JP7315571B2 - foam sheet - Google Patents

Description

The present invention relates to foam sheets, laminated foam sheets and molded containers.

Extruded foam sheets of styrene resins are widely used for food packaging containers such as food trays, lunch boxes, instant noodle containers, natto containers, and cups because of their lightness, rigidity, and moldability. In addition, styrenic resin plate-shaped extruded foam has excellent heat insulation and mechanical strength, so it is used in various fields such as flooring and wall materials for general buildings, ceiling materials, and core materials for tatami mats. However, it is feared that discarded plastic waste, which is made of styrene-based resin, may become a source of generation of "microplastics," which are attracting attention as an environmental problem.

From the viewpoint of environmental protection, it is preferable to use plastic products containing various biodegradable polymers. Moreover, the use of plant-derived raw materials is preferable from the viewpoint of saving petroleum resources. In recent years, general consumers have come to recognize these facts, and attempts to use biodegradable polymers and plant-derived polymers as raw materials for industrial products have been made widely. In particular, polylactic acid is a plant-derived and biodegradable polymer that has a relatively high melting point, toughness, transparency, and chemical resistance.

Since most of the conventional polylactic acid-based resin foamed sheet moldings are formed using an amorphous polylactic acid-based resin composition that is easily foamed as a material, the polylactic acid-based resin foamed sheet molding has a problem of low heat resistance. Therefore, a method for obtaining a molded polylactic acid resin foam sheet having excellent heat resistance by increasing the degree of crystallinity during thermoforming of the polylactic acid resin foam sheet has been proposed (Patent Document 1). However, there is a problem that the brittleness of the resin composition increases, and as a result, the sheet strength of the polylactic acid-based resin foamed sheet molding decreases. Therefore, for example, studies have been made to ensure fluidity and improve mechanical properties by blending styrene-based resins and polylactic acid (Patent Document 2). However, it has been difficult to satisfy the physical properties demanded by the market and to design products that take advantage of the properties of each resin simply by blending and melting and mixing styrene resin and polylactic acid.

Japanese Patent Application Laid-Open No. 2004-359910 JP-A-2008-50426

An object of the present invention is to provide a foamed sheet obtained from a polylactic acid-containing styrenic resin composition having an excellent balance between heat resistance and sheet strength.

An object of the present invention is to provide a foam sheet and a laminated foam sheet that have sufficient heat resistance to suppress deformation of a container even when heated in a microwave oven, and that have excellent sheet strength and are easy to handle.

The foam sheet of the present invention is
A foamed sheet obtained by foaming a styrene-based resin composition,
The styrene resin composition contains (A) 55 parts by mass to 90 parts by mass of a styrene resin, (B) 10 parts by mass to 45 parts by mass of polylactic acid , and (C) 1 part by mass to 8 parts by mass of a copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester, based on a total of 100 parts by mass of (A) a styrene resin and (B) polylactic acid.
The (C) copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester is a multi-layer structure particle in which a polymer particle containing butadiene is used as a core, and a monomer containing an ethylenically unsaturated carboxylic acid ester is graft-copolymerized on the surface thereof to form a shell,
The foam sheet has a thickness of 0.5 mm or more and 3.5 mm or less, an expansion ratio of 1.1 times or more and 20.0 times or less, an average cell diameter of 30 μm or more and 500 μm or less, a total volatile content of less than 800 μg/g, and a closed cell rate of 60% or more.

In the foamed sheet of the present invention, it is preferable that the (A) styrene-based resin and the (B) polylactic acid form a co-continuous structure.

The laminated foamed sheet of the present invention is obtained by laminating a resin non-foamed film made of a thermoplastic resin on at least one surface of the foamed sheet of the present invention.

The molded container of the present invention is obtained by molding the foamed sheet or laminated foamed sheet of the present invention.

The foamed sheet of the present invention has sufficient heat resistance to suppress deformation of the container even when heated in a microwave oven, and has excellent sheet strength and is easy to handle.

<Foam sheet>
The foamed sheet of the present invention is obtained by foaming a styrene-based resin composition.

