JP7786251B2 - packaging material - Google Patents

Description

The present invention relates to a packaging material using a gas barrier film.

Packaging materials used to package foods, pharmaceuticals, etc. are required to have gas barrier properties, i.e., the ability to prevent the intrusion of gases (water vapor, oxygen, etc.) that degrade the contents, in order to prevent deterioration and spoilage of the contents and maintain their functionality and quality. For this reason, film materials with gas barrier properties (gas barrier films) are used for these packaging materials.

Known gas barrier films include those in which a gas barrier layer made of a material with gas barrier properties is provided on the surface of a resin substrate. Known gas barrier layers include metal foils, metal vapor-deposited films, and films formed by wet coating. Films known to exhibit oxygen barrier properties include resin films formed from coating agents containing water-soluble polymers, resins such as polyvinylidene chloride, and inorganic layered mineral composite resin films formed from coating agents containing water-soluble polymers and inorganic layered minerals (Patent Document 1). Other proposed gas barrier layers include a gas barrier layer formed by sequentially laminating a vapor-deposited thin film layer made of an inorganic oxide and a gas barrier composite coating containing a water-soluble polymer, an inorganic layered compound, and a metal alkoxide (Patent Document 2), and a gas barrier layer containing a polyvalent metal salt of a carboxylic acid, which is a reaction product of the carboxyl group of a polycarboxylic acid polymer and a polyvalent metal compound (Patent Document 3).

In recent years, growing environmental awareness stemming from issues such as marine plastic waste has led to calls for even more efficient sorting and recycling of plastic materials. Packaging laminates, which have traditionally been made high-performance by combining a variety of different materials, are no exception, and there is now a growing demand for them to be made into monomaterials.

To achieve mono-material laminates, the resin materials of the films that make up each layer must be the same type. For example, polyethylene, a type of polyolefin, is widely used in packaging materials, so there are high hopes for mono-material laminates using polyethylene.

To achieve mono-materialization, for example, Patent Document 4 proposes a laminate using a polyethylene film with a vapor deposition layer on at least one side of the substrate and heat seal layer, and discloses stretched polyethylene as the substrate from the perspective of printability and bag-making suitability.

Patent No. 6191221 Japanese Patent Application Laid-Open No. 2000-254994 Patent No. 4373797 Japanese Patent Application Laid-Open No. 2020-055157

The present inventors have found that when the laminate described in Patent Document 4 is applied to packaging bags, stand-up pouches, etc., the adhesion between the stretched polyethylene substrate constituting the laminate and the linear low-density polyethylene heat-seal layer may be insufficient depending on the contents to be packaged. Insufficient adhesion may cause delamination in the laminate or tearing of the packaging material.
The inventors have solved this problem while maintaining a mono-material structure.

In view of the above circumstances, an object of the present invention is to provide a packaging material in which the substrate and the heat seal layer are sufficiently adhered to each other and which is easy to recycle.

The present invention provides a packaging material formed using a laminate comprising a gas barrier film and a heat seal layer containing polyethylene, the proportion of the polyethylene in the entire laminate being 90 mass % or more .
The laminate has only one heat seal layer.
The gas barrier film includes a substrate containing polyethylene and an inorganic oxide layer formed on a first surface side of the substrate.
The gas barrier film has a birefringence ΔN of the first surface calculated based on measurement by a parallel Nicol rotation method of 0 or more and 0.007 or less, the inorganic oxide layer contains silicon oxide, and is configured not to include an oxygen barrier coating containing water-swellable mica.
The heat seal layer is bonded to the gas barrier film so as to sandwich the inorganic oxide layer between the substrate and the heat seal layer.
This packaging material is formed by heat-sealing the heat-sealable layers of the laminate together.

The present invention provides a gas barrier film that allows for sufficient adhesion between the substrate and the heat seal layer, and can form a laminate that is easy to recycle.

1 is a schematic cross-sectional view of a laminate according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a laminate according to an example.

Hereinafter, one embodiment of the present invention will be described with reference to FIG.
1 is a schematic cross-sectional view of a laminate 1 according to this embodiment. The laminate 1 includes a gas barrier film 10 and a heat seal layer 30. The gas barrier film 10 of this embodiment is one aspect of the gas barrier film according to the present invention.
The gas barrier film 10 and the heat seal layer 30 are joined by an adhesive layer 20 .
The proportion of polyethylene in the laminate 1 is 90% by mass or more, so that the laminate 1 is configured as a mono-material with high recyclability.

The gas barrier film 10 includes a sheet-like substrate 11, and an undercoat layer 12, an inorganic oxide layer 13, and an overcoat layer 14 formed in this order on a first surface 11a of the substrate 11.
The proportion of polyethylene in the gas barrier film 10 is 90% by mass or more, which makes the gas barrier film 10 a highly recyclable mono-material.
Each component of the gas barrier film 10 will be described below.

The substrate 11 includes polyethylene (PE). The substrate 11 may be a single-layer film made of a single resin, or a single-layer or laminated film made of multiple resins. PE may also be laminated onto another substrate (metal, wood, paper, ceramics, etc.). In other words, the substrate 11 may be a single layer, or two or more layers.

