heat seal paper

Abstract

The main objective is to obtain heat-sealable paper with excellent heat-sealing properties. The present invention provides a paper sheet having a base paper and a heat-sealable coating layer located on at least one of the outermost surfaces, the coating layer contains a thermoplastic resin and a thickener, The heat seal paper, wherein the thickener comprises polyethylene oxide (PEO).

Description

[Technical Field]

[0001] The present invention relates to heat seal paper. [Background technology]

[0002] Heat-seal paper having a heat-seal layer on at least one surface of a base paper is used to form flexible packaging (pillow packaging), paper cups, paper plates, paper trays, etc. Furthermore, as such heat-seal paper, one has been proposed that reduces the amount of plastic used by laminating only a coating layer without laminating a laminate layer. For example, Patent Document 1 proposes a base paper for paper cups that has at least one heat-seal layer on at least one surface of a paper base material whose main component is pulp, and the heat-seal layer is a coating layer containing an ionomer, thereby reducing the amount of plastic used, having excellent recyclability, and being suitable for processing into three-dimensional paper containers. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2024-040898 Summary of the Invention [Problem to be solved by the invention]

[0004] In heat sealable paper, a high degree of uniformity in the coating layer is required to improve heat sealing suitability. In order to form a thick coating layer with excellent uniformity, thickeners are added to the coating solution to increase viscosity. Polyvinyl alcohol (PVA) is a common thickener, but in order for the PVA to exert its thickening effect, it must be added at a fairly high concentration. On the other hand, in Japan, items that come into direct contact with food or are held in the mouth must meet the standards for leaching tests on synthetic resin utensils, containers, and packaging set out in Ministry of Health and Welfare Notification No. 370 of 1959. However, PVA is hydrophilic and easily leaches out, resulting in increased evaporation residues during leaching tests. The primary object of the present invention is to provide heat-sealable paper with excellent heat-sealing properties, and a secondary object is to provide heat-sealable paper with little evaporation residue. [Means for solving the problem]

[0005] The means for solving the problems of the present invention are as follows. 1. A paper sheet having a base paper and a heat-sealable coating layer located on at least one of the outermost surfaces, the coating layer contains a thermoplastic resin and a thickener, A heat sealable paper characterized in that the thickener contains polyethylene oxide (PEO). 2. The heat-sealable paper described in 1., characterized in that the viscosity average molecular weight of the PEO is 1 million or more and 4.8 million or less. 3. The heat sealable paper according to 1. or 2., characterized in that the thermoplastic resin is a biodegradable resin. 4. The heat-sealable paper according to any one of 1. to 3., characterized in that an underlayer is provided between the base paper and the coating layer. 5. A heat-sealable paper according to any one of 1. to 4., characterized in that the amount of evaporation residue in an elution test (water at 95°C for 30 minutes) in accordance with Ministry of Health and Welfare Notification No. 370 of 1959 is 25 μg / ml or less. 6. A coating step of applying a coating liquid containing a thermoplastic resin and a thickener to the outermost surface of at least one side of the base paper; a drying step of drying the coating layer; in this order, A method for producing heat-sealable paper, characterized in that the thickener contains polyethylene oxide (PEO). 7. The manufacturing method according to 6, wherein the coating step is performed by curtain coating. 8. The manufacturing method according to 6. or 7., wherein the coating liquid has a Brookfield viscosity of 70 mPa·s or more and 500 mPa·s or less. [Effects of the Invention]

[0006] The heat seal paper of the present invention has excellent heat sealing properties. The heat seal paper of the present invention leaves little evaporation residue and is suitable for use in applications where it comes into contact with food or where it is held in the mouth. DETAILED DESCRIPTION OF THE INVENTION

[0007] The heat-sealable paper of the present invention has a base paper and a coating layer having heat-sealability located on at least one outermost surface thereof, the coating layer contains a thermoplastic resin and a thickener, The thickening agent includes polyethylene oxide (PEO).

