Polyvinyl chitosan tannic acid composite active film, preparation method thereof and application thereof

Abstract

The application discloses a polyvinyl chitosan tannic acid composite active film, a preparation method thereof and application thereof, and belongs to the technical field of food packaging materials. The polyethylene film is subjected to oxygen plasma treatment and silane coupling agent chemical grafting double modification in sequence to construct an amino-containing chemical anchoring layer. Citric acid aqueous solution is used to dissolve chitosan and tannic acid is added to form a ternary crosslinking coating solution. The composite active film is prepared by adopting a double-layer gradient coating process. The tensile strength of the obtained composite film reaches 26.8 MPa, the ultraviolet blocking rate is greater than 99%, the DPPH and ABTS free radical scavenging rates are 95.8% and 99.2% respectively, the composite active film has antibacterial activity, and the shelf life of small cucumbers can be significantly prolonged when the composite active film is used for small cucumber preservation.

Description

Technical Field

[0001] This invention belongs to the field of food packaging materials technology, specifically relating to a polyethylene-chitosan-tannic acid composite active film, its preparation method, and its application in the preservation of fresh fruits and vegetables. Background Technology

[0002] Polyethylene film has long been widely used as a food packaging material due to its excellent mechanical strength, good heat-sealing properties, and low production cost. Wong et al. (FoodChemistry, 2020, 329: 126989) reported a tilapia preservation film with a chitosan / gallic acid coating on a plasma-treated polyethylene film substrate. The plasma treatment improved the surface wettability of the polyethylene film; however, this study only used plasma treatment as a surface modification method and did not introduce a chemical bonding layer, so the adhesion stability of the coating during long-term storage was not fully verified. Traditional polyethylene films have several inherent defects: high oxygen permeability, making it difficult to effectively block external oxygen from oxidizing and eroding food; almost no ability to block ultraviolet radiation, allowing ultraviolet radiation to directly penetrate the film layer and irradiate the food surface, accelerating chlorophyll decomposition and lipid oxidation; and no antibacterial or antioxidant activity, failing to inhibit the growth of microorganisms during food storage. These defects make it difficult to meet the stringent requirements of high-quality preservation environments for fresh fruits and vegetables by relying solely on polyethylene films.

[0003] Chitosan is a natural cationic polysaccharide obtained by deacetylation of chitin, possessing excellent film-forming properties, biocompatibility, and biodegradability. The protonated amino groups on the chitosan molecular chain can electrostatically interact with the phospholipid bilayer of microbial cell membranes, thus exhibiting certain antibacterial activity. Cheng Long et al. (China Plastics, 2021, 35(4): 35-41) studied the effect of chitosan on the properties of corn starch edible films, showing that the tensile strength of single chitosan films was low, and their mechanical properties could not meet the actual packaging requirements. Fan Shuting et al. (China Plastics, 2024, 38(7): 38-42) prepared chitosan / corn starch crosslinked films, which improved mechanical properties, but their antioxidant capacity remained limited. Tannic acid is a natural polyphenol compound containing a large number of phenolic hydroxyl groups. It not only possesses excellent free radical scavenging ability and ultraviolet absorption characteristics, but can also form a crosslinked network with the amino groups of chitosan through hydrogen bonds and covalent bonds. Lee et al. (Food Hydrocolloids, 2023, 136: 108249) reported a chitosan / tannic acid composite film, prepared by neutralization under different pH conditions. The resulting composite film exhibited excellent UV resistance, antioxidant properties, and antibacterial properties, confirming the feasibility of crosslinking tannic acid with chitosan. However, this composite film was formed independently using a solution casting method, and its mechanical strength was insufficient to meet the tensile strength requirements of actual packaging films. Sharma et al. (International Journal of Biological Macromolecules, 2024, 260: 129317) reported a composite film crosslinked with chitosan-guar gum using tannic acid, further confirming that crosslinking with tannic acid can improve mechanical properties. However, this film was also an independently formed system, lacking the ability to be composited with an industrial polyethylene substrate.

[0004] In existing technologies, reports on directly coating chitosan or polyphenol-containing solutions onto the surface of polyethylene films typically employ single surface treatment methods such as corona discharge or plasma treatment. The oxygen-containing polar groups introduced by single surface treatments are highly susceptible to aging and degradation in air, leading to a significant decrease in the bonding strength between the coating and the polyethylene substrate with prolonged storage. Furthermore, Qiao et al. (LWT, 2021, 135: 109984) investigated the effects of different acid solvents on the properties of chitosan films. Their results showed that the mechanical and barrier properties of chitosan films were limited when using acetic acid as a solvent, while polyacid solvents such as citric acid showed promise in improving coating performance through participation in crosslinking reactions. However, this study was limited to standalone chitosan film systems and did not address the composite with a polyethylene substrate.

[0005] Therefore, how to improve the long-term interfacial bonding strength between the coating and the substrate while maintaining the mechanical strength of the polyethylene film, and endow the film with multifunctional properties such as UV blocking, anti-oxidation, and antibacterial activity, is a technical problem that urgently needs to be solved in this field. Specifically, the existing technology has three core shortcomings: First, the surface modification method of polyethylene is singular, and the polar groups introduced by plasma treatment age and decay rapidly in air, resulting in a sharp decline in coating adhesion over time; second, chitosan coatings mostly use acetic acid as a solvent, and acetic acid is a monocarboxylic acid that completely evaporates during the film-forming and drying process and does not participate in the crosslinking reaction, resulting in a limited crosslinking density of the coating; third, the coating process is mostly single-layer coating, and there is a lack of gradient buffer between the coating and the substrate, which easily leads to interfacial stress concentration in environments with alternating temperature and humidity, resulting in coating cracking or peeling. Summary of the Invention

[0006] To address the problems of poor interfacial bonding strength between polyethylene substrate and active coating, limited coating function, and insufficient durability in existing technologies, this invention provides a polyethylene-based chitosan-tannic acid composite active film, its preparation method, and its application in the preservation of fresh fruits and vegetables.

[0007] In a first aspect, the present invention provides a polyethylene-based chitosan-tannic acid composite active film, comprising, from bottom to top, a polyethylene base layer, a silane coupling agent chemical anchoring layer, a chitosan-citric acid primer layer, and a chitosan-tannic acid-citric acid functional coating. The silane coupling agent chemical anchoring layer is a polysiloxane thin layer formed by covalent bonding of γ-aminopropyltriethoxysilane on the surface of a polyethylene film treated with oxygen plasma, with free amino groups exposed on the surface of this thin layer. The chitosan-citric acid primer layer is a thin layer obtained by dissolving chitosan in an aqueous citric acid solution and then drying it to form a film. The chitosan-tannic acid-citric acid functional coating is a ternary crosslinked network thin layer formed by crosslinking chitosan, tannic acid, and citric acid under heating conditions through hydrogen bonding, Schiff base covalent bonding, and esterification, wherein the amount of tannic acid added is 2-6 wt% of the mass of chitosan.

