Mechanisms of Action of Sulphur Containing Antioxidants

Abstract

The thesis deals with the antioxidant chemistry of two classes of sulphur-containing antioxidants. Part A is concerned with the sulphoxide dimethylsulphinyldipropionate (DMSD) and its reaction with hydroperoxide. Its activity as a peroxide decomposer is explained in terms of catalytic activity of certain of its reaction products. In particular, the sulphinic acid methyl-β-sulphinopropionate (MSP) is thought to be the major contributor to the overall activity. Data obtained from oxygen absorption studies has been used to postulate reactions involving the observed decomposition products of DMSD. A scheme has been put forward to explain the complex series of reactions involved in the DMSD/hydroperoxide system. Part B is a study of the chemistry of mercaptobenzothiazole (MBT) and its zinc salt (ZMBT) in isolation from a rubber network and goes some way in explaining their antioxidant behaviour. Similar techniques to those used in Part A lead to the conclusion that decomposition products were again responsible for the observed pro-oxidant and antioxidant reactions. The most striking difference between the two systems was the decomposition modes of the derived sulphinic acids. In the case of MBT, desulphonation occurred immediately leaving a benzothiazole residue. The preferred decomposition route of MSP, however, was the formation of higher molecular weight products. Sulphur dioxide formation was not involved with MSP, but this was postulated as the immediate precursor to the major peroxide decomposing species, sulphur trioxide or sulphuric acid, in the case of MBT and ZMBT