Seven-layer corrugated board and production process thereof

Abstract

This invention provides a seven-layer corrugated board and its manufacturing process, comprising an outer protective layer, an outer core paper layer, an outer corrugated layer, an intermediate support layer, an inner corrugated layer, an inner core paper layer, and an inner protective layer stacked sequentially; wherein, the intermediate support layer is made by coating, drying, and curing an intermediate support layer slurry, and by weight, the intermediate support layer slurry comprises: 20-28 parts of enzyme-modified sisal pulp, 8-10 parts of basalt fiber, 25-30 parts of cross-linked oxidized starch, and water-based... The composition comprises: 10-12 parts polyurethane, 2-3 parts chitosan, 1.5-2.0 parts hydroxyl fluorosilicone oil, 1.0-1.5 parts nano-titanium dioxide, 3-4 parts 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 1.0-1.5 parts fumed nano-silica, 1.0-1.5 parts γ-methacryloyloxypropyltrimethoxysilane, 5-7 parts quaternary ammonium salt polyurethane-montmorillonite composite emulsion, and 25-35 parts deionized water. The components in this invention work synergistically to give the seven-layer corrugated board excellent mechanical properties, water resistance, mildew and antibacterial properties, flame retardant properties, and structural stability.

Description

Technical Field

[0001] This invention relates to the field of corrugated board technology, specifically to a seven-layer corrugated board and its manufacturing process. Background Technology

[0002] Corrugated sheets, as lightweight and high-strength packaging and building auxiliary materials, are widely used in logistics and transportation, building envelope, and other fields. Their comprehensive performance is determined by the substrate structure, adhesive system, and functional modification effects. In existing technologies, to improve the comprehensive performance of corrugated sheets, functional fillers are often added to modify the substrate or adhesive. In some scenarios, auxiliary materials such as thermoplastic polyolefin waterproof membranes are also used to enhance the protective effect. However, these modification methods are mostly single-function oriented and it is difficult to achieve a synergistic improvement in mechanical support performance and comprehensive protective performance. Moreover, traditional substrates often rely on single fibers or synthetic materials, and their structural stability is easily affected by environmental factors.

[0003] With the expansion of application scenarios, the requirements for mechanical properties, water resistance, and structural stability of corrugated sheets are gradually increasing. Even with simple filler additions to optimize performance, existing corrugated sheets still suffer from insufficient interfacial adhesion strength between the substrate and the adhesive, resulting in inadequate flexural and impact resistance, making them unable to withstand external loads under complex working conditions. Simultaneously, the traditional water-resistant design is incomplete, with insufficient adhesive layer density and a substrate prone to water absorption, making the corrugated sheets susceptible to delamination in humid environments and significantly reducing mechanical strength. This makes it impossible to balance mechanical reliability and water resistance stability, thus limiting the application of corrugated sheets. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention provides a seven-layer corrugated board and its manufacturing process.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention discloses a seven-layer corrugated board, comprising, in sequence, an outer protective layer, an outer core paper layer, an outer corrugated layer, an intermediate support layer, an inner corrugated layer, an inner core paper layer, and an inner protective layer; wherein, the intermediate support layer is made by coating, drying, and curing an intermediate support layer slurry, and by weight, the intermediate support layer slurry comprises: 20-28 parts of enzyme-modified sisal pulp, 8-10 parts of basalt fiber, 25-30 parts of cross-linked oxidized starch, 10-12 parts of waterborne polyurethane, 2-3 parts of chitosan, 1.5-2.0 parts of hydroxyl fluorosilicone oil, 1.0-1.5 parts of nano-titanium dioxide, 3-4 parts of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), 1.0-1.5 parts of fumed nano-silica, and γ-methacryloyloxypropyltrimethoxysilane (KH-570). 1.0-1.5 parts, 5-7 parts of quaternary ammonium salt polyurethane montmorillonite composite emulsion, and 25-35 parts of deionized water.

[0007] Using the above technical solutions, DOPO can improve the flame retardant performance of corrugated sheets; enzymatically modified sisal pulp and basalt fiber intertwine to form a substrate support skeleton, providing basic mechanical support for the corrugated sheets; cross-linked oxidized starch and waterborne polyurethane serve as the main adhesives, ensuring the film-forming properties, bonding strength, and flexibility of the adhesive layer, while chitosan assists in enhancing the adhesive layer's bonding effect; hydroxyl fluorosilicone oil can improve the overall water resistance of the corrugated sheets; nano-titanium dioxide can fill the gaps between the substrate and the adhesive, improving the corrugated sheets' flexural strength and impact resistance; fumed nano-silica can improve the suspension of the slurry and the density of the adhesive layer, assisting in optimizing mechanical and water resistance properties; KH-570 can connect inorganic fillers and organic adhesives, improving interfacial bonding; quaternary ammonium salt polyurethane montmorillonite composite emulsion can improve the anti-mildew and antibacterial properties, barrier properties, and flexibility of the corrugated sheets; the various components work together to give the seven-layer corrugated sheets excellent mechanical properties, water resistance, anti-mildew and antibacterial properties, flame retardant properties, and structural stability.

[0008] Preferably, the basalt fiber is a short-cut fiber with a length of 6-8 mm; the solid content of the waterborne polyurethane is 38%-42%; the degree of deacetylation of the chitosan is ≥90% and the molecular weight is 100,000-200,000 Da; the viscosity of the hydroxyl fluorosilicone oil at 25°C is 500-600 mPa·s; and the average particle size of the nano-titanium dioxide is 10-20 nm.

[0009] Using the above technical solutions, chopped basalt fibers can interweave with substrate components such as enzyme-modified silane-modified sisal pulp to form a supporting structure, improving the mechanical strength of the seven-layer corrugated board; waterborne polyurethane, with its suitable solid content, can enhance the flexibility and water resistance of the adhesive layer and improve the bonding effect between the adhesive layer and the substrate; chitosan with a deacetylation degree ≥90% and a molecular weight of 100,000-200,000 Da can strengthen the adhesive performance of the adhesive layer and also has a certain anti-mildew and antibacterial effect; hydroxyl fluorosilicone oil with a viscosity of 500-600 mPa·s at 25℃ can form a stable hydrophobic layer on the substrate surface, improving the water resistance of the corrugated board; nano-titanium dioxide with an average particle size of 10-20 nm can fill the gaps between the substrate and the adhesive, improving the bending and impact resistance of the corrugated board; the synergistic regulation of each component effectively improves the overall performance of the corrugated board.

[0010] Preferably, the raw materials for preparing enzyme-modified sisal pulp, by weight, include: 90-100 parts of sisal fiber pulp, 0.3-0.5 parts of laccase, 0.05-0.1 parts of sodium pyrophosphate, 0.1-0.2 parts of ammonium persulfate, and 3-4 parts of vinyltrimethoxysilane.

[0011] Using the above technical solution, sisal fiber pulp serves as the basic raw material, providing a fiber support framework for the formation of enzymatically modified sisal pulp. Laccase can oxidize the phenolic hydroxyl groups in lignin and cellulose on the surface of sisal fibers, generating active groups and providing reaction sites for subsequent grafting reactions. Sodium pyrophosphate can stabilize the reaction system, ensuring the orderly progress of enzymatic oxidation and grafting reactions. Ammonium persulfate can initiate a graft copolymerization reaction between vinyltrimethoxysilane and free radicals on the fiber surface, forming covalent bonds. Vinyltrimethoxysilane can introduce siloxane groups on the fiber surface through grafting reactions, improving the bonding effect between the fiber and the adhesive and crosslinking agent. The synergistic effect of each raw material enables the prepared enzymatically modified sisal pulp to possess both good structural stability and interfacial bonding performance.

[0012] Preferably, the preparation method of enzyme-catalyzed silane-modified sisal pulp includes the following steps:

[0013] 1) Adjust the pH of the sisal fiber slurry to 4.3-4.7 with 0.10-0.15 mol / L acetate-sodium acetate buffer solution, add laccase and sodium pyrophosphate, and stir the reaction at 200-300 r / min for 60-90 min at 35-40℃. During the reaction, oxygen is continuously introduced and the oxygen flow rate is controlled at 0.5-1.0 L / min.

[0014] 2) Adjust the pH of the system obtained in step 1) to 6.5-7.0 with 0.04-0.08 mol / L sodium hydroxide solution, add ammonium persulfate, heat to 50-55℃ under nitrogen protection, add vinyltrimethoxysilane, and stir the reaction at 400-500 r / min for 90-120 min.

[0015] 3) Place the reaction solution obtained in step 2) under vacuum degassing conditions of 60-65℃ and vacuum degree of -0.085MPa to -0.095MPa for 20-40 minutes. During vacuum degassing, stir continuously at a speed of 200-300r / min to obtain enzyme-modified sisal pulp.

[0016] Using the above technical solution, the acetate-sodium acetate buffer solution maintains the pH stability of the reaction system, creating a suitable environment for the laccase-catalyzed reaction. Laccase oxidizes the surface groups of sisal fibers with the participation of oxygen to generate active structures, and sodium pyrophosphate ensures the stability of the reaction system. Sodium hydroxide solution adjusts the pH to the neutral range, providing conditions for the graft copolymerization of vinyltrimethoxysilane and active fiber groups initiated by ammonium persulfate. Nitrogen protection can reduce the loss of active ingredients during the reaction and promote the formation of covalent bonds. Subsequent vacuum degassing combined with stirring can remove bubbles in the reaction solution and improve the uniformity and structural stability of the enzymatically modified silane sisal pulp.

