High-strength paper packaging material and preparation method thereof

Abstract

The invention discloses a high-strength paper packaging material and a preparation method thereof, and relates to the technical field of paper pulp molding.The preparation method comprises the following steps that waste paper is crushed and then mixed with bamboo fibers and nanocellulose for pulping, a chitosan solution, melamino-formaldehyde resin, gamma-aminopropyltriethoxysilane, alums and glycerinum are added to prepare forming slurry, and the forming slurry is prepared; performing vacuum adsorption to form a wet blank, preheating a hot press, putting the wet blank into a hot pressing mold, covering the wet blank with the polyvinyl alcohol-sodium alginate composite film, setting pressure and hot pressing time, and after hot pressing is finished, slowly relieving pressure to obtain a formed blank; and putting the formed blank into a blast oven, drying, taking out a dried sample, and standing at room temperature to balance moisture, so as to obtain the high-strength paper packaging material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of pulp molding, in particular to a high-strength paper packaging material and a preparation method thereof. BACKGROUND

[0002] With the transformation of the global packaging industry towards lightweight and green, the application demand of high-strength paper packaging materials in e-commerce logistics, precision instrument transportation and other fields has risen sharply, but the existing technical system faces multiple challenges in raw material adaptability, performance balance and process economy. Traditional paper packaging materials use wood pulp as the main raw material, which is not only limited by the sustainability of forest resources, but also has the inherent defect of insufficient fiber bonding strength, which leads to structural damage of the material in heavy load and impact resistance scenarios, making it difficult to meet the protection needs of heavy goods transportation. The packaging materials prepared from waste paper recycling have problems such as fiber aging and impurity residue, and even after conventional chemical treatment, their mechanical properties still significantly decrease, especially in key indicators such as breaking resistance and tensile strength, which are difficult to meet the high-strength application standards.

[0003] In the field of functional modification, although nano-enhanced technology is widely tried, the interface compatibility bottleneck between fillers and fiber matrix always exists - traditional modification process easily leads to nano-particle agglomeration, not only failing to achieve strength synergistic improvement, but also causing internal stress concentration of the material, resulting in decreased toughness. At the same time, the large use of chemical additives in existing enhancement processes not only increases production costs, but also may cause environmental problems such as formaldehyde release and difficulty in degradation, which is contrary to the current trend of green packaging development of "using paper instead of plastic". In addition, the traditional hot press forming process has inherent defects of high energy consumption and complex process, and the insufficient interface bonding strength of multi-layer composite structure also restricts the application of the material on high-speed automatic packaging lines.

[0004] Therefore, it is of great significance to invent a high-strength paper packaging material and a preparation method thereof. SUMMARY

