Modified ethylene-vinyl alcohol copolymer, gas barrier material, film or sheet, multilayer structure, packaging container, and method for producing modified ethylene-vinyl alcohol copolymer

Abstract

Provided is an ethylene-vinyl alcohol copolymer descried below, which achieves both moldability and gas barrier properties under high humidity conditions.　An ethylene-vinyl alcohol copolymer which has an ethylene structural unit content of 20-60 mol% and includes the structural unit (1). (In the structural unit (1), R1 represents an alkyl group having 1 to 3 carbon atoms, and X represents an alkylene group having 1 to 5 carbon atoms.)

Description

Modified ethylene-vinyl alcohol copolymer, gas barrier material, film or sheet, multilayer structure, packaging container, and method for producing modified ethylene-vinyl alcohol copolymer

[0001] The present invention relates to a modified ethylene-vinyl alcohol copolymer, a gas barrier material, a film or sheet, a multilayer structure, a packaging container, and a method for producing the modified ethylene-vinyl alcohol copolymer.

[0002] Ethylene-vinyl alcohol copolymer (hereinafter sometimes referred to as "EVOH") is a melt-moldable gas barrier resin and is widely used, particularly as a food packaging material. EVOH is a crystalline resin, and therefore must be handled at temperatures above its melting point during molding. However, since the molding processing temperature and thermal decomposition temperature of vinyl alcohol resin are close to each other, problems include resin burning and gel formation during long-term molding processing. It is also known that during film molding, crystal growth occurs in the resin as it is stretched, resulting in reduced moldability and transparency. Therefore, a gas barrier resin with a low melting point and excellent moldability has been desired.

[0003] Therefore, as a method for lowering the melting point of EVOH, a method of copolymerizing a third monomer component in addition to vinyl acetate and ethylene has been proposed. For example, Patent Document 1 shows that when EVOH having an allyl-modified group is used in the barrier layer of a multilayer film, the gas barrier property under high humidity conditions is improved while the moldability is improved.

[0004] Japanese Unexamined Patent Publication No. 62-46643

[0005] In the technology disclosed in Patent Document 1, the gas barrier property under high humidity conditions is significantly reduced as the modification rate of the allyl modifying group increases, and there is room for improvement in achieving both moldability and gas barrier property under high humidity conditions.

[0006] In view of this background, the present invention provides a modified EVOH that has both moldability and gas barrier properties under high humidity conditions.

[0007] However, in view of the above circumstances, the present inventors have conducted extensive research and found that, by using an EVOH containing a specific amount of ethylene structural units and having the following structural unit (1) as a monomer of a third component, the EVOH has a low melting point and excellent moldability, and also has excellent gas barrier properties under high humidity conditions, even when the monomer modification rate of the third component is increased.

[0008] That is, the present invention has the following aspects: [1] A modified ethylene-vinyl alcohol copolymer having an ethylene structural unit content of 20 to 60 mol % and having the following structural unit (1): [In the structural unit (1), the R 1 is an alkyl group having 1 to 3 carbon atoms, and X is an alkylene group having 1 to 5 carbon atoms.] [2] The modified ethylene-vinyl alcohol copolymer according to [1], wherein the modification rate of the structural unit (1) is 0.5 to 10 mol %. [3] The modified ethylene-vinyl alcohol copolymer according to [1] or [2], wherein the modified ethylene-vinyl alcohol copolymer is water-insoluble. [4] The modified ethylene-vinyl alcohol copolymer according to any one of [1] to [3], wherein the modified ethylene-vinyl alcohol copolymer, when formed into a monolayer film having a thickness of 200 to 300 μm, has a saturated water absorption of less than 15 mass % at 20°C and 90% RH. [5] A gas barrier material having a layer containing the modified ethylene-vinyl alcohol copolymer according to any one of [1] to [4]. [6] A film or sheet having the gas barrier material according to [5]. [7] A multilayer structure having the gas barrier material according to [5]. [8] A packaging container having the gas barrier material according to [5]. [6] A method for producing the modified ethylene-vinyl alcohol copolymer according to any one of [1] to [4], comprising the steps of copolymerizing ethylene, a vinyl ester monomer, and a compound having the following structural unit (2) to obtain a modified ethylene-vinyl ester copolymer, and saponifying the obtained modified ethylene-vinyl ester copolymer: [In the structural unit (2), the R 2 is an alkyl group having 1 to 3 carbon atoms, and the R 3is an alkyl group having 1 to 10 carbon atoms, and Y is an alkylene group having 1 to 5 carbon atoms.

[0009] The modified EVOH of the present invention is excellent in moldability and gas barrier property under high humidity conditions.

[0010] The present invention will be described below based on examples of embodiments for carrying out the present invention, but the present invention is not limited to the embodiments described below.

[0011] In this specification, "x and / or y (x and y are optional)" refers to at least one of x and y, and can mean either x alone, y alone, or both x and y. When "X to Y" (X and Y are optional numbers) is used, unless otherwise specified, it also encompasses "X or more and Y or less," as well as "preferably more than X" or "preferably less than Y." When "X or more" (X is optional number) or "Y or less" (Y is optional number) is used, it also encompasses "preferably more than X" or "preferably less than Y." In this specification, "film" also encompasses "tape" and "sheet." Regarding the numerical ranges described in stages in this specification, the upper or lower limit of one stage can be arbitrarily combined with the upper or lower limit of another stage. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of the numerical range can be replaced with the values ​​shown in the examples.

