JP7552592B2 - Kraft paper and kraft paper bags - Google Patents

Description

The present invention relates to kraft paper and kraft paper bags. In particular, the present invention relates to kraft paper and kraft paper bags suitable for heavy-duty packaging used to fill and transport powder products.

Kraft paper, obtained from kraft pulp, is used for a variety of applications, including bag manufacturing and packaging paper. Kraft paper, particularly kraft paper bags for heavy packaging used to transport heavy items, requires high strength to prevent the bags from breaking during processes such as filling, transporting, and storing the contents.

Among the industries that use kraft paper bags, in the flour milling and chemical manufacturing industries, fine particulate powder is blown into kraft paper bags. To perform blowing and filling efficiently, kraft paper with high breathability is required. However, considering the strength of the kraft paper, it is necessary to increase the number of bonding points between the pulp fibers by further beating, but further beating tends to worsen the breathability of the kraft paper. Therefore, Patent Document 1 proposes kraft paper that combines strength and breathability by using pulp with long fiber length.

In addition, kraft paper obtained from kraft pulp is strong, durable, and tear-resistant, and may be used for a variety of purposes, such as heavy-duty packaging paper bags for transporting heavy items, cardboard materials, envelopes, and adhesive tape (Patent Document 2).

Patent No. 6222946 Patent No. 6198376

Kraft paper bags are sometimes used to transport heavy objects, so they need to be strong and durable. On the other hand, when filling them with powder, the kraft paper bags need to have a certain degree of breathability. In recent years, as air-fed filling speeds have increased, there is a demand for kraft paper bags with even higher breathability.
In view of the above circumstances, an object of the present invention is to provide kraft paper that maintains its strength while also improving its breathability, and a kraft paper bag using the same.

In order to achieve the above object, the present invention employs the following configuration.
[1] Kraft paper mainly composed of kraft pulp,
The kraft paper has a Canadian standard freeness of 700 mL or more as measured in accordance with JIS P 8121-2:2012 of 700 mL or more of the disintegrated pulp obtained by disintegrating the kraft paper by a pulp disintegration method described in JIS P 8220:2012, and a fiber coarseness of 0.200 mg/m or more as measured in accordance with JIS P 8120:1998.
[2] The kraft paper according to [1], having a longitudinal tear strength index of 15.0 mN·m ² /g or more and a transverse tear strength index of 18.0 mN·m ² /g or more, as measured in accordance with JIS P 8116:2000.
[3] The kraft paper according to [1] or [2], having a tensile breaking elongation in the machine direction of 8.0% or more and a tensile breaking elongation in the transverse direction of 6.0% or more, as measured in accordance with JIS P 8113:2006.
[4] The kraft paper according to any one of [1] to [3], having a longitudinal specific tensile energy absorption of 2.5 J/g or more and a transverse specific tensile energy absorption of 2.3 J/g or more, as measured in accordance with JIS P 8113:2006.
[5] The kraft paper according to any one of [1] to [4], having an air resistance of 10 seconds or less as measured in accordance with JIS P 8117:2009.
[6] The kraft paper according to any one of [1] to [5], having a Cobb water absorbency of 15 to 28 g/ ^m2 at a temperature of 23°C and a contact time of 60 seconds, as measured in accordance with JIS P 8140:1998.
[7] A kraft paper bag made of the kraft paper described in any one of [1] to [6].
[8] The kraft paper bag according to [7], which is for containing powder.

The present invention may also have the following configuration.
[11] Kraft paper mainly composed of kraft pulp,
The basis weight measured in accordance with JIS P 8124:2011 is 70 g/m2 ^or more,
The Cobb water absorption measured in accordance with JIS P 8140:1998 at a temperature of 23°C and a contact time of 60 seconds is 15 to 28 g/ ^m2 ;
A kraft paper having a Cobb water absorbency to basis weight ratio of 0.18 to 0.35.
[12] The kraft paper according to [11], having a longitudinal tear strength index of 15.0 mN·m ² /g or more and a transverse tear strength index of 18.0 mN·m ² /g or more, as measured in accordance with JIS P 8116:2000.
[13] The kraft paper according to [11] or [12], having a tensile breaking elongation in the machine direction of 8.0% or more and a tensile breaking elongation in the transverse direction of 6.0% or more, as measured in accordance with JIS P 8113:2006.
[14] The kraft paper according to any one of [11] to [13], having a longitudinal specific tensile energy absorption of 2.5 J/g or more and a transverse specific tensile energy absorption of 2.3 J/g or more, as measured in accordance with JIS P 8113:2006.
[15] The kraft paper according to any one of [11] to [14], having an air resistance measured in accordance with JIS P 8117:2009 of 10 seconds or less.
[16] The kraft paper according to any one of [11] to [15], wherein when the kraft paper is disintegrated by the pulp disintegration method described in JIS P 8220:2012, the resulting disintegrated pulp has a Canadian standard freeness of 700 mL or more as measured in accordance with JIS P 8121-2:2012 and a fiber coarseness of 0.200 mg/m or more as measured in accordance with JIS P 8120:1998.
[17] A kraft paper bag made of the kraft paper described in any one of [11] to [16].

