JP7461799B2 - Gas barrier shrink film - Google Patents

Description

The present invention relates to a gas barrier shrink film. In particular, the present invention relates to a gas barrier shrink film that is suitable for packaging using a packaging machine and is suitable for use in the food packaging field, where gas barrier properties are primarily required.

Examples of packaging methods for covering food products include household wrap packaging, overlap packaging, twist packaging, bag packaging, skin packaging, pillow shrink packaging, stretch packaging, and top seal packaging. In particular, continuous packaging machines for pillow shrink packaging and top seal packaging are widely used because they can package at high speeds and produce good finishes.

Furthermore, in recent years, with environmental concerns, supermarkets and convenience stores have become increasingly conscious of reducing the amount of unsold food and other waste, and gas-pack packaging for long-term storage at room temperature has been attracting attention. Gas-pack packaging is a tool that achieves long-term storage by sealing the inside of a container with nitrogen gas or carbon dioxide gas to inhibit the growth of bacteria and other microorganisms. For the packaging film used, a gas-barrier film with low oxygen permeability is suitable. A known example of a gas-barrier film is a film that is a laminate of a barrier resin and a polyolefin resin with low-temperature sealing properties.

Gas pack packaging is used for meat and fish, which are stored under low temperature and high humidity conditions in supermarkets, convenience stores, etc. For this reason, gas barrier properties at a certain level of temperature and humidity are important. It is also important that the container does not deform during gas pack packaging.

For example, Patent Document 1 discloses a multilayer film composed of a polyolefin resin, a modified polyolefin adhesive resin layer, a polyamide resin layer, and a saponified ethylene-vinyl acetate copolymer layer.
Furthermore, Patent Document 2 discloses a multilayer film including a base layer, a gas barrier layer, and a heat seal layer, in which the base layer contains a thermoplastic resin having a melting point of 130° C. or higher, the gas barrier layer contains a saponified ethylene-vinyl alcohol copolymer, and the heat seal layer contains a polyethylene-based resin layer.

Patent No. 3418204 JP 2016-179648 A

However, in Patent Document 1, since a polyamide resin layer is used, the gas barrier property is poor at a certain temperature and humidity, and there is a risk of the container deforming during gas pack packaging.
Furthermore, in Patent Document 2, a higher level of mechanical properties is not achieved.

The problem that this invention aims to solve is to provide a film that has excellent gas barrier properties even under high humidity conditions, does not cause the container to deform when packaged in a gas pack, and has excellent mechanical properties.

The present inventors have conducted extensive research to solve the above problems and have completed the present invention.
[1]
The film has at least four layers: a base layer (A), an adhesive layer (D), a gas barrier layer (B), and a heat seal layer (C);
The adhesive layer (D) contains two types of thermoplastic resins (X) and (Y) and has a sea-island structure,
The thermoplastic resin (X) is a modified LLDPE,
The thermoplastic resin (Y) is HDPE,
the base layer (A) contains an aliphatic polyamide polymer, an aliphatic polyamide copolymer, or a polypropylene-based resin,
the base material layer (A), the adhesive layer (D), the gas barrier layer (B), the adhesive layer, and the heat seal layer (C) are in contact with each other in this order, and the adhesive layer between the gas barrier layer (B) and the heat seal layer (C) is the adhesive layer (D) or an adhesive layer other than the adhesive layer (D);
A gas barrier shrink film characterized by:
[2]
2. The gas barrier shrink film according to claim 1, wherein the sea-island structure is a sea-island structure consisting of a continuous phase in which the thermoplastic resin (X) and the thermoplastic resin (Y) are in a partially compatible state, and a dispersed phase made of the thermoplastic resin (X).
[3]
The gas barrier shrink film according to [1] or [2], wherein the adhesive layer (D) contains two types of thermoplastic resins (X) and (Y), and the density dx of the thermoplastic resin (X) and the density dy of the thermoplastic resin (Y) satisfy dy-dx>30 kg/ ^m3 .
[4]
The gas barrier shrink film according to any one of [1] to [3] , wherein a mass ratio of the thermoplastic resin (X) to the thermoplastic resin (Y) is 10:90 to 90:10.
[5]
The gas barrier shrink film according to any one of [1] to [4] , wherein the heat seal layer (C) contains at least a polyethylene-based resin.
[6]
6. The gas barrier shrink film according to any one of [1] to [5], which has a thermal shrinkage rate in the longitudinal direction at 100° C. of 30% or more.

The gas barrier shrink film of the present invention has the above-mentioned configuration, and therefore has excellent gas barrier properties even under high humidity conditions, and the container does not deform during gas pack packaging, and has excellent mechanical properties.

The following describes in detail the form for carrying out the present invention (hereinafter, simply referred to as the "present embodiment"). The following present embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content. The present invention can be carried out with appropriate modifications within the scope of its gist.

[Gas barrier shrink film]
The gas barrier shrink film of this embodiment comprises at least four layers, namely, a base layer (A), an adhesive layer (D), a gas barrier layer (B), and a heat seal layer (C), and the adhesive layer (D) contains a thermoplastic resin and has a sea-island structure. The gas barrier shrink film preferably comprises one base layer (A), one gas barrier layer (B), and one heat seal layer (C), and one or more adhesive layers (D). The adhesive layer may be only an adhesive layer that satisfies the requirements of the adhesive layer (D) described below, or may include other adhesive layers.

The layer structure of the gas barrier shrink film may be, for example, a layer structure having a base layer (A) and a heat seal layer (C) as the outermost layers (surface layers) and an adhesive layer (D) and a gas barrier layer (B) inside, more specifically, the following examples are given. With these layer structures, the effects of the present invention are more pronounced.
Substrate layer/gas barrier layer/adhesive layer/heat seal layer Substrate layer/adhesive layer/gas barrier layer/heat seal layer Substrate layer/adhesive layer/gas barrier layer/adhesive layer/heat seal layer

The following describes in detail the preferred aspects of each layer that makes up the gas barrier shrink film.

(Substrate layer (A))
The base layer (A) is a layer that imparts heat resistance to the gas barrier shrink film and is preferably located as the outermost layer of the gas barrier shrink film. The base layer (A) can also serve as a stretching support layer during the production of the gas barrier shrink film.

