JP7137027B1 - Paperboard - Google Patents

Abstract

An object of the present invention is to provide a paperboard capable of improving strength in a hot and humid environment while reusing waste paper pulp. A paperboard according to an aspect of the present invention is a paperboard having 4 to 6 layers, comprising at least a surface layer, a lower surface layer, a middle layer and a back layer, wherein the surface layer is made of unbleached softwood kraft. The surface layer contains pulp, the content of the softwood unbleached kraft pulp is 90% by mass or more relative to the total pulp component of the surface layer, the surface lower layer, the middle layer, and the back layer contain corrugated waste paper pulp, and the back layer contains all the pulp components The content of recycled corrugated paper pulp is 90% by mass or more, all layers contain internal sizing agents, and the total content of internal sizing agents in all layers is 2.5 kg/ton of pulp or more in terms of solid content6 0 kg/ton of pulp or less, a total basis weight of 330 g/m 2 or more and 400 g/m 2 or less, and a bump sizing degree of the surface layer and the back surface layer at a measurement time of 2 minutes of 5 g/m 2 or more and 50 g/m 2 or less. [Selection figure] None

Description

The present invention relates to paperboard.

BACKGROUND ART In recent years, corrugated board liners of general quality are used for corrugated board sheets used for packaging containers used for transportation by ship in a hot and humid environment. Corrugated cardboard liners are lighter than those made of wood or resin, and have cushioning properties that prevent the contents from being damaged. Therefore, they are used in various fields including precision equipment.

High compressive strength is required for corrugated cardboard cases used for packing packaging containers for transporting heavy objects. In order to develop high compressive strength, it is generally necessary to manufacture base paper with a high basis weight. Compressibility cannot be satisfied. Therefore, as a multi-layered paperboard used for packaging containers, for example, Patent Document 1 discloses a paperboard with a high basis weight of 447 g/m ² and 6 layers (see Patent Document 1).

Japanese Patent Application Laid-Open No. 2020-020065

However, in order to use it for transportation of heavy objects used in a hot and humid environment, it is required to improve the compressive strength during transportation even in a hot and humid environment while maintaining a low basis weight.

In recent years, paperboards are also required to be made of materials that are friendly to the recycling of waste paper and the environment. In particular, in recent years, from the standpoint of conservation of wood resources and economic efficiency, the use of waste paper as raw material pulp has been oriented.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a paperboard that can improve the strength in a hot and humid environment while promoting the reuse of waste paper pulp.

A paperboard according to an aspect of the present invention is a paperboard having 4 to 6 layers, comprising at least a surface layer, a lower surface layer, a middle layer and a back layer, wherein the surface layer contains unbleached softwood kraft pulp. The content of the unbleached softwood kraft pulp relative to the total pulp component of the surface layer is 90% by mass or more, the lower surface layer, the intermediate layer and the back layer contain recycled corrugated paper pulp, and the entire back layer The content of the corrugated waste paper pulp to the pulp component is 90% by mass or more, the internal sizing agent is contained in all layers, and the total content of the internal sizing agent in all layers is 2.5 kg / The pulp is t or more and 6.0 kg/pulp t or less, the total basis weight is 330 g/m ² or more and 400 g/m ² or less, and the surface layer and the back surface layer have a bump sizing degree of 5 g/m ² at a measurement time of 2 minutes. 50 g/m ² or less, and the difference in hump sizing degree between the surface layer and the back layer is 10 g/m ² or less.

ADVANTAGE OF THE INVENTION According to this invention, the paperboard which can improve the intensity|strength under a hot and humid environment can be provided, reusing waste paper pulp.

[Description of the embodiment of the present invention]
A paperboard according to an aspect of the present invention is a paperboard having 4 to 6 layers, comprising at least a surface layer, a lower surface layer, a middle layer and a back layer, wherein the surface layer contains unbleached softwood kraft pulp. The content of the unbleached softwood kraft pulp relative to the total pulp component of the surface layer is 90% by mass or more, the lower surface layer, the intermediate layer and the back layer contain recycled corrugated paper pulp, and the entire back layer The content of the corrugated waste paper pulp to the pulp component is 90% by mass or more, the internal sizing agent is contained in all layers, and the total content of the internal sizing agent in all layers is 2.5 kg / The pulp is t or more and 6.0 kg/pulp t or less, the total basis weight is 330 g/m ² or more and 400 g/m ² or less, and the surface layer and the back surface layer have a bump sizing degree of 5 g/m ² at a measurement time of 2 minutes. 50 g/m ² or less, and the difference in hump sizing degree between the surface layer and the back layer is 10 g/m ² or less.

