Method for graded recovery of waste cotton-polyester blended fabric by using subcritical hydration deep eutectic solvent system

Abstract

The invention discloses a method for graded recovery of waste cotton-polyester blended fabrics by using a subcritical hydration deep eutectic solvent system, and belongs to the technical field of high-valued recovery and utilization of waste textiles. According to the method, cotton fibers in the waste cotton-polyester blended fabric can be efficiently converted into different sizes of cellulose forms (cotton staple fibers, MCC, CNC and CNF) by precisely regulating and controlling the technological process, meanwhile, it is ensured that polyester fibers are kept in the original state so as to be recycled, and a perfect subcritical hydration DES system cyclic utilization system is constructed; and the recovery cost is reduced, the recovery efficiency is improved, and the additional value of the recovered product is improved, so that the high-valued, green and efficient recycling of the waste cotton-polyester blended fabric is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of recycling waste textiles, and in particular relates to a method for graded recycling of waste polyester-cotton blended fabrics by using a subcritical hydration deep eutectic solvent system. Background Art

[0002] At present, waste textiles come from a wide range of sources, covering many fields such as daily life, industrial production and service industries. China alone produces more than 23 million tons of waste textiles each year, with a cumulative storage volume of 160 million tons. Among them, the separation of polyester-cotton blended fabrics has always been a difficult problem to be solved. Traditional mechanical separation methods cannot achieve accurate and thorough separation, and are prone to damage the fibers, causing fiber performance degradation and greatly reducing their reuse value; although chemical separation can achieve a certain degree of separation, it requires a large amount of chemical reagents, which not only pushes up costs, but also causes serious environmental pollution problems, and the quality of the separated fibers is also difficult to meet expectations.

[0003] At present, the ionic liquid dissolution method can achieve separation by dissolving one component in the ionic liquid and retaining the other component. For example, by treating waste polyester-cotton blended fabrics with 1-butyl-3-methylimidazole chloride, polyester can be successfully separated and its performance can be guaranteed to be unchanged. However, the ionic liquid dissolution method is hampered by the high cost of the ionic liquid itself and the complexity of recycling and treatment. Its synthesis / preparation requires special raw materials and conditions, resulting in high initial costs. When recycling, due to its special chemical structure and properties, complex separation, purification and regeneration methods are required, which limits large-scale industrial applications. The acid-base method achieves separation of the two by relying on the difference in acid and alkali resistance of polyester and cotton. For example, using phosphoric acid or hydrochloric acid to treat waste polyester-cotton blended fabrics under specific conditions can hydrolyze cotton fibers into corresponding products, and at the same time obtain polyester fibers with stable structural morphology and physicochemical properties and a recovery rate of over 90%. However, the equipment requirements are stringent and the reaction time is long. The reaction equipment must have excellent corrosion resistance, which increases the cost of equipment purchase and maintenance, and is not conducive to large-scale and rapid treatment of waste polyester-cotton blended fabrics.

[0004] Deep eutectic solvent (DES), as an emerging green solvent, has brought hope to the solution of this problem, but it also has significant shortcomings. Conventional DES systems have high viscosity. During the separation process, the intermolecular forces are strong and the fluidity is poor, which limits the interaction between DES and the fiber, resulting in limited separation effect and difficulty in meeting actual needs.

[0005] However, the above existing technologies all have their own problems, and the in-depth development and utilization of the recycled products are insufficient, the process optimization is lacking, the high-value application of the obtained products is not enough, and the process parameters under subcritical water conditions are not accurate enough in regulating the generation of different forms of cellulose, which is difficult to meet the diversified market needs. Therefore, it is necessary to overcome the defects of the existing waste polyester-cotton blended fabric recycling technology, such as high cost, low efficiency, insufficient product utilization, and imprecise process control, and to provide a waste polyester-cotton blended fabric recycling method that is efficient, low-cost, and high-value. Summary of the invention

[0006] Based on the problems of high cost, low efficiency, insufficient product utilization and imprecise process control in the waste polyester-cotton blended fabric recycling technology in the above-mentioned background technology, the present invention proposes a method for graded recycling of waste polyester-cotton blended fabrics using a subcritical hydrated deep eutectic solvent (DES) system. The present invention can achieve efficient conversion of cotton fibers in waste polyester-cotton blended fabrics into cellulose forms of different scales (cotton staple fibers, microcrystalline cellulose (MCC), cellulose nanocrystals (CNC) and cellulose nanofibers (CNF)), while ensuring that the polyester fibers remain in their original state for recycling and reuse, and construct a complete subcritical hydrated DES system recycling cycle mechanism to reduce recycling costs, improve recycling efficiency, make full use of recycled products and accurately control the process, thereby achieving high-value and green recycling of waste polyester-cotton blended fabrics.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a method for graded recovery of waste polyester-cotton blended fabrics by using a subcritical hydration deep eutectic solvent (DES) system, comprising the following steps:

[0009] The hydrogen bond acceptor and the hydrogen bond donor are mixed, and water is added after the mixture is evenly mixed, and the mixture is stirred while adding water, and a uniform and transparent hydrated deep eutectic solvent system is obtained after the mixture is evenly stirred;

[0010] The waste polyester-cotton blended fabric is placed in the hydrated DES system, reacted at 100-150° C. (forming subcritical conditions), and the reaction product is ultrasonically crushed, centrifuged, dialyzed, and dried.

[0011] The hydrogen bond acceptor is choline chloride;

[0012] The hydrogen bond donors include formic acid, acetic acid, lactic acid, citric acid or p-toluenesulfonic acid.

