Paperboard and its manufacturing method

Abstract

To provide cardboard having excellent mechanical strength as well as excellent recyclability, and capable of reducing environmental load and a manufacturing method thereof.SOLUTION: Disclosed is cardboard involving a kraft pulp as a raw material pulp, wherein the cardboard has a thickness of 0.5 mm or over and 15.0 mm or under, a density of 1.00 g / cm3 or over, a content of an additive other than the raw material pulp in the cardboard of 1 pt.mass or under relative to 100 pts.mass of the raw material pulp, and a flexure strength in its longitudinal direction and its transverse direction of 8.0 N or over, respectively. Also disclosed is a manufacturing method of cardboard involving a kraft pulp as a raw material pulp, wherein the manufacturing method includes laminating, hot-pressing, and drying wet paper produced by paper-making of the raw material pulp, having a content of an additive other than the raw material pulp of 1 pt.mass or under relative to 100 pts.mass of the raw material pulp.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to paperboard and methods for making the same. [Background technology]

[0002] In recent years, the problem of plastic waste has become more serious. Plastic does not decompose for a long time, and the waste turns into microplastics in the natural environment, causing serious damage to the ecosystem. As a countermeasure, it has been proposed to replace plastic with paper.

[0003] For example, Patent Document 1 describes a multi-layer paper having a multiple-ply middle layer and a pair of surface layers formed on the front and back of the middle layer, in which the pulp of the surface and middle layers contains a specific proportion or more of softwood kraft pulp and hardwood kraft pulp, the basis weight of the entire middle layer is greater than the basis weight of each of the surface layers, and the basis weight of each layer and the basis weight of the entire multi-layer paper are within a specific range, and it is described that uses of this multi-layer paper can include tableware, etc.

[0004] Furthermore, for example, as a paper material with excellent mechanical strength, Patent Document 2 describes a vulcanized fiber that is manufactured by stacking surface layer base paper and back layer base paper whose main components are natural fibers and middle layer base paper containing natural fibers and carbon fibers in the order of surface layer base paper, middle layer base paper, back layer base paper, and then immersing the stacked layers in an aqueous zinc chloride solution. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-80590 [Patent Document 2] JP 2020-153052 A Summary of the Invention [Problem to be solved by the invention]

[0006] Since the applications of paper are expected to expand further, there is a demand for the development of paper with better mechanical strength, and there is room for improvement in the multilayer paper described in Patent Document 1. In addition, the paper made of vulcanized fiber described in Patent Document 2 has excellent mechanical strength, but has the problem of being unenvironmentally friendly because it is treated with an aqueous zinc chloride solution.

[0007] Therefore, an object of the present invention is to provide a paperboard that is excellent in mechanical strength and recyclability, and that can reduce the burden on the environment, and a method for producing the same. [Means for solving the problem]

[0008] The object of the present invention can be achieved by the following configuration. <1> A paperboard made from kraft pulp, with a thickness of 0.5 mm to 15.0 mm and a density of 1.00 g / cm 3 or more, wherein the amount of additives other than the raw pulp in the paperboard is 1 part by mass or less per 100 parts by mass of the raw pulp, and the bending strength in the longitudinal and transverse directions is 8.0 N or more. <2> Bending strength per basis weight is 0.010N / (g / m 2 ) or more, <1> The paperboard described in <3> The bending modulus in the longitudinal and transverse directions is 2.00 GPa or more, <1> or <2> The paperboard described in <4> The tensile strength in the longitudinal and transverse directions is 60.0 MPa or more, <1> ~ <3> 2. The paperboard according to claim 1 . <5> The raw material pulp is unbleached kraft pulp and contains 80% by mass or more of softwood kraft pulp. <1> ~ <4> 2. The paperboard according to claim 1 . <6> The composition of the paperboard through the thickness direction is the same; <1> ~ <5> 2. The paperboard according to claim 1 . <7> The paperboard has water-disintegrability. <1> ~ <6> 2. The paperboard according to claim 1 . <8> <1> ~ <7> Tableware, medical equipment, furniture, stationery, and building materials formed from the paperboard according to any one of claims 1 to 4. <9> <1> ~ <7> The method for producing paperboard according to any one of the above, wherein the additives other than the raw material pulp are 1 part by mass or less per 100 parts by mass of the raw material pulp, and wet paper produced by papermaking the raw material pulp is stacked, hot pressed, and dried. Effect of the Invention

[0009] According to the present invention, it is possible to provide a paperboard that is excellent in mechanical strength and recyclability, and that can reduce the burden on the environment, and a method for producing the same. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Preferred embodiments of the present invention will be described below. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, in this specification, unless otherwise specified, operations and measurements of physical properties are performed under conditions of room temperature (20 to 25°C) and a relative humidity of 40 to 50%.

