关键词: 再生PET, 化学法再生, 固相缩聚, 特性黏度, 反应条件, 端甲基

Abstract: High molecular weight (high intrinsic viscosity) polyethylene terephthalate (PET) has excellent mechanical properties, and is an important raw material for the production of high-strength industrial silk, engineering plastics, reinforcement materials and other products. At present, the molecular weight of PET is mainly improved by solid-state polycondensation (SSP) process, which is: the low-molecular weight polymer is heated to above the glass transition temperature and below the melting point, and glycol, water molecules, acealdehyde and other by-products are removed by vacuuming or injecting nitrogen, so that it continues to carry out chain growth reaction in the amorphous region. However, as the production of PET products continues to increase, a large amount of oil resources are consumed. To solve this problem, the researchers carried out research on the regeneration methods of PET, among which, the chemical regeneration of PET(CRPET) based on glycol alcohololysis and methanol transesterification process can meet the requirements of high-quality regeneration. Nowadays, CRPET has been industrialized in the field of flame-retardant modification, cationic dyeing and other civil grade fibers, but there are few reports on the preparation of CRPET with high molecular weight (high intrinsic viscosity).
In order to achieve the preparation of high molecular weight CRPET, solid-state polycondensation process was used to increase the viscosity of CRPET. Under vacuum conditions, the solid-state polycondensation reaction characteristics of vPET and CRPET were compared. The intrinsic viscosity of CRPET during solid-state polycondensation was investigated by changing the methyl group content, size, reaction atmosphere, vacuum, nitrogen flow rate and pre-crystallization temperature. The results show that under the same reaction conditions, the solid-state polycondensation rate of vPET is higher than that of CRPET. The presence of terminal methyl group can affect the rate of CRPET solid-state polycondensation, but this effect gradually decreases with the increase of temperature. Reducing the particle size of CRPET will increase the growth rate of intrinsic viscosity, but inter-bonding is also more likely to take place, resulting in a slowdown in the growth of intrinsic viscosity at the later stage of the reaction of small-sized CRPET, which is more obvious at high temperature. Under the premise of keeping the reaction temperature and particle size unchanged, the intrinsic viscosity increment of CRPET under vacuum condition is larger than that under nitrogen condition. The solid-state polycondensation reaction is easier to be carried out at higher vacuum or nitrogen flow rate, but when the nitrogen flow rate increases to a larger value, the influence on the intrinsic viscosity gradually decreases. The increase of pre-crystallization temperature will reduce the surface adhesion of CRPET, thus promoting the positive solid-state polycondensation reaction, but too high crystallization temperature will hinder the diffusion and escape of small molecules in CRPET, resulting in a slow down of the growth of intrinsic viscosity. Solid-state polycondensation can improve the crystallization properties of CRPET without affecting its thermal properties.
According to the analysis results of parameters and properties of CRPET before and after solid-state polycondensation, it can be found that the intrinsic viscosity of CRPET can be increased by solid-state polycondensation, and the material property requirements of industrial fibers can be reached (the intrinsic viscosity is≥1.05 dL/g). The research results provide a useful reference for the industrial production of CRPET with high viscosity.

Key words: regenerated PET, chemical regeneration, solid-state polycondensation, intrinsic viscosity, reaction condition, terminal methyl group