JP7619376B2 - Water-resistant paper, food containers, paper cups, paper lids, and method for manufacturing water-resistant paper - Google Patents

Description

The present invention relates to water-resistant paper and food containers, paper cups, and paper lids made using the same. It also relates to a method for manufacturing the water-resistant paper.

Traditionally, plastic products have been used for various containers (trays, cups, etc.) and their lids for beverages and food, as well as food cutlery (e.g. chopsticks, spoons, forks, etc.), but there is a desire to switch to paper products in order to reduce the environmental impact.

Traditionally, polyethylene laminated paper, which has a polyethylene film laminated to one side of a base paper, has been used for various containers for beverages, food, etc., but there was a problem that it was difficult to remove the polyethylene film when recycling, making it poorly recyclable.

To address these issues, efforts have been made to impart properties such as water resistance during the papermaking process or through coating.

Furthermore, Patent Document 1 describes a packaging paper having at least one heat seal layer on at least one side of a paper base material, the heat seal layer containing an ionomer, a dry coating weight of the heat seal layer being 2 to 10 g/ ^m2 in total, and two or more layers of the heat seal layer being formed on at least one side, for the purpose of providing a packaging paper that can reduce the amount of plastic used.

Patent No. 6580291

The waterproof paper described in Patent Document 1 had problems with its suitability for processing, as cracks occurred in the top curl when it was processed into paper cups.

Therefore, the present invention aims to provide a waterproof paper that is water resistant and has excellent processing suitability.

The inventors discovered that the above problems could be solved by adjusting the tensile strength of the waterproof paper within a specified range, and thus completed the present invention.

That is, the present invention relates to the following <1> to <20>.
<1> A waterproof paper having at least one waterproof layer on at least one surface of a paper base, the waterproof paper having a tensile strength in the transverse direction of 19.0 kN/m or less.
<2> The waterproof paper according to <1>, having a transverse breaking elongation of 5.3% or more.
<3> The waterproof paper according to <1> or <2>, having a tensile modulus in the transverse direction of 3.7 GPa or less.
<4> The waterproof paper according to any one of <1> to <3>, having a Taber stiffness in the lateral direction of 10 mN·m or less.
<5> The waterproof paper according to any one of <1> to <4>, wherein the content of softwood bleached kraft pulp relative to the total amount of pulp constituting the paper base material is 5 to 50 mass%.
<6> The waterproof paper according to any one of <1> to <5>, wherein the paper base material has three or more paper layers.
<7> The waterproof paper according to any one of <1> to <6>, wherein the ratio of tensile strength in the machine direction to that in the cross direction is 1.40 or more and 3.00 or less.
<8> The waterproof paper according to any one of <1> to <7>, wherein the total coating amount of the waterproof layer is 4 to 50 g/ ^m2 .
<9> The waterproof paper according to any one of <1> to <8>, wherein at least three waterproof layers are formed on at least one surface of the paper base material.
<10> The waterproof paper according to any one of <1> to <9>, which has waterproof layers on both sides of the paper base material.
<11> The waterproof paper according to any one of <1> to <10>, wherein the waterproof layer contains an ethylene-(meth)acrylic acid copolymer.
<12> The waterproof paper according to any one of <1> to <11>, which has heat sealability.
<13> The waterproof paper according to any one of <1> to <12>, having a Cobb water absorbency in water at 20°C after a contact time of 30 minutes of 20 g/m2 ^or less.
<14> The waterproof paper according to any one of <1> to <13>, having a Cobb water absorbency of 50 g/m2 ^or less in water at 90°C after a contact time of 30 minutes.
<15> The waterproof paper according to any one of <1> to <14>, which has an oil resistance of 6 or more as measured in accordance with JAPAN TAPPI No. 41.
<16> The waterproof paper according to any one of <1> to <15>, which is for use as a food container.
<17> A food container made using the waterproof paper according to any one of <1> to <15>.
<18> A paper cup made using the waterproof paper according to any one of <1> to <15>.
<19> A paper lid made using the waterproof paper according to any one of <1> to <15>.
<20> The method for producing waterproof paper according to any one of <1> to <16>, further comprising a coating step of coating a coating liquid for a waterproof layer on a paper substrate by an on-machine method, the coating step having a coating width of 2000 mm or more and a coating speed of 100 m/min or more.
<21> The waterproof paper according to any one of <1> to <16>, which satisfies at least one of the following (i) and (ii):
(i) The Taber stiffness in the machine direction is 7.0 mN·m or less. (ii) The tensile strength in the machine direction is 15.0 kN/m or more and 20.0 kN/m or less.

The waterproof paper of this embodiment is a waterproof paper having at least one waterproof layer on at least one surface of a paper base material, and has a tensile strength in the transverse direction of 19.0 kN/m or less. The waterproof paper of this embodiment is waterproof and has excellent processability.
A paper cup usually has a top curl portion at its end from the viewpoint of mouthfeel and fitting with a lid to be attached. In addition, in the case of a paper lid that is used assuming that a beverage is drunk while attached to the paper cup, a top curl portion may also be provided at the end of the paper lid. In forming such a paper cup or paper lid, the lateral direction of the water-resistant paper is formed to be the horizontal direction of the paper cup or paper lid. In this case, although the detailed mechanism is unknown, it is speculated that by adjusting the lateral tensile strength of the water-resistant paper to 19.0 kN/m or less, the flexibility of the part to be processed (for example, the part to be top curled when formed into a paper cup or paper lid) is increased, and the processing suitability is improved (cracks and wrinkles are less likely to occur in the top curl part). However, the above mechanism is based on speculation, and the mechanism by which the effects of the present invention are achieved is not limited to this. In this specification, the "lateral direction" in the water-resistant paper means the CD direction (direction perpendicular to the paper-making direction) of the constituent paper base material. In addition, the "longitudinal direction" of the waterproof paper means the MD direction (papermaking direction) of the paper substrate. Furthermore, in a preferred embodiment of the present invention, the waterproof paper has an appropriate Taber stiffness, thereby obtaining excellent side sealing properties. In a further preferred embodiment, the coating liquid for the waterproof layer is applied on the paper substrate by the on-machine method, thereby obtaining excellent production efficiency.

The structure and physical properties of the waterproof paper of this embodiment are described in further detail below.

In this specification, the numerical range represented by "X to Y" means a numerical range including X as the lower limit and Y as the upper limit. When the numerical range is described in stages, the upper and lower limits of each numerical range can be combined in any way. In addition, "(meth)acrylic" is a generic term including both acrylic and methacrylic.

<Paper base material>
The paper base material constituting the waterproof paper of this embodiment preferably has three or more paper layers, more preferably has four or more paper layers, and even more preferably has five or more paper layers. By having three or more paper layers, the desired basis weight and paper thickness can be achieved, and the physical properties of the waterproof paper can be adjusted to the desired range by adjusting the basis weight, freeness, etc. of each layer. The upper limit of the paper layers is not particularly limited, but is preferably seven layers or less, more preferably six layers or less.

Pulps constituting the paper base material include chemical pulps such as bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP); mechanical pulps such as groundwood pulp (GP), pressurized groundwood pulp (PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), chemi-mechanical pulp (CMP), and chemi-ground pulp (CGP); recycled paper pulp; non-wood fiber pulps such as kenaf, bagasse, bamboo, and cotton; and synthetic pulp. These pulps may be used alone or in combination of two or more. Of these, it is preferable to use LBKP and NBKP in combination.

The content of softwood bleached kraft pulp (NBKP) relative to the total amount of pulp constituting the paper base material is, from the standpoints of processability, smoothness and water resistance, preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, even more preferably 20% by mass or more, and is preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less, even more preferably 45% by mass or less, even more preferably 40% by mass or less, and may be 35% by mass or less or 30% by mass or less.

The content of hardwood bleached kraft pulp (LBKP) relative to the total amount of pulp constituting the paper base material is, from the standpoints of processability, smoothness and water resistance, preferably 40% by mass or more, more preferably 45% by mass or more, even more preferably 50% by mass or more, even more preferably 55% by mass or more, even more preferably 60% by mass or more, and may be 65% by mass or more or 70% by mass or more, and is preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, and even more preferably 80% by mass or less.

When the paper base material has three or more paper layers, the content of softwood bleached kraft pulp (NBKP) relative to the total amount of pulp constituting the outer layer (surface layer and back layer) of the paper base material is preferably 5% by mass or more, more preferably 10% by mass or more, even more preferably 15% by mass or more, even more preferably 20% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less, even more preferably 45% by mass or less, even more preferably 40% by mass or less, and may be 35% by mass or 30% by mass or less, from the viewpoint of processing suitability, smoothness and water resistance. When the paper base material has three or more paper layers, the content of softwood bleached kraft pulp (NBKP) relative to the total amount of pulp constituting the middle layer may be 0% by mass or more, and may be within the same numerical range as the outer layer.

When the paper base material has three or more paper layers, the content of hardwood bleached kraft pulp (LBKP) relative to the total amount of pulp constituting the outer layer (front layer and back layer) of the paper base material is preferably 40% by mass or more, more preferably 45% by mass or more, even more preferably 50% by mass or more, even more preferably 55% by mass or more, even more preferably 60% by mass or more, and may be 65% by mass or 70% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less, even more preferably 85% by mass or less, and even more preferably 80% by mass or less, from the viewpoint of processing suitability, smoothness and water resistance. When the paper base material has three or more paper layers, the content of hardwood bleached kraft pulp (LBKP) relative to the total amount of pulp constituting the middle layer may be 100% by mass or less, and may be within the same numerical range as the outer layer.