One embodiment of the present invention is a kraft paper mainly composed of kraft pulp, in which the kraft paper is disintegrated by the pulp disintegration method described in JIS P 8220:2012 to obtain disintegrated pulp, the disintegrated pulp having a Canadian standard freeness measured in accordance with JIS P 8121-2:2012 of 700 mL or more, and a fiber roughness measured in accordance with JIS P 8120:1998 of 0.200 mg/m or more. The kraft paper according to this embodiment has excellent breathability while maintaining its strength. Furthermore, the kraft paper bag according to this embodiment also enables efficient air-fed filling.

Another embodiment of the present invention is a kraft paper mainly composed of kraft pulp, the kraft paper having a basis weight of 70 g/m2 or ^more as measured in accordance with JIS P 8124:2011, a Cobb water absorbency of 5 to 40 g/ ^m2 (preferably 10 to 35 g/ ^m2 , more preferably 15 to 28 g/ ^m2 ) at a temperature of 23°C and a contact time of 60 seconds as measured in accordance with JIS P 8140:1998, and a ratio of the Cobb water absorbency to the basis weight of 0.10 to 0.45 (preferably 0.15 to 0.40, more preferably 0.18 to 0.35). Even when stored in a stacked state, the kraft paper according to this embodiment is less susceptible to the glue used during bag making penetrating to the opposite side, and the surface is less likely to become sticky and the kraft paper bags are less likely to adhere to each other.

The present invention will be described in detail below. The following description of the constituent elements may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

[Kraft paper]
The kraft paper of the present invention is mainly composed of kraft pulp. In this specification, kraft pulp is pulp obtained by treating wood with an alkaline agent mainly composed of sodium hydroxide and sodium sulfide. The main component in kraft paper refers to a component that occupies 50% by mass or more with respect to the total mass of the kraft paper. That is, the mass ratio of the kraft pulp to the total mass of the kraft paper is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more. The mass ratio of the kraft pulp to the total mass of the kraft paper is preferably 100% by mass or less. The mass ratio of the kraft pulp (solid content) to the mass of all pulps constituting the kraft paper is preferably 97% by mass or more, more preferably 98% by mass or more. The mass ratio of the kraft pulp (solid content) to the mass of all pulps constituting the kraft paper is preferably 100% by mass or less.

When the kraft paper of the present invention is disintegrated by the pulp disintegration method described in JIS P 8220:2012, the resulting disintegrated pulp (hereinafter simply referred to as "disintegrated pulp") has a Canadian standard freeness (hereinafter sometimes referred to as "freeness") measured in accordance with JIS P 8121-2:2012 of 700 mL or more, and a fiber coarseness (hereinafter simply referred to as "fiber coarseness") measured in accordance with JIS P 8120:1998 of 0.200 mg/m or more. The kraft paper of the present invention has the above-mentioned configuration and therefore has both excellent strength and breathability.

The upper limit of the freeness of the disintegrated pulp measured in accordance with JIS P 8121-2:2012 is not particularly limited, but is preferably 800 mL or less, more preferably 750 mL or less, and even more preferably 735 mL or less. By making the freeness of the disintegrated pulp 700 mL or more, excellent breathability is easily obtained. By making the freeness of the disintegrated pulp equal to or less than the above upper limit, the kraft paper is more likely to exhibit excellent strength. The freeness of the disintegrated pulp can be adjusted by beating, etc.

The basis weight of the kraft paper measured in accordance with JIS P 8124:2011 is preferably 70 g/ ^m2 or more, more preferably 75 g/m2 or ^more , and even more preferably 80 g/m2 or ^more . The upper limit of the basis weight of the kraft paper measured in accordance with JIS P 8124:2011 is not particularly limited, but is preferably 150 g/ ^m2 or less.

The Cobb water absorbency of the kraft paper, measured in accordance with JIS P 8140:1998 at a temperature of 23°C and a contact time of 60 seconds, is preferably 5 to 40 g/ ^m2 , more preferably 10 to 35 seconds, even more preferably 15 to 28 g/ ^m2 , and particularly preferably 20 to 25 g/ ^m2 .

