Biaxially stretched sheet and molded product

Abstract

To provide a biaxially stretched sheet that enables the production of a molded product while preventing excessive reduction in thickness.SOLUTION: The present invention provides a biaxially stretched sheet comprising a copolymer containing styrene and methacrylic acid as monomer units and a diene-based rubber-modified polystyrene containing a diene-based rubber component. In a stress-strain curve derived from a tensile test at 100°C, the difference between the stress at the breaking point and the stress at the descending yield point is 10 MPa or more.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to biaxially oriented sheets and molded articles. [Background technology]

[0002] Biaxially stretched sheets are widely used in the field of food packaging containers, and biaxially stretched sheets with various properties are being developed. For example, Patent Document 1 describes a biaxially stretched sheet made of a styrene-based resin composition containing a styrene-methacrylic acid copolymer (A) and a high-impact polystyrene (B) in a mass ratio (A) / (B) of 97.0 / 3.0 to 99.9 / 0.1, in which the methacrylic acid monomer unit content of the styrene-methacrylic acid copolymer (A) is 3 to 14 mass%, the Vicat softening temperature of the styrene-based resin composition is in the range of 106 to 132°C, and the orientation relaxation stresses in the longitudinal and transverse directions of the biaxially stretched sheet are both in the range of 0.5 to 1.2 MPa, and the biaxially stretched sheet is described as having good transparency, strength, heat resistance, film formability, and shapeability during secondary molding, and excellent crack resistance during trimming. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2017 / 122775 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the study by the present inventors, when a biaxially oriented sheet is molded, the thickness of the sheet varies, and the thickness may be excessively reduced at a specific portion of the molded product (for example, a corner of a container). If the thickness is excessively reduced at a specific portion, the strength of the portion is likely to be insufficient, which is undesirable. Therefore, one aspect of the present invention aims to provide a biaxially oriented sheet that can produce a molded product in which excessive reduction in thickness is suppressed. [Means for solving the problem]

[0005] As a result of intensive research, the present inventors have found that by increasing the difference between the stress at the break point and the stress at the lower yield point in a stress-strain curve when a tensile test is performed at 100° C. on a biaxially oriented sheet containing a copolymer containing styrene and methacrylic acid as monomer units and a diene-based rubber-modified polystyrene containing a diene-based rubber component, it is possible to suppress excessive reduction in thickness in a molded article molded from the biaxially oriented sheet. In some aspects, the present invention provides the following [1] to [8]. [1] A biaxially oriented sheet containing a copolymer containing styrene and methacrylic acid as monomer units and a diene-based rubber modified polystyrene containing a diene-based rubber component, wherein in a stress-strain curve obtained by performing a tensile test on the biaxially oriented sheet at 100°C, the difference between the stress at the breaking point and the stress at the lower yield point is 10 MPa or more. [2] The biaxially oriented sheet according to [1], wherein the content of methacrylic acid is 2.0 to 15.0 mass % based on the total amount of the biaxially oriented sheet. [3] The biaxially oriented sheet according to [1] or [2], wherein the weight average molecular weight (Mw) of the biaxially oriented sheet is 150,000 to 400,000. [4] The biaxially oriented sheet according to any one of [1] to [3], wherein the content of the diene rubber component is 0.005 to 0.50 mass % based on the total amount of the biaxially oriented sheet. [5] The biaxially oriented sheet according to any one of [1] to [4], wherein the mass ratio of the content of the copolymer to the content of the diene rubber modified polystyrene is 95.0 / 5.0 to 99.9 / 0.1. [6] The biaxially oriented sheet according to any one of [1] to [5], which has a thickness of 0.1 to 0.3 mm. [7] A molded product obtained by molding the biaxially oriented sheet according to any one of [1] to [6]. [8] The molded article according to [7], which is a food packaging container for microwave heating. Effect of the Invention

[0006] According to one aspect of the present invention, there is provided a biaxially oriented sheet capable of producing a molded article in which excessive thickness reduction is suppressed. In the molded article produced from the biaxially oriented sheet, excessive thickness reduction is suppressed, and therefore reduction in strength at corners and the like is also suppressed. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram showing an example of a stress-strain curve (SS curve) obtained when a tensile test is carried out on a test piece of a biaxially oriented sheet under the condition of 100°C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, embodiments of the present invention will be described in detail, however, the present invention is not limited to the following embodiments.

[0009] The biaxially oriented sheet according to one embodiment of the present invention contains a copolymer containing styrene and methacrylic acid as monomer units (hereinafter also referred to as "component (A)") and a diene-based rubber-modified polystyrene containing a diene-based rubber component (hereinafter also referred to as "component (B)").

[0010] The content of methacrylic acid (methacrylic acid unit) contained as a monomer unit of the (A) component may be 2.0 mass% or more, 2.5 mass% or more, 3.0 mass% or more, or 3.5 mass% or more based on the total amount of the biaxially stretched sheet, from the viewpoint of excellent heat resistance of the biaxially stretched sheet and the molded product. The content of methacrylic acid (methacrylic acid unit) may be 15.0 mass% or less, 12.0 mass% or less, 10.0 mass% or less, 9.0 mass% or less, 8.0 mass% or less, 7.0 mass% or less, 6.0 mass% or less, or 5.0 mass% or less based on the total amount of the biaxially stretched sheet, from the viewpoint of excellent moldability of the biaxially stretched sheet. The content of methacrylic acid (methacrylic acid unit) may be, for example, 2.0 to 15.0 mass%, 2.0 to 12.0 mass%, or 2.5 to 9.0 mass% based on the total amount of the biaxially stretched sheet.

