Single-sided multi-layer water-based high-barrier heat-sealable environment-friendly packaging paper and preparation method thereof

Abstract

The application discloses a single-sided multi-layer water-based high-barrier heat-sealable environment-friendly packaging paper and a preparation method thereof. The packaging paper comprises a pure paper base paper substrate, the basis weight of the base paper substrate is 30-100 g / m2, and the inner side surface of the base paper substrate is sequentially stacked with a water-based moisture-proof bottom layer, a water-based oxygen barrier middle layer and a water-based moisture-proof heat-sealable composite surface layer from inside to outside, so as to form a sandwiched three-layer coating structure of an inner and outer double-layer moisture-proof wrapping middle oxygen barrier layer; the water-based moisture-proof heat-sealable composite surface layer is an integrated single-layer coating, and simultaneously has the functions of long-acting moisture-proof of the outer layer and low-temperature heat sealing; the other side surface of the base paper substrate is a virgin printing surface without coating; the water-based moisture-proof bottom layer, the water-based oxygen barrier middle layer and the water-based moisture-proof heat-sealable composite surface layer are all food contact grade full water-based coating layers. The packaging paper has the functions of long-acting high barrier, low-temperature heat sealing and high-definition printing, and is plastic-free, aluminum-free and recyclable.

Description

Technical Field

[0001] This invention relates to the fields of papermaking technology and food packaging material technology. Specifically, it relates to a single-sided, multi-layer, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper, and a method for preparing such a paper. Background Technology

[0002] Currently, high-barrier sealed packaging on the market mainly adopts multi-layer plastic composite, paper-plastic composite, or paper-aluminum-plastic composite structures. In recent years, with increasingly stringent environmental protection requirements, many all-water-based barrier paper products have emerged in the industry. However, the above-mentioned existing technologies all have significant technical defects, specifically as follows: (i) The oxygen barrier layer is prone to moisture damage and failure, and its barrier performance is unstable. Most existing water-based barrier papers employ a dual-layer structure consisting of a single moisture barrier layer and a single oxygen barrier layer. In this type of solution, a moisture barrier layer is only set on one side, which cannot simultaneously isolate the bidirectional erosion of residual moisture inside the paper fibers and external moisture. This causes the hydrophilic oxygen barrier layer (such as polyvinyl alcohol) to absorb water and plasticize, resulting in a decrease in crystallinity and a rapid decline in barrier performance during storage, transportation, and use.

[0003] However, the protein oxygen barrier layer used in some technical solutions also has hydrophilic properties, making it prone to absorbing water and swelling in high humidity environments, leading to coating structure damage and a significant decrease in barrier performance. Furthermore, bio-based raw materials (proteins, chitosan) pose a risk of microbial growth under long-term storage conditions, limiting their applicability in the food packaging field.

[0004] (ii) Functional layering design leads to redundant processes and discontinuous outer moisture protection.

[0005] Separating the moisture barrier and heat sealing functions into multiple independent coatings has the following drawbacks: Firstly, it increases the coating and drying processes, raising equipment energy consumption and production cycle; secondly, the outer heat sealing layer focuses on heat sealing and bonding, without setting an independent continuous hydrophobic and moisture barrier film layer, resulting in insufficient long-term moisture protection. In humid and hot environments, water vapor can easily penetrate from the heat sealing layer interface, accelerating the failure of the intermediate oxygen barrier layer.

[0006] (iii) The full coverage of the coating results in poor printability.

[0007] Most commercially available water-based barrier paper products use a full-coverage coating structure on the paper base surface, without reserving a dedicated uncoated printing area. Due to the low surface energy and excessive smoothness of the coated surface, ink adhesion is poor and ink is prone to fading, making it only suitable for simple monochrome or low-screen printing, and unable to meet the production requirements of industrial high-definition color printing.

[0008] (iv) Traditional composite packaging materials are difficult to recycle

[0009] Multi-layered plastic composite and paper-aluminum-plastic composite packaging materials have tight interlayer adhesion, making it impossible to separate and recycle individual materials. Their natural degradation cycle is extremely long, which does not conform to the plastic restriction order and the development direction of the packaging circular economy. Summary of the Invention

[0010] To address the technical problems in existing technologies, such as the oxygen barrier layer being susceptible to dual erosion from moisture inside the base paper and external environmental water vapor leading to water absorption and plasticization failure; the redundant functional layers in existing three-layer coated paper (moisture barrier and heat sealing belong to two separate layers), multiple processes, and discontinuous outer waterproofing; and the fact that existing products have full coating coverage without reserving a dedicated printing surface, failing to meet the demands of high-definition industrial color printing, this invention provides a single-sided, multi-layer, water-based, high-barrier, heat-sealable, and printable environmentally friendly packaging paper and its preparation method. Through an inner and outer double-layer moisture-barrier sandwich structure, it bidirectionally isolates moisture to protect the oxygen barrier layer, resulting in minimal degradation of barrier performance after humid heat aging. The outer integrated coating simultaneously achieves moisture barrier and low-temperature heat sealing (≥8N / 15mm), eliminating the need for a separate heat sealing process. The original printing surface on the reverse side has a national standard level 1 adhesion, suitable for high-definition color printing. The all-water-based, plastic-free, and aluminum-free system is re-pulpable, recyclable, and completely biodegradable.

[0011] In a first aspect, the present invention provides a single-sided, multi-layered, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper, comprising: Pure paper base, wherein the basis weight of the base paper is 30-100 g / m²; The paper substrate has a water-based moisture-proof bottom layer, a water-based oxygen-proof middle layer, and a water-based moisture-proof heat-sealing composite surface layer stacked sequentially from the inside to the outside, forming a sandwich-type three-layer coating structure with inner and outer double-layer moisture-proof covering the middle oxygen-proof layer; the water-based moisture-proof heat-sealing composite surface layer is an integrated single-layer coating that simultaneously has the dual functions of long-lasting moisture-proof outer layer and low-temperature heat sealing at 100-130℃. The other side of the original paper substrate is a completely uncoated, unprinted surface. The water-based moisture-barrier bottom layer, water-based oxygen-barrier middle layer, and water-based moisture-barrier heat-sealing composite top layer are all food-contact grade all-water-based coating layers. Under standard test conditions at room temperature, the water vapor transmission rate of the packaging paper is ≤1g / (m²·24h), and the oxygen transmission rate is ≤1cm³ / (m²·24h·0.1MPa); the heat seal strength of the water-based moisture-resistant heat-sealing composite surface layer is ≥8N / 15mm.

[0012] Furthermore, The dry coating amount of the water-based moisture-barrier underlayer is 5-10 g / m², the dry coating amount of the water-based oxygen-barrier intermediate layer is 2-5 g / m², and the dry coating amount of the water-based moisture-barrier heat-sealing composite top layer is 5-10 g / m².

[0013] Furthermore, the density of the base paper is 0.60–0.70 g / cm³, and the air permeability is 25–60 s / 100ml; the base paper is any one of food-grade bleached thin packaging paper, white kraft paper, bamboo pulp paper, fluorine-free greaseproof paper, glassine paper, or food-grade specialty base paper.

[0014] Furthermore, the surface roughness of the original printed surface is 2.0 to 5.0 μm, and the surface strength is ≥1.2 m / s; the printing adhesion of the original printed surface reaches the national standard level 1, the compatible printing screen count is 80 to 150 lines / inch, and the ink shedding rate is ≤1.0%.

[0015] Furthermore, the raw material composition of the water-based moisture-proof bottom layer, by weight, is as follows: Food-grade pure acrylic acid emulsion, 65.0–78.0 parts. Bio-based wax emulsion, 8.0–15.0 parts 3.0–12.0 parts deionized water 0.3-1.0 parts of polyurethane thickener Nonionic wetting agent 0.2-0.6 parts, 0.1 to 0.4 parts of silicone defoamer.

[0016] Furthermore, the raw material composition of the aqueous oxygen barrier layer, by weight, is as follows: Polyvinyl alcohol 10.0–30.0 parts 20.0–40.0 parts of nano-montmorillonite barrier filler with a particle size of 200–500 nm. Bio-based barrier latex, 30.0–50.0 parts Nonionic wetting agent 0.2-0.6 parts, 0.1 to 0.4 parts of silicone defoamer; The aqueous oxygen barrier middle layer does not contain soy protein, chitosan, or acetylenol surfactants.

[0017] Furthermore, based on parts by weight, the raw material composition of the water-based moisture-resistant heat-sealing composite surface layer is as follows: Low-temperature heat-sealing acrylic copolymer emulsion, 75.0–85.0 parts. 3.0–12.0 parts deionized water 0.3-1.0 parts of polyurethane thickener Nonionic wetting agent 0.2-0.6 parts, 0.1 to 0.4 parts of silicone defoamer.

[0018] In a second aspect, the present invention provides a method for preparing single-sided multilayer water-based high-barrier heat-sealable environmentally friendly packaging paper as described in any one of the first aspects, comprising the following steps: S10. Coating preparation: Prepare water-based moisture-barrier undercoating liquid, water-based oxygen-barrier intermediate coating liquid and water-based moisture-barrier heat-sealing composite topcoat liquid respectively, and after curing, send them to the corresponding material tanks of the coating equipment for later use. S20, Base paper feeding: The food contact thin base paper is directly fed into the coating production line. The base paper does not undergo corona treatment, pre-drying, surface leveling and dehumidification pretreatment. S30. Coating and curing the moisture-proof underlayer: Coating the water-based moisture-proof underlayer coating liquid onto one side surface of the base paper substrate, and drying and curing it in a hot air drying zone of 110℃~150℃ for 40~100s. S40. Coating and curing the oxygen barrier intermediate layer: Coating the water-based oxygen barrier intermediate layer coating liquid onto the surface of the cured moisture barrier bottom layer, and drying and curing it in a hot air drying zone of 130℃~150℃ for 40~100s. S50. Coating and curing the moisture-proof heat-sealing composite surface layer: Coating the water-based moisture-proof heat-sealing composite surface layer coating liquid onto the surface of the cured oxygen-barrier intermediate layer, and drying and curing it in a hot air drying zone of 130℃~150℃ for 40~100s. S60. Winding and Printing: After the three-layer coating is dried and cured, the paper is continuously wound up. The original printing surface on the uncoated side of the base paper is used for offset printing, flexographic printing or gravure printing, and then cut to obtain the finished packaging paper.