The ratio of Cobb water absorbency to basis weight is preferably 0.10 to 0.45, more preferably 0.15 to 0.40, even more preferably 0.18 to 0.35, even more preferably 0.21 to 0.35, and particularly preferably 0.28 to 0.35. Here, the ratio of Cobb water absorbency to basis weight is the value of Cobb water absorbency/basis weight.

Conventionally, when kraft paper bags are stored in a stacked state, the glue used during bag making can penetrate to the opposite side, causing the surface to become sticky, or the kraft paper bags to stick together. However, by setting the basis weight, Cobb water absorbency, and ratio of Cobb water absorbency to basis weight of the kraft paper within the above range, it is possible to make it difficult for the glue used during bag making to penetrate to the opposite side even when the kraft paper bags are stored in a stacked state. This prevents the surface from becoming sticky, and further prevents the kraft paper bags from sticking together. In particular, by setting the basis weight of the kraft paper within the above range, it is easier to suppress the penetration of glue to the opposite side when the kraft paper bags are stored in a stacked state, even if a sufficient amount of glue required for adhesion is used. In addition, by setting the Cobb water absorbency to be equal to or higher than the above lower limit, it becomes easier to firmly bond the kraft paper bags together with glue, and by setting the Cobb water absorbency to be equal to or lower than the above upper limit, it is possible to more effectively suppress the seepage of glue when the kraft paper bags are stacked after bag making. Furthermore, by setting the ratio of Cobb water absorbency to basis weight to above the above-mentioned lower limit, it becomes easier to firmly bond pieces of kraft paper together with glue, and by setting the ratio of Cobb water absorbency to basis weight to below the above-mentioned upper limit, it is possible to more effectively prevent glue from seeping out when kraft paper bags are stacked after bag making.

The Cobb water absorbency can be adjusted, for example, by adding a sizing agent. Examples of sizing agents include rosin-based sizing agents, alkyl ketene dimer-based sizing agents, alkenyl succinic anhydride-based sizing agents, synthetic sizing agents, etc. The amount of sizing agent added is preferably 0.05 to 1.00% by mass, and more preferably 0.10 to 0.20% by mass, based on the mass (solid content) of the total pulp that makes up the kraft paper.

Furthermore, for kraft paper made from the same type of kraft pulp, the higher the basis weight, the higher the Cobb water absorbency.

When the kraft paper of the present invention is disintegrated by the pulp disintegration method described in JIS P 8220:2012, the fiber coarseness of the resulting disintegrated pulp as defined in JIS P 8120:1998 (hereinafter simply referred to as "fiber coarseness") is 0.200 mg/m or more. The upper limit of the fiber coarseness is not particularly limited, but is preferably 0.400 mg/m or less, more preferably 0.350 mg/m or less, even more preferably 0.300 mg/m or less, and particularly preferably 0.250 mg/m or less. The fiber coarseness is the bone dry mass of the fiber per unit length. The higher the fiber coarseness, the more the fiber curls and the easier it is to elongate.

The fiber coarseness of the disintegrated pulp can be adjusted by the Runkel ratio of the raw pulp, the kappa number (Kα number) of the raw pulp, the blade used for beating, the temperature during beating, and the like.
The Runkel ratio is twice the average fiber wall thickness (average fiber cell wall thickness) divided by the average lumen diameter. The average lumen diameter is calculated by subtracting twice the average fiber wall thickness from the average fiber width. In other words, the Runkel ratio is a value calculated from the average fiber wall thickness and the average fiber width. The average fiber wall thickness and average fiber width required to calculate the Runkel ratio can be measured using a fiber analyzer (Metso Automation, Kajaani Fiber Lab ver. 4.0).

The lower the Runkel ratio of the raw pulp, the softer the fibers are, and the more likely they are to curl due to the kneading action caused by beating, making them more likely to stretch. In other words, the fiber coarseness is likely to increase. Also, the lower the Runkel ratio, the more likely they are to be crushed by the physical force of beating, which increases inter-fiber bonds and improves strength. However, a low Runkel ratio tends to reduce tear strength. For this reason, taking into account the overall balance, the Runkel ratio of the raw pulp before beating is preferably 1.5 to 2.5, and more preferably 1.7 to 2.3.

The Runkel ratio is higher for pulp made from softwoods than for pulp made from hardwoods. Therefore, the Runkel ratio of the raw pulp can be adjusted by the mixing ratio of softwood kraft pulp and hardwood kraft pulp. However, the raw pulp may be made only from softwood kraft pulp.

