JP7529447B2 - Food container inner lining film, its manufacturing method, uses, food container - Google Patents

Description

The present invention relates to a food container lining film that forms the inner layer of a food container. More specifically, the present invention relates to a lining film that forms the inner layer of a food container such as a food tray that has a layer made of a foam sheet. The present invention also relates to a method for producing the film, uses, and food containers using the film.

In recent years, frozen foods, such as prepared dishes, have become increasingly popular. Many of these frozen foods are cooked in a microwave oven while still in the food containers used for freezing. Therefore, food containers must be (1) cold-resistant so that they do not crack during freezing, (2) heat-resistant so that they can withstand microwave cooking, and (3) insulating so that heat is not transferred to the hands and fingers when holding the container immediately after microwave cooking.

Patent Document 1 is an invention related to a microwaveable container that can also be used as a storage container for frozen storage, and aims to provide a microwaveable container that is easy to handle when freezing and heating (Patent Document 1 [0001] [0008]). In Patent Document 1, a layer (foam layer) that exhibits heat insulation is provided on the container body, and the outermost layer is formed from a resin composition whose main component is high-density polyethylene resin, thereby suppressing the occurrence of cracks when dropped (Patent Document 1 [0009]).

Patent Document 2 is an invention related to a thermoplastic polyester resin foam sheet and a thermoplastic polyester resin foam container (Patent Document 2 [0001]). The purpose of Patent Document 2 is to provide a thermoplastic polyester resin foam sheet (Patent Document 2 [0005]) that can obtain containers with high thermal insulation properties and increase container productivity. The thermoplastic polyester resin foam sheet disclosed in Patent Document 2 contains a thermoplastic polyester resin and a crystallization promoter, and is characterized by a crystallization time of 14 minutes or less (Patent Document 2 [Claim 1]). Patent Document 2 also discloses that the container obtained from the thermoplastic polyester resin foam sheet is used as a freezer microwaveable container (Patent Document 1 [Claim 13]).

JP 2011-173649 A JP 2018-90761 A

As described above, food containers for storing food in a freezer and then cooking the food in a microwave oven are required to have (1) cold resistance, (2) heat resistance, and (3) thermal insulation. As disclosed in Patent Documents 1 and 2, the use of a foam sheet (foam layer) can largely solve the problem of (3) thermal insulation, but no containers that are satisfactory in terms of (1) cold resistance and (2) heat resistance have yet been proposed.
The present invention aims to provide a food container lining film that is excellent in both (1) cold resistance and (2) heat resistance. It is also an object of the present invention to develop a food container that is excellent in all of (1) cold resistance, (2) heat resistance, and (3) heat insulation by bonding the lining film to a foamed sheet that is excellent in (3) heat insulation.

According to the present invention, as a means for solving the above problems,
(1) A food container inner lining film forming the inner layer of a food container is provided, characterized in that a surface layer (A) is made of a polypropylene-based resin, and a layer (B) in contact with the surface layer is made of a polyethylene-based resin having a density of less than 940 kg/ ^m3 .
(2) The food container inner lining film according to (1) is provided, characterized in that the polyethylene resin constituting the layer (B) in contact with the surface layer is a linear low-density polyethylene having a density of less than 918 ^kg /m3.
(3) The food container inner lining film according ^to (1) or (2) is provided, characterized in that the polyethylene resin constituting the layer (B) in contact with the surface layer is a linear low-density polyethylene having a density of less than 910 kg/m3.

(4) There is provided a food container inner lining film as described in any one of (1) to (3), characterized in that the layer (B) in contact with the surface layer has a laminate layer (C) on the side opposite the surface layer (A), and the laminate layer (C) is made of a linear low-density polyethylene having a higher density than the polyethylene resin constituting the layer (B) in contact with the surface layer.
(5) There is provided a food container inner lining film according to any one of (1) to (4), characterized in that, when the thickness of the food container inner lining film is T and the thickness of the layer (B) in contact with the surface layer is Tb, Tb ≧ 0.7T.
(6) There is provided the food container inner lining film according to any one of (1) to (5), characterized in that the polypropylene-based resin is a propylene homopolymer and/or a propylene-ethylene block copolymer.

(7) There is provided a method for producing an inner lining film for a food container according to any one of (1) to (6), characterized in that the film is produced by a T-die coextrusion method without a stretching treatment.
(8) There is provided a use of the food container inner lining film according to any one of (1) to (6), characterized in that it is used as a food container for storing food in a frozen state and then cooking the food in the food container in a microwave oven.
(9) There is provided a food container comprising at least a foamed sheet and a food container inner lining film according to any one of claims 1 to 6, characterized in that the surface layer (A) made of a polypropylene-based resin is the innermost layer.

