JP7772149B2 - White paperboard and method for manufacturing coated white paperboard - Google Patents

Description

The present invention relates to a method for producing white paperboard and coated white paperboard.

White paperboard is a thick cardboard made of 3 to 9 layers and has long been used for various packaging boxes, etc. Due to cost and resource conservation requirements, recycled paper pulp is used for white paperboard.
The middle layer is made of undeinked recycled paper pulp. The surface and subsurface layers are made of high-whiteness pulp such as virgin pulp. If whiteness is required on both sides, the back layer is made the same as the surface layer, but if not, recycled paper pulp with low whiteness is used.
The surface layer, the undersurface layer, etc. are required to conceal the color of the intermediate layer, and therefore, pigments are added to these layers to increase their opacity (Patent Document 1).

In many cases, recycled paper pulp is used in place of or in addition to virgin pulp for the surface and subsurface layers. Deinked recycled paper pulp with high whiteness (deinked pulp) is used as the recycled paper pulp for the surface and subsurface layers.
When deinked pulp is used for the stock forming the surface layer or subsurface layer, a manufacturing problem occurs in that the stock yield is likely to decrease.
This is because deinked pulp tends to contain finer fibers than virgin pulp, which tend to fall off in the wire part.

Japanese Patent Application Publication No. 6-41896

In order to improve the yield of paper stock containing deinked pulp, it is possible to add aluminum sulfate to the stock. However, when aluminum sulfate is used with paper stock containing deinked pulp, precipitates can adhere to the piping of the papermaking equipment. If these precipitates fall off the piping and get mixed into the paper stock, there is a risk that the quality of the white board will be impaired.

Such problems tend to occur particularly when calcium carbonate is added as a pigment to increase opacity in addition to aluminum sulfate to a stock containing deinked pulp.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing white paperboard or coated white paperboard of stable quality without mixing precipitates into the paperstock while maintaining the yield of paperstock containing deinked pulp.

In order to achieve the above object, the present invention employs the following configuration.
[1] A method for producing a white paperboard having multiple layers, comprising adding aluminum sulfate to a pulp slurry containing deinked pulp, then adding a paper strength agent, and then adding calcium carbonate to prepare a stock, and then making the prepared stock into paper to form at least a portion of the multiple layers.
[2] The method for producing white paperboard according to [1], wherein the white paperboard has at least a surface layer, a middle layer, and a back layer, and the surface layer is formed by papermaking the prepared paper stock.
[3] The method for manufacturing white paperboard according to [1], wherein the white paperboard has at least a surface layer, a subsurface layer, a middle layer, and a back layer, and the prepared paper stock is made into paper to form at least one of the surface layer and the subsurface layer.
[4] The method for producing white paperboard according to any one of [1] to [3], wherein 0.1 to 2.0% by mass of aluminum sulfate is added relative to 100% by mass of the solid content of the pulp slurry.
[5] The method for producing white paperboard according to any one of [1] to [4], wherein the paper strength agent is added in an amount of 0.1 to 2.0% by mass relative to 100% by mass of the solid content of the pulp slurry.
[6] The method for producing white paperboard according to any one of [1] to [5], wherein the calcium carbonate is added in an amount of 1 to 20% by mass relative to 100% by mass of the solid content of the pulp slurry.
[7] The method for producing white paperboard according to any one of [1] to [6], wherein the paper strength agent is a polyacrylamide-based paper strength agent.
[8] A method for producing coated white paperboard, comprising the steps of obtaining a white paperboard by the method for producing white paperboard according to any one of [1] to [7], and forming a pigment coating layer on at least one surface of the obtained white paperboard.

The method for producing white paperboard or coated white paperboard of the present invention makes it possible to produce white paperboard or coated white paperboard of consistent quality without introducing precipitates into the paperstock, while maintaining the yield of paperstock containing deinked pulp.

[White paperboard]
The white paperboard obtained by the method for producing white paperboard of the present invention is a multi-layer paperboard having multiple layers. The white paperboard obtained by the method for producing white paperboard of the present invention is preferably a multi-layer paperboard of three or more layers, with at least a surface layer, a middle layer, and a back layer laminated together, and more preferably a multi-layer paperboard of four or more layers, with a surface layer, a lower surface layer, a middle layer, and a back layer laminated together.