[0017] Preferably, the raw materials for preparing the quaternary ammonium salt polyurethane montmorillonite composite emulsion, by weight, include: 8-12 parts sodium montmorillonite, 45-55 parts polypropylene glycol, 20-30 parts isophorone diisocyanate, 0.05-0.1 parts organotin catalyst, 3-4 parts N-methyldiethanolamine, 1.5-2.0 parts glacial acetic acid with a concentration of 0.10-0.15 mol / L, and 60-80 parts deionized water.

[0018] Using the above technical solution, sodium-based montmorillonite, relying on its layered structure, can enhance the barrier and mechanical properties of the system, and form a stable bond with other components through charge interaction; polypropylene glycol provides flexible support for polyurethane molecular chain segments, improving the toughness and film-forming properties of the composite emulsion; isophorone diisocyanate, as a crosslinking monomer, participates in the formation of polyurethane prepolymer, laying the foundation for increasing the crosslinking density of the system; organotin catalyst accelerates the polyurethane prepolymerization process, ensuring efficient and orderly reaction; N-methyldiethanolamine introduces tertiary amine groups, providing reaction sites for subsequent cationization modification; glacial acetic acid plays a neutralizing role, causing the tertiary amine groups to undergo quaternization reaction to generate quaternary ammonium salts, adjusting the system potential to a suitable range, and promoting the electrostatic assembly and composite of cationic polyurethane and sodium-based montmorillonite; deionized water, as a dispersion medium, ensures uniform dispersion of each component, forming a stable composite emulsion system.

[0019] Preferably, the preparation method of the quaternary ammonium salt polyurethane montmorillonite composite emulsion includes the following steps:

[0020] (1) Weigh out polypropylene glycol, isophorone diisocyanate and organotin catalyst according to the proportion, and stir the reaction at 85-90℃ and under nitrogen protection for 150-180 min to obtain isocyanate-terminated polyurethane prepolymer.

[0021] (2) Cool the isocyanate-terminated polyurethane prepolymer obtained in step (1) to 70-75℃, add N-methyldiethanolamine, stir at 300-400 r / min for 50-70 min, then cool to 40-45℃, add glacial acetic acid to neutralize to pH 6.0-7.0, continue stirring for 20-30 min to generate cationic polyurethane prepolymer;

[0022] (3) Add sodium montmorillonite and deionized water to the cationic polyurethane prepolymer obtained in step (2), and shear emulsify at a speed of 6000-8000 r / min for 25-35 min, and then mature at 55-65℃ for 100-140 min to obtain quaternary ammonium salt polyurethane montmorillonite composite emulsion.

[0023] Using the above technical solution, the organotin catalyst accelerates the reaction between polypropylene glycol and isophorone diisocyanate through a prepolymerization reaction under nitrogen protection, forming a structurally stable isocyanate-terminated polyurethane prepolymer. The addition of N-methyldiethanolamine extends the chain of the prepolymer and introduces tertiary amine groups. After neutralization with glacial acetic acid, it is converted into a cationic polyurethane prepolymer, providing a charge basis for subsequent bonding with sodium-based montmorillonite. High-speed shear emulsification promotes the full bonding of negatively charged sodium-based montmorillonite and cationic polyurethane prepolymer through electrostatic interaction. The subsequent curing process further stabilizes the interfacial bonding, allowing montmorillonite to be uniformly dispersed in the polyurethane system, ultimately forming a stable, flexible, and barrier-resistant quaternary ammonium salt polyurethane-montmorillonite composite emulsion.

[0024] Preferably, the nitrogen flow rate is controlled at 0.1-0.3 L / min, and the stirring speed is 300-400 r / min.

[0025] Using the above technical solution, the nitrogen gas flow rate can completely purge the air in the reaction system, reduce the interference of oxygen on the formation process of the isocyanate-terminated polyurethane prepolymer, and ensure the smooth progress of the prepolymerization reaction and the tertiary amine chain extension reaction. The stirring speed of 300-400 r / min can fully contact and mix the components such as polypropylene glycol, isophorone diisocyanate and N-methyldiethanolamine, avoid product structural defects caused by uneven local reaction, and at the same time achieve uniform introduction of tertiary amine groups into the prepolymer, laying the foundation for subsequent neutralization to form a stable cationic polyurethane prepolymer.

[0026] Preferably, the method for preparing the intermediate support layer slurry includes the following steps:

[0027] a. Add the enzyme-modified silane sisal pulp and basalt fiber to a high-speed mixer and disperse them at 25-30℃ and a speed of 600-800 r / min for 10-15 min.

[0028] b. Add cross-linked oxidized starch, waterborne polyurethane and chitosan to the system obtained in step a, heat to 35-40℃, and stir at 400-500 r / min for 20-30 min.

[0029] c. Stir the system obtained in step b at 30-35℃ and 500-600 r / min, and add hydroxyl fluorosilicone oil, nano titanium dioxide, DOPO, fumed nano silica and KH-570 in sequence. After each addition of an additive, stir for 5-8 min to disperse it evenly.

[0030] d. Add quaternary ammonium salt polyurethane montmorillonite composite emulsion to the system obtained in step c. Under the conditions of 60-70℃ and pH adjusted to 5.0-6.0 with 0.10-0.15mol / L acetate-sodium acetate buffer solution, stir at 300-400r / min for 30-40min. Then cool to 30-35℃, add deionized water, and stir at 400-500r / min for 50-70min to obtain the intermediate support layer slurry.

[0031] Using the above technical solution, the enzyme-catalyzed silane-modified sisal pulp and basalt fiber are uniformly dispersed through gradient temperature and rotation speed control, ensuring a uniform matrix structure. Cross-linked oxidized starch, waterborne polyurethane, and chitosan efficiently wet the fibers, improving interfacial adhesion. Orderly feeding and segmented stirring ensure uniform dispersion of hydroxyl fluorosilicone oil, nano-titanium dioxide, DOPO, fumed nano-silica, and KH-570, allowing them to perform their respective functions and avoiding interference between components. Under suitable temperature, pH, and rotation speed, the quaternary ammonium salt polyurethane montmorillonite composite emulsion reacts with the system to form a pre-crosslinked structure. Subsequent water homogenization further optimizes the pulp's stability and density, ultimately yielding a balanced intermediate support layer pulp suitable for corrugated board coating requirements.

[0032] This invention also discloses a manufacturing process for a seven-layer corrugated board, comprising the following steps:

[0033] S1. Raw material pretreatment: The substrates of the outer protective layer, outer core paper layer, outer corrugated layer, inner corrugated layer, inner core paper layer and inner protective layer are placed in a constant humidity and temperature room and conditioned for 12-18 hours under the environmental conditions of 23±2℃ and 50±5% relative humidity.

[0034] S2. Corrugated Forming: The corrugated base paper of the outer and inner corrugated layers is formed into A-type corrugated paper under the conditions of preheating temperature of 150-160℃ and corrugated roll running speed of 15-20m / min, with a flute height of 4.5-5.0mm and a flute pitch of 8-9mm.

[0035] S3. Intermediate Support Layer Coating and Molding: The intermediate support layer slurry is coated using a doctor blade coating machine at a rate of 200-250 g / m². 2 The coating amount is applied to the inner surface of the outer corrugated layer and dried in a hot air drying oven at a temperature of 100-110℃ for 6-8 minutes. The thickness of the intermediate support layer after drying is controlled to be 1.5-2.0 mm.

[0036] S4. Interlayer bonding: During external lamination, the outer protective layer, outer core paper layer, and outer corrugated layer coated with the intermediate support layer are laminated using a laminating machine. The adhesive used is cross-linked oxidized starch adhesive, and the adhesive application rate is controlled at 80-100 g / m². 2 The lamination temperature is 120-130℃, the lamination pressure is 0.3-0.4MPa, and the lamination speed is 10-15m / min. During the inner lamination, the other side of the middle support layer is sequentially laminated with the inner corrugated layer, the inner core paper layer, and the inner protective layer. The adhesive and process parameters are the same as those for the outer lamination.

[0037] S5. Hot pressing and curing: The seven-layer corrugated board after step S4 is put into a hot press and pre-cured for 3-5 minutes at 100-110℃ and 0.35-0.45MPa; then the temperature is increased to 125-135℃ at a rate of 5-8℃ / min and pressure is maintained at 0.4-0.5MPa for 5-7 minutes.

[0038] S6. Post-processing: After hot pressing in step S5, the seven-layer corrugated board is air-cooled and humidified, and then cut and trimmed in sequence to obtain the finished seven-layer corrugated board.

[0039] The above technical solution ensures uniform moisture content in each layer of the substrate through raw material conditioning, reducing the risk of deformation and interlayer separation during subsequent processing. A-type corrugated forming parameters are matched to the substrate characteristics, balancing structural support and cushioning performance. The coating and drying process of the intermediate support layer controls its thickness and curing degree, ensuring tight adhesion to the outer corrugated layer and maximizing its functionality. Interlayer bonding uses cross-linked oxidized starch adhesive with suitable parameters to enhance the bonding strength and structural stability between layers. Two-stage hot-press curing, through gradient temperature and pressure control, promotes full cross-linking reaction, enhancing the overall structural density and mechanical properties. Post-processing, including air-cooling conditioning and trimming, ensures that the finished seven-layer corrugated board meets process requirements in terms of moisture content, maintains uniform dimensions, and guarantees stable overall performance.