[0005] The present application aims to provide a high-strength paper packaging material and a preparation method thereof to solve the problems in the prior art.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions: S1: Add waste paper fragments into a high-speed crusher, pour in deionized water, set the rotation speed to 3000 rpm, and crush for 30-35 min to obtain coarse paper pulp; filter the coarse paper pulp through an 80-mesh screen to remove impurities, collect the filtrate, add bamboo fibers and nano-cellulose to the filtrate, and transfer to a beater, add deionized water, set the beater rotation speed to 800 rpm, and beat for 30-35 min to obtain a uniform mixed fiber slurry; S2: add chitosan into deionized water, drop acetic acid to adjust pH to 4.9-5.1, 500 rpm magnetic stirring for 20-25 min until completely dissolved to obtain chitosan solution; add chitosan solution into the uniformly mixed fiber pulp, 400 rpm magnetic stirring for 20-25 min, then add melamine formaldehyde resin, gamma-aminopropyl triethoxysilane in turn, continue to stir for 15 min after each addition, finally add alum, glycerol, make up with deionized water, stir for 25-30 min to obtain the molding slurry; S3: clean the mold of the vacuum adsorption molding machine and lay a layer of 80 mesh filter screen, take the molding slurry and pour it evenly into the mold, start the vacuum system, set the vacuum degree to 0.08 MPa, adsorption time 5 min, make the slurry form tightly under negative pressure to obtain the wet blank; S4: add polyvinyl alcohol into deionized water, stir at 600 rpm in a 90-95℃ constant temperature water bath for 40-45 min until completely dissolved, cool to 60℃ to obtain polyvinyl alcohol solution; mix sodium alginate and deionized water, 500 rpm magnetic stirring at room temperature for 30-35 min until completely dissolved to obtain sodium alginate solution; mix polyvinyl alcohol solution and sodium alginate solution, 400 rpm magnetic stirring for 20-25 min, then slowly drop glutaraldehyde, continue to stir for 30-35 min to obtain the composite film solution; pour the composite film solution into a horizontally placed glass dish, flatten it with a spatula, stand at room temperature for 1 h to remove bubbles, put the culture dish into a blast oven, set the temperature to 50℃, dry for 3 h until the film is completely solidified and wrinkle-free, then peel off the glass dish to obtain the polyvinyl alcohol-sodium alginate composite film; S5: preheat the hot press to 120℃, put the wet blank into the hot press mold, cover the polyvinyl alcohol-sodium alginate composite film on the wet blank, set the pressure to 5 MPa, hot pressing time 20 min, after hot pressing, slowly release the pressure to obtain the shaped blank; put the shaped blank into a blast oven, set the temperature to 60℃, dry for 2 h until the water content is ≤8%, take out the dried sample, stand at room temperature for 24 h to balance the moisture, to obtain the high-strength paper packaging material.

[0007] Further, in the coarse paper pulp, the proportion of each component is 49-51 parts by mass, deionized water 196-204 parts; in the uniformly mixed fiber pulp, the proportion of each component is 245-255 parts by mass of coarse paper pulp, 19.6-20.4 parts of bamboo fiber, 4.9-5.1 parts of nano cellulose, 98-102 parts of deionized water.

[0008] Furthermore, in the chitosan solution, the proportions of each component by mass are as follows: chitosan 0.49–0.51 parts, deionized water 9.8–10.2 parts, and glacial acetic acid 0.059–0.061 parts; in the molding slurry, the proportions of each component by mass are as follows: uniformly mixed fiber slurry 318–332 parts, chitosan solution 10.3–10.7 parts, melamine-formaldehyde resin 5.6–5.8 parts, γ-aminopropyltriethoxysilane 2.84–2.96 parts, alum 1.96–2.04 parts, glycerol 6.2–6.4 parts, and deionized water 98–102 parts.

[0009] Furthermore, in the polyvinyl alcohol solution, the proportions of each component by mass are: polyvinyl alcohol 9.8–10.2 parts, deionized water 98–102 parts; in the sodium alginate solution, the proportions of each component by mass are: sodium alginate 9.8–10.2 parts, deionized water 98–102 parts; in the composite membrane solution, the proportions of each component by mass are: polyvinyl alcohol solution 108–112 parts, sodium alginate solution 108–112 parts, glutaraldehyde 2.16–2.24 parts.

[0010] Compared with the prior art, the beneficial effects of the present invention are: In this invention, waste paper fiber is used as the matrix. It is widely available and low in cost, but its strength is limited. Bamboo fiber has high crystallinity and aspect ratio. Its rigid molecular chains can serve as a "reinforcing skeleton". Through interweaving and entanglement with waste paper fiber, it reduces stress concentration when a single fiber breaks. Nanocellulose has a large specific surface area and can fill the micron-level gaps between waste paper fiber and bamboo fiber. It combines with the matrix fiber through a large number of hydrogen bonds and van der Waals forces to form a dense structure of "macro framework-micro filler", which improves the load-bearing capacity of the fiber network.