The paperboard is a paperboard having 4 or more layers and 6 or less layers, and by providing at least a surface layer, a front and lower layer, a middle layer and a back layer, it is possible to improve compressive strength and suitability for box making and suppress delamination. . In addition, the surface layer exposed as a packing material contains unbleached softwood kraft pulp, and the content of the unbleached softwood kraft pulp with respect to the total pulp component of the surface layer is 90% by mass or more, so that the high-temperature and high-humidity environment It is possible to improve the strength in the bottom and to obtain high folding endurance, so it is excellent in the effect of suppressing creases. In addition, since the surface and bottom layers, the middle layer and the back layer contain high sizing corrugated waste paper pulp, waste paper pulp can be reused without increasing the content of the internal sizing agent. sizing can be given.
In order to maintain the paperboard quality in a hot and humid environment, it is necessary to suppress the increase in humidity in the paperboard. In the paperboard, all layers contain an internal sizing agent in order to impart sizing properties to the surface layer and the back layer. It is possible to effectively impart sizing properties while suppressing the amount of the internal sizing agent added. Since the paperboard has good sizing properties, it can maintain its quality even when transported by ship or in a hot and humid environment. Further, when the basis weight of the paperboard is 330 g/m ² or more and 400 g/m ² or less, it is possible to achieve both the compression strength of the paperboard and weight reduction in transporting heavy objects. Furthermore, since the bump sizing degree of the surface layer and the back layer is 5 g/m ² or more and 50 g/m ² or less at a measurement time of 2 minutes, the moisture absorption of the paperboard can be suppressed in a hot and humid environment. Quality can be maintained. When the difference in the Cobb sizing degree between the surface layer and the back layer is 10 g/m ² or less, it is possible to suppress distortion due to the difference in moisture absorption between the surface layer and the back layer in a hot and humid environment. It is possible to improve the bursting strength in a humid environment.

At least the surface layer and the lower surface layer contain a dry paper strength agent, and the content of the dry paper strength agent in the lower surface layer is higher than the content of the dry paper strength agent in the surface layer. preferable. When at least the surface layer and the undersurface layer contain the dry paper strength agent, the bonding between the fibers of the pulp on the surface side of the paperboard is strengthened, so that the strength in a hot and humid environment can be further improved. In addition, since the content of the dry paper strength agent in the lower surface layer is greater than the content of the dry paper strength agent in the surface layer, the pulp fibers that are exhausted by the recycling process of waste paper and are relatively flat Since the strength of the surface lower layer containing corrugated waste paper pulp can be increased, the strength in a hot and humid environment can be further improved.

The content of the corrugated waste paper pulp is 90% by mass or more with respect to the total pulp components of the surface lower layer and the middle layer, the density is 0.75 g/cm ³ or more and 0.95 g/cm ³ or less, and the bursting strength is It is preferably 950 kPa or more and 1100 kPa or less. By setting the content of the corrugated waste paper pulp to the total pulp components of the lower surface layer and the middle layer to be 90% by mass or more, the sizing of the paperboard is further improved, and the recycling of the waste paper pulp is promoted. can be done. In addition, when the density of the paperboard is within the above range, cushioning properties can be imparted, and high compressive strength can be obtained without adding an excessive amount of dry paper strength enhancer. Furthermore, the bursting strength is 950 kPa or more and 1100 kPa or less, so that the paperboard can improve impact resistance in a hot and humid environment.

The paperboard is preferably used for transporting heavy loads. Since the paperboard can improve the strength in a hot and humid environment, it is suitable for use as a corrugated cardboard sheet used for packaging containers for Southeast Asia, and in various fields such as the distribution field in a hot and humid environment It can be used as industrial paper.

[Details of the embodiment of the present invention]
The present invention will be described in detail below. Although the constituent elements described below may be described based on representative embodiments and specific examples, the present invention is not limited to such embodiments.

<Paperboard>
A paperboard according to an embodiment of the present invention comprises at least a surface layer, a lower surface layer, an intermediate layer and a back layer in this order. The paperboard may also comprise a single layer or multiple layers between the middle layer and the back layer. That is, the number of layers of the paperboard is a multi-layered paper having 4 to 6 layers, and in the case of a multi-layered paper having 5 or more layers, a second middle layer between the middle layer and the back layer and a configuration including a second back layer between the intermediate layer and the back layer. Thus, by being a multi-layered paper having 4 to 6 layers, the paperboard can be improved in compressive strength and suitability for box making, and can suppress delamination. On the other hand, the upper limit of the number of layers of the paperboard is 6 layers. When the number of layers of the paperboard is 7 or more, delamination tends to occur.

