Water-resistant paper and packaging container

Abstract

To provide water-resistant paper which is excellent in water resistance, continuous moldability and heat sealability, is excellent in moldability and peeling resistance at a top curl part when formed into a packaging container (especially a paper cup), and suppresses the occurrence of appearance defects of the packaging container, and to provide a packaging container using the water-resistant paper.SOLUTION: The water-resistant paper has a water-resistant layer as the uppermost layer on one side of a paper substrate, wherein the water-resistant layer has at least two melting points, the first melting point is ≥ 75 °C and <105 °C, the second melting point is ≥ 105 °C and ≤ 120 °C, and the water-resistant layer contains an ethylene - (meth) acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene. Wherein a mass ratio of the ethylene-(meth)acrylic acid copolymer and the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene in the water-resistant layer is 10 / 90 or more and 50 / 50 or less, a coating amount of the water-resistant layer is 2.5g / m2 or more, a pigment coating layer containing a pigment and a binder is provided between the paper base material and the water-resistant layer, a coating layer containing polyethyleneimine is provided on the other surface of the paper base material, and a tensile modulus in a longitudinal direction is 4.0 GPa or more and 7.5 GPa or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a waterproof paper and a packaging container made using the same. [Background technology]

[0002] Plastic products have traditionally been used as various containers and lids for beverages, food, etc., but there is a desire to switch to paper products in order to reduce environmental impact. Conventionally, polyethylene-laminated paper, which has a polyethylene film laminated to one side of a base paper (paper substrate), has been used for various containers for beverages, food, etc., but there has been a problem in that it is difficult to remove the polyethylene film when recycling, making it poorly recyclable. In order to address these problems, attempts have been made to impart water resistance and other properties to the paper by imparting properties during the papermaking process or by coating. Patent Document 1 describes a paper container base paper for cups, cup lids, paper plates, etc., which has processing suitability such as water resistance, heat sealability, top curl suitability, and gravure printability equivalent to polyethylene laminated paper, and is also excellent in recyclability for effective use of resources. The paper container base paper has a sealing undercoat layer and a topcoat layer consisting of a water-resistant and oil-resistant layer on at least one side of the base paper, and the undercoat layer is a pigment / binder resin layer with a coating amount of 4 to 15 g / m per side. 2 The resin coating amount of the water- and oil-resistant topcoat layer is 2 g / m per side. 2 The paper container base paper is as described above and is characterized by satisfying the following requirements A to D. A. The absorbency of the base paper is 50g / m 2 Below, the smoothness is 10 seconds or more. B. The pigment used in the undercoat layer has an aspect ratio of 5 to 30 and an average particle size of 0.5 to 30 μm. C. The blending ratio of the undercoat layer is 10 to 100 parts by mass of binder resin per 100 parts by mass of pigment. D. The water- and oil-resistant resin used in the topcoat layer has a gel fraction of 70% or more and is a water-dispersible resin with a critical surface tension of 25 dyn / cm or more.

[0003] Furthermore, Patent Document 2 discloses a packaging paper having at least one heat seal layer on at least one side of a paper substrate, with the aim of providing a packaging paper that can reduce the amount of plastic used, wherein the heat seal layer contains an ionomer and the dry coating amount of the heat seal layer is 2 to 10 g / m2 in total. 2 and a packaging paper characterized in that two or more heat seal layers are formed on at least one surface. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-158089 [Patent Document 2] Patent No. 6580291 Summary of the Invention [Problem to be solved by the invention]

[0005] When waterproof paper is continuously produced, heat sealing is performed when the waterproof paper is formed into packaging containers, etc., but there are problems such as insufficient heat sealing properties and sticking to the forming machine (for example, the mandrel in the case of paper cups), resulting in poor continuous formability. The technologies described in Patent Documents 1 and 2 have room for improvement in addressing these problems. Furthermore, particularly when used for paper cup applications, cracks, insufficient curling, wrinkles, etc. occur in the top curl portion, resulting in poor top curl formability and peeling of the adhesive portion in the top curl portion. Furthermore, wrinkles and buckling may occur in the body of the paper cup, resulting in poor appearance of the body member. The present invention aims to provide a water-resistant paper that has excellent water resistance, continuous formability, and heat-sealability, and further has excellent formability and peel resistance in the top curl portion when made into a packaging container (particularly a paper cup), thereby suppressing the occurrence of defects in the appearance of the packaging container, and a packaging container made using the water-resistant paper. [Means for solving the problem]

[0006] The present inventors have found that the above problems can be solved by providing waterproof paper having a waterproof layer on one of the uppermost layers of a paper substrate, the waterproof layer having at least two melting points, the first melting point and the second melting point being within a specific range, the waterproof layer containing specific resins in a specific content ratio, the water-resistant layer having a coating weight of at least a specific value, a pigment coating layer between the paper substrate and the waterproof layer, and further a coating layer containing polyethyleneimine on the other side of the paper substrate, and the waterproof paper having a tensile modulus in the machine direction within a specific range. <1> ~ <11> Regarding. <1> A waterproof paper having a waterproof layer on the top layer of one side of a paper substrate, the water-resistant layer has at least two melting points, one of which is equal to or higher than 75°C and lower than 105°C, and the second of which is equal to or higher than 105°C and lower than 120°C; the water-resistant layer contains an ethylene-(meth)acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene; a mass ratio of the ethylene-(meth)acrylic acid copolymer to the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene (ethylene-(meth)acrylic acid copolymer / copolymer of ethylene and an unsaturated hydrocarbon other than ethylene) in the water-resistant layer is 10 / 90 or more and 50 / 50 or less; The coating weight of the water-resistant layer is 2.5 g / m 2 That's all, a pigment coating layer containing a pigment and a binder is disposed between the paper substrate and the water-resistant layer; a coating layer containing polyethyleneimine on the other surface of the paper substrate; The longitudinal tensile modulus is 4.0 GPa or more and 7.5 GPa or less. Water resistant paper. <2> the total content of the ethylene-(meth)acrylic acid copolymer and the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene in the water-resistant layer is 60% by mass or more; <1> The water-resistant paper described in <3> The unsaturated hydrocarbons other than ethylene include an α-olefin having 4 to 20 carbon atoms. <1> or <2> The water-resistant paper described in <4> In the pigment coating layer, the mass ratio of the pigment content to the binder content (pigment / binder) is 100 / 120 or more and 100 / 5 or less. <1> ~ <3> 1. The waterproof paper according to any one of the preceding items. <5> The binder contains a styrene-acrylic copolymer. <1> ~ <4> 1. The waterproof paper according to any one of the preceding items. <6> the pigment contains calcium carbonate and kaolin, and the mass ratio of the calcium carbonate content to the kaolin content in the pigment coating layer (calcium carbonate / kaolin) is 50 / 50 or more and 90 / 10 or less; <1> ~ <5> 1. The waterproof paper according to any one of the preceding items. <7> The coating weight of the pigment coating layer is 1g / m 2 More than 20g / m 2 Below is the <1> ~ <6> 1. The waterproof paper according to any one of the preceding items. <8> The specific compressive strength in the longitudinal direction is 32 N·m / g or less. <1> ~ <7> 1. The waterproof paper according to any one of the preceding items. <9> Polyethyleneimine coating amount: 0.01g / m 2 More than 0.5g / m 2 Below is the <1> ~ <8> 1. The waterproof paper according to any one of the preceding items. <10> For packaging containers, <1> ~ <9> 1. The waterproof paper according to any one of the preceding items. <11> <1> ~ <10> A packaging container made using the waterproof paper described in any one of the above. [Effects of the Invention]

[0007] According to the present invention, there are provided water-resistant paper that has excellent water resistance, continuous formability and heat sealing properties, and further, when made into a packaging container (particularly a paper cup), has excellent formability and peel resistance in the top curl portion, and suppresses the occurrence of poor appearance of the packaging container, as well as a packaging container made using said water-resistant paper. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Waterproof paper] The water-resistant paper of this embodiment is water-resistant paper having a water-resistant layer as the uppermost layer on one side of a paper base, the water-resistant layer having at least two melting points, one of which is 75°C or higher and lower than 105°C, and the second of which is 105°C or higher and 120°C or lower, the water-resistant layer containing an ethylene-(meth)acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene, the mass ratio of the ethylene-(meth)acrylic acid copolymer and the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene in the water-resistant layer (ethylene-(meth)acrylic acid copolymer / copolymer of ethylene and an unsaturated hydrocarbon other than ethylene) being 10 / 90 or higher and 50 / 50 or lower, and the coating weight of the water-resistant layer being 2.5 g / m 2 or more, and has a pigment coating layer containing a pigment and a binder between the paper substrate and the water-resistant layer, and has a coating layer containing polyethyleneimine on the other side of the paper substrate, and has a tensile modulus in the longitudinal direction of 4.0 GPa or more and 7.5 GPa or less. According to the embodiment, a waterproof paper is provided which has excellent water resistance, continuous formability and heat sealing properties, and further, when made into a packaging container (particularly a paper cup), has excellent formability and peel resistance in the top curl portion, and reduces defects in the appearance of the packaging container (particularly the body portion in the case of a paper cup). The detailed reasons why the above-mentioned effects are obtained are unclear, but part of the reason is thought to be as follows. Generally, from the viewpoint of heat-sealing properties, the easier the heat-sealing layer is to melt by heat, the better. On the other hand, from the viewpoint of preventing sticking to the molding machine, the less likely the heat-sealing layer is to melt by heat, the better. In other words, it can be said that there is a trade-off between the former and the latter. In this embodiment, the water-resistant layer has at least two melting points within the above-mentioned ranges and contains a specific resin, so that the ease (or difficulty) of melting by heat is appropriate. This is thought to have ensured heat-sealing properties while suppressing sticking to the molding machine (mandrel) during heat sealing, resulting in a water-resistant paper with excellent continuous formability. In addition, a pigment-containing layer is provided between the paper substrate and the water-resistant layer, and the coating weight of the water-resistant layer is 2.5 g / m 2It is believed that by doing the above, a waterproof paper excellent in water resistance and heat sealability was obtained. Furthermore, it is believed that the heat-sealing properties are further improved by providing a coating layer containing polyethyleneimine on the other side of the paper substrate. Also, by setting the tensile modulus within a specific range, when the waterproof paper of this embodiment is used for paper cups, it is believed that the formability at the top curl portion is excellent, the resilience of the top curl portion is not too high, the force applied to the adhesive portion of the top curl portion is reduced, peeling of the adhesive portion of the top curl portion is suppressed, and further, it has strength that can withstand forming, and wrinkles and buckling of the packaging container (especially the body portion in the case of a paper cup) are less likely to occur, thereby suppressing the occurrence of poor appearance. The effects of the present invention are not limited by the above mechanism. The machine direction of the waterproof paper means the direction corresponding to the papermaking direction (MD) of the paper base material, and the cross direction of the waterproof paper means the direction corresponding to the width direction (CD) of the paper base material. This embodiment will be described in further detail below.