The substrate 11 may be an unstretched film or a stretched film such as a uniaxially stretched or biaxially stretched film.
The density of the PE contained in the substrate 11 is preferably 0.935 or more, and more preferably 0.940 or more. When the density of the PE is within the above range, the substrate 11 is easily prevented from stretching and wrinkling during rolling, and the inorganic oxide layer 13 is easily prevented from cracking.
PE may be at least one type of polymer selected from homopolymers, random copolymers, and block copolymers. Homopolymers are polyethylenes consisting solely of polyethylene. Random copolymers are polyethylenes in which the main monomer, ethylene, and a small amount of a comonomer other than ethylene (e.g., an α-olefin) are randomly copolymerized to form a homogeneous phase. Block copolymers are polyethylenes in which the main monomer, ethylene, and the comonomer (e.g., an α-olefin) are copolymerized in blocks or polymerized in a rubbery form to form a heterogeneous phase.

The substrate 11 may have a multilayer structure including multiple layers (films) each containing PE of different densities. It is desirable to appropriately multilayer the substrate 11, taking into consideration the processability, rigidity, stiffness, heat resistance, and powder shedding during transport of the films constituting each layer. The films constituting the substrate 11 can be made using an appropriate selection of high-density polyolefin, medium-density polyolefin, low-density polyolefin, etc. In this case, too, it is preferable that the density of the substrate 11 as a whole be 0.935 or higher.
Each layer of the substrate 11 may contain a slip agent, an antistatic agent, etc., and the content or content ratio of these may vary from layer to layer. The substrate 11 having multiple layers can be produced by extrusion coating, co-extrusion coating, sheet molding, co-extrusion blow molding, etc.
The first surface 11a of the substrate 11 may be subjected to a surface treatment such as chemical treatment, solvent treatment, corona treatment, low-temperature plasma treatment, or ozone treatment in order to improve adhesion to the undercoat layer 12 or the inorganic oxide layer 13. Furthermore, a similar surface treatment may be applied to the second surface 11b opposite to the first surface 11a in order to bond it to a printing substrate.

The substrate 11 may contain additives such as fillers, antiblocking agents, antistatic agents, plasticizers, lubricants, and antioxidants. These additives may be used alone or in combination of two or more.

There are no particular restrictions on the thickness of the substrate 11, and it can be determined appropriately based on price and application, taking into account suitability as a packaging material and suitability for laminating other films. For practical purposes, the thickness of the substrate 11 is preferably 3 μm to 200 μm, more preferably 5 μm to 120 μm, even more preferably 6 μm to 100 μm, and particularly preferably 10 μm to 40 μm.

(Undercoat layer 12)
The undercoat layer 12 is a layer containing an organic polymer as a main component, and is sometimes called a primer layer. By providing the undercoat layer, the film-forming property and adhesive strength of the inorganic oxide layer 13 can be improved.
The content of the organic polymer in the undercoat layer 12 may be, for example, 70% by mass or more, or 80% by mass or more. Examples of organic polymers include polyacrylic resins, polyester resins, polycarbonate resins, polyurethane resins, polyamide resins, polyolefin resins, polyimide resins, melamine resins, and phenolic resins. In consideration of the hot water resistance of the adhesive strength between the substrate 11 and the inorganic oxide layer 13, it is preferable that the undercoat layer 12 contain at least one of the above polyacrylic resins, polyol resins, polyurethane resins, polyamide resins, or reaction products of these organic polymers.
The undercoat layer 12 may contain a silane coupling agent, an organic titanate, a modified silicone oil, or the like.

More preferably, the organic polymer used in the undercoat layer 12 is an organic polymer having a urethane bond formed by the reaction of a polyol having two or more hydroxyl groups at its terminal with an isocyanate compound, or an organic polymer containing a reaction product of a polyol having two or more hydroxyl groups at its terminal with an organic silane compound such as a silane coupling agent or its hydrolyzate. Either one or both of these may be used.

Examples of the polyols include at least one selected from acrylic polyol, polyvinyl acetal, polystyrene polyol, and polyurethane polyol. The acrylic polyol may be obtained by polymerizing an acrylic acid derivative monomer, or may be obtained by copolymerizing an acrylic acid derivative monomer with another monomer. Examples of the acrylic acid derivative monomer include ethyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and hydroxybutyl methacrylate. Examples of the monomer copolymerized with the acrylic acid derivative monomer include styrene.
The isocyanate compound reacts with the polyol to form a urethane bond, thereby enhancing the adhesion between the substrate 11 and the inorganic oxide layer 13. That is, the isocyanate compound functions as a crosslinking agent or curing agent. Examples of the isocyanate compound include aromatic monomers such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), aliphatic monomers such as xylene diisocyanate (XDI), hexamethylene diisocyanate (HMDI), and isophorone diisocyanate (IPDI), as well as polymers and derivatives thereof. The above-mentioned isocyanate compounds may be used singly or in combination of two or more.

