JP7697170B2 - Flexible packaging paper and flexible packaging bodies - Google Patents

Description

The present invention relates to flexible packaging paper and flexible packaging using the same.

In recent years, there has been a growing trend to move away from petroleum and plastic due to environmental issues such as plastic waste and global warming, and there is a demand to reduce as much as possible the amount of resin materials derived from fossil resources and non-biodegradable resin materials used in industrial products.
In this climate, it is desired to reduce the environmental impact of packaging materials. For example, methods such as reducing the thickness of packaging materials using plastic films are conceivable, but such thin packaging materials are less easy to handle during the process of forming the packaging material, and are prone to chipping and tearing at the thermocompression bonded parts. In addition, although the amount of resin used is reduced, the problem of resin remaining semi-permanently without decomposing when it leaks into the environment remains the same.

Patent Documents 1 and 2 propose using heat-seal paper, which is a paper base material laminated with a heat-seal layer, as a packaging material. These can significantly reduce the environmental impact because, depending on the type of heat-sealable resin, they can decompose in the environment. However, heat-seal paper has significantly different physical properties compared to plastic films, and so when heat-seal paper is passed through a bag-making machine under the same operating conditions as plastic films, problems such as the paper meandering or breaking, folds or wrinkles in the resulting soft packaging, and the inability to make bags in the desired shape can occur, making it necessary to optimize operating conditions such as slowing down the line speed.

JP 2020-163675 A JP 2021-046234 A

The objective of the present invention is to provide flexible packaging paper that can be made into bags using a bag making machine, and flexible packaging that uses this flexible packaging paper.

The means for solving the problems of the present invention are as follows.
1. A paper substrate and a heat seal layer on at least one outermost surface thereof;
The paper base material has a thickness of 25 μm or more and 100 μm or less, and a Young's modulus in the MD direction of 3 GPa or more and 15 GPa or less.
2. The flexible packaging paper according to 1., wherein the paper base material has a basis weight of 20 g/ ^m2 or more and 70 g/ ^m2 or less.
3. The flexible packaging paper according to 1. or 2., wherein the density of the paper base material is 0.5 g/ ^cm3 or more and 0.95 g/ ^cm3 or less.
4. A flexible packaging body, characterized in that an item to be packaged is enclosed in a packaging material made of the flexible packaging paper according to any one of 1. to 3.

The flexible packaging paper of the present invention can be used as a substitute for packaging materials made of conventional resin films. The flexible packaging paper of the present invention can be passed through a bag making machine under the same operating conditions as conventional plastic films to produce flexible packages. Since there is no need to change the operating conditions, it is easy to switch between the resin film and the flexible packaging paper of the present invention to produce flexible packages made of different materials.
Since the flexible packaging paper of the present invention is mainly made of paper, the amount of resin material used can be significantly reduced.

"Flexible packaging paper"
The flexible packaging paper has a paper base material and a heat seal layer on at least one outermost surface.

The paper base material is a base material on which a heat seal layer is formed on at least one surface. The paper base material is a sheet mainly made of pulp, and may be a base paper obtained by papermaking a paper material containing pulp, fillers, various auxiliaries, etc., as it is, or a base paper with one or more functional layers such as a sealing layer, ink receiving layer, water-resistant layer, oil-resistant layer, water vapor barrier layer, gas barrier layer, etc. formed on at least one surface of the base paper.
The paper base material used in the present invention has a thickness of 25 μm or more and 100 μm or less. The thickness of the paper base material is measured in accordance with JIS P8118:2014.
By having a thickness of 25 μm or more and 100 μm or less, it is easy to pass the paper substrate through a bag making machine that uses a conventional plastic film. The thickness of the paper substrate is preferably 30 μm or more, more preferably 35 μm or more, and even more preferably 40 μm or more, and is preferably 90 μm or less, more preferably 80 μm or less, and even more preferably 70 μm or less.

