JP2014079985A - Multilayer sheet for thermoforming and container for microwave oven heating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multilayer sheet for thermoforming capable of thermoforming a container for microwave oven heating which is excellent in heat resistance, heat insulating properties and oil resistance and suppresses the generation of a blister when heated by a microwave oven and to provide a container for microwave oven heating obtained from the multilayer sheet for thermoforming.SOLUTION: There is provided a multilayer sheet for thermoforming having a specific thickness which is formed by laminating and adhering a polyolefin-based resin film layer through a rubber-modified polystyrene resin layer laminated and adhered by extrusion lamination on one surface of a foamed sheet using a styrene-(meth)acrylic acid copolymer as the base resin. The rubber-modified polystyrene resin layer is constituted of a rubber-modified polystyrene having a rubber particle diameter of 1.5 μm or less or a rubber-modified polystyrene having a rubber particle diameter of exceeding 1.5 μm and 2.5 μm or less and a gel content of 28% or more, the apparent density of the foamed sheet is in a specific range and the basis weight of the rubber-modified polystyrene resin layer is in a specific range.

Description

  TECHNICAL FIELD The present invention relates to a thermoforming multilayer sheet and a microwave heating container obtained by thermoforming the thermoforming multilayer sheet.

  In recent years, microwave ovens are used to heat cooked foods at convenience stores and the like. Generally, a container for heating a microwave oven is required to have heat resistance and heat insulation. As a container having excellent heat resistance and heat insulation, heat-resistant polystyrene having a Vicat softening temperature of 110 ° C. or higher such as a styrene-methacrylic acid copolymer is used. A container formed by thermoforming a foam sheet as a base resin is used. Here, since some foods contain oil, such food containers are required to have oil resistance in addition to these characteristics. As such a container, for example, as described in Patent Document 1, there is one obtained by thermoforming a laminated sheet in which a polypropylene film is laminated and bonded to a heat-resistant polystyrene foam sheet through an adhesive such as ethyl acetate. .

  Recently, a heat-resistant polystyrene foam sheet is laminated with a resin layer made of rubber-modified polystyrene as a base resin by extrusion lamination, and a laminated foam sheet in which a polyolefin film such as polypropylene is laminated on the resin layer surface is thermoformed. However, containers with higher heat resistance are used.

Japanese Patent Laid-Open No. 10-86924

  However, even in the case of a container formed by thermoforming a laminated foam sheet in which a polypropylene film is laminated through the resin layer, particularly when heated in a microwave oven containing spaghetti or curry rice with a lot of oil, depending on the heating conditions The phenomenon that the surface of the container is locally heated and the resin layer and the polyolefin film are partially peeled from the foamed layer and swelled with the tertiary foaming of the foamed layer of the container (hereinafter referred to as “blister”). ) May occur. Therefore, there is a demand for a solution to the generation of blisters when a container containing food containing a lot of oil is heated in a microwave oven.

  The present invention solves the above-mentioned conventional problems, and is capable of thermoforming a container for heating a microwave oven that is excellent in heat resistance and heat insulation and oil resistance, and that suppresses the generation of blisters when heated in a microwave oven. It is an object of the present invention to provide a thermoforming multilayer sheet and a microwave heating container obtained from the thermoforming multilayer sheet.

According to this invention, the multilayer sheet | seat for thermoforming shown below and the container for microwave oven heating are provided.
[1] A polyolefin-based resin film layer is laminated and adhered to one side of a foamed sheet using a styrene- (meth) acrylic acid copolymer as a base resin via a rubber-modified polystyrene resin layer laminated and adhered by extrusion lamination. A multilayer sheet for thermoforming having a thickness of 0.5 to 3 mm,
The rubber-modified polystyrene resin layer is composed of rubber-modified polystyrene having a rubber particle size of 1.5 μm or less, or rubber-modified polystyrene having a rubber particle size of more than 1.5 μm and 2.5 μm or less and a gel content of 28% or more. And an apparent density of the foam sheet is 130 to 260 kg / m ³ , and a basis weight of the rubber-modified polystyrene resin layer is 70 to 200 g / m ² .
[2] A microwave heating container formed by thermoforming the thermoforming multilayer sheet as described in 1 above.

The multilayer sheet for thermoforming of the present invention is a specific sheet having a basis weight of 70 to 200 g / m ^{2 on} one side of a foam sheet having a specific range of apparent density styrene- (meth) acrylic acid copolymer as a base resin. A resin layer composed of rubber-modified polystyrene is laminated and adhered by extrusion lamination, and a polyolefin-based resin film layer is laminated and adhered to the surface thereof, so that the foam sheet has an excellent balance between heat resistance and heat insulation. At the same time, because it has excellent adhesion to the rubber-modified polystyrene resin layer, the container obtained by thermoforming the thermoforming multilayer sheet is heated in a microwave oven with ingredients containing a lot of oil such as curry and demiglace sauce. Even if it is done, generation | occurrence | production of a blister can be prevented effectively.

