A recycling method of polyethylene terephthalate (PET)

Abstract

This invention discloses a method for recycling polyethylene terephthalate (PET). In this invention, an organic acid is used as a catalyst to hydrolyze PET. The organic acid is selected from one or more of bisaccharide, lactic acid, and glycolic acid. Then, the hydrolysis product is directly polymerized with an added monomer and polymerization catalyst without any separation or treatment steps to prepare a biodegradable copolyester. The prepared copolyester is biodegradable. Therefore, this recycling method realizes the conversion of non-degradable PET into biodegradable copolyester, which is of great significance for alleviating environmental pollution problems.

Description

Technical Field

[0001] This invention relates to the field of recycling technology. More specifically, it relates to a method for recycling polyethylene terephthalate (PET). Background Technology

[0002] Polyethylene terephthalate (PET) is a thermoplastic polyester copolymerized from terephthalic acid (TPA) and ethylene glycol (EG). It possesses excellent mechanical properties, low permeability, high transparency, non-toxicity, and chemical stability, making it widely used in bottles, fibers, and films, and is the most produced polyester plastic. However, the rigid benzene ring structure in the PET molecular chain and the strong intermolecular interactions make it difficult to degrade in the natural environment. The large accumulation of used PET products causes significant environmental pollution, making recycling crucial. This not only reduces resource consumption and environmental pollution but also saves energy and lowers raw material costs.

[0003] Currently, PET recycling is mainly divided into physical recycling and chemical recycling. Physical recycling typically involves sorting and processing waste PET to clean and separate it, producing recyclable granules or sheets. This method is simple to operate and low in cost. However, during physical recycling, the molecular chains of PET partially break due to high-temperature melting and reprocessing, leading to a decrease in molecular weight and reduced performance of the recycled products. These recycled products can only be used to manufacture low-end materials such as non-food packaging materials, short fibers, and nonwoven fabrics. Chemical recycling includes hydrolysis, methanololysis, ethylene glycololysis, and ammonolysis. It can not only convert waste PET into new raw materials but also further convert them into other high-value-added chemicals, thus achieving efficient resource utilization. Compared with physical recycling, chemical recycling can also effectively remove impurities and contaminants, improve the quality of recycled materials, and avoid secondary pollution. Patent CN114805766A discloses a method for preparing PBT from PBT and waste PET polyester or / and PET-PBT mixed polyester, but the product requires multiple processing steps, making the process complex and energy-intensive. Patent CN112441917A discloses a method for recovering terephthalic acid esters from waste PET, which also requires separation and purification to obtain the product. Therefore, a simpler method that does not require separation and purification is needed for the recycling of PET. Summary of the Invention

[0004] Therefore, the purpose of this invention is to provide a method for recycling polyethylene terephthalate (PET). The recycling method provided by this invention uses organic acids adipic acid (AA), lactic acid (LA), and / or glycolic acid (GA) as catalysts to achieve the hydrolysis reaction of PET. The products after the hydrolysis reaction do not need to be separated and can all be used as raw materials for subsequent polymerization reactions. By adding different monomers, different types of biodegradable copolyesters can be prepared, realizing the conversion of non-degradable PET into biodegradable copolyesters, which is of great significance for alleviating environmental pollution problems.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention discloses a method for recycling polyethylene terephthalate (PET), which uses an organic acid as a catalyst to hydrolyze PET, wherein the organic acid is selected from one or more of bisaccharide, lactic acid, and glycolic acid.

[0007] The PET used in this invention is selected from recycled materials of various molecular weights obtained from different sources, such as waste PET sheets, waste PET films, and waste PET fibers.

[0008] Furthermore, based on mass percentage, the amount of organic acid used is 10-200 wt% of PET. For example, the amount of catalyst used can be 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, 100 wt%, 110 wt%, 120 wt%, 130 wt%, 140 wt%, 150 wt%, 160 wt%, 170 wt%, 180 wt%, 190 wt%, 200 wt% of PET, etc., preferably in the range of 10-100 wt%, more preferably 10-50 wt%.

