JP2024055128A - Waterproof paper - Google Patents

Abstract

[Problem] The object of the present invention is to provide paper with excellent water resistance. [Solution] The present invention provides a waterproof paper having a coating layer containing a waterproofing agent and tetrapod-shaped zinc oxide. According to the present invention, it is possible to obtain waterproof paper with excellent water repellency. [Selected Figure] None

Description

The present invention relates to paper that is water-repellent and water-resistant, and a method for producing the same.

In order to package various items, paper containers and packaging materials of various shapes using a paper base material are used. Generally, paper containers and packaging materials are based on a paper base material, and therefore may experience a decrease in strength due to moisture.

For this reason, it has been proposed to impart moisture-proof and waterproof properties to the paper by applying wax to the surface of the paper base material. For example, Patent Document 1 describes the production of a moisture-proof liner by incorporating microcapsules containing wax into the coating layer located on the outermost surface. Patent Document 2 describes the improvement of the moisture-proof and waterproof properties of paper by reducing blisters with a diameter of 50 μm or more on the surface of the coating layer.

JP 2011-162899 A JP 2020-165039 A

The objective of the present invention is to provide paper with excellent water repellency and water resistance.

After extensive research into the above-mentioned problems, the inventors have succeeded in developing paper with sufficient water repellency and water resistance by providing a pigment coating layer on the base paper that contains zinc oxide of a specific shape and a waterproofing agent.

The present invention includes, but is not limited to, the following aspects.
[1] A waterproof paper having a coating layer containing a waterproofing agent and tetrapod-shaped zinc oxide, the dynamic contact angle of which after 5 seconds of contact with water is 115° or more.
[2] The waterproof paper according to [1], wherein the dynamic contact angle is 125° or more.
[3] The waterproof paper according to [1] or [2], in which the Cobb water absorbency of the paper surface after 30 minutes is 10.0 g/ ^m2 or less.
[4] The waterproof paper according to any one of [1] to [3], wherein the waterproofing agent contains at least one of a styrene-based resin, an acrylic-based resin, and a polyolefin-based resin.
[5] The waterproof paper according to any one of [1] to [4], wherein the waterproofing agent comprises a styrene-acrylic resin.
[6] The waterproof paper according to any one of [1] to [5], wherein the coating layer further contains wax.
[7] The waterproof paper according to any one of [1] to [6], wherein the coating layer contains a sizing agent and the weight ratio of sizing agent/waterproofing agent is 0.3 or less.
[8] The waterproof paper according to [7], wherein the sizing agent comprises an AKD-based sizing agent.
[9] A method for producing the waterproof paper according to any one of [1] to [8], comprising applying a coating liquid containing a waterproofing agent and tetrapod-shaped zinc oxide to a paper substrate.

The present invention provides paper with sufficient water repellency and water resistance.

The present invention relates to a paper (waterproof paper) having excellent water resistance. In the present invention, water resistance means a property of being resistant to water and not breaking even when in contact with water for a relatively long time. Specifically, the waterproof paper according to the present invention has a water absorption amount (Cobb 1800) of 15.0 g/m ^{2 or less} after contact with water for 30 minutes based on a water absorption test method (Cobb method: JIS P 8140). In a preferred embodiment, the waterproof paper according to the present invention has a Cobb 1800 of 12.5 g/m ² or less or 10.0 g/m ² or less, and may also have a Cobb 1800 of 7.5 g/m ² or less or 5.0 g/m ² or less. There is no particular limit to the lower limit of the water absorption amount, but if the Cobb 1800 is 0.5 g/m ² or less, for example, it can be said to have waterproofness (a property in which water molecules are not easily absorbed into the inside of the paper layer).

The waterproof paper according to the present invention has a paper substrate (base paper) and a waterproof coating layer (water-resistant layer) provided on the paper substrate, and the waterproof layer contains a waterproofing agent and a pigment. In the present invention, by blending zinc oxide having a specific shape and a waterproofing agent in the pigment coating layer, not only water resistance but also excellent water repellency is achieved. Generally, when a waterproofing agent or a sizing agent is used in combination with a white pigment, the effect of the waterproofing agent or the sizing agent is not easily expressed by the pigment, so it is difficult to increase the water resistance and water repellency of pigment-coated paper. However, according to the present invention, waterproof paper with excellent water repellency can be obtained.

