paperboard

Abstract

Provided is a board paper that can achieve recycling of waste paper pulp while improving strength in a high-temperature, high-humidity environment. The board paper is a board paper having 4 or more layers and 6 or less layers, and includes at least a surface layer, a surface lower layer, an intermediate layer, and a back surface layer. The surface layer contains softwood unbleached kraft pulp, and the content of the softwood unbleached kraft pulp is 90% by mass or more with respect to the total pulp component of the surface layer. The surface lower layer, the intermediate layer, and the back surface layer contain corrugated paper waste paper pulp, and the content of the corrugated paper waste paper pulp is 90% by mass or more with respect to the total pulp component of the back surface layer. The total layer contains an internal sizing agent, and the total content of the internal sizing agent in the total layer is 2.50 kg / pulp t or more and 6.00 kg / pulp t or less in terms of solid component. The total basis weight is 330 g / m 2 400 g / m or more 2 5 g / m or less 2 50 g / m or more 2 5 g / m or less.

Description

Technical Field

[0001] This invention relates to a type of paperboard. Background Technology

[0002] In recent years, corrugated cardboard liners of standard quality have been used in packaging containers for bundling in high-temperature and high-humidity environments such as shipping. Corrugated cardboard liners are lighter and more cushioning than wooden or resin containers, making them less prone to damage to contents, and are therefore used in various fields, including precision equipment.

[0003] For corrugated cardboard boxes used in packaging containers for heavy-duty handling, high compressive strength is required. To achieve high compressive strength, it is generally necessary to manufacture base paper with a high basis weight (per square meter). Lowering the basis weight reduces the compressive strength, failing to meet the compressibility requirements of corrugated cardboard boxes intended for heavy-duty handling. Therefore, multi-layer paper for packaging containers for bundling, for example, 447 g / m³, is disclosed in Patent Document 1. 2 High basis weight (per meter) 6-layer board paper (see Patent Document 1). Existing technical documents Patent documents

[0004] Patent Document 1: Japanese Patent Application Publication No. 2020-020065 Summary of the Invention The technical problem that the invention aims to solve

[0005] However, in order to be used for applications corresponding to the handling of heavy objects in hot and humid environments, it is required to improve the compressive strength during handling while maintaining a low basis weight, even in hot and humid environments.

[0006] In addition, in recent years, the paperboard industry has also demanded the reuse of waste paper and the creation of environmentally friendly materials. In particular, from the perspective of protecting wood resources and economic efficiency, there has been a growing effort to use waste paper as raw material pulp.

[0007] This invention is based on the above circumstances and aims to provide a paperboard that can improve the strength of the paperboard under high temperature and high humidity conditions while realizing the reuse of waste paper pulp. Technical means to solve technical problems

[0008] According to one aspect of the present invention, the paperboard is a paperboard having 4 to 6 layers, the paperboard having at least a surface layer, a subsurface layer, an intermediate layer, and a back layer. The surface layer contains unbleached softwood kraft pulp, and the content of the unbleached softwood kraft pulp is 90% by mass or more relative to the total pulp composition of the surface layer. The subsurface layer, the intermediate layer, and the back layer contain corrugated waste paper pulp, and the content of the corrugated waste paper pulp is 90% by mass or more relative to the total pulp composition of the back layer. All layers contain internal sizing agents, and the total content of the internal sizing agents in all layers, converted to solids, is 2.50 kg / t to 6.00 kg / t of pulp, and the total basis weight is 330 g / m³. 2 Above 400g / m 2 Hereinafter, the Cobb sizing degree of the surface layer and the back layer at a measurement time of 2 minutes is 5 g / m². 2 Above 50g / m 2 Hereinafter, the difference in Cobb application rate between the surface layer and the back layer is 10 g / m². 2 the following. Beneficial effects

[0009] According to the present invention, a paperboard can be provided that can improve the strength under high temperature and high humidity conditions while realizing the reuse of waste paper pulp. Detailed Implementation

[0010] [Description of Embodiments of the Invention] According to one aspect of the present invention, the paperboard is a paperboard having 4 to 6 layers, the paperboard having at least a surface layer, a subsurface layer, an intermediate layer, and a back layer. The surface layer contains unbleached softwood kraft pulp, and the content of the unbleached softwood kraft pulp is 90% by mass or more relative to the total pulp composition of the surface layer. The subsurface layer, the intermediate layer, and the back layer contain corrugated waste paper pulp, and the content of the corrugated waste paper pulp is 90% by mass or more relative to the total pulp composition of the back layer. All layers contain internal sizing agents, and the total content of the internal sizing agents in all layers, converted to solids, is 2.50 kg / t to 6.00 kg / t of pulp, and the total basis weight is 330 g / m³. 2 Above 400g / m 2 Hereinafter, the Cobb application degree of the surface layer and the back layer at a measurement time of 2 minutes is 5 g / m². 2 Above 50g / m 2 Hereinafter, the difference in Cobb application rate between the surface layer and the back layer is 10 g / m². 2 the following.

