JP7805533B2 - Multilayer structure and packaging material - Google Patents

Description

The present invention relates to multilayer structures and packaging materials.

Packaging materials for long-term food storage often require gas barrier properties, including oxygen barrier properties. Using packaging materials with high gas barrier properties can prevent oxidative deterioration of food due to oxygen intrusion and microbial growth. Metal foils (e.g., aluminum foils), metal vapor deposition layers, and inorganic vapor deposition layers (e.g., silicon oxide and aluminum oxide) are commonly used to improve gas barrier properties. Meanwhile, resin layers with gas barrier properties, such as vinyl alcohol polymers and polyvinylidene chloride, are also widely used. Vinyl alcohol polymers exhibit gas barrier properties through hydrogen bonding between hydroxyl groups within the molecules, resulting in crystallization and densification. Among these, ethylene-vinyl alcohol copolymers (hereinafter sometimes abbreviated as "EVOH") are suitable for melt molding due to their excellent thermal stability. With the development of coextrusion technology, multilayer structures with an EVOH layer as an intermediate layer have become widely used as gas barrier packaging materials.

Furthermore, in recent years, driven by environmental and waste issues, there has been a growing global demand for post-consumer recycling (hereinafter simply referred to as "recycling"), which involves collecting and recycling packaging materials consumed in the market. Recycling typically involves shredding collected packaging materials, separating and cleaning them as necessary, and then melt-mixing them using an extruder. In this regard, packaging materials must be composed of as few materials as possible (mono-materialization), which allows for the production of high-purity, high-quality recycled raw materials. For example, gas-barrier packaging materials containing a polyamide layer and an EVOH layer are commonly used due to their excellent mechanical strength. However, chemical reactions between the polyamide resin and the EVOH can produce particles, which can reduce recyclability. Taking these challenges into consideration, it is necessary to achieve mono-materialization to provide gas-barrier packaging materials with excellent recyclability while maintaining gas barrier properties and mechanical strength.

Patent Document 1 describes a multilayer structure having a hard layer with a puncture strength of 40 N/mm or more and 150 N/mm or less and (1) a resin composition layer containing EVOH with a melting point of 170°C or more and EVOH with a melting point of less than 170°C, or (2) a resin composition layer containing modified EVOH containing a modifying group with a specific primary hydroxyl group, which, despite not having a polyamide layer, has excellent mechanical strength and thermoformability, and when the recovered material is melt-molded, the generation of lumps due to resin degradation (gelation) is suppressed, making it highly recyclable.

International Publication No. 2020/071513

However, when used as packaging material for heavy contents, there is a tendency for higher resistance to breakage due to dropping, and the multilayer structure described in Patent Document 1 sometimes lacks resistance to breakage due to dropping. In particular, when the temperature of the contents is high, such as when the contents are filled with heat, sterilized by heat, or cooked by heat, the resistance to breakage due to dropping can sometimes be particularly insufficient.

The present invention has been made to solve the above problems, and aims to provide a multilayer structure and packaging material that have excellent gas barrier properties, resistance to bag breakage due to dropping, and recyclability.

According to the present invention, the above object is to
[1] A film having a sealant layer (X) and a substrate layer (Y), wherein the sealant layer (X) comprises a barrier layer (A) containing, as a main component, an ethylene-vinyl alcohol copolymer (a) (hereinafter sometimes abbreviated as "EVOH (a)") having an ethylene unit content of 20 to 50 mol% and a saponification degree of 90 mol% or more, an adhesive layer (B) containing, as a main component, an adhesive resin (b), and a film having a density of 0.880 to 0.930 g/cm a multilayer structure having a heat-sealable layer (C) containing, as a main component, an ethylene-α-olefin copolymer resin (c) ( ^hereinafter , may be simply referred to as "ethylene-α-olefin copolymer resin (c)") of 3, a base layer (Y) containing, as a main component, polypropylene (y) (hereinafter, may be simply referred to as "polypropylene (y)") having a melting point measured by a differential scanning calorimeter (DSC) of 150°C or more, the base layer (Y) being a biaxially stretched layer having an elastic modulus of 2000 MPa or more in both directions under conditions of 23°C and 50% RH, and the heat-sealable layer (C) of the sealant layer (X) being on the outermost surface;
[2] The multilayer structure of [1], wherein the difference (S-S Y ) between the puncture strength S measured according to JIS Z 1707:2019 under conditions of 23°C and 50% RH and the puncture strength S _Y of the base layer ( _Y ) measured according to JIS Z 1707:2019 under conditions of 23°C and 50% RH is 3.5 N or more;
[3] The multilayer structure according to [2], wherein the puncture strength S is 12 N or more;
[4] The multilayer structure of any one of [1] to [3], wherein when the temperature of the base layer (Y) is increased from 20°C to 220°C at 10°C/min and then decreased to -50°C at 10°C/min using a differential scanning calorimeter (DSC), the difference (T1-T2) between the maximum endothermic peak temperature (T1) during temperature increase and the maximum exothermic peak temperature (T2) during temperature decrease is 56°C or less;
[5] The multilayer structure of any one of [1] to [4], wherein the sealant layer (X) and the substrate layer (Y) are laminated via an adhesive layer (Z), and the thickness of the adhesive layer (Z) is 2 to 7 μm;
[6] The multilayer structure of any one of [1] to [5], wherein the ethylene-α-olefin copolymer resin (c) is a linear low-density polyethylene copolymerized with ethylene and an α-olefin having 6 or more carbon atoms;
[7] The multilayer structure according to any one of [1] to [6], wherein the barrier layer (A) contains a boron compound in an amount of 50 to 400 ppm in terms of elemental boron;
[8] The multilayer structure according to any one of [1] to [7], wherein the ethylene unit content of the EVOH (a) is 22 mol% or more and 28 mol% or less;
[9] The multilayer structure of any one of [1] to [8], wherein the thickness of the barrier layer (A) is 5 μm or less;
[10] The multilayer structure according to any one of [1] to [9], wherein the oxygen transmission rate measured under conditions of 20°C and 65% RH according to the method described in JIS K 7126-2:2006 is 5 cc/( ^m2 ·day·atm) or less;
[11] The multilayer structure of any one of [1] to [10], wherein the water vapor transmission rate measured under conditions of 40°C and 90% RH by the method described in JIS K 7129-2:2019 is 3 g/( ^m2 ·day) or less;
[12] A packaging material comprising the multilayer structure according to any one of [1] to [11];
This is achieved by providing

The multilayer structure and packaging material of the present invention have excellent gas barrier properties, resistance to bag breakage due to dropping, and recyclability.

Embodiments of the present invention are described below. Note that, although specific materials (compounds, etc.) that exhibit specific functions are sometimes exemplified in the following description, the present invention is not limited to embodiments that use such materials. Unless otherwise specified, the exemplified materials may be used alone or in combination.

The multilayer structure of the present invention has a sealant layer (X) and a base layer (Y). The sealant layer (X) has a barrier layer (A) containing, as a main component, EVOH (a) having an ethylene unit content of 20 to 50 mol% and a degree of saponification of 90 mol% or more, an adhesive layer (B) containing, as a main component, an adhesive resin (b), and a heat-sealable layer (C) containing, as a main component, an ethylene-α-olefin copolymer resin (c) having a density ^of 0.880 to 0.930 g/cm3. The base layer (Y) is a biaxially stretched layer containing, as a main component, polypropylene (y) having a melting point as measured by differential scanning calorimetry (DSC) of 150°C or more, and has an elastic modulus in both of the biaxial directions at 23°C and 50% RH of 2000 MPa or more. The heat-sealable layer (C) of the sealant layer (X) is located on the outermost surface. "Containing as a main component" means containing more than 50% by mass, but preferably contains 70% by mass or more, more preferably contains 80% by mass or more, even more preferably contains 90% by mass or more, and may be an embodiment in which it contains 95% by mass or more, 97% by mass or more, or 99% by mass or more.

The multilayer structure of the present invention tends to have improved gas barrier properties while maintaining recyclability by providing a barrier layer (A). The multilayer structure of the present invention tends to have improved mechanical strength, including resistance to bag breakage due to dropping, and improved recyclability by providing an adhesive layer (B). The heat-sealable layer (C) mainly contains an ethylene-α-olefin copolymer resin (c) having a density of 0.880 to 0.930 g/ ^cm3 , which tends to achieve excellent mechanical strength, including resistance to bag breakage due to dropping, while maintaining heat sealability. The multilayer structure of the present invention tends to have excellent mechanical strength, including resistance to bag breakage due to dropping, even under high-temperature conditions by providing a base layer (Y) mainly containing polypropylene (y) having a melting point of 150°C or higher as measured by differential scanning calorimetry (DSC), which is a biaxially stretched layer, and having a modulus of elasticity in both directions of 2000 MPa or higher at 23°C and 50% RH.

"Bag drop resistance" can be evaluated by the number of tests required before water leakage occurs when a bag made using the multilayer structure of the present invention is filled with contents and dropped from a certain height, and can be evaluated specifically by the method described in the Examples. The term "bag drop resistance" basically refers to resistance to bag drop at room temperature and at high temperatures. "Recyclability" can be evaluated by the occurrence of lumps in a molten molded product obtained by melt-kneading crushed material from the multilayer structure, and can be evaluated specifically by the method described in the Examples. "Barrier properties" refer to oxygen barrier properties and water vapor barrier properties, and "gas barrier properties" refers to oxygen barrier properties.

"ppm" means the content by mass (mass ppm).

The "surface (or surface layer)" of a multilayer film or multilayer structure does not refer to the front or back, but rather to the exposed surface. In other words, a multilayer film or multilayer structure has two surfaces. Similarly, a multilayer film or multilayer structure has two outermost surfaces.

"Consisting essentially of" allows for the inclusion of optional components to the extent that they do not affect the effects of the present invention, while "consisting only of" means excluding optional components other than impurities that are inevitably included.

