Heat seal paper

Abstract

To provide heat seal paper with excellent heat seal strength, and in particular, heat seal paper having better heat seal strength when materials used are limited.SOLUTION: A heat seal paper comprises a paper substrate, and a heat seal layer on at least one side of the paper substrate. The contact angle of the paper substrate to water is less than 100°. The heat seal layer contains at least a water-dispersible resin binder, where the dry coating weight per side is 5 g / m2 or more. A coating agent for forming the heat seal layer has a surface tension of less than 46 mN / m at the time of coating.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to heat seal paper, and more particularly to heat seal paper having heat sealing suitability that is suitable for use in packaging applications such as packaging materials, bags, containers, boxes, cups, and lids. [Background technology]

[0002] In recent years, plastic waste has become a major issue due to concerns that plastic that has been released into the environment as garbage will not decompose for a long time and will have a negative impact on the ecosystem. As a countermeasure, it has been proposed to replace plastic with biomass-derived materials and paper, which is a biodegradable material. In particular, as an alternative to packaging materials made of plastic films, heat seal paper has been provided in which a heat seal layer is formed by coating a water or solvent dispersion containing a thermoplastic resin dispersion on a paper base material (Patent Documents 1 to 3, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2020 / 152753 [Patent Document 2] Patent Publication No. 2022-11705 [Patent Document 3] Patent Publication No. 2022-18163 Summary of the Invention [Problem to be solved by the invention]

[0004] Heat seal paper is naturally required to have excellent heat seal strength, and to achieve this, it is necessary to optimize the types of thermoplastic resin contained in the paper base material and the heat seal layer, etc. On the other hand, the types of paper base material and thermoplastic resin may be limited depending on processability, the purpose of use of the heat seal paper, etc. An object of the present invention is to provide a heat seal paper having excellent heat seal strength, particularly when the materials used are limited. [Means for solving the problem]

[0005] The means for solving the problems of the present invention are as follows. 1. A paper substrate and a heat seal layer on at least one surface of the paper substrate, The contact angle of the paper substrate with water is less than 100°; The heat seal layer contains at least a water-dispersible resin binder and has a dry coating weight of 5 g / m per side. 2 That's all. A heat seal paper characterized in that the coating agent for forming the heat seal layer has a surface tension of less than 46 mN / m when applied. 2. The heat seal paper described in 1., characterized in that the geometric mean of the tensile stiffness in the MD and CD directions of the paper base material is 200 kN / m or more. 3. The heat seal paper according to 1 or 2, wherein the heat seal layer contains an ethoxylated acetylene-based surfactant. 4. The heat-sealable paper according to any one of 1. to 3., wherein the water-dispersible resin binder is at least one selected from the group consisting of ethylene methacrylic acid copolymer resin, ethylene acrylic acid copolymer resin, and PHBH. Effect of the Invention

[0006] According to the present invention, it is possible to provide a heat seal paper having excellent heat seal strength. In particular, when either or both of the paper base material and the water-dispersible resin binder made of a thermoplastic resin are limited to specific ones, it is possible to provide a heat seal paper having even more excellent heat seal strength. [Brief description of the drawings]

[0007] [Figure 1]Graph showing the relationship between surface tension, contact angle, and heat seal strength for examples and comparative examples in which ethylene acrylic acid copolymer resin is used as a water-dispersible resin binder and the dry coating amount is 5 g / m2 or more. [Diagram 2] 1 is a graph showing the relationship between surface tension, contact angle, and heat seal strength for the results of Examples and Comparative Examples in which an ethylene methacrylic acid copolymer resin was used as a water-dispersible resin binder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] "Heat seal paper" The present invention relates to a paper substrate and a heat seal layer on at least one surface of the paper substrate, The contact angle of the paper substrate with water is less than 100°; The heat seal layer contains at least a water-dispersible resin binder and has a dry coating weight of 5 g / m per side. 2 That's all. The present invention relates to a heat seal paper in which the coating agent for forming the heat seal layer has a surface tension of less than 46 mN / m when applied.

