Anti-deformation paperboard for a packaging box and method for manufacturing the same

Abstract

The application provides a deformation-resistant paperboard for a packaging box and a preparation method thereof. The deformation-resistant paperboard comprises two layers of cardboard and a core layer arranged between the two layers of cardboard. The core layer comprises the following raw materials in parts by mass: 100 parts of wood leaf fiber; 2-4 parts of nanocellulose; 8-12 parts of polyvinyl alcohol; 1-3 parts of polyethylene imine; 4-8 parts of hydrophilic modified inorganic filler; and 1-3 parts of crosslinking agent. By optimizing the structure and component ratio of the core layer, the crease area of the paperboard can remain stable deformation after hot pressing and forming, and is not easy to rebound. Meanwhile, the paperboard can maintain good dimensional stability under humidity change conditions, thereby improving the forming precision and use durability of the packaging box.

Description

A deformation-resistant cardboard for packaging boxes and its preparation method Technical Field

[0001] This application relates to the field of packaging materials technology, specifically to a deformation-resistant cardboard for packaging boxes and its preparation method. Background Technology

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

[0003] Currently, most widely used anti-deformation cardboard on the market adopts a multi-layer composite structure or sandwich structure. The core layer is often based on ordinary wood pulp paper or recycled fiber, which is pressed together with the face paper after papermaking. However, the traditional core layer structure has two prominent problems: First, the creases are prone to springback after hot pressing. The structural rigidity or plasticity of the core layer in existing cardboard is insufficient, making it difficult to form a stable deformation structure at the crease, resulting in the inability to maintain the hot pressing marks and affecting the forming accuracy and appearance quality of the packaging box. Second, it has poor dimensional stability in humid environments. Conventional pulp materials are prone to expansion after absorbing moisture, especially the core layer, which can cause structural bulging and warping after absorbing water between layers. This leads to significant deformation of the packaging box during humid storage or transportation, affecting its performance and product image.

[0004] To address this, some technologies have attempted to improve the paper's resistance to deformation by increasing the paper basis weight, introducing a coating layer, or filling the spaces between the paperboards with foaming materials. However, these methods suffer from problems such as increased thickness, complex processes, or insufficient environmental adaptability.

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

[0006] This application provides a deformation-resistant cardboard for packaging boxes and its preparation method, aiming to solve the problems of easy springback in the crease area and poor dimensional stability in humid environments of existing packaging cardboard after thermoforming.

[0007] In a first aspect, this application provides a deformation-resistant cardboard for packaging boxes, comprising: 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; and 1-3 parts of crosslinking agent.

[0008] According to this application, by optimizing the core layer structure and component ratio, the cardboard can maintain stable deformation in the crease area after thermoforming, making it less prone to springback, and at the same time maintain good dimensional stability under humidity changes, thereby improving the forming accuracy and durability of the packaging box.

[0009] Specifically, the core layer uses wood fiber as the main structural framework, providing basic support and thickness to the paperboard. The introduced nanocellulose, with its high specific surface area and aspect ratio, forms a dense network structure between the wood fibers, limiting moisture absorption and expansion, stabilizing the spatial structure, and significantly reducing the dimensional change rate of the paperboard under humidity fluctuations. Simultaneously, nanocellulose, with its small particle size and negative charge, is easily lost with moisture in traditional pulp systems. This application introduces polyethyleneimine as a cationic polymer, which can significantly improve the retention rate of nanocellulose in the pulp through electrostatic adsorption, enhancing its reinforcing effect in the core layer.

[0010] Polyvinyl alcohol (PVA) is the main film-forming polymer. Its flexible segments can responsively deform during hot pressing and retain the indentation structure after cooling. Polyethyleneimine (PEI) not only acts as a retention aid but also forms a hydrogen-bonded system with PVA and cellulose nanoparticles. Under the action of a crosslinking agent, the two form a dense crosslinked network, effectively inhibiting segment flow and improving the overall wet stability of the core layer structure and the structural retention of the indentation area. During the hot pressing process of the core layer and cardboard, the crosslinking reaction between PVA and PVA occurs simultaneously under the action of the crosslinking agent, initially constructing a stable three-dimensional network structure, locking PVA, PVA, and cellulose nanoparticles, and promoting the stable and uniform distribution of cellulose nanoparticles in the three-dimensional network, thus maximizing their function.

[0011] Meanwhile, during the hot pressing process of bonding the core layer with the cardboard to prepare anti-deformation cardboard, the cross-linking reaction is basically completed, but a small number of cross-linking sites remain. In the subsequent hot indentation process, some of the remaining 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 shape, reduce post-indentation rebound, and improve the indentation retention rate.