[0011] In this invention, chitosan molecules contain a large number of amino and hydroxyl groups, which form hydrogen bonds with the hydroxyl groups on the fiber surface. At the same time, the small number of unremoved acetyl groups in its molecular chain are hydrophobic, which can reduce the penetration channels of water molecules in the fiber gaps. Melamine-formaldehyde resin undergoes a cross-linking reaction during hot pressing to form a three-dimensional network structure. This structure is insoluble in water and can coat the fiber surface, blocking water molecules from contacting the fiber. At the same time, melamine-formaldehyde resin and the amino groups of chitosan can undergo a condensation reaction, enhancing the overall integrity of the water-resistant network. One end of the γ-aminopropyltriethoxysilane molecule is a siloxane group, which can undergo hydrolytic condensation with the hydroxyl groups on the nanocellulose or fiber surface to form covalent bonds. The other end is an amino group, which can form hydrogen bonds with the hydroxyl groups of polyvinyl alcohol and the ether bonds of melamine-formaldehyde resin. This solves the problem of weak bonding caused by the incompatibility between inorganic fibers and organic adhesives in traditional technologies, thus improving the interfacial shear strength.

[0012] In this invention, the Al³⁺ generated by the dissociation of alum is a high-valence metal ion with strong coordination ability. It can form stable coordination bonds or ionic bonds with substances containing polar groups such as cellulose, starch, and chitosan in the system, laying the foundation for building a cross-linked network. Glycerol, as a plasticizer, can insert between the molecular chains of polyvinyl alcohol and chitosan, weakening the intermolecular forces, increasing the mobility of chain segments, avoiding the increase in brittleness of the material due to excessive cross-linking, and improving the elongation at break of the material.

[0013] In this invention, the polyvinyl alcohol-sodium alginate composite film contains a large number of hydroxyl groups, which can form hydrogen bonds with the amino groups of chitosan and undergo condensation reactions with the hydroxyl groups of melamine-formaldehyde resin; the carboxyl groups of sodium alginate can be bonded to the amino groups of chitosan through ionic bonds, and at the same time form hydrogen bonds with the amino groups of γ-aminopropyltriethoxysilane; during the hot pressing process, the film and the additives on the surface of the wet blank undergo synergistic cross-linking to form a continuous "film-substrate" bonding layer, which further improves the water resistance and surface wear resistance of the material.

[0014] In this invention, high pressure allows the fibers remaining in the wet blank to come into closer contact, increasing the hydrogen bonding points between fibers. At the same time, the rigid structure of bamboo fiber and nanocellulose forms a supporting skeleton under pressure, preventing the material from losing toughness due to excessive compression. High temperature promotes the complete cross-linking of melamine-formaldehyde resin, forming a stable three-dimensional network. It also softens polyvinyl alcohol and fills the gaps between fibers. After cooling, it solidifies into an "adhesive layer," firmly binding the dispersed fibers and additives into a whole, solving the problem of insufficient use of additives in traditional processes. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] In the following examples, polyvinyl alcohol, Mw=45000, CAS9002-89-5; chitosan, Mw=150000, CAS9012-76-4; melamine-formaldehyde resin, Mw=1600, CAS9003-08-1; bamboo fiber, length 0.5-1mm; nanocellulose, length 1-40μm; and other raw materials were commercially available.

