Paperboard

Abstract

The present invention addresses the problem of providing a high basis weight paperboard which is not susceptible to line cracking during bending and which has excellent printing quality. As a solution, provided is a cardboard which is provided with at least a surface layer containing 50-100 mass% of coniferous unbleached kraft pulp, a surface middle layer containing 95 mass% or less of waste cardboard pulp, a back middle layer containing 95 mass% or less of waste cardboard pulp, and a back layer containing 4-6 mass% or less of the surface layer and the back middle layer. The inner layer contains less than 90% by mass of corrugated board waste paper pulp, all the layers contain an internal sizing agent, the content of the total internal sizing agent is 0.2-1.0 parts by mass relative to 100 parts by mass of the total paper pulp, the total basis weight is 330 g / m2 or more, and the surface layer and the inner layer have a bothered water absorbency of 5-50 g / m2 at a measurement time of 2 minutes. The absolute value of the difference between the water absorbency of the surface layer and the water absorbency of the inner layer is 10 g / m < 2 > or less.

Description

Technical Field

[0001] The present invention relates to a cardboard, and particularly to a cardboard suitable for use as a face paper for corrugated cardboard, or for making boxes, composite paper, etc. Background Art

[0002] Corrugated cardboard is manufactured by pasting a face paper on one or both sides of a corrugated core paper formed into a waveform using a corrugating machine.

[0003] Corrugated cardboard is used for packaging various items because of its light weight and low price. Corrugated cardboard may sometimes be impacted by the internal packaging during handling, or may be impacted from the outside during storage. Therefore, for the face paper of corrugated cardboard, it is required to have a high bursting strength that does not break even when impacted. For cardboard that requires bursting strength, generally, either virgin pulp, waste pulp from corrugated cardboard made from 100% virgin pulp, or a mixture of them is used as the raw material pulp. The cardboard obtained by using these raw material pulps has high strength, is not prone to cracking along the score line (a damaged line generated from the score line (shallow groove) provided on the corrugated cardboard base paper), and is suitable for use as the face paper of corrugated cardboard.

[0004] However, the raw material cost of virgin pulp is high. Even for waste pulp from corrugated cardboard made from 100% virgin pulp, it needs to be recycled through a dedicated recycling channel to avoid mixing with other waste pulps such as waste pulp from corrugated cardboard containing waste pulp, so the cost is high and its output is also small. Therefore, if cardboard containing a large amount of virgin pulp or waste pulp from corrugated cardboard made from 100% virgin pulp is used as the raw material pulp in the manufacture of corrugated cardboard, box making, or composite paper making, it will result in high costs.

[0005] In order to reduce the raw material cost, a cardboard with an increased blending ratio of waste pulp has been proposed.

[0006] Patent Document 1 proposes a cardboard in which the blending ratio of waste pulp in the total pulp is 70% by mass or more, the basis weight is 350 g / m 2 or more, and the air permeability resistance is 25 seconds or more. In its examples, waste pulp from corrugated cardboard is used as the waste pulp.

[0007] Patent Document 2 proposes a cardboard having 4 to 6 layers. It is characterized in that the content of unbleached softwood kraft pulp in the surface layer is 90% by mass or more, the surface lower layer, middle layer, and inner layer contain waste pulp from corrugated cardboard, and the content of the waste pulp from corrugated cardboard is 90% by mass or more relative to the total pulp component of the inner layer. All layers contain an internal sizing agent, and the total content of the internal sizing agent in all layers is 2.50 kg / ton of pulp or more and 6.00 kg / ton of pulp or less in terms of solid content, and the total basis weight is 330 g / m 2 or more and 400 g / m2 The Cobb water absorbency of the above-mentioned surface layer and the above-mentioned inner layer at a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less, and the difference in Cobb water absorbency between the above-mentioned surface layer and the above-mentioned inner layer is 10 g / m 2 or less.

[0008] Patent document Patent document 1: Japanese Patent Application Laid-Open No. 2020-20065 Patent document 2: Japanese Patent No. 7137027 Summary of the invention In both Patent Documents 1 and 2, corrugated cardboard waste pulp is used as the waste pulp. This is because corrugated cardboard waste pulp has a tendency to have a longer fiber length, a higher lignin content rate and a higher starch content rate, and a lower ink content rate compared with other waste pulps, and thus it is easier to maintain strength compared with other waste pulps.

[0009] Although the paperboards described in Patent Documents 1 and 2 are known to have excellent strength, especially in the case of high basis weight, when the corrugated cardboard is bent for manufacturing a cardboard box, there is a tendency for cracking at the score line to occur, and depending on the pulp formulation, the adhesion to the printing plate may sometimes decrease due to poor cushioning properties, resulting in poor printing quality when printing a pattern or the like on the surface of the corrugated cardboard.

[0010] The present invention has been completed in view of the above circumstances, and an object thereof is to provide a high-basis-weight paperboard that is less likely to crack at the score line during bending processing and has excellent printing quality.

