Paperboard and liner for corrugated board

Abstract

The present invention addresses the problem of providing a paperboard excellent in terms of strength and scratch resistance even under a high humidity environment. As a means for solving the problem, provided is a paperboard having a surface layer, at least one intermediate layer, and a back layer, and containing 30 mass% or more of hardwood kraft pulp with respect to the total pulp mass, wherein the back layer contains 39 mass% or less of softwood kraft pulp, the surface layer contains 50-100 mass% of softwood kraft pulp, and the remaining SCT compressive strength after storage for 1 hour in a high-humidity environment (23°C-90%) is 60% or more.

Description

Liners for cardboard and corrugated cardboard

[0001] The present invention relates to paperboard, particularly paperboard suitable for corrugated cardboard liners, box making, laminated paper, etc., and a method for manufacturing the same.

[0002] Corrugated cardboard is manufactured by laminating a liner to one or both sides of a corrugated core using a corrugator. Because corrugated cardboard is lightweight and inexpensive, it is used to package a variety of goods. Corrugated cardboard may be subjected to internal collisions during transportation or external impacts during storage. Therefore, corrugated cardboard liners are required to have high burst strength to prevent tearing even when subjected to impact. Paperboard requiring high burst strength generally uses softwood kraft pulp as the raw material. This is because softwood kraft pulp has longer fibers compared to hardwood kraft pulp, resulting in paper with superior strength.

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

[0004] Japanese Patent Publication No. 2020-193396

[0005] In the transportation of various goods using cardboard boxes, crushing or bulging sometimes occurred in the bottom layer of the box. Upon diligent investigation into the cause, it was confirmed that compressive strength and other properties decreased under high humidity conditions. Furthermore, while developing paperboard that suppresses the decrease in compressive strength under high humidity conditions, although the decrease in compressive strength was suppressed, a decrease in scratch resistance was observed. This invention was made in view of these circumstances, and aims to provide paperboard that is excellent in strength and scratch resistance even under high humidity conditions.

[0006] The means for solving the above problems are as follows: 1. A cardboard having a surface layer, one or more intermediate layers, and a back layer, wherein the back layer contains 30% by mass or more of hardwood kraft pulp with respect to the total pulp mass, the back layer contains 39% by mass or less of softwood kraft pulp, the surface layer contains 50% by mass or more and 100% by mass or less of softwood kraft pulp, and the remaining percentage of SCT compressive strength after storage for 1 hour in a high humidity environment (23°C, 90%) is 60% or more. 2. The cardboard according to 1, wherein the back layer contains 40% by mass or more and 100% by mass or less of hardwood kraft pulp. 3. All layers of the surface layer, one or more intermediate layers, and the back layer contain an internal sizing agent, and the edge water absorption (2 min) is 30 g / m ２ The cardboard described in 1. or 2., characterized in that: 4. The surface layer, the one or more intermediate layers, and the back layer all contain an internally added drying agent, and the abrasion resistance is 150 mg or less, as described in any of 1. to 3. 5. The cardboard described in any of 1. to 4. contains 0.1% to 75% by mass of recycled paper pulp with respect to the total pulp mass, and the surface layer does not contain recycled paper pulp, as described in any of 1. to 4. 6. The cardboard described in any of 1. to 5. is characterized in that the ratio of the basis weight of the back layer to the total basis weight is 30% to 50% by mass, as described in any of 1. to 6. 7. The cardboard described in any of 1. to 6. is characterized in that the basis weight of the back layer is greater than the basis weight of the surface layer, as described in any of 1. to 7. A corrugated cardboard liner made of the cardboard described in any of 1. to 7.

[0007] The paperboard of the present invention exhibits suppressed reduction in strength, such as compressive strength, in high-humidity environments. The paperboard of the present invention also has excellent scratch resistance. Therefore, the paperboard of the present invention is particularly suitable for corrugated cardboard liners used in high-humidity environments.