[Styrene resin composition]
The styrene resin composition contains (A) 55 parts by mass to 90 parts by mass of styrene resin and (B) 10 parts by mass to 45 parts by mass of polylactic acid with respect to a total of 100 parts by mass of (A) styrene resin and (B) polylactic acid. When the proportion of (B) polylactic acid is 10 parts by mass or more, it is easy to obtain sufficient heat resistance to suppress deformation even when heated in a microwave oven, and when it is 45 parts by weight or less, it is easy to obtain sufficient sheet strength. The styrene resin composition preferably contains (A) 60 parts by mass to 85 parts by mass of styrene resin and (B) 15 parts by mass to 40 parts by mass of polylactic acid, more preferably (A) 60 parts by mass to 75 parts by mass of styrene resin and (B) 25 parts by mass to 40 parts by mass of polylactic acid.

The (A) styrene-based resin used in the present invention is obtained by polymerizing an aromatic vinyl compound-based monomer, and may be subjected to rubber modification by adding a conjugated diene-based rubber-like polymer as necessary. Examples of the polymerization method include known methods such as bulk polymerization, bulk/suspension two-stage polymerization, and solution polymerization. As the aromatic vinyl compound-based monomer, known monomers such as styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene and p-methylstyrene can be used, but styrene is preferred. In addition, monomers such as acrylonitrile, (meth)acrylic acid, (meth)acrylic acid ester, and maleic anhydride that can be copolymerized with these aromatic vinyl compound monomers may be used as long as they do not impair the performance of the styrene resin composition. Furthermore, in the present invention, a polymer obtained by adding a cross-linking agent such as divinylbenzene to the aromatic vinyl compound monomer may be used.

Examples of the conjugated diene-based rubbery polymer (A) used for rubber modification of the styrene resin of the present invention include polybutadiene, random or block copolymers of styrene-butadiene, polyisoprene, polychloroprene, random, block or graft copolymers of styrene-isoprene, ethylene-propylene rubber, ethylene-propylene-diene rubbers, etc. Among them, polybutadiene and random, block or graft copolymers of styrene-butadiene are particularly preferred. Also, these may be partially hydrogenated.

(A) Examples of styrene resins include polystyrene (GPPS), rubber-modified polystyrene (HIPS), ABS resin (acrylonitrile-butadiene-styrene copolymer), AS resin (acrylonitrile-styrene copolymer), MS resin (methyl methacrylate-styrene copolymer), AAS resin (acrylonitrile-acrylic rubber-styrene copolymer), AES resin (acrylonitrile-ethylene propylene-styrene copolymer), and the like. mentioned. Among these, rubber-modified polystyrene (HIPS) is particularly preferred because it can increase the impact resistance of the resin composition.

The molecular weight of the rubber-modified polystyrene (HIPS) is not particularly limited, but the reduced viscosity (η _sp /C) is preferably 0.5 dl/g or more and 1.0 dl/g or less.

The content of the conjugated diene-based rubber-like polymer in the rubber-modified polystyrene (HIPS) is not particularly limited, but is preferably 3% by mass or more and 10% by mass or less.

The (B) polylactic acid used in the present invention is preferably a plant-derived raw material from the viewpoint of reducing carbon dioxide emissions.

In the case of poly(L-lactic acid), the crystallization rate differs depending on the ratio of the D-lactic acid component. Considering the heat resistance and moldability of the resin composition of the present invention, the ratio of the D-lactic acid component is preferably 0.01 mol % or more and 5.0 mol % or less. Especially preferably, it is in the range of 0.01 mol % or more and 1.5 mol % or less.

(B) Polylactic acid preferably has a weight average molecular weight (Mw) of 50,000 to 400,000, more preferably 100,000 to 300,000.

The styrenic resin composition may contain (C) a copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester. (C) A copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester is preferably contained because the sheet strength can be dramatically improved. The (C) copolymer of butadiene and ethylenically unsaturated carboxylic acid ester of the present invention is a thermoplastic elastomer having butadiene and ethylenically unsaturated carboxylic acid as monomer units. Examples of ethylenically unsaturated carboxylic acid esters include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, and the like, and two or more of them may be used in combination. If necessary, it is also possible to combine other monomers copolymerizable with butadiene and/or ethylenically unsaturated carboxylic acid ester. Among the above ethylenically unsaturated carboxylic acid esters, methyl methacrylate is most preferred.