The waterproof paper of the present invention has not only water resistance but also excellent water repellency. In the present invention, water repellency means the property of repelling water even when it is splashed with water for a short time, and preventing the penetration of water as droplets. Specifically, the dynamic contact angle 5 seconds after the water is dropped is 115° or more. In a preferred embodiment, the waterproof paper of the present invention has a contact angle of 125° or more, 135° or more, or may be 140° or more. There is no particular upper limit to the contact angle, but it may be, for example, 175° or less or 165° or less.

The water-resistant layer according to the present invention can be formed by applying a coating agent to a paper substrate and drying the coating agent. The coating amount of the water-resistant layer is preferably 2 to 50 g/ ^m2 per side, more preferably 3 to 40 g/ ^m2 , and even more preferably 4 to 30 g/ ^m2 , and may be 5 to 20 g/ ^m2 .

The basis weight of the waterproof paper according to the present invention is not particularly limited, but may be, for example, 30 to 600 g/ ^m2 , preferably 35 to 300 g/ ^m2 , more preferably 40 to 200 g/ ^m2 , and may be 45 to 100 g/ ^m2 .

The waterproof paper of the present invention can be used for various purposes without any particular restrictions. For example, it can be used as a printing coated paper used in offset printing, gravure printing, letterpress printing, etc., and also as a white paperboard having a waterproof layer on the base paper of the paperboard. The coating layer of the present invention can also be used for labels and packaging. In addition, when only the waterproofing agent is applied, high water resistance and a certain degree of water repellency (contact angle of about 100° to 110°) are obtained, but the ink is difficult to fix in water-based printing such as inkjet printing due to the formation of a dense resin film. On the other hand, the coating layer of the present invention maintains high water resistance and water repellency while the ink pigment is easily fixed to the coating film, making water-based printing possible. As a result, it can also be used for applications such as paper bags, paper hats, paper aprons, paper ponchos, paper umbrellas, wallpaper, and cardboard helmets that have been subjected to water-based printing.

Tetrapod-shaped zinc oxide In the present invention, zinc oxide having a tetrapod shape is used in the water-resistant coating layer. Zinc oxide is an oxide of zinc represented by ZnO, and is widely used industrially as a white pigment. In the present invention, zinc oxide having a tetrapod shape is used among zinc oxides, and the tetrapod shape means a shape in which crystals extend three-dimensionally radially from the center like a tetrapod. The number of crystals extending from the center is not limited to four, and may be, for example, 2 to 8 or 3 to 5. The shape of the crystals extending from the center may be not only conical like a tetrapod, which is an actual wave-dissipating block, but also needle-shaped. For example, Pana-Tetra (Amtec Co., Ltd.) can be suitably used as zinc oxide having a tetrapod shape. In a preferred embodiment, the zinc oxide having a tetrapod shape is a needle-shaped single crystal of zinc oxide, and the length of the needle-shaped part is, for example, 1 to 50 μm, and may be 5 to 30 μm.

In the waterproof paper according to the present invention, only tetrapod-shaped zinc oxide may be used as a pigment, or other white pigments may be used in combination with tetrapod-shaped zinc oxide. Examples of white pigments to be used in combination with tetrapod-shaped zinc oxide include calcium carbonate, titanium oxide, kaolin, clay, engineered kaolin, delaminated clay, talc, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicate salts, colloidal silica, satin white, mica, and montmorillonite. Among these, kaolin, mica, or calcium carbonate having a flat shape are preferred. It is preferable that the aspect ratio of the flat inorganic pigment is 10 or more. When tetrapod-shaped zinc oxide is used in combination with other pigments in the present invention, it is preferable that 50% by weight or more of the pigment used in the waterproof coating layer is tetrapod-shaped zinc oxide.