[0009] 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 arbitrarily. Furthermore, "(meth)acrylic" is a generic term including both acrylic and methacrylic.

[0010] <Paper base material> The paper base material constituting the waterproof paper of this embodiment may be a single-layer or multi-layer structure. In the case of multi-layer paper, the number of paper layers is not particularly limited, but is preferably three or more, more preferably four or more, and even more preferably five or more. By using 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 number of paper layers is not particularly limited, but is preferably seven or less, more preferably six or less.

[0011] 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), pressure 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. Among these, a combination of LBKP and NBKP is preferred.

[0012] The content of softwood bleached kraft pulp (NBKP) relative to the total amount of pulp constituting 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, from the viewpoint of keeping the longitudinal tensile modulus within the desired range and suppressing the occurrence of poor appearance of packaging containers (particularly the body in the case of paper cups), and is preferably 60% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, and even more preferably 25% by mass or less.

[0013] The content of hardwood bleached kraft pulp (LBKP) relative to the total amount of pulp constituting the paper base material is preferably 40% by mass or more, more preferably 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 75% by mass or more, from the viewpoints of moldability, smoothness, and water resistance, and is preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 85% by mass or less, from the viewpoints of keeping the longitudinal tensile modulus within a desired range and preventing poor appearance of packaging containers (particularly the body in the case of paper cups).

[0014] The beating degree of the pulp is not particularly limited, but is preferably 200 mL or more, more preferably 350 mL or more, and even more preferably 400 mL or more in Canadian Standard Freeness (CSF), and is preferably 800 mL or less, more preferably 750 mL or less, and even more preferably 700 mL or less. If the CSF of the pulp is within the above range, the paper strength required for a packaging container is easily obtained. In order to obtain a paper strength suitable for use as a waterproof paper for packaging containers, the Canadian Standard Freeness (CSF) of LBKP is preferably 310 mL or more and 500 mL or less, more preferably 350 mL or more, even more preferably 380 mL or more, and more preferably 480 mL or less, and particularly preferably 470 mL or less. In order to obtain suitable paper strength as a waterproof paper for packaging containers, the Canadian Standard Freeness (CSF) of NBKP is preferably 350 mL or more and 700 mL or less, more preferably 400 mL or more, even more preferably 450 mL or more, and more preferably 600 mL or less, even more preferably 550 mL or less. Canadian standard freeness is measured in accordance with JIS P 8121-2:2012 "Pulp - Testing method for freeness - Part 2: Canadian standard freeness method." It is preferred that LBKP and NBKP are used in combination as the pulp constituting the paper base material, and that the Canadian Standard Freeness (CSF) of each is within the above range.

[0015] Examples of additives for paper substrates include pH adjusters (sodium bicarbonate, sodium hydroxide, etc.), dry strength agents (polyacrylamides, starches, etc.), wet strength agents (polyamide polyamine epichlorohydrin resin, melamine-formaldehyde resin, or urea-formaldehyde resin), internal sizing agents (rosin-based, alkyl ketene dimer, etc.), drainage retention aids, antifoaming agents, fillers (calcium carbonate, talc, etc.), dyes, and fixing agents (aluminum sulfate). These additives may be used alone or in combination of two or more. The combined use of an internal sizing agent and a wet strength agent is preferred because it improves the water resistance of the paper substrate. The amount of additives is not particularly limited and may be within a commonly used range.

[0016] Among these, in this embodiment, the paper base material preferably contains a dry paper strength agent, and since the tensile modulus and specific compressive strength of the waterproof paper are set within the desired range, it is preferable that the dry paper strength agent contains starches, and more preferably contains cationized starch. The content of the dry paper strength agent is preferably 0.3 parts by mass or more, more preferably 0.4 parts by mass or more, even more preferably 0.5 parts by mass or more, and even more preferably 0.6 parts by mass or more, per 100 parts by mass of pulp (dry mass), from the viewpoint of keeping the tensile modulus within the desired range and suppressing the occurrence of poor appearance of the packaging container (particularly the body in the case of a paper cup). It is also preferably 1.5 parts by mass or less, more preferably 1.2 parts by mass or less, and even more preferably 1.0 part by mass or less.

[0017] The basis weight of the paper substrate is not particularly limited, but for example, in the case of packaging containers, preferably food containers, more preferably paper cups, it is preferably 150 g / m from the viewpoint of obtaining sufficient paper strength as a packaging container. 2 More preferably, 180 g / m 2 More preferably, 200 g / m 2 More preferably, 220 g / m 2 From the viewpoint of formability, it is preferably 500 g / m 2 or less, more preferably 430 g / m2 or less, more preferably 380 g / m 2 or less, even more preferably 330 g / m 2 Below 280 g / m, particularly preferably 2 The basis weight of the paper substrate is measured in accordance with JIS P 8124:2011.

[0018] The thickness of the paper substrate is not particularly limited, but for example, in the case of packaging containers, preferably food containers, more preferably paper cups, from the viewpoint of obtaining the paper strength required for the packaging container, it is preferably 150 μm or more, preferably 210 μm or more, more preferably 230 μm or more, and even more preferably 240 μm or more, and from the viewpoint of formability, it is preferably 650 μm or less, more preferably 480 μm or less, even more preferably 410 μm or less, even more preferably 380 μm or less, and particularly preferably 330 μm or less. The thickness of the paper substrate is measured in accordance with JIS P 8118:2014.

[0019] The density of the paper substrate is preferably 0.4 g / cm from the viewpoint of obtaining the desired strength. 3 More preferably, 0.6 g / cm 3 More preferably, 0.7 g / cm 3 and preferably 1.0 g / cm 3 or less, more preferably 0.9 g / cm 3 The density of the paper substrate is calculated from the basis weight and thickness of the paper substrate obtained by the above-mentioned measurement method.

[0020] [Method for manufacturing paper substrate] An example of a method for producing a paper base material is a method of making a stock containing pulp. The stock may further contain additives. Examples of additives include those listed above. The stock can be prepared by adding the 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 stock is not particularly limited, and may be within a commonly used range. For example, it is 60% by mass or more and less than 100% by mass relative to the total mass of the stock (solid content).

[0021] Papermaking from stock can be carried out by standard methods. For example, the stock is cast onto a wire or the like, dewatered to obtain a wet paper, and multiple wet papers are stacked as necessary, and this single-layer or multi-layer wet paper is 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 multi-layer paper is obtained. When multiple wet papers are stacked, an adhesive may be applied to the surface of the wet paper (the surface on which other wet papers are stacked).

[0022] <Water resistant layer> In this embodiment, the waterproof paper has a waterproof layer on the top layer of one side of the paper base material. Preferably, the waterproof paper of this embodiment has a waterproof layer on only one side of the paper base material. The water-resistant layer contains at least a resin component and may contain various additives in addition to the resin component. The water-resistant layer is preferably formed by applying a coating liquid containing at least the resin component to a paper substrate, and the coating liquid is preferably an aqueous coating liquid.

[0023] [Melting point] The water-resistant layer has at least two melting points. It is sufficient that the water-resistant layer has two or more melting points, and it may have three or more melting points, but it preferably has two melting points. The first melting point is 75°C or higher and lower than 105°C, and from the viewpoint of improving heat sealability and blocking resistance and moldability, it is preferably 78°C or higher, more preferably 80°C or higher, even more preferably 85°C or higher, and is preferably 103°C or lower, more preferably 100°C or lower, even more preferably 95°C or lower. From the same viewpoint, the second melting point is 105°C or higher and 120°C or lower, preferably 107°C or higher, more preferably 108°C or higher, even more preferably 110°C or higher, and preferably 118°C or lower, more preferably 115°C or lower. As described above, the water-resistant layer only needs to have at least two melting points: a melting point of 75°C or higher but lower than 105°C, and a melting point of 105°C or higher but lower than 120°C. The water-resistant layer may have multiple melting points falling within the above temperature ranges, or may have an additional melting point that does not fall within the above melting points.

[0024] The melting point of the water-resistant layer is measured by the following method. Specifically, a thin layer of only the water-resistant layer was scraped off from the surface of the water-resistant paper with a razor. Approximately 5 mg of the scraped water-resistant layer was sealed in an aluminum pan and measured using a differential scanning calorimeter (NEXTA DSC600, Hitachi High-Tech Science Corporation). The temperature was raised from 30°C to 150°C at 10°C / min in a nitrogen atmosphere (first run), then cooled to 30°C at 10°C / min, and again raised from 30°C to 150°C at 10°C / min (second run). The endothermic peak temperature of the second run was read and used as the melting point. As will be described later, when the water-resistant layer has a plurality of melting points due to the inclusion of two or more resins with different melting points, the melting points may be approximated by the melting points of the resins used.

