Composition Analysis of lubricating oil:
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 appli- cable 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 test methods 279with 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 preventa-
tive 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 X-ray 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 gravi- metrically. 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 test methods 281a 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 entrainment 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).