Food-grade composite packaging material with both air permeability and waterproofness and its production process

Abstract

The present application relates to the technical field of packaging materials, in particular to a food-grade composite packaging material with air permeability and waterproofness and a production process thereof.The food-grade composite packaging material comprises a polypropylene waterproof layer, a microporous air-permeable layer, a polyvinylidene fluoride contact layer and a polyurethane adhesive binder; the microporous air-permeable layer comprises the following components: 40-60 parts by weight of polyvinyl alcohol modifier, 10-20 parts by weight of ammonium bicarbonate pore former, 2-5 parts by weight of tributyl citrate plasticizer, 0.5-2 parts by weight of butylated hydroxyl anisole antioxidant, 1-3 parts by weight of sodium dodecyl sulfate surfactant and 5-10 parts by weight of epoxy resin crosslinking agent; polyvinyl alcohol is selected as a base material, cellulose nanocrystals are selected as a particle filler, and 3-glycidyl ether oxypropyl trimethoxysilane is selected as a grafting modifier.The microporous air-permeable layer allows oxygen, carbon dioxide and other gas molecules to selectively permeate through the microporous structure formed by the pore former, maintains the gas exchange inside the package, prolongs the shelf life of food, and at the same time maintains the integrity of the package.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of packaging materials, in particular, to a food-grade composite packaging material with air permeability and waterproofness and a production process thereof. BACKGROUND

[0002] Fresh fruits and vegetables will undergo respiration during storage, releasing carbon dioxide and consuming oxygen. When the packaging material is completely air-tight, it will cause the carbon dioxide concentration inside the package to rise and the oxygen concentration to drop, causing anaerobic respiration or accelerating the ripening of fruits and vegetables, which in turn affects the quality. In addition, the loss of water in fruits and vegetables will directly affect their weight, appearance, and taste. Therefore, having a certain degree of waterproofness can effectively prevent external moisture penetration and internal moisture evaporation, maintaining the water content and tenderness of fruits and vegetables. In view of this, a packaging material is needed that allows a certain degree of gas exchange to maintain the freshness of fruits and vegetables and extend the shelf life. Therefore, a food-grade composite packaging material with air permeability and waterproofness and a production process thereof are proposed. SUMMARY

[0003] The present application aims to provide a food-grade composite packaging material with air permeability and waterproofness and a production process thereof to solve the problem of fresh fruits and vegetables undergoing respiration during storage, releasing carbon dioxide and consuming oxygen, which causes the carbon dioxide concentration inside the package to rise and the oxygen concentration to drop, causing anaerobic respiration or accelerating the ripening of fruits and vegetables, which in turn affects the quality.

[0004] To achieve the above-mentioned purpose, the present application provides a food-grade composite packaging material with air permeability and waterproofness, which comprises a polypropylene waterproof layer, a microporous air-permeable layer, a polyvinylidene fluoride contact layer, and a polyurethane adhesive.

[0005] The microporous air-permeable layer comprises the following components: polyvinyl alcohol (PVA) modifier 40-60 parts by weight, ammonium bicarbonate (NH4HCO3) pore former 10-20 parts by weight, tributyl citrate plasticizer 2-5 parts by weight, butylated hydroxyanisole antioxidant 0.5-2 parts by weight, sodium dodecyl sulfate surfactant 1-3 parts by weight, and epoxy resin crosslinking agent 5-10 parts by weight.

[0006] The ammonium bicarbonate (NH4HCO3) pore former 10-20 parts by weight is used to generate gas during heat treatment, forming a microporous structure.

[0007] The butylated hydroxyanisole antioxidant is used to prevent the oxidation and degradation of PVA during processing and subsequent storage. It may be necessary to add an appropriate amount of food-grade antioxidant.

[0008] Sodium dodecyl sulfate surfactant in the dispersion of cellulose nanocrystals (CNC), need to use surfactant to ensure the uniform dispersion of CNC in PVA matrix, to avoid agglomeration; The selected surfactant should be food grade and compatible with the system;

[0009] Epoxy resin crosslinking agent is used to enhance the mechanical strength and thermal stability of the modified PVA film, and a proper amount of crosslinking agent is needed; The crosslinking agent reacts with the hydroxyl groups on the PVA molecular chain to form a network structure;

[0010] The polyvinyl alcohol (PVA) modifier realizes composite modification of polyvinyl alcohol (PVA) by using nanoparticle filling-chemical grafting method, wherein polyvinyl alcohol (PVA) is selected as the base material, cellulose nanocrystals (CNC) are selected as the particle filler, and 3-glycidyl ether oxypropyl trimethoxysilane (KH-560) is selected as the grafting modifier.

[0011] As preferred, the weight ratio of polyvinyl alcohol (PVA), cellulose nanocrystals (CNC) and 3-glycidyl ether oxypropyl trimethoxysilane (KH-560) is 35:10:1.

