Method for preparing regenerated flame-retardant high-shrinkage polyester by methanol alcoholysis

Abstract

The application discloses a method for preparing regenerated flame-retardant high-shrinkage polyester by methanol alcoholysis, and belongs to the technical field of polyester. The method comprises the following steps: (a) mixing waste polyester fabric with methanol to perform catalytic depolymerization, to obtain a regenerated DMT mixed solution; (b) performing impurity separation and methanol recovery treatment on the regenerated DMT mixed solution, and then performing cooling crystallization, centrifugal drying and rectification purification treatment, to obtain pure DMT; (c) performing ester exchange reaction on the pure DMT, ethylene glycol, an ester exchange catalyst and a DEG inhibitor; (d) adding CEPPA-EG and IPA-EG to perform copolymerization after the ester exchange reaction; and (e) adding a polymerization catalyst, a stabilizer and an antioxidant to perform polymerization reaction, to obtain a modified polyester melt; and (f) performing pelletizing and drying, to obtain a slice. The regenerated polyester prepared by the method has the functions of flame retardation, high boiling water shrinkage, low-temperature dyeing and the like, and is suitable for fields of high-end clothes, automotive interior decoration, electronic packaging materials and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of polyester, more particularly, to a method for preparing regenerated flame-retardant high-shrinkage polyester by methanol alcoholysis. BACKGROUND

[0002] At present, the recycling methods of waste PET mainly include physical recycling and chemical recycling. In the chemical recycling method, the ethylene glycol alcoholysis method needs to be carried out at high temperature (> 190℃), which has high energy consumption and low DMT yield (< 85%). The methanolysis method has attracted much attention due to its mild reaction conditions, high product purity and other characteristics. This method degrades PET into dimethyl terephthalate (DMT) and ethylene glycol (EG) through methanolysis reaction. The high-purity product of DMT can be obtained by recrystallization, which can provide high-quality raw materials for the preparation of regenerated PET. However, the current methanolysis-regenerated PET technology still faces many challenges. First, the traditional methanolysis process has low reaction efficiency and low DMT yield, which leads to poor economic efficiency. Second, the performance of regenerated PET often cannot meet the needs of high-end applications, especially in terms of flame retardancy and other key indicators. The lack of flame retardancy limits the application of regenerated PET in electronic appliances, automotive interiors and other fields.

[0003] In view of the problem of insufficient flame retardant performance of products prepared by methanolysis, the current method for improvement is to add flame retardants by physical blending. However, this method has problems such as migration of flame retardants and poor thermal stability, which cannot meet the long-term use requirements. In recent years, some researchers have tried to introduce reactive flame retardant monomers such as 2-carboxyethyl phenyl phosphinic acid (CEPPA) through copolymerization modification to improve the persistence of flame retardant performance. However, the introduction of reactive flame retardant monomers often leads to inhibition of the DMT polymerization process, low chip viscosity, and affects the final thermal stability and physical and mechanical properties of the material.

[0004] In terms of boiling water shrinkage control, high boiling water shrinkage of conventional polyester fibers can be achieved by changing the spinning process parameters, but the stability of the product during later use will be poor.

[0005] The existing preparation process of regenerated PET mainly uses a step-by-step method for functional modification, i.e., first preparing regenerated PET, and then introducing functional properties through subsequent processing. Although this method can improve the boiling water shrinkage, it not only increases the process complexity, but also may lead to uneven performance. In addition, due to the high regularity of PET molecular chain, high-temperature dyeing above 130℃ is still required, which has high energy consumption and damages the fibers.

[0006] In recent years, some researchers have tried to combine methanol depolymerization with functional modification, and achieve the multifunctionalization of recycled PET through molecular design. For example, some patents have reported a method of introducing functional monomers during methanol depolymerization, but this method has problems such as low depolymerization efficiency and uneven distribution of functional monomers. Another study proposes a strategy of synchronously introducing multiple functional monomers during the polymerization of recycled PET, but the problem of synergistic control of flame retardant performance and shrinkage performance has not been solved, and the subsequent dyeing temperature still needs to be above 110℃.

