Recovery method of waste cotton-polyester blended fabric

Abstract

The invention belongs to the technical field of regenerated polyester, and particularly relates to a waste cotton-polyester blended fabric recycling method which comprises the following steps: firstly, decolorizing a colored waste cotton-polyester blended fabric by adopting a decolorizing agent, then mixing a depolymerizing agent, isosorbide dimethyl ether, a first catalyst and the decolorized cotton-polyester blended fabric, and then carrying out depolymerization reaction, the decolorizing agent and the depolymerizing agent are more than one of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate, and the temperature of the depolymerizing reaction is 130-150 DEG C; washing the separated cotton fibers with water, and screening to remove residual polyester powder on the surfaces; firstly, terephthalate is converted into DMT, and then DMT, ethylene glycol, a second catalyst and a functional additive are subjected to a repolymerization reaction to obtain regenerated polyethylene glycol terephthalate. According to the method, depolymerization and polyester cotton separation processes are ingeniously fused, so that the operation steps are simplified, the energy consumption is reduced, and the purity of a depolymerization product is also improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of recycled polyester, and particularly relates to a method for recycling waste polyester-cotton blended fabrics. Background Art

[0002] With the rapid development of the global textile industry, the total amount of waste textiles has increased exponentially, and among them, polyester (PET) and cotton fiber blended fabrics (polyester-cotton blended fabrics) account for more than 60%. Such fabrics are widely used in the fields of clothing, home textiles and industry due to their excellent wear resistance, strong wrinkle resistance and low cost. However, it is difficult to efficiently separate and recycle them after being discarded, which seriously threatens the ecological environment and human health.

[0003] Polyester, as a petroleum-based synthetic fiber, depends on non-renewable resources for production; cotton fiber, as a natural cellulose material, consumes a large amount of water resources and pesticides during the planting process. The abandonment of both means a huge waste of resources. In addition, each polyester-cotton blended fabric contains 0.5-4.0 wt.% of dyes and auxiliaries, and these dyes will affect the physical and chemical properties of recycled products, bringing serious obstacles to the high-value utilization of waste textiles.

[0004] Currently, the methods for recovering cotton from polyester-cotton blended fabrics mainly include selective dissolution and selective depolymerization. Selective dissolution realizes the recovery of polymer from polymer by selectively dissolving one component in the blended fiber. For example, dimethyl sulfoxide (DMSO) and N,N-dimethylcyclohexylamine (DMCHA) can be used to dissolve polyethylene terephthalate (PET) and leach out dyes, and cotton can be recovered by filtration. However, this method has a slow dissolution process, high energy consumption, and is only applicable to polyester-cotton blended fabrics with a low polyester content. In addition, cellulose solvents such as N-methylmorpholine-N-oxide (NMMO), ionic liquids and alkaline / urea solvents can also be used to selectively dissolve the cotton component in the polyester-cotton blended fabric. Some ionic liquids can dissolve cotton at 80-130 °C, and then regenerated cotton fibers can be obtained by wet spinning. However, this treatment process may lead to cellulose degradation and crystal structure transformation, reducing the quality of the regenerated cotton, and the synthesis and recovery costs of ionic liquids are relatively high, which limits their application in industrial scenarios.

[0005] Selective depolymerization is to selectively decompose the polyester in the polyester-cotton blended fabric into monomers. This method has a fast separation speed and can produce monomers that can be used to produce recycled polyester from waste textiles. However, the traditional PET depolymerization conditions are harsh, requiring high temperature and high pressure, which easily leads to carbonization of cotton fibers or a decrease in monomer purity.

[0006] In order to deal with the above-mentioned difficulties, a number of patent technologies have emerged. Chinese invention patent CN 119192672 A discloses a low-energy rapid chemical separation method for polyester-cotton fiber products. The method prepares a betaine-based low eutectic solvent by mixing a hydrogen bond donor and a hydrogen bond acceptor, and combines it with an alkali solution to form a low eutectic solvent / alkali composite system. The polyester-cotton fiber product undergoes a dissolution reaction in the composite system, and after the reaction, the undissolved components are filtered and separated, and then washed and dried. This invention can dissolve the polyester fiber component in a short time under mild conditions without damaging the cotton fiber component. It has the advantages of low cost and green environmental protection, and realizes the rapid quantitative analysis of the two fiber components of polyester and cotton. However, long-term exposure of cotton fibers to an alkaline environment may cause slow oxidation of the cellulose molecular chain or a decrease in crystallinity, affecting its respinning performance, and the high cost of betaine limits its further industrial application.

[0007] Chinese invention patent CN 117626468 A discloses a method for preparing regenerated fibers by separating and recycling waste polyester and cotton. When preparing the regenerated fibers, the waste polyester and cotton fabrics are placed in a low eutectic solvent, the cellulose is separated by ultrasonic treatment, the cellulose is pre-treated and grafted with methacrylic anhydride, and the modified cellulose is combined with diethyl allylphosphonate under the action of a photoinitiator, benzophenone. In the process of blending polylactic acid and modified cellulose, the mechanical properties of the modified blended regenerated fibers are significantly improved, and the modified fibers contain phosphorus-oxygen double bonds, which effectively enhance the flame retardant properties of the regenerated fibers. However, this method involves multiple ultrasonic treatments, centrifugation, vacuum filtration, drying, etc., which results in a long overall process and high energy consumption; in addition, this method uses N,N-dimethylformamide (DMF) as a spinning solvent, which is toxic and may pollute the environment.

[0008] Therefore, ensuring the complete separation of all polyester-cotton blended textiles without affecting the quality of cotton has become an important issue that needs to be addressed. Summary of the invention

[0009] The purpose of the invention is to solve the problems existing in the prior art and provide a method for recycling waste polyester-cotton blended fabrics.

[0010] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0011] A method for recycling waste polyester-cotton blended fabrics. First, a decolorizing agent is used to decolorize the colored waste polyester-cotton blended fabrics. The objects to be decolorized are polyester and cotton fibers in the waste polyester-ammonia blended fabrics. Then, a depolymerizing agent, dimethyl isosorbide, a first catalyst, and the decolorized polyester-cotton blended fabrics are mixed and subjected to a depolymerization reaction. The object to be depolymerized is polyester in the decolorized polyester-cotton blended fabrics. Both the decolorizing agent and the depolymerizing agent are one or more of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The mass of dimethyl isosorbide is 10-40 wt.% of the total mass of the depolymerizing agent and dimethyl isosorbide. The temperature of the depolymerization reaction is 130-150 °C.

[0012] If dimethyl isosorbide is not added, a higher temperature is required for the depolymerization reaction to ensure sufficient depolymerization of polyester. At this temperature, cotton fibers are prone to carbonization or thermal degradation reactions at high temperatures, leading to the introduction of impurities. The present invention solves this problem by adding dimethyl isosorbide. Dimethyl isosorbide and the depolymerizing agent form a co-solvent, which can reduce the temperature of the depolymerization reaction on the premise of ensuring sufficient depolymerization of polyester. Because dimethyl isosorbide and the depolymerizing agent form a co-solvent system, the solvent synergistic effect significantly enhances the swelling ability of the solvent for polyester (PET); the molecular structure of dimethyl isosorbide contains an ether bond and a rigid cyclic structure, with moderate polarity and close solubility parameters to PET, enhancing the permeability of the solvent to PET and promoting the swelling of its amorphous and crystalline regions; the ether oxygen atom of dimethyl isosorbide can form weak hydrogen bonds with the ester groups of PET, reducing the intermolecular force between PET chains, thus breaking its crystalline structure and accelerating the cleavage of ester bonds at a lower temperature; in addition, the introduction of dimethyl isosorbide also reduces the viscosity of the reaction system, improves the fluidity of the solvent, promotes the full contact of PET with the depolymerizing agent, and avoids the occurrence of local overheating or uneven reaction, thereby reducing the overall process temperature requirement; the decomposition starting temperature of cotton fibers (cellulose) is about 220 °C, and the depolymerization of PET using only the depolymerizing agent usually requires a temperature above 160 °C. By adding dimethyl isosorbide, the depolymerization temperature can be reduced to 130-160 °C, far lower than the thermal degradation threshold of natural cellulose in cotton fibers, and carbonization or the generation of by-products such as furfural can be avoided.

