JP7639290B2 - Multilayer film, food packaging sheet, food container, and method for manufacturing container - Google Patents

Description

The present invention relates to a multilayer film used for packaging food, a food packaging sheet, and a food container, as well as a method for manufacturing the food container.

The use of foam materials is being promoted to reduce the amount of packaging materials used in food packaging. Among foam materials, polystyrene foam sheets (also called polystyrene paper or PSP) are preferably used because they can be easily formed into containers of any shape. For example, changing a polypropylene container to one made of polystyrene foam sheets of the same shape can reduce the weight of the packaging material by roughly half.

When using polystyrene foam sheets for packaging food, a commonly used method is to heat-seal (laminate) a functional film onto the surface of the polystyrene foam sheet to impart functions (such as sealing properties and oil resistance) suitable for packaging food. For example, JP 2016-155584 A (Patent Document 1) discloses a packaging container in which a co-extruded laminate film having a gas barrier layer is laminated onto a substrate made of polystyrene foam. In addition, JP 09-290492 A (Patent Document 2) discloses a synthetic resin laminate sheet in which an adhesive resin layer and a film are laminated onto a substrate layer made of polystyrene resin, and discloses that the synthetic resin laminate sheet can be used for food trays, etc.

JP 2016-155584 A Japanese Patent Application Publication No. 9-290492

However, in the techniques disclosed in Patent Documents 1 and 2, it was necessary to provide a separate layer that functions as an adhesive between the base layer and the film layer. This required a step of introducing adhesive into the heat fusion process (for example, by applying an adhesive), which could make the heat fusion process complicated. In addition, it could be difficult to reuse scraps of film that have been coated with adhesive, so there were cases where such scraps had no choice but to be discarded.

Therefore, there is a need to develop a multilayer film that can be thermally bonded to a substrate without the use of adhesives.

The multilayer film according to the present invention is a multilayer film having an outer layer containing a polypropylene resin and an adhesive layer capable of adhering to a polystyrene resin, and is characterized in that the peel strength when heat-sealed to the polystyrene resin under conditions of a temperature of 180°C, a time of 1.0 second, and a pressure of 0.05 MPa is 3.0 N/15 mm or more.

The food packaging sheet according to the present invention is characterized in that it comprises a laminate portion having an outer layer containing a polypropylene-based resin and an adhesive layer capable of adhering to a polystyrene-based resin, and a substrate portion containing a foamed polystyrene-based resin and laminated by the laminate portion, and that the peel strength between the laminate portion and the substrate portion is 3.0 N/15 mm or more.

The food container according to the present invention is characterized in that it is obtained by forming the food packaging sheet into a container shape.

The container manufacturing method according to the present invention is characterized by including an extrusion process in which a polypropylene resin and an adhesive polymer containing at least one monomer unit selected from vinyl acetate and styrene are co-extruded to form a multilayer film having an outer layer containing the polypropylene resin and an adhesive layer containing the adhesive polymer, a lamination process in which a base sheet containing a foamed polystyrene resin and the adhesive layer of the multilayer film are heat-sealed to obtain a food packaging sheet, and a container forming process in which the food packaging sheet is formed into a container shape to obtain a container.

According to these configurations, since an adhesive layer capable of adhering to polystyrene-based resin is provided, the multilayer film (laminate portion) and the substrate portion containing polystyrene-based resin can be adhered without the use of adhesive. This makes it possible to simplify and conserve resources in the heat fusion process when manufacturing food packaging sheets and food containers compared to conventional methods. In other words, overall, the amount of resources required for packaging food can be reduced compared to conventional methods.

The following describes preferred embodiments of the present invention. However, the scope of the present invention is not limited to the preferred embodiments described below.

In one embodiment of the multilayer film according to the present invention, the adhesive layer contains an adhesive polymer, and the adhesive polymer preferably contains vinyl acetate as at least one monomer unit.

This configuration tends to give the multilayer film particularly high adhesion to polystyrene resins.

