Process Study On Acetaldehyde Removal In Solid-state Polycondensation Of Poly (Ethylene Terephthalate) Based On Supercritical Carbon Dioxide

Acetaldehyde(AA)concentration is one of the key quality index of poly(ethylene terephthalate)(PET)slices.It usually takes a long time in the conventional solid-phase polycondensation process of polyester to meet the requirements of bottle-grade polyester in which the value of AA is less than 1 ppm.Based on periodical renewal supercritical carbon dioxide(scC02),the effects of temperature 200-230 ?,pressure 8-12 MPa,reaction time 1-10 h and the size of pre-polymer particles 0.30-3.00 mm on AA removal in the solid-state polycondensation of PET were investigated systematically in this work.The polymerization degree,AA content,micromorphology,and thermal properties of AA-removal PET products were characterized using viscosity analysis,HS-GC,SEM and DSC,respectively.The results indicated that it is a highly efficient strategy to obtain PET products with degree of polymerization close to 150 and concentration of AA less than 1 ppm in a short time using the AA removal in the PET solid-state polycondensation process assisted by the periodical renewal supercritical carbon dioxide.Under the condition of a periodical renewal time of 1 h,the optimal operation conditions of this process is the temperature 230 ?,pressure 8 MPa,particle size 0.30-0.45 mm.SEM observations revealed 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.Thermodynamic performance tests showed that the PET crystallinity increases with extended reaction time due to the plasticization of scCO2 which induces the PET crystallinity.In summary,the use of scCO2 can effectively enhance the AA removal in the PET solid-state polycondensation process.