[Surface layer]
The surface layer contains softwood unbleached kraft pulp (NUKP).

The lower limit of the content of the unbleached softwood kraft pulp in the surface layer is 90% by mass, preferably 95% by mass. The content of unbleached softwood kraft pulp in the surface layer positioned on the outermost surface side as a packing material for corrugated cardboard is set to 90% by mass or more, and by increasing the content ratio, it is possible to improve the strength in a hot and humid environment. At the same time, since high folding endurance can be obtained, the effect of suppressing creases is excellent. If the content of unbleached softwood kraft pulp in the surface layer is less than 90%, it may be difficult to ensure the desired compressive strength. In addition, if the content of unbleached softwood kraft pulp in the surface layer is less than 80%, the content of long fibers is reduced, so the folding endurance of the surface layer is reduced, and creases are likely to occur. From the viewpoint of recyclability, the surface layer preferably contains less than 10% waste paper pulp and is used within a range that does not reduce the compressive strength.

[Surface layer]
The bottom layer contains corrugated waste paper pulp. By containing corrugated waste paper pulp with high sizing properties in the surface lower layer, it is possible to effectively impart sizing properties without increasing the content of the internal sizing agent while reusing the waste paper pulp. .

The lower limit of the content of the used corrugated paper pulp in the lower surface layer is preferably 80% by mass, more preferably 85% by mass, and more preferably 90% by mass. The upper limit of the content of the used corrugated paper pulp may be 100% by mass. By setting the content of the used corrugated paper pulp in the surface and lower layer to the above range, the paperboard can improve the sizing property and promote the reuse of the used paper pulp.

[middle class]
The middle layer contains corrugated waste paper pulp. By containing corrugated waste paper pulp with high sizing properties in the middle layer, it is possible to effectively impart sizing properties without increasing the content of the internal sizing agent while reusing the waste paper pulp.

The lower limit of the content of the used corrugated paper pulp in the middle layer is preferably 80% by mass, more preferably 85% by mass, and more preferably 90% by mass. The upper limit of the content of the used corrugated paper pulp may be 100% by mass. By setting the content of the corrugated waste paper pulp in the middle layer to the above range, the paperboard can improve the sizing property and promote the reuse of the waste paper pulp.

[Back layer]
The back layer contains corrugated waste paper pulp.

The lower limit of the content of the used corrugated paper pulp in the back layer is 90% by mass, preferably 95% by mass. The upper limit of the content of the used corrugated paper pulp may be 100% by mass. By setting the content of the corrugated paperboard waste paper pulp as the raw material pulp of the back layer located on the most back side as a packing material to the above range, the paperboard improves the strength in a hot and humid environment while recycling the waste paper pulp. Promotion of utilization can be enhanced.

In addition to the above soft tree kraft pulp, the above corrugated waste paper pulp and the above kraft waste paper pulp, each layer of the paperboard contains other pulp such as waste magazine pulp, waste tea paper, waste kraft envelope paper, waste newspaper paper, waste leaflet paper, waste office paper, De-inked waste paper pulp, de-inked/de-inked waste paper pulp, de-inked/bleached waste paper pulp, etc., which are produced from white waste paper, Kent waste paper, structural waste paper, and certificate waste paper, etc., can be used. Mechanical pulp such as ground wood pulp (GP), thermomechanical pulp (TMP), and bleached chemithermomechanical pulp (BCTMP) may also be used as necessary.

[Additive]
(Internal sizing agent)
The paperboard contains an internal size in all layers. By containing the internal sizing agent in all the layers of the paperboard, it is possible to increase the resistance to humidity in the paperboard under a hot and humid environment, and to maintain the quality of the paperboard. Examples of the internal sizing agents include styrene internal sizing agents, alkyl ketene dimers (AKD), alkenyl succinic anhydrides (ASA), neutral rosin internal sizing agents, and rosin internal sizing agents. Among these, a neutral rosin internal sizing agent is preferred.

The rosin internal sizing agent is not particularly limited, and those known in the papermaking field can be used. Rosin-based substances include, for example, reinforced rosins obtained by modifying rosins such as gum rosin, wood rosin and tall oil rosin with α,β-unsaturated carboxylic acids such as fumaric acid, maleic acid and acrylic acid or their anhydrides; Rosin esters obtained by reacting rosins with polyhydric alcohols such as glycerin, trimethylolethane, trimethylolpropane, pentaerythritol and diglycerin can be mentioned. In the present invention, the rosin-based internal sizing agent also includes emulsified products obtained by emulsifying these alone or a mixture thereof, and those emulsified alone and then mixed. Furthermore, the emulsified product to which various polymers are added in order to further improve the sizing property is also included.

