Coated white paperboard

Abstract

To provide coated white paperboard bearable against fold-up load and strong against cracking in a pigment coated layer.SOLUTION: Coated white paperboard includes two or more pigment coated layers on a surface of base paper, where each of the pigment coated layers contains ground calcium carbonate as pigment in which a ratio (D75 / D25) of a particle diameter of 75 cumulative wt.% (D75) to a particle diameter of 25 cumulative wt.% (D25) is 3.3 or more and 4.0 or less, and a content ratio of the ground calcium carbonate in a top pigment coated layer coated outermost of all the pigment contained in the top pigment coated layer exceeds 5 wt.% and is 30 wt.% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coated white paperboard, and in particular to a coated white paperboard for use in both offset and gravure printing that is resistant to coating layer cracking. [Background technology]

[0002] Whiteboard is a cardboard typically made of two to nine layers of base paper. Due to growing demands for environmental initiatives and cost reductions, recycled paper pulp is increasingly used. For example, the surface layer is made of bleached, deinked, or bleached recycled paper pulp with high whiteness, while the middle and back layers are made of lower-whiteness, less expensive deinked or non-deinked recycled paper pulp. The recycled paper pulp content has been increasing year by year, and lower-grade whiteboard now contains nearly 100% recycled paper pulp. In this context, issues with the base paper associated with increased recycled paper content include increased occurrence of defects such as dust and black spots, as well as whiteness variations. To remove dust and other foreign matter, measures have been taken to strengthen the deinking and dust removal processes. Advances in deinking technology and dust removal equipment have significantly improved the problem of dust, but it is still not completely eliminated. The more intensive the deinking and dust removal processes are, the worse the yield becomes, which is economically disadvantageous, so currently, deinking and dust removal processes are not carried out beyond a certain level.

[0003] Coated whiteboard is a type of whiteboard with a pigment coating, consisting primarily of pigments and adhesives, applied to a multi-layer base paper. It is primarily used for packaging boxes for cosmetics, pharmaceuticals, soap, gifts, and other items. To highlight the enclosed product, the surface of the coated whiteboard is printed using various printing methods, and then processed into paper containers through various post-processing processes. Based on differences in the base paper composition and coating layer formation, coated whiteboard can be classified into premium whiteboard, specialty whiteboard, and coated whiteboard. Premium whiteboard is made with chemical pulp in all layers and coated on both sides. Special whiteboard is made with chemical pulp in the top and bottom layers, recycled paper in the inner layers, and coated on one or both sides. These premium and specialty whiteboards are collectively referred to as manilaboard. Coated white board is a white paperboard with a surface layer made of chemical pulp, a back layer and layers inside made of recycled paper, and a coating layer on one side. Coated white paperboard is often processed into boxes after going through processes such as printing and die-cutting. When assembling a box, it must be folded, and the surface of the coated white paperboard is subjected to a stretching force during folding. If the strength of the surface layer of the coated white paperboard cannot withstand this force, it will break and tear. This break is called "crease cracking" or "coating layer cracking." In mild cases, the surface layer will crack, but in severe cases, large cracks will occur, including in the base paper. A coating layer crack not only looks unsightly, but also reduces the strength of the assembled box, resulting in a fatal defect in the packaging structure. As countermeasures against such coating layer cracking, technologies have been disclosed, such as a coated liner that contains 5% by weight or more of bleached softwood kraft pulp as pulp in the surface layer of the base paper, and technologies that control the density, print surface roughness, and air permeability of coated paperboard within specific ranges (see, for example, Patent Documents 1 and 2). However, these conventional technologies have not yet achieved sufficient results, and new technologies to prevent coating layer cracking are needed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-249762 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-105895 Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made in view of the above circumstances, and has as its object to provide a coated white paperboard that can withstand the load of bending and is resistant to cracking of the pigment coating layer. [Means for solving the problem]

[0006] As a result of extensive research into solving the above problems, the inventors have found that a coated white paperboard having strength sufficient to withstand bending loads can be obtained by having all pigment coating layers contain heavy calcium carbonate as a pigment, with the ratio (D75 / D25) of the particle size distribution curve's 75th cumulative weight % particle diameter (D75) to the particle size distribution curve's 25th cumulative weight % particle diameter (D25) being 3.3 to 4.0, and by setting the content of heavy calcium carbonate in the outermost top pigment coating layer to be more than 5 wt % and not more than 30 wt % of the total pigment contained in the top pigment coating layer, and have thus completed the present invention.

