JP2022165168A - white paperboard - Google Patents

Abstract

To provide a white paperboard that can prevent the occurrence of Japanese hydrangea-like spots and also has practically sufficient strength.SOLUTION: The present invention discloses a white paperboard at least comprising a surface layer, a subsurface layer, a middle layer and a back layer. The subsurface layer contains calcium carbonate. The calcium carbonate content of the subsurface layer is 3 mass% or more relative to the solid content 100 mass% of the subsurface layer. The subsurface layer has an ash content of 35% or less.SELECTED DRAWING: None

Description

The present invention relates to white paperboard.

A multi-layered white paperboard having at least a surface layer, a surface and bottom layer, a middle layer and a back layer is known (Patent Document 1). As described in Patent Document 1, waste paper pulp is generally used for the middle layer of white paperboard from the viewpoint of low cost and resource saving.
White paperboard using waste paper pulp is known to have a problem with so-called hydrangea spots. Hydrangea spots are caused when white paperboard containing sublimation ink lumps is stored at room temperature for several months. The resulting multicolored spots. White paperboard with hydrangea spots on it tends to be avoided by consumers because of its appearance, so it has no choice but to be discarded, causing the problem of wasted paper.

It has been pointed out that sublimation transfer paper is mixed into used paper, which is a raw material of used paper pulp, as a cause of the mixture of sublimable ink lumps into white paperboard (Patent Document 2). Therefore, Patent Document 2 proposes a specific sublimation transfer paper having an ink-receiving layer containing a water-soluble binder.
In the specific sublimation transfer paper described in Patent Document 2, even if it is collected as used paper, the transfer ink is likely to disperse in water together with the water-soluble binder in the defibering process for making used paper pulp. Therefore, lumps of the transfer ink are less likely to remain on the waste paper pulp, and generation of hydrangea spots is suppressed when the waste paper pulp is used as a raw material for white paperboard.

JP-A-6-41896 JP 2019-64187 A

In principle, it is prohibited to dispose of the sublimation transfer paper by separating it into used paper. In recent years, however, used sublimation transfer paper is increasingly being reused as stuffing for shoes and bags. It is difficult to distinguish whether or not the used sublimation transfer paper is sublimation transfer paper by appearance, and contamination with waste paper is inevitable.

In addition, even if there is a specific sublimation transfer paper on which measures such as those proposed in Patent Document 2 are taken, there are many general sublimation transfer papers on the market that do not take any measures against hydrangea spots. ing. Therefore, it cannot be ruled out that sublimation transfer paper, which does not take measures against hydrangea spots, is mixed with used paper. Therefore, there is still a possibility that hydrangea spots may occur in white paperboard using waste paper pulp.
The present invention provides a white paperboard that can suppress the occurrence of hydrangea spots.

The present invention has the following aspects.
[1] A white paperboard having at least a surface layer, a surface and bottom layer, a middle layer and a back layer, wherein the surface and bottom layers contain calcium carbonate, and the calcium carbonate content of the surface and bottom layers is 100 mass% of the solid content of the surface and bottom layers. white paperboard, wherein the ash content of the surface lower layer is 35% or less.
[2] The white paperboard of [1], wherein the surface layer also contains calcium carbonate.
[3] The white paperboard of [1] or [2], wherein the basis weight of the surface lower layer is 15 to 90 g/m ² .
[4] The white paperboard according to any one of [1] to [3], wherein a pigment-coated layer containing a pigment and a binder is provided on the surface of the surface layer.
[5] Further has a lower back layer between the back layer and the intermediate layer, the lower back layer also contains calcium carbonate, and the content of calcium carbonate in the lower back layer is 100% by mass of the solid content of the lower back layer The white paperboard according to any one of [1] to [4], wherein the content is 3% by mass or more.
[6] The white paperboard of [5], wherein the lower back layer has an ash content of 35% or less.
[7] The white paperboard according to any one of [1] to [6], wherein the calcium carbonate is light calcium carbonate.
[8] The white paperboard of [7], wherein the light calcium carbonate has a volume-based average primary particle diameter _D50 of 0.2 to 10 μm.

ADVANTAGE OF THE INVENTION According to this invention, the white paperboard which can suppress generation|occurrence|production of a hydrangea spot is provided.

The white paperboard of the present invention has at least a surface layer, a surface and bottom layer, a middle layer and a back layer.
The surface layer is the layer that is arranged on the frontmost side of the multi-layered white paperboard, and the back layer is the layer that is arranged on the rearmost side (opposite side to the surface layer). The subsurface layer is the layer immediately below the surface layer. The lower surface layer can also be said to be a layer that contacts the surface layer inside the white paperboard. In the white paperboard of the present invention, the middle layer is arranged between the front and bottom layers and the back layer.

