Paperboard and manufacturing method thereof

Abstract

To provide a paperboard having excellent burst strength.SOLUTION: A paperboard includes three paper layers or more and has a burst index of 3.5kPa m2 / g or over, a tensile strength in the vertical direction of 7kN / m or over and the transverse direction of 4kN / m or over, a Cobb water absorbency of its front layer surface and back layer surface 120 s of 100 g / m2 or under, respectively, a compressive strength specified by JIS P8126:2005 of 170 N m2 / g or over, and an air resistance specified by JIS P8117:2009 (Oken test machine method) of 175 s or over.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to paperboard, particularly paperboard suitable for use as a liner for corrugated cardboard, for making boxes, for laminating paper, and the like, and to a method for producing the same. [Background technology]

[0002] Corrugated cardboard is manufactured by attaching a liner to one or both sides of a corrugated core using a corrugator. Corrugated cardboard is used for packaging various items because it is lightweight and inexpensive. During transportation, packaged items may collide with the cardboard from the inside, or the cardboard may be subjected to external impact during storage. For this reason, cardboard liners may be required to have high bursting strength so as not to break even when subjected to impact, depending on the application. Conventionally, cardboard that requires bursting strength uses, as raw pulp, either virgin pulp or recycled paper pulp derived from cardboard made of 100% virgin pulp, or a mixture of these. Cardboard using such raw pulp has high strength and is less prone to crease cracks (breakage lines arising from creases (shallow grooves) provided in cardboard base paper), making it suitable for cardboard liners.

[0003] However, virgin pulp is expensive as a raw material. In addition, waste paper pulp derived from 100% virgin pulp cardboard is also expensive because it needs to be collected through a dedicated collection route to avoid mixing with other waste paper (e.g., cardboard made from recycled pulp), and the amount of waste paper generated is small. Therefore, when paperboard containing a large amount of these pulps is used as raw pulp for cardboard manufacturing, box making, and interleaving paper manufacturing, the cost is high. In order to reduce raw material costs, paperboards with a high recycled paper pulp content have been proposed. For example, the applicant has proposed a paperboard with a recycled paper pulp content of 70% or more by mass and a basis weight of 350 g / m 2 The paperboard has an air resistance of 25 seconds or more (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-20065 A DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a paperboard which contains recycled paper pulp and yet has excellent burst strength. [Means for solving the problem]

[0006] The means for solving the above problems are as follows. 1.Has 3 or more paper layers, The total pulp contained in all paper layers is 40% by weight or more and 80% by weight or less of recycled paper pulp, and 20% by weight or more and 60% by weight or less of kraft pulp, A paperboard characterized in that the surface and back layers combined contain 80% or more by weight of the total kraft pulp contained in all paper layers. 2. The paperboard according to 1, characterized in that at least one of the front and back layers contains 60% or more by weight of kraft pulp relative to the pulp contained in said one layer. 3. The paperboard according to 1 or 2, wherein either the front layer, the back layer, or both, contain recycled paper pulp. 4. The paperboard according to any one of 1. to 3., comprising 0.3% by weight or more of a paper strength agent. 5. Specific burst strength: 3.5 kPa m 2 / g or more. 6. The paperboard according to any one of 1. to 5., characterized in that the tensile strength is 7 kN / m or more in the longitudinal direction and 4 kN / m or more in the transverse direction. 7. The 120-second Cobb water absorbency of the front and back layers is 100 g / m 2 7. The paperboard according to any one of 1. to 6., characterized in that: 8. A paperboard according to any one of 1. to 7., characterized in that the ratio of the 120-second Cobb water absorbency of the front layer surface to the 120-second Cobb water absorbency of the back layer surface (front layer / back layer) is within the range of 0.5 or more and 2.0 or less. 9. The paperboard according to any one of 1. to 8., which is a liner for corrugated cardboard. 10.Has 3 or more paper layers; The total pulp contained in all paper layers is 40% by weight or more and 80% by weight or less of recycled paper pulp, and 20% by weight or more and 60% by weight or less of kraft pulp, A method for manufacturing paperboard, in which the surface layer and the back layer together contain 80% by weight or more of the total kraft pulp contained in all paper layers, A method for producing paperboard, characterized in that the CSF of the surface layer pulp and the back layer pulp immediately after mechanical treatment is greater than the CSF of at least one of the middle layer pulps. 11. The method for producing paperboard according to 10, characterized in that at least one layer of the middle layer pulp has a CSF of 150 ml or more and 550 ml or less immediately after mechanical treatment. 12. A method for producing paperboard according to claim 10 or 11, characterized in that a shaking device is used in the wire part.