Examples of silane coupling agents include vinyltrimethoxysilane, γ-chloropropylmethyldimethoxysilane, γ-chloropropyltrimethoxysilane, glycidoxypropyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, and γ-methacryloxypropylmethyldimethoxysilane. The organic silane compound may be a hydrolyzate of these silane coupling agents. The organic silane compound may contain one of the above-mentioned silane coupling agents and their hydrolyzates, or a combination of two or more of them.

The undercoat layer 12 can be formed using a mixture of the above-mentioned components in an organic solvent in any desired ratio. The mixture may contain, for example, a curing accelerator such as a tertiary amine, an imidazole derivative, a metal salt compound of a carboxylic acid, a quaternary ammonium salt, or a quaternary phosphonium salt; an antioxidant such as a phenol, sulfur, or phosphite; a leveling agent; a flow adjuster; a catalyst; a crosslinking accelerator; a filler; etc.
The mixed liquid can be disposed in the form of a layer on the substrate 11 by a known printing method such as offset printing, gravure printing, or silk screen printing, or a known coating method such as roll coating, knife edge coating, or gravure coating. After disposing, the mixture can be heated to, for example, 50 to 200° C., thereby forming the undercoat layer 12.

There are no particular restrictions on the thickness of the undercoat layer 12, and it can be, for example, 0.005 to 5 μm. The thickness can be determined appropriately depending on the application and desired properties. The thickness of the undercoat layer 12 is preferably 0.01 to 1 μm, and more preferably 0.01 to 0.5 μm. If the thickness of the undercoat layer 12 is 0.01 μm or more, sufficient adhesion strength between the substrate 11 and the inorganic oxide layer 13 is obtained, and oxygen barrier properties are also good. If the thickness of the undercoat layer 12 is 1 μm or less, it is easy to form a uniform coated surface, and drying load and manufacturing costs can be reduced.

(Inorganic oxide layer 13)
Examples of inorganic oxides constituting the inorganic oxide layer 13 include aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, tin oxide, zinc oxide, and indium oxide. Aluminum oxide or silicon oxide is particularly preferred because it has excellent productivity and excellent oxygen barrier properties and water vapor barrier properties in heat resistance and moist heat resistance. The inorganic oxide layer 13 may be formed of one type of inorganic oxide, or may be formed of two or more appropriately selected inorganic oxides.

The thickness of the inorganic oxide layer 13 can be 1 nm or more and 200 nm or less. If the thickness is 1 nm or more, excellent oxygen barrier property and water vapor barrier property can be obtained. If the thickness is 200 nm or less, the manufacturing cost can be kept low, cracks due to external forces such as bending or pulling are less likely to occur, and deterioration of the barrier property can be suppressed.
The inorganic oxide layer 13 can be formed by a known film formation method such as vacuum deposition, sputtering, ion plating, or plasma chemical vapor deposition (CVD).

(Overcoat layer 14)
A known oxygen barrier film formed by a wet coating method can be used as the overcoat layer 14. The overcoat layer has an optional configuration and does not necessarily have to be provided.
The overcoat layer 14 is obtained by forming a coating film made of a coating agent by a wet coating method on any one of the substrate 11, the undercoat layer 12, and the inorganic oxide layer 13, and then drying the coating film. In this specification, the term "coating film" refers to a wet film, and the term "film" refers to a dry film.

The overcoat layer 14 may include a film containing at least one of a metal alkoxide and its hydrolysate, or a reaction product thereof, and a water-soluble polymer (hereinafter sometimes referred to as an "organic-inorganic composite film"). It is preferable that the overcoat layer 14 further includes at least one of a silane coupling agent and its hydrolysate.

_{Examples of} metal alkoxides and hydrolysates thereof contained in the organic-inorganic composite coating include those represented by the general formula M(OR) _n , such as tetraethoxysilane [Si( _OC2H5 ) ₄ ] and triisopropoxyaluminum [Al( _OC3H7 ) ₃ ], as well as their hydrolysates. Only one of these may be contained, or two or more may be contained in appropriate combination.

In the organic-inorganic composite coating, the total content of at least one of the metal alkoxide and its hydrolysate, or its reaction product, is, for example, 40 to 70 mass%. From the perspective of further reducing oxygen permeability, the lower limit of this total content may be 50 mass% and the upper limit of this total content may be 65 mass%.

The water-soluble polymer contained in the organic-inorganic composite coating is not particularly limited, and examples include various polymers such as polyvinyl alcohol, polysaccharides such as starch, methyl cellulose, and carboxymethyl cellulose, and acrylic polyols. From the perspective of further improving oxygen gas barrier properties, it is preferable to include a polyvinyl alcohol-based polymer. The number-average molecular weight of the water-soluble polymer is, for example, 40,000 to 180,000.

Water-soluble polymers based on polyvinyl alcohol can be obtained, for example, by saponifying (including partial saponification) polyvinyl acetate. These water-soluble polymers may have several tens of percent or only a few percent of acetate groups remaining.

