A method for selectively and efficiently depolymerizing and recycling PET from waste blended textiles

Abstract

This invention pertains to the field of chemical recycling of polyester in textiles and discloses a method for selectively and efficiently depolymerizing and recycling PET from waste blended textiles. The method includes: (1) melting and quenching the waste blended textiles, followed by pulverizing and grinding to obtain waste blended textile powder; (2) subjecting the waste blended textile powder to a depolymerization reaction under methanol and guanidine catalyst conditions to obtain a crude product; and (3) separating and purifying the crude product to obtain polymer-grade dimethyl terephthalate (DMT) refined product. This method enables efficient and selective methanol depolymerization of PET in waste blended textiles under mild conditions to depolymerize the PET and obtain DMT monomers.

Description

Technical Field

[0001] This invention relates to the field of chemical recycling of polyester in textiles, and more particularly to a method for selectively and efficiently depolymerizing and recycling PET from waste blended textiles. Background Technology

[0002] Polyethylene terephthalate (PET) is the second most consumed plastic after polyolefins. Due to its difficulty in biodegradation, improper disposal of related waste not only wastes fossil resources but also causes environmental pollution. PET is mainly used in packaging materials such as beverage bottles and polyester textile fibers. Beverage bottles can be physically recycled through melt reprocessing, but repeated melting and extrusion reduce molecular weight, degrade performance, and limit the number of cycles (see the paper "State-of-the-art of industrial PET mechanical recycling: technologies, impact of contamination and guidelines for decision-making"). Therefore, this strategy is more suitable for recycling waste bottle flakes into textiles with lower mechanical requirements, which is essentially a form of downgrading recycling. Compared to bottle PET, waste textiles account for approximately 70% of waste PET. Since textiles typically contain different fibers such as PET, cotton, nylon, and spandex, as well as various dyes and additives, and these fibers are physically woven together through blending, pre-separation through manual sorting is impossible, thus hindering their recycling through melt reprocessing. Currently, most waste textiles are still landfilled or incinerated, with a recycling rate of less than 1%, which significantly increases greenhouse gas emissions and wastes non-renewable resources.

[0003] Chemical depolymerization of PET back to monomers is the optimal route for achieving closed-loop recycling. While catalytic / enzymatic hydrolysis, methanololysis, and glycololysis have been reported, they all have limitations hindering their industrialization. More importantly, these methods are typically suitable for relatively pure systems (bottle flakes, factory scraps, single / bicomponent fabrics). However, when extended to multicomponent blended fabrics, dyes, additives, and various fibers generate exceptionally complex mixtures after depolymerization, leading to excessively high costs for monomer separation and purification. This becomes the biggest bottleneck affecting the economic efficiency and practical feasibility of the route. Therefore, achieving efficient and selective depolymerization of PET in multicomponent blended fabric systems, while maintaining a simple product composition, and achieving high yield, high purity (monomer purity ≥99.9% for polymerization grade to meet repolymerization requirements), and low-cost purification of the target depolymerized monomers, has become a key technical issue restricting the economic and engineering feasibility of chemical recycling of waste blended fabrics. Simultaneously, to compete effectively with petroleum-based monomers, the process must also consider low energy consumption, low solvent consumption, and low environmental impact to meet the requirements of large-scale depolymerization and recycling.

[0004] Previous studies have attempted to break down blended fabrics back to bis(2-hydroxyethyl) terephthalate (BHET) monomers via ethylene glycol hydrolysis; however, the monomer purification process is extremely complex (paper: 混合纺织品废料的化学回收 纺织废料This is mainly because spandex also undergoes depolymerization under high-temperature catalytic conditions, significantly complicating the product composition and greatly increasing the difficulty and cost of monomer separation and purification. Furthermore, BHET is prone to repolymerization at high temperatures, making conventional distillation impossible for purification (Patent: US 7211193B2). To solve the monomer purification problem, current methods involve converting the degraded BHET into dimethyl terephthalate (DMT) before purification by distillation, which significantly increases operating costs (Patent: US7462649B2). A more optimized route is to directly methanolysis of PET into DMT, but the reaction conditions and depolymerization efficiency of methanolysis remain a difficult contradiction to reconcile. While supercritical methanolysis is highly efficient, its conditions are extremely demanding, significantly increasing the complexity of the process and equipment; simultaneously, other polymer fibers also depolymerize under these conditions, reducing selectivity, increasing product complexity, and raising separation and purification costs, leading to higher costs and safety risks during large-scale scaling. Methanololysis under mild conditions typically results in low depolymerization efficiency due to the use of heterogeneous catalysts and the hindrance of mass transfer by PET crystal regions (paper: Low-energy catalytic methanolysis of poly(ethyleneterephthalate)). Although some organic catalysts (e.g., 1,5,7-triazabicyclo[4.4.0]decen-5-ene, TBD) show great potential for PET depolymerization under mild conditions, these highly active catalysts have poor stability and can simultaneously catalyze the depolymerization of other polymer fibers (e.g., spandex), leading to decreased depolymerization selectivity, more complex depolymerization product composition, and increased subsequent separation and purification costs. Therefore, unlike other relatively simple waste systems, the chemical recycling of waste blended textiles places stringent demands on both high catalytic activity and high catalytic selectivity, which existing methods generally struggle to achieve simultaneously.

