metallic constituents (ASTM D-482, ASTM D-2415, ASTM D-4628, ASTM D-4927, ASTM D-5185, ASTM D-6443, IP 4).
Another method (ASTM D-893) covers the determination of pentaneand toluene-insoluble constituents in used lubricating oils. Pentaneinsoluble constituents include oil-insoluble materials, some oil-insoluble resinous matter originating from oil or additive degradation, or both. Tolueneinsoluble constituents can come from external contamination, fuel carbon, highly carbonized materials from degradation of fuel, oil, and additives, or engine wear and corrosion materials. A significant change in pentane- or toluene-insoluble constituents and insoluble resins indicates a change in oil that could lead to lubrication problems. The insoluble constituents measured can also assist in evaluating the performance characteristics of a used oil or in determining the cause of equipment failure.
Two test methods are used: Procedure A covers the determination of insoluble constituents without the use of coagulant in the pentane and provides an indication of the materials that can be readily separated from the oil-solvent mixture by centrifugation. Procedure B covers the determination of insoluble constituents in lubricating oil that contains detergents and uses a coagulant. In addition to the materials separated by using procedure
A, this coagulation procedure separates some finely divided materials that may be suspended in the oil. The results obtained by procedures A and B should not be compared because they usually give different values. The same procedure should be applied when comparing results obtained periodically on an oil in use, or when comparing results determined in different laboratories.
In procedure A, a sample is mixed with pentane and centrifuged, after which the oil solution is decanted and the precipitate is washed twice with pentane, dried, and weighed. For toluene-insoluble constituents, a separate sample of the oil is mixed with pentane and centrifuged. The precipitate is washed twice with pentane, once with toluene-alcohol solution, and once with toluene. The insoluble material is then dried and weighed. In procedure B, procedure A is followed except that instead of pentane, a pentanecoagulant solution is used.
- Carbonizable Substances (Acid Test)
The test for carbonizable substances with sulfuric acid is not usually applied to lubricating oil requirements. However, the need may arise and being aware of the availability of such a test is warranted. In this test method (ASTM D-565), a sample of the oil is treated with concentrated sulfuric acid under prescribed conditions and the resulting color is compared with a reference standard to determine whether it passes or fails the test. When the oil layer shows no change in color and when the acid layer is not darker than the reference standard colorimetric solution, the oil is reported as passing the test. A bluish haze or a slight pink or yellow color in the oil layer should not be interpreted as a change in color.The more fully refined the oil, the lighter the color of the acid layer.
However, with the introduction of ultraviolet absorption procedures (ASTM D-2008, ASTM D-2269), the test finds less use but still provides a useful method to determine possible contamination of lubricating oil with impurities transparent to both visible and ultraviolet light and hence not detectable by color or by ultraviolet absorption measurements (ASTM D-2008).
The test for carbonizable substances (ASTM D-565) should not be confused with the test methods for determining carbon residue (ASTM D-189, ASTM D-524, ASTM D4530, IP 13, IP 14, IP 398) (q.v.).
- Carbon Residue
Lubricating oil is not usually considered to be used under the extreme conditions under which coke is formed from, for example, fuel oil. Nevertheless, the tests that are applied to determine the carbon-forming propensity of fuel oil (and other petroleum products) are also available for application to lubricating oil should the occasion arise.
Thus the tests for the Conradson carbon residue (ASTM D-189, IP 13), the Ramsbottom carbon residue (ASTM D-524, IP 14), and the microcarbon carbon residue (ASTM D4530, IP 398) are often included in inspection data for fuel oil.
In the Conradson carbon residue test (ASTM D-189, IP 13), a weighed quantity of sample is placed in a crucible and subjected to destructive distillation for a fixed period of severe heating. At the end of the specified heating period, the test crucible containing the carbonaceous residue is cooled in a desiccator and weighed and the residue is reported as a percentage (% w/w) of the original sample (Conradson carbon residue). In the Ramsbottom carbon residue test (ASTM D-524, IP 14), the sample is weighed into a glass bulb that has a capillary opening and is placed into a furnace (at 550°C/1022°F).The volatile matter is distilled from the bulb, and the nonvolatile matter that remains in the bulb cracks to form thermal coke.
