Network structure sulfur vulcanisation

After these early studies an extensive FT-Raman study [77] was performed to bridge the gap between the low-molecular-weight ENBH model vulcanisation studies and the vulcanisation studies using high-molecular-weight EPDM. These studies will be presented in detail. First, a series of low-molecular-weight dialkenylsulfides will be discussed in order to determine the effect of sulfur vulcanisation on Raman spectra of olefins. Subsequently, the attachment of the sulfur crosslinks at the allylic positions, the conversion of ENB, the length of sulfur crosslinks and the network structure will be addressed for unfilled sulfur vulcanisates of amorphous EPDM. Some preliminary network structure/ properties relationships will also be presented. 

The analytical techniques discussed previously can be used to study the EPDM network as such or its formation in time as well as to determine relationships between the network structure and the properties of the vulcanisates. In a preliminary approach some typical vulcanised EPDM properties, i.e., hardness, tensile strength, elongation at break and tear strength, have been plotted as a function of chemical crosslink density (Figure 6.6). The latter is either determined directly via 1H NMR relaxation time measurements or calculated from the FT-Raman ENB conversion (Table 6.3). It is concluded that for these unfilled, sulfur-vulcanised, amorphous EPDM, the chemical crosslink density is the main parameter determining the vulcanisate properties. It is beyond the purpose of this review to discuss these relationships in a more detailed and theoretical way. 

Another study used 13C NMR to examine the thermal ageing of NR with both peroxide and conventional and EV sulfur vulcanised samples [59]. The samples were heat aged at 70 °C in air with 13C NMR recorded after 3 and 21 days. The vulcanisate structures vulcanised by peroxide and the EV sulfur system showed low sensitivity to ageing effects while peaks in the conventional sulfur system almost disappeared after 21 days of ageing. In addition, the main chain isoprene carbon peaks showed considerable broadening due to increasing stiffness of the network. 

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. 

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