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.

Effect of graphene treated with cyclohexyl diamine by diazonium reaction on cure kinetics, mechanical, thermal, and physical properties of natural rubber/graphene nanocomposite foam

“…Cure characteristics of the natural rubber compound were observed by rheograph from a MDR at 155°C, which was a temperature used for curing the rubber compound by compression molding process. Cure characteristics are the important factors for controlling the bubble size and microvoid structure in the rubber foam system affecting the mechanical and thermal properties of NRF . As displayed in Fig.…”

“…Although NRF/CG3 showed the highest tensile strength, it revealed slightly higher compression set than NRF/UG3 due to the slightly larger bubble size resulting in poor distributive force in the rubber foam system. It might be concluded from this finding and our previous work that the optimal mechanical properties of natural rubber nanocomposite foam will be acquired for the uniformly distributed small bubble size in natural rubber matrix.…”

“…The amount of CBA was fixed at 5 phr which was the content that produced non‐coalescent bubbles in the NRF. Although our previous work showed that the content of CBA should be fixed at 4 phr, in this work, chemical ingredients with different grades and purities were used which might affect the rate of sulfur vulcanization reaction differently. The total mixing time was 15 min.…”

Towards novel super‐elastic foams based on isoperene rubber: Preparation and characterization

“…Finally, the mold was opened and the prepared foams were kept at room temperature for 24 hours. For conventional foams, a two‐step unlimited‐expansion foaming method as a standard procedure was used . The compounds were preheated and precured in a hot press (120°C, 10 minutes) and then placed in a hot air oven (160°C, 30 minutes) for simultaneously curing and foaming.…”

“…The effect of foaming agent content on the morphology and properties of NR foams was studied by Najib et al for sodium bicarbonate and by Charoeythornkhajhornchai et al for azodicarbonamide (ADC). It was reported that foam density, mechanical properties (modulus and hardness), and curing properties (cure content and ultimate torque) were all decreasing with increasing foaming agent content.…”