[0011] [Paperboard] The paperboard according to this embodiment (hereinafter, simply referred to as paperboard) is a paperboard made of kraft pulp as a raw material pulp, has a thickness of 0.5 mm or more and 15.0 mm or less, and a density of 1.00 g / cm 3 or more, additives other than the raw pulp in the paperboard are 1 part by mass or less per 100 parts by mass of the raw pulp, and bending strengths in the longitudinal and transverse directions are 8.0 N or more. The paperboard according to the present embodiment has excellent bending strength due to the thickness being within the above range and the density being equal to or greater than the above value. This provides a material that has a texture and weight different from plastic, is very hard, and is suitable for processing such as cutting and milling with a machine tool. In addition, the paperboard according to the present embodiment has excellent recyclability and reduces the burden on the environment because the additives other than the raw pulp in the paperboard are equal to or less than a specific value. The bending strengths in the longitudinal and transverse directions are measured in accordance with JIS K 7171:2016, as described below.

[0012] <Raw pulp> In order to obtain a paperboard that can be easily reused, the paperboard according to the present embodiment uses kraft pulp as the raw material pulp. The kraft pulp as the raw material pulp is preferably one or more selected from softwood kraft pulp such as pine and cedar, and hardwood kraft pulp such as acacia and eucalyptus.

[0013] From the viewpoint of obtaining a paperboard excellent in mechanical strength, particularly bending properties such as bending strength and bending modulus, and tensile strength, and from the viewpoint of improving moldability into a shape suitable for an application, it is preferable that the raw material pulp contains softwood kraft pulp as the main component. "Containing softwood kraft pulp as the main component" means that the content of softwood kraft pulp in the raw material pulp is more than 50 mass%, and the content of softwood kraft pulp in the raw material pulp is preferably 80 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and even more preferably 100 mass%. Softwood kraft pulp has a long average fiber length, and paperboards using softwood kraft pulp as the raw material pulp tend to have excellent mechanical strength. The raw material pulp is preferably one or more types selected from the group consisting of bleached kraft pulp and unbleached kraft pulp, and more preferably unbleached kraft pulp. More preferably, since the bleaching process causes some damage to the pulp fibers, the raw pulp is mainly composed of unbleached softwood kraft pulp, which allows the production of molded products made of fibers with a large molecular weight and excellent long-term durability.

[0014] (Canadian Standard Freeness (CSF)) The raw material pulp has a CSF according to JIS P 8121-2: 2012 of preferably 150 mL or more, more preferably 175 mL or more, and preferably 600 mL or less, more preferably 550 mL or less. When the CSF of the raw material pulp is in the above range, the recyclability is excellent. When two or more kinds of raw pulp are used, the CSF of the raw pulp may be adjusted to fall within the above range by mixing pulps that have been beaten separately, or by beating pulps that have been mixed in advance. The method for beating the pulp is not particularly limited, and a general beating machine such as a refiner is used.

[0015] <Additives other than raw pulp> From the viewpoint of obtaining a paperboard that can be easily reused, the amount of additives other than the raw pulp is 1 part by mass or less per 100 parts by mass of the raw pulp, preferably 0.7 parts by mass or less, more preferably 0.5 parts by mass or less, even more preferably 0.3 parts by mass or less, still more preferably 0.1 parts by mass or less, and even more preferably 0 part by mass.

