Biaxially oriented sheet, multilayer sheet, and molded article

Abstract

To provide a biaxially oriented sheet having superior transparency.SOLUTION: A biaxially oriented sheet comprises a polystyrene-based resin and an amorphous polyester, wherein a difference in glass transition temperature between the polystyrene-based resin and the amorphous polyester is 25°C or less.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] Sheets blended with a styrenic resin and a polyester are used for packaging materials and the like. For example, Patent Document 1 discloses a resin composition containing 20% by mass or more and 99% by mass or less of a heat-resistant styrenic resin (A) and 1% by mass or more and 80% by mass or less of a polyester resin (B) containing a residue derived from biomass, wherein the heat-resistant styrenic resin (A) is a copolymer mainly composed of a styrenic monomer (a) and containing at least one monomer (b) selected from the group consisting of a styrenic monomer of a type different from (a), (meth)acrylic acid, (meth)acrylic acid ester, and maleic anhydride, and a molded article which is a film-like material having at least one layer containing the resin composition and having a haze of 10.0% or less when the thickness is 250 μm.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] According to the studies of the present inventors, it is difficult to achieve excellent transparency in a biaxially stretched sheet containing a styrenic resin and a polyester. Even if it is transparent at the time of kneading the styrenic resin and the polyester or at the time of forming into a sheet shape, the transparency may decrease after passing through the biaxial stretching process. Therefore, an aspect of the present invention aims to provide a biaxially stretched sheet having excellent transparency.

Means for Solving the Problems

[0005] The inventors have found that in a biaxially stretched sheet containing a polystyrene-based resin and an amorphous polyester, by reducing the difference between the glass transition temperature of the polystyrene-based resin and the glass transition temperature of the amorphous polyester, more specifically, by making the difference in the glass transition temperature 25°C or less, the transparency of the biaxially stretched sheet can be improved. The present invention provides the following [1] to [8] in several aspects. [1] A biaxially stretched sheet containing a polystyrene-based resin and an amorphous polyester, wherein the difference between the glass transition temperature of the polystyrene-based resin and the glass transition temperature of the amorphous polyester is 25°C or less. [2] The biaxially stretched sheet according to [1], wherein the polystyrene-based resin contains, as monomer units, 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. [3] The biaxially stretched sheet according to [1] or [2], wherein the amorphous polyester contains, as monomer units, an aromatic dicarboxylic acid and a diol. [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 polystyrene-based resin and the amorphous polyester are crosslinked with a crosslinking agent. [6] The biaxially stretched sheet according to any one of [1] to [5], wherein the root mean square slope RΔq of the surface of the biaxially stretched sheet is 4.5 degrees or less. [7] A multilayer sheet including a first layer made of the biaxially stretched sheet according to any one of [1] to [6] and a second layer provided on the first layer. [8] A molded article formed by molding the biaxially stretched sheet according to any one of [1] to [6] or the multilayer sheet according to [7].

Advantages of the Invention

[0006] According to one aspect of the present invention, a biaxially stretched sheet having excellent transparency is provided.

Embodiments 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 contains a polystyrene-based resin and an amorphous polyester. In this biaxially stretched sheet, the difference between the glass transition temperature of the polystyrene-based resin and the glass transition temperature of the amorphous polyester is 25°C or less.

[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 alkylstyrene, halogenated styrene, nitrostyrene, acetylstyrene, and methoxystyrene.

[0010] Alkylstyrene 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 alkylstyrene may be 1 or more, 8 or less, 5 or less, or 3 or less. The number of alkyl groups in alkylstyrene 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). Examples of alkylstyrenes having three alkyl groups include 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 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 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 increasing the glass transition temperature (Tg) of the polystyrene resin, 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 of the biaxially stretched sheet having excellent heat resistance, and 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 of the molded article having further excellent strength. 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 of the biaxially stretched sheet having 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 of the molded article having further excellent strength. 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 of the biaxially stretched sheet having further 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 elution time and elution volume is converted into molecular weight to obtain various average molecular weights.

[0023] The glass transition temperature (Tg) of the polystyrene-based resin may be 90°C or higher, 100°C or higher, 110°C or higher, or 120°C or higher, and may also be 150°C or lower, 140°C or lower, or 135°C or lower. In this specification, Tg is determined based on the DSC curve when a 5 mg biaxially stretched sheet is heated to 300°C at a rate of 10°C / min in a nitrogen atmosphere until melted, then rapidly cooled to 20°C at a rate of 20°C / min, and then heated again at a rate of 10°C / min.