[0012] As preferred, the preparation method of the polyvinyl alcohol (PVA) modifier is as follows:

[0013] S1.1, disperse cellulose nanocrystals (CNC) in deionized water, and use ultrasonic treatment for 30-45 min to make the cellulose nanocrystals (CNC) fully dispersed to form a cellulose nanocrystals (CNC) suspension;

[0014] S1.2, dissolve 3-glycidyl ether oxypropyl trimethoxysilane (KH-560) in ethanol to prepare a 3-glycidyl ether oxypropyl trimethoxysilane solution;

[0015] S1.3, add the above cellulose nanocrystals (CNC) suspension and 3-glycidyl ether oxypropyl trimethoxysilane solution to the PVA solution in sequence, keep constant temperature at 60℃, and continuously stir at a speed of 100-230 rpm for 2-4 hours to carry out grafting reaction, which is used to ensure the effective grafting of KH-560 with PVA and CNC;

[0016] S1.4, after the reaction is completed, cool the mixed system to room temperature, remove impurities and byproducts by centrifugation and filtration to obtain a polyvinyl alcohol (PVA) modifier solution, and obtain a solid powder of polyvinyl alcohol (PVA) modifier by freeze-drying method.

[0017] As preferred, in S1.3, the grafting reaction is specifically:

[0018] The epoxy groups on the 3-glycidyl etheroxypropyltrimethoxysilane molecule can undergo ring-opening reactions with the hydroxyl groups (-OH) on the surface of cellulose nanocrystals (CNC) to form stable Si-OC and Si-OH bonds, thereby achieving the chemical grafting of KH-560 on the CNC surface.

[0019] The epoxy group on the 3-glycidyl etheroxypropyltrimethoxysilane molecule undergoes a ring-opening reaction with the hydroxyl group (-OH) on the polyvinyl alcohol (PVA) molecular chain. The hydroxyl group (-OH) serves as a reaction site for grafting with the epoxy group, and the resulting Si-OC bond covalently links the KH-560 molecular chain segment to the polyvinyl alcohol (PVA) molecular chain. This not only increases the crosslinking degree and cohesive force of polyvinyl alcohol (PVA), but also endows the surface of polyvinyl alcohol (PVA) with a certain degree of silanization modification.

[0020] In summary, the grafting reaction specifically includes two aspects: first, the grafting of 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) to the hydroxyl groups on the surface of cellulose nanocrystals (CNC) to functionalize the CNC surface; second, the grafting of 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) to the hydroxyl groups of the polyvinyl alcohol (PVA) molecular chain to enhance the crosslinking structure of PVA and perform silanization modification. These two processes occur simultaneously, ultimately forming a polyvinyl alcohol (PVA) modifier solution, in which the CNC is tightly linked to the PVA molecular chain through KH-560 bridging, forming a composite material with excellent properties.

[0021] Preferably, the microporous breathable layer is prepared by the following method:

[0022] S2.1 Weigh the following components by weight:

[0023] Ammonium bicarbonate (NH4HCO3) pore-forming agent 10-20 parts by weight, tributyl citrate plasticizer 2-5 parts by weight, butylated hydroxyanisole antioxidant 0.5-2 parts by weight, sodium dodecyl sulfate surfactant 1-3 parts by weight and epoxy resin crosslinking agent 5-10 parts by weight;

[0024] S2.2 Slowly add 40-60 parts by weight of polyvinyl alcohol (PVA) modifier to deionized water, and stir in a mixer at a speed of 60-120 rpm until the polyvinyl alcohol (PVA) modifier is completely dissolved to form a homogeneous transparent polyvinyl alcohol (PVA) solution.

[0025] The solution in the stirrer is heated to 60-80℃, and the 3-glycidyl etheroxypropyltrimethoxysilane in the polyvinyl alcohol (PVA) modifier undergoes a grafting reaction with polyvinyl alcohol (PVA) through heating.

[0026] The glycidyl ether group in 3-glycidyl etheroxypropyltrimethoxysilane can undergo a ring-opening reaction at a certain temperature, and copolymerize with the hydroxyl groups (-OH) on the polyvinyl alcohol molecular chain or the hydroxyl groups on the surface of cellulose nanocrystals (CNC). No additional initiator is required; the reaction can be initiated simply by raising the temperature.

[0027] S2.3 After the initiation reaction is completed, add 10-20 parts by weight of ammonium bicarbonate (NH4HCO3) pore-forming agent, 2-5 parts by weight of tributyl citrate plasticizer, 0.5-2 parts by weight of butylated hydroxyanisole antioxidant and 1-3 parts by weight of sodium dodecyl sulfate surfactant to polyvinyl alcohol (PVA) solution in sequence, and continue to stir evenly to ensure that all components are fully mixed to obtain a mixture.

[0028] S2.4 Under stirring, add 5-10 parts by weight of epoxy resin crosslinking agent to the mixed solution in S2.3. The epoxy resin crosslinking agent fully contacts and reacts with the hydroxyl groups on the polyvinyl alcohol (PVA) molecular chain to form a crosslinked structure. Under stirring, the epoxy resin crosslinking agent is added to the mixed solution containing PVA to achieve effective contact between the hydroxyl groups on the PVA molecular chain and the crosslinking agent and to form a stable three-dimensional network structure.

[0029] S2.5. Let the mixed solution that has completed the cross-linking reaction stand for 10-15 minutes, remove the internal air bubbles by vacuum degassing to improve the density and uniformity of the membrane, and then cast the degassed solution evenly into a flat mold.

