Biaxially oriented sheet and container formed therefrom

Abstract

To provide a biaxially stretched sheet which is excellent in reworkability to a general-purpose polystyrene-based resin, heat resistance, transparency, rigidity and appearance, and a container obtained by secondary molding of the biaxially stretched sheet.SOLUTION: A biaxially stretched sheet contains a styrene-methacrylic acid copolymer (a) containing a styrenic monomer unit (A) and a methacrylic acid monomer unit, and a rubber-modified polystyrene-based resin (b) having a polymer matrix and rubbery polymer particles, in which when the total of the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene-based resin (b) is 100 pts.mass, a content of the styrene-methacrylic acid copolymer (a) is 93.0 pts.mass or more and less than 97.0 pts.mass; a content of the rubber-modified polystyrene-based resin (b) is more than 3.0 pts.mass and 7.0 pts.mass or less; a content of the methacrylic acid monomer unit with respect to the whole styrene-methacrylic acid copolymer (a) is 2-12 mass%; and a haze is 5% or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a biaxially stretched sheet, a container formed by secondary molding of the sheet, and a lid material therefor.

Background Art

[0002] Styrene-methacrylic acid-based resins typified by styrene-methacrylic acid copolymer resins are excellent in heat resistance, transparency, and rigidity, and are inexpensive. Therefore, they are widely used as packaging materials for food containers such as lunch boxes and prepared foods, foamed boards for heat insulating materials for houses, diffusion plates for liquid crystal TVs containing diffusing agents, etc. In particular, due to the spread of high-power microwave ovens used for business purposes in convenience stores and the like in recent years, as a material for containers having heat resistance capable of withstanding the temperature during cooking in a high-power microwave oven and a lid material for sealing or covering the container, a biaxially stretched sheet using a styrene-methacrylic acid-based resin composition is used.

[0003] On the other hand, the styrene-methacrylic acid-based resin composition contained in the biaxially stretched sheet has low compatibility with polystyrene-based resins made of general-purpose styrene homopolymers or rubber-modified polystyrene resins having a styrene homopolymer as a polymer matrix. Therefore, when reworking the end material obtained after forming a container from a biaxially stretched sheet using a styrene-methacrylic acid-based resin composition, the amount of rework is limited, and a problem occurs in that a large amount of resin waste that has to be treated as waste is generated. Further, Patent Document 1 mentions a method of copolymerizing α-methylstyrene as a heat-resistant polystyrene-based resin that can be reworked with a polystyrene-based resin made of a general-purpose styrene homopolymer or a rubber-modified polystyrene resin having a styrene homopolymer as a polymer matrix.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] While the technology described in Patent Document 1 above yields biaxially oriented sheets with excellent reworkability to polystyrene resins made from general-purpose styrene homopolymers, or rubber-modified polystyrene resins using styrene homopolymers as a polymer matrix, copolymerization of α-methylstyrene and styrene using general-purpose radical bulk polymerization is difficult, and industrially satisfactory yields cannot be secured unless living polymerization is used, resulting in poor productivity. Therefore, there has been a problem in industrially obtaining styrene-methacrylic acid resin sheets with excellent reworkability and heat resistance. Furthermore, considering food containers and their packaging materials, which are the applications of the styrene-methacrylic acid resin sheets mentioned above, transparency, rigidity, and appearance are required. Therefore, the present invention provides a biaxially oriented sheet with excellent reworkability, heat resistance, transparency, rigidity, and appearance for general-purpose polystyrene resins, and a container formed by secondary molding of the biaxially oriented sheet. [Means for solving the problem]

[0006] In view of the above problems, the inventors diligently conducted research and succeeded in realizing a biaxially oriented sheet using a styrene-based resin composition that is excellent in reworkability, heat resistance, transparency, rigidity, and appearance, as well as a container obtained by secondary molding of the biaxially oriented sheet, by kneading a styrene-methacrylic acid copolymer (a) with a rubber-modified polystyrene resin (b) having a specific amount of polymer matrix and rubber-like polymer particles, and thus completed the present invention. That is, the present invention is as described in [1] to

[11] below.

[0007] [1] A biaxially oriented sheet comprising a styrene-methacrylic acid copolymer (a) containing styrene monomer units (A) and methacrylic acid monomer units, and a rubber-modified polystyrene resin (b) having a polymer matrix and rubbery polymer particles, When the total amount of the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b) is 100 parts by mass, the content of the styrene-methacrylic acid copolymer (a) is 93.0 parts by mass or more and less than 97.0 parts by mass, and the content of the rubber-modified polystyrene resin (b) is greater than 3.0 parts by mass and 7.0 parts by mass or less. The content of the methacrylic acid monomer units relative to the entire styrene-methacrylic acid copolymer (a) is 2 to 12% by mass. A biaxially oriented sheet with a haze level of 5% or less.

[0008] [2] The distretched sheet according to [1] above, wherein the content of the methacrylic acid monomer units relative to the entire styrene-methacrylic acid copolymer (a) is 3 to 6% by mass.

[0009] [3] The biaxially oriented sheet according to [1] or [2] above, further comprising 500 to 5000 ppm of liquid paraffin.

[0010] [4] A biaxially oriented sheet according to any of [1] to [3] above, further comprising 30 to 300 ppm of metal soap.

[0011] [5] The biaxially oriented sheet according to any of [1] to [4] above, wherein the melt mass flow rate of the rubber-modified polystyrene resin (b) at 200°C is 10 to 25 g / 10 min.

[0012] [6] A biaxially oriented sheet according to any one of [1] to [5] above, wherein the rubber component contained in the rubbery polymer particles is made of rhoxybutadiene rubber.

[0013] [7] The biaxially oriented sheet according to any one of [1] to [6] above, wherein the structure of the rubbery polymer particles is a salami structure.

[0014] [8] A biaxially stretched sheet obtained by extrusion molding a styrene resin composition containing the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b), wherein the solution viscosity of the styrene resin composition is 5.0 to 15.0 cP, the biaxially stretched sheet according to any one of [1] to [7] above

[0015] [9] A molded product made of the biaxially stretched sheet according to any one of [1] to [8] above.

[0016]

[10] The food packaging container for microwave heating according to [9], wherein the molded product according to [9] is a food packaging container for microwave heating.

[0017]

[11] A hood pack having a container body and a lid material that can be fitted to the container body, wherein the shape of the fitting portion between the container body and the lid material is an internal fitting, the hood pack according to

[10]

Advantages of the Invention

[0018] According to the present invention, it is possible to provide a biaxially stretched sheet using a styrene resin composition excellent in reworkability, heat resistance, transparency, rigidity, and appearance to a general-purpose polystyrene resin, and a container formed by secondary molding of the biaxially stretched sheet.

Modes for Carrying Out the Invention

[0019] Hereinafter, the modes for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. The present invention is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist.

