JP2023023655A - Laminated foam sheet for thermoforming - Google Patents

Abstract

To develop a laminated foam sheet for thermoforming that makes it easy to peel a film from a laminated foam sheet, and prevent, after the thermoforming, a phenomenon that an adhered film floats.SOLUTION: A laminated foam sheet for thermoforming of the present invention is obtained by detachably laminating and adhering a thermoplastic resin film to a surface of a polystyrene resin foam sheet, wherein the apparent density of the laminated foam sheet is within a specific range, an area ratio occupied by specific microbubbles on the peeling surface of the polystyrene resin foam sheet obtained by peeling the thermoplastic resin film from the laminated foam sheet is within a specific range, and the average peel strength of the thermoplastic resin film is within a specific range.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a laminated foam sheet for thermoforming, and more particularly to a laminated foam sheet for thermoforming from which a laminated film can be peeled off.

A laminated foam sheet is known in which a thermoplastic resin film is laminated and adhered to a polystyrene resin foam sheet. A packaging container such as a tray obtained by thermoforming the laminated foamed sheet is widely used as a food container for storing cooked food or the like.

In recent years, in order to reduce the burden on the environment, packaging containers have been recycled by collecting, washing, melting, and pelletizing them. However, the collected packaging containers were often dirty, and had to be washed at home or at collection plants when recycling them.
In addition, since the film is colored, there is a risk that a coloring agent may be mixed in the recovered raw material, resulting in coloring of the recovered raw material. In order to solve such problems, a technique has been developed in which the film laminated on the foamed polystyrene resin sheet is peeled off, thereby eliminating the need to wash the packaging container. As the packaging container, there is one disclosed in Patent Document 1, for example.

JP-A-07-96572

The laminated foam sheet described in Patent Document 1 and the molded article obtained by thermoforming have a low peel strength when the film is peeled off from the laminated foam sheet so that the laminated film can be easily peeled off. It is.
However, in the laminated foamed sheet described in Patent Document 1, after thermoforming, the laminated film peels off from the polystyrene resin foamed sheet and floats, resulting in a gap at the interface between the polystyrene resin foamed sheet and the film. is formed (hereinafter also referred to as "film floating"), and there is a possibility that the appearance may be deteriorated.

This film floating can be prevented by increasing the adhesive strength between the film and the foamed polystyrene resin sheet. However, if the adhesive strength of the film is increased, the film cannot be peeled off from the laminated foam sheet.
Therefore, in the prior art, it was difficult to achieve both the problem of separating the film from the laminated foam sheet and the problem of preventing the film from floating.
Moreover, in the prior art, although the film can be peeled off from the laminated foam sheet, it sometimes takes time to peel off the film, leaving a problem.

It is an object of the present invention to develop a laminated foamed sheet for thermoforming that facilitates peeling of the film from the laminated foamed sheet, prevents the phenomenon that the adhered film floats after thermoforming, and has excellent appearance. Make it an issue.

According to the present invention, the following laminated foamed sheet for thermoforming is provided.
[1] A laminated foam sheet for thermoforming, comprising a polystyrene resin foam sheet and a thermoplastic resin film, wherein the thermoplastic resin film is detachably laminated and adhered to at least one side of the polystyrene resin foam sheet. ,
The laminated foam sheet has an apparent density of 50 kg/m ³ or more and 200 kg/m ³ or less,
The area ratio of fine cells having an area of 0.002 mm ² or less on the peeling surface of the polystyrene resin foam sheet obtained by peeling the film from the laminated foam sheet is 0.5% or more and 20% or less,
A laminated foam sheet for thermoforming, wherein the average value of peel strength when the thermoplastic resin film is peeled from the laminated foam sheet is 40 cN/25 mm or more and 250 cN/25 mm or less.
[2] The laminated foam sheet for thermoforming according to 1 above, wherein the polystyrene resin foam sheet is a foam sheet formed by foaming with an organic physical foaming agent containing a saturated hydrocarbon having 3 to 5 carbon atoms and dimethyl ether. .
[3] The laminated foamed sheet for thermoforming according to 2 above, wherein the residual amount of the organic physical foaming agent in the laminated foamed sheet is 1.0% by weight or more and 2.5% by weight or less.
[4] The laminated foam sheet for thermoforming according to any one of the above 1 to 3, wherein the coefficient of variation of the peel strength of the laminated foam sheet is 0.05 or more and 0.15 or less.

In the laminated foam sheet for thermoforming of the present invention, the laminated and bonded thermoplastic resin film can be easily peeled off from the polystyrene resin foam sheet. Furthermore, in the laminated foamed sheet of the present invention, fine cells are formed on the peeled surface of the polystyrene resin foamed sheet from which the thermoplastic resin film has been peeled off. The obtained molded article is prevented from the phenomenon that the laminated thermoplastic resin film is lifted from the polystyrene-based resin foam sheet.

FIG. 1 is a drawing showing an example of a state in which microbubbles are formed on the peeled surface of a polystyrene-based resin foam sheet that appears after a thermoplastic resin film is peeled from a laminated foam sheet. FIG. 2 is a photograph showing an example of a molded article in which the film float 3 is seen. FIG. 3 is an explanatory view showing how the thermoplastic resin film is peeled off from the laminated foamed sheet of the present invention to reveal the peeled surface of the polystyrene-based resin foamed sheet.

The laminated foam sheet for thermoforming of the present invention will be described in detail below.
The laminated foamed sheet for thermoforming of the present invention (hereinafter also simply referred to as laminated foamed sheet) comprises a polystyrene resin foamed sheet (hereinafter also simply referred to as foamed sheet) and a thermoplastic resin film (hereinafter also simply referred to as film). ), and the film is releasably laminated and adhered to at least one side of the foam sheet.