[0016] Additives other than raw pulp include dry strength agents, wet strength agents, water-resistant agents, water repellents, foamable microcapsules, sizing agents, dyes, retention aids, fillers, pH adjusters, slime control agents, thickeners, preservatives, fungicides, antibacterial agents, flame retardants, rodenticides, insect repellents, moisturizers, freshness-retaining agents, oxygen absorbers, microcapsules, foaming agents, surfactants, electromagnetic shielding materials, antistatic agents, rust inhibitors, fragrances, and deodorizers.

[0017] [Physical properties of paperboard] <Thickness> From the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties, and uniform density, the thickness of the paperboard is 0.5 mm or more, preferably 0.7 mm or more, more preferably 0.8 mm or more, even more preferably 1.0 mm or more, and 15.0 mm or less, preferably 12.0 mm or less, more preferably 10.0 mm or less, even more preferably 5.0 mm or less. The thickness of the paperboard is measured in accordance with JIS P 8118:2014.

[0018] <Basic weight> The basis weight of the paperboard is not particularly limited as long as the thickness and density of the paperboard are within a specific range, but is preferably 800 g / m 2 More preferably, 850 g / m 2More preferably, 950 g / m 2 and preferably 2500 g / m 2 Less than 2300g / m 2 More preferably, 2100 g / m or less 2 The basis weight of paperboard is measured in accordance with JIS P 8124:2011.

[0019] <density> The density of the paperboard is set to 1.00 g / cm from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties. 3 or more, preferably 1.02 g / cm 3 More preferably, 1.04 g / cm 3 From the viewpoint of ease of manufacture, the density is preferably 1.40 g / cm 3 Less than or equal to 1.35 g / cm 3 More preferably, 1.30 g / cm 3 The density of the paperboard is calculated from the thickness and basis weight of the paperboard, which are obtained by the measurement methods described above.

[0020] <Vertical bending strength> The bending strength of the paperboard in the longitudinal direction is 8.0N or more from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties. From the same viewpoint, the bending strength of the paperboard in the longitudinal direction is preferably 10.0N or more, more preferably 10.5N or more, even more preferably 11.0N or more, and even more preferably 15.0N or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 100N or less. The bending strength of the paperboard in the longitudinal direction is measured in accordance with JIS K 7171:2016. Note that the longitudinal direction (MD) of the paperboard is the direction of the paperboard parallel to the traveling direction when the paperboard is made by a papermaking machine, and is the flow direction. However, when it is unclear whether the direction of the paperboard is the longitudinal direction, it is measured at an angle of 22.5 degrees, and the direction showing the strongest measured value is the longitudinal direction, and the direction perpendicular to this is the transverse direction (CD). The same applies below.

[0021] <Lateral bending strength> The bending strength in the cross direction of the paperboard is 8.0 N or more from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties. From the same viewpoint, the bending strength in the cross direction of the paperboard is preferably 10.0 N or more, more preferably 15.0 N or more. The upper limit is not particularly limited, but is 100 N or less from the viewpoint of ease of production. The bending strength in the cross direction of the paperboard is measured in accordance with JIS K 7171:2016. Note that the cross direction (CD) of the paperboard refers to the direction of the paperboard perpendicular to the traveling direction of the paperboard when the paperboard is made on a papermaking machine.

[0022] <Bending strength> The bending strength of the paperboard in the present invention is 8.0 N or more in both the longitudinal and transverse directions from the viewpoint of obtaining workability as a material having excellent mechanical strength, particularly bending properties, and a certain level of isotropy. From the same viewpoint, the bending strength of the paperboard is preferably 10.0 N or more, more preferably 15.0 N or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 100 N or less.

[0023] <Geometric mean value of bending strength> From the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties, the geometric mean value of the bending strength of the paperboard is preferably 8.0 N or more, more preferably 10.0 N or more, and even more preferably 15.0 N or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 100 N or less. The geometric mean value of the bending strength of the paperboard is calculated as the square root of the product of the bending strength in the longitudinal direction of the paperboard and the bending strength in the transverse direction of the paperboard, which are obtained by the method described above.

