Preparation method and application of degradable environment-friendly polylactic acid apple preservative film

Abstract

This invention discloses a method for preparing and applying a biodegradable and environmentally friendly polylactic acid (PLA) apple preservation film, relating to the field of fruit and vegetable preservation materials technology. The invention includes the following steps: Step S1: Add chitosan to a 1-2 wt% aqueous acetic acid solution and stir for 20-30 minutes to obtain a chitosan-acetic acid solution; Step S2: Add modified zeolite and β-cyclodextrin to the chitosan-acetic acid solution, heat to 40-50°C, stir for 30-60 minutes, add tea polyphenols, and stir for 20-30 minutes to obtain a premix; Step S3: Add sodium hydroxide solution to the premix. The advantages are: compared with existing technologies, it simultaneously achieves synergistic effects of toughening film formation, modified atmosphere barrier, ethylene removal, and long-lasting antibacterial properties. The functions do not cancel each other out, and it solves the process bottleneck of high filler content preventing blown film production while maintaining full degradation. It has broad application prospects in the field of post-harvest apple preservation and biodegradable packaging.

Description

Technical Field

[0001] This invention relates to the field of fruit and vegetable preservation materials, and in particular to a method for preparing and applying a biodegradable and environmentally friendly polylactic acid apple preservation film. Background Technology

[0002] Biodegradable and environmentally friendly polylactic acid apple preservation film is a film material used for post-harvest packaging and preservation of apples. It is made by melt blending and blow molding, with polylactic acid (PLA) as the main base material and supplemented with other fully biodegradable components and functional additives. Polylactic acid is a fully biodegradable polymer material made from renewable plant resources such as corn and cassava through fermentation and polymerization. It has good transparency and processability.

[0003] However, pure polylactic acid is brittle and has low melt strength, making it prone to bubble breakage when blown into film alone. It often needs to be toughened and modified by introducing polybutylene terephthalate (PET). Apples have a high post-harvest respiration rate, are sensitive to ethylene, and are susceptible to Penicillium mold infection. Therefore, the plastic wrap needs to have modified atmosphere packaging, ethylene removal, and antibacterial functions.

[0004] In existing technologies, antibacterial preservation films mostly achieve antibacterial effects by adding inorganic antibacterial agents, but the fillers exacerbate the brittleness of polylactic acid and have no ability to remove ethylene; zeolite composite films can adsorb ethylene, but the adsorption capacity of zeolite is limited by the amount added and has no antibacterial function; chitosan coated films have natural antibacterial properties, but the hydrophilic coating has poor interfacial bonding with hydrophobic polylactic acid and is easy to peel off.

[0005] Therefore, it is necessary to design a method for preparing and applying a biodegradable and environmentally friendly polylactic acid apple preservation film. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for preparing and applying a biodegradable and environmentally friendly polylactic acid apple preservation film, thus solving the problems mentioned in the background section.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film includes the following steps: Step S1: Add chitosan to a 1-2 wt% aqueous acetic acid solution, stir at 200-400 rpm for 20-30 minutes to obtain a chitosan acetic acid solution; Step S2: Add the modified zeolite and β-cyclodextrin to the chitosan acetate solution, heat to 40-50℃, stir at 200-400 rpm for 30-60 min, add tea polyphenols, stir at 300-500 rpm for 20-30 min to obtain the premix. Step S3: Add sodium hydroxide solution to the premixed solution, adjust the pH to 8-9, filter, wash, heat to 50-60℃, vacuum dry for 12-24h, grind and pulverize, pass through a 200-mesh sieve to obtain composite loaded particles; Step S4: Vacuum dry polylactic acid at 75-85℃ for 10-14h, vacuum dry composite loaded particles and chitosan at 55-65℃ for 22-26h, transfer the dried polylactic acid, composite loaded particles and chitosan into a high-speed mixer, add polybutylene terephthalate-adipate, mix at 500-1000rpm for 10-20min, transfer into a twin-screw extruder, extrude at 160-180℃, screw speed 50-100rpm, melt blend, extrude, cool, and pelletize to obtain composite masterbatch; Step S5: Transfer the composite masterbatch into a single-screw blown film machine, blown film temperature 165-185℃, traction speed 3-6m / min, blow-up ratio 2-3, blow molding, cooling, and winding to obtain the cling film base film; Step S6: After corona treatment, the plastic wrap base film is immersed in chitosan coating solution for 1-3 minutes, pulled up at a speed of 10-30 mm / s, heated to 50-60℃, and dried for 1-2 hours to obtain biodegradable and environmentally friendly polylactic acid apple plastic wrap. This step utilizes the synergy of chitosan and tea polyphenols to form a gradient antibacterial system that ranges from immediate effect upon contact to long-term release. The synergy of modified zeolite and β-cyclodextrin complementarily broadens the concentration and humidity range adaptability for ethylene removal. Furthermore, the synergy of PLA and polybutylene terephthalate-adipate enables high-filler-content functional loaded particles to be successfully blow-molded into films, achieving the integration of mechanical properties and multiple preservation functions while maintaining full degradation.