[0008] Base paper In the present invention, the base paper is a sheet made of pulp, fillers, various auxiliaries, and the like. Pulp may be any of a variety of known pulps, including bleached softwood kraft pulp (NBKP), unbleached softwood kraft pulp (NUKP), bleached hardwood kraft pulp (LBKP), unbleached hardwood kraft pulp (LUKP), and sulfite pulp (SP) and other wood-based chemical pulps; 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 pulp obtained from kenaf, bagasse, bamboo, hemp, straw, and the like; and recycled paper pulp obtained by removing the ink contained in recycled paper through a deinking process.

[0009] Among these, it is preferable to use chemical pulp made from wood fibers or mechanical pulp made from wood fibers, and it is more preferable to use chemical pulp made from wood fibers, for reasons such as the fact that they are less likely to be contaminated with foreign matter, are less likely to discolor over time when recycled as waste paper raw material, and have a high whiteness that results in a good surface appearance when printed, making them particularly valuable when used as packaging materials. It is also preferable to use a small amount of recycled paper pulp. Specifically, the ratio of chemical pulp to the total pulp is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and most preferably 100% by mass. It is also preferable that the ratio of recycled paper pulp to the total pulp is 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, and most preferably 0% by mass.

[0010] Examples of fillers that can be used include 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, phenolic resin, and hollow microparticles. In the present invention, the base paper does not need to contain a filler, and it is preferable not to use a filler.

[0011] Examples of various auxiliaries include various internal sizing agents such as rosin, alkyl ketene dimer (AKD), and alkenyl succinic anhydride (ASA); various nonionic, cationic, and amphoteric retention aids, freeness improvers, paper strength improvers, various starches, polyacrylamide, urea resin, melamine resin, epoxy resin, polyamide resin, polyamide, polyamine resin, polyamine, polyethyleneimine, vegetable gum, polyvinyl alcohol, latex, polyethylene oxide, hydrophilic crosslinked polymer particle dispersions and derivatives or modified products thereof; basic aluminum compounds such as aluminum sulfate, aluminum chloride, sodium aluminate, basic aluminum chloride, and basic polyaluminum hydroxide; water-soluble aluminum compounds such as alumina sol that is easily decomposed in water; polyvalent metal compounds such as ferrous sulfate and ferric sulfate; silica sol, antifoaming agents, coloring dyes, coloring pigments, pH adjusters, pitch control agents, and slime control agents, and these can be appropriately selected and used as needed.

[0012] The surface of the base paper may be treated with various chemicals. Examples of chemicals include oxidized starch, hydroxyethyl etherified starch, enzyme-modified starch, polyacrylamide, polyvinyl alcohol, surface sizing agents, water-resistant agents, water-retention agents, thickeners, and lubricants. These may be used alone or in combination of two or more. Furthermore, these chemicals may 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, silicates, colloidal silica, and satin white, as well as organic pigments such as solid, hollow, and core-shell pigments, which may be used alone or in combination of two or more.

[0013] The basis weight of the base paper is not particularly limited as long as it is within the range usable as base paper for heat seal paper. For example, 2 It is preferable that the weight is 50 g / m or more. 2 More preferably, it is 100 g / m or more. 2The upper limit of the basis weight of the base paper is not particularly limited, but is preferably 500 g / m 2 Preferably, it is 450 g / m or less. 2 More preferably, it is 400 g / m or less. 2 It is even more preferable that:

[0014] The density of the base paper can be selected appropriately depending on the desired quality and handling properties, but is usually 0.5 g / cm 3 More than 1.0g / cm 3 The following are preferred: In the present invention, the base paper may be a paper consisting of only a single paper layer or a multi-layer paper having two or more paper layers. When the base paper is a multi-layer paper, the paper materials, basis weights, etc. of the paper layers may be the same or different.