[0007] 13. The paperboard according to claim 1, characterized in that it contains 40% by weight or more and 70% by weight or less of recycled paper pulp and 30% by weight or more and 60% by weight or less of kraft pulp relative to the total pulp contained in all paper layers. Effect of the Invention

[0008] The paperboard of the present invention is low-cost because it contains 40% by weight or more of recycled paper pulp relative to the total pulp. The paperboard of the present invention has excellent bursting strength and tensile strength, and is not easily broken even when subjected to impact. The paperboard of the present invention is not easily affected by moisture or humidity, and its strength does not decrease significantly depending on the usage environment. The paperboard of the present invention can be suitably used as a liner for corrugated cardboard, medium base paper, paperboard for paper containers, paperboard for box making, paperboard for laminated paper, etc. BEST MODE FOR CARRYING OUT THEINVENTION

[0009] The paperboard of the present invention has three or more paper layers, The total pulp contained in all paper layers is 40% by weight or more and 80% by weight or less of recycled paper pulp, and 20% by weight or more and 60% by weight or less of kraft pulp, The surface layer and back layer combined are characterized by containing 80% or more by weight of the total kraft pulp contained in all paper layers. The paperboard of the present invention is composed of three or more paper layers, i.e., a surface layer, one or more middle layers, and a back layer, and the surface layer and the back layer are the outermost layers. The number of layers of the paperboard of the present invention is three or more, and can be appropriately set depending on the basis weight. The back layer refers to the bottom surface in the paper flow direction during papermaking, and is the layer that is formed when the raw material is first supplied to the wire part in a multi-layer papermaking machine, while the front layer refers to the side opposite the back layer, i.e., the top surface in the paper flow direction during papermaking, and is the layer that is formed when the raw material is last supplied to the wire part in a multi-layer papermaking machine.

[0010] The paperboard of the present invention contains recycled paper pulp in an amount of 40% by weight or more and 80% by weight or less based on the total pulp. By increasing the recycled paper pulp content, the cost of raw pulp can be reduced, so the recycled paper pulp content is preferably 50% by weight or more, and more preferably 60% by weight or more based on the total pulp. The waste paper pulp may be one or more of the following: waste paper pulp obtained by disintegrating unprinted waste paper such as corrugated cardboard, white, extra white, medium white, and white damaged; waste paper printed on fine paper, fine coated paper, medium quality paper, medium quality coated paper, and recycled paper; waste paper with writing on it, papers such as discarded confidential documents, magazines, and newspapers; and deinked waste paper pulp (DIP) obtained by deinking the disintegrated waste paper pulp. Among these, it is preferable to use waste paper pulp derived from corrugated cardboard (hereinafter referred to as corrugated cardboard pulp). Corrugated cardboard mainly uses relatively long fiber pulp such as unbleached softwood kraft pulp, and the liner and medium are bonded together with a starch-based adhesive. The amount of printing is also small, so corrugated cardboard pulp tends to have longer fiber length, higher lignin content and starch content, and lower ink content compared to other waste paper pulps. In addition, many countries have established recycling systems for cardboard, and the recovery rate is high, along with that of recycled newspapers and magazines, so recycled cardboard pulp is inexpensive. Therefore, by using recycled cardboard pulp, it is possible to achieve both strength development of the paperboard and reduced production costs. The blending ratio of recycled cardboard pulp to recycled paper pulp is preferably 80% by weight or more, more preferably 90% by weight or more, even more preferably 95% by weight or more, and most preferably 100% by weight.

[0011] The paperboard of the present invention contains 20% by weight or more and 60% by weight or less of kraft pulp relative to the total pulp, and the front and back layers together contain 80% by weight or more of the total kraft pulp contained in all paper layers. In this way, by containing 20% ​​by weight or more of kraft pulp relative to the total pulp and concentrating 80% by weight or more of the total kraft pulp in the outermost layer, the bursting strength of the paperboard can be increased. In order to obtain a paperboard with excellent bursting strength, it is preferable that the blending ratio of kraft pulp relative to the total pulp is high, and the blending ratio of kraft pulp relative to the total pulp is preferably 22% by weight or more, more preferably 24% by weight or more, and even more preferably 30% by weight or more. In order to obtain a paperboard with excellent bursting strength, it is preferable that the front and back layers together contain 85% by weight or more of the total kraft pulp contained in all paper layers, more preferably 90% by weight or more, and even more preferably 95% by weight or more.

[0012] The kraft pulp may be one of bleached softwood kraft pulp (NBKP), unbleached softwood kraft pulp (NUKP), bleached hardwood kraft pulp (LBKP), and unbleached hardwood kraft pulp (LUKP), or a mixture of two or more of these. Among these, unbleached softwood kraft pulp (NUKP) is preferred because it has a long fiber length and a large fiber diameter, which contributes to improving burst strength. The blending ratio of NUKP to kraft pulp is preferably 80% by weight or more, more preferably 90% by weight or more, even more preferably 95% by weight or more, and most preferably 100% by weight.