The content of the white pigment in the water-resistant layer can be, for example, 5 to 50% by weight, and preferably 10 to 45% by weight. If the pigment content is too low, the effect of the white pigment cannot be fully obtained, and if the pigment content is too high, water resistance and water repellency may not be fully achieved. In the present invention, it is preferable that the ISO whiteness of the water-resistant paper is 1% or more higher than the whiteness of the paper base material by providing a water-resistant coating layer containing tetrapod-shaped zinc oxide on the paper base material.

Waterproofing Agent The waterproof paper according to the present invention uses a waterproofing agent in the waterproof coating layer. In a preferred embodiment, the waterproofing agent contains a synthetic resin, and preferably contains at least one of styrene-based resin, acrylic-based resin, and polyolefin-based resin. In particular, the waterproofing agent is preferably a styrene-based resin and/or an acrylic-based resin, and preferably contains a styrene-acrylic resin (styrene-acrylic acid copolymer).

The styrene-based resin that can be contained in the waterproof coating layer constituting the present invention preferably has a copolymerization ratio of 50 mass% or more of a styrene-based monomer having a styrene skeleton in its structure, and may be composed only of a polymer of a styrene-based monomer.

Examples of styrene monomers include styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, 2,4-dimethylstyrene, ethylstyrene, p-tert-butylstyrene, α-methylstyrene, and α-methyl-p-methylstyrene.

Examples of monomers copolymerizable with styrene monomers include alkyl methacrylates such as methyl methacrylate, cyclohexyl methacrylate, and methylphenyl methacrylate; alkyl acrylates such as methyl acrylate, ethyl acrylate, butyl acrylate, and cyclohexyl acrylate; unsaturated carboxylic acids such as methacrylic acid and acrylic acid; unsaturated dicarboxylic anhydrides such as maleic acid and itaconic acid; unsaturated nitriles such as acrylonitrile and methacrylonitrile; and conjugated dienes such as 1,3-butadiene and 2-methyl-1,3-butadiene. These can be used alone or in combination of two or more.

The acrylic resin that can be contained in the water-resistant layer constituting the present invention is a resin in which the copolymerization ratio of acrylic acid, methacrylic acid, and acrylic monomers that are derivatives of these is 50% by mass or more, and may be composed only of polymers of acrylic monomers.

Examples of acrylic monomers include methacrylic acid esters such as cyclohexyl methacrylate, t-butylcyclohexyl methacrylate, and methyl methacrylate, and acrylic acid esters such as methyl acrylate, ethyl acrylate, butyl acrylate, and isopropyl acrylate. The acrylic resin may be a polymer of one or more monomers selected from these acrylic monomers.

Examples of monomers that can be copolymerized with acrylic monomers include aromatic vinyl compounds such as styrene, o-methylstyrene, p-methylstyrene, 2,4-dimethylstyrene, ethylstyrene, p-tert-butylstyrene, α-methylstyrene, and α-methyl-p-methylstyrene; unsaturated nitriles such as acrylonitrile and methacrylonitrile; maleimides such as N-phenylmaleimide and N-cyclohexylmaleimide; unsaturated dicarboxylic anhydrides such as maleic anhydride; and unsaturated carboxylic acids such as methacrylic acid and acrylic acid. These can be used alone or in combination of two or more.

In a preferred embodiment of the present invention, in addition to the synthetic resins described above, the waterproof coating layer contains a wax. Examples of waxes contained in the waterproof coating layer include polyethylene wax, Fischer-Tropsch wax, synthetic oil waxes (fatty acid esters, fatty acid amides, ketones and amines), synthetic waxes such as hydrogenated oil, and natural waxes such as beeswax, wood wax, paraffin wax, and microcrystalline wax. These waxes can be used alone or in combination of two or more, and hydrocarbon waxes containing paraffin are particularly suitable.

