Kraft Paper and Packaging Bags

Abstract

To provide kraft paper having both specific tensile strength and tear strength, as well as excellent sizing performance, and a packaging bag using this paper.SOLUTION: A kraft paper according to the present invention comprises wood pulp as a main component, wherein the disintegrated pulp obtained by redefibrating the kraft paper has a Canadian Standard Freeness of 500 mL or more, and the content of a polyacrylamide-based dry paper-strengthening agent in the kraft paper is 0.05 mass% to 0.75 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to kraft paper and a packaging bag using the same. [Background technology]

[0002] Kraft paper is primarily used for large bags used to carry heavy industrial goods such as cement, fertilizer, and pesticides, as well as for packaging bags for salt, rice, wheat, and agricultural products. When manufacturing large bags to hold heavy items, the paper must be strong enough to withstand the impacts that may be applied during transportation, and must also be resistant to tearing when wet during use.

[0003] For example, Patent Document 1 discloses a heavy-duty packaging kraft paper characterized by using kraft pulp with a weight-average fiber length of 2.3 to 4.0 mm to form a paper support having a disintegration freeness of 600 to 680 mL, with the aim of reducing production costs and achieving high breathability without reducing the strength of the kraft paper. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-89889 Summary of the Invention [Problem to be solved by the invention]

[0005] The heavy-duty packaging kraft paper described in Patent Document 1 has not been sufficiently studied to achieve both specific tensile strength and specific tear strength. Furthermore, the Stockigt sizing degree as an index of sizing ability has not been sufficiently studied.

[0006] An object of the present invention is to provide kraft paper that satisfies both specific tensile strength and specific tear strength and also has excellent sizing properties, and to provide packaging bags using the same. [Means for solving the problem]

[0007] The present inventors have found that the above-mentioned problems can be solved by setting the Canadian Standard Freeness of disintegrated pulp obtained by re-disintegrating kraft paper and the content of polyacrylamide-based dry strength agent in the kraft paper within specific ranges. That is, the present invention provides the following: <1> ~ <12> Regarding. <1> Kraft paper made primarily of wood pulp, in which the Canadian standard freeness of the disintegrated pulp obtained by re-disintegrating the kraft paper is 500 mL or more, and the content of a polyacrylamide-based dry strength agent in the kraft paper is 0.05% by mass or more and 0.75% by mass or less. <2> The weight average fiber length of the pulp that makes up the kraft paper is 2.2 mm or more and 3.7 mm or less. <1> Kraft paper according to claim 1. <3> The wood pulp includes softwood kraft pulp; <1> or <2> Kraft paper according to claim 1. <4> The content of softwood kraft pulp in the wood pulp is 90% by mass or more. <3> Kraft paper according to claim 1. <5> The raw materials for softwood kraft pulp include Douglas fir, <3> or <4> Kraft paper according to claim 1. <6> The specific tensile strength is 40.0 N·m / g or more. <1> ~ <5> 1. Kraft paper according to any one of the preceding items. <7> Specific tear strength is 10.0 mN·m 2 / g or more, <1> ~ <6> 1. Kraft paper according to any one of the preceding items. <8> Basis weight: 30 g / m 2 More than 200g / m 2 Below is the <1> ~ <7> 1. Kraft paper according to any one of the preceding items. <9> The Stoeckigt size is 12 seconds or more, <1> ~ <8> 1. Kraft paper according to any one of the preceding items. <10> The elongation at break is 3.1% or more. <1> ~ <9> 1. Kraft paper according to any one of the preceding items. <11> Kraft paper for heavy-duty bags, <1> ~ <10> 1. Kraft paper according to any one of the preceding items. <12> <1> ~ <11> A packaging bag made using the kraft paper described in any one of 1 to 3. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide kraft paper that satisfies both specific tensile strength and specific tear strength, and a packaging bag using the same. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Kraft paper] The kraft paper of this embodiment is a kraft paper whose main component is wood pulp, and the Canadian standard freeness of the disintegrated pulp obtained by re-disintegrating the kraft paper is 500 mL or more, and the content of a polyacrylamide-based dry strength agent in the kraft paper is 0.05 mass% or more and 0.75 mass% or less. The kraft paper of this embodiment can achieve both high specific tensile strength and high specific tear strength. Furthermore, the kraft paper of this embodiment can achieve both high specific tensile strength and high specific tear strength, and also has excellent sizing properties. The mechanism behind this effect is unknown, but is presumed to be as follows. The kraft paper of this embodiment is mainly composed of wood pulp, and therefore can be formed from pulp with relatively long fiber length, resulting in excellent specific tear strength. On the other hand, the more the pulp is beaten (i.e., the lower the Canadian Standard Freeness of the pulp that makes up the kraft paper), the lower the tear strength index. This is thought to be because accelerated beating causes the fibers to fibrillate, increasing the interfiber bonding area and improving the tensile strength index, which increases the modulus of elasticity and makes the sheet harder, making the tear strength index more likely to decrease. It is also thought that the shortening of fibers due to accelerated beating is also a factor in the tendency for tear strength index to decrease. The kraft paper of this embodiment uses pulp that has been moderately beaten, and the Canadian standard freeness of the disintegrated pulp obtained by re-defibration is 500 mL or more, which prevents the decrease in specific tear strength due to the above phenomenon and makes it possible to achieve excellent specific tear strength. Furthermore, by setting the content of the polyacrylamide dry strength agent in the kraft paper of this embodiment to 0.05% by mass or more, the strength of the bond points between fibers is improved, thereby improving the tensile strength index and further improving the Stockigt sizing degree. On the other hand, by setting the content of the polyacrylamide dry strength agent to 0.75% by mass or less, excessive aggregation of pulp fibers can be prevented, and a significant decrease in tear strength can be suppressed. As described above, it is believed that the kraft paper of this embodiment can achieve both high specific tensile strength and high specific tear strength, and also has excellent sizing properties. However, the scope of the present invention is not limited by the above mechanism.

