Kraft paper and manufacturing method of kraft paper

Abstract

To provide kraft paper which can be manufactured at low cost and has excellent strengths such as tear strength.SOLUTION: Kraft paper includes a hardwood kraft pulp as a raw material pulp, wherein a content of the hardwood kraft pulp in the raw material pulp is 90 mass% or over, a pulp after disintegration has a kappa value of 9.5 or over, and a pulp viscosity of kraft paper measured in accordance with TAPPI T 230 is 15 mPa s or over.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to kraft paper and a method for making kraft paper. [Background technology]

[0002] Kraft paper is a strong paper made from kraft pulp. Kraft paper is used, for example, for light packaging to hold small items, and for heavy packaging to hold heavy industrial materials such as cement, fertilizer, and pesticides. Kraft paper used for the above purposes is required to be strong enough to withstand shocks from the contents caused by vibrations during transportation.

[0003] Patent Document 1 discloses a kraft paper that contains bleached kraft pulp, a sizing agent, a paper strength agent, aluminum sulfate, and a (meth)acrylamide resin, but contains substantially no fillers, with the bleached kraft pulp containing a specific range of softwood bleached kraft pulp having a CSF freeness of a specific value or more, and a specific range of hardwood bleached kraft pulp having a CSF freeness of a specific range, with the objective of providing a kraft paper that is substantially free of fillers and has excellent whiteness, and good paper strength and printability.

[0004] Patent Document 2 discloses a kraft paper mainly composed of kraft pulp, the object of which is to provide kraft paper having sufficient paper strength while using pulp with thick fiber walls and small internal voids, and a manufacturing method thereof. The kraft paper is characterized in that the disintegrated pulp obtained by disintegrating the kraft paper by the pulp disintegration method described in JIS P 8220:2012 has an overall average fiber length within a specific range, and contains a fraction (A) of fibers having a fiber length equal to or greater than a specific value and a fraction (B) of fibers having a fiber length equal to or less than a specific value, the Runkel ratio of fraction (A) being equal to or greater than a specific value, and the total content of fraction (A) and fraction (B) in the disintegrated pulp being equal to or greater than a specific value.

[0005] Patent Document 3 discloses a kraft paper made on a gap former type papermaking machine with the aim of providing a kraft paper with small difference between the front and back sides and sufficiently increased strength, the kraft paper being characterized in that the tensile strength in the MD relative to the CD is within a specific range, and the tensile strength in the CD is also within a specific range.

[0006] Patent Document 4 discloses bleached kraft paper that satisfies the following conditions (1) to (8) for the purpose of providing kraft packaging paper that has a sufficiently high sizing effect even under conditions of a high recycled paper pulp content, resolves the problems of paper dust during printing and bumps during handling, and is suitable for processing as packaging paper. (1) In the raw pulp, the content of softwood bleached kraft pulp is within a specific range, and the content of hardwood bleached kraft pulp is within a specific range. (2) The freeness of the softwood bleached kraft pulp is within a specific range, and the freeness of the hardwood bleached kraft pulp is within a specific range. (3) A specific amount or more of calcium carbonate is added as a filler, and the ash content is within a specific range. (4) A surface treatment agent mainly composed of an adhesive is applied in a specific amount per side. (5) An internal sizing agent and a surface sizing agent are used to achieve a Stockigt sizing degree of a specific value or more. (6) The whiteness is set to a specific value or more. (7) The oil absorption is set to a specific range. (8) The smoothness is set to a specific range. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2022-90158 [Patent Document 2] Patent Publication No. 2021-38487 [Patent Document 3] JP 2015-21209 A [Patent Document 4] JP 2013-100624 A Summary of the Invention [Problem to be solved by the invention]

[0008] The kraft papers described in Patent Documents 1 to 4 use softwood kraft pulp as the raw material pulp from the viewpoint of strength required for kraft paper, such as tear strength and tensile strength. However, softwood kraft pulp is more expensive than hardwood kraft pulp, so there has been a demand for the development of kraft paper that is less expensive and satisfies the strength required for kraft paper by using raw material pulp mainly composed of hardwood kraft pulp.

