Understanding Ligand Exchange in a Molybdenum Dialkyldithiocarbamate/Zinc Dialkyldithiophosphate Additive System during Inhibited Oxidation in Various Base Oils

摘要

The proposed ILSAC GF-3 engine oil standard calls for lower oil volatility and substantially improved fuel efficiency of engine oils (1), These improvements can be achieved by using very high viscosity index bas oils (current candidate demonstration oils are formulated using Group II base oils with viscosity index greater than 115 or Group III base oils) and by application of effective friction reducing additives. One additive system that can be used to achieve the required fuel efficiency, and at the same time provide good wear protection, is a combination of molybdenum dialkyldithiocarbamate, Mo(dtc)_2, and zinc dialkyldithiophosphate, Zn(dtp)_2. Investigations of the friction reducing properties of oils containing this additive system showed that their friction reducing activity is gradually depleted with mileage accumulation (2) and that some formulations using this additive system fail to maintain their friction reducing capability in the Sequence VIB test after 96 hours of oil aging (3). It has been reported previously that the fuel efficiency of Mo(dtc)_2 containing oils can be extended by sulfur containing compounds (4) and by peroxy radical trapping antioxidants (5). However, studies of retention of fuel efficiency carried out with various oils containing Mo(dtc)_2 and Zn(dtp)_2 did not always confirm that fuel efficiency can be extended by these means. Based on our previous studies (6), this could be explained by the effects of base oil composition on ligand exchange and antioxidant reactions occurring in this additive system during oxidation (7). In this work, our investigations of base oil effects are extended to include very high viscosity index base oils, that are being proposed for formulation of GF-3 oils.