Process for the degradation of pet polyester

Abstract

This invention provides a degradation process for PET polyester, relating to the field of polyester recycling technology. The degradation process involves depolymerizing a reaction system containing PET polyester material, a depolymerizing agent, and an alkaline catalyst at a degradation temperature not exceeding 100°C; the depolymerizing agent is selected from organic ester compounds. This invention uses organic ester compounds as both the degradation agent and alkylating agent for PET polyester degradation, resulting in mild reaction conditions that allow for relatively complete degradation of the PET polyester material and yield of high-yield dialkyl terephthalate.

Description

Technical Field

[0001] This invention belongs to the field of polyester recycling technology and relates to a degradation process method for PET polyester. Background Technology

[0002] Polyethylene terephthalate (PET) is the most important type of thermoplastic polyester material, widely used in bottles, fibers, films, engineering plastics, and other fields. In 2022 alone, China's apparent consumption of PET approached 50 million tons. With the rapid development of the PET industry, the amount of waste PET material is increasing, reaching tens of millions of tons annually in China, giving rise to the need for waste PET material recycling.

[0003] Currently, waste PET recycling employs physical, chemical, and biological methods. One important chemical method is methanol hydrolysis to recover dimethyl terephthalate (DMT), a raw material for PET. However, due to methanol's low boiling point, methanol hydrolysis typically requires high temperature and pressure (150-300℃, 2-20 MPa), which severely limits its industrial application. Furthermore, methanol's high toxicity further restricts the industrialization of this technology.

[0004] Chinese patent CN117326940A discloses a method for alcoholysis of polyester, in which a swelling solvent, an alcohol solvent, and an alkoxide are added to the polyester material, and the alcoholysis reaction can be carried out at a temperature not exceeding 100°C. Chinese patent CN116655465A discloses a method for methanol alcoholysis of PET, in which nitrile compounds are added as co-solvents and catalytic stabilizers during the methanol alcoholysis process, and guanidine or ether organic bases (DBU, TBD, DBN, etc.) are used as catalysts to complete the alcoholysis reaction at 40-120°C, selectively converting PET in the mixture into DMT monomers. However, the above methods still require the use of methanol, which is highly toxic, as the alcoholysis agent.

[0005] Therefore, researching PET polyester degradation reaction systems that do not contain methanol and have relatively mild reaction conditions has become a hot topic and an urgent problem to be solved in the industry. Existing technologies have disclosed reports of using carbonate solvents or carboxylic acid ester solvents as polyester degradation agents and alkylating agents, but these still have the drawbacks of not being mild enough in reaction conditions and being unfavorable for industrial production. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a degradation process for PET polyester, with at least one of the following objectives: 1) to provide a polyester degradation reaction with mild reaction conditions; 2) to provide a polyester degradation method that does not use highly toxic methanol.

[0007] The technical solution of the present invention is as follows:

[0008] A degradation process for PET polyester involves depolymerizing a reaction system containing PET polyester material, a depolymerizing agent, and an alkaline catalyst at a degradation temperature not exceeding 100°C.

[0009] The depolymerizing agent is selected from organic ester compounds.

[0010] Preferably, the PET polyester material is selected from waste PET polyester material, and the PET content in the waste PET polyester material is not less than 20%.

[0011] Preferably, the depolymerizing agent has the structure shown in formula (1).

[0012] R 1 COOR 2 (1)

[0013] Among them, R 1 Selected from H, C1-C8 alkyl, substituted C2-C8 alkyl, C6-C12 aryl, or C5-C9 cycloalkyl, R 2 Selected from C1-C4 alkyl groups.

[0014] More preferably, the R 2 Selected from methyl.

[0015] Preferably, the alkaline catalyst is selected from inorganic base catalysts and / or organic base catalysts.

[0016] Preferably, the weight ratio of the PET polyester material, the depolymerizing agent, and the alkaline catalyst is 1:1-15:0.02-0.2.

[0017] Preferably, the reaction system further contains a co-solvent, which is selected from one or a combination of two or more of halogenated hydrocarbon solvents, ether solvents, nitrile solvents and ketone solvents.

[0018] Preferably, the weight ratio of the PET polyester material, the depolymerizing agent, the alkaline catalyst, and the cosolvent is 1:0.5-5:0.02-0.2:2-5.

[0019] Preferably, the degradation temperature does not exceed 80°C.

[0020] Preferably, the depolymerization time is 0.5-5 hours.

