Sulphur vulcanization

The vulcanization process established by Goodyear in 1839 is still currently in use. Sulphur vulcanization takes place in three stages. 

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. 

It should be recognized that appreciable shifts in bonding strength are sometimes made possible by special compounding variations for instance, the heat resistance of NR vulcanizates may be improved considerably by variation of the vulcanizing system. The normal sulphur vulcanization system is capable of many variants, which will govern the chemical nature of the sulphur cross link, i.e., whether it is essentially a more, di, or poly-sulphide type. The nature of sulphur cross link can have considerable influence on the bond strength and the heat-resistance of vulcanizate... 

The saturated rubbers such as butyl or EPDM cannot be cross linked by sulphur and accelerators. Organic peroxides are necessary for the vulcanization of these rubbers. When the peroxides decompose, free radicals are formed on the polymer chains and these chains can then combine to form cross links of the type where only carbon-carbon bonds are formed, unlike in sulphur vulcanization. These carbon-carbon bonds are quite stable ones. Such bonds are also formed by vulcanization using gamma or x-ray radiation of compounded rubbers. Some rubbers can be vulcanized by the use of certain bisfunctional compounds which form bridge type cross... 

At temperatures well below Tg, when entropic motions are frozen and only elastic bond deformations are possible, polymers exhibit a relatively high modulus, called the glassy modulus (Eg) which is on the order of 3 Gpa. As the temperature is increased through Tg the stiffness drops dramatically, by perhaps two orders of magnitude, to a value called rubbery modulus Er. In elastomers that have been permanently crosslinked by sulphur vulcanization or other means, the values of Er, is determined primarily by the crosslink density the kinetics theory of rubber elasticity gives the relation as... 

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]...

Figure 6.6 Mechanical properties of unfilled sulphur vulcanized amorphous EPDM as...

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. 

Rubber has a structure intermediate between thermosets and thermoplastics, with molecular chains linked by sulphur bridges during vulcanization. In pyrolysis, the main material is tyre rubber, a compound of styrene- butadiene- and isoprene-based rubber (SBR), of carbon black, sulphur, vulcanization aids, and zinc oxide. 

Fig. 22. Relative effect of atomic oxygen on a variety of polymers 1, polysulphide 2, formaldehyde polymer 3, polypropylene 4, low density polyethylene 5, polyethylene glycol terephthalate) 6, polystyrene 7, polytetrafluoroethylene 8, sulphur-vulcanized natural rubber [268].

Table 20. Curing rate of sulphur vulcanized isobutene copolymers ...

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. 

Resole resins based on phenolic compounds with a substituted para position, such as 4-octylphenol, can be used as mbber vulcanizing agents in combination with different activators that enhance the vulcanization rate. Activators are not required if halomethylated phenolic resins are used. The application of rubber vulcanization to resole resins is limited to rubbers that are difficult to vulcanize by conventionally sulphur vulcanization. For example... 

The level and consistency of bond strength are influenced by the nature of the rubber factors include the mix viscosity, level of filler, scorch safety and, in the case of sulphur-vulcanized rubbers, the amount of elemental snlphur in the vulcanizing system. 

Brass plating Sulphur-vulcanized rubber will form a good bond to brass during vulcanization - provided... 

Due to its advanced technology and low cost the rubber industry often prefers the use of sulphur-vulcanized polyurethane elastomers even though some of their technical properties, e.g. resistance to thermal degradation, are inferior to the peroxide- and diisocyanate-cured grades which often have short processing times (i.e. they are scorchy) and whose cure may be adversely affected by the presence of moisture in the unvulcanized rubber mix water is present in rubber fillers, e.g. carbon blacks usually contain about 0-5-1% and some non-black fillers such as silicas and clays 2-10%. Also to maximize scorch time it is common practice to quench-cool the rubber after internal mixing by immersion in cold-water tanks or by cold-water spray application to the surface of the hot-milled sheet. 

Depolymerization (or reversion) occurs essentially at high temperatures, only in linear polymers having weak monomer-monomer bonds, or in tridimensional polymers having weak cross-link junctions (see Table 12.2). These are linear polymers containing the weakest aliphatic C-C bonds, i.e. involving tetrasubstituted carbon atoms, e.g. polyisobutylene (PIB), poly(methyl methacrylate) (PMMA), poly(or-methyl styrene) (PorMS), etc. These are also linear polymers containing heteroatoms, e.g. poly(oxy methylene) (POM), poly(ethylene terephthalate) (PET), poly(vinyl chloride) (PVC), etc., but also sulphur vulcanized elastomers. Cross-linking predominates mainly in unsaturated linear polymers, i.e. essentially polybutadiene and its... 

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