[0040] Preferably, in step S6, the conditions for air-cooled humidity conditioning are as follows: air-cooled humidity conditioning for 6-8 hours at a temperature of 40-50℃ and a wind speed of 3-5m / s to reduce the moisture content of the corrugated board to 8%-10%, followed by moisture content balancing for 2-4 hours at a temperature of 23±2℃ and a relative humidity of 50±5%.

[0041] Using the above technical solution, air-cooled dehumidification conditions at 40-50℃ and 3-5m / s wind speed can quickly regulate the moisture content of corrugated board to 8%-10%, balancing dehumidification efficiency and board structure stability. This prevents excessive moisture content from causing board deformation and adhesive layer failure, or excessively low moisture content from causing embrittlement. Subsequent moisture content balancing treatment at 23±2℃ and 50±5% relative humidity ensures uniform moisture content distribution within the corrugated board, reducing stress residue caused by differences in internal and external moisture content, guaranteeing the dimensional stability of the finished corrugated board, and providing suitable conditions for subsequent cutting and trimming processes.

[0042] The beneficial effects of this invention are as follows:

[0043] DOPO enhances the flame retardant properties of corrugated sheets; enzyme-modified silane-modified sisal pulp and basalt fiber intertwine to form a substrate support skeleton, providing basic mechanical support for the corrugated sheets; cross-linked oxidized starch and waterborne polyurethane serve as the main adhesives, ensuring film-forming properties, bonding strength, and flexibility of the adhesive layer, while chitosan assists in enhancing the adhesive layer's bonding effect; hydroxyl fluorosilicone oil improves the overall water resistance of the corrugated sheets; nano-titanium dioxide fills the gaps between the substrate and the adhesives, improving the corrugated sheets' flexural strength and impact resistance; fumed nano-silica enhances the suspension of the slurry and the density of the adhesive layer, assisting in optimizing mechanical and water resistance properties; KH-570 connects inorganic fillers and organic adhesives, improving interfacial bonding; quaternary ammonium salt polyurethane montmorillonite composite emulsion enhances the anti-mildew and antibacterial properties, barrier properties, and flexibility of the corrugated sheets; the synergistic effect of these components gives the seven-layer corrugated sheets excellent mechanical properties, water resistance, anti-mildew and antibacterial properties, flame retardant properties, and structural stability. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0045] The specific information on the raw materials used in the embodiments of the present invention is shown in Table 1.

[0046] Table 1

[0047] Example 1:

[0048] This embodiment discloses a seven-layer corrugated board, comprising an outer protective layer, an outer core paper layer, an outer corrugated layer, an intermediate support layer, an inner corrugated layer, an inner core paper layer, and an inner protective layer, stacked sequentially. The intermediate support layer is made by coating, drying, and curing an intermediate support layer slurry. By weight, the intermediate support layer slurry comprises: 20 parts enzymatically modified sisal slurry, 8 parts basalt fiber, 25 parts cross-linked oxidized starch, 10 parts waterborne polyurethane with a solid content of 38%, 2 parts chitosan, 1.5 parts hydroxyl fluorosilicone oil with a viscosity of 500 mPa·s at 25°C, 1 part nano-titanium dioxide with an average particle size of 10 nm, 3 parts DOPO, 1 part fumed nano-silica, 1 part KH-570, 5 parts quaternary ammonium salt polyurethane-montmorillonite composite emulsion, and 25 parts deionized water. The basalt fiber is a short-cut fiber with a length of 6 mm; the chitosan has a degree of deacetylation ≥90% and a molecular weight of 100,000 Da.

[0049] The raw materials for preparing enzyme-modified sisal pulp by weight are: 90 parts sisal fiber pulp, 0.3 parts laccase, 0.05 parts sodium pyrophosphate, 0.1 parts ammonium persulfate, and 3 parts vinyltrimethoxysilane.

[0050] The preparation method of enzyme-catalyzed silane-modified sisal pulp includes the following steps:

[0051] 1) Adjust the pH of the sisal fiber slurry to 4.3 with 0.1 mol / L acetate-sodium acetate buffer solution, add laccase and sodium pyrophosphate, and stir at 200 r / min for 60 min at 35℃. During the reaction, oxygen is continuously introduced and the oxygen flow rate is controlled at 0.5 L / min.

[0052] 2) Adjust the pH of the system obtained in step 1) to 6.5 with 0.04 mol / L sodium hydroxide solution, add ammonium persulfate, heat to 50°C under nitrogen protection, add vinyltrimethoxysilane, and stir the reaction at 400 r / min for 90 min.

[0053] 3) Place the reaction solution obtained in step 2) under vacuum degassing conditions of 60℃ and vacuum degree of -0.085MPa for 20min. During vacuum degassing, stir continuously at a speed of 200r / min to obtain enzyme-modified sisal pulp.

[0054] The raw materials for preparing the quaternary ammonium salt polyurethane montmorillonite composite emulsion, by weight, include: 8 parts sodium montmorillonite, 45 parts polypropylene glycol, 20 parts isophorone diisocyanate, 0.05 parts organotin catalyst, 3 parts N-methyldiethanolamine, 1.5 parts glacial acetic acid with a concentration of 0.1 mol / L, and 60 parts deionized water.

[0055] The preparation method of quaternary ammonium salt polyurethane montmorillonite composite emulsion includes the following steps:

[0056] (1) Weigh out polypropylene glycol, isophorone diisocyanate and organotin catalyst according to the proportion, stir and react for 150 min at 85℃ under nitrogen protection to obtain isocyanate-terminated polyurethane prepolymer.

[0057] The nitrogen flow rate was 0.1 L / min, and the stirring speed was 300 r / min.

[0058] (2) Cool the isocyanate-terminated polyurethane prepolymer obtained in step (1) to 70°C, add N-methyldiethanolamine, stir at 300 r / min for 50 min, then cool to 40°C, add glacial acetic acid to neutralize to pH 6.0, continue stirring for 20 min to generate cationic polyurethane prepolymer;

[0059] (3) Add sodium montmorillonite and deionized water to the cationic polyurethane prepolymer obtained in step (2), shear emulsify at a speed of 6000 r / min for 25 min, and then mature at 55℃ for 100 min to obtain quaternary ammonium salt polyurethane montmorillonite composite emulsion.

[0060] The preparation method of the intermediate support layer slurry includes the following steps:

[0061] a. Add the enzyme-modified silane sisal pulp and basalt fiber to a high-speed mixer and disperse them at 25°C and 600 r / min for 10 min.

[0062] b. Add cross-linked oxidized starch, aqueous polyurethane and chitosan to the system obtained in step a, heat to 35°C, and stir at 400 r / min for 20 min;

[0063] c. Stir the system obtained in step b at 30°C and 500 r / min, and add hydroxyl fluorosilicone oil, nano titanium dioxide, DOPO, fumed nano silica and KH-570 in sequence. After each addition of an additive, stir for 5 min to disperse it evenly.

[0064] d. Add quaternary ammonium salt polyurethane montmorillonite composite emulsion to the system obtained in step c, and stir at 300 r / min for 30 min at 60℃ with pH adjusted to 5.0 using 0.1 mol / L acetate-sodium acetate buffer solution. Then cool down to 30℃, add deionized water, and stir at 400 r / min for 50 min to obtain the intermediate support layer slurry.

[0065] This embodiment also discloses a manufacturing process for a seven-layer corrugated board, including the following steps:

[0066] S1. Raw material pretreatment: The substrates of the outer protective layer, outer core paper layer, outer corrugated layer, inner corrugated layer, inner core paper layer and inner protective layer are placed in a constant humidity and temperature room and conditioned for 12 hours under the environmental conditions of 23±2℃ and 50±5% relative humidity.

[0067] S2. Corrugated Forming: The corrugated base paper of the outer and inner corrugated layers is formed into A-type corrugated paper under the conditions of preheating temperature of 150℃ and corrugated roll running speed of 15m / min, with a flute height of 4.5mm and a flute pitch of 8mm.

[0068] S3. Intermediate Support Layer Coating and Molding: The intermediate support layer slurry is coated using a doctor blade coating machine at a rate of 200g / m². 2 The coating amount is applied to the inner surface of the outer corrugated layer and dried in a hot air drying oven at 100℃ for 6 minutes. The thickness of the intermediate support layer after drying is controlled to be 1.5mm.

[0069] S4. Interlayer bonding: During external lamination, the outer protective layer, outer core paper layer, and outer corrugated layer coated with the intermediate support layer are laminated using a laminating machine. The adhesive used is cross-linked oxidized starch adhesive, and the adhesive application rate is controlled at 80g / m². 2 The lamination temperature is 120℃, the lamination pressure is 0.3MPa, and the lamination speed is 10m / min. During the inner lamination, the other side of the middle support layer is sequentially laminated with the inner corrugated layer, the inner core paper layer, and the inner protective layer. The adhesive and process parameters are the same as those for the outer lamination.

[0070] S5. Hot pressing and curing: The seven-layer corrugated board after step S4 is put into a hot press and pre-cured for 3 minutes at 100℃ and 0.35MPa pressure; then the temperature is increased to 125℃ at a rate of 5℃ / min and pressure is maintained at 0.4MPa for 5 minutes.

[0071] S6. Post-processing: The seven-layer corrugated board after hot pressing in step S5 is air-cooled and humidified. The air-cooling and humidification conditions are: 6 hours at a temperature of 40℃ and a wind speed of 3m / s to reduce the moisture content of the corrugated board to 8%. Then, the moisture content is balanced for 2 hours at a temperature of 23±2℃ and a relative humidity of 50±5%. After that, the board is cut and trimmed to obtain the finished seven-layer corrugated board.