Examples of softwood kraft pulp include unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), semi-bleached softwood kraft pulp (NSBKP), and softwood sulfite pulp. These may be used in combination. Of these, it is preferable to use unbleached softwood kraft pulp (NUKP) as the raw material pulp because it has excellent tensile elongation at break and strength. Examples of wood species for softwood kraft pulp include Douglas fir, red pine, spruce, and radiata pine. Of these, Douglas fir is preferably used because it has a high Runkel ratio. Two or more of these may be used in combination as the raw material pulp.

Examples of hardwood kraft pulp include unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), semi-bleached hardwood kraft pulp (LSBKP), and sulfite hardwood pulp. Among these, it is preferable to use unbleached hardwood kraft pulp (LUKP) as the raw material pulp in terms of strength. Examples of hardwood kraft pulp species include eucalyptus, beech, and acacia. Two or more of these may be used in combination as the raw material pulp.

The Kα value is an index of the lignin content or bleachability of pulp measured in accordance with JIS P 8211:2011. The lower the Kα value (the less lignin) of the raw pulp, the softer the fibers are, and therefore the easier it is for curling to occur due to the loosening action caused by beating, and the easier it is for elongation to occur. In other words, the fiber coarseness of the disintegrated pulp can be increased. However, if the Kα value is too low, the single fiber strength may decrease. On the other hand, if the Kα value is too high, the fibers are hard, and therefore the curling may be difficult to occur, and the elongation may be difficult to occur. For this reason, taking into consideration the overall balance, the Kα value of the raw pulp is preferably 35 to 55, and more preferably 38 to 50.

Deluxe finer (basalt) blades are preferred for beating. Compared to conical type refiners, the Deluxe finer has wider blades on both the rotor and stator, so the beating is more like kneading than cutting, making it easier for curling to occur and elongation to occur. In other words, the fiber coarseness of the disintegrated pulp can be increased. In addition, the use of the Deluxe finer makes it less likely for the tear strength of the kraft paper to decrease.

The temperature during beating is preferably 30 to 90°C, more preferably 70 to 90°C. The higher the temperature, the easier it is for curling to occur and for elongation to be achieved. In other words, the fiber coarseness of the disintegrated pulp can be increased. In particular, since the softening point of lignin is 70°C or higher, beating at a temperature of 70°C or higher softens the fibers and makes them particularly susceptible to curling (coarseness).

The longitudinal tear strength index of the kraft paper measured in accordance with JIS P 8116:2000 is not particularly limited, but from the viewpoint of drop impact resistance, it is preferably 15.0 mN· ^m2 /g or more, more preferably 16.0 mN· ^m2 /g or more, even more preferably 17.0 mN· ^m2 /g or more, even more preferably 17.5 mN· ^m2 /g or more, and particularly preferably 18.0 mN· ^m2 /g or more. The upper limit of the longitudinal tear strength index is not particularly limited, but it is preferably 25.0 mN· ^m2 /g or less. In addition, the tear strength ratio in the transverse direction of the kraft paper measured in accordance with JIS P 8116:2000 is not particularly limited, but is preferably 18.0 mN·m ² /g or more, more preferably 20.0 mN·m ² /g or more, even more preferably 22.0 mN·m ² /g or more, and particularly preferably 23.0 mN·m ² /g or more. The tear strength ratio in the transverse direction is not particularly limited, but from the viewpoint of drop impact resistance, is preferably 28.0 mN·m ² /g or less, more preferably 26.0 mN·m ² /g or less, even more preferably 25.0 mN·m ² /g or less, and particularly preferably 24.0 mN·m ² /g or less. If the tear strength ratios in the longitudinal direction and transverse direction are equal to or greater than the above lower limit, a kraft paper bag that is less likely to break during the process of filling, transporting, storing, etc. of the contents can be obtained. The tear strength index in the longitudinal and transverse directions tends to be higher as the freeness of the disintegrated pulp increases. In this specification, the transverse direction refers to the width direction of the papermaking machine, and the longitudinal direction refers to the flow direction of the papermaking machine.

