JP7706532B2 - Packaging substrate, packaging material, and method for manufacturing packaging material - Google Patents

Description

The present invention relates to a packaging substrate, a packaging material having the packaging substrate, and a method for manufacturing the packaging material.

Traditionally, plastic materials have been used primarily for packaging containers such as food trays and packaging bodies such as pillowcase bags. However, in recent years, there has been a growing trend to move away from plastic, sparked by environmental issues such as the problem of plastic waste in the ocean, and there is a desire to reduce the amount of resin materials used in industrial products as much as possible. In light of this trend, consideration is being given to using paper as a packaging material to reduce the environmental impact of packaging bodies as well.

Patent Document 1 proposes a heat seal paper that is environmentally friendly, has excellent water resistance and suppresses cracks caused by folds in the area where the heat seal layer is laminated, and has a paper base mainly composed of unbleached softwood kraft pulp and a heat seal layer laminated on one side of the paper base, and has a folding endurance in the machine direction of the paper base of 500 to 1,000 times and a folding endurance in the width direction of 80 to 200 times, a Beck smoothness of the surface of the heat seal layer laminated side of ³ to ²³⁰ seconds, a main component of the heat seal layer is an alkali neutralized ethylene (meth)acrylic acid copolymer, and a coating amount of the heat seal layer of 1.5 to 10.0 g/m2.

Patent Document 2 proposes a heat seal paper that has a paper base material whose main component is pulp and a heat seal layer laminated on one side of the paper base material, the main component of the heat seal layer being an alkali neutralized ethylene (meth)acrylic acid copolymer, the Beck smoothness of the surface of the heat seal layer laminated on the paper base material is 5 seconds or more and 50 seconds or less, the Parker Print Surf smoothness is 3.8 μm or more and 8.0 μm or less, the Beck smoothness of the surface of the paper base material not laminated with the heat seal layer is 50 seconds or more and 500 seconds or less, and the Parker Print Surf smoothness is 2.0 μm or more and 4.2 μm or less, and the heat seal paper does not contain laminated paper or plastic film that has a high environmental load, and has excellent water resistance and printability of the heat seal layer non-laminated surface.

Patent Document 3 proposes a heat seal paper having one or more heat seal layers on at least one side of a paper base material, the heat seal layer containing a water-dispersible resin binder, and having an ISO stiffness of 0.55 mNm or less in the longitudinal direction of the heat seal paper measured in accordance with ISO 2493-1:2010, an ISO stiffness of 0.45 mNm or less in the transverse direction of the heat seal paper, and a puncture strength of 7.5 N or more measured in accordance with JIS Z 1707:2019, the heat seal paper having drop impact resistance, excellent flexibility, and excellent heat sealability.

Patent Document 4 proposes a flexible packaging paper having a paper base material and a heat seal layer provided on at least one side of the paper base material, in which the paper base material has a filler content of 1 mass % or less, a basis weight of ²⁵ g/m2 to 50 g/ ^m2 , and a density of 0.85 g/ ^m3 to 1.35 g/ ^m3 , the dry mass (per side) of the heat seal layer is 4 g/ ^m2 to 20 g/ ^m2 , and the flexible packaging paper has a tear strength (MD direction and CD direction) of 100 mN to 650 mN, which allows the flexible packaging paper to be easily torn in any direction and allows the packaged item to be easily removed when used as a flexible package.

Patent Document 5 proposes a heat seal sheet that includes a paper base material and a thermal adhesive layer that is provided on the outermost surface of at least one side of the paper base material and contains 50% by mass or more of an organic material, where the thermal adhesive layer contains styrene-butadiene copolymer latex as at least a part of the organic material, and the thermal adhesive surface on which the thermal adhesive layer is provided has an Oken smoothness of 50 seconds or more as measured in accordance with JIS P 8155:2010, has breathability and moisture permeability, generates little dust, and is recyclable.

JP 2023-107689 A JP 2023-107678 A Patent No. 7243901 JP 2023-81594 A JP 2018-053400 A

Packaging materials using heat-sealing paper have been proposed as an alternative to packaging materials using plastic films. However, since paper is less stretchable than plastic films, packaging materials using heat-sealing paper have inferior cushioning properties. Furthermore, since paper is less flexible than plastic films, it is not possible to package contents with complex shapes according to the shape of the contents.
An object of the present invention is to provide a packaging substrate having an excellent balance between cushioning properties and flexibility, a packaging material using the packaging substrate, and a method for producing the packaging material.

