Biaxially oriented sheet, multilayer sheet, and molded article

Abstract

To provide a biaxially oriented sheet having superior resistance to microwave heating.SOLUTION: A biaxially oriented sheet comprises a crosslinked product of a resin composition containing a polystyrene-based resin, a polyester, and a crosslinker.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially stretched sheet, a multilayer sheet, and a molded article.

Background Art

[0002] A biaxially stretched sheet containing a styrene resin is used as a packaging material. For example, when used for food packaging, a biaxially stretched sheet is required to have resistance to range heating, which is difficult to deform even when heated in a microwave oven with the oil content of the contents adhering thereto. For example, Patent Document 1 describes that a heat-resistant and oil-resistant laminated body can be obtained by applying a hydrophilic coating agent to a styrene resin sheet using a copolymer obtained by copolymerizing a styrene monomer and methacrylic acid at a specific ratio, and the laminated body can be suitably used as a food packaging material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] One aspect of the present invention aims to provide a biaxially stretched sheet having excellent resistance to range heating.

Means for Solving the Problems

[0005] The inventors have found that, even under heating conditions by a microwave oven that would cause deformation in a biaxially stretched sheet using only one of a styrene resin and a polyester, by forming a crosslinked product of a resin composition using both a styrene resin and a polyester together with a crosslinking agent, the resistance to range heating is improved, making it less likely to deform and suppressing the occurrence of perforations. The present invention provides the following [1] to [7] in several aspects. [1] A biaxially stretched sheet comprising a crosslinked product of a resin composition containing a polystyrene resin, a polyester, and a crosslinking agent. [2] The biaxially stretched sheet according to [1], wherein the polystyrene resin contains a styrene monomer and at least one monomer selected from the group consisting of (meth)acrylic acid, (meth)acrylic acid ester, and maleic anhydride as monomer units. [3] The biaxially stretched sheet according to [1] or [2], wherein the polyester contains an aromatic dicarboxylic acid and a diol as monomer units. [4] The biaxially stretched sheet according to [3], wherein at least one of the aromatic dicarboxylic acid and the diol is derived from biomass. [5] The biaxially stretched sheet according to any one of [1] to [4], wherein the crosslinking agent contains a compound having an epoxy group. [6] A multilayer sheet comprising a first layer made of the biaxially stretched sheet according to any one of [1] to [5] and a second layer provided on the first layer. [7] A molded article formed by molding the biaxially stretched sheet according to any one of [1] to [5] or the multilayer sheet according to [6]. [Advantages of the Invention]

[0006] According to one aspect of the present invention, a biaxially stretched sheet excellent in resistance to range heating is provided. [Modes for Carrying Out the Invention]

[0007] Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.

[0008] The biaxially stretched sheet according to one embodiment is composed of a crosslinked body of a resin composition containing a polystyrene-based resin, a polyester, and a crosslinking agent. According to the studies of the present inventors, when using a biaxially stretched sheet composed of a crosslinked body of a resin composition containing only either a polystyrene-based resin or a polyester as a material for a molded article for microwave heating, depending on the output and heating time of the microwave, the molded article may be deformed by heating using the microwave. However, by using a biaxially stretched sheet composed of a crosslinked body of a resin composition containing both a polystyrene-based resin and a polyester and further containing a crosslinking agent, it is possible to impart range heating resistance to the molded article, which can withstand the conditions under which the molded article is deformed when using a biaxially stretched sheet containing only either a polystyrene-based resin or a polyester.

[0009] The polystyrene-based resin is a polymer mainly containing a styrene-based monomer as a monomer unit. The styrene-based monomer may be styrene (unsubstituted styrene) or substituted styrene. Unsubstituted styrene refers to styrene having no substituent (styrene having no functional group other than one vinyl group). Substituted styrene refers to styrene in which at least one of the hydrogen atoms on the benzene ring in styrene and the hydrogen atoms constituting the vinyl group is substituted with a substituent such as an alkyl group or a halogeno group. Examples of substituted styrene include alkyl styrene, halogenated styrene, nitrostyrene, acetyl styrene, and methoxy styrene.