The longitudinal tensile breaking elongation of the kraft paper measured in accordance with JIS P 8113:2006 is preferably 6.0% or more, more preferably 7.0% or more, even more preferably 7.5% or more, even more preferably 8.0% or more, and particularly preferably 8.5% or more, from the viewpoint of drop impact resistance and/or glue penetration suppression. The longitudinal tensile breaking elongation is not particularly limited, but is preferably 12.0% or less, and more preferably 10.0% or less. The transverse tensile breaking elongation of the kraft paper measured in accordance with JIS P 8113:2006 is not particularly limited, but is preferably 4.0% or more, more preferably 5.0% or more, even more preferably 6.0% or more, and particularly preferably 6.5% or more. The transverse tensile breaking elongation is not particularly limited, but is preferably 10.0% or less, more preferably 9.0% or less, and even more preferably 8.0% or less. When the tensile elongation at break in the longitudinal and transverse directions is equal to or greater than the above lower limit, it is possible to obtain a kraft paper bag that is less likely to break during processes such as filling, transportation, storage, etc. The tensile elongation at break in the longitudinal and transverse directions tends to be higher as the fiber coarseness of the disintegrated pulp increases.

The specific tensile energy absorption in the longitudinal direction of the kraft paper as specified in JIS P 8113:2006 is preferably 2.5 J/g or more, more preferably 2.8 J/g or more, and even more preferably 3.0 J/g or more. The specific tensile energy absorption in the transverse direction of the kraft paper as specified in JIS P 8113:2006 is preferably 1.5 J/g or more, more preferably 2.0 J/g or more, even more preferably 2.3 J/g or more, even more preferably 2.5 J/g or more, and especially preferably 2.8 J/g or more, from the viewpoint of drop impact resistance and/or suppression of glue penetration. If the specific tensile energy absorption in the longitudinal direction and transverse direction is equal to or greater than the above lower limit, a kraft paper bag that is less likely to break during the process of filling, transporting, storing, etc. of the contents can be obtained. The specific tensile energy absorption in the longitudinal direction and transverse direction tends to be higher as the fiber coarseness of the disintegrated pulp increases.

The air resistance of the kraft paper measured in accordance with JIS P 8117:2009 is preferably 50 seconds or less, more preferably 30 seconds or less, even more preferably 20 seconds or less, even more preferably 10 seconds or less, and particularly preferably 7 seconds or less. The air resistance of the kraft paper measured in accordance with JIS P 8117:2009 is preferably 0 seconds or more. By having the air resistance below the above upper limit, kraft paper with excellent breathability can be obtained. The air resistance tends to be lower as the freeness of the disintegrated pulp increases.

Kraft paper may contain optional ingredients in addition to kraft pulp. Optional ingredients include sizing agents, anionic, cationic or amphoteric retention aids, drainage aids, dry strength agents, wet strength agents, internal additives such as fillers, water resistance agents, dyes, and fluorescent whitening agents.

Examples of sizing agents include internal sizing agents such as rosin and alkyl ketene dimer (AKD); and surface sizing agents such as styrene/acrylic acid copolymer and styrene/methacrylic acid copolymer. The amount of sizing agent is not particularly limited, but is preferably 1.0 part by mass or less, more preferably 0.50 parts by mass or less, even more preferably 0.30 parts by mass or less, and even more preferably 0.20 parts by mass or less, per 100 parts by mass of raw pulp (solid content equivalent). In addition, the amount of sizing agent is preferably 0 parts by mass or more per 100 parts by mass of raw pulp.

Examples of retention aids include polyacrylamide and aluminum sulfate. The amount of retention aid is not particularly limited, but is preferably 5.0 parts by mass or less, more preferably 3.0 parts by mass or less, and even more preferably 2.5 parts by mass or less, per 100 parts by mass of raw pulp (solid content equivalent). In addition, the amount of retention aid is preferably 0 parts by mass or more per 100 parts by mass of raw pulp.

Examples of dry strength enhancers include cationic starch, polyacrylamide, and carboxymethyl cellulose. The amount of dry strength enhancer is not particularly limited, but is preferably 3.0 parts by mass or less, and more preferably 2.0 parts by mass or less, per 100 parts by mass of raw pulp (solid content equivalent). In addition, the amount of dry strength enhancer is preferably 0 parts by mass or more per 100 parts by mass of raw pulp.

Examples of wet strength enhancers include polyamide polyamine epichlorohydrin (PAE), urea formaldehyde resin, melamine formaldehyde resin, etc., and the wet strength enhancer is preferably PAE. The amount of the wet strength enhancer in the paper base material is not particularly limited, but is preferably 0.05 parts by mass or more, more preferably 0.07 parts by mass or more, per 100 parts by mass of raw pulp (solid content equivalent). The amount of the wet strength enhancer is preferably 1.0 parts by mass or less, more preferably 0.7 parts by mass or less, per 100 parts by mass of raw pulp.

Examples of fillers include inorganic fillers such as clay, 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.

