Manufacturing method of white cardboard

Abstract

The object of this invention is to provide white cardboard in which both the occurrence of hydrangea spots and blistering are suppressed. [Solution] The present invention relates to a method for manufacturing white cardboard having at least a surface layer, a subsurface layer, a middle layer, and a back layer, comprising the steps of: obtaining a surface layer slurry; obtaining a subsurface layer slurry; obtaining a middle layer slurry; obtaining a back layer slurry; and multilayer papermaking of the surface layer slurry, subsurface layer slurry, middle layer slurry, and back layer slurry, wherein the step of obtaining the subsurface layer slurry and / or the step of obtaining the middle layer slurry includes a step of disintegrating laminated recycled paper containing a thermoplastic resin, the amount of laminated recycled paper included is 1% by mass or more relative to the total solid content mass in the slurry, and the basis weight of the subsurface layer is 25 to 95 g / m² 2 The present invention relates to a method for manufacturing white cardboard, wherein the addition rate of filler to the subsurface layer is 3 parts by mass or less per 100 parts by mass of pulp in the subsurface layer.

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing white cardboard. [Background technology]

[0002] Conventionally, multilayer white cardboard with a layer structure of three or more layers is known. White cardboard has long been used for various packaging boxes and the like. Bleached chemical pulp is often used for the front and back layers of white cardboard, but recycled paper pulp is generally used for the middle layer from the viewpoint of low cost and resource conservation (for example, Patent Document 1).

[0003] However, a known problem with white cardboard made from recycled pulp is the so-called "hydrangea spot" phenomenon. Hydrangea spots are multicolored spots that appear when white cardboard containing clumps of sublimable ink is stored at room temperature for several months. The ink clumps sublimate in the middle layers of the cardboard and seep out onto the surface. White cardboard with hydrangea spots tends to be undesirable to consumers due to its appearance and must be discarded, leading to a waste paper problem.

[0004] It has been pointed out that the cause of clumps of sublimable ink getting mixed into white cardboard is the mixing of sublimation transfer paper with recycled paper used as raw material for recycled paper pulp (Patent Document 2). Patent Document 2 proposes a specific sublimation transfer paper having an ink-receiving layer containing a water-soluble binder. Furthermore, Patent Document 3 proposes white cardboard having at least a surface layer, a subsurface layer, a middle layer and a back layer in order to suppress the occurrence of hydrangea spots, characterized in that the subsurface layer contains calcium carbonate, the calcium carbonate content of the subsurface layer is 3% by mass or more relative to 100% by mass of the solid content of the subsurface layer, and the ash content of the subsurface layer is 35% or less. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-41896 [Patent Document 2] Japanese Patent Publication No. 2019-64187 [Patent Document 3] Japanese Patent Publication No. 2022-165168 [Overview of the project] [Problems that the invention aims to solve]

[0006] As disclosed in Patent Document 3, it has been studied that incorporating calcium carbonate into the subsurface layer can suppress the occurrence of hydrangea spots. However, the present inventors have encountered the problem that when calcium carbonate is incorporated into the subsurface layer in amounts exceeding a predetermined level, swelling may occur in the white cardboard.

[0007] Therefore, in order to solve the problems of the prior art, the inventors of this invention proceeded with research with the aim of providing white cardboard in which both the occurrence of hydrangea spots and the occurrence of blistering are suppressed. [Means for solving the problem]

[0008] Examples of specific embodiments of the present invention are shown below.

[0009] [1] A method for manufacturing white cardboard having at least a surface layer, a subsurface layer, a middle layer and a back layer, A process to obtain a slurry for the surface layer, a process to obtain a slurry for the subsurface layer, a process to obtain a slurry for the middle layer, and a process to obtain a slurry for the back layer. The process includes a step of multi-layer papermaking using a surface slurry, a subsurface slurry, a middle layer slurry, and a back layer slurry. The process of obtaining a slurry for the sublayer and / or a slurry for the middle layer includes a step of disintegrating laminated waste paper containing a thermoplastic resin, and the amount of laminated waste paper included is 1% by mass or more relative to the total solid content mass in the slurry. The basis weight of the subsurface layer is 25-95 g / m². 2 And, A method for manufacturing white cardboard, wherein the addition rate of filler to the subsurface layer is 3 parts by mass or less per 100 parts by mass of pulp in the subsurface layer. [2] The method for producing white cardboard according to [1], wherein the step of obtaining a slurry for the sublayer and / or a slurry for the middle layer includes a step of crushing and washing laminated waste paper. [3] In the crushing and washing step, the crushing and washing is performed with a washing solution with a pH of 7 or higher, as described in [2]. [4] A method for producing white cardboard according to any one of [1] to [3], wherein in the step of obtaining a slurry for the lower layer and / or a slurry for the middle layer, laminated waste paper is decomposed at 20 to 60°C. [5] A method for producing white cardboard according to any one of [1] to [4], wherein the fiber roughness of the pulp fibers contained in the laminated recycled paper is 0.140 mg / m or less. [6] A method for manufacturing white cardboard according to any one of [1] to [5], wherein the proportion of pulp fibers with a fiber length of 0.2 mm or more and less than 0.6 mm among the pulp fibers contained in the laminated recycled paper is 8% or more. [7] A method for producing white cardboard according to any one of [1] to [6], wherein the laminated recycled paper contains a rosin-based sizing agent. [8] A method for producing white cardboard according to any one of [1] to [7], further comprising the step of beating the pulp fibers contained in the slurry, for the step of obtaining a slurry for the subsurface layer and / or a slurry for the middle layer. [9] A method for producing white cardboard according to any one of [1] to [8], further comprising a drying step after the multilayer papermaking step. [Effects of the Invention]

[0010] According to the manufacturing method of the present invention, it is possible to obtain white cardboard in which both the occurrence of hydrangea spots and the occurrence of blistering are suppressed. [Modes for carrying out the invention]

[0011] The present invention will be described in detail below. The following description may be based on representative embodiments and specific examples, but the present invention is not limited to such embodiments. In this specification, numerical ranges represented by "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits.

[0012] (Method of manufacturing white cardboard) This embodiment is a method for manufacturing white cardboard having at least a surface layer, a subsurface layer, a middle layer, and a back layer, comprising the steps of: obtaining a surface layer slurry; obtaining a subsurface layer slurry; obtaining a middle layer slurry; obtaining a back layer slurry; and multi-layer papermaking of the surface layer slurry, subsurface layer slurry, middle layer slurry, and back layer slurry, wherein the step of obtaining the subsurface layer slurry and / or the step of obtaining the middle layer slurry includes a step of disintegrating laminated recycled paper containing a thermoplastic resin, the amount of laminated recycled paper included is 1% by mass or more relative to the total solid content mass in the slurry, and the basis weight of the subsurface layer is 25 to 95 g / m² 2 The present invention relates to a method for manufacturing white cardboard, wherein the addition rate of filler to the subsurface layer is 3 parts by mass or less per 100 parts by mass of pulp in the subsurface layer.

[0013] In this embodiment, the process of obtaining a slurry for the sub-layer and / or the middle layer includes a step of disintegrating a predetermined amount or more of laminated waste paper containing thermoplastic resin. Furthermore, by setting the basis weight of the sub-layer within a predetermined range, the occurrence of hydrangea spots on the white cardboard is suppressed, and in addition, the occurrence of blistering on the white cardboard is suppressed. In this embodiment, thermoplastic resin derived from laminated waste paper is blended into the sub-layer and / or middle layer, and this thermoplastic resin can suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, by setting the basis weight of the sub-layer within a predetermined range, the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard can be effectively suppressed.

[0014] In conventional technology, thermoplastic resins tend to become foreign matter when incorporated into white cardboard, often degrading the quality of the white cardboard, such as its strength. Therefore, when manufacturing white cardboard using recycled materials, the thermoplastic resins were separated and removed. In this embodiment, for example, by selectively using recycled materials in which the thermoplastic resin has not been printed, the thermoplastic resin does not become foreign matter, and furthermore, by binding to a portion of the pulp fibers, it is possible to suppress the sublimation of ink clumps and their appearance on the surface of the white cardboard.

[0015] Furthermore, in this embodiment, the addition rate of filler to the subsurface layer is set to 3 parts by mass or less per 100 parts by mass of pulp in the subsurface layer. This means that the addition rate of fillers such as calcium carbonate in the subsurface layer is low. In the prior art, it was considered to suppress the occurrence of hydrangea spots by incorporating a predetermined amount or more of filler such as calcium carbonate into the subsurface layer, but this weakened the interlayer bonding between the subsurface layer and the surface layer, which sometimes resulted in blistering of the white cardboard. In this embodiment, the occurrence of hydrangea spots has been successfully suppressed without incorporating filler into the subsurface layer. Moreover, in this embodiment, by reducing the addition rate of fillers such as calcium carbonate in the subsurface layer, the interlayer adhesion between the subsurface layer and the surface layer can be improved, and as a result, blistering of the white cardboard can be suppressed.

[0016] In the process of obtaining the slurry for the sub-layer and / or the slurry for the middle layer, the amount of recycled laminated paper containing thermoplastic resin added may be 1% by mass or more relative to the total mass of solids contained in the slurry, preferably 1.5% by mass or more, more preferably 2% by mass or more, and even more preferably 2.5% by mass or more. Furthermore, the amount of recycled laminated paper added may be 40% by mass or less relative to the total mass of solids contained in the slurry, more preferably 30% by mass or less, even more preferably 25% by mass or less, and particularly preferably 20% by mass or less. By setting the amount of recycled laminated paper containing thermoplastic resin added in the process of obtaining the slurry for the sub-layer and / or the slurry for the middle layer within the above range, it is possible to more effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard.

[0017] In this embodiment, laminated recycled paper containing thermoplastic resin may be incorporated into both the subsurface layer slurry and the middle layer slurry. In this case, the amount of laminated recycled paper containing thermoplastic resin incorporated into the subsurface layer slurry and the middle layer slurry represents the total amount.

[0018] The manufacturing method of this embodiment includes the steps of obtaining a surface layer slurry, obtaining a sub-surface layer slurry, obtaining a middle layer slurry, obtaining a back layer slurry, and multilayer papermaking using the surface layer slurry, sub-surface layer slurry, middle layer slurry, and back layer slurry. In this embodiment, it is preferable that the sub-surface layer and / or middle layer contain a predetermined amount or more of thermoplastic resin, so the steps of obtaining the sub-surface layer slurry and / or middle layer slurry include the step of disintegrating laminated waste paper containing thermoplastic resin. Specifically, it is preferable to crush laminated waste paper containing thermoplastic resin to be used as recycled raw material and disperse it in the sub-surface layer slurry and / or middle layer slurry to obtain the sub-surface layer slurry and / or middle layer slurry containing thermoplastic resin.

[0019] In the process of obtaining the slurry for the subsurface layer and / or the slurry for the middle layer, the dissociation temperature of the pulp fibers is preferably 20°C or higher, more preferably 25°C or higher, and even more preferably 30°C or higher. Furthermore, the dissociation temperature is preferably 60°C or lower, and more preferably 55°C or lower. By setting the dissociation temperature within the above range, wax components and the like that promote the sublimation of sublimable ink can be washed away, and as a result, it is presumed that the occurrence of hydrangea spots can be more effectively suppressed.

[0020] In the process of obtaining a slurry for the sublayer and / or the middle layer, the concentration of recycled laminate paper at the time of disintegration is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more. Furthermore, the concentration of recycled laminate paper at the time of disintegration is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less. In the disintegration process, the higher the concentration of recycled laminate paper, the larger the foreign matter (e.g., resin fragments of thermoplastic resin) remains when separated from the pulp fibers, making it easier to remove these foreign matter in the dust removal process. However, the lower the concentration of recycled laminate paper, the finer the foreign matter fragments become, making them more difficult to remove in the dust removal process. Therefore, by keeping the concentration of recycled laminate paper at the time of disintegration within the above range, it becomes easier to control the content of thermoplastic resin contained in the white cardboard within the desired range.

