Multilayer structure, use for multilayer structure, and production method for multilayer structure

Abstract

A multilayer structure according to the present invention includes at least a barrier layer (A), polyolefin adhesive resin layers (B1, B2), polypropylene layers (C1, C2), a polyurethane adhesive resin layer (D), and a sealant layer (E), the layers being arranged in the order C1 / B1 / A / B2 / C2 / D / E. The barrier layer (A) includes an ethylene / vinyl alcohol copolymer (a1) and a polyamide (a2), the mass ratio (a1 / a2) of the ethylene / vinyl alcohol copolymer (a1) and the polyamide (a2) being 70 / 30–95 / 5, and the ethylene unit content of the ethylene / vinyl alcohol copolymer (a1) being 20–40 mol%. The sealant layer (E) includes polypropylene. The present invention thereby provides a multilayer structure that has excellent gas barrier properties after retort processing and also has excellent recyclability.

Description

Multilayer structure, its uses and manufacturing method

[0001] The present invention relates to a multilayer structure having a barrier layer containing an ethylene-vinyl alcohol copolymer and a polypropylene layer. It also relates to a heat-sealable packaging material comprising the multilayer structure. It also relates to a packaging container comprising the multilayer structure. It also relates to a package formed by filling the packaging container with contents. It also relates to a method for producing the multilayer structure.

[0002] Glass, metal, and metal foil are still widely used as materials for containers for retorting foods, etc. However, recently, plastic containers have become more commonly used as containers for retorting foods, etc. Among these, ethylene-vinyl alcohol copolymer (hereinafter sometimes abbreviated as EVOH) is widely used as a barrier resin for plastic containers because of its good processability and excellent gas barrier properties.

[0003] Due to its chemical structure, EVOH is known to exhibit a significant decrease in its gas barrier properties in high-temperature, high-relative-humidity environments. This is thought to be because water acts as a plasticizer for EVOH, weakening the hydrogen bonds in the amorphous regions of EVOH and increasing the free volume, thereby increasing gas diffusion within the polymer matrix. Therefore, when a package is subjected to a typical steam retort process at 110-132°C for 15-80 minutes, the oxygen transmission rate of EVOH increases dramatically, causing oxidative deterioration of food, resulting in a loss of flavor and a shortened shelf life.

[0004] When using a container made of a multilayer structure including an EVOH layer, the gas barrier properties of the container can be maintained appropriately by keeping the EVOH layer as dry as possible. Patent Document 1 describes a container for retort food, which has inner and outer layers made of polymer films and an intermediate layer made of an EVOH film, and is made of a thin multilayer structure in which the polymer film of the outer layer has higher moisture permeability than the polymer film of the inner layer. It is described that use of this container shortens the drying time of the EVOH layer after retort treatment, and the oxygen barrier properties of the container are restored in a short time. Polyamide, polyester, and polycarbonate are described as materials for the outer layer with high moisture permeability, and polyolefin is described as a material for the inner layer with low moisture permeability.

[0005] Meanwhile, driven by environmental and waste issues, there has been a global increase in demand for so-called post-consumer recycling (hereinafter sometimes simply referred to as "recycling"), which involves recovering and recycling packaging materials consumed in the market. Recycling typically involves cutting recovered packaging materials, separating and washing them as necessary, and then melt-kneading them using an extruder. However, when different types of materials are used for the inner and outer layers, as in the multilayer structure described in Patent Document 1, the materials are not compatible with each other when recovered and melt-kneaded, making recycling difficult. Furthermore, melt-kneading is even more difficult when there is a large difference in melting point between the resin contained in the outer layer and the resin contained in the inner layer. Therefore, there is a need for a multilayer structure that uses EVOH for the intermediate layer and polyolefin for the inner and outer layers, yet has a low oxygen transmission rate after retort treatment.

[0006] Patent Document 2 describes a multilayer structure having a layer containing polypropylene (hereinafter sometimes abbreviated as PP), an oxygen barrier layer containing EVOH, and a sealant layer containing polypropylene laminated in this order, and is said to have good gas barrier properties after retort treatment and good recyclability. However, the gas barrier properties after retort treatment may still be insufficient depending on the application, and improvement in this regard has been desired.

[0007] JP-A-2-231138 WO 2022 / 054887 A1

[0008] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a multilayer structure that has excellent gas barrier properties after retort treatment and is also highly recyclable, as well as a packaging container and a packaging body containing the same, and to provide a method for producing such a multilayer structure.

[0009] The present invention is as follows: [1] A multilayer structure comprising at least a barrier layer (A), polyolefin-based adhesive resin layers (B1, B2), polypropylene layers (C1, C2), a polyurethane-based adhesive resin layer (D), and a sealant layer (E), arranged in the order of C1 / B1 / A / B2 / C2 / D / E, wherein the barrier layer (A) comprises an ethylene-vinyl alcohol copolymer (a1) and a polyamide (a2), and the mass ratio (a1 / a2) of the ethylene-vinyl alcohol copolymer (a1) to the polyamide (a2) is 70 / 30 or more and 95 / 5 or less, the ethylene unit content of the ethylene-vinyl alcohol copolymer (a1) is 20 mol % or more and 40 mol % or less, and the sealant layer (E) comprises polypropylene. [2] The multilayer structure according to [1], wherein the barrier layer (A) contains a magnesium salt in an amount of 10 ppm or more and 1,400 ppm or less, calculated as magnesium. [3] The multilayer structure according to [2], wherein the magnesium salt is magnesium hydroxide. [4] The multilayer structure according to any one of [1] to [3], wherein the ratio (T1 / T2) of the total thickness of the layers on the polypropylene layer (C1) side of the barrier layer (A) to the total thickness of the layers on the sealant layer (E) side of the barrier layer (A) is 0.3 or less. [5] The multilayer structure according to any one of [1] to [4], wherein the ratio of the thickness of the barrier layer (A) to the total thickness is 10% or less. [6] The multilayer structure according to any one of [1] to [5], wherein the polyamide (a2) is nylon 6. [7] The multilayer structure according to any one of [1] to [6], wherein the sealant layer (E) is made of unstretched polypropylene. [8] The multilayer structure is subjected to retort sterilization at 120°C for 30 minutes, and then the sealant layer (E) side is maintained at 20°C and 100% RH, and the polypropylene layer (C1) side is maintained at 20°C and 65% RH for 2 weeks, respectively. After that, the oxygen transmission rate measured in accordance with JIS K7126-2 (constant pressure method; 2006) is 5 cc / m or less. ２

[10] A packaging container comprising the multilayer structure according to any one of [1] to [8], wherein the sealant layers (E) are melted and sealed together.