[0008] The paperboard of the present invention exhibits minimal reduction in compressive strength even in high-humidity environments. It also boasts excellent print quality, making it suitable for applications involving pattern and color printing. Furthermore, it is less prone to creasing along the crease lines (damage lines arising from shallow grooves in the base paper) during folding, maintaining high strength even when used as packaging material for boxes and other items. The paperboard of the present invention is suitable for use during Japan's rainy season and summer, or in warm, high-humidity regions such as Okinawa, Taiwan, Southeast Asian countries, and Central and South American countries, particularly those located between 30 degrees north and 30 degrees south latitude. In these periods and regions, it can be suitably used for applications such as corrugated cardboard liners, core paper, paperboard for paper containers, box-making paper, and laminated paper.

[0009] The paperboard of the present invention has a surface layer, one or more intermediate layers, and a back layer, and contains 30% by mass or more of hardwood kraft pulp with respect to the total pulp mass, the back layer contains 39% by mass or less of softwood kraft pulp, and the surface layer contains 50% by mass or more and 100% by mass or less of softwood kraft pulp, and the remaining percentage of SCT compressive strength after storage for 1 hour in a high humidity environment (23°C, 90%) is 60% or more. In this specification, the description "A to B" (A and B are numerical values) means a numerical range that includes both ends of that range, i.e., A or more and B or less.

[0010] The paperboard of the present invention is composed of a surface layer, one or more intermediate layers, and a back layer, with the surface layer and back layer being the outermost layers. The paperboard of the present invention has three or more layers, and the number of intermediate layers can be appropriately set according to the required basis weight, etc. The back layer is the bottom surface during papermaking and is the layer formed when raw materials are first supplied to the wire part in a multi-layer papermaking machine, while the surface layer is the opposite side of the back layer, i.e., the top surface during papermaking and is the layer formed when raw materials are last supplied to the wire part in a multi-layer papermaking machine.

[0011] The paperboard of the present invention contains 30% by mass or more of hardwood kraft pulp relative to the total pulp mass. Although the mechanism is unknown, hardwood kraft pulp exhibits excellent maintenance of SCT compressive strength under high humidity conditions. Furthermore, while a higher proportion of softwood kraft pulp reduces the likelihood of surface cracking during folding and other processes, the addition of internally added dry-strengthening agents can worsen the form and, despite the addition of these agents, can lead to a decrease in strength. By containing 30% by mass or more of hardwood kraft pulp relative to the total pulp mass, the paperboard of the present invention is less prone to form deterioration even when using internally added dry-strengthening agents, thus preventing a decrease in strength due to the addition of these agents.

[0012] The paperboard of the present invention may include, in addition to hardwood kraft pulp, various pulps derived from wood fibers such as softwood kraft pulp, recycled paper pulp, crushed wood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), and chemothermetic pulp (CTMP), as well as non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, etc. Both unbleached kraft pulp (LUKP / NUKP) and bleached kraft pulp (LBKP / NBKP) can be used as hardwood kraft pulp and softwood kraft pulp. However, for applications where whiteness is not required, unbleached kraft pulp is preferred because it offers superior strength and can be produced with low energy consumption since a bleaching process is unnecessary.

[0013] As for other pulps, it is preferable to use inexpensive recycled paper pulp from the standpoint of cost reduction. However, from the standpoint of strength in high humidity environments and scratch resistance, it is preferable that at least one of the surface layer and back layer of the paperboard of the present invention does not contain recycled paper pulp, it is more preferable that the surface layer does not contain recycled paper pulp, and it is even more preferable that both the surface layer and back layer do not contain recycled paper pulp. The Canadian standard dilution of the pulp used in each layer of the present invention is not particularly limited, but it is preferable that it is between 300 ml CSF and 600 ml CSF.

