Coated white paperboard, packaging material, and method for producing coated white paperboard

Abstract

The present invention addresses the problem of providing coated white paperboard with which occurrence of curling is suppressed and tearing of the paperboard during production is suppressed. The present invention relates to coated white paperboard having a coating layer on at least one side of the white paperboard, the white paperboard containing pulp fibers and polyethylene and having a layered structure of two or more layers, wherein the polyethylene content ratio is 50-2500 ppm with respect to the total mass of the coated white paperboard; and the basis weight is 150 g / m2 or more.

Description

Coated white cardboard, packaging material, and method for manufacturing coated white cardboard

[0001] The present invention relates to coated white cardboard, packaging materials, and a method for manufacturing coated white cardboard.

[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] In some cases, a pigment coating layer is provided on the surface of white cardboard to enhance printing effect and design. For example, Patent Document 2 discloses coated white cardboard having a base paper for coated white cardboard having at least three layers: a surface layer, a subsurface layer, and a back layer, and a pigment coating layer applied to the surface layer, characterized in that the surface layer does not contain a sizing agent, but the subsurface layer does.

[0004] Japanese Patent Publication No. Hei 6-41896 Japanese Patent Publication No. 2023-158394

[0005] However, conventional coated white cardboard is susceptible to humidity and can unintentionally curl during the manufacturing process. When curling occurs in coated white cardboard, it can lead to problems in post-processing and box-making processes.

[0006] Furthermore, in the stages prior to feeding coated white cardboard into the box-making process, etc., processes are carried out to apply tension to the coated white cardboard or to wind it up. However, if the strength of the coated white cardboard is insufficient at these stages, paper breakage can occur, which has been a problem. Paper breakage during the manufacturing process of coated white cardboard reduces the manufacturing efficiency of the coated white cardboard, which is problematic.

[0007] Therefore, in order to solve the problems of the prior art, the present inventors proceeded with research with the aim of providing coated white cardboard in which curling is suppressed and paper tearing during manufacturing is suppressed.

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

[0009] [1] Coated white cardboard having a coating layer on at least one side of the white cardboard, wherein the white cardboard contains pulp fibers and polyethylene, has a layer structure of two or more layers, the polyethylene content is 50 to 2500 ppm relative to the total mass of the coated white cardboard, and the basis weight is 150 g / m² ２ The above applies to coated white cardboard. [2] The coated white cardboard according to [1], wherein the kink index of the pulp fibers is 3100 (1 / m) or less. [3] The coated white cardboard according to [1] or [2], wherein the synergistic mean of the longitudinal tensile modulus and transverse tensile modulus is 2.0 GPa or more. [4] The coated white cardboard according to any one of [1] to [3], wherein the polyethylene is polyethylene derived from laminated recycled paper. [5] The coated white cardboard according to any one of [1] to [4], wherein the coating layer contains a pigment and a binder. [6] The coated white cardboard according to any one of [1] to [5], wherein the white cardboard has a layer structure in which the surface layer, surface sub-layer, middle layer and back layer are laminated in this order, and the coating layer is provided on both sides of the white cardboard, with the surface coating layer, surface layer, surface sub-layer, middle layer, back layer and back coating layer being laminated in this order. [7] The coated white cardboard according to [6], wherein the difference between the total amount (mass%) of ash contained in the surface coating layer, surface layer and subsurface layer and the total amount (mass%) of ash contained in the back layer and back coating layer is 20% by mass or less. [8] A packaging material obtained by processing the coated white cardboard according to any one of [1] to [7]. [9] A method for producing coated white cardboard, comprising the steps of: making white cardboard containing pulp fibers and polyethylene and having a layer structure of two or more layers; and forming a coating layer on at least one of the white cardboard, wherein the polyethylene content is 50 to 2500 ppm with respect to the total mass of the coated white cardboard, and the basis weight is 150 g / m². ２The above is a method for manufacturing coated white cardboard.

[10] The method for manufacturing coated white cardboard according to [9], comprising the steps of: dissociating pulp fibers from laminated waste paper having a paper substrate containing pulp fibers and a resin layer containing polyethylene to obtain a pulp slurry containing pulp fibers and polyethylene; and manufacturing paper from the pulp slurry.

[11] The method for manufacturing coated white cardboard according to [9] or

[10] , wherein the dissociation temperature of the pulp fibers in the step of obtaining the pulp slurry is 20 to 60°C.

[12] The method for manufacturing coated white cardboard according to any one of [9] to

[11] , wherein the step of obtaining the pulp slurry includes a step of crushing and washing laminated waste paper.

[13] A method for manufacturing coated white cardboard according to any one of [9] to

[12] , wherein the step of papermaking white cardboard comprises the steps of obtaining a surface slurry containing pulp fibers, obtaining a subsurface slurry containing pulp fibers, obtaining a middle layer slurry containing pulp fibers and polyethylene, and obtaining a back layer slurry containing pulp fibers, and white cardboard is papermade by multilayer assembly.

[14] A method for manufacturing coated white cardboard according to

[13] , wherein the step of obtaining the middle layer slurry further comprises the step of beating the middle layer slurry.

[15] A method for manufacturing coated white cardboard according to any one of [9] to

[14] , further comprising a drying step after the step of papermaking white cardboard.

[16] A method for manufacturing coated white cardboard according to any one of [9] to

[15] , wherein the step of forming the coating layer is the step of coating a coating liquid containing a pigment and a binder.

[0010] 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 expressed using "~" mean a range that includes the numbers written before and after "~" as the lower and upper limits.

[0011] (Coated White Cardboard) This embodiment is a coated white cardboard having a coating layer on at least one side of the white cardboard, wherein the white cardboard contains pulp fibers and polyethylene, has a layer structure of two or more layers, the polyethylene content is 50 to 2500 ppm relative to the total mass of the coated white cardboard, and the basis weight is 150 g / m² ２Regarding the coated white paperboard as described above. In this embodiment, by setting the polyethylene content within the above range and the basis weight of the coated white paperboard within the above range, it is possible to obtain a coated white paperboard in which curl generation is suppressed even under high humidity conditions. Further, in this embodiment, by setting the polyethylene content to be not more than the above upper limit value, it is possible to suppress the occurrence of paper breakage in the manufacturing process of the coated white paperboard. Thereby, the manufacturing efficiency of the coated white paperboard and the subsequent manufacturing efficiency of the packaging material can be enhanced.

[0012] The polyethylene content contained in the coated white paperboard of this embodiment may be 50 ppm or more, preferably 100 ppm or more, more preferably 150 ppm or more, and even more preferably 200 ppm or more. Also, the polyethylene content may be not more than 2500 ppm, preferably not more than 2250 ppm, more preferably not more than 2000 ppm, still more preferably not more than 1800 ppm, even more preferably not more than 1500 ppm, and particularly preferably not more than 1000 ppm. By setting the polyethylene content contained in the coated white paperboard to be not less than the above lower limit value, it becomes easy to suppress the occurrence of curl in the coated white paperboard even under high humidity conditions. Also, by setting the polyethylene content contained in the coated white paperboard to be not more than the above upper limit value, it is possible to suppress the occurrence of paper breakage in the manufacturing process of the coated white paperboard.

[0013] The overall basis weight of the coated white paperboard of this embodiment should be 150 g / m ２ or more, preferably 175 g / m ２ or more, more preferably 200 g / m ２ or more, still more preferably 250 g / m ２ or more, even more preferably 300 g / m ２ or more, particularly preferably 350 g / m ２ or more. Also, the basis weight of the coated white paperboard is preferably, for example, not more than 600 g / m ２ and preferably not more than 500 g / m ２The following is more preferable: By setting the basis weight of the coated white cardboard to be above the lower limit, curling can be effectively suppressed even in high-humidity environments. Furthermore, by setting the basis weight of the coated white cardboard to be below the upper limit, production efficiency in the box-making process can be increased and manufacturing costs can be reduced. The total basis weight of the coated white cardboard is measured in accordance with JIS P 8124:2011 after the coated white cardboard has been conditioned for 24 hours in a humidity-controlled environment as specified in JIS P 8111:1998.

