Biodegradable plastic and preparation method thereof

Abstract

The invention discloses biodegradable plastic and a preparation method thereof, and relates to the technical field of bio-based materials. The biodegradable plastic is prepared by adding a composite toughening agent and a self-made chain extender into polylactic acid, the composite toughening agent is prepared by mixing tributyl citrate, castor oil and catechol dopamine, and catechol dopamine is prepared by polymerization reaction of isobutyl acrylate and dopamine acrylamide. The formed long-chain multi-branch chain composite toughening agent is inserted into a macromolecular chain, so that the impact strength is improved; the self-made chain extender is prepared by reacting itaconic acid glycidyl ester with thiohydracrylic acid ester, the itaconic acid glycidyl ester is prepared by reacting itaconic acid with glycidyl methacrylate, the thiohydracrylic acid ester is prepared by reacting trimethylolpropane with 3-thiohydracrylic acid ester, the processing stability is improved, and the thermal stability is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of bio-based materials, and specifically to a biodegradable plastic and a preparation method thereof. Background Art

[0002] Biodegradable plastics are a type of polymer that can be completely degraded in a biochemical or biological environment and ultimately converted into environmentally harmless substances such as carbon dioxide and water. There are two main parts to the degradation of biodegradable plastics: microbial decomposition and hydrolysis. During degradation, under the action of microbial enzymes adhering to the surface of the material, the long polymer chains are further broken by hydrolysis and other reactions, ultimately forming low molecular weight fragments.

[0003] Polylactic acid is a new type of biodegradable material. It is made from starch raw materials extracted from renewable plant resources and synthesized through a series of reactions. However, biodegradable plastics made from polylactic acid often have problems such as low impact strength and insufficient thermal stability. Therefore, the present invention has studied and prepared a biodegradable plastic with excellent impact strength and thermal stability to solve this problem. Summary of the Invention

[0004] The purpose of the present invention is to provide a biodegradable plastic and a preparation method thereof to solve the problems raised in the above background art.

[0005] To solve the above technical problems, the present invention provides the following technical solution: A biodegradable plastic, comprising polylactic acid, a composite toughening agent, a self-made chain extender, a compatibilizer, a lubricant, and a degradation promoter.

[0006] Preferably, the composite toughening agent is prepared by mixing tributyl citrate, castor oil, and catechol dopamine; the catechol dopamine is prepared by the polymerization reaction of isobutyl acrylate and dopamine acrylamide.

[0007] Preferably, the self-made chain extender is prepared by the reaction of itaconic acid glycidyl ester and mercaptopropionate.

[0008] Preferably, the itaconic acid glycidyl ester is prepared by the reaction of itaconic acid and glycidyl methacrylate; the mercaptopropionate is prepared by the reaction of trimethylolpropane and 3-mercaptopropionic acid.

[0009] Preferably, the compatibilizer is maleic anhydride; the lubricant is glyceryl tristearate; the degradation promoter is 1,5-pentanediol.

[0010] Preferably, a preparation method of a biodegradable plastic comprises the following specific steps:

[0011] S1. Under a nitrogen atmosphere, mix dopamine acrylamide, isobutyl acrylate, and N,N-dimethylformamide in a mass ratio of 4:3:80 - 100, stir to dissolve, then add azobisisobutyronitrile in an amount of 0.01 - 0.04 times the mass of dopamine acrylamide and benzyl dithiobenzoate in an amount of 0.1 - 0.14 times the mass of dopamine acrylamide. Heat to 70 - 80 °C and react for 36 - 40 h. Dialyze with deionized water for 24 h and freeze-dry at -40 - -60 °C to obtain catechol dopamine;

[0012] S2. Mix N,N-dimethylacetamide, tributyl citrate, castor oil, and catechol dopamine in a mass ratio of 20 - 40:8 - 10:1:4, stir evenly, then heat to 80 - 90 °C, add dilute sulfuric acid with a mass fraction of 3 - 6% in an amount of 0.3 - 0.5 times the mass of tributyl citrate, and react for 12 - 16 h to obtain a composite toughening agent:

[0013] S3. Mix glycidyl itaconate and mercaptopropionate in a mass ratio of 20 - 40:3, heat to 80 - 90 °C, add dilute sulfuric acid with a mass fraction of 3 - 6% in an amount of 0.2 - 0.4 times the mass of glycidyl itaconate, and react for 12 - 16 h to obtain a self-made chain extender:

[0014] S4. Mix polylactic acid, composite toughening agent, self-made chain extender, compatibilizer, lubricant, and degradation promoter in a mass ratio of 60 - 80:8:1 - 3:2 - 4:0.2 - 0.5:1, and extrude and pelletize through a twin-screw extruder to obtain a biodegradable plastic.

[0015] Preferably, in the above step S1, the preparation method of dopamine acrylamide is as follows: Under a nitrogen atmosphere, mix borax, sodium bicarbonate, and deionized water in a mass ratio of 1:0.3 - 0.5:10, stir evenly, then add hydrochloric acid dopamine with the same mass as sodium bicarbonate, and then dropwise add a mixed solution of methylacrylic anhydride and tetrahydrofuran with a mass ratio of methylacrylic anhydride to tetrahydrofuran of 1:5 and 8 - 10 times the mass of sodium bicarbonate. Adjust the pH to 7.8 - 8.2 with sodium hydroxide, react for 12 - 14 h, wash with ethyl acetate 2 - 3 times, adjust the pH to 1 - 2 with hydrochloric acid, extract with ethyl acetate, dry with anhydrous magnesium sulfate, and finally precipitate with n-hexane, filter by suction and dry to obtain dopamine acrylamide.

[0016] Preferably, in the above step S3, the preparation method of glycidyl itaconate is as follows: Mix itaconic acid, triphenylphosphine, p-hydroxyanisole, and glycidyl methacrylate in a mass ratio of 3 - 3.6:0.2:0.1:8.4 - 8.8, heat to 110 - 120 °C, react for 6 - 8 h, and remove the remaining glycidyl methacrylate by vacuum with an oil pump to obtain glycidyl itaconate.

[0017] Preferably, in the above step S3, the preparation method of the mercaptopropionate is as follows: trimethylolpropane, 3-mercaptopropionic acid, methanesulfonic acid and toluene are mixed in a mass ratio of 31-33:100-120:2:64, heated to 110-120 °C, and refluxed for 6-8 h. After cooling to room temperature, toluene with the same mass as 3-mercaptopropionic acid is added, and it is washed 3-5 times successively with saturated sodium bicarbonate, saturated sodium chloride and deionized water, dried with anhydrous magnesium sulfate, and finally filtered and distilled under reduced pressure to obtain the mercaptopropionate.

[0018] Preferably, in the above step S4, the temperatures of each temperature zone of the twin-screw extruder are as follows: the first zone is 120-130 °C; the second zone is 115-135 °C; the third zone is 120-140 °C; the fourth zone is 135-145 °C; the fifth zone is 140-150 °C; the sixth zone is 140-150 °C; the seventh zone is 145-155 °C; the eighth zone is 145-155 °C; the ninth zone is 150-160 °C, and the screw speed is 180-220 rpm.

[0019] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0020] The biodegradable plastic prepared by the present invention is obtained by adding a composite toughening agent and a self-made chain extender to polylactic acid;

[0021] The composite toughening agent is prepared by mixing tributyl citrate, castor oil and catechol dopamine. Catechol dopamine is prepared by the polymerization reaction of isobutyl acrylate and dopamine acrylamide. The acrylamide polymer monomer containing catechol groups prepared by the reaction of isobutyl acrylate and dopamine acrylamide has more excellent compatibility and degradability. The ester butyl group at the end of tributyl citrate undergoes a transesterification reaction with the hydroxyl groups on castor oil and catechol dopamine to form a long-chain multi-branched composite toughening agent, which is inserted into the macromolecular chain, weakening the intermolecular force, making it easier to undergo elastic deformation, and thus better absorbing impact energy and improving the impact strength;

