A multi-layer moisture-proof packaging paper

Abstract

This invention relates to the field of packaging paper technology, and discloses a multi-layer moisture-proof packaging paper, comprising an inner layer, a barrier layer, and an outer layer, both of which are made of breathable materials. The barrier layer is made of a plurality of hydrophobic particles and a plurality of hydrophilic particles. The hydrophobic particles form a three-dimensional network support structure with a plurality of open pores, and the hydrophilic particles are embedded in the open pores. The hydrophilic particles have water absorption and swelling properties. After absorbing water and swelling, the hydrophilic particles squeeze and contact each other to form hydrophilic channels that penetrate the barrier layer. The ratio of the area of ​​the hydrophilic particles on the inner surface of the barrier layer to the area of ​​the hydrophilic particles on the outer surface is 30 to 1000. This multi-layer moisture-proof packaging paper, with its unique barrier layer structure and precise ratio design of hydrophilic particles on its inner and outer surfaces, achieves automatic and efficient one-way moisture permeability, solving the problem that existing paper-based packaging materials, while providing moisture protection, are unable to simultaneously achieve moisture permeability, breathability, environmental friendliness, and long-term preservation of the contents.

Description

Technical Field

[0001] This invention relates to the field of packaging paper technology, and in particular to a multi-layer moisture-proof packaging paper. Background Technology

[0002] With consumers increasingly demanding food preservation and sustainable packaging, traditional plastic packaging faces significant challenges due to its environmental pollution issues. Paper-based packaging, as a renewable and biodegradable alternative, is gaining widespread attention. However, the inherent hydrophilicity of paper-based materials makes them unsuitable for moisture barrier properties, failing to meet the packaging needs of moisture-sensitive fresh agricultural products, food, and other moisture-sensitive items.

[0003] Currently, the main technical approaches to improving the moisture-proof performance of paper-based packaging are as follows:

[0004] Surface-coated barrier layers: A polymer film, such as PE, PP, PVDC, EVOH, or inorganic oxides, such as AlOx and SiOx, is coated onto the surface of the paper-based material. While these coatings effectively block water vapor, they often sacrifice the recyclability and biodegradability of the paper-based material and may introduce microplastic problems. Furthermore, continuous barrier layers may hinder gas exchange between the inside and outside of the packaging, leading to internal moisture buildup, condensation, and ultimately accelerating spoilage of the contents.

[0005] Composite multilayer structures: These methods combine paper-based materials with barrier materials such as plastic films and aluminum foil. While this approach offers excellent barrier performance, it also faces challenges such as difficult recycling, high costs, and insufficient environmental friendliness.

[0006] Hydrophobic modification: This involves treating the pulp fibers with hydrophobic agents, such as sizing or surface impregnation with hydrophobic agents, to improve the paper's water resistance. However, this modification typically provides only limited moisture protection and is equally ineffective at allowing water vapor to escape in high-humidity environments.

[0007] While existing technologies address the moisture-proofing issue of paper-based packaging, they often fail to effectively expel internal moisture, particularly for fresh produce that needs to "breathe," leading to a "suffocating" effect and accelerated spoilage. Therefore, developing a novel paper-based packaging material that is both effectively moisture-proof and automatically wicks away moisture, while also being environmentally friendly, is a pressing technical challenge in the packaging industry. Summary of the Invention

[0008] To address the aforementioned shortcomings, the present invention aims to propose a multi-layer moisture-proof packaging paper that solves the problem that existing paper-based packaging materials, while providing moisture protection, are unable to simultaneously achieve moisture permeability, air permeability, environmental friendliness, and long-term preservation of the contents.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] A multi-layer moisture-proof packaging paper includes an inner layer, a barrier layer, and an outer layer. The inner layer is located inside the barrier layer, and the outer layer is located outside the barrier layer. Both the inner layer and the outer layer are made of breathable material.

[0011] The barrier layer is made of several hydrophobic particles and several hydrophilic particles. The hydrophobic particles form a three-dimensional network support structure with several open pores, and the hydrophilic particles are embedded in the open pores. The hydrophilic particles have water absorption and swelling properties. After absorbing water and swelling, the hydrophilic particles squeeze and contact each other to form hydrophilic channels that penetrate the barrier layer. The ratio of the area of ​​the hydrophilic particles on the inner surface of the barrier layer to the area of ​​the hydrophilic particles on the outer surface is 30 to 1000. The content of hydrophilic particles in the barrier layer is 1 to 5% by dry weight.

[0012] Preferably, the hydrophobic particles are hydrophobic lignin particles or modified lignin particles with a particle size of 5-20 micrometers.