In the present invention, it is preferable that the water-resistant layer contains a sizing agent. Generally, a sizing agent is an additive for preventing ink or water from bleeding into the paper. Examples of sizing agents include rosin-based sizing agents, rosin emulsion-based sizing agents, α-carboxymethyl saturated fatty acid, neutral rosin-based sizing agents, alkyl ketene dimer (AKD)-based sizing agents, alkenyl succinic anhydride (ASA)-based sizing agents, and cationic polymer-based sizing agents. When a waterproofing agent and a sizing agent are used in combination in the present invention, the ratio of sizing agent/waterproofing agent used is, for example, 0.5 or less or 0.4 or less, and preferably 0.3 or less or 0.2 or less. The water repellency of the paper can be improved by using a large amount of sizing agent, but if too much is used, the sizing agent may precipitate on the paper surface or the abrasion resistance of the paper may deteriorate.

In the present invention, for example, the paper substrate can be manufactured by applying a coating agent to at least one side of the paper substrate and drying the coating agent. The water-resistant layer can be formed by applying the coating agent using a known coating method, such as air knife coating, curtain coating, blade coating, gate roll coating, and die coating. The water-resistant layer may be a single layer or multiple layers, and multiple water-resistant layers may be applied sequentially, or two or more layers may be applied simultaneously by curtain coating or the like. When drying the water-resistant layer, the temperature of the coating layer at the exit of the drying process is preferably adjusted to be less than 120°C. The coating speed when applying the coating agent can be appropriately set in consideration of the viscosity of the coating agent and the target coating amount.

In a preferred embodiment, the coating agent is applied to the paper substrate using a contour coating method such as air knife coating or curtain coating, which ensures a uniform amount of coating agent on the paper substrate surface, and therefore a uniform coating thickness, which can prevent blisters from occurring in the coating layer during the subsequent drying process. In addition, the amount of coating agent used can be reduced compared to contact coating methods, which can reduce manufacturing costs.

In the present invention, the water-resistant layer can use an adhesive (binder) that has been conventionally used. Examples of adhesives include synthetic adhesives such as various copolymers (latexes) such as styrene-butadiene, styrene-acrylic, ethylene-vinyl acetate, butadiene-methyl methacrylate, and vinyl acetate-butyl acrylate, polyvinyl alcohol, maleic anhydride copolymers, and acrylic acid-methyl methacrylate copolymers; proteins such as casein, soy protein, and synthetic protein; starches other than the above starch-derived polymer compounds, such as oxidized starch, cationic starch, urea phosphate esterified starch, and etherified starch such as hydroxyethyl etherified starch; and cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose. One or more types of adhesives can be appropriately selected and used.

In the present invention, the coating solution for forming the water-resistant layer may contain, for example, a stabilizer, a defoamer, a viscosity modifier, a water retention agent, a preservative, a colorant, and the like.
In the present invention, the method for preparing the coating liquid is not particularly limited, and can be appropriately adjusted depending on the type of coater. When a blade type coater is used, the solid content concentration of the coating liquid is preferably 20 to 70% by weight, more preferably 30 to 50% by weight. The viscosity of the coating liquid can be measured by a B-type viscometer based on JIS K 7117-1, and is, for example, preferably 50 to 3500 mPa·s, more preferably 100 to 2500 mPa·s, and may be 150 to 1500 mPa·s or 200 to 500 mPa·s.

The coating agent applied to the paper substrate is dried to form a water-resistant layer. In this drying process, the temperature of the water-resistant layer at the exit is preferably less than 120°C, and may be adjusted to be 100°C or less. If the temperature of the water-resistant layer at the exit is 120°C or higher, the incidence of blisters in the water-resistant layer may increase, and blocking may occur in the water-resistant paper that is wound after the water-resistant layer is formed. On the other hand, the temperature of the water-resistant layer at the exit is preferably 60°C or higher, more preferably 70°C, and may be 80°C or higher. If the temperature of the water-resistant layer at the exit is less than 60°C, not only may blocking occur in the water-resistant paper that is wound after the water-resistant layer is formed, but the water-resistant layer may not be dried sufficiently to fully exhibit water resistance and water repellency.

