JP7567973B2 - Heat seal paper - Google Patents

Description

The present invention relates to heat seal paper.

In recent years, there has been a demand to replace various plastic products with paper products in order to reduce the environmental impact. As a countermeasure, the replacement of plastics with paper has been proposed, but when processing paper into bags and containers, heat sealability is required.
Conventionally, laminated paper, in which a polyethylene film or polypropylene film is laminated to a paper base material, has been used to impart heat sealability. However, there is a problem in that it is difficult to remove the polyethylene film when recycling, making the paper less recyclable.
In order to address such problems, Patent Document 1 describes a packaging paper having at least one heat seal layer on at least one side of a paper base material, the heat seal layer containing an ionomer, a dry coating weight of the heat seal layer being 2 to 10 g/ ^m2 in total, and two or more layers of the heat seal layer being formed on at least one side, with the aim of providing a packaging paper that can reduce the amount of plastic used.

Patent No. 6580291

Heat seal paper is required to have high heat seal peel strength depending on the application, and also required to have impact resistance when used as a packaging bag. Such issues are not discussed in Patent Document 1.
An object of the present invention is to provide a heat sealable paper which has high heat seal peel strength, excellent impact resistance when used in a packaging bag, and further has an excellent pulp recovery rate after re-defibration.

The present inventors have discovered that by having a heat seal layer containing a water-dispersible resin binder on a paper base material, the paper base material having a basis weight of a predetermined value or more, and the heat seal layer having a coating amount of a predetermined amount or more, it is possible to obtain a heat seal paper that has high heat seal peel strength, excellent impact resistance when used as a package, and further has an excellent pulp recovery rate after re-disintegration.
The present invention relates to the following items <1> to <9>.
<1> A heat seal paper having a heat seal layer containing a water-dispersible resin binder on at least one surface of a paper base material, the paper base material being unbleached kraft paper having a basis weight of 110 g/m2 or ^more , and the heat seal layer having a coating weight of 7 g/m2 ^{or more} .
<2> The heat seal paper according to <1>, wherein the heat seal layer contains 3 parts by mass or more and 70 parts by mass or less of a pigment per 100 parts by mass of the water-dispersible resin binder.
<3> The heat seal paper according to <1> or <2>, wherein the heat seal layer contains 10 parts by mass or more and 30 parts by mass or less of a lubricant per 100 parts by mass of the water-dispersible resin binder.
<4> The heat sealable paper according to any one of <1> to <3>, wherein the water-dispersible resin binder is an ethylene-vinyl acetate copolymer.
<5> The heat seal paper according to any one of <1> to <4>, wherein the heat seal layer contains 50% by mass or more of an ethylene-vinyl acetate copolymer.
<6> The heat seal paper according to any one of <1> to <5>, in which two or more heat seal layers are formed on at least one surface of a paper base material.
<7> The heat seal paper according to any one of <1> to <6>, wherein the heat seal peel strength when the heat seal layers are heat sealed together under conditions of 160°C, 0.2 MPa, and 1 second is 9.0 N/15 mm or more.
<8> The heat-sealable paper according to any one of <1> to <7>, wherein the pulp recovery rate after redisintegration of the heat-sealable paper is 85% or more.
<9> A packaging bag made using the heat seal paper according to any one of <1> to <8>.

The present invention provides a heat seal paper that has high heat seal peel strength, excellent impact resistance when used in packaging bags, and excellent pulp recovery rate after redisintegration.

[Heat seal paper]
The heat seal paper of the present invention is a heat seal paper having a heat seal layer containing a water-dispersible resin binder on at least one surface of a paper base material, the paper base material being unbleached kraft paper having a basis weight of 110 g/m2 or ^more , and the coating amount of the heat seal layer being 7 g/ ^m2 or more.
According to the present invention, it is possible to provide a heat sealable paper which has high heat seal peel strength, excellent impact resistance when made into a packaging bag, and further has an excellent pulp recovery rate after re-defibration.
The detailed reasons why the above-mentioned effects are obtained are unclear, but part of the reason is considered to be as follows. In the present invention, unbleached kraft paper with a basis weight of 110 g/ ^m2 or more is used as the paper base material, and unbleached kraft paper has excellent fiber strength. In addition, since the heat seal layer contains a water-dispersible resin binder and the coating amount of the heat seal layer is 7 g/m2 or ^more , it is considered that the heat seal layer is partially compounded with the pulp fibers of the unbleached kraft paper, which is the paper base material, to obtain a heat seal paper with excellent heat seal peel strength. In addition, by using unbleached kraft paper with a high basis weight as the paper base material, the heat seal peel progresses over the entire surface, and therefore the heat seal peel strength is considered to be high. In addition, by using a paper base material with excellent paper strength, it is considered that a packaging bag made using heat seal paper having the paper base material has excellent impact resistance.
Furthermore, the heat seal layer is formed using a coating solution containing a water-dispersible resin binder, preferably a water-based coating solution, and the heat seal paper of the present invention has excellent redisintegration properties and a high pulp recovery rate. As a result, it is believed that a reusable heat seal paper with low environmental impact has been obtained.
The present invention will now be described in further detail.