[0010] <Pulp that makes up kraft paper> (wood pulp) In this embodiment, the kraft paper is mainly composed of wood pulp. "Kraft paper is mainly composed of wood pulp" means that the wood pulp content in the kraft paper is more than 50% by mass, and the wood pulp content is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. In this embodiment, the kraft paper may contain non-wood pulp as the pulp, but from the viewpoint of improving the specific tensile strength and specific tear strength, the content of wood pulp in the total pulp constituting the kraft paper is preferably more than 50% by mass, more preferably 80% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, with the upper limit being 100% by mass. The wood pulp is not particularly limited, and known wood pulps can be used. Specific examples include chemical pulps such as hardwood kraft pulp (LKP) and softwood kraft pulp (NKP); and mechanical pulps such as groundwood pulp (GP), pressure groundwood pulp (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), chemi-mechanical pulp (CMP), and chemi-ground pulp (CGP). Examples of hardwood kraft pulp (LKP) include unbleached hardwood kraft pulp (LUKP) and bleached hardwood kraft pulp (LBKP). Examples of softwood kraft pulp (NKP) include unbleached softwood kraft pulp (NUKP) and bleached softwood kraft pulp (NBKP). Examples of non-wood pulp include recycled paper pulp, non-wood fiber pulp such as kenaf, bagasse, bamboo, and cotton, and synthetic pulp. These pulps may be used alone or in combination of two or more. Among these, from the viewpoint of improving the specific tensile strength and specific tear strength of the resulting kraft paper, softwood kraft pulp (NKP) is preferred, at least one selected from the group consisting of softwood unbleached kraft pulp (NUKP) and softwood bleached kraft pulp (NBKP) is more preferred, and softwood unbleached kraft pulp (NUKP) is even more preferred.

[0011] When the wood pulp contains softwood kraft pulp (NKP) (preferably softwood unbleached kraft pulp (NUKP)), the content of softwood kraft pulp (NKP) (preferably softwood unbleached kraft pulp (NUKP)) in the wood pulp is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass or more. %in Since softwood kraft pulp (NKP) has a long average fiber length, the content of softwood kraft pulp (NKP) in the wood pulp, which is the main component of the pulp that makes up kraft paper, is 90% by mass or more, and thus the desired specific tensile strength and specific tear strength can be obtained. RukuFurthermore, when softwood kraft pulp (NKP) is used as the pulp constituting the kraft paper, the strength of the pulp fibers themselves is higher than that of hardwood kraft pulp (LKP), which makes it easier to improve the specific tensile strength and specific tear strength of the resulting kraft paper.

[0012] From the viewpoint of improving the specific tensile strength and / or specific tear strength of the resulting kraft paper, the raw material for softwood kraft pulp (NKP) preferably contains one or more species selected from the group consisting of Caribbean pine, Radiata pine, cedar, and larch, and more preferably contains Douglas fir.

[0013] (Freeness) The freeness (Canadian standard freeness) of the pulp constituting the kraft paper may be adjusted so that the freeness of the kraft paper after re-disintegration, as described below, is 500 mL or more. The freeness of the pulp constituting the kraft paper is preferably 500 mL or more, more preferably 520 mL or more, from the viewpoint of the specific tear strength of the resulting kraft paper, and is preferably 760 mL or less, more preferably 700 mL or less, even more preferably 650 mL or less, and even more preferably 630 mL or less, from the viewpoint of the specific tensile strength and / or breaking elongation. The freeness of the pulp constituting the kraft paper is determined by a method in accordance with JIS P 8121:1995. The freeness of the pulp constituting the kraft paper can be adjusted to fall within the above range by appropriately adjusting the beating treatment conditions.

[0014] (average fiber length) The average fiber length of the pulp constituting the kraft paper is preferably 1.0 mm or more, more preferably 1.3 mm or more, and even more preferably 1.5 mm or more, from the viewpoint of achieving both the specific tensile strength and the specific tear strength of the resulting kraft paper. The upper limit is not particularly limited, but from the viewpoint of the texture of the kraft paper, it is preferably 3.0 mm or less, more preferably 2.8 mm or less, and even more preferably 2.5 mm or less. The average fiber length of the pulp that makes up the kraft paper is measured in accordance with ISO 16065-2 (2007).

[0015] (weight average fiber length) The weight-average fiber length of the pulp constituting the kraft paper is preferably 2.2 mm or more, more preferably 2.4 mm or more, even more preferably 2.5 mm or more, still more preferably 2.8 mm or more, and even more preferably 2.9 mm or more, from the viewpoint of achieving both the specific tensile strength and specific tear strength of the resulting kraft paper and also achieving the Stockigt sizing degree. There is no particular upper limit, but from the viewpoint of the texture of the kraft paper, it is preferably 3.7 mm or less, more preferably 3.5 mm or less, even more preferably 3.3 mm or less, and even more preferably 3.1 mm or less. The weight average fiber length of wood pulp is specifically measured in accordance with ISO 16065-2 (2007).