[0012] The hydrophilic modified inorganic fillers distributed in the core layer also play an important supporting role. These particles undergo functionalized coating, possessing good dispersibility and polar interfaces, enabling them to interfacially bind with the cross-linked network. Embedded within the colloidal structure, they act as microscopic fulcrums, helping to withstand external forces, preventing chain segment rebound, and diffusing stress from the indentation area to the surrounding area, effectively mitigating deformation recovery caused by stress concentration. Simultaneously, their surface structure exhibits good compatibility with 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". This achieves deformation stability in the indentation area and dimensional control capability in a wet environment, providing excellent molding retention performance and structural reliability for the packaging box.

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

[0015] In some of the above embodiments, oxidized nanocellulose (TOCN) possesses a flexible long-chain structure and a high specific surface area. Its surface contains carboxyl functional groups, which allow it to form numerous hydrogen bonds with polyvinyl alcohol (PVA) and polyethyleneimine (PEI) in the slurry, constructing a continuous and flexible expansion-limiting network. This network can be distributed among the wood fibers, effectively restricting fiber slippage and void expansion after moisture absorption, thereby improving the dimensional stability of the core layer under high humidity conditions.

[0016] Cellulose nanocrystals (CNCs) are highly crystalline, short, rod-shaped particles with high rigidity and dimensional stability, making them difficult to deform within cross-linked networks. When embedded in a TOCN-constructed network, CNCs act as a "micro-skeleton," helping to maintain the overall network's spatial structure and inhibiting chain collapse or network compression deformation under the stress of heat or moisture in the paperboard. Simultaneously, due to their small particle size and high specific surface area, CNCs can form numerous rigid confinement sites within the system and achieve 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 structural synergy of "flexible chain segment expansion limitation + rigid particle anchoring", but also improves the uniformity and compactness of the network structure and reduces the wet expansion rate through complementary distribution at the microscale. When the mass ratio of the two is controlled within the range of 1:0.5 to 1.5, it can effectively avoid excessive entanglement of TOCN leading to decreased 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 only TOCN, the composite CNC method creates more rigid deformation-resistant points during the indentation process, improving the stability of the indentation structure. Furthermore, compared to solutions using only CNC, the composite TOCN introduces a continuous expansion-limiting network, which helps improve wet expansion suppression and network flexibility. Therefore, 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 filler introduced in the core layer, CNC particles, although also rigid particles, have smaller particle sizes and are distributed closer to the interior of the cross-linked network, participating in the filling and confinement of structural gaps at the microscopic level. Meanwhile, the inorganic filler, with its larger size, is embedded in the overall network, forming macroscopic anchor points that support deformation and diffuse stress in the indentation area. The two complement and synergize in terms of particle size, distribution area, and action path, jointly constructing a structurally stable system from the microscopic to the mesoscopic level, thus providing synergistic enhancement in indentation retention and wet dimensional control.

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

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

[0022] Cellulose nanocrystals (CNCs), used as rigid reinforcing particles, are preferably 5–15 nm in diameter and 100–500 nm in length, which helps them embed into the network structure to form uniform confinement sites. Controlling their length to 100–500 nm prevents particle sedimentation or accumulation in the system, improving its dispersion stability; while controlling the crystallinity to 60%–90% retains sufficient rigidity as micro-supports and allows for some interfacial bonding with the colloidal network.

[0023] By rationally designing the particle size and morphology parameters of oxidized cellulose nanoparticles and cellulose nanocrystals, the two can achieve a synergistic effect of flexible chain segment expansion restriction and rigid particle anchoring in the core layer, thereby improving the uniformity, stability and stress response of the expansion restriction network, and 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-deposition of dopamine and tannic acid on the surface of the inorganic filler.

[0025] In some of the above embodiments, the inventors discovered that, compared to hydrophilic modified inorganic fillers obtained by self-polymerization deposition of dopamine on the surface of inorganic fillers, inorganic fillers co-deposited with dopamine and tannic acid, when applied to the core layer structure of anti-deformation paperboard, can further improve the indentation retention performance of the paperboard after hot indentation forming, as well as its dimensional stability in humid and hot environments. The reason for this may be that tannic acid molecules are rich in high-density phenolic hydroxyl structures, which can synergistically deposit with dopamine oxidation products on the particle surface during co-deposition through electrostatic adsorption, hydrogen bonding, and π–π conjugated stacking, thereby assisting in the construction of a composite coating layer with a more uniform film distribution, richer functional groups, and stronger hydrophilicity. This composite coating layer helps improve the water dispersibility and retention rate of particles during pulping, and enhances the interfacial bonding force between particles and components such as polyvinyl alcohol and polyethyleneimine in the colloidal network, thereby improving the anchoring effect of particles in 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 impart a higher negative potential to the particle surface, thereby enhancing its electrostatic adsorption with the positively charged polyethyleneimine. This helps 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 by regulating the excessive cross-linking tendency of the polydopamine network, thus improving its deformation adaptability and stress buffering capacity in the indentation area. Under high humidity conditions, tannic acid has a higher density of phenolic hydroxyl structures, which, together with the active groups on the polydopamine surface, form stable connections with the cross-linked networks of PVA, PEI, and nanocellulose, while limiting the moisture adsorption path and structural expansion space, which helps 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 modified filler surface has a higher negative potential, the deposition layer formed on the particle surface is mostly in a non-covalent adsorption form due to the lack of tannic acid's own oxidative self-polymerization ability. Lacking a stable spatial framework, the film layer is prone to detachment under shear, thermal pressure, or hygroscopic conditions. 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 provide effective support in areas of structural deformation.