Further, the (C) copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester of the present invention is preferably a multi-layer structure particle in which a polymer particle containing butadiene is used as a core and a monomer containing an ethylenically unsaturated carboxylic acid ester is graft-copolymerized on the surface to form a shell to further increase the impact strength. Examples of such (C) copolymers of butadiene and ethylenically unsaturated carboxylic acid esters include Metabrene C-223A (manufactured by Mitsubishi Chemical Corporation) and Kaneace M-511 (manufactured by Kaneka Corporation).

The amount of the (C) copolymer of butadiene and ethylenically unsaturated carboxylic acid ester added in the resin composition is not particularly limited, but is preferably 1 part by mass or more and 8 parts by mass or less, more preferably 3 parts by mass or more and 8 parts by mass or less, with respect to a total of 100 parts by mass of (A) the styrene resin and (B) polylactic acid. When the amount is 1 part by mass or more, the effect of improving the impact strength is likely to be obtained, and when the amount is 8 parts by mass or less, the effect of improving the heat resistance is likely to be obtained.

In the present embodiment, it is preferable to form a co-continuous structure by kneading (A) the styrene-based resin and (B) the polylactic acid. When (A) styrene-based resin and (B) polylactic acid form a co-continuous structure, (A) styrene-based resin and (B) polylactic acid are entangled with each other, which is preferable in terms of improving sheet strength.

A co-continuous structure refers to a structure in which multiple components are mixed with each other while forming continuous phases. Here, the phase state in the polymer alloy is roughly classified into four types: complete compatibility (single phase), sea-island structure (multiphase), bicontinuous structure (multiphase), and layered structure (multiphase). It is known that most polymer alloys are not completely compatible and form a sea-island structure, co-continuous structure or layered structure. The sea-island structure refers to a structure in which one of a plurality of components is dispersed in a continuous phase and the other is dispersed in the form of particles (islands). Further, the layered structure refers to a structure in which each component forms a continuous phase, but the components are independent without being mixed with each other.

In order to form a co-continuous structure in a polymer alloy, it was found that the resin composition and resin temperature during melt-kneading are important. Extrusion molding is preferably performed at a resin temperature of 160° C. or higher and 240° C. or lower. Within this range, a strong co-continuous structure is formed and the sheet strength is improved.

To the resin composition of the present invention, other additives such as reinforcing materials, flame retardants, dyes and pigments, coloring inhibitors, lubricants, antioxidants, anti-aging agents, light stabilizers, antistatic agents, fillers, crystallization nucleating agents, known additives such as compatibilizers, and modifiers such as coloring agents such as titanium oxide and carbon black can be added to the resin composition of the present invention. The method of adding these is not particularly limited, and they may be added by a known method. For example, it is possible to apply a method of adding in a raw material charging process, a polymerization process, and a finishing process during the production of (A) a styrene resin or (B) polylactic acid, or a process of mixing a resin composition using an extruder or a molding machine.

The method for mixing the resin composition of the present invention is not particularly limited, and known mixing techniques can be applied. For example, using a mixing device such as a mixer type mixer, a V type blender, and a tumbler type mixer, various raw materials are mixed in advance, and the mixture is melt-kneaded to produce a uniform resin composition. The melt-kneading device is also not particularly limited, but examples thereof include Banbury mixers, kneaders, rolls, single-screw extruders, special single-screw extruders, and twin-screw extruders. Furthermore, there is also a method of separately adding an additive such as a flame retardant in the middle of a melt-kneading device such as an extruder.

[Thickness]
The thickness of the foamed sheet of the present invention is 0.5 mm or more and 3.5 mm or less, preferably 1.5 mm or more and 2.5 mm or less, and more preferably 1.5 mm or more and 2.3 mm or less. If the thickness of the foam sheet is within the above range, the foam sheet will be excellent in sheet strength and easy to handle. Moreover, when the thickness of the foamed sheet is at least the lower limit, strength, heat resistance, and heat insulation are excellent. If the thickness of the foamed sheet is equal to or less than the upper limit, the thermoformability will be good.