[0013] The molar ratio of the hydrogen bond donor, the hydrogen bond acceptor and water is (1-3):1:(1-3). The molar ratio of the hydrogen bond acceptor, the hydrogen bond donor and water can be combined or used alone according to specific needs. The treatment effect of the subcritical hydration DES system on waste polyester-cotton blended fabrics can be optimized by selecting and proportioning different donors. For example, when choline chloride and citric acid are used as a combination, the molar ratio of citric acid, choline chloride and water can be (1-3):1:(1-3); when choline chloride and lactic acid are used as a combination, the molar ratio of lactic acid, choline chloride and water can be (1-2):1:(1-1.5).

[0014] The stirring speed is 300-500 rpm and the time is 30-60 minutes. The stirring ensures that the components of the subcritical hydrated DES system are fully and evenly mixed.

[0015] Before placing the waste polyester-cotton blended fabric in the hydrated deep eutectic solvent system, the method further includes the step of pretreating the waste polyester-cotton blended fabric: washing the waste polyester-cotton blended fabric with a neutral detergent, rinsing with clean water after washing, drying until the moisture content is less than 10%, and cutting into squares with a side length of 2-5 cm. The pretreatment has at least the following effects: (1) Removing non-fiber substances: Pretreatment can remove non-fiber substances in the sample, such as coatings, slurries, etc. These non-fiber substances may be present in the fiber itself or added during subsequent processing, including grease, wax or finishing agents, etc. Removing these substances can reduce their impact on the test results. (2) Improving separation efficiency: Pretreatment helps to improve the efficiency of subsequent separation processes. (3) Reducing environmental pollution: Pretreatment can remove substances that may cause environmental pollution, such as certain chemical additives, and reduce the impact of subsequent processing on the environment. (4) Improving the quality of recycled fibers: Pretreatment can remove impurities and non-fiber substances, thereby improving the purity and quality of recycled fibers and making them more suitable for subsequent reuse. (5) Reduce the cost of subsequent treatment: Pretreatment can reduce the amount of chemical reagents required in the subsequent treatment process and reduce the cost of wastewater treatment. (6) Improve resource recovery rate: Pretreatment helps to improve the recovery rate of resources. For example, after pretreatment, polyester and cotton fibers can be recycled more effectively, achieving high-value utilization of resources.

[0016] Furthermore, the step of pretreating the waste polyester-cotton blended fabric is: placing the waste polyester-cotton blended fabric in an aqueous solution containing a neutral detergent, soaking it at 40-50°C for 20-30 minutes, then mechanically stirring and cleaning it at a stirring speed of 200-300rpm for 15-20 minutes, rinsing it with clean water for 3-5 times after cleaning, and then drying it in an oven at 60-80°C for 2-3 hours to make its moisture content less than 10%, and cutting it into squares with a side length of 2-5 cm (i.e., small pieces of clean fabric).

[0017] The mass ratio of the waste polyester-cotton blended fabric to the total mass of the hydrogen bond acceptor, the hydrogen bond donor and water (ie, the DES system) is 1:(10-15).

[0018] When the reaction is carried out at 100-150°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the stirring speed is 400-600rpm, and the reaction time is 0.5-2 hours. By adjusting the process parameters such as reaction temperature, time and stirring speed in the process of treating waste polyester-cotton blended fabrics with the subcritical hydration DES system, the degree of hydrolysis of waste polyester-cotton blended fabrics can be controlled to obtain cellulose products of different forms.

[0019] Furthermore, when the reaction temperature is 100-110°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the reaction time is 0.5-1.0 hour, and the stirring speed is 400-500rpm, cotton staple fibers and polyester fibers are recovered;

[0020] When the reaction temperature is increased to 120-130°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the reaction time is extended to 1.0-1.5 hours, and the stirring speed is 500-550rpm, microcrystalline cellulose (MCC) and polyester fiber are recovered;

[0021] When the reaction temperature is further increased to 140-150°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the reaction time is further extended to 1.5-2.0 hours, and the stirring speed is 550-600rpm, cellulose nanocrystals (CNC) or cellulose nanofibers (CNF) and polyester fibers are recovered.

[0022] During the ultrasonic pulverization, the ultrasonic power is 300-900W, the ultrasonic frequency is 20-40kHz, and the ultrasonic time is 20-60 minutes. Preferably, the ultrasonic pulverization is performed in an ultrasonic cell pulverizer. The ultrasonic pulverization treatment can further refine the cellulose structure, promote the dissociation of cellulose from the fabric, and improve the uniformity and quality of cellulose products in different forms.

[0023] After ultrasonic pulverization, the mixture is cooled to room temperature and then centrifuged at a speed of 6000-8000 rpm for 20-30 minutes to separate the polyester fiber from the DES solution containing different forms of cellulose. The polyester fiber precipitate is washed with clean water several times to remove residual DES and a small amount of cellulose impurities, and then dried at 80-100°C for 1-2 hours before recycling.

[0024] The dialysis is performed using a dialysis bag with a molecular weight cutoff of 8000-10000Da, the dialysis is performed in deionized water, the dialysis time is 48-72 hours, the deionized water is continuously replaced during the dialysis, and the dialysis method is used to remove small molecular impurities and residual DES components in the DES solution containing cellulose in different forms; and / or

[0025] The drying is freeze drying or spray drying. After dialysis, cellulose products of different forms are obtained by freeze drying or spray drying.

[0026] Furthermore, during freeze drying, the material is first pre-frozen to -40 to -50°C (minus forty to minus fifty degrees Celsius), and then dried at a vacuum degree of 0.05-0.1 Pa for 24-48 hours.