[0072] Example 2:

[0073] This embodiment discloses a seven-layer corrugated board, comprising an outer protective layer, an outer core paper layer, an outer corrugated layer, an intermediate support layer, an inner corrugated layer, an inner core paper layer, and an inner protective layer, stacked sequentially. The intermediate support layer is made by coating, drying, and curing an intermediate support layer slurry. By weight, the intermediate support layer slurry comprises: 28 parts of enzyme-modified sisal slurry, 10 parts of basalt fiber, 30 parts of cross-linked oxidized starch, 12 parts of waterborne polyurethane with a solid content of 42%, 3 parts of chitosan, 2 parts of hydroxyl fluorosilicone oil with a viscosity of 600 mPa·s at 25°C, 1.5 parts of nano-titanium dioxide with an average particle size of 20 nm, 4 parts of DOPO, 1.5 parts of fumed nano-silica, 1.5 parts of KH-570, 7 parts of quaternary ammonium salt polyurethane-montmorillonite composite emulsion, and 35 parts of deionized water. The basalt fiber is a short-cut fiber with a length of 8 mm; the chitosan has a degree of deacetylation ≥90% and a molecular weight of 200,000 Da.

[0074] The raw materials for preparing enzyme-modified sisal pulp by weight are: 100 parts sisal fiber pulp, 0.5 parts laccase, 0.1 parts sodium pyrophosphate, 0.2 parts ammonium persulfate, and 4 parts vinyltrimethoxysilane.

[0075] The preparation method of enzyme-catalyzed silane-modified sisal pulp includes the following steps:

[0076] 1) Adjust the pH of the sisal fiber slurry to 4.7 with 0.15 mol / L acetate-sodium acetate buffer solution, add laccase and sodium pyrophosphate, and stir at 300 r / min for 90 min at 40℃. During the reaction, oxygen is continuously introduced and the oxygen flow rate is controlled at 1.0 L / min.

[0077] 2) Adjust the pH of the system obtained in step 1) to 7.0 with 0.08 mol / L sodium hydroxide solution, add ammonium persulfate, heat to 55°C under nitrogen protection, add vinyltrimethoxysilane, and stir the reaction at 500 r / min for 120 min.

[0078] 3) Place the reaction solution obtained in step 2) under vacuum degassing conditions of 65℃ and vacuum degree of -0.095MPa for 40min. During vacuum degassing, stir continuously at a speed of 300r / min to obtain enzyme-modified sisal pulp.

[0079] The raw materials for preparing the quaternary ammonium salt polyurethane montmorillonite composite emulsion, by weight, include: 12 parts sodium montmorillonite, 55 parts polypropylene glycol, 30 parts isophorone diisocyanate, 0.1 parts organotin catalyst, 4 parts N-methyldiethanolamine, 2 parts glacial acetic acid with a concentration of 0.15 mol / L, and 80 parts deionized water.

[0080] The preparation method of quaternary ammonium salt polyurethane montmorillonite composite emulsion includes the following steps:

[0081] (1) Weigh out polypropylene glycol, isophorone diisocyanate and organotin catalyst according to the proportion, stir and react for 180 min under nitrogen protection at 90℃ to obtain isocyanate-terminated polyurethane prepolymer.

[0082] The nitrogen flow rate was 0.3 L / min, and the stirring speed was 400 r / min.

[0083] (2) Cool the isocyanate-terminated polyurethane prepolymer obtained in step (1) to 75°C, add N-methyldiethanolamine, stir at 400 r / min for 70 min, then cool to 45°C, add glacial acetic acid to neutralize to pH 7.0, continue stirring for 30 min to generate cationic polyurethane prepolymer;

[0084] (3) Add sodium montmorillonite and deionized water to the cationic polyurethane prepolymer obtained in step (2), shear emulsify at 8000 r / min for 35 min, and then mature at 65℃ for 140 min to obtain quaternary ammonium salt polyurethane montmorillonite composite emulsion.

[0085] The preparation method of the intermediate support layer slurry includes the following steps:

[0086] a. Add the enzyme-modified silane sisal pulp and basalt fiber to a high-speed mixer and disperse them at 30°C and 800 r / min for 15 min.

[0087] b. Add cross-linked oxidized starch, aqueous polyurethane and chitosan to the system obtained in step a, heat to 40°C, and stir at 500 r / min for 30 min;

[0088] c. Stir the system obtained in step b at 35°C and 600 r / min, and add hydroxyl fluorosilicone oil, nano titanium dioxide, DOPO, fumed nano silica and KH-570 in sequence. After each addition of an additive, stir for 8 min to disperse it evenly.

[0089] d. Add quaternary ammonium salt polyurethane montmorillonite composite emulsion to the system obtained in step c, and stir at 400 r / min for 40 min at 70℃ with pH adjusted to 6.0 using 0.15 mol / L acetate-sodium acetate buffer solution. Then cool to 35℃, add deionized water, and stir at 500 r / min for 70 min to obtain the intermediate support layer slurry.

[0090] This embodiment also discloses a manufacturing process for a seven-layer corrugated board, including the following steps:

[0091] S1. Raw material pretreatment: The substrates of the outer protective layer, outer core paper layer, outer corrugated layer, inner corrugated layer, inner core paper layer and inner protective layer are placed in a constant humidity and temperature room and conditioned for 18 hours under the environmental conditions of 23±2℃ and 50±5% relative humidity.

[0092] S2. Corrugated Forming: The corrugated base paper of the outer and inner corrugated layers is formed into A-type corrugated paper under the conditions of preheating temperature of 160℃ and corrugated roll running speed of 20m / min, with a flute height of 5mm and a flute pitch of 9mm.

[0093] S3. Intermediate Support Layer Coating and Molding: The intermediate support layer slurry is coated using a doctor blade coating machine at a rate of 250g / m². 2 The coating amount is applied to the inner surface of the outer corrugated layer and dried in a hot air drying oven at 110℃ for 8 minutes. The thickness of the intermediate support layer after drying is controlled to be 2mm.

[0094] S4. Interlayer bonding: During external lamination, the outer protective layer, outer core paper layer, and outer corrugated layer coated with the intermediate support layer are laminated using a laminating machine. The adhesive used is cross-linked oxidized starch adhesive, and the adhesive application rate is controlled at 100g / m². 2 The lamination temperature is 130℃, the lamination pressure is 0.4MPa, and the lamination speed is 15m / min. During the inner lamination, the other side of the middle support layer is sequentially laminated with the inner corrugated layer, the inner core paper layer, and the inner protective layer. The adhesive and process parameters are the same as those for the outer lamination.

[0095] S5. Hot pressing and curing: The seven-layer corrugated board after step S4 is put into a hot press and pre-cured for 5 minutes at 110℃ and 0.45MPa pressure; then the temperature is increased to 135℃ at a rate of 8℃ / min and pressure is maintained at 0.5MPa for 7 minutes.

[0096] S6. Post-processing: The seven-layer corrugated board after hot pressing in step S5 is air-cooled and humidified. The air-cooling and humidification conditions are: 8 hours at a temperature of 50℃ and a wind speed of 5m / s to reduce the moisture content of the corrugated board to 10%. Then, the moisture content is balanced for 4 hours at a temperature of 23±2℃ and a relative humidity of 50±5%. After that, the board is cut and trimmed to obtain the finished seven-layer corrugated board.

[0097] Example 3:

[0098] This embodiment discloses a seven-layer corrugated board, comprising an outer protective layer, an outer core paper layer, an outer corrugated layer, an intermediate support layer, an inner corrugated layer, an inner core paper layer, and an inner protective layer stacked sequentially. The intermediate support layer is made by coating, drying, and curing an intermediate support layer slurry. By weight, the intermediate support layer slurry comprises: 24 parts of enzyme-modified sisal slurry, 9 parts of basalt fiber, 27 parts of cross-linked oxidized starch, 11 parts of waterborne polyurethane with a solid content of 40%, 2.5 parts of chitosan, 1.8 parts of hydroxyl fluorosilicone oil with a viscosity of 550 mPa·s at 25°C, 1.2 parts of nano-titanium dioxide with an average particle size of 15 nm, 3.5 parts of DOPO, 1.2 parts of fumed nano-silica, 1.2 parts of KH-570, 6 parts of quaternary ammonium salt polyurethane-montmorillonite composite emulsion, and 30 parts of deionized water. The basalt fiber is a short-cut fiber with a length of 7 mm; the chitosan has a degree of deacetylation ≥90% and a molecular weight of 150,000 Da.

[0099] The raw materials for preparing enzyme-modified sisal pulp by weight are: 95 parts sisal fiber pulp, 0.4 parts laccase, 0.08 parts sodium pyrophosphate, 0.15 parts ammonium persulfate, and 3.5 parts vinyltrimethoxysilane.

[0100] The preparation method of enzyme-catalyzed silane-modified sisal pulp includes the following steps:

[0101] 1) Adjust the pH of the sisal fiber slurry to 4.5 with 0.12 mol / L acetate-sodium acetate buffer solution, add laccase and sodium pyrophosphate, and stir at 250 r / min for 75 min at 38℃. During the reaction, oxygen is continuously introduced and the oxygen flow rate is controlled at 0.8 L / min.

