Preparation and characterization of microcellular polystyrene foams processed in supercritical carbon dioxide

The foaming of polystyrene using supercritical (SC) CO(2) has been studied to better understand the microcellular foaming process, as we plan future studies that involve the creation of composite microcellular foams. Rapid decompression of SC CO(2)-saturated polystyrene at sufficiently high temperatures (above the depressed T(g)) yields expanded microcellular foams. The resulting foam structures can be controlled by manipulating processing conditions. Experiments varying the foaming temperature while holding other variables constant show that higher temperatures produce larger cells and reduced densities. Structures range from isotropic cells in samples retaining their initial geometry to highly expanded foams recovered in the shape of the foaming vessel and having oriented, anisotropic cells and limited density reduction. Higher saturation pressures lead to higher nucleation densities and hence smaller cells. Decreasing the rate of depressurization permits a longer period of cell growth and therefore larger cell sizes. Foams having a bimodal distribution of cell sizes can be created by reducing the pressure in two stages.