JPH0698982B2 - Food container - Google Patents

Description

TECHNICAL FIELD The present invention relates to a food container.

(Prior Art) Styrene-based resins are widely used as food containers because they do not contain harmful components and have good shape stability. In addition, the styrene-based resin has a property of easily foaming, and when foamed, it gives a molded product that is lightweight and has good heat insulating properties. Therefore, the light weight is widely used as a heat insulating container for containing food.

Containers made of a styrene resin foam are roughly classified into two types according to their manufacturing methods. One is via foamed beads, which are placed in a perforated mold and heated by contact with steam to fuse the beads together to form a container. The other one is
It is passed through a foamed polystyrene sheet, which is softened by heating the foamed sheet and brought into close contact with a molding die to form a container.

Many of the containers made through the foamed beads have a large shape, and thus have a large wall thickness. Therefore, the containers made of beads are less likely to be destroyed, and are therefore used for applications such as, for example, putting and transporting fish and shellfish. On the other hand, the container made via the foamed sheet is necessarily thin and therefore small in size for the convenience of manufacturing and handling the sheet. Therefore, the container made from the sheet has a drawback that it is easily broken due to its thin wall thickness. But,
Because of its light weight, good shape stability, and high heat insulation properties, it is widely used as a food container for retail sale, particularly as a container for instant food or frozen food. Containers of this kind must withstand heating.
This is because many instant foods require addition of water to heat them, and frozen foods need to be frozen.

Ovens and microwave ovens have come to be used for heating instant foods and thawing frozen foods. However, the container made of the polystyrene foam described above is deformed when heated in an oven or a microwave oven, and thus such a heating means cannot be adopted. Especially for instant foods, the fact that such a heating means is convenient for use, but not applicable, has been a major drawback.

Therefore, abandoning the production of food containers made of expanded polystyrene, containers made of synthetic resin applied to paper containers and containers made of other resins such as polypropylene were made. However, although some of these containers have excellent heat resistance, they are not suitable for practical use because they have poor heat insulating properties. This is because the container is inferior in heat insulating property, and the surface of the container becomes hot immediately after being heated in the microwave oven, so that the container cannot be held with bare hands. In addition, some resins may be melted or deformed when heated by a microwave oven. So, in the end, it couldn't be suitable for heating in a microwave oven.

JP-A-59-135237 mainly discloses a foam of a polyester resin, but further mentions that a sheet of the foam obtained is molded into a food container. Further, the publication also explains that the food container made of the expanded polyester resin thus produced can be heated in an oven. Indeed, the container obtained by molding the Polyester resin foam sheet shows satisfactory heat resistance and does not deform even if it is put in a small amount of water and heated in an oven for a short time. If a large amount of water is put into the oven and heated in the oven, the container will be deformed. In general, in a resin foam, the cell wall becomes thin and has a non-uniform thickness, but even in the case of polyester resin foam, the cell wall is roughly the same.
It is thought that the thickness is 0.1 to 20 microns, so that water vapor easily permeates. However, the substance is not clear. In any case, there was a drawback that the container was deformed.

Japanese Unexamined Patent Publication No. 62-70037 proposes to use a foamed polystyrene sheet, to which a non-foamed film of a special polyester resin is attached to form a composite sheet, and the composite sheet thus obtained is molded into a container. is doing. This proposal does not specify the thickness ratio between the expanded polystyrene sheet and the non-expanded polyester resin film, but if the non-expanded polyester resin film has a larger thickness than the expanded polystyrene sheet Since the characteristics such as the durability, the cushioning property and the lightness are lost, the non-foamed polyester resin film has to be naturally thin. However, when the thickness of the polyester resin film is small, the ability to block heat and maintain its shape is weak, so that the expanded polystyrene sheet undergoes thermal deformation and eventually the container deforms. Therefore, even with this proposal, the container cannot withstand the heating by the microwave oven.

