Interactions Leading to Formation of Low Friction Films in Systems Containing Molybdenum Dialkyldithiocarbamate and Zinc Dialkyldithiophosphate Additives

摘要

The friction reducing capability of an additive system containing molybdenum dialkyldithiocarbamate (Mo(dtc)_2) and zinc dialkyldithiophosphate (Zn(dtp)_2), which is used in advanced low friction engine oils, is gradually depleted with mileage accumulation due to oil oxidation. In order to understand processes involved in this loss of friction reducing capability and to find a way to improve the retention of this capability, we have investigated chemical changes occurring in the Mo(dtc)_2/Zn(dtp)_2 additive system during oxidation in base oils of different composition and assessed the effects of these changes on friction reduction. It has been previously determined that under oxidative conditions Mo(dtc)_2 and Zn(dtp)_2 undergo ligand exchange reactions leading to formation of an equilibrium mixture of Mo and Zn dialkyldithiocarbamate and dialkyldithiophosphate products including molybdenum dialkyldithiophosphate, Mo(dtp)_2, and zinc dialkyldithiocarbamate, Zn(dtc)_2. All these products are known to be antioxidants which during oxidation react with peroxy radicals or hydroperoxides and at the same time undergo series of oxidative conversions producing secondary antioxidants. In our studies the Zn containing products were found to be stronger antioxidants than the corresponding Mo compounds. They effectively prevent chain oxidation and protect Mo compounds from being consumed. Zn(dtp)_2 and its conversion products were found to preferentially react during oxidation with hydroperoxides (in oils containing aromatics) and Zn(dtc)_2 with peroxy radicals (in paraffinic oils). Extensive base oil oxidation in the systems inhibited by Mo(dtc)_2/Zn(dtp)_2 begins only when Zn(dtp)_2 is completely consumed. This was found to coincide with a complete loss of friction reducing capability despite the fact that Mo compounds are still present in the system. Evidence is presented that this loss of friction reducing efficiency occurs because the polar base oil oxidation products and polar base oil components interfere with friction reducing process involving Mo(dtc)_2 and its ligand exchange products. Based on these results, the retention of friction reducing properties can be improved through the use of additional effective antioxidants which protect Mo(dtc)_2 and its ligand exchange products from being consumed and prevent formation of polar oxidation products and also through the use of base oils which do not contain polar components or are not prone to form them upon oxidation.