High-barrier degradable vegetable and fruit preservative film based on multilayer compounding and preparation method thereof

Abstract

The invention discloses a high-barrier degradable vegetable and fruit preservative film based on multi-layer compounding and a preparation method thereof, belongs to the technical field of food packaging materials, and aims to solve the technical problem that the barrier property, the mechanical property and the antibacterial property of the degradable vegetable and fruit preservative film in the prior art need to be further improved. The degradable vegetable and fruit preservative film comprises a first layer, a second layer and a third layer which are sequentially laminated, and the first layer is prepared from, by weight, 65-75 parts of polylactic acid, 4-8 parts of maleic anhydride grafted polylactic acid, 2-4 parts of modified carbon nitride, 1-3 parts of alizarin microcapsules, 3-6 parts of modified lignin and 1-2 parts of auxiliary additives; through the synergistic effect of the modified carbon nitride, the alizarin microcapsules and the modified lignin, the barrier property, the mechanical property and the antibacterial property of the degradable vegetable and fruit preservative film are further improved.

Description

Technical Field

[0001] This invention relates to the field of food packaging materials technology, specifically to a high-barrier biodegradable fruit and vegetable preservation film based on multilayer composite and its preparation method. Background Technology

[0002] With the increasing awareness of environmental protection and the promotion of plastic ban policies, the environmental pollution caused by traditional non-degradable plastic cling film has become increasingly prominent. Degradable fruit and vegetable cling film has become the core direction of industry development. At present, most of the degradable cling films on the market are based on polylactic acid, polybutylene adipate, etc., but the blending of single raw materials and additives is difficult to meet the comprehensive performance requirements of fruit and vegetable preservation.

[0003] First, the molecular structure of pure biodegradable polymers is relatively loose, and their ability to block gases such as oxygen and carbon dioxide as well as water vapor is weak. This leads to increased respiration of fruits and vegetables, rapid loss of water, and a significant shortening of the shelf life. Adding nanofillers can improve the barrier properties, but ordinary nanoparticles are prone to agglomeration and have poor compatibility with the matrix interface. Not only can they not form a dense barrier network, but they may also damage the mechanical properties of the material.

[0004] Secondly, although polylactic acid substrates have excellent degradation performance, they are brittle, have low elongation at break, and are easily damaged. Flexible substrates such as polybutylene adipate have insufficient strength, and in multilayer composite structures, peeling often occurs due to loose interlayer bonding, affecting ease of use.

[0005] Furthermore, fruits and vegetables are susceptible to contamination by microorganisms such as Escherichia coli and Staphylococcus aureus during the preservation process. Most biodegradable plastic wraps lack antibacterial design, and some products with added natural antibacterial agents are prone to loss or decomposition during preparation or use because the antibacterial components lack a stable coating structure, resulting in short-lived and unstable antibacterial effects.

[0006] To address this technical deficiency, a solution is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a high-barrier biodegradable vegetable and fruit preservation film based on multilayer composite and its preparation method, in order to solve the technical problem that the barrier performance, mechanical properties and antibacterial properties of biodegradable vegetable and fruit preservation films in the prior art need to be further improved.

[0008] The objective of this invention can be achieved through the following technical solution: a high-barrier biodegradable vegetable and fruit preservation film based on multi-layer composite, wherein the biodegradable vegetable and fruit preservation film comprises a first layer, a second layer and a third layer stacked sequentially;

[0009] The first layer comprises the following components by weight: 65-75 parts polylactic acid, 4-8 parts maleic anhydride-grafted polylactic acid, 2-4 parts modified carbon nitride, 1-3 parts alizarin microcapsules, 3-6 parts modified lignin and 1-2 parts auxiliary additives.

[0010] The second layer comprises the following components by weight: 24-28 parts thermoplastic starch, 69-72 parts vinyl alcohol and 1-2 parts auxiliary additives;

[0011] The third layer comprises the following components by weight: 46-50 parts polybutylene adipate, 50-54 parts butylene terephthalate, and 1-2 parts auxiliary additives.

[0012] Furthermore, the modified carbon nitride is prepared by the following steps:

[0013] A1. Add carbon nitride to an ethanol-water solution, ultrasonically disperse for 30-50 min, add sodium hydroxide solution dropwise to adjust the pH of the solution to 11-12, heat to 50-60℃ and stir for 2-4 h, and then perform post-treatment to obtain activated carbon nitride.

[0014] A2. Add cellulose acetate to a reaction vessel containing glacial acetic acid, ultrasonically disperse for 1-2 hours, then add activated carbon nitride, ultrasonically treat for another 2-4 hours, and then perform post-treatment to obtain modified carbon nitride.

[0015] Reaction mechanism:

[0016] Carbon nitride is uniformly dispersed in an ethanol-water solution due to the hydrophobicity of ethanol. The pH is adjusted to a strongly alkaline environment using sodium hydroxide solution. Hydroxide ions, with their strong nucleophilicity, attack the weak CN covalent bonds in defective regions on the carbon nitride surface, causing partial ring structure breakage and generating an active intermediate. This intermediate undergoes hydrolysis with water in the system, introducing stable hydroxyl groups onto the carbon nitride surface. Subsequently, glacial acetic acid, as a good solvent, helps cellulose acetate molecular chains unfold. The hydroxyl groups on the activated carbon nitride surface tightly bind with the hydroxyl and acetoxy groups on the cellulose acetate molecular chains through hydrogen bonding, resulting in a uniform coating of cellulose acetate molecular chains on the activated carbon nitride surface. This yields cellulose acetate-coated modified carbon nitride with both good dispersibility and interfacial compatibility.