[Expansion ratio]
The expansion ratio of the foamed sheet of the present invention is 1.1 times or more and 20.0 times or less, preferably 1.5 times or more and 15.0 times or less, and more preferably 1.7 times or more and 12.0 times or less. If the foaming ratio of the foamed sheet is within the above range, the foamed sheet has excellent sheet strength and is easy to handle. Further, when the expansion ratio of the foam sheet is at least the above lower limit value, the heat insulating properties are excellent, and when the expansion ratio of the foam sheet is at most the above upper limit value, the sheet strength is excellent.

[Average bubble diameter]
The foamed sheet of the present invention has an average cell diameter of 30 μm or more and 500 μm or less, preferably 50 μm or more and 350 μm or less, and more preferably 50 μm or more and 300 μm or less. If the average cell diameter of the foamed sheet is within the above range, the foamed sheet will be excellent in sheet strength and easy to handle. Moreover, when the average cell diameter of the foamed sheet is at least the above lower limit, moldability is good and a good molded product can be obtained. When the average cell diameter of the foamed sheet is equal to or less than the upper limit, the foamed sheet becomes soft and cracks are less likely to occur.

[Total amount of volatile components]
The total amount of volatile components in the foamed sheet of the present invention is less than 800 μg/g, preferably less than 700 μg/g, more preferably less than 600 μg/g. If the total amount of volatile components in the foamed sheet is equal to or less than the above upper limit, the molded article will be less likely to deform and will be excellent in heat resistance and oil resistance.

[Closed cell ratio]
The closed cell ratio of the foam sheet of the present invention is 60% or more, preferably 70% or more, more preferably 80% or more. If the closed cell content of the foam sheet is within the above range, the foam sheet will be excellent in sheet strength and easy to handle.

[Production method]
The method for producing the foamed sheet of the present invention is not particularly limited, and examples thereof include a method in which the styrene resin composition, the nucleating agent, etc. are supplied to an extruder, heated and melted, a foaming agent is added and kneaded, extruded from a mold attached to the tip of the extruder to foam, and the resulting foamed sheet is wound up and collected.

As the foaming agent, a widely used one is used. For example, chemical blowing agents such as azodicarbonamide, dinitrosopentamethylenetetramine, hydrazoyldicarbonamide, and sodium bicarbonate; saturated aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and hexane; ethers such as dimethyl ether; and physical blowing agents such as methyl chloride, carbon dioxide, and nitrogen.

Nucleating agents include, for example, talc, sodium hydrogencarbonate, ammonium hydrogencarbonate, calcium carbonate, clay, citric acid and the like. Among them, talc is preferable as the nucleating agent. A nucleating agent may be used individually by 1 type, and may use 2 or more types together. The amount of the nucleating agent to be added is preferably 0.01 parts by mass or more and 5 parts by mass or less with respect to 100 parts by mass of the styrene-based resin composition.

Examples of the die attached to the tip of the extruder include an annular die having an annular opening (circular die), a T-die, and the like. As a specific embodiment in the case of using an annular mold, for example, a cylindrical foam extruded from the annular mold is placed along a cooling mandrel, and cutters provided on both sides of the tip of the cooling mandrel cut the cylindrical foam in the axial direction to form two foam sheets.

The method for controlling the thickness of the foamed sheet, expansion ratio, average cell diameter, total amount of volatile components and closed cell ratio is not particularly limited. For example, by increasing the amount of the nucleating agent used, the expansion ratio is increased. Also, by reducing the amount of foaming agent used, the foaming ratio is reduced. By increasing the melt-kneading temperature of the resin component in the extruder, the total amount of volatile components in the foamed sheet increases. The thickness of the foam sheet can be controlled by changing the amount and type of foaming agent used. Also, by reducing the amount of the nucleating agent used, the average cell diameter increases.