<Heat seal layer>
In the present invention, the heat seal paper has a heat seal layer on at least one surface of a paper substrate.
The heat seal paper of the present invention may have heat seal layers on both sides of the paper base material. From the viewpoint of imparting heat sealability, the heat seal layer may be formed as the uppermost layer on at least one side, and two or more heat seal layers may be formed on one side.

[Water-dispersible resin binder]
In the present invention, the heat seal layer contains a water-dispersible resin binder.
The water-dispersible resin binder is not particularly limited as long as it is a water-dispersible resin and can impart heat sealability. The water-dispersible resin binder may be a dispersion-type water-dispersible resin binder or an emulsion-type water-dispersible resin binder. The water-dispersible resin binder may be used alone or in combination of two or more kinds.
The heat seal paper of the present invention is preferable because the heat seal layer contains a water-dispersible resin binder, and therefore the heat seal layer can be coated using an aqueous coating solution, thereby suppressing the problem of VOCs (Volatile Organic Compounds).Furthermore, the heat seal paper is preferable because it has an excellent pulp recovery rate after redisintegration.

In the present invention, the water-dispersible resin binder is preferably an ethylene copolymer from the viewpoint of heat sealability.
Examples of ethylene copolymers include ethylene-vinyl acetate copolymers, ethylene-aliphatic unsaturated carboxylic acid copolymers, and ethylene-aliphatic unsaturated carboxylic acid ester copolymers. Among these, ethylene-vinyl acetate copolymers are preferred from the viewpoint of obtaining heat seal paper having higher peel strength and superior impact resistance when used as a packaging bag.

(Ethylene-vinyl acetate copolymer)
The ethylene-vinyl acetate copolymer is a copolymer in which at least ethylene and vinyl acetate are copolymerized, and may further contain other monomers as the case may be. The content of structural units derived from other monomers is preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 3% by mass or less, and even more preferably 1% by mass or less of the entire copolymer.
From the viewpoint of heat seal strength, the ethylene-vinyl acetate copolymer has a ratio of ethylene in all monomers constituting the copolymer of preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 40% by mass or less, more preferably 30% by mass or less.
The glass transition temperature of the ethylene-vinyl acetate copolymer is preferably 30° C. or lower, more preferably 20° C. or lower, even more preferably 10° C. or lower, and is preferably −30° C. or higher, more preferably −20° C. or higher, even more preferably −10° C. or higher.

(Ethylene-Aliphatic Unsaturated Carboxylic Acid Copolymer)
Examples of the ethylene-aliphatic unsaturated carboxylic acid copolymer include ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, etc. The ethylene-acrylic acid copolymer and ethylene-methacrylic acid copolymer may be ionomers that are polymer aggregates by utilizing the cohesive force of cations.
Examples of the ethylene-(meth)acrylic acid copolymer ionomer include the Chemipearl series (manufactured by Mitsui Chemicals, Inc.) and the Zaixen series (manufactured by Sumitomo Seika Chemicals, Ltd.).

(Ethylene-Aliphatic Unsaturated Carboxylic Acid Ester Copolymer)
Examples of the ethylene-aliphatic unsaturated carboxylic acid ester copolymer include an ethylene-acrylic acid ester copolymer, an ethylene-methacrylic acid ester copolymer, etc. As the ethylene-aliphatic unsaturated carboxylic acid ester copolymer, an ethylene-methyl acrylate copolymer, an ethylene-ethyl acrylate copolymer, or an ethylene-methyl methacrylate copolymer is preferred in terms of versatility.

The content of the water-dispersible resin binder in the solid content of the heat seal layer is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, from the viewpoint of obtaining high heat seal peel strength, and is preferably 98% by mass or less, more preferably 95% by mass or less, even more preferably 92% by mass or less, and particularly preferably 80% by mass or less, from the viewpoint of ensuring the content of other components.
The content of the ethylene-vinyl acetate copolymer in the solid content of the heat seal layer is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, from the viewpoint of obtaining high heat seal strength, and is preferably 98% by mass or less, more preferably 95% by mass or less, even more preferably 92% by mass or less, and particularly preferably 80% by mass or less, from the viewpoint of ensuring the contents of other components.