[0019] Furthermore, The preparation method of the aqueous moisture-barrier undercoating liquid includes: Add deionized water to a stirred tank, and add nonionic wetting agent and silicone defoamer sequentially at 200-400 rpm, stirring for 3-8 minutes. Increase the speed to 600-1000 rpm, and add food-grade pure acrylic acid emulsion dropwise at a uniform rate for 10-20 minutes, stirring continuously for 15-25 minutes after the addition is complete. Maintain the speed of 600-1000 rpm and slowly add bio-based wax emulsion, stirring for 10-20 minutes until the system is homogeneous. Adjust the speed to 1000-1400 rpm, and add polyurethane thickener in portions, stirring for 20-40 minutes to adjust the viscosity. Add dilute ammonia to adjust the pH to 7.0-8.0, and stir for 5-15 minutes. Filter through an 80-120 mesh filter and allow to mature at room temperature in a sealed environment for 18-30 hours before use. The preparation method of the aqueous oxygen barrier intermediate coating solution includes: Add 75-85℃ warm water to a stirred tank, add polyvinyl alcohol powder, and dissolve it under low-speed stirring for 20-40 minutes until completely free of particles; cool to 35-45℃, add nonionic wetting agent and organosilicon defoamer at 500-700 rpm and stir for 3-8 minutes; increase the speed to 1200-1800 rpm, add nano-montmorillonite barrier filler in batches, and disperse at high speed for 30-50 minutes; reduce the speed to 600-800 rpm, slowly add bio-based barrier latex and stir for 20-30 minutes; add dilute ammonia water to adjust the pH to 7.0-8.0 and stir for 5-15 minutes; filter through a 120-180 mesh filter and mature at room temperature in a sealed container for 14-22 hours for later use. The preparation method of the aqueous moisture-resistant heat-sealing composite surface coating liquid includes: Add deionized water to a stirred tank, add nonionic wetting agent and silicone defoamer at 200-400 rpm and stir for 3-8 minutes; add low-temperature heat-sealing acrylic copolymer emulsion at 600-1000 rpm and stir for 15-25 minutes; adjust the speed to 1000-1400 rpm, add polyurethane thickener in batches and stir for 20-40 minutes to adjust the viscosity; add dilute ammonia to adjust the pH to 7.0-8.0; filter through an 80-120 mesh filter and mature at room temperature in a sealed environment for 18-30 hours for later use. The water-based moisture-barrier undercoating and water-based moisture-barrier heat-sealing composite topcoat have a solid content of 25%–40%, a viscosity of 100–300 cP at 25°C, and a pH of 7.0–8.0; the water-based oxygen-barrier intermediate coating has a solid content of 15%–25%, a viscosity of 50–250 cP at 25°C, and a pH of 7.0–8.0.

[0020] Furthermore, the water-based moisture-barrier undercoat, water-based oxygen-barrier intermediate coating, and water-based moisture-barrier heat-sealing composite topcoat are applied by any one of roller coating, blade coating, or gravure coating. When roller coating is used, the viscosity of each coating layer at 25°C is controlled to be 80–250 cP. When blade coating is used, the viscosity of each coating layer at 25°C is controlled to be 150–300 cP. When gravure coating is used, the viscosity of each coating layer at 25°C is controlled to be 50–150 cP.

[0021] Beneficial effects

[0022] This invention provides a single-sided, multi-layer, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper. Through a sandwich structure design of "inner and outer double-layer moisture-barrier wrapping with an intermediate oxygen-barrier layer," it fundamentally solves the technical problem of existing water-based barrier papers' oxygen-barrier layer being prone to moisture-induced failure. The inner moisture-barrier bottom layer seals the pores of the base paper fibers, preventing residual moisture from escaping outwards. The outer moisture-barrier heat-sealing composite layer isolates environmental moisture from penetrating inwards. This bidirectional moisture barrier keeps the intermediate polyvinyl alcohol oxygen-barrier layer dry at all times. Even after 48 hours of accelerated aging under humid heat, the water and oxygen barrier indicators remain at extremely low levels (WVTR≤1.5, OTR≤2.5), ensuring stable long-term freshness preservation performance. Furthermore, the outermost single layer of this invention simultaneously achieves long-lasting waterproofing and low-temperature heat sealing at 100~130℃ (heat-sealing strength ≥8N / 15mm), eliminating a separate heat-sealing coating process. This results in lower equipment energy consumption, higher production efficiency, and a complete and continuous outer waterproof membrane, providing stronger long-lasting moisture protection. Furthermore, the reverse side retains the original fiber layer of the paper without any coating, forming a unique high-definition printing substrate. Printing adhesion reaches the highest national standard level 1, suitable for 80-150 linewidth industrial color printing. This solves the shortcomings of existing high-barrier papers, such as poor ink adhesion and limited printing capabilities. The all-water-based, plastic-free, aluminum-free, and organic solvent-free material system supports the recycling of single-material waste paper, with a natural degradation cycle of only 120-180 days, aligning with plastic restriction and dual-carbon policies.

[0023] The present invention provides a method for preparing single-sided, multi-layer, water-based, high-barrier, heat-sealable, and environmentally friendly packaging paper. Through a complete process path of "independent gradient preparation of coating solutions—single-sided, multi-layer coating—independent, medium-high temperature hot air curing of each layer," it achieves a balance between excellent product performance and economic efficiency in mass production. First, the three types of coating solutions are independently prepared according to their optimized feeding sequence, gradient stirring speed, and temperature control conditions, ensuring the uniformity of the microstructure and batch stability of each coating material. Single-sided, multi-layer coating and independent high-temperature hot air curing of each layer at 110~150℃ ensure that the coatings do not interfere with each other during curing, resulting in tight interlayer bonding. Simultaneously, the high temperature promotes the cross-linking density of the acrylic resin, giving the coating excellent water resistance and flexural strength, completely solving the problem of delamination, blistering, and peeling defects in existing double-layer coatings during mass production. Finally, the outer integrated coating simultaneously achieves the dual functions of long-term moisture barrier and low-temperature heat sealing in a single coating and curing process. The process path is shorter and the equipment investment is lower. Combined with conventional hot air drying tunnels, there is no need for vacuum or vapor deposition equipment. Furthermore, it has wide compatibility with roller coating / scraping / microgravure coating methods, giving this invention a strong advantage in industrial mass production conversion. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0025] Figure 1 A schematic diagram of the cross-sectional structure of a single-sided, multi-layer, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper provided for an embodiment of the present invention; Figure 2 This is a schematic diagram comparing the gas barrier mechanism of a conventional PVA coating and the PVA / nanomontmorillonite composite coating provided in the embodiments of the present invention. Figure 3 This is a practical application diagram of the environmentally friendly packaging paper prepared according to the present invention in a snack bag; Figure 4 This is a schematic flowchart illustrating a method for preparing single-sided, multi-layer, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper, provided for an embodiment of the present invention. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] In this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0028] It should be noted that if the embodiments of the present invention involve directional indicators (such as up, down, left, right, front, back, etc.), the directional indicators are only used to explain the relative positional relationship and movement of the components in a specific posture. If the specific posture changes, the directional indicators will also change accordingly.

[0029] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0030] This invention provides a single-sided, multi-layered, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper and its product structure. It aims to solve technical problems such as the oxygen barrier layer's susceptibility to moisture damage and failure, functional redundancy leading to discontinuous moisture protection in the outer layer, the inability to perform high-definition industrial printing due to full coating coverage, and the difficulty in recycling and degrading traditional composite packaging. This invention utilizes a "double-layer moisture-barrier sandwich" structure design, ensuring the middle oxygen barrier layer remains dry for long-lasting high barrier performance. The integrated outer coating design allows for simultaneous waterproofing and heat-sealing functions on a single layer. By retaining the original uncoated printing surface on the reverse side, it simultaneously achieves high-definition industrial color printing and high-strength heat sealing. This is a fully water-based, plastic-free, aluminum-free, and recyclable green high-barrier packaging paper product.

[0031] To better understand the above technical solutions, the present disclosure will be further described below with reference to specific embodiments, but the scope of protection of the present disclosure is not limited thereto.

[0032] This invention provides a single-sided, multi-layer, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper, comprising: Pure paper base, wherein the basis weight of the base paper is 30-100 g / m²; The paper substrate has a water-based moisture-proof bottom layer, a water-based oxygen-proof middle layer, and a water-based moisture-proof heat-sealing composite surface layer stacked sequentially from the inside to the outside, forming a sandwich-type three-layer coating structure with inner and outer double-layer moisture-proof covering the middle oxygen-proof layer; the water-based moisture-proof heat-sealing composite surface layer is an integrated single-layer coating that simultaneously has the dual functions of long-lasting moisture-proof outer layer and low-temperature heat sealing at 100-130℃. The other side of the original paper substrate is a completely uncoated, unprinted surface. The water-based moisture-barrier bottom layer, water-based oxygen-barrier middle layer, and water-based moisture-barrier heat-sealing composite top layer are all food-contact grade all-water-based coating layers. Under standard test conditions at room temperature, the water vapor transmission rate of the packaging paper is ≤1g / (m²·24h), and the oxygen transmission rate is ≤1cm³ / (m²·24h·0.1MPa); the heat seal strength of the water-based moisture-resistant heat-sealing composite surface layer is ≥8N / 15mm.

