JP7529997B2 - Manufacturing method of white paperboard and 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 paper 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 pulp with high whiteness, 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 subsurface layer, and the like 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 the surface layer and the undersurface layer, recycled paper pulp is often used instead of or together with virgin pulp. Deinked recycled paper pulp with high whiteness (deinked pulp) is used as the recycled paper pulp used in the surface layer and the undersurface layer.
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 is more likely to contain fine fibers than virgin pulp, and these fibers are more likely to fall off in the wire part.

Japanese Patent Application Publication No. 6-41896

One way to improve the yield of stock containing deinked pulp is to add aluminum sulfate to the stock. However, when aluminum sulfate is used with stock containing deinked pulp, precipitates may adhere to the pipes of the papermaking equipment. If these precipitates fall off the pipes and get mixed into the stock, there is a risk that the quality of the white board will be impaired.

Such problems tended to occur particularly when aluminum sulfate was added to the stock containing deinked pulp, and calcium carbonate was also added as a pigment to increase opacity.
The present invention has been made in consideration 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 paper stock while maintaining the yield of paper stock containing deinked pulp.

In order to achieve the above object, the present invention employs the following configuration.
[1] A method for producing whiteboard paper having multiple layers, comprising the steps of: 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 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 producing white paperboard described in [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 the 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 stable quality without mixing precipitates into the paperstock, while maintaining the yield of the paperstock containing deinked pulp.

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

The surface layer is the layer that is placed on the outermost side of the white board, the back layer is the layer that is placed on the innermost side (opposite the surface layer), and the middle layer is the layer that is placed between the surface layer and the back layer. A subsurface layer may be provided between the surface layer and the middle layer. A subsurface layer may be provided 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.

Pulp with high whiteness is usually used for the surface layer. Examples of pulp with high whiteness include virgin pulp and deinked recycled paper pulp. When deinked pulp is used as part or all of the pulp that constitutes the surface layer, it is preferable to use a paper stock prepared by the method described below in [Preparation of Paper Stock] as the paper stock that constitutes the surface layer.

The surface layer preferably has a basis weight of 15 to 90 g/m ² , and 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, whereas 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 constituting the subsurface layer is one that has a higher whiteness than the pulp of the middle layer and a lower whiteness than the pulp of the surface layer. When deinked pulp is used as part or all of the pulp constituting the subsurface layer, it is preferable to use a stock prepared by the method described below in [Stock Preparation] as the stock constituting the subsurface layer.

The basis weight of the underside layer is preferably 15 to 90 g/ ^m2 , and more preferably 25 to 75 g/ ^m2 .
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. 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 a layer sandwiched between at least the front and back layers, it is generally made of the lowest grade pulp among the layers constituting the base paper, such as disintegrated pulp used in newspapers, magazines, tickets, medium-quality recycled paper, imitation brown paper, cardboard, mounts, land certificates, and balls.
The total basis weight of the middle layer is adjusted appropriately 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 whiteness than the pulp of the middle layer and a lower whiteness 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 generally, a lower grade waste paper than that of 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/ ^m2 , and more preferably 25 to 75 g/ ^m2 .
When 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. When 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 since 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 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 in 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 appropriately adjusted according to 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 manufacturing method of the 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 in the preparation of the present stock contains at least deinked pulp. The pulp slurry used in the preparation of the present stock may contain virgin pulp in addition to the deinked pulp.

In the pulp slurry used to prepare the present paper stock, there is no particular limitation on the ratio of deinked pulp to the total pulp, and this may be appropriately adjusted taking into consideration which layer it is used to form, the quality required for the resulting white board, manufacturing convenience, etc.
In order to easily obtain the full significance of the application of 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 waste paper pulp that has been deinked and has a high degree of whiteness. In order to increase the whiteness, it may be bleached after deinking.
Examples of waste paper to be used as the raw material for deinked pulp include papers that have been used once but with a small printed area, such as white, ruled white, extra white, medium white, and white Manila, printed matter such as cards, imitation, colored, Kent, and white art, and colored papers that have been used once, used high-quality waste paper such as coated paper for printing, beverage cartons, and office paper, as well as 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, plain brown, miscellaneous bags, and cardboard, and recovered paper containers and old boxes such as cardboard, white mounts, and mounts. Confidential office paper, tickets, and other waste paper may be shredded.
The waste paper used as the raw material for the 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 waste paper with high whiteness such as white paper, cards, special white paper, medium white paper, white Manila paper, imitation paper, and colored paper can be preferably used.
When forming the underside layer from this paper stock, it is preferable to use low-grade waste paper, i.e., waste paper containing a large amount of medium-density fiber, compared to the surface layer. For example, deinked pulp derived from newspapers, magazines, colored paper, balls, etc. is preferable. Among them, deinked pulp derived from magazines is preferable because it has excellent hiding power for the middle layer.