[0020] [Biaxially Stretched Sheet] The biaxially oriented sheet of this embodiment contains specific amounts of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) having a polymer matrix and rubber-like polymer particles, and has a specific haze value. In the biaxially oriented sheet, when the total amount of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) is 100 parts by mass, the content of styrene-methacrylic acid copolymer (a) is 93.0 parts by mass or more and less than 97.0 parts by mass, and the content of rubber-modified polystyrene resin (b) is greater than 3.0 parts by mass and 7.0 parts by mass or less. When the total amount of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) is set at 100 parts by mass, if the content of rubber-modified polystyrene resin (b) exceeds 7.0 parts by mass, the transparency of the composition containing styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) decreases, resulting in a decrease in the transparency of the biaxially oriented sheet and the molded product formed therefrom. On the other hand, if the content of rubber-modified polystyrene resin (b) is 3.0 parts by mass or less, the reworkability to general-purpose polystyrene resin decreases. Furthermore, by reducing the haze of the biaxially oriented sheet to 5% or less, a biaxially oriented sheet with excellent appearance and transparency can be obtained. More preferably, it should be 4% or less, and even more preferably 3% or less. The biaxially stretched sheet of the present invention may have a specific haze value and may contain a styrene-methacrylic acid copolymer (a) and a rubber-modified polystyrene resin (b) in respective predetermined amounts. Therefore, after preparing a styrene resin composition containing the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b), a biaxially stretched sheet may be formed from the styrene resin composition, or, the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b) may be blended in respective predetermined amounts and directly formed into a biaxially stretched sheet without passing through the styrene resin composition. In the former case, a styrene resin composition containing a specific content of the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b) having a polymer matrix and rubber-like polymer particles may be used as a molding raw material. More specifically, the biaxially stretched sheet can be obtained by extrusion molding a styrene resin composition containing the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b) having a polymer matrix and rubber-like polymer particles in a specific mass ratio, and then biaxially stretching the obtained unstretched sheet. Hereinafter, the styrene-methacrylic acid copolymer (a), the rubber-modified polystyrene resin (b) which are components of the biaxially stretched sheet, and a styrene resin composition which is one form of the raw material of the biaxially stretched sheet will be described.

[0021] <<Styrene-methacrylic acid copolymer (a)>> The styrene-methacrylic acid copolymer (a) in the present invention is a copolymer having a styrene monomer unit (A) and a methacrylic acid monomer unit as essential components (hereinafter also simply referred to as copolymer (a)), and contributes to improving the heat resistance of the biaxially stretched sheet and a molded article formed from the sheet. Further, the styrene-methacrylic acid copolymer (a) may further have monomer units other than the essential components of the styrene monomer unit (A) and the methacrylic acid monomer unit, if necessary (for example, (meth)acrylic acid ester monomer units represented by methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, etc., and other monomer units).

[0022] <Styrene-based monomer (A)> In the styrene-methacrylic acid copolymer (a) of this embodiment, the content of the styrene monomer unit (A) relative to the entire styrene-methacrylic acid copolymer (a) is 88 to 98% by mass, preferably 91 to 97.5% by mass, and more preferably 94 to 97% by mass. If the content of the styrene monomer unit (A) is less than 88% by mass, it leads to a decrease in fluidity, and if it is more than 98% by mass, it becomes difficult to include the desired amount of methacrylic acid monomer units described later, and the effect of improving heat resistance by the methacrylic acid monomer units cannot be sufficiently obtained. Furthermore, the content of styrene-methacrylic acid copolymer (a) in the biaxially oriented sheet in this embodiment is substantially the same as the content of styrene-methacrylic acid copolymer (a) contained in the styrene-based composition.

[0023] In this embodiment, the styrene monomer (A) is not particularly limited, but examples include styrene, α-methylstyrene, β-methylstyrene, paramethylstyrene, orthomethylstyrene, metamethylstyrene, chlorostyrene, bromostyrene, etc. From an industrial standpoint, styrene and α-methylstyrene are particularly preferred, and styrene is more preferred. These can be used individually or as a mixture of two or more as the styrene monomer (A). Note that the styrene monomer unit (A) refers to a repeating unit obtained by polymerizing the styrene monomer (A). The same applies to the styrene monomer unit (B) contained in the rubber-modified polystyrene resin (b) described later. <Methacrylic acid monomer> In the styrene-methacrylic acid copolymer (a) of this embodiment, the methacrylic acid monomer units play a role in improving heat resistance. The content of the methacrylic acid monomer units relative to the entire styrene-methacrylic acid copolymer (a) is 2 to 12% by mass, preferably 2.5 to 9% by mass, and more preferably 3 to 6% by mass. In particular, by adjusting the content of methacrylic acid monomer units to 3 to 6% by mass, a styrene-based resin composition or biaxially oriented sheet with excellent transparency and reworkability to general-purpose polystyrene-based resins can be obtained when kneaded with a rubber-modified polystyrene-based resin (b). If the content is less than 2% by mass, the effect of improving heat resistance is insufficient. Furthermore, if the content of methacrylic acid monomer units exceeds 12% by mass, problems such as an increase in gelled material in the resin and excessive viscosity during manufacturing occur.

[0024] <Other monomers> The styrene-methacrylic acid copolymer (a) according to the present invention may further have other monomer units other than the styrene monomer unit (A) and the methacrylic acid monomer unit described above. That is, in this embodiment, the other monomer units may be copolymerized with other monomers other than the two monomers shown above, as long as they are copolymerizable with the styrene monomer (A) and the methacrylic acid monomer, and the effects of the invention are not impaired. For example, other monomers other than the two monomers shown above include, for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, cyclohexyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, maleic anhydride, maleic acid, fumaric acid, itaconic acid, (meth)acrylonitrile, dimethyl maleate, dimethyl fumarate, diethyl fumarate, ethyl fumarate, maleimide, and nuclear-substituted maleimide. The content of styrene monomer units (A), methacrylic acid monomer units, and other monomer units in the styrene-methacrylic acid copolymer (a) according to the present invention is determined by proton nuclear magnetic resonance ( 1 It can be determined from the integral ratio of the spectrum measured with a 1H-NMR detector.

[0025] The melt flow rate of the styrene-methacrylic acid copolymer (a) according to the present invention at 200°C is preferably 0.5 to 5.0, more preferably 1.0 to 4.0, and even more preferably 1.5 to 3.0. When the melt flow rate is 0.5 or higher, it is preferred from the viewpoint of fluidity, and when it is 5.0 or lower, it is preferred from the viewpoint of the mechanical strength of the resin. In this disclosure, the melt flow rate is a value measured at 200°C and a load of 49N in accordance with ISO 1133.

[0026] The weight-average molecular weight (Mw) of the styrene-methacrylic acid copolymer (a) according to the present invention is preferably 100,000 to 400,000, more preferably 120,000 to 320,000, and even more preferably 150,000 to 270,000. When the weight-average molecular weight is 100,000 to 400,000, a resin with excellent practical balance between impact strength and fluidity can be obtained. The weight-average molecular weight can be measured on a polystyrene standard basis by gel permeation chromatography.

[0027] The Vicat softening temperature of the styrene-methacrylic acid copolymer (a) according to the present invention is preferably 104 to 130°C, more preferably 105 to 125°C, even more preferably 106 to 120°C, and even more preferably 107 to 115°C. By setting the Vicat softening temperature of the styrene-methacrylic acid copolymer (a) to 104°C or higher, an effect of improving the heat resistance of the composition can be obtained, and by setting it to 130°C or lower, it becomes easier to knead with rubber-modified polystyrene. The method for measuring the Vicat softening temperature described herein is measured in accordance with ISO 306.

[0028] <Methyrene-methacrylic acid copolymer (a) production method> The method for producing the styrene-methacrylic acid copolymer (a) of this embodiment will be described below. The method for producing the styrene-methacrylic acid copolymer (a) of the present invention preferably includes the steps of: preparing a mixed solution by mixing a styrene monomer (A), a methacrylic acid monomer, other monomers as needed, a monohydric alcohol described later, and a solvent; polymerizing the mixed solution to produce a reaction product; and recovering the reaction product. There are no particular restrictions on the polymerization method of the styrene-methacrylic acid copolymer (a), but for example, radical polymerization, and among these, bulk polymerization or solution polymerization, can be preferably employed. Specifically, the polymerization method mainly comprises a polymerization step of polymerizing polymerization raw materials (monomer components) and a defoliation step of removing unreacted monomers, polymerization solvents, and other volatile components from the polymerization product. The polymerization method according to this embodiment will be described below. In this embodiment, when polymerizing the polymerization raw materials to obtain the styrene-methacrylic acid copolymer (a), the polymerization raw material composition typically contains a polymerization initiator. Examples of polymerization initiators include organic peroxides, such as peroxyketals like 2,2-bis(t-butylperoxy)butane, 1,1-bis(t-butylperoxy)cyclohexane, and n-butyl-4,4-bis(t-butylperoxy)valerate; dialkyl peroxides like di-t-butylperoxide, t-butylcumylperoxide, and dicumylperoxide; diacyl peroxides like acetylperoxide and isobutyrylperoxide; peroxydicarbonates like diisopropylperoxydicarbonate; peroxyesters like t-butylperoxyacetate; ketone peroxides like acetylacetone peroxide; and hydroperoxides like t-butylhydroperoxide. From the viewpoint of decomposition rate and polymerization rate, 1,1-bis(t-butylperoxy)cyclohexane is preferred among them. In this embodiment, a chain transfer agent may be used as needed during the polymerization of the styrene-methacrylic acid copolymer (a). Examples of chain transfer agents include α-methylstyrene linear dimer, n-dodecyl mercaptan, t-dodecyl mercaptan, n-octyl mercaptan, and the like. Solution polymerization using a polymerization solvent can be employed as the polymerization method for the styrene-methacrylic acid copolymer (a) described above. Preferred polymerization solvents include aromatic solvents such as toluene, ethylbenzene, propylbenzene, and butylbenzene. If necessary, a solvent system may be used in which polar solvents such as alcohols or ketones are combined to adjust the solubility of the styrene-methacrylic acid copolymer (a).

