Waterproof paper and method for manufacturing the same

Abstract

To develop a technique for manufacturing a paper having excellent waterproof property.SOLUTION: A waterproof paper of the present invention has (a) a paper substrate, (b) an undercoat layer provided on at least one side of the paper substrate and containing a synthetic resin having a glass transition point of 65-260°C, and (c) a waterproof layer containing a styrene-acrylic resin and a wax and provided on the undercoat layer as an outermost layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to waterproof paper and a method for manufacturing the same. [Background technology]

[0002] Traditionally, paper containers and packaging materials, in various forms using paper as the base material, have been used to package a wide range of goods. Generally, because paper containers and packaging materials use paper as the base material, they are extremely permeable to water vapor and other moisture, which can lead to a decrease in strength due to moisture generated from the packaged goods. Furthermore, some goods being packaged or wrapped are extremely sensitive to water vapor and other moisture entering from the outside. In addition, when transporting chilled products, for example, with ice, paper containers and packaging materials need to be waterproof.

[0003] Therefore, methods have been proposed for forming a wax layer by coating the surface of a paper substrate with a water-repellent wax composition, and for forming a resin coating by laminating polyethylene film, polypropylene film, etc. For example, Patent Document 1 describes moisture-proof paper in which a mixture of a wax emulsion and a water-insoluble synthetic resin emulsion, along with a surfactant, is applied to at least one side of the paper, and then heat-treated to form a surfactant layer on the outermost layer. Patent Document 2 describes a paper having at least two coating layers on at least one side of the base paper, the outermost coating layer containing microcapsules encapsulating wax, and the coating amount is 0.5 g / m² in terms of solid content. 2 More than 2.5g / m 2 The following describes a moisture-proof liner in which the coating layer located between the outermost coating layer and the base paper contains an acrylic copolymer and / or a styrene copolymer. Furthermore, Patent Document 3 proposes producing paper with a high level of moisture resistance and waterproofing by drying the waterproof coating layer under specific drying conditions. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-266096 [Patent Document 2] Japanese Patent Publication No. 2011-162899 [Patent Document 3] Japanese Patent Publication No. 2020-165039 [Overview of the project] [Problems that the invention aims to solve]

[0005] In the context of the move away from plastics, there is a demand for paper containers to replace polystyrene foam, and water-resistant and water-repellent liners are attracting attention for manufacturing such containers. However, in the method of forming a wax layer described in Patent Document 1, simply applying a single layer of the wax composition makes it difficult to sufficiently suppress moisture permeability. To sufficiently suppress moisture permeability, it is necessary to apply the wax composition multiple times, which makes the manufacturing process extremely complicated. Furthermore, in the method of laminating with a resin film as described in Patent Document 2, in addition to the complicated manufacturing process due to the lamination process, paper or cardboard laminated with a resin film has extremely poor disintegration properties when collected and reused as waste paper after use, making recycling difficult. Moreover, the method described in Patent Document 3 requires the adoption of specific drying conditions, and therefore cannot be said to be an economically advantageous method.

[0006] Furthermore, simple moisture-proof paper does not provide sufficient waterproofing, making it unsuitable for use as a container or packaging material for transporting chilled products, for example, with ice. Conventional moisture-proof paper, which only has a surfactant layer on the outermost layer, or moisture-proof liners with at least two coating layers, did not provide sufficient waterproofing.

[0007] In view of these circumstances, the object of the present invention is to provide waterproof paper that combines moisture resistance and waterproofing properties, and a method for manufacturing the same. [Means for solving the problem]

[0008] The inventors of the present invention earnestly studied the above problems and found that by applying a specific synthetic resin to a paper substrate to adjust the water repellency and then applying a waterproof agent, it is possible to produce paper excellent in moisture resistance and waterproofness.