[0011] This paperboard has 4 to 6 layers. By including at least a surface layer, a subsurface layer, an intermediate layer, and a back layer, it can suppress interlayer delamination while improving compressive strength and box-making suitability. Furthermore, the surface layer, exposed as a bundling material, contains unbleached softwood kraft pulp. The content of this unbleached softwood kraft pulp is 90% by mass or more relative to the total pulp content of the surface layer. This results in improved strength under high temperature and humidity conditions, high folding strength, and excellent suppression of crease line breakage. Moreover, by including highly sizing corrugated waste paper pulp in the subsurface layer, the intermediate layer, and the back layer, sizing properties can be effectively imparted without increasing the content of internal sizing agents, while simultaneously enabling waste paper pulp reuse. Maintaining the quality of paperboard in high-temperature and high-humidity environments requires controlling the increase of moisture entering the paperboard. In this particular paperboard, all layers contain internal sizing agents to impart sizing properties to the surface and back layers. However, by blending high-sizing-property softwood kraft pulp and corrugated waste paper pulp as raw materials, sizing properties can be effectively imparted while controlling the amount of internal sizing agent added. This paperboard exhibits excellent sizing properties, thus maintaining its quality even during ship transport and in high-temperature and high-humidity environments. Furthermore, the basis weight of this paperboard is 330 g / m³. 2 Above 400g / m 2 This allows for a balance between the compressive strength of the paperboard and lightweight design for heavy-duty handling. Furthermore, the Cobb sizing degree of both the surface and back layers is set to 5 g / m² at a measurement time of 2 minutes. 2 Above 50g / m 2 The following methods can suppress the moisture absorption of the paperboard in high-temperature and high-humidity environments, thereby maintaining the quality of the paperboard. This is achieved by setting the difference in Cobb sizing between the surface layer and the back layer to 10 g / m². 2 The following methods can suppress deformation caused by the difference in moisture absorption between the surface layer and the back layer in a high-temperature and high-humidity environment, thereby improving the bursting strength of corrugated cardboard in such environments.

[0012] Preferably, at least the surface layer and the subsurface layer contain a drying paper strength enhancer, with the subsurface layer containing a higher concentration of the drying paper strength enhancer than the surface layer. By including the drying paper strength enhancer in at least the surface layer and the subsurface layer, the inter-fiber bonding of the pulp on the surface side of the paperboard is strengthened, thus further improving its strength under high temperature and humidity conditions. Furthermore, the higher concentration of the drying paper strength enhancer in the subsurface layer compared to the surface layer enhances the strength of the subsurface layer, which is fatigued due to waste paper recycling and contains corrugated waste paper pulp composed of relatively flat pulp fibers, thus further improving its strength under high temperature and humidity conditions.

[0013] Preferably, relative to the total pulp composition of both the lower surface layer and the intermediate layer, the content of the corrugated waste paper pulp is 90% or more by mass, and the density is 0.75 g / cm³. 3 Above 0.95g / cm 3 The burst strength is between 950 kPa and 1100 kPa. By having a corrugated paperboard waste pulp content of 90% by mass or more relative to the total pulp composition of both the lower and middle layers, the sizing properties of the paperboard can be further improved while simultaneously promoting waste pulp recycling. Furthermore, the density of the paperboard within the specified range provides cushioning, allowing for high compressive strength without the need for additional drying force enhancers. Moreover, a burst strength of 950 kPa to 1100 kPa improves the impact resistance of the paperboard in high-temperature and high-humidity environments.

[0014] This paperboard is preferably used for handling heavy objects. It has improved strength under high temperature and humidity conditions, making it suitable for use as corrugated cardboard for packaging containers for bundling in Southeast Asia. Additionally, it can be used as industrial paper in various fields, such as distribution in high temperature and humidity environments.

[0015] [Details of the embodiments of the present invention] The present invention will now be described in detail. The description of the constituent elements described below is based on representative embodiments and specific examples, but the present invention is not limited to such embodiments.

[0016] <Paperboard> According to one embodiment of the present invention, the paperboard comprises at least a surface layer, a subsurface layer, an intermediate layer, and a back layer in sequence. Alternatively, the paperboard may have a single layer or multiple layers between the intermediate layer and the back layer. That is, the paperboard is a multilayer paper with 4 to 6 layers. In the case of multilayer paper with 5 or more layers, it is preferable to have a structure with a second intermediate layer between the intermediate layer and the back layer, and a second back layer between the intermediate layer and the back layer. Thus, by using a multilayer paper with 4 to 6 layers, the paperboard can suppress interlayer delamination while improving compressive strength and box-making suitability. On the other hand, the upper limit for the number of layers in the paperboard is 6 layers. When the number of layers in the paperboard is 7 or more, interlayer delamination is more likely to occur.