[0011] The method for solving the above-mentioned problem is as follows.

[0012] 1. A paperboard, characterized in that it has at least a surface layer, a middle surface layer, an inner middle layer, and an inner layer, and has 4 or more and 6 or less layers, the surface layer contains 50 to 100% by mass of unbleached softwood kraft pulp, the middle surface layer and the inner middle layer contain 95% by mass or less of corrugated cardboard waste pulp, and the inner layer contains less than 90% by mass of corrugated cardboard waste pulp, all layers contain an internal sizing agent, and the total content of the internal sizing agent is 0.2 to 1.0 parts by mass with respect to 100 parts by mass of the total pulp, the total basis weight is 330 g / m 2 or more, the Cobb water absorbency of the surface layer and the inner layer at a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less, and the absolute value of the difference in the Cobb water absorbency between the surface layer and the inner layer is 10 g / m 2As follows.

[0013] 2. The cardboard according to 1., characterized in that the compression energy is 20 - 150 mN·cm / cm 2 .

[0014] 3. The cardboard according to 1. or 2., characterized in that the compression recovery rate is 70% or more.

[0015] 4. The cardboard according to any one of 1. - 3., characterized in that the layers other than the surface layer contain one or more selected from magazine waste pulp, liner waste pulp, softwood kraft pulp, and hardwood kraft pulp.

[0016] 5. The cardboard according to any one of 1. - 4., characterized in that the layers other than the surface layer contain waste pulp with an average fiber length of 1.05 mm or less.

[0017] 6. A cardboard, characterized in that it has at least a surface layer, a middle surface layer, a middle inner layer, and an inner layer, and has 4 or more and 6 or less layers, the surface layer contains 50 - 100% by mass of unbleached softwood kraft pulp, all layers contain internal sizing agents, and the total content of internal sizing agents is 0.2 - 1.0 parts by mass relative to 100 parts by mass of the total pulp, the total basis weight is 330 g / m 2 or more, the Cobb water absorbency of the surface layer and the inner layer at a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less, and the absolute value of the difference in the Cobb water absorbency between the surface layer and the inner layer is 10 g / m 2 or less, the compression energy is 20 - 150 mN·cm / cm 2 , and / or the compression recovery rate is 70% or more.

[0018] 7. The cardboard according to 6., characterized in that the layers other than the surface layer contain one or more selected from magazine waste pulp, liner waste pulp, softwood kraft pulp, hardwood kraft pulp, and corrugated cardboard waste pulp.

[0019] 8. The cardboard according to 6. or 7., characterized in that the layers other than the surface layer contain waste pulp with an average fiber length of 1.05 mm or less.

[0020] When the cardboard of the present invention is bent, it is not prone to cracking along the score line, and it can maintain high strength when made into a box. The cardboard of the present invention has excellent printing quality and is suitable for applications with drawn patterns or color printing. Even if the cardboard of the present invention cracks along the score line, water is not easily infiltrated from the score line cracking part or the like. Therefore, when made into corrugated cardboard, the contents can be prevented from being wetted. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is an image of the side of the bent part after the score line cracking test of the cardboard obtained from Example 1 and 17.

[0022] Figure 2 It is an image of the side of the bent part after the score line cracking test of the cardboard obtained from Comparative Example 1 and 3. DETAILED DESCRIPTION OF THE INVENTION

[0023] The first mode of the cardboard of the present invention has at least 4 or more and 6 or less layers including a surface layer, a surface middle layer, a back middle layer, and a back layer. The surface layer contains 50 to 100% by mass of unbleached softwood kraft pulp. The surface middle layer and the back middle layer contain 95% by mass or less of corrugated cardboard waste pulp, and the back layer contains less than 90% by mass of corrugated cardboard waste pulp. All layers contain an internally added sizing agent, and the total content of the internally added sizing agent is 0.2 to 1.0 parts by mass relative to 100 parts by mass of the total pulp. The total basis weight is 330 g / m 2 or more. The Cobb water absorbency of the surface layer and the back layer at a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less, and the absolute value of the difference in the Cobb water absorbency between the surface layer and the back layer is 10 g / m 2 or less.

[0024] The second mode of the cardboard of the present invention has at least a surface layer, a surface middle layer, a back middle layer, and a back layer, and has 4 or more and 6 or less layers. The surface layer contains 50 to 100% by mass of unbleached softwood kraft pulp. All layers contain an internally added sizing agent, and the total content of the internally added sizing agent is 0.2 to 1.0 parts by mass relative to 100 parts by mass of the total pulp. The total basis weight is 330 g / m 2 or more. The Cobb water absorbency of the surface layer and the back layer at a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less, and the absolute value of the difference in the Cobb water absorbency between the surface layer and the back layer is 10 g / m2 Hereinafter, the compression energy is 20 to 150 mN·cm / cm 2 , and / or the compression recovery rate is 70% or more.

