Laminated paper and liquid paper container using same

Abstract

The present invention provides: 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 could occur during transport; and a paper container for liquids employing the same. Provided is laminated paper having a paper base material and a thermoplastic resin layer that is laminated on at least one of the surfaces of the paper base material, wherein: 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, which is measured by a ring crush method defined by JIS P 8126:2005, is 3.0-6.0 kN / m; the interlayer strength I is 100-250 J / m2; and R / I is 0.020-0.060 kN / J.

Description

[Technical field]

[0001] The present invention relates to a laminated paper and a liquid paper container using the same. [Background technology]

[0002] 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 pasting a single sheet of laminated paper, so they are prone to cracks on the sides and corners, and leakage of the contents from these cracks becomes a problem.

[0003] Various technologies have been developed to prevent leakage of liquid from such paper containers for liquid.For example, Patent Document 1 discloses a base paper for liquid containers that is made up of three paper layers, an inner layer and outer layers 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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-242999 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, the base paper for liquid containers disclosed in Patent Document 1 only addresses cracks 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 liquid is actually used, if it is dropped during transportation, the impact of the drop is likely to be received at a single corner. When a strong impact is applied to a corner that is already prone to cracks, there are many accidents in which the corner breaks when dropped. However, the invention in Patent Document 1 does not take into account cracks due to impacts such as dropping, and no drop tests were conducted.

[0006] In addition, paper containers for liquids such as milk cartons are generally not blank, but often have designs or the like added to indicate the source or to stimulate purchasing desire, etc., but the base paper for liquid containers described in Patent Document 1 is considered to have insufficient printability.

[0007] The present invention has been made in consideration of the above-mentioned circumstances. That is, an 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 using the same. [Means for solving the problem]

[0008] The present inventors have found that when the ring crush (RC) compressive strength and interlaminar strength values ​​of laminated paper are within a specified range as specified in JIS P 8126:2015, and the RC compressive strength / interlaminar strength value is also within a specified range, the impact of a drop or the like can be dispersed within the paper container for liquids, preventing damage to the paper container for liquids. They have also found that such laminated paper has excellent printability.

[0009] 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 during the formation of the paper layer in the papermaking process of the paper base material (machine direction) is also referred to as the MD direction, and the direction perpendicular to the MD direction (cross direction) is also referred to as the CD direction.

[0010] (1) A laminated paper having a paper base and a thermoplastic resin layer laminated on at least one surface of the paper base, the paper base having a multilayer structure having three or more pulp layers mainly composed of cellulose pulp, 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, and an interlaminar strength I of 100 to 250 J / m. 2 and R / I is 0.020~0.060kN / J.

[0011] (2) The laminated paper according to (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.

[0012] (3) The laminated paper according to (1) or (2), wherein the content of hardwood bleached kraft pulp in the cellulose pulp is 50 to 100 mass % and the content of softwood bleached kraft pulp is 0 to 50 mass %.

[0013] (4) When Z is the Z-axis strength (kPa), Z / I is 1.8 to 4.5 kPa m 2 The laminated paper according to any one of (1) to (3), wherein

[0014] (5) A paper container for liquid using the laminated paper according to any one of (1) to (4) above. [Brief description of the drawings]

[0015] [Figure 1] 1 is a graph showing the relationship between ring crush compressive strength and interlaminar strength in laminated paper. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] 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. The paper base material has a multilayer structure having three or more pulp layers mainly composed of cellulose pulp. The laminated paper has a ring crush compressive strength R in the CD direction of 3.0 to 6.0 kN / m, as measured by the ring crush method specified in JIS P 8126:2005, and an interlaminar strength I of 100 to 250 J / m. 2 and R / I is 0.020 to 0.060 kN / J. The laminated paper of this embodiment has excellent printability, and a liquid paper container using this laminated paper is less likely to be damaged even when subjected to shock such as dropping 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 that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0017] [Paper base material] The paper base material is a multi-layered paper having three or more paper layers (pulp layers). 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 is preferable, and 30 to 200 g / m 2 More preferably, 40 to 100 g / m 2 More preferably, the basis weight of each layer may be the same or different. The basis weight of the pulp layer included in the multi-layer paper is 20 g / m 2 If the pulp fibers in the multilayer paper are sufficiently intertwined, the adhesion between the layers is strengthened and delamination between the layers can be suppressed. 2 When the thickness is less than 100 g / m, the drainage during papermaking is improved, a uniform paper layer can be formed, and the smoothness of the paper base material can be improved. The basis weight of the outer layer (front layer and back layer) is not particularly limited, but is preferably 60 to 100 g / m 2The basis weight of each intermediate layer is not particularly limited, but is preferably 40 to 70 g / m 2 The total basis weight of the middle layer is not particularly limited, but is preferably 120 to 200 g / m 2 In order to strengthen the interlayer adhesion, starch or a paper strength enhancer may be added between the layers of the multi-layer paper. When starch is used as an interlayer adhesive, the amount of starch applied is 1.0 to 3.0 g / m2 per layer. 2 It is preferable that:

