JP2011021283A - Paperboard and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a paperboard causing little surface staining (color fading) by water wetting, moisture and friction while using a waste paper pulp as a main component, having texture of a pulp material, and suitable, for example, for bag-making and cardboard case; and to provide a method for easily producing the paperboard at a low cost. <P>SOLUTION: The paperboard having two or more multilayered structure has at least a surface layer comprising a colored layer containing a sulfur dye, and a direct dye and/or a color pigment. The colored layer comprises a raw material pulp containing the waste paper pulp as a main component, and provides a hot water extract of the colored layer having pH 4.0-6.5, measured based on JIS P 8133 (1998), in the paperboard. The method for producing the paperboard includes adding the sulfur dye to a slurry of the raw material pulp comprising the waste paper pulp as the main component, further adding the direct dye and/or the colored pigment thereto, and forming the colored layer. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a paperboard and a manufacturing method thereof. More specifically, a paperboard which is mainly composed of waste paper pulp, has little surface contamination (color loss) due to water wetting, moisture, and rubbing, and has a texture of pulp material, for example, suitable for bag making and cardboard cases and It relates to the manufacturing method.

  2. Description of the Related Art Conventionally, a paperboard for bag making is used for a packaging base material for shopping such as a handbag or an envelope. In recent years, it has been desired that such paperboard not only protects the contents but also has fashionability and functionality. Especially for shopping handbags, due to recent progress in visualization, colorful and dark colors are favored, and it looks great by vividly printing on the paper surface or using paper made by coloring pulp. Paperboard with improved quality is used.

  In order to improve the appearance, printing-type paperboards with a printed layer with patterns and various information printed on the paper surface are given a glossy varnish or laminate on the printed surface, resulting in surface contamination due to water wetting, moisture, or rubbing (Color fading) is prevented.

  A paper board for printing type bag making needs to have a process of applying glossy varnish after printing on the paper surface, and in particular, to obtain a deep color tone, it is necessary to thicken the printing layer. There is a problem that the printed layer interferes with adhesiveness during bag processing, causing inadvertent peeling. For this reason, the spread of paperboard having a colored layer in which raw material pulp is colored in advance is in progress. Such paper-making type paperboard, which is pre-colored with raw pulp, has a good texture without impairing the texture of the pulp, is excellent in processability, and has less color fading.

  However, paper-made handbags of colored paperboard that are colored with raw pulp have problems that the unfixed dye in the colored pulp elutes due to inadvertent water wetting, etc. Has the problem of surface contamination (color loss) due to.

  In addition, due to the recent trend of environmental protection, waste paper pulp is frequently used as raw material pulp, and environmental protection and resource saving efforts are desired. Waste paper made from waste paper instead of conventional 100% virgin pulp paperboard Development of paperboard using pulp as a main component is desired.

  However, waste paper pulp, which has been repeatedly recycled, loses fluff on the pulp fiber surface and exhibits high anionicity. The problem is that the dye of the dye is eluted and the color is transferred to the worn clothes, and the problem of surface contamination (color loss) due to rubbing becomes more prominent.

  For example, Patent Document 1 uses a reducing agent, an oxidizing agent, and a chemical with a high electrolyte concentration for dyeing a conventional fiber in order to solve the problem caused by elution of such an unfixed dye. Thus, there has been a problem in the treatment of these waste liquids after dyeing, and a method for producing a water-soluble sulfur dye that can be dyed without adding a reducing agent is disclosed. However, Patent Document 1 discloses a method of directly dyeing a cotton fabric or a fabric such as a woven fabric in which cotton and polyester are mixed, and the dyeing temperature is 50 to 75 ° C. Accordingly, excessive energy is required to set the wet paper temperature to the above condition in the paper making process, which causes a significant problem in terms of cost.

  Patent Document 2 discloses a technique for improving the colorability in a pH range of 7.5 to 11.0 and a temperature range of 60 to 120 ° C. in order to color a polyamide or a polyamide-containing substrate with a sulfur dye. It is disclosed.

  Further, Patent Document 3 describes any combination of fibers or fiber structures (cellulose fibers such as cotton and linen, animal fibers such as wool and silk, synthetic fibers such as nylon, polyester and acetate, and regenerated fibers such as rayon. Cotton, yarn, woven fabric, knitted fabric, non-woven fabric, paper sheet), after being immersed in a mixed solution of a cationizing agent and an alkaline auxiliary agent at 60 to 80 ° C. as a pretreatment, A technique for immersing and dyeing in a mixed solution in which a fixing agent and / or a color former is mixed is disclosed.

  However, since the techniques disclosed in Patent Documents 2 and 3 both require heating, excessive energy is required, and there is still a significant problem in cost.

JP 05-140879 A Special table 2003-504532 gazette JP-A-11-158785

  The present invention has been made in view of the above-mentioned background art, and is mainly composed of waste paper pulp, but has little surface contamination (color loss) due to water wetting, moisture, and rubbing, and has a texture of pulp material, for example, It is an object of the present invention to provide a paperboard suitable for bag making or cardboard case, and a method for easily producing the paperboard at low cost.

The present invention
A paperboard having a multilayer structure of two or more layers,
At least a surface layer composed of a colored layer containing a sulfur dye and a direct dye and / or color pigment;
The colored layer is made of raw pulp mainly composed of waste paper pulp,
The present invention relates to paperboard, wherein the colored layer has a hot water extraction pH of 4.0 to 6.5 measured according to JIS P 8133 (1998).

The present invention also provides
A method for producing a paperboard having a multilayer structure of two or more layers,
The present invention relates to the above-mentioned method for producing paperboard, characterized in that a colored layer is formed by adding a dye and / or a color pigment directly after adding a sulfur dye to a slurry of raw material pulp mainly composed of waste paper pulp.

  The paperboard of the present invention is mainly composed of waste paper pulp, but has little surface contamination (color loss) due to water wetting, moisture, and rubbing, and has a texture of pulp material, for example, suitable for bag making or cardboard cases. Is. Moreover, such a paperboard can be easily manufactured at low cost by the manufacturing method of the present invention.

Schematic flow diagram showing an example of a process for producing a paperboard of the present invention

(Embodiment)
The paperboard of the present invention has a multilayer structure of two or more layers, and has at least a surface layer composed of a colored layer containing a sulfur dye and a direct dye and / or a color pigment, and the colored layer is mainly composed of waste paper pulp. The hot water extraction pH of the colored layer is 4.0 to 6.5.

