A Quantum Chemical Study of the Unimolecular Decomposition Mechanisms of Zinc Dialkyldithiophosphate Antiwear Additives

Abstract

Density functional theory calculations were applied to study a set of isomerization and decomposition reactions that zinc dialkyldithiophosphate (ZDDP) engine oil antiwear additives may take part in. The products of these reactions comprise a set of chemical species that can lead to the formation of poly(thio)phosphate films that are thought to be responsible for the antiwear protection offered by ZDDPs. The specific reactions examined involved either intramolecular alkyl group transfer within ZDDP or the intramolecular elimination of olefins from this molecule. A series of substituents were employed to examine how the nature of the substituent on the ZDDP molecule affects the thermodynamic and mechanistic details of these reactions, which in turns may have ramifications regarding various aspects of antiwear film formation. It was found that qualitative and quantitative aspects of these reactions were markedly different when hydrogen atoms were used as substituents instead of alkyl groups. The details of the reactions for the H-substituted system indicated that the precursors to the antiwear films should not be formed. When alkyl groups were employed as substituents, the energetic details of the reactions considered in this study exhibited a dependence upon the nature of these groups. An examination of these details revealed that straight-chained primary alkyl ZDDPs should decompose through reactions involving alkyl group transfer, while secondary and branched primary alkyl ZDDPs should primarily undergo olefin elimination reactions to form precursors to ZDDP antiwear films. The details of the reaction pathways that these systems follow to form the precursors shed light on the observed byproducts of antiwear film formation.

Interpretation of experiments on ZDDP anti-wear films through pressure-induced cross-linking

“…In those studies, several decomposition routes accessible to ZDDPs were identified and explored at the molecular level. It was found that the elimination of olefins and H 2 S can occur through intramolecular b-hydrogen transfer [33], while dialkyl sulfides can be formed through alkyl group transfer [32]. The previous simulations also suggest that linkage isomers of ZDDPs may decompose through the elimination of metathiophosphates [31].…”

“…Nonetheless, it is important to understand the decomposition processes in order to justify the model system used in the calculations and gain fundamental insight into processes that occur during the initial stages of film formation. This has been the focus of previous simulations [30][31][32][33]. In those studies, several decomposition routes accessible to ZDDPs were identified and explored at the molecular level.…”

On the Mechanism of ZDDP Antiwear Film Formation

“…One possibility is promotion of breakage of the bond between the alkyl (or aryl) group and O (or S after O/S exchange [46]). It has long been recognised that secondary alkyl ZDDPs show faster tribofilm formation and lower thermal stability than primary ones, suggesting that strength of this bond influences the rate of ZDDP reaction [7,51,52]. However, another simpler possibility is that applied stress acts on the ZDDP molecule to stretch S-Zn or O-Zn bonds, thus making these more polar and less covalent.…”

“…Clearly, considerable further work still needs to be done; in particular to use a test apparatus that can reach high shear stresses with steel/steel contacts and to study and compare a range of single-component ZDDPs of differing structures, unlike the mixed ZDDP employed in this study. It has been suggested that primary and secondary ZDDPs undergo different reaction routes [52] and this might be expressed in differing shear stress responses. However, the approach outlined in this study, of separating ZDDP reactivity into a thermal activation energy and an activation volume, and thus exploring the effects of both temperature and shear stress on ZDDP tribofilm formation, appears very fruitful.…”

Studies on ZDDP Anti-Wear Films Formed Under Different Conditions by XANES Spectroscopy, Atomic Force Microscopy and 31P NMR

“…5(b)). Density functional theory (DFT) calculations have been applied to ZDDPs with the different substituents [40]. It reveals that primary alkyl ZDDP and secondary ZDDP decompose through different passways.…”