[0021] The process of obtaining a slurry for the sublayer and / or the slurry for the middle layer preferably includes a step of crushing and washing laminate waste paper containing thermoplastic resin. In the crushing and washing step, a crushing and washing machine is used to perform a crushing step in which the laminate waste paper is crushed into small pieces by a rotating blade, and a washing step in which the crushed laminate waste paper is washed with washing water in a continuous manner. For example, this can be done using a crushing and washing machine (A-Tech Co., Ltd., paper container recycling device PPRS). As for the rotating blade used in the crushing step, it is preferable to use a single-screw or double-screw rotating blade, and it is more preferable to use a double-screw rotating blade. The crushing size can be adjusted by changing the number of rotating blades (number of hooks), and it is possible to crush it into smaller pieces by increasing the number of hooks. From the viewpoint of improving the efficiency of the disintegration process, the crushing size is 20 cm. 2 ~400cm 2 It is preferable to adjust it so that it results in the following.

[0022] Examples of washing water used in the washing process of the crushing and washing process include water, hot water, caustic soda solution, ozonated water, hydrogen peroxide solution, sodium hypochlorite solution, etc., and can be changed as appropriate. From the viewpoint of removing dirt from laminated waste paper, it is preferable to use any of the following: hot water, caustic soda solution, ozonated water, hydrogen peroxide solution, or sodium hypochlorite solution. Furthermore, in the crushing and washing process, it is preferable to perform the crushing and washing with a washing solution with a pH of 7 or higher, and it is even more preferable to perform the crushing and washing with caustic soda solution.

[0023] The flow rate of the washing water in the crushing and washing process can be adjusted to any desired amount, such as 0.1 m 3 It is preferable that it be 0.5m or more. 3 It is more preferable that it be 1.0m or more / h. 3 It is even more preferable that the flow rate of the washing water be 5.0 m³ or more. 3 It is preferable that the amount is less than / h. Furthermore, the crushed pieces after crushing and washing may be baled using a compressor.

[0024] In the manufacturing method of this embodiment, by providing such a crushing and washing step, wax components and the like that promote the sublimation of sublimable ink can be washed away, and as a result, it is presumed that the occurrence of hydrangea spots can be more effectively suppressed. Furthermore, by providing a crushing and washing step, it becomes easier to control the fiber roughness of the pulp fibers to a desired range. By appropriately controlling the fiber roughness of the pulp fibers, it is possible to more effectively suppress the sublimation of ink clumps and their appearance on the surface of the white cardboard.

[0025] The steps for obtaining a slurry for the subsurface layer and / or a slurry for the middle layer may include a step of beating the pulp fibers contained in each slurry. In the beating step, for example, the beating treatment can be carried out using a double disc refiner or the like. During the beating treatment, for example, laminated recycled paper pulp fibers and other recycled paper pulp fibers may be beated individually, or they may be beated after being mixed.

[0026] Examples of paper machines used to make pulp slurry in the multilayer papermaking process include long-wire paper machines, gap former type paper machines, cylinder wire paper machines, and short-wire paper machines.

[0027] A paper machine for making paper from pulp slurry generally includes a wire section, a press section, a dryer section, a calender section, and a reel section. The wire section is the process of dewatering the supplied slurry and forming it into a sheet. In the wire section, the pulp slurry is supplied to the headbox, where a multi-layered sheet that will become the base paper for white cardboard can be formed. In the headbox, the component ratios of the pulp slurry forming each layer can be different. This allows for different properties in each layer that makes up the white cardboard.

[0028] The process of obtaining the pulp slurry may include a step of bleaching the pulp fiber raw material. Known bleaching agents such as oxygen-based bleaches or chlorine-based bleaches can be used in the bleaching process.

[0029] In the process of obtaining a slurry for the sublayer and / or a slurry for the middle layer, it is preferable to further include a dust removal step after the step of disintegrating the laminated waste paper to obtain a pulp slurry. The dust removal step is a step of removing foreign matter and other substances as removal components from the pulp slurry after the disintegration step. Here, foreign matter refers to components with a particularly high specific gravity that originate from layers other than the paper base layer of the laminated waste paper. In addition, the removal components refer to all components that are removed by the dust removal step, including components that originate from layers other than the paper base layer of the laminated waste paper, as well as some laminated waste paper pulp derived from the paper base layer.

[0030] The dust removal process preferably includes a step of removing foreign matter from the pulp slurry after the disintegration process by centrifugal separation and a step of removing foreign matter from the pulp slurry after the disintegration process by screen treatment.

[0031] The process of removing foreign matter by centrifugal separation primarily involves removing large foreign matter generated after the disintegration process using a cleaner. The cleaner is cone-shaped and, by the principle of centrifugal separation, can remove foreign matter with a specific gravity greater than pulp fibers, such as sand and metal particles. From the viewpoint of efficiently removing foreign matter with a specific gravity greater than pulp fibers, heavy foreign matter cleaners and low-concentration ramole cleaners are recommended for use in the process of removing foreign matter by centrifugal separation. In the process of removing foreign matter by centrifugal separation, the concentration of the pulp slurry is preferably 0.5 to 5.0% by mass, more preferably 0.8 to 4.0% by mass, and even more preferably 1.0 to 3.0% by mass, from the viewpoint of efficiently removing foreign matter and reducing dust in the resulting recycled paper pulp.

[0032] The process of removing foreign matter by screen treatment is performed after the process of removing foreign matter by centrifugal separation, with the purpose of removing foreign matter. As screens used for screening, for example, basket-shaped screens with holes or slits forming a predetermined opening area can be used, slit screens can be used, coarse screens can be used, and fine screens can be used. In the process of removing foreign matter by screen treatment, it is preferable to perform a rough screening treatment followed by a fine screening treatment, from the viewpoint of efficiently removing foreign matter. As the coarse sorting screen, a round-hole screen or a slit screen is preferred, with a round-hole screen being more preferred. The diameter of the round holes in the round-hole screen is preferably 0.5 to 2.5 mm, more preferably 0.8 to 2.0 mm. The slit width of the slit screen is preferably 0.25 to 0.5 mm. From the viewpoint of efficiently removing foreign matter, the slit width of the selection screen is preferably 0.10 to 0.25 mm, more preferably 0.10 to 0.20 mm, and even more preferably 0.10 to 0.18 mm.

[0033] In the rough screening process, the solid content concentration of the pulp slurry is preferably 1.0 to 5.0% by mass, more preferably 1.5 to 4.0% by mass, and even more preferably 2.0 to 3.0% by mass, from the viewpoint of efficiently removing foreign matter. In the selection screen process, the solid content concentration of the pulp slurry is preferably 0.2 to 5.0% by mass, more preferably 0.5 to 3.5% by mass, and even more preferably 0.8 to 3.0% by mass, from the viewpoint of efficiently removing foreign matter.

[0034] In the manufacturing method of this embodiment, a deinking step may be included, if necessary, after the dust removal step and before the washing step described later, in which the pulp slurry is deinked. The deinking step removes the ink contained in the printing layer from the pulp slurry, as well as coarse inorganic foreign matter. The deinking process may be performed using a flotator or the like.

[0035] When deinking is performed using a flotator, the solid content concentration of the pulp slurry is preferably 0.5 to 2.0% by mass, more preferably 0.5 to 1.3% by mass.

[0036] In the deinking process, a deinking agent may be added immediately before processing with the flotator. The deinking agent used immediately before processing with the flotator should preferably have strong ink-coagulating properties. Examples of fatty acids include DI-254 (oleic acid) and DI-268 from Kao Corporation, and K-4004-D from Daiichi Kogyo Seiyaku Co., Ltd. Examples of fatty acid derivatives include DIY-23543 from Kao Corporation, and Paper Aid W and Daihope 1000 from Daiichi Kogyo Seiyaku Co., Ltd. Examples of higher alcohol derivatives include DI-7020 from Kao Corporation. When adding a deinking agent in the deinking process, the amount of deinking agent added is preferably 0.01 to 0.5 parts by mass, more preferably 0.03 to 0.3 parts by mass, per 100 parts by mass of solid content in the pulp slurry.

[0037] In the manufacturing method of this embodiment, it is even more preferable to include a washing step to wash the pulp slurry after the dust removal step. The washing step further removes foreign matter and other contaminants from the pulp slurry. The washing step may be performed by repeatedly alternating between washing the pulp slurry and dewatering the pulp slurry, as needed.

[0038] Examples of equipment used in the cleaning process include DNT washers, compact washers, fall washers, Variosplit, SP filters, DP Cosmo, gap washers, and disc filters, with disc filters being preferred. In washing pulp slurry, the solid content concentration of the pulp slurry is preferably 0.5 to 5.0% by mass, more preferably 1.0 to 3.0% by mass, from the viewpoint of efficiently removing foreign matter from the pulp slurry.

[0039] In the manufacturing method of this embodiment, a dewatering step may be included after the washing step to dewater the pulp slurry and obtain recycled paper pulp. Including a dewatering step improves the handling properties of the obtained recycled paper pulp and makes it easier to blend it with raw pulp when producing paper products using recycled paper pulp. Examples of equipment used to dewater the pulp slurry in the dewatering step include a double nip thickener, a drum thickener, a disc thickener, a valveless thickener, etc., with a disc thickener being preferred.

[0040] In the process of obtaining the pulp slurry that forms each layer, optional components may be mixed in addition to pulp fibers and thermoplastic resin. Examples of optional components include dry strength agents, wet strength agents, sizing agents, aluminum sulfate, yield improvers, filtration improvers, bulking agents, coloring dyes, coloring pigments, fluorescent whitening agents, pH adjusters, pitch control agents, preservatives, slime control agents, and softeners. Examples of dry strength agents include cationized starch, polyacrylamide (PAM), and carboxymethylcellulose (CMC). Examples of wet strength agents include polyamide epichlorohydrin, urea, melamine, and thermocrosslinkable polyacrylamide. Examples of sizing agents include rosin-based, alkyl ketene dimer-based, and alkenyl succinic anhydride-based sizing agents, but when adding a sizing agent, it is preferable to use a rosin-based sizing agent. By using a rosin-based sizing agent, the sublimation of the sizing agent together with the sublimable ink is suppressed. The optional components may be used individually or in combination of two or more. The content of the optional components is preferably 10% by mass or less, more preferably 8% by mass or less, even more preferably 6% by mass or less, and most preferably 4% by mass or less, based on the total mass of the white cardboard.

[0041] In the process of obtaining the pulp slurry that forms each layer, when calcium carbonate is added, it is preferable to add the calcium carbonate as a calcium carbonate dispersion in which calcium carbonate is dispersed in water. When calcium carbonate is added as a calcium carbonate dispersion, a dispersant may be added to the calcium carbonate dispersion to obtain good dispersibility. Examples of dispersants include anionic surfactants. When adding fillers such as calcium carbonate to the slurry for the subsurface layer, the addition rate of the filler is adjusted to be 3 parts by mass or less per 100 parts by mass of pulp for the subsurface layer.

[0042] The composition of the slurry used in papermaking for each layer can be appropriately modified considering the quality required for white cardboard, manufacturing convenience, etc. For example, the ratio of deinked pulp and bleached pulp to the total pulp in the slurry is not particularly limited. It should be adjusted as appropriate considering which layer the slurry will be used to form, etc.

[0043] In the manufacturing method of this embodiment, it is preferable to further include a drying step after the step of multi-layer papermaking of white cardboard. In the drying step, the white cardboard obtained in the papermaking step is heated and dried. The drying step is preferably, for example, a step of blowing hot air onto the wet paper.

[0044] The drying temperature in the drying process is preferably 80°C or higher, more preferably 90°C or higher, and even more preferably 100°C or higher. Furthermore, the drying temperature is preferably 160°C or lower, and more preferably 140°C or lower. The papermaking speed of the paper machine in the drying process is preferably 150 m / min or higher, and more preferably 200 m / min or higher. Furthermore, the papermaking speed of the paper machine in the drying process is preferably 1000 m / min or lower. By providing such a drying process, the thermoplastic resin melts and binds appropriately to a portion of the pulp fibers, thereby more effectively suppressing the sublimation of ink clumps and their appearance on the surface of the white cardboard.