[0008] Secondly, the present invention provides a method for preparing the above-mentioned polyethylene-based chitosan-tannic acid composite active film, comprising the following steps: plasma treatment of the polyethylene film using oxygen as the working gas, followed by immersion of the polyethylene film in an ethanol-water mixed solution of γ-aminopropyltriethoxysilane to complete chemical grafting, thereby achieving dual modification of the polyethylene film surface; adding chitosan to a citric acid aqueous solution and stirring to dissolve, then adding a plasticizer to prepare a primer; preparing a chitosan-citric acid solution using the same method, adding a tannic acid aqueous solution and stirring under heating conditions, then adding a plasticizer to prepare a functional coating solution; first coating the primer solution onto the surface of the modified polyethylene film and drying and pre-curing, then coating the functional coating solution onto the surface of the primer layer and drying and curing, thereby obtaining the composite active film.

[0009] Thirdly, the present invention provides the application of the above-mentioned polyethylene chitosan tannic acid composite active film in food preservation packaging, especially suitable for active preservation packaging of fresh fruits and vegetables with a water content greater than 90%.

[0010] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention constructs a covalent bond anchoring layer on the surface of polyethylene through dual modification of oxygen plasma treatment and chemical grafting of silane coupling agent, so that a continuous chemical bond link is formed between the coating and the substrate. This solves the problem of insufficient coating adhesion caused by the rapid aging and decay of polar groups after traditional single plasma treatment. The coating adhesion level of the composite film is still 0 after 14 days of accelerated aging in high humidity.

[0011] (2) In this invention, citric acid is used instead of traditional acetic acid as the chitosan solvent. The three carboxyl groups of citric acid undergo esterification and amidation reactions with chitosan during the film formation process, and synergistically construct a chitosan-tannic acid-citric acid ternary crosslinking network with the Schiff base of tannic acid. This results in a 33.3% increase in tensile strength of the composite film compared to the acetic acid system and a 16.3% decrease in water vapor permeability coefficient.

[0012] (3) The present invention adopts a double-layer gradient coating process. The bottom coating forms an interface transition between the silane coupling agent layer and the functional coating. The functional coating imparts ultraviolet blocking, anti-oxidation and antibacterial activities to the film. The composite film has a blocking rate of more than 99% in the ultraviolet region of 200~320nm, and the DPPH and ABTS free radical scavenging rates reach 95.8% and 99.2%, respectively.

[0013] (4) When the composite active film of the present invention is used for the preservation packaging of cucumbers, the shelf life of cucumbers under normal temperature storage conditions of 25°C can be extended from 2 to 3 days for pure polyethylene film packaging to more than 5 days. The quality loss rate during storage is less than 2%, and the hardness is maintained at more than 9.2 kgf. Attached Figure Description

[0014] Figure 1 This is a process flow diagram for preparing the polyvinyl chitosan-tannic acid composite active film of the present invention.

[0015] Figure 2 This is a schematic diagram of the layered structure of the composite active membrane of the present invention.

[0016] Figure 3 The images show the Fourier transform infrared spectra of different thin films.

[0017] Figure 4 (A) TG curves and (B) DTG curves for different thin films.

[0018] Figure 5The images show the appearance (A~C), surface SEM morphology (D~F), and cross-sectional SEM morphology (G~I) of different films. A, D, and G are PE films, B, E, and H are CCT films, and C, F, and I are PE / CCT-4 composite films.

[0019] Figure 6 (A) Stress-strain curves and (B) Tensile strength-elongation at break histograms for different films.

[0020] Figure 7 (A) Water vapor transmission rate, (B) Water vapor transmission coefficient and (C) Oxygen transmission rate of PE membrane and different PE / CCT-TA composite membranes.

[0021] Figure 8 The following are the (A) UV-Vis transmittance curves, (B) DPPH and ABTS free radical scavenging rates, and (C) inhibition zone diameters of different films.

[0022] Figure 9 The changes in (A) appearance, (B) mass loss rate, (C) hardness, and (D) soluble solids content of cucumbers packaged in different films during 5 days of storage at room temperature were investigated. Detailed Implementation

[0023] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0024] like Figure 1 As shown, the preparation process of the polyethylene-chitosan-tannic acid composite active film of the present invention includes four core steps: dual surface modification of the polyethylene film, preparation of the chitosan-tannic acid composite coating solution, double-layer gradient coating, and curing and drying. Figure 2 As shown, the resulting composite active film consists of, from bottom to top, a polyethylene substrate layer, a silane coupling agent chemical anchoring layer, a chitosan-citric acid base coating layer, and a chitosan-tannic acid-citric acid functional coating layer. For ease of comparison with the composite active film of this invention, a chitosan-citric acid-tannic acid independently cast film without a polyethylene substrate was also prepared as a reference in the following examples and comparative examples, labeled as CCT film (Chitosan-Citric acid-Tannic acid cast film). The CCT film was prepared as follows: a chitosan-tannic acid-citric acid functional coating solution was prepared according to step three of Example 1, and the coating solution was directly cast into a polytetrafluoroethylene mold and dried at 60°C to form a film. Example 1.

[0025] Step 1: Dual surface modification of polyethylene film: Take a food-grade low-density polyethylene film with a thickness of 15 μm and cut it into rectangular samples of 200 mm × 300 mm. Place the sample in the vacuum chamber of a low-temperature plasma treatment instrument and perform oxygen plasma treatment using oxygen as the working gas at a power of 40 W and a treatment time of 40 s. Immediately after treatment, immerse the polyethylene film in a 2 wt% ethanol-water mixed solution of γ-aminopropyltriethoxysilane (ethanol to water volume ratio of 9:1). The pH of the solution is pre-adjusted to 4.5-5.0 with acetic acid. Immerse for 30 min at room temperature, then rinse three times with anhydrous ethanol to remove excess physically adsorbed silane. Subsequently, dry in an 80°C forced-air drying oven for 30 min to allow the silane to complete the condensation reaction on the polyethylene surface, forming a stable Si-O-Si covalent network and exposing a large number of free amino groups on the surface. The polyethylene film with the above dual modification is labeled as mPE.

[0026] Oxygen plasma treatment introduces oxygen-containing polar groups such as hydroxyl, carboxyl, and peroxy groups onto the polyethylene film surface, reducing the water contact angle of the originally inert polyethylene surface from 95° to below 40°. This provides abundant silanol bonding sites for the subsequent hydrolysis and condensation reaction of the silane coupling agent. After the three ethoxy groups of γ-aminopropyltriethoxysilane hydrolyze to generate silanols, they undergo a dehydration condensation reaction with the hydroxyl groups on the polyethylene surface to form covalent Si-OC bonds. Simultaneously, intermolecular self-condensation reactions occur between silanol molecules to form a cross-linked polysiloxane thin layer. Compared to plasma treatment alone, the covalent bonding of the silane coupling agent layer significantly improves the durability of surface modification. Even after 30 days of storage in a high-humidity environment, the water contact angle of the modified surface remains below 45°, while the water contact angle of the polyethylene film surface treated only by plasma recovers to above 80° under the same conditions. The exposed free amino groups on the silane layer surface provide reactive sites for the subsequent chemical bonding of the chitosan coating.