In one embodiment, the multilayer film according to the present invention further comprises a retaining layer disposed between the outer layer and the adhesive layer in contact with the adhesive layer, and the retaining layer preferably contains polyethylene.

This configuration makes it difficult for the adhesive layer to peel off from the other parts of the multilayer film.

In one embodiment, the multilayer film according to the present invention preferably further includes a gas barrier layer between the outer layer and the adhesive layer.

This configuration can impart gas barrier properties to the multilayer film. Therefore, for example, when the multilayer film is applied to a food container, deterioration of the food contained in the food container can be suppressed.

In one embodiment of the multilayer film according to the present invention, the gas barrier layer preferably contains an ethylene-vinyl alcohol copolymer.

This configuration makes it easier for the multilayer film to have particularly high gas barrier properties.

In one embodiment of the multilayer film according to the present invention, the material and thickness of the gas barrier layer are preferably determined so that the oxygen transmission rate of a film formed from the material to the specified thickness, as measured according to JIS K 7126, is 5.0 ml/ ^m2 ·day·atm or less.

The material and thickness of the gas barrier layer of the multilayer film with this configuration are determined based on the specific level of gas barrier properties required for food containers, resulting in a multilayer film that is particularly suitable for food packaging applications.

In one embodiment, the multilayer film of the present invention preferably has a thickness of 40 to 100 μm.

This configuration results in a multilayer film that is easy to heat seal and manufacture.

Further features and advantages of the present invention will become more apparent from the following description of exemplary and non-limiting embodiments, which are given with reference to the drawings.

FIG. 2 is a cross-sectional view showing a layer structure of a multilayer film. FIG. 2 is a cross-sectional view showing a layer structure of the food packaging sheet. FIG. 2 is a schematic cross-sectional view showing the structure of a food container. FIG. 2 is a schematic diagram showing an example of a lamination method.

Embodiments of the multilayer film, food packaging sheet, food container, and container manufacturing method according to the present invention will be described with reference to the drawings. Below, an example will be described in which the multilayer film according to the present invention is applied to a multilayer film 10 used for laminating a polystyrene foam sheet 20 (hereinafter referred to as PSP 20). Also described will be a food packaging sheet 30 obtained by laminating the PSP 20 with the multilayer film 10, a food container 40 obtained by forming the food packaging sheet 30 into a container shape, and a manufacturing method for the food container 40.

[Construction and manufacturing method of multi-layer film]
The multilayer film 10 according to this embodiment comprises an outer layer 1 provided on a first surface side and an adhesive layer 2 provided on a second surface side ( FIG. 1 ). The multilayer film 10 also comprises a plurality of layers between the outer layer 1 and the adhesive layer 2, specifically, from the outer layer 1 side, a first intermediate adhesive layer 3, a gas barrier layer 4, a second intermediate adhesive layer 5, and a support layer 6.

When the PSP 20 is laminated with the multilayer film 10 to obtain the food packaging sheet 30, the second surface (adhesive layer 2) is brought into contact with the PSP 20 and heat-sealed, so that the first surface (outer layer 1) is exposed (Figure 2). When the food packaging sheet 30 is molded into a container shape to obtain a food container 40, the food packaging sheet 30 is molded so that the surface on the multilayer film 10 (outer layer 1) side has a shape capable of containing food F (Figure 3). Therefore, the outer layer 1 is the layer that will come into contact with food F when the food container 40 is obtained.

The thickness of the multilayer film 10 according to this embodiment is, for example, 70 μm. The thickness is broken down as follows: outer layer 1 is 25.9 μm (37% of the thickness of the multilayer film 10; the same applies below), first intermediate adhesive layer 3 is 5.6 μm (8%), gas barrier layer 4 is 8.4 μm (12%), second intermediate adhesive layer 5 is 5.6 μm (8%), retention layer 6 is 10.5 μm (15%), and adhesive layer 2 is 14.0 μm (20%). The thickness of the multilayer film 10 is not particularly limited, but may be 40 to 100 μm. If the thickness of the multilayer film 10 is less than 40 μm, the gas barrier layer 4 becomes thin and the gas barrier property may not meet the required level. In addition, 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 100 μm, heat is less likely to be transferred to the adhesive surface when heat-sealing with the PSP 20, and poor heat sealing may occur. Note that the absolute values of the thicknesses of each layer and their ratios to the thickness of the multilayer film 10 are not limited to the above examples.

