Kraft paper and method for manufacturing kraft paper

Abstract

[Problem] To provide: kraft paper in which deterioration of texture is suppressed while strength is maintained and good air permeability is provided; and a method for producing the kraft paper. [Solution] The problem is solved by a method for producing kraft paper, the method being characterized by comprising a first addition step of adding a first amphoteric polyacrylamide resin to a slurry composed mainly of unbleached kraft pulp and a second addition step of adding a second amphoteric polyacrylamide resin to the slurry after the first addition step, and being characterized in that the cation requirement of the first amphoteric polyacrylamide resin is 110-200 μeq / L, the cation requirement of the second amphoteric polyacrylamide resin is less than the cation requirement of the first amphoteric polyacrylamide resin, and the added amount of the second amphoteric polyacrylamide resin is less than the added amount of the first amphoteric polyacrylamide resin.

Description

Kraft paper and method for manufacturing the same

[0001] The present invention relates to kraft paper and a method for manufacturing the same.

[0002] Kraft paper is manufactured using kraft pulp obtained by kraft pulping wood as a raw material, and has excellent strength physical properties such as tensile strength. Therefore, it is widely used as a packaging bag for filling and transporting contents such as agricultural products, chemicals, flour, and cement. Since agricultural products, chemicals, flour, cement, etc. are heavy goods, the packaging bag is sometimes called a heavy bag. In addition, in order to further increase the strength, a multi-layer paper obtained by stacking two or more layers of kraft paper, or a kraft paper and a film may be bonded and used.

[0003] In the process of filling the packaging bag made of kraft paper with the contents, as the contents are filled by air conveyance, the air inside the packaging bag and the air sent together with the contents escape to the outside of the packaging bag. At this time, it is desirable that the contents remain inside the packaging bag and only the air permeates the packaging bag and is released to the outside.

[0004] However, if the strength of the kraft paper forming the packaging bag is weak or the air leakage is poor, the packaging bag may break in the process of filling the contents. Therefore, in order to improve the air leakage, it is preferable to use kraft paper with good air permeability for the packaging bag. If the kraft paper has good air permeability, the air inside the packaging bag can easily permeate through the kraft paper forming the bag and be released to the outside during the process of filling the contents, making the packaging bag less likely to break.

[0005] In Patent Document 1, a kraft paper is proposed in which the air permeability of the kraft paper is extremely low, so that even if fine powdery particles are filled by air conveyance, there is no breakage or contamination due to powder scattering.

[0006] Japanese Patent Laid-Open No. 7-70975

[0007] However, improving the breathability of kraft paper can sometimes lead to a deterioration of its texture. This deterioration of texture causes unevenness in the paper, resulting in an unattractive appearance when printed. Therefore, the main problem that this invention aims to solve is to provide kraft paper and a method for manufacturing kraft paper that maintain strength and good breathability while suppressing deterioration of the texture.

[0008] Amphoteric polyacrylamide resins used in paper manufacturing are typically used to improve the strength of the paper. However, the inventors of this invention focused on the fact that amphoteric polyacrylamide resins have the effect of binding pulp fibers together and forming fibrous clumps. Excessive addition of amphoteric polyacrylamide resin to the slurry is usually avoided because it causes the formation of many fibrous clumps and excessively deteriorates the texture of the paper produced. However, the inventors discovered that the texture can be easily adjusted by adding two types of amphoteric polyacrylamide resins in appropriate amounts. The embodiments of the invention completed based on this discovery are shown below. <First Embodiment> Kraft paper in which the main component of the raw pulp is unbleached kraft pulp, with a basis weight of 70 g / m² ２ 140g / m or more ２ The kraft paper is characterized by the following: it contains an amphoteric polyacrylamide resin, has an air permeability of 13 seconds or less, and has a form index of 80 or more and 110 or less. <Second embodiment> A method for producing kraft paper, comprising: a first addition step of adding a primary amphoteric polyacrylamide resin to a slurry mainly composed of unbleached kraft pulp; and a second addition step of adding a secondary amphoteric polyacrylamide resin to the slurry after the first addition step, wherein the cation requirement of the primary amphoteric polyacrylamide resin is 110 to 200 μeq / L, the cation requirement of the secondary amphoteric polyacrylamide resin is lower than the cation requirement of the primary amphoteric polyacrylamide resin, and the amount of secondary amphoteric polyacrylamide resin added is less than the amount of primary amphoteric polyacrylamide resin added.