The content of the water-soluble polymer in the organic-inorganic composite coating is, for example, 15 to 50% by mass. A water-soluble polymer content of 20 to 45% by mass is preferable, as it further reduces the oxygen permeability of the organic-inorganic composite film.

The silane coupling agents and their hydrolysates contained in the organic-inorganic composite coating include silane coupling agents having organic functional groups. Examples of such silane coupling agents and their hydrolysates include ethyltrimethoxysilane, vinyltrimethoxysilane, gamma-chloropropylmethyldimethoxysilane, gamma-chloropropyltrimethoxysilane, glycidoxypropyltrimethoxysilane, gamma-methacryloxypropyltrimethoxysilane, gamma-methacryloxypropylmethyldimethoxysilane, and hydrolysates thereof. Only one of these may be contained, or two or more may be contained in appropriate combination.

It is preferable that at least one of the silane coupling agent and its hydrolyzate has an epoxy group as the organic functional group. Examples of silane coupling agents having an epoxy group include γ-glycidoxypropyltrimethoxysilane and β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. Epoxy-containing silane coupling agents and their hydrolyzates may have an organic functional group other than the epoxy group, such as a vinyl group, amino group, methacryl group, or ureyl group.

A silane coupling agent having an organic functional group and its hydrolyzate can further improve the oxygen barrier properties of the overcoat layer 14 and its adhesion to the undercoat layer 12 or inorganic oxide layer 13 through the interaction between the organic functional group and the hydroxyl groups of the water-soluble polymer. In particular, the interaction between the epoxy groups of the silane coupling agent and its hydrolyzate and the hydroxyl groups of the polyvinyl alcohol can form an overcoat layer 14 with particularly excellent oxygen barrier properties and adhesion.

The total content of at least one of the silane coupling agent and its hydrolysate or reaction product thereof in the organic-inorganic composite coating is, for example, 1 to 15 mass%. If the total content of at least one of the silane coupling agent and its hydrolysate or reaction product thereof is 2 to 12 mass%, the oxygen permeability of the organic-inorganic composite coating can be further reduced, which is preferable.

The organic-inorganic composite coating may contain a crystalline inorganic layered compound having a layered structure. Examples of inorganic layered compounds include clay minerals such as kaolinite, smectite, and mica. These can be used alone or in appropriate combinations of two or more. The particle size of the inorganic layered compound is, for example, 0.1 to 10 μm. The aspect ratio of the inorganic layered compound is, for example, 50 to 5,000.

Smectite clay minerals are preferred as inorganic layered compounds, as they can form a coating with excellent oxygen barrier properties and adhesive strength by allowing water-soluble polymers to penetrate between the layers of the layered structure (intercalation). Specific examples of smectite clay minerals include montmorillonite, hectorite, saponite, and water-swellable synthetic mica.

The thickness of the overcoat layer 14 is set according to the required oxygen barrier properties and can be, for example, 0.05 to 5 μm, preferably 0.05 to 1 μm, and more preferably 0.1 to 0.5 μm. If the thickness of the overcoat layer 14 is 0.05 μm or more, sufficient oxygen barrier properties are likely to be obtained. If the thickness of the overcoat layer 14 is 1 μm or less, it is easy to form a uniform coated surface, and drying load and manufacturing costs can be reduced.

The overcoat layer 14, made of an organic-inorganic composite film, exhibits excellent oxygen barrier properties even after boiling or retort sterilization. The laminate 1, in which a sealant film is bonded to a gas barrier film 10, has sufficient adhesion and sealing strength for use as a packaging material for boiling or retort processing, and also combines transparency, flex resistance, and stretch resistance not found in metal foils or metal-deposited films. Another advantage is that there is no risk of generating harmful substances such as dioxins.

(Adhesive layer 20)
A known dry laminating adhesive can be used as the adhesive layer 20. There are no particular restrictions on the dry laminating adhesive that can be used, and specific examples include two-component curing ester-based adhesives, ether-based adhesives, and urethane-based adhesives.

A gas barrier adhesive that exhibits gas barrier properties after curing can also be used for the adhesive layer 20. Use of a gas barrier adhesive can improve the gas barrier properties of the laminate 1. The oxygen permeability of the gas barrier adhesive is preferably 150 cc/ ^m2 ·day·atm or less, more preferably 100 cc/ ^m2 ·day·atm or less, even more preferably 80 cc/ ^m2 ·day·atm or less, and particularly preferably 50 cc/ ^m2 ·day·atm or less. Having an oxygen permeability within the above range can sufficiently improve the gas barrier properties of the laminate 1, and can also suppress a decrease in gas barrier properties due to the gas barrier adhesive penetrating into the gaps, even if minor cracks or the like occur in the inorganic oxide layer 13 or the overcoat layer 14.
Examples of gas barrier adhesives include epoxy adhesives, polyester/polyurethane adhesives, etc. Specific examples of gas barrier adhesives include "Maxieve" manufactured by Mitsubishi Gas Chemical Company, Inc. and "Paslim" manufactured by DIC Corporation.