In this specification, the phrase “peelable” means that the average peel strength between the foam sheet and the film when the film is peeled off from the laminated foam sheet is 40 cN/25 mm or more and 250 cN/25 mm or less, and the film is It means that it can be peeled off from the laminated foam sheet.

In the present invention, the polystyrene-based resin foam sheet uses a polystyrene-based resin as a base resin. The polystyrene resin means a resin containing 50% by weight or more of styrene monomer component units, and examples thereof include polystyrene, rubber-modified polystyrene (impact polystyrene), styrene-αmethylstyrene copolymer, Methylstyrene copolymer, styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene -Methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-acrylonitrile copolymer, mixtures of polystyrene and polyphenylene ether, etc., and mixtures of two or more of these polystyrene resins good too. The polystyrene-based resin may contain polyfunctional monomer component units such as divinylbenzene and multi-branched macromonomers.

Further, the base resin may contain other components as long as the object of the present invention is not impaired. Other components include polypropylene-based resins such as propylene homopolymers and ethylene-propylene copolymers, polyethylene-based resins such as high-density polyethylene and low-density polyethylene, styrene-conjugated diene block copolymers and hydrogenated products thereof. thermoplastic elastomer, ethylene-propylene rubber, rubber such as butadiene rubber, and the like. The upper limit of the blending amount of other components is preferably 30% by mass in the base resin, more preferably 10% by mass or less, further preferably 5% by mass or less, 0% by mass, That is, it is particularly preferable to consist only of a styrene-based resin.

The melt flow rate (MFR) of the polystyrene resin constituting the foamed sheet is preferably 0.1 to 5 g/10 minutes, more preferably 1 to 3 g/10 minutes. When the MFR is within this range, a foamed sheet having a closed cell structure and excellent mechanical strength can be produced over a wide range of production conditions.
The melt flow rate in this specification means the melt mass flow rate measured by the test method A of JIS K 7210-1 (2014), and adopts the conditions of a test temperature of 200° C. and a load of 5 kg.

Typical resins constituting the thermoplastic resin film include polyolefin-based resins and polystyrene-based resins. Examples of the polyolefin-based resin include linear low-density polyethylene, high-density polyethylene, low-density polyethylene, polyethylene-based resins such as ethylene-vinyl acetate copolymer, polyethylene-based resins including biomass-derived polyethylene, propylene homopolymer, ethylene- Examples include polypropylene resins such as propylene block copolymers and ethylene/propylene random copolymers, and polypropylene resins including biomass-derived polypropylene.
Examples of the polystyrene-based resin include resins similar to the polystyrene-based resins described above. Moreover, a multilayer film in which a plurality of films using these resins are laminated can also be used.

The thickness of the thermoplastic resin film is preferably 5 μm or more and 50 μm or less, more preferably 10 μm or more and 40 μm or less, still more preferably 15 μm or more and 35 μm or less, and particularly preferably 20 μm or more and 30 μm or less. If the thickness is within this range, defects such as tearing and holes in the film can be prevented, and the film can be peeled off from the foamed sheet.

Moreover, as the film, a film provided with an adhesive layer for bonding with the polystyrene resin foam sheet may be used. Examples of adhesives constituting the adhesive layer include conventionally known adhesives such as commonly used acrylic adhesives and urethane adhesives, and conventionally known adhesive resins such as ethylene-vinyl acetate. A so-called adhesive-coated film, in which an adhesive is laminated on a resin film, can also be used.

Examples of methods for laminating and bonding the films include known methods such as thermal lamination and extrusion lamination. Above all, it is preferable to laminate and adhere a thermoplastic resin film to one or both sides of the foam sheet by thermal lamination from the viewpoints of workability, lightness of the resulting molded product, and cost.

Conventionally known methods can be used for the method of thermal lamination and the apparatus used for thermal lamination.

In the present invention, when the film is peeled from the laminated foam sheet, the average value of the peel strength measurements between the foam sheet and the film (hereinafter also referred to as the average peel strength) is 40 cN / 25 mm or more and 250 cN / 25 mm or less. If the peel strength is within this range, the film can be peeled off. In the present invention, even if the average peel strength is such a relatively small value, the occurrence of film floating in the laminated foam sheet is suppressed. From the viewpoint of ease of peeling, the average peel strength is preferably 50 cN/25 mm or more and 200 cN/25 mm or less, more preferably 60 cN/25 mm or more and 150 cN/25 mm or less, still more preferably 70 cN/25 mm or more and 130 cN/25 mm or less. be.

Measurement of peel strength is performed as follows. A test piece with a width of 25 mm is cut out from the laminated foam sheet along the direction perpendicular to the extrusion direction, and the film is peeled from the laminated foam sheet in a 90 ° peel test at a peel speed condition of 300 mm / min in accordance with JIS Z0237: 2009. and measure the peel strength at that time. The measurement is performed at least 10 times, and the arithmetic mean is taken as the average peel strength.

The variation coefficient of the peel strength is preferably 0.05 or more and 0.15 or less. The coefficient of variation is a value that represents the relative variation in the measured value of peel strength, and a coefficient of variation of 0.05 or more and 0.15 or less means that the film is laminated and bonded with more uniform strength. means That is, the smaller the coefficient of variation, the smaller the variation in individual measured values of peel strength.
As a method of laminating and bonding with such a uniform strength, as described later, the film is laminated under the conditions that microbubbles are formed in the laminated foam sheet, and the generation of the microbubbles disperses the heat during bonding. A method of laminating and adhering films while cooling can be employed. By doing so, it is considered that the variation in peel strength is reduced and the coefficient of variation is reduced.

The coefficient of variation is a value obtained by dividing the standard deviation calculated from the individual measured values of peel strength by the arithmetic mean of the individual measured values (the average peel strength), and is calculated by the following formula (1). be done.
Cv=({Σ(Di−Dav) ² /(n−1)} ^1/2 )/Dav)
... (1)
However, in the formula (1), Di is the measured value of individual peel strengths, and Dav is the average peel strength.