[0008] Furthermore, the preparation steps of the modified zeolite in step S2 are as follows: Natural zeolite was added to an 8-12 wt% hydrochloric acid solution, heated to 60-80℃, stirred at 100-200 rpm for 4-6 hours, filtered, washed, heated to 100-120℃, dried for 2-4 hours, transferred to a muffle furnace, heated to 300-400℃, calcined for 3-5 hours, and cooled to room temperature to obtain modified zeolite. The mass ratio of the natural zeolite to the 8-12wt% hydrochloric acid solution is 1:8-12; This step utilizes the synergistic effect of hydrochloric acid and natural zeolite to acid wash out impurities and non-framework aluminum within the zeolite pores, thereby clearing and expanding the microporous channels. Subsequent high-temperature calcination further activates the acidic sites on the surface, significantly enhancing the physical adsorption capacity and selectivity of the modified zeolite for ethylene, thus providing efficient ethylene removal capabilities for food preservation films.

[0009] Furthermore, the mass ratio of chitosan to 1-2wt% acetic acid aqueous solution in step S1 is 1:40-50; The mass ratio of chitosan acetate solution, modified zeolite, β-cyclodextrin and tea polyphenols in step S2 is 100:1-1.2:0.3-0.5:0.15-0.25.

[0010] Furthermore, in step S4, the mass ratio of polylactic acid, composite supported particles, polybutylene terephthalate (PET) to chitosan is 1:0.1-0.2:0.2-0.3:0.1-0.2.

[0011] Furthermore, the power of the corona treatment in step S6 is 100-200W, and the processing speed is 1-3m / min.

[0012] Furthermore, the preparation steps of the chitosan coating solution in step S6 are as follows: Add chitosan to a 1-2 wt% aqueous acetic acid solution, add glycerol, stir at 200-400 rpm for 20-30 minutes to obtain a chitosan coating solution; The mass ratio of chitosan, aqueous acetic acid solution, and glycerol is 1:20-30:0.4-0.8; This step involves the synergistic effect of chitosan and glycerol. Chitosan imparts natural antibacterial and gas barrier properties to the coating, while glycerol, as a small molecule plasticizer, inserts between the chitosan molecular chains, weakening intermolecular hydrogen bonds and enhancing chain flexibility. The two complement each other, ensuring that the coating remains flexible and non-brittle after drying, and can be bent synchronously with the base film without peeling off, thus ensuring the long-lasting stability of antibacterial and barrier functions.

