Journal of Textile Research ›› 2025, Vol. 46 ›› Issue (06): 1-7.doi: 10.13475/j.fzxb.20241201301

• Column of Youth Scientists′Salon on New Fiber Materials and Green Textile Development •     Next Articles

Transformation rule of polyester glycolysis products in methanol transesterification

XU Wenhao1, CHEN Lin2, XU Shimei2, WANG Xiuli2, WANG Yuzhong2()   

  1. 1. School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610046, China
    2. College of Chemistry, Sichuan University, Chengdu, Sichuan 610046, China
  • Received:2024-12-06 Revised:2025-03-12 Online:2025-06-15 Published:2025-07-02
  • Contact: WANG Yuzhong E-mail:yzwang@scu.edu.cn

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Abstract:

Objective Polyester is the most widely used fiber material in waste textiles, and its efficient recycling is the key to solving the problem of waste textiles. At present, the combined glycolysis-methanol transesterification process is widely used in industry. In this process, the methanol transesterification reaction of glycolysis products into dimethyl terephthalate (DMT)is the key step. Therefore, the transformation rule of glycolysis products in methanol transesterification can provide theoretical guidance for the efficient chemical recycling of waste polyester.

Method In this paper, the chemical recycling of polyester was studied according to the industrial combined glycolysis-methanol transesterification process. The depolymerization products of polyester textile during glycolysis and methanol transesterification were qualitatively and quantitatively studied by high performance liquid chromatography-mass spectrometry (HPLC-MS).

Results In the first step of glycolysis, the glycolysis products of polyester are mainly bis(2-hydroxyethyl) terephthalate (BHET) monomer, dimer, trimer, tetramer, and pentamer. The oligomer content can be controlled by reducing the ethylene glycol ratio and shortening the depolymerization time. Firstly, the glycolysis solution with a mass ratio of polyethylene terephthalate (PET) to ethylene glycol (EG) of 1∶1 and depolymerized at 190 ℃ for 1 h was selected for methanol transesterification. HPLC-MS showed that BHET monomer, dimer, and trimer could be completely converted into DMT within 3 h, while tetramer and pentamer could not be completely converted into DMT due to their large molecular weight and low reaction activity. The intermediate 2-hydroxyethyl methyl terephthalate is generated during the conversion of BHET and oligomers to DMT. The conversion rate of BHET and oligomers to DMT was very fast (within 5 min), and then remain at a very low content. The BHET in the PET glycolysis products was separated by water washing, and the conversion process of BHET oligomers to DMT was studied in detail. HPLC-MS confirmed that the separated oligomer products were mainly BHET dimer, tetramer, pentamer and trace trimer. The separated BHET oligomers were subjected to methanol transesterification reaction. It was found that the BHET dimer and trimer completely disappeared after methanol transesterification, which again showed that dimer and trimer were completely converted into DMT. However, BHET tetramer and pentamer still exist, proving that it was difficult to completely convert them into DMT. This is because BHET tetramer and pentamer have larger molecular weights and lower reactivity. They have fewer effective collisions with methanol in the transesterification reaction and cannot be completely converted into DMT. Since DMT is the final recycled monomer of polyester in this combined process, the depolymerization of polyester into BHET monomer, dimer, and trimer during glycolysis is the ″effective″ depolymerization.

Conclusion Aiming at the glycolysis-methanol transesterification combined process used in industry, the methanol transesterification process of the glycolysis depolymerization products of polyester was researched and revealed. In the first step of glycolysis, the glycolysis products of polyester are mainly BHET monomer, dimer, trimer, tetramer, and pentamer. In the second step of methanol transesterification, BHET monomer, dimer, and trimer can be completely converted into DMT at 65 ℃ for 3 h. However, BHET tetramers and pentamers exhibit low reactivity due to their large molecular weight and are difficult to be completely converted into DMT. The depolymerization of polyester into BHET monomer, dimer, and trimer during glycolysis represents the ″effective″ depolymerization. Therefore, the content of BHET monomer and its dimer and trimer can be increased by increasing the proportion of ethylene glycol and prolonging the depolymerization time, thereby improving the final DMT yield and achieving efficient chemical recycling of polyester fibers. The transformation rules of BHET and its oligomers to DMT in the glycolysis products of polyester were determined, which provides theoretical guidance for the chemical recycling of polyester textiles in glycolysis-methanol transesterification combined process.

Key words: waste textile, recycling, polyester, glycolysis, methanol transesterification, oligomer, transformation rule

CLC Number: 

  • TS102.9

Fig.1

Combined glycolysis-methanol transesterification process for polyester chemical recycling"

Fig.2

HPLC chromatogram of polyester glycolysis solution (m(PET)∶m(EG)=1∶1.5)"

Fig.3

Mass spectrum of polyester glycolysis solution. (a) Mass spectrum of BHET; (b) Mass spectrum of BHET dimer; (c) Mass spectrum of BHET trimer"

Fig.4

HPLC chromatogram of polyester glycolysis solution (m(PET)∶m(EG)=1∶1)"

Tab.1

Composition analysis of methanol transesterification at different reaction times"

反应
时间/
min		 相对含量/%
BHET 二
聚体		 三
聚体		 四
聚体		 五
聚体		 DMT MHET
0 70.68 17.16 8.06 1.82 0.90 0 0
1 2.85 0 3.21 0.39 0.70 68.19 13.36
5 0.12 0 0 0.19 0.26 84.98 14.45
10 0.29 0 0 0.18 0.55 84.00 11.60
30 0.10 0 0 0.17 0.65 80.40 18.40
60 0.41 0 0 0.10 0.24 82.84 12.78
90 0.09 0 0 0.14 0.30 90.67 8.80

Fig.5

Separation process of polyester glycolysis products"

Fig.6

HPLC chromatogram of filter residue from glycolysis products of polyester (a) and methanol transesterification product of residue in glycolysis product of polyester (b)"

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