[0029] In this embodiment, the polymerization solvent is preferably used in the range of 3 to 35 parts by mass, and more preferably in the range of 5 to 30 parts by mass, per 100 parts by mass of the total monomers constituting the styrene-methacrylic acid copolymer (a). If the amount of polymerization solvent exceeds 35 parts by mass per 100 parts by mass of the total monomers, the polymerization rate decreases and the molecular weight of the resulting resin also decreases, which tends to reduce the mechanical strength of the resin. Furthermore, if the amount of polymerization solvent is less than 3 parts by mass, it may become difficult to control heat removal during polymerization. Adding the polymerization solvent in a ratio of 3 to 35 parts by mass per 100 parts by mass of the total monomers is preferable in terms of ensuring uniform quality and controlling the polymerization temperature. Furthermore, when using a monohydric alcohol as a polymerization solvent, it is preferable to add it in a ratio of 1 to 10% by mass per 100% by mass of the total polymerization solvent. The apparatus used in the polymerization step to obtain the styrene-methacrylic acid copolymer (a) according to the present invention is not particularly limited and can be appropriately selected according to general polymerization methods for styrene-based resins. For example, in the case of bulk polymerization, a polymerization apparatus consisting of one or more fully mixed reactors can be used. There are also no particular limitations on the devolatilization step. In the case of bulk polymerization, polymerization is carried out until the amount of unreacted monomer is preferably 50% by mass or less, more preferably 40% by mass or less, and then devolatilization is performed by known methods to remove volatile components such as the unreacted monomer. For example, conventional devolatilization apparatus such as a flash drum, twin-screw devolatilizer, thin-film evaporator, or extruder can be used, but a devolatilization apparatus with a small retention area is preferred. The temperature of the devolatilization treatment is usually around 190 to 280°C, and more preferably 190 to 260°C from the viewpoint of suppressing decomposition. The pressure of the devolatilization treatment is usually around 0.13 to 4.0 kPa, preferably 0.13 to 3.0 kPa, and more preferably 0.13 to 2.0 kPa. Devolutation methods include, for example, removing volatile components by reducing the pressure under heating, and removing them by passing them through an extruder or the like designed for the purpose of removing volatile components.

[0030] <<Rubber-modified polystyrene resin (b)>> The rubber-modified polystyrene resin (b) in the present invention (also simply referred to as resin (b)) is obtained by dispersing rubbery polymer particles (= rubbery polymer particles) in a polymer matrix of a resin consisting of a styrene monomer (B), and polymerizing the styrene monomer (B) in the presence of the rubbery polymer. The lower limit of the content of rubber-modified polystyrene resin (b) in the styrene-based composition used in the biaxially oriented sheet of the present invention, or in the biaxially oriented sheet, is preferably, in order of the total amount of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) being 100 parts by mass, greater than 3.0 parts by mass, 3.1 parts by mass or more, 3.2 parts by mass or more, 3.3 parts by mass or more, 3.4 parts by mass or more, 3.5 parts by mass or more, 3.6 parts by mass or more, 3.7 parts by mass or more, 3.8 parts by mass or more, and 3.9 parts by mass or more. Similarly, the upper limit of the content of rubber-modified polystyrene resin (b) is preferably, in order of the following: 7.0 parts by mass or less, 6.8 parts by mass or less, 6.6 parts by mass or less, 6.4 parts by mass or less, 6.2 parts by mass or less, 6.0 parts by mass or less, 5.8 parts by mass or less, 5.6 parts by mass or less, 5.4 parts by mass or less, and 5.0 parts by mass or less. These upper and lower limits can be combined arbitrarily. Furthermore, the content range of the rubber-modified polystyrene resin (b) is greater than 3.0 parts by mass and less than or equal to 7.0 parts by mass, when the total amount of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) is 100 parts by mass, and is preferably in the following order: 3.1 parts by mass or more and 6.8 parts by mass or less, 3.2 parts by mass or more and 6.6 parts by mass or less, 3.3 parts by mass or more and 6.4 parts by mass or less, 3.4 parts by mass or more and 6.2 parts by mass or less, 3.5 parts by mass or more and 6.0 parts by mass or less, 3.6 parts by mass or more and 5.8 parts by mass or less, 3.7 parts by mass or more and 5.6 parts by mass or less, 3.8 parts by mass or more and 5.4 parts by mass or less, and 3.9 parts by mass or more and 5.0 parts by mass or less. By setting the content of the rubber-modified polystyrene resin (b) in the range of 3.0 to 7.0 parts by mass, a biaxially oriented sheet and molded product thereof with excellent reworkability to general-purpose polystyrene resins can be obtained, and by further reducing it to 5.0 parts by mass or less, a biaxially oriented sheet and molded product thereof with even better transparency can be obtained. Furthermore, the content of the rubber-modified polystyrene resin (b) in the biaxially oriented sheet in this embodiment is substantially the same as the content of the rubber-modified polystyrene resin (b) contained in the styrene composition.

[0031] <Rubber-like polymer particles> In this embodiment, the rubbery polymer constituting the rubbery polymer particles in the rubber-modified polystyrene resin (b) can be polybutadiene, polyisoprene, natural rubber, polychloroprene, styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, etc., but from an industrial viewpoint, polybutadiene and styrene-butadiene copolymer are preferred. For polybutadiene, high-cis polybutadiene with a high cis content, low-cis polybutadiene with a low cis content, or both can be used, but from the viewpoint of ease of industrial handling, low-cis polybutadiene is preferred. The structure of the styrene-butadiene copolymer may be a random structure, a block structure, or a combination thereof. These rubbery polymers may be used individually or in combination of two or more. Saturated rubber obtained by hydrogenating butadiene rubber can also be used. In this embodiment, the rubbery polymer particles preferably contain styrene monomer units (B) within the dispersed particles (rubbery polymer particles) of the rubbery polymer. The form of this encapsulation may be a core-shell type dispersed particle with a polymer having styrene monomer units (B) as the core and the rubbery polymer as the shell, or a so-called salami-structure type dispersed particle in which multiple domain phases of the polymer having styrene monomer units (B) are contained within the rubbery polymer. From the viewpoint of providing strength to the biaxially oriented sheet, salami-structure dispersed particles are preferred.

[0032] <Content of conjugated diene monomer units> In this embodiment, the rubbery polymer particles or rubbery polymer are preferably formed from conjugated diene monomers. In this specification, a conjugated diene monomer unit is a diolefin having a pair of conjugated double bonds among the monomer units constituting the rubbery polymer particles, and examples include 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and 1,3-hexadiene. The content of conjugated diene monomer units in the rubber-modified styrene resin (b) and the styrene resin composition can be measured by the procedure described in the Examples section below, or by an equivalent method. In this embodiment, the content of conjugated diene monomer units in the rubber-modified polystyrene resin (b) is preferably 0.5 to 15% by mass, more preferably 1.0 to 7.0% by mass, and even more preferably 1.5 to 4.0% by mass, relative to the total rubber-modified polystyrene resin (b).

