Environment-friendly plastic bag and preparation method thereof

Abstract

The present application relates to the technical field of plastic packaging materials, and discloses an environment-friendly plastic bag and a preparation method thereof, which comprises the following steps: performing surface pretreatment on a low-density polyethylene base material through corona treatment; coating a chitosan-sodium-based montmorillonite composite inner layer barrier coating on the surface of the base material; performing crosslinking pretreatment on the inner layer coating by using citric acid to form a three-dimensional network structure; coating an alginate propylene glycol ester-chitosan grafted polymer composite intermediate transition layer to realize interface bridging; coating a nano-silver-sodium alginate outer layer antibacterial coating; respectively curing each layer of coating by using a 60 DEG C-70 DEG C-80 DEG C three-section gradient curing process; and heat sealing and forming into a plastic bag. The peeling strength of the coating is improved from below 0.5 N / cm to above 2.5 N / cm, the coating integrity is maintained above 95% in a high-humidity environment, and the technical effects of high interface bonding strength, barrier-antibacterial synergistic function and recyclable utilization are realized.

Description

Technical Field

[0001] This invention relates to the field of plastic packaging materials technology, and more specifically, to an environmentally friendly plastic bag and its preparation method. Background Technology

[0002] Polyethylene (PE) plastic bags are widely used in food packaging, but plain PE film has limited barrier properties against oxygen and water vapor, and lacks antibacterial properties, making it difficult to effectively extend the shelf life of packaged food. To address this issue, existing technologies typically coat the surface of PE film with functional coatings, such as barrier coatings (to improve oxygen and water vapor barrier properties) or antibacterial coatings (to inhibit microbial growth).

[0003] However, existing functional coated plastic bags face several technical bottlenecks. First, the interfacial bonding strength between the coating and the PE substrate is insufficient, relying solely on physical adsorption or weak intermolecular forces. This makes them prone to coating peeling or delamination under high humidity or repeated rubbing conditions. Second, single-function coatings cannot simultaneously meet the dual requirements of barrier properties and antibacterial properties, while simple double-layer coating stacking suffers from poor interlayer adhesion, hindering the full synergistic effect of the two functions. Third, current coating preparation processes typically employ single-temperature curing, failing to consider the optimal curing temperature requirements of different coating materials, leading to impaired performance of some coatings. Furthermore, the coating material is difficult to separate from the PE substrate, affecting the recycling value of the plastic bags. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides an environmentally friendly plastic bag and a method for preparing the same.

[0005] A method for preparing an environmentally friendly plastic bag includes the following steps: Step 1: Perform surface pretreatment on the polyethylene substrate; Step 2: Coat the substrate surface with an inner barrier coating composed of chitosan and montmorillonite, and allow it to dry naturally until it is semi-dry. Step 3: Spray a citric acid aqueous solution onto the semi-dry inner barrier coating surface for cross-linking pretreatment, and then cure it at the first temperature. The polycarboxyl structure of citric acid undergoes a cross-linking reaction with the amino group of chitosan and the hydroxyl group on the surface of montmorillonite, thus constructing a partially cross-linked three-dimensional network structure in the inner coating. Step 4: Coat the surface of the cured inner barrier coating with an intermediate transition layer composed of propylene glycol alginate and chitosan grafted polymer, and then cure it at a second temperature. The main chain of the chitosan grafted polymer is chitosan, the side chain is polyacrylic acid, propylene glycol alginate forms a bridge with the inner coating, and the chitosan grafted polymer forms a bridge with both the inner and outer layers. Step 5: Coat the surface of the cured intermediate transition layer with an outer antibacterial coating composed of nano-silver and sodium alginate, and then cure it at the third temperature; Step 6: Heat-seal the coated multi-layer polyethylene film to form a plastic bag; The first temperature, the second temperature, and the third temperature increase sequentially to form a gradient curing process.

[0006] Preferably, in step 3, the citric acid aqueous solution has a mass fraction of 1-3%, the spraying amount is 5-15 g / m², the first temperature is 60℃, and the curing time is 3-5 minutes.

[0007] Preferably, in step 2, the chitosan has a degree of deacetylation ≥ 85%, a molecular weight of 100,000-300,000, and a viscosity of 50-200 mPa·s; the montmorillonite is sodium-based montmorillonite with a cation exchange capacity of 80-120 mmol / 100g, a particle size of 2-10μm, and an interlayer spacing of 1.0-1.5 nm; the mass ratio of chitosan to montmorillonite is 3:1 to 5:1.

[0008] Preferably, in step 4, the alginate propylene glycol solution and the chitosan graft polymer solution are mixed at a mass ratio of 3:2 to 4:1.

[0009] Preferably, the degree of esterification of the propylene glycol alginate is 30-60%, the molecular weight is 50,000-150,000, and the viscosity is 100-500 mPa·s; the grafting rate of the chitosan graft polymer is 20-50%, the molecular weight is 150,000-400,000, the viscosity is 200-800 mPa·s, and the amino content is 2.5-4.5 mmol / g.

[0010] Preferably: in step 4, the second temperature is 70°C and the curing time is 4-6 minutes; in step 5, the third temperature is 80°C and the curing time is 3-5 minutes.

[0011] Preferably, in step 5, the sodium alginate has a molecular weight of 100,000-500,000, a viscosity of 50-300 mPa·s, and an M / G ratio of 1.2-1.8; the nano-silver has a particle size of 10-50 nm and a silver content of ≥99.9%; and the mass ratio of nano-silver to sodium alginate is 1:10 to 1:20.

[0012] Preferably, in step 1, the polyethylene substrate is a low-density polyethylene film with a density of 0.91-0.93 g / cm³, a melt index of 1.5-3.5 g / 10min, and a thickness of 30-80 μm; the surface pretreatment includes: cleaning with a 75-95% ethanol solution and drying, and then using corona treatment to increase the surface energy. The corona treatment power is 300-500W, the treatment speed is 5-10m / min, and the surface tension reaches 38-42 mN / m.

[0013] Preferably: in step 2, the inner barrier coating is applied by roller coating, with a roller speed of 10-30 r / min; in step 4, the intermediate transition layer is applied by scraping, with the angle between the scraper and the coating surface being 30-45° and the scraping speed being 3-8 m / min; in step 5, the outer antibacterial coating is applied by spraying, with a spraying pressure of 0.2-0.4 MPa and a spraying distance of 15-25 cm.

[0014] Preferably, in step 6, the heat sealing conditions are: heat sealing temperature 120-150℃, heat sealing pressure 0.2-0.4MPa, and heat sealing time 1-3 seconds.

[0015] The beneficial effects of this invention are as follows: 1. Significantly improves the interfacial bonding strength of the coating. A partially cross-linked three-dimensional network structure is constructed in the inner coating through citric acid cross-linking pretreatment, providing physical anchoring sites and chemical bonding sites for subsequent coatings. A two-component material (propylene glycol alginate and chitosan grafted polymer) in the intermediate transition layer forms chemical or physical bridges with both the inner and outer layers, achieving a multi-faceted interface enhancement mechanism of "chemical cross-linking-molecular bridging-electrostatic hydrogen bonding." The coating peel strength is improved from below 0.5 N / cm in existing technologies to above 2.5 N / cm, an increase of over 400%. After 30 days in a high-humidity environment (80% relative humidity), the coating integrity remains above 95%, while the integrity of existing double-layer coatings drops to below 60% under the same conditions. This high interfacial bonding strength ensures the long-term stability and reliability of the plastic bag in practical use (high humidity environment, repeated rubbing).

[0016] 2. Achieve synergistic enhancement of barrier and antibacterial functions The inner chitosan-montmorillonite composite coating provides excellent oxygen and water vapor barrier properties, while the outer nano-silver-sodium alginate composite coating offers broad-spectrum antibacterial functionality. The intermediate transition layer not only enhances interfacial bonding, but its propylene glycol alginate component also possesses certain barrier and antibacterial properties, creating a synergistic effect with the inner and outer layers. Through high-strength interfacial bonding, the three coating layers become a unified whole, allowing each layer to fully exert its function, resulting in an overall preservation effect 3-4 times that of a single functional layer. The shelf life of fresh food is extended to 2-3 times that of traditional plastic bags, representing a further improvement of over 40% compared to traditional double-layered plastic bags, significantly reducing food spoilage and loss rates.

[0017] 3. Optimize the coating preparation process to achieve optimal performance for each layer. A three-stage gradient curing process (60℃→70℃→80℃) is employed, with optimal curing temperatures set based on the characteristics of each coating material. The inner layer is cured at 60℃ to maintain the integrity of the montmorillonite sheet structure and its barrier properties; the middle layer is cured at 70℃ to form a stable transition interface; and the outer layer is cured at 80℃ to promote uniform dispersion of nano-silver and densification of the coating. Gradient curing avoids the performance compromises caused by single-temperature curing, ensuring that each coating layer reaches its optimal performance state. Simultaneously, the temperature gradient reduces thermal stress concentration, resulting in an overall increase of over 60% in the coating's mechanical strength and flexibility, and a 25% increase in production efficiency.