After a specified heating period, the bulb is removed from the bath, cooled in a desiccator, and weighed to report the residue (Ramsbottom carbon residue) as a percentage (% w/w) of the original sample. In the microcarbon residue test (ASTM D-4530, IP 398), a weighed quantity of the sample placed in a glass vial is heated to 500°C (932°F) under an inert (nitrogen) atmosphere in a controlled manner for a specific time and the carbonaceous residue [carbon residue (micro)] is reported as a percentage (% w/w) of the original sample.
The data produced by the microcarbon test (ASTM D-4530, IP 398) are equivalent to those by the Conradson carbon method (ASTM D-189, IP 13). However, the microcarbon test method offers better control of test conditions and requires a smaller sample. Up to 12 samples can be run simultaneously. This test method is applicable to petroleum and to petroleum products that partially decompose on distillation at atmospheric pressure and is applicable to a variety of samples that generate a range of yields (0.01% w/w to 30% w/w) of thermal coke.
- Cloud Point
The cloud point is the temperature at which a cloud of wax crystal first appears in a liquid when it is cooled under conditions prescribed in the test method. This test method covers only petroleum oils that are transparent in layers 38 mm (1.5 in.) in thickness and have a cloud point below 49°C (120°F). The cloud point is an indicator of the lowest temperature of the utility of an oil for certain applications and it is usually higher than the pour point (ASTM D-97, ASTM D-5853, ASTM D-5949, ASTM D-5950,ASTM D-5985, IP 15).
The cloud point (ASTM D-2500, IP 219) of lubricating oil is the temperature at which paraffinic wax, and other components that readily solidify, begin to crystallize out and separate from the oil under prescribed test conditions. It is of importance to know when narrow clearances might be restricted by accumulation of solid material (for example, oil feed lines or filters).
Neither the cloud point nor the pour point should be confused or interchanged with the freezing point (ASTM D-D 2386, ASTM D-5901, ASTM D-5972, IP 16, IP 434, IP 435). The freezing point presents an estimate of minimum handling temperature and minimum line or storage temperature.
It is not a test for an indication of purity and has limited value for lubricating oil.
- Color
Determination of the color of petroleum products is used mainly for manufacturing control purposes and is an important quality characteristic. In some cases the color may serve as an indication of the degree of refinement of the material. However, color is not always a reliable guide to product quality and should not be used indiscriminately in product specifications (ASTM D-156, ASTM D-1209, ASTM D-1500, ASTM D-1544, ASTM D- 6045, IP 17).
In one test (ASTM D-156) for the determination of color, the height of a column of the oil is decreased by levels corresponding to color numbers until the color of the sample is lighter than that of the standard. The color number immediately above this level is recorded as the Saybolt color of the oil, and a color number of +25 corresponds to water-mineral, whereas the minimum color intensity reading on this scale is expressed by +30, a value normally attained by mineral oils. In another test (IP 17), in which the measurements are performed with an 18-in. cell against color slides on a scale, a color of 1.0 or under is considered water-mineral and medicinal oils will normally be 0.5 or less. Conversion scales for different color tests are available (ASTM D-1500). Although sometimes found in insulating oil specifications, the color characteristic is of no technical significance. Pale oils are, as a general rule, more severely refined than dark oils of the same viscosity, and color (ASTM D- 1500, IP 17) is not a guide to stability. Deterioration of color after submission of the oil to an aging test is sometimes limited, but here again extent of oil deterioration can be much better measured by some other property such as acidity development or change in electrical conductivity (ASTM D- 2624, ASTM D-4308, IP 274). About the only point that can be made in favor of color measurement on new oil is that it can give an immediate guide to a change in supply continuity.
- Composition
The importance of composition of lubricating oils lies in the effect it has on their compatibility (ASTM D-2226). This can often be determined by studies of the composition. For example, molecular type analysis separates an oil into different molecular species. One molecular type analysis is the so-called clay-gel analysis. In this method, group separation is achieved by adsorption in a percolation column with selected grades of clay and/or silica gel as the adsorption media (ASTM D-1319, ASTM D-2007, IP 156).
