Laminated paper and paper container for liquids employing 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 the 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 specifying 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 drops during transportation, it is likely to receive the impact of the drop at a single point on the corner. Even at the corners where cracking is likely to occur, a strong impact is applied, resulting in frequent accidents where the corners crack during dropping. However, the invention of Patent Document 1 does not consider cracking due to impacts such as dropping, 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 attached, rather than being plain, in order to indicate the source or improve the willingness to purchase. 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 by impacts such as dropping 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 specified 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 due to dropping or the like can be dispersed within the liquid paper container, and damage to the liquid paper container can be suppressed. It has also been found that such 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 in which the pulp slurry flows out (longitudinal direction) during the formation of the paper layer in the papermaking of the paper base material is also referred to as the MD direction hereinafter. Also, the direction perpendicular to the MD direction (lateral direction) is also referred to as the CD direction.

[0010] (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, wherein the paper base material 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 and R / I is 0.020 to 0.060 kN / J, the laminated paper.

[0011] (2) The laminated paper according to (1) above, wherein 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) above, wherein the content of hardwood kraft pulp in the cellulose pulp is 50 to 100% by mass, and the content of 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 / J, the laminated paper according to any one of (1) to (3) above.

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

Brief Description of the Drawings

[0015]

Figure 1

Embodiments for Carrying Out the Invention

[0016] The laminated paper of the present 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 the present embodiment has excellent printability, and a liquid paper container using the laminated paper is not easily damaged even by impacts such as drops 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, a multi-layer paper (multi-layer paper) having a multi-layer structure having three or more paper layers (pulp layers) is used. The number of pulp layers is usually preferably three or more, more preferably four or more, and even more preferably five or more. Also, the number of pulp layers is preferably seven or less, 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, 30 to 200 g / m 2 is more preferable, 40 to 100 g / m 2 is even more preferable, 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 multi-layer 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 multi-layer paper is 300 g / m 2 or less, the water 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 (the front layer and the back layer) are not particularly limited, but are preferably 60 to 100 g / m 2It is. The basis weight per layer of the middle layer is not particularly limited, but is preferably 40 to 70 g / m 2 It is. The total basis weight of the middle layer is not particularly limited, but is preferably 120 to 200 g / m 2 It is. Between the layers of the multi-layer paper, starch, paper strength enhancer, etc. may be contained in order to strengthen the interlayer adhesion. When using starch as the interlayer adhesive, the coating amount is preferably 1.0 to 3.0 g / m per each layer interface 2 It is preferable that it is.

[0018] (Pulp) The paper base material is mainly composed of cellulose pulp (hereinafter also referred to as pulp). Here, the main component means a component that occupies 50% by mass or more of the components constituting 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 softwood kraft pulp (NBKP), softwood kraft pulp (NKP), hardwood kraft pulp (LBKP), hardwood kraft pulp (LKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), oxygen bleached kraft pulp (OKP), etc. chemical pulps. Also, examples of wood pulp include semi-chemical pulps such as semi-chemical pulp (SCP) and chemigroundwood pulp (CGP), and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of non-wood pulp include hemp, bagasse, cotton, etc. These pulps can be used alone or in combination of two or more. As the pulp constituting the paper base material, wood-based pulps such as acacia wood and eucalyptus wood which are LKP are suitable in terms of quality and cost. Note that the raw material pulp is not limited to virgin pulp.

[0019] The inventors have repeatedly studied to realize a laminated paper capable of forming a liquid paper container that is difficult to be damaged even by impacts such as dropping assumed in transportation. As a result, it has been found that by giving flexibility to the paper base material of the laminated paper instead of making it hard and tough, the impact can be dispersed throughout the liquid paper container, thereby preventing cracking.

[0020] There are multiple methods to adjust the flexibility of the paper substrate. For example, it can also be adjusted by the blending ratio of pulp. As the pulp that imparts flexibility to the paper substrate, LBKP is suitable, specifically, wood-based pulps such as acacia wood and eucalyptus wood are applicable. By blending a large amount of LBKP, an appropriate flexibility can be given to the paper substrate.

[0021] The parameters serving as the above-mentioned flexibility indicators will be described later.