The surface layer is the layer that is located on the top side of the white paperboard, the back layer is the layer that is located on the bottom side (opposite the surface layer), and the middle layer is the layer that is located between the surface layer and the back layer. A subsurface layer may be located between the surface layer and the middle layer. A subsurface layer may be located between the back layer and the middle layer. The middle layer may be formed of one layer or multiple layers, but it is preferable to form it of multiple layers in order to increase the amount of low-quality recycled paper pulp used.

High-brightness pulp is usually used for the surface layer. Examples of high-brightness pulp include virgin pulp and deinked recycled paper pulp. When deinked pulp is used as part or all of the pulp that makes up the surface layer, it is preferable to use stock prepared by the method described below in "Stock Preparation" as the stock that makes up the surface layer.

The surface layer preferably has a basis weight of 15 to 90 g/m ² , more preferably 25 to 70 g/m ^2. The basis weight of each layer can be measured in accordance with JIS P8124.
If the basis weight of the surface layer is equal to or greater than the preferred lower limit, the blackness of the middle layer can be sufficiently concealed. If the basis weight of the surface layer is equal to or less than the preferred upper limit, swelling during papermaking can be easily suppressed.

When a subsurface layer is provided, the pulp that makes up the subsurface layer should have a higher brightness than the pulp in the middle layer, but a lower brightness than the pulp in the surface layer. When deinked pulp is used as part or all of the pulp that makes up the subsurface layer, it is preferable to use stock prepared by the method described below in [Stock Preparation] as the stock that makes up the subsurface layer.

The basis weight of the undersurface layer is preferably 15 to 90 g/m ² , and more preferably 25 to 75 g/m ² .
If the basis weight of the undersurface layer is equal to or greater than the preferred lower limit, the blackness of the middle layer can be sufficiently concealed together with the surface layer. Also, if the basis weight of the undersurface layer is equal to or less than the preferred upper limit, swelling during papermaking can be easily suppressed.

The middle layer is usually composed of a plurality of layers, but may be composed of a single layer. When the middle layer is composed of a plurality of layers, the pulp constituting each layer may be the same or different.
Since the middle layer is sandwiched between at least the front and back layers, it is generally made of the lowest quality pulp among the layers constituting the base paper, such as disintegrated pulp from newspapers, magazines, tickets, medium-quality recycled paper, imitation brown paper, corrugated cardboard, cardboard backing, land tax, and cardboard.
The total basis weight of the middle layer is adjusted as appropriate depending on the thickness required for the application of the coated paperboard, but the basis weight per layer is preferably 15 to 90 g/ ^m2 , and more preferably ²⁵ to 75 g/m2.

When a lower back layer is provided, the pulp constituting the lower back layer is pulp having a higher brightness than the pulp of the middle layer but a lower brightness than the pulp of the lower back layer. The pulp used for the above-mentioned surface layer and lower surface layer may be used for the lower back layer, but typically lower-grade waste paper than the surface layer, i.e., waste paper containing a large amount of medium-density fiber, is used. For example, unbleached deinked waste paper pulp from newspapers, magazines, colored paper, cardboard, etc. is generally used.
When deinked pulp is used as part or all of the pulp constituting the lower back layer, the stock constituting the lower back layer may be prepared by the method described below in [Preparation of Stock].

The basis weight of the lower back layer is preferably 15 to 90 g/m ² , and more preferably 25 to 75 g/m ² .
If the basis weight of the lower back layer is equal to or greater than the preferred lower limit, the colored foreign matter in the middle layer can be sufficiently concealed together with the back layer. If the basis weight of the lower back layer is equal to or less than the preferred upper limit, sufficient paper layer strength can be obtained.

The back layer does not require the same whiteness as the surface layer, but because it is visible to the human eye, pulp with a higher whiteness than the middle layer is usually used. Pulp derived from shredded office paper or the like may also be included. Pulp obtained by disintegrating shredded office paper or the like has a relatively high whiteness even without deinking or bleaching, so it is preferable to incorporate it into the back layer.
When deinked pulp is used as part or all of the pulp constituting the back layer, the stock constituting the back layer may be prepared by the method described below in [Preparation of Stock].

The basis weight of the back layer is preferably 15 to 90 g/m ² , more preferably 25 to 70 g/m ^2. The basis weight of the middle layer is adjusted appropriately depending on the thickness required for the application of the coated paperboard.
If the basis weight of the back layer is equal to or greater than the preferred lower limit, the colored foreign matter in the middle layer can be sufficiently concealed, and if the basis weight of the back layer is equal to or less than the preferred upper limit, sufficient paper layer strength can be obtained.