[0012] A modified EVOH according to one embodiment of the present invention (hereinafter, sometimes referred to as "the present modified EVOH") has the following structural unit (1) in EVOH containing 20 to 60 mol % of ethylene structural units:

[0013] In the structural unit (1), the R 1 is an alkyl group having 1 to 3 carbon atoms, and X is an alkylene group having 1 to 5 carbon atoms. A method for producing the modified EVOH will now be described.

[0014] The present modified EVOH is a water-insoluble thermoplastic resin typically obtained by copolymerizing ethylene, a vinyl ester monomer, and an unsaturated monomer convertible to structural formula (1), and then saponifying the resulting copolymer, a modified ethylene-vinyl ester copolymer. Because the present modified EVOH is used, particularly as a food packaging material, it is required to be non-elutable under cold to hot water conditions. Specifically, a water-insoluble thermoplastic resin is one that exhibits an elution amount of 3.0% by mass or less, preferably 1% by mass or less, and more preferably 0.5% by mass or less, when immersed in hot water at 95°C or less.

[0015] Examples of vinyl ester monomers used in the present modified EVOH include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl versatate, and vinyl trifluoroacetate. These may be used alone or in combination of two or more. Among these, vinyl acetate is preferred from an economical viewpoint.

[0016] The unsaturated monomer convertible to the structural formula (1) used in the present modified EVOH includes, for example, a compound having the following structural unit (2).

[0017]

[0018] In the structural unit (2), the above R 2 is an alkyl group having 1 to 3 carbon atoms, and the R 3 represents an alkyl group having 1 to 10 carbon atoms, and Y represents an alkylene group having 1 to 5 carbon atoms.

[0019] In the structural unit (2), R 2 represents an alkyl group having 1 to 10 carbon atoms. 2 The structure of R is not particularly limited, but may have a branched structure or a cyclic structure, and the functional group may be such that some of the hydrogen atoms of the alkyl group are substituted with other functional groups. Examples of such functional groups include an alkoxy group, a halogen atom, and a hydroxyl group. 2is preferably an alkyl group having 1 to 5 carbon atoms, and suitable examples thereof include linear or branched alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, and pentyl groups.

[0020] Specific examples of the unsaturated monomer represented by the structural unit (2) include 2-methyl-2-propenyl acetate, 2-ethyl-2-propenyl acetate, 3-methyl-3-butenyl acetate, 3-ethyl-3-butenyl acetate, 4-methyl-4-pentenyl acetate, 5-methyl-5-hexenyl acetate, 6-methyl-6-heptenyl acetate, etc. Among these, 2-methyl-2-propenyl acetate and 3-methyl-3-butenyl acetate are preferably used in terms of cost and ease of production.

[0021] The polymerization method for copolymerizing ethylene, a vinyl ester monomer, and a compound having the structural unit (2) may be, for example, any of batch polymerization, semi-batch polymerization, continuous polymerization, and semi-continuous polymerization, and the polymerization method may be, for example, a known method such as bulk polymerization, solution polymerization, suspension polymerization, emulsion polymerization, etc. Among these, bulk polymerization is preferably used for the purpose of controlling the molecular weight distribution and simplifying the production process.

[0022] When such copolymerization is carried out by solution polymerization, examples of the solvent to be used include lower alcohols having 1 to 5 carbon atoms, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-methyl-2-propanol; ketones, such as acetone and 2-butanone; and carbonates, such as dimethyl carbonate and diethyl carbonate. These may be used alone or in combination of two or more. Of these, methanol is preferred because of the ease of controlling the polymerization reaction. Furthermore, 2-propanol is preferred when synthesizing a copolymer with a low degree of polymerization.

[0023] The amount of the solvent used can be appropriately selected taking into consideration the degree of polymerization of the desired modified EVOH and the chain transfer constant of the solvent. When the solvent is methanol or 2-propanol, the mass ratio of S (solvent) / M (monomer) is preferably 0.01 to 10, and more preferably 0.05 to 7.

[0024] The copolymerization components in solution polymerization can be charged by any method, such as initial lump-sum charging, divided charging, continuous charging, etc. Initial lump-sum charging is particularly preferred due to its ease of operation.

[0025] A polymerization initiator is used for the copolymerization. Examples of such polymerization initiators include azo initiators such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), and 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and peroxide initiators such as acetyl peroxide, benzoyl peroxide, lauryl peroxide, t-butyl peroxyneodecanoate, 1,1,3,3-tetramethylbutyl peroxyneodecanoate, diisopropyl peroxydicarbonate, and di-2-ethylhexyl peroxydicarbonate. These initiators may be used alone or in combination of two or more.

[0026] The amount of the polymerization initiator used cannot be generally determined because it varies depending on the type of initiator, but is selected as appropriate depending on the polymerization rate. For example, when 2,2'-azobisisobutyronitrile or t-butylperoxyneodecanoate is used, the amount is usually 0.0001 to 0.10 parts by mass, and preferably 0.001 to 0.05 parts by mass, per 100 parts by mass of the vinyl ester monomer.

