A Study of Polymer Bound Sulphur Based Antioxidants in Rubber

Abstract

A commercially available hindered phenolic antioxidant, 2,6-Ditert-butyl phenol has been converted into a mercaptan derivative, viz. 3,5-Ditert-butyl-4-hydroxy benzyl mercaptan and the latter has been successfully bound to natural rubber by a free radical process. Reaction parameters such as mode of initiation, initiator concentration, mercaptan concentration, time, temperature etc., have been investigated in rubber solution, in natural rubber latex, and in the polymer melt. A masterbatch technique has been developed to saturate a small quantity of rubber with a large amount of mercaptan and to use this concentrate to dilute untreated latex to obtain a desired level of stability. Infra-red spectroscopy has been used to estimate the concentration of adduct phenol, and accelerated ageing tests, viz. oxygen absorption and continuous stress relaxation were carried out to evaluate ageing behaviour. The effects of the presence of 3,5-Ditert-butyl-4-hydroxy benzyl mercaptan on the vulcanization characteristics have been investigated in detail using a Monsanto Rheometer. To investigate the mechanism of addition, a number of mercaptans and simple unsaturated model compounds have been used and the products have been analysed. The effect of the presence of antioxidant bound rubber on the oxidation of polypropylene was studied by a rapid flowing air oven test. An attempt to add 3,5-Ditert-butyl-4—-hydroxy benzyl mercaptan to a saturated polymer, polythelene, was unsuccessful as theoretically expected. Oxidation of an unsaturated compound, tetralin, was studied in the presence of monosulphides which are similar to those obtained by adding mercaptans to olefins.