The food container lining film of the present invention has excellent heat resistance because the surface layer (A) is made of a polypropylene-based resin. In a food container in which the layer in contact with food is made of the surface layer (A), the surface of the container is unlikely to melt even when the food is heated to high temperatures in a microwave oven. In addition, the layer (B) in contact with the surface layer is made of a polyethylene-based resin with a density of less than 940 kg/ ^m3 , so the container has excellent cold resistance. The layer (B) in contact with the surface layer prevents the container from cracking even in low-temperature environments of -30°C to -20°C.
In particular, when the layer (B) is made of a linear low-density polyethylene having a density of less than 918 kg/m ³ , or even less than 910 kg/m ³ , the cold resistance is very excellent.

Furthermore, when the above-mentioned layer (B) is in contact with the surface layer (A) on one side and has a laminate layer (C) on the other side, and the laminate layer (C) is made of a linear low-density polyethylene having a higher density than the layer (B), the handleability of the film is improved.
Furthermore, when the thickness of the film lining the food container is T and the thickness of the layer (B) in contact with the surface layer is Tb, if Tb ≥ 0.7T, the cold resistance of the film becomes even more excellent.
Furthermore, when the resin constituting one of the surface layers (A) is a propylene homopolymer and/or a propylene-ethylene block copolymer, the heat resistance becomes even more excellent.

Furthermore, when the food container inner lining film of the present invention is produced by T-die coextrusion without being stretched, it has good formability into food containers such as trays.
In addition, since the food container inner lining film of the present invention has excellent (1) cold resistance and (2) heat resistance, by bonding it to a foam sheet having excellent (3) insulation properties, a food container suitable for storing food in a frozen state and then cooking it in a microwave oven can be obtained.

1 is a schematic cross-sectional view showing one embodiment of a food container inner lining film of the present invention. 1 is a DSC chart of polypropylene homopolymer. 1 is a DSC chart of a propylene-ethylene block copolymer. 1 is a DSC chart of a propylene-ethylene random copolymer. FIG. 2 is a schematic cross-sectional view showing another embodiment of the food container inner lining film of the present invention. FIG. 1 is a schematic cross-sectional view showing one embodiment of a food container of the present invention.

The present invention will be described in detail below, but the present invention is not limited thereto and various embodiments can be adopted within the scope of the same effects.
[Food container interior film]
Fig. 1 is a schematic cross-sectional view showing one embodiment of the food container lining film of the present invention. The food container lining film of the present invention (hereinafter, abbreviated as "lining film" as necessary) 1 comprises at least one surface layer (A) and a layer (B) in contact with the surface layer.

<Surface layer (A)>
As shown in Fig. 1, the surface layer (A) is a layer that forms one surface of the inner lining film 1 and contributes to improving the heat resistance of the inner lining film. The surface layer (A) is made of a conventionally known polypropylene-based resin such as a polypropylene homopolymer, a propylene-ethylene block copolymer, a propylene-ethylene random copolymer, or a propylene-butene-1 random copolymer. However, in consideration of heat resistance, the surface layer (A) is preferably made of a polypropylene homopolymer and/or a propylene-ethylene block copolymer.

Figures 2 to 4 are DSC charts of polypropylene-based resins, which were measured using a DSC6200 manufactured by Hitachi High-Tech Science Corp. The measurement temperature for Figure 2 (polypropylene homopolymer) was held at 30°C for 1 minute, then heated from 30°C to 230°C at 10°C/min, held at 230°C for 5 minutes, cooled to 0°C at 10°C/min, and then heated again from 0°C to 230°C at 10°C/min. Figure 3 (propylene-ethylene block copolymer) and Figure 4 (propylene-ethylene random copolymer) were measured in the same manner as Figure 2, except that the temperature was raised to 250°C.
2 is a DSC chart of the polypropylene homopolymer used in the examples described later, and the endothermic peak in the second heating is sharp, with a peak top at 167.1° C. Thus, when a food container is prepared in which the surface layer (A) is the innermost layer using an inner lining film made of polypropylene homopolymer, and food is placed in the container and heated in a microwave oven, the surface layer (A) does not soften until the food surface reaches 120 to 130° C., and the surface layer (A) does not melt until the food surface approaches 167.1° C.

