JP5251364B2 - Coated white paperboard - Google Patents

Description

  The present invention relates to a coated white paperboard having less unevenness of white density on the coated surface and excellent printability such as printing smoothness and ink fillability.

  White paperboard is usually a cardboard having a multilayer base paper structure of two or more layers, and is often used in various packaging boxes. Packaging boxes are required to have aesthetics and printability for the purpose of enhancing the advertising effect of products, and many coated white paperboards with at least two coating layers provided on one side to provide printability are used. . Coated white paperboard is roughly divided into two grades, coated white balls and special whiteboard paper, due to the difference in the base paper composition and the coating layer formation state, and coating with a printability and white appearance only on one side of the base paper White paperboard is “coated white ball”, and the coated whiteboard paper with a white appearance is printed on both sides of the front and back of the base paper (the front side is two coating layers and the back side is one coating layer). It is subdivided into “paper”.

  The base paper portion of these coated white paperboards is generally formed in a multilayer structure consisting of 4 to 5 layers. For example, coated white cardboard is formed from the front side in a four-layer configuration of surface layer, front layer, lower layer, middle layer, and back layer, while special white paperboard is 5 layers of surface layer, front layer, lower layer, middle layer, back layer, back layer. It is formed with a layer structure. The used pulp of each layer is often used from the viewpoint of environmental and packaging costs. For coated white balls, it is used for the middle and back layers of base paper, and for special white board, it is used for middle layer pulp. , Low whiteness waste paper pulp that has not been deinked is used. In order to conceal the color of this low-white pulp layer, high-white virgin pulp or deinked pulp is used for the surface pulp in the coated white ball, and in the special white paperboard, the surface and back layer pulp. Yes. However, due to economic problems, an intermediate white layer (front and lower layers, special white paperboard has a front layer and a back layer) between a high white pulp layer (surface layer and special white paperboard also has a back layer) and a low white pulp layer (middle layer). It is usual to provide a lower layer) to conceal the color of the low white middle layer. However, even if these pulp layers are provided, it is not sufficient to conceal the color of the low-white middle layer, and the intermediate layer and middle layer colors (gray) are projected on the surface of the base paper through the surface layer. Color difference unevenness (white light / dark unevenness) occurs in a minute area.

  Usually, the coating layer portion of the coated white paperboard is formed from two coating layers (undercoating and overcoating) on the surface, and various coating methods such as an air knife, a blade and a rod are used.

  Air knife coating is the most effective coating method for concealing unevenness of the white density of the base paper because it can form a contoured coating layer that follows the unevenness of the base paper surface. However, since the surface after coating is a surface along the unevenness of the base paper, the smoothness is low, and because it is a method of scraping off the applied paint by blowing air, it has the disadvantage that the paint concentration must be set low. ing.

  On the other hand, the blade coating method has a feature that the smoothness after coating is quite high because it forms a flat coated surface regardless of the unevenness of the surface of the base paper, but conversely, it conceals the white density unevenness of the base paper. Due to coating unevenness due to unevenness of the base paper, uneven ink absorption occurs during offset printing, and in addition to concerns about print quality such as uneven ink deposition, foreign matter contained in the paint or the surface of the base paper is caught on the blade edge during operation. There is a drawback that streak defects, which are coating defects in the shape, tend to occur.

  Recently, the smoothness of the surface is often emphasized, and the blade coating method is often selected as the top coating method. However, in the blade coating method, a smooth surface is formed, but on the other hand, the coverage of the surface of the base paper is inferior, and it is difficult to sufficiently hide the white density unevenness projected on the surface of the base paper. In order to solve this problem, a method of using titanium oxide (see Patent Document 1) or a method of increasing the coating amount has been performed for the purpose of increasing the opacity of the coating layer. However, titanium oxide is expensive, and there is concern about the effect of smoothness and printing quality deterioration, so the amount of titanium oxide is practically limited. In addition, regarding the increase in the coating amount, in the case of an increase in the coating layer of the top coating blade, it is not economical because it is a coating layer that uses a large amount of high white and expensive pigments, and it is uneven due to the flattening coating. In order to form a simple coating layer, there is a problem in that new white density unevenness occurs due to the difference in whiteness between the coating layers (for example, the undercoat layer and the top coat layer).

