Preparation method and application of PVA-PLA (Polyvinyl Acetate-Poly Lactic Acid) multi-layer high-barrier film based on metal-phenolic network enhancement

Abstract

The invention belongs to the technical field of biodegradable composite materials, and discloses a preparation method and application of a PVA-PLA (Polyvinyl Acetate-Poly Lactic Acid) multi-layer high-barrier film based on metal-phenolic network enhancement. The preparation method comprises the following steps: firstly, preparing a PVA (Polyvinyl Alcohol) composite solution containing lignin and a polyphenol compound, and carrying out film casting to obtain a base layer film; then preparing a PLA coating solution containing Femetal ions; and finally, immersing the base layer film into the PLA coating liquid, and constructing a multi-layer structure through solvent volatilization. The core of the invention is that a polyphenol compound in the base layer and Fe in the PLA layer are utilized to form a metal-phenolic aldehyde network coordination bond at an interface, and the metal-phenolic aldehyde network coordination bond is used as a molecular bridge, so that the interface bonding force between PVA and PLA is greatly enhanced, and the problem of interface layering caused by polarity difference between PVA and PLA is effectively solved. Meanwhile, due to the synergistic effect of the lignin and the polyphenol compound, the film is endowed with excellent oxygen, carbon dioxide and water vapor barrier properties and an antibacterial function. The film is green and safe, and is suitable for active packaging of food, especially fresh keeping of fruits and vegetables.

Description

Technical Field

[0001] This invention relates to the field of biodegradable composite materials technology, specifically to a method and application for preparing a PVA-PLA multilayer high-barrier film based on a metal-phenolic network reinforcement, and more specifically, to a method based on tannic acid-iron ions (TA-Fe). 3+ Polyvinyl alcohol-polylactic acid (PVA-PLA) multilayer films synergistically reinforced by coordination bonds and lignin, their preparation methods, and their application in food preservation packaging. Background Technology

[0002] With the gradual implementation of global plastic bans and the unprecedented rise in consumers' environmental and health awareness, the development of biodegradable, high-performance, and multifunctional green packaging materials has become a core focus of the food packaging industry and scientific research. Among many bio-based biodegradable materials, polyvinyl alcohol (PVA) and polylactic acid (PLA) are highly favored due to their respective excellent properties.

[0003] PVA is a water-soluble polymer with excellent gas barrier properties (especially against oxygen and carbon dioxide), high transparency, good film-forming properties, biocompatibility, and complete biodegradability. However, the abundance of hydroxyl groups in the PVA molecular chain results in strong hydrophilicity and extremely poor water resistance and wet stability, severely limiting its application in high-temperature and high-humidity environments, especially in the packaging of fresh fruits and vegetables. PLA is a polyester derived from renewable resources such as corn and cassava, possessing good biodegradability, high mechanical strength, hydrophobicity, and a certain degree of transparency. However, PLA also has significant drawbacks: it is brittle, has low elongation at break, and poor impact resistance; secondly, although its gas barrier properties (especially water vapor barrier properties) are better than PVA, they are still insufficient to meet high-end preservation requirements; furthermore, PLA has a slow crystallization rate and poor thermal stability.

[0004] To obtain packaging materials with excellent overall performance, researchers often composite PVA and PLA, aiming to combine the excellent gas barrier properties and flexibility of PVA with the hydrophobicity and rigidity of PLA, achieving complementary performance advantages. Common composite methods include blending and lamination. However, due to the strong polarity and hydrophilicity of PVA molecules and the weak polarity and hydrophobicity of PLA molecules, there is a huge difference in polarity and interfacial tension between the two, resulting in extremely poor interfacial compatibility. In the process of preparing multilayer films by solution casting, this incompatibility manifests as severe interfacial delamination and peeling, causing the mechanical properties of the composite material to be far lower than the theoretical values, and the final product to have structural defects, making it unsuitable for practical application.

[0005] To address interfacial compatibility issues, traditional methods typically employ chemical coupling agents or compatibilizers, such as maleic anhydride graft copolymers. While these methods offer some effectiveness, they often introduce chemically synthesized reagents, potentially posing risks of biotoxic residues and failing to meet food packaging safety requirements. Furthermore, the processes are complex and costly, hindering the development of green chemistry. Therefore, developing a green, safe, and efficient interfacial compatibility strategy is crucial for driving the practical application of PVA / PLA composite materials.