[0021] The beneficial effects of this invention are:

[0022] (1) The present invention found that by using organic ester compounds as alcoholysis agents, under the action of alkaline catalysts, PET can be fully degraded under relatively mild conditions (≤100℃, normal pressure), without the need for high temperature and high pressure, and without the need to add highly toxic methanol, which helps to promote the industrial development of PET degradation and recycling methods.

[0023] (2) The degradation process of PET polyester of the present invention is suitable for various waste PET materials, including but not limited to PET film, PET plastic and various fiber fabrics. For fiber fabrics, since the degradation conditions are mild and do not require high temperature and high pressure, other materials (such as cotton, spandex, nylon, polypropylene, etc.) in the fiber fabrics (other than PET polyester) are not degraded. After the PET polyester material contained therein is degraded, other materials can also be separated and recycled, which has high selectivity and realizes the directional separation of materials. Detailed Implementation

[0024] The technical solution of the present invention will be further explained and described below through specific embodiments.

[0025] In order to achieve a relatively mild degradation process for PET polyester materials, this invention proposes a degradation process method for PET polyester, which depolymerizes a reaction system containing PET polyester materials, depolymerizing agents and alkaline catalysts at a degradation temperature not exceeding 100°C.

[0026] The depolymerizing agents mentioned above are selected from organic ester compounds.

[0027] This invention discovers that using organic ester compounds as depolymerizing agents, under alkaline catalysts, allows for the degradation of PET polyester materials under relatively mild conditions. Simultaneously, these organic ester compounds also act as alkylating agents, degrading PET polyester to generate dialkyl terephthalate without requiring high temperatures and pressures, and without using or generating highly toxic methanol. When the organic ester compound is an organic methyl ester, such as methyl propionate or methyl acetate, the degradation of PET polyester to generate dimethyl terephthalate can be reused as a raw material for PET polyester synthesis.

[0028] In this invention, organic ester compounds may include carbonate compounds or carboxylic acid ester compounds. Carbonate compounds include dimethyl carbonate, diethyl carbonate, etc. Carbonate compounds are low-toxicity and environmentally friendly solvents, but their stability is relatively poor. When water is present in the system, carbonate compounds are prone to hydrolysis, which has the following adverse effects: (1) Carbonate compounds hydrolyze to generate alcohol compounds, especially when dimethyl carbonate is used as a methylating agent, it will hydrolyze to generate methanol, which cannot achieve the purpose of avoiding the use of highly toxic methanol; (2) It will lead to a poorer degradation effect on PET polyester materials; (3) The degradation rate of carbonates on PET polyester materials is significantly lower than that of carboxylic acid ester compounds, which is not the best in terms of production efficiency. Therefore, when carbonate compounds are used as degradation agents, the water content of the reaction system is strictly required, and the lower the water content, the better. However, although PET polyester materials are baked to remove water before degradation, there will still be a small amount or trace amount of water, such as 0.5-3wt%. When the organic ester compound is a carboxylic acid ester, the carboxylic acid ester has better hydrolysis resistance than the carbonate compound, so it can play a more stable role as a degrading agent. Even if the reaction system contains a small amount of water, it will not affect the degradation effect of the carboxylic acid ester compound.

[0029] In a preferred embodiment of the present invention, the PET polyester material is selected from waste PET polyester material, and the PET content in the waste PET polyester material is not less than 20%. In this invention, the waste PET polyester material can be PET film, PET plastic, and various fiber fabrics, etc. The PET polyester degradation process of the present invention is suitable for various fiber fabrics, and can degrade the PET polyester material in the fiber fabric into dialkyl terephthalate, and separate the non-PET polyester materials (such as cotton, spandex, polypropylene, nylon, etc.) from the fiber fabric. For example, the PET content in the waste PET polyester material is not less than 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, etc., without particular limitation. More preferably, the PET content in the waste PET polyester material is not less than 40%.

[0030] In a preferred embodiment of the present invention, the depolymerizing agent is a carboxylic acid ester compound having the structure shown in formula (1).

[0031] R 1 COOR 2 (1)

[0032] Among them, R 1 Selected from H, C1-C8 alkyl, substituted C2-C8 alkyl, C6-C12 aryl, or C5-C9 cycloalkyl, R 2 Selected from C1-C4 alkyl groups.