[0005] Although heating and quenching to destroy the crystalline phase structure is a common pretreatment method in the PET depolymerization process, existing technology uses it in a relatively pure enzymatic hydrolysis system (patent: CN 107236147 B) in order to improve depolymerization efficiency. However, it has not yet been applied in the depolymerization and recycling of waste blended textiles. Summary of the Invention

[0006] The purpose of this invention is to provide a method for selectively and efficiently depolymerizing and recycling PET from waste blended textiles, which enables efficient and selective methanol depolymerization of PET in waste blended textiles under mild conditions to obtain DMT monomers.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] A method for selectively and efficiently depolymerizing and recycling PET from waste blended textiles includes:

[0009] (1) Waste blended fabric is melted, quenched, pulverized and ground to obtain waste blended fabric powder;

[0010] (2) The waste blended fabric powder was subjected to a depolymerization reaction under the conditions of methanol and guanidine catalyst to obtain the crude product;

[0011] (3) The crude product is separated and purified to obtain polymer-grade dimethyl terephthalate (DMT) refined product.

[0012] This invention develops a strategy for the efficient and selective recycling of PET from waste blended fabrics under mild conditions. The resulting depolymerized monomers achieve polymerization-grade purity and can be directly used for repolymerization and reuse. Unlike existing strategies that rely on highly active catalysts, this invention employs an apparently low-activity catalyst to reduce the depolymerization tendency of non-PET components, suppressing side reactions at the source and simplifying product composition. Simultaneously, a heating-quenching phase regulation process is introduced for the PET component. By disrupting its crystal structure, the accessibility of ester bond sites and mass transfer efficiency are improved. This selectively enhances the catalyst's catalytic activity for PET methanololysis without increasing the depolymerization effect on other non-PET fibers.

[0013] It should be noted that, in addition to PET, the waste blended textiles contain one or more of the following: spandex, cotton, nylon, acrylic (polyacrylonitrile), vinylon, flax, wool, silk, and other man-made or natural fibers; as well as one or more of the following: commercial dyes, finishing agents, fire retardants, antibacterial agents, water-resistant agents, and other insoluble or soluble additives. Furthermore, the other components in the waste blended textiles besides PET do not affect the quenching, depolymerization, and subsequent purification processes.

[0014] The quenching in step (1) is carried out in a medium of -196~70°C, and the resulting PET has a crystallinity X. c The content is ≤15%, and no annealing is performed. Preferably, the quenching temperature is 0~25 °C.

[0015] It should be noted that, unlike PET flakes or granules, PET fibers have a higher degree of crystallinity due to stretching and orientation during production. Therefore, the heating-quenching process in step (1) is a necessary condition for achieving efficient and selective methanol hydrolysis of PET in waste blended fabrics under mild conditions. This process does not change the chemical structure of PET, but it can promote the mass transfer efficiency of catalyst and methanol in PET. The crystallinity of PET after quenching is usually ≤15%. In contrast, fibrous or powdered PET that has not been quenched cannot achieve similar depolymerization selectivity and efficiency under the aforementioned conditions.

[0016] In step (1), the melting temperature is 220~300 °C, which is adjusted according to the molecular weight of PET in the waste fabric. Preferably, the heating temperature in step (1) is 250-280 °C.

[0017] The particle size of the waste blended fabric powder in step (1) is 1 μm to 1000 μm. Preferably, the particle size of the powder is ≤450 μm.

[0018] In the depolymerization process described in step (2), the amount of methanol used is 3~7 mL·g based on the mass of the powder. -1 The amount of catalyst used is 1~20 wt% of the PET mass.

[0019] The depolymerization in step (2) is carried out in a closed system, under methanol autogenous pressure (autogenous pressure of methanol at the reaction temperature in a closed system) and 70~130°C conditions, and the PET depolymerization rate (calculated based on the mass loss before and after the depolymerization process) is ≥99% within 20~120 min.