  The paperboard of the present invention is more preferably composed of three paper layers, a surface layer composed of the colored layer, an intermediate layer, and a back layer. In the following, the present invention will be described in more detail by taking a paperboard composed of three paper layers as an example.

  The raw material pulp used for each layer constituting the paperboard of the present invention is preferably composed mainly of waste paper pulp made from so-called recovered recovered paper from the city. The types of waste paper pulp include, for example, pulp obtained by deinking or bleaching office waste paper including enzyme bleached pulp, milk pack waste paper, high quality cut-off waste paper, coat cut-off waste paper, fine white, color fine quality, special white, medium white, etc. Examples include waste paper pulp obtained from unprinted paper, land tickets, new corrugated paper, newspapers, craft waste paper, imitation, magazine waste paper, and the like.

  Examples of raw material pulp other than the waste paper pulp include hardwood bleached kraft pulp (LBKP), softwood bleached kraft pulp (NBKP), hardwood unbleached kraft pulp (LUKP), softwood unbleached kraft pulp (NUKP), hardwood sulfite pulp, conifer Pulp and chips chemically treated with wood fibers such as sulfite pulp as main raw materials, or pulp and chips chemically processed with non-wood fibers such as kenaf and hemp as fiber raw materials other than wood Virgin pulp such as semi-chemical pulp, pulped with refiner, etc. And kraft pulp, semi-chemical pulp, etc. That.

  The raw material pulp used in the present invention is mainly composed of waste paper pulp for environmental protection and effective utilization of resources. Above all, used white paper, waste paper pulp derived from colored fine quality used paper is a paperboard having a colored layer. In the manufacturing process, the waste paper pulp itself does not need to be bleached, the paper quality is almost uniform, the raw material composition is less uneven, and the ash content is low. It is useful for obtaining the paperboard of the present invention suitable for, for example, bag making or cardboard case, which has little surface contamination (color loss) and has a texture of pulp material, and is particularly preferably used.

  Further, the ratio of the used paper pulp in the raw pulp is preferably 60% by mass or more, more preferably 80% by mass or more from the viewpoint of environmental protection and effective use of resources, and the total amount of the raw material pulp is used. It may be.

  The raw material pulp used for the surface layer is preferably adjusted to a Canadian standard freeness (hereinafter referred to as CSF) of 200 to 370 mL, more preferably 250 to 320 mL. When the CSF of the raw material pulp of the surface layer is less than 200 mL, the fibers are likely to be dense, and there is a risk of poor dewatering of the paperboard. On the other hand, when the CSF of the raw material pulp of the surface layer exceeds 370 mL, the dehydrating property is improved, but the spaces between the fibers are apt to be sparse, and the texture of the paperboard, that is, the decorativeness of the paperboard may be lowered.

  The raw material pulp used for the middle layer and the back layer is preferably adjusted to a CSF of 250 to 500 mL, more preferably 300 to 400 mL. If the CSF of the raw pulp of the middle layer or the back layer is less than 250 mL, the fibers are likely to become dense, poorly mating with the surface layer mainly composed of waste paper pulp and poor dewatering of the paperboard, There is a fear. On the other hand, if the CSF of the raw pulp of the middle layer or the back layer exceeds 500 mL, the fibers are likely to become loose, and there is a risk that the formation unevenness of the surface layer mainly composed of waste paper pulp, or the formation failure of the paperboard will occur. is there.

  When adjusting the CSF of the raw pulp, a known beating machine such as a single disc refiner (hereinafter referred to as SDR), a double disc refiner (hereinafter referred to as DDR), or a conical refiner can be used.

  In the present specification, CSF refers to JIS P 8121 (1995) “pulp” after a sample is disaggregated by a standard disintegrator in accordance with the method described in JIS P 8220 (1998) “pulp-disaggregation method”. It is a value obtained by measuring the freeness with a Canadian standard freeness tester in accordance with the method described in “Drainage Test Method”.

  As described above, the waste paper pulp is relatively flat by removing the fluffing (fibrillation) on the fiber surface provided by the refining process repeated several times, especially during the production of the virgin raw pulp, especially through the beating process. Exhibits properties. At the same time, unlike virgin pulp, virgin pulp is difficult to dye due to its high anionic property, and the raw material pulp containing wastepaper pulp has low dyeability and low dye yield. In addition to the need for a large amount of agent and the like, there are cases where unfixed dyes are eluted and the problem of color transfer to the worn clothes and the problem of surface contamination (color loss) due to rubbing may appear more prominently.

  Then, although the detailed reason is not certain with respect to the said problem by using a waste paper pulp as a raw material pulp, it contains a sulfur dye and a direct dye and / or a color pigment in the colored layer which has a waste paper pulp as a main component. By setting the hot water extraction pH of the colored layer to 4.0 to 6.5, the pulp material is free from surface contamination (color loss) due to water wetting, moisture, and rubbing while containing wastepaper pulp as a main component. A paperboard of the present invention having the following texture has been found.

  The surface of the paperboard of the present invention is colored by forming a colored layer by mixing and dyeing a sulfur dye and a direct dye and / or a color pigment in a raw material pulp mainly composed of a surface layer of waste paper pulp. It has the characteristics to be. Usually, sulfur dyes are dyes with relatively little discoloration, but are expensive, and sulfur dioxide gas is generated in use. In addition, direct dyes tend to cause discoloration and dye elution although they are inexpensive. Furthermore, although the pigment has excellent fading properties, it has low adhesion to the raw material pulp, and there is a concern that the tensile strength and tear strength may be reduced.

  However, in the present invention, while supplementing the advantages and disadvantages of these sulfur dyes, direct dyes and color pigments, the main component is waste paper pulp, which is a problem, but there are few surface stains (color loss) due to water wetting, moisture, and rubbing, A paperboard having a texture of a pulp material, for example, a paperboard suitable for bag making or cardboard case, and a method for easily producing the paperboard at a low cost are found, and at least in the raw material pulp of the surface layer, a sulfur dye and If a direct dye and / or color pigment, preferably a sulfur dye and a direct dye and / or color pigment are contained and blended in a predetermined amount and ratio, a target paperboard in which conventional problems are solved is obtained. In addition, it has been found that when the difference between the color tone of the surface layer and the color tone of the surface layer is made small, color unevenness hardly occurs and the appearance is further improved. It should be noted that sulfur dyes and direct dyes and / or color pigments may be blended in the raw pulp of the middle layer and the back layer, which are the front and lower layers, and the middle layer and / or the back layer may be used as a colored layer.