[0009] An object of the present invention is to provide a kraft paper which can be produced at low cost and has excellent strength properties such as tear strength. [Means for solving the problem]

[0010] The present inventors have found that the above problems can be solved in kraft paper containing hardwood kraft pulp as the raw material pulp by adjusting the content of hardwood kraft pulp in the raw material pulp to at least a specific value, adjusting the kappa number of the pulp after redefibration to at least a specific value, and adjusting the pulp viscosity of the kraft paper, as measured in accordance with TAPPI T 230, to at least a specific value. <1> A kraft paper containing hardwood kraft pulp as a raw material pulp, wherein the hardwood kraft pulp content in the raw material pulp is 90 mass% or more, the kappa number of the pulp after redisintegration is 9.5 or more, and the pulp viscosity of the kraft paper, measured in accordance with TAPPI T 230, is 15 mPa s or more. <2> the kappa number of the pulp after redisintegration is 30 or less, and the viscosity of the pulp of the kraft paper is 120 mPa s or less; <1> 2. Kraft paper according to claim 1. <3> Specific tear strength in the longitudinal direction is 8.0mN·m 2 / g or more, and the transverse tear strength is 8.8mN m 2 / g or more, <1> or <2> 2. Kraft paper according to claim 1. <4> Basis weight 70g / m 2 More than 150g / m 2Below is the <1> ~ <3> 2. The kraft paper according to claim 1 . <5> Density 0.65g / cm 3 More than 0.85g / cm 3 Below is the <1> ~ <4> 2. The kraft paper according to claim 1 . <6> Used for light packaging, <1> ~ <5> 2. The kraft paper according to claim 1 . <7> A method for producing kraft paper by papermaking a raw material pulp containing hardwood kraft pulp, wherein the raw material pulp contains 90% by mass or more of hardwood kraft pulp, has a kappa number of 9.5 or more, and has a pulp viscosity of 15 mPa s or more as measured in accordance with TAPPI T 230, and the method for producing the raw material pulp includes a cooking step of cooking wood, and an active alkali concentration in the cooking step is 10% by mass or more and 30% by mass or less. Effect of the Invention

[0011] According to the present invention, there is provided a kraft paper which can be produced at low cost and has excellent strength properties such as tear strength and tensile strength. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A preferred embodiment of the present invention will be described below. In this specification, the range "X to Y" means "X or more and Y or less". In this specification, unless otherwise specified, operations and measurements of physical properties are performed under conditions of room temperature (20 to 25°C) and relative humidity of 40 to 55%. In this specification, the "longitudinal direction" of the kraft paper means the direction corresponding to the papermaking direction (MD direction) of the kraft paper. In addition, the "transverse direction" of the kraft paper means the direction corresponding to the width direction (CD direction) of the kraft paper.

[0013] [Kraft paper] The kraft paper of this embodiment (hereinafter also simply referred to as kraft paper) is a kraft paper containing hardwood kraft pulp as raw material pulp, in which the hardwood kraft pulp content in the raw material pulp is 90 mass% or more, the kappa number of the pulp after redisintegration is 9.5 or more, and the pulp viscosity of the kraft paper, measured in accordance with TAPPI T 230, is 15 mPa s or more.

[0014] The kraft paper of the present embodiment is made of raw pulp containing 90% or more by mass of hardwood kraft pulp, and therefore can be produced at a lower cost than kraft paper made of raw pulp mainly composed of softwood kraft pulp, which is conventionally used in terms of the strength required for kraft paper. When made of raw pulp containing hardwood kraft pulp, the strength of the kraft paper tends to decrease. However, even when the kraft paper is made of raw pulp containing 90% or more by mass of hardwood kraft pulp, the kappa number of the pulp after redisintegration is set to a specific value or more, and the pulp viscosity of the kraft paper is set to a specific value or more, so that the kraft paper can have excellent strength such as tear strength required for kraft paper. The mechanism by which this effect is achieved is not clear, but is presumed to be as follows. By moderately cooking the wood chips (wood chips) used as the material for the raw pulp without excessively proceeding, the disintegration of the pulp (cellulose) is suppressed, and as a result, it is considered that the kraft paper has excellent strength such as tear strength required for kraft paper. The effect of the present invention is not limited to the above mechanism.

[0015] <Raw pulp> The kraft paper of the present embodiment is made of raw material pulp containing hardwood kraft pulp. Specifically, the content of hardwood kraft pulp in the raw material pulp is 90% by mass or more, preferably 92% by mass or more, more preferably 94% by mass or more, and 100% by mass or less. As the raw material for hardwood kraft pulp, from the viewpoint of easily adjusting the kappa number of the pulp after redisintegration and the pulp viscosity of the kraft paper to the desired range, it is preferable to use one or more species selected from the group consisting of the Eucalyptus genus and the Acacia genus, and more preferably the Eucalyptus genus. The hardwood kraft pulp may be any of unbleached hardwood kraft pulp, semi-bleached hardwood kraft pulp, and bleached hardwood kraft pulp, so long as the kappa number and pulp viscosity of the re-disintegrated pulp can be within the desired range. Among these, from the viewpoint of easily adjusting the kappa number of the re-disintegrated pulp and the pulp viscosity of the kraft paper to the desired range, preferably one or more selected from the group consisting of semi-bleached hardwood kraft pulp and unbleached hardwood kraft pulp, and more preferably unbleached hardwood kraft pulp.

