JP2014208934A - Coated white paperboard and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coated white paperboard which has high strength, a high degree of whiteness of a finished product, no unevenness in white color, and excellent smoothness even when a raw paper sheet having low strength and a small degree of whiteness, and is produced by curtain coating.SOLUTION: A method of producing a coated white paperboard includes (A) a pre-calender step of calender-treating a raw paper sheet by using a heated roll provided with an induction heat generation mechanism with which the roll itself generates heat and a heat pipe mechanism for uniformizing the temperature of the whole of the roll surface and nipping and (B) a curtain coating step of coating the raw paper sheet with a coating liquid containing an adhesive and a pigment by curtain coating.

Description

  The present invention relates to a coated white paperboard in which a pigment coating layer is provided on a base paper mixed with used paper. More specifically, the present invention relates to a bulky coated white paperboard having high whiteness and little whiteness unevenness, and a method for producing the same.

  The base paper of coated white paperboard is usually manufactured by multi-layer papermaking, and the surface layer uses a lot of expensive bleached chemical pulp with high whiteness, and the lower and middle layers are cheaper and cheaper than the exposed chemical pulp. Often used are deinked and undeinked waste paper pulp. The reason why the bleached chemical pulp is used for the surface layer is to keep the white unevenness on the surface of the coated white paperboard well and make it difficult to see foreign matters such as dust contained in the used paper pulp from the surface. In addition, the use of waste paper pulp for the lower and middle layers is from the viewpoint of environmental and cost reduction.

  In recent years, demands for environmental efforts and cost reductions are increasing, and the usage rate of waste paper pulp tends to increase. Along with this, the problem that white unevenness and foreign matters such as dust become conspicuous on the surface of the coated white paperboard is increasing. In order to remove foreign matters such as dust, it is necessary to enhance the deinking and dust removal treatment, but the yield becomes worse as the treatment is enhanced. Moreover, when the usage rate of waste paper pulp is raised, the whiteness of the paperboard before coating tends to be low. As a response to these problems, for example, a method for increasing the coating amount has been proposed. However, if the coating amount is too large, the density becomes high at the same basis weight, and the bulkiness is impaired. Moreover, if the coating amount is increased, the operability is lowered, for example, the drying property at the time of coating is deteriorated. In addition, Patent Documents 1 and 2 disclose a method for producing white paperboard using a pigment having a high concealment property in the coating liquid. However, pigments with high hiding properties are expensive, and the advantage of reducing the cost by increasing the usage rate of waste paper pulp is lost.

  Patent Document 3 discloses a method of improving whiteness unevenness by making the difference in whiteness between the surface layer and the lower layer lower than a certain level, that is, by improving the paper layer structure. However, in this method, since the whiteness difference is set to a specific range, the usage rate of the used paper pulp is limited, so that a large amount of used paper pulp cannot be used.

  Patent Document 4 discloses a method of making white unevenness and dust inconspicuous by performing curtain coating so as to eliminate the occurrence of coating defects as in the roll coating method.

  In addition, when used paper is blended, there is a problem that the rigidity and strength of the paperboard base paper are lowered. For this reason, Patent Document 5 discloses that high-quality waste paper pulp having a fiber length of 1.50 mm or more is used as a base paper, or that strength reduction is suppressed by making ash content 10% or less.

  On the other hand, as a surface finishing method for coated paper, Patent Documents 6 and 7 disclose a method in which a metal roll is heated to a surface temperature of 100 to 250 ° C. by an induction heating jacket roll. A method of heating to a surface temperature of 100 to 350 ° C. by an induction heating method is disclosed. Patent Documents 6 and 7 disclose that surface finishing is performed with the metal roll after coating a working liquid containing an adhesive and a pigment, but specific examples of processing before forming the coating layer are not disclosed.

JP-A-6-166991 JP 2003-306882 A JP 2005-298998 A JP 2009-41131 A JP 2006-322097 A JP 7-70973 A Japanese Patent Laid-Open No. 2-200888

  In general, curtain coating is a pre-metering type coating method that does not scrape off the coating liquid during coating, so a uniform coating layer can be applied even if there is some unevenness on the base paper. It is considered that white unevenness is unlikely to occur. Curtain coating does not scrape off with a blade, and since the coating layer formed as a curtain film is placed on the base paper, there are many voids in the coating layer, which is preferable in terms of light scattering and whiteness. . Therefore, it is considered that curtain coating is suitable for a method for producing coated white paperboard using waste paper pulp. However, the use of waste paper pulp tends to reduce the strength and smoothness of the base paper.

  In view of such a situation, even when using a base paper having low strength and low whiteness, a coated white board having high strength, high whiteness of the product, no white unevenness and excellent smoothness is used for curtain coating. It is an object of the present invention to provide a method of manufacturing by the above.

  As a result of intensive studies, the present inventors have found that the above problem can be solved by optimizing the smoothing process of the base paper, and have completed the present invention.

That is, the said subject is solved by the following this invention.
(1) (A) a pre-calendar process in which a base paper is calendered and nipped using a heated roll provided with an induction heating mechanism for generating heat from the roll itself and a heat pipe mechanism for equalizing the temperature of the entire roll surface; B) Curtain coating process for coating the base paper with a coating liquid containing an adhesive and a pigment by curtain coating,
A method for producing coated white paperboard, comprising:
(2) Coated white paperboard produced by the above method.

  According to the present invention, it is possible to produce a bulky coated white paperboard containing a large amount of used paper in the base paper and having high rigidity and strength, high whiteness of the product and no white unevenness even if the rigidity and strength are low. .

Outline diagram showing the principle of induction heating roll

  The production method of the present invention includes: (A) a pre-calendar that calenders and nips a base paper using a heated roll having an induction heating mechanism that generates heat from the roll itself and a heat pipe mechanism that equalizes the temperature of the entire roll surface. And (B) a curtain coating step of coating the base paper with a coating liquid containing an adhesive and a pigment by curtain coating. Hereinafter, the present invention will be described in detail. In the present invention, “to” includes values at both ends.