[0025] 〔resin〕 The water-resistant layer contains two or more resins with different melting points, thereby imparting multiple melting points to the water-resistant layer. Since the water-resistant layer is preferably formed by applying an aqueous coating liquid to the paper substrate, the resin is preferably a water-dispersible resin. The resin contained in the water-resistant layer is not particularly limited, and examples thereof include water-dispersible polymers such as polycarbonate, polyacrylonitrile, polystyrene-based polymers (e.g., styrene-butadiene-based copolymers), polyurethane, polyolefins (e.g., polyethylene, polypropylene, polybutadiene, copolymers of ethylene and unsaturated hydrocarbons other than ethylene), (meth)acrylic acid-based polymers (e.g., acrylic acid esters, copolymers of (meth)acrylic acid and other unsaturated monomers, ethylene-(meth)acrylic acid copolymers), ethylene-vinyl acetate copolymers, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyesters, polyamides, polyethers, and modified products thereof. Among these, from the viewpoint of obtaining waterproof paper excellent in water resistance, continuous formability, and heat sealability, at least an ethylene-(meth)acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene are contained. In addition to these two resins, other resins such as an ethylene-vinyl acetate copolymer may also be contained.

[0026] (ethylene-(meth)acrylic acid copolymer) In this embodiment, the water-resistant layer contains an ethylene-(meth)acrylic acid copolymer, which is believed to mainly contribute to the first melting point. Here, the ethylene-(meth)acrylic acid copolymer may be blended as an ionomer. Note that "blended as an ionomer" means that the ionomer is used as a raw material at least when the water-resistant layer is produced, and the water-resistant layer does not necessarily contain the resin in the form of an ionomer after production. An ionomer is a polymer neutralized with a cation. In other words, all resins in which ethylene-(meth)acrylic acid copolymers are neutralized with a cation fall into the category of ionomers. In addition to metal ions, cations include ammonium ions (NH4 + ), and organic ammonium ions. Metal ions include lithium ions (Li + ), sodium ions (Na +), potassium ions (K + ), magnesium ions (Mg 2+ ), calcium ions (Ca 2+ ), alkaline earth metal ions such as zinc ions (Zn 2+ ), copper ions (Cu 2+ Among these, sodium ions are preferred as metal ions from the viewpoint of availability. The copolymerization ratio of ethylene and (meth)acrylic acid may be appropriately selected so as to obtain a desired melting point, but the content of ethylene-derived structural units in the ethylene-(meth)acrylic acid copolymer is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 65% ​​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. The content of ethylene-derived structural units in the ethylene-(meth)acrylic acid copolymer is, for example, 13 This can be confirmed by C-NMR analysis.

[0027] The ethylene-(meth)acrylic acid copolymer may be a commercially available product, and examples thereof include MP498345N, MP4983R, MP4990R, and MFHS1279 manufactured by Michelman Japan LLC, ZAIXXEN (registered trademark) A and ZAIXXEN (registered trademark) AC manufactured by Sumitomo Seika Chemicals Co., Ltd., the Chemipearl series (S-100, S-300, and 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 Dow Mitsui Polychemicals Co., Ltd., Hi-Tec SC-100 manufactured by Toho Chemical Industry Co., Ltd., and Aquatex AC-3100 manufactured by Chuo Rika Kogyo Co., Ltd.

[0028] From the viewpoints of improving water resistance and heat sealability, as well as blocking resistance and moldability, the melting point of the ethylene-(meth)acrylic acid copolymer is preferably 75°C or higher, more preferably 78°C or higher, even more preferably 80°C or higher, still more preferably 85°C or higher, and is preferably lower than 105°C, more preferably 103°C or lower, even more preferably 100°C or lower, and still more preferably 95°C or lower.

[0029] (copolymer of ethylene and unsaturated hydrocarbons other than ethylene) In this embodiment, the water-resistant layer contains, as a resin, a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene, in addition to the ethylene-(meth)acrylic acid copolymer, which is believed to mainly contribute to the second melting point. The unsaturated hydrocarbon other than ethylene may be a monoolefin having one unsaturated double bond in the molecule, a diolefin having two unsaturated double bonds, a triene having three unsaturated double bonds, or a tetraene having four unsaturated double bonds. In addition, it may have conjugated double bonds. Among these, from the viewpoints of water resistance, blocking resistance, moldability, and heat sealability, the unsaturated hydrocarbon other than ethylene preferably contains an α-olefin having from 4 to 20 carbon atoms, more preferably an α-olefin having from 6 to 16 carbon atoms, and even more preferably an α-olefin having from 6 to 12 carbon atoms. The types of ethylene and unsaturated hydrocarbons other than ethylene, and their copolymerization ratio may be appropriately selected so that the melting point falls within the desired range, but the content of structural units derived from ethylene in the copolymer of ethylene and unsaturated hydrocarbons other than ethylene is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 65% ​​by mass or more, and even more preferably 68% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less, and even more preferably 88% by mass or less. The content of structural units derived from ethylene in the copolymer of ethylene and unsaturated hydrocarbons other than ethylene is, for example, 13 This can be confirmed by C-NMR analysis.

[0030] From the viewpoint of improving heat sealability and continuous moldability, the melting point of the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene is preferably 105°C or higher, more preferably 107°C or higher, even more preferably 108°C or higher, and still more preferably 110°C or higher, and is preferably 120°C or lower, more preferably 118°C or lower, and even more preferably 115°C or lower.

[0031] The copolymer of ethylene and an unsaturated hydrocarbon other than ethylene may be either a commercially available product or a synthetic product. In the case of a commercially available product, for example, DIC Graphics Corporation's (DIC Seal E-806LV) or the like can be used. In the case of a synthetic product, it may be polymerized by a known method, and the synthesis method is not particularly limited. In addition, the obtained copolymer may be dispersed in an aqueous medium by a known method and used.

[0032] The water-resistant layer contains, as resin components, an ethylene-(meth)acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene. From the viewpoints of water resistance, continuous formability, and heat sealability, the mass ratio of the ethylene-(meth)acrylic acid copolymer and the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene (ethylene-(meth)acrylic acid copolymer / copolymer of ethylene and an unsaturated hydrocarbon other than ethylene) is 10 / 90 or more, preferably 12 / 88 or more, more preferably 15 / 85 or more, and 50 / 50 or less, preferably 45 / 55 or less, more preferably 40 / 60 or less, even more preferably 35 / 65, and still more preferably 30 / 70 or less.

[0033] The water-resistant layer may contain an ethylene-(meth)acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene. From the viewpoints of water resistance, continuous formability, and heat sealability, the total content of the ethylene-(meth)acrylic acid copolymer and the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene in the water-resistant layer is preferably 60% by mass or more, more preferably 65% ​​by mass or more, even more preferably 70% by mass or more, and even more preferably 75% by mass or more, and is preferably 98% by mass or less, more preferably 96% by mass or less.

[0034] In addition to the resins described above, the water-resistant layer may contain other components, such as amine-based additives that serve as neutralizing agents (aminomethylpropanol, dimethylaminomethylpropanol, dimethylaminoethanol, diethylaminoethanol, etc.), higher fatty acids (oleic acid, stearic acid, palmitic acid, etc.), anti-sagging agents / anti-settling agents, defoaming agents, viscosity modifiers, leveling agents, wetting agents, dispersants, colorants such as pigments and coloring dyes, and anti-blocking agents.

[0035] The water-resistant layer is obtained by preparing a coating liquid for the water-resistant layer containing a resin component and coating this as the uppermost layer on one side of the paper substrate. The method for applying the coating liquid for the water-resistant layer is not particularly limited, and any commonly used coating device may be appropriately selected and used, such as an air knife coater, blade coater, gravure coater, rod blade coater, roll coater, reverse roll coater, bar coater, curtain coater, die slot coater, champlex coater, metering blade type size press coater, short dwell coater, spray coater, gate roll coater, or lip coater.

[0036] [Preferred embodiment of water-resistant layer] The waterproof paper of this embodiment has a waterproof layer on the top layer of one side of the paper base, and more preferably on only one side. By having a waterproof layer on the top layer of the paper base, it is possible to impart water resistance and heat sealability as well as blocking resistance. The coating amount of the water-resistant layer is 2.5 g / m from the viewpoint of obtaining a water-resistant layer that is excellent in water resistance, heat sealing property, prevention of peeling of the adhesive portion of the top curl portion, and continuous formability. 2 or more, and preferably 3.0 g / m 2 More preferably, 3.5 g / m 2 More preferably, 4.0 g / m 2 More preferably, 4.5 g / m 2 and preferably 25 g / m 2 Less than 20 g / m, more preferably 2 or less, more preferably 16 g / m 2 It is preferable that the coating weight of the water-resistant layer is equal to or less than the above upper limit from the viewpoint of recyclability and reducing the environmental load. When the water-resistant layer is provided on both sides of the paper base material, the above coating weight is a preferred range of the coating weight per side.

[0037] The coating amount of the water-resistant layer may be measured as follows. Alternatively, the thickness of the water-resistant layer may be determined by observing the cross section of the water-resistant paper with a scanning electron microscope, and the thickness may be multiplied by the density of the solid content of the coating liquid to calculate the coating amount after drying.