The degree of beating of the pulp is not particularly limited, but is preferably 200 to 800 mL, more preferably 350 to 700 mL, in terms of Canadian Standard Freeness (hereinafter also referred to as CSF or freeness). If the CSF of the pulp is within the above range, the paper strength required for food containers or cutlery is easily obtained. If the CSF is 200 mL or more, the interfiber bond is not too high, and the stress such as compression and tension is concentrated on the outermost layer of the folded part during the folding process in the molding process, and the occurrence of the phenomenon in which the resin layer is destroyed can be suppressed, and good processing suitability can be maintained. Furthermore, if the CSF is 800 mL or less, the smoothness of the paper surface is good, and printability can be maintained. Furthermore, from the viewpoint of setting the lateral tensile strength of the water-resistant paper within the desired range and improving processing suitability, the degree of beating of the pulp is preferably 420 mL or more, more preferably 450 mL or more, in terms of CSF.

CSF is measured in accordance with JIS P 8121-2:2012 "Pulp - Freeness test method - Part 2: Canadian standard freeness method".

Additives for paper base materials include, for example, pH adjusters (sodium bicarbonate, sodium hydroxide, etc.), dry strength agents (polyacrylamide, starch, etc.), wet strength agents (polyamide polyamine epichlorohydrin resin, melamine-formaldehyde resin, urea-formaldehyde resin), internal sizing agents (rosin-based, alkyl ketene dimer, etc.), drainage yield improvers, defoamers, fillers (calcium carbonate, talc, etc.), dyes, etc. These additives may be used alone or in combination of two or more. The content of the additives is not particularly limited and may be within the range commonly used.

The basis weight of the paper substrate is not particularly limited, but for example, for paper cups or paper lids, it is preferably 180 g/m ² or more, more preferably 200 g/m ² or more, and preferably 350 g/m ² or less. Furthermore, from the viewpoint of improving side sealability, the basis weight of the paper substrate is preferably 300 g/m ² or less, more preferably 250 g/m ² or less. The thickness of the paper substrate is also not particularly limited, but for example, for paper cups or paper lids, it is preferably 220 μm or more, more preferably 300 μm or more, and preferably 450 μm or less. Furthermore, from the viewpoint of improving side sealability, the thickness of the paper substrate is preferably 350 μm or less, more preferably 300 μm or less. The basis weight of the paper substrate is measured in accordance with JIS P 8124:2011. The thickness of the paper substrate is measured in accordance with JIS P 8118:2014.

[Method of manufacturing paper base material]
A method for producing a paper base material includes a method of making a paper stock containing pulp. The paper stock may further contain additives. Examples of additives include the additives listed above. The paper stock can be prepared by adding additives to a pulp slurry. The pulp slurry is obtained by beating pulp in the presence of water. The pulp beating method and beating device are not particularly limited, and may be the same as known beating methods and beating devices. The pulp content in the paper stock is not particularly limited, and may be within a range that is usually used. For example, it is 60% by mass or more and less than 100% by mass with respect to the total mass of the paper stock (solid content).

The papermaking method is not particularly limited, and papermaking can be performed using various papermaking machines such as a Fourdrinier papermaking machine, a multi-layer Fourdrinier papermaking machine, a cylinder papermaking machine, a multi-layer cylinder papermaking machine, a multi-layer Fourdrinier cylinder papermaking machine, a twin-wire papermaking machine, etc. By performing papermaking using a papermaking machine mounted on an on-machine coater, the entire process from papermaking to coating of the water-resistant layer can be performed in an integrated manner, and production efficiency can be significantly improved.
In papermaking, the papermaking width is preferably 2000 mm or more, more preferably 3000 mm or more, and even more preferably 4000 mm or more. The upper limit of the papermaking width is not particularly limited, but is, for example, 10000 mm or less. The papermaking speed is preferably 100 m/min or more, more preferably 200 m/min or more, and even more preferably 400 m/min or more. The upper limit of the papermaking speed is also not particularly limited, but is, for example, 2000 m/min or less.

The paper base material may be a single layer, a multilayer, or a laminate of multiple layers. The paper stock can be made by a standard method. For example, the paper stock can be cast onto a wire or the like, dehydrated to obtain a wet paper, and multiple wet papers can be stacked as necessary, and this single or multilayer wet paper can be pressed and dried. In this case, if multiple wet papers are not stacked, a single layer paper is obtained, and if multiple wet papers are stacked, a multilayer paper is obtained. When multiple wet papers are stacked, an adhesive can be applied to the front surface of the wet paper (the surface on which other wet papers are stacked). In order to improve oil resistance, a pigment layer can be provided on the front surface after pressing and drying.

<Water-resistant layer>
The waterproof paper of the present embodiment has at least one waterproof layer on at least one surface of a paper base material. From the viewpoint of heat sealability, the waterproof layer is preferably provided as the uppermost layer.

The water-resistant paper of this embodiment preferably has three or more water-resistant layers on at least one side of the paper base material. By adopting such a form, even if a coating defect such as a pinhole occurs in the lower water-resistant layer, it can be covered by the upper water-resistant layer, so that good water resistance can be exhibited. For example, when used for a paper cup or paper lid, it is preferable to have three or more water-resistant layers on the inner surface (also called the liquid-contacting surface). Furthermore, when used for an application where condensation occurs, such as a cold water paper cup or paper lid, it is preferable to have one or more water-resistant layers on the outer surface (the surface opposite to the liquid-contacting surface, also called the printed surface), more preferably two or more layers, and even more preferably three or more layers. Furthermore, in the case of a paper lid that is used assuming that a beverage is drunk while attached to a paper cup, the outer surface may be the drinking mouth surface, so it is preferable to have one or more water-resistant layers on that surface, more preferably two or more layers, and even more preferably three or more layers. That is, it is preferable that the water-resistant paper of this embodiment has water-resistant layers on both sides of the paper base material. It is also preferable to have a water-resistant layer on both sides of the paper base material in that the side sealability can be improved.

From the viewpoints of water resistance and heat sealability, it is preferable that the water-resistant layer contains an ethylene-(meth)acrylic acid copolymer, and from the viewpoints of suppressing blocking and preventing equipment contamination, it is more preferable that the water-resistant layer contains an ethylene-methacrylic acid copolymer. The ethylene-(meth)acrylic acid copolymer may be either a commercially available product or a synthetic product. Specific examples of commercially available products include the Chemipearl series (S-100, S-300, S-500) manufactured by Mitsui Chemicals, Inc., MYE-30ER and MYE-30MAZ manufactured by Maruyoshi Chemical Co., Ltd., the Surlyn series manufactured by DuPont, the Himilan series manufactured by Mitsui-DuPont Polychemicals, Hi-Tec SC-100 manufactured by Toho Chemical Industry Co., Ltd., Joncryl HPB-4110 manufactured by BASF Japan, RHOBARR320 manufactured by Dow Chemical Japan, AQUEENCE EPIX BC212F manufactured by Henkel Japan, and Aquatex AC-3100 manufactured by Chuo Rika Kogyo Co., Ltd.

From the viewpoint of water resistance, the content of ethylene-(meth)acrylic acid copolymer in the water-resistant layer is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more and 100% by mass or less, of the resin components of the water-resistant layer.

Here, the resin component of the water-resistant layer means a polymer component contained in the water-resistant layer, i.e., a compound having a weight average molecular weight of 1,000 or more.

The water-resistant layer preferably contains organic particles in addition to the ethylene-(meth)acrylic acid copolymer.
The organic particles preferably have a melting point or softening point of 100° C. or higher. When the melting point or softening point of the organic particles is 100° C. or higher, the organic particles do not melt or soften during drying, and excellent blocking properties are obtained. Furthermore, roll contamination during coating can be suppressed. Furthermore, when organic particles having a melting point or softening point of 100° C. or higher are contained, water resistance is improved. Although the reason for this is unclear, it is believed that due to the difference in melting point (or softening point) between the ethylene-(meth)acrylic acid copolymer and the organic particles, cracks that occur during film formation of the ethylene-(meth)acrylic acid copolymer are filled with the organic particles that melt later, reducing cracks and pinholes in the water-resistant layer. However, the effects of the present invention are not limited by the above mechanism.
The melting point or softening point of the organic particles is preferably 110° C. or higher, more preferably 120° C. or higher, and even more preferably 125° C. or higher. There is no upper limit, but it is preferably 240° C. or lower, more preferably 200° C. or lower, and even more preferably 160° C. or lower.
The melting point or softening point of the organic particles is measured by the ring and ball softening point method, JIS K2207:2006.

From the viewpoint that it is preferable to use an aqueous coating liquid for the water-resistant layer, it is preferable that the organic particles are water-dispersible organic particles. In other words, it is preferable that the organic particles are blended into the coating liquid for the water-resistant layer as an aqueous dispersion. Also, from the viewpoint of improving heat sealability and water resistance, it is preferable that the aqueous dispersion of the organic particles contains little or no surfactants, dispersants, etc.

Examples of the organic particles include polyester particles, acrylic particles, polystyrene particles, nylon particles, silicone particles, and polyolefin particles, and from the above-mentioned viewpoint, polyolefin particles are preferred.
Examples of the polyolefin resin constituting the polyolefin particles include polymers of olefins such as ethylene, propylene, 1-butene, 1-pentene, and 1-hexene. The polyolefin resin may be a homopolymer or copolymer of the olefins listed above. The polyolefin resin may be modified. Examples of the modification include anion modification and cation modification, and it is also preferable to use polyolefin particles whose dispersibility in water has been improved by modification.
The polyolefin particles are more preferably at least one selected from the group consisting of polyethylene particles, polypropylene particles, and propylene-ethylene copolymer resin particles.
The polyethylene may be high density polyethylene or low density polyethylene, and the polypropylene may be high density polypropylene or low density polypropylene, and there is no particular limitation.