The lower limit of the total content of the internal sizing agent in all layers is 2.50 kg/t of pulp, preferably 2.55 kg/t of pulp in terms of solid content. The upper limit of the total content of the internal sizing agent in all layers is 6.00 kg/t of pulp in terms of solid content, preferably 5.85 kg/t of pulp. By setting the amount of the internal sizing agent to be within the above range, the paperboard can be imparted with sufficient water resistance, and the durability against humidity in the paperboard can be enhanced in a hot and humid environment. In addition, "kg/pulp t" represents the addition amount (kg) with respect to 1 ton of pulp. Here, the total content of the internal sizing agent in all layers refers to a value obtained by dividing the total amount of internal sizing agent added in all layers by the total amount of pulp used in all layers.

(Dry paper strength agent)
In the paperboard, at least the surface layer and the lower surface layer contain a dry paper strength agent, and the content of the dry paper strength agent in the lower surface layer is equal to the content of the dry paper strength agent in the surface layer. preferably more than When at least the surface layer and the undersurface layer contain the dry paper strength agent, the bonding between the fibers of the pulp on the surface side of the paperboard is strengthened, so that the strength in a hot and humid environment can be further improved. In addition, since the content of the dry paper strength agent in the lower surface layer is greater than the content of the dry paper strength agent in the surface layer, the pulp fibers that are exhausted by the recycling process of waste paper and are relatively flat Since the strength of the surface lower layer containing corrugated waste paper pulp can be increased, the strength in a hot and humid environment can be further improved.

As the dry paper strength enhancer, an acrylamide-based dry paper strength enhancer is preferred. Amphoteric polyacrylamide resin-based dry paper strength agents are more preferred. Amphoteric polyacrylamide is obtained by polymerizing a cationic monomer or an anionic monomer with acrylamide, and is directly fixed to pulp fibers via its own cationic group. In addition, amphoteric polyacrylamide resin-based dry paper strength agents are characterized by their ability to control the molecular structure and molecular weight.

The lower limit of the amount of dry paper strength enhancer added to the entire paperboard is preferably 2.70 kg/t of pulp, more preferably 2.75 kg/t of pulp in terms of solid content. The upper limit of the amount of the dry paper strength enhancer added to the entire paperboard is preferably 3.90 kg/t of pulp, more preferably 3.85 kg/t of pulp in terms of solid content. By setting the addition amount of the dry paper strength enhancer to the entire paperboard within the above range, the strength in a hot and humid environment can be improved while ensuring the flexibility of the paperboard.

The lower limit of the amount of dry paper strength enhancer added to the surface layer is preferably 0.8 kg/ton of pulp in terms of solid content. Moreover, the upper limit of the amount of the dry paper strength enhancer added to the surface layer is preferably 2.0 kg/ton of pulp in terms of solid content. Although the compressive strength can be improved by including the dry paper strength agent, the inclusion of the dry paper strength agent may impair the flexibility of the paper layer. The paperboard has a large amount of softwood unbleached kraft pulp added in the surface layer, which contributes to improving the compressive strength. For this reason, by setting the amount of the dry paper strength enhancer to the surface layer within the above range, it is possible to suppress breakage of the surface layer such as creases while ensuring flexibility, and further improve strength in a hot and humid environment. can. Furthermore, when the addition ratio of the dry paper strength agent to the surface layer exceeds the above upper limit, the freeness is lowered, and the raw material pulp may be easily discharged during the pressing and dewatering steps of the press part of the paper machine. In addition, in the press part, scum of the raw material pulp may adhere to the doctor blade used when removing the wet paper stuck to the roll, and the quality of the surface layer may be deteriorated.

The lower limit of the amount of dry paper strength enhancer added to the lower surface layer is preferably 2.0 kg/ton of pulp in terms of solid content. The upper limit of the amount of the dry paper strength enhancer added to the lower surface layer is preferably 7.0 kg/ton of pulp in terms of solid content. By setting the amount of the dry paper strength enhancer to the lower surface layer within the above range, the strength of the lower surface layer containing corrugated waste paper pulp in a hot and humid environment is further improved while suppressing the decrease in flexibility due to the dry paper strength enhancer. can.

The lower limit of the amount of the dry paper strength agent added to the middle layer is preferably 1.0 kg/ton of pulp in terms of solid content. Moreover, the upper limit of the amount of the dry paper strength agent added to the middle layer is preferably 6.0 kg/t of pulp in terms of solid content. By setting the amount of the dry paper strength enhancer in the middle layer within the above range, the strength of the middle layer containing corrugated waste paper pulp in a hot and humid environment can be further improved while suppressing the decrease in flexibility due to the dry paper strength enhancer.

