Preparation method of a high-barrier polyvinyl alcohol packaging film

Abstract

The present invention relates to the technical field of modification of coated polyvinyl alcohol barrier films, and discloses a preparation method of a high-barrier polyvinyl alcohol packaging film, including: preparing a carboxyl-terminated crosslinking agent; based on the esterification reaction mechanism, using the crosslinking agent to crosslink and modify the polyvinyl alcohol resin to obtain crosslinked polyvinyl alcohol; dispersing layered double metal hydroxide nanosheets modified by a coupling agent in the crosslinked polyvinyl alcohol matrix to prepare a water-resistant modified polyvinyl alcohol coating solution; using an extrusion blow molding process to prepare a PLA film, and coating the water-resistant modified polyvinyl alcohol coating solution on its surface to prepare a coated polyvinyl alcohol barrier biodegradable film. The present invention provides a preparation method of a coated polyvinyl alcohol barrier biodegradable film, and prepares a coated polyvinyl alcohol barrier biodegradable film with excellent barrier properties, and the coated polyvinyl alcohol barrier biodegradable film can be applied in the packaging field.

Description

Technical Field

[0001] The present invention relates to the technical field of modification of coated polyvinyl alcohol barrier films, and specifically to a preparation method of a high-barrier polyvinyl alcohol packaging film. Background Art

[0002] With the increasing aggravation of white pollution and the daily consumption of non-renewable resources such as petroleum, the preparation of new biodegradable packaging materials to replace traditional packaging materials has become a research hotspot in the packaging field. Polylactic acid (PLA) is a biodegradable and environmentally friendly aliphatic polyester polymer material, but its barrier performance is poor, which greatly limits its application scope in the packaging field.

[0003] Polyvinyl alcohol (PVA) is an environmentally friendly polymer. Due to its degradability, recyclability, and relatively high barrier performance, it is widely used in fields such as packaging and biomedicine. Among them, polyvinyl alcohol is often used as a barrier material. It is dissolved in water and coated on packaging films, which can significantly improve the barrier performance of packaging materials. For example, the patent with the publication number CN117510956A prepared a high-barrier film by coating a coating solution containing polyvinyl alcohol on a corona-pretreated base film. However, polyvinyl alcohol has too many hydroxyl groups and is prone to form hydrogen bonds with water molecules in the air. Under the condition of high environmental humidity, its mechanical properties and barrier properties decrease significantly.

[0004] At present, methods for enhancing the barrier performance of polyvinyl alcohol include nano-composite modification and cross-linking modification, etc. For example, the patent with the publication number CN117510956A also added graphene oxide nano-sheet materials to the coating solution containing polyvinyl alcohol to synergistically enhance the barrier performance of the base film. The patent with the publication number CN115895357A discloses a new biodegradable high-barrier film coating solution, its preparation method and application. The high-barrier coating solution is composed of polyvinyl alcohol, tetraethyl orthosilicate, urea, ethanol, and water. It is prepared by an integrated method. The active silanol groups of the hydrolysis products condense and cross-link with the hydroxyl groups of polyvinyl alcohol during the drying process, improving the barrier performance of the coating.

[0005] Search found that: Layered double metal hydroxide nanosheets can provide a longer propagation path for permeating molecules due to their relatively high aspect ratio and have good barrier performance for small molecules. Summary of the Invention

[0006] The present invention provides a preparation method of a high-barrier polyvinyl alcohol packaging film, which designs and synthesizes a novel carboxyl-terminated crosslinking agent, uses the carboxyl-terminated crosslinking agent to crosslink and modify the polyvinyl alcohol resin, reduces the hydroxyl group content in the polyvinyl alcohol molecular chain, improves the mechanical strength and water resistance of the polyvinyl alcohol, and composites the MgAl-LDH modified by a titanate coupling agent CS-311 with the crosslinked polyvinyl alcohol and then coats it on the surface of the PLA film to achieve the technical effect of synergistically enhancing the barrier performance of the PLA film.