[0011] The content of methacrylic acid (based on the total amount of the biaxially stretched sheet) is determined by neutralization titration at room temperature (25°C). Specifically, 0.5 g of the biaxially stretched sheet is first weighed and dissolved in a mixed solution of toluene / ethanol = 8 / 2 (volume ratio). The obtained solution is subjected to neutralization titration with a 0.1 mol / L ethanol solution of potassium hydroxide using an automatic potentiometric titrator (AT-510, manufactured by Kyoto Electronics Manufacturing Co., Ltd.). From the volume V (L) of the potassium hydroxide ethanol solution required to reach the end point, the mass-based content of methacrylic acid is calculated according to the following formula. Methacrylic acid content (mass%) = V × 0.1 × 86 / 0.5 × 100

[0012] The content of methacrylic acid (methacrylic acid units) may be 2.5% by mass or more, 3.0% by mass or more, or 3.5% by mass or more based on the total amount of monomer units contained in the copolymer (hereinafter, also simply referred to as "total amount of monomer units") from the viewpoint of excellent heat resistance of the biaxially stretched sheet and molded product. The content of methacrylic acid (methacrylic acid units) may be 20.0% by mass or less, 15.0% by mass or less, 12.0% by mass or less, 10.0% by mass or less, 8.0% by mass or less, 6.0% by mass or less, or 5.0% by mass or less based on the total amount of monomer units from the viewpoint of further excellent moldability of the biaxially stretched sheet.

[0013] The content of styrene (styrene units) contained as a monomer unit in component (A) may be 85.0 mass% or more, 88.0 mass% or more, 90.0 mass% or more, or 93.0 mass% or more, based on the total amount of the biaxially oriented sheet, from the viewpoint of excellent formability of the biaxially oriented sheet, and may be 98.0 mass% or less, 97.5 mass% or less, 97.0 mass% or less, or 96.5 mass% or less, from the viewpoint of excellent heat resistance of the biaxially oriented sheet and the molded product.

[0014] The content of styrene (styrene units), based on the total amount of monomer units, may be 80.0 mass% or more, 85.0 mass% or more, 88.0 mass% or more, 90.0 mass% or more, 92.0 mass% or more, 94.0 mass% or more, or 95.0 mass% or more from the viewpoint of excellent formability of the biaxially oriented sheet, and may be 97.5 mass% or less, 97.0 mass% or less, or 96.5 mass% or less from the viewpoint of excellent heat resistance of the biaxially oriented sheet and molded product.

[0015] The (A) component may contain only methacrylic acid and styrene as monomer units, or may further contain other monomer units copolymerizable with methacrylic acid and styrene. Examples of other monomer units include methyl methacrylate, maleic anhydride, ethyl methacrylate, n-butyl methacrylate, and methyl acrylate. The content of other monomer units may be, for example, 10.0 mass% or less, 5.0 mass% or less, or 3.0 mass% or less based on the total amount of monomer units. The (A) component may contain only methacrylic acid as a monomer unit having an acidic group such as a carboxyl group.

[0016] From the viewpoint of achieving superior strength in a molded article, the weight average molecular weight (Mw) of the (A) component 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 achieving superior moldability in a biaxially oriented sheet, the weight average molecular weight (Mw) of the (A) component may be 450,000 or less, 400,000 or less, 380,000 or less, or 350,000 or less.

[0017] From the viewpoint of achieving even better strength for the molded article, the Z-average molecular weight (Mz) of the (A) component may be 300,000 or more, 350,000 or more, 400,000 or more, 420,000 or more, or 450,000 or more. From the viewpoint of achieving even better moldability for the biaxially oriented sheet, the Z-average molecular weight (Mz) of the (A) component may be 600,000 or less, 550,000 or less, or 500,000 or less.

[0018] In this specification, the weight average molecular weight (Mw) and the Z average molecular weight (Mz) are determined by GPC measurement under the following conditions. Equipment: Tosoh Corporation GPC "HLC-8320GPC" Column: Shodex KF404 x 3 Temperature: 40℃ Solvent: Tetrahydrofuran Flow rate: 0.2ml / min Pressure: 10MPa Detection: RI Sample preparation method: 120 mg of a sample was dissolved in 15 mL of tetrahydrofuran, and then filtered through a syringe filter (Merck Millipore's Millex (registered trademark) 0.45 μm). Injection volume: 10μl Calibration curve: Using standard polystyrene (Polymer Laboratories), the relationship between elution time and elution amount is converted into molecular weight to determine various average molecular weights.

[0019] The component (A) is obtained by polymerizing methacrylic acid, styrene, 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 used as industrial production methods for polystyrene and the like. In terms of quality and productivity, the bulk polymerization method or the solution polymerization method is preferred. 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.

[0020] During polymerization of (A), 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-di(t-butylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, t-butylperoxyisopropylcarbonate, dicumyl peroxide, t-butylcumyl peroxide, t-butylperoxyacetate, t-butylperoxy-2-ethylhexanoate, polyethertetrakis(t-butylperoxycarbonate), ethyl-3,3-di(t-butylperoxy)butyrate, and t-butylperoxyisobutyrate. Specific examples of the chain transfer agent include aliphatic mercaptans, aromatic mercaptans, pentaphenylethane, α-methylstyrene dimer, terpinolene, etc. Specific examples of the surfactant include nonionic surfactants such as polyoxyethylene alkyl ether.

[0021] The content of the (A) component in the biaxially stretched sheet may be 95.0% by mass or more, 97.0% by mass or more, 98.0% by mass or more, or 98.5% by mass or more, and may be 99.9% by mass or less, 99.7% by mass or less, 99.5% by mass or less, or 99.2% by mass or less, based on the total amount of the biaxially stretched sheet. When the content is equal to or more than the lower limit, the biaxially stretched sheet and the molded product have excellent heat resistance and transparency. When the content is equal to or less than the upper limit, the biaxially stretched sheet and the molded product have excellent lubricity.

[0022] Diene-based rubber-modified polystyrene containing a diene-based rubber component is also called rubber-modified impact-resistant styrene-based resin, and is obtained, for example, by dissolving a rubber-like diene polymer in a styrene-based monomer and polymerizing (preferably graft polymerizing) it.