1. Coated white paperboard In the present invention, coated white paperboard is a paperboard provided with a pigment coating layer on one side or both sides of a base paper, and at least used paper pulp is blended. Although the basic weight of the coated white paperboard manufactured by this invention is not specifically limited, Generally it is about 150-800 g / m < ² >, and 200 g / m < ² > or more is preferable. Further, the whiteness of the coated white paper board produced in the present invention is preferably 75% or more, more preferably 77.5% or more, and still more preferably 80% or more. Furthermore, the ash content in the coated white paperboard is preferably 18% by weight or more, and more preferably 25% by weight or more. INDUSTRIAL APPLICABILITY The present invention can produce a bulky coated white paperboard having high rigidity and strength, high whiteness of the product, and no white unevenness even when using a base paper having high ash and low strength.

2. Pre-calendar process A
(1) Calendering In this step, nip is performed by calendering for the main purpose of smoothing the base paper. The calendar process may be performed by either a method in which a base paper is sandwiched between a metal roll and a metal roll, or a method in which a base paper is sandwiched between a metal roll and a resin roll. The base paper is nipped by being clamped between the rolls. In the present invention, a roll (hereinafter sometimes referred to as an “induction heating roll”) having an induction heating mechanism that can freely set the temperature and a heat pipe mechanism that equalizes the temperature of the entire roll surface is provided. Use. Examples of such rolls include “Jacket Roll (registered trademark)” manufactured by Tokuden Corporation.

  The induction heating roll has an induction heating type heat source inside the roll and accurately controls a desired roll surface temperature. An induction coil is an example of the induction heating method. In addition, the induction heating roll includes a heat pipe mechanism that equalizes the temperature of the entire roll surface. Examples of the heat pipe mechanism include a jacket chamber provided along the circumferential surface in the vicinity of the roll surface.

  FIG. 1 shows the principle of the induction heating roll. In FIG. 1, 1 is an induction heating roll, 10 is an induction coil as an induction heating mechanism, 12 is a power source, and 20 is a jacket chamber as a heat pipe mechanism. A gas-liquid two-phase heat medium is sealed inside the jacket chamber, and the temperature distribution on the roll surface is kept uniform by repeating the cycle of evaporation and condensation. Thus, the induction heating roll is structurally different from the heating roll that heats by introducing high-temperature steam or heat medium oil into the roll.

  The conditions for the calendar treatment are not particularly limited, but for example, the linear pressure can be about 4 to 100 kN / m at 80 to 300 ° C. More preferably, the temperature is 90 to 250 ° C. and the linear pressure is 4.5 to 50 kN / m.

  As will be described later, the base paper used for the pre-calendar process can be a paper sheet that has exited the dryer part. Although the surface of the paper base is rough, it is considered that white unevenness is improved because the surface can be smoothed and the structure can be made dense to a predetermined paper thickness by this step.

(2) Base Paper The base paper used in the present invention is not particularly limited as long as it contains at least waste paper pulp. For example, a base paper blended with bleached chemical pulp or unbleached chemical pulp can be used. Waste paper pulp may or may not be deinked. As the deinked pulp, it is possible to use deinked pulp made from selected waste paper such as high quality paper, medium quality paper, lower grade paper, newspaper, flyers, magazines, etc., or unselected waste paper mixed with these. The mixing ratio of the waste paper pulp can be 50% by weight or more, preferably 80% by weight or more in the total pulp.

  The base paper may be a mixture of the above-mentioned various pulps, may be composed of the same pulp, or may be a stack of one or more different pulps. For example, it may be a base paper that uses pulp with low whiteness for the middle layer and pulp with higher whiteness for the front and back layers, or multiple layers of the same layer of pulp. It may be a raw paper. The base paper of the present invention is preferably a multi-layered paper of two or more pulp layers (paper layers).

  The whiteness of the base paper used in the present invention is not limited, but the effect of the present invention increases as the base paper having a lower whiteness is used. The whiteness of the base paper that can be used in the present invention is usually 45 to 72%, more preferably 50 to 52%, and still more preferably 55 to 67%. Of course, the effect of the present invention can be obtained even if a base paper having a whiteness lower than the above range is used. The whiteness is the whiteness of the base paper surface on the side where the coating layer is provided. In the case of multilayer papermaking in which the base paper is composed of two or more paper layers, the whiteness of the pulp in each layer other than the outermost layer is not particularly limited.

  According to the present invention, a coated white paperboard with high whiteness can be obtained without white unevenness even when a low whiteness pulp similar to that used for the middle layer and the back layer is used for the outermost layer. Further, in the present invention, coated white paperboard with reduced whiteness unevenness is obtained even when a high whiteness pulp is used only for the outermost layer and a base paper using a low whiteness pulp is used for the middle layer or the back layer pulp. Can be manufactured. Moreover, even if the whiteness of the base paper is higher than 72%, a coated white board with reduced whiteness unevenness can be produced. The whiteness of the pulp can be measured in the form of a sheet having a thickness that does not allow the pulp to penetrate.

  As described above, according to the present invention, it is possible to produce a white paperboard excellent in whiteness and white unevenness even when a base paper having a relatively low whiteness is used. The use of a low whiteness base paper containing a large amount of waste paper pulp or containing pulp that has not been deinked is advantageous in terms of cost and can reduce the environmental burden.

The Taber stiffness of the base paper is measured according to JIS P 8125. The desired stiffness value varies depending on the basis weight, paper thickness, product usage, etc., but cannot be generally stated. For example, when the basis weight is about 300 g / m ² , the value is vertical (paper flow direction). It is preferably 12 mN · m or more and lateral (paper width direction) of 5.0 mN · m or more. The ash content of the base paper can be 18% to 35%. In the present invention, a coated white paperboard having excellent strength and rigidity can be obtained even when such low strength and high ash content base paper is used.