From the viewpoint of imparting heat resistance, the substrate layer (A) preferably contains a thermoplastic resin having a melting point of 130° C. or higher.
As the thermoplastic resin having a melting point of 130° C. or more, polyesters such as polymethylene terephthalate, polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polylactic acid, etc.; aliphatic polyamide polymers such as nylon 6, nylon 12, nylon 66, etc.; aliphatic polyamide copolymers such as nylon 6/66, nylon 6/12, etc.; aromatic polyamide polymers such as MXD6 (polymetaxylene adipamide); polymethylpentene; polypropylene-based resins; etc. are preferred, and aliphatic polyamide polymers, aliphatic polyamide copolymers, and polypropylene-based resins are more preferred, and polypropylene-based resins are even more preferred. Among these, one type may be used alone, or a combination of two or more may be used.
The melting point of the thermoplastic resin is preferably 130° C. or higher and 230° C. or lower, more preferably 130° C. or higher and 160° C. or lower, from the viewpoint of imparting heat shrinkability.
The mass ratio of the thermoplastic resin having a melting point of 130° C. or more in the base layer (A) is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, based on 100% by mass of the base layer (A). In particular, the base layer (A) preferably contains 50% by mass or more, more preferably 60% by mass or more, and even more preferably 70% by mass or more, of a polypropylene-based resin having a melting point of 130° C. or more (preferably 130° C. or more and 230° C. or less).

As the polypropylene-based resin, a propylene homopolymer or a propylene-based copolymer can be suitably used, for example, polypropylene, a propylene-α-olefin copolymer, a ternary copolymer of propylene, ethylene and an α-olefin, etc. can be suitably used.
The propylene homopolymer is a polymer obtained by polymerizing only propylene.
As the propylene-based copolymer, a copolymer of propylene and at least one selected from ethylene and an α-olefin having 4 to 20 carbon atoms can be suitably used. A copolymer of propylene and at least one selected from ethylene and an α-olefin having 4 to 8 carbon atoms is more preferred, and a copolymer of propylene and ethylene is even more preferred.

The above-mentioned α-olefins include 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, and the like. These can be used alone or in combination of two or more.

As the propylene-α-olefin copolymer, a copolymer of propylene and at least one comonomer selected from ethylene comonomer, butene comonomer, hexene comonomer, and octene comonomer is generally easily available and can be preferably used.

The polypropylene resin may be polymerized using a known catalyst such as a single-site catalyst or a multi-site catalyst, and from the viewpoint of achieving even better transparency, it is preferable that the polypropylene resin is polymerized using a single-site catalyst.

The polypropylene resin may be a resin polymerized with a catalyst such as a Ziegler-Natta catalyst, or a resin polymerized with a metallocene catalyst. That is, for example, syndiotactic polypropylene, isotactic polypropylene, etc. can also be used as the polypropylene resin.

As the polypropylene-based resin, a polypropylene-based resin whose crystal/amorphous structure (morphology) is controlled on the nano-order can also be used.
The polypropylene-based resins can be used alone or in combination. Mixing polypropylene with a propylene-α-olefin copolymer is preferred because it tends to reduce the crystallinity of the base layer and improve the heat shrinkability.

From the viewpoint of achieving even better heat resistance and strength, the base layer (A) preferably contains at least one selected from the group consisting of the polypropylene-based resin, the aliphatic polyamide polymer, and the aliphatic polyamide copolymer, and more preferably consists of only at least one selected from the group consisting of the polypropylene-based resin, the aliphatic polyamide polymer, and the aliphatic polyamide copolymer.
From the viewpoint of achieving even better heat resistance, the base layer (A) may be a layer consisting of only a propylene homopolymer and a propylene-based copolymer, and may contain 30% by mass or more and less than 50% by mass of a propylene homopolymer, or more than 50% by mass and 70% by mass or less of a propylene-based copolymer, relative to 100% by mass of the base layer.

The substrate layer (A) may contain components other than the thermoplastic resin. For example, it may contain any additives such as thermoplastic resins, various surfactants, antiblocking agents, antistatic agents, lubricants, plasticizers, antioxidants, UV absorbers, colorants, and inorganic fillers, to the extent that the properties are not impaired.

In order to impart anti-fogging properties to the gas barrier shrink film of this embodiment, a glycerin-based fatty acid ester can be added as a surfactant to the base layer (A). When a glycerin-based fatty acid ester is added, the content is preferably 0.1% by mass or more and 5.0% by mass or less based on the base layer.

Examples of glycerin-based fatty acid esters include mono-, di-, tri-, and poly-fatty acid esters of glycerin, and examples of these include mono-, di-, tri-, and tetraglycerin esters of saturated or unsaturated fatty acids having 8 to 18 carbon atoms. Among these, diglycerin oleate, diglycerin laurate, glycerin stearate, glycerin monooleate, or mixtures thereof are preferred because they do not inhibit the slipperiness or optical properties of the film. Furthermore, by adding an ethylene oxide adduct to lower the surface tension of water droplets, good anti-fogging properties can be imparted. Examples of ethylene oxide adducts include polyoxyethylene alkyl ether.

(Gas Barrier Layer (B))
The gas barrier layer (B) is a layer containing a saponified ethylene-vinyl alcohol copolymer (EVOH), and serves to improve the gas barrier properties of the gas barrier shrink film.

The peak melting temperature (hereinafter referred to as melting point) of the saponified ethylene-vinyl alcohol copolymer is preferably 180° C. or lower, more preferably 140 to 170° C., and even more preferably 150 to 160° C. If the melting point is 180° C. or lower (preferably 160° C. or lower), relaxation of molecular orientation is promoted during the heat shrinkage step in packaging, and therefore good heat shrinkability suitable for shrinkage applications can be exhibited.
In this specification, the melting point is the temperature at the top of the endothermic reaction peak appearing in the melting curve obtained by differential scanning calorimetry (DSC). When multiple melting peaks exist, it is sufficient that the melting peak temperature on the highest temperature side is within the above-mentioned numerical range (i.e., in this specification, the melting point is regarded as the melting peak temperature on the highest temperature side).

The ethylene-vinyl alcohol copolymer saponification product tends to have better stretchability because the crystallinity and melting point decrease as the content of structural units derived from ethylene (sometimes referred to as ethylene content in this specification) increases. In general, the melting point is 183°C when the ethylene content is 32 mol%, 173°C when it is 38 mol%, and 163°C when it is 44 mol%. A suitable ethylene content is 30 mol% or more and 40 mol% or less, more preferably 31 mol% or more and 39 mol% or less, and even more preferably 32 mol% or more and 38 mol% or less. When the ethylene content is in the above range, a film with excellent stretchability and gas barrier properties can be obtained.