[0017] Further, in step A1, the ratio of carbon nitride to ethanol aqueous solution is 1g:80mL, the mass fraction of ethanol aqueous solution is 70%, and the mass fraction of sodium hydroxide solution is 5%. The post-treatment operation includes: after the reaction is completed, filtering, washing the filter cake with deionized water 3-5 times, transferring it to a vacuum drying oven, and drying it at 80℃ to constant weight to obtain activated carbon nitride. In step A2, the ratio of cellulose acetate, glacial acetic acid, and activated carbon nitride is 3g:40mL:0.15g. The post-treatment operation includes: filtering, washing the filter cake with deionized water 3-5 times, transferring it to a vacuum drying oven, and drying it at 80℃ for 24h to obtain modified carbon nitride.

[0018] Furthermore, the alizarin microcapsules are prepared by the following steps:

[0019] B1. Add madder powder to anhydrous ethanol, reflux at 80℃ for 1-2 hours, seal and soak for 24 hours, filter, centrifuge the filtrate, take the supernatant, dialyze in deionized water for 24 hours using a dialysis bag with a molecular weight cutoff of 500 Da, then transfer the dialysate to a rotary evaporator, concentrate under reduced pressure at 60℃ to 1 / 3 of the original volume, and freeze-dry at -40℃ for 24 hours to obtain madder extract;

[0020] Reaction mechanism:

[0021] Using ethanol as a polar extraction solvent, the target active ingredients containing polar groups such as hydroxyl and carbonyl groups in madder powder are fully dissolved into the ethanol phase through the principle of "like dissolves like" and hydrogen bonding. Subsequent high-temperature reaction further disrupts the cell wall and cell membrane structure of madder cells, promoting the efficient release of intracellular active ingredients. At the same time, it triggers the degradation or configurational transformation of some macromolecular active ingredients, improving their solubility in the solvent. Subsequently, centrifugation is used to remove undissolved plant residues, fibers and other insoluble impurities. Then, using a semi-permeable membrane, distillation, dialysis and sieving are used to remove excess ethanol and small molecule salts and other water-soluble impurities from the system. Freeze-drying removes water through low-temperature sublimation, finally obtaining madder extract.

[0022] B2. Add the alizarin solution to the sodium alginate solution and stir at 20-30℃ for 30-60 minutes. Then add calcium chloride solution dropwise. After the addition is complete, continue stirring for 20-30 minutes. Filter the solution and wash the filter cake with deionized water 3-5 times. Transfer the filter cake to a vacuum drying oven and dry at 60℃ for 5-6 hours to obtain alizarin microcapsules.

[0023] Reaction mechanism:

[0024] When the carboxyl groups on the water-soluble sodium alginate molecular chain are mixed with the alizarin solution, they initially disperse and encapsulate the hydrophobic alizarin molecules through hydrophobic interactions and hydrogen bonds, forming a uniformly dispersed mixed system. After the addition of calcium chloride, the calcium ions act as cross-linking agents and specifically coordinate with the carboxyl groups of sodium alginate to form a three-dimensional network hydrogel structure. This transforms sodium alginate from a water-soluble polymer into insoluble gel microspheres, thereby stably encapsulating alizarin within the gel network, ultimately forming microcapsules of alizarin encapsulated in sodium alginate gel.

[0025] Furthermore, in step B1, the ratio of the amount of madder powder to anhydrous ethanol is 1g:20mL; in step B2, the ratio of the amount of the alizarin solution, sodium alginate solution, and calcium chloride solution is 40mL:100mL:14-17mL, wherein the alizarin solution is obtained by dissolving 0.2g of alizarin in 40mL of 30wt% ethanol solution, the sodium alginate solution has a mass fraction of 2%, and the calcium chloride solution has a mass fraction of 1.8%.

[0026] Furthermore, the modified lignin is prepared by the following steps:

[0027] C1. Add alkali-degraded lignin and levulinic acid to a reaction vessel, stir at 50-60℃ for 20-30 min, add trifluoroacetic anhydride, continue stirring for 1-2 h, cool to room temperature, add anhydrous ethanol, stir for 1-2 h, and then perform post-treatment to obtain esterified lignin.

[0028] C2. Esterified lignin and N,N-dimethylacetamide are added to a reaction vessel, heated to 50-60℃ and stirred for 20-30 min. Then, butylene adipate and chloroform are added, and stirring is continued at 50-60℃ for 1-2 h. After post-treatment, modified lignin is obtained.

[0029] Reaction mechanism:

[0030] After the phenolic and alcoholic hydroxyl groups on the alkali-free lignin molecular chain are fully contacted with levulinic acid, trifluoroacetic anhydride is used as an acylation activator to activate the carboxyl groups in the levulinic acid molecule, forming a highly reactive acyl intermediate. This intermediate then undergoes an esterification reaction with the hydroxyl groups of the alkali-free lignin, grafting hydrophobic levulinic acid ester groups onto the lignin molecular chain to achieve esterification modification of lignin and reduce its hydrophilicity. Anhydrous ethanol is then added to promote precipitation. After post-treatment, esterified lignin is obtained. Subsequently, N,N-dimethylacetamide is used as a polar solvent to dissolve the esterified lignin, and chloroform is used as a co-solvent to optimize the compatibility between butylene adipate and esterified lignin. The ester groups of butylene adipate and the residual hydroxyl groups of esterified lignin undergo physical blending and weak chemical interactions through hydrogen bonding and van der Waals forces, allowing the esterified lignin to be uniformly dispersed in the butylene adipate matrix. After post-treatment, modified lignin is obtained.