The means for solving the problems of the present invention are as follows.
1. A packaging substrate comprising natural fibers,
Basis weight 18g/m2 or ^more and 50g/ ^m2 or less,
A packaging substrate having a tensile strength C/M ratio of 60% or more, a tear strength of 450 mN or more in MD and 650 mN or more in CD.
2. The packaging substrate according to 1., wherein the density of the packaging substrate is 0.2 g/ ^cm3 or more and 0.5 g/ ^cm3 or less.
3. The natural fibers include pulp;
3. The packaging substrate according to 1 or 2, characterized in that it contains 50% by mass or more of non-wood pulp having an average fiber length of 2.5 mm or more, an average fiber diameter of 15 μm to 25 μm, and a Canadian Standard Freeness of 650 ml CSF or more, based on the total amount of pulp.
4. The packaging substrate according to any one of 1. to 3., wherein the packaging substrate contains abaca pulp in an amount of 50% by mass or more based on the total fibers.
5. A packaging substrate according to any one of 1. to 4., having a heat seal layer on at least one surface thereof,
A packaging material having a heat seal strength of 0.5 N/15 mm or more when heat sealed at a temperature of 130° C., a pressure of 1.0 kgf/cm ² for 1 second, and peeled off with a T-shaped strip at a tensile speed of 30 mm/min.
6. A method for producing a packaging material, comprising applying a coating liquid containing a thermoplastic resin onto the packaging substrate according to any one of 1. to 4. by a gravure method or a flexographic method to form a heat seal layer.
7. The method for producing a packaging material according to 6., wherein a contact angle between the packaging substrate and the coating liquid is 70° or more and 120° or less.

According to the present invention, it is possible to provide a packaging base material and a packaging material that combine cushioning properties and flexibility. A packaging material with excellent cushioning properties can protect the contents from shaking during transportation and from external impacts. A packaging material with excellent flexibility can be packaged to match the shape of the contents. Therefore, the packaging material of the present invention is suitable for packaging contents that are vulnerable to impacts and have complex shapes, such as electronic components.

Graph for explaining a method for calculating compressive strength from the measurement results of a KES-G5 compression tester.

"Packaging base material"
The packaging substrate of the present invention has a basis weight of 18 g/m ² or more and 50 g/m ² or less. The packaging substrate of the present invention has a basis weight of 18 g/m ² or more and 50 g/m ² or less, so that it can achieve both cushioning and flexibility at a high level. In the packaging substrate of the present invention, when emphasis is placed on cushioning, the basis weight is preferably 20 g/m ² or more, more preferably 22 g/m ² or more. In addition, when emphasis is placed on flexibility, the basis weight is preferably 40 g/m ² or less, more preferably 35 g/m ² or less, and even more preferably 30 g/m ² or less.

The packaging substrate of the present invention preferably has a density of 0.2 g/cm ³ or more and 0.5 g/cm ³ or less. If the density is less than 0.2 g/cm ³ , the bulkiness is high and the cushioning property is excellent, but the flexibility may decrease. If the density is more than 0.5 g/cm ³ , the flexibility is excellent, but the packaging material may have poor cushioning property. In the packaging substrate of the present invention, when the cushioning property is important, the density is preferably 0.4 g/cm ³ or less, more preferably 0.3 g/cm ³ or less. In addition, when the flexibility is important, the density is preferably 0.25 g/cm ³ or more, more preferably 0.3 g/cm ³ or more.

The packaging substrate of the present invention contains natural fibers, and for example, paper and wet-laid nonwoven fabric can be suitably used. The packaging substrate of the present invention may contain natural fibers, and may also contain resin fibers other than natural fibers. However, from the viewpoint of reducing the amount of resin used in the packaging substrate of the present invention, the ratio of natural fibers to all fibers is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 92% by mass or more, even more preferably 94% by mass or more, even more preferably 96% by mass or more, even more preferably 98% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass. In addition, when resin fibers are included, it is preferable to use biodegradable resin fibers made of a biodegradable resin, such as polylactic acid fibers, polycaprolactone fibers, polybutylene succinate fibers, polyethylene succinate fibers, polyvinyl alcohol fibers, polyglycolic acid fibers, poly(caprolactone/butylene succinate) fibers, poly(butylene succinate/adipate) fibers, and 3-hydroxybutyrate-co-3-hydroxyhexanoate fibers.