[0033] <Average particle size of rubbery polymer particles> In this embodiment, the rubbery polymer, which is the rubber component in the rubber-modified polystyrene resin (b), exists as particles of the rubbery polymer in the rubber-modified polystyrene resin (b), the styrene resin composition, or the biaxially oriented sheet. The average particle size of the rubbery polymer particles in the rubber-modified polystyrene resin (b) or the styrene resin composition is preferably 0.3 to 5.0 μm, more preferably 0.5 to 4.0 μm, and even more preferably 0.7 to 3.0 μm. The rubber-modified styrene resin (b) is obtained by polymerizing a styrene monomer (B) in a reactor with a stirrer in the presence of rubbery polymer particles, but the average particle size of the rubbery polymer particles can be adjusted by the rotation speed of the stirrer, the molecular weight of the rubbery polymer used, etc. In this disclosure, the average particle size of the rubbery polymer particles is a value measured from a cross-sectional observation image using a transmission electron microscope. In the biaxially oriented sheet described later, the rubbery polymer particles are stretched by the stretching ratio, and the average particle size of the rubbery polymer particles increases by up to about 400%.

[0034] <Swelling Index> In this embodiment, it is preferable that the swelling index of the toluene-insoluble portion of the rubber-modified polystyrene resin (b) is 8.0 to 14.0, and that the mass ratio of the toluene-insoluble portion to the rubber content in the toluene-insoluble portion (toluene-insoluble portion / rubber content in the toluene-insoluble portion) is 1.5 to 4.0. More preferably, this swelling index is 9.0 to 13.5, and even more preferably 9.5 to 13.0, and the ratio of toluene-insoluble portion to rubber content in the toluene-insoluble portion is more preferably 2.0 to 3.5, and even more preferably 2.5 to 3.5. When the swelling index of the toluene-insoluble portion of the rubber-modified styrene resin (b) is 8.0 to 14.0, and the ratio of toluene-insoluble portion to rubber content in the toluene-insoluble portion is 1.5 to 4.0, a resin with excellent mechanical strength can be obtained. In this disclosure, the swelling index of toluene-insoluble matter and the ratio of toluene-insoluble matter to rubber content in toluene-insoluble matter are values ​​measured by the procedures described in the Examples section or by procedures that will be understood to those skilled in the art to be equivalent thereto.

[0035] In this embodiment, the melt flow rate of the rubber-modified polystyrene resin (b) at 200°C is preferably 1.5 to 25.0 g / 10 min, more preferably 2.0 to 20.0 g / 10 min, and even more preferably 2.0 to 18.0 g / 10 min. When the melt flow rate is in the range of 1.5 to 25.0 g / 10 min, the miscibility with the styrene-methacrylic acid resin (a) is good, and the mechanical strength is also good. In this disclosure, the melt flow rate is a value measured at 200°C and a load of 49 N in accordance with ISO 1133.

[0036] <Method for producing rubber-modified polystyrene resin (b)> The method for producing rubber-modified polystyrene resin (b) is not particularly limited, but it can be produced by bulk polymerization (or solution polymerization) in which styrene monomer (B) (and solvent) is polymerized in the presence of a rubbery polymer, bulk-suspension polymerization in which the reaction transitions to suspension polymerization, or emulsion graft polymerization in which styrene monomer (B) is polymerized in the presence of rubbery polymer particles. In bulk polymerization, it can be produced by continuously supplying a mixed solution containing the rubbery polymer, styrene monomer (B), and optionally an organic solvent, organic peroxide, and / or chain transfer agent to a polymerization apparatus configured by connecting a fully mixed reactor or a tank reactor and a plurality of tank reactors in series.

[0037] <Monohydric alcohols with 16 or more carbon atoms> In a preferred embodiment of this product, the styrene-based resin composition or biaxially oriented sheet preferably contains a monohydric alcohol with 16 or more carbon atoms. Alcohols with 15 or fewer carbon atoms are highly volatile, and when molding or other processes are carried out, the alcohol generates an odor, reducing workability. However, it has been confirmed that by increasing the number of carbon atoms to 16 or more, the volatility is reduced, and off-odors during molding and other processes are suppressed.

[0038] In this embodiment, the inclusion of a monohydric alcohol having 16 or more carbon atoms suppresses the formation of gels generated by dehydration condensation between methacrylic acid monomer units derived from the styrene-methacrylic acid copolymer (a) during molding at around 250°C, resulting in a molded product with excellent appearance. The monohydric alcohol is an alcohol having 16 or more carbon atoms containing one hydroxyl group, and may contain heteroatoms such as oxygen or nitrogen in the carbon chain, and may also contain bonds other than single bonds, such as double bonds, triple bonds, ester bonds, and amide bonds. The number of carbon atoms is preferably 16 or more, more preferably 17 or more, and even more preferably 18 to 50. The monohydric alcohol may be contained in the styrene-based resin composition or the biaxially oriented sheet. Therefore, the monohydric alcohol may remain in the final resin composition by being present (or added) to the polymerization solution used when polymerizing resin (a), or it may be included by mixing it in the extruder when kneading the resin.

[0039] In this embodiment, the boiling point of the monohydric alcohol having 16 or more carbon atoms is preferably 260°C or higher, more preferably 270°C or higher, and even more preferably 290°C or higher. If the boiling point of the alcohol is below 260°C, its volatility increases, and off-odors tend to be generated during molding, etc.

[0040] The content of monohydric alcohol having 16 or more carbon atoms in the styrene-based resin composition or biaxially oriented sheet according to the present invention is preferably 0.03 to 1.0 parts by mass, more preferably 0.04 to 0.7 parts by mass, and even more preferably 0.05 to 0.4 parts by mass, when the total amount of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) is 100 parts by mass. If the content of monohydric alcohol having 16 or more carbon atoms is less than 0.1 parts by mass, the gel suppression effect during molding decreases. On the other hand, if the content exceeds 1.0 part by mass, the amount remaining in the resin increases, which greatly reduces the odor or heat resistance, and the heat resistance increase effect due to methacrylic acid modification becomes poor.

[0041] The monohydric alcohols with 16 or more carbon atoms mentioned above are not particularly limited, but examples include 1-hexadecanol, isohexadecanol, 1-octadecanol, 5,7,7-trimethyl-2-(1,3,3-trimethylbutyl)-1-octanol, isooctadecanol, 1-isoisoeicosanol, 8-methyl-2-(4-methylhexyl)-1-decanol, 2-heptyl-1-undecanol, 2-heptyl-4-methyl-1-decanol, 2-(1,5-dimethylhexyl)-(5,9-dimethyl)-1-decanol, and polyoxyethylene alkyl ethers. The above polyoxyethylene alkyl ethers are preferably compounds represented by the following general formula (2). [ka] (In the general formula (2) above, R is an alkyl group having 12 to 20 carbon atoms, and X represents the average number of ethylene oxide additions, which is an integer from 1 to 15.)

[0042] <Hindered phenol type antioxidant> In a preferred embodiment of this product, the styrene-based resin composition or biaxially oriented sheet preferably contains a hindered phenol type antioxidant. The inclusion of a hindered phenol type antioxidant prevents yellowing during kneading, extrusion sheet molding, or container molding, and prevents a decrease in strength due to a decrease in the molecular weight of the rubber dispersion component. When the total amount of the styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b) in the styrene-based resin composition or biaxially oriented sheet is 100 parts by mass, the content of the hindered phenol type antioxidant is preferably in the range of 0.001 to 0.1 parts by mass, more preferably 0.002 to 0.05 parts by mass, and even more preferably 0.003 to 0.03 parts by mass. A content of 0.001 parts by mass or more is effective in suppressing the decomposition of the rubber dispersion component, while a content of 0.1 parts by mass or more induces yellowing of the resin, which is undesirable.

