Electrical Contact Resistance Measurements. Part I. Temperature Effects on Antiwear Film Formation by Secondary, Primary, and Aryl Zinc Dithiophosphates in Fully Formulated Engine Oil

Modern Passenger Car Motor Oils (PCMOs) are formulated to provide superior wear, oxidation, and deposit control under the most demanding driving conditions. In most PCMOs, zinc dialkyldithiophosphate (ZnDTP) has been the dominant antiwear and antioxidant agent for well over 50 years. Unfortunately, the phosphorus in ZnDTP may partially deactivate the exhaust emission catalyst. To ensure that the catalysts function for at least 120,000 miles, engine manufacturers are exploring phosphorus limitations for upcoming PCMO performance categories.

This paper explores the antiwear film formation of low phosphorus engine oils using the Electrical Contact Resistance bench test. A prototype ILSAC GF-4 formulation blended with varying amounts and types of ZnDTP was tested at temperatures typical of operating engines. Secondary ZnDTP was found to produce the best films under the broadest temperature range. As the concentration of phosphorus derived from secondary ZnDTP was reduced from 0.1 wt % to 0.08 wt %, the antiwear film became unstable, sometimes forming and sometimes not. At 0.05 wt % and less phosphorus, no effective antiwear film formed.

In similar experiments with primary and aryl ZnDTP, no effective antiwear film formed at any phosphorus level up to the currently allowable 0.1 wt %.