Nano-Scale Pores are Formed between the Shish-Kebab Structures of Double-Mold Polyethylene by Supercritical Carbon Dioxide Foaming

A new method incorporating dynamic polymer processing and supercritical CO2 was adopted to foam the bimodal polyethylene (BPE) with a nano-cellular structure. The shish-kebab crystallites with high strength were introduced with a dynamic polymer processing of injection-molding. Deliberate reservation of shish-kebab crystals in the foaming was systematically investigated at different foaming temperatures and pressures. Morphology of nano-pores existing together with residual shish-kebab crystals was found. The foaming temperatures of 120, 125, 128, and 130°C were suitable for inducing the nano-cellular structures. With increasing the foaming temperature, the cell changed from a slit-like shape to a round shape, because the melting and recrystallization content of the microcrystals in the kebabs are different. The crystallinity and the melting point increased with the increasing foaming temperature, while the orientation degree showed a downward trend. Different foaming pressures of 15.4, 18.6, 20.0, and 22.5 MPa performed at 128°C were adopted to examine foaming pressure effects. With increasing the pressure, the crystallinity, the melting point, and the degree of orientation showed a slight drop. In the aspect of mechanical properties, the foamed samples exhibit higher flexibility compared with unfoamed ones.