[0023] Examples of styrene-based monomers include styrene, alkylstyrenes (e.g., o-, m-, and p-isomers such as methylstyrene, ethylstyrene, isopropylstyrene, butylstyrene, and tertiary butylstyrene), α-alkylstyrenes (e.g., α-methylstyrene, α-ethylstyrene, and the like), monohalogenated styrenes (e.g., o-, m-, and p-isomers such as chlorostyrene, bromostyrene, and fluorostyrene), dihalogenated styrenes (e.g., nuclear-substituted isomers such as dichlorostyrene, dibromostyrene, difluorostyrene, and chlorobromostyrene), trihalogenated styrenes (e.g., trichlorostyrene, tribromostyrene, Examples of the styrene monomer include styrenes having nucleus substitution such as styrene, trifluorostyrene, dichlorobromostyrene, dibromochlorostyrene, and difluorochlorostyrene, tetrahalogenated styrenes (e.g., styrenes having nucleus substitution such as styrene, tetrabromostyrene, tetrafluorostyrene, and dichlorodibromostyrene), pentahalogenated styrenes (e.g., pentachlorostyrene, pentabromostyrene, trichlorodibromostyrene, and trifluorodichlorostyrene), α- and β-halogen-substituted styrenes (e.g., α-chlorostyrene, α-bromostyrene, β-chlorostyrene, and β-bromostyrene, etc.). These monomers may be used alone or in combination of two or more.

[0024] Examples of rubber-like diene polymers that can be used include one or more conjugated 1,3-dienes (e.g., butadiene, isoprene, 2-chloro-1,3-butadiene, 1-chloro-1,3-butadiene, piperylene, etc.), butadiene-styrene copolymers, butadiene-acrylonitrile copolymers, and butadiene-styrene-acrylonitrile copolymers.

[0025] The content of the diene rubber component (hereinafter also simply referred to as "rubber component") contained in the (B) component may be 0.5 mass% or more, 1.0 mass% or more, 3.0 mass% or more, 5.0 mass% or more, or 8.0 mass% or more based on the total amount of the (B) component, and may be 15.0 mass% or less, or 12.0 mass% or less. When the content is equal to or more than the above lower limit, the biaxially oriented sheet and molded article have excellent lubricity. When the content is equal to or less than the above upper limit, the biaxially oriented sheet and molded article have excellent transparency.

[0026] The content of the rubber component may be 0.005% by mass or more, 0.01% by mass or more, 0.02% by mass or more, 0.05% by mass or more, or 0.08% by mass or more based on the total amount of the biaxially oriented sheet, from the viewpoint of excellent lubricity of the biaxially oriented sheet and molded article. The content of the rubber component may be 0.50% by mass or less, 0.30% by mass or less, 0.25% by mass or less, 0.20% by mass or less, or 0.15% by mass or less based on the total amount of the biaxially oriented sheet, from the viewpoint of excellent heat resistance and transparency of the biaxially oriented sheet and molded article. The content of the rubber component may be, for example, 0.005 to 0.50% by mass, or 0.01 to 0.30% by mass or less.

[0027] The rubber content of the biaxially oriented sheet based on the total amount can be measured by dissolving 0.25g of the biaxially oriented sheet in 50ml of chloroform, adding iodine monochloride to react with the double bonds in the rubber component, then adding potassium iodide to convert the remaining iodine monochloride to iodine, and back titrating with sodium thiosulfate (iodine monochloride method). The analysis method is described in, for example, "(3) Rubber content" on page 659 of "New edition Polymer analysis handbook" edited by the Polymer Analysis Research Roundtable of the Japan Society for Analytical Chemistry, Kinokuniya Shoten (1995 edition), and can be measured using this method.

[0028] The content of the (B) component in the biaxially stretched sheet may be 0.1% by mass or more, 0.2% by mass or more, 0.5% by mass or more, or 0.8% by mass or more, and may be 5.0% by mass or less, 3.0% by mass or less, 2.0% by mass or less, or 1.5% by mass or less, based on the total amount of the biaxially stretched sheet. When the content is equal to or more than the lower limit, the biaxially stretched sheet and the molded product have excellent lubricity. When the content is equal to or less than the upper limit, the biaxially stretched sheet and the molded product have excellent heat resistance and transparency.

[0029] The mass ratio of the content of the (A) component to the content of the (B) component (the content of the (A) component / the content of the (B) component) may be 95.0 / 5.0 or more, 96.0 / 4.0 or more, 97.0 / 3.0 or more, 98.0 / 2.0 or more, or 98.5 / 1.5 or more from the viewpoint of excellent heat resistance and transparency of the biaxially oriented sheet and the molded article, and may be 99.9 / 0.1 or less, 99.7 / 0.3 or less, or 99.5 / 0.5 or less from the viewpoint of excellent lubricity of the biaxially oriented sheet and the molded article. The mass ratio of the content of the (A) component to the content of the (B) component may be, for example, 95.0 / 5.0 to 99.9 / 0.1, 96.0 / 4.0 to 99.9 / 0.1, or 97.0 / 3.0 to 99.9 / 0.1.

[0030] The content of unreacted styrene monomer contained in the biaxially oriented sheet may be 1000 ppm or less based on the total amount of the biaxially oriented sheet. The content of unreacted methacrylic acid monomer may be 150 ppm or less based on the total amount of the biaxially oriented sheet. By making the content of these unreacted monomers equal to or less than the above upper limit, it is possible to prevent bleeding out of the sheet surface, and also to prevent the occurrence of surface roughness and dirt when contacting the rolls of the extruder and the stretching machine. In addition, when the biaxially oriented sheet is molded, it is possible to prevent the biaxially oriented sheet from adhering to the mold of the molding machine and impairing the appearance of the molded product, and also to prevent mold dirt from causing impairing the appearance of the molded product thereafter. The quantitative determination of the unreacted styrene monomer and the unreacted methacrylic acid monomer is measured by the internal standard method using gas chromatography described below. Equipment name: GC-12A (manufactured by Shimadzu Corporation) Column: Glass column φ3 [mm] × 3 [m] Quantitative method: Internal standard method (cyclopentanol)

[0031] The biaxially stretched sheet may further contain various additives depending on the application. Examples of additives include antioxidants, antigelling agents (e.g., polyoxyethylene alkyl ether (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. These additives may be added in the polymerization step, devolatilization step, or granulation step of the (A) and (B) components, or may be added when the (A) and (B) components are mixed during the production of the biaxially stretched sheet. The total content of the additives may be, for example, 0.1% by mass or more and 1.0% by mass or less based on the total amount of the biaxially stretched sheet.

