Laminated paper and liquid paper containers using the same

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

Technical Field

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

Background Art

[0002] It is widely common to manufacture a liquid paper container typified by a milk carton using laminated paper obtained by laminating a paper base material to impart water resistance. Impact resistance is important for many containers, but in a liquid paper container whose contents are liquid, the damage caused by leakage of the contents is particularly large. Since many liquid paper containers are formed by folding and bonding a single sheet of laminated paper, creases are likely to occur at the sides and corners, and leakage of the contents from these creases becomes a problem.

[0003] Various techniques for suppressing leakage of such liquid paper containers have been developed. For example, Patent Document 1 discloses a base paper for a liquid container comprising three paper layers of an inner layer and outer layers provided on both sides of the inner layer, wherein the weight ratio of outer layer: inner layer: outer layer is 1:2:1 to 1:3:1, and the internal bond strength is 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

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the base paper for liquid containers disclosed in Patent Document 1 only addresses cracking during forming by stipulating the weight ratio of each layer of the paper base material and the internal bond strength. For example, when actually operating a liquid paper container, if it falls during transportation, it is likely to receive the impact of the fall at a single point of the corner. Moreover, since the corner is already prone to cracking, when a strong impact is applied to this corner, accidents where the corner cracks during a fall frequently occur. However, the invention of Patent Document 1 does not consider cracking caused by impacts such as falls, and no drop tests or the like have been conducted.

[0006] In addition, liquid paper containers such as milk cartons generally have patterns or the like, rather than being plain, in order to indicate the source or to improve the purchasing desire. However, 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 view of the above circumstances. That is, an object of the present invention is to provide a laminated paper capable of forming a liquid paper container that has excellent printability and is difficult to be damaged even by impacts such as falls assumed during transportation, and a liquid paper container using the same.

Means for Solving the Problems

[0008] The present inventors have found that when the values of the ring crush (RC) compressive strength and the interlayer strength defined in JIS P 8126:2015 of the laminated paper are within a predetermined range, and the value of RC compressive strength / interlayer strength is within a predetermined range, the impact caused by a fall or the like can be dispersed within the liquid paper container, and damage to the liquid paper container can be suppressed. In addition, it has also been found that such a laminated paper has excellent printability.

[0009] The present invention has been completed based on such findings. That is, the present invention has the following configuration. Hereinafter, the direction (vertical direction) in which the pulp slurry flows out during the papermaking of the paper base material, and the process in which the paper layer is formed, will also be referred to as the MD direction hereinafter. In addition, the direction perpendicular to the MD direction (horizontal direction) will also be referred to as the CD direction.

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

[0011] (2) The laminate paper described in (1) above, wherein the bending strength B in the CD direction is 300 to 500 gf and the R / B ratio is 0.0100 to 0.0150 kN / m·gf.

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

[0013] (4) When the Z-axis strength (kPa) is Z, Z / I is 1.8 to 4.5 kPa·m 2 Laminating paper as described in any of (1) to (3) above, which is / J.

[0014] (5) A liquid paper container using laminated paper as described in any of (1) to (4) above. [Brief explanation of the drawing]

[0015] [Figure 1] This graph shows the relationship between ring crush compressive strength and interlayer strength in laminated paper. [Modes for carrying out the invention]

[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, and the ring crush compression strength R in the CD direction measured by the ring crush method defined in JIS P 8126:2005 is 3.0 to 6.0 kN / m, and the interlayer strength I is 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 printing suitability, and a liquid paper container using the laminated paper is difficult to be damaged even by impacts such as dropping assumed during transportation. Embodiments of the present invention will be described below. However, the embodiments of the present invention are not limited to the following embodiments. In this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0017] [Paper base material] As the paper base material, paper (multilayer paper) having a multilayer structure with 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. Also, 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 It is more preferably 30 to 200 g / m 2 and even more preferably 40 to 100 g / m 2 and the basis weights of the layers may be the same or different from each other. When the basis weight of the pulp layer contained in the multilayer paper is 20 g / m 2 or more, the pulp fibers between the layers are sufficiently intertwined, so that the adhesion between the layers is strengthened and delamination between the layers can be suppressed. Also, when the basis weight of the pulp layer contained in the multilayer paper is 300 g / m 2 or less, the drainage property 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 weights of the outer layers (front layer and back layer) are not particularly limited, but are preferably 60 to 100 g / m 2The basis weight per layer of the middle layer is not particularly limited, but is preferably 40-70 g / m². 2 The total basis weight of the middle layer is not particularly limited, but is preferably 120-200 g / m². 2 In multilayer paper, starch or paper strength enhancers may be included between the layers to strengthen interlayer adhesion. When using starch as an interlayer adhesive, the application amount should be 1.0 to 3.0 g / m² per layer. 2 It is preferable that this be the case.