[0007] Specifically, the present invention provides the following: 1. A coated white paperboard with two or more pigment coating layers on the surface of the base paper, each of the pigment coating layers contains, as a pigment, ground calcium carbonate having a ratio (D75 / D25) of the particle size at 75 cumulative weight % (D75) to the particle size at 25 cumulative weight % (D25) in a particle size distribution curve of 3.3 or more and 4.0 or less; A coated white paperboard characterized in that the content of the ground calcium carbonate in the outermost top pigment coating layer is more than 5% by weight and not more than 30% by weight of the total pigment contained in the top pigment coating layer. 2. The coated white paperboard described in 1., characterized in that the content of the heavy calcium carbonate in the top pigment coating layer is 18% by weight or more and 23% by weight or less of the total pigment contained in the top pigment coating layer. 3. The coated white paperboard according to 1. or 2., characterized in that the top coat pigment coating layer contains calcium carbonate, including the heavy calcium carbonate, and the calcium carbonate content in the top coat pigment coating layer is in the range of 15% by weight to 50% by weight of the total pigment contained in the top coat pigment coating layer. 4. The coated white paperboard described in 3., characterized in that the content of calcium carbonate in the top pigment coating layer is in the range of 19% by weight or more and less than 40% by weight of the total pigment contained in the top pigment coating layer. 5. A coated white paperboard according to any one of 1. to 4., characterized in that the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, contains kaolin as a pigment in an amount of 5% by weight or more and 25% by weight or less of the total pigment contained in the undercoat pigment coating layer. 6. The coated white paperboard according to 5., wherein the undercoat pigment coating layer contains delaminated kaolin having an average aspect ratio of 30 or more and 60 or less. 7. A coated white paperboard according to any one of 1. to 6., characterized in that the content of the heavy calcium carbonate in the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, is 60% by weight or more of the total pigment contained in the undercoat pigment coating layer. 8. A coated white paperboard according to any one of 1. to 7., characterized in that the top coat pigment coating layer contains kaolin as a pigment in an amount of 5% by weight or more and 80% by weight or less of the total pigment contained in the top coat pigment coating layer. 9. A coated white paperboard described in any one of 1. to 8., characterized in that the topcoat pigment coating layer contains a styrene-butadiene-based latex having a single glass transition temperature of 0°C or higher and 30°C or lower as an adhesive. 10. A coated white paperboard described in any one of 1. to 9., characterized in that the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, contains a styrene-butadiene-based latex having a single glass transition temperature of -50°C or higher and lower than 0°C as an adhesive. 11. A coated white paperboard described in any of 1. to 10., characterized in that each of the pigment coating layers contains a pigment and an adhesive in a range of 5 to 30 parts by weight of adhesive per 100 parts by weight of pigment. 12. The coating amount of the top pigment coating layer is 3 to 15 g / m2 in total, in dry weight, per side. 2 12. The coated white paperboard according to any one of 1. to 11., characterized in that: 13. The coating weight of the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, is 5 to 30 g / m2 in total, in dry weight, per side. 2 13. The coated white paperboard according to any one of 1. to 12., characterized in that: 14. The coated white paperboard according to any one of 1. to 13., wherein the base paper has at least three pulp layers. [Effects of the Invention]

[0008] According to the present invention, since each of the two or more pigment coating layers contains heavy calcium carbonate with a fairly wide particle size distribution, the voids in the pigment coating layer are reduced, resulting in a high-density, strong pigment coating layer. This makes it possible to provide a coated white paperboard that is resistant to cracking in the pigment coating layer and is suitable for use in packaging boxes for gifts and the like. DETAILED DESCRIPTION OF THE INVENTION

[0009] The coated white paperboard of the present invention will be described in detail below. <About the pigment coating layer> The coated white paperboard of the present invention comprises two or more pigment coating layers on the surface of a base paper, each containing a pigment containing heavy calcium carbonate and an adhesive. Among the pigment coating layers of the present invention, the outermost coating layer is also referred to as the topcoat pigment coating layer, and the coating liquid that forms the topcoat pigment coating layer is also referred to as the topcoat pigment coating liquid. The layer closer to the base paper than the outermost coating layer is also referred to as the undercoat pigment coating layer, and the coating liquid that forms the undercoat pigment coating layer is also referred to as the undercoat pigment coating liquid.

