Chemorheology of irradiation-cured natural rubbers: 1. Stress relaxation mechanisms for various curing systems in natural rubber at high temperature

Abstract

Stress relaxation mechanisms were investigated for various curing systems in natural rubber, especially the ‘complex’ crosslinking irradiation cure systems containing sulphur or tetramethylthiuram disulphide (TMTD) in both air and nitrogen mainly at 100°C. With the systems, the chemical stress relaxation in air showed that in the main oxidative scission was the major cause of stress decay. However, the number of moles of main chain scission, q m ( t) seemed to be dependent on the chemical structure of the crosslink. In the TMTD curing system and irradiation-TMTD curing systems, q m ( t) is independent of the ratio, ρ, of the effective chain density N c (0) based on the carbon-carbon crosslinkages to N m (0) based on mono- and di-sulphide linkages. Stress decay in air of both accelerated-sulphur cures and irradiation-sulphur cures was shown to be due to both the interchange reaction of polysulphide linkages and random scission of the main chain. On the other hand, in nitrogen there are two simultaneous reactions at the crosslink:interchange and thermal scission.