Removal of Oligomers from Polyethylene Terephthalate Resins by Supercritical Carbon Dioxide Extraction for Improving Properties of PET Films

摘要

PET materials contain small concentrations of monomers and low molecular-weight oligomers formed during the condensation polymerization of ethylene glycol and terephthalic acid as byproducts. All those low molecular weight oligomers can migrate to the surface if the film is treated at high temperatures for tens of minutes. The presence of these oligomers on the surface can interfere with the manufacturing process, so it is important to reduce the concentration of oligomers for improving properties of PET films. In this study, we first confirmed that the principal component of the crystalline deposits on the film surface after the thermal treatment was cyclic trimer. To remove this component from PET resins, extraction experiments of PET resins with supercritical carbon dioxide (sc-CO_2) were carried out using a semi-batch apparatus under various temperatures and extraction times. As a result, it was found that the cyclic trimer in the PET residue was partially removed by this technique for long processing times (2 - 8 h). The decrease in cyclic trimer content in the PET resins was approximately 10% of that in untreated PET resins even if the crystallinity of PET resins was changed. This finding indicates that the production of low-oligomer-content PET resins was realized regardless of the crystallinity of starting materials. It was also found that this technique may be applicable to the polyester film industry.