PREPARATION OF POLYSTYRENE FOAMS WITH BI-MODAL CELLS

Using supercritical carbon dioxide (Sc-CO2) as the physical foaming agent, polystyrene (PS) foams were prepared in a batch process via two different heating modes and depressurization. The cellular structures, foam densities and wetting properties of the PS foams were investigated by means of scanning electron microscope, electronic balance and contact angle measuring device, respectively. The results showed that a mono-modal cell structure was obtained only through depressurization,whereas a bi-modal cell structure was achieved through the synergistic effect of heating and depressurization. The large and small cells were formed by nucleation in the heating and depressurization stages, respectively. For the PS foams prepared at 100 degrees C foaming temperature and 30 similar to 70 degrees C saturation temperatures, the large and small cells had average diameters of 50 similar to 216 and 10 similar to 15 mu m, respectively. Increasing the diameter and density of the large cell was favourable to the decrease of the foam density, and the lowest foam density was 0.15 g/cm(3). The mono-modal cell structure yielded an increase in static contact angle (CA) from the intrinsic value of the PS (87.1 degrees) to 138.8 degrees on the fractured surface of the foam; the CA value on the surface with a bi-modal cell structure prepared at 70 degrees C saturation temperature increased up to 155.1 degrees, displaying the superhydrophobic wetting properties.