When the adhesive layer 20 is made of a gas barrier adhesive, its thickness is preferably at least 50 times the thickness of the inorganic oxide layer 13. When the thickness is within the above range, cracking of the inorganic oxide layer 13 can be more sufficiently suppressed and the gas barrier properties of the laminate 1 can be further improved. Furthermore, the adhesive layer 20 can be imparted with cushioning properties that absorb external impacts, thereby preventing the inorganic oxide layer 13 from cracking due to impact. From the viewpoints of maintaining the flexibility of the laminate 1, processability, and cost, the thickness is preferably no more than 300 times the thickness of the inorganic oxide layer 13.
Such thickness is expressed in numerical values, for example, from 0.1 to 20 μm, preferably from 0.5 to 10 μm, and more preferably from 1 to 5 μm.

The adhesive that forms the adhesive layer 20 can be applied by, for example, bar coating, dipping, roll coating, gravure coating, reverse coating, air knife coating, comma coating, die coating, screen printing, spray coating, gravure offset, or the like. The temperature for drying the adhesive coating can be, for example, 30 to 200°C, with 50 to 180°C being preferred. The temperature for curing the coating can be, for example, room temperature to 70°C, with 30 to 60°C being preferred. By keeping the drying and curing temperatures within the above ranges, the occurrence of cracks in the inorganic oxide layer 13 and adhesive layer 20 can be further suppressed, and excellent gas barrier properties can be achieved.

From the perspective of preventing cracking of the inorganic oxide layer 13, it is preferable that the adhesive layer 20 and the inorganic oxide layer 13 are in direct contact with each other, but other layers may be present between the adhesive layer 20 and the inorganic oxide layer 13.

(Heat seal layer 30)
The heat seal layer 30 is a layer containing polyolefin, and functions as a sealant when a packaging bag or the like is produced using the laminate 1. A polyolefin film can be used as the heat seal layer 30. By using a polyethylene film as the heat seal layer 30, the laminate 1 can be made into a mono-material.

Polyolefin-based resins that can be used for the heat-seal layer 30 include ethylene-based resins such as low-density polyethylene resin (LDPE), medium-density polyethylene resin (MDPE), linear low-density polyethylene resin (LLDPE), ethylene-vinyl acetate copolymer (EVA), ethylene-α-olefin copolymer, and ethylene-(meth)acrylic acid copolymer; blends of polyethylene and polybutene; and polypropylene-based resins such as homopolypropylene resin (PP), propylene-ethylene random copolymer, propylene-ethylene block copolymer, and propylene-α-olefin copolymer. These thermoplastic resins can be selected appropriately depending on the intended use and temperature conditions, such as boiling treatment.

The heat seal layer 30 may contain various additives such as a flame retardant, a slip agent, an antiblocking agent, an antioxidant, a light stabilizer, and a tackifier.
The thickness of the heat seal layer 30 can be appropriately set in consideration of the shape of the packaging bag to be manufactured, the mass of the contents to be contained, etc., and can be, for example, 30 to 150 μm.

When a laminate 1 having an adhesive layer 20 and a heat seal layer 30 is produced using a polyolefin film, either a dry lamination method in which the films are bonded together using an adhesive such as a one-component curing or two-component curing urethane adhesive, or a non-solvent dry lamination method in which the films are bonded together using a solvent-free adhesive can be applied.
Alternatively, the heat seal layer 30 can be formed by extrusion lamination, in which a thermoplastic resin is heated and melted, extruded into a curtain shape, and laminated together. In this case, the adhesive layer 20 may be omitted.

The above is the basic structure of the laminate 1. In the laminate 1, the inorganic oxide layer 13 is located between the substrate 11 and the heat seal layer.
The gas barrier film 10 can be used alone for various packaging materials that require gas barrier properties, but by preparing one or more laminates each having a heat seal layer 30 on the gas barrier film 10, and placing the heat seal layers 30 opposite each other and heat-sealing the edges, various packaging materials such as packaging bags and standing pouches can be formed using the laminate 1.

After conducting various studies into cases where the adhesion between the polyethylene substrate and the heat seal layer is insufficient, the inventors discovered that the birefringence of the substrate affects adhesion. Birefringence ΔN is the absolute value of the difference Nx - Ny between the refractive index Nx in the machine direction of the substrate 11 and the refractive index Ny in the transverse direction, and can be measured and calculated using the parallel Nicol rotation method.

An example of a method for measuring the birefringence ΔN is shown below.
Uniaxially polarized measurement light (wavelength 586.6 nm) is incident on the film to be measured from the normal direction of the film. The light transmitted through the film is decomposed into linearly polarized light in the MD direction and linearly polarized light in the TD direction, and the refractive indexes Nx and Ny of each linearly polarized light are measured. The birefringence ΔN is calculated based on the measured Nx and Ny.