Mass spectrometry can also be used for compositional studies of lubricating oil (ASTM D-3239). This test method covers the determination by high ionizing voltage, low-resolution mass spectrometry of 18 aromatic hydrocarbon types and three aromatic thiophene types in straight-run aromatic petroleum fractions boiling within the range from 205 to 540°C (400–1000°F). Samples must be nonolefinic, must not contain more than 1 mass % of total sulfur, and must not contain more than 5% nonaromatic hydrocarbons. The relative abundances of seven classes of aromatics in petroleum fractions are determined by using a summation of peaks most characteristic of each class. Calculations are carried out by the use of an inverted matrix derived from the published spectra of pure aromatic compounds.
The aromatic fraction needed for this analysis is obtained by using liquid elution chromatography (ASTM D-2549). Aromatic content is a key property of hydrocarbon oils insofar as the aromatic constituents can affect a variety of properties. An existing method using high-resolution nuclear magnetic resonance (ASTM D-5292) is applicable to a wide range of petroleum products that are completely soluble in chloroform and carbon tetrachloride at ambient temperature. The data obtained by this method can be used to evaluate changes in aromatic contents of hydrocarbon oils resulting from process changes. This test method is not applicable to samples containing more than 1% by weight olefinic or phenolic compounds. The hydrogen magnetic resonance spectra are obtained on sample solutions in either chloroform or carbon tetrachloride with a continuous wave or pulse Fourier transform high-resolution nuclear magnetic resonance spectrometer. Carbon magnetic resonance spectra are obtained on the sample solution in deutero-chloroform with a pulse Fourier transform high-resolution nuclear magnetic resonance spectrometer.
The total quantity of sulfur in a gear oil due to the base oil and the additives present can be determined by a bomb method (ASTM D-129, IP 61) in which the sulfur is assessed gravimetrically as barium sulfate.The copper
strip test (ASTM D-130, ASTM D-849, ASTM D-2649, IP 154) is used to simulate the tendency of the oil to attack copper, brass, or bronze. Because active sulfur is desirable for some extreme-pressure applications, a positive copper strip result can indicate that the formulation is satisfactory, but care is necessary in the interpretation of copper strip results because formulations of different chemical compositions may give different results and yet have similar performance in the intended application. Corrosion preventative properties are also measurable (ASTM D-4636).
The constituent elements (barium, calcium, magnesium, tin, silica, zinc, aluminum, sodium, and potassium) of new and used lubricating oils can also be determined (ASTM D-811). Corresponding methods for barium, calcium, and zinc in unused oils are available (IP 110, IP 111, and IP 117,
respectively. For new lubricating oils ASTM D-874/IP 163 can be used to check the concentration of metallic additives present by measuring the ash residue after ignition. This latter method is useful to check the quality of new oils at blending plants or against specifications.
The lead content of new and used gear oils can be determined by the chemical separation method (IP 120). However, there are a number of instrumental techniques that enable the results to be obtained very much more rapidly, among which are polarographic, flame photometric, and Xray fluorescence methods. Chlorine can be determined by a chemical method as silver chloride (ASTM D-808) or by a titration method (ASTM D-1317, IP 118).
Phosphorus can serve as a beneficial adjunct or as a deleterious agent. There are several test methods for the determination of phosphorus. In addition to the three test methods described here, reference should also be made to multielement analysis methods such as inductively coupled plasma atomic emission spectroscopy (ICPAES) (ASTM D-4951, ASTM D-5185) and X-ray fluorescence (XRF) (ASTM D-4927, ASTM D-6443) described above in this guide. Phosphorus can also be determined by a photometric procedure (IP 148) or by a test method (ASTM D-1091) in which the organic material in the sample is destroyed, phosphorus in the sample is converted to phosphate ion by oxidation with sulfuric acid, nitric acid, and hydrogen peroxide, and the magnesium pyrophosphate is determined gravimetrically.
Another method (ASTM D-4047, IP 149) in which the phosphorus is converted to quinoline phosphomolybdate is also available The extent and nature of the contamination of a used automotive engine oil by oxidation and combustion products can be ascertained by determining the amounts of materials present in the lubricating oil that are insoluble in n-pentane and toluene (ASTM D-893).