The paper base material of the present invention has a Young's modulus in the MD direction of 3 GPa or more and 15 GPa or less. If the Young's modulus is less than 3 GPa, the paper base material is easily deformed, and the paper base material is stretched during manufacturing, so that the paper base material is shrunk after manufacturing, and the vertical dimension is easily shifted, and wrinkles are easily generated. On the other hand, if the Young's modulus exceeds 15 GPa, the tension cannot be increased during manufacturing, so that the soft packaging paper is easily transported poorly, and the cutting interval is easily shifted. The Young's modulus is preferably 4 GPa or more, more preferably 5 GPa or more, and preferably 12 GPa or less, and more preferably 10 GPa or less. The Young's modulus in the MD direction of the paper base material is measured in accordance with JIS P8113:2006, Part 2, Constant Speed Extension Method.

From the viewpoint of flexibility and strength, the paper base material of the present invention preferably has a basis weight of 20 g/m ² or more and 70 g/m ² or less. The basis weight of the paper base material is more preferably 24 g/m ² or more, even more preferably 28 g/m ² or more, and more preferably 65 g/m ² or less, even more preferably 60 g/m ² or less, and even more preferably 55 g/m ² or less. The basis weight of the paper base material is measured in accordance with JIS P8124:2011.

From the viewpoint of flexibility and strength, the paper base material of the present invention preferably has a density of 0.5 g/cm ³ or more and 0.95 g/cm ³ or less. The density of the paper base material is more preferably 0.55 g/cm ³ or more, more preferably 0.9 g/cm ³ or less, even more preferably 0.85 g/cm ³ or less, and even more preferably 0.75 g/cm ³ or less. The thickness of the paper base material is calculated from the density and basis weight.

The paper substrate of the present invention preferably has a puncture strength of 1.0 N or more. If the puncture strength of the paper substrate is 1.0 N or more, the soft packaging body is less likely to be torn when contacted by a protrusion or the like. The puncture strength of the paper substrate is more preferably 1.1 N or more, and even more preferably 1.2 N or more. The puncture strength of the paper substrate is measured in accordance with JIS Z1707:2019 7.5 puncture strength test.

The paper base material of the present invention preferably has a bending stiffness B in the MD direction measured by the KES method of 0.10 g·cm ² /cm or more and 1.8 g·cm ² /cm or less, and a bending hysteresis 2HB in the MD direction measured by the KES method of 0.01 g·cm/cm or more and 1.2 g·cm/cm or less.
The KES method is an abbreviation for Kawabata Evaluation System, and is one of the methods for measuring the physical properties of soft materials such as nonwoven fabrics and woven fabrics. The bending rigidity B and bending hysteresis 2HB according to the KES method can be measured, for example, by an automated pure bending tester KES-FB2-S manufactured by Kato Tech Co., Ltd.

The smaller the value of the bending stiffness B, the softer it is. If the bending stiffness B is less than 0.10 g·cm ² /cm, the soft packaging paper is too soft, and when it is made into a soft packaging body, it loses tension, and it may be difficult to maintain a specified packaging shape. On the other hand, if the bending stiffness B exceeds 1.8 g·cm ² /cm, it becomes difficult to bend during bag making, and the dimensions in the lateral direction in particular may deviate from the designed dimensions. The bending stiffness B is more preferably 0.2 g·cm ² /cm or more, more preferably 0.3 g·cm ² /cm or ^more , even more preferably 0.4 g·cm ² /cm or more, more preferably 1.7 g·cm ² /cm or less, even more preferably 1.5 g·cm ² /cm or less, even more preferably 1.2 g·cm ² /cm or less, even more preferably 1.0 g·cm 2 /cm or less, and even more preferably 0.8 g·cm ² /cm or less.

The smaller the hysteresis 2HB value, the easier it is to return to the original shape after bending. If the hysteresis 2HB is less than 0.01 g cm/cm, bending, scoring, folding, etc. become difficult, and a force is generated that tries to return to the original shape (flat or rolled) after bag making, which may cause the flexible packaging to warp or round, resulting in a decrease in aesthetics and shape retention. On the other hand, if the hysteresis 2HB exceeds 1.2 g cm/cm, it becomes difficult to return to the original shape after bending, and wrinkles may easily occur, especially in parts that come into contact with the former of the bag making machine and are subjected to load during bending and folding, and the packaging may also be prone to tearing. This hysteresis 2HB is more preferably 0.02 g·cm/cm or more, even more preferably 0.025 g·cm/cm or more, even more preferably 0.035 g·cm/cm or more, even more preferably 0.055 g·cm/cm or more, and even more preferably 1.1 g·cm/cm or less, even more preferably 1.05 g·cm/cm or less, even more preferably 1.0 g·cm/cm or less, even more preferably 0.9 g·cm/cm or less, even more preferably 0.8 g·cm/cm or less, and even more preferably 0.7 g·cm/cm or less.