[0013] The paperboard of the present invention contains 80% by weight or more of the total kraft pulp contained in all paper layers in the front and back layers combined, and it is preferable that the front layer contains kraft pulp, and it is more preferable that both the front and back layers contain kraft pulp. By containing kraft pulp in both the front and back layers, the bursting strength can be increased. In the paperboard of the present invention, either or both of the pulp for the surface layer and the pulp for the back layer preferably contain 60% by weight or more of kraft pulp. By containing 60% by weight or more of kraft pulp in either or both of the pulp for the surface layer and the pulp for the back layer, the bursting strength can be further increased. This kraft pulp content is more preferably 80% by weight or more, even more preferably 90% by weight or more, and most preferably 100% by weight.

[0014] In the paperboard of the present invention, either or both of the surface layer pulp and the back layer pulp may contain recycled paper pulp. By including recycled paper pulp in either or both of the surface layer pulp and the back layer pulp, the recycled paper pulp content can be increased, thereby reducing raw material costs. The paperboard of the present invention may contain pulps other than recycled paper pulp and kraft pulp, for example, various pulps derived from wood fibers such as groundwood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), and non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, etc. The above-mentioned other pulps may also be contained in the front and back layers, so long as the effects of the present invention are not impaired.

[0015] In the paperboard of the present invention, the average fiber length of either or both of the pulp for the surface layer and the pulp for the back layer is preferably in the range of 1.2 mm (1200 μm) to 2.0 mm (2000 μm). By setting the average fiber length within the above range, it contributes to improving the burst strength. The average fiber length of either or both of the pulp for the surface layer and the pulp for the back layer is preferably 1.3 mm or more, more preferably 1.4 mm or more. The upper limit is preferably 1.9 mm or less, more preferably 1.8 mm or less. In addition, it is preferable that both of the pulp for the surface layer and the pulp for the back layer satisfy the above average fiber length. In the present invention, the average fiber length is a length-weighted average fiber length, and can be measured by observing the fibers using an image analyzer such as a Fiber Tester manufactured by ABB Corporation or a Fractionator manufactured by Valmet Co., Ltd., an optical microscope, or an electron microscope.

[0016] The paperboard of the present invention has a middle layer containing recycled paper pulp. The recycled paper pulp content in the pulp for the middle layer is preferably 80% by weight or more, more preferably 90% by weight or more, even more preferably 95% by weight or more, and most preferably 100% by weight. The recycled paper pulp in the middle layer preferably has an average fiber length in the range of 0.7 mm (700 μm) to 1.6 mm (1600 μm). By setting the average fiber length within the above range, the fibers are evenly distributed during papermaking in a papermaking machine, and good formation is formed, which contributes to improving the burst strength. The average fiber length of the recycled paper pulp in the middle layer is preferably 0.8 mm or more, more preferably 0.9 mm or more. The upper limit is preferably 1.5 mm or less, more preferably 1.4 mm or less. When multiple middle layers are provided, the recycled paper pulp content and average fiber length in each middle layer may be the same or different, but it is preferable that the recycled paper pulp in each layer satisfies the above values.

[0017] The freeness (Canadian Standard Freeness: CSF) of the surface layer pulp and the back layer pulp immediately after mechanical treatment is greater than the freeness (CSF) of at least one of the middle layer pulps immediately after mechanical treatment, specifically, it is preferably 10 ml CSF or more greater, more preferably 30 ml CSF or more greater, and even more preferably 50 ml CSF or more greater. When the freeness satisfies this relationship, the fibers for the middle layer having a lower freeness are efficiently retained between the fibers of the lower layer during production, and since the fibers have a moderate water retention property, the fibers of each layer are more entangled, which contributes to improving the burst strength. When multiple middle layers are provided, it is preferable that the middle layer pulp closest to the back layer satisfies the above freeness relationship, and the freeness of the middle layer pulp closest to the back layer among the middle layer pulps is more preferably 10 ml CSF or more less than the freeness of the other middle layer pulps.

