Mooney scorch time and cure index of epoxidized natural rubber in presenceof sodium carbonate

The effect of sodium carbonate on the Mooney scorch time and cure index of epoxidized natural rubber (ENR 50) was studied with concentrations of 0-2 p hr and 100-160 degreesC temperatures. A conventional vulcanization system b ased on an ASTM formulation was used throughout the investigation on the gu m and carbon black filled rubber compound. The results indicated that the s corch time and cure index for the gum and filled compounds increased to a m aximum value at 0.15 phr of sodium carbonate, and further loading of sodium carbonate caused it to decrease. This observation was attributed to the ne utralization of the residual acid in ENR 50 in the initial stage, thus redu cing the formation of ether crosslinks via an acid-catalyzed ring-opening r eaction with the epoxide group in ENR 50. However, as the sodium carbonate was increased beyond 0.15 phr, the excess sodium carbonate enhanced the vul canization rate as shown by the drop of the scorch time and cure index. The peak maximum was more evident at lower temperature and its peak height dec reased with increasing temperature, suggesting that the neutralization effe ct by sodium carbonate was overshadowed by the faster cure rate resulting f rom the availability of thermal energy to overcome the activation energy of vulcanization as the temperature was elevated. (C) 2001 John Wiley & Sons, Inc.