Uniaxially stretched multilayer structure and manufacturing method thereof

Abstract

Provided is a uniaxially stretched multilayer structure that is a multilayer structure having been stretched in a uniaxial direction and including a resin composition (A) layer, the resin composition (A) containing an ethylene-vinyl alcohol copolymer, the multilayer structure having excellent transparency.　This multilayer structure having been stretched in a uniaxial direction is a multilayer structure in which each layer has been stretched in a uniaxial direction, the multilayer structure including a resin composition (A) layer, the resin composition (A) containing an ethylene-vinyl alcohol copolymer, wherein the ethylene unit content of the ethylene-vinyl alcohol copolymer in the resin composition (A) layer is less than 32% by mol, and the ethylene-vinyl alcohol copolymer has a single ethylene unit content.

Description

Uniaxially stretched multilayer structure and method for producing the same

[0001] The present invention relates to a uniaxially stretched multilayer structure (hereinafter may be referred to as a "uniaxially stretched multilayer structure") having a layer of a resin composition (A) containing an ethylene-vinyl alcohol-based copolymer (hereinafter may be referred to as an "EVOH") having an ethylene unit content of less than 32 mol%, wherein the ethylene-vinyl alcohol-based copolymer has a single ethylene unit content.

[0002] EVOH has very strong intermolecular forces due to hydrogen bonds between hydroxyl groups present in the polymer side chains. Therefore, it has high crystallinity, and since the intermolecular forces are strong even in the amorphous portion, films using EVOH are difficult for gas molecules to pass through and exhibit excellent gas barrier properties. Therefore, EVOH is used as a gas barrier layer to impart gas barrier properties to multilayer structures such as films and containers in a wide range of fields, such as food packaging.

[0003] For example, Patent Document 1 describes a uniaxially stretched laminated resin film having a resin layer (α) made of an ethylene-α-olefin copolymer and polyethylene and a resin layer (β) selected from polyamide resin, aromatic polyester resin, polyvinyl alcohol, and EVOH, with the aim of providing a uniaxially stretched laminated resin film excellent in gas barrier properties and stretchability. In the examples of Patent Document 1, a uniaxially stretched film is disclosed in which a gas barrier layer made of EVOH having an ethylene content of 32 mol% is provided as an intermediate layer (fourth layer).

[0004] : JP 2006-150624 A

[0005] As described in Patent Document 1, a uniaxially stretched multilayer structure having a resin composition layer containing EVOH is known, but there is room for improvement in the transparency of the multilayer structure.

[0006] The present invention has been made in view of the above problems, and provides a uniaxially stretched multilayer structure having an excellent transparency and a resin composition layer containing EVOH.

[0007] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have found that the transparency of a uniaxially stretched multilayer structure can be improved by using EVOH having an ethylene unit content of less than 32 mol% and providing a layer of a resin composition of the EVOH having a single ethylene unit content.

[0008] That is, the present invention provides the following [1] to [8]. [1] A multilayer structure in which each layer is uniaxially stretched, the multilayer structure comprising a layer of a resin composition (A) containing an ethylene-vinyl alcohol-based copolymer, wherein the ethylene unit content of the ethylene-vinyl alcohol-based copolymer in the resin composition (A) layer is less than 32 mol%, and the ethylene-vinyl alcohol-based copolymer contains a single ethylene unit. [2] The multilayer structure of [1], which is uniaxially stretched 3 to 12 times. [3] The multilayer structure of [1] or [2], in which the resin composition (A) is primarily composed of an ethylene-vinyl alcohol-based copolymer. [4] The multilayer structure of any of [1] to [3], further comprising a layer of a thermoplastic resin (B). [5] The multilayer structure of any of [1] to [4], further comprising a layer of an adhesive resin (C). [6] The multilayer structure of any of [1] to [5], in which the multilayer structure does not include a layer primarily composed of polyamide. [7] A method for producing the multilayer structure of any one of [1] to [6], comprising the following steps (1) and (2): step (1): melt-extruding a resin composition (A) and a thermoplastic resin (B) to form an intermediate multilayer structure; and step (2): uniaxially stretching the intermediate multilayer structure to produce a uniaxially stretched multilayer structure. [8] The method for producing the multilayer structure of [7], wherein step (1) is melt-extruding a resin composition (A), a thermoplastic resin (B), and an adhesive resin (C) to form an intermediate multilayer structure.

