Stress relaxation of vulcanized rubbers III — Experimental study

Abstract

Three polysulphide crosslinked vulcanizates have been aged in air at elevated temperature and constant elongation and the consequent relaxation of stress measured continuously. The stocks were natural rubber vulcanized with sulphur and tetramethyl thiuram disulphide, sulphur and diphenyl guanidine and sulphur and N cyclohexyl benzthiazyl sulphenamide. Analysis of the experimental observations by a method due to Edelstein, Vodden and Wilson has enabled the velocity constants associated with chain scission and crosslink scission to be calculated. Experiments carried out on two of the stocks at temperatures ranging from 100° to 115°C have shown that the velocity constants obey the Arrhenius equation and activation energies have been obtained for the reactions involved. It is postulated that crosslink scission proceeds through the formation of an oxygen complex at the crosslink. An experiment in which a rubber specimen was heated in vacuo prior to relaxation in air provided confirmatory evidence for this mechanism of degradation as the specimen behaved according to theoretical predictions. It is concluded that the postulated theory fits the experimental observations well and that these polysulphide stocks degrade by a combination of chain and crosslink scission.