Analysis of thermal degradation of automotive lubricating oil using Raman spectroscopy

This work proposes the use of Raman spectroscopy to identify chemical changes during the heating of automotive lubricating oil (ALO) samples of mineral (MN), semi-synthetic (SS), and synthetic (ST) types. Each sample was heated and maintained at 140 ± 2.5 °C for a period of 8 h and resting for 16 h for three days (total heat testing period of 24 h), and aliquots collected at t = 0, 8, 16, and 24 h of heating were subjected to Raman spectroscopy. Spectral changes evidenced degradation and oxidation, with decrease of peaks assigned to C–C stretching and vibrations of methyl and methylene groups (CH3 and CH2) (1014, 1400 and 1451 cm−1) of base oils, and increase of peaks assigned to hydroxyl radicals (peroxides), carboxylates, and carboxylic acids (475, 531, 1164, 1241, and 1325 cm−1) in the MN and SS ALO samples. In the ST ALO sample, the changes observed in peaks corresponding to methyl and methylene groups were lower than MN and SS types, suggesting lower degradation, while the emergence of peak at 904 cm−1 indicates the formation of hydroperoxides. These findings suggest that thermal stress can promote oxidation in ALOs, favoring the breaking of C–C and C–H bonds and the formation of oxidative products, particularly in the MN and SS types. The strong correlation (r > 0.9) between heating time and the changes in the principal component analysis variables indicates the potential for monitoring the degradation process through alterations in the cited peaks.