Sulfur vulcanisation elastomers

Peroxides. Peroxides are probably the most common materials used after sulfur because of their abiUty to cross-link a variety of diene- and non diene-containing elastomers, and their abiUty to produce thermally stable carbon—carbon cross-links. Carbon—carbon bonds are inherently stronger than the carbon—sulfur bonds developed with sulfur vulcanisation (21). 

The technology of sulfur vulcanisation of unsaturated elastomers has evolved since Goodyear s invention in 1839. Scientific studies into the chemistry of sulfur vulcanisation started to appear in the late 1950s (for reviews see References [14-18]). Two experimental approaches can be distinguished the analysis of rubber vulcanisates themselves and the so-called low-molecular-weight model studies. 

Over the last decade the development of advanced analytical techniques, such as Fourier transform (FT) Raman and solid-state NMR spectroscopy, have been impressive, resulting in a great deal of progress in the field of the sulfur vulcanisation of unsaturated elastomers [22-25]. 

In general, most of the problems encountered in the study of the chemistry of the sulfur vulcanisation of elastomers are also encountered in the study of peroxide-curing. In comparison with sulfur vulcanisation only a limited number of spectroscopic studies on peroxide-curing have been published. 

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. 

Accelerated sulfur formulations are the most common vulcanisation systems used in commercial and industrial applications. Therefore, research on both the fundamental and applied aspects of accelerated sulfur vulcanisation is ongoing. Several reviews of the chemistry and/or physics of accelerated sulfur-vulcanisation of elastomers have been published [13, 14, 22, 23]. 

Solid-state 13C NMR spectra of carbon black filled, uncured and sulfur-vulcanised HR were recorded at 22.6 MHz. The line broadening of the filled polymer relative to the unfilled polymer is attributed to incomplete motional narrowing of the NMR lines [53, 54] Incorporation of filler also results in a decrease in the signal-to-noise ratios in the spectra, but fundamentally it does not obscure the qualitative and quantitative nature of the spectra for the moderately cured elastomer systems. 

The chains must be crosslinked to form a network (cf. Fig 7.16). In most elastomers containing double bonds, covalent bonds are introduced between chains. This can be done either with sulfur or polysulfide bonds (the well known sulfur vulcanisation of natural rubber is an example), or else by direct reactions between double bonds, initiated via decomposition of a peroxide additive into radicals. Double bonds already exist in the chemical structure of polyisoprene, polybutadiene and its copolymers. When this is not the case, as for silicones, ethylene-propylene copolymers and polyisobutylene, units are introduced by copolymerisation which have the property of conserving a double bond after incorporation into the chain. These double bonds can then be used for crosslinking. This is how Butyl rubber is made from polyisobutylene, by adding 2% isoprene. Butyl is a rubber with the remarkable property of being impermeable to air. It is used to line the interior of tyres with no inner tube. 

Ethylene propylene diene polymers (EPDM) are basic elastomers with double bonds in the side chain. They are generated by adding small amounts of diene mmiomers in the copolymerisation of ethylene and propylene. Due to the presence of this unsaturated bond in the basic elastomer both vulcanisation with peroxides and vulcanisation with sulfur are possible. Vulcanisation with peroxides is usually chosen. This generates a product that is relatively inert and weU resistant to ageing. 

As a result of the high polarisability of C—S and S—S bonds, Raman spectroscopy is especially suitable for studying sulfur vulcanisation of elastomers. However, conventional Raman studies of elastomers are limited on account of sample fluorescence (often due to impurities). 

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

Deformulation of vulcanised rubbers and rubber compounds at Dunlop (1988) is given in Scheme 2.3. Schnecko and Angerer [72] have reviewed the effectiveness of NMR, MS, TG and DSC for the analysis of rubber and rubber compounds containing curing agents, fillers, accelerators and other additives. PyGC has been widely used for the analysis of elastomers, e.g. in the determination of the vulcanisation mode (peroxide or sulfur) of natural rubbers. 

Raman and IR spectroscopic studies dealing with the qualitative and/or quantitative determination of rubber compounding ingredients, i.e., the elastomer itself [22, 26-31], fillers [32, 33], vulcanisation chemicals and other additives [34-37], are not included here. The same applies to studies dealing with the crosslinking of elastomers by means of chemicals other than sulfur or peroxide [38-41], self-crosslinking of elastomers blends [42-44], crystallisation (strain-induced) [45-48] and oxidation/ageing [49-53]. 

Swelling is strongly dependent on the chemical nature of the vulcanised elastomer and the high proportion of combined sulfur in ebonite gives it a much reduced affinity to aliphatic hydrocarbon acids and water. 

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. 

Initially, vulcanisation was accomplished by heating elemental sulfur at a concentration of 8 parts per hundred parts of rubber (phr) for 5 h at 140 °C. The addition of zinc oxide reduced the time to 3 h. Accelerators in concentrations as low as 0.5 phr have since reduced times to 1-3 min. As a result, elastomer vulcanisation by sulfur without accelerator is no longer of commercial significance. An exception is the use of about 30 or more phr of sulfur, with little or no accelerator, to produce moulded products of a hard rubber called ebonite. 

The curing systems used to vulcanise speciality elastomers such as EPDM, CR, HR and NBR are different than those used to cure NR, SBR, BR and its blends. The former elastomers are less unsaturated and therefore need a high ratio of accelerator to sulfur. 

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