Method for continuously degrading PET (Polyethylene Terephthalate) by utilizing screw extruder

Abstract

The invention discloses a method for continuously degrading PET (Polyethylene Terephthalate) by using a screw extruder, belongs to the technical field of PET recycling, and solves the problem that a polymer degradation product cannot be obtained by a traditional reaction kettle intermittent PET chemical recycling method. The method comprises the following steps: injecting PET into a first screw extruder, controlling the temperature of a cylinder of the first screw extruder at 180-320 DEG C, injecting a first degradation aid and a first catalyst into a mixing section of the first screw extruder, and carrying out homogeneous depolymerization reaction on PET and the first degradation aid to obtain a degradation intermediate product; injecting the degradation intermediate product into a second screw extruder, controlling the temperature of a cylinder of the second screw extruder to be 100-200 DEG C, injecting a second degradation aid and a second catalyst into a mixing section of the second screw extruder, and performing homogeneous depolymerization reaction on the degradation intermediate product and the second degradation aid to obtain the PET degradation product. According to the invention, the production efficiency is improved, and controllable gradient homogeneous degradation of PET is realized.

Description

Technical Field

[0001] This invention belongs to the field of PET recycling technology, specifically relating to a method for continuously degrading PET using a screw extruder. Background Technology

[0002] In 2020, China's consumption of bottle-grade PET reached nearly 10 million tons, resulting in a massive social stockpile of waste products. Given the pressures from environmental protection and resource constraints, the recycling of PET, a petroleum-based synthetic polymer that is not easily degraded, is increasingly becoming a focus of attention. PET recycling methods can be mainly categorized into: primary recycling, energy recovery, physical recycling, and chemical recycling. Chemical recycling involves depolymerizing high-molecular-weight polyester materials into monomers, oligomers, or other chemicals under the action of macromolecular plasticizers / chain extenders. The advantage of chemical methods lies in reducing downgrading of materials, thus promoting the long-term preservation of material value in the economy. It also unlocks the potential for upgrading and reusing polyester waste, enabling the utilization of value-added chemicals and improving economic efficiency. For example, separating and purifying the products obtained from chemical recycling can yield new raw materials for polymerization reactions. Furthermore, the products from chemical recycling can be further processed, and each depolymerization of polyester materials can produce new primary polymers, thus enabling resource recycling. Therefore, chemical methods are widely recognized as a crucial recycling method that aligns with the principles of "sustainable development."

[0003] Depending on the degrading agent, the chemical recycling of PET can be divided into four methods: hydrolysis, alcoholysis, ammonolysis, and glycolysis. For PET, chemical recycling can break down the polyester chains, generating low-molecular-weight polyethylene terephthalate (BHET) intermediates, or completely degrade them into monomers such as terephthalic acid (TPA) and dimethyl terephthalate (DMT). After separation and purification, the products can be reused as monomers for polyester production or as raw materials for synthesizing other chemical products. Compared to physical recycling, chemical recycling maximizes resource utilization and has greater advantages. However, current technologies focus on using large amounts of degrading agents in reactors to degrade PET with the aid of catalysts. This reaction process is a batch-type heterogeneous degradation system, and the degraded PET mainly exists in monomer or dimer / trimer forms, not in the form of polymeric degradation products. These shortcomings restrict the development of PET chemical recycling processes; only by developing economical chemical recycling technologies can the reuse of waste PET be effectively promoted. Summary of the Invention

[0004] The purpose of this invention is to provide a method for continuous degradation of PET using a screw extruder, in order to solve the problem that the traditional batch chemical recycling method of PET in a reactor cannot obtain polymer degradation products. The method of this invention improves production efficiency and achieves controllable gradient homogeneous degradation of PET.