[0014] The amount of hardwood kraft pulp relative to the total pulp mass is 30% by mass or more, and more preferably 40% by mass or more, even more preferably 50% by mass or more, even more preferably 60% by mass or more, even more preferably 65% ​​by mass or more, and even more preferably 70% by mass or more, from the viewpoint of SCT compressive strength and abrasion resistance in a high humidity environment, and improvement of paper strength by adding an internally added drying force agent. Furthermore, 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, and even more preferably 85% by mass or less. In the paperboard of the present invention, the amount of softwood kraft 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. Furthermore, the amount of softwood kraft pulp relative to the total pulp mass is preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less.

[0015] As recycled paper pulp, one or more types can be used, or a mixture of two or more types, including recycled corrugated cardboard, recycled paper pulp obtained by disintegrating unprinted recycled paper such as fine white, extra white, medium white, and damaged white paper, recycled paper printed on fine paper, fine coated paper, medium coated paper, newsprint, etc., recycled paper with writing on it, paper such as discarded confidential documents, recycled magazine paper, and pulp (DIP) obtained by disintegrating and deinking recycled newspaper. Among these, recycled paper pulp derived from recycled corrugated cardboard (hereinafter referred to as recycled corrugated cardboard pulp) and recycled magazine paper are preferred in terms of cost and strength development.

[0016] In the paperboard of the present invention, the amount of recycled paper pulp relative to the total pulp mass is not particularly limited, but can be, for example, 0.1 to 75% by mass. The lower the amount of recycled paper pulp, the higher the bursting strength, abrasion resistance, etc. can be maintained. Therefore, from the viewpoint of specific bursting strength and abrasion resistance, 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, and even more preferably 5% by mass or less. On the other hand, since recycled paper pulp is inexpensive, the cost decreases as the amount of recycled paper pulp added increases. Therefore, from the viewpoint of cost, the amount of recycled paper pulp relative to the total pulp mass is preferably 0.1% by mass or more, more preferably 1% by mass or more, even more preferably 5% by mass or more, even more preferably 10% by mass or more, and even more preferably 15% by mass or more.

[0017] The paperboard of the present invention has a softwood kraft pulp content of 39% by mass or less in the backing layer. By limiting the softwood kraft pulp content of the backing layer to 39% by mass or less, buckling is more likely to occur inward (towards the backing layer) when the paperboard is folded with the backing layer facing inward, thus making it easier to prevent cracking of the surface layer. The softwood kraft pulp content of the backing layer is more preferably 35% by mass or less, even more preferably 30% by mass or less, even more preferably 20% by mass or less, even more preferably 10% by mass or less, and most preferably 0% by mass.

[0018] The paperboard of the present invention preferably has a backing layer containing 40 to 100% by mass of hardwood kraft pulp, from the viewpoint of scratch resistance. The amount of hardwood kraft pulp in the backing layer is more preferably 61% by mass or more, even more preferably 65% ​​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. In addition to softwood kraft pulp and hardwood kraft pulp, the backing layer may also contain other pulps such as recycled paper pulp, but it is preferable that the proportion of hardwood kraft pulp is the highest among the pulps used. From the viewpoint of balancing cost and strength, the amount of recycled paper pulp in the backing layer is preferably 10% by mass or more and 49% by mass or less.

[0019] The paperboard of the present invention has a surface layer containing 50 to 100% by mass of softwood kraft pulp. As a result, the paperboard of the present invention has excellent burst strength and abrasion resistance, as well as excellent 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. Furthermore, it is preferable that the surface layer does not contain recycled paper pulp, and it is more preferable that the papermaking fibers of the surface layer consist only of softwood kraft pulp and hardwood kraft pulp. The amount 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.