[0013] As a preferred technical solution:

[0014] For the method for recycling waste polyester-cotton blended fabrics as described above, the specific process of using a decolorizing agent to decolorize the colored waste polyester-cotton blended fabrics is as follows: First, under the protection of nitrogen or an inert gas, the waste polyester-cotton blended fabrics are put into the decolorizing agent for preliminary decolorization and then subjected to solid-liquid separation to obtain a semi-finished product of decolorized polyester-cotton blended fabrics and a decolorizing solution. Then, under the protection of nitrogen or an inert gas, the semi-finished product of decolorized polyester-cotton blended fabrics is put into the decolorizing agent for deep decolorization and then subjected to solid-liquid separation to obtain decolorized polyester-cotton blended fabrics and a decolorizing solution. Finally, all the decolorizing solutions are recovered to obtain the recovered decolorizing agent and dyes.

[0015] In colored waste polyester-cotton blended fabrics, the short-range interaction between dyes and polyester mainly relies on van der Waals forces and hydrogen bonds; under high-temperature conditions of 130 - 140 °C (the dyeing temperature of polyester), the dyes will enter the amorphous region of polyester, which also means that if physical methods are to be used to decolorize such dyeing systems, the interactions between the decolorizing agent, dyes, and polyester need to be considered; during the physical decolorization process, when polyester swells in the decolorizing agent, the internal pores will expand. However, to achieve decolorization, effective interaction between dye molecules and the decolorizing agent must occur. If the decolorizing agent cannot dissolve the dyes and simultaneously break the binding force between the dyes and polyester, even if the polyester has swollen, the dyes will still remain on the polyester; therefore, during the physical decolorization process, swelling is a necessary prerequisite for decolorization but not a sufficient condition; taking toluene as an example, toluene can cause slight swelling of polyester, but due to its low solubility for dyes and its inability to break the binding force between dyes and polyester, it is difficult to decolorize waste colored polyester textiles.

[0016] The inventor of the present invention has conducted a large number of experiments and theoretical calculations. After numerous attempts and analyses, the decolorizing agent used in the present invention has been determined. The solubility parameter of this decolorizing agent is extremely close to that of polyester. This characteristic enables the decolorizing agent to smoothly penetrate into the interior of polyester fibers without dissolving the polyester, causing the polyester fibers to swell; this decolorizing agent can also dissolve the dyes and simultaneously break the binding force between the dyes and polyester, thus enabling the dyes to detach from the surface of the polyester fibers.

[0017] For the recycling method of waste polyester-cotton blended fabrics as described above, during preliminary decolorization, the liquor ratio (the mass ratio of waste polyester-cotton blended fabrics to the decolorizing agent) is 1:10 - 70, the decolorization temperature is 110 - 140 °C, and the decolorization time is 10 - 40 min;

[0018] After preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabrics calculated according to the K / S value is above 96%, the strength retention rate measured according to GB / T3923.1 - 2013 is above 95%, and the whiteness measured according to GB / T17644 - 2008 is 60 - 80;

[0019] During deep decolorization, the liquor ratio (the mass ratio of the semi-finished decolorized polyester-cotton blended fabrics to the decolorizing agent) is 1:10 - 70, the decolorization temperature is 110 - 140 °C, and the decolorization time is 10 - 40 min;

[0020] After deep decolorization, the decolorization rate of the waste polyester-cotton blended fabrics calculated according to the K / S value is above 98%, the strength retention rate measured according to GB / T3923.1 - 2013 is above 90%, and the whiteness measured according to GB / T17644 - 2008 is 70 - 80;

[0021] The solid-liquid separation method includes one or more of atmospheric pressure filtration, vacuum filtration (suction filtration), and centrifugal filtration;

[0022] The recovery treatment method includes one or more of distillation, vacuum distillation, rotary evaporation, and rectification, and the purity of the recovered decolorizing agent is 98 - 99.9%.

[0023] For the recovery method of a waste polyester-cotton blended fabric as described above, the depolymerization reaction is carried out under the protection of nitrogen or inert gas. After the depolymerization reaction, the cotton fibers are separated first, and then the depolymerization solution is separated and purified to obtain terephthalate.

[0024] For the recovery method of a waste polyester-cotton blended fabric as described above, the depolymerizing agent is the same as the decolorizing agent.

[0025] For the recovery method of a waste polyester-cotton blended fabric as described above, the molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 3 - 30:1, and the mass addition amount of the first catalyst is 0.1 - 5 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0026] Since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 50 - 90 wt.%. The first catalyst is a mixture of zinc acetate and a basic catalyst. The content of zinc acetate in the first catalyst is 50 - 100 wt.%. The basic catalyst is one or more of potassium carbonate, sodium carbonate, sodium hydroxide, and potassium hydroxide. Designing the first catalyst in this way can prevent the depolymerizing agent from decomposing;

[0027] The time of the depolymerization reaction is 30 - 200 min;

[0028] The steps for separating and purifying the depolymerization solution to obtain terephthalate are as follows:

[0029] (a) Carry out vacuum devolatilization on the depolymerization solution at 140 - 190 °C, collect the volatilized components removed, and the non-volatile components not removed contain isosorbide dimethyl ether. Utilize the characteristic that isosorbide dimethyl ether is easily soluble in water to separate it from other substances;

[0030] (b) Carry out centrifugal separation at 70 - 100 °C to obtain a solid product;

[0031] (c) Treat the solid product to obtain terephthalate;

[0032] The depolymerization rate of the polyester in the decolorized polyester-cotton blended fabric (= the mass reduction value of the polyester in the decolorized polyester-cotton blended fabric after depolymerization / the mass of the polyester in the decolorized polyester-cotton blended fabric before depolymerization × 100%) is 90 - 100%;

[0033] In the depolymerization reaction, the depolymerizing agent attacks the ester groups in the polyester molecular chain through a nucleophilic reaction, causing the polyester molecular chain to break. At the same time, the depolymerizing agent and ethylene glycol generated due to the breakage of the polyester molecular chain spontaneously form a ring to generate ethylene carbonate, thereby promoting the forward shift of the depolymerization reaction equilibrium and finally efficiently depolymerizing to obtain terephthalate; since the depolymerizing agent is inert to the natural cellulose structure of cotton fibers, the crystalline form and mechanical properties of cotton fibers are completely retained, and the original fabric form is basically maintained. This method not only realizes the rapid depolymerization of polyester but also completes the effective separation of polyester-cotton fibers.

[0034] For the recycling method of waste polyester-cotton blended fabric as described above, the separated cotton fibers are also washed with water and screened to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 98 - 100%.

[0035] For the recycling method of waste polyester-cotton blended fabric as described above, it also includes first converting terephthalate into DMT (if the terephthalate itself is DMT, no conversion is required), and then carrying out a repolymerization reaction on DMT, ethylene glycol, a second catalyst (tetrabutyl titanate, antimony glycolate, antimony acetate, antimony trioxide, acetates or alcoholates of manganese, sodium, cobalt, zinc, magnesium, potassium) and a functional additive (trimethyl phosphate, triphenyl phosphate, trimethyl phosphite, antioxidant 1010, titanium dioxide) to obtain regenerated polyethylene terephthalate (regenerated PET). The conversion method is: mix terephthalate, methanol, and sodium hydroxide (catalyst), heat to 60°C under nitrogen protection and react for 3 h, then stop heating, cool to room temperature, and wash with methanol. The mass ratio of terephthalate, methanol, and sodium hydroxide is 1:5:0.01.

