Study of the antiwear characteristics of intentionally deteriorated Zinc dialkyldithiophosphate (ZnDTP), which can simulate used engine-oil, was carried out. ZnDTPs were degraded by reacting with cumenehydroperoxide (CHP) which models the hydroperoxide present in combustion gas. Sample oils were analyzed by P-NMR, showing that ZnDTP produces various kinds of phosphorous-containing oil-soluble compounds after reacting with CHP. It was found that the antiwear characteristic of ZnDTP deteriorated due to the degradation, and an acceleration of wear was observed in the highly degraded regions. The effect of the ashless dispersant on the antiwear characteristics of both degraded and nondegraded ZnDTP was studied. It was found that the dispersant reduced wear in the low-degradation region. The addition of dispersant, however, had no effect on wear with further degradation where the dispersant was almost entirely used up to disperse Zn-containing solids when the dispersant concentration was low. When the concentration was high, the dispersant showed wear reduction in the region of greater degradation. The antiwear action of dispersant was discussed herein based on its adsorption and micelle formation.

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