[0009] According to the present invention, a uniaxially stretched multilayer structure with excellent transparency can be obtained by using EVOH having an ethylene unit content of less than 32 mol% as the EVOH and providing a layer of a resin composition of the EVOH having a single ethylene unit content. It is generally known that as the ethylene unit content of EVOH decreases, stretchability decreases. Therefore, the use of EVOH with a low ethylene unit content for stretched films tends to be avoided, and the example in the aforementioned Patent Document 1 uses EVOH with an ethylene unit content of 32 mol%. The present inventors unexpectedly found that, in a uniaxially stretched multilayer structure, when EVOH with an ethylene unit content of less than 32 mol% is used, stretchability does not decrease and the transparency of the multilayer structure is superior compared to EVOH with a high ethylene unit content of 32 mol% or more.

[0010] The following describes in detail the embodiments of the present invention, but the present invention is not limited to the following description and can be implemented with any modifications within the scope of the gist of the present invention. In this specification, when an expression using "~" is used with a numerical value or physical property value before and after it, it is used as including the values ​​before and after it. Furthermore, when an expression is expressed as "X or more" (X is any number) or "Y or less" (Y is any number), it also means that "it is preferably greater than X" or "it is preferably less than Y".

[0011] Hereinafter, the monomer units contained in the copolymer resin may be simply referred to as "units." For example, a monomer unit based on ethylene may be referred to as an "ethylene unit."

[0012] The uniaxially stretched multilayer structure of the present invention is characterized by having a layer of resin composition (A) containing EVOH having an ethylene unit content of less than 32 mol%, and the EVOH contained in resin composition (A) has a single ethylene unit content. Each component will be described below.

[0013] [Resin composition (A) layer] The resin composition (A) layer contains EVOH having an ethylene unit content of less than 32 mol%, and the EVOH contains a single ethylene unit. By providing the resin composition (A) layer, the uniaxially stretched multilayer structure of the present invention has excellent transparency. Note that a plurality of resin composition (A) layers may be provided.

[0014] The content of EVOH contained in the resin composition (A) layer is not particularly limited, but EVOH is preferably the main component (i.e., the content of EVOH contained in the resin composition (A) layer is 50% by mass or more), more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, 90% by mass or more, 95% by mass or more, or 100% by mass or more.

[0015] The resin composition (A) layer may contain components other than EVOH within a range that does not impair the effects of the present invention, generally within a range of 5 mass % or less, such as an antiblocking agent, a processing aid, a resin other than EVOH, a carboxylic acid compound, a phosphoric acid compound, a boron compound, a metal salt, a stabilizer, an antioxidant, an ultraviolet absorber, a plasticizer, an antistatic agent, a lubricant, a colorant, a filler, a surfactant, a desiccant, a crosslinking agent, and a reinforcing agent such as various fibers.

[0016] The thickness of the resin composition (A) layer is not particularly limited, but is preferably 0.5 μm or more, more preferably 0.8 μm or more, and may be 1 μm or more. When the thickness of the resin composition (A) layer is 0.5 μm or more, the gas barrier property tends to be improved. Furthermore, the thickness of the resin composition (A) layer is preferably 20 μm or less, more preferably 10 μm or less, and may be 5 μm or less. When the thickness of the resin composition (A) layer is 20 μm or less, the appearance characteristics (film surface) after stretching tend to be good. Note that the above-mentioned preferred thickness of the resin composition (A) layer means the thickness after stretching. The ratio of the thickness of the resin composition (A) layer to the total thickness of all layers of the uniaxially stretched multilayer structure of the present invention is not particularly limited, but is preferably 30% or less, more preferably 20% or less, and may be 10% or less, or 5% or less, from the viewpoints of industrial productivity and mechanical properties.