[0020] The paperboard of the present invention only needs to contain 30% by mass or more of hardwood kraft pulp relative to the total mass of pulp contained in the paperboard, and as long as this is satisfied, the pulp constituting the middle layer is not particularly limited. The middle layer may contain one or more types of pulps derived from wood fibers, such as softwood kraft pulp, hardwood kraft pulp, recycled paper pulp, crushed wood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), chemothermetic pulp (CTMP), and non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, etc. The amount of hardwood kraft pulp in the middle layer is more preferably 50% by mass or more, even more preferably 60% by mass or more, even more preferably 71% by mass or more, and even more preferably 75% by mass or more, from the viewpoint of maintaining the SCT compressive strength in a high humidity environment. The amount of softwood kraft pulp in the middle layer is preferably 29% by mass or less, and more preferably 25% by mass or less, because it prevents the form from collapsing when paper strength agents are added and minimizes the reduction in strength. Furthermore, the amount of recycled paper pulp in the middle layer is preferably 5% by mass or more and 50% by mass or less, from the standpoint of balancing cost and strength. If there are two or more middle layers, the pulp composition of each middle layer may be the same or different.

[0021] In the paperboard of the present invention, it is preferable that the surface layer, one or more intermediate layers, and back layer all contain an internal sizing agent from the viewpoint of maintaining strength in a high-humidity environment. Examples of internal 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 anhydride (ASA), cationic polymer-based sizing agents, etc., and one or more of these can be used. Among these, rosin-based sizing agents and rosin emulsion-based sizing agents are preferred from the viewpoint of not staining the paper machine and having excellent operability. From the viewpoint of maintaining strength in a high-humidity environment, the amount of internal sizing agent blended in each paper layer is preferably 0.2% by mass or more, more preferably 0.25% by mass or more, and even more preferably 0.3% by mass or more, based on 100% by mass of pulp. There is no particular upper limit to the amount of sizing agent blended, but it is about 1.0% by mass, based on 100% by mass of pulp. Even if the sizing agent is added in an amount exceeding 1.0% by mass relative to 100% by mass of pulp, the effect of maintaining strength in a high-humidity environment is almost saturated and hardly improves, resulting in high costs. In the paperboard of the present invention, the type and blending ratio of the sizing agent in each paper layer may be the same or different.

[0022] In the paperboard of the present invention, it is preferable from the viewpoint of bursting strength that the surface layer, one or more intermediate layers, and back layer all contain 0.1 to 2.0% by mass of an internally added dry strength enhancer (hereinafter also referred to as internally added dry strength enhancer) per 100% by mass of pulp. As the internally added dry strength enhancer, any substance used in the papermaking field, such as polyacrylamide (PAM) or modified starch, can be used without particular limitation, but it is preferable to use a polyacrylamide type because it has a high paper strength enhancing effect. Any anionic, cationic, or amphoteric polyacrylamide type can be used. From the viewpoint of bursting strength, the amount of internally added dry strength enhancer per 100% by mass of pulp in each paper layer is more preferably 0.15% by mass or more, even more preferably 0.2% by mass or more, even more preferably 0.22% by mass or more, and even more preferably 0.25% by mass or more. While it is possible to blend internally added dry-strengthening agent in amounts exceeding 2.0% by mass relative to 100% by mass of pulp, blending beyond 2.0% by mass will result in near saturation of the paper strength-enhancing effect and little improvement, and will also increase costs. In the paperboard of the present invention, the type and blending ratio of internally added dry-strengthening agent in each paper layer may be the same or different, but it is preferable that the amount of internally added dry-strengthening agent relative to the pulp in each paper layer is highest in the back layer, followed by the middle layer, and then the front layer. By increasing the amount of internally added dry-strengthening agent relative to the pulp in each paper layer in the order of front layer, middle layer, and back layer, it is possible to improve strength by blending internally added dry-strengthening agent while suppressing cracking of the surface layer during bending.

[0023] The paperboard of the present invention may contain internal additives such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, and silica sol, to the extent that it does not affect the quality. The type and proportion of internal additives 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 internally added wet paper strength enhancer relative to 100% by mass of pulp in the paperboard of the present invention is less than 0.1% by mass. If the amount of internally added wet paper strength enhancer relative to 100% by mass of pulp is 0.1% by mass or more, the disintegrability decreases, which may reduce the recyclability as waste paper or recycled paper.