[11] The packaging container according to

[10] , which is a pouch.

[12] The packaging container according to

[10] or

[11] , which is for use in a sterilization treatment with steam or hot water.

[13] A package obtained by filling the packaging container according to any one of

[10] to

[12] .

[14] A method for producing a multilayer structure according to any one of [1] to [8], comprising producing a multilayer film by coextrusion molding, the multilayer film including at least a barrier layer (A), polyolefin-based adhesive resin layers (B1, B2), and polypropylene layers (C1, C2), in which these layers are arranged in the order of C1 / B1 / A / B2 / C2, and bonding a polypropylene film to the polypropylene layer (C2) side of the multilayer film using a polyurethane-based adhesive resin.

[0010] The multilayer structure of the present invention has excellent gas barrier properties after retort treatment and is also highly recyclable. Therefore, a packaging container containing the multilayer structure is suitable for packaging and storing foods and other items that require treatment under high temperature and humidity, such as retort treatment, for long periods of time. Furthermore, such a multilayer structure can be produced by the production method of the present invention.

[0011] The multilayer structure of the present invention comprises at least a barrier layer (A), polyolefin-based adhesive resin layers (B1, B2), polypropylene layers (C1, C2), a polyurethane-based adhesive resin layer (D), and a sealant layer (E), with these layers arranged in the order of C1 / B1 / A / B2 / C2 / D / E. The most notable feature of the layer structure of the multilayer structure of the present invention is that the polyurethane-based adhesive resin layer (D) is located between the polypropylene layer (C2) and the sealant layer (E) containing polypropylene. The present inventors have found that the adoption of such a layer structure improves the gas barrier property after retort treatment.

[0012] This point will be explained by comparing Example 1 and Comparative Example 3 of the present specification. In the layer structure of the multilayer structure of Example 1, a polypropylene layer (C2: 15 μm), a polyurethane adhesive resin layer (D: 4 μm), and a polypropylene layer (sealant layer (E): 70 μm) are arranged from a barrier layer (A: 10 μm) via a polyolefin adhesive resin layer (B2: 5 μm), and a 4 μm polyurethane adhesive resin layer is inserted between the polypropylene layers with a total thickness of 85 μm. On the other hand, in the layer structure of the multilayer structure of Comparative Example 3, a polypropylene layer (C2: 85 μm) is arranged from a barrier layer (A: 10 μm) via a polyolefin adhesive resin layer (B2: 5 μm), and no polyurethane adhesive resin layer is included. As described above, the only difference between the layer structure of Example 1 and that of Comparative Example 3 is whether or not a thin (4 μm) polyurethane adhesive resin layer is inserted inside a thick (85 μm) polypropylene layer. However, the presence of this polyurethane adhesive resin layer reduces the oxygen transmission rate (OTR) after retort treatment to less than half.

[0013] From this, it is believed that by disposing the polyurethane adhesive resin layer (D) between the polypropylene layer (C2) and the polypropylene layer (sealant layer (E)), moisture was rapidly expelled from the barrier layer (A) (a resin composition layer containing EVOH (a1) and polyamide (a2)) after retort treatment. It is surprising that, since there is no need to provide an adhesive layer between polypropylene layers, which are layers made of the same type of resin, the OTR after retort treatment was significantly reduced by deliberately disposing a polyurethane adhesive resin layer. Although the term "retort treatment" is used in this specification, the effects of the present invention can also be achieved when other treatments using steam or hot water under high temperature and humidity are performed. The present invention will be described in detail below.

[0014] [Barrier Layer (A)] The barrier layer (A) contains EVOH (a1) and polyamide (a2). EVOH (a1) is typically obtained by saponifying an ethylene-vinyl ester copolymer obtained by polymerizing ethylene and a vinyl ester. The ethylene unit content of EVOH (a1) is 20 mol% or more and 40 mol% or less. If the ethylene unit content is less than 20 mol%, the melt moldability of the multilayer structure deteriorates. The ethylene unit content is preferably 25 mol% or more. On the other hand, if the ethylene unit content exceeds 40 mol%, the gas barrier property of the multilayer structure deteriorates. The ethylene unit content is preferably 35 mol% or less. The saponification degree of EVOH (a1) is preferably 85 mol% or more. The saponification degree refers to the ratio of the number of vinyl alcohol units to the total number of vinyl alcohol units and vinyl ester units in EVOH (a1). If the saponification degree is 85 mol% or more, the gas barrier property of the multilayer structure is further improved. The saponification degree is more preferably 95 mol % or more, and further preferably 99 mol % or more. The ethylene unit content and saponification degree of EVOH (a1) are as follows: １ It is determined by H-NMR measurement.

[0015] The MFR of EVOH (a1) (210°C, under a load of 2.16 kg) is preferably 0.1 g / 10 min or more and 50 g / 10 min or less. The MFR of EVOH (a1) is more preferably 1 g / 10 min or more, and even more preferably 2 g / 10 min or more. On the other hand, the MFR of EVOH (a1) is more preferably 30 g / 10 min or less, and even more preferably 15 g / 10 min or less. When the MFR of EVOH (a1) is in the above range, the melt moldability of the multilayer structure is further improved.

[0016] EVOH (a1) may be a mixture of multiple types of EVOH. In that case, the average values ​​of the ethylene unit content, degree of saponification, and MFR may be within the above ranges.