  The sulfur dye is a water-insoluble dye containing many sulfur bonds in the dye molecule, and is reduced by a reducing agent (usually sodium sulfide) in the dye bath, adsorbed on the fiber in the form of a leuco dye, and then oxidized. It is a dye that completes dyeing as a water-insoluble dye. Specifically, for example, vulcanized 2,4-dinitrophenol and picric acid, vulcanized diformylmetatoluidine and benzine, vulcanized metatoluylenediamine, 3-amino-7-oxy Examples include vulcanized phenazine, vulcanized 2,4-dinitro-4-oxydiphenylamine, vulcanized indophenol of phenylperic acid, and vulcanized 1,5-dinitronaphthalene. Among them, anionic sulfur dyes are preferred because they have high dyeability on waste paper pulp and less color fading due to water wetting and moisture even if they contain ash. Especially, 2,4-dinitrophenol and picric acid are preferred. A vulcanized product is preferred. Specific examples of the anionic sulfur dye include, for example, “Diresul Black PFT Liquid” (trade name) manufactured by Clariant Japan Co., Ltd., “Kayaku Sulfur” (trade name) series manufactured by Nippon Kayaku Co., Ltd., Mitsui BSF “Mitsui Sulfur” (trade name) series manufactured by Dye Co., Ltd. can be preferably used.

  As will be described later, by combining a sulfur dye with a direct dye or pigment and using an aluminum sulfate compound as a fixing agent, the dyeing property to waste paper pulp is further increased, and a certain amount of ash is contained. However, it is possible to further suppress discoloration due to water wetting and moisture.

  The direct dye is a water-soluble dye having a relatively large molecular weight and affinity for cellulose fibers. Specific examples include anionic direct dyes and cationic direct dyes. Among these, anionic direct dyes are more effective because they have excellent dyeing properties for waste paper pulp and ash components, low dye costs, excellent compatibility with sulfur dyes, and great effects of preventing discoloration due to water wetting and moisture. preferable.

  In particular, the anionic direct dye has good compatibility with the anionic sulfur dye, and it is difficult to cause a problem that only a specific raw material pulp fiber, which is said to be inadvertently aggregated or hair dyed, is darkly colored. By setting the content to 15% by mass or less, problems such as poor coloring derived from ash, elution of dye due to inadvertent wetting, and color fading due to rubbing are further improved.

  Anionic direct dyes are originally made for the purpose of dyeing cellulose fibers, and are dyed directly onto cellulose fibers from a dye bath containing a neutral salt. From its chemical structure, it is classified into azo, stilbene, thiazole, dioxazine, phthalocyanine and the like. As an anionic direct dye, those composed of an azo dye having an azo group (—N═N—) as a chromophore are generally used, but any anionic direct dye can be used in the present invention. Specific examples of the anionic direct dye include, for example, “Delma Carbon BFN” (trade name) manufactured by Clariant Japan Co., Ltd., “Cartasol Yellow 5GE Liquid” (trade name), and “Kayafect Yellow” manufactured by Nippon Kayaku Co., Ltd. G ”(trade name) and the like.

  Cationic direct dyes are those in which the end of the chromophore is a cationic group, for example, having a sulfone group, a phenolic hydroxyl group, a carboxyl group, etc. in the molecule and having a molecular structure in the form of sodium, potassium or ammonium salts. It is a dye.

  The color pigment is a general term for pigments that are insoluble in water and oil, and includes inorganic pigments, organic pigments, and lake pigments. In the present invention, an inorganic pigment can be suitably used from the viewpoint of excellent fixability to used paper pulp exhibiting a relatively flat property and a pigment test by a regenerating process that is repeated several times. Specific examples of the inorganic pigment include “Grand Black YT-100” (trade name) manufactured by Mikuni Color Co., Ltd.

  In the paperboard of the present invention, the addition amount of the sulfur dye is 0.5 to 6.0% by mass, further 0.6 to 5.5% with respect to the dry mass of the raw material pulp used for forming the colored layer as the surface layer. It is preferable that it is 8 mass%. If the amount of sulfur dye added is less than 0.5% by mass, the colored layer will be light in color and may be difficult to understand even if it is eluted. Conversely, if the addition amount of sulfur dye exceeds 6.0% by mass, it will be difficult to discolor due to the auxiliary effect of sulfur dye as a fixing agent, but it will be caused by water wetting, moisture and rubbing caused by increasing the addition amount. There is no difference in the effect of suppressing discoloration, resulting in excessive quality, which may increase manufacturing costs.

  Moreover, the addition amount (total addition amount in the case of combined use) of a direct dye and / or a color pigment is 0.3 to 8.4 mass with respect to the dry mass of the raw material pulp used for forming the colored layer as the surface layer. %, More preferably 0.5 to 8.0% by mass. If the added amount of the direct dye and / or color pigment is less than 0.3% by mass, the colored layer becomes a light color and may be difficult to understand even if it is eluted. Conversely, when the amount of the direct dye and / or color pigment exceeds 8.4% by mass, the direct dye and / or color pigment is excessive with respect to the raw material pulp, so the yield does not increase and the dyeing effect is difficult to be exhibited. There is.

  The total addition amount of the sulfur dye and the direct dye and / or the color pigment is 1.0 to 14.0% by mass relative to the dry mass of the raw material pulp used for forming the colored layer as the surface layer. It is preferable that it is 3-12.0 mass%. If the total amount added is less than 1.0% by mass, the colored layer will be light in color and may be difficult to understand even if it is eluted. On the contrary, when the total addition amount exceeds 14.0% by mass, since the sulfur dye and the direct dye and / or color pigment are excessive with respect to the raw material pulp, the yield may not be achieved and the dyeing effect may not be exhibited. By using a combination of sulfur dyes, especially anionic sulfur dyes, and direct dyes, particularly anionic direct dyes and / or color pigments, the raw material pulp is mainly composed of waste paper pulp and the fixability of the dye. Therefore, it is possible to produce a paperboard in which elution of other dyes is suppressed.

