High-efficiency acetaldehyde removal during solid-state polycondensation of poly(ethylene terephthalate) assisted by supercritical carbon dioxide

The concentration of acetaldehyde (AA) is the main quality index of poly(ethylene terephthalate) (PET) used in food and drink packaging. A new method for AA removal has been developed by using supercritical carbon dioxide (scCO2) during the solid-state polycondensation of PET. The influence factors of AA removal including the temperature, pressure, reaction time and the size of pre-polymer particles are systematically studied in this work. The results indicate that it is a highly efficient way to obtain high molecular weight PET with relative low concentration of AA. Correspondingly, the polymerization degree of PET could increase from 27.9 to 85.6 while the concentration of AA reduces from 0.229 × 10−6 to 0.055 × 10−6 under the optimal operation conditions of 230 °C, 8 MPa and size of 0.30-0.45 mm. Thermodynamic performance tests show the increasing extent of PET crystallinity due to the fact that the plasticization of scCO2 is not obvious with extended reaction time, therefore the increasing crystallinity has no significant influence on AA removal. SEM observations reveal that the effects of scCO2-induced plasticization and swelling on PET increase significantly with the decrease of prepolymer size, and the surface of PET becomes more loose and porous in favor of the AA removal.