Fabrication of microcellular polycarbonate foams with unimodal or bimodal cell-size distributions using supercritical carbon dioxide as a blowing agent

This article reports on the fabrication of microcellular polycarbonate (PC) foams with unimodal or bimodal cell-size distributions using supercritical CO2 (scCO2) as a physical blowing agent. The saturation and desorption behaviors of CO2 in PC for various pressures are investigated. The effects of key processing parameters such as saturation pressure, foaming temperature, and foaming time on the relative density, cell sizes, and the cell densities of the unimodal foams are discussed in detail. The glass transition temperature (Tg) of the polymer/gas mixture is found to present a good linear relationship with the absorbed CO2 concentration, and foaming of the samples takes place above the Tg of the polymer/gas mixture. A two-step batch depressurization process is applied to produce bimodal PC foams with both small and large cells, which exhibit significantly improved tensile properties compared to the unimodal foams. In addition, the phenomenon of extensional stress-induced nucleation promoting the generation of nanocellular structures around the expanding larger cells is observed at the increased foaming temperature of 160° in the production of bimodal foams.