The lower limit of the amount of dry paper strength agent added to the back layer is preferably 2.0 kg/ton of pulp in terms of solid content. The upper limit of the amount of the dry paper strength enhancer added to the back layer is preferably 8.0 kg/ton of pulp in terms of solid content. By setting the amount of the dry paper strength enhancer in the back layer within the above range, the strength of the back layer containing corrugated waste paper pulp in a hot and humid environment can be increased while suppressing the decrease in flexibility due to the dry paper strength enhancer. can improve.

(Other additives)
Each layer of the paperboard, such as the surface layer, the lower surface layer, the middle layer, the back layer, etc., may contain various additives for papermaking within a range that does not impair the intended effects of the present invention. Examples of various papermaking additives include chemical fixing agents such as flocculants (silica), retention aids such as polyamides, water resistance agents such as polyamines and epichlorohydrin, chemical fixing agents such as aluminum sulfate, antifoaming agents, clay, fillers such as talc and calcium carbonate; known additives such as basic dyes, acid dyes, anionic direct dyes and cationic direct dyes; They can be added in combination.

[Coating layer]
The paperboard may have a coating layer formed on the surfaces of the surface layer and the back layer, and the coating layer may contain starch and an anionic externally added sizing agent. A coating layer is formed on the surfaces of the surface layer and the back layer, and the coating layer contains starch and an anionic externally added sizing agent, thereby filling between the pulp fibers of the surface layer in contact with the coating layer. Therefore, the printability can be improved and the smoothness of the printing surface can be ensured.

Examples of starch include corn starch, potato starch, tapioca starch, wheat starch, rice starch, and various modified starches such as oxidized starch and phosphate esterified starch made from these raw materials, but are limited to these. is not. These starches may be used alone or in combination of two or more.

Examples of anionic external sizing agents include rosin soap external sizing agents, rosin emulsion external sizing agents, special modified rosin external sizing agents, alkenyl succinic acid, alkenyl succinic anhydride, anionic alkyl ketene dimers, olefin resins, Styrene-based resins, styrene-acrylic resins, styrene-maleic acid-acrylic resins, wax-based external addition sizing agents, and the like can be mentioned.

The mass ratio of the starch to the anionic externally added sizing agent in the coating layer is preferably 90.9:9.1 to 96.8:3.2, preferably 93.0:7.0 to 95.0:7.0 in terms of solid content. 0:5.0 is more preferred. When the mass ratio of the starch and the anionic externally added sizing agent is within the above range, the surface layer side of the paperboard can be improved in printability and a smooth printed surface can be ensured. Furthermore, since the rigidity of the coating layer can be improved, the compressive strength can be improved on the back layer side of the paperboard.

The coating layer contains the above starch and anionic external sizing agent, anti-slip agent, antiseptic agent, anti-rust agent, defoaming agent, viscosity modifier, dispersant, flow modifier, dye, water resistance agent, water retention agent, etc. may contain various auxiliary agents.

The lower limit of the coating amount of the coating layer is preferably 1.50 g/m ² and more preferably 1.70 g/m ² in terms of solid content. On the other hand, the upper limit of the coating amount is preferably 2.25 g/m ² , more preferably 2.17 g/m ² . When the coating amount of the coating layer is within the above range, it is possible to seal between the pulp fibers of the surface layer that is in contact with the coating layer on the surface layer side of the paperboard, so that the printability can be improved. can.

[Characteristics of paperboard]
(basis weight)
Basis weight is measured according to JIS-P8124 (2011). In a further preferred form of the invention, the total basis weight of the paperboard is 330 g/m ² or more and 400 g/m ² or less, more preferably 340 g/m ² or more and 380 g/m ² or less. If the total basis weight of the paperboard is less than 330 g/m ² , the compressive strength may be insufficient. If the total basis weight of the paperboard exceeds 400 g/m ² , it may become difficult to reduce the weight of the paperboard when transporting heavy objects. When the basis weight of the paperboard is 330 g/m ² or more and 400 g/m ² or less, it is possible to achieve both the compressive strength of the paperboard and weight reduction in transporting heavy objects.