[0010] Alkyl styrene is a compound in which at least one of the hydrogen atoms in styrene is substituted with an alkyl group. The number of carbon atoms of the alkyl group in alkyl styrene may be 1 or more, 8 or less, 5 or less, or 3 or less. The number of alkyl groups in alkyl styrene may be 1 or more, 3 or less, or 2 or less.

[0011] Examples of alkylstyrenes having one alkyl group include α-alkylstyrene, o-alkylstyrene, m-alkylstyrene, and p-alkylstyrene. Examples of α-alkylstyrene include α-methylstyrene. Examples of o-alkylstyrene include o-methylstyrene and o-ethylstyrene. Examples of m-alkylstyrene include m-methylstyrene and m-ethylstyrene. Examples of p-alkylstyrene include p-methylstyrene, p-ethylstyrene, and p-tert-butylstyrene.

[0012] Examples of alkylstyrenes having two alkyl groups include 2,4-dimethylstyrene, 2,5-dimethylstyrene, 3,4-dimethylstyrene, 3,5-dimethylstyrene, and 4,α-dimethylstyrene (1-methyl-4-isopropenylbenzene). An example of an alkylstyrene having three alkyl groups is 2,4,6-trimethylstyrene.

[0013] Halogenated styrene is a compound in which at least one of the hydrogen atoms in styrene is substituted by a halogeno group. The number of halogeno groups in the halogenated styrene may be 1 or more, and may be 3 or less, or 2 or less. Examples of halogenated styrene include fluorostyrene, chlorostyrene, dichlorostyrene, bromostyrene, dibromostyrene, and iodostyrene.

[0014] The styrenic monomer is preferably at least one selected from the group consisting of styrene and α-methylstyrene, and more preferably styrene.

[0015] The polystyrene resin may contain only styrenic monomers as monomer units, or may contain other monomers in addition to styrenic monomers. The content of styrenic monomers may be 80% by mass or more, 85% by mass or more, or 88% by mass or more, based on the total mass of the monomer units contained in the polystyrene resin, and may be 99% by mass or less, 95% by mass or less, or 93% by mass or less.

[0016] Examples of other monomers include unsaturated carboxylic acids, unsaturated carboxylic acid esters, carboxylic anhydrides, vinyl carboxylates, conjugated dienes, α-olefins, (meth)acrylonitrile, and the like.

[0017] Examples of unsaturated carboxylic acids include (meth)acrylic acid, fumaric acid, maleic acid, and itaconic acid. Examples of unsaturated carboxylic acid esters include (meth)acrylic acid esters. Examples of carboxylic anhydrides include maleic anhydride. Examples of vinyl carboxylates include vinyl acetate. Examples of conjugated dienes include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and 1,3-hexadiene. Examples of α-olefins include ethylene, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene.

[0018] From the viewpoint of further improving the range heating resistance of the biaxially stretched sheet, it is preferable that the polystyrene resin contains, as monomer units, a styrenic monomer and at least one monomer selected from the group consisting of (meth)acrylic acid, (meth)acrylic acid ester, and maleic anhydride.

[0019] When the polystyrene resin contains other monomers (preferably at least one monomer selected from the group consisting of (meth)acrylic acid, (meth)acrylic acid ester, and maleic anhydride), the content of the monomer may be 1% by mass or more, 5% by mass or more, or 7% by mass or more based on the total mass of the monomer units contained in the polystyrene resin from the viewpoint that the biaxially stretched sheet has further excellent heat resistance. Further, the content of the monomer may be 20% by mass or less, 15% by mass or less, or 13% by mass or less.

[0020] The weight average molecular weight (Mw) of the polystyrene resin may be 150,000 or more, 170,000 or more, 200,000 or more, 230,000 or more, 250,000 or more, or 260,000 or more from the viewpoint that the strength of the molded product is further excellent. The weight average molecular weight (Mw) of the polystyrene resin may be 450,000 or less, 400,000 or less, 350,000 or less, or 300,000 or less from the viewpoint that the biaxially stretched sheet has excellent moldability.

[0021] The Z average molecular weight (Mz) of the polystyrene resin may be 300,000 or more, 350,000 or more, 400,000 or more, or 420,000 or more from the viewpoint that the strength of the molded product is further excellent. The Z average molecular weight (Mz) of the polystyrene resin may be 600,000 or less, 550,000 or less, 500,000 or less, or 480,000 or less from the viewpoint that the biaxially stretched sheet has excellent moldability.

