A high-strength, chemically stable, flame-retardant closed-loop recycled plastic and its preparation and recycling method

Abstract

A high-strength, chemically stable, flame-retardant closed-loop recycled plastic and its preparation and recycling method, belonging to the technical field of closed-loop recycled plastic preparation. The present invention utilizes reversible forces to connect rigid polymer chains with multifunctional flame-retardant crosslinking agents to construct reversible crosslinked plastics with high crosslinking degree, which have excellent mechanical properties, chemical stability and flame retardancy, and its strength is comparable to that of commercial engineering plastics such as PI and PEEK, etc. It can be immersed in corrosive liquids such as acids, alkalis, organic solvents, etc. for a long time without losing mechanical strength, and its flame retardancy reaches the highest level V0 of UL94 test. Controllable depolymerization can be achieved in acidic solutions, and monomer purification can be achieved by solution precipitation method, with a monomer recovery rate of >90%. The recovered monomers can be repolymerized into new plastics identical to the original materials. The present invention provides a simple and universal method for the design of closed-loop recyclable plastics with flame retardant properties, which is expected to be widely used in packaging, home furnishings, construction and other fields.

Description

Technical Field

[0001] The invention belongs to the technical field of closed-loop recycled plastic preparation, and specifically relates to a high-strength, chemically stable, flame-retardant closed-loop recycled plastic and a preparation and recycling method thereof. Background Art

[0002] Plastics have become an indispensable part of our daily lives, industry, and agriculture. They can be seen in many aspects such as packaging, construction, medical treatment, and electronic materials. However, the prevalence of disposable plastic consumption and the difficulty of plastic degradation have led to the accumulation of ubiquitous plastic waste around the world. In order to alleviate the global plastic crisis, it is imperative to develop strategies to recycle waste plastics in a more sustainable way, because recycling is one of the best options to prevent plastics from becoming permanent waste. Closed-loop recycling completely depolymerizes plastics and recycles the depolymerized repeating units or monomers into new materials that are identical to the raw materials. Therefore, among various recycling strategies, closed-loop recycling is the most economical way to make full use of waste plastics. In addition, the highly flammable nature of plastics brings huge fire hazards, which greatly limits its application. Flame-retardant treatment of plastics can effectively delay the spread of flames and even prevent the material from being ignited. Recently, many studies have focused on the preparation of flame-retardant plastics, and a variety of flame retardants have been developed to give plastics flame-retardant properties. However, the introduction of flame retardants often increases the difficulty of plastic recycling because they will inevitably degrade and release toxic gases during the complex recycling process, while deteriorating the properties of the material. Therefore, how to achieve closed-loop recycling of flame-retardant plastics in a simpler way is the problem we need to solve.

[0003] Reversible cross-linking of polymer building blocks with supramolecular interactions and dynamic bonds provides a feasible approach for designing plastics with closed-loop recyclability. The breakage of supramolecular interactions and dynamic bonds allows reversibly cross-linked plastics to depolymerize under mild conditions, facilitating their closed-loop recycling. However, the research on the preparation of closed-loop recyclable flame-retardant plastics using reversible cross-linking strategies is still in its infancy. At present, there are two main strategies for achieving the recycling of flame-retardant plastics. The first strategy is to use reversibly cross-linked plastics with dynamic exchange reactions. Based on the exchange reaction under stimulation by reversible forces, waste flame-retardant plastics can be recycled by techniques such as thermomechanical reshaping. However, the process of high-temperature mechanical recycling often decomposes the flame retardant and deteriorates the material properties. The second strategy is to add dissociable bonds to the polymer network to prepare reversibly cross-linked flame-retardant plastics that can be depolymerized. Under certain conditions, scrapped flame-retardant plastics can be degraded by interaction dissociation, thereby achieving partial recycling of flame retardants. At present, the depolymerization and recycling of most flame-retardant plastics only stays at the recycling of flame retardants, while ignoring the recycling of polymer matrix, and the recycling rate is relatively low. It remains challenging to prepare flame-retardant closed-loop recycled plastics that can simultaneously recycle polymers and flame retardants under mild conditions. At the same time, the mechanical properties of flame-retardant plastics need to be further improved. In addition, a prominent problem with reversibly cross-linked plastics is that in an aqueous environment or specific solvents, supramolecular interactions and dynamic bonds are prone to partial breakage, resulting in deterioration of mechanical strength and functionality. If this problem is not solved, the application of reversibly cross-linked plastics will be greatly limited. Therefore, the development of closed-loop recycled plastics with good chemical stability, high strength and flame retardancy has very important social and economic value, but it is still full of challenges. Summary of the invention

[0004] The purpose of the present invention is to provide a high-strength, chemically stable, flame-retardant closed-loop recycled plastic and a preparation and recycling method thereof. The present invention utilizes a flame-retardant small molecule cross-linking agent to reversibly cross-link rigid chain polymer chains to prepare a high-performance flame-retardant plastic that can achieve closed-loop recycling.