[0009] Although not limited to the following, the present invention includes the following aspects. [1] A waterproof paper having: (a) a paper substrate; (b) an undercoat layer provided on at least one side of the paper substrate and containing a synthetic resin having a glass transition temperature of 65 to 260°C; and (c) a waterproof layer provided as the outermost layer on the undercoat layer and containing a styrene-acrylic resin and a wax, wherein the water repellency at the stage of applying the undercoat layer before applying the waterproof layer is R7 or less when measured by the method specified in Paper Pulp Test Method No. 68. [2] The waterproof paper according to [1], wherein the undercoat layer does not contain wax. [3] The waterproof paper according to [1] or [2], wherein the synthetic resin used in the undercoat layer contains polyvinyl alcohol and / or a styrene-acrylic resin. [4] The waterproof paper according to any one of [1] to [3], wherein the Cobb water absorption in 30 minutes is 7 g / m 2 or less. [5] The waterproof paper according to any one of [1] to [4], wherein the paper substrate is multi-ply board paper. [6] A method for manufacturing waterproof paper, comprising: a step of providing an undercoat layer containing a synthetic resin having a glass transition temperature of 65 to 260°C on at least one side of the paper substrate so that the water repellency specified in Paper Pulp Test Method No. 68 is R7 or less; and a step of providing a waterproof layer containing a styrene-acrylic resin and a wax on the undercoat layer. [Effect of the Invention] [[ID=二十一]] [[ID=二十二]] [[ID=二十三]]

[0010] [[ID=二十四]]<化学 [[ID=二十五]]According to the present invention, it is possible to produce paper having both moisture resistance and waterproofness. [[ID=二十六]] [[ID=二十七]][Brief Description of the Drawings] [[ID=二十八]] [[ID=二十九]] [[ID=三十]]

[0011] [[ID=三十一]] [[ID=三十二]] [Figure 1] [[ID=三十三]]It is a view (assembly view) showing a carrier used for evaluating waterproofness. [Figure 2] It is a diagram (development view) showing the carrier used for the evaluation of waterproofness. [Figure 3] It is a diagram (folded view) showing the carrier used for the evaluation of waterproofness.

Mode for Carrying Out the Invention

[0012] The present invention relates to waterproof paper. In the present invention, waterproof paper means paper that does not allow water to penetrate even when exposed to water for a long time. Specifically, the Cobb water absorption at 30 minutes described later is 40 g / m at a certain stage. 2 The following paper is referred to as waterproof paper. In a preferred embodiment, the waterproof paper according to the present invention does not cause water leakage and the shape of the container does not deform even when water is placed in the paper container and left for one week, or even if there is some deformation, the container shape is maintained. In a more preferred embodiment, the waterproof paper according to the present invention does not cause water leakage and the shape of the container does not deform even when water is placed in the paper container and left for three weeks, or even if there is some deformation, the container shape is maintained.

[0013] There is no particular limitation on the use of the waterproof paper according to the present invention. For example, it can be used as a cardboard box or a packaging box, and can contain, for example, food products such as fresh fish and vegetables, or hygroscopic substances such as detergents. The basis weight of the waterproof paper according to the present invention is not particularly limited, but can be, for example, 30 to 800 g / m. 2 When the paper base material is single-layer paper, the basis weight of the waterproof paper can be, for example, 30 to 350 g / m. 2 or 50 to 300 g / m. 2 When the paper base material is a multi-layered board paper having two or more paper layers, the basis weight of the waterproof paper can be 75 to 800 g / m. 2 or 100 to 700 g / m. 2 Furthermore, it can be 200 to 600 g / m. 2 and so on.

[0014] In a preferred embodiment, the waterproof paper of the present invention has a Cobb water absorption of 40 g / m or less on the surface for 30 minutes. 2 or less, and 25 g / m. 2The following is more preferable: 15 g / m 2 The following and 7g / m 2 The following may apply: In this invention, the Cobb water absorption is determined in accordance with the Cobb method specified in JIS P 8140, by contacting 100 ml of distilled water with the coating layer and measuring the weight of water absorbed per unit area after a specified time. The lower the Cobb water absorption even under conditions where the measurement time is extended, the lower the water absorption of the coating layer.