[0036] For the recycling method of waste polyester-cotton blended fabric as described above, the cotton content in the waste polyester-cotton blended fabric does not exceed 30 wt.%.

[0037] Beneficial effects:

[0038] The decolorizing agent adopted in the present invention has excellent solubility, and its solubility parameter is close to that of polyester, which can efficiently remove dyes and additives in waste polyester-cotton blended fabric under mild conditions while avoiding damage to the fiber structure.

[0039] The decolorizing agent adopted in the present invention has the characteristic of low boiling point, all lower than 130°C. In particular, the boiling point of dimethyl carbonate is only 90°C. Compared with traditional high-boiling decolorizing agents, such as dimethyl sulfoxide with a boiling point of 189°C, the distillation and recovery temperature of the decolorizing agent of the present invention is reduced by more than 40%, and the energy consumption is reduced by about 35%. At the same time, the decolorizing agent of the present invention also has low toxicity and high volatility, which greatly reduces its residue in the regenerated material, not only reducing environmental and health risks but also improving the quality and application performance of the regenerated material, meeting the development direction of green chemistry.

[0040] The present invention uses the same type of decolorizing agent and depolymerizing agent in the whole process. In the decolorizing stage, the decolorizing agent can efficiently remove the dye in the waste polyester-cotton blended fabric; then the decolorizing agent is recycled and reused as a depolymerizing agent in the depolymerization stage, participating in the ester exchange reaction to generate high-purity depolymerized monomers. The process realizes efficient synergy of decolorizing and depolymerization, simplifies the overall process flow, improves production efficiency, and the efficient recovery of the decolorizing agent significantly reduces its loss and environmental pollution, providing a more economical and efficient solution for the industrial closed-loop recovery of waste polyester-cotton blended fabrics.

[0041] The present invention utilizes the synergistic effect of the depolymerizing agent and dimethyl isosorbide to achieve efficient depolymerization of polyester in waste polyester-cotton blended fabrics at a relatively low depolymerization temperature, while ensuring the integrity of the cotton fibers. This method cleverly combines the depolymerization and polyester-cotton separation processes, which not only simplifies the operating steps and reduces energy consumption, but also improves the purity of the depolymerization products. The regenerated polyester monomer obtained after depolymerization can be directly used in polyester synthesis, and the cotton fibers can be directly reused for spinning without secondary treatment. The present invention effectively opens up the full-chain closed-loop path of "decolorization-separation-repolymerization" of waste polyester-cotton blended fabrics, providing a new innovative idea for the green transformation of the textile industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The present invention is a process flow chart of the method for recycling waste polyester-cotton blended fabrics;

[0043] Figure 2 The figures are scanning electron microscope images of the waste polyester-cotton blended fabric in Example 1 before and after depolymerization, wherein (a) is a scanning electron microscope image before depolymerization, and (b) is a scanning electron microscope image after depolymerization;

[0044] Figure 3 The infrared spectra of the waste polyester-cotton blended fabric in Example 1 before and after depolymerization;

[0045] Figure 4 This is the H NMR spectrum of the terephthalate obtained after depolymerization of the waste polyester-cotton blended fabric in Example 1;

[0046] Figure 5 This is an infrared spectrum of terephthalate obtained after depolymerization of waste polyester-cotton blended fabric in Example 1;

[0047] Figure 6 The decolorizing agent and dye recovered in Example 1, wherein (a) is the decolorizing agent and (b) is the dye. DETAILED DESCRIPTION

[0048] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0049] The detection methods of relevant performance indicators in the following embodiments:

[0050] Decolorization rate: The calculation of the decolorization rate is based on the K / S value. The steps to obtain the K / S value are as follows: First, use a calibrated desktop spectrophotometer (model S81 of Tianjin Trus Technology Co., Ltd.) for measurement; the test conditions are set as D65 light source, 10° viewing angle, reflection mode UV400 cut-off, and φ18; in order to ensure that the fabric sample is opaque during color measurement, the fabric sample to be measured needs to be folded twice; the fabric sample to be measured is measured five times and the average value is taken; the fabric decolorization rate is calculated according to the following formula:

[0051] ;

[0052] In the formula, is the K / S value of the fabric before decolorization, is the K / S value of the fabric after decolorization.

[0053] Strength retention rate: Use a YG 026B type electronic fabric strength tester and test according to the standard of GB / T 3923.1-2013 "Textiles - Tensile properties of fabrics - Part 1: Determination of breaking force and elongation at break (strip method)", and calculate the strength retention rate of the fabric according to the following formula:

[0054] 00%;

[0055] In the formula, is the breaking force of the textile before decolorization, is the breaking force of the textile after decolorization.

[0056] Whiteness: Use a S81 type desktop spectrophotometer of Tianjin Trus Technology Co., Ltd. to obtain the L * , a * , b * values of the fabric sample (L * , a * , b * belong to the L * a * b * index system, L * is the lightness index, a * and b * are chromaticity indices), and calculate the fabric whiteness according to the following formula:

[0057] .

[0058] Wherein, W is the whiteness;

[0059] Each fabric sample was measured 5 times and the average value was taken.

[0060] Conversion rate of terephthalate:

[0061] Conversion rate of terephthalate (%) = ;

[0062] Wherein, is the mass (g) of dimethyl terephthalate collected, is the molar mass (g / mol) of terephthalate collected, is the mass (g) of polyester in waste polyester-cotton blended fabric, is the molar mass of the repeating unit of polyethylene terephthalate ( = 192 g / mol).

[0063] Purity of terephthalate: The purity of dimethyl terephthalate collected was quantitatively analyzed using a GC-2010 Pro gas chromatograph produced by Shimadzu Corporation of Japan, equipped with a flame ionization detector (FID) and a chromatographic data processing workstation, and the purity was fitted by the internal standard method; The content of dimethyl terephthalate was determined by the internal standard standard curve method, using dipropyl phthalate as the internal standard substance and chloroform as the solvent, and quantified by the peak area; Test conditions: The carrier gas is high-purity nitrogen (≥99.95%), the split ratio is 15:1, the chromatographic injection port temperature is set at 280 °C, the column temperature is 60 °C, the total flow rate is 3.7 mL / min, and the chromatographic column flow rate is 0.7 mL / min.

[0064] Oligomer content of terephthalate: The purity and oligomer content of terephthalate collected were qualitatively and quantitatively analyzed using an LC-16 high-performance liquid chromatograph (HPLC) equipped with a WondaSil C18-WR (200 mm, packing particle size 5 μm) chromatographic column and an ultraviolet detector produced by Shimadzu Corporation of Japan. The detection wavelength was 254 nm, the detection temperature was 40 °C, a binary gradient test method was used, the mobile phases were methanol and water (3:1, V / V), the flow rate was 0.8 mL / min, and the purity was fitted by the external standard method.

[0065] Conversion rate of cotton fiber:

[0066] Conversion rate of cotton fiber (%) = ;

[0067] Wherein, is the mass (g) of cotton in the colored waste polyester-cotton blended fabric, is the mass (g) of the collected cotton fibers.

[0068] The color fastness to washing of the colored waste polyester-cotton blended fabric used in the present invention is at least 4-5 levels, and it is prepared in two ways: blending first and then dyeing, and dyeing first and then blending.

[0069] Blending first and then dyeing means that after blending polyester staple fibers and cotton fibers in proportion into yarns or fabrics, dyeing is carried out (using two-bath method or one-bath two-step method).

