Multilayer structure

Abstract

The present invention pertains to a multilayer structure containing a thermoplastic resin layer (III), a barrier layer (I), and paper (II) in this order. The barrier layer (I) is adjacent to the paper (II) and contains: a modified starch (A) having an average amylose content of 40 mass% or greater; and a water-soluble polymer (B). The paper (II) has an air permeation resistance of 30-80,000 seconds measured on the basis of ISO 5636-6.

Description

Description Title of invention: Multilayer structure Technical field [0001 I This patent application is Japanese Patent Application No. 2021-184964 (filing date: 20 This application claims priority under the Paris Convention to the above-referenced patent application Ser. No. 09 / 2002, filed on November 12, 2002, which is hereby incorporated by reference in its entirety.

[0002] The present invention relates to a multilayer structure used for food packaging containers and the like, and to a packaging material or lid material and a packaging bag for microwave heating that include the multilayer structure.

[0003] With the recent rise in environmental awareness and calls for the reduction of plastics, demand for paper packaging is on the rise. Paper is a material with excellent biodegradability and recyclability, but it has poor barrier properties and strength, and in order to fulfill the same functions as plastic packaging, it needs to be laminated or composited with other functional materials. Resin compositions containing highly biodegradable modified starch and polyvinyl alcohol are widely used in food packaging, and can be laminated with paper to add gas barrier properties (e.g., Patent Document 1). Recently, the use of paper has also been increasing in packaging bags for microwave heating (e.g., Patent Document 2). Prior Art Literature Patent Literature

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2021-123617 Patent Document 2: Japanese Patent Application Laid-Open No. 2021-54501 Summary of the invention Problems to be solved by the invention

[0005] Resin compositions containing modified starch and polyvinyl alcohol not only provide barrier properties when combined with paper, but can also be recycled (disintegrated) together with the paper, which is an advantage over other barrier materials. Therefore, according to the investigations of the present inventors, it has been found that when a multilayer structure including a barrier layer made of such a resin composition between a paper layer and a resin layer is heated by microwaves, problems such as the generation of bubbles due to foaming and voids at the interface, deformation of the multilayer structure, and deterioration of barrier properties may occur.

[0006] Therefore, an object of the present invention is to provide a multilayer structure that can suppress foaming and deformation due to microwave heating and has excellent barrier properties even after microwave heating, as well as a packaging material or lid material containing the multilayer structure, and a packaging bag for microwave heating. * Means for solving the problem [0007I] As a result of extensive research into solving the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by adjusting the average amylose content of the modified starch (A) to 40% by mass or more and arranging the barrier layer (I) directly adjacent to the paper (II) having an air resistance within a certain range in a multilayer laminate comprising a thermoplastic resin layer (IH), a barrier layer (I) containing a modified starch (A) and a water-soluble polymer (B), and paper (II) in this order, and have completed the present invention. That is, the present invention includes the following aspects.

[0008] [1] A multilayer structure comprising a thermoplastic resin layer (III), a barrier layer (I), and paper (II) in this order, wherein the barrier layer (I) is adjacent to paper (H), the barrier layer (I) comprises a modified starch (A) having an average amylose content of 40 mass% or more and a water-soluble polymer (B), and the paper (II) has an air resistance of 30 to 80,000 seconds as measured in accordance with IS0 5636-6. [2] The multilayer structure according to [1], wherein the water-soluble polymer (B) is polyvinyl alcohol and / or polyoxyalkylene. [3] The modified starch (A) content is 30-99.5 parts by mass, and the water-soluble polymer (B) content is 0.5-70 parts by mass, based on 100 parts by mass of the total of the modified starch (A) and the water-soluble polymer (B). Multilayer structure. [4] The multilayer structure according to any one of

[0001] to

[0003] , wherein the thickness of the Norlia layer (I) is 1 to 600 μm. [5] The multilayer structure according to any one of

[0001] to [4I], wherein the thickness of the thermoplastic resin layer (III) is 1 to 1,000 μm. [6] A packaging material or a lid material comprising the multilayer structure according to any one of

[0001] to [5]. 〇 [7] A packaging bag for microwave heating, comprising the multilayer structure according to any one of

[0001] to [5]. The multilayer structure of the present invention can suppress foaming and deformation due to microwave heating and has excellent barrier properties even after microwave heating. Therefore, it can be suitably used as a packaging material or a lid material for food, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a schematic diagram of a twin-screw extruder used in the examples. [Fig. 2] A schematic diagram of a coating manufacturing apparatus used in the examples.

[0011] [Multilayer structure] The multilayer structure of the present invention comprises a thermoplastic resin layer (III), a barrier layer (I), and paper (II) in this order, the barrier layer (I) being adjacent to the paper (II), the barrier layer (I) comprising 40 mass % or more of modified starch (A) and a water-soluble polymer (B), and the air resistance of the paper (II) being 30-80000 seconds. The inventors have investigated the relationship between microwave heating and the subsequent deformation, foaming, and poor barrier properties of the multilayer structure, and have found that when heated together with food during microwave heating, water vapor is generated in the central barrier layer (I), and if the rate at which this water vapor is released to the outside is slow, bubbles and deformation may occur, or heat may cause destruction within and between the layers, resulting in poor barrier properties. On the other hand, if the rate at which water vapor is released to the outside is too fast, the resin composition in the barrier layer may shrink, causing the multilayer structure to deteriorate. Therefore, the present inventors have conducted further studies and have found that, even in a layer structure in which a barrier layer (I) containing modified starch (A) and a water-soluble polymer (B) is disposed between a thermoplastic resin layer (III) and a paper (II), if the average amylose content of the modified starch (A) is adjusted to 40% by mass or more, paper (H) is placed adjacent to the barrier layer (I), and the air resistance of the paper (H) is adjusted to 30 to 80,000 seconds, it is possible to suppress foaming and deformation of the multilayer structure due to microwave heating, and the barrier property after microwave heating is excellent. In this specification, the properties capable of suppressing foaming and deformation due to microwave heating may be simply referred to as "foaming resistance" and "deformation resistance," respectively. In this specification, the term "barrier properties" refers to barrier properties including odor resistance, oil resistance, and gas barrier properties, and unless otherwise specified, this term refers to properties both before and after microwave heating. [0012I <Barrier Layer (I)>> The barrier layer (I) in the multilayer structure of the present invention is a layer having barrier properties and contains a modified starch (A) and a water-soluble polymer (B).

[0013] (Modified Starch (A)) From the viewpoint of easily improving barrier properties, foaming resistance, deformation resistance, and biodegradability, the modified starch (A) is preferably at least one selected from the group consisting of, for example, etherified starch, esterified starch, cationized starch, and crosslinked starch.

[0014] Examples of starches include starches derived from cassava, corn, potato, sweetcorn, sago, pioca, sorghum, beans, bracken, lotus, water chestnut, wheat, rice, oats, arrowroot, turmeric, and peas. Among these, starches derived from corn and cassava are preferred, and starches derived from high-amylose corn are more preferred. Starches can be used alone or in combination of two or more.

[0015] Examples of etherified starches include alkyl-etherified starches such as methyl-etherified starch; and carboxymethyl-etherified starch. Carboxyalkyl-etherified starch: for example, hydroxyalkyl-etherified starch such as etherified starch having a hydroxyalkyl group having 2 to 6 carbon atoms. Allyl-etherified starch can also be used.

[0016] Examples of esterified starches include esterified starches having structural units derived from carboxylic acids, such as esterified starches having structural units derived from acetic acid; esterified starches having structural units derived from dicarboxylic anhydrides, such as esterified starches having structural units derived from maleic anhydride, esterified starches having structural units derived from phthalic anhydride, and esterified starches having structural units derived from octenylsuccinic anhydride; and esterified starches having structural units derived from oxoacids, such as nitrate-esterified starch, phosphate-esterified starch, and urea-phosphate-esterified starch. Other examples include xanthate-esterified starch, acetoacetate-esterified starch, etc.

[0017] Examples of cationized starch include a reaction product of starch with 2-diethylaminoethyl chloride and a reaction product of starch with 2,3-epoxypropyltrimethylammonium chloride.

[0018] Examples of crosslinked starches include formaldehyde-crosslinked starch, epichlorohydrin-crosslinked starch, phosphate-crosslinked starch, and acrolein-crosslinked starch.

