Some Factors Affecting the Decomposition of Three Commercial Zinc Organodithiophosphates

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Abstract
Despite the widespread use of zinc organodithiophosphates (ZDP's) as antiwear additives, how the ZDP's perform their antiwear function is still vague. Thermal decomposition of ZDP's and the formation of a protective antiwear coating by the decomposition products is one popular theory; but the correlation between thermal decomposition temperature and antiwear performance leaves something to be desired. To determine if other factors influence ZDP behavior, bench decomposition studies were conducted on three commercial ZDP's in which the effects of oxygen, water, other additives in the blend, and the base oil were investigated. For these studies, the bulk oil changes were followed by infrared spectroscopy, volatiles formation by pressure measurements, solids formation by centrifugation and subsequent elemental analysis and surface coating formation on steel balls by x-ray fluorescence spectroscopy. It was found that the decomposition of ZDP's is affected by oxygen availability, water, other additives in the blend, and the base oil. Thus, simple thermal decomposition is not an adequate explanation; and a better theory is needed to explain the antiwear behavior of ZDP's.