3. Curtain coating process B
(1) Curtain coating In this step, after the pre-calendar step A, a coating liquid containing an adhesive and a pigment to be described later is applied by a curtain coating method. Curtain coating is a coating method in which a coating solution is formed on a base paper by forming a film by flowing a coating liquid in a curtain shape and passing the base paper through the film. Curtain coating is a contour coating in which a coating layer is formed along a base paper, and has a characteristic that the coating amount can be easily controlled because of a so-called pre-weighing system.

  In the present invention, as will be described later, one curtain coating film including a plurality of layers is formed from a plurality of coating liquids, and the curtain coating layer is formed through the base paper or the base paper pre-coated on the curtain film. It is preferable. When a curtain coating is formed by forming a plurality of curtain films from a plurality of coating liquids, it is inevitable that some disturbance will occur in the curtain film when the curtain film contacts the traveling paper. When one curtain film is formed and curtain coating is simultaneously performed on a plurality of coating layers, the layer structure between the coating layers can be maintained.

In the present invention, the coating amount of the curtain coating layer is a total of each layer, preferably 5 to 50 g / m ² , preferably 10 to 40 g / m ² , more preferably 11 to 35 g / m ² in terms of dry weight per side. . When the coating amount is less than 5 g / m ² , the coating layer becomes thin, and it may be difficult to sufficiently improve the whiteness and white unevenness of white paperboard. On the other hand, if the coating amount of one layer exceeds 45 g / m ² , the operability may be deteriorated, for example, the drying property during coating may be deteriorated, or printing unevenness due to binder migration may be caused. The coating amount of the curtain coating layer is most preferably a coating amount of 45 g / m ² or less per side in total for each layer.

In the present invention, the coating amount of the curtain undercoat coating layer and the coating amount of the curtain topcoat coating layer are not particularly limited. coating layer 10 to 30 g / ^{m 2,} it is preferred overcoat coating layer is in the range of 1 to 5 g / ^{m 2.}

  A known apparatus used for curtain coating can be used. What is known as the shape of the curtain coating apparatus is a slot type curtain coating apparatus that directly forms a curtain film by discharging the coating liquid downward from the die, and discharging the coating liquid upward from the die, There is a slide-type curtain coating apparatus that flows while forming a coating liquid film on a slope on a die, and then forms a curtain film by free-falling after leaving the die. Any device may be used in the present invention.

  The coating speed of the present invention is not particularly limited, but is generally about 100 to 800 m / min. If the coating speed is excessively high, craters tend to occur.

  In the present invention, in addition to the coating layer provided by curtain coating as described later, an additional coating layer can be provided by using a coating apparatus other than the curtain coating apparatus. For example, curtain coating may be performed after rod (bar) coating or blade coating. In this case, it is preferable that the top coat layer formed by curtain coating be the outermost coat layer. When higher smoothness is required, it is preferable to provide the outermost coating layer by blade coating after curtain coating. Moreover, you may perform wet on wet coating which performs upper layer coating, without drying a lower layer coating part.

(2) Preparation of coating liquid In the present invention, one curtain film is formed from two or more kinds of coating liquids, and two or more curtain coating layers are formed on the base paper through the curtain film. Is preferred. The curtain coating layer preferably includes an outermost coating layer that is the outermost layer of the coated white paperboard. The outermost coating layer is also referred to as a curtain top coating layer, and the coating liquid for forming the top coating layer is also referred to as a curtain top coating liquid. Further, a layer adjacent to the outermost layer and closer to the base paper is also referred to as a curtain undercoat coating layer, and a coating liquid for forming the undercoat coating layer is also referred to as a curtain undercoat coating liquid. Each curtain coating layer may have the same property or different properties. For example, two coating liquids, a top coating liquid that can achieve specific whiteness and an undercoat coating liquid that can achieve a specific specific scattering coefficient, can be curtain coated simultaneously. Thereby, even if the whiteness of the base paper is low, it is possible to produce a coated white paperboard having high whiteness and reduced whiteness unevenness. Although this reason is not limited, it is guessed as follows.

  In the first place, in order to increase the whiteness of the surface by coating, the whiteness of the coating layer needs to be higher than the surface before coating, and the coating layer needs to have light scattering properties. Generally, the higher the whiteness of the coating layer relative to the whiteness of the surface before coating, the higher the whiteness after coating. However, even when the whiteness of the coating layer is sufficiently higher than the whiteness of the surface before coating, the surface before coating cannot be sufficiently concealed unless the light scattering property is high. The effect of increasing the degree is low. Here, it is considered that whether the whiteness or light scattering property of the coating layer is more effective for improving the whiteness after coating depends on the whiteness of the surface before coating. Specifically, when the whiteness of the surface before coating is low, it is effective to have a high light scattering property because it is highly necessary to conceal the surface, while the whiteness of the surface before coating is effective. When the degree is high, it is considered that it is effective that the degree of whiteness is high because the necessity of hiding the surface is low. Based on this premise, the present invention is inferred as follows. The curtain undercoat layer is provided on a base paper with low whiteness and a lot of dust. Therefore, in order to conceal these and effectively increase the whiteness, light scattering is higher than high whiteness. Is considered to be more effective. At this time, since the whiteness of the curtain undercoat layer is sufficiently higher than the whiteness of the base paper in a normal formulation, it is considered that the required conditions are sufficiently satisfied. On the other hand, it can be considered that the curtain topcoat layer is provided on a surface having a relatively high whiteness level that is covered to some extent by the curtain undercoat layer. Therefore, the whiteness of the curtain topcoat layer needs to be one step higher than that of such a surface, but the necessity of hiding the surface is considered to be relatively low. As described above, in the present invention, it is considered that, by such a mechanism, even if the whiteness of the base paper is low, it is possible to produce a coated white board with high whiteness and reduced whiteness unevenness.

