JP2025083378A - Laminated paper and paper container for liquids employing the same - Google Patents

Abstract

To provide: laminated paper that has excellent printability and that is capable of being molded into a paper container for liquids which is unlikely to be damaged by an impact resulting from dropping or the like that may occur during transport; and a paper container for liquids employing the same.SOLUTION: The laminated paper comprises a paper base material and a thermoplastic resin layer that is laminated on at least one of the surfaces of the paper base material. The paper base material has a multilayer structure having three or more pulp layers having cellulose pulp as the principal component. The CD-direction ring crush compressive strength R measured by a ring crush method defined by JIS P 8126:2005 is 3.0-6.0 kN/m, and the interlayer strength I is 100-250 J/m2, where R/I is 0.020-0.060 kN/J.SELECTED DRAWING: None

Description

The present invention relates to laminated paper and liquid paper containers using the same.

It is common to manufacture paper containers for liquids, such as milk cartons, using laminated paper, which is made by laminating a paper base material to give it water resistance. Impact resistance is important for many containers, but for paper containers for liquids, the damage caused by leakage of the contents is particularly great. Most paper containers for liquids are formed by folding and gluing a single sheet of laminated paper, which means that fold cracks are likely to occur on the sides and corners, and leakage of the contents from these fold cracks can be a problem.

Various technologies have been developed to prevent leakage of liquid from such paper containers. For example, Patent Document 1 discloses a base paper for liquid containers that is made up of three paper layers, an inner layer and an outer layer provided on both sides of the inner layer, with a weight ratio of outer layer:inner layer:outer layer of 1:2:1 to 1:3:1, and an internal bond strength of 0.20 to 0.36 N.m for the outer layer and 0.05 to 0.28 N.m for the inner layer.

JP 2009-242999 A

However, the base paper for liquid containers disclosed in Patent Document 1 only addresses cracks caused by folding during molding by specifying the weight ratio of each layer of the paper base material and the internal bond strength. For example, when a paper container for liquids is actually used, if it is dropped during transportation, the impact of the drop is likely to be received at a single corner. Corners are already prone to cracks, so when a strong impact is applied, there are many accidents in which the corner breaks when dropped. However, the invention in Patent Document 1 does not take into account cracks caused by impacts such as dropping, and no drop tests are conducted.

In addition, paper containers for liquids, such as milk cartons, are generally not blank, but often have designs or the like to indicate the source or to increase purchasing interest, but the base paper for liquid containers described in Patent Document 1 is considered to have insufficient printability.

The present invention has been made in consideration of the above-mentioned circumstances. In other words, the object of the present invention is to provide a laminated paper capable of forming a paper container for liquid that has excellent printability and is resistant to damage even when subjected to shocks such as dropping that may occur during transportation, and a paper container for liquid that uses the same.

The inventors have found that when the ring crush (RC) compressive strength and interlayer strength values of laminated paper specified in JIS P 8126:2015 are within a specified range, and the RC compressive strength/interlayer strength value is also within a specified range, the impact of a fall or the like can be dispersed within the liquid paper container, preventing damage to the liquid paper container. They have also found that such laminated paper has excellent printability.

The present invention has been completed based on these findings. That is, the present invention has the following configuration.
Hereinafter, the direction in which the pulp slurry flows out during the formation of a paper layer in papermaking of a paper base material (longitudinal direction) is also referred to as the MD direction, and the direction perpendicular to the MD direction (transverse direction) is also referred to as the CD direction.

(1) A laminated paper having a paper base material and a thermoplastic resin layer laminated on at least one surface of the paper base material, the paper base material having a multilayer structure with three or more pulp layers whose main component is cellulose pulp, the laminated paper having a ring crush compressive strength R in the CD direction of 3.0 to 6.0 kN/m, an interlaminar strength I of 100 to 250 J/ ^m2 , and an R/I of 0.020 to 0.060 kN/J, as measured by the ring crush method specified in JIS P 8126:2005.

(2) The laminated paper described in (1) above, in which the bending strength B in the CD direction is 300 to 500 gf and R/B is 0.0100 to 0.0150 kN/m·gf.

(3) The laminated paper according to (1) or (2), in which the content of hardwood bleached kraft pulp in the cellulose pulp is 50 to 100% by mass, and the content of softwood bleached kraft pulp is 0 to 50% by mass.

(4) The laminated paper according to any one of (1) to (3) above, wherein Z/I is 1.8 to 4.5 kPa·m ² /J, where Z is the Z-axis strength (kPa).

(5) A liquid paper container using the laminated paper described in any one of (1) to (4).

1 is a graph showing the relationship between ring crush compressive strength and interlaminar strength in laminated paper.

The laminated paper of this embodiment has a paper base material and a thermoplastic resin layer laminated on at least one surface of the paper base material, and the paper base material has a multilayer structure having three or more pulp layers mainly composed of cellulose pulp, and has a ring crush compressive strength R in the CD direction measured by the ring crush method specified in JIS P 8126:2005 of 3.0 to 6.0 kN/m, an interlaminar strength I of 100 to 250 J/ ^m2 , and an R/I of 0.020 to 0.060 kN/J. The laminated paper of this embodiment has excellent printability, and a liquid paper container using the laminated paper is unlikely to be damaged even by impact such as dropping that is expected during transportation. An embodiment of the present invention will be described below. However, the embodiment of the present invention is not limited to the following embodiment. In this specification, a numerical range expressed using "to" means a range including the numerical values written before and after "to" as the lower and upper limits.