[0017] [EVOH] EVOH can be obtained by saponifying an ethylene-vinyl ester copolymer. A representative vinyl ester is vinyl acetate, but other fatty acid vinyl esters (vinyl propionate, vinyl pivalate, etc.) can also be used. The ethylene-vinyl ester copolymer can be produced by any known polymerization method, such as solution polymerization, suspension polymerization, or emulsion polymerization, and the saponification of the ethylene-vinyl ester copolymer can also be carried out by a known method.

[0018] The ethylene unit content of the EVOH used in the resin composition (A) layer is less than 32 mol%, as measured according to ISO 14663. When the ethylene unit content of the EVOH is less than 32 mol%, the transparency of the uniaxially stretched multilayer structure is improved. On the other hand, when the ethylene unit content of the EVOH is 32 mol% or more, the transparency of the uniaxially stretched multilayer structure is reduced. The ethylene unit content of the EVOH is preferably 30 mol% or less, more preferably 29 mol% or less, even more preferably 27 mol% or less, and particularly preferably 25 mol% or less. The lower limit of the ethylene unit content of the EVOH is usually 20 mol%.

[0019] The degree of saponification of the vinyl ester unit of EVOH is a value measured in accordance with JIS K6726 (wherein EVOH is a solution uniformly dissolved in a water / methanol solvent), and from the viewpoints of barrier property, thermal stability, and moisture resistance, it is preferably 80 mol% or more, more preferably 90 mol% or more, even more preferably 98 mol% or more, and particularly preferably 99 mol% or more, and may be 100 mol%.

[0020] The melt flow rate (MFR) of EVOH (210°C, load 2160 g) is usually 0.5 to 100 g / 10 min, preferably 1 to 50 g / 10 min, and particularly preferably 3 to 35 g / 10 min. If the MFR is too high, film-forming properties tend to become unstable, whereas if it is too low, the viscosity tends to become too high, making melt extrusion difficult.

[0021] EVOH may be copolymerized with a polymerizable monomer other than ethylene and vinyl ester within a range that does not impair the effects of the present invention, generally within a range of 5 mol % or less. Examples of such polymerizable monomers include α-olefins such as propylene, isobutene, α-octene, α-dodecene, and α-octadecene; hydroxyl-containing α-olefins such as 3-buten-1-ol, 4-penten-1-ol, and 3-butene-1,2-diol, and hydroxyl-containing α-olefin derivatives such as esters and acylates thereof; hydroxymethylvinylidene diacetates such as 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, and 1,3-dibutyronyloxy-2-methylenepropane; unsaturated carboxylic acids or salts thereof, partial alkyl esters, complete alkyl esters, nitriles, amides, or anhydrides; unsaturated sulfonic acids or salts thereof; vinylsilane compounds; vinyl chloride; and styrene. These may be used alone or in combination of two or more.

[0022] Furthermore, EVOH that has been "post-modified" by urethanization, acetalization, cyanoethylation, oxyalkylenation, or the like can also be used.

[0023] The EVOH contained in the resin composition (A) has a single ethylene unit content. In other words, the resin composition (A) does not contain two or more types of EVOH with different ethylene unit contents. If the resin composition (A) contains two or more types of EVOH with different ethylene unit contents, the transparency of the uniaxially stretched multilayer structure will decrease. In the present invention, "having a single ethylene unit content" means that the melting peak in differential scanning calorimetry (DSC) is a single peak having only one apex, and two points at half the height (H / 2) of the height (H) from the baseline to the apex of the peak are within ±10°C of the apex temperature. In the above definition, the DSC conditions are as follows:

[0024] DSC conditions: The sample was heated from 30°C to 230°C at a rate of 10°C / min, held at this temperature for 1 minute, cooled to 30°C at a rate of 10°C / min, held at this temperature for 1 minute, and then heated again from 30°C to 230°C at a rate of 10°C / min. The vertical axis of the second heating chart shows heat capacity, and the horizontal axis shows temperature. The shape of the melting peak was confirmed, and the apex temperature of the melting peak was read. The baseline was a straight line connecting a point 20°C higher than the apex temperature of the melting peak (point a) and a point 80°C lower than the apex temperature of the melting peak (point b). Two or more EVOHs with different vinyl ester types, physical properties, etc., may be mixed together as long as they have the same ethylene unit content.

[0025] [Thermoplastic resin (B) layer] The uniaxially stretched multilayer structure of the present invention may have a thermoplastic resin (B) layer. By having a thermoplastic resin (B) layer, the mechanical strength and barrier properties against water vapor of the uniaxially stretched multilayer structure tend to be further improved. In addition, properties such as heat sealability and mechanical strength can be imparted depending on the type of thermoplastic resin constituting the thermoplastic resin (B) layer.

[0026] Examples of thermoplastic resins used in the thermoplastic resin (B) layer include polyethylenes such as linear low-density polyethylene, low-density polyethylene, very low-density polyethylene, medium-density polyethylene, and high-density polyethylene; ethylene-vinyl acetate copolymers; ionomers; ethylene-propylene (block or random) copolymers; ethylene-(meth)acrylic acid copolymers; ethylene-(meth)acrylic acid ester copolymers; polypropylene; propylene-α-olefin copolymers; polybutene; and polypentene; homo- or copolymers of these olefins, or polyolefins graft-modified with unsaturated carboxylic acids or their esters; polyesters; polyamides (including copolymerized polyamides); polyvinyl chloride; polyvinylidene chloride; acrylic resins; polystyrene; polyvinyl esters; polyester elastomers; polyurethane elastomers; chlorinated polystyrene; chlorinated polypropylene; aromatic polyketones or aliphatic polyketones, and polyalcohols obtained by reducing these; polyacetals; and polycarbonates. Among these, polyolefins are preferred from the viewpoint of excellent heat-sealing properties and mechanical properties, and polypropylene and polyethylene are more preferred. These can be used alone or in combination of two or more.

[0027] The content of the thermoplastic resin in the thermoplastic resin (B) layer is not particularly limited, but preferably the thermoplastic resin is the main component (i.e., the content of the thermoplastic resin contained in the thermoplastic resin (B) layer is 50% by mass or more). The content of the thermoplastic resin is more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, 90% by mass or more, 95% by mass or more, or 100% by mass or more.

[0028] The melt flow rate (MFR, measured at 190°C under a load of 2160 g according to JIS K 7210:2014) of the thermoplastic resin constituting the thermoplastic resin (B) layer is not particularly limited, but is preferably 0.10 to 10.0 g / 10 min, more preferably 0.30 to 5.0 g / 10 min. When the MFR of the thermoplastic resin constituting the thermoplastic resin (B) layer is in the above range, molding stability tends to be better.

[0029] The thickness of the thermoplastic resin (B) layer is not particularly limited, but is preferably 7 to 100 μm, more preferably 10 to 50 μm, from the viewpoints of industrial productivity and mechanical properties. The preferred thickness of the thermoplastic resin (B) layer refers to the thickness after stretching. The thermoplastic resin (B) layer may be one layer or multiple layers. When multiple thermoplastic resin (B) layers are present, the total thickness thereof is preferably within the above range.

[0030] [Adhesive Resin (C) Layer] The uniaxially stretched multilayer structure of the present invention may have an adhesive resin (C) layer. By having an adhesive resin (C) layer, the appearance properties (film surface) of the uniaxially stretched multilayer structure tend to be better.

