High temperature vulcanization of elastomers: 3. Network structure of efficiently vulcanized natural rubber mixes

Abstract

The effect of vulcanization temperature (140–200°C) and time on the structures of pure gum natural rubber vulcanizates with two different N-cyclohexyl-2-benzothiazylsulphenamide (CBS): sulphur ratios (A, 3·5:1·5; B, 6·0:0·4 CBS/S) has been determined. Analyses of vulcanizates were carried out as reported in Part 2. Results show that both mixes are efficient in crosslinking, resulting in mainly monosulphidic crosslinks and relatively few modifications of the rubber chains. Raising the cure temperature from 140°C reduces the density of chemical crosslinks, particularly those of monosulphidic crosslinks, obtainable in the vulcanizates. This decrease in crosslink density has been shown to be irreversible with respect to cure temperature. The formation of intramolecular sulphidic groups and zinc sulphide increases with rising cure temperature, but this increase is small compared with that reported for the conventional CBS-accelerated system. The main difference between mixes A and B is that mix A yields a higher level of crosslinks and a major proportion of cyclic sulphides as main-chain modification. Negligible chain scission occurs during vulcanization at 140–200°C. These network results are interpreted mechanistically, and essential network features for obtaining good physical properties in high temperature vulcanizates are deduced.