The shape of the organic particles is not particularly limited and may be spherical or amorphous, but is preferably spherical. Note that spherical means an aspect ratio of 3 or less.

The average particle size of the organic particles is, from the viewpoint of blocking resistance, preferably 0.01 μm or more, more preferably 0.02 μm or more, and even more preferably 0.03 μm or more, and, from the viewpoint of maintaining heat sealability, preferably 50 μm or less, more preferably 30 μm or less, even more preferably 15 μm or less, and even more preferably 10 μm or less.
In addition, when the average particle size of the organic particles is larger than or approximately the same as the film thickness of the water-resistant layer, unevenness is formed on the coating film surface, resulting in excellent blocking resistance and suppression of roll staining. For the above reasons, the average particle size of the organic particles is preferably 1 μm or more, more preferably 3 μm or more, and even more preferably 5 μm or more.

In addition, when the average particle size of the organic particles is smaller than the film thickness, for example, less than 1.0 μm, when the water-resistant layer is provided, the organic particles are oriented on the coating film surface as the water contained in the coating liquid for the water-resistant layer evaporates, protecting the surface of the water-resistant layer, thereby providing excellent blocking resistance and suppressing roll staining. For the above reasons, the average particle size of the organic particles is preferably 0.5 μm or less, more preferably 0.3 μm or less, and even more preferably 0.1 μm or less.
The average particle size of the organic particles is measured by a laser diffraction particle size distribution measuring device based on the principle of the laser diffraction method or by a cole counter method.

From the viewpoint of blocking resistance, the content of organic particles in the water-resistant layer is preferably 0.5% by mass or more, more preferably 1.0% by mass or more, and even more preferably 1.5% by mass or more, and from the viewpoint of heat sealability, it is 40% by mass or less, preferably 35% by mass or less, more preferably 30% by mass or less, even more preferably 25% by mass or less, still more preferably 15% by mass or less, and particularly preferably 10% by mass or less.
From the viewpoint of improving blocking resistance and water resistance, the mass ratio of the ethylene-(meth)acrylic acid copolymer to the organic particles in the water-resistant layer (ethylene-(meth)acrylic acid copolymer/organic particles) is preferably 60/40 or more, more preferably 70/30 or more, even more preferably 75/25 or more, and is preferably 99.5/0.5 or less, more preferably 99/1 or less, even more preferably 98/2 or less.

The water-resistant layer may contain other components in addition to the ethylene-(meth)acrylic acid copolymer and organic particles described above.

Examples of other components include viscosity adjusters; defoamers; leveling agents such as surfactants and alcohols; colorants such as color pigments and color dyes; and anti-blocking agents such as inorganic pigments and synthetic resins.

Since these components tend to deteriorate water resistance and heat sealability, the total content of other components is preferably 50% by mass or less, more preferably 30% by mass or less, and even more preferably 10% by mass or less, of the solid content of the water-resistant layer, with the lower limit being 0% by mass.

The water-resistant layer is obtained by preparing a coating liquid for the water-resistant layer containing an ethylene-(meth)acrylic acid copolymer and, if necessary, organic particles, and applying this to at least one side of the paper base material.

The coating liquid for the water-resistant layer is preferably an aqueous dispersion, i.e., the coating liquid for the water-resistant layer preferably does not contain an emulsifier such as a surfactant or a dispersant.
The ethylene-(meth)acrylic acid copolymer blended in the coating liquid for the water-resistant layer may be in the form of an ionomer. Here, the ionomer is a copolymer neutralized with a cation. That is, synthetic resins obtained by neutralizing an ethylene-methacrylic acid copolymer, an ethylene-acrylic acid copolymer, and an ethylene-urethane copolymer with a cation all fall under the category of ionomers. The ionomer may be used alone or in combination of two or more types. Examples of the cation include metal ions such as sodium ions and potassium ions, ammonium ions (NH ₄ ⁺ ), and organic ammonium ions.

The method for applying the coating liquid for the water-resistant layer onto the paper base material is not particularly limited, and may be either a method performed continuously with the papermaking process (the so-called on-machine method), or a method in which the paper base material is once wound up and the coating liquid for the water-resistant layer is applied separately using a coating machine (the so-called off-machine method). From the viewpoint of production efficiency, the on-machine method is preferred.
When the coating liquid for the water-resistant layer is applied by the on-machine method (on-machine coater), the coating width is preferably 2000 mm or more, more preferably 3000 mm or more, and even more preferably 4000 mm or more. The upper limit of the coating width is not particularly limited, but is, for example, 10000 mm or less. The coating speed is preferably 100 m/min or more, more preferably 200 m/min or more, and even more preferably 500 m/min or more. The upper limit of the coating speed is also not particularly limited, but is, for example, 2000 m/min or less. By using an on-machine coater, the papermaking width and the coating width can be made wider, making it possible to make paper at a higher speed, and thus improving the production efficiency of the water-resistant paper.
The method for applying the coating solution for the water-resistant layer is not particularly limited, and may be appropriately selected from commonly used coating devices, such as an air knife coater, a blade coater, a gravure coater, a rod blade coater, a roll coater, a reverse roll coater, a bar coater, a curtain coater, a die slot coater, a champlex coater, a metering blade type size press coater, a short dwell coater, a spray coater, a gate roll coater, and a lip coater.

The coating weight of the water-resistant layer per layer is not particularly limited, but is preferably 1 to 10 g/ ^m2 . The total coating weight of the water-resistant layers in the water-resistant paper is preferably 8 g/ ^m2 or more, preferably 10 g/ ^m2 or more, more preferably 15 g/m2 or ^more , and even more preferably 20 g/ ^m2 or more. The upper limit is not particularly limited, but is preferably 50 g/m2 ^or less, more preferably 40 g/m2 ^or less, and even more preferably 30 g/m2 or ^less . When the water-resistant paper is used as a paper cup or a paper lid, the total coating weight of the water-resistant layers on the inner surface (liquid-contacting surface) is preferably 2 g/m2 or ^more , more preferably 5 g/m2 or ^more , and even more preferably 8 g/m2 or ^more . The upper limit is not particularly limited, but is preferably 20 g/m2 ^or less. When a water-resistant layer is also provided on the outer surface (printed surface), the total coating amount of the water-resistant layer on that side is preferably 2 g/m2 or ^more , more preferably 5 g/m2 ^or more, and even more preferably 8 g/m2 or ^more . There is no particular upper limit, but it is preferably 20 g/m2 ^or less.

<Other layers>
The waterproof paper of this embodiment may have other layers in addition to the waterproof layer described above. An example of the other layers is a pigment layer between the paper base material and the waterproof layer, which is preferable because the pigment layer functions as a sealing layer and improves waterproofness. The pigment layer contains a pigment and an adhesive (binder).

The adhesive is not particularly limited, but examples include synthetic adhesives such as styrene-butadiene, styrene-acrylic, ethylene-vinyl acetate, butadiene-methyl methacrylate, vinyl acetate-butyl acrylate copolymers, polyvinyl alcohol, maleic anhydride copolymers, and acrylic acid-methyl methacrylate copolymers; proteins such as casein, soy protein, and synthetic protein; starches such as oxidized starch, cationic starch, urea phosphate esterified starch, etherified starch such as hydroxyethyl etherified starch, and dextrin; cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose. The adhesive (binder) may be used alone or in combination of two or more types. In addition, a preferred embodiment of the adhesive is the use of a synthetic adhesive in combination with starches.

The pigments used in the pigment layer are not particularly limited, but examples include inorganic pigments such as kaolin, clay, engineered kaolin, delaminated clay, calcined clay, heavy calcium carbonate, light calcium carbonate, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica, and satin white; organic pigments such as solid, hollow, or core-shell types; and the like. The pigments may be used alone or in combination of two or more types.

The coating amount of the pigment layer is, for example, from 2 g/m ² to 40 g/m ² in view of the function of sealing.

The coating liquid for the pigment layer may be prepared by mixing the above-mentioned pigment, adhesive (binder), and aqueous medium. In addition to the pigment and adhesive (binder), various components may be added to the above-mentioned coating liquid as necessary. Examples of the various components include various auxiliaries that are typically blended into coating liquids, such as dispersants, thickeners, water retention agents, defoamers, water-resistant agents, colorants, and surfactants. The coating method for the pigment layer is not particularly limited, and the coating methods described in the section on <Water-resistant layer> can be used.

<Water-resistant paper properties>
[Tensile strength]
The water-resistant paper of this embodiment has a lateral tensile strength of 19.0 kN/m or less. It is believed that the flexibility of the part to be processed (for example, the part to be top curled when molded into a paper cup or paper lid) is increased, and the processing suitability is improved (cracks and wrinkles are less likely to occur in the top curl part). From the viewpoint of further increasing the flexibility and further improving the processing suitability, the lateral tensile strength of the water-resistant paper is preferably 17.0 kN/m or less, more preferably 15.0 kN/m or less, and even more preferably 14.0 kN/m or less. Furthermore, from the viewpoint of achieving both processing suitability and other performance, it is preferably 13.0 kN/m or less, more preferably 12.0 kN/m or less, even more preferably 11.0 kN/m or less, and even more preferably 10.0 kN/m or less. The lower limit of the tensile strength in the transverse direction of the waterproof paper is not particularly limited, but from the viewpoint of processing suitability, it is preferably 6.0 kN/m or more, more preferably 6.5 kN/m or more, even more preferably 7.0 kN/m or more, even more preferably 8.0 kN/m or more, and may be 9.0 kN/m or more. The tensile strength in the transverse direction of the waterproof paper can be adjusted to within the above range by adjusting the blending of the pulp constituting the paper base material, the beating degree (freeness) or fiber length, the amount of dry strength agent added, the papermaking speed, the J/W ratio, the shaking conditions in the CD direction of the papermaking wire (for example, shaking on/off, vibration frequency, vibration width), etc.