[0024] Furthermore, known fillers may be added to each paper layer of the paperboard of the present invention as needed. 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 dioxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, and organic fillers such as urea-formaldehyde resin, polystyrene resin, and phenolic resin, and one or more of these may be used. The proportion of fillers in each paper layer of the paperboard of the present invention may be the same or different.

[0025] The paperboard of the present invention may have a coating layer on its surface for purposes such as improving the strength of the paperboard and providing printability. Depending on the purpose, the coating layer may contain one or more known surface treatment chemicals such as externally added paper strength enhancers (hereinafter also referred to as surface paper strength enhancers), surface sizing agents, water repellents, pigments, binder resins, anti-slip agents, preservatives, defoamers, viscosity modifiers, and creasing prevention agents. From the viewpoint of improving strength, the paperboard of the present invention preferably has a coating layer containing a surface paper strength enhancer. The surface paper strength enhancer can be a starch-based one such as polyacrylamide, polyvinyl alcohol, oxidized starch, cationized starch, or amphoteric starch, or a cellulose-based one such as carboxymethylcellulose, but polyacrylamide is preferred. The amount of surface paper strength enhancer applied is 0.05 g / m². ２ Preferably, it is 0.1 g / m ２ The above is more preferable. Furthermore, to improve weather resistance, the coating layer preferably contains a surface sizing agent and a water repellent. Examples of surface sizing agents include those similar to those used for internal additives as described above, and examples of water repellents include fluororesins, polyamide resins, wax emulsions, etc.

[0026] The cardboard of the present invention has a remaining SCT compressive strength (short span compressive strength) of 60% or more after being stored for 1 hour in a high humidity environment (23°C, 90%). This remaining strength is calculated by dividing the SCT compressive strength after 1 hour of storage in a high humidity environment (23°C, 90%) by the SCT compressive strength after 24 hours of storage at 23°C, 50%. This remaining compressive strength is preferably 62% or more, more preferably 64% or more, even more preferably 66% or more, even more preferably 68% or more, even more preferably 70% or more, even more preferably 72% or more, and even more preferably 74% or more.

[0027] The paperboard of the present invention has an edge water absorption (2 min) of 30 g / m². ２ The following is preferable: The paperboard of the present invention is suitable for use in high-humidity environments, but high-humidity environments are prone to getting wet due to condensation, rain, etc. The water absorption rate (2 min) at the edges of the paperboard, or in the event of cracking along the crease lines, should be 30 g / m². ２By being as follows, it is possible to prevent the penetration of water from the ruled line cracking part. The value of the end water absorption degree (2 minutes) is 25 g / m ２ The following is more preferable, 20 g / m ２ The following is even more preferable, 15 g / m ２ The following is even more preferably, 10 g / m ２ The following is even more preferably, 5 g / m ２ The following is even more preferable.

[0028] The paperboard of the present invention preferably has a wear resistance of 150 mg or less. The value of this wear resistance conforms to JIS K7204:1999, uses an H-18 wear wheel, and is the wear mass when rotated 100 times on the surface layer. The value of the wear resistance is more preferably 140 mg or less, even more preferably 130 mg or less, even more preferably 120 mg or less, even more preferably 110 mg or less, and even more preferably 100 mg or less.

[0029] The paperboard of the present invention, in accordance with JIS-P8140 (1998) for the surface layer and the back layer, the Cobb water absorption degree measured with a measurement time of 120 seconds is preferably 60 g / m ２ or less for both. The paperboard of the present invention is easy to get wet, but by the Cobb water absorption degree of the surface layer and the back layer being 60 g / m ２ or less, it is possible to prevent the penetration of water from its surface. The Cobb water absorption degree of the surface layer and the back layer is, independently of each other, more preferably 55 g / m ２ or less, even more preferably 50 g / m ２ or less, even more preferably 45 g / m ２ or less, even more preferably 40 g / m ２ or less, even more preferably 35 g / m ２ or less is even more preferable.

[0030] The paperboard of the present invention is less likely to have ruled line cracks. The paperboard of the present invention preferably has a ruled crack rate of 50% or less when evaluated under humidity conditioning of 23°C and 50%. This ruled crack rate is preferably lower, more preferably 46% or less, even more preferably 42% or less, and even more preferably 38% or less.

