Efficient sulphur vulcanization

Rheological behaviour of the TPVs that were prepared from dynamic vulcanization of NR-g-PMMA and PMMA blends using various vulcanization systems including conventional vulcanization (CV) and efficient sulphur vulcanization (EV) systems were studied by Nakason et al The ingredient formulas of each vulcanization systems are summarized in Table 18.6. 

The Goodyear vulcanization process takes hours or even days to be produced. Accelerators can be added to reduce the vulcanization time. Accelerators are derived from aniline and other amines, and the most efficient are the mercaptoben-zothiazoles, guanidines, dithiocarbamates, and thiurams (Fig. 32). Sulphenamides can also be used as accelerators for rubber vulcanization. A major change in the sulphur vulcanization was the substitution of lead oxide by zinc oxide. Zinc oxide is an activator of the accelerator system, and the amount generally added in rubber formulations is 3 to 5 phr. Fatty acids (mainly stearic acid) are also added to avoid low curing rates. Today, the cross-linking of any unsaturated rubber can be accomplished in minutes by heating rubber with sulphur, zinc oxide, a fatty acid and the appropriate accelerator. 

An estimation of ZnCFO efficiency as vulcanization active component was carried out in modelling unfilled elastomeric compositions on the basis of isoprene, butadiene-nitrile, chloroprene and butyl rubbers of sulphur, thiuram, peroxide, metaloxide and resin vulcanization systems. 

The efficiency of the kinetic model has been demonstrated for many substrates polypropylene (2), polyethylene (3-4), poly(ethylene terephthalate) (5), polyisoprene (6), sulphur vulcanized polyisoprene (7), polybutadiene (8-9), amine crosslinked epoxy (10) and polybismaleimide (77) in large temperature, y dose rate and oxygen pressure ranges. 

Both CIS- and trans-polybutadienes present a somewhat different picture since efficiencies much greater than unity have been observed. This high efficienc tas been found to increase with an increase in the vinyl (1,2-) content. J or example it has been found (Kraus, 1963) that whereas a poly butadiene with a 10% vinyl content had a cross-linking efficiency of about 2, a 98% 1,2- polymer had a value in excess of 100 It is reasonable to presume that this high efficiency is due to a polymerization process initiated by reaction (A) but it is to be noted that there is much evidence to show that this polymerization cross-linking occurs via main chain double bonds as well as on the pendent vinyl groups. As with accelerated sulphur vulcanization there are important, but not well understood differences between polybutadiene and polyisoprene. 

The vulcanization by sulphur alone is faster for ENR than for NR as isolated double bonds react more rapidly than contiguous double bonds. The presence of isolated double bonds also increases the efficiency of vulcanization as they inhibit the formation of in-chain cyclic sulphides. ... 

The efficiency of a vulcanization is measured by the number of reticulations per atom of sulphur of the product. Up to 3 sulphur atoms per 100 carbon atoms, the efficiency of the vulcanization is of the order of 0.2. The molecular mass of the reticulated polyisoprene is infinite. In this state, it is a rubber. 

The action of accelerators is improved by the presence of activators in practice, a combination of zinc oxide and a fatty acid is nearly always used. Specifically, the activator increases the efficiency of the vulcanizing system so that, typically, only about 5 sulphur atoms are combined for each cross-link formed. As a consequence, the vulcanizate shows improved physical properties, aging characteristics and appearance (less sulphur bloom). The use of accelerator-activator systems is standard practice in rubber compounding and a typical basic tread formulation might be as follows ... 

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. 

The first practical systems were based on sulphur. On its own this was not very efficient and it has since been shown that up to 55 sulphur atoms may be combined for every cross-link formed. As the years passed it was found that the use of sulphur in conjunction with certain metal oxides, fatty acids and compounds known collectively as accelerators , gave rise to vulcanizates (the products of vulcanization) with superior properties and which required much shorter vulcanization times. Today the accelerated sulphur systems are still dominant and form the bulk of the subject matter of this chapter. Other methods of vulcanization are however known and several have been, and sometimes still are, of some industrial usage and these will be considered more briefly. 

Before actually treating the vulcanizate with a chemical probe some initial information on the fate of the sulphur combined during vulcanization may be obtained by calculating the Moore-Trego efficiency parameter E given by ... 

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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