  There is also a method of increasing the coating amount by adding a special pigment such as structured kaolin, calcined kaolin, or satin white to increase the high shear viscosity of the paint (see Patent Documents 2 and 3). However, since these coating pigments are expensive, it is not economically desirable to use them only for the purpose of increasing the high shear viscosity.

  Furthermore, as another method for increasing the coating amount of the coating layer, several methods for incorporating a polyacrylamide copolymer in the coating layer in a single-layer coated printing paper have been proposed. (Patent Documents 4, 5, and 6). These proposals are not practical because, when the applied paint is scraped off, the paint scatters and adheres to the paper surface, resulting in a defect and an operational defect. As described above, in coated white paperboard, it is difficult to achieve both whiteness unevenness and high quality, such as high smoothness and printability, and economical efficiency.

JP-A-6-166991 JP 2002-363877 A JP 2006-328570 A JP-A-53-94609 JP 2003-105689 A JP 2006-70268 A

  In the case of using a rod coating method, which is effective in reducing the generation of defects such as streaks due to wrinkles when coating on base paper and reducing the energy load during coating drying in coated white paperboard Because it is necessary to force the rigid rod into the profile control in the width direction, the control range of the coating amount is extremely narrow and the coating adhesion amount is also compared to other coating methods Coated white paperboard with excellent printability such as printing smoothness and ink fillability with less unevenness in white density on the surface of coated white paperboard. It is to provide.

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention applied a specific weight average to the coating liquid when the coating liquid mainly composed of a pigment and an adhesive is applied by rod coating. It has been found that this problem can be solved by containing an anionic polyacrylamide copolymer having a molecular weight. That is, in a coated white paperboard on which two or more layers of a coating liquid mainly composed of a pigment and an adhesive are coated, an anionic polyacrylamide-based copolymer having a weight average molecular weight of 4 to 20 million in the coating liquid And a B-type viscosity at 60 rpm of the coating solution of 1000 mPa · s or more and 3000 mPa · s or less, and a layer that contacts the base paper by a rod coating method. It is a coated white paperboard characterized by coating one or more layers .

It is preferred before Symbol anionic polyacrylamide copolymer is a polymer obtained from the water-in-oil polymerization method.

The anionic polyacrylamide copolymer is preferably a polymer obtained by a dispersion polymerization method.

The content of the anionic polyacrylamide copolymer is preferably 0.001 to 0.5% by mass with respect to the total solid content of the pigment in the coating liquid.

  It is preferable that the outermost coating layer to be applied in two or more layers is applied by a blade coating method.

It is preferable that the rice paper weight range of the coated white paperboard is 150 g / m ² or more and 700 g / m ² or less.

  According to the present invention, it is possible to obtain a coated white paperboard with less unevenness of white density on the surface of the coated white paperboard and excellent printability such as printing smoothness and ink fillability, and a rod coating method. Specific advantages such as prevention of occurrence of coating defects inherently and ease of operation adjustment have been added, and it has become possible to improve both productivity and quality of coated white paperboard.

The present invention significantly improves the smoothness in rod coating by adding an anionic polyacrylamide copolymer to the coating solution, and further applies blade coating when overcoating after undercoating with rod coating. When the coating method is used, the surface of the base paper cannot be sufficiently coated with the undercoating so far, and a uniform coating layer cannot be obtained. The uniform coating layer can be formed, and a highly smooth printing surface specific to the blade can be formed, thereby achieving the desired object of the present invention.

Here, by containing an anionic polyacrylamide copolymer having a weight average molecular weight of 4,000,000 to 20,000,000, the coating amount adherence is sufficient to satisfy the smoothness of the coating surface, the blank paper feeling and the printability. The reason for improvement can be estimated as follows. That is, the addition of 4 million to 20 million anionic polyacrylamide-based copolymers develops the function of improving the viscoelastic properties, particularly the elastic properties of the coating liquid due to its ultrahigh molecular weight effect, and the rod being coated It is presumed that the pressure pushed by the rod is reduced by the previous paint elastic resistance directly under the rod, and the amount of paint passing directly under the rod is increased, resulting in an increase in the coating amount. As a result of the increase in the coating amount, the coating of the base paper is improved, the smoothness and gloss of the coated white paperboard surface are improved, and the unevenness of white / darkness is improved. It is possible to prevent printing defects such as poor printing properties and poor printing smoothness. In addition, the same thickening effect can be obtained by increasing the addition amount even in a normal alkali thickening type fluidity modifier, but it also causes an extreme viscosity increase (low B-type viscosity of 3000 mPa · s) at low shear. Therefore, in addition to poor paint preparation and poor liquid feeding, its viscoelastic modification action is relatively weak chemical bonding such as adsorption and association, so the bonding effect at high shear is small and the coating amount increasing effect is also limited Is.