In the present invention, when the paper substrate is exposed on one side of the flexible packaging paper, the smoothness (Oken type) of the exposed surface (surface) of the paper substrate is preferably 50 seconds or more and 700 seconds or less. The flexible packaging paper of the present invention is wound into a roll, unwound from the roll, and passed through a bag making machine to be processed into a flexible package. In this case, if the smoothness (Oken type) of the exposed surface of the paper substrate is 50 seconds or more and 700 seconds or less, the friction and slipperiness between the paper substrate and the bag making machine are good, and production defects can be reduced. If this smoothness (Oken type) is less than 50 seconds, the friction is large and a large burden is placed on the flexible packaging paper during paper passing, and the paper substrate may be torn. On the other hand, if this smoothness (Oken type) exceeds 700 seconds, the friction is small and the flexible packaging paper becomes slippery, so the flexible packaging paper is likely to meander when passing through the bag making machine, and positional deviation may occur in the heat seal section or the cut section. The smoothness (Oken method) of the exposed surface of the paper base material is preferably 70 seconds or more, even more preferably 100 seconds or more, and more preferably 650 seconds or less, even more preferably 600 seconds or less, even more preferably 500 seconds or less, and even more preferably 450 seconds or less.
In addition, the smoothness (Oken type) value of the other side (rear side) of the paper base material is not particularly limited because a heat seal layer, etc. is laminated thereon. However, in order to improve adhesion with the layer laminated on the other side, it is preferable that the smoothness (Oken type) of the other side of the paper base material is smaller (rougher) than the smoothness (Oken type) of the exposed side of the paper base material, and specifically, it is preferable that the smoothness (Oken type) value is 30 seconds or more smaller.

As the pulp, chemical pulps such as bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), unbleached softwood pulp (NUKP), sulfite pulp, mechanical pulps such as stone grind pulp and thermomechanical pulp, wood fibers such as deinked pulp and waste paper pulp, non-wood fibers obtained from kenaf, bamboo, hemp, etc. can be used, and two or more types can be mixed and used. Among these, it is preferable to use chemical pulp and mechanical pulp of wood fiber, and it is more preferable to use chemical pulp, because it is difficult to cause foreign matter contamination, it is difficult to cause discoloration over time when it is used as a waste paper raw material after use and recycled, and it has a high whiteness, so that the surface texture during printing is good. Specifically, it is preferable that the amount of chemical pulp such as LBKP or NBKP is 80% by weight or more, and it is more preferable that the amount of chemical pulp is 100% by weight. The hardwood pulp content in the pulp is preferably 30% by weight or more, and more preferably 50% by weight or more.
The freeness (Canadian Standard Freeness: CSF) of the pulp is preferably 600 ml or less, more preferably 550 ml or less, and even more preferably 500 ml or less, from the standpoint of the strength of the paper base material, etc.

Fillers that can be used include inorganic fillers such as talc, kaolin, calcined kaolin, clay, heavy calcium carbonate, light calcium carbonate, white carbon, zeolite, magnesium carbonate, barium carbonate, titanium dioxide, zinc oxide, silicon oxide, amorphous silica, aluminum hydroxide, calcium hydroxide, magnesium hydroxide, zinc hydroxide, barium sulfate, and calcium sulfate, and organic fillers such as urea-formaldehyde resin, polystyrene resin, phenolic resin, and microhollow particles, all of which are known fillers for papermaking. Note that fillers are not essential materials and may not be used.

Examples of various auxiliary agents include sizing agents such as rosin, alkyl ketene dimer (AKD), and alkenyl succinic anhydride (ASA), 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, 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 selected and used as needed.

