JP7698376B2 - Manufacturing method of thermoformable foam sheet, manufacturing method of packaging container, thermoformable foam sheet, packaging container, and method of using packaging container - Google Patents

Description

The present invention relates to a method for producing a thermoformable foam sheet obtained by laminating a film to a base sheet, a method for producing a packaging container made of the thermoformable foam sheet, the thermoformable foam sheet, the packaging container, and a method for using the packaging container.

Conventionally, food packaging containers sold, for example, at supermarkets and convenience stores, have been obtained by thermoforming synthetic resin sheets. The above sheets are made by laminating a base sheet and, if necessary, a film using known methods such as co-extrusion and lamination, and the properties of the base sheet and film, such as their composition and thickness, are determined according to the application and function of the packaging container.

Patent Document 1 discloses a technology for obtaining a foam sheet for deep drawing by heat laminating a 30 μm-thick single-layer film made of high-impact polystyrene as an impact-resistant auxiliary layer to a polystyrene foam sheet as a substrate using a heat roll, and then dry laminating a thermoplastic resin multilayer film made of a modified ethylene-vinyl alcohol copolymer as an oxygen barrier layer and polypropylene as a moisture-proof layer using a urethane adhesive.

Patent Document 2 discloses a technology relating to a packaging container in which a laminate film is laminated on a polystyrene foam sheet as a substrate by a dry lamination method, in which, from the side furthest from the substrate, a coextruded laminate film including a gas barrier layer, a dry lamination adhesive as a dry lamination adhesive layer, printing ink as a printing layer, and a non-oriented polystyrene film as a polystyrene-based resin layer are laminated in this order, with the printing layer and the polystyrene-based resin layer having thicknesses of 20 μm.

JP 2011-51264 A Patent No. 6572191

However, Patent Document 1 does not disclose the peel (adhesion) strength of the laminate film to the base sheet. In other words, the addition of a barrier layer or the like increases the overall thickness of the laminate film, making it easier for the laminate film to peel off from the base sheet. Therefore, depending on the properties of the film closest to the base sheet, the lamination procedure including the film, and the correlation with the method of sealing the food in the packaging container, there is a risk that the laminate film will peel off from the base sheet when the food is opened.

In addition, in Patent Document 2, the printed polystyrene resin layer and the co-extruded laminate film are laminated by a dry lamination method using a dry lamination adhesive, which results in a thick laminate film as a whole. This narrows the range of conditions for the heating temperature and other factors when thermally laminating the laminate film to the base sheet, making it difficult to achieve the desired peel strength. This may lead to a decrease in production efficiency when processing raw sheets and packaging containers.

In order to extend the shelf life of foods, sealing lids (hereafter also referred to as "top seals") that do not allow oxygen and other gases to easily pass into the packaging container and skin films that create a vacuum inside the packaging container are increasingly being used depending on the type of food. However, compared to conventional snap-on lids that seal packaging containers, top seals and skin films are in close contact with the laminated film and are prone to the phenomenon in which the laminated film peels off from the base material when the container is opened (hereafter also referred to as "bagging phenomenon").

Furthermore, the inventors have hypothesized that one of the problems other than the bagging phenomenon is air pockets between the base sheet and the laminated film. In detail, when the base sheet is a polystyrene foam sheet (hereinafter also referred to as "PSP"), butane gas is used as a foaming agent to cause secondary expansion of the base sheet when molding the packaging container. Therefore, there is a concern that after the food is placed in the packaging container and exposed to high temperatures when tightly packaging it, the butane gas will expand again and float up, forming air pockets between the PSP and the laminated film. This led the inventors to the idea that there is room for improvement in the properties of the layer closest to the PSP as the base sheet.

The inventors are also considering maximizing cost-effectiveness by balancing the costs incurred at each process with the number of processes. In more detail, assuming that food storage periods are extended and bagging phenomenon is avoided, there may be a difference in cost-effectiveness between operating more expensive processes with fewer man-hours and operating less expensive processes with more man-hours. This led the inventors to the idea that there is room for improvement in the lamination procedure for the laminated film.

The object of the present invention is to provide a method for producing a thermoformable foam sheet that can be expected to improve cost-effectiveness by optimizing the lamination procedure of the laminated film while also achieving an adhesion state between the base sheet and the laminated film that is suitable for the application and function of the packaging container, a method for producing a packaging container made of the thermoformable foam sheet, the thermoformable foam sheet, the packaging container, and a method for using the packaging container.

