The effect of accelerator/sulphur ratio on the scorch time of epoxidized natural rubber

Abstract

The Mooney scorch time of epoxidized natural rubber (ENR) was studied with natural rubber (SMR L) as a reference. The effect of sulphur to accelerator ratio on scorch behaviour in the presence of the more common types of accelerators was investigated. Mooney scorch time was studied by using the Mooney Shearing Disk Viscometer in the temperature range 100–180 °C. The results indicate that Mooney scorch time decreases exponentially with increasing temperature for ENR and SMR L. At a fixed temperature, ENR with 25 mol % epoxidation shows shorter scorch time compared with SMR L. This has been attributed to the activation of the double bond by the oxirane group. The investigation involving different types of accelerators shows that Mooney scorch time decreases in the order; N-tert-butyl-2-benzothiazylsulphenamide (TBSS) > tetramethylthiuram disulphide (TMTD) > diphenylguanidine (DPG) > 2-mercaptobenzothiazole (MBT) > zinc dimethyldithiocarbamate (ZDMC). This accelerator dependence becomes less significant as temperature increases. Studies of the effect of varying amounts of MBT at a fixed sulphur concentration show that scorch time decreases as the accelerator concentration increases. Thus, scorch time was observed to decrease in the order; conventional vulcanization system > semi-efficient vulcanization system > efficient vulcanization system for MBT accelerated compounds. However, no such predictable patterns were shown by the other types of accelerators. It was also found that the influence of accelerator/sulfur ratio becomes less significant as vulcanization temperature increases.