[0038] <Pigment coating layer> The waterproof paper of this embodiment has a pigment coating layer between the paper substrate and the waterproof layer. The pigment coating layer seals and smooths the paper substrate. This allows for the formation of a uniform waterproof layer, improving water resistance and heat sealability. Since the waterproof paper of this embodiment preferably has a waterproof layer only on the top layer of one side, it is preferable for the waterproof paper to have a pigment coating layer only on one side of the paper substrate. The pigment coating layer contains a pigment and a binder.

[0039] The pigment coating layer is primarily composed of a pigment and a binder. The phrase "the pigment coating layer is primarily composed of a pigment and a binder" means that the total content of the pigment and binder in the pigment coating layer is, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 70% 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. The upper limit is not particularly limited, but is preferably 100% by mass or less. The pigment coating layer may further contain optional components in addition to the pigment and binder.

[0040] [Pigments] The pigment is not particularly limited, and examples thereof 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; and organic pigments such as solid, hollow, and core-shell pigments. These may be used alone or in combination of two or more. Among these, from the viewpoint of further improving water resistance and / or heat sealability, at least one pigment selected from the group consisting of kaolin and calcium carbonate is preferred, and it is more preferred to contain at least kaolin, and it is even more preferred to use a combination of kaolin and calcium carbonate.

[0041] The average particle size of the pigment is not particularly limited, but is preferably at least 0.1 μm, more preferably at least 0.5 μm, and preferably no greater than 20 μm, more preferably no greater than 10 μm, even more preferably no greater than 4 μm, even more preferably no greater than 3 μm, and particularly preferably no greater than 2.5 μm. Having an average particle size of the pigment within the above range is preferable because it results in a water-resistant layer with superior smoothness and waterproof paper with excellent water resistance and heat-sealability. The average particle size of the pigment can be determined using a laser diffraction particle size distribution analyzer based on the principles of laser diffraction, and the value measured from the measured particle size distribution is used.

[0042] When calcium carbonate and kaolin are used in combination as pigments, the mass ratio of the calcium carbonate content to the kaolin content in the pigment coating layer (calcium carbonate / kaolin) is not particularly limited, but is preferably 50 / 50 or more and 90 / 10 or less, more preferably 50 / 50 or more and 80 / 20 or less, even more preferably 60 / 40 or more and 75 / 25 or less, and still more preferably 65 / 35 or more and 75 / 25 or less. Kaolin is a layered inorganic compound, and its use as a pigment is advantageous in that it provides a pigment coating layer with superior smoothness, thereby resulting in a waterproof paper with excellent water resistance and heat sealability. On the other hand, a coating solution for a pigment coating layer containing kaolin (a coating solution for a pigment coating layer) tends to have a high viscosity, which may cause defects during coating. The combined use of calcium carbonate and kaolin as pigments in the above mass ratio provides a pigment coating layer with excellent smoothness, excellent coatability, a large amount of coating per coat, and the pigment coating layer can be formed with fewer coats, which is preferable because it provides excellent coating efficiency.

[0043] From the viewpoint of continuous moldability, the pigment content in the pigment coating layer is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, and even more preferably 45% by mass or more, and from the viewpoint of water resistance and heat sealability, it is preferably 98% by mass or less, more preferably 95% by mass or less.

[0044] 〔binder〕 The binder contained in the pigment coating layer is not particularly limited, but examples thereof include styrene-butadiene resins; acrylic resins such as methyl (meth)acrylate copolymers and styrene-acrylic copolymers; and olefin-unsaturated carboxylic acid copolymers such as ethylene-acrylic acid copolymers and ethylene-methacrylic acid copolymers; and more preferably, acrylic resins and styrene-butadiene resins, more preferably, styrene-acrylic copolymers and styrene-butadiene resins, and even more preferably, styrene-acrylic copolymers. The styrene-acrylic copolymer is preferably a copolymer of styrene and a (meth)acrylic acid ester, more preferably a copolymer of styrene and an acrylic acid ester, and even more preferably a copolymer of styrene and an acrylic acid alkyl ester, in which the number of carbon atoms in the alkyl moiety of the alkyl ester is particularly preferably 1 to 8.

[0045] The binder content in the pigment coating layer is preferably 2% by mass or more, more preferably 5% by mass or more, from the viewpoint of water resistance and heat sealability, and is preferably 80% by mass or less, more preferably 50% by mass or less, even more preferably 30% by mass or less, and even more preferably 20% by mass or more, from the viewpoint of continuous moldability.

[0046] In the pigment coating layer, the mass ratio of pigment to binder content (pigment / binder) is preferably 100 / 120 or more, more preferably 100 / 110 or more, even more preferably 100 / 100 or more, and preferably 100 / 3 or less, more preferably 100 / 4 or less, even more preferably 100 / 5 or less, and even more preferably 100 / 6 or less. A pigment to binder mass ratio of at least the above lower limit is preferred because roll contamination during production is suppressed and operability is excellent. If the binder content is too high, roll contamination may occur during production. On the other hand, if the pigment-to-binder mass ratio exceeds the upper limit, i.e., if the pigment content in the pigment coating layer is too high, pores resulting from the pigment may form in the pigment coating layer, causing the water-resistant layer to penetrate into the pigment coating layer and resulting in non-uniform thickness of the water-resistant layer. If the water-resistant layer becomes non-uniform in thickness, water may penetrate through thinner portions, reducing water resistance. Furthermore, if the water-resistant layer becomes non-uniform in thickness, thinner portions may be difficult to adhere, resulting in failure to achieve the desired heat-sealability. On the other hand, if the pigment / binder mass ratio is within the above range, i.e., if the pigment content in the pigment coating layer is appropriately low, the above phenomenon is suppressed, resulting in good thickness uniformity of the water-resistant layer, and as a result, waterproof paper with excellent water resistance and heat-sealability is obtained, which is preferable.

[0047] The coating amount of the pigment coating layer is not particularly limited, but is preferably 1 g / m2 in terms of solid content. 2 More preferably, 3 g / m 2 More preferably, 5 g / m 2 More preferably, 7 g / m 2 and preferably 20 g / m 2 Less than 15 g / m, more preferably 2 or less, more preferably 12 g / m 2 or less, even more preferably 10 g / m 2 When the waterproof paper of this embodiment has pigment coating layers on both sides, the preferred coating amount of the pigment coating layer is the preferred range of the coating amount per side. The method for forming the pigment coating layer is not particularly limited, but a method in which a dispersion containing a pigment and a binder is applied to a paper substrate and then dried to form the layer is preferred. The dispersion containing the pigment and the resin binder is preferably an aqueous dispersion. The pigment coating layer may contain, in addition to the pigment and binder, the components exemplified as other components in the water-resistant layer, and preferably contains an antifoaming agent.

[0048] The pigment coating layer may further contain components other than the pigment and binder. Examples of such components include adhesives, dispersants, thickeners, water retention agents, antifoaming agents, waterproofing agents, colorants, surfactants, etc. Examples of adhesives include proteins such as casein, soy protein, and synthetic protein; starches such as oxidized starch, cationic starch, urea phosphate esterified starch, and etherified starch such as hydroxyethyl etherified starch, and dextrin; and cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose.

[0049] <Coating layer containing polyethyleneimine> The waterproof paper of this embodiment has a coating layer containing polyethyleneimine (hereinafter also referred to as a "polyethyleneimine-containing layer") on the other side of the paper substrate, particularly on the side opposite to the side on which the water-resistant layer is provided. The polyethyleneimine-containing layer is preferably provided on the side opposite to the side on which the water-resistant layer is provided as the uppermost layer of the paper substrate (the other side), more preferably on the uppermost layer of the other side, and even more preferably on the other side of the paper substrate alone. Having a polyethyleneimine-containing layer on the waterproof paper is preferable because it results in waterproof paper with excellent heat-sealing properties. Furthermore, if a heat-sealing layer is also provided as the uppermost layer on the side opposite to the side on which the water-resistant layer is provided of the paper substrate, sticking to the molding machine may occur, but having a polyethyleneimine-containing layer is preferable because it provides excellent continuous molding properties. The number average molecular weight of the polyethyleneimine contained in the polyethyleneimine-containing layer is not particularly limited, but is preferably 30,000 or more, more preferably 50,000 or more, and is preferably 150,000 or less, more preferably 100,000 or less. The number average molecular weight of polyethyleneimine is a value determined by a viscosity method. As the polyethyleneimine, commercially available products may be used, for example, "Epomin P-1000," "Epomin P-3000," and "Epomin HM-2000" (all trade names, manufactured by Nippon Shokubai Co., Ltd.).

[0050] From the viewpoint of heat sealability, the content of polyethyleneimine in the polyethyleneimine-containing layer is preferably 10% by mass or more, more preferably 15% by mass or more, even more preferably 20% by mass or more, still more preferably 25% by mass or more, and even more preferably 30% by mass or more, and the upper limit is not particularly limited, but is 100% by mass or less. From the viewpoint of the viscosity of the coating liquid, the content of polyethyleneimine in the polyethyleneimine-containing layer is preferably 80% by mass or less, more preferably 60% by mass or less, and even more preferably 40% by mass or less.

[0051] From the viewpoints of heat sealing properties and ease of application, the polyethyleneimine-containing layer preferably contains at least one selected from the group consisting of polyvinyl alcohol and starch, and more preferably contains polyvinyl alcohol.

[0052] The degree of polymerization of polyvinyl alcohol is not particularly limited, but is preferably 1,000 or more, more preferably 1,500 or more, and is preferably 4,000 or less, more preferably 2,000 or less. The degree of saponification of polyvinyl alcohol is not particularly limited, but is preferably 85 mol % or more, more preferably 88 mol % or more, and 100 mol % or less. The degree of polymerization and the degree of saponification are measured in accordance with JIS K 6726:1994 (Testing method for polyvinyl alcohol).