[0018] (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 accounts for 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 softwood kraft pulp (NBKP), softwood kraft pulp (NKP), bleached hardwood kraft pulp (LBKP), 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), and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include hemp, bagasse, and cotton. These pulps can be used alone or in combination of two or more. From the standpoint of quality and cost, wood-based pulps such as acacia and eucalyptus, which are LKP, are suitable for use as pulp for the paper base material. The raw pulp is not limited to virgin pulp.

[0019] The inventors of the present invention have conducted extensive research to realize 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 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.

[0020] There are several ways to adjust the flexibility of the paper base material, but for example, it can be adjusted by the blending ratio of pulp. LBKP is suitable as a pulp that gives flexibility to the paper base material, specifically, wood-based pulp such as acacia wood and eucalyptus wood. By blending a large amount of LBKP, it is possible to give the paper base material an appropriate flexibility.

[0021] The parameters that serve as indicators of the flexibility will be described later.

[0022] Further effects of including a large amount of LBKP in the paper substrate include improved surface smoothness due to improved texture of the paper substrate. A paper substrate with high smoothness can further increase the adhesion of the thermoplastic resin layer described below, thereby improving the sealing property of the laminated paper, and as a result, the impact resistance can be further improved. In addition, when the paper substrate has high smoothness, the printability of the paper substrate surface can be improved, and even when the thermoplastic resin layer is adhered to the paper substrate surface, the smoothness of the thermoplastic resin layer is increased, so that the printability of the thermoplastic resin layer surface can also be improved.

[0023] On the other hand, NBKP is suitable as a pulp that imparts strength to the paper base material, and specifically, wood-based pulps such as Douglas fir, radiata pine, and cedar are suitable.

[0024] The content of LBKP 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 content of LBKP in the pulp of the paper base material is within the above range, the effects of the present invention are further improved. In addition, the content of NBKP 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 content of NBKP in the pulp of the paper base material is within the above range, the effects of the present invention are 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-defibrated.

[0025] The content of LBKP 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 content of NBKP 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.

[0026] When making the 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.

[0027] The total basis weight of the paper base material is not particularly limited, but is preferably 150 to 500 g / m 2 It is preferable to set the thickness to 200 to 400 g / m 2 More preferably, the thickness is 250 to 350 g / m 2 It is more preferable that the total basis weight of the paper base material is set to the above lower limit or more, thereby increasing the rigidity when the paper base material is formed into a paper container. Also, by setting the total basis weight of the paper base material to the above upper limit or less, it is possible to keep the amount of chemicals and raw materials used within an appropriate range, thereby suppressing the manufacturing cost.

[0028] (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 can 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 can be used.

[0029] The papermaking conditions are not particularly limited, but the pulp outflow speed is preferably 100 to 400 m / min, 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, 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.

[0030] [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.

[0031] (thermoplastic resin layer) Thermoplastic resins can be either crystalline or non-crystalline depending on the application. Thermoplastic resins 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.

[0032] 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 made 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 layers are 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 layers are 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.

[0033] As a method for laminating the thermoplastic resin layer on the paper substrate, various known methods such as extrusion lamination, dry lamination, wet lamination, thermal lamination, etc. can be appropriately used. When the thermoplastic resin layer is a single layer, the extrusion lamination method is preferred.