[0030] S2.6 Place the cast mold in a constant temperature oven for heat treatment to obtain a microporous membrane to be cooled. During this process, ammonium bicarbonate decomposes to produce gas, which forms a microporous structure in the membrane. Ammonium bicarbonate (NH4HCO3) acts as a pore-forming agent and decomposes under heating conditions to produce ammonia (NH3), carbon dioxide (CO2), and water vapor (H2O). These gases form a microporous structure in the PVA membrane.

[0031] S2.7 After the heat treatment is completed, the microporous membrane is allowed to cool naturally to room temperature in the air, and then removed from the mold to obtain a microporous breathable membrane.

[0032] Preferably, the specific reaction conditions in S2.4 are as follows:

[0033] Before adding the epoxy resin crosslinking agent, preheat the polyvinyl alcohol (PVA) mixture to 40-60℃. After adding the crosslinking agent and starting to stir and mix, raise the temperature to 60-80℃. Within this temperature range, the ring-opening reactivity of the epoxy groups is enhanced, increasing the probability of effective crosslinking with the hydroxyl groups on the PVA molecular chain. At the same time, avoid excessively high temperatures that could lead to increased side reactions or PVA degradation. Continue stirring for 15-30 minutes to ensure that the crosslinking agent and PVA solution are fully and uniformly mixed, reducing the possibility of excessively high or low local concentrations and promoting uniform crosslinking. Maintain the temperature at 60-80℃ for 2-6 hours. Keeping this temperature constant allows the epoxy groups of the epoxy resin to fully contact and undergo crosslinking with the hydroxyl groups on the PVA molecular chain within a certain time.

[0034] Preferably, in step S2.6, the heating rate of the constant temperature oven is as follows:

[0035] In the initial heating stage, the heating rate is 1-3℃ / min. This is to allow the components in the PVA solution to gradually adapt to the temperature change and avoid local overheating or premature decomposition of the pore-forming agent due to rapid heating, which would affect the uniform distribution of micropores. At the same time, low-speed heating also helps to reduce the stress accumulated inside the solution and on the membrane surface, preventing cracks or deformation of the membrane during the pore-forming process.

[0036] When the temperature approaches the critical point of 150-190℃ where ammonium bicarbonate begins to decompose in large quantities, increase the heating rate and keep it within the range of 4-5℃ / min. The purpose is to quickly cross the decomposition temperature range of ammonium bicarbonate and ensure that it can decompose fully in a short time to form a large amount of gas to generate uniform micropores.

[0037] Isothermal time: When the temperature rises to 200℃, maintain the current temperature for 15-30 minutes to ensure that ammonium bicarbonate can be completely decomposed, while avoiding the negative impact of excessively high temperature on other properties of the membrane. This ensures that ammonium bicarbonate is completely decomposed and that the generated gas diffuses fully and forms a stable microporous structure in the membrane. This ensures the effective decomposition of the pore-forming agent and avoids excessive shrinkage or performance degradation of the membrane due to prolonged high-temperature treatment.

[0038] On the other hand, the present invention provides a production process for a food-grade composite packaging material that combines breathability and waterproofness, used in any one of the above-mentioned food-grade composite packaging materials that combines breathability and waterproofness. The specific preparation method of the composite packaging material is as follows:

[0039] S3.1. The prepared microporous breathable membrane is used as the microporous breathable layer, and the polypropylene waterproof layer, the microporous breathable layer, and the polyvinylidene fluoride contact layer are arranged in sequence.

[0040] S3.2 Apply polyurethane adhesive evenly to the surfaces of the layers to be bonded.

[0041] S3.3. The adhesive-coated film layers are precisely aligned in the design sequence of polypropylene waterproof layer, microporous breathable layer, and polyvinylidene fluoride contact layer, and then fed into the laminator for lamination.

[0042] S3.3 The composite packaging material is cured in an oven to ensure that the adhesive is fully cross-linked and achieves the best bonding strength. Then, it is naturally cooled to room temperature to obtain the composite packaging material. Cooling is used to stabilize the composite material and eliminate internal stress.

[0043] Preferably, the laminator temperature is 160-180℃ and the laminator pressure is 0.8-2.0MPa.

[0044] Preferably, in step S3.3, the oven is used to cure the product for 30-45 minutes at a temperature of 140-190°C.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] 1. In this food-grade composite packaging material that combines breathability and waterproofness and its production process, cellulose nanocrystals are uniformly dispersed and appropriately arranged in the polyvinyl alcohol matrix, which can form a large number of nanochannels, which is conducive to the diffusion of gas molecules and thus improves breathability; the microporous breathable layer prepared by modified polyvinyl alcohol allows gas molecules such as oxygen and carbon dioxide to selectively permeate through the microporous structure formed by the pore-forming agent, maintains gas exchange inside the packaging, extends the shelf life of food, and maintains the integrity of the packaging.

[0047] 2. In this food-grade composite packaging material and its production process, which combines breathability and waterproofness, the microporous breathable layer and the polypropylene waterproof layer together construct a composite membrane structure that is both effectively waterproof and moderately breathable. The polypropylene waterproof layer is used to prevent direct penetration of external liquid water, protect the microporous breathable layer from damage, and provide additional gas barrier capabilities. The microporous breathable layer focuses on breathability, ensuring gas exchange and humidity management inside the fruit and vegetable packaging, which is conducive to maintaining the freshness of the fruits and vegetables. The multi-layer structure design makes full use of the advantages of each layer of materials, avoids functional conflicts, and optimizes the overall packaging performance. Detailed Implementation

[0048] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0049] The present invention provides a food-grade composite packaging material that combines breathability and waterproofness, comprising a polypropylene waterproof layer, a microporous breathable layer, a polyvinylidene fluoride contact layer, and a polyurethane adhesive.