From the viewpoint of further improving processing suitability, the longitudinal tensile strength of the waterproof paper is preferably 14.0 kN/m or more, more preferably 15.0 kN/m or more, and may be 18.0 kN/m or more, 20.0 kN/m or more, or 21.0 kN/m or more, and is preferably 30.0 kN/m or less, more preferably 28.0 kN/m or less, even more preferably 27.0 kN/m or less, even more preferably 26.0 kN/m or less, even more preferably 21.0 kN/m or less, and particularly preferably 20.0 kN/m or less. The longitudinal tensile strength can be adjusted within the above range by adjusting the composition of the pulp constituting the paper base material, the beating degree (freeness) or fiber length, the amount of dry strength agent added, the papermaking speed, the J/W ratio, the conditions of shaking in the CD direction of the papermaking wire (e.g., shaking on/off, vibration frequency, vibration width), etc.

The ratio of the tensile strength in the longitudinal direction to the transverse direction (longitudinal direction/transverse direction) of the water-resistant paper is preferably 1.40 or more, more preferably 1.45 or more, even more preferably 1.50 or more, even more preferably 1.60 or more, from the viewpoints of processing suitability, smoothness and water resistance, and is preferably 3.00 or less, more preferably 2.50 or less, and may be 2.30 or less. The tensile strength ratio can be adjusted within the above range by adjusting the papermaking speed, J/W ratio, the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration width), etc.

The transverse and longitudinal tensile strength of water-resistant paper shall be values measured in accordance with JIS P 8113:2006.

[Elongation at break]
The horizontal breaking elongation of the waterproof paper is preferably 4.5% or more, more preferably 5.0% or more, even more preferably 5.3% or more, even more preferably 5.4% or more, and may be 5.5% or more. By making the horizontal breaking elongation of the waterproof paper equal to or more than the above lower limit, it is considered that the elongation of the part to be processed (for example, the part to be top curled when molded into a paper cup or paper lid) is improved, and the processing suitability is improved (cracks and wrinkles are less likely to occur in the top curl part). In addition, the upper limit of the horizontal breaking elongation of the waterproof paper is not particularly limited, but from the viewpoint of processing suitability, it is preferably 8.0% or less, more preferably 7.5% or less, and may be 7.0% or less. The transverse breaking elongation can be adjusted to within the above range by adjusting the composition of the pulp constituting the paper base material, the degree of beating (freeness) or fiber length, the amount of dry strength agent added, the papermaking speed, the J/W ratio, and the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration amplitude), etc.

The longitudinal breaking elongation of the waterproof paper is preferably 1.5% or more, more preferably 1.8% or more, may be 2.0% or more, or 2.1% or more, and is preferably 4.0% or less, more preferably 3.0% or less, and even more preferably 2.5% or less. The longitudinal breaking elongation can be adjusted within the above range by adjusting the composition of the pulp constituting the paper base material, the beating degree (freeness) or fiber length, the amount of dry strength agent added, the papermaking speed, the J/W ratio, the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration width), etc.

From the viewpoint of processing suitability, the ratio of the longitudinal and transverse breaking elongation of the water-resistant paper (longitudinal/transverse) is preferably 0.60 or less, more preferably 0.50 or less, and even more preferably 0.45 or less. The lower limit is not particularly limited, but is preferably 0.10 or more, more preferably 0.20 or more. The breaking elongation ratio can be adjusted within the above range by adjusting the papermaking speed, J/W ratio, and the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration width), etc.

The breaking elongation in the transverse and longitudinal directions of waterproof paper shall be the values measured in accordance with JIS P 8113:2006.

[Tensile modulus]
The tensile modulus of the water-resistant paper in the lateral direction is preferably 3.7 GPa or less, more preferably 3.5 GPa or less, and even more preferably 3.2 GPa or less. By making the tensile modulus of the water-resistant paper in the lateral direction equal to or less than the above upper limit, the part to be processed (for example, the part to be subjected to the top curl treatment when molded into a paper cup or a paper lid) becomes easy to deform, and the processing suitability is improved. In addition, the lower limit of the tensile modulus of the water-resistant paper in the lateral direction is not particularly limited, but from the viewpoint of processing suitability, it is preferably 1.5 GPa or more, more preferably 1.7 GPa or more, and may be 2.0 GPa or more. The tensile modulus of the lateral direction can be adjusted within the above range by adjusting the blending of the pulp constituting the paper base material, the beating degree (freeness) or fiber length, the amount of dry paper strength agent added, the papermaking speed, the J/W ratio, the conditions of shaking in the CD direction of the papermaking wire (for example, shaking on/off, vibration frequency, vibration width), etc.

For the waterproof paper of this embodiment, the longitudinal tensile modulus is preferably 4.5 GPa or more, more preferably 5.0 GPa or more, and preferably 10.0 GPa or less, more preferably 9.0 GPa or less, and even more preferably 8.0 GPa or less, from the viewpoint of further improving processing suitability. The longitudinal tensile modulus can be adjusted within the above range by adjusting the pulp composition constituting the paper base material, the papermaking speed, the J/W ratio, the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration width), etc.

From the viewpoint of further improving processing suitability, the ratio of the tensile modulus in the longitudinal direction and the transverse direction (longitudinal direction/transverse direction) of the waterproof paper is preferably 1.60 or more, more preferably 1.80 or more, even more preferably 2.00 or more, and is preferably 4.00 or less, more preferably 3.50 or less, and may be 3.00 or less. The tensile modulus ratio can be adjusted within the above range by adjusting the papermaking speed, J/W ratio, and the shaking conditions in the CD direction of the papermaking wire (e.g., shaking on/off, vibration frequency, vibration width), etc.

The tensile modulus of elasticity in the transverse and longitudinal directions of water-resistant paper shall be the values measured in accordance with JIS P 8113:2006.

[Taber stiffness]
The Taber stiffness of the water-resistant paper in the horizontal direction is preferably 10 mN·m or less, more preferably 6.0 mN·m or less, and even more preferably 4.0 mN·m or less. By making the Taber stiffness of the water-resistant paper in the horizontal direction equal to or less than the above upper limit, excellent side sealing properties can be obtained when the water-resistant paper is rolled in the horizontal direction to be molded into a cylindrical shape. In other words, when a paper cup is molded using water-resistant paper that satisfies the above physical properties, a paper cup or paper lid that has excellent side sealing properties and is less likely to leak liquid from the side seal portion can be obtained. In addition, the lower limit of the Taber stiffness of the water-resistant paper in the horizontal direction is not particularly limited, but from the viewpoint of processing suitability, it is preferably 2.0 mN·m or more, more preferably 2.5 mN·m or more. The Taber stiffness in the lateral direction can be adjusted within the above range by adjusting the thickness of the paper base material, the composition of the pulp that constitutes the paper base material, the degree of beating (freeness), the papermaking speed, the J/W ratio, and the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration amplitude), etc.

From the viewpoint of further improving processing suitability, the Taber stiffness in the longitudinal direction of the water-resistant paper is preferably 25 mN·m or less, more preferably 15 mN·m or less, even more preferably 10 mN·m or less, even more preferably 9.0 mN·m or less, and even more preferably 7.0 mN·m or less. The lower limit of the Taber stiffness in the longitudinal direction of the water-resistant paper is not particularly limited, but is preferably 4.0 mN·m or more. The Taber stiffness in the longitudinal direction can be adjusted within the above range by adjusting the thickness of the paper base material, the composition of the pulp constituting the paper base material, the degree of beating (freeness), the papermaking speed, the J/W ratio, the shaking conditions in the CD direction of the papermaking wire (e.g., shaking on/off, vibration frequency, vibration width), etc.

The ratio of the Taber stiffness in the longitudinal and transverse directions of the water-resistant paper (longitudinal/transverse) is preferably 1.5 or more, more preferably 1.8 or more, and may be 2.5 or more, 3.0 or more, or 3.5 or more, from the viewpoint of further improving the side sealability. The upper limit is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less, from the viewpoint of further improving the processing suitability. The Taber stiffness ratio can be adjusted within the above range by adjusting the papermaking speed, J/W ratio, and the shaking conditions of the papermaking wire in the CD direction (e.g., shaking on/off, vibration frequency, vibration width), etc.

The Taber stiffness in the transverse and longitudinal directions of waterproof paper shall be values measured in accordance with JIS P 8125:2000.

[Air permeability]
The waterproof paper of the present embodiment preferably has an air permeability of 1,000 seconds or more, more preferably 3,000 seconds or more, even more preferably 10,000 seconds or more, and even more preferably 99,999 seconds or more. The upper limit of the air permeability is not particularly limited, but is, for example, 1,000,000 seconds or less.