[0009] Furthermore, the solvent used in the hydrolysis reaction is water;

[0010] The water content, by weight percentage, is 10-2000 wt% of PET. For example, the water content is 10 wt%, 50 wt%, 100 wt%, 150 wt%, 200 wt%, 250 wt%, 300 wt%, 350 wt%, 400 wt%, 450 wt%, 500 wt%, 550 wt%, 600 wt%, 650 wt%, 700 wt%, 7... 50wt%, 800wt%, 850wt%, 900wt%, 950wt%, 1000wt%, 1100wt%, 1200wt%, 1300wt%, 1400wt%, 1500wt%, 1600wt%, 1700wt%, 1800wt%, 1900wt%, 2000wt%, etc., preferably 100-1000wt%, more preferably 100-500wt%.

[0011] Furthermore, the hydrolysis reaction temperature is 50-250℃, preferably 180-220℃; the hydrolysis reaction time is 1-12h, so that the hydrolysis rate of PET in the hydrolysis reaction is 0-100%. When the hydrolysis rate of PET is 0%, the mass of the solid does not change, but the molecular weight decreases significantly, forming oligomers.

[0012] Furthermore, the hydrolysis reaction is carried out under closed conditions, such as in a closed reaction vessel.

[0013] Furthermore, the recycling method also includes polymerizing the hydrolysis reaction products with additionally added monomers and polymerization catalysts to prepare biodegradable copolyesters.

[0014] In this invention, the hydrolysis reaction product used for repolymerization can be either a monomer compound obtained after complete hydrolysis, an incompletely reacted oligomer, or a mixture of oligomers and monomers. Regardless of whether it is a monomer, oligomer, or a mixture of both, it can be successfully polymerized to obtain a copolyester. Therefore, this invention can obtain biodegradable copolyesters with relatively lower energy consumption and in a shorter time.

[0015] Furthermore, the hydrolysis reaction products include oligomers and / or monomers of PET; preferably, the monomers are selected from terephthalic acid and / or ethylene glycol.

[0016] Furthermore, the additional monomers are selected from terephthalic acid (TPA) and / or ethylene glycol (EG);

[0017] The polymerization catalyst is selected from one or more of the following: antimony-based catalysts (e.g., antimony trioxide, antimony glycolate, and antimony acetate), germanium-based catalysts (e.g., germanium oxide), rare earth catalysts, and titanium-based catalysts (e.g., titanates, titanate esters, and titanium oxides), preferably titanate ester catalysts (e.g., tetrabutyl titanate TBT).

[0018] Furthermore, the polymerization reaction includes esterification and polycondensation reactions carried out sequentially.

[0019] Furthermore, the aggregation includes the following steps:

[0020] The hydrolysis product is then subjected to esterification with additional monomers and polymerization catalysts.

[0021] The product of the esterification reaction is then subjected to a further polycondensation reaction to obtain a biodegradable copolyester.

[0022] Furthermore, the conditions for the esterification reaction are: atmospheric pressure and a reaction temperature of 160-240℃, preferably 200-240℃; the conditions for the polycondensation reaction are: a vacuum of 10-100 Pa and a reaction temperature of 220-270℃, preferably 250-270℃.

[0023] Furthermore, the molar ratio of the additionally added monomer terephthalic acid to the organic acid is 0.1-10, preferably 0.2-5. For example, the molar ratio of terephthalic acid to the organic acid can be 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc.

[0024] Furthermore, the molar ratio of the additionally added monomer ethylene glycol to the organic acid is 0.2-20, preferably 0.4-10. For example, the molar ratio of ethylene glycol to organic acid can be 0.4, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, etc.

[0025] Copolyesters with different molecular sequence structures can be prepared by varying the ratio of the additional monomers terephthalic acid and ethylene glycol.

