Supercritical CO2 in controlling phase morphology of polypropylene/polystyrene blends and the corresponding mechanical properties and foamability

The objective of this study was to investigate the effect of supercritical carbon dioxide (scCO(2)) during continuous extrusion on controlling phase structure of polypropylene (PP)/polystyrene (PS) blends and the corresponding mechanical properties and foamability. Viscosity reduction of PP and PS was studied using a slit die rheometer attached to a tandem extrusion system. The scCO(2) was injected at 2.0, 4.0 and 6.0 wt% to PP/PS blends during extrusion. It was found that a sharp decrease in the size of the dispersed phase was achieved with the injection of scCO(2). The size of the minor phase for 75/25 PP-2/PS blend with 4.0 wt% scCO(2) was significantly reduced to 50 nm, due to the viscosity ratio of PP-2 to PS close to one. The mechanical properties of the polymer blends were closely related to the phase structure, and better dispersion favored enhanced mechanical properties. Foamability of the PP/PS blends was also found to be closely dependent on phase morphology, and better dispersion endorsed uniform cell structure with smaller cell size and high cell densities.