[0045] The manufacturing method of this embodiment may further include a calendering process after the drying process. The calendering process is performed in the calender part. In the calender part, the surface of the dried sheet is stretched while being pressed to smooth the surface of the sheet.

[0046] When a coating layer is provided on the surface of the white cardboard, the manufacturing method of this embodiment may include a step of forming a coating layer on at least one side of the white cardboard. The step of forming the coating layer preferably involves applying a coating liquid containing a pigment and a binder. In the step of forming the coating layer, the coating liquid is applied to the surface of the white cardboard to form a coating film. Then, the coating film is dried to provide a coating layer on the surface of the white cardboard.

[0047] The content of the binder in the coating liquid is preferably 2 parts by mass or more, more preferably 5 parts by mass or more, with respect to 100 parts by mass of the pigment. Also, the content of the binder is preferably 50 parts by mass or less, more preferably 30 parts by mass or less, with respect to 100 parts by mass of the pigment. If the proportion of the binder is at least the above lower limit value, the strength of the coating layer tends to be sufficient. Also, if the proportion of the binder is at most the above upper limit value, the ink drying property is excellent and the suitability for bookbinding also tends to improve.

[0048] The coating amount of the coating liquid is preferably 3 g / m² or more, more preferably 5 g / m² or more, and even more preferably 10 g / m² or more in terms of the basis weight at the time of drying. Also, the coating amount of the coating liquid is preferably 50 g / m² or less, more preferably 40 g / m² or less, and even more preferably 35 g / m² or less in terms of the basis weight at the time of drying. When the coating layer is provided on both sides of the white cardboard, the coating amounts of the front coating layer and the back coating layer may be different. 2 above 2 above 2 above 2 below 2 below 2 below

[0049] The coating solution may be applied in multiple stages. When the coating solution is applied in multiple stages, for example, in two stages, the first (primer layer) and the second (topcoat layer) coating solutions may be the same or different. Furthermore, the coating solution may be applied directly to the surface and / or back layer, or to the surface layer via another coating film.

[0050] When applying the coating solution, a known coater can be used. The coater may be on-machine or off-machine, but it is preferable to use an on-machine coater that uses a known coater part attached to the paper machine. As the coating device of the coater part, 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 with a calender can be used as appropriate. Among these, rod metering coaters and curtain coaters are preferred because the coating amount is constant, thus suppressing uneven coating and other issues in the coating layer. The applied coating layer is dried using a known drying device to form the coating layer. The paper machine may have multiple coater parts. In this case, the coating layer can be applied in multiple stages.

[0051] A calendering section may be provided after the coater section, if necessary. By providing a calendering section after the coater section, the coated layer is smoothed. Any known calendering device can be used for the calendering section after the coater section, such as supercalenders, gloss calenders, soft nip calenders, thermal calenders, and shoe calenders. These may be used in combination. Among these, a soft nip calender equipped with metal rolls and elastic rolls is preferred because it can smooth the coated layer while maintaining the paper thickness. The calendering section after the coater section may be on-machine or off-machine.

[0052] The manufacturing method of this embodiment may include a step of winding up white cardboard to form a roll (wind-up). In this case, it is preferable that at the end processing section of the wind-up, the end of the long white cardboard discharged from the wind-up section is glued along the width direction to prevent the long white cardboard from unraveling.

[0053] <<Thermoplastic resin>> The subsurface and / or middle layers constituting the white cardboard contain pulp fibers and a thermoplastic resin. The thermoplastic resin may be either a natural resin or a synthetic resin, and examples include starch derivatives, casein, shellac, polyvinyl alcohol and its derivatives, acrylic resins, ionomer resins, maleic acid resins, urethane resins, polyester resins (polyethylene terephthalate, etc.), styrene-butadiene resins, vinyl chloride resins, polyolefin resins, ethylene-vinyl alcohol copolymers, ethylene-acrylic acid copolymers (EAA), ethylene-methacrylic acid copolymers (EMAA), ethylene-methyl methacrylate copolymers (EMMA), styrene-acrylic copolymers, and polyamide resins (nylon, etc.). Among these, the thermoplastic resin is preferably a polyolefin resin, and examples of polyolefin resins include polyethylene, polypropylene, and ethylene-α-olefin copolymers (ethylene-propylene copolymer, etc.). The polyolefin resin is preferably polyethylene or polypropylene, and particularly preferably polyethylene. As polyethylene, high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE) are preferred, and low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) are more preferred.

[0054] The density of polyolefin resin is 0.900 g / cm³. 3 Preferably, it should be 0.910 g / cm³ or more. 3 It is more preferable that the above conditions are met. Furthermore, the density of the polyolefin resin is 0.970 g / cm³. 3Preferably, it is 0.950 g / cm³. 3 It is more preferable that the following is the case: 0.930 g / cm³ 3 The following is even more preferable:

[0055] The melting point of the polyolefin resin is preferably 85°C or higher, more preferably 90°C or higher, and even more preferably 100°C or higher. Furthermore, the melting point of the polyolefin resin is preferably 170°C or lower, more preferably 150°C or lower, and even more preferably 130°C or lower.

[0056] <<Pulp Fiber>> Each layer constituting the white cardboard contains pulp fibers. The pulp fiber content in the white cardboard is preferably 80% by mass or more, more preferably 85% by mass or more, and even more preferably 90% by mass or more. The upper limit of the pulp fiber content in the white cardboard is the remainder excluding polyethylene.

[0057] Pulp fibers include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include hardwood pulp (hardwood kraft pulp (LKP)), softwood pulp (softwood kraft pulp (NKP)), sulfite pulp (SP), dissolved pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), oxygen-bleached kraft pulp (OKP), and other chemical pulps. Other examples include semi-chemical pulp (SCP) and chemigroundwood pulp (CGP), as well as mechanical pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include cotton pulp such as cotton linters and cotton lint, non-wood pulp such as hemp, straw, and bagasse, and cellulose, chitin, and chitosan isolated from sea squirts and seaweed. Deinked pulp can be made from recycled paper. One type of pulp may be used alone, or two or more types may be mixed together.

[0058] The pulp fibers preferably include coniferous pulp fibers and hardwood pulp fibers, and more preferably, a combination of coniferous kraft pulp (NKP) fibers and hardwood kraft pulp (LKP) fibers is used. In this embodiment, it is also preferable to use deinked pulp fibers as pulp fibers, and it is preferable that deinked pulp fibers made from recycled paper are included.

[0059] White cardboard preferably contains softwood pulp fibers and hardwood pulp fibers. Here, if N is the content of softwood pulp fibers relative to the total pulp mass (parts by mass) and L is the content of hardwood pulp fibers relative to the total pulp mass (parts by mass), then N:L is preferably 0:100 to 40:60, more preferably 0:100 to 20:80, and even more preferably 0:100 to 10:90. The content of each pulp fiber can be analyzed according to JIS P 8120:1998.

[0060] <<Recycled materials>> The white cardboard of this embodiment is preferably manufactured using recycled materials. That is, the white cardboard of this embodiment is white cardboard containing recycled materials (recycled white cardboard). The recycled materials constituting the white cardboard are preferably waste paper. In particular, the subsurface layer and / or middle layer are preferably manufactured using waste paper, and laminated waste paper containing thermoplastic resin (for example, thermoplastic resin laminated waste paper, waste paper containing thermoplastic resin sheets, or waste paper coated with thermoplastic resin) is preferably used, laminated waste paper having a paper base material containing pulp fibers and a thermoplastic resin layer (thermoplastic resin laminated waste paper) is more preferably used, and laminated waste paper derived from liquid containers (thermoplastic resin laminated waste paper) is even more preferably used. Examples of liquid containers include beverage containers, liquid seasoning containers, detergent containers, and specifically, paper cups, paper glasses, milk cartons, aseptic containers, etc. Laminated waste paper derived from liquid containers is not repeatedly recycled, so the carboxyl groups in the pulp fibers are highly active, and the hydrogen bonds between the fibers become stronger, which more effectively suppresses the sublimation of ink clumps contained in the middle layer. Therefore, it is preferable to use it as a recycled material.

[0061] The proportion of laminated waste paper to the total mass of recycled raw materials used in the manufacture of white cardboard is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1.0% by mass or more, and particularly preferably 2.0% by mass or more. Furthermore, the proportion of laminated waste paper is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less, and particularly preferably 20% by mass or less.

[0062] The recycled materials constituting the sub-layer and / or middle layer preferably include laminated waste paper. In this case, the sub-layer and / or middle layer will contain pulp fibers derived from laminated waste paper and thermoplastic resin derived from laminated waste paper.

[0063] The thermoplastic resin content in the laminated waste paper is preferably 1% by mass or more, more preferably 5% by mass or more, and particularly preferably 8% by mass or more. Furthermore, the thermoplastic resin content in the laminated waste paper is preferably 40% by mass or less, more preferably 30% by mass or less, and particularly preferably 20% by mass or less.

[0064] The laminated recycled paper preferably contains softwood pulp fibers and hardwood pulp fibers in addition to thermoplastic resin. When N (parts by mass) is the content of softwood pulp fibers relative to the total pulp mass contained in the laminated recycled paper, and L (parts by mass) is the content of hardwood pulp fibers relative to the total pulp mass, the ratio N:L is preferably 0:100 to 40:60, more preferably 0:100 to 20:80, and even more preferably 0:100 to 10:90.

[0065] The fiber roughness of the pulp fibers contained in the recycled laminated paper is preferably 0.140 mg / m or less, more preferably 0.130 mg / m or less, and even more preferably 0.120 mg / m or less. Furthermore, the fiber roughness of the pulp fibers contained in the recycled laminated paper is preferably 0.050 mg / m or more. By setting the fiber roughness of the pulp fibers contained in the recycled laminated paper to be below the above upper limit, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. This is presumably because the reduced fiber roughness of the pulp fibers creates fine voids in the subsurface and middle layers, and these voids can absorb the sublimated ink. Furthermore, by setting the fiber roughness of the pulp fibers contained in the recycled laminated paper to be above the above lower limit, the density of the subsurface and / or middle layers is increased, and as a result, the sublimated ink can be absorbed.

[0066] The fiber roughness of pulp fibers contained in laminated waste paper is calculated using the following method. First, laminated waste paper is cut into 4 cm squares and immersed in deionized water to prepare a laminated waste paper content of 2% by mass. After immersion for 24 hours, the pulp is processed using a standard disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) in accordance with JIS P 8220-1:2012 until no undisintegrated fibers remain, thereby disintegrating the pulp into fibers. The slurry (dispersion of pulp fibers) after disintegration is prepared to a solid content concentration of 0.1% by mass, and the pulp solid content concentration is calculated in accordance with JIS P 8225:2003. The slurry after disintegration is prepared to a solid content concentration of 0.004% by mass and yield 500 g of slurry, and the amount taken is recorded. The dry weight of the sample is calculated using the solid content concentration and the amount of slurry taken. Using the obtained slurry, the "fiber roughness" is measured in accordance with ISO 16065-2:2014 using a fiber length measuring instrument (model Valmet FS-5 UHD base unit, manufactured by Valmet). The fiber roughness is gravimetric coarseness, which is measured by applying the sum of fiber lengths obtained from image analysis and the dry weight of the sample input into the instrument.