[0017] Example 1: A method for preparing a high-strength paper packaging material: S1: Add 49 parts of waste paper fragments to a high-speed shredder, pour in 196 parts of deionized water, set the speed to 3000 rpm, and shred for 30 minutes to obtain coarse pulp; filter 245 parts of coarse pulp through an 80-mesh sieve to remove impurities, collect the filtrate, add 19.6 parts of bamboo fiber and 4.9 parts of nanocellulose to the filtrate, transfer to a pulper, add 98 parts of deionized water, set the pulping speed to 800 rpm, and pulp for 30 minutes to obtain a uniformly mixed fiber pulp; S2: Add 0.49 parts chitosan to 9.8 parts deionized water, add 0.059 parts glacial acetic acid to adjust the pH to 4.9, and stir magnetically at 500 rpm for 20 minutes until completely dissolved to obtain a chitosan solution; add 10.3 parts chitosan solution to 318 parts uniformly mixed fiber pulp, stir magnetically at 400 rpm for 20 minutes, then add 5.6 parts melamine-formaldehyde resin and 2.84 parts γ-aminopropyltriethoxysilane in sequence, and continue stirring for 15 minutes after each addition; finally add 1.96 parts alum and 6.2 parts glycerol, add 98 parts deionized water, and stir for 25 minutes to obtain the molding slurry; S3: Clean the mold of the vacuum adsorption molding machine and lay a layer of 80-mesh filter screen. Pour the molding slurry evenly into the mold, start the vacuum system, set the vacuum degree to 0.08MPa and the adsorption time to 5min, so that the slurry is tightly formed under negative pressure to obtain a wet blank. S4: Add 9.8 parts of polyvinyl alcohol to 98 parts of deionized water, stir at 600 rpm for 40 min in a 90℃ constant temperature water bath until completely dissolved, and cool to 60℃ to obtain a polyvinyl alcohol solution; mix 9.8 parts of sodium alginate and 98 parts of deionized water, stir magnetically at 500 rpm for 30 min at room temperature until completely dissolved to obtain a sodium alginate solution; mix 108 parts of polyvinyl alcohol solution and 108 parts of sodium alginate solution, stir magnetically at 400 rpm for 20 min, then slowly add 2.16 parts of glutaraldehyde, and continue stirring for 30 min to obtain a composite membrane solution; pour the composite membrane solution into a horizontally placed glass dish, level it with a spatula, let it stand at room temperature for 1 h to remove air bubbles, place the culture dish in a forced-air drying oven, set the temperature to 50℃, and dry for 3 h until the membrane is completely cured and wrinkle-free, remove it and peel off the glass dish to obtain a polyvinyl alcohol-sodium alginate composite membrane; S5: Preheat the hot press to 120℃, place the wet blank into the hot press mold, cover the wet blank with the polyvinyl alcohol-sodium alginate composite film, set the pressure to 5MPa, and the hot pressing time to 20min. After the hot pressing is completed, slowly release the pressure to obtain the shaped blank. Place the shaped blank into a forced-air drying oven, set the temperature to 60℃, and dry for 2h until the moisture content is ≤8%. Take out the dried sample and place it at room temperature for 24h to balance the moisture to obtain high-strength paper packaging material.

