Paperboard and method for manufacturing the same

Abstract

To provide a paperboard with an excellent strength and printing quality.SOLUTION: There is provided a paperboard, comprising a top layer, at least one or more middle layers and a bottom layer, and contains 20 mass% or more of hardwood kraft pulp relative to the total pulp mass, and the surface layer contains 50 to 100 mass% of softwood kraft pulp. The paperboard has a compression energy of 2.0 to 25.0 gf cm / cm2, and a compression recovery rate of 40 to 95%.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 are required to have high bursting strength so that they will not break even when subjected to impact. Paperboard that requires bursting strength is generally blended with softwood kraft pulp as the raw pulp. This is because softwood kraft pulp has longer fibers than hardwood kraft pulp, and thus paper with excellent strength can be obtained.

[0003] For example, Patent Document 1 proposes a cardboard liner with excellent impact resistance which has at least a surface layer, a subsurface layer and a back layer, in which the surface layer contains softwood kraft pulp, the surface layer having a softwood kraft pulp content of 80% by mass or more, the subsurface layer contains softwood kraft pulp and recycled corrugated paper pulp, the softwood kraft pulp content of the subsurface layer is 25% by mass or more and 35% by mass or less, the back layer contains recycled corrugated paper pulp and recycled magazine paper pulp, the recycled corrugated paper pulp content of the back layer is 30% by mass or more and 70% by mass or less, and the recycled magazine paper pulp content of the back layer is 30% by mass or more and 70% by mass or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-193396 A DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0005] In recent years, the volume of mail order sales has increased dramatically, and some mail order companies use cardboard packaging with original printing, but the printing design is also required to be sophisticated. However, it was found that paperboard with a high content of softwood kraft pulp has poor cushioning and therefore poor adhesion to the printing plate, resulting in poor print quality when patterns are printed on the surface of the cardboard. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a paperboard that has excellent strength and print quality. [Means for solving the problem]

[0006] The means for solving the above problems are as follows. 1. It has a surface layer, one or more middle layers, and a back layer. Contains 20% or more by mass of hardwood kraft pulp relative to the total pulp mass, The surface layer contains 50 to 100% by mass of softwood kraft pulp, Compression energy: 2.0-25.0gf cm / cm 2 A paperboard characterized in that the compression recovery rate is 40 to 95%. 2. The paperboard according to 1, wherein the back layer contains 40 to 100% by mass of hardwood kraft pulp. 3. The paperboard according to 1 or 2, wherein the back layer contains an internal paper strength agent in an amount of 0.3 to 0.8% by mass relative to the mass of the pulp. 4. The hardwood kraft pulp content is 20 to 95 mass%, the softwood kraft pulp content is 5 to 50 mass%, and the recycled paper pulp content is 0 to 75 mass%, based on the total pulp mass; The paperboard according to any one of 1. to 3., wherein the surface layer does not contain recycled paper pulp. 5. The paperboard according to any one of 1. to 4., wherein the proportion of the basis weight of the back layer to the total basis weight is 30% by mass or more and 50% by mass or less. 6. A cardboard liner made of the paperboard according to any one of 1. to 5. Summary of the Invention

[0007] The paperboard of the present invention has excellent print quality and is suitable for applications in which patterns or color printing is performed. The paperboard of the present invention is less likely to develop line cracks (breakage lines arising from lined lines (shallow grooves) imparted to the base paper) when folded, and can maintain high strength even when used as packaging material for boxes, etc. The paperboard of the present invention can be suitably used, for example, as a liner for corrugated cardboard, medium base paper, paperboard for folding cartons, paperboard for box making, laminated paper, etc. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The paperboard of the present invention has a surface layer, one or more middle layers, and a back layer, Contains 20% or more by mass of hardwood kraft pulp relative to the total pulp mass, The surface layer contains 50 to 100% by mass of softwood kraft pulp, Compression energy: 2.0-25.0gf cm / cm 2 , and the compression recovery rate is 40 to 95%. In this specification, the expression "A to B" (A and B are numerical values) means a numerical range including both ends, that is, A or more and B or less.