<Barrier Layer (A)>
The multilayer structure of the present invention has a barrier layer (A) containing EVOH (a) as a main component. EVOH (a) has excellent gas barrier properties, so a multilayer structure having a layer containing EVOH (a) as a main component is preferably used as a packaging material with excellent content preservation properties. EVOH (a) can be easily melt-mixed with polyolefin-based resins, so a packaging material with excellent recyclability can be provided.

EVOH (a) is typically obtained by saponifying an ethylene-vinyl ester copolymer obtained by polymerizing ethylene and a vinyl ester. A typical example of a vinyl ester is vinyl acetate, but other fatty acid vinyl esters (such as vinyl formate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, and vinyl versatate) can also be used.

The ethylene unit content of EVOH (a) is 20 to 50 mol%. An ethylene unit content of 20 mol% or more improves the melt moldability of EVOH (a) and the pulverized product of the multilayer structure containing EVOH (a). The ethylene unit content is preferably 21 mol% or more, more preferably 22 mol% or more, and may be 25 mol% or more. On the other hand, an ethylene unit content of 50 mol% or less improves the gas barrier properties of the multilayer structure of the present invention. The ethylene unit content is preferably 46 mol% or less, more preferably 42 mol% or less, even more preferably 39 mol% or less, even more preferably 33 mol% or less, and particularly preferably 28 mol% or less. The saponification degree of EVOH (a) is 90 mol% or more. The "saponification degree" refers to the ratio of the number of vinyl alcohol units to the total number of vinyl alcohol units and vinyl ester units in EVOH (a). A saponification degree of 90 mol% or more improves the gas barrier properties of the multilayer structure of the present invention. The saponification degree is preferably 95 mol% or more, more preferably 99 mol% or more, and even more preferably 99.9 mol% or more. The saponification degree may be 100 mol% or less. The ethylene unit content and saponification degree of EVOH (a) are determined by ¹ H-NMR measurement.

EVOH (a) may be a mixture of two or more types of EVOH with different ethylene unit contents. In this case, the difference in ethylene unit content between the EVOHs with the most different ethylene unit contents is preferably 30 mol% or less, more preferably 25 mol% or less, even more preferably 20 mol% or less, and may be 3 mol% or more. Similarly, EVOH (a) may be a mixture of two or more types of EVOH with different degrees of saponification. In this case, the difference in saponification degree between the EVOHs with the most different degrees of saponification is preferably 7 mol% or less, more preferably 5 mol% or less, and may be 0.5 mol% or more. When it is desired to achieve both thermoformability and gas barrier property at a higher level, it is preferred to use as EVOH (a) an EVOH (a1) having an ethylene unit content of 22 mol% or more and less than 34 mol% and a degree of saponification of 99 mol% or more and an EVOH (a2) having an ethylene unit content of 34 mol% or more and less than 50 mol% and a degree of saponification of 99 mol% or more mixed in a blending mass ratio (a1/a2) of 60/40 to 90/10.

EVOH (a) may contain other monomer units in addition to ethylene, vinyl ester, and vinyl alcohol, as long as the effects of the present invention are not impaired. In particular, the introduction of a modifying group containing a primary hydroxyl group with a specific structure may enable EVOH (a) to achieve high levels of both gas barrier properties and moldability. The content of other monomer units is preferably 10 mol% or less, more preferably 5 mol% or less, even more preferably 1 mol% or less, and particularly preferably substantially none. Examples of such other monomers include alkenes such as propylene, butylene, pentene, and hexene; 3-acyloxy-1-propene, 3-acyloxy-1-butene, 4-acyloxy-1-butene, 3,4-diacyloxy-1-butene, 3-acyloxy-4-methyl ...4-methyl-1-butene, 4-methyl-1-butene, 4-methyl-1-butene, 4-methyl-1-butene, 4-methyl-1-butene, 1-butene, 4-acyloxy-2-methyl-1-butene, 4-acyloxy-3-methyl-1-butene, 3,4-diacyloxy-2-methyl-1-butene, 4-acyloxy-1-pentene, 5-acyloxy-1-pentene, 4,5-diacyloxy-1-pentene, 4-acyloxy-1-hexene, 5-acyloxy-1-hexene, 6-acyloxy-1-hexene, 5,6-diacyloxy Examples of suitable olefin sulfonic acids include ester group-containing alkenes such as siloxy-1-hexene and 1,3-diacetoxy-2-methylenepropane, or saponified products thereof; unsaturated acids such as acrylic acid, methacrylic acid, crotonic acid, and itaconic acid, or anhydrides, salts, or mono- or dialkyl esters thereof; nitriles such as acrylonitrile and methacrylonitrile; amides such as acrylamide and methacrylamide; olefin sulfonic acids such as vinyl sulfonic acid, allyl sulfonic acid, and methallylsulfonic acid, or salts thereof; vinyl silane compounds such as vinyltrimethoxysilane, vinyltriethoxysilane, vinyltri(β-methoxyethoxy)silane, and γ-methacryloxypropylmethoxysilane; alkyl vinyl ethers, vinyl ketone, N-vinylpyrrolidone, vinyl chloride, and vinylidene chloride.

EVOH (a) may be modified as needed by urethanization, acetalization, cyanoethylation, oxyalkylenation, etc. Oxyalkylenation can be carried out using an epoxy compound, such as epoxyethane (ethylene oxide), epoxypropane, 1,2-epoxybutane, 2,3-epoxybutane, 3-methyl-1,2-epoxybutane, 1,2-epoxypentane, 3-methyl-1,2-epoxypentane, 1,2-epoxyhexane, 2,3-epoxyhexane, 3,4-epoxyhexane, 3-methyl-1,2-epoxyhexane, 3-methyl-1,2-epoxyheptane, 4-methyl-1,2-epoxyheptane, 1, Examples include 2-epoxyoctane, 2,3-epoxyoctane, 1,2-epoxynonane, 2,3-epoxynonane, 1,2-epoxydecane, 1,2-epoxydodecane, epoxyethylbenzene, 1-phenyl-1,2-propane, 3-phenyl-1,2-epoxypropane, various alkyl glycidyl ethers, various alkylene glycol monoglycidyl ethers, various alkenyl glycidyl ethers, various epoxy alkanols such as glycidol, various epoxy cycloalkanes, and various epoxy cycloalkenes. Among these, 1,2-epoxybutane, 2,3-epoxybutane, epoxypropane, epoxyethane, or glycidol is preferred, and epoxypropane or glycidol is more preferred.

The MFR (190°C, under a load of 2.16 kg) of EVOH (a), measured in accordance with JIS K 7210 (2014), is preferably 0.2 to 20 g/10 min. The MFR of EVOH (a) is more preferably 0.5 g/10 min or more, and even more preferably 0.8 g/10 min or more. On the other hand, the MFR of EVOH (a) is more preferably 15 g/10 min or less, even more preferably 10 g/10 min or less, even more preferably 5 g/10 min or less, and particularly preferably 3 g/10 min or less. When the MFR of EVOH (a) is within the above range, the melt moldability of the pulverized product of EVOH (a) and the multilayer structure containing EVOH (a) (the multilayer structure of the present invention) is improved.

The barrier layer (A) may contain components other than EVOH (a), as long as the effects of the present invention are not impaired. Examples of other components include alkali metal ions, polyvalent metal ions, carboxylic acids, phosphate compounds, boron compounds, prooxidants, antioxidants, plasticizers, heat stabilizers (melt stabilizers), photoinitiators, deodorizers, UV absorbers, antistatic agents, lubricants, colorants, fillers, desiccants, bulking agents, pigments, dyes, processing aids, flame retardants, and antifogging agents. The content of other components in the barrier layer (A) is typically 5% by mass or less, preferably 3% by mass or less, and more preferably 1% by mass or less.

The barrier layer (A) preferably contains 10 to 400 ppm of alkali metal ions. The lower limit of the alkali metal ion content is more preferably 100 ppm, and even more preferably 150 ppm. On the other hand, the upper limit of the alkali metal ion content is more preferably 350 ppm, and may be 250 ppm. When the alkali metal ion content is 10 ppm or more, the interlayer adhesion in the multilayer structure of the present invention, including the layer obtained by molding EVOH (a), is improved. On the other hand, when the alkali metal ion content is 400 ppm or less, discoloration tends to be suppressed. By controlling the content ratio of alkali metal ions to the carboxylic acid described below, melt moldability and discoloration resistance can be further improved.

Examples of alkali metal ions include lithium, sodium, potassium, rubidium, and cesium ions, but sodium or potassium ions are preferred from the standpoint of industrial availability. In particular, the use of sodium ions can sometimes achieve high levels of both color and interlayer adhesion with the adhesive layer. These ions may be used alone or in combination of two or more.

Examples of alkali metal salts that provide alkali metal ions include aliphatic carboxylates, aromatic carboxylates, carbonates, hydrochlorides, nitrates, sulfates, phosphates, and metal complexes of alkali metals such as sodium and potassium. Among these, at least one selected from the group consisting of sodium acetate, potassium acetate, sodium phosphate, and potassium phosphate is more preferred due to its ease of availability.

The barrier layer (A) preferably contains 10 to 200 ppm of at least one polyvalent metal ion selected from the group consisting of magnesium ions, calcium ions, and zinc ions. The inclusion of a certain amount of polyvalent metal ions suppresses thickening, gelation, and resin adhesion to the screw during melt molding of EVOH (a) and the pulverized multilayer structure containing EVOH (a). The barrier layer (A) more preferably contains magnesium ions or calcium ions as the polyvalent metal ion, and even more preferably magnesium ions. The polyvalent metal ion is preferably contained as a carboxylate salt. The carboxylic acid may be either an aliphatic or aromatic carboxylic acid, with aliphatic carboxylic acids being preferred. Examples of aliphatic carboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, lauric acid, stearic acid, myristic acid, behenic acid, and montanic acid, with higher fatty acids having 10 to 25 carbon atoms being more preferred. To suppress discoloration during melt molding, the polyvalent metal ion is also preferably contained as a salt of a polycarboxylic acid, as described below.