As the natural fiber, fibers derived from plants, microorganisms, and animals can be used without any particular restrictions. Specifically, examples include natural cellulose such as pulp, cellulose produced by microorganisms such as acetic acid bacteria, mercerized pulp obtained by treating cellulose with a high concentration of alkali, regenerated cellulose obtained by dissolving cellulose in a solvent such as a cuprammonium solution or a morpholine derivative and then reprecipitating, cellulose-based fibers such as various cellulose derivatives such as acetylated modified cellulose and carboxylated modified cellulose, and animal hair such as silk, wool, and goat hair, and one or more of these can be used. As the natural fiber, wood pulp and non-wood pulp can be preferably used.

The average fiber length of the pulp contained in the packaging substrate of the present invention is preferably 2.5 mm or more, more preferably 3.0 mm or more, and even more preferably 3.5 mm or more. If the average fiber length is less than 2.5 mm, the packaging substrate becomes flexible, but the packaging substrate becomes dense and tight, which may reduce the cushioning properties.
The average fiber width of the pulp contained in the packaging substrate of the present invention is preferably 15 μm or more and 30 μm or less. The average fiber width is a factor that determines the cushioning properties of the substrate. If the average fiber width is narrow, the fiber itself is soft but the substrate becomes tight, resulting in inferior cushioning properties, whereas if the average fiber width is wide, the fiber itself becomes difficult to bend, resulting in inferior flexibility.
In this specification, the average fiber length and the average fiber width refer to the length-weighted average fiber length and the length-weighted average fiber width, respectively, and can be measured by observing the fibers using, for example, an image analyzer such as a Fiber Tester manufactured by ABB Corporation, a Fractionator manufactured by Valmet Ltd., an FS5 manufactured by Valmet Ltd., or a Morfi manufactured by Voith Turbo K.K., an optical microscope, an electron microscope, or the like.

The degree of beating of the pulp contained in the packaging substrate of the present invention is preferably 650 ml CSF or more, more preferably 670 ml CSF or more, even more preferably 690 ml CSF or more, and most preferably unbeaten. As the pulp is beaten, the density increases and the substrate becomes softer, but the cushioning properties tend to decrease.

The packaging base material of the present invention preferably contains 50% by mass or more of non-wood pulp having an average fiber length of 2.5 mm or more, an average fiber diameter of 15 μm to 25 μm, and a Canadian Standard Freeness of 650 ml CSF or more, based on the total amount of pulp. By containing 50% by mass or more of such non-wood pulp, it is easy to obtain a packaging base material that is low in density and has excellent cushioning properties and flexibility.
The average fiber length of the non-wood pulp is more preferably 2.8 mm or more, even more preferably 3.2 mm or more, and even more preferably 3.6 mm or more.
The average fiber width of the non-wood pulp is more preferably 17 μm or more, even more preferably 19 μm or more, and more preferably 23 μm or less.
The Canadian Standard Freeness of the non-wood pulp is more preferably 670 ml CSF or more, even more preferably 690 ml CSF or more, and most preferably unbeaten.

In the present invention, the amount of non-wood pulp that satisfies the above-mentioned average fiber length, average fiber width, and Canadian Standard Freeness relative to the total fibers is more preferably 60% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 93% by mass or more, even more preferably 96% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass.
In the present invention, the amount of non-wood pulp satisfying the above-mentioned average fiber length, average fiber width, and Canadian Standard Freeness relative to the total pulp is more preferably 70% by mass or more, even more preferably 90% by mass or more, even more preferably 93% by mass or more, even more preferably 96% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass.

In the present invention, examples of wood pulp include chemical pulps such as bleached kraft pulp (BKP), unbleached kraft pulp (UKP), semi-bleached kraft pulp (SBKP), and sulfite pulp, which use coniferous and/or broad-leaved trees as the material; mechanical pulps such as stone ground pulp (SGP), pressed stone ground pulp (TGP), chemi-ground pulp (CGP), groundwood pulp (GP), and thermomechanical pulp (TMP); dissolving pulp; and mercerized pulp. One or more of these may be used.
In the present invention, examples of non-wood pulp include pulps produced from non-wood materials such as bast fibers such as flax (linen), western hemp (kenaf), jute, paper mulberry, and mitsumata, hard fibers such as bagasse, bamboo, and esparto, seed hair fibers such as cotton (linter), and leaf sheath/leaf fibers such as Manila hemp (abaca) and sisal, and one or more of these can be used.

Among these, the packaging base material of the present invention preferably contains abaca pulp (Manila hemp pulp). By blending abaca pulp, it becomes easy to obtain a packaging base material having excellent cushioning properties and flexibility.
In the present invention, the amount of abaca pulp relative to the total fibers is preferably 50% by mass or more, more preferably 60% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, even more preferably 93% by mass or more, even more preferably 96% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass.
In the present invention, the amount of abaca pulp relative to the total pulp is more preferably 70% by mass or more, even more preferably 90% by mass or more, even more preferably 93% by mass or more, even more preferably 96% by mass or more, even more preferably 99% by mass or more, and most preferably 100% by mass.