[0033] The present invention provides a single-sided multi-layer water-based high-barrier heat-sealable environmentally friendly packaging paper. The packaging paper uses pure paper as a base, and only one side of the base is provided with three functional coatings, while the other side completely retains the original appearance of the original paper fibers, forming a double-sided functional independent partition structure of "one side for barrier heat sealing and one side for high-definition printing".

[0034] The cross-sectional structure of the packaging paper of this invention is as follows: Figure 1 The diagram illustrates the overall structure of a paper base with independent functional zones on both sides. The central layer is a pure paper base. The left side represents the original printing surface of the paper (uncoated), preserving its original fiber structure for subsequent high-definition printing. The right side features three layers stacked sequentially from the inside out: a moisture-barrier bottom layer adhered to the paper surface, an oxygen-barrier middle layer in the middle, and a moisture-barrier heat-sealing composite layer on the outermost side. Moisture from the external environment is blocked by the outermost moisture-barrier heat-sealing composite layer, preventing it from penetrating the interior. Simultaneously, residual moisture inside the paper is sealed within the fiber layer by the moisture-barrier bottom layer, preventing it from escaping. This entire structure forms a two-way moisture barrier system: an outer moisture barrier → a middle oxygen barrier → an inner moisture barrier. The middle oxygen barrier layer is comprehensively protected by the moisture-barrier layers on both sides, effectively blocking moisture erosion from both the inside of the paper and the external environment.

[0035] The base paper is food-contact grade thin packaging paper with a basis weight range of 30–100 g / m², preferably 40–80 g / m². Below 30 g / m², the paper strength is insufficient, making it difficult to meet the mechanical requirements of subsequent coating, printing, and packaging forming processes; above 100 g / m², the paper stiffness is too high, reducing flexibility and hindering high-speed coating and winding operations. Density is controlled within the range of 0.60–0.70 g / cm³. Too low a density results in a loose paper fiber structure, allowing the coating liquid to penetrate excessively, leading to an uneven coating surface and wasted coating material; too high a density results in too few fiber pores, making it difficult for the coating to form sufficient mechanical anchoring, reducing interlayer adhesion. Air permeability is controlled within the range of 25–60 s / 100 ml (Schober method). Too low an air permeability indicates that the paper is too dense, making it difficult for the coating to penetrate and form anchoring; too high an air permeability indicates that the paper is too loose, allowing the coating liquid to penetrate too deeply, affecting the barrier effect and increasing the coating weight.

[0036] Optional base paper types include, but are not limited to: food-grade bleached tissue paper, white kraft paper, bamboo pulp paper, fluorine-free greaseproof paper, glassine paper, and food-grade specialty base paper. All of the above paper types possess good fiber uniformity and surface smoothness, and meet the safety requirements for raw materials in the GB 4806 national standard series for food contact materials. Before loading the base paper onto the machine, the moisture content should be controlled within the range of 4% to 8% to ensure uniform moisture release during the coating and drying process, avoiding pinhole defects in the coating due to excessive moisture content or paper brittleness due to excessive moisture content.

[0037] Three functional layers are sequentially stacked on one side of the base paper substrate from the inside out (i.e., from the direction closest to the base paper surface to the direction away from the base paper surface): a water-based moisture-barrier bottom layer, a water-based oxygen-barrier middle layer, and a water-based moisture-barrier heat-sealing composite top layer. The three layers work together to form a sandwich-type protective structure with "double-layered moisture barrier inside and out, enclosing an oxygen-barrier layer in the middle".

[0038] All three coating layers are food-contact grade, water-based coating systems, free of organic solvents, with extremely low VOC residue, meeting food contact safety requirements. The coatings are firmly bonded together through interpenetrating molecular chains and hydrogen bonds and van der Waals forces formed by polar groups (carboxyl, hydroxyl, etc.), eliminating the need for additional adhesive layers or primers.

[0039] Moisture-barrier undercoat dry coating amount: 5–10 g / m²: This coating amount is sufficient to seal the fiber pores on the surface of the base paper, forming a continuous and complete waterproof film. Oxygen-barrier intermediate layer dry coating amount: 2–5 g / m²: This thickness range is sufficient for the polyvinyl alcohol / nano-montmorillonite composite system to form a continuous and dense barrier film layer. Moisture-barrier heat-sealable composite topcoat dry coating amount: 5–10 g / m²: This coating amount ensures the formation of a complete and continuous waterproof film on the outer layer while also providing sufficient thickness for the heat-sealable fusion layer.

[0040] The moisture barrier underlayer is the first functional coating applied directly to the surface of the base paper. Its core functions include: (1) sealing the natural pores and capillary channels of the base paper fibers and cutting off the path for residual moisture inside the base paper to escape upwards; (2) forming a smooth and flat bonding surface, providing a good base for the uniform coating of the upper oxygen barrier middle layer; and (3) enhancing the interfacial bonding force between the entire coating system and the base paper substrate through the penetration and anchoring of polymer molecular chains into the fiber pores.

[0041] In one alternative embodiment, the moisture-proof underlayer comprises the following raw material components in parts by weight: Food-grade pure acrylic acid emulsion (e.g., Acronal® series or Primal® series) 65.0–78.0 parts, bio-based wax emulsion (e.g., carnauba wax emulsion or rice bran wax emulsion) 8.0–15.0 parts, deionized water 3.0–12.0 parts, polyurethane thickener (e.g., Borchigel® L 75 N or Rheolate® 255) 0.3–1.0 parts, nonionic wetting agent (e.g., Tergitol™ 15-S series, non-acetylenic diol) 0.2–0.6 parts, silicone defoamer (e.g., BYK®-024 or FoamStar® SI 2293) 0.1–0.4 parts; after preparation, adjust the pH to 7.0–8.0 with dilute ammonia water (5%–25% by mass concentration).

[0042] The functions and synergistic mechanisms of the above components are as follows: Food-grade pure acrylic emulsion (65.0–78.0 parts): This is the main film-forming material for the moisture-barrier underlayer. It uses a pure acrylic copolymer (pure acrylic) emulsion, whose molecular chains contain a large number of polar ester groups (-COOR), enabling the formation of a dense hydrophobic film layer during drying and curing through intermolecular polar attraction and van der Waals forces. This emulsion is an aqueous dispersion system, exhibiting good film continuity, high flexibility, and excellent wetting and penetration properties to the base paper fibers. The glass transition temperature (Tg) of the pure acrylic emulsion is controlled within the range of -5 to 10°C, providing the coating film with sufficient flexibility to resist the internal stress during paper bending and prevent cracking.

[0043] Bio-based wax emulsion (8.0–15.0 parts): As an auxiliary hydrophobic component, it is prepared by emulsification of naturally derived waxes (such as carnauba wax, rice bran wax, and other plant waxes or their derivatives). The wax microparticles in the wax emulsion melt during the drying process (melting point 50–70°C) and are uniformly dispersed in the continuous phase of the acrylic resin, forming a microphase-separated "island structure." These wax microparticles are highly hydrophobic, significantly reducing the surface energy and water absorption rate of the coating and blocking capillary water absorption. Simultaneously, as stress dispersion points, the wax microparticles can absorb and buffer crack propagation energy during bending, improving the coating's resistance to bending cracking.

[0044] Deionized water (3.0–12.0 parts): Used as a dispersion medium to adjust the overall solid content and viscosity of the coating, giving the coating liquid suitable leveling and application adaptability. The water completely evaporates during the drying process, leaving no residue.

[0045] Polyurethane thickener (0.3-1.0 parts): It is an associative polyurethane thickener that forms a three-dimensional network structure by adsorbing hydrophobic groups onto the surface of emulsion particles, thereby improving the low shear viscosity of the coating, preventing the coating from sagging on vertical surfaces during the coating process, and ensuring good leveling properties under high shear.

[0046] Nonionic wetting agent (0.2-0.6 parts): Reduces the interfacial tension between the coating liquid and the paper base, promotes the rapid spreading and penetration of the coating liquid on the paper fiber surface, and eliminates coating defects such as pinholes and craters.

[0047] Silicone defoamer (0.1-0.4 parts): Eliminates air bubbles generated during preparation and coating, and prevents pinhole defects from forming after the coating has cured.

[0048] The cured moisture-barrier underlayer has a continuous, dense, and flexible film layer with low water vapor permeability, effectively preventing the migration of moisture from the base paper. Testing showed that the water vapor permeability of paper coated with the moisture-barrier underlayer (without the middle and top layers) was reduced by more than 50% compared to the original paper.

[0049] The oxygen barrier middle layer is the core high oxygen barrier functional layer in the entire three-layer coating system, and its performance directly determines the oxygen barrier capability of the product. This layer is placed on top of the cured moisture barrier underlayer and is protected from both sides by the inner moisture barrier underlayer and the outer moisture barrier heat-sealing composite surface layer.

[0050] In one optional embodiment, the oxygen-barrier intermediate layer comprises the following raw material components in parts by weight: The mixture comprises 10.0–30.0 parts of polyvinyl alcohol (e.g., PVA-117 or PVA-124), 20.0–40.0 parts of organically modified nano-montmorillonite (e.g., Cloisite® 30B) with a particle size of 200–500 nm, 30.0–50.0 parts of bio-based barrier latex (e.g., corn starch-based bio-latex or cationic starch latex), 0.2–0.6 parts of nonionic wetting agent, and 0.1–0.4 parts of silicone defoamer; after preparation, the pH is adjusted to 7.0–8.0 with dilute ammonia. The oxygen-barrier middle layer does not contain soybean protein, chitosan, or acetylenic diol surfactants.