[0022] The self-made chain extender is prepared by the reaction of itaconic acid glycidyl ester and mercaptopropionate. Itaconic acid glycidyl ester is prepared by the reaction of itaconic acid and glycidyl methacrylate. Mercaptopropionate is prepared by the reaction of trimethylolpropane and 3-mercaptopropionic acid. Itaconic acid and glycidyl methacrylate undergo a ring-opening reaction, and then mercaptopropionate is introduced as a side chain to react with the terminal carboxyl groups on polylactic acid, increasing its molecular weight, improving the processing stability and enhancing the thermal stability. Detailed implementation mode

[0023] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be described clearly and completely. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0024] The polylactic acid used in the embodiments and comparative examples of the present invention was purchased from Wuhan Haishan Technology Co., Ltd., CAS: 26100-51-6.

[0025] In order to more clearly illustrate the method provided by the present invention, the following examples are used for detailed description. The test methods for each index of the biodegradable plastics prepared in the examples and comparative examples are as follows:

[0026] Impact strength: The biodegradable plastics prepared in the examples and comparative examples were tested for impact strength with reference to GB / T1043.1.

[0027] Thermal stability: The biodegradable plastics prepared in the examples and comparative examples were tested for heat distortion temperature with reference to GB / T1634.1.

[0028] Example 1

[0029] S1. Under a nitrogen atmosphere, borax, sodium bicarbonate and deionized water were mixed in a mass ratio of 1:0.3:10, and after stirring evenly, hydrochloric acid dopamine with the same mass as sodium bicarbonate was added. Then, a mixed solution of methacrylic anhydride and tetrahydrofuran with 8 times the mass of sodium bicarbonate was added dropwise. The mass ratio of methacrylic anhydride to tetrahydrofuran was 1:5. The pH was adjusted to 7.8 with sodium hydroxide. After reacting for 12 h, it was washed twice with ethyl acetate, the pH was adjusted to 1 with hydrochloric acid, extracted with ethyl acetate, dried with anhydrous magnesium sulfate, and finally precipitated with n-hexane, filtered by suction and dried to obtain dopamine acrylamide; Under a nitrogen atmosphere, dopamine acrylamide, isobutyl acrylate and N,N-dimethylformamide were mixed in a mass ratio of 4:3:80, stirred and dissolved, and then azobisisobutyronitrile with 0.01 times the mass of dopamine acrylamide and benzyl dithiobenzoate with 0.1 times the mass of dopamine acrylamide were added. The temperature was raised to 70 °C and reacted for 36 h, dialyzed with deionized water for 24 h, and freeze-dried at -40 °C to obtain catechol dopamine;

[0030] S2. N,N-dimethylacetamide, tributyl citrate, castor oil and catechol dopamine were mixed in a mass ratio of 20:8:1:4, stirred evenly, then the temperature was raised to 80 °C, and dilute sulfuric acid with a mass fraction of 3% and 0.3 times the mass of tributyl citrate was added, and reacted for 12 h to obtain a composite toughening agent;

[0031] S3. Mix itaconic acid, triphenylphosphine, p - methoxyphenol, and glycidyl methacrylate in a mass ratio of 3:0.2:0.1:8.4, heat up to 110 °C, react for 6 h, remove the remaining glycidyl methacrylate by vacuum with an oil pump to obtain glycidyl itaconate; Mix trimethylolpropane, 3 - mercaptopropionic acid, methanesulfonic acid, and toluene in a mass ratio of 31:100:2:64, heat up to 110 °C, reflux and react for 6 h, cool to room temperature and add toluene with the same mass as 3 - mercaptopropionic acid, wash 3 times successively with saturated sodium bicarbonate, saturated sodium chloride, and deionized water, dry with anhydrous magnesium sulfate, and finally filter and distill under reduced pressure to obtain mercaptopropionate; Mix glycidyl itaconate and mercaptopropionate in a mass ratio of 20:3, heat up to 80 °C, add dilute sulfuric acid with a mass fraction of 3% and 0.2 times the mass of glycidyl itaconate, react for 12 h to obtain a self - made chain extender:

[0032] S4. Mix polylactic acid, composite toughener, self - made chain extender, compatibilizer, lubricant, and degradation promoter in a mass ratio of 60:8:1:2:0.2:1, extrude and pelletize through a twin - screw extruder. The temperatures of each temperature zone of the twin - screw extruder are: zone 1: 120 °C; zone 2: 115 °C; zone 3: 120 °C; zone 4: 135 °C; zone 5: 140 °C; zone 6: 140 °C; zone 7: 145 °C; zone 8: 145 °C; zone 9: 150 °C, and the screw speed is 180 rpm to obtain biodegradable plastic.

[0033] Example 2

[0034] S1. Under a nitrogen atmosphere, mix borax, sodium bicarbonate, and deionized water in a mass ratio of 1:0.4:10, stir evenly, add hydrochloric acid dopamine with the same mass as sodium bicarbonate, and then dropwise add a mixed solution of methylacrylic anhydride and tetrahydrofuran with 9 times the mass of sodium bicarbonate. The mass ratio of methylacrylic anhydride to tetrahydrofuran is 1:5, adjust the pH to 8.0 with sodium hydroxide, after reacting for 13 h, wash 2 times with ethyl acetate, adjust the pH to 1.5 with hydrochloric acid, extract with ethyl acetate, dry with anhydrous magnesium sulfate, and finally precipitate with n - hexane, filter and dry to obtain dopamine acrylamide; Under a nitrogen atmosphere, mix dopamine acrylamide, isobutyl acrylate, and N,N - dimethylformamide in a mass ratio of 4:3:90, stir to dissolve, add azobisisobutyronitrile with 0.03 times the mass of dopamine acrylamide and benzyl dithiobenzoate with 0.12 times the mass of dopamine acrylamide, heat up to 75 °C, react for 38 h, dialyze with deionized water for 24 h, and freeze - dry at - 50 °C to obtain catechol dopamine;

[0035] S2. Mix N,N - dimethylacetamide, tributyl citrate, castor oil, and catechol dopamine in a mass ratio of 30:9:1:4, stir evenly, heat up to 85 °C, add dilute sulfuric acid with a mass fraction of 5% and 0.4 times the mass of tributyl citrate, react for 14 h to obtain a composite toughener:

[0036] S3. Mix itaconic acid, triphenylphosphine, p - hydroxyanisole and glycidyl methacrylate in a mass ratio of 3.3:0.2:0.1:8.6, heat up to 115 °C, react for 7 h, remove the remaining glycidyl methacrylate by vacuum with an oil pump to obtain glycidyl itaconate; Mix trimethylolpropane, 3 - mercaptopropionic acid, methanesulfonic acid and toluene in a mass ratio of 32:110:2:64, heat up to 115 °C, reflux and react for 7 h, cool to room temperature and add toluene with the same mass as 3 - mercaptopropionic acid, wash 4 times successively with saturated sodium bicarbonate, saturated sodium chloride and deionized water, dry with anhydrous magnesium sulfate, finally filter and distill under reduced pressure to obtain mercaptopropionate; Mix glycidyl itaconate and mercaptopropionate in a mass ratio of 30:3, heat up to 85 °C, add 5% dilute sulfuric acid with a mass 0.3 times that of glycidyl itaconate, react for 14 h to obtain a self - made chain extender:

[0037] S4. Mix polylactic acid, composite toughening agent, self - made chain extender, compatibilizer, lubricant and degradation promoter in a mass ratio of 70:8:2:3:0.4:1, extrude and pelletize through a twin - screw extruder. The temperatures of each temperature zone of the twin - screw extruder are: zone 1 at 125 °C; zone 2 at 125 °C; zone 3 at 130 °C; zone 4 at 140 °C; zone 5 at 145 °C; zone 6 at 145 °C; zone 7 at 150 °C; zone 8 at 150 °C; zone 9 at 155 °C, and the screw speed is 200 rpm to obtain a biodegradable plastic.