[0017] EVOH (a1) may contain other monomer units in addition to ethylene, vinyl ester, and vinyl alcohol, as long as the effects of the present invention are not impaired. The content of other monomer units is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and particularly preferably substantially none. Examples of other monomer units include α-olefins such as propylene, 1-butene, 1-hexene, and 4-methyl-1-pentene; (meth)acrylic acid esters; unsaturated carboxylic acids such as maleic acid, fumaric acid, and itaconic acid; alkyl vinyl ethers; N-(2-dimethylaminoethyl)methacrylamide or a quaternary product thereof, N-vinylimidazole or a quaternary product thereof, N-vinylpyrrolidone, N,N-butoxymethylacrylamide, vinyltrimethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane.

[0018] The barrier layer (A) contains a polyamide (a2) in addition to an EVOH (a1). The small amount of polyamide contained in the barrier layer (A) suppresses whitening and wrinkles even after retort treatment, improving the appearance of the multilayer structure. The mass ratio (a1 / a2) of the EVOH (a1) to the polyamide (a2) is 70 / 30 or more and 95 / 5 or less. If the mass ratio (a1 / a2) is less than 70 / 30, the recyclability and gas barrier properties are reduced. The mass ratio (a1 / a2) is preferably 75 / 25 or more, more preferably 80 / 20 or more. On the other hand, if the mass ratio (a1 / a2) exceeds 95 / 5, the appearance may be poor after retort treatment. The mass ratio (a1 / a2) is preferably 93 / 7 or less.

[0019] Examples of the polyamide (a2) include polycaproamide (nylon 6), poly-ω-aminoheptanoic acid (nylon 7), poly-ω-aminononanoic acid (nylon 9), polyundecaneamide (nylon 11), polylauryllactam (nylon 12), polyethylenediamineadipamide (nylon 26), polytetramethyleneadipamide (nylon 46), polyhexamethyleneadipamide (nylon 66), polyhexamethylenesebacamide (nylon 610), and polyhexamethylenedodecaamide (nylon 61). 2), Polyoctamethylene adipamide (Nylon 86), Polydecamethylene adipamide (Nylon 106), Caprolactam / Lauryl lactam copolymer (Nylon 6 / 12), Caprolactam / ω-aminononanoic acid copolymer (Nylon 6 / 9), Caprolactam / Hexamethylenediammonium adipate copolymer (Nylon 6 / 66), Lauryl lactam / Hexamethylenediammonium adipate copolymer (Nylon 12 / 66), Ethylenediammonium adipate / Hexamethylenediammonium ammonium adipate copolymer (nylon 26 / 66), caprolactam / hexamethylenediammonium adipate / hexamethylenediammonium sebacate copolymer (nylon 6 / 66 / 610), ethylenediammonium adipate / hexamethylenediammonium adipate / hexamethylenediammonium sebacate copolymer (nylon 26 / 66 / 610), polyhexamethylene isophthalamide (nylon 6I), polyhexamethylene terephthalamide (nylon 6T), hexamethylene Examples of the polyamide include 11-aminoundecaneamide / hexamethylene terephthalamide copolymer (nylon 6I / 6T), 11-aminoundecaneamide / hexamethylene terephthalamide copolymer, polynonamethylene terephthalamide (nylon 9T), polydecamethylene terephthalamide (nylon 10T), polyhexamethylene cyclohexylamide, polynonamethylene cyclohexylamide, and polyamides modified with aromatic amines such as methylenebenzylamine and metaxylylenediamine. Further examples include metaxylylenediammonium adipate.The polyamide is preferably an aliphatic polyamide, more preferably a polyamide mainly composed of caproamide, and even more preferably a polyamide in which 75 mol % or more of the constituent units of the polyamide are caproamide units. Among these, from the viewpoint of compatibility with EVOH, the polyamide is preferably nylon 6.

[0020] The barrier layer (A) preferably contains 10 ppm or more and 1400 ppm or less of magnesium salt in terms of magnesium. By containing 10 ppm or more of magnesium salt, gel formation can be suppressed when melt-molding for a long time, and long-run properties are improved. The content of magnesium salt is more preferably 20 ppm or more, and even more preferably 30 ppm or more, in terms of magnesium. On the other hand, if the content of magnesium salt is too high, the color may deteriorate when melt-molding for a long time. The content of magnesium salt is more preferably 500 ppm or less, and even more preferably 200 ppm or less, in terms of magnesium. In this specification, "ppm" means "ppm by mass".

[0021] Examples of magnesium salts include magnesium hydroxide, magnesium carboxylate, magnesium sulfate, magnesium carbonate, etc. From the viewpoint of long-term running properties, magnesium hydroxide and magnesium carboxylate are preferred, and magnesium hydroxide is more preferred. Examples of magnesium carboxylates include magnesium acetate, magnesium propionate, magnesium butyrate, magnesium stearate, etc.

[0022] The barrier layer (A) contains an oxygen-absorbing resin having a carbon-carbon double bond, and may also contain a transition metal catalyst such as a cobalt salt that promotes oxidation, which may enable the layer to maintain high oxygen barrier properties even after retort treatment.

[0023] [Polyolefin-Based Adhesive Resin Layer (B)] The polyolefin-based adhesive resin layer (B) (hereinafter sometimes referred to as "Tie") is disposed between the barrier layer (A) and the polypropylene layer (C) to bond the two layers together. The resin contained in the polyolefin-based adhesive resin layer (B) is a polyolefin containing polar functional groups such as carboxyl groups, epoxy groups, and hydroxyl groups. Examples include carboxylic acid-modified polyolefins. Carboxylic acid-modified polyolefins refer to modified olefins containing carboxyl groups obtained by chemically bonding (e.g., by addition reaction or graft reaction) an ethylenically unsaturated carboxylic acid or its anhydride to a polyolefin. Examples of polyolefins include polyethylene (low pressure, medium pressure, high pressure), linear low-density polyethylene, polypropylene, and polybutene; and copolymers of olefins and comonomers copolymerizable with the olefins (e.g., vinyl esters, unsaturated carboxylic acid esters), such as ethylene-vinyl acetate copolymers and ethylene-acrylic acid ethyl ester copolymers. Among these, polypropylene is preferred. Examples of the ethylenically unsaturated carboxylic acid or its anhydride include ethylenically unsaturated monocarboxylic acid, ethylenically unsaturated dicarboxylic acid, its monoester, or its anhydride, among which ethylenically unsaturated dicarboxylic acid anhydride is preferred.Specific examples include maleic acid, fumaric acid, itaconic acid, maleic anhydride, itaconic anhydride, maleic acid monomethyl ester, maleic acid monoethyl ester, fumaric acid monomethyl ester, etc., and maleic anhydride is particularly preferred.The resins contained in the polyolefin adhesive resin layer (B1) and the polyolefin adhesive resin layer (B2) may be different or the same type.