[0007] In summary, how to develop an efficient and multifunctional recycled PET preparation method based on methanol depolymerization to achieve synergistic control of flame retardant performance and boiling water shrinkage has become a technical problem to be solved in the field. SUMMARY

[0008] The purpose of the present application is to overcome the shortcomings of the prior art and provide a method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis. This method uses waste textiles as raw materials to efficiently prepare recycled DMT by methanol depolymerization, and combines the copolymerization modification of the third monomer CEPPA (2-carboxyethyl phenyl phosphinic acid) and the fourth monomer IPA (isophthalic acid) to prepare multifunctional recycled polyester (PET) with flame retardance (LOI≥30%), high boiling water shrinkage (40%), and low temperature dyeing (98℃ normal pressure dyeing), which is suitable for high-end clothing, automotive interior, electronic packaging materials, and other fields.

[0009] To achieve the above purpose, the present application adopts the following technical solutions:

[0010] A method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis, comprising the following steps:

[0011] (a) mixing the waste polyester fabric after crushing, methanol cleaning, and drying treatment with methanol, and performing depolymerization reaction under the action of a catalyst to obtain a recycled DMT mixed solution;

[0012] (b) performing impurity separation and methanol recovery treatment on the recycled DMT mixed solution to obtain a concentrated DMT mixed solution; feeding the concentrated DMT mixed solution to a crystallization kettle, adding cooled methanol for cooling crystallization, centrifuging and drying to obtain crude DMT; and performing rectification purification on the crude DMT to obtain pure DMT with a purity of ≥99.8%;

[0013] (c) adding the pure DMT, ethylene glycol, ester exchange catalyst, and DEG inhibitor into an ester exchange reaction kettle, performing ester exchange reaction while warming up to 222℃, and precipitating methanol during the reaction process;

[0014] (d) after the transesterification reaction is completed, stop heating, add CEPPA-EG and IPA-EG, and perform a copolymerization reaction while heating to 240-250 DEG C; when the temperature is raised to 240-250 DEG C, distill the excess EG, control the top temperature of the transesterification reactor to be less than 190 DEG C, and add a polymerization catalyst, a stabilizer, and an antioxidant into the transesterification reactor, filter the BHET solution system in the transesterification reactor by using nitrogen pressurization, and then deliver the system into a polymerization reactor to perform a polymerization reaction, thereby obtaining a modified polyester melt;

[0015] (e) pelletizing and drying the modified polyester melt to obtain regenerated PET chips with the functions of flame retardation, high shrinkage, and low-temperature dyeing.

[0016] The application further provides that in step (a), the waste polyester fabric and methanol are in a mass ratio of 1:4-6; the catalyst is one or more of sodium methoxide, zinc acetate, lead acetate, aluminum isopropoxide, acetonitrile, and polyionic liquid PIL-Zn 2+ , and the catalyst is added in an amount of 0.5-1.2 wt% of the waste polyester fabric after crushing, methanol cleaning, and drying treatment.

[0017] The application further provides that in step (a), the depolymerization temperature is 70-90 DEG C, the reaction pressure is 0.2-0.5 MPa, and the reaction time is 3-5 h.

[0018] The application further provides that in step (a), the specific steps of crushing, methanol cleaning, and drying treatment of the waste polyester fabric are as follows: under the protection of nitrogen, the waste polyester fabric is crushed into fragments of 5-25 mm by using a double-shaft shredder, then the fragments and methanol are put into an ultrasonic cleaning tank in a mass ratio of 1:3, ultrasonic cleaning is performed for 2-3 times at 40 DEG C, and then drying is performed at a vacuum degree of-0.08 MPa and at 80 DEG C until the moisture content is less than or equal to 0.1%.

[0019] The application further provides that in step (b), 5 DEG C cooling methanol is added in a volume ratio of 1:3-5 of the DMT mixed solution to methanol during cooling crystallization.

[0020] The application further provides that in step (c), the mass ratio of ethylene glycol, pure DMT, transesterification catalyst, and DEG inhibitor is 30-50:50-70:0.5-2:0.001-0.01; the transesterification catalyst is manganese acetate, and the DEG inhibitor is sodium acetate.