[0005] The method for preparing a high-strength, chemically stable, flame-retardant closed-loop recycled plastic of the present invention comprises the following steps:

[0006] 1) dissolving the amino-functional rigid chain polymer A and the flame retardant small molecule B in solvent C (one or more of dichloromethane, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone) respectively to prepare a solution in which the concentration of the amino-functional rigid chain polymer A is 50-100 mg / mL and the concentration of the flame retardant small molecule B is 1-30 mg / mL;

[0007] 2) The amino-functionalized rigid-chain polymer A solution obtained in step 1) and the flame-retardant small molecule B solution are uniformly mixed in a ratio of 1:1 based on the molar number of the functional groups of the raw materials, the solvent is volatilized at 30-80° C., and then cured at 180-300° C. under vacuum for 2-6 hours to obtain the high-strength, chemically stable, flame-retardant closed-loop recycled plastic of the present invention.

[0008] Selection of building blocks as described in the above steps:

[0009] The amino-functional rigid chain polymer A used in the present invention is an amino-terminated polymer, which is obtained by polycondensation of an acylated monomer and an amino monomer. The present invention has no special requirements for the polycondensation reaction method, and any polycondensation method known to those skilled in the art can be used.

[0010] Preferred acylated monomers are one or more of terephthaloyl chloride, isophthaloyl chloride, pyromellitic anhydride, hexafluoroisopropylphthalic anhydride, isophthalic acid and the like.

[0011] Preferred amino monomers are one or more of 2,2'-bis(trifluoromethyl)diaminobiphenyl, p-phenylenediamine, m-phenylenediamine, diaminodiphenyl ether, 3,3'-diaminobiphenyl, and the like.

[0012] In order to ensure amino termination, the amino monomer is always in excess, and the molar ratio of the acylated monomer to the amino monomer is 1:1.1-2.

[0013] The flame retardant small molecule B used in the present invention is an aldehyde-functionalized flame retardant crosslinker, and one or more of aldehyde-functionalized phosphorus oxychloride, cyclophosphazene, cage-type polysilsesquioxane, triazine, etc. are selected. The synthesis method is referred to in the literature (Macromolecules 2018, 51, 20, 8001-8012).

[0014] The present invention provides a method for recycling the above-mentioned high-strength chemically stable flame-retardant closed-loop recycled plastic, the steps of which are as follows:

[0015] 1) placing high-strength chemically stable flame-retardant closed-loop recycled plastic into a mixed solvent D to depolymerize and dissolve the plastic to obtain a uniform plastic solution;

[0016] 2) The plastic solution of step 1) is precipitated in solvent E, and the amino-functional rigid-chain polymer A and the flame-retardant small molecule B are precipitated from the solution. A neutralizer F is then added to the solution for purification, followed by solid-liquid separation, and the resulting solid is a mixture of the amino-functional rigid-chain polymer A and the flame-retardant small molecule B. The obtained mixture of the amino-functional rigid-chain polymer A and the flame-retardant small molecule B does not need to be separated and can be directly used for the preparation of new plastics, thereby realizing closed-loop recycling of plastics.

[0017] The mixed solvent D is a mixture of an organic solvent and an acid, the organic solvent is one or more of N-methylpyrrolidone, tetrahydrofuran, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, and acetone, the acid is one or more of concentrated hydrochloric acid, glacial acetic acid, phosphoric acid, and citric acid, and the volume ratio of the organic solvent to the acid is 20 to 1:1.

[0018] The solvent E is a poor solvent for the amino-functional rigid chain polymer A and the flame retardant small molecule B, and is preferably one or more of water, ethanol, ethyl acetate, n-hexane, and petroleum ether.