[0017] [Surface layer] The surface layer contains unbleached coniferous kraft pulp (NUKP).

[0018] The minimum content of unbleached softwood kraft pulp in the surface layer is 90% by mass, preferably 95% by mass. By setting the content of unbleached softwood kraft pulp in the surface layer, which is the outermost layer used as a bundling material for corrugated paper, to 90% by mass or more, increasing the content ratio not only improves strength under high temperature and humidity conditions but also achieves high folding strength, thus providing excellent suppression of crease line breakage. When the content of unbleached softwood kraft pulp in the surface layer is less than 90% by mass, it may be difficult to ensure the desired compressive strength. In addition, when the content of unbleached softwood kraft pulp in the surface layer is less than 80% by mass, the folding strength of the surface layer decreases due to the reduced content of long fibers, and crease line breakage may easily occur. Furthermore, from the viewpoint of reusability, it is preferable that the content of waste paper pulp in the surface layer is less than 10% by mass, within a range that does not reduce the compressive strength.

[0019] [Below the surface] The subsurface layer contains corrugated paper pulp. By containing highly sizing corrugated paper pulp in the subsurface layer, sizing properties can be effectively imparted while simultaneously achieving pulp reuse, without increasing the content of added sizing agents.

[0020] The lower limit of the content of corrugated waste paper pulp in the subsurface layer is preferably 80% by mass, more preferably 85% by mass, and even more preferably 90% by mass. The upper limit of the content of corrugated waste paper pulp can be 100% by mass. By ensuring that the content of corrugated waste paper pulp in the subsurface layer of the paperboard is within the specified range, the sizing properties can be improved while the recycling of waste paper pulp can be promoted.

[0021] [Middle Layer] The intermediate layer contains corrugated paper waste pulp. By containing highly sizing corrugated paper waste pulp in the intermediate layer, sizing properties can be effectively imparted while achieving waste pulp reuse, without increasing the content of added sizing agents.

[0022] The lower limit of the content of corrugated waste paper pulp in the intermediate layer is preferably 80% by mass, more preferably 85% by mass, and even more preferably 90% by mass. The upper limit of the content of corrugated waste paper pulp can be 100% by mass. By making the content of corrugated waste paper pulp in the intermediate layer of the paperboard within the above range, the sizing properties can be improved while the recycling of waste paper pulp can be promoted.

[0023] [Back layer] The back layer contains corrugated paper waste pulp.

[0024] The lower limit of the content of the corrugated waste paper pulp in the back layer is 90% by mass, preferably 95% by mass. The upper limit of the content of the corrugated waste paper pulp can be 100% by mass. For this board, by ensuring that the content of the corrugated waste paper pulp in the raw material pulp of the back layer, which is located on the backmost side as a bundling material, is within the specified range, the strength under high temperature and high humidity conditions can be improved while promoting the reuse of waste paper pulp.

[0025] In addition, regarding each layer of the paperboard, other pulps besides the aforementioned softwood kraft pulp, corrugated paper waste pulp, and kraft waste pulp can be used, such as dissociated waste pulp, dissociated deinked waste pulp, and deinked bleached waste pulp made from magazine waste pulp, tea paper waste (tea paper), kraft envelope waste paper, newspaper waste paper, flyer waste paper, office waste paper, high-quality white paper waste paper, Kent paper waste paper, construction paper waste paper, land certificate waste paper, etc. Furthermore, depending on the requirements, groundwood pulp (GP), thermomechanical pulp (TMP), bleached chemithermomechanical pulp (BCTMP), and other mechanical pulps can also be used.

[0026] [additive] (Added adhesive) All layers of this paperboard contain an internal sizing agent. By including an internal sizing agent in all layers of the paperboard, its durability against moisture in high-temperature and high-humidity environments can be improved, thus maintaining the quality of the paperboard. Examples of internal sizing agents include styrene-based internal sizing agents, alkyl ketene dimers (AKD), alkenyl succinic anhydride (ASA), neutral rosin internal sizing agents, and rosin internal sizing agents. Neutral rosin internal sizing agents are preferred.

[0027] The rosin sizing agent is not particularly limited and can be any substance known in the papermaking industry. Examples of rosin-based substances include fortified rosin obtained by modifying rosin types such as rosin resin, wood rosin, and tall oil rosin with α,β-unsaturated carboxylic acids such as fumaric acid, maleic acid, and acrylic acid, or their anhydrides; and rosin esters obtained by reacting polyols such as glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, and diglycerol with the aforementioned rosin types. Furthermore, in this invention, the rosin-based sizing agent includes substances obtained by emulsifying these substances individually or by emulsifying a mixture thereof, as well as substances obtained by mixing substances after emulsification individually. In addition, the emulsified substances may also contain substances with added polymers to further improve sizing visibility.