[0018] 4. Maintain good recyclability. The coating materials are all natural or modified natural polymers (chitosan, montmorillonite, sodium alginate, propylene glycol alginate, etc.). Although citric acid crosslinking and interfacial bridging enhance the interfacial bonding, citric acid crosslinking is reversible and can partially de-crosslink under alkaline conditions. The coating material and the PE substrate can still be effectively separated through appropriate recycling processes (such as alkaline immersion and mechanical separation). The recycled PE material has a purity of over 95%, and its mechanical properties are close to those of the virgin material, resulting in high recycling value and compliance with environmental protection and circular economy requirements.

[0019] 5. Packaging for fresh food suitable for high humidity environments The three-layer coating system, citric acid cross-linking network, and multiple interface reinforcements enable the plastic bags to maintain stable barrier and antibacterial properties for a long time even in high humidity environments (relative humidity 85-95%), without swelling, peeling, or functional degradation of the coating. It is particularly suitable for packaging fresh foods such as fruits, vegetables, aquatic products, and meat products in cold chain logistics, significantly reducing food spoilage rates, minimizing food loss, and extending shelf life, resulting in significant economic and social benefits.

[0020] 6. Made with natural materials, safe and environmentally friendly. The coating materials primarily consist of natural or modified natural polymers such as chitosan, montmorillonite, sodium alginate, and propylene glycol alginate, as well as food-grade antibacterial agents like nano-silver. These materials exhibit good biocompatibility, high safety, and comply with regulations for food contact materials. The citric acid crosslinking agent is also a food-grade natural organic acid, non-toxic and harmless. The overall solution aligns with green and environmentally friendly principles, contributing to the sustainable development of the plastic packaging industry. Attached Figure Description

[0021] Figure 1 This is a bar chart comparing the peel strength of different samples of the present invention. Figure 2 This is a SEM image of the coating surface of the sample of the present invention; Figure 3 This is the coating integrity change curve under high humidity conditions according to the present invention; Figure 4 This is a bar chart showing the change in peel strength retention rate under high humidity conditions according to the present invention; Figure 5 This is a curve showing the variation of the decay rate of strawberries packaged in different samples according to the present invention; Figure 6 This is a bar chart comparing the barrier and antibacterial properties of different samples of the present invention. Detailed Implementation

[0022] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples.

[0023] Example 1: This example presents a method for preparing an environmentally friendly plastic bag, including the following steps: Step 1: Perform surface pretreatment on the polyethylene substrate; The polyethylene substrate is a low-density polyethylene film with a density of 0.92 g / cm³, a melt index of 2.5 g / 10min, and a thickness of 55 μm. The surface pretreatment includes: cleaning with an 85% ethanol solution and drying, followed by corona treatment to increase the surface energy. The corona treatment power is 400W and the treatment speed is 8m / min, so that the surface tension reaches 40 mN / m.

[0024] Step 2: Coat the substrate surface with an inner barrier coating composed of chitosan and montmorillonite, and allow it to dry naturally until it is semi-dry. The chitosan has a degree of deacetylation of 85%, a molecular weight of 200,000, and a viscosity of 125 mPa·s; the montmorillonite is sodium-based montmorillonite with a cation exchange capacity of 100 mmol / 100g, a particle size of 6μm, and an interlayer spacing of 1.2 nm; the mass ratio of chitosan to montmorillonite is 4:1. The inner barrier coating is applied using a roller coating method with a roller speed of 20 r / min.

[0025] Step 3: Spray a citric acid aqueous solution onto the semi-dry inner barrier coating surface for cross-linking pretreatment, and then cure it at the first temperature. The polycarboxyl structure of citric acid undergoes a cross-linking reaction with the amino group of chitosan and the hydroxyl group on the surface of montmorillonite, thus constructing a partially cross-linked three-dimensional network structure in the inner coating. The citric acid aqueous solution has a mass fraction of 2%, the spraying amount is 10 g / m², the first temperature is 60℃, and the curing time is 4 minutes.

[0026] Step 4: Coat the surface of the cured inner barrier coating with an intermediate transition layer composed of propylene glycol alginate and chitosan grafted polymer, and then cure it at a second temperature. The main chain of the chitosan grafted polymer is chitosan, the side chain is polyacrylic acid, propylene glycol alginate forms a bridge with the inner coating, and the chitosan grafted polymer forms a bridge with both the inner and outer layers. The intermediate transition layer was applied by scraping, with the angle between the scraper and the coating surface at 38° and the scraping speed at 5 m / min. The alginate propylene glycol solution and the chitosan graft polymer solution were mixed at a mass ratio of 3.5:1.5.

[0027] The propylene glycol alginate has a degree of esterification of 45%, a molecular weight of 100,000, and a viscosity of 300 mPa·s; the chitosan graft polymer has a grafting rate of 35%, a molecular weight of 370,000, a viscosity of 500 mPa·s, and an amino content of 3.5 mmol / g. The second temperature is 70°C, and the curing time is 5 minutes; in step 5, the third temperature is 80°C, and the curing time is 4 minutes.

[0028] Step 5: Coat the surface of the cured intermediate transition layer with an outer antibacterial coating composed of nano-silver and sodium alginate, and then cure it at the third temperature; The outer antibacterial coating is applied by spraying at a pressure of 0.3 MPa and a distance of 20 cm. The sodium alginate has a molecular weight of 300,000, a viscosity of 175 mPa·s, and an M / G ratio of 1.5; the nano-silver has a particle size of 30 nm and a silver content of 99.9%; the mass ratio of nano-silver to sodium alginate is 1:15. Step 6: Heat-seal the coated multi-layer polyethylene film to form a plastic bag; The heat sealing conditions are: heat sealing temperature 135℃, heat sealing pressure 0.3MPa, and heat sealing time 2 seconds; The first temperature, the second temperature, and the third temperature increase sequentially, forming a gradient curing process.

[0029] Example 2: This example differs from Example 1 in that: The polyethylene substrate is a low-density polyethylene film with a density of 0.91 g / cm³, a melt index of 1.5 g / 10min, and a thickness of 30 μm. The surface pretreatment includes cleaning with a 75% ethanol solution and drying, followed by corona treatment to increase the surface energy. The corona treatment power is 300W and the treatment speed is 5m / min, so that the surface tension reaches 38 mN / m.

[0030] The chitosan has a degree of deacetylation of 86%, a molecular weight of 100,000, and a viscosity of 50 mPa·s; the montmorillonite is sodium-based montmorillonite with a cation exchange capacity of 80 mmol / 100g, a particle size of 2μm, and an interlayer spacing of 1.0 nm; the mass ratio of chitosan to montmorillonite is 3:1. The citric acid aqueous solution has a mass fraction of 1%, the spraying amount is 5 g / m², the first temperature is 60℃, and the curing time is 3 minutes; The alginate propylene glycol solution and the chitosan graft polymer solution were mixed at a mass ratio of 3:2.

[0031] The degree of esterification of propylene glycol alginate is 30%, the molecular weight is 50,000, and the viscosity is 100 mPa·s; the grafting rate of the chitosan graft polymer is 20%, the molecular weight is 150,000, the viscosity is 200 mPa·s, and the amino content is 2.5 mmol / g.

[0032] The second temperature is 70°C and the curing time is 4 minutes; in step 5, the third temperature is 80°C and the curing time is 3 minutes.

[0033] The sodium alginate has a molecular weight of 100,000, a viscosity of 50 mPa·s, and an M / G ratio of 1.2; the nano-silver has a particle size of 10 nm and a silver content of 99.99%; the mass ratio of nano-silver to sodium alginate is 1:10.

[0034] The heat sealing conditions are: heat sealing temperature 120℃, heat sealing pressure 0.2MPa, and heat sealing time 1 second.

[0035] The inner barrier coating was applied using a roller coating method with a roller speed of 10 r / min. The intermediate transition layer was applied by scraping, with the angle between the scraper and the coating surface at 30° and the scraping speed at 3 m / min. The outer antibacterial coating is applied by spraying at a pressure of 0.2 MPa and a distance of 15 cm.

[0036] Example 3: This example differs from Example 1 in that: The polyethylene substrate is a low-density polyethylene film with a density of 0.93 g / cm³, a melt index of 3.5 g / 10min, and a thickness of 80 μm. The surface pretreatment includes cleaning with a 95% ethanol solution and drying, followed by corona treatment to increase the surface energy. The corona treatment power is 500W and the treatment speed is 10m / min, so that the surface tension reaches 42 mN / m.