The method for making the packaging substrate is not particularly limited, and can be performed using a Fourdrinier paper machine, a cylinder paper machine, a short-wire paper machine, an inclined short-wire paper machine, etc. Among these, it is preferable to use a short-wire paper machine, an inclined short-wire paper machine, or a Fourdrinier paper machine, which can reduce fiber orientation and increase the C/M ratio (horizontal/vertical ratio) of the physical properties, and it is more preferable to use an inclined short-wire paper machine.

The drying process can be appropriately selected from a multi-cylinder dryer, a Yankee dryer, a hot air dryer, etc. Among these, it is preferable to dry the packaging substrate with a Yankee dryer that turns the substrate into one-sided glossy paper.
When the packaging substrate is one-sided glossy paper, the heat seal layer may be provided on either the glossy or rough side, but when the heat seal layer is provided on the smoother side (glossy side) that contacts the Yankee dryer, a more uniform coating layer can be formed, so that high heat seal strength can be imparted with a small amount of coating. In addition, since the rough side, which is the side opposite the glossy side, has less fuzz than the glossy side, when a heat seal layer is provided on the glossy side, the heat seal layer suppresses fuzz on the glossy side, resulting in a packaging material with less fuzz.

In the present invention, examples of various auxiliary agents include sizing agents such as rosin, alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), and styrene acrylic resins, polyacrylamide polymers, polyvinyl alcohol polymers, cationic starch, various modified starches, dry strength agents such as urea-formaldehyde resin and melamine-formaldehyde resin, wet strength agents, water repellents, retention agents, drainage improvers, coagulants, aluminum sulfate, bulking agents, dyes, fluorescent whitening agents, pH adjusters, defoamers, UV inhibitors, anti-fading agents, pitch control agents, slime control agents, etc., and can be appropriately selected and used as needed.

When a wet strength agent is used in the present invention, the type is not particularly limited, and examples of usable wet strength agents include dialdehyde guar gum, glyoxal modified polyacrylamide, polyamide epichlorohydrin, polyamide polyamine epichlorohydrin, polyamine epichlorohydrin, and polyethyleneimine.
When a dry strength agent is used in the present invention, the type is not particularly limited, and examples of the agent that can be used include carboxymethyl cellulose (CMC), carboxymethyl guar gum, polyacrylamide, glyoxal-modified polyacrylamide, and hydroxypropyl guar gum.
In the present invention, the water repellent or sizing agent is not particularly limited, and examples thereof include styrene acrylic acid ester copolymers, styrene-acrylonitrile copolymers, anionic polyurethanes, and styrene-maleic anhydride copolymers.

(Paper quality and characteristics of packaging substrates)
The packaging substrate of the present invention has a prescribed compressive strength and flexural strength, and therefore has excellent cushioning properties and flexibility as a packaging material.
(Tensile strength)
The tensile strength can be adjusted by the fiber length and beating degree of the fibers used in the packaging substrate.
The packaging substrate of the present invention has a C/M ratio (horizontal/vertical ratio) of tensile strength of 60% or more. Paper with a C/M ratio of tensile strength of 60% or more has a small difference in tear strength and bending strength in all directions and has no direction in which it is prone to tearing, so it has excellent cushioning properties and is not easily torn even when subjected to impact. The C/M ratio of tensile strength is preferably 70% or more, more preferably 75% or more, even more preferably 85% or more, even more preferably 90% or more, and even more preferably 95% or more. The upper limit of the C/M ratio of tensile strength is not particularly limited, but is about 115%. In the present invention, the tensile strength is measured in accordance with JAPAN TAPPI No. 71.
The packaging substrate of the present invention preferably has a tensile strength of 0.3 kN/m or more, more preferably 0.4 kN/m or more, and even more preferably 0.5 kN/m or more, in both MD (longitudinal direction) and CD (transverse direction).

(Tear strength)
The tear strength can be adjusted by the fiber length, beating degree, etc. of the fibers used in the packaging substrate.
The packaging substrate of the present invention has a tear strength of 450 mN or more in MD and 650 mN or more in CD. By having a tear strength of 450 mN or more in MD and 650 mN or more in CD, damage to the package when it is subjected to an impact or the like can be prevented. The tear strength is more preferably 550 mN or more in MD, even more preferably 650 mN or more, even more preferably 680 mN or more, and more preferably 700 mN or more in CD, even more preferably 750 mN or more, even more preferably 780 mN or more. In this specification, the tear strength means a value measured according to JIS P8116:2000 "Paper - Tear Strength Test Method - Elmendorf Type Tear Tester Method" with a test piece length of 63 mm, a cut of 20 mm, and a tear length of 43 mm.