[0018] (pulp) Paper substrates are primarily composed of cellulose pulp (hereinafter also referred to as pulp). Here, "primary component" refers to a component that accounts for 50% or more by mass of the components constituting the paper substrate. Examples of cellulose pulp include wood pulp and non-wood pulp. Examples of wood pulp include wood-based pulps such as bleached kraft pulp from softwood (NBKP), softwood kraft pulp (NKP), bleached kraft pulp from hardwood (LBKP), and hardwood kraft pulp (LKP), as well as chemical pulps such as sulfite pulp (SP), dissolved pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP). In addition, wood pulps include semi-chemical pulps such as semi-chemical pulp (SCP) and chemiground wood pulp (CGP), and mechanical pulps such as crushed wood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulps include hemp, bagasse, and cotton. These pulps can be used individually or in combination of two or more. As pulps for paper base materials, wood-based pulps such as acacia and eucalyptus, which are LKP (Long Kiln Pulp), are suitable in terms of quality and cost. Note that the raw material pulp is not limited to virgin pulp.

[0019] The inventors of this invention conducted extensive research to develop a laminated paper that can be molded into liquid paper containers that are resistant to damage from impacts such as drops 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 robust, the impact can be distributed throughout the entire liquid paper container, thereby preventing cracking.

[0020] There are several ways to adjust the flexibility of paper substrates, one example being by changing the pulp blending ratio. LBKP is suitable as a pulp for imparting flexibility to paper substrates; specifically, wood-based pulps such as acacia and eucalyptus pulp are appropriate. By incorporating a high proportion of LBKP, the paper substrate can be given an appropriate level of flexibility.

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

[0022] A further benefit of incorporating a high amount of LBKP into the paper substrate is improved surface smoothness due to the improved texture of the paper substrate. A highly smooth paper substrate allows for better adhesion of the thermoplastic resin layer, as described later, thereby improving the sealing properties of the laminate paper and, as a result, further enhancing its impact resistance. In addition, high smoothness of the paper substrate improves printability on the paper substrate surface, and even when a thermoplastic resin layer is adhered to the paper substrate surface, the increased smoothness of the thermoplastic resin layer also improves printability on the surface of the thermoplastic resin layer.

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

[0024] The LBKP content in the pulp of the paper substrate 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 effects of the present invention are further enhanced when the LBKP content in the pulp of the paper substrate is within the above range. The NBKP content in the pulp of the paper substrate 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. The effects of the present invention are further enhanced when the NBKP content in the pulp of the paper substrate is within the above range. Note that the content of each pulp in the pulp of the paper substrate during papermaking does not change even if it is redisintegrated.

[0025] The LBKP content in the pulp constituting the outer layer (front and back layers) of the paper substrate 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 (front and back layers) of the paper substrate 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. The effects of the present invention are further enhanced when the content of each pulp in the pulp constituting the outer layer (front and back layers) of the paper substrate is within the above ranges.

[0026] When papermaking using paper substrates, various internal additives can be selected and used as needed. Examples of internal additives include sizing agents, yield enhancers, filtration enhancers, bulk enhancers, paper strength enhancers, various starches such as cationized starch, aluminum sulfate, coloring dyes, coloring pigments, pH adjusters, pitch control agents, slime control agents, and the like. Furthermore, fillers may be added when the paper base material is made into paper. The fillers used can be those commonly used in the papermaking field, and are not particularly limited.