[0051] The functions and synergistic mechanisms of the above components are as follows: Polyvinyl alcohol (PVA) (10.0–30.0 parts): This is the core film-forming matrix material for the oxygen barrier middle layer. PVA molecular chains contain a large number of hydroxyl groups (-OH), which can form dense intermolecular hydrogen bonds between the chains. During drying, these bonds spontaneously arrange themselves into highly crystalline regional structures. These crystalline regions have extremely strong barrier properties against gas molecules (especially oxygen)—oxygen molecules have difficulty diffusing within the PVA crystalline lattice. PVA's oxygen barrier performance is far superior to most synthetic polymers, achieving extremely low OTR (overall oxygen resistance) in the dry state. However, PVA is also hydrophilic. The intrusion of water molecules weakens the hydrogen bonds between PVA molecular chains, reduces crystallinity, and enhances the mobility of molecular chain segments, making it easier for oxygen molecules to pass through, resulting in a sharp decline in barrier performance. Therefore, this solution uses an inner and outer moisture-barrier structure to physically isolate the PVA layer from water vapor, maintaining its dry and crystalline state during long-term use. This is the core technical principle for achieving long-lasting high barrier performance. The degree of polymerization of PVA is preferably 400-2000, and the degree of saponification is preferably above 85% to ensure good water solubility and film-forming properties.

[0052] Nano-montmorillonite barrier filler (particle size 200–500 nm) (20.0–40.0 parts): This is a layered inorganic nanofiller, a key component in constructing the "maze effect" barrier channels. Montmorillonite is a 2:1 type layered silicate mineral with a single layer thickness of approximately 1 nm and an aspect ratio exceeding 200. During high-speed shear dispersion, the nano-montmorillonite sheets can achieve nanoscale dispersion and oriented along the coating direction in the PVA matrix. When oxygen molecules permeate the composite film, they cannot penetrate in a straight line and must detour around these densely packed, impermeable sheet fillers, increasing the diffusion path length by several to tens of times, macroscopically manifested as an exponential decrease in oxygen permeability. The PVA matrix itself already possesses high oxygen barrier properties, and the addition of nano-montmorillonite further reduces the OTR to ≤1 cm³ / (m²·24h·0.1 MPa). The filler particle size is controlled within the range of 200-500 nm, which ensures good dispersibility in the PVA matrix (excessive particle size easily leads to sedimentation and agglomeration) and sufficient aspect ratio to extend the gas diffusion path (insufficient aspect ratio results in insufficient particle size).

[0053] Bio-based barrier latex (30.0–50.0 parts): This is a starch-based bio-latex, used as an auxiliary film-forming agent and toughening component in the PVA / montmorillonite composite system. The hydroxyl groups on its molecular chain can form intermolecular hydrogen bonds with the hydroxyl groups of PVA, improving the system's compatibility and film density, while also synergistically improving the dispersion stability of the montmorillonite sheets. The addition of bio-based latex can also regulate the rheological behavior of the coating solution, making it more adaptable to the process requirements of different coating methods, while reducing overall raw material costs.

[0054] Figure 2 This diagram illustrates a comparison of the gas barrier mechanisms of conventional PVA coatings and PVA / nano-montmorillonite composite coatings. The left side of the diagram shows the microstructure and gas barrier mechanism of a conventional pure polyvinyl alcohol (PVA) oxygen barrier coating: PVA molecular chains cross-link through hydrogen bonds to form a continuous polymer matrix, but oxygen molecules (O2) diffuse rapidly in a near-linear direction driven by a concentration gradient, resulting in a short barrier path and high permeation rate. The right side of the diagram shows the "maze effect" barrier mechanism of the oxygen barrier middle layer (PVA / nano-montmorillonite composite coating) of this invention: two-dimensional nano-montmorillonite (MMT) sheets are highly oriented, stacked parallel to each other, and staggered within the PVA matrix, forming numerous organic-inorganic hydrogen bond cross-linking points. When oxygen molecules pass through the composite coating, they must repeatedly navigate the tortuous gaps between the montmorillonite sheets, significantly extending the diffusion path, reducing the diffusion rate, and ultimately drastically decreasing the gas permeation. This comparison intuitively reveals that this invention, by introducing nano-montmorillonite sheets into the PVA matrix, transforms the rapid linear penetration of oxygen molecules into a long, tortuous path, thereby achieving highly efficient gas barrier.

[0055] The moisture-barrier heat-sealing composite surface layer is an integrated single-layer coating that simultaneously performs the functions of long-term waterproof barrier and low-temperature heat sealing. In one optional embodiment, the moisture-barrier heat-sealing composite surface layer comprises the following raw material components by weight: The following ingredients are used: 75.0-85.0 parts of low-temperature heat-sealing acrylic copolymer emulsion, 3.0-12.0 parts of deionized water, 0.3-1.0 parts of polyurethane thickener, 0.2-0.6 parts of nonionic wetting agent, and 0.1-0.4 parts of silicone defoamer; after preparation, adjust the pH to 7.0-8.0 with dilute ammonia.

[0056] The functions and synergistic mechanisms of the above components are as follows: Low-temperature heat-sealing acrylic copolymer emulsion (75.0-85.0 parts): This is the main film-forming material and functional component of this surface layer. The emulsion is a modified acrylic copolymer with a molecular structure designed. Two types of key segments are introduced into its molecular chain: (1) hydrophobic segments containing long-chain alkyl or fluorine / silicone modified segments, which are enriched on the coating surface after film formation, giving the coating low surface energy and high hydrophobicity, forming a continuous waterproof film, and blocking the penetration of external water vapor; (2) heat-sensitive segments containing polar groups (such as carboxyl groups, amide groups, etc.), which are in a glassy state at room temperature and have no surface stickiness; when heated to 100-130℃, the segments undergo a glass transition, the molecular chain mobility is enhanced, and under heat sealing pressure, they can diffuse and entangle with the molecular chains on the surface of the substrate, forming a strong adhesive interface after cooling.

[0057] The mechanism for achieving a heat-sealing strength of ≥8N / 15mm is as follows: Under the combined action of heat-sealing temperature (100~130℃) and pressure (approximately 200kPa), the surface of the topcoat softens and melts, adhering tightly to the substrate (which can be paper, coating, or the surface of a packaging container). The molecular chain segments of the acrylic copolymer migrate and penetrate into the surface of the substrate, forming an adhesive interface after cooling, which is jointly contributed by mechanical interlocking and intermolecular forces.

[0058] The composite top layer emulsion is free of harmful monomer residues such as styrene and formaldehyde, meeting food contact safety requirements. The functions of deionized water, polyurethane thickener, nonionic wetting agent, and silicone defoamer are the same as those of their corresponding components in the moisture-barrier bottom layer, respectively serving as dispersion medium, rheology modifier, interface wetting agent, and defoamer; these will not be elaborated further here.

[0059] The synergistic relationship between the surface layer and the inner moisture-barrier underlayer: the moisture-barrier underlayer blocks moisture from the base paper from the inside, while the moisture-barrier heat-sealed composite surface layer blocks ambient moisture from the outside. Together, they form a two-way moisture barrier for the intermediate oxygen barrier layer. This "double-layer sandwich" structure ensures that there is no moisture source on either side of the PVA oxygen barrier layer in the longitudinal (thickness direction). Even under accelerated aging in an extreme humid and hot environment of 55℃ / 90%RH for 48 hours, its OTR can still be maintained at a low level of 2.0 to 2.5.

[0060] This invention completely preserves the original fiber structure of the paper on the reverse side (i.e., the surface without the three-layer coating), without applying any barrier coating, primer, or surface treatment agent. The core performance parameters of this surface are as follows: Surface roughness 2.0~5.0μm: This roughness range falls within the optimal window for offset ink transfer. Surface strength ≥1.2m / s: This indicator characterizes the ability of paper surface fibers to resist peeling during printing. Low surface strength can lead to fiber fuzzing and powdering during high-speed printing, contaminating the printing plate and blanket, causing smearing and dot loss. Printing adhesion reaches national standard level 1: Tested according to GB / T 7706, this is the highest level. The binder in the ink penetrates into the pores of the paper fibers to form mechanical anchoring (“pinning effect”). After the ink cures, it is firmly bonded to the fibers, and the coating (ink layer) shows no peeling during the cross-cut test. Compatible printing screen rulings 80~150 lines / inch: Covering the screen ruling range from ordinary packaging printing to high-end color printing, meeting the graphic printing needs of most food and daily chemical packaging. Ink peeling rate ≤1.0%: Tested using the tape peeling method, after standard tape adhesion and peeling, the ink peeling area accounts for no more than 1.0%.

[0061] Through the synergistic effect of the above three-layer coating system and the original printed surface on the reverse side, the product disclosed herein can achieve the comprehensive performance shown in Table 1 below under standard test conditions at room temperature (temperature 23℃±2℃, relative humidity 50%±5%, referring to GB / T 1037 and GB / T 19466).

[0062] Table 1: Overall Performance of the Environmentally Friendly Guaranteed Paper of the Invention

[0063] The aforementioned WVTR and OTR indicators are equivalent to those of traditional paper-aluminum-plastic composite packaging, and can completely replace it in various packaging scenarios requiring moisture protection, oxidation resistance, and airtight preservation. The heat-sealing strength ≥8N / 15mm meets the sealing strength requirements of industrial automated packaging lines. Its natural degradation cycle is only 120–180 days, far superior to the 100–400 years of paper-aluminum-plastic composites, supporting the recycling of single-material waste paper and aligning with plastic restriction orders and dual-carbon policies.

[0064] The raw material system of the three-layer coating of this disclosed product does not contain chitosan, soybean protein, beeswax, polylactic acid, or acetylenol surfactants. Chitosan and protein raw materials are susceptible to water absorption and swelling, degradation of barrier properties, and microbial growth under high temperature and humidity environments; polylactic acid, although a biodegradable material, degrades slowly (requiring several months under industrial composting conditions) and is expensive; acetylenol surfactants are special additives, costly, and have limited supply channels. This disclosure achieves good coating results using conventional nonionic wetting agents and silicone defoamers, with widely available raw materials and controllable costs.