[0067] Of the pulp fibers contained in the laminated waste paper, the proportion of fibers with a fiber length of 0.2 mm or more and less than 0.6 mm is preferably 8% or more, more preferably 10% or more, even more preferably 12% or more, and particularly preferably 14% or more. By setting the proportion of fibers with a fiber length of 0.2 mm or more and less than 0.6 mm of the pulp fibers contained in the laminated waste paper to be above the above lower limit, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. It is predicted that as the proportion of fibers with a fiber length of 0.2 mm or more and less than 0.6 mm increases, the number of fibers that remain in the paper without coming loose from the wire during papermaking increases, and the density in the paper increases, thus physically suppressing the sublimation of ink clumps contained in the middle layer. Here, the above fiber length is the fiber length of the pulp fibers obtained by disintegrating the laminated waste paper. Specifically, it is measured by the following method. First, laminated waste paper is cut into 4cm squares, immersed in deionized water, and prepared to contain 2% by mass of laminated waste paper. After immersion for 24 hours, the pulp is processed using a standard disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) in accordance with JIS P 8220-1:2012 until no undisintegrated fibers remain, thus disintegrating the pulp into fibers. The resulting slurry (dispersion of pulp fibers) is prepared to a solid content concentration of 0.1% by mass, and the pulp solid content concentration is calculated in accordance with JIS P 8225:2003. The slurry is then prepared to a solid content concentration of 0.004% by mass and yield 500g of slurry, and the amount taken is recorded. The dry weight of the sample is calculated using the solid content concentration and the amount of slurry taken. Using the obtained slurry, the fiber length of fibers between 0.2 mm and 0.6 mm is measured in accordance with ISO 16065-2:2014 using a fiber length measuring instrument (model Valmet FS-5 UHD base unit, manufactured by Valmet). The ratio of small fibers is the ratio of the number of fibers (percentage of the number of fibers) weighted by length for fibers between 0.2 mm and 0.6 mm (small fibers). The number of fibers (n) for each length range is measured using the fiber length measuring instrument. i ) was measured, and the percentage of the number of fibers weighted by length (f i ') is calculated using the following formula (1).

[0068]

number

[0069] The symbols in equation (1) represent the following: f i ': Percentage of fiber count weighted by length (%) n i : The number of fibers in the i-th length range l i : Center of the i-th length range, displayed in millimeters Σn i l i n calculated for all length ranges i l i sum

[0070] The ratio of small fibers is calculated using formula (1) for the fiber length category of 0.2 mm or more and less than 0.6 mm. Specifically, the Valmet FS-5 UHD base unit used in the example measures the number of fibers in the following six categories (Fraction 1 to Fraction 6). Fraction 1: Fiber length less than 0.2 mm Fraction 2: Fiber length 0.2 mm or more and less than 0.6 mm Fraction 3: Fiber length 0.6 mm or more and less than 1.2 mm Fraction 4: Fiber length 1.2 mm or more and less than 2.0 mm Fraction 5: Fiber length 2.0 mm or more and less than 3.2 mm Fraction 6: Fiber length 3.2 mm or more and less than 7.6 mm The ratio of small fibers is the percentage of the number of fibers weighted by length (f) for Fraction 2. i Let ') be the calculated value.

[0071] The recycled materials constituting the sub-layer and / or middle layer may include both thermoplastic resin-free recycled paper and thermoplastic resin-containing recycled paper (laminated recycled paper). In this case, the proportion of thermoplastic resin-free recycled paper to the total mass of recycled materials used in the production of the sub-layer and / or middle layer is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 90% by mass or less. Furthermore, the proportion of thermoplastic resin-free recycled paper is preferably 70% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more.

[0072] Examples of waste paper that does not contain thermoplastic resins include deinked pulp fibers and corrugated cardboard pulp fibers made from waste paper other than thermoplastic resin-containing waste paper. Examples of deinked pulp include waste paper pulp (Kent waste paper pulp) collected from the trimmings of coated paper for printing generated from bookbinding and printing factories, etc., as well as magazine waste paper pulp, flyer waste paper pulp, newspaper waste paper pulp, office waste paper pulp, information paper waste paper pulp, cardboard waste paper pulp, and paper container waste paper pulp, all of which have been deinked.

[0073] Furthermore, the content of pulp fibers derived from recycled materials relative to the total mass of pulp fibers contained in the subsurface and middle layers is preferably 90% by mass or more, more preferably 94% by mass or more, and even more preferably 98% by mass or more. In addition, the content of pulp fibers derived from recycled materials relative to the total mass of pulp fibers contained in the subsurface and middle layers may be 100% by mass.

[0074] As described later, laminated recycled paper may contain optional components that white cardboard may contain. In particular, it is preferable that laminated recycled paper contains a sizing agent. Examples of sizing agents include rosin-based, alkyl ketene dimer-based, and alkenyl succinic anhydride-based sizing agents, but it is preferable that laminated recycled paper contains a rosin-based sizing agent. By using a rosin-based sizing agent, the sublimation of the sizing agent together with the sublimation ink is suppressed.

[0075] When the laminated recycled paper contains a rosin-based sizing agent, the rosin-based sizing agent content is preferably 0.001 parts by mass or more, more preferably 0.005 parts by mass or more, and even more preferably 0.01 parts by mass or more, per 100 parts by mass of pulp fibers contained in the laminated recycled paper. Furthermore, the rosin-based sizing agent content is preferably 4.0 parts by mass or less, more preferably 3.0 parts by mass or less, and even more preferably 2.0 parts by mass or less, per 100 parts by mass of pulp fibers contained in the laminated recycled paper. By keeping the rosin-based sizing agent content in the laminated recycled paper within the above ranges, the rosin-based sizing agent content in the white cardboard can be kept within an appropriate range, and as a result, the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard can be effectively suppressed.

[0076] <<Optional ingredients>> White cardboard may contain optional components. Examples of optional components include dry strength agents, wet strength agents, sizing agents, aluminum sulfate, yield enhancers, filtration enhancers, bulk enhancers, coloring dyes, coloring pigments, fluorescent whitening agents, pH adjusters, pitch control agents, preservatives, slime control agents, and softeners. Examples of dry strength agents include cationized starch, polyacrylamide (PAM), and carboxymethylcellulose (CMC). Examples of wet strength agents include polyamide epichlorohydrin, urea, melamine, and thermocrosslinkable polyacrylamide. Examples of sizing agents include rosin-based, alkyl ketene dimer-based, and alkenyl succinic anhydride-based sizing agents; however, when adding a sizing agent, it is preferable to use a rosin-based sizing agent.

[0077] (white paperboard) This embodiment may also relate to white cardboard manufactured by the manufacturing method described above. The white cardboard has at least a surface layer, a subsurface layer, a middle layer, and a back layer, and the subsurface layer and / or middle layer constituting the white cardboard contain pulp fibers and thermoplastic resin. By including a predetermined amount of thermoplastic resin in the subsurface layer and / or middle layer, it is possible to suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. This makes it possible to suppress the occurrence of hydrangea spots on the white cardboard.

[0078] The total basis weight of the white cardboard in this embodiment is 150 g / m². 2 Anything above that is acceptable, up to 175g / m². 2 Preferably, it should be 200g / m² or more. 2 It is more preferable that the amount be greater than or equal to 250 g / m². 2 It is even more preferable that the amount be greater than or equal to 300 g / m². 2 It is even more preferable that the amount be greater than or equal to 350 g / m². 2 It is particularly preferable that the above conditions are met. Furthermore, the basis weight of the white cardboard should be, for example, 600 g / m². 2 Preferably, it is 500g / m² 2 The following is more preferable: By setting the basis weight of the white cardboard to be above the lower limit, it is possible to more effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, by setting the basis weight of the white cardboard to be below the upper limit, it is possible to increase production efficiency in the box-making process and reduce manufacturing costs. The total basis weight of the white cardboard is measured in accordance with JIS P 8124:2011 after the white cardboard has been conditioned for 24 hours in a humidity-controlled environment as specified in JIS P 8111:1998.

[0079] The overall thickness of the white cardboard is preferably 160 μm or more, more preferably 200 μm or more, even more preferably 250 μm or more, and particularly preferably 300 μm or more. Furthermore, the overall thickness of the white cardboard is preferably 750 μm or less, more preferably 650 μm or less, and even more preferably 550 μm or less. By setting the thickness of the white cardboard to be above the lower limit above, it is possible to more effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, by setting the thickness of the white cardboard to be below the upper limit above, it is possible to increase production efficiency in the box-making process and reduce manufacturing costs. The overall thickness of the white cardboard is measured in accordance with JIS P 8118:2014 after the white cardboard has been conditioned for 24 hours in a humidity-controlled environment as specified in JIS P 8111:1998.

[0080] The overall density of the white cardboard is 0.60 g / cm³. 3 Preferably, it should be 0.70 g / cm³ or more. 3 It is more preferable that the value be greater than or equal to 0.80 g / cm³. 3 It is even more preferable that the above conditions are met. Furthermore, the overall density of the white cardboard should be 1.00 g / cm³. 3 The following is preferable: By setting the density of the white cardboard to be above the lower limit, it is possible to more effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, by setting the density of the white cardboard to be below the upper limit, it is possible to increase production efficiency in the box-making process and reduce manufacturing costs.

[0081] The ash content of white cardboard is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, based on the total mass of the white cardboard. Furthermore, the ash content of white cardboard is preferably 35% by mass or less, based on the total mass of the white cardboard. The ash content of white cardboard is measured in accordance with JIS P 8251:2003.

[0082] <<Layer structure of white cardboard>> White cardboard is a multi-layered white cardboard having a layered structure in which at least the front layer, front sub-layer, middle layer, and back layer are laminated in this order. The front layer is the layer located on the outermost side in multi-layered white cardboard, and the back layer is the layer located on the outermost side (opposite the front layer). The front sub-layer is the layer located immediately below the front layer and is in direct contact with the front layer. The front sub-layer can also be said to be the layer that is in contact with the front layer on the inside of the white cardboard. In the white cardboard of the present invention, a middle layer is located between the front sub-layer and the back layer. A back sub-layer may also be provided on the back side. The back sub-layer is the layer located immediately above the back layer and is in direct contact with the back layer. If the white cardboard further has a back sub-layer, it is preferable that the white cardboard of the embodiment has a structure in which the front layer, front sub-layer, middle layer, back sub-layer, and back layer are laminated in this order. In this specification, for layers other than the surface and back layers, layers with a whiteness of 50% or more are defined as the surface sublayer or back sublayer, and layers with a whiteness of less than 50% are defined as the middle layer. When measuring the whiteness of the surface sublayer, middle layer, and back sublayer, the white cardboard is immersed in 40°C hot water for 6 hours, then each layer is peeled off and dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the material is humidified for 24 hours under the humidity control conditions specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0083] (surface) The surface layer is preferably composed of pulp with high whiteness. Examples of pulp with high whiteness include deinked pulp from recycled paper and bleached pulp. Among these, bleached pulp is preferred because of its high whiteness.

[0084] Deinked pulp is deinked recycled paper pulp with a high degree of whiteness. Examples of recycled paper used as raw material for deinked pulp include: paper that has been used once but has little printing, such as white paper, ruled white paper, extra white paper, medium white paper, and white Manila paper; printed materials and colored papers that have been used once, such as cards, imitation paper, colored paper, Kent paper, and white art paper; used high-quality recycled paper such as coated printing paper, beverage cartons, and office paper; commercial medium-quality recycled paper such as tickets, medium-quality scraps, and Kent Manila paper; general medium-quality recycled paper such as newspapers and magazines; and brown recycled paper such as shredded brown paper, plain brown paper, miscellaneous bags, and cardboard. The recycled paper may also be shredded office paper or tickets that have confidentiality. These may be used individually or in combination of two or more types. In particular, as the surface deinking pulp, deinking pulp derived from recycled paper with a high degree of whiteness, such as white paper, cardboard, extra white, medium white, white Manila, imitation, and colored paper; and deinking pulp derived from recycled Kent paper are preferred.

[0085] Examples of bleached pulp that is not recycled paper pulp include, for example, bleached softwood kraft pulp (NBKP), bleached hardwood kraft pulp (LBKP), semi-bleached hardwood kraft pulp (LSBKP), semi-bleached softwood kraft pulp (NSBKP), hardwood sulfite pulp, softwood sulfite pulp, and other chemical pulps, as well as thermomechanical pulp (TMP), chemothermetic pulp (CTMP), chemigland pulp (CGP), refinergland pulp (RGP), gland pulp (GP), pressure-reduced stone gland pulp (PGW), and stone gland pulp (SGP).