[0022] As a further effect of incorporating a large amount of LBKP into the paper substrate, an improvement in the smoothness of the surface due to the improvement in the texture of the paper substrate can be mentioned. A paper substrate with high smoothness can enhance the adhesion of the thermoplastic resin layer described later, so that the sealing property of the laminated paper can be improved, and as a result, the impact resistance can be further improved. In addition, when the smoothness of the paper substrate is high, the printability on the paper substrate surface can be improved, and even when a thermoplastic resin layer is adhered to the paper substrate surface, since the smoothness of the thermoplastic resin layer becomes high, the printability on the thermoplastic resin layer surface can also be improved.

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

[0024] The content rate of LBKP in the pulp of the paper base material is preferably 50% by mass or more, more preferably 55% by mass or more, still more preferably 60% by mass or more, and is preferably 100% by mass or less, more preferably 90% by mass or less, still more preferably 80% by mass or less. When the content rate of LBKP in the pulp of the paper base material is within the above range, the effects of the present invention are further improved. Further, the content rate of NBKP in the pulp of the paper base material is preferably 0% by mass or more, more preferably 10% by mass or more, still more preferably 20% by mass or more, and is preferably 50% by mass or less, more preferably 45% by mass or less, still more preferably 40% by mass or less. When the content rate 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 rate of each pulp in the pulp of the paper base material during papermaking does not change even when redissociated.

[0025] The content rate of LBKP in the pulp constituting the outer layer (front layer and back layer) of the paper base material is preferably 50% by mass or more, more preferably 55% by mass or more, still more preferably 60% by mass or more, and is preferably 100% by mass or less, more preferably 90% by mass or less, still more preferably 80% by mass or less. Further, the content rate of NBKP in the pulp constituting the outer layer (front layer and back layer) of the paper base material is preferably 0% by mass or more, more preferably 10% by mass or more, still more preferably 20% by mass or more, and is preferably 50% by mass or less, more preferably 45% by mass or less, still more preferably 40% by mass or less. When the content rate of each pulp in the pulp constituting the outer layer (front layer and back layer) of the paper base material is within the above range, the effects of the present invention are further improved.

[0026] When papermaking the paper base material, it is possible to appropriately select and use various internal additives as required. Examples of internal additives include sizing agents, yield improvers, drainage improvers, bulking agents, paper strength improvers, various starches such as cationized starch, sulfur bands, coloring dyes, coloring pigments, pH adjusters, pitch control agents, slime control agents, and the like. Furthermore, a filler may be added when forming the paper base material. Any filler generally used in the paper manufacturing field may be used, and there is no particular limitation.

[0027] The total basis weight of the paper base material is not particularly limited, but it is preferably 150 to 500 g / m 2 more preferably 200 to 400 g / m 2 even more preferably 250 to 350 g / m 2 By setting the total basis weight of the paper base material to be equal to or greater than the above lower limit value, the rigidity can be increased when forming into a paper container. Also, by setting the total basis weight of the paper base material to be equal to or less than the above upper limit value, the usage amounts of chemicals and raw materials can be kept within an appropriate range, and as a result, the manufacturing cost can be suppressed.

[0028] (Paper making) Multi-layer paper having a structure in which a large number of pulp layers are laminated can be manufactured using paperboard manufacturing techniques, and generally, it can be manufactured using a cylinder mold fourdrinier paper machine, a long wire fourdrinier paper machine, etc. The pH during paper making has an acidic region (acidic papermaking), a weakly acidic region (weakly acidic papermaking), a neutral region (neutral papermaking), an alkaline region (alkaline papermaking), etc., and any of them may 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 wire running speed (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 wire running speed (jet-wire ratio) within the above range, it becomes easier to control the taber stiffness in the MD direction and the CD direction. 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 value of M / C within the range of 2.5 to 2.8. As a result, it becomes easier to obtain a liquid paper container with excellent formability.

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

[0031] (Thermoplastic resin layer) Depending on the application, any thermoplastic resin, either a crystalline resin or an amorphous resin, can be used as the thermoplastic resin. Examples of 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 composed of a single resin, or a single layer composed of a mixture of multiple resins, or may be 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. 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. Also, the coating amount of the thermoplastic resin layer is not particularly limited, but is usually 10 to 50 g / m 2 is. When the thermoplastic resin layer is provided on both sides of the paper substrate, 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 an extrusion lamination method, a dry lamination method, a wet lamination method, and a heat lamination method can be appropriately used. When the thermoplastic resin layer is a single layer, the extrusion lamination method is preferable.