[Paper base material]
As the paper base material, a multi-layered paper (multi-layered paper) having three or more paper layers (pulp layers) is used. The number of pulp layers is usually preferably three or more, more preferably four or more, and even more preferably five or more. The number of pulp layers is preferably seven or less, and more preferably six or less. The basis weight of each pulp layer is not particularly limited, but is preferably 20 to 300 g/m ² , more preferably 30 to 200 g/m ² , and even more preferably 40 to 100 g/m ² , and each layer may have the same or different basis weight. When the basis weight of the pulp layer contained in the multi-layered paper is 20 g/m ² or more, the pulp fibers between the layers are sufficiently entangled, thereby strengthening the adhesion between the layers and suppressing delamination. When the basis weight of the pulp layer contained in the multi-layered paper is 300 g/m ² or less, the drainage during papermaking is improved, so that a uniform paper layer can be formed and the smoothness of the paper base material can be improved. The basis weight of the outer layers (front and back layers) is not particularly limited, but is preferably 60 to 100 g/ ^m2 each. The basis weight per layer of the middle layers is also not particularly limited, but is preferably 40 to 70 g/ ^m2 . The total basis weight of the middle layers is also not particularly limited, but is preferably 120 to 200 g/ ^m2 . Starch, paper strength agents, etc. may be contained between the layers of the multi-layer paper to strengthen the interlayer adhesion. When starch is used as the interlayer adhesive, the application amount is preferably 1.0 to 3.0 g/ ^m2 between each layer.

(pulp)
The paper base material is mainly composed of cellulose pulp (hereinafter also referred to as pulp). Here, the main component refers to a component that occupies 50% by mass or more of the components that constitute the paper base material. Examples of cellulose pulp include wood pulp and non-wood pulp. Examples of wood pulp include wood-based pulps such as bleached kraft pulp (NBKP) of softwood, softwood kraft pulp (NKP), bleached kraft pulp (LBKP) of hardwood, and hardwood kraft pulp (LKP), and chemical pulps such as sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP). Examples of wood pulp include semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP), ground wood pulp (GP), and mechanical pulps such as thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include hemp, bagasse, cotton, etc. These pulps can be used alone or in combination of two or more. As the pulp constituting the paper base material, wood-based pulps such as LKP acacia wood and eucalyptus wood are suitable in terms of quality and cost. The raw material pulp is not limited to virgin pulp.

The inventors conducted extensive research to develop a laminated paper that can be used to form a paper container for liquids that is resistant to damage even when subjected to shocks such as dropping that may occur during transportation. As a result, they discovered that by making the paper base material of the laminated paper flexible rather than hard and sturdy, it is possible to prevent cracking by dispersing the shock throughout the entire paper container for liquids.

There are several ways to adjust the flexibility of the paper base material, but it can also be adjusted by the blending ratio of pulp, for example. LBKP is suitable as a pulp that gives flexibility to the paper base material, specifically wood-based pulp such as acacia or eucalyptus. By blending a large amount of LBKP, it is possible to give the paper base material the appropriate flexibility.

The parameters that indicate this flexibility will be discussed later.

An additional effect of having a paper base material contain a large amount of LBKP is the improvement of the surface smoothness due to the improved texture of the paper base material. A paper base material with high smoothness can further increase the adhesion of the thermoplastic resin layer described below, thereby improving the sealability of the laminated paper, and as a result, the impact resistance can be further improved. In addition, a paper base material with high smoothness can improve the printability of the paper base surface, and even if a thermoplastic resin layer is adhered to the paper base surface, the smoothness of the thermoplastic resin layer is increased, so the printability of the thermoplastic resin layer surface can also be improved.

On the other hand, NBKP is suitable as a pulp that gives strength to the paper base material, and specifically, wood-based pulps such as Douglas fir, radiata pine, and cedar are suitable.

The LBKP content in the pulp of the paper base material is preferably 50% by mass or more, more preferably 55% by mass or more, even more preferably 60% by mass or more, and preferably 100% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less. When the LBKP content in the pulp of the paper base material is within the above range, the effect of the present invention is further improved. In addition, the NBKP content in the pulp of the paper base material is preferably 0% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, and preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less. When the NBKP content in the pulp of the paper base material is within the above range, the effect of the present invention is further improved. Note that the content of each pulp in the pulp of the paper base material at the time of papermaking does not change even if it is re-disintegrated.

The LBKP content in the pulp constituting the outer layer (surface layer and back layer) of the paper base material is preferably 50% by mass or more, more preferably 55% by mass or more, even more preferably 60% by mass or more, and preferably 100% by mass or less, more preferably 90% by mass or less, and even more preferably 80% by mass or less. The NBKP content in the pulp constituting the outer layer (surface layer and back layer) of the paper base material is preferably 0% by mass or more, more preferably 10% by mass or more, even more preferably 20% by mass or more, and preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less. When the content of each pulp in the pulp constituting the outer layer (surface layer and back layer) of the paper base material is within the above range, the effect of the present invention is further improved.