[0024] [Polypropylene Layer] The polypropylene contained in the polypropylene layer (C) and the sealant layer (E) may be homopolypropylene or copolymer polypropylene copolymerized with other monomers such as ethylene. The content of units derived from the other monomers is usually 10% by mass or less, preferably 5% by mass or less. The copolymer polypropylene may be random copolymer polypropylene or block copolymer polypropylene. By using polypropylene, a packaging container with excellent water resistance, moisture resistance, and heat resistance can be provided. In addition, since the melting point is relatively low, it is also excellent in recyclability. The polypropylenes contained in the polypropylene layer (C1), the polypropylene layer (C2), and the sealant layer (E) may be different types or the same type.

[0025] [Polyurethane-Based Adhesive Resin Layer (D)] The polyurethane-based adhesive resin layer (D) is disposed between the polypropylene layer (C2) and the sealant layer (E) to bond the two layers together. The polyurethane contained in the layer (D) is not particularly limited as long as it is a polyurethane obtained by reacting a polyisocyanate with a polyol. It may be a one-component or two-component type, but from the viewpoint of adhesiveness, a two-component type is preferred. In the case of a two-component type, the polyisocyanate and the polyol are mixed prior to application and then applied to the surface of either or both of the polypropylene layer (C2) and the sealant layer (E). In this case, the mixture may be dissolved in an organic solvent or dispersed in water. In such cases, the organic solvent or water is removed after application, and then the polypropylene layer (C2) and the sealant layer (E) are superimposed and cured.

[0026] Each of the layers (A) to (E) included in the multilayer structure of the present invention may contain other components within a range that does not impair the effects of the present invention. Examples of other components include other resins, desiccants, dispersants, plasticizers, stabilizers, surfactants, colorants, UV absorbers, antistatic agents, crosslinking agents, metal salts, fillers, and reinforcing agents such as various fibers. The content of other components is usually less than 50% by mass, and preferably less than 10% by mass.

[0027] [Layer Structure] In the multilayer structure of the present invention, the layers (A) to (E) are arranged in the order of C1 / B1 / A / B2 / C2 / D / E. In this case, each layer may be composed of multiple layers. For example, the multilayer structure of the present invention also includes a case where the barrier layer (A) is composed of two layers, each of which contains a different ethylene unit content of EVOH, or a case where the polypropylene layer (C) is composed of two layers, each of which contains a different type of PP.

[0028] The multilayer structure of the present invention may further include layers other than the layers (A) to (E) described above, provided that the effects of the present invention are not impaired. For example, the combination of the polyurethane adhesive resin layer (D) and the sealant layer (E) may be repeated twice, resulting in a structure such as C1 / B1 / A / B2 / C2 / D / E / D / E. In this case, the incorporated polypropylene layer is no longer a sealant layer (E), but this multilayer structure is still included in the present invention.

[0029] Furthermore, to further improve the gas barrier property and water vapor barrier property, a vapor-deposited film of a metal such as aluminum, or an inorganic oxide such as silicon oxide or aluminum oxide may be formed on the surface of any of the layers (A) to (E). However, from the viewpoint of recyclability, it is preferable not to have a layer made of such an inorganic vapor-deposited film. Also, from the viewpoint of recyclability, it is preferable that the multilayer structure of the present invention does not include a layer whose main component is a resin with a melting point of 210°C or higher. Here, "mainly composed" means that the resin accounts for 50% by mass or more of the total. Furthermore, a printed layer may be formed on the surface of any of the layers (A) to (E).

[0030] The thickness of the barrier layer (A) is preferably 1 μm or more and 50 μm or less. When the barrier layer (A) is 1 μm or more, the gas barrier property is improved. The thickness of the barrier layer (A) is more preferably 2 μm or more, and even more preferably 4 μm or more. On the other hand, when the barrier layer (A) is 50 μm or less, the flexibility is improved and the weight of the multilayer structure is reduced. The thickness of the barrier layer (A) is more preferably 30 μm or less, and even more preferably 20 μm or less.

[0031] The thickness of the polypropylene layer (C1) is preferably 5 μm or more and 50 μm or less. The polypropylene layer (C1) is a layer disposed outside the barrier layer (A) when the container is made into a packaging container, and is a layer through which moisture absorbed by the barrier layer (A) after retort treatment passes. Therefore, if the polypropylene layer (C1) is too thick, it takes a long time for the moisture content of the barrier layer (A) increased during retort treatment to decrease, and during that time, the barrier property is reduced, which makes the contents, such as food, more susceptible to oxidative degradation. Therefore, it is preferable that the polypropylene layer (C1) is not too thick. The thickness of the polypropylene layer (C1) is more preferably 30 μm or less, and even more preferably 20 μm or less. On the other hand, in order to protect the barrier layer (A) from external mechanical impacts, it is preferable that the polypropylene layer (C1) is not too thin. The thickness of the polypropylene layer (C1) is more preferably 8 μm or more, and even more preferably 10 μm or more.