[0037] The chitosan has a degree of deacetylation of 87%, a molecular weight of 300,000, and a viscosity of 200 mPa·s; the montmorillonite is sodium-based montmorillonite with a cation exchange capacity of 120 mmol / 100g, a particle size of 10 μm, and an interlayer spacing of 1.5 nm; the mass ratio of chitosan to montmorillonite is 5:1. The citric acid aqueous solution has a mass fraction of 3%, the spraying amount is 15 g / m², the initial temperature is 60℃, and the curing time is 5 minutes. The alginate propylene glycol solution and the chitosan graft polymer solution were mixed at a mass ratio of 4:1.

[0038] The degree of esterification of propylene glycol alginate is 60%, the molecular weight is 150,000, and the viscosity is 500 mPa·s; the grafting rate of the chitosan graft polymer is 50%, the molecular weight is 400,000, the viscosity is 800 mPa·s, and the amino content is 4.5 mmol / g.

[0039] The second temperature is 70°C and the curing time is 6 minutes; in step 5, the third temperature is 80°C and the curing time is 5 minutes.

[0040] The sodium alginate has a molecular weight of 500,000, a viscosity of 300 mPa·s, and an M / G ratio of 1.8; the nano-silver has a particle size of 50 nm and a silver content of 99.999%; the mass ratio of nano-silver to sodium alginate is 1:20.

[0041] The heat sealing conditions are: heat sealing temperature 150℃, heat sealing pressure 0.4MPa, and heat sealing time 3 seconds.

[0042] The inner barrier coating is applied by roller coating at a speed of 30 r / min. The intermediate transition layer was applied by scraping, with the angle between the scraper and the coating surface at 45° and the scraping speed at 8 m / min. The outer antibacterial coating is applied by spraying at a pressure of 0.4 MPa and a distance of 25 cm.

[0043] Example 4: This example proposes a method for preparing an environmentally friendly plastic bag. By constructing a three-layer functional coating system on a PE substrate and employing citric acid crosslinking pretreatment and gradient curing processes, high interfacial bonding strength, synergistic barrier-antibacterial function, and recyclability are achieved. The method includes the following steps: Step 1: PE substrate preparation and surface pretreatment Recyclable low-density polyethylene (LDPE) film was selected as the substrate, with a density of 0.91-0.93 g / cm³, preferably 0.92 g / cm³, a melt index of 1.5-3.5 g / 10min (190℃, 2.16kg), preferably 2.5 g / 10min, and a thickness of 30-80 μm, preferably 50 μm. Low-density polyethylene exhibits good flexibility and processability. The density of 0.92 g / cm³ ensures the material has moderate crystallinity (approximately 50-60%), and the melt index of 2.5 g / 10min ensures moderate fluidity, facilitating subsequent heat sealing while maintaining sufficient mechanical strength. The PE film undergoes surface pretreatment. First, it is cleaned with an ethanol solution of 75-95% by mass, preferably 85%, to remove oil and impurities, and then air-dried. Next, corona treatment is applied to increase the surface energy. The corona treatment power is 300-500W, preferably 400W, and the treatment speed is 5-10m / min, preferably 8m / min, so that the surface tension reaches 38-42mN / m, preferably 40mN / m. This introduces polar groups such as hydroxyl and carbonyl groups into the PE surface to improve the adhesion of subsequent coatings.

[0044] Step 2: Preparation and application of inner barrier coating Preparation of the inner barrier coating solution: Chitosan (degree of deacetylation ≥85%, preferably 90%, molecular weight 100,000-300,000, preferably 200,000, viscosity 50-200 mPa·s, preferably 100 mPa·s) is dissolved in an acetic acid solution with a mass fraction of 1-2%, preferably 1.5%, to obtain a chitosan solution with a mass fraction of 2-4%, preferably 3%. Sodium montmorillonite powder (cation exchange capacity 80-120 mmol / 100g, preferably 100 mmol / 100g, particle size 2-10 μm, preferably 5 μm, interlayer spacing 1.0-1.5 nm, preferably 1.2 nm) is added to the solution. The mass ratio of chitosan to montmorillonite is 3:1 to 5:1, preferably 4:1. The mixture is stirred at a speed of 300-600 r / min, preferably 450 r / min, for 30-60 minutes, preferably 45 minutes, until homogeneous, to obtain the inner barrier coating solution. A stirring speed of 450 r / min ensures thorough dispersion of montmorillonite in the chitosan solution, preventing agglomeration and ensuring optimal barrier properties. The high cation exchange capacity of sodium-based montmorillonite facilitates its binding with chitosan through ion exchange, while a 1.2 nm interlayer spacing ensures the integrity of the montmorillonite's layered structure, providing excellent barrier performance. The chitosan used is a food-grade natural polymer, and the montmorillonite is a food-grade layered silicate mineral, both meeting the requirements of GB 4806.7-2016 "National Food Safety Standard for Plastic Materials and Products for Food Contact".

[0045] The coating liquid is applied to the pretreated PE substrate surface using a roller coating method. The roller speed is 10-30 r / min, preferably 20 r / min, and the coating thickness is controlled at 10-20 μm wet film, preferably 15 μm, corresponding to a dry film thickness of 3-6 μm, preferably 4.5 μm. Roller coating enables large-area uniform coating, and the roller speed of 20 r / min ensures that the coating liquid is fully spread and avoids the formation of air bubbles. After coating, it is allowed to air dry at room temperature (20-25℃, preferably 22℃) for 5-10 minutes, preferably 7 minutes, to remove most of the moisture. At this point, the coating still retains 15-25%, preferably 20%, of moisture. The molecular chains of the coating material are not yet fully fixed, and it has a certain degree of fluidity, being in a semi-dry state. Chitosan provides film-forming and antibacterial properties, while the montmorillonite lamellar structure blocks oxygen and water vapor. This semi-dry coating needs to undergo citric acid crosslinking pretreatment and curing in step 3 to form an inner coating with excellent barrier properties.

[0046] Step 3: Citric acid crosslinking pretreatment This step is one of the core aspects of this invention. In existing technologies, the inner and outer layers of multilayer coated plastic bags are only bonded by weak intermolecular forces such as hydrogen bonds and electrostatic interactions, resulting in low interfacial bonding strength and a tendency to delamination in high-humidity environments. This invention creatively introduces a citric acid crosslinking pretreatment step before the inner barrier coating cures. The multi-carboxyl group structure of citric acid reacts with the amino groups of chitosan and the hydroxyl groups on the surface of montmorillonite to form a partially crosslinked three-dimensional network structure in the inner coating. This network structure not only improves the mechanical strength and stability of the inner coating itself, but more importantly, it provides physical anchoring sites and chemical bonding sites for the subsequent intermediate transition layer and outer antibacterial coating, fundamentally solving the problem of insufficient interfacial bonding strength in existing technologies.

[0047] Existing technologies typically involve directly coating an outer antibacterial coating after the inner barrier coating has cured. The layers are bound together only by physical adsorption or weak intermolecular forces of the coating materials themselves, lacking effective interface enhancement measures. This invention, by introducing citric acid crosslinking pretreatment before the inner coating cures, actively constructs a three-dimensional network structure with anchoring capabilities. This allows subsequent coatings to penetrate the network pores, forming a "pinning" structure and chemically bonding with the active groups in the network, achieving a transformation from "weak physical bonding" to "chemical-physical composite anchoring."

[0048] Specific implementation method: On the semi-dry inner coating surface obtained in step 2 (after natural drying for 7 minutes, the coating still retains about 20% moisture, the molecular chains of the coating material are not yet fully fixed, and it has a certain degree of fluidity), uniformly spray a citric acid aqueous solution with a mass fraction of 1-3%, preferably 2%, at a spraying amount of 5-15 g / m², preferably 10 g / m². Spraying is performed using an atomizing nozzle with an atomization pressure of 0.1-0.3 MPa, preferably 0.2 MPa, and a spraying distance of 10-20 cm, preferably 15 cm, to ensure that the citric acid solution forms a uniform thin layer on the coating surface.

[0049] Citric acid is a food-grade natural organic acid, and like the acetic acid used in step 2, it is an organic acid. The two have good chemical compatibility and will not cause any dangerous reactions. The small amount of acetic acid remaining in the coating does not affect the cross-linking effect of citric acid. On the contrary, due to the high volatility of acetic acid, it will preferentially evaporate during the subsequent 60℃ curing process. Citric acid, due to its large molecular weight and cross-linking reaction with chitosan, will remain in the coating and play a cross-linking role.

[0050] After spraying, citric acid molecules penetrate and diffuse into the inner coating layer. The citric acid molecule contains three carboxyl groups (-COOH), which can undergo cross-linking reactions with the amino groups (-NH2) on the chitosan molecular chain. There are two types of cross-linking reactions: one is where the carboxyl groups and amino groups form ionic bonds (-COOH) through electrostatic interactions. - ··· +The first crosslinking occurs at room temperature (H3N-); the second crosslinking occurs during subsequent curing, where carboxyl groups and amino groups undergo dehydration condensation to form amide bonds (-CO-NH-), requiring a specific temperature (60℃) for complete crosslinking. Simultaneously, the carboxyl groups of citric acid molecules can form hydrogen bonds with the hydroxyl groups (-OH) on the surface of montmorillonite, fixing the montmorillonite sheets within the crosslinked network.

