Preparation method of a thermoplastic biodegradable plastic

Abstract

The present invention discloses a preparation method of a thermoplastic biodegradable plastic, which comprises the following steps: adipic acid and 1,4-butanediol are subjected to an esterification dehydration reaction to obtain a first product; dimethyl terephthalate or terephthalic acid and 1,4-butanediol are subjected to a reaction to obtain a second product; lactic acid monomers are pre-polycondensed to obtain a third product; the first product, the second product and the third product are jointly subjected to a melt polycondensation reaction to obtain a final product. By directly adding lactic acid for modification in the synthesis stage, the present invention introduces low-cost lactic acid monomers into the amorphous region of PBAT, and slightly replaces the BA segments, so as to realize the preparation of polybutylene adipate terephthalate with high mechanical indexes. The present invention can realize the controllable synthesis of the copolyester molecular weight, with uniform molecular weight and uniform molecular chain structure; the synthesized product has good color and luster, and has good mechanical properties, and is completely biodegradable, and can be applied to packaging materials and disposable tableware.

Description

Technical Field

[0001] The present invention relates to a method for preparing a degradable plastic, and particularly to a method for preparing a thermoplastic biodegradable plastic. Background Art

[0002] As a new type of functional plastic, degradable plastic products are characterized in that after reaching a certain service life, under specific environmental conditions, due to obvious changes in their chemical structures, some performance losses and appearance changes occur, resulting in degradation, so as to achieve the purpose of being harmless or less harmful to the natural environment.

[0003] Poly(butylene adipate terephthalate) (referred to as "PBAT" for short) is a thermoplastic biodegradable material, which is prepared from 1,4-butanediol (BDO), adipic acid (AA), terephthalic acid (PTA) or dimethyl terephthalate (DMT) through esterification and polycondensation reactions.

[0004] PBAT has good film properties, excellent flexibility and biodegradability, and has good thermal stability and excellent mechanical properties. PBAT can be processed in various forms such as injection molding, extrusion molding, and blow molding, and is widely used in material fields such as sheets, mulch films, packaging, and foaming materials, and is particularly suitable for application in packaging films, garbage bags, agricultural mulch films, dust-proof nets, etc. This material can be biodegradable, can combat white pollution, is a green material, and is one of the very active and best market-applied degradable materials in the current research on degradable plastics.

[0005] However, compared with other general plastics, PBAT shows poor performance in mechanical indexes such as elastic modulus and tensile strength. How to improve the mechanical indexes of PBAT by means of chemical synthesis modification has become an urgent technical problem to be solved. Summary of the Invention

[0006] Object of the Invention: The object of the present invention is to provide a method for preparing a thermoplastic biodegradable plastic, and solve the problem of how to improve the mechanical indexes such as elastic modulus and tensile strength of existing PBAT.

[0007] Technical Solution: A method for preparing a thermoplastic biodegradable plastic according to the present invention is characterized by comprising the following steps:

[0008] (1) Performing an esterification dehydration reaction on adipic acid and 1,4-butanediol to obtain a first product;

[0009] (2) Reacting dimethyl terephthalate or terephthalic acid with 1,4-butanediol to obtain a second product;

[0010] (3) Pre-polycondensing lactic acid monomers to obtain a third product;

[0011] (4) The first product, the second product and the third product are subjected to a melt polycondensation reaction together to obtain the final product.

[0012] In the present invention, lactic acid or a variety of diols are used to copolymerize and modify PBAT. Lactic acid monomers or diols are introduced into the amorphous region of PBAT to replace a small amount of the adipic acid-butanediol segment, and a high-barrier and high-strength modulus PBAT-based copolyester is prepared. The synthesis of the copolyester adopts the fractional esterification method. Lactic acid is dehydrated and condensed to prepare a prepolymer, adipic acid and butanediol, and terephthalic acid and butanediol are respectively esterified into prepolymers. After mixing the obtained three prepolymers, a ternary block copolyester mixture with different high molecular weights is obtained through pre-polycondensation and melt polycondensation.