[0019] The neutralizing agent F is one or more of potassium hydroxide aqueous solution (1 mol / L), sodium hydroxide aqueous solution (1 mol / L), sodium carbonate aqueous solution (1 mol / L), sodium bicarbonate aqueous solution (1 mol / L), and triethylamine;

[0020] The principle of the present invention is as follows:

[0021] The aldehyde functionalized flame retardant crosslinker and the amino functionalized rigid chain polymer are combined together by Schiff base reaction to generate imine bonds to form a highly crosslinked network. The flame retardant small molecule used in the present invention is multifunctionalized, so a polymer material highly crosslinked by dynamic imine bonds can be prepared by high temperature curing. Since the entire polymer chain and the crosslinker all use fully aromatic ring structure molecules, and the crosslinker is multifunctionalized, the prepared reversible crosslinked plastic can obtain ultra-high fracture strength. At the same time, the aromatic ring hydrophobic environment created by the polymer chain and the crosslinker can protect the dynamic imine crosslinking structure, prevent the water-sensitive imine bonds from being destroyed by corrosive liquids such as solvents, and give the plastic excellent chemical stability. Due to the flame retardant function of the crosslinker and the high aromaticity and carbon content of the polymer chain, the material is given excellent flame retardant properties, which improves the fire safety of the plastic during use. Based on the swelling ability of the polymer chain in certain solvents and the dissociation ability of the imine bond under acidic conditions, the plastic can be selectively depolymerized under the dual stimulation of a specific solvent and acid to regenerate the original amino and aldehyde molecules. The solution formed by complete depolymerization can be quickly purified and impurized by a solution precipitation method. The two recycled monomers can be used to prepare the original plastic again, thus achieving closed-loop recycling of high-strength, chemically stable flame-retardant plastics under mild conditions.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] The reversibly cross-linked closed-loop recycled plastic prepared by the present invention has ultra-high mechanical strength, excellent chemical stability and flame retardancy. The closed-loop recyclable plastic described in the present invention has solved a series of problems existing in the current plastic application field in a breakthrough way, greatly improved the mechanical properties, chemical stability and fire safety of recyclable plastics, extended the service life of plastics, and solved the post-processing problem of waste plastics. The preparation process of the present invention does not involve complex synthesis procedures, the preparation process is controllable, and it is suitable for large-scale preparation. The present invention provides a convenient and widely applicable method for the design of closed-loop recyclable plastics with flame retardant properties. There is currently no report on such high-strength, chemically stable, flame-retardant closed-loop recyclable plastics.

[0024] The specific advantages of the present invention are as follows:

[0025] 1.Excellent mechanical properties

[0026] The reversibly cross-linked closed-loop recycled plastic prepared by the method of the present invention has a breaking strength between 80 and 150 MPa, a modulus between 1.8 and 3 GPa, and mechanical properties comparable to those of commercial engineering plastics.

[0027] 2. Excellent chemical stability

[0028] The reversibly cross-linked closed-loop recycled plastic prepared by the method of the present invention is immersed in deionized water, acidic solution (1M HCl) and alkaline solution (1M NaOH) for 1 to 30 days respectively, and the mechanical properties remain unchanged. It is immersed in common organic solvents (N-methylpyrrolidone, N,N-dimethylformamide, tetrahydrofuran, ethanol, n-hexane) for 12 to 48 hours, and the mechanical properties are basically maintained. Therefore, it has good chemical stability.

[0029] 3. Unique closed-loop recycling performance

[0030] The reversibly cross-linked closed-loop recycled plastic prepared by the method of the present invention is stirred in a mixed solution of an organic solvent and an acid for 3 to 12 hours, and the imine bonds inside the plastic are gradually opened, and the plastic is depolymerized and dissolved. After solution precipitation and neutralization purification, the original amino and aldehyde molecules can be obtained, and the recycled molecules can be used to prepare new flame-retardant plastics. Such closed-loop recycling can be repeated 1 to 5 times, and the recovery rate is as high as 90 to 100%.

[0031] 4. Excellent flame retardant properties

[0032] The reversibly cross-linked closed-loop recycled plastics prepared by the method of the present invention can pass the UL94 vertical combustion test, the flame retardant level is V0 level, the limiting oxygen index is between 40% and 50%, and has self-extinguishing properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1: A photo of the plastic prepared in Example 1 of the present invention;

[0034] Figure 2 : Actual photos of the chemical stability of the plastic prepared in Example 2 of the present invention;

[0035] Figure 3 : Schematic diagram and actual photos of the preparation of closed-loop recycling of plastics in Example 3 of the present invention;

[0036] Figure 4 : Mechanical curves of recycled plastics prepared in Example 4 of the present invention.