[0007] A preparation method of a coated polyvinyl alcohol barrier biodegradable film comprises the following steps:

[0008] Step 1: Prepare a carboxyl-terminated crosslinking agent, which is a carboxyl-terminated linear crosslinking agent and / or a carboxyl-terminated branched crosslinking agent;

[0009] Step 2: Based on the esterification reaction mechanism, use the crosslinking agent to crosslink and modify the polyvinyl alcohol resin to prepare a crosslinked polyvinyl alcohol;

[0010] Step 3: Disperse the layered double metal hydroxide nanosheets modified by a coupling agent in the crosslinked polyvinyl alcohol matrix to prepare a water-resistant modified polyvinyl alcohol coating solution;

[0011] Step 4: Prepare a PLA film by an extrusion blow molding process, and coat the water-resistant modified polyvinyl alcohol coating solution on its surface to prepare a coated polyvinyl alcohol barrier biodegradable film.

[0012] Preferably, the preparation method of the carboxyl-terminated linear crosslinking agent is as follows:

[0013] Step S2-1: Using 4-hydroxy-2-butanone as a nucleophile, under the catalytic action of a phase transfer catalyst, the chlorine functional group of 1,4-dichlorobutane reacts with the hydroxyl functional group of 4-hydroxy-2-butanone to undergo a nucleophilic substitution reaction to generate a linear carbonyl monomer;

[0014] Step S2-2: Using the Schiff base reaction mechanism, through the condensation reaction of the carbonyl functional group of the linear carbonyl monomer with the amino functional group of β-alanine, a carboxyl-terminated linear crosslinking agent is generated.

[0015] Preferably, the phase transfer catalyst is one of tetrabutylammonium hydrogensulfate, tetrabutylammonium bromide, tetrabutylammonium chloride, and trioctylmethylammonium chloride.

[0016] Preferably, the preparation method of the carboxyl-terminated branched crosslinking agent is as follows:

[0017] Step S4-1: Using 4-hydroxy-2-butanone as a nucleophile, under the catalysis of a basic substance, a nucleophilic substitution reaction occurs between the chlorine functional group of cyanuric chloride and the hydroxyl functional group of 4-hydroxy-2-butanone to generate a branched carbonyl monomer;

[0018] Step S4-2: Using the Schiff base reaction mechanism, a condensation reaction occurs between the carbonyl functional group of the branched carbonyl monomer and the amino functional group of β-alanine to generate a carboxyl-terminated branched crosslinking agent.

[0019] Preferably, the basic substance is one of sodium bicarbonate, sodium hydroxide, sodium carbonate, pyridine, and triethylamine.

[0020] Preferably, the preparation method of the water-resistant modified polyvinyl alcohol coating solution in step three is as follows: Dissolve 5 to 30 parts by weight of crosslinked polyvinyl alcohol in deionized water with stirring at 85°C to 95°C to prepare a crosslinked polyvinyl alcohol mother liquor, cool it to 40 to 60°C, add 0.05 to 5 parts by weight of MgAl-LDH modified with a titanate coupling agent CS-311, and mix evenly to obtain a water-resistant modified polyvinyl alcohol coating solution.

[0021] Preferably, the preparation method of the coated polyvinyl alcohol barrier biodegradable film in step four is as follows:

[0022] Step S7-1: Put 5 to 50 parts by weight of polylactic acid resin into a twin-screw extruder for melting, extrusion, pelletizing, and drying, and then use an upward blowing method to blow mold with a single-screw extruder to obtain a 50 to 150 μm PLA film. The PLA film is pretreated by washing successively with an ethanol aqueous solution (the volume ratio of ethanol to deionized water is 0.5 to 1.5:​​1) and deionized water;

[0023] Step S7-2: Use a 20 to 50 μm metering rod to evenly apply the water-resistant modified polyvinyl alcohol coating solution on the surface of the PLA film and dry it to obtain a coated polyvinyl alcohol barrier biodegradable film.

[0024] A coated polyvinyl alcohol barrier biodegradable film prepared according to the above method.

[0025] Preferably, the water vapor transmission rate of the coated polyvinyl alcohol barrier biodegradable film is 0.5 to 1.0 [g / (m 2 ·24h)], and the oxygen transmission rate is 0.3 to 0.5 [cm 3 / (m 2 ·24h·0.1MPa)].