[0031] Further, in step C1, the ratio of the alkali-degraded lignin, acetylpropionic acid, trifluoroacetic anhydride, and anhydrous ethanol is 1g:10g:5mL:100mL; the post-treatment operation includes: after the reaction, centrifugation, washing the product with anhydrous ethanol 3-4 times, and freeze-drying at -20℃ for 12h to obtain esterified lignin; in step C2, the ratio of the esterified lignin, N,N-dimethylacetamide, butylene adipate, and chloroform is 0.5g:150mL:9.5g:100mL; the post-treatment operation includes: after the reaction, transferring to a petri dish, letting it stand for 15h, and then transferring it to a drying oven and drying at 60℃ for 24h to obtain modified lignin.

[0032] Furthermore, the preparation method of the biodegradable fruit and vegetable preservation film includes the following steps:

[0033] S1. Polylactic acid, maleic anhydride-grafted polylactic acid, modified carbon nitride, alizarin microcapsules, modified lignin and auxiliary additives are respectively used as the first layer material, thermoplastic starch, ethylene alcohol and auxiliary additives are used as the second layer material, and polybutylene adipate, butylene terephthalate and auxiliary additives are used as the third layer material. They are added to three twin-screw extruders and melt-blended for 8-10 minutes. Then, they are melt-extruded into a three-layer co-extrusion blown film extrusion blown film to obtain a three-layer co-extruded film.

[0034] S2. The three-layer co-extruded film is thermally stretched 3-4 times along its longitudinal direction at a temperature of 70-80℃. Then, compressed air is blown in, and after cooling, the film is guided to the winding machine through the idle guide roller by the film rotation traction device to obtain a biodegradable fruit and vegetable preservation film.

[0035] Further, in step S1, the thickness ratio of the first layer, the second layer, and the third layer is 2:4:4. The temperatures of the six temperature zones of the twin-screw extruder for the first layer material, from the feed end towards the die head, are sequentially set to 150℃, 155℃, 160℃, 165℃, 165℃, and 160℃; the temperatures of the six temperature zones of the twin-screw extruder for the second layer material, from the feed end towards the die head, are sequentially set to 165℃, 170℃, 175℃, 175℃, 175℃, and 170℃; the temperatures of the six temperature zones of the twin-screw extruder for the third layer material, from the feed end towards the die head, are sequentially set to 110℃, 115℃, 120℃, 120℃, 120℃, and 115℃; in step S2, the temperature of the compressed air is 20-30℃, and the wind speed is 1.0-1.5m / s.

[0036] The present invention has the following beneficial effects:

[0037] 1. The modified carbon nitride of the present invention, after being coated with cellulose acetate, has excellent dispersibility and interfacial compatibility, and can form a dense physical barrier network inside the membrane, effectively blocking gas permeation channels. The modified lignin enhances hydrophobicity through esterification modification, which can inhibit the migration and permeation of water vapor in the membrane. The alizarin microcapsules are uniformly dispersed in the form of particles, further filling the tiny gaps in the membrane and optimizing the dense structure of the membrane. Under the synergistic effect of the three, the path of gas and water vapor to penetrate the membrane is greatly blocked, which strengthens the barrier ability of the preservation film against gas and water vapor from different dimensions and significantly improves the overall barrier performance.

[0038] 2. The modified carbon nitride of this invention, as a nano-reinforcing filler, is tightly bonded to the membrane matrix through hydrogen bonds, providing rigid support for the material and helping to improve tensile strength. The modified lignin, after multi-stage modification, has both good compatibility and flexibility. Its molecular chain segments can alleviate stress concentration under stress on the membrane and improve elongation at break. The sodium alginate gel coating structure of the alizarin microcapsules can form a stable interface bond with the matrix, avoiding local peeling under stress and further optimizing mechanical transfer efficiency. The three complement each other, enhancing the tensile strength of the membrane through rigid fillers and improving toughness through flexible components and interface optimization, making the mechanical properties of the food preservation film more balanced and reliable.

[0039] 3. The alizarin microcapsules of this invention, as the core antibacterial component, continuously release alizarin active ingredients through a sustained-release mechanism, which can directly inhibit Escherichia coli and Staphylococcus aureus. The dense membrane surface constructed by modified carbon nitride reduces the adsorption sites and attachment space of bacteria, hindering bacterial reproduction. The hydrophobic properties of modified lignin reduce the wettability of the membrane surface, destroying the suitable living environment for bacteria and indirectly inhibiting bacterial growth. The three work synergistically, directly killing bacteria through antibacterial components and indirectly inhibiting bacterial growth through structural optimization and environmental regulation, significantly improving the comprehensiveness and durability of the antibacterial effect of the preservation film, and showing excellent inhibitory ability against both target bacteria. Detailed Implementation

[0040] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] In this application, polylactic acid is selected from Dongguan Yingxiang Plastic Raw Materials Co., Ltd., and the model number is 6202D.

[0042] In this application, the maleic anhydride-grafted polylactic acid is selected from Dongguan Mingyuan Plastics Co., Ltd., and its brand name is PBAT maleic anhydride grafted.

[0043] In this application, polybutylene adipate is selected from Shandong Huling New Materials Co., Ltd., and the model number is 1006.