[0010] (About heavy calcium carbonate) All of the pigment coating layers of the present invention contain heavy calcium carbonate as a pigment, in which the ratio (D75 / D25) of the particle size of 75 cumulative weight % (D75 (μm)) to the particle size of 25 cumulative weight % (D25 (μm)) measured by X-ray sedimentation particle size distribution measurement (X-ray transmission gravity sedimentation) is 3.3 or more and 4.0 or less. In the present invention, the lower limit of D75 / D25 is preferably more than 3.3, more preferably 3.4 or more, and particularly preferably 3.5 or more. The smaller the D75 / D25 value, the narrower the particle size distribution, i.e., the sharper the peak of the particle size distribution of the heavy calcium carbonate. Therefore, the heavy calcium carbonate used in the pigment coating layer of the present invention has a relatively broad particle size distribution. In the present invention, by making the particle size distribution of the heavy calcium carbonate somewhat broad and not using a uniform particle size, it is possible to reduce the voids in the pigment coating layer and obtain a pigment coating layer with high density and strength. Furthermore, by reducing the voids in the pigment coating layer, pigment particles can be bonded with a smaller amount of binder, resulting in a pigment coating layer with good adhesion efficiency. The particle size distribution measured by the X-ray sedimentation particle size distribution measurement method employs a liquid-phase sedimentation method, and particle diameters are measured based on Stokes' law. Furthermore, X-rays are irradiated onto the suspension during sedimentation, and the particle size distribution is measured based on the amount of transmitted X-rays according to the Beer-Lambert law. In the present invention, values ​​measured using an X-ray sedimentation particle size distribution measurement device (Sedigraph 5100, manufactured by Micromeritics) are used.

[0011] The topcoat pigment coating layer, which is the outermost layer of the pigment coating layers of the present invention, contains the above-mentioned heavy calcium carbonate in an amount greater than 5% by weight and less than 30% by weight of the total pigment contained in the topcoat pigment coating layer. A content within this range can significantly suppress coating layer cracking. If the amount is less than 5% by weight, the amount is too small to achieve a sufficient effect of suppressing coating layer cracking. If the amount is greater than 30% by weight, the voids between the white pigment particles are reduced, reducing the light scattering properties of the pigment coating layer, which may result in poor opacity. The ground calcium carbonate is contained in an amount of preferably 10% by weight or more, more preferably 15% by weight or more, and particularly preferably 18% by weight or more of the total pigment contained in the top pigment coating layer, and preferably 28% by weight or less, more preferably 25% by weight or less, and particularly preferably 23% by weight or less. Furthermore, in order to obtain the effect of suppressing coating layer cracking, it is preferable that the undercoat pigment coating layer close to the base paper in the pigment coating layer of the present invention contains the above-mentioned heavy calcium carbonate in an amount of 60% by weight or more of the total pigment contained in the undercoat pigment coating layer, more preferably 70% by weight or more, and particularly preferably 80% by weight or more.

[0012] The primer pigment coating layer of the present invention may further contain kaolin as a pigment. The content of kaolin is preferably in the range of 0% to 30% by weight, and more preferably 5% to 25% by weight, of the total pigment contained in the primer pigment coating layer. When the primer pigment coating layer contains kaolin, delaminated kaolin is preferred. Dellaminated kaolin is obtained by peeling off ordinary kaolin, which is composed of stacked hexagonal plate-shaped crystals, into single layers. Because of its relatively large particle size and high aspect ratio, its incorporation into the coating layer improves the coverage of the base paper and smoothness. Among these, delaminated kaolin with an average aspect ratio of 30 to 60 is preferred. Dellaminated kaolin with an average aspect ratio of 30 to 60 improves whiteness unevenness and further improves water retention. This is presumably because the flat pigment particles fill the gaps between the pigment particles, suppressing dehydration and improving water retention. However, flat pigments with high aspect ratios, such as delaminated kaolin, have acidic groups on their main surfaces, which means they have poor affinity with adhesives containing many acidic groups, such as latex, resulting in low adhesion efficiency. Therefore, the content of delaminated kaolin in the pigment coating layer is preferably 30% by weight or less, and more preferably 25% by weight or less. Furthermore, the top pigment coating layer of the present invention preferably further contains kaolin as a pigment to obtain high hiding power, smoothness, and high slipperiness during box processing, with the content of kaolin being preferably in the range of 5 to 80% by weight, more preferably 30 to 80% by weight, of the total pigment contained in the top pigment coating layer.

[0013] The pigment coating layer of the present invention may further contain conventionally known pigments in addition to those mentioned above. Specific examples include inorganic fillers such as clay, calcined clay, diatomaceous earth, talc, calcined kaolin, light calcium carbonate, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, calcium sulfite, gypsum, white carbon, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, papermaking sludge, and inorganic particles recycled from deinking froth, and organic fillers such as urea-formalin resin, polystyrene resin, vinyl chloride resin, melamine resin, phenolic resin, and plastic hollow microparticles. In particular, in order to obtain hiding power while maintaining high whiteness, it is preferable to contain light calcium carbonate in one of the pigment coating layers, and it is particularly preferable to contain it in the topcoat pigment coating layer, which is required for hiding power and high whiteness. When the topcoat pigment coating layer contains light calcium carbonate, the proportion of light calcium carbonate and heavy calcium carbonate, based on the total pigment contained in the topcoat pigment coating layer, is preferably 15% by weight or more, more preferably 18% by weight or more, and particularly preferably 19% by weight or more, and the upper limit is preferably 50% by weight or less, more preferably 45% by weight or less, particularly preferably less than 40% by weight, and most preferably 35% by weight or less.