[0070] Dyeing first and then blending means that polyester fibers and cotton fibers are dyed separately first, and then the two kinds of colored fibers are mixed and spun in proportion to make fabrics.

[0071] The polyester component is dyed by disperse dyes (azo disperse dyes, anthraquinone disperse dyes, heterocyclic disperse dyes, etc.) by high-temperature high-pressure dyeing method, carrier dyeing method, thermosol dyeing method or atmospheric pressure high-temperature dyeing method, or by cationic dyes through processes such as dipping and fixing, or by acid dyes through processes such as acid bath dyeing and fixing treatment, or by vat dyes through processes such as reduction, oxidation and soaping; the cotton component therein is dyed by direct dyes, reactive dyes, cationic dyes, vat dyes, etc. through processes such as impregnation, adsorption, fixing, soaping, water washing and drying.

[0072] In view of the fact that disperse dyes are common dyes for dyeing polyester and reactive dyes are common dyes for dyeing cotton, the following examples of the present invention are only described by taking the colored waste polyester-cotton blended fabric dyed with disperse dyes and reactive dyes as examples.

[0073] Example 1

[0074] A method for recycling waste polyester-cotton blended fabric, the process flow is as Figure 1 shown, and the specific steps are as follows:

[0075] (1) Material preparation:

[0076] Colored waste polyester-cotton blended fabric: The dyes used are disperse green 9 (azo disperse dye) and reactive green 19 (reactive dye), and the cotton content is 5wt.%.

[0077] Decolorant: Dimethyl carbonate;

[0078] Depolymerizing agent: Dimethyl carbonate;

[0079] Isosorbide dimethyl ether;

[0080] First catalyst: Zinc acetate;

[0081] Methanol;

[0082] Ethylene glycol;

[0083] Second catalyst: Zinc acetate, tetrabutyl titanate;

[0084] Functional additives: Trimethyl phosphate, antioxidant 1010;

[0085] (2) Decolorization:

[0086] (2.1) Under the protection of nitrogen or inert gas, put the waste polyester-cotton blended fabric into the decolorizing agent for preliminary decolorization and then carry out solid-liquid separation to obtain the semi-finished decolorized polyester-cotton blended fabric and the decolorizing solution;

[0087] Among them, during preliminary decolorization, the bath ratio is 1:70, the decolorization temperature is 140 °C, and the decolorization time is 40 min;

[0088] After preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 98%, the strength retention rate is 99%, and the whiteness is 80;

[0089] (2.2) Under the protection of nitrogen or inert gas, put the semi-finished decolorized polyester-cotton blended fabric into the decolorizing agent for deep decolorization and then carry out solid-liquid separation to obtain the decolorized polyester-cotton blended fabric and the decolorizing solution;

[0090] Among them, during deep decolorization, the bath ratio is 1:10, the decolorization temperature is 110 °C, and the decolorization time is 40 min;

[0091] After deep decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 99%, the strength retention rate is 96%, and the whiteness is 80;

[0092] Finally, all the decolorizing solutions in steps (2.1) and (2.2) need to be recovered by distillation to obtain the recovered decolorizing agent and dye (as shown in the physical diagram Figure 6 ), and the specific operating conditions are: under normal pressure, control the temperature at 90 - 95 °C to ensure that the decolorizing agent evaporates sufficiently until the volume of the distillate reaches more than 95% of the theoretical recovery amount;

[0093] (3) Depolymerization:

[0094] Mix the depolymerizing agent, isosorbide dimethyl ether, the first catalyst and the decolorized polyester-cotton blended fabric, and carry out a depolymerization reaction at 150 °C for 180 min (under the protection of nitrogen or inert gas). After the depolymerization reaction, first separate the cotton fibers, then carry out vacuum devolatilization of the depolymerization solution at 140 °C, and then carry out centrifugal separation at 70 °C to obtain a solid product. Mix the solid product with methanol at a mass ratio of 1:10, stir at a constant temperature of 60 °C for 30 min, selectively remove impurities by dissolution-recrystallization, and finally obtain terephthalate (i.e., DMT) through solid-liquid separation;

[0095] Among them, since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 50 wt.%, the mass of isosorbide dimethyl ether is 40 wt.% of the total mass of the depolymerizing agent and isosorbide dimethyl ether, the molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 30:1, and the mass addition amount of the first catalyst is 5 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0096] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 100%, the conversion rate of terephthalate is 100%, the purity of terephthalate is 99.9%, and the oligomer content of terephthalate is 0.01 wt.%;

[0097] The nuclear magnetic resonance hydrogen spectrum and infrared spectrum of the terephthalate separated and purified in this step are respectively as Figure 4 、as Figure 5 shown. It can be seen from Figure 4 that the chemical shifts and integral areas of each peak are all in line with the theoretical values of terephthalate; among them, the hydrogen proton peak on the benzene ring corresponds to the chemical shift of 8.11 ppm, and the methyl hydrogen proton peak directly connected to the ester group corresponds to the chemical shift of 4.02 ppm; after calculation, the ratio of the two peak areas (benzene ring hydrogen to methyl hydrogen) is 0.67 (4.00 / 6.00≈0.67), which indicates that the depolymerization reaction product is terephthalate and has a high purity;

[0098] (4)Other treatments:

[0099] Wash and screen the separated cotton fibers to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 100%;

[0100] Perform a repolymerization reaction on DMT, ethylene glycol, the second catalyst and the functional additive to obtain recycled polyethylene terephthalate. The specific steps are as follows:

[0101] Mix DMT, ethylene glycol, zinc acetate, and functional additives (trimethyl phosphate, antioxidant 1010) evenly to form reaction system A. Slowly raise the temperature of reaction system A to 220 °C at a heating rate of 30 °C / h for transesterification reaction, collect methanol, and when the methanol alcohol yield reaches 98% (methanol alcohol yield = actual alcohol yield / theoretical alcohol yield × 100%, the actual alcohol yield is the number of moles of collected methanol, and the theoretical alcohol yield is the number of moles of DMT × 2), the transesterification ends to obtain bis(2-hydroxyethyl) terephthalate; then mix the obtained bis(2-hydroxyethyl) terephthalate and tetrabutyl titanate evenly to form reaction system B. Slowly raise the temperature of reaction system B to 270 °C at a heating rate of 30 °C / h, evacuate to make the pressure of reaction system B gradually reach 15 Pa, and carry out a polycondensation reaction for 180 min to obtain recycled polyethylene terephthalate;

[0102] Among them, the mass ratio of DMT, ethylene glycol, zinc acetate, tetrabutyl titanate, trimethyl phosphate, and antioxidant 1010 is 1:0.6:0.05:0.05:0.05:0.05;

[0103] The microscopic morphology of the waste polyester-cotton blended fabric before and after depolymerization in this example is as Figure 2 shown, and the infrared spectrogram before and after depolymerization is as Figure 3 shown; from Figure 3 it can be seen that the strong absorption peak at about 1710 cm -1 disappears after depolymerization, while the broad peak at about 3300 - 3600 cm -1 , the strong absorption peak at about 1100 cm -1 , and the absorption peak at about 2900 cm -1 still exist, and there is no interference from other impurity peaks. This indicates that the polyester is completely depolymerized, and only cotton fibers remain after depolymerization, and the chemical structure of the cotton fibers is not damaged; at the same time, from Figure 2 it can be seen that after depolymerization, the microscopic morphology of the cotton fibers is basically the same as that of the original fabric, and the fabric form is still maintained. This comparison of microscopic morphology further confirms that the method for recycling waste polyester-cotton blended fabric proposed by the present invention can efficiently depolymerize polyester while perfectly retaining the fabric form of cotton fibers, laying a structural foundation for the subsequent separation and reuse of cotton fibers.

