GENERATION OF MICROCELLULAR POLYMERIC FOAMS USING SUPERCRITICAL CARBON-DIOXIDE .2. CELL-GROWTH AND SKIN FORMATION

We have generated microcellular polymeric foam structures using a pres sure induced phase separation in concentrated mixtures of supercritica l CO2 and poly(methyl methacrylate). The process typically generates a microcellular core structure encased by a nonporous skin, the thickne ss of which decreases with increasing saturation pressure. This trend can be described by a model for skin formation that is based on the di ffusion rate of gas out of the sample. Significant density reductions on the order of 30 to 70% can be achieved by changing the pressure and temperature conditions in the foaming process. There are several ways in which the saturation pressure affects the average cell size, with the net effect that cell size decreases sharply with increasing pressu re above 2000 psi, leveling out at higher pressures. Cell size increas es with increasing temperature from 40-degrees to 70-degrees-C. A mode l for cell growth, based on a cell model of Arefmanesh and Advani, mod ified to include the effect of CO2 on model parameters. reproduces the se trends.