[0009] (Paper base material) The paper base material is obtained by papermaking a paper material containing pulp, filler, various auxiliaries, etc. When the heat seal paper of the present invention is used for applications in which it comes into contact with food, it is preferable to use materials that comply with food safety standards, such as materials that are approved as food additives or have FDA certification, for each material in the paper base material.

[0010] The pulp may be any of a variety of known pulps, including chemical wood pulps such as bleached softwood kraft pulp (NBKP), unbleached softwood kraft pulp (NUKP), bleached hardwood kraft pulp (LBKP), unbleached hardwood kraft pulp (LUKP), and sulfite pulp (SP); mechanical wood pulps such as ground pulp (GP), refiner ground pulp (RGP), stone ground pulp (SGP), chemi-ground pulp (CGP), semi-chemical pulp (SCP), thermomechanical pulp (TMP), and chemi-thermomechanical pulp (CTMP); non-wood pulps obtained from kenaf, bagasse, bamboo, hemp, straw, and the like; and waste paper pulp made from waste paper, which has had the ink removed in a deinking process. Among these, chemical pulps such as LBKP and NBKP, which are less likely to be contaminated with foreign matter, are preferred, and it is also preferred that the amount of recycled paper pulp is small. Specifically, the amount of chemical pulp in the total pulp is preferably 80% by weight or more, more preferably 90% by weight or more, even more preferably 95% by weight or more, and particularly preferably 100% by weight. The amount of recycled paper pulp is preferably 10% by weight or less, more preferably 5% by weight or less, even more preferably 1% by weight or less, and most preferably none.

[0011] Examples of fillers that can be used include known fillers such as inorganic fillers such as talc, kaolin, calcined kaolin, clay, heavy calcium carbonate, light calcium carbonate, white carbon, zeolite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, barium sulfate, and calcium sulfate, and organic fillers such as urea-formalin resin, polystyrene resin, phenol resin, and hollow microparticles. Note that fillers are not essential materials and may not be used.

[0012] Examples of various auxiliary agents include sizing agents such as rosin, alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), polyacrylamide polymers, polyvinyl alcohol polymers, cationic starch, various modified starches, urea-formaldehyde resin, melamine-formaldehyde resin, and other dry strength agents, wet strength agents, retention agents, drainage improvers, coagulants, aluminum sulfate, bulking agents, dyes, fluorescent brighteners, pH adjusters, defoamers, UV inhibitors, anti-fading agents, pitch control agents, slime control agents, and the like, and can be selected and used as appropriate as needed.

[0013] The method of manufacturing (papermaking) the paper base material is not particularly limited, and known manufacturing (papermaking) methods and papermaking machines can be selected, such as a Fourdrinier papermaking machine, a cylinder papermaking machine, a short wire papermaking machine, a twin-wire papermaking machine such as a gap former type or a hybrid former type (on-top former type). The pH during papermaking may be in the acidic range (acidic papermaking), pseudo-neutral range (pseudo-neutral papermaking), neutral range (neutral papermaking), or alkaline range (alkaline papermaking), and after papermaking in the acidic range, an alkaline agent may be applied to the surface of the paper layer. The paper base material may be one layer, or may be composed of two or more layers.

[0014] Furthermore, the surface of the paper substrate can be treated with various chemicals. Examples of chemicals that can be used include oxidized starch, hydroxyethyl etherified starch, enzyme-modified starch, polyacrylamide, polyvinyl alcohol, surface sizing agents, water-resistant agents, water retention agents, thickeners, and lubricants, and these can be used alone or in combination of two or more. Furthermore, these various chemicals can be used in combination with pigments. Examples of pigments include inorganic pigments such as kaolin, clay, engineered kaolin, delaminated clay, heavy calcium carbonate, light calcium carbonate, mica, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicate salts, colloidal silica, and satin white, and organic pigments such as solid, hollow, or core-shell types can be used alone or in combination of two or more.

