High-barrier boiling-resistant primer-free polyvinyl alcohol coating liquid as well as preparation method and application thereof

Abstract

The invention discloses high-barrier boiling-resistant primer-free polyvinyl alcohol coating liquid as well as a preparation method and application thereof, and relates to the technical field of packaging materials. According to the invention, core-shell structured SiO2 / TiO2 nano composite particles are innovatively adopted, and a multi-layer barrier system is constructed by cooperating with the natural oxygen barrier advantage of a polyvinyl alcohol matrix and the physical barrier effect of the nano particles, so that efficient double barrier of oxygen and water vapor is realized; by accurately regulating and controlling the mass ratio of the nano silicon dioxide to the nano titanium dioxide and the ratio of all the raw materials, high-oxygen-barrier, high-water-barrier or balanced barrier type products can be flexibly customized, and the storage and fresh-keeping requirements of different foods are met.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, specifically to a high-barrier, water-resistant, boil-proof, primerless polyvinyl alcohol coating liquid, its preparation method, and its application. Background Technology

[0002] Barrier properties are one of the important indicators of packaging materials. During the storage of food, medicine and other products, the degree of barrier properties directly affects their shelf life. In recent years, the demand for high-barrier plastic packaging materials has continued to grow.

[0003] Currently, the main high-barrier packaging materials in China include aluminum foil, aluminized film, EVOH five-layer co-extruded film, and PVDC coated film. Aluminum foil and aluminized film offer excellent oxygen and water barrier properties, but their opacity, brittleness, and difficulty in recycling contribute to environmental pollution. While EVOH combines the easy processability of polyethylene with the gas barrier properties of polyvinyl alcohol, its hygroscopic nature under high humidity conditions leads to a decline in barrier performance. Practical applications often require five or more layers of co-extruded film, and the reliance on imported EVOH resin and the high investment in co-extrusion equipment keep costs high. Although the domestic production technology for PVDC emulsions is mature, the reliance on imported resins, the strong acidity of the emulsion, the complex coating process, and the inability to recycle, along with the harmful substances produced by waste combustion, severely restrict its development. Polyvinyl alcohol (PVA) resin is a water-soluble polymer. Under dry conditions, PVA exhibits excellent oxygen barrier properties, with the lowest oxygen permeability coefficient among all resins. However, some existing PVA coating solutions and films have several problems. For example, some modified PVA water-based coating solutions suffer from high and unstable viscosity, easy self-polymerization of crosslinking agents leading to gel formation, short shelf life, large coating thickness, low drying efficiency, and crosslinking agents containing high levels of free formaldehyde that do not meet safety standards, or catalysts that are toxic and pose health risks. Furthermore, traditional PVA coating methods require pre-coating with solvent-based or water-based polyurethane primers. These primers contain large amounts of organic solvents and additives, potentially including volatile organic compounds (VOCs) or other harmful substances, posing environmental restrictions and harming both the environment and human health. Chinese invention patent CN108410286A discloses a coating liquid for preparing a high-barrier, water-resistant film and its preparation method. Through a specific raw material formulation and preparation method, the coating liquid possesses characteristics such as high transparency, low viscosity, weak acidity, non-corrosiveness to equipment, environmental friendliness, and simple process. The resulting coating is thin, dries quickly, has high adhesion strength to the substrate, exhibits excellent barrier properties, and has stable viscosity suitable for long-term storage. After boiling in water at 100℃ for 60 minutes, the coating film still maintains good barrier properties. Chinese invention patent CN104961903A discloses a high-oxygen-barrier, primerless polyvinyl alcohol coated film and its manufacturing method. Using a modified self-adhesive polyvinyl alcohol solution as the coating, the combined use of epoxy resin and N-methylallylamine improves the adhesion between the solution and the substrate film, enabling direct coating without a primer. Simultaneously, it optimizes the oxygen barrier properties of the film, completely eliminating coating peeling, and exhibits good peel strength after printing and lamination.