  The mass ratio of the sulfur dye and the direct dye and / or color pigment contained in the colored layer is preferably 20/80 to 60/40, more preferably 30/70 to 55/45. When the mass ratio is less than 20/80, the auxiliary effect of the sulfur dye as a fixing agent is lowered, and the dye may be easily discolored due to water wetting, moisture, or rubbing. On the other hand, if the mass ratio exceeds 60/40, it is difficult for the color fading to occur due to the auxiliary effect of the sulfur dye, but the color caused by water wetting, moisture, or rubbing due to the increased ratio of the sulfur dye. There is no difference in the effect of suppressing the drop, resulting in excessive quality, which may increase the manufacturing cost.

  In addition, when both a direct dye and a color pigment are contained in a colored layer, it is preferable that both mass ratio is about 3 / 97-25 / 75.

  Moreover, in the raw material pulp of the middle layer and the back layer, which are the front and lower layers, when a sulfur dye and a direct dye and / or color pigment are blended, and the middle layer and / or the back layer are also colored layers, the addition amount of the sulfur dye, Addition amount of direct dye and / or color pigment (total addition amount in the case of combined use), total addition amount of sulfur dye and direct dye and / or color pigment, mass ratio of sulfur dye and direct dye and / or color pigment The mass ratio of the direct dye and the color pigment may be adjusted so as to be almost the same as in the case of the surface layer.

  Although the detailed reason has not been elucidated, for example, a sulfur dye is directly added to the pulp slurry in the coloring tank, and an aluminum sulfate-based compound is added in a state where the pulp slurry and the sulfide dye are in harmony, so that the pH becomes 4 After adjusting to 0.0 to 8.0, further 4.2 to 6.8, a direct dye and / or color pigment is added, and an aluminum sulfate compound is added to adjust the pH to 3.8 to 5.4. Furthermore, by adjusting to 4.0 to 5.0, the sulfur dye is fixed to waste paper pulp and ash and not dissolved in water, and the effect of assisting the fixing of the direct dye and color pigment used together, The effect of having affinity for cellulose fibers, which is a characteristic, brings about a synergistic effect, and sulfur dyes, direct dyes and color pigments and the binding of cellulose fibers become stable, so that colors due to water wetting, moisture, and rubbing The question of falling There is believed to be resolved.

  In addition to direct dyes and color pigments, it has been studied to use, for example, basic dyes, acid dyes, disperse dyes, leuco dyes and the like in combination with sulfur dyes. However, when these basic dyes, acid dyes, disperse dyes, leuco dyes, etc. are used, the dye and cellulose, especially the waste paper pulp and the ash content are dyed due to the binding power and the solubility in water. It is not possible to obtain a paperboard with a texture of a pulp material, which is poor in wearing property and has a sufficiently small surface stain (color loss) due to water wetting, moisture and rubbing. Furthermore, since the dye is eluted, a complicated work such as adding a large amount of sulfuric acid band, polymer flocculant, etc. is required. There is also a problem that the drainage load becomes high. Therefore, in the present invention, a direct dye and / or a color pigment is added to a raw material pulp mainly composed of waste paper pulp together with a sulfur dye.

  The colored layer containing a sulfur dye and a direct dye and / or color pigment in the present invention was measured according to the method described in JIS P 8251 “Paper, paperboard and pulp-ash content test method—525 ° C. combustion method”. The ash content is preferably 15% by mass or less, and more preferably 6% by mass or less.

  In addition, when producing a paperboard having a surface layer composed of a colored layer by blending a sulfur dye and a direct dye and / or a color pigment into a raw material pulp mainly composed of waste paper pulp, Due to the high anionic effect exhibited by ash, the colorability of sulfur dyes, direct dyes and color pigments to the raw pulp is likely to be hindered. For this reason, in the present invention, the ash content in the colored layer is preferably 15% by mass or less, and more preferably 6% by mass or less, including the ash content derived from waste paper pulp. Furthermore, by adjusting the hot water extraction pH of the colored layer to 4.0 to 6.5, even if calcium carbonate contained in the ash derived from waste paper elutes to some extent (even if it is converted to calcium hydroxide) ), Surface paper (color fading) due to water wetting, moisture, and rubbing is low, and a paperboard suitable for, for example, bag making or cardboard case with a texture of pulp material can be easily produced at low cost. .

  The ash contained in the raw material pulp has the effect of increasing the opacity and making the paper difficult to stretch in the addition of the sulfur dye and the direct dye and / or color pigment, but the ash content exceeds 15% by mass. Since the main components of ash are potassium, calcium, and magnesium, the alkalinity becomes strong, making it difficult to fix the dye and obtain the desired paper quality. Then, in order to adjust the hot water extraction pH of the colored layer to 4.0 to 6.5, a large amount of chemicals such as a sulfuric acid band, a paper strength enhancer, and a polymer flocculant must be added. There is a risk of rising or increasing drainage load. As a result, while the main component is waste paper pulp, the surface contamination (color loss) due to water wetting, moisture, and rubbing is small, and the texture of the pulp material is suitable, for example, for bag making or cardboard cases. Obtaining paperboard can be difficult. On the contrary, if the ash content is low, it is not always good. If the ash content is too low, it leads to an increase in costs such as selection of raw material pulp and enhancement of a selection facility. Therefore, it is preferable to use an old white paper that has a low ash content and does not increase the cost, and the ash content is 4% by mass or more.

  That is, when the amount of ash is used, since the role of sulfur dye as a fixing agent and the effect of reducing drainage load is more fully exhibited, the selection and blending of waste paper is sufficient to reduce the ash sufficiently. For bag-making and corrugated cardboard cases with the texture of the pulp material, with the surface pulp (color fading) due to water wetting, moisture, and rubbing being small, while the main component is waste paper pulp. A suitable paperboard of the present invention can be obtained more easily.

  As described above, a sulfur dye and a direct dye and / or color pigment are blended into a raw material pulp mainly composed of waste paper pulp and dyed to form a colored layer. The raw material pulp has an ash content of 15% by mass or less, and further, a predetermined amount of a sulfur dye, particularly an anionic sulfur dye, and a direct dye and / or a color pigment, particularly an anionic direct dye, in a predetermined ratio. By adding, the paperboard of the present invention can be obtained with less surface contamination (color loss) due to water wetting, moisture, and rubbing.

In the paperboard of the present invention, there is little surface contamination (color loss) due to color difference (dye elution: ΔEa).
). That is, in the paperboard of the present invention, ΔEa can be about 24 or less, and further about 15 or less. On the other hand, in the case of a conventional paperboard that does not have the configuration of the present invention as described later, the dye elution is particularly large when wet, and ΔEa exceeds 28, for example.

