The mechanism of N-cyclohexylbenzothiazole sulfenamide (CBS), 2-bisbenzothiazole- 2,2′-disulfide (MBTS) and 2-mercaptobenzothiazole (MBT) accelerated sulfur vulcanization of polyisoprene was investigated by heating compounds in a differential scanning calorimeter (DSC) at a programmed rate. The reaction was stopped at points along the thermogramme and the system analysed by estimation of the nature and amount of extractable curatives and soluble by-products and by characterization of the network produced in terms of crosslink density and crosslink type. The effect of MBT, a by-product of both CBS and MBTS vulcanization, on the mode of action of CBS and MBTS acceleration was studied. The interaction of ZnO with all three accelerator systems was investigated. It is suggested that MBT leads to crosslinking via a reaction sequence not unlike that which applies to unaccelerated sulfur vulcanization and that MBTS reacts via an additive process followed by reaction between a benzothiazole terminated polysulfidic pendent group and a neighbouring chain. The CBS reaction sequence is initiated by the substitutive interaction of the accelerator/sulfur complex with the polymer but an exchange reaction between CBS and pendent groups introduced in this process delays crosslinking. The crosslinking mechanism is initially similar to that found with a MBTS accelerator but in the latter stages depends on whether free MBT or the amine salt of MBT is present.
|Degree Type||Doctoral degree|