[0013] The reaction formulas for each step are as follows:

[0014]

[0015] The side reactions in each step are as follows:

[0016]

[0017] The final product ternary block copolyester includes polymers with the following structural formulas:

[0018]

[0019] In the present invention, by introducing lactic acid as a chain extender, a high molecular weight poly(butylene adipate terephthalate) (number average molecular weight is 30,000 g / mol - 70,000 g / mol) is directly synthesized by a melt polycondensation process. The molecular weight of the novel high-strength thermoplastic biodegradable plastic PBAT is 60,000 - 200,000.

[0020] Preferably, in step (1), the method of the esterification dehydration reaction is: adipic acid and 1,4-butanediol are mixed in a molar ratio of 1:1 - 1.5, then a titanium-based catalyst is added and heated to 130 - 180 °C for an atmospheric pressure esterification dehydration reaction. According to the water output and acid value of the esterification reaction, the end point of the esterification reaction is judged to obtain the first product.

[0021] Preferably, the end point of the esterification reaction is that the water output of the esterification reaction reaches 85% of the theoretical value and the acid value < 15 mg / g. The titanium-based catalyst includes one or more of tetra-n-butyl titanate, tetra-isopropyl titanate, titanium-phosphorus compounds, titanium-silicon compounds, and titanium-magnesium composite catalysts.

[0022] Preferably, in step (2), the reaction includes:

[0023] Dimethyl terephthalate and 1,4-butanediol are mixed in a molar ratio of 1:1 - 3, then a titanium-based catalyst is added, and the temperature is slowly raised to 200 - 260 °C. Transesterification reaction is carried out under a pressure of 40 kPa to remove methanol. The reaction end point is judged according to the output of methanol to obtain a second product;

[0024] Alternatively, terephthalic acid and 1,4-butanediol are mixed in a molar ratio of 1:1 - 3, then a titanium-based catalyst is added, and the temperature is slowly raised to 200 - 260 °C. Secondary esterification reaction is carried out under a pressure of 40 kPa to remove water. The reaction end point is judged according to the output of water in the secondary esterification reaction to obtain a second product.

[0025] Preferably, in the transesterification reaction, the dosage of the titanium-based catalyst is 0.2‰ - 0.4‰ of the theoretical discharge weight, and the reaction time is 0.5 - 2.5 h; in the secondary esterification reaction, the dosage of the titanium-based catalyst is 1‰ - 5‰ of the theoretical discharge weight, and the reaction time is 1.5 - 3.5 h; the titanium-based catalyst includes one or more of tetra-n-butyl titanate, tetra-isopropyl titanate, titanium-phosphorus compound, titanium-silicon compound, and titanium-magnesium composite catalyst. The titanium-phosphorus compound is a compound composed of titanium as the central atom and a phosphorus compound as the ligand; the titanium-silicon compound is TiO2 / SiO2, where Ti / Si is 9:1; the titanium-magnesium composite catalyst is a composite of titanate and magnesium acetate.

[0026] Preferably, in step (3), the method of pre-polycondensation of lactic acid monomer is as follows: A zinc-based catalyst is added to the aqueous lactic acid solution, heated to 130 - 170 °C, and stirred for reaction for 3 - 4 h. The reaction end point is judged according to the output of water in the reaction to obtain an intermediate; the intermediate is polycondensed under a vacuum of 150 - 200 Pa for 5 - 15 min to obtain a third product.

[0027] Preferably, the zinc-based catalyst is one or more of zinc, diethyl zinc, zinc octoate, zinc acetate, zinc oxide, and zinc chloride.

[0028] Preferably, in step (4), the melt polycondensation reaction includes: The first product, the second product, the titanium-based catalyst, the antioxidant, and the stabilizer are mixed and stirred at 190 °C for reaction for 20 - 30 min, then the third product is added and the temperature is raised to 200 - 210 °C, and the reaction is carried out under vacuum conditions. The reaction process is judged according to the instrument current, and the polycondensation reaction ends when the preset value is reached.