(Problems to be Solved by the Invention) As described above, until now, a synthetic resin food container having characteristics such as light weight, heat insulation, and impact resistance has not been able to withstand heating by a microwave oven or the like. . Therefore, the inventor of the present invention has attempted to provide a food container that is sufficiently resistant to heating by a microwave oven and the like, is lightweight, has excellent heat insulating properties, and is not easily broken by impact. The present invention has been completed based on such an idea.

(Means for Solving the Problem) As described above, the container formed by molding the foamed sheet of the polyester resin causes deformation when the container is heated in a microwave with a large amount of water. It has been found that when a non-foamed film made of the same polyester resin is stuck on the inner surface of the container, the resulting container does not deform even if a large amount of water is put in it and heated in a microwave oven.
This is because the polyester resin film generally has a uniform thickness and does not have an extremely thin portion such as a bubble wall, and therefore has sufficient power to block water vapor. It was thought that it was because water vapor could not penetrate into the sheet.

The inventor further conducted an experiment and tried to make a composite sheet by laminating a non-foamed film of another resin having poor water vapor permeability on a foamed sheet of polyester resin to form a composite sheet. As other resins, polyolefin resin, polyamide resin, polyvinyl chloride resin, polyacrylonitrile resin, polyvinylidene chloride resin, ethylene-vinyl alcohol copolymer, etc. were used. As a result, when a non-foamed film of these resins is pasted onto a polyester resin foamed sheet, the container obtained from this does not deform even if a large amount of water is put into it and heated in a microwave oven. It was confirmed. The present invention was made based on such confirmation.

(Summary of the Invention) The present invention uses a composite sheet obtained by laminating a non-foamed film of a thermoplastic resin on at least one surface of a foamed sheet of a thermoplastic polyester resin, and a container with the non-foamed film facing the inner liner. The gist is a food container formed into a shape.

(Detailed Description of the Invention) In the present invention, a thermoplastic polyester resin is used as a foamed sheet. The thermoplastic polyester resin is a high molecular weight linear polyester obtained by causing a condensation reaction of an aromatic dicarboxylic acid and a diol. As the aromatic dicarboxylic acid, terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid and the like can be used. Examples of the diol include ethylene glycol, trimethylene glycol, tetramethylene glycol, neopentylene glycol, hexamethylene glycol, cyclohexane dimethylol, tricyclodecane dimethylol, 2,2-bis (4-β-hydroxyethoxyphenyl). )propane,
4,4'-bis (β-hydroxyethoxy) diphenyl sulfone, diethylene glycol and the like can be used.

Among the polyester resins obtained from these dicarboxylic acids and diols, suitable ones are polyethylene terephthalate, polybutylene terephthalate, polybutylene terephthalate elastomer, non-crystalline polyester, polycyclohexane terephthalate and the like. These resins can be used alone or mixed with each other. Further, other resins may be added to these resins for use. When adding another resin, the amount of the other resin needs to be smaller than the amount of the polyester resin.

In this invention, the polyester resin is required to be a foamed sheet. In order to obtain an invention sheet, the above-mentioned polyester resin, in particular, a polyester resin having a high degree of polymerization is put into an extruder, and a foaming agent is contained therein, and the sheet is extruded in the form of a sheet from a die attached to the tip of the extruder, and then air Among them, it is suitable to vaporize the foaming agent to foam. It is suitable that the thickness of the foamed sheet is 0.5 to 5 mm and the expansion ratio is 2 to 20.
When forming a foamed sheet, it is desirable to add a diepoxy compound or a thickener such as an acid anhydride, for example, pyromellitic acid anhydride to the polyester resin. In addition, it is desirable to add a small amount of a substance such as talc powder, which becomes a foam nucleus.