[0034] At the time of lamination, if necessary, the thermoplastic resin layer may be subjected to corona treatment or oxidation treatment. By performing 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 having a paper base material and a thermoplastic resin layer preferably has a basis weight of 150 to 550 g / m 2 and more preferably 200 to 500 g / m 2 and even more preferably 250 to 450 g / m 2 The laminated paper preferably has a thickness of 200 to 750 μm, more preferably 250 to 600 μm, even more preferably 300 to 500 μm, and particularly preferably 400 to 500 μm. Further, the laminated paper preferably has a density of 0.60 to 1.20 g / cm 3 and more preferably 0.70 to 0.90 g / cm 3

[0036] As the layer structure of the laminated paper, as described above, a two-layer or three-layer laminated paper composed of a paper base material and a thermoplastic resin layer is basic, but in addition, various diverse layer structures can be formed according to the application.

[0037] For example, a layer of a water-soluble polymer 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 can be used alone or in combination of two or more. 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 having film-forming properties, but from the viewpoint of processing suitability, polyvinyl alcohol (PVA), starches, and polyacrylamides are preferable.

[0039] ​Examples of pigments that can be used for the coating layer include, but are not limited to, refined natural mineral pigments such as various kaolins, talcs, or heavy calcium carbonates, light calcium carbonate, or 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, or microcapsules, etc.

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

[0041] The inventors repeatedly studied various strengths of laminated paper in order to realize a liquid paper container excellent in flexibility and impact resistance. As a result, the following was found.

[0042] (Ring crush compression strength (RC compression strength)) The RC compression strength is the maximum load when a test piece is fixed in a cylindrical shape and compressed in the cylindrical axis direction with a pressure plate until it collapses. The method for measuring the RC compression strength will be described later. The inventors of the present invention have found that in laminated paper, the RC compressive strength in the CD direction has a great influence on flexibility. Usually, in order to suppress the swelling of the body of a liquid paper container formed using laminated paper after liquid filling, the CD direction of the laminated paper is often formed as the vertical direction of the liquid paper container. Here, the swelling of the body refers to the phenomenon that when a liquid paper container is filled with liquid, the body part swells due to the pressure of the contents. Therefore, when the liquid paper container after liquid filling is subjected to an impact such as dropping, 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 for absorbing the impact such as dropping of the liquid paper container after liquid filling and suppressing defects such as cracking.

[0043] The RC compressive strength in the CD direction of the laminated paper 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 defined in JIS P 8126:2005. When the RC compressive strength in the CD direction of the laminated paper is within the above range, the liquid paper container using the laminated paper has appropriate flexibility. As a result, even when the liquid paper container filled with liquid is subjected to impacts such as vibration and dropping during transportation, the liquid paper container bends appropriately in the entire vertical direction to absorb the impact energy and suppress breakage and leakage of the contents. The RC compressive strength in the CD direction of the laminated paper can be adjusted within the above range by adjusting the freeness of the pulp, the pulp composition (ratio of NBKP), the coating amount of the interlayer starch, the press line pressure, the jet wire ratio, etc.

[0044] (Interlayer strength) The interlayer strength in the laminated paper of the present embodiment refers to that measured by the method defined by JAPAN TAPPI 18-2:2007. The inventors of the present invention focused on the fact that the penetration of liquid between paper layers is a major cause of cracking of liquid paper containers, and examined the interlayer strength capable of suppressing the penetration of liquid between paper layers.

[0045] The interlayer strength of the laminated paper is 100 to 250 J / m 2 It is. The interlayer strength of the laminated paper is preferably 110 J / m 2 or more, more preferably 120 J / m 2 or more, still more preferably 125 J / m 2 or more, and preferably 200 J / m 2 or less, more preferably 160 J / m 2 or less, still more preferably 150 J / m 2 or less. When the interlayer strength is equal to or higher than the above lower limit value, damage caused by the liquid inside the liquid paper container penetrating between the layers of the paper base material can be prevented. On the other hand, when the interlayer strength is equal to or lower than the above upper limit value, appropriate interlayer peeling can occur when the laminated paper is bent along the ruled line, and the formability of the liquid paper container is improved. The interlayer strength can be adjusted within the above range by adjusting the freeness of the pulp, the coating amount of the interlayer starch, the press line pressure, etc.