The outer layer 1 includes a polypropylene-based resin. Here, the polypropylene-based resin may be polypropylene alone or may be a mixture of polypropylene and other materials. Examples of polypropylene used alone or mixed with other materials include homopolypropylene, block polypropylene, and random polypropylene. In addition, when the polypropylene-based resin includes a material other than polypropylene, examples of the material include polymers such as polyethylene and elastomers. Here, examples of polyethylene include high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDEP). When a mixture of polypropylene and other materials is used as the polypropylene-based resin, cold impact resistance can be imparted. The outer layer 1 preferably includes 10% by mass or more of polypropylene.

As mentioned above, the outer layer 1 may be made of only polypropylene-based resin, or may be made of a mixture of polypropylene-based resin and other materials. In other words, the statement in the previous paragraph that "the outer layer 1 contains a polypropylene-based resin" does not intend to exclude the outer layer 1 from containing a material other than a polypropylene-based resin. This also applies to the other layers described below, and the examples of materials constituting each layer do not prevent the layer from containing a material other than the exemplified material.

Since the outer layer 1 is the layer that will come into contact with the food F, it is preferable that it is oil-resistant. Therefore, the outer layer 1 is configured to include a polypropylene-based resin, which is one type of resin that has oil resistance. It is assumed that the food container 40 will be heated in a microwave oven together with the food F contained therein, and since the softening point of polypropylene-based resin is generally around 120°C, it has sufficient heat resistance against the temperature (approximately 100°C) of the water vapor generated when the food F is heated in a microwave oven.

The adhesive layer 2 is a layer that can adhere to polystyrene. In this embodiment, the adhesive layer 2 is configured to include an adhesive polymer that has adhesiveness to polystyrene, and the adhesive polymer includes at least vinyl acetate as a monomer unit. That is, the adhesive polymer may be composed of a single component of polyvinyl acetate or a copolymer that includes vinyl acetate as a monomer unit (hereinafter referred to as a vinyl acetate-based copolymer), or may be composed of a mixture of a resin that does not include vinyl acetate as a monomer unit (polyethylene, ethylene butene copolymer, etc.) and polyvinyl acetate or a vinyl acetate-based copolymer. In either example, the adhesiveness to polystyrene is expressed by the action of the vinyl acetate component contained in the adhesive polymer. Note that the adhesive polymer contained in the adhesive layer 2 is not limited to the polymers that include at least vinyl acetate as a monomer unit as exemplified above.

Due to the action of adhesive layer 2, multilayer film 10 has good adhesion to polystyrene resin. Specifically, when multilayer film 10 is heat-sealed to polystyrene resin under conditions of a temperature of 180°C, a time of 1.0 second, and a pressure of 0.05 MPa, the peel strength is 3.0 N/15 mm or more. Note that polystyrene resin here refers to a polymer containing 50 mol% or more of styrene as a monomer unit, and may be homopolystyrene that does not have monomer units other than styrene, or may be modified polystyrene that contains monomer units other than styrene.

The retaining layer 6 is provided in contact with the adhesive layer 2. The retaining layer 6 has the function of preventing peeling between the adhesive layer 2 and other parts of the multilayer film 10, and is made of a material having a molecular structure common to that of the material constituting the adhesive layer 2, or a material having a melting point close to that of the material constituting the adhesive layer 2, for example. In this embodiment, the retaining layer 6 contains polyethylene. Polyethylene has a molecular structure common to the main chain portion of the polyvinyl acetate or vinyl acetate copolymer used in the adhesive layer 2, and therefore has high adhesion to the adhesive layer 2, which can effectively prevent peeling of the adhesive layer 2.