[0032] In the biaxially oriented sheet, the average particle size (Ro) of the rubber particles (particles of the diene rubber component) contained in the (B) component may be 1.5 μm or more, 2.0 μm or more, or 4.0 μm or more, and may be 9.0 μm or less, 8.0 μm or less, or 6.0 μm or less. When Ro is equal to or more than the above lower limit, the biaxially oriented sheet and molded article have excellent lubricity. When Ro is equal to or less than the above upper limit, the decrease in transmitted light and the increase in scattered light are suppressed, and the biaxially oriented sheet and molded article have excellent transparency.

[0033] In this specification, the average particle size (Ro) of rubber particles is measured by cutting a biaxially oriented sheet using an ultrathin sectioning method so that the observation surface is parallel to the main surface of the sheet, and measuring the average particle size (Ro) using osmium tetroxide (OsO 4 ) after dyeing the rubber component, 100 particles were measured using a transmission microscope, and the arithmetic mean particle size was measured.

[0034] The weight average molecular weight (Mw) of the biaxially stretched sheet may be 150,000 or more, 170,000 or more, 200,000 or more, 220,000 or more, 230,000 or more, 240,000 or more, 250,000 or more, 255,000 or more, or 260,000 or more, from the viewpoint of further improving the strength of the molded product. The weight average molecular weight (Mw) of the biaxially stretched sheet may be 400,000 or less, 380,000 or less, or 350,000 or less, from the viewpoint of further improving the moldability of the biaxially stretched sheet. The Mw of the biaxially stretched sheet may be, for example, 150,000 to 400,000, 200,000 to 400,000, or 230,000 to 400,000. The weight average molecular weight (Mw) of the biaxially stretched sheet can also be expressed as the weight average molecular weight (Mw) of all components contained in the biaxially stretched sheet.

[0035] 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 from the viewpoint of further improving the strength of the molded article. The thickness of the biaxially stretched sheet may be 0.7 mm or less, 0.5 mm or less, 0.4 mm or less, or 0.3 mm or less from the viewpoint of excellent transparency of the biaxially stretched sheet and the molded article. The thickness of the biaxially stretched sheet may be, for example, 0.1 to 0.7 mm, or 0.1 to 0.3 mm.

[0036] The stretching ratios of the biaxially oriented sheet in the MD (Machine Direction; sheet flow direction) and TD (Transverse Direction; direction perpendicular to the sheet flow direction) may be 1.9 times or more, 2.0 times or more, 2.3 times or more, or 2.5 times or more, and 4.0 times or less, 3.5 times or less, or 3.0 times or less. When the stretching ratio is equal to or more than the lower limit, the strength of the molded product is further improved. When the stretching ratio is equal to or less than the upper limit, the moldability of the biaxially oriented sheet is further improved. The stretching ratios in the MD and TD may be the same or different from each other.

[0037] When the MD stretch ratio of the biaxially oriented sheet is m and the TD stretch ratio is n, the areal stretch ratio indicated by m×n may be 4 times or more, 6 times or more, or 7.5 times or more, and may be 16 times or less, or 12.5 times or less.

[0038] The stretch ratio in this specification refers to the ratio of change in a test piece of a biaxially stretched sheet before and after heating, and is specifically expressed by the following formula: Stretching ratio = Y / Z (unit [times]) In this formula, Y represents the length [mm] of a straight line drawn in MD or TD on a test piece of a biaxially oriented sheet before heating, and Z represents the length [mm] of the straight line after the test piece is allowed to stand for 60 minutes in an oven at a temperature 30°C higher than the Vicat softening point temperature of the biaxially oriented sheet measured in accordance with JIS K7206 and allowed to shrink. If a straight line is drawn in MD, the MD stretch ratio can be calculated, and if a straight line is drawn in TD, the TD stretch ratio can be calculated.

[0039] In the stress-strain curve of the biaxially oriented sheet when a tensile test is performed at 100°C, the difference between the stress at the break point and the stress at the lower yield point (hereinafter, simply referred to as "the difference between the stress at the break point and the stress at the lower yield point") is 10 MPa or more. According to the study by the present inventors, by making the difference between the stress at the break point and the stress at the lower yield point 10 MPa or more, the variation in thickness (thickness deviation depending on the part of the molded product) of the molded product obtained by molding the biaxially oriented sheet can be suppressed. When a molded product is manufactured using a biaxially oriented sheet, thickness deviation depending on the part of the molded product may occur during molding. For example, in hot plate molding (hot plate compressed air molding), the biaxially oriented sheet is heated and softened during molding, and the sheet is pressed against the mold side by vacuum suction to mold, so it is thought that the side part of the container is likely to be thinner than the bottom part. In particular, in a container formed from a biaxially stretched sheet, the corners at the connecting portions between the side and bottom are significantly deformed during molding, and are pulled from both the side and bottom. At this time, the biaxially stretched sheet is susceptible to the effects of thickness variations in the biaxially stretched sheet itself, differences in the amount of stretching and molecular orientation state between MD and TD, and variations in stretching in each direction, and therefore the thickness variations at the corners of the container are likely to be large (thinner than other parts). However, in the biaxially stretched sheet, the difference between the breaking stress and the lower yield stress is 10 MPa or more, so that excessive reduction in thickness can be suppressed even in such parts. This is one of the reasons why the biaxially stretched sheet has excellent shapeability during molding, and the molded product obtained by molding the biaxially stretched sheet also has excellent strength. The difference between the stress at break and the stress at lower yield point may be 11 MPa or more, or 12 MPa or more, from the viewpoint of further suppressing excessive reduction in thickness and further improving the strength of the molded product, and may be, for example, 30 MPa or less, or 20 MPa or less.

[0040] In this specification, the difference between the stress at break and the lower yield stress is determined by the following method. First, 10 type 5 test pieces are prepared using biaxially stretched sheets according to JIS K7127:1999. Here, five of the ten test pieces have an arbitrary direction (e.g., MD) as the longitudinal direction, and the remaining five have a direction (e.g., TD) approximately perpendicular to the arbitrary direction as the longitudinal direction. Next, according to JIS K7127:1999, a tensile test is performed on each test piece under a condition of 100°C to obtain a stress-strain curve (SS curve). Next, in the obtained SS curve, the stress at break and the stress at the lower yield stress are obtained, and the difference between the stress at break and the lower yield stress (stress at break - stress at lower yield stress) is calculated. Here, the lower yield point corresponds to the point where the stress becomes minimum on the SS curve after reaching the yield point defined in JIS K7161-1:2014.