[0027] The total basis weight of the paper substrate is not particularly limited, but is generally between 150 and 500 g / m². 2 It is preferable to use 200-400 g / m². 2 It is more preferable to use 250-350 g / m². 2 It is even more preferable to do so. By setting the total basis weight of the paper substrate to be above the lower limit, the rigidity can be increased when the paper container is formed. Furthermore, by setting the total basis weight of the paper substrate to be below the upper limit, it becomes possible to keep the amount of chemicals and raw materials used within an appropriate range, and as a result, manufacturing costs can be reduced.

[0028] (Papermaking) Multilayer paper, which consists of multiple layers of pulp, can be manufactured using paperboard manufacturing techniques and is generally produced using a cylinder wire mesh paper machine, a long wire mesh paper 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 is acceptable.

[0029] The papermaking conditions are not particularly limited, but the pulp outflow rate is preferably 100 to 400 m / min, and more preferably 150 to 350 m / min. The ratio of the outflow rate to the wire running speed (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 rate to the wire running speed (jet wire ratio) within the above range, it becomes easier to control the taper stiffness in the MD direction and the CD direction. For example, if the taper stiffness in the MD direction is M and the taper stiffness in the CD direction is C, it becomes easier to control the M / C value within the range of 2.5 to 2.8. This makes it easier to obtain liquid paper containers with excellent moldability.

[0030] [Laminating paper] Laminated paper has a thermoplastic resin layer on at least one surface of a paper substrate. For example, laminated paper may be manufactured by laminating a thermoplastic resin onto at least one surface of a paper substrate to form a thermoplastic resin layer.

[0031] (thermoplastic resin layer) Depending on the application, thermoplastic resins can be either crystalline or amorphous. Examples of thermoplastic resins include polyethylene (LDPE, HDPE, LLDPE, etc.), polyolefin resins such as 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 composed of a single resin, a single layer composed of a mixture of multiple resins, or a layer formed from 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 thick. When the thermoplastic resin layer is provided on both sides of the paper substrate, it is preferable that the thickness of each thermoplastic resin layer is within the above range. The amount of thermoplastic resin layer applied is not particularly limited, but is usually 10 to 50 g / m². 2 Furthermore, when thermoplastic resin layers are provided on both sides of the paper substrate, it is preferable that the amount of each thermoplastic resin layer applied is within the above range.

[0033] Various known methods such as extrusion lamination, dry lamination, wet lamination, and heat lamination can be used as appropriate for laminating the thermoplastic resin layer onto the paper substrate. 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 corona treatment or oxidation treatment as needed. By performing these treatments, polar groups are generated on the surface of the thermoplastic resin layer, which can improve adhesion.

[0035] (Basis weight, thickness, density) Laminated paper having a paper base and a thermoplastic resin layer has a basis weight of 150 to 550 g / m². 2 Preferably, it is 200-500 g / m 2 It is more preferable that it be 250-450 g / m 2 It is even more preferable that the laminate paper 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. Furthermore, the laminate paper has a density of 0.60 to 1.20 g / cm³. 3 Preferably, it is 0.70-0.90 g / cm³. 3 It is preferable that it be so.

[0036] As mentioned above, the basic layer structure of laminated paper is a two-layer or three-layer structure consisting of a paper base and a thermoplastic resin layer. However, various other diverse layer structures can be formed depending on the application.

[0037] For example, a layer of water-soluble polymer or a coating layer mainly composed of pigment and adhesive may be provided on at least one surface of the paper substrate. These layers can be used individually or in combination of two or more types. By providing a water-soluble polymer layer or a coating layer, it becomes possible to adjust the degree of bleeding of linear fatty acids and linear alcohols.

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

[0039] Pigments that can be used in the coating layer include, but are not limited to, various types of refined natural mineral pigments such as kaolin, talc, or heavy calcium carbonate; light calcium carbonate; composite synthetic pigments of calcium carbonate and other hydrophilic organic compounds; satin white; lithopon; titanium dioxide; silica; alumina; aluminum hydroxide; zinc oxide; magnesium carbonate; calcined kaolin; hollow organic pigments; dense organic pigments; plastic pigments; binder pigments; plastic beads; or microcapsules.