In the present specification, ΔEa refers to a value measured as follows. First 5cm x
After immersing a 5 cm square paperboard in water at 20 ° C. for 60 seconds, the paperboard was compliant with the method described in JIS P 8222 (1998) “Pulp-Preparation of handsheets for testing” for 30 seconds with a sheet press. The filter paper (ADVANTEC, Φ18) is sandwiched, dried, and the filter paper in contact with the colored layer surface side of the paperboard is used. The method described in JIS Z 8730 (2002) “Color display method—color difference of object color” is used. The color difference is calculated by a compliant color difference calculation method, and is a difference between the color tone of the filter paper before contact with the paperboard and the color tone of the filter paper after contact with the paperboard, which is obtained based on the following formula. Note that a smaller value of ΔEa indicates that color fading due to water wetting is less likely to occur. The color tone of each filter paper was measured using a colorimetric color difference meter (Nippon Denshoku Industries Co., Ltd., ZE2000).
ΔEa = {(L1-L2) ² (a1-a2) ² (b1-b2) ² } ^1/2
L1, a1, b1: Color tone of filter paper before contact with paperboard L2, a2, b2: Color tone of filter paper after contact with paperboard

Furthermore, as described above, the paperboard of the present invention does not easily fade even when exposed to light, and this can be evaluated by a color difference (light resistance: ΔEb). That is, in the paperboard of the present invention, ΔEb can be 1.7 or less, and further 1.5 or less. On the other hand, in the case of a conventional paperboard not having the configuration of the present invention as described later, ΔEb exceeds 2.0, for example.

In this specification, ΔEb refers to a value measured as follows. JIS B
7751 (2007) UV light arc tester (model: FAL-3D, manufactured by Suga Test Instruments Co., Ltd.) in accordance with “Ultraviolet carbon arc lamp type light resistance tester and weather resistance tester”. 400-500 W / m ² ) is irradiated onto a 6 cm × 4.5 cm or larger paperboard at 45 ° C. for 4 hours, and JIS Z 8730 (2002) “Color display method—color difference of object color” on the colored layer surface side of the paperboard It is calculated by the color difference calculation method based on the method described in 1. and is a difference between the color tone of the colored layer before ultraviolet irradiation and the color tone of the colored layer after ultraviolet irradiation, which is obtained based on the following formula: . In addition, it shows that it is hard to fade by exposure, so that the value of (DELTA) Eb is small. Also,
The color tone of each colored layer was measured using the colorimetric color difference meter.
ΔEb = {(L1-L2) ² (a1-a2) ² (b1-b2) ² } ^1/2
L1, a1, b1: Color tone of the colored layer of the paperboard before UV irradiation L2, a2, b2: Color tone of the colored layer of the paperboard after UV irradiation

  The colored layer of the paperboard of the present invention is formed by adding a sulfur dye and a direct dye and / or a color pigment to a raw material pulp slurry mainly composed of waste paper pulp, and dyeing the raw material pulp. In addition to adjusting the ash content of the colored layer as described above, in addition to adjusting the ash content of the colored layer, for example, a method for adding an aluminum sulfate compound or the hot water extraction pH of the paperboard can be used to obtain a paperboard with less surface contamination (color loss) due to rubbing It is possible to adjust.

  The aluminum sulfate-based compound is not particularly limited as long as it has an effect as a fixing agent for sulfur dyes. For example, a sulfate band is preferably used.

  Here, the method for producing a paperboard of the present invention is characterized in that after a sulfur dye is added to a raw pulp slurry mainly composed of waste paper pulp, a colored layer is formed by directly adding a dye and / or a color pigment. For example, by using the above-mentioned aluminum sulfate-based compound and separately adding a sulfur dye, a direct dye, and a color pigment in the following order, a temperature condition of about 20 ° C. to less than 50 ° C. In the paperboard, the sulfur dye and the direct dye and / or color pigment are more firmly fixed to the raw pulp fiber, and as a result, the paper board is further reduced in surface contamination (color loss) due to water wetting, moisture and rubbing. Can do.

One example of the process for producing the paperboard of the present invention will be described with reference to the schematic flowchart of FIG.
In addition, in FIG. 1, code | symbol AF shows the addition order 1st-6th of each component.

  First, the case where a direct dye and a color pigment are used in combination will be described. After the raw pulp is dissolved by the pulper 1, the cleaned and beaten pulp slurry is transferred to the coloring tank 2. Next, a sulfur dye (A: first addition order) is added to the coloring tank 2, and an aluminum sulfate compound (i) (B: second addition order) is added and stirred. Next, from the dyeing tank 3, the pigment fixing agent (C: addition order third), the color pigment (D: addition order fourth) and the direct dye (E: addition order fifth) are sequentially placed in the coloring tank 2. Add and stir. Further, an aluminum sulfate compound (ii) (F: addition order No. 6) is added to the coloring tank 2 and stirred, and the resulting mixture is transferred to the seed box 4 and then supplied to the paper machine 5 for paper making. .

  When only a direct dye is used without using a color pigment, a sulfur dye and an aluminum sulfate compound (i) are sequentially added to the coloring tank 2 and stirred, and then the dye is directly colored from the dyeing tank 3. The mixture is added to the tank 2 and stirred, and the aluminum sulfate compound (ii) is further added and stirred.

  When only a color pigment is used without using a direct dye, a sulfur dye and an aluminum sulfate compound (i) are sequentially added to the coloring tank 2 and stirred, and then the pigment fixing agent is added from the dyeing tank 3. Then, the color pigment is sequentially added to the coloring tank 2 and stirred, and the aluminum sulfate compound (ii) is further added and stirred.

  In addition, after adding dye etc. to raw material pulp slurry, it is preferable to fully stir, for example for 3 minutes or more. When an anionic sulfur dye is used as the sulfur dye, the pH is preferably adjusted to 4.0 to 8.0 and left for 3 minutes or longer.

  The aluminum sulfate-based compound (i) is suitably blended as a fixing agent for sulfur dyes, and the addition amount thereof is 1.5 to 4.0% by mass relative to the dry mass of the raw material pulp, It is preferable that it is 2.0-3.5 mass%. When the added amount of the aluminum sulfate compound (i) is less than 1.5% by mass, the auxiliary effect of the sulfur dye as a fixing agent is reduced, so that the color of the dye is liable to be lost due to water wetting, moisture, or rubbing. There is a fear. On the contrary, if the amount of aluminum sulfate compound (i) added exceeds 4.0% by mass, discoloration hardly occurs due to the auxiliary effect of the sulfur dye as a fixing agent, but even if the amount added is further increased. There is no difference in suppression of discoloration due to water wetting, moisture, and rubbing, and there is a risk that the manufacturing cost increases due to excessive quality.