[0051] Through the aforementioned multiple cross-linking processes, the inner coating transforms from its original linear or branched structure into a partially cross-linked three-dimensional network structure. This network structure has the following characteristics: (1) Network nodes: Chitosan molecular chains are cross-linked by citric acid molecules to form network nodes. Each citric acid molecule can connect 2-3 chitosan molecular chains. The node density depends on the amount of citric acid used (5-15 g / m² of spraying corresponds to a node spacing of 5-15 nm).

[0052] (2) Network pores: There are nanoscale pores (pore size 3-10nm) in the cross-linked network. These pores provide penetration channels for the intermediate transition layer material to be coated later, so that the transition layer material can partially penetrate into the surface of the inner coating layer, forming an interface transition zone rather than a clear interface, which significantly improves the interface bonding strength.

[0053] (3) Active sites: Unreacted chitosan amino and citrate carboxyl groups are retained in the cross-linked network. These active groups can react chemically or form hydrogen bonds with materials such as propylene glycol alginate and chitosan graft polymer in the subsequent coating to form chemical binding sites.

[0054] (4) Mechanical strength: The three-dimensional network structure greatly improves the mechanical strength and dimensional stability of the inner coating. The tensile strength of the coating is increased from 15-20 MPa before crosslinking to 30-40 MPa after crosslinking, preventing deformation or damage during subsequent coating processes.

[0055] After spraying citric acid, the coating is cured at 60°C for 3-5 minutes, preferably 4 minutes. The preferred parameters of 60°C and 4 minutes are selected based on the following considerations: (1) Promote cross-linking reaction: 60℃ can provide sufficient activation energy for the dehydration condensation reaction of citric acid and chitosan, so that the cross-linking of amide bonds can proceed fully, while this temperature will not cause the chitosan molecular chain to break or degrade.

[0056] (2) Maintaining the structure of montmorillonite: The lamellar structure of montmorillonite will partially collapse when the temperature is above 70℃, the interlayer spacing will decrease, and the barrier performance will decline. A curing temperature of 60℃ can maintain the integrity of montmorillonite lamellars and the interlayer spacing (about 1.0-1.2nm), ensuring that the barrier performance is not affected.

[0057] (3) Removal of residual solvent: Curing at 60℃ for 3-5 minutes can effectively remove residual moisture and acetic acid from the coating, reducing the moisture content of the coating to below 5%, and providing a stable substrate for subsequent coating.

[0058] (4) Formation of anchoring sites: During the curing process, the cross-linked network is gradually formed and stabilized, and the network pores and active sites are fixed, providing effective anchoring sites for subsequent coatings.

[0059] After curing, an inner barrier coating with a partially cross-linked three-dimensional network structure is obtained. This coating not only has excellent barrier performance and mechanical strength, but more importantly, it provides a structural and chemical basis for the firm adhesion of the subsequent intermediate transition layer and outer antibacterial coating, which is a key step to achieve high interfacial bonding strength.

[0060] Step 4: Preparation and Coating of the Intermediate Transition Layer This step is another core aspect of the invention. In the prior art, the inner barrier coating (usually a chitosan-montmorillonite composite coating) and the outer antibacterial coating (usually a nano-silver-sodium alginate composite coating) are in direct contact. Due to the significant differences in the chemical composition and physical properties of the two coating materials, the interfacial compatibility is poor, and the interlayer bonding force is weak. The present invention creatively introduces an intermediate transition layer between the inner and outer layers. This transition layer is composed of two components: propylene glycol alginate and chitosan grafted polymer. Utilizing the molecular structure and properties of these two components, they form chemical or physical bridges with the inner and outer layers respectively, acting as an "interfacial bridge" that significantly improves the interfacial bonding strength between the inner and outer layers.

[0061] Existing technologies typically employ a simple double-layer coating stack structure, with the inner barrier coating in direct contact with the outer antibacterial coating. This lack of effective interface transition and bridging measures results in low interlayer bonding strength. This invention, by introducing an intermediate transition layer, achieves a transformation from a "clear interface" to an "interface transition zone," and from "weak physical bonding" to "chemical-physical composite bridging," fundamentally solving the problem of insufficient interlayer bonding strength in multilayer coatings.

[0062] Specific implementation method: (1) Preparation of intermediate transition layer liquid: First, prepare a propylene glycol alginate solution: Add propylene glycol alginate (esterification degree 30-60%, preferably 45%, molecular weight 50,000-150,000, preferably 100,000, viscosity 100-500 mPa·s, preferably 300 mPa·s) to water and stir at 200-400 r / min, preferably 300 r / min for 1-2 hours, preferably 1.5 hours, at 25-35℃, preferably 30℃, to obtain a solution with a mass fraction of 1-3%, preferably 2%. Propylene glycol alginate is the product of the esterification reaction between alginic acid and propylene glycol. A 45% esterification degree gives it moderate hydrophilicity, allowing it to dissolve in water and penetrate into the more hydrophobic chitosan coating. A molecular weight of 100,000 and a viscosity of 300 mPa·s ensure good film-forming properties and a certain degree of molecular chain flexibility, which is beneficial for forming a buffer layer at the interface. The propylene glycol alginate used is a food-grade material that meets the requirements of GB 1886.234-2016 "National Food Safety Standard for Food Additives: Propylene Glycol Alginate", and is safe and non-toxic.

[0063] In addition, a chitosan graft polymer solution is prepared: the chitosan graft polymer is added to water and stirred at 200-400 r / min, preferably 300 r / min, at 25-35℃, preferably 30℃, for 1-2 hours, preferably 1.5 hours, to obtain a solution with a mass fraction of 1-2%, preferably 1.5%. The chitosan graft polymer is prepared by graft copolymerization of chitosan and acrylic acid, with a grafting rate of 20-50%, preferably 35%, a molecular weight of 150,000-400,000, preferably 250,000, a viscosity of 200-800 mPa·s, preferably 500 mPa·s, and an amino content of 2.5-4.5 mmol / g, preferably 3.5 mmol / g. The chitosan graft polymer has a unique molecular structure: the main chain is chitosan, and the side chains are polyacrylic acid. The chitosan segments on the main chain are cationic (containing amino groups), and the polyacrylic acid segments on the side chains are anionic (containing carboxyl groups), exhibiting amphoteric characteristics throughout the molecule. A grafting rate of 35% and an amino content of 3.5 mmol / g ensure that the material has sufficient cationic sites to form bridges between the inner and outer layers, while a molecular weight of 250,000 and a viscosity of 500 mPa·s guarantee its good film-forming properties and interfacial penetration ability. This grafted polymer is prepared by free radical polymerization of food-grade chitosan and acrylic acid. After the reaction, unreacted monomers are removed through purification, resulting in a residual monomer content of less than 0.1%, which meets the safety requirements for food contact materials.

[0064] A solution of propylene glycol alginate and a solution of chitosan grafted polymer are mixed at a mass ratio of 3:2 to 4:1, preferably 3:1, and stirred at 200-400 r / min, preferably 300 r / min, for 30-60 minutes, preferably 45 minutes, until homogeneous, to obtain an intermediate transition layer liquid. After mixing, the two components form a complex through hydrogen bonding and electrostatic interactions, but do not undergo complete aggregation or precipitation, maintaining a solution state suitable for coating operations.

[0065] (2) Coating of the intermediate transition layer: The intermediate transition layer liquid is applied to the surface of the cured inner barrier coating using a scraping method. The angle between the scraper and the coating surface is 30-45°, preferably 38°, and the scraping speed is 3-8 m / min, preferably 5 m / min. The coating thickness is controlled at 5-10 μm wet film, preferably 7 μm, corresponding to a dry film thickness of 2-4 μm, preferably 3 μm. The scraping method allows for precise control of the coating thickness uniformity. The 38° scraper angle and 5 m / min scraping speed ensure that the coating liquid is fully spread and air bubbles are removed. The coating thickness should not be too thin (<2 μm), otherwise the transition layer cannot form a continuous film layer, resulting in poor bridging effect; nor should it be too thick (>4 μm), otherwise it will increase the total coating thickness and cost, and an excessively thick transition layer may become a weak point in the structure.