[0043] Examples of hinder-tophenol type antioxidants according to the present invention include 2,6-di-t-butyl-p-cresol, 2,6-diphenyl-4-octadecyloxyphenol, distearyl(3,5-di-t-butyl-4-hydroxybenzyl)phosphonate, 1,6-hexamethylenebis[(3,5-di-t-butyl-4-hydroxyphenyl)propionamide], 4,4'-thiobis(6-t-butylm-cresol), 2,2'-methylenebis(4-methyl-6-t-butylphenol), and 2,2'-methylenebis Tylenebis(4-ethyl-6-t-butylphenol), 4,4'-butylidenebis(6-t-butyl m-cresol), 2,2'-ethylidenebis(4,6-di-t-butylphenol), 2,2'-ethylidenebis(4-sec-butyl-6-t-butylphenol), 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-t-butylbenzyl)isocyanurate, 1,3,5-tris(3,5-di-t-butylphenyl) (Tyl 4-hydroxybenzyl) isocyanurate, 1,3,5-tris(3,5-di-t-butyl4-hydroxybenzyl)-2,4,6-trimethylbenzene, 2-t-butyl4-methyl-6-(2-acryloyloxy-3-t-butyl5-methylbenzyl)phenol, 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate stearyl, 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-t-pentylphenyl=acrylate, tetra Kiss[3-(3,5-di-t-butyl4-hydroxyphenyl)propionate methyl]methane, 2-t-butyl-6-methyl-4-{3-[(2,4,8,10-tetra-t-butyldibenzo[d,f][1,3,2]dioxaphosfepin-6-yl)oxy]propyl}phenol, thiodiethylene glycol bis[(3,5-di-t-butyl4-hydroxyphenyl)propionate], 1,6-hexamethylene bis[(3,5-di-t-butyl-4-hydroxyphenyl)propionate], bis[3,Examples include 3-bis(4-hydroxy-3-t-butylphenyl)butyric acid glycol ester, bis[2-t-butyl4-methyl-6-(2-hydroxy-3-t-butyl-5-methylbenzyl)phenyl] terephthalate, 1,3,5-tris[(3,5-di-t-butyl-4-hydroxyphenyl)propionyloxyethyl] isocyanurate, 3,9-bis[1,1-dimethyl-2-{(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy}ethyl]-2,4,8,10-tetraoxaspiro[5,5]undecane, and triethylene glycol bis[(3-t-butyl-4-hydroxy-5-methylphenyl)propionate]. These may be used individually or in combination of two or more. In this embodiment, the hindered phenol type antioxidant may be added during the polymerization of each resin, or it may be added during the mixing of the styrene-methacrylate copolymer (a) and the rubber-modified polystyrene resin (b).

[0044] <Metal soap> In a preferred embodiment of this product, the styrene-based resin composition or biaxially oriented sheet preferably contains metal soaps. The inclusion of metal soaps in the styrene-based resin composition has the effect of reducing the discharge pressure during sheet extrusion. Furthermore, when the biaxially oriented sheet is secondarily molded and processed into containers or the like, the release properties from the mold can be improved, and a highly productive oriented sheet can be obtained. The metal soap content is preferably in the range of 10 to 1000 ppm, more preferably 20 to 800 ppm, even more preferably 30 to 600 ppm, and even more preferably 50 to 300 ppm relative to the total amount of the styrene-based resin composition or biaxially oriented sheet. A metal soap content of 10 ppm or more can impart release properties to the biaxially oriented sheet, but if it exceeds 1000 ppm, it will bleed out to the resin surface and contaminate the mold surface that comes into contact with it during secondarily molding.

[0045] Examples of metal soaps according to the present invention include zinc stearate, calcium stearate, magnesium stearate, etc., but zinc stearate is preferred.

[0046] <Liquid paraffin> The preferred styrene-based resin composition or biaxially oriented sheet of this embodiment preferably further contains liquid paraffin. The styrene-based resin composition or biaxially oriented sheet containing liquid paraffin exhibits an effect of improving fluidity and the strength of the sheet molded product, and an effect of reducing cracking during winding. The content of liquid paraffin in the styrene-based resin composition or biaxially oriented sheet is preferably 500 to 5000 ppm, more preferably 600 to 4000 ppm, and even more preferably 650 to 3000 ppm. Below 500 ppm, no fluidity improvement effect is obtained, and above 3000 ppm, a decrease in heat resistance occurs.

[0047] The liquid paraffin used in this invention may also be referred to as white mineral oil, mineral oil, MO, white mineral oil, etc., depending on the degree of refinement and commercial practice. Liquid paraffin is preferably preferred if it has a naphthene component ratio of 20% or more, more preferably 30% or more, as determined by ndM ring analysis, due to its excellent compatibility with components in styrene-based resin compositions or biaxially oriented sheets. The ndM ring analysis method is a compositional test method for high-boiling point petroleum fractions, and by determining the refractive index (n), density (d), and molecular weight (M), it is possible to determine the aromatic ring ratio (%Ca), naphthene ring ratio (%Cn), and paraffin chain ratio (%Cp) in the oil (ASTM D3238). From the viewpoint of product color, the polycyclic aromatic components in white mineral oil must be 3% or less, preferably 0.5% or less. In liquid paraffin, the aromatic rings are usually 0%. Having a low concentration of low-boiling-point components in liquid paraffin is effective in avoiding volatile content problems during extrusion molding. It is preferable that the 5% distillation temperature is 400°C or higher, calculated at atmospheric pressure from the vacuum distillation method or gas chromatography method according to JIS K2254. The kinematic viscosity of the liquid paraffin is preferably in a viscosity range that has a low amount of the low-boiling-point components mentioned above, effectively lowers the Vicat softening temperature, and is easy to handle. (40 mm² / s to 120 mm² at 40°C) 2 A range of / s is preferred, and more preferably 60 to 80 mm² / s.

[0048] There are no particular limitations on the method of adding liquid paraffin, and methods include adding liquid paraffin during the polymerization process, or kneading using known kneaders such as single-screw extruders, twin-screw extruders, and Banbury mixers. It is especially preferable to add it during the production of each resin because it improves dispersibility, and it is particularly preferable to add it during the production of rubber-modified polystyrene resin (b).

[0049] The quantitative determination and identification of liquid paraffin in this embodiment can be easily confirmed by methods common to those skilled in the art. For example, a styrene-based resin composition or a fragment of a molded article of the composition (e.g., a biaxially oriented sheet) is dissolved in a solvent that dissolves the matrix resin, such as tetrahydrofuran, to prepare a solution. Then, while stirring this solution with a stirrer, n-hexane is added dropwise in small amounts to precipitate the polymer matrix and rubbery polymer. After that, the filtrate filtered through a glass filter is evaporated to dryness, then the volume is adjusted with n-hexane, passed through a polytetrafluoroethylene membrane filter, and separated by liquid chromatography to calculate the liquid paraffin content in the composition or molded article. Furthermore, for the analysis of liquid paraffin, pyrolysis GC-MS is used. 1 H-NMR or 13 Identification, quantification, and molecular weight measurement can be performed using various analytical instruments such as 1C-NMR.

[0050] <Other ingredients> The styrene-based resin composition or biaxially oriented sheet of this embodiment may also be made by adding various additives commonly used in styrene-based resins to achieve known effects, in addition to the monohydric alcohol, hinder-tophenol type antioxidant, metal soap, and liquid paraffin mentioned above. Examples include stabilizers, antioxidants, UV absorbers, lubricants, mold release agents, plasticizers, anti-blocking agents, antistatic agents, anti-fogging agents, or mineral oil. Reinforcing materials such as styrene-butadiene block copolymer or MBS resin may also be added to the extent that they do not impair the physical properties. There are no specific regulations regarding the compounding method, but examples include adding the additives during polymerization, or mixing the additives in a blender before melt-kneading after polymerization, and then melt-kneading them in an extruder or Banbury mixer.

