Biaxially oriented syndiotactic polystyrene film

Abstract

Provided is a biaxially oriented syndiotactic polystyrene film excellent in transparency and easy to cut. The biaxially oriented syndiotactic polystyrene film contains a syndiotactic polystyrene resin. The film has a haze of 25.0% or less, and a refractive index Nx in the MD direction of 1.580 or more. The film preferably contains at least one of an antioxidant and lubricant particles. The antioxidant preferably contains at least one of a hindered phenol antioxidant and a phosphorus antioxidant. The film preferably has a thickness of 10 μm-80 μm inclusive.

Description

Biaxially stretched syndiotactic polystyrene film

[0001] The present invention relates to a biaxially oriented syndiotactic polystyrene film.

[0002] Biaxially oriented syndiotactic polystyrene film has excellent heat resistance, electrical properties, and transparency, and is expected to be used in a variety of film applications, including electrical and electronic equipment materials (especially circuit boards, resin panels for millimeter-wave radomes, resin panels with good radio wave transmission, magnetic tape, and capacitors), automotive and electrical components, home appliances, various machine parts, industrial materials, photography and platemaking, and packaging. When used in these film applications, smooth and thin film surfaces are required, and particularly for photography and platemaking, transparency and lack of cloudiness are required. However, simply making biaxially oriented syndiotactic polystyrene film smooth and transparent results in poor handling during film production and processing.

[0003] A film with good slipperiness is known that contains inorganic particles and has a surface roughness Ra limited to a specific range (Patent Document 1). The film described in Patent Document 1 contributes to improving handleability by roughening the film surface through the addition of inorganic particles.

[0004] Japanese Patent Application Publication No. 2018-080261

[0005] However, the transparency of the film described in Patent Document 1 is significantly reduced due to the addition of a large amount of inorganic particles, and therefore the quality of the film described in Patent Document 1 is such that it cannot be used in applications requiring transparency (for example, photographic applications, platemaking applications, optical film applications, etc.).

[0006] Therefore, an object of the present invention is to provide a biaxially stretched syndiotactic polystyrene film that is excellent in transparency and ease of cutting.

[0007] As a result of intensive research to solve the above problems, the inventors have succeeded in obtaining a biaxially stretched syndiotactic polystyrene film with excellent transparency even in a composition containing lubricant particles by adopting a sequential biaxial method.

[0008] More specifically, by adopting a sequential biaxial stretching process in which a raw material containing syndiotactic polystyrene resin and, if necessary, an antioxidant and lubricant particles is melt-extruded, the material is formed into a sheet by a die and a cooling roll, which is then stretched in the longitudinal direction and then in the transverse direction, we have succeeded in producing a biaxially stretched syndiotactic polystyrene film with excellent transparency.

[0009] The present invention provides the following aspects. [Item 1] A biaxially stretched syndiotactic polystyrene film comprising a syndiotactic polystyrene resin, having a haze of 25.0% or less, and having a refractive index Nx in the MD direction of 1.580 or more. [Item 2] The biaxially stretched syndiotactic polystyrene film according to Item 1, in which the difference between the refractive index Nx and the refractive index Ny in the TD direction is 0.04 or more. [Item 3] The biaxially stretched syndiotactic polystyrene film according to Item 1 or 2, in which at least one surface of the biaxially stretched syndiotactic polystyrene film has a maximum protrusion height SRp of 400.0 nm or less, and an arithmetic mean roughness SRa of the surface of 40.0 nm or less. [Item 4] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 3, in which the maximum protrusion height SRp is 80.0 nm or more, and the arithmetic mean roughness SRa is 4.0 nm or more. [Item 5] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 4, having a thickness of 80 μm or less. [Item 6] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 5, further comprising at least one of an antioxidant and lubricant particles. [Item 7] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 6, comprising at least one of a hindered phenol-based antioxidant and a phosphorus-based antioxidant. [Item 8] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 7, having a ratio of the breaking strength in the TD direction to the breaking strength in the MD direction of 1.25 or more. [Item 9] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 8, having a ratio of the breaking strength in the TD direction to the TD50% tensile strength of 1.3 or more, wherein the TD50% tensile strength is a tensile strength at a tensile strain equivalent to half the tensile strain corresponding to the TD breaking strength. [Item 10] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 9, having a TD breaking strength of 100 MPa or more. [Item 11] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 10, having MD breaking strength of 90 MPa or more.[Item 12] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 11, having a refractive index Nz in the thickness direction of 1.628 or less. [Item 13] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 12, having a dielectric loss tangent at 10 GHz of 0.0003 or less.