[0016] The raw material pulp may be hardwood kraft pulp, or may be kraft pulp such as softwood kraft pulp, mechanical pulp, recycled paper pulp, or herbaceous pulp such as hemp. When the raw material pulp contains pulp other than hardwood kraft pulp (preferably hardwood semi-bleached kraft pulp or hardwood unbleached kraft pulp), among these, softwood kraft pulp is preferred, and softwood unbleached kraft pulp is more preferred. When softwood kraft pulp is used, the raw material for the softwood kraft pulp is preferably one or more species selected from the group consisting of the genus Fir and the genus Pinus, more preferably the genus Fir, from the viewpoint of making it easy to adjust the kappa number of the pulp after redisintegration and the pulp viscosity of the kraft paper to the desired ranges.

[0017] <Kappa number> From the viewpoint of improving strength such as tear strength required for kraft paper, the kappa number of the pulp constituting the kraft paper after re-disintegration (re-disintegrated pulp) is 9.5 or more, preferably 9.7 or more, more preferably 10 or more, and from the viewpoint of pulp yield, it is preferably 50 or less, more preferably 40 or less, and even more preferably 30 or less. The kappa number of the re-disintegrated pulp is measured in accordance with JIS P 8211:2011 using kraft paper pulp disintegrated in accordance with JIS P 8220-1:2012 as a sample. The kappa number of the pulp that makes up kraft paper after re-disintegration (re-disintegrated pulp) can be adjusted by adjusting the type of raw material for the raw pulp, the active alkali concentration during the cooking treatment described below, the cooking treatment time described below, and the cooking treatment temperature described below.

[0018] <Pulp viscosity> From the viewpoint of improving strength such as tear strength required of kraft paper, the pulp viscosity of the kraft paper is 15 mPa·s or more, preferably 16 mPa·s or more, more preferably 17 mPa·s or more, and from the viewpoint of runnability during the production of the kraft paper, it is preferably 120 mPa·s or less, more preferably 100 mPa·s or less, even more preferably 80 mPa·s or less, still more preferably 70 mPa·s or less, still more preferably 60 mPa·s or less, still more preferably 50 mPa·s or less, and still more preferably 40 mPa·s or less. The pulp viscosity of the kraft paper is measured in accordance with TAPPI T 230. The pulp viscosity of kraft paper can be adjusted by adjusting the type of raw material pulp, the active alkali concentration during the cooking treatment described below, the cooking treatment time described below, and the cooking treatment temperature described below.

[0019] <Tear strength ratio: longitudinal direction> The longitudinal tear strength of the kraft paper is preferably 8.0 mN m from the viewpoint of effectively suppressing bag breakage when the kraft paper is used for light packaging, for example. 2 / g or more, preferably 8.3 mN m 2 / g or more, and more preferably 8.5 mN m2 / g or more, and the upper limit is not particularly limited, but is preferably 30 mN m 2 / g or less, preferably 20 mN m 2 The longitudinal tear strength of kraft paper is measured in accordance with JIS P 8116:2000.

[0020] <Tear strength: transverse direction> The transverse tear strength of the kraft paper is preferably 8.8 mN m from the viewpoint of effectively suppressing bag breakage when the kraft paper is used for light packaging, for example. 2 / g or more, preferably 9.0 mN m 2 / g or more, and the upper limit is not particularly limited, but is preferably 30 mN m 2 / g or less, preferably 20 mN m 2 The transverse tear index of kraft paper is measured in accordance with JIS P 8116:2000.

[0021] <Basic weight> The basis weight of the kraft paper is not particularly limited, but from the viewpoint of strength when the kraft paper is used for light packaging, for example, it is preferably 70 g / m 2 More preferably, 90 g / m 2 More preferably, 100 g / m 2 More preferably, 110 g / m 2 From the viewpoint of ease of production of the kraft paper and processability of the kraft paper, it is preferably 150 g / m 2 Less than 140 g / m 2 More preferably, 130 g / m 2 The basis weight of kraft paper is measured in accordance with JIS P 8124:2011.

