Multilayer film, food packaging sheet, food container, and method for manufacturing container

Abstract

To provide a multilayer film which can be heat-welded to a base material without introducing an adhesive thereto.SOLUTION: A multilayer film 10 has an outer layer 1 containing a polypropylene-based resin and an adhesive layer 2 capable of being bonded to a polystyrene-based resin, in which a peel strength when heat-fused to the polystyrene-based resin under a temperature of 180°C, time of 1.0 second and a pressure of 0.05 MPa is 3.0 N / 15 mm or more.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] 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. [Background technology]

[0002] 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, if a polypropylene container is replaced with a polystyrene foam sheet container of the same shape, the weight of the packaging material can be reduced by about half.

[0003] When using a polystyrene foam sheet for packaging food, a method of heat fusing (laminating) a functional film on the surface of the polystyrene foam sheet is commonly used 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 laminated film having a gas barrier layer is laminated on a substrate made of polystyrene foam. In addition, JP 9-290492 A (Patent Document 2) discloses a synthetic resin laminate sheet in which an adhesive resin layer and a film are laminated on a substrate layer made of a polystyrene resin, and discloses that the synthetic resin laminate sheet can be used for food trays, etc. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2016-155584 A [Patent Document 2] Japanese Patent Application Publication No. 9-290492 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the techniques disclosed in Patent Documents 1 and 2, it was necessary to provide a layer that functions as an adhesive between the base layer and the film layer. Therefore, it was necessary to provide a step of introducing an adhesive (for example, by applying an adhesive) in the heat-sealing process, which could make the heat-sealing process complicated. In addition, it was sometimes difficult to reuse the scraps of the film that had been coated with an adhesive, so there were cases where the scraps had to be discarded.

[0006] Therefore, it is necessary to develop a multi-layer film that can be heat-sealed to a substrate without using an adhesive. [Means for solving the problem]

[0007] The multilayer film of the present invention is a multilayer film comprising an outer layer containing a polypropylene-based resin and an adhesive layer capable of adhering to a polystyrene-based resin, and is characterized in that the peel strength when heat-sealed to the polystyrene-based 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.

[0008] In addition, the food packaging sheet according to the present invention 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 base portion containing a foam of a polystyrene-based resin and laminated by the laminate portion, and is characterized in that the peel strength between the laminate portion and the base portion is 3.0 N / 15 mm or more.

[0009] The food container according to the present invention is characterized in that it is obtained by forming the food packaging sheet described above into a container shape.

[0010] In addition, the container manufacturing method of the present invention is characterized by including an extrusion process in which a polypropylene-based 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-based resin and an adhesive layer containing the adhesive polymer, a lamination process in which a base sheet containing a polystyrene-based resin foam and the adhesive layer of the multilayer film are heat-fused 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.

[0011] 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 producing food packaging sheets and food containers compared to conventional methods. In other words, the amount of resources required for food packaging can be reduced compared to conventional methods.

[0012] Preferred embodiments of the present invention will be described below. However, the scope of the present invention is not limited to the preferred embodiments described below.

[0013] In one embodiment of the multi-layer film according to the present invention, the adhesive layer preferably contains an adhesive polymer, and the adhesive polymer preferably contains vinyl acetate as at least one monomer unit.

[0014] This structure tends to provide particularly high adhesion to the multi-layer film with respect to the polystyrene resin.

[0015] In one embodiment, the multilayer film according to the present invention further comprises a support layer provided between the outer layer and the adhesive layer in contact with the adhesive layer, and the support layer preferably contains polyethylene.

[0016] With this configuration, the adhesive layer is less likely to peel off from the other portions of the multi-layer film.

[0017] In one embodiment, the multi-layer film according to the present invention preferably further comprises a gas barrier layer between the outer layer and the adhesive layer.

[0018] This structure can impart gas barrier properties to the multilayer film, and therefore, when the multilayer film is applied to a food container, for example, deterioration of the food contained in the food container can be suppressed.

[0019] In one embodiment of the multilayer film according to the present invention, the gas barrier layer preferably contains an ethylene-vinyl alcohol copolymer.

[0020] This structure tends to provide the multi-layer film with particularly high gas barrier properties.

[0021] In one embodiment of the multi-layer film according to the present invention, the material and thickness of the gas barrier layer are such that the oxygen permeability of a film made of the material and having the thickness is 5.0 ml / m or less, as measured according to JIS K 7126. 2 It is preferable that the temperature is determined to be equal to or less than 1·day·atm.

