Molecular Characterization of Lubricant Base Oil Oxidation Using <i>Operando</i> Headspace Chromatography and Rancimat

In internal combustion engines, lubricants undergo significant thermal, mechanical, and chemical stresses, which vary depending on the type of fuel used and ultimately deteriorate their performance over time. Among the degradation mechanisms of lubricants, liquid-phase oxidation plays a critical role, resulting in the accumulation of oxygenated byproducts that affect viscosity and tribological properties. While antioxidant additives are commonly used to mitigate oxidation, the emergence of new fuels such as natural gas, hydrogen, or ammonia raises new questions of fuel/lubricant compatibility and lubricant formulation. In this study, a portfolio of analytical techniques was developed and optimized, taking into account the specificity of the lubricants. For the first time, headspace gas chromatography (GC) was used to identify more than one hundred light oxidation products, categorized into six main families: alkanes (C4–C17), 2-ketones (C5–C13), 1-alcohols (C4–C10), carboxylic acids (C2–C11), and dihydrofuranones (C5–C10). The effect of heating temperature on product detection and product types was investigated, as well as the effect of the boiling point on product detection. Additionally, Rancimat equipment, traditionally used to assess oxygenated hydrocarbons, was set up to evaluate the induction periods of base oils, including mineral oils, at varying temperatures. Changes in density, viscosity, and infrared spectra were also monitored during continuous air exposure and heating. These methodologies were applied to eight different base oils, and the combination of both techniques enabled a comprehensive understanding of their oxidation behavior under controlled conditions.