(Compressive strength)
The compressive strength is correlated with the density (porosity) and thickness of the packaging substrate, and the degree of beating and thickness of the natural fibers used. In particular, the compressive strength value can be adjusted by the thickness and degree of beating of the natural fibers used in the packaging substrate.
The packaging substrate of the present invention preferably has a compressive strength of 1.5 or less (measurement number: 1 sheet, deformation speed: 0.02 mm/sec, initial compression load: 50 gf/cm ² , maximum compression load: 300 gf/cm ² ) measured using a KES-G5 compression tester manufactured by Kato Tech Co., Ltd. The smaller this compressive strength value, the better the cushioning properties. In the present invention, the compressive strength of the packaging substrate is more preferably 1.45 or less, even more preferably 1.4 or less, and even more preferably 1.35 or less. The lower limit of this compressive strength is not particularly limited, but is, for example, about 1.1.

(Bending strength)
The bending strength is correlated with the basis weight, thickness, fiber thickness, etc. of the packaging substrate, and the value of the bending strength can be adjusted in particular by the thickness, etc. of the natural fibers blended into the packaging substrate.
The packaging substrate of the present invention preferably has a bending strength in both MD and CD directions of 0.8 g·cm ² /cm or less, as measured using a KES-FB2-A pure bending tester manufactured by Kato Tech Co., Ltd. The smaller the bending strength value, the better the flexibility. In the present invention, the bending strength of the packaging substrate is preferably 0.6 g·cm ² /cm or less, more preferably 0.4 g·cm ² /cm or less, and even more preferably 0.2 g·cm ² /cm or less. The lower limit of this bending strength is not particularly limited, but is, for example, about 0.02 g·cm ² /cm.

(Size)
When the heat seal layer is formed by coating, the packaging substrate preferably has a suitable sizing property or water repellency suitable for coating. Specifically, it is preferable that the pen-writing sizing degree described in JAPAN TAPPI No. 12 is 1 or more, since it can be suitably coated with a gravure coater, a flexo coater, etc. The pen-writing sizing degree is more preferably 2 or more, even more preferably 3 or more, even more preferably 4 or more, and even more preferably 5 or more.

"Packaging materials"
The packaging base material of the present invention can be made into a heat-sealable packaging material by providing a heat-sealable layer on at least one surface thereof.
The packaging material of the present invention has a heat seal strength of 0.5 N/15 mm or more when heat sealed at a temperature of 130° C., a pressure of 1.0 kgf/cm ² for 1 second, and peeled off with a T-shaped strip at a tensile speed of 30 mm/min. This heat seal strength is preferably 0.8 N/15 mm or more, more preferably 1.5 N/15 mm or more, even more preferably 2 N/15 mm or more, still more preferably 2.5 N/15 mm or more, even more preferably 3 N/15 mm or more, and still more preferably 3.5 N/15 mm or more.

The heat seal layer is provided on at least one surface of the packaging substrate, and may be provided on both surfaces.
The dry mass of the heat seal layer per side is preferably 1 g/m ² or more and 20 g/m ² or less. If the dry mass of the heat seal layer (per side) is less than 1 g/m ² , the heat sealability may deteriorate. If the dry mass of the heat seal layer (per side) exceeds 20 g/m ² , the heat sealability is saturated and hardly improved, and the amount of resin used increases, resulting in high costs and being undesirable from the standpoint of environmental load. The dry mass of the heat seal layer (per side) is preferably 1.5 g/m ² or more, more preferably 2 g/m ² or more. The dry mass of the heat seal layer (per side) is preferably 15 g/m ² or less, more preferably 12 g/m ² or less, and even more preferably 9 g/m ² or less. In particular, a packaging substrate having excellent sizing properties, for example, a packaging substrate having a pen size degree of 2 or more, can easily form a heat seal layer even with a small amount of coating, and therefore the dry mass of the heat seal layer (per side) can be, for example, 6 g/ ^m2 or less, 5 g/m2 or ^less , 4 g/m2 or ^less , 3 g/m2 or ^less , etc.

