Laminated foam sheets for thermoforming and thermoformed containers

Abstract

This technology maintains peel strength that facilitates the peeling of multilayer resin films from laminated foam sheets, while suppressing ink residue on the foam sheet after peeling and preventing static charge buildup during peeling of the multilayer resin film. [Solution] A thermoforming laminated foam sheet is provided, in which a multilayer resin film is peelably laminated and bonded to at least one side of a polystyrene foam sheet, wherein the multilayer resin film is laminated in the order of an adhesive layer, an intermediate layer, a printed layer printed with ink, and a thermoplastic resin layer, the adhesive layer and the polystyrene foam sheet are bonded, the average peel strength between the multilayer resin film and the polystyrene foam sheet is 40 cN / 25 mm or more and 250 cN / 25 mm or less, and the intermediate layer contains an inorganic pigment.

Description

[Technical Field]

[0001] This invention relates to a laminated foam sheet for thermoforming and a thermoformed container. [Background technology]

[0002] Laminated foam sheets are known in which a thermoplastic resin film is laminated and bonded to a polystyrene foam sheet. Packaging containers such as trays obtained by thermoforming these laminated foam sheets are widely used as food containers for storing cooked foods and the like.

[0003] In recent years, in order to reduce the burden on the environment, recycling of such packaging containers by melting them down and turning them into pellets has been promoted. However, collected packaging containers are often dirty and had to be washed at home or at collection plants before recycling. Furthermore, the films often have decorations printed with ink, and there was a risk that the ink would mix with the collected materials and stain them. If the collected materials are stained, the number of uses for which they can be reused is limited.

[0004] To solve the problems described above, a laminated foam sheet has been developed in which the film laminated onto the polystyrene foam sheet can be peeled off. Being able to peel off the film eliminates the need to wash the packaging container. Furthermore, even if the film is printed with ink, it prevents the ink from mixing with the recovered raw materials.

[0005] As such a laminated foam sheet, Patent Document 1 discloses a peelable laminate in which a plastic film, an intermediate layer containing a polyurethane resin, a printed layer printed with ink, and a polystyrene foam sheet are laminated by thermal lamination, and the peel strength between the intermediate layer and the polystyrene foam sheet is smaller than the peel strength between other layers. Patent Document 1 aims to maintain sufficient adhesion between the film and the polystyrene foam sheet while easily peeling the printed layer from the polystyrene foam sheet when the film is peeled off. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2003-251763 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the technology described in Patent Document 1, some ink sometimes remained on the polystyrene foam sheet side when the film was peeled off. Furthermore, in the technology described in Patent Document 1, peeling off the film generated static electricity, causing the film to stick to its surroundings (such as hands), resulting in poor handling. The surface of the film may have food residue or other contaminants attached to it, and the film sticking to these surfaces can cause contamination.

[0008] In consideration of the above circumstances, the present invention aims to provide a laminated foam sheet for thermoforming and a thermoforming container that maintains peel strength that facilitates the peeling of the film from the laminated foam sheet, while suppressing ink residue on the foam sheet after peeling and suppressing electrostatic charge on the film after peeling. [Means for solving the problem]

[0009] A laminated foamed sheet for thermoforming, in which a multilayer resin film is detachably laminated and adhered to at least one side of a polystyrene-based resin foamed sheet, wherein the multilayer resin film is laminated in the order of an adhesive layer, an intermediate layer, a printed layer printed using ink, and a thermoplastic resin layer, the adhesive layer is adhered to the polystyrene-based resin foamed sheet, the average value of the peel strength between the multilayer resin film and the polystyrene-based resin foamed sheet is 40 cN / 25 mm or more and 250 cN / 25 mm or less, and the intermediate layer contains an inorganic pigment.

[0010] [2] The laminated foamed sheet for thermoforming according to [1], wherein the inorganic pigment is at least one selected from titanium oxide and talc.

[0011] [3] The laminated foamed sheet for thermoforming according to [1] or [2], wherein the content of the inorganic pigment contained in the intermediate layer is 10% by mass or more.

[0012] [4] The laminated foamed sheet for thermoforming according to [1] to [3], wherein the thickness of the intermediate layer is 0.2 μm or more and 5 μm or less.

[0013] [5] The laminated foamed sheet for thermoforming according to [1] to [4], wherein the intermediate layer contains a polyurethane-based resin.

[0014] [6] The laminated foamed sheet for thermoforming according to [1] to [5], wherein the adhesive layer contains a polyolefin-based adhesive.

[0015] [7] The laminated foamed sheet for thermoforming according to [1] to [6], wherein the multilayer resin film is laminated and adhered to the polystyrene-based resin foamed sheet by thermal lamination.

[0016] [8] A container obtained by thermoforming the laminated foamed sheet for thermoforming according to [1] to [7], wherein the multilayer resin film is located inside the container. [Effect of the Invention]

[0017] According to the present invention, it is possible to suppress ink residue on the foam sheet after peeling and to suppress electrostatic charge of the multilayer resin film after peeling, while maintaining peel strength that facilitates peeling of the multilayer resin film from the laminated foam sheet. [Brief explanation of the drawing]

[0018] [Figure 1] This is a cross-sectional view of a laminated foam sheet for thermoforming according to an example of the present invention. [Figure 2] This is a cross-sectional view of a laminated foam sheet for thermoforming according to an example of the present invention. [Figure 3] These are electron images (left: secondary electron image, right: backscattered electron image) of the surface of the adhesive layer side of the multilayer resin film according to Comparative Example 3, taken using a scanning electron microscope. [Figure 4] These are electron images (left: secondary electron image, right: backscattered electron image) of the surface on the adhesive layer side of the multilayer resin film according to Example 1, taken using a scanning electron microscope. [Modes for carrying out the invention]

[0019] [1] Laminated foam sheet for thermoforming The thermoforming laminated foam sheet according to the present invention (hereinafter also referred to as "laminated foam sheet") is a sheet in which a multilayer resin film is peelably laminated and bonded to at least one side of a polystyrene-based resin foam sheet, wherein the multilayer resin film is laminated on the polystyrene-based resin foam sheet in the order of adhesive layer, intermediate layer, printed layer printed with ink, and thermoplastic resin layer. The average peel strength between the multilayer resin film and the polystyrene-based resin foam sheet is 40 cN / 25 mm or more and 250 cN / 25 mm or less, and the intermediate layer contains an inorganic pigment.

[0020] Figure 1 is a cross-sectional view of a thermoforming laminated foam sheet (100) according to an example of the present invention. Figure 1 illustrates a thermoforming laminated foam sheet (100) in which a multilayer resin film (10) is laminated and bonded to one surface of a polystyrene resin foam sheet (20). As illustrated in Figure 1, the adhesive layer (11), intermediate layer (12), printed layer (13), and thermoplastic resin layer (14) are laminated in this order from the bonding surface of the multilayer resin film (10) with the polystyrene resin foam sheet (20) to form the multilayer resin film (10). As illustrated in Figure 2, the thermoforming laminated foam sheet (100) allows for the peeling off of the multilayer resin film (10).