[0050] The microporous breathable layer comprises the following components: 40-60 parts by weight of polyvinyl alcohol (PVA) modifier, 10-20 parts by weight of ammonium bicarbonate NH4HCO3 pore-forming agent, 2-5 parts by weight of tributyl citrate plasticizer, 0.5-2 parts by weight of butylated hydroxyanisole antioxidant, 1-3 parts by weight of sodium dodecyl sulfate surfactant, and 5-10 parts by weight of epoxy resin crosslinking agent;

[0051] The polyvinyl alcohol (PVA) modifier uses a nanoparticle filling-chemical grafting method to achieve composite modification of PVA. In this method, PVA is selected as the base material, cellulose nanocrystals (CNC) are used as particle fillers, and 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) is used as the grafting modifier.

[0052] The weight ratio of polyvinyl alcohol (PVA), cellulose nanocrystals (CNC), and 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) is 35:10:1.

[0053] 3-Glycidyl etheroxypropyltrimethoxysilane (KH-560) has epoxy groups that react with the hydroxyl groups of PVA to form stable Si-OC bonds, which enhance the cohesion and water resistance of the film. At the same time, its methoxy groups can be hydrolyzed to generate silanol groups, which participate in the further construction of siloxane networks.

[0054] Among them, ammonium bicarbonate (NH4HCO3) pore-forming agents have the following advantages:

[0055] Ammonium bicarbonate undergoes thermal decomposition at certain temperatures (usually above 100°C), producing ammonia (NH3), carbon dioxide (CO2), and water vapor (H2O). These gases can escape during heat treatment, leaving pores, and the decomposition products pose no risk of residue.

[0056] As an inorganic salt, ammonium bicarbonate has significant differences in chemical properties from components such as PVA, tributyl citrate, butylated hydroxyanisole, sodium dodecyl sulfate, and epoxy resin crosslinking agents, making it less prone to chemical reactions and exhibiting good compatibility.

[0057] By adjusting the amount of ammonium bicarbonate added and the heat treatment conditions (such as temperature and time), its decomposition rate and gas release can be controlled, thereby affecting the size and distribution of the formed pores and achieving regulation of air permeability.

[0058] Ammonium bicarbonate is a commonly used food additive. Its decomposition products are non-toxic and harmless, and meet the requirements of food safety regulations.

[0059] The preparation method of the polyvinyl alcohol (PVA) modifier is as follows:

[0060] S1.1 Disperse 10 parts by weight of cellulose nanocrystals (CNC) in 90 parts by weight of deionized water and sonicate for 30-45 minutes to fully disperse the cellulose nanocrystals (CNC) and form a cellulose nanocrystals (CNC) suspension with a dispersion concentration of 10%.

[0061] S1.2 Dissolve 1 part by weight of 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) in 99 parts by weight of ethanol to prepare a 1% 3-glycidyl etheroxypropyltrimethoxysilane solution.

[0062] S1.3. The above-mentioned cellulose nanocrystal (CNC) suspension and 3-glycidyl etheroxypropyltrimethoxysilane solution were added sequentially to 35 parts by weight of PVA solution. The temperature was kept constant at 60°C, and the mixture was stirred continuously at 100-230 rpm for 2-4 hours to carry out the grafting reaction; to ensure the effective grafting of KH-560 with PVA and CNC.

[0063] S1.4 After the reaction is complete, the mixture is cooled to room temperature, and impurities and byproducts are removed by centrifugation and filtration to obtain a polyvinyl alcohol (PVA) modifier solution. Solid powdered polyvinyl alcohol (PVA) modifier is obtained by freeze drying.

[0064] Cellulose nanocrystals (CNCs), as nanoscale reinforcing fillers, can significantly affect the microstructure of composite materials, especially gas transport pathways, due to their high orientation and high aspect ratio. When cellulose nanocrystals (CNCs) are uniformly dispersed in a polyvinyl alcohol (PVA) matrix, they form a large number of nanochannels, which are conducive to the diffusion of gas molecules and thus improve air permeability.

[0065] Meanwhile, 3-glycidyl etheroxypropyltrimethoxysilane (KH-560), as a silane coupling agent, forms a stable graft structure by chemically bonding with the PVA and CNC surfaces through its reactive groups at both ends, thereby improving the interfacial bonding force between the two. At the same time, it helps to improve the dispersibility of CNC in the PVA matrix and avoid agglomeration.

[0066] Example 1: A production process for a food-grade composite packaging material that combines breathability and waterproofness, comprising the following steps:

[0067] The specific preparation method of composite packaging materials is as follows:

[0068] S3.1. The prepared microporous breathable membrane is used as the microporous breathable layer, and the polypropylene waterproof layer, the microporous breathable layer, and the polyvinylidene fluoride contact layer are arranged in sequence.

[0069] S3.2 Apply polyurethane adhesive evenly to the surfaces of the layers to be bonded.