[0020] The guanidine catalyst mentioned in step (2) is a guanidine and its salt formed with an organic / inorganic acid. The guanidine is selected from one or more of guanidine, dicyandiamide, 1,1-dimethylguanidine, 1,3-dimethylguanidine, 1,3-diphenylguanidine, 1,3-di-o-tolueneguanidine, 1,1,3,3-tetramethylguanidine, 2-tert-butyl-1,1,3,3-tetramethylguanidine, metformin, phenformin, or butylguanidine. The salt formed with the organic / inorganic acid is an onium salt [GH]. + X - X represents an inorganic or organic acid anion. The reaction equation for the depolymerization process is shown in Figure I:

[0021] Reaction formula I.

[0022] Preferably, the amount of methanol used is 4~5 mL·g -1 The catalyst was 1,1,3,3-tetramethylguanidine powder, with a catalyst dosage of 10 wt% of PET. The reaction temperature was 100 °C, the reaction pressure was the autogenous pressure of methanol at that temperature in a closed system, and the reaction time was 60 min.

[0023] It should be noted that, apart from PET, the mass loss of other man-made or natural fibers in waste blended fabrics before and after the depolymerization process is less than 6%, while the mass loss of PET is ≥99%. Therefore, this depolymerization process has obvious selectivity for PET in waste blended fabrics.

[0024] It should be noted that, under the aforementioned conditions, the kinetics of methanololysis are superior to those of ethylene glycololysis. This is mainly because: during ethylene glycololysis, each chain breakage of PET generates two terminal hydroxyl functional groups (reaction formula II), and since this depolymerization reaction is a heterogeneous reaction, under certain catalyst concentration conditions, the terminal hydroxyl groups and the hydroxyl groups in the alcoholysis agent compete with each other. The enhanced interchain transesterification (reaction formula III) inhibits the transesterification reaction between the alcoholysis agent and PET, thereby slowing down the depolymerization kinetics; while during methanololysis, each chain breakage of PET generates only one terminal hydroxyl functional group (reaction formula II), therefore the inhibitory effect of interchain transesterification (reaction formula III) on the transesterification between the alcoholysis agent and PET is weaker than that of ethylene glycololysis.

[0025] Reaction II;

[0026] Reaction III.

[0027] The separation and purification method in step (3) is distillation and repeated recrystallization, specifically: the vacuum distillation temperature of DMT is 120~140 ºC, the vacuum degree is 1~50 mbar, the recrystallization heating temperature of DMT is 80~120 ºC, the pressure is the autogenous pressure of methanol in the closed system at the recrystallization heating temperature, the recrystallization cooling temperature of DMT is -20~25 ºC, and the solvent required for DMT recrystallization is methanol, with a volume of 2~6 mL·g. -1 DMT.

[0028] Preferably, the solid component containing DMT, after being washed with water, is subjected to vacuum distillation at 130 °C and 25 mbar to obtain crude DMT. Methanol is then added to the crude DMT product at a volume of 4 mL·g. -1 The crude DMT product was sealed and heated to 90-110 °C at the autogenous pressure of methanol in the closed system at that temperature. After the solid was completely dissolved, the temperature was lowered to -5-5 °C, filtered, and dried to obtain the refined DMT product.

[0029] It should be noted that the DMT in the depolymerization product can reach the polymerization grade purity (≥99.9%) after distillation and recrystallization (≥1 time), with a single total yield of ≥90%, and its final purity is not affected by the components and contents other than PET in the initial waste blended fabric.

[0030] It should be noted that PET polymerization is essentially a condensation reaction, and the molecular weight of the polymer is extremely sensitive to monomer purity. For example, monomers with 99% and 99.9% purity often yield significantly different molecular weights of PET, leading to a marked difference in the mechanical properties of the resulting PET. Therefore, the purity of the depolymerized and recovered monomers is a key indicator determining the molecular weight and reuse performance of the PET in subsequent repolymerization.

[0031] It should be noted that the methanol solvent and catalyst used in the depolymerization and separation purification process can be separated or regenerated by methods such as vacuum distillation, vacuum sublimation, absorption, condensation, adsorption, membrane separation, and acid-base neutralization, so as to achieve the recycling of solvent and catalyst.

[0032] Preferably, the method includes the following steps:

[0033] (1) The waste blended fabric is coarsely crushed and heated to above the PET melting temperature, and then quenched in a refrigerant;

[0034] (2) The material obtained in step (1) is crushed, ground and dried to obtain the material to be depolymerized;

[0035] (3) The material to be depolymerized obtained in step (2) is mixed with methanol and catalyst and subjected to a depolymerization reaction;

[0036] (4) Neutralize, concentrate, filter, wash with water and dry the mixture obtained in step (3) to obtain the crude product;

[0037] (5) The crude product obtained in step (4) is distilled and repeatedly recrystallized to obtain refined DMT.