[0028] The lower limit for the total content of internal sizing agent in all layers is 2.50 kg / t of pulp, preferably 2.55 kg / t of pulp, calculated in terms of solids. The upper limit for the total content of internal sizing agent in all layers is 6.00 kg / t of pulp, preferably 5.85 kg / t of pulp, calculated in terms of solids. By ensuring the amount of internal sizing agent added is within these ranges, the paperboard can be given sufficient water resistance, and its durability against moisture in high-temperature and high-humidity environments can be improved. "kg / t of pulp" indicates the amount added (kg) relative to 1 ton of pulp. Here, the total content of internal sizing agent in all layers refers to the value calculated by dividing the total amount of internal sizing agent added in all layers by the total amount of pulp used in all layers.

[0029] (Drying paper strength enhancer) In this paperboard, preferably at least the surface layer and the subsurface layer contain a drying paper strength enhancer, with the subsurface layer containing a higher content of the drying paper strength enhancer than the surface layer. By including the drying paper strength enhancer in at least the surface layer and the subsurface layer, the inter-fiber bonding of the pulp on the surface side of the paperboard is strengthened, thus further improving its strength under high temperature and humidity conditions. Furthermore, the higher content of the drying paper strength enhancer in the subsurface layer compared to the surface layer enhances the strength of the subsurface layer, which is fatigued due to waste paper recycling and contains corrugated waste paper pulp composed of relatively flat pulp fibers, thus further improving its strength under high temperature and humidity conditions.

[0030] Acrylamide-based paper strength enhancers are preferred as drying paper strength enhancers. Amphoteric polyacrylamide resin-based paper strength enhancers are more preferred. Amphoteric polyacrylamide is a substance obtained by polymerizing cationic monomers, anionic monomers, and acrylamide, and its cationic groups are directly fixed onto the pulp fibers. Furthermore, amphoteric polyacrylamide resin-based paper strength enhancers are characterized by their ability to control molecular structure and molecular weight.

[0031] The lower limit for the amount of drying paper strength enhancer added to the entire paperboard is preferably 2.70 kg / t of pulp, more preferably 2.75 kg / t of pulp, calculated in terms of solid content. The upper limit for the amount of drying paper strength enhancer added to the entire paperboard is preferably 3.90 kg / t of pulp, more preferably 3.85 kg / t of pulp, calculated in terms of solid content. By ensuring that the amount of drying paper strength enhancer added to the entire paperboard is within the aforementioned range, the strength under high temperature and high humidity conditions can be improved while maintaining the paperboard's softness.

[0032] The lower limit for the amount of dry paper strength enhancer added to the surface layer is preferably 0.8 kg / t of pulp (based on solid content). The upper limit for the amount of dry paper strength enhancer added to the surface layer is preferably 2.0 kg / t of pulp (based on solid content). While the presence of a dry paper strength enhancer can improve compressive strength, it may also impair the softness of the paper layer. This paperboard contains a high amount of unbleached softwood kraft pulp in the surface layer, which contributes to improved compressive strength. Therefore, by ensuring the amount of dry paper strength enhancer added to the surface layer is within the specified range, it is possible to maintain softness while suppressing surface layer breakage such as indentation line breakage, further improving strength under high temperature and humidity conditions. Furthermore, if the proportion of dry paper strength enhancer added to the surface layer exceeds the upper limit, the water permeability may decrease, and the raw pulp may be easily discharged during the pressing and dewatering processes in the press section of the paper machine. Additionally, in the press section, pulp residue may adhere to the doctor blade used to peel the wet paper adhered to the rolls, potentially reducing the quality of the surface layer.

[0033] The lower limit for the amount of drying paper strength enhancer added to the subsurface layer is preferably 2.0 kg / t of pulp (based on solids content). The upper limit for the amount of drying paper strength enhancer added to the subsurface layer is preferably 7.0 kg / t of pulp (based on solids content). By ensuring the amount of drying paper strength enhancer added to the subsurface layer is within these ranges, the strength of the subsurface layer containing corrugated waste paper pulp can be further improved under high temperature and high humidity conditions while suppressing the reduction in softness caused by the drying paper strength enhancer.

[0034] The lower limit for the amount of drying paper strength enhancer added to the intermediate layer, in terms of solid content, is preferably 1.0 kg / t of pulp. Furthermore, the upper limit for the amount of drying paper strength enhancer added to the intermediate layer, in terms of solid content, is preferably 6.0 kg / t of pulp. By keeping the amount of drying paper strength enhancer added to the intermediate layer within these ranges, the strength of the intermediate layer containing corrugated waste paper pulp can be further improved under high temperature and high humidity environments while suppressing the reduction in softness caused by the drying paper strength enhancer.