[0044] In this application, carbon nitride is selected from Hubei Yihua New Material Technology Co., Ltd., CAS No. 143334-20-7;

[0045] In this application, cellulose acetate is selected from Dongguan Kaiyuan Plastic Raw Materials Co., Ltd., and the model is CM155.

[0046] In this application, the madder powder is selected from Sanyuan Longsheng Biotechnology Co., Ltd., and the product mesh size is 80 mesh.

[0047] In this application, the alkali-degraded lignin is selected from Wuhan Kemike Biomedical Technology Co., Ltd., with product code 962-476-2 and model number Cl31439.

[0048] Example 1

[0049] This embodiment provides a method for preparing a high-barrier, biodegradable fruit and vegetable preservation film based on multilayer composites, including the following steps:

[0050] S1. Preparation of modified carbon nitride

[0051] Weigh 10g of carbon nitride and add it to 800mL of 70wt% ethanol aqueous solution. Disperse it by sonication for 30min. Add 5wt% sodium hydroxide solution dropwise to adjust the pH of the solution to 11. Heat the solution to 50℃ and stir for 2h. After the reaction is complete, filter the solution. Wash the filter cake three times with deionized water and transfer it to a vacuum drying oven. Dry it at 80℃ to constant weight to obtain activated carbon nitride.

[0052] Weigh out 30g of cellulose acetate and add it to a reaction vessel containing 400mL of glacial acetic acid. After ultrasonic dispersion for 1h, add 1.5g of activated carbon nitride and ultrasonically treat for another 2h. Filter the mixture and wash the filter cake three times with deionized water. Transfer the cake to a vacuum drying oven and dry it at 80℃ for 24h to obtain modified carbon nitride.

[0053] S2. Preparation of alizarin microcapsules

[0054] Weigh 100g of madder powder and add it to 2L of anhydrous ethanol. Reflux at 80℃ for 1 hour, then seal and soak for 24 hours. Filter the solution and centrifuge the filtrate. Take the supernatant and dialyze it in deionized water for 24 hours using a dialysis bag with a molecular weight cutoff of 500 Da. Transfer the dialysate to a rotary evaporator and concentrate it under reduced pressure at 60℃ to 1 / 3 of the original volume. Then freeze-dry it at -40℃ for 24 hours to obtain madder extract.

[0055] Weigh out 2g of alizarin and dissolve it in 400mL of 30wt% ethanol solution to obtain alizarin solution;

[0056] Weigh 40 mL of alizarin solution and add it to 100 mL of 2 wt% sodium alginate solution. Stir at 20 °C for 30 min, then add 14 mL of 1.8 wt% calcium chloride solution dropwise. After the addition is complete, continue stirring for 20 min. Filter the solution, wash the filter cake three times with deionized water, transfer it to a vacuum drying oven, and dry it at 60 °C for 5 h to obtain alizarin microcapsules.

[0057] S3, Preparation of modified lignin

[0058] Weigh out 10g of dealkalized lignin and 100g of levulinic acid and add them to the reaction vessel. Stir at 50℃ for 20min, then add 50mL of trifluoroacetic anhydride and continue stirring for 1h. Cool to room temperature, then add 1L of anhydrous ethanol and stir for 1h. After the reaction is complete, centrifuge to separate the product. Wash the product three times with anhydrous ethanol and freeze-dry at -20℃ for 12h to obtain esterified lignin.

[0059] Weigh out 5g of esterified lignin and 1.5L of N,N-dimethylacetamide and add them to the reaction vessel. Heat the mixture to 50℃ and stir for 20min. Then add 95g of butylene adipate and 1L of chloroform. Continue stirring at 50℃ for 1h. After the reaction is complete, transfer the mixture to a petri dish and let it stand for 15h. Then transfer it to a drying oven and dry it at 60℃ for 24h to obtain modified lignin.

[0060] S4. Prepare biodegradable fruit and vegetable preservation film.

[0061] Weigh out the following components by weight: 65 parts polylactic acid, 4 parts maleic anhydride-grafted polylactic acid, 2 parts modified carbon nitride, 1 part alizarin microcapsules, 3 parts modified lignin, and 1 part auxiliary additive as the first layer material; 24 parts thermoplastic starch, 69 parts vinyl alcohol, and 1 auxiliary additive as the second layer material; and 46 parts polybutylene adipate, 50 parts butylene terephthalate, and 1 auxiliary additive as the third layer material. Add these components to three twin-screw extruders. For the first layer material, set the temperature of the six temperature zones in the twin-screw extruder from the feed end towards the die head to 150℃, 155℃, and so on. The temperatures of the six temperature zones of the twin-screw extruder for the second layer of material, set from the feed end towards the die head, are 165℃, 170℃, 175℃, 175℃, 175℃, and 170℃ respectively. The temperatures of the six temperature zones of the twin-screw extruder for the third layer of material, set from the feed end towards the die head, are 110℃, 115℃, 120℃, 120℃, 120℃, and 115℃ respectively. The materials are melt-blended for 8 minutes each, and then melt-extruded into a three-layer co-extrusion blown film mill to obtain a three-layer co-extruded film with a thickness ratio of 2:4:4.

[0062] The three-layer co-extruded film is thermally stretched three times along its longitudinal direction at a temperature of 70℃. Then, compressed air is blown in at a wind speed of 1.0m / s. After cooling, the film is guided to the winding machine via a film rotation traction device through an idle guide roller to obtain a biodegradable fruit and vegetable preservation film.