The basis weight of the surface layer constituting the paperboard is preferably 30.0 g/m ² to 80.0 g/m ² , more preferably 40.0 g/m ² to 50.0 g/m ² . . The basis weight of the surface lower layer is preferably 50.0 g/m ² to 90.0 g/m ² , more preferably 60.0 g/m ² to 80.0 g/m ² . The basis weight of the middle layer is preferably 50.0 g/m ² to 90.0 g/m ² , more preferably 60.0 g/m ² to 80.0 g/m ² . The basis weight of the back layer is preferably 140.0 g/m ² to 190.0 g/m ² , more preferably 150.0 g/m ² to 180.0 g/m ² . In the paperboard of the present invention, by keeping the overall basis weight within the above range, it is possible to improve the moldability during box making, and even in a hot and humid environment, when the contents are filled after molding, Damage to the packaging container can be suppressed.

(density)
The lower limit of the density of the paperboard is preferably 0.75 g/cm ³ , more preferably 0.78 g/cm ³ . The upper limit of the density is preferably 0.95 g/cm ³ , more preferably 0.85 g/cm ³ . When the density of the paperboard is within the above range, it is possible to impart cushioning properties and exhibit high compressive strength without adding an excessive amount of dry paper strength enhancer.

(paper thickness)
The paper thickness is measured according to “Paper and paperboard—Testing methods for thickness and density” described in JIS-P8118 (2014). The total paper thickness of the paperboard of the present invention is preferably from 300 μm to 550 μm, more preferably from 350 μm to 500 μm.

(Cobb size degree)
The Cobb size degree is measured in accordance with JIS-P8140 (1998) with a measurement time of 2 minutes. The lower limit of the Cobb sizing degree of the paperboard is 5 g/m ² , preferably 30 g/m ² . If the Cobb size degree of the paperboard is less than the above lower limit, the humidity in the paperboard may not be sufficiently suppressed in a hot and humid environment, and lamination failure may occur during processing into paper containers and the like. On the other hand, the upper limit of the Cobb sizing degree of the paperboard is preferably 50 g/m ² , more preferably 40 g/m ² . If the bump sizing degree of the paperboard is equal to or higher than the above upper limit, the paperboard may absorb moisture and cause waviness or wrinkles. When the Cobb sizing degree of the surface layer and the back layer at a measurement time of 2 minutes is within the above range, moisture absorption of the paperboard can be suppressed in a hot and humid environment, and the quality of the paperboard can be maintained.

The upper limit of the difference in bump sizing degree between the surface layer and the back layer is 10 g/m ² , preferably 7 g/m ² . On the other hand, the lower limit of the difference in Cobb sizing degree is preferably 1 g/m ² . If the difference in bump sizing exceeds 10 g/m ² , it becomes difficult to suppress distortion due to the difference in moisture absorption between the surface layer and the back layer in a hot and humid environment, and the bursting strength of the corrugated cardboard sheet may decrease in a hot and humid environment. If the difference in the Cobb sizing degree is less than 1 g/m ² , the Cobb sizing degree will be excessively adjusted, which may increase operational fluctuations and increase the cost required for quality control.

(Compressive strength in width direction)
The compressive strength in the width direction is measured according to JIS-P8126 (2005). As a paperboard for transporting heavy loads, the compressive strength in the width direction perpendicular to the papermaking direction of the paperboard is 680 N or more, so that the strength suitable for use as a paperboard for transporting heavy loads can be maintained. On the other hand, when the compressive strength is 850 N or less, the processing suitability of the paperboard is excellent.

(Burst strength)
The bursting strength was measured according to JIS-P8131 (2009) paper and paperboard bursting strength test method using a Mullen high-pressure tester.

(Rupture Index)
The rupture index is specified in JIS-P8131 (2009). The rupture index is a value obtained by dividing the rupture strength of paper expressed in kilopascals (kPa) as measured according to JIS-P8131 (2009) by the basis weight. The lower limit of the rupture index of the paperboard in an atmosphere of 60° C. and 80% RH is preferably 2.60 kPa·m ² /g, more preferably 2.70 kPa·m ² /g. When the rupture index is within the above range, the paperboard can have improved impact resistance in a hot and humid environment. The above "60° C., 80% RH" is measured according to JIS-P8111 (1998).

The paperboard is preferably used for transporting heavy loads. Since the paperboard has excellent strength in a hot and humid environment, it is suitable for use as a corrugated cardboard sheet used for packaging containers for Southeast Asia. It can be used as paper.

[Paperboard manufacturing method]
The method for producing the paperboard is not particularly limited, but includes, for example, a beating step, a papermaking step, a coating step, and a drying step.

(Beating process)
In the beating step, the raw material pulp constituting each layer is beaten to a desired freeness. The raw material pulp includes soft tree kraft pulp, corrugated waste paper pulp, and the like. First, a slurry obtained by dispersing pulp fibers in water is mixed with an additive corresponding to each paper layer as necessary to prepare a stock for each paper layer.