[0040] There are no particular restrictions on the method of forming the water-soluble polymer layer or the coating layer, and blade coaters, rod coaters, etc., can be used. In addition, layers other than the thermoplastic resin layer, such as a metal film layer such as aluminum foil (Al foil), can be provided. Furthermore, different types of thermoplastic resin layers can be provided on the laminate paper having a paper substrate and a thermoplastic resin layer, or layers other than the thermoplastic resin layer can be provided.

[0041] The inventors conducted extensive research on various strengths of laminated paper in order to realize a liquid paper container with excellent flexibility and impact resistance. As a result, the following was discovered.

[0042] (Ring crush compressive strength (RC compressive strength)) RC compressive strength is the maximum load when a test specimen is fixed in a cylindrical shape and compressed in the axial direction of the cylinder with a pressure plate, resulting in collapse. 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 significantly affects flexibility. Typically, liquid paper containers formed using laminated paper are often formed with the CD direction of the laminated paper aligned with the vertical direction of the liquid paper container in order to suppress bulging of the container body after liquid filling. Bulging refers to the phenomenon in which the body of a liquid paper container bulges due to the pressure of the contents when it is filled with liquid. Therefore, when a liquid paper container is subjected to impact such as dropping after being filled with liquid, a compressive load is applied to the CD direction of the laminated paper. Thus, in laminated paper, the RC compressive strength in the CD direction is important for the liquid paper container to absorb impacts such as dropping after being filled with liquid and to suppress defects such as cracking.

[0043] The RC compressive strength of laminated paper in the CD direction is 3.0 to 6.0 kN / m, preferably 3.2 to 5.5 kN / m. The above 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. Because the RC compressive strength of laminated paper in the CD direction is within the above range, liquid paper containers using this laminated paper have appropriate flexibility. As a result, even if the liquid paper container, filled with liquid, is subjected to vibrations or drops during transport, the container flexes appropriately in the vertical direction, absorbing the impact energy and suppressing damage and leakage of contents. The RC compressive strength of laminated paper in the CD direction can be adjusted within the above range by adjusting the pulp freeness, pulp composition (NBKP ratio), amount of interlayer starch coating, press line pressure, jet wire ratio, etc.

[0044] (Interlayer strength) In this embodiment, the interlayer strength in the laminated paper refers to the strength measured by the method specified in JAPAN TAPPI 18-2:2007. The inventors focused on the fact that the penetration of liquid into the interlayers of paper is a major cause of cracking in liquid paper containers, and investigated an interlayer strength that can suppress the penetration of liquid into the interlayers of paper.

[0045] The interlayer strength of laminated paper is 100-250 J / m². 2 The interlayer strength of the laminate paper is preferably 110 J / m². 2 More 120 J / m 2 More preferably 125 J / m 2 The above, and preferably 200 J / m 2 More preferably 160 J / m 2 More preferably, 150 J / m 2 The following applies: If the interlaminar strength is above the lower limit, damage caused by the liquid inside the liquid paper container penetrating between the layers of the paper substrate can be prevented. On the other hand, if the interlaminar strength is below the upper limit, appropriate delamination can occur when the laminated paper is folded along the crease lines, improving the moldability of the liquid paper container. The interlaminar strength can be adjusted within the above range by adjusting the pulp freeness, the amount of interlaminar starch applied, the press line pressure, etc.

[0046] (RC compressive strength / interlayer strength) The inventors investigated the balance between RC compressive strength and interlayer strength in order to realize a liquid paper container that achieves both high flexibility and strong interlayer bonding. Hereafter, RC compressive strength may be referred to as R and interlayer strength as I.

[0047] Figure 1 shows a graph illustrating the relationship between ring crush compressive strength and interlayer 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 achieve good 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 higher, even more preferably 0.025 kN / J or higher, and even more preferably 0.045 kN / J or lower, and even more preferably 0.040 kN / J or lower.