That is, the method for producing a foam sheet for thermoforming a packaging container having a drawing ratio of 0.1 to 0.45 in the present invention is characterized by comprising: a step of laminating a thin first film onto a foam base sheet by thermocompression bonding without a drying step of an adhesive to obtain a foam sheet with a film; and a step of laminating a second film including a barrier layer onto the first film side of the foam sheet with a film by thermocompression bonding without a drying step of an adhesive to obtain a foam sheet for thermoforming.

It is preferable that the manufacturing method of the above-mentioned thermoforming foam sheet further includes a step of obtaining a thin first film with a pattern by printing.

It is desirable that the first film have a thickness of 10 μm or more and less than 30 μm, and the second film have a thickness of 20 μm or more and less than 60 μm .

The method for producing a packaging container according to the present invention is characterized by including a step of molding the foam sheet for thermoforming produced by the method for producing a foam sheet for thermoforming described above.

In addition, the present invention provides a foam sheet for thermoforming a packaging container having a drawing ratio of 0.1 to 0.45 , which is characterized by comprising a foam base sheet, a first film having a thickness of 10 μm or more and less than 30 μm that is thermally laminated to the foam base sheet, and a second film having a thickness of 20 μm or more and less than 60 μm that includes a barrier layer that is thermally laminated to the first film.

It is preferable that the first film is a printed patterned film.

The packaging container of the present invention is characterized in that it is obtained by molding the above-mentioned thermoformable foam sheet.

The method of using the packaging container in the present invention is characterized by gas replacement packaging or tightly sealed packaging.

According to the present invention, it is possible to achieve an adhesion state between the foam base sheet and the laminated film that is suitable for the application and function of the packaging container, and it is also possible to optimize the lamination procedure of the laminated film, thereby improving cost-effectiveness.

FIG. 2 is a partially enlarged cross-sectional view of a foamed sheet for thermoforming according to one embodiment of the present invention. FIG. 2 is another partially enlarged cross-sectional view of the foam sheet for thermoforming. 1 is an example of a packaging container made of a thermoformable foam sheet according to one embodiment of the present invention.

Below, we will explain a method for producing a thermoformable foam sheet in one embodiment of the present invention, a method for producing a packaging container made of the thermoformable foam sheet, and a method for using the thermoformable foam sheet, packaging container, and packaging container, with reference to Figures 1 to 3. In these figures, for ease of explanation, certain parts and their leading lines are shown with dashed lines or imaginary lines (two-dot chain lines), and some cross-sectional parts are shown with hatching.

<Outline of manufacturing method of foamed sheet for thermoforming>
The manufacturing method of the thermoforming foam sheet 1 shown in FIG. 1 includes a step of laminating a thin first film 12 onto a foam base sheet 11 to obtain a foam sheet with a film (hereinafter also referred to as a "first lamination step"), and a second step of laminating a second film 13 including a barrier layer onto the first film 12 side of the foam sheet with a film to obtain a foam sheet for thermoforming (hereinafter also referred to as a "second lamination step").

The thermoforming foam sheet 1 manufactured by these processes has a thin first film 12 that is reliably thermocompressed to the foam base sheet, and the first film 12 easily adheres to the foam base sheet 11, making it easy to laminate. Because it is easy to laminate, the peel strength between the first film 12 and the foam base sheet 11 is improved, so the foaming agent is less likely to rise from the foam base sheet 11 during molding and is less likely to accumulate between the first film 12 and the foam base sheet 11. Furthermore, since the process does not include a dry lamination process, the peel strength between the film made up of the first film 12 and the second film 13 (hereinafter referred to as the "laminate film 14") and the foam base sheet 11 is also improved, so the laminate film 14 is less likely to peel off from the foam base sheet 11, and the bagging phenomenon can be easily avoided.

Although each of these processes may be performed within the same production line for the thermoforming foam sheet, one of the processes may be performed within the production line and the other outside the production line. This makes it easier to rearrange the processes and change the equipment associated with each process, and is expected to improve cost-effectiveness by eliminating the need to use, for example, the dry lamination method, which is expensive due to the adhesive drying process among lamination methods.

The method for producing a foam sheet for thermoforming further includes a step of printing to obtain a thin first film 12 with a pattern (hereinafter also referred to as the "printing step").

The thermoforming foam sheet manufactured including this process can produce a packaging container with a pattern while exhibiting the desired peel strength between the first film 12 and the foam base sheet 11, and between the second film 13 and the first film 12, so it is expected to have the effect of improving the product value depending on the type of food to be packaged.