(Papermaking process)
Next, these raw material slurries are used to make paper with a paper machine in a neutral range so that the pH of the paperboard is 6 or more and 8 or less. In the papermaking process, the raw material pulp is made into paper using a papermaking machine that makes paper from the pulp raw material jetted onto the running wire. The paper machine is not particularly limited, and known paper machines such as fourdrinier paper machines, cylinder paper machines, hybrid formers, gap formers, etc. can be used. Paper is made with the following multilayers.

(Coating process)
A step of applying a coating liquid to the surface layer and the back layer can be provided. A known coating machine can be used in the coating step.

(Drying process)
The paper is then dried in a dryer cylinder.

Finally, it is wound on a reel to obtain a paperboard.

According to the paperboard, it is possible to improve the strength in a hot and humid environment while reusing the waste paper pulp.

<Other embodiments>
The present invention is not limited to the above-described embodiments, and can be implemented in various modified and improved modes in addition to the above-described modes.

For example, the paperboard may comprise a single or multiple intermediate layers between the intermediate layer and the back layer.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the present invention is not limited to the following examples.

[Example 1]
Raw materials shown in Tables 1 and 2 were added to the pulp slurry. The freeness of the unbleached softwood pulp and the corrugated waste paper pulp in the surface layer is obtained by disintegrating the pulp in accordance with the pulp disintegration method of JIS-P8220-1 (2012). 2 (2012) Canadian standard freeness test method.
(Additive)
(1) Rosin-based internal sizing agent “NS-77P” from Harima Kasei Co., Ltd. Special rosin internal sizing agent (solid content: 50%)
(2) Acrylamide-based dry paper strength agent Harima Kasei Co., Ltd. “Hermid RB-550”: amphoteric polyacrylamide (solid content 20%)

[Examples 2 to 18, Comparative Examples 1 to 6 and Reference Example (commercially available)]
Paperboards of Examples 2 to 18 and Comparative Examples 1 to 6 were prepared in the same manner as in Example 1, except that the types, contents, and physical properties of the raw materials were as shown in Tables 1 and 2. Obtained. Also, as a reference example, a commercial product with a basis weight of 400 g/m ² was evaluated.

[evaluation]
Each paperboard obtained was evaluated by the following method. Table 3 shows the evaluation results.
(1) Basis weight (g/m ² )
It was measured in accordance with "Paper and paperboard- Basis weight measurement method" described in JIS-P8142 (1998).
(2) Paper thickness (μm)
It was measured in accordance with "Paper and paperboard-Testing methods for thickness and density" described in JIS-P8118 (2014).
(3) Density (g/cm ³ )
It was measured in accordance with "Paper and paperboard-Testing methods for thickness and density" described in JIS-P8118 (2014).
(4) Compressive Strength in the Width Direction The compressive strength in the width direction under normal temperature and humidity (25° C., 65% RH) was measured according to JIS-P8126 (2005).
(5) Cobb size degree Measured according to JIS-P8140 (1998). Incidentally, the Cobb size degree referred to in the present invention was measured by contacting the surface of the surface layer or the back surface layer with water for 2 minutes.
(6) Bursting strength and bursting index Bursting strength and bursting index in an atmosphere of normal temperature and humidity (25 ° C., 65% RH) are measured according to JIS-P8131 (2009), the surface in contact with the rubber diaphragm , and the front and back surfaces were each measured 10 times (average value). The bursting index was calculated by dividing the bursting strength by the basis weight.
(7) Folding Endurance Folding endurance was measured in the vertical direction according to "Paper and paperboard--Testing method for folding endurance--MIT tester method" described in JIS-P8115 (2001). When the vertical folding endurance of the paperboard is 490 times or more, creases can be suppressed.
(8) Strength in a hot and humid environment (bursting strength in a hot and humid environment and burst strength residual rate in a hot and humid environment)
First, the obtained paperboard is left in a dryer at 105° C. for 2 hours, and then immersed in water for 30 minutes with a weight placed on the test piece in a container containing distilled water. was turned upside down and immersed in water for 30 minutes with a weight placed thereon. Thereafter, the water on the paperboard was wiped off with a cloth, and the bursting strength was measured according to JIS-P8131 (2009). Then, the bursting strength under the above test conditions was divided by the bursting strength under the atmosphere of normal temperature and humidity (23° C., 50% RH) to obtain the residual burst strength [%] under the high temperature and high humidity environment. If the burst strength residual rate [%] of the paperboard under the hot and humid environment is less than 20%, sufficient impact resistance under the hot and humid environment cannot be obtained, and the box is likely to collapse during transportation. Moreover, when the burst strength residual rate [%] in the above-mentioned high-temperature and high-humidity environment is 38% or more, the solubility in water decreases, resulting in poor recyclability.

