Anti-deformation paperboard for packaging box and preparation method of anti-deformation paperboard

Abstract

The invention provides an anti-deformation paperboard for a packaging box and a preparation method thereof.The anti-deformation paperboard comprises two layers of paperboard and a core layer arranged between the two layers of paperboard, and the core layer comprises, by mass, 100 parts of wood leaf fibers; 2 to 4 parts of nano cellulose; 8-12 parts of polyvinyl alcohol; 1 to 3 parts of polyethyleneimine; 4-8 parts of a hydrophilic modified inorganic filler; and 1-3 parts of a cross-linking agent. By optimizing the structure and component proportion of the core layer, the paperboard can keep stable deformation and is not easy to rebound in a crease area after hot press molding, and meanwhile, good dimensional stability is kept under the condition of humidity change, so that the molding precision and the use durability of the packaging box are improved.

Description

Technical Field

[0001] The present application relates to the technical field of packaging materials, and in particular to a deformation-resistant cardboard for packaging boxes and a preparation method thereof. Background Art

[0002] Packaging boxes are widely used in high-end consumer goods, tobacco products, and gift decorations. Their structural design not only requires excellent support strength and formability, but also requires stability in the crease area after hot pressing, without significant springback. Furthermore, during transportation and use, packaging boxes must also exhibit strong environmental adaptability, especially to cope with dimensional changes and structural deformation caused by humidity fluctuations.

[0003] At present, the deformation-resistant cardboards widely used on the market mostly adopt a multi-layer composite structure or a sandwich structure, in which the core layer is often based on ordinary wood pulp paper or recycled fiber, and is pressed with the surface cardboard after being formed by papermaking. However, there are two prominent problems with the traditional core layer structure. First, the crease is easy to rebound after hot pressing. The structural rigidity or plasticity of the core layer in the existing cardboard is insufficient, making it difficult to form a stable deformation structure at the crease position, resulting in difficulty in maintaining the hot pressing mark, affecting the molding accuracy and visual quality of the packaging box; second, the dimensional stability is poor in a wet environment. Conventional pulp materials are prone to expansion after absorbing moisture, especially the core layer will cause structural bulging, warping and other problems after absorbing water between layers, resulting in obvious deformation of the packaging box during humid storage or transportation, affecting the performance and product image.

[0004] To this end, some technologies have attempted to improve its anti-deformation performance by increasing the paper weight, introducing a coating layer, or filling foaming materials between cardboards. However, the above methods have problems such as increased thickness, complex processes, or insufficient environmental adaptability.

[0005] Therefore, how to improve the deformation retention ability of cardboard in the crease area after hot pressing while reducing its dimensional change rate in a wet environment is a technical problem that needs to be urgently solved in the current field of deformation-resistant cardboard. Summary of the Invention

[0006] The present application provides a deformation-resistant cardboard for packaging boxes and a preparation method thereof, aiming to solve the problem that the crease area of ​​the existing packaging cardboard is easy to rebound after hot pressing and has poor dimensional stability in a wet environment.

[0007] In the first aspect, the present application provides a deformation-resistant cardboard for packaging boxes, comprising: two layers of cardboard, and a core layer arranged between the two layers of cardboard, wherein the core layer comprises the following raw materials in parts by mass: 100 parts of wood leaf fibers; 2 to 4 parts of nanocellulose; 8 to 12 parts of polyvinyl alcohol; 1 to 3 parts of polyethyleneimine; 4 to 8 parts of hydrophilic modified inorganic fillers; and 1 to 3 parts of a cross-linking agent.

[0008] According to the present application, by optimizing the core layer structure and component ratio, the cardboard can maintain stable deformation in the crease area after hot pressing and is not easy to rebound. At the same time, it maintains good dimensional stability under humidity changes, thereby improving the molding accuracy and durability of the packaging box.

[0009] Specifically, the core layer uses wood leaf fibers as its primary structural framework, providing the paperboard's basic support and thickness. The introduced nanocellulose, with its high specific surface area to aspect ratio, forms a dense network between the wood fibers, limiting hygroscopic expansion and stabilizing the spatial structure, significantly reducing the paperboard's dimensional change rate under humidity fluctuations. Furthermore, nanocellulose, with its small particle size and negative charge, is susceptible to water loss in traditional pulp systems. This application introduces polyethyleneimine as a cationic polymer, which significantly increases the retention of nanocellulose in the pulp through electrostatic adsorption, enhancing its reinforcing effect within the core layer.

[0010] Polyvinyl alcohol (PVA) is the primary film-forming polymer, whose flexible chains responsively deform during hot pressing and maintain the indented structure after cooling. Polyethyleneimine not only acts as a retention aid but also forms a hydrogen-bonding system with the PVA and nanocellulose. Together, these two, under the action of a crosslinker, form a dense crosslinked network, effectively inhibiting chain flow and improving the wet stability of the core layer's overall structure and the structural retention of the indented area. During the hot pressing process of laminating the core layer to the cardboard, the crosslinking agent causes a simultaneous crosslinking reaction between the PVA and PVA, initially establishing a stable three-dimensional network structure. This locks the PVA, PVA, and PVA nanocellulose together, promoting a stable and uniform distribution of the nanocellulose within the three-dimensional network and allowing it to fully function.

[0011] At the same time, during the hot pressing process of laminating the core layer and the cardboard to prepare anti-deformation cardboard, the cross-linking reaction is basically completed, but a small amount of cross-linking sites will remain. In the subsequent hot indentation treatment, some of the residual cross-linking sites can continue to react. At the same time, the chain segments are rearranged in the indentation area and shaped by the network structure, which helps to stabilize the indentation morphology, reduce rebound after pressing, and improve the indentation retention rate.