Hereinafter, the multilayer sheet for thermoforming according to the present invention will be described in detail.
The thermoforming multilayer sheet of the present invention (hereinafter also simply referred to as “multilayer sheet”) is a foamed sheet (hereinafter also simply referred to as “foamed sheet”) using a specific polystyrene-based resin as a base resin. A rubber-modified polystyrene resin layer (hereinafter also simply referred to as “resin layer”) laminated and adhered to one side of a foam sheet, and a polyolefin resin film layer (hereinafter simply referred to as “film”) laminated and adhered to the outer surface of the resin layer. A layer ”).

The foam sheet in the present invention uses a styrene- (meth) acrylic acid copolymer as a base resin.
In the present specification, styrene-methacrylic copolymer acid, styrene-acrylic acid copolymer, and mixtures thereof are collectively referred to as styrene- (meth) acrylic acid copolymer.

  The content of the (meth) acrylic acid component in the styrene- (meth) acrylic acid copolymer is about 5 to 25% by weight in the copolymer. The copolymer component to be copolymerized with styrene may be a mixture of methacrylic acid and acrylic acid. Further, in order to improve moldability and the like, a small amount of (meth) acrylic acid alkyl ester such as methyl methacrylate may be copolymerized as a third component.

In the present invention, the styrene- (meth) acrylic acid copolymer may be blended with a polystyrene resin other than the styrene- (meth) acrylic acid copolymer. Examples of the polystyrene resin include polystyrene, rubber-modified polystyrene (impact polystyrene), styrene-acrylonitrile copolymer, styrene-methyl methacrylate copolymer and the like.
The blending amount of the polystyrene resin is preferably 20 parts by weight or less, more preferably 17 parts by weight or less, further preferably 10 parts by weight or less, particularly 100 parts by weight of the styrene- (meth) acrylic acid copolymer. The amount is preferably 5 parts by weight or less.

  A styrene elastomer can be added to the styrene- (meth) acrylic acid copolymer. That is, since styrene- (meth) acrylic acid copolymer has strong brittleness, impact resistance can be improved by adding a styrene elastomer. Specific examples of the styrenic elastomer include styrene-butadiene block copolymers, styrene-isoprene block copolymers, and complete or partial hydrogenated products thereof. The addition amount of the polystyrene-based elastomer is preferably 10 parts by weight or less, more preferably 7 parts by weight or less with respect to 100 parts by weight of the styrene- (meth) acrylic acid copolymer.

In the multilayer sheet of the present invention, a rubber-modified polystyrene resin layer is laminated and adhered to the foamed sheet by extrusion lamination.
The rubber-modified polystyrene resin is usually obtained by polymerizing a styrene monomer in the presence of a rubber-like polymer such as polybutadiene. The rubber-like polymer is grafted with a styrene-based polymer, and a dispersed phase is obtained. It is composed of rubber-like polymer particles that form a styrene polymer that forms a continuous phase.

  As the styrene monomer constituting the rubber-modified polystyrene resin, known ones such as styrene, α-methyl styrene, o-methyl styrene, m-methyl styrene, and p-methyl styrene can be used. is there. Styrenic monomers can be used alone or as a mixture.

  The rubber-like polymer constituting the rubber-modified polystyrene resin includes polybutadiene, styrene-butadiene random or block copolymer, polyisoprene, styrene-isoprene random or block copolymer, ethylene-propylene rubber, ethylene-propylene. -A diene rubber etc. are mentioned, Especially the random or block copolymer of a polybutadiene and a styrene-butadiene is used suitably.

In the present invention, the rubber-modified polystyrene resin constituting the rubber-modified polystyrene resin layer to be laminated and adhered to the foamed sheet having a styrene- (meth) acrylic acid copolymer as a base resin is specified among the rubber-modified polystyrene resins. Is used.
That is, the present inventors laminated a rubber-modified polystyrene resin layer by extrusion lamination on a foamed sheet using a styrene- (meth) acrylic acid copolymer as a base resin. Even if they are bonded, the adhesive force may decrease between the foam sheet and the resin layer after a long period of time after lamination, and this decrease in adhesive force is one of the causes of blistering. I found it. Furthermore, the present inventors have found that if a specific rubber-modified polystyrene resin is used, sufficient adhesive strength can be maintained and blistering can be prevented.

  The rubber-modified polystyrene resin used in the present invention has a rubber particle size of 1.5 μm or less. The multilayer sheet in which the resin layer is formed of this has high adhesive strength between the foamed sheet and the resin layer, and the high adhesive strength is maintained even after a long time after lamination, so a container obtained from the multilayer sheet Is difficult to generate blisters when heated in a microwave oven.