[0027] The present invention also provides a method for recovering a hydrated deep eutectic solvent, wherein the system to be recovered is the system after the waste polyester-cotton blended fabric is recovered according to the above method, and the system after the waste polyester-cotton blended fabric is recovered according to the above method is subjected to reduced pressure distillation, so that the components of the hydrated DES (hydrogen bond donors, hydrogen bond acceptors and water) reach the original proportion (that is, the hydrogen bond acceptors and hydrogen bond donors of the DES system are basically not consumed after the reaction, and after the excess water added in the cleaning process is removed by reduced pressure distillation, the hydrated DES system before the reaction (original proportion) can be obtained).

[0028] When the DES system recycled by the above method is used again to recycle waste polyester-cotton blended fabrics, the composition of the DES solution is analyzed to restore its composition and performance to the original subcritical hydrated DES system state. This recycling and recycling step ensures the sustainable use of the system, so that the subcritical hydrated DES system can maintain stable processing performance in multiple cycles.

[0029] Compared with the prior art, the present invention has the following advantages and technical effects:

[0030] The present invention utilizes a combination of a specific hydrogen bond acceptor (choline chloride) and a variety of selectable hydrogen bond donors (formic acid, acetic acid, lactic acid, citric acid, p-toluenesulfonic acid, etc.) in a subcritical hydrated DES system, and accurately controls process parameters such as hydration amount, reaction temperature, pressure, time, and stirring speed. The subcritical hydrated DES selectively interacts with waste polyester-cotton blended fabrics under specific conditions, destroys the cellulose crystal structure and promotes its hydrolysis. At the same time, the principle of the influence of different process parameters on the degree of hydrolysis (the inherent connection between each process and parameter) is utilized to achieve the technical effect of accurately controlling the hydrolysis of waste polyester-cotton blended fabrics to generate different forms of cellulose products such as cotton staple fibers, MCC, CNC and CNF, and solves the technical problems of single product form and inaccurate process control in the existing waste polyester-cotton blended fabric recycling technology. In addition, an ultrasonic cell crushing step is introduced after the subcritical hydrated DES treatment, and the cavitation effect and mechanical action of the ultrasound further refine the cellulose structure, thereby improving the uniformity and quality of the product. In addition, through the recycling and reuse of the subcritical hydration DES system, including operations such as reduced pressure distillation to remove water, the economy and sustainability of the entire recycling process are ensured, which effectively reduces costs, improves resource utilization, and solves the problems of high recycling costs and waste of resources in waste polyester-cotton blended fabrics. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0032] Figure 1 The scanning electron microscope images of waste polyester-cotton blended fabrics before and after treatment, wherein: a is untreated waste polyester-cotton blended fabric, b1 is cotton staple fiber (50 μm) produced in Example 1, b2 is polyester fiber (50 μm) produced in Example 1, c1 is MCC (20 μm) produced in Example 2, c2 is polyester fiber (50 μm) produced in Example 2, d1 is CNF (200 nm) produced in Example 3, d2 is polyester fiber (50 μm) produced in Example 3, e1 is CNC (200 nm) produced in Example 4; e2 is polyester fiber (50 μm) produced in Example 4;

[0033] Figure 2 is the recovery rate of the subcritical hydrated DES system in Example 3;

[0034] Figure 3 It is the number of recycling times of the subcritical hydration DES system in Example 1 and the separation and recovery yield of waste polyester-cotton blended fabric. DETAILED DESCRIPTION

[0035] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0036] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0037] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0038] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0039] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0040] Unless otherwise specified, the normal temperature in the present invention is 25±2°C.

[0041] All raw materials used in the examples of the present invention are commercially available.

[0042] Existing technologies, such as using deep eutectic solvents to extract and efficiently recover polyester and microcrystalline cellulose from waste polyester-cotton blended fabrics in one step (DOI: 10.1007 / s11696-022-02246-5), have achieved certain results in the recovery of waste polyester-cotton blended fabrics, but the conventional DES system is used to treat the fabrics. The high solvent viscosity leads to limited processing efficiency, and the ability to control the product morphology is insufficient, making it difficult to meet the diversified market needs. The present invention uses a hydrated DES system and operates under subcritical conditions, which has significant advantages. Hydrated DES effectively reduces the solvent viscosity, and combined with subcritical conditions, it enhances the molecular activity, and can quickly and efficiently separate polyester-cotton components under mild conditions, greatly improving the separation efficiency. By precisely controlling the process parameters, cellulose products with various forms (such as fibers, powders, etc.) and different crystallinity and polymerization degrees, such as CNC or CNF, can be prepared, and the existing technology cannot achieve this diversified regulation. In addition, the present invention focuses on overall process integration innovation, with each link closely connected and coordinatedly optimized to form a continuous and stable process flow, which greatly improves the resource recovery rate, reduces energy consumption and costs, and shows broad application prospects in high-end and emerging fields such as biomedical materials and high-performance composite materials. The economic and environmental benefits are significantly improved, effectively overcoming the defects of the existing technology.

