Multilayer film, container, and method for manufacturing the container

Abstract

To achieve a multilayer film that has resistance to bending when thermally fused to a previously three-dimensionally molded substrate.SOLUTION: A multilayer film 10 is thermally fused to a previously three-dimensionally molded, paper-made substrate with an adhesion layer 16 in contact with the same. The multilayer film 10 includes an outermost layer 11, an intermediate layer 13, a holding layer 15, and the adhesion layer 16 in contact with the holding layer 15, in the stated order. The outermost layer 11 and the holding layer 15 include the same kind of resin.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multilayer film, a container, and a method for manufacturing a container. [Background technology]

[0002] Multilayer films that are bonded to a pre-formed three-dimensional substrate are known. For example, Patent Document 1 discloses a laminate film for paper containers that is bonded to paper containers. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-146996 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In the technology disclosed in Patent Document 1, a multilayer film is heated and then adhered to a pre-formed three-dimensional substrate, such as a paper cup. However, because the multilayer film has multiple layers, differences in the behavior of these layers during heating sometimes caused the multilayer film to bend during heating. As a result, the multilayer film was heat-sealed to the flange portion of the three-dimensional substrate in a folded state, leading to a problem of poor sealing with the lid material.

[0005] Therefore, there is a need to realize a multilayer film that exhibits minimal curvature when heat-sealed to a pre-formed three-dimensional substrate. [Means for solving the problem]

[0006] The multilayer film according to the present invention is A multilayer film that is heat-sealed to a pre-formed three-dimensional paper substrate with an adhesive layer in contact with it, The outermost layer, the intermediate layer, the retaining layer, and the adhesive layer in contact with the retaining layer are provided in the order described above. The outermost layer and the retaining layer are characterized by containing the same type of resin.

[0007] Furthermore, the container according to the present invention is The invention comprises a pre-formed three-dimensional substrate and the above-mentioned multilayer film, The multilayer film is characterized in that it is heat-fused to the substrate.

[0008] Furthermore, the container manufacturing method according to the present invention is A base material molding process for forming a base material having a three-dimensional shape from a flat paper material, A multilayer film forming step comprising forming a multilayer film having, in the order described above, an outermost layer, an intermediate layer, a retaining layer containing the same type of resin as the outermost layer, and an adhesive layer in contact with the retaining layer, A heating and softening step in which the multilayer film is heated and softened, The present invention is characterized by including a lamination step, in which the multilayer film softened by the heating and softening step is heat-fused to the substrate by being brought into close contact with the adhesive layer.

[0009] With these configurations, the outermost layer and the retaining layer of the multilayer film contain the same type of resin, which increases the symmetry of the layer structure in the lamination direction. As a result, the difference in behavior between one side and the other side in the lamination direction is reduced during heating. Therefore, it is possible to reduce the curvature of the multilayer film when heat-sealing it to a pre-formed three-dimensional substrate.

[0010] Preferred embodiments of the present invention will be described below. However, the scope of the present invention is not limited by the examples of preferred embodiments described below.

[0011] The above multilayer film, in one embodiment, It is preferable that each layer from the outermost layer to the intermediate layer and each layer from the retaining layer to the intermediate layer have symmetry in terms of thickness and material.

[0012] The above multilayer film, in one embodiment, The resin of the same kind is preferably a polyolefin resin of the same kind.

[0013] As one aspect, the above multilayer film The thickness of the entire multilayer film is preferably 200 μm or less, and the ratio of the thickness of the adhesive layer to the thickness of the entire multilayer film is preferably 25% or less.

[0014] As one aspect, the above multilayer film The adhesive layer preferably contains a modified polyolefin resin having a polar group.

[0015] As one aspect, the above multilayer film The intermediate layer is preferably a gas barrier layer containing an ethylene-vinyl alcohol copolymer.

[0016] As one aspect, the above multilayer film The outermost layer contains an ethylene-based resin material. When the 10 cm × 3 cm multilayer film with the short side as the flow direction is placed on the hot plate with the outermost layer in contact with the surface of the hot plate and held at 145 °C for 1 minute, the heating curvature amount, which is the maximum distance from the outermost layer of the multilayer film to the surface of the hot plate, is preferably 0.5 cm or less.

[0017] According to these configurations, it becomes possible to further reduce the curvature of the multilayer film when heat-sealing to a three-dimensionally pre-formed substrate.

[0018] As one aspect, the above multilayer film It is preferably irradiated with an electron beam under the condition of an absorbed dose of 15 to 250 kGy, and more preferably irradiated with an electron beam under the condition of an absorbed dose of 30 to 125 kGy.

[0019] These configurations allow for high heat resistance because the material is irradiated with an absorbed dose of 15 kGy or more, and also prevent excessive thermal contraction because the material is irradiated with an absorbed dose of 250 kGy or more.

[0020] The above multilayer film, in one embodiment, Preferably, the heating curvature, which is the maximum distance from the outermost layer of the multilayer film to the surface of the heating plate, is 1 cm or less after placing the 10 cm x 3 cm multilayer film, with the flow direction as the shorter side, on the heating plate with the outermost layer in contact with the surface of the heating plate and holding it at 145°C for 1 minute.

[0021] The above multilayer film, in one embodiment, It is preferable that the heating curvature angle, which is the angle between the surface of the heating plate and the tangent to the edge of the multilayer film, is 45° or less after placing the 10cm x 3cm multilayer film, with the flow direction as the shorter side, on the heating plate with the outermost layer in contact with the surface of the heating plate and holding it at 145°C for 1 minute.

