Liner base paper

Abstract

To provide liner base paper capable of suppressing rule crack formation and having improved hardness even when a recycle raw material with a trend of less environmental load yet low strength is used.SOLUTION: Provided is liner base paper in which a water-soluble resin layer is laminated on a paper substrate. When a face in contact with a middle core is regarded as a rear face, the liner base paper has a water-soluble resin layer on the rear face, wherein a burst index of the liner base paper measured in accordance of JIS P 8131:2009 is 2.00 kPa / (g / m2) or over, and a relative moisture zero-spun tensile strength of a disintegrated pulp using the liner base paper and measured in accordance of JIS P 8227:2008 is 85 N m / g or under.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a liner paper and a corrugated cardboard sheet made using the liner paper. [Background technology]

[0002] A liner paper is a paperboard used as the cover or backing paper of a corrugated board or solid fiberboard. When the liner paper is used as a corrugated board sheet that constitutes a box-shaped corrugated board, for example, a crease is formed in order to fold the liner paper. When the liner paper with the crease is folded, the front side of the liner paper is subjected to a stretching force, which may cause cracks. Such a cracked state is called a crease crack, and not only does the crease reduce the aesthetic appeal, but it also leads to a problem of insufficient strength of the corrugated board.

[0003] In order to solve the above problems, Patent Document 1 discloses a liner paper having two or more layers, at least a surface layer and a back layer, in which at least the surface layer contains a moisturizing agent, and describes that this effectively inhibits the liner paper from releasing moisture, thereby preventing the occurrence of crease cracks. Furthermore, Patent Document 2 discloses a cardboard sheet having a liner and a corrugated core bonded together, the cardboard being characterized in that the liner is coated with a creasing prevention agent that is a mixture of water, a specific amount of a moisturizer, a surfactant, and a specific amount of a penetrating agent. It states that because the penetrating agent is mixed with the creasing prevention agent, the moisturizer quickly penetrates the liner, and the effect of preventing creasing can be reliably achieved in accordance with the box-making speed. Furthermore, Patent Document 3 describes that by adding a water-soluble lignin-based substance and a copolymer having a specific diallylamine-based monomer as a constituent monomer to a pulp slurry containing recycled paper pulp, it is possible to produce paper of excellent quality, such as paper with high dry strength and less tendency to develop groove cracks (crease cracks). Moreover, Patent Document 4 discloses a paperboard having two or more paper layers, in which the fiber roughness of a first paper layer located on the surface is greater than the fiber roughness of a second paper layer laminated on the back surface of the first paper layer, and describes that this can effectively prevent creasing. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-345696 A [Patent Document 2] JP 2010-260181 A [Patent Document 3] Japanese Patent Application Publication No. 6-184983 [Patent Document 4] JP 2014-70318 A Summary of the Invention [Problem to be solved by the invention]

[0005] Traditionally, cardboard sheets containing linerboard have been made from a high proportion of recycled paper, but in recent years, they have been recycled more and more, which has led to shorter fiber lengths and a decline in strength such as burst strength and compressive strength.

[0006] As in Patent Documents 1 and 2, there is a problem that the decrease in strength and the occurrence of crease cracks cannot be sufficiently prevented by simply adding a moisturizing agent to the surface layer of the liner base paper, especially in a liner base paper that has been repeatedly recycled. In addition, in the internal addition method of adding a paper strength enhancer to the paper stock as in Patent Document 3, when the water quality of the white water deteriorates, for example, in a high-temperature and high-humidity environment, the fixation of the paper strength enhancer deteriorates, and the crease cracks of the liner base paper cannot be sufficiently suppressed. Furthermore, as in Patent Document 4, if the content of kraft pulp is increased instead of waste paper to increase the fiber coarseness of the first paper layer, there is a problem that the cost increases and a huge amount of energy is used to manufacture the liner base paper, which is an environmental burden.

[0007] The present invention aims to provide a liner base paper that has improved strength while suppressing the occurrence of creases, even when using recycled raw materials such as office waste paper and cardboard waste paper, which tend to have low environmental impact but low strength. [Means for solving the problem]

[0008] The inventors have discovered that even if the wet zero span tensile strength of the pulp obtained by disintegrating the liner base paper is below a specific value, by forming a liner base paper in which a water-soluble resin layer is laminated on a paper base material and the water-soluble resin layer is on the back surface, it is possible to provide a liner base paper with a tensile strength ratio of a specific value or above, which suppresses the occurrence of creasing and has improved strength. That is, the present invention provides the following: <1> ~ <8> Regarding. <1> A liner base paper having a water-soluble resin layer laminated on a paper base material, the liner base paper having a water-soluble resin layer on the back surface when the surface of the liner base paper that contacts the core is the back surface, and the burst strength of the liner base paper measured in accordance with JIS P 8131:2009 is 2.00 kPa / (g / m 2 ) or more, and the specific wet zero span tensile strength of the disintegrated pulp using the liner base paper is 85 N·m / g or less, as measured in accordance with JIS P 8227:2008. <2> The coating amount of the water-soluble resin layer is 0.1 g / m 2 More than 4.0g / m 2 Below is the <1> The liner base paper described in. <3> In the length-weighted fiber length distribution of the pulp in the surface layer of the liner base paper, measured in accordance with JIS P 8226-2:2011, the ratio of fibers having a fiber length of 0.5 mm or more and less than 1.2 mm is 35% or more, and the ratio of fibers having a fiber length of 1.2 mm or more is 25% or less. <1> or <2> The liner base paper described in. <4> The paper base material has two or more paper layers including at least a front layer and a back layer. <1> ~ <3> 3. A liner base paper according to any one of the preceding claims. <5> The paper base material has one or more intermediate layers in addition to the front and back layers, <4> The liner base paper described in. <6> The whiteness of the surface of the liner base paper is 10% or more and 30% or less. <1> ~ <5> 3. A liner base paper according to any one of the preceding claims. <7> The main component of the water-soluble resin is starch or polyacrylamide. <1> ~ <6> 3. A liner base paper according to any one of the preceding claims. <8> <1> ~ <7> 2. A cardboard sheet comprising the liner paper according to claim 1. Effect of the Invention