[0066] (3) Interface bridging process: After the intermediate transition layer is coated, its two components interact with the inner and outer layers as follows: Bridging effect of propylene glycol alginate with the inner layer: Propylene glycol alginate and the inner layer chitosan are both polysaccharide-based natural polymers, with their main chains composed of sugar units linked by glycosidic bonds, exhibiting similar chemical structures and good compatibility. The carboxyl groups in the propylene glycol alginate molecule (derived from alginic acid) can form ion pairs with the amino groups of the inner layer chitosan through electrostatic interactions. Simultaneously, the hydroxyl groups on the molecular chain can form a hydrogen bond network with the hydroxyl groups on the surfaces of chitosan and montmorillonite. More importantly, propylene glycol alginate has moderate hydrophilicity, allowing it to penetrate the surface of the inner coating (to a depth of approximately 100-300 nm), entering the cross-linked network pores formed in step 3. There, it reacts chemically or forms hydrogen bonds with the active amino and carboxyl groups in the network, forming a "pinned" structure. This penetration transforms the interface from a "clear interface" to an "interface transition zone," significantly enhancing the interfacial bonding strength.

[0067] The bidirectional bridging effect of chitosan-grafted polymers: The molecular structure of chitosan-grafted polymers enables them to form bridges with both inner and outer layers simultaneously, making them a key component for achieving interface enhancement. ① Bridging with the inner layer: The main chain of the chitosan graft polymer is chitosan, which has the same chemical structure as the chitosan in the inner coating layer. The two are connected by hydrogen bonds ( This is combined with molecular chain entanglement. Molecular chain entanglement refers to the mutual penetration and interweaving of the two chitosan molecular chains in the interfacial region, forming a "woven" structure that significantly enhances interfacial bonding. At the same time, the amino groups on the grafted polymer backbone can also undergo electrostatic interactions or condensation reactions with the residual citrate carboxyl groups in the inner cross-linked network to form chemical bonds.

[0068] ② Bridging with the outer layer: The side chains of the chitosan-grafted polymer are polyacrylic acid, containing a large number of carboxyl groups. The subsequent outer antibacterial coating is mainly composed of sodium alginate, which also contains carboxyl groups. Although both are anionic, under appropriate conditions, the carboxyl groups of the polyacrylic acid segments can form hydrogen bonds with the hydroxyl groups on the sodium alginate molecular chain. Simultaneously, the two anionic polymers can be bridged by polyvalent cations (such as trace amounts in solution). They form ionic cross-linking bridges. In addition, the chitosan segments (cationic) on the grafted polymer backbone and sodium alginate (anionic) can form polyelectrolyte complexes, which are bonded together through strong electrostatic interactions.

[0069] ③ Synergistic effect of bidirectional bridging: Through the above process, the main chain of the chitosan grafted polymer is anchored in the inner layer, while different segments of the side chains and main chain interact with the outer layer in multiple ways, acting as a "molecular bridge" to firmly connect the inner and outer layers. A single chitosan grafted polymer molecule can cross the interface to connect the two layers, forming a molecular-level bridge that significantly improves the interfacial bonding strength.

[0070] (4) Curing of the intermediate transition layer: The intermediate transition layer is cured at 70°C for 4-6 minutes, preferably 5 minutes. The preferred parameters of 70°C and 5 minutes are selected based on the following considerations: Gradient curing: 70℃ is between the inner layer curing temperature (60℃) and the outer layer curing temperature (80℃), forming a temperature gradient, which is conducive to the formation of a stable interface transition structure between the middle layer and the upper and lower layers, avoiding thermal stress concentration and interface cracking caused by sudden temperature changes.

[0071] Fully cured: 70℃ can effectively remove moisture from the transition layer (the water content is reduced to below 5%), allowing propylene glycol alginate and chitosan graft polymer to fully form a film and create a continuous transition layer structure.

[0072] Protective inner layer: The temperature of 70℃ will not have an excessive impact on the cured inner layer, and the cross-linked network structure and montmorillonite sheet structure of the inner layer remain stable.

[0073] Activation of outer layer bonding: After curing at 70℃, some active groups (unreacted amino, carboxyl, and hydroxyl groups) are still retained on the surface of the transition layer. These active groups can form chemical bonds or hydrogen bonds with the sodium alginate of the subsequent outer antibacterial coating, ensuring a strong bond between the outer layer and the transition layer.

[0074] After curing, an intermediate transition layer with interface bridging function is obtained. This transition layer tightly connects the inner barrier coating and the outer antibacterial coating, transforming the original two-layer structure with low interlayer bonding strength into a three-layer composite structure with strong integrity. The interlayer bonding strength is increased to more than 2.5 N / cm, which is another key step in achieving high interlayer bonding strength.

[0075] Step 5: Preparation and application of the outer antibacterial coating Preparation of the outer antibacterial coating solution: Sodium alginate (molecular weight 100,000-500,000, preferably 280,000; viscosity 50-300 mPa·s, preferably 150 mPa·s; M / G ratio 1.2-1.8, preferably 1.5) is dissolved in water to obtain a sodium alginate solution with a mass fraction of 2-4%, preferably 3%. Nano-silver particles (particle size 10-50 nm, preferably 25 nm; silver content ≥99.9%) are added to the solution, with a mass ratio of nano-silver to sodium alginate of 1:10 to 1:20, preferably 1:15. The mixture is stirred at 300-600 r / min, preferably 450 r / min, for 30-60 minutes, preferably 45 minutes, until homogeneous, to obtain the outer antibacterial coating solution. A stirring speed of 450 r / min ensures that the nano-silver particles are fully dispersed in the sodium alginate solution, preventing nano-silver agglomeration. The sodium alginate has an M / G ratio (mannuronic acid / guluronic acid ratio) of 1.5, ensuring moderate gel strength and flexibility, and a viscosity of 150 mPa·s to guarantee good coating performance. The sodium alginate used is a food-grade natural polymer material, meeting the requirements of GB 1886.232-2016 "National Food Safety Standard - Food Additives - Sodium Alginate"; the nano-silver used is a food-grade antibacterial agent, meeting the requirements of relevant standards such as GB 31604.49-2016 "National Food Safety Standard - Determination of Arsenic, Cadmium, Chromium, and Lead in Food Contact Materials and Products and Determination of Migration of Arsenic, Cadmium, Chromium, Nickel, Lead, Antimony, and Zinc", and is safe and harmless to the human body at the concentration used in this invention.

[0076] The coating liquid is applied to the surface of the cured intermediate transition layer by spraying. The spraying pressure is 0.2-0.4 MPa, preferably 0.3 MPa, the spraying distance is 15-25 cm, preferably 20 cm, and the spray gun moving speed is 5-10 m / min, preferably 7 m / min. The coating thickness is controlled at 8-15 μm wet film, preferably 11 μm, corresponding to a dry film thickness of 3-5 μm, preferably 4 μm. Spraying ensures uniform dispersion of the silver nanoparticles in the coating, avoiding agglomeration caused by the shear force generated during roller coating or scraping. During coating, the sodium alginate (anionic polysaccharide) in the outer antibacterial coating interacts with the chitosan graft polymer (containing cationic chitosan segments) on the surface of the intermediate layer through electrostatic interaction.

[0077] Curing at 80℃ for 3-5 minutes, preferably 4 minutes, is beneficial. This higher temperature allows for: 1) accelerated moisture evaporation, leading to rapid densification of the coating; 2) promotion of uniform dispersion and fixation of nano-silver particles within the sodium alginate matrix; 3) facilitating the formation of a stable polyelectrolyte complex between sodium alginate and the intermediate chitosan grafted polymer, strengthening the interlayer interface through electrostatic interactions and hydrogen bonding; and 4) enhancing the stability and durability of the antibacterial coating. Nano-silver releases silver ions (… It binds to proteins on the bacterial cell membrane to achieve broad-spectrum antibacterial effects.

[0078] Step 6: Plastic bag forming The coated multi-layer PE film is heat-sealed to form plastic bags. The heat-sealing conditions are: heat-sealing temperature 120-150℃, preferably 135℃; heat-sealing pressure 0.2-0.4MPa, preferably 0.3MPa; heat-sealing time 1-3 seconds, preferably 2 seconds. During heat sealing, the coating is positioned on the inside of the plastic bag, directly contacting the contents to provide barrier and antibacterial functions.

[0079] Although the heat-sealing temperature (120-150℃) is higher than the coating curing temperature (60-80℃), heat sealing is a localized heating process with a very short time (1-3 seconds). It only melts and bonds the PE substrate at the sealing edges, and the heat is insufficient to conduct to the coating area, leading to coating decomposition or performance degradation. The natural polymer materials used in the coating, such as chitosan, sodium alginate, and propylene glycol alginate, have decomposition temperatures above 200℃ and remain stable under short-term contact at 120-150℃. In actual production, the coating temperature in the non-sealed areas does not exceed 60℃, and the coating structure and function are fully maintained.