[0021] The application further provides that in step (d), the addition amount of CEPPA-EG is 3.0-4.5% of the mass of the pure DMT in step (4), and the addition amount of IPA-EG is 0.8-2.0 wt% of the mass of the pure DMT in step (4).

[0022] The application is further provided that, in step (d), the CEPPA-EG is prepared by mixing CEPPA and EG in a molar ratio of 1:2 under nitrogen protection and esterifying at 210 DEG C for 2 hours to generate CEPPA-EG.

[0023] The IPA-EG is prepared by mixing IPA and EG in a molar ratio of 1:2 and esterifying at 230 DEG C for 1.5 hours to generate IPA-EG.

[0024] The application is further provided that, in step (d), the polymerization catalyst, the stabilizer and the antioxidant account for 1-3%, 1-3% and 0.0001-0.0008% of the mass of the pure DMT in step (4) respectively; the polymerization catalyst is antimony trioxide, the stabilizer is trimethyl phosphate and the antioxidant is a steric phenolic antioxidant.

[0025] The application is further provided that, in step (d), the polymerization reaction is specifically as follows: the liquid in the polymerization kettle is heated to 268-270 DEG C by using a heat medium heating method, then low vacuum is extracted to make the vacuum degree reach 500-1000 Pa, then high vacuum is extracted to make the polymerization kettle reach a high vacuum state of 10-20 Pa, high and low speed stirring is started, high speed stirring is carried out at a stirring frequency of 99.9% in the initial stage of the reaction, low speed stirring is carried out at a stirring frequency of 45% when the temperature in the polymerization kettle reaches 281-286 DEG C, and the stirring is stopped when the stirring power of the polymerization kettle reaches 19 kw and the temperature is controlled to 281-286 DEG C, so that the modified polyester melt is obtained.

[0026] In summary, the application has the following beneficial effects:

[0027] (1) Low-temperature and high-efficiency depolymerization regeneration system: the methanol depolymerization temperature is 110 DEG C lower than that of the ethylene glycol alcoholysis method, the DMT yield is increased to 92-99%, and the raw material cost is reduced by 40%;

[0028] (2) Precise regulation of molecular chain structure: the cyclic phosphonate structure of CEPPA is bonded by copolymerization, the retention rate of phosphorus element is > 90%, and the long-term distribution of flame retardant is realized; the IPA meta-substitution destroys the crystalline region, the crystallinity is reduced from 35% to 18-25%, the glass transition temperature (Tg) is reduced to 65-68 DEG C, and low-temperature and normal-pressure dyeing is realized;

[0029] (3) Process coupling innovation: staged polymerization makes the phosphorus element gradiently distributed in the molecular chain, and improves the flame retardant efficiency; pre-esterification of CEPPA and IPA into CEPPA-EG and IPA-EG avoids agglomeration of modified monomers, and further improves the product performance. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1A molecular chain structure model diagram of the regenerated flame-retardant high-boiling shrinkage polyester of the application. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the application.

[0032] The method for preparing the regenerated flame-retardant high-shrinkage polyester by the methanol alcoholysis method of the application comprises the following steps:

[0033] (1) Under the protection of nitrogen, a double-shaft shredder is used to shred the waste polyester fabric (PET content ≥ 80 wt%) into fragments of 5-25 mm; then the fragments and methanol are put into an ultrasonic cleaning tank at a mass ratio of 1:3, and ultrasonic cleaning is performed 2-3 times at 40 ℃ (ultrasonic frequency 40 kHz, power 500 W / m 3 , and each cleaning time is 30 min), and then drying is performed at a vacuum degree of-0.08 MPa and 80 ℃ until the moisture content is ≤0.1%.

[0034] (2) The waste polyester fabric after the shredding, methanol cleaning and drying treatment is mixed with methanol at a mass ratio of 1:4-6, and depolymerization is performed at 70-90 ℃ and 0.2-0.5 MPa under the action of a catalyst for 3-5 h to obtain a regenerated DMT mixed solution. The catalyst is one or more of sodium methoxide, zinc acetate, lead acetate, aluminum isopropoxide, acetonitrile, polyionic liquid PIL-Zn 2+ , and triazabicyclodecene, and the catalyst addition amount is 0.5-1.2 wt% of the waste polyester fabric after the shredding, methanol cleaning and drying treatment.