[0021] In this invention, it is possible to suppress ink residue on the foam sheet after peeling and to suppress electrostatic charge of the multilayer resin film after peeling, while maintaining peel strength that facilitates the peeling of the multilayer resin film from the laminated foam sheet. Furthermore, this invention can also suppress the phenomenon of void formation at the interface between the polystyrene-based resin foam sheet and the multilayer resin film (film lifting).

[0022] In this invention, by allowing the multilayer resin film to be peeled from the laminated foam sheet, the need to wash the packaging container is eliminated, and even if ink is printed on the film, it is possible to prevent the ink from mixing with the foam sheet after peeling. Therefore, the foam sheet after peeling can be suitably used as a recycled polystyrene raw material by melting and pelletizing it after collection. The recycled polystyrene raw material can be used as a recycled material for polystyrene-based resin foam sheets, recycled material for polystyrene-based resin foam particle molded products, recycled material for polystyrene-based resin extruded foam boards, and other polystyrene raw materials for foam products.

[0023] <Polystyrene foam sheet> Polystyrene-based resin foam sheets used in laminated foam sheets primarily consist of polystyrene-based resin. Polystyrene-based resin refers to a resin containing 50% by weight or more of styrene-based monomer component units. Examples include polystyrene, rubber-modified polystyrene (impact-resistant polystyrene), styrene-α-methylstyrene copolymer, styrene-p-methylstyrene copolymer, styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-acrylonitrile copolymer, and mixtures of polystyrene and polyphenylene ether. A mixture of two or more of these polystyrene-based resins may also be used. Polystyrene-based resin may also contain polyfunctional monomer component units such as divinylbenzene and polybranched macromonomers.

[0024] "Mainly composed of polystyrene resin" means that 50% by mass or more of the resin constituting the polystyrene resin foam sheet is polystyrene resin, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and particularly preferably 90% by mass or more is polystyrene resin.

[0025] Other components may be blended into the resin constituting the polystyrene foam sheet, as long as they do not hinder the objectives of the present invention. Examples of other components include polypropylene resins such as propylene homopolymers and ethylene-propylene copolymers, polyethylene resins such as high-density polyethylene and low-density polyethylene, thermoplastic elastomers such as styrene-conjugated diene block copolymers and their hydrogenated products, and rubbers such as ethylene-propylene rubber and butadiene rubber. The amount of other components blended into the polystyrene foam sheet is preferably 30% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 0% by mass, i.e., consisting only of polystyrene resin.

[0026] The melt flow rate (MFR) of the polystyrene resin constituting the polystyrene foam sheet is preferably 0.1 g / 10 min to 5 g / 10 min, and more preferably 1 g / 10 min to 3 g / 10 min. Having the polystyrene resin's MFR within this range allows for the production of polystyrene foam sheets with a closed-cell structure and excellent mechanical strength over a wide range of manufacturing conditions.

[0027] The melt flow rate of polystyrene resin refers to the melt mass flow rate measured according to Test Method A of JIS K7210-1 (2014), using test conditions of a temperature of 200°C and a load of 5 kg.

[0028] <Multilayer resin film> The details of the multilayer resin film (adhesive layer, intermediate layer, printed layer, thermoplastic resin layer) used in laminated foam sheets are as follows.

[0029] [Thermoplastic resin layer] Typical resins that make up the thermoplastic resin layer include polyolefin resins and polystyrene resins.

[0030] Examples of polyolefin resins include polyethylene resins such as linear low-density polyethylene, high-density polyethylene, low-density polyethylene, and ethylene-vinyl acetate copolymers; polyethylene resins including biomass-derived polyethylene; polypropylene resins such as propylene homopolymer, ethylene-propylene block copolymer, and ethylene-propylene random copolymer; and polypropylene resins including biomass-derived polypropylene.

[0031] Examples of polystyrene-based resins include those similar to the polystyrene-based resin used in the polystyrene-based foam sheet described above. The thermoplastic resin layer is not limited to a single layer; a multilayer film formed by laminating multiple films using these resins may also be used as the thermoplastic resin layer.

[0032] The thickness of the thermoplastic resin layer is preferably 5 μm to 50 μm, more preferably 10 μm to 40 μm, even more preferably 15 μm to 35 μm, and particularly preferably 20 μm to 30 μm. If the thickness of the thermoplastic resin layer is within this range, defects such as tearing or perforation of the film are prevented, and the multilayer resin film can be peeled from the polystyrene foam sheet.

[0033] [Print layer] The ink used in the printing layer is typically a pigment dispersed in a solvent. A commercially available, general-purpose ink suitable for printing on thermoplastic resin layers can be used.

[0034] Pigments are changed as appropriate depending on the color of the printed design or text, and inorganic and organic pigments are used. Examples of inorganic pigments include titanium dioxide, red iron oxide, antimony red, cadmium yellow, cobalt blue, Prussian blue, ultramarine, carbon black, graphite, calcium carbonate, kaolin, clay, barium sulfate, aluminum hydroxide, and talc. Examples of organic pigments include soluble azo pigments, insoluble azo pigments, azo lake pigments, condensed azo pigments, copper phthalocyanine pigments, and condensed polycyclic pigments.

[0035] The solvent is not particularly limited as long as it can disperse the pigment, and examples include aromatic solvents such as toluene and xylene, alicyclic solvents such as cyclohexane and methylcyclohexane, ester solvents such as ethyl acetate, propyl acetate and amyl acetate, ketone solvents such as acetone, methyl ethyl ketone and methyl isobutyl ketone, alcohol solvents such as ethanol and propanol, vegetable oils such as soybean oil and linseed oil, and water. Two or more types may be used in combination. The ink may also contain binders such as resins and various other additives.

[0036] The thickness of the printed layer is preferably 0.1 μm to 10 μm, more preferably 0.5 μm to 5 μm, and even more preferably 0.7 μm to 3 μm. Having the printed layer thickness within this range makes it easier to obtain molded articles with excellent moldability and a superior appearance. Furthermore, having the printed layer thickness within this range prevents problems such as the formation of steps when winding the multilayer resin film into a roll. The printed layer thickness refers to the average thickness. Note that the printed layer may have areas where ink is not applied and no patterns or text are printed.

[0037] [Middle class] The intermediate layer is a layer containing inorganic pigment. By placing an intermediate layer containing inorganic pigment between the printing layer and the adhesive layer, it is possible to suppress the residue of ink on the polystyrene foam sheet side when the multilayer resin film is peeled off. In addition, the presence of inorganic pigment in the intermediate layer creates irregularities on the surface of the intermediate layer (the surface opposite to the printing layer). As will be described in detail later, the formation of irregularities on the surface of the intermediate layer makes it possible to suppress the peeling charge of the multilayer resin film when it is peeled off from the laminated foam sheet. Furthermore, the presence of inorganic pigment in the intermediate layer also has the effect of blocking the ink color of the printing layer and stabilizing the conditions for thermal lamination, which will be described later.

