JP7728320B2 - 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 this 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 pillow packaging bags. However, in recent years, environmental issues such as those related to plastic waste in the ocean have led to a growing trend toward eliminating plastic, and there is a desire to reduce the amount of resin 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.

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

Patent Document 2 proposes heat-seal paper that includes a paper base primarily composed of pulp and a heat-seal layer laminated on one side of the paper base, where the heat-seal layer is primarily composed of an alkali-neutralized ethylene (meth)acrylic acid copolymer, and where the surface of the paper base on which the heat-seal layer is laminated has a Beck smoothness of 5 seconds to 50 seconds and a Parker Print Surf smoothness of 3.8 μm to 8.0 μm, and the surface of the paper base on which the heat-seal layer is not laminated has a Beck smoothness of 50 seconds to 500 seconds and a Parker Print Surf smoothness of 2.0 μm to 4.2 μm. The paper does not contain environmentally hazardous laminated paper or plastic film, and has excellent water resistance and printability on the surface on which the heat-seal layer is not laminated.

Patent Document 3 proposes heat-seal paper having one or more heat-seal layers on at least one side of a paper substrate, 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, measured in accordance with ISO 2493-1:2010, an ISO stiffness of 0.45 mNm or less in the transverse direction, and a puncture strength of 7.5 N or more, measured in accordance with JIS Z 1707:2019, resulting in heat-seal paper with excellent drop impact resistance, flexibility, and heat-sealing properties.

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% by 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 of the heat seal layer (per side) 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, and can be easily torn from any direction, allowing packaged items to be easily removed when used as a flexible package.

Patent Document 5 proposes a heat-sealable 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% or more by mass of an organic material, where the thermal adhesive layer contains styrene-butadiene copolymer latex as at least 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, and the sheet is breathable, moisture permeable, has low dust generation, and is recyclable.

JP 2023-107689 A Japanese Patent Application Laid-Open No. 2023-107678 Patent No. 7243901 JP 2023-81594 A JP 2018-053400 A

Packaging materials using heat-sealable paper have been proposed as an alternative to packaging materials using plastic film. However, because paper is less stretchable than plastic film, packaging materials using heat-sealable paper have inferior cushioning properties. Furthermore, because paper is less flexible than plastic film, 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 excellent cushioning properties, a packaging substrate having excellent flexibility, a packaging material using these packaging substrates, 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 characterized in that the compressive strength measured using a KES-G5 compression tester (measurement number: 1 sheet, deformation speed: 0.02 mm/sec, initial compressive load: 50 gf/cm ² , maximum compressive load: 300 gf/cm ² ) is 1.5 or less.
2. The packaging substrate according to 1., characterized in that the bending strength in both the MD and CD directions measured with a KES-FB2-A pure bending tester is 0.8 g·cm ² /cm or less.
3. The packaging substrate according to 1. or 2., wherein the density of the packaging substrate is 0.2 g/cm ³ or more and 0.5 g/cm ³ or less.
4. The natural fibers include pulp,
4. The packaging substrate according to any one of 1. to 3., 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, relative to the total amount of pulp.
5. The packaging substrate according to any one of 1. to 4., wherein the packaging substrate contains abaca pulp in an amount of 50% by mass or more based on the total fibers.
6. A packaging substrate according to any one of 1. to 5., having a heat seal layer on at least one surface thereof;
A packaging material characterized in that the heat seal strength when heat sealed at a temperature of 130°C, a pressure of 1.0 kgf/cm ² for 1 second, and peeled off using a T-type strip at a pulling rate of 30 mm/min is 0.5 N/15 mm or more.
7. 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 5. by a gravure method or a flexographic method, thereby forming a heat seal layer.
8. The method for producing a packaging material according to 7., wherein the contact angle between the packaging substrate and the coating liquid is 70° or more and 120° or less.