[0022] <Thickness> The thickness of the kraft paper is not particularly limited, but from the viewpoint of strength when the kraft paper is used for light packaging, for example, it is preferably 100 μm or more, more preferably 120 μm or more, and even more preferably 140 μm or more, and from the viewpoint of ease of production of the kraft paper and processability of the kraft paper, it is preferably 200 μm or less, more preferably 180 μm or less, and even more preferably 170 μm or less. The thickness of the kraft paper is measured in accordance with JIS P 8118:2014.

[0023] <density> The density of the kraft paper is not particularly limited, but from the viewpoint of strength when the kraft paper is used for light packaging, for example, the density is preferably 0.60 g / cm 3 More preferably, 0.63 g / cm 3 More preferably, 0.65 g / cm 3 More preferably, 0.67 g / cm 3 More preferably, 0.70 g / cm 3 From the viewpoint of ease of production of the kraft paper and processability of the kraft paper, it is preferably 0.90 g / cm 3 Less than or equal to 0.85 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.

[0024] <Tensile strength index: longitudinal direction> The longitudinal tensile index of kraft paper is preferably 55 N·m / g or more, more preferably 60 N·m / g or more, and even more preferably 65 N·m / g or more, from the viewpoint of effectively preventing bag breakage when the kraft paper is used for light packaging, for example, and the upper limit is not particularly limited, but from the viewpoint of ease of production of the kraft paper, it is preferably 150 N·m / g or less, more preferably 120 N·m / g or less. The longitudinal tensile index of kraft paper is measured in accordance with JIS P 8113:2006.

[0025] <Tensile strength index: transverse direction> The tensile index of kraft paper in the cross direction is preferably 25 N·m / g or more, more preferably 30 N·m / g or more, and even more preferably 32 N·m / g or more, from the viewpoint of effectively preventing bag breakage when the kraft paper is used for light packaging, for example, and the upper limit is not particularly limited, but from the viewpoint of ease of production of the kraft paper, it is preferably 100 N·m / g or less, more preferably 80 N·m / g or less, and even more preferably 70 N·m / g or less. The tensile index of kraft paper in the cross direction is measured in accordance with JIS P 8113:2006.

[0026] <Specific burst strength> The burst strength of the kraft paper is preferably 3.0 kPa·m from the viewpoint of effectively suppressing bag breakage when the kraft paper is used for light packaging, for example. 2 / g or more, preferably 3.5 kPa m 2 / g or more, and the upper limit is not particularly limited, but from the viewpoint of ease of production of kraft paper, it is preferably 10.0 kPa m 2 / g or less, more preferably 8.0 kPa m 2 / g or less, and more preferably 7.0 kPa m 2 The burst strength of kraft paper is measured in accordance with JIS P 8112:2008.

[0027] [Kraft paper manufacturing method] The method for producing kraft paper of this embodiment is a method for producing kraft paper produced by papermaking a raw material pulp containing hardwood kraft pulp, wherein the hardwood kraft pulp content in the raw material pulp is 90% by mass or more, the kappa number of the raw material pulp is 9.5 or more, and the pulp viscosity of the raw material pulp, measured in accordance with TAPPI T 230, is 15 mPa s or more, the method for producing the raw material pulp includes a cooking step of cooking wood, and the active alkali concentration in the cooking step is 10% by mass or more and 30% by mass or less.

[0028] <Cooking process> The cooking process is a process in which wood is cooked. By cooking the wood chips used as the raw pulp material under conditions of an active alkali concentration of 10% by mass to 30% by mass, raw pulp with a kappa number of 9.5 or more and a pulp viscosity of 15 mPa s or more is obtained.

[0029] The apparatus used for the cooking treatment is not particularly limited, but may be a general-purpose continuous cooker, batch cooker, etc. As the cooking method, a known cooking method such as kraft cooking, polysulfide cooking, soda cooking, alkali sulfite cooking, etc. may be used, but from the viewpoints of pulp quality, energy efficiency, etc., the kraft cooking method is preferred.

[0030] (Raw pulp) In this embodiment, the wood chips used as the raw material pulp material are chips mainly composed of broad-leaved trees. The broad-leaved trees used for the wood chips are preferably at least one species selected from the group consisting of Eucalyptus and Acacia, more preferably Eucalyptus, from the viewpoint of facilitating adjustment of the kappa number and pulp viscosity of the raw material pulp to the desired ranges.