[0019] From the viewpoint of easily improving the barrier property, foaming resistance, deformation resistance and biodegradability, the modified starch (A) is preferably at least one selected from the group consisting of etherified starches having a hydroxyalkyl group having 2 to 6 carbon atoms and esterified starches having structural units derived from dicarboxylic acid anhydrides, and examples thereof include hydroxyethyl etherified starch, hydroxypropyl etherified starch, hydroxybutyl etherified starch, esterified starch having structural units derived from maleic anhydride, esterified starch having structural units derived from phthalic anhydride and esterified starch having structural units derived from octenylsuccinic anhydride. The modified starch (A) is preferably at least one selected from the group consisting of esterified starches having the structural unit. The modified starch (A) can be used alone or in combination of two or more. In this specification, the number of carbon atoms listed before "starch" indicates the number of carbon atoms in the group substituted with one hydroxyl group in the starch (the group formed by modifying one hydroxyl group in the starch). For example, etherified starch having a hydroxyalkyl group with 2 to 5 carbon atoms indicates that the hydroxyalkyl group formed by modifying one hydroxyl group in the starch has 2 to 5 carbon atoms. The etherified starch having a hydroxyalkyl group having 2 to 6 carbon atoms may be obtained by reacting starch with an alkylene oxide such as ethylene oxide, propylene oxide, or butylene oxide. The average number of hydroxy groups used for modification is preferably 0.05 to 2 per glucose unit in the starch. [0021I] The average amylose content of the modified starch (A) contained in the barrier layer (I) is 40% by mass or more. If the average amylose content of the modified starch (A) is less than 40% by mass, the barrier properties and deformation resistance tend to decrease. In the multilayer structure of the present invention, the average amylose content of the modified starch (A) contained in the barrier layer (I) is 40% by mass or more, and therefore the barrier properties and deformation resistance can be improved. The average amylose content of the modified starch (A) is preferably 45% by mass or more, more preferably 50% by mass or more, even more preferably 55% by mass or more, still more preferably 60% by mass or more, and particularly preferably 65% ​​by mass or more. When the average amylose content is equal to or greater than the above lower limit, the barrier properties and deformation resistance tend to be improved. The average amylose content of the modified starch (A) is usually 90% by mass or less. In this specification, the amylose content can be measured, for example, by the iodine coloring method described in "Starch 50 No. 4158-163 (1998) J.K." When one type of modified starch is used, the average amylose content indicates the amylose content of the one type of modified starch, and when two or more types of modified starch are used, the average amylose content indicates the amylose content of the two or more types of modified starch. Therefore, for example, when two or more types of modified starch are used to achieve an average amylose content of 40% by mass or more, the starch may contain modified starch with an amylose content of less than 40% by mass. [0022I] The moisture content in the modified starch (A) may be 5 to 15% by mass.

[0023] Commercially available modified starch (A) can also be used. Typical commercially available examples of modified starch (A) include ECO FILM (trademark) and National 1658 (trademark), which are hydroxypropyl etherified starches manufactured by Ingredion.

[0024] The content of the modified starch (A) is preferably 30 parts by mass or more, more preferably 40 parts by mass or more, even more preferably 50 parts by mass or more, even more preferably 60 parts by mass or more, particularly preferably 70 parts by mass or more, especially more preferably 75 parts by mass or more, and is preferably 99.5 parts by mass or less, more preferably 99 parts by mass or less, even more preferably 98 parts by mass or less, especially preferably 95 parts by mass or less, based on 100 parts by mass of the total of the modified starch (A) and the water-soluble polymer (B). When the content of the modified starch (A) is at least the above lower limit, biodegradability is easily improved, and when the content of the modified starch (A) is at most the above upper limit, barrier property, oil resistance, and deformation resistance are easily improved.

[0025] (Water-soluble polymer (B)) The water-soluble polymer (B) is a polymer compatible with the modified starch (A). There are no particular limitations on the water-soluble polymer (B), but it is preferable that it has a melting point suitable for the processing temperature of the modified starch (A). From the viewpoint of easily improving the barrier properties, foaming resistance, and deformation resistance, it is preferably polyvinyl alcohol and / or polyoxyalkylene, and more preferably polyvinyl alcohol.

[0026] The degree of salination of the polyvinyl alcohol is preferably 80 mol% or more, and may be 100 mol% or less, 99.9 mol% or less, or 99.8 mol% or less. When the degree of salination of the polyvinyl alcohol is within the above range, the barrier The degree of saponification is more preferably 85 mol% or more, even more preferably 88 mol% or more, and particularly preferably 90 mol% or more. The degree of saponification indicates the molar fraction of hydroxyl groups relative to the total of hydroxyl groups and ester groups in polyvinyl alcohol. The degree of saponification can be measured in accordance with JSK 6726 (Testing Methods for Polyvinyl Alcohol), for example, by the method described in the Examples. Polyvinyl alcohol is produced, for example, by hydrolysis of polyvinyl acetate obtained by polymerization of vinyl acetate monomer. [0027I] The viscosity of a 4% aqueous solution of polyvinyl alcohol at 20°C, measured in accordance with JIS Z 8803, is preferably 1 to 50 mPa•s. When the viscosity of polyvinyl alcohol is within the above range, the barrier properties, foaming resistance, and deformation resistance tend to be improved. The viscosity is more preferably 2 mPa•s or more, even more preferably 3 mPa•s or more, more preferably 45 mPa•s or less, and even more preferably 40 mPa•s or less.

[0028] The polyvinyl alcohol (B) may further contain other monomer units in addition to vinyl alcohol units. Examples of the other monomer units include monomer units derived from ethylenically unsaturated monomers. Examples of ethylenically unsaturated monomers include α-olefins such as ethylene, propylene, n-butene, isobutylene, and 1-hexene; acrylic acid and its salts; unsaturated monomers having an acrylate group; methacrylic acid and its salts; unsaturated monomers having a methacrylate group; acrylamide, N-methyl acrylamide, N-ethyl acrylamide, N,N-dimethyl acrylamide, diacetone acrylamide, acrylamide propane sulfonic acid and its salts, acrylamide propyl dimethylamine and its salts (e.g., quaternary salts); methacrylamide, N-methyl methacrylamide, N-ethyl methacrylamide, methacrylamidopropane sulfonic acid and its salts, and methacrylamide propyl dimethylamine. vinyl ether and its salts (e.g., quaternary salts); methyl vinyl ether, ethyl vinyl ether vinyl ethers such as n-propyl vinyl ether, i-propyl vinyl ether, n-butyl vinyl ether, i-butyl vinyl ether, t-butyl vinyl ether, dodecyl vinyl ether, stearyl vinyl ether, and 2,3-diacetoxy-1-vinyloxypropane; vinyl cyanides such as acrylonitrile and methacrylonitrile; vinyl halides such as vinyl chloride and vinyl fluoride; vinyl halides such as vinylidene chloride and vinylidene fluoride; allyl compounds such as allyl acetate, 2,3-diacetoxy-1-allyloxypropane, and allyl chloride; unsaturated dicarboxylic acids such as maleic acid, itaconic acid, and fumaric acid, and their salts or esters; vinyl silyl compounds such as vinyltrimethoxysilane; isopropenyl acetate; vinyl formate, vinyl acetate, vinyl propionate, Examples of such monomers include vinyl ester monomers such as vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl versatate, vinyl caproate, vinyl carboxylate, vinyl laurate, vinyl palmitate, vinyl stearate, vinyl oleate, and vinyl benzoate. The content of other monomer units is preferably 10 mol % or less, and more preferably 5 mol % or less, of the total molar amount of the structural units constituting the polyvinyl alcohol. The method for producing polyvinyl alcohol is not particularly limited. For example, there is a method in which vinyl acetate monomer and, optionally, other monomers are polymerized, and the resulting polymer is chlorinated to convert it into vinyl alcohol units. Polymerization methods include batch polymerization, semi-batch polymerization, continuous polymerization, and semi-continuous polymerization. Examples of polymerization methods include known methods such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. The chlorination of the polymer can be carried out by known methods. For example, the chlorination can be carried out in a state in which the polymer is dissolved in alcohol or aqueous alcohol. Examples of alcohols that can be used in this case include lower alcohols such as methanol and ethanol. Polyvinyl alcohols can be used alone or in combination of two or more types. [0030I] Polyoxyalkylene includes polyalkylene oxide and polyalkylene. The polyoxyalkylene represents a glycol and has a structural unit represented by the following formula (1) (also referred to as structural unit (1)). The polyoxyalkylene may have two or more different types of structural unit (1). [Chemical 1] Mo R ー 〇 ±7 ( 1) [wherein R is an alkylene group, and n is 1 or more]

[0031] In formula (1), examples of the alkylene group include alkylene groups having 2 to 10 carbon atoms, such as ethylene, propylene, trimethylene, butylene, isobutylene, pentylene, hexylene, heptylene, octylene, nonylene, and decylene. Among these, from the viewpoint of easily improving barrier properties, foaming resistance, and deformation resistance, alkylene groups having 2 to 6 carbon atoms are preferred, and ethylene and / or propylene are more preferred. When n is 2 or more, these alkylene groups can be used alone or in combination.

[0032] In order to facilitate improving the barrier properties, foaming resistance, and deformation resistance, n in formula (1) is preferably at least 5, more preferably at least 50, even more preferably at least 100, and is preferably at most 120,000, more preferably at most 70,000. When the polyoxyalkylene contains different structural units (1), the number of repeats n of each structural unit may be the same or different.

[0033] Examples of polyalkylene oxides include polymers having structural units derived from alkylene oxides having 2 to 6 carbon atoms, such as polyethylene oxide, polypropylene oxide, polytrimethylene oxide (polyoxetane), polybutylene oxide, polyisobutylene oxide, or copolymers of monomers constituting these. Examples of polyalkylene glycols include polymers having structural units derived from alkylene glycols having 2 to 6 carbon atoms, such as polyethylene glycol, polypropylene glycol, polytrimethylene oxide, polyisobutylene oxide, or copolymers of monomers constituting these. Examples of suitable polyoxyalkylenes include methylene glycol, polybutylene glycol, polyisobutylene glycol, and copolymers of monomers constituting these. Among these, from the viewpoint of easily improving barrier properties, foaming resistance, and deformation resistance, the polyoxyalkylene is preferably polyethylene oxide, polypropylene oxide, polyethylene glycol, polypropylene glycol, or a copolymer of monomers constituting these. Preferred examples of such copolymers include a copolymer of ethylene oxide and propylene oxide, and a copolymer of ethylene glycol and propylene glycol. [0034I] The polyoxyalkylene may contain structural units derived from monomers other than the structural unit (1), so long as the effects of the present invention are not impaired. When the polyoxyalkylene is a copolymer, the polymerization form of the copolymer is not particularly limited and may be any of random, block, graft, or tapered. The polyoxyalkylene may be used alone or in combination of two or more types. From the viewpoint of easily improving barrier properties, foaming resistance, and deformation resistance, the weight-average molecular weight of the polyoxyalkylene is preferably 10,000 or more, more preferably 50,000 or more, and preferably 5,000,000 or less, more preferably 3,000,000 or less.