[0102] 2) Adjust the pH of the system obtained in step 1) to 6.8 with 0.06 mol / L sodium hydroxide solution, add ammonium persulfate, heat to 52°C under nitrogen protection, add vinyltrimethoxysilane, and stir the reaction at 450 r / min for 105 min.

[0103] 3) Place the reaction solution obtained in step 2) under vacuum degassing conditions of 62℃ and vacuum degree of -0.09MPa for 30min. During vacuum degassing, stir continuously at a speed of 250r / min to obtain enzyme-modified sisal pulp.

[0104] The raw materials for preparing the quaternary ammonium salt polyurethane montmorillonite composite emulsion, by weight, include: 10 parts sodium montmorillonite, 50 parts polypropylene glycol, 25 parts isophorone diisocyanate, 0.08 parts organotin catalyst, 3.5 parts N-methyldiethanolamine, 1.8 parts glacial acetic acid with a concentration of 0.12 mol / L, and 70 parts deionized water.

[0105] The preparation method of quaternary ammonium salt polyurethane montmorillonite composite emulsion includes the following steps:

[0106] (1) Weigh out polypropylene glycol, isophorone diisocyanate and organotin catalyst according to the proportion, stir and react for 165 min at 88℃ under nitrogen protection to obtain isocyanate-terminated polyurethane prepolymer.

[0107] The nitrogen flow rate was 0.2 L / min, and the stirring speed was 350 r / min.

[0108] (2) Cool the isocyanate-terminated polyurethane prepolymer obtained in step (1) to 72°C, add N-methyldiethanolamine, stir at 350 r / min for 60 min, then cool to 42°C, add glacial acetic acid to neutralize to pH 6.5, continue stirring for 25 min to generate cationic polyurethane prepolymer;

[0109] (3) Add sodium montmorillonite and deionized water to the cationic polyurethane prepolymer obtained in step (2), shear emulsify at 7000 r / min for 30 min, and then mature at 60℃ for 120 min to obtain quaternary ammonium salt polyurethane montmorillonite composite emulsion.

[0110] The preparation method of the intermediate support layer slurry includes the following steps:

[0111] a. Add the enzyme-modified silane sisal pulp and basalt fiber to a high-speed mixer and disperse them at 700 r / min for 12 min at 28℃;

[0112] b. Add cross-linked oxidized starch, aqueous polyurethane and chitosan to the system obtained in step a, heat to 38°C, and stir at 450 r / min for 25 min;

[0113] c. Stir the system obtained in step b at 32℃ and 550 r / min, and add hydroxyl fluorosilicone oil, nano titanium dioxide, DOPO, fumed nano silica and KH-570 in sequence. After each addition of an additive, stir for 6.5 min to disperse it evenly.

[0114] d. Add quaternary ammonium salt polyurethane montmorillonite composite emulsion to the system obtained in step c, and stir at 350 r / min for 35 min at 65℃ with pH adjusted to 5.5 using 0.12 mol / L acetate-sodium acetate buffer solution. Then cool to 32℃, add deionized water, and stir at 450 r / min for 60 min to obtain the intermediate support layer slurry.

[0115] This embodiment also discloses a manufacturing process for a seven-layer corrugated board, including the following steps:

[0116] S1. Raw material pretreatment: The substrates of the outer protective layer, outer core paper layer, outer corrugated layer, inner corrugated layer, inner core paper layer and inner protective layer are placed in a constant humidity and temperature room and conditioned for 15 hours under the environmental conditions of 23±2℃ and 50±5% relative humidity.

[0117] S2. Corrugated Forming: The corrugated base paper of the outer and inner corrugated layers is formed into A-type corrugated paper under the conditions of preheating temperature of 155℃ and corrugated roll running speed of 18m / min, with a flute height of 4.8mm and a flute pitch of 8.5mm.

[0118] S3. Intermediate Support Layer Coating and Molding: The intermediate support layer slurry is coated using a doctor blade coating machine at a rate of 225g / m². 2 The coating amount is applied to the inner surface of the outer corrugated layer and dried in a hot air drying oven at 105℃ for 7 minutes. The thickness of the intermediate support layer after drying is controlled to be 1.8mm.

[0119] S4. Interlayer bonding: During external lamination, the outer protective layer, outer core paper layer, and outer corrugated layer coated with the intermediate support layer are laminated using a laminating machine. The adhesive used is cross-linked oxidized starch adhesive, and the adhesive application rate is controlled at 90g / m². 2 The lamination temperature is 125℃, the lamination pressure is 0.35MPa, and the lamination speed is 12m / min. During the inner lamination, the other side of the middle support layer is sequentially laminated with the inner corrugated layer, the inner core paper layer, and the inner protective layer. The adhesive and process parameters are the same as those for the outer lamination.

[0120] S5. Hot pressing and curing: The seven-layer corrugated board after step S4 is put into a hot press and pre-cured for 4 minutes at 105℃ and 0.4MPa pressure; then the temperature is increased to 130℃ at a rate of 6.5℃ / min and pressure is maintained at 0.45MPa for 6 minutes.

[0121] S6. Post-processing: The seven-layer corrugated board after hot pressing in step S5 is air-cooled and humidified. The air-cooling and humidification conditions are: air-cooled and humidified for 7 hours at a temperature of 45℃ and a wind speed of 4m / s to reduce the moisture content of the corrugated board to 9%. Then, the moisture content is balanced for 3 hours at a temperature of 23±2℃ and a relative humidity of 50±5%. After that, it is cut and trimmed to obtain the finished seven-layer corrugated board.

[0122] Comparative Example 1:

[0123] A seven-layer corrugated board and its production process are disclosed, the only difference between this and Example 3 is that no enzymatically modified sisal pulp is added.

[0124] Comparative Example 2:

[0125] A seven-layer corrugated board and its production process are disclosed, the only difference between this and Example 3 is that no quaternary ammonium salt polyurethane montmorillonite composite emulsion is added.

[0126] Comparative Example 3:

[0127] A seven-layer corrugated board and its production process are different from those in Example 3 only in that KH-570 is not added.

[0128] Comparative Example 4:

[0129] A seven-layer corrugated board and its production process are disclosed, the only difference between this and Example 3 is that unmodified sisal fiber pulp is used to replace the enzyme-modified silane sisal pulp in an equal amount.

[0130] Comparative Example 5:

[0131] A seven-layer corrugated board and its manufacturing process are described, which differ from Example 3 only in that hydroxyl fluorosilicone oil is not added.

[0132] Comparative Example 6:

[0133] A seven-layer corrugated board and its production process are disclosed, which differ from Example 3 only in that DOPO is not added.

[0134] Comparative Example 7:

[0135] A seven-layer corrugated board and its production process are disclosed, which differ from Example 3 only in that the hot-pressing curing temperature of both stages is set to 105°C.

[0136] Comparative Example 8:

[0137] A seven-layer corrugated board and its manufacturing process differ from Example 3 only in that: in step S3, the coating amount of the intermediate support layer is changed to 125 g / m². 2 The thickness of the intermediate support layer after drying is controlled to be 0.9mm.

[0138] Comparative Example 9:

[0139] It adopts a conventional five-layer corrugated board structure (without an intermediate support layer), and the basis weight of the face paper is 200g / m². 2 Core paper basis weight 150g / m 2 The total thickness is 12mm, which is consistent with the total weight of the seven-layer paper.

[0140] The corrugated boards obtained in Examples 1-3 and Comparative Examples 1-9 were tested for mechanical properties, water resistance and mildew resistance, flame retardancy and weather resistance. The testing methods and standards for each property are as follows:

[0141] (I) Methods for testing mechanical properties

[0142] The testing was conducted according to the methods specified in GB / T 6544-2020, "Corrugated Cardboard".

[0143] Flat compressive strength determination: The specimen size is 30mm×30mm. A compression testing machine is used to apply pressure at a speed of 10±2mm / min. The maximum load when the specimen fails is recorded. Flat compressive strength is calculated according to the formula: maximum load / specimen area. The unit is kPa.

[0144] Edge crush strength determination: The specimen size is 25mm×100mm. The edge crush strength tester is used to apply pressure at a speed of 12.5±2.5mm / min. The maximum load when the specimen fails is recorded. The edge crush strength is calculated according to the formula: edge crush strength = maximum load / specimen width. The unit is N / m.

[0145] Flexural strength determination: The specimen size is 100mm×25mm. A bending load is applied using a flexural testing machine at a speed of 5±1mm / min. The maximum force value at which the specimen breaks is recorded, and the unit is N.

[0146] Interlayer bond strength determination: The sample size is 25mm×25mm. The peeling force is applied at a speed of 25±5mm / min using an interlayer bond strength tester. The maximum load when the sample delaminates is recorded. The interlayer bond strength is calculated according to the formula: interlayer bond strength = maximum load / sample area, and the unit is kPa.

[0147] (II) Test methods for water resistance and mildew resistance

[0148] Determination of burst strength retention rate after immersion: The initial burst strength was determined according to GB / T1539-2021 "Determination of Bursting Strength of Paperboard". The sample diameter was 50±2mm, and a burst strength tester was used to apply pressure at a rate of 170±15mL / min. The sample was then immersed in deionized water at room temperature for 24±0.5h. After removal, the surface moisture was quickly wiped dry, and the burst strength was immediately measured. The burst strength retention rate was calculated using the formula: Bursting strength retention rate = (Burning strength after immersion / Initial burst strength) × 100%, in percentages (%).

