JP7841361B2 - Laminated sheets for lids, lids, food packaging containers, and packaged foods - Google Patents

Description

This invention relates to a laminated sheet for lids, a lid, a food packaging container, and packaged food.

In recent years, demand for pre-cooked or processed chilled and frozen foods sold in convenience stores and supermarkets has been growing, supported by changes in household structure due to the rise of nuclear families, lifestyle changes, and advancements in distribution and refrigeration technologies. Simultaneously, demand for packaging containers for chilled and frozen foods is also increasing.

Meanwhile, with efforts underway to reduce plastic waste, there is a growing demand for food packaging containers made from paper, a renewable resource with a low environmental impact. There is also a growing demand for paper-based packaging containers for chilled foods.

Furthermore, some packaging containers consist of a container body with an opening and a lid that covers this opening.

For example, Patent Document 1 discloses a film used for a lid, comprising a base layer, a cohesive breaking layer, and a heat-seal layer in that order.

Japanese Patent Publication No. 2016-210063

The present invention aims to provide a lid for food packaging containers that includes a paper base material and offers excellent openability.

According to one aspect of the present invention, a laminated sheet for a lid is provided for use on the lid of a food packaging container comprising a container body having an opening and a lid covering the opening. The laminated sheet comprises a water-resistant functional layer, a paper substrate, a support layer, and a heat-seal layer in this order, wherein the mass of the paper substrate is greater than the mass of any other layer included in the laminated sheet, the heat-seal layer has a thickness of 1 μm to 5.5 μm, and the composite elastic modulus of the heat-seal layer is 190 MPa or less.

According to another aspect of the present invention, a laminated sheet for a lid is provided, wherein the composite elastic modulus is 100 MPa or more.

According to yet another aspect of the present invention, a laminated sheet for a lid is provided, further comprising a printed layer between the functional layer and the paper substrate, as described above.

According to yet another aspect of the present invention, a laminated sheet for a lid is provided, further comprising a gas barrier layer having gas barrier properties between the printed layer and the heat-seal layer.

According to yet another aspect of the present invention, a laminated sheet for a lid is provided, wherein the gas barrier layer comprises at least one of an inorganic oxide layer and a resin-containing layer.

According to yet another aspect of the present invention, when the layers other than the paper substrate included in the laminated sheet for the lid are classified into layers made of plastic and other layers, the mass of the paper substrate is greater than the total mass of the plastic layers and the total mass of the other layers, in any of the above embodiments.

According to yet another aspect of the present invention, a laminated sheet for a lid is provided, wherein the paper substrate is coated paper having a coating layer on one side, according to any of the above embodiments.

According to yet another aspect of the present invention, a laminated sheet for a lid is provided, wherein the heat seal layer has a glass transition temperature in the range of 20 to 55°C and comprises a heat seal varnish containing an ethylene-vinyl acetate copolymer, as described above.

According to yet another aspect of the present invention, a laminated sheet for a lid is provided, further comprising an anchor coat layer having a thickness of 0.5 μm or more and 2.5 μm or less between the support layer and the heat seal layer, as described above.

According to yet another aspect of the present invention, a lid made of a laminated sheet for lids according to any of the above aspects is provided.

According to yet another aspect of the present invention, a food packaging container is provided comprising a container body having an opening and a lid covering the opening, wherein the support layer is disposed between the paper substrate and the internal space of the food packaging container.

According to yet another aspect of the present invention, a food packaging container is provided in which the container body has a flange around the opening, and the lid is heat-sealed to the flange via the heat-seal layer.

According to yet another aspect of the present invention, a food packaging container is provided in which the internal space of the food packaging container is filled with a mixed gas containing oxygen, nitrogen, and carbon dioxide.

According to yet another aspect of the present invention, a food packaging container is provided that is either a chilled food packaging container or a frozen food packaging container, according to any of the above aspects.

According to yet another aspect of the present invention, a packaged food is provided comprising a food packaging container according to any of the above aspects and food contained in the food packaging container.

According to the present invention, a lid is provided for use in food packaging containers, containing a paper base material and offering excellent openability.

A schematic partial cross-sectional view showing an example of a laminated sheet for a lid according to the first embodiment of the present invention. A cross-sectional diagram illustrating one step in the measurement of the composite modulus of elasticity. A graph showing the load-displacement curve. A schematic cross-sectional view showing a food packaging container according to a third embodiment of the present invention.

The embodiments of the present invention will be described below with reference to the drawings. The embodiments described below are more specific to any of the aspects described above. Elements having similar or identical functions are denoted by the same reference numerals, and redundant descriptions are omitted.

[First Embodiment]
Figure 1 is a schematic cross-sectional view showing an example of a laminated sheet for a lid according to the first embodiment of the present invention.
The laminated sheet 10 for the lid shown in Figure 1 is used as the lid in a food packaging container that comprises a container body having an opening and a lid that covers the opening. In other words, the laminated sheet 10 for the lid is a lid material that is used either as the lid itself or as a portion cut out from it.

The laminated sheet 10 for the lid includes, in this order, a heat-seal layer 1, an anchor coat layer 2, a support layer 3, a gas barrier layer 4, a paper substrate 5, a printed layer 6, and a water-resistant functional layer (water-resistant layer) 7.

The layers contained in the lid laminate sheet 10 are described below.

(Water-resistant functional layer)
The water-resistant functional layer (water-resistant layer) 7, as described later in packaged food, is a layer that prevents liquids from outside the container, such as moisture or oil due to condensation, from penetrating the lid, thereby preventing these liquids from reaching layers such as the printing layer 6, paper substrate 5, and gas barrier layer 4. By preventing liquids from outside the container from reaching layers such as the printing layer 6, paper substrate 5, and gas barrier layer 4, the functional layer 7 prevents, for example, deterioration, damage, or a decrease in the adhesion of these layers.

For example, the functional layer 7 is formed on the printed layer 6 to control the water absorption of the partially laminated sheet, which is the portion of the laminated sheet 10 for the lid from the functional layer 7 to the gas barrier layer 4. Preferably, the functional layer 7 has water resistance that reduces the water absorption of the laminated sheet for the lid by the Cobb method described below to 20 g/ ^m² or less.

Here, water absorption refers to the water absorption obtained when the measurement surface is the surface of the functional layer 7 and the contact time between the test piece and water is 300 seconds, according to the method specified in JIS P8140:1998 "Paper and cardboard - Water absorption test method - Cobb method". As mentioned above, this water absorption is preferably 20 g/ ^m² or less, more preferably 10 g/ ^m² or less, and even more preferably ⁵ g/m² or less. The lower limit of this water absorption is ideally 0 g/ ^m² . In one example, this water absorption is 1 g/ ^m² or more.

The functional layer 7 is preferably an overprint varnish layer (hereinafter referred to as the "OP varnish layer").
The functional layer 7, in one example, contains a water-resistant resin. The water-resistant resin can be any resin capable of achieving the above-mentioned water absorption level, without limitation. Examples of water-resistant resins include polyolefin resins such as polyethylene, polypropylene, polyvinyl chloride, polyvinylidene chloride, polyvinyl alcohol, and vinyl chloride-vinyl acetate copolymer, as well as silicone resins, acrylic resins, epoxy resins, polyester resins, cellulose resins, or urethane resins. The functional layer 7 can be obtained, for example, by coating a coating containing a water-resistant resin onto a paper substrate 5 on which the printed layer 6 is formed, using a known method. In addition to the water-resistant resin, the coating may further contain additives such as pigments, dyes, curing agents, leveling agents, anti-blocking agents, and lubricants, as well as solvents.

The functional layer 7 preferably has high abrasion resistance and scratch resistance in order to maintain sufficient water resistance. From this viewpoint, the thickness of the functional layer 7 and the amount of paint applied to it are preferably greater than the thickness of a normal OP varnish layer and the amount of normal OP varnish applied. Here, "amount applied" refers to the mass of solids per unit area.