[0009] According to the present invention, kraft paper and a method for manufacturing kraft paper are provided that maintain strength and good breathability while suppressing deterioration of the paper's texture.

[0010] This is a diagram illustrating the paper manufacturing process.

[0011] This section describes embodiments for carrying out the present invention. Note that this embodiment is just one example of the present invention. The scope of the present invention is not limited to this embodiment.

[0012] The kraft paper of this embodiment is characterized in that the main component of the raw material pulp is unbleached kraft pulp, it contains a polyacrylamide-based amphoteric polyacrylamide resin, has an air permeability of 13 seconds or less, and has a form index of 80 to 110. Furthermore, the method for manufacturing the kraft paper of this embodiment comprises a first addition step of adding a primary amphoteric polyacrylamide resin to a slurry mainly composed of unbleached kraft pulp, and a second addition step of adding a secondary amphoteric polyacrylamide resin to the slurry after the first addition step, characterized in that the cation requirement of the primary amphoteric polyacrylamide resin is 110 to 200 μeq / L, the cation requirement of the secondary amphoteric polyacrylamide resin is lower than that of the primary amphoteric polyacrylamide resin, and the amount of secondary amphoteric polyacrylamide resin added is less than that of the primary amphoteric polyacrylamide resin. First, the kraft paper of this embodiment will be described in detail below.

[0013] Examples of raw pulp for manufacturing kraft paper in this embodiment include virgin pulp, which is pulp made directly from wood, and recycled paper pulp.

[0014] As virgin pulp, for example, wood pulp made from hardwoods, softwoods, etc., can be used.

[0015] Wood pulp includes, for example, hardwood kraft pulp (LKP), softwood kraft pulp (NKP), sulfite pulp (SP), chemical pulp such as dissolved pulp (DP), and mechanical pulp (TMP, etc.). One or more of these can be selected and used.

[0016] Chemical pulps include, for example, hardwood kraft pulp, which may be bleached hardwood kraft pulp, unbleached hardwood kraft pulp, or semi-bleached hardwood kraft pulp. Similarly, softwood kraft pulp may be bleached softwood kraft pulp, unbleached softwood kraft pulp, or semi-bleached softwood kraft pulp. Dissolved pulps include, for example, pulp produced by the sulfite process or the kraft process, as well as rayon pulp.

[0017] Examples of mechanical pulps include stone gland pulp (SGP), pressure stone gland pulp (PGW), refiner gland pulp (RGP), chemigland pulp (CGP), thermo gland pulp (TGP), gland pulp (GP), thermomechanical pulp (TMP), chemothermetic pulp (CTMP), refiner mechanical pulp (RMP), and bleached thermomechanical pulp (BTMP).

[0018] As the raw material pulp for the kraft paper in this embodiment, unbleached kraft pulp is preferable among the virgin pulps mentioned above, and unbleached hardwood kraft pulp and unbleached softwood kraft pulp are particularly preferred. Since unbleached kraft pulp has excellent strength, if the main component of the raw material pulp used for kraft paper is unbleached kraft pulp, it is suitable for filling with heavy contents, etc. Furthermore, if the main component of the raw material pulp used for kraft paper is unbleached softwood kraft pulp, it is preferable.

[0019] Recycled paper pulp includes, for example, disintegrated and deinked recycled paper pulp and disintegrated, deinked and bleached recycled paper pulp (DIP), which are manufactured from brown recycled paper, kraft envelope recycled paper, magazine recycled paper, newspaper recycled paper, newsprint recycled paper, flyer recycled paper, office recycled paper, corrugated cardboard recycled paper, high-grade white recycled paper, medium-grade recycled paper, low-grade recycled paper, Kent recycled paper, imitation recycled paper, land deed recycled paper, unsorted recycled paper, etc., which originally contain a large amount of mechanical pulp. One or more of these can be selected and used.