[0036] Commercially available polyoxyalkylenes can also be used. Typical examples of commercially available polyoxyalkylenes include ALKOX (trademark) E-75G, ALKOX (trademark) L-11, ALKOX (trademark) L-6, and ALKOX (trademark) EP 101 ON, all manufactured by Meisei Chemical Industry Co., Ltd., and PEO (trademark) PE0-1 and PE0-2, all manufactured by Sumitomo Seika Chemicals Co., Ltd.

[0037] The content of the water-soluble polymer (B) is preferably 0.5 parts by mass or more, more preferably 1 part by mass or more, even more preferably 2 parts by mass or more, particularly preferably 5 parts by mass or more, based on 100 parts by mass of the total of the modified starch (A) and the water-soluble polymer (B). It is preferably 70 parts by mass or less, more preferably 60 parts by mass or less, even more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less. When the content of the water-soluble polymer (B) is at least the above lower limit, the barrier property, oil resistance, and deformation resistance are likely to be improved, and when the content of the water-soluble polymer (B) is at most the above upper limit, the biodegradability is likely to be improved. In the barrier layer (I), the total proportion of the modified starch (A) and the water-soluble polymer (B) is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, and may be 90% by mass or more, 95% by mass or more, or 98% by mass or more, based on the mass of the barrier layer (I). The total proportion of the modified starch (A) and the water-soluble polymer (B) may be 100% by mass or less. When the total proportion of the modified starch (A) and the water-soluble polymer (B) is within the above ranges, the barrier properties, foaming resistance, deformation resistance, and biodegradability are likely to be improved.

[0039] The total proportion of the modified starch (A) and the water-soluble polymer (B) is preferably 60% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, and may be 90% by mass or more, 95% by mass or more, 98% by mass or more, or 99% by mass or more, based on the mass of the resin constituting the barrier layer (I). The total proportion of the modified starch (A) and the water-soluble polymer (B) may be 100% by mass or less.

[0040] (Other Components) In the multilayer structure of the present invention, the barrier layer (I) may further contain a fatty acid and / or a fatty acid salt thereof having 12 to 22 carbon atoms. Examples of fatty acids and fatty acid salts thereof having 12 to 22 carbon atoms include stearic acid, calcium stearate, sodium stearate, palmitic acid, lauric acid, myristic acid, linoleic acid, and behenic acid. Of these, from the viewpoint of processability, stearic acid, calcium stearate, and sodium stearate are preferred. The fatty acids and fatty acid salts thereof having 12 to 22 carbon atoms can be used alone or in combination of two or more.

[0041] The barrier layer (I) is a fatty acid having 12 to 22 carbon atoms and / or a fatty acid thereof. When a salt is contained, the content in the barrier layer (I) is preferably 0.01 to 3 mass%, more preferably 0.03 to 2 mass%, and even more preferably 0.1 to 1 mass%, based on the mass of the barrier layer (I). When the content of the fatty acid having 12 to 22 carbon atoms and / or the fatty acid salt thereof is within the above range, processability tends to be advantageous. [0042I] The barrier layer (I) may further contain clay. Examples of clay include synthetic or natural layered silicate clays such as montmorillonite, bentonite, beidelite, mica, hectorite, saponite, nontronite, sauconite, vermiculite, ledikite, magadite, kenyaite, stevensite, and vulkonskoite. The clays can be used alone or in combination. When the barrier layer (I) contains clay, the content of the clay in the barrier layer (I) is preferably 0.1 to 5 mass %, more preferably 0.1 to 3 mass %, and even more preferably 0.5 to 2 mass %, based on the mass of the barrier layer (I). When the clay content is within the above range, there is a tendency for the transparency and strength to be advantageous.

[0044] If the water-containing composition (described below) that forms the barrier layer (I) contains a plasticizer, the film-forming and coating properties are improved when the barrier layer (I) is formed directly on the paper (II), and the adhesion between the paper (II) and the barrier layer (I) is easily increased, which makes it easier to suppress the generation of voids and deformation at the interface due to microwave heating. Therefore, it is preferable that the barrier layer (I) in the multilayer structure contains a plasticizer. Examples of plasticizers include water, sorbitol, glycerol, maltitol, xylitol, mannitol, glycerol trioleate, epoxidized linseed oil, epoxidized soybean oil, tributyl citrate, acetyltriethyl citrate, glyceryl triacetate, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, polyethylene oxide, and polyethylene glycol. The plasticizers can be used alone or in combination of two or more. Among these plasticizers, water is preferred from the viewpoint of easily improving the barrier property. When the barrier layer (I) contains water, the total ratio of the modified starch (A), the water-soluble polymer (B), and the water in the barrier layer (I) is is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more, based on the mass of the barrier layer (I), and may be 90% by mass or more, 95% by mass or more, 98% by mass or more, 99% by mass or more, or 99.9% by mass or more. [0045I] The moisture content (water content) in the barrier layer (I) is preferably 3% by mass or more, more preferably 4% by mass or more, even more preferably 7% by mass or more, and preferably 20% by mass or less, more preferably 18% by mass or less, and even more preferably 15% by mass or less, based on the mass of the barrier layer (I). When the moisture content is equal to or higher than the above lower limit, the barrier layer (I) is likely to have appropriate bending resistance as a food packaging material, and when the moisture content is equal to or lower than the above upper limit, foaming resistance, deformation resistance, and barrier properties are likely to be improved. The moisture content is determined by crushing the material to a maximum particle size of 1 mm or less using a Wonder Blender WB-1, and measuring the moisture content at a temperature of 130°C using a heat-drying moisture meter. 0 This is the moisture content measured at C for 60 minutes.

[0046] The barrier layer (I) may further contain, as necessary, additives such as fillers, processing stabilizers, weather resistance stabilizers, colorants, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, other thermoplastic resins, lubricants, fragrances, antifoaming agents, deodorizers, extenders, release agents, mold release agents, reinforcing agents, crosslinking agents, mildew inhibitors, preservatives, and crystallization rate retarders. These additives may be used alone or in combination.

[0047] The barrier layer (I) may be one or more, and may be a single layer or multiple layers. When there are two or more barrier layers (I), it is sufficient that at least one barrier layer (I) is disposed between the paper (II) and the thermoplastic resin layer (III) and adjacent to the paper (II), and the thickness and type of each layer may be the same or different.

[0048] The barrier layer (I) is preferably in the form of a film or sheet. From the viewpoint of easily improving barrier properties, foaming resistance, deformation resistance, and biodegradability, the thickness of the barrier layer (I) is preferably 1 Atm or more, more preferably 3 mm or more, even more preferably 5 mm or more, particularly preferably 100 μm or more, and preferably 600 μm or more. The thickness of the barrier layer (I) is preferably 0.00 μm or less, more preferably 500 μm or less, even more preferably 400 μm or less, still more preferably 300 μm or less, and particularly preferably 250 μm or less. When the multilayer structure has two or more barrier layers (I), the thickness of the barrier layer (I) indicates the thickness of one barrier layer (I). The thickness of the barrier layer (I) can be measured by observation with a microscope, for example, by the method described in the Examples.

[0049] <Paper (II)> The multilayer structure of the present invention includes paper (II) adjacent to the barrier layer (I). In this specification, "adjacent" means that the layers are adjacent to each other and in direct contact with each other. That is, in the present invention, the barrier layer (1) is laminated directly on the surface of the paper (II) without any other layer interposed therebetween. The paper may be, for example, a film or sheet containing pulp, fillers, chemicals, and pigments. Examples of pulp include chemical pulps such as bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), unbleached softwood pulp (NUKP), and sulfite pulp; mechanical pulps such as stone-ground pulp and thermomechanical pulp; wood fibers such as deinked pulp and recycled paper pulp; and non-wood fibers obtained from kenaf, bamboo, hemp, etc. These pulps can be used alone or in combination of two or more. Among these, chemical pulp, mechanical pulp, and wood fiber are preferred, with chemical pulp being more preferred, from the viewpoints of easily suppressing the incorporation of foreign matter into the base paper and the occurrence of discoloration over time when used paper containers are recycled, and of easily achieving a good surface texture when printed.