[0031] The adhesive resin constituting the adhesive resin (C) layer is not particularly limited, but is preferably a polyolefin having a carboxy group, a carboxylic anhydride group, or an epoxy group, and more preferably a polyolefin having a carboxylic anhydride group. Such adhesive resins tend to have excellent adhesion to the resin composition (A) layer or the thermoplastic resin (B) layer.

[0032] Examples of polyolefins containing carboxyl groups include polyolefins copolymerized with acrylic acid or methacrylic acid. In this case, all or part of the carboxyl groups contained in the polyolefin may be present in the form of a metal salt, as typified by ionomers. Examples of polyolefins having carboxylic acid anhydride groups include polyolefins graft-modified with maleic anhydride or itaconic acid. Examples of polyolefins having epoxy groups include polyolefins copolymerized with glycidyl methacrylate. Among these, polyolefins having carboxylic acid anhydride groups such as maleic anhydride are preferred, and polyethylenes having carboxylic acid anhydride groups are particularly preferred. These can be used alone or in combination of two or more.

[0033] The content of the adhesive resin in the adhesive resin (C) layer is not particularly limited, but it is preferable that the adhesive resin be the main component (i.e., the content of the adhesive resin contained in the adhesive resin (C) is 50% by mass or more). The content of the adhesive resin is more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, 90% by mass or more, 95% by mass or more, or 100% by mass or more.

[0034] The melt flow rate (MFR, measured at 190°C under a load of 2160 g according to JIS K 7210:2014) of the adhesive resin constituting the adhesive resin (C) is not particularly limited, but is preferably 0.1 to 20.0 g / 10 min, more preferably 1.0 to 10.0 g / 10 min. When the MFR of the adhesive resin (C) is in the above range, molding stability tends to be better.

[0035] The thickness of the adhesive resin (C) layer is not particularly limited, but from the viewpoints of industrial productivity and quality stability, it is preferably 0.5 to 20 μm, more preferably 1 to 10 μm. Note that the above-mentioned preferred thickness of the adhesive resin (C) layer means the thickness after stretching. When the uniaxially stretched multilayer structure of the present invention has a plurality of resin composition (A) layers and thermoplastic resin (B) layers, the adhesive resin (C) layer may be provided between the respective layers, and the number of adhesive resin (C) layers in the uniaxially stretched multilayer structure of the present invention is not particularly limited.

[0036] [Uniaxially Stretched Multilayer Structure] The layer structure of the uniaxially stretched multilayer structure of the present invention is not particularly limited as long as it includes a resin composition (A) layer. By providing a resin composition (A) layer, a uniaxially stretched multilayer structure with excellent transparency can be obtained. For example, the following layer structures are exemplified as the layer structure of the uniaxially stretched multilayer structure. The resin composition (A) layer is referred to as the "(A) layer," the thermoplastic resin (B) layer as the "(B) layer," and the adhesive resin (C) layer as the "(C) layer." In the examples below, thermoplastic resin (B) and adhesive resin (C) layers are exemplified as layers other than the resin composition (A) layer, but the layers other than the resin composition (A) layer are not limited to these. It is also preferable not to include a layer whose main component is polyamide (i.e., the polyamide content in the layer is 50% by mass or more). The " / " symbol indicates that the layers on both sides of the symbol are directly laminated. Examples of the layer configuration of the uniaxially stretched multilayer structure of the present invention include (B) layer / (A) layer, (B) layer / (A) layer / (B) layer, (B) layer / (C) layer / (A) layer / (B) layer, (B) layer / (C) layer / (A) layer / (C) layer / (B) layer, (B) layer / (A) layer / (B) layer / (A) layer / (B) layer, (B) layer / (B) layer / (C) layer / (A) layer / (C) layer / (B) layer / (B) layer, and the like.