[0015] The method for treating the surface of the paper substrate is not particularly limited, but known coating devices such as a rod metering size press, a pond type size press, a gate roll coater, a spray coater, a blade coater, and a curtain coater can be used. Examples of the paper base material include various known paper such as fine paper, medium quality paper, coated paper, one-sided glossy paper, kraft paper, one-sided glossy kraft paper, bleached kraft paper, glassine paper, paperboard, white paperboard, and liner.

[0016] The paper substrate used in the present invention has a contact angle with water (pure water, 2 μL, 0.5 seconds after contact) of less than 100°. By using a paper substrate with a contact angle of less than 100°, a heat seal paper with excellent heat seal strength can be obtained compared to a paper substrate with a contact angle of 100° or more. The contact angle of the paper substrate is preferably 90° or less, more preferably 85° or less, and even more preferably 80° or less. The contact angle of the paper substrate is preferably 30° or more, and more preferably 35° or more. The contact angle of the paper substrate can be adjusted, for example, by the hydrophilicity of the auxiliary or agent used, or by using a surfactant as an external agent, particularly when performing surface treatment.

[0017] The paper substrate preferably has a geometric mean of tensile stiffness in the MD and CD directions of 200 kN / m or more. When the geometric mean of tensile stiffness is large, the deformation (elongation) of the paper when a load is applied is small, so that the heat seal layer cannot follow the deformation of the paper when a load is applied, and it is possible to prevent the occurrence of destruction such as cracks. In addition, in the heat seal layer, breakage starting from the destruction point is unlikely to occur, so that the heat seal strength is improved. The geometric mean of this tensile stiffness is more preferably 300 kN / m or more, more preferably 400 kN / m or more, even more preferably 500 kN / m or more, even more preferably 600 kN / m or more, and even more preferably 700 kN / m or more. The upper limit of the geometric mean of tensile stiffness is not particularly limited, but is, for example, about 800 kN / m or less.

[0018] The basis weight of the paper base material can be appropriately selected depending on the desired quality and handling properties, but is usually 20 g / m 2 More than 600g / m 2 The following are preferred: For heat seal paper used for packaging purposes such as food packaging, bags, containers, boxes, cups, and lids, 25 g / m 2 More than 600g / m 2 The following is more preferable, and in particular for heat seal paper used for bags, lids, or soft packaging applications described below, 30 g / m 2 More than 150g / m 2 The following are also heat-sealable paper for containers, boxes, and cups: 150 g / m 2 More than 350g / m 2 The following are more preferable. In terms of composition, the flexible packaging material is a packaging material made of a highly flexible material, and generally refers to a packaging material made of thin, flexible materials such as paper, film, and aluminum foil, either alone or in combination. In terms of shape, it refers to a packaging material such as a bag that maintains a three-dimensional shape when the contents are placed inside. The density of the paper base material can be appropriately selected depending on the desired qualities and ease of handling, but is usually 0.5 g / cm 3 More than 1.0g / cm 3 The following are preferred:

[0019] (Heat seal layer) The heat seal layer contains at least a water-dispersible resin binder and is formed directly on the surface of the paper substrate having a contact angle of less than 100°. The heat seal layer is provided on at least one surface of the paper substrate. When the heat seal layer is provided on both surfaces of the paper substrate, the contact angle of the other surface of the paper substrate is not particularly limited, and one or more functional layers such as a sealing layer, an ink receiving layer, a water-resistant layer, an oil-resistant layer, a water vapor barrier layer, a gas barrier layer, etc. can be formed between the paper substrate and the other heat seal layer.