When making a paper base material, various internal additives can be appropriately selected and used as necessary. Examples of internal additives include sizing agents, retention improvers, freeness improvers, bulking agents, paper strength improvers, various starches such as cationic starch, aluminum sulfate, coloring dyes, coloring pigments, pH adjusters, pitch control agents, slime control agents, etc.
Furthermore, a filler may be blended when the paper base material is made. The filler may be any filler commonly used in the papermaking field, and is not particularly limited.

The total basis weight of the paper base material is not particularly limited, but is preferably 150 to 500 g/m ² , more preferably 200 to 400 g/m ² , and even more preferably 250 to 350 g/m ^2. By making the total basis weight of the paper base material equal to or greater than the above lower limit, the rigidity can be increased when the paper base material is molded into a paper container. In addition, by making the total basis weight of the paper base material equal to or less than the above upper limit, it is possible to keep the amount of chemicals and raw materials used within an appropriate range, thereby reducing manufacturing costs.

(Papermaking)
Multilayer paper having a structure in which many pulp layers are laminated can be manufactured using manufacturing techniques for paperboard, etc., and can generally be manufactured using a cylinder-cylinder combined papermaking machine, a Fourdrinier combined papermaking machine, etc. The pH during papermaking may be in the acidic range (acidic papermaking), weakly acidic range (weakly acidic papermaking), neutral range (neutral papermaking), alkaline range (alkaline papermaking), etc., and any of these may be used.

The papermaking conditions are not particularly limited, but the pulp outflow speed is preferably 100 to 400 m/min, and more preferably 150 to 350 m/min. The ratio of the outflow speed to the running speed of the wire (jet wire ratio) is preferably 0.850 to 1.150, and more preferably 0.900 to 1.100. By setting the ratio of the outflow speed to the running speed of the wire (jet wire ratio) within the above range, it becomes easier to control the Taber stiffness in the MD and CD directions. For example, when the Taber stiffness in the MD direction is M and the Taber stiffness in the CD direction is C, it becomes easier to control the M/C value to a range of 2.5 to 2.8. This makes it easier to obtain a liquid paper container with excellent moldability.

[Laminated paper]
The laminated paper has a thermoplastic resin layer on at least one surface of a paper substrate. For example, the laminated paper may be produced by laminating a thermoplastic resin on at least one surface of a paper substrate to form a thermoplastic resin layer.

(Thermoplastic resin layer)
The thermoplastic resin may be either a crystalline resin or a non-crystalline resin depending on the application. Examples of the thermoplastic resin include polyolefin resins such as polyethylene (LDPE, HDPE, LLDPE, etc.), polypropylene, and polymethylpentene, polyester resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), polyamide resins, biodegradable resins such as polylactic acid and polybutylene succinate (PBS), polystyrene resins, polyvinyl chloride resins, acrylonitrile butadiene styrene (ABS) resins, acrylic resins, and modified polyphenylene ether (PPE) resins.

The thermoplastic resin layer may be a single layer made of a single resin, a single layer made of a mixture of multiple resins, or a layer made of multiple layers of the same or different resins. The thickness of the thermoplastic resin layer is not particularly limited, but is usually about 10 to 50 μm. When the thermoplastic resin layer is provided on both sides of the paper base material, it is preferable that the thickness of each thermoplastic resin layer is within the above range. The coating amount of the thermoplastic resin layer is not particularly limited, but is usually 10 to 50 g/m ^2. When the thermoplastic resin layer is provided on both sides of the paper base material, it is preferable that the coating amount of each thermoplastic resin layer is within the above range.

As a method for laminating the thermoplastic resin layer onto the paper substrate, various known methods such as extrusion lamination, dry lamination, wet lamination, and thermal lamination can be appropriately used. When the thermoplastic resin layer is a single layer, the extrusion lamination method is preferred.

During lamination, if necessary, the thermoplastic resin layer may be subjected to a corona treatment or oxidation treatment. By carrying out these treatments, polar groups are generated on the surface of the thermoplastic resin layer, improving adhesion.

(grammage, thickness, density)
The laminated paper having a paper base material and a thermoplastic resin layer preferably has a basis weight of 150 to 550 g/m ² , more preferably 200 to 500 g/m ² , and even more preferably 250 to 450 g/m ^2. The laminated paper preferably has a thickness of 200 to 750 μm, more preferably 250 to 600 μm, even more preferably 300 to 500 μm, and particularly preferably 400 to 500 μm. The laminated paper also preferably has a density of 0.60 to 1.20 g/cm ³ , and even more preferably 0.70 to 0.90 g/cm ³ .

As mentioned above, the basic layer structure of laminated paper is a two-layer or three-layer structure consisting of a paper base material and a thermoplastic resin layer, but a variety of other layer structures can be formed depending on the application.

For example, a water-soluble polymer layer or a coating layer mainly composed of a pigment and an adhesive may be provided on at least one surface of the paper base material. These layers may be used alone or in combination of two or more. By providing a water-soluble polymer layer or a coating layer, it is possible to adjust the degree of bleeding of the straight-chain fatty acid or straight-chain alcohol.