[0104] Comparative Example 1

[0105] A method for recycling waste polyester-cotton blended fabric is basically the same as Example 1, except that isosorbide dimethyl ether is not used.

[0106] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 70%, far lower than 100% in Example 1. The conversion rate of terephthalate is 65%, the purity of terephthalate is 89%, and the oligomer content of terephthalate is 10 wt.%.

[0107] It can be seen from the comparison that without adding isosorbide dimethyl ether, even if the same depolymerization temperature and time are maintained, the depolymerization rate of polyester will be significantly reduced, and some polyester is not completely decomposed into monomers, resulting in the mixing of unreacted oligomer impurities and by-products in the product. These factors together make the subsequent separation of polyester and cotton difficult.

[0108] Example 2

[0109] A method for recycling waste polyester-cotton blended fabric is as follows:

[0110] (1) Material preparation:

[0111] Colored waste polyester-cotton blended fabric: The dyes used are Disperse Yellow 3 (azo disperse dye) and Reactive Yellow M-3RE (reactive dye), and the cotton content is 8 wt.%;

[0112] Decolorizing agent: A mixture of dimethyl carbonate and diethyl carbonate with a molar ratio of 2:1;

[0113] Depolymerizing agent: A mixture of dimethyl carbonate and diethyl carbonate with a molar ratio of 2:1;

[0114] Isosorbide dimethyl ether;

[0115] First catalyst: A mixture of zinc acetate and a basic catalyst, where the content of zinc acetate is 90 wt.%, and the basic catalyst is sodium carbonate;

[0116] Methanol;

[0117] Sodium hydroxide;

[0118] Ethylene glycol;

[0119] Second catalyst: Manganese acetate, antimony glycolate;

[0120] Functional additive: Triphenyl phosphate, antioxidant 1010;

[0121] (2) Decolorization:

[0122] (2.1) Under the protection of nitrogen or inert gas, put the waste polyester-cotton blended fabric into the decolorizing agent for preliminary decolorization and then carry out solid-liquid separation to obtain a semi-finished product of decolorized polyester-cotton blended fabric and a decolorizing solution;

[0123] Among them, during preliminary decolorization, the bath ratio is 1:60, the decolorization temperature is 130 °C, and the decolorization time is 30 min;

[0124] After preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 98%, the strength retention rate is 98%, and the whiteness is 80;

[0125] (2.2) Under the protection of nitrogen or inert gas, put the semi-finished product of decolorized polyester-cotton blended fabric into the decolorizing agent for deep decolorization and then carry out solid-liquid separation to obtain a decolorized polyester-cotton blended fabric and a decolorizing solution;

[0126] Among them, during deep decolorization, the bath ratio is 1:20, the decolorization temperature is 110 °C, and the decolorization time is 30 min;

[0127] After deep decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 99%, the strength retention rate is 95%, and the whiteness is 80;

[0128] Finally, all the decolorizing solutions in steps (2.1) and (2.2) need to be recovered by distillation to obtain the recovered decolorizing agent and dye. The specific operating conditions are as follows: Under normal pressure, the temperature is controlled at 130 - 135 °C to ensure that the decolorizing agent evaporates sufficiently until the volume of the distillate reaches more than 95% of the theoretical recovery amount;

[0129] (3) Depolymerization:

[0130] After mixing the depolymerizing agent, isosorbide dimethyl ether, the first catalyst, and the decolorized polyester-cotton blended fabric, carry out a depolymerization reaction at 145 °C for 200 min (protected by nitrogen or inert gas). After the depolymerization reaction, first separate the cotton fibers, then carry out vacuum devolatilization of the depolymerization solution at 160 °C, and then carry out centrifugal separation at 90 °C to obtain a solid product. Mix the solid product, methanol, and sodium hydroxide in a mass ratio of 1:5:0.01, and react at 65 °C for 3 h (protected by nitrogen) to obtain a transesterification solution. The transesterification solution is cooled to 25 °C to precipitate a solid, and after filtration and washing with methanol, terephthalate is obtained;

[0131] Among them, since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 50 wt.%, the mass of isosorbide dimethyl ether is 30 wt.% of the total mass of the depolymerizing agent and isosorbide dimethyl ether, the molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 25:1, and the mass addition amount of the first catalyst is 3.5 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0132] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 100%;

[0133] (4) Other treatments:

[0134] Wash and screen the separated cotton fibers to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 100%;

[0135] First convert terephthalate into DMT, and then carry out a repolymerization reaction on DMT, ethylene glycol, the second catalyst, and the functional additive to obtain regenerated polyethylene terephthalate. The specific steps are as follows:

[0136] Mix DMT, ethylene glycol, manganese acetate, and functional additives (triphenyl phosphate, antioxidant 1010) evenly to form reaction system A. Slowly raise the temperature of reaction system A to 220 °C at a heating rate of 30 °C / h for transesterification reaction, and collect methanol. When the methanol yield reaches 98% (methanol yield = actual methanol output / theoretical methanol output × 100%, the actual methanol output is the molar number of collected methanol, and the theoretical methanol output is the molar number of DMT × 2), the transesterification ends, and bis(2-hydroxyethyl) terephthalate is obtained. Then, mix the obtained bis(2-hydroxyethyl) terephthalate and antimony glycolate evenly to form reaction system B. Slowly raise the temperature of reaction system B to 270 °C at a heating rate of 30 °C / h, evacuate to make the pressure of reaction system B gradually reach 15 Pa, and carry out polycondensation reaction for 180 min to obtain recycled polyethylene terephthalate;

[0137] Among them, the mass ratio of DMT, ethylene glycol, manganese acetate, antimony glycolate, triphenyl phosphate, and antioxidant 1010 is 1:0.6:0.05:0.05:0.05:0.05.

[0138] Example 3

[0139] A method for recycling waste polyester-cotton blended fabrics, the process flow is as Figure 1 shown, and the specific steps are as follows:

[0140] (1) Material preparation:

[0141] Colored waste polyester-cotton blended fabric: The dyes used are Disperse Blue 183 (azo disperse dye) and Reactive Blue 19 (reactive dye), and the cotton content is 10 wt.%.

[0142] Decolorant: A mixture of dimethyl carbonate and ethyl methyl carbonate with a molar ratio of 1:1;

[0143] Depolymerizer: A mixture of dimethyl carbonate and ethyl methyl carbonate with a molar ratio of 1:1;

[0144] Isosorbide dimethyl ether;

[0145] First catalyst: A mixture of zinc acetate and a basic catalyst, where the content of zinc acetate is 80 wt.%, and the basic catalyst is sodium hydroxide;

[0146] Methanol;

[0147] Sodium hydroxide;

[0148] Ethylene glycol;

[0149] Second catalyst: Zinc acetate, antimony trioxide;

[0150] Functional additive: Trimethyl phosphate, antioxidant 1010;

[0151] (2)Decolorization:

[0152] (2.1)Under the protection of nitrogen or inert gas, put the waste polyester-cotton blended fabric into the decolorizing agent for preliminary decolorization, and then carry out solid-liquid separation to obtain the semi-finished decolorized polyester-cotton blended fabric and the decolorizing solution;

[0153] Among them, during preliminary decolorization, the bath ratio is 1:50, the decolorization temperature is 130 °C, and the decolorization time is 25 min;

[0154] After preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 98%, the strength retention rate is 98%, and the whiteness is 80;

[0155] (2.2)Under the protection of nitrogen or inert gas, put the semi-finished decolorized polyester-cotton blended fabric into the decolorizing agent for deep decolorization, and then carry out solid-liquid separation to obtain the decolorized polyester-cotton blended fabric and the decolorizing solution;