  In this invention, water and a required component are mixed and a coating liquid is adjusted. In the preparation of the coating liquid, ordinary mixing means such as a mixer may be used. The pigment, adhesive, and other additives in the curtain coating liquid are not particularly limited, but the following is a top coating liquid that can achieve a specific whiteness and an undercoat coating liquid that can achieve a specific specific scattering coefficient. Each of the coating liquids will be described by taking as an example the case where the two coating liquids are simultaneously curtain-coated.

(2-1) Curtain Undercoat Coating Solution 1) Light Scattering Property of Curtain Undercoat Coating Layer The curtain undercoat coating layer preferably has a high light scattering property in order to conceal the base paper more effectively. The light scattering property of the coating layer can be based on the scattering property per unit coating amount, that is, the specific scattering coefficient. The higher the specific scattering coefficient, the higher the concealability of the coating layer.

The specific scattering coefficient of the coating layer can be measured as follows.
i) On a transparent polyethylene terephthalate (PET) film (thickness 100 μm, opacity of 5% or less), the coating solution is applied by a Mayer bar so that the weight of the coating layer after drying is about 20 g / m ^2. The weight W [g / m ² ] of the coating layer is accurately measured with a balance. Drying is carried out at 105 ° C. for 30 minutes or more until there is no change in the amount of moisture reduction.
ii) Next, this coated film is backed with a black plate, and the reflectance of the coating layer is measured using a reflectance meter to obtain reflectance R0.
iii) Using an oven, the coating liquid is dried at 105 ° C. for 30 minutes or more until the variation in the amount of moisture disappears, and a pellet having a sufficient thickness (1 mm or more) that does not show through is obtained. R∞ is measured as described above.
iv) Using these values, the specific scattering coefficient s is calculated according to the following equation defined in TAPPI T425 (ISO 9426).

a = 0.5 [(1 / _R∞ ) + _R∞ ]
b = 0.5 [(1 / _R∞ ) _−R∞ )]
X = [1−aR ₀ ] / [bR ₀ ]
s = 1 / W x (0.5 / b) ln [(X + 1) / (X-1)]

The ratio scattering coefficient curtain subbing coating layer is preferably not less than ^{80 m} 2 / kg, more preferably at least ^90m 2 / kg, still more preferably at least 120 m ² / kg. When the specific scattering coefficient is lower than 80 m ² / kg, it is difficult to obtain sufficient concealability. Moreover, when the component used for the coating layer of normal white paperboard is contained, it is thought that the upper limit of a specific scattering coefficient is about 250 m < ² > / kg.

  The whiteness of the curtain undercoat coating layer is preferably higher than the whiteness of the base paper on the surface to be coated in order to conceal the low whiteness base paper and increase the whiteness after coating. The preferable range of the whiteness of the undercoat coating layer is 75 to 95%, more preferably 80 to 95%.

  The whiteness measured from the coating layer side after coating the curtain undercoat coating layer on the base paper is preferably lower than the whiteness of the topcoat coating layer. Specifically, the whiteness is preferably less than 85%.

2) Each component of undercoat coating liquid In order to obtain the undercoat coating layer which has said property, it is preferable that an undercoat coating liquid contains a specific pigment and an adhesive agent (binder). As the pigment, a pigment having high concealability is preferable. In order to make the undercoating liquid into the range of the above-mentioned specific scattering coefficient, it is preferable to mainly use light calcium carbonate having a narrow particle size distribution and high light scattering properties. The content is not particularly limited, but the concealing effect becomes significant when the light calcium carbonate content is 20 to 100 parts by weight in 100 parts by weight of the pigment. More preferably, it is 50 parts by weight or more. The light calcium carbonate has a cubic shape, a spindle shape, a needle shape, and the like, and any of them may be used. However, when a spindle shape or a needle shape is used, the concealment effect is particularly high. The particle diameter of light calcium carbonate is preferably 0.2 to 0.8 μm, and more preferably 0.2 to 0.5 μm in terms of average particle diameter (D50). As an alternative to light calcium carbonate, engineered heavy calcium carbonate or engineered kaolin controlled to have a narrow particle size distribution may be included, but the concealability is inferior to light calcium carbonate.

  Further, it is preferable to use a certain amount of calcined kaolin having a high light scattering property by including voids in the particles as a pigment. The particle diameter of the calcined kaolin is preferably 1.0 to 3.0 μm, more preferably 1.5 to 2.5 μm in terms of average particle diameter (D50). The content of the calcined kaolin is preferably 5 to 20 parts by weight in 100 parts by weight of the pigment. If the amount is less than 5 parts by weight, the concealing effect may be insufficient. Further, when the amount is more than 20 parts by weight, the solid content concentration of the coating liquid tends to be lowered for the following reason. The calcined kaolin, like other kaolins, must be dispersed in water before being contained in the coating liquid to form a slurry. In the case of calcined kaolin, the solid content of the slurry is due to the characteristics of including voids in the particles. The concentration cannot be increased. Accordingly, when a large amount of such a slurry is contained in the coating liquid, the solid content concentration of the coating liquid decreases. As a substitute for the calcined kaolin, titanium dioxide having a high light scattering property due to its high refractive index may be used. However, since the amount of the adhesive is larger than that of the calcined kaolin, the surface strength tends to be inferior. Further, the calcined kaolin has an advantage that the smoothness of the coated white paper board is improved as compared with the titanium dioxide.