(surface)
The surface layer is composed of pulp with high whiteness. Pulp having a high degree of whiteness includes deinked pulp of waste paper and bleached pulp. Among them, bleached pulp is preferable because of its high degree of whiteness.
Deinked pulp is deinked white waste paper pulp. Waste paper used as a raw material for deinked pulp includes, for example, paper that has been used once but has a small amount of printing such as super white, ruled white, special white, medium white, and white manila; Printed matter such as white art and paper that has been colored and used once; Used high-quality waste paper such as coated paper for printing, beverage packs, and office paper; waste paper; general waste paper such as newspapers and magazines; The used paper may be confidential office paper or shredded materials such as tickets. These may be used singly or in combination of two or more.
As the deinked pulp for the surface layer, deinked pulp derived from waste paper with a high degree of whiteness such as fine white/card, extra white/medium white/white Manila, imitation/high color, etc.; and deinked pulp derived from Kent paper are preferable.

Examples of so-called bleached pulp that is not waste paper pulp include unbleached softwood kraft pulp (NUKP), unbleached hardwood kraft pulp (LUKP), bleached softwood kraft pulp (NBKP), bleached hardwood kraft pulp (LBKP), and semi-bleached hardwood pulp. Kraft pulp (LSBKP), softwood semi-bleached kraft pulp (NSBKP), hardwood sulfite pulp, chemical pulp such as softwood sulfite pulp, thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), chemiground pulp (CGP), refiner Mechanical pulps such as ground pulp (RGP), ground pulp (GP), pressure stone ground pulp (PGW), stone ground pulp (SGP) and the like can be mentioned.

It is preferable that the surface layer of the white paperboard of the present invention also contains calcium carbonate. Details and preferred aspects of calcium carbonate will be described later in the section (lower surface layer).
Calcium carbonate is often used in waste paper recovered as a raw material for waste paper pulp. Therefore, when waste paper pulp is used to make the surface layer, calcium carbonate derived from waste paper is often mixed in the surface layer. From the above, the content of calcium carbonate in the surface layer is the amount of calcium carbonate added to the paper material used to make the surface layer (hereinafter referred to as "surface layer material") when the waste paper pulp is not used for the surface layer. When using waste paper pulp for the paper making of the surface layer, it is the total amount of calcium carbonate derived from waste paper pulp and calcium carbonate added to the surface layer paper material.
The content of calcium carbonate in the surface layer is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and even more preferably 3 to 12% by mass based on 100% by mass of the solid content of the surface layer. When the content of calcium carbonate in the surface layer is at least the above lower limit, the darkness of the middle layer can be sufficiently concealed. If the content of calcium carbonate in the surface layer is equal to or less than the above upper limit, it is easy to ensure the strength of the white paperboard.

The ash content of the surface layer is preferably 3 to 35%, more preferably 5 to 30%, even more preferably 10 to 25%. When the ash content of the surface layer is at least the lower limit, the surface whiteness of the white paperboard can be easily increased. If the ash content of the surface layer is equal to or less than the upper limit, it is easy to ensure the strength of the white paperboard.
The ash content of each layer can be measured according to JIS P-8251. Ash is the amount of residue after burning, and corresponds to the amount of calcium carbonate, other inorganic pigments, and the like. When waste paper pulp is used, it corresponds to the amount of inorganic pigments including those contained in the waste paper pulp.

The basis weight of the surface layer is preferably 15-90 g/m ² , more preferably 25-70 g/m ² . The basis weight of each layer can be measured according to JIS P8124.
If the basis weight of the surface layer is at least the above lower limit, the darkness of the middle layer can be sufficiently hidden. Further, when the basis weight of the surface layer is equal to or less than the upper limit, it is easy to suppress swelling during papermaking.

(Surface layer)
The lower surface layer is composed of pulp having a lower whiteness than the pulp of the surface layer and a higher whiteness than the pulp of the middle layer. As the pulp constituting the lower surface layer, deinked pulp derived from waste paper, which is lower grade than that of the surface layer, that is, deinked pulp derived from waste paper containing a large amount of medium-quality fibers can be preferably used. For example, deinked pulp derived from newspapers, magazines, color, balls and the like is preferred. Among them, deinked pulp derived from waste magazine paper is preferred.