[0050] Examples of fillers include known fillers such as white carbon, talc, kaolin, clay, heavy calcium carbonate, light calcium carbonate, titanium oxide, zeolite, and synthetic resin fillers. Fillers can be used alone or in combination of two or more. Examples of chemicals include oxidized starch, hydroxyethyl etherified starch, enzyme-modified starch, polyacrylamide, polyvinyl alcohol, surface sizing agents (e.g., neutral sizing agents), water-resistant agents, water-retention agents, thickeners, lubricants, retention aids, drainage aids, and paper strength agents, and these can be used alone or in combination of two or more. The above may be used in combination. Examples of retention aids include aluminum sulfate and various anionic, cationic, nonionic, or amphoteric agents. Examples of dry strength agents include polyacrylamide and cationized starch, and examples of wet strength agents include polyamideamine epichlorohydrin. These agents are added to the extent that they do not affect the formation or runnability. Examples of neutral sizing agents include alkyl ketene dimers, alkenyl succinic anhydrides, and neutral rosin sizing agents. Examples of pigments include inorganic pigments such as kaolin, clay, engineered kaolin, delaminated clay, heavy calcium carbonate, light calcium carbonate, mica, talc, titanium dioxide, barium sulfate, calcium sulfate, zinc oxide, silicic acid, silicates, colloidal silica, and satin white, as well as organic pigments such as solid, hollow, and core-shell pigments, which can be used alone or in combination. Furthermore, dyes, fluorescent whitening agents, pH adjusters, antifoaming agents, pitch control agents, slime control agents, and the like can also be added as needed. The surface of the paper may also be treated with various chemicals or pigments. The method for producing paper (papermaking) is not particularly limited, and paper can be produced by acidic, neutral, or alkaline papermaking methods using a known Fourdrinier former, on-top hybrid former, or gear former machine, etc. [0052I] The method for treating the surface of paper is not particularly limited, and known coating devices such as a rod metering size press, a pond type size press, a gate roll coater, a spray coater, a blade coater, and a curtain coater can be used.

[0053] Examples of the paper obtained in this manner include fine paper, medium-quality paper, coated paper, semi-glossy paper, kraft paper, semi-glossy kraft paper, bleached kraft paper, unbleached kraft paper, semi-glossy bleached kraft paper, rayon paper, tissue paper, glassine paper, paperboard, white paperboard, waterproof paper, cellophane, liner, etc. The paper may be used alone or in combination of two or more types. It can be used. The paper may have a transparent coating layer on one or both sides of the base paper described above as part of the paper. Applying a transparent coating to the base paper facilitates improving the surface strength and smoothness of the base paper and also facilitates improving the coatability when applying a pigment. The transparent coating layer may contain a starch-derived polymer compound as a binder. The amount of the transparent coating is preferably 0.1 to 4.0 g / m2, more preferably 0.5 to 2.5 g / m2, in solids per side. For example, a coating liquid containing as its main component starch, various starches such as oxidized starch, polyacrylamide, polyvinyl alcohol, or other water-soluble polymers may be applied to the base paper using a coater (applicator) such as a size press, gate roll coater, premetalling size press, curtain coater, or spray coater. Furthermore, it is preferable to pre-calender the base paper before coating using an online soft calender, online chilled calender or the like to smooth the base paper in advance, in order to make the coating layer uniform after coating. The paper may be subjected to a smoothing treatment as required. For the smoothing treatment, a normal smoothing treatment device such as a supercalender, a gross calender, a soft calender, a heat calender, or a shoe calender can be used. The smoothing treatment device is used on-machine or off-machine as appropriate, and the shape of the pressure device, the number of pressure nips, heating, etc. are also adjusted as appropriate. In the multilayer structure of the present invention, the air resistance of the paper (II) is 30 to 80,000 seconds. In the present invention, the barrier layer (I) and the paper (II) are adjacent to each other without the inclusion of another layer (for example, an adhesive or film) between them, and the air resistance is adjusted to a predetermined range. Therefore, water vapor generated in the barrier layer (I) is easily released to the outside through the paper (II) at an optimum speed. Therefore, the foaming resistance, deformation resistance, and barrier properties after microwave heating of the multilayer structure can be effectively improved.

[0057] The air resistance of the paper (II) in the multilayer structure of the present invention is preferably 35 seconds or more. More preferably, it is 40 seconds or more, even more preferably 45 seconds or more, even more preferably 50 seconds or more, particularly preferably 55 seconds or more, and preferably 600 seconds or more. The air resistance is preferably 0.000 seconds or less, more preferably 40,000 seconds or less, even more preferably 3,000 seconds or less, and even more preferably 20,000 seconds or less. When the air resistance is at least the lower limit, deformation resistance is easily improved. When the air resistance of paper (II) is at most the upper limit, foaming resistance, deformation resistance, and barrier properties after microwave heating are easily improved. The air resistance is measured in accordance with SO5636-6 and can be measured, for example, by the method described in the Examples. The air resistance can be adjusted within the above range by appropriately adjusting the basis weight, the type and amount of filler, the components constituting the coating layer and their amounts, the method of smoothing, etc. The air resistance tends to increase with increasing basis weight, increasing the amount of filler added, increasing the coating weight of the coating layer, and increasing the finished density of the smoothing treatment.

[0059] The basis weight of the paper (II) is preferably 1 g / m 2 More preferably, 10 g / m or more, and even more preferably, 20 g / m 2 More preferably, it is 30 g / m² or more, and more preferably 1,000 g / m² or more. 2Less than 700 g / m 2 or less, more preferably 500 g / m 2 Less than 400 g / m 2 Less than 300 g / m 2 The basis weight of paper (II) within the above ranges tends to improve barrier properties, foaming resistance, deformation resistance, and biodegradability. When two or more papers (II) are used, the basis weight of paper (II) refers to the basis weight of one paper (II). The basis weight of paper (II) can be measured by the method described in the examples.

[0060] As the paper (II), a commercially available product satisfying the above-mentioned air resistance range may be used. One or more papers (II) may be provided, and the paper (II) may be single-layered or multi-layered. When the multilayer structure has two or more papers (II), at least one paper (II) may be disposed adjacent to the surface of the barrier layer (I) opposite the side where the thermoplastic resin layer (III) is located. For example, when the multilayer structure has two papers (II), the papers (II) may be disposed adjacent to both sides of the barrier layer (I). Furthermore, when two or more papers (II) are provided, the thicknesses and types of the layers may be different or the same. The multilayer structure of the present invention has a thermoplastic resin layer (III) on the side of the barrier layer (I) opposite to the side where the paper (II) is located. The presence of the thermoplastic resin layer (III) in the multilayer structure of the present invention can improve deformation resistance and effectively suppress curling after microwave heating. Furthermore, when the multilayer structure of the present invention is used in contact with a moisture-containing substance, the presence of the thermoplastic resin layer (III) makes it easier to maintain odor resistance and oil resistance even after microwave heating. The thermoplastic resin layer (III) may be adjacent to the barrier layer (I), or may be laminated on the barrier layer (I) via another layer described below. The thermoplastic resin layer (III) contains a thermoplastic resin. One or more types of thermoplastic resins may be contained. Examples of thermoplastic resins include polyester resins such as polyethylene terephthalate, polybutylene terephthalate, polybutylene succinate (PBS), and polybutylene succinate adipate (PBSA); polyolefin resins such as polyethylene and polypropylene; polyvinyl alcohol resins; polystyrene resins; polymethylpentene resins; polyvinyl chloride resins; polyvinylidene chloride (PVDC) resins; polyamide resins such as 6-nylon and 6,6-nylon; polyacetal resins; polycarbonate resins; polylactic acid (PLA): esterified starch; cellulose acetate resins; polyurethane resins such as biopolyurethane; poly(meth)acrylic resins such as polymethyl methacrylic; ionomer resins; and copolymers thereof. Examples of copolymers include styrene-(meth)acrylic copolymers, acrylonitrile-butadiene-styrene copolymers, and acrylonitrile-styrene copolymers. These thermoplastic resins can be used alone or in combination. The term "(meth)acrylic resin" encompasses methacrylic resins and acrylic resins. Fossil resource-derived resins and bio-derived resins may be used as thermoplastic resins.Among the thermoplastic resins exemplified above, examples of bio-based resins include polylactic acid (PLA), esterified starch, cellulose acetate, and polybutylene succinate. Examples of bio-based resins include biopolymers such as poly(ethylene glycol terephthalate), poly(ethylene glycol terephthalate), and biopolyurethane. Bio-based resins are polymeric materials that contain substances derived from renewable organic resources as raw materials and are preferably obtained by chemical or biological synthesis, with a number average molecular weight (Mn) of 1.00 or more. In particular, the use of biodegradable resins such as PLA, esterified starch, cellulose acetate, PBS, and PBSA can improve biodegradability. Note that biodegradable resins are resins that can be decomposed to the molecular level by the action of microorganisms, ultimately becoming carbon dioxide and water and circulating in nature. Among these thermoplastic resins, from the viewpoint of facilitating enhanced resistance to foaming, deformation, and barrier properties, at least one resin selected from the group consisting of polyolefin resins, polyester resins, polystyrene resins, poly(meth)acrylic resins, and copolymers thereof is preferred, and at least one resin selected from the group consisting of polyethylene, polypropylene, and styrene-(meth)acrylic copolymers is more preferred. In one embodiment of the present invention, the glass transition temperature (also referred to as Tg) of the thermoplastic resin layer (III) is preferably -150°C or higher, more preferably -130°C or higher, even more preferably -120°C or higher, still more preferably -110°C or higher, and is preferably 150°C or lower, more preferably 120°C or lower, and even more preferably 100°C or lower. When the Tg of the thermoplastic resin layer (III) is within the above range, the thermoplastic resin layer (III) is likely to have heat sealability suitable for use as a food packaging material. The Tg of the thermoplastic resin layer (III) can be measured by a differential scanning calorimeter (DSC), for example, by the method described in the Examples. The above Tg indicates the Tg of the thermoplastic resin layer (III) itself, but may also be the Tg of a resin contained in the thermoplastic resin layer (III). That is, the Tg of the resin contained in the thermoplastic resin layer (III) can be selected from the above-mentioned range of Tg of the thermoplastic resin layer (III). When the thermoplastic resin layer (III) contains two or more resins, the Tg of the resin can be the average value of all the resins.