[0031] The basis weight of the paperboard of the present invention is not particularly limited. For example, it can be 50 g / m ２ or more and 800 g / m ２ or less. The paperboard of the present invention can be suitably used, for example, as corrugated cardboard, particularly as a liner for corrugated cardboard for packaging, medium liner paper, paperboard for paper containers, paperboard for making boxes, paperboard for laminated paper, etc. In the case of multi-layered paper made of two or more paper layers, the total of all layers can be appropriately set within the range of 50 g / m ２ or more and 800 g / m ２ or less. For example, when used as a liner for corrugated cardboard, it can be 70 g / m ２ or more and 550 g / m ２ or less.

[0032] From the viewpoint of burst strength, the ratio of the basis weight of the back layer to the total basis weight is preferably 30% by mass or more and 50% by mass or less, more preferably 31% by mass or more and 48% by mass or less, and even more preferably 32% by mass or more and 45% by mass or less. From the viewpoint of burst strength, the ratio of the basis weight of the surface layer to the total basis weight is preferably 10% by mass or more and 30% by mass or less, more preferably 11% by mass or more and 29% by mass or less, and even more preferably 12% by mass or more and 25% by mass or less.

[0033] From the viewpoint of burst strength, it is preferable that the basis weight of the back layer containing a large amount of hardwood kraft pulp is larger than the basis weight of the surface layer containing a large amount of softwood kraft pulp. The ratio of the basis weight of the back layer to the total basis weight is preferably 1% by mass or more larger than the ratio of the basis weight of the surface layer, and more preferably 5% by mass or more larger. From the viewpoint of burst strength, the ratio of the basis weight of all middle layers to the total basis weight is preferably 30% by mass or more and 50% by mass or less, and even more preferably 35% by mass or more and 45% by mass or less. When there are two or more middle layers, the ratio of the basis weight of each middle layer may be the same or different.

[0034] The paperboard according to the present invention has three or more paper layers, and known manufacturing (paper-making) methods using a combined paper-making machine such as a fourdrinier paper-making machine, a twin-wire paper-making machine, a cylinder paper-making machine, a gap former, a hybrid former (ontopper), etc. can be selected. Also, the pH during paper-making may be in any of the acidic region (acidic papermaking), pseudo-neutral region (pseudo-neutral papermaking), neutral region (neutral papermaking), and alkaline region (alkaline papermaking). After papermaking in the acidic region, an alkaline agent may be applied to the surface of the paper layer.

[0035] Since the formulation of raw material pulp etc. is different for each layer of the paperboard of the present invention, a paper stock corresponding to each layer is prepared and then paper-making is carried out. When paper-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 in 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 becomes uniform (the formation is improved), so the thin parts are reduced compared to other parts that become the starting points of breakage, and the burst strength is improved.

[0036] Furthermore, by adjusting the papermaking conditions for each layer, the strength of the paperboard can also be improved. For example, by making the J / W ratio (= J / W × 100), which is the ratio of the velocity (J) of the jet liquid (paper stock) ejected from the headbox to the velocity (W) of the wire, less than 100%, the jet liquid is pulled by the wire, so the pulp can be oriented in the papermaking direction (flow direction). And the paperboard in which the pulp is oriented in the longitudinal direction (lengthwise direction) tends to have an improved specific 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. Also, by changing the basis weight distribution of each layer to increase the basis weight of the surface layer and / or the back layer, or by adjusting the compounding ratio of the paper strength enhancer for each layer to make the compounding ratio of the paper strength enhancer for the surface layer and / or the back layer higher than that of the middle layer, etc., the strength can be improved.