In this test, a solution of the used lubricating oil in pentane is centrifuged, the oil solution is decanted, and the precipitate is washed, dried and weighed. Insoluble constituents (precipitate) are expressed as a percentage by weight of the original amount of used oil taken and include the resinous material resulting from the oxidation of the oil in service, together with the benzene-insoluble constituents. The latter are determined on a separate portion of sample that is weighed, mixed with pentane, and centrifuged. The precipitate is washed twice with pentane, once with benzene-alcohol solution, and once with benzene. The insoluble material is then dried and weighed to give the percentage of benzene insoluble constituents that contain wear debris, dirt, carbonaceous matter from the combustion products, and decomposition products of the oil, additives, and fuel.
Where highly detergent/dispersant oils are under test, coagulated pentane-insoluble constituents and coagulated benzene-insoluble constituents may be determined by using methods similar to those just described but employing a coagulant to precipitate the very finely divided materials that may otherwise be kept in suspension by the detergent/ dispersant additives.
Size discrimination of insoluble matter may be made to distinguish between finely dispersed, relatively harmless matter and the larger, potentially harmful particles in an oil (ASTM D-4055). The method uses filtration through membranes of known pore size. Membrane filtration techniques are increasingly being used.
The metallic constituents (barium, boron, calcium, magnesium, tin, silicon, zinc, aluminum, sodium, potassium, etc.) of new and used lubricating oils can be determined by a comprehensive system of chemical analysis (ASTM D-874, IP 163).
Turbine oil systems usually contain some free water as a result of steam leaking through glands and then condensing. Marine systems may also have salt water present because of leakage from coolers. Because of this, rust inhibitors are usually incorporated. The rust-preventing properties of turbine oils are measured by a method (ASTM D-665, IP 135) that uses synthetic seawater or distilled water in the presence of steel.The oil should also be noncorrosive to copper (ASTM D-130, IP 154).
The presence of water in turbine systems tends to lead to the formation of emulsions and sludge containing water, oil, oil oxidation products, rust particles, and other solid contaminants that can seriously impair lubrication.
The lubricating oil, therefore, should have the ability to separate from water readily and to resist emulsification during passage of steam into the oil until a predetermined volume has condensed, and the time required for separation is measured (IP 19). Alternatively, the rate of separation of oil that has been stirred with an equal volume of water is measured (ASTM D-1401).
These test methods are only approximate guides to the water-separating characteristics of modern inhibited turbine oils, and the results should be used in conjunction with experience gained of the particular service conditions encountered.
Although systems should be designed to avoid entertainment of air in the oil, it is not always possible to prevent this (ASTM D-892, IP 146). The formation of a stable foam (ASTM D-892, ASTM D-3519, ASTM D-3601, ASTM D-6082, IP 146) increases the surface area of the oil that is exposed to small bubbles of air, thus assisting oxidation. The foam can also cause loss of oil from the system by overflow. Defoaming agents are usually incorporated in turbine oils to decrease their foaming tendency.
Air release is also an important property if a soft or spongy governor system is to be avoided. A careful choice of type and amount of defoaming agent will provide the correct balance of foam protection and air release properties.
Dilution of an oil by fuel under low-temperature or short-distance stop start operation can occur frequently. Dilution of engine oil by diesel fuel can be estimated from gas chromatography (ASTM D-3524), and gasoline dilution can also be measured by gas chromatography (ASTM D-3525).
Low-temperature service conditions may also result in water vapor from combustion products condensing in the crankcase (ASTM D-95, IP 74).
- Density (Specific Gravity)
There are alternative but related means of expressing the weight of a measured volume of a product.
Both density (specific gravity) and API gravity measurements are used as manufacturing control tests and, in conjunction with other tests, are also used for characterizing unknown oils because they correlate approximately with hydrocarbon composition and, therefore, with the nature of the crude source of the oil (ASTM D-l298, IP 160).
For lubricating oil, the purpose of limiting density range is to provide a check on oil composition. In addition, a minimum density may offer some indication of solvent power as well as guarding against excessive paraffin content. In this respect, the inclusion of density in a mineral oil specification may duplicate the aniline point (ASTM D-611, IP 2) requirement.