[0022] The material and thickness of the gas barrier layer of the multi-layer film of this construction are determined based on the specific level of gas barrier properties required for food containers, so that a multi-layer film that is particularly suitable for food packaging applications can be obtained.

[0023] In one embodiment, the multilayer film according to the present invention preferably has a thickness of 40 to 100 μm.

[0024] This structure makes it possible to obtain a multi-layer film that is easy to heat seal and to manufacture.

[0025] 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. [Brief description of the drawings]

[0026] [Figure 1] FIG. 2 is a cross-sectional view showing a layer structure of a multilayer film. [Diagram 2] FIG. 2 is a cross-sectional view showing a layer structure of the food packaging sheet. [Diagram 3] FIG. 2 is a schematic cross-sectional view showing the structure of a food container. [Figure 4] FIG. 2 is a schematic diagram showing an example of a lamination method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The 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. In the following, an example 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 PSP20) will be described. In addition, a food packaging sheet 30 obtained by laminating the PSP20 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 will also be described.

[0028] [Construction and manufacturing method of multi-layer film] The multilayer film 10 according to this embodiment comprises an outer layer 1 provided on the first surface side and an adhesive layer 2 provided on the second surface side (FIG. 1). The multilayer film 10 also comprises multiple 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.

[0029] When PSP 20 is laminated with multilayer film 10 to obtain food packaging sheet 30, the second surface (adhesive layer 2) is brought into contact with PSP 20 and heat-sealed, so that the first surface (outer layer 1) is exposed (FIG. 2). When food packaging sheet 30 is molded into a container shape to obtain food container 40, molding is performed so that the surface of food packaging sheet 30 facing multilayer film 10 (outer layer 1) has a shape capable of containing food F (FIG. 3). Therefore, outer layer 1 is the layer that will come into contact with food F when food container 40 is obtained.

[0030] The thickness of the multilayer film 10 according to the present 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%), support 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 a required level. If the thickness of the multilayer film 10 is less than 40 μm, the yield during the production of the multilayer film 10 may be low. On the other hand, if the thickness of the multilayer film 10 exceeds 100 μm, heat may not be easily transferred to the bonding surface during heat fusion with the PSP 20, which may result in poor heat fusion. The absolute thicknesses of the layers and their ratios to the thickness of the multilayer film 10 are not limited to the above examples.

[0031] 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. When the polypropylene-based resin includes a material other than polypropylene, examples of the material 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). 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.

[0032] As described above, the outer layer 1 may be made of only a polypropylene-based resin, or may be made of a mixture of a polypropylene-based resin and another material. That is, the description 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.

[0033] Since the outer layer 1 is the layer that will come into contact with the food F, it is preferable that the outer layer 1 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, the polypropylene-based resin has sufficient heat resistance against the temperature (about 100°C) of the steam generated when the food F is heated in a microwave oven.

[0034] 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. The adhesive polymer contained in the adhesive layer 2 is not limited to the polymers that include vinyl acetate as at least a monomer unit as exemplified above.

[0035] The adhesive layer 2 provides the multilayer film 10 with good adhesion to polystyrene resin. Specifically, when the multilayer film 10 is 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, the peel strength is 3.0 N / 15 mm or more. Note that the polystyrene resin refers to a polymer containing 50 mol % or more of styrene as a monomer unit, and may be a homopolystyrene having no monomer unit other than styrene, or a modified polystyrene containing a monomer unit other than styrene.

[0036] 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, for example, a material having a molecular structure in common with 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. In this embodiment, the retaining layer 6 contains polyethylene. Since polyethylene has a molecular structure in common with the main chain portion of the polyvinyl acetate or vinyl acetate copolymer used in the adhesive layer 2, it has high adhesion to the adhesive layer 2, which can effectively prevent peeling of the adhesive layer 2.

[0037] The gas barrier layer 4 is a layer for imparting 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 having an ethylene content of 38 mol % or less, since the gas barrier properties are particularly high. In addition to the ethylene-vinyl alcohol copolymer, examples of materials suitable for the material contained in the gas barrier layer 4 include polyamide MXD6 and polyvinylidene chloride. In addition, the gas barrier layer 4 may contain additives having gas barrier properties (mica, clay, etc.).