The gas barrier layer 4 is a layer for imparting the gas barrier properties (particularly oxygen barrier properties) required for the food container 40 to the multilayer film 10. In this embodiment, the gas barrier layer 4 is configured to contain an ethylene-vinyl alcohol copolymer. Here, it is particularly preferable to use an ethylene-vinyl alcohol copolymer with an ethylene content of 38 mol% or less, since this will result in particularly high gas barrier properties. In addition to the ethylene-vinyl alcohol copolymer, examples of materials suitable for use in the gas barrier layer 4 include polyamide MXD6 and polyvinylidene chloride. The gas barrier layer 4 may also contain additives (mica, clay, etc.) that have gas barrier properties.

The material and thickness of the gas barrier layer 4 greatly affect the gas barrier properties of the multilayer film 10, so the material and thickness of the gas barrier layer 4 must be selected within a range that allows the gas barrier properties required for the food container 40 to be exhibited. The material and thickness of the gas barrier layer 4 are determined so that the oxygen transmission rate of a film formed from the material to the specified thickness, as measured according to JIS K 7126, is 5.0 ml/ ^m2 ·day·atm or less. For example, if the gas barrier layer 4 is made of an ethylene-vinyl alcohol copolymer having an ethylene content of 38 mol% or less and has a thickness of 6 μm or more, the standard of an oxygen transmission rate of 5.0 ml/ ^m2 ·day·atm or less can be easily achieved. It is more preferable that the material and thickness of the gas barrier layer 4 are determined so that the oxygen transmission rate of a film formed from the material to the specified thickness is 3.0 ml/ ^m2 ·day·atm or less, and even more preferable that the oxygen transmission rate of the film is 1.0 ml/ ^m2 ·day·atm or less.

The first intermediate adhesive layer 3 and the second intermediate adhesive layer 5 are layers that function to bond adjacent layers on both sides of each layer. That is, the first intermediate adhesive layer 3 is a layer that bonds the outer layer 1 and the gas barrier layer 4, and the second intermediate adhesive layer 5 is a layer that bonds the retaining layer 6 and the gas barrier layer 4. Known adhesive polymers can be used as materials constituting the first intermediate adhesive layer 3 and the second intermediate adhesive layer 5, and the materials used are selected according to the combination of the two layers to be bonded by each layer.

The multilayer film 10 having the above configuration can be obtained, for example, by co-extrusion molding. That is, the materials constituting the outer layer 1, the first intermediate adhesive layer 3, the gas barrier layer 4, the second intermediate adhesive layer 5, the retaining layer 6, and the adhesive layer 2 are extruded from an extruder and discharged from a T-die to obtain the multilayer film 10 having the above configuration (an example of an extrusion process). Note that the multilayer film 10 can also be obtained by a method in which the outer layer 1, the first intermediate adhesive layer 3, the gas barrier layer 4, the second intermediate adhesive layer 5, the retaining layer 6, and the adhesive layer 2, which have been formed into a film-like shape on a layer-by-layer basis, are heat-sealed to each other.

[Configuration and manufacturing method of food packaging sheet]
The food packaging sheet 30 according to this embodiment is a multi-layered food packaging sheet obtained by laminating a PSP 20 (an example of a substrate portion) with the above-mentioned multi-layer film 10 (an example of a laminate portion) (FIG. 2).

The PSP 20 is a sheet containing a foamed polystyrene resin, and a known polystyrene foam sheet can be used. For example, a polystyrene foam sheet having a thickness of 3 mm or less, a density of 0.05 to 0.5 g/cm ³ , and an expansion ratio of 2 to 20 times can be used as the PSP 20. The definition of "polystyrene resin" is as described above.

