"How do you check the quality of a lubricant additive? Which parameters are most important?"
There are many tests that can help determine the quality of a lubricant additive. The importance of the parameters is prioritized by the specific role that the lubricant will fill. One thing to remember is that lubricant performance is not solely governed by additives. The base oil also plays a major part.
Following are some of the most critical parameters, along with the standardized testing methods, in no particular order:
Viscometrics – ASTM D445 (or Modified)
This test method specifies a procedure for the determination of the kinematic viscosity of liquid petroleum products, both transparent and opaque, by measuring the time for a volume of liquid to flow under gravity through a calibrated glass capillary viscometer. The dynamic viscosity can be obtained by multiplying the kinematic viscosity by the density of the liquid.
Wear and Friction Control – ASTM D5182 or D4998
This test method evaluates gear-tooth face scuffing resistance of fluids using A-profile gears. The rig is operated at 1,450 rpm up to 12 progressive load stages at 15-minute intervals. Gear teeth are inspected after each load stage for scuffing. In addition to a visual evaluation of gear-tooth condition, gear weight loss is measured.
Oxidation Resistance — ASTM D943
This test method is widely used for specification purposes and is considered valuable in estimating the oxidation stability of lubricants, especially those that are prone to water contamination. However, it should be recognized that the correlation between the results of this method and the oxidation stability of a lubricant in field service may vary markedly with field service conditions and with various lubricants. The precision statement for this method was based on steam turbine oils.
Dispersancy — ASTM D1401
This test method provides a guide for determining the water separation characteristics of oils subject to water contamination and turbulence. Dispersancy is used for specification of new oils and monitoring of in-service oils.
Base Number — ASTM D2896
New and used petroleum products can contain basic constituents that are present as additives. The relative amounts of these materials can be determined by titration with acids. The base number is a measure of the amount of basic substance in the oil, always under the test conditions. It is sometimes used as a measure of lubricant degradation in service. However, any condemning limits must be empirically established.
Detergency – ASTM D4951-09
Additive packages are blends of individual additives that can act as detergents, antioxidants, anti-wear agents and so forth. Many additives contain one or more elements covered by this test method. Additive package specifications are based in part on elemental composition. Lubricating oils are typically blends of additive packages, and their specifications are also based in part on elemental composition. This test method can be used to determine if additive packages and unused lubricating oils meet specifications with respect to elemental composition. Specifically looking at detergents would require investigation of calcium, phosphorous, magnesium, barium, etc.
Demulsibility – ASTM D2711
This test provides a guide for determining the demulsibility characteristic of lubricating oils that are prone to water contamination and may encounter the turbulence of pumping and circulation, which is capable of producing water-in-oil emulsions.
Corrosion Resistance – ASTM D665
In many instances, such as in steam turbine gears, water can become mixed with the lubricant, and rusting of ferrous parts can occur. This test indicates how well inhibited mineral oils aid in preventing this type of rusting. The method is also used for testing hydraulic and circulating oils, including heavier-than-water fluids, as well as for the specification of new oils and monitoring of in-service oils.
Pour Point – ASTM D97
The pour point of a liquid is the lowest temperature at which it becomes semi-solid and loses its flow characteristics.