[0086] The whiteness of the surface layer is preferably 70% or higher, more preferably 75% or higher, even more preferably 80% or higher, and particularly preferably 85% or higher. If the whiteness of the surface layer is above the lower limit of the above value, the surface whiteness of the white cardboard will be high, which will improve its appearance. When measuring the whiteness of the surface layer, the white cardboard is immersed in 40°C hot water for 6 hours, the surface layer is peeled off, and the cardboard is dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the cardboard is humidified for 24 hours under the humidity control environment specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0087] The surface layer has a basis weight of 10 g / m². 2 Preferably, it is 15 g / m 2 It is more preferable that the amount be greater than or equal to 20 g / m 2 It is even more preferable that the above conditions are met. Furthermore, the basis weight of the surface layer is 80 g / m². 2 Preferably, it is 65 g / m 2 More preferably, the following is true: 50 g / m 2 It is even more preferable that the following conditions apply: 35 g / m 2 The following conditions are particularly preferable: If the basis weight of the surface layer is equal to or greater than the lower limit above, the color (darkness) of the middle layer can be sufficiently concealed. Also, if the basis weight of the surface layer is equal to or less than the upper limit above, swelling during papermaking can be easily suppressed. When measuring the basis weight of the surface layer, the white cardboard is immersed in 40°C hot water for 6 hours, the surface layer is peeled off, and the paper is dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the paper is humidified for 24 hours under the humidity control conditions specified in JIS P 8111:1998, and the measurement is performed in accordance with JIS P 8124:2011.

[0088] The thickness of the surface layer is preferably 15 μm or more, more preferably 20 μm or more, and even more preferably 25 μm or more. Furthermore, the thickness of the surface layer is preferably 100 μm or less, more preferably 85 μm or less, and even more preferably 70 μm or less. If the thickness of the surface layer is above the lower limit, the color (darkness) of the middle layer can be sufficiently concealed. Furthermore, if the thickness of the surface layer is below the upper limit, swelling during papermaking can be easily suppressed.

[0089] The density of the surface layer is 0.60 g / cm³. 3 Preferably, it should be 0.70 g / cm³ or more. 3 It is more preferable that the value be greater than or equal to 0.80 g / cm³. 3 It is even more preferable that the above conditions are met. Furthermore, the density of the surface layer is 1.00 g / cm³. 3 The following is preferable: If the density of the surface layer is within the above range, the color (darkness) of the middle layer can be sufficiently concealed.

[0090] The surface layer may contain the optional components mentioned above, and may also contain calcium carbonate. The calcium carbonate may be light calcium carbonate or heavy calcium carbonate, but light calcium carbonate is preferred. In addition, light calcium carbonate and heavy calcium carbonate may be used in combination as the calcium carbonate. Since calcium carbonate is often used in recycled paper recovered as a raw material for recycled paper pulp, if recycled paper pulp is used for papermaking of the surface layer, the surface layer may contain calcium carbonate derived from recycled paper. The addition rate of the filler in the surface layer is preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the surface layer pulp. Furthermore, the addition rate of the filler in the surface layer is preferably 3 parts by mass or more per 100 parts by mass of the surface layer pulp.

[0091] The surface ash content is preferably 1% by mass or more, more preferably 4% by mass or more, and even more preferably 8% by mass or more, relative to the total mass of the surface layer. Furthermore, the surface ash content is preferably 45% by mass or less, more preferably 35% by mass or less, and even more preferably 25% by mass or less, relative to the total mass of the surface layer. If the surface ash content is above the lower limit, it is easier to increase the surface whiteness of the white cardboard. Also, if the surface ash content is below the upper limit, it is easier to ensure the strength of the white cardboard. The surface ash content is measured in accordance with JIS P 8251:2003 after immersing the white cardboard in 40°C hot water for 6 hours and peeling off the surface layer.

[0092] (lower surface layer) The subsurface layer is composed of pulp with a lower whiteness than the surface layer pulp and a higher whiteness than the middle layer pulp. In this specification, the subsurface layer is defined as a layer with a whiteness of 50% or more. As the pulp constituting the subsurface layer, it is preferable to use deinked pulp derived from recycled paper of a lower grade compared to the surface layer, that is, deinked pulp derived from recycled paper that contains a large amount of medium-density fibers. For example, deinked pulp derived from newspapers, magazines, colored paper, balls, etc., is preferred. Among these, deinked pulp derived from recycled magazine paper is preferred.

[0093] The whiteness of the subsurface layer is preferably 50% or higher, more preferably 55% or higher, and even more preferably 60% or higher. Furthermore, the whiteness of the subsurface layer is preferably 85% or lower. If the whiteness of the subsurface layer is above the lower limit, it is easier to increase the surface whiteness of the white cardboard. Also, if the whiteness of the subsurface layer is below the upper limit, manufacturing costs can be reduced. When measuring the whiteness of the subsurface layer, the white cardboard is immersed in 40°C hot water for 6 hours, the subsurface layer is peeled off, and the cardboard is dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the cardboard is humidified for 24 hours under the humidity control conditions specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0094] In this embodiment, the subsurface layer may contain a thermoplastic resin. The thermoplastic resin contained in the subsurface layer is preferably a thermoplastic resin derived from thermoplastic resin-containing recycled paper (for example, recycled paper containing thermoplastic resin or a thermoplastic resin sheet), and is particularly preferably a thermoplastic resin derived from recycled paper laminated with thermoplastic resin.

[0095] The content of thermoplastic resin in the subsurface layer is preferably 50 ppm or more, more preferably 100 ppm or more, and even more preferably 200 ppm or more. Furthermore, the content of thermoplastic resin in the subsurface layer is preferably 3000 ppm or less, more preferably 2500 ppm or less, and even more preferably 2000 ppm or less. By setting the content of thermoplastic resin in the subsurface layer to be above the lower limit, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, by setting the content of thermoplastic resin in the subsurface layer to be below the upper limit, it is possible to suppress the occurrence of paper breaks during the manufacturing process of the white cardboard.

[0096] When measuring the content of thermoplastic resin contained in the subsurface layer, an extraction step to separate the thermoplastic resin contained in the subsurface layer, as described below, and an analysis step to quantify the thermoplastic resin by thermal decomposition GC / MS are performed. In the extraction process, the subsurface layer peeled from the white cardboard is collected, and the thermoplastic resin in the subsurface layer is extracted using a high-speed solvent extraction apparatus (machine: Büch, E-916). The extraction conditions are an extraction temperature of 180°C, an extraction pressure of 150 bar, and an extraction solvent of xylene (Wako Pure Chemical Industries, special grade). The sample is set in the extraction apparatus, and extracts equivalent to two extraction cycles are collected. After extraction, the solvent is removed using a rotary evaporator (water bath at 60°C), and the sample is dried at 105°C for 12 hours to obtain the thermoplastic resin in the subsurface layer. Next, in the analysis step, an appropriate amount of xylene is added to the extracted thermoplastic resin to a concentration of 2-5 mg / ml, and the mixture is dissolved at 130°C for 2 hours while stirring. Then, the xylene solution of the dissolved extract, whose tare weight has been measured, is added to the eco-cup so that the extract volume is 50-200 μg, and the solvent is evaporated at 105°C for 2 hours, and the amount of extract to be analyzed is measured. The content of the thermoplastic resin in the extract is quantified by pyrolysis GC / MS (Shimadzu GC / MS-QP2010). Furthermore, if the thermoplastic resin is polyethylene, the measurement conditions for pyrolysis GC / MS shall be as follows. • Polyethylene standard: (Tosoh 07C03C, LDPE) • Column: HP-5MS (Length: 30m, Inner diameter: 0.250mm, Thickness: 0.25μm) • Analysis conditions: Pyrolysis temperature = 600°C, Inlet temperature = 320°C, Split ratio = 1:60, Interface temperature = 280°C GC column temperature conditions = 40℃ (held for 3 minutes) → 10℃ / min → 325℃ (held for 15 minutes), ion source temperature = 200℃ Detection: m / z 29-600 (Quantification is calculated using the mass chromatogram area value of 1,19-eicosadiene (C20) at m / z 82) The above analytical method applies when the thermoplastic resin is polyethylene. However, when quantifying the content of thermoplastic resins other than polyethylene, the analysis can be performed similarly by appropriately changing the sample, analytical conditions, and quantitative m / z.

[0097] When a thermoplastic resin is contained in the subsurface layer, it is preferable that the thermoplastic resin is uniformly dispersed throughout the entire subsurface layer. For example, when the subsurface layer is divided into three equal parts in the thickness direction, it is preferable that the content ratio of the thermoplastic resin in the three regions is within ±10%.

[0098] The basis weight of the lower layer is 25 g / m². 2 Anything above that is acceptable, 30g / m 2 Preferably, it is 32 g / m 2 It is more preferable that the above conditions are met. Furthermore, the basis weight of the lower layer is 95 g / m². 2 The following is acceptable: 85g / m 2 Preferably, it is 80 g / m 2 More preferably, the following is 70g / m 2 It is even more preferable that the following conditions apply: 60 g / m 2 It is even more preferable that the following conditions are met: 50 g / m 2 The following conditions are particularly preferable. If the basis weight of the sub-layer is equal to or greater than the lower limit, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Also, if the basis weight of the sub-layer is equal to or less than the upper limit, it is easier to suppress swelling during papermaking. Furthermore, it is preferable that the basis weight of the sub-layer be greater than that of the surface layer, as this can suppress the occurrence of cracks on the surface of the white cardboard. When measuring the basis weight of the sub-layer, the white cardboard is immersed in 40°C hot water for 6 hours, the sub-layer is peeled off, and the paper is dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the paper is humidified for 24 hours under the humidity control conditions specified in JIS P 8111:1998, and the measurement is performed in accordance with JIS P 8124:2011.

[0099] The thickness of the subsurface layer is preferably 25 μm or more, more preferably 30 μm or more, even more preferably 35 μm or more, and particularly preferably 40 μm or more. Furthermore, the thickness of the subsurface layer is preferably 115 μm or less, more preferably 100 μm or less, and even more preferably 85 μm or less. If the thickness of the subsurface layer is above the lower limit, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, if the thickness of the subsurface layer is below the upper limit, it is easier to suppress swelling during papermaking.

[0100] The density of the subsurface layer is 0.60 g / cm³. 3 Preferably, it should be 0.70 g / cm³ or more. 3 It is more preferable that the value be greater than or equal to 0.80 g / cm³. 3 It is even more preferable that the above conditions are met. Furthermore, the density of the subsurface layer is 1.00 g / cm³. 3 The following is preferable. If the density of the lower layer is within the above range, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard.

[0101] The subsurface layer, like the surface layer, may contain the optional components mentioned above, and is preferably composed of calcium carbonate. The calcium carbonate may be light calcium carbonate or heavy calcium carbonate, but light calcium carbonate is preferred. In addition, light calcium carbonate and heavy calcium carbonate may be used in combination as the calcium carbonate. Calcium carbonate is often used in recycled paper recovered as a raw material for recycled paper pulp. When recycled paper pulp is used in the papermaking of the subsurface layer, calcium carbonate derived from recycled paper may be mixed into the subsurface layer. In addition, calcium carbonate may be added to the subsurface layer separately. The addition rate of filler to the subsurface layer should be 3 parts by mass or less per 100 parts by mass of pulp in the subsurface layer, preferably 2.5 parts by mass or less, and more preferably 2 parts by mass or less. Furthermore, it is particularly preferable that the addition rate of filler to the subsurface layer be 0 parts by mass per 100 parts by mass of pulp in the subsurface layer. In this embodiment, the occurrence of hydrangea spots has been successfully suppressed even without blending filler into the subsurface layer. In this embodiment, by keeping the addition rate of fillers such as calcium carbonate in the subsurface layer below the above upper limit, the interlayer adhesion between the subsurface layer and the surface layer can be improved, and as a result, the occurrence of blistering in the white cardboard can be suppressed.

[0102] The ash content of the subsurface layer is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, relative to the total mass of the subsurface layer. Furthermore, the ash content of the subsurface layer is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less, relative to the total mass of the subsurface layer. If the ash content of the subsurface layer is above the lower limit, it is easier to increase the surface whiteness of the white cardboard. Furthermore, if the ash content of the subsurface layer is below the upper limit, it is easier to ensure the strength of the white cardboard. The ash content of the subsurface layer is measured in accordance with JIS P 8251:2003 after immersing the white cardboard in 40°C hot water for 6 hours and peeling off the subsurface layer.