[0013] Preferably, the hydrophilic particles are cross-linked starch particles, calcium alginate gel particles, or cellulose and starch-based superabsorbent resins with a particle size of 0.5 to 3 micrometers.

[0014] Preferably, it also includes adhesive particles, which are polybutylene succinate particles or their derivatives, with a particle size of 1 to 5 micrometers.

[0015] Preferably, the hydrophobic particles are epoxy-modified lignin particles.

[0016] Preferably, the adhesive particles are maleic anhydride-grafted modified polybutylene succinate particles.

[0017] Preferably, the method for adding hydrophilic particles is as follows: hydrophilic particles and amphiphilic cellulose are added to water and mixed to obtain a slurry. The slurry is poured from the top into a three-dimensional network composed of hydrophobic particles, and a concentration gradient is formed through infiltration. The slurry is then dried to obtain a single-layer barrier layer.

[0018] Preferably, the barrier layer is a multi-layer barrier layer, which is formed by stacking several single-layer barrier layers along its thickness direction, and the content of hydrophilic particles in the several single-layer barrier layers gradually decreases from the inside to the outside.

[0019] Preferably, the method for stacking single-layer barrier layers is as follows: using a solvent that can dissolve hydrophobic particles, coating the stacking surfaces with the solvent, then bonding the stacking surfaces together, and drying the solvent to obtain a stacked multilayer barrier layer.

[0020] Preferably, the inner layer is made of hydrophilic bamboo pulp paper, the outer layer is made of hydrophobic wood pulp paper, and the wood pulp paper is made by adding a hydrophobic sizing agent for in-pulp sizing. The inner layer, the outer layer and the barrier layer are all pressed and bonded together with a mesh pressure-sensitive adhesive.

[0021] The technical solution provided by this invention may include the following beneficial effects:

[0022] 1. Leveraging its unique barrier layer structure and the precise ratio of hydrophilic particles on its inner and outer surfaces, this packaging achieves automatic and efficient one-way moisture permeability. When excess moisture is released from the contents, such as fresh fruit, due to respiration or environmental changes, the high proportion of hydrophilic particles on the inner side of the barrier layer quickly senses and absorbs this moisture. They then expand and come into contact with each other, forming hydrophilic channels that connect the entire barrier layer. These channels act like automatic valves, actively expelling excess water vapor from the packaging, effectively preventing condensation and high humidity inside. Furthermore, before the hydrophilic particles absorb water and expand to completely connect the hydrophilic channels, the packaging paper maintains a certain level of humidity. The required humidity can be precisely set by controlling the ratio of hydrophilic particles on the inner and outer sides of the barrier layer during production. This responsive moisture removal mechanism greatly reduces the risk of microbial growth and spoilage, significantly extending the shelf life and freshness of the contents. The ability to maintain a certain level of humidity significantly improves the storage quality of the contents, preventing excessive drying and water loss, making it particularly suitable for humidity-sensitive fresh agricultural products.

[0023] 2. The low content of hydrophilic particles (1-5%) in the barrier layer, along with the extremely low proportion of hydrophilic particles on the outer side of the barrier layer, endows the packaging paper with excellent external moisture-proof capabilities. This means it can effectively resist the intrusion of liquid water or high-humidity air from the external environment, preventing external moisture from seeping back into the packaging, thus comprehensively protecting the contents from external moisture damage. Combined with the fact that both the inner and outer layers are made of breathable materials, the entire packaging system achieves efficient unidirectional moisture removal and external moisture protection while still maintaining necessary gas exchange. This delicate balance ensures the dynamic stability of the internal environment of the packaging, providing ideal microclimate conditions for perishable contents. It can effectively "breathe" and expel moisture while resolutely "resisting" external moisture, offering advantages unmatched by traditional moisture-proof packaging.

[0024] 3. The use of lignin in hydrophobic particles offers significant social and environmental benefits. Lignin is a major byproduct of the papermaking process, with large yields and stable sources. Its high-value utilization can improve the comprehensive utilization efficiency of forestry resources, promote the upgrading of the papermaking industry from traditional pulping to biomass refining and material manufacturing, and drive the development of related industrial chains such as forest processing and material production, increasing employment and regional economic vitality. In terms of environmental protection, lignin itself is a natural macromolecule that is renewable and biodegradable. Using lignin to prepare materials can reduce the incineration and emission of waste lignin, lowering wastewater load and air pollution. At the same time, lignin has low toxicity and good biocompatibility; its use in materials can reduce the amount of potentially harmful chemical additives and improve the environmental friendliness of the final material. Overall, the utilization of lignin aligns with the direction of green manufacturing, resource recycling, and sustainable development.

