Thermal, spectroscopic and rheological study of mineral base lubricating oils

The thermal degradation process of mineral base lubricating oils was studied in this work by means of thermal, spectroscopic and rheologic analysis. The lubricating oils were degraded at temperatures varying from 150 to 210 degrees C, and for degradation times from 1 to 48 h. After the degradation, the lubricating oils were characterized by X-ray fluorescence, IR and NMR spectroscopies, rheological properties and thermal analyses (TG/DSC). The spectroscopic analyses determined the oxidation reaction products. TG curves indicate that the thermal stability of lubricating oils is below 161 degrees C. TG curves in air present three mass loss stages, whereas in nitrogen there are only two mass loss steps. DSC analyses in air indicate two highly exothermic peaks related to hydrocarbon oxidation and combustion processes, while in nitrogen only two endothermic peaks were observed. The decrease in the degradation temperature led to a decrease of the lubricant viscosity.