[0009] According to the present invention, even when using recycled raw materials, which have a low environmental impact but tend to have low strength, a liner base paper is provided that has improved strength while suppressing the occurrence of creasing. [Brief description of the drawings]

[0010] [Figure 1] FIG. 2 is a cross-sectional schematic diagram showing one embodiment of a liner base paper of the present embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [Liner base paper] The liner base paper of the present invention (hereinafter, also simply referred to as "liner base paper") is a liner base paper in which a water-soluble resin layer is laminated on a paper base material, and when the surface of the liner base paper that contacts the core is the back surface, the liner base paper has a water-soluble resin layer on the back surface, and the burst strength of the liner base paper measured in accordance with JIS P 8131:2009 is 2.00 kPa / (g / m 2 ) or more, and the specific wet zero span tensile strength of the disintegrated pulp, measured using the liner base paper in accordance with JIS P 8227:2008, is 85 N·m / g or less. The liner base paper of the present invention has a wet zero span tensile strength of the disintegrated pulp that is below a specific value, and even if pulp with inferior strength is used, the liner base paper has a water-soluble resin layer on the back side rather than the front side, which can suppress the occurrence of creasing while improving strength such as burst strength.

[0012] The structure of the liner paper of the present invention will be described with reference to FIG. The liner base paper 1 of the present invention has at least a paper substrate 10 and a water-soluble resin layer 20, and is suitable for use as the outermost liner base paper of a cardboard sheet. The liner base paper has a front surface and a back surface, and when the cardboard sheet is made, the surface that contacts the core is referred to as the "back surface of the liner base paper," while the opposite surface is referred to as the "front surface of the liner base paper." The front surface of the liner base paper is in contact with an air layer. The liner base paper 1 also has a water-soluble resin layer 20 on the back surface. The paper substrate 10 of the liner base paper 1 similarly has a front and back surface, with the same side as the "front surface of the liner base paper" being the "front surface of the paper substrate" and the same side as the "back surface of the liner base paper" being the "back surface of the paper substrate."

[0013] The paper substrate 10 may be a single layer or may have two or more paper layers, but it is preferable that the paper substrate 10 has two or more paper layers including at least a front layer and a back layer, so that each layer can have the desired characteristics as a liner base paper, and it is preferable that the paper substrate 10 has one or more middle layers in addition to the front and back layers. The number of middle layers is not particularly limited, but is preferably 1 to 7 layers, more preferably 1 to 5 layers, and even more preferably 1 to 3 layers. In Fig. 1, the paper base material 10 has a three-layer paper layer structure, including a surface layer 11, one middle layer 12, and a back layer 13. Needless to say, the surface layer of the paper base material is the paper layer that includes the front surface of the paper base material, and the back layer of the paper base material is the paper layer that includes the back surface of the paper base material. When the paper substrate is a single layer, the "surface layer of the paper substrate" and the "back layer of the paper substrate" refer to the same layer.

[0014] Furthermore, in the liner base paper, the layer including the "surface layer of the paper substrate" is referred to as the "surface layer of the liner base paper," and the layer including the "back layer of the paper substrate" is referred to as the "back layer of the liner base paper." Therefore, when the paper substrate has two or more layers having at least a surface layer and a back layer, the "surface layer of the liner base paper" is synonymous with the "surface layer of the paper substrate." The water-soluble resin layer is laminated on the "back layer of the paper substrate" so as to be in contact with the "back layer of the paper substrate". Therefore, the "back layer of the liner base paper" includes at least the "back layer of the paper substrate" and the water-soluble resin layer. It is considered that the water-soluble resin layer partially penetrates into the "back layer of the paper substrate". The penetration is advantageous for increasing the strength of the paper substrate and does not interfere with the achievement of the effects of the present invention. Therefore, the occurrence of the penetration is an expected aspect and does not deviate from the spirit of the present invention. When the paper base material is a single layer, the "surface layer of the liner base paper" is synonymous with the paper base material, and the "back layer of the liner base paper" is a layer including the paper base material and the water-soluble resin layer.

[0015] <Paper base material> The liner base paper of the present invention has a water-soluble resin layer laminated on the back surface of a paper substrate. The paper substrate may be a single-layer paper substrate as described above, but preferably has two or more paper layers having at least a front layer and a back layer, and more preferably has one or more middle layers in addition to the front layer and back layer. The number of layers of the middle layer is preferably 1 to 7, more preferably 1 to 5, and even more preferably 1 to 3.