[0022] These configurations make it possible to reduce the curvature of the multilayer film when heat-sealing it to a pre-formed three-dimensional substrate.

[0023] These configurations make it possible to reduce the curvature of the multilayer film when heat-sealing it to a pre-formed three-dimensional substrate.

[0024] The above multilayer film, in one embodiment, It is preferable that it is intended for microwaveable foods.

[0025] The above-mentioned multilayer film, which has high heat resistance, can be suitably used for the above-mentioned applications.

[0026] The above container, in one embodiment, It is preferable that it has resistance to range expansion.

[0027] Because it is microwave-safe, the above container can be suitably used for microwaveable foods.

[0028] Further features and advantages of the present invention will become clearer through the following description of exemplary and non-limiting embodiments, with reference to the drawings. [Brief explanation of the drawing]

[0029] [Figure 1] This is a schematic cross-sectional diagram showing the layer structure of a multilayer film. [Figure 2] This is a schematic cross-sectional view showing the structure of the container. [Figure 3] This figure shows a method for measuring the curvature of a multilayer film. [Figure 4] This is a flowchart showing the method for manufacturing a container. [Modes for carrying out the invention]

[0030] 1. First Embodiment In the following description, a multilayer film 10 according to the first embodiment will be explained with reference to the drawings. Figure 1 is a schematic cross-sectional view showing the layer structure of the multilayer film 10. Figure 2 is a schematic cross-sectional view showing the structure of the container 20. The multilayer film 10 is a film that is heat-sealed to a pre-formed three-dimensional substrate 21 with an adhesive layer 16 in contact with it. The multilayer film 10 comprises, in the order described, an outermost layer 11 provided on the first surface side, an intermediate layer 13, a retaining layer 15, and an adhesive layer 16 provided on the second surface side. The adhesive layer 16 is in contact with the retaining layer 15. The container 20 is formed when this multilayer film 10 is heat-sealed to the substrate 21. In the illustrated example, the multilayer film 10 comprises, in order from the outermost layer 11 side, a first intermediate adhesive layer 12, an intermediate layer 13, a second intermediate adhesive layer 14, a retaining layer 15, and an adhesive layer 16.

[0031] The outermost layer 11 and the retaining layer 15 each contain the same type of resin. Here, "same type of resin" means resins that have common constituent units, and in both cases, the ratio of the amount of common constituent units (moles) to the total amount of constituent units (moles) is 20 mol% or more.

[0032] To explain in detail using ethylene-based resins as an example, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), metallocene-catalyzed linear low-density polyethylene (mLLDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), ethylene-methyl acrylate copolymer (EMA resin), ethylene-methyl methacrylate copolymer (EMMA resin), and ethylene-vinyl alcohol copolymer are considered the same type because the ratio of the amount of constituent units derived from ethylene (moles) to the total amount of constituent units (moles) is 20 mol% or more. On the other hand, among propylene-ethylene random copolymers and propylene-ethylene block copolymers, etc., those in which the ratio of the amount of constituent units derived from ethylene (moles) to the total amount of constituent units (moles) is less than 20 mol% are not considered the same type as the low-density polyethylene mentioned above.

[0033] For resins of the same type, the ratio of the amount of common constituent units (moles) to the total amount of constituent units (moles) is preferably 30 mol% or more, more preferably 40 mol% or more, and even more preferably 50 mol% or more.

[0034] In this embodiment, the outermost layer 11 and the retaining layer 15 each contain the same type of polyolefin resin. The definition of "same type of polyolefin resin" is as described above. Furthermore, "polyolefin resin" refers to a polymer containing units derived from olefins, and examples include homopolymers of olefins such as polyethylene, polypropylene, polybutene, and poly-methylpentene, copolymers of two or more olefins, or copolymers of one or more olefins with other heterogeneous components.

[0035] The outermost layer 11 is a layer that comes into direct contact with the contents of the container 20 and is preferably composed of various thermoplastic resins. The outermost layer 11 is composed of, for example, polyethylene resin, polypropylene resin, polyamide resin, polyester resin, polycarbonate resin, polystyrene resin, and thermoplastic elastomer. Preferably, the outermost layer 11 is composed of the aforementioned polyolefin resin. The outermost layer 11 may contain only one of the aforementioned polyolefin resins, or it may contain two or more. The outermost layer 11 may be composed of a polyolefin resin alone, or it may be composed of a mixture of a polyolefin resin and other materials. The other materials are not particularly limited and can be arbitrarily selected depending on the purpose.

[0036] In this embodiment, the outermost layer 11 is composed of the same polyethylene resin as the retaining layer 15. "Polyethylene resin" refers to a polymer containing ethylene as at least one monomer unit. Examples of the aforementioned polyethylene resins include polyethylene, ethylene-vinyl acetate resin, ethylene-vinyl alcohol resin, ethylene-aliphatic unsaturated carboxylic acid resin, ethylene-aliphatic unsaturated carboxylic acid ester resin, and ionomer resin. The polyethylene is not particularly limited, but examples include high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDEP). The outermost layer 11 may contain only one type of polyethylene resin, or it may contain two or more types. Preferably, the outermost layer 11 contains 10% by mass or more of polyethylene. More preferably, it contains 25% by mass or more of polyethylene. Even more preferably, it contains 50% by mass or more of polyethylene.

[0037] If the outermost layer 11 contains polyethylene, the crosslinking density of the outermost layer 11 can be improved by irradiating the multilayer film 10 with an electron beam from the outermost layer 11 side. Preferably, the outermost layer 11 contains low-density polyethylene. In this way, the crosslinking density of the outermost layer 11 can be further improved by irradiating the multilayer film 10 with an electron beam from the outermost layer 11 side.