[0026] Beneficial effects

[0027] The present invention designed and synthesized two novel carboxyl-terminated crosslinking agents. Further, based on the esterification reaction mechanism, polyvinyl alcohol resin was crosslinked and modified using them to prepare crosslinked polyvinyl alcohol. Then, MgAl-LDH modified with titanate coupling agent CS-311 was compounded with the crosslinked polyvinyl alcohol to prepare a water-resistant modified polyvinyl alcohol coating solution. Finally, the water-resistant modified polyvinyl alcohol coating solution was coated on the surface of a PLA film prepared by an extrusion blow molding process to prepare a coated polyvinyl alcohol barrier biodegradable film.

[0028] It was found through experiments that: compared with the polylactic acid film, the two coated polyvinyl alcohol barrier biodegradable films prepared by the present invention can significantly reduce their water vapor transmission rate and oxygen transmission rate, showing excellent barrier properties.

[0029] And it was found that the coated polyvinyl alcohol barrier biodegradable film prepared by the present invention also has a slight improvement in mechanical properties and can be applied in the packaging field. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is the synthesis route of the carboxyl-terminated linear crosslinking agent;

[0031] Figure 2 is the synthesis route of the carboxyl-terminated branched crosslinking agent;

[0032] Figure 3 is the experimental result of the barrier properties of the coated polyvinyl alcohol barrier biodegradable film. DETAILED DESCRIPTION OF THE INVENTION

[0033] Experimental Example 1:

[0034] Synthesize the carboxyl-terminated linear crosslinking agent, as Figure 1 shown, including two synthesis processes, specifically as follows:

[0035] (1) Synthesize the linear carbonyl monomer: Using 4-hydroxy-2-butanone as the nucleophile, under the catalysis of a phase transfer catalyst, the chlorine functional group of 1,4-dichlorobutane undergoes a nucleophilic substitution reaction with the hydroxyl functional group of 4-hydroxy-2-butanone to generate the linear carbonyl monomer.

[0036] Among them, the phase transfer catalyst is one of tetrabutylammonium hydrogensulfate, tetrabutylammonium bromide, tetrabutylammonium chloride, and trioctylmethylammonium chloride; in this experimental example, tetrabutylammonium hydrogensulfate was selected for use.

[0037] The specific experimental steps for synthesizing the linear carbonyl monomer are as follows: Add 1.8 g of 4-hydroxy-2-butanone, 5 mL of 30 wt% aqueous sodium hydroxide solution, and 50 mL of N,N-dimethylformamide into a three-necked flask. Under the protection of nitrogen and mechanical stirring, stir and dissolve for 30 min. Then, sequentially add 2.5 mL of tetrabutylammonium hydrogen sulfate and 1.2 mL of 1,4-dichlorobutane to the three-necked flask. Heat up to 70 °C and stir for 6 h. After cooling, distill off the solvent and dry to obtain the linear carbonyl monomer;

[0038] (2) Synthesis of carboxyl-terminated linear crosslinker: Using the Schiff base reaction mechanism, a condensation reaction occurs between the carbonyl functional group of the linear carbonyl monomer and the amino functional group of β-alanine to generate the carboxyl-terminated linear crosslinker;

[0039] The specific experimental steps for synthesizing the carboxyl-terminated linear crosslinker are as follows: Add 2.3 g of the linear carbonyl monomer and 30 mL of absolute ethanol into a three-necked flask equipped with a water separator. Stir at room temperature until completely dissolved. Then, add 2 mL of glacial acetic acid and 20 mL of β-alanine solution (prepared from 1.8 g of β-alanine and 20 mL of absolute ethanol) dropwise to the three-necked flask. Heat up to 85 °C and reflux for 3 h. After cooling, perform suction filtration, recrystallize with absolute ethanol, and dry under vacuum to obtain the carboxyl-terminated linear crosslinker;

[0040] 1H NMR characterization of the carboxyl-terminated linear crosslinker is as follows: 1 H NMR (CDCl3, 400 MHz) δ: 1.61 - 1.65 (t, 4H), 1.95 (s, 6H), 2.40 - 2.43 (t, 4H), 2.65 - 2.68 (t, 4H), 3.46 - 3.52 (m, 8H), 3.83 - 3.88 (t, 4H); The active hydrogen in the carboxyl functional group of the carboxyl-terminated linear crosslinker does not show a peak in the CDCl3 solvent.