[0006] On the other hand, modern food packaging not only requires materials to have basic protective functions, but also places higher demands on active preservation functions. Active packaging, by incorporating antibacterial and antioxidant components, actively delays and inhibits food spoilage, thereby significantly extending shelf life. Introducing natural plant extracts (such as polyphenolic compounds) into packaging films is currently a research hotspot. However, these small molecules are prone to migration and volatility, leading not only to a rapid decline in preservation function but also the potential for food contamination and compromised safety. How to stably and persistently immobilize active ingredients within the packaging matrix is ​​a major technical challenge in the development of bio-based active packaging. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the above-mentioned background technology and provide a PVA-PLA multilayer film based on TA-Fe³⁺ coordination bonds and lignin synergistic reinforcement, which solves the problem of poor interfacial compatibility between PVA and PLA, and at the same time endows the film with excellent mechanical properties, thermal stability, barrier properties and antibacterial properties, making it suitable for active food packaging.

[0008] Tannic acid (TA) and other polyphenolic compounds are rich in catechol groups, which can react with Fe. 3+ Rapid coordination reactions occur between polyvalent metal ions, forming a strong, dynamically reversible cross-linked network. This network exhibits excellent adhesion, mechanical strength, and antibacterial properties. When metal-phenolic networks (MPNs) are introduced into the PVA / PLA interface, the phenolic hydroxyl groups of TA can form numerous hydrogen bonds with the hydroxyl groups of PVA, while its catechol units can interact with Fe... 3+ Coordination occurs between the benzene ring structure of TA and the PLA segments, which in turn generate hydrophobic interactions and π-π stacking, thus constructing a strong "molecular bridge" at the interface and fundamentally enhancing the interfacial bonding force. Simultaneously, the formation of the MPN network restricts the thermal movement of the polymer segments, improving the thermal stability of the composite material.

[0009] Furthermore, lignin, as the second most abundant natural polyphenol polymer, exhibits a synergistic effect with TA. Its three-dimensional network structure and high molecular weight effectively fill the free volume of polymer molecules, extending the permeation path of gas molecules and thus significantly improving the barrier properties of the film. Lignin itself also possesses excellent UV shielding capabilities and antioxidant activity. Combining it with TA-Fe... 3+ The system coupling can solve interface problems while giving the film multiple functions such as reinforcement, barrier, antibacterial, anti-ultraviolet, and anti-oxidation in one stop, achieving "one material with multiple effects".

[0010] Based on this, the present invention innovatively proposes to utilize TA-Fe 3+ The coordination system synergistically interacts with plant polyphenols such as lignin to construct high-performance PVA-PLA multilayer composite films. This strategy not only effectively solves the common problem of interfacial compatibility in bio-based composite materials, but also provides a new technological path for developing next-generation green, safe, and multifunctional active food packaging materials.

[0011] To achieve the objectives of this invention, the method for preparing a metal-phenolic network-reinforced PVA-PLA multilayer high-barrier film includes the following steps: (1) Preparation of PVA / lignin / polyphenol composite solution: a. Slowly add PVA powder to room temperature water, stir and disperse until there are no lumps, and let it stand to swell to obtain a mixture; b. Heat and stir the mixture obtained in step a until the PVA is completely dissolved to obtain a transparent PVA solution; c. Cool the PVA solution to 60-70°C, add the plasticizer, and stir to mix thoroughly; d. Cool the solution obtained in step c to room temperature, add lignin and polyphenol compound in sequence, stir evenly and degas to obtain composite solution; (2) Forming of PVA / lignin / polyphenol base film: The composite solution obtained in step (1) is cast onto a substrate (such as a glass plate), dried, and the film is peeled off to obtain a PVA / lignin / polyphenol compound composite film (denoted as PVA / Lign / PP film). (3) Preparation of PLA coating solution: Dissolve PLA in an organic solvent to prepare PLA solution, and add metal salt to PLA solution; (4) Construction of multilayer film: The PVA / Lign / PP film obtained in step (2) is immersed in the PLA coating solution prepared in step (3). After immersion, the solvent is allowed to evaporate naturally, thus forming a dense PLA coating on the surface of the PVA base layer, and finally the multilayer film is obtained.