[0024] The polystyrene-based resin is obtained by polymerizing a styrene-based 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 production by continuous polymerization or 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.

[0025] When polymerizing the polystyrene-based 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-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxy-2-ethylhexanoate, polyether tetrakis(t-butyl peroxycarbonate), 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.

[0026] From the viewpoint that the biaxially stretched sheet has excellent heat resistance, the content of the polystyrene-based resin in the biaxially stretched sheet 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 biaxially stretched sheet. In the description of the above numerical range, "or more" may mean "more than". From the viewpoint that the biaxially stretched sheet has excellent oil resistance, the content of the polystyrene-based resin in the biaxially stretched sheet 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 biaxially stretched sheet. In the description of the above numerical range, "or less" may mean "less than".

[0027] An amorphous 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.

[0028] 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, and the like. 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, and the like.

[0029] Examples of the diol include diols having a cyclic skeleton and linear aliphatic diols. Examples of the diols 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, and the like. The diol having a cyclic skeleton preferably contains at least one selected from the group consisting of alicyclic diols and heterocyclic diols, and more preferably contains at least one of isosorbide and 1,4-cyclohexanedimethanol.

[0030] Examples of the linear aliphatic diol include 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, and the like. The linear aliphatic diol preferably contains ethylene glycol.

[0031] The amorphous polyester preferably contains a biomass-derived compound as a monomer unit. At least one of the above polyvalent carboxylic acid and polyhydric alcohol may be derived from biomass. For example, the amorphous 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.

[0032] The Tg of the amorphous polyester may be 70 °C or higher, 80 °C or higher, 90 °C or higher, 100 °C or higher, 109 °C or higher, or 110 °C or higher, and may be 140 °C or lower, 135 °C or lower, or 130 °C or lower. The Tg of the amorphous polyester can be adjusted, for example, by adjusting the ratio of the mass of the diol having a cyclic skeleton (preferably an alicyclic diol or a heterocyclic diol) based on the total mass of the diol. For example, by increasing the ratio of the mass of the diol having a cyclic skeleton (preferably an alicyclic diol or a heterocyclic diol) based on the total mass of the diol, the Tg of the amorphous polyester can be increased.

[0033] The amorphous polyester can be 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.

[0034] Examples of the amorphous polyester include commercially available products such as amorphous polyesters containing spiroglycol (e.g., "ALTESTER (registered trademark)" manufactured by Mitsubishi Gas Chemical Company, Inc.), amorphous polyesters containing 2,2,4,4-tetramethyl-1,3-cyclobutanediol (e.g., "TRITAN (registered trademark)" manufactured by Eastman Chemical Company), and amorphous polyesters containing isosorbide.

[0035] The content of the amorphous polyester in the biaxially stretched sheet 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 biaxially stretched sheet, 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 excellent in oil resistance. When the content is at most the above upper limit value, the biaxially stretched sheet is excellent in heat resistance.

[0036] In the biaxially stretched sheet according to one embodiment, the difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester (hereinafter, also simply referred to as "Tg difference") is 25°C or less. The Tg difference may be 20°C or less, 15°C or less, 12°C or less, or 10°C or less. The inventors have found that by making the Tg difference smaller, the generation of surface irregularities in the biaxially stretched sheet can be suppressed (specifically, the value of the root mean square slope becomes smaller). When the generation of surface irregularities is suppressed, the haze can be kept low and the transparency can be improved. It is considered that the generation of surface irregularities in the biaxially stretched sheet is suppressed by making the Tg difference smaller because the difference in the elongational viscosity of the polystyrene-based resin and the amorphous polyester in the stretching temperature range becomes smaller, and when stretching the unstretched sheet, the polystyrene-based resin and the amorphous polyester are stretched to the same extent. The Tg of the polystyrene-based resin may be equal to or higher than the Tg of the amorphous polyester, or may be less than the Tg of the amorphous polyester.

[0037] The difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester is determined based on the DSC curve when, for a 5 mg biaxially stretched sheet, after melting by heating to 300 °C at a rate of 10 °C / min in a nitrogen atmosphere, it is rapidly cooled to 20 °C at a rate of 20 °C / min and then heated again at a rate of 10 °C / min. From the obtained DSC curve, the Tg of each component is determined, and by calculating the difference, the difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester can be obtained.