[0041] Experimental Example 2:

[0042] Synthesis of the carboxyl-terminated branched crosslinker, as Figure 2 shown, includes two synthesis processes, specifically as follows:

[0043] (1) Synthesis of the branched carbonyl monomer: Using 4-hydroxy-2-butanone as the nucleophile, under the catalytic action of a basic substance, a nucleophilic substitution reaction occurs between the chlorine functional group of cyanuric chloride and the hydroxyl functional group of 4-hydroxy-2-butanone to generate the branched carbonyl monomer;

[0044] Among them, the basic substance is one of sodium bicarbonate, sodium hydroxide, sodium carbonate, pyridine, and triethylamine; In this experimental example, triethylamine is selected for use;

[0045] The specific experimental steps for synthesizing the branched carbonyl monomer are as follows: Add 1.8 g of cyanuric chloride, 2.7 g of 4-hydroxy-2-butanone, and 80 mL of N,N-dimethylformamide into a three-necked flask. Under the protection of nitrogen and mechanical stirring, add 5 mL of triethylamine dropwise to the three-necked flask, and stir and react at room temperature for 12 h. Remove the solvent by rotary evaporation and dry to obtain the branched carbonyl monomer;

[0046] (2) Synthesize the carboxyl-terminated branched crosslinker: Using the Schiff base reaction mechanism, a condensation reaction occurs between the carbonyl functional group of the branched carbonyl monomer and the amino functional group of β-alanine to generate the carboxyl-terminated branched crosslinker;

[0047] The specific experimental steps for synthesizing the carboxyl-terminated branched crosslinker refer to the preparation experiment of the carboxyl-terminated linear crosslinker. The only difference from the preparation experiment of the carboxyl-terminated linear crosslinker is that 2.3 g of the branched carbonyl monomer is used to replace 2.3 g of the linear carbonyl monomer;

[0048] The 1H NMR characterization of the carboxyl-terminated branched crosslinker is as follows: 1 H NMR(CDCl3, 400 MHz) δ: 1.95(s, 9H), 2.53 - 2.57(t, 6H), 2.71 - 2.76(t, 6H), 3.85 - 3.90(t, 6H), 4.20 - 4.24(t, 6H); The active hydrogen in the carboxyl functional group of the carboxyl-terminated branched crosslinker does not show a peak in the CDCl3 solvent.

[0049] Experimental Example 1:

[0050] (1) Prepare crosslinked polyvinyl alcohol Ⅰ. The preparation process is as follows: Based on the esterification reaction mechanism, use the carboxyl-terminated linear crosslinker to crosslink and modify the polyvinyl alcohol resin to obtain crosslinked polyvinyl alcohol Ⅰ. The specific experimental steps are as follows: Add 10 g of polyvinyl alcohol resin and 100 mL of deionized water into a three-necked flask, heat up to 95 °C and stir to dissolve for 4 h to prepare the polyvinyl alcohol mother liquor. Cool to 65 °C, and sequentially add 2 mL of concentrated sulfuric acid and 20 mL of the carboxyl-terminated linear crosslinker solution (prepared from 4 g of the carboxyl-terminated linear crosslinker, 10 mL of N,N-dimethylformamide, and 10 mL of deionized water) dropwise to the three-necked flask. Keep stirring and reacting at 65 °C for 3 h, cool to room temperature, pour it into ethanol for sedimentation, filter by suction, and dry to obtain crosslinked polyvinyl alcohol Ⅰ;

[0051] (2) Preparation of crosslinked polyvinyl alcohol II. The preparation process is as follows: Based on the esterification reaction mechanism, the polyvinyl alcohol resin is crosslinked and modified using a carboxyl-terminated branched crosslinking agent to obtain crosslinked polyvinyl alcohol II. The specific experimental steps can be referred to the preparation experiment of crosslinked polyvinyl alcohol I. The difference between it and the preparation experiment of crosslinked polyvinyl alcohol I is only that: the carboxyl-terminated branched crosslinking agent is used to replace the carboxyl-terminated linear crosslinking agent;

[0052] Among them, the polyvinyl alcohol resin is Nichigo G-PolymerTM purchased from Mitsubishi Chemical Group.