In general, the greater the interaction between molecular chains, the better the gas barrier performance tends to be, and saponified ethylene-vinyl alcohol copolymers in particular are known to have high gas barrier performance due to the large interactions between molecules. However, because the interactions between molecular chains are large, the heat shrinkage stress is large, and when used as a shrink (heat shrinkable) film, deformation of the packaging container is likely to occur.

By controlling the crystal structure, the saponified ethylene-vinyl alcohol copolymer can achieve both a melting point of 180°C or less (preferably 160°C or less) and an ethylene content of 40 mol% or less. Furthermore, by using a saponified ethylene-vinyl alcohol copolymer with a larger supercooling temperature difference, which is the difference between the melting point and the crystallization temperature when cooled, compared to a typical saponified ethylene-vinyl alcohol copolymer, the thickness of the lamella is small and the crystals are easily uniformed. In the case of a gas barrier layer containing such a saponified ethylene-vinyl alcohol copolymer, stress concentration is reduced and container deformation during gas pack packaging can be suppressed. The supercooling temperature difference is preferably 25°C or more, more preferably 27°C or more, and even more preferably 29°C or more.

The reason why the above effect is achieved is not entirely clear, but it is thought to be as follows. That is, the heat shrinkage properties of gas barrier shrink films are expressed when the amorphous parts, which have been stretched due to the molecular orientation of the constituent resin, relax and return to a non-oriented state, and the crystalline parts play a role in preventing the shrinkage of the amorphous parts up to near the melting temperature. Generally, saponified ethylene-vinyl alcohol copolymers have strong intermolecular interactions and large heat shrinkage stress, which tends to dominate the heat shrinkage stress of the entire laminate film, but by controlling the crystal structure, it is thought that it is possible to suppress stress concentration due to relaxation of the orientation of the amorphous and crystalline parts, and provide a gas barrier shrink film with excellent barrier properties.

(Heat seal layer (C))
The heat seal layer (C) is a layer that imparts heat sealability to the gas barrier shrink film and is preferably located as the outermost layer of the gas barrier shrink film. The heat seal layer can also serve as a stretching support layer during the production of the gas barrier shrink film.

The heat seal layer (C) preferably contains a polyethylene-based resin containing a structural unit derived from ethylene, and more preferably consists of an ethylene-based resin such as an ethylene homopolymer or an ethylene-α-olefin copolymer.
Examples of the polyethylene resin include high density polyethylene, medium density polyethylene, low density polyethylene (LDPE) (preferably linear low density polyethylene (LLDPE)), and very low density polyethylene. Examples of the very low density polyethylene include linear very low density polyethylene (also called "VLDPE" or "ULDPE").

Here, polyethylene resins can be classified by density based on JIS K6922. Specifically, those with a density of 942 kg/m3 or more are called high-density polyethylene, those with a density of 930 kg/ ^m3 or ^more and less than 942 kg/ ^m3 are called medium-density polyethylene, those with a density of 910 kg/ ^m3 or more and less than 930 kg/ ^m3 are called low-density polyethylene, and those with a density less than 910 kg/ ^m3 are called ultra-low-density polyethylene.

The ethylene-α-olefin copolymer refers to a copolymer of ethylene and an α-olefin, such as propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 4-methyl-1-pentene, 3-methyl-1-pentene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, and 1-eicosene.
As the ethylene-α-olefin copolymer, a copolymer of ethylene and at least one comonomer selected from a propylene comonomer, a butene comonomer, a hexene comonomer and an octene comonomer is generally easily available and can be suitably used.

The ethylene-α-olefin copolymer is preferably a soft copolymer in which the proportion of α-olefin in the total monomers constituting the copolymer (based on the charged monomers) is 5% by mass or more and 30% by mass or less.

The polyethylene resin may be polymerized using a known catalyst such as a single-site catalyst or a multi-site catalyst, and from the viewpoint of achieving even better transparency, it is preferable to polymerize using a single-site catalyst.

From the viewpoint of improving the heat sealability at low temperatures, the polyethylene resin preferably has a density of 860 kg/m ³ or more and 925 kg/m ³ or less, more preferably 870 kg/m ³ or more and 920 kg/m ³ or less, and even more preferably 880 kg/m ³ or more and 915 kg/m ³ or less. The lower the density of the polyethylene resin, the more the heat sealability at low temperatures tends to improve, and if the density is 925 kg/m ³ or less, the heat sealability tends to improve.

As the polyethylene-based resin, a polyethylene-based resin whose crystal/amorphous structure (morphology) is controlled at the nano-order can also be used.

Polyethylene resins can be used alone or in combination, and mixing ethylene homopolymers with ethylene-α-olefin copolymers is preferred because it tends to reduce the crystallinity of the heat seal layer and improve heat shrinkability and heat sealability at low temperatures.

The heat seal layer may contain components other than the polyethylene resin. For example, it may contain any additives such as thermoplastic resins, various surfactants, antiblocking agents, antistatic agents, lubricants, plasticizers, antioxidants, UV absorbers, colorants, and inorganic fillers, to the extent that the properties of the layer are not impaired.

To impart anti-fogging properties to the gas barrier shrink film of this embodiment, the above-mentioned glycerin-based fatty acid ester can be added as a surfactant to the heat seal layer.

(Adhesive Layer (D))
The gas barrier shrink film of the present embodiment has at least one adhesive layer (D) containing a thermoplastic resin and having a sea-island structure. For example, when there are multiple adhesive layers, one layer may be the adhesive layer (D) and the other layers may be other adhesive layers. Among them, from the viewpoint of achieving both excellent mechanical properties and flexibility, it is preferable that one adhesive layer is the adhesive layer (D).
In order to achieve both excellent mechanical properties and extensibility, it is preferable that the gas barrier shrink film of this embodiment has an adhesive layer (D) between one of the surface layers, the base layer (A), and an internal layer (e.g., the gas barrier layer (B)).

The above-mentioned sea-island structure is preferably formed from two or more types of thermoplastic resins. For example, it can be configured to have a sea-island structure consisting of a continuous layer consisting of a portion in which the thermoplastic resin (X) and the thermoplastic resin (Y) are compatible with each other, and a dispersed layer consisting of the thermoplastic resin (X). However, it is not limited to such a configuration.

The sea-island structure means a structure in which one of the components is dispersed in an island-like form in a continuous phase, and the island portions, which are the dispersed phase, are usually discontinuous. The islands are preferably uniformly distributed throughout the adhesive layer (D). It is preferable that two or more islands are present.
The area ratio of the islands is preferably 15 to 45%. The area ratio of the islands can be measured by the method described in the Examples below.