[0004] However, there is currently no polyvinyl alcohol coating film technology that combines the advantages of high barrier properties, water resistance, and no adhesive base. Summary of the Invention

[0005] The technical problem to be solved by this invention is to overcome the shortcomings of the prior art and provide a high-barrier, water-resistant, boil-resistant, primer-free polyvinyl alcohol coating liquid, its preparation method and application. The polyvinyl alcohol coating liquid is coated on different film substrates, so that the coated base film has excellent water and oxygen barrier properties and boil-resistant properties. No pre-coating primer is required, which can simplify the production process, reduce costs, reduce environmental pollution and harm to the health of operators, improve the overall competitiveness of products, and meet higher packaging requirements.

[0006] The technical solution of this invention is as follows: In a first aspect, the present invention provides a high-barrier, water-resistant, adhesive-free polyvinyl alcohol coating liquid, which is composed of the following raw materials in parts by weight: 15-50 parts polyvinyl alcohol, 150-185 parts deionized water, 5-8 parts catalytic crosslinking agent, 0.1-1 parts defoamer, 0.1-1 parts slip agent, 1-5 parts nanocomposite particles, 5-10 parts regulator, and 0-5 parts antifreeze agent; wherein, the catalytic crosslinking agent is a combination of one of gallic acid and tannic acid with one of hydrochloric acid, citric acid, maleic acid, and phosphoric acid, and one of gallic acid and tannic acid accounts for 30-50% of the total mass of the catalytic crosslinking agent; the slip agent is polylactic acid microsphere slip agent or beeswax powder; the nanocomposite particles are core-shell structured nanocomposite particles constructed from nano-silica and nano-titanium dioxide.

[0007] Preferably, the defoamer is a polyether-modified silicone defoamer.

[0008] Preferably, the regulator is isopropanol, ethanol, or methanol.

[0009] Preferably, the antifreeze is a polyol antifreeze, an alcohol ether antifreeze, or a compound antifreeze.

[0010] On the other hand, the present invention provides a method for preparing the above-mentioned high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid, comprising the following steps: S1 Preparation of polyvinyl alcohol solution: Heat deionized water to 50-60℃, add polyvinyl alcohol, continue stirring and heating to 85-95℃, continue stirring and keep the temperature constant for 40-45 minutes to dissolve the polyvinyl alcohol, and cool naturally to room temperature (take care to avoid skin formation on the surface of the solution during the cooling process) to obtain polyvinyl alcohol solution. Preparation of S2 nanocomposite particles: Nano-silica was added to anhydrous ethanol, stirred, and ultrasonically dispersed to obtain a nano-silica suspension; nano-titanium dioxide was added to anhydrous ethanol, ultrasonically dispersed, with a mass ratio of nano-silica to nano-titanium dioxide of 1:(0.2-1.5) to obtain a nano-titanium dioxide suspension; the nano-titanium dioxide suspension was dropped into the nano-silica suspension at a rate of 1-2 mL / min, and ultrasonically coated at 30-40℃ to obtain a composite suspension; the composite suspension was centrifuged, washed, vacuum dried to constant weight, cooled, ground, and sieved to obtain SiO2@TiO2 nanocomposite particles; Preparation of polyvinyl alcohol coating solution in step S3: Heat the polyvinyl alcohol solution prepared in step S1 to 50-60℃, add 40-50wt.% catalytic crosslinking agent, and stir to carry out pre-crosslinking; then raise the temperature to 70-75℃, add the remaining catalytic crosslinking agent and SiO2@TiO2 nanocomposite particles prepared in step S2, and stir to carry out crosslinking; cool down to 45-50℃, add defoamer, lubricant, regulator and antifreeze agent, and mix evenly to obtain polyvinyl alcohol coating solution.

[0011] Preferably, in step S1, the stirring speed is 400-800 r / min.

[0012] Preferably, in step S2, when preparing the nano-silica suspension and the nano-titanium dioxide suspension, the ultrasonic dispersion conditions are 200-300W, 40-50kHz, and 30-60min; when preparing the composite suspension, the ultrasonic coating conditions are 250-350W, 40-60kHz, and 60-120min; the centrifugation speed of the composite suspension is 10000-12000r / min, the centrifugation time is 15-20min, and it is washed with acetone 3-4 times to remove free particles and impurities. The vacuum drying conditions are 60-80℃, -0.09MPa, and 8-12h.