The content of polyvalent metal ions in the barrier layer (A) is preferably 10 to 200 ppm in terms of metal atoms. A content of 10 ppm or more improves the viscosity stability of EVOH (a) and the ground product of the multilayer structure containing EVOH (a), suppressing gelation of the resin and adhesion of the resin to the extruder screw. The lower limit of the polyvalent metal ion content is more preferably 20 ppm. On the other hand, a polyvalent metal ion content of 200 ppm or less suppresses excessive decomposition of the ground product of the multilayer structure containing EVOH (a), improving the color of the recovered composition. The upper limit of the polyvalent metal ion content is more preferably 160 ppm, and even more preferably 120 ppm.

The barrier layer (A) preferably further contains a boron compound. The lower limit of the boron compound content is preferably 50 ppm, more preferably 100 ppm, in elemental boron equivalent. Meanwhile, the upper limit of the boron compound content is preferably 400 ppm, more preferably 300 ppm, in elemental boron equivalent. By including a boron compound within this range, the mechanical properties of the multilayer structure may be improved. Furthermore, the thermal stability of EVOH (a) and the pulverized product of the multilayer structure during melt molding may be improved, and the occurrence of gels and lumps may be suppressed. Drawdown resistance and neck-in resistance during film formation may be improved. These effects are presumably due to the chelating interaction between EVOH (a) and the boron compound.

Examples of boron compounds include boric acid, boric acid esters, borate salts, _and boron hydrides.Specific examples include boric acids such as orthoboric acid ( _H3BO3 ), metaboric acid, and tetraboric acid; boric acid esters such as trimethyl borate and triethyl borate; alkali metal salts or alkaline earth metal salts of the above boric acids; and borate salts such as borax.Of these, orthoboric acid is preferred.

The barrier layer (A) preferably contains a carboxylic acid. The lower limit of the carboxylic acid content is preferably 50 ppm, more preferably 100 ppm. On the other hand, the upper limit of the carboxylic acid content is preferably 400 ppm, more preferably 350 ppm. A carboxylic acid content within the above range tends to suppress deterioration of hue. The carboxylic acid content is determined by extracting 10 g of the resin composition constituting the barrier layer (A) with 50 ml of pure water at 95°C for 8 hours, and then titrating the resulting extract. Note that the carboxylic acid content in the resin composition does not take into account carboxylic acids present as salts in the extract. If the resin composition contains acidic compounds other than carboxylic acids, the carboxylic acid content in the resin composition can be determined by subtracting the contribution of these acidic compounds from the measured value obtained by titration.

The pKa of the carboxylic acid is preferably 3.5 to 5.5. When the pKa of the carboxylic acid is within this range, the pH buffering ability in the weakly acidic range is enhanced, further improving melt processability and further reducing the discoloration effects caused by acidic and basic substances.

The carboxylic acid may be a monocarboxylic acid. These may be used alone or in combination of two or more. A "monocarboxylic acid" is a compound containing one carboxy group in the molecule. Monocarboxylic acids with a pKa in the range of 3.5 to 5.5 are not particularly limited, and examples include formic acid (pKa = 3.77), acetic acid (pKa = 4.76), propionic acid (pKa = 4.85), and acrylic acid (pKa = 4.25). These carboxylic acids may further contain substituents such as hydroxyl groups, amino groups, and halogen atoms. Among these, acetic acid is preferred due to its high safety and ease of availability and handling.

The carboxylic acid may be a polycarboxylic acid. When the carboxylic acid is a polycarboxylic acid, the discoloration resistance of EVOH (a) at high temperatures and the discoloration resistance of a melt-molded product of crushed material of a multilayer structure containing EVOH (a) may be further improved. It is also preferable that the polycarboxylic acid compound has three or more carboxy groups. In this case, discoloration resistance may be more effectively improved. A "polycarboxylic acid" is a compound having two or more carboxy groups in the molecule. In this case, it is preferable that the pKa of at least one carboxy group is in the range of 3.5 to 5.5, and examples thereof include oxalic acid (pKa2 = 4.27), succinic acid (pKa1 = 4.20), fumaric acid (pKa2 = 4.44), malic acid (pKa2 = 5.13), glutaric acid (pKa1 = 4.30, pKa2 = 5.40), adipic acid (pKa1 = 4.43, pKa2 = 5.50), and the like. = 5.41), pimelic acid (pKa1 = 4.71), phthalic acid (pKa2 = 5.41), isophthalic acid (pKa2 = 4.46), terephthalic acid (pKa1 = 3.51, pKa2 = 4.82), citric acid (pKa2 = 4.75), tartaric acid (pKa2 = 4.40), glutamic acid (pKa2 = 4.07), aspartic acid (pKa = 3.90), etc.

The barrier layer (A) may further contain a phosphate compound. The lower limit of the content of the phosphate compound is preferably 5 ppm in terms of phosphate radicals. On the other hand, the upper limit of the content of the phosphate compound is preferably 100 ppm in terms of phosphate radicals. By containing the phosphate compound within this range, discoloration of the melt-molded product of the EVOH (a) and the pulverized product of the multilayer structure may be suppressed, and thermal stability may be improved. "In terms of phosphate radicals" refers to the value obtained by converting the content of phosphorus element (P) into the content of phosphate ions (PO ₄ ³⁻ ).

Phosphate compounds include various acids such as phosphoric acid and phosphorous acid, as well as their salts. The phosphate may be a primary phosphate, a secondary phosphate, or a tertiary phosphate. The cation species of the phosphate is not particularly limited, but alkali metals or alkaline earth metals are preferred. Of these, sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, and dipotassium hydrogen phosphate are preferred as phosphate compounds.

The barrier layer (A) may further contain, as an antioxidant, for example, a hindered phenol compound having an ester bond or an amide bond. The content of the hindered phenol compound is preferably 1,000 to 10,000 ppm. A content of 1,000 ppm or more can suppress discoloration, thickening, and gelation of the resin when the pulverized product of the multilayer structure is melt-molded. The content of the hindered phenol compound is more preferably 2,000 ppm or more. On the other hand, a content of the hindered phenol compound of 10,000 ppm or less can suppress discoloration and bleed-out resulting from the hindered phenol compound. The content of the hindered phenol compound is more preferably 8,000 ppm or less.

Hindered phenol compounds have at least one hindered phenol group. A "hindered phenol group" refers to a group in which a bulky substituent is bonded to at least one carbon atom adjacent to the carbon atom to which the hydroxyl group of a phenol is bonded. As the bulky substituent, an alkyl group having 1 to 10 carbon atoms is preferred, and a t-butyl group is more preferred.

The hindered phenol compound is preferably in a solid state at or near room temperature. From the viewpoint of suppressing bleed-out of the compound, the melting point or softening temperature of the hindered phenol compound is preferably 50°C or higher, more preferably 60°C or higher, and even more preferably 70°C or higher. From the viewpoint of suppressing bleed-out, the molecular weight of the hindered phenol compound is preferably 200 or higher, more preferably 400 or higher, and even more preferably 600 or higher. Meanwhile, the molecular weight is typically 2000 or lower. From the viewpoint of facilitating mixing with EVOH (a), the melting point or softening temperature of the hindered phenol compound is preferably 200°C or lower, more preferably 190°C or lower, and even more preferably 180°C or lower.

The hindered phenol compound has an ester bond or an amide bond. Examples of hindered phenol compounds having an ester bond include esters of aliphatic carboxylic acids having a hindered phenol group and aliphatic alcohols, while examples of hindered phenol compounds having an amide bond include amides of aliphatic carboxylic acids having a hindered phenol group and aliphatic amines. Among these, from the viewpoint of facilitating mixing with EVOH (a), it is preferable that the hindered phenol compound have an amide bond.

Specific structures of hindered phenol compounds include pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] commercially available from BASF as Irganox 1010, stearyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate commercially available as Irganox 1076, 2,2'-thiodiethylbis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] commercially available as Irganox 1035, and 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate commercially available as Irganox 1135. octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate, ethylene bis(oxyethylene) bis(3-tert-butyl-4-hydroxy-5-methylbenzenepropanoate) commercially available as Irganox 245, 1,6-hexanediol bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] commercially available as Irganox 259, and N,N'-hexamethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanamide] commercially available as Irganox 1098. Among these, N,N'-hexamethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanamide] commercially available as Irganox 1098 and pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] commercially available as Irganox 1010 are preferred, with Irganox 1098 being more preferred.

The barrier layer (A) may further contain a thermoplastic resin other than EVOH (a). Examples of thermoplastic resins other than EVOH (a) include various polyolefins (polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, ethylene-propylene copolymers, copolymers of ethylene and α-olefins having 4 or more carbon atoms, copolymers of polyolefins and maleic anhydride, ethylene-vinyl ester copolymers, ethylene-acrylic acid ester copolymers, and modified polyolefins obtained by graft-modifying these with unsaturated carboxylic acids or their derivatives), various polyamides (nylon 6, nylon 6/6, nylon 6/66 copolymer, nylon 11, nylon 12, polymetaxylylene adipamide, etc.), various polyesters (polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc.), polyvinyl chloride, polyvinylidene chloride, polystyrene, polyacrylonitrile, polyurethane, polycarbonate, polyacetal, polyacrylate, and modified polyvinyl alcohol resins. The content of the thermoplastic resin in the barrier layer (A) is less than 50% by mass, preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and may be 1% by mass or less.

The proportion of EVOH (a) in the resin constituting the barrier layer (A) is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or more, 97% by mass or more, or 99% by mass or more. The resin constituting the barrier layer (A) may consist solely of EVOH (a). The proportion of EVOH (a) in the barrier layer (A) is greater than 50% by mass, preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or more, 97% by mass or more, or 99% by mass or more. The barrier layer (A) may be composed essentially of EVOH (a) alone.