The temperature of the water-resistant layer at the exit of the drying process can be set taking into consideration the basis weight and thickness of the paper substrate. Here, the exit of the drying process refers to the exit of the drying zone if there is one drying zone in the drying process, and refers to the exit of the most downstream drying zone if there are multiple drying zones in the drying process.

The temperature of the water-resistant layer at the exit of the drying process can be adjusted by adjusting the drying time and the temperature of the drying zone. The drying time is determined by the feed speed of the paper substrate, the number and length of the drying zones, the equipment capacity of the drying zones (air volume, infrared output), etc. In addition, the drying method can be a known drying method, such as a steam cylinder heating drying method, a hot air drying method, a gas-type infrared drying method, an electric infrared drying method, etc., and any one of these can be used, or a combination of two or more of them can be used.

Paper base material (base paper)
The waterproof paper of the present invention has at least a base paper layer. The base paper can be manufactured by a known method, for example, a papermaking raw material (paper material) is made in a wire part, and then the base paper can be manufactured by passing it through a press part and a pre-drier part. The base paper used in the present invention may be made of a single layer or a multi-layer, but when manufacturing a white paperboard, it is preferable to use a multi-layer base paper. The method for manufacturing the base paper of the present invention is not particularly limited, and it can be manufactured by a known method using a known raw material. The base paper used in the present invention is not particularly limited, and commonly used fine paper, medium quality paper, wood paper, kraft paper, one-sided glazed paper, glassine paper, machine coated paper, art paper, cast coated paper, synthetic paper, resin coated paper, etc. can be used without exception.

The basis weight of the base paper of the present invention is not particularly limited and can be appropriately selected depending on the application, but can be, for example, 30 to 500 g/ ^m2 , or may be 33 to 200 g/ ^m2 or 35 to 75 g/ ^m2 .

Chemical pulp can be used as the pulp raw material for the base paper of the present invention. In addition to chemical pulp, various pulps can be used depending on the application, such as deinked pulp (DIP), groundwood pulp (GP), refiner groundwood pulp (RGP), thermomechanical pulp (TMP), chemi-thermomechanical pulp (CTMP), chemi-ground pulp (CGP), and semi-chemical pulp (SCP). As deinked pulp, deinked pulp made from sorted waste paper such as fine paper, medium-quality paper, low-quality paper, newspapers, flyers, and magazines, or unsorted waste paper that is a mixture of these, can be used.

In the present invention, any known filler can be used as the filler for the base paper. For example, inorganic fillers such as heavy calcium carbonate, light calcium carbonate, clay, silica, light calcium carbonate-silica composite, kaolin, calcined kaolin, delaminated kaolin, white carbon, talc, magnesium carbonate, barium carbonate, barium sulfate, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, zinc oxide, titanium oxide, and amorphous silica produced by neutralization with minerals of sodium silicate, and organic fillers such as urea-formaldehyde resin, melamine resin, polystyrene resin, and phenol resin can be used alone or in combination. Among these, heavy calcium carbonate and light calcium carbonate, which are typical fillers in neutral papermaking and alkaline papermaking, are preferably used to improve opacity. The filler content in the paper is not particularly limited, but may be, for example, 1 to 40% by solids weight or 10 to 35% by solids weight.

In the present invention, known papermaking additives can be used. For example, aluminum sulfate and various anionic, cationic, nonionic or amphoteric retention improvers, drainage improvers, various paper strength enhancers, internal sizing agents, and other papermaking internal additives can be used as necessary. Examples of dry strength improvers include polyacrylamide and cationized starch, and examples of wet strength improvers include polyamidoamine epichlorohydrin. These chemicals are added to the extent that they do not affect the formation or runnability. Examples of neutral sizing agents include alkyl ketene dimers, alkenyl succinic anhydrides, and neutral rosin sizing agents. In addition, dyes, fluorescent whitening agents, pH adjusters, defoamers, pitch control agents, slime control agents, and the like can be added as necessary.

The method for making the base paper in the present invention is not particularly limited, and can be carried out using a Fourdrinier paper machine including a top wire, an on-top former, a gap former, a cylinder paper machine, a board paper machine using a combination of a Fourdrinier paper machine and a cylinder paper machine, a Yankee dryer machine, etc. The pH during papermaking may be acidic, neutral, or alkaline, but neutral or alkaline is preferred. The papermaking speed is not particularly limited.