[0053] Experimental Example Two:

[0054] (1) Preparation of water-resistant modified polyvinyl alcohol coating solution I. The preparation process is as follows: The layered double hydroxide nanosheets modified with titanate coupling agent CS-311 are dispersed in the matrix of crosslinked polyvinyl alcohol I to prepare water-resistant modified polyvinyl alcohol coating solution I;

[0055] Among them, the layered double hydroxide nanosheets are one of ZnNiAl-LDH, MgAl-LDH, ZnAl-LDH, NiAl-LDH; In this example, MgAl-LDH is selected for use;

[0056] The specific preparation steps of water-resistant modified polyvinyl alcohol coating solution I are as follows: 10 g of crosslinked polyvinyl alcohol I and 110 mL of deionized water are added to a three-necked flask, heated to 90 °C and stirred for dissolution for 4 h to prepare a mother liquor of crosslinked polyvinyl alcohol I. After cooling to 50 °C, 0.5 g of MgAl-LDH modified with titanate coupling agent CS-311 is added to the three-necked flask, and stirred at 50 °C for 3 h until evenly mixed to obtain water-resistant modified polyvinyl alcohol coating solution I;

[0057] (2) Preparation of water-resistant modified polyvinyl alcohol coating solution II. The preparation process is as follows: The layered double hydroxide nanosheets modified with titanate coupling agent CS-311 are dispersed in the matrix of crosslinked polyvinyl alcohol II to prepare water-resistant modified polyvinyl alcohol coating solution II;

[0058] Among them, the layered double hydroxide nanosheets are one of ZnNiAl-LDH, MgAl-LDH, ZnAl-LDH, NiAl-LDH; In this example, MgAl-LDH is selected for use;

[0059] The specific preparation steps of water-resistant modified polyvinyl alcohol coating solution II can be referred to the preparation experiment of water-resistant modified polyvinyl alcohol coating solution I. The difference between it and the preparation experiment of water-resistant modified polyvinyl alcohol coating solution I is only that: crosslinked polyvinyl alcohol II is used to replace crosslinked polyvinyl alcohol I;

[0060] Among them, MgAl-LDH is a two-dimensional layered double metal hydroxide of MgAl-LDH purchased from Jiangsu Xianfeng Nano Materials Technology Co., Ltd., with the product number: 102911, and its specification: sheet diameter 1-4μm;

[0061] The preparation method of MgAl-LDH modified by titanate coupling agent CS-311 is as follows: Add 3g of MgAl-LDH, 1.5g of titanate coupling agent CS-311 and 60mL of ethanol aqueous solution (the volume ratio of ethanol to deionized water is 9:1) into a three-necked flask. Under mechanical stirring, add acetic acid to the three-necked flask to adjust the pH to 4, heat up to 80°C and reflux for 24h, and vacuum dry to obtain MgAl-LDH modified by titanate coupling agent CS-311.

[0062] Experimental Example Three:

[0063] (1) Prepare polyvinyl alcohol-based film I, which includes two preparation steps, specifically as follows:

[0064] Step 1: Prepare PLA film: Using polylactic acid resin as raw material, prepare PLA film by extrusion blow molding process. The specific preparation steps are as follows:

[0065] Put 10g of polylactic acid resin into a twin-screw extruder for melting and extrusion, then cut it with a pelletizer to obtain polylactic acid resin particles of 1-5mm, dry them, and use an up-blowing method with a single-screw extrusion blow molding machine to blow mold to obtain a 70μm PLA film; Fix the PLA film on a wire frame, put it into an ethanol aqueous solution (the volume ratio of ethanol to deionized water is 1:1) for cleaning for 2h, then clean it with deionized water, and dry it at 45°C for standby;