As the foaming agent, a solid compound that decomposes to generate a gas when heated to a temperature higher than the softening temperature of the resin, a liquid that vaporizes at the softening temperature of the resin, a gas that can be dissolved in the resin under pressure, or the like. Can also be used. Examples of the above solid compounds are azodicarbonamide, dinitrosopentamethylenetetramine, hydrazodicarbonamide, sodium bicarbonate and the like. Examples of the above liquid are saturated aliphatic hydrocarbons such as hexane, pentane and butane, saturated alicyclic hydrocarbons such as cyclohexane, aromatic hydrocarbons such as benzene and xylene, methyl chloride, Freon (registered trademark). And halogenated hydrocarbons. Examples of inert gases are carbon dioxide, nitrogen and the like.

In the present invention, a non-foamed film of thermoplastic resin is used. As the thermoplastic resin forming the non-foamed film, the thermoplastic polyester resin forming the foam seal can be used. In addition, various thermoplastic resins having low water vapor permeability can be used. Examples include polyolefin resins, polyamide resins, polyvinyl chloride resins, polyacrylonitrile resins, polyvinylidene chloride resins, and ethylene-vinyl alcohol copolymer resins.

When these resins are formed into a film by an ordinary method such as an extrusion method, all of them have a low water vapor permeability. For example, polyethylene terephthalate film is said to have a water vapor permeability of 0.22 g · mm / m ² · 24 hr (hereinafter, this unit is omitted), and polypropylene film is
The value is 0.084, and the value is 0.00083 for polyvinylidene chloride film. Therefore, when water is put in this as a food container and heated in a microwave oven, the film having a small thickness is sufficient. The thickness of the film is usually 10-500 microns, preferably 20-400 microns.

The non-foamed resin film may be unstretched, uniaxially stretched or biaxially stretched.
Therefore, various methods can be used to make the resin film. For example, it can be rolled by a calender roll or extruded from an extruder to obtain a film. Among them, the method using an extruder is preferable.

When the non-foamed film of thermoplastic resin is attached to the foamed sheet of polyester resin, an adhesive is used if necessary. When the non-foamed film is made of the same kind of thermoplastic polyester resin as the foamed seal, both the film surface and the seal surface are softened, and the softened surfaces are simply opposed to each other and extruded. , No glue is required as it can be glued.

However, when the film is made of a resin having no adhesiveness to the polyester resin, for example, a polypropylene resin, an adhesive is used. An ethylene / vinyl acetate copolymer is suitable as the adhesive. In addition, as the adhesive, acryl-based, urethane-based, ionomer, thermoplastic elastomer, acid-modified polyolefin resin, or the like can be used.

At the time of bonding, the non-foamed film may be bonded as a single layer or may be bonded as a plurality of layers.
The non-foamed film layer may be as thin as about 10 to 500 microns, but the foamed polyester resin sheet layer should be thicker than this, and the thickness is usually within the range of 0.5 to 5 mm, and It is suitable to be within the range of 2 to 500 times the thickness of the film layer.

A suitable method for attaching the non-foamed film to the foamed sheet is a method using a plurality of extruders. More specifically, one extruder is supplied with a thermoplastic polyester resin, from which an expandable polyester resin is extruded, and another extruder is supplied with a thermoplastic resin, from which a non-expandable resin is extruded, Lead these resins to the same one base,
This is a method in which both resins are combined in a die to form a laminated sheet. In addition, the resin extruded from each extruder may be separately formed into a sheet and a film in a separate die, and the sheets and the films may be overlapped and pressed between rolls to be bonded.

Alternatively, the foamed sheet and the non-foamed film may be separately formed, and they may be separately wound into rolls, stacked while being rewound, and passed through a pair of rolls to be bonded. At this time, the roll, in the foamed sheet, in order to prevent the polyester resin from crystallizing, it is good to have a low temperature, for example, normal temperature, while on the other hand, in the non-foamed film, the film surface is softened, It is suitable to maintain high temperature.

The composite sheet produced by pasting is heated again to be formed into a container. For this molding, a molding die is required. The die may be composed of both a convex mold and a concave mold, or may be composed of only one of them. . When using a mold consisting of both molds, it is possible to mold by simply sandwiching and pressing the composite sheet between both molds, but in the case of only one mold, the air between the sheet and the mold is It must be removed or pressed from above the sheet and pressed into the mold. In this case, the non-foamed film is placed so that the inside of the container is on the crocodile.