Examples of so-called virgin pulp that is not recycled 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, and mechanical pulps such as thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), chemi-grand 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 suppress the deinked pulp containing fine fibers from falling off in the wire part, improving the retention of the pulp slurry.

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 more 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 generation of precipitates can be more reliably suppressed.

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

Examples of the dry strength agent 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 among these, polyacrylamide strength agents are preferred for their strength development effect and improved pulp yield.

The polyacrylamide-based paper strength 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 the inclusion of precipitates in 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, based on 100% by mass of the solid content of the pulp slurry.
By setting the amount of the paper strength agent to be equal to or greater than the above-mentioned preferable lower limit, the generation of precipitates can be more reliably suppressed. On the other hand, by setting the amount of the paper strength agent to be 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 a particle size measured by dropping an aqueous dispersion of precipitated calcium carbonate into the circulation system of a laser diffraction particle size distribution analyzer, and then subjecting the dispersion to ultrasonic treatment. The volume-based average primary particle size _D50 is the 50% primary particle size based on volume.

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 more than the preferable lower limit, the opacity of the formed layer is easily increased, whereas when the amount is equal to or less than the preferable upper limit, the occurrence of precipitation can be more reliably suppressed.
It is preferable that calcium carbonate is 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 as long as they do not impair the effects of the present invention.
Other components include pigments other than calcium carbonate, water-resistant agents, water-repellents, foaming microcapsules, sizing agents, dyes, fillers, pH adjusters, slime control agents, thickeners, preservatives, antifungal agents, antibacterial agents, flame retardants, preservatives, rodenticides, insect repellents, moisturizers, freshness-retaining agents, oxygen absorbers, microcapsules, foaming agents, surfactants, electromagnetic shielding materials, antistatic agents, rust inhibitors, fragrances, deodorants, etc., which can be selected and blended as necessary. These can also be used in combination.
Examples of pigments other than calcium carbonate include white clay, clay, calcined clay, talc, titanium oxide, and the like.

(Preparation method of paper material)
In the present invention, aluminum sulfate is first added to a pulp slurry containing deinked pulp, then a paper strength agent is added, and then calcium carbonate is added 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 onto the wire before calcium sulfate, which causes precipitation, is formed.

(Papermaking process)
In the manufacturing method of the white paperboard of the present invention, at least a part of the layers is 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 from this paper stock.
When manufacturing 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 this paper stock. In other words, 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 this paper stock. It is particularly preferable to form both the surface layer and the subsurface layer by papermaking this 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 part, a press part, and a dryer part. Size pressing may also be performed in the dryer part. A papermaking machine equipped with a calendar part (pre-calender) after the dryer part 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 the cylinder type, the fourdrinier type, the short wire type, the inclined type, and the twin wire type.
In multi-stage wire parts, these methods can be combined in multiple stages. For example, there are Fourdrinier combination, short net combination, short net cylinder combination, and Fourdrinier cylinder combination.
Among these, the use of short net paper is preferred in that it improves the yield of deinked pulp containing a large amount of fine fibers.

As the press part, any known press machine can be used as appropriate, but it is preferable to include a shoe press. It is also preferable to combine a roll press with a shoe press. When a shoe press is used, water can be sufficiently squeezed out in the press part, making it easier to control the amount of water when size pressing in the dryer part.

As the dryer section, any known dryer can be used as appropriate. For example, a cylinder dryer that comes into contact with and heats the wet white paperboard, or a non-contact dryer such as an infrared dryer or a 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 used in the general coated paper manufacturing field 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. In particular, calcium carbonate is preferred because it brings about excellent printability.

