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Compatibility of Lubricant Mixtures
David Doyle, OMA, CLS
As product formulations in industrial oil applications become more complex, lubricant compatibility has become an increasing concern. Mixing of incompatible lubricants in a circulation system can lead to the formation of insoluble materials which plug flow passageways, lead to additive dropout, and reservoir sludge build up. Mixing of two incompatible lubricants can compromise the overall performance of the combined products in service. This can result in a fluid which is inferior to the original products before they were combined.
The most common problem occurs when a lubricant which has an acidic additive package is combined with a lubricant using an alkaline based additive package. This acid / based mixture will result in insoluble soap materials being created, which cause filter and passageway plugging. Most lubricant additives exist in balanced concentrations. Lubricants with dissimilar additive chemistry can reduce the overall effectiveness of the additives or cause them to fall out of solution. Component materials such as seals can also be negatively affected.
One of the challenges today is the mixing of lubricants formulated with Group I base oils with products formulated with Group II base oils or mineral oils with synthetic based products. Many additives that can remain in suspension in Group I base oils do not work as well in Group II base oils. Also, the type of additives that perform a particular function in a Group I base oil may have different chemistry in Group II base oil for the same function. Certain synthetic lubricants cannot be mixed with mineral oil based products. Many products simply do not blend and separate when given the chance, especially in the reservoir. Seal degradation, bearing and pump failures can results from these mixtures.
When changing from one lubricant product to another in an industrial application it’s recommend that end-users consult their lubricant supplier for guidance. Not all lubricants may be incompatible just because they are from a different supplier. Oils with similar chemistry would not cause a concern. As an example, many AW Hydraulic oils are similar in chemistry and would not cause a problem if mixed. Lubricants with dissimilar chemistry in either the additive packages or base oil formulations are the concern.
Products can be tested for compatibility by a laboratory. Test Method ASTM D7155 provides a standard practice for evaluating the mixture of two lubricants for compatibility. Generally two products are mixed at varying concentrations and observed for clarity and the formation of precipitates and sediment. The observations are conducted at varying temperatures for established lengths of time. Combined product performance can be looked at by employing additional subsequent testing for foaming, rust prevention, corrosion, water separability, etc.
Always consult with your lubricant supplier when changing to a different product in your lubrication circulating system. When still in doubt request additional testing of the mixture before combining products. Even an on-site visual observation with a simple mixture may show signs of incompatibility after setting for a few days to a week.
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