[0013] Compared with existing technologies, this invention provides a method for preparing a biodegradable and environmentally friendly polylactic acid (PLA) apple preservation film. This invention utilizes a blend of PLA, polybutylene terephthalate (PET), and chitosan, along with the design of composite loaded particles. A dual-mode ethylene-targeted removal system is constructed using modified zeolite and β-cyclodextrin. Chitosan substrate provides antibacterial properties, while tea polyphenols provide sustained release, creating a gradient antibacterial effect. Corona treatment enhances the interfacial bonding between the coating layer and the base film. Compared with existing technologies, this invention simultaneously achieves synergistic effects of toughening film formation, modified atmosphere barrier, ethylene removal, and long-lasting antibacterial properties, with no mutual cancellation among the functions. It solves the process bottleneck of high-filler-content blown film production while maintaining full biodegradability, and has broad application prospects in the fields of post-harvest apple preservation and biodegradable packaging. Attached Figure Description

[0014] Figure 1 This is a flowchart of a method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film proposed in this invention; Figure 2The bar chart shows the tensile strength and elongation at break test results of the biodegradable and environmentally friendly polylactic acid apple preservation films prepared in Examples 7-9 and Comparative Examples 1-4 of this invention. Figure 3 The bar chart shows the oxygen permeability test results of the biodegradable and environmentally friendly polylactic acid apple preservation films prepared in Examples 7-9 and Comparative Examples 1-4 of this invention. Figure 4 The bar chart shows the water vapor transmission rate test results of the degradable and environmentally friendly polylactic acid apple preservation films prepared in Examples 7-9 and Comparative Examples 1-4 of this invention. Detailed Implementation

[0015] Reference Figure 1-4 To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments.

[0016] Example 1: Preparation of modified zeolite Add 500g of natural zeolite to 4kg of 12wt% hydrochloric acid solution, heat to 60℃, stir at 200rpm for 4h, filter, wash, heat to 120℃, dry for 2h, transfer to muffle furnace, heat to 400℃, calcine for 3h, cool to room temperature to obtain modified zeolite.

[0017] Example 2: Preparation of modified zeolite Add 500g of natural zeolite to 5kg of 10wt% hydrochloric acid solution, heat to 70℃, stir at 150rpm for 5h, filter, wash, heat to 110℃, dry for 3h, transfer to muffle furnace, heat to 350℃, calcine for 4h, cool to room temperature to obtain modified zeolite.

[0018] Example 3: Preparation of modified zeolite Add 500g of natural zeolite to 6kg of 8wt% hydrochloric acid solution, heat to 80℃, stir at 100rpm for 6h, filter, wash, heat to 100℃, dry for 4h, transfer to muffle furnace, heat to 300℃, calcine for 5h, cool to room temperature to obtain modified zeolite.

[0019] Example 4: Preparation of Chitosan Coating Solution Add 1 kg of chitosan to 20 kg of 2 wt% acetic acid aqueous solution, add 400 g of glycerol, stir at 400 rpm for 20 min to obtain chitosan coating solution.

[0020] Example 5: Preparation of Chitosan Coating Solution Add 1 kg of chitosan to 25 kg of 1.5 wt% acetic acid aqueous solution, add 600 g of glycerol, stir at 300 rpm for 25 min to obtain chitosan coating solution.

[0021] Example 6: Preparation of Chitosan Coating Solution Add 1 kg of chitosan to 30 kg of 1 wt% acetic acid aqueous solution, add 800 g of glycerol, stir at 200 rpm for 30 min to obtain chitosan coating solution.