The two thermoplastic resins (X) and (Y) forming the sea-island structure preferably have a density dx of the thermoplastic resin (X) and a density dy of the thermoplastic resin (Y) such that dx < dy and dy - dx > 30 kg/m ^3. With this combination, a sea-island structure is easily formed by a continuous layer made of the portion where the thermoplastic resin (X) and the thermoplastic resin (Y) are compatible with each other and a dispersed layer made of the thermoplastic resin (X).
The density difference dy-dx is more preferably 40 kg/m3 or more, and even more preferably 45 kg/ ^m3 or more, from the viewpoint that a continuous phase in which the thermoplastic resin is compatible is easily formed throughout the adhesive layer, and mechanical strength and flexibility are further improved. Also, it is preferably 60 kg/ ^m3 or ^less .
For example, when the adhesive layer contains three or more thermoplastic resins, the thermoplastic resin having the highest density may be designated as thermoplastic resin (Y) and the thermoplastic resin having the lowest density may be designated as thermoplastic resin (X).

The mass ratio of the thermoplastic resin (X) to the thermoplastic resin (Y) is preferably 10:90 to 90:10, more preferably 20:80 to 80:20, and even more preferably 30:70 to 70:30. Within this range, a sea-island structure is easily formed, and the mechanical strength and flexibility are further improved.

The thermoplastic resin (X) may be an olefin resin having a polar group, and is preferably modified LLDPE. Modified LLDPE is difficult to crystallize, and therefore easily forms island structures in a sea-island structure. For example, modified LLDPE is preferably a modified polymer obtained by graft-copolymerizing an unsaturated carboxylic acid such as maleic acid or fumaric acid or an acid anhydride thereof with a polyethylene resin such as an ethylene homopolymer, an ethylene-propylene copolymer, or an ethylene-α-olefin copolymer, and more preferably a modified polymer obtained by graft-copolymerizing maleic anhydride. This makes it possible to fully develop adhesiveness. In addition, by using a thermoplastic resin having a polar group, the adhesive layer has a suitable degree of flexibility, making it easier to perform stretching processing.

The modified LLDPE preferably has a density of 910 kg/ ^m3 or less.

The thermoplastic resin (Y) is preferably HDPE.
HDPE is an abbreviation for high density polyethylene, and preferably has a density of 942 kg/ ^m3 or more.

When the thermoplastic resin (X) is modified LLDPE and the thermoplastic resin (Y) is HDPE, the modified LLDPE is difficult to crystallize, and the HDPE is easy to crystallize, so they are not completely uniformly compatible. A part of the modified LLDPE and the HDPE are partially compatible to form a eutectic structure. This eutectic structure part becomes the sea component of the sea-island structure, and the remaining uncrystallized modified LLDPE becomes the island component. This structure makes it possible to achieve both mechanical strength and flexibility. Usually, when two or more thermoplastic resins with different density differences are mixed in the adhesive layer, the adhesiveness decreases. However, the present inventors have found that by providing a sea-island structure in the adhesive layer (particularly, by using two or more thermoplastic resins that are difficult to be compatible due to a large density difference, a sea-island structure is provided in which a part of the thermoplastic resins is compatible to form a sea structure and a part of the thermoplastic resins is not compatible to form an island structure), the mechanical properties and flexibility are improved.
Although polyamide resins have excellent mechanical strength, they are prone to moisture absorption, which may reduce the gas barrier properties. By providing an island-in-a-sea structure in the adhesive layer using a thermoplastic resin other than polyamide resins (for example, a combination of an olefin resin having a polar group, such as the above-mentioned modified LLDPE, and high-density polyethylene), the gas barrier properties can be maintained for a long period of time even under high humidity conditions.
This crystal structure is particularly suitable for the adhesive layer of a shrink film, which requires barrier properties.

Generally, when HDPE is used as a film component, the film becomes hard due to its crystallinity, and it is difficult to stretch it, making it difficult to use as a gas barrier shrink film. However, it is presumed that the influence of some of the island components in the modified LLDPE made it possible to stretch the film, despite the use of HDPE to create a sea-island structure.
Furthermore, it is believed that the high mechanical strength of HDPE makes it possible to obtain sufficient mechanical strength for the entire film.

In this way, by containing thermoplastic resin and having an island-sea structure, it is possible to achieve both strength and elongation.

The gas barrier shrink film of the present embodiment may have a single adhesive layer or two or more adhesive layers, for example, a first adhesive layer provided between the heat seal layer (C) and the gas barrier layer (B) and a second adhesive layer provided between the gas barrier layer (B) and the base layer (A).
The first adhesive layer and the second adhesive layer may be made of the same resin or different resins, and may have the same thickness or different thicknesses.
Furthermore, in this embodiment, it is sufficient that at least one adhesive layer contains a thermoplastic resin and has a sea-island structure, and the first adhesive layer may contain a thermoplastic resin and have a sea-island structure, or the second adhesive layer may contain a thermoplastic resin and have a sea-island structure.

The adhesive layer other than the adhesive layer (D) is not particularly limited and may be any known adhesive layer.

The characteristics of the gas barrier shrink film of this embodiment will be described below.
(Maximum heat shrinkage stress)
In order to suppress stress concentration, the gas barrier shrink film of this embodiment preferably has a maximum heat shrinkage stress of 3.0 MPa or less, and more preferably 2.5 MPa or less, for up to 3 minutes at a measurement temperature of 100° C. in order to suppress stress concentration.
In pillow shrink packaging, which uses a shrink method to tightly wrap the packaged item, fresh meat, seafood, etc. are often packed into plastic foam trays or molded trays, and the trays are prone to deformation due to the thermal shrinkage stress caused by the thermal shrinkage of the film, which can result in packages with poor appearance.
When the gas barrier shrink film of this embodiment has a maximum thermal shrinkage stress in the longitudinal direction at 100°C of 3.0 MPa or less, when food or the like is packaged using an automatic packaging machine, tray deformation is more effectively suppressed, resulting in a tight and attractive package.
In addition, food packaging trays used in automatic packaging machines are generally rectangular trays in which the machine direction, i.e., the length direction, is longer than the width direction, and therefore the width direction is particularly vulnerable to deformation due to heat shrinkage stress and is prone to container deformation. Therefore, it is preferable that the maximum heat shrinkage stress in the width direction of the film is 2.5 MPa or less, since container deformation is easily suppressed, and it is more preferable that the maximum heat shrinkage stress in both the length direction and the width direction is 3.0 MPa or less, since container deformation of a square tray can also be suppressed.
When packaging is performed using an automatic packaging machine, the time it takes for the package to pass through the shrink tunnel is only a few seconds, so the film surface temperature tends to be about 10° C. to 20° C. lower than the hot air temperature set in the tunnel. Therefore, for the gas barrier shrink film of this embodiment, the heat shrinkage stress, which is an index of container deformation, is measured and evaluated by immersing the film in an oil bath at 100° C., assuming a hot air temperature set at 120° C. The maximum heat shrinkage stress is measured in accordance with ASTM-D2838.