[0037] Figure 5 : Actual photos of the plastic UL94 test prepared in Example 5 of the present invention;

[0038] Figure 6 : SEM images of the surface (a) and cross-section (b) of the char layer of the plastic UL94 after combustion prepared in Example 5 of the present invention. DETAILED DESCRIPTION

[0039] The following examples are provided to further illustrate the specific implementation and results of the present invention, but are not intended to limit the present invention. The vertical combustion test was performed in a CZF-2 horizontal vertical combustion tester according to ASTM D4084-98. The limiting oxygen index test was performed in a HC-2C oxygen index flammability tester according to GBT 2406.2-2009.

[0040] Example 1: A method for preparing a high-strength, chemically stable, flame-retardant closed-loop recycled plastic. The amount of each raw material is calculated by mole fraction as follows:

[0041] The amino-functionalized rigid-chain polymer A is prepared by the polycondensation of terephthaloyl chloride and p-phenylenediamine, with the molar ratio of diacyl chloride to diamino monomer being 1:1.5. The method is referred to in the literature (ACS Sustainable Chem. Eng. 2018, 6, 1362-1369). The flame-retardant small molecule B is an aldehyde-functionalized triazine. Polymer A and aldehyde-functionalized triazine were dissolved in N,N-dimethylformamide, respectively, to prepare 60 mg / mL and 20 mg / mL solutions, respectively. Polymer A and aldehyde molecular solution were mixed at room temperature in a volume ratio of 3:1, and continued stirring for 5 hours to obtain a uniform and transparent solution. The mixed solution was poured into a 20*30*3cm glass mold and placed on a hot plate at 60°C overnight to allow the solvent to evaporate completely. The material was then placed in a 180°C vacuum oven for curing for 3 hours, and finally a slightly yellow transparent film was obtained ( Figure 1), that is, the high-strength, chemically stable, flame-retardant closed-loop recycled plastic described in the present invention. The tensile strength of the obtained material was tested by a universal material testing machine to be 108.2MPa, which is comparable to commercial engineering plastics such as PI and PEEK. The plastic can still maintain its original shape after being soaked in deionized water, acidic (1M HCl) and alkaline (1M NaOH) aqueous solutions at room temperature for 20 days, showing excellent water resistance. The plastic was soaked in a variety of organic solvents for 30 hours, and the plastic did not depolymerize and dissolve, showing excellent resistance to organic solvents. The plastic was cut into pieces, put into a mixed solution of N,N-dimethylformamide and glacial acetic acid (volume ratio of 10:1), stirred for 12 hours, and the plastic was completely depolymerized and dissolved in the mixed solution to obtain a transparent and uniform solution. The depolymerized solution was precipitated in n-hexane, purified by adding triethylamine, filtered and dried to obtain a mixture of high-purity polymer A and aldehyde-functionalized triazine, with a yield of about 92.3%. The reversible cross-linked plastic was re-prepared with the recycled monomers, and its tensile strength was 107.8MPa, which is basically the same as the mechanical properties of the original material. The plastic was tested for flame retardancy, and it showed excellent flame retardancy. The vertical burning test reached UL94-V0 level, and the limiting oxygen index was 43.5%. Example 2: A method for preparing high-strength, chemically stable flame-retardant closed-loop recycled plastics. The amount of each raw material is calculated by molar fraction as follows:

[0042] The amino-functionalized rigid chain polymer A is prepared by polycondensation of hexafluoroisopropylphthalic anhydride and diaminodiphenyl ether, with the molar ratio of dianhydride to diamino monomer being 1:1.2. The method is based on the literature (RSC Adv., 2015, 5, 103246–103254). The flame retardant small molecule B is aldehyde-functionalized cyclophosphazene. Polymer A and aldehyde-functionalized cyclophosphazene are dissolved in tetrahydrofuran to prepare 100 mg / mL and 20 mg / mL solutions, respectively. Then, the volume ratio of polymer A to aldehyde molecule solution is 5:1, and stirring is continued for 30 minutes to obtain a uniform and transparent solution. The mixed solution is poured into a 10*10*3 cm glass mold and placed on a hot plate at 30°C overnight to completely evaporate the solvent. The material is then placed in a vacuum oven at 220°C for 5 hours to cure, and a yellow transparent film is finally obtained. The obtained material has a tensile strength of 115.6MPa when tested by a universal material testing machine, which is comparable to commercial engineering plastics such as PI and PEEK. The plastic can still maintain its original shape after being immersed in deionized water, acidic (1M HCl) and alkaline (1MNaOH) aqueous solutions at room temperature for 10 days ( Figure 2). The plastic was immersed in a variety of organic solvents for 24 hours, and the plastic did not depolymerize or dissolve, showing excellent resistance to organic solvents. The plastic was cut into pieces and placed in a mixed solution of tetrahydrofuran and concentrated hydrochloric acid (volume ratio of 20:1), stirred for about 5 hours, and the plastic was completely depolymerized and dissolved in the mixed solution to obtain a transparent and uniform solution. The depolymerization solution was precipitated in water, neutralized and purified by adding sodium hydroxide solution, and filtered and dried to obtain high-purity polymer A and aldehyde-functionalized cyclophosphazene, with a yield of about 97.8%. The reversible cross-linked plastic was re-prepared with the recovered monomers, and its tensile strength was basically the same as that of the original material. The plastic was flame retardant tested, and it showed excellent flame retardant properties. The vertical burning test reached the UL94-V0 level, and the limiting oxygen index was 48.8%.

[0043] Example 3: A method for preparing a high-strength, chemically stable, flame-retardant closed-loop recycled plastic. The amount of each raw material is calculated by mole fraction as follows:

[0044] The amino-functionalized rigid chain polymer A is prepared by polycondensation of isophthalic acid and 3,3'-diaminobiphenyl, with the molar ratio of carboxylic acid monomer to amino monomer being 1:1.5. The method is based on the literature (DOI:10.1016 / S1872-2040(09)60019-0). The flame retardant small molecule B is aldehyde-functionalized phosphorus oxychloride. Polymer A and aldehyde-functionalized phosphorus oxychloride are dissolved in dimethyl sulfoxide to prepare 60 mg / mL and 10 mg / mL solutions, respectively. Then, the volume ratio of polymer A to aldehyde molecule solution is 6:1, and stirring is continued for 5 hours to obtain a uniform and transparent solution. The mixed solution is poured onto a 5*5 cm glass plate and placed on a hot plate at 80°C overnight to allow the solvent to evaporate completely. The material is then placed in a vacuum oven at 180°C for 3 hours to obtain a yellow transparent film. The tensile strength of the obtained material is 125.2 MPa when tested by a universal material testing machine, which is comparable to commercial engineering plastics. After being immersed in deionized water, acidic (1M HCl) and alkaline (1M NaOH) aqueous solutions at room temperature for 14 days, the plastic still maintained its original strength, showing excellent water resistance. After being immersed in a variety of organic solvents for 12 hours, the plastic did not depolymerize or dissolve, showing excellent resistance to organic solvents. The plastic was cut into pieces and placed in a mixed solution of dimethyl sulfoxide and citric acid (volume ratio of 8:1), stirred for about 24 hours, and the plastic was completely depolymerized and dissolved in the mixed solution, and a transparent and uniform solution was obtained ( Figure 3 The depolymerization solution was precipitated in ethyl acetate, neutralized and purified by adding triethylamine, filtered and dried to obtain high-purity polymer A and aldehyde-functionalized phosphorus oxychloride, with a yield of about 91.2% ( Figure 3). The reversibly cross-linked plastic was re-prepared with the recycled monomers, and its tensile strength was basically the same as that of the original material. The plastic was tested for flame retardancy, and it showed excellent flame retardancy, reaching UL94-V0 grade in the vertical burning test, and the limiting oxygen index was 49.2%.