[0038] As inorganic pigments, inorganic white pigments are preferred because they effectively block the ink color of the printing layer and make it easier to further stabilize the conditions for thermal lamination. Examples of inorganic white pigments include titanium dioxide, zinc oxide, zirconium oxide, silicon dioxide, barium sulfate, zinc sulfide, glass, clay, mica, talc, kaolinite (kaolin), halloysite, zeolite, acid clay, activated clay, boehmite, pseudoboehmite, inorganic oxides, and metal salts (e.g., alkaline earth metal salts), and two or more may be used in combination. Among inorganic white pigments, from the viewpoint of effectively blocking the ink color of the printing layer, one or more selected from titanium dioxide, zinc oxide, talc, and metal salts are preferred, one or more selected from titanium dioxide and talc are more preferred, and titanium dioxide is particularly preferred.

[0039] However, inorganic pigments are not limited to white, and may be pigments of other colors. For example, they may be inorganic pigments such as red iron oxide, antimony red, cadmium yellow, cobalt blue, Prussian blue, ultramarine, carbon black, or graphite.

[0040] The average particle size of the inorganic pigment is preferably 0.01 μm to 15 μm, more preferably 0.05 μm to 8 μm, and even more preferably 0.1 μm to 4 μm. Having the average particle size of the inorganic pigment within this range allows for a moderate level of surface irregularity, maintaining peel strength that facilitates the peeling of the film from the laminated foam sheet, while also making it easier to suppress static charge buildup during film removal. Furthermore, having the average particle size of the inorganic pigment within this range allows for effective shielding of the ink color in the printed layer.

[0041] The average particle size of inorganic pigments refers to the median diameter, which corresponds to 50% of the cumulative value in the volume-based particle distribution measured by the laser diffraction method specified in JIS-Z8825-1:2001.

[0042] The inorganic pigment content in the intermediate layer is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. This range of inorganic pigment content in the intermediate layer significantly suppresses ink residue on the polystyrene-based resin foam sheet after peeling of the multilayer resin film, and also suppresses electrostatic charging of the multilayer resin film after peeling. The upper limit of the inorganic pigment content in the intermediate layer is not particularly limited, but is generally around 80% by mass.

[0043] The intermediate layer contains a resin in addition to the inorganic pigment. Examples of resins included in the intermediate layer include polyurethane resins, polyolefin resins, polyester resins, polyether resins, acrylic resins, and ethylene-vinyl acetate, and may also be a mixture of two or more of these resins. From the viewpoint of excellent dispersibility of the inorganic pigment, as well as excellent adhesion to the film and durability (chemical resistance and abrasion resistance), it is preferable that the intermediate layer contains a polyurethane resin.

[0044] The resin content in the intermediate layer is preferably 20% by mass or more and 90% by mass or less, more preferably 20% by mass or more and 85% by mass or less, and even more preferably 20% by mass or more and 80% by mass or less. Having the resin content in the intermediate layer within this range makes it easier to obtain excellent dispersibility of inorganic pigments, as well as excellent adhesion to the film and durability (chemical resistance and abrasion resistance).

[0045] The intermediate layer may further contain various other components, as long as they do not hinder the objectives and effects of the present invention. Examples of other components include aromatic solvents such as benzene, toluene, and xylene; ester solvents such as ethyl acetate, propyl acetate, and butyl acetate; alcohol solvents such as methanol, ethanol, isopropanol, and n-butanol; ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; N-methyl-2-pyrrolidone; and water. These can be used individually or in combination of two or more.

[0046] The above-mentioned content of inorganic pigments, resins, and other components is based on the entire intermediate layer being considered as 100% by mass. However, if the intermediate layer contains volatile components as other components, the entire intermediate layer, including those volatile components, shall be considered as 100% by mass.

[0047] The thickness of the intermediate layer is preferably 0.2 μm to 5 μm, more preferably 0.5 μm to 3 μm, and even more preferably 0.7 μm to 2 μm. By keeping the thickness of the intermediate layer within this range, it is possible to more reliably suppress the residue of ink remaining on the polystyrene-based resin foam sheet side when the multilayer resin film is peeled off.

[0048] The ratio of the thickness of the intermediate layer (μm) to the average particle size (μm) of the inorganic pigment is preferably 2 to 10, more preferably 3 to 8, and even more preferably 4 to 7. Having this ratio of the intermediate layer thickness (μm) to the average particle size (μm) of the inorganic pigment effectively creates irregularities on the surface of the intermediate layer (the surface opposite to the printed layer), thereby more effectively suppressing the electrostatic discharge of the multilayer resin film.

[0049] [Adhesive layer] Various known adhesives and adhesive resins can be used for the adhesive layer. For example, adhesives such as polyolefin adhesives, polyurethane adhesives, polyester adhesives, polyether adhesives, and acrylic adhesives, as well as adhesive resins such as ethylene vinyl acetate, can be used. From the viewpoint of easily bonding with polystyrene foam sheets by heat lamination or the like, it is preferable to construct the adhesive layer with a polyolefin adhesive.

[0050] The thickness of the adhesive layer is preferably 1 μm to 50 μm, more preferably 10 μm to 40 μm, and even more preferably 15 μm to 35 μm. Having the adhesive layer thickness within this range allows for sufficient strength bonding with the polystyrene foam sheet, more reliably suppressing the lifting of the multilayer resin film.

[0051] <Method for manufacturing laminated foam sheets for thermoforming> The method for manufacturing the laminated foam sheet of the present invention will now be described. The laminated foam sheet of the present invention can be obtained by forming a polystyrene-based resin foam sheet by a known extrusion foaming method, and then laminating and bonding a multilayer resin film to this polystyrene-based resin foam sheet. An example of a specific method for manufacturing the laminated foam sheet is as follows.

[0052] [Method for manufacturing polystyrene foam sheets] First, the method for manufacturing a polystyrene foam sheet will be explained. Specifically, the polystyrene resin mentioned above, along with foam regulators such as talc added as needed, are supplied to an extruder, heated, melted, and kneaded to form a molten resin. Next, a physical foaming agent is injected into the molten resin under pressure, and the mixture is further kneaded to adjust the resin temperature to a foamable temperature to form a foamable resin molten product. Next, the foamable resin molten product is introduced into an annular die attached to the downstream side of the extruder and extruded into the atmosphere to form a tubular foam. Then, the tubular foam is pulled along a cylindrical cooling device and cut open to obtain a polystyrene foam sheet.

[0053] For obtaining polystyrene-based foamed sheets, one or more foaming agents can be used, for example, selected from organic physicoblasting agents and inorganic physicoblasting agents.

[0054] Examples of organic physical blowing agents include aliphatic alcohols with 1 to 5 carbon atoms, aliphatic hydrocarbons with 3 to 5 carbon atoms, dialkyl ethers with 1 to 3 carbon atoms in the alkyl chain, alkyl chlorides, hydrofluoroolefins, and fluorinated hydrocarbons.