[0043] CN112920454A (A method for separating and recovering waste polyester-cotton blended fabrics using a low eutectic solvent) discloses a method for obtaining polyester fiber and cellulose by placing waste polyester-cotton blended fabrics in a low eutectic solvent, subjecting them to ultrasonic treatment, heating and heat preservation degradation, ultrasonic treatment again, and filtration separation. This method has mild preparation conditions and simple operation, and achieves the rapid degradation of cotton fibers under mild conditions, while achieving the purpose of high cellulose yield and no destruction of polyester. Although the patent uses low eutectic solvents and ultrasound and other means to separate and recover waste polyester-cotton blended fabrics, the low eutectic solvent used in the patent has a high viscosity, resulting in poor performance of subsequent cellulose products. The present invention adopts a subcritical hydrated deep eutectic solvent (DES) system, and a low-viscosity hydrated DES is formed by optimizing the ratio. Subcritical conditions can more accurately control the hydrolysis process of waste cotton, and it can be hydrolyzed into a variety of cellulose forms with different scales and better performance, such as cotton fragments, MCC, CNC, CNF, etc., to achieve customized production of cellulose forms, which is not achievable in the patent. In addition, the present invention designs a complete recycling and recycling scheme for the subcritical hydrated DES system, including a precise dehydration step of vacuum distillation, to ensure that the system has stable performance when used repeatedly, and can continuously and efficiently hydrolyze new waste polyester-cotton blended fabrics, while the recycling technology in the above patents in this regard is relatively simple and lacks effective guarantee for the long-term stability of the system performance. The present invention focuses on the innovative application of the subcritical hydrated DES system, and through the unique DES construction and process parameter control, it realizes the efficient conversion of waste polyester-cotton blended fabrics into polymorphic cellulose and the lossless recycling and recycling of polyester fibers.

[0044] 1. The present invention is based on a multi-parameter coordinated control process in the process of treating waste polyester-cotton blended fabrics using a subcritical hydration DES system, specifically including a refined control strategy for key process parameters such as hydration amount (viscosity value is 50-100mPa·s), reaction temperature (accurate setting within the range of 100-150°C), pressure (strictly controlled within the range of 0.8-1.5MPa), reaction time (accurate control of 0.5-2.0 hours) and stirring speed (specific range of 400-600rpm), so as to achieve precise control of the degree of hydrolysis of cotton fibers, and further achieve key technical means for the directional preparation of cellulose products of different forms and high added value such as cotton staple fibers, MCC, CNC and CNF.

[0045] 2. The ultrasonic cell pulverization treatment process introduced after the subcritical hydrated DES system treatment of the present invention and its precise process parameter setting, that is, the ultrasonic power is set in the range of 300-900W, the ultrasonic frequency is controlled in the interval of 20-40kHz, and the ultrasonic time is limited to 20-60 minutes. With the help of the cavitation effect and mechanical action mechanism of ultrasound, the deep refinement of the cellulose structure is achieved, and the uniformity and quality of cellulose products of different forms are effectively improved, which is an innovative link.

[0046] 3. The complete process of recycling and reusing the subcritical hydrated DES system of the present invention includes the specific distillation temperature (60-80°C) and vacuum degree (0.08-0.12Pa) control in the reduced pressure distillation and water removal step, as well as the component replenishment link (accurate choline chloride and corresponding donor replenishment operations based on the specific hydrogen bond donor selected in the early stage), to ensure that the system can maintain stable performance and sustainable high-efficiency processing capacity after multiple cycles.

[0047] In the examples and comparative examples of the present invention, the neutral detergent used is sodium alkylbenzene sulfonate.

[0048] In the present invention, the calculation method of the purity of the cellulose product is: cellulose product purity (%) = mass of pure cellulose component / total mass of recovered cellulose product*100%.

[0049] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in the art and are not the focus of the present invention.

[0050] The technical solution of the present invention is further illustrated by the following embodiments.

[0051] Example 1

[0052] Choline chloride is selected as a hydrogen bond acceptor and formic acid is selected as a hydrogen bond donor. After stirring at room temperature, deionized water is added. The molar ratio of formic acid, choline chloride and deionized water is 2:1:1. Stirring is performed while adding deionized water. The stirring speed is 300rpm. The stirring time is 30 minutes. A uniform and transparent hydrated DES system is formed. The viscosity value is 51.4mPa·s. 10g (polyester-cotton ratio is 65 / 35, mass ratio) of waste polyester-cotton blended fabric is sorted and placed in 50mL of an aqueous solution containing 0.5g of a neutral detergent. It is soaked at 40°C for 20 minutes, and then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes. After cleaning, it is rinsed with clean water 3 times, dried in a 60°C oven for 2 hours, so that the water content is less than 10%, and cut into small pieces with a side length of 2cm*2cm (i.e., small pieces of pretreated fabric, the same below). The small pieces of pretreated fabric are placed in an autoclave and 100mL of a hydrated DES system is added. After sealing the reactor, the temperature was raised to 100°C, the heating rate was 5°C / min, the pressure was maintained at 0.8MPa during the reaction, the stirring speed was 400rpm, and the reaction time was 0.5 hours (under these conditions, cotton staple fibers and polyester fibers can be obtained). The mixture after the reaction was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 300W, the ultrasonic frequency was set to 2kHz, and the ultrasonic time was set to 20 minutes. After the ultrasonic treatment, centrifugal separation was performed, the centrifugal speed was 6000rpm, and the centrifugal time was 20 minutes. The solid precipitate (i.e., polyester fiber and cotton staple fiber) was dialyzed in deionized water for 24 hours using a dialysis bag with a molecular weight cutoff of 5000Da, during which the deionized water was continuously replaced, and freeze-dried after dialysis. During freeze-drying, it was pre-frozen to -40°C, dried at a vacuum degree of 0.05Pa for 48 hours, and the product was sieved using a 150-mesh sieve to obtain 3.33g of cellulose product in the form of cotton staple fibers with a purity of 96%; 6.37g of polyester fiber was obtained with a recovery rate of 98%. The water content in the DES solution after separation of nanocellulose was removed by reduced pressure distillation at a distillation temperature of 60°C and a vacuum degree of 0.08 Pa, so that the water content was restored to the initial level (i.e., the viscosity value was 51.4 mPa·s), and the DES recovery rate was 92%.