[0034] During lamination, the thermoplastic resin layer may be subjected to a corona treatment or an oxidation treatment, if necessary. By carrying out these treatments, polar groups are generated on the surface of the thermoplastic resin layer, and the adhesiveness can be improved.

[0035] (basis weight, thickness, density) The laminated paper, which has a paper base material and a thermoplastic resin layer, has a basis weight of 150 to 550 g / m 2 It is preferable that the thickness is 200 to 500 g / m 2 More preferably, it is 250 to 450 g / m 2 The laminated paper has a thickness of preferably 200 to 750 μm, more preferably 250 to 600 μm, further preferably 300 to 500 μm, and particularly preferably 400 to 500 μm. The laminated paper also has a density of 0.60 to 1.20 g / cm 3 and preferably 0.70 to 0.90 g / cm 3 It is more preferable that:

[0036] As described 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 in addition to that, a variety of different layer structures can be formed depending on the application.

[0037] 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.

[0038] The water-soluble polymer is not particularly limited as long as it is a water-soluble polymer having film-forming properties, but from the viewpoint of processability, polyvinyl alcohol (PVA), starches, and polyacrylamides are preferred.

[0039] 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.

[0040] There is no particular limitation on the method of forming the water-soluble polymer layer or the coating layer, and a blade coater, a 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.

[0041] The present inventors have conducted extensive research into the strength of various types of laminated paper in order to realize a paper container for liquids that is excellent in flexibility and impact resistance. As a result, they have discovered the following.

[0042] (Ring crush compressive strength (RC compressive strength)) RC compressive strength is the maximum load when a test piece is fixed in a cylindrical shape and compressed in the axial direction with a pressure plate until it is crushed. The method for measuring 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 prevent the container from swelling after filling with liquid. The 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 is filled into the liquid paper container. 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 to prevent defects such as cracks.

[0043] 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, a liquid paper container using the laminated paper has appropriate flexibility. As a result, even if the liquid paper container filled with liquid is subjected to an impact such as vibration or dropping during transportation, the liquid paper container can bend appropriately in the entire vertical direction to absorb the impact energy and prevent breakage or leakage of the 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.

[0044] (Interlaminar strength) The interlayer strength of the laminated paper of this embodiment refers to that measured by the method specified by 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 cracks in liquid paper containers, and have investigated interlayer strength that can prevent liquid from penetrating between paper layers.

[0045] The interlaminar strength of laminated paper is 100~250J / m 2 The interlaminar strength of the laminated paper is preferably 110 J / m 2 More preferably, 120 J / m 2 More preferably, 125 J / m 2 and preferably 200 J / m 2 Less than or equal to 160 J / m 2 Less than 150 J / m, more preferably 2 The following is the result. 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.

[0046] (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.

[0047] A graph showing the relationship between ring crush compressive strength and interlaminar strength in laminated paper is shown in Figure 1. By setting the R / I value within the range of 0.020 to 0.060 kN / J, it is possible to improve all of the impact resistance, prevention of bulging, sealing strength of the laminate, and printability. The R / I value of the 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.

[0048] (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 for liquid from being crushed even when the paper container for liquid is filled with liquid and is subjected to shock such as vibration or dropping. As a result, the leakage of the liquid filled in the paper container for liquid can be prevented. In addition, when the paper container for liquid is molded, the occurrence of folding processing 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 impact of the drop can be received by the entire surface, not by a single point on a corner or side. 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.

[0049] The bending strength of laminated paper in the CD direction is measured by the following method, and the bending strength can be measured using a bending stiffness measuring instrument "BST-150M" (manufactured by NALL Asahi Research Institute Co., Ltd.). The sample used to measure bending strength is a laminated paper test piece cut to a width of 38 mm and a length of 70 mm. 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 with different test pieces, and the average value is taken as the bending strength in the CD direction of the laminated paper.