[0055] Examples of inorganic physical blowing agents that can be used include oxygen, nitrogen, carbon dioxide, air, and water. Among these, water and carbon dioxide are preferred because they reduce environmental impact and dissipate quickly from the extruded foam, thus allowing the dimensions of the resulting extruded foam to stabilize quickly.

[0056] As a physical blowing agent, a physical blowing agent containing an aliphatic hydrocarbon having 3 to 5 carbon atoms and an early dissipation blowing agent is preferably used.

[0057] Aliphatic hydrocarbons with 3 to 5 carbon atoms have a relatively slow volatilization rate from polystyrene foam sheets and a slower permeation rate into polystyrene resin than air. Therefore, the secondary foaming and plasticization properties of the polystyrene foam sheet during thermoforming can be ensured over a long period, and the life cycle of the resulting laminated foam sheet can be extended. From this viewpoint, n-butane, isobutane, and a mixture of n-butane and isobutane are preferred as aliphatic hydrocarbons with 3 to 5 carbon atoms, with isobutane being more preferred.

[0058] Examples of early-dissipating blowing agents include those among the various physical blowing agents mentioned above that have a permeation rate to polystyrene resin faster than air. For example, ethers with a boiling point of 140°C or less, dialkyl carbonates with a boiling point of 140°C or less, alcohols, carbon dioxide, and water are examples of early-dissipating blowing agents. Among these, it is preferable to use one or more of ethyl methyl ether, dimethyl ether, and diethyl ether as early-dissipating blowing agents. It is even more preferable to use dimethyl ether as an early-dissipating blowing agent because it facilitates the production of low-apparent-density polystyrene resin foam sheets and can sufficiently plasticize the polystyrene resin.

[0059] Furthermore, when a polystyrene foam sheet is manufactured using a mixed blowing agent of dimethyl ether, n-butane, and isobutane, the dimethyl ether contributes to reducing the apparent density but volatilizes quickly, while n-butane and isobutane (especially isobutane) remain in the polystyrene foam sheet for a long period of time, contributing to improved thermoformability. By using a fast-dissipating blowing agent, the amount of residual blowing agent in the laminated foam sheet can be reduced, thereby effectively suppressing the lifting of the multilayer resin film.

[0060] The total amount of organic physical blowing agent is preferably 1 part by weight or more and 8 parts by weight or less, and preferably 2 parts by weight or more and 5 parts by weight or less, per 100 parts by weight of polystyrene resin.

[0061] The following describes the case in which isobutane and dimethyl ether are used as physical blowing agents. However, the physical blowing agents used in the present invention are not limited to this combination, and one or more of the various blowing agents described above can be used in combination.

[0062] The amount of isobutane added is preferably 1 part by weight or more and 4 parts by weight or less per 100 parts by weight of polystyrene resin. By adding isobutane within this range, a polystyrene resin foam sheet with the desired thickness and apparent density can be easily formed. Furthermore, a polystyrene resin foam sheet with excellent moldability, particularly excellent secondary foaming properties and excellent moldability into molds can be obtained. From the above viewpoint, it is more preferable that the amount of isobutane added is 2 parts by weight or more and 3.5 parts by weight or less per 100 parts by weight of polystyrene resin.

[0063] The amount of dimethyl ether added is preferably 0.5 parts by weight or more and 2 parts by weight or less per 100 parts by weight of polystyrene resin. Since dimethyl ether has a fast dissipation rate from the polystyrene resin foam sheet, using dimethyl ether as a physical blowing agent allows for sufficient plasticization of the polystyrene resin during the production of the polystyrene resin foam sheet, while also contributing to a lower apparent density of the polystyrene resin foam sheet. Furthermore, since the amount of residual blowing agent in the resulting polystyrene resin foam sheet can be reduced, the laminated foam sheet in which a multilayer resin film is laminated after thermoforming will have suppressed lifting of the multilayer resin film after thermoforming. From these viewpoints, the amount of dimethyl ether added is preferably 0.6 parts by weight or more and 1.8 parts by weight or less per 100 parts by weight of polystyrene resin, and more preferably 0.8 parts by weight or more and 1.5 parts by weight or less.

[0064] The weight ratio of isobutane to dimethyl ether is preferably 55:45 to 80:20. By setting the weight ratio of isobutane to dimethyl ether within this range, a polystyrene-based resin foam sheet can be obtained that exhibits excellent thermoformability and can prevent lifting of the multilayer resin film. From this viewpoint, a weight ratio of isobutane to dimethyl ether of 60:40 to 75:25 is more preferable.

[0065] The polystyrene resin used in polystyrene foam sheets may contain various additives as needed, such as foam regulators, colorants including pigments and dyes, heat stabilizers, and fillers.

[0066] As a foam regulator, inorganic powders such as talc, kaolin, mica, silica, calcium carbonate, barium sulfate, titanium dioxide, aluminum oxide, clay, bentonite, and diatomaceous earth, as well as conventionally known chemical foaming agents such as azodicarbodiamide, can be used. From the viewpoint of handling and cost, talc is preferred. The amount of foam regulator added varies depending on the type of foam regulator and the desired bubble size, but is preferably 0.01 parts by weight to 8 parts by weight, more preferably 0.02 parts by weight to 5 parts by weight, and especially more preferably 0.05 parts by weight to 3 parts by weight per 100 parts by weight of polystyrene resin.

[0067] [Manufacturing method for multilayer resin film] Next, a method for manufacturing a multilayer resin film (adhesive layer, intermediate layer, printed layer, thermoplastic resin layer) will be explained. A multilayer resin film can be made by laminating the printed layer, intermediate layer, and adhesive layer in this order from one surface of the thermoplastic resin layer.

[0068] Various known techniques can be used to form the printed layer, intermediate layer, and adhesive layer. For example, lamination can be performed by gravure printing or flexographic printing. However, the method of forming each layer is not limited to the examples above, and various other coating methods can also be used.

[0069] Next, a method for laminating and bonding a polystyrene foam sheet and a multilayer resin film will be described.

[0070] Known methods for laminating and bonding a multilayer resin film to a polystyrene foam sheet include, for example, thermal lamination and extrusion lamination. It is preferable to laminate and bond the multilayer resin film to one or both sides of the polystyrene foam sheet by thermal lamination because it is easy to adjust the average peel strength. As illustrated in Figure 1, the adhesive layer (11) of the multilayer resin film (10) is bonded to the polystyrene foam sheet (20).

[0071] The lamination and bonding of multilayer resin films to polystyrene foam sheets by thermal lamination is typically performed by first winding the extruded polystyrene foam sheet into a roll, then, in a separate process from the foaming process, unwinding the polystyrene foam sheet from the roll, overlapping the multilayer resin films, and passing it through a thermal lamination device (offline processing).