[Pigments]
In the present invention, the heat seal layer preferably contains a pigment in addition to the water-dispersible resin binder. By containing the pigment, the problem of the heat seal layer coating surface sticking to the back surface of the heat seal paper and causing peeling (blocking) during production of the heat seal paper is suppressed, and a heat seal paper with excellent blocking resistance can be obtained.
The pigment is not particularly limited, and various pigments used in conventional pigment coating layers are exemplified. The pigment may be used alone or in combination of two or more. From the viewpoint of heat seal peel strength and blocking resistance, the pigment preferably has an aspect ratio of 20 or more. The aspect ratio of the pigment is more preferably 25 or more, even more preferably 30 or more, and particularly preferably 60 or more, and from the viewpoint of availability and smoothness of the heat seal layer surface, it is preferably 10,000 or less, more preferably 1,000 or less, and even more preferably 300 or less.
The aspect ratio of a pigment means the major axis/minor axis, and may be measured by the following method.

The pigment is preferably a layered inorganic compound with an aspect ratio of 20 or more. The layered inorganic compound has a flat plate-like shape. If the pigment is flat, the pigment is prevented from protruding from the surface of the heat seal layer, and a heat seal layer with excellent blocking resistance can be obtained while maintaining heat sealability.

The length (average particle size) of the pigment is preferably 0.1 μm or more and 100 μm or less. If the length is 0.1 μm or more, the pigment is likely to be aligned parallel to the paper base material. Also, if the length is 100 μm or less, there is little concern that a part of the pigment will protrude from the heat seal layer. The length of the pigment is more preferably 0.3 μm or more, even more preferably 0.5 μm or more, particularly preferably 1.0 μm or more, and more preferably 30 μm or less, even more preferably 20 μm or less, particularly preferably 15 μm or less.
Here, the length of the pigment contained in the heat seal layer is determined as follows. A magnified photograph is taken of the cross section of the heat seal layer using an electron microscope. At this time, the magnification is set so that about 20 to 30 pigment particles are contained within the image. The length of each pigment within the image is measured. The average value of the obtained lengths is then calculated to be the length of the pigment. The length of the pigment is sometimes expressed as particle diameter.

The pigment preferably has a thickness of 200 nm or less. The thickness of the pigment is more preferably 100 nm or less, even more preferably 80 nm or less, even more preferably 50 nm or less, and particularly preferably 30 nm or less. Also, it is preferably 5 nm or more, more preferably 10 nm or more. The smaller the average thickness of the pigment, the higher the heat seal peel strength that can be obtained. Here, the thickness of the pigment contained in the heat seal layer is determined as follows. A magnified photograph is taken of the cross section of the heat seal layer using an electron microscope. At this time, the magnification should be such that about 20 to 30 pigments are contained within the screen. The thickness of each pigment within the screen is measured. The average value of the obtained thicknesses is then calculated to be the thickness of the pigment.

Specific examples of pigments include mica, bentonite, kaolin, pyrophyllite, talc, smectite, vermiculite, chlorite, septe chlorite, serpentine, stilpnomelane, montmorillonite, heavy calcium carbonate (ground calcium carbonate), light calcium carbonate (synthetic calcium carbonate), composite synthetic pigments of calcium carbonate and other hydrophilic organic compounds, satin white, lithopone, titanium dioxide, silica, barium sulfate, calcium sulfate, alumina, aluminum hydroxide, zinc oxide, magnesium carbonate, silicates, colloidal silica, hollow or solid organic pigments, plastic pigments, binder pigments, plastic beads, and microcapsules.
Specific examples of mica include synthetic mica (e.g., swellable synthetic mica), white mica (muscovite), sericite (sericite), phlocopite, biotite, fluorine mica (artificial mica), red mica, soda mica, vanadium mica, illite, chin mica, paragonite, brittle mica, etc. Specific examples of bentonite include montmorillonite.
Specific examples of kaolin include various types of kaolin such as kaolin, calcined kaolin, structured kaolin, and delaminated kaolin.
Among these, from the viewpoints of heat seal peel strength, blocking resistance, and economy, pigments having an aspect ratio of 20 or more are preferred, and it is more preferred to contain one or more of mica, bentonite, kaolin, and talc, with kaolin being even more preferred.

In the present invention, from the viewpoint of blocking resistance, the content of the pigment relative to 100 parts by mass of the water-dispersible resin binder in the heat seal layer is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, and even more preferably 8 parts by mass or more. Also, from the viewpoint of obtaining high heat seal peel strength, the content of the pigment having an aspect ratio of 20 or more relative to 100 parts by mass of the water-dispersible resin binder is preferably 70 parts by mass or less, more preferably 60 parts by mass or less, even more preferably 40 parts by mass or less, and particularly preferably 20 parts by mass or less.
The water-dispersible resin binder means a solid content amount, and specifies a content of a binder having an aspect ratio of 20 or more relative to 100 parts by mass of the solid content of the water-dispersible resin binder.