[0066] Among them, the temperatures of the 1-3 zones of the twin-screw extruder are 160°C, 180°C, and 190°C respectively, and the rotation speed is 100r / min; the rotation speed of the pelletizer is 200r / min; the temperatures of the 1-3 zones of the single-screw extrusion blow molding machine are 180°C, 190°C, and 195°C respectively, the rotation speed is 50r / min, the traction speed is 6m / min, the die head diameter of the blow molding machine is 65mm, and the blow-up ratio is 3.5;

[0067] Step 2: Fix the PLA film on a coater, use a 30μm metering rod to evenly coat the water-resistant modified polyvinyl alcohol coating solution I on the surface of the PLA film, and put it into an oven at 70°C for drying for 3h to obtain polyvinyl alcohol-based film I;

[0068] (2) Prepare polyvinyl alcohol-based film II: Use the water-resistant modified polyvinyl alcohol coating solution II to replace the water-resistant modified polyvinyl alcohol coating solution I, and refer to the preparation steps and experimental conditions of polyvinyl alcohol-based film I to prepare polyvinyl alcohol-based film II;

[0069] Among them, the polylactic acid resin was purchased from Suzhou Jiangcangfa Plastic Co., Ltd., and its grade was 4032D.

[0070] Performance test:

[0071] (1) Water barrier performance: The water vapor transmission rate of the sample was tested according to GB / T 1037-2021. The specific test steps were as follows: Place a 30 cm circular sample in a glass dryer with anhydrous calcium chloride as a desiccant at an ambient temperature of 25°C for 72 h, and then use a W3 / 060 type water vapor transmission rate tester to test the water barrier performance of the sample;

[0072] (2) Oxygen barrier performance: The oxygen transmission rate of the sample was tested according to GB / T 1038-2000. The specific test steps were as follows: Place a 30 cm circular sample in a glass dryer with anhydrous calcium chloride as a desiccant at an ambient temperature of 25°C for 72 h, and then use a Y110 type oxygen transmission rate tester to test the oxygen barrier performance of the sample;

[0073] (3) Mechanical properties: The tensile strength of the sample was tested using an Instron 5565 universal tensile testing machine. The specific test steps were as follows: Fix an 80 mm × 15 mm sample on the tensile testing machine and conduct a tensile test at a tensile rate of 5 mm / min, and record the longitudinal and transverse tensile strengths of the sample;

[0074] The above experimental results are shown in Table 1 below.

[0075] Table 1 Performance experimental results of the coated polyvinyl alcohol barrier biodegradable film

[0076]

[0077] Obtained by plotting the test data in Table 1 Figure 3 , and the following conclusions can be drawn through comprehensive analysis of the above experimental results:

[0078] (1) Compared with the polylactic acid film, both of the two polyvinyl alcohol-based films prepared in the present invention can significantly reduce their water vapor transmission rate and oxygen transmission rate, showing excellent barrier performance;

[0079] (2) The polyvinyl alcohol-based film prepared in the present invention also has a slight improvement in mechanical properties and can be applied in the packaging field.

Claims

1. A preparation method of a coated polyvinyl alcohol barrier biodegradable film, characterized in that, It includes the following steps: Step 1: Prepare a carboxyl-terminated crosslinker, which is a carboxyl-terminated linear crosslinker and / or a carboxyl-terminated branched crosslinker; The chemical structural formula of the carboxyl-terminated linear crosslinker is: ； The chemical structural formula of the carboxyl-terminated branched crosslinker is: ； Step 2: Based on the esterification reaction mechanism, use the crosslinker to crosslink and modify the polyvinyl alcohol resin to prepare crosslinked polyvinyl alcohol; Step 3: Disperse the layered double metal hydroxide nanosheets modified by a coupling agent in the crosslinked polyvinyl alcohol matrix to prepare a water-resistant modified polyvinyl alcohol coating solution; Step 4: Use the extrusion blow molding process to prepare a PLA film, and coat the water-resistant modified polyvinyl alcohol coating solution on its surface to prepare a coated polyvinyl alcohol barrier biodegradable film.