[0048] (Bending strength) The bending strength of the laminated paper in the CD direction 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. If the bending strength of the laminated paper in the CD direction is above the lower limit, even if the liquid paper container using the laminated paper is subjected to impacts such as vibration or dropping while filled with liquid, its rigidity can prevent it from collapsing. As a result, leakage of the liquid filled in the liquid paper container can be suppressed. Furthermore, during the molding of the liquid paper container, defects in the bending process due to insufficient rigidity can be suppressed. On the other hand, if the bending strength of the laminated paper in the CD direction is below the upper limit, the liquid paper container using the laminated paper will have appropriate rigidity, so that when the liquid paper container is subjected to impacts such as vibration or dropping while filled with liquid, the impact of the drop can be absorbed by the entire surface, rather than being absorbed by a single point such as a corner or edge. This prevents the container from cracking and leaking its contents. Furthermore, it helps to suppress cracking caused by excessive rigidity during the molding of liquid paper containers.

[0049] The bending strength of laminated paper in the CD direction is measured using the following method, and the bending stiffness measuring instrument "BST-150M" (manufactured by NALL Asahi Research Institute Co., Ltd.) can be used to measure the bending strength. The sample used for measuring the bending strength is a test piece of laminated paper cut to a width of 38 mm and a length of 70 mm. The bending length (distance between the clamp and the load point) is set to 13 mm, and the clamping allowance is set to 24 mm. After bending the test piece from 90° to 180°, the maximum bending strength during the bending process up to 90° 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 of the laminated paper in the CD direction.

[0050] The flexural strength in the CD direction of the laminated paper in this embodiment contributes to the durability of the liquid paper container using the laminated paper when subjected to vibrations, drops, and other impacts while filled with liquid. Normally, when forming a liquid paper container using laminated paper, the CD direction of the laminated paper is often aligned with the vertical direction of the liquid paper container. Therefore, the flexural strength in the CD direction is important for absorbing vibrations, drops, and other impacts during transportation of a liquid-filled liquid paper container, thereby suppressing defects such as cracking. The flexural strength in the CD direction can be adjusted within the above range by adjusting the paper thickness (basis weight), pulp freeness, pulp composition, jet wire ratio, etc.

[0051] (RC compressive strength / flexural strength) The inventors investigated the balance between RC compressive strength and bending strength in order to realize a liquid paper container that combines high flexibility with strength against bending. Hereafter, RC compressive strength may be referred to as R and bending strength as B.

[0052] By setting the R / B ratio within the range of 0.0100 to 0.0150 kN / m·gf, it is possible to achieve good 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 be 0.0100 to 0.0150 kN / m·gf.

[0053] (Z-axis strength) In this embodiment, the Z-axis strength of the laminated paper refers to the strength measured by the method specified in JAPAN TAPPI 18-1:2007. The inventors focused on the fact that the penetration of liquid between paper layers is a major cause of cracking in liquid paper containers, and investigated a Z-axis strength that can suppress 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, even more preferably 340 kPa or more, and preferably 550 kPa or less, more preferably 500 kPa or less, and even more preferably 450 kPa or less.

[0054] Because the Z-axis strength of the laminated paper is within the above range, the liquid paper container made from the laminated paper of this embodiment has appropriate interlayer strength, making it less likely for the container to malfunction due to delamination of layers even when subjected to vibrations or impacts such as dropping during transportation. Furthermore, laminated paper with a Z-axis strength within the above range exhibits appropriate delamination of layers when molded into a container, resulting in excellent processability. Specifically, if the Z-axis strength is above the lower limit, the adhesion between layers is enhanced, suppressing delamination of layers when a liquid-filled paper container is subjected to vibrations or impacts such as dropping during transportation. This improves the durability of the paper container, suppresses the penetration of liquid into the interlayers, and prevents the container from collapsing. On the other hand, if the Z-axis strength is below the upper limit, the interlayer adhesion strength can be kept within an appropriate range, thereby improving the moldability when the laminated paper is folded along the crease lines. 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 investigated the balance between Z-axis strength and interlayer strength in order to realize a liquid paper container that can suppress the penetration of liquid into the interlayers. Hereafter, Z-axis strength may be referred to as Z. Z / I is 1.8~4.5 kPa·m 2 By keeping the Z / I value within the range of / J, it is possible to achieve good 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 higher, more preferably 2.5kPa·m 2 It is greater than or equal to / J, and more preferably 4.0kPa·m 2 / J or less, more preferably 3.5kPa·m 2 It is less than or equal to / J.