[0025] Hereinafter, the cardboard of the first method and the cardboard of the second method are collectively referred to as the cardboard of the present invention.

[0026] In addition, in this specification, the expression "A to B" (A and B are numerical values) means a numerical range including the values of A and B, that is, A or more and B or less.

[0027] The cardboard of the present invention has at least a surface layer, a surface middle layer, a back middle layer, and a back layer, and has 4 or more and 6 or less layers. In addition, the back layer refers to the bottom during papermaking and is the layer formed by first supplying raw materials to the wire part in a multi-layer paper machine, and the surface layer is the opposite side of the back layer, that is, the top during papermaking and is the layer formed by finally supplying raw materials to the wire part in a multi-layer paper machine.

[0028] Regarding the cardboard of the present invention, relative to the total pulp contained in the surface layer, the surface layer contains 50 to 100% by mass of unbleached softwood kraft pulp. Thereby, the bursting strength of the cardboard of the present invention is excellent, and at the same time, cracking at the score line on the surface layer can be prevented. The blending amount of unbleached softwood kraft pulp in the surface layer is preferably 60% by mass or more, more preferably 80% by mass or more, further preferably 90% by mass or more, still further preferably 95% by mass or more, and most preferably 100%. In addition to unbleached softwood kraft pulp, the surface layer may contain bleached softwood kraft pulp, unbleached hardwood kraft pulp, bleached hardwood kraft pulp, waste paper pulp, etc. Relative to the total pulp contained in the surface layer, it is preferably contains 80% by mass or more of kraft pulp (collectively referring to bleached kraft pulp and unbleached kraft pulp), more preferably contains 90% by mass or more, and further preferably contains 95% by mass or more.

[0029] In the cardboard of the first embodiment of the present invention, with respect to the total pulp contained in each of the surface middle layer and the back middle layer, the surface middle layer and the back middle layer contain 95% by mass or less of corrugated cardboard waste pulp, and with respect to the total pulp contained in the back layer, the back layer contains less than 90% by mass of corrugated cardboard waste pulp. Hereinafter, the surface middle layer, the back middle layer, and the back layer are collectively referred to as other layers. Since corrugated cardboard waste pulp is easier to maintain strength compared to other waste pulps, it has always been in a high ratio in cardboard using waste pulp in the past. However, for a cardboard that satisfies either one of the surface middle layer and the back middle layer containing more than 95% by mass of corrugated cardboard waste pulp and the back layer containing 90% by mass or more of corrugated cardboard waste pulp, or satisfies both, since it is hard and has low cushioning properties, it is prone to score cracking, and in addition, its adhesion to the printing plate is low, resulting in poor printing suitability. In the cardboard of the first embodiment of the present invention, by the surface middle layer and the back middle layer containing 95% by mass or less of corrugated cardboard waste pulp and the back layer containing less than 90% by mass of corrugated cardboard waste pulp, the strength can be maintained and the cushioning properties can be improved. As a result, score cracking can be prevented, and at the same time, the printing suitability is excellent.

[0030] There are no particular restrictions on the pulp other than the corrugated cardboard waste pulp contained in the other layers, and it may contain one or more of various pulps derived from wood fibers such as softwood kraft pulp, hardwood kraft pulp, waste pulp other than corrugated cardboard waste pulp, groundwood pulp (GP), refiner groundwood pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), etc., and non-wood pulp obtained from kenaf, bagasse, bamboo, hemp, straw, etc. Since the corrugated cardboard waste pulp solidifies due to the continuous progress of fiber keratinization, when the surface layer is bent outward, there is a tendency to cause score cracking of the surface layer. However, since the fibers of virgin pulp such as softwood kraft pulp or hardwood kraft pulp are soft, by blending them, the strength of the paper itself can be improved, and at the same time, even when the surface layer is bent outward, it is possible to more effectively prevent score cracking of the surface layer.

[0031] As waste paper pulp other than corrugated cardboard waste paper pulp (hereinafter also referred to as other waste paper pulp), waste paper pulp obtained by dissociating unprinted waste paper such as super white, extra white, medium white, and white waste can be used in combination; waste paper printed on high-quality paper, high-quality coated paper, medium-quality paper, medium-quality coated paper, rough paper, etc., waste paper with writing, paper such as discarded confidential documents, magazine waste paper, newspaper waste paper after dissociation and deinking pulp (DIP), etc., one or more of them. As other waste paper pulp, pulp with an average fiber length of 1.05 mm or less is preferred. As described above, the fiber length of corrugated cardboard waste paper pulp is longer than that of other waste paper pulp. By containing other waste paper pulp with an average fiber length of 1.05 mm or less, the strength of other layers is reduced. When the surface layer is bent outward, other layers are prone to buckling inward, and it is possible to more effectively prevent the surface layer from cracking at the score line. The average fiber length of other waste paper pulp is preferably 0.7 mm or more, more preferably 0.8 mm or more, and further preferably 0.9 mm or more. In addition, in the present invention, the average fiber length refers to the length-weighted average fiber length. For example, an image analysis device such as an optical fiber tester manufactured by ABB Ltd. or a separator manufactured by Valmet Ltd., an optical microscope, or an electron microscope can be used to measure by observing the fibers.