In the white paperboard of the present invention, the lower surface layer contains calcium carbonate. Presumably, most of the fine calcium carbonate in the lower surface layer moves below the lower surface layer (on the wire side) when the lower surface layer is made into paper. When the paper-made surface lower layer and the middle layer are superimposed and then dried, calcium carbonate is porous, so it absorbs the sublimation transfer ink component dissolved in the water of the middle layer and dries in a captured state. be. Furthermore, since calcium carbonate is opaque, the transfer ink captured by calcium carbonate can be hidden. Therefore, the occurrence of hydrangea spots is suppressed.

The content of calcium carbonate in the lower surface layer is the carbonic acid added to the paper material used for making the lower surface layer (hereinafter referred to as "lower surface layer paper material") when the waste paper pulp is not used for the paper making of the lower surface layer. It is the amount of calcium, and when waste paper pulp is used to make the lower surface layer, it is the total amount of calcium carbonate derived from the waste paper pulp and calcium carbonate added to the lower surface layer paper material.
The content of calcium carbonate in the lower surface layer is 3% by mass or more, preferably 3 to 15% by mass, more preferably 3 to 10% by mass, based on 100% by mass of the solid content of the lower surface layer. When the content of calcium carbonate in the surface lower layer is at least the above lower limit, the effect of absorbing and trapping the sublimable transfer ink by calcium carbonate is sufficiently obtained. In addition, the surface whiteness of white paperboard tends to increase. Therefore, it is possible to reduce the amount of pulp having a high degree of whiteness used in the surface layer.
If the content of calcium carbonate in the surface lower layer is equal to or less than the above upper limit, it is easy to ensure the strength of the white paperboard.

The ash content of the surface lower layer is 35% or less, preferably 5 to 30%, more preferably 10 to 28%, even more preferably 15 to 25%. If the ash content of the surface lower layer is at least the lower limit, it is easy to suppress the occurrence of hydrangea spots. If the ash content of the surface lower layer is equal to or less than the above upper limit, the strength of the white paperboard tends to be sufficient for practical use.

Calcium carbonate may be light calcium carbonate or heavy calcium carbonate, but light calcium carbonate is preferred. As calcium carbonate, light calcium carbonate and heavy calcium carbonate may be used in combination.

The volume-based average primary particle diameter D ₅₀ of precipitated calcium carbonate is preferably 0.2 to 10 μm, more preferably 1 to 5 μm, even more preferably 2 to 3 μm.
Here, the primary particle size refers to a particle size measured after an aqueous dispersion of light calcium carbonate is dropped into the circulation system of a laser diffraction particle size distribution analyzer and then subjected to ultrasonic treatment. The volume-based average primary particle diameter _D50 is the 50% volume-based primary particle diameter.

The basis weight of the surface lower layer is preferably 15 to 90 g/m ² , more preferably 20 to 70 g/m ² and even more preferably 25 to 55 g/m ² .
If the basis weight of the surface and bottom layers is at least the above lower limit, the darkness of the middle layer can be sufficiently concealed together with the surface layers. In addition, it is easy to obtain the effect of capturing the transfer ink by the calcium carbonate in the lower surface layer. When the basis weight of the surface and bottom layer is equal to or less than the upper limit, it is easy to suppress swelling during papermaking.
In the white paperboard of the present invention, the basis weight of the lower surface layer is preferably larger than the basis weight of the surface layer because cracks on the surface of the white paperboard can be suppressed.

(middle class)
The middle layer is generally made of the lowest grade pulp among the layers constituting the white paperboard. For example, disaggregated pulp such as newspapers, magazines, tickets, medium-quality rolls, tea imitations, corrugated cardboard, mounts, coupons, and balls can be mentioned.
The middle layer may be formed with one layer or with a plurality of layers. However, in order to increase the amount of low-quality waste paper pulp used, it is preferable to form the middle layer with a plurality of layers. When the middle layer is composed of a plurality of layers, the pulp constituting each layer of the middle layer may be the same or different.
The total basis weight of the middle layer is appropriately adjusted according to the thickness required by the use of the white paperboard. The basis weight per layer of the intermediate layer is, for example, preferably 15 to 90 g/m ² , more preferably 25 to 75 g/m ² .

(lower back layer)
The white paperboard of the present invention may further have a lower back layer in addition to the surface layer, lower surface layer, middle layer and back layer. A lower backing layer is disposed between the backing layer and the middle layer. The lower back layer can also be said to be a layer that contacts the back layer inside the white paperboard.
The pulp constituting the lower back layer has a higher degree of whiteness than the pulp of the middle layer and a lower degree of whiteness than the pulp of the back layer. The pulp used for the surface and bottom layers may be used for the back and bottom layers, but usually low grade waste paper compared to the surface layer, that is, waste paper containing a large amount of medium-density fibers is used. For example, it is common to use unbleached deinked waste paper pulp such as newspapers, magazines, colored papers, balls, and the like.