[0020] Water-dispersible resin binder The water-dispersible resin binder can be any thermoplastic resin used in the papermaking field for forming a heat seal layer, and can be used without any particular limitation. For example, a resin having a glass transition temperature of 100°C or less can be used. The glass transition temperature of the thermoplastic resin is preferably -20°C or more and 85°C or less. The melting point of the thermoplastic resin is preferably 80°C or more and 120°C or less. As the water-dispersible resin binder (thermoplastic resin), for example, polyolefin resins such as low-density polyethylene (LDPE), high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and polypropylene (PP), polyester resins such as polyvinyl acetate (PVAc), PET, and polylactic acid, ethylene methacrylic acid copolymer resin (EMAA), ethylene methyl acrylate copolymer resin (EMA), ethylene acrylic acid copolymer resin (EAA), ethylene vinyl acetate copolymer resin (EVA), and styrene acrylic acid ester copolymer resin can be used, and one or more of these can be mixed and used. Among these, ethylene methacrylic acid copolymer resin, ethylene acrylic acid copolymer resin, and polyolefin resin are preferred because of their excellent heat seal strength, and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), a type of polyester, is preferred because of its biodegradability. The resin having an acrylic acid or methacrylic acid unit may be an ionomer.

[0021] Surfactants The heat seal layer may contain a surfactant. By including a surfactant in the heat seal layer, it becomes easier to make the surface tension of the coating agent for forming the heat seal layer less than 46 mN / m during application. The ionicity of the surfactant is not limited, and any of anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants can be used alone or in combination of two or more.Specific examples of the surfactants include silicone surfactants, fluorine surfactants, alcohol surfactants, acetylene surfactants having an acetylene group, acetylene diol surfactants having an acetylene group and two hydroxyl groups, ethoxylated acetylene surfactants, ethoxylated acetylene diol surfactants, alkylsulfonic acid surfactants having an alkyl group and sulfonic acid, ester surfactants, amide surfactants, amine surfactants, alkyl ether surfactants, phenyl ether surfactants, sulfate ester surfactants, and phenol surfactants.Among these, acetylene surfactants, acetylene diol surfactants, ethoxylated acetylene surfactants, and ethoxylated acetylene diol surfactants are preferred, and ethoxylated acetylene surfactants are more preferred. The amount of surfactant in the heat seal layer is not particularly limited, but is preferably 0.01% by weight or more and 5% by weight or less based on the entire heat seal layer. If the amount of surfactant is less than 0.01% by weight, the effect of the surfactant may not be exhibited. If the amount of surfactant is more than 5% by weight, the heat seal strength may be reduced. The amount of surfactant in the heat seal layer is more preferably 3% by weight or less based on the entire heat seal layer, more preferably 2% by weight or less, and even more preferably 1% by weight or less.

[0022] In addition to the water-dispersible resin binder and the surfactant, the heat seal layer may contain wax, an antiblocking agent such as a pigment, a dispersant, a thickener, a dye, etc., within the range not impairing the effects of the present invention.

[0023] The heat seal layer is formed by directly coating at least one surface of the paper substrate with a coating agent which contains at least a water-dispersible resin binder and may contain a surfactant and the like, and then drying the coating agent. The coating method of the heat seal layer is not particularly limited, and can be applied by a known coating device and coating system. For example, the coating device can be a blade coater, a bar coater, an air knife coater, a curtain coater, a spray coater, a roll coater, a reverse roll coater, a size press coater, a gate roll coater, etc. In addition, the coating system can be an aqueous coating using a solvent such as water, a solvent-based coating using a solvent such as an organic solvent, etc., and the aqueous coating is preferable from the viewpoint of safety and low environmental load.

[0024] In the present invention, the coating agent forming the heat seal layer has a surface tension of less than 46 mN / m when applied. By directly applying a coating agent having a surface tension of less than 46 mN / m when applied onto a paper substrate having a contact angle of less than 100°, a heat seal paper having excellent heat seal strength can be obtained. The surface tension of the coating agent when applied is preferably 42 mN / m or less, more preferably 38 mN / m or less, and preferably 30 mN / m or more, more preferably 32 mN / m or more.

