OXIDATIVE DEGRADATION OF PHOSPHATE ESTERS.

A thin film oxidation (microoxidation) test has been developed to simulate severe liquid phase oxidation in typical lubrication systems. Tributyl phosphate, tricresyl phosphate, trixylyl phosphate and t-butylphenyl diphenyl phosphate have been evaluated and compared. At low rates, oxidation is not limited by oxygen diffusion at the air-liquid interface. Gel permeation chromatography has been adapted to the microoxidation test with the phosphate esters to provide a quantitative analysis of the reaction products as a function of molecular weight. The hydrocarbon portion of the phosphate appears to oxidize as the first step. The oxidized material then reacts further to form a product of about twice the molecular weight of the original phosphate. The formation of polymers appears to be a result of condensation through the oxidized hydrocarbon portion of the molecule. There is little evidence of condensation through the phosphate groups to form P-O-P linkages. The P-O-P condensation appears to be a result of thermal reactions of the phosphate esters. Relative oxidation stability of a phosphate ester is a function of the ease of oxidation of the hydrocarbon structure of the alcohols from which the ester is prepared.