[0022] Example 7: Preparation of biodegradable and environmentally friendly polylactic acid apple preservation film S1: Add 1 kg of chitosan to 40 kg of 2 wt% acetic acid aqueous solution, stir at 200 rpm for 30 min to obtain chitosan acetic acid solution. S2: Add 200g of modified zeolite (Example 1) and 100g of β-cyclodextrin to 20kg of chitosan acetate solution, heat to 40℃, stir at 400rpm for 30min, add 50g of tea polyphenols, stir at 300rpm for 30min to obtain premixed solution. S3: Add sodium hydroxide solution to the premixed solution, adjust the pH to 8.4-8.6, filter, wash, heat to 50℃, vacuum dry for 24h, grind and pulverize, pass through a 200-mesh sieve to obtain composite loaded particles; S4: Dry 1000g of polylactic acid under vacuum at 75℃ for 14h, dry 100g of composite loaded particles and 100g of chitosan under vacuum at 65℃ for 22h, transfer to a high-speed mixer, add 300g of polybutylene terephthalate-adipate, mix at 1000rpm for 10min, transfer to a twin-screw extruder, extrude at 180℃ and screw speed of 50rpm, melt blend, extrude, cool, and pelletize to obtain composite masterbatch; S5: Transfer the composite masterbatch into a single-screw blown film machine, blown film temperature 165℃, traction speed 6m / min, blow-up ratio 2, blow molding, cooling, and winding to obtain the cling film base film; S6: Power 100W, processing speed 3m / min, corona treatment is performed on the plastic wrap base film. After treatment, it is immersed in chitosan coating solution (Example 4) for 1 minute, lifting speed 30mm / s, temperature is raised to 50℃, and dried for 2 hours to obtain biodegradable and environmentally friendly polylactic acid apple plastic wrap.

[0023] Example 8: Preparation of biodegradable and environmentally friendly polylactic acid apple preservation film S1: Add 1 kg of chitosan to 45 kg of 1.5 wt% acetic acid aqueous solution, stir at 300 rpm for 25 min to obtain chitosan acetic acid solution; S2: Add 220g of modified zeolite (Example 2) and 80g of β-cyclodextrin to 20kg of chitosan acetate solution, heat to 45℃, stir at 300rpm for 45min, add 40g of tea polyphenols, stir at 400rpm for 25min to obtain premixed solution. S3: Add sodium hydroxide solution to the premixed solution, adjust the pH to 8.4-8.6, filter, wash, heat to 55℃, vacuum dry for 18h, grind and pulverize, pass through a 200-mesh sieve to obtain composite loaded particles; S4: Dry 1000g of polylactic acid under vacuum at 80℃ for 12h, dry 150g of composite loaded particles and 150g of chitosan under vacuum at 60℃ for 24h, transfer to a high-speed mixer, add 250g of polybutylene terephthalate-adipate, mix at 750rpm for 15min, transfer to a twin-screw extruder, extrude at 170℃ and screw speed at 75rpm, melt blend, extrude, cool, and pelletize to obtain composite masterbatch; S5: Transfer the composite masterbatch into a single-screw blown film machine, blown film temperature 175℃, traction speed 4.5m / min, blow-up ratio 2.5, blow molding, cooling, and winding to obtain the cling film base film; S6: Power 150W, processing speed 2m / min, corona treatment is performed on the plastic wrap base film. After treatment, it is immersed in chitosan coating solution (Example 5) for 2 minutes, lifting speed 20mm / s, temperature is raised to 55℃, and dried for 1.5 hours to obtain biodegradable and environmentally friendly polylactic acid apple plastic wrap.

