The Rheological Behavior And Extrusion Foaming With Supercritical CO₂of High Melt Strength Poly（Ethylene Terephthalate）

Polyethylene terephthalate (PET) foams have many excellent properties, such as good mechanical properties, high temperature resistance, good chemical resistance, recyclability etc, which makes them find a broad applications. Conventional PET with linear molecular structure is not suitable for extrusion foaming process due to its poor melt strength. In this work, the melt strength and viscoelasticity of four types of PET was analyzed through characterization of shear rheology and extensional rheology of PETs, including the dynamic complex viscosity, storage and loss modulus, relaxation spectrum, and extensional viscosity. It was shown that did only PET4, prepared through in situ modification combined with solid-state condensation polymerization, succeed in the extensional characterization and was observed the phenomenon of strain hardening (SH), which was supposed to stabilize process of cell growth and prevent cell coalescence.PET4was used for extrusion foaming with supercritical CO2. It was shown that a suitable die temperature was favorable for preparing low density PET foams, which had the maximum expansion ratio of9.37, and well-distribution pores with average diameter of500μm; Both the foaming pressure and the expansion ratio of PET foams increased with increasing screw rotation speed, and the pore diameter could decrease to be as low as450μm; In the gas input range, the expansion ratio of PET foams increased linearly with increasing the gas input. When the gas input was higher than5ml/min, the expansion ratio reached10and then leveled off, but the cell size decreased to be400μm and cell density increased. Adding0.1wt%Nano SiCO2dramatically deduced the average cell size and increased the cell density significantly so that the PET foams were prepared with expansion ratio of9.45, average cell size of265μm and cell density of4.58X105cells/cm3.