[0037] The total thickness of the uniaxially stretched multilayer structure of the present invention is not particularly limited and can be appropriately set depending on the application. The total thickness is preferably 10 μm or more, more preferably 15 μm or more. A total thickness of 10 μm or more tends to further improve industrial productivity and mechanical properties. Furthermore, the total thickness is preferably 100 μm or less, more preferably 50 μm or less. A total thickness of 100 μm or less tends to further improve industrial productivity and economic efficiency. The preferred total thickness of the uniaxially stretched multilayer structure above refers to the thickness after stretching.

[0038] The internal haze value of the uniaxially stretched multilayer structure of the present invention is preferably less than 0.4%, more preferably 0.3% or less, and particularly preferably 0.2% or less, 0%. Such measurement can be performed using a multilayer structure having a thickness of 33 μm and a haze meter in accordance with ASTM D1003.

[0039] The method for producing the uniaxially stretched multilayer structure of the present invention is not particularly limited, and generally, a conventional coextrusion method can be used in which each resin is extruded through a separate die or a common die and laminated. Examples of dies that can be used include an annular die and a T-die. Examples of molding methods include extrusion molding, blow molding, injection molding, thermoforming, cast molding, and inflation molding.

[0040] An example of a method for producing a uniaxially stretched multilayer structure of the present invention includes the following steps (1) and (2): step (1): melt-extruding a resin composition (A) and a thermoplastic resin (B) to form a multilayer structure intermediate; and step (2): uniaxially stretching the multilayer structure intermediate formed in step (1) to produce a uniaxially stretched multilayer structure. Note that step (1) may also be melt-extruding a resin composition (A), a thermoplastic resin (B), and an adhesive resin (C) to form a multilayer structure intermediate. When the multilayer structure intermediate formed in step (1) is cylindrical, the method may further include, in addition to steps (1) and (2), cutting at least a portion of the cylindrical multilayer structure to obtain a planar multilayer structure. Note that the multilayer structure intermediate in the present invention refers to a multilayer structure before being stretched in the uniaxial direction.

[0041] The uniaxially stretched multilayer structure of the present invention is uniaxially stretched. The stretching ratio is not particularly limited, but a stretching ratio of 3 to 12 times is preferred. If the multilayer structure of the present invention is stretched less than 3 times, thickness unevenness due to stretching tends to occur and gas barrier properties tend to be reduced. On the other hand, if the multilayer structure of the present invention is stretched more than 12 times, the film surface after stretching tends to deteriorate. The uniaxially stretched multilayer structure of the present invention is preferably stretched uniaxially by 4 times or more, more preferably by 5 times or more. Furthermore, the uniaxially stretched multilayer structure of the present invention is preferably stretched uniaxially by 10 times or less, more preferably by 8 times or less. The uniaxially stretched multilayer structure of the present invention has advantages such as economic efficiency, ease of tearing the multilayer structure (making it easy to open the packaging material when used as a packaging material), and improved gas barrier properties. The stretching direction of the uniaxially stretched multilayer structure is not particularly limited and may be either the machine direction (MD) or the width direction (TD), with uniaxial stretching in the machine direction (MD) being preferred. It is preferred that the structure is not substantially stretched in a direction different from the stretching direction. The stretched state of the multilayer structure (unstretched, uniaxially stretched, biaxially stretched, etc.) can be confirmed by using a general method for analyzing the orientation of a resin (e.g., wide-angle X-ray scattering (WAXS)).

[0042] The stretching method for the uniaxially stretched multilayer structure of the present invention is not particularly limited, and examples thereof include tenter stretching, tubular stretching, and roll stretching. From the viewpoint of production cost, uniaxial stretching by roll stretching is preferred. Furthermore, when the multilayer structure of the present invention is an inflation-molded article, roll stretching is preferred.

[0043] The stretching temperature when stretching the multilayer structure is not particularly limited, but a temperature range of 50 to 130°C is generally adopted.

[0044] [Multilayer structure] The uniaxially stretched multilayer structure of the present invention may be a multilayer structure in which another layer is further laminated. The other layer to be laminated is not particularly limited, and may be a layer containing a thermoplastic resin exemplified as the thermoplastic resin (B) or an adhesive resin exemplified as the adhesive resin (C) described above.