The API gravity (ASTM D-287, IP 192) is also used for lubricating oil and is based on a hydrometer scale that may be readily converted to a relative density basis API gravity, deg = (141.5/sp gr 60/60°F) – 131.5
API density is also a critical measure reflecting the quality of lubricating oil.
- Flash Point and Fire Point
The flash point gives an indication of the presence of volatile components in an oil and is the temperature to which the oil must be heated under specified test conditions to give off sufficient vapor to form a flammable mixture with air.
The fire point is the temperature to which the product must be heated under the prescribed test conditions to cause the vapor-air mixture to burn continuously on ignition.The Cleveland open cup method (ASTM D-92, IP
36) can be used to determine both flash and fire points of lubricating oils, and the Pensky–Martens closed (ASTM D-93, IP 34) and open (IP 35) flash points are also widely used.
The flash and fire points are significant in cases where high-temperature operations are encountered, not only because of the hazard of fire but also as an indication of the volatility of an oil. In the case of used oils, the flash point is used to indicate the extent of contamination with more volatile oils or with fuels such as gasoline (ASTM D-3607). The flash point can also be used to assist in the identification of different types of base oil blend.
For used automotive engine oils that can be contaminated by a variety of materials, the presence of diesel fuel constituents, resulting from low temperature or short-distance stop-start operation, can be approximately estimated from measurements of the flash point of the oil (ASTM D-92, IP 36) that is appreciably lowered by small quantities of fuel. The presence of gasoline constituents can be measured by distillation (ASTM D-322, IP 23) or by infrared spectroscopy. Fire-resistant lubricating oil is used widely in the coal mining industry.
The use of such fluids also is expanding in the metal cutting and forming, lumber, steel, aluminum, and aircraft industries. A test is also available to evaluate the fire-resistant properties of lubricating oil under a variety of conditions (ASTM D-5306). Most tests involve dripping, spraying, or pouring the liquid into a flame or onto a hot surface of molten metal, but in this test the fluid is impregnated into ceramic fiber media and the linear flame propagation rate, used for the comparison of relative flammability is measured.
- Oxidation Stability
Oxidation results in the development of acidic products that can lead to corrosion and can also affect the ability of the oil to separate from water Oxidation can also lead to an increase in viscosity and the formation of sludge that can restrict oil paths, thus impairing circulation of the oil and interfering with the function of governors and oil relays. Correctly formulated turbine oils have excellent resistance to oxidation and will function satisfactorily for long periods without changing the system charge. Oxidation stability can be assessed by various tests (ASTM D-943, IP 114, IP 157) that use copper as well as iron as catalysts in the presence of water to simulate metals present in service conditions.
Although systems are usually designed to avoid entrainment of air in the oil, it is not always possible to prevent this, and the formation of a stable foam increases the surface area of the oil that is exposed to small bubbles of air, thus assisting oxidation. Defoaming agents are usually incorporated in turbine oils to decrease their foaming tendency, and this can be measured (ASTM D-892, IP 146). Air release is also an important property, and a careful choice of type and amount of defoaming agents is necessary to provide the correct balance of foam protection and air release properties.
- Pour Point
The pour point (ASTM D-97, IP 15) is the lowest temperature at which an oil will flow under specified test conditions, and it is roughly equivalent to the tendency of the oil to cease to flow from a gravity-fed system or from a container and is a guide to, but not an exact measure of, the temperature at which flow ceases under the service conditions of a specific system.
The pour point of wax-containing oils can be reduced by the use of special additives known as pour point depressants that inhibit the growth of wax crystals, thus preventing the formation of a solid structure. It is a recognized property of oil of this type that previous thermal history may affect the measured pour point. The test procedure (ASTM D-97, IP 15) also permits some measurement of the effect of thermal conditions on waxy oils.
The importance of the pour point to the user of lubricants is limited to applications where low temperatures are likely to influence oil flow. Obvious examples are refrigerator lubricants and automotive engine oils in cold climates. Any pump installed in outside locations where temperatures periodically fall below freezing should utilize lubricants with a pour point below those temperatures or the borderline pumping temperature can be
- Thermal Stability
The panel coking test, when used in conjunction with other tests, can be used to assess the deposit-forming tendencies due to thermal instability, and the available alkalinity of these oils can be measured (ASTM D-66, IP 177 IP 276).