The kraft paper may have a coating layer. The coating layer is preferably a coating layer mainly composed of a natural adhesive such as starch or a synthetic adhesive such as polyvinyl alcohol, polyacrylamide, styrene-butadiene copolymer resin, acrylic resin, etc. When the kraft paper has a coating layer, the coating weight of the coating layer is preferably 0.3 to 3.0 g/ ^m2 .

[Kraft paper manufacturing method]
The method for producing kraft paper of the present invention preferably includes the steps of cooking wood chips as a main raw material by the kraft process to obtain unbleached kraft pulp, beating the unbleached kraft pulp to obtain a pulp slurry, and making paper from the pulp slurry. Note that pulp other than unbleached kraft pulp may be added to the pulp slurry. Furthermore, the above-mentioned internal additives may be added to the pulp slurry within a range that does not impair the effects of the present invention.

As the sizing agent, it is preferable to add the sizing agents described above. The amount of sizing agent added is preferably 0.05 to 1.00% by mass, and more preferably 0.10 to 0.20% by mass, based on the mass (solid content) of the total pulp contained in the slurry.

In the process of beating unbleached kraft pulp to obtain a pulp slurry, it is preferable to add a dry strength agent to the pulp slurry after beating. The amount of dry strength agent added is preferably 0.5 to 3.0% by mass based on the mass (solids content) of the total pulp contained in the slurry. By adding cationic starch, the strength of the kraft paper is increased, and kraft paper with good breaking elongation can be obtained.

For example, in order to make the freeness of the disintegrated pulp 700 to 750 mL, beating is preferably performed until the finished freeness is 670 to 750 mL. Also, in order to make the fiber coarseness of the disintegrated pulp 0.200 mg/m or more, beating is preferably performed until the finished fiber coarseness is 0.160 to 0.250 mg/m.

In the process of beating unbleached kraft pulp to obtain a pulp slurry, the beating temperature is preferably 30 to 90°C, and more preferably 70 to 90°C. The higher the temperature, the easier it is for curling to occur and for elongation to be achieved. In other words, the fiber coarseness of the disintegrated pulp can be increased. In particular, since the softening point of lignin is 70°C or higher, beating at a temperature of 70°C or higher softens the fibers and makes them particularly susceptible to curling (coarseness).

The method and type of papermaking machine for making kraft paper can be selected from known methods and machines such as a Fourdrinier papermaking machine, a short wire papermaking machine, a twin-wire papermaking machine, a cylinder papermaking machine, a gap former papermaking machine, etc. Special treatments such as creping can also be carried out if necessary.

During the manufacturing process of kraft paper, in order to improve surface properties, it is possible to pass the paper through various finishing devices, such as a machine calender or soft nip calender, to apply a product finish.

[Kraft paper bag]
The kraft paper bag of the present invention is made from the above-mentioned kraft paper. The shape of the bag is not particularly limited, but examples thereof include machine-sewn bags such as pleat-free opening type machine-sewn bags and pleat-free valve-equipped machine-sewn bags described in JIS Z 0102:2004, and gusseted bags such as pleat-free opening type bottom-flared bags and pleat-free valve-equipped bottom-flared bags.

Glue may be used when making bags from kraft paper. Examples of glue used when making bags include vinyl acetate copolymer, ethylene-vinyl acetate copolymer, acrylic copolymer, hot melt resin, carboxymethyl cellulose, and the like. Water is preferred as the solvent for the glue used when making bags. In this case, the dry coating amount of the glue used when making bags is preferably 15 to 23 g/ ^m2 . If the dry coating amount is equal to or greater than the above lower limit, the glue can be firmly bonded, making bags easier. Also, if the dry coating amount is equal to or less than the above upper limit, the glue is less likely to seep out even when the kraft paper bags are stacked after making bags.

In order to further increase the strength of the kraft paper bag of the present invention, it is preferable to make it into a multi-layered paper bag of two or more layers. When making a kraft paper bag into a multi-layered paper bag, for example, it may be configured by laminating a synthetic resin layer onto the above-mentioned kraft paper, or it may be configured as a three-layered structure of kraft paper / synthetic resin layer / kraft paper. Also, when making a kraft paper bag into a multi-layered paper bag, an adhesive layer may be provided between each layer.

The kraft paper bag of the present invention is a strong paper bag, so it can be used as a heavy-duty paper bag that can package contents by weight and withstand the weight and handling. There are no particular limitations on the contents of the kraft paper bag of the present invention, but the contents are preferably powder or granular, as they have excellent breathability and are suitable for air-fed filling. In other words, the kraft paper bag of the present invention is preferably for containing powder.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples. Note that % is mass % unless otherwise specified.