[0028] Based on the aforementioned synergistic effects, the composite coating layer constructed by the synergistic co-deposition of dopamine and tannic acid can more effectively improve the distribution stability of particles in the network structure and the interfacial forces compared with the use of dopamine or tannic acid-modified fillers alone, thereby improving the structural stability and moisture resistance of paperboard during use.

[0029] In some embodiments, the hydrophilic modified inorganic filler is prepared by the following method:

[0030] The inorganic filler was dispersed in a Tris-HCl buffer solution with a pH of 8-9 containing 0.5-2 g / L dopamine and 0.5-2 g / L tannic acid, and reacted at 20-25 °C for 12-24 h under aerobic conditions to obtain the hydrophilic modified inorganic filler.

[0031] In some of the above embodiments, the pH value of the Tris-HCl buffer solution is controlled at 8-9, which is conducive to the smooth progress of the dopamine auto-oxidation reaction. This pH range can also increase the negative charge on the surface of the inorganic filler, promoting the adhesion stability of the coating film. The concentrations of both dopamine and tannic acid are controlled within the range of 0.5-2 g / L, which can balance reaction efficiency and film quality: if the concentration is too low, the coating will be insufficient and it will be difficult to form a continuous film; if the concentration is too high, it will easily lead to the precipitation of free polymerization products, reduce the coating uniformity and affect the particle dispersibility. The reaction temperature is controlled at 20-25℃, close to room temperature, which helps to avoid excessive cross-linking of the polymerized film, particle agglomeration or structural expansion at high temperatures; the reaction time is controlled at 12-24 hours to ensure that the film is fully and stably formed and that a dense coating is formed on the particle surface.

[0032] Through co-deposition reactions under the aforementioned controlled conditions, hydrophilic modified inorganic fillers with uniform surface, abundant functional groups, and stable structure can be obtained. These particles exhibit good dispersibility and retention in pulp, readily synergistically forming colloidal networks with matrix components such as PVA, PEI, and TOCN, and embedding themselves into cross-linked structures during papermaking. Ultimately, this endows the core layer with stronger interfacial adhesion and multi-scale support effects, thereby improving the indentation retention performance and wet dimensional stability of the deformation-resistant paperboard.

[0033] In some embodiments, the inorganic filler includes at least one of silicon dioxide and calcium carbonate; the average particle size of the inorganic filler is 2~5μm.

[0034] In some of the above embodiments, silica and calcium carbonate, as common inorganic fillers, are abundant, low in cost, and have good thermal stability, and are widely used in paper reinforcement and coating. These inorganic fillers have high specific surface area and surface hydroxyl groups, which can form stable bonds with dopamine and tannic acid, facilitating the formation of a uniform and dense coating layer. Simultaneously, they provide a supporting structure in the form of rigid particles, and their high bulk modulus helps to enhance the structural rigidity of the core layer during compression.

[0035] The inorganic filler preferably has an average particle size of 2-5 μm. Fillers within this particle size range exhibit good dispersion stability, are not prone to agglomeration or sedimentation, and are easily co-formed with colloidal materials during the papermaking process, and stably embedded in the cross-linked network during hot pressing. After the aforementioned dopamine-tannic acid co-deposition modification, its surface possesses good hydrophilicity and chemical reactivity, enabling it to form multi-point interfacial bonds with materials such as polyvinyl alcohol, polyethyleneimine, and nanocellulose, thereby enhancing the consistency and toughness of the core layer structure and effectively suppressing springback deformation and wet dimensional changes in the indentation area.

[0036] In some embodiments, the cardboard comprises a surface density of 80~160 g / m². 2 Coated paper.

[0037] In some of the above embodiments, coated paper possesses excellent surface density, stiffness, and printability, making it a commonly used 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, which is beneficial for subsequent hot creasing, folding, and surface printing processes.

[0038] The areal density is controlled at 80~160 g / m³. 2 By combining coated paper with a core material that has expansion control and indentation stabilization functions, a sandwich-like layered structure with coordinated structure and balanced stress transmission can be constructed. The coated paper provides good compressive strength and surface stability, while the core layer provides buffering and limiting functions. The two work together in the indentation area to help form a stable and uniform folded structure and maintain good shape retention in humid and hot environments.

