Ti-Si composite glycol salts: depolymerization and repolymerization studies of PET

In this study, a Ti-Si-ethylene glycol salt (Ti/Si-EG) was synthesized and used as a catalyst for the depolymerization of PET-ethylene glycol to produce bis(hydroxyethyl)terephthalate (BHET), and the optimum conditions for the depolymerization were determined by response surface analysis: w(EG) : w(PET) was 4 : 1, the catalyst mass dosage was 0.56% (in terms of the mass of PET), the depolymerization temperature was 203 °C, the depolymerization time was 3.8 h, and the highest yield of the product (BHET) was 90.10%. Furthermore, the depolymerization product BHET was used as the raw material and Ti/Si-EG was used as the catalyst for the polycondensation reaction to synthesize PET, and the amount of the catalyst added was 0.015% by mass (in terms of BHET). The performance of the synthesized PET was tested to be the same as that of PET synthesized by the PTA method under the same conditions, thus achieving the resourceful treatment and high-value recycling of waste PET. The introduction of Si in the catalyst reduced the catalytic activity of Ti and avoided the problem of yellowing of polyester products, and the use of the glycol salt as a catalyst in the depolymerization and polycondensation reaction process did not introduce impurity groups.