[0012] Furthermore, in some embodiments of the present invention, the static swelling time in step a is 30 minutes to 1 hour.

[0013] Furthermore, in some embodiments of the present invention, the solid content of PVA in the solution during step a is 1-10%.

[0014] Preferably, in some embodiments of the present invention, the solid content of PVA in the solution during step a is 4-8%.

[0015] Furthermore, in some embodiments of the present invention, step b involves heating and stirring at 85-95°C.

[0016] Furthermore, in some embodiments of the present invention, the plasticizer in step c is glycerol.

[0017] Furthermore, in some embodiments of the present invention, the amount of plasticizer used in step c is 0.5-1.5% of the PVA solid content.

[0018] Preferably, in some embodiments of the present invention, the amount of plasticizer used in step c is 0.9-1.1% of the PVA solid content, for example 1%.

[0019] Furthermore, in some embodiments of the present invention, the amount of lignin added in step d is 10-40% of the mass of PVA.

[0020] Preferably, in some embodiments of the present invention, the amount of lignin added in step d is 15-30% of the mass of PVA.

[0021] Furthermore, in some embodiments of the present invention, the amount of polyphenol compound added in step d is 0.5-5% of the mass of PVA.

[0022] Preferably, in some embodiments of the present invention, the amount of polyphenol compound added in step d is 1-3% of the mass of PVA.

[0023] Furthermore, in some embodiments of the present invention, the polyphenolic compound in step d is selected from one or more of tannic acid, quercetin, and gallocatechin gallate (EGCG).

[0024] Furthermore, in some embodiments of the present invention, step (2) involves drying at 60-70°C.

[0025] Furthermore, in some embodiments of the present invention, the drying time in step (2) is 4-8 hours.

[0026] Furthermore, in some embodiments of the present invention, the organic solvent in step (3) is selected from dichloromethane and / or chloroform.

[0027] Furthermore, in some embodiments of the present invention, the concentration of PLA solution in step (3) is 3-10% (w / v), preferably 4%-8% (w / v).

[0028] Furthermore, in some embodiments of the present invention, the metal salt in step (3) is a ferric salt (such as ferric salt, such as FeCl3). Fe³⁺ can coordinate with polyphenolic compounds in the base film, enhancing interlayer bonding and significantly improving interlayer interface strength.

[0029] Furthermore, in some embodiments of the present invention, the concentration of the metal salt in step (3) is 0.1-0.8 mol / L, preferably 0.2-0.6 mol / L.

[0030] Furthermore, in some embodiments of the present invention, the immersion time in step (4) is 5-30 seconds, preferably 8-12 seconds.

[0031] On the other hand, the present invention also provides a PVA-PLA multilayer high-barrier film, wherein the film has a multilayer structure, including a PVA / lignin / polyphenol composite base layer and upper and lower PLA surface layers, wherein the PLA surface layers contain Fe³⁺, and the film is prepared by the aforementioned method of the present invention.

[0032] In another aspect, the present invention also provides an application of the aforementioned PVA-PLA multilayer high-barrier film based on metal-phenolic network reinforcement, wherein the application is to use the film for food packaging.

[0033] Compared with the prior art, the advantages of the present invention are as follows: (1) This invention introduces lignin and polyphenol compounds into the PVA matrix at the same time, making full use of the antioxidant and antibacterial functions of natural substances to prepare a high-value-added functional base film.

[0034] (2) The present invention utilizes the PLA layer to provide the film with excellent hydrophobicity and water vapor barrier properties, which makes up for the shortcomings of PVA hydrophilicity; at the same time, the PVA base layer enhances the toughness of PLA and overcomes its brittleness.

[0035] (3) The present invention innovatively introduces Fe³⁺ into the PLA coating, which enables it to have a highly efficient metal-polyphenol coordination with the polyphenols in the base layer, forming a strong "bridge" between the two layers, greatly improving the interlayer bonding force and avoiding delamination.