[0027] Step 2, Preparation of Chitosan-Citrate Primer: Weigh 2.0 g of chitosan with a degree of deacetylation of 85% and a viscosity-average molecular weight of 300 kDa, and add it to 100 mL of a 1.5 wt% citric acid aqueous solution. Stir magnetically at 300 r / min for 4 hours in a 60°C water bath until the chitosan is completely dissolved, obtaining a uniform and transparent chitosan-citric acid solution. Add 0.5 mL of glycerol as a plasticizer to this solution and continue stirring for 30 min to ensure uniform dispersion. The resulting primer is labeled CCT-CA.

[0028] Citric acid offers dual advantages over traditional acetic acid as a dissolving acid for chitosan: Citric acid molecules contain three carboxyl groups. While protonating the amino groups of chitosan to dissolve it, the excess carboxyl groups can undergo esterification and amidation reactions with the hydroxyl and amino groups of chitosan. This allows for the in-situ construction of citric acid-chitosan covalent crosslinking sites during subsequent drying and curing, thereby improving the mechanical strength and water resistance of the coating. Furthermore, as a food-grade organic acid, citric acid residues do not pose a risk to food safety.

[0029] Step 3: Preparation of the chitosan-tannic acid-citric acid functional coating solution: Take another 2.0 g of chitosan and dissolve it in 100 mL of a 1.5 wt% citric acid aqueous solution using the same method as in Step 2. After the chitosan is completely dissolved, weigh 0.08 g of tannic acid and dissolve it in 20 mL of deionized water, stirring at room temperature until completely dissolved. Slowly add the tannic acid aqueous solution to the chitosan-citric acid solution, and stir at 300 r / min for 1 h in a 60°C water bath to allow the phenolic hydroxyl groups of tannic acid to fully react with the amino groups of chitosan to form hydrogen bonds and covalent crosslinks. Add 0.5 mL of glycerol to the solution and continue stirring for 30 min. The resulting functional coating solution is labeled CCT-TA-CA. In this example, the amount of tannic acid added is 4 wt% of the chitosan mass.

[0030] Tannic acid molecules contain 25 phenolic hydroxyl groups and 10 ester bonds. The phenolic hydroxyl groups undergo Michael addition and Schiff base reactions with the primary amine groups of chitosan to form CN covalent bonds. Simultaneously, the phenolic hydroxyl groups of tannic acid form numerous intermolecular hydrogen bonds with the amide and hydroxyl groups of chitosan. The carboxyl groups of citric acid participate in the esterification reaction, forming chitosan-citric acid ester crosslinking points. Together, these three components construct a chitosan-tannic acid-citric acid ternary crosslinking network. The crosslinking density of this ternary crosslinking network is significantly higher than that of the chitosan-tannic acid binary system, resulting in reduced coating swelling, increased mechanical strength, and decreased water vapor permeability.

[0031] Step 4, Double-Layer Gradient Coating: The mPE film is fixed on the glass plate of a benchtop coating machine with the modified side facing up. First, CCT-CA primer is poured into the reservoir of the coating machine, with the reservoir opening width set to 0.5 mm and the coating speed set to 80 mm / min, to apply the first layer of coating to the mPE film. After coating, the primer is pre-cured by drying in a 60°C oven for 1 hour. Then, CCT-TA-CA functional coating liquid is poured into the reservoir, with the opening width set to 1.0 mm and the coating speed set to 100 mm / min, to apply the second layer of coating to the primer surface. After coating, the second layer is cured by drying in a 60°C oven for 3 hours. The resulting polyvinyl chitosan-tannic acid composite active film is labeled PE / CCT-4.

[0032] The CCT-CA thin layer of the base coating is approximately 3–5 μm thick, and its main function is to form an interface transition between the silane coupling agent layer and the functional coating. Amide bonds are formed between the amino groups of chitosan in the base coating and the amino groups on the surface of the silane coupling agent layer through citric acid bridging. Simultaneously, since both the base coating and the functional coating are chitosan matrices, they exhibit good molecular chain entanglement and hydrogen bond interpenetration, thus achieving a gradient transition from the polyethylene substrate to the functional coating. The functional coating is approximately 8–12 μm thick, and the chitosan-tannic acid-citric acid ternary crosslinked network endows the coating with UV blocking, antioxidant, and antibacterial activities. The total thickness of the overall composite film is approximately 26–32 μm.

[0033] Step 5: Perform performance testing on the prepared PE / CCT-4 composite active membrane.

[0034] Observation of appearance and microstructure: such as Figure 5 As shown in (A) to (C), it can be visually observed that pure polyethylene film ( Figure 5 (A) is colorless and transparent, CCT film ( Figure 5 (B) has a uniform yellowish-brown transparent appearance; the PE / CCT-4 composite film ( Figure 5 The (C) film exhibits a uniform yellowish-brown translucent appearance with a smooth, flat surface free of bubbles and pinholes. The surface and cross-sectional microstructure of the film were observed using a scanning electron microscope at an accelerating voltage of 15 kV. Figure 5 As shown in (D)~(F), the surface of the pure polyethylene film ( Figure 5 (D) Slight stretching texture is visible on the surface of the CCT film. Figure 5 The surface of the PE / CCT-4 composite film (E) is continuous, smooth, and crack-free, indicating good compatibility between chitosan and tannic acid. Figure 5 The (F) matrix is ​​uniform and dense, with no cracks or particle agglomeration, indicating that tannic acid is evenly dispersed in the chitosan matrix. Figure 5 As shown in (G)~(I), the cross-section of pure polyethylene film ( Figure 5 The thickness of the (G) is uniform, approximately 15 μm, and the cross-section of the CCT membrane ( Figure 5 (H)) Continuous, dense, and without delamination, PE / CCT-4 composite film cross-section ( Figure 5 (I) shows a clear double-layer structure, with a polyethylene base layer on top and a CCT coating layer on the bottom. The interface between the base layer and the functional coating is continuous and there are no signs of delamination. The interface between the functional coating and the polyethylene base is tightly bonded. Although the silane coupling agent layer is only nanometer thick and difficult to distinguish directly under a scanning electron microscope, its presence prevents any debonding gaps between the coating and the base.

[0035] Thermal stability analysis: Thermogravimetric analysis was performed using a thermogravimetric analyzer to heat the sample from 28°C to 800°C at a heating rate of 10°C / min under a nitrogen atmosphere. Figure 4 In this context, PE is a pure polyethylene film (Comparative Example 1), CCT is a chitosan-citric acid-tannic acid independently cast film, and PE / CCT is a PE / CCT-4 composite film (Example 1). Figure 4 As shown in (A), pure polyethylene film exhibits a single weight loss phase in the temperature range of 410–475°C, with residual mass approaching zero. Both CCT film and PE / CCT-4 composite film underwent three stages of degradation. Figure 4 As shown in (B), the DTG curve of the pure polyethylene film exhibits a single sharp weight loss rate peak at 452°C. The thermal decomposition of the PE / CCT-4 composite film occurs in three stages: the first stage, from 50 to 120°C, results in a mass loss of approximately 5%, attributed to the evaporation of bound water and glycerol plasticizers in the film; the second stage, from 120 to 400°C, results in a mass loss of approximately 25%, attributed to the thermal decomposition of the chitosan backbone and tannic acid side groups; the third stage, from 410 to 480°C, results in a mass loss attributed to the thermal degradation of the polyethylene substrate, with the DTG curve showing a weight loss rate peak at 449°C. The initial decomposition temperature of the composite film is above 200°C, meeting the heat resistance requirements of food packaging under short-term heat-sealing conditions from room temperature to high temperature.