[0035] (3) The regenerated DMT mixed solution is subjected to impurity separation (i.e., coarse filtration of the regenerated DMT mixed solution is performed by using a 200-mesh stainless steel filter screen to remove fiber residues, then 1 wt% of activated carbon is added to the filtrate, stirring is performed at 60 ℃ for 1 h for activated carbon adsorption and decolorization, and then 0.22-μm ceramic membrane filtration is performed to remove colloidal particles) and methanol recovery (recovery is performed by using a falling-film vacuum distillation column, the number of column plates is 10, the reflux ratio is 2:1, the column top temperature is controlled at 60 ℃, and the pressure is-0.095 MPa) to obtain a concentrated DMT mixed solution; the concentrated DMT mixed solution is delivered to a crystallization kettle, 5 ℃ cooling methanol is added at a volume ratio of 1:3-5 of the DMT mixed solution to methanol, stirring and mixing are performed for 30 min, then crystallization is performed by standing, centrifugal drying is performed, and coarse DMT with a purity of ≥99.3% and ash content of ≤0.01% is obtained; the coarse DMT is subjected to rectification and purification at 180 ℃ and a vacuum degree of 0.1 Pa to obtain pure DMT with a purity of ≥99.8%.

[0036] (4) adding pure DMT, ethylene glycol, transesterification catalyst, DEG inhibitor into the transesterification reactor, carrying out transesterification reaction while heating to 222℃, and methanol is separated out during the reaction, wherein the mass ratio of ethylene glycol, pure DMT, transesterification catalyst and DEG inhibitor is 30-50: 50-70: 0.5-2: 0.001-0.01; the transesterification catalyst is manganese acetate, and the DEG inhibitor is sodium acetate.

[0037] (5) after the transesterification reaction is completed, stopping heating, adding CEPPA-EG and IPA-EG, carrying out copolymerization reaction while heating to 240-250℃ (the addition amount of CEPPA-EG is 3.0-4.5% of the mass of pure DMT in step (4), and the addition amount of IPA-EG is 0.8-2.0% of the mass of pure DMT in step (4)), wherein the preparation method of CEPPA-EG is: mixing CEPPA and EG in a molar ratio of 1:2 under nitrogen protection and esterifying at 210℃ for 2h to generate CEPPA-EG, and the hydroxyl value of the product is ≥98%; the preparation method of IPA-EG is: mixing IPA and EG in a molar ratio of 1:2 and esterifying at 230℃ for 1.5h to generate IPA-EG, and the acid value of the product is ≤2mgKOH / g, and the light transmittance (450nm) is ≥90%; when the temperature is heated to 240-250℃, distilling the excess EG, controlling the top temperature of the transesterification reactor to be within 190℃, and adding polymerization catalyst, stabilizer and antioxidant into the transesterification reactor (the polymerization catalyst, stabilizer and antioxidant are 1-3%, 1-3% and 0.0001-0.0008% of the mass of pure DMT in step (4) respectively, the polymerization catalyst is antimony trioxide, the stabilizer is trimethyl phosphate, and the antioxidant is a steric phenolic antioxidant), filtering the BHET solution system in the transesterification reactor by nitrogen pressurization, and then conveying it to the polymerization reactor to carry out polymerization reaction, to obtain modified polyester melt (the intrinsic viscosity is 0.60-0.70dL / g); wherein the specific steps of the polymerization reaction are as follows:

[0038] heating the liquid in the polymerization reactor to 268-270℃ by heat medium heating, then carrying out low vacuum treatment to make the vacuum degree reach 500-1000Pa, then carrying out high vacuum treatment to make the polymerization reactor reach a high vacuum state of 10-20Pa, and starting high and low speed stirring reaction (in the initial stage of the reaction, high speed stirring reaction is carried out at a stirring frequency of 99.9%, and when the temperature in the polymerization reactor reaches 281-286℃, low speed stirring reaction is carried out at a stirring frequency of 45%), when the stirring power of the polymerization reactor reaches 19kw and the temperature is controlled to 281-286℃, stopping stirring, to obtain modified polyester melt.