[0014] The adhesive (binder) contained in the pigment coating solution of the present invention is not particularly limited, and adhesives conventionally used in coated paper can be used. Specific examples include various copolymers or latexes such as styrene-butadiene, styrene-acrylic, ethylene-vinyl acetate, butadiene-methyl methacrylate, acrylic acid-methyl methacrylate, vinyl acetate-butyl acrylate, styrene-maleic anhydride, and ethylene-maleic anhydride; polyvinyl alcohols such as fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, acetoacetylated polyvinyl alcohol, and carboxy-modified polyvinyl alcohol; proteins such as casein, soy protein, and synthetic protein; starches such as oxidized starch, cationized starch, urea phosphate esterified starch, and etherified starch such as hydroxyethyl etherified starch; dextrin; and cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, and nanocellulose. One or more of these adhesives can be appropriately selected and used. In particular, the adhesive used in the pigment coating solution of the present invention preferably contains latex and starch. The latex used is preferably a styrene-butadiene latex (hereinafter sometimes referred to as "SBR") having a single glass transition temperature (hereinafter sometimes referred to as "Tg"). In the present invention, the primer pigment coating layer preferably contains an SBR with a low Tg, and the top pigment coating layer preferably contains an SBR with a high Tg. Specifically, the Tg of the SBR used in the primer pigment coating layer is preferably -50°C or higher and lower than 0°C, and the Tg of the SBR used in the top pigment coating layer is preferably 0°C or higher and 30°C or lower. Since SBRs with lower Tg are generally softer, using SBRs with the above-mentioned Tg in each pigment coating layer can impart cushioning to the primer pigment coating layer and strength to the top pigment coating layer. Furthermore, the use of starch in the pigment coating liquid can increase the water retention of the pigment coating liquid. The use of a pigment coating liquid with high water retention suppresses the penetration of adhesive components into the paper during coating, allowing a large amount of adhesive to be retained in the pigment coating layer, thereby enhancing the effectiveness of the adhesive components in the pigment coating layer and resulting in a pigment coating layer with excellent pigment coating layer strength, surface strength, and cushioning properties.

[0015] The blending ratio of pigment to adhesive in the pigment coating liquid forming each pigment coating layer of the present invention is adjusted appropriately within a range that allows the desired pigment coating liquid to be obtained. Typically, the solids content is preferably 5 to 30 parts by weight of adhesive per 100 parts by weight of pigment, and more preferably 8 to 20 parts by weight. If the solids content is less than 5 parts by weight, the strength of the pigment coating layer is weakened, and problems such as paper dust generation and poor print strength are likely to occur. Furthermore, if the solids content is greater than 30 parts by weight, the gaps between the white pigment particles in the coating layer are reduced, improving adhesion efficiency, but the light scattering properties of the pigment coating layer are reduced, leading to problems such as poor opacity and increased costs.

[0016] In addition to the pigment and adhesive, the pigment coating liquid that forms the pigment coating layer of the present invention may contain, as needed, various auxiliary agents that are typically blended into pigment coating liquids for coated paper, such as dispersants, viscosity improvers, water retention agents, defoamers, water resistance agents, fluorescent dyes, coloring dyes, surfactants, pH adjusters, cationic resins, anionic resins, ultraviolet absorbers, and metal salts. The solids concentration of the pigment coating liquid is preferably 58% by weight or more, more preferably 62% by weight or more. If the solids content is lower than 58% by weight, the water retention capacity decreases, and excessive penetration of the pigment coating liquid into the base paper may result in a decrease in the quality of the coated white paperboard. On the other hand, there is no particular upper limit to the solids concentration, but considering factors such as liquid transportability, it is preferably 75% by weight or less, more preferably 70% by weight or less.