When the whiteboard of the present invention further has an under-back layer, the under-back layer may also contain calcium carbonate. The calcium carbonate in the lower back layer can function as an adsorbent for the sublimation transfer ink in the same way as the calcium carbonate in the front and lower layer, and can also suppress the generation of hydrangea spots on the surface of the back layer by trapping the transfer ink. Especially when it is desired to suppress the occurrence of hydrangea spots in the back layer, it is preferred that the under back layer also contain calcium carbonate.

The content of calcium carbonate in the lower back layer is the calcium carbonate added to the paper stock used for making the lower back layer (hereinafter referred to as "lower back layer paper material") if waste paper pulp is not used to make the lower back layer. When waste paper pulp is used to make the back and bottom layer, it is the total amount of the calcium carbonate derived from the waste paper pulp and the calcium carbonate added to the back and bottom layer stock.
The content of calcium carbonate in the lower back layer is preferably 3% by mass or more, more preferably 2 to 15% by mass, and even more preferably 3 to 10% by mass, based on 100% by mass of the solid content of the lower back layer. When the content of calcium carbonate in the lower back layer is at least the above lower limit, there is a tendency that calcium carbonate has a sufficient effect of absorbing and trapping the sublimation transfer ink. In addition, the surface whiteness of the back layer tends to be high. Therefore, when a pulp having a high degree of whiteness is used for the backing layer, the amount used can be reduced.
When the content of calcium carbonate in the lower back layer is equal to or less than the above upper limit, it is easy to ensure the strength of the white paperboard.

The ash content of the underside layer is preferably 35% or less, more preferably 10 to 30%, even more preferably 15 to 25%. If the ash content of the surface lower layer is at least the lower limit, it is easy to suppress the occurrence of hydrangea spots. When the ash content of the back lower layer is equal to or less than the upper limit, it is easy to ensure the strength of the white paperboard.

The basis weight of the lower back layer is preferably 15 to 90 g/m ² , more preferably 25 to 75 g/m ² .
If the basis weight of the lower back layer is at least the above lower limit, the colored foreign matter in the middle layer can be sufficiently concealed together with the back layer. In addition, it is easy to obtain the effect of capturing the transfer ink by the calcium carbonate in the lower back layer. If the basis weight of the lower back layer is equal to or less than the above upper limit, sufficient strength of the paper layer can be obtained.

(back layer)
The back layer is not required to be as white as the surface layer, but since it is visible to the human eye, pulp with a higher whiteness than that of the middle layer is usually used. The backing layer may comprise pulp from shredded material such as office paper. Pulp obtained by defibrating shredded materials such as office paper has a relatively high degree of whiteness even without deinking and bleaching, so it is preferably blended in the backing layer.

The basis weight of the backing layer is preferably 15-90 g/m ² , more preferably 25-70 g/m ² .
If the basis weight of the back layer is at least the above lower limit, colored foreign matter in the middle layer can be sufficiently concealed. Further, when the basis weight of the backing layer is equal to or less than the above upper limit, sufficient strength of the paper layer can be obtained.

Each layer of the white paperboard may further contain a paper strength enhancer since the strength of the white paperboard tends to be higher. Details and preferred embodiments of the paper strength agent will be described in the section (Manufacturing method) below.

(Pigment coating layer)
In the white paperboard of the present invention, a pigment coating layer containing a pigment and a binder may be provided on the surface of the surface layer.
Examples of pigments include those commonly used in the field of coated paper production, such as kaolin, calcium carbonate (heavy calcium carbonate, light calcium carbonate), titanium dioxide, aluminum hydroxide, silica, satin white and talc. Among these, calcium carbonate is preferable because it exhibits excellent printability.

A water-based adhesive is preferable as the binder. Examples of water-based adhesives include starches such as oxidized starch, phosphate esterified starch, hydroxyethyl etherified starch, dextrin, enzyme-modified starch, and water-soluble starch; styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer. latex, conjugated diene copolymer latex such as styrene-methyl methacrylate-butadiene copolymer latex, acrylic copolymer latex such as acrylic acid ester and/or methacrylic acid ester copolymer latex; casein; , gelatin, soybean protein and other proteins, various polyvinyl alcohols, various polyacrylamides, synthetic resin adhesives such as melamine resins, and various cellulose derivatives such as carboxymethyl cellulose. One or more of these adhesives can be used as the binder.