[0045] As a method for producing the multilayer structure, various known production methods can be used, such as dry lamination, sand lamination, extrusion lamination, coextrusion lamination, and solution coating.

[0046] [Uses] The uniaxially stretched multilayer structure of the present invention and a multilayer structure containing the same can be suitably used as packaging materials such as various packaging containers and packaging films for general foods, as well as seasonings such as mayonnaise and dressings, fermented foods such as miso, oily foods such as salad oil, snacks, beverages, cosmetics, pharmaceuticals, etc., or as part of laminates thereof.

[0047] Specific embodiments of the present invention will be described in more detail below using examples, but the present invention is not limited to the following examples as long as the gist of the invention is not exceeded. Note that the values ​​of various production conditions and evaluation results in the following examples represent preferred upper or lower limit values ​​in the embodiments of the present invention, and preferred ranges may be defined by a combination of the above-mentioned upper or lower limit values ​​and the values ​​in the following examples or values ​​between examples.

[0048] The materials used in the following Examples, Comparative Examples and Reference Examples are as follows:

[0049] <EVOH> EVOH (A-1): ethylene unit content 29 mol%, MFR (210 ° C, load 2.16 kg): 3.8 g / 10 min, density: 1.21 g / cm 3 EVOH (A-2): ethylene unit content 25 mol%, MFR (210 ° C, load 2.16 kg): 4.0 g / 10 min, density: 1.22 g / cm 3 EVOH (A-3): ethylene unit content 32 mol%, MFR (210 ° C, load 2.16 kg): 3.8 g / 10 min, density: 1.19 g / cm 3 EVOH (A-4): ethylene unit content 38 mol%, MFR (210 ° C, load 2.16 kg): 4.0 g / 10 min, density: 1.17 g / cm 3, saponification degree 99.9 mol% <Thermoplastic resin> Polyethylene (B): "Novatec (registered trademark) HY540" manufactured by Mitsubishi Chemical Corporation, MFR (190°C, load 2.16 kg): 1.0 g / 10 min <Adhesive resin> Acid-modified polyethylene resin (C): "Modic (registered trademark) M545" manufactured by Mitsubishi Chemical Corporation, MFR (190°C, load 2.16 kg): 6.0 g / 10 min

[0050] [Production of Uniaxially Stretched Multilayer Structure] (A-1), (B), and (C) were fed into a multilayer cast film apparatus (manufactured by Plastics Engineering Research Institute Co., Ltd.), and multilayer coextrusion was performed to obtain a multilayer structure intermediate having a three-type, five-layer structure of (B) layer / (C) layer / (A-1) layer / (C) layer / (B layer). The thicknesses (μm) of the individual layers of the obtained multilayer structure intermediate were 80 / 10 / 20 / 10 / 80. Next, using a stretching apparatus (manufactured by MEC Techno Co., Ltd.), the obtained multilayer structure intermediate was passed between a roll heated to a surface temperature (stretching temperature) of 120°C and another roll rotating at a different speed, and stretched 6 times in the longitudinal direction (MD direction), thereby obtaining a uniaxially stretched multilayer structure. The thicknesses (μm) of the individual layers of the obtained uniaxially stretched multilayer structure were 13.3 / 1.6 / 3.3 / 1.6 / 13.3.

[0051] (Multilayer co-extrusion molding conditions) (A-1) layer: 40 mmφ single screw extruder (barrel temperature: 210°C) (B) layer: 40 mmφ single screw extruder (barrel temperature: 230°C) (C) layer: 32 mmφ single screw extruder (barrel temperature: 200°C) Die: 4-type 5-layer feed block type T die (die temperature: 230°C) Take-up speed: 4 m / min

[0052] Examples 2 to 5, Comparative Examples 1 and 2 Uniaxially stretched multilayer structures were produced in the same manner as in Example 1, except that the types of EVOH and stretching conditions were as shown in Table 1.