[0016] (Raw pulp) Examples of raw pulp for paper substrate include wood pulp, non-wood pulp, and waste paper pulp. Examples of wood pulp include, but are not limited to, chemical pulp such as hardwood kraft pulp (LKP), softwood kraft pulp (NKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), bleached kraft pulp (BKP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP), mechanical pulp such as groundwood pulp (GP), thermomechanical pulp (TMP), and chemi-thermomechanical pulp (CTMP), and the like. Examples of non-wood pulp include, but are not limited to, cotton-based pulp such as cotton linters and cotton lint, and non-wood-based pulp such as hemp, wheat straw, and bagasse. The waste paper pulp is not particularly limited, but examples thereof include pulp made from waste paper such as recycled corrugated paper (corrugated paper), office paper, white recycled paper, brown recycled paper, Kraft envelope paper, magazine paper, newspaper paper, flyer paper, white recycled paper, Kent recycled paper, imitation recycled paper, and land certificate paper. The raw material pulp may be one of the above types alone or a mixture of two or more types. Among these, from the viewpoint of environmental load, the raw material pulp is preferably recycled paper pulp, and more preferably recycled paper is used as the raw material. Among recycled paper, from the viewpoint of obtaining a high-strength liner base paper, recycled corrugated paper, recycled office paper, recycled white board paper, and recycled kraft paper are preferred. The raw pulp may be mixed with organic synthetic fibers such as polyamide fibers and polyester fibers, regenerated fibers such as polynosic fibers, and inorganic fibers such as glass fibers, ceramic fibers, and carbon fibers.

[0017] If necessary, optional components such as sizing agents, anionic, cationic or amphoteric retention aids, drainage aids, dry strength agents, wet strength agents, internal additives such as fillers, water-resistant agents, dyes, and fluorescent whitening agents may be added to the raw pulp. Examples of the sizing agent include rosin sizing agent and alkyl ketene dimer. Examples of retention aids include aluminum sulfate and polyacrylamide. Dry strength agents include cationic starch, polyacrylamide, carboxymethyl cellulose, and the like. Examples of the wet strength agent include polyamide polyamine epichlorohydrin, urea formaldehyde resin, and melamine formaldehyde resin. Examples of the filler include inorganic fillers such as clay, talc, kaolin, calcined kaolin, calcium carbonate, calcium sulfate, barium sulfate, titanium dioxide, zinc oxide, alumina, magnesium carbonate, magnesium oxide, silica, white carbon, bentonite, zeolite, sericite, and smectite; and organic fillers such as acrylic resins and vinylidene chloride resins.

[0018] The method for producing the paper base material is not particularly limited, but includes a step of making a pulp slurry containing raw pulp and drying it. The papermaking method is also not particularly limited, and examples thereof include an acidic papermaking method in which papermaking is performed at a pH of about 4.5, and a neutral papermaking method in which papermaking is performed at a pH of about 6 to about 9. The papermaking machine is also not particularly limited, and examples thereof include a continuous papermaking machine such as a fourdrinier type, a cylinder type, or a tilt type, or a multi-layer papermaking machine that combines these. The drying method is also not particularly limited, and a known drying device can be used.

[0019] <Water-soluble resin layer> The liner base paper of the present invention has a water-soluble resin layer laminated on the back surface of the paper substrate. The water-soluble resin is a resin that can be dissolved in water. Specifically, a resin that is confirmed to dissolve in an amount of 1 g or more in 100 g of water by visual observation at any temperature between room temperature and 100°C can be determined to be a water-soluble resin. Examples of the water-soluble resin include starch; hydroxyethylated starch, oxidized starch, etherified starch, phosphate esterified starch, enzyme-modified starch, and modified starch obtained by flash drying thereof; and synthetic resins such as melamine resin, urea resin, epoxy resin, polyacrylamide (also called PAM), polyvinyl alcohol (also called PVA), and carboxymethyl cellulose. Among these, the water-soluble resin is preferably one or more selected from the group consisting of starch, modified starch, polyacrylamide, and polyvinyl alcohol, more preferably one or more selected from the group consisting of starch and polyacrylamide, and even more preferably starch, from the viewpoint of improving the strength of the liner base paper and suppressing crease cracking. The starch includes self-modified starch. The polyacrylamide may be modified, and examples thereof include cation-modified polyacrylamide, anion-modified polyacrylamide, and amphoteric polyacrylamide, and anion-modified polyacrylamide is preferable. The water-soluble resins mentioned above have -OH groups and -CONH groups that can form hydrogen bonds with the pulp on the surface of the paper base material. 2 group, -NH 2 It is believed that these compounds have the effect of suppressing the occurrence of creases in the liner base paper while improving its strength because they contain -OH groups. Note that starch, modified starch, epoxy resin, polyvinyl alcohol, and carboxymethyl cellulose have -OH groups, and polyacrylamide has -CONH 2 The urea resin has a -NH 2 It has a group.

[0020] In the present invention, the main component of the water-soluble resin is preferably starch or polyacrylamide, and being the main component means that at least 50% by mass of the water-soluble resin is starch or polyacrylamide, preferably 70% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, with the upper limit being 100% by mass.