[0033] The substituents in the aforementioned substituted C2-C8 alkyl groups do not contain active hydrogen, such as -OH, -SH, -COOH, -NH2, -NH-, etc., organic groups. The substituents in the substituted C2-C8 alkyl groups can be, for example, halogens, ether bonds, etc. For example, the depolymerizing agent of the present invention can be butyl acetate, ethyl acetate, methyl acetate, methyl propionate, ethyl benzoate, methyl benzoate, methyl butyrate, methyl cyclohexanoate, etc. When R... 2 When R is methyl, the product obtained after the degradation of PET polyester is dimethyl terephthalate; when R 2 When the ethyl group is ethyl, the product obtained after the degradation of PET polyester is diethyl terephthalate, and so on.

[0034] More preferably, R 2 The depolymerizing agent can be methyl acetate, methyl butyrate, methyl benzoate, methyl chloroacetate, methyl 2-chloropropionate, methyl bromoacetate, etc., selected from methyl groups. In this case, the degradation product of the PET polyester material is dimethyl terephthalate, which, after separation and purification, can be reused to prepare PET polyester, achieving the recycling and reuse of PET polyester. More preferably, R... 1 Selected from C1-C8 alkyl or substituted C2-C8 alkyl, for example, R 1 It can be ethyl, propyl, butyl, chloroethyl, etc.

[0035] In a preferred embodiment of the present invention, the alkaline catalyst is selected from inorganic alkaline catalysts and / or organic alkaline catalysts, such as using an inorganic alkaline catalyst alone, an organic alkaline catalyst alone, or a combination of an inorganic alkaline catalyst and an organic alkaline catalyst. For example, the inorganic alkaline catalyst may be sodium aluminate, potassium aluminate, sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, sodium methoxide, lithium methoxide, potassium methoxide, sodium ethoxide, potassium ethoxide, etc., and the organic alkaline catalyst may be 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 1,5-diazabicyclo[4.3.0]-5-nonene (DBN), 1,1,3,3-tetramethylguanidine (TMG), 4-dimethylaminopyridine (DMAP), triethylamine, etc.

[0036] In a preferred embodiment of the present invention, the weight ratio of PET polyester material, depolymerizing agent, and alkaline catalyst is 1:1-15:0.02-0.2. In this invention, the depolymerizing agent is in excess relative to the PET polyester material. The depolymerizing agent functions to depolymerize, dissolve (or swell), and alkylate, improving the solubility or swelling of the PET polyester material and facilitating the degradation reaction, accelerating the degradation rate, and resulting in dialkyl terephthalate as the degradation product. An excess of depolymerizing agent also helps to accelerate the depolymerization reaction rate and promote complete depolymerization of the PET polyester material. For example, the weight ratio of PET polyester material, depolymerizing agent, and alkaline catalyst can be 1:1:0.02, 1:1:0.05, 1:1:0.08, 1:1:0.1, 1:1:0.12, 1:1:0.15, 1:1:0.18, 1:1:0.2, 1:3:0.02, 1:3:0.05, 1:3:0.08, 1:3:0.1, 1:3:0.12, 1:3:0.15, 1:3:0.18, 1:3:0.2, 1:5:0.02, 1:5:0.05, 1:5:0.08, 1:5:0.1, 1:5:0.12, 1:5:0.15, 1:5:0.18, 1:5:0.2, Any value from 1:8:0.02, 1:8:0.05, 1:8:0.08, 1:8:0.1, 1:8:0.12, 1:8:0.15, 1:8:0.18, 1:8:0.2, 1:10:0.02, 1:10:0.05, 1:10:0.08, 1:10:0.1, 1:10:0.12, 1:10:0.15, 1:10:0.18, 1:10:0.2, 1:15:0.02, 1:15:0.05, 1:15:0.08, 1:15:0.1, 1:15:0.12, 1:15:0.15, 1:15:0.18, 1:15:0.2, etc., without any particular restrictions.

[0037] In a preferred embodiment of the present invention, the reaction system further contains a co-solvent, which is selected from one or a combination of two or more of halogenated hydrocarbon solvents, ether solvents, nitrile solvents, and ketone solvents. Adding a co-solvent to the reaction system has the following effects: (1) it can better dissolve or swell PET polyester material; (2) it can reduce the amount of degradation agent used; (3) it helps to regulate the solubility parameters of the system, thereby improving the reaction rate and selectivity, and reducing the occurrence of side reactions. In the present invention, there are no particular limitations on halogenated hydrocarbon solvents, which can be dichloromethane, trichloromethane, carbon tetrachloride, dichloroethane, etc.; there are no particular limitations on ether solvents, which can be tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether, propylene glycol dimethyl ether, isopropyl ether, methyl tert-butyl ether, etc.; there are no particular limitations on nitrile solvents, which can be acetonitrile, propionitrile, benzonitrile, etc.; and there are no particular limitations on ketone solvents, which can be acetone, methyl ethyl ketone, cyclohexanone, etc. From an environmental perspective, co-solvents can be selected from acetone, carbon tetrachloride, dichloromethane, tetrahydrofuran, ethylene glycol dimethyl ether, etc.