The food packaging sheet 30 is obtained by laminating the PSP 20 with the multilayer film 10. The lamination method may be a known method, for example, a melt-press method using a roll device 50 shown in FIG. 4 (an example of a lamination process). The raw rolls of the multilayer film 10 and the PSP 20 are placed on the raw material rolls 51 and 52, respectively. The multilayer film 10 and the PSP 20 sent from the raw material rolls 51 and 52 come into contact with each other before the melt-press roll 53, and are melt-pressed by the melt-press roll 53. More specifically, the surface of the multilayer film 10 on the adhesive layer 2 side comes into contact with the PSP 20 before the melt-press roll 53. The PSP 20 may be configured to be preheated before it comes into contact with the multilayer film 10.

Of the melt-pressing rolls 53, the heating roll 53a is heated, and the opposing roll 53b opposite the heating roll 53a may be heated or unheated as appropriate. The multilayer film 10 and the PSP 20 are pressed together by the heating roll 53a and the opposing roll 53b, and the food packaging sheet 30 obtained by the pressing is wound up on the product roll 54.

The lamination method described above can improve the adhesion between the multilayer film 10 and the PSP 20. Specifically, the peel strength between the multilayer film 10 and the PSP 20 is 3.0 N/15 mm or more. It is preferable that the peel strength between the multilayer film 10 and the PSP 20 is 5.0 N/15 mm or more.

In addition, instead of the lamination method of the melt-press bonding type exemplified above, an extrusion lamination method may be used. When lamination is performed by the extrusion lamination method, the multilayer film 10 is extruded in a molten state onto the PSP 20, and then cooled and pressed to complete the lamination. In this case, the extrusion method of the multilayer film 10 may be the same as the method described above as an example of the manufacturing method of the multilayer film 10.

[Container configuration and manufacturing method]
The food container 40 is a container obtained by forming the food packaging sheet 30 into a deep dish shape (FIG. 3). FIG. 3 shows a state in which the food container 40 is filled with food F and the opening of the food container 40 is sealed with a top film 41. In this embodiment, the top film 41 is a film made of polyethylene terephthalate, polybutylene terephthalate, oriented nylon, or the like. Note that, although FIG. 3 shows an example of the food container 40 formed into a deep dish shape, the shape of the food container obtained by forming the food packaging sheet 30 is not particularly limited.

As shown in FIG. 3, the food container 40 has a shape that allows food F to be contained on the multilayer film 10 (outer layer 1) side of the food packaging sheet 30, and the outer layer 1 contacts the food F. Here, the outer layer 1 contains a polypropylene-based resin, which provides oil resistance, and therefore prevents the PSP 20 from being damaged by the oil contained in the food F. In addition, the softening point of the polypropylene-based resin used in the outer layer 1 is generally around 120°C, so that it has sufficient heat resistance to food F heated in a microwave oven.

In addition, the top film 41 and the multilayer film 10 (outer layer 1) of the food packaging sheet 30 are heat-sealed at the edge 42 of the food container 40. Therefore, the food F is contained in a sealed space defined by the multilayer film 10 and the top film 41. Here, the multilayer film 10 has high gas barrier properties (particularly oxygen barrier properties, with an oxygen permeability of 5.0 ml/ ^m2 ·day·atm or less) derived from the gas barrier layer 4, and the top film 41 also has high gas barrier properties. That is, since the food F is sealed by a member having high gas barrier properties, deterioration of the food F is suppressed.

When the food F is removed from the food container 40, the top film 41 is peeled off from the food packaging sheet 30. At this time, it is required that the top film 41 is peeled off from the multilayer film 10 (outer layer 1), while the multilayer film 10 (adhesive layer 2) is not peeled off from the PSP 20. Therefore, the food container 40 is configured so that the peel strength between the top film 41 and the multilayer film 10 (outer layer 1) is smaller than the peel strength between the multilayer film 10 (adhesive layer 2) and the PSP 20. In addition, since the operation of peeling the top film 41 from the food packaging sheet 30 is generally performed manually, it is preferable that the peel strength between the top film 41 and the multilayer film 10 (outer layer 1) is set to a level that allows easy peeling by hand, specifically, 10 N/15 mm or less. For these reasons, it is preferable that the peel strength between the top film 41 and the multilayer film 10 (outer layer 1) be 10 N/15 mm or less, and be smaller than the peel strength between the multilayer film 10 (adhesive layer 2) and the PSP 20.