In general, anionic polyacrylamide copolymers exhibit high viscosity when dissolved in water when the weight average molecular weight exceeds 4 million. It causes operational troubles such as defects and poor liquid delivery. However, polymer polymers obtained by water-in-oil polymerization or dispersion polymerization are preferred because they can increase the high shear viscosity while keeping the B-type viscosity relatively low.

The weight average molecular weight of the anionic polyacrylamide copolymer used in the present invention is 4 million to 20 million, more preferably 9 million to 17 million. Incidentally, if the weight average molecular weight is less than 4,000,000, the viscoelasticity at the time of high shear is inferior, and thus the coating amount cannot be increased. On the other hand, if it exceeds 20 million, the viscosity at the time of low shear becomes extremely high (B-type viscosity of 3000 mPa · s or more), and there is a possibility that the coating itself cannot be performed, and the weight average molecular weight is 20 million. In addition, the weight average molecular weight said here is the value of polystyrene conversion measured by gel permeation chromatography (GPC method).

The content of the anionic polyacrylamide copolymer is preferably 0.001 to 0.5% by mass with respect to the total solid content of the pigment from the viewpoint of the viscosity of the coating liquid and coating suitability, Preferably it is 0.005-0.2 mass%. As physical properties of the coating liquid at this time, the high shear viscosity of the coating liquid at a shear rate of 9 × 10 ⁴ sec ⁻¹ is preferably 35 mPa · s or more, and more preferably 40 mPa · s or more. In addition, the upper limit of the high shear viscosity of the coating liquid is preferably 110 mPa · s or less, and more preferably 100 mPa · s or less. When the high shear viscosity is less than 35 mPa · s, it is difficult to obtain the effect of increasing the coating amount of the present invention, and when it exceeds 110 mPa · s, coating defects (rod wrinkles) are generated, which is not preferable. As the B-type viscosity, the viscosity at 60 rpm is 1000 mPa · s or more and 3000 mPa · s or less, and more preferably 1200 mPa · s or more and 2500 mPa · s or less. When the pressure is less than 1000 mPa · s, as described above, paint flaws are generated when the applied paint is scraped off with a blade or a rod, and stains on the coated surface and coating defects are likely to occur. On the other hand, if it exceeds 3000 mPa · s, it is not preferable because of poor stirring during coating preparation and poor circulation of the coating.

  The solid content concentration of the coating liquid of the present invention is preferably a solid content concentration in which the high shear viscosity of the coating liquid falls within the above-described viscosity range, but it is possible to reduce drying addition or coating in the drying step after coating. From the viewpoint of improving the production efficiency of coated white paperboard by increasing the speed, the solid content concentration is preferably as high as possible, and is preferably 50 to 70% by weight. The high shear viscosity (mPa · s) referred to here is a viscosity measured using a Hercules high shear viscometer (manufactured by Kumagai Riki Kogyo Co., Ltd.) under the conditions of Bob F and a rotational speed of 4400 rpm.

The coating amount of the coated white paperboard produced in the present invention is adjusted to 15 to 30 g / m ² per side as a total dry weight on the base paper layer by multiple coatings. In addition, when it is less than 15 g / m ^2, it becomes easy to cause white unevenness and poor printability due to insufficient coverage of the base paper. Moreover, when it exceeds 30 g / m < ² >, it will become easy to cause workability, especially ruled line cracking. Usually, the coated white paperboard is often formed from two coating layers. At this time, the coating amount of the first undercoat layer to be applied in contact with the base paper is 8.5 to 15 g / m. ^2. The coating amount of the top coat layer to be applied second is preferably adjusted to 6.5 to 15 g / m ² from the viewpoint of cost and quality.