[0036] (4) The process of this invention is simple and environmentally friendly. It adopts solution casting and impregnation processes, the equipment is simple and easy to operate, and it is suitable for laboratory and small-scale production. Moreover, the main components of the whole system are bio-based or biodegradable materials, which are environmentally friendly. Attached Figure Description

[0037] Figure 1 This is the preparation process of the PVA / lignin / tannic acid composite film in a mold in Example 1 of the present invention; Figure 2 This is an image of the apparent morphology of the multilayer thin film in Embodiment 1 of the present invention; Figure 3 , Figure 4 The images show the contact angle and electron microscope cross-sectional view of the multilayer thin film in Embodiment 2 of the present invention, respectively. Figure 5 Cell proliferation and toxicity detection of the multilayer film in Example 3 of this invention; Figure 6 These are experimental results of the application of the multilayer film of Example 4 and Comparative Example 1 in the preservation of longan.

[0038] Figure 7 This is a diagram of the apparent morphology of the multilayer thin film in Comparative Example 1 of the present invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Additional aspects and advantages of this invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. It should be understood that the following description is merely illustrative and not intended to limit the invention.

[0040] The terms “comprising,” “including,” “having,” “containing,” or any other variations thereof, as used herein, are intended to cover a non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0041] The indefinite articles “a” and “an” preceding an element or component of this invention do not impose any limitation on the quantity (i.e., number of times) of the element or component. Therefore, “an” or “a” should be interpreted as including one or at least one, and the singular form of an element or component also includes the plural form, unless the quantity clearly refers only to the singular form.

[0042] Furthermore, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., described below refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms are not necessarily directed at the same embodiment or example. Moreover, the technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other. Example 1: PVA / lignin / tannic acid / PLA multilayer film containing FeCl3

[0043] Weigh 5g of PVA powder and slowly add it to 95g of room temperature deionized water. Stir magnetically for 30 minutes until no lumps remain. Allow to swell for 45 minutes. Place the mixture in a 90°C oil bath and heat at 400 rpm for approximately 3 hours until the PVA is completely dissolved, yielding a transparent solution. Cool to 65°C, add 0.05g of glycerol, and continue stirring for 30 minutes. Remove the solution, cool to room temperature, add 0.05g of tannic acid and 1g of lignin, stir for 1 hour, and allow to stand to remove bubbles. Cast the mixture onto a clean 20cm × 20cm glass plate and dry in a 65°C oven for 6 hours to obtain a PVA / lignin / tannic acid composite film. Figure 1 As shown. An 8% (w / v) PLA / dichloromethane solution was prepared, and FeCl3 was added to it to achieve a concentration of 0.2 mol / L. The composite film obtained above was immersed in the PLA / FeCl3 solution for 10 seconds, then slowly lifted out and placed in a fume hood to allow the dichloromethane to completely evaporate, ultimately obtaining a multilayer film, as shown. Figure 2 As shown. Example 2: PVA / lignin / quercetin / PLA multilayer film containing FeCl3

[0044] Weigh 6g of PVA powder and slowly add it to 95g of room temperature deionized water. Stir magnetically for 30 minutes until no lumps remain. Allow to stand and swell for 45 minutes. Place the mixture in a 90°C oil bath and heat at 400 rpm for approximately 3 hours until the PVA is completely dissolved, yielding a transparent solution. Cool to 65°C, add 0.05g of glycerol, and continue stirring for 30 minutes. Remove the solution, cool to room temperature, add 0.18g of quercetin and 1.8g of lignin, stir for 1 hour, and allow to stand to remove bubbles. Cast the mixed solution onto a clean 20cm × 20cm glass plate and dry in a 65°C oven for 6 hours to obtain a PVA / lignin / quercetin composite film. Prepare a 6% (w / v) PLA / dichloromethane solution and add FeCl3 to it to achieve a concentration of 0.4mol / L. The composite film obtained above was immersed in the PLA / FeCl3 solution for 10 seconds, then slowly lifted out and placed in a fume hood to allow the dichloromethane to completely evaporate, ultimately yielding a multilayer film. Figure 3 , Figure 4 The images show the contact angle and electron microscope cross-section of the multilayer thin film. Figure 3 The contact angle of the multilayer film is 94.7°, and the film exhibits hydrophobicity due to the presence of ester bonds (-COO-) and terminal methyl groups (-CH3) on the PLA molecular chain. Although ester bonds have a certain degree of polarity, the nonpolar portion (methyl and methylene groups) of the entire molecular chain is dominant, which results in a lower surface energy of PLA and weaker interaction with water molecules, thus exhibiting weak hydrophobicity. Figure 4 The cross-sectional electron microscope image clearly shows a three-layer structure. The middle PVA layer is about 100 μm thick, while the two outer PLA layers are 15.45 μm and 4.17 μm thick, respectively. The reason for this uneven thickness of the two PLA film layers may be that during the drying process of PVA, quercetin is deposited on the lower surface of PVA by gravity. Due to the high concentration of quercetin on the lower surface, its interaction with FeCl3 is stronger, resulting in a thicker PLA layer after immersion. Example 3: PVA / lignin / tannic acid / quercetin / PLA multilayer film containing FeCl3

