The Thermal Degradation and EP Performance of Zinc Dialkyldithiophosphate Additives in White Oil

The oil-soluble degradation products formed in heated samples of oil containing zinc di(n-, iso- or s-butyl)dithiophosphate (ZDTP) additives have been identified using 1H and 31P nuclear magnetic resonance spectroscopy. The final degradation product in each case is S,S,S-trialkyltetrathiophosphate, SP(SR)3. The intermediate degradation products are O,S,S-trialkyltrithiophosphate, SP(SR)2(OR) and O,O,S-trialkyldithiophosphate, SP(SR)(OR)2. The rate of thermal degradation of the ZDTPs follows the order s-butyl > > n-butyl > isobutyl. A white, noncrystalline precipitate is formed in the heated oils which is rich in zinc and oxygen and low in phosphorus and sulfur compared with the original ZDTP. In four-ball tests, synthesized degradation products were shown to have EP activity comparable with that of the ZDTP additives.