[0018] The surface layer pulp preferably has a CSF immediately after the mechanical treatment in the range of 400 ml to 650 ml, more preferably 420 ml or more, and even more preferably 440 ml or more, and more preferably 600 ml or less, and even more preferably 550 ml or less. The freeness (CSF) of the pulp for the backing layer immediately after the mechanical treatment is preferably in the range of 300 ml to 600 ml. The lower limit is preferably 320 ml or more, more preferably 350 ml or more, and even more preferably 370 ml or more. The upper limit is preferably 580 ml or less, and more preferably 550 ml or less. The pulp for the middle layer preferably has a freeness (CSF) in the range of 150 ml to 550 ml immediately after mechanical treatment. The lower limit is preferably 200 ml or more, more preferably 250 ml or more. The upper limit is preferably 500 ml or less, more preferably 450 ml or less. In particular, by highly beating the waste corrugated cardboard pulp to a CSF of 150 ml to 550 ml, the bursting strength can be improved. When multiple middle layers are provided, it is preferable that the pulp for the middle layer closest to the back layer has the smallest freeness immediately after mechanical treatment.

[0019] The average fiber length and freeness (CSF) of pulp can be adjusted by mechanical treatment. The method of mechanical treatment is not particularly limited, and examples thereof include beating, defibration, dispersion, kneading, etc., among which beating is preferred. The equipment used for mechanical treatment is not particularly limited, and examples include high-speed rotary type, colloid mill type, high-pressure type, roll mill type, ultrasonic type, etc., and refiner, beater, PFI mill, kneader, disperser, high-speed disintegrator, conifer, top finer, etc., which act on metal or blades and pulp fibers around a rotating shaft, or those which use friction between pulp fibers can be used. In the present invention, the mechanical treatment is preferably beating using a disc refiner, conifer, or top finer, since it can efficiently beat the fibers. The concentration when performing mechanical treatment is not particularly limited, and it may be performed under high concentration conditions of solid concentration of 15% by weight or more and 50% by weight or less, or under low concentration conditions of 0.5% by weight or more and less than 15% by weight. The number of times of mechanical treatment is not limited, and the treatment may be performed once or multiple times. When the mechanical treatment is performed multiple times, different devices and solid concentration conditions may be combined.

[0020] In order to improve the strength of the paperboard of the present invention, a conventionally used paper strength enhancer such as polyacrylamide (PAM) or modified starch can be added internally and / or externally as necessary, and it is preferable to add it internally. The externally added paper strength enhancer is also called a surface paper strength agent. The paper strength enhancer is preferably contained in an amount of 0.3% by weight or more, more preferably 0.4% by weight or more, based on the entire paperboard. The upper limit of the blending rate of the paper strength enhancer is not particularly limited, but it can be, for example, 1.5% by weight or less. Even if the paper strength enhancer is blended in an amount of more than 1.5% by weight, the effect is saturated and further strengthening of the paper strength is hardly expected, resulting in high costs. When the paper strength enhancer is added internally, it is preferable to use polyacrylamide because it has a high paper strength strengthening effect. As the polyacrylamide, any of anionic, cationic, and amphoteric can be used. When the paper strength enhancer is added internally, the blending rate of the paper strength enhancer in each paper layer of the paperboard of the present invention may be the same or different. In the present invention, it is preferable that an internal paper strength agent is blended in the surface layer and / or back layer, and it is more preferable that the blending ratio of the internal paper strength agent in the surface layer is equal to or lower than the blending ratio of the internal paper strength agent in the other paper layers.

[0021] The paperboard of the present invention may be internally added with a sizing agent for preventing moisture absorption in order to improve the weather resistance of the paperboard. Examples of sizing agents include rosin-based sizing agents, rosin emulsion-based sizing agents, α-carboxymethyl saturated fatty acids, neutral rosin-based sizing agents, alkyl ketene dimers (AKD), alkenyl succinic anhydrides (ASA), cationic polymer-based sizing agents, etc. Furthermore, to the extent that it does not affect the quality, internal additive chemicals such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, silica sol, etc. may be internally added. The blending ratios of the sizing agents and internal additive chemicals in each paper layer of the paperboard of the present invention may be the same or different.

[0022] Furthermore, if necessary, known fillers can be added internally. Examples of fillers include inorganic fillers such as kaolin, calcined kaolin, delaminated kaolin, clay, calcined clay, delaminated clay, illite, heavy calcium carbonate, light calcium carbonate, light calcium carbonate-silica composite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, and organic fillers such as urea-formaldehyde resin, polystyrene resin, and phenol resin. The filler content in each paper layer of the paperboard of the present invention may be the same or different.

