Accelerated Sulphur Vulcanization

Figure 6.2 Simplified scheme of the mechanism of accelerated sulphur vulcanization of...

Figure 6.3 Accelerated sulphur vulcanization of EPDM as demonstrated by low-molecular weight model olefin studies [75,76]...

Ostromyslensky [90] observed that rubber could be vulcanized in the absence of sulphur or its compounds if small quantities of aromatic nitro compounds, e.g. nitrobenzene, dinitro- or sym- trinitro-benzene, tetranitronaphthalene plus certain oxidizing compounds, such as benzoyl peroxide, were added to it. The resultant vulcanized rubber had mechanical properties not inferior to those of a product vulcanized by sulphur alone, and when free from accelerators it exhibited a greater resistance to ageing. Some metal oxides, such as PbO, CaO, BaO, promoted vulcanization by nitro compounds. Urea played the role of an accelerator of vulcanization. These observations have been confirmed in more recent studies by other workers. 

Bhatnagar S.K. and S. Banergie.1968. Kinetics of accelerated vulcanizates. Sulphur vulcanization accelerated with 2-mercatobenzothiazole and its Zn salts in presence of ZnO and stearic acid. Angewandte Makromol. Chemie. 3(1) 177-121. 

Bhatnagar S.K. and S. Baneijee. 1968. Kinetics of accelerated vulcanization. El. V-cyclohexyl-benzothiazole-2-sulfenamide accelerated sulphur vulcanization of rubbers. J. Inst. Rubber Ind. 2 177-193. 

Mukhopadhyay R. and S.K. De. 1978. Effect of elevated temperature on the unaccelerated and accelerated sulphur vulcanization of natural rubber. Rubber Chem. Technol. 51 704-17. 

Technologically, the most important methods of vulcanizing natural rubber are by the use of accelerated sulphur systems and organic peroxides and only these methods are considered in this section. 

It is now supposed that accelerated sulphur vulcanization follows the general pathway shown in Fig. 18.1. The various steps involved in this scheme are considered below. 

Fig. 18.1 Pathway of accelerated sulphur vulcanization of natural rubber (after 3)...

Fig. 18.2 Structural features of networks formed in accelerated sulphur vulcanization after J, 4)...

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. 

During the quarter of a century following World War II chemists at the British Rubber Producers Research Association (subsequently renamed the Natural Rubber Producers Research Association and, later, the Malaysian Rubber Producers Research Association) developed this work and using a variety of techniques have largely established the nature of the accelerated sulphur vulcanization of natural rubber. Important contributions have also been made by smaller research teams, for example Scheele and co-workers in Germany, and Craig, Juve, Campbell, Coran and Wise, and others in the United States. These studies have shown, not only that there are a variety of chemical structures present in a vulcanized natural rubber... 

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... 

The use of spectrographic analysis to study accelerated sulphur vulcanization has proved very limited. In the case of infra-red analysis the absorption of infra-red by sulphur bonds is very weak and no peaks have been found that can be unequivocally associated with sulphur cross-links (see reviews by Linnig and Stewart, 1958 Linnig et al., 1964). Some interesting results have however been obtained more recently using Raman spectroscopy (Stewart and Linnig, 1967 Coleman et al., 1973). 

Perthiomercaptides are also believed to be formed when certain sulphur-donors such as tetramethyl thiuram disulphide reacts with the zinc complex. It is thus possible to vulcanize a rubber by a mechanism broadly similar to that of an accelerated sulphur system without the use of elemental sulphur. 

Fig. 8.6. Course of the CBS-accelerated sulphur vulcanization of natural rubber at 140T with a high sulphur accelerator ratio (2-5 0-6). (x) Total cross-links ( ) poly sulphide ( ) monosulphide ( ) disulphide. The upper and lower figures on the curve relating to total cross-links are values of E and E, respectively. (After Moore, 1965.)...

It is not difficult to relate these regions to stages in the course of accelerated sulphur vulcanization as discussed in the previous... 

Fig. 8.11. Tensile strength of pure gum natural rubber vulcanizates plotted against 1/Mc for various vulcanizing systems. O accelerated sulphur x TMT sulphurless peroxide A high energy radiation. (After Greensmith et al,...

The use of sulphur donors in place of elemental sulphur has been practised since the early 1920s when it was found that the accelerators tetramethyl thiuram disulphide and tetraethyl thiuram disulphide in conjunction with zinc oxide gave vulcanizates with improved ageing properties when compared with conventional accelerated sulphur systems. For many years it was believed that vulcanization was brought about by the abstraction of one sulphur atom from the disulphide to yield the corresponding monosulphide... 

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. 

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 urethane vulcanization systems are compatible with accelerated sulphur systems and part of the former may be replaced (up to 50%) by the less expensive sulphur system with little detriment to the good resistance to reversion and oxidation conferred by the urethane system. 

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. 

The acrylic rubbers, being saturated polymers, cannot be vulcanized by conventional accelerated sulphur systems. On the other hand the homopolymers, such as poly(ethyl acrylate) and such copolymers as those of n-butyl acrylate with acrylonitrile could be cross-linked by such agents as ... 

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). 

The rubbers may be vulcanized by conventional accelerated sulphur systems and are characterized by good low temperature and dynamic properties with a constancy of many properties over a wide temperature range. They do however possess only moderate oil resistance. First introduced as Dynagen XP-139 (General Tire and Rubber Co.) they are now marketed by Hercules as Parel. 

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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