[0038] The outermost layer 11 is preferably a layer that can be heat-sealed to an ethylene-based resin material. "Ethylene-based resin material" refers to a resin material that contains ethylene as at least one monomer unit. Examples of ethylene-based resin materials include members composed of polyethylene resin, ethylene-vinyl acetate resin, etc. Examples of polyethylene resins include homopolymers of ethylene, block copolymers of ethylene and other monomers, or random copolymers of ethylene and other monomers. Examples of ethylene-vinyl acetate resins include mixtures or copolymers of ethylene and vinyl acetate.

[0039] The first intermediate adhesive layer 12 and the second intermediate adhesive layer 14 are layers that have the function of bonding adjacent layers on both sides. That is, the first intermediate adhesive layer 12 is a layer that bonds the outermost layer 11 and the intermediate layer 13, and the second intermediate adhesive layer 14 is a layer that bonds the intermediate layer 13 and the retaining layer 15. Known adhesive polymers can be used as the materials constituting the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14, and the material used is selected according to the combination of two layers to be bonded by each layer. It is desirable that the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14 contain the same type of resin.

[0040] The function of the intermediate layer 13 is not particularly limited, but preferably it imparts gas barrier properties to the multilayer film 10. "Gas barrier properties" refer to the resistance to gas permeation. The intermediate layer 13 imparts gas barrier properties to the multilayer film 10 against, for example, carbon dioxide (CO2), nitrogen gas (N2), and oxygen gas (O2). In this embodiment, the intermediate layer 13 is a gas barrier layer that imparts gas barrier properties to the multilayer film 10 against oxygen gas (O2) in particular, and is configured to include an ethylene-vinyl alcohol copolymer. Here, it is preferable to use an ethylene-vinyl alcohol copolymer with an ethylene content of 38 mol% or less, as this results in particularly high gas barrier properties. In addition to the ethylene-vinyl alcohol copolymer, examples of suitable materials to be included in the intermediate layer 13 include polyamide MXD6 and polyvinylidene chloride. Furthermore, the intermediate layer 13 may contain additives with gas barrier properties (such as mica or clay).

[0041] The material and thickness of the intermediate layer 13 significantly affect the gas barrier properties of the multilayer film 10; therefore, the material and thickness of the intermediate layer 13 must be selected within a range that exhibits the required gas barrier properties. The material and thickness of the intermediate layer 13 are determined by the oxygen permeability measured according to JIS K 7126-2:2006 (isobaric method) for a film formed with the material and thickness, which should be 5.0 ml / m². 2 It is determined to be less than or equal to ·day·atm. For example, if the intermediate layer 13 is made of an ethylene-vinyl alcohol copolymer with an ethylene content of 38 mol% or less, and its thickness is 6 μm or more, the oxygen permeability will be 5.0 ml / m 2 It is easy to achieve the standard of 3.0 ml / m² or less. The material and thickness of the intermediate layer 13 are such that the oxygen permeability of the film formed from the material to the specified thickness is 3.0 ml / m². 2 It is more preferable that it be determined to be less than or equal to 1.0 ml / m³ / day·atm. 2 It is even more preferable that it be determined to be less than or equal to the day·atm.

[0042] The retaining layer 15 is provided in contact with the adhesive layer 16. As described above, the retaining layer 15 contains the same type of resin as the outermost layer 11. The retaining layer 15 is a layer that holds the adhesive layer 16 and has the function of preventing the adhesive layer 16 from peeling off from the other parts of the multilayer film 10. For example, the retaining layer 15 can be made of a material having a molecular structure common to the material constituting the adhesive layer 16, or a material with a melting point close to the material constituting the adhesive layer 16. Preferably, the retaining layer 15 contains the same type of polyolefin resin as the outermost layer 11. The retaining layer 15 may contain only one type of polyolefin resin, or it may contain two or more types. Furthermore, the retaining layer 15 may be made of a polyolefin resin alone, or it may be made of a mixture of a polyolefin resin and other materials. The other materials are not particularly limited and can be arbitrarily selected depending on the purpose.

[0043] In this embodiment, the retaining layer 15 is made of the same polyethylene resin as the outermost layer 11. Since polyethylene has a molecular structure common to modified polyolefin resins suitably used in the adhesive layer 16, it has high adhesion to the adhesive layer 16, thereby suitably preventing the peeling of the adhesive layer 16. The retaining layer 15 may contain only one type of polyethylene resin, or it may contain two or more types. Preferably, the retaining layer 15 contains 10% by mass or more of polyethylene. More preferably, it contains 25% by mass or more of polyethylene. Even more preferably, it contains 50% by mass or more of polyethylene.

[0044] The adhesive layer 16 is a layer that can be adhered to a pre-formed three-dimensional substrate 21. Preferably, the adhesive layer 16 is a heat-sealable layer that can be heat-sealed to the substrate 21. The substrate 21 is a paper substrate made of paper. It is desirable that the adhesive layer 16 contains a modified polyolefin resin having polar groups. The inclusion of a modified polyolefin resin in the adhesive layer 16 provides excellent adhesion to the paper substrate 21. It is preferable that the modified polyolefin resin has constituent units derived from monomers having carboxyl groups, or groups in which two carboxyl groups have been anhydroused.

