A KINETIC-STUDY OF ANTIOXIDATION MECHANISMS

A polyol ester containing phenyl alpha naphthylamine (PAN) oxidation i nhibitor has been studied in the Penn State microoxidation test. Parti cular emphasis has been placed on the reaction kinetics of this mixtur e during and after the induction period (stable life). The behavior of PAN in the oxidation can be monitored by gel permeation chromatograph y (GPC) using the ultra violet (UV) detector without interference from the oxidation products of the ester base oil. The results of the oxid ation studies show that the PAN oxidizes preferentially to form primar y oxidation products which oxidize further by condensation polymerizat ion to form high molecular weight polymers. The ester base oil does no t start to oxidize until the PAN has been consumed by the oxidation pr ocess. Once the ester begins to oxidize, the degradation proceeds at a rate equivalent to the rate of the ester base oil without an oxidatio n inhibitor. The net result is that under the 200-degrees-C conditions of this study, the inhibitor molecules which might be expected to be less susceptible to oxidation than the base oil are preferentially oxi dized at a slow rate. After complete oxidation of PAN, the base oil is oxidized at a rapid rate. Both oxidation rates follow first-order kin etics. The ability of the PAN to exist in several energy states due to resonance appears to be a key factor in the oxidation behavior of the mixture.