  The reason for this is that if the rubber particle size is 1.5 μm or less, the unevenness of the surface of the resin layer at the time of lamination becomes small, and sufficient adhesive strength is developed and maintained in order for the resin layer and the foamed sheet to be sufficiently adhered It is inferred that The lower limit of the rubber particle size is approximately 0.1 μm, and preferably 0.2 μm from the viewpoint of impact resistance.

  The gel content of the rubber-modified polystyrene resin having a rubber particle size of 1.5 μm or less is preferably 15% or more. If the gel content is too low, the desired impact resistance may not be obtained. From this viewpoint, the gel content of the rubber-modified polystyrene resin having a rubber particle size of 1.5 μm or less is more preferably 18% or more. The upper limit of the gel content is approximately 35%, and is preferably 33% from the viewpoint of the appearance of the resin layer.

In the present invention, a rubber-modified polystyrene resin having a rubber particle size of 2.5 μm or less and a gel content of 28% or more is used even if the rubber particle size exceeds 1.5 μm. Despite the fact that the rubber particle size is large to some extent, this is because the gel content is high, so there is little slip at the interface between the heat-resistant foam sheet and the resin layer, and the adhesiveness is high. It is presumed that high adhesive strength is obtained as in the following, and that the adhesive strength is maintained.
However, if the rubber particle size exceeds 2.5 μm, even if the gel content is 28% or more, the adhesive strength decreases with time, so it is difficult to prevent the generation of blisters.
The upper limit of the gel content is approximately 35%, and is preferably 33% from the viewpoint of the appearance of the resin layer.

The rubber particle size in this specification is a value calculated from the following equation (1) by measuring the particle size of 500 dispersed rubber particles in a transmission electron micrograph of an ultrathin section of rubber-modified polystyrene resin. In the case where the rubber particles are oriented, the average value of the minor axis and the major axis is taken as the particle size.
Rubber particle size = ΣniDi ² / ΣniDi (1)
In the above formula (1), ni is the number of rubber particles having a particle diameter Di.

In the present specification, the gel content is measured as follows.
First, weigh accurately about 1.5 g of the sample, put it into a 100 mL conical flask with a stopper, add 30 mL of methyl ethyl ketone (MEK), leave it for a whole day and night, and confirm that the sample is dissolved in MEK for 10 minutes. Shake. This is put into a precisely weighed centrifuge tube and centrifuged at 4000 rpm for 40 minutes. The supernatant liquid centrifuged by decantation is discarded, and the inner wall of the centrifuge tube is washed with a small amount of MEK. The centrifuge tube is pre-dried in a fume hood for 1 day, and then dried in a vacuum dryer at 70 ° C. for 15 hours or more. After drying, after cooling to normal temperature in a desiccator, the centrifuge tube is precisely weighed, and the gel content is determined by the following equation (2). When measuring the gel content of the resin layer, use a sample obtained by cutting out the resin layer from the multilayer sheet as a sample. When measuring the gel content of the rubber-modified polystyrene resin raw material, Use pellets.

Gel content (% by weight) = (ba) / S × 100 (2)
Where S: weight of sample, a: weight of centrifuge tube, b: total weight of dried gel and centrifuge tube.

  The melt flow rate (MFR) of the rubber-modified polystyrene resin is preferably 1 to 15 g / 10 minutes, more preferably 2 to 10 g / 10 minutes. If the melt flow rate is within the above range, the resin layer can be easily formed on the foamed sheet with sufficient adhesive strength by extrusion lamination.

  The melt flow rate of the rubber-modified polystyrene resin is a value measured under the condition H (test temperature 200 ° C., nominal load 5.00 kg) in Table 1 based on JIS K7210 (1999).

  In the resin layer, various additives such as an antioxidant, a heat stabilizer, an inorganic filler, a colorant and the like can be added as necessary.

In the multilayer sheet of the present invention, a polyolefin resin film layer is laminated and bonded to the rubber-modified polystyrene resin layer. Examples of the polyolefin resin constituting the film layer include polyethylene resins such as high-density polyethylene and linear low-density polyethylene, polypropylene such as polypropylene homopolymer, propylene-ethylene block copolymer, and propylene-ethylene random copolymer. Based resins and the like. In these, the polypropylene resin which is excellent in oil resistance and heat resistance is preferable, and in it, since it is excellent in thermoformability, an ethylene-propylene random copolymer is preferable.
By providing the film layer, the multilayer sheet of the present invention is excellent in oil resistance, and the container obtained by thermoforming has a foamed layer that is oily when heated by a microwave oven while containing food containing oil. Can prevent dissolution.