[0045] Preferred modified polyolefin resins include modified polypropylene resins and modified polyethylene resins. Examples of modified polypropylene resins include modified polymers obtained by graft copolymerizing polypropylene resins such as polypropylene, propylene-α-olefin copolymers, and terpolymers of propylene, ethylene, and α-olefin with unsaturated carboxylic acids such as maleic acid and fumaric acid, or their acid anhydrides. Examples of modified polyethylene resins include polyethylene, ethylene-unsaturated carboxylic acid copolymers, and copolymers. The adhesive layer 16 may contain only one type of modified polyolefin resin, or it may contain two or more types. Furthermore, the adhesive layer 16 may consist of a modified polyolefin resin alone, or it may be composed of a mixture of a modified polyolefin resin and other materials. The other materials are not particularly limited and can be arbitrarily selected depending on the purpose.

[0046] The ratio of the content of the modified polyolefin resin having polar groups to the total mass of the adhesive layer 16 is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, and even more preferably 70% by mass or more and 100% by mass or less. When the ratio is above the lower limit, the adhesion between the multilayer film 10 and the paper substrate 21 is further improved.

[0047] The ratio of the thickness of the adhesive layer 16 to the total thickness of the multilayer film 10 is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less. The thickness of the adhesive layer 16 is not particularly limited as long as the ratio of the thickness of the adhesive layer 16 to the total thickness of the multilayer film 10 is within the above range, but is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 30 μm or less.

[0048] It is desirable that the multilayer film 10 has symmetry in thickness and material between each layer from the outermost layer 11 to the intermediate layer 13, and between each layer from the retaining layer 15 to the intermediate layer 13. Symmetry in thickness means that the thickness of corresponding layers is within ±30%. Preferably, it is within ±10%, and even more preferably within ±5%. Symmetry in material means that the thicknesses of corresponding layers contain the same type of resin, i.e., the ratio of the amount of common constituent units (moles) to the total amount of constituent units (moles) is 20 mol% or more. A corresponding layer means a layer located symmetrically to the intermediate layer 13 in the lamination direction (thickness direction).

[0049] The overall thickness of the multilayer film 10 is not particularly limited, but is preferably 200 μm or less. More preferably, it is 150 μm or less, and even more preferably, it is 40 μm or more and 140 μm or less. If the thickness of the multilayer film 10 is less than 40 μm, the intermediate layer 13 becomes thin, and it may not meet the required level of gas barrier properties. Also, if the thickness of the multilayer film 10 is less than 40 μm, the yield when manufacturing the multilayer film 10 may be low. On the other hand, if the thickness of the multilayer film 10 exceeds 200 μm, heat may not be easily transferred to the bonding surface when the adhesive layer 16 is heat-fused to other components, which may lead to poor heat fusion. The thickness of the multilayer film 10 according to this embodiment is, for example, 100 μm.

[0050] The ratio of the thickness of the outermost layer 11 and the retaining layer 15 to the total thickness of the multilayer film 10 is preferably 10% or more, more preferably 12% or more, and even more preferably 25% or more. The ratio of the thickness of the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14 to the total thickness of the multilayer film 10 is preferably 20% or less, preferably 10% or less, and even more preferably 8% or less. In the illustrated example, the ratio of the thickness of each layer to the total thickness of the multilayer film 10 is 28% and 30% for the outermost layer 11 and the retaining layer 15, 8% for the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14, and 12% for the intermediate layer 13.

[0051] Figure 3 shows a method for measuring the heating curvature ΔWh of the multilayer film 10. The multilayer film 10 has a flow direction and a transverse direction. The flow direction is the direction in which each material flowed in the multilayer film formation process S12 described later (so-called Machine Direction), and the transverse direction of the multilayer film is the direction that crosses the flow direction (so-called Transverse Direction).

[0052] The heating curvature ΔWh is the maximum distance from the outermost layer 11 of the multilayer film 10 to the surface of the heating plate 30 after placing a 10cm × 3cm multilayer film 10, with its flow direction as the shorter side, on the heating plate 30 with the outermost layer 11 in contact with the surface of the heating plate 30, and holding it for 1 minute. This heating curvature ΔWh is a quantity that indicates the degree of curvature of the multilayer film 10 in the heated state. The heating curvature ΔWh of the multilayer film 10 is preferably 0.5cm or less, more preferably 0.2cm or less, and even more preferably 0.1cm or less. Furthermore, the heating curvature angle Δθ, which is the angle between the surface of the heating plate 30 and the tangent to the edge of the multilayer film 10, is 45° or less, preferably 40° or less, and more preferably 35° or less. Preferably, the edge of the multilayer film 10 is the part furthest from the heating plate 30. The tangent at the end of the multilayer film 10 may be the tangent on the outermost layer 11 side or the tangent on the adhesive layer 16 side.

[0053] Furthermore, the multilayer film 10 is preferably irradiated with an electron beam at an absorbed dose of 15 to 250 kGy, and more preferably at an absorbed dose of 30 to 125 kGy. Being within this range allows for high heat resistance and prevents excessive thermal shrinkage.

[0054] It is preferable that the multilayer film 10 is irradiated with an electron beam at an absorbed dose of 15 kGy or more and 130 kGy or less. By irradiating the multilayer film 10 with an electron beam at 20 kGy or more and 130 kGy or less, the crosslinking density of the multilayer film 10 (particularly the outermost layer 11, the intermediate layer 13, and the holding layer 15) can be improved. As a result, the heat resistance and melt tension of the multilayer film 10 as a whole can be improved.

[0055] The reason why electron beam irradiation improves the crosslinking density of the multilayer film 10 is not entirely clear, but it is thought to be as follows: When the multilayer film 10 is irradiated with an electron beam, the carbon-hydrogen bonds in the polyethylene of the outermost layer 11 of the multilayer film 10 are broken, and radicals are generated at the broken bond ends. The generated radicals, through the molecular motion of the molecular chains, come into contact with other polyethylene molecular chains, extract hydrogen atoms, and bond with carbon atoms in the polyethylene molecular chains, resulting in the formation of a crosslinked structure. It is presumed that a similar phenomenon occurs in the intermediate layer 13.