[0041] FIG. 1 is an example of a stress-strain curve (SS curve) obtained when a tensile test is performed on a test piece of a biaxially oriented sheet under conditions of 100°C. In the SS curve shown in FIG. 1, P1 is the point (break point) at which the biaxially oriented sheet breaks, and P2 is the first point (yield point) at which an increase in strain occurs without an increase in stress. If the strain is further increased after reaching the yield point P2, the stress decreases and then increases, as shown in FIG. 1. The minimum point observed here (the point at which the stress value is smallest between the break point P1 and the yield point P2) is the lower yield point P3. In the example shown in FIG. 1, the stress S at the break point P1 is 1 and the stress S at the lower yield point P3 3 Difference with (S 1 -S 3 ) is calculated as the difference between the stress at break and the lower yield stress.

[0042] As described above, the difference between the breaking stress and the lower yield stress is calculated for each of the 10 test pieces, and the average of the 10 values ​​is determined as the difference between the breaking stress and the lower yield stress for each biaxially oriented sheet.

[0043] The biaxially stretched sheet can be produced by the following method. First, the (A) component, the (B) component, and optional components (the various additives mentioned above) used as necessary are mixed (e.g., melt-kneaded) and extruded from a die (e.g., a T-die) by an extruder under known conditions to obtain an unstretched sheet. Next, the unstretched sheet is stretched sequentially or simultaneously in the biaxial directions to obtain a biaxially stretched sheet.

[0044] A method for obtaining a biaxially stretched sheet from an unstretched sheet may include, for example, a first stretching step of stretching a first sheet (unstretched sheet) containing component (A), component (B), and optional components used as necessary in a first direction while heating it to a first temperature to obtain a second sheet, a second stretching step of stretching the second sheet in a second direction approximately perpendicular to the first direction while heating it to a second temperature to obtain a third sheet, a step of placing the third sheet at a temperature 23 to 33°C lower than the second temperature (hereinafter also referred to as the "annealing temperature") (hereinafter also referred to as the "annealing step"), and a step of placing the third sheet at a temperature 40°C or more lower than the second temperature (hereinafter also referred to as the "cooling temperature") (hereinafter also referred to as the "cooling step").

[0045] The composition of the first sheet (unstretched sheet) may be the same as that of the biaxially stretched sheet. Specifically, the first sheet may contain the above-mentioned components (A) and (B), and various additives as necessary.

[0046] In the first stretching step, the first sheet is stretched in a first direction while being heated to a first temperature to obtain a second sheet. The first temperature is defined as the surface temperature of the first sheet when the first sheet is stretched in the first direction. An example of a method for heating the first sheet to the first temperature is a method in which the temperature around the first sheet (for example, the ambient temperature) is set to about the first temperature. The first temperature may be within the range of Tg of the first sheet ±20°C, may be within the range of Tg of the first sheet ±15°C, may be within the range of Tg of the first sheet ±13°C, or may be within the range of Tg of the first sheet ±10°C. The first temperature may be, for example, 90°C or more, 100°C or more, or 105°C or more, and may be 150°C or less, 140°C or less, or 135°C or less.

[0047] The stretching speed in the first stretching step (the stretching speed when the first sheet is stretched in the first direction) may be 110% / sec or more, 130% / sec or more, or 140% / sec or more, and may be 230% / sec or less, 220% / sec or less, or 210% / sec or less.

[0048] The first direction may be, for example, MD. For example, a method of stretching the first sheet in MD may be a method of making a difference in the rotation speed between a downstream roll and an upstream roll. Specifically, the first sheet is nipped using two upstream rolls and two downstream rolls, and the rotation speed of the downstream roll is made faster than the rotation speed of the upstream roll, thereby stretching the first sheet in MD.

[0049] The stretching speed in the first stretching step (MD stretching speed) is calculated by the following formula (1). {(So-Si) / L} × 100 (1) In equation (1), Si is the rotational speed of the upstream roll (m / sec), So is the rotational speed of the downstream roll (m / sec), and L is the distance between the upstream roll and the downstream roll.

[0050] By adjusting So / Si, the stretching ratio of the biaxially oriented sheet can be adjusted. For example, the stretching in the first direction (MD) may be performed so that the stretching ratio in the MD of the biaxially oriented sheet falls within the above-mentioned numerical range.

[0051] The first stretching step described above provides a second sheet (a sheet stretched uniaxially). In the second stretching step, the second sheet is stretched in a second direction substantially perpendicular to the first direction while being heated to a second temperature to provide a third sheet. The second temperature is defined as the surface temperature of the second sheet when the second sheet is stretched in the second direction. As a method for heating the second sheet to the second temperature, a method in which the temperature around the second sheet (for example, the atmospheric temperature) is set to the second temperature can be mentioned.

[0052] The second temperature may be higher than the Tg of the first sheet. The second temperature may be 10° C. or more, 20° C. or more, or 25° C. or more higher than the Tg of the first sheet, and 50° C. or less, 40° C. or less, or 35° C. or less higher than the Tg of the first sheet. The second temperature may be 100° C. or more, 110° C. or more, or 120° C. or more, and 180° C. or less, 160° C. or less, or 155° C. or less.

[0053] The stretching speed in the second stretching step (the stretching speed when stretching the second sheet in the second direction) may be 1% / sec or more, 5% / sec or more, or 7% / sec or more, and may be 20% / sec or less, 15% / sec or less, or 10% / sec or less.

[0054] The second direction may be, for example, TD. For example, the method of stretching the second sheet in TD includes a method of gripping the end of the second sheet while the second sheet is flowing and stretching it in TD. Specifically, the second sheet can be stretched in TD by gripping the end of the second sheet with a chuck using a tenter and expanding the chuck width while the second sheet is flowing.

[0055] The stretching speed in the second stretching step (stretching speed in TD) is calculated by the following formula (2). {(Wi-Wo) / Wi}×100 / (L / S) ···(2) In equation (2), Wo is the width of the sheet before TD stretching (m), Wi is the width of the sheet after TD stretching (m), L is the MD distance the sheet flows during TD stretching (m), and S is the MD flow velocity of the sheet (m / sec).