[0038] In a preferred embodiment of the present invention, the weight ratio of PET polyester material, depolymerizing agent, alkaline catalyst, and cosolvent is 1:0.5-5:0.02-0.2:2-5. For example, the weight ratio of PET polyester material, depolymerizing agent, alkaline catalyst, and cosolvent can be 1:0.5:0.02:2, 1:5:0.2:5, 1:1:0.05:3, 1:2:0.1:4, 1:1:0.1:2, 1:1:0.15:4, 1:2:0.12:3, 1:1.5:0.1:3, 1:0.8:0.1:5, 1:0.5:0.1:5, 1:1: Any value from 0.15:3, 1:3:0.18:3, 1:2:0.15:3, 1:2:0.1:3, 1:2:0.1:2, 1:3:0.1:3, 1:3:0.15:3, 1:4:0.1:3, 1:4:0.1:4, 1:4:0.12:4, 1:4:0.15:4, 1:4:0.0.08:4, 1:3:0.12:3, etc., without any particular restrictions.

[0039] Further preferably, when the PET polyester material contains spandex, the reaction system contains a co-solvent, particularly selected from ketone and nitrile solvents such as acetone, methyl ethyl ketone, cyclohexanone, and acetonitrile. The weight of the co-solvent is greater than the weight of the depolymerizing agent. In this case, the degradation reaction will not degrade the spandex, and the spandex will maintain its original state. However, when the PET polyester material contains spandex, if the reaction system does not contain a co-solvent or the weight of the co-solvent is less than the weight of the depolymerizing agent, the spandex will also degrade. During the reaction, the entire PET polyester material dissolves and disappears, affecting the recovery of DMT and preventing the high-selectivity separation of different materials.

[0040] In a preferred embodiment of the present invention, the degradation temperature does not exceed 80°C. For example, when the reaction system does not contain a co-solvent, the reaction temperature can be 50-80°C; when the reaction system contains a co-solvent, the reaction temperature can be 35-80°C. Furthermore, the reaction system is in an atmospheric pressure environment. Therefore, the degradation reaction of the present invention can be carried out under relatively mild conditions, without the need for high temperature and high pressure.

[0041] In a preferred embodiment of the present invention, the depolymerization time is 0.5-5 hours. The depolymerization time can be adjusted according to the amount of alkaline catalyst, depolymerization temperature, amount of depolymerizing agent, amount of co-solvent, etc. For example, the depolymerization time can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, etc. Further, the depolymerization time is 0.5-4h.

[0042] Depending on the type of carboxylic acid ester compound selected, especially R in the structure shown in formula (1) 2 The structure can be adjusted to control the type of dialkyl terephthalate, a product obtained after the degradation of PET polyester materials. For example, when R... 2 The methyl group is used, and the degradation product obtained is dimethyl terephthalate. Furthermore, when the PET polyester material contains materials that are not degraded by the degradation agent, these materials can be separated and recycled. For example, when the PET polyester material is waste textile fabric containing PET, the method of this invention can achieve the recycling and reuse of materials other than PET, thus better realizing the effective utilization of waste resources.

[0043] In this invention, after the PET polyester material is degraded, it can be separated and collected by cooling and crystallizing out the obtained dialkyl terephthalate, which has low solubility in the degradation agent (or co-solvent). The collected crystallized solid is then washed, dried, weighed to measure the yield, and its purity is tested using methods such as gas chromatography.

[0044] The technical solutions of the present invention will be further described and explained below with reference to various embodiments.

[0045] In the following embodiments, the PET polyester materials of various states and compositions are pre-dried to a moisture content of no more than 1 wt%.

[0046] Example 1

[0047] 50 g of PET film was mixed with a catalyst (potassium carbonate) and a degrading agent (methyl butyrate) at a weight ratio of 1:0.06:3. After reacting at 50°C for 3 hours, the PET film disappeared. The reaction system was filtered while hot, and the filtrate was cooled to 5°C, resulting in the precipitation of a solid. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 82.3%, and the purity was 98.6%.