In the laminated foam sheet of the present invention, when the film is peeled from the laminated foam sheet, the area ratio of cells having an area of 0.002 mm ² or less existing on the peeling surface of the foam sheet (hereinafter, also referred to as the fine bubble area ratio ) is 0.5% or more and 20% or less.

The fine cells can be formed, for example, by laminating a film by thermal lamination using a foam sheet formed using a specific organic physical foaming agent to be described later. It is believed that when the film is laminated and adhered to the foam sheet by thermal lamination, the surface of the foam sheet is heated to generate fine air bubbles. Then, in the obtained laminated foamed sheet, fine air bubbles are observed on the peeled surface of the polystyrene-based resin foamed sheet from which the thermoplastic film has been peeled from the laminated foamed sheet.

When the microbubble area ratio is 0.5% or more, film floating is less likely to occur. The reason for this is thought to be that when microbubbles are generated on the surface of the foam sheet, volatilization of the foaming agent near the surface is accelerated, making it more difficult for the foaming agent to stay, thereby suppressing film floating.
In addition, the film floating tends to become severe as the amount of the foaming agent is increased in an attempt to produce a foamed sheet having a low apparent density. From this, it is considered that the physical foaming agent remaining in the foam sheet volatilizes after thermoforming and accumulates at the interface between the foam sheet and the film, causing the film to float and form a space.

In addition, when the microbubbles are generated, heat is dissipated due to changes in the state of the sheet surface, so that the adhesion state between the film and the foam sheet changes at the portion where the microbubbles are generated, thereby improving the ease of peeling. In particular, the microbubbles improve the easiness of peeling at the stage when the film starts to be peeled off, thereby facilitating the peeling of the film.

On the other hand, if the area ratio of the microbubbles exceeds 20%, the peel strength may be too low, and the film may easily float.
From this point of view, the microbubble area ratio is preferably 1% or more and 15% or less, more preferably 2% or more and 10% or less.

FIG. 1 shows an example of a state in which microbubbles are formed on the peeled surface of the foam sheet that appears when the film of the laminated foam sheet is peeled off. As shown in FIG. 1, microbubbles 2 are formed on the peeling surface in a mixture of normal bubbles 1 . In addition, FIG. 3 is an explanatory view showing how the film is peeled off from the laminated foam sheet to reveal the peeling surface of the foam sheet.

The measurement of the fine bubble area ratio is carried out by manually peeling off the film from a test piece cut out from the center in the width direction of the laminated foam sheet, and then photographing the peeled surface using a CCD camera or the like to obtain an enlarged image. The area ratio of the microbubbles generated in the area of ² is measured. The microbubble area ratio is calculated using image editing software as described later.

Next, various physical properties of the laminated foam sheet for thermoforming of the present invention will be described.
The thickness of the laminated foam sheet of the present invention is preferably 0.5 mm or more and 5 mm or less. When the thickness is within this range, the thermoformability of the laminated foam sheet will be good, and the container obtained by thermoforming the laminated foam sheet will be excellent in rigidity, heat insulation, handleability and the like. From this point of view, the thickness is preferably 0.7 mm or more and 4 mm or less, more preferably 1 mm or more and 3 mm or less.

The thickness of the laminated foam sheet is determined by taking three measurement samples of 260 mm on a side at random from the laminated foam sheet, measuring the thickness of the measurement samples by the method described above, and taking the arithmetic mean value.

The basis weight of the laminated foam sheet is preferably 50 g/m ² or more and 300 g/m ² or less. When the basis weight of the laminated foamed sheet is within this range, the laminated foamed sheet is superior in lightness while maintaining rigidity, and the resulting molded article is expected to withstand use in food packaging containers and the like. Become. From the above viewpoint, the basis weight is more preferably 80 g/m ² or more and 250 g/m ² or less, and even more preferably 100 g/m ² or more and 200 g/m ² or less.

In this specification, the basis weight of the laminated foam sheet is measured as follows. First, a test piece of 100 mm length × 100 mm width × thickness of the laminated foam sheet is cut out from the laminated foam sheet, the weight (g) of the test piece is measured, the weight is multiplied by 100, and the basis weight is converted into units. (g/m ² ) is obtained.
The basis weight of the foam sheet to which the film is not laminated and adhered can also be measured in the same manner as the basis weight of the laminated foam sheet.

The apparent density of the laminated foam sheet is 50 kg/m ³ or more and 200 kg/m ³ or less. If the apparent density is within this range, the thermoformed container will be excellent in strength and lightness. From this point of view, the apparent density is preferably 60 kg/m ³ or more and 150 kg/m ³ or less, more preferably 70 kg/m ³ or more and 100 kg/m ³ or less.

As used herein, the apparent density of the laminated foam sheet is measured as follows. First, the basis weight (g/m ² ) of the laminated foam sheet is determined by the method described above. Next, the obtained basis weight (g/m ² ) of the laminated foam sheet is divided by the thickness (mm) of the laminated foam sheet, and the value is converted into units to obtain the apparent density (kg/m ³ ) of the laminated foam sheet. .
The apparent density of the foam sheet to which the film is not laminated and adhered can also be measured in the same manner as the laminated foam sheet.

In the present invention, the number of internal cells (A) present in a cross section of 4 mm ² passing through the central portion in the thickness direction of the laminated foam sheet and perpendicular to the thickness direction of the laminated foam sheet in the horizontal direction is 100 or less. is preferably
In addition, the ratio (B/A) of the number of surface cells (B) present in the peeled surface portion 4 mm ² of the polystyrene resin foam sheet from which the film of the laminated foam sheet is peeled to the number of cells (A) is It is preferably 1.5 or more and 2.3 or less.
A laminated foam sheet having cells within these ranges is excellent in appearance and can be more suitably used as a food container. Furthermore, even when used in combination with wrap packaging, an outer fitting type lid, or an inner fitting type lid as the packaging form of the container, it is possible to cope well.
From this point of view, the number of internal cells (A) existing in a cross section of 4 mm ² perpendicular to the thickness direction of the laminated foam sheet is preferably 80 or more and 95 or less, and 85 or more and 93 or less. is more preferred.