[0020] In this embodiment, the percentage of unbleached kraft pulp in the raw pulp used for the kraft paper is 70% or more, preferably 80% or more, and more preferably 95% or more. When this percentage is 70% or more, when the kraft paper is processed into a bag, the bag will not tear when the contents are filled into the bag by air or when the bag containing the contents is transported, and will have good stiffness and flexibility. Here, virgin pulp can be used as the unbleached kraft pulp.

[0021] Furthermore, among unbleached kraft pulps, softwood unbleached kraft pulp may be the main component, for example, with the percentage of softwood unbleached kraft pulp in the raw pulp being 50% or more, preferably 90% or more. In this case, the raw pulp may also contain at least one of hardwood bleached kraft pulp and recycled paper pulp in addition to softwood unbleached kraft pulp. For example, it may contain 0 to 25% hardwood unbleached kraft pulp, or 0 to 25% deinked recycled paper pulp or undeinked recycled paper pulp. This is preferable because it maintains the strength of the kraft paper.

[0022] The kraft paper according to this embodiment contains an amphoteric polyacrylamide resin. The amphoteric polyacrylamide resin contained in the kraft paper consists of a primary amphoteric polyacrylamide resin and a secondary amphoteric polyacrylamide resin. By adjusting the amount of primary and secondary amphoteric polyacrylamide resins added, the texture of the kraft paper produced can be adjusted. The mechanism for adjusting the texture can be considered as follows. First, regarding the relationship between the amphoteric polyacrylamide resin and pulp fibers, when the amphoteric polyacrylamide resin is added to the slurry, the amphoteric polyacrylamide resin forms hydrogen bonds with the pulp fibers. Since the amphoteric polyacrylamide resin has multiple bonding points, multiple pulp fibers form hydrogen bonds with a certain amphoteric polyacrylamide resin, and the pulp fibers are bonded together by the amphoteric polyacrylamide resin, forming a fiber mass. Specifically, by adding primary amphoteric polyacrylamide resin to the slurry in the kraft paper manufacturing process, the fibers contained in the slurry aggregate to form a fiber mass. Subsequently, the slurry containing the formed fiber masses is subjected to shear force in each step, which disperses the aggregated pulp fibers. However, by adding a secondary amphoteric polyacrylamide resin, which has a lower cation requirement than the primary polyacrylamide resin, the fiber masses re-bond and grow larger. If only the primary amphoteric polyacrylamide resin is added to the slurry, relatively small fiber masses are dispersed, resulting in kraft paper with relatively good strength, but poor air permeability. On the other hand, if a secondary amphoteric polyacrylamide resin with a relatively lower cation requirement is added after the addition of the primary amphoteric polyacrylamide resin, the secondary amphoteric polyacrylamide resin further bonds to the surface of the dispersed fiber masses, forming larger fiber masses. Kraft paper manufactured from a slurry having these larger fiber masses has a form index within a predetermined range and exhibits density differences. Here, kraft paper with density differences refers to paper in which areas of locally high and low fiber density coexist within the kraft paper, and which has voids of various sizes.Thus, the kraft paper according to this embodiment has a density difference due to the action of two types of amphoteric polyacrylamide resins, resulting in excellent air permeability. On the other hand, since the amphoteric polyacrylamide resin has the effect of strengthening the bonds between fibers contained in the slurry, the strength of the kraft paper produced is improved.

[0023] The inventors have found that if, for example, only a deuterated polyacrylamide resin is added to the slurry without adding a primary amphoteric polyacrylamide resin, or if only a primary amphoteric polyacrylamide resin is added, the resulting fibrous mass is smaller than that formed by adding both a primary and a deuterated polyacrylamide resin, making it difficult to obtain kraft paper with the desired texture index.

[0024] Amphoteric polyacrylamides are polymerized with acrylamide using cationic and anionic monomers, and they directly adhere to pulp fibers via their own cationic groups. Another characteristic of amphoteric polyacrylamide resins is that their molecular structure and molecular weight can be controlled. While not particularly limited, amphoteric polyacrylamide resins can be those in which the cationic group is diallyldimethylammonium chloride and the anionic group is sulfoethyl methacrylate, those in which the cationic group is aminoethylacrylamide and the anionic group is a carboxyl group, or those in which the cationic group is a quaternary ammonium base and the anionic group is a phosphate group.