[0027] The reaction temperature for the copolymerization reaction cannot be generally determined depending on the polymerization solvent and pressure used, but is usually preferably 40 to 80° C., particularly preferably 55 to 80° C. If the temperature is too low, the polymerization takes a long time, and an attempt to shorten the polymerization time tends to require a large amount of initiator, whereas if the temperature is too high, the polymerization rate becomes so fast that it tends to be difficult to control the polymerization.

[0028] The method for introducing ethylene into the copolymer may be a conventional pressurized polymerization of ethylene, and the amount of ethylene introduced can be controlled by the pressure of ethylene, which is usually selected from the range of 2.0 to 8.0 MPa, although it depends on the target content of ethylene structural units.

[0029] In addition, in the case of a batch system, the polymerization time is usually 4 to 10 hours, and more preferably 6 to 9 hours. If the polymerization time is too short, it tends to be necessary to increase the polymerization temperature or set a large amount of initiator, while conversely, if the polymerization time is too long, it is not preferable because it causes problems in terms of productivity. In the case of a continuous system, the average residence time in the polymerization vessel is usually 2 to 8 hours, and more preferably 2 to 6 hours. If the residence time is too short, it tends to be necessary to increase the polymerization temperature or set a large amount of catalyst, while conversely, if the polymerization time is too long, it is not preferable because it causes problems in terms of productivity.

[0030] Furthermore, copolymerization may be carried out in the presence of a chain transfer agent, provided that the effects of the present invention are not impaired. Examples of chain transfer agents include aldehydes such as acetaldehyde, propionaldehyde, and crotonaldehyde; and mercaptans such as 2-hydroxyethanethiol. These may be used alone or in combination of two or more. Among these, aldehydes are preferred. The amount of chain transfer agent added during copolymerization is determined depending on the chain transfer constant of the chain transfer agent and the desired degree of polymerization of EVOH, but generally, 0.1 to 10 parts by mass per 100 parts by mass of vinyl ester monomer is preferred.

[0031] After copolymerization of the copolymerization components, it is also preferable to add a conjugated polyene such as sorbic acid as a polymerization inhibitor to reliably terminate the reaction. In this way, a modified ethylene-vinyl ester copolymer is obtained, and the modified EVOH can be obtained by saponifying the modified ethylene-vinyl ester copolymer.

[0032] The saponification can be carried out by any known method, for example, by dissolving the modified ethylene-vinyl ester copolymer obtained above in alcohol or aqueous alcohol and using a saponification catalyst.

[0033] Examples of the alcohol include lower alcohols having 1 to 5 carbon atoms, such as methanol, ethanol, and propanol. These may be used alone or in combination of two or more. Of these, methanol is preferred.

[0034] The concentration of the modified ethylene-vinyl ester copolymer in the alcohol is appropriately selected depending on the viscosity, and is usually 5 to 60% by mass.

[0035] Examples of the saponification catalyst include alkali catalysts such as hydroxides and alcoholates of alkali metals, such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and potassium ethylate; and acid catalysts such as sulfuric acid, hydrochloric acid, nitric acid, metasulfonic acid, zeolite, and cation exchange resins.

[0036] The temperature at which the saponification is carried out is not limited, but a range of 20 to 140°C is suitable. As the saponification proceeds, particulate matter is produced, indicating that the reaction has progressed. If a gel-like product precipitates at this time, the product can be pulverized. The produced particulate matter can be washed, dried, neutralized, and desalted to obtain modified EVOH.

[0037] The modified EVOH produced in this manner mainly comprises ethylene structural units, vinyl alcohol units, and structural units (1), and when the degree of saponification is less than 100 mol%, it contains a small amount of vinyl ester structural units remaining as unsaponified portions.

[0038] The modified EVOH may further contain other monomer units besides the structural units (1) and (2), vinyl alcohol units, vinyl ester units, and ethylene units, to the extent that the effects of the present invention are not impaired. The other monomer units are monomer units derived from ethylenically unsaturated monomers copolymerizable with vinyl esters. Examples of the ethylenically unsaturated monomer unit include α-olefins such as propylene, n-butene, isobutylene, and 1-hexene; acrylic acid and its salts; unsaturated monomers having an acrylic ester group; methacrylic acid and its salts; unsaturated monomers having a methacrylic ester group; acrylamide, N-methylacrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, diacetoneacrylamide, acrylamidopropanesulfonic acid and its salts, acrylamidopropyldimethylamine and its salts (e.g., quaternary salts); methacrylamide, N-methylmethacrylamide, N-ethylmethacrylamide, methacrylamidopropanesulfonic acid and its salts, methacrylamidopropyldimethylamine and its salts (e.g., quaternary salts); methylvinyl ether Examples of the other monomer units include vinyl ethers such as ethyl vinyl ether, 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; vinylidene 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 and salts or esters thereof such as maleic acid, itaconic acid, and fumaric acid; vinyl silyl compounds such as vinyltrimethoxysilane; and isopropenyl acetate. Furthermore, structures derived from the unsaturated monomer units represented by structural unit (2) that have not been saponified are also included in the other monomer units. The content of the other monomer units is preferably 10 mol % or less, and more preferably 5 mol % or less.