[0021] The coating amount of the water-soluble resin layer is preferably 0.1 g / m in terms of solid content. 2 More preferably, 0.2 g / m2 More preferably, 0.3 g / m 2 More preferably, 1.0 g / m 2 and preferably 4.0 g / m 2 Less than or equal to 3.5 g / m 2 Less than 3.0 g / m, more preferably 3.0 g / m 2 When the coating amount of the water-soluble resin layer is within the above range, the strength of the liner base paper can be improved while suppressing the occurrence of creases. When the main component of the water-soluble resin is starch, the coating amount of the water-soluble resin layer is preferably 0.5 g / m2 in terms of solid content. 2 More preferably, 1.0 g / m 2 More preferably, 1.2 g / m 2 More preferably, 1.4 g / m 2 and preferably 4.0 g / m 2 Less than or equal to 3.5 g / m 2 Less than 3.0 g / m, more preferably 3.0 g / m 2 The following is the result. When the main component of the water-soluble resin is polyacrylamide, the coating amount of the water-soluble resin layer is preferably 0.1 g / m2 in terms of solid content. 2 More preferably, 0.2 g / m 2 More preferably, 0.3 g / m 2 and preferably 3.0 g / m 2 Less than or equal to 2.5 g / m 2 Less than 2.0 g / m, more preferably 2.0 g / m 2 The following is the result.

[0022] The water-soluble resin layer is formed by applying a coating solution containing a water-soluble resin to the back layer of a paper substrate and then drying. The concentration of the water-soluble resin in the coating solution can be any concentration as long as the coating can be performed, but it is preferable to set the concentration to about 3% by mass or more and 15% by mass or less depending on the coating amount. The coating solution may be applied two or more times. The coating method is not particularly limited, and a known coating method can be used. The coating method may be an on-machine method in which papermaking and coating are performed in a single process, or an off-machine method in which papermaking and coating are performed in separate processes. Examples of the coating method include a method of immersing the back layer of the paper substrate in a coating liquid, a method of applying the coating liquid to the back layer of the paper substrate with a roll or blade, and a method of spraying the coating liquid to the back layer of the paper substrate. Among these, the method of applying the coating liquid to the back layer of the paper substrate with a roll or blade is preferred from the viewpoint of easy adjustment of the coating amount. The coating machine is not particularly limited, and for example, a two-roll size press coater, a gate roll coater, a rod metering size press coater, a blade coater, a bar coater, a spray coater, an air knife coater, a curtain coater, or the like can be used. The drying method is not particularly limited, and a known dryer can be used. For example, a steam heating cylinder, a heated hot air air dryer, a gas heater dryer, an electric heater dryer, an infrared heater dryer, etc. can be used alone or in combination.

[0023] The liner base paper of the present invention has a water-soluble resin layer on the back surface, and the coating amount of the water-soluble resin layer is preferably within the above range, so that the strength of the liner base paper can be improved while suppressing crease cracks. Therefore, according to the present invention, even when using recycled raw materials that are repeatedly recycled and tend to have shorter fiber lengths and lower strength, it is possible to provide a liner base paper that can impart the required strength to the liner base paper while suppressing the occurrence of crease cracks, and has a low environmental impact. Furthermore, according to the liner base paper of the present invention, even when the quality of the white water deteriorates, for example, in a high-temperature and high-humidity environment, it is possible to impart the required strength to the liner base paper while suppressing the occurrence of crease cracks.

[0024] <Physical properties of liner paper> (Specific burst strength) The burst strength of the liner paper is set to 2.00 kPa / (g / m from the viewpoint of obtaining a liner paper suitable for use in corrugated cardboard sheets. 2 ) or more, preferably 2.15 kPa / (g / m2 ) or more, more preferably 2.25 kPa / (g / m 2 The upper limit is not particularly limited, but from the viewpoint of production, it is preferably 10 kPa / (g / m 2 ) or less. The burst strength of the liner paper is 2.00 kPa / (g / m 2 ), the strength of the liner paper is inferior and it is not suitable for use as a corrugated cardboard sheet. The burst strength index of the liner base paper is measured in accordance with JIS P 8131:2009.

[0025] (Wet Zero Span Tensile Strength) The specific wet zero span tensile strength of the pulp obtained by disintegrating the liner base paper is 85 N·m / g or less, preferably 80 N·m / g or less, more preferably 75 N·m / g or less, and even more preferably 70 N·m / g or less, from the viewpoint of reducing the environmental impact. Also, from the viewpoint of obtaining the strength required for the liner base paper, it is preferably 50 N·m / g or more. According to the present invention, even if the specific wet zero span tensile strength of the pulp obtained by disintegrating the liner base paper is within the above range, the occurrence of creasing can be suppressed while providing the strength required for the liner base paper. Therefore, according to the present invention, a liner base paper with low environmental impact can be provided. The specific wet zero span tensile strength of the pulp obtained by disintegrating the liner base paper is measured by the following method. First, the liner base paper is disintegrated to obtain disintegrated pulp in accordance with JIS P 8220-1:2012. A pulp sheet is produced using the obtained disintegrated pulp in accordance with JIS P 8222:2015, and the specific wet zero span tensile strength of the pulp sheet in a rewetted state is measured in accordance with JIS P 8227:2008.