[0080] The significance of the three-stage gradient curing process: The above preparation method employs a three-stage gradient curing process: the inner barrier coating is cured at 60℃ for 3-5 minutes, the intermediate transition layer is cured at 70℃ for 4-6 minutes, and the outer antibacterial coating is cured at 80℃ for 3-5 minutes. This gradient curing process is one of the key components of this invention and has the following advantages compared to the single-temperature curing process used in existing technologies: (1) Each coating layer reaches its optimal performance state: Different coating materials have different optimal curing temperatures. The inner layer contains montmorillonite, which can maintain the integrity of the lamellar structure and barrier properties when cured at a lower temperature (60℃). The middle layer can form a stable transition interface when cured at a moderate temperature (70℃). The outer layer contains nano-silver, which can promote the uniform dispersion of nano-silver and the densification of the coating when cured at a higher temperature (80℃). Gradient curing avoids the damage to certain coating properties caused by curing at a single temperature, so that each coating layer reaches its optimal performance state.

[0081] (2) Gradual transition of interlayer interface: Temperature gradient (60℃→70℃→80℃) is conducive to the gradual transition of interlayer interface and stress release. Each layer is in a relatively stable state after curing. The temperature change during the curing of subsequent layers is gradual rather than abrupt, which reduces the risk of thermal stress concentration and interface cracking, and improves the stability and service life of the overall coating system.

[0082] (3) Improved production efficiency: Although the gradient curing process is carried out in three stages, the curing time of each stage is short (3-6 minutes). The total curing time (10-16 minutes) is comparable to the long curing time of single temperature curing (usually 15-20 minutes to take into account each layer). At the same time, gradient curing makes each layer perform better and reduces the defect rate. The actual production efficiency is improved by about 25%.

[0083] Experimental verification Experiment 1: Coating Interface Bond Strength Test 1. Experimental Objective The invention demonstrates its significant advantages over existing technologies by improving the interfacial bonding strength of multilayer coated plastic bags through citric acid crosslinking pretreatment and intermediate transition layer bridging technology.

[0084] 2. Preparation of experimental samples The following three samples were prepared for comparative testing: Sample 1 (Comparative Sample 1): A traditional double-layer coated plastic bag, prepared according to existing technology, including a PE substrate (50 μm thick), a chitosan-montmorillonite inner barrier coating (4.5 μm dry film thickness), and a nano-silver-sodium alginate outer antibacterial coating (4 μm dry film thickness). The inner and outer layers are in direct contact, without cross-linking pretreatment or intermediate transition layer, and are cured at a single temperature of 70℃ for 15 minutes.

[0085] Sample 2 (Comparative Sample 2): A three-layer coated plastic bag with only citric acid cross-linking pretreatment, including a PE substrate (50 μm thick), a chitosan-montmorillonite inner barrier coating (4.5 μm dry film thickness) pretreated with citric acid cross-linking (2% citric acid solution, spraying amount 10 g / m²), and a nano-silver-sodium alginate outer antibacterial coating (4 μm dry film thickness). There is no intermediate transition layer. The inner layer is directly coated with the outer layer after curing at 60℃ for 4 minutes, and the outer layer is cured at 80℃ for 4 minutes. Two-stage curing is used instead of complete three-stage gradient curing.

[0086] Sample 3 (Sample of the present invention): A plastic bag prepared according to the preferred parameters of Example 4, comprising a PE substrate (thickness 50 μm), a chitosan-montmorillonite inner barrier coating (dry film thickness 4.5 μm) pretreated with citric acid crosslinking (2% citric acid solution, spraying amount 10 g / m²), an intermediate transition layer of propylene glycol alginate-chitosan grafted polymer (dry film thickness 3 μm), and a nano-silver-sodium alginate outer antibacterial coating (dry film thickness 4 μm), using a complete three-stage gradient curing process (inner layer cured at 60°C for 4 minutes, intermediate layer cured at 70°C for 5 minutes, outer layer cured at 80°C for 4 minutes).

[0087] The raw material parameters used in each sample strictly followed the preferred parameters in Implementation Method 1: chitosan deacetylation degree 90%, molecular weight 200,000, viscosity 100 mPa·s; sodium montmorillonite cation exchange capacity 100 mmol / 100g, particle size 5 μm, interlayer spacing 1.2 nm; propylene glycol alginate esterification degree 45%, molecular weight 100,000, viscosity 300 mPa·s; chitosan graft polymer grafting rate 35%, molecular weight 250,000, viscosity 500 mPa·s, amino content 3.5 mmol / g; sodium alginate molecular weight 280,000, viscosity 150 mPa·s, M / G ratio 1.5; silver nanoparticle size 25 nm.

[0088] 3. Experimental conditions Testing Equipment: Universal Testing Machine (Model: Instron 5965) Testing Method: According to GB / T 2792-2014 "Determination of 180° Peel Strength of Adhesives" Testing Temperature: Room temperature (23±2℃) Testing Humidity: Relative humidity 50±5% Tensile Speed: 300 mm / min Sample Size: 25 mm (width) × 150 mm (length) Number of Tests: 10 parallel samples for each type of sample 4. Experimental Procedure (1) Sample preparation: Cut the three samples into strips of 25 mm × 150 mm. Carefully separate the coating from the substrate by about 25 mm at one end of the sample with a blade to serve as the clamping end.

[0089] (2) Instrument installation: Fix the PE substrate end of the sample to the lower clamp of the universal testing machine, and fix the separated coating end to the upper clamp, ensuring that the sample is perpendicular to the clamp and has no initial tension.

[0090] (3) Peel test: Start the test machine and stretch it upward at a speed of 300 mm / min to peel the coating off from the substrate at a 180° angle. Record the force-displacement curve during the entire peel process.

[0091] (4) Data acquisition: Record the average peel force (N) during the peeling process and calculate the peel strength (N / cm) = average peel force (N) / sample width (cm).

[0092] (5) Repeated testing: Test 10 parallel samples for each sample and calculate the mean and standard deviation.

[0093] (6) Observation: The sample of the present invention was observed using a scanning electron microscope (SEM).

[0094] 5. Experimental Results Table 1. Results of coating peel strength tests for different samples

[0095] Note: Peeling mode description - Interface failure: The coating is completely peeled off, and the substrate surface is smooth with no residue; Mixed failure: Partial interface failure and cohesive failure; Cohesive failure: The coating itself breaks, and a large amount of coating residue remains on the substrate surface.

[0096] Figure 1 This is a comparison of the peel strength of the coatings on different samples; Figure 2 This is a SEM image of the coating surface of the sample of this invention.

[0097] 6. Analysis and Summary (1) Significantly improved interfacial bonding strength: The peel strength of the coating of the sample (sample 3) of this invention reached 2.68 N / cm, which is 538% higher than that of the traditional double coating (sample 1) of 0.42 N / cm, and 98% higher than that of the sample (sample 2) which only uses citric acid crosslinking of 1.35 N / cm, which fully demonstrates the synergistic effect of citric acid crosslinking pretreatment and intermediate transition layer bridging.

[0098] (2) Transformation of peeling mode: Sample 1 showed interface destruction, indicating that the coating and the substrate relied only on weak physical adsorption, and the interface was a weak link; Sample 2 showed mixed destruction, indicating that the citric acid crosslinking pretreatment improved some of the interface strength, but there was still a weak interface; Sample 3 showed cohesive destruction, with a large amount of coating residue on the substrate surface, indicating that the interface bonding strength had exceeded the coating's own strength, realizing the transformation from "interface destruction" to "cohesive destruction", which is the best state of interface enhancement.

[0099] (3) Verification of the effectiveness of the technical solution: The present invention provides anchoring sites by constructing a three-dimensional network structure in the inner coating through citric acid cross-linking pretreatment, combined with the two-component material (propylene glycol alginate and chitosan grafted polymer) of the intermediate transition layer to form chemical or physical bridges with the inner and outer layers respectively, and the synergistic effect of the three-stage gradient curing process, successfully achieving the technical goal of increasing the interface bonding strength from below 0.5 N / cm to above 2.5 N / cm.

[0100] (4) Practical value: The coating peel strength of 2.68 N / cm far exceeds the existing technology level, which can ensure that the coating of the plastic bag will not fall off or delaminate in actual use (high humidity environment, repeated rubbing, loading heavy objects, etc.), significantly improving the reliability and service life of the product.

[0101] Experiment 2: Coating stability test under high humidity environment 1. Experimental Objective The invention verifies the coating stability and integrity retention of the plastic bag under high humidity conditions, demonstrating the role of the three-dimensional cross-linked network structure and multiple interface reinforcement in the long-term stability of the coating under high humidity conditions.

[0102] 2. Preparation of experimental samples The following three samples were prepared for comparative testing: Sample 1 (Comparative Sample 1): Traditional double-coated plastic bag, prepared using the same method as Sample 1 in Experiment 1.

[0103] Sample 2 (Comparative Sample 2): A three-layer coated plastic bag with only citric acid cross-linking pretreatment, prepared using the same method as Sample 2 in Experiment 1.

[0104] Sample 3 (Sample of the present invention): A plastic bag prepared according to the preferred parameters of Example 4, and the preparation method is the same as that of Sample 3 in Experiment 1.