[0056] (Taber stiffness) Taber stiffness refers to the force required to bend a test specimen to a 15-degree angle. The method for measuring Taber stiffness will be described later. The inventors hypothesized that laminated paper, which requires a lot of force to fold during the molding of liquid paper containers, is prone to cracking, and therefore investigated the Taber stiffness of laminated paper.

[0057] The following was found as a result of the investigation: The Taber stiffness of the laminated paper in the MD direction 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 of the laminated paper in the CD direction 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 within the above ranges by adjusting the paper thickness (basis weight), pulp freeness, etc. If M is the taper stiffness in the MD direction of the laminated paper and C is the taper stiffness in the CD direction, then M / C is preferably 2.5 to 2.8, and more preferably 2.6 to 2.7.

[0058] (Taber stiffness / bending strength) The inventors investigated the balance between tapered stiffness in the CD direction and bending strength in order to realize a liquid paper container that can prevent cracking. When forming a liquid paper container using laminated paper, tapered stiffness in the CD direction is important to improve the container's durability when subjected to impacts such as vibration and dropping.

[0059] By setting the C / B ratio within the range of 0.020 to 0.030 mN·m / gf, it is possible to achieve good impact resistance, prevention of bulging, sealing strength of the laminate, and printability. The C / B value of the laminate 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 container] Various liquid paper containers can be manufactured using the laminated paper of this embodiment. Known methods can be appropriately selected and used for manufacturing the liquid paper containers.

[0061] The laminated paper of this embodiment can be used for liquid paper containers such as milk cartons, coffee containers, aseptic containers, paper cups, ice cream 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 "%" refer to "parts by mass" and "% by mass," respectively, 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 paper are described below. Unless otherwise specified, measurements were performed in an environment with a temperature of 23°C ± 1°C and a humidity of 50% ± 2%, as specified in JIS P8111:1998.

[0065] (1) Basis weight: Measured in accordance with JIS P8124:2011. (2) Thickness: In accordance with JIS P8118:2014, apply a pressure of 100kPa ± 10kPa to the circular area (200mm) of the test specimen. 2 The thickness was measured when it was added to ). (3) Density: Measured in accordance with JIS P8118:2014. The thickness was measured by applying a pressure of 100kPa ± 10kPa to the circular area (200mm) of the test specimen. 2 The thickness was measured when it was added to ). (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 - Delamination strength test method for laminated layers - 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 cardboard - Test method for folding strength - IT testing machine method. (8) Taber stiffness: Measured in accordance with JIS P 8125:2000 Paper and cardboard - Stiffness test method - Taber stiffness tester method.

[0066] (Example 1) To 100 parts by mass (solid content) of pulp slurry obtained by beating 100 parts of LBKP, 0.50 parts by mass of cationized starch, 0.3 parts by mass of sizing agent, 0.10 parts by mass of paper strength enhancer, 0.07 parts by mass of wet paper strength enhancer, and 0.25 parts by mass of aluminum sulfate were added to prepare five layers of paper stock: surface layer, subsurface layer, middle layer, subback layer, and back layer. Using this paper stock, the target basis weight for all five layers was set to 64 g / m². 2 Using a five-layer long-wire paper machine, paper was made under the conditions of a pulp flow rate of 160 m / min and a jet wire ratio of 1.020, with 2.0 g / m² of potato starch used as interlayer starch. 2 The paper substrate of Example 1 was obtained by applying the material between each layer, pressing, and heat-drying.

[0067] On the paper substrate of Example 1, commercially available polyethylene (low-density polyethylene, density 0.92 g / m²) was laminated using the extrusion lamination method at a temperature of 320°C. 2 ) 20g / m² on the surface 2 Laminate it in this way, and then apply 30g / m² to the back. 2The paper was laminated in the manner described above to obtain laminated paper having the parameters shown in Example 1 of Table 1. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, and its performance was evaluated.

[0068] (Example 2) Laminated paper having the parameters shown in Example 2 of Table 1 was obtained in the same manner as in Example 1, except that the addition rate of cationized starch was 0.40 parts by mass and the addition rate of paper strength enhancer was 0 parts by mass. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed by heat sealing, and its performance was evaluated.