[0024] Example 9: Preparation of biodegradable and environmentally friendly polylactic acid apple preservation film S1: Add 1 kg of chitosan to 50 kg of 1 wt% acetic acid aqueous solution, stir at 400 rpm for 20 min to obtain chitosan acetic acid solution. S2: Add 240g of modified zeolite (Example 3) and 60g of β-cyclodextrin to 20kg of chitosan acetate solution, heat to 50℃, stir at 200rpm for 60min, add 30g of tea polyphenols, stir at 500rpm for 20min to obtain premixed solution. S3: Add sodium hydroxide solution to the premixed solution, adjust the pH to 8.4-8.6, filter, wash, heat to 60℃, vacuum dry for 12h, grind and pulverize, pass through a 200-mesh sieve to obtain composite loaded particles; S4: Dry 1000g of polylactic acid under vacuum at 85℃ for 10h, dry 200g of composite loaded particles and 200g of chitosan under vacuum at 55℃ for 26h, transfer to a high-speed mixer, add 200g of polybutylene terephthalate-adipate, mix at 500rpm for 20min, transfer to a twin-screw extruder, extrude at 160℃ and screw speed of 100rpm, melt blend, extrude, cool, and pelletize to obtain composite masterbatch; S5: Transfer the composite masterbatch into a single-screw blown film machine, blown film temperature 185℃, traction speed 3m / min, blow-up ratio 3, blow molding, cooling, and winding to obtain the cling film base film; S6: Power 200W, processing speed 1m / min, corona treatment is performed on the base film of the plastic wrap. After the treatment is completed, it is immersed in chitosan coating solution (Example 6) for 3 minutes, the lifting speed is 10mm / s, the temperature is raised to 60℃, and it is dried for 1 hour to obtain biodegradable and environmentally friendly polylactic acid apple plastic wrap.

[0025] Comparative Example 1: Compared with Example 7, this comparative example did not add polybutylene terephthalate in step S4, and the remaining steps and parameters were the same. This comparative example will not be repeated here. Finally, a biodegradable and environmentally friendly polylactic acid apple preservation film was obtained.

[0026] Comparative Example 2: Compared with Example 7, this comparative example did not add β-cyclodextrin in step S2, and the remaining steps and parameters were the same. This comparative example will not be repeated here. Finally, a biodegradable and environmentally friendly polylactic acid apple preservation film was obtained.

[0027] Comparative Example 3: Compared with Example 7, this comparative example did not add tea polyphenols in step S2, and the remaining steps and parameters were the same. This comparative example will not be repeated here. Finally, a biodegradable and environmentally friendly polylactic acid apple preservation film was obtained.

[0028] Comparative Example 4: Compared with Example 7, this comparative example did not undergo corona treatment in step S6, and all other steps and parameters were the same. This comparative example will not be repeated here. The final result is a biodegradable and environmentally friendly polylactic acid apple preservation film.

[0029] Performance testing: Tensile strength and elongation at break tests: 1. Referring to the test standard GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", the plastic wrap obtained in Examples 7-9 and Comparative Examples 1-4 was cut into strips with a width of 15 mm and a length of not less than 150 mm. Two parallel lines with a spacing of 50 mm were marked in the middle of the sample. Five parallel samples of each type were taken and placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±5% for no less than 4 hours to adjust the condition. 2. Clamp both ends of the specimen in the pneumatic fixtures of the electronic tensile testing machine, align the long axis of the specimen with the axis of the testing machine, and set the initial spacing of the fixtures to 100 mm. Perform tensile testing at a test speed of 50 mm / min until the specimen breaks. 3. After the test, record the tensile strength and elongation at break of each specimen, and calculate the arithmetic mean of the five specimens. The tensile strength is expressed in MPa and the elongation at break is expressed in %.

[0030] Table 1. Tensile strength and elongation at break test results of the examples and comparative examples Example 7 32.5 185 Example 8 35.8 210 Example 9 33.2 195 Comparative Example 1 28.6 45 Comparative Example 2 31.8 178 Comparative Example 3 32.1 182 Comparative Example 4 31.5 180 Oxygen permeability test 1. Referring to the test standard GB / T 1038-2000 "Test method for gas permeability of plastic films and sheets - differential pressure method", three undamaged and creased circular samples were randomly cut from the surface of the plastic wrap obtained in Examples 7-9 and Comparative Examples 1-4. The samples were placed in a desiccator at a temperature of 23±2℃ for 72 hours to adjust their condition. 2. Measure the thickness of each sample, evenly measure 5 points, and take the average value; evenly apply a layer of vacuum grease to the surface of the test chamber of the differential pressure gas permeation instrument, place the sample between the upper and lower test chambers respectively, press and seal tightly, and there should be no air bubbles; evacuate the entire system until the pressure in the low-pressure chamber is ≤10Pa, close the lower chamber, fill the high-pressure chamber with oxygen, maintain the pressure in the high-pressure chamber at 0.1MPa, and test at 23℃; 3. During the test, the instrument automatically records the pressure change in the low-pressure chamber. After the test, it automatically calculates the oxygen permeability and takes the arithmetic mean of three samples, in cm. 3 / (m 2 ·24h·0.1MPa).