[0012] The hydrophilically modified inorganic fillers distributed within the core layer also play a crucial supporting role. These particles, treated with a functionalized coating, possess excellent dispersibility and a polar interface, enabling interfacial bonding with the cross-linked network. Embedded within the colloidal structure, they serve as microscopic fulcrums, helping to withstand external forces, prevent chain segment rebound, and diffuse stress from the indentation area to the surrounding area, effectively mitigating deformation recovery caused by stress concentration. Furthermore, their surface structure exhibits good compatibility with the nanocellulose and polyvinyl alcohol networks, contributing to improved network stability.

[0013] In summary, the components in the core layer work synergistically through spatial configuration and chemical structure to form a composite structural system of "fiber skeleton + expansion-limiting network + cross-linked colloid + anchoring filler", which achieves deformation stability in the indentation area and dimensional control ability in a wet environment, providing the packaging box with excellent molding retention performance and structural reliability.

[0014] In some embodiments, the nanocellulose includes oxidized nanocellulose and cellulose nanocrystals, and the mass ratio of the oxidized nanocellulose to the cellulose nanocrystals is 1:0.5-1.5.

[0015] In some of the aforementioned embodiments, oxidized nanocellulose (TOCN) possesses a flexible long-chain structure and a high specific surface area. Its surface contains carboxyl functional groups, which form numerous hydrogen bonds with polyvinyl alcohol (PVA) and polyethyleneimine (PEI) in the slurry, creating a continuous and flexible expansion-limiting network. This network is distributed between the wood leaf fibers, effectively limiting interfiber slippage and void expansion after moisture absorption, thereby improving the dimensional stability of the core layer in high-humidity environments.

[0016] Cellulose nanocrystals (CNCs) are short, rod-shaped particles with high crystallinity. They are rigid, dimensionally stable, and resistant to deformation within a cross-linked network. When embedded within the TOCN network, CNCs act as a "microscopic skeleton," helping to maintain the spatial structure of the overall network and inhibiting chain collapse or network compression deformation when the cardboard is subjected to heat, pressure, or swelling stress. Furthermore, due to their small particle size and high specific surface area, CNCs form a large number of rigid limiting points within the system, forming stable connections with the cross-linked network through physical entanglement or electrostatic adsorption, further enhancing structural strength.

[0017] The combination of TOCN and CNC not only achieves the structural synergy of "flexible chain segment expansion limitation + rigid particle anchoring", but also improves the uniformity and density of the network structure through complementary distribution at the microscale, reducing the rate of expansion. When the mass ratio of the two is controlled within the range of 1:0.5-1.5, it can effectively avoid excessive entanglement of TOCN and the resulting decrease in fluidity, and also prevent excessive accumulation of CNC in the network, resulting in uneven distribution or lack of continuity, thus achieving a balance between film-forming properties, mechanical stability and interfacial compatibility.

[0018] Compared to solutions using TOCN alone, the addition of CNC creates more rigid deformation-resistant points during the indentation process, improving the stability of the indentation structure. Compared to solutions using CNC alone, the addition of TOCN introduces a continuous expansion-limiting network, which helps improve wet expansion resistance and network flexibility. Consequently, this composite structure exhibits better overall performance in terms of indentation retention and wet expansion rate control.

[0019] Furthermore, compared to the hydrophilic-modified inorganic fillers introduced into the core layer, CNC, while also rigid particles, has a smaller particle size and is distributed closer to the interior of the cross-linked network, filling and limiting structural gaps at the microscopic level. Meanwhile, the inorganic fillers, at a larger size, are embedded within the overall network, forming macroscopic anchor points that support deformation and diffuse stress in the indentation area. The two complement each other in terms of particle size, distribution, and action pathways, jointly constructing a structurally stable system from the microscopic to the mesoscopic level, thereby providing a synergistic enhancement in indentation retention and wet-state dimensional control.

[0020] In some embodiments, the average diameter of the oxidized nanocellulose is 10-20 nm, and the average length is 500-1000 nm; the average diameter of the cellulose nanocrystals is 5-15 nm, the average length is 100-500 nm, and the crystallinity is 60%-90%.

[0021] In some of the above-mentioned embodiments, the oxidized nanocellulose (TOCN) is in the form of flexible filaments, and its diameter is controlled within the range of 10 to 20 nm, which is conducive to its uniform dispersion in the slurry while retaining a high specific surface area; the length is controlled within the range of 500 to 1000 nm, which can bridge between fibers to form a continuous expansion-limiting network, enhance network connectivity, and improve dimensional stability in the hygroscopic state.

[0022] Cellulose nanocrystals (CNCs), used as rigidity-enhancing particles, are preferably 5-15 nm in diameter and 100-500 nm in length, which facilitates their embedding into the network structure and forms uniform confinement points. A controlled length of 100-500 nm prevents particle sedimentation or accumulation within the system, improving dispersion stability. A controlled crystallinity of 60%-90% maintains sufficient rigidity to serve as microsupport points while also allowing for interfacial bonding with the colloidal network.

[0023] By rationally designing the particle size and morphology parameters of oxidized nanocellulose and cellulose nanocrystals, the two can achieve a synergistic effect of flexible chain segment expansion limitation and rigid particle anchoring in the core layer, thereby improving the uniformity, stability and stress response capability of the expansion limitation network, thereby further improving the indentation retention effect and wet dimensional stability of the paperboard.

[0024] In some embodiments, the hydrophilic modified inorganic filler is obtained by co-depositing dopamine and tannic acid on the surface of the inorganic filler.