[0069] (Example 3) The basis weight of the surface and back layers is 80.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 53.3 g / m². 2 Laminate paper having the parameters shown in Example 3 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow rate 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

[0070] (Example 4) A pulp slurry 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 the pulp slurry. Using this pulp, the basis weight of the surface and back layers was set to 90.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 46.7 g / m². 2 Laminate paper having the parameters shown in Example 4 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow rate of 250 m / min and a jet wire ratio of 1.000. The total basis weight of the five layers was the same as in Example 1. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

[0071] (Example 5) A pulp slurry 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 the pulp slurry. Using this pulp, the basis weight of the surface and back layers was set to 70.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 60.0 g / m². 2 Laminate paper having the parameters shown in Example 5 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow rate of 250 m / min and a jet wire ratio of 1.000. The total basis weight of the five layers was the same as in Example 1. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed using heat sealing. Performance evaluation was then conducted.

[0072] (Example 6) A pulp slurry 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 the pulp slurry. Using this pulp, the basis weight of the surface and back layers was set to 90.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 46.7 g / m². 2 Laminate paper having the parameters shown in Example 6 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed using heat sealing. Performance evaluation was then conducted.

[0073] (Example 7) A pulp slurry 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 the pulp slurry. Using this pulp, the basis weight of the surface and back layers was set to 88.8 g / m². 2 The basis weight of the top, middle, and bottom layers is 47.5 g / m². 2 Laminate paper having the parameters shown in Example 7 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed using heat sealing. Performance evaluation was then conducted.

[0074] (Example 8) A pulp slurry 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 the pulp slurry. Using this pulp, the basis weight of the surface and back layers was set to 80.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 53.3 g / m². 2 Laminate paper having the parameters shown in Example 5 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed using heat sealing. Performance evaluation was then conducted.

[0075] (Example 9) A pulp slurry 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 the pulp slurry. Using this pulp, the basis weight of the surface and back layers was set to 70.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 60.0 g / m². 2Laminate paper having the parameters shown in Example 9 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed using heat sealing. Performance evaluation was then conducted.

[0076] (Comparative Example 1) A pulp was obtained in the same manner as in Example 1, except that 100 parts of NBKP were beaten. Using this pulp, the basis weight of the surface and back layers was set to 100.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 40.0 g / m². 2 Laminate paper having the parameters shown in Comparative Example 1 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with the settings configured as shown and the pulp flow rate at 250 m / min. The total basis weight of the five layers was the same as in Example 1. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

[0077] (Comparative Example 2) A pulp was obtained in the same manner as in Example 1, except that 100 parts of NBKP were beaten. Using this pulp, the basis weight of the surface and back layers was set to 120.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 26.7 g / m². 2 Laminate paper with the parameters shown in Comparative Example 2 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with the settings configured as shown and the pulp flow rate at 250 m / min. The total basis weight of the five layers was the same as in Example 1. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

[0078] (Comparative Example 3) A pulp was obtained in the same manner as in Example 1, except that 100 parts of NBKP were beaten. Using this pulp, the basis weight of the surface and back layers was set to 80.0 g / m².2 The basis weight of the top, middle, and bottom layers is 53.3 g / m². 2 Laminate paper having the parameters shown in Comparative Example 3 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow rate 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

[0079] (Comparative Example 4) A pulp 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 pulp, the basis weight of the surface and back layers was set to 90.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 46.7 g / m². 2 Laminate paper with the parameters shown in Comparative Example 4 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow rate 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

[0080] (Comparative Example 5) A pulp 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 pulp, the basis weight of the surface and back layers was set to 70.0 g / m². 2 The basis weight of the top, middle, and bottom layers is 60.0 g / m². 2 Laminate paper with the parameters shown in Comparative Example 5 of Table 1 was obtained in the same manner as in Example 1, except that the papermaking process was carried out under the same conditions as in Example 1, with a pulp outflow 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. Using this laminated paper, a 1L liquid paper container (milk carton) was formed, filled with liquid (water), and then sealed with heat seal, followed by performance evaluation.

