Effect of azodicarbonamide on microstructure, cure kinetics and physical properties of natural rubber foam

Abstract

The effect of azodicarbonamide as chemical blowing agent on the morphology, cure kinetics and physical properties of natural rubber foam is investigated. From the morphology, when the amount of chemical blowing agent increases from 3 to 4 phr, the bubble size in the rubber matrix slightly decreases due to the increase of vulcanization reaction rate from the presence of amine fragment species as by-products from the decomposition of azodicarbonamide. The coalescence between bubbles is observed in the specimen with 5 and 6 phr of azodicarbonamide owing to high gas content in the rubber matrix. Moreover, the scorch time slightly reduces and cure rate increases as a function of azodicarbonamide content. The autocatalytic model can be used to explain the curing reaction and mechanism of this natural rubber foam. Furthermore, the activation energy (Ea) directly relates to the bubble size and microvoid structure of natural rubber foam. When compared with the vulcanized natural rubber without adding chemical blowing agent, it is found that the bulk density of natural rubber foam significantly decreases and the volumetric expansion ratio of natural rubber foam increases at high content of chemical blowing agent. Moreover, natural rubber foam at 4 phr of azodicarbonamide exhibits the lowest thermal expansion coefficient due to the smallest bubble size with less coalescence.