[0063] Example 2

[0064] This embodiment provides a method for preparing a high-barrier, biodegradable fruit and vegetable preservation film based on multilayer composites, including the following steps:

[0065] S1. Preparation of modified carbon nitride

[0066] Weigh 10g of carbon nitride and add it to 800mL of 70wt% ethanol aqueous solution. Disperse it by sonication for 40min. Add 5wt% sodium hydroxide solution dropwise to adjust the pH of the solution to 11.5. Heat the solution to 55℃ and stir for 3h. After the reaction is complete, filter the solution. Wash the filter cake with deionized water 4 times and transfer it to a vacuum drying oven. Dry it at 80℃ to constant weight to obtain activated carbon nitride.

[0067] Weigh 30g of cellulose acetate and add it to a reaction vessel containing 400mL of glacial acetic acid. After ultrasonic dispersion for 1.5h, add 1.5g of activated carbon nitride and ultrasonic treatment for another 3h. Filter the mixture and wash the filter cake 4 times with deionized water. Transfer the cake to a vacuum drying oven and dry it at 80℃ for 24h to obtain modified carbon nitride.

[0068] S2. Preparation of alizarin microcapsules

[0069] Weigh 100g of madder powder and add it to 2L of anhydrous ethanol. Reflux at 80℃ for 1.5h, then seal and soak for 24h. Filter, add the filtrate to a centrifuge and centrifuge. Take the supernatant and dialyze it in deionized water for 24h using a dialysis bag with a molecular weight cutoff of 500Da. Then transfer the dialysate to a rotary evaporator and concentrate it under reduced pressure at 60℃ to 1 / 3 of the original volume. Finally, freeze-dry it at -40℃ for 24h to obtain madder extract.

[0070] Weigh out 2g of alizarin and dissolve it in 400mL of 30wt% ethanol solution to obtain alizarin solution;

[0071] Weigh 40 mL of alizarin solution and add it to 100 mL of 2 wt% sodium alginate solution. Stir at 25 °C for 45 min, then add 15 mL of 1.8 wt% calcium chloride solution dropwise. After the addition is complete, continue stirring for 25 min. Filter the solution, wash the filter cake four times with deionized water, transfer it to a vacuum drying oven, and dry it at 60 °C for 5.5 h to obtain alizarin microcapsules.

[0072] S3, Preparation of modified lignin

[0073] Weigh out 10g of dealkalized lignin and 100g of levulinic acid and add them to the reaction vessel. Stir at 55℃ for 25min, then add 50mL of trifluoroacetic anhydride and continue stirring for 1.5h. Cool to room temperature, then add 1L of anhydrous ethanol and stir for 1.5h. After the reaction is complete, centrifuge to separate the product. Wash the product three times with anhydrous ethanol and freeze-dry at -20℃ for 12h to obtain esterified lignin.

[0074] Weigh out 5g of esterified lignin and 1.5L of N,N-dimethylacetamide and add them to the reaction vessel. Heat the mixture to 55℃ and stir for 25min. Then add 95g of butylene adipate and 1L of chloroform. Continue stirring at 55℃ for 1.5h. After the reaction is complete, transfer the mixture to a petri dish and let it stand for 15h. Then transfer it to a drying oven and dry it at 60℃ for 24h to obtain modified lignin.

[0075] S4. Prepare biodegradable fruit and vegetable preservation film.

[0076] Weigh out the following components by weight: 70 parts polylactic acid, 6 parts maleic anhydride-grafted polylactic acid, 3 parts modified carbon nitride, 2 parts alizarin microcapsules, 4 parts modified lignin, and 1.5 parts auxiliary additives, as the first layer material; 26 parts thermoplastic starch, 70 parts ethylene alcohol, and 1.5 parts auxiliary additives, as the second layer material; and 48 parts polybutylene adipate, 52 parts butylene terephthalate, and 1.5 parts auxiliary additives, as the third layer material. Add these components to three twin-screw extruders. For the first layer material, the twin-screw extruders have six temperature zones set sequentially from the feed end towards the die head: 150℃, 15 ... The temperatures of the six temperature zones of the twin-screw extruder for the second layer of material, set from the feed end towards the die head, are 165℃, 170℃, 175℃, 175℃, 175℃, and 170℃ respectively. The temperatures of the six temperature zones of the twin-screw extruder for the third layer of material, set from the feed end towards the die head, are 110℃, 115℃, 120℃, 120℃, 120℃, and 115℃ respectively. The materials are melt-blended for 9 minutes each, and then melt-extruded into a three-layer co-extrusion blown film mill to obtain a three-layer co-extruded film with a thickness ratio of 2:4:4.

[0077] The three-layer co-extruded film is thermally stretched three times along its longitudinal direction at a temperature of 75°C. Then, compressed air is blown in at a wind speed of 1.2 m / s. After cooling, the film is guided to the winding machine via a film rotation traction device through an idle guide roller to obtain a biodegradable fruit and vegetable preservation film.

[0078] Example 3

[0079] This embodiment provides a method for preparing a high-barrier, biodegradable fruit and vegetable preservation film based on multilayer composites, including the following steps:

[0080] S1. Preparation of modified carbon nitride

[0081] Weigh 10g of carbon nitride and add it to 800mL of 70wt% ethanol aqueous solution. Disperse it by sonication for 50min. Add 5wt% sodium hydroxide solution dropwise to adjust the pH of the solution to 12. Heat the solution to 50-60℃ and stir for 4h. After the reaction is complete, filter the solution. Wash the filter cake with deionized water 5 times and transfer it to a vacuum drying oven. Dry it at 80℃ to constant weight to obtain activated carbon nitride.

