An X-ray absorption spectroscopy examination of structural changes to zinc di-alkyl-di-thiophosphate (ZDDP) following milling in the presence of iron oxides and subsequent thermal processing

Solid-state mixtures of the lubricating oil additive zinc di-alkyl-di-thiophosphate (ZDDP) with two different iron oxide phases (FeO and Fe2O3) prepared by milling are examined using X-ray absorption spectroscopy (EXAFS). The changes of the local atomic environment around the zinc atomic site within ZDDP, both in the presence of iron and following subsequent temperature profiling, are discussed. The thermal decomposition behaviour of the milled powder is in general consistent with a two-stage reaction process: an initial loss of the alkyl chains of the di-thiophosphate groups, followed by the breakdown of the ZDDP molecule. Whilst the specific iron oxide phase used does not appear to significantly influence these decomposition reactions, changes are also seen in the local atomic structure of both the ZDDP and the iron oxides following milling and after subsequent thermal reaction. Drawing down on previously published work, the results of this work are represented via a tentative molecular-scale model based upon the known structural chemistry of ZDDP and rationalised with differential scanning calorimetry (DSC) analysis.