[0025] 4. All hydrophilic particles selected are biodegradable and environmentally friendly materials:

[0026] Cross-linked starch possesses excellent water absorption while avoiding the solubility issues associated with starch after water absorption. It swells into stable gel particles upon absorbing water, rather than completely dissolving. Cross-linking strengthens the cross-linked starch structure, allowing it to maintain its particle shape after swelling and forming independent water transfer channels. As a raw material for cross-linked starch, starch is derived from renewable resources such as corn, potatoes, and cassava. It is completely biodegradable, non-toxic, harmless, and inexpensive. Natural starch can be modified through chemical cross-linking agents, such as environmentally friendly cross-linking agents like epichlorohydrin, phosphates, and citric acid, or through physical cross-linking methods such as heat treatment. The cross-linked starch is then processed into micron or submicron-sized particles, offering good economic efficiency, ease of large-scale production, and adjustable performance.

[0027] Alginate is derived from plants or seaweed, is renewable, biodegradable, and non-toxic. Sodium alginate, as a natural gum, is itself a highly efficient thickener and gelling agent with strong water absorption capacity. Through cross-linking or ionic cross-linking, especially the cross-linking of sodium alginate with calcium ions to obtain calcium alginate gel particles, the preparation process is mature, the resulting gel structure is stable, and it has good shape retention and mechanical strength. This allows it to form stable gel particles after absorbing water, thereby forming stable hydrophilic channels.

[0028] Using cellulose and starch-based superabsorbent resins as hydrophilic particles in the barrier layer, instead of traditional petroleum-based products, although their water absorption performance may be slightly inferior in other applications, for packaging materials, they can provide sufficient and effective moisture absorption and expansion capacity to ensure the formation of hydrophilic channels. More importantly, they give packaging paper significant environmental advantages as a single-use packaging product. These natural polymer materials are renewable and biodegradable, greatly reducing the environmental burden caused by packaging paper waste. This perfectly matches the current global demand and development trend for green and sustainable packaging, providing consumers with a more environmentally friendly and responsible choice.

[0029] 5. Polybutylene succinate (PBS) is used as the bonding particle, and the hydrophobic particles are bonded by heating to 120℃~125℃ using a hot press. PBS itself is hydrophobic, and its melting point is 110~115℃, which is lower than lignin's 150℃. This avoids the hydrophobic particles from being thermally decomposed at temperatures above 150℃, thus preventing damage to the hydrophobic particle structure and affecting its mechanical properties.

[0030] Molten PBS can penetrate into the micropores and surface irregularities of lignin particles. After cooling and solidification, these penetrated polymers form mechanical interlocks, causing the lignin particles to bond integrally with the PBS to form a three-dimensional network support structure.

[0031] 6. Use amphiphilic cellulose, which is both lipophilic and hydrophilic, to mix with hydrophilic particles and form a slurry. Amphiphilic cellulose can act as a dispersant to prevent the hydrophilic particles from agglomerating, further stabilizing the dispersion effect, and can also form hydrogen bonds or physical entanglement with the surface of hydrophilic particles through its hydrophilic groups.

[0032] During the infiltration process, amphiphilic cellulose can make good contact with the surface of hydrophobic particles, forming an effective interfacial bridge. Due to its interfacial activity, the hydrophobic end of the amphiphilic cellulose is adsorbed on the surface of the hydrophobic particles, thereby forming a bonding interface layer in the entire three-dimensional network. This reduces the interfacial tension between the slurry and the hydrophobic network, promotes wetting and infiltration, and allows the hydrophilic particles to penetrate and adhere to the hydrophobic network, effectively bridging the gap between two incompatible materials. After the dehydration and drying process, the amphiphilic cellulose binds and fixes the hydrophilic particles to the three-dimensional network formed by the hydrophobic particles.

[0033] 7. By employing a water-based wet process, the water absorption performance of the hydrophilic particles is ensured to remain unaffected. After the hydrophilic particles naturally dry and shrink, they form a superior hydrophobic barrier effect. By allowing the hydrophilic particles to permeate into the three-dimensional network formed by the hydrophobic particles while in a water-absorbing and swelling state, the problem of excessive local accumulation of hydrophilic particles, which could then burst the hydrophobic network support structure after water absorption and swelling, is avoided.