[0029] Preferably, the method of reaction under vacuum conditions is: React under a vacuum of 100 - 1000 Pa for 0.2 - 1.6 h; then react under a vacuum of less than 100 Pa for 0.2 - 1.6 h.

[0030] Preferably, the stabilizer is 0.3‰ - 2‰ of organophosphite esters of the theoretical discharge weight, and the antioxidant is a hindered phenol.

[0031] In some embodiments, the organic phosphites are one or more of triphenyl phosphate, triethyl phosphite, pentaerythritol bisphosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, and distearyl pentaerythritol diphosphite; the hindered phenols are one or both of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and N,N'-bis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoyl)hexanediamine.

[0032] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages:

[0033] (1) In the present invention, lactic acid modification is directly added during the synthesis stage, introducing low-cost lactic acid monomers into the amorphous region of PBAT, and slightly replacing the BA segment to achieve the preparation of high molecular weight and high strength modulus poly(butylene adipate terephthalate). The present invention can achieve the controllable synthesis of the copolyester molecular weight, with uniform molecular weight and uniform molecular chain structure;

[0034] (2) The added lactic acid or polyol has easily available raw materials and a small addition amount, which can not only reduce the production cost of the product but also improve the mechanical property indexes such as the elastic modulus and tensile strength of the product;

[0035] (3) Through the control of the process and the optimization of the reaction aids, the reaction time is greatly shortened, the synthesized product has good color and excellent mechanical properties, and is completely biodegradable, and can be widely applied to consumer product fields such as packaging materials and disposable tableware. Description of the Drawings

[0036] Figure 1 It is the material balance diagram of the design formula PBA30LA20T50;

[0037] Figure 2 It is the 1H NMR spectrum of the esterification reaction product of terephthalic acid and 1,4-butanediol with different alcohol-acid ratios;

[0038] Figure 3 It is the 1H NMR spectrum of PBLA10T50 - PBLA30T50;

[0039] Figure 4 It is the stress-strain curve of the PBALA10T50 - PBALA30T50 copolyester. Detailed Embodiments

[0040] The technical solution of the present invention will be further described below with reference to the drawings.

[0041] Example 1: A method for preparing a thermoplastic biodegradable plastic is as follows: dimethyl terephthalate (DMT)

[0042] (1) Esterification reaction of terephthalic acid and 1,4-butanediol

[0043] First, add 10.814 kg of 1,4-butanediol (BDO) and then 9.968 kg of terephthalic acid (PTA) into a 30L stainless steel esterification kettle. The molar ratio of alcohol to acid is 2:1, and 30.6 g of tetrabutyl titanate (TBT) as the catalyst is added. Mix them into a slurry. The stirring speed is 600 rpm. At the initial stage of the reaction, heat it to 190°C, and then raise the temperature to 220°C after about half an hour. Control the pressure at 40 kPa. The overall reaction time is 3 - 4 hours. To avoid the solidification of butanediol in the condenser, set the top temperature of the condenser at 150°C. Detect, separate, and weigh the by-products. The by-product esterification water is 2376 g, and tetrahydrofuran is 864 g. According to the experimental water production and reaction time, judge the end point of the esterification reaction. The end point of the reaction is that the actual weighed value of the water production is close to the calculated theoretical value. It can be judged that the reaction has reached the end point by observing that the water production rate of the reaction decreases to 1 drop / second. In order to reduce the generation amount of tetrahydrofuran (THF) in the esterification reaction of terephthalic acid and 1,4-butanediol and reduce the consumption of 1,4-butanediol, the esterification reaction of PBAT needs to be carried out under negative pressure; the polycondensation kettle operates under vacuum, and a cold trap device is installed in front of the screw vacuum pump for deep cooling to trap tetrahydrofuran as much as possible.