[0039] The order of arrangement of the structural unit (1), vinyl alcohol unit, vinyl ester unit, ethylene structural unit and other monomer units in the present modified EVOH is not particularly limited, and may be random, block, alternating or the like.

[0040] The present modified EVOH may further contain additives, such as fillers, processing stabilizers such as copper compounds, weathering stabilizers, colorants, ultraviolet absorbers, light stabilizers, antioxidants, antistatic agents, flame retardants, plasticizers, 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, as required.

[0041] The modification rate of the structural unit (1) in the present modified EVOH is usually 0.1 to 10 mol%, preferably 0.5 to 9 mol%, more preferably 1 to 8 mol%, and even more preferably 2 to 6 mol%. If the modification rate is too low, moldability tends to decrease, while if it is too high, gas barrier properties tend to deteriorate. The modification rate of the structural unit (1) can be calculated, for example, by the method described in the Examples below.

[0042] The saponification degree of the present modified EVOH is usually 95 mol% or more, preferably 98.5 mol% or more, and more preferably 99 mol% or more. If the saponification degree is too low, the gas barrier property, aroma retention, solvent resistance, and oil resistance tend to decrease. The upper limit of the saponification degree is 100 mol%.

[0043] The content of ethylene structural units in the present modified EVOH is 20 to 60 mol%, preferably 25 to 50 mol%, and more preferably 25 to 45 mol%. If the content of ethylene structural units is too low, the gas barrier properties under high humidity conditions tend to decrease, while if the content is too high, the molecular weight tends to decrease and the mechanical strength during molding tends to decrease. The content of ethylene structural units in the present modified EVOH can be controlled by adjusting the ethylene pressure during copolymerization. The saponification degree and the content of ethylene structural units can be calculated, for example, by the method described in the Examples below.

[0044] The melt flow rate (MFR) of the modified EVOH (210°C, 2160 g load) is usually 1 to 100 g / 10 min, preferably 1 to 50 g / 10 min, and more preferably 3 to 35 g / 10 min. If the MFR is too high, processing defects during extrusion molding tend to occur, while if it is too low, poor flowability tends to occur during molding. In this specification, the MFR is determined by measuring the rate at which a sample flows through an orifice with a length of 8 mm and a hole diameter of 2.095 mm under conditions of a temperature of 210°C and a load of 2160 g using a semi-automatic melt flow rate tester (manufactured by Toyo Seiki Seisakusho, Ltd.).

[0045] The melting point of the present modified EVOH is usually 100 to 200° C., preferably 100 to 190° C., and more preferably 100 to 180° C. If the melting point is too high, moldability tends to deteriorate, while if it is too low, the mechanical strength of the resulting molded product tends to decrease.

[0046] The glass transition temperature (Tg) of the present modified EVOH is usually 50 to 70°C, preferably 50 to 65°C, and more preferably 55 to 65°C. If the glass transition temperature is too high, molding processability tends to decrease, while if it is too low, gas barrier properties under high humidity conditions tend to decrease. The melting point and glass transition temperature can be calculated using a differential scanning calorimeter (DSC) by the method described in the Examples below.

[0047] The modified EVOH can be blended with other components to form a resin composition. Examples of such other components include other thermoplastic resins, plasticizers, lubricants, stabilizers, surfactants, colorants, UV absorbers, antistatic agents, desiccants, crosslinking agents, metal salts, fillers, and various fibers. These can be used alone or in combination of two or more.

[0048] The content of the modified EVOH in the resin composition is usually 80% by mass or more, preferably 90% by mass or more, and more preferably 95% by mass or more, with the upper limit being 100% by mass.

[0049] The modified EVOH can be formed into a film and suitably used as a gas barrier material for food packaging, etc. The method for obtaining a gas barrier material from modified EVOH is not particularly limited, and examples thereof include (i) a method in which a solution of modified EVOH is applied to a base resin film and dried to form a layer made of modified EVOH (hereinafter referred to as "EVOH layer") to obtain a gas barrier material, and (ii) a method in which the modified EVOH is melt-molded to form a modified EVOH layer to obtain a gas barrier material.

[0050] In the above method (i), examples of the solvent used for the modified EVOH solution include water and lower alcohols having 1 to 5 carbon atoms, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, and 2-butanol. These may be used alone or in combination of two or more. Of these, a mixed solvent of water and 1-propanol is preferred.

[0051] The solid content of the modified EVOH in the solution is usually 0.5 to 30% by mass, preferably 5 to 20% by mass.

[0052] Examples of methods for applying the modified EVOH solution include known methods such as bar coating, roll coating, die coating, gravure coating, comma coating, and screen printing, with the bar coating being preferred.

[0053] After coating, the gas barrier material made of modified EVOH can be obtained by drying it by heat treatment, etc. The heat treatment is preferably performed at 60 to 105° C. for 0.5 to 10 minutes, for example.

[0054] In this way, a film having the present modified EVOH layer is obtained. The film may be a gas barrier material having a single layer structure or a multilayer structure, but a multilayer structure is preferred. The multilayer gas barrier material preferably has at least one modified EVOH layer. Furthermore, the multilayer gas barrier material may be laminated with the modified EVOH layer or may be laminated with another base resin.