[0015] The waterproof paper of the present invention also has excellent moisture resistance, and in a preferred embodiment, the moisture permeability is 100 g / m². 2 • Less than 24 hours, more preferably 75 g / m² 2 • 24 hours or less, more preferably 50 g / m² 2 • Less than 24 hours. Here, the moisture permeability of the paper can be measured from the coated layer side of the waterproof paper in accordance with JIS Z 0208, and a smaller value means higher moisture resistance.

[0016] The waterproof paper of the present invention preferably has a coating surface smoothness of 15 seconds or more, more preferably 20 seconds or more, and even more preferably 25 seconds or more. By having the surface smoothness of the coating layer of the waterproof paper within the above range, a high gloss is obtained on the surface of the coating layer, resulting in a waterproof paper with superior aesthetic appeal.

[0017] paper base material The waterproof paper according to the present invention comprises at least a paper substrate and a primer layer and a waterproof coating layer provided on at least one surface of the paper substrate. In the present invention, the basis weight of the paper substrate is not particularly limited, for example, 10 to 800 g / m². 2 This is possible. If the paper substrate is single-layer paper, the basis weight is 10-300 g / m². 2 It can be set appropriately within this range; for example, if the paper base material is kraft paper, the basis weight can be set from 30 to 250 g / m². 2 It can be set within this range. Furthermore, if the paper substrate is multilayer paperboard with two or more layers, its basis weight is 70-800 g / m². 2 It can be set appropriately within this range; for example, in the case of corrugated cardboard liners, the basis weight can be set to 80-600 g / m².2 It can be set within the range.

[0018] There are no particular restrictions on the paper substrate used in the present invention, but preferably the surface on which the waterproof coating layer is applied has a 120-second Cobb water absorption of 100 g / m². 2 Preferably 75 g / m 2 More preferably 50g / m 2 The following range applies. Furthermore, the paper substrate used in this invention has a 120-second Cobb water absorption rate of 5 g / m². 2 The above is preferable, with a density of 7 g / m². 2 Above, a comfortable 10g / m 2 This concludes the explanation. In the present invention, the 120-second Cobb water absorption of the paper substrate can be adjusted by applying a water-repellent agent such as wax, or a coating agent (sealant) that does not have waterproofing properties such as a resin-containing varnish. However, by keeping the 120-second Cobb water absorption of the paper substrate within the above range, it is possible to prevent a decrease in paper strength due to excessive penetration of moisture contained in the solvent of the waterproof coating, and to ensure reliable coating by allowing solid components in the waterproof coating to remain on the surface of the paper layer, thereby achieving both improved waterproofing and moisture resistance.

[0019] As raw material pulp for paper substrates, any known material can be used without particular restrictions. Specifically, examples include various wood-derived pulps such as bleached softwood kraft pulp (NBKP), bleached hardwood kraft pulp (LBKP), unbleached softwood kraft pulp (NUKP), unbleached hardwood kraft pulp (LUKP), crushed wood pulp (GP), refiner ground pulp (RGP), chemical pulp (CP), thermomechanical pulp (TMP), and chemothermetic pulp (CTMP), as well as non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, etc.

[0020] The paper substrate may contain recycled paper pulp, or it may not contain recycled paper pulp. When recycled paper pulp is included, for example, if the paper substrate is single-layer paper, the proportion of recycled paper pulp to the total pulp is preferably 10% by mass or more, more preferably 25% by mass or more, even more preferably 50% by mass or more, and most preferably 70% by mass or more, and it can also be 100% by mass (consisting only of recycled paper pulp). In addition, kraft pulp may be blended as pulp other than recycled paper pulp, or the entire composition may be kraft pulp. Furthermore, if the paper substrate is multilayer paperboard having two or more paper layers, the proportion of recycled paper pulp per layer can be as described above, and the proportion of recycled paper pulp in each layer may differ.

[0021] As recycled paper pulp, you can use recycled corrugated cardboard, recycled paper pulp obtained by disintegrating unprinted recycled paper such as fine white, extra white, medium white, and damaged white paper, recycled paper printed on fine paper, fine coated paper, medium-quality paper, medium coated paper, newsprint, etc., recycled paper with writing on it, paper such as discarded confidential documents, recycled magazine paper, and recycled newspaper pulp (DIP) obtained by disintegrating and deinking recycled newspaper.

