Foaming of reactively modified polypropylene: Effects of rheology and coagent type

Abstract

A series of linear controlled rheology polypropylenes were produced through reaction with dicumyl peroxide in the melt state, to investigate the effect of molecular weight and viscosity on foaming. Foaming experiments by compression molding, using a chemical blowing agent (azodicarbonamide), and by batch foaming using nitrogen-blowing agent, revealed that lower viscosity promoted higher expansion ratios and larger cells, due to reduced resistance to cell growth. Linear polypropylene was further modified using trimethylolpropane trimethacrylate and triallyl trimesate coagents. Coagent modification resulted in pronounced changes in the shear thinning and elasticity of the modified polypropylenes. However, evidence of long-chain branching was only present in polypropylene modified by triallyl trimesate. Foams based on coagent-modified polypropylenes had higher expansion ratios than their degraded counterparts. This was ascribed in part to the lower viscosities, and to a nucleating effect, arising from the presence of a finely dispersed phase of coagent-rich nanoparticles. Strain hardening in polypropylenes modified by triallyl trimesate further resulted in a finer cell structure, due to suppressed coalescence.