The present invention provides packaging substrates and packaging materials with excellent cushioning properties. In particular, it provides packaging substrates and packaging materials that combine cushioning and flexibility. Packaging materials with excellent cushioning properties can protect contents from shaking during transportation and from external impacts. Packaging materials with excellent flexibility can be packaged to fit the shape of the contents.

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/ ^m2 or more and 50 g/ ^m2 or less. By having a basis weight of 18 g/ ^m2 or more and 50 g/ ^m2 or less, the packaging substrate of the present invention can achieve both cushioning and flexibility at a high level. When cushioning is emphasized in the packaging substrate of the present invention, the basis weight is preferably 20 g/m2 or ^more , and more preferably 22 g/ ^m2 or more. Furthermore, when flexibility is emphasized, the basis weight is preferably 40 g/m2 ^or less, more preferably 35 g/m2 or ^less , and even more preferably 30 g/m2 or ^less .

The packaging substrate of the present invention preferably has a density of 0.2 g/ ^cm3 or more and 0.5 g/ ^cm3 or less. If the density is less than 0.2 g/ ^cm3 , the packaging material will be bulky and have excellent cushioning properties, but flexibility may be reduced. If the density exceeds 0.5 g/ ^cm3 , the packaging material will have excellent flexibility but poor cushioning properties. When cushioning properties are important in the packaging substrate of the present invention, the density is preferably 0.4 g/cm3 or ^less , and more preferably 0.3 g/ ^cm3 or less. Furthermore, when flexibility is important, the density is preferably 0.25 g/cm3 or ^more , and more preferably 0.3 g/ ^cm3 or more.

The packaging substrate of the present invention contains natural fibers, and for example, paper and wetlaid nonwoven fabrics are suitable. 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 proportion 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. Furthermore, when resin fibers are included, it is preferable to use biodegradable resin fibers made from biodegradable resins 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.

Natural fibers can be derived from plants, microorganisms, or animals, without any particular restrictions. Specific examples include natural celluloses 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 cuprammonium solution or a morpholine derivative and then reprecipitating, cellulosic fibers such as various cellulose derivatives such as acetylated cellulose and carboxylated cellulose, and animal hair such as silk, wool, and goat hair. One or more of these can be used. Wood pulp and non-wood pulp are suitable natural fibers.

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 will be flexible, but the packaging substrate will have a high density and become a tight substrate, which may result in reduced 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 fibers themselves are soft but the substrate becomes tight, resulting in poor cushioning properties, while if the average fiber width is thick, the fibers themselves become difficult to bend, resulting in poor 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 Corporation, an FS5 manufactured by Valmet Corporation, or a Morfi manufactured by Voith Turbo K.K., an optical microscope, an electron microscope, or the like.

The beating degree 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 becomes more beaten, its density increases and the substrate becomes more flexible, but its cushioning properties tend to decrease.

The packaging substrate 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 substrate that is low in density and has excellent cushioning 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 that satisfies 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 are made from softwood and/or hardwood; 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, and 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), kenaf, jute, mulberry, and mitsumata; hard fibers such as bagasse, bamboo, and esparto; seed fibers such as cotton (linter); and leaf sheath and leaf fibers such as abaca and sisal. One or more of these may be used.

Among these, the packaging substrate of the present invention preferably contains abaca pulp (Manila hemp pulp). By incorporating abaca pulp, it becomes easier to obtain a packaging substrate with excellent cushioning properties and flexibility.
In the present invention, the amount of abaca pulp relative to the total fiber content 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 packaging substrates is not particularly limited, and can be carried out using a Fourdrinier paper machine, cylinder paper machine, short-wire paper machine, inclined short-wire paper machine, etc. Among these, it is preferable to use a short-wire paper machine, inclined short-wire paper machine, or Fourdrinier paper machine, which can reduce fiber orientation and increase the C/M ratio (width/length ratio) of 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, Yankee dryer, hot air dryer, etc. Among these, it is preferable to dry using a Yankee dryer, which turns the packaging 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 if the heat seal layer is provided on the smoother side (glossy side) that comes into contact with the Yankee dryer, a more uniform coating layer can be formed, thereby imparting high heat seal strength with a smaller coating amount. Furthermore, since the rough side, which is the side opposite the glossy side, has less fuzz than the glossy side, if a heat seal layer is provided on the glossy side, the heat seal layer will suppress fuzz on the glossy side, resulting in a packaging material with less fuzz.