[0064] In one embodiment of the present invention, the weight average molecular weight (also referred to as Mw) of the thermoplastic resin layer (III) is preferably 10,000 or more, more preferably 20,000 or more, and preferably 1,000,000 or less, more preferably 800,000 or less. When the Mw of the thermoplastic resin layer (III) is within the above range, it is easy to achieve both melt moldability and mechanical strength. The above Mw indicates the Mw of the thermoplastic resin layer (III) itself, but it may also be the Mw of the resin contained in the thermoplastic resin layer (III). That is, the Mw of the resin contained in the thermoplastic resin layer (III) can be selected from the above Mw range of the thermoplastic resin layer (III). Furthermore, when the thermoplastic resin layer (III) contains two or more resins, the Mw of the resins can be the average value of all the resins. Mw can be determined by gel permeation chromatography (GPC) measurement and conversion to standard polystyrene.

[0065] The thermoplastic resin layer (III) may further contain an additive. Examples of the additive include those exemplified in the section <Barrier layer (I)>. These additives can be used alone or in combination of two or more. One or two or more thermoplastic resin layers (III) may be provided, and the multilayer structure may be a single layer or multiple layers. When the multilayer structure has two or more thermoplastic resin layers (III), it is sufficient that at least one thermoplastic resin layer (III) is disposed on the side of the barrier layer (I) opposite to the side where the paper (II) is located. The thickness and type of the two or more thermoplastic resin layers (III) may be the same or different.

[0066] The thermoplastic resin layer (III) may be a layer having sealing properties (e.g., heat sealing properties), water vapor barrier properties, light-shielding properties, etc., and is preferably a layer having sealing properties. When the thermoplastic resin layer (III) has sealing properties, it improves the airtightness and protects the contents from deterioration due to oxidation by oxygen, etc., and tends to enable an extension of the storage period.

[0067] The form of the thermoplastic resin layer (III) is not particularly limited, but is preferably a film or sheet. The thickness of the thermoplastic resin layer (III) is preferably 1 μm or more, more preferably 3 μm or more, even more preferably 5 μm or more, and particularly preferably 10 μm or more, from the viewpoint of easily improving barrier properties, foaming resistance, and deformation resistance. The thickness of the thermoplastic resin layer (III) is preferably 1,000 μm or less, more preferably 500 μm or less, even more preferably 200 μm or less, still more preferably 100 μm or less, particularly preferably 70 μm or less, and particularly preferably 50 μm or less. When the thickness of the thermoplastic resin layer (III) is within the above range, the barrier property, foaming resistance, and deformation resistance are likely to be improved. When the multilayer structure has two or more thermoplastic resin layers (III), the thickness of the thermoplastic resin layer (III) indicates the thickness of one thermoplastic resin layer (III). The thickness of the thermoplastic resin layer (III) can be measured by microscopic observation of a cross-sectional image obtained by cutting the multilayer structure perpendicular to the thickness direction, and can be measured, for example, by the method described in the Examples. [0068I] A commercially available product can also be used as the thermoplastic resin layer (III). When extrusion coating, which will be described later, is used to form the thermoplastic resin layer (III), a composition capable of forming a resin layer by extrusion coating (for example, a resin emulsion) may be used. When a lamination method, which will be described later, is used, a film or sheet may be used as the thermoplastic resin layer (III).

[0069] <Multilayer Structure> The multilayer structure of the present invention comprises a thermoplastic resin layer (IH), a barrier layer (I), and paper (II) in this order, wherein the barrier layer (I) is adjacent to the paper (II) and comprises a modified starch (A) having an average amylose content of 40% by mass or more and a water-soluble polymer (B). The air resistance of the paper (II) is 30 to 80,000 seconds, so that foaming and deformation due to microwave heating can be suppressed and the multilayer structure has excellent barrier properties before and after microwave heating. Preferably, the multilayer structure has excellent odor resistance and oil resistance in addition to gas barrier properties before and after microwave heating. Therefore, the multilayer structure of the present invention can be suitably used as a packaging material or lid material for foods and the like, and is particularly suitable as a multilayer structure for microwave heating, i.e., a multilayer structure to be used for microwave heating.

[0070] If foaming caused by microwave heating can be suppressed, the barrier layer after microwave heating It is possible to prevent or suppress the generation of voids at the interface between the barrier layer (I) and the paper (II), the thermoplastic resin layer (III), or other layers, and the generation of bubbles in the barrier layer (I). In addition, if deformation due to microwave heating can be suppressed, for example, the generation of curling can be effectively suppressed, and the barrier layer (I) can be easily heated by microwave heating. The shape of the packaging bag (or container) to which the multilayer structure is applied can be maintained even when heated. In one embodiment of the present invention, the amount of curl after microwave heating is preferably 0.5 cm or less, more preferably 0.4 cm or less, even more preferably 0.3 cm or less, and even more preferably 0.2 cm or less. When the amount of curl after microwave heating is not more than the above upper limit, deformation due to microwave heating can be effectively suppressed, and the shape of the packaging bag (or container) containing the multilayer structure can be easily maintained. The lower limit of the amount of curl after microwave heating is 0 cm or more. The amount of curl after microwave heating can be measured, for example, by the method described in the Examples.

[0072] The multilayer structure of the present invention has excellent odor prevention and oil resistance both before and after microwave heating. Therefore, even when a packaging bag (or container) containing the multilayer structure containing smelly food, oil, or the like is irradiated with microwaves to heat the food, oil, or the like, the generation and permeation of odors can be effectively prevented or suppressed.

[0073] The multilayer structure of the present invention may contain layers other than the barrier layer (I), the paper (II), and the thermoplastic resin layer (III).

[0074] Examples of other layers include an adhesive layer, a primer layer, a metal layer, an inorganic vapor deposition layer, a heat seal layer, a moisture-proof layer, and a light-shielding layer. Examples of adhesives that constitute the adhesive layer include acrylic adhesives, urethane adhesives, epoxy adhesives, vinyl acetate adhesives, ethylene-vinyl acetate adhesives, vinyl chloride adhesives, silicone adhesives, nitrile cellulose adhesives, phenolic adhesives, polyvinyl alcohol adhesives, melamine adhesives, and styrene adhesives. Urethane adhesives are preferred from the viewpoint of adhesiveness. The thickness of the adhesive layer is preferably 0.1 to 30 μm, more preferably 1 to 20 μm. The thickness of the adhesive layer can be measured using an optical microscope, a film thickness meter, or the like.

[0075] The metal layer may be a layer made of a metal foil made of various metals such as aluminum, and the inorganic vapor deposition layer may be a layer made of a vapor-deposited inorganic oxide such as silicon oxide or aluminum oxide. The heat seal layer is a layer that can be heat-sealed, and the moisture-proof layer is a layer that has a moisture-proof effect. do.

[0076] The other layers include a thermoplastic resin layer (III), a barrier layer (I), and a paper (II) in this order, and the lamination position is not particularly limited as long as the barrier layer (I) is adjacent to the paper (II). However, from the viewpoint of easily releasing water vapor generated in the barrier layer (I) to the outside, it is preferable that the other layers be provided on the side of the barrier layer (I) opposite to the side where the paper (II) is located, for example, between the barrier layer (I) and the thermoplastic resin layer (III), or on the side of the thermoplastic resin layer (III) opposite to the side where the barrier layer (I) is located. From the viewpoint of improving foaming resistance, it is preferable that the paper (II) be the outermost layer of the multilayer structure. The multilayer structure is The film may have one or more other layers, and if it has two or more layers, the thickness and type of the other layers may be the same or different.

[0077] Specific examples of the layer structure of the multilayer structure of the present invention are shown below: Barrier layer (I), paper (II), and thermoplastic resin layer (III). The following structure can be given as I): Thermoplastic resin layer (III) / Barrier layer (1) / Paper (II); Thermoplastic resin layer (III ) / adhesive layer / barrier layer (I) / paper (II) ; thermoplastic resin layer (III) / adhesive layer / paper (II) / barrier layer (I) / paper (II) ; thermoplastic resin layer (III) / paper (II ) / Barrier layer (I) / Paper (II) ; Thermoplastic resin layer (III) / Paper (II) / Adhesive layer / Barrier layer (I) / Paper (II) ; Thermoplastic resin layer (III) / Adhesive layer / Paper (II ) / Adhesive layer / Barrier layer (I) / Paper (II) 〇

[0078] In the multilayer structure having the above layer configuration, the paper (II) and the barrier layer (I ) are adjacent to each other, but the other layer may be included at a position other than between these layers.

[0079] [Method for Producing Multilayer Structure] The method for producing the multilayer structure of the present invention is not particularly limited, and examples thereof include a method comprising the steps of: a step of directly laminating a barrier layer (I) on paper (II) to obtain a laminate (a) (referred to as step (X)); and a step of laminating a thermoplastic resin layer (III) on the side of the barrier layer (I) in the obtained laminate (a) opposite to the side where the paper (II) is located, to obtain a multilayer structure (referred to as step (X)).

