Chemorheology of irradiation-cured natural rubbers: 3. Stress relaxation mechanisms in nitrogen at elevated temperature

Abstract

Chemorheological studies were carried out on three kinds of natural rubber vulcanizates, differing in the structure of their crosslinks, in nitrogen in the temperature range 100° to 230°C. The crosslinking site in these polymers consists of a carbon-carbon bond (sample 1), a carbon-sulphur-carbon bond (sample 2), and carbon-carbon and carbon-sulphur-carbon bonds (sample 3), respectively. The stress decay curve of all the samples was expressed by the sum of two exponential terms, that is, two different scission reactions took place in the polymer networks at the same time. Sample 1 underwent two different random scissions in the main chains above 180°C: one was the oxidative scission due to the oxygen molecules in the sample and the other was the thermal scission involving the peroxy and/or carbonyl groups on the polymer chains. The stress decay of sample 2 was based on the cleavages of two different types of crosslink which were mono- and disulphide linkages, whereas that of sample 3 was attributed to the scission both of crosslinkages and of the main chain. The scission of crosslinkages occurred at lower temperatures (above 120°C).