As the base paper used in the coated white paperboard of the present invention, it is preferable to use a base paper of two or more layers as the base paper of the coated white paperboard, and the base weight of the base paper is 150 g / m ² or more, 700 g / m A range of m ² is preferred.

  As a raw material of the above-mentioned base paper, general coated white paperboard, or various pulps used for coated white paperboard can be used, although not particularly limited, for example, bleached or unbleached chemical pulp, One kind or two or more kinds of mechanical pulp, deinked or undeinked waste paper pulp, and the like are appropriately mixed and used. By the way, as coated white paperboard using base paper of multi-layer paper, generally bleached chemical pulp or deinked wastepaper pulp or mixed pulp is used on the surface layer, medium deinked wastepaper pulp on the lower surface layer, and middle and backside layers. Often uses undeinked waste paper pulp to make up the base paper. In addition, a sizing agent, a paper strength agent, a chemical stabilizer, a filtering agent, a bulking agent, a filler, a dye, and the like can be appropriately added to the above-described raw materials as necessary.

  The base paper making method is not particularly limited. For example, a round net paper machine, a short net paper machine, a round short net or long net short combination paper machine, a long net paper machine Commercial paper machines such as long web multilayer paper machines and twin-wire paper machines can be used, and general coated paperboards and various papermaking methods that are base paper manufacturing methods for coated paperboards. Can be appropriately selected and used.

  In addition, for the above-mentioned base paper, there is no problem even if the paper base paper is directly subjected to the coating of the pigment coating layer, but before applying the pigment coating layer, a machine calendar, a soft calendar, or a Yankee dryer Etc. can be used to smooth the base paper in advance.

  Furthermore, before the pigment coating layer is applied, the surface of the base paper can be subjected to surface treatment using a transfer roll coater such as a two roll size press, a gate roll size press coater, a metal ring size press coater, or a shim sizer. The surface treatment agent used for the surface treatment such as size press is not particularly limited, but various starches generally used for improving or improving paper strength, coating suitability, printing suitability, etc., polyvinyl Coating liquids mainly composed of alcohols, polyacrylamides, various surface sizing agents and the like can be used. For the coating solution, various pigments commonly used for coating include heavy calcium carbonate, light calcium carbonate, talc, clay, kaolin, titanium dioxide, synthetic silica, aluminum hydroxide, and the like. One or more inorganic pigments and synthetic polymer fine particles such as polystyrene resin and urea formaldehyde resin can be blended as necessary.

  In the present invention, as the coating method, a known coating method such as rod coating and other air knives, bars, curtains and sprays may be combined, but the outermost coating layer is preferably a blade coating method. . For example, in the case of two layers, it is particularly preferable to use a rod coating method for the undercoat layer and a blade coating method for the overcoat layer.

  The coating liquid used when producing the coated white paperboard of the present invention is a pigment coating liquid mainly composed of a pigment and an adhesive. As the pigment used in the pigment coating liquid, pigments generally used for paper processing can be used, and are not particularly limited. For example, heavy calcium carbonate, light calcium carbonate, clay, calcined kaolin , Engineered kaolin, deramikaolin, talc, calcium sulfate, barium sulfate, calcium silicate, aluminum hydroxide, titanium dioxide, zinc oxide, alumina, magnesium carbonate, magnesium oxide, silica, alumina magnesium silicate, calcium silicate, bentonite, zeolite, Known and publicly used in the general coated paper manufacturing field, including inorganic pigments such as sericite, smectite, satin white, and talc, and organic pigments such as solid, hollow, and through-hole type plastic pigments and binder pigments One or more of the above pigments And it can be appropriately selected and used within a range that does not impair the effects of the present invention.

  Further, the adhesive is not particularly limited and can be appropriately used for each coating layer as long as the adhesive used in the field of manufacturing general coated paper does not impair the effects of the present invention. For example, various starches such as oxidized starch, esterified starch, cold water soluble starch, cationic starch, enzyme-modified starch, proteins such as casein, soy protein, synthetic protein, cellulose derivatives such as carboxymethylcellulose and methylcellulose, polyvinyl alcohol and the like Modified products, etc., and dispersion type adhesives such as styrene-butadiene copolymer, conjugated diene polymer latex such as methyl methacrylate-butadiene copolymer, acrylic polymer latex, ethylene-vinyl acetate copolymer, etc. A vinyl polymer latex or the like can be used, and one or more of these can be appropriately selected and used as necessary.

