Diene rubbers accelerated sulphur vulcanization

Of the dienes, butadiene appears to have been the earliest to have been evaluated. Although it allowed accelerated sulphur vulcanization this was to the detriment of heat and oil resistance. More recently, non-conjugated dienes such as those used with EPDM rubbers have been quoted in the patent literature. These include dicyclopentadiene and methyl cyclopentadiene. A French patent (Tellier and Grimaud, 1968) quotes the use of tetrahydrobenzyl acrylate (VIII). 

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. 

Fig. 8.1. Typical chemical groupings present in a sulphur-vulcanized natural rubber network, (a) Monosulphide cross-link (b) Disulphide cross-link (c) Poly sulphide cross-link (jc = 3-6) (d) Parallel vicinal cross-link (n = 1-6) attached to adjacent main chain atoms and which have the same influence as a single cross-link (e) Cross-links attached to common or adjacent carbon atom (f) Intra-chain cyclic monosulphide (g) Intra-chain cyclic disulphide (h) Pendent sulphidic group terminated by moiety X derived from accelerator (i) Conjugated diene (j) Conjugated triene (k) Extra-network material (1) Carbon-carbon cross-links (probably absent).

The determination of cross-link density or of Me by a chemical method is virtually impossible with an accelerated sulphur-cured diene rubber vulcanizate because of the complexity of the system and the formidable obstacles to analysis. Certain vulcanizing systems do however appear to be simpler and capable of such treatment. If for such a system Me is determined by a physical method then it is to be expected that it might be possible to obtain a meaningful calibration between the two values of Me obtained. It is then reasonable to assume that this calibration may then enable, in the case of a sulphur-vulcanized diene rubber, an equivalent of the chemically... 

These two elements occur in the same group of the Periodic Table as sulphur and are capable of vulcanizing diene rubbers. They have found some limited use in the production of natural rubber vul-canizates showing a measure of heat resistance above that normally expected of a conventional accelerated sulphur, natural rubber vulcanizate. Cost and handling problems such as odour and toxicity have discouraged the use of these materials as commercial vulcanizing agents and their application for this purpose is now minimal. 

Whilst butyl rubber may be vulcanized using the conventional sulphur-based systems used with diene rubbers the low degree of unsaturation requires that a much more active accelerator system be used. For this reason the powerful thiuram and thiocarbamate accelerators are generally preferred to the more conventional thiazole and sulphenamide types. As may be expected, both the rate and extent of reaction depend on the degree of unsaturation. 

The long established use of accelerated sulphur systems for vulcanization of general purpose rubbers has always led to a reluctance of rubber processors to accept alternative vulcanization systems unless this was absolutely necessary. This is not simply innate conservatism but as much due to the anticipation of problems of contamination of stocks during processing and of difficulties in reclaiming. For this reason modified ethylene-propylene rubbers were developed in which units from a third monomer were present and which provided unsaturation in the polymer (EPDM rubbers). These termonomers, invariably dienes, used in amounts of 3-8% of the total monomer weight are generally expensive, relative to more common monomers and adversely affect the price structure but nevertheless the EPDM type of rubber is now dominant over the EPM copolymers. 

Vulcanization is similar to that of the diene rubbers and can be brought about by the use of accelerated sulphur, sulphur-donor and by peroxide systems. Where the reactions have been studied in detail it is reported that there is a high cross-linking efficiency. 

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