The MD direction and the TD direction are two mutually perpendicular directions defined in a resin film. Generally, in a resin film distributed in a roll state, the longitudinal direction is the MD direction and the width direction is the TD direction. In a resin film distributed in a rectangular or square shape, the direction in which one side extends is the MD direction, and the direction in which another side perpendicular to the one side extends is the TD direction.
Usually, when a packaging material is manufactured using a resin film and the shape of the packaging material viewed in the normal direction of the resin film is rectangular or square, the set of MD and TD directions specified in the above manner coincides with the set of MD and TD directions of the resin film used in the manufacture. Therefore, when measuring the birefringence ΔN using such a packaging material, the MD and TD directions can be specified in the above manner.

The inventors' investigations have revealed that when the birefringence ΔN of the substrate 11 is 0 or more and 0.007 or less, sufficient adhesive strength can be ensured between the substrate 11 provided with the inorganic oxide layer 13 and the heat seal layer 30.
In this specification, "sufficient adhesion strength" means that the lamination strength between the substrate 11 and the heat seal layer 30 measured in accordance with JIS K6854 is 2N or more. As will be shown later using examples, in the laminate 1 according to this embodiment, sufficient adhesion strength between the substrate 11 and the heat seal layer 30 is ensured.

The birefringence ΔN shows a certain correlation with the degree of orientation of the resin film, and tends to be smaller for unstretched films, but the birefringence ΔN of a very small number of stretched films may also fall within the above numerical range. In other words, the birefringence ΔN is a parameter independent of the general distinction between stretched and unstretched resin films, and its relationship with the adhesion strength to the heat seal layer 30 was discovered for the first time by the inventors.

An example of a manufacturing procedure for the gas barrier film 10 and the laminate 1 will be described.
First, a substrate 11 having a birefringence ΔN of 0 or more and 0.007 or less is selected. The substrate 11 may be a commercially available product or may be produced by a known method.
The substrate 11 may be formed by laminating a plurality of resin films. In this case, it is preferable that the birefringence ΔN of all of the plurality of resin films is 0 or more and 0.007 or less, since this allows the interlayer bonding strength of the substrate to be maintained high.

Next, an undercoat layer 12 (if necessary) and an inorganic oxide layer 13 are formed on the substrate 11 .
When forming the undercoat layer 12, for example, a mixed liquid for forming the undercoat layer 12 may be applied to the first surface 11a to form a coating film, and the coating film may be dried (to remove the solvent).
The mixed solution can be applied by a known wet coating method, such as roll coating, gravure coating, reverse coating, die coating, screen printing, or spray coating.
The coating film made from the mixed solution can be dried by known drying methods such as hot air drying, heat roll drying, infrared irradiation, etc. The drying temperature for the coating film can be, for example, 50 to 200° C. The drying time varies depending on the thickness of the coating film, the drying temperature, etc., but can be, for example, 1 second to 5 minutes.
The inorganic oxide layer 13 can be formed by the above-mentioned vacuum deposition method, sputtering method, ion plating method, plasma chemical vapor deposition (CVD) method, or the like.

If necessary, an overcoat layer 14 is formed on the inorganic oxide layer 13 .
The overcoat layer 14 can be formed, for example, by applying a mixed liquid for forming the overcoat layer 14 to form a coating film, and then drying the coating film.
The method of applying and drying the mixed liquid can be the same as those described in the step of forming the undercoat layer 12 .
The overcoat layer 14 may be formed by a single coating and drying process, or by repeated coating and drying processes using the same or different mixed liquids multiple times.

The laminate 1 may further include a printed layer, a protective layer, a light-shielding layer, and other functional layers, as required.
The printed layer can be provided in a position visible from the outside of the laminate or packaging material in order to display information about the contents, identify the contents, or improve the design of the packaging bag. The printing method and printing ink are not particularly limited, and can be appropriately selected from known printing methods and printing inks in consideration of printability on the film, design such as color tone, adhesion, safety as a food container, etc.
Examples of printing methods include gravure printing, offset printing, gravure offset printing, flexographic printing, and inkjet printing. Among these, gravure printing is preferred from the viewpoints of productivity and high definition of the image. To improve the adhesion of the printed layer, the surface of the layer on which the printed layer is formed may be subjected to various pretreatments such as corona treatment, plasma treatment, and flame treatment, or a coating layer such as an easy-adhesion layer may be provided.
The printing layer can be provided between the inorganic oxide layer and the adhesive layer, between the overcoat layer and the adhesive layer, etc. Furthermore, as will be described later, when the substrate has a multi-layer structure, the printing layer can also be provided within the substrate.

The gas barrier film of this embodiment will be further described using examples and comparative examples. The present invention is not limited in any way by the specific content of the examples and comparative examples.

