The Antioxidant Function of Dithiophosphoryl Compounds in Rubbers

Abstract

A quantitative study of the reaction between 0,0-dialkyldithiophosphoricacids and natural rubber latex, showed thata moderate yield of adduct was obtained using low concen~trations of the acids. High concentration of the acids causedcoagulation of the latex due to their acidity. Attempts tobind the dithiophosphoryl compounds mechanochemically undershearing condition in the polymer melt resulted in a lowerextent of binding. The adduct. however made. became unboundduring vulcanization. All the dithiophosphoryl compoundsexamined were found to exhibit a pro-oxidant effect in conventionalaccelerated sulphur vulcanizates, but bis-(0,0-dibutyl)thiophosphoryl disulphide was shown to improve oxidationresistance in low sulphur (CBS-EV) and sulphurless(TMTD) vulcanizates. Bis-(0,0-diisopropyl) thiophosphoryl disulphide and bis-(0,0-dibutyl) thiophosphoryl disulphide have been found to possessant ifatigue activity in peroxide vulcanizate cured at 160°C.The activity increased considerably when the stocks were cured at 180°C. An infra-red examination of the reaction using squalene as a model compound revealed that a transformation product characterized as a sulphenic acid was formed. This compound is believed to be involved in the regenerative process during fatiguing and the corresponding sulphinyl radical acts as an alkyl radical trap. Ultraviolet spectroscopic analysis showed the formation of conjugated diene and triene supporting the above mechanism.