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

Furthermore, the surface of the base paper can be treated with various chemicals. Examples of the chemicals used include oxidized starch, hydroxyethyl etherified starch, enzyme-modified starch, polyacrylamide, polyvinyl alcohol, surface sizing agents, water-resistant agents, water retention agents, thickeners, lubricants, etc., which can be used alone or in combination of two or more. Furthermore, these various chemicals can be used in combination with pigments. Examples of the pigments include inorganic pigments such as kaolin, clay, engineered kaolin, delaminated clay, heavy calcium carbonate, light calcium carbonate, mica, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicate salts, colloidal silica, and satin white, and organic pigments such as solid, hollow, or core-shell types, which can be used alone or in combination of two or more.
The method for surface treatment of the base paper is not particularly limited, but known coating devices such as a rod metering size press, a pond type size press, a gate roll coater, a spray coater, a blade coater, and a curtain coater can be used.

Furthermore, one or more functional layers, such as a sealing layer, ink receiving layer, water-resistant layer, oil-resistant layer, water vapor barrier layer, gas barrier layer, etc., can be formed on at least one surface of the base paper. The functional layer may be either a coating layer or a laminate layer, but a coating layer is preferred from the viewpoint of maintaining the flexibility of the paper substrate.

Heat-seal layer The heat-seal layer is a layer that imparts heat-sealing suitability, specifically, a layer that can be attached to an object by applying heat and pressure. The heat-sealing suitability of the flexible packaging paper of the present invention makes it easy to form the flexible packaging paper into flexible packaging materials, maintain the shape, and ensure the sealability.

The heat seal layer is provided on at least one outermost surface of the flexible packaging paper, and may be either a coating layer or a laminate layer.
The thermoplastic resin contained in the heat seal layer is not particularly limited, and thermoplastic resins used for heat seal applications such as ethylene-vinyl acetate resins, styrene-acrylic acid ester copolymer resins, acrylic resins, ethylene-acrylic resins, polyolefin resins (polyethylene, polypropylene, etc.), polyester resins (polyethylene terephthalate, polyethylene succinate, polybutylene terephthalate, polyethylene naphthalate, etc.), polyvinyl alcohol resins, polyvinyl acetate resins, and polylactic acid resins can be used without any particular limitation. Among these, ethylene-vinyl acetate resins, styrene-acrylic acid ester copolymer resins, acrylic resins, and ethylene-acrylic resins are preferred from the viewpoint of heat seal strength. In addition, biodegradable resins such as polyvinyl alcohol, polylactic acid, and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) are preferred from the viewpoint of reducing the environmental load when the resin is discharged as waste.
The heat seal layer may contain additives such as an antiblocking agent, a silane coupling agent, etc. As the antiblocking agent, pigments, waxes, metal soaps, etc. may be used without any particular limitation. However, in the flexible packaging paper of the present invention, it is preferable that the heat seal layer does not contain additives from the viewpoint of cost.

When the heat seal layer is a coating layer, the dry weight of the heat seal layer is preferably 3 g/m ² or more and 20 g/m ² or less per side. If the dry weight is less than 3 g/m ² , the heat sealability may decrease. If the dry weight exceeds 20 g/m ² , the heat sealability is hardly improved and the cost increases. The heat seal layer may be a single layer or may be composed of two or more layers. By forming the heat seal layer into a multi-layer of two or more layers, defects such as coating unevenness can be reduced compared to the case of a single layer. When the heat seal layer is composed of two or more layers, it is preferable that the total dry weight of all the heat seal layers is within the above range, and it is preferable that the coating amount of the dry weight is 2 g/m ² or more.
When the heat seal layer is a laminate layer, the thickness of the heat seal layer is preferably 20 μm or more and 100 μm or less. If the thickness is less than 20 μm, the heat sealability may not be ensured. Moreover, if the thickness exceeds 100 μm, it is not preferable from the viewpoint of cost.

When the heat seal layer is a coating layer, the coating method is not particularly limited, and coating can be performed using a known coating device and coating system, such as a blade coater, a bar coater, an air knife coater, a curtain coater, a spray coater, a roll coater, a reverse roll coater, a size press coater, a gate roll coater, etc.
The coating system may be either aqueous coating using a solvent such as water or solvent-based coating using a solvent such as an organic solvent, but aqueous coating is preferred from the viewpoint of safety and hygiene. In the case of aqueous coating, a water-dispersible resin or a water-soluble resin is used as the thermoplastic resin, so that the heat seal layer is preferably a coating layer of a water-dispersible resin or a water-soluble resin.
When the heat seal layer is a laminate layer, the method for forming the layer is not particularly limited, and the layer can be laminated by appropriately using various methods such as extrusion lamination, wet lamination, and dry lamination.