[Lubricant]
The heat seal layer preferably contains a lubricant in addition to the above-mentioned water-dispersible resin binder and pigment. By containing a lubricant, a heat seal layer with better blocking resistance can be obtained. In addition, by containing a lubricant, the slipperiness of the surface is improved, and the stain resistance is also improved, which is preferable.
Specific examples of the lubricant include hydrocarbon-based lubricants such as paraffin and wax, fatty acid-based lubricants such as higher fatty acids and oxy fatty acids, fatty acid amide-based lubricants such as fatty acid amides and alkylene bis fatty acid amides, ester-based lubricants such as fatty acid lower alcohol esters, fatty acid polyhydric alcohol esters, and fatty acid polyglycol esters, alcohol-based lubricants such as fatty alcohols, polyhydric alcohols, polyglycols, and polyglycerols, metal soaps, and mixed lubricants. These lubricants may be naturally derived or synthetic, and may be used alone or in combination of two or more. Among these, from the viewpoint of dispersibility in the heat seal layer, hydrocarbon-based lubricants are preferred, polyolefin waxes are more preferred, polyethylene waxes and polypropylene waxes are even more preferred, and low molecular weight polyethylene waxes and low molecular weight polypropylene waxes are even more preferred. In addition, in order to make the lubricant into a solution or dispersion, a solubilizer, a dispersant, or an emulsifier may be used in combination.
As the lubricant, commercially available products may be used, and specific examples thereof include low molecular weight polyethylene wax dispersions (manufactured by Mitsui Chemicals, Inc., Chemipearl W-310, Chemipearl W-410, and Chemipearl W-800).

The content of the lubricant in the heat seal layer is preferably 10 parts by mass or more, more preferably 14 parts by mass or more, and even more preferably 18 parts by mass or more, per 100 parts by mass of the water-dispersible resin binder, from the viewpoint of improving blocking resistance, surface slipperiness, and stain resistance, and is preferably 30 parts by mass or less, more preferably 26 parts by mass or less, and even more preferably 22 parts by mass or less, from the viewpoint of maintaining a high heat seal peel strength.

In the present invention, the heat seal layer may contain other components in addition to the above-mentioned water-dispersible resin binder, pigment, and lubricant.
Examples of other components include antifoaming agents; viscosity modifiers; leveling agents such as surfactants and alcohols; and colorants such as coloring dyes.
Since these other components tend to deteriorate the heat sealability, the total content of the other components is preferably 30 mass % or less, more preferably 10 mass % or less, and even more preferably 5 mass % or less, of the solid content of the heat seal layer.

[Method of forming heat seal layer]
The heat seal layer can be obtained by preparing a coating liquid for the heat seal layer, which contains at least a water-dispersible resin binder, and applying this to at least one surface of the paper substrate.
The coating liquid for the heat seal layer may be a self-emulsifying dispersion, and may contain an emulsifier such as a surfactant and a dispersant.

The method for applying the coating liquid for the heat seal layer is not particularly limited, and may be appropriately selected from commonly used coating devices, such as air knife coaters, blade coaters, gravure coaters, rod blade coaters, roll coaters, reverse roll coaters, bar coaters, curtain coaters, die slot coaters, champlex coaters, metering blade type size press coaters, short dwell coaters, spray coaters, gate roll coaters, and lip coaters.
In addition, after the coating liquid is applied and before drying, it is preferable to smooth the air bubbles generated in the coated coating film and the traces of the air bubbles broken by a smoothing bar. Specifically, after the coating liquid is applied and before it reaches the drying device, a smoothing bar is provided so as to be in contact with the surface of the coating film. The smoothing bar is provided so as to be in contact with the surface of the coating film by rotating it as necessary. This makes it possible to smooth the surface of the heat seal layer, and also to suppress the occurrence of pinholes, etc., and to obtain high heat seal strength. In addition, the orientation of the pigment is improved, and the detachment of the pigment from the surface of the heat seal layer can also be suppressed.

[Preferred embodiment of heat seal layer]
The heat seal paper of the present invention may have a heat seal layer on at least one side of the paper base material, or may have a heat seal layer on both sides of the paper base material. From the viewpoint of imparting heat sealability, the heat seal layer is preferably provided at least as the uppermost layer.

The heat seal paper has a heat seal layer on at least one side, and preferably has two or more heat seal layers formed on at least one side of the paper base material. By forming two or more heat seal layers, the occurrence of pinholes and the like is suppressed, and a smooth surface is easily obtained. As a result, a heat seal paper with few defects is obtained.
In addition, when there are two or more heat seal layers, it is preferable that the two or more heat seal layers are provided as layers that are in direct contact with each other, and it is preferable that the uppermost seal layer constitutes the uppermost layer of the heat seal paper.
It is preferable that the heat seal layer is formed in two or more layers, and from the viewpoint of ease of production and the effects of having multiple layers, it is preferably five layers or less, more preferably four layers or less, even more preferably three layers or less, and it is most preferable that it is two layers.