[0065] After all three coating layers have cured and the reverse side has been printed, this product can be directly used in various scenarios requiring high-barrier, heat-sealed packaging. In use, the side with the three coating layers serves as the heat seal, which is then heat-sealed against the other coated side or the opening of the packaging container (100–130℃, pressure approximately 200 kPa, time 0.3–1.0 s). The heat seal strength is ≥8 N / 15 mm, ensuring reliable sealing. The reverse printed side is used to display product information, brand logos, barcodes, nutrition facts, and other graphic content. The 80–150 line density printing clarity meets the needs of high-end consumer product packaging.

[0066] Specific application categories include, but are not limited to: tea bags (moisture-proof and freshness-preserving), nut and snack bags (oxygen barrier to prevent spoilage), meat product packaging (oxygen barrier to delay oxidation and discoloration), pharmaceutical aluminum-plastic alternative packaging (moisture-proof and oxygen barrier to ensure efficacy), cosmetic outer packaging (moisture-proof and deterioration-proof), and instant beverage packaging (moisture-proof and clumping-proof). Figure 3 This invention relates to a practical application of the packaging paper produced in snack bags. These snack bags can be printed in high definition and also have excellent barrier heat-sealing properties.

[0067] The present invention also provides a method for preparing single-sided multilayer water-based high-barrier heat-sealable environmentally friendly packaging paper as described in any of the preceding claims, such as... Figure 4 As shown, it includes the following steps: S10. Coating preparation: Prepare water-based moisture-barrier undercoating liquid, water-based oxygen-barrier intermediate coating liquid and water-based moisture-barrier heat-sealing composite topcoat liquid respectively, and after curing, send them to the corresponding material tanks of the coating equipment for later use. S20, Base paper feeding: The food contact thin base paper is directly fed into the coating production line. The base paper does not undergo corona treatment, pre-drying, surface leveling and dehumidification pretreatment. S30. Coating and curing the moisture-proof underlayer: Coating the water-based moisture-proof underlayer coating liquid onto one side surface of the base paper substrate, and drying and curing it in a hot air drying zone of 110℃~150℃ for 40~100s. S40. Coating and curing the oxygen barrier intermediate layer: Coating the water-based oxygen barrier intermediate layer coating liquid onto the surface of the cured moisture barrier bottom layer, and drying and curing it in a hot air drying zone of 130℃~150℃ for 40~100s. S50. Coating and curing the moisture-proof heat-sealing composite surface layer: Coating the water-based moisture-proof heat-sealing composite surface layer coating liquid onto the surface of the cured oxygen-barrier intermediate layer, and drying and curing it in a hot air drying zone of 130℃~150℃ for 40~100s. S60. Winding and Printing: After the three-layer coating is dried and cured, the paper is continuously wound up. The original printing surface on the uncoated side of the base paper is used for offset printing, flexographic printing or gravure printing, and then cut to obtain the finished packaging paper.

[0068] The preparation method of the present invention mainly includes three steps: (1) independent preparation of three types of coating liquids; (2) sequential coating and curing of three coating layers on one side of the paper base; (3) winding and reverse printing. The entire process adopts conventional hot air drying tunnel, without the need for vacuum equipment or metal vapor deposition process, and can be implemented in existing water-based coating production lines.

[0069] The three types of coating solutions (moisture-barrier undercoating solution, oxygen-barrier intermediate coating solution, and moisture-barrier heat-sealing composite topcoat solution) must be prepared separately and independently, and must not be mixed or used in the same preparation equipment (to prevent cross-contamination between different systems). The ambient temperature in the preparation workshop should be controlled between 20 and 28°C to ensure batch stability of the viscosity and rheological behavior of the coating solutions.

[0070] 1. Preparation of moisture-barrier undercoating solution

[0071] Add the prescribed amount of deionized water to a stirred tank, start the stirrer, and sequentially add the nonionic wetting agent and silicone defoamer under low-speed shear conditions of 200-400 rpm, stirring continuously for 3-8 minutes to ensure the additives are fully dispersed in the aqueous phase. Then, increase the stirring speed to 600-1000 rpm and add food-grade pure acrylic acid emulsion dropwise at a uniform rate, completing the entire dropwise addition process within 10-20 minutes. After the dropwise addition is complete, continue stirring for 15-25 minutes to ensure complete dispersion of the emulsion particles. While maintaining a speed of 600-1000 rpm, slowly add the bio-based wax emulsion, continuing to stir for 10-20 minutes until the system is homogeneous and shows no visible layering. Then, increase the speed to 1000-1400 rpm and add the polyurethane thickener in small amounts in 2-3 portions, with intervals of approximately 5-8 minutes between each addition, for a total stirring time of 20-40 minutes, allowing the system viscosity to gradually increase to the target range. After preparation, slowly add dilute ammonia (5%–25% by mass) to adjust the pH to 7.0–8.0, and stir continuously for 5–15 minutes to ensure the system is fully homogeneous. Finally, filter through an 80–120 mesh filter to remove any possible small amounts of insoluble matter or gel particles, transfer to a sealed container, and let it stand and mature at room temperature (20–28°C) for 18–30 hours before use.

[0072] Key points for preparation: Thickener should be added in stages rather than all at once to prevent excessively high local concentrations from causing emulsion flocculation; dilute ammonia should be added slowly to avoid drastic pH fluctuations that could cause emulsion demulsification.

[0073] 2. Preparation of oxygen-barrier intermediate coating solution

[0074] Because polyvinyl alcohol (PVA) dissolves extremely slowly at room temperature, a hot water system must be prepared beforehand for dissolution. Add warm water (75–85°C) to a stirred tank, then add the PVA powder. Continue dissolving for 20–40 minutes under low-speed stirring until the system is completely transparent and free of visible undissolved particles. This step is crucial to ensure complete dissolution of the PVA; otherwise, residual particles will severely affect the coating's density and barrier properties.

[0075] After dissolution, cool the system to 35–45°C (to avoid poor dispersion of the filler caused by adding nano-montmorillonite at high temperatures). Add the nonionic wetting agent and silicone defoamer at 500–700 rpm and stir for 3–8 minutes. Then increase the speed to 1200–1800 rpm and add the nano-montmorillonite barrier filler (200–500 nm) in batches, with an interval of about 3–5 minutes between each batch. The total high-speed dispersion time is 30–50 minutes, using strong shear force to break the montmorillonite agglomerates into nano-sized primary particles. After dispersion, reduce the speed to 600–800 rpm and slowly add the bio-based barrier latex, stirring continuously for 20–30 minutes to ensure uniform mixing of all components. Finally, adjust the pH to 7.0–8.0 with dilute ammonia, stir for 5–15 minutes, filter through a 120–180 mesh fine filter, and transfer to a sealed container to mature at room temperature for 14–22 hours for later use.

[0076] Key points for preparation: The dispersion of nano-montmorillonite is the key to this step. The rotation speed and dispersion time must ensure that the filler is fully exfoliated into nanosheets (XRD detection interlayer spacing ≥3nm), otherwise the maze effect cannot be fully utilized; the PVA solution must be cooled to about 40℃ before adding the nanofiller. At high temperatures, the movement of PVA molecular chains is too violent, which is not conducive to the orientation and arrangement of montmorillonite sheets.

[0077] 3. Preparation of moisture-resistant heat-sealing composite coating liquid

[0078] The preparation process is similar to that of the moisture-barrier bottom layer. Add deionized water to a stirred tank, and then add a nonionic wetting agent and an organosilicon defoamer at 200–400 rpm, stirring for 3–8 minutes. Increase the speed to 600–1000 rpm, and add the low-temperature heat-sealing acrylic copolymer emulsion at a uniform rate, continuing to stir for 15–25 minutes. Increase the speed to 1000–1400 rpm, and add the polyurethane thickener in portions, stirring for 20–40 minutes to adjust the viscosity to the target range. Adjust the pH to 7.0–8.0 with dilute ammonia, stir thoroughly, filter through an 80–120 mesh filter, and transfer to a sealed container to mature at room temperature for 18–30 hours for later use.

[0079] The solid content, viscosity, and pH of the three coating solutions are as follows: the solid content of the moisture-barrier undercoat and the moisture-barrier heat-sealing composite topcoat are each 25%–40%, the viscosity at 25°C is 100–300 cP, and the pH is 7.0–8.0; the solid content of the oxygen-barrier intermediate coating is 15%–25%, the viscosity at 25°C is 50–250 cP, and the pH is 7.0–8.0. The above indicators were determined using conventional methods in this field. Solid content was measured using a 105°C oven drying method; viscosity was measured using a rotational viscometer (such as a Brookfield type) with a corresponding rotor in a 25°C constant temperature water bath; and pH was measured using a standard pH meter.

[0080] The three types of coating solutions are prepared without the addition of chitosan, soy protein, beeswax, polylactic acid, or acetylation diol surfactants. The curing process must be carried out in a sealed container to prevent moisture evaporation from causing surface skin formation and viscosity changes. The three types of coating solutions should be used within 72 hours after preparation. If the storage period is exceeded, the solid content and viscosity should be retested, and if necessary, the solution should be finely adjusted to the specified range with deionized water or thickener.

[0081] Raw paper feeding

[0082] Food contact thin base paper that meets the parameter requirements (basis weight 30-100 g / m², density 0.60-0.70 g / cm³, air permeability 25-60 s / 100ml) is directly fed into the coating production line. The base paper does not undergo any pretreatment operations such as corona treatment, pre-drying, surface leveling, or dehumidification.