[0031] The content of hardwood kraft pulp in the raw material pulp obtained from wood chips is 90% by mass or more, preferably 92% by mass or more, more preferably 94% by mass or more, and 100% by mass or less. The raw material pulp may be hardwood kraft pulp (preferably hardwood semi-bleached kraft pulp or hardwood unbleached kraft pulp), as well as kraft pulp such as softwood kraft pulp, mechanical pulp, waste paper pulp, and herbaceous pulp such as hemp. When the raw material pulp contains pulp other than hardwood kraft pulp, among these, softwood kraft pulp is preferable, and softwood unbleached kraft pulp is more preferable. When the raw material pulp contains coniferous kraft pulp, the coniferous tree used for the wood chips is preferably one or more species selected from the group consisting of the genus Fructus and the genus Pinus, more preferably the genus Fructus, from the viewpoint of making it easier to adjust the kappa number and pulp viscosity of the raw material pulp to the desired ranges.

[0032] <Kappa number> From the viewpoint of improving strength such as tear strength required for kraft paper, the kappa number of the raw material pulp is 9.5 or more, preferably 9.7 or more, more preferably 10 or more, and from the viewpoint of pulp yield, it is preferably 50 or less, more preferably 40 or less, and even more preferably 30 or less. The kappa number of the raw material pulp is measured in accordance with JIS P 8211:2011.

[0033] <Pulp Viscosity> From the viewpoint of improving strength such as tear strength required for kraft paper, the pulp viscosity of the raw material pulp is 15 mPa·s or more, preferably 16 mPa·s or more, more preferably 17 mPa·s or more, and from the viewpoint of runnability during the production of kraft paper, it is preferably 120 mPa·s or less, more preferably 100 mPa·s or less, even more preferably 80 mPa·s or less, still more preferably 70 mPa·s or less, still more preferably 60 mPa·s or less, still more preferably 50 mPa·s or less, and still more preferably 40 mPa·s or less. The pulp viscosity of the raw material pulp is measured in accordance with TAPPI T 230.

[0034] (Active alkali concentration) The active alkali concentration in the cooking step is 10% by mass or more and 30% by mass or less, preferably 27% by mass or less, and more preferably 24% by mass or less, from the viewpoint of facilitating adjustment of the kappa number and pulp viscosity of the raw pulp to the desired ranges. The active alkali concentration in the cooking white liquor is the active alkali concentration relative to the wood chips (bone dry mass) used as the material for the raw pulp, and is converted as the addition rate of Na2O. Specifically, it is calculated by the following formula. Active alkali concentration = NaOH addition rate x 0.775 + Na2S addition rate x 0.795

[0035] (sulfidity) The sulfidity of the cooking white liquor is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 20% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. In the case of the kraft cooking method, the sulfidity of the cooking white liquor is determined by the percentage of Na2S converted into Na2O relative to the active alkali, and is specifically calculated by the following formula (JIS P 0001:1998). Sulfidity (mass%)=Na2S / (NaOH+Na2S)×100

[0036] (liquid ratio) The ratio of the mass of the cooking white liquor to the wood chips (bone dry mass) (liquid ratio) is preferably 1 or more, more preferably 2 or more, and even more preferably 3 or more from the viewpoint of uniformity of pulp quality, and is preferably 10 or less, more preferably 8 or less, and even more preferably 6 or less from the viewpoint of production efficiency of kraft paper.

[0037] (Cooking temperature) The treatment temperature (cooking temperature) in the cooking step is preferably 100°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher, from the viewpoint of making it easier to adjust the kappa number and pulp viscosity of the raw material pulp to the desired ranges, and is preferably 250°C or lower, more preferably 200°C or lower, and even more preferably 180°C or lower.

[0038] (Cooking time) The treatment time (cooking time) in the cooking step is preferably 30 minutes or more, more preferably 1 hour or more, and even more preferably 1.5 hours or more, from the viewpoint of making it easier to adjust the kappa number and pulp viscosity of the raw material pulp to the desired ranges, and is preferably 4 hours or less, and more preferably 3 hours or less.

[0039] <Refining process> The cooked raw pulp is preferably beaten in order to more effectively increase the tear strength of the kraft paper. The method of beating is not particularly limited, but it is preferable to disperse the cooked raw pulp in water to prepare a pulp dispersion and beat the pulp. The method of beating is not particularly limited, but it can be performed using a refiner such as a double disc refiner, a single disc refiner, or a conical refiner.

[0040] The Canadian Standard Freeness (CSF) of the beaten raw pulp is not particularly limited, but from the viewpoint of more effectively increasing the tear strength index of the kraft paper, it is preferably 300 mL or more, more preferably 330 mL or more, and preferably 700 mL or less, more preferably 600 mL or less, and further preferably 550 mL or less. The CSF of the raw pulp is measured in accordance with JIS P 8121-2:2012.