In addition, the number of cells present in the cross section of the laminated foam sheet obtained by cutting the laminated foam sheet along the thickness direction (the number of cells in the longitudinal section) is preferably 10 or more and 25 or less, more preferably 15 or more and 20. It is below.
The number of air bubbles in the longitudinal section is obtained by photographing, with a microscope or the like, cross sections cut along the direction perpendicular to the extrusion direction and the thickness direction at 10 points equally spaced in the width direction of the laminated foam sheet to obtain an enlarged image. Next, a straight line l is drawn in the thickness direction of each cross-sectional photograph, and the number of all cells n in the foam sheet that intersect with the straight line l is counted. The average number of cells thus obtained is taken as the average number of cells in the thickness direction of the laminated foam sheet.

The laminated foam sheet of the present invention has a closed cell content of 60% or more. If the closed cell content is too small, the rigidity and thermoformability of the obtained container may be lowered. Moreover, there is a possibility that the rigidity of the container may be lowered. From this point of view, the closed cell ratio is preferably 70% or more, more preferably 80% or more.

In this specification, the closed cell rate of the laminated foam sheet or foam sheet is measured as follows. According to ASTM-D2856-70 procedure C, measure using Toshiba Beckman Co., Ltd. air comparison type densimeter 930 (A cut sample cut randomly from a laminated foam sheet or foam sheet to 25 mm × 40 mm × sheet thickness , So that the total thickness of the sample approaches 20 mm (but does not exceed 20 mm.) Multiple sheets are stacked and placed in a sample cup for measurement.) Laminated foam sheet or foam sheet (cut sample) Using the true volume Vx, the closed cell ratio S (%) is calculated by the following formula (2), and obtained as an average value of n=5.
S(%)=(Vx−W/ρ)×100/(Va−W/ρ) (2)

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

Next, the remaining amount of the organic physical foaming agent in the laminated foam sheet of the present invention will be explained.
The residual amount of the organic physical foaming agent in the laminated foam sheet is preferably 1.0% by weight or more and 2.5% by weight or less. If the residual amount is within this range, desirable secondary foaming occurs during in-mold molding, a molded article having the shape of the mold can be obtained, and floating of the film after thermoforming is prevented. . For this reason, the residual amount is more preferably 1.2% by weight or more and 2.3% by weight or less, and still more preferably 1.5% by weight or more and 2.1% by weight or less. In addition, the foaming agent remaining amount in the laminated foam sheet becomes smaller than the foaming agent content in the foam sheet by passing through the film lamination process by thermal lamination.

When the residual amount is within this range, it is possible to easily separate the film from the laminated foam sheet and prevent the film from floating. In order to prevent film floating, the smaller the remaining amount, the better. On the other hand, in order to develop good thermoformability, the organic physical foaming agent must remain in the foam sheet and contribute to secondary foaming during thermoforming. Therefore, as described above, it is desirable that the organic physical foaming agent remains in the foamed sheet constituting the laminated foamed sheet to such an extent that film floating does not occur.

Next, the physical properties of the foam sheet used for producing the laminated foam sheet of the present invention will be described.
The apparent density of the foamed sheet is preferably 50 kg/m ³ or more and 200 kg/m ³ or less. If the apparent density is within this range, the resulting molded article obtained by thermoforming the laminated foam sheet will be excellent in strength and lightweight. From this point of view, the apparent density is preferably 60 kg/m ³ or more and 150 kg/m ³ or less, more preferably 65 kg/m ³ or more and 100 kg/m ³ or less. The apparent density of the foam sheet can be measured in the same manner as for the laminated foam sheet.

The thickness of the foam sheet is preferably 0.5 mm or more and 5 mm or less. When the thickness is within this range, a molded article obtained by thermoforming a laminated foam sheet obtained by laminating and bonding a film to the foam sheet has excellent rigidity, heat insulation, handleability, etc., and is suitable for various containers. can be used. From this point of view, the thickness is preferably 0.7 mm or more and 4 mm or less, more preferably 1 mm or more and 3 mm or less. The thickness of the foam sheet can be measured in the same manner as the method for measuring the laminated foam sheet.

The basis weight of the foamed sheet is preferably 50 g/m ² or more and 250 g/m ² or less. When the basis weight of the foam sheet is within this range, a molded article obtained by thermoforming the laminated foam sheet obtained from the foam sheet can withstand use as a food packaging container or the like. From the above viewpoint, the basis weight is more preferably 80 g/m ² or more and 200 g/m ² or less, and still more preferably 100 g/m ² or more and 150 g/m ² or less. The basis weight of the foam sheet can be measured in the same manner as the method for measuring the laminated foam sheet.

Next, the content of the organic physical foaming agent in the foam sheet used for producing the laminated foam sheet will be explained.
The content of the organic physical foaming agent in the foam sheet used for producing the laminated foam sheet is preferably 1.5% by weight or more and 3% by weight or less.
If the content is within this range, desirable secondary foaming occurs and the polystyrene resin is plasticized during in-mold molding of the obtained laminated foam sheet, so that the shape of the mold is shaped. A molded article can be obtained, and the phenomenon of film floating after thermoforming is prevented. For this reason, the content is more preferably 1.8% by weight or more and 2.8% by weight or less, and still more preferably 1.9% by weight or more and 2.5% by weight or less.

In order to obtain a laminated foamed sheet with a residual amount of the foaming agent within the above range, the organic physical foaming agent used in the production of the foamed sheet should be an organic It is preferred to use a mixed physical blowing agent.