[0026] (Length-weighted fiber length distribution) In the length-weighted fiber length distribution of the pulp in the surface layer of the liner base paper, the ratio of fibers having a fiber length of 0.5 mm or more and less than 1.2 mm is preferably 35% or more, more preferably 38% or more, from the viewpoint of reducing the environmental impact, and is preferably 80% or less, from the viewpoint of obtaining the strength required for the liner base paper. In addition, in the length-weighted fiber length distribution of the pulp in the surface layer of the liner base paper, the ratio of fibers having a fiber length of 1.2 mm or more is preferably 30% or less, more preferably 25% or less, and even more preferably 22% or less, from the viewpoint of reducing the environmental load and ensuring the effects of the present invention, and is preferably 10% or more, from the viewpoint of obtaining the strength required for the liner base paper. Furthermore, in the length-weighted fiber length distribution of the pulp in the surface layer of the liner base paper, the ratio of fibers having a fiber length of 0.2 mm or more and less than 0.5 mm is preferably low from the viewpoint of the strength of the liner base paper, and is preferably 30% or less, more preferably 20% or less, and even more preferably 15% or less. There is no lower limit and it may be 0%, but from the viewpoint of reducing the environmental load, it may be 5% or more, or 10% or more. In the length-weighted fiber length distribution of the pulp in the surface layer of the liner base paper, the ratio of fibers with a fiber length of less than 0.2 mm is preferably low, preferably 35% or less, more preferably 30% or less, and even more preferably 25% or less. There is no lower limit and it may be 0%, but from the viewpoint of reducing the environmental load, it may contain 10% or more, or may contain 20% or more.

[0027] The length-weighted average fiber length of the pulp in the surface layer of the liner base paper is preferably long from the viewpoint of the strength of the liner base paper, and is preferably 0.90 mm or more, more preferably 0.95 mm or more, and even more preferably 0.98 mm or more, and from the viewpoint of reducing the environmental load, is preferably 1.20 mm or less, more preferably 1.10 mm or less, and even more preferably 1.05 mm or less. Furthermore, the length-weighted average fiber length of the pulp in all layers of the liner base paper is preferably long from the viewpoint of the strength of the liner base paper, and specifically, is preferably 0.90 mm or more, more preferably 1.00 mm or more, and even more preferably 1.05 mm or more. There is no upper limit, but from the viewpoint of reducing the environmental load, it is preferably 1.50 mm or less, more preferably 1.35 mm or less, and even more preferably 1.20 mm or less. According to the present invention, even if the fiber length distribution of the pulp in the surface layer and all layers of the liner base paper is within the above range, the occurrence of creasing can be suppressed while providing the required strength to the liner base paper. Therefore, according to the present invention, a liner base paper with low environmental impact can be provided. The length-weighted fiber length distribution of each layer and all layers of the liner base paper is measured by the following method. First, each layer or all layers of the liner base paper is defibrated to obtain defibrated pulp in accordance with JIS P 8220-1:2012. The length-weighted fiber length distribution of the obtained defibrated pulp is measured in accordance with JIS P 8226-2:2011.

[0028] (Recycled paper content) The content of corrugated waste paper in the raw pulp constituting the surface layer of the liner base paper, i.e., in the pulp constituting the surface layer of the paper base material, is preferably 55% by mass or more, more preferably 60% by mass or more, even more preferably 65% ​​by mass or more, and may be 100% by mass, from the viewpoint of obtaining pulp having the above-mentioned fiber length distribution and obtaining high-strength liner base paper. When office waste paper is used as the raw pulp that constitutes the surface layer of the liner base paper, i.e., the pulp that constitutes the surface layer of the paper base material, the content of office waste paper in the raw pulp may be 0% by mass from the viewpoint of obtaining a high-strength liner base paper, but from the viewpoint of obtaining pulp having the above-mentioned fiber length distribution, from the viewpoint of low environmental impact, and from the viewpoint of obtaining an appropriate whiteness, it is preferably 1% by mass or more, more preferably 2% by mass or more, even more preferably 3% by mass or more, and preferably 40% by mass or less, more preferably 30% by mass or less. The blending ratio of white recycled paper in the raw pulp constituting the surface layer of the liner base paper, i.e., in the pulp constituting the surface layer of the paper base material, is preferably 20% by mass or less from the viewpoints of obtaining pulp having the above-mentioned fiber length distribution, of reducing the environmental load, and of obtaining an appropriate degree of whiteness. From the viewpoint of obtaining a high-strength liner base paper, the blending ratio is more preferably 15% by mass or less, further preferably 10% by mass or less, and may be 0% by mass.

[0029] In the raw material pulp constituting the back layer of the liner base paper, the blending ratio of recycled corrugated paper is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 85% by mass or more, and there is no particular upper limit and it may be 100% by mass. Note that the back layer of the liner base paper is a layer consisting of the back layer of the paper base material and the water-soluble resin layer, and since the water-soluble resin layer does not contain raw material pulp, the raw material pulp constituting the back layer of the liner base paper is essentially synonymous with the raw material pulp constituting the back layer of the paper base material. The blending ratio of office waste paper in the raw pulp constituting the back layer of the liner base paper may be 0% by mass from the viewpoint of obtaining a high-strength liner base paper, but is preferably 40% by mass or less, more preferably 30% by mass or less, from the viewpoint of obtaining pulp having the above-mentioned fiber length distribution and from the viewpoint of low environmental load. Also, for the above-mentioned reasons, the raw pulp constituting the back layer of the liner base paper is essentially synonymous with the raw pulp constituting the back layer of the paper substrate. In terms of obtaining pulp having the above-mentioned fiber length distribution and reducing the environmental load, the blending ratio of white recycled paper in the raw material pulp constituting the back layer of the liner base paper is preferably 40% by mass or less, more preferably 30% by mass or less. Also, for the above-mentioned reasons, the raw material pulp constituting the back layer of the liner base paper is essentially synonymous with the raw material pulp constituting the back layer of the paper base material.