[0015] The base paper manufacturing (papermaking) method and the type of papermaking machine are not particularly limited, and known manufacturing (papermaking) methods and papermaking machines can be selected, such as a Fourdrinier papermaking machine, a twin-wire papermaking machine, a cylinder papermaking machine, a gap former, a hybrid former (on-top former), and a multilayer papermaking machine that combines these. Furthermore, 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 base material. Furthermore, when treating the surface of a paper substrate with a chemical, the method of surface treatment is not particularly limited, and 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.

[0016] Coating layer The heat-sealable paper of the present invention has a coating layer with heat-sealability (hereinafter also referred to as a heat-sealable coating layer) on at least one outermost surface. The heat-sealable paper of the present invention only needs to have a heat-sealable coating layer on at least one outermost surface, and can have a heat-sealable coating layer on both surfaces. In addition, other layers such as an underlayer, a water vapor barrier layer, a gas barrier layer, or an ink-receiving layer may be present between the base paper and the heat-sealable coating layer.

[0017] The heat-sealable coating layer contains at least a thermoplastic resin and a thickener. The thermoplastic resin contained in the heat-sealable coating layer can be any known heat-sealable resin without any particular limitations. Examples include polylactic acid (PLA), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), polybutylene succinate (PBS), polyvinyl acetate (PVAc), polyvinyl chloride (PVC), ethylene-vinyl acetate copolymer (EVA), polyolefin, acrylic, ethylene-(meth)acrylic acid copolymer, styrene-acrylic acid copolymer, and modified products thereof. One or more of these can be used. Among these, biodegradable resins are preferred, with PLA or PHBH being more preferred, and PHBH being even more preferred. A biodegradable resin is a resin that can be decomposed by microorganisms into at least water and carbon dioxide, and has an aerobic biodegradation rate of 50% or more in 6 months, as measured according to ISO-14855-2 (2018).

[0018] The present invention uses at least polyethylene oxide (PEO) as a thickener. By including PEO in the thickener, it is possible to reduce elution from the heat-sealable coating layer. More preferably, the PEO has a viscosity-average molecular weight of 1,000,000 or more and 4,800,000 or less. In addition to the thermoplastic resin and thickener, the coating layer may contain various auxiliaries that are blended into coating solutions in the papermaking field, such as surfactants, dispersants, water retention agents, antifoaming agents, water resistance agents, pH adjusters, UV absorbers, metal salts, lubricants, coloring dyes, and pigments.

[0019] Underlayer The heat-sealable paper of the present invention preferably has an underlayer between the base paper and the outermost heat-sealable coating layer. By forming the heat-sealable coating layer on the underlayer, a more uniform coating layer with fewer coating defects can be formed. The under layer is preferably a coating layer containing a thermoplastic resin. When the under layer contains a thermoplastic resin, the under layer and the coating layer work together to exhibit heat sealing properties, thereby improving heat sealing suitability. The thermoplastic resin contained in the under layer may be the same as the thermoplastic resin contained in the coating layer located on the outermost surface. The under layer may also contain various auxiliaries that are blended into coating solutions in the papermaking field, such as thickeners, surfactants, dispersants, water retention agents, antifoaming agents, water resistance agents, pH adjusters, UV absorbers, metal salts, lubricants, coloring dyes, and pigments. The compositions of the under layer and the coating layer may be the same or different.

[0020] The amount of heat-sealable coating per side is 6 g / m2 in dry weight. 2 More than 40g / m 2 The coating weight per side is preferably 6 g / m2 or less in dry weight. 2 By setting the coating amount at 6 g / m or more, it becomes easy to obtain a coating layer with excellent heat-sealing properties. 2 If the coating density is less than 40 g / m, the coating layer may not be formed uniformly on the paper surface. 2 Above this coating amount, the heat sealability is almost saturated and there is almost no further improvement, and the energy cost and time required for drying increase. 2 More preferably, 15 g / m 2 More preferably, 20 g / m 2 The above is even more preferable. The coating amount of the undercoat per side is 5 g / m2 in dry weight. 2 More than 20g / m 2 The coating weight per side is preferably 5 g / m2 or less in dry weight. 2 By setting the coating density to 20 g / m or more, coating defects can be reduced. 2Above this coating amount, the effect is almost saturated and there is almost no further improvement, and the energy cost and time required for drying increase. 2 More preferably, 7 g / m 2 The above is even more preferable.