[0034] 8. By using multi-layer composites, the content of hydrophilic particles in each single barrier layer can be controlled to solve the problem of difficulty in controlling the concentration gradient formed by the penetration of hydrophilic particles when manufacturing a barrier layer with a large thickness. Detailed Implementation

[0035] To facilitate understanding of the present invention, a more complete description is provided below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0036] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0038] A multi-layer moisture-proof packaging paper includes an inner layer, a barrier layer, and an outer layer. The inner layer is located inside the barrier layer, and the outer layer is located outside the barrier layer. Both the inner layer and the outer layer are made of breathable material.

[0039] The barrier layer is made of several hydrophobic particles and several hydrophilic particles. The hydrophobic particles form a three-dimensional network support structure with several open pores, and the hydrophilic particles are embedded in the open pores. The hydrophilic particles have water absorption and swelling properties. After absorbing water and swelling, the hydrophilic particles squeeze and contact each other to form hydrophilic channels that penetrate the barrier layer. The ratio of the area of ​​the hydrophilic particles on the inner surface of the barrier layer to the area of ​​the hydrophilic particles on the outer surface is 30 to 1000. The content of hydrophilic particles in the barrier layer is 1 to 5% by dry weight.

[0040] This invention's multi-layer moisture-proof packaging paper achieves automatic and efficient one-way moisture permeability through its unique barrier layer structure and precise design of the ratio of hydrophilic particles on its inner and outer surfaces. When excess moisture is generated inside the packaging due to the respiration of the contents, such as fresh fruit, or environmental changes causing water loss from the food, the high proportion of hydrophilic particles on the inner side of the barrier layer quickly senses and absorbs the moisture, then expands and comes into contact with each other, forming hydrophilic channels that run through the entire barrier layer. These channels act like automatic valves, actively expelling excess water vapor from the packaging, effectively preventing the formation of condensation and a high-humidity environment inside. Furthermore, before the hydrophilic particles absorb water and expand to completely connect the hydrophilic channels, the packaging paper can still maintain a certain level of humidity. The required humidity can be precisely set by controlling the ratio of hydrophilic particles on the inner and outer sides of the barrier layer during production. This responsive dehumidification mechanism greatly reduces the risk of microbial growth and spoilage, significantly extends the shelf life and freshness of the contents, and the ability to maintain a certain level of humidity significantly improves the storage quality of the contents, preventing excessive drying and water loss, making it particularly suitable for humidity-sensitive fresh agricultural products.

[0041] Meanwhile, the low content of hydrophilic particles (1-5%) in the barrier layer and the extremely low proportion of hydrophilic particles on the outer side of the barrier layer endow the packaging paper with excellent external moisture-proof capabilities. This means it can effectively resist the intrusion of liquid water or high-humidity air from the external environment, preventing external moisture from seeping back into the packaging, thus comprehensively protecting the contents from external moisture damage. Combined with the fact that both the inner and outer layers are made of breathable materials, the entire packaging system achieves efficient unidirectional moisture removal and external moisture protection while still maintaining necessary gas exchange. This delicate balance ensures the dynamic stability of the internal environment of the packaging, providing ideal microclimate conditions for perishable contents. It can effectively "breathe" and expel moisture while resolutely "resisting" external moisture, offering advantages unmatched by traditional moisture-proof packaging.

[0042] Preferably, the hydrophobic particles are hydrophobic lignin particles or modified lignin particles with a particle size of 5-20 micrometers.

[0043] Although hydrophobic lignin or modified lignin contains a small amount of hydrophilic hydroxyl groups, it has high hydrophobicity due to its large aromatic structure and highly branched polymer characteristics. Lignin is the world's second largest biomass resource after cellulose, and it is abundant and inexpensive.

[0044] The use of lignin offers significant social and environmental benefits. Lignin is a major byproduct of the papermaking process, with large yields and stable sources. Its high-value utilization can improve the comprehensive utilization efficiency of forestry resources, promote the upgrading of the papermaking industry from traditional pulping to biomass refining and material manufacturing, and drive the development of related industrial chains such as forest processing and material production, increasing employment and regional economic vitality. In terms of environmental protection, lignin is a natural macromolecule that is renewable and biodegradable. Using lignin to prepare materials can reduce the incineration and emission of waste lignin, lowering wastewater load and air pollution. At the same time, lignin has low toxicity and good biocompatibility; its use in materials can reduce the amount of potentially harmful chemical additives and improve the environmental friendliness of the final material. Overall, the utilization of lignin aligns with the directions of green manufacturing, resource recycling, and sustainable development.

[0045] Preferably, the hydrophilic particles are cross-linked starch particles, calcium alginate gel particles, or cellulose and starch-based superabsorbent resins with a particle size of 0.5 to 3 micrometers.