[0036] Fourier transform infrared spectroscopy analysis: such as Figure 3 As shown, CCT is a pure chitosan membrane, TA is tannic acid powder, CCT is a chitosan-citric acid-tannic acid independently cast membrane, and PE / CCT is a PE / CCT-4 composite membrane (Example 1). Infrared spectroscopy of the films was performed using ATR mode, with a scanning range of 4000~500 cm⁻¹. -1 4cm resolution -1 Pure polyethylene film at 2916cm -1 and 2848cm -1 Absorption peaks for both symmetric and asymmetric stretching vibrations of CH appear at 1462 cm⁻¹. -1 A CH bending vibration absorption peak appears at 1100 cm⁻¹. In addition to retaining the characteristic peaks of polyethylene, the mPE film also exhibits an absorption peak at 1100 cm⁻¹. -1 An absorption peak for the Si-O-Si stretching vibration appears at 1040 cm⁻¹. -1 The presence of a Si-OC stretching vibration absorption peak at 3350 cm⁻¹ confirms that the silane coupling agent has been successfully bonded to the polyethylene surface. The PE / CCT-4 composite film exhibits an absorption peak at 3350 cm⁻¹. -1 A broad and strong absorption peak appears at 1720 cm⁻¹, which is attributed to the superposition of OH and NH stretching vibrations in chitosan and tannic acid; at 1720 cm⁻¹... -1 A new C=O stretching vibration peak appears at 1635 cm⁻¹, which is attributed to the ester bond formed between citric acid and chitosan;-1 The C=N stretching vibration peak appears at 1560 cm⁻¹, which is attributed to the Schiff base structure formed between the amino group of chitosan and the phenolic hydroxyl group of tannic acid. -1 The original NH bending vibration peak was significantly weakened, further confirming that the amino groups of chitosan participated in the cross-linking reaction.

[0037] Mechanical property testing: Tensile testing of the film was conducted using a universal testing machine, referring to GB / T1040.1-2018. The film was cut into 50mm × 10mm specimens with a clamping distance of 30mm. The tensile rate was 100mm / min, and each group underwent five parallel tests. Figure 6 As shown in (A), the stress-strain curve of pure polyethylene film exhibits typical characteristics of a highly elastic polymer, with an elongation at break as high as 290%, but a tensile strength of only 3.2 MPa. The stress-strain curves of each mPE / CCT-TA composite film all show a significant increase in tensile strength and a decrease in elongation at break. Figure 6 As shown in (B), the PE / CCT-4 composite film exhibits a tensile strength of 26.8 MPa and an elongation at break of 145%. Compared to pure polyethylene film, the tensile strength is increased by 7.4 times, and although the elongation at break is slightly lower, it remains above 100%, meeting the requirements for film extensibility in food packaging. The significant improvement in mechanical properties is attributed to the formation of the chitosan-tannic acid-citric acid ternary crosslinking network and the improved load transfer efficiency between the coating and the substrate due to the silane coupling agent layer.

[0038] Water vapor barrier performance test: The test was conducted using a water vapor transmission rate tester in accordance with GB / T1037-2021. The membrane was cut into circular samples with a diameter of 65 mm. The test temperature was 38°C and the relative humidity was 90%. Each group was tested in parallel three times. Figure 7 The figures represent (A) water vapor transmission rate, (B) water vapor transmission coefficient, and (C) oxygen transmission rate of PE film and different PE / CCT composite films, where PE is pure polyethylene film (Comparative Example 1), PE / CCT-0 is Comparative Example 3, PE / CCT-2 is Example 2, PE / CCT-4 is Example 1, and PE / CCT-6 is Example 3. Figure 7 (A) and Figure 7As shown in (B), the chitosan-tannic acid-citric acid functional coating reduces the water vapor transmission rate and water vapor transmission coefficient of the PE film, and the moisture barrier capacity of the composite film is further improved with the increase of tannic acid content. The water vapor transmission coefficient of pure polyethylene film is 5.2×10-14 g·cm / (cm2·s·Pa), while that of PE / CCT-4 composite film is reduced to 3.6×10-14 g·cm / (cm2·s·Pa), a reduction of 30.8% compared with pure polyethylene film. The dense structure of chitosan-tannic acid crosslinked network increases the tortuous path of water vapor molecule diffusion, while the silane coupling agent layer itself has a certain hydrophobic barrier effect. The two work together to improve the moisture barrier capacity of the composite film.

[0039] Oxygen barrier performance test: The membrane was cut into 40mm × 40mm squares and sealed in a weighing bottle with a diameter of 40mm and a depth of 25mm. 3g of reduced iron powder was pre-placed inside the bottle. After the initial weighing, the bottle was placed in a desiccator containing a saturated sodium chloride solution and left at room temperature for 48 hours before being weighed again. Figure 7 As shown in (C), the pure polyethylene film has the highest oxygen permeability of 8.2 × 10⁻⁶. -3 g / (m 2 The oxygen permeability of the film after CCT coating decreased significantly, and showed a continuous decreasing trend with increasing tannic acid content. The oxygen permeability of the PE / CCT-4 composite film decreased to 2.8 × 10⁻⁶. -3 g / (m 2 The carbon dioxide content (·s) was reduced by 65.9% compared to pure polyethylene film. The orderly arrangement of chitosan molecular chains and the space-filling effect of tannic acid aromatic rings in the cross-linked network jointly hindered the diffusion channels of oxygen molecules through the coating.

[0040] UV blocking performance testing: such as Figure 8 As shown, PE is pure polyethylene film (Comparative Example 1), PE / CCT-0 is Comparative Example 3, PE / CCT-2 is Example 2, PE / CCT-4 is Example 1, and PE / CCT-6 is Example 3. Figure 8 As shown in (A), the transmittance of the film in the 200–800 nm wavelength range was measured using a UV-Vis spectrophotometer. Pure polyethylene film has nearly 100% transmittance across the entire wavelength range and lacks UV blocking capability. The PE / CCT-4 composite film has less than 1% transmittance in the 200–320 nm UV region, achieving near-complete blocking of UVB and UVC ultraviolet rays; less than 8% transmittance in the 320–400 nm UV region, also showing significant blocking effect against UVA ultraviolet rays; and maintains over 50% transmittance in the 400–800 nm visible light region. This excellent UV blocking performance stems from the strong absorption of ultraviolet light by the numerous phenolic hydroxyl groups and conjugated aromatic ring structure in the tannic acid molecule.