[0032] Among these, other layers preferably contain one or more selected from magazine waste paper pulp, liner waste paper pulp, softwood kraft pulp, and hardwood kraft pulp, and more preferably contain only one or more selected from the above and corrugated cardboard waste paper pulp.

[0033] In the cardboard of the first embodiment of the present invention, with respect to the total pulp contained in the surface middle layer and the inner middle layer respectively, the surface middle layer and the inner middle layer only need to contain 95% by mass or less of corrugated cardboard waste paper pulp, and preferably less than 95% by mass. In addition, there is no particular limitation on its lower limit value. From the perspective of the balance between cost and strength, it is preferably 20% by mass or more, more preferably 30% by mass or more, and further preferably 40% by mass or more.

[0034] In the cardboard of the first embodiment of the present invention, with respect to the total pulp contained in the inner layer, the inner layer only needs to contain less than 90% by mass of corrugated cardboard waste paper pulp, and preferably 89% by mass or less. In addition, there is no particular limitation on its lower limit value. From the perspective of the balance between cost and strength, it is preferably 20% by mass or more, more preferably 30% by mass or more, and further preferably 40% by mass or more.

[0035] In the cardboard of the second embodiment of the present invention, from the perspectives of cushioning property, preventing cracking at the score line, and printing applicability, with respect to the total pulp contained in the surface middle layer and the inner middle layer respectively, the surface middle layer and the inner middle layer preferably contain 95% by mass or less of corrugated cardboard waste paper pulp, and more preferably contain less than 95% by mass. In addition, there is no particular limitation on its lower limit value. From the perspective of the balance between cost and strength, it is preferably 20% by mass or more, more preferably 30% by mass or more, and further preferably 40% by mass or more.

[0036] In the cardboard of the second mode of the present invention, from the perspectives of cushioning property, prevention of cracking at the score line, and printability, the inner layer preferably contains less than 90% by mass of corrugated cardboard waste pulp, more preferably 89% by mass or less, relative to the total pulp contained in the inner layer. In addition, there is no particular limitation on its lower limit value. From the perspective of the balance between cost and strength, it is preferably 20% by mass or more, more preferably 30% by mass or more, and further preferably 40% by mass or more.

[0037] In the cardboard of the present invention, relative to the total pulp contained in other layers, other layers preferably contain 10% by mass or more of one or more selected from magazine waste pulp, liner waste pulp, softwood kraft pulp, and hardwood kraft pulp, more preferably 20% by mass or more, further preferably 30% by mass or more, and still further preferably 40% by mass or more.

[0038] In the cardboard of the present invention, the compositions of the respective pulps of the other layers other than the surface layer may be the same or different. However, since the inner layer is located on the outermost surface, strength is required. Therefore, in the other layers, the inner layer preferably has the least amount of corrugated cardboard waste pulp formulated, and in addition, preferably contains one or more of softwood kraft pulp and hardwood kraft pulp.

[0039] Each layer of the cardboard of the present invention can be internally added with internal sizing agents such as polyacrylamide (PAM) or modified starch, rosin sizing agents, neutral rosin sizing agents, rosin emulsion sizing agents, α-carboxymethyl saturated fatty acids, alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), cationic polymer sizing agents, etc., as needed, and known internal additives such as barium sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, and silica sol.

[0040] All layers of the cardboard of the present invention contain internal sizing agents, and the content of the total internal sizing agents contained in the cardboard is 0.2 to 1.0 parts by mass relative to 100 parts by mass of the total pulp contained in the cardboard. Thereby, the end water absorption of the cardboard can be reduced, and it is possible to prevent water from seeping in from the side end of the cardboard or from the score line breakage part in case of score line breakage. The end water absorption (2 minutes) of the cardboard of the present invention is preferably 150 g / m 2 Hereinafter, more preferably 130 g / m 2 Hereinafter, further preferably 110 g / m 2 Hereinafter, still further preferably 90 g / m 2 Hereinafter. The value of the end water absorption (2 minutes) is preferably as small as possible, but the lower limit value in the cardboard of the present invention is 40 g / m 2Around. When sizing agents are added to all layers, the blending ratios of the sizing agents added to each layer can be the same or different. The blending ratios of the sizing agents in the outermost surface layer and the innermost layer are preferably higher than those of the other layers.