Examples of various auxiliary agents that can be used in the present invention 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-formalin resin and melamine-formalin resin, wet strength agents, water repellents, retention agents, drainage aids, coagulants, aluminum sulfate, bulking agents, dyes, fluorescent whitening agents, pH adjusters, defoamers, UV inhibitors, anti-fading agents, pitch control agents, and slime control agents, and can be selected and used as needed.

When a wet strength agent is used in the present invention, the type thereof is not particularly limited, and examples that can be used 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 thereof is not particularly limited, and examples 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 copolymer, styrene-acrylonitrile copolymer, anionic polyurethane, and styrene-maleic anhydride copolymer.

(Paper quality and characteristics of packaging substrate)
The packaging substrate of the present invention has a predetermined compressive strength and bending strength, and therefore has excellent cushioning properties and flexibility as a packaging material.
(Compression 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 blended therein. In particular, the compressive strength value can be adjusted by the thickness and degree of beating of the natural fibers blended into the packaging substrate.
The packaging substrate of the present invention has a compressive strength of 1.5 or less (measured sheet: 1, deformation speed: 0.02 mm/sec, initial compressive load: 50 gf/cm ² , maximum compressive 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 preferably 1.45 or less, more preferably 1.4 or less, and even more preferably 1.35 or less. There is no particular restriction on the lower limit of this compressive strength, but it 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 particularly 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 the 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 this 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.

(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 preferably has a C/M ratio (width/length ratio) of tensile strength of 60% or more, more preferably 70% or more, and even more preferably 75% or more. By setting the C/M ratio of tensile strength to 30% or more, the difference in tear strength and bending strength in all directions is reduced, making it possible to produce paper suitable for packaging applications. The upper limit of the C/M ratio of tensile strength is not particularly limited, but is approximately 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 (machine direction) and CD (cross 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 preferably has a tear strength of 450 mN or more in the MD and 650 mN or more in the CD, from the viewpoint of preventing breakage of the package when subjected to impact, etc. The tear strength in the MD is more preferably 550 mN or more, even more preferably 650 mN or more, and even more preferably 680 mN or more, and the CD is more preferably 700 mN or more, even more preferably 750 mN or more, and even more preferably 780 mN or more. In this specification, tear strength means the value measured in accordance with JIS P8116:2000 "Paper - Tear strength test method - Elmendorf tear tester method" using a test piece 63 mm long, a 20 mm incision, and a tear length of 43 mm.

(Size)
When a heat seal layer is formed by coating, the packaging substrate preferably has appropriate sizing or water repellency suitable for coating. Specifically, a pen-writing sizing degree of 1 or more according to JAPAN TAPPI No. 12 is preferred because it allows for suitable coating using a gravure coater, flexo coater, or the like. 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 substrate 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/ ^cm2 , and a time of 1 second, and peeled off using a T-type seal 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 even 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/m2 or ^more and 20 g/ ^m2 or less. If the dry mass (per side) of the heat-seal layer is less than 1 g/ ^m2 , the heat-sealability may deteriorate. If the dry mass (per side) of the heat-seal layer is more than 20 g/ ^m2 , the heat-sealability will saturate and not improve much, and the amount of resin used will increase, resulting in high costs and being undesirable from the standpoint of environmental impact. The dry mass (per side) of the heat-seal layer is preferably 1.5 g/ ^m2 or more, and more preferably 2 g/ ^m2 or more. The dry mass (per side) of the heat-seal layer is preferably 15 g/ ^m2 or less, more preferably 12 g/m2 or ^less , and even more preferably 9 g/ ^m2 or less. In particular, a packaging substrate with excellent sizing properties, for example, a packaging substrate with 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 sometimes be set to, for example, 6 g/ ^m2 or less, 5 g/m2 or ^less , 4 g/m2 or ^less , 3 g/m2 or ^less , etc.