[0046] Cross-linked starch possesses excellent water absorption while avoiding the solubility issues associated with starch after water absorption. It swells into stable gel particles upon absorbing water, rather than completely dissolving. Cross-linking strengthens the cross-linked starch structure, allowing it to maintain its particle shape after swelling and forming independent water transfer channels. As a raw material for cross-linked starch, starch is derived from renewable resources such as corn, potatoes, and cassava. It is completely biodegradable, non-toxic, harmless, and inexpensive. Natural starch can be modified through chemical cross-linking agents, such as environmentally friendly cross-linking agents like epichlorohydrin, phosphates, and citric acid, or through physical cross-linking methods such as heat treatment. The cross-linked starch is then processed into micron or submicron-sized particles, offering good economic efficiency, ease of large-scale production, and adjustable performance.

[0047] Alginate is derived from plants or seaweed, is renewable, biodegradable, and non-toxic. Sodium alginate, as a natural gum, is itself a highly efficient thickener and gelling agent with strong water absorption capacity. Through cross-linking or ionic cross-linking, especially the cross-linking of sodium alginate with calcium ions to obtain calcium alginate gel particles, the preparation process is mature, the resulting gel structure is stable, and it has good shape retention and mechanical strength. This allows it to form stable gel particles after absorbing water, thereby forming stable hydrophilic channels.

[0048] Using cellulose and starch-based superabsorbent resins as hydrophilic particles in the barrier layer, instead of traditional petroleum-based products, although their water absorption performance may be slightly inferior in other applications, for packaging materials, they can provide sufficient and effective moisture absorption and expansion capacity to ensure the formation of hydrophilic channels. More importantly, they give packaging paper significant environmental advantages as a single-use packaging product. These natural polymer materials are renewable and biodegradable, greatly reducing the environmental burden caused by packaging paper waste. This perfectly matches the current global demand and development trend for green and sustainable packaging, providing consumers with a more environmentally friendly and responsible choice.

[0049] In one embodiment, direct heating is used to heat the mixed lignin particles to 150°C, causing them to soften and intertwine to form a three-dimensional network support structure.

[0050] Preferably, it also includes adhesive particles, which are polybutylene succinate particles or their derivatives, with a particle size of 1 to 5 micrometers.

[0051] In a preferred embodiment, polybutylene succinate (PBS) is used as the bonding particle, and the hydrophobic particles are bonded by heating to 120°C~125°C using a hot press. PBS itself is hydrophobic, and its melting point is 110~115°C, which is lower than lignin's 150°C. This avoids thermal decomposition of the hydrophobic particles at temperatures above 150°C, which would damage the structure of the hydrophobic particles and affect their mechanical properties.

[0052] Molten PBS can penetrate into the micropores and surface irregularities of lignin particles. After cooling and solidification, these penetrated polymers form mechanical interlocks, causing the lignin particles to bond integrally with the PBS to form a three-dimensional network support structure.

[0053] Preferably, the hydrophobic particles are epoxy-modified lignin particles.

[0054] Epoxy groups are introduced onto the hydroxyl groups of lignin through reaction with epichlorohydrin or other epoxy compounds. These epoxy groups are highly reactive and can undergo ring-opening reactions with the carboxyl and hydroxyl groups at the ends of the PBS chains during hot pressing, forming covalent bonds and improving interfacial adhesion strength. Furthermore, the introduced epoxy groups themselves have an affinity for PBS, improving the compatibility between lignin and PBS, thereby enhancing mixing and bonding efficiency.

[0055] In addition, lignin can be hydrophobically modified by fatty acids or other materials with hydrophobic groups, which can also increase the compatibility of lignin and PBS.

[0056] Preferably, the adhesive particles are maleic anhydride-grafted modified polybutylene succinate particles.

[0057] By grafting maleic anhydride (MAH) onto PBS, groups that can react with lignin hydroxyl groups are introduced to improve the compatibility of PBS with lignin and the interfacial bonding strength.

[0058] MAH is an unsaturated cyclic anhydride that can be grafted onto the molecular chain of PBS via a free radical reaction under the action of a peroxide initiator. The grafted MAH undergoes ring-opening to form carboxylic acid and anhydride groups. These groups can react with the hydroxyl groups of lignin to form covalent bonds, enhancing the interfacial adhesion strength.

[0059] It should be noted that although the carboxylic acid and anhydride groups of MAH-grafted modified PBS increase the hydrophilicity of PBS, the overall hydrophobicity will further increase after they combine with the hydroxyl groups on lignin, further enhancing the hydrophobic and moisture-proof properties of the three-dimensional network formed by the hydrophobic particles.