[0030] Furthermore, when the liner base paper has a water-soluble resin layer laminated on a paper base material having two or more paper layers including at least a surface layer and a back layer, it is preferable from the viewpoint of obtaining a liner base paper with excellent strength that the content ratio of corrugated recycled paper in the raw pulp constituting the back layer is larger than the content ratio of corrugated recycled paper in the raw pulp constituting the surface layer. When the content ratio of corrugated recycled paper in the back layer is A mass % and the content ratio of corrugated recycled paper in the surface layer is B mass %, it is preferably AB>0, more preferably AB≧1, and even more preferably AB≧2, and from the viewpoint of increasing the content ratio of corrugated recycled paper in both layers, it is preferably AB≦40, more preferably AB≦30, and even more preferably AB≦25.

[0031] (whiteness) The whiteness of the surface of the liner base paper is preferably 30% or less, more preferably 25% or less, from the viewpoint of reducing the environmental load by using a large amount of recycled raw materials, and is preferably 10% or more, more preferably 13% or more, and even more preferably 15% or more, from the viewpoint of obtaining a cardboard sheet with excellent appearance. The whiteness of the surface of the liner base paper is measured in accordance with JIS P 8148:2018.

[0032] (ash) The ash content of all layers of the liner base paper is preferably 5.0% or more, more preferably 8.0% or more, even more preferably 12.0% or more, from the viewpoint of reducing the environmental impact by using more recycled raw materials and suppressing creasing, and is preferably 23.0% or less, more preferably 20.0% or less, even more preferably 18.0% or less, even more preferably 16.5% or less, and particularly preferably 15.5% or less. The ash content of all layers of the liner paper is measured in accordance with JIS P 8251:2003.

[0033] (Thickness) The thickness of the liner base paper is not particularly limited, but taking into consideration strength, ease of handling during production and when used to make cardboard sheets, etc., it is preferably 0.10 mm or more, more preferably 0.15 mm or more, even more preferably 0.20 mm or more, and is preferably 0.50 mm or less, more preferably 0.40 mm or less, even more preferably 0.35 mm or less. The thickness of the liner base paper is measured in accordance with JIS P 8118:2014.

[0034] (Basic weight) The basis weight of the liner base paper is not particularly limited, but taking into consideration strength, ease of handling during manufacture, and ease of use in making cardboard sheets, it is preferably 100 g / m 2 More preferably, 200 g / m 2 More preferably, 250 g / m 2 and preferably 500 g / m 2 Less than 400 g / m 2 More preferably, 300 g / m or less 2 The following is the result. The basis weight of the liner base paper is measured in accordance with JIS P 8124:2011.

[0035] The basis weight of the liner base paper is the basis weight of the paper base material (g / m 2 ) and the coating weight of the water-soluble resin layer (g / m 2 Therefore, the basis weight of the paper substrate can be appropriately set in consideration of the coating weight of the water-soluble resin layer so that the basis weight of the liner base paper falls within the above range. Furthermore, when the paper base has two or more paper layers, the basis weight of each paper layer is not particularly limited, but may be appropriately selected, preferably within the range of 10% to 90% of the basis weight of the entire paper base.

[0036] [Corrugated cardboard sheet] The liner paper of the present invention is used for a corrugated cardboard sheet, and is preferably used as the outermost surface of the corrugated cardboard sheet. The corrugated cardboard sheet includes a liner paper and a corrugated core, and the liner paper is disposed on one or both sides of the core. In the present invention, the core is disposed on the back side of the liner paper. The cardboard sheet may be a single-sided cardboard sheet in which liner paper is placed on only one side of the core, or a double-sided cardboard sheet in which liner paper is placed on both sides of the core. Furthermore, the cardboard sheet may be made up of multiple layers of core and liner paper laminated together, or may be a double-sided cardboard sheet (a cardboard sheet in which the corrugated top of a single-sided cardboard sheet is glued to one side of a double-sided cardboard sheet) or a quadruple-double-sided cardboard sheet (a cardboard sheet in which the corrugated top of a single-sided cardboard sheet is glued to one side of a double-sided cardboard sheet).

[0037] The core constituting the cardboard sheet is not particularly limited, and may be one that is used in general cardboard sheets. The raw pulp for the core may be any of the raw pulps for the liner base paper listed above. The core may be a laminate of multiple layers of core base paper, and may have an adhesive layer between the core base papers.

