Cure and tear properties of ENR 25/SMR L and ENR 50/SMR L blends

Curing characteristics of two rubber blends, namely, epoxidized natural rubber (ENR) 25/SMR L and ENR 50/SMR L were studied using a Monsanto Moving Die Rheometer (MDR 2000), and the blend ratio was varied from 0–100% ENR. Also, a systematic study of tear strength of the blends was carried out before and after ageing at 100°C for 48 h. A Monsanto Tensometer (Model T10) operating at 50 cm/min was used to determine the tear strength of the standard crescent test piece. The results indicate that scorch time and cure time decrease with increasing ENR contents, whereas maximum torque and torque difference show the contrary. This observation is attributed to the activation of an adjacent double bond by the epoxide group in ENR (shorter scorch and cure time) and consequently enhances cure to produce higher crosslink density (higher torque values). The effect of activation is more significant in ENR 50 than in ENR 25, due to the presence of more epoxide groups in the former. From viscous torque and loss tangent data, better resilience property of the blend can be obtained if the ENR content is less than 25%, or more than 50%. The tear strength increases with the ENR content up to a maximum value at about 50% ENR in the blend. This finding is ascribed to the increased energy dissipation, as indicated by the increase in tan δ with % ENR. However, tear strength of aged sample decreases with increasing ENR contents, due to the poor aging property of the ENR, especially ENR 50.