Integrated process of supercritical CO2-assisted melt polycondensation modification and foaming of poly(ethylene terephthalate)

An integrated process of melt polycondensation modification and foaming of poly(ethylene terephthalate) (PET) was performed in a high pressure vessel assisted by supercritical carbon dioxide (scCO2). ScCO2 was firstly employed to sweep PET melt, i.e., high pressure CO2 continuously flows through the vessel at a fixed flow rate to remove small molecules for higher molecular weight PET, then this modified PET melt was directly foamed through a rapid depressurization process using scCO2 as blowing agent. In this integrated process, PET with high melt strength after polycondensation modification could be foamed directly in molten state. Therefore, re-molten process of solid modified PET pellets was canceled to avoid its degradation and CO2 saturation time could be saved in foaming process, thus processing time could be shortened and energy efficiency could be improved. The influences of scCO2 sweeping treatment time, pressure and flow rate on properties of the modified PETs and cell morphologies of the foamed PETs were investigated respectively. The results showed that CO2 sweeping treatment could effectively enhance PET melt polycondensation modification process, which was superior to that of N2 treatment. PET foams with average cell diameter of 32–62 μm and cell density of 1 × 107 to 4 × 107 cells/cm3 have been obtained in the integrated process. Compared with the process of only foaming modified PET by scCO2 or performing scCO2 assisted modified PET further melt polycondensation process and scCO2 foaming process separately, this integrated process produced better cell morphology.

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► An integrated process of PET melt polycondensation and foaming is performed by CO2.
► The integrated process can reduce PET degradation and shorten processing time.
► The melt strength of the modified PET can be improved with CO2 sweeping.
► Better cell morphology can be obtained in the integrated process.