[0039] Therefore, 80~160g / m 2 Coated paper, as a surface material for cardboard, can improve the forming quality and service life of cardboard while ensuring its structural strength.

[0040] In some embodiments, the wood fibers are derived from at least one of softwood pulp and hardwood pulp. Based on the above embodiments, softwood pulp fibers are longer and more flexible, which helps to construct a skeletal support structure and improve the overall toughness and indentation retention of the core layer; while hardwood pulp fibers are shorter and have a larger surface area, which is beneficial for colloid adhesion and network uniformity control. The two types of fibers, used in combination or alone, can construct a stable multi-scale fiber structure, effectively improving the core layer structural strength and paper density.

[0041] 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 higher. Based on the above embodiments, this molecular weight range can balance the film-forming ability and water solubility of PVA, allowing it to exist stably in the paper pulp and participate in the construction of the paper network. The higher degree of hydrolysis results in a higher hydroxyl density, which is beneficial for forming hydrogen bond networks with polyethyleneimine, nanocellulose, and modified fillers, thereby improving the compactness and crosslinking stability of the core structure and enhancing its wet dimensional control capability.

[0042] 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 ability and a certain network bonding force. It can promote the effective retention of nanocellulose and crosslinking agents during papermaking, and further participate in crosslinking and curing after papermaking, improving the interfacial bonding force and structural integrity of the colloidal network, thereby synergistically enhancing indentation retention and moisture resistance.

[0043] In some embodiments, the crosslinking agent includes glutaraldehyde. Based on the above embodiments, glutaraldehyde is a commonly used small-molecule dialdehyde crosslinking 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 hot pressing, it can further react with the hydroxyl and amine groups in PVA, PEI, nanocellulose, and hydrophilic modified inorganic fillers to form stable chemical connections such as ether bonds and imine structures, thereby forming a spatial crosslinking network in the core layer.

[0044] In some embodiments, the areal density of the core layer is 120~250 g / m³. 2 Based on the above embodiments, the core layer with this areal density range, while possessing sufficient strength and compressive strength, can generate controllable directional deformation during hot indentation, forming an indentation limiting structure in conjunction with the cross-linked network and inorganic fillers, thereby improving the stability of the fold lines and the ability to retain wet dimensions.

[0045] Secondly, this application provides a method for preparing a deformation-resistant cardboard for packaging boxes, comprising:

[0046] Provide the raw materials included in the core layer of the deformation-resistant paperboard according to any embodiment of the first aspect;

[0047] The raw materials are dispersed in water to obtain a core layer slurry;

[0048] The core layer slurry is formed into a core layer;

[0049] The core layer is placed between two sheets of cardboard and then hot-pressed to obtain a deformation-resistant cardboard.

[0050] According to this application, the method involves forming a core layer slurry with expansion-limiting and multi-point anchoring functions, and then hot-pressing it with surface cardboard to construct a deformation-resistant paperboard with stable structure, strong resistance to wet deformation, and excellent indentation retention.

[0051] Specifically, the method includes the following steps: First, wood leaf fibers, nanocellulose, polyvinyl alcohol, polyethyleneimine, hydrophilic modified inorganic fillers, and crosslinking agents are dispersed in water to obtain a stable and uniform core layer slurry. During the papermaking process, this slurry promotes the retention of nanocellulose and crosslinked small molecules through the retention mechanism of polyethyleneimine, forming a uniformly structured wet paper sheet. In the subsequent hot pressing process, under the action of temperature and pressure conditions, the crosslinking agent reacts chemically with PVA, PEI, nanocellulose, etc., to construct a three-dimensional crosslinked network; at the same time, the hydrophilic modified inorganic filler forms multi-point interfacial bonding with the network structure, realizing particle anchoring and stress transmission functions.

[0052] The core paper is placed between two sheets of cardboard and then heat-pressed together, achieving both structural integration and simultaneous crease formation on the surface cardboard. The resulting cardboard exhibits excellent crease retention and wet dimensional stability, making it widely applicable in packaging boxes requiring precise crease positioning and moisture protection, particularly suitable for demanding applications such as cigarette boxes, gift boxes, and folding boxes.

[0053] In some embodiments, the method specifically includes:

[0054] Wood fibers derived from softwood pulp are dispersed in water and pulped to a freeness of 30-45°SR. Nanocellulose, pre-dissolved polyvinyl alcohol aqueous solution, polyethyleneimine aqueous solution, hydrophilic modified inorganic filler, and crosslinking agent are then added to the pulp. 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. Polyvinylimine is first dissolved in water and then added dropwise to the pulp. After mixing all components and stirring for 30-60 minutes, water is added to obtain a core layer pulp with a solid content of 2%-5%.