<Overview of Lamination>
The lamination is to laminate the first film 12 to the foam base sheet 11, or to laminate the second film 13 to the film-attached foam sheet to which the first film 12 is laminated. Here, the foam base sheet 11 and the first film 12 may be laminated by thermocompression bonding with a heating roll, or may be laminated with an adhesive interposed between the foam base sheet 11 and the first film. In addition, in laminating the film-attached foam sheet to which the first film 12 is laminated and the second film 13, the lamination may be performed by thermocompression bonding, or an adhesive may be interposed between the film-attached foam sheet and the second film 13. Examples of such lamination means include a thermal lamination method and an extrusion lamination method, and the thermal lamination method, which is relatively cost-effective, is preferable.

The bonding temperature of the laminate may be set appropriately depending on the properties of the foam base sheet 11, the first film 12, and the second film 13, and may be within a range that allows the desired peel strength to be exhibited between the foam base sheet 11 and the first film 12, and between the first film 12 and the second film 13.

When the material of the foam base sheet 11 is a polystyrene resin, the temperature may be, for example, 100°C to 300°C, preferably 130°C to 270°C, and more preferably 160°C to 240°C. When the material of the foam base sheet 11 is a polypropylene resin, the temperature may be, for example, 100°C to 300°C, preferably 130°C to 270°C, and more preferably 160°C to 240°C. When the material of the foam base sheet 11 is a polyethylene resin, the temperature may be, for example, 90°C to 280°C, preferably 100°C to 230°C, and more preferably 110°C to 190°C.

<Peel strength>
According to the peel strength test method for adhesives specified in JIS K6854-2, the peel strength between the foam base sheet 11 and the first film 12 and/or the peel strength between the first film 12 and the second film 13 is 1.5 N/15 mm or more, preferably 2.0 N/15 mm or more, more preferably 4.0 N/15 mm or more accompanied by impact resistance against vibration, dropping, etc. during distribution, even more preferably 5.0 N/15 mm or more, and even more preferably 6.0 N/15 mm or more. In addition, it is acceptable if the films are sufficiently bonded and difficult to peel.

<Thickness of foam sheet for thermoforming>
The thickness of the thermoforming foam sheet 1 may be greater than the sum of the thickness of the foam base sheet 11, the thickness of the first film 12, and the thickness of the second film 13, and may be 3.0 mm to 4.5 mm, and preferably 3.8 mm to 4.0 mm, when the material of the foam base sheet 11 is a polystyrene-based resin. When the material of the foam base sheet 11 is a polyolefin-based resin, the thickness may be 0.5 mm to 2.5 mm, and preferably 0.6 mm to 1.5 mm, and more preferably 0.8 mm to 1.2 mm.

<Thickness of laminated film>
The thickness of the laminate film 14 may be 40 μm to 100 μm, and is preferably 50 μm to 70 μm which is shock resistant to vibrations and drops during distribution and which exhibits the desired peel strength in the range of lamination adhesion temperatures, and if it is less than 40 μm, it is too thin and it is difficult to obtain the effects of the first film 12 and the second film 13, and if it exceeds 100 μm, it is too thick and is easily peeled off from the foam base sheet 11. In addition to the first film and the second film, for example, a third film may be laminated at any location as necessary.

The first film is not particularly limited, but may be colorless and transparent, opaque, or translucent, and may have a printed surface as described below, or may be colored with a kneaded pigment or the like. Such a printed surface may be formed by printing on the surface of the first film, for example, on the surface that is to be laminated with the foam base sheet.

<Printing Overview>
The printed surface may be on both sides of the first film 12, but preferably only on the surface that is not adhered to the second film 13 (the surface that is adhered to the foam base sheet 11) so that the peel strength is not relatively low. The printing process may be set after the first lamination process, but is preferably set before the first lamination process so that the printed surface is the surface that is adhered to the foam base sheet 11.

<Combination of the first lamination step and the second lamination step>
The combination patterns of the first lamination step and the second lamination step for the production line of the thermoforming foam sheet include a case where the first lamination step is set outside the production line and the second lamination step is set inside the production line (hereinafter also referred to as "process pattern 1"), a case where the first lamination step and the second lamination step are set inside the production line (hereinafter also referred to as "process pattern 2"), a case where the first lamination step is set inside the production line and the second lamination step is set outside the production line (hereinafter also referred to as "process pattern 3"), and a case where the first lamination step and the second lamination step are set outside the production line of the thermoforming foam sheet (hereinafter also referred to as "process pattern 4"). Process pattern 1 or process pattern 2 is preferable, and process pattern 1 is more preferable. Outside the production line is, for example, a different production line in the same building, a special production area in the same building, or a different building.