[0048] Example 2

[0049] 50 g of PET film was mixed with a catalyst (potassium carbonate), a degradation agent (methyl acetate), and a co-solvent (acetonitrile) in a weight ratio of 1:0.06:3:2. The mixture was reacted at 50°C for 2.2 hours, after which the PET film disappeared. The reaction system was filtered while hot, and the filtrate was cooled to 5°C, resulting in the precipitation of a solid. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 86.3%, and the purity was 98.8%.

[0050] Example 3

[0051] The difference between this embodiment and Example 1 is that in Example 1, methyl acetate was replaced with an equal weight of dimethyl carbonate, and the PET film disappeared after reacting at 50°C for 4.5 hours. The remaining steps remained unchanged. The yield of DMT was measured to be 70.4%, and the purity was 96.7%.

[0052] Example 4

[0053] 50 grams of polyester / nylon (70 / 30) fabric were used. The weight ratio of fabric to catalyst (TMG) and degradation agent (methyl chloroacetate) was 1:0.1:7. The mixture was reacted at 65°C for 3 hours. After the reaction, the reaction system was filtered while hot. The filtrate was nylon. The filtrate was cooled to 5°C, and a solid precipitated. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 78.8%, and the purity was 93.9%. The recovered nylon was washed with water, dried, and weighed, with a recovery rate of 95.4%.

[0054] Example 5

[0055] 50 g of polyester / nylon (70 / 30) fabric was used. The weight ratio of the fabric to the catalyst (TMG), the degradation agent (methyl chloroacetate), and the cosolvent tetrahydrofuran was 1:0.1:4:4. The mixture was reacted at 65°C for 2 hours. After the reaction was completed, the reaction system was filtered while hot. The filtrate was nylon. The filtrate was cooled to 5°C, and a solid precipitated. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 83.5%, and the purity was 96.1%. The recovered nylon was washed with water, dried, and weighed, with a recovery rate of 98.8%.

[0056] Example 6

[0057] 50 grams of pure polyester fabric was taken, and the weight ratio of polyester fabric to catalyst (sodium methoxide) and degradation agent (methyl propionate) was 1:0.15:10. After reacting at 60°C for 2.5 hours, the polyester fabric disappeared. The reaction system was filtered while hot, and the filtrate was cooled to 5°C, resulting in the precipitation of a solid. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 81.4%, and the purity was 95.9%.

[0058] Example 7

[0059] 50 grams of pure polyester fabric was taken, and the polyester fabric, catalyst (sodium methoxide), degradation agent (methyl propionate), and cosolvent (tetrahydrofuran) were reacted in a weight ratio of 1:0.15:3:3. After reacting at 60°C for 2 hours, the polyester fabric disappeared. The reaction system was filtered while hot, and the filtrate was cooled to 5°C, resulting in the precipitation of a solid. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 85.0%, and the purity was 97.3%.

[0060] Example 8

[0061] The difference between this embodiment and Example 6 is that in Example 6, methyl propionate was replaced with an equal weight of dimethyl carbonate. The remaining steps remained unchanged. Experiments showed that the polyester fabric disappeared after reacting with dimethyl carbonate at 60°C for 3.5 hours, significantly slower than the 2.5 hours with methyl propionate. The DMT yield was measured to be 68.3%, and the purity was 94.7%.

[0062] Comparative Example 1

[0063] The difference between this comparative example and Example 6 is that in Example 6, sodium methoxide was replaced with an equal weight of ZnO, and the polyester fabric disappeared after reacting at 60°C for 28 hours. The remaining steps remained unchanged. The DMT yield was measured to be 10.4%, and the purity was 90.5%.

[0064] Example 9

[0065] 50 g of polyester / cotton (50 / 50) fabric was mixed with a catalyst (sodium methoxide and DMAP in a 1:1 weight ratio) and a methylating agent (methyl propionate) in a 1:0.2:15 weight ratio. The mixture was reacted at 70 °C for 2.5 hours. After the reaction, the reaction system was filtered while hot. The filtrate was cooled to 5 °C, and a solid precipitated. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was 84.7%, and the purity was 93.3%. The cotton recovered after solid-liquid separation was washed, dried, and weighed; the recovery rate was 93.2%.