[0055] The core pulp is formed using a conventional wet paper machine, flowing onto the forming wire mesh. After vacuum dewatering and pressing, a wet paper sheet is obtained. The vacuum dewatering time is 10-30 seconds, and the pressing pressure is 0.2-0.4 MPa. The pressed wet paper sheet is then fed into a drying section and dried at 70-100°C until the moisture content is below 10%, yielding the core layer. The areal density of the dried core layer is controlled at 120-250 g / m². 2 ;

[0056] The dried core layer is placed between two sheets of cardboard and then hot-pressed at 120~160℃ and 0.5~1.5MPa for 0.5~2 minutes on a hot press to obtain the anti-deformation cardboard.

[0057] Thirdly, this application provides a packaging box, which is obtained by heat-pressing and folding a deformation-resistant cardboard prepared according to any embodiment of the first aspect or according to any embodiment of the second aspect.

[0058] According to this application, the packaging box is made by heat-pressing and folding anti-deformation cardboard, which can maintain the stability of the indentation structure and clear fold lines in long-term use and high humidity environment, significantly improving the forming accuracy, structural strength and environmental adaptability of the packaging box.

[0059] The packaging box uses the deformation-resistant cardboard provided in this application as the base material. This cardboard consists of two layers of coated paperboard and a functional core layer sandwiched between them. The core layer incorporates nano-cellulose, polyvinyl alcohol, polyethyleneimine, crosslinking agents, and hydrophilic modified inorganic fillers. Through hot pressing, a three-dimensional crosslinked network and particle anchoring structure are formed, providing localized rigid support and chain segment restraint in the crease areas. In actual processing, the cardboard can be pressed with a pre-set fold line in one go using a hot-pressing mold, and after die-cutting, folded into a box with a stable geometric structure. This is suitable for scenarios requiring high-precision positioning and shape maintenance, such as cigarette boxes, gift boxes, and folding packaging boxes.

[0060] Compared with traditional packaging cardboard, the forming structure of the packaging box is less prone to springback, bulging or dimensional deformation after being subjected to stress or moisture absorption, which can significantly improve the appearance consistency, service life and user experience of the finished packaging.

[0061] In some embodiments, the hot indentation treatment conditions include: using a linear die to hot press at 110~130°C and 2~4MPa for 2~5s to form a hot indentation with a width of 0.3~0.6mm.

[0062] Compared with the prior art, the beneficial effects of this application are at least as follows:

[0063] 1. By introducing a core structure containing nanocellulose, polyvinyl alcohol, polyethyleneimine, crosslinking agent and hydrophilic modified inorganic filler, a dense and stable three-dimensional crosslinking network and multi-point anchoring structure are constructed during interlayer hot pressing, which can significantly improve the resilience of the paperboard indentation area and solve the problems of easy recovery of creases and blurred lines in traditional packaging paperboard.

[0064] 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 of conventional cardboard caused by moisture absorption and expansion in a humid environment.

[0065] 3. After being modified by synergistic coating of dopamine and tannic acid, the inorganic filler has good hydrophilicity and interfacial compatibility. It can not only achieve stable dispersion and effective retention, but also serve as a local support point during hot indentation, enhancing the retention force and deformation resistance of the indentation structure. Detailed Implementation

[0066] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0067] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the 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 one or more embodiments or examples.

[0068] 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 technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0069] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0070] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0071] Oxidized cellulose nanoparticles, TEMPO-oxidized cellulose nanoparticles, with an average diameter of approximately 15 nm;

[0072] Cellulose nanocrystals have an average diameter of approximately 10 nm and a crystallinity of 60%–90%.

[0073] Calcium carbonate particles, with an average particle size of approximately 3 μm;

[0074] Double-sided coated paper with an surface density of 100g / m² 2 Purchased from Renyi Paper Industry;

[0075] Polyvinyl alcohol has a weight-average molecular weight of approximately 50,000 and a degree of hydrolysis of approximately 90%.

[0076] Polyethyleneimine, with a weight-average molecular weight of approximately 2000;

[0077] Bleached softwood pulp, Canadian Northwood bleached softwood pulp.

[0078] Preparation Example 1

[0079] Preparation of hydrophilic modified inorganic fillers:

[0080] Ten parts of calcium carbonate particles were weighed and dispersed in 500 parts of Tris-HCl buffer solution with pH 8.5 containing 1.0 g / L dopamine hydrochloride and 1.0 g / L tannic acid. The mixture was ultrasonically dispersed for 5 min. Then, the mixture was stirred and reacted in air at 25 °C for 18 h. After the reaction was completed, the mixture was centrifuged and the supernatant was discarded. The mixture was washed three times with water until the pH of the washing solution returned to neutral. The precipitate was vacuum dried at 60 °C for 8 h to obtain hydrophilic modified inorganic filler A.