[0082] Weigh 30g of cellulose acetate and add it to a reaction vessel containing 400mL of glacial acetic acid. After ultrasonic dispersion for 2 hours, add 1.5g of activated carbon nitride and ultrasonically treat for another 4 hours. Filter the mixture and wash the filter cake 5 times with deionized water. Transfer the cake to a vacuum drying oven and dry it at 80℃ for 24 hours to obtain modified carbon nitride.

[0083] S2. Preparation of alizarin microcapsules

[0084] Weigh 100g of madder powder and add it to 2L of anhydrous ethanol. Reflux at 80℃ for 2 hours, then seal and soak for 24 hours. Filter the solution and centrifuge the filtrate. Take the supernatant and dialyze it in deionized water for 24 hours using a dialysis bag with a molecular weight cutoff of 500 Da. Transfer the dialysate to a rotary evaporator and concentrate it under reduced pressure at 60℃ to 1 / 3 of the original volume. Then freeze-dry it at -40℃ for 24 hours to obtain madder extract.

[0085] Weigh out 2g of alizarin and dissolve it in 400mL of 30wt% ethanol solution to obtain alizarin solution;

[0086] Weigh 40 mL of alizarin solution and add it to 100 mL of 2 wt% sodium alginate solution. Stir at 30 °C for 60 min, then add 17 mL of 1.8 wt% calcium chloride solution dropwise. After the addition is complete, continue stirring for 30 min. Filter the solution, wash the filter cake 5 times with deionized water, transfer it to a vacuum drying oven, and dry it at 60 °C for 6 h to obtain alizarin microcapsules.

[0087] S3, Preparation of modified lignin

[0088] Weigh out 10g of dealkalized lignin and 100g of levulinic acid and add them to the reaction vessel. Stir at 60℃ for 30min, then add 50mL of trifluoroacetic anhydride and continue stirring for 2h. Cool to room temperature, then add 1L of anhydrous ethanol and stir for 2h. After the reaction is complete, centrifuge to separate the product. Wash the product 4 times with anhydrous ethanol and freeze-dry at -20℃ for 12h to obtain esterified lignin.

[0089] Weigh out 5g of esterified lignin and 1.5L of N,N-dimethylacetamide and add them to the reaction vessel. Heat the mixture to 60℃ and stir for 30min. Then add 95g of butylene adipate and 1L of chloroform. Continue stirring at 60℃ for 2h. After the reaction is complete, transfer the mixture to a petri dish and let it stand for 15h. Then transfer it to a drying oven and dry at 60℃ for 24h to obtain modified lignin.

[0090] S4. Prepare biodegradable fruit and vegetable preservation film.

[0091] Weigh out the following components by weight: 75 parts polylactic acid, 8 parts maleic anhydride-grafted polylactic acid, 4 parts modified carbon nitride, 3 parts alizarin microcapsules, 6 parts modified lignin, and 2 parts auxiliary additives as the first layer material; 28 parts thermoplastic starch, 72 parts ethylene glycol, and 2 auxiliary additives as the second layer material; and 50 parts polybutylene adipate, 54 parts butylene terephthalate, and 2 auxiliary additives as the third layer material. Add these components to three twin-screw extruders. For the first layer material, the twin-screw extruders have six temperature zones set sequentially from the feed end towards the die head: 150℃, 155℃, 1... The temperatures of the six temperature zones of the twin-screw extruder for the second layer of material, set from the feed end towards the die head, are 165℃, 170℃, 175℃, 175℃, 175℃, and 170℃ respectively. The temperatures of the six temperature zones of the twin-screw extruder for the third layer of material, set from the feed end towards the die head, are 110℃, 115℃, 120℃, 120℃, 120℃, and 115℃ respectively. The materials are melt-blended for 10 minutes each, and then melt-extruded into a three-layer co-extrusion blown film mill to obtain a three-layer co-extruded film with a thickness ratio of 2:4:4.

[0092] The three-layer co-extruded film is thermally stretched four times along its longitudinal direction at a temperature of 70-80℃. Then, compressed air is blown in at a wind speed of 1.5m / s. After cooling, the film is guided to the winding machine via a film rotation traction device through an idle guide roller to obtain a biodegradable fruit and vegetable preservation film.

[0093] Comparative Example 1

[0094] The difference between this comparative example and Example 3 is that step S1 is omitted, and the modified carbon nitride in step S4 is replaced with carbon nitride from step S1.

[0095] Comparative Example 2

[0096] The difference between this comparative example and Example 3 is that the alizarin in step S2 is used instead of the alizarin microcapsules in step S4.

[0097] Comparative Example 3

[0098] The difference between this comparative example and Example 3 is that step S3 is omitted, and the dealkalized lignin in step S3 is used instead of the modified lignin in step S4.

[0099] Performance testing:

[0100] The gas permeability of the biodegradable fruit and vegetable preservation films prepared in Examples 1-3 and Comparative Examples 1-3 was determined in accordance with the standard GB / T 1038.1-2022 "Test methods for gas permeability of plastic films and sheets - Part 1: Differential pressure method" to measure the gas barrier performance of the preservation films.

