Time-Temperature Studies of High Temperature Deterioration Phenomenon in Lubricant Systems: Synthetic Ester Lubricants.

Abstract

Abstract : Kinetic and mechanistic investigations of the effects of oxygen pressure on liquid phase autoxidation reactions in model lubricants at elevated temperatures were carried out with n-hexadecane at 160 to 190 C and at oxygen pressures from 4 to 120 kPa. Resultes of these investigations showed that intramolecular alpha, gamma and alpha, delta hydrogen abstraction reactions of peroxy radicals are highly reversible and that the intermediate hydroperoxyalkyl radicals formed from these abstractions, besides addition of oxygen and reverse intramolecular hydrogen abstraction (isomerization) reactions, undergo cyclization reactions leading to formation of cyclic ether products. A general reaction scheme for autoxidation of any system containing alkyl chains with carbon number greater than four is proposed. Absolute rate constants for formation, isomerization, and cyclization of hydroperoxyalkyl radicals have been determined for the first time for liquid-phase autoxidation and the corresponding Arrhenius parameters have been derived. Their values are in general agreement with those previously determined or estimated in gas phase combustion studies. Results of kinetic analysis of cleavage product formation are consistent with two modes of their formation in the entire range of oxygen pressures studied. The first mode involves decomposition of alpha, gamma hydroperoxyketone species and the second reactions of alkoxy radicals.