Temperature dependence of the oxidative stability of corn oil and polyalphaolefin in the presence of sulfides

The effect of sulfide-modified corn oil (SMCO) and ditertiary dodecyl pentasulfide (PS) additives on the oxidative stability of corn oil (CO) and polyalphaolefin oil (PAO) was investigated using pressurized differential scanning calorimetry in dynamic (DDSC) and isothermal (IDSC) modes. DDSC showed a weak pro-oxidant effect of SMCO and PS in CO, and antioxidant behavior in PAO. Analysis of the IDSC data showed the existence of isokinetic temperature in both oils. Below the isokinetic temperature the sulfides behaved as antioxidants while above it they behaved as pro-oxidants. The isokinetic temperature was found to depend on the chemical structure of the oils. For the highly unsaturated CO, the isokinetic temperature was 100–135 °C, while for the fully saturated PAO it was above 200 °C. The existence of isokinetic temperature provides consistent explanation to our and literature data for oxidation behaviors of sulfide additives in oils, which appears to be contradictory (pro-oxidant vs. antioxidant) depending on experimental conditions. The isokinetic temperature suggests that accelerated oxidation test methods at elevated temperatures are poor predictors of the low-temperature performance of sulfide-containing antioxidants.