[0038] Example 3

[0039] S1. Under a nitrogen atmosphere, mix borax, sodium bicarbonate and deionized water in a mass ratio of 1:0.5:10, stir evenly, add hydrochloric acid dopamine with the same mass as sodium bicarbonate, and then dropwise add a mixed solution of methylacrylic anhydride and tetrahydrofuran with a mass 10 times that of sodium bicarbonate. The mass ratio of methylacrylic anhydride to tetrahydrofuran is 1:5, adjust the pH to 8.2 with sodium hydroxide, after reacting for 14 h, wash 3 times with ethyl acetate, adjust the pH to 2 with hydrochloric acid, extract with ethyl acetate, dry with anhydrous magnesium sulfate, finally precipitate with n - hexane, filter and dry to obtain dopamine acrylamide; Under a nitrogen atmosphere, mix dopamine acrylamide, isobutyl acrylate and N,N - dimethylformamide in a mass ratio of 4:3:100, stir and dissolve, add azobisisobutyronitrile with a mass 0.04 times that of dopamine acrylamide and benzyl dithiobenzoate with a mass 0.14 times that of dopamine acrylamide, heat up to 80 °C, react for 40 h, dialyze with deionized water for 24 h, and freeze - dry at - 60 °C to obtain catechol dopamine;

[0040] S2. Stir N,N-dimethylacetamide, tributyl citrate, castor oil, and catechol dopamine evenly according to a mass ratio of 40:10:1:4. After that, raise the temperature to 90 °C, add dilute sulfuric acid with a mass fraction of 6% that is 0.5 times the mass of tributyl citrate, and react for 16 h to obtain a composite toughening agent:

[0041] S3. Mix itaconic acid, triphenylphosphine, p-hydroxyanisole, and glycidyl methacrylate according to a mass ratio of 3.6:0.2:0.1:8.8. Raise the temperature to 120 °C and react for 8 h. Use an oil pump to remove the remaining glycidyl methacrylate under vacuum to obtain glycidyl itaconate; mix trimethylolpropane, 3-mercaptopropionic acid, methanesulfonic acid, and toluene according to a mass ratio of 33:120:2:64. Raise the temperature to 120 °C and reflux for 8 h. Cool to room temperature and add toluene with the same mass as 3-mercaptopropionic acid. Wash it 5 times successively with saturated sodium bicarbonate, saturated sodium chloride, and deionized water, dry it with anhydrous magnesium sulfate, and finally filter and distill it under reduced pressure to obtain mercaptopropionate; mix glycidyl itaconate and mercaptopropionate according to a mass ratio of 40:3. Raise the temperature to 90 °C, add dilute sulfuric acid with a mass fraction of 6% that is 0.4 times the mass of glycidyl itaconate, and react for 16 h to obtain a self-made chain extender:

[0042] S4. Mix polylactic acid, composite toughening agent, self-made chain extender, compatibilizer, lubricant, and degradation promoter according to a mass ratio of 80:8:3:4:0.5:1. Extrude and pelletize through a twin-screw extruder. The temperatures of each temperature zone of the twin-screw extruder are: zone 1: 130 °C; zone 2: 135 °C; zone 3: 140 °C; zone 4: 145 °C; zone 5: 150 °C; zone 6: 150 °C; zone 7: 155 °C; zone 8: 155 °C; zone 9: 160 °C. The screw speed is 220 rpm to obtain a biodegradable plastic.

[0043] Comparative Example 1

[0044] The composition of Comparative Example 1 is the same as that of Example 2. The difference between this biodegradable plastic and its preparation method and those of Example 2 is that the composite toughening agent only includes tributyl citrate and catechol dopamine.