[0025] The weight-average molecular weight of the primary amphoteric polyacrylamide resin is preferably 2.5 million to 4 million, more preferably 2.5 million to 3.5 million. The amount of primary amphoteric polyacrylamide resin added is preferably 1.5 kg / pulpt to 5.0 kg / pulpt, more preferably 3.0 kg / pulpt to 4.5 kg / pulpt, on a solid content basis.

[0026] Furthermore, the cation requirement of the primary amphoteric polyacrylamide resin is preferably greater than 110 μeq / L and less than or equal to 200 μeq / L, more preferably greater than 115 μeq / L and less than or equal to 150 μeq / L. On the other hand, the anion requirement of the primary amphoteric polyacrylamide resin is preferably greater than 0 μeq / L and less than or equal to 20 μeq / L, more preferably greater than 5 μeq / L and less than or equal to 18 μeq / L. If the cation requirement and anion requirement are within the above ranges, a suitable amount of fiber clumps will be formed, and in combination with the fiber clump growth effect of the secondary amphoteric polyacrylamide resin, a kraft paper with a suitable texture and excellent air permeability will be produced.

[0027] The B-type viscosity of the primary amphoteric polyacrylamide resin is preferably 5,000 to 15,000 mPa·s. If the B-type viscosity is within this range, it will result in kraft paper of appropriate strength. Here, the B-type viscosity can be measured under conditions of 25°C and 12 rpm in accordance with the "Method for Measuring the Viscosity of Liquids" of JIS-Z8803 (2011). The B-type viscosity is the resistance torque when the dispersion is stirred, and a higher value means that more energy is required for stirring.

[0028] On the other hand, the weight-average molecular weight of the amphoteric polyacrylamide resin is preferably 1 million to 2.4 million, more preferably 1 million to 2 million. The amount of amphoteric polyacrylamide resin added is preferably 0.2 kg / pulpt to 1.4 kg / pulpt, more preferably 0.4 kg / pulpt to 1.2 kg / pulpt, and particularly preferably 0.8 kg / pulpt to 1.0 kg / pulpt, in terms of solid content. If the amount of amphoteric polyacrylamide resin added is within the above range, the size of the fiber mass can be adjusted to achieve the desired texture index.

[0029] Furthermore, the cation requirement of the secondary amphoteric polyacrylamide resin is preferably 17 μeq / L or more and 110 μeq / L or less, more preferably 20 μeq / L or more and 100 μeq / L or less. On the other hand, the anion requirement of the secondary amphoteric polyacrylamide resin is preferably greater than 17 μeq / L and 110 μeq / L or less, more preferably greater than 18 μeq / L and 50 μeq / L or less. If the cation requirement and anion requirement are within the above ranges, the fibrous mass formed by the addition of the primary amphoteric polyacrylamide resin will grow, and after growth, the fibrous mass will be scattered inside the kraft paper, resulting in a suitable texture and breathability.

[0030] The weight-average molecular weight of amphoteric polyacrylamide resins can be measured using the GPC-MALS method, which involves connecting a multi-angle light scattering detector to a GPC, under the following conditions: The GPC unit is an Agilent Technologies "LC1100 series," the column is a Showa Denko K.K. "SHODEX SB806MHQ," the eluent is an N / 15 phosphate buffer (pH 3) containing N / 10 sodium nitrate, the flow rate is 1.0 ml / min, detector 1 is a Wyatt Technology "Multi-angle light scattering detector DAWN," and detector 2 is a Showa Denko K.K. "Suggested refractive index detector RI-101."

[0031] The cation demand of amphoteric polyacrylamide resins can be measured, for example, by the following method: The amphoteric polyacrylamide resin is filtered through a 150 μm pass filter cloth, and the filtrate is collected. The filtrate is then placed in a flow potentiometer (PCD (Particle Change Detector)-03 model, manufactured by Mutec Co., Ltd.), and the cation demand can be measured from the volume of titrant (Poly-DADMAC, manufactured by Kishida Chemical Co., Ltd.).