[0044] (2) Esterification reaction of adipic acid and 1,4-butanediol

[0045] Add 3.894 kg of BDO and 5.26 kg of adipic acid (AA) into a 30L stainless steel esterification kettle. The molar ratio of alcohol to acid is 1.2:1, and 18.36 g of TBT as the catalyst is added. Mix them into a slurry. The stirring speed is 600 rpm. Heat the reaction to 170°C, and the overall reaction time is 3 - 4 hours. Detect and weigh the by-products. The by-product esterification water is 2304 g. According to the experimental water production and reaction time, judge the end point of the esterification reaction. The end point of the esterification reaction is that the water production of the esterification reaction reaches 85% of the theoretical value and the acid value < 15 mg / g.

[0046] (3) Prepolymerization reaction of lactic acid

[0047] Add 6.006 kg of 90% lactic acid aqueous solution and 16.4 g of zinc acetate into a 30L stainless steel esterification kettle. Heat the reaction to 150°C, and the overall reaction time is 3 - 4 hours. The stirring speed is 600 rpm. Detect and weigh the by-products. The by-product esterification water is 1680 g. According to the experimental water production, judge the end point of the lactic acid esterification reaction. Then carry out the pre-polycondensation of lactic acid, and carry out the polycondensation for 10 minutes under the vacuum condition of 150 - 200 Pa to end the reaction.

[0048] (4) Polycondensation reaction

[0049] The products obtained in steps (1) and (2) are first introduced into a 100 L stainless steel polycondensation kettle. Add 44 g of antioxidant pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), 21 g of tetrabutyl titanate, and 50 g of stabilizer triphenyl phosphate. Control the temperature at 190 °C, the stirring speed at 600 rpm, and stir for 20 - 30 minutes. Then introduce the lactic acid prepolymer obtained in step (3), mix and stir for about 20 minutes, and start pre-polycondensation. Gradually raise the temperature to 200 °C, and control the temperature and vacuum degree below 50 Pa. The reaction time is about 5 - 6 hours. As the reaction progresses, the stirring speed gradually decreases, and the later speed is 300 rpm. Judge the reaction progress according to the instrument current. According to the change of the reaction current, when the current increases significantly and lasts for a period of time and then gradually decreases, it can be judged that the polycondensation reaction ends. When it reaches the preset value, end the polycondensation reaction. The polycondensation reaction is carried out under vacuum. When the molecular chain increases to the specified molecular weight range, the reaction process is completed, stop the vacuum, and use nitrogen to press out the materials in the polycondensation kettle, cast the belt through the casting head and then cool and pelletize.

[0050] At the same time, filter and separate the solid and liquid of the by-products. Considering that the melting point of lactide is 93 - 100 °C, the pipeline temperature can be considered to be raised to 100 °C. 4.758 kg of by-product BDO and 2.594 kg of lactide / lactic acid mixture can be obtained.

[0051] Finally, about 21.428 kg of copolyester is obtained, and the intrinsic viscosity > 1.0 dL / g at 30 °C in a mixed solution of phenol / tetrachloroethane (1:1 wt / wt).

[0052] Example 2: A preparation method of a thermoplastic biodegradable plastic is as follows:

[0053] (1) Esterification reaction of terephthalic acid and 1,4-butanediol

[0054] First add 11.896 kg of BDO and then 10.965 kg of PTA to a 30 L stainless steel esterification kettle. The molar ratio of alcohol to acid is 2:1, and add 33.66 g of catalyst tetrabutyl titanate (TBT), and mix into a slurry. The stirring speed is 600 rpm. Heat to 200 °C at the beginning of the reaction, and raise the temperature to 230 °C after about half an hour. Control the pressure at 40 kPa, and the overall reaction time is 3 - 4 hours. To avoid the solidification of butanediol in the condenser, set the top temperature of the condenser at 150 °C. Detect, separate and weigh the by-products. The by-product esterification water is 2614 g, and tetrahydrofuran is 952 g. Judge the end point of the esterification reaction according to the experimental water output and reaction time.

[0055] (2) Esterification reaction of adipic acid and 1,4-butanediol

[0056] Add 3.894 kg of BDO, 3.507 kg of AA, with an alcohol-to-acid ratio of 1.2:1, and 12.24 g of catalyst TBT into a 30L stainless steel esterification kettle, and mix them into a slurry. The stirring speed is 600 rpm. Heat the reaction to 180 °C, and the overall reaction time is 3 - 4 hours. Detect and weigh the by-products, and the by-product esterification water is 864 g. Determine the end point of the esterification reaction based on the experimental water output and reaction time.