[0103] (middle layer) The middle layer typically uses the lowest-grade pulp among the layers that make up white cardboard. In this specification, the middle layer is a layer with a whiteness of less than 50%. Examples include disintegrated pulp from newspapers, magazines, tickets, medium-quality scraps, imitation brown paper, corrugated cardboard, backing paper, land deeds, and balls.

[0104] The intermediate layer may consist of only one layer or multiple layers. In this embodiment, it is preferable that the intermediate layer consists of multiple layers. When the intermediate layer consists of multiple layers, the pulp constituting each layer of the intermediate layer may all be the same or may be different.

[0105] The whiteness of the middle layer is preferably less than 50%. Furthermore, the whiteness of the middle layer is preferably 30% or more, more preferably 35% or more, and even more preferably 40% or more. If the whiteness of the middle layer is above the above lower limit, it is easier to increase the surface whiteness of the white cardboard. When measuring the whiteness of the middle layer, the white cardboard is immersed in 40°C hot water for 6 hours, the middle layer is peeled off, and the measurement is performed using a sample that has been dried in a dryer set to 105°C for 30 minutes. Before measurement, the sample is humidified for 24 hours under the humidity control environment specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0106] In this embodiment, the intermediate layer may contain a thermoplastic resin. If multiple intermediate layers are provided, all intermediate layers may contain a thermoplastic resin, or only some of the intermediate layers may contain a thermoplastic resin. The thermoplastic resin contained in the intermediate layer is preferably a thermoplastic resin derived from thermoplastic resin-containing recycled paper (for example, recycled paper containing thermoplastic resin or a thermoplastic resin sheet), and is particularly preferably a thermoplastic resin derived from recycled paper laminated with a thermoplastic resin.

[0107] The content of thermoplastic resin in the middle layer is preferably 50 ppm or more, more preferably 100 ppm or more, and even more preferably 200 ppm or more. Furthermore, the content of thermoplastic resin is preferably 3000 ppm or less, more preferably 2500 ppm or less, and even more preferably 2000 ppm or less. By setting the content of thermoplastic resin in the middle layer to be above the lower limit, it is possible to effectively suppress the sublimation of ink clumps contained in the middle layer and their appearance on the surface of the white cardboard. Furthermore, by setting the content of thermoplastic resin in the middle layer to be below the upper limit, it is possible to suppress the occurrence of paper breaks during the manufacturing process of the white cardboard.

[0108] The thermoplastic resin content in the middle layer can be measured using the same method as the thermoplastic resin content in the subsurface layer.

[0109] When a thermoplastic resin is included in the middle layer, it is preferable that the thermoplastic resin is uniformly dispersed throughout the entire middle layer. For example, when the middle layer is divided into three equal parts in the thickness direction, it is preferable that the content ratio of the thermoplastic resin in the three regions is within ±10%.

[0110] The basis weight of the middle layer is 50 g / m². 2 Preferably, it should be 100g / m² or more. 2 It is more preferable that the amount be greater than or equal to 150 g / m². 2 It is even more preferable that the amount be greater than or equal to 200 g / m². 2 It is even more preferable that the amount be greater than or equal to 250 g / m². 2 It is particularly preferable that the above conditions are met. Furthermore, the basis weight of the middle layer should be 500 g / m². 2 Preferably, it is 400g / m² 2 It is more preferable that the following conditions apply: 360 g / m² 2 It is even more preferable that the following conditions apply: 330 g / m² 2The following is particularly preferable. If the basis weight of the middle layer is within the above range, swelling during papermaking can be suppressed. If multiple middle layers are provided, the basis weight of the middle layer is the total basis weight of the middle layers. When measuring the basis weight of the middle layer, the white cardboard is immersed in 40°C hot water for 6 hours, the middle layer is peeled off, and the paper is dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the paper is humidified for 24 hours under the humidity control conditions specified in JIS P 8111:1998, and the measurement is performed in accordance with JIS P 8124:2011.

[0111] The thickness of the middle layer is preferably 60 μm or more, more preferably 100 μm or more, and even more preferably 150 μm or more. Furthermore, the thickness of the middle layer is preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 450 μm or less. If the thickness of the middle layer is within the above range, swelling during papermaking can be suppressed.

[0112] The density of the middle layer is 0.60 g / cm³. 3 Preferably, it should be 0.70 g / cm³ or more. 3 It is more preferable that the value be greater than or equal to 0.80 g / cm³. 3 It is even more preferable that the above conditions are met. Furthermore, the density of the middle layer is 1.00 g / cm³. 3 The following is preferable. If the density of the middle layer is within the above range, swelling during papermaking can be suppressed.

[0113] The middle layer may contain the aforementioned optional components, similar to the surface layer.

[0114] The ash content of the middle layer is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, relative to the total mass of the middle layer. Furthermore, the ash content of the middle layer is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less, relative to the total mass of the middle layer. If the ash content of the middle layer is within the above range, it is easier to ensure the strength of the white cardboard. The ash content of the middle layer is measured in accordance with JIS P 8251:2003 after immersing the white cardboard in 40°C hot water for 6 hours and peeling off the middle layer.

[0115] (Underlayer) The back layer is the layer located on the innermost side (opposite the top layer). Therefore, the back layer does not require the same degree of whiteness as the top layer, but because it is visible to the human eye, pulp with a higher degree of whiteness than the middle layer is usually used. The back layer may also contain pulp derived from shredded office paper, etc. Pulp obtained by disintegrating shredded office paper, etc., has a relatively high degree of whiteness even without deinking or bleaching, so it is preferable to incorporate it into the back layer.

[0116] The whiteness of the backing layer is preferably 50% or higher, more preferably 55% or higher, and even more preferably 60% or higher. If the whiteness of the backing layer is above the lower limit of the above value, the appearance of the back surface of the white cardboard will not be significantly impaired. When measuring the whiteness of the backing layer, the white cardboard is immersed in 40°C hot water for 6 hours, the backing layer is peeled off, and the cardboard is dried in a dryer set to 105°C for 30 minutes before measurement. Before measurement, the cardboard is humidified for 24 hours under the humidity control conditions specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0117] The basis weight of the backing layer is 10g / m². 2 Preferably, it is 15 g / m 2 It is more preferable that the amount be greater than or equal to 20 g / m 2 It is even more preferable that the amount be greater than or equal to 25 g / m². 2 It is particularly preferable that the above conditions are met. Also, the basis weight of the back layer should be 90 / m². 2It is preferably the following, 70 / m 2 More preferably, it is the following, 55 / m 2 Even more preferably, it is the following, 40 g / m 2 It is particularly preferable that it is the following. If the basis weight of the back layer is within the above range, the colored foreign substances in the middle layer can be sufficiently concealed. Also, if the basis weight of the back layer is within the above range, sufficient paper layer strength can be obtained. When measuring the basis weight of the back layer, use the one obtained by immersing the white cardboard in hot water at 40°C for 6 hours, peeling off the back layer, and drying it in a dryer set at 105°C for 30 minutes. Before measurement, condition it for 24 hours in the conditioning environment specified in JIS P 8111:1998 and perform the measurement in accordance with JIS P 8124:2011.

[0118] The thickness of the back layer is preferably 15 μm or more, more preferably 20 μm or more, even more preferably 25 μm or more, and particularly preferably 30 μm or more. Also, the thickness of the back layer is preferably 100 μm or less, more preferably 90 μm or less, and even more preferably 80 μm or less. If the thickness of the back layer is within the above range, the colored foreign substances in the middle layer can be sufficiently concealed.

[0119] The density of the back layer is preferably 0.60 g / cm 3 or more, more preferably 0.70 g / cm 3 or more, and even more preferably 0.80 g / cm 3 or more. Also, the density of the back layer is preferably 1.00 g / cm 3 or less. If the density of the back layer is within the above range, the colored foreign substances in the middle layer can be sufficiently concealed.

[0120] The back layer may contain any of the above-mentioned components as in the surface layer.

[0121] The ash content of the backing layer is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, relative to the total mass of the backing layer. Furthermore, the ash content of the backing layer is preferably 25% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, relative to the total mass of the backing layer. If the ash content of the backing layer is within the above range, the colored foreign matter in the middle layer can be sufficiently concealed. When measuring the ash content of the backing layer, the white cardboard is immersed in 40°C hot water for 6 hours, the backing layer is peeled off, and it is dried in a dryer set to 105°C for 30 minutes, and measured in accordance with JIS P 8251:2003.

[0122] (Underly layer) White cardboard may have a surface layer, a subsurface layer, a middle layer, a back layer, and a subsurface layer. The subsurface layer is located between the back layer and the middle layer. The subsurface layer can also be described as the layer that is in contact with the back layer on the inside of the white cardboard. In this specification, the subsurface layer is a layer with a whiteness of 50% or more.

[0123] For the pulp constituting the sub-back layer, it is preferable to use pulp that has a higher degree of whiteness than the pulp in the middle layer but a lower degree of whiteness than the pulp in the back layer. The pulp used for the top layer and sub-top layer may be used for the sub-back layer, but usually, a lower-grade recycled paper, i.e., recycled paper containing a large amount of medium-density fibers, is used compared to the top layer. For example, unbleached and deinked recycled paper pulp from newspapers, magazines, colored paper, and balls is commonly used.

[0124] If the white cardboard further has a back layer, the back layer may contain the optional components mentioned above, similar to the surface layer, and may also contain calcium carbonate. The calcium carbonate may be light calcium carbonate or heavy calcium carbonate, but light calcium carbonate is preferred. In addition, light calcium carbonate and heavy calcium carbonate may be used in combination as the calcium carbonate. Calcium carbonate is often used in recycled paper recovered as a raw material for recycled paper pulp. If recycled paper pulp is used in the papermaking of the back layer, calcium carbonate derived from recycled paper may be mixed into the back layer. In addition, calcium carbonate may be added separately to the back layer. The calcium carbonate content of the back layer is preferably 0.1 to 20% by mass, more preferably 0.5 to 15% by mass, and even more preferably 1 to 10% by mass, relative to the total mass of the back layer. If the calcium carbonate content of the back layer is above the lower limit above, the design quality of the white cardboard can be more effectively enhanced. If the calcium carbonate content of the back layer is below the upper limit above, it is easier to ensure the strength of the white cardboard.

[0125] The ash content of the underlayer is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more, relative to the total mass of the underlayer. Furthermore, the ash content of the underlayer is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less, relative to the total mass of the underlayer. If the ash content of the underlayer is above the lower limit, it is easier to adequately conceal colored foreign matter in the middle layer. Also, if the ash content of the underlayer is below the upper limit, it is easier to ensure the strength of the white cardboard. The ash content of the underlayer is measured in accordance with JIS P 8251:2003 after immersing the white cardboard in 40°C hot water for 6 hours and peeling off the underlayer.

[0126] (Coated white cardboard) This embodiment may also relate to coated white cardboard having a coating layer on at least one side of the white cardboard. The coating layer may be provided on only one of the surfaces of the front layer or the back layer, or it may be provided on both sides. When the coating layer is provided on both sides, the coating layer provided on the front layer is called the front coating layer, and the coating layer provided on the back layer is called the back coating layer. The coating layer is preferably a layer containing pigment and binder. By providing a coating layer on the surface of the white cardboard, the printability of the front side can be improved and the whiteness can be increased.

[0127] Examples of pigments include kaolin, calcium carbonate (heavy calcium carbonate, light calcium carbonate), titanium dioxide, aluminum hydroxide, silica, satin white, and talc, which are commonly used in the field of coated paper manufacturing. Among these, calcium carbonate is preferred, and heavy calcium carbonate is particularly preferred, due to its excellent printability.

[0128] The binder is preferably a water-based adhesive. 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; latexes such as conjugated diene copolymer latex such as styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer latex, and styrene-methyl methacrylate-butadiene copolymer latex, and acrylic copolymer latex 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 carboxymethylcellulose. One or more of these adhesives can be selected and used as the binder.

[0129] Among these, latex with 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 resistance to breakage. More preferably, the glass transition temperature of the latex is -50 to 0°C. It is also preferable to blend starch together with the latex. When latex and starch are used together, a balance is achieved 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.