[Preparation of paper stock]
In the method for manufacturing white paperboard of the present invention, in order to form some of the multiple paper layers, aluminum sulfate is added to a pulp slurry containing deinked pulp, followed by the addition of a paper strength agent, and then calcium carbonate to prepare a stock. Hereinafter, the stock prepared by this method may be referred to as the "main stock."

(pulp slurry)
The pulp slurry used to prepare the present stock contains at least deinked pulp. The pulp slurry used to prepare the present stock may contain virgin pulp in addition to the deinked pulp.

In the pulp slurry used to prepare this paper stock, there is no particular limitation on the ratio of deinked pulp to the total pulp, and it may be adjusted appropriately taking into consideration which layer it is used to form, the quality required for the resulting white paperboard, manufacturing convenience, etc.
In order to fully realize the benefits of applying the present invention, the ratio of deinked pulp to the total pulp is preferably 40% or more, and more preferably 60% or more.

Deinked pulp is deinked waste paper pulp with high brightness. To increase brightness, it may be bleached after deinking.
Examples of waste paper that can be used as a raw material for deinked pulp include paper that has been used once but has a small printed area, such as white, ruled white, extra white, medium white, and white Manila; printed materials and colored papers that have been used once, such as cards, imitation paper, colored paper, Kent, and white art; used high-quality waste paper such as coated printing paper, beverage cartons, and office paper; medium-quality business waste paper such as tickets, medium-quality waste rolls, Kent, and Manila; general medium-quality waste paper such as newspapers, magazines, and miscellaneous paper; brown waste paper such as cut brown paper, plain brown paper, miscellaneous bags, and cardboard; and recovered paper containers and old boxes such as cardboard, white backing paper, and backing paper. Confidential office paper, tickets, and other waste paper may also be shredded.
The waste paper used as the raw material for deinked pulp may be one type or two or more types in combination.

When forming the surface layer with this paper stock, deinked pulp derived from high-whiteness waste paper such as white paper, card paper, special white paper, medium white paper, white Manila paper, imitation paper, and colored paper can be preferably used.
When forming the undersurface layer from this paper stock, it is preferable to use low-grade recycled paper compared to the surface layer, i.e., recycled paper containing a large amount of medium-density fiber. For example, deinked pulp derived from newspapers, magazines, colored paper, cardboard, etc. is preferable. In particular, deinked pulp derived from magazines is preferable because it provides excellent hiding power for the middle layer.

Examples of so-called virgin pulp that is not recycled paper pulp include chemical pulps such as softwood unbleached kraft pulp (NUKP), hardwood unbleached kraft pulp (LUKP), softwood bleached kraft pulp (NBKP), hardwood bleached kraft pulp (LBKP), hardwood semi-bleached kraft pulp (LSBKP), softwood semi-bleached kraft pulp (NSBKP), hardwood sulfite pulp, and softwood sulfite pulp, as well as mechanical pulps such as thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), chemi-ground pulp (CGP), refiner ground pulp (RGP), ground pulp (GP), pressed stone ground pulp (PGW), and stone ground pulp (SGP).

(Aluminum sulfate)
Aluminum sulfate is the common name for aluminum sulfate. Aluminum sulfate is a typical inorganic flocculant. Adding aluminum sulfate can prevent deinked pulp containing fine fibers from falling off in the wire part, improving the pulp slurry yield.

The amount of aluminum sulfate added is preferably 0.1 to 2.0% by mass, and more preferably 0.4 to 1.0% by mass, based on 100% by mass of the solid content of the pulp slurry.
When the amount of aluminum sulfate added is equal to or greater than the above-mentioned preferable lower limit, the effect of improving the pulp slurry retention is easily obtained, while when the amount is equal to or less than the above-mentioned upper limit, the formation of precipitates can be more reliably suppressed.

(Paper strength agent)
Strengthening agents are papermaking chemicals used to give strength to the paper product. They are divided into wet strengthening agents and dry strengthening agents, but the strength agent used in this paper stock is preferably a dry strengthening agent, as it improves internal strength, surface strength, and interlaminar strength.