[0045] Comparative Example 2

[0046] The composition of Comparative Example 2 is the same as that of Example 2. The difference between this biodegradable plastic and its preparation method and those of Example 2 is that the composite toughening agent only includes tributyl citrate and castor oil.

[0047] Comparative Example 3

[0048] The composition of Comparative Example 3 is the same as that of Example 2. The difference between this biodegradable plastic and its preparation method and those of Example 2 is that the self-made chain extender is only glycidyl itaconate.

[0049] Comparative Example 4

[0050] The composition of Comparative Example 4 is the same as that of Example 2. The difference between this biodegradable plastic and its preparation method and those of Example 2 lies in that the self-made chain extender is only glycidyl methacrylate.

[0051] Effect Example

[0052] Table 1 below gives the performance analysis results of the biodegradable plastics prepared by using Examples 1 to 3 and Comparative Examples 1 to 4 of the present invention:

[0053] Table 1

[0054]

[0055]

[0056] It can be clearly found by comparing the experimental data of the examples and comparative examples in Table 1 that the biodegradable plastics prepared by using Examples 1, 2, and 3 have higher impact strength and thermal stability.

[0057] From the comparison of the experimental data of Example 1, Example 2, Example 3 and Comparative Example 1, Comparative Example 2, it can be found that the acrylamide polymer monomer containing catechol groups prepared by the reaction of isobutyl acrylate and dopamine acrylamide has more excellent compatibility and degradability. The ester butyl group at the end of tributyl citrate undergoes transesterification reaction with the hydroxyl groups on castor oil and catechol dopamine to form a long-chain multi-branched composite toughening agent, which is inserted into the macromolecular chain, weakens the intermolecular force, can more easily undergo elastic deformation, and thus better absorbs impact energy and improves the impact strength.

[0058] From the comparison of the experimental data of Example 1, Example 2, Example 3 and Comparative Example 3, Comparative Example 4, it can be found that itaconic acid reacts with glycidyl methacrylate to open the ring, and then mercaptopropionate is introduced as a side chain to react with the terminal carboxyl group on polylactic acid, increasing its molecular weight and improving the processing stability and thermal stability.

[0059] Obviously, the above embodiments are merely examples given for clearly explaining the embodiments of the present invention, rather than limiting the embodiments of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made on the basis of the above description. It is not necessary and impossible to enumerate all the embodiments here. And these obvious changes or modifications derived from the spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A biodegradable plastic, characterized in that, It includes polylactic acid, a composite toughening agent, a self-made chain extender, a compatibilizer, a lubricant, and a degradation promoter.

2. The biodegradable plastic according to claim 1, characterized in that, The composite toughening agent is prepared by mixing tributyl citrate, castor oil, and catechol dopamine; the catechol dopamine is prepared by the polymerization reaction of isobutyl acrylate and dopamine acrylamide.

3. A biodegradable plastic according to claim 1, wherein, The self-made chain extender is prepared by the reaction of itaconic acid glycidyl ester and mercaptopropionate.

4. A biodegradable plastic according to claim 1, characterized in that, The itaconic acid glycidyl ester is prepared by the reaction of itaconic acid and glycidyl methacrylate; the mercaptopropionate is prepared by the reaction of trimethylolpropane and 3-mercaptopropionic acid.

5. A biodegradable plastic according to claim 1, characterized in that, The compatibilizer is maleic anhydride; the lubricant is glyceryl tristearate; the degradation promoter is 1,5-pentanediol.