[0025] In some of the aforementioned embodiments, the inventors discovered that, compared to hydrophilically modified inorganic fillers obtained by self-polymerization and deposition of dopamine on the surface of inorganic fillers alone, inorganic fillers treated with co-deposition and coating with dopamine and tannic acid, when used in the core structure of deformation-resistant paperboard, can further improve the paperboard's indentation retention after hot indentation and its dimensional stability in hot and humid environments. This may be due to the high density of phenolic hydroxyl groups in tannic acid molecules, which can co-deposit with dopamine oxidation products on the particle surface during the co-deposition process through electrostatic adsorption, hydrogen bonding, and π-π conjugated stacking. This helps to form a composite coating with a more uniform film distribution, richer functional groups, and stronger hydrophilicity. This composite coating helps improve the water dispersibility and retention of the particles during the slurry papermaking process, and strengthens the interfacial bonding between the particles and components such as polyvinyl alcohol and polyethyleneimine within the colloidal network, thereby enhancing the anchoring effect of the particles within the network structure.

[0026] Specifically, the weak acidity of tannic acid causes it to partially dissociate into a negatively charged form under alkaline conditions, which may give the particle surface a higher negative potential, thereby enhancing the electrostatic adsorption between it and the positively charged polyethyleneimine, which helps to improve the retention efficiency and network integration ability of the particles during the papermaking process; at the same time, the flexible aromatic branched structure of tannic acid may alleviate the rigidity of the polydopamine film layer by regulating the excessive cross-linking tendency of the polydopamine network, thereby improving its deformation adaptability and stress buffering capacity in the indentation area; and under high humidity conditions, tannic acid has a higher density of phenolic hydroxyl structure, which cooperates with the active groups on the surface of polydopamine to form a stable connection with the cross-linked network of PVA, PEI and nanocellulose, while limiting the moisture adsorption path and structural expansion space, which helps to improve the wet dimensional stability of the paperboard.

[0027] On the other hand, if tannic acid is used alone to coat inorganic fillers, although the surface potential of the modified filler is higher, due to the lack of oxidative self-polymerization ability of tannic acid itself, the deposited layer formed on the particle surface is mostly in the form of non-covalent adsorption, lacking a stable spatial framework, and the film layer is prone to shear, hot pressing, or wet expansion. However, by forming a synergistic polymerization structure with dopamine, the adhesion and mechanical strength of the film layer can be significantly improved, ensuring that the particles can still play an effective supporting role in the deformation area of ​​the structure.

[0028] Based on the above-mentioned multi-faceted synergistic effects, the composite coating layer constructed by the synergistic co-deposition of dopamine and tannic acid can more effectively improve the distribution stability and interfacial force of particles in the network structure compared to fillers modified with dopamine or tannic acid alone, thereby improving the structural stability and anti-wetting performance of the paperboard during use.

[0029] In some embodiments, the hydrophilic modified inorganic filler is prepared by the following method: The inorganic filler is dispersed in a Tris-HCl buffer solution with a pH of 8 to 9 and includes 0.5 to 2 g / L dopamine and 0.5 to 2 g / L tannic acid, and reacted at 20 to 25° C. for 12 to 24 hours under aerobic conditions to obtain a hydrophilic modified inorganic filler.

[0030] In some of the aforementioned embodiments, the pH of the Tris-HCl buffer is controlled between 8 and 9, which facilitates the smooth progress of the dopamine auto-oxidation reaction. This pH range also increases the negative charge on the surface of the inorganic filler, promoting the adhesion stability of the coating. The concentrations of both dopamine and tannic acid are controlled within the range of 0.5 to 2 g / L, ensuring a balance between reaction efficiency and film quality. A concentration that is too low results in insufficient coating and difficulty forming a continuous film layer; a concentration that is too high tends to cause precipitation of free polymer products, reducing coating uniformity and affecting particle dispersibility. The reaction temperature is controlled between 20 and 25°C, close to room temperature, to help avoid excessive cross-linking of the polymerized film layer, particle agglomeration, or structural expansion at high temperatures. The reaction time is controlled between 12 and 24 hours to ensure sufficient and stable film formation and a dense coating on the particle surface.

[0031] Through the co-precipitation reaction under the above-mentioned conditions, a hydrophilic-modified inorganic filler with a uniform surface, rich functional groups, and stable structure is obtained. These particles exhibit excellent dispersibility and retention in the slurry, easily forming a colloidal network with matrix components such as PVA, PEI, and TOCN, and becoming embedded within the cross-linked structure during the papermaking process. Ultimately, this imparts stronger interfacial adhesion and multi-scale support to the core layer, thereby improving the indentation retention and wet dimensional stability of the deformation-resistant paperboard.

[0032] In some embodiments, the inorganic filler includes at least one of silicon dioxide and calcium carbonate; and the average particle size of the inorganic filler is 2 to 5 μm.

[0033] In some of the aforementioned embodiments, silica and calcium carbonate, as common inorganic fillers, are abundant, low-cost, and thermally stable, and are widely used in paper reinforcement and coating. These inorganic fillers possess a high specific surface area and surface hydroxyl groups, which form stable bonds with dopamine and tannic acid, facilitating the formation of a uniform and dense coating layer. Furthermore, these rigid particles provide structural support, and their high bulk modulus helps enhance the structural rigidity of the core layer under compression.