[0026] Furthermore, the polymerization catalyst accounts for 0.2-0.5 wt% of the total acid in the polymerization system, preferably 0.4-0.5 wt%. In this invention, the total acid in the polymerization system refers to the sum of the amount of acid added by the organic acid catalyst and the amount of TPA theoretically obtained after complete hydrolysis of PET.

[0027] Furthermore, the biodegradable copolyester is prepared using the recycling method described above. Exemplarily, the biodegradable copolyester is selected from one of the following structures:

[0028]

[0029] Where x is 1-10, y is 1-3, and z is 1-10.

[0030] The beneficial effects of this invention are as follows:

[0031] 1. The recycling method provided by this invention uses organic acids adipic acid (AA), lactic acid (LA), and / or glycolic acid (GA) as catalysts to achieve the hydrolysis reaction of polyethylene terephthalate (PET). The hydrolysis product requires no separation step and can be directly used in the next polymerization reaction, greatly simplifying the process and reducing production costs. Furthermore, the three organic acids all have high catalytic performance, resulting in a high hydrolysis rate of PET and a high yield of the hydrolysis product TPA.

[0032] 2. Based on the hydrolysis reaction, this invention mixes the hydrolysis products with different additional monomers, and completes the two-step reaction of esterification and polycondensation in sequence under the action of a polymerization catalyst to prepare copolyesters with different properties, thus expanding the application range. At the same time, the prepared copolyesters are biodegradable, realizing the transformation of non-degradable PET into biodegradable copolyesters, which is of great significance for alleviating environmental pollution problems. Attached Figure Description

[0033] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0034] Figure 1 The FTIR spectrum of TPA, the hydrolysis product in Example 1, is shown.

[0035] Figure 2 The hydrolysis product TPA in Example 1 is shown. 1 H NMR spectrum. Detailed Implementation

[0036] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further explains the invention. Similar components in the drawings are indicated by the same reference numerals. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0037] The structure of the hydrolysis products was characterized using FTIR (FTIR-6800, Jasco, Japan) in the wavenumber range of 3500–600 cm⁻¹. -1 The instrument resolution is 0.1 cm. -1 .

[0038] use 1 The structure of the hydrolysis products was analyzed by 1H NMR (Avance 400, Bruker, Germany) in deuterated dimethyl sulfoxide (DMSO) as the solvent.

[0039] The molecular weight and distribution of the polymer were tested using GPC (e2695, Waters, USA) with chloroform and o-chlorophenol as a mixed solvent, chloroform as the mobile phase, and PMMA as the reference material.

[0040] Example 1

[0041] LA-catalyzed PET hydrolysis:

[0042] PET particles (1.0 g), a certain amount of LA and water were added to a 50 mL reaction vessel and heated and stirred at a certain temperature for a certain time at a stirring speed of 25 rpm. After the reaction was completed, the reaction vessel was allowed to cool naturally to room temperature, the reaction solution was filtered, and the remaining solid 1 and filtrate 1 were collected separately. The remaining solid 1 was placed in NaOH solution and reacted for a period of time, then filtered again, and the remaining solid 2 (i.e., unreacted PET) and filtrate 2 were collected. The remaining solid 2 was washed three times with deionized water and dried in a vacuum oven at 60 °C for 12 h until constant weight. H2SO4 was added to filtrate 2, filtered, and the precipitated remaining solid 3 (i.e., product TPA) was collected and dried in a vacuum oven at 60 °C for 12 h until constant weight. The hydrolysis rate of PET was calculated using formula (1), and the yield of product TPA was calculated using formula (2). The specific product results under different hydrolysis conditions are shown in Table 1.

[0043] PET hydrolysis rate (%) = (m0-m1)×100 / m0 (1)

[0044] Where m0 is the initial mass of PET and m1 is the mass of the remaining solid 2.

