Synergistic effect of antiwear additives and antioxidants in vegetable oil

Certain vegetable oils have been recognised as promising candidates for environmentally friendly lubricants. Their lubricity can be optimised using antiwear additives. However, decomposition of the additives, accompanied by auto-oxidation of the vegetable oils, has been found to occur. In other words, the antiwear additives may degrade in vegetable oils, and give less oxidative stability. In order to understand the mechanism, oxidation testing of a vegetable oil was performed by monitoring the peroxide value (POV). It was observed that POV increases as soon as oxidation starts. The value reached 1400–1500 ppm within 50 h of reaction time. Then it decreased to an equilibrium value of 400–500 ppm within 200 h. Antioxidants to prevent peroxide generation were examined. Phenolic antioxidants and zinc dialkyldithiocar-bamate exhibited good results with regard to preventing peroxide generation, whereas aromatic amines gave poor results. A synergistic effect that prevents the decomposition of the antiwear additives was found.