The resin films used in the examples and comparative examples are shown below.
α1: Unstretched polyethylene film (thickness 32 μm, density 0.950 g/cm ³ , corona treated on one side)
α2: Unstretched polyethylene film (thickness 25 μm, density 0.952 g/cm ³ , corona treated on one side)
α3: Uniaxially stretched polyethylene film (thickness 25 μm, density 0.950 g/cm ³ , corona treated on one side)
α4: Biaxially oriented polyethylene film (thickness 25 μm, density 0.950 g/cm ³ , one side corona treated)

(Preparation of Mixture for Undercoat Layer)
Acrylic polyol and tolylene diisocyanate were mixed so that the number of NCO groups in tolylene diisocyanate was equal to the number of OH groups in the acrylic polyol, and the mixture was diluted with ethyl acetate to a total solids content (total amount of acrylic polyol and tolylene diisocyanate) of 5% by mass. β-(3,4-epoxycyclohexyl)trimethoxysilane was then added and mixed in an amount of 5 parts by mass per 100 parts by mass of the total amount of acrylic polyol and tolylene diisocyanate.
In this way, a mixed solution for forming an undercoat layer was obtained.

(Preparation of Mixture for Overcoat Layer)
Liquids A, B, and C shown below were mixed in a mass ratio of 70/20/10, respectively, to obtain a mixed liquid for an overcoat layer.
Solution A: A hydrolysis solution with a solid content of 5% by mass ( _{SiO2 equivalent} ) obtained by adding 72.1 g of 0.1N hydrochloric acid to 17.9 g of tetraethoxysilane (Si( _OC2H5 ) ₄ ) and 10 g of methanol and stirring for 30 minutes to hydrolyze the mixture. Solution B: A 5% by mass water/methanol solution of polyvinyl alcohol (water:methanol mass ratio 95:5).
Liquid C: A hydrolysis solution obtained by diluting 1,3,5-tris(3-trimethoxysilylpropyl)isocyanurate with a mixture of water and isopropyl alcohol (a mass ratio of water:isopropyl alcohol of 1:1) to a solid content of 5% by mass.

The two types of adhesives used in the adhesive layer are shown below.
(urethane adhesive)
An adhesive (gas barrier adhesive) obtained by mixing 100 parts by mass of Takelac A525 manufactured by Mitsui Chemicals, Inc., 11 parts by mass of Takenate A52 manufactured by Mitsui Chemicals, Inc., and 84 parts by mass of ethyl acetate.
An adhesive prepared by mixing 23 parts by mass of a solvent made by mixing ethyl acetate and methanol in a mass ratio of 1:1 with 16 parts by mass of Maxieve C93T manufactured by Mitsubishi Gas Chemical Company, Inc. and 5 parts by mass of Maxieve M-100 manufactured by Mitsubishi Gas Chemical Company, Inc.

Example 1
The resin film α1 was used as the substrate. The mixture for the undercoat layer was applied to the corona-treated surface (first surface) of the substrate by gravure coating, followed by drying and curing to form an undercoat layer with a coating amount of 0.1 g/ ^m2 .
Next, a transparent inorganic oxide layer (silica vapor deposition film) made of silicon oxide was formed on the undercoat layer to a thickness of 30 nm using an electron beam heating vacuum deposition apparatus, with an O/Si ratio of 1.8.
Furthermore, the mixed solution for the overcoat layer was applied onto the inorganic oxide layer by gravure coating, and then dried and cured to form an overcoat layer having a thickness of 0.3 μm.
In this way, a gas barrier film according to Example 1 was obtained.
The above-mentioned urethane adhesive was applied by gravure coating and dried onto the overcoat layer of the gas barrier film according to Example 1 to form a 3 μm thick adhesive layer. A 60 μm thick unstretched film made of LLDPE (manufactured by Mitsui Chemicals Tocello, Inc., product name: TUX-MCS) was attached to this adhesive layer by dry lamination as a heat seal layer. This was then subjected to aging at 40°C for 4 days. This yielded a laminate according to Example 1.

Example 2
A gas barrier film and a laminate according to Example 2 were obtained in the same manner as in Example 1, except that resin film α2 was used as the substrate.
Example 3
A gas barrier film according to Example 3 was obtained in the same manner as in Example 1, except that no overcoat layer was provided.
The gas barrier adhesive was applied by gravure coating and dried onto the inorganic oxide layer of the gas barrier film of Example 3 to form a 3 μm thick adhesive layer. A 60 μm thick unstretched film made of LLDPE (manufactured by Mitsui Chemicals Tocello, Inc., product name: TUX-MCS) was attached to this adhesive layer by dry lamination as a heat seal layer. This was then subjected to aging at 40°C for 4 days. This gave a laminate of Example 3.
Example 4
A gas barrier film and a laminate according to Example 4 were obtained in the same manner as in Example 3, except that resin film α2 was used as the substrate.

Example 5
A pattern was printed on one surface of the resin film α1 by gravure printing to form a printed layer. The urethane adhesive was applied to the printed layer, and the printed layer was attached by dry lamination to the surface (second surface) of the substrate of the gas barrier film of Example 1 on which the inorganic oxide layer was not provided, to produce a gas barrier film of Example 5. Similarly, the printed resin film α1 was attached to the laminate of Example 1 to produce a laminate of Example 5.
The layer structure of the laminate according to Example 5 is shown in Figure 2. The substrate of the laminate 1A shown in Figure 2 is configured by bonding a first polyethylene film 111 having a printed layer 115 to the second surface 11b of the substrate 11 with an adhesive layer 120. That is, the substrate according to Example 5 has a configuration in which two polyethylene layers are bonded together with an adhesive layer.