[0045] Weigh 8g of PVA powder and slowly add it to 95g of room temperature deionized water. Stir magnetically for 30 minutes until no lumps remain. Allow to stand and swell for 45 minutes. Place the mixture in a 90°C oil bath and heat at 400 rpm for approximately 3 hours until the PVA is completely dissolved, yielding a transparent solution. Cool to 65°C, add 0.05g of glycerol, and continue stirring for 30 minutes. Remove the solution, cool to room temperature, add 0.1g of tannic acid, 0.06g of quercetin, and 2.4g of lignin, stir for 1 hour, and allow to stand to remove bubbles. Cast the mixed solution onto a clean 20cm × 20cm glass plate and dry in a 65°C oven for 6 hours to obtain a PVA / lignin / tannic acid / quercetin composite film. Prepare a 4% (w / v) PLA / dichloromethane solution and add FeCl3 to it to achieve a concentration of 0.3 mol / L. The composite film obtained above was immersed in the PLA / FeCl3 solution for 10 seconds, then slowly lifted out and placed in a fume hood to allow the dichloromethane to completely evaporate, ultimately yielding a multilayer film. Figure 5 The study investigated cell proliferation and toxicity of multilayer films. Results showed that the samples did not exhibit cytotoxicity to HepG2 cells during a culture period of up to 168 hours; in fact, they showed significant cell proliferation-promoting effects at all tested concentrations. Example 4: PVA / lignin / EGCG / PLA multilayer film containing FeCl3

[0046] Weigh 4g of PVA powder and slowly add it to 95g of room temperature deionized water. Stir magnetically for 30 minutes until no lumps remain. Allow to stand and swell for 45 minutes. Place the mixture in a 90°C oil bath and heat at 400 rpm for approximately 3 hours until the PVA is completely dissolved, yielding a transparent solution. Cool to 65°C, add 0.05g of glycerol, and continue stirring for 30 minutes. Remove the solution, cool to room temperature, add 0.04g of EGCG and 0.6g of lignin, stir for 1 hour, and allow to stand to remove bubbles. Cast the mixed solution onto a clean 20cm × 20cm glass plate and dry in a 65°C oven for 6 hours to obtain a PVA / lignin / EGCG composite film. Prepare a 4% (w / v) PLA / dichloromethane solution and add FeCl3 to it to achieve a concentration of 0.5 mol / L.

[0047] The composite film obtained above was immersed in the PLA / FeCl3 solution for 10 seconds, then slowly lifted out and placed in a fume hood to allow the dichloromethane to completely evaporate, ultimately yielding a multilayer film with an oxygen permeability of 3.7 cc / (m²). 2 (24h), carbon dioxide transmission rate was 15.3 cc / (m³). 2 •24h), water vapor transmission rate was 4.91 g / (m 2The mechanical tensile strength is 46.2 MPa (24h). Figure 6 The images show the application test results of the multilayer film in this embodiment and the multilayer film in Comparative Example 1 on the preservation of longan. The film preservation test conditions were: 0-4°C, 20 days. Comparative Example 1: Pure PVA / PLA bilayer film (free of lignin, polyphenols and FeCl3)

[0048] This comparative example aims to illustrate that the pure PVA and PLA bilayer films prepared without lignin, polyphenolic compounds, and FeCl3 have problems such as weak interfacial bonding and poor functionality.