"Soft packaging"
The flexible packaging body of the present invention has an item to be packaged enclosed in a packaging material made of the flexible packaging paper of the present invention described above.
The shape of the flexible packaging body is not particularly limited, and may be a vertical pillow packaging bag, a horizontal pillow packaging bag, a side seal bag, a two-sided seal bag, a three-sided seal bag, a gusset bag, a bottom gusset bag, a stand bag, etc. The flexible packaging body of the present invention is made mainly of paper, and therefore is easier to tear than conventional packaging bodies using resinous packaging materials, and can be easily torn from any point.
In addition, the flexible packaging paper of the present invention can be used in place of a resin film in a packaging machine that uses a conventional resin film with almost no change in the manufacturing conditions. The flexible packaging material of the present invention can be manufactured using conventional manufacturing machines as is, so no new equipment is required.

"Example 1"
As the pulp raw material, 80 parts by weight of hardwood bleached kraft pulp (LBKP, CSF: 500 ml) and 20 parts by weight of softwood bleached kraft pulp (NBKP, CSF 530 ml) were used. A stock was prepared by blending 0.1 parts by weight (relative to the dry weight of the pulp) of polyacrylamide (PAM) with a molecular weight of 2.5 million as a dry strength agent, 0.35 parts by weight of alkyl ketene dimer (AKD) as a sizing agent, 0.15 parts by weight of polyamide epichlorohydrin (PAEH) resin as a wet strength agent, and 0.08 parts by weight of polyacrylamide (PAM) with a molecular weight of 10 million as a retention agent, with respect to 100 parts by weight of this mixed pulp. A wet paper was made from this stock using a papermaking machine, and the wet paper was dried using a Yankee dryer to obtain a paper base material (base paper). The smoothness (Oken method) of the obtained paper base material was 83 seconds on the surface and 10 seconds on the back side.
A heat seal layer made of low density polyethylene having a thickness of 25 μm was provided on this paper base material by extrusion lamination to obtain a flexible packaging paper.

"Example 2"
As the pulp raw material, 50 parts by weight of softwood bleached kraft pulp (NBKP, CSF: 460 ml) and 50 parts by weight of hardwood bleached kraft pulp (LBKP, CSF: 300 ml) were used. A stock was prepared by mixing 0.9 parts by weight (based on the dry weight of the pulp) of polyamine epichlorohydrin resin (WS4010, a wet strength agent manufactured by Seiko PMC) activated with caustic soda (based on the dry weight of the pulp), 0.3 parts by weight of polyacrylamide (dry strength agent), aluminum sulfate, and a sizing agent with 100 parts by weight of this mixed pulp. A wet paper was made from this stock using a papermaking machine, and the wet paper was dried using a Yankee dryer to obtain a paper base material. The smoothness (Oken method) of the obtained paper base material was 197 seconds on the surface and 9 seconds on the back surface.
A heat seal layer made of low density polyethylene having a thickness of 25 μm was provided on this paper base material by extrusion lamination to obtain a flexible packaging paper.

"Example 3"
As the pulp raw material, a mixed pulp using 60 parts by weight of softwood bleached kraft pulp (NBKP, CSF: 40 ml) and 40 parts by weight of hardwood bleached kraft pulp (LBKP, CSF: 40 ml) was used as the raw material, and a base paper with a basis weight of 30.0 g/m ² was made using a fourdrinier papermaking machine, and an on-machine 2-roll size press was used to make it contain modified PVA (RS4104 (Kuraray) content 1.0 g/m ² ) and wet strength agent (WS-4020 (Seiko PMC) content 0.05 g/m ² ), and dried with a dryer to obtain a base paper with a moisture content of 8.0%. Thereafter, moisture was added to this base paper to make it a wet paper with a moisture content of 15.0%, and the paper base material was obtained by supercalendering at a temperature of 130 ° C. and a linear pressure of 250 kg/cm. The smoothness (Oken method) of the obtained paper base was 1234 seconds on the front side and 1085 seconds on the back side.
A heat seal layer made of low density polyethylene having a thickness of 25 μm was provided on this paper base material by extrusion lamination to obtain a flexible packaging paper.