[0018] Example 2: A method for preparing a high-strength paper packaging material: S1: Add 50 parts of waste paper fragments to a high-speed shredder, pour in 199 parts of deionized water, set the speed to 3000 rpm, and shred for 32 minutes to obtain coarse pulp; filter 248 parts of coarse pulp through an 80-mesh sieve to remove impurities, collect the filtrate, add 19.8 parts of bamboo fiber and 5 parts of nanocellulose to the filtrate, transfer to a pulper, add 99 parts of deionized water, set the pulping speed to 800 rpm, and pulp for 32 minutes to obtain a uniformly mixed fiber pulp; S2: Add 0.5 parts chitosan to 10 parts deionized water, add 0.06 parts glacial acetic acid to adjust the pH to 4.9, and stir magnetically at 500 rpm for 22 min until completely dissolved to obtain a chitosan solution; add 10.4 parts chitosan solution to 324 parts uniformly mixed fiber pulp, stir magnetically at 400 rpm for 22 min, then add 5.7 parts melamine-formaldehyde resin and 2.87 parts γ-aminopropyltriethoxysilane in sequence, and continue stirring for 15 min after each addition; finally add 1.99 parts alum and 6.3 parts glycerol, add 100 parts deionized water, and stir for 27 min to obtain the molding slurry; S3: Clean the mold of the vacuum adsorption molding machine and lay a layer of 80-mesh filter screen. Pour the molding slurry evenly into the mold, start the vacuum system, set the vacuum degree to 0.08MPa and the adsorption time to 5min, so that the slurry is tightly formed under negative pressure to obtain a wet blank. S4: Add 10 parts of polyvinyl alcohol to 100 parts of deionized water, stir at 600 rpm for 42 min in a 90℃ constant temperature water bath until completely dissolved, and cool to 60℃ to obtain a polyvinyl alcohol solution; mix 10 parts of sodium alginate and 100 parts of deionized water, and stir magnetically at 500 rpm for 32 min at room temperature until completely dissolved to obtain a sodium alginate solution; mix 110 parts of polyvinyl alcohol solution and 110 parts of sodium alginate solution, stir magnetically at 400 rpm for 22 min, then slowly add 2.18 parts of glutaraldehyde, and continue stirring for 32 min to obtain a composite membrane solution; pour the composite membrane solution into a horizontally placed glass dish, smooth it with a spatula, let it stand at room temperature for 1 h to remove air bubbles, place the culture dish in a forced-air drying oven, set the temperature to 50℃, and dry for 3 h until the membrane is completely cured and wrinkle-free, remove it and peel off the glass dish to obtain a polyvinyl alcohol-sodium alginate composite membrane; S5: Preheat the hot press to 120℃, place the wet blank into the hot press mold, cover the wet blank with the polyvinyl alcohol-sodium alginate composite film, set the pressure to 5MPa, and the hot pressing time to 20min. After the hot pressing is completed, slowly release the pressure to obtain the shaped blank. Place the shaped blank into a forced-air drying oven, set the temperature to 60℃, and dry for 2h until the moisture content is ≤8%. Take out the dried sample and place it at room temperature for 24h to balance the moisture to obtain high-strength paper packaging material.

[0019] Example 3: A method for preparing a high-strength paper packaging material: S1: Add 50 parts of waste paper fragments to a high-speed shredder, pour in 202 parts of deionized water, set the speed to 3000 rpm, and shred for 34 minutes to obtain coarse pulp; filter 253 parts of coarse pulp through an 80-mesh sieve to remove impurities, collect the filtrate, add 20.1 parts of bamboo fiber and 5 parts of nanocellulose to the filtrate, transfer to a pulper, add 101 parts of deionized water, set the pulping speed to 800 rpm, and pulp for 34 minutes to obtain a uniformly mixed fiber pulp; S2: Add 0.5 parts chitosan to 10.1 parts deionized water, add 0.06 parts glacial acetic acid to adjust the pH to 5, and stir magnetically at 500 rpm for 24 min until completely dissolved to obtain a chitosan solution; add 10.6 parts chitosan solution to 328 parts uniformly mixed fiber pulp, stir magnetically at 400 rpm for 24 min, then add 5.7 parts melamine-formaldehyde resin and 2.91 parts γ-aminopropyltriethoxysilane in sequence, and continue stirring for 15 min after each addition; finally add 2.02 parts alum and 6.3 parts glycerol, add 100 parts deionized water, and stir for 29 min to obtain the molding slurry; S3: Clean the mold of the vacuum adsorption molding machine and lay a layer of 80-mesh filter screen. Pour the molding slurry evenly into the mold, start the vacuum system, set the vacuum degree to 0.08MPa and the adsorption time to 5min, so that the slurry is tightly formed under negative pressure to obtain a wet blank. S4: Add 10.2 parts of polyvinyl alcohol to 102 parts of deionized water, stir at 600 rpm for 44 min in a 93℃ constant temperature water bath until completely dissolved, and cool to 60℃ to obtain a polyvinyl alcohol solution; mix 10.2 parts of sodium alginate and 102 parts of deionized water, and stir magnetically at 500 rpm for 34 min at room temperature until completely dissolved to obtain a sodium alginate solution; mix 112 parts of polyvinyl alcohol solution and 112 parts of sodium alginate solution, stir magnetically at 400 rpm for 24 min, then slowly add 2.23 parts of glutaraldehyde, and continue stirring for 34 min to obtain a composite membrane solution; pour the composite membrane solution into a horizontally placed glass dish, level it with a spatula, let it stand at room temperature for 1 h to remove air bubbles, place the culture dish in a forced-air drying oven, set the temperature to 50℃, and dry for 3 h until the membrane is completely cured and wrinkle-free, remove it and peel off the glass dish to obtain a polyvinyl alcohol-sodium alginate composite membrane; S5: Preheat the hot press to 120℃, place the wet blank into the hot press mold, cover the wet blank with the polyvinyl alcohol-sodium alginate composite film, set the pressure to 5MPa, and the hot pressing time to 20min. After the hot pressing is completed, slowly release the pressure to obtain the shaped blank. Place the shaped blank into a forced-air drying oven, set the temperature to 60℃, and dry for 2h until the moisture content is ≤8%. Take out the dried sample and place it at room temperature for 24h to balance the moisture to obtain high-strength paper packaging material.