(Thermal shrinkage rate)
The gas barrier shrink film of this embodiment has a heat shrinkage rate at 100°C, measured in accordance with the measurement method ASTM-D2732, of preferably 30% or more in the length direction, more preferably 25% or more in the width direction, and even more preferably 30% or more in both the length direction and the width direction. In order to obtain a package with a tight finish in pillow shrink packaging and top seal packaging using the gas barrier shrink film of this embodiment, a heat shrinkage rate of 25% or more will allow a package with a tight and neat finish to be obtained.

To obtain a heat shrinkage rate within the above range, the melting point of the resin constituting the gas barrier shrink film is preferably 165°C or less, more preferably 160°C or less, and even more preferably less than 160°C. If the melting point of the resin constituting the gas barrier shrink film is 165°C or less, relaxation of molecular orientation is promoted, and good heat shrinkability suitable for shrinkage applications can be achieved.

In addition, the gas barrier shrink film preferably has a thermal shrinkage rate in the length direction of 10% or less at 60°C, and more preferably in both the length direction and width direction of 10% or less. Such gas barrier shrink films are preferable because they can suppress dimensional changes in the film during transportation and/or storage.

The gas barrier properties of the gas barrier shrink film against oxygen are measured by oxygen permeability under oxygen conditions of 65% RH and a measurement temperature of 23° C. From the viewpoint of being able to suppress oxygen permeation and being suitable for gas pack packaging, the oxygen permeability is preferably 17.0 cm ³ ·20 μm/m ² ·24 h ·atm or less, and more preferably 16.0 cm ³ ·20 μm/m ² ·24 h ·atm or less. A gas barrier shrink film having such gas barrier properties can sufficiently suppress oxygen permeation.

The tensile stress at break in the MD direction of the gas barrier shrink film of this embodiment is preferably 85 MPa or more, more preferably 90 MPa or more, from the viewpoint of mechanical properties.The tensile stress at break in the TD direction of the gas barrier shrink film of this embodiment is preferably 65 MPa or more, more preferably 70 to MPa or more, from the viewpoint of mechanical properties.
The tensile breaking stress can be measured by the method described in the Examples below.

The tensile elongation at break in the MD direction of the gas barrier shrink film of this embodiment is preferably 145% or more, more preferably 150% or more, from the viewpoints of tear resistance and flexibility. The tensile elongation at break in the TD direction of the gas barrier shrink film of this embodiment is preferably 160% or more, more preferably 180% or more, from the viewpoints of tear resistance and flexibility.
The tensile elongation at break can be measured by the method described in the Examples below.

The heat shrinkage rate of the gas barrier shrink film of this embodiment in the MD direction at 100°C is preferably 30% or more, more preferably 30 to 45%, and even more preferably 35 to 45%, in order to improve the finish of the package. The heat shrinkage rate of the gas barrier shrink film of this embodiment in the TD direction at 100°C is preferably 25% or more, more preferably 25 to 40%, and even more preferably 30 to 40%, in order to improve the finish of the package.
The heat shrinkage rate can be measured by the method described in the Examples below.

The maximum heat shrinkage stress in the MD direction of the gas barrier shrink film of this embodiment at 100°C is preferably 3.0 MPa or less, more preferably 1.0 to 2.5 MPa, in order to suppress deformation of the tray of the package. The maximum heat shrinkage stress in the TD direction of the gas barrier shrink film of this embodiment at 100°C is preferably 2.5 MPa or less, more preferably 1.0 to 2.0 MPa, in order to suppress deformation of the tray of the package.
The heat shrinkage stress can be measured by the method described in the Examples below.

The nitrogen gas permeability of the gas barrier shrink film of this embodiment at 23°C and 65% RH is preferably 1.6 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, more preferably 1.5 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, from the viewpoint of gas barrier properties. The nitrogen gas permeability of the gas barrier shrink film of this embodiment at 5°C and 90% RH is preferably 0.2 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, more preferably 0.1 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, from the viewpoint of gas barrier properties.
The nitrogen gas permeability can be measured by the method described in the Examples below.

The carbon dioxide gas permeability of the gas barrier shrink film of this embodiment at 23°C and 65% RH is preferably 52.0 ^cm3 ·20μm/ ^m2 ·24h·atm or less, more preferably 51.0 ^cm3 ·20μm/ ^m2 ·24h·atm or less, from the viewpoint of gas barrier properties. The carbon dioxide gas permeability of the gas barrier shrink film of this embodiment at 5°C and 60% RH is preferably 10.0 ^cm3 ·20μm/ ^m2 ·24h·atm or less, more preferably 9.0 ^cm3 ·20μm/ ^m2 ·24h·atm or less, from the viewpoint of gas barrier properties.
The carbon dioxide gas permeability can be measured by the method described in the Examples below.

The oxygen gas permeability of the gas barrier shrink film of this embodiment at 23°C and 65% RH is preferably 17.0 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, more preferably 16.0 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, from the viewpoint of gas barrier properties. The oxygen gas permeability of the gas barrier shrink film of this embodiment at 5°C and 90% RH is preferably 7.5 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, more preferably 6.5 ^cm3 ·20 μm/ ^m2 ·24 h·atm or less, from the viewpoint of gas barrier properties.
The oxygen gas permeability can be measured by the method described in the Examples below.

The gas barrier shrink film of this embodiment can be used for food packaging, and is particularly suitable for food packaging applications when storing at low temperatures (e.g., above 0°C and below 10°C).

[Method of producing heat shrinkable film]
The method for producing the heat-shrinkable laminated film is not particularly limited, but may be, for example, the following method.
The method for producing the gas barrier shrink film according to the present embodiment preferably includes a step of laminating a laminate having at least four resin layers (hereinafter sometimes referred to as "unstretched raw sheet") by a co-extrusion method and heat-stretching the laminate. The co-extrusion method will be described below.