[0031] Table 2. Oxygen permeability test results of the examples and comparative examples Example 7 680 Example 8 620 Example 9 710 Comparative Example 1 1520 Comparative Example 2 665 Comparative Example 3 635 Comparative Example 4 640 Water vapor transmission rate test 1. Referring to the test standard GB / T 21529-2008 "Determination of Water Vapor Transmission Rate of Plastic Films and Sheets by Electrolytic Sensor Method", three circular samples were randomly cut from the surface of the plastic wrap obtained in Examples 7-9 and Comparative Examples 1-4. Three samples of each type were placed in an environment with a temperature of 23±2℃ and a relative humidity of 50±5% for no less than 4 hours. 2. Place the porous disk filled with distilled water into the wet chamber of the permeation chamber to create a 100% relative humidity environment; clamp the sample between the dry and wet chambers of the permeation chamber, close and seal the permeation chamber; adjust the reversing valve to the position where the carrier gas bypasses the electrolytic cell and flows directly to the atmosphere to prevent moisture from entering the electrolytic cell during clamping; apply DC voltage to the electrolytic cell, and after about 30 minutes, adjust the reversing valve to the test position where the carrier gas passes through the electrolytic cell, and test the temperature at 38±2℃; 3. Measure the change in electrolytic current at regular time intervals. When the fluctuation range of three consecutive current sampling values ​​is no greater than 5%, it is considered that water vapor permeation has reached a steady state. Record the current value and calculate the water vapor transmission rate. Take the arithmetic mean of three samples, with the unit being g / (m³). 2 •24h).

[0032] Table 3. Water vapor transmission rate test results of the examples and comparative examples Example 7 42.5 Example 8 38.6 Example 9 44.8 Comparative Example 1 95.2 Comparative Example 2 40.8 Comparative Example 3 39.5 Comparative Example 4 41.2 Data Analysis: As can be seen from Tables 1-3, the biodegradable and environmentally friendly polylactic acid apple preservation film prepared by this invention has significant advantages in terms of mechanical properties, gas barrier properties, and water vapor barrier properties, and its overall performance is significantly better than that of the comparative examples.

[0033] In Comparative Example 1, due to the removal of polybutylene terephthalate (PET), the tensile strength and elongation at break decreased, while the oxygen permeability and water vapor permeability soared. This was because the pure PLA melt strength was extremely low, and during the blown film process, bubble rupture and micropore defects occurred, making it impossible to form a complete and dense film structure, resulting in a comprehensive deterioration of mechanical properties and barrier properties.

[0034] In Comparative Example 2, the tensile strength, elongation at break, oxygen permeability, and water vapor permeability did not change significantly compared to Example 8 due to the removal of β-cyclodextrin. This is because the amount of β-cyclodextrin added was only 0.3%-0.5% of the chitosan acetate solution, which did not have a substantial impact on the matrix mechanical properties and barrier structure of the film.

[0035] Comparative Example 3, due to the omission of tea polyphenols, showed no significant changes in various mechanical properties and barrier properties compared to Example 8. This is because tea polyphenols, as a small molecule antibacterial agent, were added at only 0.15%-0.25% of the chitosan acetate solution, and had no significant impact on the mechanical properties and barrier structure of the PLA / polybutylene terephthalate / chitosan blend system.

[0036] Comparative Example 4, due to the elimination of corona treatment, exhibited mechanical properties and barrier properties that were essentially the same as those of Example 8. This is because corona treatment only affects the hydrophilicity of the base film surface and does not alter the mechanical properties and barrier structure of the film substrate.