[0038] In this invention, heating and quenching, along with a catalyst, are necessary conditions for achieving efficient and selective depolymerization of PET under mild conditions. The obtained DMT has a polymerization grade purity and can be directly used for the repolymerization of polyester polymer products. The process can be carried out continuously: heating and extrusion - water bath quenching - granulation and grinding - drying - continuous reactor - filtration - neutralization - water washing - drying - distillation - recrystallization.

[0039] The DMT monomer obtained by this invention can be repolymerized and used to prepare packaging materials such as clothing and textile fibers or plastic bottles.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] (1) This invention achieves high catalytic selectivity for PET depolymerization in complex mixed systems by destroying the crystalline phase structure. Through the dual-path synergy of the above-mentioned low apparent activity catalytic system design and PET crystalline phase structure regulation, the high activity and high selectivity of PET in complex waste blended systems can be taken into account. The depolymerization products have simple composition and are easy to separate and purify, which reduces the separation and purification cost and improves the purity of the products.

[0042] (2) This invention is applicable to a variety of complex waste blended fabrics, and can selectively depolymerize PET into DMT. The reaction conditions are mild, the reaction efficiency is high, and the equipment requirements are simple.

[0043] (3) The depolymerization product DMT has high purity. DMT monomer with polymer grade purity (≥99.9%) can be obtained by distillation and recrystallization without the need for adsorbent materials such as activated carbon. Moreover, the obtained DMT monomer with polymer grade purity (≥99.9%) can be directly used for repolymerization and reuse, which has higher economic and environmental benefits compared with its petroleum-based production process.

[0044] (4) The solvent and catalyst are easy to recycle, the depolymerization process does not produce toxic and harmful waste, and the cost of DMT is reduced, which has good economic benefits and social value. Attached Figure Description

[0045] Figure 1 This is a flowchart of the depolymerization and recycling of waste blended fabrics in Example 4;

[0046] Figure 2 This is a purity characterization diagram of the DMT monomer recovered in Example 4. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and are not intended to limit it in any way.

[0048] Example 1 (Recycling PET from a mixture of colorless polyester, cotton, spandex, nylon, vinylon and other fibers)

[0049] raw material:

[0050] Colorless polyester fiber, cotton fiber, spandex fiber, nylon fiber, flax fiber, acrylic fiber, wool fiber, silk fiber, Taobao platform; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), sodium hydroxide (NaOH), Sinopharm Chemical Reagent Co., Ltd.; 3Å molecular sieve, Sigma-Aldrich reagent.

[0051] Preprocessing:

[0052] First, 100 g of the mixed fiber (containing 52% polyester) was heated to 270 °C until the PET component melted. The material was then rapidly transferred to room temperature water (25 °C) for quenching. The quenched material was then maintained at its glass transition temperature (T0). g Grind at a temperature of ≈ 70°C or below, and sieve the grinding material. Re-grind materials that do not meet the particle size requirements until all materials have a particle size of 450 μm or below.

[0053] Depolymerization and post-processing:

[0054] First, 100 g of the ground material was added to a 500 mL pressure-resistant bottle, followed by 260 mL of methanol and 5.2 g of TMG. The pressure-resistant bottle was then sealed and heated to 100 °C, and stirred at this temperature for 1 hour. After the reaction was complete, the pressure-resistant bottle was cooled, and 4.5 g of concentrated hydrochloric acid was added to neutralize the TMG. The neutralized depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining the non-PET component. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated precipitated solid was then washed with water, filtered, and dried to obtain a washing liquid and a pale yellow crude dimethyl terephthalate (DMT) product, respectively.

[0055] The washing solution was distilled under reduced pressure at 60 °C and 120 °C to obtain water and ethylene glycol (EG). Then, 27 mL of methanol and 1.9 g of NaOH were added to the distillation residue and stirred overnight at room temperature. After the reaction was complete, the mixture was filtered to obtain sodium chloride solid and filtrate. The filtrate was further fractionally distilled under reduced pressure to obtain methanol and TMG, respectively. Alternatively, the filtrate could be dried using a 3 Å molecular sieve and used directly in the next depolymerization process. Crude DMT product and 210 mL of methanol were added to a pressure-resistant bottle, which was sealed and stirred at 100 °C. After the solid was almost completely dissolved, the pressure-resistant bottle was cooled to -5 °C and its contents were filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain purified DMT product with a yield of 94% and a purity of 99.98%.