[0041] Antioxidant performance test: such as Figure 8 As shown in (B), the free radical scavenging rates of the membranes were tested using the DPPH and ABTS methods, respectively. DPPH method: 50 mg of the membrane sample was weighed and placed in 4 mL of 0.1 mmol / L DPPH methanol solution. After standing in the dark for 30 min, the absorbance was measured at 517 nm. ABTS method: An equal volume of 7.4 mmol / L ABTS solution and 2.6 mmol / L potassium persulfate solution were mixed. After reacting in the dark for 14 h, the solution was diluted to an absorbance of 0.70 at 734 nm. 5 mg of the membrane sample was weighed and placed in 4 mL of ABTS working solution. After standing in the dark for 10 min, the absorbance was measured at 734 nm. The DPPH and ABTS free radical scavenging rates of the pure polyethylene membrane were both below 5%, indicating almost no antioxidant capacity. The DPPH free radical scavenging rate of the PE / CCT-4 composite membrane was 95.8%, and the ABTS free radical scavenging rate was 99.2%. The phenolic hydroxyl groups of tannic acid can act as hydrogen atom donors to reduce free radicals into stable molecules, while the amino groups of chitosan can also chelate metal ions and block the free radicals generated by the Fenton reaction chain. The two work together to give the composite membrane excellent antioxidant activity.

[0042] Antibacterial performance test: such as Figure 8 As shown in (C), the antibacterial activity of the film was evaluated using the agar diffusion method. *Escherichia coli* and *Staphylococcus aureus* were used as representatives of Gram-negative and Gram-positive bacteria, respectively, and a concentration of 1.0 × 10⁻⁶ was used. 6 ~1.0×10 8 A bacterial suspension of CFU / mL was evenly spread onto nutrient agar plates, and a 10 mm diameter film disc was placed in the center of the plate. The plates were then incubated at 37°C and 90% relative humidity for 24 hours. Pure polyethylene film did not form an inhibition zone. The inhibition zone diameters of the PE / CCT-4 composite film against *Escherichia coli* and *Staphylococcus aureus* were 3.5 mm and 4.2 mm, respectively. The protonated amino groups of chitosan disrupted the integrity of microbial cell membranes through electrostatic interactions, tannic acid denatured microbial proteins by binding to them, and the acidic microenvironment of citric acid further inhibited microbial growth. The synergistic effect of these three components demonstrated superior antibacterial efficacy compared to any single component. Example 2.

[0043] The operating steps in this embodiment are basically the same as in Embodiment 1, except that the amount of tannic acid added in step three is adjusted to 2wt% of the chitosan mass, i.e., 0.04g of tannic acid is weighed and dissolved in 20mL of deionized water. The resulting composite active membrane is labeled PE / CCT-2.

[0044] The PE / CCT-2 composite membrane has a tensile strength of 21.5 MPa and an elongation at break of 168%; its water vapor transmission coefficient is 4.1 × 10⁻⁶. -14 g·cm / (cm2 (·s·Pa); oxygen permeability was 4.3×10 -3 g / (m 2 •s); UV transmittance (200~320nm) was less than 5%; DPPH radical scavenging rate was 78.6%, and ABTS radical scavenging rate was 82.3%; the inhibition zone diameters against Escherichia coli and Staphylococcus aureus were 1.5mm and 2.0mm, respectively. Compared with Example 1, all performance indicators were reduced, indicating that increasing the tannic acid content from 2wt% to 4wt% can significantly enhance the cross-linking network density and improve mechanical strength and barrier properties. In terms of antioxidant and antibacterial properties, the number of phenolic hydroxyl groups provided by 2wt% tannic acid is insufficient to achieve saturation scavenging, and there is room for further improvement in the free radical scavenging rate. In the infrared spectrum, PE / CCT-2 showed a high efficiency at 1635cm⁻¹. -1 The intensity of the characteristic peak of the C=N Schiff base at this location is weaker than that of PE / CCT-4, indicating that the lower amount of tannic acid added corresponds to a lower degree of Schiff base crosslinking. Example 3.

[0045] The operating steps in this embodiment are basically the same as those in Embodiment 1, except that the amount of tannic acid added in step three is adjusted to 6 wt% of the chitosan mass, i.e., 0.12 g of tannic acid is weighed and dissolved in 20 mL of deionized water. The resulting composite active membrane is labeled PE / CCT-6.

[0046] The PE / CCT-6 composite membrane has a tensile strength of 24.3 MPa and an elongation at break of 132%; its water vapor transmission coefficient is 3.5 × 10⁻⁶. -14 g·cm / (cm 2 (·s·Pa); oxygen permeability is 2.6×10 -3 g / (m 2 •s); UV transmittance (200~320nm) was less than 1%; DPPH radical scavenging rate was 96.5%, and ABTS radical scavenging rate was 99.5%; the inhibition zone diameters against Escherichia coli and Staphylococcus aureus were 4.0mm and 5.0mm, respectively. Compared with Example 1, the barrier properties and active functions were slightly improved, but the tensile strength decreased from 26.8MPa to 24.3MPa, and the elongation at break decreased from 145% to 132%, showing a downward trend in mechanical properties. Scanning electron microscopy revealed a small number of granular protrusions with a diameter of about 5~10μm on the surface of the PE / CCT-6 functional coating. These were phase separation domains formed by the local aggregation of excess tannic acid in the crosslinking network. These phase separation domains generated stress concentration effects, becoming the starting point for crack initiation during tensile testing, thus reducing the fracture toughness of the coating. Considering both mechanical properties and active functions, the composite film exhibited the best overall performance when the tannic acid addition was 4wt% of the chitosan mass. This conclusion is completely consistent with the changing trends of the indicators in the performance data table. Example 4

[0047] The operation steps in this embodiment are basically the same as those in Embodiment 1, except that: in step one, the silane coupling agent treatment is omitted, and only oxygen plasma treatment is performed. In step four, the base coating is omitted, and only a single layer of CCT-TA-CA functional coating liquid is coated. The resulting composite active film is labeled PE-p / CCT-4.

[0048] The PE-p / CCT-4 composite film showed an adhesion grade of 2 in the tape peel test (according to GB / T9286-2021 cross-cut adhesion test), while the PE / CCT-4 composite film in Example 1 showed an adhesion grade of 0. After being placed in an accelerated aging environment of 50°C and 95% relative humidity for 14 days, the coating of the PE-p / CCT-4 composite film showed obvious blistering and local peeling, while the coating of the PE / CCT-4 composite film showed no visible changes. This result confirms that the silane coupling agent chemical anchoring layer and the primer layer play an irreplaceable role in improving the long-term interfacial bonding strength between the coating and the polyethylene substrate.

[0049] Comparative Example 1.

[0050] Untreated pure polyethylene film was used as a control group and labeled as PE.

[0051] Comparative Example 2.

[0052] The polyethylene film is treated with oxygen plasma (40W, 40s) without any coating. Marked as PE-p.

[0053] Comparative Example 3.

[0054] The polyethylene film underwent dual modification and primer preparation according to steps one and two of Example 1. In step three, tannic acid was not added; only a chitosan-citric acid solution was used as the functional coating liquid for double-layer coating. The resulting film was labeled PE / CCT-0.