[0045] TPA yield (%) = m a ×100 / m b (2)

[0046] Where, m a The mass of the remaining solid 3, m b This is the theoretical mass of TPA that can be obtained from the complete hydrolysis of PET.

[0047] Table 1. Results of products under different hydrolysis reaction conditions

[0048]

[0049]

[0050] Note: The LA and water dosages in Table 1 are based on mass percentages of LA or water relative to PET.

[0051] As shown in Table 1, LA has a significant promoting effect on the hydrolysis of PET. After the addition of LA, the hydrolysis rate of PET and the yield of product TPA are significantly improved. This is mainly because the H2 produced by LA... +The hydrolysis of PET was promoted. By adjusting factors such as temperature, reaction time, and catalyst dosage, the degree of PET hydrolysis can be controlled to obtain oligomers and monomers. After optimizing the reaction conditions, the optimal reaction conditions were determined to be a hydrolysis temperature of 200℃, a hydrolysis reaction time of 5 h, a LA dosage of 100 wt%, and a water dosage of 300 wt%. Under these conditions, the hydrolysis rate of PET was 100%, and the yield of TPA was 81.3%.

[0052] Figure 1 The FTIR spectrum of TPA, the hydrolysis product in Example 1, is shown. Figure 2 The hydrolysis product TRA in Example 1 is shown. 1 HNMR spectrum. (From) Figure 1 and Figure 2 It can be seen that the hydrolysis reaction yielded TPA.

[0053] Example 2

[0054] 1.0 g of PET plastic bottle fragments, 1.0 g of LA, and 3.0 g of water were added to a 50 mL reactor and heated and stirred at 200 °C for 5 h at a stirring speed of 25 rpm. After the reaction was completed, the hydrolysis rate of the PET plastic bottle was determined to be 100%, and the yield of TPA was 79.3%, referring to the yield calculation method in Example 1. Comparison with the results of Example 1 revealed that, under the same process conditions, the hydrolysis results obtained from PET from different sources were essentially consistent.

[0055] Examples 3-5

[0056] LA-catalyzed PET hydrolysis, followed by polymerization of the hydrolysis products to prepare PETL copolyester:

[0057] Hydrolysis: PET granules (50.0 g), LA (50.0 g), and water (150.0 g) were added to a 1.2 L reactor and heated and stirred at 200 °C for a period of time at a stirring speed of 25 rpm. After the reaction was completed, the reactor was allowed to cool naturally to room temperature, and all the products were added to a 500 mL three-necked flask.

[0058] Polymerization: TPA (141.4 g), EG (121.8 g), and polymerization catalyst TBT (1.17 g) were added to a three-necked flask. The flask was then placed in an oil bath equipped with a top stirrer at a stirring speed of 450 rpm. The reaction was then carried out at 200-240 °C for approximately 5-7 hours until the esterification reaction was complete. The pressure of the reaction system was reduced to below 100 Pa, and the oil bath temperature was increased to 250 °C to initiate the polycondensation reaction. The polycondensation reaction continued for 3-4 hours. The reaction was considered complete when the viscosity of the polymer significantly increased and the torque of the stirrer reached 30 N·cm. The hydrolysis reaction time and the molecular weight of the final product PETL are shown in Table 2.

[0059] Table 2. Molecular weight of PETL final product under different conditions

[0060]

[0061] Example 6

[0062] GA-catalyzed PET hydrolysis:

[0063] PET particles (1.0 g), a certain amount of GA and water were added to a 50 mL reaction vessel and heated and stirred at a certain temperature for a certain time at a stirring speed of 25 rpm. After the reaction was completed, the reaction vessel was allowed to cool naturally to 60 °C. The reaction solution was filtered, and the remaining solid 1 and filtrate 1 were collected separately. The remaining solid 1 was placed in NaOH solution and reacted for a period of time, and then filtered again to collect the remaining solid 2 (i.e., unreacted PET) and filtrate 2. The remaining solid 2 was washed three times with deionized water and dried in a vacuum oven at 60 °C for 12 h until constant weight. H2SO4 was added to filtrate 2, and the precipitated remaining solid 3 (i.e., product TPA) was collected by filtration and dried in a vacuum oven at 60 °C for 12 h until constant weight. The hydrolysis rate of PET was calculated using formula (1), and the yield of product TPA was calculated using formula (2). The specific product results under different hydrolysis conditions are shown in Table 3.