[0034] The inorganic filler preferably has an average particle size of 2-5 μm. Fillers within this particle size range exhibit excellent dispersion stability, are less prone to agglomeration or sedimentation, are easily co-molded with the colloid material during papermaking, and are stably embedded within the cross-linked network during hot pressing. After modification through the aforementioned dopamine-tannic acid co-precipitation, its surface possesses excellent hydrophilicity and chemical reactivity, enabling multi-point interfacial bonding with materials such as polyvinyl alcohol, polyethyleneimine, and nanocellulose. This enhances the consistency and toughness of the core structure, effectively suppressing rebound deformation and wet dimensional change in the indented area.

[0035] In some embodiments, the cardboard includes a surface density of 80 to 160 g / m 2 coated paper.

[0036] In some of the aforementioned embodiments, coated paper exhibits excellent surface density, stiffness, and embossability, making it a common outer layer material for high-end printed packaging such as boxes and cigarette cases. Its surface, after coating and calendering, exhibits good ink adaptability and mechanical strength, facilitating subsequent hot stamping, folding, and surface printing.

[0037] The surface density is controlled at 80-160 g / m 2 By selecting coated paper with appropriate surface density and combining it with a core layer material with expansion control and indentation stabilization functions, a sandwich-type layered structure with coordinated structure and balanced stress conduction can be constructed. The coated paper provides good compression resistance and surface stability, and the core layer provides buffering and limiting functions. The two work synergistically in the indentation area, helping to form a stable and uniform fold line structure and maintain good shape retention in a hot and humid environment.

[0038] Therefore, 80~160g / m 2 As the surface material of cardboard, coated paper can ensure the structural strength of the cardboard while improving its molding quality and service life.

[0039] In some embodiments, the wood fiber is derived from at least one of softwood pulp and hardwood pulp. Based on these embodiments, softwood pulp fibers are longer and more flexible, contributing to the structural support structure and improving the core layer's overall toughness and indentation retention. Hardwood pulp fibers, on the other hand, are shorter and have a larger surface area, facilitating colloid attachment and controlling network uniformity. A blend of these two types of fibers, or their use alone, can create a stable multi-scale fiber structure, significantly improving core layer strength and paper density.

[0040] In some embodiments, the polyvinyl alcohol has a weight-average molecular weight of 30,000 to 80,000 and a degree of hydrolysis of 88% or greater. Based on the aforementioned embodiments, this molecular weight range balances the film-forming ability and water solubility of PVA, enabling its stable presence in papermaking slurry and its participation in papermaking network construction. A higher degree of hydrolysis results in a higher hydroxyl density, which facilitates the formation of a hydrogen bonding network with polyethyleneimine, nanocellulose, and modified fillers, thereby improving the core layer's structure density and crosslinking stability, and enhancing its wet-state dimensional control capabilities.

[0041] In some embodiments, the weight-average molecular weight of the polyethyleneimine is 1000-3000. Based on the above embodiments, PEI in this molecular weight range, while maintaining good solubility and cationic density, possesses strong retention and a certain degree of network cohesion. This promotes the effective retention of nanocellulose and crosslinking agents during the papermaking process and further participates in crosslinking and curing after papermaking, enhancing the interfacial bonding and structural integrity of the colloidal network, thereby synergistically enhancing indentation stability and resistance to moisture deformation.

[0042] In some embodiments, the cross-linking agent includes glutaraldehyde. Based on the above embodiment, glutaraldehyde is a commonly used small molecule dialdehyde cross-linking agent. Although glutaraldehyde is a small molecule, it has good reactivity with the amine groups on PEI in the slurry and can be retained by grafting with PEI. Therefore, it is not easily lost during the papermaking process. During the hot pressing process, it can further react with the hydroxyl and amine groups in PVA, PEI, nanocellulose, and hydrophilically modified inorganic fillers to form stable chemical linkages such as ether bonds and imine structures, thereby forming a spatial cross-linked network in the core layer.

[0043] In some embodiments, the surface density of the core layer is 120-250 g / m 2 Based on the above implementation, the core layer in this surface density range can produce controllable directional deformation during the hot indentation process while having sufficient strength and compressive resistance, and cooperate with the cross-linked network and inorganic filler to form an indentation limiting structure, thereby improving the fold line stability and wet dimensional retention ability.

[0044] In a second aspect, the present application provides a method for preparing a deformation-resistant paperboard for a packaging box, comprising: Providing raw materials for the core layer of the deformation-resistant paperboard according to any embodiment of the first aspect; Dispersing the raw materials in water to obtain a core layer slurry; Sheet-forming the core layer slurry to obtain a core layer; The core layer is placed between two sheets of cardboard, and hot pressed to obtain a deformation-resistant cardboard.

[0045] According to the present application, the method forms a core layer pulp with expansion limiting and multi-point anchoring functions, and hot-presses it with surface cardboard to construct a deformation-resistant cardboard with stable structure, strong resistance to moisture deformation and excellent indentation retention.

[0046] Specifically, the method includes the following steps: first, dispersing wood leaf fibers, nanocellulose, polyvinyl alcohol (PVA), polyethyleneimine (PEI), a hydrophilically modified inorganic filler, and a crosslinking agent in water to produce a stable and uniform core layer slurry. During the papermaking process, this slurry promotes the retention of nanocellulose and crosslinked small molecules through the polyethyleneimine retention mechanism, forming a uniformly structured wet paper sheet. Subsequently, during hot pressing, under the influence of temperature and pressure, the crosslinking agent chemically reacts with PVA, PEI, and nanocellulose, forming a three-dimensional crosslinked network. Simultaneously, the hydrophilically modified inorganic filler forms multiple interfacial bonds with the network structure, achieving particle anchoring and stress conduction.

[0047] This core paper sheet is placed between two sheets of cardboard and hot-pressed to achieve structural integration while simultaneously creating indentations in the surface cardboard. The resulting paperboard exhibits excellent indentation retention and wet dimensional stability, making it suitable for a wide range of packaging applications requiring precise crease positioning and moisture resistance, particularly in demanding applications such as cigarette boxes, gift boxes, and folding boxes.