[0046] (RC compressive strength / interlayer strength) In order to realize a liquid paper container that combines high flexibility and strong interlayer bonding, the present inventors examined the balance between the RC compressive strength and the interlayer strength. Hereinafter, the RC compressive strength may also be referred to as R, and the interlayer strength may be referred to as I.

[0047] A graph showing the relationship between the ring crush compressive strength and the interlayer strength in the laminated paper is shown in FIG. 1. By setting the R / I value within the range of 0.020 to 0.060 kN / J, all of the impact resistance, prevention of barrel swelling, seal strength of the laminate, and printability can be made good. 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, still more preferably 0.025 kN / J or more, and more preferably 0.045 kN / J or less, still more preferably 0.040 kN / J or less.

[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, still more preferably 400 gf or less, and even more preferably 360 gf or less. When the bending strength of the laminated paper in the CD direction is equal to or higher than the above lower limit value, even when the liquid paper container using the laminated paper is subjected to impacts such as vibration and dropping in a state filled with liquid, it can be suppressed from being crushed due to its rigidity. As a result, leakage of the liquid filled in the liquid paper container can be suppressed. Also, even during the molding of the liquid paper container, it is possible to suppress the occurrence of defective bending due to insufficient rigidity. On the other hand, when the bending strength of the laminated paper in the CD direction is equal to or lower than the above upper limit value, the liquid paper container using the laminated paper will have appropriate rigidity. Therefore, when the liquid paper container filled with liquid is subjected to impacts such as vibration and dropping, it is possible to receive the impact of dropping over the entire surface without receiving the impact of dropping at a single point on a corner or side. Thereby, it is possible to prevent cracking and leakage of the contents. Also, even during the molding of the liquid paper container, it is possible to suppress the occurrence of cracking due to excessive rigidity.

[0049] The bending strength of the laminated paper in the CD direction is measured by the following method. For the measurement of the bending strength, a bending stiffness measuring machine "BST-150M" (manufactured by NALL Asahi Research Co., Ltd.) can be used. As the sample used for the measurement of the bending strength, a test piece of laminated paper cut out to a width of 38 mm and a length of 70 mm is used. Then, with the bending length (distance between the clamp part and the load point) being 13 mm and the clamp margin being 24 mm, after bending the test piece to 90° or more and 180° or less, the maximum value of the bending strength in the bending process of 90° or less is taken as the bending strength. The measurement of the bending strength is performed 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 bending strength of the laminate paper in the CD direction in this embodiment contributes to the durability of a liquid paper container made of the laminate paper when it is subjected to impacts such as vibration and dropping in a state filled with liquid. Usually, when forming a liquid paper container using laminate paper, the CD direction of the laminate paper is often formed as the vertical direction of the liquid paper container. Therefore, the bending strength in the CD direction is important for absorbing impacts such as vibration and dropping and suppressing defects such as cracks when a liquid-filled liquid paper container is subjected to impacts such as vibration and dropping during transportation. The bending strength in the CD direction can be adjusted within the above range by adjusting the paper thickness (tsubo), pulp freeness, pulp composition, jet wire ratio, etc.

[0051] (RC Compressive Strength / Bending Strength) The inventors of the present invention examined the balance between the RC compressive strength and the bending strength in order to realize a liquid paper container that combines high flexibility and strength against bending. Hereinafter, the RC compressive strength may also be referred to as R, and the bending strength may be referred to as B.

[0052] By setting R / B within the range of 0.0100 to 0.0150 kN / m·gf, it is possible to make all of the impact resistance, prevention of barrel swelling, seal strength of the laminate, and printability good. It is more preferable that the value of R / B of the laminate paper is 0.0100 to 0.0150 kN / m·gf.

[0053] (Z-axis Strength) The Z-axis strength of the laminate paper in this embodiment refers to the one measured by the method defined by JAPAN TAPPI 18-1:2007. The inventors of the present invention focused on the fact that the penetration of liquid between paper layers is a major cause of cracking of liquid paper containers, and examined the Z-axis strength capable of suppressing the penetration of liquid between paper layers. The Z-axis strength of the laminate paper is preferably 260 kPa or more, more preferably 300 kPa or more, still more preferably 340 kPa or more, and preferably 550 kPa or less, more preferably 500 kPa or less, still more preferably 450 kPa or less.

