Supercritical Carbon Dioxide Induced Foaming of Highly Oriented Isotactic Polypropylene

A highly oriented isotactic polypropylene (iPP) with a shish-kebab crystalline structure is used as a template to achieve nanocellular foams by CO2 foaming in the solid state. The shish-kebab crystalline structure is obtained by tuning injection-molding conditions. Nanocells are generated in amorphous domains confined by shish-kebab crystalline domains which cannot foam. The highly oriented iPP consists of both shish-kebab crystalline structure and spherulites structure and is used to investigate the effect of the crystalline morphology on the cell formation at a given CO2 saturation pressure (15 MPa) and various foaming temperatures (135, 140, 143, and 146 degrees C). When the foaming temperature is 143 degrees C, a uniform open nanocellular morphology is obtained. The cell size and cell density both increase with foaming temperature from 135 to 143 degrees C due to the fact that more crystals are molten, providing more space for cell nucleation and growth. However, when the foaming temperature is increased to 146 degrees C, the cell size abruptly increases from dozens of nanometers to dozens of micrometers, and the corresponding cell density decreases by several orders of magnitude.