[0014] The overall thickness of the coated 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 coated 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 coated white cardboard to be above the lower limit above, curling can be effectively suppressed even in high humidity environments. Furthermore, by setting the thickness of the coated white cardboard to be below the upper limit above, production efficiency in the box-making process can be increased and manufacturing costs can be reduced. The overall thickness of the coated white cardboard is measured in accordance with JIS P 8118:2014 after the coated white cardboard has been conditioned for 24 hours in a humidity-controlled environment as specified in JIS P 8111:1998.

[0015] The overall density of the coated 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 amount 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 coated white cardboard is 1.00 g / cm³. ３ The following is preferable: By setting the density of the coated white cardboard to be above the lower limit, curling can be effectively suppressed even in high-humidity environments. Furthermore, by setting the density of the coated white cardboard to be below the upper limit, production efficiency in the box-making process can be increased and manufacturing costs can be reduced.

[0016] The ash content of coated white cardboard is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, based on the total mass of the coated white cardboard. Furthermore, the ash content of coated white cardboard is preferably 35% by mass or less, more preferably 30% by mass or less, and even more preferably 27% by mass or less. The ash content of coated white cardboard is measured in accordance with JIS P 8251:2003.

[0017] The synergistic mean of the longitudinal and transverse tensile moduli of coated white cardboard is preferably 2.0 GPa or higher, more preferably 2.5 GPa or higher, and even more preferably 3.0 GPa or higher. Furthermore, the synergistic mean of the longitudinal and transverse tensile moduli of coated white cardboard is preferably 4.5 GPa or lower. By keeping the synergistic mean of the longitudinal and transverse tensile moduli of coated white cardboard within the above range, curling can be effectively suppressed even in high-humidity environments. The synergistic mean of the longitudinal and transverse tensile moduli of coated white cardboard is measured in accordance with JIS P 8113:2006 after the coated white cardboard has been conditioned for 24 hours in a humidity-controlled environment as specified in JIS P 8111:1998. The test was performed using a transverse tensile testing machine (Lorentzen & Wattre, CODE SE-064), and the geometric mean was calculated from the obtained longitudinal and transverse tensile moduli.

[0018] <White cardboard> <<Polyethylene>> The white cardboard constituting coated white cardboard contains pulp fibers and polyethylene. As polyethylene, it is preferable to use high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), or linear low-density polyethylene (LLDPE), and it is more preferable to use low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE).

[0019] The density of polyethylene is 0.900 g / cm³. ３ Preferably, it should be 0.910 g / cm³ or more. ３It is more preferable that the above values ​​are met. Furthermore, the density of polyethylene is 0.970 g / cm³. ３ Preferably, 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:

[0020] The melting point of polyethylene 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 polyethylene is preferably 170°C or lower, more preferably 150°C or lower, and even more preferably 130°C or lower.

[0021] The polyethylene content is preferably 50 ppm or more, more preferably 100 ppm or more, even more preferably 150 ppm or more, even more preferably 180 ppm or more, and particularly preferably 200 ppm or more, based on the total mass of the coated white cardboard. Furthermore, the polyethylene content is preferably 2500 ppm or less, more preferably 2200 ppm or less, even more preferably 2000 ppm or less, even more preferably 1800 ppm or less, even more preferably 1500 ppm or less, and particularly preferably 1000 ppm or less. By setting the polyethylene content to above the lower limit, the polyethylene can bind to a portion of the pulp fibers, thereby suppressing the expansion and contraction of the pulp fibers, and as a result, the occurrence of curling in the coated white cardboard can be suppressed. On the other hand, by setting the polyethylene content to below the upper limit, the occurrence of paper breaks during manufacturing can be suppressed, and operability can be improved.

[0022] The polyethylene content in coated white cardboard can be controlled by adjusting the polyethylene content in the pulp slurry used to manufacture the white cardboard. For example, a predetermined amount of polyethylene may be added to the pulp slurry. Alternatively, as described later, the polyethylene content may be controlled by appropriately controlling the type and amount of polyethylene-containing waste paper (e.g., waste paper containing polyethylene laminate or polyethylene sheets) used as recycled raw material. In this embodiment, the polyethylene contained in the white cardboard is preferably derived from polyethylene-containing waste paper, and particularly preferably derived from laminate waste paper.

[0023] Furthermore, the polyethylene content in coated white cardboard can also be controlled by adjusting the waste paper concentration during the disintegration of polyethylene-containing waste paper. For example, in the disintegration process, the higher the concentration of polyethylene-containing waste paper, the larger the foreign matter (e.g., polyethylene resin fragments) 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 polyethylene-containing waste paper, the smaller the foreign matter fragments become, making them more difficult to remove in the dust removal process. For this reason, it is preferable to determine the disintegration conditions so that the waste paper concentration during the disintegration of polyethylene-containing waste paper is within an appropriate range.

[0024] In this embodiment, the white cardboard has a layered structure of two or more layers. In this case, polyethylene may be contained in all of the two or more paper layers, or polyethylene may be contained in only one of the layers. In particular, it is preferable that the white cardboard has three or more paper layers, in which case polyethylene may be contained in all of the layers, but it is especially preferable that polyethylene is contained in the middle layer. In the paper layers containing polyethylene, it is preferable that the polyethylene is uniformly dispersed throughout the entire paper layer. For example, when the paper layer is divided into three equal parts in the thickness direction, it is preferable that the polyethylene content ratio in the three regions is within ±10%.

[0025] To measure the polyethylene content in coated white cardboard, the following steps are performed: an extraction step to separate the polyethylene contained in the coated white cardboard, and an analysis step to quantify the polyethylene by pyrolysis GC / MS. In the extraction step, coated white cardboard is collected, and polyethylene is extracted from the coated white cardboard 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 60°C), and the sample is dried at 105°C for 12 hours to obtain polyethylene from the coated white cardboard. Subsequently, in the analysis step, an appropriate amount of xylene is added to the extracted polyethylene to a concentration of 2-5 mg / ml, and the mixture is dissolved at 130°C for 2 hours while stirring. Next, the xylene solution of the dissolved extract, whose tare weight has already been measured, is added to the eco-cup so that the extract volume is between 50 and 200 μg. The solvent is evaporated at 105°C for 2 hours, and the amount of extract to be analyzed is measured. The polyethylene content of the extract is quantified by pyrolysis GC / MS (Shimadzu Corporation GC / MS-QP2010). The measurement conditions for pyrolysis GC / MS are as follows. • Polyethylene standard: (Tosoh 07C03C, LDPE) • Column: HP-5MS (Length: 30m, Inner diameter: 0.250mm, Thickness: 0.25μm) • Analytical conditions: Pyrolysis temperature = 600°C, Inlet temperature = 320°C, Split ratio = 1:60, Interface temperature = 280°C, GC column temperature conditions = 40°C (hold for 3 minutes) → 10°C / min → 325°C (hold for 15 minutes), Ion source temperature = 200°C • Detection: m / z 29-600 (Quantitative analysis is calculated using the mass chromatogram area value of 1,19-eicosadiene (C2O) at m / z 82)

[0026] <<Pulp Fibers>> The white cardboard that makes up coated 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.

[0027] 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), chemiground wood pulp (CGP), and mechanical pulp (GP), as well as 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.

[0028] 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.

[0029] 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.

[0030] The kink index of the pulp fibers contained in the white cardboard is preferably 3100 (1 / m) or less, more preferably 3050 (1 / m) or less, even more preferably 3000 (1 / m) or less, even more preferably 2900 (1 / m) or less, even more preferably 2800 (1 / m) or less, and particularly preferably 2700 (1 / m) or less. Furthermore, the kink index of the pulp fibers is preferably 2000 (1 / m) or more, more preferably 2100 (1 / m) or more, and even more preferably 2200 (1 / m) or more. By keeping the kink index of the pulp fibers below the above upper limit, the decrease in strength of the coated white cardboard can be suppressed, and the occurrence of curl in the coated white cardboard can be effectively suppressed. Furthermore, by setting the kink index of the pulp fibers to above the lower limit mentioned above, the degree of bending of the pulp fibers can be increased, thereby suppressing the shedding of pulp fibers from coated white cardboard.