  Moreover, it is preferable to adjust so that the addition amount of an aluminum sulfate type compound may become in the said range, when it also considers affecting the pH of the pulp slurry of raw material pulp. That is, when the addition amount is within the above range, a synergistic effect is exhibited both in the role of the aluminum sulfate-based compound as a fixing agent for the sulfur dye and in the dyeability in the pH range described later.

  The pH of the pulp slurry after the addition of the aluminum sulfate compound (i) is preferably 4.0 to 8.0, more preferably 4.2 to 6.8. If the pH of the pulp slurry is less than 4.0, the oxidation reaction is instantaneously carried out, and the reaction does not proceed as a whole, resulting in hair dyeing or fixing failure, and the dye discoloration due to water wetting, moisture, or rubbing. Is likely to occur. Conversely, if the pH of the pulp slurry exceeds 8.0, the oxidizing agent (aluminum sulfate compound) necessary for the oxidation reaction will be insufficient, leaving unreacted parts or poor fixing, resulting in water wetting, moisture, or rubbing. Dye discoloration may occur easily.

  The aluminum sulfate compound (ii) is suitably blended as a fixing agent for sulfur dyes and as a fixing agent for direct dyes, and the amount added is 3.8 to the dry mass of the raw pulp. It is preferably 5.4% by mass, more preferably 4.0-5.0% by mass. When the added amount of the aluminum sulfate compound (ii) is less than 3.8% by mass, the auxiliary effect of the sulfur dye as a fixing agent is lowered, so that the color of the dye is likely to be lost due to water wetting, moisture, or rubbing. There is a fear. Conversely, if the amount of aluminum sulfate compound (ii) added exceeds 5.4% by mass, the acidity is high, so the action of hydrolyzing cellulose molecules with sulfate ions generated from the aluminum sulfate compound is strong, and the pulp fibers are The brittleness tends to cause discoloration due to water wetting, moisture, and rubbing, resulting in a decrease in storability and a cause of ruled cracking. For example, it tends to be unsuitable as a packaging base material for shopping such as handbags.

  The pH of the pulp slurry after adding the aluminum sulfate compound (ii) is preferably 3.8 to 5.4, more preferably 4.0 to 5.0. When the pH of the pulp slurry is less than 3.8, the acidity is high, so that the cellulose molecule is strongly hydrolyzed by sulfate ions generated from the aluminum sulfate compound, and the pulp fiber becomes brittle. They tend to cause discoloration due to rubbing, deteriorate storage stability and cause cracks, and tend to be unsuitable as a packaging base material for shopping such as handbags. Conversely, when the pH of the pulp slurry exceeds 5.4, the oxidant (aluminum sulfate compound) necessary for the direct dye is insufficient, the unreacted portion remains, and the color of the dye is lost due to water wetting, moisture, and rubbing. This is likely to occur, and the auxiliary effect of the sulfur dye as a fixing agent may be reduced.

  In the present invention, for example, the aluminum sulfate compound is used as a main pH adjuster and an oxidizing agent for sulfur dyes, and the method described in JIS P 8133 (1998) “Testing method for pH of paper, paperboard, and pulp-water extract”. By conducting acid papermaking so that the hot water extraction pH of the colored layer measured in accordance with the standard is 4.0 to 6.5, preferably 4.2 to 5.6, surface contamination due to water wetting, moisture, and rubbing ( It is possible to produce a paperboard with less discoloration) and a texture of pulp material.

  When the hot water extraction pH of the colored layer is less than 4.0, the action of hydrolyzing cellulose molecules is strong, and the pulp fibers become brittle, so that color fading is likely to occur due to water wetting, moisture, and rubbing, and the storage stability is poor. It becomes a factor of ruled cracks. On the other hand, when the hot water extraction pH of the colored layer exceeds 6.5, there is a problem that the complementary color, the paper strength enhancer, and the sizing agent are poorly fixed, and the color is easily lost.

  Originally, when using raw pulp containing waste paper pulp, it is common to produce paper with neutral papermaking because the waste paper pulp raw material becomes high pH through the waste paper treatment process. In the invention, the sulfur dye and the direct dye and / or color pigment are fixed to the raw material pulp, and there are few surface stains (color loss) due to water wetting, moisture and rubbing, which are conventional problems, and the texture of the pulp material is provided. In order to provide a paperboard having a multilayer structure, a sulfur dye and a direct dye and / or color pigment are fixed to the raw material pulp, and the hot water extraction pH of the colored layer containing the sulfur dye and the direct dye and / or color pigment is adjusted. For example, it is adjusted to 4.0 to 6.5 with acidic papermaking.

  Examples of the pigment fixing agent added in advance when the color pigment is used include polyamine compounds, polyvinylamine compounds, polyacrylamide compounds, and the like.

  Furthermore, for each layer constituting the paperboard of the present invention, if necessary, for example, a filler, a yield improver, a drainage improver, a paper strength enhancer, a fixing agent, a fluorescent whitening agent, an antifoaming agent, a pitch control agent. Further, a papermaking internal additive such as a slime control agent, a bulking agent, and a foaming agent can be appropriately blended.

  As the paper strength enhancer, any of dry strength enhancer and wet strength enhancer can be used. For example, polyacrylamide (hereinafter referred to as PAM), oxidized starch, modified starch, polyvinyl alcohol (hereinafter PVA). Polyethyleneimine, epichlorohydrin (hereinafter referred to as EP), and the like. In view of the fact that the manufacturing cost does not increase and excessive quality design is used, and especially the surface layer, considering that the main component is waste paper pulp, which tends to decrease the tensile strength and tear strength, a dry paper strength enhancer And the wet paper strength enhancer in a ratio (mass ratio) of 0.4: 0.6 to 0.6: 0.4, and the combined amount of both is based on the dry mass of the raw pulp It is preferable to adjust so that it may become 0.05-0.8 mass% by solid content.

In the present invention, the water absorption is adjusted by internally adding a known sizing agent such as an acidic rosin sizing agent, a neutral rosin sizing agent, and an alkyl ketene dimer. JIS P 8140 (1998) “Paper and paperboard—water absorption The cobb sizing degree of the paperboard measured in accordance with the method described in “Degree Test Method—Cobb Method” is preferably 15 to 80 g / m ² , more preferably ²⁰ to 50 g / m ² at a contact time of 120 seconds.