The coating amount of the heat seal layer is preferably 7 g/m2 or ^more , more preferably 7.5 g/m2 or more, and is preferably 20 g/m2 ^{or less} , more preferably 15 g/m2 ^{or less} , and even more preferably 10 g/m2 ^{or less} , from the viewpoint of obtaining a heat seal paper having excellent heat seal peel strength and excellent impact resistance when made into a packaging ^bag .
The coating amount of the heat seal layer is the coating amount in solid content, and when the heat seal paper has two or more heat seal layers, it means the total coating amount.
When the heat seal film has two heat seal layers, the coating amount of the lower layer is preferably equal to or greater than the coating amount of the upper layer, and more preferably the coating amount of the lower layer is equal to the coating amount of the upper layer.

<Other demographics>
In the present invention, the heat seal paper may have, for example, an undercoat layer between the paper substrate and the heat seal layer in addition to the heat seal layer. The undercoat layer can be provided for the purpose of improving the adhesion between the paper substrate and the heat seal layer, or for the purpose of imparting water resistance to the heat seal paper. That is, when the heat seal paper has an undercoat layer, it is preferable that the heat seal paper has the undercoat layer and the heat seal layer in this order.

<Paper base material>
The heat seal paper of the present invention has a heat seal layer on the uppermost layer of at least one surface of a paper substrate.
In the present invention, unbleached kraft paper having a basis weight of 110 g/m ² or more is used as the paper base material.
Unbleached kraft paper uses unbleached pulp, and examples of unbleached pulp include unbleached softwood pulp (NUKP) and unbleached hardwood pulp (LUKP). These pulps may be used alone or in combination of two or more.

The degree of beating of the pulp is not particularly limited, but the Canadian Standard Freeness (CSF) of the disintegrated pulp is preferably 300 mL or more and 800 mL or less, more preferably 400 mL or more and 750 mL or less, and even more preferably 500 mL or more and 700 mL or less. If the CSF of the pulp is within the above range, the necessary paper strength is easily obtained when the pulp is made into a packaging bag, and a packaging bag having excellent impact resistance can be obtained.
The CSF of disintegrated pulp is measured in accordance with JIS P 8121-2:2012 "Pulp - Freeness test method - Part 2: Canadian standard freeness method" for pulp obtained by disintegrating a paper base material in accordance with JIS P 8220-1:2012.

The kappa number of the disintegrated pulp is preferably from 20 to 80, more preferably from 25 to 70, and even more preferably from 30 to 60. When the kappa number is within the above range, the pulp has excellent impact resistance when made into a packaging bag, and is also excellent in processability into a packaging bag, which is preferable.
The kappa number of disintegrated pulp is measured in accordance with JIS P 8211:2011 for pulp obtained by disintegrating a paper base material in accordance with JIS P 8220-1:2012.

Examples of additives to paper base materials include pH adjusters (sodium bicarbonate, sodium hydroxide, etc.), dry strength agents (polyacrylamide, starch, etc.), wet strength agents (polyamide polyamine epichlorohydrin resin, melamine-formaldehyde resin, urea-formaldehyde resin), internal sizing agents (rosin-based, alkyl ketene dimer, etc.), drainage retention aids, antifoaming agents, fillers (calcium carbonate, talc, etc.), dyes, etc. These additives may be used alone or in combination of two or more.
The content of the additives is not particularly limited and may be within a range that is usually used.

The basis weight of the paper base material is 110 g/m2 or ^more , preferably 115 g/m2 ^{or more} , and more preferably 120 g/m2 or ^more , from the viewpoint of forming a heat seal paper having excellent impact resistance when made into a packaging bag. The upper limit is not particularly limited, but from the viewpoint of processability and manufacturing when made into a packaging bag, it is preferably 200 g/m2 or ^less , more preferably 180 g/m2 ^{or less} , even more preferably 160 g/m2 ^{or less} , and particularly preferably 150 g/m2 or ^less .
The basis weight of the paper substrate is measured in accordance with JIS P 8124:2011.

From the viewpoints of processability and production, the thickness of the paper base material is preferably 100 μm or more, more preferably 110 μm or more, and is preferably 250 μm or less, more preferably 230 μm or less.
The thickness of the paper base material is measured in accordance with JIS P 8118:2014.