A known method can be used as the processing method (container forming process) for obtaining the food container 40 from the food packaging sheet 30. Examples of such processing methods include vacuum forming, pressure forming, vacuum pressure forming, match forming, and press forming.

Other embodiments
Finally, other embodiments of the multilayer film, food packaging sheet, food container, and container manufacturing method according to the present invention will be described. Note that the configurations disclosed in each of the following embodiments can be combined with the configurations disclosed in other embodiments as long as no contradiction occurs.

In the above embodiment, an example has been described in which the multilayer film 10 includes an outer layer 1, a first intermediate adhesive layer 3, a gas barrier layer 4, a second intermediate adhesive layer 5, a retaining layer 6, and an adhesive layer 2. However, in the multilayer film according to the present invention, the presence or absence of layers other than the outer layer containing a polypropylene-based resin and the adhesive layer capable of adhering to a polystyrene-based resin is optional. For example, the outer layer may have a multilayer structure, in which case it is sufficient that the outermost layers of the multiple layers constituting the outer layer contain a polypropylene-based resin.

In the above embodiment, the food packaging sheet 30 is obtained by laminating the PSP 20 with the multilayer film 10. However, the base material of the food packaging sheet according to the present invention may not be foamed in part or in whole. For example, as a modification of the above embodiment, an impact-resistant layer made of high impact polystyrene (HIPS) may be provided. The impact-resistant layer may be provided between the PSP 20 and the adhesive layer 2, or on a side of the PSP 20 different from the side that contacts the adhesive layer 2. By providing the impact-resistant layer, further impact resistance can be imparted to the multilayer film 10.

The polymeric materials used in each layer constituting the multilayer film of the present invention may contain known additives. Examples of such additives include plasticizers, antioxidants, flame retardants, stabilizers, UV absorbers, antistatic agents, lubricants, water repellents, and oil repellents.

As for other configurations, it should be understood that the embodiments disclosed in this specification are illustrative in all respects and that the scope of the present invention is not limited thereto. Those skilled in the art will easily understand that appropriate modifications are possible without departing from the spirit of the present invention. Therefore, other embodiments that are modified without departing from the spirit of the present invention are naturally included in the scope of the present invention.

The present invention will be further described below with reference to examples. However, the following examples are not intended to limit the present invention.

〔sample〕
The examples and reference examples are the multilayer film 10 according to the above embodiment. However, reference examples 2 to 5 are examples in which the outer layer has a multilayer structure, and the layer provided between the outer layer 1 and the first intermediate adhesive layer 3 is referred to as the second outer layer. The multilayer films according to comparative examples 1 and 2 are examples of films conventionally used for laminating a polystyrene foam sheet when the polystyrene foam sheet is used for food packaging, and are multilayer films that do not have an adhesive layer that can adhere to a polystyrene resin. Comparative example 3 is an example that differs from example 1 in that it does not have an adhesive layer 2. Comparative example 4 is an example that differs from example 1 in that the outer layer 1 does not contain a polypropylene resin. The specific configurations of the films of each example are shown in Table 1.

The polystyrene foam sheet to be laminated with the multi-layer film of the Examples , Reference Examples and Comparative Examples had a thickness of 2 mm, a density of 0.1 g/cm ³ and an expansion ratio of 10 times.