[0024] <Bending strength per basis weight> The bending strength per basis weight of the paperboard is preferably 0.010 N / (g / m) from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly excellent bending properties. 2 ) or more, more preferably 0.015N / (g / m 2 ) or more. There is no particular upper limit to the value, but from the viewpoint of ease of manufacture, it is preferably 0.050 N / (g / m 2) or less. The bending strength per basis weight of the paperboard is determined by dividing the geometric mean value of the bending strength of the paperboard obtained by the above-mentioned method by the basis weight of the paperboard measured in accordance with JIS P 8124:2011. Increasing the surface roughness of the paperboard tends to reduce the stress in a thinner paperboard. Therefore, by adjusting the bending strength per basis weight, the practical strength can be adjusted.

[0025] <Vertical bending elastic modulus> The bending modulus of the paperboard in the longitudinal direction is preferably 2.00 GPa or more, more preferably 2.70 GPa or more, even more preferably 2.80 GPa or more, and even more preferably 3.00 GPa or more, from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties. The upper limit is not particularly limited, but is 10.0 GPa or less from the viewpoint of ease of production. The bending modulus of the paperboard in the longitudinal direction is measured in accordance with JIS K 7171:2016.

[0026] <Lateral bending modulus> The bending modulus of the paperboard in the transverse direction is preferably 2.00 GPa or more, more preferably 2.20 GPa or more, and even more preferably 3.00 GPa or more, from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties. The upper limit is not particularly limited, but is 10.0 GPa or less from the viewpoint of ease of production. The bending modulus of the paperboard in the transverse direction is measured in accordance with JIS K 7171:2016.

[0027] <Flexural modulus> The bending modulus of the paperboard in the present invention is preferably 2.00 GPa or more, more preferably 3.00 GPa or more, in both the longitudinal and transverse directions, from the viewpoint of improving the processability as a material having a certain level of isotropy. The upper limit is not particularly limited, but is 10.0 GPa or less from the viewpoint of ease of production.

[0028] <Geometric mean value of flexural modulus> From the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly excellent bending properties, the geometric mean value of the bending modulus of the paperboard is preferably 2.00 GPa or more, more preferably 2.30 GPa or more, even more preferably 2.50 GPa or more, and even more preferably 3.00 GPa or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 10.0 GPa or less. The geometric mean value of the bending modulus of the paperboard is calculated as the square root of the product of the bending modulus of the paperboard in the longitudinal direction and the bending modulus of the paperboard in the transverse direction, which are obtained by the above-mentioned method.

[0029] <Flexural modulus per basis weight> The bending modulus per basis weight of the paperboard is preferably 0.0025 GPa / (g / m) from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly bending properties, and from the viewpoint of ease of production. 2 ) or more, more preferably 0.0030GPa / (g / m 2 ) or more, and preferably 0.010 GPa / (g / m 2 ) or less, more preferably 0.0045GPa / (g / m 2 The bending modulus per basis weight of the paperboard is determined by dividing the geometric mean value of the bending modulus of the paperboard obtained by the above-mentioned method by the basis weight of the paperboard measured in accordance with JIS P 8124:2011.

[0030] <Longitudinal tensile strength> From the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly tensile strength, the tensile strength in the longitudinal direction of the paperboard is preferably 60.0 MPa or more, more preferably 70.0 MPa or more, even more preferably 90.0 MPa or more, even more preferably 100.0 MPa or more, and even more preferably 110.0 MPa or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 150 MPa or less. The tensile strength in the longitudinal direction of the paperboard is measured in accordance with JIS P 8113:2006.

[0031] <Transverse tensile strength> The tensile strength in the transverse direction of the paperboard is preferably 60.0 MPa or more, more preferably 70.0 MPa or more, from the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly tensile strength. The upper limit is not particularly limited, but is 100 MPa or less from the viewpoint of ease of production. The tensile strength in the transverse direction of the paperboard is measured in accordance with JIS P 8113:2006.

[0032] <Tensile strength> The tensile strength of the paperboard in the present invention is preferably 60.0 MPa or more, more preferably 70.0 MPa or more, in both the machine and cross directions, from the viewpoint of obtaining excellent mechanical strength, particularly tensile strength, and processability as a material having a certain level of isotropy. The upper limit is not particularly limited, but is 150 MPa or less from the viewpoint of ease of production.