[0072] However, a method may be adopted in which a multilayer resin film is layered onto an extruded polystyrene foam sheet on the manufacturing line of the polystyrene foam sheet, and the sheet is passed through a thermal lamination device to form a laminated foam sheet, which is then wound into a roll (online processing).

[0073] In thermal lamination, for example, a thermal lamination apparatus having a heating roll and a clamping roll capable of pressing a polystyrene foam sheet is used. Specifically, it is preferable to overlap a multilayer resin film and a polystyrene foam sheet, supply them between the heating roll and the clamping roll so that the multilayer resin film is in contact with the heating roll, and sandwich the multilayer resin film and the polystyrene foam sheet so that the multilayer resin film is laminated and bonded to the polystyrene foam sheet.

[0074] When obtaining a laminated foam sheet, the peel strength can be adjusted by changing, for example, the heating conditions during bonding of the polystyrene-based resin foam sheet and the multilayer resin film, as well as the line speed. As for the heating conditions, for example, the temperature of the heating roll is preferably set to 160 to 185°C, more preferably to 170 to 190°C. Also, the line speed is preferably set to 5 to 20 m / min, more preferably to 6 to 18 m / min.

[0075] Furthermore, various known methods and apparatuses can be used for the thermal lamination method and the apparatus used for thermal lamination.

[0076] As described above, irregularities are formed on the surface of the intermediate layer by inorganic pigments. By laminating and bonding the polystyrene foam sheet and the multilayer resin film by thermal lamination, the irregularities of the intermediate layer are reflected in the adhesive layer. In other words, irregularities corresponding to those of the intermediate layer are formed on the surface of the adhesive layer. The irregularities on the surface of the adhesive layer reduce the contact area between the adhesive layer and the polystyrene foam sheet. This is thought to reduce the resistance when peeling the multilayer resin film, thereby suppressing peeling charge, while maintaining the peel strength necessary to easily peel the multilayer resin film from the laminated foam sheet. Furthermore, the reduced resistance when peeling also contributes to improved ease of peeling.

[0077] The laminated foam sheet according to the present invention has excellent thermoformability, and by thermoforming the laminated foam sheet, molded articles with various mold shapes can be obtained.

[0078] Thermoforming methods include vacuum forming and pressure forming, as well as their applications such as free drawing forming, plug-and-ridge forming, ridge forming, matched mold forming, straight forming, drape forming, reverse draw forming, air slip forming, plug-assisted forming, plug-assisted reverse draw forming, and combinations thereof. These thermoforming methods are preferred because they allow for the continuous production of molded bodies in a short amount of time.

[0079] Since thermoplastic resin layers have excellent oil resistance, it is preferable that the laminated foam sheet for thermoforming be thermoformed so that the multilayer resin film faces the inside of the container. In particular, when the multilayer resin film is made of a polyolefin resin or the like, which has excellent oil resistance, it is even more preferable to thermoform the multilayer resin film so that it is located on the inside of the container.

[0080] Molded products obtained by thermoforming are used as various food containers, such as food trays and natto containers. After using these food containers, the cleaning process can be simplified by peeling off only the multilayer resin film from the laminated foam sheet (i.e., by separating the multilayer resin film from the polystyrene-based resin foam sheet). Therefore, the recyclability of the polystyrene-based resin foam sheet is improved.

[0081] Furthermore, the recyclability of the laminated foam sheets is further improved because the multilayer resin film can be peeled off the remaining sheets that are punched out during the molding process, just like the molded parts themselves.

[0082] <Physical properties of laminated foam sheets for thermoforming> The physical properties of the thermoforming laminated foam sheet according to the present invention will be described.

[0083] [Thickness of laminated foam sheet] The thickness of the laminated foam sheet is preferably 0.5 mm to 5 mm. When the thickness of the laminated foam sheet is within this range, the thermoformability of the laminated foam sheet is good, and the container obtained by thermoforming the laminated foam sheet will have excellent rigidity, heat insulation, and handling properties. From the above viewpoint, the thickness of the laminated foam sheet is preferably 0.7 mm to 4 mm, and more preferably 1 mm to 3 mm.

[0084] The thickness of the laminated foam sheet is determined by randomly selecting three 260mm-sided measurement samples from the laminated foam sheet, measuring the thickness of each sample using the following method, and taking the arithmetic mean.

[0085] [Grammage of the laminated foamed sheet] The grammage of the laminated foamed sheet is 50 g / m 2 or more and 300 g / m 2 or less, preferably. When the grammage of the laminated foamed sheet is within this range, the laminated foamed sheet will maintain rigidity while being more excellent in lightness, and the obtained molded body will be able to withstand use such as in food packaging containers. From the above viewpoints, the grammage of the laminated foamed sheet is 80 g / m 2 or more and 250 g / m 2 or less, more preferably, and 100 g / m 2 or more and 230 g / m 2 or less, even more preferably.

[0086] The grammage of the laminated foamed sheet is measured as follows. First, a test piece with a length of 100 mm × width of 100 mm × thickness of the laminated foamed sheet is cut out from the laminated foamed sheet, the weight (g) of the test piece is measured, and the weight is multiplied by 100 and unit conversion is performed to obtain the grammage (g / m 2 ).

[0087] [Apparent density of the laminated foamed sheet] The apparent density of the laminated foamed sheet is 50 kg / m 3 or more and 200 kg / m 3 or less, preferably. If the apparent density of the laminated foamed sheet is within this range, the strength and lightness of the container formed by thermoforming will be excellent. From the above viewpoints, the apparent density of the laminated foamed sheet is 60 kg / m 3 or more and 150 kg / m 3 or less, more preferably, and even more preferably 70 kg / m 3 or more and 100 kg / m 3 or less.

[0088] The apparent density of the laminated foamed sheet is measured as follows. First, the grammage (g / m 2 ) of the laminated foamed sheet is obtained by the method described above. Next, the value obtained by dividing the grammage (g / m 2 ) of the obtained laminated foamed sheet by the thickness (mm) of the laminated foamed sheet is subjected to unit conversion to obtain the apparent density (kg / m3 )

[0089] [Closed-cell ratio of laminated foam sheets] The closed-cell ratio of the laminated foam sheet is preferably 60% or more. If the closed-cell ratio of the laminated foam sheet is within this range, the resulting container will have excellent rigidity and thermoformability. From this viewpoint, the closed-cell ratio of the laminated foam sheet is more preferably 70% or more, and even more preferably 80% or more.

[0090] The closed-cell ratio in laminated foam sheets is measured as follows: Following procedure C of ASTM-D2856-70, the closed-cell ratio S (%) is calculated using the following formula (1) and obtained as the average value for n=5. S(%)=(Vx-W / ρ)×100 / (Va-W / ρ) ···(1)

[0091] Vx: True volume (cm³) of the cut sample measured by the above method. 3 This corresponds to the sum of the volume of the resin constituting the laminated foam sheet and the total volume of the closed-cell portions within the cut sample. Va: The apparent volume (cm³) of the cut sample calculated from the external dimensions of the cut sample used for measurement. 3 ). W: Total weight (g) of the cut sample used for measurement. ρ: Density of the resin constituting the laminated foam sheet (g / cm³) 3 )

[0092] <Physical properties of polystyrene foam sheets> The physical properties of the polystyrene-based resin foam sheet according to the present invention will be described.