[0013] Thirdly, the present invention provides the application of the above-mentioned high-barrier, water-resistant, and adhesive-free polyvinyl alcohol coating liquid. The polyvinyl alcohol coating liquid is applied to the substrate by a machine, and then dried, wound up, and microwave cured to obtain food packaging material.

[0014] Preferably, the coating amount of the polyvinyl alcohol coating liquid on the substrate is 0.7-2 g / m². 2 The coating speed is 300-400 m / min; the substrate is a PET film, a BOPP film, a BOPP film, a BOPP film, an MDOPE film, or a PA film that has been pre-treated by vapor deposition; the drying temperature is 100-120℃ and the drying time is 15-25 s; the microwave curing power is 500-600W and the time is 8-10 min.

[0015] Compared with the prior art, the present invention has the following advantages: 1. The polyvinyl alcohol coating solution of this invention is a highly transparent water-based system, non-corrosive, odorless, and generates no harmful substances throughout the preparation process. The use of bio-based catalytic crosslinking agents and biodegradable slip agents (polylactic acid microsphere slip agents, beeswax powder) endows the coating solution with excellent environmental compatibility and biodegradability, avoiding the environmental and operator health hazards posed by VOCs and harmful additives in traditional base adhesives from the source. Simultaneously, through strict control of raw material selection and preparation processes, the migration of harmful substances in the coating solution and finished packaging materials meets the requirements of GB 4806.7-2023 "National Food Safety Standard Requirements for Plastic Materials and Articles for Food Contact," ensuring food contact safety.

[0016] 2. This invention innovatively employs core-shell structured SiO2@TiO2 nanocomposite particles, synergizing the natural oxygen barrier advantages of the polyvinyl alcohol matrix with the physical barrier effect of the nanoparticles to construct a multi-layered barrier system, achieving highly efficient dual barrier against oxygen and water vapor. By precisely controlling the mass ratio of nano-silica to nano-titanium dioxide and the proportions of each raw material, high oxygen barrier, high water barrier, or balanced barrier products can be flexibly customized to meet the storage and preservation needs of different foods.

[0017] 3. This invention employs a gradient temperature crosslinking process (50-60℃ pre-crosslinking → 70-75℃ crosslinking), ultrasonic-assisted dispersion, and microwave curing, effectively solving problems such as unstable viscosity of traditional PVA coating solutions, easy self-polymerization of crosslinking agents, and low drying efficiency. The solvent evaporates quickly, resulting in a thin and uniform film thickness, avoiding hardening and yellowing of the coating substrate, and not affecting subsequent printing, lamination, and other processing steps. The coating solution exhibits excellent storage stability and can withstand temperatures as low as -5℃ without freezing, adapting to the production and transportation needs of different regions and seasons.

[0018] 4. This invention eliminates the pre-coating step in traditional coating processes, simplifying the production process, reducing the investment in supporting equipment, manpower, and raw materials, significantly lowering production costs, and fundamentally eliminating the environmental pollution problems caused by the use of primer, thus aligning with the development trend of the green packaging industry. Furthermore, the finished packaging material is compatible with various substrates (vapor-deposited PET / BOPP film, corona-treated BOPP / MDOPE / PA film), with coating speeds up to 300-400 m / min and drying time of only 15-25 seconds. It boasts high production efficiency, is easy to achieve industrial-scale mass production, and possesses both economic value and environmental significance.

[0019] 5. The coated film of this invention exhibits a low increase in oxygen permeability after being boiled at 121°C for 30 minutes, maintaining excellent barrier properties. After microwave heating at 700W for 5 minutes, the coating shows no discoloration or cracking, making it suitable for applications such as food heating and sterilization. Furthermore, the addition of a slip agent optimizes the surface properties of the coating, improving the film's processing adaptability and user experience. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention.

[0021] Example 1 The preparation method of the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid in this embodiment includes the following steps: S1 Preparation of polyvinyl alcohol solution: 184g of deionized water was heated to 50℃, and 16g of polyvinyl alcohol (degree of polymerization 1800, degree of alcoholysis 99.3%, Chongqing Chuanwei Chemical Co., Ltd.) was added. The mixture was stirred and heated to 95℃ at 600r / min for 2 hours and then kept at a constant temperature for 40 minutes to ensure that the polyvinyl alcohol was fully dissolved. The mixture was then allowed to cool naturally to room temperature to obtain a polyvinyl alcohol solution.