The thermoplastic resin forming the heat seal layer is not particularly limited, and thermoplastic resins used for heat seal applications such as ethylene-vinyl acetate resins, styrene-acrylate copolymer resins, acrylic resins, polyester resins such as polyethylene, polypropylene, and polyethylene terephthalate, polyvinyl alcohol, polyvinyl acetate, polylactic acid, polyhydroxyalkyl acid (PHBH), polybutylene succinate, and polyglycolic acid can be used without any particular restrictions. Among these, ethylene-vinyl acetate resins and acrylic resins are preferred in terms of heat seal strength. Also, biodegradable resins such as polyvinyl alcohol, polylactic acid, polyhydroxyalkyl acid, polybutylene succinate, and polyglycolic acid are preferred in terms of reducing the environmental load if they are discharged as waste.

The heat seal layer may be either a coating layer or a laminate layer, but is preferably a coating layer from the viewpoints of production efficiency and recyclability (redisintegrability). When the heat seal layer is a coating layer, it may be either an aqueous coating using a solvent such as water or a solvent-based coating using a solvent such as an organic solvent, but is preferably an aqueous coating from the viewpoint of safety and hygiene. When the aqueous coating is used, it is preferable to use an aqueous dispersion of a thermoplastic resin or a water-soluble thermoplastic resin.
The coating method is not particularly limited, and coating can be performed using a known coating device and coating system. Examples of coating devices include gravure coaters, flexo coaters, blade coaters, bar coaters, air knife coaters, curtain coaters, spray coaters, roll coaters, reverse roll coaters, size press coaters, and gate roll coaters. As described later, coating is preferably performed using a gravure coater or a flexo coater.
When the heat seal layer is formed by lamination, either of extrusion lamination and dry lamination may be used.

The packaging substrate of the present invention is made mainly of natural fibers, has excellent cushioning properties and flexibility, and is porous and water-absorbent.
Bar coaters and air knife coaters supply excess coating liquid to a substrate with an applicator roll and then scrape off the excess coating liquid with a Mayer bar or air knife. However, if the packaging substrate is porous, not only will the coating liquid penetrate into the substrate when it is supplied to the substrate, resulting in an excessive amount of coating, but it may also bleed through to the opposite side of the substrate, contaminating the process.
On the other hand, a gravure coater or a flexo coater measures out a specified amount to be applied in the embossed pattern of a gravure roll, and either transfers the coating liquid directly to the substrate and applies it thereto, or transfers the coating liquid to a coating roll and then applies it to the substrate, so that a coating layer can be formed limited to the surface of the substrate. For this reason, the coating liquid transferred by a gravure coater or the like does not easily penetrate into the substrate even if the substrate has good water absorption, and remains on the substrate surface, so that sufficient heat seal strength can be obtained even if the dry mass (per side) of the heat seal layer is small.
The embossing pattern of the gravure roll can be appropriately determined in terms of the mesh number and cup depth depending on the concentration of the aqueous dispersion or solution forming the heat seal layer and the dry mass of the heat seal layer.

When a water-based coating is used to form the heat seal layer as a coating layer, a water dispersion of a thermoplastic resin or an aqueous solution of a thermoplastic resin is used.
If the packaging substrate is porous and has good water absorption, the aqueous dispersion or solution of the thermoplastic resin may penetrate into the substrate, and the thermoplastic resin may not be localized on the substrate surface, resulting in a weakened heat seal strength. In addition, in order to achieve sufficient heat seal strength, it is necessary to increase the dry mass of the heat seal layer.
Therefore, in particular, when coating with a gravure coater, a flexo coater, or the like, it is preferable to combine the packaging substrate and the coating liquid so that the contact angle between them is 70° or more and 120° or less. By having the contact angle between the packaging substrate and the coating liquid be 70° or more and 120° or less, the coating liquid remains appropriately on the packaging substrate, and the thermoplastic resin forms a uniform layer on the packaging substrate surface, so that a packaging material with good heat seal strength can be obtained while suppressing the amount of thermoplastic resin applied. If the contact angle is less than 70°, the coated thermoplastic resin does not penetrate into the packaging substrate and is not localized on the packaging substrate surface, so the heat seal strength may be weakened, and in order to obtain sufficient heat seal strength, it is necessary to increase the dry mass of the heat seal layer. On the other hand, if the contact angle exceeds 120°, the coating liquid is repelled from the packaging substrate surface, causing unevenness in the heat seal layer, and uniform heat seal strength may not be obtained. This contact angle is preferably 80° or more, and preferably 110° or less. When coating is performed using a gravure coater, a flexo coater or the like, the viscosity of the coating liquid (B-type viscosity, No. 2 rotor, 60 rpm) is preferably 500 mPa·s or less, and more preferably 200 mPa·s or less.