By setting the air permeability within the above range, the air barrier properties are improved, and water-resistant paper with even better water resistance can be obtained. In particular, when there are two or more water-resistant layers, or when a pigment layer is provided as an underlayer of a water-resistant layer, the air permeability can be increased.

The air permeability shall be the value measured in accordance with JIS P 8117:2009.

[Cobb Water Absorbency]
From the viewpoint of cold water resistance, the water-resistant paper of this embodiment has a Cobb water absorbency in water at 20°C after a contact time of 30 minutes of preferably 20 g/m2 ^{or less} , more preferably 15 g/m2 or ^less , even more preferably 10 g/m2 ^{or less} , still more preferably 8 g/m2 or ^less , and even more preferably 5 g/m2 ^or less (the lower limit is 0 g/ ^m2 ). The Cobb water absorbency is a value measured in accordance with JIS P 8140:1998.

From the viewpoint of hot water resistance, the water-resistant paper of this embodiment has a Cobb water absorbency in 90°C water after a contact time of 30 minutes of preferably 50 g/m2 ^{or less} , more preferably 35 g/m2 or ^less , even more preferably 25 g/m2 or ^less , even more preferably 15 g/m2 ^or less, and even more preferably 10 g/m2 or ^less (the lower limit is 0 g/ ^m2 ). The Cobb water absorbency is a value measured in accordance with JIS P 8140:1998.

[Oil resistance]
From the viewpoint of application to foods containing oil (soup, coffee, etc.), the water-resistant paper of this embodiment preferably has an oil resistance of 6 or more, more preferably 8 or more, and even more preferably 10 or more (maximum oil resistance of 12). The oil resistance of the water-resistant paper is a value measured in accordance with JAPAN TAPPI No. 41 (kit method). The oil resistance can be adjusted to a desired value by providing a pigment layer between the paper base material and the water-resistant layer, or by adjusting the number and type of layers of the water-resistant layer.

[Recycling rate]
From the viewpoint of environmental load, the recycle rate of the waterproof paper of the present embodiment is preferably 90% or more, and more preferably 93% or more. The recycle rate is a value measured by the method described in the examples.

[Basis weight/paper thickness]
The basis weight of the waterproof paper of this embodiment is not particularly limited, but for example, for paper cups or paper lids, it is preferably 200 g/ ^m2 or more and preferably 400 g/m2 or less. Furthermore, from the viewpoint of improving side sealing properties, the basis weight of the waterproof paper is preferably 300 g/m2 ^{or less} , more preferably 260 g/m2 ^or less. The basis weight of the waterproof paper is measured in accordance with JIS P 8124:2011.

The thickness of the water-resistant paper of this embodiment is not particularly limited, but for example, for paper cups or paper lids, it is preferably 220 μm or more and preferably 500 μm or less. Furthermore, from the viewpoint of improving side sealing properties, the thickness of the water-resistant paper is preferably 400 μm or less, more preferably 300 μm or less. The thickness of the water-resistant paper is measured in accordance with JIS P 8118:2014.

<Uses of water-resistant paper>
The water-resistant paper of the present embodiment is preferably used for liquid containers such as paper cups, etc. When used for paper containers, it is preferable from the viewpoint of formability that the water-resistant layer has heat sealability.

As an example of molding, the surface of the water-resistant paper is printed as necessary, punched out into a shape corresponding to the shape of the liquid container to be manufactured, folded, and the overlapping parts are heat sealed to form a liquid container. The water-resistant paper of this embodiment has excellent water resistance and heat sealability, and is therefore also excellent in moldability.

The water-resistant paper of this embodiment has excellent processing suitability and is therefore suitable for use as a food container. More specifically, examples include paper cups (for iced drinks, for hot drinks), paper plates, paper trays, paper food containers, paper lids, spoons, forks, knives, chopsticks, straws, and the like. In particular, the water-resistant paper of this embodiment has processing suitability that can withstand high curvature processing such as top curl processing of paper cups and paper lids (particularly paper lids for paper cups), and is therefore suitable for use as paper cups and paper lids (particularly paper lids for paper cups). Therefore, as another aspect of the present invention, food containers (particularly paper cups and paper lids) made of the above-mentioned water-resistant paper are also provided.

The features of the present invention are explained in more detail below with reference to examples and comparative 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, unless otherwise specified, "%" and "parts" represent "% by mass" and "parts by mass", respectively. Furthermore, the operations of the examples and comparative examples were carried out at room temperature (20-25°C) and normal humidity (40-50% RH) unless otherwise specified.

The raw materials used in the examples and comparative examples are as follows.
Ethylene-methacrylic acid copolymer ionomer: aqueous dispersion of ethylene-methacrylic acid copolymer metal salt, trade name "CHEMIPEARL S300", manufactured by Mitsui Chemicals, Inc. Defoamer: trade name "BISMAR KS-38E", manufactured by Nissin Chemical Laboratory Co., Ltd. Organic particle A: high density PE (HDPE), trade name "AQUACER 272N", manufactured by BYK, average particle size 30 μm
Organic particles B: low molecular weight PE + modified PO (modified polyolefin, low molecular weight PO), product name "Chemipearl W400", manufactured by Mitsui Chemicals, Inc., average particle size 4 μm

[Example 1]
<Preparation of paper substrate>
To 100 parts by mass (solid content equivalent) of pulp slurry in which beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed at a ratio of 70% and 30%, 0.8 parts by mass of weak acidic rosin sizing agent, 0.15 parts by mass of wet strength agent, and 1.45 parts by mass of aluminum sulfate were added to prepare a stock for the outer layers (1st layer, 5th layer). Similarly, to 100 parts by mass (solid content equivalent) of pulp slurry in which beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed at a ratio of 70% and 30%, 0.8 parts by mass of weak acidic rosin sizing agent, 0.15 parts by mass of wet strength agent, and 1.7 parts by mass of aluminum sulfate were added to prepare a stock for the middle layers (2nd layer to 4th layer). These paper stocks were used to make paper using a 5-layer Fourdrinier paper machine to obtain paper base materials. The papermaking speed was 600 m/min.

<Preparation of Water-Resistant Coating Fluid>
(a) Preparation of Coating Solution A (Coating Solution for Gravure Coater) An antifoaming agent was blended with an ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution A having a solid content of 30 mass%, with the ethylene-methacrylic acid copolymer ionomer content being 97.5 mass% and the antifoaming agent content being 2.5 mass% in the solid content.

(b) Preparation of Coating Solution B (Coating Solution for Air Knife Coater) An ethylene-methacrylic acid copolymer ionomer and an antifoaming agent were mixed to prepare Coating Solution B having a solid content of 28 mass%, with the ethylene-methacrylic acid copolymer ionomer content being 97.5 mass% and the antifoaming agent content being 2.5 mass% in the solid content.

<Preparation of waterproof paper>
The above-mentioned Coating Liquid A was applied to one side (front side) of a paper base material having a basis weight of 320 g/ ^m2 and a paper thickness of 415 μm using a gravure coater so that the coating amount (solid content) was 2.5 g/ ^m2 , and then dried to form a water-resistant layer (hereinafter referred to as the first front layer). Thereafter, Coating Liquid A was again applied to the same side using a gravure coater so that the coating amount (solid content) was 2.5 g/ ^m2 , and then dried to form a water-resistant layer (hereinafter referred to as the second front layer). On the same surface, the above-mentioned Coating Liquid B was applied using an air knife coater so that the coating amount (solid content) was 5 g/ ^m2 , and then dried to form a water-resistant layer (hereinafter referred to as the third layer on the front surface). Thereafter, Coating Liquid B was again applied using an air knife coater so that the coating amount (solid content) was 5 g/ ^m2 , and then dried to form a water-resistant layer (hereinafter referred to as the fourth layer on the front surface), thereby forming a total of four water-resistant layers.

The other side (rear side) of the paper substrate was coated with the above-mentioned coating liquid A using a gravure coater so that the coating amount (solid content) was 2.5 g/m ² , and then dried to form a water-resistant layer (hereinafter referred to as the first back layer). The same side was coated with the above-mentioned coating liquid A again using a gravure coater so that the coating amount (solid content) was 2.5 g/m ² , and then dried to form a water-resistant layer (hereinafter referred to as the second back layer). The same side was coated with the above-mentioned coating liquid B using an air knife coater so that the coating amount (solid content) was 5 g/m ^2, and then dried to form a water-resistant layer (hereinafter referred to as the third back layer), forming a total of three water-resistant layers. The coating speed was 120 m/min.

[Example 2]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 80% and 20%. Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 80% and 20%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. The coating liquid was applied to the front surface of the obtained paper base material up to the first to third layers in the same manner as in Example 1 to form water-resistant layers (first to third layers on the surface), and water-resistant paper was produced.

[Example 3]
A paper substrate was obtained in the same manner as in Example 2. The coating liquid was applied to the front surface of the paper substrate in the same manner as in Example 1 up to the fourth layer to form water-resistant layers (first to fourth layers on the front surface), thereby producing water-resistant paper.

[Example 4]
A paper substrate was obtained in the same manner as in Example 2. In the same manner as in Example 1, a water-resistant layer (front 1st to 3rd layers) was formed on the front surface of the paper substrate, and then a water-resistant layer (reverse 1st to 3rd layers) was formed on the reverse surface of the paper substrate to produce a water-resistant paper.

[Example 5]
<Preparation of base paper>
A paper substrate was obtained in the same manner as in Example 2. In the same manner as in Example 1, a water-resistant layer (front 1st to 4th layers) was formed on the front surface of the paper substrate, and then a water-resistant layer (reverse 1st to 3rd layers) was formed on the reverse surface of the paper substrate to produce a water-resistant paper.