  For the pigment coating solution, if necessary, a pH adjuster such as a dispersant, caustic soda, aqueous ammonia, an antifoaming agent, an antiseptic, a fluorescent dye, a release agent, a dye, a water-resistant agent, Various auxiliary agents such as coloring pigments, antioxidants, anti-aging agents, ultraviolet absorbers and water repellents can be added as appropriate.

  In addition, before applying a plurality of coating layers, or after applying and drying an undercoat layer that is an intermediate stage, and / or after applying and drying the uppermost layer of a final stage that provides a plurality of coating layers, It is desirable to apply a smoothing treatment to the surface of the coating layer using a render, soft calender, super calender or the like.

  The present invention will be specifically described below with reference to examples. Of course, the present invention is not limited to these examples. In addition, unless otherwise indicated, the part in an example and% show a mass part and mass%, respectively.

Example 1
(Preparation of coating solution for undercoat coating layer)
As a pigment, 100 parts of heavy calcium carbonate (trade name: FMT90, manufactured by Phimatech Co., Ltd.) is added, and as a dispersant, 0.2 part of sodium polyacrylate is added to the above-mentioned all pigments. A pigment slurry having a partial concentration of 74% was prepared. To this slurry, 5 parts of oxidized starch (trade name: Ace Y, manufactured by Oji Cornstarch Co., Ltd.) (in terms of solid content) and a styrene-butadiene copolymer latex having a solid concentration of 50% (trade name: X-400B, manufactured by JSR Corporation) 15 parts (in terms of solid content), 0.05 part of an anionic polyacrylamide copolymer obtained by a water-in-oil polymerization method (trade name: Somalex 530, manufactured by Somare, weight average molecular weight 15 million) (active ingredient) Conversion) was added, and water was further added to prepare a paint having a solid concentration of 64%.

(Preparation of coating solution for top coat layer)
As a pigment, kaolin (trade name: UW-90, manufactured by BASF) 65 parts, engineered heavy calcium carbonate (trade name: FMT-OP2A, manufactured by Pimatech) 35 parts, as a dispersant, for all the above pigments Then, 0.2 part of sodium polyacrylate was added, and a pigment slurry having a solid content concentration of 71% was prepared using a Coreless disperser. To this slurry, 1 part of oxidized starch (trade name: Ace Y, supra) (solid content conversion), 15 parts of styrene-butadiene copolymer latex (trade name: L-1825, manufactured by Asahi Kasei Co., Ltd.) with a solid concentration of 50% ( (Solid content conversion) is added, 0.2 parts of caustic soda as pH adjuster, 0.05 parts of alkali thickening fluidity modifier (trade name: Somalex 270K, manufactured by Somar), and water are added. Thus, a paint having a solid content concentration of 65% was prepared.

(Preparation of coated white paperboard)
Deinked waste paper pulp on the front and lower layers (surface layer: pulp whiteness 76%, front lower layer: pulp whiteness 55%), undeinked wastepaper pulp (pulp whiteness 50%) on the middle and back (3-7) layers ) Was used to obtain a base paper of 290 g / m ² which was made into 7 layers.

(Coating of white paperboard undercoat)
The base paper obtained above was coated with a coating solution for an undercoat coating layer by a rod coater and dried at a coating amount of 10 g / m ² to obtain an undercoat layer coated paper.

(Coating of coated white paperboard top coat)
Further, on the undercoat coating layer, the coating solution for the topcoat layer was applied with a blade coater to a dry coating amount of 10 g / m ² and dried to obtain a coated white paperboard of the surface 2-layer type. Next, two stacks of metal rolls having a surface temperature of 150 ° C. and passing through a 2-nip soft calender were used to obtain coated white paperboard having both sides smoothed.