[0009] The paperboard of the present invention is composed of a surface layer, one or more middle layers, and a back layer, with the surface layer and the back layer being the outermost layers. The paperboard of the present invention has three or more layers, and the number of middle layers can be appropriately set according to the required basis weight, etc. The back layer refers to the bottom surface during papermaking, and is the layer formed when the raw material is first supplied to the wire part in a multi-layer papermaking machine, and the surface layer refers to the side opposite the back layer, i.e., the top surface during papermaking, and is the layer 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 20% by mass or more of hardwood kraft pulp relative to the total pulp mass. At least the surface layer of the paperboard of the present invention contains 50 to 100% by mass of softwood kraft pulp. As the hardwood kraft pulp and the softwood kraft pulp, either unbleached kraft pulp (LUKP / NUKP) or bleached kraft pulp (LBKP / NBKP) can be used, but when used in applications that do not require whiteness, unbleached kraft pulp is preferred because it has excellent strength and does not require a bleaching process, allowing it to be produced with low energy.

[0011] The paperboard of the present invention may contain various pulps derived from wood fibers, such as hardwood kraft pulp and softwood kraft pulp, waste paper pulp, 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, and the like. As the other pulp, it is preferable to use inexpensive waste paper pulp. However, it is preferable that at least one of the front layer and back layer of the paperboard of the present invention does not contain waste paper pulp, it is more preferable that the front layer does not contain waste paper pulp, and it is even more preferable that both the front layer and back layer do not contain waste paper pulp. The Canadian Standard Freeness of the pulp used in each layer of the present invention is not particularly limited, but is preferably 300 ml CSF or more and 600 ml CSF or less.

[0012] In the paperboard of the present invention, the amount of hardwood kraft pulp relative to the total pulp mass is preferably 20-95 mass%, the amount of softwood kraft pulp is preferably 5-50 mass%, and the amount of recycled paper pulp is preferably 0-75 mass%. The amount of hardwood kraft pulp relative to the total pulp mass is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 60% by mass or more, even more preferably 65% ​​by mass or more, even more preferably 70% by mass or more. The amount of hardwood kraft pulp relative to the total pulp mass is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less. In the paperboard of the present invention, the content of softwood kraft pulp relative to the total pulp mass is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 45% by mass or less, more preferably 40% by mass or less.

[0013] The waste paper pulp may be one or a mixture of two 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 paper; waste paper printed on fine paper, fine coated paper, medium quality paper, medium quality coated paper, and recycled paper; written waste paper; papers such as discarded confidential documents; magazine paper; and deinked pulp (DIP) obtained by disintegrating and disintegrating waste newspaper. Of these, waste paper pulp derived from corrugated cardboard (hereinafter referred to as corrugated cardboard pulp) and magazine paper are preferred from the standpoint of cost and strength.

[0014] In the paperboard of the present invention, the amount of recycled paper pulp relative to the total pulp mass is not particularly limited, but the smaller the amount of recycled paper pulp, the higher the bursting strength can be maintained. Therefore, from the viewpoint of bursting strength ratio, the amount of recycled paper pulp relative to the total pulp mass is preferably 75% by mass or less, more preferably 70% by mass or less, even more preferably 50% by mass or less, even more preferably 30% by mass or less, even more preferably 10% by mass or less, even more preferably 5% by mass or less, and most preferably 0% by mass (not included). On the other hand, recycled paper pulp is inexpensive, so the more the amount of recycled paper pulp is added, the lower the cost. Therefore, from the viewpoint of cost, the amount of recycled paper pulp relative to the total pulp mass is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more.

[0015] The surface layer of the paperboard of the present invention contains 50 to 100% by mass of softwood kraft pulp. This gives the paperboard of the present invention excellent burst strength and printability. The surface layer may contain hardwood kraft pulp, recycled paper pulp, etc., but it is preferable that the proportion of softwood kraft pulp is the highest. It is also preferable that the surface layer does not contain recycled paper pulp, and it is more preferable that the papermaking fibers of the surface layer are only softwood kraft pulp and hardwood kraft pulp. The content of softwood kraft pulp in the surface layer is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 95% by mass or more, and most preferably 100% by mass.

[0016] In the paperboard of the present invention, the back layer preferably contains 40 to 100% by mass of hardwood kraft pulp. The back layer may contain softwood kraft pulp, recycled paper pulp, etc., but preferably does not contain recycled paper pulp, and more preferably the papermaking fibers in the back layer are only hardwood kraft pulp and softwood kraft pulp. The amount of hardwood kraft pulp in the back layer is more preferably 60% by mass or more, even more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and most preferably 100% by mass.