Aliphatic hydrocarbons with 3 to 5 carbon atoms are relatively slow to volatilize from the foam sheet, and the permeation rate to polystyrene resin is slower than that of air. It can be ensured for a long period of time, and the life cycle of the resulting laminated foam sheet can be lengthened. For this reason, butane is more preferably used, and isobutane is more preferably used.
On the other hand, as the early-dissipative foaming agent, those having a higher permeation rate through polystyrene-based resin than air can be mentioned. Examples thereof include ethers having a boiling point of 140° C. or lower, dialkyl carbonates having a boiling point of 140° C. or lower, alcohols, carbon dioxide, and water. Among these, ethyl methyl ether, dimethyl ether and diethyl ether are preferred. It is more preferable to use dimethyl ether because it facilitates the production of foamed sheets with a low apparent density and can sufficiently plasticize polystyrene-based resins.
The permeation rate of dimethyl ether through polystyrene resin is 77 times that of normal butane, and the permeation rate of isobutane is 0.1 times that of normal butane. Therefore, when a foamed sheet is produced using a mixed blowing agent of dimethyl ether, normal butane, and isobutane, dimethyl ether contributes to lowering the apparent density, but volatilizes early, and normal butane and isobutane, especially isobutane, take a long time. It remains in the foamed sheet for a long period of time and can contribute to the improvement of thermoformability.

Note that the use of dimethyl ether tends to increase the size of the cells inside the foam sheet.
Therefore, since the number of internal cells (A) described above tends to be small, the number of surface cells present in 4 mm ² of the peeled surface of the foam sheet side from which the film of the laminated foam sheet is peeled relative to the number of cells (A) The ratio (B/A) of (B) is more preferably 1.8 or more and 2.8 or less.
It is considered that the reason why the cells inside the foamed sheet become larger is that the speed of dimethyl ether permeating through the cell walls is high, which increases the cell growth rate in the initial stage of foaming.

The residual amount (% by weight) of the organic physical foaming agent in the laminated foam sheet and the content ratio (% by weight) of the organic physical foaming agent in the foam sheet can be measured by an internal standard method using a gas chromatograph. can. Specifically, about 1 g of a sample is cut from the vicinity of the center of the entire width of the laminated foam sheet or foam sheet, immediately put into toluene containing an internal standard substance in a sample bottle with a lid, close the lid, and then thoroughly A gas chromatograph analysis is performed on a measurement sample of a solution in which the foaming agent in the sample is dissolved in toluene by stirring, and the content of the organic physical foaming agent is determined.

By the method described above, the saturated hydrocarbon content (% by weight) in the laminated foam sheet and the foam sheet can be determined. Moreover, the content ratio (% by weight) of dimethyl ether in the laminated foam sheet and the foam sheet can also be obtained.

The residual amount of dimethyl ether in the laminated foam sheet is preferably 1% by weight or less, more preferably 0.5% by weight or less, 0.3% by weight or less, and still more preferably 0.1% by weight or less. , particularly preferably 0.05% by weight or less.

Next, a method for manufacturing the laminated foam sheet of the present invention will be described.
The laminated foamed sheet of the present invention can be obtained by forming a foamed sheet by a conventionally known so-called extrusion foaming method, and laminating and bonding a thermoplastic resin film to this foamed sheet. Specifically, for example, it can be produced as follows.

First, a polystyrene-based resin foam sheet is manufactured. Specifically, a polystyrene resin and, if necessary, a cell control agent such as talc are supplied to an extruder, heated, melted, and kneaded to form a molten resin. Next, a physical foaming agent is injected into the molten resin, and the mixture is further kneaded and adjusted to a foamable resin temperature to obtain a foamable resin melt. The foamable resin melt is then introduced into an annular die mounted downstream of the extruder and extruded and foamed into the atmosphere to form a tubular foam. Next, a foam sheet can be obtained by cutting open the tubular foam while pulling it along a cylindrical cooling device.

As the foaming agent for obtaining the foamed sheet used in the present invention, as described above, an organic physical foaming agent containing an aliphatic hydrocarbon having 3 to 5 carbon atoms and an early dissipative foaming agent is preferably used. As aliphatic hydrocarbons having 3 to 5 carbon atoms, normal butane, isobutane, and normal butane are used because the dissipation rate from the foam sheet is slow and the secondary foamability of the foam sheet in thermoforming can be ensured over a long period of time. Mixed butanes of isobutane are preferred, with isobutane being more preferred.

On the other hand, as the early dissipative foaming agent, dimethyl ether is preferable because it facilitates production of a low apparent density foam sheet and can sufficiently plasticize a polystyrene resin. In addition, by using an early fugitive foaming agent, it is possible to reduce the amount of the foaming agent remaining in the laminated foam sheet, so that film floating can be effectively suppressed.
The total amount of the organic physical blowing agent compounded is preferably 1 to 8 parts by weight, preferably 2 to 5 parts by weight, per 100 parts by weight of the polystyrene resin.

The case where isobutane and dimethyl ether are used in combination as the organic physical blowing agent will be described below. However, the present invention is not limited to this combination, and other saturated hydrocarbons having 3 to 5 carbon atoms and early dissipative blowing agents can be used. Also, one or two or more saturated hydrocarbons having 3 to 5 carbon atoms and one or two or more early-dissipative blowing agents may be used in combination.

The amount of isobutane compounded is preferably 1 to 4 parts by weight with respect to 100 parts by weight of the polystyrene resin. A foamed sheet having a desired thickness and apparent density can be easily formed by using isobutane in a blending amount within the above range. In addition, it is possible to obtain a foamed sheet having excellent moldability, particularly excellent secondary foamability, and excellent moldability. From this point of view, the blending amount is more preferably 2 to 3.5 parts by weight.