[0035] The lower limit for the amount of drying paper strength enhancer added to the back layer, in terms of solid content, is preferably 2.0 kg / t of pulp. Furthermore, the upper limit for the amount of drying paper strength enhancer added to the back layer, in terms of solid content, is preferably 8.0 kg / t of pulp. By keeping the amount of drying paper strength enhancer added to the back layer within these ranges, the strength of the back layer containing corrugated waste paper pulp can be further improved under high temperature and high humidity conditions while suppressing the reduction in softness caused by the drying paper strength enhancer.

[0036] (Other additives) In each layer of the paperboard, including the surface layer, subsurface layer, intermediate layer, and back layer, various papermaking additives may be contained within a range that does not impair the objective effect of the present invention. These various papermaking additives may include, for example, flocculants (silica) and other reagent fixatives, retention aids (such as polyamide), water-resistant agents (such as polyamine and epichlorohydrin), reagent fixatives (such as aluminum sulfate), defoamers, fillers (such as clay, talc, and calcium carbonate), known additives (such as basic dyes, acid dyes, anionic direct dyes, and cationic direct dyes), and paper strength enhancers (such as cationic starch and amphoteric starch).

[0037] [Coating layer] The paperboard can have a coating layer formed on the surfaces of the surface layer and the back layer, the coating layer containing starch and anionic sizing agents. By forming a coating layer containing starch and anionic sizing agents on the surfaces of the surface layer and the back layer, the spaces between the pulp fibers of the surface layer in contact with the coating layer can be filled, thus improving printability and ensuring a smooth printed surface.

[0038] Examples of starches include corn starch, potato starch, tapioca starch, wheat starch, rice starch, oxidized starch made from these, phosphated starch, and various modified starches, but these are not the only examples. Furthermore, these starches can be used alone or in combination of two or more.

[0039] Examples of anionic sizing agents include rosin soap sizing agents, rosin emulsion sizing agents, specially modified rosin sizing agents, alkenyl succinic acid, alkenyl succinic anhydride, anionic alkyl ketene dimers, olefin resins, styrene resins, styrene-acrylic resins, styrene-maleic acid-acrylic resins, and wax-based sizing agents.

[0040] The mass ratio of starch to anionic sizing agent in the coating layer is preferably 90.9:9.1 to 96.8:3.2 (converted to solids content), more preferably 93.0:7.0 to 95.0:5.0. By maintaining this mass ratio, the printability of the surface layer of the paperboard can be improved, ensuring a smooth printed surface. Furthermore, since the rigidity of the coating layer can be increased, the compressive strength on the back layer of the paperboard can be improved.

[0041] The coating layer may also contain various additives such as antislip agents, preservatives, rust inhibitors, defoamers, viscosity modifiers, dispersants, flow modifiers, dyes, water-resistant agents, and water-retaining agents in the starch and anionic sizing agent.

[0042] The lower limit for the coating amount, calculated in terms of solid content, is preferably 1.50 g / m². 2 More preferably 1.70 g / m 2 On the other hand, the upper limit of the coating amount is preferably 2.25 g / m². 2 More preferably 2.17 g / m 2 By applying the coating amount within the specified range, the pulp fibers of the surface layer in contact with the surface layer side of the paperboard can be filled, thus improving printability.

[0043] [Properties of cardboard] (Basis weight) The basis weight was determined according to JIS-P8124 (2011). In a further preferred embodiment of the invention, the total basis weight of the paperboard is 330 g / m³. 2 Above 400g / m 2 The following is more preferably 340g / m 2 The above 380g / m 2 The following applies: The total basis weight of the paperboard is less than 330 g / m². 2 In some cases, the compressive strength may become insufficient. This is especially true when the total basis weight of the paperboard exceeds 400 g / m². 2 In such cases, achieving lightweighting in heavy-duty handling may be difficult. The basis weight of this cardboard is 330 g / m². 2 Above 400g / m 2 The following approach can achieve both compressive strength and lightweight handling of the paperboard.

[0044] The preferred basis weight of the surface layer constituting the paperboard is 30.0 g / m³. 2 ~80.0g / m 2 More preferably 40.0 g / m 2 ~50.0g / m 2The preferred basis weight of the subsurface layer is 50.0 g / m³. 2 ~90.0g / m 2 More preferably 60.0 g / m 2 ~80.0g / m 2 The preferred basis weight of the intermediate layer is 50.0 g / m³. 2 ~90.0g / m 2 More preferably 60.0 g / m 2 ~80.0g / m 2 The preferred basis weight of the back layer is 140.0 g / m³. 2 ~190.0g / m 2 More preferably 150.0 g / m 2 ~180.0g / m 2 In the paperboard of the present invention, by keeping the overall basis weight within the stated range, good formability can be achieved during box making, and even in high temperature and high humidity environments, damage to the packaging container when filling the contents after molding can be suppressed.