- Viscosity
The viscosity of lubricating oil is a measure of its flow characteristics. It is generally the most important controlling property for manufacture and for selection to meet a particular application. The viscosity of a mineral oil changes with temperature but not normally with high stress and shear rate (ASTM D-5275,ASTM D-5481,ASTM D-5684,IP 294), unless specific additives that may not be shear stable are included to modify the viscosity temperature characteristics. Explosions can also result when lubricating oil is in contact with certain metals under high shear conditions (ASTM D-3115).
Thus for base oils, the rate of flow of the oil through a pipe or capillary tube is directly proportional to the pressure applied.This property is measured for most practical purposes by timing the flow of a fixed amount of oil through a calibrated glass capillary tube under gravitational force at a standard temperature and is known as the kinematic viscosity of the oil (ASTM D-445, IP 71). The unit of viscosity used in conjunction with this method is the centistoke (cSt), but this may be converted into the other viscosity systems (Saybolt, Redwood, Engler) by means of conversion formula. At very high pressures, the viscosity of mineral oils increases considerably with increase in pressure, the extent depending on the crude source of the oil and on the molecular weight (ASTM D-2502, ASTM D-2878) of the constituent components.
Because the main objective of lubrication is to provide a film between load-bearing surfaces, the selection of the correct viscosity for the oil is aimed at a balance between a viscosity high enough to prevent the lubricated surfaces from contacting and low enough to minimize energy losses through excessive heat generation caused by having too viscous a lubricant (ASTM D-2422, BS-4231).
The standard viscosity temperature charts (ASTM D-341) are useful for estimating viscosity at the various temperatures that are likely to be encountered in service.
The viscosity of automotive engine oil is the main controlling property for manufacture and for selection to meet the particular service condition using the American Society of Automotive Engineers (SAE) viscosity classification.
The higher-viscosity oils are standardized at 210°F (99°C), and the lighter oils that are intended for use in cold weather conditions are standardized at 0°F (–18°C). The principal difference between the requirements of gas and other internal combustion engine oils is the necessity to withstand the degradation that can occur from accumulation of oxides of nitrogen in the oil that are formed by combustion.The condition of gas engine oils in large engines can be followed by measuring oil viscosity increase (ASTM D-66, ASTM D-97, IP 177, IP 139) to determine changes in the neutralization value resulting from oxidation. In addition, analytical techniques such as infrared spectroscopy and membrane filtration can be used to check for nitration of the oil and buildup of suspended carbonaceous material.
The viscosity index is an empirical number that indicates the effect of change of temperature on the viscosity of an oil. Multigrade motor oils do not behave as Newtonian oils, and the improved viscosity-temperature characteristics of multigrade oils enables, for example, an oil to be formulated to have mixed characteristics (ASTM D-2602, ASTM D-3829).
The viscosity index is important in applications in which an appreciable change in temperature of the lubricating oil could affect the operating characteristics of the equipment. Automatic transmissions for passenger vehicles
are an example of this, where high-viscosity-index oils with improvers are used to minimize differences between a viscosity low enough to permit a sufficiently rapid gear shift when starting under cold conditions and a viscosity adequate at the higher temperatures encountered in normal running. Paraffinic oils have the lowest rate of change of viscosity with temperature (highest viscosity index), whereas the naphthenic/aromatic oils have the highest rate of change (lowest viscosity index) (ASTM D-567, ASTM D-2270, IP 73, IP 226).
The viscosity index of multigrade automotive engine oils is typically in the range of 130–190, whereas monograde oils are usually between 85 and The improved viscosity-temperature characteristics of multigrade oils enables, for example, an SAE 20W/50 oil to be formulated that spans SAE 20W viscosity characteristics at low temperatures and SAE 40 to 50 characteristics at the working temperature.