[0041] Regarding the cardboard of the present invention, relative to 100 parts by mass of the total pulp contained in the cardboard, it is preferably to contain 0.3 parts by mass or more of an internally added paper strength enhancer, and more preferably 0.4 parts by mass or more. There is no particular limitation on the upper limit of the blending ratio of the internally added paper strength enhancer. For example, it can be set to 1.5 parts by mass or less relative to 100 parts by mass of the total pulp. Even if more than 1.5 parts by mass of the internally added paper strength enhancer is blended, the effect has been saturated and hardly any further paper strength enhancement can be expected above this amount. Instead, it will lead to an increase in cost. When blending the internally added paper strength enhancer, due to its high paper strength enhancing effect, polyacrylamide is preferably used. As polyacrylamide, any of anionic, cationic, and amphoteric types can be used. In addition, the blending ratios of the internally added paper strength enhancer in each layer of the cardboard of the present invention can be the same or different, and there can also be a paper layer that does not contain the internally added paper strength enhancer.

[0042] Each layer of the cardboard of the present invention can be internally added with known fillers as needed. As fillers, for example, there can be listed: inorganic fillers such as kaolin, calcined kaolin, exfoliated kaolin, clay, calcined clay, exfoliated clay, illite, heavy calcium carbonate, light calcium carbonate, light calcium carbonate - silica composite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silica, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, etc., and organic fillers such as urea - formalin resin, polystyrene resin, phenolic resin, etc. One or more than two kinds can be used. In addition, the blending ratios of the fillers in each paper layer of the cardboard of the present invention can be the same or different.

[0043] For the purpose of improving the strength of the cardboard or imparting printing applicability, etc., the cardboard of the present invention can have a coating layer on the surface. The coating layer can contain one or more than two kinds of known surface treatment chemicals such as surface paper strength agents, surface sizing agents, waterproof agents, pigments, binder resins, anti - slip agents, preservatives, defoaming agents, viscosity modifiers, anti - cracking agents, etc. according to its purpose.

[0044] From the perspective of improving strength, the cardboard of the present invention preferably has a coating layer containing a surface paper strength agent.

[0045] As the surface paper strength agent, polyacrylamide - based, polyvinyl alcohol - based, oxidized starch, cationized starch, starch - based such as amphoteric starch, cellulose - based such as carboxymethyl cellulose, etc. can be used, and polyacrylamide - based is preferred. The coating amount of the surface paper strength agent is preferably 0.05 g / m 2 or more, and more preferably 0.1 g / m 2 or more.

[0046] In addition, for the purpose of improving weather resistance, the coating layer preferably contains a surface sizing agent and a water repellent. Examples of the surface sizing agent include the same substances as those used for internal addition described above. Examples of the water repellent include fluororesins, polyamide resins, wax emulsions, etc.

[0047] The total basis weight of the paperboard of the present invention is 330 g / m 2 or more. When the paperboard is bent, the inner side is compressed and the outer side is stretched. Therefore, the greater the basis weight (the thicker) of the paperboard, the greater the difference in the amount of deformation between the inner and outer sides during bending, and the more likely it is to occur creasing and cracking during bending processing. Since the paperboard of the present invention is not likely to occur creasing and cracking, it is suitable for paperboards with a high basis weight, and the total basis weight is preferably 340 g / m 2 or more, more preferably 360 g / m 2 or more, and further preferably 380 g / m 2 or more. In the paperboard of the present invention, the upper limit of the basis weight is not particularly limited as long as it is within the range that can be manufactured by a paper machine. For example, it is 600 g / m 2 or less, preferably 550 g / m 2 or less, and more preferably 500 g / m 2 or less.

[0048] The basis weight of the inner layer is preferably 15% by mass or more and 50% by mass or less, more preferably 17% by mass or more, more preferably 48% by mass or less, and further preferably 46% by mass or less with respect to the basis weight of the entire paperboard. In addition, among the 4 to 6 paper layers of the paperboard of the present invention, it is preferred that the basis weight of the inner layer is the largest.

[0049] The basis weight of the surface layer is preferably 10% by mass or more and 25% by mass or less, more preferably 12% by mass or more, and more preferably 23% by mass or less with respect to the basis weight of the entire paperboard.

[0050] For the paperboard of the present invention, the Cobb sizing degree measured in accordance with Japanese Industrial Standard JIS-P8140 (1998) for a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less for both the surface layer and the inner layer, and the absolute value of the difference in the Cobb sizing degree between the surface layer and the inner layer is 10 g / m 2 or less.