[0039] (6) The modified polyester melt is cut into granules and dried to obtain regenerated PET chips with functions of flame retardation, high shrinkage and low temperature dyeing, and the performance indexes are: limiting oxygen index (LOI) ≥ 30%; boiling water shrinkage 25%-40%; 98°C normal pressure dyeing dyeing rate ≥ 90%.

[0040] The regenerated flame-retardant high-boiling-water-shrinkage polyester molecular chain structure model of the application is shown as Figure 1

[0041] Example 1

[0042] (1) Under the protection of nitrogen, the waste polyester fabric (PET content ≥ 80wt%) is broken into 5-25mm pieces by a double-shaft shredder; then the pieces are put into an ultrasonic cleaning tank with methanol at a mass ratio of 1:3, ultrasonic cleaning is carried out twice at 40°C (ultrasonic frequency 40kHz, power 500W / m 3 , each time for 30min), and then dried at a vacuum degree of-0.08MPa and 80°C until the moisture content is ≤0.1%.

[0043] (2) The waste polyester fabric after being broken, methanol cleaned and dried is mixed with methanol at a mass ratio of 1:5, depolymerization is carried out at 80°C and 0.3MPa for 4h under the action of catalyst sodium methoxide, and regenerated DMT mixed liquor is obtained; the catalyst addition amount is 0.8wt% of the waste polyester fabric after being broken, methanol cleaned and dried.

[0044] (3) The regenerated DMT mixed liquor is treated by impurity separation (i.e. coarse filtration of the regenerated DMT mixed liquor by using a 200-mesh stainless steel filter screen to remove fiber residues, then 1wt% of activated carbon is added to the filtrate, stirred at 60°C for 1h for activated carbon adsorption decolorization, and then 0.22μm ceramic membrane filtration is carried out to remove colloidal particles) and methanol recovery (recovery is carried out by using a falling film vacuum distillation column, with plate number 10, reflux ratio 2:1, control of column top temperature 60°C and pressure-0.095MPa), and concentrated DMT mixed liquor is obtained; the concentrated DMT mixed liquor is sent to a crystallization kettle, 5°C cooling methanol is added at a ratio of DMT mixed liquor to methanol volume ratio 1:4, stirred and mixed for 30min, then crystallization is carried out by standing, centrifugal drying is carried out, and coarse DMT with purity ≥99.3% and ash content ≤0.01% is obtained; the coarse DMT is subjected to rectification purification at 180°C and a vacuum degree of 0.1Pa, and pure DMT with purity ≥99.8% is obtained.

[0045] (4) The pure DMT, ethylene glycol, ester exchange catalyst manganese acetate and DEG inhibitor sodium acetate are added to an ester exchange reaction kettle, the ester exchange reaction is carried out while being heated to 222°C, and methanol is precipitated during the reaction, wherein the mass ratio of ethylene glycol, pure DMT, ester exchange catalyst and DEG inhibitor is 32:67.2:1:0.007.​

[0046] (5) After the completion of the transesterification reaction, stop heating, add CEPPA-EG and IPA-EG, and perform a copolymerization reaction while heating to 245°C (the addition amount of CEPPA-EG is 3.0% of the mass of the pure DMT in step (4), and the addition amount of IPA-EG is 1.2 wt% of the mass of the pure DMT in step (4)), wherein the preparation method of CEPPA-EG is: under nitrogen protection, CEPPA and EG are mixed at a molar ratio of 1:2 and esterification is performed at 210°C for 2h to generate CEPPA-EG, and the product has a hydroxyl value of ≥98%. The preparation method of IPA-EG is: IPA and EG are mixed at a molar ratio of 1:2 and esterification is performed at 230°C for 1.5h to generate IPA-EG, and the product has an acid value of ≤2mg KOH / g and a light transmittance (450nm) of ≥90%. When the temperature is raised to 245°C, the excess EG is distilled off, the top temperature of the transesterification reactor is controlled to be within 190°C, and a polymerization catalyst, a stabilizer, and an antioxidant (the polymerization catalyst, the stabilizer, and the antioxidant are 1.2%, 1%, and 0.0005% of the mass of the pure DMT in step (4), respectively, the polymerization catalyst is antimony trioxide, the stabilizer is trimethyl phosphate, and the antioxidant is antioxidant 1010) are added to the transesterification reactor. The BHET solution system in the transesterification reactor is filtered and then transported to the polymerization reactor for polymerization reaction by using a nitrogen pressurization method, and a modified polyester melt is obtained; wherein the specific steps of the polymerization reaction are as follows:

[0047] The liquid in the polymerization reactor is heated to 268°C by using a heat medium heating method, then low vacuum is extracted to make the vacuum degree reach 670Pa, high vacuum is extracted to make the polymerization reactor reach a high vacuum state of 14Pa, and high and low speed stirring is started (in the initial stage of the reaction, high speed stirring is performed at a stirring frequency of 99.9%, and when the temperature in the polymerization reactor reaches 285°C, low speed stirring is performed at a stirring frequency of 45%). When the stirring power of the polymerization reactor reaches 19kw and the temperature is controlled to 285°C, the stirring is stopped, and a modified polyester melt is obtained.

[0048] (6) The modified polyester melt is cut and dried to obtain regenerated PET chips with the functions of flame retardation, high shrinkage, and low temperature dyeing.

[0049] Example 2

[0050] The regenerated flame-retardant high-shrinkage polyester is prepared according to the method of Example 1, but in step (2), the polyionic liquid PIL-Zn 2+ is used as the catalyst, and the addition amount of the catalyst is 0.6wt% of the waste polyester fabric after crushing, methanol cleaning, and drying treatment.

[0051] Comparative Example 1

[0052] The polyester is prepared by the glycol alcoholysis-methanol ester exchange method, i.e. prepared according to the method of Example 1, but step (2) is as follows: the waste polyester fabric after crushing, methanol cleaning and drying treatment is mixed with glycol at a mass ratio of 1:5, and depolymerization is carried out at 220℃, 0.3MPa for 4h under the action of catalyst potassium carbonate to obtain a regenerated DMT mixture; the catalyst addition amount is 0.8wt% of the waste polyester fabric after crushing, methanol cleaning and drying treatment.

[0053] Comparative Example 2

[0054] The polyester is prepared according to the method of Example 1, but no CEPPA-EG is added in step (5).

[0055] The properties of the polyesters prepared in Example 1-Example 2, Comparative Example 1-Comparative Example 2 are tested, and the results are shown in the following table:

[0056]

[0057] It is also found by experiment that when the polyester is prepared according to the method of Example 1 but the methanol alcoholysis temperature in step (2) of Example 1 is reduced to 70℃, the DMT yield is reduced by 6.7% compared with Example 1, and the tensile strength of the finally obtained polyester is reduced to 3.6cN / dtex; when the polyester is prepared according to the method of Example 2 but the methanol alcoholysis temperature in step (2) is reduced to 70℃, the DMT yield is reduced by 2.1% compared with Example 2, and the tensile strength of the finally prepared polyester is 4.3cN / dtex.

[0058] The above only describes the preferred embodiments of the present application, and the protection scope of the present application is not limited to the above examples only, and any technical solution falling within the concept of the present application belongs to the protection scope of the present application. It should be noted that for ordinary skilled persons in the art, some improvements and refinements without departing from the principles of the present application are also considered to be within the protection scope of the present application.