[0005] The technical solution of this invention is: a method for continuous degradation of PET using a screw extruder, comprising the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder. The barrel temperature of the first screw extruder is controlled at 180-320℃. The PET is conveyed, compressed, and melted into a molten state. The first degradation aid and the first catalyst are injected into the mixing section of the first screw extruder. The PET and the first degradation aid undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder, control the barrel temperature of the second screw extruder to 100-200℃, and inject the second degradation aid and the second catalyst into the mixing section of the second screw extruder. The degradation intermediate and the second degradation aid undergo a homogeneous depolymerization reaction. After exhausting the gas to remove by-products and cooling, the PET degradation product is obtained.

[0006] As a further improvement of the present invention, in step S2, the conveying section temperature of the first screw extruder is 180-240℃, the compression section temperature is 190-250℃, the melting section temperature is 250-320℃, the mixing section temperature is 250-320℃, and the exhaust section temperature is 180-240℃.

[0007] As a further improvement of the present invention, in step S2, the first degradation aid is one or a mixture of several of glycerol, neopentyl glycol, and pentaerythritol; the first catalyst is one or a mixture of several of zinc acetate, lead acetate, and stannous chloride.

[0008] As a further improvement of the present invention, in step S2, the mass ratio of PET, the first degradation aid, and the first catalyst is 100:2-10:1.

[0009] As a further improvement of the present invention, in step S3, the conveying section temperature of the second screw extruder is 160-200℃, the compression section temperature is 150-190℃, the melting section temperature is 150-190℃, the mixing section temperature is 150-190℃, and the exhaust section temperature is 100-150℃.

[0010] As a further improvement of the present invention, in step S3, the second degradation aid is one or a mixture of several of ethylene glycol, hexanediol, octanediol, and decanediol; the second catalyst is one or a mixture of several of zinc acetate, lead acetate, and stannous chloride.

[0011] As a further improvement of the present invention, in step S3, the mass ratio of PET, the second degradation aid, and the second catalyst is 100:6-14:2.

[0012] As a further improvement of the present invention, the first screw extruder is a co-rotating twin-screw extruder or a co-rotating three-screw extruder.

[0013] As a further improvement of the present invention, the second screw extruder is a co-rotating twin-screw extruder or a co-rotating three-screw extruder.

[0014] The beneficial effects of this invention are: this invention utilizes a screw extruder to continuously degrade PET. The first step utilizes a first screw extruder to convey, compress, and melt PET into a fluid dynamic state. In the mixing section, a small amount of high-boiling-point, non-volatile branched polyol is injected to achieve molecular-level miscibility with PET. Under the action of a catalyst, homogeneous degradation occurs. Because the branched polyol and PET achieve molecular-level miscibility, the molecular weight of PET can be controlled for homogeneous degradation by adjusting the amount of polyol added and the process conditions. The product resulting from the reaction of the branched polyol added in the first stage with PET exhibits a significantly reduced melting point, as the original crystalline morphology of PET is destroyed into an amorphous state. The second step utilizes a second screw extruder to convey, compress, and melt the PET degradation product obtained in the first stage into a fluid dynamic state at a relatively low temperature. In the mixing section, a low-boiling-point straight-chain diol is injected to achieve molecular-level miscibility with the PET degradation product prepared in the first stage. By controlling the amount of diol added and the process conditions, the PET degradation product prepared in the first step undergoes a homogeneous transesterification reaction, achieving controllable gradient homogeneous degradation of PET and obtaining polymer degradation products, providing a new option for subsequent reuse product design. Attached Figure Description

[0015] Figure 1 This is the 1H NMR spectrum of the PET degradation product obtained in Example 1 of the present invention; Figure 2 This is a molecular weight distribution diagram of the PET degradation products obtained in Examples 1-5 of the present invention; Figure 3 This is a molecular weight distribution diagram of the PET degradation products obtained in Examples 5-9 of the present invention. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0017] Example 1 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The temperature of the conveying section of the first screw extruder is set to 180°C, the temperature of the compression section is 190°C, the temperature of the melting section is 250°C, the temperature of the mixing section is 250°C, and the temperature of the exhaust section is 180°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:2:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 160℃, the temperature of the compression section is 150℃, the temperature of the melting section is 150℃, the temperature of the mixing section is 150℃, and the temperature of the exhaust section is 100℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:10:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 1.