[0017] The paperboard of the present invention contains 20% by mass or more of hardwood kraft pulp relative to the total pulp mass contained in the paperboard, and the surface layer contains 50 to 100% by mass of softwood kraft pulp, and the pulp constituting the middle layer is not particularly limited as long as this is satisfied. The middle layer can contain one or more of various pulps derived from wood fibers, such as softwood kraft pulp, hardwood kraft pulp, waste paper pulp, 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. When the middle layer has two or more layers, the pulp composition constituting each middle layer may be the same or different.

[0018] In the paperboard of the present invention, the back layer preferably contains 0.3 to 0.8% by mass of an internal strength enhancer (hereinafter also referred to as internal strength agent) relative to the pulp mass from the viewpoint of burst strength. As the internal strength agent to be blended in the back layer, polyacrylamide (PAM)-based agents and modified starch-based agents used in the papermaking field can be used without any particular restrictions, but it is preferable to use polyacrylamide-based agents because of their high paper strength enhancing effect. As the polyacrylamide-based agents, any of anionic, cationic, and amphoteric agents can be used. From the viewpoint of burst strength, the blending amount of the internal strength agent relative to the pulp mass in the back layer is more preferably 0.35% by mass or more, even more preferably 0.4% by mass or more, and even more preferably 0.45% by mass or more. The internal strength agent can be blended in an amount of more than 0.8% by mass relative to the pulp mass, but even if it is blended in an amount of more than 0.8% by mass, the paper strength enhancing effect is almost saturated and hardly improved, and the cost becomes high. The paperboard of the present invention may contain the above-mentioned internal strength agent in layers other than the back layer. In this case, the amount of the internal strength agent contained in layers other than the back layer is not particularly limited, but is about 0.01% by mass or more and 0.5% by mass or less. The type and amount of the internal strength agent in each paper layer of the paperboard of the present invention may be the same or different. However, it is preferable that the amount of the internal strength agent relative to the pulp in each layer is the highest in the back layer.

[0019] The paperboard of the present invention may be internally added with a sizing agent 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, and the like, and one or more of these may be used. In addition, internally added chemicals such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, and silica sol may be internally added within a range that does not affect the quality. The blending ratio of the sizing agent and other internally added chemicals in each paper layer of the paperboard of the present invention may be the same or different.

[0020] Furthermore, known fillers can be added to each layer of the paperboard of the present invention as necessary. 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, and one or more of these can be used. The filler content in each paper layer of the paperboard of the present invention may be the same or different.

[0021] The paperboard of the present invention may have a coating layer on its surface for the purpose of improving the strength of the paperboard, imparting printability, etc. Depending on the purpose, the coating layer may contain one or more of known surface treatment chemicals such as an external paper strength agent (hereinafter also referred to as a surface strength agent), a surface sizing agent, a water repellent, a pigment, a binder resin, an anti-slip agent, an antiseptic, an antifoaming agent, a viscosity modifier, and an anti-crease agent. From the viewpoint of improving strength, the paperboard of the present invention preferably has a coating layer containing a surface strength agent. The surface strength agent may be a polyacrylamide-based agent, a polyvinyl alcohol-based agent, a starch-based agent such as oxidized starch, cationic starch, amphoteric starch, or a cellulose-based agent such as carboxymethyl cellulose, but a polyacrylamide-based agent is preferred. The coating amount of the surface strength agent is 0.05 g / m 2 It is preferable that the content is 0.1 g / m or more. 2 More preferably, it is equal to or greater than this. For the purpose of improving weather resistance, the coating layer preferably contains a surface sizing agent and a water repellent agent. Examples of the surface sizing agent include those similar to those for internal addition described above, and examples of the water repellent agent include fluorine-based resins, polyamide-based resins, wax emulsions, etc.