  As a method for forming the film layer, a polyolefin resin film provided with an adhesive layer in advance can be laminated and adhered on the resin layer by thermal lamination, or a polyolefin provided with an adhesive layer in advance during extrusion lamination of the resin layer. Laminate adhesion by laminating a resin-based resin film over a resin layer and passing between a pair of rolls, or laminating and bonding by co-extrusion of an adhesive layer and a polyolefin-based resin layer onto the resin layer The three layers of the rubber-modified polystyrene resin layer, the adhesive layer, and the polyolefin resin film layer can be coextruded and laminated on the foamed sheet by laminating and bonding.

  In the case of thermal lamination, a polyolefin-based resin film provided with an adhesive layer in advance is used, or a polystyrene-based resin / polyolefin-based resin multilayer film that is laminated and adhered in advance by dry lamination via an adhesive layer is used. As the film, a stretched film or an unstretched film can be used, but an unstretched film is preferably used because of excellent thermoformability.

  As the adhesive layer, any material can be used as long as the rubber-modified polystyrene resin layer and the polyolefin resin film can be bonded and laminated. For example, an acrylic adhesive, a urethane adhesive, ethyl acetate, or the like can be used. A known adhesive or a conventionally known adhesive resin such as ethylene-vinyl acetate is used. In the case where three layers of a resin layer, an adhesive layer, and a polyolefin resin film layer are coextruded, a mixed resin of a polystyrene resin and a polyolefin resin can also be used as the adhesive layer.

The basis weight of the film layer, from the viewpoint of achieving both thermoformability and oil resistance imparting, preferably 10 to 50 g / ^{m 2,} more preferably from 20 to 30 g / ^{m 2.}

In the present invention, the apparent density of the foam sheet is 130 to 260 kg / m ³ . Although the foam sheet constituting the multilayer sheet of the present invention has improved adhesiveness with a resin layer comprising a rubber-modified polystyrene resin as a base resin, in order to effectively prevent the generation of blisters, the foam sheet The apparent density must be within the above range. If the apparent density is too small, the heat resistance of the resulting container is reduced, it is easily deformed when heated, and a relatively large amount of foaming agent remains in the bubbles. Even if the adhesive strength is sufficient, bubbles in the foam layer of the container are likely to swell during microwave heating, and blisters are likely to occur. On the other hand, if the apparent density is too high, the heat insulating property is lowered, and if the microwave oven is heated, the entire container may be too warm and deformed. From this point of view,該見seat density is preferably ^{135~255kg / m 3, 140~250kg / m} 3 and more preferably.

  In this specification, the apparent density of the foam sheet is determined by cutting out a measurement sample of 10 cm in the extrusion direction × full width from the multilayer sheet, cutting out only the foam sheet from the measurement sample, and calculating the weight of the cut foam sheet as the outer dimension of the foam sheet. It can be obtained by dividing by the volume obtained from

The basis weight of the foamed sheet is preferably 100 to 400 g / m ² . If the basis weight is too small, the heat resistance of the resulting container is lowered and there is a risk of deformation when heated. On the other hand, if the basis weight is too large, the cost may increase. From this viewpoint, the basis weight is more preferably 150 to 380 g / m ² .

In the present invention, the rubber-modified polystyrene resin layer has a basis weight of 70 to 200 g / m ² . If the basis weight is too small, there is a possibility that generation of blisters cannot be prevented. On the other hand, if the basis weight is too large, there is a possibility that the cooling time at the time of thermoforming becomes longer and the productivity is lowered, or depending on the container shape, it cannot be thermoformed as desired. From this viewpoint, the basis weight is preferably 80 to 190 g / m ² , more preferably 90 to 180 g / m ² .

  The thickness of the multilayer sheet of the present invention is 0.5 to 3 mm. If the thickness is too thin, there is a risk that the heat resistance and mechanical strength such as rigidity of the resulting container will decrease. If the thickness is too thick, thermoforming may be reduced. From this viewpoint, the thickness of the multilayer sheet is preferably 0.8 to 2.7 mm, more preferably 0.9 to 2.5 mm.

  In the multilayer sheet of the present invention, the closed cell ratio is preferably 60% or more. If the closed cell ratio is low, the rigidity of the container obtained by thermoforming may be lowered. From this viewpoint, it is preferably 70% or more, more preferably 80% or more, and particularly preferably 90% or more. Usually, when the closed cell ratio is high, bubbles are liable to be generated due to the tertiary foaming during heating. However, since the multilayer sheet of the present invention has a specific configuration, blisters can be effectively suppressed.