[Measurement method, evaluation method]
(Compared to Runkel)
The Runkel ratio of the disintegrated pulp was calculated from the average fiber width and the average fiber wall thickness by dividing twice the average fiber wall thickness (average thickness of the cell walls of the fibers) by the average lumen diameter.

(Freeness of disintegrated pulp)
The kraft paper produced in the following Examples and Comparative Examples was disintegrated to obtain disintegrated pulp by the pulp disintegration method described in JIS P 8220: 2012. The freeness of the disintegrated pulp was measured by the method specified in JIS P 8121-2: 2012.

(Kα value)
The Kα value of the disintegrated pulp was measured by the method specified in JIS P 8211:2011.

(Fiber coarseness of disintegrated pulp)
The kraft paper produced in the following Examples and Comparative Examples was disintegrated to obtain disintegrated pulp by the pulp disintegration method described in JIS P 8220: 2012. Disintegrated pulp was diluted with water to 0.02% by mass in accordance with JIS P 8120: 1998, and 25 mL (equivalent to 5 mg of bone-dry pulp) was accurately taken from the slurry, and the fiber coarseness of the disintegrated pulp was measured using Kajaani Fiber Lab ver. 4.0 manufactured by Metso Automation.

(basis weight, density)
The basis weight of the kraft paper was measured according to the method specified in JIS P 8124: 2011. Then, the thickness of the kraft paper was measured according to the method specified in JIS P 8118: 2014. The basis weight divided by the paper thickness was determined as the density of the kraft paper.

(Specific tear strength)
The tear strength of the kraft paper in the longitudinal direction (MD) and transverse direction (CD) was measured using a tear tester (Toyo Seiki Co., Ltd., Elmendorf tear tester (SA-W type)) according to the method specified in JIS P 8116:2000. The specific tear strength of the kraft paper in the longitudinal direction (MD) and transverse direction (CD) was calculated by dividing each tear strength by the basis weight.

(Tensile elongation at break)
The tensile elongation at break in the machine direction (MD) and the cross direction (CD) of the kraft paper was measured using a horizontal tensile tester (manufactured by L&W, CODE SE-064) according to the method specified in JIS P 8113:2006.

(Tensile Energy Absorption Index (TEAI))
The tensile energy absorption of the kraft paper in the machine direction (MD) and the cross direction (CD) was measured using a horizontal tensile tester (manufactured by L&W, CODE SE-064) according to the method specified in JIS P 8113:2006. The specific tensile energy absorption (TEAI) in the machine direction (MD) and the cross direction (CD) of the kraft paper was calculated by dividing each tensile energy absorption by the basis weight.

(Cobb absorbency (60 seconds))
The Cobb water absorbency of the kraft paper was measured according to the method specified in JIS P 8140:1998 at a temperature of 23° C. for a contact time of 60 seconds.

(Air permeability resistance)
The air resistance of the kraft paper was measured using an Oken type testing machine (manufactured by Asahi Seiko Co., Ltd., digital type Oken type air permeability/smoothness testing machine) according to the method specified in JIS P 8117:2009.

(Filling ability/bag breaking suitability)
The kraft paper was processed into a pleated, valved, bottom-flared bag as specified in JIS Z 1505:2004, measuring 370 mm in height, 95 mm in depth, and 320 mm in width. 25 kg or 40 kg of cement was filled into the bag by air blowing in 5 seconds, and the bag was evaluated according to the following criteria.
⊚: The air escaped quickly and the bag was filled extremely well without breaking.
○: Some bags were broken, but no problems were observed in use.
△: The bag broke, causing problems in use.
×: Many bags were broken.

(Drop impact characteristics)
Kraft paper was processed into a pleated, valved, bottom-flared bag with a height of 370 mm, depth of 95 mm, and width of 320 mm as specified in JIS Z 1505: 2004, and 25 kg of cement was filled therein by air blowing. The bag filled with cement was dropped horizontally from a height of 1.2 m according to JIS Z 0217: 1998, and the number of times it took for the bag to break was counted and evaluated according to the following criteria.
⊚: The number of times until the bag broke was 10 or more.
△: The number of times until the bag broke was 8 to 9.
×: The number of times until the bag broke was 7 or less.

(Gluing suitability A (adhesiveness))
The craft paper of each example was cut to a width of 30 mm and a length of 150 mm, and 0.3 mL of water-soluble glue (Lifebond P-8100, manufactured by Nichiei Kako Co., Ltd., concentration 48% by mass) was thinly spread and applied to a section of 7 mm width and 10 cm length, and another piece of craft paper of the same example was layered and bonded. Three sets of this were stacked, a 5 kg weight was placed on top, and the pieces were left for one day, after which the peeling of the bonded parts was attempted and evaluated.
⊚: The kraft paper in the glued portion is completely bonded to each other.
△: Generally adhered, but some areas are not adhered.
x: Almost no adhesion and easy to peel off.

