EVALUATION OF DEPOSIT FORMING TENDENCY OF MINERAL AND SYNTHETIC BASE OILS USING THE PENN STATE MICROOXIDATION TEST

Deposit formation in the Penn State microoxidation test is the combined result of the lubricant oxidation and the secondary oxidation-condensation polymerization reactions to form high molecular weight products. Results of this test have been previously shown to give excellent correlation with the results from Sequence IIID engine tests and heavy-duty diesel engine tests. In the current study, the Penn State test was used to evaluate a series of mineral and synthetic base oils with and without oxidation inhibitors. The oils selected for this study cover the range of viscosity and volatility typically used in formulating SAE 5W-30 to 15W-40 crankcase lubricants. The experiments were designed to show the relative contributions of volatility, thermal stability, oxidative stability, type of refining, and type of oxidation inhibitor on the process of deposit formation in a thin lubricating film on a hot ferrous surface. This study shows that the influence of volatility, base stock refining, and inhibitor effectiveness on deposit formation of mineral oil and synthetic base oils can be determined by using the Penn State microoxidation test. Generally, data show that for the same oxidation stability, volatility of the base stock plays an important role in controlling deposit formation. The evaluation of two oxidation inhibitors with different inhibiting mechanisms shows that the additive response as a function of base oil refining may be significantly different. It is concluded that the addition of relatively high concentrations of copper as an additive, 2000 ppm Cu, can significantly reduce the deposit formation tendencies of all the base oils considered in this study.