[0017] <About the base paper> The base paper in the coated white paperboard of the present invention is preferably a multi-layered paper made of two or more pulp layers (paper layers). The number of layers in the base paper is preferably three or more, and more preferably five or more. The outermost paper layers of the base paper are called the surface layer and back layer, and the inner layer sandwiched between the surface layer and back layer is called the middle layer. The second layer adjacent to the surface layer and facing the middle layer is called the lower surface layer. The middle layer may be one layer or two or more layers. The layers of the base paper of the present invention may also be called the first layer, second layer, and third layer, counting from the side where the pigment coating layer is provided. The base paper of the present invention can be any type commonly used for coated whiteboard, without exception. However, it is preferable that recycled paper pulp be blended in at least one layer of the multi-layer base paper. Using a base paper containing a large amount of recycled paper pulp is advantageous in terms of cost and also reduces the environmental impact. The recycled paper pulp can be deinked pulp (DIP) or undeinked pulp. The recycled paper pulp can be made from sorted recycled paper such as fine paper, medium-grade paper, low-grade paper, newspaper, flyers, and magazines, or a mixture of these. Pulps other than recycled paper pulp are not particularly limited, and include chemical pulps such as kraft pulp and sulfite pulp made from hardwood or softwood; mechanical pulps such as thermomechanical pulp and pressure-ground wood pulp; and semi-chemical pulps such as chemical thermomechanical pulp. The pulp used can be a mixture of these various pulps, or the same pulp can be used. Furthermore, one or more paper layers containing different pulps may be stacked. The base paper of the present invention can use, for example, a pulp with low brightness for the middle layer and a pulp with higher brightness for the front and back layers, or all layers can be stacked with the same pulp. In the present invention, when the base paper has three or more layers, the front and back layers preferably contain 30% by weight or more, and more preferably 50% by weight or more, of hardwood bleached kraft pulp (hereinafter sometimes referred to as "LBKP"), which has high brightness and low impurity content, relative to 100% by weight of the pulp in the front or back layer. The middle layer preferably contains 10% by weight or more, and more preferably 30% by weight or more, of recycled paper pulp, which has low brightness and relatively high impurity content, relative to 100% by weight of the pulp in the middle layer.

[0018] The base paper of the present invention typically contains a filler. The filler is not particularly limited, but examples include inorganic fillers such as clay, calcined clay, diatomaceous earth, talc, kaolin, calcined kaolin, delaminated kaolin, heavy calcium carbonate, light calcium carbonate, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, calcium sulfite, gypsum, white carbon, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, papermaking sludge, and inorganic particles recycled from deinking floss; organic fillers such as urea-formaldehyde resin, polystyrene resin, vinyl chloride resin, melamine resin, phenolic resin, and hollow plastic microparticles; and fillers contained in waste paper and broken paper, which can be used alone or in combination of two or more. Regarding the filler content of the base paper, in order to conceal dust and color unevenness present in the middle layer, it is preferable to use 2% by weight or more of the above fillers in the back layer based on 100% by weight of the pulp in the back layer. There are no particular restrictions on the amount of filler that can be added to the surface and middle layers, but if the filler content is too high, the interlaminar strength of the base paper will decrease, so it is necessary to adjust it appropriately according to the desired quality.

[0019] The method for making the base paper in the present invention is not particularly limited, and can be carried out using a Fourdrinier paper machine including a top wire, an on-top former, a gap former, a cylinder paper machine, a board paper machine using a Fourdrinier paper machine and a cylinder paper machine in combination, a Yankee dryer machine, or a hybrid paper machine that combines these. The pH during papermaking may be acidic, neutral, or alkaline. In the case of a multi-layer base paper, the pH during papermaking may be the same for each layer or may be different for each layer. The papermaking speed is not particularly limited. The basis weight of the base paper in the coated white paperboard of the present invention is usually 150 to 650 g / m 2 That's about it. When a coated white paperboard is produced according to the present invention, the base paper may be smoothed in advance using an online soft calender, online chilled calender or the like before the pigment coating layer is formed.

[0020] In the present invention, the method for drying the base paper is not limited, and various methods such as a steam heating cylinder, a heated hot air dryer, a gas heater dryer, an electric heater dryer, and an infrared heater dryer can be used alone or in combination.

[0021] In the present invention, a base paper with low smoothness may be used, but a base paper with high smoothness may be used as long as it does not cause pooling of the coating liquid, known as puddling, during the formation of the pigment coating layer. In order to improve the smoothness of the base paper, as described above, a treatment such as pre-calendering may be carried out before the formation of the pigment coating layer. Furthermore, as a means of improving the smoothness of the base paper, a clear coating liquid containing starch as a main component or a pigment coating liquid containing a pigment can be pre-coated onto the base paper before the formation of the pigment coating layer. This pre-coated base paper can be used to form the pigment coating layer without undergoing a drying process, i.e., while the coating liquid on the base paper is still wet. In this way, there are no restrictions on the state of the base paper after pre-coating before being used to form the pigment coating layer.