[0060] Preferably, the method for adding hydrophilic particles is as follows: hydrophilic particles and amphiphilic cellulose are added to water and mixed to obtain a slurry. The slurry is poured from the top into a three-dimensional network composed of hydrophobic particles, and a concentration gradient is formed through infiltration. The slurry is then dried to obtain a single-layer barrier layer.

[0061] In aqueous media, hydrophilic particles can be well dispersed. By mixing hydrophilic cellulose with hydrophilic particles and forming a slurry, the amphiphilic cellulose can act as a dispersant to prevent the hydrophilic particles from agglomerating, further stabilizing the dispersion effect, and through hydrogen bonds or physical entanglement formed between its hydrophilic groups and the surface of the hydrophilic particles.

[0062] Hydrophilic particles and amphiphilic cellulose slurry are poured from above into a three-dimensional network formed by hydrophobic particles. Relying on gravity or external air pressure from above, the slurry will penetrate downwards along the pores of the hydrophobic network. Due to the filtering and adsorption effect of the pores formed by the three-dimensional network, the hydrophilic particles are intercepted and retained in the pores during the penetration process. As the penetration depth increases, the number of hydrophilic particles gradually decreases, forming a concentration gradient. This results in a higher concentration of hydrophilic particles on the top side, with larger particles remaining at the top, and a lower concentration of hydrophilic particles at the bottom, with only small particles penetrating to the bottom.

[0063] During the infiltration process, amphiphilic cellulose can make good contact with the surface of hydrophobic particles, forming an effective interfacial bridge. Due to its interfacial activity, the hydrophobic end of the amphiphilic cellulose is adsorbed on the surface of the hydrophobic particles, thereby forming a bonding interface layer in the entire three-dimensional network. This reduces the interfacial tension between the slurry and the hydrophobic network, promotes wetting and infiltration, and allows the hydrophilic particles to penetrate and adhere to the hydrophobic network, effectively bridging the gap between two incompatible materials. After the dehydration and drying process, the amphiphilic cellulose binds and fixes the hydrophilic particles to the three-dimensional network formed by the hydrophobic particles.

[0064] By employing an aqueous wet process, the water absorption performance of the hydrophilic particles is ensured to remain unaffected. After the hydrophilic particles naturally dry and shrink, they form a superior hydrophobic barrier effect. By allowing the hydrophilic particles to permeate into the three-dimensional network formed by the hydrophobic particles while in a water-absorbing and swelling state, the problem of excessive local accumulation of hydrophilic particles, which could then burst the hydrophobic network support structure upon water absorption and swelling, is avoided.

[0065] By controlling the amount of slurry applied, the slurry concentration, the pore structure of the hydrophobic network, and the drying conditions, the hydrophilic / hydrophobic gradient and overall properties of the final material can be precisely controlled. The main components used are all bio-based, conforming to the principles of green manufacturing.

[0066] Preferably, the barrier layer is a multi-layer barrier layer, which is formed by stacking several single-layer barrier layers along its thickness direction, and the content of hydrophilic particles in the several single-layer barrier layers gradually decreases from the inside to the outside.

[0067] By using multi-layer composites, the content of hydrophilic particles in each single barrier layer can be controlled, thus solving the problem of difficulty in controlling the concentration gradient formed by the infiltration of hydrophilic particles when manufacturing a barrier layer with a large thickness.

[0068] Preferably, the method for stacking single-layer barrier layers is as follows: using a solvent that can dissolve hydrophobic particles, coating the stacking surfaces with the solvent, then bonding the stacking surfaces together, and drying the solvent to obtain a stacked multilayer barrier layer.

[0069] When the hydrophobic particles are lignin particles, an aqueous ethanol solution is preferred as the solvent. Ethanol is a bio-based solvent with low toxicity, easy biodegradability, moderate volatility, and leaves no harmful residue after evaporation. It is a recognized "green solvent" and has low cost. Water is also non-toxic and harmless. Pure ethanol has limited solubility for high molecular weight lignin. Adding an appropriate amount of water can adjust the polarity and hydrogen bonding ability, thereby improving the solubility of lignin.

[0070] When the hydrophobic particles are epoxy-modified lignin particles, acetone is preferred as the solvent. The introduction of epoxy groups reduces the polarity of lignin and decreases the strong hydrogen bonding between lignin molecules, improving its solubility in organic solvents. Acetone is a common organic solvent with low toxicity, is volatile, recyclable, and easily degraded in the environment, making it considered one of the relatively environmentally friendly solvents. Epoxy-modified lignin exhibits good solubility in acetone. Acetone's moderate polarity and affinity for epoxy groups result in ideal dissolution efficiency. Acetone also has a low boiling point of only 56°C, making it highly volatile. After lamination, it can be quickly dried and ventilated to remove residues, further reducing usage costs through solvent recovery.