[Test Method]
For the multilayer films of Example 1 , Reference Examples 1 to 5 , and Comparative Examples 2 to 4, the adhesive layer 2 was heat-sealed to the polystyrene foam sheet in a state where it was in contact with the polystyrene foam sheet without performing a treatment such as applying an adhesive. For the multilayer film of Comparative Example 1, a mixture of Takelac (registered trademark) A520 (manufactured by Mitsui Chemicals, Inc.), Takenate (registered trademark) A10 (manufactured by Mitsui Chemicals, Inc.), and ethyl acetate in a weight ratio of 6:1:6 was applied as an adhesive to the surface where the polyethylene terephthalate (PET) was exposed, and then the surface to which the adhesive was applied was heat-sealed in a state where it was in contact with the polystyrene foam sheet. For all of the examples, the conditions for heat-sealing were a temperature of 180° C., a time of 1.0 second, and a pressure of 0.05 MPa.

The peel strength of the sample (hereafter referred to as the heat-sealed sample) obtained by heat-sealing the multilayer film and the polystyrene sheet under the above conditions was measured in accordance with JIS Z 0238:1998. However, the sample width was 15 mm, and the peel speed was 200 mm/min. The peel strength was measured when tensile stress was applied along the extrusion direction of the multilayer film (MD) and when tensile stress was applied along the direction perpendicular to the extrusion direction of the multilayer film (TD).

The peelability of the top film (lid material) of the heat-sealed samples was also evaluated. A top film (polypropylene-based easy-peel sealant) was welded to the multilayer film side of the heat-sealed sample at 160°C for 2 seconds, and then the top film was peeled off by hand. After peeling, the sample was visually inspected to see whether peeling had occurred between the multilayer film and the polystyrene foam sheet.

In addition, the thermal resistance of the multilayer film was evaluated. Thermal resistance refers to the phenomenon in which the multilayer film sticks to the hot plate that heats the sheet when the multilayer film and the polystyrene foam sheet are thermally bonded together. It is desirable to avoid thermal resistance, since it can cause problems such as insufficient thermal bonding between the multilayer film and the polystyrene foam sheet and poor appearance of the food packaging sheet after thermal bonding. When the multilayer film was pressed against the polystyrene foam sheet using a hot plate (175°C, 0.5 MPa), it was visually confirmed whether or not a part of the outer layer 1 of the multilayer film peeled off from the polystyrene foam sheet and adhered to the hot plate.

〔result〕
The configurations of the Examples , Reference Examples and Comparative Examples and the results of the peel strength tests are shown in Tables 1 and 2.

The abbreviations of the resin names in the table represent the following resins. Note that when the names of multiple resins for the outer layer 1 are shown together with percentage values, this means that the outer layer 1 was formed by co-extrusion of the indicated resins in the indicated weight ratio.
PP: Polypropylene PE: Polyethylene EMAA: Ethylene-methacrylic acid copolymer ION: Ionomer EVOH: Ethylene-vinyl alcohol copolymer EVA: Ethylene-vinyl acetate copolymer BS: Styrene-butadiene copolymer PETG: Polyethylene terephthalate glycol copolymer Note that the first intermediate adhesive layer 3 and the second intermediate adhesive layer 5 were made from products in the Admer (registered trademark) series (Mitsui Chemicals, Inc.).

As shown in Tables 1 and 2, the multilayer films of the Examples and Reference Examples showed peel strengths equal to or greater than that of the multilayer film of Comparative Example 1 in both the MD and TD directions. That is, the multilayer films of the Examples and Reference Examples were able to achieve peel strengths equal to or greater than that of the multilayer films conventionally used for laminating a polystyrene foam sheet when the polystyrene foam sheet is used for food packaging purposes, without using an adhesive. In addition, among the examples in which the multilayer films were heat-sealed to the polystyrene foam sheet without using an adhesive, the phenomenon of the multilayer film peeling off from the polystyrene foam sheet when the lid material was peeled off was not observed in Example 1 and Reference Examples 1 to 5 , which had high peel strengths, whereas in Comparative Examples 2 and 3, which had low peel strengths, the multilayer film peeled off from the polystyrene foam sheet when the lid material was peeled off. Note that, even when the peel strength was high, heat traction was observed during heat fusion in Comparative Example 4.