[0149] Anti-mold grade determination: The determination was conducted according to GB / T 19273-2023, "Test Method for Anti-mold Performance of Wood Preservatives". The samples were placed in a mold incubator at 28±2℃ and 95±5% relative humidity for 28±1 days. After incubation, mold growth was observed and graded according to the standard: Grade 0 indicates no mold growth; Grade 1 indicates mold coverage ≤10%; Grade 2 indicates coverage 10-30%; Grade 3 indicates coverage 30-60%; and Grade 4 indicates coverage >60%.

[0150] Antibacterial rate determination: The determination was conducted according to GB / T 21551.2-2010, "Antibacterial, sterilizing and purifying functions of household and similar electrical appliances - Part 2: Antibacterial materials". The test bacteria were Escherichia coli and Staphylococcus aureus, with the bacterial concentration controlled at 1.0 × 10⁻⁶. 5 -9.9×10 5CFU / mL. After contacting the sample with the bacterial solution for 24±1h, the viable count was determined. The antibacterial rate was calculated using the formula: (Viable count of blank sample - Viable count of sample) / Viable count of blank sample × 100%, with units of %.

[0151] (III) Test methods for flame retardancy and weather resistance

[0152] Vertical flammability rating determination: Conducted according to GB / T 14656-2009, "Test Methods for Flame Retardant Paper and Paperboard Combustion Performance". Sample size: 125mm × 13mm, thickness: 1.5-2.0mm. A vertical flammability tester is used, with a flame height of 20±1mm and an application time of 10±0.5s. The rating (V-0, V-1, V-2, or HB) is determined based on factors such as burning speed and whether dripping material ignites absorbent cotton.

[0153] Determination of strength retention rate after thermal aging: The sample was placed in a forced-ventilation oven and aged at 70±2℃ for 72±1h. After removal, it was cooled in a standard environment for 2-4h. The strength after aging was determined according to the aforementioned edge crush strength determination method. The strength retention rate after thermal aging was calculated using the formula: Strength retention rate after thermal aging = (Edge crush strength after thermal aging / Initial edge crush strength) × 100%, with units of %.

[0154] The results are shown in Tables 2, 3 and 4.

[0155] Table 2 Mechanical property test results

[0156] Group Compressive strength (kPa) Edge compressive strength (N / m) Flexural strength (N) Interlayer bond strength (kPa) Example 1 2000 18000 300 520 Example 2 2250 19500 325 550 Example 3 2500 21000 350 580 Comparative Example 1 1800 15500 250 450 Comparative Example 2 1750 16000 265 480 Comparative Example 3 1650 14500 230 380 Comparative Example 4 1950 17500 285 530 Comparative Example 5 2400 20000 335 560 Comparative Example 6 2450 20500 340 570 Comparative Example 7 1900 15800 270 470 Comparative Example 8 1750 15300 255 440 Comparative Example 9 1400 12500 210 500

[0157] Table 3. Test results of water resistance and mildew resistance

[0158] Group Bursting strength retention rate after immersion in water (%) Anti-mildew rating Antibacterial rate of Escherichia coli (%) Antibacterial rate against Staphylococcus aureus (%) Example 1 75 1 94.0 96.0 Example 2 80 0 95.5 97.5 Example 3 86 0 97.0 99.0 Comparative Example 1 70 1 93.5 95.5 Comparative Example 2 65 2 82.5 84.2 Comparative Example 3 60 1 90.2 89.5 Comparative Example 4 73 1 93.8 95.8 Comparative Example 5 52 0 96.4 98.1 Comparative Example 6 84 0 96.5 98.2 Comparative Example 7 72 1 95.2 94.8 Comparative Example 8 68 0 96.8 97.5 Comparative Example 9 55 3 81.0 82.0

[0159] Table 4. Test results of flame retardancy and weather resistance

[0160] Group Vertical flammability rating Edge crush strength retention rate after heat aging (%) Example 1 V-1 80 Example 2 V-1 83 Example 3 V-0 85 Comparative Example 1 V-1 72 Comparative Example 2 V-1 74 Comparative Example 3 V-1 68 Comparative Example 4 V-1 76 Comparative Example 5 V-0 84 Comparative Example 6 HB 78 Comparative Example 7 V-1 70 Comparative Example 8 V-1 74 Comparative Example 9 V-2 65

[0161] Using Example 3 as the control group, the performance differences and causes of Comparative Examples 1-9 are analyzed as follows:

[0162] Comparative Example 1: Sisal pulp without added enzymatic silane modification

[0163] The flat compressive strength decreased from 2500 kPa to 1800 kPa (a decrease of 28.0%), the edge compressive strength decreased from 21000 N / m to 15500 N / m (a decrease of 26.2%), the flexural strength decreased from 350 N to 250 N (a decrease of 28.6%), and the interlaminar bond strength decreased from 580 kPa to 450 kPa (a decrease of 22.4%). The retention rate of burst strength after water immersion decreased from 86% to 70% (a decrease of 18.6%), and the retention rate of edge compressive strength after heat aging decreased from 85% to 72% (a decrease of 15.3%). The mildew resistance rating remained at Level 1, the antibacterial rate against Escherichia coli decreased from 97.0% to 93.5% (a decrease of 3.5 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 95.5% (a decrease of 3.5 percentage points). The vertical burning rating remained at Level V-1. Enzyme-modified silane-modified sisal pulp is one of the core skeletal components of the intermediate support layer. Through laccase oxidation and silane grafting, a large number of organic active groups are introduced onto the fiber surface, which can form chemical covalent bonds with the adhesive and crosslinking agent. Without this component, the intermediate support layer relies solely on basalt fibers for basic support. The interfacial bonding between the fiber and the adhesive decreases from chemical bonding to physical adsorption, resulting in poor fiber dispersibility and easy agglomeration. The rigid-toughness complementary structure of the substrate skeleton cannot be formed, leading to a significant decrease in mechanical properties. Simultaneously, the insufficient density of the fiber-adhesive interface allows water to easily penetrate and damage the adhesive layer structure, consequently reducing water resistance and thermal aging performance.

[0164] Comparative Example 2: Polyurethane-montmorillonite composite emulsion without quaternary ammonium salt addition

[0165] The flat compressive strength decreased from 2500 kPa to 1750 kPa (a decrease of 30.0%), the edge compressive strength decreased from 21000 N / m to 16000 N / m (a decrease of 23.8%), the flexural strength decreased from 350 N to 265 N (a decrease of 24.3%), and the interlaminar bond strength decreased from 580 kPa to 480 kPa (a decrease of 17.2%). The retention rate of burst strength after immersion in water decreased from 86% to 65% (a decrease of 24.4%), and the retention rate of edge compressive strength after heat aging decreased from 85% to 74% (a decrease of 12.9%). The mildew resistance rating decreased from level 0 to level 2, the antibacterial rate against Escherichia coli decreased from 97.0% to 82.5% (a decrease of 14.5 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 84.2% (a decrease of 14.8 percentage points). The vertical burning rating remained at level V-1. Quaternary ammonium salt polyurethane-montmorillonite composite emulsion is a multifunctional innovative component that combines antifungal and antibacterial properties, cross-linking reinforcement, and waterproofing. Its cationic quaternary ammonium salt groups can achieve highly efficient antifungal and antibacterial effects through electrostatic adsorption, while the montmorillonite layered structure can form a physical barrier to enhance water resistance. The terminal isocyanate groups can react with the hydroxyl and amino groups in the adhesive to participate in the construction of a three-dimensional cross-linked network. Without this component, on the one hand, the core function of antifungal and antibacterial properties is lost; relying solely on the basic antifungal effect of chitosan is insufficient to inhibit mold growth, resulting in a significant decrease in the antibacterial rate. On the other hand, the cross-linking density of the cross-linked network is significantly reduced, the physical barrier effect of montmorillonite disappears, water easily penetrates the adhesive layer, and the adhesive layer lacks density, leading to a decline in mechanical properties, water resistance, and thermal aging performance.

[0166] Comparative Example 3: No KH-570 added

[0167] The flat compressive strength decreased from 2500 kPa to 1650 kPa (a decrease of 34.0%), the edge compressive strength decreased from 21000 N / m to 14500 N / m (a decrease of 30.9%), the flexural strength decreased from 350 N to 230 N (a decrease of 34.3%), and the interlaminar bond strength decreased from 580 kPa to 380 kPa (a decrease of 34.5%). The retention rate of burst strength after immersion in water decreased from 86% to 60% (a decrease of 30.2%), and the retention rate of edge compressive strength after heat aging decreased from 85% to 68% (a decrease of 20.0%). The mildew resistance rating decreased from level 0 to level 1, the antibacterial rate against Escherichia coli decreased from 97.0% to 90.2% (a decrease of 6.8 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 89.5% (a decrease of 9.5 percentage points). The vertical burning rating decreased from level V-0 to level V-1. KH-570 is the core crosslinking agent for constructing covalent bonds between the inorganic and organic phases. Its siloxane groups, after hydrolysis, can dehydrate and condense with the hydroxyl groups of inorganic fillers such as basalt fiber, nano-titanium dioxide, and fumed nano-silica. Simultaneously, its double bonds can undergo free radical addition reactions with the vinyl groups on the surface of enzymatically modified sisal pulp under the initiation of residual ammonium persulfate. This dual action constructs a dense three-dimensional crosslinked network, which is crucial for interfacial bonding. Without this component, covalent bonds cannot form between the inorganic and organic phases, resulting in extremely poor interfacial bonding, failure to construct the three-dimensional crosslinked network, and numerous microscopic voids in the adhesive layer. This leads to a significant decrease in mechanical properties and interlayer bonding. These microscopic voids provide channels for water and mold invasion, significantly degrading water resistance and antifungal / antibacterial properties. Furthermore, the voids facilitate the migration of the flame retardant DOPO and result in uneven dispersion of flame-retardant groups, drastically reducing flame retardant performance and lowering the vertical burning rating to V-1. Insufficient adhesive layer density also leads to easy destruction of the adhesive layer structure by free radicals during thermo-oxidative aging, consequently reducing thermal aging performance.

