Isolation and Chemical Characterization of a Zinc Dialkyldithiophosphate-Derived Antiwear Agent

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Abstract
A secondary-alkyl zinc dialkyldithiophosphate (ZDP) was thermally decomposed in the presence of air. The resulting precipitate, when blended in a base grease, demonstrated better antiwear performance in a 4-ball wear machine than the parent additive. When extracted with acetone, a white powder with excellent antiwear properties was obtained from the precipitate. Chemical analysis indicated that this white powder was approximately 86 mass percent zinc pyrophosphate, 11 mass percent zinc orthophosphate, and 3 mass percent of an unknown sulfur compound. The antiwear effectiveness of a commercial zinc pyrophosphate and a zinc orthophosphate was also measured, but neither provided comparable antiwear protection. The white powder was amorphous/microcrystalline, whereas the commercial zinc pyrophosphate and zinc pyrophosphate were highly crystalline. These results indicate that zinc pyrophosphate is one of the thermal degradation products of ZDP. In addition, zinc pyrophosphate is suspected of being primarily responsible for ZDP's antiwear properties. The degree of amorphism/microcrystallinity may be a critical property for good antiwear performance of zinc pyrophosphates.