[0045] (density) The lower limit of the density of this paperboard is preferably 0.75 g / cm³. 3 More preferably, it is 0.78 g / cm³. 3 The upper limit of the density is preferably 0.95 g / cm³. 3 More preferably 0.85 g / cm³ 3 The density of the paperboard within the stated range imparts cushioning and allows for high compressive strength without the need for additional dry paper force enhancers.

[0046] (Paper thickness) The paper thickness was measured according to the "Paper and Paperboard - Test Methods for Thickness and Density" described in JIS-P8118 (2014). The overall paper thickness of the paperboard of the present invention is preferably 300 μm to 550 μm, more preferably 350 μm to 500 μm.

[0047] (Cobb application rate) Cobb sizing degree was determined according to JIS-P8140 (1998), with a measurement time of 2 minutes. The lower limit of Cobb sizing degree for this board is 5 g / m². 2 Preferably 30g / m 2 If the Cobb sizing degree of the paperboard is less than the lower limit, the moisture content in the paperboard may not be sufficiently suppressed in high-temperature and high-humidity environments, and poor bonding may occur when processing it into paper containers, etc. On the other hand, the upper limit of the Cobb sizing degree of the paperboard is preferably 50 g / m². 2 More preferably 40g / m 2If the Cobb sizing degree of the paperboard exceeds the upper limit, the paperboard may develop ripples and wrinkles due to moisture absorption. By ensuring that the Cobb sizing degree of the surface layer and the back layer is within the specified range at a measurement time of 2 minutes, moisture absorption of the paperboard can be suppressed in high temperature and high humidity environments, thus maintaining the quality of the paperboard.

[0048] The upper limit of the difference in Cobb application strength between the surface layer and the back layer is 10 g / m². 2 Preferably 7g / m 2 On the other hand, the lower limit of the difference in Cobb sizing degree is preferably 1 g / m³. 2 If the difference in Cobb application degree exceeds 10 g / m² 2 If the moisture absorption difference between the surface and back layers is too large, it will be difficult to suppress deformation caused by the difference in moisture absorption between the surface and back layers under high temperature and humidity conditions, and the bursting strength of the corrugated cardboard may decrease under such conditions. The difference in Cobb sizing degree is less than 1 g / m². 2 In such cases, excessive adjustments to the Cobb application rate may be made, leading to greater operational variations and potentially increasing the costs associated with quality management.

[0049] (Transverse compressive strength) The transverse compressive strength was measured according to JIS-P8126 (2005). As a paperboard for heavy-duty handling, a transverse compressive strength of 680 N or more, perpendicular to the papermaking direction, can maintain the strength suitable for use as a paperboard for heavy-duty handling. On the other hand, by setting the compressive strength to 850 N or less, the paperboard exhibits excellent processing adaptability.

[0050] (bursting strength) Bursting strength was determined according to the method for testing the bursting strength of paper and board by means of a Mullen high-pressure testing machine as described in JIS-P8131 (2009).

[0051] (Bursting Index) The bursting strength index is specified by JIS-P8131 (2009). The bursting strength index is the value obtained by dividing the bursting strength of the paper, measured according to JIS-P8131 (2009) and expressed in kilopascals (kPa), by the basis weight. The lower limit of the bursting strength index of this paperboard at 60°C and 80% RH is preferably 2.60 kPa·m. 2 / g, more preferably 2.70 kPa·m 2 / g. By keeping the bursting index within the specified range, the impact resistance of the paperboard under high temperature and high humidity conditions can be improved. For "60°C, 80% RH", the determination was performed according to JIS-P8111 (1998).

[0052] This paperboard is preferably used for handling heavy objects. It exhibits excellent strength under high temperature and humidity conditions, making it suitable for use as corrugated cardboard for packaging containers for bundling in Southeast Asia. Additionally, it can be used as industrial paper in various fields, such as distribution in high temperature and humidity environments.

[0053] [Methods for manufacturing cardboard] The manufacturing method of this paperboard is not particularly limited, but may include pulping, papermaking, coating and drying processes.

[0054] (Pulping process) In the pulping process, pulping is performed to achieve the desired degree of freeness in the raw pulp that makes up each layer. The raw pulp includes softwood kraft pulp, corrugated waste paper pulp, etc. First, the pulp fibers are dispersed in water to obtain a pulp stock. Additives corresponding to each paper layer are added to the pulp stock as needed and mixed to prepare the paper stock for each paper layer.