3 is a DSC chart of the propylene-ethylene block copolymer used in the examples described later. The chart shows that the endothermic peak in the second heating is a very small peak at 115.9° C. due to the ethylene component, but the main peak is a sharp peak topped at 164.6° C. Thus, the surface layer (A) hardly melts until the food surface approaches 164.9° C.
Fig. 4 is a DSC chart of the propylene-ethylene random copolymer. From the chart, the endothermic peak in the second heating is broad with a peak top at 139.0°C. Therefore, it is considered that the surface layer (A) made of the resin starts to melt before the food surface reaches 139.0°C.

The thickness of the surface layer (A) is not particularly limited, but is preferably 2 μm or more, and particularly 3 μm or more, in terms of heat resistance. If the surface layer (A) is too thick, the inner lining film curls and becomes difficult to attach to the foam sheet. Therefore, when the thickness of the surface layer (A) is Ta and the thickness of the film is T, it is preferable that Ta≦0.4T, particularly Ta≦0.3T, and even more preferably Ta≦0.2T.

<Layer (B) in contact with surface layer>
The inner lining film of the present invention has excellent cold resistance (impact resistance in a low-temperature environment) because the layer (B) in contact with the surface layer is made of a polyethylene resin having a density of less than 940 kg/ ^m3 . As the polyethylene resin, conventionally known polyethylene resins such as low-density polyethylene, high-density polyethylene, and linear low-density polyethylene can be blended as necessary. Among them, linear low-density polyethylene having a density of less than 918 kg/ ^m3 , particularly linear low-density polyethylene having a density of less than 910 kg/ ^m3 , is suitable as a resin for forming the layer (B) because of its excellent cold resistance.
If the density of the polyethylene resin is 940 kg/ ^m3 or more, the inner lining film cannot exhibit good cold resistance. Although there is no particular lower limit for the density of the polyethylene resin constituting layer (B), if the density is too low, there is a risk of the impact strength in a room temperature environment decreasing, so that the density is preferably 860 kg/ ^m3 or more, particularly 880 kg/ ^m3 or more.

A propylene-based elastomer can be added to layer (B) up to a maximum of 30% by weight. Adding a propylene-based elastomer improves the cold resistance of the inner lining film and also increases the adhesion between surface layer (A) and layer (B).

The layer (B) improves the cold resistance of the inner lining film, but if the layer (B) is too thin, this effect is poor. If the thickness of the layer (B) is Tb and the thickness of the film is T, it is preferable that Tb ≥ 1/2T, and more preferably Tb ≥ 0.6T, and even more preferably Tb ≥ 0.7T.

<Laminate layer (C)>
Fig. 5 is a schematic cross-sectional view showing another embodiment of the inner lining film of the present invention. The above-mentioned layer (B) has excellent cold resistance, but is poor in handling. In particular, when the layer (B) is made of linear low-density polyethylene having a density of less than 918 kg/ ^m3 , especially linear low-density polyethylene having a density of less than 910 kg/ ^m3 , the inner lining film may be difficult to peel off from the conveying roll, or may not be able to be smoothly unwound after being stored in a roll. In order to solve such problems, it is effective to provide a laminate layer (C) made of linear low-density polyethylene having a higher density than the layer (B), as shown in Fig. 5.
In order to obtain a reliable effect, the thickness of the laminate layer (C) is preferably 1 μm or more, more preferably 2 μm or more, but if it exceeds 10 μm, no improvement in effect is observed.

Similarly to the above-mentioned layer (B), a propylene-based elastomer can be added to the surface layer (A) and laminate layer (C) to improve cold resistance. However, the amount of propylene-based elastomer in the surface layer (A) and laminate layer (C) is preferably 20% by weight or less, and more preferably 10% by weight or less. If more than 20% by weight of elastomer is added, the handling properties of the inner lining film may deteriorate.

The above-mentioned surface layer (A), the layer (B) in contact with the surface layer, and the laminate layer (C) may contain various additives such as lubricants, antiblocking agents, antistatic agents, antifogging agents, and antioxidants, as well as very small amounts of resins other than the above-mentioned resins, to the extent that the object of the present invention is not impeded. Furthermore, the present invention does not prevent the provision of very thin other resin layers between the surface layer (A), the layer (B) in contact with the surface layer, and the laminate layer (C), as long as they do not affect the cold resistance or heat resistance of the film.