[0038] The material and thickness of the gas barrier layer 4 have a large effect on the gas barrier properties of the multilayer film 10, and therefore the material and thickness of the gas barrier layer 4 must be selected within a range that provides the gas barrier properties required for the food container 40. The material and thickness of the gas barrier layer 4 must be selected so that the oxygen transmission rate of a film made of the material and having a thickness of 5.0 ml / m or more, as measured according to JIS K 7126, is 5.0 ml / m or more. 2 For example, if the gas barrier layer 4 is made of an ethylene-vinyl alcohol copolymer with an ethylene content of 38 mol % or less and has a thickness of 6 μm or more, the oxygen permeability is determined to be 5.0 ml / m 2 The material and thickness of the gas barrier layer 4 are such that the oxygen permeability of a film made of the material and having the specified thickness is 3.0 ml / m 2 More preferably, the concentration is determined to be equal to or less than 1.0 ml / m 2 It is even more preferred that the temperature is determined to be equal to or less than 1·day·atm.

[0039] The first intermediate adhesive layer 3 and the second intermediate adhesive layer 5 are layers that have the function of bonding 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 depending on the combination of the two layers to be bonded by each layer.

[0040] The multilayer film 10 having the above-mentioned structure can be obtained, for example, by co-extrusion. 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-mentioned structure (an example of an extrusion process). Alternatively, the multilayer film 10 can 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 films individually, are heat-sealed to each other.

[0041] [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).

[0042] PSP20 is a sheet containing a foamed polystyrene resin, and a known polystyrene foam sheet can be used. For example, PSP20 has a thickness of 3 mm or less and a density of 0.05 to 0.5 g / cm. 3 In this case, a polystyrene foam sheet having an expansion ratio of 2 to 20 times can be used. The definition of "polystyrene resin" is as described above.

[0043] The food packaging sheet 30 is obtained by laminating the PSP20 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). Raw material rolls 51 and 52 are provided with original rolls of the multilayer film 10 and the PSP20, respectively. The multilayer film 10 and the PSP20 fed 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 PSP20 before the melt-press roll 53. The PSP20 may be preheated before coming into contact with the multilayer film 10.

[0044] Among the melt-bonding rolls 53, the heating roll 53a is heated, and the opposing roll 53b disposed opposite the heating roll 53a may be heated or unheated as appropriate. The multilayer film 10 and the PSP 20 are bonded together by the heating roll 53a and the opposing roll 53b, and the food packaging sheet 30 obtained by the bonding is taken up on a product roll 54.

[0045] The lamination method as 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. The peel strength between the multilayer film 10 and the PSP 20 is preferably 5.0 N / 15 mm or more.

[0046] 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 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.

[0047] [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 food F is placed in the food container 40 and then 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.

[0048] 3, the food container 40 has a shape capable of containing food F on the surface of the food packaging sheet 30 facing the multilayer film 10 (outer layer 1), and the outer layer 1 comes into contact with 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 oil contained in the food F. In addition, the polypropylene-based resin used in the outer layer 1 generally has a softening point of about 120° C., and therefore has sufficient heat resistance for food F heated in a microwave oven.

[0049] Additionally, 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. The multilayer film 10 has high gas barrier properties (particularly oxygen barrier properties, with an oxygen permeability of 5.0 ml / m) derived from the gas barrier layer 4. 2 1·day·atm or less, 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.

[0050] When the food F is taken out of 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. 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 manual peeling, 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 ) is 10 N / 15 mm or less, and is smaller than the peel strength between the multilayer film 10 (adhesive layer 2 ) and the PSP 20 .

[0051] A known method can be used as a processing method (container forming step) for obtaining a 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.

[0052] 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.

[0053] In the above embodiment, the multilayer film 10 is described as having an outer layer 1, a first intermediate adhesive layer 3, a gas barrier layer 4, a second intermediate adhesive layer 5, a support 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 outer layer contain a polypropylene-based resin.

[0054] In the above embodiment, the food packaging sheet 30 is obtained by laminating the PSP 20 with the multilayer film 10. However, the substrate 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 in contact with the adhesive layer 2. By providing the impact-resistant layer, the multilayer film 10 can be given further impact resistance.

[0055] The polymeric materials used in each layer constituting the multi-layer film of the present invention may contain known additives, such as plasticizers, antioxidants, flame retardants, stabilizers, UV absorbers, antistatic agents, lubricants, water repellents, oil repellents, etc.

[0056] Regarding 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. A person skilled in the art would easily understand that appropriate modifications are possible without departing from the spirit of the present invention. Therefore, other embodiments modified without departing from the spirit of the present invention are naturally included in the scope of the present invention. EXAMPLES

[0057] The present invention will be further described below with reference to examples, but the following examples are not intended to limit the present invention.