[0093] [Apparent density of polystyrene foam sheets] The apparent density of the polystyrene foam sheet is 50 kg / m³. 3 More than 200kg / m 3 The following is preferable. If the apparent density is within this range, the molded body obtained by thermoforming the resulting laminated foam sheet will have excellent strength and lightweight properties. From this viewpoint, the apparent density is 60 kg / m³. 3 More than 150kg / m 3 It is more preferable, and even more preferably, that the following is true: 65 kg / m 3 More than 100kg / m 3 The following applies. The apparent density of polystyrene foam sheets can be measured in the same manner as that of laminated foam sheets.

[0094] [Thickness of polystyrene foam sheet] The thickness of the polystyrene foam sheet is preferably 0.5 mm to 5 mm. When the thickness of the polystyrene foam sheet is within this range, the molded body obtained by thermoforming the laminated foam sheet, which is formed by laminating and bonding a film to the polystyrene foam sheet, has excellent rigidity, heat insulation, and handling properties, and can be suitably used as various containers. From the above viewpoint, the thickness of the polystyrene foam sheet is more preferably 0.7 mm to 4 mm, and even more preferably 1 mm to 3 mm. The thickness of the polystyrene foam sheet can be measured in the same manner as the measurement method for the laminated foam sheet.

[0095] [Basis weight of polystyrene foam sheets] The basis weight of the polystyrene foam sheet is 50 g / m². 2 More than 250g / m 2 The following is preferable. When the basis weight of the polystyrene foam sheet is within this range, the molded article obtained by thermoforming the laminated foam sheet obtained from the polystyrene foam sheet will be able to withstand use as a food packaging container or the like. From the above viewpoint, the basis weight of the polystyrene foam sheet is 80 g / m². 2 More than 200g / m2 It is more preferable that the following conditions apply: 100g / m 2 More than 180g / m 2 The following is even more preferable. The basis weight of the polystyrene foam sheet can be measured in the same manner as the measurement method for the laminated foam sheet.

[0096] [Closed-cell ratio of polystyrene foam sheets] The closed-cell ratio of the polystyrene foam sheet is preferably 60% or higher. If the closed-cell ratio of the polystyrene foam sheet is within this range, the resulting container will have excellent rigidity and thermoformability. From this viewpoint, the closed-cell ratio of the laminated foam sheet is more preferably 70% or higher, and even more preferably 80% or higher. The closed-cell ratio of the polystyrene foam sheet can be measured in the same manner as the measurement method for the laminated foam sheet.

[0097] [Peel strength in laminated foam sheets] As mentioned above, the average peel strength between the multilayer resin film and the polystyrene-based resin foam sheet when peeling the multilayer resin film from the laminated foam sheet (hereinafter referred to as "average peel strength") is 40 cN / 25 mm or more and 250 cN / 25 mm or less. Having the average peel strength within this range allows for maintaining a peel strength that facilitates the peeling of the multilayer resin film while suppressing the lifting of the film. From this perspective, the average peel strength is preferably 50 cN / 25 mm or more and 200 cN / 25 mm or less, more preferably 60 cN / 25 mm or more and 150 cN / 25 mm or less, and even more preferably 70 cN / 25 mm or more and 130 cN / 25 mm or less. Furthermore, from the viewpoint of ensuring good peeling of the multilayer resin film when peeling it from the laminated foam sheet, it is preferable that the peel strength across the entire range be 300 cN / 25 mm or less.

[0098] The peel strength of laminated foam sheets is measured as follows: After removing 10 mm from both ends in the width direction of the laminated foam sheet, a 25 mm wide test piece is cut out perpendicular to the extrusion direction. The multilayer resin film is peeled from the laminated foam sheet in a 90° peel test at a peeling speed of 300 mm / min in accordance with JIS Z0237:2022, and the peel strength is measured. This measurement is performed at least 10 times, and the arithmetic mean is taken as the average peel strength. If a weak peel section with a deliberately lower peel strength is provided in the laminated foam sheet, the measurement shall be performed excluding the weak peel section.

[0099] Furthermore, in a molded article formed by thermoforming a laminated foam sheet, the average peel strength of the multilayer resin film is preferably 40 cN / 25 mm or more and 250 cN / 25 mm or less. Having an average peel strength within this range allows for easy peeling of the multilayer resin film while suppressing lifting. From this viewpoint, the average peel strength is more preferably 50 cN / 25 mm or more and 200 cN / 25 mm or less, even more preferably 60 cN / 25 mm or more and 150 cN / 25 mm or less, and particularly preferably 70 cN / 25 mm or more and 130 cN / 25 mm or less.

[0100] The average peel strength of a molded article is determined as follows: A 25 mm long test piece is cut from the molded article perpendicular to the extrusion direction. The multilayer resin film is peeled from the molded article in accordance with JIS Z0237:2022, using a 90° peel test with a peeling speed of 300 mm / min, and the peel strength is measured. This measurement is performed at least three times, and the arithmetic mean is taken as the average peel strength.

[0101] As can be understood from the above explanation, according to the present invention, it is possible to maintain peel strength that facilitates the peeling of the multilayer resin film from the laminated foam sheet, suppress ink residue on the foam sheet after peeling, and suppress electrostatic charge of the multilayer resin film after peeling. [Examples]

[0102] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to these examples.

[0103] For forming the foamed sheet, a tandem extruder was used, consisting of a first single-screw extruder with an inner diameter of 115 mm and a second single-screw extruder with an inner diameter of 180 mm connected in series.

[0104] The thermal lamination apparatus used included a heating roll (upper) and a clamping roll (lower) capable of pressing the foam sheet and film together, and two cooling rolls, one above and one below, capable of pressing and cooling the laminated foam sheet, which consisted of a foam sheet (lower) and a film (upper).

[0105] The various raw materials used in the examples and comparative examples are as follows.

