关键词: 钛系催化剂, 聚对苯二甲酸乙二醇酯, 催化性能, 缩聚反应

Abstract: Titanium-based catalysts have the advantages of green and high efficiency, and have broad prospects in replacing antimony-based catalysts. To investigate the stability and catalytic properties of a self-made titanium-phosphorus catalyst during the polycondensation of polyethylene terephthalate (PET), the structure of the catalyst was tested with ultraviolet visible spectrophotometer (UV-Vis) and Fourier transform infrared spectroscopy (FT-IR). The esterification products of purified terephthalic acid (PTA) and ethylene glycol (EG) were used for polycondensation to verify the hydrolysis resistance of the catalyst, and to make the catalyst more evenly dispersed and the reaction situation more suitable for actual production. It is also possible to achieve a longer period of polycondensation to prepare PET with high intrinsic viscosity. In addition, the molecular structure and properties of PET synthesized over titanium-phosphorus catalysts were characterized by ultra high performance polymer chromatography, multi angle laser light scattering instrument, differential refractometer (APC-MALLS-RID), differential scanning calorimetry (DSC), and thermogravimetric analyzer (TG). 
We made a comparison between the titanium-phosphorus catalyst and the antimony-based catalyst, and added a catalyst before esterification. The addition amount of titanium-based catalyst was 5 ppm, and the addition amount of antimony-based catalyst (antimony trioxide) was 180 ppm. The prepolymer was placed in a multifunctional polycondensation reactor, and PET was synthesized through polycondensation at a temperature of 270 ℃ and a vacuum of 20–60 Pa. The results show that the titanium mass fraction of the self-made titanium-phosphorus catalyst was 17.10%. In the FT-IR spectrum, there are symmetric and antisymmetric stretching vibrations of the ester group (C=O) at positions 1,567 cm-1 and 1,438 cm-1, and the stretching vibration peak of the hydroxyl group (−OH) appears near 3,424 cm-1, indicating a strong interaction between Ti ions and ligands in the catalyst. The above indicates that Ti ions are well coordinated and the catalyst has been successfully prepared. After 103 min of esterification, the titanium-phosphorus catalyst catalyzes the polycondensation of the prepolymer for 180 min to obtain PET with an intrinsic viscosity of 0.907 dL/g and a weight average molecular weight (Mw) of 57,240 g/mol. Its catalytic activity is still higher than that of antimony-based catalysts, and exhibits good catalytic stability, which also indicates that the catalyst has good ability to prepare high viscosity PET. The polydispersity index of titanium-phosphorus PET is higher than that of antimony-based PET at the same polycondensation time due to the fact that titanium-phosphorus catalysts lead to more side reactions, fewer additions, and fewer active sites compared to antimony-based catalysts. After 120 minutes of reactive polycondensation of esterification products catalyzed by the titanium-phosphorus catalyst, the rate of broadening the molecular weight distribution of the products increases, while the degree of yellowing of PET increases. Therefore, the polycondensation time should be controlled within 120 minutes as much as possible in the process. 
In terms of thermal properties, the melting peak temperature of titanium-phosphorus PET and antimony-based PET increases to the highest value at 90 min of polycondensation, with titanium-phosphorus PET being at 250.9℃ and antimony-based PET being at 249.8 ℃. And then, the melting peak temperature  gradually decreases. After 180 min of polycondensation, the temperature of 5% thermal weight loss for titanium-phosphorus PET and antimony-based PET is 413.3℃ and 412.3 ℃, respectively. The thermal stability of the titanium-phosphorus catalyst for synthesizing PET is better than that of antimony-based PET.

Key words: titanium catalyst, PET, catalytic properties, polycondensation reaction