[0049] 1. Preparation of PVA solution: Weigh 5g of PVA powder and slowly add it to 95g of room temperature deionized water. Stir magnetically for 30 minutes until no lumps remain. Allow to stand and swell for 45 minutes. Place the mixture in a 90°C oil bath and heat at 400 rpm with stirring for approximately 3 hours until the PVA is completely dissolved, yielding a clear solution. Cool to 65°C, add 0.05g of glycerin (as a plasticizer), and continue stirring for 30 minutes.

[0050] Remove the solution, cool it to room temperature, and allow it to stand to remove bubbles.

[0051] 2. Molding of pure PVA film: The pure PVA solution was cast onto a clean glass plate measuring 20cm × 20cm and dried in a 65°C forced-air drying oven for 6 hours to obtain a transparent and flexible pure PVA film.

[0052] 3. Preparation of PLA coating solution: Prepare an 8% (w / v) PLA / dichloromethane solution. (Note: Do not add FeCl3 to this solution.) 4. Construction of multilayer thin films: The pure PVA film obtained in step 2 is immersed in the PLA solution prepared in step 3 for 10 seconds, then slowly lifted out and placed in a fume hood to allow the dichloromethane to completely evaporate, thus obtaining a pure PVA / PLA multilayer film. Figure 7 The image shows the apparent morphology of the multilayer film in Comparative Example 1, confirming that the prepared pure PVA and PLA bilayer film has problems such as weak interfacial bonding and easy peeling. Comparative Example 2: PVA / PLA bilayer film containing FeCl3 (lignin-free, polyphenol-free)

[0053] The only difference between this comparative example and Example 1 is that tannins and lignin were not added. Comparative Example 3: PVA / lignin / PLA bilayer film (polyphenol-free, FeCl3-free)

[0054] The difference between this comparative example and Example 1 is that tannic acid was not added, and FeCl3 was not added to the PLA coating solution. This is intended to demonstrate that adding only lignin without polyphenolic compounds and FeCl3 did not improve the interfacial bonding between the PVA / lignin layer and the PLA layer. Comparative Example 4: PVA / lignin / PLA bilayer film containing FeCl3 (polyphenol-free)

[0055] The only difference between this comparative example and Example 1 is that tannic acid was not added.

[0056] Based on the provided examples and comparative examples, the expected performance is compared and analyzed below in five performance indicators: water vapor barrier performance, oxygen barrier performance, antibacterial durability, mechanical properties, and interlayer delamination. The analysis is based on the material composition, process conditions, and relevant test results of the examples and comparative examples. Overall, Examples 1-4 (containing FeCl3 and polyphenolic compounds) are expected to outperform Comparative Examples 1-4 (without polyphenols or FeCl3) in all performance aspects, mainly due to the metal-polyphenol coordination network formed by FeCl3 and polyphenolic compounds (such as tannic acid, quercetin, and EGCG), which enhances interfacial bonding, density, and functionality.

[0057] Table 1. Comparative analysis of expected performance indicators for different examples and comparative examples (out of 5 points)

[0058] Summary of water vapor barrier performance comparison: Examples 1-4 > Comparative Example 4 ≈ Comparative Example 2 > Comparative Example 3 > Comparative Example 1, indicating that the synergistic effect of polyphenols and FeCl3 can significantly improve water vapor barrier performance.

[0059] Summary of oxygen barrier performance comparison: Examples 1-4 > Comparative Example 4 > Comparative Example 3 ≈ Comparative Example 2 > Comparative Example 1, indicating that the coordination bond between polyphenols and FeCl3 is the key to improving oxygen barrier performance.

[0060] Summary of antibacterial durability comparison: Examples 1-4 > Comparative Example 4 > Comparative Example 3 > Comparative Example 1 ≈ Comparative Example 2, indicating that the combination of polyphenols and FeCl3 is the core to achieving high antibacterial durability.

[0061] Summary of mechanical property comparison: Examples 1-4 > Comparative Example 4 > Comparative Example 3 > Comparative Example 2 > Comparative Example 1, indicating that the coordination bond between polyphenols and FeCl3 can effectively improve mechanical properties.

[0062] Summary of interlayer delamination comparison: Examples 1-4 > Comparative Example 4 > Comparative Example 3 > Comparative Example 2 > Comparative Example 1. This shows that Examples 1-4, by introducing polyphenolic compounds (tannic acid, quercetin, EGCG) and FeCl3, formed a stable metal-polyphenol network, which significantly improved water vapor barrier, oxygen barrier, antibacterial durability, mechanical properties, and interlayer bonding.