[0022] Preparation of S2 nanocomposite particles: Nano-silica (99.8% purity, Hubei Huifu E-commerce Co., Ltd.) and hydrophobic nano-titanium dioxide (99.8% purity, Hubei Huifu E-commerce Co., Ltd.) were added to anhydrous ethanol at a mass ratio of 1:0.2. Suspensions of nano-silica and nano-titanium dioxide were prepared by ultrasonic dispersion (200W, 40kHz, 60min). Subsequently, the nano-titanium dioxide suspension was added dropwise to the nano-silica suspension at a rate of 1mL / min, and ultrasonically coated for 120min at 250W, 40kHz, and 30℃ to obtain a composite suspension. The composite suspension was centrifuged at 10000r / min for 20min, the precipitate was collected, washed three times with acetone, and then vacuum dried at 60℃ and -0.09MPa for 12h to constant weight. After cooling, it was ground through a 200-mesh sieve to obtain SiO2@TiO2 nanocomposite particles.

[0023] Preparation of polyvinyl alcohol coating solution S3: After heating the polyvinyl alcohol solution to 50°C, add 2g of citric acid and 2g of gallic acid, and stir continuously for 1h for pre-crosslinking; then raise the temperature to 70°C, add 2g of citric acid, 2g of gallic acid and 1g of SiO2@TiO2 nanocomposite particles, and stir for 3h for crosslinking; then cool naturally to 50°C, add 0.5g of polylactic acid microsphere lubricant, 0.1g of water-based coating defoamer (DT-650, Daejeon Chemical Co., Ltd.) and 10g of isopropanol, and mix evenly to obtain the polyvinyl alcohol coating solution.

[0024] The polyvinyl alcohol coating solution prepared in this embodiment was applied onto a pre-deposited PET film using a coating machine, with a coating amount of 1.8 g / m². 2 The coating speed is 300 m / min; the coated PET film is dried at 100℃ for 25 seconds; it is then wound up and microwave-cured for 10 minutes at a power of 600W.

[0025] Example 2 The difference from Example 1 is that in step S2, the mass ratio of nano-silica to nano-titanium dioxide is 1:1.5.

[0026] Example 3 The difference from Example 1 is that in step S2, the mass ratio of nano-silica to nano-titanium dioxide is 1:1.

[0027] Example 4 The difference from Example 1 is that the polyvinyl alcohol coating liquid prepared in Example 1 is coated onto a BOPP film that has been pre-treated by vapor deposition.

[0028] Example 5 The difference from Example 1 is that the polyvinyl alcohol coating liquid prepared in Example 1 is coated onto a BOPP film that has been pre-treated with corona by a machine.

[0029] Example 6 The difference from Example 1 is that the polyvinyl alcohol coating liquid prepared in Example 1 is coated onto the MDOPE film that has been pre-treated with corona by a machine.

[0030] Example 7 The difference from Example 1 is that the polyvinyl alcohol coating liquid prepared in Example 1 is coated onto a pre-corona-treated PA film using a machine.

[0031] Example 8 The preparation method of the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid in this embodiment includes the following steps: S1 Preparation of polyvinyl alcohol solution: Heat 150g of deionized water to 55℃, then add 50g of polyvinyl alcohol (degree of polymerization 1800, degree of alcoholysis 99.3%, Chongqing Chuanwei Chemical Co., Ltd.), and continuously stir and heat to 90℃ at 400r / min. After stirring continuously for 2h, keep the temperature constant for 42min to ensure that the polyvinyl alcohol is fully dissolved. Let it cool naturally to room temperature to obtain a polyvinyl alcohol solution.

[0032] Preparation of S2 nanocomposite particles: Nano-silica and hydrophobic nano-titanium dioxide were added to anhydrous ethanol at a mass ratio of 1:0.2. Suspensions of nano-silica and nano-titanium dioxide were prepared by ultrasonic dispersion (250 W, 45 kHz, 50 min). Subsequently, the nano-titanium dioxide suspension was added dropwise to the nano-silica suspension at a rate of 1 mL / min, and ultrasonically coated for 100 min at 300 W, 50 kHz, and 35 °C to obtain a composite suspension. The composite suspension was centrifuged at 11000 r / min for 18 min, the precipitate was collected, washed three times with acetone, and then vacuum dried at 70 °C and -0.09 MPa for 10 h to constant weight. After cooling, it was ground through a 200-mesh sieve to obtain SiO2@TiO2 nanocomposite particles.