[0054] Since the Z-axis strength of the laminated paper is within the above range, the liquid paper container made from the laminated paper of the present embodiment has appropriate interlayer strength and is less likely to cause defects in the container due to delamination even when subjected to impacts such as vibration and dropping during transportation. Further, laminated paper with a Z-axis strength within the above range has excellent processability because appropriate delamination occurs when formed into a container. Specifically, when the Z-axis strength is equal to or greater than the above lower limit value, the adhesion between layers increases, and when a liquid paper container filled with liquid is subjected to impacts such as vibration and dropping during transportation, delamination of the layers can be suppressed. Thereby, the durability of the paper container can be improved, penetration of liquid between layers can be suppressed, and crushing of the container can be suppressed. On the other hand, when the Z-axis strength is equal to or less than the above upper limit value, the adhesive strength between layers can be set within an appropriate range, thereby enhancing the formability when the laminated paper is bent along ruled lines. The Z-axis strength can be adjusted within the above range by adjusting the coating amount of the interlayer starch.

[0055] (Z-axis strength / interlayer strength) The inventors of the present invention examined the balance between the Z-axis strength and the interlayer strength in order to realize a liquid paper container capable of suppressing penetration of liquid between layers. Hereinafter, the Z-axis strength may also be referred to as Z. By setting Z / I within the range of 1.8 to 4.5 kPa·m 2 / J, it is possible to achieve good impact resistance, prevention of barrel swelling, sealing strength of the laminate, and printability. The value of Z / I of the laminated paper is more preferably 2.2 kPa·m 2 / J or more, even more preferably 2.5 kPa·m 2 / J or more, and more preferably 4.0 kPa·m 2 / J or less, even more preferably 3.5 kPa·m 2 / J or less.

[0056] (Taber stiffness) The Taber stiffness refers to the force required to bend a test piece at an angle of 15 degrees. The method for measuring the Taber stiffness will be described later. The inventors hypothesized that in laminated paper, which requires a lot of force to be bent during the formation of a liquid paper container, folding breakage might easily occur, and thus examined the Taber stiffness of laminated paper.

[0057] As a result of the examination, 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 still more preferably 9.0 to 9.5 mN·m. The Taber stiffness in the MD direction and the CD direction can be adjusted within the above ranges by adjusting the paper thickness (tsubo), the freeness of the pulp, etc. Assuming that the Taber stiffness in the MD direction of the laminated paper is M and the Taber stiffness in the CD direction is C, M / C is preferably 2.5 to 2.8, and more preferably 2.6 to 2.7.

[0058] (Taber stiffness / flexural strength) The inventors examined the balance between the Taber stiffness in the CD direction and the flexural strength in order to realize a liquid paper container that can prevent folding breakage. When forming a liquid paper container using laminated paper, the Taber stiffness in the CD direction is important to enhance the container durability when it receives impacts such as vibration and dropping.

[0059] By setting C / B within the range of 0.020 to 0.030 mN·m / gf, all of the impact resistance, prevention of barrel swelling, seal strength of the laminate, and printing suitability can be made good. The value of C / B 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 container] Using the laminate paper of this embodiment, various liquid paper containers can be manufactured. As the method for manufacturing the liquid paper container, a known method can be appropriately selected and used.

[0061] The laminate paper of this embodiment can be used for liquid paper containers such as milk cartons, coffee containers, aseptic containers, paper cups, ice cups, foam cups, and insulated cups.

Example

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

[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, Sugi

[0064] The following shows the measurement methods performed on the paper base material and the laminate paper. Unless otherwise specified, the measurement was carried out in an environment of a temperature of 23°C ± 1°C and a humidity of 50% ± 2% as described in JIS P8111:1998.

