Graft polymerisation of antioxidants into natural rubber

Abstract

Antioxidants containing polymerisable vinyl groups have been successfully bound to natural rubber in the presence of suitable free radical generators. Reaction parameters such as the mode of initiation, initiator system, reaction time, pH of the reaction medium, swelling time have been investigated in natural rubber latex. Of the free radical generating systems examined the redox system consisted of tertiary butyl hydroperoxide and tetra ethylene pentamine (TBHP/TEP) appeared to be most effective in natural rubber latex. Infra-red spectroscopy has been used to estimate the concentration of the bound anlioxidants and ageing behaviour was assessed by oxygen absorption and continuous stress relaxation. The curing characteristics of the grafted rubber samples were studied in detai1 using the Monsanto rheometer. It was found that antioxidants which do not contain vinyl groups also react under the same conditions with natural rubber latex. Some of these appeared to be marginally more effective than the corresponding vinyl antioxidants. It was observed that in the case of the vinyl antioxidants only a part of the added antioxidant grafted into the rubber and the rest remained unchanged or was oxidised to coloured products. The by-products formed during the process of grafting were identified and analysed. A masterbatch technique has been developed to produce concentrates of bound antioxidants in rubber latex and these have been used as additives to untreated latex to obtain grafted rubber latices with a known level of oxidative stability. It was found that irrespective of the presence or absence of vinyl group in the antioxidants phenols containing one tertiary butyl and one methyl group in ortho position were most effective. Followed by phenols with two tertiary butyl groups. This is the order of activity previously observed for rubber antioxidants. The effectiveness of these antioxidant structures appears to be similar when present in a rubber bound form to that of Lhe free antioxidant.