[0056] The yield is calculated based on the initial mass of the PET component in the powder. In this example, it is (the actual mass of DMT obtained from depolymerization) / (the theoretical mass of DMT obtained from depolymerization of 52g PET) = 94%.

[0057] Example 2 (Recycling PET from dyed blends of polyester, cotton, spandex, nylon, vinylon, etc.)

[0058] raw material:

[0059] Dyeing (including more than 20 common clothing colors such as red, orange, yellow, green, cyan, blue, and purple, all of which are commercial dyes with unknown specific components) Polyester fiber, cotton fiber, spandex fiber, nylon fiber, linen fiber, acrylic fiber, wool fiber, and silk fiber, Taobao platform; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), and sodium hydroxide (NaOH) from Sinopharm Chemical Reagent Co., Ltd.; 3Å molecular sieve, Sigma-Aldrich reagent.

[0060] Preprocessing:

[0061] First, 100 g of colored blended fibers (containing 50% polyester) were heated to 270 °C until the PET component melted. The material was then rapidly transferred to room temperature water (25 °C) for quenching. The quenched material was then maintained at its glass transition temperature (T0). g Grind at a temperature of ≈70 °C or below, and sieve the grinding material. Re-grind materials that do not meet the particle size requirements until all materials have a particle size of 450 μm or below.

[0062] Depolymerization and post-processing:

[0063] First, 100 g of the grinding material was added to a 500 mL pressure-resistant bottle, followed by 250 mL of methanol and 5.0 g of TMG. The pressure-resistant bottle was then sealed and heated to 100 °C, and stirred at this temperature for 1 hour. After the reaction was complete, the pressure-resistant bottle was cooled, and 4.3 g of concentrated hydrochloric acid was added to neutralize the TMG. The neutralized depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining the non-PET component. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated precipitated solid was then washed with water, filtered, and dried to obtain a washing liquid and a crude dimethyl terephthalate (DMT) product containing various dye molecules.

[0064] The washing solution was distilled under reduced pressure at 60 °C and 120 °C to obtain water and ethylene glycol (EG). Then, 25 mL of methanol and 1.8 g of NaOH were added to the distillation residue, and the mixture was stirred overnight at room temperature. After the reaction was complete, the mixture was filtered to obtain solid sodium chloride and a filtrate. The filtrate was further fractionally distilled under reduced pressure to obtain methanol and TMG, respectively. Alternatively, the filtrate could be dried using a 3 Å molecular sieve and used directly in the next depolymerization process. The crude DMT product was distilled under reduced pressure at 130 °C and 25 mbar to remove the dye and obtain a pale yellow DMT product. The pale yellow DMT product and 200 mL of methanol were added to a pressure-resistant flask, which was sealed and stirred at 105 °C. After the solid was almost completely dissolved, the pressure-resistant flask was cooled to 0 °C and its contents were filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain purified DMT with a yield of 92% and a purity of 99.95%.

[0065] Example 3 (Recycling PET from dyed pure polyester, pure cotton, polyester-cotton, polyester-spandex, and polyester-nylon blended fabrics)

[0066] raw material:

[0067] Commercially available black pure polyester fabric (100%), green pure cotton fabric (100%), white polyester-cotton blended fabric (60% / 40%), gray polyester-ammonia blended fabric (95% / 5%), and blue polyester-nylon blended fabric (29% / 71%) (specific dye compositions unknown), available on Taobao platform; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), and sodium hydroxide (NaOH) from Sinopharm Chemical Reagent Co., Ltd.; 3Å molecular sieve from Sigma-Aldrich reagents.

[0068] Preprocessing:

[0069] First, 100 g of commercially available blended fabric (containing various dyed blends such as pure polyester, polyester-cotton, polyester-spandex, and polyester-nylon, with a total polyester content of 60%) was coarsely broken into strips 2 cm in length and width. The fabric strips were then heated to 270 °C until the PET component melted. The material was then rapidly transferred to room temperature water (25 °C) for quenching. The quenched material was maintained at its glass transition temperature (T0). g Grind at a temperature of ≈ 70 °C or below, and sieve the grinding material. Re-grind materials that do not meet the particle size requirements until all materials have a particle size of 450 μm or below.

[0070] Depolymerization and post-processing:

[0071] First, 100 g of the grinding material was added to a 500 mL pressure-resistant bottle, followed by 300 mL of methanol and 6.0 g of TMG. The pressure-resistant bottle was then sealed and heated to 100 °C, and stirred at this temperature for 1 hour. After the reaction was complete, the pressure-resistant bottle was cooled, and 5.2 g of concentrated hydrochloric acid was added to neutralize the TMG. The neutralized depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining the non-PET component. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated precipitated solid was then washed with water, filtered, and dried to obtain a washing liquid and a crude dimethyl terephthalate (DMT) product containing various dye molecules.