[0050] The CD bending strength of the laminated paper of this embodiment contributes to the durability of a paper container for liquid using the laminated paper when the paper container for liquid is subjected to shock such as vibration or dropping while filled with liquid. Usually, when a paper container for liquid is formed using laminated paper, the CD direction of the laminated paper is often formed as the vertical direction of the paper container for liquid. Therefore, the CD bending strength is important for absorbing the shock of vibration or dropping and suppressing defects such as cracking when a paper container for liquid filled with liquid is subjected to shock such as vibration or 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 blending, jet wire ratio, etc.

[0051] (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.

[0052] By setting R / B in 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.

[0053] (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 the penetration of liquid between paper layers is a major cause of cracking of liquid paper containers, and have studied Z-axis strength that can prevent the penetration of liquid 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.

[0054] By the Z-axis strength of the laminated paper being within 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 subjected to vibration during transportation or impact such as dropping. In addition, the laminated paper having a Z-axis strength within the above range has excellent processability because moderate interlayer peeling occurs when 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 the liquid paper container filled with liquid is subjected to impact such as vibration during transportation or dropping. This can improve the durability of the paper container, suppress the penetration of the 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 within the above range by adjusting the amount of interlayer starch applied.

[0055] (Z-axis strength / interlaminar strength) The inventors of the present invention have studied the balance between Z-axis strength and interlaminar strength in order to realize a paper container for liquids that can prevent liquids from penetrating between the layers. Hereinafter, Z-axis strength is sometimes referred to as Z. Z / I is set to 1.8 to 4.5 kPa m 2 By setting the Z / I value within the range, it is possible to improve the 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, and more preferably 2.5 kPa m 2 / J or more, and more preferably 4.0 kPa m 2 / J or less, and more preferably 3.5 kPa m 2 / J or less.

[0056] (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.

[0057] 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 and CD directions can be adjusted to within the above ranges by adjusting the paper thickness (basis weight), pulp freeness, 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 is preferably 2.5 to 2.8, and more preferably 2.6 to 2.7.

[0058] (Taber stiffness / flexural strength) The inventors of the present invention have studied the balance between the Taber stiffness in the CD direction and the bending strength in order to realize a paper container for liquids that can prevent cracks when folded. 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.

[0059] By setting C / B in the range of 0.020 to 0.030 mN·m / gf, it is possible to improve all of the following: impact resistance, prevention of bulging, sealing strength of the laminate, and printability. The C / B value of the laminated paper is more preferably 0.023 to 0.028 mN·m / gf, and even more preferably 0.025 to 0.028 mN·m / gf.

[0060] [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.

[0061] 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. EXAMPLES

[0062] The effects of the present invention will be described in detail below with reference to examples. In the examples and comparative examples, "parts" and "%" respectively refer to "parts by mass" and "% by mass" unless otherwise specified.

[0063] 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

[0064] The measurement methods used for paper substrates and laminated papers are described below. Unless otherwise specified, measurements were performed in an environment of temperature 23°C ± 1°C and humidity 50% ± 2% as specified in JIS P8111:1998.

[0065] (1) Basis weight: Measured in accordance with JIS P8124:2011. (2) Thickness: According to JIS P8118:2014, a pressure of 100 kPa ± 10 kPa is applied to the circular area (200 mm 2 ) was applied and the thickness was measured. (3) Density: Measured according to JIS P8118:2014. The thickness was measured by applying a pressure of 100 kPa ± 10 kPa to the circular area (200 mm 2 ) was applied and the thickness was measured. (4) Z-axis strength: Measured in accordance with 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 in accordance with JIS P 8115:2001 Paper and paperboard-Determination of folding endurance-IT testing machine method. (8) Taber stiffness: Measured in accordance with JIS P 8125:2000 Paper and paperboard - Stiffness test method - Taber stiffness tester method.

[0066] 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 stock for the surface layer, the undersurface layer, the middle layer, the undersurface layer, and the undersurface layer. Using this stock, the set basis weight of all five layers was 64 g / m. 2 The paper was made using a five-layer fourdrinier paper machine at a pulp flow rate of 160 m / min and a jet wire ratio of 1.020. Potato starch was used as the interlayer starch at 2.0 g / m 2 was applied between each layer, and pressed and dried by heating to obtain the paper base material of Example 1.