[0051] In this embodiment, as described above, various additives can be added to the styrene-based resin composition or the biaxially oriented sheet. However, the total content of the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b) in the styrene-based resin composition or the biaxially oriented sheet is not particularly limited, but is preferably 90% by mass or more, more preferably 92% by mass, and even more preferably 95% by mass or more. In other words, the above-mentioned other components may be present in the styrene-based resin composition at a concentration of less than 10% by mass.

[0052] [Physical properties of styrene-based resin compositions] The physical properties of the styrene-based resin composition in this embodiment are described below. In this embodiment, the Vicat softening temperature of the styrene resin composition is preferably 103°C to 120°C, more preferably 105°C to 115°C, and even more preferably 106°C to 112°C. Setting the Vicat softening temperature to 103°C or higher suppresses dimensional deformation when in contact with hot water, and setting it to 105°C or higher yields sheets and containers suitable for heating and cooking in a microwave oven. Keeping the Vicat softening temperature below 115°C yields biaxially oriented sheets that are easy to mold, similar to general-purpose polystyrene resins, and setting it below 112°C yields biaxially oriented sheets with excellent reworkability to general-purpose polystyrene. The Vicat softening temperature can be measured in accordance with ISO 306 under conditions of a load of 50N and a heating rate of 50°C / h. In this embodiment, the melt flow rate of the styrene-based resin composition at 200°C is preferably in the range of 0.5 to 5.0 g / 10 min, more preferably 1.0 to 4.0 g / 10 min, and even more preferably 1.5 to 3.0 g / 10 min. By setting the melt flow rate to 0.5 g / 10 min or higher, good moldability can be obtained, and by setting it to 5.0 g / 10 min or lower, a stretched sheet with excellent strength can be obtained.

[0053] [Biaxially oriented sheet] This embodiment is a biaxially oriented sheet formed using a styrene-methacrylic acid copolymer (a) and a rubber-modified polystyrene resin (b), or the styrene resin composition of the present invention described above. A commonly known method can be used to manufacture the biaxially oriented sheet. The biaxially oriented sheet is produced by stretching it in the flow direction (MD) with a roll, and then stretching it in the vertical direction (TD) with a tenter. Alternatively, a styrene resin composition molded into a plate may be heated to approximately the Vicat softening temperature of the composition + 10 to 40°C and then sequentially or simultaneously biaxially stretched with a tenter. Furthermore, the biaxially oriented sheet may be used in multilayer formation with styrene resins other than the styrene resin composition, such as polystyrene resin. It may also be used in multilayer formation with resins other than styrene resins. Examples of resins other than styrene resins include PET resin and nylon resin.

[0054] In this embodiment, it is preferable in terms of strength to stretch the biaxially oriented sheet by approximately 1.3 to 7.0 times in the MD direction and 1.3 to 7.0 times in the TD direction.

[0055] In the embodiment, the thickness of the biaxially oriented sheet is preferably 0.1 mm or more, more preferably 0.15 mm or more, and even more preferably 0.2 mm or more, in order to ensure the strength, especially the rigidity, of the sheet and the container. On the other hand, from the viewpoint of economy, it is preferably 0.7 mm or less, more preferably 0.6 mm or less, and even more preferably 0.5 mm or less.

[0056] In this embodiment, the orientation relaxation stress in the longitudinal and transverse directions of the biaxially oriented sheet is preferably in the range of 0.4 to 1.3 MPa, and the strength of the molded product of the biaxially oriented sheet can be maintained by adjusting the orientation relaxation stress to this range.

[0057] In a preferred embodiment of this product, a known anti-fogging agent may be applied to one side of the biaxially oriented sheet to prevent fogging caused by moisture volatilizing from food when used as a food packaging container. Examples of such anti-fogging agents include nonionic surfactants such as sucrose fatty acid esters and polyglycerin fatty acid esters, and polyether-modified silicone oil. The method for applying the above-mentioned anti-fogging agent to the biaxially oriented sheet is not particularly limited, and simple methods include using a roll coater, knife coater, gravure roll coater, etc. Spraying and immersion can also be used. Furthermore, the wettability of the sheet surface may be improved by surface treatment such as corona treatment, ozone treatment, or primer treatment before application.

[0058] Another aspect of the present invention provides a molded article formed using the biaxially oriented sheet described above. The biaxially oriented sheet or a multilayer containing the same can be formed, for example, by vacuum forming to produce a lid for a bento box or a container for side dishes, etc. In particular, a transparent lid for a food packaging container that is compatible with microwave heating is preferred because the features of the present invention are fully demonstrated. [Examples]

[0059] The present invention will now be described in detail with reference to examples and comparative examples, but the present invention is not limited to these examples. The analysis and evaluation methods for resins, compositions, and biaxially oriented sheets in the examples and comparative examples are as follows.

[0060] [Characterization of each resin and composition] (1) Measurement of weight-average molecular weight The weight-average molecular weight (Mw) of each resin and resin composition was measured using gel permeation chromatography (GPC) under the following conditions. Measuring instrument: Tosoh HLC-8220 Sorting column: Two TSK gel Super HZM-H columns (4.6mm inner diameter) manufactured by Tosoh are connected in series. Guard column: Tosoh TSK guard column Super HZ-H Solvent used for measurement: Tetrahydrofuran (THF) Sample concentration: 5 mg of the sample was dissolved in 10 mL of solvent and filtered through a 0.45 μm filter. Injection volume: 10μL Measurement temperature: 40℃ Flow rate: 0.35mL / min Detector: Ultraviolet absorption detector (Tosoh UV-8020, wavelength 254nm) Eleven types of TSK standard polystyrene manufactured by Tosoh Corporation (F-850, F-450, F-128, F-80, F-40, F-20, F-10, F-4, F-2, F-1, A-5000) were used to create the calibration curve. The calibration curve was created using an approximation formula for a linear curve.

[0061] (2) Measurement of meltmass flow rate (MFR) The melt mass flow rate (g / 10 min) of each resin and resin composition was measured in accordance with ISO 1133 under load conditions of 200°C and 49 N.

[0062] (3) Measurement of Vicat softening temperature The Vicat softening temperature of each resin and resin composition was measured in accordance with ISO 306. The load was 50 N and the heating rate was 50 °C / h. Molded biaxially oriented sheets using materials with a Vicat softening temperature exceeding 105 °C exhibited excellent dimensional stability (heat resistance) at temperatures simulating microwave heating (100 °C and above).

[0063] (4) Measurement of the alcohol content in the resin composition The content of monohydric alcohols with 16 or more carbon atoms was measured using gas chromatography under the following conditions. Sample preparation: 1.0 g of resin was dissolved in 5 mL of methyl ethyl ketone, then 5 mL of hexane containing the standard substance (p-diethylbenzene) was added to re-precipitate the polymer components, and the supernatant was collected and used as the measurement solution. Measurement equipment: Agilent 6850 series GC system Detector: FID Column: HP-1 (100% dimethylpolysiloxane) 30m, Film thickness 0.25 μm, 0.32 mmφ Injection volume: 1 μL (splitless) Column temperature: Hold at 40°C for 2 minutes → Increase temperature to 320°C at a rate of 20°C / min → Hold at 320°C for 15 minutes. Inlet temperature: 250℃ Detector temperature: 280℃ Carrier gas: Helium

[0064] (5) Measurement of the content of each additive and conjugated diene monomer unit The content of hindered phenol-type antioxidants and conjugated diene monomer units in the resin composition was measured by thermal GC to satisfy the following conditions. Measurement conditions Pyrolysis unit Equipment: Frontier Lab PY-3030D Furnace temperature: 600℃ GC Equipment: Shimadzu Corporation GCMS-GP2020NX Column: Ultra Alloy-5 Column temperature: The column was held at 50°C for 5 minutes, then the temperature was increased at a rate of 10°C / min, and from 100°C onwards, it was increased at a rate of 70°C / min, and held at 300°C for 10 minutes. Inlet temperature: 300℃ Detector temperature: 300℃ Split ratio: 1 / 30 Carrier gas: Helium Detection method: Mass spectrometer (MSD) Furthermore, when detecting each additive, sample pretreatment and detection conditions may be adjusted as appropriate, taking into account peak overlap and intensity.