As shown in Table 3, the content of the unbleached softwood kraft pulp relative to the total pulp component of the surface layer is 90% by mass or more, and the lower surface layer, the middle layer, and the back layer contain corrugated waste paper pulp. The content of the corrugated waste paper pulp to the total pulp component of the back layer is 90% by mass or more, all the layers contain an internal sizing agent, and the total content of the internal sizing agent in all the layers is solid It is 2.50 kg/ton or more and 6.00 kg/ton or less of pulp in terms of minutes, the total basis weight is 330 g/m ² or more and 400 g/m ² or less, and the surface layer and the back layer have a bump sizing degree of 5 g/ Examples 1 to 18 in which m ² or more and 50 g/m ² or less and the difference in hump sizing degree between the surface layer and the back surface layer is 10 g/m ² or less have a compressive strength in the width direction of 680 N or more. , and it can be seen that the strength suitable for use as paperboard for transporting heavy objects can be maintained. In addition, all of the burst strength in a hot and humid environment, the burst strength residual ratio in a hot and humid environment, the folding endurance, and the folding endurance were good.

On the other hand, in Comparative Examples 1, 2, and 6, in which the total basis weight is less than 330 g/m ² , the total basis weight is as low as less than 330 g/m ² , and therefore the compressive strength in the width direction is low, and it is difficult to transport heavy objects. It was not suitable as paperboard for paper. In Comparative Example 3, the content of softwood kraft pulp relative to the total pulp component of the surface layer was less than 90% by mass, so the compressive strength and folding endurance in the width direction were low, making it unsuitable as a paperboard for transporting heavy loads. rice field. Comparative Example 4, in which the total content of the internal sizing agent is 2.50 kg/ton of pulp or less and the difference in the hump sizing degree between the surface layer and the back surface layer is 10 g/m ² or more, has high burst strength under high temperature and high humidity. Survival rate decreased. Comparative Example 5, in which the total content of internal sizing agents is 2.50 kg/ton of pulp or less and the hump sizing degree of the surface layer and back layer is 50 g/m ² or more, has a low burst strength residual rate under high temperature and high humidity. did.
In addition, although the commercial product of the reference example has a difference of 10 g/m ² or more in the degree of bump sizing between the surface layer and the back layer, the total basis weight is as high as 410 g/m ² , so it has excellent compressive strength and high temperature and humidity. Bursting strength residual rate increased. However, since the total basis weight is as high as 410 g/m ² , it becomes difficult to achieve weight reduction in transporting heavy objects.

As a result, it was shown that the strength in a hot and humid environment can be improved while reusing the waste paper pulp. The paperboard can be used as industrial paper in various fields such as a distribution field in a hot and humid environment as a paperboard for transporting heavy objects.

Claims (4)

A paperboard having 4 to 6 layers,
At least a surface layer, a lower surface layer, a middle layer and a back layer,
The surface layer contains unbleached softwood kraft pulp,
The content of the softwood unbleached kraft pulp with respect to the total pulp component of the surface layer is 90% by mass or more,
The surface and bottom layer, the middle layer and the back layer contain corrugated waste paper pulp,
The content of the corrugated waste paper pulp to the total pulp component of the back layer is 90% by mass or more,
all layers contain an internal sizing agent,
The total content of the internal sizing agent in all the layers is 2.50 kg/t or more and 6.00 kg/t or less of pulp in terms of solid content,
a total basis weight of 330 g/m ² or more and 400 g/m ² or less;
The surface layer and the back surface layer have a bump sizing degree of 5 g/m ² or more and 50 g/m ² or less at a measurement time of 2 minutes,
A paperboard in which the difference in the Cobb sizing degree between the surface layer and the back surface layer is 10 g/m ² or less. At least the surface layer and the undersurface layer contain a dry paper strength agent,
2. The paperboard according to claim 1, wherein the content of said dry strength agent in said lower surface layer is higher than the content of said dry strength agent in said surface layer. The content of the corrugated waste paper pulp is 90% by mass or more with respect to the total pulp components of the lower surface layer and the middle layer,
Density is 0.75 g/cm ³ or more and 0.95 g/cm ³ or less,
3. The paperboard according to claim 1, wherein the bursting strength is 950 kPa or more and 1100 kPa or less. The paperboard according to any one of claims 1 to 3, which is used for transporting heavy objects.