The present invention can be used, for example, in food packaging applications.

10: Multilayer film 1: Outer layer 2: Adhesive layer 3: First intermediate adhesive layer 4: Gas barrier layer 5: Second intermediate adhesive layer 6: Retaining layer 20: Styrofoam sheet 30: Food packaging sheet 40: Food container 41: Top seal 42: Edge 50: Rolling device 51: Raw material roll 52: Raw material roll 53: Melt compression roll 53a: Heating roll 53b: Counter roll 54: Product roll F: Food

Claims (7)

A multilayer film comprising, in this order, an outer layer containing a polypropylene-based resin, a first intermediate adhesive layer, a gas barrier layer, a second intermediate adhesive layer, a support layer, and an adhesive layer capable of adhering to a polystyrene-based resin,
The adhesive layer includes an adhesive polymer including at least vinyl acetate as a monomer unit,
the retaining layer includes polyethylene, is provided in contact with the adhesive layer and the second intermediate adhesive layer , and retains the adhesive layer against the second intermediate adhesive layer ;
A multilayer film having a peel strength of 3.0 N/15 mm or more when the adhesive layer is heat-sealed to a polystyrene foam sheet, which is a foam of polystyrene-based resin containing 50 mol % or more of styrene as a monomer unit and has a density of 0.05 to 0.5 g/cm ³ , under conditions of a temperature of 180°C, a time of 1.0 second, and a pressure of 0.05 MPa. The multilayer film according to claim 1, wherein the gas barrier layer contains an ethylene-vinyl alcohol copolymer. 3. The multilayer film according to claim 1, wherein the material and thickness of the gas barrier layer are determined so that a film made of the material and having the specified thickness has an oxygen permeability of 5.0 ml/ ^m2 ·day·atm or less as measured according to JIS K 7126. The multilayer film according to any one of claims 1 to 3 has a thickness of 40 to 100 μm. a laminate portion having, in this order, an outer layer containing a polypropylene-based resin, a first intermediate adhesive layer, a gas barrier layer, a second intermediate adhesive layer, a retaining layer, and an adhesive layer capable of adhering to a polystyrene-based resin, the adhesive layer containing an adhesive polymer containing at least vinyl acetate as a monomer unit, the retaining layer containing polyethylene, and being a layer provided in contact with the adhesive layer and the second intermediate adhesive layer and retaining the adhesive layer against the second intermediate adhesive layer ;
a substrate portion in which a polystyrene foam sheet, which is a foam of a polystyrene-based resin containing 50 mol % or more of styrene as a monomer unit and has a density of 0.05 to 0.5 g/cm ³ , is laminated by the adhesive layer of the laminate portion;
The food packaging sheet, wherein the peel strength between the laminate portion and the base portion is 3.0 N/15 mm or more. A food container obtained by forming the food packaging sheet according to claim 5 into a container shape. an extrusion process of co-extrusion molding a polypropylene-based resin, a first intermediate adhesive polymer, an ethylene-vinyl alcohol copolymer, a second intermediate adhesive polymer, polyethylene, and an adhesive polymer containing vinyl acetate as a monomer unit, to form a multilayer film comprising, in this order, an outer layer containing the polypropylene-based resin, a first intermediate adhesive layer containing the first intermediate adhesive polymer, a gas barrier layer containing the ethylene-vinyl alcohol copolymer, a second intermediate adhesive layer containing the second intermediate adhesive polymer, a retention layer containing the polyethylene, and an adhesive layer containing the adhesive polymer containing vinyl acetate as a monomer unit , the retention layer being provided in contact with the adhesive layer and the second intermediate adhesive layer and holding the adhesive layer against the second intermediate adhesive layer ;
a lamination step of heat-sealing a base sheet containing a polystyrene-based resin foam and the adhesive layer of the multilayer film to obtain a food packaging sheet;
and a container forming step of forming the food packaging sheet into a container shape to obtain a container.

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