[0045] Example 4: A method for preparing a high-strength, chemically stable flame-retardant closed-loop recycled plastic. The amount of each raw material is calculated by mole fraction as follows:

[0046] The rigid chain polymer chain A is formed by polycondensation of isophthaloyl chloride and m-phenylenediamine, with the molar ratio of dichloride to diamine monomer being 1:1.12, and the method is referred to the literature (ACS Sustainable Chem. Eng. 2018, 6, 1362-1369). The flame retardant small molecule B is an aldehyde-functionalized silsesquioxane. The polymer A and the aldehyde-functionalized silsesquioxane are dissolved in dichloromethane to prepare 80 mg / mL and 5 mg / mL solutions, respectively. Subsequently, the volume ratio of polymer A to the aldehyde molecule solution is 4:1, and stirring is continued for 1 hour to obtain a uniform and transparent solution. The mixed solution is poured into a 5*5*3cm polytetrafluoroethylene mold and placed on a hot plate at 30℃ overnight to completely evaporate the solvent. The material is then placed in a vacuum oven at 200℃ for 6 hours to obtain a yellow translucent film. The tensile strength of the obtained material is tested by a universal material testing machine and is 116.3MPa, which is comparable to commercial engineering plastics. After being immersed in deionized water, acidic (1M HCl) and alkaline (1M NaOH) aqueous solutions at room temperature for 3 days, the plastic can still maintain its original shape and mechanical properties, showing the excellent water resistance of the plastic. After being immersed in a variety of organic solvents for 48 hours, the plastic did not depolymerize and dissolve, showing excellent resistance to organic solvents. The plastic was cut into pieces and placed in a mixed solution of dichloromethane and phosphoric acid (volume ratio of 12:1), and stirred for about 4 hours. The plastic was completely depolymerized and dissolved in the mixed solution to obtain a transparent and uniform solution. The depolymerized solution was precipitated in petroleum ether, neutralized and purified by adding triethylamine, and filtered and dried to obtain high-purity polymer A and aldehyde-functionalized silsesquioxane with a yield of about 95.0%. The reversible cross-linked plastic was re-prepared with the recycled monomer, and its tensile strength was basically the same as that of the original material ( Figure 4 ). The plastic was tested for flame retardancy and showed excellent flame retardancy, with the vertical burning test reaching UL94-V0 grade and the limiting oxygen index being 46.2%.

[0047] Example 5: A method for preparing a high-strength, chemically stable, flame-retardant closed-loop recycled plastic. The amount of each raw material is calculated by mole fraction as follows:

[0048] The amino-functionalized rigid chain polymer A is prepared by polycondensation of pyromellitic acid dianhydride and 2,2'-bis(trifluoromethyl)diaminobiphenyl, with the molar ratio of dianhydride to diamino monomer being 1:1.25, and the method is referred to the literature (RSC Adv., 2015, 5, 103246–103254). The flame retardant small molecule B is aldehyde-functionalized phosphorus oxychloride. Polymer A and aldehyde-functionalized phosphorus oxychloride are dissolved in N-methylpyrrolidone to prepare 50 mg / mL and 10 mg / mL solutions, respectively. Then, the volume ratio of polymer A to aldehyde molecule solution is 10:1, and stirring is continued for 2 hours to obtain a uniform and transparent solution. The mixed solution is poured into a 8*8*3cm polytetrafluoroethylene mold and placed on a hot plate at 80℃ overnight to completely evaporate the solvent. The material is then placed in a vacuum oven at 240℃ for curing for 1 hour, and a yellow transparent film is finally obtained. The obtained material was tested with a universal material testing machine and its tensile strength was 104.8MPa, which is comparable to commercial engineering plastics. The plastic can still maintain its original shape after being immersed in deionized water, acidic (1M HCl) and alkaline (1MNaOH) aqueous solutions at room temperature for 2 days. The plastic was immersed in a variety of organic solvents for 24 hours, and the plastic did not depolymerize or dissolve, showing excellent resistance to organic solvents. The plastic was cut into pieces and placed in a mixed solution of N-methylpyrrolidone and concentrated hydrochloric acid (volume ratio of 5:1), and stirred for about 8 hours. The plastic was completely depolymerized and dissolved in the mixed solution to obtain a transparent and uniform solution. The depolymerization solution was precipitated in ethanol, neutralized and purified by adding sodium carbonate aqueous solution, and filtered and dried to obtain high-purity polymer A and aldehyde-functionalized phosphorus oxychloride, with a yield of about 93.5%. The recycled monomer can be used to re-prepare new plastics identical to the original material. The plastic was flame retardant tested, and the plastic showed excellent flame retardant effect ( Figure 5 ). The vertical combustion test reached UL94-V0 level, and the limiting oxygen index was 46.8%. From the SEM image, it can be seen that the surface of the carbon layer after the UL94 vertical combustion test is dense, while its interior is fluffy and porous. Such a fluffy and porous closed carbon layer can effectively isolate heat and oxygen, making the material have excellent flame retardant properties ( Figure 6 ).