[Method of manufacturing paper base material]
The method for producing the paper base material includes a method of making a paper stock containing pulp. The paper stock may further contain additives. Examples of the additives include the additives listed above.
The stock can be prepared by adding additives to the pulp slurry.
The pulp slurry can be obtained by beating pulp in the presence of water. The method and apparatus for beating the pulp are not particularly limited, and may be the same as known beating methods and apparatuses.
The content of pulp in the stock is not particularly limited and may be within a range that is usually used, for example, 60% by mass or more and less than 100% by mass based on the total mass of the stock.

The papermaking process can be carried out by a standard method. For example, the papermaking process can be carried out by casting the paper onto a wire or the like, dehydrating the paper to obtain a wet paper, stacking multiple wet papers as necessary, pressing the single-layer or multi-layer wet paper, and drying the resulting paper. 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).

<Physical properties of heat seal paper>
The heat seal paper of the present invention has heat sealability.
The conditions for heat sealing are not particularly limited, but the temperature during heat sealing is, for example, preferably 60° C. to 300° C., more preferably 70° C. to 200° C., and even more preferably 80° C. to 180° C. The pressure during heat sealing is, for example, preferably 0.05 MPa to 10 MPa, and more preferably 0.1 MPa to 2 MPa. The pressurization time is, for example, preferably 0.1 seconds to 15 seconds, and more preferably 0.2 seconds to 5 seconds.

From the viewpoint of excellent heat seal peel strength, the heat seal paper of the present invention has a heat seal peel strength when the heat seal layers are heat sealed together under conditions of 160°C, 0.2 MPa, and 1 second of preferably 9.0 N/15 mm or more, more preferably 9.5 N/15 mm or more, and even more preferably 10.0 N/15 mm or more. There is no particular upper limit, but from the viewpoint of ease of achievement, it is preferably 20.0 N/15 mm or less, more preferably 18.0 N/15 mm or less, and even more preferably 15.0 N/15 mm or less.
The peel strength is measured by the method described in the Examples.

The heat seal paper of the present invention has an excellent pulp recovery rate by re-disintegration, and the pulp recovery rate after re-disintegration of the heat seal paper is preferably 85% or more, more preferably 90% or more, even more preferably 95% or more, and particularly preferably 98% or more. There is no particular upper limit, and it is 100% or less. When the pulp recovery rate after re-disintegration is within the above range, the paper is excellent in recyclability and excellent in reducing the environmental load.
The heat-sealable paper of the present invention has a heat-sealable layer formed on a paper base material, preferably by coating, and the coating liquid is preferably an aqueous coating liquid, so that the pulp recovery rate after re-disintegration is significantly superior to cases in which a resin film is laminated.
The recovery rate of the pulp after redefibration is measured by the method described in the Examples.

The heat seal paper of the present invention has heat sealability and excellent processability, and is therefore particularly suitable for packaging bags, and is particularly suitable for packaging bags that require impact resistance.
More specifically, it is suitable for use as a packaging bag when the mass of the contents is large or for use in transportation, etc.

The features of the present invention are explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, processing contents, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below.

The raw materials used in the examples and comparative examples are as follows.
[Ethylene-vinyl acetate copolymer]
S-470HQ: Ethylene-vinyl acetate copolymer (Sumikaflex S-470HQ, manufactured by Sumika Chemtex Co., Ltd., solid content 55%)
[Lubricant]
W-310: Low molecular weight polyethylene wax dispersion (Mitsui Chemicals, Inc., Chemipearl W-310, solid content 38.5%)
[Pigment]
Kaolin A: average particle size 8 μm, aspect ratio 80 to 100
Kaolin B: average particle size 1.5 μm, aspect ratio 30

[Example 1]
<Preparation of Heat Seal Layer Coating>
182 parts of ethylene-vinyl acetate copolymer (Sumikaflex S-470HQ, solids content 55%, manufactured by Sumika Chemtex Co., Ltd.), 52 parts of low molecular weight polyethylene wax dispersion (Chemipearl W-310, solids content 38.5%, manufactured by Mitsui Chemicals, Inc.), and 20 parts of a 50% aqueous dispersion of kaolin A (average particle size 8 μm) were mixed, water was added to the mixture so that the solids content became 40%, and the mixture was stirred to obtain a heat seal layer coating material (concentration 40%).
<Production of heat seal paper>
The obtained heat seal layer coating material was applied to unbleached kraft paper with a basis weight of 120 g/ ^m2 (manufactured by Oji Materia Co., Ltd., thickness 170 μm, CSF of disintegrated pulp 630 mL) using a gravure coater (using a smoothing bar) so that the coating amount of the heat seal layer after drying would be 4 g/ ^m2 , forming a first heat seal layer. Thereafter, the same surface was again coated with a gravure coater (using a smoothing bar) so that the coating amount of the heat seal layer after drying would be 4 g/ ^m2 , forming a second heat seal layer.