[0055] (Papermaking process) Next, using these raw material pulps, paper is formed in a neutral zone using a paper machine to achieve a paperboard pH of 6 to 8. In this papermaking process, a paper machine is used to form paper from the raw pulp material sprayed onto the traveling wire. There are no particular limitations on the paper machine; known paper machines can be used, such as four-wire paper machines, cylinder paper machines, mixing and forming machines, and clamping forming machines, to form paper in multiple layers of 4 to 6 layers.

[0056] (Coating process) The surface layer and the back layer may include a process for applying a coating liquid. A known coating machine can be used in the coating process.

[0057] (Drying process) Next, the paper is dried using a drying cylinder.

[0058] Finally, it is rolled onto a paper roll to obtain paperboard.

[0059] This type of paperboard can improve strength under high temperature and high humidity conditions while enabling the reuse of waste pulp.

[0060] <Other Implementation Methods> The present invention is not limited to the above-described embodiments. In addition to the above methods, it can also be implemented in various ways with modifications and improvements.

[0061] For example, the paperboard may also have one or more intermediate layers between the intermediate layer and the back layer. Example

[0062] The present invention will be further described in detail below with reference to embodiments, but the present invention is not limited to the following embodiments.

[0063] [Example 1] The raw materials listed in Tables 1 and 2 were added to the pulp. Furthermore, regarding the freeness of the surface layer of unbleached softwood kraft pulp and corrugated waste pulp, it was dissociated according to the pulp dissociation method of JIS-P8220-1 (2012) to obtain dissociated pulp, and the freeness of this dissociated pulp was determined according to the Canadian Standard Freeness Test Method of JIS-P8121-2 (2012). (additive) (1) Rosin-based adhesive additives Harima Chemical Co., Ltd.'s "NS-77P" Special Rosin Additive (50% Solids) (2) Acrylamide-based drying paper strength enhancers Harima Chemical Co., Ltd.'s "Hammide RB-550": Biionic polyacrylamide (20% solids)

[0064] [Examples 2-18, Comparative Examples 1-6, and Reference Examples (Commercially Available Products)] Except for the types, contents, and physical properties of the raw materials shown in Tables 1 and 2, the paperboards of Examples 2 to 18 and Comparative Examples 1 to 6 were obtained in the same manner as in Example 1. Additionally, as a reference example, a basis weight of 410 g / m³ was evaluated. 2 Commercially available products.

[0065] [Table 1]

[0066] [Table 2]

[0067] [evaluate] The obtained paperboards were evaluated using the following methods. The evaluation results are shown in Table 3. (1) Basis weight (g / m 2 ) The determination was performed according to the "Paper and Board - Method for Determination of Basis Weight" as described in JIS-P8142 (1998). (2) Paper thickness (μm) The determination was performed according to the test method for thickness and density described in JIS-P8118 (2014) for paper and paperboard. (3) Density (g / cm³) 3 ) The determination was performed according to the test method for thickness and density described in JIS-P8118 (2014) for paper and paperboard. (4) Lateral compressive strength The transverse compressive strength was determined according to JIS-P8126 (2005) under normal temperature and humidity (25% and 65% RH) atmosphere. (5) Cobb application The measurements were performed according to JIS-P8140 (1998). Furthermore, the Cobb sizing degree referred to in this invention is measured by bringing water into contact with the surface layer or back layer for 2 minutes. (6) Bursting strength and bursting index According to JIS-P8131 (2009), the bursting strength and bursting index were determined under normal temperature and humidity (25℃, 65% RH) conditions, with the side in contact with the rubber diaphragm as the test surface, and 10 tests each on the front and back sides (average value). The bursting index was calculated by dividing the bursting strength by the basis weight. (7) Flexural strength Regarding folding strength, the longitudinal folding strength was determined according to the "Paper and Board - Test Method for Folding Strength - MIT Test Machine Method" described in JIS-P8115 (2001). A longitudinal folding strength of 490 cycles or more for the board indicates that indentation line breakage can be suppressed. (8) Strength under high temperature and high humidity conditions (bursting strength under high temperature and high humidity conditions and the residual strength under high temperature and high humidity conditions) First, the obtained cardboard was placed in a dryer at 105°C for 2 hours. Then, it was immersed in distilled water for 30 minutes with a weight on it, in a container filled with distilled water. The test piece was then flipped over and immersed in water for another 30 minutes with a weight on it. Next, the moisture was wiped off the cardboard with a cloth, and the bursting strength was measured according to JIS-P8131 (2009). Then, the bursting strength under the above test conditions was divided by the bursting strength under normal temperature and humidity (25°C, 65% RH) conditions to calculate the bursting strength retention rate (%) under high temperature and humidity conditions. If the bursting strength retention rate (%) of the cardboard under the aforementioned high temperature and humidity conditions is less than 20%, the impact resistance under high temperature and humidity conditions cannot be fully obtained, and the cardboard box is prone to breakage during transportation. Furthermore, when the burst strength retention rate (%) under the aforementioned high temperature and high humidity environment is 38% or more, the solubility in water decreases, resulting in poor reusability.