[0022] Furthermore, in the papermaking process, sizing agents and water-repellent agents can be added internally or externally, and paper strength enhancers can be added internally to improve strength. Examples of sizing agents include rosin-based sizing agents, rosin emulsion-based sizing agents, α-carboxymethyl saturated fatty acids, neutral rosin-based sizing agents, alkyl ketene dimers (AKD), alkenyl succinic anhydride (ASA), and cationic polymer-based sizing agents. Examples of water-repellent agents include fluoropolymer resins, polyamide resins, and waxes. Examples of paper strength enhancers include conventionally used paper strength enhancers such as polyacrylamide (PAM) and modified starch.

[0023] Furthermore, known fillers can be added to the paper substrate as needed, and inorganic and organic fillers can be used without restriction. Examples of inorganic fillers include kaolin, calcined kaolin, delaminated kaolin, clay, calcined clay, delaminated clay, illite, heavy calcium carbonate, light calcium carbonate, light calcium carbonate-silica composite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon dioxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, and zinc hydroxide. Examples of organic fillers include urea-formaldehyde resin, polystyrene resin, and phenolic resin.

[0024] Furthermore, aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum compounds, water-soluble aluminum compounds, polyvalent metal compounds, silica sol, etc., may be added as internal additives, to the extent that they do not affect the quality of the paper substrate.

[0025] Paper substrates can be manufactured using known papermaking methods. For example, they can be manufactured using, but are not limited to, wire mesh paper machines, gap former paper machines, hybrid former paper machines, on-top former paper machines, and circular wire mesh paper machines.

[0026] Furthermore, to adjust the smoothness of the paper substrate of the present invention, a smoothing treatment may be performed as necessary. For the smoothing treatment, smoothing devices such as a regular calender, super calender, gloss calender, soft calender, hot calender, and shoe calender can be used. The smoothing device may be appropriately adjusted in terms of the form of the pressurizing device, the number of pressurizing nips, heating, linear pressure, etc.

[0027] Primer layer (sealing layer) The waterproof paper according to the present invention is characterized in that a specific synthetic resin is applied to the paper substrate so that the water repellency is R7 or less before the waterproof coating layer is applied. By applying the synthetic resin, the paper surface can be covered, improving the surface smoothness and suppressing the penetration of the waterproof coating layer into the paper surface, thereby improving moisture resistance and waterproofing.

[0028] The water repellency of a paper substrate coated with an undercoat layer can be measured by the method specified in Paper Pulp Test Method No. 68. In the present invention, the water repellency at the stage when the undercoat layer is applied before applying the waterproof layer is R7 or less, preferably R5 or less, and more preferably R3 or less.

[0029] The waterproof paper according to the present invention has an undercoat layer provided on a paper substrate, and the amount of the undercoat layer can be applied in any amount, taking into consideration the final waterproofness and economic efficiency. In a preferred embodiment, the amount of undercoat layer applied is 0.1 to 15 g / m² per side. 2 More preferably 0.5~10 g / m 2 It is 1-7 g / m 2 That's fine.

[0030] The undercoat layer according to the present invention contains a synthetic resin having a glass transition temperature of 65 to 260°C. The glass transition temperature of the synthetic resin can be measured by known methods, but it is preferably measured by thermogravimetric analysis (TGA). The glass transition temperature of the synthetic resin is preferably 70 to 240°C, and more preferably 75 to 220°C. Furthermore, if the glass transition temperature of the synthetic resin is higher than that of the resin of the waterproofing layer, the waterproofing coating layer is less likely to peel off when heat treatment is performed, which is preferable.

[0031] The synthetic resin contained in the undercoat layer according to the present invention includes a synthetic resin having a melting point of 65 to 260°C. The melting point of the synthetic resin can be measured by known methods, but it is preferably measured by differential scanning calorimetry (DSC). The melting point of the synthetic resin is preferably 70 to 240°C, more preferably 75 to 220°C, and even more preferably 80 to 200°C.