The heating temperature of the composite sheet at the time of molding into a container is appropriately determined depending on the temperatures of the thermoplastic polyester resin forming the foamed sheet and the thermoplastic resin forming the non-foamed film. Depending on the heating temperature at this time, the foaming rate of the foamed sheet is further increased or decreased, so that even if foamed sheets having the same thickness are used, a difference occurs in the thickness of the composite sheet. Further, depending on the heating temperature and heating time at this time, the crystallinity of the foamed sheet is promoted. This crystallinity is
15% or more is desirable.

The composite sheet thus formed is made into a container by removing a portion not required as a container. This container is the container according to the present invention, in which the non-foamed film is located on the inner surface and the foamed thermoplastic polyester sheet is located on the outer surface.

Although the case where the non-foamed film is bonded to only one surface of the foamed sheet has been described above as an example, the non-foamed film may be bonded to both surfaces of the foamed sheet.

(Effect of the Invention) According to the present invention, since the foamed sheet made of the thermoplastic polyester resin is used, the container is lightweight as a whole, has heat insulating properties, and is not easily broken by impact. Not only does it have much better heat resistance than those using polystyrene resin foam sheets. In addition, since the non-foamed film of thermoplastic resin is attached to one surface of the foamed sheet of thermoplastic polyester resin, and the non-foamed film is positioned toward the inner wall of the container, the non-foamed film has a uniform thickness. Have a
In addition, since it has a thickness sufficient to prevent the permeation of water vapor, the invasion of water vapor into the foam sheet layer is prevented, and accordingly, it is prevented that the foam sheet swells due to the water vapor at the time of heating to cause the deformation of the container. Be done. Thus, according to the present invention, a food container that can withstand heating by a microwave oven or the like can be obtained, and the food container retains the conventional advantages. The present invention has many advantages in this respect.

(Examples) Next, the advantages of the present invention will be specifically described with reference to Examples and Comparative Examples. In the following, "parts" means "parts by weight".

Example 1 In this example, both the foamed sheet and the non-foamed film,
The same kind of thermoplastic polyester resin was used.

As the polyester resin, 100 parts of polyethylene terephthalate pellets (TR8580 manufactured by Teijin Ltd.) with an intrinsic viscosity of 0.81 were used, dried with hot air having a dew point of -20 ° C and a temperature of 160 ° C for 5 hours, and then pyromellitic dianhydride. 0.3 part, 0.1 part of sodium carbonate and 0.6 part of talc were added thereto, and the mixture was uniformly mixed with a tumbler, and then this was put into an extruder.

The extruder has a cylinder temperature of 274-287 ℃ and a die temperature of 2
It was set to 77 ° C. Also, butane as a foaming agent was pressed in at a ratio of about 1.0 part from the middle of the cylinder.

A die with an annular gap is attached to the tip of the extruder, and a polyester resin containing butane is extruded into a cylindrical shape from the annular gap, and while the resin is being foamed, it is advanced over a cylindrical mandrel and foamed. It was a sheet. Then, this was cut open and wound into a roll as a flat sheet. The resulting foamed sheet had a density of 0.262 g / cm ³ and a thickness of
The width was 1.45 mm and the width was 640 mm.

As the non-foamed thermoplastic resin film, a polyethylene terephthalate resin film (FFL manufactured by Teijin Ltd.) with a thickness of 50 μm was used. This film was laminated on the above-mentioned sheet, and sandwiched between a pair of rolls to be laminated.
At this time, the roll was such that the foamed sheet was kept at room temperature and the non-foamed film was kept at 135 ° C. Thus, a composite sheet was obtained in which the non-foamed film was attached to only one surface of the foamed sheet.