2. The preparation method of a coated polyvinyl alcohol barrier biodegradable film according to claim 1, characterized in that, The preparation method of the carboxyl-terminated linear crosslinker is: Step S2-1: Using 4-hydroxy-2-butanone as a nucleophile, under the catalytic action of a phase transfer catalyst, the chlorine functional group of 1,4-dichlorobutane reacts with the hydroxyl functional group of 4-hydroxy-2-butanone to undergo a nucleophilic substitution reaction to generate a linear carbonyl monomer; Step S2-2: Using the Schiff base reaction mechanism, through the condensation reaction of the carbonyl functional group of the linear carbonyl monomer with the amino functional group of β-alanine, a carboxyl-terminated linear crosslinker is generated.

3. The preparation method of a coated polyvinyl alcohol barrier biodegradable film according to claim 2, characterized in that, The phase transfer catalyst is one of tetrabutylammonium hydrogensulfate, tetrabutylammonium bromide, tetrabutylammonium chloride, and trioctylmethylammonium chloride.

4. The preparation method of a coated polyvinyl alcohol barrier biodegradable film according to claim 1, characterized in that, The preparation method of the carboxyl-terminated branched crosslinker is: Step S4-1: Using 4-hydroxy-2-butanone as a nucleophile, under the catalytic action of an alkaline substance, the chlorine functional group of cyanuric chloride reacts with the hydroxyl functional group of 4-hydroxy-2-butanone to undergo a nucleophilic substitution reaction to generate a branched carbonyl monomer; Step S4-2: Using the Schiff base reaction mechanism, through the condensation reaction of the carbonyl functional group of the branched carbonyl monomer with the amino functional group of β-alanine, a carboxyl-terminated branched crosslinker is generated.

5. The preparation method of a coated polyvinyl alcohol barrier biodegradable film according to claim 4, characterized in that, The alkaline substance is one of sodium bicarbonate, sodium hydroxide, sodium carbonate, pyridine, and triethylamine.

6. The preparation method of a coated polyvinyl alcohol barrier biodegradable film according to claim 1, characterized in that, The preparation method of the water-resistant modified polyvinyl alcohol coating solution in Step 3 is: Dissolve 5-30 parts by weight of crosslinked polyvinyl alcohol in deionized water by stirring at 85°C - 95°C to prepare a crosslinked polyvinyl alcohol mother liquor, cool it to 40 - 60°C, and add 0.05 - 5 parts by weight of MgAl-LDH modified by a titanate coupling agent CS-311, and mix evenly to prepare a water-resistant modified polyvinyl alcohol coating solution.

7. The preparation method of a coated polyvinyl alcohol barrier biodegradable film according to claim 1, characterized in that, The preparation method of the coated polyvinyl alcohol barrier biodegradable film in Step 4 is: Step S7-1: Put 5-50 parts by weight of polylactic acid resin into a twin-screw extruder for melting and extrusion, pelletize and dry it, and then use a single-screw extrusion blow molding machine to blow mold by the up-blowing method to obtain a 50-150 μm PLA film, and pre-treat the PLA film. The specific steps of this pre-treatment are: First, clean it with an ethanol aqueous solution and then clean it with deionized water; among them, the ethanol aqueous solution is composed of 0.5 - 1.5 parts by volume of ethanol and 1 part by volume of deionized water; Step S7-2: Use a 20-50 μm metering rod to evenly apply the water-resistant modified polyvinyl alcohol coating solution on the surface of the PLA film, and dry it to obtain a coated polyvinyl alcohol barrier biodegradable film.

8. A coated polyvinyl alcohol barrier biodegradable film prepared by the method according to any one of claims 1-7.

9. The coated polyvinyl alcohol barrier biodegradable film according to claim 8, wherein the water vapor transmission rate of the film is 0.5 to 1.0 [g / (m 2 •24 h)], and the oxygen transmission rate is 0.3 to 0.5 [cm 3 / (m 2 •24 h•0.1 MPa)].

10. Application of the coated polyvinyl alcohol barrier biodegradable film prepared by the method according to any one of claims 1-7 in the packaging field.

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