[0038] In the biaxially stretched sheet, the polystyrene-based resin and the amorphous polyester may be crosslinked by a crosslinking agent. In one embodiment, in addition to the crosslinking between the polystyrene-based resin and the polyester, the polystyrene-based resins may be crosslinked by a crosslinking agent, the polyesters may be crosslinked by a crosslinking agent, and the molecules of the polystyrene resin or polyester may also be crosslinked by a crosslinking agent. Examples of the crosslinking agent include epoxy-based crosslinking agents, oxazoline-based crosslinking agents, maleic acid-based crosslinking agents, etc. The crosslinking agent may be, for example, a compound having an epoxy group, a compound having an oxazoline group, etc.

[0039] 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. Also, 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.).

[0040] 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.

[0041] A 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 be mentioned.

[0042] 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 styrenic monomer as monomer units. As the styrenic monomer, those described above can be used without particular limitation.

[0043] 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, and 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 with respect to 100 parts by mass in total of the content of the above polystyrene resin and the amorphous polyester.

[0044] The biaxially stretched sheet may further contain other components. Examples of the other components include antioxidants, anti-gelling agents (for example, 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 amorphous polyester, or may be added when the polystyrene resin and the amorphous polyester are mixed 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 biaxially stretched sheet.

[0045] From the viewpoint of further improving the heat resistance of the polystyrene resin, 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.

[0046] 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.

[0047] The root mean square slope RΔq of the surface of the biaxially stretched sheet may be 5.0 degrees or less, 4.5 degrees or less, 4.0 degrees or less, 3.5 degrees or less, or 3.0 degrees or less. The root mean square slope RΔq of the surface of the biaxially stretched sheet is the root mean square slope calculated from the roughness curve of the surface of the biaxially stretched sheet in accordance with JIS B0601:2013.

[0048] 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.

[0049] The draw ratios in the MD (Machine Direction; sheet flow direction) and TD (Transverse Direction; direction perpendicular to the sheet flow direction) of the biaxially stretched sheet may each be 1.9 times or more, 2.0 times or more, 2.3 times or more, or 2.4 times or more, and may be 4.0 times or less, 3.5 times or less, or 3.0 times or less. When the draw ratio is at least the above lower limit value, the strength of the molded product is excellent. Also, when the draw ratio is at most the above upper limit value, the formability 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.

[0050] 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, or 13 times or less, or 10 times or less.

[0051] 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 calculated 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 direction with respect to the test piece of the biaxially stretched sheet before heating, and Z represents the length [mm] of the above straight line after the test piece is allowed to stand for 60 minutes and shrink 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. If a straight line is drawn in the MD direction, the MD draw ratio is calculated, and if a straight line is drawn in the TD direction, the TD draw ratio is calculated.

[0052] The above biaxially stretched sheet can be manufactured by the following method. First, a polystyrene-based resin, an amorphous polyester, and optional components (the crosslinking agent and other components described above) used as necessary are mixed to obtain a resin composition. After melting and 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.

[0053] Another embodiment of the present invention is a resin composition comprising a polystyrene-based resin and an amorphous polyester, wherein the difference between the glass transition temperature of the polystyrene-based resin and the glass transition temperature of the amorphous polyester is 25°C or less. By using such a resin composition, the above-mentioned biaxially stretched sheet can be produced. As the polystyrene-based resin and the amorphous polyester in the resin composition, those described above can be used without particular limitation. The contents of the polystyrene-based resin and the amorphous polyester in the resin composition may be within the numerical ranges described above as the contents of the polystyrene-based resin and the amorphous polyester in the biaxially stretched sheet. Here, the phrase "based on the total mass of the biaxially stretched sheet" shall be construed as "based on the total mass of the resin composition".

[0054] The difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester in the resin composition may be within the numerical range described above as the difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester in the biaxially stretched sheet. The resin composition may further contain a crosslinking agent and may further contain other components. The crosslinking agent and other components may be those described above as the crosslinking agent and other components in the biaxially stretched sheet, respectively. The contents of the crosslinking agent and other components may be within the ranges described above as the contents of the crosslinking agent and other components in the biaxially stretched sheet. Here, the phrase "based on the total mass of the biaxially stretched sheet" shall be construed as "based on the total mass of the resin composition".

[0055] 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 opposite to the second layer of the first 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 decorative layer containing a colorant (a layer for patterning the multilayer sheet), an antibacterial layer containing an antibacterial agent, and a gas barrier layer containing a gas barrier substance (e.g., ethylene-vinyl alcohol copolymer) that suppresses the permeation of gas (such as oxygen).