The waterproof paper of the present invention has one or more waterproof layers containing a pigment on the base paper, but the waterproof paper of the present invention may have a clear (transparent) coating layer not containing a pigment on one or both sides of the above-mentioned base paper. By providing a clear coating layer by applying a coating liquid (size press liquid) not containing a pigment on the base paper, the surface strength and smoothness of the base paper can be improved, and the coatability when applying a pigment can be improved. In the present invention, the clear coating layer may contain the starch-derived polymer compound of the present invention as a binder. The amount of the clear coating may be 0.1 to 8.0 g/m ² or 0.5 to 6.5 g/m ² in terms of solid content per side.

In the present invention, clear coating refers to the application (size press) of a coating liquid (surface treatment liquid) whose main components are starch, various starches such as oxidized starch, water-soluble polymers such as polyacrylamide and polyvinyl alcohol, and emulsions of styrene-acrylic resins, etc., onto base paper using a coater (applicator) such as a size press, gate roll coater, premetering size press, curtain coater, or spray coater.

In the present invention, it is preferable to pre-calender the base paper before coating using an online soft calendar, online chilled calendar, or the like to smooth the base paper in advance in order to make the coating layer uniform after coating. In this case, the processing line pressure is preferably 30 to 100 kN/m, more preferably 50 to 100 kN/m. The moisture content of the base paper during pre-calendering is also important, and a moisture content of 3 to 5% is preferable.

The present invention will be described in more detail below with reference to specific examples, but the present invention is not limited to these examples. In this specification, concentrations and the like are based on weight unless otherwise specified, and numerical ranges include their endpoints.

Manufacturing of waterproof paper (1) Sample 1
A coating solution (paint) was applied to one side (glossy side) of bleached glossy paper (manufactured by Nippon Paper Industries Co., Ltd.) using a Mayer bar, and then dried with hot air to produce paper samples. The bleached glossy paper used as the paper base material (base paper) was produced using a Yankee machine from paper stock containing bleached chemical pulp, internal sizing agents, and paper strength agents, and was not pigment- or clear-coated (basis weight: 45 g/ ^m2 , water contact angle: 104.4°).
The composition of the coating liquid (solid content ratio) is as shown in the table below, and was prepared by stirring and dispersing 58 parts by weight of tetrapod-shaped zinc oxide (Panatetra WZ-0501, manufactured by Amtec) in 100 parts by weight of a waterproofing agent containing styrene-acrylic resin and wax (VaporCoat 2220, manufactured by Michaelman).

(2) Sample 2
Paper samples were produced in the same manner as Sample 1, except that an AKD-based sizing agent (SE2384 manufactured by Seiko PMC) was added to the coating liquid according to the formulation shown in the table below.
(3) Samples 3-4
Paper samples were prepared in the same manner as Sample 2, except that the coating amount was changed.
(4) Samples 5-6
Paper samples were produced in the same manner as Sample 2, except that the ratio of tetrapod-like zinc oxide and surface sizing agent to the waterproofing agent was changed as shown in the table below.
(5) Sample 7
Paper samples were produced in the same manner as sample 2, except that tetrapod-shaped zinc oxide and kaolin (KCS manufactured by Imerys) were used in combination as the white pigment, and the ratio of the white pigment and surface sizing agent used relative to the waterproofing agent was changed as shown in the table below.
(6) Sample 8 (Comparative Example)
A paper sample was prepared in the same manner as Sample 1, except that no sizing agent and no white pigment (tetrapod-like zinc oxide) were used.
(7) Sample 9 (Comparative Example)
Paper samples were produced in the same manner as Sample 2, except that no white pigment (tetrapod-shaped zinc oxide) was used and the ratio of surface sizing agent to waterproofing agent was changed as shown in the table below.
(8) Sample 10 (Comparative Example)
Paper samples were produced in the same manner as sample 2, except that kaolin (KCS manufactured by Imerys) was used as the white pigment instead of tetrapod-shaped zinc oxide, and the ratio of the white pigment and surface sizing agent used relative to the waterproofing agent was changed as shown in the table below.
(9) Sample 11 (Comparative Example)
A paper sample was produced in the same manner as sample 10, except that granular zinc oxide (Sakai Chemical Industry, fine zinc oxide, particle size: 0.28 μm) was used as the white pigment. The zinc oxide used in sample 11 did not have a tetrapod-like shape and was irregularly granular.
(10) Sample 12 (Comparative Example)
A paper sample was produced in the same manner as Sample 10, except that precipitated calcium carbonate (Okutama Kogyo, Tamapearl TP221GS) was used as the white pigment.