<Combination of printing process and first lamination process>
As a combination of the printing process and the first lamination process for the production line of the thermoforming foamed sheet 1, the printing process is preferably set together with the first lamination process, specifically, in process pattern 1 or process pattern 4, it is set outside the production line, and in process pattern 2 or process pattern 3, it is set inside the production line.

<Details of foam base sheet>
The thickness of the foam substrate sheet 11 may be 1.8 mm to 2.5 mm, preferably 2.0 mm to 2.2 mm, when the material of the foam substrate sheet 11 is a polystyrene resin. Also, when the material of the foam substrate sheet 11 is a polyolefin resin, the thickness may be 0.5 mm to 2.5 mm, preferably 0.6 mm to 1.5 mm, more preferably 0.8 mm to 1.2 mm. If the thickness is below the lower limit, the strength and rigidity of the laminate sheet or packaging container is too low and it is easily broken, and if the thickness is above the upper limit, it is difficult to mold the packaging container so that the thickness distribution is as designed, and the remaining amount of the foaming agent added during production is too large, so that floating or post-deformation is likely to occur.

The raw material of the foam base sheet 11 is, for example, a thermoplastic resin such as a polyolefin resin, such as a polypropylene resin or a polyethylene resin, or a polystyrene resin, and may be one type or a mixture of two or more types in a predetermined ratio, or may be a mixture of these with a filler (inorganic filler) in a predetermined weight ratio.

The inorganic filler may be, for example, talc, calcium carbonate, silica, clay, wollastonite, potassium titanate, xonotlite, gypsum fiber, aluminum borate, fibrous magnesium compound (MOS), aramid fiber, carbon fiber, glass fiber, mica, glass flakes, or polyoxybenzoyl whiskers. One type or a mixture of two or more types in a specified ratio may be used, and talc, which has a proven track record in the food packaging field, is preferred.

The foam base sheet 11 may be manufactured, for example, by adding a foaming agent to the raw material polystyrene resin, melt-kneading the mixture in an extruder, and then extruding and foaming the mixture to obtain a polystyrene foam sheet (PSP).

The blowing agent may be any of the conventional compounds, such as volatile blowing agents, e.g., propane, butane, pentane, and hexane; chemical blowing agents (decomposition type blowing agents), e.g., ammonium carbonate, ammonium bicarbonate, sodium bicarbonate, and sodium citrate; carbon dioxide, nitrogen gas, water, and other applicable blowing agents. These blowing agents may be used alone or in combination.

<Details of the first film>
The first film 12 is preferably easily adhered to the foam base sheet 11 and contains 50% or more of the same raw materials as the raw materials of the surface of the foam base sheet 11. For example, when the foam base sheet 11 is a polystyrene foam sheet, the first film 12 is an unstretched polystyrene film (CPS film) or a biaxially oriented polystyrene film (OPS film); when the foam base sheet 11 is a polypropylene foam sheet, the first film 12 is an unstretched polyolefin film (CPP film) or a biaxially oriented polyolefin film (OPP film); and when the foam base sheet 11 is a polyethylene foam sheet, the first film 12 is an inflation polyethylene film (IPE film) or a unstretched polyethylene film (CPE film).

The thickness of the first film 12 may be 10 μm or more but less than 30 μm, preferably 10 μm to 28 μm, which is shock resistant to vibrations and drops during distribution and exhibits the desired peel strength within the range of lamination adhesion temperatures, more preferably 15 μm to 26 μm, and even more preferably 18 μm to 25 μm. If the thickness is less than 10 μm or more than 30 μm, the lamination adhesion temperature range is likely to be narrow, and if it is thinner than 10 μm, the amount of components that function as a sealant will be reduced, and there is a concern that the desired peel strength improvement effect will not be achieved. If it is thicker than 30 μm, there is a concern that heat will not be transferred uniformly when adhering the first film 12 to the foam base sheet, resulting in variation in peel strength.

The first film 12 may be laminated to both sides of the foam base sheet, but from an economical perspective, it is preferable that it is laminated and bonded only to the side where the second film is to be laminated.

<Details of the second film>
2, the second film 13 includes, in addition to the barrier layer, a resin layer made of a thermoplastic resin such as a polypropylene-based resin or a polyethylene-based resin, and an adhesive layer that bonds the barrier layer and the resin layer, and is laminated by co-extrusion. The resin layers may be made of the same material or may be laminated at the same position, or may include a first resin layer and a second resin layer that are different from each other. The order of the layers included in the second film 13, from the side closest to the first film 12, is, for example, a first resin layer 131, an adhesive layer 132, a barrier layer 133, an adhesive layer 134, and a second resin layer 135.

