Synthesis and Evaluation of Bound Thiol Antioxidants

Abstract

The reactions of hindered phenolic antioxidants with NR latex in the presence of a free radical generating system have been studied. 3,5-Di-tert-butyl-4-hydroxybenzyl carboxy methyl (BTGA), and ethyl (BTPA) sulphides and 3,5-di-tert-butyl-4-hydroxybenzyl mercaptoglycollate (BTGE) have been found to react with NR in the presence of tert-butyl hydroperoxide (TBH) and tetraethylenepentamine (TEPA). The concentrations of bound antioxidants were estimated by an infra-red technique after exhaustive extraction. The reactions of 4-mercaptoacetamidodiphenyl amine (MADPA) and 3,5-di-tert-butyl-4-hydroxybenzyl mercaptan (BHBM) with natural and styrene-butadiene rubbers have also been investigated by a mechanochemical process in a torque rheometer. Masterbatch concentrates containing high levels of bound antioxidants in natural and styrene-butadiene rubbers have been used as additives for untreated rubbers to obtain rubbers with high levels of oxidative stability. Oxygen absorption and stress relaxation techniques were used to evaluate the effectiveness of the bound antioxidants. The performances of bound antioxidants produced both by latex and mechanochemical binding have been found to be superior to conventionally added Nonox WSP, which is almost completely removed during extraction. A major advantage of the bound antioxidant concentrates studied in this work has been found to be their suitability for the preparation of solid masterbatches. The effects of various rubber-bound systems on the curing characteristics of NR and SBR rubbers have been assessed using the Monsanto rheometer. In the case of MADPA and BHBM, these effects have been found to be insignificant, particularly in the extracted rubbers.