When the barrier layer (A) contains components other than EVOH (a), the resin composition constituting the barrier layer (A) can be produced by melt-kneading EVOH (a) and, if necessary, other additives (such as metal ions). The other additives may be blended in a solid state, such as a powder, or as a melt. They may also be blended as solutes in a solution or dispersoids in a dispersion. Aqueous solutions and aqueous dispersions are preferred as solutions and dispersions, respectively. Melt-kneading can be performed using known mixing or kneading equipment, such as a kneader-ruder, extruder, mixing roll, or Banbury mixer. The temperature range during melt-kneading can be adjusted appropriately depending on the melting point of the EVOH (a) used, and is typically between 150 and 300°C.

In another embodiment, a masterbatch containing a high concentration of other additives relative to EVOH(a) can be produced by melt-kneading, and the masterbatch can then be dry-blended with EVOH(a) that is substantially free of other additives for use in the production of a multilayer structure. In yet another embodiment, EVOH(a) and other additives can be dry-blended for use in the production of a multilayer structure. "Dry-blending" refers to mechanically mixing EVOH(a) in granular or pelletized form. Mixing can be performed using a mixing device such as a tumbler, ribbon mixer, or Henschel mixer, or by manual stirring or shaking in a sealed container. The mixing temperature may be between room temperature and below the melting point of EVOH(a), and mixing can be performed under an air or nitrogen atmosphere. EVOH(a) may also be produced by pre-adding some components to EVOH(a) and then melt-kneading additional components as described above. An example of a method for pre-adding some components to EVOH(a) is immersing EVOH(a) in the form of pellets or powder in a solution containing the dissolved additives. The solution is preferably an aqueous solution.

<Adhesive layer (B)>
The multilayer structure of the present invention has an adhesive layer (B) containing an adhesive resin (b) as a main component. The adhesive layer (B) has a function of bonding the barrier layer (A) and the thermal seal layer (C). Therefore, the adhesive layer (B) is preferably provided between the barrier layer (A) and the thermal seal layer (C), and is preferably directly laminated on the barrier layer (A) and the thermal seal layer (C).

Examples of adhesive resin (b) include modified olefin polymers containing carboxy groups, obtained by chemically bonding an unsaturated carboxylic acid or its anhydride to an olefin polymer via addition reaction, grafting, or other methods. Examples of unsaturated carboxylic acids or their anhydrides include maleic acid, maleic anhydride, fumaric acid, acrylic acid, methacrylic acid, crotonic acid, itaconic acid, citraconic acid, and hexahydrophthalic anhydride, with maleic anhydride being preferred. Specifically, suitable examples include one or a mixture of two or more selected from maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, maleic anhydride-grafted ethylene-propylene copolymer, maleic anhydride-grafted ethylene-ethyl acrylate copolymer, and maleic anhydride-grafted ethylene-vinyl acetate copolymer, with maleic anhydride-grafted polyethylene being more preferred. The acid value of such adhesive resin (b) is preferably 0.5 to 5 mgKOH/g, more preferably 1 to 4 mgKOH/g. The acid value of the adhesive resin (b) can be measured in accordance with JIS K 0070:1992 using xylene as a solvent.

The adhesive resin (b) may be a mixture of an unmodified resin (bx) and an acid-modified resin (by). In this case, from the viewpoint of further increasing mechanical strength, the unmodified resin (bx) preferably contains the ethylene-α-olefin copolymer resin (c) described below, and is more preferably ethylene-α-olefin copolymer resin (c). When the unmodified resin (bx) contains ethylene-α-olefin copolymer resin (c), the ethylene-α-olefin copolymer resin (c) contained in the adhesive layer (B) and the ethylene-α-olefin copolymer resin (c) contained in the heat-sealable layer (C) may be the same or different, but are preferably the same. The ratio (bx/by) of the unmodified resin (bx) to the acid-modified resin (by) in the adhesive resin (b) is preferably 55/45 to 95/5, and more preferably 65/35 to 90/10. In this case, a resin with a relatively high degree of acid modification can be preferably used as the acid-modified resin (by), and its acid value is preferably 5 to 30 mgKOH/g, more preferably 8 to 20 mgKOH/g. This may further improve the mechanical strength of the resulting multilayer structure while maintaining the required interlayer adhesive strength. When the adhesive resin (b) of the present invention is a mixture of an unmodified resin (bx) and an acid-modified resin (by), the unmodified resin (bx) and the acid-modified resin (by) may be melt-kneaded in advance, or a dry blend of the unmodified resin (bx) and the acid-modified resin (by) may be used. For melt-kneading, known mixing or kneading devices such as a kneader-ruder, extruder, mixing roll, or Banbury mixer can be used. The temperature range during melt-kneading can be appropriately adjusted depending on the melting points of the unmodified resin (bx) and the acid-modified resin (by) used, and is typically 150 to 300°C. The mixing may be carried out using a mixing device such as a tumbler, a ribbon mixer, or a Henschel mixer, or may be carried out in a sealed container by manually stirring, shaking, etc. The mixing temperature may be from room temperature to below the melting points of the unmodified resin (bx) and the acid-modified resin (by), and the mixing may be carried out in an air atmosphere or a nitrogen atmosphere.

The adhesive layer (B) may contain components other than the adhesive resin (b), as long as the effects of the present invention are not impaired. Examples of other components include alkali metal ions, polyvalent metal ions, carboxylic acids, phosphate compounds, boron compounds, prooxidants, antioxidants, plasticizers, heat stabilizers (melt stabilizers), photoinitiators, deodorizers, UV absorbers, antistatic agents, lubricants, colorants, fillers, desiccants, bulking agents, pigments, dyes, processing aids, flame retardants, and antifogging agents. The content of other components in the adhesive layer (B) is typically 5% by mass or less, preferably 3% by mass or less, and more preferably 1% by mass or less. The adhesive layer (B) may further contain a thermoplastic resin other than the adhesive resin (b). Examples of the thermoplastic resin that may be contained in the barrier layer (A) include the resins listed above. The content of the thermoplastic resin in the adhesive layer (B) is less than 50% by mass, preferably less than 30% by mass, more preferably less than 10% by mass, even more preferably 5% by mass or less, and may be 1% by mass or less.

The proportion of adhesive resin (b) in the resin constituting adhesive layer (B) is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or more, 97% by mass or more, or 99% by mass or more. The resin constituting adhesive layer (B) may consist solely of adhesive resin (b). The proportion of adhesive resin (b) in adhesive layer (B) is greater than 50% by mass, preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or more, 97% by mass or more, or 99% by mass or more. The adhesive layer (B) may be composed essentially of adhesive resin (b) alone.

<Heat-sealing layer (C)>
The multilayer structure of the present invention has a heat-sealable layer (C) containing, as a main component, an ethylene-α-olefin copolymer resin (c) having a density of 0.880 to 0.930 g/cm ^3. The heat-sealable layer (C) not only functions as a sealing layer when forming a packaging material, but also has the functions of increasing various mechanical strengths such as drop-breakage resistance, puncture strength and elongation, and tensile strength and elongation.

The density of the ethylene-α-olefin copolymer resin (c) is 0.880 to 0.930 g/cm ^3. When the density is within this range, the resulting multilayer structure is flexible and has excellent handleability, and in addition to drop-breakage resistance, various mechanical strengths such as puncture strength and elongation and tensile strength and elongation are improved. The lower limit of the density is preferably 0.890 g/cm ³ , more preferably 0.900 g/cm ³ , even more preferably 0.905 g/cm ³ , and may be 0.910 g/cm ^3. The upper limit of the density is preferably 0.925 g/cm ³ , more preferably 0.920 g/cm ³ .

The MFR (190°C, under a load of 2.16 kg) of the ethylene-α-olefin copolymer resin (c) is preferably 0.5 to 5.0 g/10 min. When the MFR is within the above range, the ethylene-α-olefin copolymer resin (c) has excellent melt processability, and the resulting multilayer structure has improved mechanical strength, such as drop-breakage resistance, puncture strength and elongation, and tensile strength and elongation. The lower limit of the MFR is more preferably 0.7 g/10 min. The upper limit of the MFR is more preferably 4.0 g/10 min, and even more preferably 2.0 g/10 min. The MFR is measured at 190°C under a load of 2.16 kg in accordance with JIS K 7210 (2014).

Ethylene-α-olefin copolymer resin (c) is a resin obtained by polymerizing ethylene with an α-olefin having 3 or more carbon atoms. Examples of α-olefins having 3 or more carbon atoms include propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, and 4-methyl-1-pentene. Ethylene-α-olefin copolymer resin (c) is preferably a linear low-density polyethylene obtained by polymerizing ethylene with an α-olefin having 6 or more carbon atoms, and may also be a linear low-density polyethylene obtained by polymerizing ethylene with an α-olefin having 8 or more carbon atoms. When the α-olefin copolymerized with ethylene has a relatively large carbon number, various mechanical strengths such as pin puncture strength and tensile strength and elongation may be particularly improved. There is no particular upper limit on the carbon number of the α-olefin having 3 or more carbon atoms, but a carbon number of 12 or less is common.

A metallocene catalyst is preferably used as the polymerization catalyst. Linear low-density polyethylene polymerized using a metallocene catalyst is produced by copolymerizing ethylene and an α-olefin in the presence of a catalyst formed from a compound of a transition metal of Group 4 of the periodic table, preferably zirconium, having at least one ligand with a cyclopentadienyl skeleton, an organoaluminum oxy compound, and various other components added as needed. Linear low-density polyethylene polymerized using a metallocene catalyst has excellent melt moldability, and the resulting multilayer structure has an excellent balance of heat resistance, flexibility, and mechanical strength.

Linear low-density polyethylene, which is produced by polymerizing ethylene and an α-olefin having six or more carbon atoms using a metallocene catalyst, is commercially available as an industrially produced product, such as "Evolue™" (Prime Polymer Co., Ltd.), "Sumikasen™" (Sumitomo Chemical Co., Ltd.), "Yumerit™" (Ube Maruzen Polyethylene Co., Ltd.), and "Elite™" (Dow Chemical Co., Ltd.).