[0130] The coating layer may further contain one or more of the following, as needed: dispersants, pH adjusters (such as sodium hydroxide and ammonia water), defoamers, fluorescent dyes, mold release agents, water-resistant agents, fluidity improvers, slime control agents, preservatives, dyes, coloring pigments, etc.

[0131] The basis weight of the coating layer is 3 g / m². 2 Preferably, it is 5 g / m 2 It is more preferable that the amount be greater than or equal to 10 g / m 2 It is even more preferable that the above conditions are met. Furthermore, the basis weight of the coating layer is 50 g / m². 2 Preferably, it is 40 g / m 2 It is more preferable that the following conditions apply: 35 g / m 2 The following is even more preferable. Furthermore, if the coating layer is provided on both sides of the white cardboard, the basis weight of the front coating layer and the back coating layer may be different.

[0132] (packaging material) The white cardboard of this embodiment can be used not only for general applications such as printing cardboard and paper packaging cardboard, but also for applications such as drawing sheets that utilize pressure deformation and food packaging. In particular, the white cardboard of this embodiment is preferably a packaging material made by processing white cardboard, and more preferably a packaging material made by processing white cardboard into a box. [Examples]

[0133] The features of the present invention will be further specifically described below by way of examples and comparative examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be appropriately changed as long as they do not depart from the gist of the present invention. Therefore, the scope of the present invention should not be construed in a limited manner by the specific examples shown below. Also, the operations in the examples and comparative examples were carried out under the conditions of 23 ± 1°C and a relative humidity of 50 ± 2% unless otherwise specified.

[0134] <Example 1> [Production of laminated waste paper pulp] (Crushing and washing process) As laminated waste paper, laminated waste paper A (printing layer / paper base layer / LDPE layer, pulp blend in the paper base layer N:L = 15:85, freeness 515 mL, rosin sizing agent content 0.23 parts by mass, alkyl ketene dimer sizing agent content 0 parts by mass, basis weight 200 g / m 2 , packaging material for paper cups) was used, and it was crushed and washed with a crushing and washing machine (manufactured by A-Tech Co., Ltd., paper container recycling device PPRS, number of hooks: 6, washing water: water, washing water temperature: 25°C, flow rate 1.5 m 3 / h) to a size of 20 cm 2 or more and 400 cm 2 or less (average size: 60 cm 2 ).

[0135] (Dissociation process) 100 kg of the crushed and washed laminated waste paper and water were charged into a low-concentration pulper (manufactured by Aikawa Iron Works Co., Ltd., AHX helical pulper), and dissociation treatment was carried out under the conditions of a laminated waste paper concentration of 4% by mass, a treatment time of 10 minutes, a treatment temperature of 40°C, and a pH of 6 to obtain a laminated waste paper pulp slurry.

[0136] (Dust removal process) After adjusting the concentration of the laminated recycled paper pulp slurry to 2.5% by mass solids, it was processed with a weight-based foreign matter cleaner (Aikawa Iron Works Co., Ltd., FC100 type), and then further processed with a coarse sorting screen (Aikawa Iron Works Co., Ltd., MaxFlow-1, Model 1000 type, 1.2 mm round hole). After the coarse sorting screen treatment, the laminated recycled paper pulp slurry was adjusted to 1.0% by mass solids and processed with a fine sorting screen (Aikawa Iron Works Co., Ltd., MaxFlow-1, Model 1000 type, 0.15 mm slit). After the fine sorting screen treatment, the laminated recycled paper pulp slurry was processed with a low-concentration rameau cleaner (Aikawa Iron Works Co., Ltd., Bycorn 150).

[0137] (Washing process, dewatering process) The slurry after the dust removal process was washed using a disc filter (DF520, manufactured by IHI Voith Paper Technology Co., Ltd.), and then dewatered using a disc thickener (EFK1310, manufactured by IHI Voith Paper Technology Co., Ltd.) to obtain laminated recycled paper pulp.

[0138] [Preparing white cardboard] (Preparation of surface paper material) As raw material pulp, a mixture of 80% by mass of deinked pulp obtained by deinking recycled Kent paper and 20% by mass of bleached hardwood kraft pulp was used. This mixture was disintegrated under the conditions of a recycled paper concentration of 4% by mass, a processing time of 10 minutes, a processing temperature of 40°C, and a pH of 6 to obtain a pulp slurry for the surface layer. To 100 parts by mass of pulp (solid content) in this pulp slurry, 0.2 parts by mass of a sizing agent (product name: Sizing Pine N-817, manufactured by Arakawa Chemical Industries, Ltd.), 1.0 part by mass of aluminum sulfate, 0.4 parts by mass of a dry paper strength enhancer (product name: Polystron 1276, manufactured by Arakawa Chemical Industries, Ltd.), and 8 parts by mass of light calcium carbonate (product name: Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd.) were added to prepare the surface layer (first layer) paper material (pulp slurry).

[0139] (Preparation of the paper material for the lower layer of the table) As raw material pulp, deinked pulp obtained by deinking waste magazine paper was used, and a pulp slurry for the subsurface layer was obtained by disintegrating it under the conditions of a waste paper concentration of 4% by mass, a processing time of 10 minutes, a processing temperature of 40°C, and a pH of 6. To 100 parts by mass of pulp (solid content) in this pulp slurry, 0.5 parts by mass of aluminum sulfate, 0.1 parts by mass of a dry paper strength enhancer (product name: Polystron 1276, manufactured by Arakawa Chemical Industries, Ltd.), and 3 parts by mass of light calcium carbonate (product name: Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd.) were added to prepare the subsurface layer (second layer) paper material (pulp slurry).

[0140] (Preparation of mid-layer paper materials) As raw material pulp, recycled magazine pulp (mass ratio of recycled magazine pulp not used with sublimation ink to recycled magazine pulp used with sublimation ink = 99.9:0.1) was used, and the recycled magazine pulp slurry obtained by disintegrating under conditions of recycled paper concentration of 4% by mass, processing time of 10 minutes, processing temperature of 40°C, and pH 6 was mixed with a laminated recycled magazine pulp slurry with a laminated recycled magazine pulp concentration of 4% by mass obtained by adding water to the laminated recycled magazine pulp obtained above, in a mass ratio of 97:3. This pulp slurry was beaten to a freeness of 340±5 ml to obtain a pulp slurry for the middle layer. To 100 parts by mass of pulp (solid content) in this pulp slurry, 1.5 parts by mass of aluminum sulfate was added to prepare the paper material (pulp slurry) for the middle layer (3rd and 4th layers).

[0141] (Preparation of backing paper material) As raw material pulp, a mixture of 50% by mass of recycled magazine pulp and 50% by mass of recycled newspaper pulp was used, and a pulp slurry for the backing layer was obtained by disintegrating it under the conditions of a recycled paper concentration of 4% by mass, a processing time of 10 minutes, a processing temperature of 40°C, and a pH of 6. To 100 parts by mass of pulp (solid content) in this pulp slurry, 1.0 part by mass of aluminum sulfate and 0.1 parts by mass of a dry paper strength enhancer (product name: Polystron 1276, manufactured by Arakawa Chemical Industries, Ltd.) were added to prepare the backing layer (5th layer) paper material (pulp slurry).

[0142] (Manufacturing of white cardboard) Using the prepared pulp for each layer as described above, the basis weight of the surface layer (first layer) was set to 25 g / m². 2The basis weight of the lower layer (second layer) is 35 g / m². 2 The basis weight of the middle layer (3rd layer) is 140g / m². 2 The basis weight of the middle layer (4th layer) is 170g / m². 2 The basis weight of the back layer (5th layer) is 30g / m². 2 The paper was then made by combining five layers using a short-wire paper machine, resulting in a basis weight of 400 g / m². 2 We obtained white cardboard. In the papermaking process, a total length of 2000m was wound up after drying in a multi-cylinder dryer at a speed of 240m / min and a temperature of 125°C.

[0143] <Example 2> In the preparation of the intermediate layer of white cardboard in the [Preparation of White Cardboard] section, white cardboard was obtained in the same manner as in Example 1, except that a pulp slurry was used which was a mixture of magazine waste pulp slurry and laminate waste pulp slurry in a mass ratio of 90:10.

[0144] <Example 3> In the preparation of the lower layer of white cardboard, a pulp slurry was used, which consisted of a deinked pulp slurry obtained by deinking magazine waste paper and a laminated waste paper pulp slurry with a laminated waste paper pulp concentration of 4% by mass, obtained by adding water to laminated waste paper pulp, in a mass ratio of 97:3. In the preparation of the middle layer of white cardboard, a pulp slurry was used, which consisted of a magazine waste paper pulp slurry and a laminated waste paper pulp slurry, in a mass ratio of 100:0. Otherwise, white cardboard was obtained in the same manner as in Example 1.

[0145] <Example 4> In the preparation of the sub-layer paper stock for the white cardboard, white cardboard was obtained in the same manner as in Example 3, except that a pulp slurry was used which consisted of a deinked pulp slurry obtained by deinking magazine waste paper and a laminated waste paper pulp slurry with a laminated waste paper pulp concentration of 4% by mass, obtained by adding water to laminated waste paper pulp, in a mass ratio of 90:10.

[0146] <Example 5> In the preparation of the lower layer of white cardboard, a pulp slurry was used, which consisted of a deinked pulp slurry obtained by deinking magazine waste paper and a laminated waste paper pulp slurry with a laminated waste paper pulp concentration of 4% by mass, obtained by adding water to laminated waste paper pulp, in a mass ratio of 97:3. In the preparation of the middle layer of white cardboard, a pulp slurry was used, which consisted of a magazine waste paper pulp slurry and a laminated waste paper pulp slurry, in a mass ratio of 90:10. Otherwise, white cardboard was obtained in the same manner as in Example 1.

[0147] <Example 6> White cardboard was obtained in the same manner as in Example 3, except that the laminated recycled paper was not crushed or washed in the (crushing and washing process) of the [production of laminated recycled paper pulp].

[0148] <Example 7> White cardboard was obtained in the same manner as in Example 3, except that the washing water temperature of the crushing and washing machine was set to 40°C in the (crushing and washing process) of the [production of laminated recycled paper pulp].

[0149] <Example 8> White cardboard was obtained in the same manner as in Example 3, except that the washing water for the crushing and washing machine in the (crushing and washing process) of the [production of laminated recycled paper pulp] was changed to caustic soda water.

[0150] <Example 9> In the production of laminated recycled paper pulp, the laminated recycled paper used is laminated recycled paper B (printing layer / paper base material layer / LDPE layer, pulp composition of the paper base material layer N:L=0:100, disintegration-freeness 515 mL, rosin sizing agent content 0.23 parts by mass, alkyl ketene dimer sizing agent content 0 parts by mass, basis weight 200 g / m²). 2 White cardboard was obtained in the same manner as in Example 3, except that ) was used.

[0151] <Example 10> In the production of laminated recycled paper pulp, the laminated recycled paper is laminated recycled paper C (printing layer / paper base layer / LDPE layer, pulp composition of the paper base layer N:L=25:75, disintegration-freeness 515 mL, rosin sizing agent content 0.23 parts by mass, alkyl ketene dimer sizing agent content 0 parts by mass, basis weight 200 g / m²). 2 White cardboard was obtained in the same manner as in Example 3, except that ) was used.

[0152] <Example 11> In the production of laminated recycled paper pulp, the laminated recycled paper used is laminated recycled paper D (printing layer / paper base material layer / LDPE layer, pulp composition of the paper base material layer N:L=15:85, disintegration-freeness 630 mL, rosin sizing agent content 0.23 parts by mass, alkyl ketene dimer sizing agent content 0 parts by mass, basis weight 200 g / m²). 2 White cardboard was obtained in the same manner as in Example 3, except that ) was used.

[0153] <Example 12> In the production of laminated recycled paper pulp, the laminated recycled paper is laminated recycled paper E (printing layer / paper base layer / LDPE layer, pulp composition of the paper base layer N:L=15:85, disintegration-freeness 405 mL, rosin sizing agent content 0.23 parts by mass, alkyl ketene dimer sizing agent content 0 parts by mass, basis weight 200 g / m²). 2 White cardboard was obtained in the same manner as in Example 3, except that ) was used.

