关键词:  , 废旧聚酯, 醇解, 酯交换, 再生DMT, 减压升华

Abstract: Polyester fibers are widely used in textiles and apparel, automotive interiors, civil engineering and other fields because of their excellent mechanical properties, chemical stability and processability. As the application fields of polyester fibers continue to expand and production capacity continues to increase, the cumulative amount of waste polyester textiles is also growing steadily. A large amount of waste polyester textiles are landfilled or incinerated, causing serious waste of resources and environmental pollution. The glycolysis method, which can depolymerize waste polyester into bis(hydroxyethyl) terephthalate (BHET), has mild reaction conditions, good safety and high product yield, but its traditional depolymerization catalysts are based on heavy metal salts such as zinc, manganese, cobalt, antimony and other heavy metal salts. Although their catalytic effects are significant, they can easily cause heavy metal pollution.
Furthermore, waste polyester textiles come from a wide range of sources and often contain impurities such as cotton, oil stains and dyes. Moreover, the product BHET is prone to polycondensation under high temperature conditions, makeing it extremely difficult to remove impurities and decolorize, thus limiting this method to the treatment of pure bottle flakes and films and hindering its application in the recycling of waste polyester textiles. Utilizing the inherent advantages of the depolymerization system of the glycolysis method, the waste polyester is depolymerized to produce glycolysis products, which can then be esterified with methanol to produce dimethyl terephthalate (DMT), one of the most promising process routes at present. However, the traditional strong alkaline transesterification catalyst NaOH is easy to cause equipment corrosion, and the conventional recrystallization method of purification is time-consuming and energy-intensive, and consumes a large amount of methanol consumption. In this way, the quality of DMT is difficult to reach high purity grade.
Based on the above, glycolysis products of waste polyester textiles were prepared by using K2CO3 as catalyst, and then K2CO3 was used as catalyst for methanol transesterification to transform the glycolysis products into recycled DMT. The recycled DMT containing impurities was purified by decompression sublimation method, and the specific components of the glycolysis products and transesterification products were analyzed in depth. The results indicated that the main component of the glycolysis product was BHET, with trace amounts of BHET oligomer and monohydroxyethyl terephthalate. After optimization, the optimal reaction parameters of the transesterification process were as follows: the reaction temperature was 70 ℃, the reaction time was 118 min, the mass ratio of methanol to the glycolysis products was 2.13:1, and the addition ratio of K2CO3 was 0.94% of the mass of the glycolysis products, the actual yield of recycled DMT under this condition was 88.50%. After three times of purification by decompression sublimation method, impurities such as 2-hydroxyethyl methyl terephthalate, monomethyl terephthalate and dimethyl isophthalate were effectively removed from recycled DMT, and the purity of recycled DMT was as high as 99.89%. The results provide useful reference for the selection of catalysts, the regulation of transesterification reaction parameters and the purification of products in the recovery of waste polyester textiles based on the glycolysis-transesterification method.

Key words: waste polyester, glycolysis, transesterification, recycled DMT, decompression sublimation