[0037] The manufacturing (papermaking) method and papermaking machine after the wire part are not particularly limited as long as they do not impair the required strength of the paperboard and production efficiency, and those commonly used in papermaking machines can be used. The type of press in the press part process of the papermaking machine is not particularly limited, such as straight-through press, reverse press, inbar press, twin-bar press, pickup press, uni-press, tri-nip press, tri-vent press, shoe press, mini-shoe press, etc., but known press devices can be used. Furthermore, there are no particular limitations on the press conditions, and they can be set appropriately within the normal operating range, but it is preferable to use low-speed papermaking as long as it does not impair production efficiency, as increasing the press time makes it easier for hydrogen bonds to form between fibers and improves the strength of the paperboard.

[0038] The type of dryer (drying device) used in the dry part process of the paper machine is not particularly limited as long as it does not impair production efficiency or the strength of the paperboard. Known drying devices such as the hot air drying method and the multi-cylinder drying method commonly used in paper machines can be used, and the number of drying devices can be as few as one, or two or more, such as pre-dryers and after-dryers. Furthermore, there are no particular limitations on the drying conditions, and they can be set appropriately within the normal operating range.

[0039] Furthermore, when a coating layer is provided, a coating solution containing surface treatment chemicals such as surface strength agents, surface sizing agents, and water repellents is applied after papermaking. There are no particular limitations on the method of applying the coating solution, and known devices such as two-roll size presses, gate roll coaters, sim sizers, and sprayers can be used as appropriate. The calender may be bypassed or processed within the normal operating range.

[0040] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. The cardboard obtained in the following examples and comparative examples was measured by the following methods. The results are shown in Tables 3 to 6. (Basis weight) Measured with reference to JIS P 8124:2011. (Paper thickness) Measured with reference to JIS P 8118:2014. (Density) Determined from basis weight and paper thickness with reference to JIS P 8118:2011 and JIS P 8124:2014.

[0041] (Cobb water absorption) Measured in accordance with JIS P 8140:1998. Specifically, 100 ml of distilled water was brought into contact with the paper surface, and after 2 minutes, the weight of water absorbed per unit area was measured. (Edge water absorption) A paperboard sample was cut into a 5 cm square, polyethylene tape was attached to the front and back surfaces (so that only the edges of the paper would absorb water), and it was immersed in a 0.05% brilliant blue aqueous solution for 2 minutes. The water absorption was calculated from the weight difference before and after immersion. (Creasing rate) A 5cm square sheet of cardboard sample 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 pressing time of 0.3 s, with the surface side facing outwards and the fold line oriented in the CD direction of the cardboard. 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 determine the creasing rate.

[0042] (Abrasion resistance) In accordance with JIS K7204:1999, an H-18 abrasion wheel was used, and the abrasion mass was measured after 100 rotations on the surface. (SCT compressive strength) Measured in accordance with JIS P 8156:2012. (Remaining SCT compressive strength) The SCT compressive strength of samples that were settling for 1 hour, 5 hours, and 24 hours in an environment of 23°C and 90% humidity was measured, and the value was calculated by dividing it by the SCT compressive strength of a sample settling for 24 hours at 23°C and 50% humidity.

[0043] (Transportation test: 9 layers stacked, bottom layer) Sample and core paper 170g / m ２A box measuring 500 mm (length) x 350 mm (width) x 250 mm (height) was created using a laminated corrugated cardboard sheet. The surface layer of the sample (cardboard obtained in the example and comparative example) was positioned on the surface of the corrugated cardboard box. Nine boxes containing 10 kg of contents were stacked on a pallet, and after 30 minutes in an environment of 23°C and 90% humidity, they were dropped from a height of 5 cm to apply impact. After being left undisturbed for 5 days, the deformation of the bottom box was visually inspected and evaluated according to the following criteria.

[0044] (Transportation test: After unloading, bottom layer condition) After checking the degree of crushing of the boxes as described above, the boxes were lowered while rubbing them against each other, and the degree of peeling of the top surface (surface layer) of the bottom layer of boxes was visually checked and evaluated according to the following criteria.