[0032] The thickness of the polypropylene layer (C2) is preferably 2 μm or more and 80 μm or less. The polypropylene layer (C2) is a layer disposed inside the barrier layer (A) and outside the polyurethane adhesive resin layer (D) when the packaging container is formed. It does not function like the polypropylene layer (C1) described above, and its thickness has a high degree of freedom. However, as described below, if it is co-extruded with the polypropylene layer (C1) and the barrier layer (A), it is easier to mold it with a thickness equal to that of the polypropylene layer (C1) in terms of balance. Therefore, the thickness of the polypropylene layer (C2) is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. The thickness of the polypropylene layer (C2) is preferably 5 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more.

[0033] The thickness of each of the polyolefin adhesive resin layers (B1, B2) is preferably 1 μm or more and 20 μm or less. From the viewpoint of good adhesion between the barrier layer (A) and the polypropylene layer (C), the thickness of the polyolefin adhesive resin layer (B) is preferably 1 μm or more, more preferably 2 μm or more. On the other hand, if the polyolefin adhesive resin layer (B) is too thick, the cost and mass of the multilayer structure increase. The thickness of the polyolefin adhesive resin layer (B) is preferably 20 μm or less, more preferably 10 μm or less.

[0034] The thickness of the sealant layer (E) is preferably 20 μm or more and 150 μm or less. When the sealant layer (E) is 20 μm or more, the adhesive strength during heat sealing can be increased. The thickness of the sealant layer (E) is more preferably 40 μm or more. On the other hand, when the sealant layer (E) is 150 μm or less, the flexibility is improved and the weight of the multilayer structure is reduced. The thickness of the sealant layer (E) is more preferably 120 μm or less, and even more preferably 100 μm or less.

[0035] The thickness of the polyurethane adhesive resin layer (D) is preferably 1 μm or more and 20 μm or less. From the viewpoint of good adhesion between the polypropylene layer (C) and the sealant layer (E), the thickness of the polyurethane adhesive resin layer (D) is preferably 1 μm or more, more preferably 2 μm or more. On the other hand, if the polyurethane adhesive resin layer (D) is too thick, the cost and mass of the multilayer structure increase. The thickness of the polyurethane adhesive resin layer (D) is preferably 20 μm or less, more preferably 10 μm or less.

[0036] Furthermore, the ratio (T1 / T2), where T1 is the total thickness of the layers on the polypropylene layer (C1) side of the barrier layer (A) and T2 is the total thickness of the layers on the sealant layer (E) side of the barrier layer (A), is preferably 0.3 or less. That is, when the multilayer structure is made into a packaging container, it is preferable that the barrier layer (A) be located closer to the outer side. Generally, foods to be retorted often contain a large amount of moisture, and the inner surface of the packaging container is often wet with water. On the other hand, the outer surface of the packaging container is often exposed to the same humidity as indoors, and therefore has a lower humidity than the inner surface. In such cases, when the moisture distribution in the multilayer structure reaches equilibrium, the moisture content of the barrier layer (A) is lower closer to the outer surface and higher closer to the inner surface. Therefore, from the viewpoint of maintaining good gas barrier properties, a small ratio (T1 / T2) is preferable. The ratio (T1 / T2) is more preferably 0.25 or less. The ratio (T1 / T2) is preferably 0.1 or more.

[0037] The total thickness T1 of the layers on the polypropylene layer (C1) side of the barrier layer (A) is preferably 6 μm or more and 70 μm or less. If T1 is too thick, the moisture content of the barrier layer (A), which increased during the retort treatment, takes a long time to decrease thereafter, and the barrier properties decrease during that time, which makes the contents, such as food, more susceptible to oxidative deterioration. Therefore, it is preferable that T1 is not too thick. The thickness of T1 is more preferably 40 μm or less, and even more preferably 30 μm or less. On the other hand, in order to protect the barrier layer (A) from external mechanical impacts, etc., it is also preferable that T1 is not too thin. The thickness of T1 is more preferably 10 μm or more, and even more preferably 12 μm or more.

[0038] The total thickness T2 of the layers on the sealant layer (E) side of the barrier layer (A) is preferably 30 μm or more and 250 μm or less. If T2 is too thick, the mass of the multilayer structure increases, and the cost also increases. Therefore, the thickness of T2 is more preferably 200 μm or less, and even more preferably 150 μm or less. On the other hand, in order to reduce the moisture content of the barrier layer (A) and maintain good gas barrier properties of the multilayer structure, it is better that T2 is not too thin. The thickness of T2 is more preferably 50 μm or more, and even more preferably 60 μm or more.

[0039] The ratio of the thickness of the barrier layer (A) to the total thickness of the multilayer structure is preferably 10% or less. This improves the recyclability of the multilayer structure. The ratio of the thickness of the barrier layer (A) is more preferably 8% or less, and even more preferably 6% or less. From the viewpoint of gas barrier properties, the ratio of the thickness of the barrier layer (A) is usually 1% or more.

[0040] The multilayer structure of the present invention is subjected to retort sterilization at 120°C for 30 minutes, and then the sealant layer (E) side is maintained at 20°C and 100% RH, and the polypropylene layer (C1) side is maintained at 20°C and 65% RH for 2 weeks, respectively. After this, the oxygen transmission rate measured in accordance with JIS K7126-2 (constant pressure method; 2006) is 5 cc / m or less. ２In this way, by having an oxygen transmission rate (OTR) after retort of not more than a certain level, the container can be suitably used as a packaging container for storing contents such as food for a long period of time. The oxygen transmission rate is more preferably 4 cc / m ２ ·day·atm or less, and more preferably 3 cc / m ２ ・day・atm or less.

[0041] [Method for producing multilayer structure] The method for producing the multilayer structure of the present invention is not particularly limited, but a suitable method is to produce by coextrusion a multilayer film containing at least a barrier layer (A), polyolefin-based adhesive resin layers (B1, B2), and polypropylene layers (C1, C2), with these layers arranged in the order C1 / B1 / A / B2 / C2, and then adhere a polypropylene film to the polypropylene layer (C2) side of the multilayer film using a polyurethane-based adhesive resin. Since polyurethane-based adhesive resins are difficult to melt-form, they are preferably used as an adhesive for dry lamination of films together.