[0080] <Step (X)> Step (X) is a step of directly laminating a barrier layer (I) on paper (II) to obtain a laminate (a), and preferably includes a step (referred to as step (i)) of coating paper (II) with an aqueous composition containing the modified starch (A) and the water-soluble polymer (B), and more preferably includes a step of using an extruder to coat paper (II) transported by a take-up machine with the aqueous composition. The inclusion of such a step tends to improve the barrier properties, foaming resistance, and deformation resistance of the resulting multilayer structure.

[0081] (Production of Water-Containing Composition) The water-containing composition comprises a resin composition containing the modified starch (A) and the water-soluble polymer (B), and has a water content of 1 to 50% by mass. The water content is preferably 5% by mass or more, more preferably 8% by mass or more, and preferably 45% by mass or less, more preferably 40% by mass or less. When the water content is within the above range, the application property and film-forming property when coating the water-containing composition on paper (II) are easily improved, and the adhesion between paper (II) and the barrier layer (I) in the resulting multilayer structure is easily increased. This makes it easy to suppress the generation of voids and deformation at the interface due to microwave heating, and also makes it easy to improve barrier properties. The water content of the water-containing composition can be measured, for example, using a heat-drying moisture meter at a temperature of 130°C. 0The water content is the water content measured at 100°C for 60 minutes, and can be measured by the method described in the Examples. In this specification, the water-containing composition means all water-containing resin compositions having a water content of 1 to 50% by mass measured by the above method. In other words, the water-containing composition is preferably a resin composition obtained by adding water to the resin composition to adjust the water content to the above range, but also includes resin compositions whose water content is within the above range at the time of production.

[0082] The resin composition can be produced, for example, by a method including at least the steps of: (1) mixing the modified starch (A) and the water-soluble polymer (B) to obtain a mixture; (2) extruding the mixture; and (3) cooling and drying the extruded mixture. The components contained in the resin composition are the same as those contained in the barrier layer (I), but their moisture contents may be the same or different. The water content may be selected from the same range as that of the barrier layer (I). [0083I] Step (1) is a step of mixing at least the modified starch (A) and the water-soluble polymer (B), and optionally other components, such as the fatty acid having 12 to 22 carbon atoms and / or its fatty acid salt, the clay, the plasticizer, and the additives, can be mixed together.

[0084] Step (1) is usually carried out using an extruder, in which the components are subjected to shear stress by a screw and mixed homogeneously while being heated by applying external heat to the barrel.

[0085] The extruder may be, for example, a twin-screw extruder. Twin-screw extruders may be either co-rotating or counter-rotating. The screw diameter may be, for example, 20 to 150 mm, and the ratio of the extruder length (L) to the screw diameter (D), L / D, may be, for example, 20-50. The screw rotation speed is preferably 80 rpm or higher, more preferably 100 rpm or higher. The extrusion pressure is preferably 5 bar (0.5 MPa) or higher, more preferably 10 bar (1 MPa) or higher. Each component can be directly introduced into the extruder. Alternatively, the components may be premixed using a mixer and then introduced into the extruder.

[0086] In step (1), from the viewpoint of easily improving film-forming properties and gas barrier properties, it is preferable to mix a plasticizer, preferably water, in an amount of preferably 0.1% by mass or more, more preferably 1% by mass or more, even more preferably 10% by mass or more, even more preferably 15% by mass or more, and particularly preferably 20% by mass or more, with an upper limit of preferably 50% by mass or less, more preferably 45% by mass or less, and even more preferably 40% by mass or less, relative to the mass of the mixture. Here, the mass of the mixture refers to the total mass of the mixture including the plasticizer. In step (1), the plasticizer may be introduced at an early stage of extrusion, before the heating temperature is reached, for example, when the temperature is 10°C or below. The modified starch (A) can be subjected to a cooking treatment using a combination of moisture, heat, and shear stress to form gelatin (gel). Alternatively, a plasticizer, preferably water, may be added separately. By introducing the water-soluble polymer (B), the water-soluble polymer (B) can be dissolved, the resin composition can be softened, and the modulus and brittleness can be reduced. In step (1), the cooking treatment is preferably carried out by heating to a temperature of more than 100°C and not more than 150°C, more preferably from 115°C to 140°C. Here, the cooking treatment is a treatment for crushing and gelling starch granules. Heating can be carried out by applying heat from the outside to the barrels of the extruder. By applying a temperature that is changed stepwise to each barrel, heating to the desired temperature is possible. Cooking at a temperature above 120°C is advantageous in terms of processability. The cooked mixture is preferably heated for 85-180°C to prevent foaming. 20°C, preferably 90 to 11°C 0 It is preferable to push the mixture toward the die while lowering the temperature to 100°C. In addition, foaming can be prevented and moisture can be removed by venting the barrel.

[0089] The residence time in the extruder can be set depending on the temperature profile and screw speed, and is preferably 1 to 2.5 minutes.

[0090] In the step (2) of extruding the mixture, the molten mixture that has been forced through the extruder while being melt-kneaded is extruded through a die. The die temperature is preferably 85 to 120° C., more preferably 90 to 110° C.

[0091] In the step (3) of cooling and drying the extruded mixture (melt), The melt can be extruded into a film or sheet, or into strands.

[0092] When the mixture is extruded into a film, the mixture can be extruded through a film-forming die and then cooled and dried while being taken up by a take-up roller. It is preferable to cool the area between the die and the roller to prevent the mixture from adhering to the roller. A forming roll may be installed between the die and the roller. The forming roll may be made of, for example, rubber, resin, or metal. For drying, the roll may be heated, and dehumidified air may be supplied during the take-up. In the case of the blown tube method, dehumidified air is supplied when the film exits the die. It can be used to expand the film. Talc can also be entrained in the air stream to prevent film blocking. [0093I] When the mixture is extruded into strands, the strands can be formed into pellets by extruding them through a strand nozzle with multiple holes and cutting them with a rotary cutter. To prevent the pellets from sticking together, vibration can be applied periodically or constantly, and moisture in the pellets can be removed by hot air, dehumidified air, or an infrared heater.

[0094] In a preferred embodiment of the present invention, after the resin composition is formed, water is added to form a water-containing composition, so the resin composition is preferably in the form of pellets.

[0095] In a preferred embodiment of the present invention, water is added to the obtained resin composition (preferably a pellet-shaped resin composition) and, for example, stirred and mixed to obtain a water-containing composition. In order to prevent the resin composition from sticking together and to allow the water to be absorbed by the entire pellets, it is preferable to add water in two or more portions while stirring. In addition, in order to maintain a constant water content, the water-containing composition may be stored in a sealed container.

[0096] In step (X) (production of laminate (a)), the water-containing composition is preferably introduced into an extruder. Examples of extruders include single-screw extruders and twin-screw extruders. The extruder has a screw diameter of, for example, 20 to 150 mm, a ratio (L / D) of the extruder length (L) to the screw diameter (D) of, for example, 15 to 50, and a screw rotation speed of, preferably, 80 rpm or higher, more preferably 100 rpm or higher. The cylinder temperature in the extruder may be, for example, 80 to 120°C, and preferably 90 to 110°C.

[0097] The aqueous composition introduced into the extruder is plasticized and discharged from the die outlet. Meanwhile, the paper (II) is conveyed by a take-up machine, preferably a roller-type take-up machine. The aqueous composition discharged from the die outlet is coated onto the conveyed paper (II), thereby obtaining a laminate (a). The obtained laminate (a) is conveyed between a plurality of rolls, including a metal roll, while being pressed against the paper (II), and is wound into a roll by a winder. The plurality of rolls may include, for example, a pressure roll, a cast roll, a touch roll, etc. In this way, a laminate (a) having a barrier layer (I) and a paper (II) adjacent to the barrier layer (I) can be obtained. In the present invention, since the water in the water-containing composition evaporates during the above-mentioned production process, the moisture content of the barrier layer (I) in the obtained laminate (a) is lower than that of the water-containing composition. The moisture content may also be adjusted by drying the obtained laminate (a). 〇 <Step (Y)> Step (Y) is a step of laminating a thermoplastic resin layer (III) on the side of the barrier layer (I) in the obtained laminate (a) opposite to the side where the paper (II) is located, to obtain a multilayer structure.

[0099] Examples of the lamination method for the thermoplastic resin layer (III) in step (ii) include extrusion coating, extrusion lamination, and film application. The extrusion coating method may be a method in which a thermoplastic resin and, optionally, additives are extrusion coated or extrusion laminated onto the barrier layer (I) in the laminate (a) (e.g., paper (II) / barrier layer (I)), or, when the laminate (a) contains other layers (e.g., paper (II) / barrier layer (I) / other layers), onto other layers such as an adhesive layer or a primer layer. The film application method may be a method in which a film-like thermoplastic resin layer (III) is dry-laminated or sand-laminated onto the barrier layer (I) in the laminate (a).

[0100] When the multilayer structure of the present invention includes other layers, the other layers may be laminated by the following method: For example, the method for laminating the thermoplastic resin layer (III) includes the methods exemplified above.

