High temperature vulcanization of elastomers: 2. Network structures in conventional sulphenamide-sulphur natural rubber vulcanizates

A natural rubber (NR) gum mix with a conventional N-cyclohexyl-2-benzothiazylsulphenamide (CBS) accelerated sulphur system (0.5:2.5 CBS/S) was vulcanized at temperatures from 140°C to 200°C. The influence of cure temperature on (a) the chemical crosslink density, (b) the distribution of crosslink types, (c) the extent of sulphidic main-chain modifications, and (d) the zinc sulphide formation was investigated. Results show that elevated cure temperatures produce a network with a lower crosslink density, in particular a lower polysulphidic crosslink density. The formation of intramolecular sulphidic groups and zinc sulphide increases with increasing temperatures. The possibility of chain scission during vulcanization was examined by a quantitative analysis of the sol-gel data. Less than 1 site of scission per 100 crosslinked isoprene units was established in the temperature range of 140–200°C. The network results can be satisfactorily correlated with the physical properties of a tyre tread mix of NR as reported in Part 1. Mechanistic interpretations are made to account for the network results.