[0156] Among them, during deep decolorization, the bath ratio is 1:30, the decolorization temperature is 120 °C, and the decolorization time is 30 min;

[0157] After deep decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 99%, the strength retention rate is 95%, and the whiteness is 80;

[0158] Finally, all the decolorizing solutions in steps (2.1) and (2.2) need to be recovered by vacuum distillation to obtain the recovered decolorizing agent and dye. The specific operating conditions are: under reduced pressure, control the temperature at 70 - 80 °C to ensure that the decolorizing agent evaporates sufficiently until the volume of the distillate reaches more than 95% of the theoretical recovery amount;

[0159] (3)Depolymerization:

[0160] Mix the depolymerizing agent, isosorbide dimethyl ether, the first catalyst and the decolorized polyester-cotton blended fabric, and carry out the depolymerization reaction at 140 °C for 120 min (under the protection of nitrogen or inert gas). After the depolymerization reaction, first separate the cotton fibers, then carry out vacuum devolatilization of the depolymerization solution at 180 °C, and then carry out centrifugal separation at 100 °C to obtain a solid product. Mix the solid product, methanol, and sodium hydroxide in a mass ratio of 1:5:0.01, and react at 65 °C for 3 h (under nitrogen protection) to obtain the transesterification solution. Cool the transesterification solution to 25 °C to precipitate a solid, and after filtration and washing with methanol, obtain terephthalate;

[0161] Among them, since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 60 wt.%. The mass of isosorbide dimethyl ether is 25 wt.% of the total mass of the depolymerizing agent and isosorbide dimethyl ether. The molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 20:1. The mass addition amount of the first catalyst is 2 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0162] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 99%;

[0163] (4) Other treatments:

[0164] The separated cotton fibers are washed with water and screened to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 99%;

[0165] First, convert terephthalate into DMT, and then carry out a repolymerization reaction on DMT, ethylene glycol, a second catalyst, and a functional additive to obtain recycled polyethylene terephthalate. The specific steps are as follows:

[0166] Mix DMT, ethylene glycol, zinc acetate, and functional additives (trimethyl phosphate, antioxidant 1010) evenly to form reaction system A. Slowly raise the temperature of reaction system A to 220 °C at a heating rate of 30 °C / h for transesterification reaction, and collect methanol. When the alcohol yield of methanol reaches 98% (the alcohol yield of methanol = actual alcohol yield / theoretical alcohol yield × 100%, the actual alcohol yield is the number of moles of collected methanol, and the theoretical alcohol yield is the number of moles of DMT × 2), the transesterification ends, and bis(2-hydroxyethyl) terephthalate is obtained. Then, mix the obtained bis(2-hydroxyethyl) terephthalate and antimony trioxide evenly to form reaction system B. Slowly raise the temperature of reaction system B to 270 °C at a heating rate of 30 °C / h, evacuate to make the pressure of reaction system B gradually reach 15 Pa, and carry out a polycondensation reaction for 180 min to obtain recycled polyethylene terephthalate;

[0167] Among them, the mass ratio of DMT, ethylene glycol, zinc acetate, antimony trioxide, trimethyl phosphate, and antioxidant 1010 is 1:0.6:0.05:0.05:0.05:0.05.

[0168] Example 4

[0169] A method for recycling waste polyester-cotton blended fabric, the specific steps are as follows:

[0170] (1) Material preparation:

[0171] Colored waste polyester-cotton blended fabric: The dyes used are Disperse Red 60 (anthraquinone disperse dye) and Reactive Red 195 (reactive dye), and the cotton content is 15 wt.%;

[0172] Decolorizing agent: ethyl methyl carbonate;

[0173] Depolymerizing agent: ethyl methyl carbonate;

[0174] Isosorbide dimethyl ether;

[0175] First catalyst: a mixture of zinc acetate and a basic catalyst, wherein the content of zinc acetate is 70 wt.%, and the basic catalyst is potassium hydroxide;

[0176] Methanol;

[0177] Sodium hydroxide;

[0178] Ethylene glycol;

[0179] Second catalyst: manganese acetate, tetrabutyl titanate;

[0180] Functional additive: triphenyl phosphate, antioxidant 1010;

[0181] (2) Decolorization:

[0182] (2.1) Under the protection of nitrogen or inert gas, put the waste polyester-cotton blended fabric into the decolorizing agent for preliminary decolorization, and then carry out solid-liquid separation to obtain a semi-finished product of decolorized polyester-cotton blended fabric and a decolorizing solution;

[0183] Among them, during the preliminary decolorization, the bath ratio is 1:40, the decolorization temperature is 120 °C, and the decolorization time is 20 min;

[0184] After the preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 97%, the strength retention rate is 97%, and the whiteness is 75;

[0185] (2.2) Under the protection of nitrogen or inert gas, put the semi-finished product of decolorized polyester-cotton blended fabric into the decolorizing agent for deep decolorization, and then carry out solid-liquid separation to obtain a decolorized polyester-cotton blended fabric and a decolorizing solution;

[0186] Among them, during the deep decolorization, the bath ratio is 1:50, the decolorization temperature is 130 °C, and the decolorization time is 20 min;

[0187] After the deep decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 98%, the strength retention rate is 93%, and the whiteness is 78;

[0188] Finally, all the decolorizing solutions in steps (2.1) and (2.2) need to be recovered by vacuum distillation to obtain the recovered decolorizing agent and dye. The specific operating conditions are: under reduced pressure, control the temperature at 70 - 80 °C to ensure that the decolorizing agent evaporates fully until the volume of the distillate reaches more than 95% of the theoretical recovery amount;

[0189] (3) Depolymerization:

[0190] After mixing the depolymerizing agent, dimethyl isosorbide, the first catalyst and the decolorized polyester-cotton blended fabric, carry out the depolymerization reaction at 140 °C for 90 min (under the protection of nitrogen or inert gas). After the depolymerization reaction, first separate the cotton fibers, then carry out devolatilization under reduced pressure on the depolymerization solution at 190 °C, and then carry out centrifugal separation at 80 °C to obtain a solid product. After mixing the solid product, methanol and sodium hydroxide in a mass ratio of 1:5:0.01, react at 65 °C for 3 h (under nitrogen protection) to obtain a transesterification solution. The transesterification solution is cooled to 25 °C to precipitate a solid, and after filtration and washing with methanol, terephthalate is obtained;

[0191] Among them, since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 70 wt.%. The mass of dimethyl isosorbide is 20 wt.% of the total mass of the depolymerizing agent and dimethyl isosorbide. The molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 15:1. The mass addition amount of the first catalyst is 1 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0192] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 98%;

[0193] (4) Other treatments:

[0194] Wash and screen the separated cotton fibers to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 99%;

[0195] First convert terephthalate into DMT, and then carry out a repolymerization reaction on DMT, ethylene glycol, the second catalyst and the functional additive to obtain regenerated polyethylene terephthalate. The specific steps are as follows:

[0196] Mix DMT, ethylene glycol, manganese acetate and functional additives (triphenyl phosphate, antioxidant 1010) evenly to form reaction system A. Slowly raise the temperature of reaction system A to 220 °C at a heating rate of 30 °C / h for the transesterification reaction, and collect methanol. When the methanol alcohol yield reaches 98% (methanol alcohol yield = actual alcohol yield / theoretical alcohol yield × 100%, the actual alcohol yield is the number of moles of methanol collected, and the theoretical alcohol yield is the number of moles of DMT × 2), the transesterification ends, and bis(2-hydroxyethyl) terephthalate is obtained; then mix the obtained bis(2-hydroxyethyl) terephthalate and tetrabutyl titanate evenly to form reaction system B. Slowly raise the temperature of reaction system B to 270 °C at a heating rate of 30 °C / h, evacuate to make the pressure of reaction system B gradually reach 15 Pa, and carry out a polycondensation reaction for 180 min to obtain regenerated polyethylene terephthalate;

[0197] Among them, the mass ratio of DMT, ethylene glycol, manganese acetate, tetrabutyl titanate, triphenyl phosphate, and antioxidant 1010 is 1:0.6:0.05:0.05:0.05:0.05.