[0056] The above multilayer sheet can be produced, for example, by applying a coating liquid containing components (anti-fogging agent, release agent, antistatic agent, colorant, antibacterial agent, gas barrier substance, etc.) according to the purpose on at least one surface of the biaxially stretched sheet to form the second layer and 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.

[0057] 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.

[0058] The method for obtaining a molded article from the biaxially stretched sheet is not particularly limited, and a method commonly used in the secondary molding method of a conventional biaxially stretched sheet can be used. For example, secondary molding 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 Ltd. (1995).

Examples

[0059] Hereinafter, the present invention will be described more specifically based on examples. Note that the present invention is not limited to these examples.

[0060] [Synthesis Example 1: Synthesis of Polystyrene-based Resin (A-1)] The polystyrene-based resin (A-1) was produced by the following procedure. An apparatus in which a first reactor and a second reactor, which are completely mixed type stirring tanks, were 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.

[0061] Subsequently, the solution containing the 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. Thereafter, 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-based resin (A-1) 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-based resin (A-1) was 90% by mass of styrene and 10% by mass of methacrylic acid based on the total mass of the monomer units contained in (A-1). Also, the weight average molecular weight (Mw) and Z average molecular weight (Mz) determined by GPC measurement were 280,000 and 450,000, respectively.

[0062] In addition, the POE content in the obtained pellet-shaped polystyrene-based resin (A-1) was 0.20% by mass based on the total mass of the polystyrene-based resin (A-1) 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-based resin (A-1) was precisely weighed and dissolved in THF. Methanol and a small amount of hydrochloric acid were added to the solution, and the polymer component was reprecipitated, and the precipitate was removed by filtration. The filtrate was concentrated to obtain a 10 ml concentrated solution. The 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 a known concentration was used for the quantification. The HPLC conditions are as follows. HPLC model: Alliance system 2695 separation module manufactured by Waters Corporation, 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 (mass %) = (Quantitative value of POE) / 5 × 100

[0063] [Amorphous polyester] The amorphous polyesters used in the examples and comparative examples are as follows. (B-1) to (B-3) are amorphous polyesters containing terephthalic acid as an aromatic dicarboxylic acid and ethylene glycol, isosorbide, and 1,4-cyclohexanedimethanol as monomer units, and a part of ethylene glycol and isosorbide is derived from biomass. Also, (B-4) is a petroleum-derived amorphous polyester containing terephthalic acid as an aromatic dicarboxylic acid and ethylene glycol and 1,4-cyclohexanedimethanol as monomer units.

[0064] [Crosslinking agent] The crosslinking agents used in the examples are as follows. (C-1) CESA Extend1598 manufactured by Clariant (C-1) 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.

[0065] [Measurement of glass transition temperature (Tg)] The glass transition temperatures of the polystyrene resin (A-1) and the amorphous polyesters (B-1) to (B-4) were measured by a differential scanning calorimeter (DSC). As the apparatus, Q2000 manufactured by TA Instruments was used. 5 mg of the polystyrene resin or amorphous polyester to be measured was placed in a cell, melted by heating to 300 °C at a rate of 10 °C / min under a nitrogen atmosphere, then rapidly cooled to 20 °C at a rate of 20 °C / min, and a DSC curve was obtained when heating was carried out again at a rate of 10 °C / min. At the time of the second heating, the temperature at the midpoint of the step generated in the DSC curve was determined as the glass transition temperature (Tg). The glass transition temperatures of the polystyrene resin (A-1) and the amorphous polyesters (B-1) to (B-4) are shown in Table 1.

[0066]

Table 1

[0067] <Examples 1 to 4, 7 to 9 and Comparative Examples 1 to 2> The components of the types and amounts shown in Table 1 were melt-kneaded using a small twin-screw extruder (Process11 manufactured by ThermoFisher 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 Ever Kikai Co., Ltd.). The thickness of the biaxially stretched sheet was 0.25 mm.

[0068] <Examples 5, 6> Biaxially stretched sheets of Examples 5 and 6 were obtained by the same method as the above <Examples 1 to 4, 7 to 9 and Comparative Examples 1 to 2>, except that the stretching temperatures were 142 °C and 152 °C in order. The thickness of the biaxially stretched sheet was 0.25 mm.

[0069] <Physical properties of the biaxially stretched sheet> Regarding each of the manufactured biaxially stretched sheets of the examples and comparative examples, the physical properties were examined as follows.