[0106] [Polystyrene foam sheet] (1) Polystyrene resin Abbreviation "Resin 1": Polystyrene resin manufactured by PSJ Corporation: Product name "GX154" (MFR 1.5g / 10min) (2) Bubble regulator Masterbatch of talc "Matsumura Sangyo Co., Ltd. High Filler #12" (35% talc by weight) (3) Organic physical blowing agents Isobutane Dimethyl ether

[0107] [Multilayer resin film] (1) Thermoplastic resin layer Abbreviation "CPP": Unoriented polypropylene film "KT" manufactured by Santox Co., Ltd. (thickness 25μm) (2) Printing layer Ink (Botanical Ink®, Sakata Inx Corporation "Bellflora" (polyurethane type)) Printing area: 70% (printed on thermoplastic resin layer) (3) Middle class Inorganic pigment (titanium dioxide: average particle size 0.2 μm) 15% by mass Resin (Sakata Inx Corporation "Bellflora" (polyurethane type)) 85% by mass Printing area: 100% (printed on the printing layer) (4) Adhesive layer Polyolefin adhesive (Sakata Inx Corporation "XGL-8320") Printing area: 100% (printed on the intermediate layer)

[0108] First, a multilayer resin film was manufactured by the following method. First, a printed layer was printed onto the thermoplastic resin layer using a gravure proofing machine to create a pattern of fried chicken (printed area 70%), and then dried to form the printed layer on the thermoplastic resin layer. Next, an intermediate layer was solid-printed onto the surface of the thermoplastic resin layer where the printed layer was formed using a gravure proofing machine, and then dried to form the intermediate layer on the surface of the printed layer. Furthermore, an adhesive layer was solid-printed onto the surface of the laminated resin film where the intermediate layer was formed using a gravure proofing machine, and then dried to manufacture a multilayer resin film having a thermoplastic resin layer, a printed layer, an intermediate layer, and an adhesive layer. The thickness of the printed layer was set to 1 μm, the thickness of the intermediate layer to 1 μm, and the thickness of the adhesive layer to 20 μm, so that the total thickness of the multilayer resin film was 47 μm. The ratio of the thickness of the intermediate layer (μm) to the average particle size (μm) of the inorganic pigment was set to 5. In Comparative Example 3, a transparent intermediate layer consisting only of resin (Sakata Inx Corporation's "Bellflora") was provided without adding an inorganic pigment to the intermediate layer.

[0109] Furthermore, a polystyrene resin foam sheet was manufactured by the following method. 0.4 parts by weight of talc per 100 parts by weight of polystyrene resin (resin 1) was supplied to the first extruder of a tandem extruder, heated, melted, and kneaded. Organic physical blowing agents were then injected under pressure in the order of 3.0 parts by weight of butane and 1.0 part by weight of dimethyl ether per 100 parts by weight of polystyrene resin, and the mixture was further kneaded. The mixture was then transferred to the second extruder, where the extrusion resin temperature was adjusted to 145°C to obtain a molten resin for foam sheet formation. The molten resin for foam sheet formation was extruded from an annular die into the atmosphere at a discharge rate of 370 kg / hr to form a foamed cylindrical body. Immediately afterward, 23°C air was blown into the inside of the foamed cylindrical body (mandrel side (M side)) at an airflow rate of 4.8 m³. 3The foamed cylindrical body was sprayed at a rate of / min and rapidly cooled. Then, while being pulled along the outer surface of a cooling cylinder (mandrel) with an outer diameter of 670 mm at a speed of 18.5 m / min, it was cut into two pieces along the extrusion direction and wound up, obtaining two polystyrene foam sheets (width 1050 mm).

[0110] The physical blowing agent was injected using separate diaphragm pumps, with each pump having its own gas flow rate (gas weight per hour) set, and was simultaneously injected into the molten resin. The amount of physical blowing agent was calculated from the total discharge rate (kg / hr) and the individual gas injection rates (kg / hr). The resulting polystyrene foam sheet had a basis weight of 160 g / m². 2 The thickness is 1.9 mm, and the apparent density is 84 kg / m³. 3 The closed-cell ratio was 86%.

[0111] Next, the obtained polystyrene foam sheet was cured at room temperature, and 21 days after production, a multilayer resin film was laminated and bonded to the S side (the side opposite to the M side, which is the side taken up along the mandrel) of the polystyrene foam sheet by thermal lamination. In thermal lamination, the multilayer resin film was laminated and bonded to one side of the foam sheet using the thermal lamination apparatus described above, under thermal lamination conditions of heating temperature shown in Table 1, pinch clearance of 0.3 mm, and line speed shown in Table 1.

[0112] After heat lamination, the materials were cured in an indoor environment (23°C, 50% relative humidity) for 24 hours. Then, thermoforming was performed using a rectangular tray molding die measuring 100 mm × 185 mm × 18 mm in height to obtain a tray-shaped molded body. The physical properties of the laminated foam sheets and molded bodies were measured and evaluated for the examples and comparative examples. The results are shown in Table 1. The measurement and evaluation methods for each physical property for the examples and comparative examples are as follows.

[0113] <Laminated foam sheet> [Thickness of laminated foam sheet] The thickness of the laminated foam sheet was calculated by randomly selecting three 260mm-sided measurement samples from the laminated foam sheet, measuring the thickness of each sample using the following method, and then calculating the arithmetic mean.

[0114] [Basis weight of laminated foam sheets] The basis weight of a laminated foam sheet is calculated by first cutting a test piece measuring 100mm x 100mm x the thickness of the laminated foam sheet from the sheet, measuring the weight (g) of the test piece, multiplying the weight by 100, and then converting the units to obtain the basis weight (g / m²). 2 ) was sought.

[0115] [Apparent density of laminated foam sheets] The apparent density of the laminated foam sheet was measured as follows: First, the basis weight (g / m²) of the laminated foam sheet was measured using the method described above. 2 Next, the basis weight (g / m²) of the laminated foam sheet was determined. 2 The apparent density (kg / m³) of the laminated foam sheet is converted to units by dividing the value by the thickness (mm) of the laminated foam sheet. 3 )

[0116] [Average peel strength in laminated foam sheets] In laminated foam sheets, the average peel strength (average value of peel strength) when the multilayer resin film is peeled off was measured as follows. First, after removing 10 mm from both ends in the width direction of the laminated foam sheet, a 25 mm wide test piece was cut out from the laminated foam sheet perpendicular to the extrusion direction. In accordance with JIS Z0237:2022, the multilayer resin film was peeled off the laminated foam sheet in a 90° peel test with a peeling speed condition of 300 mm / min, and the peel strength at that time was measured. This measurement was performed 10 times, and the arithmetic mean was taken as the average peel strength.

[0117] [Closed-cell ratio of laminated foam sheets] The closed-cell ratio in laminated foam sheets was measured as follows. Following procedure C of ASTM-D2856-70, measurements were taken using a Toshiba Beckman Corporation air-comparison hydrometer 930 (cut samples were randomly cut from the laminated foam sheet to 25 mm × 40 mm × sheet thickness, and multiple samples were stacked in a sample cup so that the sum of the sample thicknesses was as close to 20 mm as possible (but not exceeding 20 mm)). The closed-cell ratio S (%) was calculated using the following equation (1) and obtained as the average value for n=5. S(%)=(Vx-W / ρ)×100 / (Va-W / ρ) ···(1)

[0118] Vx: True volume (cm³) of the cut sample measured by the above method. 3 This corresponds to the sum of the volume of the resin constituting the laminated foam sheet and the total volume of the closed-cell portions within the cut sample. Va: The apparent volume (cm³) of the cut sample calculated from the external dimensions of the cut sample used for measurement. 3 ). W: Total weight (g) of the cut sample used for measurement. ρ: Density of the resin constituting the laminated foam sheet (g / cm³) 3 )

[0119] Figure 3 shows the electron image (left: secondary electron image, right: backscattered electron image) of the adhesive layer side surface of the multilayer resin film peeled from the thermoforming laminated foam sheet for Comparative Example 3. Figure 4 shows the electron image (left: secondary electron image, right: backscattered electron image) of the adhesive layer side surface of the multilayer resin film peeled from the thermoforming laminated foam sheet for Example 1. The electron images in Figures 3 and 4 were acquired using a scanning electron microscope (FE-SEM SU8220, Hitachi High-Tech). The backscattered electron images were acquired with an acceleration voltage of 5kV / emission current of 10μA / detector PDBSE (phase imaging mode). The phase imaging mode is an observation method that emphasizes the surface irregularities of the sample.