[0055] Examples of the base resin include polyethylene-based resins such as linear low-density polyethylene, low-density polyethylene, very low-density polyethylene, medium-density polyethylene, high-density polyethylene, ethylene-propylene (block and random) copolymers, and ethylene-α-olefin (α-olefin having 4 to 20 carbon atoms) copolymers; polypropylene-based resins such as polypropylene and propylene-α-olefin (α-olefin having 4 to 20 carbon atoms) copolymers; (unmodified) polyolefin-based resins such as polybutene, polypentene, and polycyclic olefin-based resins (polymers having a cyclic olefin structure in at least one of the main chain and side chain); and polyolefins obtained by dissolving these polyolefins in an unsaturated carboxylic acid or Examples of suitable materials include polyolefin resins in the broad sense, including modified olefin resins such as unsaturated carboxylic acid-modified polyolefin resins graft-modified with olefin esters or ionomers, ionomers, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, ethylene-acrylic acid ester copolymers, polyester resins, polyamide resins (including copolymerized polyamides), polyvinyl chloride, polyvinylidene chloride, acrylic resins, polystyrene, vinyl ester resins, polyester elastomers, polyurethane elastomers, polystyrene elastomers, halogenated polyolefins such as chlorinated polyethylene and chlorinated polypropylene, and aromatic or aliphatic polyketones. These materials may be used alone or in combination of two or more. Biodegradable resins may be used to obtain gas barrier materials with biodegradable substrates. These substrate resins may also be subjected to surface treatments such as corona treatment. The terms linear low-density polyethylene, low-density polyethylene, very low-density polyethylene, medium-density polyethylene, and high-density polyethylene are commonly used terms to represent types of polyethylene.

[0056] The multilayer structure is subjected to a (heat) stretching treatment as necessary. The stretching treatment may be either uniaxial stretching or biaxial stretching, and in the case of biaxial stretching, it may be simultaneous stretching or sequential stretching. Furthermore, the stretching method may be a roll stretching method, tenter stretching method, tubular stretching method, stretch-blow method, vacuum pressure forming, or the like, whichever provides a higher stretch ratio. The stretching temperature is a temperature near the melting point of the multilayer structure, and is usually selected from the range of 40 to 170°C, preferably about 60 to 160°C. If the stretching temperature is too low, the stretchability will be poor, and if it is too high, it will be difficult to maintain a stable stretched state.

[0057] For the purpose of imparting dimensional stability, heat setting may be performed after the stretching treatment. Heat setting can be performed by known means, for example, by heat treating the stretched film while maintaining it in a tensile state, typically at 80 to 180°C, preferably 100 to 165°C, for typically 2 to 600 seconds. When the multilayer stretched film obtained from the present resin composition is used as a shrink film, heat shrinkability can be imparted by not performing the heat setting described above, but by performing a treatment such as cooling and setting the stretched film by blowing cold air on it.

[0058] In some cases, the multilayer structure can be used to obtain cup- or tray-shaped multilayer containers (packaging containers). In such cases, a drawing method is usually employed, specifically vacuum forming, pressure forming, vacuum pressure forming, plug-assisted vacuum pressure forming, etc. Furthermore, when a tube- or bottle-shaped multilayer container (laminate structure) is obtained from a multilayer parison (a hollow tubular preform before blowing), a blow molding method is employed.

[0059] The thickness of the multilayer structure, and further the thickness of the modified EVOH layer and base resin layer constituting the multilayer structure, cannot be generally determined depending on the layer configuration, type of base resin, type of adhesive resin, intended use, packaging form, required physical properties, etc., but the thickness of the multilayer structure is usually 10 to 5,000 μm, preferably 30 to 3,000 μm, and particularly preferably 50 to 2,000 μm. The thickness of the layer made of the modified EVOH is usually 1 to 200 μm, preferably 1 to 100 μm, and particularly preferably 1 to 50 μm, and the thickness of the base resin layer is usually 5 to 3,000 μm, preferably 10 to 2,000 μm, and particularly preferably 20 to 1,000 μm.

[0060] When the present modified EVOH is formed into a monolayer film having a thickness of 200 to 300 μm, the saturated water absorption at 20°C x 90% RH is usually less than 15% by mass, preferably less than 13%, and more preferably less than 12% by mass. If the saturated water absorption of such a monolayer film is too high, the resin tends to expand, causing structural deformation or peeling from the substrate of a multilayer structure. If the saturated water absorption is too low, the laminated film tends to become brittle, such as cracking when bent. The saturated water absorption of such a monolayer film can be calculated, for example, by the method described in the Examples below.

[0061] The oxygen permeability of the modified EVOH layer is 20 cc. 20 μm / m under an environment of 20°C and 90% RH. 2 ・It is preferable that the concentration is 15 cc or less atm. 2 ・It is more preferable that the concentration is 14 cc. atm or less, and 20 μm / m 2 The oxygen permeability can be calculated using an oxygen permeability measuring device by the method described in the Examples below.

[0062] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples as long as it does not depart from the gist of the invention. In the examples, "parts" and "%" are by mass.