Cut out a sample of size 250 x 250 mm from the composite sheet,
This is preheated by contacting it on a heating plate whose surface temperature is 140 ° C for 6 seconds, and then it is sandwiched between a convex mold and a concave mold heated at 180 ° C for 8 seconds to form a container and promote the crystallization. Then, immediately thereafter, it was sandwiched between a convex mold and a concave mold having the same shape at room temperature for 6 seconds and cooled. At that time, the non-foamed film layer was placed on the inside of the container. The shape of the obtained container was as shown in FIG. 1 and FIG. 1 is a side view of the container, and FIG. 2 is a plan view. During this molding, the composite sheet increased the expansion ratio and the thickness of the bottom of the container was 3.80 mm.

After putting 150 cc of water in this container and sealing it with a lid to prevent steam from leaking, the container was put in a microwave oven with an output of 500 W and heated for 3 minutes to boil the water. This container is
Immediately after heating, it could be taken out of the microwave with bare hands. The container did not deform due to this heating.
As a reminder, the thickness of the bottom of the container was measured and found to be 3.95 mm. Therefore, this container was increased in thickness by only 4% by heating in the microwave oven and could be treated as having no deformation.

Example 2 In this example, the foamed sheet obtained in Example 1 was used, the non-foamed film was a polypropylene resin film having a thickness of 100 μm, and ethylene-vinyl acetate copolymer was used as an adhesive for bonding. Resin was used.

First, an ethylene-vinyl acetate copolymer resin was applied to one side of the polypropylene resin film, and the foamed sheet of the polyester resin obtained in Example 1 faced the coated surface, and the film and the sheet were superposed, It pasted between the paired rolls and stuck together. At this time, the foamed sheet is rolled at room temperature and the non-foamed film is rolled at 125
℃ was made.

A sample of 250 × 250 mm was cut out from the composite sheet thus obtained, and the foamed sheet had a heating plate with a surface temperature of 140 ° C.
The non-foamed film was preheated by simultaneously contacting a heating plate at 100 ° C. for 6 seconds. Subsequently, the foamed sheet was brought into contact with the concave mold heated to 140 ° C. and the non-foamed film was contacted with the convex mold heated to 100 ° C., and the composite sheet was sandwiched between both sides for 8 seconds, It was molded into the same container shape as in Example 1 and promoted crystallization. Immediately thereafter, it was sandwiched for 6 seconds between molds at room temperature having the same shape as the above mold, and cooled to obtain a container. At that time, the non-foamed film was made to come to the inside of the container. By this molding, the foamed sheet was swollen, and the container had a bottom thickness of 2.94 mm.

The container thus obtained was filled with water and heated in a microwave oven in the same manner as in Example 1. Immediately after heating, it was confirmed that there was no deformation even after heating by holding this container with bare hands. As a precaution, the thickness of the bottom of the container was measured and found to be 3.05 mm. Since this increase in thickness was only 4%, it was confirmed that it can sufficiently withstand heating by a microwave oven.

Example 3 In this example, the foamed sheet obtained in Example 1 was used, and the non-foamed film was a 150-micron-thick polyethylene terephthalate resin film, which was laminated without an adhesive to obtain a composite sheet. . A pair of rolls were used for bonding, the roll of the foamed sheet was at room temperature, and the roll of the non-foamed film was at 155 ° C.

A sample of 250 × 250 mm was cut out from the composite sheet, and this sample was used as a container in exactly the same manner as in Example 1. This container is
The thickness of the bottom was swollen to 4.39 mm.

Water was put in this container and heated in a microwave oven in the same manner as in Example 1. Immediately after heating, the container could be held with bare hands and removed from the range. The removed container is
It was recognized that it was not deformed. As a precaution, the thickness of the bottom of the container after heating was measured, and it was found that the thickness was 4.44 mm and the deformation rate was as small as 1%.

Example 4 This example was carried out in exactly the same manner as Example 3, except that the polyethylene terephthalate resin film used in Example 3 was replaced with a polybutylene terephthalate resin film having a thickness of 30 μm. The thickness of the bottom of the molded container was 4.20 mm.