[0053] Example 2

[0054] Choline chloride was selected as the hydrogen bond acceptor and lactic acid as the hydrogen bond donor. After stirring at room temperature, deionized water was added. The molar ratio of lactic acid, choline chloride and deionized water was 1.5:1:1.25. Stirring was performed while adding deionized water. The stirring speed was 400rpm and the stirring time was 30 minutes. A uniform and transparent hydrated DES system was formed. The viscosity value was 73.2mPa·s. 10g (polyester-cotton ratio 65 / 35, mass ratio) of waste polyester-cotton blended fabric was sorted and placed in 50mL of an aqueous solution containing 0.5g of a neutral detergent. It was soaked at 40°C for 20 minutes, and then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes. After cleaning, it was rinsed with clean water for 3 times, dried in a 60°C oven for 2 hours to make the water content less than 10%, and cut into small pieces with a side length of 2cm*2cm. The pretreated small pieces of fabric were placed in an autoclave and 100mL of the hydrated DES system was added. After sealing the reactor, the temperature was raised to 120°C at a heating rate of 8°C / min. During the reaction, the pressure was maintained at 1.2MPa, the stirring speed was 500rpm, and the reaction time was 1 hour (under these conditions, it is beneficial to obtain microcrystalline cellulose (MCC) and polyester fiber). The reaction mixture was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 450W, the ultrasonic frequency was set to 30kHz, and the ultrasonic time was set to 40 minutes. After the ultrasonic treatment, centrifugal separation was performed, the centrifugal speed was 7000rpm, and the centrifugal time was 25 minutes. The solid precipitate (i.e., polyester fiber and MCC) was dialyzed in deionized water for 48 hours using a dialysis bag with a molecular weight cutoff of 9000Da, during which the deionized water was continuously replaced, and freeze-dried after dialysis. During freeze-drying, it was pre-frozen to -40°C and dried at a vacuum degree of 0.05Pa for 48 hours. The product was sieved using a 150-mesh sieve to obtain 6.42g of polyester fiber with a recovery rate of 96.8%; 3.29g of cellulose product in the form of MCC was obtained with a purity of 97%. The water content in the DES solution after separation of nanocellulose was removed by reduced pressure distillation at a distillation temperature of 60°C and a vacuum degree of 0.08 Pa, so that the water content was restored to the initial level (i.e., the viscosity value was 73.2 mPa·s), and the DES recovery rate was 90%.

[0055] Example 3

[0056] Choline chloride was selected as the hydrogen bond acceptor and citric acid as the hydrogen bond donor. After stirring at room temperature, deionized water was added. The molar ratio of citric acid, choline chloride and deionized water was 3:1:3. Stirring was performed while adding deionized water. The stirring speed was 500rpm and the stirring time was 30 minutes. A uniform and transparent hydrated DES system was formed with a viscosity value of 82.8mPa·s. 10g (polyester-cotton ratio 65 / 35, mass ratio) of waste polyester-cotton blended fabric was sorted and placed in 50mL of an aqueous solution containing 0.5g of a neutral detergent. It was soaked at 40°C for 20 minutes, then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes. After cleaning, it was rinsed with clean water for 3 times. It was dried in a 60°C oven for 2 hours to make the water content less than 10%, and cut into small pieces with a side length of 2cm*2cm. The pretreated small pieces of fabric were placed in an autoclave and 100mL of a hydrated DES system was added. After sealing the reactor, the temperature was raised to 130°C at a heating rate of 10°C / min. During the reaction, the pressure was maintained at 1.5MPa, the stirring speed was 600rpm, and the reaction time was 2 hours (cellulose nanofibers (CNF) and polyester fibers can be generated under these conditions). The reaction mixture was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 600W, the ultrasonic frequency was set to 40kHz, and the ultrasonic time was set to 60 minutes. After the ultrasonic treatment, centrifugal separation was performed at a centrifugal speed of 8000rpm and a centrifugal time of 30 minutes to separate the polyester fiber from the DES solution containing CNF. The crude polyester fiber was washed 2-3 times with clean water and ethanol to remove residual DES, and dried at 80°C for 1 hour to recover 6.31g of polyester fiber with a recovery rate of 97%. The dialysis method was used to remove small molecular impurities and residual DES components in the DES solution containing CNF. A dialysis bag with a molecular weight cutoff of 10,000 Da was selected and dialyzed in deionized water for 48 hours. The deionized water was continuously replaced during the period. After dialysis, the nanocellulose powder product was obtained by freeze drying. During freeze drying, it was pre-frozen to -50°C and dried at a vacuum degree of 0.1 Pa for 48 hours to obtain 2.76 g of cellulose product in the form of CNF with a purity of up to 95%. The water content in the DES solution after separating the nanocellulose was removed by reduced pressure distillation. The distillation temperature was 60°C and the vacuum degree was 0.08 Pa. The water content was restored to the initial level (i.e., the viscosity value was 82.8 mPa·s), and the DES recovery rate was 96%.

