Molecular-level insights into the thermo-oxidative aging of lubricant base oils during the hot-tube test

The hot-tube test is a widely used industrial method for evaluating the thermal stability of lubricant base oils based on color changes and deposit formation. However, the underlying chemical processes remain poorly understood at the molecular level. In this study, two mineral-based base oils (designated as API group I base oils) with distinctly different bulk properties were subjected to the hot-tube test at 280 °C for 16 h. The aged oils and deposits were comprehensively characterized by elemental analysis, SARA (Saturates Aromatics Resins Asphaltenes) fractionation, high-temperature simulated distillation, and high-resolution mass spectrometry coupled with electrospray ionization (ESI). The results show that thermo-oxidative aging leads to a marked increase in oxygen and resin contents, with trace asphaltene formation. The deposit yields differ significantly between the two oils, and deposits consist of highly condensed and/or oxygenated compounds without enrichment of sulfur or nitrogen. Molecular analysis reveals that the oxidation of hydrocarbons generates a wide range of polar oxygenates (containing one or more oxygen atoms in each molecule). The solubility of these oxidation products in the oil matrix is a key determinant of deposit formation; a low saturates-to-aromatics ratio reduces their solubility. Regarding sulfur-containing compounds, sulfides are more readily oxidized than thiophenic compounds, but a direct causal link between sulfur oxidation and deposit yield is not established from the current data. This study provides molecular-level insight into the hot-tube test process, suggesting that deposit formation is primarily driven by hydrocarbon oxidation and likely controlled by the solubility limit of the resulting polar products in the base oil matrix. These findings offer a chemical basis for interpreting hot-tube test results and guiding base oil refining and additive design.