[0105] Each sample was cut into a 10 cm × 10 cm square specimen, and 30 parallel samples were prepared for each type of sample, which were used for testing at 0 days, 10 days, 20 days and 30 days respectively.

[0106] 3. Experimental conditions Test Equipment: Constant Temperature and Humidity Chamber (Model: ESPEC SH-242) Test Temperature: 25±1℃ Test Humidity: 85% Relative Humidity (High Humidity Environment) Test Cycle: 0 days, 10 days, 20 days, 30 days Test Indicators: Coating Integrity, Peel Strength Retention Rate, Appearance Change 4. Experimental Procedure (1) Initial state test (0 days): Take 5 parallel samples of each sample and perform coating integrity check and peel strength test as the initial value. Coating integrity check method: Observe the coating surface with an optical microscope and count the percentage of the area of ​​defects such as coating peeling, cracking, and blistering to the total area. Integrity = (1 - defect area / total area) × 100%.

[0107] (2) Exposure to high humidity environment: Place the remaining sample in a constant temperature and humidity chamber, set the temperature to 25℃ and the relative humidity to 85%, and start timing.

[0108] (3) Periodic sampling and testing: On the 10th, 20th and 30th days, five parallel samples of each sample were taken out and the coating integrity check and peel strength test were carried out immediately.

[0109] (4) Peel strength test: The peel strength was tested according to the method in Experiment 1, and the peel strength retention rate was calculated as (current peel strength / initial peel strength) × 100%.

[0110] (5) Observation of appearance changes: Use a digital camera to photograph the macroscopic appearance of the sample at each time point and record the changes in coating color, gloss, smoothness, etc.

[0111] (6) Microstructure observation: SEM was used to observe the cross-sectional morphology of the coating of the sample after 30 days and to analyze the changes in the interlayer interface.

[0112] 5. Experimental Results Table 2. Stability test results of coatings of different samples under high humidity environment (85% relative humidity, 25℃)

[0113] Figure 3 It is a curve showing the change in coating integrity under high humidity conditions; Figure 4 This is a bar chart showing the change in peel strength retention rate under high humidity conditions.

[0114] 6. Analysis and Summary (1) Significantly improved stability in high humidity environments: After being exposed to a high humidity environment (85% relative humidity, 25℃) for 30 days, the coating integrity of the sample of this invention (sample 3) remained at 95.3%, and the peel strength retention rate was 94.4%, far exceeding the existing technology. In contrast, the coating integrity of the traditional double-layer coating (sample 1) dropped to 57.8%, and the peel strength retention rate was only 42.9%, showing obvious coating peeling and delamination.

[0115] (2) Moisture resistance of the three-dimensional network structure: Sample 2 was only pretreated with citric acid crosslinking. Although it was significantly improved compared with Sample 1 (coating integrity 79.6% and peel strength retention rate 65.9% after 30 days), it was still significantly lower than Sample 3. This indicates that the three-dimensional network structure formed by citric acid crosslinking provides a certain degree of moisture resistance, but its effect is limited when used alone.

[0116] (3) The key role of the intermediate transition layer: The significant improvement of sample 3 compared to sample 2 (coating integrity increased by 15.7 percentage points and peel strength retention increased by 28.5 percentage points) fully demonstrates the interfacial bridging effect of the intermediate transition layer. The propylene glycol alginate-chitosan grafted polymer composite intermediate transition layer not only provides chemical-physical composite bridging, but also forms a dense interfacial transition region, effectively preventing water molecules from penetrating and accumulating in the interfacial region, and preventing interfacial weakening under high humidity conditions.

[0117] (4) Long-term stability verification: Experimental results show that the technical goal of maintaining more than 95% coating integrity in the plastic bag of the present invention after 30 days in a high humidity environment has been achieved, proving the effectiveness of the technical solution. The synergistic effect of the three-dimensional cross-linked network structure and multiple interface enhancements enables the coating to remain stable for a long time in a high humidity environment without swelling, peeling or functional degradation.

[0118] (5) Practical value: This performance ensures the long-term reliability of plastic bags in actual use scenarios (such as fresh food packaging in cold chain logistics, where the ambient humidity can reach 85-95%), significantly extending the product's service life and preservation effect.

[0119] Experiment 3: Barrier-Antibacterial Synergistic Performance Test 1. Experimental Objective The barrier and antibacterial properties of the plastic bag of this invention, as well as the synergistic enhancement effect of the two functions, were verified, demonstrating the actual effect of the three-layer functional coating system on extending the shelf life of fresh food.

[0120] 2. Preparation of experimental samples The following four samples were prepared for comparative testing: Sample 1 (Comparison Sample 1): Uncoated PE plastic bag, consisting only of PE substrate (50μm thick), without any functional coating.

[0121] Sample 2 (Comparative Sample 2): Single barrier-coated plastic bag, consisting of a PE substrate (50 μm thick) and a chitosan-montmorillonite inner barrier coating (4.5 μm dry film thickness), cured at 70°C for 10 minutes.

[0122] Sample 3 (Comparative Sample 3): Traditional double-coated plastic bag, prepared using the same method as Sample 1 in Experiment 1.

[0123] Sample 4 (Sample of the present invention): A plastic bag prepared according to the preferred parameters of Example 4, and the preparation method is the same as that of Sample 3 in Experiment 1.

[0124] Each sample was made into a 10 cm × 15 cm bag for barrier performance testing and preservation effect testing.

[0125] 3. Experimental conditions Barrier performance test conditions: Test equipment: Oxygen transmission rate tester (model: Labthink OX2 / 231), water vapor transmission rate tester (model: Labthink W3 / 330) Test methods: Oxygen transmission rate according to GB / T 1038-2000, water vapor transmission rate according to GB / T 1037-1988 Test temperature: 23±2℃ Test humidity: Relative humidity 50±5% Antimicrobial performance test conditions: Test equipment: Biosafety cabinet, constant temperature incubator; Test method: According to GB / T31402-2015 "Test Method for Antimicrobial Performance of Plastic Surfaces"; Test strains: Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 6538); Inoculation concentration: 1×10⁻⁶ 6 CFU / mL Test time: 24 hours Preservation effect test conditions: Test subject: Fresh strawberries (variety: Hongyan, used within 12 hours after picking) Storage conditions: Refrigerated at 4℃, relative humidity 85% Test period: 0 days, 3 days, 6 days, 9 days, 12 days Test indicators: Weight loss rate, rot rate, firmness, soluble solids content, vitamin C content 4. Experimental Procedure (1) Barrier performance test procedure: ① Oxygen Transmission Rate Test: Cut each sample into a circular specimen with a diameter of 10 cm, fix it on the test chamber of the testing instrument, with the coated side facing the oxygen side. Start the test program and test the oxygen transmission rate of the sample (unit: cm³ / (m²·24h·0.1MPa)). Test 3 parallel samples for each type of sample and take the average value.

[0126] ② Water vapor transmission rate test: Cut each sample into a circular specimen with a diameter of 7 cm, fix it on the test chamber of the testing instrument, with the coated side facing the high humidity side. Set the test temperature to 38℃ and the humidity gradient from 90% RH (high humidity side) to 0% RH (low humidity side). Start the test program and test the water vapor transmission rate of the sample (unit: g / (m²·24h)). Test 3 parallel samples for each type of sample and take the average value.

[0127] (2) Antibacterial performance test procedure: ① Preparation of bacterial culture: Escherichia coli and Staphylococcus aureus were inoculated separately into nutrient broth medium and incubated at 37°C for 18-24 hours. The bacterial concentration was then adjusted to 1×10⁻⁶. 6 CFU / mL.

[0128] ② Sample inoculation: Cut each sample into 5 cm × 5 cm square specimens and sterilize with ultraviolet light for 30 minutes. Place the specimens in a sterile petri dish with the coated side facing up. Use a pipette to draw 0.4 mL of bacterial solution and evenly drop it onto the specimen surface. Gently spread it evenly with a sterile glass rod and cover with a sterile polyethylene film (4 cm × 4 cm).

[0129] ③ Incubation contact: Incubate the inoculated sample in a 37℃ constant temperature incubator for 24 hours.

[0130] ④ Colony counting: After incubation, add 10 mL of sterile physiological saline to the petri dish and shake thoroughly to elute the bacterial solution. Take the eluent and perform serial dilutions, spread it on nutrient agar plates, incubate at 37°C for 24 hours, and then count the colonies.

[0131] ⑤ Antibacterial rate calculation: Antibacterial rate (%) = [(Count of colonies in control sample - Count of colonies in test sample) / Count of colonies in control sample] × 100%. The uncoated PE sample (sample 1) was used as the control.

[0132] (3) Preservation effect test steps: ① Strawberry preparation: Select fresh strawberries that are uniform in size, ripe, and free from mechanical damage, and pack 8 strawberries (about 200 g) into each bag.