[0041] <Paper making process> The papermaking process is preferably a process of making paper from beaten raw pulp (pulp slurry). The papermaking method is not particularly limited, and examples thereof include an acidic papermaking method in which papermaking is performed at a pH of about 4.5, and a neutral papermaking method in which papermaking is performed at a pH of about 6 to about 9. The papermaking machine is also not particularly limited, and a known wet papermaking machine, such as a Fourdrinier papermaking machine, a gap former papermaking machine, a cylinder papermaking machine, or a short wire papermaking machine, can be appropriately selected and used.

[0042] In the manufacturing method of the kraft paper of this embodiment, known internal additives can be added appropriately in the papermaking process, such as sizing agents, dry strength agents, wet strength agents, retention aids, pH adjusters, drainage improvers, water resistance agents, softeners, antistatic agents, defoamers, slime control agents, dyes, and pigments.

[0043] (sizing agent) In order to obtain kraft paper having superior water resistance, it is preferable to add a sizing agent to the pulp slurry. Examples of the sizing agent include various known paper sizing agents such as rosin-based, alkyl ketene dimer-based, alkenyl succinic anhydride-based, maleic anhydride-based, styrene-acrylic acid-based, and styrene-acrylic-based. When a sizing agent is added to the pulp slurry, the content of the sizing agent (converted to solids) is preferably 0.001 parts by mass or more, more preferably 0.01 parts by mass or more, even more preferably 0.05 parts by mass or more, and preferably 4.0 parts by mass or less, more preferably 2.0 parts by mass or less, even more preferably 1.0 part by mass or less, and even more preferably 0.5 parts by mass or less, relative to 100 parts by mass (converted to solids) of the pulp fibers constituting the pulp slurry (raw pulp).

[0044] When a sizing agent is added to the pulp slurry, it is preferable to further add a retention aid such as aluminum sulfate to the pulp slurry from the viewpoint of fixing the added sizing agent. When a retention aid is added to the pulp slurry, the content (solid content equivalent) of the retention aid is preferably 0.001 parts by mass or more, more preferably 0.01 parts by mass or more, even more preferably 0.1 parts by mass or more, still more preferably 0.3 parts by mass or more, and preferably 4.0 parts by mass or less, more preferably 2.0 parts by mass or less, and even more preferably 1.0 parts by mass or less, relative to 100 parts by mass (solid content equivalent) of pulp fibers constituting the pulp slurry (raw pulp).

[0045] (Dry strength agent) In order to more effectively increase the strength of the kraft paper, it is preferable to add a dry strength agent to the pulp slurry. Examples of the dry strength agent include polyacrylamide polymers, starches such as cationic starch, urea resins, polyamide-polyamine resins, polyethyleneimine, polyamines, polyvinyl alcohol, and polyethylene oxide. When a dry strength agent is added to the pulp slurry, the content of the dry strength agent (converted to solids) is preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, even more preferably 0.1 parts by mass or more, still more preferably 0.5 parts by mass or more, and is preferably 8.0 parts by mass or less, more preferably 7.0 parts by mass or less, even more preferably 5.0 parts by mass or less, and still more preferably 3.0 parts by mass or less, relative to 100 parts by mass (converted to solids) of pulp fibers constituting the pulp slurry (raw pulp).

[0046] (Wet strength agent) In order to more effectively increase the strength of the kraft paper, it is preferable to add a wet strength agent to the pulp slurry. Examples of the wet strength agent include melamine resin, urea resin, polyamide-polyamine-epichlorohydrin resin, etc. When a wet strength agent is added to the pulp slurry, the content of the wet strength agent (converted to solids) is preferably 0.001 parts by mass or more, more preferably 0.01 parts by mass or more, even more preferably 0.1 parts by mass or more, and preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, even more preferably 1.0 part by mass or less, and even more preferably 0.5 parts by mass or less, relative to 100 parts by mass (converted to solids) of pulp fibers constituting the pulp slurry (raw material pulp).

[0047] <Surface treatment process> The method for producing kraft paper according to the present embodiment may include a surface treatment step in which a chemical is applied to the surface of the kraft paper. Examples of the chemical used in the surface treatment step include a surface sizing agent and a surface strength agent.

[0048] Examples of surface sizing agents include alkylketene dimer compounds. Examples of surface strength agents include modified starches such as oxidized starch, enzyme-treated starch, and acid-treated starch, aqueous resins such as polyvinyl alcohol (PVA), styrene-acrylic acid copolymers, and styrene-methacrylic acid copolymers. These may be used alone or in combination of two or more. The amount of the surface sizing agent and the surface strength agent to be applied is preferably 0.01 g / m in terms of solid content from the viewpoint of increasing the surface strength of the kraft paper. 2 More preferably, 0.1 g / m 2 More preferably, 0.3 g / m 2 More preferably, 0.4 g / m 2 From the viewpoint of making the surface strength of the kraft paper appropriate, it is preferably 5.0 g / m 2 Less than or equal to 3.0 g / m 2 More preferably, 1.0 g / m or less 2 When a surface sizing agent and a surface strength agent are used in combination, it is preferable that the amount of each applied is within the above range.