[0022] The paperboard of the present invention has a compression energy of 2.0 to 25.0 gf cm / cm 2 and the compression recovery rate is 40-95%. Compression energy indicates the energy required for compression, and the larger this value is, the easier it is to compress. Compression recovery rate indicates the ability to recover when returning to the original state from a compressed state, and the closer it is to 100(%), the greater the recovery rate is. The paperboard of the present invention satisfies the above compression energy and compression recovery rate, and therefore has excellent cushioning properties and can adhere closely to a printing plate, resulting in excellent printability. In addition, because the paperboard of the present invention has excellent cushioning properties, it is less likely to break at the crease when folded. Compression energy is 2.2gf cm / cm 2 More than 2.4gf cm / cm is preferable. 2 More than 2.6gf cm / cm is preferable. 2 The above is even more preferable. The compression recovery rate is preferably 43% or more, more preferably 46% or more, and even more preferably 50% or more.

[0023] The paperboard of the present invention is less susceptible to crease cracks. The paperboard of the present invention preferably has a crease crack rate of 50% or less when evaluated under humidity conditions of 23°C and 50%. The lower the crease crack rate, the more preferable, with a crease crack rate of 46% or less being more preferable, a crease crack rate of 42% or less being even more preferable, and a crease crack rate of 38% or less being even more preferable.

[0024] The paperboard of the present invention has a bursting strength index (burst strength divided by basis weight) of 2.2 kPa m 2 / g or more. The specific burst strength is 2.4 kPa m 2 / g or more, and 2.6 kPa m 2 / g or more, and more preferably 2.8 kPa m 2 / g or more, and more preferably 3.0 kPa m 2 The upper limit of the burst strength of the paperboard of the present invention is not particularly limited, but is, for example, 4.0 kPa m 2 / g or less.

[0025] The basis weight of the paperboard of the present invention is not particularly limited, but is, for example, 50 g / m 2 More than 800g / m 2 The paperboard of the present invention can be suitably used, for example, as cardboard, particularly as a liner for cardboard used in packaging, corrugating medium, paperboard for paper containers, paperboard for box making, paperboard for laminated paper, etc. In the case of multi-layer paper having two or more paper layers, the total weight of all layers can be 50 g / m 2 More than 800g / m 2 The thickness can be appropriately set within the following ranges. For example, when used as a cardboard liner, it is set to 70 g / m 2 More than 550g / m 2 It can be as follows:

[0026] The ratio of the basis weight of the back layer to the entire basis weight is preferably from 30% by mass to 50% by mass, and more preferably from 32% by mass to 45% by mass, from the viewpoint of burst strength. The ratio of the surface layer basis weight to the entire basis weight is preferably from 10% by mass to 30% by mass, more preferably from 12% by mass to 25% by mass, from the viewpoint of burst strength. The ratio of the basis weight of all the intermediate layers to the entire basis weight is preferably 30% by mass or more and 50% by mass or less, and more preferably 35% by mass or more and 45% by mass or less, from the viewpoint of burst strength. When there are two or more intermediate layers, the basis weight ratios of the intermediate layers may be the same or different.

[0027] 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.

[0028] In the paperboard of the present invention, the blending of raw pulp and other ingredients differs for each layer, so that the paper stock is prepared according to each layer before papermaking. When making the paperboard of the present invention, 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 stock (also called the machine width direction). By using the shaking device, the basis weight is made uniform (the formation is improved), so that the number of thin parts compared to other parts that are the starting points of breakage is reduced, and the bursting strength is improved.

[0029] 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 is pulled by the wire, so that 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 make the strength agent content of the surface layer and / or back layer higher than that of the middle layer, etc.

[0030] 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.

[0031] There is no particular restriction on the type of dryer (drying device) in the dry part process of the papermaking machine, so 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, after-dryers, etc. There is also no particular restriction on the drying conditions, and they can be set appropriately within the normal operating range.

[0032] 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

[0033] 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 with reference to JIS P 8124. (Thickness) Measurements were performed with reference to JIS P 8118. (density) The value was calculated from the basis weight and thickness with reference to JIS P 8118 and JIS P 8124.

[0034] (Specific burst strength) The bursting strength was measured with reference to JIS P 8131, and the bursting strength ratio was calculated by dividing the bursting strength by the basis weight. (Cracked lines) A paperboard sample cut into a 5 cm square was folded in half using a screw servo press (Muto Design Co., Ltd., MSV-10-200S) at a speed of 175 mm / min, a pressure of 5.0 kN, and a pressure time of 0.3 s, with the surface side facing outward and the fold line in the CD direction of the paperboard.The length of the cracked area on the surface of the sample was then measured and divided by the total length of the sample (5 cm) to obtain the crack rate.