[0032] The synthetic resin used in the undercoat layer of the present invention is not particularly limited as long as its glass transition temperature is between 65 and 260°C. Examples include polyvinyl alcohol, polyvinyl alcohols such as carboxy-modified polyvinyl alcohol, styrene-acrylic resin, acrylic resin, styrene-butadiene copolymer, vinyl acetate resin, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, vinyl chloride resin, polyvinyl chloride, water-dispersible polymers such as polyvinylidene chloride, styrene-acrylic acid, styrene-maleic acid, olefin compounds, alkyl (meth)acrylate compounds, casein, polyvinylpyrrolidone, gelatin urea resin, alkyd resin, urethane resin, polyethylene, etc. The synthetic resin used in the undercoat layer may be used individually or in combination of two or more. In the present invention, among these, it is preferable to use a synthetic resin containing polyvinyl alcohol, styrene-based resin, or acrylic-based resin in the undercoat layer because it facilitates uniform application when providing the waterproof coating layer described later.

[0033] Furthermore, if the undercoat layer contains wax, the water-repellent properties of the wax may make it difficult to apply the waterproof coating layer described later uniformly; therefore, it is preferable that the undercoat layer does not contain wax.

[0034] Waterproof coating layer (topcoat layer) The waterproof paper according to the present invention has a waterproof coating layer, and the waterproof coating layer according to the present invention contains a synthetic resin and a wax. The synthetic resin preferably contains at least one of styrene resins, acrylic resins, and polyolefin resins. In particular, it is preferable that the synthetic resin is a styrene resin and / or an acrylic resin.

[0035] The styrene-based resin that can be contained in the waterproof coating layer of the present invention preferably has a copolymerization ratio of 50% by mass or more of styrene-based monomers having a styrene skeleton in its structure, and may consist only of polymers of styrene-based monomers.

[0036] 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.

[0037] Furthermore, monomers copolymerizable with styrene monomers include, for example, 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 acid 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 individually or in combination of two or more.

[0038] The acrylic resin that can be contained in the waterproof coating layer of the present invention is a resin in which the copolymerization ratio of acrylic monomers, which are acrylic acid, methacrylic acid, and their derivatives, is 50% by mass or more, and may consist only of polymers of acrylic monomers.

[0039] 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 obtained by polymerizing one or more monomers selected from these acrylic monomers.

[0040] Furthermore, monomers copolymerizable 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 acid anhydrides such as maleic anhydride; and unsaturated carboxylic acids such as methacrylic acid and acrylic acid. These can be used individually or in combination of two or more.

[0041] The resin used in the waterproof coating layer preferably has a glass transition temperature of 20°C or higher, more preferably 25°C or higher, and even more preferably 30°C or higher. While there is no particular upper limit to the glass transition temperature, it is preferably 260°C or lower, more preferably 240°C or lower, and even more preferably 220°C or lower. Furthermore, the resin used in the waterproof coating layer preferably has a melting point of 65°C or higher, more preferably 70°C or higher, and even more preferably 75°C or higher. While there is no particular upper limit to the melting point of the waterproof resin layer, it is preferably 200°C or lower, more preferably 190°C or lower, and even more preferably 180°C or lower. In this invention, it is preferable that the glass transition temperature of the resin used in the waterproof layer is lower than that of the resin used in the undercoat layer.

[0042] In the present invention, the waterproof coating layer contains wax. The wax contained in the waterproof coating layer may be a synthetic wax or a natural wax, and may be used alone or in combination of two or more. Examples of synthetic waxes include polyethylene wax, Fischer-Tropsch wax, polypropylene wax, oil-based synthetic waxes (fatty acid esters, fatty acid amides, ketones and amines), and hydrogen-hardened oils. Examples of natural waxes include animal-derived waxes such as beeswax, shellac, and privet wax, plant-derived waxes such as wood wax, carnauba wax, candelilla wax, and rice wax, paraffin wax, microcrystalline wax, slack wax, montan wax, ceresin, and ozokerite. Hydrocarbon waxes containing paraffin are particularly preferred.