For example, in the laminated sheet 10 for the lid shown in Figure 1, the coating for forming the functional layer 7 is preferably applied in an amount of 0.2 g/ ^m² or more, and more preferably in an amount of 2.0 g/ ^m² or more. The coating is applied in an amount of, for example, 10 g/ ^m² or less. The thickness of the functional layer 7 is preferably 0.2 μm or more, and more preferably 2.0 μm or more. The thickness of the functional layer 7 is, for example, 10 μm or less. The functional layer 7 may also be provided on the printed layer 6 by lamination.

(Printing layer)
The printed layer 6 is a layer formed to make the laminated sheet 10 for the lid or the lid a practical commercial product. The printed layer 6 is a layer composed of ink in which various pigments, extender pigments, plasticizers, desiccants, and stabilizers are added to conventionally used ink binder resins such as urethane, acrylic, nitrocellulose, rubber, and vinyl chloride, and displays patterns such as characters and images. As a method for forming the printed layer 6, well-known printing methods such as offset printing, gravure printing, and silkscreen printing, or well-known coating methods such as roll coating, knife-edge coating, and gravure coating can be used.

The thickness of the printed layer 6 is not particularly limited; in one example, it is in the range of 0.1 to 5 μm, and in another example, it is in the range of 0.2 to 1 μm.

(Paper base material)
The lid laminate sheet 10 contains a paper substrate 5. The mass of the paper substrate 5 is greater than the mass of any other layer contained in the lid laminate sheet 10. The ratio of the mass of the paper substrate 5 to the mass of the lid laminate sheet 10 is preferably 40% or more, more preferably 45% or more, even more preferably 50% or more, and even more preferably more than 50%. In one example, this ratio is 80% or less, in another example, 70% or less, and in yet another example, 65% or less.

When the layers of the laminated lid sheet 10, other than the paper base material 5, are classified into layers made of plastic and other layers, it is preferable that the mass of the paper base material 5 is greater than the total mass of the plastic layers and the total mass of the other layers. In this case, in Japan, the laminated lid sheet 10 can be treated as paper under the Container and Packaging Recycling Law.

Here, the above classification follows the "Explanatory Materials for the Container and Packaging Recycling Law." Specifically, "plastic" refers to a material that contains polymers as an essential component and is shaped and manufactured into a product using its fluidity during processing. Paints and adhesives are not included in plastic because they are unrelated to the concept of "shaping." Therefore, in the example shown in Figure 1, the support layer 3 is a "layer made of plastic." Also, in the example shown in Figure 1, the heat seal layer 1, the anchor coat layer 2, the printing layer 6 formed from ink, the functional layer 7 formed by coating, and the adhesive layer (not shown) are "other layers."

On the other hand, the gas barrier layer 4 is classified according to the following cases. Specifically, in the example shown in Figure 1, if the gas barrier layer 4 is a layer formed on the paper substrate 5 by coating or vapor deposition, it could be treated as a "paper substrate" according to the above explanatory materials, but here it is treated as an "other layer." For example, if the paper substrate 5 is a barrier paper with the gas barrier layer 4 coated or vapor-deposited on one side, the gas barrier layer 4 is an "other layer." Also, if the gas barrier layer 4 is a layer formed on the support layer 3 by coating or vapor deposition, it is treated as an "other layer." Furthermore, if the gas barrier layer 4 uses a polymer film produced by melt molding such as extrusion, it is a "layer made of plastic."

The basis weight of the paper substrate 5, i.e., the mass per unit area, is in the range of 20 to 500 g/ ^m² in one example, and in the range of 40 to 100 g/ ^m² in another example. Increasing the basis weight of the paper substrate 5 makes the lid harder and tends to reduce its ease of opening. Decreasing the basis weight reduces the strength of the lid.

Furthermore, increasing the basis weight of the paper substrate 5 increases the proportion of the paper substrate 5's mass to the total mass of the laminated lid sheet 10. However, increasing the basis weight of the paper substrate 5 increases carbon dioxide emissions associated with the manufacturing of the paper substrate 5 and the disposal of the laminated lid sheet 10.

The paper substrate 5 is not particularly limited as long as it is primarily composed of plant-derived pulp. Examples of paper substrate 5 include high-quality paper, medium-quality paper, coated paper such as lightly coated paper, glossy paper, bleached and unbleached kraft paper (acidic or neutral paper).

The paper substrate 5 is preferably coated paper having a coating layer on at least one side. That is, the paper substrate 5 is preferably single-sided coated paper or double-sided coated paper. The surface of the coated paper with the coating layer has superior smoothness compared to the surface of paper without a coating layer.

If the paper substrate 5 is coated paper having a coating layer on one side, the printing layer 6 can be provided, for example, on the coating layer. In this case, excellent adhesion can be achieved between the paper substrate 5 and the printing layer 6.

If the paper substrate 5 is coated paper having a coating layer on one side, the gas barrier layer 4 can be provided, for example, on the coating layer. This improves the adhesion of the gas barrier layer 4 to the paper substrate 5. Furthermore, it reduces the thickness of the gas barrier layer 4 required for barrier properties to be achieved.

By using coated paper with coating layers on both sides as the paper substrate 5, excellent adhesion can be achieved between the paper substrate 5 and the printing layer 6. Furthermore, it becomes easier to achieve excellent adhesion between the paper substrate 5 and the gas barrier layer 4.

The coating layer contains a resin. Examples of resins included in the coating layer include low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ethylene-α-olefin copolymer polymerized using a metallocene catalyst (single-site catalyst), polypropylene, ethylene-vinyl acetate copolymer, ionomer resin, ethylene-ethyl acrylate copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-propylene copolymer, methylpentene polymer, polyolefin resins such as polyethylene and polypropylene modified with unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic anhydride, and fumaric acid, acid-modified polyolefin resins, polyethylene terephthalate resin, polybutylene terephthalate resin, nylon resin, and thermoplastic resins such as styrene-butadiene rubber. These resins may be used in combination of two or more, or copolymerized. The coating layer may further contain additives, such as fillers including silica, clay, kaolin, calcium carbonate, talc, mica, and titanium dioxide.

The thickness of the coating layer is preferably in the range of 0.5 to 50 μm, and more preferably in the range of 1 to 15 μm.

(Gas barrier layer)
The gas barrier layer 4 has gas barrier properties such as oxygen barrier properties and water vapor barrier properties. In packaged food described later, the gas barrier layer 4 suppresses the intrusion of gases such as oxygen, water vapor, and aroma components from outside the container into the container. As a result, the gas barrier layer 4 suppresses the deterioration of the food contents in packaged food. In addition, the gas barrier layer 4 suppresses the diffusion of odor components of the contents to the outside of the container in packaged food. For example, the gas barrier layer 4 has an oxygen permeability of 0.1 to 100 cc/ ^m² /day/atm in an atmosphere of 30°C and 70% relative humidity.

The gas barrier layer 4 is, for example, a metal layer, an inorganic oxide layer, a resin-containing layer, or a combination of two or more of these. When microwave heating by a microwave oven is anticipated, the gas barrier layer 4 is preferably an inorganic oxide layer, a resin-containing layer, or a combination of these.

The gas barrier layer 4 may be formed by coating, by melt molding, or by depositing an inorganic oxide. Alternatively, the gas barrier layer 4 may be a metal foil such as aluminum foil, or by depositing a metal such as aluminum.

Examples of inorganic oxides that can be used include silicon oxide, boron oxide, or metal oxides such as aluminum oxide, magnesium oxide, calcium oxide, potassium oxide, tin oxide, sodium oxide, titanium oxide, lead oxide, zirconium oxide, and yttrium oxide.

The resin-containing layer can be formed, for example, by coating. In this case, a coating solution containing resins such as polyvinyl alcohol (PVA), ethylene-vinyl alcohol copolymer, ethylene-vinyl acetate copolymer, polyvinylidene chloride, polyacrylonitrile, and epoxy resin can be used. Additives such as organic or inorganic particles, layered compounds, and curing agents may be added to this coating solution. Various coating methods such as gravure coating, die coating, blade coating, knife coating, and bar coating can be used as coating methods.