Example 2
In the preparation of the coating liquid for the undercoat layer of Example 1, an anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by a water-in-oil polymerization method was obtained by a water-in-oil polymerization method. A coated white paperboard was obtained in the same manner as in Example 1 except that the obtained anionic polyacrylamide copolymer (trade name: High Holder C503, Kurita Industries, Ltd., weight average molecular weight 5 million) was used. At this time, the dry coating amount of the coating liquid for undercoat coating layer on the rod coater was 9 g / m ² .

Example 3
In the preparation of the undercoat layer coating solution of Example 1, an anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by a water-in-oil polymerization method was obtained by a dispersion polymerization method. A coated white paperboard was obtained in the same manner as in Example 1 except that it was changed to an anionic polyacrylamide copolymer (trade name: Hymolock FA230, manufactured by Hymo Co., Ltd., weight average molecular weight 14 million). At this time, the dry coating amount of the coating liquid for the undercoat coating layer on the rod coater was 10 g / m ² .

Example 4
In the preparation of the coating liquid for the undercoat layer of Example 1, 0.02 part of the anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by the water-in-oil polymerization method was used. A coated white paperboard was obtained in the same manner as in Example 1 except that At this time, the dry coating amount of the coating liquid for undercoat coating layer on the rod coater was 9 g / m ² .

Example 5
In the preparation of the coating solution for the undercoat coating layer of Example 1, the addition amount of the anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by the water-in-oil polymerization method was set to 0. A coated white paperboard was obtained in the same manner as in Example 1 except that the amount was changed to 2 parts. At this time, the dry coating amount of the coating liquid for undercoat coating layer on the rod coater was 11 g / m ² .

Example 6
In the preparation of the coating solution for the undercoat coating layer of Example 1, 0.6 part of an anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by the water-in-oil polymerization method was used. However, the viscosity of the paint was high at the time of preparing the paint and the return of the paint was slightly bad at the time of coating, but the coating could be performed without any problems. At this time, the dry coating amount of the coating liquid for undercoat coating layer on the rod coater was 13 g / m ² .

Reference Example Example 1 In the preparation of a coating solution for an undercoat coating layer, an anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by a water-in-oil polymerization method was not added. Furthermore, an anionic polyacrylamide-based co-polymer obtained by adding 0.05 part of an alkali thickening type fluidity modifier (trade name: Somalex 270K, supra) to the coating solution for the topcoat layer by a water-in-oil polymerization method. A coated white paperboard was obtained in the same manner as in Example 1 except that the polymer was changed to a polymer (trade name: Somalex 530, supra) and coated with a rod coater. At this time, the dry coating amount of the undercoat layer coating solution with the rod coater was 7 g / m ² , and the dry coating amount of the top coat layer coating solution with the rod coater was 9 g / m ² .

Example 7
A coated white paperboard was obtained in the same manner as in Example 1 except that the topcoat layer coating solution of Example 1 was coated with a rod coater. At this time, the dry coating amount of the coating liquid for the topcoat layer on the rod coater was 9 g / m ² .

Comparative Example 1
Except that the anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by the water-in-oil polymerization method was not added in the preparation of the coating solution for the undercoat coating layer of Example 1. Coated white paperboard was obtained in the same manner as in Example 1. At this time, the dry coating amount of the coating liquid for the undercoat coating layer on the rod coater was 7 g / m ² .

Comparative Example 2
A coated white paperboard was obtained in the same manner as in Comparative Example 1 except that the coating amount of the coating solution for the topcoat coating layer in Comparative Example 1 was increased to 13 g / m ² . At this time, the dry coating amount of the coating liquid for the undercoat coating layer on the rod coater was 7 g / m ² .

Comparative Example 3
In the preparation of the coating solution for the undercoat coating layer of Example 1, an anionic polyacrylamide copolymer obtained by a water-in-oil polymerization method (trade name: Somalex 530, manufactured by Somare, weight average molecular weight 15 million) ) Was used in the same manner as in Example 1 except that 0.05 part of an alkali thickening type fluidity modifier (trade name Somalex 270K, supra) was used. At this time, the dry coating amount of the coating liquid for undercoat coating layer on the rod coater was 7.5 g / m ² .