[0025] The dry coating weight of the heat seal layer on each side is 5g / m 2 That's it. The dry coating weight per side is 5g / m 2 If the dry coating amount is less than 20 g / m2, it is difficult to form a heat seal layer of sufficient thickness on the paper substrate, and sufficient heat seal strength may not be obtained. The higher the dry coating amount per side of the heat seal layer, the higher the heat seal strength tends to be. However, if it is too high, the energy cost and material cost for drying will increase, so it is recommended to use a dry coating amount of 20 g / m2 or less. 2 Less than 10 g / m is preferable. 2 The following is more preferable. The heat seal layer may be one layer or may be composed of two or more layers. When the heat seal layer is composed of two or more layers, it is preferable that the total coating amount of all the coating layers is within the above range. EXAMPLES

[0026] (Evaluation method) Surface tension of coating agent Measurement was performed by the Wilhelmy method (plate method, vertical plate method) using an automatic surface tensiometer (Kyowa Interface Science Co., Ltd., CBVP-Z). Contact angle of paper substrate In accordance with the static sampling method of JIS R3257 (test method for wettability of glass substrate surfaces), 2 μL of pure water was sampled, and the moment the sample contacted the paper substrate was set as 0 seconds. The contact angle was measured 0.5 seconds later, and the average value of 5 measurements was calculated. Tensile stiffness Measured using an L&W tensile tester in accordance with ISO / DIS1924-3.

[0027] Heat seal strength Two sheets of 100 mm wide heat seal paper were placed together with the heat seal layers facing each other, and the heat seal tester (Tester Sangyo, TP-701-S) was used to measure the heat seal strength at 120°C and 2 kgf / cm 2 The heat sealing was performed under the conditions of 0.5 seconds. The heat-sealed test pieces were left to stand in an environment with a temperature of 23°C ± 1°C and a humidity of 50 ± 2% for at least 12 hours, and then, in accordance with JIS Z1707, the test pieces were cut to a width of 15 mm and subjected to a T-peel at a tensile speed of 200 mm / min using a Tensilon universal material testing machine (TC-1250A, manufactured by A&D), and the maximum load recorded was taken as the heat seal peel strength.

[0028] "Examples 1 to 5, Comparative Examples 1 to 8" ·Paper base material Base paper (basis weight 50g / m 2 The base paper was coated with an ethoxylated acetylene diol surfactant (Surfynol-465, manufactured by Nissin Chemical Industry Co., Ltd.) in different amounts to give contact angles of 95.5°, 77.9°, 52.0°, and 40.2°, respectively, and used as the paper substrate.

[0029] An ethylene acrylic acid copolymer resin (Michaelman: MICHEM PRIME 498345N.S), which is a thermoplastic resin, and a surfactant 1 (Nissin Chemical Industry Co., Ltd.: Surfynol-440, ethoxylated acetylene type) were mixed and dispersed to obtain the composition (solid content) shown in Table 1, to obtain an aqueous dispersion with a solid content concentration of 42 to 45%, and a coating agent for a heat seal layer was obtained. This coating agent was applied to one side of a paper substrate so as to obtain the dry coating amount shown in Table 1, and then dried to obtain a heat seal paper.

[0030] [Table 1]

[0031] ·result From Comparative Examples 2, 6 to 8, the dry coating amount was 5 g / m 2 When the dry coating amount was less than 5 g / m, the heat seal strength was poor, regardless of the contact angle of the base paper and the surface tension of the coating agent. 2 It is presumed that if the thickness is less than this, a heat seal layer of sufficient thickness is not formed. Dry coating weight: 5g / m 2 The results obtained above are shown in FIG. 1, which shows the relationship between surface tension, contact angle, and heat seal strength. As shown in Figure 1, by applying a coating agent with a surface tension of less than 46 mN / m to a paper substrate with a contact angle with water of less than 100°, heat seal paper with excellent heat seal strength could be obtained. It was also confirmed that the smaller the contact angle of the paper substrate and the smaller the surface tension of the coating agent, the higher the heat seal strength tends to be. From this, it was confirmed that when using a specific paper substrate or a specific thermoplastic resin, the heat seal strength can be increased by adjusting the contact angle and the surface tension of the coating agent.