[Example 6]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 90% and 10%. Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 90% and 10%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. In the same manner as in Example 1, a water-resistant layer (front first to fourth layers) was formed on the front surface of the paper base material, and then a water-resistant layer (back first to third layers) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Example 7]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 45% and 55%. Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 45% and 55%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. In the same manner as in Example 1, a water-resistant layer (front layers 1 to 4) was formed on the front surface of the paper base material, and then a water-resistant layer (back layers 1 to 3) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Example 8]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 50%:50%. Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 50%:50%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. In the same manner as in Example 1, a water-resistant layer (front layers 1 to 4) was formed on the front surface of the paper base material, and then a water-resistant layer (back layers 1 to 3) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Example 9]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 40% and 60%. Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed in a ratio of 40% and 60%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. In the same manner as in Example 1, a water-resistant layer (front layers 1 to 4) was formed on the front surface of the paper base material, and then a water-resistant layer (back layers 1 to 3) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Comparative Example 1]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that the pulp slurry was prepared from 100% beaten LBKP (CSF 460 mL). Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that the beaten LBKP (CSF 460 mL) was mixed at a ratio of 100%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. In the same manner as in Example 1, a water-resistant layer (front layers 1 to 4) was formed on the front surface of the paper base material, and then a water-resistant layer (back layers 1 to 3) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Comparative Example 2]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 1, except that the pulp slurry was prepared from 100% beaten LBKP (CSF 400 mL). Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that the beaten LBKP (CSF 400 mL) was mixed at a ratio of 100%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. In the same manner as in Example 1, a water-resistant layer (front layers 1 to 4) was formed on the front surface of the paper base material, and then a water-resistant layer (back layers 1 to 3) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Evaluation method]
The following measurements and evaluations were carried out on the waterproof papers of Examples 1 to 9 and Comparative Examples 1 and 2. The results are shown in Tables 1 and 2.
<Basance weight>
The basis weight of the waterproof paper was measured in accordance with JIS P 8124:2011.

<Paper thickness>
The thickness of the waterproof paper was measured in accordance with JIS P 8118:2014.

<Air permeability>
The air permeability of the waterproof paper was measured in accordance with JIS P 8117:2009.

<Tensile strength, elongation at break, tensile modulus>
The tensile strength, breaking elongation, and tensile modulus of the waterproof paper were measured in accordance with JIS P 8113:2006. The longitudinal direction of the waterproof paper refers to the papermaking direction (MD direction) of the paper base material. The transverse direction of the waterproof paper refers to the direction perpendicular to the papermaking direction of the paper base material (CD direction).
<Taber stiffness>
The Taber stiffness of the waterproof paper was measured in accordance with JIS P 8125:2000.

<Cobb Water Absorbency>
The obtained waterproof paper was cut into a 10 cm square, and the surface of the waterproof paper was brought into contact with ion-exchanged water at 20° C. or 90° C. for 30 minutes in accordance with JIS P 8140:1998, and the Cobb water absorbency was measured from the difference in mass before and after the contact in accordance with JIS P 8140:1998.

<Recyclability>
The obtained waterproof paper was weighed out to about 50 g in bone dry weight, cut into pieces of about 2 cm each by hand without using a blade, and tap water was added to make the concentration 3%, and the paper was treated at 21°C and 3000 rpm for 10 minutes using a disintegrator (No. 2532, manufactured by Kumagai Riki Kogyo Co., Ltd.) to prepare a pulp slurry in which the pulp fibers were completely disintegrated. The pulp slurry was screened for 10 minutes with a slit width of 0.2 mm (8 cuts) and a flow rate of 10 L/min to obtain a residue. The ratio of the bone dry residue weight to the charged bone dry weight was defined as the residue rate, and the pulp transmittance was calculated by subtracting this from 100%, which was defined as the recycle rate.

<Processing suitability>
The obtained waterproof paper was molded into a cup shape with the front side (liquid-contacting surface) on the inside and the back side (printing surface) on the outside, and the CD direction of the constituent paper substrate was the horizontal direction of the cup, and the moldability of the top curl part of the cup when top curled was evaluated based on the following criteria. The top curling was performed in the following manner. The opening was widened by applying a metal mold to the periphery of the opening at the top end of the cup, and a metal mold with a curl shape of about 3 mm in diameter was applied to curl the cup toward the outside. The cup was then pushed downward, and the substrate was rolled inward along the curved surface of the metal mold with a curl shape set below. The processing suitability was evaluated based on the following criteria.
Using the above method, 10,000 paper cups were molded and the top curl portion was inspected for defects (cracks, paper layer peeling, insufficient curl, wrinkles). The defect rate of the top curl portion was calculated using the formula: Defective rate = number of defective cups / number of paper cups molded x 100.
[Evaluation Criteria]
A: The defect rate of the top curl portion is 1% or less (no problem in practical use)
B: The defect rate of the top curl portion is more than 1% and 2% or less (no problem in practical use)
C: The defect rate of the top curl portion is more than 2% and 3% or less (no problem in practical use)
D: The defect rate of the top curl portion is more than 3% and 10% or less (problematic in practical use)
E: The defect rate of the top curl portion exceeds 10% (problematic in practical use)

<Blocking resistance>
The sample was cut into a 5 cm square, overlapped so that the front and back surfaces (paper substrate in the case of one-sided coating) were in close contact, and then sandwiched between a blocking tester and heated at 65°C for 30 minutes, followed by application of a pressure of 0.5 kg/ ^cm2 for 30 minutes. After cooling to room temperature, the sample was evaluated as follows.
[Evaluation Criteria]
A: Peels off without resistance B: There is slight resistance C: There is a lot of resistance, and the coated surface peels off slightly D: There is considerable resistance, and the coated surface peels off (paper peels off)

<Production efficiency>
The production efficiency of waterproof paper was evaluated according to the following criteria.
[Evaluation Criteria]
The amount of waterproof paper produced per minute is expressed in m (meter)/min.
A: The production speed is 400 m/min or more, and the production efficiency is very good.
B: The production speed is 200 m/min or more and less than 400 m/min, and the production efficiency is somewhat good.
C: The production speed is less than 200 m/min, and the production efficiency is very poor.

<Smoothness>
The smoothness of the back surface (outer surface of the cup) of the obtained waterproof paper was visually evaluated based on the following criteria.
[Evaluation Criteria]
A: High smoothness, high printability, no coating unevenness. B: Low smoothness, slightly poor printability, coating unevenness. C: Very low smoothness, poor printability, very much coating unevenness.

<Side sealing performance>
The obtained waterproof paper was cut into a length of 15 cm x width of 25 cm, and rolled horizontally with the surface facing inward to form a cylinder with a diameter of 7.5 cm. The heat sealability of the overlapping part of the waterproof paper (corresponding to the side of the body of the paper cup, side seal width 0.7 cm) was evaluated based on the following criteria. Heat sealing was performed using "CS-205" manufactured by Chubu Sogyo Co., Ltd., at a temperature of 250°C, a pressure of 0.1 MPa, and a bonding time of 2 seconds.
[Evaluation Criteria]
A: The adhesive strength is strong, and the film does not easily peel off even if the film is pushed by hand from the outside of the tube and the shape is distorted.
B: The adhesive strength is slightly weak, and when the tube is pushed by hand from the outside and the shape is distorted, part of the adhesive part peels off.
C: The adhesive strength is weak, and when the shape of the tube is distorted by pressing with a hand from the outside, about half of the adhesive portion peels off.

<Oil resistance>
The oil resistance of the waterproof paper surface was evaluated according to JAPAN TAPPI No. 41 (kit method). A higher value (kit value) indicates higher oil repellency.

As can be seen from the results in Tables 1 and 2, by having an appropriate value for the tensile strength in the transverse direction, when the obtained waterproof paper was molded into a cup shape, waterproof paper having excellent processability without cracks or wrinkles at the top curl portion was obtained. As a preferred embodiment, according to Examples 1 to 5 and 8 to 9 in which the NBKP content relative to the total amount of pulp constituting the paper base material was 20 to 50%, waterproof paper having further appropriate values for elongation and tensile modulus was obtained, and thus having better processability.
On the other hand, in Example 6, in which the NBKP content relative to the total amount of pulp constituting the paper base material was 10%, the paper was water-resistant with high tensile strength in the lateral direction and excellent processing suitability, but some cracks and wrinkles occurred in the top curl portion.
In addition, in Example 7, in which the NBKP content relative to the total amount of pulp constituting the paper base material was 55%, the processing suitability was good, but the surface smoothness was slightly inferior, so the printability tended to deteriorate when used for cups. Furthermore, unevenness occurred during application of the coating liquid, and the Cobb water absorbency tended to deteriorate.
On the other hand, in Comparative Example 1, in which the NBKP content relative to the total amount of pulp constituting the paper base material was 0%, the lateral tensile strength and tensile modulus were too high, and the lateral breaking elongation was too low, causing large cracks and wrinkles in the top curl portion. In Comparative Example 2, which has the same formulation as Comparative Example 1 except that the beating degree of LBKP was increased, the lateral tensile strength was high, causing large cracks and wrinkles in the top curl portion.