[0033] <Geometric mean value of tensile strength> From the viewpoint of obtaining a paperboard having excellent mechanical strength, particularly tensile strength, the geometric mean value of the tensile strength of the paperboard is preferably 70.0 MPa or more, more preferably 80.0 MPa or more, even more preferably 90.0 MPa or more, and even more preferably 95.0 MPa or more. The upper limit is not particularly limited, but from the viewpoint of ease of production, it is 120 MPa or less. The geometric mean value of the tensile strength of the paperboard is calculated as the square root of the product of the tensile strength in the longitudinal direction of the paperboard and the tensile strength in the transverse direction of the paperboard, which are obtained by the above-mentioned method.

[0034] <Moisture> The moisture content of the paperboard is preferably 3.0% or more, and preferably 8.5% or less, more preferably 8.0% or less, even more preferably 7.5% or less, and even more preferably 7.0% or less. The moisture content of the paperboard is measured in accordance with JIS P 8127:2010.

[0035] <Water degradability> The paperboard according to the present embodiment is preferably water-disintegrable. Water-disintegrable refers to the property that, when the paperboard is immersed in water and stirred with a standard pulp disintegrator, the compressed pulp can be loosened without retaining the original shape of the paperboard. The paperboard according to the present embodiment contains additives other than the raw pulp in an amount of 1 part by mass or less per 100 parts by mass of the raw pulp, and further has water-disintegrability, so that it can be more easily reused as pulp.

[0036] <CSF of disintegrated pulp> The CSF of the disintegrated pulp obtained by disintegrating paperboard in accordance with JIS P 8220: 2021 is preferably 200 mL or more and preferably 650 mL or less from the viewpoint of productivity when recycling the paperboard as pulp. The CSF of the disintegrated pulp is measured in accordance with JIS P 8121-2: 2012.

[0037] <Composition through thickness> The paperboard according to the present embodiment preferably has a uniform composition in the thickness direction, which makes it easier to obtain a paperboard having an excellent manufacturing method and uniform density.

[0038] <Application> The paperboard according to this embodiment has excellent mechanical strength, such as bending strength, bending modulus, and tensile strength, and further has excellent recyclability, and is therefore suitable for use as tableware, such as plates, bowls, chopsticks, cups, coasters, trays, stirrers, straws, spoons, forks, and knives; medical instruments, such as tongue depressors, earpicks, combs, and pus trays; furniture, such as hangers, storage boxes, and storage shelves; stationery, such as clear boards, clips, and pens; and building materials, such as interior materials.

[0039] [Paperboard manufacturing method] In the paperboard manufacturing method according to the present embodiment, it is preferable that the raw pulp is kraft pulp, additives other than the raw pulp are 1 part by mass or less per 100 parts by mass of the raw pulp, and wet papers manufactured by papermaking the raw pulp are laminated, hot pressed, and dried. Specifically, for example, it is preferable that the wet papers obtained by passing through the wire section of a papermaking machine are laminated using a pulp slurry prepared from the raw pulp, and the laminated wet papers are hot pressed and dried.

[0040] Examples of papermaking machines include Fourdrinier papermaking machines, cylinder papermaking machines, twin-wire machines, and combination machines. From the viewpoint of efficiently achieving a paperboard thickness within the desired range, it is preferable to use a cylinder papermaking machine. The method of hot pressing the wet paper is not particularly limited, but for example, it is preferably at 80°C to 200°C and 10 to 100 kgf / cm 2 The wet paper can be pressed by hot pressing under the above conditions for several minutes to several hours. The method for drying the wet paper is not particularly limited, but for example, a Yankee dryer, a multi-cylinder dryer, or the like can be used.

[0041] As additives other than the raw pulp, those listed as additives other than the raw pulp for the paperboard according to the above-mentioned embodiment can be used. From the viewpoint of producing a paperboard that can be easily reused, the amount of additives other than the raw pulp is 1 part by mass or less, preferably 0.7 parts by mass or less, more preferably 0.5 parts by mass or less, even more preferably 0.3 parts by mass or less, still more preferably 0.1 parts by mass or less, and even more preferably 0 part by mass, relative to 100 parts by mass of the raw pulp.