The water-soluble polymer is not particularly limited as long as it has film-forming properties, but from the viewpoint of processability, polyvinyl alcohol (PVA), starches, and polyacrylamides are preferred.

Examples of pigments that can be used in the coating layer include, but are not limited to, various kaolins, talc, refined natural mineral pigments such as heavy calcium carbonate, light calcium carbonate, composite synthetic pigments of calcium carbonate and other hydrophilic organic compounds, satin white, lithopone, titanium dioxide, silica, alumina, aluminum hydroxide, zinc oxide, magnesium carbonate, calcined kaolin, hollow organic pigments, solid organic pigments, plastic pigments, binder pigments, plastic beads, and microcapsules.

There are no particular limitations on the method for forming the water-soluble polymer layer or coating layer, and a blade coater, rod coater, or the like can be used. In addition, a metal film layer such as an aluminum foil (Al foil) can be provided as a layer other than the thermoplastic resin layer. Furthermore, a different type of thermoplastic resin layer can be provided on the laminated paper having a paper base material and a thermoplastic resin layer, or a layer other than the thermoplastic resin layer can be provided.

The inventors of the present invention have conducted extensive research into the strength of various types of laminated paper in order to create a paper container for liquids that is flexible and impact resistant. As a result, they have discovered the following:

(Ring crush compressive strength (RC compressive strength))
The RC compressive strength is the maximum load when a test piece is fixed in a cylindrical shape and compressed in the axial direction of the cylinder with a pressure plate until it is crushed. The method for measuring the RC compressive strength will be described later.
The inventors have found that in laminated paper, the RC compressive strength in the CD direction has a large effect on flexibility. Usually, liquid paper containers formed using laminated paper are often formed with the CD direction of the laminated paper as the vertical direction of the liquid paper container in order to suppress the container's body from swelling after filling with liquid. The body swelling refers to a phenomenon in which the body of a liquid paper container swells due to the pressure of the contents when the liquid paper container is filled with liquid. Therefore, when the liquid paper container after filling with liquid is subjected to an impact such as being dropped, a compressive load is applied in the CD direction of the laminated paper. Therefore, in laminated paper, the RC compressive strength in the CD direction is important in order for the liquid paper container after filling with liquid to absorb the impact of being dropped and suppress defects such as cracks.

The RC compressive strength of the laminated paper in the CD direction is 3.0 to 6.0 kN/m, and preferably 3.2 to 5.5 kN/m. The RC compressive strength is the ring crush compressive strength in the CD direction measured by the ring crush method specified in JIS P 8126:2005. When the RC compressive strength of the laminated paper in the CD direction is within the above range, the liquid paper container using the laminated paper has moderate flexibility. As a result, even if the liquid paper container filled with liquid is subjected to vibration during transportation or impact such as dropping, the liquid paper container can absorb the impact energy by bending moderately in the entire vertical direction, and can suppress breakage and leakage of contents. The RC compressive strength of the laminated paper in the CD direction can be adjusted to within the above range by adjusting the pulp freeness, pulp blending (ratio of NBKP), amount of interlayer starch applied, press line pressure, jet wire ratio, etc.

(Interlaminar strength)
The interlayer strength of the laminated paper of this embodiment refers to that measured by the method specified in JAPAN TAPPI 18-2: 2007. The inventors of the present invention have focused on the fact that penetration of liquid between paper layers is a major cause of cracking of liquid paper containers, and have investigated interlayer strength that can prevent liquid from penetrating between paper layers.

The interlayer strength of the laminated paper is 100 to 250 J/m ^2. The interlayer strength of the laminated paper is preferably 110 J/m ² or more, more preferably 120 J/m ² or more, even more preferably 125 J/m ² or more, and is preferably 200 J/m ² or less, more preferably 160 J/m ² or less, even more preferably 150 J/m ² or less.
When the interlayer strength is equal to or greater than the lower limit, damage caused by the liquid inside the paper container for liquid penetrating between the layers of the paper base material can be prevented. On the other hand, when the interlayer strength is equal to or less than the upper limit, appropriate interlayer peeling can be caused when the laminated paper is folded along the score line, improving the moldability of the paper container for liquid. The interlayer strength can be adjusted to within the above range by adjusting the freeness of the pulp, the amount of interlayer starch applied, the line pressure of the press, etc.

(RC compressive strength/interlayer strength)
The inventors of the present invention have studied the balance between RC compressive strength and interlayer strength in order to realize a paper container for liquids that has both high flexibility and strong interlayer bonding. Hereinafter, RC compressive strength will sometimes be referred to as R and interlayer strength as I.

Figure 1 shows a graph showing the relationship between ring crush compressive strength and interlaminar strength in laminated paper. By setting the R/I value within the range of 0.020 to 0.060 kN/J, it is possible to improve impact resistance, prevention of bulging, sealing strength of the laminate, and printability. The R/I value of laminated paper is preferably 0.015 to 0.050 kN/J. The R/I value is more preferably 0.020 kN/J or more, even more preferably 0.025 kN/J or more, and more preferably 0.045 kN/J or less, even more preferably 0.040 kN/J or less.