In the co-extrusion method, each material is melt-extruded from a separate extruder, laminated in a multi-layer die, melt-extruded, and quenched to obtain an unstretched raw material. The method of melt-extrusion is not particularly limited, and examples include a method using a multi-layer T-die or a multi-layer circular die. Among these, a method using a multi-layer circular die is preferred. The use of a multi-layer circular die is advantageous in terms of the space required for equipment and the investment amount, is suitable for small-lot production of a wide variety of products, and makes it easier to obtain the desired heat shrinkage rate.

The coolant used for rapid cooling is usually water at 60°C or less. The coolant can be brought into direct contact with the molten resin or used indirectly as an internal coolant for the metal roll. When used as an internal coolant, other known coolants such as oil can be used in addition to water, and in some cases it can be used in combination with blowing cold air.

In the stretching process, the unstretched raw film obtained is heated, for example, to a temperature equal to or higher than the softening temperature of the resin constituting the unstretched raw film, and stretched, for example, 1.5 times or more in the MD (machine direction: longitudinal direction (length direction)) and 3 times or more in the TD (transverse direction: transverse direction (width direction)). By using such a stretching process, a gas barrier shrink film having the above-mentioned specified heat shrinkage rate can be easily obtained.

The stretching ratio is appropriately selected depending on the purpose, and if necessary, heat treatment (thermal relaxation treatment) can be performed after stretching. Thermal relaxation treatment relaxes the molecular orientation of the gas barrier shrink film, further suppressing dimensional changes during transportation and/or storage.

The stretching process can also be carried out by a direct inflation method, in which air or nitrogen is blown into the tube immediately after melt extrusion to stretch it. This method also makes it easy to obtain a gas barrier shrink film with a specified heat shrinkage rate. However, in order to more reliably achieve an appropriate heat shrinkage rate, a biaxial stretching method is preferred, and a tubular method (also called a double bubble method) in which the unstretched raw material obtained by the circular die described above is heated and biaxially stretched is more preferred. In other words, the gas barrier shrink film of this embodiment is preferably a biaxially stretched multilayer film produced by the tubular method in which biaxial stretching is performed.

The above manufacturing method may include a crosslinking step in which the resin is crosslinked before or after stretching.

When crosslinking is performed, it is preferable to perform the crosslinking treatment by irradiating with energy rays before heating and stretching the resin. This increases the melt tension of the laminate during heating and stretching, making it possible to further stabilize the stretching. Note that the laminate after stretching may be irradiated with energy rays to crosslink the resin. Examples of energy rays that can be used include ionizing radiation such as ultraviolet rays, electron beams, X-rays, and gamma rays. Of these, electron beams are preferred.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[evaluation]
The melting points of the resins constituting the laminated films, and the tensile stress at break, tensile elongation at break, heat shrinkage rate, heat shrinkage stress, nitrogen gas permeability, carbon dioxide gas permeability, and oxygen gas permeability of the laminated films of the examples and comparative examples were measured and evaluated by the following methods. The adhesive layers were also observed by the following methods.

(Melting Point)
The melting point is measured using differential scanning calorimetry (DSC). 5 mg of resin was weighed, and in order to cancel the thermal history of the resin, the temperature was increased from 0°C to 300°C at a heating rate of 10°C/min, and then decreased from 300°C to 0°C at a heating rate of 10°C/min. After that, the endothermic peak when the temperature was increased from 0°C to 300°C at a heating rate of 10°C/min was measured as the melting point.

(Tensile breaking stress/tensile breaking elongation)
The tensile breaking stress is measured in accordance with ASTM-D882. Using a tensile tester, the sample is pulled at a speed of 200 mm/min, and the stress (the value obtained by dividing the tensile load value by the cross-sectional area of the test piece) and the elongation (elongation) are determined when the sample breaks (breaks). The tensile elongation is calculated by the following formula.
Tensile elongation (%)=100×(L−L ₀ )/L ₀
L ₀ : Sample length before test L : Sample length at break

(Heat shrinkage rate at 100°C)
A film cut into a square having a length and width of 100 mm is placed in a thermostatic chamber set at an atmospheric temperature of 100° C. for 30 minutes, and the elasticity is calculated according to the following formula.
Heat shrinkage rate (%) = 100 x (100 - W) / 100
W: Dimension of the film after removal from the thermostatic chamber

(Maximum heat shrinkage stress at 100°C)
The maximum heat shrinkage stress is measured in accordance with ASTM-D2838. The film is sampled in a strip of 90 mm in the length direction/or width direction (measurement length 50 mm + chuck grip 40 mm) and 10 mm in the width direction/or length direction, and the maximum heat shrinkage stress after immersion for 3 minutes in an oil bath at 100°C is measured.

(Nitrogen gas permeability)
The nitrogen gas permeability was measured in accordance with JIS K7126-1 (differential pressure method).
The measuring device used was a differential pressure gas/vapor transmission rate measuring device [GTR-30XAD2, G2700T・F], [GTR-30XAD, G6800T・F(S)] (manufactured by GTR Tech Co., Ltd. and Yanaco Technical Science Co., Ltd.). The detector used was a gas chromatograph [thermal conductivity detector (TCD)]. The test conditions were: temperature 23°C, humidity 65% RH, partial pressure 74.6 cmHg for gas, 1.4 cmHg for water vapor, and temperature 5°C, humidity 90% RH, partial pressure 75.4 cmHg for gas, 0.6 cmHg for water vapor. The permeation direction was from the food contact side. The permeation area was 15.2×10 ^-4 m ² (permeation part diameter φ4.4×10 ^-2 m), and the number of tests was n=1.
<Evaluation criteria for nitrogen gas permeability under conditions of 23°C and 65% RH>
◯ (Good): 1.5 cm ³ 20 μm/m ² 24 h atm or less △ (Slightly poor): Over 1.5 cm ³ 20 μm/m ² 24 h atm 1.6 cm ³ 20 μm/m ² 24 h atm or less × (Poor): Over 1.6 cm ³ 20 μm/m ² 24 h atm <Evaluation criteria for nitrogen gas permeability under conditions of 5°C and 90% RH>
◯ (Good): 0.1 cm ³ 20 μm/m ² 24 h atm or less △ (Slightly poor): 0.1 cm ³ 20 μm/m ² 24 h atm or more 0.2 cm ³ 20 μm/m ² 24 h atm or less × (Poor): 0.2 cm ³ 20 μm/m ² 24 h atm or more