Among these, a latex having a glass transition temperature of −50 to 30° C. as measured by Vibron viscoelasticity is preferred because it increases the flexibility of the coated surface and improves the resistance to bending and cracking. More preferably, the latex has a glass transition temperature of -50 to 0°C.
It is also preferred to incorporate starch with the latex. The combination of latex and starch balances the fixation of fine fibers within the backing layer and the surface strength of the surface of the backing layer.
The weight ratio of latex to starch is preferably from 100:0 to 5:50.

Pigment coating liquid, if necessary, a dispersing agent, sodium hydroxide, a pH adjuster such as aqueous ammonia; an antifoaming agent; a fluorescent dye; It may further contain one or more of preservatives, dyes, color pigments, and the like.

(Production method)
The white paperboard of the present invention can be produced, for example, by multi-layer papermaking of paper stock containing pulp slurry. Calcium carbonate is added to the surface and bottom layer stock. In the case of producing white paperboard containing calcium carbonate in both the surface layer and the back and bottom layers, calcium carbonate is added to the surface layer stock and the back and bottom layer stock. Calcium carbonate is preferably added as a calcium carbonate dispersion in which calcium carbonate is dispersed in water. This makes it possible to add a fixed amount, and it becomes easy to suppress fluctuations in the amount to be added. When calcium carbonate is added as a calcium carbonate dispersion, a dispersant may be added to the calcium carbonate dispersion in order to obtain good dispersibility. Dispersants include anionic surfactants and the like.
In adjusting the stock used for making paper for each layer, the amount of calcium carbonate to be added is appropriately adjusted so that the content of calcium carbonate in each layer is as described above.

The paper material used for making paper for each layer can be prepared by adding a pulp slurry constituting each layer and, if necessary, calcium carbonate, a paper strength agent, aluminum sulfate, and other components.
The composition of the stock used for making paper for each layer can be appropriately changed in consideration of the quality required for the white paperboard, convenience in manufacturing, and the like. For example, the ratio of deinked pulp and bleached pulp to the total pulp in the stock are not particularly limited. It is appropriately adjusted in consideration of which layer the stock is to be used for forming.

A paper strength agent is a papermaking chemical for giving strength to paper products. Strength agents are divided into wet strengthening agents and dry strengthening agents. The paper strength agent is preferably a dry paper strength agent in order to improve internal strength, surface strength and interlaminar strength.

Among the dry paper strength agents, it is preferable to use a polyacrylamide-based paper strength agent for the effect of developing strength and improving the yield of pulp. The proportion of the polyacrylamide-based paper strength agent in the paper strength agent used in the stock is preferably 0.1% or more, more preferably 0.4% or more, and particularly preferably only the polyacrylamide-based paper strength agent.

The polyacrylamide-based paper strength agent is not particularly limited, and any of anionic, amphoteric and cationic agents can be used. Among these, an amphoteric polyacrylamide-based paper strength agent is preferable because of its wide fixing area.

Examples of paper strength agents other than polyacrylamide-based paper strength agents include natural paper strength agents such as starch and vegetable gums, polyvinyl alcohol resins, styrene-butadiene latex, urea-formaldehyde resins, melanin resins, polyamide resins, and ketone resins. , polyamide epichlorohydrin resins, polyamine epichlorohydrin resins, polyamide epoxy resins, glycerol polyglycidyl ether resins, polyethyleneimine resins and other synthetic paper strength agents. These can be used singly or in combination.
Among these, starches, polyvinyl alcohol resins, and styrene-butadiene latex are preferred, and starches are particularly preferred. Examples of starches include oxidized starch, esterified starch such as urea phosphate esterified starch, enzyme-modified starch, etherified starch such as hydroxyethyl etherified starch, and the like, and these can be used alone or in combination. can do. Among them, it is preferable to use oxidized starch.

The amount of the paper strength agent added is preferably 0.1 to 1.5% by mass, more preferably 0.4 to 1.0% by mass, based on 100% by mass of the solid content of the pulp slurry.
When the addition amount of the paper strength agent is equal to or more than the lower limit, it is possible to more reliably suppress the formation of precipitates. In addition, when it is equal to or less than the above upper limit, it is easy to suppress the deterioration of drainage due to over-aggregation.