[0070] S3.3. The adhesive-coated film layers are precisely aligned in the design sequence of polypropylene waterproof layer, microporous breathable layer, and polyvinylidene fluoride contact layer, and then fed into the laminator. The lamination is carried out under the conditions of laminator temperature of 160-180℃ and laminator pressure of 0.8-2.0MPa.

[0071] S3.3. The composite packaging material is cured in an oven at 140-190℃ for 30-45 minutes to ensure complete cross-linking of the adhesive. Then, it is naturally cooled to room temperature to obtain the composite packaging material.

[0072] The preparation method of the microporous breathable layer is as follows:

[0073] S2.1 Weigh the following components by weight:

[0074] Ammonium bicarbonate (NH4HCO3) pore-forming agent 15 parts by weight, tributyl citrate plasticizer 3 parts by weight, butylated hydroxyanisole antioxidant 1 part by weight, sodium dodecyl sulfate surfactant 2 parts by weight and epoxy resin crosslinking agent 8 parts by weight.

[0075] S2.2 Slowly add 40 parts by weight of polyvinyl alcohol (PVA) modifier to deionized water, and stir in a mixer at a speed of 60-120 rpm until the polyvinyl alcohol (PVA) modifier is completely dissolved to form a homogeneous transparent polyvinyl alcohol (PVA) solution.

[0076] The solution in the stirrer is heated to 60-80℃, and the 3-glycidyl etheroxypropyltrimethoxysilane in the polyvinyl alcohol (PVA) modifier undergoes a grafting reaction with polyvinyl alcohol (PVA) through heating.

[0077] S2.3 After the initiation reaction is completed, 15 parts by weight of ammonium bicarbonate (NH4HCO3) pore-forming agent, 3 parts by weight of tributyl citrate plasticizer, 1 part by weight of butylated hydroxyanisole antioxidant and 2 parts by weight of sodium dodecyl sulfate surfactant are added sequentially to polyvinyl alcohol (PVA) solution and stirred until homogeneous to ensure that all components are fully mixed to obtain a mixture.

[0078] S2.4 Under stirring, add 8 parts by weight of epoxy resin crosslinking agent to the mixed solution in S2.3. The epoxy resin crosslinking agent fully contacts and reacts with the hydroxyl groups on the polyvinyl alcohol (PVA) molecular chain to form a crosslinked structure.

[0079] Specifically, before adding the epoxy resin crosslinking agent, the polyvinyl alcohol (PVA) mixture solution is preheated to 40-60℃; after the crosslinking agent is added and stirring begins, the temperature is raised to 60-80℃ and stirred continuously for 15-30 minutes, and maintained at 60-80℃ for 2-6 hours.

[0080] S2.5. Let the mixed solution that has completed the cross-linking reaction stand for 10-15 minutes, remove the internal air bubbles by vacuum degassing, and then pour the degassed solution evenly into a flat mold.

[0081] S2.6 Place the cast mold in a constant temperature oven for heat treatment to obtain a microporous membrane to be cooled;

[0082] The heating rate of the constant temperature oven is specifically as follows:

[0083] In the initial heating stage, the heating rate is 1-3℃ / min; when the temperature approaches the critical point of 150-190℃ where ammonium bicarbonate begins to decompose in large quantities, the heating rate is increased and maintained in the range of 4-5℃ / min.

[0084] Temperature holding time: When the temperature rises to the range of 200℃, maintain the current temperature for 15-30 minutes.

[0085] S2.7 After the heat treatment is completed, the microporous membrane is allowed to cool naturally to room temperature in the air, and then removed from the mold to obtain a microporous breathable membrane.

[0086] Example 2: A production process for a food-grade composite packaging material that combines breathability and waterproofness, using the method of Example 1, the difference being that...

[0087] In the preparation method of the microporous breathable layer, the following components by weight are used:

[0088] 50 parts by weight of polyvinyl alcohol (PVA) modifier, 15 parts by weight of ammonium bicarbonate (NH4HCO3) pore-forming agent, 3 parts by weight of tributyl citrate plasticizer, 1 part by weight of butylated hydroxyanisole antioxidant, 2 parts by weight of sodium dodecyl sulfate surfactant and 8 parts by weight of epoxy resin crosslinking agent.

[0089] Example 3: A production process for a food-grade composite packaging material that combines breathability and waterproofness, using the method of Example 1, the difference being that...

[0090] In the preparation method of the microporous breathable layer, the following components by weight are used:

[0091] The mixture contains 60 parts by weight of polyvinyl alcohol (PVA) modifier, 15 parts by weight of ammonium bicarbonate (NH4HCO3) pore-forming agent, 3 parts by weight of tributyl citrate plasticizer, 1 part by weight of butylated hydroxyanisole antioxidant, 2 parts by weight of sodium dodecyl sulfate surfactant, and 8 parts by weight of epoxy resin crosslinking agent.

[0092] Example 4: A production process for a food-grade composite packaging material that combines breathability and waterproofness, using the method of Example 1, the difference being that...

[0093] In the preparation method of the microporous breathable layer, the following components by weight are used:

[0094] 50 parts by weight of polyvinyl alcohol (PVA) modifier, 10 parts by weight of ammonium bicarbonate (NH4HCO3) pore-forming agent, 2 parts by weight of tributyl citrate plasticizer, 0.5 parts by weight of butylated hydroxyanisole antioxidant, 1 part by weight of sodium dodecyl sulfate surfactant, and 5 parts by weight of epoxy resin crosslinking agent.