[0071] Preferably, the inner layer is made of hydrophilic bamboo pulp paper, the outer layer is made of hydrophobic wood pulp paper, and the wood pulp paper is made by adding a hydrophobic sizing agent for in-pulp sizing. The inner layer, the outer layer and the barrier layer are all pressed and bonded together with a mesh pressure-sensitive adhesive.

[0072] Thanks to the fibrous structure of wood pulp or bamboo pulp paper, both the inner and outer layers provide excellent strength and toughness for support. The hydrophilicity of the inner layer allows it to quickly absorb small amounts of moisture from the surface of fruits or food, reducing internal condensation and keeping the food surface relatively dry. Its fibrous structure ensures good breathability, and the natural antibacterial properties of bamboo pulp paper greatly reduce the risk of bacterial growth inside the packaging paper. Using wood pulp paper for the outer layer reduces overall material costs. The hydrophobicity of the sizing agent effectively blocks external liquid moisture, such as rain and splashes, protecting the food and fruit inside the packaging paper. However, its breathability still allows internal moisture to diffuse and escape. Furthermore, the internal sizing agent does not alter the surface structure of the outer layer, has minimal impact on printability, maintains the ink absorption of wood pulp paper, and preserves good printability.

[0073] Through the mesh pressure-sensitive adhesive bonding process, the mesh structure ensures that the layers are firmly bonded while retaining a large number of open areas, thereby maintaining the moisture permeability and breathability of the entire packaging paper, preventing the formation of a water vapor barrier layer, allowing the barrier layer to fully perform its function, and avoiding the use of hot pressing or heat-sensitive adhesive processes that would affect the structure and performance of hydrophilic particles.

[0074] Example 1

[0075] The hydrophobic particles are hydrophobic lignin particles produced as a byproduct of papermaking, with an average particle size of 10 micrometers. The hydrophilic particles are cross-linked starch particles with an average particle size of 1 micrometer. Maleic anhydride grafted and modified polybutylene succinate particles with an average particle size of 1 micrometer are used as adhesive particles to form a three-dimensional network by hot pressing. Amphiphilic cellulose is prepared by using polycaprolactone and aminosulfonic acid modified microcrystalline cellulose. The hydrophilic particles and amphiphilic cellulose are added to water and mixed to obtain a pulp. The pulp is poured from the top into the three-dimensional network composed of hydrophobic particles. After drying, a barrier layer is formed. The ratio of the area occupied by hydrophilic particles on the inner surface to the outer surface of the barrier layer is about 800. The hydrophilic particles account for 1% of the dry weight of the barrier layer. The inner layer is made of hydrophilic bamboo pulp paper, and the outer layer is made of wood pulp paper with rosin-based sizing agents inside. Food-grade natural rubber is used as pressure-sensitive adhesive for mesh bonding to obtain moisture-proof packaging paper.

[0076] The resulting moisture-proof packaging paper has a larger ratio of hydrophilic particles on the inner and outer sides, and a lower content of hydrophilic particles. Water vapor is more easily absorbed by the inner surface, and the hydrophilic channels are connected more quickly, resulting in better moisture-proof performance.

[0077] The resulting moisture-proof packaging paper is used to package potato chips. In a simulated "moist consumption – dry storage" scenario, the potato chips are opened at 25-28℃ and 80-90% relative humidity to simulate the consumption environment, where external moisture instantly enters the packaging. The opening is then clamped shut, and the chips are placed at 18-20℃ and 45-55% relative humidity to simulate the cool, dry storage environment of a home where humidity naturally decreases at night. Conventional composite plastic packaging bags struggle to expel moisture quickly enough during this process, causing the potato chips to regain moisture noticeably within 6-12 hours of opening, and their crispness to significantly decrease within 1-2 days. However, the moisture-proof packaging paper used in this embodiment absorbs water vapor that enters after opening due to its high hydrophilic particle surface area ratio. During the subsequent dry storage stage, the moisture is gradually expelled through rapidly connected hydrophilic channels, allowing the humidity inside the bag to drop more quickly to a safe level sufficient to keep the potato chips dry, thus significantly delaying moisture regain. Tests showed that, under the same conditions, the crispness of potato chips could be maintained for 2–4 days without significant decrease, which was significantly better than the performance of conventionally packaged chips after opening and storage.