  The closed cell ratio in this specification is the true volume Vx of the multilayer sheet (cut sample) measured using the air comparison type hydrometer 930 type of Toshiba Beckman Co., Ltd. according to the procedure C of ASTM-D2856-70. Using, the closed cell ratio S (%) is calculated by the following formula (3), and the arithmetic average value of N = 3 is obtained. For the measurement, a plurality of cut samples of 25 mm × 25 mm × “sheet thickness” randomly cut from the multilayer sheet are used so as to have a thickness of about 20 mm.

      S (%) = (Vx−W / ρ) × 100 / (Va−W / ρ) (3)

Vx: the true volume (cm ³ ) of the cut sample measured by the above method, which corresponds to the sum of the volume of the resin constituting the multilayer sheet and the total cell volume of the closed cell portion in the cut sample.
Va: Apparent volume of the cut sample calculated from the outer dimensions of the cut sample used for measurement (cm ³ )
W: Total weight of cut sample used for measurement (g)
ρ: Density of resin constituting the laminated sheet (g / cm ³ )

In order to prevent the generation of blisters, it is preferable that the adhesive strength between the foamed sheet and the resin layer is strong. The adhesive strength can be evaluated by determining the peel strength in a 90 degree peel test. Specifically, in a 90-degree peel test (JIS Z0237, peel rate condition 400 mm / min) performed by grasping the foam sheet on the one hand and the resin layer and the film layer on the other hand, the peel strength is 300 gf / 25 mm or more. More preferably, it is 350 gf / 25 mm or more, More preferably, it is 400 gf / 25 mm or more, Most preferably, it is 500 gf / 25 mm or more.
The upper limit of the peel strength is about 1000 gf / 25 mm.

In the multilayer sheet of the present invention, as described above, the film layer is laminated on one surface of the foamed sheet via the resin layer. On the other hand, on the other side, unstretched polystyrene resin film, stretched polystyrene resin film, unstretched polyethylene resin film, stretched polyethylene resin film, unstretched polypropylene resin film, stretched polypropylene resin, unstretched polyester Resin films, stretched polyester resin films, and the like can be bonded and laminated by thermal lamination, and the design can be improved by printing these films.
In addition, as for the thickness of these films, 10-50 micrometers is preferable, More preferably, it is 20-30 micrometers.

In the multilayer sheet of the present invention, the total lamination amount of the resin layer, the film layer, and the adhesive layer laminated on the foam sheet is preferably 90 g / m ² or more per one atmospheric pressure of the foam sheet. More preferably, it is 100 g / m ² or more. When the total amount of lamination per internal pressure is within the above range, expansion due to the third foaming of bubbles can be effectively suppressed during heating, and blistering can be easily prevented. In addition, the upper limit of the total stacking amount per one atmospheric pressure is approximately 200 g / m ² .

The internal pressure of the foam sheet is determined as the sum of the partial pressures of air in the bubbles and the remaining foaming agent. The volume of bubbles in the foam sheet can be obtained by subtracting the volume of the resin constituting the foam sheet from the total volume of the foam sheet.
The internal pressure of air is measured as follows.
A sample having a length of 100 mm × width of 100 mm × sheet thickness is cut out from the center of the multilayer sheet. Next, the sample cut into an appropriate size is put into a container filled with ethanol, and the air in the container is discharged. Next, toluene is put in the container so that air is not mixed, the sample is dissolved in toluene, the volume of air in the bubbles at 20 ° C. is measured, and the partial pressure of air in the bubbles is obtained. If hydrocarbon compounds such as butane used as the foaming agent for the foam sheet remain in the sample, dissolve the sample using a sufficient amount of toluene to completely dissolve the hydrocarbon compound. I will do it.

  The internal pressure of the blowing agent is measured as follows. First, the residual foaming agent amount of the foam sheet is measured. Assuming that all of the foaming agent is present in the bubbles, the partial pressure of the foaming agent in the bubbles at 20 ° C. is calculated from the obtained measurement value using the gas equation of state pV = nRT. When a hydrocarbon compound such as butane is used as the foaming agent, the foaming agent is partially dissolved in the base resin constituting the foamed sheet, but most of it is in the resin by heating during thermoforming. Therefore, in the state of the container, the foaming agent is present in the bubbles. In addition, the foaming agent partially dissolves in the resin in the container as the foaming agent partially permeates through the gas. However, the foaming agent again moves into the bubbles by heating the microwave oven. Therefore, as described above, in consideration of the tertiary foaming force of the bubbles, it is desirable to estimate the internal pressure on the assumption that all of the foaming agent is present in the bubbles.

  The foaming agent amount is a value measured by an internal standard method using a gas chromatograph. Specifically, an appropriate amount of a sample is cut out from the foamed sheet of the multilayer sheet, put in a sample bottle with a lid containing an amount of toluene and an internal standard substance capable of completely dissolving the sample, and then fully closed. The content of the foaming agent in the foamed sheet is determined by conducting gas chromatographic analysis using a solution obtained by stirring and dissolving the foaming agent in the foamed plate in toluene.