Test 1: Packaging substrate "Examples 1 to 3"
Commercially available abaca pulp was disintegrated using a pulper. The freeness was CSF 730 ml.
The disintegrated abaca pulp was fed into a short wire papermaking machine in an unbeaten state to make paper of a predetermined basis weight so that the CD/MD ratio of the tensile strength was about 80 to 110%, and then dried in a Yankee dryer to obtain a packaging base material.
"Example 4"
A packaging base material was obtained by papermaking using a short wire papermaking machine in the same manner as in Example 1, except that a mixed pulp (CSF 650 ml) was used, which was a 50:50 mass ratio mixture of disintegrated abaca pulp (CSF 730 ml) and commercially available NBKP pulp beaten to a CSF of 500 ml.

"Comparative Example 1"
Commercially available NBKP was beaten to a freeness CSF of 650 ml, made into paper of a predetermined basis weight using a Fourdrinier papermaking machine, and dried in a multi-cylinder dryer to obtain a packaging substrate.
"Comparative Example 2"
A pulp obtained by mixing commercially available NBKP and LBKP in a mass ratio of 10:90 was beaten to a freeness CSF of 650 ml, made into paper of a predetermined basis weight using a Fourdrinier papermaking machine, and dried using a Yankee dryer to obtain a packaging substrate.

The obtained packaging substrates were evaluated as follows. The results are shown in Table 1.
(fiber length, fiber width)
The length-weighted average fiber length and length-weighted average fiber width were measured using an FS5 manufactured by Valmet Co., Ltd.
(basis weight, thickness, density)
Basis weight: Measured in accordance with JIS P8124.
Thickness: Measured in accordance with JIS P8118.
Density: Calculated from basis weight and thickness.

(Tensile strength)
The measurement was performed in accordance with JAPAN TAPPI No. 71 under the conditions of a test piece width of 15 mm, a test length of 180 mm, and a tensile speed of 180 mm/min.
(Tear strength)
The measurement was performed using an Elmendorf tear tester (Kumagaya Riki Kogyo Co., Ltd.) in accordance with JIS P8116:2000. The length of the test piece was 63 mm, the cut was 20 mm, and the tear length was 43 mm.

(Compressive strength)
As a measuring instrument, a KES-G5 compression tester manufactured by Kato Tech Co., Ltd. was used, and measurements were performed under the measurement conditions of a single support, a pressure plate area of 2.0 cm ² , a compression deformation rate (pressure plate descent rate) of 0.02 mm/sec (0.002 cm/sec), and a maximum compression load of 300 gf/cm ² , to produce the graph shown in FIG. 1. In FIG. 1, the y-axis direction (vertical axis direction) indicates the pressure P (gf/cm ² ) on the support, and the x-axis direction (horizontal axis direction) in the figure indicates the thickness T (mm) of one support. In FIG. 1, T ₀ indicates the thickness of one support when the pressure P is 50 gf/cm ² , and T _M indicates the thickness of one support when the pressure P is 300 gf/cm ² , which is the maximum compression load. The compressive strength is the integral of the change in pressure P (gf/ ^cm2 ) applied to the support from 50 gf/ ^cm2 to 300 gf/ ^cm2 (area S of the shaded area in Figure 1) divided by the area of triangle ABC.

(Bending strength)
Using a Kato Tech Co., Ltd. KES-FB2-A pure bending tester, measurements were made in both the MD and CD directions under conditions of a test piece of 100 mm×100 mm, a bending speed of 0.5 cm ⁻¹ /sec, and a maximum curvature of 2.5 cm ⁻¹ .

The packaging substrates obtained in Examples 1 to 4 of the present invention had a C/M ratio of tensile strength of 60% or more and a tear strength of 650 mN or more in both MD and CD, and were excellent in cushioning property and flexibility.
The packaging substrates obtained in Comparative Examples 1 and 2 had poor cushioning properties, with tear strengths of less than 650 mN in both MD and CD, while the packaging substrate obtained in Comparative Example 2 had poor flexibility, with bending strength in the MD exceeding 0.8 g·cm ² /cm.
The packaging base material in Example 4 had a lower compressive strength and excellent cushioning properties compared to the comparative examples. In Examples 1 to 3, the compressive strength was further reduced and the cushioning properties were significantly improved.

Test 2: Packaging material (packaging substrate)
The packaging substrate obtained in Example 1 was designated as packaging substrate 1.
A sizing agent (manufactured by Seiko PMC Corporation, AD1653, AKD/solid content 20%) was applied to the packaging substrate 1 at a coating concentration of 3.5% so that the coating amount was approximately 0.5 g/ ^m2 in solid content, and the substrate was dried using an air heater and a rotary dryer to produce a packaging substrate 2.

