Reactivity of Rubbers

In comparison with the developments in new rubbers that have occurred this century, developments concerned with the chemistry of the reactions of the already-formed rubbery polymers have been less immediately spectacular. It has already been pointed out that for about ISO years sulphur has been the dominant vulcanizing agent, almost exclusively used with diene rubbers. It must however be stressed that the efficiency with which the sulphur is used and the quality of the vulcanizates is today vastly superior. In part this is due to systematic semi-empirical studies which led to the development of a wide range of accelerators of vulcanization. It is also in part due to the excellent scientific studies undertaken by many chemists throughout the world but particularly by the Malaysian Rubber Producers Research Association (and its forerunners the Natural Rubber Producers Research Association and the British Rubber Producers Research Association). As a consequence of this work the mechanism of vulcanization and its control, at least in the major diene rubbers, is reasonably well understood. 

Parallel remarks can be made concerning the ageing of rubbers. Mechanisms of oxidation and of ozone attack are also reasonably well understood and antioxidant and antiozonant systems have been improved considerably. 

Over the years many chemical derivatives of rubbers have been introduced. These include chlorinated rubber, rubber hydrochloride and cyclized rubber. Whilst little work was done in this area in the period 1955-75 the renewed appreciation that natural rubber, unlike coal and oil, is a replaceable raw material has stimulated further study of the products to be derived from it. 

Important developments in rubber chemistry have taken place in every decade for the past hundred years. The future of rubber 

It is also a cause for concern that several of the chemicals used in the rubber industry are being found to exhibit toxic hazards. This is bound to place restrictions on the use of such materials and the rubber chemist will be called on to produce alternative systems or perhaps to eliminate the use of additives by some form of radiation vulcanization or the greater use of block copolymers. 

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Crosslinking of CR, chlorobutyl rubber, chlorinated acryl rubber, epichlorohydrin rubber, ethylene-epichlorohydrin copolymer rubber, PVC, chlorinated ethylene and vinylidene fluoride-hexafluoropropene copolymer rubbers in presence of metal compounds such as MgO was studied in detail to elucidate mechanism, variables affecting reaction, points of crosslinking and relative reactivity of rubbers. Control techniques using retarders or accelerators, depending on halogen activity, were proposed. 17 refs. 

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