Abstract:

Objective With the rapid growth of textile consumption, its effective recycling has become an urgent challenge. Polyethylene terephthalate (PET) is a major commercial fabric material, with significant annual production and consumption. Due to production requirements, a variety of dyes and auxiliaries are usually added to PET fabrics, and these additives cause degradation of product quality in the chemical recycling process, which is not conducive to achieving high-value recycling and closed-loop recycling. Therefore, it is necessary to pre-decolorize the fabrics so as to reduce the burden for the subsequent purification of bis(hydroxyethyl terephthalate) (BHET). In this study, it was proposed to decolorize PET fabrics using solvent extraction method, and the influence of decolorization on PET glycolysis performance were further examined.

Method The PET fabrics samples were cut into small pieces of approximately 2 cm×2 cm beforehand, and the PET pieces were washed and dried. Then, a certain amount of PET samples was added into solvent in a three-neck flask, which was connected with a condenser. The flask was placed in an oil bath to maintain the required temperature. After decolorization, the fabrics pieces were taken out and washed, dried in an oven at 60 ℃. The L^*,a^*,b^* values before and after decolorization were determined by chromameter, and then the decolorization ratio was calculated. The gravimetric method was adopted to calculate the BHET recovery rate of PET glycolysis. The purity of the recovered BHET was determined by high-performance liquid chromatography.

Results Solvent screening experiments among different types of solvents (i.e, single organic solvent, mixed solvent, and deep eutectic solvent (DES)) were conducted based on the decolorization ratio and PET textile mass loss for two single-color PET fabrics. Among them, DES was found not suitable for use as a decolonization solvent because of its high viscosity. Considering the general decolorization effect of solvents on PET fabrics, acetic acid and chlorobenzene, which are more effective in decolorization and easy to recycle and use, were finally selected as the optimal solvents. Then, the operating parameters (i.e., operating temperature, solid-liquid mass ratio and decolorization time) optimization were carried out through single-factor experiments for acetic acid and chlorobenzene decolorization, and the optimal operating condition for acetic acid was identified to be temperature 130 ℃, solid-liquid ratio 1:30, and treatment time 20 min for acetic acid. For chlorobenzene decolorization, it was evident that the concerned parameters had little effect on the decolorization performance. In order to test the universality and suitability of the two types of solvents, nine PET fabrics with different colors were decolorized using acetic acid and chlorobenzene under the optimal conditions, and the optimal decolorization ratios were all higher than 80%, reaching 82.9% and 88.4%, respectively. In particular, the general applicability of chlorobenzene decolorization was better, with decolorization ratios higher than 75% for most samples. Decolorization with acetic acid and chlorobenzene did not affect glycolysis and even contributed to the increase in BHET recovery rate. Especially, after decolorization with chlorobenzene, a general increase of 5%-12% was achieved in BHET recovery rate. In addition, the BHETs recovered through the decolorized fabrics were still of high quality, with purity greater than 90% for all.

Conclusion Different types of solvents (i.e, single organic solvent, mixed solvent, and deep eutectic solvent) were used for PET fabrics decolorization, and acetic acid and chlorobenzene were chosen as the optimal solvents. The selected solvents can be used for PET fabrics with other colors, and the optimal decolorization ratio for the nine fabrics tested was more than 80%. Moreover, decolorization did not affect the glycolysis reaction, ensuring high recovery and purity of BHET. Removal part of the dye from textiles by extraction before glycolysis can greatly reduce the burden of subsequent purification of BHET, which is more economical and has greater potential for industrialization.

Key words: waste fabric, polyester fabric, solvent extraction, decolorization, acetic acid, chlorobenzene, glycolysis, recycling