The amount of dimethyl ether compounded is preferably 0.5 to 2 parts by weight with respect to 100 parts by weight of the polystyrene resin. Since dimethyl ether dissipates rapidly from the foam sheet, the use of dimethyl ether as a physical foaming agent contributes to the reduction of the apparent density of the foam sheet while sufficiently plasticizing the polystyrene resin during the production of the foam sheet. be able to. In addition, since the residual amount of the foaming agent in the obtained foamed sheet can be reduced, the laminated foamed sheet obtained by laminating the film on the obtained foamed sheet can suppress film floating after thermoforming. . From this point of view, the blending amount is preferably 0.6 to 1.8 parts by weight, more preferably 0.8 to 1.5 parts by weight.

When a mixed foaming agent of isobutane and dimethyl ether is used as the physical foaming agent, the weight ratio of isobutane and dimethyl ether is preferably 55:45 to 80:20. By setting the weight ratio within this range, it is possible to obtain a foamed sheet which is excellent in thermoformability and capable of producing a laminated foamed sheet capable of preventing the film floating phenomenon. From this point of view, the weight ratio is more preferably 60:40 to 75:25.

In the present invention, various additives such as cell control agents, colorants such as pigments and dyes, heat stabilizers and fillers can be appropriately blended with the base resin, if necessary.

Examples of the foam control agent include inorganic powders such as talc, kaolin, mica, silica, calcium carbonate, barium sulfate, titanium oxide, aluminum oxide, clay, bentonite, and diatomaceous earth, and conventionally known chemical foaming agents such as azodicarbodiamide. agents and the like can be used. Among them, talc is preferable from the viewpoint of handleability, cost, and the like. The amount of the cell control agent to be added varies depending on the type of the cell control agent, the target cell diameter, etc., but is generally 0.01 to 8 parts by weight, further 0.02 to 5 parts by weight, per 100 parts by weight of the base resin. Parts by weight, especially 0.05 to 3 parts by weight are preferred.

Next, a method for laminating and adhering a film to a foam sheet will be described.
Examples of methods for laminating and adhering the thermoplastic resin film to the obtained foamed sheet include known methods such as thermal lamination and extrusion lamination. Among them, it is preferable to laminate and adhere a thermoplastic resin film to one or both sides of the foam sheet by thermal lamination because it facilitates adjustment of the average peel strength and tends to generate fine air bubbles.

Lamination adhesion of a film to a foam sheet by heat lamination is usually performed by first winding the foam sheet that has been extruded and taken up into a roll, and then winding the foam sheet from the roll in a separate process from the foaming process. This is done by putting the films back on top of each other and passing them through a hot lamination machine (off-line processing).
However, a method (on-line processing) may also be adopted in which a laminated foam sheet is obtained by superimposing a film on an extruded foam sheet and passing it through a heat lamination device on a foam sheet production line, and winding it into a roll (online processing). .

In the thermal lamination, for example, a thermal lamination apparatus having a heating roll capable of pressure-bonding the foam sheet and a pinching roll are used, the film and the foam sheet are overlapped, and the heating roll is applied so that the film is in contact with the heating roll. It is preferable that the film is laminated and adhered to the foamed sheet by supplying the film and the foamed sheet between and a pinching roll to sandwich the film and the foamed sheet.

When obtaining the laminated foamed sheet of the present invention, for example, the temperature of the heating roll is set to 155 to 190°C, particularly preferably 158 to 180°C. , the line speed, etc. can be adjusted to adjust the peel strength.

Conventionally known methods can be used for the method of thermal lamination and the apparatus used for thermal lamination.

The laminated foam sheet of the present invention is excellent in thermoformability, and by thermoforming, molded articles having various mold shapes can be obtained.

Thermoforming methods include vacuum forming and pressure forming, and further applications of these include free drawing forming, plug and ridge forming, ridge forming, matched mold forming, straight forming, drape forming, reverse draw forming, air slip forming, Examples include plug-assisted molding, plug-assisted reverse draw molding, etc., and molding methods in which these are combined. Such a thermoforming method is a preferable method because it can continuously obtain molded articles in a short period of time. In addition, when thermoforming a laminated foam sheet in which a film with excellent oil resistance such as a film of polyolefin resin is laminated and bonded, the thermoplastic resin film with excellent oil resistance is formed inside the resulting molded product. preferably.

Molded bodies obtained by thermoforming are used as various food containers including natto containers and food trays.
By peeling off only the film after using these containers, the cleaning process can be simplified, thereby improving the recyclability of the foamed sheet.
In addition, since the film can be peeled off in the same manner as the molded body, the sheet remaining after the molded body is punched out when molding the laminated foamed sheet is further improved in recyclability of the laminated foamed sheet of the present invention. are doing.

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

The following apparatus was used in the examples and comparative examples.
A tandem extruder in which a first single-screw extruder with an inner diameter of 115 mm and a second single-screw extruder with an inner diameter of 180 mm were connected in series was used as an apparatus for forming the foamed sheet.
As a thermal lamination device, a heating roll (upper side) and a pinching roll (lower side) capable of pressure bonding the foamed sheet and the film, and a laminated foamed sheet in which the foamed sheet (lower side) and the film (upper side) are laminated are pressed. , a device with two upper and lower cooling rolls that can be cooled was used.

Various raw materials used in Examples and Comparative Examples are shown below.