However, multigrade oils do not behave as Newtonian fluids and this is primarily due to the presence of polymeric viscosity index improvers. The result is that the viscosity of multigrade oils is generally higher at –18°C (0°F) than is predicted by extrapolation from 38°C (100°F) and 99°C (210°F) data, the extent of the deviation varying with the type and amount of viscosity index improver used. To overcome this, the SAE classification is based on a measured viscosity at –18°C (0°F) using a laboratory test apparatus known as a cold cranking simulator (ASTM D-2602).
- Volatility
The volatility of lubricating oil is not usually an issue for multitesting. Nevertheless, tests are available so that specification and purity checks can be made (ASTM D-5480). A method that is used to determine pitch volatility (ASTM D-4893) might also be used, on occasion, to determine the nonvolatility of lubricating oil. In the method, an aluminum dish containing about 15 g of accurately weighed sample is introduced into the cavity of a metal block heated and maintained at 350°C (662°F). After 30 min, during which the volatiles are swept away from the surface of the sample by preheated nitrogen, the residual sample is taken out and allowed to cool down in the desiccator.
Nonvolatility is determined by the sample weight remaining and is reported as percent w/w residue. A test is also available for the determination of engine oil volatility at 371°C (700°F), which is actually a requirement in some lubricant specifications (ASTM D-6417). This test method can be used on lubricant products not within the scope of other test methods with simulated distillation methodologies (ASTM D-2887). Applicability of this test method is limited to samples with an initial boiling point higher than 126°C. This test method may be applied to both lubricant oil base stocks and finished lubricants containing additive packages.
In the test, a sample aliquot diluted with a viscosity-reducing solvent is introduced into the gas hromatographic system, which uses a nonpolar open tubular capillary gas chromatographic column for eluting the hydrocarbon components of the sample in the order of increasing boiling point. The column oven temperature is raised at a reproducible linear rate to effect separation of the hydrocarbons. Quantitation is achieved with a flame ionization detector.The sample retention times are compared with those of known hydrocarbon mixtures, and the cumulative corrected area of the sample determined to the 371°C (700°F) retention time is used to calculate the percentage of oil volatilized at 371°C (700°F).
- Water and Sediment
Knowledge of the water content of petroleum products is important in refining, purchase and sale, and transfer of products and is useful in predicting the quality and performance characteristics of the products. The Karl Fischer test method ASTM D-6304) can be applied to the direct determination of water in lubricating oil. In this method, the sample injection in the titration vessel can be done volumetrically or gravimetrically.
The instrument automatically titrates the sample and displays the result at the end of the titration. Viscous samples can be analyzed by using a water vaporizer accessory that heats the sample in the evaporation chamber, and the vaporized water is carried into the Karl Fischer titration cell by a dry, inert carrier gas.
Sediment in lubricating oil can lead to system malfunction in critical applications, and determination of the amount of sediment is a necessity. In the test method (ASTM D-2273), a 100-ml sample of oil is mixed with 50ml of ASTM precipitation naphtha and is heated in a water bath at 32–35°C (90–95°F) for 5 min. The centrifuge tube containing the heated mixture is centrifuged for 10 min at a rate of between 600 and 700 relative centrifuge force (rcf). After the mixture is decanted carefully, the procedure is repeated with another portion of naphtha and oil. The final reading of sediment is recorded. This test method is not applicable in cases in which precipitated oil-soluble components will appreciably contribute to the sediment yield.
Insoluble material may form in lubricating oil in oxidizing conditions, and a test method is available (ASTM D-4310) to evaluate the tendency of lubricating oil to corrode copper catalyst metal and to form sludge during oxidation in the presence of oxygen, water, and copper and iron metals at an elevated temperature.This test method is a modification of another test method (ASTM D-943) in which the oxidation stability of the same kind of oils is determined by following the acid number of the oil. In the test method (ASTM D-4310), an oil sample is contacted with oxygen in the presence of water and iron-copper catalyst at 95°C (203°F) for 100 h. The weight of the insoluble material is determined gravimetrically by filtration of the oxidation tube contents through a 5-mm-pore size filter disk. The total amount of copper in the oil, water, and sludge phases is also determined.
References:
1. Handbook of Petroleum Product Analysis – JAMES G. SPEIGHT.
2. Petroleum Production Engineering Handbook – PEH.
