Studies on additive-additive interactions: Effects of dispersant and antioxidant additives on the synergistic combination of overbased sulphonate and ZDDP

A strong synergism between overbased sulphonate (OBS) and zinc dialkyldithiophosphate (ZDDP) in a proven diesel engine oil formulation has been reported in an earlier communication.¹ In continuation of these studies, the effect of different types of dispersant, of varying nitrogen content, and an antioxidant, on a synergistic combination of OBS-ZDDP has been investigated. Both detergency and dispersancy of the optimised formulation were improved with high nitrogen-content dispersants, which also increased strongly the wear tendencies. The antioxidant, though it improved detergency, did not show any positive effect on antiwear performance. Detailed IR and ³¹p-NMR spectroscopic analyses on interactive additive pairs, namely, ZDDP-PIBS and ZDDP-antioxidant, indicated the formation of association complexes and hydrogen bonding respectively.