[0020] Example 4: A method for preparing a high-strength paper packaging material: S1: Add 51 parts of waste paper fragments to a high-speed shredder, pour in 204 parts of deionized water, set the speed to 3000 rpm, and shred for 35 minutes to obtain coarse pulp; filter 255 parts of coarse pulp through an 80-mesh sieve to remove impurities, collect the filtrate, add 20.4 parts of bamboo fiber and 5.1 parts of nanocellulose to the filtrate, transfer to a pulper, add 102 parts of deionized water, set the pulping speed to 800 rpm, and pulp for 35 minutes to obtain a uniformly mixed fiber pulp; S2: Add 0.51 parts chitosan to 10.2 parts deionized water, add 0.061 parts glacial acetic acid to adjust the pH to 5.1, and stir magnetically at 500 rpm for 25 min until completely dissolved to obtain a chitosan solution; add 10.7 parts chitosan solution to 332 parts uniformly mixed fiber pulp, stir magnetically at 400 rpm for 25 min, then add 5.8 parts melamine-formaldehyde resin and 2.96 parts γ-aminopropyltriethoxysilane in sequence, and continue stirring for 15 min after each addition; finally add 2.04 parts alum and 6.4 parts glycerol, add 102 parts deionized water, and stir for 30 min to obtain the molding slurry; S3: Clean the mold of the vacuum adsorption molding machine and lay a layer of 80-mesh filter screen. Pour the molding slurry evenly into the mold, start the vacuum system, set the vacuum degree to 0.08MPa and the adsorption time to 5min, so that the slurry is tightly formed under negative pressure to obtain a wet blank. S4: Add 10.2 parts of polyvinyl alcohol to 102 parts of deionized water, stir at 600 rpm for 45 min in a 95℃ constant temperature water bath until completely dissolved, and cool to 60℃ to obtain a polyvinyl alcohol solution; mix 10.2 parts of sodium alginate and 102 parts of deionized water, stir magnetically at 500 rpm for 35 min at room temperature until completely dissolved to obtain a sodium alginate solution; mix 112 parts of polyvinyl alcohol solution and 112 parts of sodium alginate solution, stir magnetically at 400 rpm for 25 min, then slowly add 2.24 parts of glutaraldehyde, and continue stirring for 35 min to obtain a composite membrane solution; pour the composite membrane solution into a horizontally placed glass dish, smooth it with a spatula, let it stand at room temperature for 1 h to remove air bubbles, place the culture dish in a forced-air drying oven, set the temperature to 50℃, and dry for 3 h until the membrane is completely cured and wrinkle-free, remove it and peel off the glass dish to obtain a polyvinyl alcohol-sodium alginate composite membrane; S5: Preheat the hot press to 120℃, place the wet blank into the hot press mold, cover the wet blank with the polyvinyl alcohol-sodium alginate composite film, set the pressure to 5MPa, and the hot pressing time to 20min. After the hot pressing is completed, slowly release the pressure to obtain the shaped blank. Place the shaped blank into a forced-air drying oven, set the temperature to 60℃, and dry for 2h until the moisture content is ≤8%. Take out the dried sample and place it at room temperature for 24h to balance the moisture to obtain high-strength paper packaging material.