[0043] In this invention, pigments may be included in the waterproof coating layer to improve whiteness, etc., as long as the waterproofing properties are not impaired. In this case, it is preferable that the whiteness of the surface of the waterproof coating layer is 1% or more higher than that of the paper substrate due to the inclusion of pigments. Examples of such pigments include inorganic pigments such as calcium carbonate, titanium dioxide, kaolin, clay, engineered kaolin, delaminated clay, talc, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica, satin white, mica, and montmorillonite, and these pigments can be used individually or in combination of two or more. Among these pigments, kaolin, calcium carbonate, or mica, which have flattened particle shapes, are particularly suitable because they do not easily impair waterproofing properties. Such flattened inorganic pigments preferably have an aspect ratio of 10 or more. The pigment content in the waterproof coating layer is preferably 5 to 40% by mass or less, and more preferably 10% to 35% by mass or less. If the pigment content is less than 5% by mass, the effect of improving whiteness will not be sufficiently obtained, and if it exceeds 40% by mass, the moisture-proof and waterproof functions of the waterproof coating layer provided by the synthetic resin component may not be fully exhibited, which is undesirable. In addition, other coating agents such as binders, stabilizers, defoamers, viscosity modifiers, water-retaining agents, preservatives, and colorants may be included.

[0044] In the present invention, the waterproof coating layer can be formed by applying a coating agent containing the above-mentioned components onto a paper substrate and drying it. The amount of waterproof coating layer to be applied is 4 to 20 g / m². 2 Preferably, this would be 5-15 g / m 2 It is more preferable to do so. 20g / m 2 Beyond this point, further improvements in waterproofing cannot be expected, and manufacturing costs may increase.

[0045] The waterproof paper of the present invention can be manufactured by coating a waterproofing agent onto at least one surface of a paper substrate and drying the coated waterproofing agent. The waterproof coating layer can be formed by applying the coating agent using known coating methods, such as air knife coating, curtain coating, blade coating, gate roll coating, and die coating. The coating layer may be a single layer or multiple layers, multiple coating layers may be applied sequentially, or two or more layers may be applied simultaneously by curtain coating or the like. When multiple coating layers are provided, at least one layer should be a waterproof coating layer, and it is preferable to apply the waterproofing agent as the outermost coating layer. When drying the coating layer, it is preferable to adjust the temperature of the coating layer at the drying process exit to be less than 120°C. The coating speed when applying the coating agent can be appropriately set considering the viscosity of the coating agent and the target coating amount.

[0046] The coating agent applied to the paper substrate is dried to form a coating layer. In this drying process, it is preferable that the temperature of the coating layer at the exit be less than 120°C, and it may be adjusted to be 100°C or lower. If the temperature of the coating layer at the exit is 120°C or higher, the rate of blistering in the coating layer may increase, and blocking may occur in the waterproof paper wound after the coating layer has been formed. On the other hand, the temperature of the coating layer at the exit is preferably 60°C or higher, more preferably 70°C or higher, and can also be 80°C or higher. If the temperature of the coating layer at the exit is less than 60°C, in some cases, blocking may occur in the waterproof paper wound after the coating layer has been formed, and the coating layer may not be sufficiently dried, resulting in insufficient waterproofing and moisture-proofing performance.

[0047] The coating layer temperature at the drying process exit can be set considering the basis weight and thickness of the paper substrate. For example, in the case of corrugated cardboard liners made of multilayer paperboard with high basis weight and thickness, blistering tends to occur more easily on the surface of the coating layer compared to kraft paper, which is single-layer paper with relatively low basis weight and thickness. The reason for this is not limited to this, but in the case of corrugated cardboard liners, the basis weight and thickness are higher than that of kraft paper, and the air permeability is often lower. Even with the same moisture content in the paper as kraft paper, a large amount of moisture vaporized inside the paper substrate during the drying process cannot escape completely, which is thought to make blistering more likely to occur on the surface of the coating layer. For this reason, the higher the basis weight and thickness of the paper substrate, the more preferable it is to adjust the coating layer temperature at the drying process exit to a lower level within the above range.