[0055] The PE / CCT-0 film has a tensile strength of 14.8 MPa and an elongation at break of 195%; its water vapor transmission coefficient is 4.6 × 10⁻⁶. -14 g·cm / (cm 2 (·s·Pa); oxygen permeability is 5.8×10 -3 g / (m 2 The transmittance in the ultraviolet region (200~320nm) is approximately 40%; the DPPH free radical scavenging rate is 12.3%, and the ABTS free radical scavenging rate is 15.6%; the inhibition zone diameters against Escherichia coli and Staphylococcus aureus are 0.5mm and 0.8mm, respectively.

[0056] Comparative Example 4.

[0057] A 1.0 wt% aqueous acetic acid solution was used instead of an aqueous citric acid solution as the solvent for chitosan, and the remaining steps were the same as in Example 1. The resulting film was labeled PE / CCT-4-A.

[0058] The PE / CCT-4-A film has a tensile strength of 20.1 MPa and an elongation at break of 172%; its water vapor transmission coefficient is 4.3 × 10⁻⁶. -14 g·cm / (cm 2 Compared to PE / CCT-4 (tensile strength 26.8 MPa) in Example 1, the tensile strength decreased by 25%, and the water vapor transmission coefficient increased by 19.4%. This result demonstrates that the co-crosslinking effect of citric acid in the ternary crosslinking network significantly contributes to improving the mechanical and barrier properties of the coating. Acetic acid, as a monocarboxylic acid, cannot participate in the crosslinking reaction and only plays a role in dissolving chitosan.

[0059] Comparative Example 5.

[0060] The composite membrane was prepared according to the method of Example 1, but the base coating in step four was omitted. That is, after completing the dual modification in step one, a single layer of CCT-TA-CA functional coating liquid was directly coated on the mPE surface. The resulting film was labeled PE / CCT-4-S.

[0061] The tape peel adhesion grade of the PE / CCT-4-S film was grade 1, lower than grade 0 (best) in Example 1. After 14 days of accelerated aging at 50°C and 95% relative humidity, slight lifting appeared at the coating edges. This result indicates that the base coating plays a crucial interfacial transition role between the silane coupling agent layer and the functional coating. Its chitosan matrix has good chemical affinity with both interfaces, effectively buffering interfacial stress caused by differences in the thermal expansion coefficients of the materials.

[0062] The thin film performance tests in the above embodiments and comparative examples were all performed according to the following standard methods.

[0063] Tensile property testing: Following the method specified in GB / T1040.1-2018, before testing, the film thickness was precisely measured at five randomly selected locations on the film surface using a digital thickness gauge, with an accuracy of 0.001 mm. The average value was taken as the thickness of the sample. The film was cut into strips of 50 mm × 10 mm and placed in a constant temperature and humidity chamber at 25°C and 50% relative humidity for 24 hours to eliminate the influence of stress history. The two ends of the sample were fixed to the clamps of a universal testing machine with a clamping distance of 30 mm and a tensile rate of 100 mm / min. The stress-strain curve was recorded until the sample broke. Each group of tests was performed in parallel for 5 times, and the average value was taken.

[0064] Water vapor barrier performance testing: Following the cup-type weight gain method specified in GB / T1037-2021, the membrane was cut into circular samples with a diameter of 65 mm. The test temperature was 38°C, and the relative humidity was 90%. Each group was tested in triplicate. The water vapor transmission coefficient was calculated using the formula WVP=WVTR×d / ΔP, where WVTR is the water vapor transmission rate, d is the membrane thickness, and ΔP is the water vapor partial pressure difference across the membrane.

[0065] Oxygen barrier performance testing: The membrane was cut into 40mm × 40mm squares and sealed in a weighing bottle with a diameter of 40mm and a depth of 25mm. 3g of reduced iron powder was pre-placed inside the bottle. After the initial weighing, the weighing bottle was placed in a desiccator containing a saturated sodium chloride solution and kept at a constant temperature of 25°C for 48 hours, then weighed again. The oxygen permeability was calculated using the formula OP = Δm / (A × t), where Δm is the mass difference, A is the test area, and t is the test time.

[0066] UV blocking performance testing: The membrane was cut into rectangular samples of 20mm × 30mm, and scanned in the range of 200~800nm ​​using a UV-3600 UV-Vis spectrophotometer at a medium scanning rate and a sampling interval of 1nm. Air was used as a reference, and the transmittance at each wavelength was recorded.

[0067] Water contact angle test: The hydrophilicity and hydrophobicity of each film surface were characterized using a contact angle meter. At room temperature, 2 μL of deionized water was dropped onto the film surface, and the droplet morphology was photographed after 3 seconds. The contact angle was calculated using software. The water contact angle of the untreated pure polyethylene film was 95.2°, which decreased to 38.5° after oxygen plasma treatment, and recovered to 62.3° after treatment with a silane coupling agent (the hydrocarbon segments of the silane layer have a certain degree of hydrophobicity). The water contact angle of the PE / CCT-4 composite film coated with the CCT-TA-CA functional coating was 71.8°. Moderate hydrophilicity of the coating surface is beneficial for forming a thin and uniform water film in food preservation packaging, preventing water droplets from condensing on the film surface and dripping onto the food surface, thus promoting microbial growth.

[0068] Adhesion test: The adhesion test is conducted according to GB / T9286-2021 using the cross-cut test method. A 6×6 grid is drawn on the coating surface using a multi-blade cutter with 1mm spacing. 3M 610 tape is applied and then peeled off at a uniform speed at a 90° angle. The adhesion level is determined based on the area of ​​coating peeling. Level 0 indicates that the cut edges are complete and smooth with no coating peeling; Level 1 indicates that there is a small amount of coating peeling at the intersections, with the peeling area not exceeding 5%; Level 2 indicates that the coating peeling area at the cut edges and intersections is 5~15%.

[0069] The methods for testing antioxidant and antibacterial properties are as described in detail in Example 1.

[0070] Table 1 shows a comparison of the overall performance of the films in each embodiment and comparative example.

[0071] As shown in Table 1, the tensile strength of the composite membrane first increases and then decreases with increasing tannic acid content, reaching a maximum of 26.8 MPa at 4 wt%. The elongation at break continuously decreases with increasing tannic acid content, but remains at 132% at 6 wt%. The water vapor transmission coefficient and oxygen transmission rate continuously decrease with increasing tannic acid content, indicating that the increased cross-linking degree continuously fills the micropores and intersegmental gaps in the polymer matrix. The UV blocking rate approaches 100% at 4 wt% tannic acid content, and further increases to 6 wt% show no significant improvement. The DPPH and ABTS free radical scavenging rates continuously increase with increasing tannic acid content, reaching 95.8% and 99.2% respectively at 4 wt%, with very limited improvement after further increases to 6 wt%. Based on the above data, the composite membrane exhibits the best performance in mechanical properties, barrier properties, and active functions when the tannic acid content is 4 wt% of the chitosan mass (Example 1).

[0072] Comparative Example 4 further confirms that replacing acetic acid with citric acid as the chitosan solvent can increase the tensile strength by 33.3% (from 20.1 MPa to 26.8 MPa) and decrease the water vapor permeability coefficient by 16.3% (from 4.3 × 10⁻⁶ MPa) through the co-crosslinking effect of citric acid. -14 Reduced to 3.6×10 -14） The difference was statistically significant.