[0063] Example 1 Synthesis of Modified EVOH 828 parts of vinyl acetate, 21.9 parts of 2-methyl-2-propenyl acetate, and 0.3 parts of azobisisobutyronitrile as a polymerization initiator (0.0357 parts per 100 parts of vinyl acetate monomer charged) were placed in an induction stirring autoclave under a nitrogen atmosphere. After the atmosphere inside the induction stirring autoclave was replaced with ethylene, the reaction solution was heated to 67°C with stirring. The pressure on the autoclave was then increased with ethylene until the autoclave pressure gauge reached 4 MPa, after which polymerization was initiated. After stirring for 4 hours, a solution of 1 part of sorbic acid dissolved in 100 parts of methanol was added, and the mixture was cooled to room temperature (23°C) to terminate the polymerization. The reaction solution was removed and dried under reduced pressure to remove volatile components, yielding a modified ethylene-vinyl acetate copolymer. Next, a methanol solution of sodium hydroxide was added to a 20% methanol solution of the resulting modified ethylene-vinyl acetate copolymer, and the mixture was heated to reflux with stirring to carry out saponification. The crude modified EVOH was extracted from the resulting polymer solution and further subjected to neutralization and desalting treatment to obtain the desired modified EVOH.

[0064] [Preparation of Modified EVOH Single-Layer Film] The modified EVOH obtained above was used in a hot press to prepare a single-layer film at a heating temperature of 210° C. and a designed film thickness of about 200 to 300 μm.

[0065] [Preparation of Modified EVOH Laminated Film] 5 parts of the modified EVOH obtained above was added to 45 parts of a mixed solvent of water / 1-propanol = 1 / 1 (volume ratio), and the mixture was heated and stirred at 70°C for 2 hours to completely dissolve the modified EVOH. This solution was allowed to cool to 55°C to prepare a modified EVOH resin composition (coating liquid). The resulting coating liquid was applied to a corona-treated PET substrate having a thickness of 12 μm using a wire bar #24, and then dried at 80°C for 5 minutes. This process was repeated twice to obtain a laminated film in which a 6 μm-thick modified EVOH film layer was laminated on the PET substrate.

[0066] Example 2 A modified EVOH was obtained in the same manner as in Example 1, except that the amount of 2-methyl-2-propenyl acetate used was changed to 54.9 parts and the amount of azobisisobutyronitrile added as a polymerization initiator was changed to 0.0535 parts per 100 parts of the charged vinyl acetate monomer. Furthermore, a monolayer and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Example 2.

[0067] Example 3 A modified EVOH was obtained in the same manner as in Example 1, except that the amount of 2-methyl-2-propenyl acetate used in Example 1 was changed to 109.8 parts and the amount of azobisisobutyronitrile added as a polymerization initiator was changed to 0.0535 parts per 100 parts of the vinyl acetate monomer charged. A monolayer film was obtained using the modified EVOH obtained in Example 3 in the same manner as in Example 1. 3 parts of the modified EVOH obtained in Example 3 was added to 27 parts of a mixed solvent, and the mixture was heated and stirred at 87°C for 1 hour to completely dissolve the modified EVOH, preparing a modified EVOH resin composition. A laminate film was obtained using this in the same manner as in Example 1.

[0068] Example 4 A modified EVOH was obtained in the same manner as in Example 1, except that the amount of vinyl acetate used was 460 parts and the amount of 2-methyl-2-propenyl acetate was changed to 13.7 parts of 3-methyl-3-butenyl acetate. Furthermore, a monolayer film and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Example 4.

[0069] Example 5 A modified EVOH was obtained in the same manner as in Example 1, except that the amount of 3-methyl-3-butenyl acetate used in Example 4 was changed to 34.2 parts and the amount of azobisisobutyronitrile added as a polymerization initiator was changed to 0.0714 parts per 100 parts of the charged vinyl acetate monomer. Furthermore, a monolayer and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Example 5.

[0070] Example 6 A modified EVOH was obtained in the same manner as in Example 1, except that the amount of 3-methyl-3-butenyl acetate used in Example 4 was changed to 54.8 parts and the amount of azobisisobutyronitrile added as a polymerization initiator was changed to 0.1071 parts per 100 parts of the charged vinyl acetate monomer. Furthermore, a monolayer and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Example 6.

[0071] Comparative Example 1: 460 parts of vinyl acetate and 9.3 parts of allyl acetate were added to an induction stirring autoclave under a nitrogen atmosphere, and 10 parts of a methanol solution containing 0.08 parts of azobisisobutyronitrile (0.0178 parts per 100 parts of vinyl acetate monomer charged) dissolved as a polymerization initiator was added to the inner cylindrical tube of the autoclave. After the atmosphere inside the induction stirring autoclave was replaced with ethylene, the entire amount of azobisisobutyronitrile was introduced by pressure from the inner cylindrical tube, and the reaction solution was heated to 67°C while stirring. Thereafter, the pressure of the autoclave was increased with ethylene until the pressure gauge reached 4 MPa, and polymerization was initiated. After stirring for 4 hours, a solution of 1 part of sorbic acid dissolved in 100 parts of methanol was added, and the mixture was cooled to room temperature (23°C) to terminate the polymerization. The reaction solution was removed, and the volatile components were removed by vacuum drying to obtain a modified ethylene-vinyl acetate copolymer. Next, a methanol solution of sodium hydroxide was added to a methanol solution containing 20% ​​of the obtained modified ethylene-vinyl acetate copolymer, and the mixture was heated under reflux with stirring to carry out saponification. Crudely modified EVOH was extracted from the resulting polymer solution, and further neutralized and desalted to obtain the desired modified EVOH. Then, using the modified EVOH obtained in Comparative Example 1, monolayer and laminate films were obtained in the same manner as in Example 1.