The multilayer sheet of the present invention is preferably used for thermoforming containers having a development ratio of 1.1 to 2.0. In that case, it is preferable that the total lamination amount of the resin layer, the film layer, and the adhesive layer is a desired expansion ratio of the container × 80 g / m ² or more.

  The expansion ratio is (B) and (A) where the area of the multilayer sheet in the molded part is (A) and the area after molding of the part corresponding to the area (A) of the molded part is (B). Ratio: It is calculated | required by (B) / (A).

Next, the preferable example of the manufacturing method of the multilayer sheet for thermoforming of this invention is demonstrated.
The multilayer sheet of the present invention is prepared by, for example, producing a foamed sheet in advance, then laminating and adhering a rubber-modified polystyrene resin layer on one side of the foamed sheet by extrusion lamination, and then laminating a film layer on the resin layer by thermal lamination. Manufactured by bonding. For the production of foam sheets, the lamination adhesion of resin layers, and the lamination adhesion of film layers can be performed on the same line, each can be performed on a separate line, any two can be on the same line It is also possible to adopt the method performed in

  Moreover, while extruding and laminating a rubber-modified polystyrene resin layer on one side of a prefabricated foam sheet, the resin layer and the film layer are passed through a pair of rolls by overlaying a polyolefin resin film on the resin layer. It can also be laminated and adhered. Furthermore, the resin layer and the film layer can be laminated and bonded to the foamed sheet by co-extrusion laminating three layers of a rubber-modified polystyrene resin layer, an adhesive layer, and a polyolefin resin film layer on one surface of the foamed sheet.

The foam sheet is manufactured as follows.
Foam sheet containing the above-mentioned styrene- (meth) acrylic acid copolymer, polystyrene resin other than styrene- (meth) acrylic acid copolymer added as necessary, styrene elastomer, bubble regulator such as talc, etc. It is supplied to a forming extruder, heated and melted and kneaded, and then a physical foaming agent is press-fitted, and further kneaded and adjusted to a foamable resin temperature to obtain a resin melt for forming a foamed sheet.

The foamed sheet forming resin melt is introduced into an annular die and extruded and foamed into the body to form a cylindrical foam, and then the cylindrical foam is cut open while being taken along a cylindrical cooling device. Let it be a sheet-like foam sheet.
When a foam sheet is manufactured using an annular die, a wide foam sheet having a width of 1000 mm or more can be easily manufactured while suppressing the generation of a corrugated pattern called a corrugate.

  Examples of the foaming agent used to form the foam sheet include saturated hydrocarbons having 3 to 5 carbon atoms, physical foaming agents such as aliphatic ethers, aliphatic alcohols, carbon dioxide, nitrogen and water having 2 to 4 carbon atoms, carbonic acid Chemical blowing agents such as sodium hydrogen can be used. Among these foaming agents, from the viewpoint of foamability and thermoformability, the saturated hydrocarbon compound having 3 to 5 carbon atoms is preferably used, and the injection amount thereof is the carbonization in the finally obtained foamed sheet. It is preferable that the content of the hydrogen compound is 3.5% by weight or less (excluding 0).

When a film layer is formed on the resin layer by thermal lamination, or when the resin layer is extrusion laminated and the film is superimposed on the resin layer to form a film layer on the resin layer, an adhesive layer is provided in advance. A polyolefin resin film is used, or a polystyrene resin / polyolefin resin multilayer film is used.
A conventionally well-known technique can be used for the apparatus used for the method of extrusion lamination and a thermal lamination, and extrusion lamination and a thermal lamination.

  When a film layer, or a resin layer and a film layer are formed by coextrusion lamination, the polyolefin resin is supplied to an extruder for film layer formation, heated and melted and kneaded to obtain a resin melt for film layer formation, and the adhesion When the agent is supplied to an adhesive layer forming extruder, heated and melted and kneaded to form an adhesive layer forming resin melt, and simultaneously forming a resin layer, the rubber-modified polystyrene resin is added to the resin layer forming extruder. Supply, heat melt and knead to obtain resin melts for resin layer formation, supply each layer formation resin melt to a co-extrusion T-die, laminate and extrude, on the resin layer or foam sheet Laminate and adhere to. A conventionally well-known technique can be used for the method of coextrusion lamination and the apparatus used for coextrusion lamination.

  If necessary, an unstretched polystyrene-based resin film or the like can be laminated on the opposite surface of the foam sheet to which the resin layer and the film layer are bonded by thermal lamination. In this case, the film layer and the resin layer can be laminated and then laminated, or the opposite film can be laminated first and then the film layer and the resin layer can be laminated.