When forming the resin-containing layer by melt molding, extrusion molding techniques such as T-die or inflation can be used. In melt molding, for example, the resin or a mixture of the resin and additives is heated and melted, and the gas barrier layer 4 is processed into a film or sheet using a T-die or inflation. This film or sheet is then laminated to the paper substrate 5. Lamination methods include dry lamination using solvent-based adhesives, non-solvent lamination using solvent-free adhesives, and sand lamination using molten resin as an adhesive. Alternatively, the layer can be formed directly on the paper substrate by extrusion lamination, and in this case, an adhesive layer may be formed on the paper substrate if necessary.

The gas barrier layer 4 may be interposed between the paper substrate 5 and the support layer 3 by forming it on one surface of the support layer 3. Alternatively, the gas barrier layer 4 may be interposed between the paper substrate 5 of the lid laminate sheet 10 and the support layer 3 by using barrier paper made of a paper substrate 5 having the gas barrier layer 4 on one surface. The paper substrate 5 constituting the barrier paper may be coated paper having a coating layer on at least one surface. If the paper substrate 5 constituting the barrier paper has a coating layer on only one surface, the gas barrier layer 4 may be provided on the coating layer or on the surface of the paper substrate 5 where the coating layer is not formed.

The thickness of the gas barrier layer 4 is, in one example, in the range of 0.01 to 30 μm, and in another example, in the range of 0.1 μm to 12 μm.

(Support layer)
The support layer 3 improves the strength of the laminated sheet 10 for the lid.
The support layer 3 contains polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyamide, ethylene vinyl alcohol copolymer, polyacrylonitrile (PAN), polymethylpentene (PMP), polyvinyl alcohol resin, olefin resin, or unsaturated polyester resin. The support layer 3 may have a single-layer structure or a multi-layer structure.

The support layer 3 may be an unstretched film or a stretched film such as a biaxially oriented film. If a stretched film is used, it is preferable to use a biaxially oriented film. This is because biaxial stretching reduces the variation in various physical properties, such as breaking strength in the direction within the film plane, compared to uniaxial stretching.

The support layer 3 may further contain additives such as a curing agent, filler, antiblocking agent, and antistatic agent. Furthermore, the material of the support layer 3 may be one that hardens upon irradiation with active energy rays such as ultraviolet light and electron beams.

The thickness of the support layer 3 is preferably in the range of 3 to 60 μm, and more preferably in the range of 10 to 30 μm. If the support layer 3 is too thick, it becomes difficult to increase the ratio of the mass of the paper substrate 5 to the mass of the laminated sheet 10 for the lid.

As a means of forming the support layer, methods of laminating it onto a paper substrate via an adhesive can be used. These include dry lamination using solvent-based adhesives, non-solvent lamination using solvent-free adhesives, and sand lamination using molten resin as an adhesive. Alternatively, the support layer composition can be extruded in a molten state using methods such as extrusion lamination to form it directly onto the paper substrate, and in this case, an adhesive layer may be formed on the paper substrate if necessary.
(Anchor coat layer)
The anchor coat layer 2 is a layer that improves the adhesion between the heat seal layer 1 and the support layer 3.
The material for the anchor coat layer 2 is selected appropriately from adhesive resins or adhesives that provide the necessary adhesive strength, depending on the material of the layer to be bonded through it.

As adhesive resins, one or more resins selected from the following can be used: polyethylene such as low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, and copolymers of ethylene-α-olefin polymerized using metallocene catalysts; ethylene-unsaturated carboxylic acid copolymers such as ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer, ethylene-ethyl acrylate copolymer, ethylene-methacrylic acid copolymer, ethylene-methyl methacrylate copolymer, and ethylene-maleic acid copolymer; and ionomer resins.

The adhesive is, for example, an adhesive composition obtained by mixing a first composition containing a main component and a solvent with a second composition containing a curing agent and a solvent. The adhesive layer obtained from this adhesive contains a cured product formed by the reaction of the main component and the curing agent in the adhesive composition.

Examples of main components include polyols. Examples of curing agents include isocyanate compounds. Examples of adhesives include ether-based two-component reactive adhesives or ester-based two-component reactive adhesives.

The cured product of an ether-based two-component reactive adhesive is, for example, polyether polyurethane. Polyether polyurethane is produced by the reaction of a polyether polyol (the main component) and an isocyanate compound (the curing agent).

The cured products of ester-based two-component reactive adhesives are, for example, polyester polyurethane and polyester. Polyester polyurethane is produced by the reaction of a polyester polyol as the main component and an isocyanate compound as the curing agent.

In two-component reactive adhesives, acrylic polyol may be used as the main component. Furthermore, the above adhesive composition does not need to contain a solvent, as long as it melts or becomes less viscous upon heating.

The thickness of the anchor coat layer 2 is preferably within the range of 0.5 μm to 2.5 μm, and more preferably within the range of 0.8 to 2.0 μm. When the thickness of the anchor coat layer 2 is within the above range, high adhesion can be achieved despite the thinness of the anchor coat layer 2.

The dry mass per unit area of the anchor coat layer 2, i.e., the coating amount, is preferably in the range of 0.5 to 2.5 g/ ^m² , more preferably in the range of 0.8 to 2.0 g/ ^m² , and even more preferably in the range of 1.0 to 1.8 g/ ^m² . Various coating methods such as gravure coating, die coating, blade coating, knife coating, and bar coating can be used as coating means.

(Heat seal layer)
The heat seal layer 1 enables the heat sealing of the lid 21 to the container body 22 of the food packaging container 20 shown in Figure 4, which will be described later.

The heat seal layer 1 can, in one example, be formed on the anchor coat layer 2 by lamination. In this case, the heat seal layer 1 is made of, for example, an easy-peel sealant. Methods for bonding the heat seal layer to the anchor coat layer by lamination include dry lamination using solvent-based adhesives, non-solvent lamination using solvent-free adhesives, and sand lamination using molten resin as an adhesive. Furthermore, when the heat seal layer is extruded using molten resin, extrusion lamination can also be used.

The heat seal layer 1 can also be formed on the anchor coat layer 2 by coating, according to other examples. In this case, the heat seal layer 1 is made of, for example, heat seal varnish. When the heat seal layer 1 is made of heat seal varnish, a thinner heat seal layer can be easily obtained compared to when the heat seal layer 1 is made of easy-peel sealant. This is because easy-peel sealant has a multilayer structure. When the heat seal layer 1 is thin, it is easier to reduce the amount of petroleum-derived compounds used. Also, when using a heat seal layer 1 made of heat seal varnish, it is possible to reduce costs compared to when using a heat seal layer 1 made of easy-peel sealant. Here, as an example, we assume that the heat seal layer 1 is made of heat seal varnish. Various coating methods such as gravure coating, die coating, blade coating, knife coating, and bar coating can be used as coating means.

The heat seal layer 1 includes, for example, ethylene-vinyl acetate copolymer (EVA), ionomer resin, or polyolefins such as polypropylene, linear low-density polyethylene (LLDPE), and very low-density linear polyethylene (VLDPE). The heat seal layer 1 may also contain additives such as ultraviolet absorbers.

The heat seal layer 1 is preferably a layer containing an ethylene-vinyl acetate copolymer, more preferably a layer mainly composed of an ethylene-vinyl acetate copolymer, and even more preferably a layer made of an ethylene-vinyl acetate copolymer.

The glass transition temperature of the heat seal layer 1 is preferably in the range of 20 to 55°C, and more preferably in the range of 25 to 50°C. If the glass transition temperature is too low, blocking is likely to occur. If the glass transition temperature is too high, damage to other layers may occur when heat-sealing the lid to the container body.