[0056] The absorbed dose of electron beam irradiation is more preferably 20 kGy or more and 130 kGy or less, and even more preferably 25 kGy or more and 125 kGy or less. If the absorbed dose of electron beam irradiation is above the lower limit, the crosslinking density of the multilayer film 10 can be further improved. If the absorbed dose of electron beam irradiation is below the upper limit, the multilayer film 10 will not become excessively strong.

[0057] The acceleration voltage for electron beam irradiation is preferably 100kV to 300kV, more preferably 120kV to 280kV, and even more preferably 140kV to 260kV. An acceleration voltage of electron beam irradiation above the lower limit can further improve the crosslinking density of the multilayer film 10. An acceleration voltage of electron beam irradiation below the upper limit can prevent the multilayer film 10 from becoming excessively strong.

[0058] The container 20 comprises a pre-formed three-dimensional base material 21 and a multilayer film 10, the multilayer film 10 being heat-sealed to the base material 21. In this embodiment, the container 20 further comprises a lid material 22. The container 20 contains contents such as food or medicine. As the lid material 22, for example, a top film for top-seal packaging made of polyethylene resin material or a skin film for skin-pack packaging made of ethylene-vinyl acetate copolymer resin material can be used. "Top-seal packaging" is a packaging method in which the contents are filled into a container and the top film is heat-sealed to the container to preferably seal it. "Skin-pack packaging" is a packaging method in which the product is placed in a container such as a tray, cardboard backing, air cap, or film, a heated skin film is placed over it, and at the same time air is removed from below, so that the skin film adheres tightly to the surface of the food and the container to preferably seal it. In the illustrated example, the pre-formed three-dimensional base material 21 is a paper container for top-seal packaging, and the lid material 22 is a top film.

[0059] If the outermost layer 11 contains the same type of polyethylene resin as the retaining layer 15, the outermost layer 11 has oil resistance, thus preventing damage to the base material 21 from oil in food, etc. Furthermore, since the outermost layer 11 is given heat resistance by electron beam crosslinking, it has sufficient heat resistance to, for example, food heated in a microwave oven. Preferably, the multilayer film 10 is a multilayer film for microwaveable food that can be used for microwaveable food. Here, microwaveable food means food that is heated and cooked in a microwave oven. Preferably, the container 20 has microwave-safe properties. Microwave-safe properties mean that food can be heated and cooked in a microwave oven. Preferably, microwave-safe properties mean that it conforms to the "microwave oven high-frequency suitability test" and "microwave oven durability test" specified in JIS S2029:2002.

[0060] The outermost layer 11 is heat-sealed to the lid material 22. Therefore, it is possible to seal food and other items within the space defined by the multilayer film 10 and the lid material 22. For example, the multilayer film 10 has high gas barrier properties (particularly oxygen barrier properties, with an oxygen permeability of 5.0 ml / m²) derived from the intermediate layer 13. 2 It has a gas barrier function of less than or equal to day·atm. Since the lid material 22 also has high gas barrier properties, food can be sealed with a component that has high gas barrier properties, thus suppressing deterioration of food.

[0061] To prevent the lid material 22 from peeling off during transportation and display, the peel strength between the lid material 22 and the outermost layer 11 of the multilayer film 10 must be 1N / 15mm or higher. Furthermore, when removing food or other items from the container 20, the lid material 22 is peeled off from the outermost layer 11 of the multilayer film 10. At this time, while the lid material 22 is peeled off from the outermost layer 11 of the multilayer film 10, the adhesive layer 16 of the multilayer film 10 must not peel off from the base material 21. For this reason, it is preferable that the peel strength between the lid material 22 and the outermost layer 11 is smaller than the peel strength between the base material 21 and the adhesive layer 16. Also, since the operation of peeling the lid material 22 from the outermost layer 11 of the multilayer film 10 is generally performed manually, it is preferable that the peel strength between the lid material 22 and the outermost layer 11 be such that it can be easily peeled off manually, specifically 11N / 15mm or less.

[0062] As shown in Figure 4, the manufacturing method for the container 20 includes a base material molding step S11, a multilayer film formation step S12, a heat softening step S13, and a lamination step S14.

[0063] The base material molding process S11 is a process of forming a base material 21 having a three-dimensional shape from a flat paper material. Known molding methods can be used for the base material molding process S11. Examples of such molding methods include vacuum forming, pressure forming, vacuum pressure forming, match forming, and press forming. As the paper material, recycled paper pulp, virgin pulp, or pulp mixed in appropriate amounts may be used. The container 20 may be formed into a three-dimensional shape by cutting and assembling cardboard to the desired shape, or it may be formed by a mold.

[0064] The multilayer film forming step S12 is a step of forming a multilayer film 10 comprising an outermost layer 11, an intermediate layer 13, a retaining layer 15 containing the same type of resin as the outermost layer 11, and an adhesive layer 16 in contact with the retaining layer 15, in the order described above. For example, co-extrusion molding can be used as the multilayer film forming step S12. That is, the materials constituting the outermost layer 11, the first intermediate adhesive layer 12, the intermediate layer 13, the second intermediate adhesive layer 14, the retaining layer 15, and the adhesive layer 16 are extruded from an extruder and discharged from a t-die to form the multilayer film 10 with the above configuration. As an alternative forming method, the multilayer film 10 may be formed by heat-sealing the outermost layer 11, the first intermediate adhesive layer 12, the intermediate layer 13, the second intermediate adhesive layer 14, the retaining layer 15, and the adhesive layer 16, which have been formed into film layers one by one. Alternatively, the multilayer film 10 may be formed by bonding the layers together using an adhesive by any of the dry lamination method, hot melt lamination method, or wet lamination method. In the illustrated example, the multilayer film formation process S12 is performed after the substrate molding process S11, but the order may be reversed or they may be performed in parallel.