The heat-sealable layer (C) may contain components other than the ethylene-α-olefin copolymer resin (c), as long as the effects of the present invention are not impaired. Examples of other components include alkali metal ions, polyvalent metal ions, carboxylic acids, phosphate compounds, boron compounds, prooxidants, antioxidants, plasticizers, heat stabilizers (melt stabilizers), photoinitiators, deodorizers, UV absorbers, antistatic agents, lubricants, colorants, fillers, desiccants, bulking agents, pigments, dyes, processing aids, flame retardants, and anti-fogging agents. The content of other components in the heat-sealable layer (C) is typically 5% by mass or less, preferably 3% by mass or less, and more preferably 1% by mass or less. The heat-sealable layer (C) may further contain a thermoplastic resin other than the ethylene-α-olefin copolymer resin (c). Examples of the thermoplastic resin that may be contained in the barrier layer (A) include the resins listed above. The content of the thermoplastic resin in the heat-sealable layer (C) is less than 50% by mass, preferably less than 30% by mass, more preferably less than 10% by mass, even more preferably 5% by mass or less, and may be 1% by mass or less.

The proportion of ethylene-α-olefin copolymer resin (c) in the resin constituting the heat-sealable layer (C) is preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or more, 97% by mass or more, or 99% by mass or more. The resin constituting the heat-sealable layer (C) may consist solely of ethylene-α-olefin copolymer resin (c). The proportion of ethylene-α-olefin copolymer resin (c) in the heat-sealable layer (C) is greater than 50% by mass, preferably 60% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or more, 97% by mass or more, or 99% by mass or more. The heat-sealable layer (C) may be composed substantially solely of ethylene-α-olefin copolymer resin (c).

The resin composition constituting the heat-sealable layer (C) can be produced by melt-kneading the ethylene-α-olefin copolymer resin (c) and, if necessary, other additives. Melt-kneading can be performed using known mixing or kneading equipment, such as a kneader-ruder, extruder, mixing roll, or Banbury mixer. The temperature range during melt-kneading can be adjusted appropriately depending on the melting point of the ethylene-α-olefin copolymer resin (c) used, and is typically between 150 and 300°C.

In another embodiment, a masterbatch containing a high concentration of other additives, as needed, is produced by melt-kneading ethylene-α-olefin copolymer resin (c), and this masterbatch is dry-blended with ethylene-α-olefin copolymer resin (c) that is substantially free of other additives and used to produce a multilayer structure. In yet another embodiment, ethylene-α-olefin copolymer resin (c) and other additives can be dry-blended to produce the heat-sealable layer (C). Mixing may be carried out using a mixing device such as a tumbler, ribbon mixer, or Henschel mixer, or by manual stirring, shaking, or the like in a sealed container. The mixing temperature may be from room temperature to below the melting point of ethylene-α-olefin copolymer resin (c), and mixing may be carried out in an air or nitrogen atmosphere.

The sealant layer (X) has a barrier layer (A), an adhesive layer (B), and a heat-sealing layer (C). It is preferable that at least one pair of the barrier layer (A) and adhesive layer (B) are laminated adjacent to each other. This makes it possible to obtain a multilayer structure that has high gas barrier properties and recyclability, as well as excellent mechanical strength and stability.

The lamination method for producing the sealant layer (X) can be a conventional co-extrusion method in which each resin is extruded through separate dies or a common die and laminated. The die can be either a circular die or a T-die. The molding temperature during melt molding can be adjusted appropriately based on the melting point and melt viscosity of the resins used, and is often selected from the range of 150 to 300°C. From the perspective of maintaining heat seal strength when molding the multilayer structure of the present invention into flexible packaging materials, it is preferable that the sealant layer (X) be an unstretched layer.

The total thickness of the sealant layer (X) is preferably 15 to 300 μm, more preferably 25 to 250 μm, even more preferably 35 to 200 μm, and particularly preferably 45 to 150 μm. With a total thickness within the above range, the sealant layer (X) is lightweight and flexible, making it suitable for flexible packaging applications. The amount of resin used in the sealant layer (X) is small, reducing the environmental impact.

The lower limit of the thickness of the barrier layer (A) is preferably 1 μm. When the lower limit of the thickness of the barrier layer (A) is within the above range, the multilayer film exhibits sufficient gas barrier properties. The upper limit of the thickness of the barrier layer (A) is preferably 30 μm, more preferably 25 μm, even more preferably 20 μm, and particularly preferably 15 μm. When the upper limit of the thickness of the barrier layer (A) is within the above range, the recyclability of the sealant layer (X) or the multilayer structure when recovered and reused is improved. From the viewpoint of improving resistance to drop rupture, particularly at high temperatures, the thickness of the barrier layer (A) may be preferably 5 μm or less, more preferably 4 μm or less. When the thickness of the barrier layer (A) is 5 μm or less, the ethylene unit content of the EVOH (a) is preferably 33 mol% or less, more preferably 28 mol% or less, in order to maintain good gas barrier properties. When the sealant layer (X) has multiple barrier layers (A), the thickness of the barrier layer (A) refers to the total thickness of the multiple barrier layers (A).

The thickness of the adhesive layer (B) is preferably 1 to 20 μm, more preferably 2 to 10 μm. When the thickness of the adhesive layer (B) is within the above range, the interlayer adhesion between the barrier layer (A) and the heat-sealing layer (C) is enhanced, and the recyclability when the sealant layer (X) or the multilayer structure is recovered and reused is improved. When the sealant layer (X) has multiple adhesive layers (B), the thickness of the adhesive layer (B) mentioned above means the total thickness of the multiple adhesive layers (B).

The thickness of the heat-sealing layer (C) is preferably 15 to 300 μm, more preferably 30 to 200 μm, and even more preferably 40 to 150 μm. When the thickness of the heat-sealing layer (C) is within the above range, recyclability, mechanical strength, and water vapor barrier properties are improved. When the sealant layer (X) has multiple heat-sealing layers (C), the thickness of the heat-sealing layer (C) mentioned above means the total thickness of the multiple heat-sealing layers (C).

In the sealant layer (X), the ratio of the thickness of the barrier layer (A) to the total thickness of all layers is preferably 0.10 or less. When this ratio is within the above range, recyclability and mechanical strength are improved. The upper limit of the ratio of the thickness of the barrier layer (A) to the total thickness of all layers is more preferably 0.08, even more preferably 0.05, and particularly preferably 0.04. The lower limit of the ratio of the thickness of the barrier layer (A) to the total thickness of all layers is not particularly limited, but 0.005 is preferred to achieve sufficient gas barrier properties. Meanwhile, in the multilayer structure of the present invention, the ratio of the thickness of the heat-sealable layer (C) to the total thickness of all layers is preferably 0.60 or more, more preferably 0.70 or more, even more preferably 0.80 or more, and may be 0.99 or less. When this ratio is within the above range, recyclability, mechanical strength, and water vapor barrier properties are improved.

The layer structure of the sealant layer (X) is not particularly limited as long as it has a barrier layer (A), an adhesive layer (B), and a heat-sealing layer (C), and the heat-sealing layer (C) is exposed. If the heat-sealing layer (C) is expressed as (C), the adhesive resin layer (B) as (B), and the barrier layer (A) as (A), and a "/" indicates direct lamination, examples of layer structures include (C)/(B)/(A)/(B)/(C) and (C)/(B)/(A). In addition to the above layer structures, the sealant layer (X) may also have other layers. When multiple barrier layers (A), adhesive layers (B), and heat-sealing layers (C) are used, different resins may be used for each.

<Base material layer (Y)>
The multilayer structure of the present invention has a base layer (Y) that is a biaxially stretched layer containing polypropylene (y) as a main component and having a melting point of 150°C or higher as measured by differential scanning calorimetry (DSC), and that has a modulus of elasticity in both directions of 2000 MPa or higher at 23°C and 50% RH. "Polypropylene" refers to a propylene homopolymer, a copolymer of 80 mol% or more propylene and 20 mol% or less ethylene or α-olefin monomer, and a copolymer of 90 mol% or more propylene and less than 10 mol% non-olefin monomers whose functional groups contain atoms other than carbon, oxygen, and hydrogen atoms. By including polypropylene (y) as a main component and having a melting point of 150°C or higher as measured by differential scanning calorimetry (DSC), the multilayer structure of the present invention can have improved resistance to bag breakage due to dropping under high temperature conditions. The melting point is preferably 155°C or higher, more preferably 160°C or higher, and even more preferably 165°C or higher. The melting point may be 170°C or lower. When the base layer (Y) is a biaxially stretched layer and the biaxial modulus of elasticity at 23°C and 50% RH is 2000 MPa or higher in both directions, a multilayer structure having excellent portability and improved resistance to drop-induced bag breakage under high-temperature conditions can be obtained. The biaxial stretching ratio is not particularly limited, but it is preferably stretched 3 times or more but less than 12 times in each direction, more preferably 4 times or more but less than 10 times, and even more preferably 4 times or more but less than 6 times in the machine direction (MD) and 8 times or more but less than 12 times in the transverse direction (TD). The elastic modulus in the MD direction is preferably 2400 MPa or more, and may be 2600 MPa or more. The elastic modulus in the MD direction may be 4000 MPa or lower. The elastic modulus in the TD direction is preferably 3500 MPa or more, and may be 5000 MPa or higher. The elastic modulus in the TD direction may be 7000 MPa or lower. When the substrate layer (Y) is heated from 20°C to 220°C at 10°C/min using a differential scanning calorimeter (DSC) and then cooled to -50°C at 10°C/min, the difference (T1-T2) between the maximum endothermic peak temperature (T1) during heating and the maximum exothermic peak temperature (T2) during cooling is preferably 56°C or less. When the difference (T1-T2) between the maximum exothermic peak temperature (T2) during cooling is 56°C or less, the water vapor barrier properties of the resulting multilayer structure are significantly improved, and the drop-breakage resistance of the bag under high-temperature conditions may be further improved. The difference (T1-T2) is preferably 55°C or less, more preferably 54°C or less, even more preferably 53°C or less, and particularly preferably 52°C or less. The difference (T1-T2) may be 50°C or more, or 51°C or more.