[0022] The pigment coating layer of the present invention is formed by coating the surface of the base paper with a pigment coating liquid prepared by mixing necessary ingredients such as a pigment containing heavy calcium carbonate and an adhesive with water using a conventional mixing means such as a mixer. <About the coating method> There are no particular limitations on the coating method for forming the pigment coating layer of the present invention, and various common coating devices such as a blade coater, a rod coater, an air knife coater, a roll coater, and a curtain coater can be used. Among these, coating methods using a blade coater or rod coater are suitable as the coating method of the present invention because they can increase the density of the pigment coating layer and obtain a pigment coating layer that is less likely to crack. Rod coaters are also called bar coaters or bar blade coaters.

[0023] In the present invention, the coating amount of the pigment coating layer is 8 to 50 g / m2 in total for each layer on one side in dry weight. 2 is preferred, and 45 g / m 2It is more preferable that the thickness is 10 to 40 g / m or less. 2 is more preferably 11 to 35 g / m 2 The total coating amount of each layer is particularly preferably 8 g / m 2 If the coating weight is less than 50g / m, the pigment coating layer will be too thin and the whiteness and whiteness unevenness of the coated white board may not be improved sufficiently. 2 If the temperature exceeds this range, the drying property in the drying process may be reduced, which may decrease the operability, and binder migration may cause uneven printing. In the present invention, the coating weight of the undercoat pigment coating layer and the coating weight of the topcoat pigment coating layer are not particularly limited, but in order to achieve both hiding power and whiteness, the undercoat pigment coating layer should be 5 to 30 g / m2 in total in dry weight per side. 2 , top coat pigment coating layer is 3 to 15 g / m 2 The coating weight of the undercoat pigment coating layer and the topcoat pigment coating layer is preferably in the range of 5 g / m 2 , 3g / m 2 If the coating weight is less than this, the whiteness and whiteness unevenness of the coated white board may not be sufficiently improved.On the other hand, if the coating weight is too high, the drying process may become slower, which reduces workability, and cracks in the pigment coating layer may become more likely to occur.

[0024] The coated white paperboard of the present invention is produced by providing a pigment coating layer on a base paper and then undergoing a conventional drying process, but may also be subjected to a smoothing treatment such as a surface treatment process, if necessary. In a preferred embodiment, the moisture content of the produced coated white paperboard is adjusted to about 3 to 10% by weight, more preferably about 4 to 8% by weight. For the smoothing treatment, conventional smoothing treatment devices such as super calenders, gloss calenders, soft calenders, thermal calenders, and shoe calenders can be used. Smoothing treatment devices can be used on or off the machine as appropriate, and the type of pressure device, the number of pressure nips, heating, etc. can also be adjusted as appropriate. In a preferred embodiment of the present invention, the smoothness of the coated white paperboard after production is adjusted so that the PPS roughness under a clamping pressure of 2000 kPa is 0.5 μm or more and 1.6 μm or less. The PPS roughness is preferably 0.8 μm or more. If the PPS roughness is less than 0.5 μm, the pigment coating layer becomes significantly denser during the smoothing process, reducing its opacity and its ability to conceal dust and color unevenness on the base paper. On the other hand, if the PPS roughness exceeds 1.6 μm, the smoothness tends to be low, resulting in a decrease in the appearance of the blank paper surface and the printed surface.

[0025] In the present invention, a clear coating layer may be provided on the surface of the backing layer to impart stiffness and suppress curling. The clear coating liquid for forming the clear coating layer on the surface of the backing layer may contain, as necessary, various auxiliaries that are conventionally blended in ordinary coating liquids, such as conventionally known and used binders, dispersants, viscosity improvers, water retention agents, antifoaming agents, water resistance agents, surfactants, pH adjusters, cationic resins, anionic resins, ultraviolet absorbers, and metal salts. [Example]

[0026] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. In each example and comparative example, "parts" means "parts by weight" and "%" means "% by weight" unless otherwise specified.

[0027] [Example 1] <Base paper> LBKP 100% pulp used, basis weight 33g / m 2 The surface layer (first layer) is made of 100% deinked recycled paper pulp with a basis weight of 36 g / m 2 The second layer is made of 100% undeinked recycled magazine pulp, weighing 45g / m 2 The middle and back layers (3rd to 7th layers) are made separately, combined, pressed and dried, and the resulting paper has a basis weight of 294 g / m 2 The pulp slurry for each layer contained 0.6% by weight of a strength agent and 1.5% by weight of aluminum sulfate relative to 100% by weight of pulp.