[0057] (3) Lactic acid prepolymerization reaction

[0058] Add 6.756 kg of 90% lactic acid aqueous solution and 20.5 g of zinc acetate into a 30L stainless steel esterification kettle. Heat the reaction to 150 °C, and the overall reaction time is 3 - 4 hours. The stirring speed is 600 rpm. Detect and weigh the by-products, and the by-product esterification water is 2101 g. Determine the end point of the lactic acid esterification reaction based on the experimental water output. Then carry out lactic acid pre-polycondensation, and carry out polycondensation for 10 minutes under the condition of a vacuum degree of 150 - 200 Pa to end the reaction.

[0059] (4) Polycondensation reaction

[0060] First, introduce the products prepared in steps (1) and (2) into a 100L stainless steel polycondensation kettle, add 44 g of antioxidant pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate), 25 g of stabilizer triphenyl phosphate, and 20 g of tetrabutyl titanate. Control the temperature at 190 °C, with a stirring speed of 600 rpm, and stir for 20 - 30 minutes. Then introduce the lactic acid prepolymer prepared in step (3), mix and stir for about 20 minutes, and start pre-polycondensation. Gradually raise the temperature to 200 - 210 °C, and the vacuum degree is below 50 Pa. The reaction time is about 5 - 6 hours. As the reaction progresses, the stirring speed gradually decreases, and the later speed is 300 rpm. Determine the reaction progress based on the instrument current, and end the polycondensation reaction when the preset value is reached.

[0061] At the same time, filter and separate the solid and liquid of the by-products. Considering that the melting point of lactide is 93 - 100 °C, the pipeline temperature can be considered to be raised to 100 °C. By-products BDO of 4.953 kg and a lactide / lactic acid mixture of 3.244 kg can be obtained.

[0062] Finally, about 21.5 kg of copolyester is obtained, and the intrinsic viscosity > 1.0 dL / g at 30 °C in a phenol / tetrachloroethane mixed solution (1:1 wt / wt).

[0063] Example 3: The rest is the same as Example 1, except that:

[0064] Replace terephthalic acid with dimethyl terephthalate, and the molar ratio of dimethyl terephthalate to 1,4-butanediol is 1:3. The molar ratio of adipic acid to 1,4-butanediol is 1:1. Replace the catalyst TBT in steps (1) and (2) with a titanium phosphorus compound, and the dosage is 0.2‰ of the theoretical discharge weight.

[0065] In step (3), the method for pre-polycondensation of lactic acid monomers is as follows: Add diethyl zinc to the aqueous lactic acid solution, heat to 130 °C, stir and react for 3 - 4 h, judge the end point of the reaction according to the amount of water produced in the reaction to obtain an intermediate; Condense the intermediate under a vacuum of 150 Pa for 10 min to end the reaction.

[0066] In step (4), the temperature is gradually raised to 225 °C and reacted under a vacuum of 1000 Pa for 0.2 h; Subsequently, react under a vacuum of less than 100 Pa for 1.6 h.

[0067] Replace pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate) with N,N'-bis-(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyl)hexanediamine, and replace triphenyl phosphate with triethyl phosphite.

[0068] Example 4: The rest are the same as in Example 1, except that:

[0069] Replace terephthalic acid with dimethyl terephthalate, and the molar ratio of dimethyl terephthalate to 1,4-butanediol is 1:1. The molar ratio of adipic acid to 1,4-butanediol is 1:1.5. Replace the catalyst TBT in steps (1) and (2) with a titanium silicon compound, and the dosage is 0.4‰ of the theoretical discharge weight.

[0070] In step (3), the method for pre-polycondensation of lactic acid monomers is as follows: Add zinc octoate to the aqueous lactic acid solution, heat to 170 °C, stir and react for 3 - 4 h, judge the end point of the reaction according to the amount of water produced in the reaction to obtain an intermediate; Condense the intermediate under a vacuum of 200 Pa for 15 min to end the reaction.