[0095] Example 5: A production process for a food-grade composite packaging material that combines breathability and waterproofness, using the method of Example 1, the difference being that...

[0096] In the preparation method of the microporous breathable layer, the following components by weight are used:

[0097] The mixture contains 50 parts by weight of polyvinyl alcohol (PVA) modifier, 20 parts by weight of ammonium bicarbonate (NH4HCO3) pore-forming agent, 5 parts by weight of tributyl citrate plasticizer, 2 parts by weight of butylated hydroxyanisole antioxidant, 3 parts by weight of sodium dodecyl sulfate surfactant, and 10 parts by weight of epoxy resin crosslinking agent.

[0098] Comparative Example 1: The method of Example 2 was used, with unmodified polyvinyl alcohol (PVA).

[0099] This invention employs a nanoparticle-filled, chemically grafted method to achieve composite modification of polyvinyl alcohol (PVA), preparing a food-grade composite packaging material that combines breathability and waterproofness. The performance indicators and testing standards for the composite packaging material are as follows:

[0100] Referring to GB / T 1038-2022 "Test Method for Gas Permeability of Plastic Films and Sheets", the sample is fixed on the test device to form a sealed chamber. Dry, constant-pressure air is introduced into one side, while a vacuum or nitrogen environment is maintained on the other side to promote gas permeation. The permeability is calculated by monitoring the pressure change in the chamber or collecting the volume of permeated gas.

[0101] Referring to GB / T 1038-2022 "Test Method for Gas Permeability of Plastic Films and Sheets", the water droplet contact angle test is conducted. The contact angle of the water droplet on the film surface is measured using a contact angle measuring instrument. The larger the contact angle, the easier it is for the plastic film surface to form a spherical shape, indicating good hydrophobicity. The smaller the contact angle, the easier it is for the plastic film surface to spread the water droplet, indicating good hydrophilicity. The larger the angle, the better the hydrophobicity of the material surface and the stronger the waterproof performance.

[0102] The composite packaging materials prepared in Examples 1-5 and Comparative Example 1 were tested according to the above standards, and the data obtained are shown in Table 1:

[0103] Table 1 Performance data of the composite packaging materials of Examples 1-5 and Comparative Example 1

[0104]

[0105] The above data fully demonstrates that, compared to Comparative Example 1, in Examples 1-5, fresh fruits and vegetables undergo respiration during storage, releasing carbon dioxide and consuming oxygen. Therefore, packaging materials need to allow a certain degree of gas exchange to maintain the freshness of fruits and vegetables and extend their shelf life. Oxygen permeability represents the rate at which oxygen molecules can pass through a unit area of ​​packaging material, which is used to create a relatively stable microclimate for fruits and vegetables, slow down their metabolic rate, thereby extending their shelf life and maintaining good quality.

[0106] Carbon dioxide permeability refers to the amount of carbon dioxide molecules that a material per unit area allows to pass through it per unit time. It is an important indicator for evaluating a material's ability to block or allow air to pass through carbon dioxide gas. For fresh fruit and vegetable packaging, a moderate carbon dioxide permeability helps regulate the gas composition inside the packaging, promotes the balance of respiration in fruits and vegetables, and prevents physiological damage caused by excessive carbon dioxide concentration.

[0107] This invention employs a nanoparticle-filled, chemically grafted method to achieve composite modification of polyvinyl alcohol (PVA), and uses PVA to prepare a microporous breathable layer, ultimately producing a food-grade composite packaging material that combines breathability and waterproofness. This effectively improves the breathability and waterproof performance of the composite packaging material, as detailed below:

[0108] Examples 1-3 show that, with other material components remaining constant, the oxygen permeability, carbon dioxide permeability, and contact angle of the composite packaging material are significantly improved with the continuous increase of polyvinyl alcohol (PVA) modifier. The polyvinyl alcohol (PVA) modifier is achieved by using polyvinyl alcohol (PVA) as the base material, cellulose nanocrystals (CNC) as the particle filler, and 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) as the grafting modifier. The composite modification of polyvinyl alcohol (PVA) is achieved by using nanoparticle filling and chemical grafting.

[0109] Cellulose nanocrystals (CNCs) possess high crystallinity and a high aspect ratio. When uniformly dispersed and appropriately arranged within a polyvinyl alcohol (PVA) matrix, they can form numerous nanochannels, facilitating the diffusion of gas molecules (such as oxygen and carbon dioxide) and thus improving gas permeability. However, to prevent excessive CNC content from causing nanoparticle aggregation and hindering gas transport, 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) is selected as a grafting modifier. One end of 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) reacts with the hydroxyl groups on the surface of the CNCs. The reaction forms stable siloxane bonds, while the glycidyl ether oxygen group at the other end can react with the hydroxyl groups on the polyvinyl alcohol (PVA) molecular chain to form ether bonds. This helps to enhance the interfacial bonding between PVA and cellulose nanocrystals (CNC), reduce interfacial defects, and thus reduce the possibility of gas molecules penetrating through interfacial defects, indirectly improving the waterproof performance of the composite material. At the same time, 3-glycidyl etheroxypropyltrimethoxysilane (KH-560) helps to improve the dispersibility of cellulose nanocrystals (CNC) in the PVA matrix, avoid agglomeration, and maintain the good air permeability of PVA.