  In addition to the above pigment, a pigment having an average aspect ratio of 30 to 60, such as delaminated kaolin, can be used in combination in the undercoat coating layer. A pigment having an average aspect ratio of 30 to 60 further improves white unevenness and water retention. This is presumed to be because dehydration is suppressed and water retention is improved by filling the gaps between the pigments with flat pigments. The amount of the pigment having an average aspect ratio of 30 to 60 is not particularly limited, but is preferably 20 to 100% by weight, more preferably 40% by weight or more based on the total pigment.

  Adhesives are not particularly limited. For example, various copolymers such as styrene / butadiene, styrene / acryl, ethylene / vinyl acetate, butadiene / methyl methacrylate, vinyl acetate / butyl acrylate, polyvinyl alcohol, and maleic anhydride. Synthetic adhesives such as acid copolymers and acrylic acid / methyl methacrylate copolymers; proteins such as casein, soy protein, synthetic proteins; oxidized starch, positive starch, urea phosphated starch, hydroxyethyl etherified Examples include etherified starch such as starch, starches such as dextrin; cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose. In the present invention, a synthetic adhesive having a low viscosity increase of the paint is suitable. Further, polyvinyl alcohol having a low degree of polymerization (degree of polymerization of about 500) is preferably used in combination because the adhesion effect can be enhanced without significantly increasing the viscosity.

3) Characteristics of Curtain Undercoat Coating Liquid The solid content concentration of the curtain undercoat coating liquid is preferably 40 to 75% by weight, more preferably 50 to 70% by weight, and further preferably 60 to 70% by weight. is there. When the solid content concentration of the undercoat coating liquid is less than 40% by weight, the volume change of the coating layer during coating drying increases, and as a result, the surface smoothness after coating may decrease. On the other hand, if the concentration is more than 75% by weight, the fluidity of the coating solution may deteriorate, and it may be difficult to form a uniform curtain film.

  The B-type viscosity of the curtain undercoat coating liquid should be in the range of 100 to 2000 mPa · s at 30 ° C. so as to be suitable for forming a uniform and stable curtain film and for delivering the coating liquid. Is preferred. If the viscosity is 100 mPa · s or less, moisture in the paint will soak into the base paper excessively and the base paper may swell, resulting in a decrease in surface smoothness after coating, and an increase in energy required for drying. It tends to be. On the other hand, if the viscosity is greater than 2000 mPa · s, the fluidity of the paint is inferior and it is difficult to form a stable curtain film.

  Further, the surface tension of the curtain undercoat coating liquid is preferably 40 mN / m or less, more preferably 35 mN / m or less, and more preferably 30 mN / m in order to produce a uniform and stable curtain film even at a small flow rate. More preferably, it is as follows. The surface tension can be adjusted by adding a surfactant as described later.

(2-2) Curtain topcoat coating solution 1) Whiteness of curtain topcoat coating layer The whiteness of the curtain topcoat coating layer is 85% or more, and preferably 90% or more. By providing the overcoat layer on the surface that is concealed by the curtain undercoat layer and has improved whiteness, the whiteness of the coated white paperboard can be further increased.

The concealability of the curtain topcoat coating layer does not affect the quality of the white paperboard of the present invention, so it is not necessary to be as high as the undercoat coating layer and is not particularly limited, but in the normal range it is 70 m ² / kg or more. Is preferred.

2) Curtain top coating liquid component The curtain top coating liquid contains a pigment and an adhesive. As the pigment, those having high whiteness are preferable, and examples thereof include heavy calcium carbonate, light calcium carbonate, high white kaolin, titanium dioxide, and plastic pigment. These pigments can be used alone, but if necessary, two or more kinds may be mixed and used. Among these, it is preferable to use heavy calcium carbonate or light calcium carbonate, which is relatively inexpensive, as a main component. The average particle diameter (D50) of heavy calcium carbonate is preferably 0.3 to 2.0 μm, and more preferably 0.5 to 1.0 μm. When heavy calcium carbonate is used, the specific scattering coefficient is not high, but the whiteness is high. Furthermore, since heavy calcium carbonate is inexpensive, it is suitable as a pigment used for the overcoat layer. The light calcium carbonate has already been described. The content of heavy calcium carbonate or light calcium carbonate is not particularly limited, but if the content of calcium carbonate is 100 parts by weight or more, preferably 70 parts by weight or more, the effect of improving whiteness is remarkable. It becomes.

  Titanium dioxide has the highest whiteness among the pigments usually used in coating liquids, so it is preferably contained in a certain amount. The content of titanium dioxide is preferably 5 to 20 parts by weight in 100 parts by weight of the pigment. When the amount is less than 5 parts by weight, the effect of improving the whiteness of the coating layer is not sufficient. When the amount is more than 20 parts by weight, the required amount of the adhesive increases, and the surface strength may be lowered.

  The adhesive is not particularly limited, and an adhesive conventionally used for coated paper can be used. Examples of adhesives include styrene / butadiene, styrene / acrylic, ethylene / vinyl acetate, butadiene / methyl methacrylate, vinyl acetate / butyl acrylate, and other copolymers, polyvinyl alcohol, maleic anhydride copolymer And synthetic adhesives such as acrylic acid / methyl methacrylate copolymers; proteins such as casein, soy protein, synthetic protein; oxidized starch, positive starch, urea phosphated starch, hydroxyethyl etherified starch, etc. Conventional adhesives for coated paper such as starches such as etherified starch and dextrin; cellulose derivatives such as carboxymethylcellulose, hydroxyethylcellulose, and hydroxymethylcellulose are included. One or more types of adhesives can be appropriately selected and used. In a preferred embodiment, these adhesives are used in the range of about 5 to 50 parts by weight, more preferably about 8 to 30 parts by weight per 100 parts by weight of the pigment. Synthetic adhesives are preferred because they do not significantly increase the viscosity of the paint.