The thickness of the heat seal layer 1 is within the range of 1 μm to 5.5 μm. Preferably, the thickness of the heat seal layer 1 is within the range of 1.2 μm to 5.0 μm, and more preferably, within the range of 1.5 μm to 4.5 μm. If the thickness of the heat seal layer 1 is less than 1 μm, it is difficult to achieve high heat seal strength between the lid and the container body. In other words, it is difficult to achieve high airtightness. If the thickness of the heat seal layer 1 is greater than 5.5 μm, fraying and stringing are likely to occur when peeling the lid from the container body. Fraying and stringing will be discussed later. Furthermore, if the heat seal layer 1 is too thick, drying takes longer, which tends to reduce productivity.

The dry mass per unit area of the heat seal layer 1, i.e., the coating amount, is preferably in the range of 0.5 to 5.0 g/ ^m² , more preferably in the range of 1.0 to 4.5 g/ ^m² , and even more preferably in the range of 1.5 to 4.0 g/ ^m² .

The composite modulus of the heat seal layer 1 is 190 MPa or less. Preferably, the composite modulus of the heat seal layer 1 is 185 MPa or less, more preferably 180 MPa or less. Preferably, the composite modulus of the heat seal layer 1 is 100 MPa or more, more preferably 115 MPa or more, and even more preferably 125 MPa or more.

If the composite modulus of heat seal layer 1 is too low, blocking is likely to occur. If the composite modulus of heat seal layer 1 is greater than 190 MPa, fraying and stringing are likely to occur when peeling the lid from the container body.

The composite modulus of the heat seal layer 1 is preferably in the range of 100 to 190 MPa, more preferably in the range of 115 to 185 MPa, and even more preferably in the range of 125 to 180 MPa.

The composite modulus is obtained by the nanoindentation method. The measurement method for the composite modulus is described below.

First, strips of sheet material, 3 mm in length and 1 mm in width, are cut from the laminated sheet 10 for the lid. Next, the surface of the obtained sheet material on the heat-seal layer 1 side and the surface on the functional layer 7 side are coated with resin. This embeds the sheet material in resin. As the resin, a photocurable resin such as an ultraviolet-curable resin or a visible-light-curable resin is used.

Next, using a microtome, the resin-embedded sheet piece is cut in a direction parallel to the thickness and width of the laminated lid sheet 10. The finishing conditions are a cutting thickness of 300 nm and a cutting speed of 1 mm/second. This process ensures a uniform cross-section. Furthermore, this process prevents the heat-sealing varnish from dripping or falling off. The cutting is performed so that the portion of the paper substrate 5 in which the embedding resin has permeated is exposed. This prevents fraying of paper fibers from interfering with the measurement results. A test specimen is then obtained.

Next, the obtained test specimen is placed in a nanoindenter. Here, the test specimen is placed in the nanoindenter so that the indenter of the nanoindenter contacts the cross-section of the heat-seal layer 1 perpendicularly. A nanoindenter capable of surface detection at 1 μN should be used. If necessary, the embedding resin may be trimmed during placement.

Next, as shown in Figure 2, the indenter 30 is pressed into the heat seal layer 1. The pressing speed is set to 100 nm/second. Figure 2 is a schematic cross-sectional view showing the state when the indenter 30 is pressed into the heat seal layer 1 at its maximum depth. In Figure 2, hmax represents the maximum depth of indentation into the heat seal layer 1, i.e., the maximum displacement. Ac represents the contact projected area. hc represents the contact depth. During the process of pressing the indenter 30, the load applied to the heat seal layer 1 and the indentation depth are measured using a nanoindenter.

A diamond Berkovich indenter is used as the indenter 30. The tip of the Berkovich indenter consists of three faces, each having a triangular shape. One vertex of each triangle coincides with one vertex of the other two triangles. Furthermore, the angles formed by the two sides extending from the aforementioned vertices of these triangles are equal. That is, these three faces form a roughly equilateral triangular pyramid. The indenter 30 used here has an angle of 115° in each triangle. Also, the angle between the center line of the indenter 30 and the plane containing one of the aforementioned triangles is 65.27°. The nanoindenter is set so that after the surface detection load reaches 1 μN, the indenter 30 indents the surface of the heat seal layer 1 by a further 200 nm. In this setting, the actual indentation depth is approximately 260 nm. The holding time at the maximum depth is set to 2 seconds.

Next, the indenter 30 is withdrawn from the heat seal layer 1. The withdrawal speed is set to 100 nm/second. During this process, the load applied to the heat seal layer 1 and the indentation depth are measured using a nanoindenter. In Figure 2, Pmax represents the maximum load on the unloading curve.

The operation described above yields the graph shown in Figure 3. Figure 3 is a graph showing the load-displacement curve. In Figure 3, the vertical axis represents the load applied to the heat seal layer 1, and the horizontal axis represents the indentation depth of the indenter 30, i.e., the displacement. In Figure 3, a load of 0 N indicates that the indenter 30 is not in contact with the heat seal layer 1. The position of displacement 0 represents the point where the load applied to the heat seal layer 1 begins to increase from 0 N. In Figure 3, the curve starting at 0 μN and ending at approximately 13 μN shows the relationship between load and displacement during the process of indenting the indenter 30 into the heat seal layer 1. The curve starting at approximately 9 μN and ending at approximately -1 μN is the unloading curve. The unloading curve shows the relationship between load and displacement during the process of withdrawing the indenter 30 from the heat seal layer 1.

Next, the contact depth hc is determined using the Oliver-Pharr method.

The contact depth HC can be calculated using the following formula (1).

Here, ε is a constant relating to the indenter shape. For a Berkovich indenter, this constant is 0.75. The maximum load Pmax and maximum displacement hmax of the unloading curve can be determined based on the graph shown in Figure 3. S is the contact stiffness. The contact stiffness S is the slope immediately after withdrawal of the approximate curve obtained by fitting the range of 60 to 95% of the maximum load in the unloading curve in Figure 3 with the function of equation (2) below. Here, P is the load and h is the indentation depth. A, h, _f , and m are fitting parameters during the fitting process.

Next, the contact projection area Ac is determined based on the shape of the indenter and the contact depth hc. The contact projection area Ac can be expressed as a function of the contact depth hc, as shown in equation (3) below. Note that equation (3) includes terms called correction terms C1 to C5 to correct for the effect of the indenter shape. _C1 to _C5 are values determined by measuring with a maximum load of 20 μN to 10 mN using fused silica as a test specimen, so that the composite modulus of elasticity at each maximum load is 69.6 GPa, which is the composite modulus of elasticity Er of fused silica.

Next, the composite modulus Er is determined based on the contact projection area Ac and contact stiffness S. The composite modulus Er can be calculated using the following equation (4).

The composite modulus Er is obtained by measuring it at multiple locations on a single test specimen, for example, 25 locations, and averaging the obtained composite modulus Er values.
The above describes the method for measuring the composite modulus of elasticity.

(Adhesive layer)
The laminated sheet 10 for the lid may further include one or more adhesive layers.
For example, the laminated lid sheet 10 may include an adhesive layer between the gas barrier layer 4 and the paper substrate 5 to bond them together. Alternatively, the laminated lid sheet 10 may include an adhesive layer between the support layer 3 and the gas barrier layer 4 to bond them together. Alternatively, the laminated lid sheet 10 may include two or more of the above-described adhesive layers.

The adhesive layer material should be appropriately selected to provide the required adhesive strength, depending on the material of the layer to be bonded via it. For example, the same material used for the anchor coat layer 2 can be used for the adhesive layer. Various coating methods can be used, such as gravure coating, die coating, blade coating, knife coating, and bar coating.

The thickness of the laminated sheet 10 for the lid is preferably in the range of 40 to 170 μm, and more preferably in the range of 45 to 160 μm.

The laminated sheet 10 for the lid preferably has a mass per unit area in the range of 45 to 160 g/ ^m² , and more preferably in the range of 50 to 150 g/ ^m² . If this value is too low, the strength of the lid decreases. If this value is too high, the lid becomes harder and its ease of opening tends to decrease. In addition, if this value is too high, the cost will increase, as will the carbon dioxide emissions associated with manufacturing and exhaust.