Claims

1. A method for preparing recycled flame-retardant high-shrinkage polyester via methanol alcoholysis, characterized in that, Includes the following steps: (a) Waste polyester fabrics that have been crushed, washed with methanol, and dried are mixed with methanol and subjected to a depolymerization reaction under the action of a catalyst to obtain a recycled DMT mixture; the catalyst is a polyionic liquid PIL-Zn. 2+ The depolymerization reaction temperature is 70~90℃, the reaction pressure is 0.2~0.5MPa, and the reaction time is 3~5h. (b) The regenerated DMT mixture is subjected to impurity separation and methanol recovery treatment to obtain a concentrated DMT mixture; the concentrated DMT mixture is transported to a crystallization kettle, cooled methanol is added for cooling crystallization, and the mixture is centrifuged and dried to obtain crude DMT; the crude DMT is purified by distillation to obtain pure DMT with a purity ≥99.8%; (c) Pure DMT, ethylene glycol, transesterification catalyst, and DEG inhibitor were added to the transesterification reactor, and the transesterification reaction was carried out while the temperature was raised to 222°C, during which methanol was precipitated. (d) After the transesterification reaction is completed, stop heating and add CEPPA-EG and IPA-EG while heating to 240~250℃ to carry out copolymerization reaction; when the temperature reaches 240~250℃, distill off the excess EG, control the top temperature of the transesterification reactor to below 190℃, and add polymerization catalyst, stabilizer and antioxidant to the transesterification reactor. After filtering the BHET solution system in the transesterification reactor under nitrogen pressure, it is transported to the polymerization reactor for polymerization reaction to obtain modified polyester melt; (e) The modified polyester melt is pelletized and dried to obtain recycled PET chips that have flame retardant, high shrinkage and low temperature dyeing functions.

2. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (a), the waste polyester fabric and methanol are in a mass ratio of 1:4-6; the amount of catalyst added is 0.5-1.2 wt% of the waste polyester fabric after crushing, methanol washing and drying.

3. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (a), the specific steps for crushing, methanol cleaning, and drying of waste polyester fabric are as follows: Under nitrogen protection, the waste polyester fabric is crushed into 5-25mm fragments using a biaxial shredder. Then, the fragments and methanol are added to an ultrasonic cleaning tank at a mass ratio of 1:

3. The fabric is ultrasonically cleaned 2-3 times at 40℃, and then dried at a vacuum of -0.08MPa and 80℃ until the moisture content is ≤0.1%.

4. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (b), during cooling crystallization, methanol cooled at 5°C is added at a volume ratio of 1:3-5 between the DMT mixture and methanol.

5. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (c), the mass ratio of ethylene glycol, pure DMT, transesterification catalyst and DEG inhibitor is 30~50:50~70:0.5~2:0.001~0.01; the transesterification catalyst is manganese acetate and the DEG inhibitor is sodium acetate.

6. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (d), the amount of CEPPA-EG added accounts for 3.0 to 4.5% of the mass of pure DMT in step (c), and the amount of IPA-EG added accounts for 0.8 to 2.0 wt% of the mass of pure DMT in step (c).

7. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (d), CEPPA-EG is prepared by the following method: under nitrogen protection, CEPPA and EG are mixed in a molar ratio of 1:2 and esterified at 210°C for 2 hours to generate CEPPA-EG; IPA-EG was prepared by mixing IPA and EG in a 1:2 molar ratio and esterifying them at 230°C for 1.5 h to generate IPA-EG.

8. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (d), the polymerization catalyst, stabilizer, and antioxidant account for 1-3%, 1-3%, and 0.0001-0.0008% of the mass of pure DMT in step (c), respectively; the polymerization catalyst is antimony trioxide, the stabilizer is trimethyl phosphate, and the antioxidant is a sterically hindered phenolic antioxidant.

9. The method for preparing recycled flame-retardant high-shrinkage polyester by methanol alcoholysis according to claim 1, characterized in that, In step (d), the specific steps of the polymerization reaction are as follows: the liquid in the polymerization reactor is heated to 268-270°C using a heat transfer medium, then a low vacuum treatment is performed to achieve a vacuum degree of 500-1000 Pa, followed by a high vacuum treatment to achieve a high vacuum state of 10-20 Pa in the polymerization reactor. High and low speed stirring reaction is started. In the initial stage of the reaction, high speed stirring reaction is performed at a stirring frequency of 99.9%. When the temperature in the polymerization reactor reaches 281-286°C, the stirring frequency is switched to 45% for low speed stirring reaction. When the stirring power of the polymerization reactor reaches 19 kW and the temperature is controlled at 281-286°C, stirring is stopped, and modified polyester melt is obtained.

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