Volume 6, Issue 2 (February 2009)
Contaminants in Used Oils and Their Behavior
Either particle count or gravimetric analysis is commonly used for assessing oil cleanliness. These analytical methods are useful and practical. However, the former provides the particle number of defined size groups and the latter the gross weight of the contaminants in a certain volume of oil, but they cannot inform us of the characteristics of contamination. To solve hydraulic and lubrication problems due to oil contamination, it is imperative to know the characteristics of contamination. Solvent extraction methods have been used to separate contaminants from used oils. Solvent extraction analysis showed that the majority of contaminants in used oils were polymerized oil oxidation products, which were not soluble in oil. Recently, problems of varnish formation in gas turbines has been of great interest. Varnish is the polymerized oil oxidation product. Such polymerized oil oxidation products have polarity, although base mineral oils are non-polar. Polar and non-polar materials are basically incompatible with each other. Polar materials behave independently in oils. The molecular weight of oil oxidation products was determined by GPC, and it was found that they were good for lubrication when they were soluble in oil but harmful when they became insoluble. However, contaminants must be considered on the level of molecular size in order to solve hydraulic and lubrication problems. This paper discusses the behavior of oil oxidation products in oils.