[0042] To manufacture paperboard having a thickness of 0.5 mm to 15.0 mm, the following method can be adopted, although it is not particularly limited. A layer of wet paper made by a cylinder paper machine is continuously transported on a felt and wound up by a cylinder wet paper laminator until it has the desired thickness. The wet paper wound on the cylinder wet paper laminator can be cut with a cutter and dried with hot air while being pressed by a press dryer. Alternatively, hot pressing and drying can be performed by, for example, a roll press.

[0043] The paperboard manufacturing method according to the present embodiment may include a step of subjecting the paperboard to a surface treatment, such as embossing to impart a relief pattern, or calendaring to impart gloss, smoothness or transparency to the paper, or to adjust the thickness.

[0044] The paperboard manufacturing method according to the present embodiment may include a step of bonding a plurality of paperboards with an adhesive. The adhesive is preferably a water-based adhesive. The water-based adhesive is not particularly limited, but examples thereof include starch (including various modified starches such as phosphated starch and cationic starch), latexes (conjugated diene-based polymer latexes such as styrene-butadiene copolymers, acrylic polymer latexes such as acrylic acid ester and / or methacrylic acid ester polymers or copolymers, ethylene-vinyl acetate copolymer latexes, etc.), proteins such as casein, gelatin, and soybean protein, synthetic resin-based adhesives such as various polyvinyl alcohols, various polyacrylamides, and melamine resins, and various cellulose derivatives such as carboxymethyl cellulose. EXAMPLES

[0045] Examples are given below to specifically explain the present invention, but the present invention is not limited to these examples. Unless otherwise specified, the following operations were performed under conditions of 23°C and 50% RH. Furthermore, in the examples and comparative examples, "parts" and "%" respectively refer to "parts by mass" and "% by mass" unless otherwise specified.

[0046] [analysis] The raw pulp and paperboard of the examples and comparative examples were subjected to the following analyses.

[0047] <Raw pulp> (CSF) The CSF of the raw pulp was measured in accordance with JIS P 8121-2:2012.

[0048] <Paperboard> (Basic weight) The basis weight of the paperboard was measured in accordance with JIS P 8124:2011.

[0049] (Thickness) The thickness of the paperboard was measured in accordance with JIS P 8118:2014.

[0050] (density) The density of the paperboard was calculated from the thickness and basis weight obtained by the measurement methods described above.

[0051] (Bending strength) The bending strength in the longitudinal and transverse directions of the paperboard was measured in accordance with JIS K 7171:2016 by cutting 80 mm x 10 mm test pieces from the paperboard.

[0052] (geometric mean value of bending strength) The geometric mean value of the bending strength of the paperboard was determined as the square root of the product of the bending strength in the machine direction of the paperboard and the bending strength in the cross direction of the paperboard, obtained by the method described above.

[0053] (Bending strength per basis weight) The bending strength per basis weight of the paperboard was determined by dividing the geometric mean value of the bending strength obtained by the method described above by the basis weight.

[0054] (Flexural modulus) The bending elastic modulus of the paperboard in the longitudinal and transverse directions was measured in accordance with JIS K 7171:2016 by cutting a test piece of 80 mm x 10 mm from the paperboard.

[0055] (Geometric mean value of flexural modulus) The geometric mean value of the bending elastic modulus of the paperboard was calculated as the square root of the product of the bending elastic modulus in the machine direction of the paperboard and the bending elastic modulus in the cross direction of the paperboard, both of which were obtained by the method described above.

[0056] (Flexural modulus per basis weight) The bending modulus per basis weight of the paperboard was determined by dividing the geometric mean value of the bending modulus obtained by the above-mentioned method by the basis weight.

[0057] (Tensile strength) The tensile strength of the paperboard in the machine and cross directions was measured according to JIS P 8113:2006.

[0058] (geometric mean value of tensile strength) The geometric mean value of the paperboard tensile strength was determined as the square root of the product of the paperboard tensile strength in the machine direction and the paperboard tensile strength in the cross direction, obtained by the method described above.