As the binder, a water-based adhesive is preferred. Examples of water-based adhesives include starches such as oxidized starch, phosphated starch, hydroxyethyl etherified starch, dextrin, enzyme-modified starch, and water-soluble starch; latexes such as 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 copolymer latexes of acrylic acid esters and/or methacrylic acid esters; proteins such as casein, gelatin, and soybean 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 use 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 mix starch together with the latex, since the combined use of latex and starch is preferable since it provides a good balance between the fixation of the fine fibers inside the back layer and the surface strength of the back 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 proportion of the binder is equal to or greater than the preferred lower limit, the strength of the pigment coating layer is easily sufficient, whereas if the proportion of the binder is equal to or less than the preferred upper limit, the ink drying property and the suitability for box making are 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 flow improver, a slime control agent, a preservative, a dye, a coloring pigment, and the like.

The coating amount of the pigment coating liquid is preferably 5 to 50 g/ ^m2 , more preferably 8 to 40 g/ ^m2 , and further preferably 10 to 35 g/ ^m2 .
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 may be applied 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 successively form the undercoat layer and the topcoat layer.
By applying a pigment coating liquid, it is possible to improve the printability of the surface layer side and to increase the whiteness.

The coater may be on-machine or off-machine, but it is preferable to use an on-machine coater part that is a known part attached to the papermaking machine. Coating devices for the coater part include, for example, a blade coater, air knife coater, roll coater, reverse roll coater, bar coater, curtain coater, slot die coater, gravure coater, champlex coater, brush coater, slide bead coater, two-roll or metering blade type size press coater, bill blade coater, short dwell coater, gate roll coater, and a nip coater using a calendar.

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

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.
A known calendaring device can be appropriately used for the calendaring part after the coater part, and examples of such devices include a super calendar, a gloss calendar, a soft nip calendar, a heat calendar, and a shoe calendar. These may be used in combination. Among these, a soft nip calendar equipped with a metal roll and an elastic roll is preferred because it can smooth the coating layer while maintaining the paper thickness. The calendaring 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 the 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 that deinked pulp contains calcium carbonate derived from waste paper, which reacts with aluminum sulfate to produce calcium sulfate.
The calcium sulfate produced accumulates in the piping of the papermaking machine, and when it aggregates to a certain size, it peels off from the piping and transfers to the paper layer as a precipitate, causing major quality problems.
The present inventors considered that the produced calcium sulfate would aggregate and become precipitates over time, but that it 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 form of fine particles, so that when the strength agent binds the pulp fibers, it can capture the fine calcium sulfate dispersed in the slurry and fix it to the pulp fibers, preventing the fine calcium sulfate particles from agglomerating and forming precipitates.
In addition, 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 rapidly and cause the formation of precipitates.

In the present invention, after the addition of the paper strength agent, new calcium carbonate is added to prepare the paper stock, but the newly added calcium carbonate does not react with aluminum sulfate to produce precipitates, which is believed to be because most of the sulfate ions derived from aluminum sulfate have already been consumed.
In addition, since the new calcium carbonate is added last, the time from the addition of the new calcium carbonate to the papermaking process can be shortened, which is also believed to suppress the reaction between the newly added calcium carbonate and aluminum sulfate and the subsequent coagulation.

Claims (6)

A method for producing white paperboard having at least a surface layer, a middle layer, and a back layer, comprising the steps of: adding aluminum sulfate to a pulp slurry containing deinked pulp, adding a paper strength agent, and then adding 1 to 20% by mass of calcium carbonate relative to 100% by mass of solids in the pulp slurry to prepare a stock; and making paper from the prepared 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 the steps of: adding aluminum sulfate to a pulp slurry containing deinked pulp, adding a paper strength agent, and then adding 1 to 20% by mass of calcium carbonate relative to 100% by mass of solids in the pulp slurry to prepare a stock; and making paper from the prepared 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 solids in the pulp slurry. The method for producing white paperboard according to any one of claims 1 to 3 , wherein the strength agent is added in an amount of 0.1 to 2.0% by mass relative to 100% by mass of solids in the pulp slurry. The method for producing white paperboard according to any one of claims 1 to 4 , wherein the paper strength agent is a polyacrylamide-based paper strength agent. A method for producing a coated white paperboard, comprising the steps of obtaining a white paperboard by the method for producing a white paperboard according to any one of claims 1 to 5 , and forming a pigment coating layer on at least one surface of the obtained white paperboard.