Foaming of Low Density Polyethylene with Carbon Dioxide Based on Its in Situ Crystallization Behavior Characterized by High-Pressure Rheometer

The crystallization behaviors of low density polyethylene (LDPE) under ambient N2 and compressed CO2 were characterized using high-pressure differential scanning calorimetry (CO2 pressure ranging from 0.1 to 8 MPa) and high-pressure rheometer (CO2 pressure ranging from 0.1 to 30 MPa), respectively. The modulus variation during the crystallization characterized by HP-rheometer indicated that there were three typical regions where the modulus was nearly invariable with time corresponding to the crystallization beginning, the modulus increased dramatically corresponding to the crystallization ongoing, and the modulus stabilized at a higher value corresponding to the crystallization ending. The foaming was conducted at different storage modulus while other operation factors such as saturation pressure and temperature were maintained consistent. The results suggested that the crystal would affect the bubble-size distribution and even constrain the bubble growth when the relative crystallinity was more than 50%. Below this value, the crystal could facilitate cell nucleation and improve the melt strength.