Incidentally, to prevent deterioration of the contents during storage and detachment of the lid during transportation, high heat seal strength is required between the lid and the container body. However, increasing the heat seal strength increases the load on the lid and container body when opening. If this load is too great, problems are more likely to occur when opening. For example, when opening, a part of the lid may remain on the container body, causing fraying or stringing.

"Fuzzing" refers to the retention of a film-like layer of the lid, such as the heat-seal layer 1, on the container body upon opening. "Stringing" refers to the retention of a thread-like layer of the lid, such as the heat-seal layer 1, on the container body upon opening.

For example, due to fuzzing and stringing, a portion of the lid may protrude from the opening of the container body. In this case, the contents may be difficult to remove, and a portion of the lid may become mixed into the contents. Such packaging containers have an undesirable appearance and may lead to a negative impression on consumers.

Fraying and stringing occur at the boundary between the heat-sealed and unheat-sealed portions of the packaging container. It is believed that this fraying and stringing occur because, upon opening, a portion of the lid, such as the heat-seal layer 1, does not instantly break at the aforementioned boundary, but rather stretches into a film-like or thread-like structure before breaking. Therefore, to suppress fraying and stringing, it is conceivable, for example, to cause the heat-seal layer 1 to break before it stretches.

Furthermore, in particular, in lids where delamination is prone to occur, it is considered that rupture of the heat-seal layer is less likely to occur. Therefore, in order to suppress fraying and stringing, it is preferable that the two adjacent layers constituting the lid have high adhesion.

The laminated sheet 10 for the lid described above has a heat-seal layer 1 thickness within the specified range, and the composite elastic modulus of the heat-seal layer 1 is within the specified range, making the heat-seal layer 1 prone to breakage. Therefore, the laminated sheet 10 for the lid described above is less prone to fraying and stringing. Consequently, the laminated sheet 10 for the lid described above offers excellent ease of opening.

Furthermore, the aforementioned laminated sheet 10 for the lid achieves high heat seal strength between the container body and the lid, thus providing excellent sealing performance.

Furthermore, in the aforementioned laminated lid sheet 10, the mass of the paper substrate 5 is greater than the mass of any other layer contained in the laminated lid sheet 10, thus reducing the amount of petroleum-derived compounds used. Therefore, the aforementioned laminated lid sheet 10 can reduce the environmental impact.

Furthermore, this laminated lid sheet 10 not only offers excellent openability but also possesses high gas barrier properties. Moreover, this laminated lid sheet 10 is less prone to a decrease in gas barrier properties, particularly oxygen barrier properties. This will be explained below.

Food packaging containers are sometimes required to have excellent oxygen barrier properties to prevent oxygen from entering from the outside, thereby suppressing oxidation of the food they contain. In such food packaging containers, the lid also needs to have oxygen barrier properties.

To impart gas barrier properties against oxygen and other gases to paper-based lids, a metal foil or metal-deposited film made of aluminum or other metals is often applied as a gas barrier layer on the paper substrate. However, food packaging containers with lids containing a metal layer have several drawbacks: they cannot be inspected for metal foreign matter using metal detectors after filling; they cannot be incinerated as paper due to the metal content and cannot be recycled as waste paper; and they cannot be used as packaging containers for chilled foods that are expected to be heated in a microwave oven.

As mentioned above, some gas barrier layers do not contain a metal layer. Such gas barrier layers often contain polyamides such as polyvinyl alcohol, ethylene-vinyl alcohol copolymer, polyvinylidene chloride, and nylon MXD-6, as well as resins such as polyacrylonitrile. A lid without a metal layer can avoid the problems described above.

The optimal temperature range for distribution and storage of chilled foods is set for each type of food, but generally it is within the range of 0 to 10°C. Packaged foods, containing chilled foods in food packaging containers, reach consumers through various distribution channels after their manufacture. During this process, for example, from the time a consumer purchases the packaged food at a store until they store it in their refrigerator at home, and from the time they take the packaged food out of the refrigerator until they begin cooking, the packaged food is kept in a room temperature environment.

The inventors have found that packaged foods containing chilled food in a food packaging container with a lid comprising a paper substrate and a gas barrier layer, particularly packaged foods in which the gas barrier layer is a resin-containing layer, experience a significant decrease in the gas barrier properties, especially the oxygen barrier properties, of the lid during the first few hours after exposure from a refrigerated state to a room temperature environment. This is particularly pronounced when the proportion of the paper substrate's mass to the lid's mass is large.

The inventors have determined that the above problem is caused by condensation occurring on the lid surface. Specifically, when packaged items that have been in a refrigerated environment are exposed to a room temperature environment, condensation occurs on the outer surface of the lid. This moisture reaches the gas barrier layer, damaging it. As a result, the oxygen barrier properties of the lid decrease.

The laminated lid sheet 10 described above includes a water-resistant functional layer 7. Therefore, in packaged foods using this laminated lid sheet 10 as the lid material, moisture generated on the outer surface of the lid due to condensation is less likely to reach the gas barrier layer 4. Consequently, in packaged foods using this laminated lid sheet 10 as the lid material, damage to the gas barrier layer 4 due to condensation on the outer surface of the lid is less likely, and a decrease in oxygen barrier performance is less likely to occur.

The laminated sheet 10 for the lid has been described above.
In Figure 1, a gas barrier layer 4 is provided between the paper substrate 5 and the support layer 3, but the gas barrier layer 4 may be provided at any position between the functional layer 7 and the heat seal layer 1. For example, the gas barrier layer 4 may be provided at any position between the printing layer 6 and the heat seal layer 1. Also, the anchor coat layer 2, gas barrier layer 4, and printing layer 6 may be omitted.

[Second Embodiment]
The lid according to the second embodiment of the present invention is a lid obtained from the laminated sheet for lids according to the first embodiment described above. An example of the lid according to the second embodiment is lid 21, which will be described later with reference to Figure 4. As described in relation to the laminated sheet for lids 10, the lid according to this embodiment has excellent openability.

[Third Embodiment]
Figure 4 is a schematic cross-sectional view showing a food packaging container according to a third embodiment of the present invention. The food packaging container 20 shown in Figure 4 comprises a container body 22 having an opening and a lid 21 covering the opening.

The container body 22 is, for example, a bottomed cylindrical shape. Here, the container body 22 comprises a bottom, a body (or side wall), and a flange 22a. The flange 22a flares outward at the upper opening of the body.

The container body 22 contains, for example, an olefin resin such as polypropylene. The container body 22 may further contain components such as ethylene-vinyl alcohol copolymer to enhance its gas barrier properties. Furthermore, the container body 22 may further contain additives, such as additives aimed at improving processability, design, and chemical durability.

The container body 22 may have a single-layer structure or a multi-layer structure. This multi-layer structure may be a two-layer structure or may include three or more layers. In the latter case, the multi-layer structure may include an intermediate layer containing a gas barrier layer, such as the ethylene-vinyl alcohol copolymer mentioned above.

The container body 22 can also be made of paper. If the contents include a liquid, the container body 22 can employ a multilayer structure including a paper base material and a layer made of resin or the like on the side facing the contents to prevent the liquid from seeping into it. For the material of the container body 22 containing the paper base material, for example, paper leaves, paper dust, pulp, or recycled paper can be used. For molding the container body 22, general-purpose technologies such as folding and laminating sheets containing paper leaves (as used in paper carton manufacturing), press molding of sheets using molds, and pulp molding can be utilized. By using paper for the container body 22, it is possible to reduce the overall carbon dioxide emissions associated with the manufacturing and disposal of the food packaging container 20, thus reducing the environmental burden.