(Bending strength)
The bending strength in the CD direction of the laminated paper is preferably 300 gf or more, more preferably 320 gf or more, and preferably 500 gf or less, more preferably 450 gf or less, even more preferably 400 gf or less, and even more preferably 360 gf or less. When the bending strength in the CD direction of the laminated paper is equal to or greater than the lower limit, the rigidity of the laminated paper can prevent the paper container from being crushed even when the paper container is filled with liquid and is subjected to shock such as vibration or dropping. As a result, leakage of the liquid filled in the paper container can be prevented. In addition, when the paper container is molded, folding defects due to insufficient rigidity can be prevented. On the other hand, when the bending strength in the CD direction of the laminated paper is equal to or less than the upper limit, the paper container for liquid using the laminated paper has an appropriate rigidity, so that when the paper container for liquid is filled with liquid and is subjected to shock such as vibration or dropping, the entire surface can absorb the impact of the drop without absorbing the impact at a single point on a corner or edge. This can prevent the contents from leaking due to breakage, and can also prevent the occurrence of fold cracks due to excessive rigidity during molding of the paper container for liquids.

The bending strength of laminated paper in the CD direction is measured using the following method. A bending stiffness measuring instrument "BST-150M" (manufactured by NALL Asahi Research Institute, Ltd.) can be used to measure bending strength. A laminated paper test piece cut to a width of 38 mm and a length of 70 mm is used as the sample used to measure bending strength. The bending length (distance between the clamp part and the load point) is 13 mm, the clamping allowance is 24 mm, and the test piece is bent from 90° to 180°, and the maximum bending strength in the bending process of 90° or less is taken as the bending strength. The bending strength is measured 10 times using different test pieces, and the average value is taken as the bending strength of the laminated paper in the CD direction.

The CD bending strength of the laminated paper of this embodiment contributes to the durability of a paper container for liquids using the laminated paper when the paper container for liquids is subjected to shocks such as vibration and dropping while filled with liquid. Usually, when a paper container for liquids is formed using laminated paper, the CD direction of the laminated paper is often formed as the vertical direction of the paper container for liquids. Therefore, the CD bending strength is important in absorbing the shocks of vibration and dropping and suppressing defects such as cracks when a paper container for liquids filled with liquid is subjected to shocks such as vibration and dropping during transportation. The CD bending strength can be adjusted to within the above range by adjusting the paper thickness (basis weight), pulp freeness, pulp blend, jet wire ratio, etc.

(RC compressive strength/flexural strength)
The inventors of the present invention have studied the balance between RC compressive strength and bending strength in order to realize a paper container for liquids that is both highly flexible and strong against bending. Hereinafter, RC compressive strength will sometimes be referred to as R and bending strength as B.

By setting R/B within the range of 0.0100 to 0.0150 kN/m·gf, it is possible to improve impact resistance, prevention of bulging, sealing strength of the laminate, and printability. It is more preferable that the R/B value of the laminate paper is 0.0100 to 0.0150 kN/m·gf.

(Z-axis strength)
The Z-axis strength of the laminated paper of this embodiment refers to that measured by the method specified by JAPAN TAPPI18-1:2007. The inventors of the present invention have focused on the fact that penetration of liquid between paper layers is a major cause of cracking of liquid paper containers, and have investigated Z-axis strength that can prevent liquid from penetrating between paper layers. The Z-axis strength of the laminated paper is preferably 260 kPa or more, more preferably 300 kPa or more, and even more preferably 340 kPa or more, and is preferably 550 kPa or less, more preferably 500 kPa or less, and even more preferably 450 kPa or less.

By having the Z-axis strength of the laminated paper in the above range, the liquid paper container made from the laminated paper of this embodiment has an appropriate interlayer strength, and is unlikely to cause defects in the container due to peeling of layers even when it is subjected to vibration during transportation or impact such as dropping. In addition, laminated paper with a Z-axis strength in the above range has excellent processability because moderate interlayer peeling occurs when it is molded into a container. Specifically, when the Z-axis strength is equal to or greater than the above lower limit, the adhesion between the layers is increased, and peeling of the layers can be suppressed when a liquid paper container filled with liquid is subjected to vibration during transportation or impact such as dropping. This can improve the durability of the paper container, suppress the penetration of liquid between the layers, and suppress the crushing of the container. On the other hand, when the Z-axis strength is equal to or less than the above upper limit, the adhesive strength between the layers can be set to an appropriate range, and this can improve the moldability when the laminated paper is folded along the crease. The Z-axis strength can be adjusted to within the above range by adjusting the amount of interlayer starch applied.

(Z-axis strength/interlaminar strength)
The present inventors have studied the balance between Z-axis strength and interlayer strength in order to realize a paper container for liquid that can prevent liquid from penetrating between the layers. Hereinafter, Z-axis strength may be referred to as Z. By setting Z/I within the range of 1.8 to 4.5 kPa·m ² /J, it is possible to improve impact resistance, prevention of bulging, sealing strength of the laminate, and printability. The Z/I value of the laminate paper is more preferably 2.2 kPa·m ² /J or more, even more preferably 2.5 kPa·m ² /J or more, and more preferably 4.0 kPa·m ² /J or less, even more preferably 3.5 kPa·m ² /J or less.

(Taber stiffness)
Taber stiffness refers to the force required to bend a test piece at an angle of 15 degrees. The method for measuring Taber stiffness will be described later. The inventors hypothesized that laminated paper, which requires a large force to bend when forming a liquid paper container, is more likely to crack when folded, and investigated the Taber stiffness of laminated paper.