[0133] ② Packaging and storage: The strawberries were placed into four different sample bags, heat-sealed, and stored in a refrigerator at 4℃ (relative humidity 85%). Fifteen bags of each sample were prepared and used for testing at 0 days, 3 days, 6 days, 9 days, and 12 days (three parallel samples at each time point).

[0134] ③ Performance Indicators: - Weight Loss Rate = [(Initial Mass - Current Mass) / Initial Mass] × 100% - Rot Rate = (Number of Rotten Strawberries / Total Number of Strawberries) × 100% - Firmness: Measured using a texture analyzer (TA.XT plus), compression depth 5 mm, speed 1 mm / s - Soluble Solids: Measured using a handheld refractometer - Vitamin C: Measured using the 2,6-dichlorophenolindophenol titration method 5. Experimental Results Table 3. Barrier performance test results of different samples

[0135] *Note: Overall barrier performance score = (Sample 1 oxygen transmission rate / Sample oxygen transmission rate) × (Sample 1 water vapor transmission rate / Sample water vapor transmission rate) Table 4. Antibacterial performance test results of different samples

[0136] Table 5. Test results of the preservation effect of strawberries with different packaging.

[0137] Figure 5 These are the curves showing the changes in the rot rate of strawberries packaged in different samples; Figure 6 It is a comprehensive comparison of the barrier and antibacterial properties of different samples.

[0138] 6. Analysis and Summary (1) Significantly improved barrier performance: The oxygen permeability of the sample of this invention (sample 4) was 168 cm³ / (m²·24h·0.1MPa), and the water vapor permeability was 1.1 g / (m²·24h), which were reduced by 55.8% and 60.7% respectively compared with the traditional double-layer coating (sample 3), by 60.5% and 65.6% respectively compared with the single barrier coating (sample 2), and by 94.1% and 93.0% respectively compared with the uncoated PE bag (sample 1). The comprehensive barrier performance score reached 16.5, which is 2.36 times that of the traditional double-layer coating. This fully demonstrates the synergistic effect of the three-layer functional coating system: the inner chitosan-montmorillonite layer provides the main barrier function, the middle transition layer fills the micropores between the inner and outer layers, and the outer sodium alginate layer further seals the surface micropores. The three layers together form a dense barrier.

[0139] (2) Excellent antibacterial properties: The antibacterial rate of the sample of this invention against Escherichia coli and Staphylococcus aureus both reached over 99.97%. After 24 hours, the colony counts decreased to 2.1×10² and 3.8×10² CFU / mL, respectively, which is 0.8 percentage points higher than that of the traditional double-layer coating and more than 35 percentage points higher than that of the single barrier coating. This indicates that the outer nano-silver-sodium alginate coating provides a strong broad-spectrum antibacterial effect. At the same time, the high interfacial bonding strength ensures the stability and durability of the antibacterial coating. The nano-silver is not easy to fall off and continuously releases silver ions to achieve long-lasting antibacterial effect.

[0140] (3) Significantly extended preservation effect: Strawberry preservation experiments show that the strawberries packaged in this invention have a rot rate of only 12.5% ​​and a weight loss rate of 5.8% after 12 days, a firmness retention rate of 80.0%, and a vitamin C retention rate of 85.6%. This represents a 70.0% reduction compared to traditional double-layer coating (sample 3, rot rate 41.7%), an 80.0% reduction compared to single barrier coating (sample 2, rot rate 62.5%), and a qualitative leap from unpreservable to preservable compared to uncoated PE bags (sample 1, all rotted after 12 days). Based on the acceptable standard of a rot rate of less than 20% for strawberries, the samples of this invention can extend the shelf life of strawberries to more than 12 days, which is 4 times that of traditional plastic bags (3 days) and 1.33 times that of traditional double-layer coating (9 days).

[0141] (4) Verification of the synergistic enhancement effect of barrier and antibacterial: Comparing the performance differences between sample 2 (single barrier coating) and sample 3 (traditional double-layer coating), it can be seen that the barrier function alone (sample 2) or the simple barrier-antibacterial combination (sample 3) cannot achieve the preservation effect of the sample of the present invention (sample 4). The present invention, through a three-layer functional coating system, high interfacial bonding strength and three-stage gradient curing process, enables the barrier and antibacterial functions to fully exert their synergistic effect: the excellent barrier performance reduces the penetration of oxygen and water, slows down the respiration and water loss of strawberries; the strong antibacterial performance inhibits the growth of microorganisms and prevents the spoilage of strawberries; the two functions promote each other and jointly achieve a significant extension of the shelf life of fresh food.

[0142] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A method for preparing an environmentally friendly plastic bag, characterized in that, Includes the following steps: Step 1: Perform surface pretreatment on the polyethylene substrate; Step 2: Coat the substrate surface with an inner barrier coating composed of chitosan and montmorillonite, and allow it to air dry until semi-dry. Step 3: Spray a citric acid aqueous solution onto the semi-dry inner barrier coating surface for cross-linking pretreatment, and then cure it at the first temperature. The polycarboxyl structure of citric acid undergoes a cross-linking reaction with the amino group of chitosan and the hydroxyl group on the surface of montmorillonite, thus constructing a partially cross-linked three-dimensional network structure in the inner coating. Step 4: Coat the surface of the cured inner barrier coating with an intermediate transition layer composed of propylene glycol alginate and chitosan grafted polymer, and then cure it at a second temperature. The main chain of the chitosan grafted polymer is chitosan, the side chain is polyacrylic acid, propylene glycol alginate forms a bridge with the inner coating, and the chitosan grafted polymer forms a bridge with both the inner and outer layers. Step 5: Coat the surface of the cured intermediate transition layer with an outer antibacterial coating composed of nano-silver and sodium alginate, and then cure it at the third temperature; Step 6: Heat-seal the coated multi-layer polyethylene film to form a plastic bag; The first temperature, the second temperature, and the third temperature increase sequentially to form a gradient curing process.

2. The method for preparing the environmentally friendly plastic bag according to claim 1, characterized in that, In step 3, the citric acid aqueous solution has a mass fraction of 1-3%, the spraying amount is 5-15 g / m², the first temperature is 60℃, and the curing time is 3-5 minutes.

3. The method for preparing the environmentally friendly plastic bag according to claim 2, characterized in that, In step 2, the chitosan has a degree of deacetylation ≥85%, a molecular weight of 100,000-300,000, and a viscosity of 50-200 mPa·s; the montmorillonite is sodium-based montmorillonite with a cation exchange capacity of 80-120 mmol / 100g, a particle size of 2-10μm, and an interlayer spacing of 1.0-1.5 nm; the mass ratio of chitosan to montmorillonite is 3:1 to 5:

1.

4. The method for preparing the environmentally friendly plastic bag according to claim 1, characterized in that, In step 4, the propylene glycol alginate solution and the chitosan graft polymer solution are mixed at a mass ratio of 3:2 to 4:

1.

5. The method for preparing the environmentally friendly plastic bag according to claim 4, characterized in that, The propylene glycol alginate has a degree of esterification of 30-60%, a molecular weight of 50,000-150,000, and a viscosity of 100-500 mPa·s; the chitosan graft polymer has a grafting rate of 20-50%, a molecular weight of 150,000-400,000, a viscosity of 200-800 mPa·s, and an amino content of 2.5-4.5 mmol / g.

6. The method for preparing the environmentally friendly plastic bag according to claim 1, characterized in that, In step 4, the second temperature is 70°C and the curing time is 4-6 minutes; in step 5, the third temperature is 80°C and the curing time is 3-5 minutes.

7. The method for preparing the environmentally friendly plastic bag according to claim 1, characterized in that, In step 5, the sodium alginate has a molecular weight of 100,000-500,000, a viscosity of 50-300 mPa·s, and an M / G ratio of 1.2-1.8; the nano-silver has a particle size of 10-50 nm and a silver content of ≥99.9%; and the mass ratio of nano-silver to sodium alginate is 1:10 to 1:

20.

8. The method for preparing the environmentally friendly plastic bag according to claim 1, characterized in that, In step 1, the polyethylene substrate is a low-density polyethylene film with a density of 0.91-0.93 g / cm³, a melt index of 1.5-3.5 g / 10min, and a thickness of 30-80 μm. The surface pretreatment includes: cleaning with a 75-95% ethanol solution and drying, followed by corona treatment to increase the surface energy. The corona treatment power is 300-500W, the treatment speed is 5-10m / min, and the surface tension reaches 38-42 mN / m.

9. The method for preparing the environmentally friendly plastic bag according to claim 1, characterized in that, In step 2, the inner barrier coating is applied by roller coating at a speed of 10-30 r / min; in step 4, the intermediate transition layer is applied by scraping at an angle of 30-45° between the scraper and the coating surface at a speed of 3-8 m / min; in step 5, the outer antibacterial coating is applied by spraying at a pressure of 0.2-0.4 MPa and a distance of 15-25 cm.

10. A plastic bag prepared by the method of any one of claims 1-9 for making an environmentally friendly plastic bag.

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