[0022] In this specification, the weight average molecular weight (Mw) and the Z average molecular weight (Mz) are determined by performing GPC measurement under the following conditions. Apparatus: GPC "HLC-8320GPC" manufactured by Tosoh Corporation Column: shodex KF404×3 Temperature: 40 °C Solvent: Tetrahydrofuran Flow rate: 0.2 ml / min Pressure: 10 MPa Detection: RI Sample preparation method: After dissolving 120 mg of the sample in 15 mL of tetrahydrofuran, filtration is performed through a syringe filter (Millex (registered trademark) 0.45 μm manufactured by Merck Millipore). Injection volume: 10 μl Calibration curve: Using standard polystyrene (manufactured by Polymer Laboratories), the relationship between the elution time and the elution volume is converted into molecular weight to determine various average molecular weights.

[0023] The polystyrene resin is obtained by polymerizing a styrene monomer and, if necessary, other monomers. Examples of the polymerization method include known polymerization methods such as bulk polymerization, solution polymerization, and suspension polymerization, which are adopted as industrial production methods for polystyrene and the like. In terms of quality and productivity, bulk polymerization or solution polymerization is preferred. Also, in terms of quality and productivity, continuous polymerization or continuous production by continuous polymerization is preferred. Examples of the solvent that can be used include alkylbenzenes such as benzene, toluene, ethylbenzene, and xylene, ketones such as acetone and methyl ethyl ketone, aliphatic hydrocarbons such as hexane and cyclohexane, and alcohols such as octanol.

[0024] When polymerizing the polystyrene resin, a polymerization initiator, a chain transfer agent, and a surfactant can be used as necessary. As the polymerization initiator, for example, an organic peroxide can be used. Specific examples of the organic peroxide include benzoyl peroxide, t-butyl peroxybenzoate, 1,1-bis(t-butylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, t-butyl peroxyisopropyl carbonate, dicumyl peroxide, t-butylcumyl peroxide, t-butyl peroxyacetate, t-butyl peroxy-2-ethylhexanoate, polyether tetrakis(t-butylperoxycarbonate), ethyl-3,3-di(t-butylperoxy)butyrate, t-butyl peroxyisobutyrate, and the like. Specific examples of the chain transfer agent include aliphatic mercaptan, aromatic mercaptan, pentaphenylethane, α-methylstyrene dimer, and terpinolene. Specific examples of the surfactant include nonionic surfactants such as polyoxyethylene alkyl ether.

[0025] From the viewpoint that the biaxially stretched sheet has further excellent heat resistance, the content of the polystyrene-based resin in the resin composition may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more based on the total mass of the resin composition. In the description of the above numerical range, "or more" may mean "exceeding". From the viewpoint that the biaxially stretched sheet has further excellent oil resistance, the content of the polystyrene-based resin in the resin composition may be 99% by mass or less, 95% by mass or less, 90% by mass or less, 80% by mass or less, 76% by mass or less, or 75% by mass or less based on the total mass of the resin composition. In the description of the above numerical range, "or less" may mean "less than".

[0026] Polyester is a polymer having an ester bond in the main chain and contains a polyvalent carboxylic acid and a polyhydric alcohol as monomer units. The polyvalent carboxylic acid may be a dicarboxylic acid, and the polyhydric alcohol may be a diol. The polyester may be an amorphous polyester or a crystalline polyester.

[0027] Examples of the dicarboxylic acid include aromatic dicarboxylic acids and aliphatic dicarboxylic acids. Examples of the aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid, etc. Examples of the aliphatic dicarboxylic acids include malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dimer acid, dodecanedioic acid, cyclohexanedicarboxylic acid, methylmalonic acid, ethylmalonic acid, etc.