Examples of dry strength agents include natural strength agents such as starch and vegetable gum, and synthetic strength agents such as polyacrylamide resins, polyvinyl alcohol resins, styrene butadiene latex, urea formaldehyde resins, melamine resins, polyamide resins, ketone resins, polyamide epichlorohydrin resins, polyamine epichlorohydrin resins, polyamide epoxy resins, glycerol polyglycidyl ether resins, and polyethyleneimine resins. These can be used alone or in combination.
Among these, starches, polyvinyl alcohol resins, styrene-butadiene latex, and polyacrylamide resins are preferred, and polyacrylamide strength agents are particularly preferred for their strength development and improved pulp yield.

The polyacrylamide-based paper strengthening agent is not particularly limited, and any of anionic, amphoteric, and cationic types can be used.
Among these, amphoteric polyacrylamide-based paper strength agents are preferred because they are excellent in improving paper strength and preventing deposits from being mixed into the paper stock.

The amount of the paper strength agent added is preferably 0.1 to 2.0% by mass, and more preferably 0.2 to 1.5% by mass, relative to 100% by mass of the solid content of the pulp slurry.
When the amount of the paper strength agent added is equal to or greater than the above-mentioned preferable lower limit, the generation of precipitates can be more reliably suppressed. When the amount is equal to or less than the above-mentioned upper limit, it is easy to suppress deterioration of drainage due to excessive aggregation.

(Calcium carbonate)
The calcium carbonate may be light calcium carbonate, heavy calcium carbonate, or a combination of these.
In the case of precipitated calcium carbonate, the volume-based average primary particle diameter D ₅₀ is preferably 0.2 to 10 μm, more preferably 1 to 5 μm, and even more preferably 2 to 3 μm.
Here, the primary particle size refers to the particle size measured by dropping an aqueous dispersion of precipitated calcium carbonate into the circulation system of a laser diffraction particle size analyzer and then subjecting it to ultrasonic treatment. The volume-based average primary particle size _D50 is the 50% primary particle size on a volume basis.

The amount of calcium carbonate added is preferably 1 to 20% by mass, and more preferably 3 to 15% by mass, based on 100% by mass of the solid content of the pulp slurry.
When the amount of calcium carbonate added is equal to or greater than the above-mentioned preferable lower limit, the opacity of the formed layer is easily increased, and when the amount is equal to or less than the above-mentioned upper limit, the formation of precipitates can be more reliably suppressed.
Calcium carbonate is preferably added as an aqueous dispersion, which provides excellent dispersibility of calcium carbonate. A dispersant may be added to the aqueous dispersion.

(Other ingredients)
Other ingredients may be added to the present paper stock within the range that does not impair the effects of the present invention.
Other components may be selected and blended as needed, such as pigments other than calcium carbonate, water-resistant agents, water-repellents, foamable microcapsules, sizing agents, dyes, fillers, pH adjusters, slime control agents, thickeners, preservatives, mildew-proofing agents, antibacterial agents, flame retardants, preservatives, rodenticides, insect repellents, moisturizers, freshness-preserving agents, oxygen absorbers, microcapsules, foaming agents, surfactants, electromagnetic shielding materials, antistatic agents, rust inhibitors, fragrances, deodorizers, etc. These may also be used in combination.
Examples of pigments other than calcium carbonate include white clay, clay, calcined clay, talc, and titanium oxide.

(Preparation method of paper stock)
In the present invention, aluminum sulfate is first added to a pulp slurry containing deinked pulp, followed by the addition of a paper strength agent, and then calcium carbonate to prepare a paper stock.
It is preferable to add calcium carbonate immediately before papermaking, so that even if sulfate ions from aluminum sulfate remain in the pulp slurry, calcium carbonate can be easily supplied to the wire before calcium sulfate, which causes precipitation, is formed.

(Paper making process)
In the method for producing the white paperboard of the present invention, at least some of the layers are formed by papermaking the present paper stock.
When producing a white paperboard having at least a surface layer, a subsurface layer, a middle layer, and a back layer, particularly when producing a white paperboard having at least a surface layer, a middle layer, and a back layer but no subsurface layer, it is preferable to form the surface layer by papermaking this paper stock.
When producing a white paperboard having at least a surface layer, a subsurface layer, a middle layer, and a back layer, it is preferable to form at least one of the surface layer and the subsurface layer by papermaking using the present paper stock. That is, it is preferable to form both the surface layer and the subsurface layer, or one of the surface layer and the subsurface layer, by papermaking using the present paper stock. It is particularly preferable to form both the surface layer and the subsurface layer by papermaking using the present paper stock.

