Effects of Comonomer Composition on Adhesion and Foaming Behavior of Polypropylene Particles during Water-Free Supercritical CO2 Bead Foaming

Abstract

Water-free supercritical CO2 bead foaming offers a cleaner route to expanded polypropylene beads, but the absence of water intensifies interparticle adhesion. Four polypropylene resins with different comonomer compositions were investigated to clarify the effects of molecular structure on adhesion and foaming. Compared with homopolypropylene, the comonomer-containing resins showed lower apparent crystallinity, higher CO2 uptake under their respective processing conditions, and different melt responses. Simulations and adhesion measurements indicated that cohesive energy density relates to intrinsic particle adhesion, whereas PP-CO2 interaction energy reflects PP-CO2 affinity. The observed foaming behavior resulted from the combined effects of apparent crystallinity, melt viscoelasticity, CO2 uptake, molecular interactions, and adhesion. The ethylene–propylene copolymer provided the best balance, producing relatively uniform, dense microcellular structures with controlled adhesion. These results guide the selection of polypropylene resins for water-free supercritical CO2 bead foaming.