The proportion of polypropylene (y) in the resin constituting the base layer (Y) is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and may be 97% by mass or more or 99% by mass or more. The resin constituting the base layer (Y) may consist solely of polypropylene (y). The content of polypropylene (y) in the base layer (Y) must be greater than 50% by mass, preferably 70% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and may be 97% by mass or more or 99% by mass or more. The base layer (Y) may be composed essentially of polypropylene (y) alone.

The method for producing the base layer (Y) is not particularly limited, but it is generally produced by melt extrusion using an extruder. Either a circular die or a T-die can be used as the die. The method for biaxial stretching is also not particularly limited, and the film can be produced by stretching in the machine direction and/or the direction perpendicular to the machine direction, i.e., the width direction, using conventional stretching methods such as tubular simultaneous biaxial stretching, tenter type sequential biaxial stretching, and tenter type simultaneous biaxial stretching.

From the viewpoint of industrial productivity, the thickness of the base layer (Y) is preferably 10 to 100 μm. Furthermore, from the viewpoint of achieving a balance between the mechanical strength, water vapor barrier properties, and manufacturing costs of the multilayer structure, the thickness of the base layer (Y) is more preferably 12 to 55 μm, even more preferably 14 to 35 μm, and particularly preferably 16 to 25 μm. Note that, while a thick multilayer structure generally tends to have higher resistance to bag breakage due to dropping, if the base layer (Y) has a thickness above a certain level, resistance to bag breakage due to dropping, particularly at room temperature, tends to decrease.

The layer structure of the base layer (Y) is not particularly limited as long as it contains polypropylene (y) as the main component, and may be a single layer or multiple layers, but from the viewpoint of manufacturing costs and imparting functionality, it is generally a single to five layer structure.

<Multilayer structure>
The multilayer structure of the present invention is a multilayer structure in which a sealant layer (X) and a base layer (Y) are laminated, and has a heat-sealable layer (C) of the sealant layer (X) on the outermost surface. Since the heat-sealable layer (C) is disposed on the outermost surface of the multilayer structure, the multilayer structure of the present invention can be easily heat-sealed when used as a packaging material, etc. The multilayer structure of the present invention preferably does not have a resin layer with a thickness of 10 μm or more containing a resin with a melting point of 200°C or more as its main component, or a metal layer with a thickness of 1 μm or more. That is, it is preferable that the EVOH (a), adhesive resin (b), ethylene-α-olefin copolymer (c), and polypropylene (y) all have melting points below 200°C. The phrase "does not have a resin layer with a thickness of 10 μm or more containing a resin with a melting point of 200°C or more as its main component, or a metal layer with a thickness of 1 μm or more" means that the multilayer structure does not have a resin layer with a thickness of 10 μm or more containing a resin with a melting point of 200°C or more as its main component, and does not have a metal layer with a thickness of 1 μm or more. By not having a resin layer having a thickness of 10 μm or more containing a resin having a melting point of 200° C. or more as a main component and a metal layer having a thickness of 1 μm or more, it is possible to prevent uneven mixing with other components when the pulverized product of the multilayer structure is melt-molded. In other words, recyclability is improved. A "metal layer" is a layer having continuous and discontinuous surfaces made of metal, such as aluminum foil. From the viewpoint of further improving recyclability, the thickness per layer containing a resin having a melting point of 200° C. or more as a main component is preferably 7 μm or less, and may be 5 μm or less.

The upper limit of the total thickness of the multilayer structure of the present invention is preferably 300 μm. With a total thickness within this range, the multilayer structure of the present invention is lightweight and flexible, making it suitable for flexible packaging applications. The amount of resin used in the multilayer structure is small, thereby reducing environmental impact. The upper limit of the total thickness of the multilayer structure of the present invention is more preferably 250 μm, and even more preferably 200 μm. From the viewpoint of maintaining the mechanical strength and water vapor barrier properties of the multilayer structure of the present invention, the lower limit of the total thickness is preferably 30 μm, more preferably 50 μm, even more preferably 100 μm, and particularly preferably 150 μm. While drop-breakage resistance can usually be improved by increasing the total thickness of the multilayer structure, the present invention makes it possible to increase drop-breakage resistance without increasing the total thickness of the multilayer structure. It has been found that increasing the thickness of the base layer (Y) results in a decrease in drop-breakage resistance, particularly at room temperature, which is the opposite of the usual trend. It has also been found that adjusting the thickness of each layer allows for the production of a multilayer structure with specifically superior drop-breakage resistance. It has generally been thought that the presence of the barrier layer (A) reduces resistance to bag breakage due to dropping. However, a surprising effect has been found that in a region where the thickness of the barrier layer (A) is very thin (for example, 5 μm or less), resistance to bag breakage due to dropping, particularly at high temperatures, tends to be further improved.

The thickness of each layer in the multilayer structure of the present invention may be adjusted appropriately depending on the application. However, from the viewpoints of suppressing discoloration when the pulverized material is melt-molded, improving thermal stability during melt-molding, and suppressing the occurrence of lumps, the ratio of the total thickness of the layers containing polyolefin-based resin as a main component to the total thickness of the multilayer structure is preferably 0.80 or greater, more preferably 0.85 or greater, even more preferably 0.90 or greater, and particularly preferably 0.95 or greater. Preferred polyolefin-based resins include polyethylene, ethylene-α-olefin copolymer resins, polypropylene, and acid-modified resins thereof. Examples of polyolefin-based resins include ethylene-α-olefin copolymer resin (c), polypropylene (y), and adhesive resin (b) in the case of polyolefin-based resins.

In the multilayer structure of the present invention, the difference (S-S Y ) between the puncture strength S measured according to JIS Z 1707:2019 under conditions of 23°C and 50% RH and the puncture strength S _Y of the base layer (Y) measured according to JIS _Z 1707:2019 under conditions of 23°C and 50% RH is preferably 3.5 N or more, more preferably 4.0 N or more. The difference (S-S _Y ) may be 10 N or less. By satisfying this condition, the drop-breakage resistance of the obtained multilayer structure tends to be further improved. For the same reason, the puncture strength S of the multilayer structure of the present invention is preferably 12 N or more, more preferably 15 N or more, and even more preferably 18 N or more. The puncture strength S may be 30 N or less. The puncture strength can be adjusted, for example, by the type of polypropylene (y) used in the base layer (Y), the thickness of each layer of the multilayer structure, the layer configuration of the multilayer structure, etc.

The oxygen transmission rate (OTR) of the multilayer structure of the present invention under conditions of 20°C and 65% RH is not particularly limited and may be adjusted depending on the application, but is preferably 5 cc/( ^m2 ·day·atm) or less. A multilayer structure having an OTR in this range has excellent gas barrier properties and is suitable for use as a packaging material. The OTR is more preferably 2 cc/( ^m2 ·day·atm) or less, even more preferably 1.5 cc/( ^m2 ·day·atm) or less, and may be 0.5 cc/( ^m2 ·day·atm) or less. The OTR may be 0.1 cc/( ^m2 ·day·atm) or more. The OTR is measured in accordance with JIS K 7126-2 (isobaric method; 2006), and specifically, the method described in the examples is employed.

The water vapor transmission rate (WVTR) of the multilayer structure of the present invention under conditions of 40°C and 90% RH can be adjusted depending on the application and is not particularly limited, but is preferably 4 g/( ^m2 ·day) or less. Multilayer films with a WVTR in this range have excellent water vapor barrier properties and are suitable for use as packaging materials. The WVTR is preferably 3.5 g/( ^m2 ·day) or less, more preferably 3.0 g/( ^m2 ·day) or less, and may be 2.5 g/( ^m2 ·day) or less. The WVTR may be 1 g/( ^m2 ·day) or more. The WVTR is measured in accordance with JIS K 7129-2 (infrared sensor method; 2019), and specifically, the method described in the examples is used.

The method for laminating the base layer (Y) onto the sealant layer (X) is not particularly limited, and examples include extrusion lamination, coextrusion lamination, and dry lamination. However, from the perspective of handleability, dry lamination is preferred. An adhesive layer may be provided when laminating the base layer (Y) onto the sealant layer (X). The adhesive layer may be adhesive layer (B), or adhesive layer (Z), which can be formed by applying and drying a known adhesive. A preferred known adhesive is, for example, a two-component reactive polyurethane adhesive, which is formed by mixing and reacting a polyisocyanate component and a polyol component. The thickness of the adhesive layer (Z) is not particularly limited, but is preferably 2 to 7 μm, and more preferably 3 to 5 μm.

The multilayer structure of the present invention may include layers other than those described above, provided that the effects of the present invention are not impaired. The other layers are preferably other than a resin layer having a thickness of 10 μm or more and containing a resin having a melting point of 200°C or higher as its primary component, or a metal layer having a thickness of 1 μm or more. Examples of such layers include a resin layer containing a thermoplastic resin as its primary component. The thermoplastic resin is not particularly limited, and examples include polyolefin, polyamide, polyester, polystyrene, polyvinyl chloride, polyvinylidene chloride, acrylic resin, polycarbonate, chlorinated polyethylene, and chlorinated polypropylene. Another example of such other layer is a recovered layer. It is particularly preferable to reuse a recovered composition containing the recovered material of the multilayer structure of the present invention, as described below, as part or all of the recovered layer. Another example of such other layer is a printed layer. The printed layer may be included at any position in the multilayer structure of the present invention. Examples of such a printed layer include a film obtained by applying and drying a solution containing a pigment or dye, and optionally a binder resin. Coating methods for the printed layer include gravure printing and various coating methods using a wire bar, spin coater, die coater, etc. The thickness of the printed layer is not particularly limited, but is preferably 0.5 to 10 μm, more preferably 1 to 4 μm.

It is preferable to recover and reuse any end pieces or defective products generated during the production of the multilayer structure of the present invention. It is also preferable to recover and reuse multilayer structures that have been distributed on the market. In this case, a common recovery method is to crush the multilayer structure and then melt-mold it.