[Effect]
The foamed sheet of the present invention described above controls the ratio of (A) styrene-based resin and (B) polylactic acid, thickness, expansion ratio, average cell diameter, total volatile component content, and independent cell content within specific ranges, so that it has sufficient heat resistance to prevent deformation of the container even when heated in a microwave oven, and has excellent sheet strength and is easy to handle.

<Laminated foam sheet>
The laminated foamed sheet of the present invention has a non-foamed resin film laminated on at least one surface of the foamed sheet of the present invention. The surface on which the non-foamed resin film is laminated may be the front surface, the back surface, or the side surface of the foamed sheet, but it is preferable to be either one or both of the front surface and the back surface.

A non-foamed resin film is a non-foamed film made of a thermoplastic resin. By laminating the non-foamed resin film, the surface of the laminated foamed sheet becomes more beautiful, the rigidity becomes higher, and the heat resistance and oil resistance are further improved. Examples of thermoplastic resins that form non-foamed resin films include polystyrene resins, impact-resistant polystyrene resins, polyethylene resins, polypropylene resins, polyethylene terephthalate resins, polyphenylene ether resins, polyvinylidene chloride resins, ethylene-vinyl alcohol copolymer resins, and the like. Moreover, these may be used individually by 1 type, and may laminate|stack two or more types by a dry lamination etc. The non-foamed resin film may be colored in various tones by adding a coloring agent (pigment, dye, etc.), and may display various patterns and designs by printing on the surface.

The thickness of the non-foamed resin film is preferably 10 μm or more and 150 μm or less, more preferably 10 μm or more and 100 μm or less. When the thickness of the non-foamed resin film is at least the above lower limit, the film tends to stretch during thermoforming, and defects are less likely to occur. If the thickness of the non-foamed resin film is equal to or less than the upper limit, the cost does not increase, and the film can be laminated at a low temperature without loss of glossiness.

Examples of the method of laminating the non-foamed resin film include a method of co-extrusion and lamination of the foamed sheet and the non-foamed resin film, and a method of laminating the non-foamed resin film on the foamed sheet using a heating roll, a binder, an adhesive, or the like.

[Effect]
The laminated foam sheet of the present invention described above has a sufficient heat resistance that can suppress deformation of the container even when heated in a microwave oven, and has excellent sheet strength and is easy to handle.

<Formed container>
The molded container of the present invention is a molded container obtained by molding the foamed sheet or laminated foamed sheet of the present invention.

The shape and dimensions of the molded container of the present invention are not particularly limited, and examples thereof include cups and bowls for instant noodles in cups. The molded container of the present invention is obtained by thermoforming the foamed sheet or laminated foamed sheet of the present invention. At that time, when a laminated foam sheet is used, it is preferable to perform thermoforming so that at least the inside of the container becomes a non-foamed resin film in order to further improve the mechanical strength, heat resistance, and oil resistance of the container.

Examples of the thermoforming method include conventionally known general forming methods such as vacuum forming and pressure forming, or free drawing forming as an application of these, plug and ridge forming, ridge forming, matched mold forming, straight forming, drape forming, reverse draw forming, air slip forming, plug assist forming, and plug assist reverse load forming.

[Effect]
The molded container of the present invention described above uses a foam sheet in which the ratio of (A) styrene-based resin and (B) polylactic acid, thickness, expansion ratio, average cell diameter, total volatile component content, and closed cell rate are controlled within specific ranges, so that the container has sufficient heat resistance to suppress deformation of the container even when heated in a microwave oven, and is excellent in sheet strength and easy to handle.

EXAMPLES The present invention will be described in detail below with reference to Examples, but the present invention is not limited by the following description.
[material]
<Styrene resin (A)>
(A-1); Rubber-modified polystyrene manufactured by Toyo Styrene Co., Ltd. Reduced viscosity of 0.70 dl / g, rubber-like polymer content of 9.2% by mass
<Polylactic acid (B)>
(B-1); "REVODA190" manufactured by Zhejiang Hisun Biomaterials Co., Ltd. D-lactic acid component ratio of 0.5 mol%, weight average molecular weight (Mw) of 200,000 <Copolymer of butadiene and ethylenically unsaturated carboxylic acid ester (C)>
(C-1); "METABLEN C-223A" manufactured by Mitsubishi Chemical Co., Ltd. Multilayer structure particles in which a shell layer is formed by graft-copolymerizing methyl methacrylate and styrene to a polybutadiene (core).