[0078] Example 2

[0079] Unlike Example 1, the hydrophobic particles are epoxy-modified lignin with an average particle size of 15 micrometers, the hydrophilic particles are calcium alginate gel particles with an average particle size of 0.5 micrometers, and the barrier layer is a multilayer barrier layer made by bonding three single-layer barrier layers together with acetone. The ratio of the area occupied by the hydrophilic particles on the innermost surface to the outermost surface of the multilayer barrier layer is approximately 100, and the hydrophilic particles account for 4% of the dry weight of the barrier layer.

[0080] The resulting moisture-proof packaging paper has a high content of hydrophilic particles, making the hydrophilic channels less likely to connect, resulting in better moisture retention. At the same time, due to the increased hydrophilic particles on the outer surface, water vapor can be quickly released when the hydrophilic channels open, preventing further increase in humidity and ensuring the storage quality of the contents.

[0081] Fresh strawberries packaged in the moisture-proof packaging paper prepared in this embodiment were tested in an environment of 0-5℃ and 85-95% relative humidity. The key to preserving fresh strawberries lies in maintaining suitable humidity; they should neither be too dry nor too wet. Through a humidity buffering and balancing mechanism, the rate of water loss from the strawberries can be effectively slowed down, while inhibiting mold growth, thereby extending their freshness and shelf life. The tested storage period reached 7-14 days. In contrast, when using conventional plastic sealed boxes or cardboard boxes for packaging and storage, due to mold or dehydration, the storage period of fresh strawberries under the same conditions is only 3-7 days.

[0082] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0083] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A multi-layer moisture-proof packaging paper, characterized in that: It includes an inner layer, a barrier layer, and an outer layer. The inner layer is located inside the barrier layer, and the outer layer is located outside the barrier layer. Both the inner layer and the outer layer are made of breathable material. The barrier layer is made of several hydrophobic particles and several hydrophilic particles. The hydrophobic particles form a three-dimensional network support structure with several open pores, and the hydrophilic particles are embedded in the open pores. The hydrophilic particles have water absorption and swelling properties. After absorbing water and swelling, the hydrophilic particles squeeze and contact each other to form hydrophilic channels that penetrate the barrier layer. The ratio of the area of ​​the hydrophilic particles on the inner surface of the barrier layer to the area of ​​the hydrophilic particles on the outer surface is 30 to 1000. Calculated by dry weight, the content of hydrophilic particles in the barrier layer is 1% to 5%. The inner layer is made of hydrophilic bamboo pulp paper, and the outer layer is made of hydrophobic wood pulp paper.

2. The multi-layer moisture-proof packaging paper according to claim 1, characterized in that: The hydrophobic particles are hydrophobic lignin particles or modified lignin particles with a particle size of 5-20 micrometers.

3. The multi-layer moisture-proof packaging paper according to claim 1, characterized in that: The hydrophilic particles are cross-linked starch particles, calcium alginate gel particles, or cellulose and starch-based superabsorbent resins with a particle size of 0.5 to 3 micrometers.

4. The multi-layer moisture-proof packaging paper according to claim 1, characterized in that: It also includes adhesive particles, which are polybutylene succinate particles or their derivatives, with a particle size of 1 to 5 micrometers.

5. The multi-layer moisture-proof packaging paper according to claim 1, characterized in that: The hydrophobic particles are epoxy-modified lignin particles.

6. The multi-layer moisture-proof packaging paper according to claim 4, characterized in that: The adhesive particles are maleic anhydride-grafted modified polybutylene succinate particles.

7. The multi-layer moisture-proof packaging paper according to claim 1, characterized in that, The method for adding hydrophilic particles is as follows: hydrophilic particles and amphiphilic cellulose are added to water and mixed to obtain a slurry. The slurry is poured from the top into a three-dimensional network composed of hydrophobic particles, and a concentration gradient is formed through infiltration. The slurry is then dried to obtain a single-layer barrier layer.

8. The multi-layer moisture-proof packaging paper according to claim 7, characterized in that: The barrier layer is a multi-layer barrier layer, which is formed by stacking several single-layer barrier layers along its thickness direction, and the content of hydrophilic particles in the several single-layer barrier layers gradually decreases from the inside to the outside.

9. A multi-layer moisture-proof packaging paper according to claim 8, characterized in that, The method for stacking single-layer barrier layers is as follows: a solvent that can dissolve hydrophobic particles is used to coat the stacking surfaces, and then the stacking surfaces are bonded together. After drying the solvent, a multi-layer barrier layer is obtained.

10. The multi-layer moisture-proof packaging paper according to claim 1, characterized in that: The wood pulp paper is prepared by adding a hydrophobic sizing agent for in-pulp sizing, and the inner layer, the outer layer and the barrier layer are all bonded together by a mesh pressure-sensitive adhesive.