[Example 2]
A heat seal paper was obtained in the same manner as in Example 1, except that the coating amount of the first heat seal layer was 8 g/ ^m2 and the second heat seal layer was not coated.

[Example 3]
A heat seal paper was obtained in the same manner as in Example 1, except that the amount of W-310 was changed to 0 parts.

[Example 4]
A heat seal paper was obtained in the same manner as in Example 1, except that the substrate was changed to unbleached kraft paper having a basis weight of 130 g/ ^m2 (manufactured by Oji Materia Co., Ltd., thickness 200 μm, CSF of disintegrated pulp 630 mL).

[Example 5]
A heat seal paper was obtained in the same manner as in Example 4, except that kaolin A was changed to kaolin B (average particle size: 1.5 μm).

[Example 6]
A heat seal paper was obtained in the same manner as in Example 1, except that the substrate was changed to unbleached kraft paper having a basis weight of 150 g/ ^m2 (manufactured by Oji Materia Co., Ltd., thickness 230 μm, CSF of disintegrated pulp 630 mL).

[Comparative Example 1]
A heat seal paper was obtained in the same manner as in Example 1, except that the substrate was changed to unbleached kraft paper having a basis weight of 100 g/ ^m2 (manufactured by Oji Materia Co., Ltd., thickness 140 μm, CSF of disintegrated pulp 630 mL).

[Comparative Example 2]
A heat seal paper was obtained in the same manner as in Example 1, except that the coating amount of the first heat seal layer was changed to 3 g/m ² and the coating amount of the second heat seal layer was changed to 3 g/m ² .

[Comparative Example 3]
A heat seal paper was obtained in the same manner as in Example 1, except that the coating amount of the first heat seal layer was changed to 2 g/m ² and the coating amount of the second heat seal layer was changed to 2 g/m ² .

[Comparative Example 4]
A heat seal paper was obtained in the same manner as in Example 1, except that the substrate was changed to bleached kraft paper having a basis weight of 120 g/ ^m2 (manufactured by Oji Materia Co., Ltd., thickness 170 μm, CSF of disintegrated pulp 630 mL).

[Comparative Example 5]
A low-density polyethylene (Nippon Polyethylene Co., Ltd., Novatec LD LC600A) was extrusion laminated on unbleached kraft paper (Oji Materia Co., Ltd., thickness 170 μm, CSF of disintegrated pulp 630 mL) with a basis weight of 120 g/ ^m2 using an extrusion laminator at a T-die temperature of 320° C. and a processing speed of 80 m/min so that the coating amount was 19 g/ ^m2, to obtain a heat seal paper.

[Measurement and Evaluation]
<Basis weight and thickness of paper base material>
The basis weight of the paper base material was measured in accordance with JIS P 8124: 2011. The thickness of the paper base material was measured in accordance with JIS P 8118: 2014.

<CSF of disintegrated pulp of paper base material>
The CSF of the disintegrated pulp of the paper base material was measured by disintegrating the paper base material in accordance with JIS P 8220-1:2012, and measuring the CSF of the resulting disintegrated pulp in accordance with JIS P 8121-2:2012 "Pulp-Freeness test method-Part 2: Canadian standard freeness method."

<Measurement of heat seal peel strength>
A pair of heat seal papers was stacked so that the heat seal layers faced each other, and heat sealed using a heat seal tester (TP-701-B, manufactured by Tester Sangyo) under conditions of 160°C, 0.2 MPa, and 1 second. Subsequently, the heat seal peel strength was measured in accordance with the description of JIS Z 0238:1998. Specifically, the heat sealed test piece was cut to a width of 15 mm, and T-peeled using a tensile tester at a tensile speed of 300 mm/min, and the recorded maximum load was taken as the heat seal peel strength.

<Evaluation of redefibration property (pulp recovery rate after redefibration)>
A 30 g bone dry heat seal paper was torn into 3-4 cm squares by hand and soaked in tap water at 20° C. overnight. The heat seal paper was diluted to a concentration of 2.5%, and then disintegrated for 20 minutes at 3,000 rpm using a TAPPI standard disintegrator (Kumagaya Riki Co., Ltd.). The resulting pulp slurry was passed through a flat screen (Kumagaya Riki Co., Ltd.) equipped with a 6-cut (slit width: 0.15 mm) screen plate, and refined in a water flow of 8.3 L/min. The undisintegrated material remaining on the screen plate was collected and dried in an oven at 105° C., and the weight was measured, and the weight was calculated using the following formula:
Pulp recovery rate (%)={bone dry weight (g) of heat seal paper used in the test−bone dry weight (g) of undisintegrated material}/bone dry weight of heat seal paper used in the test×100
The pulp recovery rate was calculated from the above.