[0051] The Cobb sizing degree of the surface layer and the inner layer being 5 g / m 2 or more and 50 g / m 2 or less for the paperboard is less likely to be affected by the use environment such as moisture or humidity, with small changes in physical properties, and in addition, the bursting strength or tensile strength can be maintained. The absolute value of the difference in the Cobb sizing degree between the surface layer and the inner layer is 10 g / m 2The following cardboard is not easily peeled between paper layers due to moisture or water, thus preventing the reduction of cardboard strength including burst strength caused by the peeling between paper layers. The Cobb water absorbency of the surface layer and the inner layer is preferably 10 g / m 2 or more, more preferably 15 g / m 2 or more, further preferably 20 g / m 2 or more. In addition, it is preferably 45 g / m 2 or less, more preferably 40 g / m 2 or less, further preferably 35 g / m 2 or less. The absolute value of the difference in Cobb water absorbency between the surface layer and the inner layer is preferably 8 g / m 2 or less, more preferably 6 g / m 2 or less, further preferably 4 g / m 2 or less.

[0052] The cardboard of the present invention is not easily cracked at the scored line. The scored line cracking rate of the cardboard of the present invention evaluated under the conditions of 23°C and 50% humidity adjustment is preferably 55% or less. The scored line cracking rate is more preferably 50% or less, further preferably 45% or less, still further preferably 40% or less, still further preferably 35% or less, still further preferably 30% or less. The lower the scored line cracking rate, the better. The lower limit value of the cardboard of the present invention is about 5%.

[0053] The cardboard of the first mode of the present invention has excellent cushioning properties and can be adhered to the printing plate, so the printing applicability is excellent. In addition, since the cardboard of the first mode of the present invention has excellent cushioning properties, when bent, the inner side is easily compressed and buckled, suppressing the stretching of the outer side, so it is not easily cracked at the scored line. The compression energy of the cardboard of the first mode of the present invention is preferably 20 - 150 mN·cm / cm 2 . The compression energy represents the energy required for compression. The larger this value, the easier it is to compress. The compression energy is more preferably 120 mN·cm / cm 2 or less, further preferably 90 mN·cm / cm 2 or less. The compression recovery rate of the cardboard of the first mode of the present invention is preferably 70% or more. The compression recovery rate refers to the recovery property when recovering from the compressed state to the original state. The closer it is to 100%, the more recovery property it has. The compression recovery rate is more preferably 72% or more, further preferably 74% or more, still further preferably 76% or more.

[0054] The compression energy of the cardboard of the second mode of the present invention is 20 - 150 mN·cm / cm 2 , and / or the compression recovery rate is 70% or more. Preferably, the compression energy is 20 - 150 mN·cm / cm 2 , and the compression recovery rate is 70% or more. The compression energy of the cardboard of the second mode of the present invention is preferably 120 mN·cm / cm2 Hereinafter, more preferably 90 mN·cm / cm 2 Hereinafter. The compression recovery rate of the cardboard of the second embodiment of the present invention is preferably 72% or more, more preferably 74% or more, and further preferably 76% or more.

[0055] The use of the cardboard of the present invention is not particularly limited. For example, it can be suitably used for the facing paper for corrugated cardboard, the core base paper, the cardboard for paper containers, the cardboard for box making, the cardboard for composite paper, etc. Among these, the facing paper for corrugated cardboard is particularly preferred.

[0056] The cardboard of the present invention has 4 to 6 paper layers, and known manufacturing (paper making) methods and paper making machines such as a multi-layer paper making machine combined with a fourdrinier paper machine, a twin-wire paper machine, a cylinder paper machine, a gap former, a hybrid former (top wire former), etc. can be selectively used, and paper making is carried out by preparing paper stocks corresponding to each layer. In addition, the pH during paper making can be any of an acidic region (acidic paper making), a pseudo-neutral region (pseudo-neutral paper making), a neutral region (neutral paper making), and an alkaline region (alkaline paper making). After paper making in the acidic region, an alkaline agent can also be coated on the surface of the paper layer.

[0057] In addition, a shaking device is preferably used in the forming section. The so-called shaking device is a device that slides the breast roll in the forming section in a direction perpendicular to the paper stock flow direction (also called the machine width direction). By using the shaking device, the basis weight can be made more uniform (the paper formation is improved), thereby reducing the thin areas compared with other parts as the starting point of breakage, and improving the bursting strength.

[0058] Furthermore, by adjusting the papermaking conditions of each layer, the strength of the cardboard can also be improved. For example, by setting the ratio of the speed (J) of the jet liquid (paper stock) ejected from the headbox to the speed (W) of the wire, that is, the J / W ratio (= J / W × 100), to less than 100%, the jet liquid can be pulled by the wire, so that the pulp can be oriented in the paper making direction (flow direction). And for the cardboard with the pulp oriented in the longitudinal direction (length direction), the bursting index tends to increase. The J / W ratio is preferably 81% or more and 99% or less, more preferably 84% or more and 98% or less. In addition, by changing the basis weight distribution of each layer to increase the basis weight of the surface layer and / or the back layer, and adjusting the ratio of the paper strength enhancer in each layer so that the ratio of the paper strength enhancer in the surface layer and / or the back layer is more than that in the middle layer, etc., the strength can be improved.