[0048] In some embodiments, the method specifically comprises: Dispersing leaf fibers from softwood pulp in water and beating the pulp to a beating degree of 30-45°SR, nanocellulose, a pre-dissolved polyvinyl alcohol aqueous solution, a polyethyleneimine aqueous solution, a hydrophilic modified inorganic filler, and a cross-linking agent are then added to the pulp; the polyvinyl alcohol is pre-dissolved in water at 80-90°C to obtain a polyvinyl alcohol aqueous solution, which is then cooled to 20-25°C before being added to the pulp; the polyethyleneimine is first dissolved in water and then added dropwise to the pulp; the components are mixed and stirred for 30-60 minutes before adding water to obtain a core layer pulp with a solid content of 2%-5%; The core layer slurry is papered on a conventional wet-laid paper machine, flowed onto a forming mesh, and subjected to vacuum dehydration and pressing to obtain a wet paper sheet. The vacuum dehydration time is 10-30 seconds, and the pressing pressure is 0.2-0.4 MPa. The pressed wet paper sheet is sent to a drying section and dried at 70-100°C until the moisture content is less than 10%, thereby obtaining a core layer. The surface density of the dried core layer is controlled to be 120-250 g / m 2 ; Place the dried core layer between two pieces of cardboard, and perform lamination pressing on a hot press at 120-160°C and 0.5-1.5 MPa for 0.5-2 minutes to obtain the deformation-resistant cardboard.

[0049] In a third aspect, the present application provides a packaging box obtained by subjecting the anti-deformation cardboard according to any embodiment of the first aspect or the anti-deformation cardboard prepared according to the method according to any embodiment of the second aspect to hot-stamping treatment and folding.

[0050] According to the present application, the packaging box is made by subjecting the anti-deformation cardboard to hot indentation treatment and folding molding. It can maintain a stable indentation structure and clear fold lines under long-term use and high-humidity environment, significantly improving the molding accuracy, structural strength and environmental adaptability of the packaging box.

[0051] The packaging box uses the deformation-resistant cardboard provided in this application as its base material. This cardboard is composed of two layers of coated cardboard with a functional core layer sandwiched between them. Nanocellulose, polyvinyl alcohol, polyethyleneimine, a crosslinker, and hydrophilically modified inorganic fillers are introduced into the core layer. Through hot pressing, a three-dimensional crosslinked network and particle anchoring structure are formed, providing localized rigid support and segment-limiting capabilities in the crease area. During actual processing, the cardboard can be pressed into a preset fold line in one go using a hot pressing mold. After die-cutting, it is folded and formed into a box body with a stable geometric structure. It is suitable for use in applications such as cigarette boxes, gift boxes, and folding boxes that require high-precision positioning and shape retention.

[0052] Compared with traditional packaging cardboard, the molding structure of the packaging box is less likely to rebound, bulge or deform in size after being subjected to force or absorbing moisture, which can significantly improve the appearance consistency, service life and user experience of the finished packaging product.

[0053] In some embodiments, the hot indentation treatment conditions include: using a linear die to perform hot pressing at 110-130° C. and 2-4 MPa for 2-5 seconds to form hot indentations with a width of 0.3-0.6 mm.

[0054] Compared with the prior art, the present invention has the following advantages: 1. By introducing a core layer structure containing nanocellulose, polyvinyl alcohol, polyethyleneimine, a crosslinking agent, and a hydrophilically modified inorganic filler, a dense and stable three-dimensional crosslinked network and multi-point anchoring structure are constructed during the interlayer hot pressing process. This significantly improves the rebound resistance of the indented area of ​​the cardboard, solving the problem of easy crease recovery and blurred line shape in traditional packaging cardboard. 2. The synergistic combination of oxidized nanocellulose and cellulose nanocrystals in the core layer constructs a flexible expansion-limiting network and a rigid embedded skeleton, which improves wet dimensional stability while maintaining sufficient flexibility, effectively alleviating the bulging and deformation caused by moisture absorption and expansion of conventional paperboard in humid environments; 3. After being modified by the synergistic coating of dopamine and tannic acid, the inorganic filler has good hydrophilicity and interface adaptability, which can not only achieve dispersion stability and effective retention, but also serve as a local support point during the hot indentation process to enhance the retention and deformation resistance of the indentation structure. DETAILED DESCRIPTION

[0055] The various embodiments or implementation schemes in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments.

[0056] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with an embodiment or example is included in at least one embodiment or example of the present application. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0057] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0058] In the description of this specification, unless otherwise specified, "parts" refer to "parts by mass".

[0059] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0060] Oxidized nanocellulose, TEMPO-oxidized nanocellulose, has an average diameter of approximately 15 nm; Cellulose nanocrystals have an average diameter of about 10 nm and a crystallinity of 60% to 90%; Calcium carbonate particles, with an average particle size of about 3 μm; Double-sided coated paper, surface density 100g / m 2 , purchased from Yu Renyi Paper Industry; Polyvinyl alcohol, with a weight-average molecular weight of approximately 50,000 and a degree of hydrolysis of approximately 90%; Polyethyleneimine, weight average molecular weight about 2000; Bleached softwood pulp, Canadian Northern Bleached softwood pulp.