[Confirmation method of co-continuous structure]
Add 50 g of methyl ethyl ketone (MEK) to 0.5 g of the foam sheet to prepare an undissolved material after one week, and use a scanning electron microscope (manufactured by JEOL Ltd., JSM-6510) at a magnification of 2000 times.
With co-continuous structure: Has a continuous network structure Without co-continuous structure: Has a discontinuous structure

[Method for measuring thickness]
Excluding 20 mm at both ends of the foam sheet in the width direction, 21 points were measured at intervals of 50 mm in the width direction. The thickness at this measurement point was measured to the minimum unit of 0.1 mm using a thickness gauge 547-313 (manufactured by Mitutoyo). The average value of these measured values was taken as the thickness T (mm) of the foamed sheet.

[Expansion ratio]
Ten pieces of 10 cm×10 cm were cut out at equal intervals in the width direction except for 20 mm at both ends in the width direction of the foam sheet, and the mass (g) of each piece was measured to the nearest 0.001 g. The basis weight M (g/m ² ) was obtained by converting the average value of the mass (g) of each section into the mass per 1 m ² . From the thickness T (mm) and the basis weight M, the apparent density ρ was determined by the following formula (1), and the expansion ratio was determined by the following formula (2).
Apparent density ρ (g/cm ³ )=M/(T×10 ³ ) (1)
Foaming ratio = true density/apparent density ρ (2)

[Method for measuring average bubble diameter]
The average bubble diameter in the thickness direction of the sheet was obtained. A vertical section of the foamed sheet in the extrusion direction was observed using a scanning electron microscope (JSM-6510, manufactured by JEOL Ltd.). A vertical straight line is drawn over the entire thickness of the foam sheet, the average chord length is obtained using the following formula (3) from the length of the straight line and the number of cells intersecting the straight line based on ASTM D2842-06, and the average cell diameter is calculated using the following formula (4).
Average chord length = length of straight line / number of bubbles (3)
Average bubble diameter (μm)=average chord length/0.616 (4)

[Measurement method of total volatile content]
500 mg of the foam sheet was dissolved in 10 ml of dimethylformamide (DMF) containing cyclopentanol as an internal standard substance, and the content of volatile components (styrene monomer, toluene, ethylbenzene, cyclohexane, xylene) was measured using a gas chromatograph.
(Measurement condition)
Gas chromatograph: HP-5890 (manufactured by Hewlett-Packard)
Column: HP-WAX, 0.25 mm × 30 m, film thickness 0.5 μm
Injection temperature: 220°C
Column temperature: 60°C to 150°C, 10°C/min
Detector temperature: 220°C
Split ratio: 30/1

[Measurement method of closed cell ratio]
The foamed sheet is cut into test pieces of 25 mm long and 25 mm wide, and the number of test pieces is prepared such that when the test pieces are piled up, they are about 25 mm long. The volume (V1) of the prepared test piece was determined using a pneumatic hydrometer (Shimadzu Corporation Accupic II1340TC-100cc). The total weight (M0) and apparent volume (V0) of each test piece were measured, and the closed cell ratio was calculated by the following formula (5).
Closed cell ratio (%) = (V0-V1)/(V0-M0/true density) x 100 (5)

[Heat resistance evaluation method]
A heat-resistant container obtained by heat-molding a circular bowl-shaped container having a diameter of 100 mm and a depth of 60 mm using a foam sheet was placed in an oven at 100° C. and heated for 1 hour. After that, in the bowl container, the dimensional change amount was obtained by measuring the diameters in mutually orthogonal directions, and the ratio of the orthogonal dimensions (minor diameter/major diameter) was determined according to the following criteria.
○: The dimension ratio is 0.7 or more.
x: The dimensional ratio is less than 0.7, and the deformation is large and cannot be used.