[0033] Preparation of polyvinyl alcohol coating solution S3: After heating the polyvinyl alcohol solution to 55°C, add 1.2g hydrochloric acid and 1.2g tannic acid, and stir continuously for 1h for pre-crosslinking; then raise the temperature to 72°C, add 2.4g hydrochloric acid, 1.2g tannic acid and 3g SiO2@TiO2 nanocomposite particles, and stir for 3h for crosslinking; then cool naturally to 47°C, add 0.1g polylactic acid microsphere lubricant, 0.5g water-based coating defoamer (DT-650, Daejeon Chemical Co., Ltd.) and 8g ethanol, and mix evenly to obtain the polyvinyl alcohol coating solution.

[0034] The polyvinyl alcohol coating solution prepared in this embodiment was applied onto a pre-deposited PET film using a coating machine, with a coating amount of 2 g / m². 2 The coating speed is 350 m / min; the coated PET film is dried at 110℃ for 20 seconds; it is then wound up and microwave cured for 9 minutes at a power of 550W.

[0035] Example 9 The preparation method of the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid in this embodiment includes the following steps: S1 Preparation of polyvinyl alcohol solution: Heat 165g of deionized water to 60℃, then add 35g of polyvinyl alcohol (degree of polymerization 1800, degree of alcoholysis 99.3%, Chongqing Chuanwei Chemical Co., Ltd.), and continuously stir and heat to 85℃ at 800r / min. After stirring continuously for 2h, keep the temperature constant for 45min to ensure that the polyvinyl alcohol is fully dissolved. Let it cool naturally to room temperature to obtain a polyvinyl alcohol solution.

[0036] Preparation of S2 nanocomposite particles: Nano-silica and hydrophobic nano-titanium dioxide were added to anhydrous ethanol at a mass ratio of 1:0.5. Suspensions of nano-silica and nano-titanium dioxide were prepared by ultrasonic dispersion (300W, 50kHz, 30min). Subsequently, the nano-titanium dioxide suspension was added dropwise to the nano-silica suspension at a rate of 2mL / min, and ultrasonically coated at 350W, 60kHz, and 40℃ for 60min to obtain a composite suspension. The composite suspension was centrifuged at 12000r / min for 15min, the precipitate was collected, washed four times with acetone, and then vacuum dried at 80℃ and -0.09MPa for 8h to constant weight. After cooling, it was ground through a 200-mesh sieve to obtain SiO2@TiO2 nanocomposite particles.

[0037] Preparation of polyvinyl alcohol coating solution S3: After heating the polyvinyl alcohol solution to 60°C, add 1.4g maleic acid and 0.6g gallic acid, and stir continuously for 1h for pre-crosslinking; then raise the temperature to 75°C, add 2.1g maleic acid, 0.9g gallic acid and 5g SiO2@TiO2 nanocomposite particles, and stir for 3h for crosslinking; then cool naturally to 45°C, add 1g beeswax powder, 1g water-based coating defoamer (DT-650, Daejeon Chemical Co., Ltd.) and 5g methanol, and mix evenly to obtain the polyvinyl alcohol coating solution.

[0038] The polyvinyl alcohol coating solution prepared in this embodiment was applied onto a pre-deposited PET film using a coating machine, with a coating amount of 0.7 g / m². 2The coating speed is 400 m / min; the coated PET film is dried at 120℃ for 15 seconds; it is then wound up and microwave-cured for 8 minutes at a power of 500W.

[0039] Example 10 The preparation method of the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid in this embodiment includes the following steps: S1 Preparation of polyvinyl alcohol solution: Heat 180g of deionized water to 50℃, add 20g of polyvinyl alcohol (degree of polymerization 1800, degree of alcoholysis 99.3%, Chongqing Chuanwei Chemical Co., Ltd.), stir continuously at 600r / min and heat to 95℃, stir continuously for 2h and then keep at a constant temperature for 40min to ensure that the polyvinyl alcohol is fully dissolved, and then cool naturally to room temperature to obtain a polyvinyl alcohol solution.