[0058] 〔sample〕 Working Example , reference example is the multilayer film 10 according to the above embodiment, where Reference examples 2~5 is an example in which the outer layer has a multilayer structure, and here, 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 purposes, and are multilayer films that do not have an adhesive layer that can adhere to polystyrene resins. 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.

[0059] Working Example , reference example The polystyrene foam sheet to be laminated with the multi-layer film of the comparative example has a thickness of 2 mm and a density of 0.1 g / cm. 3 A foam with a foaming ratio of 10 times was used.

[0060] [Test Method] Example 1 , Reference examples 1~5For the multilayer films of 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 any 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.

[0061] The peel strength of the sample (hereinafter referred to as the heat-sealed sample) obtained by heat-sealing the multilayer film and the polystyrene foam 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 a tensile stress was applied along the extrusion direction of the multilayer film (MD) and when a tensile stress was applied along the direction perpendicular to the extrusion direction of the multilayer film (TD).

[0062] The peelability of the top film (covering material) of the heat-sealed sample 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 observed to see whether peeling occurred between the multilayer film and the polystyrene foam sheet.

[0063] 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 to each other. The occurrence of thermal resistance 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, so it is desirable to prevent this from occurring. When the multilayer film was pressed onto 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.

[0064] 〔result〕 Working Example , reference example The configurations of the comparative examples and the results of the peel strength tests are shown in Tables 1 and 2. [Table 1] [Table 2]

[0065] The abbreviations of the resin names in the table represent the following resins. When multiple resin names are shown for the outer layer 1 together with percentage values, this indicates 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 As the first intermediate adhesive layer 3 and the second intermediate adhesive layer 5, products of the Admer (registered trademark) series (Mitsui Chemicals, Inc.) were used.

[0066] As shown in Tables 1 and 2, in Example and reference examples The multilayer film of Example 1 exhibited peel strengths in both the MD and TD directions equal to or greater than that of the multilayer film of Comparative Example 1. and reference examples The multi-layer film of Example 1 had a peel strength equal to or greater than that of the multi-layer film conventionally used for laminating a polystyrene foam sheet when the polystyrene foam sheet is used for food packaging, without using an adhesive. , Reference examples 1~5 In Comparative Example 4, the multilayer film did not peel off from the polystyrene foam sheet when the lid was peeled off, whereas in Comparative Examples 2 and 3, which had low peel strength, the multilayer film peeled off from the polystyrene foam sheet when the lid was peeled off. Even in the case of a high peel strength, heat failure was observed during heat fusion in Comparative Example 4. [Industrial Applicability]

[0067] The present invention can be used, for example, in food packaging applications. [Explanation of symbols]

[0068] 10: Multi-layer film 1: Outer layer 2: Adhesive layer 3: First intermediate adhesive layer 4: Gas barrier layer 5: Second intermediate adhesive layer 6: Retention layer 20: Styrofoam sheet 30: Food packaging sheets 40: Food containers 41: Top seal 42: Edge 50: Roll device 51: Raw material roll 52: Raw material roll 53: Melt pressing roll 53a: Heating roll 53b: opposing roll 54: Product roll F:Food

Claims

1. 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; The density of polystyrene resin containing 50 mol% or more of styrene as a monomer unit under the conditions of temperature 180°C, time 1.0 second, and pressure 0.05 MPa is 0.05 to 0.5 g / cm ３ The multi-layer film has a peel strength of 3.0 N / 15 mm or more when the foamed polystyrene sheet and the adhesive layer are heat-sealed.

2. 2. The multilayer film according to claim 1, wherein the gas barrier layer comprises an ethylene-vinyl alcohol copolymer.

3. The material and thickness of the gas barrier layer are such that the oxygen permeability measured in accordance with JIS K 7126 for a film formed from the material to the thickness is 5.0 ml / m ２ 3. The multilayer film according to claim 1, wherein the viscosity is determined to be equal to or less than 1.times.day.atm.

4. The multilayer film according to any one of claims 1 to 3, which has a thickness of 40 to 100 µm.

5. 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; Polystyrene-based resin containing 50 mol% or more of styrene as a monomer unit has a density of 0.05 to 0.5 g / cm ３ and a base material portion in which a polystyrene foam sheet, which is a foamed body, 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.

6. A food container obtained by forming the food packaging sheet according to claim 5 into a container shape.

7. 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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