Aluminum sulfate is the common name for aluminum sulfate. Aluminum sulfate is a representative inorganic flocculant. By adding aluminum sulfate, even when waste paper pulp is used, it is possible to suppress the dropping of fine fibers in the wire part and improve the yield of the pulp slurry.

The amount of aluminum sulfate added is preferably 0.1 to 2.0% by mass, more preferably 0.4 to 1.0% by mass, based on 100% by mass of the solid content of the pulp slurry.
When the addition amount of aluminum sulfate is equal to or more than the above lower limit, it is easy to obtain the effect of improving the yield of the pulp slurry. In addition, when the content is equal to or less than the upper limit, it is possible to more reliably suppress the formation of precipitates.

Other components can be added to the paper material within limits that do not impair the effects of the present invention. Other ingredients include pigments other than calcium carbonate, waterproofing agents, water repellents, foaming microcapsules, sizing agents, dyes, fillers, pH adjusters, slime control agents, thickeners, preservatives, and antifungal agents. , antibacterial agents, flame retardants, rodenticides, insect repellents, moisturizers, freshness agents, oxygen absorbers, microcapsules, foaming agents, surfactants, electromagnetic shielding materials, antistatic agents, rust inhibitors, fragrances, deodorants agents and the like. These may be used individually by 1 type, and may use 2 or more types together.
Pigments other than calcium carbonate include clay, clay, calcined clay, talc, titanium oxide, and the like.

The papermaking method for forming each layer is not particularly limited. It is preferable to use a paper machine having at least a wire part, a press part and a dryer part. Also, a size press may be performed in the dryer part. A paper machine having a calendering part (pre-calendering) after the dryer part may also be used.

As the wire part, a known multistage wire part can be used as appropriate. A multistage wire part has at least two or more wire part portions.
Types of wire parts generally used for papermaking include a circular net type, a fourdrinier type, a short net type, an inclined type, and a twin wire type.
In a multistage wire part, these methods can be combined in multiple stages. Examples include fourdrinier mesh, short mesh mesh, short mesh combination, fourdrinier mesh combination, and the like.

As the press part, a known press machine can be used as appropriate, but a shoe press is preferably included. It is also preferable to combine a roll press and a shoe press. When a shoe press is used, water can be sufficiently squeezed in the press part, so it is easy to control the amount of water when size-pressing in the dryer part.

A known drier can be appropriately used as the drier part. For example, a non-contact dryer such as a cylinder dryer that contacts and heats the white paperboard in a wet state, an infrared dryer, a hot air dryer, or the like can be used alone or in combination.

When the pigment coating layer is provided on the surface of the surface layer of white paperboard, the coating layer is provided by applying a pigment coating liquid to the surface of the surface layer. Next, the coating layer is dried to form a pigment coating layer on the surface of the surface layer.
The pigment coating liquid preferably contains 2 to 50 parts by mass, more preferably 5 to 25 parts by mass, of the binder with respect to 100 parts by mass of the pigment.
If the proportion of the binder is at least the above lower limit, the strength of the pigment coating layer tends to be sufficient. In addition, if the ratio of the binder is equal to or less than the above upper limit value, the ink drying property is excellent and the suitability for box-making is also excellent.
The coating amount of the pigment coating liquid is preferably 5 to 50 g/m ² , preferably 8 to 40 g/m ² , more preferably 10 to 35 g/m ² in terms of dry mass.

The pigment coating liquid may be applied in multiple times. When the pigment coating liquid is applied a plurality of times, for example, two times, the first coating liquid (undercoat layer) and the second coating liquid (topcoat layer) may be the same or different.
The pigment coating liquid may be applied directly to the surface layer, or may be applied via a coating film of a size press liquid applied to the surface layer. Before applying the pigment coating liquid, the surface layer surface may be smoothed by pre-calendering. The pigment coating liquid may be applied only once or may be applied multiple times. In particular, it is preferable to apply the pigment coating liquid for the undercoat layer and the pigment coating liquid for the topcoat layer to sequentially form the undercoat layer and the topcoat layer.
By applying the pigment coating liquid, it is possible to improve the printability of the surface layer side and increase the degree of whiteness.

The coater may be either on-machine or off-machine, but is preferably on-machine using a known coater part attached to the paper machine. Examples of coating equipment for the coater part include blade coaters, air knife coaters, roll coaters, reverse roll coaters, bar coaters, curtain coaters, slot die coaters, gravure coaters, champlex coaters, brush coaters, slide bead coaters, two-roll or A metering blade type size press coater, a bill blade coater, a short dwell coater, a gate roll coater, a calender nip coater, and the like are appropriately used.
Among these, the rod metering coater and the curtain coater are preferable because the coating amount is constant and a coating layer with less coating unevenness can be obtained. The applied coating layer is dried with a known drying device to form a pigment coating layer. The paper machine may have multiple coater parts. In this case, the coating layer can be applied in a plurality of times.