[Method of manufacturing food container inner film]
The method for producing the food container lining film of the present invention is not particularly limited, but for example, the above-mentioned resin for the surface layer (A), the resin set for the layer (B) in contact with the surface layer, and optionally the resin for the laminate layer (C) can be fed into separate extruders and extruded from one die by inflation co-extrusion or T-die co-extrusion. As described below, when the lining film is laminated with a foamed sheet and molded into a food container, it is stretched along the mold. Considering the moldability at that time, it is preferable that the lining film is formed by the T-die co-extrusion method without being stretched. If the lining film is stretched during film formation, it is easy for the lining film to peel off from the foamed sheet when it is molded to fit the mold.

[Food containers]
Fig. 6 is a schematic cross-sectional view showing one embodiment of a food container of the present invention. The food container shown in Fig. 6 can be obtained, for example, by laminating the inner lining film 1 of the present invention with a foamed sheet 2 and heat molding the laminate using a mold.
The foamed sheet 2 may be any foamed sheet that has been used in conventional food containers, such as a foamed polypropylene sheet made by foaming polypropylene with a foaming agent, a heat-resistant polystyrene sheet made by foaming heat-resistant polystyrene with a foaming agent, etc. Among them, a foamed polypropylene sheet is more preferable because it has higher heat resistance than a polystyrene foam sheet and therefore has the effect of preventing the container from deforming even when overheated in a microwave oven.

The inner lining film 1 and the foam sheet 2 can be laminated with a conventionally known adhesive such as an olefin-based, polyurethane-based, polyester-based, or ether-based adhesive, or can be laminated by a thermal lamination method.
Furthermore, prior to lamination, printing can be applied to the inner lining film. Although the food container shown in Fig. 6 is composed only of the inner lining film 1 and the foamed sheet 2, if necessary, an outer lining film can be laminated on the outside of the foamed sheet 2, a gas barrier film can be laminated between the inner lining film 1 and the foamed sheet 2, or a coating layer can be provided on the surface layer (A) of the inner lining film 1.

The effects of the present invention will be confirmed by the following examples. The films are evaluated by the following methods.
[Impact strength]
Using a pendulum impact tester (with thermostatic chamber) manufactured by Tester Sangyo Co., Ltd., the pendulum impact strength (J) is measured at ambient temperatures of 23° C. and -30° C. The pendulum used for the measurement has a tip needle of 0.5 inches, and is punched from the surface layer (A) side of the film. In order to prevent damage to food containers, an impact strength of at least 0.8 J is necessary, and it is desirable for it to exceed 1.5 J.
[Surface tackiness]
If the film surface was touched with the hand and felt sticky, it was rated as X, if the film did not feel sticky when touched with the hand but felt heavy when unrolled after storage in a roll, it was rated as △, and if there were no particular problems, it was rated as O. If the film remains tacky on the surface, it is difficult to handle (transport, stack, store, etc.).
[Curl state]
The film was visually inspected, and evaluated as × if curling occurred to the extent that it interfered with use, △ if curling occurred but not to the extent that it interfered with use, and ◯ if no curling occurred. If the inner lining film curls, it becomes difficult to attach it to the foam sheet. The curling occurred so that the laminate layer (C) was on the inner surface.

Details of the resins and films used in the examples and comparative examples are as follows.
[Resin used]
LLDPE1: Linear low density polyethylene (density 904 kg/m ³ , melting point 111° C.)
LLDPE2: Linear low density polyethylene (density 931 kg/m ³ , melting point 123° C.)
LLDPE3: Linear low density polyethylene (density 866 kg/m ³ , melting point 119° C.)
LLDPE4: Linear low density polyethylene (density 944 kg/m ³ , melting point 128° C.)
PP1: Polypropylene homopolymer (density 900 kg/m ³ , melting point 167.1° C. (from the peak top at the second heating in the DSC chart in FIG. 2 ))
PP2: propylene-ethylene book copolymer (density 900 kg/m ³ , melting point 164.6° C. (from the peak top at the second heating in the DSC chart in FIG. 3))
TPO: Propylene-based elastomer (ethylene content 16%, melting point 103°C)
[Film used]
CPP: Polypropylene film (FAK#50) manufactured by Futamura Chemical Co., Ltd.
PBT: Polybutylene terephthalate film (ESRM) manufactured by Okura Kogyo Co., Ltd.

[Example 1]
A linear low density polyethylene (LLDPE1) having a density of 904 kg/ ^m3 , a linear low density polyethylene (LLDPE2) having a density of 931 kg/ ^m3 , and a polypropylene homopolymer (PP1) were molded into a three-layer sheet of 50 μm by a T-die coextrusion method. This is the inner lining film of Example 1. The thickness ratio of each layer is surface layer (A)/layer in contact with surface layer (B)/laminate layer (C) = 15:80:5. The impact strength, surface tackiness, and curl state of the film were measured, and the results are shown in Table 1.