[0067] A commercially available polyethylene (low density polyethylene, density 0.92 g / m) was laminated on the paper substrate of Example 1 at 320°C by extrusion lamination. 2 ) on the surface at 20g / m 2 Then, laminate it so that it becomes like this, and then apply 30g / m 2Thus, a laminated paper having the parameters shown in Example 1 of Table 1 was obtained. This laminated paper was used to form a liquid paper container (milk carton) with a capacity of 1 L, which was then filled with liquid (water) and then sealed by heat sealing, and performance evaluation was performed.

[0068] 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 liquid paper container (milk carton), which was filled with liquid (water) and then heat sealed to perform a performance evaluation.

[0069] Example 3 The basis weight of the front and back layers is 80.0g / m 2 The basis weight of the surface layer, middle layer and back layer is 53.3g / m 2 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 paper was made under the conditions of setting the temperature at which the pulp flow rate was 250 m / min and the jet wire ratio at 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 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0070] Example 4 A stock was obtained in the same manner as in Example 1, except that 10 parts of NBKP and 90 parts of LBKP were beaten separately and mixed to obtain a pulp slurry as a raw material. 2 The basis weight of the surface layer, middle layer and back layer is 46.7g / m 2 A laminated paper having the parameters shown in Example 4 in Table 1 was obtained in the same manner as in Example 1, except that the paper was made under the conditions of setting the temperature at which the pulp flow rate was 250 m / min and the jet wire ratio at which the pulp flow rate was 250 m / min and the jet wire ratio at which the pulp flow rate was 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 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0071] Example 5 A stock was obtained in the same manner as in Example 1, except that 20 parts of NBKP and 80 parts of LBKP were beaten separately and mixed to obtain a pulp slurry as a raw material. Using this stock, the surface layer and the back layer were made to have a basis weight of 70.0 g / m 2 The basis weight of the surface layer, middle layer and back layer is 60.0 g / m 2 A laminated paper having the parameters shown in Example 5 in Table 1 was obtained in the same manner as in Example 1, except that the paper was made under the conditions of setting the temperature at which the pulp flow rate was 250 m / min and the jet wire ratio at which the pulp flow rate was 250 m / min and the jet wire ratio at which the pulp flow rate was 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 then filled with liquid (water) and then heat sealed to evaluate its performance.

[0072] Example 6 A stock was obtained in the same manner as in Example 1, except that 30 parts of NBKP and 70 parts of LBKP were beaten separately and mixed to obtain a pulp slurry as a raw material. 2 The basis weight of the surface layer, middle layer and back layer is 46.7g / m 2 A laminated paper having the parameters shown in Example 6 in Table 1 was obtained in the same manner as in Example 1, except that the paper was made under the conditions of a pulp flow rate of 210 m / min and a jet wire ratio of 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 then filled with liquid (water) and then heat sealed to evaluate its performance.

[0073] Example 7 A stock was obtained in the same manner as in Example 1, except that 35 parts of NBKP and 65 parts of LBKP were beaten separately and mixed to obtain a pulp slurry as a raw material. Using this stock, the surface layer and the back layer were made to have a basis weight of 88.8 g / m 2 The basis weight of the surface layer, middle layer and back layer is 47.5g / m 2 A 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 paper was made under the conditions of a pulp flow rate of 210 m / min and a jet wire ratio of 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 then filled with liquid (water) and then heat sealed to evaluate its performance.

[0074] Example 8 A stock was obtained in the same manner as in Example 1, except that 40 parts of NBKP and 60 parts of LBKP were beaten separately and mixed to obtain a pulp slurry as a raw material. Using this stock, the surface layer and the back layer were made to have a basis weight of 80.0 g / m 2 The basis weight of the surface layer, middle layer and back layer is 53.3g / m 2 A laminated paper having the parameters shown in Example 5 in Table 1 was obtained in the same manner as in Example 1, except that the paper was made under the conditions of a pulp flow rate of 210 m / min and a jet wire ratio of 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 then filled with liquid (water) and then heat sealed to evaluate its performance.