[0016] (average fiber width) From the viewpoint of improving the specific tensile strength and / or specific tear strength of the resulting kraft paper, the average fiber width of the pulp constituting the kraft paper is preferably 20 μm or more, more preferably 25 μm or more, even more preferably 30 μm or more, and is preferably 45 μm or less, more preferably 40 μm or less, even more preferably 35 μm or less. The average fiber width of wood pulp is specifically measured in accordance with ISO 16065-2 (2007).

[0017] (Fine fiber content) The fine fiber content in the pulp constituting the kraft paper is preferably 18% or less, more preferably 15% or less, and even more preferably 12% or less, from the viewpoint of improving the specific tensile strength, specific tear strength, and / or sizing property of the resulting kraft paper. The lower limit is not particularly limited, but from the viewpoint of productivity, it is preferably 1% or more, more preferably 3% or more, and even more preferably 5% or more. In this embodiment, the term "fine fibers" refers to fine fibers with a fiber length of 0.1 mm or less among the pulp constituting the kraft paper. The fine fiber content in the pulp constituting the kraft paper refers to the ratio (%) of the number of fine fibers to the total number of pulp constituting the kraft paper. The fines content in wood pulp is specifically measured in accordance with ISO 16065-2 (2007).

[0018] <Polyacrylamide-based dry strength agent> The kraft paper of this embodiment contains 0.05% by mass or more and 0.75% by mass or less of a polyacrylamide-based dry strength agent. By setting the content of the polyacrylamide dry strength agent in the kraft paper to 0.05% by mass or more, the tensile strength index and Stockigt sizing degree can be improved. On the other hand, by setting the content of the polyacrylamide dry strength agent in the kraft paper to 0.75% by mass or less, aggregation of pulp fibers can be prevented, and a decrease in the tear strength index can be suppressed. The content of the polyacrylamide-based dry strength agent in the kraft paper is preferably 0.03% by mass or more, more preferably 0.10% by mass or more, even more preferably 0.20% by mass or more, and still more preferably 0.30% by mass or more, from the viewpoint of improving the specific tensile strength, specific tear strength and / or sizing property of the resulting kraft paper; and from the viewpoint of improving the breaking elongation, it is preferably 0.70% by mass or less, more preferably 0.60% by mass or less, even more preferably 0.50% by mass or less, still more preferably 0.40% by mass or less, and still more preferably 0.30% by mass or less. In this embodiment, in order to keep the polyacrylamide-based dry strength agent in the kraft paper within the above range, the polyacrylamide-based dry strength agent may be added to the stock used to manufacture the kraft paper during production of the kraft paper. Generally, the content of polyacrylamide-based dry strength agent in the kraft paper tends to be lower than the amount of polyacrylamide-based dry strength agent added to the stock. Therefore, the amount of polyacrylamide-based dry strength agent added to the stock may be appropriately adjusted taking this into consideration. The amount of polyacrylamide-based dry strength agent added to the stock is 0.10 parts by mass or more and 0.78 parts by mass or less per 100 parts by mass of raw pulp in the stock.

[0019] The polyacrylamide-based dry strength agent is preferably one or more selected from the group consisting of anionic polyacrylamide, cationic polyacrylamide, and amphoteric polyacrylamide. These polyacrylamide-based dry strength agents can be used alone or in combination of two or more.

[0020] (anionic polyacrylamide) Examples of anionic polyacrylamides include water-soluble copolymers containing (meth)acrylamide and anionic monomers as constituent components, hydrolyzed polyacrylamides, etc. These anionic polyacrylamides can be used alone or in combination of two or more. In this specification, (meth)acrylamide refers to acrylamide and / or methacrylamide.

[0021] Examples of anionic monomers that can be used as components of the water-soluble copolymer include anionic vinyl monomers such as unsaturated monocarboxylic acids, unsaturated dicarboxylic acids, unsaturated tricarboxylic acids, unsaturated tetracarboxylic acids, unsaturated sulfonic acids, unsaturated phosphonic acids, and salts thereof. These anionic monomers can be used alone or in combination of two or more. In particular, acrylic acid and its salts are preferably used. When polymerizing the anionic water-soluble copolymer, an auxiliary agent such as a chain transfer agent commonly used in polymerization may be added.

[0022] Hydrolyzed polyacrylamide is a decomposition product obtained by decomposing polyacrylamide by a conventionally known hydrolysis reaction. Hydrolyzed polyacrylamide can be obtained, for example, by subjecting a (meth)acrylamide copolymer and / or homopolymer to partial hydrolysis with an alkaline substance such as sodium hydroxide, thereby converting amide groups to carboxy groups.

[0023] (cationic polyacrylamide) Examples of cationic polyacrylamides include water-soluble copolymers containing (meth)acrylamide and cationic monomers as constituent components, Mannich-modified polyacrylamides, and Hoffmann-decomposed polyacrylamides. These cationic polyacrylamides can be used alone or in combination of two or more.