The thickness of the second film 13 may be 20 μm to 60 μm, and is preferably 30 μm to 50 μm, which is shock resistant to vibrations and drops during distribution and provides the desired peel strength within the range of lamination adhesion temperatures. If it is less than 20 μm, it is too thin and it is difficult to obtain the effects of the barrier layer and resin layer, and if it is more than 60 μm, it is too thick and easily peels off from the first film 12.

The first resin layer 131 is preferably one that is easily adhered to the first film 12, and may be, for example, only polypropylene or only polyethylene, or a mixture of these in a specified ratio, where the ratio of polypropylene:polyethylene may be 90:10 to 10:90, 80:20 to 20:80, or 70:30 to 30:70, or 1% by mass or less of polystyrene per 100% by mass of polypropylene.

The adhesive layers 132 and 134 need only be capable of adhering the first resin layer 131 to the barrier layer 133 and the barrier layer 133 to the second resin layer 135, and may be, for example, a polyolefin-based adhesive resin.

The barrier layer 133 is made of a resin that is difficult for gases that affect food quality, such as oxygen gas, water vapor gas, and carbon dioxide gas, to pass through, and is preferably made of a resin that has an oxygen transmission rate according to JIS K7126 of 1000 ml/ ^m2 ·24 hr·MPa (75% RH at 20°C) or less at a thickness of 25 μm and that can be co-extruded, such as an ethylene-vinyl alcohol copolymer (EVOH) or polyamide (PA).

The thickness of the barrier layer is not particularly limited, but may be, for example, 1 μm to 15 μm, more preferably 4 μm to 10 μm, and even more preferably 5 μm to 8 μm. If the thickness of the barrier layer is less than 1 μm, there is a concern that the desired gas barrier properties may not be obtained, and if it exceeds 15 μm, there is a concern that costs will increase due to the high material cost of the gas barrier resins such as the ethylene-vinyl alcohol copolymer (EVOH) and polyamide (PA).

The second resin layer 135 is preferably one that is easily heat-pressed to the top seal and that allows the pressed top seal to be easily removed (has easy-to-open properties). For example, it may be made of only polypropylene or only polyethylene, or a mixture of these in a specified ratio, where the ratio of polypropylene:polyethylene may be 90:10 to 10:90, 80:20 to 20:80, or 70:30 to 30:70, or 100% by weight of polypropylene to 1% by weight or less of polystyrene.

In the present invention, it is preferable that a coating agent is interposed between the first film and the second film as an adhesive. Such a coating agent can reliably bond different materials together. There are no particular limitations on the components of the coating agent, but a urethane resin adhesive is preferable. Such a coating agent may be applied to the surface of the second film or the first film in advance, or may be supplied between the first film and the second film when the first film and the second film are laminated.

The adhesion initiation temperature of the coating agent that functions as an adhesive between the first film and the second film is preferably 150°C to 180°C, and more preferably 160°C to 170°C. With this configuration, less heat is required when, for example, thermally laminating the second film onto the first film. Here, the adhesion initiation temperature refers to the temperature required for the coating agent to exert its effect.

<Printing details>
The printing surface may be, for example, a printing layer printed with a metallic ink made of glossy metallic powder. Examples of the metallic powder include aluminum powder, gold powder, silver powder, copper powder, bronze powder, and zinc powder. The metallic powder may be one type or a mixture of two or more types. The metallic powder may be a metallic powder having various forms conventionally known as a metallic gloss component such as powder of other metals or alloys or metal-deposited flakes. The metallic powder may be a metallic powder mixed with a colorant such as yellow or red to adjust the color tone, or a metallic film formed on the surface of an inorganic filler. The powder content is 5 mg/m ² or more to fully express the gloss, but is preferably 5 mg/m ² to 500 mg/m ² to avoid a decrease in moldability due to a printing layer that is too thick, more preferably 10 mg/m ² to 200 mg/m ² , and even more preferably 20 mg/m ² to 100 mg/m ² .

<Overview of packaging containers>
The packaging container 2 shown in Figure 3 is obtained by molding the thermoformable foam sheet 1 shown in Figure 1, and is used to package food with a lid or in a vacuum state, and has, for example, a bottom 21 on which the food is placed, a side portion 22 extending upward from the peripheral edge of the bottom 21, and a flange portion 23 extending outward from the entire upper edge of the side portion 22.