[0072] The washing solution was distilled under reduced pressure at 60 °C and 120 °C to obtain water and ethylene glycol (EG). Then, 30 mL of methanol and 2.1 g of NaOH were added to the distillation residue, and the mixture was stirred overnight at room temperature. After the reaction was complete, the mixture was filtered to obtain solid sodium chloride and a filtrate. The filtrate was further fractionally distilled under reduced pressure to obtain methanol and TMG, respectively. Alternatively, the filtrate could be dried using a 3 Å molecular sieve and used directly in the next depolymerization process. The crude DMT product was distilled under reduced pressure at 140 °C and 50 mbar to remove the dye and obtain a pale yellow DMT product. The pale yellow DMT product and 300 mL of methanol were added to a pressure-resistant bottle, which was sealed and stirred at 100 °C. After the solid was almost completely dissolved, the pressure-resistant bottle was cooled to -10 °C and its contents were filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain purified DMT with a yield of 91% and a purity of 99.97%.

[0073] Example 4 (Recycling PET from waste blended fabrics)

[0074] raw material:

[0075] Waste blended textiles (70% polyester, other components unknown), waste textile recycling plant; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), sodium hydroxide (NaOH), Sinopharm Chemical Reagent Co., Ltd.; 3Å molecular sieve, Sigma-Aldrich reagent.

[0076] Preprocessing:

[0077] First, 100 g of waste blended fabric strips (containing 70% polyester, with the remaining components unknown) were heated to 270 °C until the PET component melted. The material was then rapidly transferred to room temperature water (25 °C) for quenching. The quenched material was then maintained at its glass transition temperature (T0). gGrind at a temperature of ≈ 70 °C or below, and sieve the grinding material. Re-grind materials that do not meet the particle size requirements until all materials have a particle size of 450 μm or below.

[0078] Depolymerization and post-processing:

[0079] First, 100 g of the grinding material was added to a 500 mL pressure-resistant bottle, followed by 350 mL of methanol and 7.0 g of TMG. The pressure-resistant bottle was then sealed and heated to 100 °C, and stirred at this temperature for 1 hour. After the reaction was complete, the pressure-resistant bottle was cooled, and 6.0 g of concentrated hydrochloric acid was added to neutralize the TMG. The neutralized depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining the non-PET component. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated precipitated solid was then washed with water, filtered, and dried to obtain a washing liquid and a crude dimethyl terephthalate (DMT) product containing various dye molecules.

[0080] The washing solution was distilled under reduced pressure at 60 °C and 120 °C to obtain water and ethylene glycol (EG). Then, 35 mL of methanol and 2.5 g of NaOH were added to the distillation residue, and the mixture was stirred overnight at room temperature. After the reaction was complete, the mixture was filtered to obtain solid sodium chloride and a filtrate. The filtrate was further fractionally distilled under reduced pressure to obtain methanol and TMG, respectively. Alternatively, the filtrate could be dried using a 3 Å molecular sieve and used directly in the next depolymerization process. The crude DMT product was distilled under reduced pressure at 135 °C and 35 mbar to remove the dye and obtain a pale yellow DMT product. The pale yellow DMT product and 240 mL of methanol were added to a pressure-resistant bottle, which was sealed and stirred at 110 °C. After the solid was almost completely dissolved, the pressure-resistant bottle was cooled to -20 °C and its contents were filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain purified DMT with a yield of 92% and a purity of 99.92%.

[0081] Example 5 (Recycling PET from waste blended fabrics)

[0082] raw material:

[0083] Waste blended textiles (60% polyester, other components unknown), waste textile recycling plant; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), sodium hydroxide (NaOH), Sinopharm Chemical Reagent Co., Ltd.; 3Å molecular sieve, Sigma-Aldrich reagent.

[0084] Preprocessing:

[0085] First, 100 g of waste blended fabric strips (containing 60% polyester, with the remaining components unknown) were heated to 270 °C until the PET component melted. The material was then rapidly transferred to room temperature water (25 °C) for quenching. The quenched material was then maintained at its glass transition temperature (T0). g Grind at a temperature of ≈ 70 °C or below, and sieve the grinding material. Re-grind materials that do not meet the particle size requirements until all materials have a particle size of 450 μm or below.