[Examples 2 to 11, Comparative Examples 1 to 4]
The inner lining films of Examples 2 to 11 and Comparative Examples 1 and 2 were obtained in the same manner as in Example 1, except that the resins constituting each layer and the thickness ratio were changed as shown in Table 1. Examples 6 to 8 and Comparative Example 1 are two-layer films not provided with a laminate layer (C), and Comparative Example 2 is a single-layer film. Comparative Examples 3 and 4 are commercially available single-layer films. The impact strength, surface tackiness, and curl state of each film were measured and are shown in Tables 1 to 4.

The films of Examples 1 to 5, in which a linear low-density polyethylene having a density of less than 918 kg/ ^m3 is used for the layer (B) in contact with the surface layer and a linear low-density polyethylene having a density higher than that of the layer (B) is used for the laminate layer (C), all showed excellent impact strength both in a low-temperature environment of -30°C and at room temperature of 23°C. As described above, the peak tops of both PP1 and PP2 at the time of the second heating in the DSC chart exceed 160°C, and all of the films of Examples 1 to 5 have excellent heat resistance. Of the inner lining films of Examples 2 and 5, in which the thickness ratio of the surface layer is 30%, the film of Example 2, which uses a polypropylene homopolymer, showed some curling. In addition, as the thickness ratios of PP1 and PP2 increased, the impact strength at room temperature tended to decrease.

Looking at Examples 6 to 8 and Comparative Example 1, the impact strength at room temperature (23° C.) increases as the density of the layer (B) in contact with the surface layer increases. The impact strength in a low-temperature environment (−30° C.) is highest for a density of 904 kg/m ³ (Example 7).
Further, the films of Examples 6 and 7 in which linear low-density polyethylene having a relatively low density appeared on the film surface showed some surface tackiness.

A comparison of Examples 8, 9, and 10 confirmed that the impact strength in a low-temperature environment (-30°C) improved as the amount of TPO added increased. A comparison of Examples 9 and 11 also demonstrated that the addition of TPO to the surface layer (A) significantly improved low-temperature impact strength.

The film of Comparative Example 2 has excellent impact strength, but the DSC peak top is low at 123°C, and there is a problem with heat resistance. On the other hand, the films of Comparative Examples 3 and 4 both have low impact strength.

The inner lining film of the present invention has excellent low-temperature impact resistance and heat resistance sufficient to withstand use in a microwave oven, making it particularly suitable for forming the inner layer of a food container in which the food contained therein is frozen and then cooked in a microwave oven.

Claims (8)

In a food container inner lining film that forms the inner layer of a food container,
The surface layer (A) is made of a polypropylene-based resin, and the layer (B) in contact with the surface layer is made of a polyethylene-based resin having a density of less than 940 kg/ ^m3 ,
The film for lining a food container is used for food containers in which food is stored in a freezer and then cooked in a microwave oven . 2. The food container inner lining film according to claim 1, wherein the polyethylene resin constituting the layer (B) in contact with the surface layer is a linear low-density polyethylene having a density of less than 918 kg/ ^m3. 3. The food container inner lining film according to claim 1, wherein the polyethylene resin constituting the layer (B) in contact with the surface layer is a linear low-density polyethylene having a density of less than 910 kg/ ^m3. the layer (B) in contact with the surface layer has a laminate layer (C) on the surface opposite to the surface layer (A);
A food container inner lining film as described in any one of claims 1 to 3, characterized in that the laminate layer (C) is made of linear low-density polyethylene having a higher density than the polyethylene resin constituting the layer (B) in contact with the surface layer. The food container inner lining film according to any one of claims 1 to 4, characterized in that Tb ≧ 0.7T, where T is the thickness of the food container inner lining film and Tb is the thickness of the layer (B) in contact with the surface layer. The food container inner lining film according to any one of claims 1 to 5, characterized in that the polypropylene-based resin is a propylene homopolymer and/or a propylene-ethylene block copolymer. The method for producing a food container inner lining film according to any one of claims 1 to 6, characterized in that the film is produced by T-die co-extrusion without stretching. A food container comprising at least a foamed sheet and the food container inner lining film according to any one of claims 1 to 6, characterized in that the surface layer (A) made of a polypropylene-based resin is the innermost layer.

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