Influence of sulfur content and curing time on the glass transition parameters in SBR vulcanizates: an ultrasonic study

Abstract

The ultrasonic attenuation coefficient and the ultrasonic velocity are measured in SBR vulcanizates as functions of temperature at two ultrasonic frequencies of 2 and 5 MHz in the temperature range from 180 K to 340 K, where the glass-rubber transition peak (α-relaxation) dominates at these frequencies. A group of SBR samples, S samples, which contain different sulfur contents of 0, 1, 2, 5, 10 and 15 phr (part per hundred parts of rubber), were cured by press molding for 30 min at 142 °C. Another group of samples, T samples, which contain 5 phr of sulfur, were cured at the same temperature for different times of 10, 20, 30, 40, 50 and 60 min. From the positions of the attenuation peaks on the temperature axis at the two adopted frequencies, the variation of the dynamic glass transition temperature T g was investigated with crosslinking density in S samples and with curing time in T samples. The variation with crosslinking density of the apparent activation energy of the α-relaxation ( E a) was also investigated in all samples. The results show that two different mechanisms of crosslinking take place in the two groups of samples.