[0053] Commercially available polyvinyl alcohol products may be used, such as "J-Poval JM-17" (trade name) and "J-Poval JF-17" (trade name) manufactured by Nippon Vinyl Acetate & Poval Co., Ltd., and "Kuraray Poval PVA-117" (trade name) and "Exeval RS-2117" (trade name) manufactured by Kuraray Co., Ltd. Similarly, commercially available starch products may be used, such as "GRS-T110, Ace K, Cornstarch K, Ace A, Ace D" (trade names) manufactured by Oji Cornstarch Co., Ltd., and "Film Coat 370" (trade name) manufactured by Ingredion Inc.

[0054] In the polyethyleneimine-containing layer, the mass ratio of polyvinyl alcohol and starch to polyethyleneimine (polyvinyl alcohol and starch (total) / polyethyleneimine) is not particularly limited, but from the viewpoint of water resistance and heat sealability, it is preferably 30 / 70 or more, more preferably 40 / 60 or more, even more preferably 50 / 50 or more, and preferably 80 / 20 or less, more preferably 75 / 25 or less, even more preferably 70 / 30 or less. In addition, in the polyethyleneimine-containing layer, it is also preferable that the mass ratio of polyvinyl alcohol to polyethyleneimine (polyvinyl alcohol / polyethyleneimine) is within the above range.

[0055] In the polyethyleneimine-containing layer, the total content of polyethyleneimine, polyvinyl alcohol, and starch is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and may be 100% by mass, from the viewpoints of operability and cost.

[0056] The polyethyleneimine-containing layer may contain, in addition to polyethyleneimine and, if necessary, polyvinyl alcohol and / or starch, other components such as melamine resin, urea resin, epoxy resin, and polyacrylamide (also referred to as PAM).

[0057] The coating amount (solid content) of the polyethyleneimine-containing layer is preferably 0.1 g / m from the viewpoints of water resistance, heat sealability, and suppressing peeling of the adhesive portion of the top curl portion. 2 More preferably, 0.2 g / m 2 More preferably, 0.3 g / m 2 From the viewpoint of heat sealing property and cost, it is preferably 4 g / m 2 Less than 3g / m, more preferably 2 More preferably, 2 g / m or less 2or less, even more preferably 1 g / m 2 The following is the result.

[0058] In the polyethyleneimine-containing layer, the coating amount of polyethyleneimine is preferably 0.01 g / m from the viewpoints of water resistance, heat sealability, and suppressing peeling of the adhesive portion of the top curl portion. 2 More preferably, 0.02 g / m 2 More preferably, 0.03 g / m 2 More preferably, 0.05 g / m 2 and preferably 0.5 g / m 2 or less, more preferably 0.45 g / m 2 or less, more preferably 0.4 g / m 2 The coating amount of polyethyleneimine is calculated from the coating amount (solid content) of the polyethyleneimine-containing layer and the content of polyethyleneimine in the layer.

[0059] The method for forming the polyethyleneimine-containing layer is not particularly limited, but for example, the polyethyleneimine-containing layer can be obtained by preparing a coating liquid for the polyethyleneimine-containing layer containing polyethyleneimine and, if necessary, other components such as polyvinyl alcohol, applying the coating liquid onto a paper substrate, and drying the coating liquid.

[0060] The waterproof paper of this embodiment may have a configuration in which, for example, a polyethyleneimine-containing layer, a paper base material, a pigment coating layer, and a water-resistant layer are laminated in this order, or a configuration in which a polyethyleneimine-containing layer, a paper base material, a pigment coating layer, a water-resistant undercoat layer, and a water-resistant layer are laminated in this order. The waterproof paper of this embodiment may have other layers in addition to the pigment coating layer and the waterproof layer. Examples of other layers include a printed layer, a water vapor barrier layer, an oxygen barrier layer, and an oxygen absorbing layer. The printed layer may be formed using a known ink such as an oil-based ink, a water-based ink, or a biomass ink. The printed layer may be formed on one surface of the waterproof paper, or on a portion of the surface. That is, the waterproof paper of this embodiment may be printed on all or part of at least one surface (e.g., the printing surface). The printed content may be a pattern, a design, or information (such as ingredients, expiration date, or QR code (registered trademark)).

[0061] <Water-resistant paper properties> (longitudinal tensile modulus) The waterproof paper of this embodiment has a longitudinal tensile modulus of elasticity of 4.0 GPa or more and 7.5 GPa or less, preferably 4.2 GPa or more, more preferably 4.4 GPa or more, even more preferably 4.6 GPa or more, still more preferably 4.8 GPa or more, and preferably 7.3 GPa or less, more preferably 7.0 GPa or less, even more preferably 6.5 GPa or less, still more preferably 6.0 GPa or less, and even more preferably 5.5 GPa or less. When the longitudinal tensile modulus is equal to or greater than the lower limit, a water-resistant paper with excellent strength can be obtained, and when the water-resistant paper is used for paper cups, buckling and wrinkling of the body member during molding are suppressed, which is preferable. When the longitudinal tensile modulus is equal to or less than the upper limit, the water-resistant paper has excellent formability, and when the water-resistant paper is used for paper cups, the top curl formability is excellent, and the repulsive force of the top curl portion is not too high, which reduces the force applied to the adhesive portion of the top curl portion and suppresses peeling of the adhesive portion of the top curl portion, which is preferable. The longitudinal tensile modulus of waterproof paper is highly dependent on the longitudinal tensile modulus of the paper base material, and is adjusted appropriately by the basis weight and thickness of the paper base material, the type and content of the dry strength agent contained in the paper base material, and the type and degree of beating of the pulp fibers that make up the paper base material. Specifically, increasing the amount of dry paper strength agent tends to increase the tensile modulus, increasing the LBKP content in the pulp fiber tends to decrease the tensile modulus, and increasing the NBKP content tends to increase the tensile modulus. The tensile modulus is measured in accordance with JIS P 8113:2006.

[0062] (Specific compressive strength in the longitudinal direction) The waterproof paper of this embodiment has a longitudinal specific compressive strength of preferably 32 N·m / g or less, more preferably 31 N·m / g or less, and even more preferably 30 N·m / g or less. There is no particular lower limit, but the specific compressive strength is preferably 18 N·m / g or more, more preferably 20 N·m / g or more, even more preferably 22 N·m / g or more, and even more preferably 24 N·m / g or more. When the specific compressive strength in the longitudinal direction is equal to or less than the upper limit, excellent formability is obtained, and particularly when the waterproof paper is used for paper cups, excellent top curl formability is obtained, and peeling of the adhesive portion of the top curl is suppressed, which is preferable. On the other hand, when the specific compressive strength in the longitudinal direction is equal to or greater than the lower limit, excellent strength is obtained, and particularly when the waterproof paper is used for paper cups, buckling and wrinkling of the body member during forming are suppressed, which is preferable. The longitudinal specific compressive strength of waterproof paper is largely dependent on the longitudinal specific compressive strength of the paper base material, and is adjusted appropriately depending on the basis weight and thickness of the paper base material, the type and content of the dry strength agent contained in the paper base material, and the type and degree of beating of the pulp fibers that make up the paper base material. Specifically, increasing the amount of dry strength enhancer tends to increase the specific compressive strength, and increasing the LBKP content in the pulp fiber tends to decrease the specific compressive strength, while increasing the NBKP content tends to increase the specific compressive strength. The specific compressive strength is measured by the short span method, and is calculated by measuring the compressive strength in accordance with JIS P 8156:2012 and dividing it by the basis weight.

[0063] (Cobb water absorbency in water at 23°C) From the viewpoint of cold water resistance, the waterproof paper of the present embodiment has a Cobb water absorbency of 40 g / m2 in water at 23°C for 30 minutes on the surface on which the waterproof layer is provided. 2 Less than 20 g / m, more preferably 2 or less, more preferably 15 g / m 2 or less, even more preferably 10 g / m 2 More preferably 5 g / m or less 2 More preferably, 3 g / m or less 2 The lower limit is not particularly limited. The Cobb water absorbency is measured in accordance with JIS P 8140:1998. The Cobb water absorbency for water at 23°C can be adjusted to a desired range by forming the pigment coating layer and water-resistant layer of this embodiment, and a lower Cobb water absorbency tends to be obtained by increasing the coating weight of the pigment coating layer and water-resistant layer.

[0064] (Cobb water absorbency in 90°C water) From the viewpoint of water resistance to hot water, the water-resistant paper of the present embodiment has a Cobb water absorbency of 100 g / m2 or less in water at 90°C for 30 minutes on the surface on which the water-resistant layer is provided. 2 Less than 60 g / m 2 or less, more preferably 30 g / m 2 More preferably, 20 g / m or less 2 The lower limit is not particularly limited. The Cobb water absorbency is measured in accordance with JIS P 8140:1998. The Cobb water absorbency for water at 90°C can be adjusted to a desired range by forming the pigment coating layer and water-resistant layer of this embodiment, and a lower Cobb water absorbency tends to be obtained by increasing the coating weight of the pigment coating layer and water-resistant layer.

[0065] (heat sealability) In the waterproof paper of this embodiment, the waterproof layer preferably has heat sealing properties. The heat sealing conditions are not particularly limited, but the temperature during heat sealing is, for example, 60°C or higher and 300°C or lower, the pressure during heat sealing is, for example, 0.05 MPa or higher and 10 MPa or lower, and the pressure application time is, for example, 0.1 seconds or higher and 15 seconds or lower.