[0154] <Example 13> In the production of laminated recycled paper pulp, the laminated recycled paper is laminated recycled paper F (printing layer / LDPE layer / paper base material layer / LDPE layer, pulp composition of the paper base material layer N:L=15:85, disintegration-freeness 515 mL, rosin sizing agent content 0 parts by mass, alkyl ketene dimer sizing agent content 0.22 parts by mass, basis weight 200 g / m²). 2 White cardboard was obtained in the same manner as in Example 3, except that milk cartons were used.

[0155] <Example 14> In the preparation of the sub-layer paper material for the white cardboard in the [Preparation of White Cardboard] section, white cardboard was obtained in the same manner as in Example 3, except that 2 parts by mass of light calcium carbonate (product name: Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd.) were added to 100 parts by mass of pulp (solid content) in the pulp slurry.

[0156] <Comparative Example 1> In the preparation of the intermediate layer of white cardboard in the [Preparation of White Cardboard] section, white cardboard was obtained in the same manner as in Example 1, except that a pulp slurry was used which was a mixture of magazine waste pulp slurry and laminate waste pulp slurry in a mass ratio of 100:0.

[0157] <Comparative Example 2> In the preparation of the intermediate layer of the white cardboard in the [Preparation of White Cardboard] section, a pulp slurry was used which was a mixture of magazine waste pulp slurry and laminate waste pulp slurry in a mass ratio of 100:0. White cardboard was obtained in the same manner as in Example 1, except that 8 parts by mass of light calcium carbonate (product name: Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd.) were added to 100 parts by mass of pulp (solids) in the pulp slurry.

[0158] <Comparative Example 3> In the (manufacturing of white cardboard) section of [Production of white cardboard], the basis weight of the bottom layer (second layer) is 20 g / m². 2 The total basis weight of the five layers is 385 g / m². 2 White cardboard was obtained in the same manner as in Example 5, except as described above.

[0159] <Comparative Example 4> In the (manufacturing of white cardboard) section of [Production of white cardboard], the basis weight of the bottom layer (second layer) is 100 g / m². 2 The total basis weight of the five layers is 465 g / m². 2 White cardboard was obtained in the same manner as in Example 5, except for the aforementioned difference. In this papermaking process, the white cardboard was wound at a speed of 200 m / min.

[0160] <Measurement and Evaluation Methods> (Basic weight) The white cardboard obtained in the examples and comparative examples was conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998. The basis weight of the conditioned white cardboard was measured in accordance with JIS P 8124:2011. When measuring the basis weight per layer of white cardboard, the white cardboard was pre-treated by soaking it in 40°C water for 6 hours, then carefully peeling off each layer by hand, and drying it in a dryer set to 105°C for 30 minutes. After that, it was conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998, and then the basis weight per layer of the conditioned white cardboard was measured in accordance with JIS P 8124:2011.

[0161] (Fiber roughness) The laminated waste paper used in the examples and comparative examples was cut into 4 cm squares, immersed in deionized water to prepare a solution containing 2% by mass of laminated waste paper, and then immersed for 24 hours. After 24 hours of immersion, the pulp was processed in accordance with JIS P 8220-1:2012 using a standard disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) until no undisintegrated fibers remained, thereby disintegrating the pulp into fibers. The slurry (dispersion of pulp fibers) after disintegration was prepared to a solid content concentration of 0.1% by mass, and the pulp solid content concentration was calculated in accordance with JIS P 8225:2003. The slurry after disintegration was prepared to a solid content concentration of 0.004% by mass and yield 500 g of slurry, and the amount taken was recorded. The dry weight of the sample was calculated using the solid content concentration and the amount of slurry taken. Using the obtained slurry, the "fiber roughness" was measured in accordance with ISO 16065-2:2014 using a fiber length measuring instrument (model Valmet FS-5 UHD base unit, manufactured by Valmet). The fiber roughness is gravimetric coarseness, which is measured by applying the sum of fiber lengths obtained from image analysis and the dry weight of the sample input into the instrument.

[0162] (Percentage of small fibers) The laminated waste paper used in the examples and comparative examples was cut into 4 cm squares, immersed in deionized water to prepare a solution containing 2% by mass of laminated waste paper, and then immersed for 24 hours. After 24 hours of immersion, the pulp was processed in accordance with JIS P 8220-1:2012 using a standard disintegrator (manufactured by Kumagai Riki Kogyo Co., Ltd.) until no undisintegrated fibers remained, thereby disintegrating the pulp into fibers. The slurry (dispersion of pulp fibers) after disintegration was prepared to a solid content concentration of 0.1% by mass, and the pulp solid content concentration was calculated in accordance with JIS P 8225:2003. The slurry after disintegration was prepared to a solid content concentration of 0.004% by mass and yield 500 g of slurry, and the amount taken was recorded. The dry weight of the sample was calculated using the solid content concentration and the amount of slurry taken. Using the obtained slurry, the "percentage of small fibers (fibers with a length of 0.2 mm or more and less than 0.6 mm)" was measured in accordance with ISO 16065-2:2014 using a fiber length measuring instrument (model Valmet FS-5 UHD base unit, manufactured by Valmet). The ratio of small fibers is the ratio of the number of fibers (percentage of the number of fibers) weighted by length, for fibers with a fiber length of 0.2 mm or more and less than 0.6 mm (small fibers). The number of fibers (n) for each length range is measured using a fiber length measuring machine. i ) was measured, and the percentage of the number of fibers weighted by length (f i ') is calculated using the following formula (1).

[0163]

number

[0164] The symbols in equation (1) represent the following: f i ': Percentage of fiber count weighted by length (%) n i : The number of fibers in the i-th length range l i : Center of the i-th length range, displayed in millimeters Σn i l i n calculated for all length rangesi l i sum

[0165] The ratio of small fibers is calculated using formula (1) for the fiber length category of 0.2 mm or more and less than 0.6 mm. Specifically, the Valmet FS-5 UHD base unit used in the example measures the number of fibers in the following six categories (Fraction 1 to Fraction 6). Fraction 1: Fiber length less than 0.2 mm Fraction 2: Fiber length 0.2 mm or more and less than 0.6 mm Fraction 3: Fiber length 0.6 mm or more and less than 1.2 mm Fraction 4: Fiber length 1.2 mm or more and less than 2.0 mm Fraction 5: Fiber length 2.0 mm or more and less than 3.2 mm Fraction 6: Fiber length 3.2 mm or more and less than 7.6 mm The ratio of small fibers is the percentage of the number of fibers weighted by length (f) for Fraction 2. i Let ') be the calculated value.

[0166] (Rosin sizing agent content) After sampling 150 μg of the laminated recycled paper used in the examples and comparative examples, the rosin sizing agent content was measured by mass spectrometry using a gas chromatography / MS analyzer (GC-2025, manufactured by Shimadzu Corporation) under the following measurement conditions. The numerical values ​​represent the content (parts by mass) per 100 parts by mass of pulp constituting the paper substrate.

[0167] [Measurement conditions] • Sample: "Size Pine N-817" manufactured by Arakawa Chemical Industries, Ltd. • Column: Agilent Technologies, Inc. "HP-5MS" (length 30m, inner diameter 0.25μm, outer diameter 0.25mm) Carrier gas: Nitrogen • Measurement temperature conditions: Heating from 100°C to 325°C at a rate of 15°C / min, holding at 325°C for 5 minutes. • Pyrolysis apparatus: "PY-2010SL" manufactured by Frontier Lab Co., Ltd. ·Pyrolysis furnace temperature: 450℃

[0168] (Alkyl ketene dimer sizing agent content) After sampling 150 μg of the laminated recycled paper used in the examples and comparative examples, the alkyl ketene dimer sizing agent content was measured by mass spectrometry using a gas chromatography / MS analyzer (GCMS-QP2010, manufactured by Shimadzu Corporation) under the following measurement conditions. The numerical values ​​represent the content (parts by mass) per 100 parts by mass of pulp constituting the paper substrate.

[0169] [Measurement conditions] • Sample: "Size Pine K-287" manufactured by Arakawa Chemical Industries, Ltd. • Column: Agilent Technologies, Inc. "HP-5MS" (length 30m, inner diameter 0.25μm, outer diameter 0.25mm) Carrier gas: Helium • Measurement temperature conditions: Heating from 100°C to 325°C at a rate of 10°C / min, holding at 325°C for 15 minutes. • Pyrolysis apparatus: "PY-3030D" manufactured by Frontier Lab Co., Ltd. ·Pyrolysis furnace temperature: 500℃

[0170] (Hydrangea spots) The white cardboard obtained in the examples and comparative examples was cut to 297 mm in length and 210 mm in width, placed in a 160°C dryer for 15 minutes, and then the surface of the white cardboard was observed. 2 The number of spots (number / m) 2 The spots were counted and evaluated according to the following criteria. The area of ​​the spots was determined using the "Measurement Chart for Miscellaneous Matter" produced by the National Printing Bureau, and the number of spots was the average value when n10 sheets were evaluated. [Evaluation Criteria] A: The number of spots is 1.5 per square meter. 2 less than B: Number of spots: 1.5 per square meter 2 More than 4.0 pieces / m 2 less than C: Number of spots: 4.0 per square meter2 More than 6.5 pieces / m 2 less than D: Number of spots: 6.5 / m 2 That's all.

[0171] (Swelling) The frequency of blistering was counted when winding a total length of 2000m of white cardboard in the examples and comparative examples, and the blistering was evaluated according to the following criteria. The presence or absence of blistering was determined by visually checking the areas that reacted to the defect inspection machine (Omron SUPER-NASPλ) installed on the paper machine. [Evaluation Criteria] A: No swelling occurred. B: One or more blisters have appeared.

[0172] [Table 1]

[0173] In the experiment, the occurrence of hydrangea spots and swelling was suppressed.

Claims

1. A method for manufacturing white cardboard having at least a surface layer, a subsurface layer, a middle layer and a back layer, A process to obtain a slurry for the surface layer, a process to obtain a slurry for the subsurface layer, a process to obtain a slurry for the middle layer, and a process to obtain a slurry for the back layer. The process includes a step of multi-layer papermaking using the surface slurry, the subsurface slurry, the middle slurry, and the backing slurry, The step of obtaining the slurry for the lower layer and / or the slurry for the middle layer includes a step of disintegrating laminated waste paper containing a thermoplastic resin, wherein the amount of laminated waste paper included is 1% by mass or more relative to the total solid content mass in the slurry. The basis weight of the lower layer in the table above is 25-95 g / m². ２ And, A method for manufacturing white cardboard, wherein the addition rate of the filler in the lower layer of the surface is 3 parts by mass or less per 100 parts by mass of the pulp in the lower layer of the surface.

2. The method for manufacturing white cardboard according to claim 1, wherein the step of obtaining the slurry for the lower layer and / or the slurry for the middle layer includes a step of crushing and washing the laminated waste paper.

3. The method for manufacturing white cardboard according to claim 2, wherein the crushing and washing step is performed with a washing solution with a pH of 7 or higher.

4. A method for manufacturing white cardboard according to any one of claims 1 to 3, wherein in the step of obtaining the slurry for the lower layer and / or the slurry for the middle layer, the laminated waste paper is disintegrated at 20 to 60°C.

5. A method for producing white cardboard according to any one of claims 1 to 3, wherein the fiber roughness of the pulp fibers contained in the laminated waste paper is 0.140 mg / m or less.

6. A method for producing white cardboard according to any one of claims 1 to 3, wherein the proportion of pulp fibers with a fiber length of 0.2 mm or more and less than 0.6 mm among the pulp fibers contained in the laminated waste paper is 8% or more.

7. The method for producing white cardboard according to any one of claims 1 to 3, wherein the laminated recycled paper contains a rosin-based sizing agent.

8. A method for producing white cardboard according to any one of claims 1 to 3, wherein the step of obtaining the slurry for the lower layer and / or the slurry for the middle layer further includes a step of beating the pulp fibers contained in the slurry.

9. A method for producing white cardboard according to any one of claims 1 to 3, further comprising a drying step after the multilayer papermaking step.

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