ZDDP-Additive-Catalyst Interactions in Engine Oil

Abstract

The phosphorous content in engine oil is found to be the major cause of the poisoning of Catalytic converters in automobiles, hence environmental regulations limiting the phosphorous level in GF4 and GF5 oils have been introduced. Zinc dialkyl dithiophosphate (ZDDP) being the only major source of phosphorous in current engine oils, is also an indispensable component of the additive package in these oils for it has been the primary anti wear — as well as anti oxidant — additive for over fifty years. Efforts are made to replace the ZDDP with other materials with the same properties which would not be harmful to the environment and also an economically feasible substitute. Another solution to this problem is to reduce the amount of ZDDP used while improving its antiwear performance. Anti-wear action of ZDDP, involves its break down reaction with the steel surface by Zn Fe ion exchange and subsequent formation of an amorphous chemisorbed film containing zinc, phosphorus, oxygen and sulfur and also iron in the form of polyphosphates and sulfates of zinc and iron. The efficiency of this mechanism is reduced by parallel reactions between ZDDP and other additives as well as their antagonistic effects. Introduction of a material with catalytic properties which would reduce the negative effects of the presence of the other additives on the anti wear properties of ZDDP is an option that was explored in this paper. Both triboligical wear tests (Ball on cylinder lubricity evaluation tests) as well as mechanism studies (DSC, FT-IR and NMR) were used to evaluate the performance of ZDDP in the presence of the most common additives (i.e. Anti-oxidants, Detergents and Dispersant). Iron Fluoride is also introduced as a potential additive to improve the efficiency of wear protection mechanism of ZDDP. The improvements observed in the presence of the Iron Fluoride will allow further reducing the amount of ZDDP in engine oils containing this material as an additive [1].