[0198] Example 5

[0199] A method for recycling waste polyester-cotton blended fabrics, the specific steps are as follows:

[0200] (1) Material preparation:

[0201] Colored waste polyester-cotton blended fabrics: The dyes used are Disperse Yellow 64 (heterocyclic disperse dye) and Reactive Yellow 4 (reactive dye), and the cotton content is 20 wt.%.

[0202] Decolorizing agent: A mixture of ethyl methyl carbonate and diethyl carbonate with a molar ratio of 2:1;

[0203] Depolymerizing agent: A mixture of ethyl methyl carbonate and diethyl carbonate with a molar ratio of 2:1;

[0204] Isosorbide dimethyl ether;

[0205] First catalyst: A mixture of zinc acetate and a basic catalyst, among which the content of zinc acetate is 60 wt.%, and the basic catalyst is a mixture of potassium carbonate and potassium hydroxide with a molar ratio of 1:1;

[0206] Methanol;

[0207] Sodium hydroxide;

[0208] Ethylene glycol;

[0209] Second catalyst: Magnesium acetate, ethylene glycol antimonate;

[0210] Functional additives: Trimethyl phosphate, antioxidant 1010;

[0211] (2) Decolorization:

[0212] (2.1) Under the protection of nitrogen or inert gas, put the waste polyester-cotton blended fabric into the decolorizing agent for preliminary decolorization and then carry out solid-liquid separation to obtain a semi-finished product of decolorized polyester-cotton blended fabric and a decolorized solution;

[0213] Among them, during the preliminary decolorization, the bath ratio is 1:20, the decolorization temperature is 120 °C, and the decolorization time is 15 min;

[0214] After the preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 97%, the strength retention rate is 96%, and the whiteness is 70;

[0215] (2.2) Under the protection of nitrogen or inert gas, put the semi-finished decolorized polyester-cotton blended fabric into the decolorizing agent for deep decolorization, and then carry out solid-liquid separation to obtain the decolorized polyester-cotton blended fabric and the decolorizing solution;

[0216] Among them, during deep decolorization, the bath ratio is 1:60, the decolorization temperature is 130 °C, and the decolorization time is 20 min;

[0217] After deep decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 98%, the strength retention rate is 92%, and the whiteness is 75;

[0218] Finally, all the decolorizing solutions in steps (2.1) and (2.2) need to be recovered by rotary evaporation to obtain the recovered decolorizing agent and dye. The specific operating conditions are as follows: under reduced pressure, control the temperature at 90 - 100 °C to ensure that the decolorizing agent evaporates sufficiently until the volume of the distillate reaches more than 95% of the theoretical recovery amount;

[0219] (3) Depolymerization:

[0220] Mix the depolymerizing agent, isosorbide dimethyl ether, the first catalyst and the decolorized polyester-cotton blended fabric, and carry out a depolymerization reaction at 135 °C for 60 min (under the protection of nitrogen or inert gas). After the depolymerization reaction, first separate the cotton fibers, then carry out vacuum devolatilization of the depolymerization solution at 150 °C, and then carry out centrifugal separation at 80 °C to obtain a solid product. Mix the solid product, methanol, and sodium hydroxide in a mass ratio of 1:5:0.01, and react at 65 °C for 3 h (under nitrogen protection) to obtain a transesterification solution. Cool the transesterification solution to 25 °C to precipitate a solid, and after filtration and washing with methanol, obtain terephthalate;

[0221] Among them, since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 80 wt.%, the mass of isosorbide dimethyl ether is 15 wt.% of the total mass of the depolymerizing agent and isosorbide dimethyl ether, the molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 10:1, and the mass addition amount of the first catalyst is 0.5 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0222] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 95%;

[0223] (4) Other treatments:

[0224] Wash and screen the separated cotton fibers to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 98%;

[0225] First convert terephthalate into DMT, and then carry out a repolymerization reaction on DMT, ethylene glycol, the second catalyst and the functional additive to obtain recycled polyethylene terephthalate. The specific steps are as follows:

[0226] Mix DMT, ethylene glycol, magnesium acetate, and functional additives (trimethyl phosphate, antioxidant 1010) evenly to form reaction system A. Slowly raise the temperature of reaction system A to 220°C at a heating rate of 30°C / h for transesterification reaction, and collect methanol. When the methanol yield reaches 98% (methanol yield = actual methanol output / theoretical methanol output × 100%, the actual methanol output is the number of moles of collected methanol, and the theoretical methanol output is the number of moles of DMT × 2), the transesterification ends, and bis(2-hydroxyethyl) terephthalate is obtained. Then mix the obtained bis(2-hydroxyethyl) terephthalate and antimony glycolate evenly to form reaction system B. Slowly raise the temperature of reaction system B to 270°C at a heating rate of 30°C / h, evacuate to make the pressure of reaction system B gradually reach 15 Pa, and carry out polycondensation reaction for 180 min to obtain recycled polyethylene terephthalate;

[0227] Among them, the mass ratio of DMT, ethylene glycol, magnesium acetate, antimony glycolate, trimethyl phosphate, and antioxidant 1010 is 1:0.6:0.05:0.05:0.05:0.05.

[0228] Example 6

[0229] A method for recycling waste polyester-cotton blended fabrics, the specific steps are as follows:

[0230] (1) Material preparation:

[0231] Colored waste polyester-cotton blended fabric: The dyes used are Disperse Blue 73 (anthraquinone disperse dye) and Reactive Blue 19 (reactive dye), and the cotton content is 30 wt.%.

[0232] Decolorant: Diethyl carbonate;

[0233] Depolymerizer: Diethyl carbonate;

[0234] Isosorbide dimethyl ether;

[0235] First catalyst: A mixture of zinc acetate and a basic catalyst, where the content of zinc acetate is 50 wt.%, and the basic catalyst is a mixture of sodium carbonate and sodium hydroxide with a molar ratio of 1:1;

[0236] Methanol;

[0237] Sodium hydroxide;

[0238] Ethylene glycol;

[0239] Second catalyst: Sodium acetate, antimony trioxide;

[0240] Functional additive: Triphenyl phosphate, antioxidant 1010;

[0241] (2) Decolorization:

[0242] (2.1) Under the protection of nitrogen or inert gas, put the waste polyester-cotton blended fabric into the decolorizing agent for preliminary decolorization, and then carry out solid-liquid separation to obtain the semi-finished product of decolorized polyester-cotton blended fabric and the decolorizing solution;

[0243] Among them, during the preliminary decolorization, the liquor ratio is 1:10, the decolorization temperature is 110 °C, and the decolorization time is 10 min;

[0244] After the preliminary decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 96%, the strength retention rate is 95%, and the whiteness is 60;

[0245] (2.2) Under the protection of nitrogen or inert gas, put the semi-finished product of decolorized polyester-cotton blended fabric into the decolorizing agent for deep decolorization, and then carry out solid-liquid separation to obtain the decolorized polyester-cotton blended fabric and the decolorizing solution;

[0246] Among them, during the deep decolorization, the liquor ratio is 1:70, the decolorization temperature is 140 °C, and the decolorization time is 10 min;

[0247] After the deep decolorization, the decolorization rate of the waste polyester-cotton blended fabric is 98%, the strength retention rate is 90%, and the whiteness is 70;