[0040] Preparation of S2 nanocomposite particles: Nano-silica and hydrophobic nano-titanium dioxide were added to anhydrous ethanol at a mass ratio of 1:1.02. Suspensions of nano-silica and nano-titanium dioxide were prepared by ultrasonic dispersion (200W, 40kHz, 60min). Subsequently, the nano-titanium dioxide suspension was added dropwise to the nano-silica suspension at a rate of 1mL / min, and ultrasonically coated for 120min at 250W, 40kHz, and 30℃ to obtain a composite suspension. The composite suspension was centrifuged at 10000r / min for 20min, the precipitate was collected, washed three times with acetone, and then vacuum dried at 60℃ and -0.09MPa for 12h to constant weight. After cooling, it was ground through a 200-mesh sieve to obtain SiO2@TiO2 nanocomposite particles.

[0041] Preparation of polyvinyl alcohol coating solution S3: After heating the polyvinyl alcohol solution to 50°C, add 2g of phosphoric acid and 2g of gallic acid, and stir continuously for 1h for pre-crosslinking; then raise the temperature to 70°C, add 2g of phosphoric acid, 2g of gallic acid and 1g of SiO2@TiO2 nanocomposite particles, and stir for 3h for crosslinking; then cool naturally to 50°C, add 0.5g of polylactic acid microsphere lubricant, 0.1g of water-based coating defoamer (DT-650, Daejeon Chemical Co., Ltd.) and 10g of isopropanol, and then add 5g of water-based antifreeze (W-7010, Nanjing Baiju Technology Co., Ltd.). After mixing evenly, the polyvinyl alcohol coating solution is obtained.

[0042] The coating solution of this embodiment was placed at -5℃, 0℃, 5℃ and 10℃ for 48 hours respectively. The coating solution did not have any layering or freezing phenomenon. The appearance was consistent with the sample at normal room temperature. The pH value was around 6.2 and the viscosity was 92.15 mPa·s.

[0043] Comparative Example 1 The difference from Example 1 is that nano-silica is used instead of the SiO2@TiO2 nanocomposite particles in step S2.

[0044] Comparative Example 2 The difference from Example 1 is that step S2 is omitted, and SiO2@TiO2 nanocomposite particles are not added in step S3.

[0045] Comparative Example 3 The difference from Example 1 is that in step S3, hydrochloric acid is used instead of gallic acid.

[0046] Comparative Example 4 The difference from Example 1 is that after the coating liquid is applied to the PET film that has been pre-treated by vapor deposition, the curing method is high-temperature curing, with a curing temperature of 45°C and a curing time of 48 hours.

[0047] Comparative Example 5 Comparative Example 5 used PVA coating liquid (NF101, Tianjin Nuochen Materials Technology Co., Ltd.) instead of the polyvinyl alcohol coating liquid prepared in Example 1, and coated it on a PET film that had been pre-treated by vapor deposition in the manner of Example 1.

[0048] The performance of the films coated with the coating liquid in Examples 1-9 and Comparative Examples 1-5 was tested using the following methods: Oxygen permeability: Select three test samples of suitable size with uniform thickness, flat surface, and free from wrinkles, creases, pinholes, and other defects. Mark the side of the test sample facing the test gas. Place the samples in a desiccator at 23±2℃ for at least 48 hours. Determine the oxygen permeability according to the first method (differential pressure method) or the second method (coulometric method) in General Chapter 4007 of Part IV of the Chinese Pharmacopoeia 2025.

[0049] Water vapor transmission rate: Select three test samples of suitable size with uniform thickness, free from wrinkles, creases, pinholes, and other defects. The test samples should be conditioned at 23±2℃ and relative humidity of 50±10% for at least 4 hours. Place the samples in the instrument and determine the water vapor transmission rate according to Method III (infrared detector method) in General Chapter 4010, Part IV, Chinese Pharmacopoeia 2025 Edition, under the following experimental conditions: temperature 38±0.5℃, relative humidity 90±2%.