[0028] Examples of the diol include a diol having a cyclic skeleton and a linear aliphatic diol. Examples of the diol having a cyclic skeleton include cyclobutanediol, cyclopentanediol, cyclohexanediol, cycloheptanediol, cyclooctanediol, cyclopropanedimethanol, cyclobutanedimethanol, cyclopentanedimethanol, cyclohexanedimethanol (1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, etc.), 2,2,4,4-tetramethyl-1,3-cyclobutanediol, isosorbide, 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane, 5-methylol-5-ethyl-2-(1,1-dimethyl-2-hydroxyethyl)-1,3-dioxane, spiroglycol, etc. The diol having a cyclic skeleton preferably contains at least one selected from the group consisting of an alicyclic diol and a heterocyclic diol, and more preferably contains at least one of isosorbide and 1,4-cyclohexanedimethanol.

[0029] Examples of the linear aliphatic diol include, for example, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, ethylene glycol, diethylene glycol, triethylene glycol, polyalkylene glycol, hydroquinone, bisphenol, etc. The linear aliphatic diol preferably contains ethylene glycol.

[0030] The polyester preferably contains a biomass-derived compound as a monomer unit. At least one of the above polyvalent carboxylic acid and polyvalent alcohol may be derived from biomass. For example, the polyester preferably contains an aromatic dicarboxylic acid and a diol as monomer units, and at least one of the aromatic dicarboxylic acid and the diol is preferably derived from biomass.

[0031] Polyester is obtained by polymerizing a polyvalent carboxylic acid and a polyhydric alcohol. As the polymerization method, for example, a condensation polymerization method or a transesterification method using an ester of a polyvalent carboxylic acid and a polyhydric alcohol can be used.

[0032] As the polyester, commercially available products such as amorphous polyester containing spiroglycol (for example, "ALTESTER (registered trademark)" manufactured by Mitsubishi Gas Chemical Company, Inc.), amorphous polyester containing 2,2,4,4-tetramethyl-1,3-cyclobutanediol ( "TRITAN (registered trademark)" manufactured by Eastman Chemical Company), and amorphous polyester containing isosorbide may be used.

[0033] The content of the polyester in the resin composition may be 1% by mass or more, 3% by mass or more, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or more than 25% by mass, based on the total mass of the resin composition, and may be 70% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, 40% by mass or less, or 30% by mass or less. When the content is at least the above lower limit value, the biaxially stretched sheet is further excellent in oil resistance. When the content is at most the above upper limit value, the biaxially stretched sheet is further excellent in heat resistance.

[0034] The crosslinking agent is a crosslinking agent capable of crosslinking the resins (polymers) contained in the resin composition. The crosslinking agent may be, for example, a crosslinking agent in which the crosslinking reaction proceeds by heating. In one embodiment, when the polystyrene-based resin contains, as monomer units, monomers having a carboxyl group such as (meth) acrylic acid and maleic anhydride in addition to the styrene-based monomer, the crosslinking agent may be a crosslinking agent capable of crosslinking the carboxyl groups of the polystyrene-based resin, a crosslinking agent capable of crosslinking the hydroxyl groups present at the terminals of the polyester, or a crosslinking agent capable of crosslinking between the carboxyl group of the polystyrene-based resin and the hydroxyl group present at the terminal of the polyester.

[0035] Examples of the crosslinking agent include epoxy-based crosslinking agents, oxazoline-based crosslinking agents, maleic acid-based crosslinking agents, and the like. The crosslinking agent may be, for example, a compound having an epoxy group, a compound having an oxazoline group, or the like. The compound having an epoxy group may be, for example, a polymer containing a monomer having an epoxy group as a monomer unit. The monomer having an epoxy group may be, for example, a monomer having an epoxy group and a (meth)acryloyl group. Examples of the monomer having an epoxy group and a (meth)acryloyl group include glycidyl (meth)acrylate. Further, the compound having an epoxy group may be an epoxidized oil. Examples of the epoxidized oil include epoxidized vegetable oils (epoxidized soybean oil, epoxidized linseed oil, etc.).

[0036] The polymer containing a monomer having an epoxy group as a monomer unit may be a copolymer containing a monomer having an epoxy group and a styrene-based monomer as monomer units. As the styrene-based monomer, those described above can be used without particular limitation.

[0037] The compound having an oxazoline group may be, for example, a polymer containing a monomer having an oxazoline group as a monomer unit. Examples of the monomer having an oxazoline group include 2-isopropenyl-2-oxazoline. may include.