[0065] (1) Grammage: Measured according to JIS P8124:2011. (2) Thickness: According to JIS P8118:2014, the thickness was measured when a pressure of 100 kPa ± 10 kPa was applied to the circular region (200 mm 2 ) of the test piece. (3) Density: Measured according to JIS P8118:2014. The thickness was measured when a pressure of 100 kPa ± 10 kPa was applied to the circular region (200 mm 2 ) of the test piece. (4) Z - axis strength: Measured in accordance with JAPAN TAPPI Paper Pulp Test Method No. 18 - 1:2007 Paper and Paperboard - Internal Bond Strength Test Method - Part 1: Z - axis Direction Tensile Test Method. (5) Inter - layer strength: Measured in accordance with JAPAN TAPPI Paper Pulp Test Method No. 18 - 2:2007 Paperboard - Peel 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 Paperboard - Fold Resistance Strength Test Method - IT Tester 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) To 100 parts by mass (in terms of solid content) of the pulp slurry obtained by beating 100 parts of LBKP, 0.50 part by mass of cationized starch, 0.3 part by mass of sizing agent, 0.10 part by mass of paper strength enhancer, 0.07 part by mass of wet paper strength enhancer, and 0.25 part by mass of aluminum sulfate were added to prepare paper materials for five layers: surface layer, under - surface layer, middle layer, under - back layer, and back layer. Using this paper material, with the basis weight of all 5 layers set to 64 g / m 2 and using a fourdrinier paper machine for five - layer forming, papermaking was carried out under the conditions of a pulp outflow speed of 160 m / min and a jet - wire ratio of 1.020. As the inter - layer starch, 2.0 g / m 2 of potato starch was applied between each layer, followed by pressing and heat - drying to obtain the paper substrate of Example 1.

[0067] To the paper substrate of Example 1, a commercially available polyethylene (low - density polyethylene, density 0.92 g / m 2 ) was laminated on the surface at 320 °C using the extrusion lamination method to a thickness of 20 g / m 2 , and further 30 g / m 2They were laminated so as to obtain laminated paper having the parameters in Example 1 of Table 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with a liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[0068] (Example 2) A laminated paper having the parameters 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 liquid paper container (milk carton) with a capacity of 1 L was formed, filled with a liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[0069] (Example 3) The basis weight of the surface layer and the back layer was set to 80.0 g / m 2 , and a laminated paper having the parameters in Example 3 of Table 1 was obtained in the same manner as in Example 1, except that the basis weight of the surface lower layer, the middle layer, and the back lower layer was set to 53.3 g / m 2 and 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 value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with a liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[0070] (Example 4) A papermaking stock was obtained in the same manner as in Example 1, except that the pulp slurry obtained by separately beating 10 parts of NBKP and 90 parts of LBKP and then mixing them was used as the raw material. Using this papermaking stock, the basis weight of the surface layer and the back layer was set to 90.0 g / m 2 , and a laminated paper having the parameters in Example 4 of Table 1 was obtained in the same manner as in Example 1, except that the basis weight of the surface lower layer, the middle layer, and the back lower layer was set to 46.7 g / m 2 and the paper was made under the conditions of a pulp outflow speed of 250 m / min and a jet wire ratio of 1.000. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a 1-L liquid paper container (milk carton) was formed, filled with a liquid (water), sealed by heat sealing, and subjected to performance evaluation.

[0071] (Example 5) Except that the pulp slurry obtained by separately beating 20 parts of NBKP and 80 parts of LBKP and then mixing them was used as the raw material, a paper stock was obtained in the same manner as in Example 1. Using this paper stock, the basis weights of the surface layer and the back layer were set to 70.0 g / m 2 , and the basis weights of the surface underlayer, the middle layer, and the back underlayer were set to 60.0 g / m 2 . A laminated paper having the parameters in Example 5 of Table 1 was obtained in the same manner as in Example 1, except that the paper was made at a pulp outflow rate of 250 m / min and a jet wire ratio of 1.000. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a 1-L liquid paper container (milk carton) was formed, filled with a liquid (water), sealed by heat sealing, and subjected to performance evaluation.

[0072] (Example 6) Except that the pulp slurry obtained by separately beating 30 parts of NBKP and 70 parts of LBKP and then mixing them was used as the raw material, a paper stock was obtained in the same manner as in Example 1. Using this paper stock, the basis weights of the surface layer and the back layer were set to 90.0 g / m 2 , and the basis weights of the surface underlayer, the middle layer, and the back underlayer were set to 46.7 g / m 2 . A laminated paper having the parameters in Example 6 of Table 1 was obtained in the same manner as in Example 1, except that the paper was made at a pulp outflow rate of 210 m / min and a jet wire ratio of 1.080. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a 1-L liquid paper container (milk carton) was formed, filled with a liquid (water), sealed by heat sealing, and subjected to performance evaluation.