[0059] Example 1 <Paperboard manufacturing> A raw pulp consisting of 100% softwood unbleached kraft pulp was prepared so that the CSF was 330 mL, and 100 g of the raw pulp (bone dry mass) was dispersed in 20 L of water. The pulp slurry containing the raw pulp was stirred for 30 minutes in a pulp disintegrator, and then paper was made using a two-layer cylinder paper machine to obtain a wet paper. Sixteen sheets of the wet paper were mixed at 140°C and 40 kgf / cm 2 The paperboard was prepared by pressing for 10 minutes under the above conditions. The obtained paperboard was subjected to the above-mentioned analyses. The results are shown in Table 1. 40±1 g of the obtained paperboard was sampled, cut into pieces of approximately 2 cm square, and placed in 2 L of water at 20±5°C. The mixture was then mixed for 10 minutes using a standard pulp disintegrator. The compressed pulp was loosened without losing the original shape of the paperboard, and the paperboard had good water disintegrability.

[0060] Example 2 Paperboards were produced in the same manner as in Example 1, except that the thickness of the paperboard was changed to the value shown in Table 1.

[0061] Example 3 Paperboards were produced in the same manner as in Example 1, except that the thickness of the paperboard was changed to the value shown in Table 1.

[0062] Comparative Example 1 For the front and back layers, raw pulp consisting of softwood bleached kraft pulp: hardwood bleached kraft pulp = 10% by mass: 90% by mass was prepared so that the CSF was 440 mL, and for the middle layer, raw pulp consisting of softwood bleached kraft pulp: hardwood bleached kraft pulp = 35% by mass: 65% by mass was prepared so that the CSF was 440 mL. Each was made into 7 layers using paper stock with dry strength agent and sizing agent added to the slurry to produce paperboard. The obtained paperboard was analyzed as described above. The results are shown in Table 1.

[0063] [Table 1]

[0064] From the results of the Examples and Comparative Examples, it can be seen that the paperboard of the present invention has excellent mechanical strength in terms of bending strength, bending modulus, and tensile strength. The paperboard of the present invention also has excellent water disintegrability, and further, since the amount of additives other than the raw pulp in the paperboard is 1 part by mass or less per 100 parts by mass of the raw pulp, it has excellent recyclability. [Industrial Applicability]

[0065] The paperboard of the present invention has excellent mechanical strength such as bending strength, bending modulus, and tensile strength, and further has excellent recyclability, and is therefore suitable for use as tableware such as plates, bowls, chopsticks, cups, coasters, trays, stirrers, straws, spoons, forks, knives, and other such items; medical instruments such as tongue depressors, earpicks, combs, and pus trays; furniture such as hangers, storage boxes, and storage shelves; stationery such as clear boards, clips, and pens; and building materials such as interior materials.

Claims

1. A paperboard made from kraft pulp, The thickness is 1.77 mm or more and 15.0 mm or less, Density is 1.00 g / cm ３ That's all. The amount of additives other than the raw pulp in the paperboard is 1 part by mass or less per 100 parts by mass of the raw pulp, The bending strength in the longitudinal direction and the transverse direction is 8.0 N or more, Paperboard for tableware, medical equipment, furniture or building materials.

2. The bending strength per basis weight is 0.010 N / (g / m ２ 2. The paperboard of claim 1, wherein the thickness of the paperboard is equal to or greater than 100 mm.

3. 3. The paperboard of claim 1 or 2, wherein the bending modulus in the machine direction and the cross direction is each 2.00 GPa or more.

4. The paperboard of any one of claims 1 to 3, having a tensile strength in each of the machine and cross directions of 60.0 MPa or more.

5. The paperboard according to any one of claims 1 to 4, wherein the raw pulp is unbleached kraft pulp and contains 80% by mass or more of softwood kraft pulp.

6. The paperboard of any one of claims 1 to 5, wherein the paperboard has multiple paper layers, and the multiple paper layers have the same composition.

7. The paperboard according to any one of claims 1 to 6, wherein the paperboard is water-disintegratable.

8. Tableware, medical equipment, furniture, and building materials formed from the paperboard of any one of claims 1-7.

9. A method for producing paperboard according to any one of claims 1 to 7, The additive other than the raw pulp is 1 part by mass or less relative to 100 parts by mass of the raw pulp, The wet paper produced by papermaking the raw pulp is laminated, hot pressed, and dried. How paperboard is made.

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