Study of additive-additive interactions in a lubricant system by NMR, ESCA, and thermal techniques

Abstract

Engine oil lubricants are formulated with a variety of additive components at different dosages to obtain the desired physico-chemical characteristics. Antiwear, friction modification/energy efficiency, dispersancy, and detergency properties are normally achieved by the use of zinc dialkyldithiophosphate (ZDDP), molybdenum dithiocarbamate (MoDTC) and ashless alkyl phosphorodithioate, polyisobutylene succinimide (PIBS), and metal sulphonates / phenates, respectively. It has been reported that these additives interact with each other and affect the overall performance of a lubricant system. The additive-additive interactions have been studied by nuclear magnetic resonance (NMR) and infrared spectroscopic techniques, where attention has been mainly focused on the ZDDP-PIBS additive system in the presence or absence of other additives. The results have been used to relate the synergistic or antagonistic effects of such interactions to the overall performance of a lubricant. Recently, MoDTC has found wider application in lubricants as a friction modifier and energy-efficient additive. However, no studies of the additive-additive interactions of the PIBS-MoDTC-ZDDP additive system using analytical techniques have been reported. The present paper covers the fundamental and mechanistic aspects of additive-additive interactions of ZDDP, MoDTC, PIBS, and sulphonate / phenate additives present in a lubricant system as studied by 31P NMR, electron spectroscopy for chemical analysis (ESCA), and thermogravimetric analysis (TGA) techniques. ESCA, which is a surface analytical technique, has been used to provide basic evidence for the formation of various complexes through interactions occurring in the electronic binding energies of orbitals of various atoms of the additives. The ESCA studies have also revealed the actual atomic sites of interaction between the additives responsible for the formation of adducts or complexes. The differential scanning calorimetry profiles of blends have verified the interactions among the additives and shown that the stability of the additive system is quite different from that of the additives alone. The shifts in the 31P NMR signals, the changes in the binding energies of the s, p, and d orbitals of additive elements, and the multistage decomposition profiles in the TGA thermograms of interacting systems due to complexion and adduct formation have enabled a mechanism of interaction to be proposed.