[0066] Example 10

[0067] 50 g of polyester / cotton (50 / 50) fabric was mixed with a catalyst (sodium methoxide and DMAP in a 1:1 weight ratio), a methylating agent (methyl butyrate), and methyl ethyl ketone in a 1:0.2:5:5 weight ratio, and reacted at 70 °C for 1.5 hours. After the reaction was complete, the reaction system was filtered while hot. The filtrate was cooled to 5 °C, and a solid precipitated. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was 86.7%, and the purity was 96.4%. The recovered cotton after solid-liquid separation was washed, dried, and weighed; the recovery rate was 97.2%.

[0068] Example 11

[0069] 50 grams of waste PET bottle filaments were taken, and the weight ratio of the filaments to the catalyst (sodium methoxide) and the degradation agent (methyl propionate) was 1:0.12:10. After reacting at 70°C for 2 hours, the filaments disappeared. The reaction system was filtered while hot, and the filtrate was cooled to 5°C, resulting in the precipitation of a solid. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 85.8%, and the purity was 96.0%.

[0070] Example 12

[0071] The difference between this embodiment and Example 11 is that in Example 11, the degrading agent methyl propionate was replaced with an equal weight of methyl benzoate, and it was found that the filaments disappeared after reacting at 70°C for 4.5 hours. The remaining steps remained unchanged. The yield of DMT was measured to be 72.3%, and the purity was 95.2%.

[0072] Comparative Example 2

[0073] The difference between this comparative example and Example 11 is that in Example 11, sodium methoxide was replaced with an equal weight of ZnO, and the filaments disappeared after reacting at 70°C for 21 hours. The remaining steps remained unchanged. The DMT yield was measured to be 12.5%, and the purity was 75.3%.

[0074] Example 13

[0075] 50 g of polyester / ammonia (85 / 15) fabric was used. The weight ratio of fabric to catalyst (potassium carbonate), degradation agent (propyl acetate), and co-solvent (acetone) was 1:0.8:3:4.5. The mixture was reacted at 55°C for 3 hours. After the reaction, the reaction system was filtered while hot. The filtrate was spandex. The spandex was washed with water, dried, and weighed. The recovery rate was 98.6%. Notably, the spandex retained its elasticity without loss. The filtrate was cooled to 5°C, and a solid precipitated. After solid-liquid separation, the filter cake was washed with water and dried to obtain dimethyl terephthalate (DMT). The yield of DMT was measured to be 81.3%, and the purity was 96.9%.

[0076] As described above, the basic principles, main features, and advantages of the present invention have been shown and described. Those skilled in the art should understand that the present invention is not limited to the above embodiments, which are merely preferred embodiments and should not be construed as limiting the scope of the invention. All equivalent changes and modifications made in accordance with the scope of the patent and the description should still fall within the scope of the present invention. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A degradation process for PET polyester, characterized in that, The reaction system containing PET polyester material, depolymerizing agent and alkaline catalyst is depolymerized at a degradation temperature not exceeding 100°C; The depolymerizing agent is selected from organic ester compounds.

2. The degradation process method for PET polyester according to claim 1, characterized in that, The PET polyester material is selected from waste PET polyester material, and the PET content in the waste PET polyester material is not less than 20%.

3. The degradation process method for PET polyester according to claim 1, characterized in that, The depolymerizing agent has the structure shown in formula (1). R 1 COOR 2 (1) Among them, R 1 Selected from H, C1-C8 alkyl, substituted C2-C8 alkyl, C6-C12 aryl, or C5-C9 cycloalkyl, R 2 Selected from C1-C4 alkyl groups.

4. The degradation process method for PET polyester according to claim 3, characterized in that, The R 2 Selected from methyl.

5. The degradation process method for PET polyester according to claim 1, characterized in that, The alkaline catalyst is selected from inorganic alkaline catalysts and / or organic alkaline catalysts.

6. The degradation process method for PET polyester according to claim 1, characterized in that, The weight ratio of the PET polyester material, the depolymerizing agent, and the alkaline catalyst is 1:1-15:0.02-0.

2.

7. The degradation process method for PET polyester according to claim 1, characterized in that, The reaction system also contains a co-solvent, which is selected from one or a combination of two or more of the following: halogenated hydrocarbon solvents, ether solvents, nitrile solvents, and ketone solvents.

8. The degradation process method for PET polyester according to claim 1, characterized in that, The weight ratio of the PET polyester material, the depolymerizing agent, the alkaline catalyst, and the cosolvent is 1:0.5-5:0.02-0.2:2-5.

9. The degradation process method for PET polyester according to claim 1, characterized in that, The degradation temperature does not exceed 80℃.

10. The degradation process method for PET polyester according to claim 1, characterized in that, The depolymerization time is 0.5-5 hours.