Abstract: Since its invention in 1948, polyester has been widely used in different areas, including fibers, films, polyesters bottle and so on, because of its excellent fibre forming property, thermal stability and mechanical properties. The total production of polyester fibers for 2020 in China exceeded 60 million tons. However, it's hard to ignore the recycling of waste polyester with the development of the industry. For example, only on the production line will produce about 2% waste fibers, which has caused a great burden on enterprises and the environment. The molecular weight of PET can be increased, and its utilization value can be improved by polycondensation. In this study, melt polycondensation was used to increase the viscosity of waste PET fibers, and the characteristics were investigated in order to provide support for the recovery of waste PET fibers.
The polycondenstion reactions were conducted at different temperature and time conditions, and the influence on molecular weight changes were investigated by testing the characteristic viscosity. The molecular weight and distribution were tested by using APC-MALLS-RID, so as to show the change of molecular weight in the process of viscosity increasing. APC-PDA-RID was used to calculate the absolute content of oligomers and determine the composition of different cyclic oligomers. The thermal properties were tested by DSC to analyze the influence of viscosity increasing behavior on thermal properties. It was found that the waste PET powder, coarse cut fibers and virgin PET powder were melted and thickened at different temperatures and times. The results showed that at 270~280 ℃, the intrinsic viscosity increases with reaction time, and can increase to more than 1.0 dL/g within 50 min. The rate of melt polycondensation is obviously accelerated after the temperature increases, but when the temperature is higher than 270 ℃, the thermal degradation reaction will be promoted, causing the viscosity of the polyester to decline instead of rising. Therefore, 270 ℃ is an appropriate reaction temperature. The viscosity increasing time at 270 ℃ is positively correlated with the molecular weight, and the molecular weight distribution becomes wider after viscosity increasing. Cyclic trimers are the most abundant component of oligomers. The content of oligomers decreases with the increase of the reaction time. The content of oligomers decreases after reaction at 270 ℃ for 40 min. The content of oligomers in the waste PET powder is always higher than that in the virgin PET under the same conditions due to the intrinsic difference. The molecular weight and chain of the waste PET powder increase. The entanglement of the polymer chains hinders their mobility, leading to a reduction in chain regularity and a decrease in crystallization temperatures. The presence of small molecules, such as diethylene glycol generated through side reactions, exerts an influence on the melting point. In this study, characteristics of melt polycondensation and viscosification of waste polyester fibers were manifested from various aspects and compared with the virgin PET. It is found that the waste fibers have better viscosification effect, which id of great significance for recycling. 

Key words: waste polyester fibers, melt polycondensation, recycling, oligomer, advanced polymer chromatography

摘要： 为了提高废旧聚酯的循环再利用价值，以涤纶废丝为原料进行了熔融增黏，考察并分析了不同温度和时间条件下特性黏度、相对分子质量及其分布、低聚物含量和热性能的变化。结果表明：270 ℃下，涤纶废丝的特性黏度随反应时间的延长而增大，但当温度高于270 ℃时，会促进热降解反应，使聚酯黏度不升反降；270 ℃下增黏时间与相对分子质量呈正相关关系，且增黏后大分子链所占比例减小，分子量分布变宽；随着增黏反应时间的延长，低聚物总含量减小；相对分子质量的增大及副反应中小分子的生成是导致结晶温度与熔点降低的主要原因。涤纶废丝粉末可在270 ℃下50 min后增黏到1.09 dL/g，数均相对分子质量达到45100 g/mol，对其熔融增黏反应的探究可为涤纶废丝的绿色高价值回收及应用提供参考。

关键词: 涤纶废丝, 熔融增黏, 循环利用, 低聚物, 超高效聚合物色谱