[0065] (6) Average particle size of rubbery polymer particles in rubber-modified polystyrene resin (b) The average particle size (μm) of rubbery polymer particles in rubber-modified polystyrene resin (b) (so-called HIPS resin) is given by the following formula (2) for 200 rubbery elastic particles observed by cross-sectional observation using a transmission electron microscope: Average particle diameter=Σ(ni×Di 4 ) / Σ(ni×Di 3 ) {In equation (2) above, ni is the number of rubbery elastic particles having particle size Di, and Di is the average value of the major and minor axes of the rubbery elastic particles.} It was calculated using the method described above.

[0066] (7) Measurement of the swelling index of toluene-insoluble components of rubber-modified polystyrene resin (b) 1 g of rubber-modified styrene resin (b) was accurately weighed into a sedimentation tube (W1), 20 ml of toluene was added, and the mixture was shaken at 23°C for 2 hours. Then, the mixture was centrifuged in a centrifuge (Hitachi, Ltd. himac, CR-20 (rotor: R20A2)) at 10°C or below and 20,000 rpm for 60 minutes. The sedimentation tube was slowly tilted to approximately 45 degrees, and the supernatant was removed by decantation. The mass of the insoluble matter containing toluene was accurately weighed (W2), and it was vacuum-dried at 160°C and 3 kPa or below for 1 hour. The dried toluene-insoluble matter was cooled to room temperature in a desiccator, and then its mass was accurately weighed (W3). The swelling index and toluene-insoluble content were determined using the following formula. Toluene-insoluble content (mass%) = ((W3) / (W1)) × 100 Swelling index of toluene-insoluble components = (W2 / W3)

[0067] (8) Measurement of solution viscosity 4.0 g of styrene resin composition was weighed and dissolved in 36.0 g of methyl ethyl ketone to prepare the test solution. An Ostwald-Cannonfenske type (#300) viscosity system was immersed in a viscosity constant temperature bath controlled at 25 ± 0.2 °C, 10 mL of the test solution was added, and after standing for at least 5 minutes to stabilize the temperature of the sample solution, the viscosity of the solution was measured and determined.

[0068] [Characterization of Biaxially Oriented Sheet Molded Products] (9) Visual inspection (foreign objects) Five sheets measuring 10cm x 20cm were cut from a biaxially oriented sheet with a thickness of 0.25mm. The number of gel-like foreign objects on the surface of the five sheets, whose average diameter (major axis + minor axis) / 2 was 0.5mm or larger, was counted, and the appearance was judged using the following method. ○: Number of gel items is 2 or less △: Number of gel items: 3-9 ×: The number of gel items is 10 or more.

[0069] (10) Measurement of mechanical strength (film impact) The above biaxially oriented sheet was cut into 8cm x 8cm pieces, and the film impact was measured using a Toyo Seiki film impact tester (No. 195), with the n8 average value taken as the result.

[0070] (11) Heat resistance of biaxially oriented sheets The biaxially oriented sheet described above was placed in an oven set to 110°C for 60 minutes. After that, the deformation of the sheet was visually observed, and the heat resistance was evaluated based on the thermal deformation as follows. ○: Dimensional change of 1% or less △: Dimensional change of 1% or more and 3% or less ×: Dimensional deformation of 3% or more

[0071] (12) Release properties of biaxially oriented sheets Ten pieces were prepared by cutting the above biaxially oriented sheet into 3cm x 3cm sections, sandwiching them between two 5mm thick sheets of pre-hardened steel, and securing them with clips. After heating them in a 150°C oven for 5 minutes, the ease of peeling them from the sheet metal was evaluated from the following perspectives. ○: We were able to peel the sheets from all 10 samples. △: 1 to 3 samples remained attached to the sheet metal. ×: More than 5 samples did not detach from the sheet metal.

[0072] (13) Measurement of haze of biaxially oriented sheets The haze at 0.25 mm was measured for biaxially oriented sheets prepared in the examples and comparative examples using a haze meter (NDH-2000) manufactured by Nippon Denshoku Industries Co., Ltd., and the average value for n3 was measured. In particular, by keeping the haze value below 3%, a biaxially oriented sheet with good visual transparency was achieved.

[0073] (14) HIPS rework properties (mechanical strength of HIPS during mixing) The HIPS resin B-2 described later and the styrene-based resin composition prepared in each example or comparative example were mixed and extruded in a twin-screw extruder at a mass ratio of 73:27, and then pelletized. These pellets were then molded in an injection molding machine (EC60N, manufactured by Toshiba Machine Co., Ltd.) at a cylinder temperature of 230°C, a mold temperature of 45°C, an injection pressure of 80 MPa, and an injection speed of 26 mm / s to obtain ISO mold type A test specimens. The Charpy impact strength (kJ / m²) of the obtained test specimens was measured. 2 The measurement was performed with a notch, in accordance with ISO 179.

[0074] [Evaluation of the properties of molded products obtained by secondary molding of biaxially oriented sheets] Using a hot plate forming machine, a lid measuring 160mm (length) x 140mm (width) x 40mm (height) was fabricated under the conditions of a hot plate temperature of 145°C and a heating time of 2.0 seconds, and was subjected to the following evaluation. (15) Oil resistance when heated in a microwave oven A circular area of ​​salad oil, approximately 5 mm in diameter, was applied to the center of the lid. After heating in a 1500W microwave for 90 seconds, the condition of the area with the salad oil was visually evaluated. ○: No change △: Whitening present, no tears. ×: Whitening occurred, and the coated area tore.

[0075] (16) Transparency (after molding) The haze of molded products was measured using a NDH-2000 haze meter manufactured by Nippon Denshoku Industries Co., Ltd., and the n3 average value was measured and evaluated according to the following criteria. ○: Haze 2.0% or less △: Haze above 2.0% and below 5.0% ×: Haze exceeds 5.0%

[0076] (17) Strength of molded product (strength of container) A test piece measuring 80 x 80 mm was cut from the center of the lid, and the film impact was measured using a Toyo Seiki film impact tester (No. 195). The n8 average value was used, and the following criteria were used for evaluation. ○: 5.0 kgf or more ×: Less than 5.0 kgf Containers with a load of less than 5.0 kgf were found to break during transport.

[0077] [Examples of manufacturing each resin and resin composition] The following describes specific manufacturing examples of styrene-methacrylic acid copolymer (a) and rubber-modified polystyrene resin (b). <<Example of Styrene-Methacrylic Acid Copolymer (a) Production Example>> <Preparation of resin a-1> A polymerization raw material composition liquid consisting of 82.4 parts by mass of styrene, 2.9 parts by mass of methacrylic acid, 15.0 parts by mass of ethylbenzene, and 0.025 parts by mass of 1,1-bis(t-butylperoxy)cyclohexane was supplied at a rate of 0.8 liters / hour to a 3.6-liter fully mixed reactor, and then continuously supplied to a defloration unit connected to a single-screw extruder to remove volatile components such as unreacted monomers and polymerization solvents. The polymerization temperature of the fully mixed reactor was set to 125°C. The single-screw extruder was set to a temperature of 200-250°C and a pressure of 10 torr to deflorate the volatile components such as unreacted monomers and polymerization solvents. The deflorated volatile components were condensed in a condenser through a refrigerant at -5°C and recovered as unreacted liquid, and the styrene-methacrylic acid copolymer (a-1) was recovered as resin pellets. The physical properties of resin a-1 obtained by the above analytical method are shown in Table 1 below.

