Sulfur vulcanisation natural rubber

Hard rubber or ebonite whether from natural rubber or from synthetic rubber, can be defined as highly vulcanised rubber, containing a large proportion of combined sulfur. Hard rubbers made from natural rubber have vulcanisation coefficients between 25 and 47. The theoretical vulcanisation coefficient value for natural rubber is 47 and for synthetic rubbers it is in the range of 35 to 47. The coefficient of vulcanisation is usually defined as the number of units by weight of sulfur combined with 100 units by weight of unsaturated hydrocarbon. The theoretical coefficients are corrected for impurities/non rubber constituents in the raw rubber. 

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. 

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. 

Figure 15.15 Natural abundance 2H MAS spectra observed in a series of vulcanised natural rubbers with various vulcaniser (sulfur 1 or 3 wt%) and/or filler (carbon black 0 or 40 wt%) contents. The spinning speed is 0.5 kHz. The number of scans is about 300000. Spectra are simulated with two components (a mobile and a rigid one) with various residual quadrupolar interactions...

In this chapter, some of these uses are explored in greater detail. Goodyear and Hancock in 1847 discovered that, when natural rubber was heated with a small amount of sulfur, the physical properties of the resultant rubber were improved the material became tougher and more resistant to changes in temperature. This process of vulcanisation is also useful for the treatment of synthetic rubbers, and as well as sulfur, many sulfur donors such as symmetrical diphenylthiourea, tetraalkylthiuram disulfides (1) and 2-mercaptobenzothia-zole (2) (Figure 1) can be used.1 These compounds act as accelerators of the process of polymerisation of the diene monomers in synthetic rubbers for this purpose, the additional presence of zinc oxide and preferably a carboxylic acid, e.g. stearic acid, is required. 

It should be recognised that appreciable shifts in properties are sometimes made possible by special compounding variations. For instance, the heat resistance of natural rubber vulcanisates may be improved considerably by variation of the vulcanising recipe. The normal sulfur vulcanisation system is capable of many variants which will govern the chemical nature of sulfur crosslinks, i.e., whether it is essentially a mono, di or polysulfide linkage. The nature of sulfur crosslinks can have considerable influence on the heat and chemical resistance of vulcanisates. 

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. 

Waddell and co-workers [64] applied this technique to Neoprene rubber compound surfaces. The LD-MS of the sulfur-vulcanised natural rubber (NR) Compounds 1 and... 

For natural rubber, by hi the most important industrial cross-linking process is vulcanisation. The rubber is processed mixed with sulfur and a catalyst (as well... 

The monomer of natural rubber is 2-methyl-l,3-butadiene, which is very similar to the monomer for neoprene. The intermolecular forces between natural rubber polymers are van der Waals forces. A process called vulcanisation was invented to make rubber tyres more resilient and hardwearing. This links rubber polymer chains by covalent bonds across sulfur bridges (Figure 28.22). 

Some of these chemical agents (e.g., the amine/thiol mixtures) were used in the chemical probe rubber chemistry research work being undertaken at TARRC in the 1950s and 1960s to establish the chemical nature and processes involved during the sulfur vulcanisation of diene rubbers, such as NR. 

Guzman and co-workers [27] investigated whether it is possible to use waste tyre crumb as a replacement for zinc oxide as an activator in the sulfur vulcanisation of natural rubber (NR). They used the unsaturated organic compound squalene as a model compound for NR in their work, and followed the course of the vulcanisation reaction using the analytical technique high-performance liquid chromatography. The results confirmed that waste rubber crumb was an alternative to zinc oxide as an activator in the curing of NR compounds by sulfur-based cure systems. 

Since these early days, the process and the resulting vulcanised articles have been greatly improved. In addition to natural rubber, many synthetic rubbers have been introduced over the years. Furthermore, many substances other than sulfur have been introduced as components of curing (vulcanisation) systems. 

The accelerated sulfur vulcanisation of general purpose diene rubbers (e.g., natural rubber (NR), Styrene butadiene rubber (SBR), and butadiene rubber (BR)) in the presence of organic accelerators and other rubbers, which are vulcanised by closely related technology (e.g., ethylene-propylene-diene terpolymer (EPDM) rubber, butyl rubber (HR), halobutyl rubber (XIIR), nitrile rubber (NBR)) comprises more than 90% of all vulcanisations. 

What is known as the vulcanisation of rubber is an excellent example of this in 1839, the American chemist Charles Nelson Goodyear (1800-1860) succeeded in transforming the rubber that occurs naturally into the polymer product that we now call rubber by adding sulfur and applying heat. The undesirable tendency of the rubber to become sticky when heated and crumbly when cooled was overcome as a result. 

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