There are no particular limitations on the papermaking method, but it is preferable to use a papermaking machine equipped with at least a wire section, press section, and dryer section. Size pressing may also be performed in the dryer section. A papermaking machine equipped with a calendar section (pre-calender) after the dryer section may also be used.

As the wire part, a known multi-stage wire part can be appropriately used.
The multi-stage wire part has at least two or more wire part sections.
The types of wire parts generally used in papermaking include cylinder, fourdrinier, short wire, tilted, and twin wire types.
In multi-stage wire parts, these methods can be combined in multiple stages. Examples include Fourdrinier combination, short net combination, short net cylinder combination, and Fourdrinier cylinder combination.
Among these, the use of short-wire pulp is preferred in that it improves the yield of deinked pulp containing a large amount of fine fibers.

Any known press machine can be used as the press part, but it is preferable to include a shoe press. It is also preferable to combine a roll press with a shoe press. Using a shoe press allows for sufficient water extraction in the press part, making it easier to control the amount of moisture when performing size pressing in the dryer part.

Any known dryer can be used as appropriate for the dryer section. For example, a cylinder dryer, which heats wet white paperboard by contacting it, or a non-contact dryer such as an infrared dryer or hot air dryer can be used alone or in combination.

[Manufacturing method of coated white paperboard]
The method for producing a coated white paperboard of the present invention comprises the steps of obtaining a white paperboard by the method for producing a white paperboard of the present invention, and forming a pigment coating layer on at least one surface of the obtained white paperboard.
The pigment coating liquid for forming the pigment coating layer contains a pigment and a binder.
As the pigment, one or more of the well-known pigments commonly used in the field of general coated paper manufacturing, such as kaolin, heavy calcium carbonate, light calcium carbonate, titanium dioxide, aluminum hydroxide, silica, satin white, talc, etc., can be appropriately used within the range that does not impair the effects of the present invention. Calcium carbonate is particularly preferred because it brings about excellent printability.

Aqueous adhesives are preferred as binders. Examples of aqueous adhesives include starches such as oxidized starch, phosphated starch, hydroxyethyl etherified starch, dextrin, enzyme-modified starch, and water-soluble starch; conjugated diene copolymer latexes such as styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer latex, and styrene-methyl methacrylate-butadiene copolymer latex; and acrylic copolymer latexes such as acrylic acid ester and/or methacrylic acid ester copolymer latex; proteins such as casein, gelatin, and soy protein; synthetic resin adhesives such as various polyvinyl alcohols, various polyacrylamides, and melamine resins; and various cellulose derivatives such as carboxymethyl cellulose. One or more of these adhesives can be selected and used.

Among these, it is preferable to blend a latex having a glass transition temperature of -50 to 30°C as measured by Vibron viscoelasticity, since this increases the flexibility of the coated surface and improves resistance to cracking.
The glass transition temperature of the latex is more preferably from -50 to 0°C.
It is also preferable to blend starch together with the latex, since the combined use of latex and starch is preferable because it achieves a balance between fixing the fine fibers inside the backing layer and the surface strength of the backing layer.
The mass ratio of latex to starch is preferably 100:0 to 5:50.

The pigment coating liquid preferably contains 2 to 50 parts by mass, and more preferably 5 to 25 parts by mass, of the binder per 100 parts by mass of the pigment.
If the binder ratio is equal to or greater than the preferred lower limit, the strength of the pigment coating layer is likely to be sufficient, whereas if the binder ratio is equal to or less than the preferred upper limit, the ink drying property and box-making suitability will be excellent.
If necessary, one or more of the following may be appropriately contained: a dispersant, a pH adjuster such as sodium hydroxide or aqueous ammonia, an antifoaming agent, a fluorescent dye, a release agent, a water-resistant agent, a flowability improver, a slime control agent, a preservative, a dye, a coloring pigment, etc.

The coating amount of the pigment coating liquid is preferably 5 to 50 g/m ² , more preferably 8 to 40 g/m ² , and even more preferably 10 to 35 g/m ² .
When the pigment coating layer is applied multiple times, for example, in two separate applications, the coating liquids in the first application (undercoat layer) and the second application (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 formed by a size press liquid applied to the surface layer. Before applying the pigment coating liquid, the surface of the surface layer may be smoothed by pre-calendering. The pigment coating liquid may be applied only once or multiple times. In particular, it is preferable to apply a pigment coating liquid for the undercoat layer and a pigment coating liquid for the topcoat layer to form the undercoat layer and the topcoat layer in that order.
By applying a pigment coating liquid, it is possible to improve the printability of the surface layer side and increase the whiteness.