[0050] Migration of hazardous substances: determined in accordance with GB 4806.7-2023 "National Food Safety Standard Requirements for Plastic Materials and Products for Food Contact".

[0051] Coating surface resistance: Tested in accordance with IEC61340-5-1 "Surface resistance test of electrostatic protective materials" and ISO3915 "Determination of surface resistivity of plastics".

[0052] Microwave heating resistance: After microwave heating at 700W for 5 minutes, observe whether the coating of the film material shows any discoloration or cracking.

[0053] Resistant to boiling: After boiling the membrane material at 121℃ for 30 minutes, calculate the percentage change in oxygen permeability before and after boiling. If it is less than 10%, it is considered qualified and has the effect of resisting boiling.

[0054] The test results are shown in Table 1-2: Table 1 Performance test results of membrane materials in Examples 1-9

[0055] Table 2 Performance test results of the membrane materials in Comparative Examples 1-5

[0056] The test data above shows that Comparative Example 1 uses nano-silica instead of the SiO2@TiO2 nanocomposite particles prepared in step S2 of Example 1. Because the SiO2@TiO2 nanocomposite particles are designed with a core-shell structure, they are adapted to the polarity of the PVA coating liquid and have a stronger interfacial bonding force with the substrate. On the other hand, the nano-silica has a dense hydroxyl group on its surface and is too polar. In the aqueous coating system, it is easy to agglomerate due to hydrogen bonding. Agglomerated SiO2 particles will cause pinholes and microcracks in the coating film, destroy the film density, and directly cause an increase in oxygen permeability and water vapor permeability. The core-shell structure of SiO2@TiO2 nanocomposite particles can inhibit particle agglomeration and make the coating more uniform and dense.

[0057] Comparative Example 2 did not include SiO2@TiO2 nanocomposite particles. Because the SiO2@TiO2 nanocomposite particles can extend the gas permeation path through the "maze effect" and interact with the coating liquid film-forming resin (PVA), increasing the coating crosslinking density and exhibiting higher crystallinity and compactness, they can further block gas molecule penetration, forming a double barrier. Therefore, the gas barrier performance of the PVA resin-only barrier in Comparative Example 2 is significantly inferior to the composite system in Example 1 that included SiO2@TiO2 nanocomposite particles.

[0058] Organic and inorganic acids modify PVA through different mechanisms. Gallic acid, as an organic acid, can achieve a dual effect of "chemical cross-linking + physical reinforcement," while hydrochloric acid (an inorganic acid) used in Comparative Example 3 can only induce limited molecular chain degradation and cannot improve the strength and toughness of the PVA coating. Secondly, the ester and hydrogen bond network formed by the cross-linking of gallic acid and PVA can reduce the hydrophilicity of the PVA coating and decrease the swelling effect of water molecules on the PVA molecular chains. Even in humid environments, the coating structure remains stable, and the strength does not decrease significantly. However, the hydrochloric acid-modified PVA coating, due to molecular chain degradation, does not improve hydrophilicity. Upon contact with water, the molecular chains are easily swollen and dissociated, leading to a rapid decrease in the strength of the coating in Comparative Example 3.

[0059] Example 1 uses microwave curing to achieve synchronous and uniform cross-linking of the coating inside and outside, constructing a defect-free dense network structure and improving the structural stability of the coating under repeated microwave irradiation; while Comparative Example 4 uses high-temperature curing, and its heat conduction mechanism leads to cross-linking gradient and thermal stress defects in the coating. Under heating, the defects will continue to expand, eventually leading to performance degradation.

[0060] Comparative Example 5 uses commercially available PVA coating liquid, and the resulting coating film has inferior functionality compared to Example 1 of the present invention. The present invention is specifically designed for the interface characteristics and high barrier requirements of vapor-deposited PET substrate, which greatly improves interface compatibility and performance.