[0042] Coextrusion is a suitable method for producing a multilayer film having layers arranged in the order C1 / B1 / A / B2 / C2. The polyolefin adhesive resin layers (B1, B2) are thermoplastic resins, like the barrier layer (A) and the polypropylene layers (C1, C2), and have relatively low melting points. Therefore, coextrusion of these layers together to produce a multilayer film is suitable from the standpoint of productivity. The multilayer film may be formed by extrusion through a T-die, or may be extruded through a circular die and inflation-molded. The resulting multilayer film may be dry-laminated with a polypropylene film without stretching. Alternatively, the resulting multilayer film may be stretched uniaxially or biaxially before dry lamination.

[0043] The method for producing the polypropylene film for forming the sealant layer (E) is not particularly limited. It may be extruded through a T-die to form a monolayer film, or may be extruded through a circular die to form an inflation film. The resulting polypropylene film may be unstretched, or may be uniaxially or biaxially stretched. From the viewpoint of obtaining good heat sealability, an unstretched polypropylene film is preferred.

[0044] The multilayer film thus produced, arranged in the order C1 / B1 / A / B2 / C2, and a polypropylene monolayer film are preferably dry-laminated using a polyurethane adhesive resin. For dry lamination, a mixture of polyisocyanate and polyol is applied to the bonding surfaces, then the films are laminated, and, if necessary, heated and cured to produce the multilayer structure of the present invention. The polyisocyanate and polyol mixture may further contain a solvent. In this case, the solvent is removed after application and then the film is heated and cured.

[0045] [Heat-sealable packaging material] A suitable application of the multilayer structure of the present invention is a heat-sealable packaging material. The multilayer structure can be sealed by applying heat to the sealant layer (E) arranged on the outermost surface of the multilayer structure to melt it. In this case, the multilayer structures of the present invention may be overlapped with each other with the sealant layer (E) on the inside and heat-sealed, or the sealant layer (E) of the multilayer structure of the present invention may be brought into contact with the surface of another molded product and heat-sealed. The heat-sealing method is not particularly limited, and heat-sealing may be performed by pressing a heated hot plate against the multilayer structure, or by using ultrasound.

[0046] [Packaging Container] A suitable application of the multilayer structure of the present invention is a packaging container. That is, a preferred embodiment of the present invention is a packaging container comprising the multilayer structure and formed by melting and sealing the sealant layers (E) together. The packaging container of the present invention has excellent gas barrier properties after retort treatment and is therefore suitable for use in sterilization treatment using steam or hot water. The packaging container of the present invention, which has excellent gas barrier properties after retort treatment and is also highly recyclable, is used for packaging various contents that require sterilization treatment, such as food, beverages, and infusion bags. Examples of the container form include pouches such as stand-up pouches, pouches with spouts, pouches with zipper seals, flat pouches, and horizontal form-fill-seal pouches, as well as lids for deep-draw cup containers, with pouches being preferred.

[0047] When producing a pouch, it is preferable to overlap and seal the multilayer structures so that the sealant layers (E) of the multilayer structures are in contact with each other. For example, a three-sided bag can be produced by sealing three sides using two multilayer structures. Alternatively, a bag can be produced by sealing two opposing sides of a single multilayer structure folded with the sealant layer (E) on the inside. Furthermore, sealing can also be achieved by joining the sealant layers (E) at both ends of a rolled multilayer structure together.

[0048] The packaging body obtained by filling the packaging container of the present invention with contents is preferably subjected to a sterilization treatment using steam or hot water. It may be brought into contact with high-temperature water or steam at normal pressure, or it may be brought into contact with pressurized steam, a so-called retort treatment. Since the packaging body of the present invention has excellent gas barrier properties after retort treatment, it can effectively prevent oxidative deterioration of the sterilized contents. Furthermore, since the packaging container of the present invention has excellent recyclability, it can be collected after use, cut, and washed, and then melt-molded again with polyolefin, enabling post-consumer recycling.

[0049] Example 1 (Preparation of oxygen barrier resin EVOH-1) 90 parts by mass of an ethylene-vinyl alcohol copolymer (EVAL L171B manufactured by Kuraray Co., Ltd., ethylene unit content 27 mol%, saponification degree 99.9 mol%, MFR (temperature 210°C, load 2160 g) 4 g / 10 min, melting point 190°C) was added to 100 parts by mass of nylon 6 (UBE nylon manufactured by Ube Industries, Ltd.) as a polyamide. 10 parts by mass of ethylene-vinyl alcohol copolymer (SF1018A, melting point 223°C) and 0.01 parts by mass of magnesium hydroxide (42 ppm in terms of magnesium ions) were mixed and melt-kneaded in a twin-screw kneading extruder (screw diameter 25 mmΦ, L / D=30, manufactured by Toyo Seiki Seisaku-sho, Ltd.) under conditions of a cylinder temperature of 230°C and a screw rotation speed of 100 revolutions per minute. The mixture was then extruded through a die into a cooling water bath at 5°C in the form of strands and pelletized with a strand cutter to obtain pellets of a barrier resin EVOH-1 containing an ethylene-vinyl alcohol copolymer and a polyamide.

[0050] (Preparation of Co-extruded Film) EVOH-1 pellets, polypropylene resin (Novatec PP manufactured by Japan Polypropylene Corporation) and the like were fed to each extruder of a three-kind, five-layer multi-layer extruder. EA7AD" melting point 159 ° C., propylene unit content 99.2 mass%, ethylene unit content 0.8 mass%, hereinafter referred to as PP), polyolefin adhesive resin (maleic anhydride modified polypropylene "Admer QF500" manufactured by Mitsui Chemicals, Inc., melting point 161 ° C., hereinafter referred to as Tie), and extrusion temperature 210 to 235 ° C., die temperature 235 ° C., and cast from the die onto a cooling roll at 80 ° C. so that the polypropylene resin contacted the cooling roll, to produce a three-kind, five-layer co-extruded film with a layer structure consisting of PP (15 μm, layer (C1)) / Tie (5 μm, layer (B1)) / EVOH-1 (10 μm, layer (A)) / Tie (5 μm, layer (B2)) / PP (15 μm, layer (C2)).