(polystyrene resin)
(1) Abbreviated name “Resin 1”: Polystyrene resin manufactured by PS Japan Co., Ltd.: Product name “GX154” (MFR 1.5 g / 10 min)

Air bubble control agent: Masterbatch of talc "High Filler #12 manufactured by Matsumura Sangyo Co., Ltd." (35% by weight of talc)

Organic physical blowing agent (1) isobutane (2) dimethyl ether

(Thermoplastic resin film)
(1) Abbreviated name “CPP”: Unstretched polypropylene film “KT” (thickness 25 μm) manufactured by Sun Tox Co., Ltd.
(2) Abbreviated name “CPS”: Styrophane SPH manufactured by Oishi Sangyo Co., Ltd. (thickness 25 μm)

Examples 1-3, Comparative Examples 1-5
The type and amount of polystyrene resin shown in Table 1 and the amount of talc shown in Table 1 (cell regulator: high filler #12 manufactured by Matsumura Sangyo Co., Ltd.) are supplied to the first extruder of the tandem extruder and heated. After melting and kneading, an organic physical foaming agent of the type and amount shown in Table 1 was press-fitted to 100 parts by weight of the polystyrene resin so that the amount shown in Table 1 was obtained, further kneaded, and fed to the second extruder. The extruded resin temperature was adjusted to 145° C. to obtain a resin melt for forming a foamed sheet. The resin melt for forming a foamed sheet is extruded from an annular die into the atmosphere at a discharge rate of 300 kg/hr to form a foamed tubular body, and immediately after that, the inner side of the foamed tubular body (mandrel surface side (M surface side)). 23° C. air is blown at a flow rate of 3.0 m ³ /min to rapidly cool the foamed tubular body, and then the foamed tubular body is pulled along the outer surface of a cooling cylinder (mandrel) with an outer diameter of 670 mm at a speed of 17 m / min. , along the direction of extrusion and wound up to obtain two foam sheets (width 1050 mm).

Injection of the physical foaming agents was carried out by using independent diaphragm pumps, setting respective gas flow rates (injection gas weight per time), and injecting them simultaneously into the resin in a molten state. The blending amount of the foaming agent was calculated from the total discharge amount (kg/hr) and the injection amount of each gas (kg/hr).

Table 1 shows the results of measuring the basis weight, thickness, apparent density, foaming agent content, and fine cell area ratio of the resulting foamed sheet.

Next, the obtained foam sheet was cured at room temperature, and after 20 days from the production, on the S side of the foam sheet (the side opposite to the M side, which was taken along the mandrel), The films were laminated together by thermal lamination.
In the thermal lamination, using the above-described apparatus, on one side of the foam sheet, the thermal lamination conditions shown in Table 2 were applied, with a heating roll having a heating temperature shown in Table 2, a pinch clearance of 0.3 mm, and a line speed shown in Table 2. The films were laminated together.

After thermal lamination, it is cured in an indoor environment (23 ° C., relative humidity of 50%), and after 24 hours, it is thermoformed using a rectangular tray molding mold of 100 mm × 185 mm × height 18 mm to obtain a tray shape. was obtained.

Table 2 shows physical properties such as thickness of the obtained laminated foam sheet.

In the laminated foamed sheets obtained in Examples 1 to 3, the film was peelable from the laminated foamed sheet, generation of moderate microbubbles was observed, and an effect of preventing the film from floating was also confirmed. In addition, the peelability of the film was also improved, and in particular, a laminated foam sheet from which the film could be easily peeled was obtained.

Comparative Example 1 is an example using only butane as a foaming agent. A large amount of foaming agent remained, and the problem of film floating could not be solved.
Comparative Example 2 is an example in which the blending weight ratio of butane and dimethyl ether is 50:50. The film was easily peeled off, but too many microbubbles were generated, the peel strength was lowered, and the film lifted.
Comparative Example 3, like Comparative Example 1, is an example in which only butane was used as the foaming agent and the amount of the cell regulator was changed. No microbubbles were generated during thermal lamination, and film floating occurred.
Comparative Example 4 is an example in which the blending weight ratio of butane and dimethyl ether is 90:10. The amount of microbubbles generated was small, and film floating occurred.
Comparative Example 5 is an example in which the foam sheet and the film are adhered beyond the peel strength range. Although the film was prevented from floating and microbubbles were generated, the peel strength of the laminated foam sheet was too high, making it difficult to peel off the film. Therefore, it can be seen that the effect of the ease of film peeling at the beginning of peeling is exhibited when the average peel strength of the laminated foam sheet is in a relatively low range.

Next, the method for measuring each physical property value in the table and the evaluation method for each evaluation will be described.

(Thickness of foam sheet and laminated foam sheet)
The thickness of the foam sheet and laminated foam sheet is obtained by randomly taking three measurement samples of 260 mm on a side from the foam sheet and laminated foam sheet, measuring the thickness by the above method using the measurement sample, and calculating the arithmetic mean. The value (n=3) was taken as the thickness.

(Basis weight of foam sheet and laminated foam sheet)
The basis weight of the foam sheet or laminated foam sheet was measured by the method described above. Specifically, three rectangular test pieces of 100 mm were cut out over the entire width of the foam sheet or laminated foam sheet. The weight (g) of the test piece is divided by the area of the test piece (sheet width (mm) × 100 mm), converted to the weight (g) per 1 m ² , and the arithmetic average value is the foam sheet, laminated foam sheet, etc. basis weight (g/m ² ).

(Laminated foam sheet, apparent density of foam sheet)
The apparent density of the laminated foam sheet was measured by the method described above. First, the basis weight (g/m ² ) of the laminated foam sheet was determined by the method described above. Next, the obtained basis weight (g/m ² ) of the laminated foam sheet was divided by the thickness (mm) of the laminated foam sheet, and the unit was converted to obtain the apparent density (kg/m ³ ) of the laminated foam sheet. . Also, foam sheets were similarly measured.

(Laminate foam sheet, fine bubble area ratio of foam sheet)
The fine bubble area ratio of the laminated foam sheet was obtained by cutting out a test piece from the central part in the width direction of the laminated foam sheet, peeling off the film by hand, and photographing the exposed peeled surface of the foam sheet with a CCD camera at a magnification of 200. Then, select a part with an area of 4 mm ² on the obtained screen, mark the fine cell part in the 4 mm ² part, and use image editing software so that only the image after marking emerges. After binarization, the area ratio of the marking portion was calculated as the ratio of pixels.
In the above measurement, microbubbles were defined as bubbles having a bubble area of 0.002 mm ² or less.
Regarding the microbubble area ratio of the foamed sheet, the surface of the foamed sheet on the side where the film is to be laminated was observed using a CCD camera, and microbubbles were similarly observed.