[0021] The coating method is not particularly limited, and coating can be performed using known coating equipment and coating systems. Examples of coating equipment include blade coaters, bar coaters, air knife coaters, curtain coaters, spray coaters, roll coaters, reverse roll coaters, size press coaters, and gate roll coaters. Among these, the heat-sealable coating layer is preferably coated by a pre-metered coating method, and curtain coating is more preferred. By coating the outermost heat-sealable coating layer by a pre-metered coating method, a very flat and uniform coating layer can be obtained. Furthermore, when forming two or more coating layers, such as an underlayer and a heat-sealable coating layer, coating can be performed either offline or online. The coating system may be either aqueous coating using a solvent such as water or solvent-based coating using a solvent such as an organic solvent, but aqueous coating is preferred from the viewpoint of safety and hygiene. In the case of aqueous coating, a water-dispersible resin or a water-soluble resin is used as the thermoplastic resin, and therefore the heat-sealable coating layer is preferably a coating layer of a water-dispersible resin or a water-soluble resin.

[0022] The amount of thickener in the coating solution that forms the heat-sealable coating layer is not particularly limited, and is adjusted to obtain an appropriate viscosity depending on the coating system, desired coating amount, etc. For example, in the case of curtain coating, the thickener is blended so that the B-type viscosity is approximately 70 mPa·s to 500 mPa·s at 23°C, a solids concentration of 48% by mass, and a No. 3 rotor at 60 rpm.

[0023] Heat seal paper The heat seal paper of the present invention has excellent heat sealability and produces a small amount of elution (evaporation residue). The heat seal paper of the present invention preferably has an evaporation residue of 25 μg / ml or less, more preferably 20 μg / ml or less, even more preferably 15 μg / ml or less, and even more preferably 10 μg / ml or less, in an elution test (water at 95°C for 30 minutes) in accordance with Ministry of Health and Welfare Notification No. 370 of 1959.

[0024] Furthermore, the heat seal paper of the present invention preferably satisfies the following food hygiene tests (1) to (3). (1) No PCBs were detected using the test method specified in Kanshokuka No. 385 of 1972. (2) No phenol, formaldehyde, or heavy metals are detected in the test method conforming to the Ministry of Health and Welfare Notification No. 370 of 1959. (3) The fluorescent dye must not be eluted in the test method specified in Kanshoku No. 244 of 1971 and Shokuanki No. 0107001 of 2004.

[0025] The gloss of the coating surface of the heat-sealable paper is preferably 85 or higher. High gloss correlates with the smoothness of the coating surface, and a heat-sealable coating layer with a gloss of 85 or higher can be efficiently heat-sealed to the opposing base paper or coated paper during heat-sealing. Furthermore, high gloss allows the inner surface of the heat-sealable paper to reflect light after processing, giving the user a visually more luxurious feel.