[0024] Examples of cationic monomers that can be used as components of the water-soluble copolymer include cationic vinyl monomers such as vinyl monomers having a tertiary amino group or a quaternary ammonium salt. These cationic monomers can be used alone or in combination of two or more. When polymerizing the cationic water-soluble copolymer, an auxiliary agent such as a chain transfer agent generally used in polymerization may be added.

[0025] Mannich-modified polyacrylamide is a reaction product obtained by modifying polyacrylamide using the conventionally known Mannich reaction. Mannich-modified polyacrylamide can be obtained, for example, by reacting a (meth)acrylamide copolymer and / or homopolymer with formaldehyde and a secondary amine to cationize some of the amide groups. Either an aqueous solution (formalin) or paraformaldehyde can be used as formaldehyde. Examples of secondary amines include dimethylamine, diethylamine, dipropylamine, dibutylamine, diethanolamine, and dipropanolamine. These secondary amines can be used alone or in combination.

[0026] Hoffmann-degraded polyacrylamide is a degradation product obtained by decomposing polyacrylamide by the conventionally known Hoffmann reaction. Hoffmann-degraded polyacrylamide can be obtained, for example, by reacting a (meth)acrylamide copolymer and / or homopolymer with a hypohalite such as hypochlorite (NaOCl) in the presence of an alkaline substance such as sodium hydroxide (NaOH) to convert amide groups (-C=ONH-) to amino groups (-NH2).

[0027] (amphoteric polyacrylamide) Examples of amphoteric polyacrylamides include water-soluble copolymers containing (meth)acrylamide, anionic monomers, and cationic monomers as constituent components. These amphoteric polyacrylamides can be used alone or in combination of two or more.

[0028] Examples of the anionic monomer and cationic monomer used as the constituent components of the amphoteric water-soluble copolymer include the same as those exemplified as the constituent components of the anionic polyacrylamide and cationic polyacrylamide described above. In polymerizing the amphoteric water-soluble copolymer, an auxiliary agent such as a chain transfer agent generally used in polymerization may be added.

[0029] In this embodiment, the polyacrylamide-based dry strength agent is preferably an amphoteric polyacrylamide, from the viewpoint of improving the specific tensile strength and sizing ability of the resulting kraft paper, and more preferably a water-soluble copolymer having (meth)acrylamide, an anionic monomer, and a cationic monomer as constituent components.

[0030] <Optional ingredients> Kraft paper may contain optional components as needed, such as anionic, cationic or amphoteric retention aids, drainage aids, wet strength agents, sizing agents, fixing agents, internal additives such as fillers, water-resistant agents, dyes, and fluorescent whitening agents.

[0031] Examples of the wet strength agent include polyamide polyamine epichlorohydrin, urea formaldehyde resin, melamine formaldehyde resin, etc. The content of the wet strength agent in the kraft paper is not particularly limited, but is preferably 3.0 mass % or less.

[0032] Examples of sizing agents include internal sizing agents such as rosin sizing agents, synthetic sizing agents, and petroleum resin-based sizing agents, and surface sizing agents such as styrene / acrylic acid copolymers and styrene / methacrylic acid copolymers. The content of the sizing agent in the kraft paper is not particularly limited, but is preferably 3.0% by mass or less.

[0033] Examples of fixing agents include aluminum sulfate, polyethyleneimine, etc. The content of the fixing agent in the kraft paper is not particularly limited, but is preferably 3.0% by mass or less.

[0034] Examples of fillers include inorganic fillers such as talc, kaolin, calcined kaolin, calcium carbonate, calcium sulfate, barium sulfate, titanium dioxide, zinc oxide, alumina, magnesium carbonate, magnesium oxide, silica, white carbon, bentonite, zeolite, sericite, and smectite, and organic fillers such as acrylic resins and vinylidene chloride resins.

[0035] <Physical properties of kraft paper> (Basic weight) The basis weight of the kraft paper is not particularly limited, but in the case of packaging bags, it is preferably 30 g / m 2 More preferably, 40 g / m 2 More preferably, 50 g / m 2 and preferably 200 g / m 2 or less, more preferably 150 g / m 2 More preferably, 100 g / m or less 2 or less, even more preferably 80 g / m 2 The basis weight of kraft paper is measured in accordance with JIS P 8124:2011.

[0036] (Thickness) The thickness of the kraft paper is not particularly limited, but when used for packaging bags, it is preferably 60 μm or more, more preferably 80 μm or more, even more preferably 100 μm or more, and preferably 200 μm or less, more preferably 180 μm or less, even more preferably 160 μm or less. The thickness of the kraft paper is measured in accordance with JIS P 8118:2014.

[0037] (density) The density of the kraft paper is not particularly limited, but in the case of packaging bags, it is preferably 0.3 g / cm 3 More preferably, 0.4 g / cm 3 and preferably 1.0 g / cm 3 or less, more preferably 0.8 g / cm 3 or less, more preferably 0.7 g / cm 3 or less, even more preferably 0.6 g / cm 3 The density of the kraft paper is calculated from the basis weight and thickness of the kraft paper obtained by the above-mentioned measurement method.