[0083] The technical significance of this operation is: (1) saving equipment investment and process energy consumption; (2) avoiding damage to paper fibers during pretreatment; (3) preserving the original appearance of the fibers on the reverse side of the original paper to ensure good adhesion of the subsequent printing surface; (4) the original paper has a suitable moisture content and surface condition when it leaves the factory, which is sufficient to meet the requirements of the subsequent coating process. Before the original paper is put into the machine, it should be placed in the workshop environment for more than 24 hours to allow its moisture content to naturally balance with the workshop environment (temperature 20~28℃, relative humidity 40%~60%), so as to avoid deformation of the paper after coating due to temperature or humidity difference.

[0084] Multi-coating and layered curing

[0085] The process adopts a single-sided, multi-layer coating method, with each layer being dried and cured independently, which is a cyclical pattern of "coating one layer → drying and curing → coating the next layer".

[0086] Moisture-barrier undercoating and curing: Apply a water-based moisture-resistant undercoating to the reverse side (uncoated side) of the paper or a single-sided surface set according to the paper feed direction of the equipment. Any of the following methods can be used: roller coating, blade coating, or gravure coating. The coating amount should be controlled at 5–10 g / m² (dry coating weight). After coating, the paper immediately enters the hot air drying zone, setting the drying temperature to 110℃–150℃ and the dwell time to 40–100 seconds (corresponding to the drying tunnel length and paper feed speed). Within this temperature range, moisture evaporates fully, acrylic emulsion particles melt and polymerize to form a continuous transparent film, and wax particles in the wax emulsion melt and disperse evenly in the film layer. After drying and curing, the coating surface should be smooth, free of pinholes, flow marks, and sticky to the touch, with no dampness.

[0087] Oxygen barrier intermediate coating and curing: After the moisture-barrier underlayer has cured, a water-based oxygen-barrier intermediate coating is applied, with a coating amount controlled at 2–5 g / m² (dry coating weight). Since the previous layer is fully cured, the oxygen-barrier intermediate coating will not miscible with the underlayer, forming a clear interlayer boundary at the interface. The coating then enters the second hot air drying zone, with a drying temperature set at 130℃–150℃ and a dwell time of 40–100 seconds. This temperature range is higher than the curing temperature of the moisture-barrier underlayer, which is beneficial for the formation of a highly crystalline, dense barrier structure between the PVA and nano-montmorillonite composite system, while avoiding thermal damage to the already cured underlayer. After drying, the coating should be transparent or translucent, with a uniform surface free of cracks.

[0088] Moisture-resistant heat-sealing composite coating and curing: After the oxygen-barrier intermediate layer has cured, a water-based moisture-barrier heat-sealing composite topcoat is applied, with a coating amount controlled at 5–10 g / m² (dry coating weight). It then enters the third stage hot air drying zone, where the drying temperature is set at 130℃–150℃ and the dwell time is 40–100 seconds. Within this temperature range, the low-temperature heat-sealing acrylic copolymer emulsion particles fully melt and cross-link, forming a continuous hydrophobic film. After drying, the coating should be smooth and glossy, free of pinholes and orange peel texture.

[0089] Each layer has an independently set drying temperature range (110–150°C) and is cured layer by layer, ensuring that each coating does not interfere with the others during the curing process. Moisture in each layer can fully evaporate without being hindered by the upper layer, and the film-forming substances in each layer can complete cross-linking at the optimal temperature. Compared to the "wet-on-wet" process, layered curing avoids interlayer miscibility and moisture trapping at the interlayer interface, significantly reducing drying shrinkage stress and the risk of microcracks. The incremental design of the drying temperature range from low to high (bottom layer 110–150°C, middle and top layers 130–150°C) ensures both sufficient film formation of the bottom resin and dense cross-linking of the upper materials at higher temperatures.

[0090] Rewinding, Curing and Printing

[0091] After all three coating layers have cured, the product is cooled to room temperature (not exceeding 40℃) in a cooling section and then wound up continuously. The winding tension should be moderate (approximately 10–30 N / m) to avoid excessive tension causing the coating to stick or the paper to stretch and deform, and also to avoid insufficient tension causing the core to become soft and subsequent cutting to go astray. The semi-finished product after winding can be placed in an environment with a temperature of 20–28℃ and a relative humidity of 40%–60% for 24–48 hours for equilibration and curing (optional step, to be implemented or not depending on product requirements and production line pace). This allows the molecular chains within the coating to further relax and rearrange their orientation, achieving optimal mechanical balance and stable barrier properties.

[0092] During printing, offset, flexographic, or gravure printing is performed directly on the uncoated printing surface of the paper base. High-quality ink adhesion is achieved without the need for any primer, pre-coating, or corona treatment. Before printing, appropriate cleaning and dust removal (such as using electrostatic dust removal sticks or brush dust removal devices to remove surface dust) can be performed to ensure printing cleanliness. After printing, post-processing such as slitting, rewinding, or bag making is carried out according to customer requirements to obtain the finished packaging paper.

[0093] This method is applicable to any one of the coating methods, such as roller coating, blade coating, or gravure coating. The selection criteria include: product width, coating accuracy requirements, rheological properties of the coating liquid, and existing equipment configuration of the production line.

[0094] Different coating methods have different adaptability to coating liquid viscosity. In actual operation, adjustments can be made according to the following principles: When using roller coating, the viscosity of the coating liquid at 25°C should be controlled between 80 and 250 cP. Roller coating is a medium-to-high shear coating method. The shear force between the rollers helps the coating liquid spread evenly. If the viscosity is too low, the coating amount will be unstable, and if it is too high, roller marks will easily occur.

[0095] When using a blade coating method, the viscosity of the coating liquid at 25°C should be controlled between 150 and 300 cP. The coating amount is controlled by the gap between the blade and the paper. Too low a viscosity can easily cause seepage and streaks, while a slightly higher viscosity is beneficial for a smoother blade application.

[0096] When using gravure coating, the viscosity of each coating liquid at 25°C should be controlled between 50 and 150 cP. Gravure coating is a reverse transfer coating method that relies on the gravure roller to transfer the coating material. Low viscosity is beneficial for the filling and uniformity of the gravure pattern.

[0097] If the viscosity of the prepared coating solution deviates from the target range mentioned above, it can be finely adjusted by adding a small amount of deionized water (to reduce viscosity) or by adding polyurethane thickener (to increase viscosity). After adjustment, it should be stirred again for 10-15 minutes and the viscosity should be measured again. It can only be used on the machine after confirming that it meets the standard. The standard operating speed of the production line is 15m / min, which can be adjusted appropriately within the range of 2-20m / min according to the coating drying effect. If the speed is too low, the production capacity will decrease and the paper will be over-dried in the drying tunnel; if the speed is too high, the moisture will not evaporate fully before leaving the drying tunnel, and the residual moisture in the coating will reduce the barrier performance.

[0098] In the above steps, the independent gradient formulation of the coating solution ensures that each layer of material has its own optimal microstructure (continuous film formation of the bottom acrylic resin, dense barrier crystalline region formation of the middle PVA / montmorillonite layer, and thermosensitive active state of the top low-temperature heat-sealing resin); the pretreatment of the base paper simplifies the process and avoids fiber damage; the multi-coating and layer-curing ensures clear interfaces, strong adhesion, and no moisture residue in each coating layer; high-quality printing can be achieved without additional treatment on the reverse side. Each step is interconnected and together constitutes the technical integrity of this preparation method. The comprehensive performance of the product is reflected in: WVTR≤1g / (m²·24h), OTR≤1cm³ / (m²·24h·0.1MPa), heat-sealing strength≥8N / 15mm, and printing adhesion grade 1 according to national standards.

[0099] To further illustrate the effect of the dosage of each key component in this embodiment on the performance of bio-based fluorine-free oil-resistant starch paper, packaging papers prepared in Examples 1 to 6 and Comparative Examples 1 to 5 as shown below were provided and tested.

[0100] Example 1

[0101] Base paper: food-grade white kraft paper, basis weight 50g / m², density 0.65g / cm³, air permeability 40s / 100ml, moisture content controlled at 5%~7%.

[0102] The three-layer coating solution is prepared according to the optimal parameters: (1) Preparation of moisture-barrier undercoating liquid By weight: 72 parts food-grade pure acrylic emulsion, 12 parts bio-based wax emulsion, 8 parts deionized water, 0.6 parts polyurethane thickener, 0.4 parts nonionic wetting agent, and 0.3 parts silicone defoamer.

[0103] Preparation steps: Add all deionized water to a mixing tank and stir at a low speed of 300 rpm. Add nonionic wetting agent and silicone defoamer in sequence, and stir for 5 minutes until fully mixed. Increase the speed to 800 rpm and add food-grade pure acrylic acid emulsion dropwise at a uniform speed over 15 minutes. After the dropwise addition is complete, continue stirring for 20 minutes. Maintain the speed of 800 rpm and slowly add bio-based wax emulsion, stirring for 15 minutes until the system is homogeneous and does not separate into layers. Adjust the speed to 1200 rpm and stir at a high speed. Add polyurethane thickener in small amounts in batches, and stir for 30 minutes. Adjust the pH of the system to 7.0-8.0 with food-grade dilute ammonia water and continue stirring for 10 minutes. Filter through a 100-mesh filter to remove impurities and let it mature at room temperature (20-28℃) in a sealed container for 24 hours before use.

[0104] (2) Preparation of oxygen barrier intermediate coating solution

[0105] By weight: 20 parts polyvinyl alcohol, 30 parts nano montmorillonite (particle size 200-500nm), 40 parts bio-based barrier latex, 0.4 parts nonionic wetting agent, and 0.3 parts silicone defoamer.