As a result of the investigation, the following was found. The Taber stiffness in the MD direction of the laminated paper is preferably 23.0 to 30.0 mN·m, more preferably 24.0 to 28.0 mN·m, and even more preferably 24.0 to 26.0 mN·m. The Taber stiffness in the CD direction of the laminated paper is preferably 8.5 to 10.7 mN·m, more preferably 9.0 to 10.7 mN·m, even more preferably 9.0 to 10.0 mN·m, and even more preferably 9.0 to 9.5 mN·m. The Taber stiffness in the MD direction and the CD direction can be adjusted within the above ranges by adjusting the paper thickness (basis weight), the freeness of the pulp, etc.
Assuming that the Taber stiffness of the laminate paper in the MD direction is M and that in the CD direction is C, the M/C ratio is preferably 2.5 to 2.8, and more preferably 2.6 to 2.7.

(Taber stiffness/flexural strength)
The present inventors have studied the balance between Taber stiffness in the CD direction and bending strength in order to realize a paper container for liquids that can prevent fold cracks. When forming a paper container for liquids using laminated paper, the Taber stiffness in the CD direction is important in order to increase the durability of the container when it is subjected to shocks such as vibration and dropping.

By setting C/B within the range of 0.020 to 0.030 mN·m/gf, it is possible to improve impact resistance, prevention of bulging, sealing strength of the laminate, and printability. It is more preferable for the C/B value of the laminated paper to be 0.023 to 0.028 mN·m/gf, and even more preferable to be 0.025 to 0.028 mN·m/gf.

[Liquid paper containers]
Various paper containers for liquids can be manufactured using the laminated paper of this embodiment. The method for manufacturing the paper containers for liquids can be appropriately selected from known methods.

The laminated paper of this embodiment can be used for paper liquid containers such as milk cartons, coffee containers, aseptic containers, paper cups, ice cups, foam cups, and insulated cups.

The effects of the present invention will be described in detail below with reference to examples. In the examples and comparative examples, "parts" and "%" refer to "parts by mass" and "% by mass", respectively, unless otherwise specified.

The materials and manufacturing conditions used in the examples and comparative examples are as follows.
LBKP: Acacia pulp, eucalyptus pulp NBKP: Douglas fir, radiata pine, cedar

The following describes the measurement methods used for paper substrates and laminated paper. Unless otherwise specified, measurements were performed in an environment of 23°C ± 1°C and 50% ± 2% humidity as specified in JIS P8111:1998.

(1) Basis weight: Measured in accordance with JIS P8124:2011.
(2) Thickness: In accordance with JIS P8118:2014, the thickness was measured when a pressure of 100 kPa±10 kPa was applied to a circular area (200 mm ² ) of the test piece.
(3) Density: Measured in accordance with JIS P8118: 2014. The thickness was measured when a pressure of 100 kPa±10 kPa was applied to a circular area (200 mm ² ) of the test piece.
(4) Z-axis strength: Measured according to JAPAN TAPPI Paper and Pulp Test Method No. 18-1:2007 Paper and Paperboard - Internal Bond Strength Test Method - Part 1: Z-axis Tensile Test Method.
(5) Interlaminar strength: Measured in accordance with JAPAN TAPPI Paper and Pulp Test Method No. 18-2:2007 Paperboard-Interlaminar Layer Peel Strength Test Method-Part 1: Maximum Load Measurement Method.
(6) Ring crush (RC) compressive strength: Measured in accordance with JIS P 8126:2015 Paper and paperboard-Compressive strength test method-Ring crush method.
(7) Bending strength: Measured according to JIS P 8115:2001 Paper and paperboard-Determination of folding endurance-IT tester method.
(8) Taber stiffness: Measured according to JIS P 8125:2000 Paper and paperboard - Stiffness test method - Taber stiffness tester method.

Example 1
100 parts of LBKP were beaten to obtain 100 parts of pulp slurry (solid content equivalent), and 0.50 parts of cationic starch, 0.3 parts of sizing agent, 0.10 parts of strength agent, 0.07 parts of wet strength agent, and 0.25 parts of aluminum sulfate were added to prepare five layers of paper stock for the surface layer, the surface underlayer, the middle layer, the back underlayer, and the back layer. Using this paper stock, the five layers were all set to a basis weight of 64 g/ ^m2 , and a five-layer papermaking machine was used to make paper under the conditions of a pulp outflow speed of 160 m/min and a jet wire ratio of 1.020. 2.0 g/ ^m2 of potato starch was applied between each layer as interlayer starch, and the paper base material of Example 1 was obtained by pressing and heating and drying.

A commercially available polyethylene (low density polyethylene, density 0.92 g/ ^m2 ) was laminated to the front surface of the paper base material of Example 1 at 20 g/ ^m2 using an extrusion lamination method at 320°C, and further laminated to the back surface at 30 g/ ^m2 to obtain laminated paper having the parameters shown in Example 1 in Table 1. This laminated paper was used to form a 1 L paper container for liquids (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

Example 2
Except for changing the addition rate of the cationic starch to 0.40 parts by mass and the addition rate of the paper strength agent to 0 parts by mass, a laminated paper having the parameters shown in Example 2 of Table 1 was obtained in the same manner as in Example 1. This laminated paper was used to form a 1 L paper container for liquids (milk carton), which was filled with liquid (water) and then heat sealed to perform a performance evaluation.

