Azodicarbonamide and sodium bicarbonate blends as blowing agents for crosslinked polyethylene foam

Thermal decomposition characteristics of chemical blowing agent systems comprising of commercially available blowing agents based on azodicarbonamide and sodium bicarbonate were investigated. After characterising the blowing agent systems and postulating possible reaction mechanisms, they were compounded into a crosslinked polyethylene foam formulation and expanded using compression moulding variations or at atmospheric pressure. Density and in-mould temperature profiles were interpreted in the light of blowing agent system properties, detailed process parameters and permeability of individual gaseous decomposition products through polyethylene. It was concluded that as the bicarbonate proportion increased in the ADC/bicarbonate system, the higher carbon dioxide concentration in the decomposition gases tended to diffuse through the system (and potentially out of the foam) resulting in lower foaming efficiency. It was shown that systems incorporating sodium bicarbonate were entirely unsuitable for continuous atmospheric foaming of chemically crosslinked LDPE foams due to gaseous products being liberated prior to a sufficient build up of crosslinking. Conversely, processing the same formulations by compression moulding techniques produced similar densities (using equal proportions of sodium bicarbonate and azodicarbonamide) under appropriate process conditions. The advantage was a significantly reduced exotherm than that obtained with pure or activated azodicarbonamide. This would appear to suggest advantages in processing more highly temperature sensitive foams such as those based on ethylene copolymers or the even more stringent requirements for minimal heat build-up PVC foaming. Optimisation of foaming efficiency seemed to lie in investigation of modifying the diffusion rate of carbon dioxide through nonpolar polymers.