[0168] Comparative Example 4: Replacing Enzyme-Modified Sisal Pulp with Unmodified Sisal Fiber Pulp

[0169] The flat compressive strength decreased from 2500 kPa to 1950 kPa (a decrease of 22.0%), the edge compressive strength decreased from 21000 N / m to 17500 N / m (a decrease of 16.7%), the flexural strength decreased from 350 N to 285 N (a decrease of 18.6%), and the interlaminar bond strength decreased from 580 kPa to 530 kPa (a decrease of 8.6%). The retention rate of burst strength after water immersion decreased from 86% to 73% (a decrease of 15.1%), and the retention rate of edge compressive strength after heat aging decreased from 85% to 76% (a decrease of 10.6%). The mildew resistance rating remained at Level 1, the antibacterial rate against Escherichia coli decreased from 97.0% to 93.8% (a decrease of 3.2 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 95.8% (a decrease of 3.2 percentage points). The vertical burning rating remained at Level V-1. Unmodified sisal fiber pulp has no active group modification on its surface and can only form physical adsorption with the adhesive. However, enzyme-modified silane sisal pulp introduces a large number of organic active groups on the fiber surface through laccase oxidation and silane grafting, which can form chemical covalent bonds with the adhesive and crosslinking agent. The interfacial bonding force is much higher than that of physical adsorption.

[0170] Comparative Example 5: No hydroxyl fluorosilicone oil added

[0171] The flat compressive strength decreased from 2500 kPa to 2400 kPa (a decrease of 4.0%), the edge compressive strength decreased from 21000 N / m to 20000 N / m (a decrease of 4.8%), the flexural strength decreased from 350 N to 335 N (a decrease of 4.3%), and the interlaminar bond strength increased from 580 kPa to 560 kPa (a decrease of 3.4%). The retention rate of burst strength after immersion in water decreased from 86% to 52% (a decrease of 40.7%). The mildew resistance rating remained at level 0, the antibacterial rate against Escherichia coli increased from 97.0% to 96.4% (a decrease of 0.6 percentage points), and the antibacterial rate against Staphylococcus aureus increased from 99.0% to 98.1% (a decrease of 0.9 percentage points). The retention rate of edge compressive strength after heat aging decreased from 85% to 84% (a decrease of 1.2%), and the vertical burning rating remained at level V-0. The core function of hydroxyl fluorosilicone oil is to form a hydrophobic film on the substrate surface, achieving a waterproof effect by reducing surface energy. It does not directly contribute to mechanical properties, antifungal and antibacterial properties, flame retardancy, or thermal aging properties. Without this component, the hydrophobic film cannot form, allowing water to directly contact the adhesive layer and fibers, damaging the adhesive layer structure and reducing the bonding force between the fibers and the adhesive, resulting in a significant decrease in the retention rate of burst strength after immersion in water. The hydrophobic film of hydroxyl fluorosilicone oil slightly reduces the interfacial bonding stiffness between the adhesive layer and the substrate (without significant mechanical attenuation). Its absence slightly enhances the rigid bonding effect of the adhesive layer, resulting in a slight change in interlayer bonding strength and only a slight decrease in mechanical properties. However, its hydrophobic function is completely lost, leading to a significant deterioration in water resistance.

[0172] Comparative Example 6: No DOPO added

[0173] The flat crush strength decreased from 2500 kPa to 2450 kPa (a decrease of 2.0%), the edge crush strength decreased from 21000 N / m to 20500 N / m (a decrease of 2.4%), the flexural strength decreased from 350 N to 340 N (a decrease of 2.9%), and the interlaminar bond strength decreased from 580 kPa to 570 kPa (a decrease of 1.7%). The edge crush strength retention rate after heat aging decreased from 85% to 78% (a decrease of 8.2%). The vertical fire rating decreased from V-0 to HB. The mildew resistance rating remained at 0. The antibacterial rate against Escherichia coli increased from 97.0% to 96.5% (a decrease of 0.5 percentage points), and the antibacterial rate against Staphylococcus aureus increased from 99.0% to 98.2% (a decrease of 0.8 percentage points). The burst strength retention rate after water immersion decreased from 86% to 84% (a decrease of 2.3%). DOPO is a phosphorus-based flame retardant. Its core function is to achieve flame retardancy through both gas-phase and condensed-phase action. Simultaneously, the phosphate groups produced during its decomposition can capture free radicals generated during thermo-oxidative aging, inhibiting adhesive layer degradation and further improving thermal aging performance. Without this component, the flame retardant system completely fails, and the vertical flammability rating drops to HB. Thermo-oxidative aging free radicals cannot be effectively captured, and the adhesive layer structure is easily damaged by aging, resulting in a slight decrease in edge crush strength retention after thermal aging.

[0174] Comparative Example 7: The curing temperature for both stages of hot pressing was set to 105℃.

[0175] The flat compressive strength decreased from 2500 kPa to 1900 kPa (a decrease of 24.0%), the edge compressive strength decreased from 21000 N / m to 15800 N / m (a decrease of 24.8%), the flexural strength decreased from 350 N to 270 N (a decrease of 22.9%), and the interlaminar bond strength decreased from 580 kPa to 470 kPa (a decrease of 19.0%). The retention rate of burst strength after immersion in water decreased from 86% to 72% (a decrease of 16.3%), and the retention rate of edge compressive strength after heat aging decreased from 85% to 70% (a decrease of 17.6%). The mildew resistance rating decreased from level 0 to level 1, the antibacterial rate against Escherichia coli decreased from 97.0% to 95.2% (a decrease of 1.8 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 94.8% (a decrease of 4.2 percentage points). The vertical burning rating remained at level V-1. In Example 3, the hot-press curing temperature was 130°C, which is close to the optimal temperature for the cross-linking reaction between the adhesive and the cross-linking agent. When both stages of hot-press curing were set to 105°C, the curing reaction of the adhesive and the condensation reaction of the cross-linking agent could not proceed sufficiently. The three-dimensional cross-linking network of the core functional layer was not fully formed, resulting in a large number of microscopic voids in the adhesive layer. This led to a significant decrease in mechanical properties and interlayer bonding strength. These microscopic voids provided channels for water and mold invasion, significantly degrading water resistance and antifungal / antibacterial properties. Insufficient adhesive layer density also made it easier for free radicals to damage the adhesive layer structure during thermo-oxidative aging, thus reducing its thermal aging performance.

[0176] Comparative Example 8: The coating amount of the intermediate support layer was changed to 125 g / m².2 The thickness of the dried intermediate support layer is controlled to be 0.9mm.

[0177] The flat crush strength decreased from 2500 kPa to 1750 kPa (a decrease of 30.0%), the edge crush strength decreased from 21000 N / m to 15300 N / m (a decrease of 27.1%), the flexural strength decreased from 350 N to 255 N (a decrease of 27.1%), and the interlaminar bond strength decreased from 580 kPa to 440 kPa (a decrease of 24.1%). The edge crush strength retention rate after heat aging decreased from 85% to 74% (a decrease of 12.9%). The mildew resistance rating remained at level 0, the antibacterial rate against Escherichia coli decreased from 97.0% to 96.8% (a decrease of 0.2 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 97.5% (a decrease of 1.5 percentage points). The burst strength retention rate after water immersion decreased from 86% to 68% (a decrease of 20.9%), and the vertical burning rating remained at level V-1. The coating amount of the intermediate support layer directly determines its layer thickness and the supporting effect of the skeleton. In Example 3, the coating amount was 225 g / m². 2 It can form a layer thickness of 1.5-2.0 mm, providing sufficient core support for seven-layer corrugated boards. The coating weight is reduced to 125 g / m². 2 Subsequently, the thickness of the intermediate support layer becomes significantly thinner, and the skeletal support is significantly insufficient, failing to provide effective mechanical support for the seven-layer structure, resulting in a significant decrease in mechanical properties and interlayer bonding strength; the thinner layer thickness makes the adhesive layer less dense, allowing water to easily penetrate and thermal aging to easily damage the adhesive layer structure, thus reducing water resistance and thermal aging performance.

[0178] Comparative Example 9: A conventional five-layer corrugated board structure (without an intermediate support layer) is used.