[0049] The method for producing kraft paper of this embodiment includes the above-mentioned cooking step and papermaking step, preferably includes the above-mentioned cooking step, beating step, and papermaking step, and more preferably includes the above-mentioned cooking step, beating step, papermaking step, and surface treatment step.

[0050] The kraft paper of the present embodiment has excellent strength, such as tear strength, which is required of kraft paper, and can therefore be suitably used for light packaging applications. EXAMPLES

[0051] The effects of the present invention will be described in detail below with reference to examples. The materials, amounts used, ratios, processing contents, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below. Furthermore, "parts" and "%" in the examples and comparative examples refer to "parts by mass" and "% by mass", respectively, unless otherwise specified. Furthermore, the operations of the examples and comparative examples were carried out under conditions of room temperature (20 to 25°C) and normal humidity (40 to 55% RH), unless otherwise specified.

[0052] [measurement] <Raw pulp> (Canadian Standard Freeness) The Canadian standard freeness of the raw pulp was measured in accordance with JIS P 8121-2:2012.

[0053] (Kappa number) The kappa number of the raw pulp was measured in accordance with JIS P 8211:2011.

[0054] (Pulp Viscosity) The pulp viscosity of the raw pulp was measured in accordance with TAPPI T 230.

[0055] <Kraft paper> (Kappa number) The kappa number of the pulp constituting the kraft paper was measured in accordance with JIS P 8211:2011 using the kraft paper pulp disintegrated in accordance with JIS P 8220-1:2012 as a sample.

[0056] (Pulp Viscosity) The pulp viscosity of the pulp constituting the kraft paper was measured in accordance with TAPPI T 230.

[0057] (Basic weight) The basis weight of the kraft paper was measured in accordance with JIS P 8124:2011.

[0058] (Thickness) The thickness of the kraft paper was measured in accordance with JIS P 8118:2014.

[0059] (density) The density of the kraft paper was calculated from the basis weight and thickness of the kraft paper obtained by the above-mentioned measurement method.

[0060] (specific tear strength) The longitudinal and transverse tear strength indexes of the kraft paper were measured in accordance with JIS P 8116:2000.

[0061] (tensile strength index) The tensile index in the machine direction and the cross direction of the kraft paper was measured in accordance with JIS P 8113:2006.

[0062] (Specific burst strength) The burst strength index of the kraft paper was measured in accordance with JIS P 8112:2008.

[0063] Example 1 Wood chips were made from hardwoods of the genus Eucalyptus, and kraft cooking (batch cooking) was carried out for 2 hours at a cooking temperature of 160°C using cooking white liquor adjusted to a liquor ratio of 4, a sulfidity of 28% by mass, and an active alkali concentration of 12.0% by mass (as Na2O). After kraft cooking, the black liquor was separated, and the resulting chips were defibrated using a high-concentration disintegrator, then centrifuged and washed three times using a filter cloth, and then uncooked material was removed using a screen. The resulting chips were centrifuged and dehydrated to obtain unbleached hardwood kraft pulp with a kappa number of 28.7 and a viscosity of 45.5 mPa s. This unbleached hardwood pulp was then beaten using a refiner to a CSF of 350 mL. To 100 parts (solids) of the beaten pulp slurry (100% unbleached hardwood kraft pulp), 0.6 parts (solids), 0.2 parts (solids), 0.1 parts (solids) of polyacrylamide-based dry strength agent (PAM), 0.1 parts (solids), 0.1 parts (solids) of alkylketene dimer-based sizing agent (AKD), and 1.0 parts (solids) of cationic starch were added in sequence with stirring to prepare a paper stock. The paper stock was made on a gap former paper machine, and a surface sizing agent of 0.02 g / m was added. 2 (based on pulp, solid content), surface strength agent 0.8g / m 2 (relative to pulp, solids content equivalent) was applied to obtain kraft paper with the basis weight shown in Table 1. The measurement results are shown in Table 1.