[0081] Preparation Example 2

[0082] Preparation of hydrophilic modified inorganic fillers:

[0083] Ten parts of calcium carbonate particles were weighed and dispersed in 500 parts of Tris-HCl buffer solution with pH 8.5 containing 2.0 g / L dopamine hydrochloride. The mixture was ultrasonically dispersed for 5 min. Then, the mixture was stirred and reacted in air at 25 °C for 18 h. After the reaction was completed, the mixture was centrifuged and the supernatant was discarded. The mixture was washed three times with water until the pH of the washing solution returned to neutral. The precipitate was vacuum dried at 60 °C for 8 h to obtain hydrophilic modified inorganic filler B.

[0084] Preparation Example 3

[0085] Preparation of hydrophilic modified inorganic fillers:

[0086] Ten parts of calcium carbonate particles were weighed and dispersed in 500 parts of Tris-HCl buffer solution with pH 8.5 containing 2.0 g / L tannic acid. The mixture was ultrasonically dispersed for 5 min. Then, the mixture was stirred and reacted in air at 25 °C for 18 h. After the reaction was completed, the mixture was centrifuged and the supernatant was discarded. The mixture was washed three times with water until the pH of the washing solution returned to neutral. The precipitate was vacuum dried at 60 °C for 8 h to obtain hydrophilic modified inorganic filler C.

[0087] Preparation Example 4

[0088] Preparation of hydrophilic modified inorganic fillers:

[0089] Weigh 10 parts of calcium carbonate particles and disperse them in 250 parts of ethanol aqueous solution (ethanol and water volume ratio of 7:3). Disperse the mixture by ultrasonication for 5 min. Add 1 part of γ-aminopropyltriethoxysilane and heat under reflux at 70 °C for 3 h. After the reaction is completed, centrifuge the mixture, discard the supernatant, and wash it repeatedly with ethanol and water. Dry the precipitate under vacuum at 60 °C for 8 h to obtain hydrophilic modified inorganic filler D.

[0090] Example 1

[0091] Preparation of deformation-resistant cardboard for packaging boxes:

[0092] Weigh out 100 parts of bleached softwood pulp containing leaf fibers, disperse it in deionized water, and beat it until the freeness reaches 35°SR, resulting in a pulp with uniform fiber dispersion. Then, add the following components sequentially: 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 hydrophilic modified inorganic filler A, and 2 parts of glutaraldehyde.

[0093] Polyvinyl alcohol (weight average molecular weight approximately 50,000, degree of hydrolysis 88%) was dissolved in water at 90°C, stirred thoroughly until dissolved, cooled to 25°C, and then added to the slurry. Polyethyleneimine was slowly added dropwise to the slurry in the form of a 10wt% aqueous solution. Glutaraldehyde was slowly added dropwise to the slurry in the form of a 25wt% aqueous solution. After mixing the above components, the mixture was stirred at room temperature for 45 minutes, and then deionized water was added until the solid content of the system reached 3.5%, thus obtaining the core layer slurry.

[0094] The obtained core pulp is fed to a conventional wet paper machine, flows to the forming wire, dewaters under vacuum for 20 seconds, and then presses under 0.3 MPa pressure for 30 seconds to obtain a wet paper sheet. The wet paper sheet is then dried at 85°C for 6 minutes until the moisture content is below 10%, forming a dense core paper sheet. The post-drying density is controlled at 180 g / m³. 2 ;

[0095] The core paper is placed between two sheets with an surface density of 100 g / m² 2 The coated paper is fed into a flatbed hot press and hot-pressed at 140℃ and 1.2MPa for 90 seconds to obtain a structurally stable anti-deformation paperboard.

[0096] Example 2

[0097] Preparation of deformation-resistant cardboard for packaging boxes:

[0098] Similar to Example 1, except that 6 parts of oxidized cellulose nanoparticles with an average length of about 800 nm were used instead of 3 parts of oxidized cellulose nanoparticles with an average length of about 800 nm and 3 parts of cellulose nanocrystals with an average length of about 300 nm.

[0099] Example 3

[0100] Preparation of deformation-resistant cardboard for packaging boxes:

[0101] The method is largely the same as in 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 cellulose nanocrystals with an average length of about 800 nm and 3 parts of cellulose nanocrystals with an average length of about 300 nm.

[0102] Example 4

[0103] The method is largely the same as in Example 1, except that oxidized cellulose nanoparticles with an average length of about 800 nm are replaced with oxidized cellulose nanoparticles with an average length of about 300 nm in equal amounts.

[0104] Example 5

[0105] Preparation of deformation-resistant cardboard for packaging boxes:

[0106] It is largely the same as Example 1, except that hydrophilic modified inorganic filler B is used instead of hydrophilic modified inorganic filler A.

[0107] Example 6

[0108] Preparation of deformation-resistant cardboard for packaging boxes:

[0109] It is largely the same as Example 1, except that hydrophilic modified inorganic filler C is used instead of hydrophilic modified inorganic filler A.