[0048] Here, the outlet of the drying process refers to the outlet of a single drying zone in the drying process, or the outlet of the furthest downstream drying zone if there are multiple drying zones in the drying process.

[0049] The temperature of the coating layer at the drying process exit can be adjusted by controlling the drying time and the temperature of the drying zone. The drying time is determined by the paper substrate feed speed, the number and length of the drying zones, and the equipment capacity of the drying zones (airflow, infrared output). As for the drying method, known drying methods can be used, such as steam cylinder heating drying, hot air drying, gas infrared drying, and electric infrared drying, and any one of these or a combination of two or more can be used. [Examples]

[0050] The present invention will be described in more detail below with specific examples, but the present invention is not limited to these examples. In this specification, unless otherwise specified, concentrations and the like are based on weight, and numerical ranges include their endpoints.

[0051] Manufacturing of waterproof paper A paper base material (liner) was manufactured by combining a backing layer made of 100% corrugated recycled paper pulp, a middle layer made of 100% recycled paper pulp, and a top layer made of 70% unbleached kraft pulp and 30% corrugated recycled paper pulp in a weight ratio of backing layer:middle layer:top layer = 25:60:15. After drying with a dryer, the paper base material (liner) was smoothed using a calender (basis weight: approximately 280 g / m²). 2 Paper moisture content: 7.5%, surface surface water absorption rate (120 seconds): 30g / m² 2 Cobb water absorption rate after 30 minutes: 173.3 g / m² 2 (Water repellency: R0).

[0052] A base coat (sealing layer) and a top coat (waterproofing layer) were applied to the surface of a paper substrate using a bar blade, as shown in the table below. The waterproofing paper samples were then produced by hot-air drying so that the temperature of the coated layer at the exit of the drying process reached 80°C. (Coating agent) The following coating agents were used. The melting point of the resin was measured using a differential scanning calorimetry system (TA Instruments, DSC Q200), and the glass transition point was measured using a thermogravimetric analyzer (TA Instruments, TGA Q50). • Synthetic resin A: Styrene-acrylic resin (concentration: 10%, glass transition temperature: 120℃) • Synthetic resin B: Styrene-acrylic resin (concentration: 10%, glass transition temperature: 60°C) Synthetic resin C: Polyvinyl alcohol (Kuraray, RS-2117, concentration: 10%, glass transition temperature: 80℃, melting point: 230℃) • Paraffin-based wax (manufactured by Seikou PMC) • Waterproofing agent: A paint containing styrene-acrylic resin (glass transition temperature: 39°C) and paraffin wax (Michaelman, VaporCoat 2200)

[0053] [Table 1]