[0065] The heating and softening step S13 is a step in which the multilayer film 10 is heated and softened. In this embodiment, the base material 21 is placed in the lower mold of the molding die, and the multilayer film 10 is placed on the heating plate 30, which is the upper mold of the molding die, with the adhesive layer 16 side facing the base material 21, and heated and softened at 120°C to 150°C.

[0066] The lamination process S14 is a process of heat-sealing the multilayer film 10, which has been softened by the heat-softening process S13, by bringing it into close contact with the substrate 21 so that the adhesive layer 16 is in contact with it. In this embodiment, the upper mold on which the multilayer film 10 is placed is lowered, bringing the adhesive layer 16 of the multilayer film 10 into close contact with the substrate 21 placed in the lower mold, and pressurizing it with a pressure of 1.0 kgf to 2.5 kgf, preferably 1.5 kgf to 2.0 kgf, and holding it in that state for 1 to 5 seconds, thereby heat-sealing the multilayer film 10 and the substrate 21.

[0067] The multilayer film 10 of this embodiment is a multilayer film 10 that is heat-fused to a pre-formed three-dimensional substrate 21 with an adhesive layer 16 in contact with it, comprising an outermost layer 11, an intermediate layer 13, a retaining layer 15, and an adhesive layer 16 in contact with the retaining layer 15, in the order described above, wherein the outermost layer 11 and the retaining layer 15 contain the same type of resin. With this configuration, since the outermost layer 11 and the retaining layer 15 of the multilayer film 10 contain the same type of resin, the curvature of the multilayer film 10 in the heat softening process S13 can be reduced. In particular, by preparing a 10cm x 3cm test piece of the multilayer film 10 with the flow direction of the short side, placing it on the heated hot plate 30 with the outermost layer 11 in contact with the surface of the hot plate 30, and holding it at 145°C for 1 minute, the maximum distance from the outermost layer 11 to the surface of the hot plate 30 (heating curvature ΔWh) is 0.5cm or less, and the heating curvature angle Δθ, which is the angle between the surface of the hot plate 30 and the tangent to the edge of the multilayer film 10, is 45° or less, the curvature can be effectively suppressed.

[0068] 2. Second Embodiment The following describes the multilayer film 10 according to the second embodiment. In the first embodiment described above, the outermost layer 11 and the retaining layer 15 of the multilayer film 10 contained the same type of polyethylene resin, but in this embodiment, the outermost layer 11 and the retaining layer 15 contain the same type of polypropylene resin. The following description will focus on the differences from the first embodiment. Points that are not specifically described are the same as in the first embodiment.

[0069] Here, "polypropylene resin" refers to a polymer containing propylene as at least one monomer unit. Polypropylene resin may be polypropylene alone, or it may be a mixture of polypropylene and other materials. Examples of polypropylene used alone or in mixtures with other materials include homopolypropylene, block polypropylene, and random polypropylene. Furthermore, when polypropylene resin contains materials other than polypropylene, examples of such materials include polymers such as polyethylene and elastomers. Examples of polyethylene include high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDEP). Using a mixture of polypropylene and other materials as the polypropylene resin can impart cold shock resistance.

[0070] The outermost layer 11 may contain only one type of polypropylene resin, or it may contain two or more types. When the outermost layer 11 contains the same type of polypropylene resin as the retaining layer 15, the outermost layer 11 has oil resistance, so that the base material 21 is not damaged by oil contained in food, etc. Furthermore, since the softening point of the polypropylene resin used in the outermost layer 11 is generally around 120°C, it has sufficient heat resistance to food heated in a microwave oven, for example. Therefore, the multilayer film 10 of this embodiment is also preferably a multilayer film for microwaveable food. The outermost layer 11 preferably contains 10% by mass or more of polypropylene. More preferably, it contains 25% by mass or more of polypropylene. Even more preferably, it contains 50% by mass or more of polypropylene.

[0071] In this embodiment, the retaining layer 15 contains the same type of polypropylene resin as the outermost layer 11, as described above. Since polypropylene has a molecular structure common to modified polyolefin resins suitably used in the adhesive layer 16, it has high adhesion to the adhesive layer 16, thereby suitably preventing the peeling of the adhesive layer 16. The retaining layer 15 may contain only one type of polypropylene resin, or it may contain two or more types. Preferably, the retaining layer 15 contains 10% by mass or more of polypropylene. More preferably, it contains 25% by mass or more of polypropylene. Even more preferably, it contains 50% by mass or more of polypropylene.

[0072] Furthermore, in this embodiment, the heating curvature ΔWh of the multilayer film 10 is preferably 1.0 cm or less, more preferably 0.8 cm or less, and even more preferably 0.6 cm or less. Also, in this embodiment, the heating curvature angle Δθ, which is the angle formed by the surface of the heating plate 30 and the tangent to the edge of the multilayer film 10, is 45° or less, and preferably 40° or less.

[0073] [Other Embodiments] Finally, other embodiments of the multilayer film 10 and container 20 according to the present invention will be described. Note that the configurations disclosed in each of the following embodiments can be applied in combination with configurations disclosed in other embodiments, as long as no inconsistencies arise.