[0110] Example 7

[0111] Preparation of deformation-resistant cardboard for packaging boxes:

[0112] It is largely the same as Example 1, except that hydrophilic modified inorganic filler D is used instead of hydrophilic modified inorganic filler A.

[0113] Comparative Example 1

[0114] Preparation of deformation-resistant cardboard for packaging boxes:

[0115] Weigh out bleached softwood pulp containing 106 parts of wood fiber, disperse it with deionized water, and beat it until the freeness reaches 35°SR to obtain a pulp with uniform fiber dispersion. Then, add the following components in sequence: 10 parts of polyvinyl alcohol, 2 parts of polyethyleneimine, 6 parts of hydrophilic modified inorganic filler A, and 2 parts of glutaraldehyde;

[0116] Polyvinyl alcohol (weight average molecular weight approximately 50,000, degree of hydrolysis 88%) was dissolved in water at 90°C, stirred thoroughly until dissolved, cooled to 25°C, and then added to the slurry. Polyethyleneimine was slowly added dropwise to the slurry in the form of a 10wt% aqueous solution. Glutaraldehyde was slowly added dropwise to the slurry in the form of a 25wt% aqueous solution. After mixing the above components, the mixture was stirred at room temperature for 45 minutes, and then deionized water was added until the solid content of the system reached 3.5%, thus obtaining the core layer slurry.

[0117] The obtained core pulp is fed to a conventional wet paper machine, flows to the forming wire, dewaters under vacuum for 20 seconds, and then presses under 0.3 MPa pressure for 30 seconds to obtain a wet paper sheet. The wet paper sheet is then dried at 85°C for 6 minutes until the moisture content is below 10%, forming a dense core paper sheet. The post-drying density is controlled at 180 g / m³. 2 ;

[0118] The core paper is placed between two sheets with an surface density of 100 g / m² 2 The coated paper is fed into a flatbed hot press and hot-pressed at 140℃ and 1.2MPa for 90 seconds to obtain a structurally stable anti-deformation paperboard.

[0119] Comparative Example 2

[0120] Preparation of deformation-resistant cardboard for packaging boxes:

[0121] Weigh out 100 parts of bleached softwood pulp containing leaf fibers, disperse it with deionized water, and beat it until the freeness reaches 35°SR to obtain a pulp with uniform fiber dispersion. Then, add the following components in sequence: 3 parts oxidized nanocellulose, 3 parts cellulose nanocrystals, 10 parts polyvinyl alcohol, 2 parts polyethyleneimine, 6 parts calcium carbonate particles, and 2 parts glutaraldehyde;

[0122] Polyvinyl alcohol (weight average molecular weight approximately 50,000, degree of hydrolysis 88%) was dissolved in water at 90°C, stirred thoroughly until dissolved, cooled to 25°C, and then added to the slurry. Polyethyleneimine was slowly added dropwise to the slurry in the form of a 10wt% aqueous solution. Glutaraldehyde was slowly added dropwise to the slurry in the form of a 25wt% aqueous solution. After mixing the above components, the mixture was stirred at room temperature for 45 minutes, and then deionized water was added until the solid content of the system reached 3.5%, thus obtaining the core layer slurry.

[0123] The obtained core pulp is fed to a conventional wet paper machine, flows to the forming wire, dewaters under vacuum for 20 seconds, and then presses under 0.3 MPa pressure for 30 seconds to obtain a wet paper sheet. The wet paper sheet is then dried at 85°C for 6 minutes until the moisture content is below 10%, forming a dense core paper sheet. The post-drying density is controlled at 180 g / m³.2 ;

[0124] The core paper is placed between two sheets with an surface density of 100 g / m² 2 The coated paper is fed into a flatbed hot press and hot-pressed at 140℃ and 1.2MPa for 90 seconds to obtain a structurally stable anti-deformation paperboard.

[0125] Test section

[0126] Hot crease retention test: Cut the anti-deformation cardboard into 50mm×100mm samples and measure its original thickness as H0; set the temperature to 120℃, pressure to 2.5 MPa, and time to 5 seconds on the hot pressing device, and hot press a crease along the short side; immediately measure the thickness H1 (thickness after compression) of the crease area; after the sample has been left to stand for 24 hours, measure the thickness H2 after springback at the same position;

[0127] Calculate the indentation retention rate (%) using the following formula:

[0128]

[0129] Where H0 is the original thickness, H1 is the thickness after indentation, and H2 is the thickness after springback.

[0130] Wet expansion rate test: Cut a 30mm×100mm cardboard sample and record the initial thickness h0; place the sample in a constant temperature and humidity chamber (25℃, 90%RH) for 24 hours; measure the thickness h1 immediately after taking it out;

[0131] Calculate the wet expansion rate (%) using the following formula:

[0132]

[0133] Where h0 is the original thickness and h1 is the thickness after moisture absorption.