[0031] In this specification, the kink index of pulp fibers described above is the kink index of pulp fibers contained in a pulp slurry obtained by dissociating coated white cardboard in accordance with JIS P 8220-1:2012. Specifically, dissociation is performed in accordance with JIS P 8220-1:2012 to obtain a pulp slurry with a coated white cardboard content of 2% by mass. The pulp slurry is then adjusted 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. Next, the slurry after dissociation is adjusted to a solid content concentration of 0.004% by mass and a slurry volume of 500 g, and the amount taken is recorded. The dry weight of the sample is then calculated using the solid content concentration and the amount of slurry taken. The kink index is measured using the obtained slurry with a fiber length measuring instrument (model Valmet FS-5 UHD base unit, manufactured by Valmet). In this specification, the kink index (1 / m) is Kibblewhite's kink index calculated by the following formula (1). The kink index is calculated by weighting the number of kinks for each certain angle range according to the increase in angle, and dividing by the sum of the fiber lengths.

[0032]

[0033] The symbols in equation (1) represent the following: n １ = Number of kinks (bends) between 21° and 45° in the measurement sample n ２ = Number of kinks (bends) between 46° and 90° in the measurement sample n ３ = Number of kinks (bends) between 91° and 180° in the measurement sample L ｃ = Sum of fiber lengths of the measurement sample (m)

[0034] The kink index described above can be adjusted by appropriately controlling the processing temperature when disintegrating the pulp and the crushing and washing conditions of the polyethylene-containing waste paper before disintegrating the pulp. Specifically, by adopting the disintegration temperature and crushing and washing conditions described later, the kink index of the pulp fibers can be controlled to a desired range.

[0035] <<Optional Components>> White cardboard may contain optional components. 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. Optional components may be used individually or in combination of two or more. However, the content of 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 even more preferably 4% by mass or less, based on the total mass of the white cardboard.

[0036] <<Recycled Materials>> The white cardboard of this embodiment is preferably manufactured using recycled materials. That is, the coated white cardboard of this embodiment is coated white cardboard containing recycled materials (recycled coated white cardboard). Here, the recycled materials constituting the white cardboard are preferably waste paper, and the waste paper preferably includes polyethylene-containing waste paper (for example, waste paper containing polyethylene laminate or polyethylene sheets), more preferably includes laminate waste paper having a paper base material containing pulp fibers and a polyethylene layer (polyethylene laminate waste paper), and particularly preferably includes laminate waste paper derived from liquid containers (polyethylene laminate waste paper). Examples of liquid containers include beverage containers, liquid seasoning containers, detergent containers, etc. Specifically, examples include paper cups, paper glasses, milk cartons, aseptic containers, etc.

[0037] The proportion of polyethylene-containing recycled paper to the total mass of recycled raw materials used in the manufacture of white cardboard is preferably 0.5% by mass or more, more preferably 0.7% 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 polyethylene-containing recycled paper is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less, even more preferably 20% by mass or less, and particularly preferably 15% by mass or less.

[0038] The recycled materials that make up white cardboard preferably include polyethylene-containing waste paper. In this case, the white cardboard will contain pulp fibers derived from polyethylene-containing waste paper and polyethylene derived from polyethylene-containing waste paper. In particular, it is preferable that the white cardboard contains pulp fibers derived from laminated waste paper and polyethylene derived from laminated waste paper.

[0039] The polyethylene content in polyethylene-containing recycled 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 polyethylene content in polyethylene-containing recycled paper is preferably 40% by mass or less, more preferably 30% by mass or less, and particularly preferably 20% by mass or less.

[0040] Polyethylene-containing recycled paper preferably contains, in addition to polyethylene, softwood pulp fibers and hardwood pulp fibers. When N (parts by mass) is the content of softwood pulp fibers relative to the total pulp mass contained in polyethylene-containing 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.

[0041] As recycled materials for white cardboard, polyethylene-free recycled paper and polyethylene-containing recycled paper may be used in combination. In this case, the proportion of polyethylene-free recycled paper to the total mass of recycled materials used in the manufacture of white cardboard is preferably 99% by mass or less, and more preferably 95% by mass or less. Furthermore, the proportion of polyethylene-free recycled paper is preferably 70% by mass or more, more preferably 75% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more.

[0042] Polyethylene-free recycled paper includes deinked pulp fibers and corrugated cardboard pulp fibers made from recycled paper. Deinked pulp includes recycled paper pulp (Kent paper pulp) collected from the trimmings of coated paper for printing generated at bookbinding and printing plants, as well as magazine pulp, flyer pulp, newspaper pulp, office pulp, information paper pulp, cardboard pulp, and paper container pulp, all of which have been deinked.

[0043] Furthermore, the content of pulp fibers derived from recycled materials relative to the total mass of pulp fibers contained in the white cardboard 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 white cardboard may be 100% by mass.

[0044] <<Layer Structure of White Cardboard>> White cardboard is preferably made of multiple layers, and preferably has a layer 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 innermost 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. In this specification, for layers other than the front and back layers, layers with a whiteness of 50% or more are defined as the front sub-layer or back sub-layer, and layers with a whiteness of less than 50% are defined as the middle layer. When measuring the whiteness of the lower surface layer, middle layer, and lower back layer, coated 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. Prior to measurement, the cardboard is conditioned 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.

[0045] When white cardboard has a layered structure in which the surface layer, surface sub-layer, middle layer, and back layer are laminated in this order, and the coating layer is provided on both sides of the white cardboard, the coated white cardboard of this embodiment preferably has a structure in which the surface coating layer, surface layer, surface sub-layer, middle layer, back layer, and back coating layer are laminated in this order. Furthermore, the difference between the total amount of ash (mass%) contained in the surface coating layer, surface layer, and surface sub-layer and the total amount of ash (mass%) contained in the back layer and back coating layer is preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 16% by mass or less. Note that the difference in the total amount of ash (mass%) may be 0% by mass. By keeping the difference in the total amount of ash (mass%) below the above upper limit, the difference in ash content between the surface and back can be reduced, thereby more effectively suppressing curling in the coated white cardboard. Here, the total amount of ash contained in the surface coating layer, surface layer, and subsurface layer (mass%) is the total amount of ash contained in the surface coating layer, surface layer, and subsurface layer relative to the total mass of the surface coating layer, surface layer, and subsurface layer, and the total amount of ash contained in the back coating layer and back layer (mass%) is the total amount of ash contained in the back coating layer and back layer relative to the total mass of the back coating layer and back layer.

[0046] Furthermore, if the white cardboard has a back sublayer, the coated white cardboard of the embodiment is preferably configured such that the front coating layer, front layer, front sublayer, middle layer, back sublayer, back layer, and back coating layer are laminated in this order. In this case, the difference between the total amount of ash (mass%) contained in the front coating layer, front layer, and front sublayer and the total amount of ash (mass%) contained in the back sublayer, back layer, and back coating layer is preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 16% by mass or less. However, the difference between the total amount of ash (mass%) may be 0% by mass.

[0047] In this specification, the surface coating layer, surface layer, and surface sub-layer may be referred to as the surface layer group, and the back layer and back coating layer may be referred to as the back layer group. If the white cardboard further has a back sub-layer, the back sub-layer, back layer, and back coating layer may be referred to as the back layer group.

[0048] (Surface layer) 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. Bleached pulp is preferred among these due to its high whiteness.

[0049] 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 cut 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.

[0050] Examples of so-called 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), chemical pulps such as hardwood sulfite pulp and softwood sulfite pulp, as well as pulps such as thermomechanical pulp (TMP), chemothermetic pulp (CTMP), chemigland pulp (CGP), refiner gland pulp (RGP), gland pulp (GP), pressure stone gland pulp (PGW), and stone gland pulp (SGP).

[0051] 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, the surface whiteness of the white cardboard will be high, which will improve its appearance. When measuring the whiteness of the surface layer, coated white cardboard is immersed in 40°C hot water for 6 hours, the surface layer with the coating is peeled off, and the cardboard is dried in a dryer set to 105°C for 30 minutes, after which the coating layer is removed and the measurement is performed. Before measurement, the cardboard is humidified for 24 hours in a humidity-controlled environment as specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0052] The surface layer's basis weight is 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 / m². 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, it is 50 g / m ２ It is even more preferable that the following conditions apply: 35 g / m ２ The following is 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, coated white cardboard is immersed in 40°C hot water for 6 hours, the surface layer with the coating is peeled off, and the paper is dried in a dryer set to 105°C for 30 minutes before the coating layer is removed and the measurement is performed using this dried paper. 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.

[0053] 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.

[0054] 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 amount 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.