[Example 10]
<Preparation of paper substrate>
To 100 parts by mass (solid content equivalent) of pulp slurry made of beaten LBKP (CSF 400 mL) and beaten NBKP (CSF 530 mL) mixed at a ratio of 75% and 25%, 0.67 parts by mass of weak acidic rosin sizing agent, 0.84 parts by mass of wet strength agent, and 1.9 parts by mass of aluminum sulfate were added to prepare a stock for the outer layers (first layer, fifth layer). Similarly, to 100 parts by mass (solid content equivalent) of pulp slurry made of beaten LBKP (CSF 390 mL), 0.5 parts by mass of weak acidic rosin sizing agent, 0.3 parts by mass of wet strength agent, and 1.8 parts by mass of aluminum sulfate were added to prepare a stock for the middle layers (second layer to fourth layer). Using these stocks, paper was made using a five-layer papermaking Fourdrinier machine to obtain a paper base material with a basis weight of 224 g/m ² and a paper thickness of 268 μm. The papermaking speed was 600 m/min.

<Preparation of Water-Resistant Coating Fluid>
(a) Preparation of Coating Solution C (Coating Solution for Gravure Coater) An antifoaming agent was added to the ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution C having a solid content of 30 mass% with an ethylene-methacrylic acid copolymer ionomer content of 98.9 mass% and an antifoaming agent content of 1.1 mass% in the solid content.
(b) Preparation of Coating Solution D (Coating Solution for Air Knife Coater) An antifoaming agent was blended with the ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution D having a solid content of 28 mass% with an ethylene-methacrylic acid copolymer ionomer content of 98.9 mass% and an antifoaming agent content of 1.1 mass% in the solid content.
(c) Preparation of Coating Liquid E (Pigment Layer Coating Liquid) A defoamer was added to VB2160 (manufactured by Michelman Japan) having a kaolin/SBR (styrene-butadiene rubber) ratio of 4/6 to prepare a pigment layer coating liquid (Coating Liquid E) having a solids concentration of 40.0% by mass, with the VB2160 content being 98.9% by mass and the defoamer content being 1.1% by mass in the solids.
(d) Preparation of Coating Fluid F (Pigment Layer Undercoat Coating Fluid) A pigment layer undercoat coating fluid (Coating Fluid F) having a solids concentration of 45.0 mass % and a kaolin/heavy carbon/latex/starch content ratio of 25/75/10.5/4 was prepared.
(e) Preparation of Coating Fluid G (Pigment Layer Topcoat Coating Fluid) A pigment layer topcoat coating fluid (Coating Fluid G) having a kaolin/heavy carbon/latex/starch content of 24/76/12.5/1.0 and a solids concentration of 45.0 mass % was prepared.
(f) Preparation of Coating Solution H (Coating Solution for Gravure Coater) An antifoaming agent was added to the ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution H having a solid content of 28 mass% with an ethylene-methacrylic acid copolymer ionomer content of 98.9 mass% and an antifoaming agent content of 1.1 mass% in the solid content.
(g) Preparation of Coating Fluid I (Coating Fluid for Air Knife Coater) Organic particles B and an antifoaming agent were blended with an ethylene-methacrylic acid copolymer ionomer to prepare Coating Fluid I having a solid content of 28 mass%, where the content of the ethylene-methacrylic acid copolymer ionomer in the solid content was 96.9 mass%, the content of the low-molecular-weight PE + modified PO was 2.0 mass%, and the content of the antifoaming agent was 1.1 mass%.
(h) Preparation of Coating Solution J The organic particles B and an antifoaming agent were blended with the ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution J having a solid content of 28 mass%, in which the ethylene-methacrylic acid copolymer ionomer content in the solid content was 92.9 mass%, the low-molecular-weight PE + modified PO content was 6.0 mass%, and the antifoaming agent content was 1.1 mass%.
(i) Preparation of Coating Solution K Organic particles A and an antifoaming agent were blended with an ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution K having a solid content of 28 mass%, in which the ethylene-methacrylic acid copolymer ionomer content in the solid content was 93.9 mass%, the high-density PE content was 5.0 mass%, and the antifoaming agent content was 1.1 mass%.
(j) Preparation of Coating Solution L The organic particles A and an antifoaming agent were blended with the ethylene-methacrylic acid copolymer ionomer to prepare Coating Solution L having a solid content of 28 mass%, in which the ethylene-methacrylic acid copolymer ionomer content in the solid content was 78.9 mass%, the high-density PE content was 20.0 mass%, and the antifoaming agent content was 1.1 mass%.

<Preparation of waterproof paper>
The coating liquid C was applied to one side (front side) of the paper substrate using a gravure coater so that the coating amount (solid content) was 2.5 g/m ² , and then dried to form a water-resistant layer (hereinafter referred to as the first surface layer). The same side was again coated with the coating liquid C using a gravure coater so that the coating amount (solid content) was 2.5 g/m ² , and then dried to form a water-resistant layer (hereinafter referred to as the second surface layer). The same side was coated with the coating liquid D using an air knife coater so that the coating amount (solid content) was 5 g/m ² , and then dried to form a water-resistant layer (hereinafter referred to as the third surface layer), forming a total of three water-resistant layers. The coating speed was 120 m/min.

[Example 11]
The coating liquid was applied to the front surface of the paper substrate produced in Example 10 in the same manner as in Example 10, up to the 1st to 3rd layers, to form a water-resistant layer (1st to 3rd layers), to produce a water-resistant paper. The other surface (rear surface) of the paper substrate was coated with the above-mentioned coating liquid C using a gravure coater so that the coating amount (solid content) was 2.5 g/m ² , and dried to form a water-resistant layer (hereinafter referred to as the back first layer). The ^same surface was coated with the above-mentioned coating liquid D using an air knife coater so that the coating amount (solid content) was 5 g/m ² , and dried to form a water-resistant layer (hereinafter referred to as the back third layer), forming a total of three water-resistant layers.

[Example 12]
<Preparation of paper substrate>
To 100 parts by mass (solids equivalent) of pulp slurry made of beaten LBKP (CSF 400 mL) and beaten NBKP (CSF 530 mL) mixed at a ratio of 60% and 40%, 2.4 parts by mass of weak acidic rosin sizing agent, 0.14 parts by mass of wet strength agent, and 1.25 parts by mass of aluminum sulfate were added to prepare a stock for the outer layers (first and fifth layers). Similarly, to 100 parts by mass (solids equivalent) of pulp slurry made of beaten LBKP (CSF 330 mL), 1.5 parts by mass of weak acidic rosin sizing agent, 0.14 parts by mass of wet strength agent, and 1.5 parts by mass of aluminum sulfate were added to prepare a stock for the middle layers (second to fourth layers). These paper materials were used to make a five-layer paper using a fourdrinier paper machine, and then coating solution F was applied to the front surface using a rod coater to a coating amount (solid content) of 15 g/ ^m2 , and coating solution G was further applied thereon using a blade coater to a coating amount (solid content) of 10 g/ ^m2 , to obtain a paper base material having a pigment layer on the front surface, a basis weight of 229 g/ ^m2 , and a paper thickness of 257 μm. The papermaking speed was 600 m/min. The coating speed was 120 m/min.

<Preparation of waterproof paper>
The coating liquid was applied to one side (front side) of the above paper base material in the same manner as in Example 10 to form water-resistant layers (front 1st to 3rd layers) to produce water-resistant paper.

[Example 13]
Using the paper material prepared in the same manner as in Example 12, a paper base material with a basis weight of 225 g/ ^m2 and a thickness of 263 μm was produced using a fourdrinier paper machine with five layers. In the same manner as in Example 10, the coating liquid was applied to one side (front side) of the paper base material up to the first to third layers to form water-resistant layers (front first to third layers) to produce water-resistant paper.

[Example 14]
Using the paper stock prepared in the same manner as in Example 12, paper was made using a five-layer Fourdrinier paper machine to obtain a paper base material having a pigment layer on the front surface, a basis weight of 225 g/m ² , and a paper thickness of 263 μm.
On one side (front side) of the paper substrate, the coating liquid was applied for 1 to 3 layers to form water-resistant layers (front 1 to 3 layers) in the same manner as in Example 10. On the other side (reverse side) of the paper substrate, water-resistant layers (reverse 1 to 3 layers) were formed in the same manner as in Example 11 to produce water-resistant paper.

[Example 15]
Using the paper material prepared in the same manner as in Example 12, a five-layer papermaking machine was used to make a paper base material with a basis weight of 220 g/ ^m2 and a paper thickness of 252 μm. The front surface of the paper base material was coated with the above-mentioned coating liquid E using an air knife coater so that the coating amount (solid content) was 4.5 g/ ^m2 , and dried to form a pigment layer. Then, water-resistant layers (front 1st to 3rd layers) were formed on the same surface in the same manner as in Example 10 to produce a water-resistant paper.

[Example 16]
<Preparation of waterproof paper>
Using the same paper material prepared as in Example 12, a paper base material with a basis weight of 228 g/ ^m2 and a thickness of 260 μm was obtained using a 5-layer papermaking machine. The above-mentioned coating liquid E was applied to the front surface of the paper base material using an air knife coater so that the coating amount (solid content) was 4.5 g/m2 ^, and dried to form a pigment layer. After that, the above-mentioned coating liquid H was applied to the same surface using a gravure coater so that the coating amount (solid content) was 2.0 g/ ^m2 , and dried to form a water-resistant layer (hereinafter referred to as the front first layer), and then the same surface was again coated with the coating liquid H using a gravure coater so that the coating amount (solid content) was 2.0 g/ ^m2 , and dried to form a water-resistant layer (hereinafter referred to as the front second layer). On the same surface, the above-mentioned Coating Solution I was applied using an air knife coater so that the coating amount (solid content) was 4 g/ ^m2 , and then dried to form a water-resistant layer (hereinafter referred to as the third layer on the front side), thereby forming a total of three water-resistant layers.