6. The preparation method of a biodegradable plastic according to claim 1, characterized in that, It includes the following specific steps: S1. Under a nitrogen atmosphere, mix dopamine acrylamide, isobutyl acrylate, and N,N-dimethylformamide in a mass ratio of 4:3:80 - 100, stir and dissolve, then add azobisisobutyronitrile in an amount of 0.01 - 0.04 times the mass of dopamine acrylamide and benzyl dithiobenzoate in an amount of 0.1 - 0.14 times the mass of dopamine acrylamide, heat up to 70 - 80 °C, react for 36 - 40 h, dialyze with deionized water for 24 h, and freeze-dry at -40 - 60 °C to obtain catechol dopamine; S2. Mix N,N-dimethylacetamide, tributyl citrate, castor oil, and catechol dopamine in a mass ratio of 20 - 40:8 - 10:1:4, stir evenly, then heat up to 80 - 90 °C, add dilute sulfuric acid with a mass fraction of 3 - 6% in an amount of 0.3 - 0.5 times the mass of tributyl citrate, and react for 12 - 16 h to obtain the composite toughening agent; S3. Mix itaconic acid glycidyl ester and mercaptopropionate in a mass ratio of 20 - 40:3, heat up to 80 - 90 °C, add dilute sulfuric acid with a mass fraction of 3 - 6% in an amount of 0.2 - 0.4 times the mass of itaconic acid glycidyl ester, and react for 12 - 16 h to obtain the self-made chain extender; S4. Mix polylactic acid, the composite toughening agent, the self-made chain extender, the compatibilizer, the lubricant, and the degradation promoter in a mass ratio of 60 - 80:8:1 - 3:2 - 4:0.2 - 0.5:1, and extrude and pelletize through a twin-screw extruder to obtain the biodegradable plastic.

7. The preparation method of a biodegradable plastic according to claim 6, characterized in that, In the above step S1, the preparation method of dopamine acrylamide is as follows: Under a nitrogen atmosphere, mix borax, sodium bicarbonate, and deionized water in a mass ratio of 1:0.3 - 0.5:10, stir evenly, then add hydrochloric acid dopamine with the same mass as sodium bicarbonate, and then dropwise add a mixed solution of methylacrylic anhydride and tetrahydrofuran with a mass ratio of methylacrylic anhydride to tetrahydrofuran of 1:5 and a mass 8 - 10 times that of sodium bicarbonate, adjust the pH to 7.8 - 8.2 with sodium hydroxide, react for 12 - 14 h, wash with ethyl acetate 2 - 3 times, adjust the pH to 1 - 2 with hydrochloric acid, extract with ethyl acetate, dry with anhydrous magnesium sulfate, and finally precipitate with n-hexane, filter by suction and dry to obtain dopamine acrylamide.

8. The preparation method of a biodegradable plastic according to claim 6, characterized in that, In the above step S3, the preparation method of itaconic acid glycidyl ester is as follows: Mix itaconic acid, triphenylphosphine, p-hydroxyanisole and glycidyl methacrylate in a mass ratio of 3 - 3.6:0.2:0.1:8.4 - 8.8, heat up to 110 - 120 °C, react for 6 - 8 h, remove the remaining glycidyl methacrylate by vacuum with an oil pump to obtain itaconic acid glycidyl ester.

9. The preparation method of a biodegradable plastic according to claim 6, wherein, In the above step S3, the preparation method of mercaptopropionate is as follows: Mix trimethylolpropane, 3-mercaptopropionic acid, methanesulfonic acid and toluene in a mass ratio of 31 - 33:100 - 120:2:64, heat up to 110 - 120 °C, carry out reflux reaction for 6 - 8 h, cool to room temperature and add toluene with the same mass as 3-mercaptopropionic acid, wash 3 - 5 times successively with saturated sodium bicarbonate, saturated sodium chloride and deionized water, dry with anhydrous magnesium sulfate, finally filter and carry out vacuum distillation to obtain mercaptopropionate.

10. The preparation method of a biodegradable plastic according to claim 6, wherein, In the above step S4, the temperatures of each temperature zone of the twin-screw machine are as follows: Zone 1: 120 - 130 °C; Zone 2: 115 - 135 °C; Zone 3: 120 - 140 °C; Zone 4: 135 - 145 °C; Zone 5: 140 - 150 °C; Zone 6: 140 - 150 °C; Zone 7: 145 - 155 °C; Zone 8: 145 - 155 °C; Zone 9: 150 - 160 °C, and the screw speed is 180 - 220 rpm.

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