[0066] Two sheets of the waterproof paper of this embodiment were stacked together so that the front surface (liquid-contacting surface, the surface on which the waterproof layer is formed) and the back surface (outer surface, the surface on which the polyethyleneimine-containing layer is formed) faced each other, and then heated at 140°C and 2 kgf / cm 2 The heat seal peel strength when heat sealed for 2 seconds is preferably 5.5 N / 15 mm or more, more preferably 6.0 N / 15 mm or more, and although there is no particular upper limit, it is preferably 25 N / 15 mm or less, more preferably 20 N / 15 mm or less, and even more preferably 15 N / 15 mm or less. The heat seal peel strength is measured by the method described in the examples. The heat seal peel strength tends to increase when a polyethyleneimine-containing layer is provided, and also tends to increase when the coating amount of the water-resistant layer is increased.

[0067] (Basic weight) The basis weight of the waterproof paper of this embodiment is not particularly limited, but for example, in the case of a packaging container, preferably a food container, more preferably, for example, a paper cup, from the viewpoint of obtaining strength as a packaging container, it is preferably 150 g / m 2 More preferably, 180 g / m 2 More preferably, 220 g / m 2 More preferably, 240 g / m 2 More preferably, 250 g / m 2 From the viewpoint of formability, it is preferably 500 g / m 2 or less, more preferably 450 g / m 2 or less, more preferably 400 g / m 2 or less, even more preferably 350 g / m 2 Below 300 g / m, particularly preferably 2The basis weight of waterproof paper is measured in accordance with JIS P 8124:2011.

[0068] (Thickness) The thickness of the waterproof paper of this embodiment is not particularly limited, but for applications such as packaging containers, preferably food containers, and more preferably paper cups, from the viewpoint of obtaining the strength of the packaging container, it is preferably 150 μm or more, more preferably 180 μm or more, even more preferably 230 μm or more, even more preferably 250 μm or more, and particularly preferably 270 μm or more, and from the viewpoint of formability, it is preferably 650 μm or less, more preferably 500 μm or less, even more preferably 430 μm or less, even more preferably 400 μm or less, and particularly preferably 350 μm or less. The thickness of the waterproof paper is measured in accordance with JIS P 8118:2014.

[0069] (Water-resistant paper applications) The waterproof paper of this embodiment has excellent heat-sealing properties and water resistance, and therefore can be suitably used for packaging containers such as cups, plates, trays, lids, pouches, and tube-shaped containers; cutlery such as spoons, forks, knives, and chopsticks; straws; and flexible packaging materials such as wrapping paper, packaging bags, lids, and labels. Packaging containers can be formed, for example, by printing on the surface of the waterproof paper as needed, punching it into a shape corresponding to the shape of the packaging container to be manufactured, folding it, and heat-sealing the overlapping portions. Therefore, the present invention also provides packaging containers (particularly paper cups) made using the waterproof paper. Furthermore, the waterproof paper of this embodiment has excellent oil resistance, and therefore can be suitably used for food packaging applications. The contents of the packaging container may be either food or non-food. The contents of the packaging container may also be liquid, solid, or gel. The contents of the packaging container are not particularly limited, and examples thereof include beverages such as coffee, tea, black tea, juice, and carbonated drinks; alcoholic beverages such as sake, shochu, and wine; dairy beverages such as milk; foods such as instant foods (instant ramen, etc.), microwaveable foods, beverages (yogurt, ice cream, jelly, pudding, etc.), and prepared dishes; pharmaceuticals; and chemical products such as car wax, shampoo, conditioner, detergent, bath additives, hair dye, and toothpaste. The waterproof paper of this embodiment is preferably used for liquid containers such as paper cups or their lids, and other paper containers that require waterproofing (for example, paper trays). When used for liquid containers, it is preferable from the viewpoint of formability that the waterproof layer has heat sealing properties (particularly in the case of paper cups, side sealing properties). To give an example of how liquid containers (paper cups) are formed, waterproof paper is printed on the front or back as needed, punched into a shape that corresponds to the shape of the liquid container to be manufactured, wrapped around a mandrel mold and side-sealed to form the body, and then the bottom is formed and the top is curled to make a paper cup. The waterproof paper of this embodiment has excellent water resistance, heat sealing properties, and moldability. The waterproof paper of this embodiment is not limited to the above uses, and may also be used as wrapping paper. Packaging containers made from the waterproof paper of this embodiment use less plastic than packaging containers made from conventional laminated paper. Furthermore, while packaging containers made from conventional laminated paper must be disintegrated using a pulper with high disintegration performance when recycled into waste paper, packaging containers made from the waterproof paper of this embodiment can be disintegrated using a pulper with normal disintegration performance, making them highly recyclable. Packaging containers made from the waterproof paper of this embodiment can be washed, cut, disintegrated, etc. to prepare pulp slurry, and paper can be manufactured using the obtained pulp slurry. The type of paper to be made is not particularly limited, and examples include printing paper, packaging paper, sanitary paper, and cardboard. Furthermore, the paper made can be processed to manufacture packaging containers (e.g., tissue boxes, paper cup sleeves, etc.). [Example]

[0070] The waterproof paper of the present invention will be described in more detail below with reference to examples, but the present invention is not limited to these. In the examples and comparative examples, "parts" and "%" refer to "parts by mass" and "% by mass," respectively, unless otherwise specified.

[0071] [Measurement method] The measurement method is as follows: Unless otherwise specified, measurements were carried out in an environment of a temperature of 23°C ± 1°C and a relative humidity of 50 ± 2% as specified in JIS P8111:1998.

[0072] <Melt point measurement> The melting point of the dried and solidified aqueous resin emulsion was measured by the following procedure. The aqueous resin emulsion was applied to a PET film using a bar coater and dried at 120°C to form a film. After repeating the application and drying process until the film reached a thickness of approximately 20 to 50 μm, the film was peeled off from the PET film to obtain a dried and solidified aqueous resin emulsion. Five mg of the dried solid was sealed in an aluminum pan and measured using a differential scanning calorimeter (NEXTA DSC600, Hitachi High-Tech Science Corporation). The temperature was raised from 30°C to 150°C at 10°C / min under a nitrogen atmosphere (first run), then cooled to 30°C at 10°C / min, and again raised from 30°C to 150°C at 10°C / min (second run). The endothermic peak temperature of the second run was read and used as the melting point.

[0073] [Preparation of coating liquid] Coating liquid A: The following water-based resin emulsion A was used as coating liquid A as is. Water-based resin emulsion A Solid concentration: 43% by mass, B-type viscosity at 23°C: 3,000 mP·s, specific gravity: 0.97, melting point of dried solid: 86.8°C (melting point 1), 112.7°C (melting point 2) The components contained in Water-based Resin Emulsion A were analyzed using the following procedure. Water-based Resin Emulsion A was dried at 120°C for 2 hours to obtain a solidified product. The solidified product was immersed in a sufficient amount of heptane and heated at 120°C for 1 hour to separate it into a soluble and an insoluble portion. The soluble portion was subjected to vacuum distillation using an evaporator to remove the solvent, and then vacuum dried at 40°C to obtain a hot heptane-soluble portion. The insoluble portion was washed with a small amount of hot heptane and then vacuum dried at 40°C to obtain a hot heptane-insoluble portion. The components were analyzed using a gas chromatography mass spectrometer (Shimadzu Corporation, GCMS-QP2010) connected to a pyrolysis sample introduction device (FRONTIER LAB, PY-3030D). The analysis revealed that the hot heptane soluble fraction contained a copolymer of ethylene and an α-olefin with a carbon number of 6 to 10, and the hot heptane insoluble fraction of ethylene contained an ethylene-acrylic acid copolymer and dimethylaminoethanol. The mass ratio of the ethylene-acrylic acid copolymer to the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms (ethylene-acrylic acid copolymer:copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms) was 27:73. The total content of the ethylene-acrylic acid copolymer and the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms in the solid content of aqueous resin emulsion A was 68.9 mass%, and the total content of the ethylene-acrylic acid copolymer and the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms in the resin component of aqueous resin emulsion A was 78.1 mass%. 13 As a result of C-NMR analysis, the content of structural units derived from ethylene in the ethylene-(meth)acrylic acid copolymer was 65 to 85% by mass. 13 As a result of C-NMR analysis, the content of structural units derived from ethylene in the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms was 68 to 88 mass %.

[0074] Coating liquid B: The following water-based resin emulsion B was used as coating liquid B as is. Water-based resin emulsion B Solid concentration: 47% by mass, B-type viscosity at 23°C: 1,500 mP·s, specific gravity: 0.94, melting points of dried solid: 65.1°C, 99.9°C (melting point 1), 117.6°C (melting point 2) The components contained in aqueous resin emulsion B were analyzed using the following procedure. Aqueous resin emulsion B was dried at 120°C for 2 hours to obtain a solidified product. The solidified product was immersed in a sufficient amount of heptane and heated at 120°C for 1 hour to separate it into a soluble and insoluble portion. The soluble portion was subjected to reduced pressure distillation using an evaporator to remove the solvent, and then vacuum dried at 40°C to obtain a hot heptane-soluble portion. The insoluble portion was washed with a small amount of hot heptane and then vacuum dried at 40°C to obtain a hot heptane-insoluble portion. The components were analyzed using a gas chromatography mass spectrometer (Shimadzu Corporation, GCMS-QP2010) connected to a pyrolysis sample introduction device (FRONTIER LAB, PY-3030D). The analysis revealed that the hot heptane soluble fraction contained a copolymer of ethylene and an α-olefin with a carbon number of 6 to 10, and the hot heptane insoluble fraction of ethylene contained an ethylene-methacrylic acid copolymer. The mass ratio of the ethylene-methacrylic acid copolymer to the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms (ethylene-methacrylic acid copolymer:copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms) was 18:82. The total content of the ethylene-methacrylic acid copolymer and the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms in the solid content of aqueous resin emulsion B was 95.0 mass%, and the total content of the ethylene-acrylic acid copolymer and the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms in the resin component of aqueous resin emulsion A was 97.6 mass%. 13 As a result of C-NMR analysis, the content of structural units derived from ethylene in the ethylene-(meth)acrylic acid copolymer was 60 to 85% by mass. 13 As a result of C-NMR analysis, the content of structural units derived from ethylene in the copolymer of ethylene and an α-olefin having 6 to 10 carbon atoms was 68 to 88 mass %.