[0101] The water vapor transmission rate of the biodegradable fruit and vegetable preservation films prepared in Examples 1-3 and Comparative Examples 1-3 was determined according to the standard GB / T 26253-2010 "Determination of water vapor transmission rate of plastic films and sheets by infrared detector method" to measure the water vapor barrier performance of the preservation films.

[0102] The tensile strength and elongation at break of the biodegradable fruit and vegetable preservation films prepared in Examples 1-3 and Comparative Examples 1-3 were determined in accordance with the standard GB / T 1040.1-2025 "Determination of tensile properties of plastics - Part 1: General".

[0103] The antibacterial properties of the biodegradable fruit and vegetable preservation films prepared in Examples 1-3 and Comparative Examples 1-3 were determined according to standard GB / T 31402-2023 "Determination of antibacterial activity of plastics and other non-porous materials". The specific test results are shown in Table 1 below:

[0104] Table 1 - Performance Test Data of Samples

[0105]

[0106] Data Analysis:

[0107] Comparative analysis of the data in Table 1 above shows that the biodegradable fruit and vegetable preservation film prepared by this invention has a gas permeability of 7.5%, a water vapor permeability of 11.8%, a tensile strength of 16.7 MPa, an elongation at break of 160%, an antibacterial rate of 96.8% against Escherichia coli, and an antibacterial rate of 95.4% against Staphylococcus aureus.

[0108] In Comparative Example 1, the unmodified carbon nitride lacked the coating modification of cellulose acetate, resulting in poor dispersibility and weak interfacial bonding with the membrane matrix. This prevented the formation of a dense physical barrier network and effectively filled the membrane pores, leading to an increase in gas permeability and water vapor permeability to 9.5% and 14.4%, respectively. Furthermore, the unmodified carbon nitride struggled to bond tightly with the matrix via hydrogen bonds, failing to exert its nano-reinforcing effect. Aggregation also easily caused stress concentration, resulting in a decrease in tensile strength and elongation at break to 13.6 MPa and 130%, respectively. The modification state of the carbon nitride did not directly affect the antibacterial effect; only the insufficient membrane density slightly weakened the bacterial adhesion inhibition effect, leading to a decrease in the anti-Escherichia coli rate and anti-Staphylococcus aureus rate to 83.1% and 82.7%, respectively.

[0109] In Comparative Example 2, alizarin was not microencapsulated with sodium alginate gel, lacking a stable protective structure. It was easily lost or decomposed during film preparation and use, and its dispersion uniformity was poor, failing to achieve the sustained-release effect of the antibacterial component. This resulted in a decrease in the anti-Escherichia coli and anti-Staphylococcus aureus rates to 77.6% and 76.9%, respectively. Alizarin itself did not participate in the construction of the membrane's barrier structure or mechanical reinforcement. Due to slight changes in dispersion, it slightly affected the membrane's compactness, leading to an increase in gas permeability and water vapor permeability to 9.2% and 13.9%, respectively. The absence of the microcapsule structure only slightly affected the membrane's interfacial bonding uniformity, without destroying the core of the mechanical reinforcement system, resulting in a decrease in tensile strength and elongation at break to 15.0 MPa and 146%, respectively.

[0110] In Comparative Example 3, the alkali-reduced lignin, without esterification modification and subsequent composite treatment, lacked sufficient hydrophobicity and had poor compatibility with the membrane matrix. It could not effectively inhibit the migration and permeation of water vapor within the membrane, nor could it fully fill the tiny pores within the membrane. This resulted in an increase in water vapor permeability to 16.2% and gas permeability to 10.1%. Because the unmodified lignin lacked the good flexibility and interfacial bonding force after modification, it could not alleviate stress concentration under membrane stress and could not form a synergistic reinforcing effect with other components. This led to a decrease in tensile strength and elongation at break to 14.3 MPa and 138%, respectively. Due to the insufficient hydrophobicity of the unmodified lignin, it was difficult to disrupt the humid environment suitable for bacterial survival, indirectly weakening the antibacterial auxiliary effect. As a result, the decrease in the anti-Escherichia coli rate and the anti-Staphylococcus aureus rate was between that of Comparative Example 1 and Comparative Example 2.

[0111] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to specific implementations. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A high-barrier, biodegradable fruit and vegetable preservation film based on multi-layer composite, characterized in that, Biodegradable fruit and vegetable preservation film consists of a first layer, a second layer, and a third layer stacked in sequence; The first layer comprises the following components by weight: 65-75 parts polylactic acid, 4-8 parts maleic anhydride-grafted polylactic acid, 2-4 parts modified carbon nitride, 1-3 parts alizarin microcapsules, 3-6 parts modified lignin and 1-2 parts auxiliary additives. The second layer comprises the following components by weight: 24-28 parts thermoplastic starch, 69-72 parts vinyl alcohol and 1-2 parts auxiliary additives; The third layer comprises the following components by weight: 46-50 parts polybutylene adipate, 50-54 parts butylene terephthalate, and 1-2 parts auxiliary additives.

2. The high-barrier biodegradable fruit and vegetable preservation film based on multi-layer composite as described in claim 1, characterized in that, The modified carbon nitride is prepared by the following steps: A1. Add carbon nitride to an ethanol-water solution, ultrasonically disperse for 30-50 min, add sodium hydroxide solution dropwise to adjust the pH of the solution to 11-12, heat to 50-60℃ and stir for 2-4 h, and then perform post-treatment to obtain activated carbon nitride. A2. Add cellulose acetate to a reaction vessel containing glacial acetic acid, ultrasonically disperse for 1-2 hours, then add activated carbon nitride, ultrasonically treat for another 2-4 hours, and then perform post-treatment to obtain modified carbon nitride.