[0101] The multilayer structure of the present invention can be used, for example, as a barrier packaging material used for packaging food products, containers, cups, etc., or as an industrial material. Among these, it can be used as a barrier packaging material used for packaging food products, containers, cups, etc., and can be used as a soft packaging material for food products, etc. The multilayer structure of the present invention can be used for packaging food products, containers, cups, etc., and can be used as a soft packaging material for food products, etc. Since the material can suppress foaming and deformation and has excellent barrier properties even after microwave heating, it can be particularly suitably used as a microwave heating packaging material that can be used in a microwave oven. The term "flexible packaging material" refers to a packaging material made of a highly flexible material, and generally refers to a packaging material made of thin, flexible materials such as paper, film, and aluminum foil, either alone or bonded together. The term "flexible packaging material" refers to a packaging material that maintains a three-dimensional shape by placing contents inside, such as a bag. Even when used as a multilayer structure for industrial materials, it is expected to prevent spoilage and deterioration by suppressing oxygen penetration, as well as to have effects such as flavor barrier properties that prevent solvent odors from leaking out. When the multilayer structure of the present invention is used as a packaging material, a lid material, or the like, it is preferable that the paper (II) side is on the outside. By placing the paper (II) side on the outside, water vapor generated in the barrier layer (I) is easily released to the outside through the paper (II) at an optimum speed. Therefore, foaming and deformation due to microwave heating can be suppressed, and excellent barrier properties can be maintained even after the heating.

[0103] [Packaging or Lidding Material, Microwave Heating Packaging Bag] The present invention encompasses a packaging or lid material comprising the multilayer structure of the present invention. Examples of packaging materials include, but are not limited to, the barrier packaging materials described above. Examples of lid materials include, but are not limited to, container lids. When used as a container lid, it can seal the inside of the container by combining it with the container body.

[0104] The present invention includes a packaging bag for microwave heating that includes the multilayer structure of the present invention. A packaging bag for microwave heating refers to a packaging bag that can heat contents such as food by irradiating it with microwaves, and is particularly suitable for use in a microwave oven. Because the packaging bag for microwave heating of the present invention includes the multilayer structure of the present invention, it can suppress foaming and deformation due to microwave heating and maintains excellent barrier properties even after heating. Moreover, it is also highly biodegradable, which can reduce the environmental burden. Examples [0105I] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0106] [Test method]

[0107] (1) Measurement method of air resistance The air resistance of the papers used in the examples and comparative examples was measured in accordance with ISO 563 6-6 (Oken air permeability) using a submersible Oken air permeability and smoothness tester KY-6J manufactured by Asahi Seiko Co., Ltd.

[0108] (2) Evaluation Method of Deodorizing Properties (Before Microwave Heating) The multilayer structures obtained in the Examples and Comparative Examples were cut into 10 x 10 cm pieces, and the thermoplastic resin layer (III) of each of the two cut multilayer structures was placed inside, and three sides of the cut pieces were heat-sealed to a width of 5 mm. 20 g of fried chicken was placed inside, and the remaining side was heat-sealed to prepare a four-sided sealed bag. The bag was placed in a 500 ml airtight glass bottle and stored at 3°C ​​for one week. After storage, the bottle was opened, and the headspace was subjected to a sensory test to confirm odor leakage. The evaluation criteria were as follows: A: There was no smell of fried chicken. B: There was a slight smell of fried chicken. C: There was a strong smell of fried chicken.

[0109] (3) Microwave Heating Method The multilayer structures obtained in the Examples and Comparative Examples were cut into 5 x 5 cm pieces, and the cut multilayer structures were each stored at 23°C and 50% RH for 2 weeks to condition the humidity, and then heated at 500 W for 30 seconds using a microwave oven RET 2-W6J manufactured by SHARP.

[0110] (4) Evaluation of foaming After microwave heating by the above method, the multilayer structure was cut using a microtome and the cross section was observed under an optical microscope. The voids at the interface where the barrier layer (I) contacts other layers and the area occupied by bubbles with a long side of 100 mm or more in a 1 mm square area in the barrier layer (I) were measured and evaluated according to the following criteria. A: No voids at the interface and less than 1% air bubbles B: There are voids at the interface, or there are bubbles between 1% and 20% C: More than 20% bubbles.

[0111] (5) Evaluation method for curl of multilayer structure After microwave heating by the above method, the multilayer structure was left to stand at 23°C and 50% RH for 5 minutes and then cut into a 5 x 5 cm piece. With the thermoplastic resin layer (III) facing up, a paper weight was placed on one side with a width of 1 cm, and the height of curl of the film was measured. Three samples were measured in both the MD and TD directions, and the average value of a total of six points was defined as the curl amount.

[0112] (6) Method for evaluating oil resistance The multilayer structures obtained in the examples and comparative examples were cut into 10 x 10 cm pieces, and the thermoplastic resin layer (III) of each of the two cut multilayer structures was placed on the inner surface, and three sides of the cut pieces were heat-sealed to a width of 5 mm. 20 g of fried chicken was placed inside, and the remaining side was heat-sealed to prepare a four-sided sealed bag. The prepared bag was heated at 500 W for 1 minute in a SHARP microwave oven RET 2-W6J, and then the bleeding of vegetable oil onto the paper surface was evaluated as follows. A: No bleeding was observed B: Bleeding occurred in an area of ​​less than 5% of the entire paper C: Bleeding occurred over an area of ​​5% or more of the entire paper.

[0113] (7) Evaluation method for deodorizing properties (after microwave heating) The multilayer structure was cut into a 10 x 10 cm piece, and two of the cut multilayer structures were placed with the thermoplastic resin layer facing inward, and three sides of the edge were heat-sealed to a width of 5 mm. 20 g of fried chicken was placed inside, and the remaining side was heat-sealed to create a four-sided sealed bag. The bag was heated for 1 minute at 500 W using a SHARP microwave oven RET 2-W6J, and then placed in a 500 mL airtight glass bottle and stored at 23°C for 1 hour. After storage, the bag was opened, and the headspace was subjected to a sensory test to check for odor leakage. The evaluation criteria were as follows: A: There was no smell of fried chicken. B: There was a slight smell of fried chicken. C: There was a strong smell of fried chicken.

[0114] (8) Measurement of the degree of salinity of polyvinyl alcohol (B) In accordance with JIS K 672 6 (Testing method for polyvinyl alcohol), dissolution titration of the polyvinyl alcohol used in the Examples and Comparative Examples was carried out, and the degree of salinity was calculated.

[0115] (9) Viscosity measurement of polyvinyl alcohol (B) In accordance with JIS Z 880 3 (falling ball viscometer) and J SK 672 6 (polyvinyl alcohol test method), a 4% aqueous solution of polyvinyl alcohol used in the examples and comparative examples was prepared, and the viscosity at 20°C was measured using a Hoebler viscometer. The viscosity of the 4% aqueous solution of polyvinyl alcohol (B) (20°C) was 0 C) (1 ○) Measurement of Glass Transition Temperature (Tg) of Thermoplastic Resin Layer (III) The Tg of the thermoplastic resin layer (III) in the Examples and Comparative Examples was measured using a differential scanning calorimeter (TA Instruments, Model TA Q 2000) at a heating rate of 10°C / min.

[0116] (11) Measurement of thickness The thicknesses of the barrier layer (I) and the thermoplastic resin layer (III) in the examples and comparative examples were measured with a digital thickness gauge (Mitutoyo Corporation's Digimatic Micrometer MDC-25MX) for single-layer bodies. The thicknesses of the respective layers in the laminate and the multilayer structure were measured using an optical microscope (Nikon Corporation's ECLIPSE Ci-E) for a cross-section of a sample piece whose cross-section had been smoothed by microtomy.

[0117] (12) Measurement of basis weight of paper The basis weights of the paper (II) in the examples and comparative examples were measured by a method conforming to ISO 536:2019 (Paper and board - Method for determining basis weight).

[0118] [Materials Used] (1) High-amylose modified starch (A-1) • ECO FILM (trademark): Waxy corn starch modified with propylene oxide, amylose content 70% by mass, manufactured by Ingredion (2) Low-amylose modified starch (A-2) • National 1658 (trademark): Waxy corn starch modified with propylene oxide, amylose content 20% by mass, manufactured by Ingredion

[0119] (3) Polyvinyl alcohol (B) • Kuraray Poval (trademark) 4-98: Polyvinyl alcohol resin, degree of saponification 98 mol%, viscosity 4 mPa·s (20°C, 4% aqueous solution), manufactured by Kuraray 8 m. 丨%, viscosity 4 mPa - s (20°C, 4% aqueous solution), manufactured by Kuraray

[0120] (4) Paper (II) • Kraft paper, basis weight 100 g / m²: Taiyo Atlas manufactured by Daio Paper Corporation • One-sided glazed kraft paper, basis weight 40 g / m²: Star White manufactured by Maruishi Paper Co., Ltd. • White cardboard, basis weight 233 g / cm 2 : Hokuriku Corporation's Hokuriku Art Post Coated paper, basis weight 55 g / cm 2 : Ryuwo Coat manufactured by Daio Paper Corporation Glassine paper, basis weight 3.1 g / cm 2 : Nippon Paper Industries Co., Ltd. Pressure Loguracin m •Tissue paper, basis weight 2 1 g / m 2 : Shirakawa Paper Co., Ltd. Food-grade tissue paper Waterproof paper, basis weight 100 g / m²: San-ei Kaken TG B 70 Brown, thickness 70 H, m

[0121] (5) Thermoplastic resin layer (III) ■ PE (Polyethylene): Mitsui Chemicals Tocello MC-S; 25 mm thick; Tg - 1 1 〇 0 C ■ CPP (non-oriented polypropylene): Toyobo Pylenfil ACT P 1 1 28; 20um thickness; Tg-1 8 °C Styrene-acrylic emulsion: Toyo Chemical Philharmonic T4 〇 4 ; T g 22 °C Vapor-deposited PE (vapor-deposited polyethylene): Mitsui Chemicals Tocello TUX-F; 25 Um thickness; TgT 1 〇 °C Example 1 (Resin Composition) Raw materials ECO FILM (trademark) (8,000 kg) and Kuraray Poval (trademark) 4-98 (2,000 kg) were mixed in a tumbler mixer for 2 hours, and the resulting mixture was fed to a twin-screw extruder connected to a liquid pump. Figure 2 shows a schematic diagram of the twin-screw extruder used in Example 1, and the screw diameter, L / D ratio, screw rotation speed, operating mode, and temperature profile (Table 1) of the extruder are shown below.