[Method for evaluating sheet strength]
Using a foam sheet, measurement was performed with a film impact tester BU-302 (manufactured by Tester Sangyo Co., Ltd.) with an impact spherical surface R of 12.7 mm. The measurement was performed 20 times each on the front and back sides of the foamed sheet, and the average value of all measurements was taken as the sheet strength (kJ/m).

<Example 1 (reference example) >
[Production of resin composition]
Each reagent (styrene resin (A-1) 70 parts by mass, polylactic acid (B-1) 30 parts by mass) was premixed in a Henschel mixer (manufactured by Nippon Coke Kogyo Co., Ltd., FM20B), and then a twin-screw extruder (manufactured by Toshiba Machine Co., Ltd., TEM26SS) to form strands, water-cooled, and led to a pelletizer to prepare pellets. At this time, it was heated at a cylinder temperature of 200° C. and the feed rate was set at 30 kg/h.

[Production of foam sheets]
To 100 parts by mass of the pellets, 1.0 parts by mass of a nucleating agent (manufactured by Toyo Styrene Co., Ltd.: "DSM1401A") was added, put into a first extruder (manufactured by Toshiba Machine Co., Ltd., diameter 40 mm), and the cylinder temperature was 210 ° C. Heated, kneaded and melted. Then, 2.5 parts by mass of carbon dioxide as a foaming agent was injected into 100 parts by mass of the resin composition through an injection port provided in the middle of the first extruder, and mixed with the resin composition. Then, the resin composition kneaded with the nucleating agent and the foaming agent was supplied from the first extruder to the second extruder (manufactured by Toshiba Machine Co., Ltd., diameter 50 mm), cooled to a resin temperature of 170 ° C. near the die outlet, and extruded and foamed using a circular die to obtain a cylindrical body at a discharge rate of 10 kg / h. A foam sheet was obtained by cutting the obtained cylindrical body at one point along the direction of extrusion and taking it off at 3.5 m/min. The resulting foamed sheet had a co-continuous structure, a total volatile content of 400 µg/g, a thickness of 2.0 mm, an expansion ratio of 10.9, an average cell diameter of 300 µm, and a closed cell ratio of 80%.

<Examples 2 to 6, Comparative Examples 1 to 7 (Examples 2, 3, 5, and 6 are reference examples) >
A foam sheet was produced and evaluated in the same manner as in Example 1, except that the composition of the resin composition, the conditions for producing the pellets, and the conditions for producing the foam sheet were changed as shown in Table 1. Table 1 shows the results. In Comparative Example 2, the foam sheet could not be extruded, and the foam sheet could not be produced.

As shown in Table 1, the examples using the foamed sheet of the present invention are excellent in sheet strength and heat resistance.

Claims (4)

A foamed sheet obtained by foaming a styrene-based resin composition,
The styrene resin composition contains (A) 55 parts by mass to 90 parts by mass of a styrene resin, (B) 10 parts by mass to 45 parts by mass of polylactic acid , and (C) 1 part by mass to 8 parts by mass of a copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester, based on a total of 100 parts by mass of (A) a styrene resin and (B) polylactic acid.
The (C) copolymer of butadiene and an ethylenically unsaturated carboxylic acid ester is a multi-layer structure particle in which a polymer particle containing butadiene is used as a core, and a monomer containing an ethylenically unsaturated carboxylic acid ester is graft-copolymerized on the surface thereof to form a shell,
The foam sheet has a thickness of 0.5 mm or more and 3.5 mm or less, an expansion ratio of 1.1 times or more and 20.0 times or less, an average cell diameter of 30 μm or more and 500 μm or less, a total volatile content of less than 800 μg/g, and a closed cell rate of 60% or more. The foam sheet according to claim 1, wherein the (A) styrene-based resin and the (B) polylactic acid form a co-continuous structure. A laminated foam sheet comprising a non-foamed resin film made of a thermoplastic resin laminated on at least one surface of the foam sheet according to claim 1 or 2 . A molded container obtained by molding the foamed sheet according to claim 1 or 2 or the laminated foamed sheet according to claim 3 .