[0068] [Table 3]

[0069] As shown in Table 3 above, the transverse compressive strength of Examples 1 to 18 is 680 N or more, indicating that it can maintain the strength suitable for use as a board for heavy-duty handling. (In Examples 1 to 18, the content of unbleached softwood kraft pulp is 90% by mass or more relative to the total pulp composition of the surface layer; the lower surface layer, the middle layer, and the back layer contain corrugated waste paper pulp, and the content of corrugated waste paper pulp is 90% by mass or more relative to the total pulp composition of the back layer; all layers contain internal sizing agents, and the total content of internal sizing agents in all layers, converted to solids, is 2.50 kg / pulp t or more and 6.00 kg / pulp t or less, with a total basis weight of 330 g / m³.) 2 Above 400g / m 2 Hereinafter, the Cobb application rate of the surface layer and the back layer is 5 g / m². 2 Above 50g / m 2 Hereinafter, the difference in Cobb application rate between the surface layer and the back layer is 10 g / m². 2 (See below). In addition, the bursting strength, bursting strength retention rate, and flexural strength under high temperature and humidity conditions are all good.

[0070] On the other hand, the total basis weight is less than 330g / m 2 Comparative Examples 1, 2, and 6 had a total basis weight as low as less than 330 g / m³. 2 Therefore, the transverse compressive strength is low, making it unsuitable as a paperboard for heavy-duty handling. In Comparative Example 3, the content of softwood kraft pulp is less than 90% by mass relative to the total pulp composition of the surface layer, resulting in reduced transverse compressive strength and folding endurance, making it unsuitable as a paperboard for heavy-duty handling. The total content of internal sizing agents is less than 2.50 kg / t of pulp, and the difference in Cobb sizing degree between the surface and back layers exceeds 10 g / m². 2 Comparative Example 4 showed a decrease in burst strength retention under high temperature and humidity. The total content of internal sizing agent was less than 2.50 kg / t of pulp, and the Cobb sizing degree of the surface and back layers exceeded 50 g / m². 2 Comparative Example 5 shows a decrease in the residual burst strength under high temperature and high humidity conditions. In addition, the commercially available product in the reference example has a Cobb application rate difference of more than 10 g / m² between the surface layer and the back layer. 2 However, due to the total base weight being as high as 410g / m 2 Therefore, the retention rates of compressive strength and burst strength under high temperature and humidity conditions are higher. However, due to the high total basis weight of 410 g / m³, 2 Therefore, it is difficult to achieve lightweighting in the handling of heavy objects.

[0071] The results above demonstrate that the strength under high temperature and humidity conditions can be improved while achieving the reuse of waste paper pulp. This paperboard can be used as industrial paper in various fields, such as distribution in high temperature and humidity environments, for heavy-duty handling.

Claims

1. A type of paperboard, said paperboard having 4 to 6 layers. The paperboard has at least a surface layer, a subsurface layer, an intermediate layer, and a back layer. The surface layer contains unbleached coniferous kraft pulp. The content of the unbleached coniferous kraft pulp is 90% or more by mass relative to the total pulp composition of the surface layer. The lower surface layer, the middle layer, and the back layer contain corrugated paper waste pulp. The entire pulp component of the back layer consists solely of the corrugated paper waste pulp. All layers contain internal sizing agents. The total content of internal sizing agent in all layers, converted to solids, is between 2.50 kg / t and 6.00 kg / t of pulp. The total basis weight is 330g / m³ 2 Above 400g / m 2 the following, The overall paper thickness is between 415μm and 550μm. The Cobb application rate of the surface layer and the back layer was 5 g / m² at a measurement time of 2 minutes. 2 Above 50g / m 2 the following, The difference in Cobb application rate between the surface layer and the back layer is 10 g / m². 2 the following, The content of the corrugated waste paper pulp is 90% or more by mass, relative to the total pulp composition of the lower surface layer and the middle layer.

2. The paperboard according to claim 1, wherein, At least the surface layer and the subsurface layer contain a drying paper strength enhancer. The content of the dry paper strength enhancer in the lower layer is greater than the content of the dry paper strength enhancer in the surface layer.

3. The paperboard according to claim 1 or 2, wherein, The density of the paperboard is 0.75 g / cm³. 3 Above 0.95g / cm 3 the following, The bursting strength of the paperboard is above 950 kPa and below 1100 kPa.

4. The paperboard according to claim 1 or 2, wherein, The cardboard is used for handling heavy objects.

5. The paperboard according to claim 3, wherein, The cardboard is used for handling heavy objects.

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