EPR Spin-Trapping Study of Free Radical Intermediates in Polyalphaolefin Base Oil Autoxidation

Abstract

Autoxidation is a set of spontaneous oxygen-mediated radical reactions common to degradation processes of all hydrocarbon-based materials. In this work, EPR (electron paramagnetic resonance) spin-trapping technique was used to study radical species involved in the autoxidation of a sample of synthetic base oil. A sample of polyalphaolefin base oil was aged at 140°C in the presence of oxygen and the difference in the tendency to generate radical species between the aged and non-aged samples of the same base oil was investigated using DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin-trap. The ageing was performed in such a way that a small amount of hydroperoxides, the primary and key intermediate in autoxidation reactions, accumulated in the sample. Subsequently, the degradation of hydroperoxides was triggered by three different means: thermal treatment, introduction of soluble transition metal ions, and UV irradiation. Radical intermediates were monitored by EPR spin trapping; mixtures of peroxyl, alkoxyl, and carbon-centered radicals were detected and their origin was discussed.