[0055] The surface layer may contain the above-mentioned optional components, and among them, it is preferable to 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 in the papermaking of the surface layer, calcium carbonate derived from recycled paper may be mixed into the surface layer. In addition, calcium carbonate may be added to the surface layer separately. The calcium carbonate content of the surface layer is preferably 1 to 35% by mass, more preferably 2 to 25% by mass, and even more preferably 3 to 15% by mass, based on the total mass of the surface layer. If the calcium carbonate content of the surface layer is above the lower limit above, the design quality of the coated white cardboard can be more effectively enhanced. If the calcium carbonate content of the surface layer is below the upper limit above, it is easier to ensure the strength of the white cardboard.

[0056] 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. When measuring the surface ash content, coated white cardboard is immersed in 40°C hot water for 6 hours, the surface layer with the coating is peeled off, it is dried in a dryer set to 105°C for 30 minutes, and then the coating layer is removed, and the measurement is performed in accordance with JIS P 8251:2003.

[0057] (Sub-layer) The sub-layer is composed of pulp that has a lower whiteness than the pulp of the surface layer but a higher whiteness than the pulp of the middle layer. In this specification, the sub-layer is a layer with a whiteness of 50% or more. As the pulp that constitutes the sub-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.

[0058] 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, coated white cardboard is immersed in 40°C hot water for 6 hours, the subsurface 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 whiteness is measured in accordance with JIS P 8148:2018.

[0059] The basis weight of the lower layer of the table is 10 g / m². ２Preferably, it is 15 g / m ２ It is more preferable that the amount be greater than or equal to 20 g / m². 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. Furthermore, the basis weight of the lower layer is 95 g / m². ２ Preferably, it is 75 g / m ２ More preferably, the following is true: 60 g / m ２ It is even more preferable that the following conditions apply: 45 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, the dark appearance of the middle layer can be sufficiently concealed. Also, if the basis weight of the sub-layer is equal to or less than the upper limit, swelling during papermaking can be easily suppressed. Furthermore, it is preferable that the basis weight of the sub-layer is 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, coated 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.

[0060] The thickness of the subsurface 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. 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, the color (darkness) of the middle layer can be sufficiently concealed. Furthermore, if the thickness of the subsurface layer is below the upper limit, swelling during papermaking can be easily suppressed.

[0061] 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 amount be greater than or equal to 0.80 g / cm³. ３ It is even more preferable that the above is true. Furthermore, the density of the subsurface layer is 1.00 g / cm³. ３The following is preferable: If the density of the subsurface layer is within the above range, the color (darkness) of the middle layer can be sufficiently concealed.

[0062] 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. 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 calcium carbonate content of the subsurface 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 subsurface layer. If the calcium carbonate content of the subsurface layer is above the lower limit, the design quality of the coated white cardboard can be more effectively enhanced. If the calcium carbonate content of the subsurface layer is below the upper limit, it is easier to ensure the strength of the white cardboard.

[0063] 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 coated white cardboard in 40°C hot water for 6 hours and peeling off the subsurface layer.

[0064] (Middle Layer) The middle layer is generally made of the lowest grade pulp among the layers that make up the 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 waste, imitation brown paper, corrugated cardboard, backing paper, land deeds, and balls. In this embodiment, it is preferable that the middle layer contains polyethylene.

[0065] 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.

[0066] 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 coated 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 environment specified in JIS P 8111:1998, and the whiteness is measured in accordance with JIS P 8148:2018.

[0067] The basis weight of the middle layer is 50 g / m². ２ Preferably, it is 75 g / 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 270 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 400 g / m 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 intermediate layer is within the above range, it becomes easier to control the polyethylene content in the coated white cardboard to the desired range, and as a result, curling of the coated white cardboard can be effectively suppressed. If multiple intermediate layers are provided, the basis weight of the intermediate layer is the total basis weight of the intermediate layers. When measuring the basis weight of the intermediate layer, the coated white cardboard is immersed in 40°C hot water for 6 hours, the intermediate 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 measurement is performed in accordance with JIS P 8124:2011.

[0068] 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 above the lower limit, curling of the coated white cardboard can be effectively suppressed. Furthermore, if the thickness of the middle layer is below the upper limit, production efficiency in the box-making process can be increased.

[0069] 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 amount 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, production efficiency in the box-making process can be increased.

[0070] The middle layer is preferably a layer containing polyethylene. The polyethylene contained in the middle layer is preferably polyethylene derived from polyethylene-containing recycled paper (for example, recycled paper containing polyethylene laminate or polyethylene sheets), and is particularly preferably polyethylene derived from recycled polyethylene laminate.

[0071] The polyethylene content 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 polyethylene content in the middle layer is preferably 5000 ppm or less, more preferably 3000 ppm or less, and even more preferably 2000 ppm or less. By setting the polyethylene content in the middle layer to be above the lower limit, it becomes easier to suppress curling of the coated white cardboard even under high humidity conditions. Furthermore, by setting the polyethylene content in the middle layer to be below the upper limit, it is possible to suppress paper tearing during the manufacturing process of coated white cardboard.

[0072] The middle layer may contain the optional components mentioned above, similar to the surface layer. Furthermore, the middle layer may contain thermoplastic resins other than polyethylene.

[0073] 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 coated white cardboard in 40°C hot water for 6 hours and peeling off the middle layer.

[0074] (Back Layer) The back layer is the layer located on the furthest back side (opposite the top layer). For this reason, 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.

[0075] 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, the appearance of the back surface of the coated white cardboard will not be significantly impaired. When measuring the whiteness of the backing layer, the coated white cardboard is immersed in 40°C hot water for 6 hours, the backing layer with the coating is peeled off, and the cardboard is dried in a dryer set to 105°C for 30 minutes before the coating layer is removed and the measurement is performed using the resulting sample. Before measurement, the sample 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.

[0076] The basis weight of the backing layer is 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 / m². 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 backing layer is 90 g / m². ２ The following is preferable: 70 g / m ２ More preferably, the following is true: 55 g / m ２ It is even more preferable that the following conditions apply: 40 g / m ２ The following conditions are particularly preferable: If the basis weight of the backing layer is within the above range, colored foreign matter in the middle layer can be sufficiently concealed. Furthermore, if the basis weight of the backing layer is within the above range, sufficient paper layer strength can be obtained, and as a result, curling of the coated white cardboard can be effectively suppressed. When measuring the basis weight of the backing layer, the coated white cardboard is immersed in 40°C hot water for 6 hours, the backing layer with the coating is peeled off, and the paper is dried in a dryer set to 105°C for 30 minutes before the coating layer is removed and the measurement is performed using this paper. 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.

[0077] The thickness of the backing 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. Furthermore, the thickness of the backing 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 backing layer is within the above range, the colored foreign matter in the middle layer can be sufficiently concealed.

[0078] The density of the backing layer is 0.60 g / cm³. ３ Preferably, it should be 0.70 g / cm³ or more. ３ It is more preferable that the amount 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 backing layer is 1.00 g / cm³. ３ The following is preferable: If the density of the backing layer is within the above range, colored foreign matter in the middle layer can be sufficiently concealed, and curling of the coated white cardboard can be effectively suppressed.

[0079] The underlayer may contain the same optional components as the surface layer.

[0080] 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, curling of the coated white cardboard can be effectively suppressed. When measuring the ash content of the backing layer, the coated white cardboard is immersed in 40°C hot water for 6 hours, the backing layer with the coating layer is peeled off, it is dried in a dryer set to 105°C for 30 minutes, and then the coating layer is removed, and the measurement is performed in accordance with JIS P 8251:2003.

[0081] (Under-back layer) In addition to the surface layer, under-surface layer, middle layer, and back layer, the white cardboard may further have an under-back layer. The under-back layer is located between the back layer and the middle layer. The under-back 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 under-back layer is a layer with a whiteness of 50% or more.

[0082] 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.

[0083] 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 the back layer 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, the design quality of the coated white cardboard can be more effectively enhanced. If the calcium carbonate content of the back layer is below the upper limit, it is easier to ensure the strength of the white cardboard.

[0084] 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, curling of the coated white cardboard can be effectively suppressed. Furthermore, 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 coated white cardboard in 40°C hot water for 6 hours and peeling off the underlayer.