[0023] The paperboard of the present invention may have a coating layer on the surface for the purpose of improving the strength of the paperboard and imparting printability. The coating layer may contain one or more of known surface treatment chemicals such as surface strength agents (external paper strength enhancers), surface sizing agents, water repellents, pigments, binder resins, antislip agents, preservatives, defoamers, viscosity modifiers, and anti-crease agents, depending on the purpose. From the viewpoint of improving strength, the paperboard of the present invention preferably has a coating layer containing a surface strength agent. Surface strength agents that can be used include polyacrylamide-based, polyvinyl alcohol-based, starch-based agents such as oxidized starch, cationic starch, and amphoteric starch, and cellulose-based agents such as carboxymethyl cellulose, but polyacrylamide-based agents are preferred. When a coating layer containing a surface strength agent is provided, the coating amount of the surface strength agent contained in the coating layer is 0.05 g / m. 2 It is preferable that the content is 0.1 g / m or more. 2 More preferably, the above is the case. In addition, for the purpose of improving weather resistance, it is preferable to have a coating layer containing a surface sizing agent and a water repellent agent. Examples of the surface sizing agent include the same agents as those for internal addition described above, and examples of the water repellent agent include fluorine-based resins, polyamide-based resins, wax emulsions, etc.

[0024] From the viewpoint of strength, the paperboard of the present invention has a basis weight of 70 g / m 2 It is preferable that the thickness is 100 g / m or more. 2 More preferably, it is 120 g / m or more. 2The upper limit of the basis weight is not particularly limited as long as it can be produced by a papermaking machine, but is preferably 350 g / m 2 It is preferable that the thickness is less than 320 g / m 2 More preferably, it is 300 g / m or less. 2 It is even more preferable that: From the viewpoint of burst strength, the ratio of the basis weight of the front layer to the total basis weight of all the paper layers is preferably 20% by weight or more and 35% by weight or less, more preferably 22% by weight or more and 32% by weight or less, and even more preferably 24% by weight or more and 30% by weight or less. From the viewpoint of burst strength, the ratio of the basis weight of the back layer to the total basis weight of all the paper layers is preferably 25% by weight or more and 55% by weight or less, and more preferably 30% by weight or more and 50% by weight or less.

[0025] The paperboard of the present invention has a bursting strength index (burst strength divided by basis weight) of 3.5 kPa m 2 / g or more. This specific burst strength is preferably 3.5 kPa m 2 Paperboard with a specific burst strength of 3.6 kPa·m is more suitable for use as a cardboard liner, as it is less likely to break when subjected to impact. 2 / g or more is more preferable, and 3.8 kPa m 2 / g or more, and 4.0 kPa m 2 / g or more is most preferred.

[0026] The paperboard of the present invention preferably has a tensile strength, as specified by JIS P8113, of 7.0 kN / m or more in the machine direction (MD) and 4.0 kN / m or more in the cross direction (CD). With the tensile strength within the above range, the liner is less likely to be damaged by the weight of the contents. The tensile strength is more preferably 9.0 kN / m or more in the machine direction, and even more preferably 11.0 kN / m or more in the cross direction. Also, the tensile strength is more preferably 4.5 kN / m or more in the cross direction, and even more preferably 5.0 kN / m or more.

[0027] The paperboard of the present invention has a surface layer with a 120-second Cobb water absorbency of 100 g / m2 as specified in JIS P8140. 2Less than 90 g / m 2 Less than 80 g / m is more preferable. 2 It is more preferable that the 120-second Cobb water absorbency of the back layer is 100 g / m or less. 2 Less than 90 g / m 2 Less than 80 g / m is more preferable. 2 More preferably, the 120-second Cobb water absorbency of both surfaces is 100 g / m 2 The following paperboard is more suitable as a liner for corrugated cardboard because it is less susceptible to the effects of the environment, such as moisture and humidity, and is able to maintain its bursting strength and tensile strength.

[0028] In addition, it is preferable that the paperboard of the present invention has a small difference in Cobb water absorbency between the front and back sides. Specifically, it is preferable that the ratio of the 120-second Cobb water absorbency value of the front layer surface to the 120-second Cobb water absorbency value of the back layer surface (front layer surface / back layer surface) is within the range of 0.50 to 2.00. Paperboards in which the ratio of the 120-second Cobb water absorbency of the front layer surface to the back layer surface is outside the above range (i.e., the difference between the 120-second Cobb water absorbency between the front and back layers is large) are prone to peeling between the paper layers due to moisture or water, and the paperboard strength, including the burst strength, may be easily reduced due to peeling between the paper layers. It is more preferable that this Cobb water absorbency ratio is 0.60 to 1.67, and even more preferably 0.70 to 1.43.

[0029] The paperboard of the present invention preferably has an air resistance (Oken type testing machine method) of 150 seconds or more as specified in JIS P8117: 2009. A paperboard with an air resistance of 150 seconds or more can reduce the suction pressure when transported by a vacuum suction pad, improving workability and energy saving when cardboard using this paperboard as a cardboard liner is automatically transported in a logistics warehouse, etc. The air resistance is more preferably 175 seconds or more, and even more preferably 200 seconds or more.