[0056] By adjusting Wi / Wo, the stretching ratio of the biaxially oriented sheet can be adjusted. For example, the stretching in the second direction (TD) may be performed so that the stretching ratio in the TD of the biaxially oriented sheet falls within the above-mentioned numerical range.

[0057] An example of the combination of the first direction and the second direction is a combination of MD and TD. As described above, the first direction may be MD and the second direction may be TD, or the first direction may be TD and the second direction may be MD.

[0058] The above-described second stretching step provides a third sheet (a biaxially stretched sheet, a precursor of the above-described "biaxially stretched sheet"). The third sheet is then placed at a temperature (annealing temperature) 23 to 33°C lower than the second temperature in the annealing step, and then placed at a temperature (cooling temperature) 40°C or more lower than the second temperature in the cooling step. If at least one of the annealing step and the cooling step is not performed, or if at least one of the annealing temperature and the cooling temperature is not within the above-described numerical range, the molecular orientation in the sheet caused by the first stretching step and the second stretching step may be relaxed, and the difference between the above-described breaking stress and the lower yield stress may become small. The above-described method includes both an annealing step at a specific temperature and a cooling step at a specific temperature, so that relaxation of the molecular orientation is unlikely to occur, and a biaxially stretched sheet having a large difference between the above-described breaking stress and the lower yield stress is obtained.

[0059] The annealing temperature may be 24° C. or more, 25° C. or more, 26° C. or more, or 27° C. or more lower than the second temperature, and may be 32° C. or less, 31° C. or less, or 30° C. or less lower than the second temperature.

[0060] The time for which the third sheet is exposed to the annealing temperature can be appropriately set depending on the composition, thickness, etc. of the sheet, and may be, for example, from 1 second to 6 seconds.

[0061] An example of a method for subjecting the third sheet to the annealing temperature is to pass the third sheet through a chamber in which the atmospheric temperature is set to the annealing temperature.

[0062] The cooling temperature may be 41° C. or more, 42° C. or more, 43° C. or more, or 45° C. or more lower than the second temperature, and may be 100° C. or less, 80° C. or less, 70° C. or less, 60° C. or less, 55° C. or less, or 50° C. or less lower than the second temperature.

[0063] The time for which the third sheet is kept at the above cooling temperature can be appropriately set depending on the thickness of the sheet, and may be, for example, 1 second or more and 6 seconds or less.

[0064] The third sheet may be subjected to the cooling temperature by, for example, passing the third sheet through a tank in which the atmospheric temperature is set to the cooling temperature.

[0065] At least one surface of the biaxially stretched sheet may be coated with a coating agent containing one or more selected from known release agents (peeling agents), antifogging agents, and antistatic agents. The method for coating the biaxially stretched sheet with the coating agent is not particularly limited, and may be a method using a roll coater, a knife coater, a gravure roll coater, or the like, or may be a method of spraying, immersion, or the like.

[0066] The biaxially stretched sheet described above can be used, for example, to manufacture a molded product. As described above, the biaxially stretched sheet according to this embodiment has a large difference between the breaking stress and the lower yield stress, so that the thickness variation (excessive reduction in thickness at a specific portion) is suppressed in the molded product obtained by secondary molding (particularly thermoforming). Due to this, the molded product obtained by molding the biaxially stretched sheet has excellent strength. The molded product may be, for example, a container, or may be a food packaging container (food pack). In particular, when the molded product is a food packaging container for microwave heating, the characteristics of the biaxially stretched sheet according to this embodiment are fully exhibited. The molded product may be, for example, a food pack consisting of a main body portion and a lid material that can be fitted to the main body portion, and the shape of the fitting portion is an internal fitting.

[0067] The method for obtaining a molded product from the biaxially oriented sheet is not particularly limited, and any method commonly used in the secondary molding of conventional biaxially oriented sheets can be used. For example, secondary molding can be performed by a thermoforming method such as a vacuum molding method or a pressure molding method. These methods are described, for example, in "Plastic Processing Technology Handbook" edited by the Society of Polymer Science, Nikkan Kogyo Shimbun (1995). EXAMPLES

[0068] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0069] In Examples 1 to 7 and Comparative Examples 1 to 4, styrene-methacrylic acid copolymers and diene rubber modified polystyrenes produced in the manners of Experimental Examples 1 to 4 below were used.

[0070] {Experimental Example 1: Preparation of styrene-methacrylic acid copolymer (A-1)} A styrene-methacrylic acid copolymer was produced using an apparatus in which the first and second reactors, which are complete mixing type stirring vessels, were connected in series. The volumes of the first and second reactors were both 39 liters. A raw material solution was prepared by mixing 80.7% by mass of styrene, 2.8% by mass of methacrylic acid, 14.1% by mass of ethylbenzene, and 2.4% by mass of octanol, 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 Nippon Oil & Fats Co., Ltd.)) was added and mixed into the raw material solution from the inlet of the first reactor so that the added concentration (mass-based concentration relative to the raw material solution) was 200 μg / g. Polyoxyethylene alkyl ether (POE) (manufactured by Kao Corporation) was added and mixed into the raw material solution from the inlet of the first reactor. The mixture of materials fed to the first reactor was stirred in the first reactor, continuously fed 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.

[0071] Next, the solution containing the copolymer was continuously taken out from the second reactor and introduced in series into a vacuum degassing tank equipped with a preheater, which was composed of two stages, the first degassing tank and the 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, and unreacted styrene, methacrylic acid and ethylbenzene were separated. Thereafter, the mixture was extruded into a strand shape from a multi-hole die, and the strand was cooled and cut by a cold cut method to obtain a pellet-shaped styrene-methacrylic acid copolymer (A-1). As a result of analysis using pyrolysis gas chromatography, the styrene content was 96.0 mass% and the methacrylic acid content was 4.0 mass% based on the total amount of monomer units contained in the copolymer (A-1). In addition, the weight average molecular weight (Mw) and Z average molecular weight (Mz) determined by GPC measurement were 260,000 and 470,000, respectively.