[0028] <Topcoat pigment coating liquid 1> A pigment slurry was prepared from 60 parts by weight of fine kaolin (Kaofine, manufactured by Thiele), 20 parts by weight of heavy calcium carbonate (Carbital 97, manufactured by Imerys, D75 / D25=3.87), 15 parts by weight of spindle-shaped precipitated calcium carbonate (TP-221F, manufactured by Okutama Kogyo Co., Ltd., D75 / D25=2.5), and 5 parts by weight of titanium dioxide. To 100 parts by weight of the pigment, 14.5 parts by weight of styrene-butadiene copolymer latex A (ALB1443, manufactured by Asahi Kasei Corporation, Tg=17°C) and 2 parts by weight of urea phosphate esterified starch were added as adhesives. Water was then added to obtain a topcoat pigment coating solution with a solids concentration of 60%. <Primer pigment coating liquid 1> A pigment slurry was prepared from 80 parts by weight of heavy calcium carbonate (Imerys, Carbital 90, D75 / D25 = 3.82) and 20 parts by weight of delaminated kaolin (Imerys, Contour 1500, average aspect ratio 60). To this slurry, 12 parts by weight of styrene-butadiene copolymer latex B (Asahi Kasei, ALB1735, Tg = -8°C) and 3 parts by weight of urea phosphate esterified starch were added per 100 parts by weight of the pigment. Water was then added to obtain a primer pigment coating solution with a solids concentration of 60%.

[0029] <Coating> On the surface of the base paper, primer pigment coating solution 1 is applied using a rod coater to a coating amount of 8 g / m2 on one side in dry weight. 2 After coating so that the total weight of the top pigment coating layer 1 was 12 g / m2 on each side by a blade coater without a drying step. 2 After that, the coating was smoothed with a calendar to a basis weight of 314 g / m2. 2 The resulting coated white paperboard 1 had a paper thickness of 375 μm.

[0030] [Example 2] Coated white paperboard 2 was produced in the same manner as in Example 1, except that the amount of heavy calcium carbonate in the "Top Coat Pigment Coating Liquid 1" was 25 parts by weight and the amount of light calcium carbonate was 20 parts by weight. The basis weight of the resulting coated white paperboard 2 was 314 g / m 2 The paper thickness was 387 μm.

[0031] [Example 3] Coated white paperboard 3 was produced in the same manner as in Example 1, except that the amount of heavy calcium carbonate in the "Top Coat Pigment Coating Liquid 1" was 15 parts by weight and the amount of light calcium carbonate was 30 parts by weight. The basis weight of the resulting coated white paperboard 3 was 314 g / m 2 The paper thickness was 375 μm.

[0032] [Example 4] Coated white paperboard 4 was produced in the same manner as in Example 1, except that the amount of heavy calcium carbonate in the "Top Coat Pigment Coating Liquid 1" was 30 parts by weight and the amount of light calcium carbonate was 15 parts by weight. The basis weight of the resulting coated white paperboard 4 was 314 g / m 2 The paper thickness was 375 μm.

[0033] [Example 5] Coated white paperboard 5 was produced in the same manner as in Example 1, except that the amount of fine kaolin, heavy calcium carbonate, and light calcium carbonate in the "Top Coat Pigment Coating Liquid 1" was changed to 86 parts by weight, 9 parts by weight, and 5 parts by weight, respectively. The basis weight of the resulting coated white paperboard 5 was 314 g / m 2 The paper thickness was 375 μm.

[0034] [Example 6] Coated white paperboard 6 was produced in the same manner as in Example 1, except that the amount of heavy calcium carbonate in the "primer pigment coating liquid 1" was changed to 70 parts by weight and the amount of kaolin to 30 parts by weight. The basis weight of the resulting coated white paperboard 6 was 314 g / m 2 The paper thickness was 375 μm.

[0035] [Comparative Example 1] Coated white paperboard a was produced in the same manner as in Example 1, except that the heavy calcium carbonate (Imerys, Carbital 97, D75 / D25 = 3.87) in the "Top Coat Pigment Coating Solution 1" was replaced with heavy calcium carbonate (Fimatec, FMT-OP, D75 / D25 = 3.0). The basis weight of the resulting coated white paperboard a was 314 g / m 2 The paper thickness was 386 μm.

[0036] Comparative Example 2 Coated white paperboard b was produced in the same manner as in Example 1, except that the amount of heavy calcium carbonate (Imerys, Carbital 97, D75 / D25=3.87) in the "Top Coat Pigment Coating Liquid 1" was 4 parts by weight and 16 parts by weight of heavy calcium carbonate (Fimatec, FMT-OP, D75 / D25=3.0) was added. The basis weight of the resulting coated white paperboard b was 314 g / m 2 The paper thickness was 375 μm.