[0085] <Coating Layer> The coated white cardboard of this embodiment has 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, but it is preferable that it 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. In this embodiment, by providing a coating layer on the surface of the white cardboard, the printability of the front layer can be improved and the whiteness can be increased.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] The basis weight per side 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 is true. Furthermore, the basis weight per side of the coating layer should be 50 g / m². ２ Preferably, it is 40 g / m ２ It is more preferable that the following is the case: 30 g / m ２It is even more preferable that the following conditions apply: 20 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 weights of the front coating layer and the back coating layer may be different. For example, in order to suppress curling of the coated white cardboard, it is also preferable to make the basis weight of the front coating layer smaller than that of the back coating layer.

[0091] The ash content of the surface coating layer is preferably 75% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the total mass of the surface coating layer. Furthermore, the ash content of the surface coating layer is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 92% by mass or less, based on the total mass of the surface coating layer. If the ash content of the surface coating layer is above the lower limit, the design quality of the coated white cardboard can be more effectively enhanced. Furthermore, if the ash content of the surface coating layer is below the upper limit, it becomes easier to suppress curling of the coated white cardboard. When measuring the ash content of the surface coating layer, coated white cardboard is immersed in 40°C hot water for 6 hours, the surface layer with the coating is peeled off, and it is dried in a dryer set to 105°C for 30 minutes. Then, it is immersed in a 1 mol / L copper ethylenediamine solution (manufactured by Kanto Chemical Co., Ltd.) for 3 hours to dissolve the surface layer, and the remaining surface coating layer is used for measurement in accordance with JIS P 8251:2003.

[0092] The ash content of the back coating layer is preferably 75% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, relative to the total mass of the back coating layer. Furthermore, the ash content of the back coating layer is preferably 99% by mass or less, more preferably 95% by mass or less, and even more preferably 92% by mass or less, relative to the total mass of the back coating layer. If the ash content of the back coating layer is above the lower limit, curling of the coated white cardboard can be suppressed, and the design quality of the coated white cardboard can be more effectively enhanced. Furthermore, if the ash content of the back coating layer is below the upper limit, manufacturing costs can be reduced. When measuring the ash content of the back coating layer, coated white cardboard is immersed in 40°C hot water for 6 hours, the back coating layer is peeled off, and the cardboard is dried in a dryer set to 105°C for 30 minutes. Then, it is immersed in a 1 mol / L copper ethylenediamine solution (manufactured by Kanto Chemical Co., Ltd.) for 3 hours to dissolve the back coating layer, and the remaining back coating layer is used for measurement in accordance with JIS P 8251:2003.

[0093] (Packaging Material) The coated white cardboard of this embodiment can be used for general purposes such as printing cardboard and paper packaging cardboard, as well as for drawing sheets that utilize pressure deformation, food packaging, etc. In particular, the coated white cardboard of this embodiment is preferably a packaging material made by processing coated white cardboard, and more preferably a packaging material made by box-making coated white cardboard.

[0094] (Method for manufacturing coated white cardboard) This embodiment may also relate to a method for manufacturing coated white cardboard, comprising the steps of: papermaking white cardboard containing pulp fibers and polyethylene and having a layer structure of two or more layers; and forming a coating layer on at least one of the white cardboard layers. The polyethylene content in the coated white cardboard manufactured by the above manufacturing method is 50 to 2500 ppm relative to the total mass of the coated white cardboard, and the basis weight is 150 g / m². ２ That's all.

[0095] In the papermaking process for white cardboard according to this embodiment, it is preferable to disperse a predetermined amount of polyethylene in the pulp slurry. For example, by crushing polyethylene-containing waste paper used as a recycled material and dispersing it in the pulp slurry, a pulp slurry containing pulp fibers and polyethylene can be obtained. In particular, the papermaking process for white cardboard preferably includes the steps of: separating pulp fibers from laminated waste paper having a paper base material containing pulp fibers and a resin layer containing polyethylene to obtain a pulp slurry containing pulp fibers and polyethylene; and papermaking the pulp slurry. In this case, the polyethylene content can be controlled by appropriately controlling the type and amount of polyethylene-containing waste paper used as a recycled material.

[0096] In conventional technology, polyethylene tends to become a foreign substance when incorporated into coated white cardboard, often degrading the quality of the coated white cardboard, such as its strength. Therefore, when manufacturing coated white cardboard using recycled materials, polyethylene was separated and removed. In this embodiment, for example, by selectively using recycled materials in which polyethylene has not been printed, polyethylene does not become a foreign substance, and furthermore, by binding to a portion of the pulp fibers, curling of the coated white cardboard can be suppressed.

[0097] The process of making white cardboard preferably includes the steps of obtaining a surface slurry containing pulp fibers, obtaining a subsurface slurry containing pulp fibers, obtaining a middle slurry containing pulp fibers and polyethylene, and obtaining a backing slurry containing pulp fibers, and further preferably includes the step of making white cardboard by multi-layer assembly.

[0098] In this embodiment, since the intermediate layer slurry preferably contains polyethylene, it is preferable to disperse polyethylene in the intermediate layer slurry. Specifically, it is preferable to obtain an intermediate layer slurry containing polyethylene by crushing polyethylene-containing waste paper used as a recycled material and dispersing it in the intermediate layer slurry.

[0099] In the process of obtaining a pulp slurry containing pulp fibers and polyethylene, 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. By setting the dissociation temperature within the above range, it becomes easy to control the kink index of the pulp fibers to a desired range. This makes it possible to suppress the decrease in strength of the coated white cardboard and effectively suppress the occurrence of curl in the coated white cardboard.

[0100] In the process of obtaining a pulp slurry containing pulp fibers and polyethylene, the recycled paper concentration 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 recycled paper concentration 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 recycled paper concentration, the larger the foreign matter (e.g., polyethylene resin fragments) remains when separated from the pulp fibers, making it easier to remove these foreign matter in the dust removal process. However, the lower the recycled paper concentration, the finer the foreign matter fragments become, making them more difficult to remove in the dust removal process. Therefore, by keeping the recycled paper concentration at the time of disintegration within the above range, it becomes easier to control the polyethylene content in coated white cardboard within the desired range.

[0101] In the process of obtaining the pulp slurry, in addition to pulp fibers and polyethylene, optional components may be mixed into the pulp slurry. The optional components can be any of the above-mentioned optional components as appropriate.

[0102] If each layer constituting the white cardboard contains calcium carbonate or other optional components, these are added to the pulp slurry (slurry for each layer). In this case, it is preferable to add the calcium carbonate as a calcium carbonate dispersion in which calcium carbonate is dispersed in water. When adding calcium carbonate 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.

[0103] 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.

[0104] 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.

[0105] The process of obtaining a pulp slurry may include a step of beating the pulp fibers. In particular, the process of obtaining the middle layer slurry preferably further includes a step of beating the middle layer 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. In addition, in the process of obtaining the surface layer, subsurface layer, and back layer slurry, the pulp fibers constituting each layer may also be beated.

[0106] The freeness of pulp fibers in the intermediate layer slurry after beating is preferably 200 ml or more, more preferably 210 ml or more, and even more preferably 220 ml or more. Furthermore, the freeness of pulp fibers in the intermediate layer slurry after beating is preferably 500 ml or less, more preferably 450 ml or less, and even more preferably 400 ml or less. By keeping the freeness of pulp fibers in the intermediate layer slurry within the above range, the tensile modulus of the coated white cardboard can be increased, and as a result, curling in the coated white cardboard can be effectively suppressed. The freeness of pulp fibers in the intermediate layer slurry is defined as the Canadian standard filtration rate value measured in accordance with JIS P 8121-2:2012.

[0107] Examples of paper machines used to make paper from pulp slurry include long-wire paper machines, gap former type paper machines, cylinder wire paper machines, and short-wire paper machines.

[0108] 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, and a multi-layered sheet that will become the base paper for coated white cardboard can be formed. In the headbox, the component ratios of the pulp slurry that form each layer can be different. This makes it possible to make the properties of each layer that makes up the white cardboard different.

[0109] In the manufacturing method of this embodiment, it is preferable to further include a step of crushing and washing the laminated waste paper containing pulp fibers and polyethylene before the step of disintegrating the laminated waste paper to obtain a pulp slurry. In the crushing and washing step, a crushing and washing machine is used to perform a crushing step in which the laminated waste paper is crushed into small pieces by a rotating blade, and a washing step in which the crushed laminated 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 ~400 ２ It is preferable to adjust it so that it results in the following.