[0021] Comparative Example 1: A method for preparing a high-strength paper packaging material: S1: 49 parts of waste paper fragments were added to a high-speed shredder, 196 parts of deionized water were added, the speed was set to 3000 rpm, and the shredding was carried out for 30 minutes to obtain coarse pulp; 245 parts of coarse pulp were filtered through an 80-mesh sieve to remove impurities, the filtrate was collected, 19.6 parts of bamboo fiber were added to the filtrate, the mixture was transferred to a pulper, 98 parts of deionized water were added, the pulping speed was set to 800 rpm, and the pulping was carried out for 30 minutes to obtain a uniformly mixed fiber pulp; The remaining steps are the same as in Example 1.

[0022] Comparative Example 2: A method for preparing a high-strength paper packaging material: S2: Add 0.49 parts of chitosan to 9.8 parts of deionized water, add 0.059 parts of glacial acetic acid to adjust the pH to 4.9, and stir magnetically at 500 rpm for 20 min until completely dissolved to obtain a chitosan solution; add 10.3 parts of chitosan solution to 318 parts of uniformly mixed fiber pulp, stir magnetically at 400 rpm for 20 min, then add 5.6 parts of melamine-formaldehyde resin and 2.84 parts of γ-aminopropyltriethoxysilane in sequence, and continue stirring for 15 min after each addition; finally add 1.96 parts of alum, add 98 parts of deionized water, and stir for 25 min to obtain a molding slurry; The remaining steps are the same as in Example 1.

[0023] Comparative Example 3: A method for preparing a high-strength paper packaging material: S5: Preheat the hot press to 120°C, place the wet blank into the hot press mold, cover the wet blank with a polyvinyl alcohol-sodium alginate composite film, set the pressure to 3MPa, and the hot pressing time to 20min. After the hot pressing is completed, slowly release the pressure to obtain the shaped blank; place the shaped blank into a forced-air drying oven, set the temperature to 60°C, and dry for 2h until the moisture content is ≤8%. Take out the dried sample and place it at room temperature for 24h to balance the moisture to obtain a high-strength paper packaging material.

[0024] The remaining steps are the same as in Example 1.

[0025] experiment: The tensile strength of paper materials was tested according to the GB / T 12914-2018 standard.

[0026] The water resistance of paper materials was tested according to the GB / T 22897-2008 standard.

[0027] Table 1 Performance Test Data of High-Strength Paper Packaging Materials

[0028] Conclusion: The high-strength paper packaging material prepared by this invention has excellent mechanical properties and water resistance.

[0029] In Comparative Example 1, no nanocellulose was added to the filtrate, resulting in the loss of the rigid framework effect of nanocellulose, a decrease in the tensile strength of the material, and a tendency for swelling and deformation in humid environments due to its loose structure and insufficient cross-linking. In Comparative Example 2, no glycerol was added, preventing the chain segments from extending and leading to increased brittleness and decreased tensile strength. Furthermore, during hot pressing, the flow of glycerol could not fill the voids, increasing the internal porosity of the material and allowing water molecules to penetrate more easily. In Comparative Example 3, the reduced hot pressing pressure prevented the expulsion of air from the material, resulting in increased porosity. Additionally, the actual contact area between the polyvinyl alcohol-sodium alginate composite membrane and the wet blank decreased, creating interfacial voids in the non-contact areas, making delamination more likely.

[0030] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a high-strength paper packaging material, characterized in that: Includes the following steps: Preheat the hot press, place the wet blank into the mold, cover the wet blank with a polyvinyl alcohol-sodium alginate composite film, and hot press to obtain a molded blank; dry the molded blank, take out the sample, and place it at room temperature to obtain a high-strength paper packaging material.

