Effect of curing time and sulfur content on the dielectric relaxation of styrene butadiene rubber

Dielectric investigations were carried out on styrene-butadiene rubber cured by an increase in either the sulfur content or curing time in a wide range of frequencies from 100 Hz up to 10 MHz and at room temperature (approximate to25degreesC). After subtraction, the losses due to the Maxwell-Wagner effect, the dielectric data showed deviation from the Debye formula but were found to be fitted in the frequency domain by the Havriliak-Nagami function, which is a combination of Cole-Cole and Cole-Davidson functions. This function describes the relaxation mechanisms of the aggregates caused by the movement of the main chain that were expected to be formed by the addition of the different ingredients to the investigated rubber. These mechanisms are also discussed in terms of the distribution function of Frohlich between two limiting values of relaxation times tau(1) and tau(2). The relaxation times obtained from both functions increased with an increase in either sulfur content or curing time. The data obtained are discussed in terms of the crosslinking density determined from the Flory-Rehner relation. The effect of thermal aging on such systems was also studied, and the data obtained were compared with those before aging. This investigation led us to conclude that to get an end product that is highly resistant to thermal aging, the system should be cured for a time greater than 30 min with the addition of more than 5 phr sulfur. (C) 2003 Wiley Periodicals, Inc.