[0075] Coating liquid C: Product name "Chemipearl S300" (manufactured by Mitsui Chemicals, Inc., ethylene-methacrylic acid copolymer sodium salt, solid content concentration 35% by mass, melting point of dried solid 92.8°C) was used as coating liquid C as is.

[0076] Coating liquid D: Product name "201103PX.S" (manufactured by Michelman Japan LLC, ethylene-acrylic acid copolymer amine salt, solid content concentration 20% by mass, melting point of dried solid 91.5°C) was used as is as coating liquid D.

[0077] Coating liquid E: Product name "Zaixen AC" (manufactured by Sumitomo Seika Chemicals Co., Ltd., ethylene-acrylic acid copolymer ammonium salt, solid content concentration 28% by mass, melting point of dried solid 82.2°C) was used as is as coating liquid E.

[0078] Preparation of primer coating solution F A pigment dispersion with a solids concentration of 70% was prepared by using 70 parts of heavy calcium carbonate (particle size: 1.2 μm) and 30 parts of kaolin (particle size: 0.9 μm) as the pigment and adding 0.1 parts of sodium polyacrylate to 100 parts of the pigment as the dispersant. 15 parts of styrene-acrylic copolymer latex was blended with this pigment dispersion to prepare a coating liquid with a solids concentration of 62% by mass.

[0079] Preparation of primer coating solution G A pigment dispersion with a solids concentration of 70% was prepared by using 70 parts of heavy calcium carbonate (particle size: 1.2 μm) and 30 parts of kaolin (particle size: 0.9 μm) as the pigment and adding 0.1 parts of sodium polyacrylate to 100 parts of the pigment as the dispersant. 7 parts of styrene-acrylic copolymer latex was blended with this pigment dispersion to prepare a coating liquid with a solids concentration of 62% by mass.

[0080] Preparation of primer coating solution H A pigment dispersion with a solids concentration of 70% was prepared by using 70 parts of heavy calcium carbonate (particle size: 1.2 μm) and 30 parts of kaolin (particle size: 0.9 μm) as the pigment and adding 0.1 parts of sodium polyacrylate to 100 parts of the pigment as the dispersant. 100 parts of styrene-acrylic copolymer latex was blended with this pigment dispersion to prepare a coating liquid with a solids concentration of 62% by mass.

[0081] Preparation of backside coating liquid I 67 parts (solid content) of polyvinyl alcohol (saponification degree: 90 to 99 mol%, polymerization degree: 1,700) and 33 parts (solid content) of polyethyleneimine (number average molecular weight: 70,000) were mixed and diluted with water to prepare a coating liquid with a solid content concentration of 4 mass%.

[0082] Preparation of backside coating liquid J Polyethyleneimine was diluted with water to prepare a coating liquid with a solid content concentration of 4 mass %.

[0083] [Example 1] A paper stock was prepared by adding 0.85 parts by mass of a weakly acidic rosin sizing agent, 0.14 parts by mass of a wet strength agent, and 0.7 parts by mass of a cationic starch as a dry strength agent to 100 parts by mass (solid content equivalent) of a pulp slurry made by mixing beaten LBKP (CSF 450 mL) and beaten NBKP (CSF 500 mL) in a ratio of 80% by mass and 20% by mass. This paper stock was used to make a five-layer paper using a fourdrinier paper machine. The paper was then coated on-machine using a rod coater at the rear of the paper machine. Primer coating solution F was applied to the surface (liquid-contacting surface) of the paper substrate at a coating amount (solid content) of 8 g / m. 2 Then, the back surface (outer surface of the cup) was coated with Back Coating Solution I in an amount (solid content) of 0.36 g / m and dried to form an undercoat layer. 2 The coating is then dried to a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the undercoat layer of the paper substrate at a coating amount (solid content) of 10 g / m 2 The coating was then dried to form a water-resistant layer, thereby producing water-resistant paper.

[0084] [Example 2] A waterproof paper was prepared in the same manner as in Example 1, except that the top coating liquid was changed to Coating Liquid B.

[0085] [Example 3] The coating weight (solid content) of the top coat (water-resistant layer) is 3 g / m 2 A waterproof paper was prepared in the same manner as in Example 1 except for the above change.

[0086] [Example 4] The coating weight (solid content) of the top coat (water-resistant layer) is 5 g / m 2 A waterproof paper was prepared in the same manner as in Example 1 except for the above change.

[0087] [Example 5] The coating weight (solid content) of the top coat (water-resistant layer) is 15 g / m 2 A waterproof paper was prepared in the same manner as in Example 1 except for the above change.

[0088] [Example 6] A waterproof paper was prepared in the same manner as in Example 1, except that the undercoat coating liquid was changed to Coating Liquid G.

[0089] [Example 7] A waterproof paper was prepared in the same manner as in Example 1, except that the undercoat coating liquid was changed to Coating Liquid H.

[0090] [Example 8] The coating amount (solid content) of the back coating layer is 0.1 g / m 2 A waterproof paper was prepared in the same manner as in Example 1 except for the above change.

[0091] [Example 9] A waterproof paper was prepared in the same manner as in Example 1, except that the backside coating liquid was changed to backside coating liquid J.

[0092] [Example 10] A paper stock was prepared by adding 0.85 parts by mass of a weakly acidic rosin sizing agent, 0.14 parts by mass of a wet strength agent, and 1.0 part by mass of a cationic starch as a dry strength agent to 100 parts by mass (solid content equivalent) of a pulp slurry prepared by mixing beaten LBKP (CSF 450 mL) and beaten NBKP (CSF 500 mL) in a ratio of 80% by mass and 20% by mass. This paper stock was used to make a five-layer paper using a fourdrinier paper machine. The paper was then coated on-machine using a rod coater at the rear of the paper machine. Primer coating solution F was applied to the surface (liquid-contacting surface) of the paper substrate at a coating amount (solid content) of 8 g / m. 2 Then, the back surface (outer surface of the cup) was coated with Back Coating Solution I in an amount (solid content) of 0.36 g / m and dried to form an undercoat layer. 2 The coating is then dried to a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the undercoat layer of the paper substrate at a coating amount (solid content) of 10 g / m 2 The coating was then dried to form a water-resistant layer, thereby producing water-resistant paper.

[0093] [Comparative Example 1] A paper stock was prepared by adding 0.85 parts by mass of a weakly acidic rosin sizing agent, 0.14 parts by mass of a wet strength agent, and 0.7 parts by mass of a cationic starch as a dry strength agent to 100 parts by mass (solid content equivalent) of a pulp slurry made by mixing beaten LBKP (CSF 450 mL) and beaten NBKP (CSF 500 mL) in a ratio of 80% by mass and 20% by mass. This paper stock was used to make a five-layer paper using a fourdrinier paper machine. The paper was then coated on-machine using a rod coater at the rear of the paper machine. Primer coating solution F was applied to the surface (liquid-contacting surface) of the paper substrate at a coating amount (solid content) of 8 g / m. 2 The coating is then dried to form a primer layer, with a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the undercoat layer of the paper substrate at a coating amount (solid content) of 10 g / m 2 Then, the coating solution A was applied to the back surface (outer surface of the cup) in an amount (solid content) of 8 g / m2 and dried to form a water-resistant layer. 2 The paper was then dried to prepare a waterproof paper.

[0094] Comparative Example 2 Using the paper stock prepared in the same manner as in Comparative Example 1, a five-layer paper was made using a Fourdrinier paper machine, and then coated directly on the paper machine using a rod coater at the rear stage of the paper machine. The surface primer coating solution F was applied to the surface (liquid-contacting surface) of the paper substrate in an amount (solid content) of 8 g / m. 2 The coating is then dried to form a primer layer, with a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the undercoat layer of the paper substrate at a coating amount (solid content) of 10 g / m 2 The coating was then dried to form a water-resistant layer, thereby producing water-resistant paper.

[0095] Comparative Example 3 Using the paper stock prepared in the same manner as in Comparative Example 1, a five-layer paper was made using a Fourdrinier paper machine, and then coated directly on the paper machine using a rod coater at the rear stage of the paper machine. Primer coating liquid F was applied to the surface (liquid-contacting surface) of the paper substrate at a coating amount (solid content) of 8 g / m. 2 The undercoat layer was then formed by applying the undercoat coating solution F to the back surface (outer surface of the cup) in an amount (solid content) of 10 g / m. 2 The coating is then dried to a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the undercoat layer of the paper substrate at a coating amount (solid content) of 10 g / m 2 The coating was then dried to form a water-resistant layer, thereby producing water-resistant paper.

[0096] Comparative Example 4 The coating weight (solid content) of the top coat (water-resistant layer) is 2 g / m 2 A waterproof paper was prepared in the same manner as in Example 1 except for the above change.

[0097] Comparative Example 5 A paper stock was prepared by adding 0.85 parts by mass of a weakly acidic rosin sizing agent, 0.14 parts by mass of a wet strength agent, and 0.7 parts by mass of a cationic starch as a dry strength agent to 100 parts by mass (solid content equivalent) of a pulp slurry made by mixing beaten LBKP (CSF 450 mL) and beaten NBKP (CSF 500 mL) in a ratio of 80% by mass and 20% by mass. This paper stock was used to make paper using a 5-layer Fourdrinier paper machine. The paper was then coated on-machine using a rod coater at the rear of the paper machine. The backside coating liquid was applied to the backside of the paper substrate (outside of the cup) at a coating amount (solids) of 0.36 g / m. 2 The coating is then dried to a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the surface of the paper substrate (liquid contact surface) at a coating amount (solid content) of 10 g / m 2 The paper was then dried to prepare a waterproof paper.