Claims

1. A method for recycling high-strength chemically stable flame-retardant closed-loop recycled plastics, the steps of which are as follows: 1) Put the high-strength chemically stable flame-retardant closed-loop recycled plastic into the mixed solvent D to depolymerize and dissolve the plastic to obtain a uniform plastic solution; 2) The plastic solution of step 1) is precipitated in solvent E, and the amino-functional rigid-chain polymer A and the flame-retardant small molecule B are precipitated from the solution, and then a neutralizer F is added to the solution for purification, followed by solid-liquid separation, and the obtained solid is a mixture of the amino-functional rigid-chain polymer A and the flame-retardant small molecule B; the obtained mixture of the amino-functional rigid-chain polymer A and the flame-retardant small molecule B does not need to be separated and is directly used for the preparation of plastics, thereby realizing closed-loop recycling of plastics; in, High-strength, chemically stable, flame-retardant closed-loop recycled plastics are prepared by the following steps: S1: dissolving the amino-functionalized rigid chain polymer A and the flame-retardant small molecule B in solvent C respectively to prepare a solution with the concentration of the amino-functionalized rigid chain polymer A being 50-100 mg / mL and the concentration of the flame-retardant small molecule B being 1-30 mg / mL; the amino-functionalized rigid chain polymer A is an amino-terminated polymer, which is formed by condensation of an acylated monomer and an amino monomer, and the molar ratio of the acylated monomer to the amino monomer is 1:1.1-2; the acylated monomer is one or more of terephthaloyl chloride, isophthaloyl chloride, pyromellitic anhydride, hexafluoroisopropylphthalic anhydride, and isophthalic acid; the amino monomer is one or more of 2,2'-di(trifluoromethyl)diaminobiphenyl, p-phenylenediamine, m-phenylenediamine, diaminodiphenyl ether, and 3,3'-diaminobiphenyl; the flame-retardant small molecule B is aldehyde-functionalized phosphorus oxychloride, and the flame-retardant small molecule B is multifunctional; S2: The amino-functionalized rigid-chain polymer A solution and the flame-retardant small molecule B solution obtained in step S1 are uniformly mixed in a ratio of 1:1 based on the molar number of the functional groups of the raw materials, the solvent is evaporated at 30-80° C., and then cured at 180-300° C. under vacuum for 2-6 hours to obtain a high-strength, chemically stable flame-retardant closed-loop recycled plastic.

2. A method for recycling high-strength chemically stable flame-retardant closed-loop recycled plastics as claimed in claim 1, Features: The solvent C in step S1 is one or more of dichloromethane, tetrahydrofuran, N,N-dimethylformamide, dimethyl sulfoxide, and N-methylpyrrolidone.

3. A method for recycling high-strength chemically stable flame-retardant closed-loop recycled plastics as claimed in claim 1, Features: In step 1), the mixed solvent D is a mixture of an organic solvent and an acid, the organic solvent is one or more of N-methylpyrrolidone, tetrahydrofuran, dichloromethane, dimethyl sulfoxide, N,N-dimethylformamide, and acetone, the acid is one or more of concentrated hydrochloric acid, glacial acetic acid, phosphoric acid, and citric acid, and the volume ratio of the organic solvent to the acid is 20 to 1:

1.

4. A method for recycling high-strength chemically stable flame-retardant closed-loop recycled plastics as claimed in claim 1, Features: In step 2), the solvent E is a poor solvent for the amino-functionalized rigid chain polymer A and the flame retardant small molecule B, and is one or more of water, ethanol, ethyl acetate, n-hexane, and petroleum ether.

5. A method for recycling high-strength chemically stable flame-retardant closed-loop recycled plastics as claimed in claim 1, Features: The neutralizing agent F in step 2) is one or more of potassium hydroxide aqueous solution, sodium hydroxide aqueous solution, sodium carbonate aqueous solution, sodium bicarbonate aqueous solution, and triethylamine.

6. A method for recycling high-strength chemically stable flame-retardant closed-loop recycled plastics as claimed in claim 5, Features: In step 2), the concentration of the potassium hydroxide aqueous solution, the sodium hydroxide aqueous solution, the sodium carbonate aqueous solution or the sodium bicarbonate aqueous solution is 1 mol / L.

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