[0072] The concentration of POE in the pellet-shaped styrene-methacrylic acid copolymer (A-1) obtained was 0.20% by mass based on the total amount of the copolymer. The concentration of POE was determined by the following procedure. 5 g of the pellet-shaped styrene-methacrylic acid copolymer (A-1) was precisely weighed and dissolved in THF. Methanol and a small amount of hydrochloric acid were added to the solution to reprecipitate the polymer, and the reprecipitate 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). For the quantification, a calibration curve prepared using three methanol solutions of POE with known concentrations was used. The HPLC conditions were as follows. HPLC model: Alliance System 2695 Separation Module manufactured by Nihon Waters Co., Ltd. Detector: Differential refractometer (RI) Column: Tosoh Corporation TSKgel ODS-120T 4.6mm (ID) x 15cm (L) Mobile phase: Methanol / water = 80 / 20 (volume ratio) with 0.2% phosphoric acid added Flow rate: 1.0ml / min Column oven temperature: 40℃ Detector temperature: 30℃ The POE concentration was calculated using the quantitative value (g) of POE according to the following formula. POE concentration (mass%) = (quantitative value of POE) / 5 x 100

[0073] {Experimental Example 2: Preparation of styrene-methacrylic acid copolymer (A-2)} The composition of the raw material solution in Experimental Example 1 (particularly the ratio of styrene to methacrylic acid) was changed to adjust the monomer content (the styrene content and the methacrylic acid content based on the total amount of monomer units contained in the copolymer), and the reaction temperature, residence time, type and amount of polymerization initiator during polymerization were changed to adjust the weight average molecular weight and Z average molecular weight, thereby obtaining a styrene-methacrylic acid copolymer (A-2). The monomer content in the copolymer (A-2) was 90.0 mass% styrene and 10.0 mass% methacrylic acid based on the total amount of monomer units contained in the copolymer (A-2). The weight average molecular weight (Mw) and Z average molecular weight (Mz) determined by GPC measurement were 170,000 and 310,000, respectively.

[0074] {Experimental Example 3: Production of diene rubber modified polystyrene (B-1)} 6.7% by mass of low-cis polybutadiene rubber (Diene 55AS (Asahi Kasei Corp.)) as a rubber-like polymer and 88.3% by mass of styrene were dissolved in 5.0% by mass of ethylbenzene. In addition, 0.1 parts by mass of a rubber antioxidant (Irganox 1076 (BASF Corp.)) was added to a total of 100 parts by mass of the rubber-like polymer, styrene, and ethylbenzene. This polymerization raw material was supplied at 12.5 [kg / hr] to a 14-liter jacketed reactor (R-01) equipped with an anchor-shaped stirring blade with a blade diameter of d = 0.285 [m]. The reaction temperature was 140°C, and the rotation speed was N [s -1 ], N 3 d 2 is 0.83 [m 2 / s 3] and the resin ratio was 25%. The obtained resin liquid was introduced into two 21-liter jacketed plug-flow reactors arranged in series. In the first plug-flow reactor (R-02), the reaction temperature was adjusted to 120-140°C in the direction of the resin liquid flow, and in the second plug-flow reactor (R-03), the jacket temperature was adjusted so that the reaction temperature had a gradient of 130-160°C in the direction of the resin liquid flow. The resin ratio at the outlet of R-02 was 50%, and the resin ratio at the outlet of R-03 was 70%. The obtained resin liquid was heated to 230°C and then sent to a volatilization tank with a vacuum degree of 5 [torr], and after separating and recovering the unreacted monomer and solvent, it was extracted from the volatilization tank with a gear pump, passed through a die plate to form strands, and then pelletized through a water tank to recover the product. The content of diene rubber components based on the total amount of the obtained diene rubber-modified polystyrene was 10.0% by mass.

[0075] {Experimental Example 4: Production of diene rubber modified polystyrene (B-2)} The amounts of various raw materials charged in Experimental Example 3 were adjusted to obtain a diene rubber modified polystyrene (B-2) having a diene rubber component content of 1.0 mass %.

[0076] <Example 1> 99.0% by mass of the styrene-methacrylic acid copolymer (A-1) of Experimental Example 1 and 1.0% by mass of the diene rubber modified polystyrene (B-1) of Experimental Example 3 were fed to a pellet extruder (vacuum vent equipped twin screw co-rotating extruder HyperKTX140MX (manufactured by Kobe Steel, Ltd.)), melt-kneaded, and then extruded from a T-die having a width of 1200 mm at an extrusion temperature of 230°C, a discharge rate of 2600 kg / h, a rotation speed of 110 rpm, and a vent pressure of the vacuum vent (absolute pressure relative to vacuum) of 1.8 kPaA to obtain a first sheet. The Tg of the first sheet was 110°C. The glass transition temperature (Tg) of the first sheet was determined by measuring in accordance with JIS K-7121 using a DHR-2 (TA Instruments rheometer).

[0077] The first sheet was cooled by a cooling roll, and then stretched in a first direction (MD) to obtain a second sheet (first stretching step). The stretching in the first direction was performed by nipping the sheet using two rolls on the upstream side and two rolls on the downstream side. At this time, the rotation speeds of the rolls on the downstream side and the upstream side were adjusted so that the stretching ratio in MD was the value shown in Table 1. The stretching speed and the first temperature in the first stretching step were as shown in Table 1. Next, the second sheet was stretched in a second direction (TD) to obtain a third sheet (second stretching step). The stretching in the second direction was performed in a tenter in which the end of the sheet was held by a chuck and the atmospheric temperature in the tank was set to 138°C. At this time, the chuck width at the exit part of the tenter and the chuck width at the entrance part were adjusted so that the stretching ratio in TD was the value shown in Table 1. The stretching speed and the second temperature in the second stretching step were as shown in Table 1, and the MD speed at the tenter outlet (the line speed shown in Table 1) was 69 m / min.

[0078] Furthermore, the third sheet was passed through an annealing section (MD tank length: 3.2 m) in which the atmospheric temperature in the tank was set to 110° C. (annealing temperature listed in Table 1) at the above line speed, followed by a cooling section (MD tank length: 3.5 m) in which the atmospheric temperature in the tank was set to 95° C. (cooling temperature listed in Table 1), thereby placing the sheet at a temperature 28° C. lower than the second temperature, and then at a temperature 43° C. lower than the second temperature, to obtain a biaxially oriented sheet. The thickness of the obtained biaxially oriented sheet was 0.238 mm, and the average particle diameter of the rubber particles in the biaxially oriented sheet was 5.0 μm. The content of unreacted styrene monomer in the obtained biaxially oriented sheet was 200 ppm.