  The top coat and the undercoat coating liquid can adjust the dynamic surface tension of the coating liquid using a surfactant. The surfactant includes an anionic surfactant, a cationic surfactant, and a nonionic surfactant. In the present invention, an anionic surfactant is preferable. The cationic surfactant tends to aggregate the pigment in the coating solution. Further, the nonionic surfactant is difficult to give sufficient wettability to the coating solution. Examples of the anionic surfactant include sulfonic acid surfactants, sulfate ester surfactants, and carboxylic acid surfactants. Among these, sulfonic acid-based surfactants are preferable and alkylsulfosuccinic acid is particularly preferable because the wettability of the coating liquid can be improved. In a preferred embodiment, these surfactants are used in the range of about 0.1 to 2 parts by weight, more preferably about 0.2 to 1 part by weight per 100 parts by weight of the pigment.

  In the topcoat and undercoat coating liquids, various assistants blended in ordinary coating paper pigments, such as a dispersant, a thickener, a water retention agent, an antifoaming agent, a water-resistant agent, and a colorant, as necessary. Agents can be used as appropriate.

3) Characteristics of Curtain Topcoat Coating Liquid The solid content concentration of the curtain topcoat coating liquid is preferably 40 to 75% by weight, more preferably 50 to 70% by weight, and further preferably 60 to 70% by weight. is there. When the solid content concentration of the topcoat coating liquid is less than 40% by weight, the volume change of the coating layer during coating drying becomes large, and as a result, the surface smoothness after coating may be inferior. On the other hand, if the concentration is more than 75% by weight, the fluidity of the coating solution may deteriorate, and it may be difficult to form a uniform curtain film.

  The B-type viscosity of the curtain top coating solution should be in the range of 100 to 2000 mPa · s at 30 ° C. so as to be suitable for forming a uniform and stable curtain film and for delivering the coating solution. Is preferred. When the viscosity is 100 mPa · s or less, the water in the paint soaks into the base paper excessively and the base paper swells, so that the surface smoothness after coating may be lowered. The energy required for drying also increases. On the other hand, if it is greater than 2000 mPa · s, the fluidity of the paint is inferior and a stable curtain film may not be formed.

  The surface tension of the curtain coating liquid is preferably 40 mN / m or less, more preferably 35 mN / m or less, and even more preferably 30 mN / m or less in order to produce a uniform and stable curtain film even at a small flow rate. .

4). Other Steps The present invention preferably further includes the following steps in addition to the steps A and B.

4-1. Before the pre-calender step A (1) Paper layer forming step In this step, two or more paper layers are formed by ejecting paper stock from the head box. If the paper stock contains the pulp mentioned above, other additives etc. will not be specifically limited. The paper machine is not particularly limited, and examples thereof include a long net type, a circular net type, a short net type, and a hybrid type combining these, and any of these may be used. This step is preferably carried out using a paper machine including a circular net. Specifically, two or more paper layers are formed by a circular net paper machine of a method in which the stock is directly discharged from the head box to the rotating circular net and the formed thin paper layers are successively stacked. Is preferred. Each paper layer may be formed from the same stock or may be formed from different stocks.

(2) Wet paper forming step In this step, the paper layer obtained in the previous step is dehydrated to form wet paper. In this step, the paper layer is preferably dehydrated to a moisture content of about 50 to 58% through a plurality of presses. One or more of the presses are preferably shoe presses.

(3) First drying step In this step, the wet paper is dried to obtain a base paper. This step is preferably performed using a cylinder-type dryer. It is particularly preferable to use a single deck dryer. This process is also called a predryer process.

(4) Size pressing step In this step, a coating liquid containing a water-soluble polymer is applied to the base paper obtained in the first drying step. The size press is a treatment for imparting water resistance to the paper surface, and in the present invention, a coating liquid mainly composed of a water-soluble polymer is applied. A size press process is implemented between the 1st drying process (pre-dryer process) and the 2nd drying process (after-dryer process) mentioned later. Examples of the size press include a two-roll size press, a gate roll coater, and a rod metering size press, and any of them may be used. The coating liquid used for the size press may be a known coating liquid. The size pressing step may be omitted depending on the properties of the product to be manufactured.

(5) Second drying step In this step, the base paper subjected to the size press is dried. This process is also called an after dryer process. This step is preferably performed using a cylinder-type dryer, and it is particularly preferable to use a single deck dryer.

4-2. Immediately before the curtain coating process B (6) Blade or rod coating process Further, after the pre-calendar process A and immediately before the curtain coating process B, an aqueous coating mainly composed of an adhesive and a pigment A step of applying the liquid by a coating method other than curtain coating may be provided. This step is preferably carried out using a blade or rod (bar) coating method. The coating liquid used in this step is preferably a coating liquid equivalent to the curtain coating undercoat coating liquid because the effects of the present invention can be easily obtained.

4-3. After the curtain coating step B (7) Third drying step In this step, the curtain coating layer is dried. Generally, as a dryer for a coating layer, there are a steam cylinder method, an air dryer method, an infrared (heat ray) drying method, a dielectric heating method, a microwave method, and the like, which are used alone or in combination.

  In the present invention, it is preferable to dry the curtain coating film provided on the base paper by combining the infrared drying method and the thermal convection drying method. The infrared drying method is a method of drying by infrared rays, and specifically generates infrared rays by electricity, gas combustion or the like. In the present invention, infrared rays may be generated by any method. The thermal convection drying method is a method in which hot air is blown onto the coating layer for drying, and specifically, there are a single-sided drying method in which hot air is blown only on the coated surface and a double-sided drying method in which both sides are sprayed. In the present invention, the double-sided drying method is preferred because of the good drying efficiency and operational stability. Actually, it is preferable to dry using a combined dryer combining an infrared drying method and a thermal convection drying method. The combined dryer has an advantage that it can be installed even if the installation space is small because the energy density is high and the space for the circulating air duct is unnecessary.