[0026] The heat seal paper of the present invention has a low evaporation residue and is therefore suitable for applications in which it comes into contact with food or is held in the mouth, such as sticks for supporting food such as dumplings, frankfurters, and frozen desserts, eating utensils such as spoons, forks, skewers, toothpicks, and chopsticks, stirring sticks such as stirrers and spatulas, and food sheets such as paper linings, paper plates, and paper cups. [Example]

[0027] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, parts and % in the examples represent parts by mass and % by mass, respectively. (Evaluation method) ·B type viscosity The viscosity of the heat seal layer coating liquid at a temperature of 23°C and a solids concentration of 48% by mass was measured using a Brookfield viscometer (Tokyo Keiki Co., Ltd., BII type viscometer) with a No. 3 rotor at a rotation speed of 60 rpm. Curtain suitability (curtain film formation, curtain film stability) The heat-seal layer coating solution was evaluated using a slot-type curtain coater with a total volumetric flow rate of 7.2 L / min / m. Curtain film formation was evaluated by visual inspection to determine whether the coating solution formed a uniform film across the entire width of the slot without concentrating in the center of the slot (evaluated on a two-point scale: good or bad). Curtain film stability was evaluated by visual inspection to determine whether the curtain film flowing down from the curtain coater could be uniformly coated on the paper surface without being broken by the air currents generated around the base paper when the base paper was run under the coater at a speed of 150 m / min (evaluated on a two-point scale: good or bad).

[0028] Dissolution test The amount of elution from heat seal paper in water at 95°C for 30 minutes was evaluated in terms of potassium permanganate consumption, in accordance with Notification No. 370 of the Ministry of Health and Welfare in 1959. Although the heat seal paper has not been subjected to the food hygiene tests (1) to (3) below, all of the raw materials used in this example satisfy (1) to (3), and therefore the heat seal paper also satisfies (1) to (3). (1) No PCBs were detected using the test method specified in Kanshokuka No. 385 of 1972. (2) No phenol, formaldehyde, or heavy metals are detected in the test method conforming to the Ministry of Health and Welfare Notification No. 370 of 1959. (3) The fluorescent dye must not be eluted in the test method specified in Kanshoku No. 244 of 1971 and Shokuanki No. 0107001 of 2004.

[0029] Heat seal strength The test was conducted in accordance with JIS Z1707:2019 7.4 "Heat seal strength test." Two square test pieces, each 100 mm on a side, were cut out from the obtained heat seal paper, and the heat seal coating layers were brought into contact with each other. The test pieces were then pressed at a temperature of 170°C or 180°C and a pressure of 0.2 MPa (20.0 N / cm). 2 ), and heat-sealed for 1.0 second, then left to stand in an environment of 23°C and 50% humidity for 24 hours. A measurement sample with a long side of 100 mm and a short side of 15 mm was then cut out from the heat-sealed 100 mm square test piece. The peeled long edge was then clamped between the upper and lower jigs of a vertical tensile tester (Tensilon, manufactured by A&D Co., Ltd.), and the measurement sample was peeled (T-type) from the long edge at a speed of 200 mm / min. The peeled surface was visually observed and evaluated according to the following criteria. 〇: Breakage on the entire heat-sealed surface △: Partial damage on the heat-sealed surface ×: Peeling occurs between coating layers (no material breakage) -: No heat seal Glossiness The samples coated in this order with the underlayer and heat-sealable coating layer were dried and then left to stand for one day in an environment of 23°C and a relative humidity of 50%, after which the gloss was measured according to JIS P8142.

[0030] (material) base paper Nippon Paper Industries Co., Ltd., CUP-HD, basis weight 220g / m 2 thermoplastic resin PHBH: Kaneka Corporation, mass average molecular weight 600,000 EVA: Sumika Chemtex Co., Ltd., Sumika Flex S-355HQ thickener PVA1: Kuraray Co., Ltd., 22-88 PVA2: Kuraray Co., Ltd., RS-2117 PEO1: Sumitomo Seika Chemicals Co., Ltd., PEO-4, viscosity average molecular weight 1.1 million to 1.5 million PEO2: Sumitomo Seika Chemicals Co., Ltd., PEO-8, viscosity average molecular weight 1.7 million to 2.2 million PEO3: Sumitomo Seika Chemicals Co., Ltd., PEO-15, viscosity average molecular weight 3.3 million to 3.8 million surfactants Surfynol 440, manufactured by Nissin Chemical Industry Co., Ltd.