[0030] The paperboard of the present invention has a specific compressive strength of 170 N·m as specified in JIS P8126:2005. 2 / g or more. The specific compressive strength of the paperboard is preferably 170 N·m2 / g or more, the paperboard is less likely to be crushed, making it more suitable for use as a cardboard liner for packaging heavy items. The specific compressive strength is 175 N·m 2 / g or more is preferable, and 180 N·m 2 It is more preferable that the molecular weight is 1 / g or more.

[0031] The applications of the paperboard of the present invention are not particularly limited, and the paperboard can be suitably used, for example, as a liner for corrugated cardboard, corrugating medium base paper, paperboard for paper containers, paperboard for box making, paperboard for laminated paper, etc. Among these, a liner for corrugated cardboard is particularly preferred.

[0032] The paperboard according to the present invention has three or more paper layers, and a known manufacturing (papermaking) method and papermaking machine can be selected using a combination papermaking machine such as a Fourdrinier papermaking machine, a twin-wire papermaking machine, a cylinder papermaking machine, a gap former, a hybrid former (on-top former), etc. The pH during papermaking may be in the acidic range (acidic papermaking), pseudo-neutral range (pseudo-neutral papermaking), neutral range (neutral papermaking), or alkaline range (alkaline papermaking), and after papermaking in the acidic range, an alkaline agent may be applied to the surface of the paper layer. Since the composition of the raw pulp, etc. differs for each layer of the paperboard of the present invention, the paper stock is prepared according to each layer and then paper-made. In this case, as described above, it is preferable that the freeness (CSF) of the pulp for the surface layer and the pulp for the back layer immediately after mechanical treatment is higher than the freeness (CSF) of at least one layer of the pulp for the middle layer immediately after mechanical treatment.

[0033] In addition, it is preferable to use a shaking device in the wire part. The shaking device is a device that slides the breast roll of the wire part in a direction perpendicular to the flow direction of the paper material (also called the machine width direction). By using a shaking device, the basis weight is made uniform (improved formation), and the number of thin parts compared to other parts that are the starting point of breakage is reduced, thereby improving the bursting strength.

[0034] Furthermore, the strength of the paperboard can be improved by adjusting the papermaking conditions of each layer. For example, by setting the J / W ratio (=J / W×100), which is the ratio of the speed (J) of the jet liquid (paper material) ejected from the headbox to the speed (W) of the wire, to less than 100%, the jet liquid can be pulled by the wire and the pulp can be oriented in the papermaking direction (flow direction). Paperboard in which the pulp is oriented in the longitudinal direction (longitudinal direction) tends to have improved burst strength. The J / W ratio is preferably 81% or more and 99% or less, and more preferably 84% or more and 98% or less. In addition, the strength can be improved by changing the basis weight distribution of each layer to increase the basis weight of the surface layer and / or back layer, adjusting the strength agent content of each layer to increase the strength agent content of the surface layer and / or back layer compared to the middle layer, etc.

[0035] There are no particular limitations on the manufacturing (papermaking) method and papermaking machine used after the wire part as long as they do not impair the required strength of the paperboard and production efficiency, and machines used in ordinary papermaking machines can be used. The type of press in the press part of the papermaking machine is not particularly limited, and any known press device can be used, for example, a straight-through press, a reverse press, an inbar press, a twin bar press, a pickup press, a unipress, a tri-nip press, a tri-vent press, a shoe press, a mini shoe press, etc. There are also no particular limitations on the press conditions, and they can be set appropriately within the normal operating range, but since a longer press time makes it easier for the fibers to hydrogen bond and improves the strength of the paperboard, it is preferable to make the paper at a low speed within a range that does not impair production efficiency.

[0036] The type of dryer (drying device) in the drying part of the papermaking machine is not particularly limited as long as it does not impair production efficiency or the strength of the paperboard, and any known drying device such as a hot air drying method or a multi-cylinder drying method used in normal papermaking machines can be used, and the number of drying devices installed may be one, or two or more pre-dryers and after-dryers. There are also no particular limitations on the drying conditions, and they can be set appropriately within the normal operating range.

[0037] In addition, when a coating layer is provided, a coating liquid containing surface treatment chemicals such as a surface strength agent, a surface sizing agent, and a water repellent is applied after papermaking. There is no particular limitation on the method for applying the coating liquid, and known devices such as a two-roll size press, a gate roll coater, a shim sizer, and a spray can be appropriately used. The calender may be bypassed, or processing may be performed within the normal operating range. EXAMPLES

[0038] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. The paperboards obtained in the following Examples and Comparative Examples were measured by the following methods. The results are shown in Table 1. (Basic weight) Measurements were performed in accordance with JIS P 8124. (Thickness) Measurements were performed in accordance with JIS P 8118 and JIS P 8124. (density) It was calculated from the basis weight and thickness measured in accordance with JIS P 8118 and JIS P 8124.