[0074] In the above embodiment, a multilayer film 10 was described as comprising an outermost layer 11, a first intermediate adhesive layer 12, an intermediate layer 13, a second intermediate adhesive layer 14, a holding layer 15, and an adhesive layer 16. However, in the multilayer film 10 according to the present invention, the presence or absence of the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14 is optional. Furthermore, for example, two layers having different functions from the intermediate layer 13 may be provided on both sides of the intermediate layer 13 in the lamination direction, symmetrically with respect to the intermediate layer 13.

[0075] The polymer material used in each layer constituting the multilayer film 10 according to the present invention may contain known additives. Examples of such additives include plasticizers, antioxidants, flame retardants, stabilizers, ultraviolet absorbers, antistatic agents, lubricants, water repellents, and oil repellents.

[0076] In the above embodiment, a container manufacturing method was exemplified that included a base material molding step S11, a multilayer film forming step S12, a heat softening step S13, and a lamination step S14, but other steps may also be included. For example, a lid material molding step or a gravure printing step may be further provided.

[0077] With regard to other configurations, the embodiments disclosed herein are illustrative in all respects, and it should be understood that the scope of the present invention is not limited thereto. Those skilled in the art will readily understand that modifications can be made as appropriate without departing from the spirit of the invention. Therefore, other embodiments modified without departing from the spirit of the invention are naturally included within the scope of the present invention.

[0078] The present invention will be described in more detail below with reference to specific examples. However, the present invention is not limited in any way to the examples shown below.

[0079] The following multilayer films, Samples 1 to 9, were manufactured using the procedure described below.

[0080] [Preparation of Samples 1 to 4] The resin constituting the outermost layer 11 is linear low-density polyethylene (density 0.938 / cm³). 3 , manufactured by Ube Maruzen Polyethylene Co., Ltd., 4040FC) and linear low-density polyethylene (density 0.913 / cm³). 3 We prepared 1520F (manufactured by Ube Maruzen Polyethylene Co., Ltd.). The outermost layer 11 consists of two layers, with 4040FC in the first layer and 1520F in the second layer. The weight ratio of the first and second layers is 1:1.

[0081] Modified polyolefin resin (NF536, manufactured by Mitsui Chemicals, Inc.) was prepared as the resin constituting the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14.

[0082] As the resin constituting the intermediate layer 13, ethylene-vinyl alcohol copolymer (Mitsubishi Chemical Corporation, GH3804B) was prepared.

[0083] The resin constituting the retaining layer 15 is linear low-density polyethylene (density 0.913 / cm³). 3 We prepared (1520F) manufactured by Ube Maruzen Polyethylene Co., Ltd.

[0084] As the resin constituting the adhesive layer 16, ethylene vinyl acetate copolymer (ethylene:vinyl acetate copolymer ratio = 84:16, manufactured by Mitsui Dow Polychemical Co., Ltd., V5714RC) was prepared.

[0085] Next, a multilayer film was manufactured by co-extruding the outermost layer 11, the first intermediate adhesive layer 12, the intermediate layer 13, the second intermediate adhesive layer 14, the retaining layer 15, and the adhesive layer 16 in this order. The resulting multilayer film has a thickness of 100 μm and is composed of the outermost layer 11 (thickness = 28 μm), the first intermediate adhesive layer 12 (thickness = 8 μm), the intermediate layer 13 (thickness = 12 μm), the second intermediate adhesive layer 14 (thickness = 8 μm), the retaining layer 15 (thickness = 30 μm), and the adhesive layer 16 (thickness = 14 μm) being laminated in this order in the thickness direction.

[0086] Next, electron beam irradiation was performed from the outermost layer 11 of the multilayer film with an acceleration voltage of 150kV and an absorbed dose of 30kGy to 60kGy. Of these multilayer films, the one irradiated with an electron beam at an absorbed dose of 60kGy was designated as Sample 1, the one irradiated with an electron beam at an absorbed dose of 45kGy was designated as Sample 2, the one irradiated with an electron beam at an absorbed dose of 30kGy was designated as Sample 3, and the multilayer film that was not irradiated with an electron beam was designated as Sample 4.

[0087] [Preparation of Sample 5] As the resin constituting the holding layer 15, instead of linear low-density polyethylene, an ionomer (density 0.940 / cm 3 , manufactured by Mitsui Dow Polychemical Co., Ltd., 1601) was used. A multilayer film was produced in the same manner as in Sample 1 except that electron beam irradiation was not performed, and it was designated as Sample 5.

[0088] 〔Production of Sample 6〕 As the resins constituting the outermost layer 11 and the holding layer 15, instead of linear low-density polyethylene, an ionomer (density 0.940 / cm 3 , manufactured by Mitsui Dow Polychemical Co., Ltd., 1601) was used. A multilayer film was produced in the same manner as in Sample 1 except that electron beam irradiation was not performed, and it was designated as Sample 6.

[0089] 〔Production of Sample 7〕 As the resins constituting the outermost layer 11 and the holding layer 15, polypropylene (density 0.900 / cm 3 , manufactured by Sumitomo Chemical Co., Ltd., WF836DG3) was prepared.

[0090] As the resins constituting the first intermediate adhesive layer 12 and the second intermediate adhesive layer 14, a modified polyolefin resin (manufactured by Mitsubishi Chemical Corporation, GH3804B) was prepared.

[0091] As the resin constituting the intermediate layer 13, an ethylene-vinyl alcohol copolymer (manufactured by Kuraray Co., Ltd., J171B) was prepared.