[0031] ·Heat seal paper manufacturing The thermoplastic resin and thickener were mixed and stirred in the parts by mass shown in Tables 1 and 2 to obtain a coating liquid for an under layer having a solid content concentration of 48% by mass. The thermoplastic resin, thickener, and surfactant were mixed and stirred in the parts by mass shown in Tables 1 and 2 to obtain a coating solution for a heat-sealable coating layer with a solid content of 48% by mass. On one side of the paper substrate, the undercoat coating solution is applied in a dry weight of 7 g / m 2 The coated surface was then dried at 105°C for 1 minute. A coating solution for a heat-sealable coating layer was applied to the coated surface at a dry weight of 10 g / m. 2 or 23 g / m 2 The paper was then dried at 105°C for 1 minute to obtain heat seal paper. Coating was carried out using an actual machine in the following order: coating (underlayer), drying (underlayer), coating (top layer), drying (top layer). The coating amount of the top layer was adjusted by changing the paper running speed at a constant flow rate using a curtain coater (23 g / m). 2 is 150m / min, 10g / m 2 The coating amount of the undercoat was changed to 7 g / m by changing the bar size and pressing conditions. 2 was adjusted to.

[0032] [Table 1]

[0033] [Table 2]

[0034] In Comparative Examples 1, 2, and 4, the viscosity of the coating solution for forming the heat-sealable coating layer was low, and the stability of the curtain film was poor, so the heat-sealable paper was not evaluated. In Comparative Example 3, in which PVA1 was used as the thickener, the curtain film stability was good, but the amount of elution was large. In Comparative Example 5, in which PVA2 was used as the thickener, the amount of elution was slightly reduced but still large. Furthermore, the heat-sealable papers obtained in Comparative Examples 3 and 5 had a low gloss of the coating layer of 83 or less and poor HS properties at 170°C. The heat seal paper obtained in Example 1-6, which used PEO as a thickener, had high gloss and excellent HS properties at 170°C, and also had a small amount of elution. As described above, adding PEO to the coating solution allows the viscosity of the coating solution to be maintained relatively high. After the coating solution comes into contact with the paper, it can be slowly sunk in the vertical direction perpendicular to the paper's surface, forming a thick heat-sealable layer on the surface of the paper. The heat-sealable layer remains on the paper surface, resulting in a high gloss on the surface, allowing for the production of heat-sealable paper with a luxurious feel. Furthermore, heat-sealable paper with minimal evaporation residue, a level not achievable with PVA, was obtained.

Claims

1. The paper has a base paper and a heat-sealable coating layer located on at least one of its outermost surfaces, the coating layer contains a thermoplastic resin and a thickener, A heat sealable paper characterized in that the thickener contains polyethylene oxide (PEO).

2. 2. The heat seal paper according to claim 1, wherein the viscosity average molecular weight of the PEO is 1,000,000 or more and 4,800,000 or less.

3. 3. The heat seal paper according to claim 1, wherein the thermoplastic resin is a biodegradable resin.

4. 3. The heat seal paper according to claim 1, further comprising an underlayer between the base paper and the coating layer.

5. 3. The heat seal paper according to claim 1, wherein the amount of evaporation residue in an elution test (water at 95°C for 30 minutes) in accordance with Ministry of Health and Welfare Notification No. 370 of 1959 is 25 μg / ml or less.

6. a coating step of applying a coating liquid containing a thermoplastic resin and a thickener to the outermost surface of at least one side of the base paper; a drying step of drying the coating layer; in this order, A method for producing heat-sealable paper, characterized in that the thickener contains polyethylene oxide (PEO).

7. 7. The method according to claim 6, wherein the coating step is performed by curtain coating.

8. 8. The method according to claim 6, wherein the coating liquid has a Brookfield viscosity of 70 mPa·s or more and 500 mPa·s or less.

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