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Kinetic approaches are commonly used to evaluate the thermal stability of motor oils. Hence it is crucial to identify the optimal kinetic method to gain the acceptable accuracy of the result. Thermal stability of motor oil was studied under a thermal degradation time of 12 months with thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC). Stability was determined by model-fitting (Coats-Redfern) and model-free (Friedman, Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO) and Vyazovkin) methods. According to model-fitting, motor oil oxidation could be described by the first-order reaction while model-free methods revealed the existence of multiple reactions at the initial of the reaction process. All model-free methods had a similar tendency in stability throughout the conversion extent. A sudden decrease in the activation energy was apparent in the conversion range between 0.1 and 0.2 while a relative plateau in the stability was found in the conversion extent from 0.2 to 0.9. During the thermal degradation period, KAS was considered as the optimal kinetic approach for motor oil oxidation study, which proposed the highest fitting precision and a closer result to the average activation energy between 90.54-92.60 kJ/mol. The optimal kinetic model promotes well understanding of oxidation behavior by providing accurate stability prediction.

Kinetic analysis of oxidation characteristics in synthetic motor oil