[0032] On the other hand, the anion demand of amphoteric polyacrylamide resins can be measured, for example, by the following method: Take 20 mL of tap water, add 2 mL of 1 / 1000 N potassium polyvinyl sulfate solution to it, stir for 1 minute, and then filter through filter paper to obtain a filtrate. Take 10 mL of this filtrate and measure it in the same way as the method for measuring cation demand described above, and use this measurement as the blank value. Also, take 20 mL of the filtrate obtained by filtering the amphoteric polyacrylamide resin through a 150 μm pass filter cloth, and measure the cation demand in the same way as the method for measuring cation demand described above. The anion demand can be calculated using the following formula: Anion demand = (Blank value + Cation demand × 0.909 - Measurement value with potassium polyvinyl sulfate added) / 0.909

[0033] The B-type viscosity of the secondary amphoteric polyacrylamide resin is preferably 2,000 to 10,000 mPa·s. If the B-type viscosity is within this range, kraft paper with appropriate strength will be produced.

[0034] The kraft paper of this embodiment contains a primary amphoteric polyacrylamide resin and a secondary amphoteric polyacrylamide resin. The kraft paper contains 1.5 to 5.0 kg of primary amphoteric polyacrylamide resin per ton of pulp, while the content of secondary amphoteric polyacrylamide resin is less than that of primary amphoteric polyacrylamide resin. The content of primary amphoteric polyacrylamide resin in the kraft paper can be adjusted during the manufacturing process by adjusting the amounts of primary and secondary amphoteric polyacrylamide resins added.

[0035] Furthermore, the first amphoteric polyacrylamide resin contained in the kraft paper of this embodiment has a molecular weight peak between 2.5 million and 4 million, and the second amphoteric polyacrylamide resin contained in the kraft paper of this embodiment may have a molecular weight peak between 1 million and 2.4 million. In other words, the kraft paper of this embodiment contains an amphoteric polyacrylamide resin having a molecular weight peak between 2.5 million and 4 million, and an amphoteric polyacrylamide resin having a molecular weight peak between 1 million and 2.4 million.

[0036] On the other hand, the basis weight of the kraft paper in this embodiment is 70 to 140 g / m². ２ Preferably 70 to 100 g / m ２ Therefore, it has moderate rigidity and is durable, making it easy to fill with contents. Below the lower basis weight limit, manufacturing strength cannot be ensured, and above the upper basis weight limit, even if the surface quality index is lowered, the air permeability does not decrease and filling properties cannot be ensured.

[0037] The tensile strength of the kraft paper in this embodiment is 5.5 to 15.0 kN / m in the longitudinal direction, preferably 6.0 to 14.0 kN / m, and 2.6 to 8.0 kN / m in the transverse direction, preferably 2.6 to 7.0 kN / m. The tensile strength can be measured in accordance with JIS-P-8113 (2006).

[0038] The tear strength of the kraft paper in this embodiment is 900 to 2200 mN for the vertical tear strength, preferably 950 to 1500 mN, and 750 to 2000 mN for the horizontal tear strength, preferably 1000 to 1900 mN. The tear strength can be measured in accordance with JIS-P-8116 (2000).

[0039] The elongation of the kraft paper in this embodiment is 1.5 to 3.0% in the vertical direction, preferably 2.0 to 2.5%, and 2.3 to 7.0% in the horizontal direction, preferably 3.0 to 6.0%. The elongation can be measured in accordance with JIS-P-8113 (2006).

[0040] The kraft paper of this embodiment has a toughness of, for example, 7.0 to 35.0, preferably 15.0 to 35.0. Note that toughness is also referred to as tensile work, and can be obtained by (toughness) = (tensile strength (kN / m)) × (elongation (%)).

[0041] If the tensile strength, tear strength, elongation, and toughness of the kraft paper of this embodiment are respectively within the above ranges, it is preferable that the kraft paper does not stretch too much or break during the operation of filling the bag-shaped kraft paper with contents.

[0042] The kraft paper of this embodiment has a specific tensile strength (longitudinal) of 0.078 to 0.093 (kN·m / g), preferably 0.080 to 0.090 (kN·m / g), and a specific tensile strength (transverse) of 0.033 to 0.046 (kN·m / g), preferably 0.034 to 0.045 (kN·m / g). Also, the kraft paper of this embodiment has a specific tear strength (longitudinal) of 12 to 20 (mN·m ２ / g), preferably 12 to 17 (mN·m ２ / g), and a specific tear strength (transverse) of 16 to 25 (mN·m ２ / g), preferably 16 to 24 (mN·m ２ / g). If the specific tensile strength and specific tear strength of the kraft paper of this embodiment are within the above ranges, it is difficult to break during the operation of filling the bag-shaped kraft paper with contents or during the transportation of the kraft paper bag filled with contents.