[0045] Tables 3 to 6 show the pulp used in each layer of the examples and comparative examples. The recycled paper pulp consisted of corrugated cardboard pulp and magazine waste paper in a mass ratio of 80:20. For mechanical processing of the pulp, a double conifer was used for the surface layer, and a double disc refiner was used for the middle and back layers. The filtration efficiency (CSF) at the outlets of the double conifer and double disc refiner is shown in each table as the filtration efficiency (CSF) immediately after mechanical processing. In each layer, internal dry strength agent (PAM) and internal sizing agent (rosin-based) were added to the pulp in the amounts shown in each table to create the paper stock. The pulp for each layer was assembled using a multi-layer papermaking machine in the following order: back layer (35% by mass), two middle layers (25% by mass in the back layer, 20% by mass in the front layer), and top layer (20% by mass). The J / W ratio for the top layer, two middle layers, and back layer was all set to 90%, and in the wire section, a shaking device was activated during the back layer formation to obtain the paperboard.

[0046]

[0047]

[0048]

[0049]

[0050] - Results The cardboard obtained in the examples of the present invention maintained its strength even when used in a high-humidity environment, the boxes did not collapse, and surface wear was minimal. From a comparison of the results of Examples 1-4 and Comparative Example 1, Example 13 and Comparative Example 5, and Example 14 and Comparative Example 7, it was confirmed that as the amount of internal sizing agent increased, the SCT compressive strength was maintained and the abrasion resistance and edge water absorption improved. Furthermore, from a comparison of the results of Example 3 and Comparative Example 1, Example 13 and Comparative Example 5, and Example 14 and Comparative Example 7, it was confirmed that the internal sizing agent in the middle layer greatly improved the SCT compressive strength and edge water absorption. From a comparison of the results of Examples 3, 5, and 6 and Comparative Examples 2 and 3, Example 10 and Comparative Example 4, Example 13 and Comparative Example 6, and Example 14 and Comparative Example 8, it was confirmed that as the ratio of LKP to total pulp mass increased, the SCT compressive strength was maintained and the abrasion resistance was excellent. A comparison of the results from Examples 3, 7-9 confirmed that the lower the ratio of NKP in the surface layer, the more likely creasing is to occur, and that the ratio of NKP in the surface layer affects abrasion resistance. A comparison of the results from Examples 3, 10-12 confirmed that as the amount of paper strengthening agent increases, SCT compressive strength, abrasion resistance, and edge water absorption improve, but creasing becomes more likely.

Claims

1. A cardboard having a surface layer, one or more middle layers, and a back layer, wherein it contains 30% by mass or more of hardwood kraft pulp with respect to the total pulp mass, the back layer contains 39% by mass or less of softwood kraft pulp, the surface layer contains 50% by mass or more and 100% by mass or less of softwood kraft pulp, and the remaining percentage of SCT compressive strength after storage for 1 hour in a high-humidity environment (23°C, 90%) is 60% or more.

2. The cardboard according to claim 1, characterized in that the backing layer contains 40% by mass or more and 100% by mass or less of hardwood kraft pulp.

3. All layers, including the surface layer, the one or more middle layers, and the back layer, contain an internal sizing agent, and the edge water absorption (2 min) is 30 g / m². ２ The cardboard according to claim 1 or 2, characterized in that it is as follows:

4. The paperboard according to claim 1 or 2, characterized in that all layers, including the surface layer, the one or more intermediate layers, and the back layer, contain an internally added drying agent and have an abrasion resistance of 150 mg or less.

5. The paperboard according to claim 1 or 2, characterized in that it contains 0.1% by mass or more and 75% by mass or less of recycled paper pulp with respect to the total pulp mass, and the surface layer does not contain recycled paper pulp.

6. The cardboard according to claim 1 or 2, characterized in that the ratio of the basis weight of the backing layer to the total basis weight is 30% by mass or more and 50% by mass or less.

7. The paperboard according to claim 1 or 2, characterized in that the basis weight of the backing layer is greater than the basis weight of the fronting layer.

8. A corrugated cardboard liner made of cardboard according to claim 1 or 2.

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