2. The method for preparing a high-strength paper packaging material according to claim 1, characterized in that: The preparation method of the polyvinyl alcohol-sodium alginate composite membrane includes the following steps: Polyvinyl alcohol was added to deionized water and stirred at 90–95°C for 40–45 min, then cooled to 60°C to obtain a polyvinyl alcohol solution. Sodium alginate and deionized water were mixed and magnetically stirred at room temperature for 30–35 min to obtain a sodium alginate solution. The polyvinyl alcohol solution and sodium alginate solution were mixed and magnetically stirred for 20–25 min, then glutaraldehyde was added dropwise, and stirring was continued for 30–35 min to obtain a composite membrane solution. The composite membrane solution was poured into a glass dish, leveled with a spatula, allowed to stand at room temperature, and dried to obtain a polyvinyl alcohol-sodium alginate composite membrane.

3. The method for preparing a high-strength paper packaging material according to claim 2, characterized in that: In the polyvinyl alcohol solution, the proportions of each component by mass are: polyvinyl alcohol 9.8–10.2 parts, deionized water 98–102 parts; in the sodium alginate solution, the proportions of each component by mass are: sodium alginate 9.8–10.2 parts, deionized water 98–102 parts; in the composite membrane solution, the proportions of each component by mass are: polyvinyl alcohol solution 108–112 parts, sodium alginate solution 108–112 parts, glutaraldehyde 2.16–2.24 parts.

4. The method for preparing a high-strength paper packaging material according to claim 1, characterized in that: The method for preparing the wet blank includes the following steps: The slurry is poured evenly into the mold and vacuum-adsorbed, so that the slurry is tightly formed under negative pressure to obtain a wet blank.

5. The method for preparing a high-strength paper packaging material according to claim 4, characterized in that: The method for preparing the molding slurry includes the following steps: Add chitosan to deionized water, adjust the pH to 4.9–5.1 by adding glacial acetic acid dropwise, and stir magnetically for 20–25 minutes to obtain a chitosan solution. Add the chitosan solution to the uniformly mixed fiber slurry, stir magnetically for 20–25 minutes, then add melamine-formaldehyde resin and γ-aminopropyltriethoxysilane in sequence, continuing to stir after each addition. Finally, add alum and glycerin, replenish with deionized water, and stir for 25–30 minutes to obtain the molding slurry.

6. The method for preparing a high-strength paper packaging material according to claim 5, characterized in that: In the chitosan solution, the components, by mass parts, are: chitosan 0.49–0.51 parts, deionized water 9.8–10.2 parts, and glacial acetic acid 0.059–0.061 parts; in the molding slurry, the components, by mass parts, are: uniformly mixed fiber slurry 318–332 parts, chitosan solution 10.3–10.7 parts, melamine-formaldehyde resin 5.6–5.8 parts, γ-aminopropyltriethoxysilane 2.84–2.96 parts, alum 1.96–2.04 parts, glycerol 6.2–6.4 parts, and deionized water 98–102 parts.

7. The method for preparing a high-strength paper packaging material according to claim 5, characterized in that: The method for preparing the uniformly mixed fiber pulp includes the following steps: Waste paper fragments are mixed with deionized water and crushed for 30-35 minutes to obtain coarse pulp. The coarse pulp is sieved to remove impurities and the filtrate is collected. Bamboo fiber and nanocellulose are added to the filtrate, and deionized water is added. The pulp is beaten for 30-35 minutes to obtain a uniformly mixed fiber pulp.

8. The method for preparing a high-strength paper packaging material according to claim 7, characterized in that: In the coarse pulp, the components are expressed in the following proportions by mass: 49-51 parts waste paper and 196-204 parts deionized water. In the uniformly mixed fiber pulp, the components are expressed in the following proportions by mass: 245-255 parts coarse pulp, 19.6-20.4 parts bamboo fiber, 4.9-5.1 parts nanocellulose, and 98-102 parts deionized water.

9. A high-strength paper packaging material prepared by a method for preparing a high-strength paper packaging material according to any one of claims 1-8.

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