[0179] The flat compressive strength decreased from 2500 kPa to 1400 kPa (a decrease of 44.0%), the edge compressive strength decreased from 21000 N / m to 12500 N / m (a decrease of 40.5%), the flexural strength decreased from 350 N to 210 N (a decrease of 40.0%), and the interlaminar bond strength increased from 580 kPa to 500 kPa (a decrease of 13.8%). The retention rate of burst strength after immersion in water decreased from 86% to 55% (a decrease of 36.0%), and the retention rate of edge compressive strength after heat aging decreased from 85% to 65% (a decrease of 23.5%). The mildew resistance rating decreased from level 0 to level 3, the antibacterial rate against Escherichia coli decreased from 97.0% to 81.0% (a decrease of 16.0 percentage points), and the antibacterial rate against Staphylococcus aureus decreased from 99.0% to 82.0% (a decrease of 17.0 percentage points). The vertical burning rating decreased from V-0 to V-2. The intermediate support layer is the core functional layer of the seven-layer corrugated board. Its three-dimensional cross-linked network provides sufficient mechanical support for the structure, while also imparting comprehensive properties such as water resistance, mildew resistance, antibacterial properties, and flame retardancy. In contrast, the conventional five-layer corrugated board structure completely lacks this intermediate support layer. Mechanical support relies solely on the traditional structure of the face paper, core paper, and corrugated layers, without the reinforcing effect of the three-dimensional cross-linked network, resulting in a significant decrease in mechanical properties. The interlayer bond strength of the five-layer structure is also somewhat lower than that of the seven-layer corrugated board. Without the water-resistant, mildew-resistant, antibacterial, and flame-retardant system provided by the intermediate support layer, water easily penetrates, mold easily grows, and flame-retardant groups are missing, thus significantly deteriorating water resistance, mildew resistance, antibacterial properties, and flame retardancy. Furthermore, without the protection of the three-dimensional cross-linked network, the adhesive layer is easily damaged by free radicals during thermal aging, resulting in a significant decrease in thermal aging performance.

[0180] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A seven-layer corrugated board, characterized in that, The material comprises, in sequence, an outer protective layer, an outer core paper layer, an outer corrugated layer, an intermediate support layer, an inner corrugated layer, an inner core paper layer, and an inner protective layer; wherein, the intermediate support layer is made by coating, drying, and curing an intermediate support layer slurry, and by weight, the intermediate support layer slurry comprises: 20-28 parts of enzyme-modified sisal pulp, 8-10 parts of basalt fiber, 25-30 parts of cross-linked oxidized starch, 10-12 parts of waterborne polyurethane, 2-3 parts of chitosan, 1.5-2.0 parts of hydroxyl fluorosilicone oil, 1.0-1.5 parts of nano-titanium dioxide, 3-4 parts of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, 1.0-1.5 parts of fumed nano-silica, 1.0-1.5 parts of γ-methacryloyloxypropyltrimethoxysilane, 5-7 parts of quaternary ammonium salt polyurethane montmorillonite composite emulsion, and 25-35 parts of deionized water.

2. The seven-layer corrugated board according to claim 1, characterized in that, The basalt fiber is a short-cut fiber with a length of 6-8 mm; the solid content of the waterborne polyurethane is 38%-42%; the degree of deacetylation of the chitosan is ≥90% and the molecular weight is 100,000-200,000 Da; the viscosity of the hydroxyl fluorosilicone oil at 25°C is 500-600 mPa·s; and the average particle size of the nano-titanium dioxide is 10-20 nm.

3. The seven-layer corrugated board according to claim 1, characterized in that, The raw materials for preparing the enzyme-modified silane sisal pulp, by weight, include: 90-100 parts of sisal fiber pulp, 0.3-0.5 parts of laccase, 0.05-0.1 parts of sodium pyrophosphate, 0.1-0.2 parts of ammonium persulfate, and 3-4 parts of vinyltrimethoxysilane.

4. The seven-layer corrugated board according to claim 3, characterized in that, The preparation method of the enzyme-modified silane sisal pulp includes the following steps: 1) Adjust the pH of the sisal fiber slurry to 4.3-4.7 with 0.10-0.15 mol / L acetate-sodium acetate buffer solution, add laccase and sodium pyrophosphate, and stir the reaction at 200-300 r / min for 60-90 min at 35-40℃. During the reaction, oxygen is continuously introduced and the oxygen flow rate is controlled at 0.5-1.0 L / min. 2) Adjust the pH of the system obtained in step 1) to 6.5-7.0 with 0.04-0.08 mol / L sodium hydroxide solution, add ammonium persulfate, heat to 50-55℃ under nitrogen protection, add vinyltrimethoxysilane, and stir the reaction at 400-500 r / min for 90-120 min. 3) Place the reaction solution obtained in step 2) under vacuum degassing conditions of 60-65℃ and vacuum degree of -0.085MPa to -0.095MPa for 20-40 minutes to obtain enzyme-modified silane sisal pulp.

5. The seven-layer corrugated board according to claim 1, characterized in that, The raw materials for preparing the quaternary ammonium salt polyurethane montmorillonite composite emulsion, by weight, include: 8-12 parts sodium montmorillonite, 45-55 parts polypropylene glycol, 20-30 parts isophorone diisocyanate, 0.05-0.1 parts organotin catalyst, 3-4 parts N-methyldiethanolamine, 1.5-2.0 parts glacial acetic acid with a concentration of 0.10-0.15 mol / L, and 60-80 parts deionized water.

6. The seven-layer corrugated board according to claim 5, characterized in that, The preparation method of the quaternary ammonium salt polyurethane montmorillonite composite emulsion includes the following steps: (1) Weigh out polypropylene glycol, isophorone diisocyanate and organotin catalyst according to the proportion, and stir the reaction at 85-90℃ and under nitrogen protection for 150-180 min to obtain isocyanate-terminated polyurethane prepolymer. (2) Cool the isocyanate-terminated polyurethane prepolymer obtained in step (1) to 70-75℃, add N-methyldiethanolamine, stir at 300-400 r / min for 50-70 min, then cool to 40-45℃, add glacial acetic acid to neutralize to pH 6.0-7.0, continue stirring for 20-30 min to generate cationic polyurethane prepolymer; (3) Add sodium montmorillonite and deionized water to the cationic polyurethane prepolymer obtained in step (2), and shear emulsify at a speed of 6000-8000 r / min for 25-35 min, and then mature at 55-65℃ for 100-140 min to obtain quaternary ammonium salt polyurethane montmorillonite composite emulsion.

7. The seven-layer corrugated board according to claim 6, characterized in that, In step (1), the nitrogen flow rate is controlled at 0.1-0.3 L / min and the stirring speed is 300-400 r / min.

8. The seven-layer corrugated board according to claim 1, characterized in that, The preparation method of the intermediate support layer slurry includes the following steps: a. Add the enzyme-modified silane sisal pulp and basalt fiber to a high-speed mixer and disperse them at 25-30℃ and a speed of 600-800 r / min for 10-15 min. b. Add cross-linked oxidized starch, waterborne polyurethane and chitosan to the system obtained in step a, heat to 35-40℃, and stir at 400-500 r / min for 20-30 min. c. Stir the system obtained in step b at 30-35℃ and 500-600 r / min, and add hydroxyl fluorosilicone oil, nano titanium dioxide, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, fumed nano silica and γ-methacryloyloxypropyltrimethoxysilane in sequence. Stir for 5-8 min after each addition of an additive. d. Add quaternary ammonium salt polyurethane montmorillonite composite emulsion to the system obtained in step c, and stir at 300-400 r / min for 30-40 min at 60-70℃ and pH 5.0-6.

0. Then cool down to 30-35℃, add deionized water, and stir at 400-500 r / min for 50-70 min to obtain the intermediate support layer slurry.

9. A manufacturing process for a seven-layer corrugated board according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Raw material pretreatment: The substrates of the outer protective layer, outer core paper layer, outer corrugated layer, inner corrugated layer, inner core paper layer and inner protective layer are placed in a constant humidity and temperature room and conditioned for 12-18 hours under the environmental conditions of 23±2℃ and 50±5% relative humidity. S2. Corrugated Forming: The corrugated base paper of the outer and inner corrugated layers is formed into A-type corrugated paper under the conditions of preheating temperature of 150-160℃ and corrugated roll running speed of 15-20m / min, with a flute height of 4.5-5.0mm and a flute pitch of 8-9mm. S3. Intermediate Support Layer Coating and Molding: The intermediate support layer slurry is coated using a doctor blade coating machine at a rate of 200-250 g / m². 2 The coating amount is applied to the inner surface of the outer corrugated layer and dried in a hot air drying oven at a temperature of 100-110℃ for 6-8 minutes. The thickness of the intermediate support layer after drying is controlled to be 1.5-2.0 mm. S4. Interlayer bonding and lamination: For external lamination, the outer protective layer, outer core paper layer, and outer corrugated layer coated with the intermediate support layer are laminated using a laminating machine. The lamination temperature is 120-130℃, the lamination pressure is 0.3-0.4MPa, and the lamination speed is 10-15m / min. For internal lamination, the other side of the intermediate support layer is laminated sequentially with the inner corrugated layer, inner core paper layer, and inner protective layer. The adhesive and process parameters are the same as for external lamination. S5. Hot pressing and curing: The seven-layer corrugated board after step S4 is put into a hot press and pre-cured for 3-5 minutes at 100-110℃ and 0.35-0.45MPa; then the temperature is increased to 125-135℃ at a rate of 5-8℃ / min and pressure is maintained at 0.4-0.5MPa for 5-7 minutes. S6. Post-processing: After hot pressing in step S5, the seven-layer corrugated board is air-cooled and humidified, and then cut and trimmed in sequence to obtain the finished seven-layer corrugated board.

10. The production process of the seven-layer corrugated board according to claim 9, characterized in that, In step S6, the conditions for air-cooled humidity conditioning are as follows: air-cooled humidity conditioning for 6-8 hours at a temperature of 40-50℃ and a wind speed of 3-5m / s to reduce the moisture content of the corrugated board to 8%-10%, followed by moisture content equilibration for 2-4 hours at a temperature of 23±2℃ and a relative humidity of 50±5%.

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