[Example 17]
A paper base material prepared in the same manner as in Example 12 was made on a 5-layer Fourdrinier paper machine to obtain a paper base material with a basis weight of 224 g/ ^m2 and a paper thickness of 268 μm. The front surface of the paper base material was coated with the above-mentioned Coating Liquid E using an air knife coater so that the coating amount (solid content) was 4.5 g/ ^m2 , and dried to form a pigment layer. Then, the above-mentioned Coating Liquid H and Coating Liquid J were used to form water-resistant layers (front 1st to 3rd layers) on the same surface in the same manner as in Example 16, to produce a water-resistant paper.

[Example 18]
A paper base material prepared in the same manner as in Example 12 was made on a 5-layer Fourdrinier paper machine to obtain a paper base material with a basis weight of 229 g/ ^m2 and a paper thickness of 257 μm. The front surface of the paper base material was coated with the above-mentioned Coating Liquid E using an air knife coater so that the coating amount (solid content) was 4.5 g/ ^m2 , and dried to form a pigment layer. Then, on the same surface, the above-mentioned Coating Liquid H and Coating Liquid K were used to form water-resistant layers (front 1st to 3rd layers) in the same manner as in Example 16, to produce a water-resistant paper.

[Example 19]
A paper base material prepared in the same manner as in Example 12 was made on a 5-layer Fourdrinier paper machine to obtain a paper base material with a basis weight of 225 g/ ^m2 and a paper thickness of 263 μm. The front surface of the paper base material was coated with the above-mentioned Coating Liquid E using an air knife coater so that the coating amount (solid content) was 4.5 g/ ^m2 , and dried to form a pigment layer. Then, the above-mentioned Coating Liquid H and Coating Liquid L were used on the same surface to form water-resistant layers (front 1st to 3rd layers) in the same manner as in Example 16, to produce a water-resistant paper.

[Example 20]
The stock for the outer layers (first layer, fifth layer) was prepared in the same manner as in Example 12, except that the pulp slurry was prepared from 100% beaten LBKP (CSF 350 mL). Similarly, the stock for the middle layers (second to fourth layers) was prepared in the same manner as in Example 1, except that the beaten LBKP (CSF 350 mL) was mixed at a ratio of 100%. Using these stocks, paper was made using a five-layer fourdrinier papermaking machine to obtain a paper base material. At this time, the J/W ratio was adjusted to be high so that the aspect ratio of the tensile strength of the water-resistant paper was 3.06. In the same manner as in Example 1, a water-resistant layer (front layers 1 to 4) was formed on the front surface of the paper base material, and then a water-resistant layer (back layers 1 to 3) was formed on the back surface of the paper base material to produce a water-resistant paper.

[Example 21]
<Preparation of paper substrate>
To 100 parts by mass (solid content equivalent) of pulp slurry in which beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed at a ratio of 60% and 40%, 0.8 parts by mass of weak acidic rosin sizing agent, 0.15 parts by mass of wet strength agent, and 1.45 parts by mass of aluminum sulfate were added to prepare a stock for the outer layers (1st layer, 5th layer). Similarly, to 100 parts by mass (solid content equivalent) of pulp slurry in which beaten LBKP (CSF 460 mL) and beaten NBKP (CSF 650 mL) were mixed at a ratio of 60% and 40%, 0.8 parts by mass of weak acidic rosin sizing agent, 0.15 parts by mass of wet strength agent, and 1.7 parts by mass of aluminum sulfate were added to prepare a stock for the middle layers (2nd layer to 4th layer). Using these paper materials, a paper was made using a 5-layer Fourdrinier papermaking machine with an on-machine coater (manufactured by Voith Paper Technology Co., Ltd., model number: Fourdrinier multilayer) at a paper width of 4200 mm and a papermaking speed of 200 m/min. Subsequently, a water-resistant layer was coated using the on-machine coater as follows. At a coating width of 4,200 mm and a coating speed of 200 m/min, Coating Liquid F was simultaneously applied to both sides using a rod coater to a coating amount (solid content) of 4.5 g/ ^m2 to form a pigment layer, and Coating Liquid J was then applied to the front side of the pigment layer using a blade coater to a coating amount (solid content) of 5.0 g/ ^m2 , and Coating Liquid J was then applied to the back side using a blade coater to a coating amount (solid content) of 5.0 g/ ^m2 to produce a waterproof paper with water-resistant layers on both sides, a basis weight of 260 g/ ^m2 , and a paper thickness of 340 μm.

[Example 22]
A waterproof paper having a basis weight of 258 g/m ² and a thickness of 338 μm was prepared in the same manner as in Example 2-1 , except that the paper making speed and coating speed of the on-machine coater were changed to 400 m/min.

[Example 23]
A waterproof paper having a basis weight of 256 g/m ² and a thickness of 337 μm was prepared in the same manner as in Example 2-1 , except that the paper making speed and coating speed of the on-machine coater were changed to 600 m/min.

[Example 24]
A waterproof paper having a basis weight of 263 g/m ² and a thickness of 344 μm was prepared in the same manner as in Example 2-1 , except that the paper making speed and coating speed of the on-machine coater were changed to 800 m/min.

[Example 25]
A waterproof paper having a basis weight of 259 g/m ² and a thickness of 337 μm was prepared in the same manner as in Example 2-1 , except that the paper making speed and coating speed of the on-machine coater were changed to 1000 m/min.

[Example 26]
Waterproof paper with a basis weight of 250 g/m ² and a thickness of 327 μm was produced in the same manner as in Example 21, except that the papermaking conditions were changed so that the waterproof paper had the following basis weight and thickness.

[Example 27]
Waterproof paper with a basis weight of 344 g/m ² and a thickness of 468 μm was produced in the same manner as in Example 23, except that the papermaking conditions were changed so that the waterproof paper had the following basis weight and thickness.

Examples 10 to 27 were evaluated in the same manner as Examples 1 to 9 and Comparative Example 1. The results are shown in Tables 3 and 4, as well as Tables 5 and 6.

As can be seen from the results in Tables 3 and 4, in Examples 10 to 15, when the waterproof paper was rolled horizontally to form a cylindrical shape, the waterproof paper had an appropriate lateral Taber stiffness value compared to Examples 1 to 9, and therefore had a small repulsive force, resulting in excellent side sealing properties.
In addition, according to Examples 16 to 19, excellent blocking resistance was obtained due to the inclusion of organic particles. On the other hand, in Example 20, the Taber stiffness in the longitudinal direction was too high compared to the other Examples, and therefore the occurrence rate of defects in the top curl portion was high, and the processability was somewhat poor.
Furthermore, in Examples 21 to 27, the coating liquid was applied by an on-machine method, and therefore excellent production efficiency was obtained.

Although the present invention will be described in detail with reference to specific examples, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention.

Water-resistant paper can be used for various containers for food (trays, cups, etc.) and their lids, as well as food cutlery (e.g. chopsticks, spoons, forks, etc.).

Claims (13)

A waterproof paper having at least one waterproof layer on at least one surface of a paper substrate,
The tensile strength in the transverse direction is 19.0 kN/m or less ,
The ratio of tensile strength in the longitudinal direction to that in the transverse direction (longitudinal direction/transverse direction) is 1.40 or more and 3.00 or less,
The transverse elongation at break is 4.5% or more;
The content of softwood bleached kraft pulp relative to the total amount of pulp constituting the paper base material is 20 to 50 mass%;
The water-resistant paper has a hardwood bleached kraft pulp content of 50 to 80% by mass relative to the total amount of pulp constituting the paper base material . 2. The waterproof paper according to claim 1, wherein the ratio of breaking elongation in the machine direction to the cross direction (machine direction/cross direction) is 0.60 or less. Water-resistant paper according to claim 1 or 2, having a tensile modulus in the transverse direction of 3.7 GPa or less. Water-resistant paper according to any one of claims 1 to 3, having a Taber stiffness in the lateral direction of 10 mN·m or less. 5. The waterproof paper according to claim 1, wherein the ratio of tensile modulus in the machine direction to that in the cross direction (machine direction/cross direction) is 2.00 or more. The water-resistant paper according to any one of claims 1 to 5, wherein the paper base material has three or more paper layers. The waterproof paper according to any one of claims 1 to 6 , which satisfies at least one of the following (i) and (ii):
(i) The Taber stiffness in the machine direction is 7.0 mN·m or less. (ii) The tensile strength in the machine direction is 15.0 kN/m or more and 20.0 kN/m or less. The waterproof paper according to any one of claims 1 to 7 , wherein the waterproof layer contains an ethylene-(meth)acrylic acid copolymer. The waterproof paper according to any one of claims 1 to 8 , which has a Cobb water absorption of 20 g/ ^m2 or less in water at 20°C after a contact time of 30 minutes. The waterproof paper according to any one of claims 1 to 9 , which has a Cobb water absorption of 50 g/ ^m2 or less in water at 90°C after a contact time of 30 minutes. The waterproof paper according to any one of claims 1 to 10 , which has an oil resistance of 6 or more as measured in accordance with JAPAN TAPPI No. 41. A food container, a paper cup or a paper lid made using the waterproof paper according to any one of claims 1 to 11 . The method for producing waterproof paper according to any one of claims 1 to 11, comprising a coating step of coating a coating liquid for a waterproof layer on a paper substrate by an on-machine method, the coating step having a coating width of 2000 mm or more and a coating speed of 100 m/ min or more.