A calendar part may be provided after the coater part, if necessary. By providing a calendar part after the coater part, the pigment coating layer is smoothed.
For the calendering part after the coater part, a known calendering device can be appropriately used, and examples thereof include a super calender, a gloss calender, a soft nip calender, a heat calender, and a shoe calender. These may be used in combination. Among them, a soft nip calender provided with metal rolls and elastic rolls is preferable because the coating layer can be smoothed while maintaining the thickness of the paper. The calendar part after the coater part can be either on-machine or off-machine.

(Mechanism of action)
The white paperboard of the present invention described above contains calcium carbonate in the lower surface layer. The calcium carbonate in the lower surface layer can function as an adsorbent for the sublimable transfer ink. Therefore, even if the sublimable transfer ink sublimates from the middle layer or the like when the white paperboard is stored at room temperature for a long period of time, the transfer ink can be caught by the calcium carbonate in the surface and lower layers before the transfer ink reaches the surface layer.
Furthermore, since the content of calcium carbonate is 3% by mass or more with respect to 100% by mass of the solid content of the lower surface layer, a sufficient effect of trapping the sublimation transfer ink by calcium carbonate can be obtained.
Therefore, according to the white paperboard of the present invention, it is possible to suppress the occurrence of hydrangea spots on the surface of the white paperboard.
As described above, in the white paperboard of the present invention, the lower surface layer contains calcium carbonate, but since the ash content of the lower surface layer is 35% or less, the content of inorganic components in the lower surface layer is appropriate, and the occurrence of hydrangea spots is suppressed. However, the strength of the white paperboard is sufficient for practical use.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following description.

<Method for measuring ash>
The ash content of white paperboard was measured according to JIS P-8251.

<Example 1>
A multi-layer paper machine was used to obtain a white paperboard having a top layer, a bottom layer, a middle layer and a back layer. Here, the multilayer paper was made so that the basis weight of the surface layer was 40 g/m ² , the basis weight of the upper and lower layers was 45 g/m ² , the basis weight of the middle layer was 135 g/m ² , and the basis weight of the back layer was 45 g/m ² . I did
As surface paper material, 100 parts by mass of pulp with a whiteness of 85%, which is a mixture of 80% by mass of deinked pulp obtained by deinking high-quality waste paper (collected from printing and bookbinding companies) and 20% by mass of bleached hardwood kraft pulp. In addition, 8 parts by mass of light calcium carbonate (trade name “Tamapearl TP121-6S”, manufactured by Okutama Kogyo Co., Ltd., volume-based average primary particle size 2 to 3 μm) was used.
As the surface and lower layer paper material, light calcium carbonate (trade name “Tamapearl TP121-6S”, manufactured by Okutama Kogyo Co., Ltd., volume-based average primary particle size 2 to 3 μm) is added to 100 parts by mass of deinked magazine pulp with a whiteness of 70%. 11 parts by mass of was used.
Used magazine pulp having a whiteness of 40% was used as the pulp for the middle layer. The waste magazine pulp contained 0.1% by mass of pulp derived from sublimation transfer paper.
As the pulp for the back layer, a pulp having a whiteness of 51% was used, which was blended at a ratio of 50% by mass of waste newspaper, 40% by mass of waste magazine paper, and 10% by mass of pulp derived from shredded office waste paper.
The ash content of the lower surface layer of the white paperboard of Example 1 was 15.0%.

<Example 2>
Multi-layer paper was made in the same manner as in Example 1 except that 100 parts by mass of deinked recycled magazine pulp with a whiteness of 70% and 4 parts of light calcium carbonate were blended as the surface and lower layer paper material. A white paperboard was obtained. The ash content of the lower surface layer of the white paperboard of Example 2 was 11.2%.

<Example 3>
Multi-layer papermaking was performed in the same manner as in Example 1, except that 100 parts by mass of deinked waste magazine pulp with a whiteness of 70% and 13 parts by mass of light calcium carbonate were blended as the surface and lower layer paper material. , obtained a white paperboard. The ash content of the lower surface layer of the white paperboard of Example 3 was 16.8%.

<Comparative Example 1>
Multi-layer papermaking was carried out in the same manner as in Example 1, except that light calcium carbonate was not blended in the surface and lower layer paper stock, to obtain white paperboard. The ash content of the lower surface layer of the white paperboard of Comparative Example 1 was 9.5%.