  The reason why the desired effect is achieved by such a drying method is not limited, but is considered as follows. When the curtain coating layer immediately after coating is dried with an air dryer or the like, problems such as damage to the surface of the coating layer, strong binder migration, and printing unevenness occur. However, in the infrared drying method, the energy penetrates uniformly into the coating layer and the base paper, so that the surface is not damaged even though it has a high drying power. Therefore, if the curtain coating layer immediately after coating is dried by an infrared method, a bulky coating layer can be maintained. Moreover, since the roughness of the coating layer surface by wind can be suppressed, smoothness is also improved. However, with the infrared drying method alone, sufficient drying is not possible when the coating amount is large. Therefore, drying efficiency can be improved by combining a thermal convection drying method with an infrared ray method. As a result, the coating layer having a large coating amount can be dried to an optimal moisture amount while maintaining the bulkiness of the curtain coating layer. Furthermore, since the drying time can be shortened, uneven penetration of the coating liquid can be suppressed.

  In addition, in addition to the above-described combined dryer, various publicly known and publicly known drying methods can be combined as long as the effects of the present invention are not impaired.

(8) Calendar process In this process, the paper having the curtain coating layer is smoothed by calendaring. In this step, it is possible to use a smoothing processing apparatus such as a normal super calendar, gloss calendar, soft calendar, thermal calendar, shoe calendar. It is more preferable to use an apparatus including an apparatus for imparting gloss such as a raster press. You may adjust suitably the form of a pressurization apparatus, the number of pressurization nips, a heating.

These additional steps, steps A and B, are preferably performed on-machine. The following are examples of preferred embodiments.
(1) Paper layer forming step → (2) Wet paper forming step → (3) First drying step → (4) Size pressing step → (5) Second drying step → Precalender processing step A → (6) Blade or Rod coating process → Curtain coating process B → (7) Third drying process → (8) Calendar process Among these, (4) Size press process, (5) Second drying process, and (6) Blade or rod coating The construction process may be omitted. Moreover, this invention may also include processes other than (1)-(8).

(9) Others In the present invention, a clear or pigment coating layer may be provided on the surface where the curtain coating layer is not provided in order to reduce rigidity and curl. In the coating liquid for forming the back coating layer, if necessary, conventionally known and publicly used pigments and binders, and dispersants, viscosity improvers, water retention agents, antifoaming agents, water resistance agents, fluorescent dyes, Various auxiliaries blended in normal coating liquids such as colored dyes, colored pigments, surfactants, pH adjusters, cationic resins, anionic resins, ultraviolet absorbers, metal salts, and the like can be used as appropriate.
The coating amount of the clear or pigment coating layer on the back of the base paper is not particularly limited, but is generally about 0.1 to 5 g / m ² in terms of solid content on one side. Moreover, the coating method and drying method of a back surface coating layer are not restrict | limited. For example, using a known size press apparatus, for example, a 2-roll type, a 3-roll type, a gate roll type, a film transfer type, a calendar type, or a coater (coating machine) such as a curtain coater, a spray coater, or a blade coater. After applying, various drying processes such as a steam heating cylinder, heated hot air air dryer, gas heater dryer, electric heater dryer, infrared heater dryer, etc. may be used alone or in combination, or the drying process may not be performed. .

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples. Unless otherwise specified, parts and% in the present specification represent parts by weight and% by weight, respectively, and numerical ranges include their end points. Table 1 shows the composition of the coating layer, and Table 2 shows the operating conditions and evaluation results.

<Evaluation method>
(1) Whiteness: Measured according to JIS P8148 “Paper, paperboard and pulp—Measurement method of ISO whiteness (diffuse blue light reflectance)”. The whiteness of the coating layer was measured by drying the coating solution in an oven and forming a pellet with a sufficient thickness (1 mm or more). Drying was carried out using an oven at 105 ° C. for 30 minutes or longer until there was no change in the amount of water loss.
(2) White unevenness: The following criteria:
◎: Unevenness is inconspicuous ○: Mild unevenness is observed △: Unevenness is recognized and may cause a practical problem ×: Remarkable unevenness is recognized and not suitable for practical use evaluated.

(3) Specific scattering coefficient of the coating layer: As described above, the specific scattering coefficient was calculated based on the formula defined in TAPPI T425 (ISO 9426).
(4) PPS smoothness: In accordance with JIS P8151, “Paper and paperboard—Surface roughness and smoothness test method (air leak method) —Print surf tester method”, the pressure difference of the measuring head is 19.6 ± The smoothness in the case of 0.1 kPa was measured.
(5) Taber stiffness: Measured according to JIS P8125 "Paper and paperboard-Stiffness test method-Taber stiffness tester method" with respect to the paper flow direction (vertical) and the paper width direction (horizontal).

(6) Density The density was determined from the basis weight and paper thickness according to JIS P 8118 by measuring the basis weight and paper thickness according to JIS P 8124 and JIS P 8118, respectively.

[Example 1]
<Base paper>
70% whiteness using pulp blended at a rate of 100% deinked wastepaper, surface layer of 40g / m ² of rice tsubo, 55% whiteness using pulp blended with 100% magazine wastepaper not deinked, rice Using the same pulp as the middle layer and middle layer of tsubo 220 g / m ² , each of the back layers of 45% whiteness and 40 g / m ² of rice tsubo was made, combined, pressed and dried, A coated white paperboard m ² was obtained. The whiteness of the base paper was 65%. The paper making speed was 300 m / min.

<Pre-calendar processing>
A roll (“Jacket Roll (registered trademark)” manufactured by Tokuden Co., Ltd.) having an induction heating mechanism that generates heat from the roll itself and a heat pipe mechanism that equalizes the temperature of the entire roll surface was prepared. The dried base paper was passed through the roll under the conditions of 100 ° C. and linear pressure of 5 kg / cm (4.9 kN / m).