[0032] "Examples 6 to 10, Comparative Examples 9 and 10" A thermoplastic resin, ethylene methacrylic acid copolymer resin (ionomer) (Mitsui Chemicals: Chemipearl S500) and surfactant 2 (Nissin Chemical Industry Co., Ltd.: Surfynol-465, ethoxylated acetylene diol type) were mixed and dispersed to obtain the composition (solid content) shown in Table 2, to obtain an aqueous dispersion with a solid content concentration of 42 to 43%, to obtain a coating agent for the heat seal layer. This coating agent was applied to one side of the paper substrates with different contact angles obtained above in the dry coating amounts shown in Table 2, and then dried to obtain heat seal papers.

[0033] [Table 2]

[0034] ·result The relationship between surface tension, contact angle, and heat seal strength is shown in Figure 2. As shown in Figure 2, it was confirmed that even when ethylene methacrylic acid copolymer resin, an ionomer, was used as the water-dispersible resin binder, a paper substrate with a smaller contact angle with water had better heat seal strength, and the lower the surface tension of the coating agent, the better the heat seal strength. This confirmed that when using a specific paper substrate or a specific thermoplastic resin, the heat seal strength can be increased by adjusting the contact angle and the surface tension of the coating agent.

[0035] "Examples 11 and 12" A thermoplastic resin (Green Planet (registered trademark), PHBH, manufactured by Kaneka Corporation) and a surfactant 2 (Surfynol-465, ethoxylated acetylene diol, manufactured by Nissin Chemical Industry Co., Ltd.) were mixed and dispersed to obtain the composition (solid content) shown in Table 3, to obtain an aqueous dispersion with a solid content concentration of 42 to 44%, thereby obtaining a coating agent for a heat seal layer. This coating agent was applied to one side of a paper substrate (basis weight 40 g, contact angle 83.9°) to the dry coating amount shown in Table 3, and then dried to obtain a heat seal paper.

[0036] [Table 3]

[0037] ·result As shown in Table 3, even when PHBH was used as the water-dispersible resin binder, it was confirmed that the lower the surface tension of the coating agent, the better the heat seal strength tended to be.

[0038] "Examples 13 to 16" The coating materials used in Comparative Examples 9 and 10 were applied to different paper substrates and dried to obtain the dry coating amounts shown in Table 4, to obtain heat seal papers. The results are shown in Table 4 together with Comparative Examples 9 and 10 in Table 2.

[0039] [Table 4]

[0040] ·result It was confirmed from Examples 13 to 16 and Comparative Examples 9 and 10 that the heat seal strength improves as the tensile stiffness of the paper base material increases.

Claims

1. A paper substrate and a heat seal layer on at least one surface of the paper substrate, The contact angle of the paper substrate with water is less than 100°; The heat seal layer contains at least a water-dispersible resin binder and has a dry coating weight of 5 g / m per side. ２ That's all. A heat seal paper characterized in that the coating agent for forming the heat seal layer has a surface tension of less than 46 mN / m when applied.

2. 2. The heat seal paper according to claim 1, wherein the geometric mean of the tensile stiffness in the MD and CD directions of the paper base material is 200 kN / m or more.

3. 3. The heat seal paper according to claim 1, wherein the heat seal layer contains an ethoxylated acetylenic surfactant.

4. 3. The heat seal paper according to claim 1, wherein the water-dispersible resin binder is at least one selected from the group consisting of ethylene methacrylic acid copolymer resin, ethylene acrylic acid copolymer resin, and PHBH.

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