[0018] The characterization results of the NMR spectra of the PET degradation products obtained in this embodiment are as follows: Figure 1As shown, the monomers, dimers, and trimers of polyethylene terephthalate (BHET) are used as examples for illustration. The hydroxyl peak (d peak) does not appear in deuterated trifluoroacetic acid reagent. It can be observed that the two methylene groups of BHET (b peak and c peak) show signals at 4.34 ppm and 3.87 ppm, respectively, and their relative positions are the same as the actual relative positions of the two methylene groups. Simultaneously, the two methylene groups in the BHET dimer and trimer show signals at the same positions. The difference is that a new signal appears at approximately 4.55 ppm, namely the methylene group between the two BHET units (labeled as e peak). While both peaks are linked to ester groups, the b-peak and e-peak exhibit different chemical shifts. This difference can be attributed to the fact that the intermediate methylene group is located between two ester groups, and the combined effect of the diester groups generates a strong electron-withdrawing (deshielding) effect, resulting in a low-field shift (i.e., a shift to higher ppm). In contrast, in BHET monomers, although one side of the methylene group is close to an ester group while the other side is a hydroxyl group, its electron-withdrawing ability is weaker. It may even provide some shielding through hydrogen bonding, leading to a lower degree of deshielding of the methylene group and a higher-field (i.e., lower ppm) signal. Furthermore, the terminal hydroxyl group in BHET monomers readily forms hydrogen bonds with the solvent. This interaction typically causes some protons to deshield, resulting in a higher chemical shift. As the dimer content increases, some hydroxyl groups participate in intramolecular linkages or aggregation, weakening the hydrogen bonding with the solvent and enhancing the overall shielding effect, thus shifting the signal to a higher field (lower ppm).

[0019] Example 2 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The temperature of the conveying section of the first screw extruder is set to 180°C, the temperature of the compression section is 190°C, the temperature of the melting section is 250°C, the temperature of the mixing section is 250°C, and the temperature of the exhaust section is 180°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:4:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 160℃, the temperature of the compression section is 150℃, the temperature of the melting section is 150℃, the temperature of the mixing section is 150℃, and the temperature of the exhaust section is 100℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:10:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 2.

[0020] Example 3 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The temperature of the conveying section of the first screw extruder is set to 180°C, the temperature of the compression section is 190°C, the temperature of the melting section is 250°C, the temperature of the mixing section is 250°C, and the temperature of the exhaust section is 180°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:6:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 160℃, the temperature of the compression section is 150℃, the temperature of the melting section is 150℃, the temperature of the mixing section is 150℃, and the temperature of the exhaust section is 100℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:10:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 3.

[0021] Example 4 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The temperature of the conveying section of the first screw extruder is set to 180°C, the temperature of the compression section is 190°C, the temperature of the melting section is 250°C, the temperature of the mixing section is 250°C, and the temperature of the exhaust section is 180°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:8:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 160℃, the temperature of the compression section is 150℃, the temperature of the melting section is 150℃, the temperature of the mixing section is 150℃, and the temperature of the exhaust section is 100℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:10:2. The degradation intermediates and ethylene glycol underwent a homogeneous depolymerization reaction. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 4.

[0022] Example 5 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The temperature of the conveying section of the first screw extruder is set to 180°C, the temperature of the compression section is 190°C, the temperature of the melting section is 250°C, the temperature of the mixing section is 250°C, and the temperature of the exhaust section is 180°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:10:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 160℃, the temperature of the compression section is 150℃, the temperature of the melting section is 150℃, the temperature of the mixing section is 150℃, and the temperature of the exhaust section is 100℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:10:2. The degradation intermediates and ethylene glycol underwent a homogeneous depolymerization reaction. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 5.