[0038] [Method of manufacturing cardboard sheets] The manufacturing method of the cardboard sheet in which the liner base paper of the present invention can be used can be a known manufacturing method of cardboard sheet, for example, it can be manufactured by carrying out a corrugator process in which the core and the liner base paper are bonded together with an adhesive. The type of corrugated cardboard sheet is not particularly limited, and examples thereof include single-faced corrugated cardboard sheets, double-faced corrugated cardboard sheets, multi-faced corrugated cardboard sheets, and quadruple-faced corrugated cardboard sheets. The thickness of the cardboard sheet may be A-flute, B-flute, C-flute, E-flute, F-flute, G-flute, W-flute, 2-ply AA-coated, 3-ply AAA-coated, etc. The liner base paper of the present invention can be applied to cardboard (coated cardboard) in addition to cardboard sheets.

[0039] Adhesives used to bond the liner base paper to the core include synthetic resins such as starch, polyethylene resin, polypropylene resin, polyamide resin, polyester resin, ethylene / unsaturated carboxylic acid copolymer, styrene / butadiene copolymer, butadiene / acrylonitrile copolymer, styrene / butadiene / acrylonitrile copolymer, polyvinyl acetate resin, ethylene / vinyl acetate copolymer, polyacrylic acid ester copolymer, and styrene / acrylic acid ester copolymer. The adhesive may be applied to the flute top of the core or to the back layer of the liner paper, and the core and liner paper may be overlapped and bonded together by applying pressure and heat; alternatively, the adhesive may be formed as a film and placed between the core and the back layer of the liner paper, and the core and liner paper may be bonded together by applying pressure and heat. EXAMPLES

[0040] The features of the present invention will be described in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, processing contents, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below. In the examples and comparative examples, "parts" and "%" respectively refer to "parts by mass" and "% by mass" unless otherwise specified.

[0041] [Analysis and Evaluation] <Isolation of the surface, middle and back layers of liner paper> The surface layer, middle layer, and back layer of the liner base paper were obtained by immersing the liner base paper in water and peeling off each layer by hand while the liner base paper was in a swollen state with the water.

[0042] <Wet zero span tensile strength> The specific wet zero span tensile strength of the pulp obtained by disintegrating the liner base paper was measured by the following method. The liner base paper was disintegrated to obtain disintegrated pulp in accordance with JIS P 8220-1:2012. The obtained disintegrated pulp was used to prepare a pulp sheet (basis weight: 60 g / m2) in accordance with JIS P 8222:2015. 2 The resulting pulp sheet was subjected to measurement of specific wet zero span tensile strength in a rewetted state in accordance with JIS P 8227:2008.

[0043] <Length-weighted fiber length distribution and length-weighted average fiber length> The length-weighted fiber length distribution of the surface layer of the liner base paper and the length-weighted average fiber length of each layer were measured by the following method. In accordance with JIS P 8220-1:2012, the liner base paper (all layers) or each layer of the liner base paper was disintegrated to obtain disintegrated pulp. The length-weighted fiber length distribution of the obtained disintegrated pulp was measured in accordance with JIS P 8226-2:2011. The length-weighted average fiber length was calculated from the obtained results.

[0044] <Specific burst strength> The burst strength index of the liner base paper was measured and calculated in accordance with JIS P 8131: 2009. The back surface of the liner base paper was placed in contact with the rubber diaphragm.

[0045] <Specific compressive strength> The specific compressive strength of the liner base paper was measured and calculated in accordance with JIS P 8126:2005.

[0046] <Whiteness (ISO whiteness)> The whiteness (ISO whiteness) of the surface of the liner base paper was measured in accordance with JIS P 8148:2018.

[0047] <ash content> The ash content of the liner base paper was measured in accordance with JIS P 8251:2003.

[0048] <Cracked lines> The crease cracking of the liner base paper was evaluated as follows. The liner paper was laid on top of the lined underlay board so that the back side of the liner paper was the fixing surface to the underlay board (hereinafter, the laminate of the underlay board and the liner paper is called the test sample). The size of the underlay board was the same as that of the liner paper, and was 15 cm x 15 cm. The above-mentioned ruled line was a line (also called a center line) drawn so as to pass through the midpoint of one side of the underlay board and the center of the underlay board, and its length was 15 cm. The moisture content of the liner base paper in the test sample was adjusted to 7.0% or 6.0%. The test sample was then folded along the crease so that the surface layer of the liner base paper was the crest of the crease and the back layer of the liner base paper was the valley of the crease. The folded test sample was in a state where two planes including the crease had a certain angle with the crease as the boundary (in other words, the test sample was V-shaped with the crease as the boundary). Note that the test sample was in a state where it could be folded further. The test sample was then placed on a table surface such that one of the two planes of the folded test sample was parallel to the table surface. Next, tape was applied to fix three edges of one of the two flat surfaces to the table. Next, a metal roller with a mass of 13 kg and a width of 15 cm or more was placed on the roller so that the width direction of the roller (the length direction of the roller) was parallel to the crease of the test sample. Next, the roller was rolled on the table so as to pass over the test sample. The roller was passed over the entire width of the test sample while the test sample was pressed down from the crease side. In addition, only the force in the forward direction was applied to the roller while it was moving forward. The test sample, which still had room to be folded, was further folded by the operation of the roller. Thereafter, the test sample was released from its fixation, and the liner base paper was taken out. The total length (TL) of the folded liner paper and the length (BL) of the broken part of the liner paper when the liner paper was broken along the crease were measured. The ratio of the length of the broken part of the liner paper to the total length of the folded liner paper (BL / TL) x 100 [%] was calculated and evaluated according to the following criteria. A: BL / TL is between 0% and 25% B: BL / TL is more than 25% and less than 50% C: BL / TL is over 50% and below 75% D: BL / TL exceeds 75%