[0123] [Table 1]

[0124] Screw diameter: 27 mm L / Dh: 48 Screw rotation speed: 500 rpm Operation method: Co-rotating (interlocking self-wiping) method

[0125] Specifically, the resulting mixture was fed into the barrel through a hopper at C1 via a weight feeder of the twin-screw extruder at a rate of 3.5 kg / hr. Water was injected into the barrel at a flow rate of 26 g / min through a liquid pump (L) at C4. The temperature range from C5 to C9 was the cooking range, and complete pregelatinization of starch was achieved within this range. The mixture was extruded through a multi-hole strand nozzle installed after C11 and cut with a rotary cutter to form pellets. Because the pellets contained excess moisture, they were constantly vibrated to prevent sticking, and the moisture was removed with hot air.

[0126] (Laminate) Water was added to the obtained resin composition in pellet form until the content of water reached 35% by mass relative to the mass of the resin composition. When adding water, the pellets were prevented from sticking together, and To ensure that the water was uniformly absorbed throughout the pellets, water was added in multiple batches while stirring for 15 minutes in a tumbler mixer. After stirring, the pellets were placed in a polyethylene bag, sealed, and left to stand at room temperature for 6 hours to prevent water evaporation. In this way, a hydrous composition (hydrous pellets) with a water content of 35% by mass was obtained. The moisture content was confirmed by measuring for 60 minutes at 130°C using a Mettler-Toledo heat-drying moisture meter HR 73J. The obtained moisture-containing pellets were then fed into a single-screw extruder 2 shown in Figure 1 and extruded through a film-forming die 3. Next, the moisture-containing composition 4 extruded from the outlet of the die 3 was coated onto paper 5, which was being transported by a roller-type take-up machine (not shown). The coated laminate 6 (also referred to as a coating) was immediately pressed onto the paper 5 through a pressure roll (made of rubber) 7a, a cast roll (made of metal) 7b, and a touch roll (made of rubber) 7c, and then taken up into a roll by a take-up machine (not shown). In this way, a laminate in which the barrier layer (1) / paper (II) were laminated in this order was obtained. Details of the single-screw extruder used and its operating conditions, as well as the temperature profile (Table 2) are shown below. is shown below. Single-screw extruder: Extruder manufactured by the Institute of Plastics Engineering (40mm diameter, L / D = 2.5) [Table 2] •Discharge rate: 20 kg / hr Die: 450mm wide coat hanger die, lip opening: 0.2mm •Die-to-cast roll distance (air gap): 150mm Paper: Kraft paper, 100 g / m², Taio Atlas manufactured by Daio Paper Co., Ltd. The obtained laminate had no uneven thickness or foreign matter, and a coated paper with good appearance was obtained.

[0128] (Multilayer structure) An adhesive (a 10:1 (weight ratio) mixture of Takelac A520 and Takenate A50 manufactured by Mitsui Chemicals) was applied to the surface of the barrier layer (I) of the obtained laminate, and then the laminate was heated for 600 After drying with hot air at 23°C for 1 minute to remove the solvent, the laminate was laminated with a 25" meter PE film. After storing at 23°C and 50% RH for 3 days, a multilayer structure was obtained in which the thermoplastic resin layer (III) / adhesive layer / barrier layer (1) / paper (H) were laminated in this order. The adhesive layer was a polyurethane adhesive.

[0129] <Examples 2 to 10, 12 and Comparative Examples 1 to 4> Multilayer structures were obtained in the same manner as in Example 1, except that the average amylose content (%) of the modified starch (A) in the barrier layer (I), the content of the water-soluble polymer (B), and the thickness (mm) of the barrier layer (I); the type, basis weight, and air resistance (sec) of the paper (II); the components constituting the thermoplastic resin layer (III) and its thickness (mm); and the layer structure of the multilayer structure were changed as shown in Table 1. In Example 2, 4,000 kg of EC 0 FILM (trademark) and 4,000 kg of National 1658 (trademark) were used, and in Comparative Example 1, 8,000 kg of National 1658 (trademark) was used.

[0130] <Example 11> A styrene-acrylic emulsion solution was applied to the surface of the barrier layer (I) of a laminate obtained in the same manner as in Example 1, and then dried at 80°C for 1 minute. The application and drying were repeated twice in total so that the film thickness after drying became 5 μm, thereby obtaining a multilayer structure (thermoplastic resin layer (111) / barrier layer (1) / paper (II)).

[0131] <Example 13> When preparing the laminate of Example 1, the same type of kraft paper was also transported from the cast roll 7b side together with the paper 5 in Figure 2, to obtain a laminate in which paper (II) / barrier layer (1) / paper (II) were laminated in this order. A 25 µm PE film was attached to one side of this laminate in the same manner as in Example 1, to obtain a multilayer structure (thermoplastic resin layer (111) / adhesive layer / paper (II) / barrier layer (1) / paper (II)).

[0132] <Comparative Example 5> In the production of the laminate in Example 1, the paper 5 in Fig. 2 was not conveyed, and only the water-containing composition 4 was taken up as a single layer to obtain a film of 20 fjbm. An adhesive (a 10:1 (weight ratio) mixture of Takelac A520 and Takenate A50 manufactured by Mitsui Chemicals, Inc.) was applied to one side of the obtained film of 20 fjbm, and then the film was heated to 60°C. 0 After removing the solvent by hot air drying at RT for 1 minute, the laminate was laminated with kraft paper to obtain a laminate consisting of barrier layer (1) / adhesive layer / paper (II). A 25 μm PE film was laminated to the surface of the barrier layer (I) in the same manner as in Example 1 to obtain a multilayer structure (thermoplastic resin layer (111) / adhesive layer / barrier layer (1) / adhesive layer / paper (ID). <Comparative Example 6> The laminate (barrier layer (1) / paper (II)) obtained in Example 1 was used as a multilayer structure as it was.

[0134] The deodorizing properties, foaming, curl amount, and oil resistance of the multilayer structures obtained in the examples and comparative examples before microwave heating, and after microwave heating were evaluated according to the above-mentioned methods, and the results are shown in Table 3. Table 3 also shows the average amylose content (%) of the modified starch (A) in the barrier layer (I), the content of the water-soluble polymer (B), and the thickness ("m) of the barrier layer (I); the type, basis weight, and air resistance (seconds) of the paper (II); the components constituting the thermoplastic resin layer (III) and its thickness ("m"); and the layer structure of the multilayer structure. In the layer structure, "P" represents paper (II), "B" represents the barrier layer (I), "A" represents the adhesive layer, and "H" represents the thermoplastic resin layer (III).

[0135] [Lake] 3 [0136I] As shown in Table 3, the multilayer structures obtained in Examples 1 to 13 were evaluated as A in terms of odor resistance, oil resistance, and foaming after microwave heating, and it was confirmed that they had a smaller amount of curling than Comparative Examples 1 to 6. In contrast, it was confirmed that Comparative Examples 1 to 6 were inferior in at least one of the evaluations of odor resistance, oil resistance, foaming, and amount of curling after microwave heating. Therefore, it was found that the multilayer structures obtained in Examples 1 to 13 could suppress foaming and deformation due to microwave heating, and had excellent barrier properties even after microwave heating. Explanation of Symbols

[0137] 1. Hydrated composition (pellet form) 2. Single-screw extruder 3...Die 4 ■■・Water-containing composition 5 ■■・Paper 6 ■■・Laminate 7 a...Pressure roll 7 b •• Cast Roll 7 c...Touch Roll 8. Twin-screw extruder 9. Hopper ! 〇 ••・Liquid addition nozzle 1 1 •••Resin thermometer 1 2 •••Resin pressure gauge 1 3 Adapter 1 4 ... Die

Claims

1. A multilayer structure comprising a thermoplastic resin layer (III), a barrier layer (I), and a paper (II) in this order, The barrier layer (I) is adjacent to the paper (II), the barrier layer (I) comprises a modified starch (A) having an average amylose content of 40% by mass or more and a water-soluble polymer (B); The paper (II) is a multilayer structure having an air resistance of 30 to 80,000 seconds as measured in accordance with ISO 5636-6.

2. The multilayer structure according to claim 1 , wherein the water-soluble polymer (B) is polyvinyl alcohol and / or polyoxyalkylene.

3. 2. The multilayer structure according to claim 1, wherein the content of the modified starch (A) is 30 to 99.5 parts by mass and the content of the water-soluble polymer (B) is 0.5 to 70 parts by mass, based on 100 parts by mass in total of the modified starch (A) and the water-soluble polymer (B).

4. 2. The multilayer structure according to claim 1, wherein the barrier layer (I) has a thickness of 1 to 600 μm.

5. 2. The multilayer structure according to claim 1, wherein the thermoplastic resin layer (III) has a thickness of 1 to 1000 μm.

6. A packaging material or lid material comprising the multilayer structure according to claim 1.

7. A packaging bag for microwave heating, comprising the multilayer structure according to claim 1.