[0110] 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 one of the following: hot water, caustic soda solution, ozonated water, hydrogen peroxide solution, or sodium hypochlorite solution. The flow rate of the washing water can be adjusted to any amount, such as 0.1 m³. ３ It is preferable that it be 0.5 m or more per hour. ３ It is more preferable that it be 1.0 m or more per hour. ３It is even more preferable that the flow rate of the washing water be 5.0 m³ / h or higher. ３ It is preferable that the rate is less than or equal to / h. Furthermore, the crushed fragments after crushing and washing may be baled using a compressor.

[0111] In the manufacturing method of this embodiment, by providing such a crushing and washing step, it becomes easy to control the kink index of the pulp fibers within a desired range. This makes it possible to suppress the decrease in strength of the coated white cardboard and effectively suppress the occurrence of curl in the coated white cardboard.

[0112] In the manufacturing method of this embodiment, 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 that originate from layers other than the paper base layer of the laminated waste paper, and which have a particularly high specific gravity. Furthermore, the removal components refer to all components that have been 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.

[0113] 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.

[0114] 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 conical in shape 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 rameau cleaners are recommended as cleaners used 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.

[0115] The process of removing foreign matter by screening is performed after the process of removing foreign matter by centrifugal separation, for the purpose of removing foreign matter. As the screen used for screening, for example, a basket-type screen with holes or slits that open to a predetermined opening area can be used, or a slit screen can be used, and either a coarse screen or a fine screen may be used. From the viewpoint of efficiently removing foreign matter, it is preferable to perform the fine screening process after the coarse screening process. As the coarse screen, a round-hole screen or a slit screen is preferred, and a round-hole screen is 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 fine 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.

[0116] In the coarse 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 refined screening 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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 cleaning the 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.

[0122] 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.

[0123] In the manufacturing method of this embodiment, it is preferable to further include a drying step after the papermaking step 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.

[0124] 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 polyethylene melts and binds appropriately to a portion of the pulp fibers, thereby more effectively suppressing curling in the coated white cardboard.

[0125] The manufacturing method of this embodiment may further include a calendering step after the drying step. The calendering step is performed in the calendering part. In the calendering part, the surface of the dried sheet is pressed and stretched to make the surface of the sheet smooth.

[0126] The manufacturing method of this embodiment has a step of forming a coating layer on at least one side of the white cardboard. The step of forming the coating layer is a step of applying a coating liquid containing a pigment and a binder. Among them, in the manufacturing method of this embodiment, it is preferable to have a step of forming coating layers on both sides of the white cardboard. 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, by drying the coating film, a coating layer is provided on the surface of the white cardboard.

[0127] 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, based on 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, based on 100 parts by mass of the pigment. If the ratio of the binder is at least the above lower limit value, the strength of the coating layer tends to be sufficient. Also, if the ratio 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.

[0128] The coating amount per side of the coating liquid is preferably 3 g / m ２ or more, more preferably 5 g / m ２ or more, and even more preferably 10 g / m ２ or more, in terms of the basis weight after drying. Also, the coating amount per side of the coating liquid is preferably 50 g / m ２ or less, more preferably 40 g / m ２ or less, and even more preferably 35 g / m ２ or less, in terms of the basis weight after drying. When the coating layer is provided on both sides of the white cardboard, the coating amounts of the surface coating layer and the back coating layer may be different.

[0129] The coating liquid may be applied in multiple portions. When the coating liquid is applied in multiple portions, for example, in two portions, the coating liquids for the first time (the undercoat layer) and the second time (the topcoat layer) may be the same or different. Also, the coating liquid may be applied directly to the surface layer and / or the back layer, or may be applied to the surface layer through another coating film.

[0130] 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.

[0131] 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.

[0132] The manufacturing method of this embodiment may include a step of winding up coated white cardboard to form a roll (winding). In this case, it is preferable that at the end processing section of the winding, the end of the long coated white cardboard discharged from the winding section is glued along the width direction to prevent the long coated white cardboard from unraveling.

[0133] The features of the present invention will be further specifically described below with reference to Examples and Comparative Examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited 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 the laminated waste paper, a packaging material for paper cups (printing layer / paper base material layer / LDPE layer, pulp blending ratio of the paper base material layer N:L = 0:100, basis weight 173 g / m ２ ) was used and 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, flow rate 1.5 m ３ / h) to a size of 20 cm ２ or more and 400 cm ２ or less (average size: 53 cm ２ ).

[0135] (Disintegration 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 disintegration 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 waste paper pulp slurry to 2.5% by mass of solids, it was treated with a heavy foreign matter cleaner (manufactured by Aikawa Iron Works Co., Ltd., FC100 type), and further treated with a coarse screening screen (manufactured by Aikawa Iron Works Co., Ltd., MaxFlow-1, Model 1000 type, round holes 1.2 mm). After the treatment with the coarse screening screen, the concentration of the laminated waste paper pulp slurry was adjusted to 1.0% by mass of solids and treated with a fine screening screen (manufactured by Aikawa Iron Works Co., Ltd., MaxFlow-1, Model 1000 type, slit 0.15 mm). The laminated waste paper pulp slurry after the treatment with the fine screening screen was treated with a low-concentration Lamo cleaner (manufactured by Aikawa Iron Works Co., Ltd., Bycone 150).

[0137] (Washing and 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] [Preparation of Coated White Cardboard] (Preparation of Surface Layer Slurry) As raw material pulp, a mixture of 80% by mass of deinked pulp obtained by deinking Kent waste paper and 20% by mass of bleached hardwood kraft pulp was used. The mixture was disintegrated 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 obtain a surface layer pulp slurry. To 100 parts by mass of pulp (solids) 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.) was added to prepare a surface layer (first layer) slurry (pulp slurry).

[0139] (Preparation of Slurry for Subsurface Layer) 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 a slurry for the subsurface layer (second layer) (pulp slurry).

[0140] (Preparation of intermediate layer slurry) Magazine waste paper pulp was used as the raw material pulp, and the magazine waste paper pulp slurry obtained by disintegrating 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 was mixed with a laminated waste paper pulp slurry with a laminated waste paper pulp concentration of 4% by mass, obtained by adding water to the laminated waste paper pulp obtained above, in a mass ratio of 99:1. This pulp slurry was beaten to a freeness of 340 ± 5 ml to obtain the intermediate layer pulp slurry. 1.5 parts by mass of aluminum sulfate was added to 100 parts by mass of pulp (solid content) in this pulp slurry to prepare the intermediate layer (3rd and 4th layers) slurry (pulp slurry).

[0141] (Preparation of backing layer slurry) 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 a slurry for the backing layer (5th layer) (pulp slurry).

[0142] (Manufacturing of white cardboard) Using the slurry prepared above for each layer, the basis weight of the surface layer (first layer) is 25 g / m². ２ The basis weight of the lower layer (second layer) is 35 g / m². ２ The basis weight of the middle layer (third layer) is 145 g / m². ２ The basis weight of the middle layer (fourth layer) is 172 g / m². ２ The basis weight of the back layer (5th layer) is 30 g / m². ２ The paper was then assembled using a short-wire paper machine, producing five layers of paper with a basis weight of 407 g / m². ２ White cardboard was obtained. In the papermaking process, a drying process was carried out in a multi-cylinder dryer at a speed of 240 m / min and a temperature of 125°C, and a total length of 2000 m was wound up.

[0143] (Preparation of coating solution for surface coating layer) To an aqueous solution to which 0.1 parts by mass of sodium polyacrylate (product name: Poise 520, manufactured by Kao Corporation) was added as a dispersant, 100 parts by mass of heavy light calcium carbonate (product name: FMT90, manufactured by Fimatec, average particle size 0.78 μm) was added as a pigment, and a pigment dispersion with a solid content of 70% by mass was prepared using a Koles disperser. Next, to 100 parts by mass of the pigment (solid content) in this pigment dispersion, 3 parts by mass of oxidized starch (product name: Ace Y, manufactured by Oji Corn Starch Co., Ltd.) and 14 parts by mass of styrene-butadiene copolymer latex (product name: BA025, manufactured by Asahi Kasei Chemicals Corporation) were added to obtain a coating solution for surface coating layer with a solid content of 50% by mass and a B-type viscosity of 50 mPa·s.

