Chemorheology of irradiation-cured natural rubbers: 2. Stress relaxation mechanisms of sulphur-containing cured systems in air

Abstract

Stress relaxation mechanisms in air were investigated for irradiation-cured natural rubbers (NR) containing the various levels of sulphur, in relation to the crosslinked structure. All the stress relation curves of these vulcanizates except the sulphur-free sample were the sum of two exponential terms. The relaxation rate in the long time region increased with increasing sulphur contents, in spite of having the same initial chain density. This behaviour could be explained by introducing the assumption, that this system has two independent networks, i.e. one ( N p (O)) based on the polysulphide cross-linkages and another ( N c, m (O)) based on the mono- and di-sulphide linkages and the carbon—carbon bonds. N p (O) increased with increasing sulphur contents and N c, m (O) decreased with increasing sulphur contents. The q m, c ( t) (the number of moles of main chain scission) were in agreement with each other, independent of the sulphur contents. However, this value was larger than that of the sulphur-free sample. This was similar to irradiation-cured tetramethylthiuram disulphide NR, in which the crosslinking sites consisted of mono- and di-sulphides and carbon—carbon bonds.