Regulating foamability of crosslinked EVA through network design and gas expansion force control during physical foaming

Abstract

Physical foaming of ethylene-vinyl acetate (EVA) elastomers for high expansion ratio remains challenging due to the narrow processing window and low matrix modulus. This work systematically investigates the topological influence of chemical crosslinking on the viscoelasticity and subsequent foaming behavior of EVA. Crosslinking transforms the EVA melt into a robust elastic network, significantly extending stress relaxation times and inhibiting viscous flow. An optimal sparse network containing 0.2 – 0.6 phr crosslinker effectively balances melt strength against chain mobility, facilitating stable cell growth. Notably, by leveraging the synergistic effect of a CO₂/N₂ mixture, an ultrahigh expansion ratio of 30–fold (density: 0.031 g/cm³) with uniform cell morphology was achieved in the lightly crosslinked sample. In contrast, excessive crosslinking induces brittle fracture due to the restricted chain extensibility under high expansion force. The foaming mechanism is governed by the competition between the gas expansion force and the confinement of the crosslinked network. This work provides a strategic framework for designing high-performance elastomeric foams through the precise modulation of network architecture and blowing agent composition.