There are no particular restrictions on the thermoplastic resin that forms the heat seal layer, and thermoplastic resins used in heat seal applications, such as ethylene-vinyl acetate resins, styrene-acrylate copolymer resins, acrylic resins, polyester resins such as polyethylene, polypropylene, and polyethylene terephthalate, as well as polyvinyl alcohol, polyvinyl acetate, polylactic acid, polyhydroxyalkyl acid (PHBH), polybutylene succinate, and polyglycolic acid, can be used without particular restrictions. Of these, ethylene-vinyl acetate resins and acrylic resins are preferred in terms of heat seal strength. Furthermore, biodegradable resins such as polyvinyl alcohol, polylactic acid, polyhydroxyalkyl acid, polybutylene succinate, and polyglycolic acid are preferred in terms of reducing the environmental impact if they are disposed of 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 using a solvent such as an organic solvent, but is preferably an aqueous coating from the viewpoint of safety and hygiene. When using an aqueous coating, 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 below, coating is preferably performed using a gravure coater or flexo coater.
When the heat seal layer is formed by lamination, either extrusion lamination or 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 use an applicator roll to supply excess coating liquid to a substrate, and then scrape off the excess coating liquid with a Mayer bar or air knife. However, if the packaging substrate is porous, the coating liquid may penetrate the substrate when it is supplied to the substrate, resulting in an excessive amount of coating and may even bleed through to the other side of the substrate, contaminating the process.
On the other hand, gravure coaters or flexo coaters measure out a specified amount to be coated using the embossed pattern of a gravure roll, and either transfer the coating liquid directly to the substrate and apply it, or first transfer the coating liquid to a coating roll and then apply it to the substrate, so that the coating layer can be formed only on the surface of the substrate. For this reason, the coating liquid transferred with 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 surface of the substrate, making it possible to obtain sufficient heat seal strength even with a small dry mass (per side) of the heat seal layer.
The number of meshes and cup depth of the embossing pattern of the gravure roll can be appropriately selected 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 aqueous coating is used to form the heat seal layer as a coating layer, an aqueous 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 deep into the substrate, preventing the thermoplastic resin from being localized on the surface of the substrate, which may result in a weakened heat seal strength. Furthermore, to achieve sufficient heat seal strength, the dry mass of the heat seal layer must be increased.
Therefore, particularly when coating using a gravure coater, 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. When the contact angle between the packaging substrate and the coating liquid is 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 surface of the packaging substrate, thereby reducing the amount of thermoplastic resin applied and obtaining a packaging material with good heat seal strength. If this contact angle is less than 70°, the coated thermoplastic resin will not penetrate into the packaging substrate and will not be localized on the surface of the packaging substrate, which may weaken the heat seal strength. Furthermore, to achieve sufficient heat seal strength, the dry mass of the heat seal layer must be increased. On the other hand, if this contact angle exceeds 120°, the coating liquid will be repelled from the surface of the packaging substrate, causing unevenness in the heat seal layer and making it difficult to obtain uniform heat seal strength. This contact angle is preferably 80° or more and 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 in a pulper. The freeness was CSF 730 ml.
The disintegrated abaca pulp was fed into a short wire paper machine in an unbeaten state to make paper of a predetermined basis weight so that the CD/MD ratio of the tensile strength was approximately 80 to 110%, and the paper was dried in a Yankee dryer to obtain a packaging substrate.
"Example 4"
A packaging substrate was obtained by papermaking using a short wire paper machine in the same manner as in Example 1, except that a mixed pulp (CSF 650 ml) was used, which was a 50:50 mixture by mass of disaggregated 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 with a predetermined basis weight using a Fourdrinier paper 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 paper 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.