[0059] Regarding the manufacturing (paper making) methods and paper making machines after the forming section, there is no particular limitation within the range that does not impair the required strength and production efficiency of the cardboard, and the manufacturing (paper making) methods and paper making machines used in ordinary paper making machines can be used.

[0060] The form of pressing in the pressing section process of a paper machine, for example, includes straight-through pressing, reverse pressing, inverted pressing, double-roll pressing, pick-up pressing, single nip pressing, triple nip pressing, triple nip grooved pressing, shoe pressing, small shoe pressing, etc., but there is no particular limitation, and known pressing devices can also be used. In addition, the pressing conditions are not particularly limited and can be appropriately set within the normal operating range. However, by extending the pressing time, hydrogen bonds can be more easily formed in the fibers, and the strength of the cardboard can be improved. Therefore, low-speed papermaking is preferably carried out within the range that does not damage the production efficiency.

[0061] Regarding the form of the dryer (drying device) in the drying section process of a paper machine, there is no particular limitation within the range that does not damage the production efficiency and the strength of the cardboard. Known drying devices such as hot air drying method and multi-cylinder cylindrical drying method used in ordinary paper machines can be used. The number of drying devices installed can be one, or two or more such as a pre-dryer and a post-dryer can be installed. In addition, the drying conditions are not particularly limited and can be appropriately set within the normal operating range.

[0062] In addition, in the case of setting a coating layer, after papermaking, a coating liquid containing surface sizing agents, surface sizing agents, waterproof agents and other surface treatment chemicals can be coated. The method of coating the coating liquid is not particularly limited, and known devices such as double-roll sizing press, gate roll coater, SYM coater, sprayer, etc. can be appropriately used. Calendering can be bypassed or processed within the normal operating range.

[0063] Examples The present invention will be described in more detail below through examples, but the present invention is not limited thereto.

[0064] · Examples, Comparative Examples The pulp used in each layer is shown in Tables 1 to 4. The average fiber length of corrugated cardboard waste pulp is 1.07 mm, and the average fiber length of magazine waste pulp is 0.99 mm. Regarding the mechanical treatment of the pulp, a double-cone refiner is used for the surface layer, and a double-disc refiner is used for other layers. The drainage degree (CSF) at the outlet of the double-cone refiner and the double-disc refiner is shown in each table in the form of the drainage degree (CSF) just after mechanical treatment. In each layer, an internal sizing agent (PAM) and an internal sizing agent (rosin type) are added in the amounts shown in each table relative to the amount of pulp to make the furnish.

[0065] The furnishes of each layer are laminated together using a multi-layer cardboard paper machine so that the J / W ratio of the surface layer, the middle layer (2 layers or 4 layers), and the inner layer is all 90%. When making the inner layer in the forming section, the shaking device is started to obtain the cardboard.

[0066] [Physical Property Measurement] For the cardboard of the examples and comparative examples, the following methods were used for measurement. The results are shown in Tables 1 to 4.

[0067] (Grammage) Measurement was carried out in accordance with Japanese Industrial Standard JIS P 8124.

[0068] (Paper thickness) Measurement was carried out in accordance with Japanese Industrial Standard JIS P 8118.

[0069] (Density) It was obtained from the grammage and thickness measured in accordance with Japanese Industrial Standards JIS P 8118 and JIS P 8124.

[0070] (Bursting strength, bursting index) In accordance with Japanese Industrial Standard JIS P 8131, the bursting strength was measured by pressing from the surface side of the cardboard, and the bursting index was calculated by dividing it by the grammage.

[0071] (Cobb sizing degree) Measurement was carried out in accordance with Japanese Industrial Standard JIS P 8140. Specifically, 100 ml of distilled water was brought into contact with the paper surface, and after 2 minutes, the weight of the water absorbed per unit area was measured.

[0072] (Edge water absorption) The cardboard sample was cut into a square with a side length of 5 cm, polyethylene tape was pasted on the front and back surfaces (so that only the edge of the paper could absorb water), and then it was immersed in a 0.05% brilliant blue aqueous solution for 2 minutes, and calculated from the weight difference before and after immersion.

[0073] (Score breakage) The cardboard sample cut into a square with a side length of 5 cm was folded in half using a screw servo press (Muto Design Co., Ltd., MSV-10-200S) at 175 mm / min, a pressure of 5.0 kN, and a pressing time of 0.3 s, with the surface layer of the cardboard on the outside and the fold line in the CD direction (lateral direction) of the cardboard. Then, the length of the crack on the surface layer of the sample was actually measured, and the score breakage rate was obtained by dividing this length by the total length of the sample (5 cm).

[0074] In addition, for the cardboard obtained in Examples 1, 17, Comparative Example 1, and 3, the images of the side of the bent part after the score breakage test are shown in Figure 1 , 2 .