[0023] The PET degradation products from Examples 1-5 were analyzed by gel permeation chromatography (GPC), and the molecular weights of the products are as follows: Figure 2As shown. In five consecutive gradient degradation examples, by precisely controlling the screw temperature and alcohol / catalyst ratio, the number-average molecular weight of the obtained PET degradation products gradually decreased from 3.16 kg / mol in Example 1 to 0.55 kg / mol in Example 5, forming a stepwise distribution of 3.16 ~ 0.55 kg / mol. The molecular weight distribution of the PET degradation products obtained by this technology is between that of BHET monomer and PET, indicating that this technology can achieve controlled depolymerization of PET, which is mainly composed of polymers, and can provide diverse molecular weight samples for subsequent repolymerization, plasticization or functional modification.

[0024] Example 6 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The conveying section temperature of the first screw extruder is set to 240°C, the compression section temperature to 250°C, the melting section temperature to 320°C, the mixing section temperature to 320°C, and the exhaust section temperature to 240°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:10:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 200℃, the temperature of the compression section is 190℃, the temperature of the melting section is 190℃, the temperature of the mixing section is 190℃, and the temperature of the exhaust section is 150℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:6:2. The degradation intermediates and ethylene glycol underwent a homogeneous depolymerization reaction. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 6.

[0025] Example 7 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The conveying section temperature of the first screw extruder is set to 240°C, the compression section temperature to 250°C, the melting section temperature to 320°C, the mixing section temperature to 320°C, and the exhaust section temperature to 240°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:10:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 200℃, the temperature of the compression section is 190℃, the temperature of the melting section is 190℃, the temperature of the mixing section is 190℃, and the temperature of the exhaust section is 150℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:8:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 7.

[0026] Example 8 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The conveying section temperature of the first screw extruder is set to 240°C, the compression section temperature to 250°C, the melting section temperature to 320°C, the mixing section temperature to 320°C, and the exhaust section temperature to 240°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:10:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 200℃, the temperature of the compression section is 190℃, the temperature of the melting section is 190℃, the temperature of the mixing section is 190℃, and the temperature of the exhaust section is 150℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:10:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 8.

[0027] Example 9 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The conveying section temperature of the first screw extruder is set to 240°C, the compression section temperature to 250°C, the melting section temperature to 320°C, the mixing section temperature to 320°C, and the exhaust section temperature to 240°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:10:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 200℃, the temperature of the compression section is 190℃, the temperature of the melting section is 190℃, the temperature of the mixing section is 190℃, and the temperature of the exhaust section is 150℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:12:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 9.

[0028] Example 10 A method for continuously degrading PET using a screw extruder includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder via a feeding device. The first screw extruder is a co-rotating twin-screw extruder. The conveying section temperature of the first screw extruder is set to 240°C, the compression section temperature to 250°C, the melting section temperature to 320°C, the mixing section temperature to 320°C, and the exhaust section temperature to 240°C. The PET is conveyed, compressed, and melted into a molten state. In the mixing section of the first screw extruder, the first degradation aid glycerol and the first catalyst zinc acetate are injected. The mass ratio of PET, the first degradation aid and the first catalyst is 100:10:1. PET and glycerol undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder. The second screw extruder is a second screw extruder. The temperature of the conveying section of the second screw extruder is set to 200℃, the temperature of the compression section is 190℃, the temperature of the melting section is 190℃, the temperature of the mixing section is 190℃, and the temperature of the exhaust section is 150℃. The second degradation aid, ethylene glycol, and the second catalyst, zinc acetate, were injected into the mixing section of the second screw extruder. The mass ratio of PET, the second degradation aid, and the second catalyst was 100:14:2. The degradation intermediate product underwent a homogeneous depolymerization reaction with ethylene glycol. After exhausting the gas to remove byproducts and cooling, the PET degradation product was obtained and named Sample 10.