[0039] (Burst strength, burst strength index) According to JIS P 8131, bursting strength was measured by applying pressure from the surface layer side of the paper, and the bursting strength ratio was calculated by dividing the bursting strength by the basis weight. (Compressive strength, specific compressive strength) The compressive strength was measured by the ring crush method in accordance with JIS P 8126, and the specific compressive strength was calculated by dividing the compressive strength by the basis weight. (Air resistance) The measurements were made from the surface side using an Oken type smoothness and air permeability tester in accordance with JIS P 8117. (Tensile strength) The measurements were performed using a tensile tester in accordance with JIS P 8113. (Cobb absorbency) Measurements were made in accordance with JIS P 8140. Specifically, 100 ml of distilled water was brought into contact with the paper surface, and the weight of water absorbed per unit area was measured after 120 seconds had elapsed.

[0040] Example 1 The pulp used for each layer is shown below. The mechanical treatment of the pulp was performed using a double conifer for the surface layer and a double disc refiner for the middle and back layers. The freeness (CSF) at the outlet of the double conifer and double disc refiner is shown as the freeness (CSF) immediately after mechanical treatment. Back layer: 70% recycled cardboard pulp and 30% unbleached softwood kraft pulp by weight CSF480ml Middle layer: 100% recycled cardboard pulp by weight CSF 330ml (back middle layer), 350ml (front middle layer) Surface layer: 100% by weight unbleached softwood kraft pulp CSF500ml

[0041] The paper stock was added with 1.0% by weight of internal paper strength agent (PAM) based on solid content for all layers, and 0.45% by weight of internal sizing agent (rosin-based) based on solid content for the surface and back layers. The paper stock was added in the order of back layer (40% by weight), two middle layers (total 35% by weight), and surface layer (25% by weight), totaling 150g / m 2 The board was then laminated using a multi-layer board machine so that the thickness of the board was 0.1 g / m2 in terms of solid content on the surface layer of a surface strength agent (PAM, an external paper strength enhancer). 2 The paperboard was coated to obtain a paperboard. The paperboard contained 63% by weight of recycled paper pulp and 37% by weight of kraft pulp relative to the total pulp contained in all layers, and the surface and back layers contained all kraft pulp. The J / W ratio of the surface layer, the two middle layers, and the back layer was all 90%, and a shaking device was operated in the wire part when the back layer was made.

[0042] Example 2 The pulps used in each layer are shown below. Back layer: 85% recycled corrugated cardboard pulp and 15% unbleached softwood kraft pulp by weight CSF540ml Middle layer: 100% recycled cardboard pulp by weight CSF 350ml (back middle layer), 460ml (front middle layer) Surface layer: 100% by weight unbleached softwood kraft pulp CSF500ml

[0043] The paper stock was added with 0.4% by weight of internal paper strength enhancer (PAM) based on solids in the surface layer, 0.8% in the other layers, and 0.35% by weight of internal sizing agent (rosin-based) based on solids in the surface and back layers. The back layer (45% by weight), two middle layers (total 30% by weight), and the surface layer (25% by weight) were mixed together in a total of 150g / m 2 The layers were combined using a multi-layer paperboard machine to obtain a paperboard. This paperboard contained 68% by weight of recycled paper pulp and 32% by weight of kraft pulp relative to the total pulp contained in all paper layers, and the surface and back layers contained all kraft pulp. The J / W ratio of the surface layer was 93%, and the J / W ratio of the two middle layers and the back layer was 95%, and a shaking device was operated in the wire part when the back layer was made.

[0044] Example 3 The pulps used in each layer are shown below. Back layer: 100% recycled cardboard pulp by weight CSF390ml Middle layer: 100% recycled cardboard pulp by weight CSF 330ml (back middle layer), 380ml (front middle layer) Surface layer: 100% by weight unbleached softwood kraft pulp CSF500ml

[0045] The paper stock was added with 0.6% by weight of internal paper strength enhancer (PAM) based on solids in the surface layer, 1.0% in the other layers, and 0.25% by weight of internal sizing agent (rosin-based) based on solids in the surface and back layers. The back layer (40% by weight), two middle layers (total 35% by weight), and the surface layer (25% by weight) were mixed together in a total of 160g / m 2 The layers were combined using a multi-layer paperboard machine to obtain a paperboard. This paperboard contains 75% by weight of recycled paper pulp and 25% by weight of kraft pulp relative to the total pulp contained in all paper layers, and the surface layer contains all kraft pulp. The J / W ratio of the surface layer was 90%, and the J / W ratio of the two middle layers and the back layer was 95%, and a shaking device was operated in the wire part when the back layer was made.