[0064] Example 2 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 19.8 and a viscosity of 65.9 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0065] Example 3 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 13.2 and a viscosity of 30.3 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0066] Example 4 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 10.9 and a viscosity of 24.3 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0067] Example 5 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 10.6 and a viscosity of 21.2 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0068] Example 6 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 10.1 and a viscosity of 17.6 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0069] Example 7 As wood chips, hardwoods of the Eucalyptus genus were kraft-cooked for 2 hours at a cooking temperature of 160°C using cooking white liquor adjusted to a liquor ratio of 4, a sulfidity of 28% by mass, and an active alkali concentration of 19.0% (as Na2O). As wood chips, softwoods of the Aconitum genus were kraft-cooked (batch cooking method) for 2 hours at a cooking temperature of 170°C using cooking white liquor adjusted to a liquor ratio of 4, a sulfidity of 28% by mass, and an active alkali concentration of 22.0% (as Na2O). After kraft cooking, the black liquor was separated, and the resulting chips were defibrated using a high-concentration disintegrator, then centrifugal dehydration and water washing were repeated three times using a filter cloth, and then undigested material was removed using a screen, and centrifugal dehydration was performed to obtain unbleached kraft pulp. The unbleached kraft pulp was beaten in a refiner, and 950 parts (solids) of hardwood unbleached kraft pulp with a CSF of 350 mL was mixed with 50 parts (solids) of softwood unbleached kraft pulp with a CSF of 500 mL to obtain a pulp slurry with a kappa number of 14.2 and a viscosity of 30.7 mPa·s. To 100 parts (solids) of this pulp slurry (LUKP 95%, NUKP 5%), 0.6 parts (solids), aluminum sulfate, 0.2 parts (solids), polyamide-polyamine-epichlorohydrin wet strength agent (PAE), 0.1 parts (solids), polyacrylamide dry strength agent (PAM), 0.1 parts (solids), alkylketene dimer sizing agent (AKD), and 1.0 parts (solids) of cationic starch were added in this order with stirring to prepare a paper stock. The above paper stock was used to make paper using a gap former type papermaking machine, and a surface sizing agent of 0.02 g / m 2 (based on pulp, solid content), surface strength agent 0.8g / m 2 (relative to pulp, solids content equivalent) was applied to obtain kraft paper with the basis weight shown in Table 1. The measurement results are shown in Table 1.

[0070] Comparative Example 1 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 8.9 and a viscosity of 11.9 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0071] Comparative Example 2 Kraft paper was obtained in the same manner as in Example 1, except that the active alkali concentration was set to the value shown in Table 1, unbleached hardwood kraft pulp having a kappa number of 8.1 and a viscosity of 11.3 mPa s was obtained, and the unbleached hardwood kraft pulp was used to obtain kraft paper with the basis weight shown in Table 1.

[0072] [Table 1]

[0073] As can be seen from Table 1, the kraft papers of Examples 1 to 7 were particularly excellent in specific tear strength, whereas the kraft papers of Comparative Examples 1 and 2 were particularly poor in specific tear strength. [Industrial Applicability]

[0074] The kraft paper of the present embodiment has excellent strength, such as tear strength, which is required of kraft paper, and can therefore be suitably used for light packaging applications.

Claims

1. A kraft paper containing hardwood kraft pulp as raw pulp, The content of hardwood kraft pulp in the raw material pulp is 90% by mass or more, A kraft paper having a kappa number of 9.5 or more after redefibration and a pulp viscosity of 15 mPa·s or more, measured in accordance with TAPPI T 230.

2. 2. The kraft paper of claim 1, wherein the Kappa number of the redisintegrated pulp is 30 or less, and the pulp viscosity of the kraft paper is 120 mPa·s or less.

3. The longitudinal tear strength is 8.0 mNm. ２ / g or more, and the tear strength in the transverse direction is 8.8 mN·m ２ 3. The kraft paper according to claim 1, wherein the kraft paper has a viscosity of 1 / g or more.

4. Basis weight: 70 g / m ２ 150g / m or more ２ 3. The kraft paper of claim 1, wherein:

5. Density is 0.65 g / cm ３ 0.85g / cm or more ３ 3. The kraft paper of claim 1, wherein:

6. 3. The kraft paper according to claim 1 or 2, which is used for light packaging.

7. A method for producing kraft paper by making raw pulp containing hardwood kraft pulp, comprising the steps of: The content of hardwood kraft pulp in the raw material pulp is 90% by mass or more, The raw pulp has a kappa number of 9.5 or more and a pulp viscosity of 15 mPa s or more, as measured in accordance with TAPPI T 230; The method for producing raw pulp includes a cooking step of cooking wood, The method for producing kraft paper, wherein the active alkali concentration in the cooking step is 10% by mass or more and 30% by mass or less.

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