[0054] Evaluation of waterproof paper The paper quality of each sample was evaluated according to the following procedure. (Basis weight) Measured in accordance with JIS P 8124. (Cobb water absorption) Measurement was performed using the Cobb method in accordance with JIS P 8140. Specifically, 100 ml of distilled water was brought into contact with the surface (coated layer side), and the weight of water absorbed per unit area was measured after a specified time. The measurement time was set to 30 minutes, rather than the usual specified time of 120 seconds (2 minutes), in order to evaluate the water resistance. (Air permeability resistance) In accordance with JIS P 8117, the air permeability was measured from the coating layer side (front side) using an Ogane-type air permeability tester (manufactured by Asahi Seiko, EYO-55-1MN). Samples that exceeded the upper limit of the tester were evaluated as "100,000 seconds or more". (Glossiness) Measured from the front side (coated layer side) in accordance with JIS P 8142. (Moisture permeability) Measured from the front side (coated layer side) in accordance with JIS Z 0208. (Water repellency) JAPAN TAPPI Paper and pulp test method No. 68 "Paper and cardboard - The measurement was taken on the coating layer side in accordance with the "Water-based Test Method". (Waterproofing) One A4-sized sample was folded so that the front side (coated layer side) was facing inward to create a box-shaped carrier as shown in Figure 1. 200 ml of 0.005% brilliant blue solution was placed inside, and the sample was left standing for 3 weeks at 23°C and 50% RH. After that, the brilliant blue solution was discarded, and the waterproofing was evaluated based on the degree of discoloration of the carrier according to the following criteria. ○: No coloring observed, △: Coloring observed in folded areas, ×: Coloring observed throughout (Bottom moisture) For each sample, the bottom moisture was determined by measuring the paper moisture content of a paper sample taken from the area near the center of the bottom of the carrier immediately after the waterproof performance evaluation, in accordance with JIS P 8203. (Heat Resistance) An aluminum foil of the same size was placed in contact with the front side (coated layer side) of one A5-sized sample. A heat sealer heated to 120°C was used to apply a force of 2 kgf / cm over a 10 mm width in the center, and the sample was heated and pressurized for 3 seconds. The heat resistance was then evaluated based on the peelability when peeled off manually, according to the following criteria. ○: The aluminum foil is not adhering to the coated surface. △: A thin layer of the coated surface is attached to the aluminum foil. ×: The aluminum foil is adhered to the coated surface and is difficult to remove.

[0055] [Table 2]

[0056] As is clear from the results shown in the table above, the present invention made it possible to obtain paper with low moisture permeability and high waterproofness. For samples 17 and 18, the water repellency was too high at the stage when the undercoat layer (sealing layer) was applied, causing the coating liquid containing the waterproofing agent to be repelled, and it was not possible to provide a uniform topcoat layer (waterproof layer). [Explanation of Symbols]

[0057] 1 carrier 2 sheets 3. First reference line 4 1st fold line part 5. Second reference line 6 2nd fold line part 7 Square part 8 intersection 9 corners 10 3rd fold line part 11. Overlapping section 12. Overlapping quadrilateral sections 13. Seal part 14 Bottom 15, 16 Side wall section L1 Depth Dimension L2 width dimension L3, L4 Height dimensions

Claims

1. (a) Paper substrate and (b) A primer layer provided on at least one side of a paper substrate, comprising a synthetic resin having a glass transition temperature of 65 to 260°C, (c) A waterproof layer containing styrene-acrylic resin and wax, which is provided as the outermost layer on top of the undercoat layer, A waterproof paper having a 30-minute Cobb water absorption rate of 7 g / m² or less, The waterproof paper described above, wherein the water repellency at the stage when the undercoat layer is applied, before the waterproof layer is applied, is R7 or less when measured by the method specified in Paper Pulp Test Method No. 68, and the waterproof layer does not contain pigment.

2. The waterproof paper according to claim 1, wherein the undercoat layer does not contain wax, the coated surface with the waterproof layer has a Wanggan smoothness of 15 seconds or more, and the coated surface with the waterproof layer has a gloss.

3. The waterproof paper according to claim 1 or 2, wherein the synthetic resin used in the undercoat layer comprises polyvinyl alcohol and / or a styrene-acrylic resin.

4. The waterproof paper according to any one of claims 1 to 3, wherein the synthetic resin used in the undercoat layer includes a styrene-acrylic resin.

5. The waterproof paper according to any one of Claims 1 to 4, wherein the water repellency at the stage when the undercoat layer is applied, before the waterproof layer is applied, is R3 or less.

6. The waterproof paper according to any one of claims 1 to 5, wherein the paper base material is multi-layered cardboard.

7. A method for manufacturing waterproof paper according to any one of claims 1 to 6, The process involves applying a primer layer containing a synthetic resin with a glass transition temperature of 65 to 260°C to at least one side of a paper substrate so that the water repellency, as specified in Paper Pulp Test Method No. 68, is R7 or less, and A step of applying a waterproof layer containing styrene-acrylic resin and wax on top of the undercoat layer, The above method, including.

8. The method according to claim 7, further comprising the step of drying the coating layer under conditions that the temperature of the coating layer is 60°C or higher and less than 120°C.

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