[0049] [Example 1] The pulp slurry for the surface layer was made from 25% office waste paper, 6% white paperboard waste paper, and 69% corrugated paper, while the pulp slurry for the middle and back layers was made from 10% white paperboard waste paper and 90% corrugated paper. The pulp slurry had a basis weight of 275 g / m. 2 (Surface layer (total ratio 25%): 68.75g / m 2 , Middle layer (total ratio 25%): 68.75g / m 2 , Back layer (total contrast 50%): 137.5g / m 2 A paper substrate having a coating weight (solid content equivalent) of 2.0 g / m was obtained by applying a two-roll size press coater to the back layer of the obtained paper substrate. 2 The starch aqueous solution was applied to the liner base paper so that the composition satisfies the following criteria:

[0050] [Example 2] A liner base paper was obtained in the same manner as in Example 1, except that the blending ratio of the surface layer raw pulp slurry was changed as shown in Table 1.

[0051] [Example 3] Instead of the starch solution, a solution of polyacrylamide (type: anion-modified PAM, viscosity of 25% aqueous solution = 1,000 to 5,000 mPa s, pH = 7.0 to 8.0) was applied at a coating amount (solid content equivalent) of 1.0 g / m 2 A liner base paper was obtained in the same manner as in Example 1, except that coating was performed so as to obtain the following:

[0052] [Examples 4 to 7] A liner base paper was obtained in the same manner as in Example 1, except that the coating amount (converted into solid content) of the starch aqueous solution was changed as shown in Table 1. The analysis and evaluation results are shown in Table 1.

[0053] [Examples 8 to 11] A liner base paper was obtained in the same manner as in Example 3, except that the coating amount (converted into solid content) of the aqueous polyacrylamide solution was changed as shown in Table 1. The analysis and evaluation results are shown in Table 1.

[0054] [Comparative Example 1] A liner base paper was obtained in the same manner as in Example 1, except that starch was coated on the surface layer of the paper base material.

[0055] [Comparative Example 2] A liner base paper was obtained in the same manner as in Example 1, except that no starch was applied.

[0056] [Table 1]

[0057] As can be seen from Table 1, the liner base papers of Examples 1 to 11 had excellent burst strength and compressive strength while suppressing the occurrence of crease cracks. Therefore, it is expected that the cardboard sheets containing the liner base papers of Examples 1 to 11 will also suppress the occurrence of crease cracks. On the other hand, the liner base paper of Comparative Example 1 had a water-soluble resin layer on the surface layer of the liner base paper, but no water-soluble resin layer on the back layer of the liner base paper, so it was rated poor in terms of creasing and was prone to creasing.The liner base paper of Comparative Example 2 had no water-soluble resin layers on the back and surface layers of the liner base paper, so it was poor in burst strength and compression strength. [Industrial Applicability]

[0058] The liner base paper of the present invention has improved strength while suppressing the occurrence of creases, and can therefore be suitably used for corrugated cardboard sheets and the like. [Explanation of symbols]

[0059] 1 Liner base paper 10 Paper base material 11 Surface layer of paper substrate 12 Middle layer of paper base material 13 Back layer of paper substrate 20 Water-soluble resin layer

Claims

1. A liner base paper having a water-soluble resin layer laminated on a paper base material, In the liner base paper, when a surface in contact with the core is defined as a back surface, the liner base paper has a water-soluble resin layer on the back surface, The paper base material has two or more paper layers including at least a front layer and a back layer, The raw pulp constituting the back layer has a blending ratio of corrugated waste paper of 85% by mass or more, When the blending ratio of the corrugated waste paper in the back layer is A mass% and the blending ratio of the corrugated waste paper in the surface layer is B mass%, A-B>0; The burst strength of the liner base paper measured in accordance with JIS P 8131:2009 is 2.00 kPa / (g / m ２ ) and The specific wet zero span tensile strength of the disintegrated pulp, measured in accordance with JIS P 8227:2008 using the liner base paper, is 85 N m / g or less. Liner base paper (excluding those having a surface coating layer on the surface of which is applied surface chemicals containing at least a sizing agent and a dye).

2. The coating amount of the water-soluble resin layer is 0.1 g / m ２ 4.0g / m or more ２ 2. The liner base paper according to claim 1, wherein:

3. 3. The liner base paper according to claim 1 or 2, wherein in a length-weighted fiber length distribution of the pulp in the surface layer of the liner base paper, measured in accordance with JIS P 8226-2:2011, the abundance ratio of fibers having a fiber length of 0.5 mm or more and less than 1.2 mm is 35% or more, and the abundance ratio of fibers having a fiber length of 1.2 mm or more is 25% or less.

4. The liner base paper according to any one of claims 1 to 3, wherein the paper substrate has one or more intermediate layers in addition to the front and back layers.

5. The liner base paper according to any one of claims 1 to 4, wherein the whiteness of the surface of the liner base paper is 10% or more and 30% or less.

6. The liner base paper according to any one of claims 1 to 5, wherein the main component of the water-soluble resin layer is starch or polyacrylamide.

7. A cardboard sheet comprising the liner paper according to any one of claims 1 to 6.

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