Sulfur vulcanisation mechanism

Although sulfur vulcanisation was discovered over one hundred and fifty years ago, the exact mechanism of vulcanisation is still being examined. This arises not only from the complexity of the reactions and products formed but also to the fact that the mechanism of accelerated sulfur vulcanisation changes is dependent on the class of accelerators/ activators used. Typically, benzothiazole or sulfenamide are used as accelerators, zinc 

To aid in the NMR peak assignments of those 13C NMR spectra, the NR model compounds, 2-methyl-2-pentene, 2-methyl-l-pentene, and 4-methyl-2-pentene, were 

The chemical shifts due to the monosulfidic crosslinks are influenced not only by the position on the monomeric unit to which it belongs, but also by the position of the carbon atom of the monomeric unit on the other side of the bridge. The shielding parameters of monosulfide substitution on the individual carbons of the isoprene unit have also been determined. It is shown that resolvable polysulfidic crosslink resonances exist in all positions of the backbone carbons while monosulfidic crosslinks appear only between C-l and C-4 carbons with detectable intensity [18]. 

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The results of the optical spectroscopy studies into sulfur vulcanisation of polydiene rubbers correspond well with the results obtained via low molecular weight model olefin studies and solid state 13C NMR studies. From all these studies the mechanism for accelerated sulfur vulcanisation as shown in Figure 6.2 has emerged [14-18], which is... 

The mechanism of the accelerated sulfur vulcanisation of EPDM is probably similar to that of the highly unsaturated polydiene rubbers. The vulcanisation of EPDM has been studied with emphasis on the cure behaviour and mechanical and elastic properties of the crosslinked EPDM. Hardly any spectroscopic studies on the crosslinking chemistry of EPDM have been published, not only because of the problems discussed in Section 6.1.3 but also because of the low amount of unsaturation of EPDM relative to the sensitivity of the analytical techniques. For instance, high-temperature magic-angle spinning solid-state 13C NMR spectroscopy of crosslinked EPDM just allows the identification of the rubber type, but spectroscopic evidence for the presence of crosslinks is not found [72]. 

The mechanism of peroxide crosslinking of elastomers is much less intricate than that of sulfur vulcanisation. Crosslinking is initiated by the thermal decomposition of a peroxide, which is the overall cure rate determining step. Next, the active radicals thus formed abstract hydrogen from elastomer chains to form macroradicals. Finally, crosslinking results either from the combination of two macroradicals or from the addition of a macroradical to an unsaturated moiety of another primary elastomer chain. 

For sulfur vulcanisation of EPDM it was shown that the relative ENB conversion (20 to 60%) is higher than often assumed. The absolute ENB conversion was shown to be governed by the vulcanisation recipe and to be independent of the EPDM type. For the ISO 4097 [82] recipe the average length of the sulfur crosslinks is 2.7 sulfur atoms. The number of converted ENB units per sulfur bridge is 2.0, indicating that crosslinks are formed predominantly. In a preliminary study it was shown that the mechanical properties of unfilled sulfur-vulcanised amorphous EPDM are determined by the chemical crosslink density. Clearly, these studies should be extended to other vulcanisation recipes and completely formulated compounds. Vulcanisation kinetics should be studied, preferably at different temperatures. 

There is wide variety of vulcanisation agents and methods available for crosslinking rubber materials including peroxide, radiation, urethane, amine-boranes, and sulfur compounds [20]. Because of its superior mechanical and elastic properties, ease in use, and low cost, sulfur vulcanisation is the most widely used. Although vulcanisation with sulfur alone is not practical compared to the accelerated sulfur vulcanisation in terms of the slower cure rate and inferior physical properties of the end products, many fundamental aspects can be learned from such a simply formulated vulcanisation system. The use of sulfur alone to cure NR is typically inefficient, i.e., requiring 45-55 sulfur atoms per crosslink [21], and tends to produce a large portion of intramolecular (cyclic) crosslinks. However, such ineffective crosslink structures are of interest in the understanding of complex nature of vulcanisation reactions. 

The 13C NMR crosslink density results were compared with the crosslink density obtained by the mechanical measurements. In the determination of the crosslink density by mechanical methods, the contributions of the topological constraints on the results were neglected and the density was expressed as G/2RT. The 13C and mechanical-crosslink densities were obtained for both sulfur and dicumyl peroxide (DCP)-cured samples to ensure the effect of wasted crosslinks (pendent or intramolecular type sulfurisations), which are expected in the typical sulfur-vulcanisation of NR. In the major range of crosslink densities, the crosslink densities for those two systems are described by the same linear function with a slope of 1.0. Based on these observations, it is shown that the crosslink density of the sulfur-vulcanised NR as determined by 13C is identical with the true crosslink density, and the influence of the wasted or ineffective crosslinks (pendent and cyclic crosslinks) and chain ends is negligible. However, this conclusion seems to be only valid if the effect of topological constraints or entrapped entanglements on the mechanical modulus is negligible which is rarely the case in real systems. 

Yong, K.C. and Saad, C.S.M. (2010) High temperature-mechanical mixing to prepare electrically conductive sulfur-vulcanised poly(butadiene-co-acrylonitrile)-polyaniIine dodecylbenzenesulfonate blends. J. Rubber Res., 13,1-17. 

Elementary sulfur or compounds that can be used as a source of sulfur form together with suitable additives at higher temperatures thio-ether-, disulfide- or polysulfide-bridges in and between chains. This vulcanisation method is primarily suitable for those elastomers that have unsaturated bonds. The rubber produced by this method has good mechanical characteristics. However, a disadvantageous chemical characteristic of rubber vulcanised with sulfur is that additives can leach into the product. An example is the release of thiol compounds, which are incompatible with some mercury compounds. 

Balasubramanian [16] has described a devulcanisation process that uses a counter-rotating twin-screw extruder to devulcanise GTR. The DR was then blended with virgin NR in various proportions and the blends revulcanised using a sulfur cure system. The Mooney viscosity, cure characteristics and mechanical properties of the resulting vulcanisates were characterised and a four-parameter rheometric equation, based on the standard logistical model for the curing behaviour of extrusion processed blends, was derived and validated for the different levels of virgin NR. 

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