[0038] (specific tensile strength) From the viewpoint of improving the strength when used as a packaging bag, the tensile strength index of the kraft paper is preferably 40.0 N·m / g or more, more preferably 45.0 N·m / g or more, even more preferably 50.0 N·m / g or more, and even more preferably 52.0 N·m / g or more. There is no particular upper limit to the tensile strength index, but it is preferably 100 N·m / g or less, more preferably 90.0 N·m / g or less. Specific tensile strength (N·m / g) is the ratio of tensile strength (N / m) measured in accordance with JIS P 8113:2006 to basis weight (g / m 2 ) is the value divided by In addition, for oriented kraft paper, the specific tensile strength (N·m / g) is calculated by measuring the tensile strength in the machine direction and the tensile strength in the cross direction in accordance with JIS P 8113:2006, calculating the geometric mean of the tensile strength in the machine direction and the tensile strength in the cross direction, and dividing this by the basis weight. In this specification, the term "longitudinal direction" refers to a direction parallel to the paper-making direction, and the term "transverse direction" refers to a direction perpendicular to the paper-making direction. The tensile strength index of kraft paper can be adjusted by the content of polyacrylamide-based dry strength agent in the kraft paper (i.e., the amount of polyacrylamide-based dry strength agent added to the paper stock), etc. Increasing the content of polyacrylamide-based dry strength agent in the kraft paper tends to increase the tensile strength index of the kraft paper.

[0039] (Specific tear strength) The tear strength of the kraft paper is preferably 10.0 mN m from the viewpoint of improving the strength when used as a packaging bag. 2 / g or more, preferably 11.0 mN m 2 / g or more, and more preferably 12.0 mN m 2 The upper limit of the specific tear strength is not particularly limited, but is preferably 40.0 mN m 2 / g or less, more preferably 30.0 mN m 2 / g or less. Specific tear strength (mN m 2 / g) is the tear strength (mN) measured in accordance with JIS P 8116:2000, multiplied by the basis weight (g / m 2 ) is the value divided by In addition, when the kraft paper is oriented, the specific tear strength (mN m 2 / g) is the value obtained by measuring the tear strength in the machine direction and the tear strength in the cross direction in accordance with JIS P 8116:2000, calculating the geometric mean of the tear strength in the machine direction and the tear strength in the cross direction, and dividing this by the basis weight. The tear strength index of kraft paper can be adjusted by the content of polyacrylamide-based dry strength agent in the kraft paper (i.e., the amount of polyacrylamide-based dry strength agent added to the paper stock), the post-redisintegration freeness of the kraft paper (i.e., the freeness of the pulp that makes up the kraft paper), etc. Decreasing the content of polyacrylamide-based dry strength agent in the kraft paper tends to increase the tear strength index of the kraft paper. Increasing the post-redisintegration freeness of the kraft paper tends to increase the tear strength index of the kraft paper.

[0040] (Stöckigt size) The Stockigt sizing degree of the kraft paper is preferably 12 seconds or more, more preferably 14 seconds or more, even more preferably 15 seconds or more, still more preferably 17 seconds or more, and even more preferably 18 seconds or more, from the viewpoints of improving sizing properties, suppressing ink bleeding during printing and improving printability, and improving strength when wetted with water. The upper limit of the Stockigt sizing degree is not particularly limited, but is preferably 40 seconds or less, more preferably 30 seconds or less. The Stockigt sizing degree is measured in accordance with JIS P 8122:2004. The Stoeckigt sizing degree of kraft paper can be adjusted by the content of polyacrylamide-based dry strength agent in the kraft paper (i.e., the amount of polyacrylamide-based dry strength agent added to the paper stock), the post-redisintegration freeness of the kraft paper (i.e., the freeness of the pulp that makes up the kraft paper), etc. Increasing the content of polyacrylamide-based dry strength agent in the kraft paper tends to increase the Stoeckigt sizing degree of the kraft paper. Decreasing the post-redisintegration freeness of the kraft paper tends to increase the Stoeckigt sizing degree of the kraft paper.

[0041] (Elongation at break) From the viewpoint of improving the strength when used as a packaging bag, the breaking elongation of the kraft paper is preferably 3.1% or more, more preferably 3.5% or more, even more preferably 3.8% or more, and even more preferably 4.0% or more. The upper limit of the breaking elongation is not particularly limited, but from the viewpoint of manufacturability, it is preferably 20% or less, more preferably 10% or less. The elongation at break is measured in accordance with JIS P 8113:2006. In addition, when the kraft paper is oriented kraft paper, the breaking elongation is the geometric mean of the breaking elongation in the longitudinal direction and the breaking elongation in the transverse direction measured in accordance with JIS P 8113:2006. The breaking elongation of kraft paper can be adjusted by the content of polyacrylamide-based dry strength agent in the kraft paper (i.e., the amount of polyacrylamide-based dry strength agent added to the paper stock), the post-redisintegration freeness of the kraft paper (i.e., the freeness of the pulp that makes up the kraft paper), etc. Increasing the content of polyacrylamide-based dry strength agent in the kraft paper tends to increase the breaking elongation of the kraft paper. Decreasing the post-redisintegration freeness of the kraft paper tends to increase the breaking elongation of the kraft paper.