Comparative Example 4
In the preparation of the coating liquid for the undercoat coating layer of Example 1, carboxymethylcellulose (trade name: Somalex 530, supra) was used instead of the anionic polyacrylamide copolymer obtained by the water-in-oil polymerization method. Coated white paperboard was obtained in the same manner as in Example 1 except that 0.05 part of trade name: LC coat, manufactured by Daiichi Kogyo Seiyaku Co., Ltd. was added. At this time, the dry coating amount of the coating liquid for undercoat coating layer on the rod coater was 8 g / m ² .

Comparative Example 5
In the preparation of the coating solution for the topcoat coating layer of the reference example , except that the anionic polyacrylamide copolymer (trade name: Somalex 530, supra) obtained by the water-in-oil polymerization method was not added. Coated white paperboard was obtained in the same manner as in the reference example . At this time, the dry coating amount with the rod coater of the coating liquid for top coat layers was 6 g / m ² .

  The coated white paperboard obtained above was evaluated by the following evaluation method, and the results are shown in Table 1. All the evaluations were performed in an environment of 23 ° C. and 50% RH.

(B type viscosity)
According to TAPPI standard T648 Su-72, the B-type viscosity (mPa · s) at 20 ° C. was measured at 60 rpm.

(High shear viscosity)
The high shear viscosity (mPa · s) was measured using a Hercules high shear viscometer (manufactured by Kumagai Riki Kogyo Co., Ltd.) under the conditions of Bob F and a shear rate of 9.0 × 10 ⁴ sec ⁻¹ (rotation speed: 4400 rpm).

(PPS smoothness)
PPS smoothness (μm) was measured using a Parker Print Surf (PPS) surface smoothness tester (model name: MODEL M-569, manufactured by MESSMER BUCHEL, UK), backing disk: soft rubber, clamp pressure: 2 MPa. Then, the surface was measured 5 times and the average was obtained.

(White light and shade unevenness)
For the white shading unevenness, the degree of surface unevenness was visually evaluated.
(Double-circle): It is uniform and there is no white shading unevenness.
○: Some white shading unevenness is recognized, but there is no practical problem.
(Triangle | delta): White shading unevenness is conspicuous and there is a problem in practical use.
X: White shading unevenness is very conspicuous.

(Printability evaluation)
For printing smoothness and ink fillability as a printability evaluation, RI printer uses 0.1 ml of printing ink (trade name: Values-G black S type, manufactured by Dainippon Ink & Chemicals, Inc.). Then, printing was performed on each coated white paperboard, and the ink transfer surface was observed with the naked eye, and the transferred printing smoothness (uniformity of ink density) and ink fillability (transferred ink density) were evaluated in four stages. .
(Double-circle): Printing smoothness and ink inking property are especially excellent.
○: Excellent printing smoothness and ink fillability.
Δ: Slightly inferior in printing smoothness and ink fillability.
X: Print smoothness and ink inking property are inferior.

As is clear from Table 1, in the examples in which the coating liquid added with the anionic polyacrylamide copolymer according to the present invention was applied by rod coating, a sufficient coating amount could be secured. Thus, a coated white paperboard having less unevenness of white density on the coated surface and excellent printability such as printing smoothness and ink fillability has been obtained.

Claims (6)

An anionic polyacrylamide having a weight average molecular weight of 4,000,000 to 20,000,000 in a coated white paperboard on which two or more layers of a coating liquid mainly composed of a pigment and an adhesive are coated on a base paper A layer containing a copolymer and having a B-type viscosity of 1000 mPa · s or more and 3000 mPa · s or less at 60 rpm of the coating liquid, and a layer in contact with the base paper by the rod coating method is essential. A coated white paperboard , wherein at least one layer is applied. Coated white paperboard of claim 1 Symbol placement, wherein the anionic polyacrylamide copolymer is a polymer obtained from the water-in-oil polymerization method. Coated white paperboard of claim 1 Symbol placement, wherein the anionic polyacrylamide copolymer is a polymer obtained by dispersion polymerization. The content of the anionic polyacrylamide copolymer, the pigment total solid content in the coating liquid, according to any one of claims 1 to 3 is 0.001 to 0.5 mass% Coated white paperboard. The coated white paperboard according to any one of claims 1 to 4 , wherein the outermost coating layer to be coated with two or more layers is coated by a blade coating method. The basis weight of base paper 150 g / ^{m 2} or more, coated white paperboard of any of claims 1-5, characterized in that it is 700 g / ^{m 2} or less.