"Example 4"
As the pulp raw material, 50 parts by weight of softwood bleached kraft pulp (NBKP, CSF: 500 ml) and 50 parts by weight of hardwood bleached kraft pulp (LBKP, CSF: 350 ml) were used. A stock was prepared by mixing 0.9 parts by weight (based on the dry weight of the pulp) of polyamine epichlorohydrin resin (WS4010, a wet strength agent manufactured by Seiko PMC) activated with caustic soda (based on the dry weight of the pulp), 0.3 parts by weight of polyacrylamide (dry strength agent), aluminum sulfate, and a sizing agent with 100 parts by weight of this mixed pulp. A wet paper was made from this stock using a papermaking machine, and the wet paper was dried using a Yankee dryer to obtain a paper base material. The smoothness (Oken method) of the obtained paper base material was 299 seconds on the surface and 10 seconds on the back surface.
A heat seal layer made of low density polyethylene having a thickness of 25 μm was provided on this paper base material by extrusion lamination to obtain a flexible packaging paper.

"Example 5"
Sodium polyacrylate was added as a dispersant to engineered kaolin (Imerys, Barisurf HX, average particle size 9.0 μm, aspect ratio 80-100) (0.2% relative to pigment), and dispersed in a Serie mixer to prepare a kaolin slurry with a solids concentration of 55%. 200 parts (solids) of styrene-butadiene latex (Zeon Corporation, PNT7868) was blended as a water vapor barrier resin into the resulting kaolin slurry per 100 parts (solids) of pigment, to obtain a coating liquid for a water vapor barrier layer with a solids concentration of 50%.
An aqueous solution of polyvinyl alcohol (PVA117, manufactured by Kuraray Co., Ltd.) was prepared to have a solids concentration of 10%, to obtain a coating liquid for a gas barrier layer.

The coating liquid for the water vapor barrier layer was coated on one side of the back surface of the base paper obtained in Example 1 using a blade coater to give a coating amount of 12 g/ ^m2 in dry weight and dried at 105°C for 2 minutes, and then the coating liquid for the gas barrier layer was coated on one side thereof using a roll coater to give a coating amount of 3.0 g/ ^m2 in dry weight and dried at 105°C for 2 minutes, obtaining a paper substrate with a functional layer. The smoothness (Oken method) of the obtained paper substrate was 83 seconds on the front surface and 34 seconds on the back surface.
A heat seal layer of 25 μm thickness made of low density polyethylene was provided by extrusion lamination on the surface of the paper base material on the functional layer side, to obtain a flexible packaging paper.

"Comparative Example 1"
The paper substrate was a commercially available kraft paper (Shin Tokai Paper Co., Ltd.). The smoothness (Oken method) of this paper substrate was 14 seconds on the front side and 4 seconds on the back side.
A heat seal layer made of low density polyethylene having a thickness of 25 μm was provided on this paper base material by extrusion lamination to obtain a flexible packaging paper.
"Comparative Example 2"
The paper substrate was a commercially available glassine paper (Nippon Paper Papylia Co., Ltd.). The smoothness (Oken method) of this paper substrate was 1756 seconds on the front side and 1424 seconds on the back side.
A heat seal layer made of low density polyethylene having a thickness of 25 μm was provided on this paper base material by extrusion lamination to obtain a flexible packaging paper.

The obtained paper base material or flexible packaging paper was evaluated as follows. The results are shown in Table 1.
・Smoothness (Ouken style)
The surface of the paper base opposite to the surface on which the heat seal layer was to be formed (front surface) and the surface on which the heat seal layer was to be formed (back surface) were measured using a digital Oken type air permeability and smoothness tester (manufactured by Asahi Seiko Co., Ltd.) in accordance with JIS P8155.
- Puncture strength The puncture strength was measured from the paper base material side of the soft packaging paper in accordance with JIS Z1707:2019 7.5 Puncture Strength Test using an IMADA Texture Analyzer.
Young's modulus was measured using an L&W tensile tester in accordance with JIS P8113:2006, Part 2, Constant Speed Extension Method.
・B, 2HB according to the KES method
Using an automated pure bending tester KES-FB2-S manufactured by Totec Co., Ltd., the measurements were performed 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 .