[0061] Preparation Example 1 Preparation of hydrophilic modified inorganic fillers: 10 parts of calcium carbonate particles were weighed and dispersed in 500 parts of Tris-HCl buffer (pH 8.5) containing 1.0 g / L dopamine hydrochloride and 1.0 g / L tannic acid, and ultrasonically dispersed for 5 minutes. The mixture was then stirred and reacted in an air environment at 25°C for 18 hours. After the reaction, the mixture was centrifuged, the supernatant was discarded, and the mixture was washed with water three times until the pH of the washing solution returned to neutral. The resulting precipitate was vacuum dried at 60°C for 8 hours to obtain a hydrophilic modified inorganic filler A.

[0062] Preparation Example 2 Preparation of hydrophilic modified inorganic fillers: 10 parts of calcium carbonate particles were weighed and dispersed in 500 parts of Tris-HCl buffer at pH 8.5 containing 2.0 g / L dopamine hydrochloride, and ultrasonically dispersed for 5 minutes. The mixture was then stirred and reacted in an air environment at 25°C for 18 hours. After the reaction, the mixture was centrifuged, the supernatant was discarded, and the mixture was washed with water three times until the pH of the washing solution returned to neutral. The resulting precipitate was vacuum dried at 60°C for 8 hours to obtain a hydrophilic modified inorganic filler B.

[0063] Preparation Example 3 Preparation of hydrophilic modified inorganic fillers: 10 parts of calcium carbonate particles were weighed and dispersed in 500 parts of Tris-HCl buffer at pH 8.5 containing 2.0 g / L tannic acid, and ultrasonically dispersed for 5 minutes. The mixture was then stirred and reacted in an air environment at 25°C for 18 hours. After the reaction, the mixture was centrifuged, the supernatant was discarded, and the mixture was washed with water three times until the pH of the washing solution returned to neutral. The resulting precipitate was vacuum dried at 60°C for 8 hours to obtain a hydrophilic modified inorganic filler C.

[0064] Preparation Example 4 Preparation of hydrophilic modified inorganic fillers: 10 parts of calcium carbonate particles were weighed and dispersed in 250 parts of ethanol-water solution (the volume ratio of ethanol to water was 7:3), and ultrasonic dispersion was performed for 5 minutes. 1 part of γ-aminopropyltriethoxysilane was added, and the mixture was heated under reflux at 70°C for 3 hours. After the reaction, the mixture was centrifuged, the supernatant was discarded, and the mixture was repeatedly washed with ethanol and water. The resulting precipitate was vacuum dried at 60°C for 8 hours to obtain a hydrophilic modified inorganic filler D.

[0065] Example 1 Preparation of anti-deformation cardboard for packaging boxes: 100 parts of bleached softwood pulp containing leaf fibers was weighed, dispersed with deionized water, and then beaten until the beating degree reached 35° SR, resulting in a slurry with uniform fiber dispersion. Subsequently, the following components were added in sequence: 3 parts of oxidized nanocellulose with an average length of approximately 800 nm, 3 parts of cellulose nanocrystals with an average length of approximately 300 nm, 10 parts of polyvinyl alcohol, 2 parts of polyethyleneimine, 6 parts of hydrophilically modified inorganic filler A, and 2 parts of glutaraldehyde. Polyvinyl alcohol (weight-average molecular weight approximately 50,000, degree of hydrolysis 88%) was dissolved in 90°C water, stirred thoroughly, cooled to 25°C, and then added to the slurry. Polyethyleneimine was slowly added dropwise to the slurry as a 10wt% aqueous solution. Glutaraldehyde was slowly added dropwise to the slurry as a 25wt% aqueous solution. After mixing, the above components were stirred at room temperature for 45 minutes, and deionized water was added to the system to a solid content of 3.5% to obtain the core layer slurry. The obtained core layer slurry is transported to a conventional wet paper machine, slurried to a forming mesh, dehydrated under vacuum conditions for 20 seconds, and then pressed at a pressure of 0.3 MPa for 30 seconds to obtain a wet paper sheet. The wet paper sheet is dried at 85°C for 6 minutes until the moisture content is less than 10%, forming a dense core layer paper sheet. The density after drying is controlled to 180g / m 2 ; The core paper is placed between two sheets with a surface density of 100 g / m 2 The paper is placed between double-sided coated papers and sent to a flat hot press, where it is hot-pressed at 140°C and 1.2 MPa for 90 seconds to obtain a deformation-resistant cardboard with a stable structure.

[0066] Example 2 Preparation of anti-deformation cardboard for packaging boxes: It is substantially the same as Example 1, except that 6 parts of oxidized nanocellulose with an average length of about 800 nm are used instead of 3 parts of oxidized nanocellulose with an average length of about 800 nm and 3 parts of cellulose nanocrystals with an average length of about 300 nm.

[0067] Example 3 Preparation of anti-deformation cardboard for packaging boxes: The method is substantially the same as Example 1, except that 6 parts of cellulose nanocrystals with an average length of about 300 nm are used instead of 3 parts of oxidized nanocellulose with an average length of about 800 nm and 3 parts of cellulose nanocrystals with an average length of about 300 nm.

[0068] Example 4 It is substantially the same as Example 1, except that an equal amount of oxidized nanocellulose with an average length of about 300 nm is used to replace the oxidized nanocellulose with an average length of about 800 nm.

[0069] Example 5 Preparation of anti-deformation cardboard for packaging boxes: The process is substantially the same as Example 1, except that the hydrophilic modified inorganic filler B is used instead of the hydrophilic modified inorganic filler A.

[0070] Example 6 Preparation of anti-deformation cardboard for packaging boxes: The process is substantially the same as Example 1, except that the hydrophilic modified inorganic filler C is used instead of the hydrophilic modified inorganic filler A.