The lid 21 is made from a laminated sheet 10 for lids, or a piece cut from it. After the contents are placed inside the container body 22, the lid 21 is heat-sealed to the flange 22a via the heat-seal layer 1. In this heat sealing process, the sealing temperature, sealing pressure, and sealing time can be set as appropriate.

[Fourth Embodiment]
A packaged food according to the fourth embodiment of the present invention is a food packaging container according to the third embodiment described above, in which food is contained. The food contained is not particularly limited, but it is preferably a chilled food or a frozen food. Chilled foods and frozen foods are, for example, cooked or processed foods. Chilled foods and frozen foods are, for example, grilled fish, boiled fish, or prepared foods.

In this packaged food product, the heat seal strength between the lid 21 and the container body 22 is preferably 5 N/15 mm or higher. Here, the heat seal strength is the value obtained by the method specified in JIS Z0238:1998 "Test Method for Heat-Sealable Flexible Packaging Bags and Semi-Rigid Containers".

In the manufacturing of this packaged food, before heat-sealing the lid 21 to the container body 22, for example, after the contents have been placed inside the container body 22 but before heat-sealing the lid 21 to the container body 22, the gas inside the container body 22 may be replaced by a known method. For example, an inert gas may be filled into the container body 22. By appropriately changing the gas composition inside the container, it is possible to suppress bacterial growth and extend the shelf life, maintain the flavor and color of the food for a longer period by preventing oxidation, and prevent the loss of vitamins. The replacement gas is appropriately selected according to the type of food contents. A mixture of oxygen gas, nitrogen gas, and carbon dioxide gas is preferably used as the replacement gas.

The lid of this packaged food product is easy to open. Therefore, this packaged food product is less prone to fraying and stringing when opened.

The tests conducted in connection with the present invention are described below.
<1> Manufacturing of laminated sheets for lids (Example 1)
The laminated sheet 10 for the lid shown in Figure 1 was manufactured by the following method.
First, a single-sided coated paper with a basis weight of 52.3 g/ ^m² was prepared as the paper substrate 5. This single-sided coated paper was obtained by coating imitation paper with a mass per unit area of 37.3 g/ ^m² with a coating solution containing polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), silica, and layered silicate as the main components, and water as the main solvent. The resulting coating layer had a solid content mass per unit area of 15 g/ ^m² .

Next, a printed layer 6 and a functional layer 7 were sequentially formed on the coated surface of the paper substrate 5 using a gravure multi-color printing press. The printed layer 6 was formed using ordinary printing ink. The amount of printing ink applied was 1.0 g/ ^m² . The functional layer 7 was formed using an OP varnish agent mainly composed of nitrocellulose resin and polyethylene granular wax. The amount of OP varnish agent applied was 0.6 g/ ^m² .

Next, a polyethylene terephthalate (PET) film with a thickness of 12 μm and a mass per unit area of 16.8 g/ ^m² was prepared as the support layer 3. An inorganic oxide film was formed on one side of this support layer 3, and then a coating solution mainly containing polyvinyl alcohol (PVA) was applied to form a gas barrier layer 4. A gas barrier film was obtained. The mass per unit area of this gas barrier film was in the range of 17.0 to 17.4 g/ ^m² .

Next, the gas barrier film was bonded to the laminate, which consisted of a paper substrate 5, a printed layer 6, and a functional layer 7, by dry lamination. For dry lamination, first, a dry laminating agent was applied to the surface of the gas barrier layer 4 of the gas barrier film using a gravure coater to form an adhesive layer. A two-component reactive adhesive containing a polyester-based main agent and an isocyanate-based curing agent was used as the dry laminating agent. The amount of dry laminating agent applied was 3.0 g/ ^m² . Then, with this adhesive layer in between, the laminate and the gas barrier film were bonded together so that the gas barrier layer 4 faced the paper substrate 5.

Subsequently, a heat seal varnish A was applied to the support layer 3 of the laminate, which includes a support layer 3, a gas barrier layer 4, a paper substrate 5, a printing layer 6, and a functional layer 7, by gravure printing to form a heat seal layer 1. The heat seal varnish A was applied so that its mass per unit area was 3.2 g/ ^m² in a dry state. The heat seal varnish A is an aqueous emulsion mainly composed of ethylene-vinyl acetate copolymer. The solvents contained in the heat seal varnish A are water and isopropyl alcohol (IPA). The glass transition temperature of the solids contained in the heat seal varnish A is 35°C. The melting point of the heat seal varnish A is in the range of 70 to 100°C.
In this manner, a laminated sheet for the lid was obtained.

(Example 2)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish C was used instead of heat seal varnish A, and heat seal varnish C was applied so that the mass per unit area in the dry state was 3.3 g/ ^m² . Heat seal varnish C is a mixture of heat seal varnish A and heat seal varnish B as described above. In this mixture, the mass ratio of the solid content of heat seal varnish A to heat seal varnish B is 7:3. Heat seal varnish B is an aqueous emulsion mainly composed of ethylene-vinyl acetate copolymer. The solvents contained in heat seal varnish B are water and isopropyl alcohol (IPA). The glass transition temperature of the solid content contained in heat seal varnish B is 50°C. The melting point of heat seal varnish B is in the range of 70 to 100°C.

(Example 3)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish D was used instead of heat seal varnish A, and heat seal varnish D was applied so that the mass per unit area in the dry state was 3.3 g/ ^m² , and an anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3. Heat seal varnish D is a mixture of the heat seal varnish A and heat seal varnish B described above. In this mixture, the mass ratio of the solid content of heat seal varnish A to heat seal varnish B is 3:7. The anchor coat layer 2 was formed on the support layer 3 by gravure printing, applying a solvent-based coating liquid mainly composed of an ester resin and further containing an isocyanate curing agent so that the mass per unit area in the dry state was 1.5 g/ ^m² . The thickness of the anchor coat layer 2 was 1.3 μm.

(Example 4)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish B was used instead of heat seal varnish A, and heat seal varnish B was applied so that the mass per unit area in the dry state was 2.5 g/ ^m² , and the above-described anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3.

(Example 5)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish E was used instead of heat seal varnish A, and heat seal varnish E was applied so that the mass per unit area in the dry state was 3.7 g/ ^m² , and the above-described anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3. Heat seal varnish E is an aqueous emulsion mainly composed of ethylene-vinyl acetate copolymer. The solvents contained in heat seal varnish E are water and isopropyl alcohol (IPA). The glass transition temperature of the solids contained in heat seal varnish E is 52°C. The melting point of heat seal varnish E is in the range of 80 to 110°C.

(Example 6)
The laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following: In this example, the mass per unit area of the heat seal varnish A in its dry state was changed from 3.2 g/ ^m² to 4.5 g/ ^m² .

(Example 7)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following point. Specifically, in this example, heat seal varnish C was used instead of heat seal varnish A, and heat seal varnish C was applied so that the mass per unit area in the dry state was 4.2 g/ ^m² .

(Example 8)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish D was used instead of heat seal varnish A, and heat seal varnish D was applied so that the mass per unit area in the dry state was 4.1 g/ ^m² , and the above-mentioned anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3.

(Example 9)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish B was used instead of heat seal varnish A, and heat seal varnish B was applied so that the mass per unit area in the dry state was 4.1 g/ ^m² , and the above-described anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3.

(Example 10)
The laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following: In this example, the mass per unit area of the heat seal varnish A in its dry state was changed from 3.2 g/ ^m² to 1.1 g/ ^m² .

(Example 11)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following point. Specifically, in this example, heat seal varnish B was used instead of heat seal varnish A, and heat seal varnish B was applied so that the mass per unit area in the dry state was 1.1 g/ ^m² .

(Example 12)
The laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following: In this example, the mass per unit area of the heat seal varnish A in its dry state was changed from 3.2 g/ ^m² to 4.5 g/ ^m² , and the printed layer was omitted.

(Comparative Example 1)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following point. Specifically, in this comparative example, the mass per unit area of the heat seal varnish A in its dry state was changed from 3.2 g/ ^m² to 5.4 g/ ^m² .