The manufacturing method of the packaging container 2 specifically includes a process (hereinafter also referred to as the "foaming process") of adding a foaming agent to the thermoforming foam sheet 1 after production but before molding (hereinafter also referred to as the "first thermoforming foam sheet") to create a foamed state (hereinafter also referred to as the "second thermoforming foam sheet"), and a process (hereinafter also referred to as the "molding process") of vacuum molding the second thermoforming foam sheet from one side (downward) or both sides (upward and downward) in a specified mold.

There are no particular limitations on the molding process as long as the desired shape of packaging container 2 can be molded from thermoforming foam sheet 1. Examples include double-sided vacuum molding and single-sided vacuum molding, but it is preferable to select single-sided vacuum molding, in which the mold does not come into contact with the laminated film. With single-sided vacuum molding, the laminated film side does not need to be pulled, so there is less impact on the film side caused by vacuuming, reducing adhesion variations during the packaging process where top sealing and skin packing are performed, and allowing for stable adhesion.

When the lid is a top seal, the flange portion 23 to which the top seal is heat-pressed preferably has a flat portion that is approximately horizontal when the packaging container 2 is placed on a desk, or a curved portion that is arch-shaped in end view and becomes the flat portion when pressurized during heat-pressing, between the inner peripheral edge corresponding to the upper edge of the side portion 22 and the outer peripheral edge located at the outermost position, in order to facilitate adhesion to the top seal.

Furthermore, the flange portion 23 may be thinner than the bottom portion 21 and the side portion 22, preferably in an unfoamed state or with fine irregularities on the surface smoothed out, so as to be easily attached to the top seal. In the molding process, the flange portion 23 may be made thinner than the bottom portion 21 and the side portion 22 by vacuum molding from both sides using a mold in which the thickness of the space corresponding to the flange portion 23 is thinner than the thickness of the space corresponding to the bottom portion 21 and the side portion 22.

The thickness of the flange of the packaging container 2 may be 2.0 mm to 5.0 mm, preferably 2.5 mm to 4.0 mm, and more preferably 2.7 mm to 3.5 mm, if the material of the foam base sheet 11 is a polystyrene-based resin, and may be 0.5 mm to 2.5 mm, preferably 0.6 mm to 1.5 mm, and more preferably 0.8 mm to 1.2 mm, if the material of the foam base sheet 11 is a polyolefin-based resin.

The drawing ratio (S) of the packaging container of the present invention is not particularly limited, but the effect of the present invention is more effectively exhibited when it is preferably 0.1 to 0.45, more preferably 0.15 to 0.4, and even more preferably 0.2 to 0.35.
Here, the drawing ratio (S) refers to a value expressed by the following formula (1).
S = (depth of container) / (diameter of the largest circle inscribed in the opening of the container) (1)
That is, the drawing ratio (S) is the depth of the deepest part of the container divided by the diameter of the largest inscribed circle that touches the shape of the recess (opening) formed on the plane of the sheet. For example, if the shape of the recess is a circle, the diameter is the diameter of the largest inscribed circle, if the shape is an ellipse, the short diameter is the short side, and if the shape is a rectangle, the length of the short side is the diameter of the largest inscribed circle.

<How to use packaging containers>
The food may be gas-modified packaged or sealed using the packaging container 2. Modified atmosphere packaging (MAP) is a gas-modified packaging method in which the air inside the container is removed and filled with nitrogen, carbon dioxide, or a mixture of these, and the food is sealed with a top seal. Sealed packaging is a vacuum packaging method in which a heated skin film is attached to the inner surface of the container together with the food in a vacuum state to seal the food.

In the case of gas replacement packaging, food is placed in the packaging container 2, and after replacing the gas inside the container, the top seal is heat-pressed from above onto the flange portion 23 using a top sealing machine to seal it. The order of the layers included in the top seal, from the side furthest from the flange portion 23, i.e., from the outside not in contact with the food, is, for example, polyamide (PA), adhesive layer, barrier layer, adhesive layer, sealant layer (thermoplastic resin). The sealant layer is preferably easy to open. The adhesive layer is, for example, a polyolefin adhesive resin. The barrier layer is, for example, an ethylene-vinyl alcohol copolymer or polyamide.

In the case of close-contact packaging, the food is placed in the packaging container 2, the heated skin film is moved above the packaging container 2, and then the container is evacuated using a skin pack packaging machine, the air inside the container is degassed, and the container is returned to atmospheric pressure, and the skin film is attached to the inner surface of the packaging container 2 together with the food, and sealed using the differential pressure. The skin film, for example, comprises an inner layer that is attached to the inner surface of the packaging container 2, and an outer layer that is laminated on the inner layer. The outer layer may have a shape-retaining property that stretches three-dimensionally along the shape of the inner surface when heated and softened, and retains that shape when cooled. A functional intermediate layer such as a barrier layer or a reinforcing layer may be interposed between the inner layer and the outer layer.