[0098] Comparative Example 6 A waterproof paper was prepared in the same manner as in Example 1, except that the top coating liquid was changed to Coating Liquid C.

[0099] Comparative Example 7 A waterproof paper was prepared in the same manner as in Example 1, except that the top coating liquid was changed to Coating Liquid D.

[0100] [Comparative Example 8] A waterproof paper was prepared in the same manner as in Example 1, except that the top coating liquid was changed to Coating Liquid E.

[0101] Comparative Example 9 To 100 parts by mass (solids equivalent) of pulp slurry, which was a mixture of beaten LBKP (CSF 450 mL) and beaten NBKP (CSF 500 mL) in a ratio of 80% by mass and 20% by mass, 0.85 parts by mass of weakly acidic rosin sizing agent, 0.14 parts by mass of wet strength agent, and 0.7 parts by mass of amphoteric PAM (amphoteric polyacrylamide) as a dry strength agent were added to prepare a paper stock. This paper stock was used to make paper using a 5-layer Fourdrinier paper machine. The paper was then coated on-machine using a rod coater at the rear of the paper machine. The surface primer coating liquid was applied to the surface (liquid-contacting surface) of the paper substrate at a coating amount (solid content) of 8 g / m. 2 The back surface coating liquid was then applied to the back surface (outer surface of the cup) in a coating amount (solid content) of 0.36 g / m and dried to form an undercoat layer. 2 The coating is then dried to a basis weight of 250 g / m 2 A paper substrate with a thickness of 280 μm was prepared. Coating solution A was applied to the undercoat layer of the paper substrate at a coating amount (solid content) of 10 g / m 2 The coating was then dried to form a water-resistant layer, thereby producing water-resistant paper.

[0102] [Comparative Example 10] Waterproof paper was prepared in the same manner as in Example 1, except that the dry paper strength agent was changed to 0.2 parts by mass of cationized starch.

[0103] [Comparative Example 11] A water-resistant paper was produced in the same manner as in Example 1, except that only LBKP (450 mL of CSF) was used.

[0104] [Evaluation Method] Unless otherwise specified, the tests were conducted under the conditions of a temperature of 23°C ± 1°C and a humidity of 50% ± 2%. [Basis weight] Measured in accordance with JIS P 8124:2011. [Paper thickness] Measured in accordance with JIS P 8118:2014.

[0105] [Cobb water absorbency] Measured in accordance with JIS P 8140:1998 under the conditions of a temperature of 23°C or 90°C and a contact time of 30 minutes.

[0106] [Tensile modulus] Measured in accordance with JIS P 8113:2006.

[0107] [Specific compressive strength] The compressive strength was measured in accordance with JIS P 8156:2012, and the specific compressive strength was calculated by dividing the compressive strength by the basis weight.

[0108] [Heat seal peel strength] A pair of water-resistant papers was overlapped with their surfaces facing each other, and heat-sealed at 140°C, 2 kgf / cm​​​​​​​The obtained waterproof paper was molded into paper cups with an opening diameter of 80 mm and a height of 95 mm, with the front side facing inward (liquid-contacting surface) and the back side facing outward, and with the longitudinal direction of the constituent paper base material aligned vertically to the cup. Continuous moldability over the course of molding 1,000 paper cups was evaluated based on the following criteria. [Evaluation criteria] A: No sticking to the molding machine occurs, allowing for continuous molding (no practical problems) B: Occasional sticking to the molding machine occurred (problems in practical use) C: Frequent sticking to the molding machine makes continuous molding impossible (problems in practical use)

[0110] <Heat sealability> The heat-sealed portion of the body of the resulting paper cup was peeled off by hand, and the degree of adhesion was confirmed using a dye solution. [Evaluation criteria] A: The entire adhesive area is destroyed from the base material. B: There are a few areas that are not destroyed from the substrate. C: There are some parts that are not destroyed from the base material.

[0111] <Top curl forming ability> The top curled portion of the obtained paper cup was observed to check for molding defects (cracks, insufficient curl, wrinkles). [Evaluation criteria] A: Defective rate less than 3% (no practical problems) B: Defective rate 3% or more and less than 10% (no practical problems) C: Defective rate 10% or more (problems in practical use)

[0112] <Whether or not the adhesive part of the top curl has peeled off> The top curled portion of the obtained paper cup was observed, and peeling of the adhesive portion of the top curled portion was evaluated. [Evaluation criteria] A: Defective rate less than 3% (no practical problems) B: Defective rate 3% or more and less than 10% (no practical problems) C: Defective rate 10% or more (problems in practical use)

[0113] <Poor appearance of the body> The body of the obtained paper cup was observed to check for defects in appearance (wrinkles, buckling). [Evaluation criteria] A: Defective rate less than 3% (no practical problems) B: Defective rate 3% or more and less than 10% (no practical problems) C: Defective rate 10% or more (problems in practical use)

[0114] [Table 1-1]

[0115] [Table 1-2]

[0116] The results in Table 1 show that the waterproof paper of the present invention has excellent water resistance and heat sealing properties. When used for paper cups, it has excellent continuous formability, heat sealing properties, and top curl formability, and peeling of the adhesive part of the top curl and poor appearance of the body were suppressed. In contrast, in Comparative Example 1, in which a water-resistant layer was also formed on the back surface, the paper stuck to the molding machine, making continuous molding impossible. Also, in Comparative Example 2, in which a polyethyleneimine-containing layer was not formed on the back surface, and Comparative Example 3, in which a pigment-containing layer was formed, sufficient heat-seal peel strength was not obtained, and when forming paper cups, the heat-sealability was low and peeling occurred at the adhesive portion of the top curl portion. The coating weight of the water-resistant layer is 2.5 g / m 2 In Comparative Example 4, where the thickness was less than 1 / 3 of the total thickness, sufficient water resistance and heat sealability were not obtained. In Comparative Example 5, where no pigment coating layer was formed, a uniform water-resistant layer was not formed, and sufficient heat seal strength was not obtained. When a paper cup was formed, the heat sealability was poor and peeling occurred at the adhesive portion of the top curl portion. In Comparative Examples 6 to 8, in which the water-resistant layer had only one melting point and did not have a melting point in the range of 105° C. or higher and 120° C. or lower, it was difficult to achieve both water resistance and continuous moldability. As a result of using amphoteric polyacrylamide as a dry strength agent for the paper base, the water-resistant paper of Comparative Example 9, which had a longitudinal tensile modulus of more than 7.5 GPa, exhibited poor top curl formability and peeling occurred at the adhesive portion of the top curl. Furthermore, the water-resistant paper of Comparative Example 10, which had a small amount of dry strength agent added, and Comparative Example 11, which used only LBKP as the pulp raw material, had a longitudinal tensile modulus of less than 4.0 GPa, low strength, and when the paper was molded into a paper cup, wrinkles and buckling occurred in the body, resulting in a poor appearance.

Claims

1. A waterproof paper having a waterproof layer on the top layer of one side of a paper substrate, the water-resistant layer has at least two melting points, the first melting point being equal to or higher than 75°C and lower than 105°C, and the second melting point being equal to or higher than 105°C and lower than 120°C; the water-resistant layer contains an ethylene-(meth)acrylic acid copolymer and a copolymer of ethylene and an unsaturated hydrocarbon other than ethylene; a mass ratio of the ethylene-(meth)acrylic acid copolymer to the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene (ethylene-(meth)acrylic acid copolymer / copolymer of ethylene and an unsaturated hydrocarbon other than ethylene) in the water-resistant layer is 10 / 90 or more and 50 / 50 or less; The coating amount of the water-resistant layer is 2.5 g / m ２ That's all, a pigment coating layer containing a pigment and a binder is disposed between the paper substrate and the water-resistant layer; a coating layer containing polyethyleneimine on the other surface of the paper substrate; The tensile modulus in the longitudinal direction is 4.0 GPa or more and 7.5 GPa or less. Water resistant paper.

2. 2. The waterproof paper according to claim 1, wherein the total content of the ethylene-(meth)acrylic acid copolymer and the copolymer of ethylene and an unsaturated hydrocarbon other than ethylene in the waterproof layer is 60% by mass or more.

3. 2. The waterproof paper according to claim 1, wherein the unsaturated hydrocarbon other than ethylene comprises an α-olefin having from 4 to 20 carbon atoms.

4. 2. The waterproof paper according to claim 1, wherein the mass ratio of the pigment content to the binder content (pigment / binder) in the pigment coating layer is 100 / 120 or more and 100 / 3 or less.

5. 2. The waterproof paper of claim 1, wherein the binder comprises a styrene-acrylic copolymer.

6. 2. The waterproof paper according to claim 1, wherein the pigment contains calcium carbonate and kaolin, and the mass ratio of the calcium carbonate content to the kaolin content in the pigment coating layer (calcium carbonate / kaolin) is 50 / 50 or more and 90 / 10 or less.

7. The coating amount of the pigment coating layer is 1 g / m ２ 20g / m or more ２ 2. The waterproof paper of claim 1, wherein:

8. 2. The waterproof paper according to claim 1, wherein the specific compressive strength in the machine direction is 32 N·m / g or less.

9. The coating amount of polyethyleneimine is 0.01 g / m ２ 0.5g / m or more ２ 2. The waterproof paper of claim 1, wherein:

10. The waterproof paper according to claim 1, which is for use in packaging containers.

11. A packaging container made using the waterproof paper according to any one of claims 1 to 10.

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