THE EFFECT OF ACCELERATOR SULPHUR RATIO ON THE SCORCH TIME OF EPOXIDIZED NATURAL-RUBBER/

The Mooney scorch time of epoxidized natural rubber (ENR) was studied with natural rubber (SMR L) as a reference. The effect of sulphur to a ccelerator ratio on scorch behaviour in the presence of the more commo n types of accelerators was investigated. Mooney scorch time was studi ed by using the Mooney Shearing Disk Viscometer in the temperature ran ge 100-180 degrees C. The results indicate that Mooney scorch time dec reases exponentially with increasing temperature for ENR and SMR L. At a fixed temperature, ENR with 25 mol % epoxidation shows shorter scor ch time compared with SMR L. This has been attributed to the activatio n of the double bond by the oxirane group. The investigation involving different types of accelerators shows that Mooney scorch time decreas es in the order; N-tert-butyl-2-benzothiazylsulphenamide (TBSS) > tetr amethylthiuram disulphide (TMTD) > diphenylguanidine (DPG) > 2-mercapt obenzothiazole (MBT) > zinc dimethyldithiocarbamate (ZDMC). This accel erator dependence becomes less significant as temperature increases. S tudies of the effect of varying amounts of MBT at a fixed sulphur conc entration show that scorch time decreases as the accelerator concentra tion increases. Thus, scorch time was observed to decrease in the orde r; conventional vulcanization system > semi-efficient vulcanization sy stem > efficient vulcanization system for MBT accelerated compounds. H owever, no such predictable patterns were shown by the other types of accelerators. It was also found that the influence of accelerator/sulf ur ratio becomes less significant as vulcanization temperature increas es. (C) 1997 Elsevier Science Ltd.