Example 6
A gas barrier film and a laminate according to Example 6 were obtained in the same manner as in Example 5, except that the gas barrier film and the laminate according to Example 3 were used.

Example 7
A gas barrier film and a laminate according to Example 7 were obtained in the same manner as in Example 6, except that no undercoat layer was provided.

(Example 8)
A gas barrier film and a laminate according to Example 8 were obtained in the same manner as in Example 3, except that the undercoat layer and the overcoat layer were not provided.

(Comparative Example 1)
A gas barrier film and a laminate according to Comparative Example 1 were obtained in the same manner as in Example 1, except that resin film α3 was used as the substrate.

(Comparative Example 2)
A gas barrier film and a laminate according to Comparative Example 2 were obtained in the same manner as in Example 3, except that resin film α3 was used as the substrate.

(Comparative Example 3)
A gas barrier film and a laminate according to Comparative Example 3 were obtained in the same manner as in Example 3, except that resin film α4 was used as the substrate.

The gas barrier films and laminates of the Examples and Comparative Examples were evaluated for the following items.
(Measurement of birefringence ΔN of substrate)
Before producing the gas barrier film, the birefringence ΔN of the substrate was calculated under the following measurement conditions. In all cases, the corona-treated surface was measured.
Apparatus: Phase difference measuring device (KOBRA-WR, manufactured by Oji Scientific Instruments Co., Ltd.)
Light source wavelength: 586.6nm
Measurement method: Parallel Nicol rotation method, direct measurement

(Recyclability)
The proportion (wt %) of polyethylene in the laminate of each example was calculated based on the following formula (1).
(mass of resin films α1 to α4 used + mass of heat seal layer) / mass of entire laminate × 100 (1)
The evaluation was made in the following two stages.
◯ (good): The proportion of polyethylene in the laminate is 90 wt% or more. × (bad): The proportion of polyethylene in the laminate is less than 90 wt%.

(Lamination strength between substrate and heat seal layer)
In accordance with JIS Z1707, a 15 mm wide rectangular test piece was cut out from the laminate of each example, and the lamination strength between the substrate and the heat seal layer was measured using an Orientec Tensilon universal testing machine RTC-1250. The conditions were as follows:
T-type peeling normal (DryT)
T-type peeling Measurement site wetness (WetT)

(Oxygen Transmission Rate: OTR)
The laminate of each example was measured under conditions of 30° and 70% RH (relative humidity) by the Mocon method.
(Water vapor transmission rate: WVTR)
The laminate of each example was measured under conditions of 40° and 90% RH by the Mocon method.
The results are shown in Table 1.

All of the Examples had good recyclability and sufficient adhesive strength between the substrate and the heat seal layer. All of the Examples showed good gas barrier properties, but Examples 3, 4, and 6, which used a gas barrier adhesive in the adhesive layer, had particularly excellent gas barrier properties.
On the other hand, although the comparative examples had good recyclability, the adhesive strength between the substrate and the heat seal layer was insufficient, and there was concern that delamination or bag breakage might occur depending on the application.

The above describes one embodiment of the present invention and examples, but the specific configuration is not limited to this embodiment, and configuration changes and combinations that do not deviate from the gist of the present invention are also included.

1, 1A Laminate 10 Gas barrier film 11, 11A Substrate 11a First surface 12 Undercoat layer 13 Inorganic oxide layer 14 Overcoat layer 20 Adhesive layer 30 Heat seal layer

Claims (7)

A packaging material formed using a laminate comprising a gas barrier film and a heat seal layer containing polyethylene, wherein the polyethylene accounts for 90% by mass or more of the entire laminate ,
The laminate has only one heat seal layer,
The gas barrier film is
a substrate comprising polyethylene;
an inorganic oxide layer formed on the first surface side of the substrate,
the birefringence ΔN of the first surface calculated based on measurement by a parallel Nicol rotation method is 0 or more and 0.007 or less,
the inorganic oxide layer comprises silicon oxide;
Does not have an oxygen barrier coating containing water-swellable mica
It has a structure
the heat seal layer is bonded to the gas barrier film so as to sandwich the inorganic oxide layer between the heat seal layer and the base material,
The heat seal layers are formed by heat fusing together.
packaging materials. The birefringence ΔN is 0.0029 or more and 0.007 or less,
The packaging material of claim 1. The inorganic oxide film further includes an undercoat layer provided between the substrate and the inorganic oxide layer.
The packaging material according to claim 1 or 2. The substrate has a plurality of layers containing polyethylene.
The packaging material according to any one of claims 1 to 3. the gas barrier film and the heat seal layer are bonded together by an adhesive layer;
The packaging material of claim 1 . The adhesive layer is made of a gas barrier adhesive.
The packaging material of claim 1 . an overcoat layer provided between the inorganic oxide layer and the heat seal layer;
The packaging material of claim 1 .