[0038] The polymer containing a monomer having an oxazoline group as a monomer unit may be a copolymer containing a monomer having an oxazoline group and a styrene-based monomer as monomer units. As the styrene-based monomer, those described above can be used without particular limitation.

[0039] The content of the crosslinking agent may be 0.01 part by mass or more, 0.05 part by mass or more, 0.1 part by mass or more, or 0.3 part by mass or more, from the viewpoint of further suppressing the generation of holes during range heating, based on 100 parts by mass in total of the contents of the polystyrene resin and the polyester. The content of the crosslinking agent may be 10 parts by mass or less, 5 parts by mass or less, 3 parts by mass or less, or 2 parts by mass or less, based on 100 parts by mass in total of the contents of the polystyrene resin and the polyester.

[0040] The resin composition may further contain other components. Examples of other components include antioxidants, anti-gelling agents (e.g., polyoxyethylene alkyl ethers (POE)), ultraviolet absorbers, light stabilizers, lubricants, plasticizers, colorants, antistatic agents, flame retardants, mineral oils, reinforcing fibers such as glass fibers, carbon fibers, and aramid fibers, and fillers such as talc, silica, mica, and calcium carbonate. The other components may be added in the polymerization step, devolatilization step, or granulation step of the polystyrene resin and the polyester, or may be added when mixing the polystyrene resin and the polyester during the production of the biaxially stretched sheet. The total content of the other components may be, for example, 0.1% by mass or more and 1.0% by mass or less, based on the total mass of the resin composition.

[0041] The content of the anti-gelling agent may be less than 0.5% by mass, 0.4% by mass or less, or 0.3% by mass or less, based on the total mass of the polystyrene resin, from the viewpoint of further improving the heat resistance of the polystyrene resin.

[0042] The biaxially stretched sheet according to one embodiment is composed of a crosslinked product of the above resin composition. In other words, the biaxially stretched sheet is a sheet in a state where the crosslinked product of the above resin composition is biaxially stretched. Here, the crosslinked product of the resin composition means a product in which the resin (specifically, a polystyrene-based resin and a polyester) is crosslinked by at least a part of the crosslinking agent contained in the resin composition. The crosslinking of the resin by the crosslinking agent includes at least one of crosslinking between the polystyrene-based resin and the polyester, crosslinking between polystyrene-based resins, crosslinking between polyesters, and intramolecular crosslinking of the polystyrene resin or the polyester. In one embodiment, the crosslinking of the resin by the crosslinking agent may include crosslinking between the polystyrene-based resin and the polyester, and may include crosslinking between the polystyrene-based resin and the polyester, at least one of crosslinking between polystyrene-based resins, crosslinking between polyesters, and intramolecular crosslinking of the polystyrene resin or the polyester.

[0043] The thickness of the biaxially stretched sheet may be 0.1 mm or more, 0.15 mm or more, or 0.2 mm or more, and may be 0.7 mm or less, 0.5 mm or less, 0.4 mm or less, or 0.3 mm or less. The thickness of the biaxially stretched sheet may be, for example, 0.1 to 0.7 mm, or 0.1 to 0.3 mm.

[0044] The thermal shrinkage stress of the biaxially stretched sheet may be 0.2 MPa or more, 0.3 MPa or more, 0.4 MPa or more, 0.5 MPa or more, or 0.6 MPa or more, and may be 1.0 MPa or less, 0.9 MPa or less, or 0.8 MPa or less.

[0045] The draw ratios in the MD (Machine Direction; the sheet flow direction) and TD (Transverse Direction; the direction perpendicular to the sheet flow direction) of the biaxially stretched sheet may be 1.9 times or more, 2.0 times or more, 2.3 times or more, or 2.4 times or more, respectively, and may be 4.0 times or less, 3.5 times or less, or 3.0 times or less. When the draw ratio is equal to or higher than the above lower limit value, the strength of the molded product is excellent. Also, when the draw ratio is equal to or lower than the above upper limit value, the moldability of the biaxially stretched sheet is excellent. The draw ratio in the MD and the draw ratio in the TD may be the same as each other or may be different.