[0075] Example 9 A stock was obtained in the same manner as in Example 1, except that 50 parts of NBKP and 50 parts of LBKP were beaten separately and mixed to obtain a pulp slurry as a raw material. 2 The basis weight of the surface layer, middle layer and back layer is 60.0 g / m 2A 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 paper was made under the conditions of a pulp flow rate of 210 m / min and a jet wire ratio of 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 then filled with liquid (water) and then heat sealed to evaluate its performance.

[0076] Comparative Example 1 A paper stock was obtained in the same manner as in Example 1, except that 100 parts of NBKP was beaten. Using this paper stock, the basis weight of the front layer and the back layer was 100.0 g / m 2 The basis weight of the surface layer, middle layer and back layer is 40.0 g / m 2 A 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 papermaking was performed under conditions of a pulp flow rate of 250 m / min and a temperature setting of 100° C. The total basis weight of the five layers was the same as in Example 1. This laminated paper was used to form a 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0077] Comparative Example 2 A paper stock was obtained in the same manner as in Example 1, except that 100 parts of NBKP was beaten. Using this paper stock, the basis weight of the front layer and the back layer was 120.0 g / m 2 The basis weight of the surface layer, middle layer and back layer is 26.7g / m 2 A 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 papermaking was performed under conditions of a pulp flow rate of 250 m / min and a temperature setting of 100° C. The total basis weight of the five layers was the same as in Example 1. This laminated paper was used to form a 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0078] Comparative Example 3 A paper stock was obtained in the same manner as in Example 1, except that 100 parts of NBKP was beaten. Using this paper stock, the basis weight of the front layer and the back layer was 80.0 g / m2 The basis weight of the surface layer, middle layer and back layer is 53.3g / m 2 A 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 paper was made under the 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 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0079] Comparative Example 4 A paper 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 paper stock, the basis weight of the front layer and the back layer was 90.0 g / m 2 The basis weight of the surface layer, middle layer and back layer is 46.7g / m 2 A 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 paper was made under the 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 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0080] Comparative Example 5 A paper 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 paper stock, the basis weight of the front layer and the back layer was 70.0 g / m 2 The basis weight of the surface layer, middle layer and back layer is 60.0 g / m 2 A 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 paper was made under the conditions of a pulp flow rate of 260 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 1L liquid paper container (milk carton), which was then filled with liquid (water) and then heat sealed to evaluate its performance.

[0081] The laminated paper and liquid paper container obtained in the above manner were subjected to the following performance evaluations. The evaluation results are shown in Table 1.

[0082] (Drop test) The liquid paper containers (milk cartons) containing 1 L of water (23°C) produced in the examples and comparative examples were dropped vertically onto a concrete floor from a height of 30 cm based on the bottom surface height with the bottom surface facing vertically downward. The liquid 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 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 or more times B: 12~14 times C:9~11 times D: 8 times or less

[0083] (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 each liquid container base paper, and the average value was calculated. The evaluation criteria for trunk bulge were as follows: A, B, or C was considered to be acceptable. A: 79.99mm or less B: 80.00~80.99mm C: 81.00~81.99mm D:82.00mm or more

[0084] (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 30kPa 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

[0085] (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 there are no ink marks at all. B: The print is dense, but there are some ink marks. C: There are some areas where the print 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 compromised.

[0086] [Table 1]

[0087] [Table 2]

[0088] 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 laminated on the surface. The laminated paper of Examples 1 to 9 has 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 container of the examples formed from such laminated paper can disperse the impact of a drop and prevent damage due to the paper base material having appropriate flexibility, and does not bulge despite its flexibility.

[0089] The liquid paper containers made from the laminated papers 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, and therefore were insufficient in terms of printability.

Claims

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 has a multi-layer structure having three or more pulp layers mainly composed of cellulose pulp, 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; Interlaminar strength I is 100 to 250 J / m ２ and A laminated paper having an R / I of 0.020 to 0.060 kN / J.

2. The bending strength B in the CD direction is 300 to 500 gf, The laminated paper according to claim 1, wherein R / B is 0.0100 to 0.0150 kN / m·gf.

3. 3. The laminated paper according to claim 1, 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 in the cellulose pulp is 0 to 50% by mass.

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

5. A paper container for liquids using the laminated paper according to any one of claims 1 to 4.

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