[0086] Depolymerization and post-processing:

[0087] First, 100 g of the grinding material was added to a 500 mL pressure-resistant bottle, followed by 300 mL of methanol and 6.0 g of TMG. The pressure-resistant bottle was then sealed and heated to 80 °C, and stirred at this temperature for 2 hours. After the reaction was complete, the pressure-resistant bottle was cooled, and 5.2 g of concentrated hydrochloric acid was added to neutralize the TMG. The neutralized depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining the non-PET component. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated precipitated solid was then washed with water, filtered, and dried to obtain a washing liquid and a crude dimethyl terephthalate (DMT) product containing various dye molecules.

[0088] The washing solution was distilled under reduced pressure at 60 °C and 120 °C to obtain water and ethylene glycol (EG). Then, 30 mL of methanol and 2.1 g of NaOH were added to the distillation residue, and the mixture was stirred overnight at room temperature. After the reaction was complete, the mixture was filtered to obtain solid sodium chloride and a filtrate. The filtrate was further fractionally distilled under reduced pressure to obtain methanol and TMG, respectively. Alternatively, the filtrate could be dried using a 3 Å molecular sieve and used directly in the next depolymerization process. The crude DMT product was distilled under reduced pressure at 125 °C and 10 mbar to remove the dye and obtain a pale yellow DMT product. The pale yellow DMT product and 180 mL of methanol were added to a pressure-resistant bottle, which was sealed and stirred at 120 °C. After the solid was almost completely dissolved, the pressure-resistant bottle was cooled to -10 °C and its contents were filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain purified DMT with a yield of 91% and a purity of 99.93%.

[0089] Example 6 (Recycling PET from waste blended fabrics)

[0090] raw material:

[0091] Waste blended textiles (containing 50% polyester, the remaining components are unknown), waste textile recycling plant; tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; methanol, concentrated hydrochloric acid (HCl), sodium hydroxide (NaOH), Sinopharm Chemical Reagent Co., Ltd.; 3Å molecular sieve, Sigma-Aldrich reagent.

[0092] Preprocessing:

[0093] First, 100 g of waste blended fabric strips (containing 50% polyester, with the remaining components unknown) were heated to 270 °C until the PET component melted. The material was then rapidly transferred to room temperature water (25 °C) for quenching. The quenched material was then maintained at its glass transition temperature (T0). g Grind at a temperature of ≈ 70 °C or below, and sieve the grinding material. Re-grind materials that do not meet the particle size requirements until all materials have a particle size of 450 μm or below.

[0094] Depolymerization and post-processing:

[0095] First, 100 g of the grinding material was added to a 500 mL pressure-resistant bottle, followed by 200 mL of methanol and 7.5 g of TMG. The pressure-resistant bottle was then sealed and heated to 120 °C, and stirred at this temperature for 40 minutes. After the reaction was complete, the pressure-resistant bottle was cooled, and 6.5 g of concentrated hydrochloric acid was added to neutralize the TMG. The neutralized depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining the non-PET component. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated precipitated solid was then washed with water, filtered, and dried to obtain a washing liquid and a crude dimethyl terephthalate (DMT) product containing various dye molecules.

[0096] The washing solution was distilled under reduced pressure at 60 °C and 120 °C to obtain water and ethylene glycol (EG). Then, 38 mL of methanol and 2.7 g of NaOH were added to the distillation residue, and the mixture was stirred overnight at room temperature. After the reaction was complete, the mixture was filtered to obtain solid sodium chloride and a filtrate. The filtrate was further fractionally distilled under reduced pressure to obtain methanol and TMG, respectively. Alternatively, the filtrate could be dried using a 3 Å molecular sieve and used directly in the next depolymerization process. The crude DMT product was distilled under reduced pressure at 130 °C and 25 mbar to remove the dye and obtain a pale yellow DMT product. The pale yellow DMT product and 230 mL of methanol were added to a pressure-resistant flask, which was sealed and stirred at 105 °C. After the solid was almost completely dissolved, the pressure-resistant flask was cooled to 4 °C and its contents were filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain purified DMT with a yield of 91% and a purity of 99.91%.

[0097] Example 7 (Separate recycling of single fibers such as colorless polyester, cotton, spandex, nylon, and vinylon)

[0098] raw material:

[0099] Colorless polyester fiber, cotton fiber, spandex fiber, nylon fiber, flax fiber, acrylic fiber, wool fiber, silk fiber, Taobao platform; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), Sinopharm Chemical Reagent Co., Ltd.

[0100] Preprocessing:

[0101] First, 52 g of each of polyester, cotton, spandex, nylon, linen, acrylic, wool, and silk fibers were individually heated at 270 °C for 5 min. Then, each material was transferred to room temperature water (25 °C) for quenching. Next, each quenched material was ground under the same conditions (below 70 °C) to obtain abrasives for each fiber.