[0042] (Canadian Standard Freeness (post-redefibration freeness) of disintegrated pulp obtained by redefibrating kraft paper) In the kraft paper of this embodiment, the Canadian standard freeness of the disintegrated pulp obtained by disintegrating the kraft paper (freeness of the kraft paper after disintegration) is 500 mL or more. When the Canadian standard freeness of the disintegrated pulp obtained by re-disintegrating kraft paper is 500 mL or more, the decrease in specific tear strength can be suppressed as described above, and the specific tear strength can be made excellent. The Canadian Standard Freeness of the disintegrated pulp obtained by redisintegrating the kraft paper is preferably 520 mL or more, more preferably 540 mL or more, from the viewpoint of the tear strength index of the resulting kraft paper, and is preferably 800 mL or less, more preferably 780 mL or less, even more preferably 760 mL or less, still more preferably 700 mL or less, and even more preferably 650 mL or less, from the viewpoint of the tensile strength index and / or breaking elongation. The Canadian Standard Freeness of disintegrated pulp obtained by re-defibrating kraft paper is determined by disintegrating the kraft paper using a method in accordance with JIS P 8220-1:2012 and then using the resulting disintegrated pulp using a method in accordance with JIS P 8121:1995. The freeness of the kraft paper after defibration can be adjusted within the above range depending on the freeness of the pulp that constitutes the kraft paper.

[0043] [Kraft paper manufacturing method] In this embodiment, the method for producing kraft paper is not particularly limited. For example, the method may include a cooking step of cooking pulp constituting the kraft paper (hereinafter also referred to as "raw material pulp"), a beating step of beating a dispersion containing the cooked raw material pulp, and a papermaking step of making paper from the beaten raw material pulp. Each step of the production method is described below.

[0044] (Cooking process) The cooking step is a step of cooking raw pulp. In this step, although not particularly limited, raw chips used as a material for the raw pulp are preferably treated with a chemical solution containing sodium hydroxide. As a treatment method with a chemical solution containing sodium hydroxide, a known treatment method using a known chemical solution can be used.

[0045] The raw material chips used as the raw material pulp preferably contain softwood pulp as the main component. "Softwood pulp-based raw material chips" refers to raw material chips whose softwood chip content is more than 50% by mass, and the softwood chip content is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 100% by mass.

[0046] The raw material pulp may be unbleached or may be bleached. The raw material pulp is preferably one or more selected from the group consisting of bleached kraft pulp and unbleached kraft pulp, and more preferably unbleached kraft pulp.

[0047] (beating process) The beating process is a process of beating a dispersion containing the cooked raw pulp. In the beating process of this embodiment, the resulting kraft paper is re-disintegrated to obtain a disintegrated pulp having a Canadian standard freeness of 500 mL or more. By beating the disintegrated pulp obtained by re-disintegrating the kraft paper to obtain a Canadian standard freeness within the above range, the decrease in tear strength index can be suppressed and the tear strength index can be improved, as described above. In the beating process, the raw pulp concentration of the dispersion is not particularly limited, but is preferably 1% by mass or more, and preferably 35% by mass or less, and more preferably 15% by mass or less. The beating method is not particularly limited, but it is preferable to disperse the cooked raw pulp in water to prepare a dispersion having the above raw pulp concentration, and then beat the dispersion. The beating method is not particularly limited, but can be performed using a beater such as a double-disc refiner, a single-disc refiner, a conical refiner, or a deluxe refiner.

[0048] By beating a dispersion containing cooked raw pulp, kraft paper having excellent specific tensile strength, specific tear strength and sizing properties can be obtained, and productivity is also excellent.

[0049] (Paper making process) The papermaking process is a process of making paper from beaten raw pulp. In the papermaking process of this embodiment, a polyacrylamide-based dry strength agent is added so that the content of the polyacrylamide-based dry strength agent in the resulting kraft paper is 0.05% by mass or more and 0.75% by mass or less. In order to ensure that the content of the polyacrylamide-based dry strength agent in the resulting kraft paper is within the above range, the amount of polyacrylamide-based dry strength agent added in the papermaking process is preferably 0.10 parts by mass or more and 0.78 parts by mass or less per 100 parts by mass of raw pulp. By making paper so that the content of the polyacrylamide-based dry strength agent in the kraft paper is within the above range, both the specific tensile strength and specific tear strength can be improved, and further, the Stockigt sizing degree and sizing ability can be improved. The papermaking method is not particularly limited, and examples thereof include an acidic papermaking method in which papermaking is carried out at a pH of around 4.5, and a neutral papermaking method in which papermaking is carried out at a pH of about 6 to about 9. In the papermaking process, chemicals for the papermaking process such as pH adjusters, antifoaming agents, pitch control agents, slime control agents, etc. can be added as needed. The papermaking machine is also not particularly limited, and examples thereof include continuous papermaking machines such as Fourdrinier, cylinder, and tilting types, or multi-layer papermaking machines that combine these.

[0050] <Application> As described above, the kraft paper of this embodiment has excellent specific tensile strength, specific tear strength, and sizing properties, and therefore can be suitably used as kraft paper for packaging bags (especially heavy-duty packaging bags). In particular, it can be suitably used as kraft paper for heavy packaging bags for industrial use, such as cement, fertilizer, and pesticides.

