ZDDP-Additive Interactions in Engine Lubricant Formulations

Abstract

This thesis describes the investigation of the solution phase interactions, and resulting coordination-, thermo- and tribo-chemistry, in binary antiwear (Zinc Dialkyl DithioPhosphate – ZDDP) and organic friction modifier (OFM) additive formulations. The work probed in particular, the importance of OFM structure in dictating the thermolysis and tribochemical performance of ZDDP-OFM formulations, with the aim of facili- tating the selection or design of optimal additive combinations for engine lubricant applications. In this regard, Chapter 1 provides an introduction to the fundamentals of lubricant chemistry and lubricant formulation, with Chapter 2 briefly reviewing the current drivers that necessitate a more scientific approach to developing next-generation lubricant formulations.

Chapter 3 describes the investigation of model ZDDP-pyridine interactions, whereby the pyridine scaffold served as an easily tunable N-donor moiety. Both 1:1 and 1:2 ZDDP-Py’ complexation reactions were identified, both of which occur via nitrogen-zinc coordination. Quantification of the contribution of steric (%Vbur) and electronic factors (pKaH) on the propensity of solution complexation (Ka), was achieved using 31P NMR spectroscopic titration data. Complexation was accompanied by a significant change in the zinc-bound dithiophosphate (DTP) binding mode, something that was monitored using Raman spectroscopy.

Chapter 4 identified that the nature of ZDDP-OFM interactions mirror those found in the previously studied ZDDP-Py’ systems (via zinc-nitrogen complexation that is accompanied by a significant change in DTP binding bode). X-Ray crystallographic analysis of [Zn(k1-S-S2P(OiBu)2)2(C18-NH2)2] (7) unequivocally identified both the nitrogen-zinc coordination pathway and the OFM-induced change in binding mode previously inferred from solution 31P NMR spectroscopic data. The use of Raman spectroscopy for diagnosing DTP binding modes for ZDDP-OFM complexes was verified using the molecular structure of 7. Subsequent 31P NMR spectroscopic studies of other ZDDP-OFM combinations demonstrated that the strength of ZDDP-OFM solution interactions were highly dependent on the steric demands and presence of additional OFM donor sites.

Chapter 5 describes the impact of the complexation reactions discussed, on the thermolysis and tribological performance of ZDDP-OFM systems. Thermal degradation path- ways were monitored using 31P NMR spectroscopy, while the tribological performance was assessed using the mini traction machine-space layer interferometry method (MTM-SLIM). The presence of amine-functional OFMs was observed to enhance the rate of ZDDP thermal degradation in all cases studied, with the extent of the rate enhancement observed correlating with the strength of ZDDP-OFM interaction. Subsequently, it was found that the ZDDP tribofilm formation rate did not correlate to the observed thermal stability in ZDDP-OFM systems.

Chapter 6 explored some possible alternative synthetic methods for the preparation of zinc dialkyl diselenophosphates in an attempt to make use of the additional 77Se NMR spectroscopic handle that they would provide. Finally, in Chapter 7, the results obtained in Chapters 3-5 are described in a holistic manner, with the benefits of studying simplified single-component ZDDP-additive systems highlighted.