[0063] Comparative Examples 1-4 exhibited poor interfacial bonding and functionality due to the lack of polyphenols or FeCl3. Comparative Example 4 (containing FeCl3 but lacking polyphenols) showed slight improvement in some properties, but still failed to reach the levels of the examples. Therefore, the synergistic effect of polyphenols and FeCl3 is crucial for achieving high-performance multilayer films. In practical applications, selecting specific polyphenol combinations (such as the polyphenol mixture in Example 3) can optimize overall performance based on specific needs.

[0064] Those skilled in the art will readily understand that the above description is only a part of the embodiments of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a PVA-PLA multilayer high-barrier film based on a metal-phenolic network reinforcement, characterized in that, The preparation method includes the following steps: (1) Preparation of PVA / lignin / polyphenol composite solution: a. Slowly add PVA powder to room temperature water, stir and disperse until there are no lumps, and let it stand to swell to obtain a mixture; b. Heat and stir the mixture obtained in step a until the PVA is completely dissolved to obtain a transparent PVA solution; c. Cool the PVA solution to 60-70°C, add the plasticizer, and stir to mix thoroughly; d. Cool the solution obtained in step c to room temperature, add lignin and polyphenol compound in sequence, stir evenly and degas to obtain composite solution; (2) Forming of PVA / lignin / polyphenol base film: The composite solution obtained in step (1) is cast onto the substrate, dried, and the film is peeled off to obtain a PVA / lignin / polyphenol compound composite film, namely PVA / Lign / PP film. (3) Preparation of PLA coating solution: Dissolve PLA in an organic solvent to prepare PLA solution, and add metal salt to PLA solution; (4) Construction of multilayer film: The PVA / Lign / PP film obtained in step (2) is immersed in the PLA coating solution prepared in step (3). After immersion, the solvent is allowed to evaporate naturally, thus forming a dense PLA coating on the surface of the PVA base layer, and finally the multilayer film is obtained.

2. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, The static swelling time in step a is 30 minutes to 1 hour; preferably, the solid content of PVA in the solution in step a is 1-10%; preferably, the solid content of PVA in the solution in step a is 4-8%.

3. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, In step b, the mixture is heated and stirred at 85-95°C.

4. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, In step c, the plasticizer is glycerin; preferably, the amount of plasticizer used in step c is 0.5-1.5% of the PVA solid content; preferably, the amount of plasticizer used in step c is 0.9-1.1% of the PVA solid content, for example, 1%.

5. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, In step d, the amount of lignin added is 10-40% of the mass of PVA; preferably, the amount of lignin added in step d is 15-30% of the mass of PVA.

6. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, The amount of polyphenol compound added in step d is 0.5-5% of the mass of PVA; preferably, the amount of polyphenol compound added in step d is 1-3% of the mass of PVA; preferably, the polyphenol compound in step d is selected from one or more of tannic acid, quercetin, and gallic catechin gallate.

7. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, In step (2), the drying is carried out at 60-70°C; preferably, the drying time in step (2) is 4-8 hours; preferably, the organic solvent in step (3) is selected from dichloromethane and / or chloroform; preferably, the concentration of PLA solution in step (3) is 3-10% (w / v), more preferably 4%-8% (w / v).

8. The method for preparing PVA-PLA multilayer high-barrier thin films based on metal-phenolic network reinforcement according to claim 1, characterized in that, In step (3), the metal salt is a ferric salt; preferably, the concentration of the metal salt in step (3) is 0.1-0.8 mol / L, more preferably 0.2-0.6 mol / L; preferably, the immersion time in step (4) is 5-30 seconds, more preferably 8-12 seconds.

9. A PVA-PLA multilayer high-barrier film based on a metal-phenolic network reinforcement, characterized in that, The film has a multilayer structure, including a PVA / lignin / polyphenol composite base layer and upper and lower PLA surface layers, wherein the PLA surface layers contain Fe³⁺; preferably, the film is prepared by the method according to any one of claims 1-8.

10. The application of the PVA-PLA multilayer high-barrier film based on metal-phenolic network reinforcement as described in claim 9, characterized in that, The application is to use the film for food packaging.

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