The microwave oven heating container of the present invention is obtained by thermoforming the thermoforming multilayer sheet, and is suitably used for microwave oven heating.
Thermoforming methods include vacuum forming and pressure forming, as well as free drawing forming, plug and ridge forming, ridge forming, matched mold forming, straight forming, drape forming, reverse draw forming, air slip forming, Examples thereof include plug-assist molding, plug-assist reverse draw molding, and a molding method combining these.
Of these, matched mold forming is preferred because it can make the stacked height of the obtained containers uniform and can thermo-form containers having complicated shapes such as inner fitting containers.

In the container of the present invention, the total lamination amount of the resin layer, the film layer, and the adhesive layer laminated on the foam layer of the container is preferably 50 g / m ² or more, and the multilayer sheet is most stretched Also in the thin wall portion such as the side surface, the total lamination amount is preferably 50 g / m ² or more. If the total basis weight is too small, it becomes difficult to suppress the expansion force of the bubbles during thermoforming, and blisters are likely to occur. From the viewpoint of preventing the generation of blisters, the total amount of lamination is preferably 80 g / m ² or more even in the thin portion.

Further, in the container of the present invention, the total amount of lamination is preferably 50 g / m ² or more, more preferably 60 g / m ² or more per one atmospheric pressure of the foam sheet. When the basis weight per internal pressure of the atmospheric pressure is within the above range, expansion due to tertiary foaming of bubbles can be effectively suppressed during heating, and blistering can be more easily prevented.
Like the internal pressure of the foam sheet, the internal pressure is the sum of the air partial pressure in the bubbles and the partial pressure determined from the amount of residual foaming agent, and each internal pressure is determined in the same manner as in the multilayer sheet.

  The expansion ratio of the container of the present invention is preferably 1.1 to 2.0, more preferably 1.2 to 1.8.

  The container of the present invention contains food containing oil, and even when heated in a microwave oven or the like, partial peeling between the foamed sheet and the resin layer and blistering (blister) are unlikely to occur. It is expensive.

  Hereinafter, the multilayer sheet of the present invention will be described in detail with reference to examples. However, the present invention is not limited to the examples.

  In Examples and Comparative Examples, as a foam sheet forming apparatus, a tandem extruder for forming foam sheets in which a single-screw first extruder having an inner diameter of 115 mm and a second single-screw extruder having an inner diameter of 150 mm are connected in series. An extrusion apparatus in which an annular die (lip diameter: 180 mm, gap: 0.4 mm) was attached to the outlet of the second extruder was used.

  Table 1 shows styrene- (meth) acrylic acid copolymers and polystyrene used in Examples and Comparative Examples, Table 2 shows styrene elastomers, and Table 3 shows rubber-modified polystyrene resins.

Example 1
The first raw material of the tandem extruder is the resin raw material having the composition shown in Table 4 and the talc (high filler # 12 manufactured by Matsumura Sangyo Co., Ltd.) having the compounding amount shown in Table 4 with respect to 100 parts by weight of the resin raw material as a bubble regulator , Heated and melted and kneaded, and then mixed butane of normal butane / isobutane = 35/65 (weight ratio) (shown as “Bu” in Table 4) with respect to 100 parts by weight of the resin raw material. Then press-fit to the amount shown in Table 4, further knead, transfer to the second extruder and adjust to the resin temperature ° C. shown in Table 4 to form a foamed sheet-forming resin melt, forming the foamed sheet The resin melt is extruded from the annular die into the atmosphere at a discharge rate of 300 kg / hr, taken along the outer surface of a cooling cylinder (mandrel) having an outer diameter of 670 mm at the speed shown in Table 4, and further along the extrusion direction. And cut it into two pieces (Width 1050mm) was prepared.
In Example 3, the mixed foaming agent of the above mixed butane / dimethyl ether = 50/50 (weight ratio) was used as the foaming agent.

  Next, using an extrusion laminating apparatus with a T-die attached to the outlet of the extruder, the rubber-modified polystyrene resin shown in Table 4 is supplied to the extruder, heated and melted and kneaded to obtain a resin melt for forming a resin layer. The resin melt for forming the resin layer is extruded from a T-die and laminated on the foamed sheet, and the film shown in Table 4 is overlapped on the resin layer with the polyolefin surface being the outer surface, between a pair of rolls. 3 were laminated and bonded to each other to produce a multilayer sheet (foamed sheet / resin layer / film layer) having a resin layer and a film layer having a basis weight shown in Table 4. At this time, the extrusion temperature of the resin melt for forming a resin layer was 250 ° C., and the line speed was 15 m / min.