[0120] As can be seen from Figures 3 and 4, it was confirmed that irregularities were formed on the surface of the intermediate layer in Example 1 compared to Comparative Example 3.

[0121] <Molded body> [Average peel strength in molded articles] The average peel strength (average value of peel strength) when the multilayer resin film is peeled off the molded article was measured as follows. The peel strength of the molded article was measured as follows: A test piece measuring 25 mm vertically, 100 mm horizontally, and the thickness of the molded article was cut out from the molded article perpendicular to the extrusion direction. The multilayer resin film was peeled off the molded article in accordance with JIS Z0237:2022, using a 90° peel test with a peeling speed condition of 300 mm / min, and the peel strength at that time was measured. This measurement was performed three times, and the arithmetic mean was taken as the average peel strength.

[0122] [Ease of peeling off the film] The ease of peeling off the multilayer resin film was evaluated as follows: Three individuals performed the task of cutting a 25mm wide test piece from a laminated foam sheet perpendicular to the extrusion direction using a cutter, and peeling the film off the cut edge with their fingers. The ease of peeling the film, particularly at the beginning of the peeling process, was evaluated using the following evaluation method. ○: The average evaluation score of the three individuals is 2.0 or higher. ×: The average score of the three reviewers is less than 2.0. Rating 3. The film can be easily started to peel off. Two points...It is possible to start peeling off the film within an average of 10 seconds. One point to note... You can start peeling off the film by carefully scratching the cut edge with your fingernail, but it takes an average of over 10 seconds to begin peeling.

[0123] [Film lifting] The lifting of the multilayer resin film was evaluated as follows. After thermoforming a laminated foam sheet into a tray-shaped molded body, the molded body was evaluated based on whether or not voids (film lifting) were observed at the interface between the multilayer resin film and the polystyrene-based resin foam sheet on the surface of the molded body after 30 days. ○: No film lifting was observed. △: Slight gaps are visible in the uneven areas of the bottom surface. ×: Clear film lifting is visible to the naked eye.

[0124] [Film peeling and electrolysis] The electrostatic discharge that occurs when peeling off a multilayer resin film was evaluated as follows. ○: The multi-layer resin film did not stick to my hands or the sheet when peeled off. △: When peeled off, the multi-layer resin film stuck to my hand or the sheet and detached within 5 seconds. ×: When peeled off, the multi-layer resin film stuck to my hands and the sheet and did not come off even after more than 5 seconds.

[0125] [Ink residue on polystyrene foam sheets] The following evaluation was conducted to determine whether or not ink remained on the polystyrene foam sheet after the multilayer resin film had been peeled off. None: No ink residue was found on the surface of the polystyrene foam sheet upon visual inspection. Yes: Ink residue was visually confirmed on the surface of the polystyrene foam sheet.

[0126] [Table 1]

[0127] In the laminated foam sheets obtained in Examples 1-3, the effect of preventing film lifting was confirmed by having an appropriate average peel strength. Furthermore, in Examples 1-3, the ease of peeling the multilayer resin film was improved, and ink residue on the polystyrene-based resin foam sheet after peeling was suppressed. Moreover, peeling static electricity when the multilayer resin film was peeled was suppressed, resulting in a laminated foam sheet in which the film did not stick to hands or the sheet when peeled.

[0128] In contrast, in Comparative Example 1, the average peel strength was low, causing the multilayer resin film to lift. In Comparative Example 2, the average peel strength was too high, making the multilayer resin film difficult to peel off, and peeling static electricity occurred. As a result, the multilayer resin film stuck to the hand after being completely peeled from the polystyrene foam sheet. Furthermore, due to the excessively high average peel strength, ink residue was observed on the surface of the polystyrene foam sheet. Note that ink residue refers to the fact that the adhesive layer did not peel off at the interface with the polystyrene foam sheet, but rather the print layer broke inside the print layer and peeled off from the print layer, leaving the print layer and adhesive layer on the surface of the polystyrene foam sheet, and the ink was visible to the naked eye. In Comparative Example 3, although it had an appropriate average peel strength, peeling static electricity occurred because an inorganic pigment was not provided in the intermediate layer. As a result, the peeled multilayer resin film stuck to the hand. [Explanation of Symbols]

[0129] 10 Multilayer resin film 11 Adhesive layer 12 Middle Class 13 Printing layer 14 Thermoplastic resin layer 20 Polystyrene foam sheet 100 Laminated foam sheets for thermoforming

Claims

1. A thermoforming laminated foam sheet, wherein a multilayer resin film is peelably laminated and bonded to at least one side of a polystyrene-based resin foam sheet, The multilayer resin film is laminated in the following order: an adhesive layer, an intermediate layer, a printed layer printed with ink, and a thermoplastic resin layer, wherein the adhesive layer and the polystyrene-based resin foam sheet are bonded together, the average peel strength between the multilayer resin film and the polystyrene-based resin foam sheet is 40 cN / 25 mm or more and 250 cN / 25 mm or less, and the intermediate layer contains an inorganic pigment, wherein the laminated foam sheet is for thermoforming.

2. The laminated foam sheet for thermoforming according to claim 1, wherein the inorganic pigment is one or more selected from titanium dioxide and talc.

3. The laminated foam sheet for thermoforming according to claim 1 or 2, wherein the content of the inorganic pigment contained in the intermediate layer is 10% by mass or more.

4. The laminated foam sheet for thermoforming according to claim 1 or 2, wherein the thickness of the intermediate layer is 0.2 μm or more and 5 μm or less.

5. The laminated foam sheet for thermoforming according to claim 1 or 2, wherein the intermediate layer contains a polyurethane resin.

6. The laminated foam sheet for thermoforming according to claim 1 or 2, wherein the adhesive layer contains a polyolefin-based adhesive.

7. The laminated foam sheet for thermoforming according to claim 1 or 2, wherein the multilayer resin film is laminated and bonded to the polystyrene-based resin foam sheet by thermal lamination.

8. A container obtained by thermoforming the thermoforming laminated foam sheet described in claim 1 or 2, A thermoformable container in which the aforementioned multilayer resin film is located on the inside of the container.

Images