[0092] As the resin constituting the adhesive layer 16, an ethylene-vinyl acetate copolymer (ethylene:vinyl acetate copolymerization ratio = 84:16, manufactured by Mitsui Dow Polychemical Co., Ltd., V5714RC) was prepared.

[0093] Next, a multilayer film (sample 7) was manufactured by co-extruding the outermost layer 11, the first intermediate adhesive layer 12, the intermediate layer 13, the second intermediate adhesive layer 14, the retaining layer 15, and the adhesive layer 16 in this order. The resulting multilayer film (sample 7) has a thickness of 100 μm and is composed of the outermost layer 11 (thickness = 28 μm), the first intermediate adhesive layer 12 (thickness = 8 μm), the intermediate layer 13 (thickness = 12 μm), the second intermediate adhesive layer 14 (thickness = 8 μm), the retaining layer 15 (thickness = 30 μm), and the adhesive layer 16 (thickness = 14 μm) being laminated in this order in the thickness direction.

[0094] [Preparation of Sample 8] Instead of polypropylene, ionomer (density 0.940 / cm³) is used as the resin constituting the retaining layer 15. 3 A multilayer film was manufactured using the same method as for sample 7, except that it used Mitsui Dow Polychemical Co., Ltd., 1601), and this was designated as sample 8.

[0095] [Preparation of Sample 9] As the resin constituting the retaining layer 15, linear low-density polyethylene (density 0.913 / cm³) is used instead of polypropylene. 3 Sample 9 was produced using the same method as Sample 7, except that it used 1520F (manufactured by Ube Maruzen Polyethylene Co., Ltd.).

[0096] [Measurement of curvature] The aforementioned heating curvature ΔWh was measured for the multilayer films of Samples 1 to 9. The aforementioned heating and bending angle Δθ was measured for the multilayer films of Samples 1 to 9.

[0097] Table 1 below shows an overview of the composition of Samples 1 to 9, along with the measurement results of the gel fraction and curvature. The abbreviations for the resin names in the table represent the following resins, respectively. LLPDE: Linear low-density polyethylene ION: Ionomer PP: Polypropylene [Table 1] [Industrial applicability]

[0098] This invention can be used, for example, in food packaging applications. [Explanation of Symbols]

[0099] 10: Multilayer film 11: Outer layer 12: First intermediate adhesive layer 13: Middle Class 14:Second intermediate adhesive layer 15: Retention layer 16: Adhesive layer 20: Container 21: Base material 22: Lid material 30: Hot plate

Claims

1. A multilayer film that is heat-sealed to a pre-formed three-dimensional paper substrate with an adhesive layer in contact with it, The material comprises, in the order described above, an outermost layer containing a polyethylene resin material, an intermediate layer, a retaining layer, and an adhesive layer in contact with the retaining layer. The outermost layer and the retaining layer each contain the same type of resin. A multilayer film having dimensions of 10 cm x 3 cm with the flow direction as the shorter side, placed on a heating plate with the outermost layer in contact with the surface of the heating plate, and held at 145°C for 1 minute, wherein the heating curvature, which is the maximum distance from the outermost layer of the multilayer film to the surface of the heating plate, is 0.5 cm or less.

2. The multilayer film according to claim 1, wherein each layer from the outermost layer to the intermediate layer and each layer from the retaining layer to the intermediate layer have symmetry in terms of thickness and material.

3. The multilayer film according to claim 1 or 2, wherein the multilayer film is irradiated with an electron beam under conditions of an absorbed dose of 15 to 250 kGy.

4. The multilayer film according to any one of claims 1 to 3, wherein the multilayer film measuring 10 cm x 3 cm with the flow direction as the shorter side is placed on the heating plate with the outermost layer in contact with the surface of the heating plate and held at 145°C for 1 minute, and the heating curvature angle, which is the angle between the surface of the heating plate and the tangent to the edge of the multilayer film, is 45° or less.

5. The multilayer film according to any one of claims 1 to 4, wherein the total thickness of the multilayer film is 200 μm or less, and the ratio of the thickness of the adhesive layer to the total thickness of the multilayer film is 25% or less.

6. The adhesive layer comprises a modified polyolefin resin having polar groups, as described in any one of claims 1 to 5.

7. The multilayer film according to any one of claims 1 to 6, wherein the intermediate layer is a gas barrier layer containing an ethylene-vinyl alcohol copolymer.

8. A multilayer film according to any one of claims 1 to 7, for use in microwaveable foods.

9. The invention comprises a substrate that has been pre-formed in a three-dimensional shape, and a multilayer film according to any one of claims 1 to 8, A container in which the multilayer film is heat-sealed to the substrate.

10. The container according to claim 9, wherein the container is microwave-safe.

11. A base material molding process for forming a base material having a three-dimensional shape from a flat paper material, A multilayer film forming step comprising: an outermost layer containing a polyethylene resin material, an intermediate layer, a holding layer containing the same type of resin as the outermost layer, and an adhesive layer in contact with the holding layer, wherein the multilayer film, measuring 10 cm x 3 cm with the flow direction as the shorter side, is placed on the hot plate with the outermost layer in contact with the surface of the hot plate, and held at 145°C for 1 minute, thereby forming a multilayer film in which the heating curvature, which is the maximum distance from the outermost layer of the multilayer film to the surface of the hot plate, is 0.5 cm or less; A heating and softening step in which the multilayer film is heated and softened, A method for manufacturing a container, comprising a lamination step of heat-sealing the multilayer film softened by the heat-softening step by bringing it into close contact with the substrate so that the adhesive layer is in contact with it.

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