[0144] (Preparation of coating solution for back coating layer) To an aqueous solution to which 0.1 parts by mass of sodium polyacrylate (product name: Poise 520, manufactured by Kao Corporation) was added as a dispersant, 5 parts by mass of heavy calcium carbonate (product name: FMT-OP, manufactured by Fimatec, average particle size 0.60 μm), 90 parts by mass of kaolin (product name: Ultra White 90, manufactured by BASF, average particle size 0.32 μm), and 5 parts by mass of titanium dioxide (product name: KA-100, manufactured by Cosmo Chemical Co., Ltd., Korea) were added as pigments, and a pigment dispersion with a solid content of 68% by mass was prepared using a Koles disperser. Next, to 100 parts by mass of the pigment (solids) in this pigment dispersion, 3 parts by mass of oxidized starch (product name: Ace Y, manufactured by Oji Corn Starch Co., Ltd.) and 16 parts by mass of styrene-butadiene copolymer latex (product name: B1840, manufactured by Asahi Kasei Chemicals Co., Ltd.) were added to obtain a coating solution for the back coating layer with a solids content of 42% by mass and a B-type viscosity of 40 mPa·s.

[0145] (Manufacturing of coated white cardboard) In the process of manufacturing white cardboard, a coating liquid for the surface coating layer is applied to the surface (surface layer side) of the white cardboard obtained in the (manufacturing of white cardboard) process using a bar coater, with a dry mass of 15 g / m². ２ The surface coating was applied and dried at 120°C to form the surface coating layer. Next, a bar coater was used to apply the back coating liquid to the back surface (back layer side) of the white cardboard, with a dry mass of 3 g / m². ２The white cardboard was coated and dried at 120°C to form a back coating layer. Then, the white cardboard with the front and back coating layers was passed through a 2-nip soft calender at a metal roll surface temperature of 200°C to obtain coated white cardboard.

[0146] <Example 2> In the preparation of the intermediate layer slurry for the [production of coated white cardboard], coated 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 95:5.

[0147] <Example 3> In the preparation of the intermediate layer slurry for the [production of coated white cardboard], coated 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 85:15.

[0148] <Example 4> In the (disintegration process) of [Production of laminated recycled paper pulp] and [Production of coated white cardboard], coated white cardboard was obtained in the same manner as in Example 2, except that the disintegration treatment of all slurries used was carried out at a processing temperature of 25°C.

[0149] <Example 5> In the (disintegration process) of [Production of laminated recycled paper pulp] and [Production of coated white cardboard], coated white cardboard was obtained in the same manner as in Example 2, except that the disintegration treatment of all slurries used was carried out at a processing temperature of 55°C.

[0150] <Example 6> In the preparation of the intermediate layer slurry in [Preparation of coated white cardboard], coated white cardboard was obtained in the same manner as in Example 1, except that the intermediate layer pulp slurry was obtained by beating the material to a freeness of 290 ± 5 ml.

[0151] <Example 7> In the preparation of the intermediate layer slurry in [Preparation of coated white cardboard], coated white cardboard was obtained in the same manner as in Example 1, except that the intermediate layer pulp slurry was obtained by beating the material to a freeness of 230 ± 5 ml.

[0152] <Example 8> In the (manufacturing of white cardboard) of [Preparation of coated white cardboard], the basis weight of the middle layer (4th layer) is 167 g / m². ２ In the manufacture of coated white cardboard, the coating liquid for the back coating layer is 8 g / m² (dry mass).２ Coated white cardboard was obtained in the same manner as in Example 1, except that a back coating layer was formed by coating.

[0153] <Example 9> In the (Preparation of the backing slurry) of [Preparation of coated white cardboard], 10 parts by mass of titanium dioxide (product name: R-630, manufactured by Ishihara Sangyo Co., Ltd.) was added to 100 parts by mass of pulp (solid content) in the backing pulp slurry, and in (Production of white cardboard), the basis weight of the middle layer (4th layer) was 162 g / m². ２ In the manufacture of coated white cardboard, the coating liquid for the back coating layer has a dry mass of 13 g / m². ２ Coated white cardboard was obtained in the same manner as in Example 1, except that a back coating layer was formed by coating.

[0154] <Example 10> In the (manufacturing of white cardboard) of [Production of coated white cardboard], the basis weight of the middle layer (4th layer) is 77 g / m². ２ The total basis weight of the five layers is 312 g / m². ２ Coated white cardboard was obtained in the same manner as in Example 3, except that white cardboard was used. In this papermaking process, the white cardboard was wound at a speed of 300 m / min.

[0155] <Example 11> In the (manufacturing of white cardboard) of [Production of coated white cardboard], the basis weight of the middle layer (3rd layer) is 72 g / m². ２ Therefore, no middle layer (fourth layer) is provided, resulting in a total of four layers with a basis weight of 162 g / m². ２ Coated white cardboard was obtained in the same manner as in Example 3, except that white cardboard was used. In this papermaking process, the white cardboard was wound at a speed of 410 m / min.

[0156] <Comparative Example 1> In the preparation of the intermediate layer slurry in the [Preparation of Coated White Cardboard] section, a slurry (pulp slurry) for the intermediate layer (3rd and 4th layers) was prepared using 100% recycled magazine pulp slurry. Otherwise, coated white cardboard was obtained in the same manner as in Example 1.

[0157] <Comparative Example 2> In the preparation of the intermediate layer slurry for the production of coated white cardboard, coated 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 80:20.

[0158] <Comparative Example 3> In the (manufacturing of white cardboard) of [Production of Coated White Cardboard], the basis weight of the middle layer (third layer) is 22 g / m². ２ Therefore, no middle layer (fourth layer) is provided, resulting in a total of four layers with a basis weight of 112 g / m². ２ Coated white cardboard was obtained in the same manner as in Example 3, except that white cardboard was used. In this papermaking process, the white cardboard was wound at a speed of 450 m / min.

[0159] The manufacturing conditions for the middle layer in the examples and comparative examples are as follows:

[0160]

[0161] <Measurement and Evaluation Methods> (Basis Weight) The coated 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 coated white cardboard was measured in accordance with JIS P 8124:2011. As a pretreatment, the coated white cardboard was soaked in 40°C hot water for 6 hours, then carefully peeled off one layer at a time by hand, dividing it into four parts: top coating layer / top layer, bottom top layer, middle layer, and back layer / back coating layer. Each part was then dried in a dryer set to 105°C for 30 minutes. After that, the conditioned white cardboard was conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998, and the basis weight of each of the four divided coated white cardboard parts was measured in accordance with JIS P 8124:2011. Next, the surface coating layer / surface and back coating layer samples were immersed in a 1 mol / L copper ethylenediamine solution (manufactured by Kanto Chemical Co., Ltd.) for 3 hours to dissolve the surface and back coating layers. The remaining surface and back coating layers were washed with water and dried in a dryer set to 105°C for 30 minutes. After that, the samples were conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998. Then, in accordance with JIS P 8124:2011, the basis weight of the conditioned surface and back coating layers was measured, and then subtracted from the previously measured basis weight of the surface coating layer / surface and back coating layers to determine the basis weight of each layer of coated white cardboard.

[0162] (Thickness) The coated 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 thickness of the conditioned coated white cardboard was measured in accordance with JIS P 8118:2014.

[0163] (Density) From the measured basis weight and thickness, the basis weight (g / m²) ２ The density was calculated using the formula: (μm) / thickness (μm).

[0164] (Ash Content) The ash content of the coated white cardboard obtained in the examples and comparative examples was measured in accordance with JIS P 8251:2003. When measuring the ash content of the surface and back layers, the coated white cardboard was pre-treated by soaking it in 40°C hot water for 6 hours, and then carefully peeling off each layer by hand to separate the surface and back layers. For ashing, a muffle furnace "FO610" manufactured by Yamato Scientific Co., Ltd. was used, and for ash quantification, a "Sartorius Cubis MSU524S" manufactured by Sartorius Japan Co., Ltd. was used.