The coater may be on-machine or off-machine, but an on-machine coater using a known coater part attached to the paper machine is preferred. Coating devices for the coater part include, for example, 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 metering blade type size press coaters, bill blade coaters, short dwell coaters, gate roll coaters, and calendered nip coaters.

Of these, rod metering coaters and curtain coaters are preferred because they ensure a consistent coating amount and can produce a coating layer with reduced coating unevenness. The applied coating layer is then dried using a known drying device to form a pigment coating layer. The machine may have multiple coater parts. In this case, the pigment coating layer can be applied in multiple steps.

A calender part may be provided after the coater part, if necessary. By providing a calender part after the coater part, the pigment coating layer is smoothed.
A known calendering device can be used as appropriate for the calendering part after the coater part, such as a super calender, a gloss calender, a soft nip calender, a thermal calender, or a shoe calender. A combination of these may also be used. Of these, a soft nip calender equipped with a metal roll and an elastic roll is preferred because it can smooth the coating layer while maintaining the paper thickness. The calendering part after the coater part may be on-machine or off-machine.

[Mechanism of action]
The precipitates that form when aluminum sulfate and calcium carbonate are used in combination in paper stock are thought to be calcium sulfate produced by the reaction between calcium carbonate and aluminum sulfate. Also, the reason that precipitates form even without adding calcium carbonate is thought to be that calcium carbonate from waste paper is contained in deinked pulp, and this reacts with aluminum sulfate to produce calcium sulfate.
The calcium sulfate produced accumulates in the pipes inside the paper machine, and when it aggregates to a certain size, it peels off from the pipes and transfers to the paper layer as a precipitate, causing major quality problems.
The present inventors considered that the produced calcium sulfate would aggregate over time to form precipitates, but would be in the form of fine particles immediately after production, and investigated measures to prevent the aggregation of calcium sulfate at this stage.

In the present invention, the strength agent is added while the calcium sulfate is still in the fine particle form, so that when the strength agent binds the pulp fibers, it can capture and fix the fine calcium sulfate particles dispersed in the slurry to the pulp fibers, preventing the fine calcium sulfate particles from agglomerating and forming precipitates.
Furthermore, since no new calcium carbonate is added at the stage of adding the paper strength agent, there is little possibility that the reaction between aluminum sulfate and a large amount of calcium carbonate will proceed all at once and cause the formation of precipitates.

In the present invention, new calcium carbonate is added after the addition of the paper strength agent to prepare the paper stock, but precipitates due to the reaction between the newly added calcium carbonate and aluminum sulfate are unlikely to form. This is thought to be because most of the sulfate ions derived from aluminum sulfate have already been consumed.
Furthermore, since the new calcium carbonate is added last, the time from the addition of the new calcium carbonate to papermaking can be shortened, which is also thought to be effective in suppressing the reaction between the newly added calcium carbonate and aluminum sulfate and the subsequent coagulation.

Claims (5)

A method for producing white paperboard having at least a surface layer, a middle layer, and a back layer , comprising adding aluminum sulfate to a pulp slurry containing deinked pulp , then adding 0.1 to 2.0 mass% of a paper strength agent relative to 100 mass% of the solids content of the pulp slurry, and then adding calcium carbonate to prepare a paper stock, and then making paper from the prepared paper stock to form the surface layer . A method for producing white paperboard having at least a surface layer, a subsurface layer, a middle layer, and a back layer , the method comprising adding aluminum sulfate to a pulp slurry containing deinked pulp , then adding 0.1 to 2.0 mass% of a paper strength agent relative to 100 mass% of the solids content of the pulp slurry , and then adding calcium carbonate to prepare a paper stock, and then making paper from the prepared paper stock to form the surface layer . The method for producing white paperboard according to claim 1 or 2 , wherein the aluminum sulfate is added in an amount of 0.1 to 2.0% by mass relative to 100% by mass of the solid content of the pulp slurry. The method for producing white paperboard according to any one of claims 1 to 3 , wherein the paper strength agent is a polyacrylamide-based paper strength agent. A method for producing coated white paperboard, comprising the steps of obtaining a white paperboard by the method for producing white paperboard according to any one of claims 1 to 4 , and forming a pigment coating layer on at least one surface of the obtained white paperboard.