Solid state foaming of poly(ethylene terephthalate) based on periodical CO2-renewing sorption process

The periodical CO2-renewing sorption process was applied in the solid state foaming of poly(ethylene terephthalate) (PET), in order to decrease the sorption time, increase the concentration of CO2 in PET matrix, and ultimately increase the expansion ratio of PET foams. The cell nucleation and growth were initiated after every depressurization in the sorption process, and the concentration of the dissolved CO2 was reduced by the consumption of cells nucleation and cells formation, which inhibited CO2-induced crystallization. As a result, the CO2 concentration of 150-160 mg/g PET was obtained, which was much higher than the reported CO2 solubility in PET matrix (∼110 mg/g PET). With different sorption temperatures and CO2-renewing periods, PET foams with the cell size between 5 and 22 μm, the cell density between 2.42 × 108 and 2.93 × 109 cells/cm3, and the expansion ratio between 3 and 6 times were controllably prepared. Only 1 h was required to enhance the crystallinity of PET foams to about 30% by the means of annealing treatment at 110-130 °C. And the cell morphology was still well kept during the annealing process.