[0043] The kraft paper of this embodiment has an air permeability of 13 seconds or less, preferably 11 seconds or less, more preferably 9 seconds or less. For example, the lower limit of the air permeability can be 3 seconds. If the air permeability is within the above range, during the operation of filling the contents, air easily permeates the kraft paper, so the contents are less likely to flutter inside the kraft paper bag, and filling can be easily performed. Note that the air permeability can be measured in accordance with JIS-P-8117 (2009) Paper and Paperboard - Test Method for Air Permeability and Air Resistance (Intermediate Range) - Gurley Method.

[0044] The formation index of the kraft paper of this embodiment is, for example, 80 or more, preferably 85 or more, more preferably 90 or more, and also 110 or less, preferably 108 or less, more preferably 107 or less. If the formation index of the kraft paper is within the above range, formation spots will not be excessively visible on the kraft paper, and the density of the fibers forming the kraft paper will not become too high and uniform. If the formation index is too high, the strength of the kraft paper will be excessively reduced. If the formation index is too low, the density of the fibers forming the kraft paper will be high and uniform, leading to deterioration of the permeability of the kraft paper. The formation index of the kraft paper can be measured using the Formation Tester FMT-MIII manufactured by Nomura Securities Co., Ltd. The measurement area was 250 mm × 180 mm, the measurement time was 35 seconds / sample, and 3 samples were taken. The formation index of both sides of the sample was measured, and the average value was taken as the measured value. The smaller the measured value of the formation index, the better the formation and the fewer the formation spots.

[0045] The paper thickness of the kraft paper of this embodiment is not particularly limited, but for example, when it is 70 to 150 μm, it is preferable because the strength is sufficient and it is easy to handle.

[0046] On the other hand, the kraft paper of this embodiment may be a single layer, or may be a plurality of layers, for example, 2 to 8 layers, with the kraft paper stacked. When it is a plurality of layers, the layers can be adhered with a commercially available adhesive.

[0047] (Manufacturing process) The kraft paper according to the present invention can be manufactured in the following manufacturing process using, for example, the manufacturing equipment shown in FIG. 1. Raw material pulp M mainly composed of unbleached kraft pulp is prepared, and this raw material pulp M is supplied to the blending box 10. The blending box 10 mixes water and the raw material pulp M to form a slurry. Further, in the blending box 10, a dye, a coagulant, etc. may be added to the raw material pulp M and slurried. The concentration of the raw material pulp M contained in the slurry is preferably 1.0 to 5.0% by mass. Within this range, it is difficult to cause a decrease in yield and clogging of the slurry during the manufacturing process. The raw material pulp M is slurried in the blending box 10 and led to the blending chest 20.

[0048] The mixing chest 20, also called a mixing tank or mixing box, is a device that adds and mixes papermaking aids into the slurry. Examples of papermaking aids include fillers, internal sizing agents, fixatives, yield improvers, cationizing agents, and defoaming agents. Papermaking aids should be added as appropriate and mixed with the slurry depending on the intended use of the kraft paper being manufactured, although in some cases it is not necessary to add papermaking aids to the slurry. The slurry from the mixing chest 20 is supplied from the mixing chest 20 to the machine chest 30.

[0049] The machine chest 30, also called the machine tank or finished tank, is a device that can stir the slurry to homogenize its concentration. The machine chest 30 is also a device that can temporarily store the slurry and adjust the amount supplied to the seed box 40, which is downstream equipment. In this embodiment, the first amphoteric polyacrylamide resin can be added without particular restriction at any location upstream of the addition point of the second amphoteric polyacrylamide resin (i.e., any of locations B1, B2, or B3 in Figure 1) (i.e., location A in Figure 1), but it is preferable to add it in the machine chest 30. By adding the first amphoteric polyacrylamide resin in the machine chest 30, the slurry and the first amphoteric polyacrylamide resin are mixed appropriately, and the fibers contained in the slurry aggregate to form fibrous clumps. Aluminum sulfate may also be added to the slurry in the machine chest 30.