[0071] Example 7 Preparation of anti-deformation cardboard for packaging boxes: The process is substantially the same as Example 1, except that the hydrophilic modified inorganic filler D is used instead of the hydrophilic modified inorganic filler A.

[0072] Comparative Example 1 Preparation of anti-deformation cardboard for packaging boxes: 106 parts of bleached softwood pulp containing leaf fibers was weighed, dispersed with deionized water, and then beaten until the beating degree reached 35° SR, resulting in a slurry with uniform fiber dispersion. Subsequently, the following components were added in sequence: 10 parts polyvinyl alcohol, 2 parts polyethyleneimine, 6 parts hydrophilic modified inorganic filler A, and 2 parts glutaraldehyde. Polyvinyl alcohol (weight-average molecular weight approximately 50,000, degree of hydrolysis 88%) was dissolved in 90°C water, stirred thoroughly, cooled to 25°C, and then added to the slurry. Polyethyleneimine was slowly added dropwise to the slurry as a 10wt% aqueous solution. Glutaraldehyde was slowly added dropwise to the slurry as a 25wt% aqueous solution. After mixing, the above components were stirred at room temperature for 45 minutes, and deionized water was added to the system to a solid content of 3.5% to obtain the core layer slurry. The obtained core layer slurry is transported to a conventional wet paper machine, slurried to a forming mesh, dehydrated under vacuum conditions for 20 seconds, and then pressed at a pressure of 0.3 MPa for 30 seconds to obtain a wet paper sheet. The wet paper sheet is dried at 85°C for 6 minutes until the moisture content is less than 10%, forming a dense core layer paper sheet. The density after drying is controlled to 180g / m 2 ; The core paper is placed between two sheets with a surface density of 100 g / m 2 The paper is placed between double-sided coated papers and sent to a flat hot press, where it is hot-pressed at 140°C and 1.2 MPa for 90 seconds to obtain a deformation-resistant cardboard with a stable structure.

[0073] Comparative Example 2 Preparation of anti-deformation cardboard for packaging boxes: A bleached softwood pulp containing 100 parts of leaf fiber was weighed, dispersed with deionized water, and then beaten until the beating degree reached 35° SR, resulting in a slurry with uniform fiber dispersion. Subsequently, the following components were added in sequence: 3 parts of oxidized nanocellulose, 3 parts of cellulose nanocrystals, 10 parts of polyvinyl alcohol, 2 parts of polyethyleneimine, 6 parts of calcium carbonate particles, and 2 parts of glutaraldehyde. Polyvinyl alcohol (weight-average molecular weight approximately 50,000, degree of hydrolysis 88%) was dissolved in 90°C water, stirred thoroughly, cooled to 25°C, and then added to the slurry. Polyethyleneimine was slowly added dropwise to the slurry as a 10wt% aqueous solution. Glutaraldehyde was slowly added dropwise to the slurry as a 25wt% aqueous solution. After mixing, the above components were stirred at room temperature for 45 minutes, and deionized water was added to the system to a solid content of 3.5% to obtain the core layer slurry. The obtained core layer slurry is transported to a conventional wet paper machine, slurried to a forming mesh, dehydrated under vacuum conditions for 20 seconds, and then pressed at a pressure of 0.3 MPa for 30 seconds to obtain a wet paper sheet. The wet paper sheet is dried at 85°C for 6 minutes until the moisture content is less than 10%, forming a dense core layer paper sheet. The density after drying is controlled to 180g / m 2 ; The core paper is placed between two sheets with a surface density of 100 g / m 2 The paper is placed between double-sided coated papers and sent to a flat hot press, where it is hot-pressed at 140°C and 1.2 MPa for 90 seconds to obtain a deformation-resistant cardboard with a stable structure.

[0074] Test section Hot indentation retention test: Cut the deformation-resistant cardboard into 50mm x 100mm specimens, and measure their original thickness as H0. Set the temperature to 120°C, the pressure to 2.5 MPa, and the time to press for 5 seconds to create a crease along the short edge. Immediately measure the thickness of the indented area (H1) (the thickness after compression). After the sample is allowed to rest for 24 hours, measure the thickness (H2) at the same location after rebound. Use the following formula to calculate the indentation retention (%):

[0075] Where H0 is the original thickness, H1 is the thickness after indentation, and H2 is the thickness after rebound.

[0076] Wet expansion test: Cut a 30mm x 100mm cardboard sample and record the initial thickness h0. Place the sample in a constant temperature and humidity chamber (25°C, 90% RH) for 24 hours. Immediately measure the thickness h1 after removal. Use the following formula to calculate the moisture expansion rate (%):

[0077] Where h0 is the original thickness and h1 is the thickness after moisture absorption.

[0078] The test results are shown in Table 1.

[0079] Table 1

[0080] According to Table 1, each embodiment performs better than Comparative Example 1 and Comparative Example 2 in terms of indentation retention and wet expansion rate, indicating that the deformation-resistant paperboard provided by the present application has significant advantages in structural stability and adaptability to wet environments, and is particularly suitable for packaging box application scenarios with high requirements for indentation forming retention and wet state dimensional control. The possible reason for this is that in Comparative Example 1, the nanocellulose structure was not introduced, resulting in the core layer lacking an effective expansion-limiting network and microscopic support skeleton. The paperboard rebounded significantly after hot pressing and was prone to expansion and bulging in the wet state. In Comparative Example 2, although oxidized nanocellulose and cellulose nanocrystals were compounded, the calcium carbonate used was not surface-modified. The particles had poor dispersibility and low retention rate in the slurry, making it difficult to form an effective anchoring effect in the structure, resulting in a reduced indentation retention rate and a high wet expansion rate.