(Average bubble area of laminated foam sheet)
The average cell area of the laminated foam sheet is obtained by manually peeling off the film from a test piece cut out from the central portion of the laminated foam sheet in the width direction, and then observing the surface of the foam sheet using a CCD camera ^. , the area of the bubble portion excluding the fine bubble portion was measured. The area was calculated in the same manner as described above.

(Amount of foaming agent remaining in laminated foam sheet, foaming agent content in foam sheet)
The residual amount (% by weight) of the foaming agent in the laminated foam sheet was measured by an internal standard method using a gas chromatograph. Specifically, about 1 g of a sample is cut from the vicinity of the center of the entire width of the laminated foam sheet, immediately put into toluene containing an internal standard substance in a sample bottle with a lid, and after closing the lid, it is thoroughly stirred and laminated. A solution obtained by dissolving the foaming agent in the foamed sheet in toluene was used as a measurement sample and gas chromatograph analysis (GC-4000 manufactured by GL Sciences Co., Ltd.) was performed to determine the content of the foaming agent.
In the case of the laminated foam sheet, the weight of the film was subtracted to obtain the residual amount of the foaming agent in the foam sheet constituting the laminated foam sheet. Moreover, the content of the foaming agent in the foam sheet was measured immediately after the foam sheet was produced. On the other hand, the residual amount of the foaming agent in the laminated foam sheet was measured 14 days after the production of the laminated foam sheet.
Also, isobutane, which is a saturated hydrocarbon having 3 to 5 carbon atoms, and dimethyl ether, which has an early dissipative property, were measured separately, and the total amount was taken as the remaining amount of the blowing agent.

(Average peel strength of laminated foam sheet, coefficient of variation of measured value of peel strength)
The peel strength was measured for 20 samples by the method described above, and the average peel strength and coefficient of variation were calculated.

(Peeling strength of compact)
The average peel strength of the molded article was measured by the method described above for 10 samples taken from the flat bottom portion of the obtained molded article.

(Easy to peel off the film)
The laminated foam sheet was cut into 100 mm squares using a cutter, and the work of peeling off the film from the end of the cut with fingers was carried out by three people. evaluated.
○: The average evaluation score of 3 people is 2.0 points or more ×: The average evaluation score of 3 people is less than 2.0 Evaluation points 3 points: 2 points that can easily start peeling the film 2 points: average It is possible to start peeling the film within 10 seconds 1 point: The film can be started to peel off by carefully scratching the cut with a nail, but it takes more than 10 seconds on average to start peeling.

(film floating)
After 30 days have passed since the laminated foam sheet was thermoformed into a tray-shaped molded body, evaluation is made by whether or not voids are observed at the interface between the film and the foamed sheet on the surface of the molded body (film floating is observed). gone.
○: No film floating is observed △: Slight voids can be seen in the uneven part of the bottom ×: Clear film floating can be visually confirmed

The number of longitudinal cross-sectional cells, the number of surface cells, and the number of internal cells were measured for the laminated foam sheets obtained in Examples and Comparative Examples. Table 3 shows the results.

In Table 3, each item was measured as follows.
(Number of bubbles in longitudinal section)
Cross sections perpendicular to the extrusion direction were photographed with a microscope at 10 locations at equal intervals in the width direction of the laminated foam sheet. Next, a straight line l was drawn in the thickness direction of each cross-sectional photograph, and the number of all cells n in the foam sheet that crossed the straight line l was counted. The average number of cells thus obtained was taken as the average number of cells in the thickness direction of the foamed sheet.

(Number of surface bubbles B)
On the surface of the laminated foam sheet (S surface, M surface), a 4 mm ² area part of the surface is observed using a CCD camera, and the number of bubbles (excluding fine bubbles) present within a range of 4 mm ² is counted. counted.

(Internal bubble number A)
The number of cells in the central part of the longitudinal section of the laminated foam sheet (the number of internal cells (A)) is obtained by measuring the area of 4 mm ² in the central part excluding the part of 200 μm from the surface in the cross section cut in the thickness direction of the laminated foam sheet. A camera was used to observe and count the number of air bubbles present within an area of 4 mm ² .

1 Ordinary air bubbles 2 Fine air bubbles 3 Film float 4 Laminated foam sheet for thermoforming
5 Polystyrene resin foam sheet
6 thermoplastic film 7 release surface

Claims (4)

A laminated foam sheet for thermoforming, comprising a polystyrene resin foam sheet and a thermoplastic resin film, wherein the thermoplastic resin film is detachably laminated and adhered to at least one side of the polystyrene resin foam sheet,
The laminated foam sheet has an apparent density of 50 kg/m ³ or more and 200 kg/m ³ or less,
The area ratio of fine cells having an area of 0.002 mm ² or less on the peeling surface of the polystyrene resin foam sheet obtained by peeling the thermoplastic resin film from the laminated foam sheet is 0.5% or more and 20% or less,
A laminated foam sheet for thermoforming, wherein the average value of peel strength when the thermoplastic resin film is peeled from the laminated foam sheet is 40 cN/25 mm or more and 250 cN/25 mm or less.
The laminated foamed sheet for thermoforming according to claim 1, wherein the polystyrene resin foamed sheet is a foamed sheet formed by foaming with an organic physical foaming agent containing a saturated hydrocarbon having 3 to 5 carbon atoms and dimethyl ether.
3. The laminated foamed sheet for thermoforming according to claim 2, wherein the residual amount of said organic physical foaming agent in said laminated foamed sheet is 1.0% by weight or more and 2.5% by weight or less.
The laminated foam sheet for thermoforming according to any one of claims 1 to 3, wherein the laminated foam sheet has a peel strength variation coefficient of 0.05 or more and 0.15 or less.

Images