(Gluing suitability B (glue permeability))
The craft paper of each example was cut into 50 mm squares, and 1.5 mL of water-soluble glue (Lifebond P-8100, manufactured by Nichiei Kako Co., Ltd., concentration 48% by mass) was applied to a 3 ^cm2 area in the center of the craft paper, and other craft paper of the same example was layered and bonded. Three sets of this were stacked, a 5 kg weight was placed on top, and the pieces were left for 6 hours, after which the penetration of the glue was evaluated.
⊚: The glue has not penetrated to the opposite side of the glued portion.
Δ: The glue has slightly penetrated to the opposite side of the glued portion, but no stickiness is observed on the opposite side.
×: The glue has penetrated to the opposite side of the glued part and has bonded to other paper.

[Raw pulp]
Kraft pulp (1): Softwood unbleached kraft pulp (NUKP) made from Douglas fir.
Kraft pulp (2): Softwood unbleached kraft pulp (NUKP) made from domestic pine.
Kraft pulp (3): Unbleached softwood kraft pulp (NUKP) made from Japanese cedar.

[Kraft paper manufacturing]
A synthetic sizing agent (Sizepine MXE2, manufactured by Arakawa Chemical Industries Co., Ltd.) was added as an internal additive to the pulp having the composition shown in Tables 1 and 2 in an amount of 0.15% by mass based on the solid content of the pulp. The pulp was then beaten at 80° C. using a Deluxe Finer. The properties of the pulp after beating are shown in Tables 1 and 2.
To the beaten pulp, aluminum sulfate was added as an internal additive in an amount of 1.2% by mass based on the solid content of the pulp, and cationic starch and polyacrylamide were added as dry strength agents in an amount of 0.9% by mass based on the solid content of the pulp and 1.0% by mass based on the solid content of the pulp, respectively, and then paper was made (one layer) using an open top former type papermaking machine to obtain kraft paper of each example. The properties of the kraft paper and its disintegrated pulp of each example are shown in Tables 1 and 2.

[evaluation]
The evaluation results of the kraft paper of each example are shown in Tables 1 and 2. As shown in Tables 1 and 2, the kraft paper of the examples had low air resistance and excellent breathability. Furthermore, the bags obtained with the kraft paper of the examples had good filling properties, bag breaking suitability, and drop impact resistance, and had excellent strength. Furthermore, the kraft paper of the examples also had good gluing suitability (good adhesiveness, and the penetration of glue into other kraft paper when weighted was suppressed).

Claims (8)

Kraft paper made mainly of kraft pulp,
When the kraft paper is disintegrated by the pulp disintegration method described in JIS P 8220:2012, the resulting disintegrated pulp has a Canadian standard freeness measured in accordance with JIS P 8121-2:2012 of 700 mL or more, a fiber coarseness measured in accordance with JIS P 8120:1998 of 0.200 mg/m or more, a Runkel ratio of 1.98 to 2.12 , and a Kα number of 38 to 46 . The basis weight measured in accordance with JIS P 8124:2011 is 70 g/m2 ^or more,
The Cobb water absorption measured in accordance with JIS P 8140:1998 at a temperature of 23°C and a contact time of 60 seconds is 15 to 28 g/ ^m2 ;
2. The kraft paper of claim 1, wherein the ratio of said Cobb Water Absorbency to said basis weight is from 0.18 to 0.35. 3. The kraft paper according to claim 1 or 2, having a longitudinal tear strength index of 15.0 mN·m ² /g or more and a transverse tear strength index of 18.0 mN·m ² /g or more, measured in accordance with JIS P 8116:2000. The kraft paper according to any one of claims 1 to 3, which has a longitudinal tensile elongation at break of 8.0% or more and a transverse tensile elongation at break of 6.0% or more, as measured in accordance with JIS P 8113:2006. The kraft paper according to any one of claims 1 to 4, which has a longitudinal specific tensile energy absorption of 2.5 J/g or more and a transverse specific tensile energy absorption of 2.3 J/g or more, as measured in accordance with JIS P 8113:2006. The kraft paper according to any one of claims 1 to 5, which has an air resistance of 10 seconds or less as measured in accordance with JIS P 8117:2009. A kraft paper bag made of the kraft paper described in any one of claims 1 to 6. The kraft paper bag according to claim 7, which is for containing powder.