(Carbon dioxide gas permeability)
The carbon dioxide gas permeability was measured in accordance with JIS K7126-1 (differential pressure method).
The measuring device used was a differential pressure type gas/vapor transmission rate measuring device [GTR-30XAD2, G2700T・F], [GTR-30XAD, G6800T・F(S)] (manufactured by GTR Tech Co., Ltd. and Yanaco Technical Science Co., Ltd.). The detector used was a gas chromatograph [thermal conductivity detector (TCD)]. The test conditions were: temperature 23°C, humidity 65% RH, partial pressure 74.6 cmHg for gas, 1.4 cmHg for water vapor, and temperature 5°C, humidity 60% RH, partial pressure 75.6 cmHg for gas, 0.4 cmHg for water vapor. The permeation direction was from the food contact side. The permeation area was 15.2×10 ^-4 m ² (permeation part diameter φ4.4×10 ^-2 m), and the number of tests was n=1.
<Evaluation criteria for carbon dioxide gas permeability under conditions of 23°C and 65% RH>
◯ (Good): 51.0 cm ³ 20 μm/m ² 24 h atm or less △ (Slightly poor): Over 51.0 cm ³ 20 μm/m ² 24 h atm 52.0 cm ³ 20 μm/m ² 24 h atm or less × (Poor): Over 52.0 cm ³ 20 μm/m ² 24 h atm <Evaluation criteria for carbon dioxide gas permeability under conditions of 5°C 60% RH>
◯ (Good): 9.0 cm ³ 20 μm/m ² 24 h atm or less △ (Slightly poor): 9.0 cm ³ 20 μm/m ² 24 h atm or more 10.0 cm ³ 20 μm/m ² 24 h atm or less × (Poor): 10.0 cm ³ 20 μm/m ² 24 h atm or more

(Oxygen Gas Permeability) The oxygen gas permeability was measured in accordance with JIS K7126-1 (differential pressure method).
The measuring device used was a differential pressure type gas/vapor transmission rate measuring device [GTR-30XAD2, G2700T・F], [GTR-30XAD, G6800T・F(S)] (manufactured by GTR Tech Co., Ltd. and Yanaco Technical Science Co., Ltd.). The detector used was a gas chromatograph [thermal conductivity detector (TCD)]. The test conditions were: temperature 23°C, humidity 65% RH, partial pressure 74.6 cmHg for gas, 1.4 cmHg for water vapor, and temperature 5°C, humidity 90% RH, partial pressure 75.4 cmHg for gas, 0.6 cmHg for water vapor. The permeation direction was from the food contact side. The permeation area was 15.2×10 ^-4 m ² (permeation part diameter φ4.4×10 ^-2 m), and the number of tests was n=1.
<Evaluation criteria for oxygen gas permeability under conditions of 23°C and 65% RH>
◯ (Good): 16.0 cm ³ 20 μm/m ² 24 h atm or less △ (Slightly poor): Over 16.0 cm ³ 20 μm/m ² 24 h atm 17.0 cm ³ 20 μm/m ² 24 h atm or less × (Poor): Over 17.0 cm ³ 20 μm/m ² 24 h atm <Evaluation criteria for oxygen gas permeability under conditions of 5°C and 90% RH>
◯ (Good): 6.5 cm ³ 20 μm/m ² 24 h atm or less △ (Slightly poor): 6.5 cm ³ 20 μm/m ² 24 h atm or more 7.5 cm ³ 20 μm/m ² 24 h atm or less × (Poor): 7.5 cm ³ 20 μm/m ² 24 h atm or more

(Cross-section observation)
The cross sections of the heat-shrinkable laminated films obtained in the Examples and Comparative Examples were observed by the following method.
Measuring device: JEM-2100F (FE-TEM, manufactured by JEOL Ltd.)
Measurement conditions: Acceleration voltage 200 kV
Pretreatment conditions: The sample was embedded in a room temperature curing epoxy resin to prepare a cutting block. After staining the cutting block with RuO4, an ultrathin section with a thickness of 100 nm was prepared by wet cutting using an ultramicrotome (with a diamond knife).
Observation conditions: Ultrathin sections were observed using a TEM and enlarged photographs were taken (magnifications: 20,000 to 40,000 times).
Two points on the cross section of the adhesive layer (D) were observed, and when a phase separation structure was confirmed, a plurality of island structures were spread uniformly throughout the continuous phase, and the ratio of the total area of each island structure to the total area (island area ratio) was 15 to 45%, it was judged that the sea-island structure was "present," and when it was more than 0% and less than 15%, it was judged that the sea-island structure was "few." Similarly, two points on the cross section of the adhesive layer (D) were observed, and when a phase separation structure was not confirmed and no island structures were observed, it was evaluated as "absent."
The island area ratio was determined by the following method.
An image with a viewing angle of 1350 nm x 900 nm was divided vertically and horizontally at 50 nm intervals, and the number of squares occupied by island structures was counted out of the total number of 50 nm x 50 nm squares formed. The number of squares occupied by island structures was divided by the total number of squares to calculate the island area ratio at one point on the cross section. The average value of the island area ratios at two points on the cross section was then taken as the island area ratio.