[0051] The packaging body of this embodiment is a packaging body made of the above-mentioned kraft paper. Since the above-mentioned kraft paper has excellent specific tensile strength, specific tear strength, and sizing properties, the packaging body of this embodiment can be particularly suitably used as a packaging bag for carrying heavy industrial goods such as cement, fertilizer, and pesticides. [Example]

[0052] Examples are given below to specifically explain the present invention, but the present invention is not limited to these examples. Unless otherwise specified, the following operations were carried out under conditions of 23°C and a relative humidity of 50%RH. Furthermore, in the examples and comparative examples, "parts" and "%" refer to "parts by mass" and "% by mass," respectively, unless otherwise specified.

[0053] [Kraft paper manufacturing] Example 1 Kraft cooking was carried out using softwood chips including Douglas fir. After kraft cooking, the black liquor was separated, and the resulting chips were disintegrated using a disintegrator. Then, the chips were centrifuged and washed with water three times using a filter cloth. The undigested material was then removed using a screen, and the resulting chips were centrifuged and dewatered to obtain unbleached softwood kraft pulp (NUKP). The resulting NUKP was beaten using a refiner. The properties of the pulp obtained after beating are shown in Tables 1 and 2. To the NUKP slurry obtained after beating, 0.04 parts of rosin as a sizing agent, 1.2 parts of aluminum sulfate as a fixing agent, and 0.75 parts of amphoteric polyacrylamide (OKS-20 manufactured by Arakawa Chemical Industries Co., Ltd.) as a paper strength agent were added with stirring per 100 parts of NUKP content (dry mass) to prepare a paper stock. Using the above paper stock, hand-made sheets were prepared according to JIS P 8222:2015, with a basis weight of 62.2 g / m 2 Kraft paper was obtained.

[0054] <Example 2> A hand-made sheet was prepared in the same manner as in Example 1, except that the amount of amphoteric polyacrylamide added to the paper stock was changed to 0.60 parts. 2 Kraft paper was obtained.

[0055] Example 3 A hand-made sheet was prepared in the same manner as in Example 1, except that the degree of beating of the pulp was increased and the amount of amphoteric polyacrylamide added to the paper stock was changed to 0.60 parts. 2 Kraft paper was obtained.

[0056] Example 4 A hand-made sheet was prepared in the same manner as in Example 1, except that the degree of beating of the pulp was increased and the amount of amphoteric polyacrylamide added to the paper stock was changed to 0.30 parts. 2 Kraft paper was obtained.

[0057] <Example 5> A hand-made sheet was prepared in the same manner as in Example 4, except that the amount of amphoteric polyacrylamide added to the paper stock was changed to 0.10 parts. The basis weight was 63.5 g / m. 2 Kraft paper was obtained.

[0058] <Comparative Example 1> A hand-made sheet was prepared in the same manner as in Example 1, except that no amphoteric polyacrylamide was added to the paper stock. 2 Kraft paper was obtained.

[0059] <Comparative Example 2> A hand-made sheet was prepared in the same manner as in Example 4, except that the amount of amphoteric polyacrylamide added to the paper stock was changed to 1.00 parts. The basis weight was 63.6 g / m. 2 Kraft paper was obtained.

[0060] <Comparative Example 3> A hand-made sheet was prepared in the same manner as in Example 1, except that the degree of beating of the pulp was increased and the amount of amphoteric polyacrylamide added to the paper stock was changed to 0.30 parts. The basis weight was 64.5 g / m. 2 Kraft paper was obtained.

[0061] <Comparative Example 4> A hand-made sheet was prepared in the same manner as in Comparative Example 3, except that no amphoteric polyacrylamide was added to the paper stock, and the basis weight was 64.4 g / m. 2 Kraft paper was obtained.

[0062] Example 6 Using the paper stock prepared in the same manner as in Example 1, kraft paper was produced on a Fourdrinier paper machine at a papermaking speed of 700 m / min.

[0063] [Measurement method] <Measurement of pulp properties> (Freeness) The pulp in the pulp slurry obtained after beating was measured for Canadian standard freeness according to the method of JIS P 8121:1995.

[0064] (length average fiber length, weight average fiber length, average fiber width and fine fiber content) The length-average fiber length, weight-average fiber length, average fiber width, and fine fiber content of the pulp in the pulp slurry obtained after beating were measured using a fiber length measuring instrument (model: FS-5 (with UHD base unit), manufactured by Valmet) in accordance with ISO 16065-2 (2007).

[0065] <Measurement of Kraft Paper Properties> (specific tensile strength) The tensile strength of the kraft paper obtained in the examples and comparative examples was measured by a method in accordance with JIS P 8113:2006, and the specific tensile strength was calculated by dividing the obtained tensile strength (for Example 6, the geometric mean of the tensile strength in the machine direction and the tensile strength in the cross direction) by the basis weight.

[0066] (Specific tear strength) The tear strength of the kraft paper obtained in the examples and comparative examples was measured according to JIS P 8116:2000, and the specific tear strength was calculated by dividing the obtained tear strength (for Example 6, the geometric mean of the tear strength in the longitudinal direction and the tear strength in the transverse direction) by the basis weight.

[0067] (Stöckigt size) The Stockigt sizing degree of the kraft papers obtained in the examples and comparative examples was measured by a method in accordance with JIS P 8122:2004.

[0068] (Elongation at break) In accordance with JIS P 8113:2006, the kraft paper obtained in the examples and comparative examples was left to stand in an environment of 23±5°C and 50±10% humidity for at least one day to adjust the temperature and humidity, and then cut into samples 15 mm wide and 150 mm long. The sample was attached to a tensile testing machine (model RTC-1210A, manufactured by A&D Co., Ltd.) with a chuck distance of 100 mm, and a tensile test was performed at a speed of 10 mm / min to measure the breaking elongation.