[0081] Examples 1-3 demonstrate that using different nanocellulose combinations significantly impacts paperboard performance. In Example 1, oxidized nanocellulose and cellulose nanocrystals were blended in a 1:1 ratio, forming a synergistic network structure of "flexible expansion-limiting segments + rigid particle anchors." The resulting indentation retention reached 90.2%, while the wet expansion rate was only 2.4%. This performance is superior to that achieved in Example 2 using only oxidized nanocellulose and in Example 3 using only cellulose nanocrystals. This demonstrates that the combination of the two nanocellulose types can enhance paperboard molding stability and dimensional control through structural complementarity and optimized spatial distribution.

[0082] According to Examples 1 to 4, the average length of oxidized nanocellulose has a significant impact on the performance of paperboard. In Example 1, the longer TOCN can form a stronger entanglement network with polyvinyl alcohol molecules, form a multi-point cross-linked structure with polyethyleneimine and glutaraldehyde, and be embedded around CNC and hydrophilic modified inorganic fillers through coating or bridging, forming a more stable flexible expansion-limiting-rigid position-limiting composite system in the cross-linked network. Although the shorter TOCN has better dispersibility, it lacks effective cross-chain entanglement and spatial anchoring capabilities, resulting in reduced network density and weakened structural support, thereby showing a certain performance degradation in both indentation rebound and wet dimensional stability. Therefore, it is preferred to use longer-chain oxidized nanocellulose, which can play a more significant synergistic role in improving network density and suppressing structural rebound and wet expansion.

[0083] Examples 1, 5, and 7 demonstrate that the surface structure of hydrophilically modified inorganic fillers also significantly impacts paperboard performance. In Example 1, where dopamine and tannic acid were co-deposited for coating, the particles exhibited abundant surface functional groups, optimal hydrophilicity, and interfacial bonding, resulting in the best indentation retention and wet expansion. However, in Example 4 (coating with dopamine alone) or Example 5 (coating with tannic acid alone), both indentation retention and wet expansion decreased. In Example 6, modified with an aminosilane coupling agent, performance declined due to the relatively weak hydrophilicity, dispersibility, and insufficient bonding strength of the coating layer. The performance of the coating using tannic acid alone was even worse, indicating that the stability of the coating formed using tannic acid alone was relatively poor, leading to reduced performance. This indicates that the hydrophilic coating formed using co-deposition of dopamine and tannic acid exhibits superior network embedding and stress distribution, resulting in improved indentation retention and wet dimensional stability.

[0084] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A deformation-resistant cardboard for packaging boxes, characterized in that: include: Two layers of cardboard, and a core layer disposed between the two layers of cardboard, wherein the core layer comprises the following raw materials in parts by weight: 100 parts of wood leaf fiber; 2-4 parts of nanocellulose; 8-12 parts of polyvinyl alcohol; 1-3 parts of polyethyleneimine; 4-8 parts of hydrophilic modified inorganic filler; 1-3 parts of cross-linking agent.

2. The anti-deformation paperboard according to claim 1, characterized in that: The nanocellulose includes oxidized nanocellulose and cellulose nanocrystals, and the mass ratio of the oxidized nanofibers to the cellulose nanocrystals is 1:0.5-1.

5.

3. The anti-deformation paperboard according to claim 2, characterized in that: The average diameter of the oxidized nanocellulose is 10-20 nm, and the average length is 500-1000 nm; the average diameter of the cellulose nanocrystals is 5-15 nm, the average length is 100-500 nm, and the crystallinity is 60%-90%.

4. The anti-deformation paperboard according to claim 1, characterized in that: The hydrophilic modified inorganic filler is obtained by co-depositing dopamine and tannic acid on the surface of the inorganic filler.

5. The anti-deformation paperboard according to claim 4, characterized in that: The hydrophilic modified inorganic filler is prepared by the following method: The inorganic filler is dispersed in a Tris-HCl buffer solution with a pH of 8 to 9 and includes 0.5 to 2 g / L dopamine and 0.5 to 2 g / L tannic acid, and reacted at 20 to 25° C. for 12 to 24 hours under aerobic conditions to obtain a hydrophilic modified inorganic filler.

6. The anti-deformation paperboard according to claim 5, characterized in that: The inorganic filler includes at least one of silicon dioxide and calcium carbonate; the average particle size of the inorganic filler is 2-5 μm.

7. The anti-deformation paperboard according to claim 1, characterized in that: The cardboard includes a surface density of 80-160 g / m 2 coated paper.

8. The deformation-resistant paperboard according to any one of claims 1 to 7, characterized in that: The anti-deformation paperboard meets at least one of the following conditions: 1) The wood fiber is derived from at least one of softwood pulp and hardwood pulp; 2) The polyvinyl alcohol has a weight average molecular weight of 30,000 to 80,000 and a degree of hydrolysis of 88% or more; 3) The weight average molecular weight of the polyethyleneimine is 1000-3000; 4) the cross-linking agent includes glutaraldehyde; 5) The surface density of the core layer is 120~250g / m 2 .

9. A method for preparing deformation-resistant cardboard for packaging boxes, characterized in that: include: Providing raw materials for the core layer of the deformation-resistant paperboard according to any one of claims 1 to 8; Dispersing the raw materials in water to obtain a core layer slurry; Sheet-forming the core layer slurry to obtain a core layer; The core layer is placed between two sheets of cardboard, and hot pressed to obtain a deformation-resistant cardboard.

10. A packaging box, characterized in that: The anti-deformation paperboard is obtained by subjecting the anti-deformation paperboard according to any one of claims 1 to 8 or the anti-deformation paperboard prepared by the method according to claim 9 to hot embossing treatment and folding.

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