[0110] Furthermore, a comparison of Examples 2, 4, and 5 shows that:

[0111] With the polyvinyl alcohol (PVA) modifier composition remaining unchanged, the oxygen permeability, carbon dioxide permeability, and contact angle of the composite packaging material changed little with the continuous increase of ammonium bicarbonate (NH4HCO3) pore-forming agent, tributyl citrate plasticizer, butylated hydroxyanisole antioxidant, sodium dodecyl sulfate surfactant, and epoxy resin crosslinking agent. This indicates that the polyvinyl alcohol (PVA) modifier plays a dominant role in the preparation method of the microporous breathable layer.

[0112] Based on the above test experiments, Example 2 is taken as the optimal example. By comparing Example 2 with Comparative Example 1, it can be seen that:

[0113] A microporous breathable layer was prepared using unmodified polyvinyl alcohol (PVA), resulting in a food-grade composite packaging material that combines breathability and waterproofness. The oxygen permeability, carbon dioxide permeability, and contact angle of the composite packaging material were significantly reduced, which in turn led to a significant reduction in the breathability and waterproofness of the food-grade composite packaging material.

[0114] 3-Glycidyl etheroxypropyltrimethoxysilane (KH-560), as a silane coupling agent, chemically bonds with the surfaces of polyvinyl alcohol (PVA) and cellulose nanocrystals (CNC) through its reactive groups at both ends, forming a stable grafted structure and improving the interfacial bonding between the two. The siloxane network formed by grafting can reduce the sensitivity of the composite material to moisture to a certain extent. On the one hand, the hydrophobicity of the siloxane network can reduce the direct contact between moisture and the PVA-CNC interface; on the other hand, the chemically bonded and strengthened interface can inhibit phase separation or interfacial slip between PVA and CNC caused by moisture, thereby improving the dimensional stability and water resistance of the composite material in humid environments.

[0115] In summary, the polypropylene waterproof layer provides an excellent waterproof barrier, effectively preventing external moisture penetration and protecting the packaged food from moisture. Meanwhile, the microporous breathable layer made of modified polyvinyl alcohol (PVA) allows gas molecules such as oxygen and carbon dioxide to selectively permeate through the microporous structure formed by the pore-forming agent, maintaining gas exchange inside the packaging, extending the shelf life of the food, and maintaining the integrity of the packaging.

[0116] The polypropylene waterproof layer provides an effective barrier against liquid water in the external environment, preventing moisture from penetrating directly into the packaging, keeping fruits and vegetables dry, and avoiding problems such as microbial growth and spoilage caused by excessive moisture.

[0117] The microporous breathable layer and the polypropylene waterproof layer together construct a composite membrane structure that is both effectively waterproof and moderately breathable. The polypropylene waterproof layer focuses on preventing direct penetration of external liquid water, protecting the microporous breathable layer from damage, and providing additional gas barrier capabilities. The microporous breathable layer focuses on breathability, ensuring gas exchange and humidity management inside the fruit and vegetable packaging, which helps maintain the freshness of the fruits and vegetables. The multi-layer structure design makes full use of the advantages of each layer of materials, avoids functional conflicts, and optimizes the overall packaging performance.

[0118] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A food-grade composite packaging material that combines breathability and waterproofness, characterized in that, It includes a polypropylene waterproof layer, a microporous breathable layer, a polyvinylidene fluoride contact layer, and a polyurethane adhesive. The microporous breathable layer comprises the following components: 40-60 parts by weight of polyvinyl alcohol modifier, 10-20 parts by weight of ammonium bicarbonate pore-forming agent, 2-5 parts by weight of tributyl citrate plasticizer, 0.5-2 parts by weight of butylated hydroxyanisole antioxidant, 1-3 parts by weight of sodium dodecyl sulfate surfactant, and 5-10 parts by weight of epoxy resin crosslinking agent. The polyvinyl alcohol modifier uses a nanoparticle filling-chemical grafting method to achieve composite modification of polyvinyl alcohol, wherein polyvinyl alcohol is selected as the base material, cellulose nanocrystals as the particle filler, and 3-glycidyl etheroxypropyltrimethoxysilane as the grafting modifier. The preparation method of the polyvinyl alcohol modifier is as follows: S1.1 Disperse cellulose nanocrystals in deionized water and treat with ultrasound for 30-45 minutes to fully disperse the cellulose nanocrystals and form a dispersed cellulose nanocrystal suspension. S1.2 Dissolve 3-glycidyl etheroxypropyltrimethoxysilane in ethanol to prepare a 3-glycidyl etheroxypropyltrimethoxysilane solution; S1.

3. The above cellulose nanocrystal suspension and 3-glycidyl etheroxypropyltrimethoxysilane solution were added to the PVA solution in sequence, and the temperature was kept constant at 60°C. The mixture was stirred continuously at a speed of 100-230 rpm for 2-4 hours to carry out the grafting reaction. S1.4 After the reaction is complete, the mixture is cooled to room temperature, and impurities and byproducts are removed by centrifugation and filtration to obtain a polyvinyl alcohol modifier solution. Solid powdered polyvinyl alcohol modifier is obtained by freeze drying.