[0046] When the draw ratio in the MD of the biaxially stretched sheet is m and the draw ratio in the TD is n, the area draw ratio represented by m × n may be 4 times or more, 5 times or more, or 6 times or more, and may be 16 times or less, 13 times or less, or 10 times or less.

[0047] The draw ratio in this specification is calculated as the ratio of the change in the test piece of the biaxially stretched sheet before and after heating. Specifically, the draw ratio is expressed by the following formula: Draw ratio = Y / Z (unit [times]) This formula means the value calculated by this formula. In this formula, Y represents the length [mm] of the straight line drawn in the MD or TD with respect to the test piece of the biaxially stretched sheet before heating, and Z represents the length [mm] of the straight line after the test piece is left in an oven at a temperature 30°C higher than the Vicat softening point temperature of the biaxially stretched sheet measured in accordance with JIS K7206 for 60 minutes to shrink. When a straight line is drawn in the MD, the draw ratio in the MD is calculated, and when a straight line is drawn in the TD, the draw ratio in the TD is calculated.

[0048] The above biaxially stretched sheet can be manufactured by the following method. First, a polystyrene-based resin, a polyester, a crosslinking agent, and optional components (the other components described above) used as necessary are mixed to obtain a resin composition. After melt-kneading the resin composition, it is molded (for example, extrusion molding or press molding) to obtain an unstretched sheet. Next, the unstretched sheet is stretched sequentially or simultaneously in the biaxial direction to obtain a biaxially stretched sheet. The stretching of the unstretched sheet may be performed while heating. The stretching temperature may be 90 °C or higher, 100 °C or higher, or 105 °C or higher, and may be 150 °C or lower, or 145 °C or lower. In the production of the biaxially stretched sheet, a crosslinked product of the resin composition is formed by heating a resin composition containing a polystyrene-based resin, a polyester, and a crosslinking agent (for example, during the above melt-kneading).

[0049] Another embodiment of the present invention is a multilayer sheet including a layer made of the above biaxially stretched sheet. The multilayer sheet includes a first layer made of a biaxially stretched sheet and a second layer provided on the first layer. The multilayer sheet may further include a third layer provided on the side of the first layer opposite to the second layer, a fourth layer provided on the second layer, and the like. Examples of the second layer, the third layer, and the fourth layer include an anti-fogging layer containing an anti-fogging agent, a release layer containing a release agent, an antistatic layer containing an antistatic agent, a design layer containing a coloring agent (a layer for patterning the multilayer sheet), an antibacterial layer containing an antibacterial agent, and a gas barrier layer containing a gas barrier substance (for example, ethylene-vinyl alcohol copolymer) that suppresses the permeation of a gas (such as oxygen).

[0050] The above multilayer sheet can be manufactured, for example, by applying a coating liquid containing components (anti-fogging agent, release agent, antistatic agent, coloring agent, antibacterial agent, gas barrier substance, etc.) according to the purpose to at least one surface of the biaxially stretched sheet to form a second layer or the like. The method of applying the coating liquid is not particularly limited, and may be a method using a roll coater, a knife coater, a gravure roll coater, etc., or may be spraying, dipping, or the like.

[0051] The biaxially stretched sheet and the multilayer sheet described above can be used, for example, to produce molded articles. The molded article may be, for example, a container, and may be a food packaging container (food pack). The molded article may be a food packaging container for microwave heating. The molded article may be, for example, a lid material for a food packaging container that houses food.

[0052] The method for obtaining a molded article from a biaxially stretched sheet is not particularly limited, and a method commonly used in conventional secondary forming methods for biaxially stretched sheets can be used. For example, secondary forming can be performed by a thermoforming method such as a vacuum forming method or a pressure air forming method. These methods are described, for example, in "Plastic Processing Technology Handbook" edited by the Polymer Society, Nikkan Kogyo Shimbun (1995).

Examples

[0053] Hereinafter, the present invention will be described more specifically based on examples. Note that the present invention is not limited to these examples.