[0073] The superior performance of the polyvinyl chitosan-tannic acid composite active membrane of this invention originates from a multi-level synergistic mechanism, encompassing three dimensions: interfacial chemical bonding, cross-linked network topology, and active function.

[0074] At the interfacial bonding level, the oxygen-containing groups introduced on the polyethylene surface by oxygen plasma treatment provide covalent bonding sites for the silane coupling agent. γ-aminopropyltriethoxysilane is firmly anchored to the polyethylene surface through Si-OC covalent bonds. Its exposed free amino groups undergo amidation reactions with the carboxyl groups of citric acid in the chitosan undercoat, achieving a continuous chemical bond transition from polyethylene to the functional coating. This PE-SiO-NH2-NHCO-CCT-TA covalent bond chain eliminates the interfacial fragility caused by relying solely on physical adsorption and hydrogen bonding in traditional coating processes, allowing the coating to maintain good adhesion even under long-term high humidity environments. The polysiloxane network formed by the self-condensation reaction of the silane coupling agent layer also acts as a flexible buffer layer. When interfacial shear stress occurs between the polyethylene substrate and the chitosan coating due to the difference in thermal expansion coefficients, the flexible segments of the polysiloxane network can undergo conformational adjustments to disperse the stress and prevent interfacial debonding. The comparative experimental data from Example 4 and Comparative Example 5 fully demonstrate the effectiveness of this interface design strategy.

[0075] At the coating structure level, the chitosan-tannic acid-citric acid ternary crosslinking network simultaneously contains three types of covalent bonds (Schiff base C=N bond between chitosan amino group and tannic acid phenolic hydroxyl group, Michael addition CN bond between chitosan amino group and tannic acid quinone group, and ester bond between citric acid carboxyl group and chitosan hydroxyl group) and two types of non-covalent bonds (hydrogen bond between tannic acid phenolic hydroxyl group and chitosan amide group, and π-π stacking between tannic acid aromatic rings), forming a richly layered crosslinking topology. Covalent crosslinking nodes provide a high-strength framework, while non-covalent crosslinking imparts a certain energy dissipation capacity to the network. Together, they enable the coating to possess both high strength and moderate ductility. In Comparative Example 4, the tensile strength decreased by 25% after replacing citric acid with acetic acid, directly demonstrating the irreplaceable contribution of citric acid esterification crosslinking to the mechanical properties of the ternary network. The short-range ordered structure formed by the π-π stacking of tannic acid aromatic rings increases the coating's barrier effect on gas molecule diffusion, which is a key reason why tannic acid surpasses other polyphenol compounds in improving barrier performance. From a molecular dynamics perspective, the increased crosslinking density restricts the thermal freedom of chitosan molecular chain segments, reduces the free volume between molecular chain segments, and thus lowers the diffusion coefficient of water vapor and oxygen molecules in the coating.

[0076] At the level of bioactivity, antioxidant activity is mainly achieved through two mechanisms: first, the hydrogen atom transfer mechanism, where the 25 phenolic hydroxyl groups of tannic acid can act as hydrogen atom donors, reducing reactive free radicals such as DPPH and ABTS to stable molecules, while simultaneously being oxidized to a quinone structure; second, the metal ion chelation mechanism, where the amino groups of chitosan and the carboxyl groups of citric acid can synergistically chelate transition metal ions such as iron and copper ions, thus cleaving the Fenton reaction (Fe2+). 2+ +H₂O₂→Fe 3+ +·OH+OH -） The catalytic chain of the Haber-Weiss reaction reduces the generation of hydroxyl radicals at the source. Experimental data show that the DPPH scavenging rate (95.8%) and ABTS scavenging rate (99.2%) of the PE / CCT-4 composite membrane are much higher than those of PE / CCT-0 without added tannic acid (12.3% and 15.6%), indicating that tannic acid is the main contributor to the antioxidant activity of the composite membrane.

[0077] In terms of antibacterial properties, the protonated amino groups (NH3) of chitosan... +） Tannic acid adsorbs onto the negatively charged cell membrane surface of microorganisms through electrostatic interactions, altering membrane permeability and causing leakage of intracellular substances. It then penetrates the damaged cell membrane into the cell, binding with microbial membrane proteins and intracellular enzymes through hydrogen bonds and hydrophobic interactions, causing denaturation of the protein's tertiary structure and loss of catalytic activity. Citric acid releases H+... +Lowering the microenvironment pH to below 4.5 deviates from the optimal pH range for most foodborne pathogens (Escherichia coli: 6.0-7.0; Staphylococcus aureus: 6.0-7.5), thus inhibiting microbial proliferation at the metabolic level. The three active components work synergistically to exert antibacterial effects through cell membrane disruption, intracellular protein denaturation, and metabolic inhibition, with significantly better results than any single component. In Comparative Example 3, the inhibition zone of the PE / CCT-0 film without tannic acid was only 0.5-0.8 mm, while in Example 1, the inhibition zone expanded to 3.5-4.2 mm after adding 4 wt% tannic acid, an increase of more than four times, fully demonstrating the significant synergistic effect of tannic acid and chitosan in antibacterial activity.

[0078] like Figure 9 As shown, PE is pure polyethylene film (Comparative Example 1), PE / CCT-2 is Example 2, PE / CCT-4 is Example 1, and PE / CCT-6 is Example 3. Freshly picked Jixi cucumbers with uniform appearance and no mechanical damage were selected. After rinsing with tap water, they were rinsed twice with deionized water and air-dried in a cool, ventilated place at 25°C until no residual water droplets remained on the surface. Five cucumbers were individually packaged using pure polyethylene film and PE / CCT-4 composite active film, respectively, and stored in a constant temperature and humidity chamber at 25°C and 60% relative humidity for 5 days. Appearance changes, mass loss rate, hardness, and soluble solids content were observed and recorded at the same time each day.

[0079] Appearance Changes: Cucumbers packaged with pure polyethylene film maintained a bright green surface on day 1, but began to turn yellow in some areas on day 2, with a significant decrease in skin gloss. On day 3, obvious brown spots appeared, and the cucumbers began to wilt. On day 4, the browning area expanded, and the surface became sunken and wrinkled. On day 5, the cucumbers softened and deformed severely, turning a dark yellowish-brown. Cucumbers packaged with PE / CCT-4 composite active film maintained a bright green color for days 1-3, with good skin gloss and a firm feel. On day 4, only a very slight lightening of color occurred, with no brown spots. On day 5, the cucumbers maintained a relatively upright shape, with only slight wilting at the top; their overall appearance was far superior to those packaged with pure polyethylene film.

[0080] Weight loss rate: The weight loss rate of cucumbers packaged in pure polyethylene film was 0.6% on day 1, 1.8% on day 2, 3.2% on day 3, and reached 4.8% on day 4, approaching the 5% warning threshold for weight loss in fresh fruits and vegetables. The weight loss rate of the PE / CCT-4 group was 0.2% on day 1, 0.5% on day 2, 0.9% on day 3, 1.4% on day 4, and only 1.8% on day 5, far below the warning threshold. The excellent moisture and oxygen barrier properties of the composite active film effectively slowed down the transpiration and aerobic respiration of cucumbers, while its antibacterial function inhibited the leakage of tissue fluid caused by microbial growth.