[0069] (Post-redefibration freeness (Canadian standard freeness of disintegrated pulp obtained by redefibrating kraft paper)) The kraft papers of the Examples and Comparative Examples were disintegrated by a method in accordance with JIS P 8220-1:2012, and the Canadian standard freeness of the resulting disintegrated pulp was measured by a method in accordance with JIS P 8121:1995.

[0070] (Measurement of residual dry strength agent amount (method for measuring polyacrylamide compound content)) The content of polyacrylamide compounds (converted to solid content) contained in the kraft paper of the Examples and Comparative Examples was measured using a pyrolysis GC / MS analyzer under the following conditions, and the content was determined by quantifying the detected ion m / z 125 (3-Ethylidene-2,5-pyrrolidinedione) using a calibration curve. [Measurement sample] About 100 to 150 μg of the kraft paper of the examples and comparative examples was precisely weighed out and used as a measurement sample. [Measurement conditions] GC: Column HP-INNOWAX (length 30 m, inner diameter 0.25 μm, outer diameter 0.25 mm) Carrier gas: He MS:EI(+) Conditions: 50℃→(10℃ / min)→260℃held for 15 minutes Pyrolysis equipment: Frontier Lab PY-3030D, decomposition temperature 500℃ Analysis: Calculate the content of polyacrylamide compounds using the selected ion m / z 125

[0071] [Creating a calibration curve] (1) Measurement of the concentration of the undiluted polyacrylamide compound solution 1 g of a polyacrylamide compound stock solution (OKS-20, manufactured by Arakawa Chemical Industries, Ltd.) was weighed out and placed in an aluminum cup of known mass, and dried to dryness at 105°C for 2 hours or more. The total mass of the aluminum cup and the dried polyacrylamide compound stock solution was then measured, and the concentration of the polyacrylamide compound stock solution was calculated using the following formula. Concentration of the polyacrylamide compound stock solution (mass%) = {Total mass (g) of the dried aluminum cup and the dried polyacrylamide compound stock solution - Mass (g) of the aluminum cup} ÷ Mass (g) of the polyacrylamide compound stock solution × 100 (2) Preparation of standard solutions of polyacrylamide compounds A predetermined amount of the polyacrylamide compound stock solution was placed in a 50 mL cup and diluted to 10 g with ion-exchanged water to prepare a standard solution of the polyacrylamide compound. The amounts of the polyacrylamide compound stock solution were 1.0 g, 2.5 g, and 5.0 g, yielding three standard solutions with different concentrations. The concentrations of the resulting standard solutions were measured using the same method as in (1) above. (3) Preparation of standard filter paper samples After measuring the dry mass of the filter paper, the filter paper was impregnated with the target amount of a standard solution of a polyacrylamide compound and dried in a dryer at 105°C for 1 hour to prepare a standard filter paper sample. The standard filter paper sample was then pulverized in a grinder and weighed out to approximately 100-150μg, which was used to create a calibration curve. The target amount of the standard solution of polyacrylamide compound to be added to the filter paper to prepare the standard filter paper sample was calculated using the following formula. Target amount of standard solution added (g) = dry mass of filter paper (g) × target polyacrylamide compound content (mass%) ÷ standard solution concentration of polyacrylamide compound (mass%) (4) Preparation of a calibration curve The obtained standard filter paper sample was measured using a pyrolysis GC / MS analyzer, and a calibration curve was created from the measurement results of the detected ion m / z 125 (3-Ethylidene-2,5-pyrrolidinedione).

[0072] [Table 1]

[0073] [Table 2]

[0074] The results in Tables 1 and 2 show that the kraft papers of Examples 1 to 6 are excellent in both specific tensile strength and specific tear strength, as well as in sizing ability.

Claims

1. Kraft paper made primarily of wood pulp, The content of softwood kraft pulp in the wood pulp is 100% by mass, The Canadian standard freeness of the disintegrated pulp obtained by redisintegrating the kraft paper is 520 mL or more; The content of the polyacrylamide-based dry strength agent in the kraft paper is 0.20% by mass or more and 0.75% by mass or less, Specific tear strength is 10.0 mN m ２ / g or more, Basis weight: 30 g / m ２ More than 100g / m ２ or less (however, the basis weight is 95 g / m ２ (excluding the above) Kraft paper (excluding kraft paper filled with light calcium carbonate).

2. 2. The kraft paper according to claim 1, wherein the weight average fiber length of the pulp constituting the kraft paper is 2.2 mm or more and 3.7 mm or less.

3. 10. The kraft paper of claim 1, wherein the softwood kraft pulp source comprises Douglas fir.

4. 2. The kraft paper according to claim 1, having a tensile strength index of 40.0 N·m / g or more.

5. 2. The kraft paper of claim 1, having a Stockigt sizing degree of 12 seconds or more.

6. 2. The kraft paper of claim 1, having an elongation at break of 3.1% or more.

7. 2. The kraft paper of claim 1, which is a kraft paper for heavy-duty bags.

8. A packaging bag made using the kraft paper according to any one of claims 1 to 7.

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