[0102] Depolymerization and post-processing:

[0103] 52 g of polyester, cotton, spandex, nylon, linen, acrylic, wool, and silk abrasives were added to separate 500 mL pressure-resistant bottles. Then, 260 mL of methanol and 5.2 g of TMG were added to each bottle. The bottles were then sealed and heated to 100 °C, and stirred at this temperature for 1 hour. After the reaction was complete, the bottles were cooled, and 4.5 g of concentrated hydrochloric acid was added to each to neutralize the TMG. The neutralized depolymerized materials were filtered, and the resulting solids were repeatedly washed with methanol until their mass no longer decreased, yielding the undepolymerized components. Each undepolymerized component was dried and weighed, and the depolymerization rates (expressed as mass loss before and after the depolymerization process) of each fiber were calculated as follows: polyester ~99.4%, spandex ~5.7%, cotton ~3.2%, nylon ~1.1%, wool ~1.1%, linen ~1.2%, acrylic ~0.1%, and silk ~0.1%.

[0104] It can be clearly seen that the method provided by the present invention has obvious selectivity for PET in waste blended fabrics during the depolymerization process: the mass loss of other man-made or natural fibers in waste blended fabrics other than PET before and after the depolymerization process is less than 6%, while the mass loss of PET is ≥99%.

[0105] Comparative Example 1 (Recycling unmelted and quenched colorless polyester fibers)

[0106] raw material:

[0107] Colorless polyester fiber, Taobao platform; Tetramethylguanidine (TMG), TCI (Shanghai) Chemical Industry Development Co., Ltd.; Methanol, concentrated hydrochloric acid (HCl), Sinopharm Chemical Reagent Co., Ltd.

[0108] Depolymerization and post-processing:

[0109] First, 52 g of polyester fiber was added to a 500 mL pressure-resistant bottle, followed by 260 mL of methanol and 5.2 g of TMG. The pressure-resistant bottle was then sealed and heated to 100 °C, and stirred at this temperature for 1 hour. After the reaction was complete, the pressure-resistant bottle was cooled, and 4.5 g of concentrated hydrochloric acid was added to neutralize the TMG. The depolymerized material was then filtered. The filtered solid was repeatedly washed with methanol until its mass no longer decreased, thus obtaining undepolymerized polyester fiber. The filtrate was then subjected to vacuum distillation at 40 °C to concentrate the depolymerized product and recover the methanol. The concentrated solid was then washed with water, filtered, and dried to obtain a pale yellow crude dimethyl terephthalate (DMT) product.

[0110] The crude DMT product and 70 mL of methanol were added to a pressure-resistant bottle, which was then sealed and stirred at 100 °C. After the solid was almost completely dissolved, the pressure-resistant bottle was cooled and filtered. The filtered solid was washed with methanol and dried under reduced pressure to obtain the purified DMT product with a yield of 30% and a purity of 99.98%.

Claims

1. A method for selectively and efficiently depolymerizing and recycling PET from waste blended fabrics, characterized in that, include: (1) Waste blended fabric is heated until the PET component melts, then quenched, and then pulverized and ground to obtain waste blended fabric powder; the quenching is carried out in room temperature water, and the resulting PET crystallinity X c ≤15%, and no annealing treatment is performed; (2) The waste blended fabric powder is subjected to depolymerization reaction under the conditions of methanol and tetramethylguanidine to obtain crude product; the depolymerization is carried out in a closed system, at 70~130°C and under the conditions of methanol self-generated pressure; (3) The crude product was separated and purified to obtain polymer-grade dimethyl terephthalate (DMT) refined product. After vacuum distillation and recrystallization, the purity of DMT obtained was ≥99.9%, and the total yield in a single operation was ≥90%. The separation and purification method in step (3) is vacuum distillation and recrystallization, specifically: the vacuum distillation temperature of DMT is 120~140 ºC, the vacuum degree is 1~50 mbar, the recrystallization heating temperature of DMT is 80~120 ºC, the pressure is the autogenous pressure of methanol in the closed system at the recrystallization heating temperature, the recrystallization cooling temperature of DMT is -20~25 ºC, and the solvent required for DMT recrystallization is methanol, with a volume of 2~6 mL·g. -1 DMT.

2. The method according to claim 1, characterized in that, The particle size of the waste blended fabric powder in step (1) is 1μm~1000 μm.

3. The method according to claim 1, characterized in that, In the depolymerization process described in step (2), the amount of methanol used is 3~7 mL·g based on the mass of the powder. -1 The amount of catalyst used is 1-20 wt% of the PET mass.

4. The method according to claim 1, characterized in that, In step (2), the waste blended fabric achieves a PET depolymerization rate of ≥99% within 20~120 min.

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