Preferential Adsorption in the Lubrication Process of Zinc Dialkyldithiophosphate

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Abstract
Paper chromatography has been used in combination with neutron activation analysis to analyze the polar lubricity additive zinc dihexyldithiophosphate. This additive is shown to contain small but significant quantities of both phosphorous and zinc-containing polar impurities. Results of lubricity tests suggest that these polar impurities are adsorbed preferentially on a metal surface and therefore are available for the lubrication reaction in preference to the zinc dihexyldithiophosphate molecule itself. An acid phosphate-type compound is believed to be the active polar impurity controlling the wear behavior of zinc dihexyldithiophosphate. A chemical lubrication mechanism for zinc dialkyldithiophosphate is proposed. This lubrication mechanism involves the formation of a metal phosphate surface film. The agreement between this mechanism and lubrication data presented by previous investigators is discussed.