[0057] Example 4

[0058] Choline chloride is selected as a hydrogen bond acceptor and p-toluenesulfonic acid is selected as a hydrogen bond donor. After stirring at room temperature, deionized water is added. The molar ratio of p-toluenesulfonic acid, choline chloride and deionized water is 2:1:1.5. Stirring is performed while adding deionized water. The stirring speed is 500rpm and the stirring time is 30 minutes to form a uniform and transparent hydrated DES system with a viscosity value of 62.5mPa·s. 10g (polyester-cotton ratio 65 / 35, mass ratio) of waste polyester-cotton blended fabric is sorted and placed in 50mL of an aqueous solution containing 0.5g of a neutral detergent. It is soaked at 40°C for 20 minutes, then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes, and then rinsed with clean water 3 times. Dry in a 60°C oven for 2 hours to make the water content less than 10%, and cut into small pieces with a side length of 2cm*2cm. The pretreated fabric pieces are placed in an autoclave and 100g of a hydrated DES system is added. After sealing the reactor, the temperature was raised to 150°C at a heating rate of 10°C / min. During the reaction, the pressure was maintained at 1.5MPa, the stirring speed was 600rpm, and the reaction time was 2.5 hours (cellulose nanocrystals (CNC) and polyester fibers can be generated under these conditions). The mixture after the reaction was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 600W, the ultrasonic frequency was set to 40kHz, and the ultrasonic time was set to 60 minutes. After the ultrasonic treatment, centrifugal separation was performed at a centrifugal speed of 8000rpm for 30 minutes to separate the polyester fiber from the DES solution containing CNC. The crude polyester fiber was washed 2-3 times with clean water and ethanol to remove residual DES, and dried at 100°C for 2 hours for recycling to obtain 6.31g of polyester fiber with a recovery rate of 97%. The dialysis method was used to remove small molecular impurities and residual DES components in the DES solution containing CNC. A dialysis bag with a molecular weight cutoff of 15000Da was selected and dialyzed in deionized water for 48 hours. The deionized water was continuously replaced during the period. After dialysis, the nanocellulose powder product was obtained by freeze drying. During freeze drying, it was pre-frozen to -50°C and dried at a vacuum degree of 0.1Pa for 48 hours to obtain 3.05g of CNC cellulose product with a purity of up to 95%. The water in the DES solution after separating the nanocellulose was removed by reduced pressure distillation. The distillation temperature was 60°C and the vacuum degree was 0.08Pa. The water content was restored to the initial level (i.e., the viscosity value was 62.5mPa·s), and the DES recovery rate was 96%.

[0059] Scanning electron microscope images of waste polyester-cotton blended fabrics before and after treatment are shown in Figure 1, wherein: a is untreated waste polyester-cotton blended fabric (polyester-cotton ratio 65 / 35, mass ratio), b1 is cotton staple fiber (50 μm) produced in Example 1, b2 is polyester fiber (50 μm) produced in Example 1, c1 is MCC (20 μm) produced in Example 2, c2 is polyester fiber (50 μm) produced in Example 2, d1 is CNF (200 nm) produced in Example 3, d2 is polyester fiber (50 μm) produced in Example 3, e1 is CNC (200 nm) produced in Example 4; e2 is polyester fiber (50 μm) produced in Example 4.

[0060] Example 5

[0061] 10g (polyester-cotton ratio 65 / 35, mass ratio) of waste polyester-cotton blended fabric was sorted, placed in 50mL of aqueous solution containing 0.5g of neutral detergent, soaked at 40℃ for 20 minutes, then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes, and rinsed with clean water 3 times after cleaning. Dry in a 60℃ oven for 2 hours to make the water content less than 10%, and cut into small pieces with a side length of 2cm*2cm. The pretreated fabric pieces were placed in a high-pressure reactor and 100mL of the DES solvent recovered in Example 3 was added. After sealing the reactor, the temperature was raised to 130℃, the heating rate was 10℃ / min, the pressure was maintained at 1.5MPa during the reaction, the stirring speed was 600rpm, the reaction time was 2 hours, and cellulose nanofibers (CNF) were generated. The reacted mixture was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 600W, the ultrasonic frequency was 40kHz, and the ultrasonic time was 60 minutes. After the ultrasonic treatment, the polyester fiber and the DES solution containing CNF were separated by centrifugation at a speed of 8000 rpm and a centrifugation time of 30 minutes. The crude polyester fiber was washed 2-3 times with clean water and ethanol to remove the residual DES, and then dried at 100°C for 2 hours for recycling to obtain 6.24 g of polyester fiber with a recovery rate of 96%. The small molecule impurities and the remaining DES components in the DES solution containing CNF were removed by dialysis. A dialysis bag with a molecular weight cutoff of 10000Da was selected and dialyzed in deionized water for 48 hours. The deionized water was continuously replaced during the period. After dialysis, the nanocellulose powder product was obtained by freeze drying. The freeze drying was pre-frozen to -50°C and dried at a vacuum degree of 0.1Pa for 48 hours to obtain 2.73 g of cellulose product in the form of CNF with a purity of up to 95%. The water in the DES solution after separating the nanocellulose was removed by reduced pressure distillation. The distillation temperature was 60°C and the vacuum degree was 0.08Pa to restore the water content to the initial level. The DES recovery rate was 92%.