(Comparative Example 2)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish B was used instead of heat seal varnish A, and heat seal varnish B was applied so that the mass per unit area in the dry state was 5.4 g/ ^m² , and the above-described anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3.

(Comparative Example 3)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following point. Specifically, in this comparative example, the mass per unit area of the heat seal varnish A in its dry state was changed from 3.2 g/ ^m² to 0.4 g/ ^m² .

(Comparative Example 4)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish E was used instead of heat seal varnish A, and heat seal varnish E was applied so that the mass per unit area in the dry state was 5.6 g/ ^m² , and the above-described anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3.

(Comparative Example 5)
A laminated sheet for the lid was manufactured in the same manner as in Example 1, except for the following points. Specifically, in this example, heat seal varnish E was used instead of heat seal varnish A, and heat seal varnish E was applied so that the mass per unit area in the dry state was 2.3 g/ ^m² , and the above-described anchor coat layer 2 was provided between the heat seal layer 1 and the support layer 3.

<2> Evaluation (Measurement of composite modulus of elasticity)
The composite elastic modulus was measured for the laminated sheets for lids according to Examples 1 to 12 and Comparative Examples 1 to 5 using the method described above. A TI Premier (manufactured by Bruker Japan Ltd.) was used as the nanoindenter. A visible light-curable resin (product name "Aronics® LCR D-800" (manufactured by Toagosei Co., Ltd.)) was used as the resin to embed the sheet pieces. A halogen lamp light source device (product name "KTX-100R" (manufactured by Kenko Tokina Co., Ltd.)) was used to cure this resin. The resin was cured by irradiating it with light at the maximum light intensity of this light source device for 1.5 minutes. An ultramicrotome (product name "Leica EM UC7" (manufactured by Leica Microsystems Ltd.)) with a diamond knife attached was used as the microtome. In addition, to ensure that the indenter penetrated the cross-section perpendicularly during measurement, the sheet pieces were pre-fixed to a holder for fixing them before cutting.

(Measurement of the thickness of the anchor coat layer)
First, test specimens with lengths ranging from 5 to 10 mm were cut from each of the laminated lid sheets according to Examples 1 to 12 and Comparative Examples 1 to 5. Next, the outermost surface of the obtained test specimens was coated with an ultraviolet-curable resin. Then, using a microtome, the resin-coated laminated lid sheet 10 was cut in the thickness direction of the laminated lid sheet. Next, using a scanning electron microscope (SEM), images were taken at five arbitrary locations in the obtained cross-section. Then, the thickness of the anchor coat layer was measured at five arbitrary locations in each of the five obtained images, and the average value of the obtained values was taken as the thickness of the anchor coat layer. The scale indicated on the obtained images was used to measure the thickness.

(Measurement of the thickness of the heat seal layer)
For each of the laminated sheets for lids according to Examples 1 to 12 and Comparative Examples 1 to 5, the thickness of the heat seal layer was measured using the same method as the method for measuring the thickness of the anchor coat layer described above.

(Measurement of heat seal strength)
The heat seal (HS) strength of the laminated sheets for lids according to Examples 1 to 12 and Comparative Examples 1 to 5 was measured against the resin sheet using the method described above.

Here, a three-layer sheet was used as the resin sheet, comprising a pair of polypropylene layers and a layer interposed between them, consisting of a mixture of polypropylene and 4% by mass of ethylene-vinyl alcohol copolymer.

Each lid laminate sheet and resin sheet were heat-sealed using a TP-701-B heat seal tester manufactured by Tester Sangyo Co., Ltd. The heat seal tester used had a sealing bar width of 5 mm. The length of the sealing bar was perpendicular to the median diameter (MD). Heat sealing was performed by applying a temperature of 210°C and a pressure of 0.2 MPa for 2 seconds to the laminate of the lid laminate sheet and resin sheet at each heat-sealing position. From each partially heat-sealed laminate, three test pieces were cut out, each having a strip shape with a width of 15 mm, a length parallel to the MD, and one end unheat-sealed while the other end was heat-sealed for a length of 30 to 50 mm.

Separately, each lid laminate sheet and resin sheet were heat-sealed using the same method as described above, except that the length direction of the sealing bar was perpendicular to the TD. From each laminate partially heat-sealed in this way, three test pieces were cut out, each having a strip shape with a width of 15 mm, a length parallel to the TD, and one end unheat-sealed while the other end was heat-sealed over a length of 30 to 50 mm.

Next, the heat seal strength of each test specimen was measured using the method described above. Specifically, a Tensilon universal tester was used to measure the heat seal strength. The unheat-sealed laminated sheet portion and resin sheet portion of each test specimen were gripped by the grips of the tester, and these grips were moved away from each other. The relative movement speed of the grips, i.e., the peeling speed, was set to 300 mm/min. For each test specimen, the maximum tensile load applied until fracture occurred was recorded. The grip spacing was set to 50 mm.

For each laminated sheet used for the lid, the heat seal strength at MD was obtained by arithmetic mean of the maximum tensile loads obtained from three test specimens whose length direction was parallel to MD. Similarly, for each laminated sheet used for the lid, the heat seal strength at TD was obtained by arithmetic mean of the maximum tensile loads obtained from three test specimens whose length direction was parallel to TD.

(Opening test)
Lids were cut from the laminated sheets for lids according to Examples 1 to 12 and Comparative Examples 1 to 5. Using these lids, a food packaging container 20 as shown in Figure 4 was manufactured. Here, the container body 22 was made by molding the resin sheet used for measuring the heat seal strength into a tray shape. The container body 22 had a roughly rectangular opening with a length of 120 mm in the long side direction and a length of 90 mm in the short side direction, and a height of 30 mm. Heat sealing of the lid 21 to the flange 22a was performed by using a 5 mm wide sealing bar manufactured to conform to the shape of the flange 22a, and applying a temperature of 210°C and a pressure of 0.2 MPa for 1.5 seconds.

Next, for each food packaging container 20, the lid 21 was manually peeled off from the corner of the container body 22 for any 10 containers. Afterward, it was checked for fuzzing or stringing. It was also checked for paper peeling. Here, "paper peeling" refers to the phenomenon where, when the lid is peeled from the container body, cohesive failure of the paper substrate occurs, leaving a portion of the lid attached to the container body.

(Water absorption)
The water absorption of the laminated sheets for lids according to Examples 1 to 12 and Comparative Examples 1 to 5 was measured using the method described above.

The results of the above measurements and tests are summarized in Tables 1 and 2 below.

In Tables 1 and 2 above, "mass" refers to the mass per unit area. The classifications of "paper," "plastic," and "other" in the column labeled "mass ratio" follow the "Explanatory Materials for the Container and Packaging Recycling Law," as explained above. The column labeled "difference" shows the value obtained by subtracting the heat seal strength between the lid laminate sheet and the resin sheet from the breaking strength of the support layer.

In the "Opening Test" column in Tables 1 and 2 above, "A" indicates that no visible fuzzing or stringing occurred. "B" indicates that at least one of the visible fuzzing or stringing occurred locally, but no part of the lid protruded into the container opening due to the fuzzing or stringing. "C" indicates that at least one of the fuzzing or stringing occurred, and a part of the lid protruded into the container opening due to the fuzzing or stringing. In the case of "C," there is a risk of fuzzing and stringing contaminating the contents.

In the "Heat Seal Strength" column in Tables 1 and 2 above, "A" indicates that the heat seal strength for both MD and TD was 5 N/15 mm or higher. "C" indicates that at least one of the heat seal strengths for MD and TD was less than 5 N/15 mm.

As shown in Tables 1 and 2 above, when the laminated sheets for lids according to Examples 1 to 12 were used, no fuzzing or stringing occurred. In contrast, when the laminated sheets for lids according to Comparative Examples 1, 2, 4, and 5 were used, at least one of fuzzing or stringing occurred.