[0078] <Preparation of resins a-2, a-3, a-5, and a-6> By partially adjusting the amount of methacrylic acid feed, resins a-2, a-3, a-5, and a-6 were prepared using the same procedure as for resin a-1, with the raw material compositions shown in Table 1. The physical properties of each resin are as shown in Table 1.

[0079] <Preparation of resin a-4> Methyl methacrylate was added as a monomer, and resin a-4 was prepared using the same procedure as above, with the raw material composition shown in Table 1. The physical properties of resin a-4 are as shown in Table 1.

[0080] <Example of manufacturing rubber-modified polystyrene resin (b)> <<Preparation of Resin b-1>> Rubber-modified polystyrene resin b-1 (hereinafter referred to as resin b-1) was produced using a polymerization apparatus consisting of three laminar flow reactors (1.5 liters each) equipped with stirrers connected in series, followed by a two-stage vented extruder. 82.6 parts by mass of styrene, 12.0 parts by mass of ethylbenzene, 2.2 parts by mass of diene 35AE (a low-cis-butadiene rubber manufactured by Asahi Kasei Corporation) as the rubber component, 3.2 parts by mass of liquid paraffin, and 0.02 parts by mass of 1,1-bis(t-butylperoxy)cyclohexane were added to a raw material tank equipped with a stirrer. After dissolving the rubber component with a stirrer, this raw material solution was supplied to the reactors at a rate of 0.75 liters / hr, and polymerization was carried out at temperatures of 110-120°C for the first stage reactor, 120-130°C for the second stage reactor, and 140-150°C for the third stage reactor. The extruder temperature was 210-240°C, the vacuum level was 3 kPa, and the total solid content of the polymerization liquid discharged from the final reactor was 70.1% by mass. The rubbery polymer particle size was controlled by adjusting the rotation speed of the stirrer in the first-stage laminar flow reactor to 150 rpm. In the addition step, 0.05 parts by mass of Irganox 1076 (chemical structure: octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate) manufactured by BASF was added as a hindered phenol type antioxidant. The composition and properties of the obtained rubber-modified styrene resin b-1 (hereinafter referred to as resin b-1) are shown in Table 2.

[0081] <Preparation of resin b-2> Resin b-2 was prepared in the same manner as resin b-1, with some adjustments to the conditions. The composition and properties of the obtained resin b-2 are shown in Table 2.

[0082] <Preparation of resin b-3> Resin b-3 was prepared in the same manner as described above, using high-cis-butadiene rubber as the rubber component. The composition and properties of the obtained resin b-3 are shown in Table 2.

[0083] <Preparation of resin b-4> Resin b-4 was prepared in the same manner as described above, using a styrene-butadiene block copolymer as the rubber component. The composition and properties of the obtained resin b-4 are shown in Table 2.

[0084] <Method for manufacturing styrene-based resin compositions and biaxially oriented sheets> The detailed manufacturing method for the styrene-based resin composition is shown below. [Example 1] 96.5 parts by mass of the obtained styrene-methacrylic acid copolymer (resin a-1), 3.5 parts by mass of rubber-modified polystyrene resin (resin b-1), 0.16 parts by mass of a monohydric alcohol with 18 carbon atoms (FO-180, manufactured by Nissan Chemical Corporation), and 0.0055 parts by mass of zinc stearate were kneaded at 230°C and 80 rpm using a 30 mmφ twin-screw extruder, and then pelletized to obtain a pelletized styrene resin composition. The obtained pellets were processed into 10 cm × 10 cm plates by press molding. The plates were placed in a Toyo Seiki EX6-S1 batch twin-screw stretcher with a chuck distance of 85 mm, preheated for 10 minutes at the Vicat softening temperature of the styrene resin composition + 30°C, and then stretched at 170 mm / min with an X-axis magnification of 2.5 and a Y-axis magnification of 2.5 to obtain a biaxially stretched sheet with a thickness of 0.25 mm. Table 3 shows the evaluation results for the obtained resin composition and biaxially oriented sheet, as well as the evaluation results for the molded product.

[0085] [Examples 2-14] A resin composition and a biaxially oriented sheet were obtained in the same manner as in Example 1, except that the formulation was changed as shown in Table 3. The evaluation results of the resin composition and the biaxially oriented sheet, and the evaluation results of the molded product are shown in Table 3.

[0086] [Comparative Examples 1-7] A resin composition and a biaxially oriented sheet were obtained in the same manner as in Example 1, except that the formulation was changed as shown in Table 4. The evaluation results of the resin composition and the biaxially oriented sheet, and the evaluation results of the molded product are shown in Table 4.

[0087] [Table 1]

[0088] [Table 2]

[0089] [Table 3]

[0090] [Table 4] [Industrial applicability]

[0091] The biaxially oriented sheet using the styrene-based resin composition of the present invention and the container formed therefrom exhibit excellent reworkability on general-purpose polystyrene resins, heat resistance, transparency, rigidity, and appearance. Therefore, the biaxially oriented sheet of the present invention and the container formed therefrom can be used for food packaging containers, are particularly useful as transparent lids for microwave-safe food packaging, have excellent reworkability with general-purpose polystyrene resins, and have a low environmental impact, thus playing a significant role in industry.

Claims

1. A biaxially oriented sheet containing a styrene-methacrylic acid copolymer (a) containing styrene monomer units (A) and methacrylic acid monomer units, and a rubber-modified polystyrene resin (b) having a polymer matrix and rubbery polymer particles, When the total amount of the styrene-methacrylic acid copolymer (a) and the rubber-modified polystyrene resin (b) is 100 parts by mass, the content of the styrene-methacrylic acid copolymer (a) is 93.0 parts by mass or more and less than 97.0 parts by mass, and the content of the rubber-modified polystyrene resin (b) is greater than 3.0 parts by mass and less than or equal to 7.0 parts by mass. The content of the styrene monomer unit (A) relative to the entire styrene-methacrylic acid copolymer (a) is 84 to 98% by mass. The content of the methacrylic acid monomer units relative to the entire styrene-methacrylic acid copolymer (a) is 2 to 12% by mass. A biaxially oriented sheet with a haze level of 5% or less.

2. The biaxially oriented sheet according to claim 1, wherein the content of the methacrylic acid monomer units relative to the entire styrene-methacrylic acid copolymer (a) is 3 to 6% by mass.

3. The biaxially oriented sheet according to claim 1 or 2, further comprising 500 to 5000 ppm of liquid paraffin.

4. A biaxially oriented sheet according to any one of claims 1 to 3, further comprising 30 to 300 ppm of metal soap.

5. The biaxially oriented sheet according to any one of claims 1 to 4, wherein the melt mass flow rate of the rubber-modified polystyrene resin (b) at 200°C is 10 to 25 g / 10 min.

6. The biaxially oriented sheet according to any one of claims 1 to 5, wherein the rubber component contained in the rubbery polymer particles is made of hydroxybutadiene rubber.

7. The biaxially oriented sheet according to any one of claims 1 to 6, wherein the structure of the rubbery polymer particles is a salami structure.

8. The biaxially oriented sheet according to any one of claims 1 to 7, wherein the weight-average molecular weight (Mw) of the styrene-methacrylic acid copolymer (a) is 100,000 to 400,000.

9. A molded article made from a biaxially oriented sheet according to any one of claims 1 to 8.

10. A food packaging container for microwave heating, wherein the molded article described in claim 9 is used for microwave heating cooking.

11. The microwaveable food packaging container described in claim 10 is a food pack, A food pack comprising a container body having a recess and a lid material that can be fitted onto the container body, wherein the shape of the fitting portion between the container body and the lid material is an internal fitting.

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