[0062] Comparative Example 1 (normal pressure DES system)

[0063] Choline chloride is selected as a hydrogen bond acceptor and citric acid is selected as a hydrogen bond donor. After stirring at room temperature, deionized water is added. The molar ratio of citric acid, choline chloride and water is 3:1:3. Stirring is performed while adding deionized water. The stirring speed is 500rpm and the stirring time is 30 minutes to form a uniform and transparent hydrated DES system. 10g (polyester-cotton ratio is 65 / 35, mass ratio) of waste polyester-cotton blended fabric is sorted and placed in 50mL of an aqueous solution containing 0.5g of a neutral detergent. It is soaked at 40°C for 20 minutes, and then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes. After cleaning, it is rinsed with clean water 3 times, dried in a 60°C oven for 2 hours to make the water content less than 10%, and cut into small pieces with a side length of 2cm*2cm (i.e., small pieces of pretreated fabric, the same below). The small pieces of pretreated fabric are placed in an oil bath pot, 100mL of hydrated DES system is added, and the reaction is carried out at 130°C for 2 hours. The reaction mixture was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 300W, the ultrasonic frequency was set to 2kHz, and the ultrasonic time was set to 20 minutes. After the ultrasonic treatment, centrifugal separation was performed, the centrifugal speed was 6000rpm, and the centrifugal time was 20 minutes. The solid precipitate (i.e., polyester fiber and cotton staple fiber) was dialyzed in deionized water for 24 hours using a dialysis bag with a molecular weight cutoff of 5000Da, during which the deionized water was continuously replaced, and freeze-dried after dialysis. When freeze-dried, it was pre-frozen to -40°C and dried at a vacuum degree of 0.05Pa for 48 hours. The product was sieved using a 150-mesh sieve. After treatment, 2.5g of polyester fiber was obtained with a recovery rate of 62.5%; 3.2g of cellulose product was obtained with a purity of 70%, and the cellulose product was mainly in the form of larger particles, and the cotton fiber morphology was not effectively formed, and most of them were cotton staple fibers that were not fully hydrolyzed.

[0064] Comparative Example 2 (anhydrous DES system)

[0065] Choline chloride is selected as a hydrogen bond acceptor and citric acid is selected as a hydrogen bond donor. The molar ratio of citric acid to choline chloride is 3:1. The waste polyester-cotton blended fabric is sorted into 10g (polyester-cotton ratio 65 / 35, mass ratio), placed in 50mL of an aqueous solution containing 0.5g of neutral detergent, soaked at 40°C for 20 minutes, and then mechanically stirred and cleaned at a stirring speed of 200rpm for 15 minutes. After cleaning, rinse with clean water for 3 times. Dry in a 60°C oven for 2 hours to make the water content less than 10%, and cut into small pieces with a side length of 2cm*2cm. The pretreated fabric pieces are placed in a high-pressure reactor and 100mL of anhydrous DES system is added. After sealing the reactor, the temperature is raised to 130°C, the heating rate is 10°C / minute, the pressure is maintained at 1.5MPa during the reaction, the stirring speed is 600rpm, and the reaction time is 2 hours. The mixture after the reaction was transferred to an ultrasonic cell crusher, and the ultrasonic power was set to 600W, the ultrasonic frequency was set to 40kHz, and the ultrasonic time was set to 60 minutes. After the ultrasonic treatment, centrifugation was performed, and the centrifugal speed was 8000rpm and the centrifugal time was set to 30 minutes. However, due to the synergistic effect of the absence of subcritical water, the system had poor fluidity and uneven reaction. After the reaction was completed, 5.52g of polyester fiber was separated and the recovery rate was 85%. The obtained cellulose product 1.82g, with a purity of 75%, was mainly blocky cellulose aggregates, and the microcrystalline cellulose (MCC) form could not be effectively obtained, and the DES system was difficult to recycle and reuse after the reaction, and serious component agglomeration and denaturation occurred.

[0066] Result analysis:

[0067] By comparing Examples 1-5 with Comparative Examples 1-2, it can be seen that the subcritical hydration DES system of the present invention has significant advantages in the recycling of waste polyester-cotton blended fabrics. In terms of polyester fiber recovery rate, each embodiment can reach more than 95%, while the polyester fiber recovery rate of Comparative Example 1 is only 62.5%, and the polyester fiber recovery rate of Comparative Example 2 is only 85%. For cellulose products, each embodiment not only has a high yield, but also can accurately control and obtain cellulose of different forms (cotton staple fibers, MCC, CNC / CNF) and high purity (90%-95%), while each comparative example has a low yield and cannot effectively control the cellulose form, and the purity is only about 70%-75%.

[0068] The recovery rate of the hydrated DES system, the number of cycles of the subcritical hydrated DES system and the separation and recovery yield of waste polyester-cotton blended fabrics were tested. The specific test methods are as follows:

[0069] 1. Recovery rate of hydrated DES system = (m2 / m1) x 100%.

[0070] Wherein, m1 is the mass of the DES system before the reaction, and m2 is the mass of the DES system recovered by vacuum distillation after the reaction.

[0071] 2. Test method for the number of cycles of subcritical hydration DES system

[0072] According to the optimized process, waste polyester-cotton blended fabrics are treated with subcritical hydrated DES, and DES is recovered after treatment and regenerated by vacuum distillation, and then the treatment, recovery and regeneration process are repeated for multiple cycles (such as 0-6 times or more). The fabric separation recovery yield (comparing the mass of each recovered component with the initial yield) and the DES system performance indicators (such as viscosity, pH value, hydrogen bond donor and acceptor concentration changes) are closely monitored in each cycle, and the experimental conditions must be stable, and the average of multiple experiments is taken.