[0071] In step (4), the temperature is gradually raised to 225 °C and reacted under a vacuum of 500 Pa for 1.6 h; Subsequently, react under a vacuum of less than 100 Pa for 0.2 h. Replace triphenyl phosphate with pentaerythritol bisphosphite.

[0072] Example 5: The rest are the same as in Example 1, except that:

[0073] Replace terephthalic acid with dimethyl terephthalate, and the molar ratio of dimethyl terephthalate to 1,4-butanediol is 1:3. Replace the catalyst TBT in steps (1) and (2) with a titanium magnesium composite catalyst, and the dosage is 0.5‰ of the theoretical discharge weight.

[0074] In step (3), the method for pre-condensation of lactic acid monomer is: adding zinc chloride to the lactic acid aqueous solution, heating to 160° C., stirring the reaction for 3-4 hours, judging the reaction endpoint based on the amount of water produced by the reaction, and obtaining an intermediate; condensing the intermediate under a vacuum condition of 200 Pa for 5 minutes to terminate the reaction.

[0075] In step (4), the temperature is gradually raised to 225° C., and the reaction is carried out for 3 hours under a vacuum degree of 100 Pa. Triphenyl phosphate is replaced by bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite.

[0076] The product performance indicators obtained from different embodiments are as follows:

[0077] Elastic modulus (MPa) Tensile strength (MPa) Elongation at break (%) PBAT 85±1 17.6±0.2 743±5 Example 1 152±5 47±5 750±30 Example 2 124±2 32±3 753±105 Example 3 141±4 45±2 784±57 Example 4 139±6 39±4 739±79 Example 5 148±3 51±3 793±41

[0078] Compared with common PBAT, the elastic modulus of the thermoplastic biodegradable plastic prepared by the present invention is increased by 30-50%, the tensile strength is increased to above 25 MPa, and the elongation at break is maintained at 650-850%.

[0079] Embodiment 6: The rest is the same as Embodiment 1, except that:

[0080] Different copolyester final products were prepared by taking the ratio of lactic acid segments and terephthalic acid segments as variables. The ratio was precisely controlled by adjusting the raw material ratio.

[0081] The nuclear magnetic hydrogen spectra and stress-strain curves of different final products were measured respectively. The results are as follows Figure 3 and Figure 4 shown. Figure 3 PBLA10T50 represents the composition of the copolyester, in which the lactic acid (LA) segment accounts for 10% and the terephthalic acid (T) segment accounts for 50%, and the same applies to other figures. Figure 4 In the figure, PBALA10T50 represents the component composition of the copolyester, with the lactic acid (LA) segment accounting for 10% and the terephthalic acid (T) segment accounting for 50%, and the other legends are similar.

[0082] Embodiment 7: The rest is the same as Embodiment 1, except that:

[0083] In step (1), different esterification products were prepared with the molar ratio of 1,4-butanediol to terephthalic acid as a variable, and the nuclear magnetic resonance hydrogen spectrum was measured. The results are as follows: Figure 3 As shown, Figure 3The molar ratio of diol to acid input refers to the molar ratio of 1,4-butanediol and terephthalic acid weighed by theoretical calculation during raw material feeding. The actual diol to acid ratio is the actual ratio of 1,4-butanediol and terephthalic acid in the product detected by nuclear magnetic resonance hydrogen spectrum after the polycondensation reaction, and is judged according to the characteristic peak position of H in the nuclear magnetic resonance hydrogen spectrum. The supplementary addition of BDO is carried out after the esterification reaction and before the start of the polycondensation reaction. The polycondensation catalyst and a part of BDO are added to the polycondensation kettle through the auxiliary material feeding tank. To ensure the vacuum degree of the polycondensation kettle, the feeding pipeline should adopt double-valve control for closed operation.