[0054] [Synthesis Example 1: Synthesis of Polystyrene Resin] The polystyrene resin was produced according to the following procedure. An apparatus in which a first reactor and a second reactor, which are completely mixed type stirring tanks, are connected in series was used. The capacities of both the first reactor and the second reactor were 39 liters. Styrene 76.4% by mass, methacrylic acid 7.1% by mass, ethylbenzene 14.1% by mass, and octanol 2.4% by mass were mixed to prepare a raw material solution, and the raw material solution was continuously supplied to the first reactor at a flow rate of 12 kg / h. A polymerization initiator (1,1-bis(t-butylperoxy)cyclohexane (manufactured by NOF Corporation)) was added and mixed with the raw material solution from the inlet of the first reactor so that the addition concentration (concentration based on mass with respect to the raw material solution) was 200 μg / g. Polyoxyethylene alkyl ether (POE) (manufactured by Kao Corporation) was added and mixed with the raw material solution from the inlet of the first reactor. The mixture of materials supplied to the first reactor was stirred in the first reactor, continuously supplied to the second reactor, and stirred in the second reactor. At this time, the reaction temperatures in the first reactor and the second reactor were 125°C and 135°C, respectively. By the above method, a copolymer of styrene and methacrylic acid was produced.

[0055] Subsequently, the solution containing the above copolymer was continuously taken out from the second reactor and introduced in series into a vacuum devolatilization tank with a preheater composed of two stages, a first degassing tank and a second degassing tank. In the first degassing tank, the resin temperature was adjusted to 172°C and the pressure to 65 kPa, and in the second degassing tank, the resin temperature was adjusted to 218°C and the pressure to 1 kPa to separate unreacted styrene, methacrylic acid, and ethylbenzene. Then, it was extruded in a strand shape from a porous die, and the strand was cooled and cut by a cold cut method to obtain a pellet-shaped polystyrene resin containing styrene and methacrylic acid as monomer units. As a result of analysis using thermal decomposition gas chromatography, the monomer content in the synthesized polystyrene resin was 90% by mass for styrene and 10% by mass for methacrylic acid based on the total mass of the monomer units contained. Also, the weight average molecular weight (Mw) and Z average molecular weight (Mz) determined by GPC measurement were 280,000 and 450,000, respectively.

[0056] Incidentally, the content of POE in the obtained pellet-shaped polystyrene resin was 0.20% by mass based on the total mass of the polystyrene resin as a result of measurement by high performance liquid chromatography. The concentration of POE was specifically determined by the following procedure. 5 g of the pellet-shaped polystyrene resin was precisely weighed and dissolved in THF. Methanol and a small amount of hydrochloric acid were added to the solution to reprecipitate the polymer component, and the precipitate was removed by filtration. The filtrate was concentrated to obtain 10 ml of a concentrated solution. The concentration of POE in the concentrated solution was quantified by high performance liquid chromatography (HPLC). A calibration curve prepared using three points of a methanol solution of POE with known concentrations was used for the quantification. The HPLC conditions are as follows. HPLC model: Alliance system 2695 separation module manufactured by Waters K.K., Japan Detector: Differential refractometer (RI) Column: TSKgel ODS-120T 4.6 mm (ID) × 15 cm (L) manufactured by Tosoh Corporation Mobile phase: Methanol / water = 80 / 20 (volume ratio) with 0.2% by mass of phosphoric acid added Flow rate: 1.0 ml / min Column oven temperature: 40 °C Detector temperature: 30 °C Using the determined quantitative value (g) of POE, the concentration of POE was determined according to the following formula. Concentration of POE (% by mass) = (Quantitative value of POE) / 5 × 100

[0057] [Polyester] As the polyester, an amorphous polyester containing terephthalic acid as an aromatic dicarboxylic acid and ethylene glycol, isosorbide, and 1,4-cyclohexanedimethanol as monomer units was used, and a part of ethylene glycol and isosorbide was derived from biomass.

[0058] [Crosslinking agent] As the crosslinking agent, CESA Extend 1598 manufactured by Clariant was used. This crosslinking agent is a crosslinking agent masterbatch in which a polymer crosslinking agent obtained by copolymerizing a styrene monomer and an acrylic monomer containing an epoxy group is dispersed in a polystyrene resin.