[0072] Comparative Example 2 A modified EVOH was obtained in the same manner as in Comparative Example 1, except that the amounts of vinyl acetate and allyl acetate used were changed to 828 parts and 50.4 parts, respectively. Furthermore, a monolayer film and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Comparative Example 2.

[0073] Comparative Example 3 A modified EVOH was obtained in the same manner as in Comparative Example 1, except that the amount of allyl acetate used was changed to 103.8 parts and the amount of azobisisobutyronitrile used as a polymerization initiator was changed to 0.0535 parts per 100 parts of the charged vinyl acetate monomer. Furthermore, a monolayer and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Comparative Example 3.

[0074] Comparative Example 4 A modified EVOH was obtained in the same manner as in Comparative Example 1, except that 12.2 parts of 3-butenyl acetate was used instead of allyl acetate and azobisisobutyronitrile was used as a polymerization initiator in an amount of 0.0357 parts per 100 parts of the charged vinyl acetate monomer. Furthermore, a monolayer and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Comparative Example 4.

[0075] Comparative Example 5 A modified EVOH was obtained in the same manner as in Comparative Example 1, except that the amount of 3-butenyl acetate used in Comparative Example 4 was changed to 30.5 parts and the amount of azobisisobutyronitrile used as a polymerization initiator was changed to 0.0714 parts per 100 parts of the charged vinyl acetate monomer. Furthermore, a monolayer and a laminate film were obtained in the same manner as in Example 1 using the modified EVOH obtained in Comparative Example 5.

[0076] Reference Example: As the unmodified EVOH, ethylene-vinyl alcohol "Soarnol D2908" manufactured by Mitsubishi Chemical Corp. was used. Furthermore, using this unmodified EVOH, a single layer film and a laminate film were obtained in the same manner as in Example 1.

[0077] The modification rate, degree of saponification, ethylene structural unit content, melting point, and glass transition temperature when dried of the obtained EVOH of Examples 1 to 6, Comparative Examples 1 to 5, and Reference Example were measured by the following measurement methods. Furthermore, the saturated water absorption and glass transition temperature when saturated with water of the obtained monolayer films of Examples 1 to 6, Comparative Examples 1 to 5, and Reference Example, and the oxygen permeability of the obtained laminate films of Examples 1 to 6, Comparative Examples 1 to 5, and Reference Example were measured by the following measurement methods. The results are shown in Table 1 below.

[0078] [Modification rate of structural unit (1) in EVOH] The obtained modified EVOH and unmodified EVOH were dissolved in N,N-dimethylformamide, and then acetic anhydride and N-methylindazole were added and reacted to convert the hydroxyl groups of the EVOH to acetate groups, thereby obtaining an ethylene-vinyl acetate copolymer. The obtained ethylene-vinyl acetate copolymer was dissolved in deuterated chloroform and measured under the following conditions: 1 The modification rate of the structural unit (1) was calculated by measuring H-NMR. (Measurement conditions) Apparatus: Ascend-400 (manufactured by Bruker) Measurement temperature: 25°C Number of accumulations: 16

[0079] [Degree of Saponification and Ethylene Structural Unit Content of EVOH] The obtained modified EVOH and unmodified EVOH were dissolved in deuterated DMSO and subjected to the following procedure. 1 The degree of saponification and the content of ethylene structural units were calculated by measuring H-NMR. (Measurement conditions) Apparatus: Ascend-400 (manufactured by Bruker) Measurement temperature: 50°C Number of accumulations: 16

[0080] [Melting Point and Glass Transition Temperature of EVOH when Drying] The melting point and glass transition temperature of the obtained modified EVOH and unmodified EVOH were measured using a differential scanning calorimeter (DSC) under the following measurement conditions: (Measurement Conditions) Apparatus: ThermoPlus EV02 (manufactured by Rigaku Corporation) Temperature range: -30 to 210°C Heating rate: 10°C / min Number of sweeps: 2 (The melting point and glass transition temperature values ​​during the second heating run were used.)

[0081] [Saturated Water Absorption of EVOH Monolayer Film] A predetermined mass (approximately 0.1 g) (W1) of the obtained monolayer film was precisely weighed into an aluminum cup and placed in an environmental tester (set temperature 20°C, relative humidity 90% RH) to start the water absorption test. The mass of the monolayer film was measured at regular intervals, and the point at which the mass no longer changed was taken as a constant value (W2). Thereafter, the saturated water absorption of the monolayer film was calculated using the following formula (a): Saturated Water Absorption (mass%) = (W2 - W1) / W1 × 100 (a)

[0082] [Glass Transition Temperature of EVOH Monolayer Film at Saturated Water Absorption] The glass transition temperature of the EVOH monolayer film at saturated water absorption obtained by the method described above was measured using differential scanning calorimetry (DSC) under the following measurement conditions: Apparatus: DSC7000X (manufactured by Hitachi High-Tech Science Corporation) Measurement method: Temperature modulation measurement in AC temperature control mode Applied frequency: 0.02 Hz Temperature amplitude: ±3.0°C Temperature range: -50 to 80°C Heating rate: 5°C / min Number of sweeps: 1 (The phase transition point measured from the specific heat component (reversing heat flow) of the measurement data was used as the glass transition temperature).