[0165] (Polyethylene Content) For the measurement of polyethylene content, an extraction step was performed to separate the polyethylene contained in the coated white cardboard, and an analysis step was performed to quantify the polyethylene by pyrolysis GC / MS. [Extraction Step] 3 to 5 g of the coated white cardboard obtained in the examples and comparative examples was taken by dry weight and placed in an extraction cell (20 ml container). Next, polyethylene was extracted from the coated white cardboard using a high-speed solvent extraction apparatus (machine: Büch, E-916). The extraction conditions were an extraction temperature of 180°C, an extraction pressure of 150 bar, and an extraction solvent of xylene (Wako Pure Chemical Industries, special grade). A 100 ml vial (Maruemu, Mighty Vial No. 8) was set in the extraction apparatus, and extracts equivalent to two extraction cycles were collected. After extraction, the solvent was removed using a rotary evaporator (water bath 60°C), and the polyethylene in the coated white cardboard was obtained by drying at 105°C for 12 hours until completely dry. [Analysis Procedure] An appropriate amount of xylene was added to the dry polyethylene in the vial extracted above to a concentration of 2-5 mg / ml, and dissolved at 130°C for 2 hours with stirring during the process. Next, the xylene solution of the dissolved extract was added to an eco cup whose tare weight had been measured, so that the amount of extract was 50-200 μg, and the solvent was evaporated at 105°C for 2 hours, and the amount of extract to be analyzed was measured. The polyethylene content of the obtained extract was quantified using pyrolysis GC / MS (Shimadzu Corporation GC / MS-QP2010), and the polyethylene content in coated white cardboard was calculated from the polyethylene content of the extract. The measurement conditions for pyrolysis GC / MS were as follows. • Polyethylene standard: (Tosoh 07C03C, LDPE) • Column: HP-5MS (Length: 30m, Inner diameter: 0.250mm, Thickness: 0.25μm) • Analytical conditions: Pyrolysis temperature = 600°C, Inlet temperature = 320°C, Split ratio = 1:60, Interface temperature = 280°C, GC column temperature conditions = 40°C (hold for 3 minutes) → 10°C / min → 325°C (hold for 15 minutes), Ion source temperature = 200°C • Detection: m / z 29-600 (Quantification is calculated using the mass chromatogram area value of 1,19-eicosadiene (C2O) at m / z 82)

[0166] (Kink Index) Coated white cardboard obtained 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 coated white cardboard, and 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, and the pulp was disintegrated 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 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. The resulting slurry was used to measure the "kink index" using a fiber length measuring instrument (Valmet FS-5 UHD base unit, manufactured by Valmet).

[0167] The kink index (1 / m) is Kibblewhite's kink index calculated by the following formula (1). The kink index was calculated by weighting the number of kinks for each angle range according to the increase in angle, and dividing by the sum of the fiber lengths. The measurements were performed in accordance with ISO 16065-2:2014. The device is capable of detecting and measuring each individual fiber using its attached camera, and images were taken within a measurement cell with a depth of field of 0.5 mm. Fibers with a length of 0.01 mm to 10.00 mm were photographed using this device.

[0168]

[0169] The symbols in equation (1) represent the following: n １ = Number of kinks (bends) between 21° and 45° in the measurement sample n ２ = Number of kinks (bends) between 46° and 90° in the measurement sample n ３ = Number of kinks (bends) between 91° and 180° in the measurement sample L ｃ = Sum of fiber lengths of the measurement sample (m)

[0170] (Tensile Modulus) The coated 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 tensile modulus of the conditioned coated white cardboard was measured in accordance with JIS P 8113:2006. The tensile modulus was measured in the longitudinal direction (papermaking direction, MD direction) and transverse direction (direction perpendicular to the papermaking direction, CD direction). The test was performed using a transverse tensile testing machine (Lorentzen & Wattre, CODE SE-064), and the synergistic mean of the obtained longitudinal and transverse tensile moduli was calculated.

[0171] (Curling at High Humidity) Coated white cardboard obtained in the examples and comparative examples was cut into 10 cm x 10 cm sections and placed in a constant temperature and humidity chamber set to 40°C and 90% relative humidity for 6 hours. After that, it was moved to an environment of 23°C and 50% relative humidity, and the curl height of the coated white cardboard was measured after 1 minute. The coated white cardboard was laid flat on a horizontal hard surface, and the lift at each of the four corners of the coated white cardboard was measured, and the average value was evaluated according to the following criteria. When laying the coated white cardboard flat on a horizontal hard surface, the surface side was placed on the hard surface and the lift was measured. If no lift occurred, the back side was placed on the hard surface and the lift was measured, and the result was adopted. [Evaluation Criteria] A: Average lift at the four corners is less than 2 mm B: Average lift at the four corners is 2 mm or more and less than 4 mm C: Average lift at the four corners is 4 mm or more and less than 6 mm D: Average lift at the four corners is 6 mm or more

[0172] (Operability) When a total length of 2000 m of coated white cardboard was obtained in the examples and comparative examples, the frequency of paper breaks was counted and the operability was evaluated according to the following criteria. [Evaluation Criteria] A: No paper breaks occurred. B: One paper break occurred. C: Two or more paper breaks occurred.

[0173]

[0174]

[0175] In the examples, curling under high humidity conditions was suppressed, and the operability during the production of coated white cardboard was good.

Claims

1. Coated white cardboard having a coating layer on at least one side of the white cardboard, wherein the white cardboard contains pulp fibers and polyethylene and has a layer structure of two or more layers, the polyethylene content is 50 to 2500 ppm relative to the total mass of the coated white cardboard, and the basis weight is 150 g / m². ２ That concludes the description of coated white cardboard.

2. The coated white cardboard according to claim 1, wherein the kink index of the pulp fibers is 3100 (1 / m) or less.

3. The coated white cardboard according to claim 1, wherein the synergistic mean of the tensile modulus in the longitudinal direction and the tensile modulus in the transverse direction is 2.0 GPa or more.

4. The coated white cardboard according to claim 1, wherein the polyethylene is polyethylene derived from laminated recycled paper.

5. The coated white cardboard according to claim 1, wherein the coating layer comprises a pigment and a binder.

6. The coated white cardboard according to claim 1, wherein the white cardboard has a layered structure in which a surface layer, a surface sub-layer, a middle layer, and a back layer are laminated in this order, and the coating layer is provided on both sides of the white cardboard, and the surface coating layer, surface layer, surface sub-layer, middle layer, back layer, and back coating layer are laminated in this order.

7. The coated white cardboard according to claim 6, wherein the difference between the total amount (mass%) of ash contained in the surface coating layer, the surface layer, and the subsurface layer and the total amount (mass%) of ash contained in the back layer and the back coating layer is 20% by mass or less.

8. A packaging material obtained by processing coated white cardboard according to any one of claims 1 to 7.

9. A method for manufacturing coated white cardboard, comprising the steps of: making white cardboard containing pulp fibers and polyethylene and having a layer structure of two or more layers; and forming a coating layer on at least one of the white cardboard, wherein the polyethylene content is 50 to 2500 ppm relative to the total mass of the coated white cardboard, and the basis weight is 150 g / m². ２ The above describes the method for manufacturing coated white cardboard.

10. The method for manufacturing coated white cardboard according to claim 9, wherein the step of making the white cardboard comprises the steps of: separating pulp fibers from laminated recycled paper having a paper substrate containing pulp fibers and a resin layer containing polyethylene to obtain a pulp slurry containing pulp fibers and polyethylene; and making paper from the pulp slurry.

11. The method for producing coated white cardboard according to claim 9, wherein the dissociation temperature of the pulp fibers in the step of obtaining the pulp slurry is 20 to 60°C.

12. A method for manufacturing coated white cardboard according to any one of claims 9 to 11, wherein the step of obtaining the pulp slurry includes a step of crushing and washing the laminated waste paper.

13. The method for manufacturing coated white cardboard according to any one of claims 9 to 11, wherein the step of making the white cardboard comprises the steps of obtaining a surface slurry containing pulp fibers, obtaining a subsurface slurry containing pulp fibers, obtaining a middle slurry containing pulp fibers and polyethylene, and obtaining a backing slurry containing pulp fibers, and the white cardboard is made by multi-layer assembly.

14. The method for manufacturing coated white cardboard according to claim 13, wherein the step of obtaining the intermediate layer slurry further includes a step of beating the intermediate layer slurry.

15. A method for manufacturing coated white cardboard according to any one of claims 9 to 11, further comprising a drying step after the step of papermaking the white cardboard.

16. The method for manufacturing coated white cardboard according to any one of claims 9 to 11, wherein the step of forming the coating layer is a step of applying a coating liquid containing a pigment and a binder.

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