The following describes evaluation tests of a thermoformable foam sheet manufactured by the manufacturing method of a thermoformable foam sheet according to one embodiment of the present invention, and a packaging container manufactured by the manufacturing method of a packaging container made of the above-mentioned thermoformable foam sheet.

<Evaluation test methods in the examples>
The method for producing a foamed sheet for thermoforming includes at least a first lamination step and a second lamination step, in which the components of the foam base sheet (PSP or PP foam) are individual conditions, the lamination in the first lamination step and the second lamination step is performed by a thermal lamination method (roll speed = 8.5 m/min, pressure = 0.5 MPa), and a combination of the first lamination step and the second lamination step or another lamination step is individual conditions.

The first film is a non-oriented polystyrene film (CPS film) or a non-oriented polyolefin film (CPP film), and the presence or absence of printing on the foam base sheet side is an individual condition. The second film is laminated from the side closest to the first film with a coating agent, a polypropylene-based resin layer (PP), a polyolefin-based adhesive resin layer (Adh), an ethylene-vinyl alcohol copolymer layer (EVOH), a polyolefin-based adhesive resin layer (Adh), and an easy-peel layer (EP).

The manufacturing method for the packaging container in the embodiment includes a foaming process and a molding process. The molding process employs a double-sided vacuum molding method. The manufactured packaging container has a bottom, sides, and a flange, is for gas replacement packaging, and is sealed by heat-pressing a top seal onto the flange, which has a flat portion.

The peel strength test is performed using an Instron universal testing machine (chuck distance = 25 mm, peel speed = 200 mm/min) based on the adhesive peel strength test method specified in JIS K6854-2. The test pieces are a portion of the first cut foam sheet for thermoforming, a portion of the second cut foam sheet for thermoforming, and a portion of the flange of the packaging container (15 mm wide), with a portion of the laminate film peeled off from each of the above portions.

The peel strength test is carried out by clamping the peeled laminate film in one chuck of the testing machine, clamping the foam base sheet to which the laminate film was attached in the other chuck, and widening the gap between the two chucks until the laminate film peels off from the base sheet.

The individual conditions for Examples 1 to 4 and Comparative Examples 1 to 3 are as follows:

Example 1
Foam base sheet = PSP
First film = (from the foam base sheet side) printing, CPS film Second film = (from the first film side) coating agent, PP, Adh, EVOH, Adh, EP
Thickness of the first film = 20 μm, thickness of the second film = 40 μm
Combination of the first lamination process and the second lamination process=Process pattern 1
Lamination temperature during thermal lamination in the first lamination step and the second lamination step: 175° C.

Example 2
The conditions are the same as in Example 1, except that the combination of the first lamination process and the second lamination process is process pattern 2.

Example 3
The conditions were the same as in Example 1, except that the first film had no printing.

Example 4
Foam base sheet = PP foam First film = (from foam base sheet side) coating agent, printing, CPP film Second film = same conditions as in Example 1 Thickness of first film = 25 μm, thickness of second film = 40 μm
Lamination process: A first film is laminated in the production line when the foam base sheet is extruded, and a second film is laminated in the production line before molding.
Lamination temperature during thermal lamination in the first lamination step: 170° C.
Lamination temperature during thermal lamination in the second lamination step: 175° C.

<Comparative Example 1>
The conditions were the same as in Example 1, except that the dry lamination method was used in the second lamination step, the lamination temperature during thermal lamination was 205° C., and printing was performed on the second film side.

<Comparative Example 2>
The conditions are the same as in Example 1, except that the first film is not used.

<Comparative Example 3>
Foam base sheet = PP foam First film = (from foam base sheet side) coating agent, CPP film, printing Second film = same conditions as Comparative Example 1 Thickness of first film = 25 μm, thickness of second film = 40 μm
Lamination step: the first film and the second film are dry laminated outside the production line, and the first film side is laminated in the production line when the foam base sheet is extruded.
Lamination temperature during thermal lamination: 170°C

<Evaluation Results>
The evaluation results of Examples 1 to 3 and Comparative Examples 1 and 2 are shown in Table 1, and the evaluation results of Example 4 and Comparative Example 3 are shown in Table 2. For ease of reference, these tables also list the individual conditions described above.