[0134] The test results are shown in Table 1.

[0135] Table 1

[0136]

[0137] According to Table 1, each embodiment performs better than Comparative Example 1 and Comparative Example 2 in terms of indentation retention rate and wet expansion rate, indicating that the deformation-resistant cardboard provided in this application has significant advantages in structural stability and adaptability to humid environments, and is particularly suitable for packaging box applications with high requirements for indentation retention and wet dimensional control. The possible reason is that in Comparative Example 1, the lack of nanocellulose structure resulted in a lack of effective expansion-limiting network and microscopic support skeleton in the core layer, leading to significant rebound of the cardboard after hot pressing and easy expansion and bulging under wet conditions. In Comparative Example 2, although oxidized nanocellulose and cellulose nanocrystals were compounded, the calcium carbonate used was not surface-modified, resulting in poor particle dispersion and low retention in the slurry, making it difficult to form an effective anchoring effect in the structure, leading to a reduced indentation retention rate and a high wet expansion rate.

[0138] As demonstrated in Examples 1-3, using different combinations of nanocellulose significantly affects the performance of paperboard. 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 anchoring," achieving an indentation retention rate of 90.2% and a wet expansion rate of only 2.4%, which is superior to using only oxidized nanocellulose in Example 2 and only cellulose nanocrystals in Example 3. This indicates that the blending of two types of nanocellulose can jointly improve the forming stability and dimensional control capability of paperboard through structural complementarity and spatial distribution optimization.

[0139] As demonstrated in Examples 1-4, the average length of oxidized nanocellulose has a significant impact on paperboard performance. 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 embed itself around CNC and hydrophilic modified inorganic fillers through coating or bridging, forming a more stable flexible-limited-rigid-limited composite system within the cross-linked network. While 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, thus exhibiting a certain degree of performance degradation in terms of indentation rebound and wet dimensional stability. Therefore, the use of longer-chain oxidized nanocellulose is preferred, as it can exert a more significant synergistic effect in improving network density and suppressing structural rebound and wet swelling.

[0140] As shown in Examples 1, 5-7, the surface structure of hydrophilic modified inorganic fillers also has a significant impact on the performance of paperboard. In Example 1, which uses a co-deposition coating of dopamine and tannic acid, the particle surface has abundant functional groups, optimal hydrophilicity and interfacial bonding ability, resulting in the best indentation retention and wet swelling rate. However, in Examples 4 and 5, which use only dopamine coating or only tannic acid coating, both indentation retention and wet swelling rate decrease. In Example 6, which uses an aminosilane coupling agent for modification, the performance decreases due to the relatively weak hydrophilicity, dispersibility, and binding force of the coating layer. The performance is even worse when using only tannic acid coating, indicating that the coating layer formed by using only tannic acid has relatively poor stability, leading to reduced performance. This shows that the hydrophilic coating formed by the co-deposition of dopamine and tannic acid has better performance in terms of network embedding and stress dispersion, resulting in better indentation retention and wet dimensional stability.

[0141] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application 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 or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A type of 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 parts by weight of raw material: 100 parts of softwood fiber; 2-4 parts nanocellulose; 8-12 parts polyvinyl alcohol; 1-3 parts polyethyleneimine; 4-8 parts hydrophilic modified inorganic filler; 1-3 parts crosslinking agent; wherein 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; 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%; 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 8-9 containing 0.5-2 g / L dopamine and 0.5-2 g / L tannic acid, and reacted at 20-25 °C for 12-24 h under aerobic conditions to obtain the hydrophilic modified inorganic filler.

2. The anti-deformation cardboard according to claim 1, 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 2~5μm.

3. The anti-deformation paperboard according to claim 1, characterized in that, The cardboard includes materials with an areal density of 80~160g / m³. 2 Coated paper.

4. The anti-deformation paperboard according to any one of claims 1 to 3, characterized in that, The deformation-resistant paperboard meets at least one of the following conditions: 1) the polyvinyl alcohol has a weight-average molecular weight of 30,000 to 80,000 and a degree of hydrolysis of 88% or more; 2) the polyethyleneimine has a weight-average molecular weight of 1,000 to 3,000; 3) the crosslinking agent includes glutaraldehyde; 4) the areal density of the core layer is 120 to 250 g / m³. 2 .

5. A method for preparing deformation-resistant cardboard for packaging boxes, characterized in that, include: Provide the raw materials included in the core layer of the anti-deformation paperboard according to any one of claims 1 to 4; disperse the raw materials in water to obtain a core layer slurry; form the core layer slurry into a core layer; place the core layer between two sheets of cardboard and hot press to obtain the anti-deformation paperboard.

6. A packaging box, characterized in that, The deformation-resistant paperboard prepared according to any one of claims 1 to 4 or according to the method of claim 5 is obtained by hot embossing and folding.

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