[0050] The slurry obtained by mixing in the machine chest 30 is supplied to the seed box 40. A portion of the slurry supplied to the seed box 40 may be supplied back to the machine chest 30. By circulating the slurry between the machine chest 30 and the seed box 40, the amphoteric polyacrylamide resin, papermaking aids, etc., are thoroughly mixed with the slurry in the seed box 40, ensuring uniformity of concentration.

[0051] The seed box 40 is a device that mixes dyes and sizing agents using the convection of the slurry while adjusting the concentration of the dyes and sizing agents added to the slurry as needed. It also flows the slurry downstream at a predetermined pressure.

[0052] The slurry, prepared after passing through the seed box 40, is supplied to the cleaner 50. At this time, filler may be added to the slurry before it is supplied to the cleaner 50.

[0053] The cleaner 50 is a device that separates and removes heavy objects from the slurry based on mass. The slurry from which the heavy objects have been removed by the cleaner 50 is then supplied to the screen 60.

[0054] The screen 60 is a device that separates and removes large-sized particles based on their size. The slurry that passes through the screen 60 is supplied to the downstream agitator 70, where it is stirred together with the amphoteric polyacrylamide resin B to achieve a uniform concentration. At this time, the amphoteric polyacrylamide resin B and the slurry are thoroughly mixed, causing fibrous clumps contained in the slurry to grow. The size of the fibrous clumps can be adjusted by the amount of amphoteric polyacrylamide resin B added.

[0055] The second polyacrylamide resin can be added, for example, to the slurry just before it is supplied to the screen 60 (location B1 in Figure 1), but similar effects can be obtained by adding it to the slurry after it has passed through the screen 60 (location B2 in Figure 1) or to the stirring device 70 (location B3 in Figure 1).

[0056] The slurry that has passed through the agitator 70 is supplied to the wire section 80 from the inlet 71. The wire section 80 is responsible for dewatering the slurry ejected from the inlet 71 of the paper machine and forming wet paper. The wet paper formed in the wire section 80 then passes through, for example, a press section, a dryer section, etc. (not shown) to become kraft paper.

[0057] Examples of the present invention will now be described. Kraft paper of the examples and comparative examples shown in Table 1 was manufactured by following the procedure below. (1) Unbleached softwood kraft pulp (NUKP), unbleached hardwood kraft pulp (LUKP), and recycled paper pulp (recycled corrugated cardboard) were mixed with water in the proportions shown in Table 1 to prepare a slurry. (2) The slurry was passed through the mixing box 10 and the mixing chest 20 and supplied to the machine chest 30, where a primary amphoteric polyacrylamide resin (Harima Chemicals "Hermid DN-760" polyacrylamide resin) was added and mixed with the slurry. The primary amphoteric polyacrylamide resin used had a molecular weight of 2.5 to 4 million, a cation demand of 128 μeq / L, and an anion demand of 16 μeq / L. (3) The slurry that passed through the machine chest 30 was passed through the seed box 40 and the cleaner 50, and just before being supplied to the screen 60, a diamphoteric polyacrylamide resin (polyacrylamide resin Seikoh PMC "T-DS482") was added, and after passing through the screen 60, it was stirred with the stirring device 70 until homogenized. The diamphoteric polyacrylamide resin used had a molecular weight of 2 million, a cation demand of 90 μeq / L, and an anion demand of 21 μeq / L. (4) The slurry that passed through the stirring device 70 was ejected from the inlet 71 of the paper machine, dewatered in the wire part 80, and dried in a dryer to obtain kraft paper for the example and comparative example. Table 1 is shown below.

[0058] For the obtained examples and comparative examples, basis weight, tensile strength, elongation, and tear strength were measured, and toughness, specific tensile strength, specific tear strength, air permeability, and form index were also measured. In addition, fillability, strength, and appearance were evaluated. These results are shown in Table 2. Here, the longitudinal direction refers to the direction of travel of the paper machine (MD), and the transverse direction refers to the direction perpendicular to the longitudinal direction (CD).