[0106] Preparation steps: Add 80℃ warm water to a mixing tank, add polyvinyl alcohol powder, and stir at low speed for 30 minutes until completely dissolved and free of particles; cool to 40℃, add nonionic wetting agent and silicone defoamer at 600 rpm, and stir for 5 minutes; increase the speed to 1500 rpm, add nano-montmorillonite barrier filler in batches, and disperse at high speed for 40 minutes until the filler is fully exfoliated and free of agglomerates; reduce the speed to 700 rpm, slowly add bio-based barrier latex, and stir for 25 minutes until uniformly mixed; adjust the pH to 7.0-8.0 with dilute ammonia water and stir for 10 minutes; filter through a 150-mesh fine filter and mature at room temperature in a sealed container for 18 hours for later use. This middle layer does not contain soybean protein, chitosan, or acetylenic diol surfactants.

[0107] (3) Preparation of moisture-resistant heat-sealing composite surface coating liquid

[0108] By weight: 80 parts low-temperature heat-sealing acrylic copolymer emulsion, 8 parts deionized water, 0.6 parts polyurethane thickener, 0.4 parts nonionic wetting agent, and 0.3 parts silicone defoamer.

[0109] Preparation steps: Add all deionized water to a mixing tank, add nonionic wetting agent and silicone defoamer at a low speed of 300 r / min, and stir for 5 min; increase the speed to 800 r / min, add low-temperature heat-sealing acrylic copolymer emulsion at a uniform speed, and stir for 20 min; adjust to 1200 r / min high speed, add polyurethane thickener in small batches, and stir for 30 min to adjust the viscosity to the target range; adjust the pH to 7.0-8.0 with dilute ammonia water, and stir thoroughly; filter through a 100-mesh filter, and mature at room temperature in a sealed environment for 24 h for later use.

[0110] Coating and curing: Coating method: micro-gravure coating, production line speed 15m / min.

[0111] Three layers of coating are applied sequentially to one side of the food-grade white kraft paper: Moisture-barrier undercoat: Apply the above moisture-barrier undercoat liquid, with a dry coating amount of 6g / ㎡, hot air drying temperature of 130℃, and drying time of 60s. Oxygen barrier intermediate layer: The above-mentioned oxygen barrier intermediate layer coating liquid is applied to the surface of the cured moisture barrier base layer. The dry coating amount is 3g / ㎡, the hot air drying temperature is 140℃, and the drying time is 60s. Moisture-barrier heat-sealing composite surface layer: The above-mentioned moisture-barrier heat-sealing composite surface layer coating liquid is applied to the surface of the cured oxygen barrier intermediate layer. The dry coating amount is 7g / ㎡, the hot air drying temperature is 140℃, and the drying time is 60s.

[0112] The drying temperature of each layer is independently controlled, with fluctuations not exceeding ±5℃. The entire process is hot air curing, eliminating the need for vacuum drying and metal vapor deposition.

[0113] Rewinding and Printing: After all three coating layers have cured, the paper is cooled to room temperature (not exceeding 40°C) in a cooling section before continuous winding. Offset, flexographic, or gravure printing is performed directly on the uncoated side of the base paper substrate, without the need for any primer or pre-coating. Before printing, an electrostatic dust removal device cleans the printing surface to remove surface dust, ensuring high-definition printing. After printing, the paper is slit to obtain the finished packaging paper.

[0114] Example 2

[0115] 1. Base paper: 70g / ㎡ bamboo pulp thin paper, density 0.68g / cm³, air permeability 50s / 100ml; 2. Moisture barrier undercoat dry coating amount 8g / ㎡, drying at 120℃ for 80s; 3. Oxygen barrier middle layer dry coating amount 4g / ㎡, drying at 135℃ for 80s; 4. Moisture barrier heat-sealing composite top layer dry coating amount 8g / ㎡, drying at 135℃ for 80s; 5. Coating method: roller coating, production line speed 12m / min; the rest is the same as in Example 1.

[0116] Example 3

[0117] 1. Base paper: 40g / ㎡ glassine paper, density 0.62g / cm³, air permeability 30s / 100ml; 2. Moisture barrier undercoat dry coating weight 5g / ㎡, drying at 140℃ for 50s; 3. Oxygen barrier middle layer dry coating weight 2g / ㎡, drying at 145℃ for 50s; 4. Moisture barrier heat-sealing composite topcoat dry coating weight 6g / ㎡, drying at 145℃ for 50s; 5. Coating method: blade coating, production line speed 18m / min; the rest is the same as in Example 1.

[0118] Example 4

[0119] 1. Base paper: 30g / ㎡ food-grade bleached thin packaging paper, density 0.63g / cm³, air permeability 35s / 100ml; 2. Moisture-barrier bottom layer dry coating: 6g / ㎡, drying at 130℃ for 60s; 3. Oxygen-barrier middle layer dry coating: 3g / ㎡, drying at 140℃ for 60s; 4. Moisture-barrier heat-sealing composite top layer dry coating: 7g / ㎡, drying at 140℃ for 60s; 5. Coating method: micro-gravure coating, production line speed 15m / min; the rest is the same as in Example 1.

[0120] Example 5

[0121] 1. Base paper: 100g / ㎡ food-grade specialty base paper, density 0.67g / cm³, air permeability 45s / 100ml; 2. Moisture-barrier undercoat dry coating amount 8g / ㎡, drying at 130℃ for 70s; 3. Oxygen-barrier middle layer dry coating amount 4g / ㎡, drying at 140℃ for 70s; 4. Moisture-barrier heat-sealing composite top layer dry coating amount 8g / ㎡, drying at 140℃ for 70s; 5. Coating method: roller coating, machine speed 12m / min, the rest is the same as in Example 1.

[0122] Example 6: 1. Base paper: 70g / ㎡ food-grade white kraft paper, density 0.65g / cm³, air permeability 40s / 100ml; 2. Moisture-barrier undercoat dry coating: 10g / ㎡, dried at 150℃ for 50s; 3. Oxygen-barrier middle layer dry coating: 5g / ㎡, dried at 150℃ for 50s; 4. Moisture-barrier heat-sealing composite topcoat: 10g / ㎡, dried at 150℃ for 50s; 5. Coating method: scraping, machine speed 20m / min; the rest is the same as in Example 1.

[0123] Comparison Sample Settings

[0124] Comparative Example 1: Single-sided double-layer water-based barrier paper (oxygen barrier layer + composite heat-sealed moisture barrier layer, no moisture barrier bottom layer), other conditions are the same as in Example 1.

[0125] Comparative Example 2: Double-sided three-layer water-based barrier paper, with a moisture barrier layer coated on one side of the paper and an oxygen barrier layer + composite moisture barrier heat seal layer coated on the other side, other conditions are the same as in Example 1.

[0126] Comparative Example 3: Single-layer water-based moisture-barrier coated paper, without a dedicated oxygen barrier layer.

[0127] Comparative Example 4: Oxygen Barrier Intermediate Layer: 10-30 parts of soy protein isolate (instead of PVA), 20-40 parts of nano-montmorillonite, and an appropriate amount of water (excluding PVA) were used. Other conditions were the same as in Example 1.

[0128] Comparative Example 5: The outer layer on the oxygen barrier middle layer is (independent heat-sealing layer): non-hydrophobic modified heat-sealing acrylic resin (ordinary heat-sealing acrylic emulsion, without hydrophobic modified segments), dry coating amount 7 g / m², drying at 140°C for 60 s, other conditions are the same as in Example 1.

[0129] Performance testing.

[0130] 1. Water vapor transmission rate (WVTR): GB / T 1037-2021; 2. Oxygen permeability (OTR): GB / T 19466-2014; 3. Printing performance: GB / T 7706-2017; 4. Food contact safety: GB 4806 series national standards; 5. Accelerated aging conditions under damp heat: Placed in a constant temperature and humidity environment of 55℃ and 90% RH for 48 hours; 6. Heat sealing performance: Under heat sealing conditions of 120℃, 0.5s, and 200kPa, the heat sealing strength is tested at 8N / 15mm.

[0131] The packaging papers prepared in Examples 1-6 and Comparative Examples 1-5 were subjected to performance tests, and the results are shown in Table 2.

[0132] Table 2: Performance test results of the examples and comparative examples

[0133] Wherein, the initial WVTR unit is g / (m²·24h), and the initial OTR unit is cm³ / (m²·24h·0.1MPa).

[0134] Analysis of Table 1 shows that: The "double-layer moisture barrier" structure is irreplaceable. Comparative Example 1 (without bottom layer) and Comparative Example 5 (without moisture barrier top layer) both showed a sharp increase in OTR after hygrothermal aging (reaching 13.2 and 11.3 respectively), while Examples 1-4 only had an OTR of 2.0-2.5 after aging. This proves that single-sided moisture barrier cannot simultaneously resist the bidirectional erosion of internal and external moisture; only the synergy of the inner and outer layers can achieve comprehensive protection of the oxygen barrier layer.

[0135] The PVA + nano-montmorillonite system is superior to the biological protein system. Comparative Example 4 (protein replacing PVA) showed an OTR of 10.6 after aging, approximately four times that of Examples 1-4, and exhibited no heat-sealing ability. This demonstrates that the dense crystalline structure formed by the PVA and nano-montmorillonite composite provides a stable and high-barrier barrier, while biological proteins, due to their hydrophilic nature, cannot solve the problem of damp-heat aging failure.

[0136] While Comparative Example 2 (double-sided coating) exhibited excellent barrier properties, its ink shedding rate was as high as 20%, and its adhesion was only at the national standard level 4, failing to meet the requirements of industrial color printing. Examples 1-6 all had ink shedding rates ≤0.9% and adhesion at the national standard level 1. This demonstrates that functional partitioning (one coated side and one uncoated printing side) is the preferred solution that simultaneously achieves both barrier and printing performance; applying a coating to the printing side sacrifices print quality.