[0053] Reference Example 1 A uniaxially stretched multilayer structure was produced in the same manner as in Example 1, except that a tenter-type biaxial stretching machine (manufactured by Brukner) was used and the stretching conditions were as shown in Table 1. The stretching ratio "2 x 3" shown in Table 1 means that the film was stretched 2 times in the MD direction and 3 times in the TD direction, for a total stretching of 6 times.

[0054] The multilayer structure intermediates of Examples 1 to 5, Comparative Examples 1 and 2, and Reference Example 1 were evaluated for stretchability and the internal haze of the uniaxially stretched multilayer structures by the following methods. The results are shown in Table 1.

[0055] [Evaluation Method] (Possibility of Stretching) The uniaxially stretched multilayer structure (length 300 mm, width 210 mm) prepared above was visually inspected for holes, and the presence of holes was evaluated as × (Poor), and the absence of holes was evaluated as ○ (Good).

[0056] (Transparency: Internal Haze) Using the uniaxially stretched multilayer structure having a thickness of 33 μm prepared above, the internal haze was measured three times using a haze meter in accordance with ASTM D1003, and the average value was calculated. The internal haze is an index for evaluating the degree of white turbidity, and a smaller internal haze value indicates better transparency of the uniaxially stretched multilayer structure.

[0057]

[0058] Examples 1 to 5 show that the use of EVOH having an ethylene unit content of less than 32 mol% results in uniaxially stretched multilayer structures with superior transparency compared to Comparative Examples 1 and 2, which used EVOH having an ethylene unit content of 32 mol% or more. Furthermore, in biaxial stretching, when EVOH having an ethylene unit content of less than 32 mol% was used, holes were generated and stretching was impossible (Reference Example 1). Therefore, it is clear that the effects of the present invention obtained by using EVOH having an ethylene unit content of less than 32 mol% are unique to uniaxially stretched multilayer structures.

[0059] EVOH with a low ethylene unit content tends to have a higher melting point than EVOH with a high ethylene unit content, which is thought to make it easier to stretch and align the molecules neatly, resulting in excellent transparency.On the other hand, EVOH with a high ethylene unit content is less likely to stretch and align the molecules neatly, resulting in poor transparency.

[0060] 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.

Claims

1. A multilayer structure in which each layer is uniaxially stretched, The multilayer structure has a layer of a resin composition (A) containing an ethylene-vinyl alcohol copolymer, the ethylene unit content of the ethylene-vinyl alcohol copolymer in the resin composition (A) layer is less than 32 mol%, The ethylene-vinyl alcohol copolymer has a single ethylene unit content and is a uniaxially stretched multilayer structure.

2. 10. The multilayer structure according to claim 1, wherein the multilayer structure is uniaxially stretched 3 to 12 times.

3. 3. The multilayer structure according to claim 1, wherein the resin composition (A) is mainly composed of an ethylene-vinyl alcohol copolymer.

4. The multilayer structure according to claim 1 or 2, further comprising a thermoplastic resin (B) layer.

5. The multilayer structure according to claim 1 or 2, further comprising an adhesive resin (C) layer.

6. The multilayer structure according to claim 1 or 2, wherein the multilayer structure does not include a layer containing polyamide as a main component.

7. A method for producing the multilayer structure according to claim 1 or 2, comprising the following steps (1) and (2): Step (1): A step of melt-extruding the resin composition (A) and the thermoplastic resin (B) to form a multilayer structure intermediate. Step (2): A step of uniaxially stretching the multilayer structure intermediate to produce a uniaxially stretched multilayer structure.

8. 8. The method for producing a multilayer structure according to claim 7, wherein the step (1) is a step of melt-extruding the resin composition (A), the thermoplastic resin (B), and the adhesive resin (C) to form an intermediate multilayer structure.