Cure index and activation energy of ENR 25 compared with SMR L in various vulcanization systems

Abstract

Cure index of standard Malaysian rubber (SMR L) and epoxidized natural rubber (ENR 25) was determined in the temperature range from 100 to 180°C using a Monsanto Automatic Mooney Viscometer MV2000. Five accelerators, viz. 2-mercaptobenzothiazole (MBT), tetramethylthiuram disulphide (TMTD), zinc dimethyldithiocarbamate (ZDMC), N- tert-butyl-2-benzothiazyl sulphenamide (TBBS) and diphenylguanidine (DPG) were used in the study and the vulcanization systems used were conventional vulcanization (CV), semi-efficient vulcanization (semi-EV) and efficient vulcanization (EV). Results indicate that the cure index and apparent activation energy for SMR L is greater than that of ENR 25 and it decreases exponentially with increasing temperature. The lower values of the cure index and apparent activation energy in ENR 25 is attributed to the activation of a double bond by the adjacent epoxide group in ENR 25. The cure index also depends on the accelerator and vulcanization systems. Highly substituted accelerators (e.g. TBBS) tend to give a higher cure index and apparent activation energy which suggests that steric hindrance plays an important role in accelerated sulphur vulcanization of rubbers.