[0083] [Oxygen permeability of EVOH laminated film] The oxygen permeability of the laminated film prepared by the above-mentioned method was measured under conditions of 20°C and 90% RH using an oxygen permeability measuring device (OX-TRAN2 / 21). The oxygen permeability of an uncoated PET film (12 μm) was 90 cc / m 2 Since the oxygen permeability was measured at 1000 kJ / day, the oxygen permeability of the PET film was subtracted from the measured oxygen permeability, and the value per 20 μm of EVOH layer thickness (cc. 20 μm / m 2 The smaller the oxygen permeability value, the better the gas barrier property.

[0084]

[0085] The results in Table 1 above show that, as the modification rate of the modified EVOHs of Comparative Examples 1 to 5 increased, their melting points decreased compared to the unmodified EVOH of the Reference Example, but their gas barrier properties remained the same or worsened. On the other hand, the melting points of the modified EVOHs of Examples 1 to 6 decreased with increasing modification rate, but their gas barrier properties at high humidity tended to remain the same or improve, demonstrating that they were able to achieve both moldability and gas barrier properties at high humidity. Furthermore, the modified EVOHs of Examples 1 to 6 had higher glass transition temperatures in the dried state than the modified EVOHs of Comparative Examples 1 to 5 when compared at similar modification rates, indicating that the cohesive strength of the amorphous portions was not significantly reduced even when modifying groups were introduced. It is generally believed that gases such as oxygen pass through the amorphous portions of resins, and the high cohesive strength of these amorphous portions is thought to have contributed to the improvement in gas barrier properties. Furthermore, focusing on Example 3 and Comparative Example 3, both of which have modification rates exceeding 5 mol%, it was found that, despite the saturated water absorption rates of the EVOH monolayer films relative to the modification rate being similar, Example 3 had the advantageous effect of having a higher glass transition temperature at saturated water absorption. Generally, as the modification rate increases, crystallinity decreases and amorphousness increases, tending to deteriorate gas barrier properties under high humidity. However, the present modified EVOH is less likely to lose cohesive strength in the amorphous portion even when placed under high humidity, which is thought to result in improved gas barrier properties under high humidity. It is generally known that when a hydrophilic film such as polyvinyl alcohol is placed under high humidity, its crystallinity decreases with increasing modification rate, increasing its solubility in water, making it difficult to use as a gas barrier material. On the other hand, it was found that the present modified EVOH, by having both the structural unit (1) and an ethylene structural unit, suppresses moisture absorption and reduces the cohesive strength of the amorphous portion, making it usable as a gas barrier material even under high humidity.

[0086] Although the above examples show specific embodiments of the present invention, the examples are merely illustrative and should not be construed as limiting. Various modifications that are obvious to those skilled in the art are intended to fall within the scope of the present invention.

[0087] The modified EVOH has excellent moldability and gas barrier properties under high humidity conditions, and is therefore useful as a packaging material for a variety of foods, as well as seasonings such as mayonnaise and dressings, fermented foods such as miso, oily foods such as salad oil, beverages, cosmetics, pharmaceuticals, etc.

Claims

1. A modified ethylene-vinyl alcohol copolymer having an ethylene structural unit content of 20 to 60 mol % and having the following structural unit (1): [In the structural unit (1), the above R 1 is an alkyl group having 1 to 3 carbon atoms, and X is an alkylene group having 1 to 5 carbon atoms.

2. The modified ethylene-vinyl alcohol copolymer according to claim 1, wherein the modification rate of the structural unit (1) is 0.5 to 10 mol %.

3. The modified ethylene-vinyl alcohol copolymer of claim 1, wherein said modified ethylene-vinyl alcohol copolymer is water insoluble.

4. The modified ethylene-vinyl alcohol copolymer according to claim 1, wherein the modified ethylene-vinyl alcohol copolymer has a saturated water absorption rate of less than 15% by mass at 20°C x 90% RH when formed into a monolayer film having a thickness of 200 to 300 μm.

5. A gas barrier material having a layer containing the modified ethylene-vinyl alcohol copolymer according to any one of claims 1 to 4.

6. A film or sheet having the gas barrier material according to claim 5.

7. A multi-layer structure comprising the gas barrier material according to claim 5.

8. A packaging container comprising the gas barrier material according to claim 5.

9. A method for producing the modified ethylene-vinyl alcohol copolymer according to any one of claims 1 to 4, comprising the steps of: copolymerizing ethylene, a vinyl ester monomer, and a compound having the following structural unit (2) to obtain a modified ethylene-vinyl ester copolymer; and saponifying the obtained modified ethylene-vinyl ester copolymer. [In the structural unit (2), the above R 2 is an alkyl group having 1 to 3 carbon atoms, and the R 3 is an alkyl group having 1 to 10 carbon atoms, and Y is an alkylene group having 1 to 5 carbon atoms.

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