  In particular, for example, acid papermaking, adjusting the hot water extraction pH of the paperboard to 4.0 to 6.5, securing sticking at the time of drying on the surface of the Yankee dryer described later, and appropriate releasability, Among the sizing agents, it is preferable to use an acidic rosin sizing agent, and it is preferable to blend in the range of 0.5 to 1.0% by mass in terms of solid content with respect to the dry mass of the raw material pulp.

  The paper machine for producing the paperboard of the present invention is not particularly limited. For example, a long net paper machine, a twin wire paper machine, a circular net paper machine, a short net combination machine, a Yankee paper machine, an inclined machine Various known paper machines in this field such as a wire paper machine can be used as appropriate. The paperboard of the present invention prepares a raw material pulp slurry for a colored layer (surface layer) and a sulfur dye, a direct dye and a color pigment, and prepares a raw material pulp slurry according to each remaining layer, and in the paper machine It can manufacture by making the raw material pulp slurry of each layer, and press-drying the colored layer surface with a dryer.

  In the board manufacturing method of the present invention, the wet paper moisture immediately after the pressing step is 55 to 65 measured in accordance with the method described in JIS P 8127 “Paper and paperboard—Moisture test method—Method using a dryer”. It is preferable to adjust so that it may become%. If the moisture content of the wet paper is less than 55%, curling may occur after drying in a subsequent dryer process. If the moisture content of the wet paper exceeds 65%, color transfer may occur due to insufficient drying when winding the paper with a reel.

  A suitable drying method in the present invention is, for example, a so-called Yankee type dryer having a cylindrical surface of 3 m or more with a mirror finish on the side of the colored layer of the wet paper immediately after the pressing step, and preferably 180 to 270 ° on the dryer surface. It is a method in which contact is made at a hugging angle, and a method in which an after dryer is provided for drying, and when obtaining the paperboard of the present invention having a colored layer mainly composed of waste paper pulp having high hydrophilicity, the conventional problems are efficiently Can be solved. When the drip angle of the dryer is less than 180 °, it is difficult to press and dry the paperboard, and when the paper is wound with a reel, the paper may be transferred due to insufficient drying. On the other hand, when the holding angle of the dryer exceeds 270 °, curling may occur on the paperboard after drying due to the problem of the time and angle of the pressure drying process.

The basis weight of the paperboard of the present invention thus obtained is not particularly limited, but is measured according to the method described in JIS P 8124 (1998) “Paper and paperboard—basis weight measurement method”. ~320g / m ^2, preferably further is 120~280g / m ^2. When the basis weight is less than 80 g / m ² , it may be difficult to ensure the strength when the paperboard of the present invention is processed into, for example, a corrugated cardboard. On the other hand, if the basis weight exceeds 320 g / m ² , not only will the quality be excessive, but the production cost may increase.

  As described above, the case where the paperboard of the present invention is constituted by three paper layers of the surface layer, the middle layer and the back layer composed of the colored layer has been described, but the number of paper layers constituting the paperboard of the present invention is not particularly limited. Instead, the middle layer may be composed of two or more layers, and may be composed of four or more paper layers.

  Next, the paperboard and the method for producing the same according to the present invention will be described in more detail based on the following examples, but the present invention is not limited to these examples.

Examples 1-22 and Comparative Examples 1-3 (Manufacture of paperboard)
<Surface>
Using the raw material pulp of the surface layer shown in Table 1, the CSF was adjusted to 300 mL by DDR to prepare a raw material pulp slurry. To this raw pulp slurry, dye a, sulfuric acid band a, pigment fixing agent (Examples 2 to 5), color pigment (Examples 2 to 5), and dye b (Example 2 and Comparative Example) shown in Tables 2 to 5 1 and 3) and sulfuric acid band b (other than Comparative Examples 1 and 3) were sequentially added and stirred. The temperature in the reaction system when the dye a and the dye b were added was 45 ° C. Furthermore, an acidic rosin sizing agent was added in an amount of 0.75% by mass with respect to the dry mass of the raw material pulp, and a paper strength enhancer shown in Table 6 was added in an amount (solid content) with respect to the dry mass of the raw material pulp. A raw material pulp slurry for the surface layer was prepared.

  In Tables 2 to 5, for the dye a, the sulfuric acid band a, the color pigment, the dye b, and the sulfuric acid band b, each addition amount (relative to the dry mass of the raw pulp) and addition place, the sulfuric acid band a and sulfuric acid The pH of the reaction system after addition of band b (in Comparative Examples 1 and 3, the pH of the reaction system left after addition of sulfuric acid band a) is also shown. Table 6 shows the total addition amount of the dye and the pigment and the mass ratio of the dye a and the dye b and / or the color pigment.

<Middle layer>
Using 100% by weight of the coated cut-off waste paper pulp as the raw material pulp, the raw material pulp slurry was prepared by adjusting the CSF to 350 mL by DDR. In this raw material pulp slurry, the sulfuric acid band (fixing agent) is 0.4% by mass, the acidic rosin sizing agent (sizing agent) is 0.75% by mass and PAM (paper strength) with respect to the dry mass of the raw material pulp. 0.08% by mass of the reinforcing agent) was added as a solid content to prepare a raw material pulp slurry for the middle layer.

<Back layer>
Using 100% by mass of old paper pulp as a raw material pulp, the CSF was adjusted to 400 mL by DDR to prepare a raw material pulp slurry. In this raw material pulp slurry, the sulfuric acid band (fixing agent) is 0.4% by mass, the acidic rosin sizing agent (sizing agent) is 0.75% by mass and PAM (paper strength) with respect to the dry mass of the raw material pulp. (Enhancer) was added in a solid content of 0.08% by mass to prepare a raw material pulp slurry for the back layer.

