Studies of the oxidation of zinc dialkyldithiophosphates and related compounds

Abstract

Raman spectroscopic analysis of Zn[(R0)2 PS2]2 (ZDDP) and Pb[(RO)2 PS2]2 (R=ethyl or n-butyl) indicate that four sulphur atoms co-ordinate to each zinc atom whilst only two co-ordinate to each lead atom. Dialkyl dithiophosphoric arids (RO)2P(:S)SH exhibit rotational isomerism about P-S and P-0 bonds. Temperature-dependence studies of 1 r (SH) and -(P-S) band intensities indicate LH for the equilibria to be 2.5 kJ mol-1,R= iso-propyl ; 3 kJmol-1 nice, R=n-butyl; 4.5 kJ mol-1, R= n-octyl. Thin film high pressure EEC analysis of solutions containing ZDDP's or related compounds reveal the following trend with respect to antioxidant effectiveness (R = n-octyl , R’ = iso-octyl):

(RO)2 P(:S)SH < [(RO2) PS2]2 < Zn [(RO)2 PS2]2 < commercial Zn[(R’O)2 PS2]2

Successive twofold dilutions of 0.08 M organothiophosphate antioxidant in squalane reduce the observed oxidation onset teTeratures and increase the heats of reaction these tend towards values determined for pure squalane. Dynamic DSC studies of cumene hydroperoxide (CHP) reduction (0.063 IA) by n-butyl ZDDP and notable related compounds (0.C41, ) in squalane produce the following trend of reactivity with respect to the decomposition of CHP (R.= n-butyl)

Zn[(RO)2 PS2]2 > [ (RO)2 PS2]2 > RSH > R2S2 > (RS)3 > R2S > (RO)2 (RS)P=S.

Detailed investigations by dynamic and isothermal DSC on the reduction of CHP by [ R0)2 PS2]2 (R= n-octyl , n-butyl ) indicate that the mechanism of hydroperoxide decomposition is autocatalytic. The half-lives determined by F=31 NMR spectroscopic investigations on the oxidation of Zn[(RO)2 PS2 ]2 (R= n-octyl) to [(RO)2 PS2]2 and ClZn4[(RO)2 PS2 ]6 (basic ZDDP) by excess CHP (0.12 m ) are i) in squalane 10 min ; ii) in cyclohonane 7 min ; iii) in nitrobenzene 46 min. In cyclohexane and squalane the remaining hydroperoxide oxidises n-octyl basic ZDDP to an uncharacterised basic salt that produces a P-31 NMR signal 0.2 ppm lower than n-octyl basic MEP, and [(RO)2 PS2]2. The basic ZDDP salts are more resistant to oxidation by CHP than ZDDP and major products from their oxidation include [(R0)2 PS2]2, an unidentified dithiophosphate (with a P-31 RC signal of 84.4 ppm w.r.t tributyl phosphate) and an unidentified monothiophosphate (49 ppm).