[0037] <Evaluation method for cracking of pigment coating layer> The pigment coating layers of coated white paperboards 1 to 6 and a, b produced in Examples 1 to 6 and Comparative Examples 1 and 2 above were printed with 100% indigo on a sheet-fed press, and then the printed surface was creasing-processed using a laboratory creaser. The creasing was then folded, and the pigment coating layer cracking at the folded area was visually evaluated according to the following evaluation criteria. Only coated white paperboards with evaluation results of "◎" and "◯" were deemed to be practical. The evaluation results are shown in Table 1 below. [Evaluation criteria] ◎: No cracks in the pigment coating layer are observed ○: Almost no cracking of the pigment coating layer is observed △: Some cracking of the pigment coating layer is observed ×: Significant cracking of the pigment coating layer is observed

[0038] [Table 1] [Industrial Applicability]

[0039] The coated white paperboard of the present invention has two or more pigment coating layers, each of which contains heavy calcium carbonate with a fairly wide particle size distribution, which reduces voids in the pigment coating layer and results in a high-density, strong pigment coating layer, making it suitable for use in packaging boxes for gifts and the like, and useful because the pigment coating layer is resistant to cracking.

Claims

1. A coated white paperboard having two or more pigment coating layers on the surface of a base paper consisting of a surface layer, a subsurface layer, a middle layer, and a back layer, The surface layer contains 30% by weight or more of hardwood kraft pulp relative to 100% by weight of pulp, each of the pigment coating layers contains, as a pigment, ground calcium carbonate having a ratio (D75 / D25) of the particle diameter of 75 cumulative weight % (D75) to the particle diameter of 25 cumulative weight % (D25) in a particle size distribution curve of 3.3 or more and 4.0 or less; A coated white paperboard characterized in that the content of the heavy calcium carbonate in the outermost top pigment coating layer is more than 5% by weight and not more than 30% by weight of the total pigment contained in the top pigment coating layer (however, this does not include coated white paperboard for laser marking that includes a base coating layer containing 1 to 40 parts by weight of calcium carbonate per 100 parts by weight of pigment).

2. 2. The coated white paperboard according to claim 1, wherein the content of the ground calcium carbonate in the top pigment coating layer is 18% by weight or more and 23% by weight or less of the total pigment contained in the top pigment coating layer.

3. 3. The coated white paperboard according to claim 1, wherein the top coat pigment coating layer contains calcium carbonate, including the heavy calcium carbonate, and the calcium carbonate content in the top coat pigment coating layer is in the range of 15% by weight to 50% by weight of the total pigment contained in the top coat pigment coating layer.

4. 4. The coated white paperboard according to claim 3, wherein the content of calcium carbonate in the top pigment coating layer is in the range of 19% by weight or more and less than 40% by weight of the total pigment contained in the top pigment coating layer.

5. A coated white paperboard according to any one of claims 1 to 4, characterized in that the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, contains kaolin as a pigment in an amount of 5% by weight to 25% by weight of the total pigment contained in the undercoat pigment coating layer.

6. 6. The coated white paperboard according to claim 5, wherein the undercoat pigment coating layer contains delaminated kaolin having an average aspect ratio of 30 to 60 as the kaolin.

7. 7. The coated white paperboard according to claim 1, wherein the content of the heavy calcium carbonate in the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, is 60% by weight or more of the total pigment contained in the undercoat pigment coating layer.

8. The coated white paperboard according to any one of claims 1 to 7, characterized in that the top-coat pigment coating layer contains kaolin as a pigment in an amount of 5% by weight to 80% by weight of the total pigment contained in the top-coat pigment coating layer.

9. The coated white paperboard according to any one of claims 1 to 8, characterized in that the top pigment coating layer contains, as an adhesive, a styrene-butadiene-based latex having a single glass transition temperature of 0°C or higher and 30°C or lower.

10. The coated white paperboard according to any one of claims 1 to 9, characterized in that the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, contains a styrene-butadiene-based latex having a single glass transition temperature of -50°C or higher and lower than 0°C as an adhesive.

11. A coated white paperboard according to any one of claims 1 to 10, characterized in that each of the pigment coating layers contains a pigment and an adhesive in a range of 5 to 30 parts by weight of adhesive per 100 parts by weight of pigment.

12. The coating amount of the top coating pigment layer is 3 to 15 g / m2 in total on one side in dry weight. ２ The coated white paperboard according to any one of claims 1 to 11, characterized in that

13. The coating amount of the undercoat pigment coating layer, which is closer to the base paper than the topcoat pigment coating layer, is 5 to 30 g / m2 in total per side in dry weight. ２ The coated white paperboard according to any one of claims 1 to 12, characterized in that

14. The coated white paperboard according to any one of claims 1 to 13, characterized in that the base paper has at least three pulp layers.

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