[0137] An integrated outer layer (moisture barrier + heat seal) is superior to functional layering. Comparative Example 5 (without a moisture barrier layer) only retains the heat seal function but lacks continuous moisture barrier capability; after damp heat aging, its OTR rises to 11.3, close to that of Comparative Example 4. Examples 1-3 have both moisture barrier and heat seal functions in their outer layers; after aging, their OTR is only 2.0-2.5, and their heat seal strength reaches 8.7-9.5 N / 15 mm. This demonstrates that integrating moisture barrier and heat seal into a single coating simplifies the process while ensuring the integrity and continuity of the outer waterproof membrane.

[0138] Example 6 (vehicle speed 20m / min, upper limit of coating amount) WVTR / OTR reached 0.6 / 0.5 respectively, heat sealing strength 9.8, and printing national standard level 1, proving that the product performance is stable under high speed and high production conditions; Example 4 (30g / m² ultrathin base paper) heat sealing strength 6.8N / 15mm is slightly lower, but barrier properties and printability are good, indicating that the sealing temperature should be appropriately increased when heat sealing thin paper.

[0139] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

[0140] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, preferred embodiments are described in order to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art can make other changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of this invention.

[0141] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from the spirit and scope of this invention; if these modifications and variations of this invention fall within the scope of the claims of this invention and their equivalents, then this invention also intends to include these modifications and variations.

Claims

1. A single-sided, multi-layered, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper, characterized in that: include: Pure paper base, wherein the basis weight of the base paper is 30-100 g / m²; The paper substrate has a water-based moisture-proof bottom layer, a water-based oxygen-proof middle layer, and a water-based moisture-proof heat-sealing composite surface layer stacked sequentially from the inside to the outside, forming a sandwich-type three-layer coating structure with inner and outer double-layer moisture-proof covering the middle oxygen-proof layer; the water-based moisture-proof heat-sealing composite surface layer is an integrated single-layer coating that simultaneously has the dual functions of long-lasting moisture-proof outer layer and low-temperature heat sealing at 100-130℃. The other side of the original paper substrate is a completely uncoated, unprinted surface. The water-based moisture-barrier bottom layer, water-based oxygen-barrier middle layer, and water-based moisture-barrier heat-sealing composite top layer are all food-contact grade all-water-based coating layers. Under standard test conditions at room temperature, the water vapor transmission rate of the packaging paper is ≤1g / (m²·24h), and the oxygen transmission rate is ≤1cm³ / (m²·24h·0.1MPa); the heat seal strength of the water-based moisture-resistant heat-sealing composite surface layer is ≥8N / 15mm.

2. The environmentally friendly packaging paper according to claim 1, characterized in that, The dry coating amount of the water-based moisture-barrier underlayer is 5-10 g / m², the dry coating amount of the water-based oxygen-barrier intermediate layer is 2-5 g / m², and the dry coating amount of the water-based moisture-barrier heat-sealing composite top layer is 5-10 g / m².

3. The environmentally friendly packaging paper according to claim 1, characterized in that, The density of the base paper is 0.60-0.70 g / cm³, and the air permeability is 25-60 s / 100ml; the base paper is any one of food-grade bleached thin packaging paper, white kraft paper, bamboo pulp paper, fluorine-free oil-resistant paper, glassine paper, or food-grade specialty base paper.

4. The environmentally friendly packaging paper according to claim 1, characterized in that, The surface roughness of the original printed surface is 2.0 to 5.0 μm, and the surface strength is ≥1.2 m / s; the printing adhesion of the original printed surface reaches the national standard level 1, the compatible printing screen count is 80 to 150 lines / inch, and the ink shedding rate is ≤1.0%.

5. The environmentally friendly packaging paper according to claim 1, characterized in that, The raw material composition of the water-based moisture-proof bottom layer, by weight, is as follows: Food-grade pure acrylic acid emulsion, 65.0–78.0 parts. Bio-based wax emulsion, 8.0–15.0 parts 3.0–12.0 parts deionized water 0.3-1.0 parts of polyurethane thickener Nonionic wetting agent 0.2-0.6 parts, 0.1 to 0.4 parts of silicone defoamer.

6. The environmentally friendly packaging paper according to claim 1, characterized in that, The raw material composition of the aqueous oxygen barrier middle layer, by weight, is as follows: Polyvinyl alcohol 10.0–30.0 parts 20.0–40.0 parts of nano-montmorillonite barrier filler with a particle size of 200–500 nm. Bio-based barrier latex, 30.0–50.0 parts Nonionic wetting agent 0.2-0.6 parts, 0.1 to 0.4 parts of silicone defoamer; The aqueous oxygen barrier middle layer does not contain soy protein, chitosan, or acetylenol surfactants.

7. The environmentally friendly packaging paper according to claim 1, characterized in that, The raw material composition of the water-based moisture-resistant heat-sealing composite surface layer, by weight, is as follows: Low-temperature heat-sealing acrylic copolymer emulsion, 75.0–85.0 parts. 3.0–12.0 parts deionized water 0.3-1.0 parts of polyurethane thickener Nonionic wetting agent 0.2-0.6 parts, 0.1 to 0.4 parts of silicone defoamer.

8. A method for preparing a single-sided, multi-layer, water-based, high-barrier, heat-sealable, environmentally friendly packaging paper as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S10. Coating preparation: Prepare water-based moisture-barrier undercoating liquid, water-based oxygen-barrier intermediate coating liquid and water-based moisture-barrier heat-sealing composite topcoat liquid respectively, and after curing, send them to the corresponding material tanks of the coating equipment for later use. S20, Base paper feeding: The food contact thin base paper is directly fed into the coating production line. The base paper does not undergo corona treatment, pre-drying, surface leveling and dehumidification pretreatment. S30. Coating and curing the moisture-proof underlayer: Coating the water-based moisture-proof underlayer coating liquid onto one side surface of the base paper substrate, and drying and curing it in a hot air drying zone of 110℃~150℃ for 40~100s. S40. Coating and curing the oxygen barrier intermediate layer: Coating the water-based oxygen barrier intermediate layer coating liquid onto the surface of the cured moisture barrier bottom layer, and drying and curing it in a hot air drying zone of 130℃~150℃ for 40~100s. S50. Coating and curing the moisture-proof heat-sealing composite surface layer: Coating the water-based moisture-proof heat-sealing composite surface layer coating liquid onto the surface of the cured oxygen-barrier intermediate layer, and drying and curing it in a hot air drying zone of 130℃~150℃ for 40~100s. S60. Winding and Printing: After the three-layer coating is dried and cured, the paper is continuously wound up. The original printing surface on the uncoated side of the base paper is used for offset printing, flexographic printing or gravure printing, and then cut to obtain the finished packaging paper.

9. The preparation method according to claim 8, characterized in that, The preparation method of the aqueous moisture-barrier undercoating liquid includes: Add deionized water to a stirred tank, and add nonionic wetting agent and silicone defoamer sequentially at 200-400 rpm, stirring for 3-8 minutes. Increase the speed to 600-1000 rpm, and add food-grade pure acrylic acid emulsion dropwise at a uniform rate for 10-20 minutes, stirring continuously for 15-25 minutes after the addition is complete. Maintain the speed of 600-1000 rpm and slowly add bio-based wax emulsion, stirring for 10-20 minutes until the system is homogeneous. Adjust the speed to 1000-1400 rpm, and add polyurethane thickener in portions, stirring for 20-40 minutes to adjust the viscosity. Add dilute ammonia to adjust the pH to 7.0-8.0, and stir for 5-15 minutes. Filter through an 80-120 mesh filter and allow to mature at room temperature in a sealed environment for 18-30 hours before use. The preparation method of the aqueous oxygen barrier intermediate coating solution includes: Add 75-85℃ warm water to a stirred tank, add polyvinyl alcohol powder, and dissolve it under low-speed stirring for 20-40 minutes until completely free of particles; cool to 35-45℃, add nonionic wetting agent and organosilicon defoamer at 500-700 rpm and stir for 3-8 minutes; increase the speed to 1200-1800 rpm, add nano-montmorillonite barrier filler in batches, and disperse at high speed for 30-50 minutes; reduce the speed to 600-800 rpm, slowly add bio-based barrier latex and stir for 20-30 minutes; add dilute ammonia water to adjust the pH to 7.0-8.0 and stir for 5-15 minutes; filter through a 120-180 mesh filter and mature at room temperature in a sealed container for 14-22 hours for later use. The preparation method of the aqueous moisture-resistant heat-sealing composite surface coating liquid includes: Add deionized water to a stirred tank, add nonionic wetting agent and silicone defoamer at 200-400 rpm and stir for 3-8 minutes; add low-temperature heat-sealing acrylic copolymer emulsion at 600-1000 rpm and stir for 15-25 minutes; adjust the speed to 1000-1400 rpm, add polyurethane thickener in batches and stir for 20-40 minutes to adjust the viscosity; add dilute ammonia to adjust the pH to 7.0-8.0; filter through an 80-120 mesh filter and mature at room temperature in a sealed environment for 18-30 hours for later use. The water-based moisture-barrier undercoating and water-based moisture-barrier heat-sealing composite topcoat have a solid content of 25%–40%, a viscosity of 100–300 cP at 25°C, and a pH of 7.0–8.0; the water-based oxygen-barrier intermediate coating has a solid content of 15%–25%, a viscosity of 50–250 cP at 25°C, and a pH of 7.0–8.

0.

10. The preparation method according to claim 8, characterized in that, The water-based moisture-barrier undercoat, water-based oxygen-barrier intermediate coating, and water-based moisture-barrier heat-sealing composite topcoat are applied by any one of roller coating, blade coating, or gravure coating. When roller coating is used, the viscosity of each coating layer at 25°C is controlled to be 80–250 cP. When blade coating is used, the viscosity of each coating layer at 25°C is controlled to be 150–300 cP. When gravure coating is used, the viscosity of each coating layer at 25°C is controlled to be 50–150 cP.

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