[0035] (Compression energy, compression recovery rate) The paperboard sample was cut into a rectangle 50 mm long and 100 mm wide, and four sheets of the same shape were stacked together to a thickness of about 1 mm. The samples were then compressed to 2 cm using a KES-G5 compression tester (manufactured by Kato Tech). 2 The specimen was compressed to a maximum stress of 5 kPa at a speed of 50 sec / mm using steel plates with a circular flat surface, and the compression energy and compression recovery rate were measured.

[0036] (Print density) A solid cyan print (size: 2 cm length x 3 cm width) was printed on the surface of the prepared paperboard using a flexographic printing machine (printability tester F1, manufactured by IGT Testing Systems Co., Ltd.). After one day, the print density (Macbeth density) after printing was measured using a Macbeth densitometer (Gretag Macbeth RD-19). The higher the print density, the more preferable it is. Under these conditions, the Macbeth density after printing is preferably 1.2 or more. (Print gloss) In accordance with JIS Z8741, the gloss (light incidence angle 75 degrees) of the solid cyan portion of the paperboard surface after the above flexographic printing was measured using a gloss meter (True GLOSS GM-26PRO, manufactured by Murakami Color Research Laboratory). The higher the print gloss value, the more preferable it is. Under these conditions, the preferred gloss value after printing is 12.5% ​​or more. (Print quality: visual) The print quality (print unevenness) of the sample after the above flexographic printing was visually evaluated according to the following criteria. ◯: There is almost no unevenness on the printed surface, and a uniform image is obtained. △: There are some unevenness on the printed surface, and the image is not uniform. ×: The printed surface is uneven, and the image is very non-uniform.

[0037] Examples and Comparative Examples The pulp used in each layer is shown in Tables 1 to 3. The recycled paper pulp contained recycled corrugated cardboard pulp and recycled magazine paper in a mass ratio of 80:20. 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 in each table as the freeness (CSF) immediately after mechanical treatment. In each layer, an internal paper strength agent (PAM) and an internal sizing agent (rosin-based) were added in the amounts shown in each table relative to the pulp to prepare paper stock. The paper materials for each layer were combined using a multi-layer boardmaking machine in the following order to obtain the blending ratios and total layer basis weights shown in each table: back layer (35% by mass), two middle layers (25% by mass in the back layer, 20% by mass in the front layer), and front layer (20% by mass).The J / W ratios of the front layer, middle layer (two layers), and back layer were all 90%, and a shaking device was operated in the wire part when the back layer was made, to obtain paperboard.

[0038] [Table 1]

[0039] [Table 2]

[0040] [Table 3]

[0041] The paperboard obtained in the examples of the present invention had excellent cushioning properties, excellent print quality, and a creasing rate of less than 50%. In contrast, the paperboard obtained in the comparative example had poor cushioning properties, resulting in poor print quality and a creasing rate of over 50%, and in some cases over 60%.

Claims

1. It has a surface layer, one or more middle layers, and a back layer, The pulp contains 20% by mass or more of hardwood kraft pulp and 0 to 75% by mass of recycled paper pulp relative to the total pulp mass, The surface layer contains 50 to 100% by mass of softwood kraft pulp, A paperboard characterized in that the rate of creasing evaluated under conditions of 23°C and 50% humidity is 50% or less.

2. 2. The paperboard of claim 1, wherein the backing layer comprises 40 to 100% by weight of hardwood kraft pulp.

3. 3. The paperboard according to claim 1, wherein the middle layer contains an internal paper strength agent in an amount of 0.01 to 0.5% by mass relative to the mass of the pulp.

4. With respect to the total pulp mass, the hardwood kraft pulp content is 20 to 95 mass%, the softwood kraft pulp content is 5 to 50 mass%, and the recycled paper pulp content is 0 to 75 mass%, 3. The paperboard according to claim 1 or 2, wherein the surface layer does not contain recycled paper pulp.

5. The paperboard according to claim 1 or 2, characterized in that the ratio of the basis weight of the back layer to the total basis weight is 32% by mass or more and 50% by mass or less, and the ratio of the basis weight of the surface layer is 10% by mass or more and 30% by mass or less.