<Comparative Example 2>
Multi-layer papermaking was carried out in the same manner as in Example 1, except that 33 parts by mass of light calcium carbonate was blended with 100 parts by mass of deinked recycled magazine pulp with a whiteness of 70% as the surface and lower layer paper material. , obtained a white paperboard. The ash content of the lower surface layer of the white paperboard of Comparative Example 2 was 38.2%.

<Comparative Example 3>
Multi-layer paper in the same manner as in Example 1 except that synthetic silica (trade name “Fine Seal X-37” manufactured by Tokuyama, average particle size 6 μm) was used as the surface lower layer paper material instead of light calcium carbonate. A white paperboard was obtained. The ash content of the lower surface layer of the white paperboard of Comparative Example 3 was 15.4%.

<Evaluation method>
The white paperboards obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were evaluated by the following methods. Table 1 shows the results.

(Surface strength of surface layer)
Red ink (trade name “PrintInk [paper test, SD50, red B, T=13]”, manufactured by T&K TOKA Co., Ltd.) was applied to the surface of the obtained white paperboard using an RI printing tester (manufactured by Akira Seisakusho Co., Ltd.). After printing, the presence or absence of peeling (picking) on the surface of the ink-printed portion was visually observed, and the surface strength of the surface layer was determined according to the following criteria.
⊚: Very good with no picking.
Good: Almost no picking occurs, good.
Δ: Some picking occurred.
x: Remarkable picking occurred.

(surface quality of the surface layer)
The surface quality of the coated surface on the surface layer side of the coated paperboard obtained was visually observed, and the surface quality of the surface layer was evaluated according to the following criteria.
⊚: Excellent in smoothness, glossy, and excellent in whiteness.
Good: Slightly inferior in one or two of smoothness, glossiness, and whiteness, but no problem in practical use.
Δ: Poor in one of smoothness, glossiness, and whiteness, causing practical problems.
x: The surface is rough, and there is a problem in practical use.

(hydrangea spots)
Using the obtained white paperboard, a thermal transfer press (Mini Test Press MP-2F manufactured by Toyo Seiki Seisakusho Co., Ltd.) in which the upper and lower press plates are heated to 180 ° C. is used. was placed and hot pressed for 15 minutes under a pressure of 6 MPa. After standing at room temperature for 10 minutes, the state of the surface was visually observed.
A: Hydrangea spots are not observed at all.
◯: Hydrangea spots are slightly visible, but there is no problem.
Δ: Hydrangea spots can be visually recognized, but there is no problem depending on the application.
x: Hydrangea spots are clearly visible, which is problematic.

In Examples 1 to 3, it was confirmed that generation of hydrangea spots could be suppressed and white paperboard having practically sufficient strength could be provided.
In Comparative Example 1, the lower surface layer did not contain calcium carbonate, and the occurrence of hydrangea spots could not be suppressed.
In Comparative Example 2, the ash content of the surface lower layer was large, and the surface strength was lowered.
In Comparative Example 3, the lower surface layer did not contain calcium carbonate, and the surface quality was insufficient when the surface layer was visually observed.

Claims (8)

A white paperboard having at least a surface layer, a surface and bottom layer, a middle layer and a back layer,
The lower surface layer contains calcium carbonate, the content of calcium carbonate in the lower surface layer is 3% by mass or more relative to 100% by mass of the solid content of the lower surface layer, and the ash content of the lower surface layer is 35% or less. A white paperboard characterized by: 2. The white paperboard of claim 1, wherein said surface layer also comprises calcium carbonate. The white paperboard according to claim 1 or 2, wherein the basis weight of the surface lower layer is 15 to 90 g/m ² . The white paperboard according to any one of claims 1 to 3, wherein a pigment coating layer containing a pigment and a binder is provided on the surface of the surface layer. further comprising a lower back layer between the back layer and the middle layer;
5. The method according to any one of claims 1 to 4, wherein the lower back layer also contains calcium carbonate, and the content of calcium carbonate in the lower back layer is 3% by mass or more relative to 100% by mass of the solid content of the lower back layer. white paperboard. 6. The white paperboard of claim 5, wherein the underside layer has an ash content of 35% or less. White paperboard according to any one of claims 1 to 6, wherein the calcium carbonate is light calcium carbonate. The white paperboard according to claim 7, wherein the light calcium carbonate has a volume-based average primary particle diameter _D50 of 0.2 to 10 µm.