After heating in the microwave oven, the container could be taken out with bare hands, and it was confirmed that the taken out container did not change at all compared to before heating. When the thickness of the bottom of the container after trial heating was measured, the thickness of the bottom was 4.34 mm, and the deformation ratio of the thickness was a small value of 3%.

Comparative Example 1 In this comparative example, the non-foamed film was not laminated,
A container was made only from the foamed sheet of the polyester resin obtained in Example 1. More specifically, the foamed sheet of polyester resin obtained in Example 1 was preheated by contacting it with a heating plate at 140 ° C. for 6 seconds, and subsequently, it was placed between a concave mold and a convex mold heated at 180 ° C. The mixture was sandwiched for 2 seconds to accelerate the crystallization, and immediately thereafter, sandwiched between a concave mold and a convex mold having the same shape at room temperature for 6 seconds and cooled to obtain the same-shaped container obtained in Example 1. The container had a bottom wall thickness of 3.39 mm.

Put water in this container, and in exactly the same way as in Example 1,
The container was heated in the microwave. Immediately after heating, the container could be removed from the microwave with bare hands. The container taken out was greatly foamed and had irregularities on the inner surface. When the thickness of the bottom of the container after heating was measured in an attempt, the bottom thickness was 4.04 mm. This is due to the fact that the thickness of the bottom was increased by 19%, which proved the large deformation. Therefore, it was recognized that this container cannot withstand heating in a microwave oven.

Comparative Example 2 In this comparative example, a foamed sheet of polystyrene was used, a non-foamed film of a thermoplastic polyester resin was used, and these ethylene-vinyl acetate copolymer resins were laminated to form a composite sheet. This composite sheet corresponds to that disclosed in JP-A-62-70037.

More specifically, the above copolymer resin was applied to one surface of a non-foamed polyethylene terephthalate resin film having a thickness of 50 μm. This coated surface is then 1m thick with a thickness of 2.4mm
^The film and the sheet were overlapped with each other toward the expanded polystyrene sheet having a weight of 200 g per ² to be a composite sheet. A pair of rolls was used for bonding, and this roll was heated to 150 ° C. and used for bonding.

A 250 × 250 mm sample is cut out from this composite sheet, preheated by contacting this sample on a heating plate at 140 ° C. for 8 seconds, and immediately thereafter, sandwiched between a concave mold and a convex mold at room temperature for 6 seconds to cool. Then, a container having the same shape as that obtained in Example 1 was obtained. In the container, the composite sheet was positioned such that the non-foamed film was on the inside. The container had a bottom wall thickness of 4.2 mm.

Water was put in this container, and this container was heated in the microwave oven in the same manner as in Example 1. The container could be taken out of the microwave with bare hands immediately after heating, but the container was considerably deformed by heating. In particular, as a result of the polystyrene foam layer of the outer shell swelling and deformed, even the non-foamed film of the inner shell had irregularities.
When the thickness of the bottom of the container after heating was measured in an attempt, the bottom thickness was 4.88 mm, and the result was that the thickness increased by 16%. Therefore, this container was found to be unsuitable for heating in a microwave oven.

[Brief description of drawings]

FIG. 1 is a side view of the containers obtained in Examples and Comparative Examples of the present invention, and FIG. 2 is a plan view of the same container.

Claims (2)

[Claims] 1. A composite sheet obtained by laminating a non-foaming film of a thermoplastic resin on at least one side of a foaming sheet of a thermoplastic polyester resin, and molding the non-foaming film into a container shape with the inner foam facing the inner line. Food container. 2. The non-foamed film is made of a thermoplastic polyester resin, a polyolefin resin, a polyamide resin, a polyvinyl chloride resin, a polyacrylonitrile resin, a polyvinylidene chloride resin, or an ethylene-vinyl alcohol copolymer. The food container according to claim 1, wherein the food container is provided.