[Resin used]
The resins used in the examples and comparative examples are as follows.
Ethylene-vinyl alcohol copolymer EVOH1: saponified ethylene-vinyl alcohol copolymer (product name "Soarnol AT4403B", manufactured by Mitsubishi Chemical Corporation, MFR 3.5 g/10 min, ethylene content 44 mol%)
EVOH2: saponified ethylene-vinyl alcohol copolymer (product name "G-Soarnol GH3804B", manufactured by Mitsubishi Chemical Corporation, MFR 4.0 g/10 min, ethylene content 38 mol%)
Ethylene-α-olefin copolymer LLDPE1: ethylene-α-olefin copolymer (product name "Yumerit 0520F", manufactured by Ube Industries, Ltd., density 904 kg/m ³ , MFR 2.0 g/10 min, melting point 118° C.)
LLDPE2: ethylene-α-olefin copolymer (product name "Sumikathene E FV203", manufactured by Sumitomo Chemical Co., Ltd., density 913 kg/m ³ , MFR 2.0 g/10 min, melting point 115° C.)
LLDPE3: ethylene-α-olefin copolymer (product name "SLH218", manufactured by Braskem, density 916 kg/m ³ , MFR 2.3 g/10 min, melting point 125° C.)
Polypropylene PP1: propylene homopolymer (product name "SunAllomer PL500A", manufactured by SunAllomer Co., Ltd., density 900 kg/m ³ , MFR 3.5 g/10 min, melting point 160° C.)
PP2: Ethylene-propylene copolymer (product name "SunAllomer PC540R", manufactured by SunAllomer Co., Ltd., density 900 kg/m ³ , MFR 5.3 g/10 min, melting point 132° C.)
Polyolefin elastomer POP1: ethylene-1-octene copolymer (product name "Affinity PT1450G1", manufactured by Dow Chemical Japan, density 902 kg/m ³ , MFR 7.5 g/10 min, melting point 97° C.)
POP2: Ethylene-propylene copolymer (product name "Versify3200", manufactured by Dow Chemical Japan, density 876 kg/m ³ , MFR 8.0 g/10 min, melting point 81° C.)
Adhesive resin PEGL1: modified ethylene-α-olefin copolymer (product name "Admer NF587", manufactured by Mitsui Chemicals, Inc., density 907 kg/m ³ , MFR 2.3 g/10 min, melting point 120° C.)
PEGL2: modified ethylene-α-olefin copolymer (product name "Admer NF307", manufactured by Mitsui Chemicals, Inc., density 922 kg/m ³ , MFR 1.3 g/10 min, melting point 120° C.)
PEGL3: modified ethylene-α-olefin copolymer (product name "Admer NF550", manufactured by Mitsui Chemicals, Inc., density 910 kg/m ³ , MFR 6.2 g/10 min, melting point 120° C.)
PPGL1: Modified polypropylene polymer (product name "Admer QF580", manufactured by Mitsui Chemicals, Inc., density 896 kg/m ³ , MFR 7.7 g/10 min, melting point 140° C.)
PPGL2: Modified polypropylene polymer (product name "Admer QF500", manufactured by Mitsui Chemicals, Inc., density 901 kg/m ³ , MFR 3.0 g/10 min, melting point 165° C.)
Polyamide NY1: Polyamide 6 (product name "UBE NYLON 1022B", manufactured by Ube Industries, Ltd., density 1140 kg/m ³ , melting point 225° C.)
NY2: Polyamide 6/66 copolymer (product name "UBE NYLON 5034B", manufactured by Ube Industries, Ltd., density 1140 kg/m ³ , melting point 192° C.)
NY3: Hexamethylenediamine/phthalic acid copolymer nylon (amorphous nylon) (product name "Sealer PA3426", manufactured by Dow Mitsui Polychemicals, density 1190 kg/m ³ )
High density polyethylene HDPE1: high density polyethylene (product name "Hi-Zex 5000SF", manufactured by Prime Polymer Co., Ltd., density 956 kg/m ³ , MFR 0.66 g/10 min, melting point 133° C.)
HDPE2: High density polyethylene (product name "Hi-Zex 3300F", manufactured by Prime Polymer Co., Ltd., density 950 kg/m ³ , MFR 1.1 g/10 min, melting point 132° C.)
HDPE3: High density polyethylene (product name "Suntech HD S362", manufactured by Asahi Kasei Corporation, density 952 kg/m ³ , MFR 0.8 g/10 min, melting point 132° C.)
HDPE4: High density polyethylene (product name "Suntech HD B161", manufactured by Asahi Kasei Corporation, density 963 kg/m ³ , MFR 1.35 g/10 min, melting point 134° C.)
HDPE5: High density polyethylene (product name "SGF4950", manufactured by Asahi Kasei Corporation, density 956 kg/m ³ , MFR 0.34 g/10 min, melting point 133° C.)
The MFR is a value measured in accordance with JIS K7210 or ASTM D1238. The melting point is a value measured by differential scanning calorimetry (DSC).

[Examples 1 to 11, Comparative Examples 1 to 3]
A five-layer film was molded by double bubble inflation using the resins shown in Table 1. The film was then biaxially stretched in the MD and TD directions at the stretch ratios shown in Table 1 to obtain a stretched laminate film. The thickness of each layer is as shown in Table 1.
The stretch ratio in the MD direction was adjusted by the speed ratio of the pinch rolls between the bubbles, and the stretch ratio in the TD direction was adjusted by the volume of air enclosed in the bubbles. The MD direction here refers to the longitudinal direction (flow direction) when the laminated film is extruded. On the other hand, the TD direction refers to the width direction (transverse direction) when the laminated film is extruded.

In the gas barrier shrink films of Examples 1 to 12, the adhesive layer between the base layer (A) and the gas barrier layer (B) was confirmed to have a sea-island structure and was the adhesive layer (D), but the adhesive layer between the gas barrier layer (B) and the heat seal layer (C) was not confirmed to have a sea-island structure and was another adhesive layer.
In addition, in the heat-shrinkable laminate film of Example 13, the adhesive layer between the gas barrier layer (B) and the heat seal layer (C) was confirmed to have a sea-island structure and was the adhesive layer (D), but the adhesive layer between the base material layer (A) and the gas barrier layer (B) was not confirmed to have a sea-island structure and was another adhesive layer.
In Comparative Examples 1 to 4, no sea-island structure was observed in any of the adhesive layers.

Claims (6)

The film has at least four layers: a base layer (A), an adhesive layer (D), a gas barrier layer (B), and a heat seal layer (C);
The adhesive layer (D) contains two types of thermoplastic resins (X) and (Y) and has a sea-island structure,
The thermoplastic resin (X) is a modified LLDPE,
The thermoplastic resin (Y) is HDPE,
the base layer (A) contains an aliphatic polyamide polymer, an aliphatic polyamide copolymer, or a polypropylene-based resin,
the base material layer (A), the adhesive layer (D), the gas barrier layer (B), the adhesive layer, and the heat seal layer (C) are in contact with each other in this order, and the adhesive layer between the gas barrier layer (B) and the heat seal layer (C) is the adhesive layer (D) or an adhesive layer other than the adhesive layer (D);
A gas barrier shrink film characterized by: 2. The gas barrier shrink film according to claim 1, wherein the sea-island structure is a sea-island structure consisting of a continuous phase in which the thermoplastic resin (X) and the thermoplastic resin (Y) are in a partially compatible state, and a dispersed phase made of the thermoplastic resin (X). 3. The gas barrier shrink film according to claim 1, wherein the adhesive layer (D) contains two thermoplastic resins (X) and (Y), and the density dx of the thermoplastic resin (X) and the density dy of the thermoplastic resin ( Y ) satisfy dy-dx>30 kg/ ^m3 . 4. The gas barrier shrink film according to claim 1 , wherein a mass ratio of the thermoplastic resin (X) to the thermoplastic resin (Y) is 10:90 to 90:10. 5. The gas barrier shrink film according to claim 1, wherein the heat seal layer (C) contains at least a polyethylene resin. 6. The gas barrier shrink film according to claim 1, which has a thermal shrinkage rate in the longitudinal direction at 100°C of 30% or more.