[0079] <Examples 2 to 7 and Comparative Examples 1 to 4> Biaxially oriented sheets (Examples 2 to 7 and Comparative Examples 1 to 4) were obtained in the same manner as in Example 1, except that the types and contents of the styrene-methacrylic acid copolymer and diene rubber modified polystyrene, and the conditions for stretching in the first direction, stretching in the second direction, annealing, and cooling were as shown in Table 1. The Tg of the first sheet and the thickness of the biaxially oriented sheet in each example were as shown in Table 1, and the average particle size of the rubber particles contained in the biaxially oriented sheet was as shown in Table 1.

[0080] The biaxially stretched sheets of Examples 1 to 7 and Comparative Examples 1 to 4 were analyzed and evaluated as follows. The results are shown in Table 1.

[0081] [Stretching ratio] A straight line Y of 100 mm was drawn in the MD and TD on a test piece of the biaxially oriented sheet, and the test piece was allowed to stand for 60 minutes in an oven at a temperature 30° C. higher than the Vicat softening point temperature of the biaxially oriented sheet measured in accordance with JIS K7206 to cause it to shrink. After standing for 1 hour at 23° C., the length Z [mm] of the straight line was measured at room temperature and calculated using the following formula: Stretching ratio = Y / Z, unit [times] The MD stretch ratio and the TD stretch ratio were calculated by the above.

[0082] [Difference between breaking stress and lower yield stress] Using the biaxially stretched sheets of each Example and Comparative Example, a type 5 test piece described in JIS K7127:1999 was prepared in accordance with the same JIS. For each biaxially stretched sheet, a total of 10 test pieces were prepared, including 5 test pieces with MD as the longitudinal direction and 5 test pieces with TD as the longitudinal direction. In accordance with the same JIS, a tensile test was performed on each test piece under the condition of 100°C using an autograph (AGS-100NX (manufactured by Shimadzu Corporation)) to obtain a stress-strain curve (SS curve). In the obtained SS curve, the stress at the break point and the stress at the lower yield point were obtained. Then, the difference between the stress at the break point and the stress at the lower yield point (stress at the break point - stress at the lower yield point) was calculated. The average value of the "difference between the stress at break and the lower yield stress" determined for the 10 test pieces was calculated as the "difference between the stress at break and the lower yield stress" for each biaxially oriented sheet.

[0083] <Evaluation of molded products (containers)> [Container thickness and thickness reduction rate] A food pack (dimensions: lid: length 150 mm × width 130 mm × height 30 mm, body: length 150 mm × width 130 mm × height 20 mm) was molded using a hot plate molding machine (HPT-400A (Wakisaka Engineering Co., Ltd.)) at a hot plate temperature of 135°C and a heating time of 2.0 seconds. The thickness of each of the four corners of the top surface (lid) of the obtained food pack was measured, and the smallest value among these was taken as the thickness of the container (T 1 The thickness of this container (T 1 ) and the thickness of the biaxially oriented sheet before molding (T 0 ) was used to calculate the thickness reduction rate according to the following formula. When the thickness reduction rate is less than 50%, it can be said that an excessive reduction in thickness is suppressed, and the smaller the value, the more preferable it is. Thickness reduction rate (%)=(1-T 1 / T 0 ) x 100

[0084] [Container strength (compressive load at the time of container buckling)] In the food pack molded in the above [Container thickness and thickness reduction rate], the corner that showed the smallest thickness among the four corners where the thickness was measured was pressed with a compression tool (φ50 mm fixed pressure plate) at a speed of 15 mm / sec using a bench-top precision universal testing machine (AGS-100NX (Shimadzu Corporation)) to measure the compression load when the food pack buckled (compression load when container buckled) (ambient temperature 23°C). Note that if the compression load when the container buckles is 10 N or more, it can be said that the strength is good, and the higher the load, the better.

[0085] [Heat resistance of container (temperature at which container begins to deform)] Food packs molded in the same manner as in [Container thickness and thickness reduction rate] above were placed in a tank set at 100°C using a constant temperature incubator DKN812 (manufactured by Yamato Scientific Co., Ltd.) for 15 minutes, then removed and checked for deformation of the food pack. The set temperature was increased by 1°C at a time and the same operation was repeated to determine the minimum set temperature at which deformation of the food pack was confirmed (container deformation start temperature). In order to stabilize the temperature, the food pack was placed in the tank one hour after the temperature in the tank reached each set temperature. If the container deformation start temperature is 105°C or higher, it can be said to have excellent heat resistance, and the higher the container deformation start temperature, the better.

[0086] [Table 1] [Explanation of symbols]

[0087] P1...breaking point, P2...yield point, P3...lower yield point.

Claims

1. A copolymer containing styrene and methacrylic acid as monomer units; a diene-rubber-modified polystyrene containing a diene-rubber component; A biaxially oriented sheet comprising: A biaxially oriented sheet, wherein in a stress-strain curve obtained by subjecting the biaxially oriented sheet to a tensile test at 100° C., the difference between the stress at the breaking point and the stress at the lower yield point is 10 MPa or more.

2. The biaxially oriented sheet according to claim 1, wherein the content of the methacrylic acid is 2.0 to 15.0% by mass based on the total amount of the biaxially oriented sheet.

3. The biaxially oriented sheet according to claim 1 or 2, wherein the weight average molecular weight (Mw) of the biaxially oriented sheet is 150,000 to 400,000.

4. The biaxially oriented sheet according to claim 1 or 2, wherein the content of the diene rubber component is 0.005 to 0.50 mass% based on the total amount of the biaxially oriented sheet.

5. The biaxially oriented sheet according to claim 1 or 2, wherein the mass ratio of the content of the copolymer to the content of the diene rubber modified polystyrene is 95.0 / 5.0 to 99.9 / 0.

1.

6. 3. The biaxially oriented sheet according to claim 1, wherein the thickness of the biaxially oriented sheet is 0.1 to 0.3 mm.

7. A molded article obtained by molding the biaxially oriented sheet according to claim 1 or 2.

8. 8. The molded article according to claim 7, which is a food packaging container for heating in a microwave oven.

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