(thermoplastic resin)
Thermoplastic resin 1: Chemipearl S-500 (Mitsui Chemicals, Inc., ethylene acrylic type)
Thermoplastic resin 2: Arrowbase SE-1015J2 (Unitika Ltd., cross-linked polyolefin-based) 100 parts thickened with 2.3 parts of thickener SN Thickener 929S (San Nopco Ltd., sodium polycarboxylate) Thermoplastic resin 3: Arrowbase AA-1462 (Unitika Ltd., cross-linked polyolefin-based)

"Examples 5 to 7, Comparative Examples 3 and 4"
Using the combination of packaging substrate, thermoplastic resin, and gravure plate shown in Table 2, a coating liquid (aqueous dispersion of thermoplastic resin) was applied to the packaging substrate by gravure coating and dried to obtain a packaging material.
The viscosity of the coating liquid was measured using a Brookfield viscometer with a No. 2 rotor at 60 rpm.

The packaging substrates and packaging materials were evaluated, and the results are shown in Table 2.
(Pen size)
The heat sealant coated surface of the packaging substrate was measured in accordance with JAPAN TAPPI Paper and Pulp Test Method No. 12.
(contact angle)
A dynamic contact angle meter FIBRO 100 DAT MKII was used.
The coating liquid was dropped onto the packaging substrate in a drop size of 12 μL, and the contact angle 0.1 seconds after dropping was recorded as the measurement value. The measurement was repeated three times to obtain the average value. (Test environment: 23° C.)
(Heat seal strength)
The sealing agent-coated surfaces of the packaging substrates were placed together, heat-sealed at a temperature of 130°C, a pressure of 1.0 kgf/ ^cm2 , and a time of 1 second, and then peeled off using a universal tensile tester (IM20-ST model, manufactured by Intesco) with a test width of 15 mm and a tensile speed of 30 mm/min using a T-type peeler.

In Comparative Example 3, in which the contact angle between the packaging substrate and the coating liquid was less than 70°, coating was possible, but most of the coating liquid soaked into the packaging substrate, and a sufficient heat seal layer could not be formed on the surface, making heat sealing impossible.
In Comparative Example 4, in which the mesh of the gravure plate was coarser than that of Comparative Example 3, the coating liquid bleed through the packaging substrate, staining the impression roll and making it impossible to coat continuously. In addition, although the amount of coating was greater than in Comparative Example 3, much of the coating liquid soaked into the packaging substrate as in Comparative Example 3, and a sufficient heat seal layer could not be formed on the surface, making it impossible to perform heat sealing.
The packaging materials obtained in Examples 5 to 7, in which the contact angle between the packaging substrate and the coating liquid was 70° or more, had excellent heat seal strength. In particular, Example 6 used the same coating liquid as Comparative Examples 3 and 4, and a gravure plate with a coarser mesh, but the packaging substrate had a large size and was able to suppress penetration of the coating liquid, so heat sealing was possible.

Claims (7)

A packaging substrate comprising natural fibers,
Basis weight 18g/m2 or ^more and 50g/ ^m2 or less,
A packaging substrate having a tensile strength C/M ratio of 60% or more, a tear strength of 450 mN or more in MD and 650 mN or more in CD. 2. The packaging substrate according to claim 1, wherein the density of the packaging substrate is 0.2 g/cm3 or ^more and 0.5 g/ ^cm3 or less. The natural fibers include pulp,
2. The packaging substrate according to claim 1, characterized in that the packaging substrate contains 50% by mass or more of non-wood pulp having an average fiber length of 2.5 mm or more, an average fiber diameter of 15 μm to 25 μm, and a Canadian Standard Freeness of 650 ml CSF or more, based on the total amount of pulp. The packaging substrate according to claim 1, characterized in that the packaging substrate contains abaca pulp at 50% by mass or more relative to the total fibers. A packaging substrate according to any one of claims 1 to 4, comprising a heat seal layer on at least one surface thereof,
A packaging material having a heat seal strength of 0.5 N/15 mm or more when heat sealed at a temperature of 130° C., a pressure of 1.0 kgf/cm ² for 1 second, and peeled off with a T-shaped strip at a tensile speed of 30 mm/min. A method for producing a packaging material, comprising applying a coating liquid containing a thermoplastic resin onto the packaging substrate according to any one of claims 1 to 4 by a gravure method or a flexography method to form a heat seal layer. The method for producing a packaging material according to claim 6, characterized in that the contact angle between the packaging substrate and the coating liquid is 70° or more and 120° or less.

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