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Mineral Base Oil Specifications
Lubricants are formulated by blending base oils and additives to meet a series of performance specifications. These specifications relate to the chemical and physical properties of the formulated oil when it is new and also ensure that the oil continues to function and protect the engine or machinery in service. Self-evidently, lubricant performance is determined by the base oils and the additives used in the formulation.
A range of properties can be measured and used to predict performance when selecting an appropriate base oil for use in formulation. Many of these properties are used as quality control checks in the manufacturing process to ensure uniformity of product quality. Although many of these properties are modified or enhanced by the use of additives, knowledge of the base oil characteristics, especially any limitations, is vital for the effective formulation of any lubricant.
The complexity of the chemical composition of the base oils requires that most measurements are of overall, bulk, physical or chemical properties which indicate the average performance of all the molecular types in the base oil. Many tests are empirically based and are used to predict, or correlate with, the real-field performance of the lubricant. Although not rigorously scientific, the importance of such tests should not be underestimated.
A wide range of tests was developed by different companies and different countries in the early days of the oil industry. Many tests are now standardized and controlled on an international basis by organizations such as the following:
| International | International Organization for Standards, ISO. |
| USA | American Society for Testing and Materials, ASTM. |
| UK | Institute of Petroleum, IP. |
| Germany | Deutches Institut für Normung, DIN. |
| Europe | Association des Constructeurs Européens d’Automobiles, ACEA. |
| Japan | Japanese Automotive Standards Organization, JASO. |
Mineral Base Oil Physical Properties:
Viscosity
Viscosity measures the internal friction within a liquid, reflecting the way molecules interact to resist motion. It is a vital lubricant property, influencing the ability of the oil to form a lubricating film or to minimize friction and reduce wear.
Absolute Viscosity
Newton defined the absolute viscosity of a liquid as the ratio between the applied shear stress and the resulting shear rate. The unit of absolute viscosity is the pascal second (Pa.s), but centipoise (cP) is generally used as the alternative unit, where 1 Pa.s = 103 cP. Absolute viscosity is an important measurement for the lubricating properties of oils used in gears and bearings.
Kinematic Viscosity
Kinematic viscosity is the measurement of liquid flow rate through a capillary tube under the constant influence of force of gravity. The unit of kinematic viscosity is m2/s but for practical reasons it is more common to use the centistoke, cSt, where 1 cSt = 10–6 m2/s.
There are other, empirical, scales in use such as SUS (Saybolt Universal Seconds). Base oil grades are sometimes referred to by their SUS viscosities.
Viscosity Index
Viscosity/temperature relationship – the viscosity index: The most frequently used method for comparing the variation of viscosity with temperature between different oils calculates a dimensionless number. The kinematic viscosity of the sample oil is measured at two different temperatures, 40 and 100◦C, and the viscosity change is compared with an empirical reference scale.
The VI scale is a useful tool in comparing base oils, but it is vital to recognize its arbitrary base and limitations. Extrapolation outside the measured temperature range of 40–100◦C may lead to false conclusions, especially as wax crystals form at low temperatures. VI is also used as a convenient measure of the degree of aromatics removal during the base oil manufacturing process. But comparison of VIs of different oil samples is realistic only if they are derived from the same distillate feed stock. Therefore, great care should be used in applying VI measurements as indicators of base oil quality.
Low Temperature Properties
When a sample of oil is cooled, its viscosity increases predictably until wax crystals start to form. The matrix of wax crystals becomes sufficiently dense with further cooling to cause apparent solidification of the oil. But this is not a true phase change in the sense that a pure compound, such as water, freezes to form ice. Although the ‘solidified’ oil will not pour under the influence of gravity, it can be moved if sufficient force is applied, e.g. by applying torque to a rotor suspended in the oil. Further decrease in temperature causes more wax formation, increasing the complexity of the wax/oil matrix and requiring still more torque to turn the rotor. Many lubricating oils have to be capable of flow at low temperatures and a number of properties should be measured.
Cloud point
It is the temperature at which the first signs of wax formation can be detected. A sample of oil is warmed sufficiently to be fluid and clear. It is then cooled at a specified rate. The temperature at which haziness is first observed is recorded as the cloud point.
Pour point
It is the last temperature before movement ceases, not the temperature at which solidification occurs. This is an important property of diesel fuels as well as lubricant base oils. High-viscosity oils may cease to flow at low temperatures because their viscosity becomes too high rather than because of wax formation. In these cases, the pour point will be higher than the cloud point.
High Temperature Properties
High-temperature properties of a base oil are governed by its distillation or boiling range characteristics.
Volatility
Volatility is important because it indicates the tendency of oil loss in service by vaporization, e.g. in a hot engine.
Flash Point
The flash point of an oil is an important safety property because it is the lowest temperature at which auto-ignition of the vapor occurs above the heated oil sample.
Other Physical Properties
Various other physical properties may be measured, most of them relating to specialized lubricant applications. A list of the more important measurements includes the following:
Density: important, because oils may be formulated by weight but measured by volume.
Demulsification: the ability of oil and water to separate.
Foam characteristics: the tendency to foam formation and the stability of the foam that results.
Pressure/viscosity characteristics: the change of viscosity with applied pressure.
Thermal conductivity: important for heat transfer fluids.
Electrical properties: resistivity, dielectric constant.
Surface properties: surface tension, air separation.
Mineral Base Oil Chemical Properties:
Oxidation
Degradation of lubricants by oxidative mechanisms is potentially a very serious problem. Although the formulated lubricant may have many desirable properties when new, oxidation can lead to a dramatic loss of performance in service by reactions such as:
Corrosion due to the formation of organic acids.
Formation of polymers leading to sludge and resins.
Viscosity changes.
Loss of electrical resistivity.
The sulphur content of base oils is often regarded as a useful indicator of natural oxidation resistance. This is because many naturally occurring organo-sulphur compounds in crude oil are moderately effective in destroying organic peroxide intermediates and breaking the oxidation chain mechanism. However, the effectiveness of these natural inhibitors is usually rather inferior to synthesized additives which can be much more specific in their action.
Corrosion
A lubricant base oil must not contain components which corrode metal parts of an engine or a machine. Corrosion tests usually involve bringing the base oil sample into contact with a metal surface (copper and silver are often used) under controlled conditions. Discoloration of the metal, changes in surface condition or weight loss may be used to measure the corrosion tendency of the oil.
Carbon residue
A test used to measure the tendency of a base oil to form carbonaceous deposits at elevated temperatures. E.g. The Conradson carbon residue test, determines the residue which remains after pyrolytic removal of volatile compounds in the absence of air.
Seal compatibility
Lubricants come into contact with rubber or plastic seals in machines. The strength and degree of ‘swell’ of these seals may be affected by interaction with the oil. Various tests measure the effects of base oils on different seals and under different test conditions.
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