STUDY OF COPPER-SALTS AS HIGH-TEMPERATURE OXIDATION INHIBITORS

Oil-soluble copper salts such as copper naphthenate have been evaluated as additives to mineral oil, synthetic hydrocarbon, and polyol ester base stocks in the Penn State microoxidation test. These tests were carried out at 225-degrees-C under test conditions which show good agreement with the behavior of automotive crankcase oils in the III-series hot engine test. At concentrations of 200 ppm of copper in a mineral oil, the primary oxidation rate of the mineral oil is increased with the conversion of these primary oxidation products, but further reaction to condensation polymers is substantially decreased. These effects can be seen in both low-carbon steel systems and glass systems. At concentration levels of 2000 ppm copper in the mineral oil, copper is a very effective oxidation inhibitor, and also inhibits condensation polymerization reactions of the primary oxidation products. At these high concentrations, copper also is an effective metal deactivator. Iron corrosion products from a low-carbon steel catalyst surface are essentially stopped by the action of the copper additive. Soluble copper salts are equally effective in the synthetic hydrocarbon and polyol ester studied. The use of 2000 ppm soluble copper in these base stocks produces a lubricant that compares favorably in oxidative stability with the best commercial SAE-grade automotive crankcase oils. In the prevention or delay of deposit formation, 2000 ppm of soluble copper in a mineral oil appears to be superior to typical state-of-the-art SAE crankcase oils.