Claims

1. A high-barrier, water-resistant, primer-free polyvinyl alcohol coating liquid, characterized in that, It is composed of the following raw materials in parts by weight: 15-50 parts polyvinyl alcohol, 150-185 parts deionized water, 5-8 parts catalytic crosslinking agent, 0.1-1 part defoamer, 0.1-1 part slip agent, 1-5 parts nanocomposite particles, 5-10 parts regulator, and 0-5 parts antifreeze agent; wherein, the catalytic crosslinking agent is a combination of one of gallic acid and tannic acid with one of hydrochloric acid, citric acid, maleic acid, and phosphoric acid, and one of gallic acid and tannic acid accounts for 30-50% of the total mass of the catalytic crosslinking agent; the slip agent is polylactic acid microsphere slip agent or beeswax powder; the nanocomposite particles are core-shell structured nanocomposite particles constructed from nano-silica and nano-titanium dioxide.

2. The high-barrier, water-resistant, boil-proof, primerless polyvinyl alcohol coating liquid as described in claim 1, characterized in that, The defoamer is a polyether-modified silicone defoamer.

3. The high-barrier, water-resistant, boil-proof, primerless polyvinyl alcohol coating liquid as described in claim 1, characterized in that, The regulator is isopropanol, ethanol, or methanol.

4. The method for preparing the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid according to any one of claims 1-3, characterized in that, Includes the following steps: S1 Preparation of polyvinyl alcohol solution: Heat deionized water to 50-60℃, add polyvinyl alcohol, continue stirring and heat to 85-95℃, continue stirring and keep the temperature constant for 40-45 minutes to dissolve the polyvinyl alcohol, and cool naturally to room temperature to obtain polyvinyl alcohol solution. S2 Preparation of nanocomposite particles: Add nano-silica to anhydrous ethanol and disperse by ultrasonication to obtain a nano-silica suspension; add nano-titanium dioxide to anhydrous ethanol and disperse by ultrasonication, with a mass ratio of nano-silica to nano-titanium dioxide of 1:(0.2-1.5) to obtain a nano-titanium dioxide suspension. The nano-titanium dioxide suspension was dropped into the nano-silica suspension at a rate of 1-2 mL / min, and ultrasonic coating was performed at 30-40℃ to obtain a composite suspension. The composite suspension was centrifuged, washed, vacuum dried to constant weight, cooled, ground and sieved to obtain SiO2@TiO2 nanocomposite particles. S3 Preparation of polyvinyl alcohol coating solution: Heat the polyvinyl alcohol solution prepared in step S1 to 50-60℃, add 40-50wt.% catalytic crosslinking agent, and stir to carry out pre-crosslinking; Then, the temperature is raised to 70-75℃, the remaining catalytic crosslinking agent and the SiO2@TiO2 nanocomposite particles prepared in step S2 are added, and crosslinking is carried out by stirring; the temperature is lowered to 45-50℃, and defoamer, lubricant, regulator and antifreeze are added, and after mixing evenly, polyvinyl alcohol coating liquid is obtained.

5. The method for preparing the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid as described in claim 4, characterized in that, In step S1, the stirring speed is 400-800 r / min.

6. The method for preparing the high-barrier, water-resistant, primerless polyvinyl alcohol coating liquid as described in claim 4, characterized in that, In step S2, when preparing nano-silica suspension and nano-titanium dioxide suspension, the ultrasonic dispersion conditions are 200-300W, 40-50kHz, and 30-60min; when preparing composite suspension, the ultrasonic coating conditions are 250-350W, 40-60kHz, and 60-120min; the centrifugation speed of composite suspension is 10000-12000r / min, the centrifugation time is 15-20min, it is washed with acetone 3-4 times, and the vacuum drying conditions are 60-80℃, -0.09MPa, and 8-12h.

7. The application of the high-barrier, water-resistant, boil-proof, primerless polyvinyl alcohol coating liquid as described in any one of claims 1-3, characterized in that, Polyvinyl alcohol coating liquid is applied to the substrate through a machine, followed by drying, winding, and microwave curing to obtain food packaging material.

8. The application of the high-barrier, water-resistant, boil-proof, primerless polyvinyl alcohol coating liquid as described in claim 7, characterized in that, The coating amount of polyvinyl alcohol coating liquid on the substrate is 0.7-2 g / m². 2 The coating speed is 300-400 m / min; the substrate is a PET film, a BOPP film, a BOPP film, a BOPP film, an MDOPE film, or a PA film that has been pre-treated by vapor deposition; the drying temperature is 100-120℃ and the drying time is 15-25 s; the microwave curing power is 500-600W and the time is 8-10 min.

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