The dyes, pigments, sulfuric acid bands, pigment fixing agents, acidic rosin sizing agents, and paper strength enhancers used in preparing the raw material pulp slurry for each layer are as follows.
・ Anionic sulfur dye (manufactured by Clariant Japan Co., Ltd.)
(Product name: Diresul Black PFT Liquid)
・ Anionic direct dye (Clariant Japan Co., Ltd., trade name: Derma Carbon BFN)
Cationic direct dye (Clariant Japan Co., Ltd., trade name: Cartazole Turkis K-RL)
・ Color pigment (made by Mikuni Color Co., Ltd., trade name: Grand Black YT100)
・ Sulfate band (manufactured by Taki Chemical Co., Ltd., concentration: 5%)
・ Fixing agent for pigment (manufactured by Clariant Japan Co., Ltd., trade name: Cartaletin F Liquid)
・ Acid rosin sizing agent (manufactured by Modern Chemical Industry Co., Ltd., trade name: R50)
-Paper strength enhancer (PAM, Harima Kasei Co., Ltd., trade name: Hermide RB32)
-Paper strength enhancer (EP, manufactured by Seiko PMC Co., Ltd., trade name: WS2042)

Yankee dryer using mirror surface finish using these surface layer, middle layer and back layer raw material pulp slurries, using a circular net five-layer paper machine, combining the surface layer, three layer middle layer and back layer raw material pulp slurries. The surface of the colored layer was pressure-dried to obtain a paper board having a five-layer structure having a basis weight of about 170 g / m ² and a surface layer composed of the colored layer.

  The physical properties and characteristics of the obtained paperboard were measured by the following methods, respectively. These results are shown in Table 7.

  The physical properties and characteristics of the commercially available paperboard (5-layer structure: surface layer / middle layer 3 layer / back layer, raw material pulp: all layers LBKP 100% by mass, colored layer: only anionic direct dye) were also measured. The results are shown in Table 7 as Comparative Example 4.

(A) Hot water extraction pH
After immersing the paperboard in distilled water and thoroughly infiltrating the distilled water into the paper, the surface layer is peeled off and dried with a drum-type sheet dryer, and JIS P 8133 “Paper, paperboard and pulp-water extract pH test The measurement was performed according to the method described in “Method”.

(B) Ash content After immersing the paperboard in distilled water and thoroughly infiltrating the distilled water to the inside of the paper, the surface layer was peeled off and dried with a drum-type sheet dryer. JIS P 8251 “Paper, paperboard and pulp-ash content test It measured based on the method as described in "Method-525 degreeC combustion method."

(C) Color difference (a) Dye elution: ΔEa
After immersing a 5 cm × 5 cm square paperboard in water at 20 ° C. for 30 seconds, it was compliant with the method described in JIS P 8222 (1998) “Pulp-Preparation of handsheets for testing” and 30 seconds by sheet press. The paperboard is sandwiched with filter paper (ADVANTEC, Φ18), dried, and the color tone of the filter paper in contact with the colored layer surface side of the paperboard is measured using a colorimetric color difference meter (Nippon Denshoku Industries Co., Ltd., ZE2000). It was measured. From the measured values obtained and the color tone of the filter paper before contact with the paperboard measured in advance, the color difference in accordance with the method described in JIS Z 8730 (2002) “Color display method—color difference of object color” The calculation method was based on the following formula.
ΔEa = {(L1-L2) ² (a1-a2) ² (b1-b2) ² } ^1/2
L1, a1, b1: Color tone of filter paper before contact with paperboard L2, a2, b2: Color tone of filter paper after contact with paperboard

(A) Light resistance: ΔEb
In the light resistance tester (Suga Test Instruments Co., Ltd., model: FAL-3D) in accordance with JIS B 7751 (2007) “Ultraviolet carbon arc lamp type light resistance tester and weather resistance tester”, ultraviolet carbon An arc (400 to 500 W / m ² ) was irradiated onto a paper board of 6 cm × 4.5 cm or more at 45 ° C. for 4 hours, and the color tone of the colored layer surface side of the paper board was measured using the colorimetric color difference meter. From the obtained measured value and the color tone of the colored layer before ultraviolet irradiation which had been measured in advance, the color difference was calculated based on the following formula using the color difference calculation method based on JIS Z 8730 (2002).
ΔEb = {(L1-L2) ² (a1-a2) ² (b1-b2) ² } ^1/2
L1, a1, b1: Color tone of the colored layer of the paperboard before UV irradiation L2, a2, b2: Color tone of the colored layer of the paperboard after UV irradiation

(D) Manufacturing cost About the cost reduction at the time of manufacturing a paperboard, it evaluated in four steps of (double-circle), (circle), (triangle | delta), and x in the order which could be reduced.

Each of the paperboards of Examples 1 to 22 has a colored layer (surface layer) in which a raw material pulp mainly composed of waste paper pulp is blended with a specific dye called a sulfur dye and a direct dye and / or a color pigment. The pH is adjusted to a range of 4.0 to 6.5, and the ash content is also adjusted to 15% by mass or less. Therefore, in all the paperboards of Examples 1 to 22, dye elution (ΔEa) is as small as about 24 or less, surface contamination (color loss) is small, and light resistance (ΔEb) is as small as 1.7 or less. It is difficult to fade even after exposure.

On the other hand, all of the paperboards of Comparative Examples 1 to 4 are blended only with the direct dye or once with the raw paper pulp or the raw material pulp mainly composed of LBKP and blended with the sulfur dye. It has the colored layer (surface layer) obtained without doing. Therefore, in all the paperboards of Comparative Examples 1 to 4, dye elution (ΔEa) is larger than 28, surface contamination (color loss) is remarkable, and light resistance (ΔEb) is larger than 2.0, Fading easily due to exposure to light.

  The paperboard of the present invention is suitable as a paperboard for bag making, cardboard case, and the like. In addition, the manufacturing method of the present invention can greatly reduce the cost for manufacturing such paperboard.

1 Pulper 2 Coloring tank 3 Dyeing tank 4 Seed box 5 Paper machine

Claims (4)

A paperboard having a multilayer structure of two or more layers,
At least a surface layer composed of a colored layer containing a sulfur dye and a direct dye and / or color pigment;
The colored layer is made of raw pulp mainly composed of waste paper pulp,
A hot paper extraction pH of the colored layer measured in accordance with JIS P 8133 (1998) is 4.0 to 6.5.   The paperboard of Claim 1 whose ash content of the colored layer measured based on JISP8251 is 15 mass% or less.   The paperboard according to claim 1 or 2, wherein at least an aluminum sulfate-based compound is blended as a fixing agent for the sulfur dye, and is made by acidic papermaking. A method for producing a paperboard having a multilayer structure of two or more layers,
2. The paperboard production according to claim 1, wherein a colored layer is formed by adding a dye and / or a color pigment directly after adding a sulfur dye to a slurry of raw material pulp mainly composed of waste paper pulp. Method.

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