[0064] Table 3. Results of products under different hydrolysis reaction conditions

[0065]

[0066] Note: The dosage of GA and water in Table 3 are based on mass percentage of GA or water relative to PET.

[0067] As shown in Table 3, GA has a significant promoting effect on the hydrolysis of PET. After the addition of GA, the hydrolysis rate of PET and the yield of product TPA are significantly improved. This is mainly because the H2 produced by GA... + This promotes the hydrolysis of PET. By adjusting factors such as temperature, reaction time, and catalyst dosage, the degree of hydrolysis of PET can be controlled to obtain oligomers and monomers.

[0068] Examples 7-9

[0069] GA catalyzes PET hydrolysis, and the hydrolysis products are polymerized to prepare PETG copolyester:

[0070] Hydrolysis: PET granules (50.0 g), GA (50.0 g), and water (150.0 g) were added to a 1.2 L reactor and heated at 200 °C with stirring for a certain period of time at a stirring speed of 25 rpm. After the reaction was completed, the reactor was allowed to cool naturally to room temperature, and all the products were added to a 500 mL three-necked flask.

[0071] Polymerization: TPA (175.4 g), EG (147.2 g), and polymerization catalyst TBT (1.34 g) were added to a three-necked flask. The flask was then placed in an oil bath equipped with a top stirrer at a stirring speed of 450 rpm. The reaction was then carried out at 200-240 °C for approximately 5-7 hours until the esterification reaction was complete. The pressure of the reaction system was reduced to below 100 Pa, and the oil bath temperature was increased to 250 °C to initiate the polycondensation reaction. The polycondensation reaction continued for 3-4 hours. The reaction was considered complete when the viscosity of the polymer significantly increased and the torque of the stirrer reached 30 N·cm. The hydrolysis reaction time and the molecular weight of the final product PETG are shown in Table 4.

[0072] Table 4. Molecular weight of the final product PETG under different conditions.

[0073]

[0074] Example 10

[0075] AA-catalyzed PET hydrolysis:

[0076] PET particles (1.0 g), a certain amount of AA, and water were added to a 50 mL reaction vessel. The mixture was heated and stirred at a certain temperature for a certain time at a stirring speed of 25 rpm. After the reaction was completed, the reaction vessel was allowed to cool naturally to 60 °C. The reaction solution was filtered, and the remaining solid 1 and filtrate 1 were collected separately. The remaining solid 1 was placed in NaOH solution and reacted for a period of time. Then it was filtered again, and the remaining solid 2 (i.e., unreacted PET) and filtrate 2 were collected. The remaining solid 2 was washed three times with deionized water and dried in a vacuum oven at 60 °C for 12 h until constant weight. H2SO4 was added to filtrate 2, and the precipitated remaining solid 3 (i.e., product TPA) was collected by filtration and dried in a vacuum oven at 60 °C for 12 h until constant weight. The hydrolysis rate of PET was calculated using formula (1), and the yield of product TPA was calculated using formula (2). The specific product results under different hydrolysis conditions are shown in Table 5.

[0077] Table 5. Product results under different hydrolysis reaction conditions.

[0078]

[0079] Note: The dosage of AA and water in Table 5 is based on the mass percentage of AA or water relative to PET.

