Reaction Pathways in Lubricant Degradation. 2. n-Hexadecane Autoxidation

We oxidized n-hexadecane in an isothermal batch reactor at temperatures ranging from 140 to 180 °C for times ranging from 10 min to 24 h. The viscosity of the n-hexadecane oxidate was measured, and the average molecular weight of the oxidate was determined with the use of gel permeation chromatography. The oxidate viscosity and the average molecular weights increased with both reaction time and temperature, but the viscosity increase was much more dramatic. The maximum viscosity measured was 219 cP, which resulted from oxidation at 180 °C for 11 h. This high viscosity constituted a 6500% increase. By comparison, the increase in the average molecular weights of the oxidate never exceeded 50%. Analyses using ¹H and ¹³C NMR spectroscopies, acid-base and iodometric titrations, and gas chromatography provided the total concentrations of hydroperoxides, carboxylic acids, ketones, aldehydes, alcohols, and esters present in the reaction mixture. Monitoring the temporal variations of the concentrations of these different functional groups facilitated resolution of the autoxidation pathways for paraffin oxidation under relatively severe conditions. These reaction pathways provide insight into the chemical transformations that occur during the degradation of a lubricating oil under service conditions.