Soybean Oil-Based Lubricants:  A Search for Synergistic Antioxidants

Vegetable oils can contribute toward the goal of energy independence and security due to their naturally renewable resource. They are promising candidates as base fluids for ecofriendly lubricants because of their excellent lubricity, biodegradability, good viscosity−temperature characteristics, and low evaporation loss. Their use, however, is restricted due to low thermo-oxidative stability and poor cold-flow behavior. This paper presents a systematic approach to improve their oxidation behavior by searching for a suitable additive combination. The study of antioxidant/antiwear additive synergism was investigated on a set of four antioxidants and three antiwear additives in vegetable oils using pressure differential scanning calorimetry (PDSC) and a rotary bomb oxidation test (RBOT). The results indicate that dialkyldithiocarbamate antioxidant performed better than diphenylamine and hindered phenol. The zinc dialkyldithiocarbamate antioxidant showed excellent synergism with antiwear additive antimony dithiocarbamates.