When recovering the multilayer structure of the present invention, the recovered multilayer structure of the present invention is first pulverized. The pulverized recovered material may be melt-molded directly to obtain a recovered composition, or, if necessary, may be melt-molded together with other components to obtain a recovered composition. When the recovered composition is obtained by melt-molding together with other components, the other components are preferably polyolefin resins, more preferably polypropylene resins, polyethylene resins, or mixtures thereof. The pulverized recovered material may be directly used to manufacture molded articles such as multilayer structures, or the pulverized recovered material may be melt-pelletized to obtain pellets of the recovered composition, which may then be used to manufacture molded articles. Possible melt-molding methods for the recovered composition include extrusion molding, inflation molding, blow molding, melt spinning, and injection molding. The molding temperature during melt molding may be adjusted appropriately based on the melting point and melt viscosity of the resin used, and is often selected from the range of 150 to 300°C. The recovered composition may contain unused resin, but the content of the recovered material in the recovered composition is preferably 10% by mass or more, more preferably 20% by mass or more, and may even be 30% by mass or more. The content of EVOH (a) in the recycled composition is preferably 20% by mass or less, more preferably 10% by mass or less, and may be 5% by mass or less.

It is preferable to manufacture the multilayer structure of the present invention by coextrusion molding, as this reduces the amount of particles in the recovered layers after the multilayer structure of the present invention is recovered.

The multilayer structure of the present invention has excellent gas barrier properties, resistance to bag breakage due to dropping, and recyclability, and can therefore be suitably used as a variety of packaging materials such as food packaging, pharmaceutical packaging, industrial chemical packaging, and pesticide packaging, but can also be used for a wider range of applications and is not limited to these.

A suitable embodiment of the packaging material is a package in which the contents are filled. Examples of contents that can be filled include, but are not limited to, beverages such as wine and fruit juice; foods such as fruits, nuts, vegetables, meat products, baby foods, coffee, jam, mayonnaise, ketchup, cooking oil, dressings, sauces, foods boiled in soy sauce, and dairy products; and other contents that are susceptible to deterioration in the presence of oxygen, such as pharmaceuticals, cosmetics, and gasoline.

The present invention will be explained in more detail below using examples. The present invention is not limited to these examples in any way.

<Materials used>
・Base material layer (Y)
Y-1: Biaxially stretched PP film "U-1 (20 μm)" manufactured by Mitsui Chemicals Tocello Co., Ltd.
Y-2: Biaxially stretched PP film "U-1 (40 μm)" manufactured by Mitsui Chemicals Tocello Co., Ltd.
Y-3: Biaxially stretched PP film "WH-OP-HE-1 (20 μm)" manufactured by Mitsui Chemicals Tocello Co., Ltd.
YC-1: Unstretched PP film "SC (40 μm)" manufactured by Mitsui Chemicals Tocello Co., Ltd.
YC-2: Biaxially oriented HDPE film "Ethy-Lyte 25HD200" manufactured by Jindal Poly Films Ltd.
EVOH (a)
a-1: EVOH, ethylene unit content 32 mol%, saponification degree 99.9 mol%, sodium acetate 250 ppm in sodium ion equivalent, magnesium acetate 40 ppm in magnesium ion equivalent, phosphate ion 20 ppm in phosphate radical equivalent, boric acid 200 ppm in boron element equivalent, MFR (190 ° C, 2.16 kg load) 1.6 g / 10 min a-2: EVOH, ethylene unit content 32 mol%, saponification degree 99.9 mol%, sodium acetate 250 ppm in sodium ion equivalent, magnesium acetate 40 ppm in magnesium ion equivalent, phosphate ion 20 ppm in phosphate radical equivalent, MFR (190 ° C, 2.16 kg load) 1.6 g / 10 min a-3: EVOH, ethylene unit content 24 mol%, saponification degree 99.9 mol%, sodium acetate 250 ppm in sodium ion equivalent, a-4: EVOH, ethylene unit content 27 mol%, saponification degree 99.9 mol%, sodium acetate 250 ppm (calculated as sodium ion), magnesium acetate 40 ppm (calculated as magnesium ion), and phosphate ion 20 ppm (calculated as phosphate radical), MFR (210°C, 2.16 kg load) 3.0 g/10 min a-5: EVOH, ethylene unit content 44 mol%, saponification degree 99.9 mol%, sodium acetate 250 ppm (calculated as sodium ion), magnesium acetate 40 ppm (calculated as magnesium ion), and phosphate ion 20 ppm (calculated as phosphate radical), MFR (190°C, 2.16 kg load) 5.0 g/10 min. Adhesive resin (b)
b-1: Maleic anhydride modified polyethylene "Admer (trademark) NF518" manufactured by Mitsui Chemicals, Inc. (MFR (190°C, 2.16 kg load) 3.1 g/10 min, density 0.91 g/cm ³ , acid value 1.10 mgKOH/g)
b-2: Maleic anhydride modified polypropylene "Admer (trademark) QF500" manufactured by Mitsui Chemicals, Inc. (MFR (230°C, 2.16 kg load) 3.0 g/10 min, density 0.90 g/cm ³ , acid value 1.12 mgKOH/g)
Ethylene-α-olefin copolymer resin (c)
c-1: Evolue™ SP1510 linear low-density polyethylene manufactured by Prime Polymer Co., Ltd. (ethylene and 1-hexene polymerized with a metallocene catalyst, MFR (190°C, 2.16 kg load) 1.0 g/10 min, density 0.915 g/cm ³ )
c-2: Evolue™ SP0510 linear low-density polyethylene manufactured by Prime Polymer Co., Ltd. (ethylene and 1-hexene polymerized with a metallocene catalyst, MFR (190°C, 2.16 kg load) 1.2 g/10 min, density 0.903 g/cm ³ )
c-3: Evolue™ SP2510 linear low-density polyethylene manufactured by Prime Polymer Co., Ltd. (ethylene and 1-hexene polymerized with a metallocene catalyst, MFR (190°C, 2.16 kg load) 1.5 g/10 min, density 0.923 g/cm ³ )
cC-1: Evolue™ SP4030 linear low-density polyethylene manufactured by Prime Polymer Co., Ltd. (ethylene and 1-hexene polymerized with a metallocene catalyst, MFR (190°C, 2.16 kg load) 3.8 g/10 min, density 0.938 g/cm ³ ).
cC-2: Polypropylene "Novatec (trademark) PP EA7AD" manufactured by Japan Polypropylene Corporation (MFR (230°C, 2.16 kg load) 1.4 g/10 min, density 0.90 g/cm ³ ).

<Evaluation method>
(1) Differential Scanning Calorimetry Analysis of Base Material Layer (Y) The base material layer (Y) used in the Examples and Comparative Examples was heated from 20°C to 220°C at 10°C/min and then cooled to -50°C at 10°C/min using a differential scanning calorimeter DSC ("Q2000" manufactured by TA Instruments). The difference (T1-T2) between the maximum endothermic peak temperature (T1) during heating and the maximum exothermic peak temperature (T2) during cooling was calculated using a differential scanning calorimeter DSC ("Q2000" manufactured by TA Instruments). The maximum endothermic peak temperature (T1) during heating was taken as the melting point of the polypropylene (y) constituting the base material layer (Y).

(2) Measurement of Elastic Modulus of Base Material Layer (Y) The base material layer (Y) used in the Examples and Comparative Examples was conditioned at 23°C and 50% RH for 24 hours, and then cut into a 15 mm width, and the elastic modulus was measured at a tensile speed of 5 mm/min in accordance with ASTM D-638. The measurement was carried out on 10 samples each in the machine direction (MD) and the cross direction (TD), and the average value was calculated.

(3) Measurement of puncture strength of substrate layer (Y) and multilayer structure The substrate layer (Y) used in the examples and comparative examples and the multilayer structures obtained in the examples and comparative examples were conditioned for 24 hours under conditions of 23°C and 50% RH, and then the breaking strength was measured when pierced with a needle having a tip diameter of 1 mm at a speed of 50 mm/min under the same conditions. The measurement was performed 10 times at different locations, and the average value was used as the measurement result. Note that the needle was pierced from the substrate layer (Y) side in the multilayer structures.

(4) Measurement of Oxygen Transmission Rate of Multilayer Structure Using the multilayer structures obtained in the examples and comparative examples, the oxygen transmission rate was measured with one side as the oxygen supply side and the other as the carrier gas side. Specifically, using an oxygen transmission rate measuring device ("MOCON OX-TRAN2/21" manufactured by Modern Controls), the oxygen transmission rate (unit: cc/(m2 day atm)) was measured in accordance with JIS K 7126-2 (isobaric method; 2006) under the following conditions: temperature ²⁰ °C, humidity 65% RH on the oxygen supply side, humidity 65% RH on the carrier gas side, oxygen pressure 1 atmosphere, and carrier gas pressure 1 atmosphere. During the measurement, the substrate layer (Y) side of the multilayer structure was the oxygen supply side. Nitrogen gas containing 2% by volume of hydrogen gas was used as the carrier gas. Note that if the oxygen transmission rate was 5.0 cc/( ^m2 day atm) or more, it was determined that the gas barrier properties were insufficient.

(5) Measurement of Water Vapor Transmission Rate of Multilayer Structure Using the multilayer structures obtained in the examples and comparative examples, the water vapor transmission rate was measured with one side as the water vapor supply side and the other as the carrier gas side. Specifically, using a water vapor transmission rate measuring device ("MOCON PERMATRAN W3/33" manufactured by Modern Controls), the water vapor transmission rate (unit: g / (m 2 · day)) was measured in accordance with JIS K 7129-2 (infrared sensor method; ²⁰¹⁹ ) under conditions of a temperature of 40 ° C, a humidity of 90% RH on the water vapor supply side, and a humidity of 0% RH on the carrier gas side. During the measurement, the substrate layer (Y) side of the multilayer structure was the carrier gas side. Nitrogen gas was used as the carrier gas.