Furthermore, as shown in Tables 1 and 2 above, the laminated sheets for lids according to Examples 1 to 12 and Comparative Examples 1, 2, 4, and 5 achieved high heat seal strength. That is, high airtightness was achieved. On the other hand, the laminated sheet for lid according to Comparative Example 3 did not achieve high heat seal strength.

Furthermore, when using the laminated lid sheets according to Examples 1 to 12 and Comparative Examples 1 to 5, no paper peeling occurred. In addition, the water absorption of the laminated lid sheets according to Examples 1 to 12 and Comparative Examples 1 to 5 was 20 g/ ^m² or less in all cases.

<3> Evaluation of oxygen barrier properties <Example>
A laminated sheet was prepared in which a gas barrier layer consisting of a coating film mainly composed of polyvinyl alcohol (coating amount 13 g/ ^m² , thickness 10 μm) was laminated onto the main matte surface of a glossy paper (basis weight 65 g/ ^m² ) used as a paper base material.

On the main surface of the paper substrate opposite to the main surface where the gas barrier layer was formed, printing ink was applied at a rate of 1 g/ ^m² by gravure printing, and a printed layer was laminated. On top of the printed layer, an OP varnish agent mainly composed of nitrocellulose resin was applied by gravure coating, and a functional layer consisting of an OP varnish layer at a rate of 10 g/ ^m² (dry state) was laminated.

Next, an adhesive composition containing a polyester-based main component and an aliphatic isocyanate-based curing agent was applied to the gas barrier layer by gravure coating, and an adhesive layer with an application amount of 2 g/ ^m² (dry state) was laminated. A non-stretched film mainly composed of linear low-density polyethylene (LLDPE) with a thickness of 30 μm and ^a density of 27 g/m² was laminated on the adhesive layer as a heat seal layer to obtain a laminated sheet for a lid.

<Comparative Example 6>
The laminated sheet for the lid was manufactured using the same method as in Reference Example 1, except that a functional layer consisting of an OP varnish layer was not provided compared to the Reference Example.

(Preparation of test specimens for measuring oxygen permeability)
The laminated sheets for the lids obtained from the above-mentioned Reference Example and Comparative Example 6 were cut into 4cm x 4cm shapes to serve as test specimens. Two test specimens were prepared for each of the Reference Example and Comparative Example 6. The test specimens were sandwiched between two aluminum films, each having a 25mm diameter hole in the center, and fixed with adhesive so that the two holes overlapped. By laminating these films, an aluminum laminate (hereinafter also referred to as the "aluminum laminate") was obtained to hold the laminated sheets for the lids. This aluminum laminate was used as the lid for an aluminum cup in the oxygen permeability measurement test described later.

Aluminum cups were prepared in accordance with the aluminum moisture-permeable cup specified in JAPAN TAPPI Paper and Pulp Testing Method No. 7:2000 Paper and Cardboard - Moisture Permeability Testing Method B. A total of four test specimens for environmental storage, used in the tests described below, were prepared by placing an aluminum laminate containing the test specimens into the opening of these cups, securing it with fasteners, and then placing the lid on top.

(Environmental storage of test specimens and measurement of oxygen permeability)
Each test specimen obtained above was first stored for 12 hours in a refrigerated environment at a temperature of 5°C and humidity-free. Next, one of the two test specimens in each of Reference Example 1 and Comparative Example 6 was subjected to environmental storage for 1 hour in a high-temperature, high-humidity environment at a temperature of 40°C and relative humidity of 90%, thereby forcibly inducing condensation on the surface of the test specimen located on the outside of the cup. Then, as a static adjustment before measuring oxygen permeability, each test specimen was stored for 24 hours in an environment at a temperature of 24°C and relative humidity of 55%, after which oxygen permeability was measured (Condition 2). Furthermore, for the other test specimen in each of Reference Example and Comparative Example 6, after being stored in the refrigerated environment as described above, the static adjustment was performed without the environmental storage in the high-temperature, high-humidity environment described above, and then oxygen permeability was measured (Condition 1).

Oxygen permeability was measured using a MOCON OX-TRAN2/20 oxygen permeability analyzer under conditions of 30°C and 70% relative humidity. The results are shown in Table 3. Lower oxygen permeability indicates superior oxygen barrier properties.

As shown in Table 3, the measurements indicate that lids using laminated sheets for lids, which incorporate both a gas barrier layer and a functional layer, effectively control the degradation of barrier performance due to condensation even when exposed to high-temperature, high-humidity environments after being refrigerated. Furthermore, the degradation of oxygen barrier performance is dramatically improved. Considering that consumers typically spend about one hour between purchasing chilled food packaging at a store and storing it in their home refrigerator, and between removing the packaging from the refrigerator and cooking, lids using laminated sheets for lids incorporating both a gas barrier layer and a functional layer are extremely effective as lids for chilled food packaging containers.

Furthermore, the present invention is not limited to the embodiments described above, and can be modified in various ways during implementation without departing from its essence. The embodiments may also be combined as appropriate, and combined effects can be obtained in such cases. Moreover, the above embodiments include various inventions, and various inventions can be extracted by selecting combinations from the multiple disclosed constituent elements. For example, if the problem can be solved and effects obtained even if some constituent elements are deleted from all the constituent elements shown in the embodiments, then the configuration with these deleted constituent elements can be extracted as an invention.

1…Heat seal layer, 2…Anchor coat layer, 3…Support layer, 4…Gas barrier layer, 5…Paper substrate, 6…Printed layer, 7…Water-resistant functional layer, 10…Laminated sheet for lid, 20…Food packaging container, 21…Lid, 22…Container body, 22a…Flange.

Claims (15)

A laminated sheet for a lid used in the lid of a food packaging container comprising a container body having an opening and a lid covering the opening, comprising a water-resistant functional layer, a paper substrate, a support layer, and a heat-seal layer in this order, wherein the mass of the paper substrate is greater than the mass of any other layer included in the laminated sheet for the lid, the heat-seal layer has a thickness in the range of 1 μm to 5.5 μm, and the composite elastic modulus of the heat-seal layer is 190 MPa or less. The laminated sheet for a lid according to claim 1, wherein the composite elastic modulus is 100 MPa or more. The laminated sheet for a lid according to claim 1, further comprising a printed layer between the functional layer and the paper substrate. The laminated sheet for a lid according to claim 3, further comprising a gas barrier layer having gas barrier properties between the printed layer and the heat-seal layer. The laminated sheet for a lid according to claim 4, wherein the gas barrier layer comprises at least one of an inorganic oxide layer and a resin-containing layer. The laminated sheet for a lid according to claim 1, wherein, when the layers other than the paper substrate included in the laminated sheet for the lid are classified into layers made of plastic and other layers, the mass of the paper substrate is greater than the total mass of the layers made of plastic and the total mass of the other layers. The laminated sheet for a lid according to claim 1, wherein the paper substrate is coated paper having a coating layer on one side. The laminated sheet for a lid according to claim 1, wherein the heat seal layer has a glass transition temperature in the range of 20 to 55°C and comprises a heat seal varnish containing an ethylene-vinyl acetate copolymer. The laminated sheet for a lid according to claim 1, further comprising an anchor coat layer having a thickness of 0.5 μm or more and 2.5 μm or less between the support layer and the heat seal layer. A lid made of a laminated sheet for lids as described in any one of claims 1 to 9. A food packaging container comprising a container body having an opening and a lid according to claim 10 covering the opening, wherein the support layer is disposed between the paper substrate and the internal space of the food packaging container. The food packaging container according to claim 11, wherein the container body has a flange around the opening, and the lid is heat-sealed to the flange via the heat-seal layer. The food packaging container according to claim 11, wherein the internal space of the food packaging container is filled with a mixed gas containing oxygen, nitrogen, and carbon dioxide. The food packaging container according to claim 11, wherein the food packaging container is a packaging container for chilled foods or a packaging container for frozen foods. A packaged food comprising a food packaging container according to claim 11, and food contained in the food packaging container.