[0248] Finally, all the decolorizing solutions in steps (2.1) and (2.2) need to be recovered by rectification to obtain the recovered decolorizing agent and dye. The specific operating conditions are as follows: set the reflux ratio (the volume ratio of the reflux liquid to the distillate) to 3:1, control the top temperature of the tower at 75 - 85 °C, maintain the bottom temperature of the tower at 90 - 100 °C, and set the temperature of the top condenser of the tower at 5 - 10 °C to ensure sufficient condensation of the vapor;

[0249] (3) Depolymerization:

[0250] Mix the depolymerizing agent, isosorbide dimethyl ether, the first catalyst and the decolorized polyester-cotton blended fabric, and carry out the depolymerization reaction at 130 °C for 30 min (under the protection of nitrogen or inert gas). After the depolymerization reaction, first separate the cotton fibers, then carry out vacuum devolatilization of the depolymerization solution at 170 °C, and then carry out centrifugal separation at 70 °C to obtain a solid product. Mix the solid product, methanol, and sodium hydroxide in a mass ratio of 1:5:0.01, and react at 65 °C for 3 h (under the protection of nitrogen) to obtain a transesterification solution. Cool the transesterification solution to 25 °C to precipitate a solid, and after filtration and washing with methanol, obtain terephthalate;

[0251] Among them, since the depolymerizing agent is the same as the decolorizing agent, the recovered decolorizing agent can be used as a component of the depolymerizing agent. The proportion of the recovered decolorizing agent in the depolymerizing agent is 90 wt.%, the mass of isosorbide dimethyl ether is 10 wt.% of the total mass of the depolymerizing agent and isosorbide dimethyl ether, the molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 3:1, and the mass addition amount of the first catalyst is 0.1 wt.% of the mass of the decolorized polyester-cotton blended fabric;

[0252] The depolymerization rate of polyester in the decolorized polyester-cotton blended fabric is 90%;

[0253] (4) Other treatments:

[0254] Wash and screen the separated cotton fibers to remove the residual polyester powder on the surface; the conversion rate of cotton fibers is 98%;

[0255] First, convert terephthalate to DMT, and then carry out a repolymerization reaction on DMT, ethylene glycol, the second catalyst, and the functional additive to obtain regenerated polyethylene terephthalate. The specific steps are as follows:

[0256] Mix DMT, ethylene glycol, sodium acetate, and functional additives (triphenyl phosphate, antioxidant 1010) evenly to form reaction system A, and slowly raise the temperature of reaction system A to 220 °C at a heating rate of 30 °C / h for transesterification reaction, collect methanol. When the methanol yield reaches 98% (methanol yield = actual methanol yield / theoretical methanol yield × 100%, the actual methanol yield is the number of moles of collected methanol, and the theoretical methanol yield is the number of moles of DMT × 2), the transesterification ends, and bis(2-hydroxyethyl) terephthalate is obtained; then mix the obtained bis(2-hydroxyethyl) terephthalate and antimony trioxide evenly to form reaction system B, and slowly raise the temperature of reaction system B to 270 °C at a heating rate of 30 °C / h, evacuate to make the pressure of reaction system B gradually reach 15 Pa, and carry out a polycondensation reaction for 180 min to obtain regenerated polyethylene terephthalate;

[0257] Among them, the mass ratio of DMT, ethylene glycol, sodium acetate, antimony trioxide, triphenyl phosphate, and antioxidant 1010 is 1:0.6:0.05:0.05:0.05:0.05.

Claims

1. A method for recycling waste polyester-cotton blended fabrics, characterized in that: First, a decolorizing agent is used to decolorize colored waste polyester-cotton blended fabrics, and the decolorizing objects are polyester and cotton fibers in the waste polyester-spandex blended fabrics. Then, a depolymerizing agent, dimethyl isosorbide, a first catalyst and the decolorized polyester-cotton blended fabric are mixed and depolymerized, and the depolymerization object is the polyester in the decolorized polyester-cotton blended fabric. The decolorizing agent and the depolymerizing agent are both one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. The mass of dimethyl isosorbide is 10-40wt.% of the total mass of the depolymerizing agent and dimethyl isosorbide. The temperature of the depolymerization reaction is 130-150°C.

2. The method for recycling waste polyester-cotton blended fabric according to claim 1, characterized in that: The specific process of using a decolorizing agent to decolorize colored waste polyester-cotton blended fabrics is as follows: first, under the protection of nitrogen or an inert gas, the waste polyester-cotton blended fabrics are placed in a decolorizing agent for preliminary decolorization, followed by solid-liquid separation to obtain a decolorized polyester-cotton blended fabric semi-finished product and a decolorization solution; then, under the protection of nitrogen or an inert gas, the decolorized polyester-cotton blended fabric semi-finished product is placed in a decolorizing agent for deep decolorization, followed by solid-liquid separation to obtain a decolorized polyester-cotton blended fabric and a decolorization solution; finally, all the decolorization solutions are recovered to obtain recovered decolorizing agents and dyes.

3. The method for recycling waste polyester-cotton blended fabric according to claim 2, characterized in that: During the initial decolorization, the bath ratio is 1:10-70, the decolorization temperature is 110-140℃, and the decolorization time is 10-40min; After preliminary decolorization, the decolorization rate of waste polyester-cotton blended fabrics calculated according to the K / S value is more than 96%, the strength retention rate measured according to GB / T3923.1-2013 is more than 95%, and the whiteness measured according to GB / T17644-2008 is 60-80; For deep decolorization, the bath ratio is 1:10-70, the decolorization temperature is 110-140℃, and the decolorization time is 10-40min; After deep bleaching, the bleaching rate of waste polyester-cotton blended fabric calculated according to the K / S value is more than 98%, the strength retention rate measured according to GB / T3923.1-2013 is more than 90%, and the whiteness measured according to GB / T17644-2008 is 70-80.

4. The method for recycling waste polyester-cotton blended fabric according to claim 1, characterized in that: The depolymerization reaction is carried out under the protection of nitrogen or inert gas. After the depolymerization reaction, the cotton fiber is firstly separated, and then the depolymerization solution is separated and purified to obtain terephthalate.

5. The method for recycling waste polyester-cotton blended fabric according to claim 4, characterized in that: Depolymerizing agent is the same as decolorizing agent.

6. The method for recycling waste polyester-cotton blended fabric according to claim 5, characterized in that: The molar ratio of the depolymerizing agent to the polyester component in the decolorized polyester-cotton blended fabric is 3-30:1, and the mass addition amount of the first catalyst is 0.1-5wt.% of the mass of the decolorized polyester-cotton blended fabric; The decolorizing agent recovered in the depolymerizing agent accounts for 50-90wt.%, the first catalyst is a mixture of zinc acetate and an alkaline catalyst, the content of zinc acetate in the first catalyst is 50-100wt.%, and the alkaline catalyst is one or more of potassium carbonate, sodium carbonate, sodium hydroxide and potassium hydroxide; The depolymerization reaction time is 30-200min; The depolymerization rate of polyester in the bleached polyester-cotton blended fabric is 90-100%.

7. The method for recycling waste polyester-cotton blended fabric according to claim 4, characterized in that: The separated cotton fibers are also washed and screened to remove the polyester powder remaining on the surface; the conversion rate of cotton fibers is 98-100%.

8. The method for recycling waste polyester-cotton blended fabric according to claim 4, characterized in that: The method also includes the steps of first converting terephthalate into DMT, and then subjecting DMT, ethylene glycol, a second catalyst and a functional auxiliary agent to a repolymerization reaction to obtain regenerated polyethylene terephthalate.

9. The method for recycling waste polyester-cotton blended fabric according to claim 1, characterized in that: The cotton content in waste polyester-cotton blended fabrics shall not exceed 30wt.%.

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