3. The high-barrier biodegradable fruit and vegetable preservation film based on multi-layer composite as described in claim 2, characterized in that, In step A1, the ratio of carbon nitride to ethanol aqueous solution is 1g:80mL, the mass fraction of ethanol aqueous solution is 70%, and the mass fraction of sodium hydroxide solution is 5%; in step A2, the ratio of cellulose acetate, glacial acetic acid, and activated carbon nitride is 3g:40mL:0.15g.

4. The high-barrier biodegradable fruit and vegetable preservation film based on multi-layer composite as described in claim 1, characterized in that, The alizarin microcapsules were prepared by the following steps: B1. Add madder powder to anhydrous ethanol, reflux at 80℃ for 1-2 hours, seal and soak for 24 hours, filter, centrifuge the filtrate, take the supernatant, dialyze in deionized water for 24 hours using a dialysis bag with a molecular weight cutoff of 500 Da, then transfer the dialysate to a rotary evaporator, concentrate under reduced pressure at 60℃ to 1 / 3 of the original volume, and freeze-dry at -40℃ for 24 hours to obtain madder extract; B2. Add the alizarin solution to the sodium alginate solution and stir at 20-30℃ for 30-60 minutes. Then add calcium chloride solution dropwise. After the addition is complete, continue stirring for 20-30 minutes. Filter the solution and wash the filter cake with deionized water 3-5 times. Transfer the filter cake to a vacuum drying oven and dry at 60℃ for 5-6 hours to obtain alizarin microcapsules.

5. The high-barrier biodegradable fruit and vegetable preservation film based on multi-layer composite as described in claim 4, characterized in that, In step B1, the ratio of the amount of madder powder to anhydrous ethanol is 1g:20mL; in step B2, the ratio of the amount of the alizarin solution, sodium alginate solution, and calcium chloride solution is 40mL:100mL:14-17mL, wherein the alizarin solution is obtained by dissolving 0.2g of alizarin in 40mL of 30wt% ethanol solution, the sodium alginate solution has a mass fraction of 2%, and the calcium chloride solution has a mass fraction of 1.8%.

6. The high-barrier biodegradable fruit and vegetable preservation film based on multi-layer composite as described in claim 1, characterized in that, The modified lignin is prepared by the following steps: C1. Add alkali-degraded lignin and levulinic acid to a reaction vessel, stir at 50-60℃ for 20-30 min, add trifluoroacetic anhydride, continue stirring for 1-2 h, cool to room temperature, add anhydrous ethanol, stir for 1-2 h, and then perform post-treatment to obtain esterified lignin. C2. Esterified lignin and N,N-dimethylacetamide are added to a reaction vessel, heated to 50-60℃ and stirred for 20-30 min. Then, butylene adipate and chloroform are added, and stirring is continued at 50-60℃ for 1-2 h. After post-treatment, modified lignin is obtained.

7. The high-barrier biodegradable fruit and vegetable preservation film based on multi-layer composite as described in claim 6, characterized in that, In step C1, the ratio of the amount of alkali-degraded lignin, acetopropionic acid, trifluoroacetic anhydride and anhydrous ethanol is 1g:10g:5mL:100mL; in step C2, the ratio of the amount of esterified lignin, N,N-dimethylacetamide, butylene adipate and chloroform is 0.5g:150mL:9.5g:100mL.

8. The method for preparing a high-barrier biodegradable fruit and vegetable preservation film based on multilayer composite according to any one of claims 1-7, characterized in that, The preparation method of the biodegradable fruit and vegetable preservation film includes the following steps: S1. Polylactic acid, maleic anhydride-grafted polylactic acid, modified carbon nitride, alizarin microcapsules, modified lignin and auxiliary additives are respectively used as the first layer material, thermoplastic starch, ethylene alcohol and auxiliary additives are used as the second layer material, and polybutylene adipate, butylene terephthalate and auxiliary additives are used as the third layer material. They are added to three twin-screw extruders and melt-blended for 8-10 minutes. Then, they are melt-extruded into a three-layer co-extrusion blown film extrusion blown film to obtain a three-layer co-extruded film. S2. The three-layer co-extruded film is thermally stretched 3-4 times along its longitudinal direction at a temperature of 70-80℃. Then, compressed air is blown in, and after cooling, the film is guided to the winding machine through the idle guide roller by the film rotation traction device to obtain a biodegradable fruit and vegetable preservation film.

9. The method for preparing a high-barrier biodegradable fruit and vegetable preservation film based on multilayer composite according to claim 8, characterized in that, In step S1, the thickness ratio of the first, second, and third layers is 2:4:

4. The temperatures of the six temperature zones of the twin-screw extruder for the first layer of material, from the feed end towards the die head, are sequentially set to 150℃, 155℃, 160℃, 165℃, 165℃, and 160℃. The temperatures of the six temperature zones of the twin-screw extruder for the second layer of material, from the feed end towards the die head, are sequentially set to 165℃, 170℃, 175℃, 175℃, 175℃, and 170℃. The temperatures of the six temperature zones of the twin-screw extruder for the third layer of material, from the feed end towards the die head, are sequentially set to 110℃, 115℃, 120℃, 120℃, 120℃, and 115℃. In step S2, the temperature of the compressed air is 20-30℃, and the wind speed is 1.0-1.5m / s.

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