[0029] To demonstrate the effect of the proportion of the second degradation aid on the molecular weight of the product, the molecular weight of the samples in Examples 6-10 was also characterized, and the results are as follows: Figure 3 As shown in the figure, under the condition that other factors remain unchanged, the molecular weight of the product decreases with the increase of the proportion of the second degradation aid, ethylene glycol. When the mass ratio of PET, ethylene glycol, and zinc acetate is 100:10:2, the molecular weight of PET reaches its lowest point, and its alcoholysis rate reaches its highest point. Further increasing the amount of ethylene glycol added has limited effect on improving the alcoholysis rate of PET. Therefore, from the perspective of cost and actual production, it is sufficient to maintain the mass ratio of PET, the second degradation aid, and the second catalyst at 100:10:2 in the mixing section of the second screw extruder. Using more catalyst not only has limited effect on improving reaction efficiency but also increases production costs.

Claims

1. A method for continuous degradation of PET using a screw extruder, characterized in that... Includes the following steps: S1: The waste PET is crushed, washed, and dried in sequence; S2: The PET processed in step S1 is injected into the first screw extruder. The barrel temperature of the first screw extruder is controlled at 180-320℃. The PET is conveyed, compressed, and melted into a molten state. The first degradation aid and the first catalyst are injected into the mixing section of the first screw extruder. The PET and the first degradation aid undergo a homogeneous depolymerization reaction to obtain the degradation intermediate product. S3: Inject the degradation intermediate into the second screw extruder, control the barrel temperature of the second screw extruder to 100-200℃, and inject the second degradation aid and the second catalyst into the mixing section of the second screw extruder. The degradation intermediate and the second degradation aid undergo a homogeneous depolymerization reaction. The PET degradation products are obtained by removing byproducts through exhaust and cooling.

2. The method for continuous degradation of PET using a screw extruder according to claim 1, characterized in that: In step S2, the temperature of the conveying section of the first screw extruder is 180-240℃, the temperature of the compression section is 190-250℃, the temperature of the melting section is 250-320℃, the temperature of the mixing section is 250-320℃, and the temperature of the exhaust section is 180-240℃.

3. A method for continuous degradation of PET using a screw extruder according to claim 1 or 2, characterized in that: In step S2, the first degradation aid is one or a mixture of several of glycerol, neopentyl glycol, and pentaerythritol; the first catalyst is one or a mixture of several of zinc acetate, lead acetate, and stannous chloride.

4. The method for continuous degradation of PET using a screw extruder according to claim 3, characterized in that: In step S2, the mass ratio of PET, the first degradation aid, and the first catalyst is 100:2-10:

1.

5. The method for continuous degradation of PET using a screw extruder according to claim 1, characterized in that: In step S3, the conveying section temperature of the second screw extruder is 160-200℃, the compression section temperature is 150-190℃, the melting section temperature is 150-190℃, the mixing section temperature is 150-190℃, and the exhaust section temperature is 100-150℃.

6. A method for continuous degradation of PET using a screw extruder according to claim 1 or 5, characterized in that: In step S3, the second degradation aid is one or a mixture of several of ethylene glycol, hexanediol, octanediol, and decanediol; the second catalyst is one or a mixture of several of zinc acetate, lead acetate, and stannous chloride.

7. The method for continuous degradation of PET using a screw extruder according to claim 6, characterized in that: In step S3, the mass ratio of PET, the second degradation aid, and the second catalyst is 100:6-14:

2.

8. The method for continuous degradation of PET using a screw extruder according to claim 1, characterized in that: The first screw extruder is a co-rotating twin-screw extruder or a co-rotating three-screw extruder.

9. The method for continuous degradation of PET using a screw extruder according to claim 1, characterized in that: The second screw extruder is a co-rotating twin-screw extruder or a co-rotating three-screw extruder.

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