Bag-making properties The obtained soft packaging paper was cut to a width of 250 mm and wound into a roll. This was used in a horizontal pillow packaging machine (Omori Machinery Industry Co., Ltd., S-5000X BX) with a product dummy width of 75 mm x length of 75 mm x height of 28 mm (weight 45 g), with the width of the bag making machine opening set to 80 to 85 mm x height of 35 mm, a cutting pitch of 150 mm, and a rotation speed of 60 cpm to make 1,000 horizontal pillow bags, which were evaluated according to the following criteria. The center seal temperature was 140°C, and the top seal temperature was 110°C. If the soft packaging paper broke, production was stopped at that point.

Breaking OK: The flexible packaging paper does not break during production.
NG: The flexible packaging paper breaks during production.
Transportability: OK: The soft packaging paper can be transported as specified.
NG: Misalignment occurs in the conveyance of the soft packaging paper.
・Vertical dimensional stability (N=30)
1: All vertical dimensions are less than ±3% from the design.
2: All vertical dimensions are less than ±5% from the design.
3: One or more items have a vertical dimension that deviates from the design by 5% or more.

Results The flexible packaging papers obtained in Examples 1 to 5 could be conveyed as specified without breaking. The horizontal pillow bags obtained also had excellent dimensional stability in the vertical direction.
In contrast, the flexible packaging paper obtained in Comparative Example 1 broke during production, so the production of the horizontal pillow bags was stopped midway. In addition, when the horizontal pillow bags produced before the break were inspected, the vertical dimension was short and shrunk from the design dimension. This is presumably because the Young's modulus of the paper base material was small at 2.81 GPa, the paper was conveyed in a vertically stretched state by the bag making machine, and shrunk when tension was no longer applied. The flexible packaging paper of Comparative Example 2 was cut at 150 mm intervals, but had good vertical dimensional stability, but some pieces were not cut at 150 mm intervals in the vertical direction due to poor conveyance in the bag making machine. This is presumably because the Young's modulus of the paper base material was large at 15.1 GPa, so strong tension could not be applied, resulting in poor conveyance.

Claims (9)

A paper substrate and a heat seal layer on at least one outermost surface thereof,
The flexible packaging paper (excluding those containing core-sheath fibers) is characterized in that the paper base material has a thickness of 30 μm or more and 100 μm or less, a Young's modulus in the MD direction of 5 GPa or more and 12.8 GPa or less, and a bending rigidity B in the MD direction measured by the KES method of 0.10 g cm ² /cm or more and 0.80 g cm ² /cm or less . 2. The flexible packaging paper according to claim 1, wherein the paper base material has a basis weight of 20 g/m2 or ^more and 70 g/ ^m2 or less. 2. The flexible packaging paper according to claim 1 , wherein the density of the paper base material is 0.5 g/cm3 or ^more and 0.95 g/ ^cm3 or less. The flexible packaging paper according to claim 1, wherein the paper substrate has a functional layer selected from a sealing layer, an ink-receiving layer, a water-resistant layer, an oil-resistant layer, a water vapor barrier layer, and a gas barrier layer on at least one surface of the base paper. The heat seal layer is a coating layer, and the dry weight of the coating layer is 3 g/m ² 20g/m or more ² The flexible packaging paper according to claim 1, wherein: The flexible packaging paper according to claim 1, characterized in that the heat seal layer contains a thermoplastic resin selected from ethylene-vinyl acetate resins, styrene-acrylate copolymer resins, acrylic resins, ethylene-acrylic resins, polyolefin resins, polyester resins, polyvinyl alcohol resins, polyvinyl acetate resins, polylactic acid resins, and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). 2. The paper for flexible packaging material according to claim 1, wherein the bending hysteresis 2HB in the machine direction as measured by the KES method is from 0.01 g·cm/cm to 1.2 g·cm/cm. 2. The flexible packaging paper according to claim 1, wherein the paper substrate is exposed on one side of the flexible packaging paper, and the smoothness (Oken method) of the exposed side of the paper substrate is 50 seconds or more and 700 seconds or less. A flexible packaging body, comprising an article to be packaged, the article being enclosed in a packaging material made of the flexible packaging paper according to any one of claims 1 to 8 .