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

[0082] (Drop test) In the examples and comparative examples, 1L liquid paper containers (milk cartons) filled with water (23°C) were prepared. With the bottom surface facing vertically downwards, the liquid paper containers were dropped vertically onto a concrete floor from a height of 30 cm relative to the height of the bottom surface. The drop test was repeated multiple times until leakage of the contents was observed from the liquid container, and the number of drops required until leakage was recorded. Five drop tests were performed for each type of liquid container, and the average number of drops was calculated. A Lansmont PDT-56ED drop test machine was used for the drop tests. The evaluation criteria for the drop tests were as follows: A, B, or C was considered a pass. A: 15 times or more B: 12~14 times C:9~11 times D: 8 times or less

[0083] (Swollen body) Liquid paper containers (milk cartons) filled with 1 liter of water (23°C) prepared 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%. The maximum diameter of the body was measured with an accuracy of 0.01 mm using calipers 24 hours and 168 hours after filling with water. The amount of body bulge was measured for each liquid container base paper, with five measurements taken and the average value calculated. The evaluation criteria for body swelling are as follows: A grade of A, B, or C was considered acceptable. A: 79.99mm or less B: 80.00~80.99mm C: 81.00~81.99mm D: 82.00mm or more

[0084] (Laminate sealing strength) Using the laminated paper of the examples and comparative examples, 1L liquid paper containers (milk cartons) were formed, and then empty containers were created by heat sealing without filling them with liquid (water). The sealing strength of these containers was measured according to the container burst strength test described in JIS Z0238:1998. The evaluation criteria for the sealing strength of the laminate were as follows: A, B, or C was considered acceptable. A: The peak pressure of the sealing strength is 30kPa or higher. B: The peak pressure of the sealing strength is 25kPa or more and less than 30kPa. C: Peak pressure of sealing strength is 20kPa or more but less than 25kPa D: Peak pressure of sealing strength is less than 20kPa

[0085] (Printability) Offset printing was performed on the laminated paper of the examples and comparative examples, and the print density and ink unevenness were visually evaluated. The evaluation criteria for printability were as follows: A, B, or C was considered acceptable. A: The printing is meticulous, and there is absolutely no ink bleeding. B: The printing is detailed, but there is some ink unevenness. C: There are some areas where the printing is slightly unclear, and some ink unevenness is visible, but there are no problems with the overall appearance. D: There are clearly blurry areas in the print, resulting in a lack of aesthetic appeal.

[0086] [Table 1]

[0087] [Table 2]

[0088] As can be seen from the results in Table 1, the paper substrates of the examples have 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 had high sealing strength, the laminate was difficult to peel off even when subjected to impact, and damage due to the penetration of the internal liquid into the paper substrate was less likely to occur. Furthermore, the liquid paper containers of the examples formed from such laminated paper were able to disperse the impact of drops and prevent damage due to the paper substrate's appropriate flexibility, and despite being flexible, no bulging occurred.

[0089] The liquid paper containers made from laminated paper in Comparative Examples 1-5 could not withstand being dropped. Furthermore, the paper substrates in Comparative Examples 1-3 had a high NBKP content, resulting in insufficient printability.

Claims

1. A laminated paper having a paper substrate and a thermoplastic resin layer laminated on at least one surface of the paper substrate, The aforementioned paper substrate has a multilayer structure having three or more pulp layers mainly composed of cellulose pulp, Laminated paper in which, when the tapered stiffness in the CD direction is C and the bending strength in the CD direction is B, the ratio C / B is 0.020 to 0.030 mN·m / gf.

2. The laminate paper according to Claim 1, satisfying at least one of the following (1) to (3): (1) 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. (2) Interlayer strength I is 100 to 250 J / m ２ That is, (3) R / I is 0.020 to 0.060 kN / J.

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

4. The laminated paper according to any one of claims 1 to 3, wherein the content of bleached hardwood kraft pulp in the cellulose pulp is 50 to 100% by mass, and the content of bleached softwood kraft pulp is 0 to 50% by mass.

5. When the Z-axis intensity (kPa) is denoted as Z, the Z / I ratio is between 1.8 and 4.5 kPa·m. ２ A laminate paper according to any one of claims 1 to 4, wherein the laminate paper is / J.

6. The laminate paper according to any one of claims 1 to 5, wherein the taper stiffness in the MD direction is 23.0 to 30.0 mN·m.

7. A liquid paper container using laminated paper according to any one of claims 1 to 6.

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