Ester Oxidation Under Simulated Boundary-Lubrication Conditions

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Abstract
Measurement of lubricant oxidation rates at high temperatures is complicated by the influence of oxygen diffusion. Consequently, determination of kinetic rates of oxidation above 450 K has previously been limited to reactions conducted to only 2-percent conversion of the reactant. Using the Penn State microoxidation test, we quantified the effect of oxygen diffusion on the oxidation of two esters at temperatures of 448 to 518 K. Reaction rates and product compositions were followed for reactant conversions of up to 50 percent. The rate coefficients and activation energies for the initial reactions (approximately first order) were determined using a diffusion-reaction model to separate kinetics from oxygen diffusion. Oxidation intermediates react with the iron surface, which catalyzes formation of a polymeric precursor of sludge and varnish. Proximity of the primary oxidation reaction to the iron surface influences the amount of iron-corrosion product and the molecular weight of the polymer. Differences are shown between the two esters, both in oxidation resistance and in product reactivity with the metal surface. Implications of these results for bóundary lubrication are discussed.