[0037] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

[0038] This invention is intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film, characterized in that, Includes the following steps: Step S1: Add chitosan to a 1-2 wt% aqueous acetic acid solution, stir at 200-400 rpm for 20-30 minutes to obtain a chitosan acetic acid solution; Step S2: Add the modified zeolite and β-cyclodextrin to the chitosan acetate solution, heat to 40-50℃, stir at 200-400 rpm for 30-60 min, add tea polyphenols, stir at 300-500 rpm for 20-30 min to obtain the premix. Step S3: Add sodium hydroxide solution to the premixed solution, adjust the pH to 8-9, filter, wash, heat to 50-60℃, vacuum dry for 12-24h, grind and pulverize, pass through a 200-mesh sieve to obtain composite loaded particles; Step S4: Vacuum dry polylactic acid at 75-85℃ for 10-14h, vacuum dry composite loaded particles and chitosan at 55-65℃ for 22-26h, transfer the dried polylactic acid, composite loaded particles and chitosan into a high-speed mixer, add polybutylene terephthalate-adipate, mix at 500-1000rpm for 10-20min, transfer into a twin-screw extruder, extrude at 160-180℃, screw speed 50-100rpm, melt blend, extrude, cool, and pelletize to obtain composite masterbatch; Step S5: Transfer the composite masterbatch into a single-screw blown film machine, blown film temperature 165-185℃, traction speed 3-6m / min, blow-up ratio 2-3, blow molding, cooling, and winding to obtain the cling film base film. Step S6: After corona treatment, the plastic wrap base film is immersed in chitosan coating solution for 1-3 minutes, pulled up at a speed of 10-30 mm / s, heated to 50-60℃, and dried for 1-2 hours to obtain biodegradable and environmentally friendly polylactic acid apple plastic wrap.

2. The method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film according to claim 1, characterized in that, The preparation steps of the modified zeolite in step S2 are as follows: Natural zeolite was added to an 8-12 wt% hydrochloric acid solution, heated to 60-80℃, stirred at 100-200 rpm for 4-6 hours, filtered, washed, heated to 100-120℃, dried for 2-4 hours, transferred to a muffle furnace, heated to 300-400℃, calcined for 3-5 hours, and cooled to room temperature to obtain modified zeolite. The mass ratio of the natural zeolite to the 8-12 wt% hydrochloric acid solution is 1:8-12.

3. The method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film according to claim 1, characterized in that, The mass ratio of chitosan to 1-2wt% acetic acid aqueous solution in step S1 is 1:40-50; The mass ratio of chitosan acetate solution, modified zeolite, β-cyclodextrin and tea polyphenols in step S2 is 100:1-1.2:0.3-0.5:0.15-0.

25.

4. The method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film according to claim 1, characterized in that, In step S4, the mass ratio of polylactic acid, composite supported particles, polybutylene terephthalate (PET) adipate, and chitosan is 1:0.1-0.2:0.2-0.3:0.1-0.

2.

5. The method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film according to claim 1, characterized in that, The power of the corona treatment in step S6 is 100-200W, and the processing speed is 1-3m / min.

6. The method for preparing a biodegradable and environmentally friendly polylactic acid apple preservation film according to claim 1, characterized in that, The preparation steps of the chitosan coating solution in step S6 are as follows: Add chitosan to a 1-2 wt% aqueous acetic acid solution, add glycerol, stir at 200-400 rpm for 20-30 minutes to obtain a chitosan coating solution; The mass ratio of chitosan, aqueous acetic acid solution and glycerol is 1:20-30:0.4-0.

8.

7. A biodegradable and environmentally friendly polylactic acid apple preservation film prepared by the preparation method according to any one of claims 1-6.

8. The application of the biodegradable and environmentally friendly polylactic acid apple preservation film according to claim 7 in apple preservation.

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