Example 3
A laminated paper having the parameters shown in Example 3 in Table ¹ was obtained in the same manner as in Example ¹ , except that the basis weight of the front layer and back layer was set to 80.0 g/m2, the basis weight of the front lower layer, middle layer and back lower layer was set to 53.3 g/m2, and the paper was made under conditions of a pulp outflow speed of 250 m/min and a jet wire ratio of 0.980. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

Example 4
A stock was obtained in the same manner as in Example 1, except that a pulp slurry obtained by beating 10 parts of NBKP and 90 parts of LBKP separately and mixing them was used as the raw material. Using this stock, laminated paper having the parameters in Example 4 of Table 1 was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 90.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 46.7 g/m2, the pulp outflow speed was 250 m/min, and the jet wire ratio was 1.000. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

Example 5
A stock was obtained in the same manner as in Example 1, except that a pulp slurry obtained by separately beating and mixing 20 parts of NBKP and 80 parts of LBKP was used as the raw material. Using this stock, laminated paper having the parameters in Example 5 of Table 1 was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 70.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 60.0 g/m2, the pulp outflow speed was 250 m/min, and the jet wire ratio was 1.000. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then sealed by heat sealing, and performance evaluation was performed.

Example 6
A stock was obtained in the same manner as in Example 1, except that a pulp slurry obtained by separately beating and mixing 30 parts of NBKP and 70 parts of LBKP was used as the raw material. Using this stock, laminated paper having the parameters in Example 6 of Table 1 was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 90.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 46.7 g/m2, the pulp outflow speed was 210 m/min, and the jet wire ratio was 1.080. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then sealed by heat sealing, and performance evaluation was performed.

(Example 7)
A stock was obtained in the same manner as in Example 1, except that a pulp slurry obtained by separately beating and mixing 35 parts of NBKP and 65 parts of LBKP was used as the raw material. Using this stock, laminated paper having the parameters shown in Example 7 in Table 1 was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 88.8 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 47.5 g/m2, the pulp outflow speed was 210 m/min, and the jet wire ratio was 1.080. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then sealed by heat sealing, and performance evaluation was performed.

(Example 8)
A stock was obtained in the same manner as in Example 1, except that a pulp slurry obtained by separately beating and mixing 40 parts of NBKP and 60 parts of LBKP was used as the raw material. Using this stock, laminated paper having the parameters in Example 5 of Table 1 was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 80.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 53.3 g/m2, the pulp outflow speed was 210 m/min, and the jet wire ratio was 1.080. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then sealed by heat sealing, and performance evaluation was performed.

Example 9
A stock was obtained in the same manner as in Example 1, except that a pulp slurry obtained by separately beating and mixing 50 parts of NBKP and 50 parts of LBKP was used as the raw material. Using this stock, laminated paper having the parameters shown in Example 9 in Table 1 was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 70.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 60.0 g/m2, the pulp outflow speed was 210 m/min, and the jet wire ratio was 1.080. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then sealed by heat sealing, and performance evaluation was performed.

(Comparative Example 1)
A stock was obtained in the same manner as in Example 1, except that 100 parts of NBKP were beaten. Using this stock, laminated paper having the parameters shown in Comparative Example 1 in Table ¹ was obtained in the same manner as in Example ¹ , except that the basis weight of the surface layer and the back layer was set to 100.0 g/m2, the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 40.0 g/m2, and the paper was made under the condition of a pulp outflow speed of 250 m/min. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

(Comparative Example 2)
A stock was obtained in the same manner as in Example 1, except that 100 parts of NBKP were beaten. Using this stock, laminated paper having the parameters shown in Comparative Example ² in Table 1 was obtained in the same manner as in Example 1, except that the basis weight of the surface layer and the back layer was set to 120.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 26.7 g/m2, and the paper was made under the condition of a pulp outflow speed of 250 m/min. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

(Comparative Example 3)
A stock was obtained in the same manner as in Example 1, except that 100 parts of NBKP were beaten. Using this stock, laminated paper having the parameters shown in Comparative Example 3 in Table ¹ was obtained in the same manner as in Example ^{1, except that the basis weight of the surface layer and the back layer was set to 80.0 g/m2, the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 53.3 g/m2} , the pulp outflow speed was 250 m/min, and the jet wire ratio was 0.980. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

(Comparative Example 4)
A stock was obtained in the same manner as in Example 1, except that 90 parts of NBKP and 10 parts of LBKP were mixed and beaten. Using this stock, laminated paper having the parameters shown in Comparative Example 4 in Table ¹ was obtained in the same manner as in Example 1, except that the basis weight of the surface layer and the back layer was set to 90.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 46.7 g/m2, the pulp outflow speed was 250 m/min, and the jet wire ratio was 0.980. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

(Comparative Example 5)
A stock was obtained in the same manner as in Example 1, except that 80 parts of NBKP and 20 parts of LBKP were mixed and beaten. Using this stock, laminated paper having the parameters shown in Comparative Example 5 in Table ¹ was obtained in the same manner as in Example 1, except that the basis weight of the surface layer and the back layer was set to 70.0 g/ ^m2 , the basis weight of the surface lower layer, the middle layer and the back lower layer was set to 60.0 g/m2, the pulp outflow speed was 260 m/min, and the jet wire ratio was 0.980. The total basis weight of the five layers was the same as in Example 1.
This laminated paper was used to form a 1 L liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to evaluate its performance.