[0051] (Preparation of Multilayer Structure) A two-component reactive polyurethane adhesive (65 parts by mass of "Takelac A-520" and 10 parts by mass of "Takenate A-50" manufactured by Mitsui Chemicals, Inc.) was mixed with 100 parts by mass of ethyl acetate to prepare an adhesive solution. Next, the adhesive solution was applied using a wire bar to the corona-treated surface of a 70 μm-thick unstretched polypropylene (CPP) film ("RXC-22" manufactured by Mitsui Chemicals Tohcello, Inc.), and dried at 100°C for 5 minutes to form a 4 μm-thick polyurethane adhesive resin (LA) layer. This was then laminated with the co-extruded film obtained above to produce a multilayer film (multilayer structure) having a thickness and layer structure of PP (15 μm, layer (C1)) / Tie (5 μm, layer (B1)) / EVOH-1 (10 μm, layer (A)) / Tie (5 μm, layer (B2)) / PP (15 μm, layer (C2)) / LA (4 μm, layer (D)) / CPP (70 μm, layer (E)). The adhesion temperature (heating roll temperature) during lamination was 80°C, and the film was then stored in a thermostatic chamber at 40°C for 3 days for aging to cure the polyurethane adhesive resin. In the resulting multilayer film, the total thickness of the layers on the layer (C1) side of the layer (A) was T1, and the total thickness of the layers on the layer (E) side of the layer (A) was T2. The ratio T1 / T2 was 0.21. The proportion of the thickness of layer (A) to the total thickness was 8.1%.

[0052] The obtained multilayer film was cut into two 11 cm square pieces, which were overlapped so that the laminated sealant layers (E) were in contact with each other, and each of the three sides of the multilayer film was heat-sealed for 3 seconds using a 0.5 cm wide hot plate sealer heated to 160°C, to produce 10 multilayer film bags (pouches).

[0053] Next, using a high-pressure cooking sterilizer (manufactured by Hisaka Works, Ltd.), the pouch was retorted in hot water for 30 minutes at a temperature of 120 ° C. and a gauge pressure of 0.17 MPa. After retort treatment, a portion of the pouch was cut out, and any water adhering to the surface was wiped off. Then, using an oxygen transmission rate measuring device OX-TRAN2 / 21 model manufactured by MOCON INC., the outer side of the pouch (layer (C1) side) was heated to 20 ° C. and 65% RH, and the inner side of the pouch (layer (E) side) was heated to 20 ° C. and 100% humidity. The oxygen transmission rate (OTR) after 2 weeks was measured according to the method described in ISO14663-2 Annex C (1999). The measurement results are shown in Table 1.

[0054] Example 2 The same unstretched polypropylene film as used for Layer (E) was further laminated to the Layer (E) side of the multilayer structure obtained in Example 1 via a polyurethane adhesive resin (LA) layer in the same manner as in Example 1, to produce a multilayer structure having a thickness and layer structure of PP (15 μm, Layer (C1)) / Tie (5 μm, Layer (B1)) / EVOH-1 (10 μm, Layer (A)) / Tie (5 μm, Layer (B2)) / PP (15 μm, Layer (C2)) / LA (4 μm, Layer (D)) / CPP (70 μm, Layer (E)) / LA (4 μm, Layer (D)) / CPP (70 μm, Layer (E)). Retort treatment was carried out in the same manner as in Example 1, and the oxygen transmission rate (OTR) was evaluated. The results are shown in Table 1.

[0055] Example 3 A multilayer structure having a thickness and layer structure of PP (15 μm, layer (C1)) / Tie (5 μm, layer (B1)) / EVOH-1 (10 μm, layer (A)) / Tie (5 μm, layer (B2)) / PP (15 μm, layer (C2)) / LA (4 μm, layer (D)) / CPP (30 μm, layer (E)) was produced in the same manner as in Example 1, except that a 30 μm thick unstretched polypropylene film ("RXC-22" manufactured by Mitsui Chemicals Tohcello, Inc.) was used instead of the 70 μm thick unstretched polypropylene film. Retort treatment was carried out in the same manner as in Example 1, and the oxygen transmission rate (OTR) was evaluated. The results are shown in Table 1.

[0056] Example 4 An oxygen barrier resin EVOH-2 was produced in the same manner as in Example 1, except that an ethylene-vinyl alcohol copolymer (EVAL F171B manufactured by Kuraray Co., Ltd., ethylene unit content 32 mol%, degree of saponification 99.9 mol%, MFR (temperature 210°C, load 2160 g) 3.7 g / 10 min, melting point 190°C) was used when producing the oxygen barrier resin. A multilayer structure and a pouch were produced in the same manner as in Example 1 and evaluated in the same manner as in Example 1. Retort treatment was carried out and the oxygen transmission rate (OTR) was evaluated. The results are shown in Table 1. In this example, no gel was generated during the production of the co-extruded film, even 8 hours after the start of operation.

[0057] Example 5 An oxygen barrier resin EVOH-3 was produced in the same manner as in Example 4, except that magnesium hydroxide was not added when producing the oxygen barrier resin, and a multilayer structure and a pouch were produced in the same manner as in Example 1, and evaluated in the same manner as in Example 1. A retort treatment was carried out, and the oxygen transmission rate (OTR) was evaluated. The results are shown in Table 1. In this example, during the production of the co-extruded film, good products were obtained up to 3 hours after the start of operation, but gel occurred thereafter.

[0058] Comparing Example 4 with Example 5, it is clear that the inclusion of a magnesium salt in the oxygen barrier resin improved the long-run properties during extrusion molding.

[0059] Comparative Example 1 A three-kind, five-layer multilayer film having a layer structure of PP (15 μm, layer (C1)) / Tie (5 μm, layer (B1)) / EVOH-1 (10 μm, layer (A)) / Tie (5 μm, layer (B2)) / PP (15 μm, layer (C2)) was produced by coextrusion in the same manner as in Example 1, except that a 70 μm-thick unstretched polypropylene film was not laminated. Two pieces of the multilayer film were cut out in the same manner as in Example 1, and the two pieces were overlapped with each other so that the PP layers (C2) were in contact with each other, followed by heat sealing to produce a pouch. The film was then subjected to retort treatment in the same manner as in Example 1, and the oxygen transmission rate (OTR) was evaluated. The results are shown in Table 1.