(Compression strength)
Using a Kato Tech Co., Ltd. KES-G5 compression tester as the measuring instrument, measurements were carried out with a single support under the following measurement conditions: pressure plate area 2.0 cm ² , compression deformation rate (pressure plate descent rate) 0.02 mm/sec (0.002 cm/sec), and maximum compression load 300 gf/cm ² , and the graph shown in Figure 1 was produced. In Figure 1, the y-axis direction (vertical axis) indicates the pressure P (gf/cm ² ) on the support, and the x-axis direction (horizontal axis) indicates the thickness T (mm) of one support. In Figure 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 KES-FB2-A pure bending tester, measurements were made in both the MD and CD directions under the 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 .

(Tensile strength)
The measurement was carried out 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)
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 incision was 20 mm, and the tear length was 43 mm.

The packaging substrates obtained in Examples 1 to 4 of the present invention had compressive strengths of 1.5 or less (measured sheet: 1, 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, demonstrating excellent cushioning properties. Furthermore, the packaging substrates obtained in Examples 1 to 4 of the present invention had bending strengths of 0.8 g·cm ² /cm or less in both the MD and CD directions measured using a KES-FB2-A pure bending tester, demonstrating excellent flexibility.
The packaging substrates obtained in Comparative Examples 1 and 2 had a compressive strength greater than 1.5 and were poor in cushioning properties. The packaging substrate obtained in Comparative Example 2 had a flexural strength in the MD direction greater than 0.8 g·cm ² /cm and was poor in flexibility.
The packaging substrate of Example 4 had lower compressive strength and better cushioning properties than 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 (AD1653, AKD, 20% solids, manufactured by Seiko PMC Corporation) 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 terms of solids, and the coating was dried using an air heater and rotary dryer to produce the packaging substrate 2.

(thermoplastic resin)
Thermoplastic resin 1: Chemipearl S-500 (Mitsui Chemicals, Inc., ethylene acrylic)
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 combinations 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 substrate and packaging material were evaluated, and the results are shown in Table 2.
(Pen writing size)
The heat sealant coated surface of the packaging substrate was measured in accordance with JAPAN TAPPI Paper 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 and the average value was calculated (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 using a universal tensile tester (IM20-ST model, manufactured by Intesco Co., Ltd.) with a test width of 15 mm and a pulling 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, preventing the formation of a sufficient heat-sealing layer on the surface, and 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 bled through to the back of the packaging substrate, staining the impression roll and making it impossible to coat continuously. Furthermore, although the coating amount 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 heat sealing impossible.
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 sizing property 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,
The compressive strength measured using a KES-G5 compression tester (measurement number: 1, deformation speed: 0.02 mm/sec, initial compressive load: 50 gf/cm ² , maximum compressive load: 300 gf/cm ² ) is 1.5 or less,
A packaging substrate characterized in that the bending strength in both the MD and CD directions measured with a KES-FB2-A pure bending tester is 0.8 g·cm ² /cm or less . 2. The packaging substrate according to claim 1, wherein the density of the packaging substrate is 0.2 g/cm ^<3> or more and 0.5 g/cm ^<3> or less. the natural fibers include pulp;
2. The packaging substrate according to claim 1, 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. The packaging substrate according to claim 1, characterized in that the packaging substrate contains abaca pulp in an amount of 50% by mass or more relative to the total fiber content. A packaging substrate according to any one of claims 1 to 4 , having a heat seal layer on at least one surface thereof,
A packaging material characterized in that the heat seal strength when heat sealed at a temperature of 130°C, a pressure of 1.0 kgf/cm ² for 1 second, and peeled off using a T-type strip at a pulling rate of 30 mm/min is 0.5 N/15 mm or more. 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 flexographic method to form a heat seal layer. 7. The method for producing a packaging material according to claim 6 , wherein the contact angle between the packaging substrate and the coating liquid is 70° or more and 120° or less.