[0046] Example 4 The pulps used in each layer are shown below. Back layer: 85% recycled corrugated cardboard pulp and 15% unbleached softwood kraft pulp by weight CSF390ml Middle layer: 100% recycled cardboard pulp by weight CSF 330ml (back middle layer), 380ml (front middle layer) Surface layer: 100% by weight unbleached softwood kraft pulp CSF500ml

[0047] The other chemical usage amounts, paper layer weight ratios, and operating conditions were the same as in Example 3, and the total was 160 g / m 2 The paperboard was obtained by combining the layers using a multi-layer paperboard machine so that the total pulp content of all layers was 68% by weight of recycled paper pulp and 32% by weight of kraft pulp, and the front and back layers were all kraft pulp.

[0048] Example 5 The pulp composition, chemical addition amount, and operating conditions were the same as in Example 4, except that an internal paper strength agent (PAM) was added to all layers so that the solid content was 0.85% by weight. 2 The multi-layered sheets were laminated using a multi-layer paperboard machine to obtain a paperboard.

[0049] Example 6 The pulp composition, chemical addition amount, and operating conditions were the same as in Example 4, except that an internal paper strength agent (PAM) was added to all layers so that the solid content was 0.9% by weight. 2 The multi-layered sheets were laminated using a multi-layer paperboard machine to obtain a paperboard.

[0050] Example 7 The pulp composition, chemical addition amount, and operating conditions were the same as in Example 4, except that an internal paper strength agent (PAM) was added to all layers so that the solid content was 0.5% by weight. 2 The multi-layered paper was then laminated using a multi-layer paperboard machine to obtain a paperboard.

[0051] Example 8 The pulps used in each layer are shown below. Back layer: 70% recycled cardboard pulp and 30% unbleached softwood kraft pulp by weight CSF390ml Middle layer: 100% recycled cardboard pulp by weight CSF 330ml (back middle layer), 380ml (front middle layer) Surface layer: 100% by weight unbleached softwood kraft pulp CSF500ml

[0052] The paper stock was added with 1.0% by weight of internal paper strength agent (PAM) based on solids only to the surface layer (no addition to other layers), and 0.5% by weight of internal sizing agent (rosin-based) based on solids to the surface and back layers. The paper stock was added in the order of back layer (40% by weight), two middle layers (total 35% by weight), and surface layer (25% by weight), totaling 160g / m 2 The board was then laminated using a multi-layer board machine so that the thickness of the board was 0.2g / m2 in terms of solid content. 2 The paperboard was coated to obtain a paperboard. The paperboard contained 63% by weight of recycled paper pulp and 37% by weight of kraft pulp relative to the total pulp contained in all paper layers, and the surface and back layers contained all kraft pulp. The J / W ratio of the surface layer was 90%, the J / W ratio of the two middle layers was 97%, and the J / W ratio of the back layer was 97.5%, and a shaking device was operated in the wire part when the back layer was made.

[0053] Comparative Example 1 The pulps used in each layer are shown below. Back layer: 100% recycled cardboard pulp by weight CSF520ml Middle layer: 100% recycled cardboard pulp by weight CSF 350ml (back middle layer), 450ml (front middle layer) Surface layer: 100% by weight unbleached softwood kraft pulp CSF440ml

[0054] The paper stock was added with 0.1% by weight of solid content of internal paper strength enhancer (PAM) for all layers and 0.5% by weight of internal sizing agent (rosin-based) for the surface layer, in the order of back layer (40% by weight), two middle layers (total 40% by weight), and surface layer (20% by weight), totaling 160g / m 2 The layers were combined using a multi-layer paperboard machine to obtain a paperboard. This paperboard contained 82% by weight of recycled paper pulp and 18% by weight of kraft pulp relative to the total pulp contained in all paper layers, and the surface and back layers contained all kraft pulp. The J / W ratio of the surface layer was 90%, and the J / W ratio of the two middle layers and the back layer was 95%, and a shaking device was operated in the wire part when the back layer was made.

[0055] [Table 1]

Claims

1. It has three or more paper layers, Specific burst strength: 3.5 kPa.m ２ / g or more Tensile strength: 7 kN / m or more vertically and 4 kN / m or more horizontally The 120-second Cobb water absorbency of the front and back surfaces is 100 g / m ２ below The specific compressive strength as specified in JIS P8126:2005 is 170N・m ２ / g or more Air resistance (Oken test method) specified in JIS P8117:2009 is 175 seconds or more A paperboard characterized in that:

2. 2. The paperboard according to claim 1, characterized in that the ratio of the 120-second Cobb water absorbency of the front layer to the 120-second Cobb water absorbency of the back layer (front layer / back layer) is in the range of 0.5 or more and 2.0 or less.

3. 3. The paperboard of claim 1 or 2 which is a liner for corrugated board.

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