Evaluation of Waterproof Paper The produced samples were evaluated for waterproofness etc. according to the following procedure.
(1) Water resistance (Cobb water absorbency)
According to the water absorption test method (Cobb method: JIS P 8140), the water absorption amount (Cobb 1800) was measured when the water-resistant layer was brought into contact with water for 30 minutes.
(2) Water repellency (dynamic contact angle)
3.5 μl of pure water was dropped onto the paper surface (water-resistant layer), and the dynamic contact angle 5 seconds after the dropping was measured using a contact angle measuring device (DAT1100, manufactured by FIBRO System).
(3) Abrasion resistance (evaluation of black cloth transfer)
Using a Gakushin abrasion resistance tester (manufactured by Tester Sangyo Co., Ltd.), black rag paper was placed against the sample and the sample was run back and forth five times with a load of 200 g. The degree of transfer of the water-resistant layer to the black rag paper was then visually evaluated on a five-point scale based on the following criteria.
(Evaluation criteria) No adhesion at all: 5 points, thin adhesion: 4 points, partially thick adhesion: 3 points to 2 points, thick adhesion all over: 1 point

Water resistance could be imparted by applying a waterproofing agent to the paper base material (Sample 8), and furthermore, the use of a sizing agent made it possible to obtain paper that was both water resistant and water repellent (Sample 9). However, when common white pigments such as kaolin, granular zinc oxide, and calcium carbonate were added to the coating liquid, the water resistance and water repellency were significantly reduced (Samples 10-12). Furthermore, although the water repellency could be improved by adding a large amount of sizing agent, white powder precipitated on the paper surface and abrasion resistance was significantly reduced (Sample 9).

On the other hand, by using tetrapod-shaped zinc oxide in combination with a waterproofing agent based on the present invention, paper with excellent water resistance and water repellency could be obtained without compromising abrasion resistance. When using general pigments such as granular zinc oxide or calcium carbonate, the effects of the waterproofing agent and sizing agent are greatly impaired, suggesting that the special shape of zinc oxide (tetrapod-shaped) contributes greatly to improving water repellency and water resistance.

Claims (9)

Water-resistant paper that has a coating layer containing a waterproofing agent and tetrapod-shaped zinc oxide, and has a dynamic contact angle of 115° or more 5 seconds after contact with water. The waterproof paper according to claim 1, wherein the dynamic contact angle is 125° or more. 2. The waterproof paper according to claim 1, wherein the Cobb water absorbency of the paper surface after 30 minutes is 10.0 g/ ^m2 or less. The waterproof paper according to claim 1, wherein the waterproofing agent contains at least one of a styrene-based resin, an acrylic-based resin, and a polyolefin-based resin. The waterproof paper according to claim 1, wherein the waterproofing agent comprises a styrene-acrylic resin. The waterproof paper according to claim 1, wherein the coating layer further comprises wax. The waterproof paper according to claim 1, wherein the coating layer contains a sizing agent and the weight ratio of sizing agent/waterproofing agent is 0.3 or less. The waterproof paper according to claim 7, wherein the sizing agent comprises an AKD-based sizing agent. A method for producing the waterproof paper according to any one of claims 1 to 8, comprising the steps of:
The method comprises applying a coating liquid containing a waterproofing agent and zinc oxide in tetrapod form to a paper substrate.