2. The food-grade composite packaging material with both breathability and waterproofness according to claim 1, characterized in that, The weight ratio of polyvinyl alcohol, cellulose nanocrystals and 3-glycidyl etheroxypropyltrimethoxysilane is 35:10:

1.

3. The food-grade composite packaging material with both breathability and waterproofness according to claim 1, characterized in that, In S1.3, the grafting reaction specifically includes: The epoxy groups on the 3-glycidyl etheroxypropyltrimethoxysilane molecule can undergo ring-opening reactions with the hydroxyl groups on the surface of cellulose nanocrystals to form Si-OC and Si-OH bonds; The epoxy group on the 3-glycidyl etheroxypropyltrimethoxysilane molecule undergoes a ring-opening reaction with the hydroxyl group on the polyvinyl alcohol molecular chain. The hydroxyl group serves as a reaction site for grafting with the epoxy group, and the resulting Si-OC bond covalently links the KH-560 molecular chain segment to the polyvinyl alcohol molecular chain.

4. The food-grade composite packaging material with both breathability and waterproofness according to claim 1, characterized in that, The microporous breathable layer is prepared as follows: S2.1 Weigh the following components by weight: Ammonium bicarbonate pore-forming agent 10-20 parts by weight, tributyl citrate plasticizer 2-5 parts by weight, butylated hydroxyanisole antioxidant 0.5-2 parts by weight, sodium dodecyl sulfate surfactant 1-3 parts by weight, and epoxy resin crosslinking agent 5-10 parts by weight; S2.2 Slowly add 40-60 parts by weight of polyvinyl alcohol modifier to deionized water, and stir in a mixer at a speed of 60-120 rpm until the polyvinyl alcohol modifier is completely dissolved to form a homogeneous transparent polyvinyl alcohol solution. The solution in the stirrer is heated to 60-80℃, and the 3-glycidyl etheroxypropyltrimethoxysilane in the polyvinyl alcohol modifier undergoes a grafting reaction with polyvinyl alcohol through heating. S2.3 After the initiation reaction is completed, add 10-20 parts by weight of ammonium bicarbonate pore-forming agent, 2-5 parts by weight of tributyl citrate plasticizer, 0.5-2 parts by weight of butylated hydroxyanisole antioxidant and 1-3 parts by weight of sodium dodecyl sulfate surfactant to the polyvinyl alcohol solution in sequence, and continue to stir evenly to ensure that each component is fully mixed to obtain a mixture. S2.4 Under stirring, add 5-10 parts by weight of epoxy resin crosslinking agent to the mixed solution in S2.

3. The epoxy resin crosslinking agent fully contacts and reacts with the hydroxyl groups on the polyvinyl alcohol molecular chain to form a crosslinked structure. S2.

5. Let the mixed solution that has completed the cross-linking reaction stand for 10-15 minutes, remove the internal air bubbles by vacuum degassing, and then pour the degassed solution evenly into a flat mold. S2.

6. Place the cast mold in a constant temperature oven for heat treatment to obtain the microporous membrane to be cooled. S2.7 After the heat treatment is completed, the microporous membrane is allowed to cool naturally to room temperature in the air, and then removed from the mold to obtain a microporous breathable membrane.

5. The food-grade composite packaging material with both breathability and waterproofness according to claim 4, characterized in that, In S2.4, the specific reaction conditions are as follows: Before adding the epoxy resin crosslinking agent, preheat the polyvinyl alcohol mixture to 40-60℃; after adding the crosslinking agent and starting to stir and mix, raise the temperature to 60-80℃ and continue stirring for 15-30 minutes, and maintain the temperature in the range of 60-80℃ for 2-6 hours.

6. The production process of the food-grade composite packaging material with both breathability and waterproofness according to claim 4, characterized in that, In step S2.6, the heating rate of the constant temperature oven is specifically as follows: In the initial heating stage, the heating rate is 1-3℃ / min; When the temperature approaches the critical point of 150-190℃ where ammonium bicarbonate begins to decompose in large quantities, increase the heating rate and maintain it within the range of 4-5℃ / min. Temperature holding time: When the temperature rises to the range of 200℃, maintain the current temperature for 15-30 minutes.

7. A production process for a food-grade composite packaging material that combines breathability and waterproofness, used in any one of claims 1-6, characterized in that, The specific method for preparing the composite packaging material is as follows: S3.

1. The prepared microporous breathable membrane is used as the microporous breathable layer, and the polypropylene waterproof layer, the microporous breathable layer, and the polyvinylidene fluoride contact layer are arranged in sequence. S3.2 Apply polyurethane adhesive evenly to the surfaces of the layers to be bonded. S3.

3. The adhesive-coated film layers are precisely aligned in the design sequence of polypropylene waterproof layer, microporous breathable layer, and polyvinylidene fluoride contact layer, and then fed into the laminator for lamination. S3.

3. The composite packaging material is cured in an oven to ensure that the adhesive is fully cross-linked, and then naturally cooled to room temperature to obtain the composite packaging material.

8. The production process of the food-grade composite packaging material with both breathability and waterproofness according to claim 7, characterized in that, The laminator temperature is 160-180℃, and the laminator pressure is 0.8-2.0MPa.

9. The production process of the food-grade composite packaging material with both breathability and waterproofness according to claim 7, characterized in that, In step S3.3, the oven is used to cure the product for 30-45 minutes at a temperature of 140-190°C.

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