<Evaluation of impact resistance>
A heat seal paper cut to 400 mm long x 760 mm wide was folded in half with the heat seal layer surfaces facing each other to 400 mm long x 380 mm wide, and 3 kg of gravel was placed inside and sealed with an impulse sealer (VG-400, manufactured by Fuji Impulse Co., Ltd.) with a seal width of 10 mm to produce five three-sided sealed bags. The three-sided sealed bags were dropped from a height of 60 cm onto a concrete floor in the order of "1: bottom corner → 2: bottom → 3: side → 4: side → 5: top → 6: front surface → 7: back surface," and the state of damage to the bags was visually evaluated according to the following criteria.
A: The above items 1 to 7 are considered as one set. Even if two sets are dropped, the bag will not be damaged (all five bags are two sets and will not be damaged)
B: The above items 1 to 7 are considered as one set. No bag will be damaged within one set, but some bags will be damaged within two sets (more than one bag out of five will be damaged within two sets).
C: The above items 1 to 7 are considered as one set, and within one set, there is a bag that is damaged (one to four bags out of five bags are damaged within one set)
D: The above steps 1 to 7 are considered as one set, and all five bags are damaged within one set.

As shown in Examples 1 to 6, when the heat seal layer had a heat seal layer containing a water-dispersible resin binder, the paper base material was unbleached kraft paper with a basis weight of 110 g/m2 or ^more , and the heat seal layer had a coating weight of 7 g/m2 or ^more , the heat seal peel strength was high, and when made into a packaging bag, the heat seal paper had excellent impact resistance and further had an excellent pulp recovery rate after re-disintegration.
On the other hand, in Comparative Example 1, in which the basis weight of the paper base material was less than 110 g/ ^m2 , when it was made into a packaging bag, sufficient heat seal peel strength was not obtained, and the impact resistance was also poor.
In addition, in Comparative Example 2 (coating amount = 6 g/ ^m2 ) and Comparative Example 3 (coating amount = 4 g/ ^m2 ), in which the coating amount of the heat seal layer was less than 7 g/ ^m2 , sufficient heat seal peel strength was not obtained, and when made into a packaging bag, the impact resistance was poor.
In Comparative Example 4, in which bleached kraft paper was used as the paper base material instead of unbleached kraft paper, even though the basis weight was 110 g/m2 ^or more and the coating amount of the heat seal layer was 7 g/m2 ^{or more} , sufficient heat seal peel strength was not obtained and the impact resistance was poor when used as a packaging bag.
Furthermore, in Comparative Example 5 in which a low-density polyethylene (LDPE) film was laminated to unbleached kraft paper having a basis weight of 110 g/ ^m2 or more, pulp could not be recovered after re-defibration.

According to the present invention, a heat-sealable paper having excellent heat-seal peel strength and an excellent pulp recovery rate after re-defibration can be obtained, and further, a heat-sealable paper having excellent impact resistance for packaging bags made from the heat-sealable paper can be obtained.
The heat seal paper of the present invention is particularly suitable for use in various packaging bags which require impact resistance.

Claims (7)

The heat seal paper has a heat seal layer containing a water-dispersible resin binder and a pigment on at least one surface of a paper base material,
the pigment comprises a pigment having an aspect ratio of 20 or more;
The paper base material is unbleached kraft paper having a basis weight of 110 g/m2 ^{or more} ,
the water-dispersible resin binder comprises an ethylene-vinyl acetate copolymer;
The pigment comprises kaolin,
The heat seal layer contains 50% by mass or more of an ethylene-vinyl acetate copolymer,
The heat seal layer contains 3 parts by mass or more and 20 parts by mass or less of a pigment relative to 100 parts by mass of a water-dispersible resin binder,
The coating amount of the heat seal layer is 7 g / m ² or more.
Heat seal paper. 2. The heat seal paper according to claim 1 , wherein the heat seal layer contains 10 parts by mass or more and 30 parts by mass or less of a lubricant per 100 parts by mass of the water-dispersible resin binder. 3. The heat-sealable paper according to claim 1 , wherein two or more heat-sealable layers are formed on at least one surface of a paper substrate. The heat-sealable paper according to any one of claims 1 to 3, which has the heat-sealable layer as an uppermost layer on at least one surface of a paper substrate. The heat seal paper according to any one of claims 1 to 4 , wherein the heat seal peel strength when the heat seal layers are heat sealed together under conditions of 160°C, 0.2 MPa, and 1 second is 9.0 N/15 mm or more. 6. The heat-sealable paper according to claim 1 , wherein the pulp recovery rate after redisintegration of the heat-sealable paper is 85% or more. A packaging bag made using the heat seal paper according to any one of claims 1 to 6 .