[0070] [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 samples were gripped with chucks and immersed in an oil bath at 150 °C for heating. The molecular orientation was relaxed by thermal motion, and the load applied to the chucks when 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 serves as an index of the molecular orientation degree of the biaxially stretched sheet.

[0071] [Evaluation of Transparency] The transparency was evaluated by measuring the total light transmittance, haze, and internal haze using a NDH5000 manufactured by Nippon Denshoku. Haze represents the cloudiness of both the surface and the interior of the sample, and internal haze represents the cloudiness of only the interior of the sample after subtracting the light scattering due to surface irregularities. The internal haze was measured according to the following method. A quartz cell was filled with a liquid (cedar oil) having a refractive index close to that of the sample, and the haze value (Hb) was measured with the sample not placed inside. Next, the sample was placed in the cell and immersed in the liquid, and the haze value (Hs) was measured. Then, the internal haze was determined according to the following formula (1). Internal haze [%] = Hs - Hb Formula (1)

[0072] [Surface Roughness] Using a laser microscope OLS5100 manufactured by Olympus, a one-line scan was performed at an objective lens magnification of 50 times to measure the line roughness. The measurement length and cut-off period were set according to the JIS B0601:2013 standard, and the arithmetic mean roughness Ra and the root mean square slope RΔq were calculated to evaluate the magnitude of the surface irregularities. Note that for Example 8, the measurement length was set to 4 mm and the cut-off period was set to 0.8 mm. For cases other than Example 8, the measurement length was set to 1.25 mm and the cut-off period was set to 0.25 mm. The root mean square slope is known to be correlated with the strength of light scattering due to surface irregularities.

[0073] [Evaluation of Oil Resistance] A biaxially stretched sheet was cut into strips measuring 30 mm in width and 300 mm in length to prepare samples. Holes were drilled at both ends in the major axis direction of the samples and they were tied with metal rings to form loops, which were then hung on a stainless steel pipe with a diameter of 25 mm. A weight of 375 g was suspended from the tied portion at both ends. In the samples, gauze cut to a size of 10 mm × 10 mm was placed on the back side of the portion in contact with the pipe. Two drops of salad oil were dropped onto the gauze and the samples were left at room temperature. After 24 hours, the appearance was observed. If no change was observed, it was evaluated as "A"; if only very small cracks less than 5 mm were observed, it was evaluated as "B"; if cracks of 5 mm or more occurred or it was completely broken, it was evaluated as "C".

[0074] [Tensile Test] The biaxially stretched sheet was cut into dumbbell-shaped No. 1 specimens as specified in JIS K6251:2017 so that the longitudinal direction was the MD (Machine Direction; sheet flow direction) of the biaxially stretched sheet to prepare samples. 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 tensile elastic modulus and elongation were determined according to JIS K6251:2017. The test environment was 23°C and 50% RH.

[0075]

Table 2

[0076] From the results in Table 2, it can be seen that in the biaxially stretched sheet, the smaller the difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester, the smaller the haze and the higher the transparency. The biaxially stretched sheets of Examples 1 to 9 have a smaller haze and are superior in transparency compared to the biaxially stretched sheets of Comparative Examples 1 to 2.

[0077] Incidentally, for example, when comparing Examples 1 to 9 with Comparative Examples 1 and 2, there is no significant difference in the value of internal haze. Therefore, when the difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester is small, the main reason for the improvement in transparency is considered to be that the surface roughness of the biaxially stretched sheet becomes small. Actually, as shown in Table 2, the smaller the difference between the Tg of the polystyrene-based resin and the Tg of the amorphous polyester, the smaller the value of the root mean square slope. In addition, such a small surface roughness is considered to make it difficult for oil to penetrate into the biaxially stretched sheet when oil adheres to the surface of the biaxially stretched sheet, contributing to the improvement of oil resistance.

Claims

1. comprising a polystyrene resin and an amorphous polyester, A biaxially stretched sheet in which the difference between the glass transition temperature of the polystyrene resin and the glass transition temperature of the amorphous polyester is 25°C or less.

2. The biaxially stretched sheet according to claim 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 claim 1 or 2, wherein the amorphous 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 polystyrene resin and the amorphous polyester are crosslinked with a crosslinking agent.

6. The biaxially stretched sheet according to claim 1 or 2, wherein the root mean square slope RΔq of the surface of the biaxially stretched sheet is 4.5° or less.

7. 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 same.

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

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