[0073] 3. Test method for separation and recovery yield of waste polyester-cotton blended fabrics

[0074] First, the composition of waste fabrics was accurately analyzed to determine the content of polyester fiber (mass fraction a%) and cotton fiber (mass fraction b%), and a certain mass (m0 grams) of samples was weighed. After subcritical hydration DES treatment and separation and purification, the recovered polyester fiber (m1 grams) and cellulose products (such as cotton staple fibers, microcrystalline cellulose, CNF, CNC) (m2 grams) were accurately weighed. The recovery yield of each component was calculated according to the corresponding formula (polyester fiber: (m1 / (m0×a%))×100%; cellulose products such as microcrystalline cellulose (m2 / (m0×b%))×100%), and the experiment was repeated 3-5 times to take the average and strictly control the conditions.

[0075] The recovery rate of the subcritical hydrated DES system in Example 3 is shown in Figure 2 It can be seen that when recycling waste polyester-cotton blended fabrics, the recovery rate of the DES system is always higher than 85% as the number of cycles increases. For example, the recovery rate of the first cycle reaches 98%, which ensures the stability and efficiency of DES in multiple cycles. The recycling cycle with high recovery rate has significant advantages, reducing costs, reducing pollution, stabilizing the process, and effectively promoting the sustainable recycling of waste polyester-cotton blended fabrics, which is of great significance to the economy, environment and process stability.

[0076] The number of cycles of the subcritical hydrated DES system and the recovery rate of waste polyester-cotton blended fabrics in Example 1 are shown in Figure 3It can be seen that during the multiple cycles of DES (such as within the set range of 6 cycles), the polyester fiber recovery rate is always maintained at a high level (more than 90%), indicating that the recycling of the subcritical hydrated DES system has no significant effect on the recovery effect of polyester fibers, and the recovery and recycling system of the present invention is stable in ensuring the recovery of polyester fibers. At the same time, although the output of cellulose products in different forms will fluctuate slightly, the overall yield fluctuation does not exceed 5%, indicating that the subcritical hydrated DES system of the present invention can still effectively realize the conversion of waste cotton fibers into various forms of cellulose after multiple cycles, and the conversion efficiency is relatively stable, reflecting the reliability and sustainability of the entire recovery process and recycling system.

[0077] The subcritical hydrated DES system of the present invention can be effectively recycled, while the anhydrous DES system has serious composition problems after the reaction and is difficult to recycle. These data fully prove that the present invention solves the technical problems of high cost, low efficiency, insufficient product utilization and imprecise process control in the waste polyester-cotton blended fabric recycling technology mentioned in the background technology, and has good application prospects and economic benefits.

[0078] The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system, characterized in that: The following steps are involved: The hydrogen bond acceptor and the hydrogen bond donor are mixed, and water is added after the mixture is evenly mixed, and the mixture is stirred while adding water, and a hydrated deep eutectic solvent system is obtained after the mixture is evenly stirred; The waste polyester-cotton blended fabric is placed in the hydrated deep eutectic solvent system, reacted at 100-150° C., and the reaction product is ultrasonically crushed, centrifuged, dialyzed and dried.

2. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 1, characterized in that: The hydrogen bond acceptor is choline chloride; The hydrogen bond donors include formic acid, acetic acid, lactic acid, citric acid or p-toluenesulfonic acid.

3. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 2, characterized in that: The molar ratio of the hydrogen bond donor, the hydrogen bond acceptor and water is (1-3):1:(1-3).

4. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 1, characterized in that: The stirring speed is 300-500 rpm, and the time is 30-60 minutes.

5. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 1, characterized in that: Before placing the waste polyester-cotton blended fabric in the hydrated deep eutectic solvent system, the method further includes the step of pretreating the waste polyester-cotton blended fabric: washing the waste polyester-cotton blended fabric with a neutral detergent, rinsing the waste polyester-cotton blended fabric with clean water after washing, drying the waste polyester-cotton blended fabric to a moisture content of less than 10%, and cutting the waste polyester-cotton blended fabric into blocks with a side length of 2-5 cm.

6. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 1, characterized in that: The mass ratio of the waste polyester-cotton blended fabric to the total mass of the hydrogen bond acceptor, the hydrogen bond donor and water is 1:(10-15).

7. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 1, characterized in that: When the reaction is carried out at 100-150°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5 MPa, the stirring speed is 400-600 rpm, and the reaction time is 0.5-2 hours.

8. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 7, characterized in that: When the reaction temperature is 100-110°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the reaction time is 0.5-1.0 hour, and the stirring speed is 400-500rpm, cotton staple fibers and polyester fibers are recovered; When the reaction temperature is 120-130°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the reaction time is 1.0-1.5 hours, and the stirring speed is 500-550rpm, microcrystalline cellulose and polyester fiber are recovered; When the reaction temperature is 140-150°C, the heating rate is 5-10°C / min, the pressure is 0.8-1.5MPa, the reaction time is 1.5-2.0, and the stirring speed is 550-600rpm, cellulose nanocrystals or cellulose nanofibers and polyester fibers are recovered.

9. The method for graded recovery of waste polyester-cotton blended fabrics using a subcritical hydration deep eutectic solvent system according to claim 1, characterized in that: The centrifugal speed is 6000-8000rpm / min, and the centrifugal time is 20-30 minutes; The dialysis is performed using a dialysis bag with a molecular weight cutoff of 8000-10000Da, the dialysis is performed in deionized water, and the dialysis time is 48-72 hours; and / or The drying is freeze drying or spray drying.

10. A method for recovering a hydrated deep eutectic solvent, characterized in that: The system to be recycled is a system after waste polyester-cotton blended fabrics are recycled according to the method described in any one of claims 1 to 9, and the system after waste polyester-cotton blended fabrics are recycled according to the method described in any one of claims 1 to 9 is subjected to reduced pressure distillation to restore the water content to the initial level.

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