[0059] The filling performance was measured using the following procedure. Three layers of kraft paper were stacked to create three-layer kraft paper bags for both the example and comparative example. 40 kg of contents were automatically filled into the kraft paper bags by airflow at a speed of 80 g / second using an automatic filling machine. The filling performance was evaluated according to the following criteria. ◎, ○, and △ indicate a quality that is acceptable for practical use, while × indicates a quality that is not acceptable for practical use. ◎: Gas escaped quickly, and automatic filling was performed quickly. ○: Gas escaped, but automatic filling took some time. △: It took time for the gas to escape, but automatic filling was possible. ×: Gas did not escape smoothly, the bag tore, and filling was not possible.

[0060] The manufacturing strength was measured using the following procedure. Heavy bags were manufactured using an automatic bag-making machine from kraft paper of both the example and comparative example, and the resulting heavy bags were evaluated based on the following evaluation criteria. ◎, ○, and △ indicate quality suitable for practical use, while × indicates quality unsuitable for practical use. ◎: The resulting heavy bags had good stiffness and flexibility, as well as good lamination properties, and the manufacturing of heavy bags using the automatic bag-making machine was carried out very smoothly. ○: The resulting heavy bags had moderate stiffness and flexibility, as well as good lamination properties, and the manufacturing of heavy bags using the automatic bag-making machine was carried out smoothly. △: The resulting heavy bags lacked sufficient stiffness and flexibility, and the manufacturing of heavy bags using the automatic bag-making machine could not be carried out smoothly. ×: The heavy bags lacked stiffness and flexibility, and tears occurred during the manufacturing operation of the heavy bags using the automatic bag-making machine.

[0061] Visual measurements were performed using the following procedure. For both the example and comparative example, the kraft paper was held up to a fluorescent light and visually inspected according to the following evaluation criteria. ◎, ○, and △ indicate a quality that is acceptable for practical use, while × indicates a quality that is unacceptable for practical use. ◎: The kraft paper has no mottling or flocking and is in very good condition. ○: The kraft paper has little mottling or flocking and is in good condition. △: The kraft paper has noticeable mottling and flocking, but is not problematic for practical use. ×: The kraft paper has many mottling and flocking patterns and is problematic for practical use.

[0062] This invention can be used as a method for producing kraft paper and kraft paper.

[0063] 10 Mixing box 20 Mixing chest 30 Machine chest 40 Seed box 50 Cleaner 60 Screen 70 Agitator 71 Inlet 80 Wire part A Addition site for primary amphoteric polyacrylamide resin B Addition site for secondary amphoteric polyacrylamide resin M Raw pulp

Claims

1. Kraft paper whose main component is unbleached kraft pulp, with a basis weight of 70 g / m². ２ 140g / m or more ２ Kraft paper characterized by the following: containing an amphoteric polyacrylamide resin, having an air permeability of 13 seconds or less, and having a form index of 80 to 110.

2. The kraft paper according to claim 1, wherein the tensile strength (longitudinal) is 5.5 to 15.0 kN / m and the tensile strength (transverse) is 2.6 to 8.0 kN / m.

3. Kraft paper according to claim 1, wherein the toughness (vertical) is 7 to 35 and the toughness (horizontal) is 15 to 35.

4. The kraft paper according to claim 1, wherein the tear strength (longitudinal) is 900 to 2200 mN and the tear strength (transverse) is 750 to 2000 mN.

5. A method for producing kraft paper, comprising: a first addition step of adding a primary amphoteric polyacrylamide resin to a slurry mainly composed of unbleached kraft pulp; and a second addition step of adding a secondary amphoteric polyacrylamide resin to the slurry after the first addition step, wherein the cation requirement of the primary amphoteric polyacrylamide resin is 110 to 200 μeq / L, the cation requirement of the secondary amphoteric polyacrylamide resin is lower than that of the primary amphoteric polyacrylamide resin, and the amount of secondary amphoteric polyacrylamide resin added is less than that of the primary amphoteric polyacrylamide resin.

6. A method for producing kraft paper according to claim 5, wherein the amount of primary amphoteric polyacrylamide resin added is 1.5 to 5.0 kg / t, and the amount of secondary amphoteric polyacrylamide resin added is 0.2 to 1.4 kg / t.

7. A method for producing kraft paper according to claim 5, wherein the weight-average molecular weight of the primary amphoteric polyacrylamide resin is 2.5 million to 4 million, and the weight-average molecular weight of the secondary amphoteric polyacrylamide resin is 1 million to 2.4 million.

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