[0059] <Comparative Examples 1 to 4 and Examples 1 to 4> The components of the types and amounts shown in Table 1 were melt-kneaded using a small twin-screw extruder (Process 11 manufactured by Thermo Fisher Scientific). The extrusion temperature was 250 °C and the screw rotation speed was 250 RPM. The composition obtained by kneading and extruding was press-molded into a plate shape, and a biaxially stretched sheet was obtained by sequentially biaxially stretching it 2.45 × 2.45 times at a stretching temperature of 144 °C using a batch stretching machine (SDR-507D manufactured by EVA Measuring Instruments). The thickness of the biaxially stretched sheet after stretching was 0.25 mm. In Comparative Example 3, it was impossible to stretch under the same conditions because it broke at the portion held by the chuck during stretching.

[0060] <Physical properties of the biaxially stretched sheet> For each of the biaxially stretched sheets of Comparative Examples 1 to 2, 4 and Examples 1 to 4 produced, the physical properties were examined as follows.

[0061] [Thermal shrinkage stress] The biaxially stretched sheet was cut into strips with a width of 20 mm and a length of 150 mm to prepare samples. Both ends of the sample were gripped with chucks and immersed in an oil bath at 150 °C for heating. The load applied to the chuck when the molecular orientation was relaxed by thermal motion and the sheet thermally shrank was measured. The thermal shrinkage stress was determined by dividing the maximum load by the cross-sectional area of the sample. Note that the thermal shrinkage stress is an index of the degree of molecular orientation of the biaxially stretched sheet.

[0062] [Evaluation of resistance to range heating] (Presence or absence of deformation) Using a biaxially stretched sheet, a lid material (dimensions: 15 cm in length × 15 cm in width × 1.2 cm in height) was formed by a hot plate pressure air forming machine (HPT-400S manufactured by Wakizaka Engineering) under the conditions of a forming temperature of 145°C and a mold temperature of 95°C. The obtained lid material was placed on a container containing a commercially available food product (mapo donburi), and the food product was brought into contact with the surface of the lid material at 50 cm 2 to prepare a sample. The sample was heated in a commercial microwave oven at an output of 1500 W for 1 minute and 45 seconds. Then, the lid material was removed from the sample and washed, and the dimensions of the lid material were measured. The dimensions after microwave heating were calculated based on the dimensions before heating. If the longitudinal or transverse dimension was deformed by 5% or more, or if the height dimension was deformed by 10% or more, it was judged as "having deformation", and if not, it was judged as "no deformation". The results are shown in Table 1.

[0063] (perforated) In the same manner as above (regarding the presence or absence of deformation), a sample heated in a commercial microwave oven was prepared. The lid material was removed from the sample and washed, and the number of holes formed in the lid material was examined and evaluated as follows. A: No holes B: Less than 5 holes C: 5 or more holes

[0064] [Tensile test] The biaxially stretched sheet was cut into a dumbbell-shaped No. 1 according to JIS K6251:2017 so that the longitudinal direction was the MD (Machine Direction; sheet flow direction) of the biaxially stretched sheet to prepare a sample. Using an Autograph AGS-X manufactured by Shimadzu Corporation, a tensile test was conducted under the condition of a tensile speed of 5 mm / min, and the yield stress and breaking stress were determined according to JIS K6251:2017. The test environment was 23°C and 50% RH.

[0065]

Table 1

Claims

1. A biaxially stretched sheet comprising a crosslinked product of a resin composition containing a polystyrene-based resin, a polyester, and a crosslinking agent.

2. The biaxially stretched sheet according to claim 1, wherein the polystyrene-based resin contains a styrene-based monomer and at least one monomer selected from the group consisting of (meth)acrylic acid, (meth)acrylic acid ester, and maleic anhydride as monomer units.

3. The biaxially stretched sheet according to claim 1 or 2, wherein the polyester contains an aromatic dicarboxylic acid and a diol as monomer units.

4. The biaxially stretched sheet according to claim 3, wherein at least one of the aromatic dicarboxylic acid and the diol is derived from biomass.

5. The biaxially stretched sheet according to claim 1 or 2, wherein the crosslinking agent contains a compound having an epoxy group.

6. A first layer made of the biaxially stretched sheet according to claim 1, A second layer provided on the first layer, A multilayer sheet comprising the above.

7. A molded article formed by molding the biaxially stretched sheet according to claim 1 or 2, or the multilayer sheet according to claim 6.

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