[0081] Hardness Changes: The hardness of cucumbers was measured using a digital fruit hardness meter with a probe diameter of 3.5 mm and a puncture depth of 5 mm. The initial hardness of the pure polyethylene film packaging group was 10.0 kgf, decreasing to 8.5 kgf on day 2, 7.2 kgf on day 3, and 5.8 kgf on day 4, with significant tissue collapse and softening. The initial hardness of the PE / CCT-4 group was 10.1 kgf, remaining at 9.8 kgf on day 2, 9.5 kgf on day 3, 9.3 kgf on day 4, and still maintaining at 9.2 kgf on day 5. The oxygen barrier and antibacterial functions of the composite active film effectively maintained the turgor pressure and cell wall integrity of the cucumber pulp cells, delaying the hydrolytic degradation of cell wall polysaccharides by pectinase.

[0082] Soluble solids content: This was determined using a handheld digital saccharimeter. A suitable amount of cucumber was crushed, and the juice was collected for testing. The initial soluble solids content of the pure polyethylene film packaging group was 2.6%, increasing to 3.2% on day 2 and 3.8% on day 4, showing a rapid initial increase followed by a slower increase. The initial content of the PE / CCT-4 group was 2.6%, increasing to 3.5% on day 2 and 4.6% on day 4. Cucumbers are relatively young at harvest, and nutrients continue to accumulate during storage. The composite active film group, due to the effective inhibition of respiration and microbial metabolism, experienced a slower rate of nutrient consumption and a greater net accumulation, thus resulting in a consistently higher soluble solids content than the pure polyethylene film group.

[0083] Color difference analysis: The L (lightness), a (red-green), and b (yellow-blue) values ​​of the cucumber surface were measured using a colorimeter. In the pure polyethylene film packaging group, the L value continuously decreased from 65.3 to 48.7 during storage, the a value increased from an initial -8.2 to -1.5 (significant loss of greenness), and the b value increased from 35.2 to 52.6 (significant increase in yellowness), indicating severe chlorophyll degradation and yellowing of the cucumbers. In the PE / CCT-4 group, the L value slowly decreased from 65.1 to 60.3, the a value only changed from -8.3 to -6.1, and the b value increased from 35.0 to 40.2; the color difference variation was much smaller than that of the pure polyethylene film group. The color difference data quantitatively confirmed the significant advantage of the composite active film in delaying the deterioration of the cucumber's appearance quality.

[0084] Based on the above preservation test data, the PE / CCT-4 composite active film of this invention, through the synergistic effect of five functions—oxygen barrier, moisture barrier, ultraviolet barrier, antioxidant, and antibacterial—delays the dehydration and wilting, chlorophyll degradation, oxidative browning, and microbial spoilage of cucumbers from two dimensions: material transport barrier and active function intervention. This extends the shelf life of cucumbers under normal temperature storage conditions from 2-3 days for pure polyethylene film packaging to more than 5 days.

[0085] The embodiments of the present invention are not limited to the specific embodiments described above. Those skilled in the art can make various equivalent changes or substitutions based on the technical solutions of the present invention, and all such changes or substitutions should be included within the protection scope of the present invention.

Claims

1. A polyvinyl chitosan-tannic acid composite active film, characterized in that, From bottom to top, the coating consists of a polyethylene base layer, a silane coupling agent chemical anchoring layer, a chitosan-citric acid base layer, and a chitosan-tannic acid-citric acid functional coating. The silane coupling agent chemical anchoring layer is a polysiloxane thin layer formed by Si-OC covalent bonding of γ-aminopropyltriethoxysilane on the surface of a polyethylene film treated with oxygen plasma, with free amino groups exposed on the surface of this thin layer. The chitosan-citric acid base layer is a thin layer with a thickness of 3-5 μm obtained by dissolving chitosan in an aqueous citric acid solution and then drying it. The chitosan-tannic acid-citric acid functional coating is a ternary crosslinked network thin layer with a thickness of 8-12 μm formed by crosslinking chitosan, tannic acid, and citric acid under heating conditions through hydrogen bonding, Schiff base covalent bonding, and esterification, wherein the amount of tannic acid added is 2-6 wt% of the mass of chitosan.

2. The composite active membrane according to claim 1, characterized in that, The polyethylene base layer is made of low-density polyethylene and has a thickness of 10~20μm.

3. The composite active membrane according to claim 1, characterized in that, The amount of tannic acid added is 4 wt% of the chitosan mass.

4. The composite active membrane according to claim 1, characterized in that, The degree of deacetylation of the chitosan is 80-95%, and the viscosity-average molecular weight is 200-500 kDa.

5. A method for preparing the polyvinyl chitosan-tannic acid composite active film according to any one of claims 1 to 4, characterized in that, Includes the following steps: Step 1, Dual modification of polyethylene film surface: Using oxygen as the working gas, the polyethylene film is subjected to plasma treatment at a power of 30~50W and a time of 20~60s; after treatment, the polyethylene film is immersed in an ethanol-water mixed solution of γ-aminopropyltriethoxysilane with a concentration of 1~3wt% at room temperature for 20~40min, then removed, washed, and dried at 70~90°C for 20~40min to allow the silane to complete the condensation reaction; Step 2, preparation of chitosan-citric acid primer: add chitosan to a citric acid aqueous solution with a concentration of 1.0~2.0wt%, the concentration of chitosan is 1.5~2.5wt%, stir at 50~70°C for 3~5h until completely dissolved, and then add plasticizer; Step 3, preparation of chitosan-tannic acid-citric acid functional coating solution: Prepare chitosan-citric acid solution according to the method in step 2. Dissolve tannic acid in deionized water and add it to chitosan-citric acid solution. Stir at 50~70°C for 0.5~1.5h and then add plasticizer. Step 4, double-layer gradient coating: First, apply the primer liquid from Step 2 to the surface of the modified polyethylene film from Step 1, and allow it to dry and pre-cur; then apply the functional coating liquid from Step 3 to the surface of the primer layer, and allow it to dry and cure.

6. The preparation method according to claim 5, characterized in that, The volume ratio of the ethanol-water mixture in step one is (8~9.5):1, and the pH value of the solution is adjusted to 4.0~5.5 with acetic acid.

7. The preparation method according to claim 5, characterized in that, In step four, the coating thickness of the primer is 0.3~0.8mm and the coating speed is 60~100mm / min; the coating thickness of the functional coating liquid is 0.8~1.5mm and the coating speed is 80~120mm / min; the drying temperature is 50~70°C.

8. The preparation method according to claim 5, characterized in that, The plasticizer mentioned in steps two and three is glycerol, and the amount added is 0.3~1.0% of the volume of the chitosan-citric acid solution.

9. The application of the polyvinyl chitosan-tannic acid composite active film according to any one of claims 1 to 4 in food preservation packaging.

10. The application according to claim 9, characterized in that, The food in question is fresh fruits and vegetables with a water content greater than 90%.

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