[0075] (Compression energy, compression recovery rate) Cut the cardboard sample into a rectangle with a length of 50 mm and a width of 100 mm. Take 2 pieces of the same shape and stack them to make the thickness reach about 1 mm. Use a KES compression testing machine (manufactured by Kato Technology Co., Ltd.) with a steel plate gap having a circular plane of 2 cm 2 and compress at a rate of 50 sec / mm until the maximum stress reaches 5 kPa, and then measure the compression energy and compression recovery rate.

[0076] (Printing density) On the surface layer of the prepared cardboard, use a flexographic printing press (manufactured by IGT TESTING SYSTEMS Co., Ltd., printing suitability testing machine F1) to perform solid printing of cyan (size: 2 cm in length × 3 cm in width). One day later, use a spectrophotometer (manufactured by X-Rite Inc., eXact) to measure the density after printing.

[0077] Under these conditions, the Macbeth density after printing is preferably 1.2 or more.

[0078] (Printing gloss) According to Japanese Industrial Standard JIS Z8741, use a gloss meter (manufactured by Murakami Color Technology Research Institute, TrueGLOSS GM-26PRO) to measure the gloss (light incident angle 75 degrees) of the surface layer of the cyan solid part of the cardboard after the above flexographic printing, and measure the gloss after printing.

[0079] Under these conditions, the gloss after printing is preferably 12.8% or more.

[0080] (Printing quality: visual inspection) Evaluate the printing quality (printing unevenness) of the above sample after flexographic printing according to the following criteria by visual inspection.

[0081] ○: There is almost no unevenness on the printing surface, and a uniform image can be obtained.

[0082] △: There is some unevenness on the printing surface, and the image is uneven.

[0083] ×: There is unevenness on the printing surface, and the image is very uneven.

[0084] [Table 1]

[0085] [Table 2]

[0086] [Table 3]

[0087] [Table 4]

[0088] The creasing breakage rate of the cardboard of the present invention is low, and the printing quality is excellent. As Figure 1 shown, for the cardboard of the present invention obtained from Examples 1 and 17, delamination occurred between the paper layers and buckled toward the inner layer side, presumably thereby suppressing the stretching of the surface layer.

[0089] The creasing breakage rate of the cardboard obtained from the comparative examples is high, and in addition, the printing quality is poor. As Figure 2 shown, for the cardboard obtained from Comparative Example 1, the paper layer strength was high and no buckling toward the inner layer side occurred. Therefore, due to the bending process, the surface layer was stretched, resulting in creasing breakage. In addition, although the cardboard obtained from Comparative Example 3 buckled toward the inner layer side, since the strength of the surface layer was insufficient originally, creasing breakage occurred.

Claims

1. A cardboard, characterized in that, it has at least a surface layer, a middle surface layer, a middle inner layer and an inner layer, and has more than 4 layers and less than 6 layers, the surface layer contains 50-100% by mass of unbleached softwood kraft pulp, the middle surface layer, the middle inner layer and the inner layer contain corrugated cardboard waste pulp, the middle surface layer and the middle inner layer contain 95% by mass or less of corrugated cardboard waste pulp relative to the total pulp, and the inner layer contains less than 90% by mass of corrugated cardboard waste pulp relative to the total pulp, all layers contain an internal sizing agent, and the content of the total internal sizing agent is 0.2-1.0 parts by mass relative to 100 parts by mass of the total pulp, The total basis weight is 330 g / m 2 or more. The Cobb water absorbency of the surface layer and the inner layer at a measurement time of 2 minutes is 5 g / m 2 or more and 50 g / m 2 or less, and the absolute value of the difference in the Cobb water absorbency between the surface layer and the inner layer is 10 g / m 2 or less.

2. The cardboard according to claim 1, characterized in that, Cut it into a rectangle with a length of 50 mm and a width of 100 mm. Take two pieces of the same shape and stack them to make the thickness reach about 1 mm. Pass them through a KES compression testing machine (manufactured by Kato Technology Co., Ltd.) and use a steel plate gap with a circular plane of 2 cm 2 to compress at a rate of 50 sec / mm until the maximum stress reaches 5 kPa. The compression energy measured at this time is 20 - 150 mN·cm / cm 2 .

3. The cardboard according to claim 1 or 2, characterized in that, Cut it into a rectangle with a length of 50 mm and a width of 100 mm. Take two pieces of the same shape and stack them to make the thickness reach about 1 mm. Through a KES compression testing machine (manufactured by Kato Technology Co., Ltd.), use a steel plate gap with a circular plane of 2 cm 2 and compress it at a rate of 50 sec / mm until the maximum stress reaches 5 kPa. At this time, the measured compression recovery rate is more than 70%.

4. The cardboard according to claim 1 or 2, characterized in that, the layers other than the surface layer contain one or more selected from magazine waste pulp, liner waste pulp, softwood kraft pulp, and hardwood kraft pulp.

5. The cardboard according to claim 1 or 2, characterized in that, the layers other than the surface layer contain waste pulp with an average fiber length of 1.05 mm or less.

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