In Example 5, three extruders were provided, and the rubber-modified polystyrene was bonded to the extruder for resin layer formation using an extrusion laminating apparatus in which the outlet of each extruder was coupled to a T-die for coextrusion. A mixture of 65% by weight of rubber-modified polystyrene and 35% by weight of polypropylene is supplied to an extruder for forming a layer, and polypropylene is supplied to an extruder for forming a film layer and melt-kneaded. , A mixture of rubber-modified polystyrene and polypropylene, laminated in the order of polypropylene, and coextruded so that the basis weights are 150 g / m ² , 30 g / m ² , and 30 g / m ² , respectively. By extrusion lamination, a multilayer sheet (foamed sheet / resin layer / film layer) having a resin layer and a film layer having a basis weight shown in Table 4 was produced. As the rubber-modified polystyrene, HIPS1 shown in Table 3 was used, and “J900GP” manufactured by Prime Polymer Co., Ltd. was used as the polypropylene.

  Table 4 shows the physical properties of the multilayer sheets obtained in Examples and Comparative Examples.

  The apparent density of the foamed sheet, the thickness of the foamed sheet, and the overall thickness were obtained by randomly collecting three measurement samples from the multilayer sheet and performing the above measurement using the measurement sample. Average values (n = 3) are shown in the table.

  The internal pressure of the foam sheet was determined as follows. First, measurement samples were collected from three randomly selected multi-layer sheets. The air partial pressure was calculated by the above method using the measurement sample, and the air partial pressure of the foamed sheet was obtained by arithmetically averaging the calculated values (n = 3). Next, three pieces of measurement samples were newly collected from the vicinity of the sample collection point. Using the measurement sample, the partial pressure of the remaining foaming agent was calculated by the above-described method, and the respective calculated values were arithmetically averaged (n = 3) to obtain the foaming agent partial pressure of the foamed sheet. The internal pressure of the foamed sheet was determined by summing the air partial pressure and the foaming agent partial pressure obtained, and the results were shown in the table.

Peel strength In accordance with JIS Z0237, for the multilayer sheet immediately after lamination, on the one hand, the foamed sheet and on the other side the resin layer and the film layer are each gripped, and 90 ° peel is performed at a peel speed condition of 400 mm / min. (Arithmetic mean value of n = 3) was determined.

The obtained multilayer sheet was thermoformed using a single molding machine manufactured by Asano Laboratories.
As the mold, a matched mold mold (under the plug) for container molding with the expansion ratio shown in Table 5 was used, the resin layer side of the multilayer sheet was the lower side, and the heating conditions were an upper heater temperature of 310 ° C., a lower heater The multilayer sheet was thermoformed at a temperature of 390 ° C. and a heating time of 15 seconds.

Table 5 shows the results of measuring the physical properties of the obtained container.

  The internal pressure of the container was determined as follows. First, measurement samples were collected from three randomly selected locations on the side wall of the container. The air partial pressure was calculated by the above method using the measurement sample, and the calculated partial value was arithmetically averaged (n = 3) to determine the air partial pressure of the container foam layer. Next, three pieces of measurement samples were newly collected from the vicinity of the sample collection point. The partial pressure of the remaining foaming agent was calculated by the above method using the measurement sample, and the foaming agent partial pressure of the container foaming layer was determined by arithmetically averaging each calculated value (n = 3). The internal pressure of the container was determined by summing the air partial pressure and the foaming agent partial pressure obtained, and the results are shown in Table 5.

  The total basis weight of the resin layer and the film layer in the side wall portion of the container is obtained by randomly collecting three measurement samples from the container side wall portion, and using the measurement sample, An arithmetic average value (n = 3) of each measured value is shown in Table 5 by the same method.

Presence or absence of blister generation was evaluated by the following criteria by observing the presence or absence of blister generation after putting about 75 g of cooked curry in a container obtained by thermoforming and heating in a 700 W microwave oven for 120 seconds. In addition, the boundary part of Carreru is the place which is most easily overheated.
○: No blister occurred.
X: Blister occurred.

Claims (2)

A polyolefin-based resin film layer is laminated and adhered to one side of a foamed sheet using a styrene- (meth) acrylic acid copolymer as a base resin via a rubber-modified polystyrene resin layer laminated and adhered by extrusion lamination. A multilayer sheet for thermoforming having a thickness of 0.5 to 3 mm,
The rubber-modified polystyrene resin layer is composed of rubber-modified polystyrene having a rubber particle size of 1.5 μm or less, or rubber-modified polystyrene having a rubber particle size of more than 1.5 μm and 2.5 μm or less and a gel content of 28% or more. And an apparent density of the foam sheet is 130 to 260 kg / m ³ , and a basis weight of the rubber-modified polystyrene resin layer is 70 to 200 g / m ² .
A container for heating a microwave oven obtained by thermoforming the multilayer sheet for thermoforming according to claim 1.