[0060] Comparing Comparative Example 1 with Example 1, it can be seen that by laminating a non-oriented polypropylene film via a polyurethane adhesive resin (LA) layer, the oxygen transmission rate after retort treatment could be reduced.

[0061] Comparative Example 2 A three-kind, five-layer co-extruded film and pouch having a layer structure of PP (90 μm, layer (C1)) / Tie (5 μm, layer (B1)) / EVOH-1 (10 μm, layer (A)) / Tie (5 μm, layer (B2)) / PP (90 μm, layer (C2)) were prepared in the same manner as in Comparative Example 1, except that the thicknesses of Layer (C1) and Layer (C2) were 90 μm, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0062] Comparing Comparative Example 2 with Comparative Example 1, even though the thicknesses of both the outer PP layer (C1) and the inner PP layer (C2) were increased, the oxygen transmission rate of the multilayer film of Comparative Example 2 after retort treatment was significantly increased compared to Comparative Example 1. It is thought that the increased thickness of the outer polypropylene layer reduced the rate at which water absorbed in EVOH-1 during retort treatment was released to the outside, and moisture remained in EVOH-1, thereby increasing the oxygen transmission rate.

[0063] Comparative Example 3 A three-kind, five-layer co-extruded film and pouch having a layer structure of PP (15 μm, layer (C1)) / Tie (5 μm, layer (B1)) / EVOH-1 (10 μm, layer (A)) / Tie (5 μm, layer (B2)) / PP (85 μm, layer (C2)) were prepared in the same manner as in Comparative Example 1, except that the thickness of Layer (C2) was 85 μm, and evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0064] Comparing the layer structures of the multilayer films of Comparative Example 3 and Example 1, T1 / T2 is almost the same, but the difference is that in Example 1, a polyurethane-based adhesive layer (D) is inserted between the polypropylene layer (C2) and the sealant layer (E), whereas in Comparative Example 3, the polypropylene layer (C2) is thick and does not have the polyurethane-based adhesive layer (D). As a result, the oxygen transmission rate after retort treatment in Comparative Example 3 was significantly higher than in Example 1. From this, it is presumed that the polyurethane-based adhesive layer (D) inserted between the PP layers promotes the release of water absorbed by the EVOH-1 layer during retort treatment.

[0065] [Comparative Example 4] An oxygen barrier resin EVOH-4 was produced in the same manner as in Example 1, except that nylon 6 was not added when producing the oxygen barrier resin, and a multilayer structure and a pouch were produced in the same manner as in Example 1, and evaluated in the same manner as in Example 1. After retort treatment, the produced pouch turned white, and its appearance deteriorated.

[0066]

Claims

1. A multilayer structure comprising at least a barrier layer (A), polyolefin adhesive resin layers (B1, B2), polypropylene layers (C1, C2), a polyurethane adhesive resin layer (D), and a sealant layer (E), which are arranged in the order of C1 / B1 / A / B2 / C2 / D / E, wherein the barrier layer (A) comprises an ethylene-vinyl alcohol copolymer (a1) and a polyamide (a2), and the mass ratio (a1 / a2) of the ethylene-vinyl alcohol copolymer (a1) to the polyamide (a2) is 70 / 30 or more and 95 / 5 or less, the ethylene unit content of the ethylene-vinyl alcohol copolymer (a1) is 20 mol% or more and 40 mol% or less, and the sealant layer (E) comprises polypropylene.

2. The multilayer structure according to claim 1, wherein the barrier layer (A) contains 10 ppm or more and 1,400 ppm or less of magnesium salts calculated as magnesium.

3. The multilayer structure of claim 2, wherein said magnesium salt is magnesium hydroxide.

4. The multilayer structure according to claim 1 or 2, wherein the ratio (T1 / T2) of the total thickness of the layers on the polypropylene layer (C1) side of the barrier layer (A) to the total thickness of the layers on the sealant layer (E) side of the barrier layer (A) is 0.3 or less, where T1 is the total thickness of the layers on the sealant layer (E) side of the barrier layer (A).

5. The multilayer structure according to claim 1 or 2, wherein the ratio of the thickness of the barrier layer (A) to the total thickness is 10% or less.

6. The multilayer structure according to claim 1 or 2, wherein the polyamide (a2) is nylon 6.

7. The multilayer structure according to claim 1 or 2, wherein the sealant layer (E) is made of unstretched polypropylene.

8. The multilayer structure is subjected to retort sterilization at 120°C for 30 minutes, and then the sealant layer (E) side is maintained at 20°C and 100% RH, and the polypropylene layer (C1) side is maintained at 20°C and 65% RH for 2 weeks, respectively. After this, the oxygen transmission rate measured in accordance with JIS K7126-2 (constant pressure method; 2006) is 5 cc / m or less. ２ 3. The multilayer structure according to claim 1, wherein the viscosity is 0.05 sq. m / s or less.

9. A heat-sealable packaging material comprising the multilayer structure according to claim 1 or 2.

10. A packaging container comprising the multilayer structure according to claim 1 or 2, wherein the sealant layers (E) are melted and sealed together.

11. The packaging container according to claim 10, which is a pouch.

12. The packaging container according to claim 10, which is for sterilization treatment with steam or hot water.

13. A package comprising the packaging container according to claim 10 filled with contents.

14. A method for producing a multilayer structure according to claim 1 or 2, comprising producing a multilayer film by coextrusion molding, the multilayer film comprising at least a barrier layer (A), polyolefin adhesive resin layers (B1, B2), and polypropylene layers (C1, C2), with these layers arranged in the order C1 / B1 / A / B2 / C2, and adhering a polypropylene film to the polypropylene layer (C2) side of the multilayer film using a polyurethane adhesive resin.

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