[0073] (Example 7) A pulp slurry obtained by separately beating 35 parts of NBKP and 65 parts of LBKP and then mixing them was used as a raw material, and a paper stock was obtained in the same manner as in Example 1. Using this paper stock, except that the basis weights of the surface layer and the back layer were set to 88.8 g / m 2 and the basis weights of the sub-surface layer, the middle layer, and the sub-back layer were set to 47.5 g / m 2 A laminated paper having the parameters in Example 7 of Table 1 was obtained in the same manner as in Example 1, except that papermaking was carried out under the conditions of a pulp outflow rate of 210 m / min and a jet wire ratio of 1.080. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with a liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[0074] (Example 8) A pulp slurry obtained by separately beating 40 parts of NBKP and 60 parts of LBKP and then mixing them was used as a raw material, and a paper stock was obtained in the same manner as in Example 1. Using this paper stock, except that the basis weights of the surface layer and the back layer were set to 80.0 g / m 2 and the basis weights of the sub-surface layer, the middle layer, and the sub-back layer were set to 53.3 g / m 2 A laminated paper having the parameters in Example 5 of Table 1 was obtained in the same manner as in Example 1, except that papermaking was carried out under the conditions of a pulp outflow rate of 210 m / min and a jet wire ratio of 1.080. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with a liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[0075] (Example 9) A pulp slurry obtained by separately beating 50 parts of NBKP and 50 parts of LBKP and then mixing them was used as a raw material, and a paper stock was obtained in the same manner as in Example 1. Using this paper stock, except that the basis weights of the surface layer and the back layer were set to 70.0 g / m 2 and the basis weights of the sub-surface layer, the middle layer, and the sub-back layer were set to 60.0 g / m 2Set it to this value, and a laminated paper having the parameters in Example 9 of Table 1 was obtained in the same manner as in Example 1 except that papermaking was performed under the conditions of a pulp outflow speed of 210 m / min and a jet-wire ratio of 1.080. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[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 weights of the surface layer and the back layer were 100.0 g / m 2 , and the basis weights of the sub-surface layer, the middle layer, and the sub-back layer were 40.0 g / m 2 Set, and a laminated paper having the parameters in Comparative Example 1 of Table 1 was obtained in the same manner as in Example 1 except that papermaking was performed under the condition of a pulp outflow speed of 250 m / min. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[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 weights of the surface layer and the back layer were 120.0 g / m 2 , and the basis weights of the sub-surface layer, the middle layer, and the sub-back layer were 26.7 g / m 2 Set, and a laminated paper having the parameters in Comparative Example 2 of Table 1 was obtained in the same manner as in Example 1 except that papermaking was performed under the condition of a pulp outflow speed of 250 m / min. The total value of the basis weights of the five layers is the same as in Example 1. Using this laminated paper, a liquid paper container (milk carton) with a capacity of 1 L was formed, filled with liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[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 weights of the surface layer and the back layer were 80.0 g / m2 Set the basis weights of the top layer, middle layer, and bottom layer to 53.3 g / m 2 A laminated paper having the parameters in Comparative Example 3 of Table 1 was obtained in the same manner as in Example 1, except that the paper was formed under the conditions of a pulp outflow rate of 250 m / min and a jet wire ratio of 0.980. The total basis weight of the five layers is the same as in Example 1. Using this laminated paper, a paper container for 1 L of liquid (milk carton) was formed, filled with liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[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 weights of the top layer and the bottom layer were set to 90.0 g / m 2 and the basis weights of the top lower layer, middle layer, and bottom lower layer were set to 46.7 g / m 2 A laminated paper having the parameters in Comparative Example 4 of Table 1 was obtained in the same manner as in Example 1, except that the paper was formed under the conditions of a pulp outflow rate of 250 m / min and a jet wire ratio of 0.980. The total basis weight of the five layers is the same as in Example 1. Using this laminated paper, a paper container for 1 L of liquid (milk carton) was formed, filled with liquid (water), sealed by heat sealing, and performance evaluation was carried out.

[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 weights of the top layer and the bottom layer were set to 70.0 g / m 2 and the basis weights of the top lower layer, middle layer, and bottom lower layer were set to 60.0 g / m 2 A laminated paper having the parameters in Comparative Example 5 of Table 1 was obtained in the same manner as in Example 1, except that the paper was formed under the conditions of a pulp outflow rate of 260 m / min and a jet wire ratio of 0.980. The total basis weight of the five layers is the same as in Example 1. Using this laminated paper, a paper container for 1 L of liquid (milk carton) was formed, filled with liquid (water), sealed by heat sealing, and performance evaluation was carried out.

