Mechanisms of Antioxidant Decay in Gasoline Engines: Investigations of Zinc Dialkyldithiophosphate Additives

Selected engine oils and zinc dialkyldithiophosphate additive concentrates have been added to used oils in various engines and subjected to engine testing. Oil samples obtained as a function of mileage accumulation have been analyzed using the peroxy radical titration method and ³¹P NMR. The engine studies were supplemented by laboratory investigations of antioxidant behavior of pure neutral and basic zinc dialkyldithiophosphates, dialkyldithiophosphoric acid, tetraalkylthioperoxydiphosphate (disulfide), and of neutral zinc dialkyldithiophosphate in combination with a hindered phenol antioxidant, 4,4′-methylenebis(2,6-di-tert-butylphenol). These investigations included studies of the effects of hydroperoxides and hydrocarbon oxidation products on the radical scavenging activity of the above compounds. The results obtained from engine and laboratory studies were used to assess the role and chemical changes of zinc dialkyldithiophosphate additives in radical scavenging processes in the engine.