Fabrication of Microcellular HDPE Foams in a Sub-Critical CO ₂ Process

In this paper processing conditions are presented to fabricate HDPE foams with a microcellular structure. The conventional microcellular foams are normally produced by heating a gas-saturated thermoplastic to a foaming temperature near the glass transition of the gas-polymer system. Efforts to foam HDPE in the conventional process have largely been unsuccessful, due to the low glass transition temperatures and low gas solubility in these polymers. We have found that HDPE can be successfully foamed with a microcellular structure at foaming temperatures above the melting point. Density reductions of up to 65% were obtained using sub-critical carbon dioxide as blowing agent. A unique bimodal structure was observed, consisting of closed microcells with an interconnected nanoporous interior.