<Undercoating liquid>
Spindle-shaped light calcium carbonate (Okutama Kogyo, TP-221-70GS, D50 = 0.5 μm, D75 / D25 = 2.5) 75 parts by weight, delaminated kaolin (Imers, Contour 1500, average aspect ratio) 50) After preparing a pigment slurry comprising 15 parts by weight and calcined kaolin (manufactured by Imerys, Alphatex) 10 parts by weight, styrene / butadiene copolymer latex (ALB1735, manufactured by Asahi Kasei Chemical Co., Ltd.) 14 parts by weight with respect to 100 parts of the pigment Part, 0.5 part by weight of PVA (polyvinyl alcohol, POVAL 105 manufactured by Kuraray Co., Ltd.) and 0.5 part by weight of a surfactant (manufactured by Nippon Emulsifier Co., Ltd., Newcol 291-PG) were added. Water was further added to this mixture, and the B-type viscosity at 30 ° C. and 60 rpm was adjusted to 1000 mPa · s to obtain an undercoat coating solution having a solid concentration of 63% and a static surface tension of 30 mN / m. .

<Topcoat coating solution>
Prepare a pigment slurry consisting of 95 parts by weight of heavy calcium carbonate (Fimatec, FMT97, D50 = 0.7 μm, D75 / D25 = 3.8) and 5 parts by weight of titanium dioxide (DPS, RPS-V) Then, 14 parts by weight of a styrene / butadiene copolymer latex (ALB1443 manufactured by Asahi Kasei Chemical Co., Ltd.) and 0.5 parts by weight of a surfactant (Newcol 291-PG manufactured by Nippon Emulsifier Co., Ltd.) are added to 100 parts by weight of the pigment. Further, water was added to adjust the B-type viscosity at 60 rpm to 1000 mPa · s to obtain a top coating solution having a solid content concentration of 66% and a static surface tension of 30 mN / m.

<Curtain coating>
A curtain film coating apparatus was used to form one curtain film having a plurality of layers from the two coating liquids, and the curtain film was subjected to simultaneous two-layer coating through a base paper and dried. Amount of coating is subbing coating layer 18 g / ^{m 2} in weight after drying, overcoating a coating layer was 4g / ^{m 2.} The basis weight of the obtained coated white paperboard was 322 g / m ² . The coating speed was 300 m / min, which was the same as the papermaking speed because it was performed consistently with the papermaking by on-machine.

<Finishing process>
The coated white paperboard obtained was calendered under pressure conditions of 3 nips and 3 kg / cm to obtain coated white paperboard. The processing speed was 300 m / min, the same as the papermaking speed and the coating speed, because the papermaking and coating were performed consistently by on-machine.

[Examples 2 to 7]
A coated white paperboard was produced in the same manner as in Example 1 except that coating liquids C and D were prepared with the formulation shown in Table 1 and the coating conditions were as shown in Table 2. ,evaluated.
Examples 4 to 6 are examples in which the temperature and linear pressure of the pre-calender were increased and the pressurization during the finishing process was accordingly suppressed. Example 7 is an example in which pre-coating was performed before curtain coating, and the pre-coating layer was applied to the base paper by a vent blade coating method.

[Comparative Example 1]
A coated white paperboard was produced and evaluated in the same manner as in Example 1 except that the precalender treatment was not performed.
[Comparative Example 2]
A coated white paperboard was produced and evaluated in the same manner as in Example 1 except that the pre-calendar treatment was not performed and the finishing treatment was performed under a pressure condition of 5 kg / cm.

From Table 1, it is clear that the coated white board of the present invention is superior to the white board obtained in the comparative example in the balance of whiteness, white unevenness, rigidity, and bulkiness.
Comparative Example 1 is an example without pre-calendar processing and is inferior in smoothness. Comparative Example 2 is an example in which there was no pre-calendar process and the pressurization of the finishing process was increased in order to increase the smoothness, but sufficient smoothness was not obtained as compared with Example 1, and the white unevenness deteriorated. The bulkiness also decreased.

1 Induction Heating Roll 10 Induction Coil 12 Power Supply 20 Jacket Room

Claims (8)

(A) a pre-calendar process in which a base paper is calendered and nipped using a heated roll having an induction heating mechanism for generating heat in the roll itself and a heat pipe mechanism for equalizing the temperature of the entire roll surface, and (B) Curtain coating process, where the coating liquid containing adhesive and pigment is applied to the base paper by curtain coating,
A method for producing coated white paperboard, comprising: On machine
Before the pre-calendar process A,
A paper layer forming process in which the paper stock is ejected from the head box to form two or more paper layers;
A wet paper web forming process for dehydrating the paper layer to form wet paper
A first drying step of drying the wet paper to obtain a base paper,
After the curtain coating process B,
A third drying step for drying the curtain coating layer, and a calendar step for calendering the paper having the curtain coating layer,
The manufacturing method of Claim 1 provided with these. After the first drying step,
The size press process of applying the coating liquid containing a water-soluble polymer to the base paper obtained in the first drying process, and the second drying process of drying the base paper subjected to the size press. Production method. Immediately before the curtain coating process B,
The method according to any one of claims 1 to 3, further comprising a step of applying a blade or a rod with a coating liquid containing an adhesive and a pigment.   The method according to any one of claims 1 to 4, wherein the curtain coating step forms a plurality of layers including a topcoat layer which is an outermost coating layer through a base paper through one curtain film having a plurality of layers. . The method according to claim 5, wherein the whiteness of the overcoat layer is 85% or more, and the specific scattering coefficient of the undercoat layer adjacent to the overcoat layer is 80 m ² / kg or more. The method according to claim 5 or 6, wherein a specific scattering coefficient of an undercoat layer adjacent to the overcoat layer is 120 m ² / kg or more.   Coated white paperboard produced by the method according to any one of claims 1 to 7.