Claims

1. A preparation method of a thermoplastic biodegradable plastic, characterized in that, It includes the following steps: (1) Esterify and dehydrate adipic acid and 1,4-butanediol to obtain a first product; (2) React dimethyl terephthalate or terephthalic acid with 1,4-butanediol to obtain a second product; (3) Pre-polycondense lactic acid monomers to obtain a third product; (4) Carry out melt polycondensation reaction on the first product, the second product and the third product together to obtain a final product; In step (1), the method of the esterification dehydration reaction is: mix adipic acid and 1,4-butanediol according to a molar ratio of 1:1 - 1.5, add a titanium-based catalyst, heat to 130 - 180 °C, carry out esterification dehydration reaction under normal pressure, judge the end point of the esterification reaction according to the water output and acid value of the esterification reaction, and obtain the first product; the end point of the esterification reaction is that the water output of the esterification reaction reaches 85% of the theoretical value and the acid value < 15 mg / g; In step (2), the reaction includes: Mix dimethyl terephthalate and 1,4-butanediol according to a molar ratio of 1:1 - 3, add a titanium-based catalyst, slowly heat up to 200 - 260 °C, carry out transesterification reaction under a pressure of 40 kPa to remove methanol, judge the end point of the reaction according to the methanol output, and obtain the second product; Or mix terephthalic acid and 1,4-butanediol according to a molar ratio of 1:1 - 3, add a titanium-based catalyst, slowly heat up to 200 - 260 °C, carry out secondary esterification reaction under a pressure of 40 kPa to remove water, judge the end point of the reaction according to the water output of the secondary esterification reaction, and obtain the second product; In step (3), the method of pre-polycondensing lactic acid monomers is: add a zinc-based catalyst to an aqueous lactic acid solution, heat to 130 - 170 °C, stir and react for 3 - 4 h, judge the end point of the reaction according to the water output of the reaction, and obtain an intermediate; carry out polycondensation of the intermediate under a vacuum of 150 - 200 Pa for 5 - 15 min to obtain the third product.

2. The preparation method of the thermoplastic biodegradable plastic according to claim 1, characterized in that, The titanium-based catalyst includes one or more of tetra-n-butyl titanate, tetra-isopropyl titanate, titanium-phosphorus compounds, titanium-silicon compounds, and titanium-magnesium composite catalysts.

3. The preparation method of the thermoplastic biodegradable plastic according to claim 1, wherein In the transesterification reaction, the dosage of the titanium-based catalyst is 0.2‰ - 0.4‰ of the theoretical discharge weight, and the reaction time is 0.5 - 2.5 h; in the secondary esterification reaction, the dosage of the titanium-based catalyst is 1‰ - 5‰ of the theoretical discharge weight, and the reaction time is 1.5 - 3.5 h; the titanium-based catalyst includes one or more of tetra-n-butyl titanate, tetra-isopropyl titanate, titanium-phosphorus compounds, titanium-silicon compounds, and titanium-magnesium composite catalysts.

4. The preparation method of the thermoplastic biodegradable plastic according to claim 1, wherein The zinc-based catalyst is one or more of zinc, diethyl zinc, zinc octanoate, zinc acetate, zinc oxide, and zinc chloride.

5. The preparation method of the thermoplastic biodegradable plastic according to claim 1, characterized in that In step (4), the melt polycondensation reaction includes: mix the first product, the second product, a titanium-based catalyst, an antioxidant, and a stabilizer, stir and react at 190 °C for 20 - 30 min, then add the third product and mix, heat up to 200 - 210 °C, react under vacuum conditions, judge the reaction process according to the instrument current, and end the polycondensation reaction when the preset value is reached.

6. The preparation method of the thermoplastic biodegradable plastic according to claim 5, characterized in that, The method for the reaction under vacuum conditions is as follows: reacting at a vacuum degree of 100 - 1000 Pa for 0.2 - 1.6 h; subsequently reacting at a vacuum degree below 100 Pa for 0.2 - 1.6 h.

7. The preparation method of the thermoplastic biodegradable plastic according to claim 5, characterized in that, The stabilizer is an organic phosphite ester at 0.3‰ - 2‰ of the theoretical discharge weight, and the antioxidant is a hindered phenol.

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