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

[0082] (Drop Test) With the bottom surface of the liquid paper containers (milk cartons) containing 1 L of water (23°C) prepared in the examples and comparative examples facing vertically downward, the liquid paper containers were vertically dropped onto a concrete floor from a height of 30 cm based on the bottom surface height. The dropping of the liquid paper containers was repeated multiple times until leakage of the contents from the liquid container was observed, and the number of drops required until the contents leaked was recorded. The drop test was performed 5 pieces at a time for each liquid container base paper, and the average value of the number of drops was determined. The drop tester PDT-56ED manufactured by Lansmont was used for the drop test. The evaluation criteria for the drop test are as follows. It was determined to be qualified when it was A, B, or C. A: 15 times or more B: 12 - 14 times C: 9 - 11 times D: 8 times or less

[0083] (Barrel Bulging) The liquid paper containers (milk cartons) containing 1 L of water (23°C) prepared in the examples and comparative examples were stored in an environment of 23°C ± 1°C and humidity 50% ± 2%. Using a vernier caliper, the maximum diameter of the barrel part was measured with an accuracy of 0.01 mm 24 hours and 168 hours after filling with water. The measurement of the barrel bulging amount was performed 5 pieces at a time for each liquid container base paper, and the average value was determined. The evaluation criteria for barrel bulging are as follows. It was determined to be qualified when it was A, B, or C. A: 79.99 mm or less B: 80.00 - 80.99 mm C: 81.00 - 81.99 mm D: 82.00 mm or more

[0084] (Laminating Sealing Strength) Using the laminated papers of the examples and comparative examples, a liquid paper container (milk carton) with a capacity of 1 L was formed. Then, without filling the liquid (water), an empty container with heat sealing was created. For the same container, the sealing strength was measured according to the container bursting strength test described in JIS Z0238:1998. The evaluation criteria for the sealing strength of the laminate are as follows. It was determined to be qualified when it was A, B, or C. A: The peak pressure of the sealing strength is 30 kPa or more B: The peak pressure of the sealing strength is 25 kPa or more and less than 30 kPa C: The peak pressure of the sealing strength is 20 kPa or more and less than 25 kPa D: The peak pressure of the sealing strength is less than 20 kPa

[0085] (Printing suitability) The laminated papers of the examples and comparative examples were subjected to offset printing, and the fineness of the printing and the ink mura were visually evaluated. The evaluation criteria for printing suitability are as follows. It was determined to be qualified when it was A, B, or C. A: The printing is fine and no ink mura is observed at all. B: The printing is fine, but a little ink mura is observed. C: There are slightly unclear parts in the printing, and a little ink mura is also observed, but there is no problem with the beauty. D: There are clearly unclear parts in the printing, and there is a problem with the beauty.

[0086]

Table 1

[0087]

Table 2

[0088] As can be seen from the results in Table 1, the paper substrates of the examples are excellent in surface smoothness, which not only improves the printability of the laminated paper, but also can improve the sealing strength of the laminate laminated on the surface. The laminated papers of Examples 1 to 9 have high sealing strength, and it is difficult for the laminate to peel off even when subjected to impact, and it is difficult for damage to occur due to the penetration of internal liquid into the paper substrate. And the liquid paper containers of the examples formed from such laminated paper can disperse the impact of dropping and prevent damage because the paper substrate has appropriate flexibility, and no barrel swelling occurred despite having flexibility.

[0089] The liquid paper containers made from the laminated papers of Comparative Examples 1 to 5 could not withstand dropping. Also, since the paper substrates of Comparative Examples 1 to 3 had a large amount of NBKP blended, the printability was also insufficient.

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, A laminated paper having a Taber stiffness in the CD direction of C and a bending strength in the CD direction of B, in which C / B is 0.020 to 0.030 mN·m / gf.

2. 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 The laminated paper according to claim 1, wherein 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 laminated paper according to claim 1 or 2, wherein R / B 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 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.

5. 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 4, wherein:

6. A paper container for liquid, using the laminated paper according to any one of claims 1 to 5.

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