(6) Drop-breakage resistance of multilayer structure (room temperature)
Two A4-sized sheets were cut out from the multilayer structures obtained in the Examples and Comparative Examples, and the sealant layers (X) were stacked together. Three sides were heat-sealed at a width of 5 mm. Next, 1.2 L of water was filled through the opening, and the remaining side was heat-sealed to create a water-filled bag. This water-filled bag was repeatedly free-falling upright from a height of 1.5 m under conditions of 25°C and 70% RH, and the number of times water leakage was confirmed was counted. The same test was performed five times, and the average value was used as the measurement result. The results were evaluated according to the following criteria. Note that D and E are unacceptable criteria.
Judgment: Criteria A: 12 or more times B: 8 or more times but less than 12 times C: 4 or more times but less than 8 times D: 2 or more times but less than 4 times E: Less than 2 times

(7) Drop-breakage resistance of multilayer structure (high temperature)
Two A4-sized sheets of the multilayer structures obtained in the Examples and Comparative Examples were cut out, the sealant layers (X) overlapping each other, and three sides were heat-sealed at a width of 5 mm. Next, 1.2 L of water was filled through the opening, and the remaining side was heat-sealed to create a water-filled bag. This water-filled bag was immersed in hot water at 95°C for 15 minutes, and then repeatedly free-falled upright from a height of 1.5 m under conditions of 25°C and 70% RH. The number of times water leakage was confirmed was counted. The same test was performed five times, and the average value was used as the measurement result. The results were evaluated according to the following criteria. D and E are unacceptable criteria.
Judgment: Criteria A: 8 times or more B: 5 times or more but less than 8 times C: 3 times or more but less than 5 times D: 2 times or more but less than 3 times E: Less than 2 times

(8) Evaluation of Recyclability of Multilayer Structures The multilayer structures obtained in the Examples and Comparative Examples were crushed into pieces measuring 4 mm square or less. This crushed material was blended with a low-density polyethylene resin "Novatec LD LJ400" manufactured by Japan Polyethylene Corporation (MFR (190°C, 2.16 kg load) 1.5 g/10 min, density 0.921 g/cm ³ ) and a polypropylene "Novatec™ PP EA7AD" manufactured by Japan Polypropylene Corporation (MFR (230°C, 2.16 kg load) 1.4 g/10 min, density 0.90 g/cm ³ ) in a mass ratio (crushed material/polyethylene resin/polypropylene resin) of 40/30/30. A monolayer film having a thickness of 50 μm was produced by extrusion under the following conditions. The thickness of the monolayer film was adjusted by appropriately changing the screw rotation speed and take-up roll speed. As a control, a single layer film having a thickness of 50 μm was similarly obtained without blending the pulverized material ((polyethylene resin/polypropylene resin) 50/50).
Extruder: Single-screw extruder manufactured by Toyo Seiki Seisakusho Co., Ltd. Screw diameter: 20 mmφ (L/D = 20, compression ratio = 3.5, full flight type)
Extrusion temperature: feeding section/compression section/metering section/die = 180/230/230/230°C
Take-up roll temperature: 80°C
The occurrence of lumps in the obtained monolayer film was evaluated on the following five levels, A to E, where E is an unacceptable standard.
Judgment: Criteria A: The amount of particles was almost the same as the control. B: The amount of small particles was slightly more than the control. C: The amount of small particles was more than the control. D: The amount of large particles was more than the control. E: The amount of large particles was significantly more than the control.

<Example>
Example 1
Using EVOH (a-1) as the material for the barrier layer (A), adhesive resin (b-1) as the material for the adhesive layer (B), and ethylene-α-olefin copolymer resin (c-1) as the material for the heat-sealable layer (C), a multilayer film (sealant layer (X-1)) having a layer thickness and layer structure of (C)/(B)/(A)/(B)/(C)=60 μm/4 μm/6 μm/4 μm/60 μm was produced using a 300 mm wide five-layer co-extrusion cast film production facility. The film production conditions at this time are shown below.
Extrusion temperature of barrier layer (A): feeding section/compression section/metering section/die = 170/220/220/220°C
Extrusion temperature of adhesive layer (B): feeding section/compression section/metering section/die=170/220/220/220°C
Extrusion temperatures of the heat-sealing layer (C): feeding section/compression section/metering section/die=170/220/220/220°C
Chill roll temperature: 40°C
Take-up speed: 1.5 m/min

An adhesive solution was prepared by mixing 24 parts by weight of a two-component reactive polyurethane adhesive (Mitsui Chemicals' "Takelac A-520" and 4 parts by weight of "Takenate A-50") with 37 parts by weight of ethyl acetate. The adhesive solution was then applied to the substrate layer (Y-1) using a bar coater to a dry thickness of 4 μm, and dried at 80°C for 3 minutes to form the adhesive layer (Z). This was then laminated with the multilayer film (sealant layer (X-1)) obtained above to produce a multilayer structure with a layer thickness and layer structure of (Y-1)/adhesive layer (Z)/(C)/(B)/(A)/(B)/(C) = 20 μm/4 μm/60 μm/4 μm/6 μm/4 μm/60 μm.

The substrate layer (Y-1) used and the resulting multilayer structure were evaluated according to the evaluation methods described above. The results are shown in Table 1.

Examples 2 to 14, Comparative Examples 3 to 6
Multilayer structures were produced and evaluated in the same manner as in Example 1, except that the type of substrate layer (Y), the type of EVOH (a), the thickness of the barrier layer (A), the type of ethylene-α-olefin copolymer resin (c), and the thickness of the adhesive layer (Z) were changed as shown in Table 1. The results are shown in Tables 1 and 2.

Example 15
Using EVOH (a-1) as the material for the barrier layer (A), adhesive resin (b-1) as the material for the adhesive layer (B), and ethylene-α-olefin copolymer resin (c-1) as the material for the heat-sealable layer (C), a multilayer film (sealant layer (X-2)) having a layer thickness and layer structure of (A)/(B)/(C)=6 μm/8 μm/120 μm was produced using a 300 mm wide five-layer co-extrusion cast film production equipment under the same film formation conditions as in Example 1. Thereafter, a multilayer structure having a layer thickness and layer structure of (Y-1)/adhesive layer (Z)/(A)/(B)/(C)=20 μm/4 μm/6 μm/8 μm/120 μm was produced and evaluated in the same manner as in Example 1. The results are shown in Table 1.

Comparative Example 1
A multilayer structure was produced and evaluated in the same manner as in Example 1, except that the base layer (Y-1) was not laminated and the sealant layer (X-1) was used as it was as a multilayer structure. The results are shown in Table 2.

Comparative Example 2
A multilayer structure was produced and evaluated in the same manner as in Example 1, except that a single-layer film of the heat-sealing layer (C) having a thickness of 134 μm was produced and used instead of the multilayer film. The results are shown in Table 2.

Claims (12)

A sealant layer (X) and a substrate layer (Y) are included.
the sealant layer (X) comprises a barrier layer (A) containing, as a main component, an ethylene-vinyl alcohol copolymer (a) having an ethylene unit content of 20 to 50 mol% and a saponification degree of 90 mol% or more, an adhesive layer (B) containing, as a main component, an adhesive resin (b), and a heat-sealable layer (C) containing, as a main component, an ethylene-α-olefin copolymer resin (c) having ^a density of 0.880 to 0.930 g/cm3;
the base layer (Y) is a biaxially stretched layer containing, as a main component, polypropylene (y) having a melting point of 150°C or higher as measured by a differential scanning calorimeter (DSC), and the biaxially stretched layer has an elastic modulus of 2000 MPa or higher under conditions of 23°C and 50% RH;
A multilayer structure having a heat-sealable layer (C) of a sealant layer (X) on the outermost surface. 2. The multilayer structure according to claim 1, wherein the difference (S-S Y ) between the puncture strength S measured in accordance with JIS Z 1707:2019 under conditions of 23°C and 50% RH and the puncture strength S _Y of the base layer ( _Y ) measured in accordance with JIS Z 1707:2019 under conditions of 23°C and 50% RH is 3.5 N or more, and the puncture strength S is measured by piercing the multilayer structure from the base layer (Y) side . The multilayer structure according to claim 2, wherein the puncture strength S is 12 N or more. The multilayer structure according to any one of claims 1 to 3, wherein when the substrate layer (Y) is heated from 20°C to 220°C at 10°C/min and then cooled to -50°C at 10°C/min using a differential scanning calorimeter (DSC), the difference (T1-T2) between the maximum endothermic peak temperature (T1) during heating and the maximum exothermic peak temperature (T2) during cooling is 56°C or less. The multilayer structure according to any one of claims 1 to 3, wherein the sealant layer (X) and the substrate layer (Y) are laminated via an adhesive layer (Z), and the thickness of the adhesive layer (Z) is 2 to 7 μm. The multilayer structure according to any one of claims 1 to 3, wherein the ethylene-α-olefin copolymer resin (c) is a linear low-density polyethylene copolymerized with ethylene and an α-olefin having 6 or more carbon atoms. The multilayer structure according to any one of claims 1 to 3, wherein the barrier layer (A) contains a boron compound in an amount of 50 to 400 ppm in terms of elemental boron. The multilayer structure according to any one of claims 1 to 3, wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer (a) is 22 mol% or more and 28 mol% or less. The multilayer structure according to any one of claims 1 to 3, wherein the thickness of the barrier layer (A) is 5 μm or less. The multilayer structure according to any one of claims 1 to 3, wherein the oxygen transmission rate measured under conditions of 20°C and 65% RH by the method described in JIS K 7126-2:2006 is 5 cc/( ^m2 day atm) or less. The multilayer structure according to any one of claims 1 to 3, wherein the water vapor transmission rate measured by the method described in JIS K 7129-2:2019 under conditions of 40°C and 90% RH is 3 g/( ^m2 ·day) or less. A packaging material comprising the multilayer structure described in any one of claims 1 to 3.