The laminated paper and liquid paper containers obtained in the above manner were subjected to the following performance evaluations. The evaluation results are shown in Table 1.

(Drop test)
With the bottom surface of the paper containers (milk cartons) containing 1 L of water (23°C) produced in the examples and comparative examples facing vertically downward, the paper containers were dropped vertically onto a concrete floor from a height of 30 cm based on the height of the bottom surface. The paper containers were dropped multiple times until leakage of the contents was observed from the liquid container, and the number of drops required until the contents leaked was recorded. The drop test was performed on five pieces of paper for each liquid container base paper, and the average number of drops was calculated. The drop test was performed using a Lansmont drop tester PDT-56ED. The evaluation criteria for the drop test are as follows. A, B, or C was judged to be pass.
A: 15 times or more B: 12-14 times C: 9-11 times D: 8 times or less

(Body swelling)
The liquid paper containers (milk cartons) filled with 1 L of water (23°C) produced in the examples and comparative examples were stored in an environment with a temperature of 23°C ± 1°C and a humidity of 50% ± 2%, and the maximum diameter of the body was measured with a vernier caliper to an accuracy of 0.01 mm 24 hours and 168 hours after filling with water. The amount of body bulge was measured for five pieces of liquid container base paper, and the average value was calculated.
The evaluation criteria for trunk bulge were as follows: A, B or C was judged as acceptable.
A: 79.99mm or less B: 80.00-80.99mm
C: 81.00-81.99mm
D: 82.00mm or more

(sealing strength of laminate)
Using the laminated paper of the examples and comparative examples, a liquid paper container (milk carton) with a capacity of 1 L was molded, and then an empty container was made by heat sealing without filling it with liquid (water). The sealing strength of the container was measured according to the container burst strength test described in JIS Z0238:1998. The evaluation criteria for the sealing strength of the laminate are as follows. A, B, or C was judged to be acceptable.
A: Peak pressure of sealing strength is 30 kPa or more. B: Peak pressure of sealing strength is 25 kPa or more and less than 30 kPa. C: Peak pressure of sealing strength is 20 kPa or more and less than 25 kPa. D: Peak pressure of sealing strength is less than 20 kPa.

(Printability)
The laminated paper of the examples and comparative examples was subjected to offset printing, and the printing density and ink marks were visually evaluated. The printability was evaluated according to the following criteria. A, B, or C was judged as acceptable.
A: The printing is dense and no ink marks are visible.
B: The printing is dense, but some ink marks are visible.
C: There are some areas where the printing is slightly unclear and there are some ink marks, but there are no problems with the beauty of the print.
D: There are clearly unclear areas in the print, and the aesthetic quality is poor.

As can be seen from the results in Table 1, the paper base material of the examples has excellent surface smoothness, which not only improves the printability of the laminated paper, but also improves the sealing strength of the laminate layered on the surface. The laminated papers of Examples 1 to 9 have high sealing strength, the laminate is not easily peeled off even when subjected to impact, and damage due to internal liquid penetrating into the paper base material is unlikely to occur. Furthermore, the liquid paper containers of the examples formed from such laminated paper are able to disperse the impact of a drop and prevent damage due to the moderate flexibility of the paper base material, and despite its flexibility, no bulging occurred.

The liquid paper containers made from the laminated paper of Comparative Examples 1 to 5 could not withstand being dropped. In addition, the paper base materials of Comparative Examples 1 to 3 contained a large amount of NBKP, so they also lacked printability.

Claims (6)

A laminated paper having a paper base material and a thermoplastic resin layer laminated on at least one surface of the paper base material,
The paper base material has a multi-layer structure having three or more pulp layers mainly composed of cellulose pulp,
A laminated paper having a Taber stiffness in the CD direction of C and a bending strength in the CD direction of B, where C/B is 0.020 to 0.030 mN·m/gf. The ring crush compressive strength R in the CD direction measured by the ring crush method specified in JIS P 8126:2005 is 3.0 to 6.0 kN/m;
The interlaminar strength I is 100 to 250 J / ^m2 ,
The laminated paper according to claim 1, wherein R/I is 0.020 to 0.060 kN/J. The bending strength B in the CD direction is 300 to 500 gf,
The laminated paper according to claim 1 or 2, wherein R/B is 0.0100 to 0.0150 kN/m·gf. The laminated paper according to any one of claims 1 to 3, wherein the content of hardwood bleached kraft pulp in the cellulose pulp is 50 to 100% by mass, and the content of softwood bleached kraft pulp is 0 to 50% by mass. 5. The laminated paper according to claim 1, wherein Z/I is 1.8 to 4.5 kPa·m ² /J, where Z is the Z-axis strength (kPa). A liquid paper container using the laminated paper according to any one of claims 1 to 5.