[0080] As shown in Table 5, AA has a significant promoting effect on the hydrolysis of PET. After the addition of AA, the hydrolysis rate of PET and the yield of product TPA are significantly improved. This is mainly because the H produced by AA +This promotes the hydrolysis of PET. By adjusting factors such as temperature, reaction time, and catalyst dosage, the degree of hydrolysis of PET can be controlled to obtain oligomers and monomers.

[0081] Examples 11-13

[0082] AA-catalyzed PET hydrolysis, followed by polymerization of the hydrolysis products to prepare PEAT copolyester:

[0083] Hydrolysis: PET granules (50.0 g), AA (50.0 g), and water (150.0 g) were added to a 1.2 L reactor and heated at 210 °C with stirring for a certain period of time at a stirring speed of 25 rpm. After the reaction was completed, the reactor was allowed to cool naturally to room temperature, and all the products were added to a 500 mL three-necked flask.

[0084] Polymerization: TPA (13.6 g), EG (68.8 g), and polymerization catalyst TBT (0.53 g) were added to a three-necked flask. The flask was then placed in an oil bath equipped with a top stirrer at a stirring speed of 450 rpm. The reaction was then carried out at 200-240 °C for approximately 5-7 hours until the esterification reaction was complete. The pressure of the reaction system was reduced to below 100 Pa, and the oil bath temperature was raised to 250 °C to initiate the polycondensation reaction. The polycondensation reaction continued for 3-4 hours. The reaction was considered complete when the viscosity of the polymer significantly increased and the torque of the stirrer reached 30 N·cm. The hydrolysis reaction time and the molecular weight of the final product PEAT are shown in Table 6.

[0085] Table 6. Molecular weight of the final product PEAT under different conditions.

[0086]

[0087]

[0088] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. For those skilled in the art, other variations or modifications can be made based on the above description. It is impossible to exhaustively list all the implementation methods here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Claims

1. A method for recycling polyethylene terephthalate (PET), characterized in that, The hydrolysis of PET is carried out using organic acids as catalysts, wherein the organic acids are selected from one or more of bisaccharide, lactic acid and glycolic acid; The amount of organic acid used, by weight percentage, is 10-200 wt% of PET. The solvent used in the hydrolysis reaction is water; the amount of water used is 10-2000 wt% of PET. The recycling method also includes polymerizing the hydrolysis reaction products with additional monomers and polymerization catalysts to prepare biodegradable copolyesters; The hydrolysis products after hydrolysis do not require any separation steps and can be directly used in the next polymerization reaction; The hydrolysis reaction temperature is 50-250℃; the hydrolysis reaction time is 1-12 hours. The hydrolysis products include oligomers and / or monomers of PET; The additional monomers are selected from terephthalic acid and / or ethylene glycol; The polymerization catalyst is selected from one or more of antimony-based catalysts, germanium-based catalysts, rare earth catalysts, and titanium-based catalysts; The polymerization reaction includes esterification and polycondensation reactions carried out sequentially; The conditions for the esterification reaction are: atmospheric pressure and a reaction temperature of 160-240℃; the conditions for the polycondensation reaction are: a vacuum of 10-100 Pa and a reaction temperature of 220-270℃.

2. The recycling method according to claim 1, characterized in that, The amount of organic acid used is 10-100 wt% of PET, based on mass percentage.

3. The recycling method according to claim 1, characterized in that, The amount of water used is 100-1000 wt% of PET, based on mass percentage.

4. The recycling method according to claim 1, characterized in that, The monomer is selected from terephthalic acid and / or ethylene glycol.

5. The recycling method according to claim 1, characterized in that, The polymerization catalyst is a titanate catalyst.

6. The recycling method according to claim 5, characterized in that, The molar ratio of the additional monomer terephthalic acid to the organic acid is 0.1-10; the molar ratio of the additional monomer ethylene glycol to the organic acid is 0.2-20.

7. The recycling method according to claim 1, characterized in that, The biodegradable copolyester is selected from one of the following structures: PETL; PETG; PEAT; Where x is 1-10, y is 1-3, and z is 1-10.

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