<Examples 1 to 3>
It was confirmed from Example 1 that the peel strength of the second foamed sheet for thermoforming was the highest. In addition, when comparing the peel strengths of the first foamed sheet for thermoforming, the second foamed sheet for thermoforming, and the molded product with Example 1 as a reference, it was confirmed from Example 2 that the peel strength was improved by setting the first lamination step and the second lamination step in the same production line, and from Example 3 that the peel strength was equivalent even without printing if the process pattern was the same. That is, it was confirmed from Example 3 that the presence or absence of a printed surface is irrelevant to the peel strength between the foam base sheet and the first film.

<Comparative Examples 1 and 2>
In contrast to Examples 1 to 3, in Comparative Example 1, the first film and the second film are laminated by a dry lamination method using a dry lamination adhesive, and it is confirmed that the peel strength of the first foamed sheet for molding, the second foamed sheet for molding, and the molded product is all reduced because it is difficult to bond them at the desired heating temperature, and in Comparative Example 1, the peel strength of the molded product is the lowest, so it is presumed that when the first film and the second film are dry laminated and the laminated film with an increased thickness is thermally laminated, the heat required for adhesion is not sufficiently transmitted, resulting in a decrease in peel strength and causing a decrease in the quality of the molded product. In Comparative Example 2, even if the overall thickness of the laminated film is thin, there is a concern that it is difficult to achieve the desired peel strength without the first film.

That is, the only difference between Example 3 and Comparative Example 2 is the presence or absence of the first film, but this difference results in a large difference in peel strength. A stronger peel strength is achieved by using a coating agent to bond a second film including a barrier layer to a film-attached foam sheet in which a transparent plain CPS film as the first film is heat-sealed to the PSP, rather than using a coating agent to bond a second film including a barrier layer directly to the PSP as the foam base sheet. This is presumably because the surface roughness of the PSP, which is a foam, is relatively high, and even if a coating agent is used to bond the film, the adhesive surface area is reduced, resulting in a decrease in peel strength.

<Example 4 and Comparative Example 3>
In Example 4, the first film having a printed surface on the foam base sheet side is laminated to a foam base sheet made of PP foam with a coating agent to obtain a first foam sheet for thermoforming, and then the second film is laminated to the first film with a coating agent without obtaining a second foam sheet for thermoforming. On the other hand, in Comparative Example 3, the first film and the second film are dry laminated to obtain a laminate film, and then a coating agent is applied to the foam base sheet side of the laminate film, and the laminate film is heat laminated to the foam base sheet to obtain a foam sheet for molding. That is, when the first film and the second film are dry laminated, the thickness of the film laminated to the foam base sheet at one time becomes thick, and it is presumed that the heat required for adhesion is not sufficiently applied, resulting in a decrease in peel strength.

Reference Signs List 1: Foam sheet for thermoforming, 11: Foam base sheet, 12: First film, 13: Second film, 2: Packaging container, 21: Bottom, 22: Side, 23: Flange

Claims (5)

A method for producing a foamed sheet for thermoforming of a packaging container for gas replacement packaging or close contact packaging having a drawing ratio of 0.1 to 0.45, comprising the steps of:
a step of laminating the thin first film onto a foam substrate sheet by thermocompression bonding without a drying step of an adhesive to obtain a foam sheet with a film;
and laminating a second film including a barrier layer to the first film side of the film-attached foam sheet by thermocompression bonding without a drying step of an adhesive to obtain a foam sheet for thermoforming,
The thickness of the first film is equal to or greater than 10 μm and less than 30 μm,
the second film has a thickness of 20 μm or more and less than 60 μm. 2. The method for producing a foam sheet for thermoforming according to claim 1, wherein the first film contains 50% or more of the same material as that of the surface of the foam base sheet. 3. The method for producing a foam sheet for thermoforming according to claim 2, wherein the foam base sheet is a polystyrene foam sheet, and the first film is a non-oriented polystyrene film. a first lamination step of laminating the first film onto the foam base sheet to obtain the film-attached foam sheet;
A second lamination step of laminating the second film on the first film side of the film-attached foam sheet to obtain a foam sheet for thermoforming,
4. The method for producing a foamed sheet for thermoforming according to claim 1, wherein the first lamination step and the second lamination step are both set in a production line for the foamed sheet for thermoforming. a first lamination step of laminating the first film onto the foam base sheet to obtain the film-attached foam sheet;
A second lamination step of laminating the second film on the first film side of the film-attached foam sheet to obtain a foam sheet for thermoforming,
4. The method for producing a foamed sheet for thermoforming according to claim 1, wherein only one of the first lamination step and the second lamination step is set in the production line for the foamed sheet for thermoforming.

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