[0010] The following aspects are preferred. [Item 14] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 13, wherein the haze is 20% or less, or 12% or less. [Item 15] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 14, wherein the haze is 0.1% or more, or 0.5% or more. [Item 16] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 15, wherein the refractive index Nx is 1.585 or more, or 1.589 or more. [Item 17] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 16, wherein the refractive index Nx is 1.650 or less, or 1.600 or less. [Item 18] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 17, wherein the refractive index Nx is 1.595 or less, or 1.591 or less. [Item 19] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 18, wherein the refractive index Nz in the thickness direction is 1.605 or more, or 1.620 or more. [Item 20] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 19, wherein the maximum projection height SRp is 300.0 nm or less, or 280.0 nm or less. [Item 21] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 20, wherein the maximum projection height SRp is 90.0 nm or more, or 100.0 nm or more. [Item 22] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 21, wherein the arithmetic mean roughness SRa is 30.0 nm or less, or 20.0 nm or less. [Item 23] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 22, wherein the arithmetic mean roughness SRa is 5.0 nm or more, or 6.0 nm or more. [Item 24] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 23, wherein the thickness is 70 μm or less, or 60 μm or less. [Item 25] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 24, wherein the thickness is 10 μm or more, or 20 μm or more.[Item 26] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 25, wherein the content of the syndiotactic polystyrene resin is 50% by mass or more, or 60% by mass or more, relative to 100% by mass of the biaxially stretched syndiotactic polystyrene film. [Item 27] ​​The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 26, wherein the content of the syndiotactic polystyrene resin is 75% by mass or more, or 90% by mass or more. [Item 28] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 27, wherein the content of the syndiotactic polystyrene resin is 95% by mass or more, or 98% by mass or more. [Item 29] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 28, further comprising lubricant particles. [Item 30] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 29, wherein the content of the lubricant particles is 0.005% by mass or more, or 0.01% by mass or more, based on 100% by mass of the biaxially stretched syndiotactic polystyrene film. [Item 31] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 30, wherein the average particle size of the lubricant particles is 2.0 μm or less, or 1.5 μm or less. [Item 32] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 31, wherein the content of the antioxidant is 0.01% by mass or more, or 0.05% by mass or more, based on 100% by mass of the biaxially stretched syndiotactic polystyrene film. [Item 33] The biaxially stretched syndiotactic polystyrene film according to any one of Items 1 to 32, wherein the content of the antioxidant is 0.10% by mass or more, or 0.12% by mass or more.

[0011] The present invention can provide a biaxially stretched syndiotactic polystyrene film that is excellent in transparency and ease of cutting. Therefore, the film of the present invention has excellent processability and low dielectric loss, making it suitable for use in circuit boards and the like.

[0012] In this specification, the refractive index in the MD direction may be referred to as Nx or the refractive index Nx, the refractive index in the TD direction may be referred to as Ny or the refractive index Ny, and the refractive index in the thickness direction may be referred to as Nz or the refractive index Nz.

[0013] Hereinafter, the MD direction, i.e., Machine Direction, may be referred to as the longitudinal direction. The MD direction may also be referred to as the vertical direction. The TD direction, i.e., Transverse Direction, may also be referred to as the width direction. The TD direction may also be referred to as the horizontal direction.

[0014] The biaxially stretched syndiotactic polystyrene film according to an embodiment of the present invention (hereinafter sometimes referred to as "the film of the present invention" or "the biaxially stretched syndiotactic polystyrene film of the present invention") contains a syndiotactic polystyrene resin, has a haze of 25.0% or less, and a refractive index Nx in the MD direction of 1.580 or more. Because the refractive index Nx is 1.580 or more, the film of the present invention can be torn by hand along the TD direction. In other words, it has excellent cuttability. This is thought to be because, since the refractive index Nx is 1.580 or more, the molecular chains of the syndiotactic polystyrene resin extend roughly in the TD direction. Furthermore, because the haze is 25.0% or less, it has excellent transparency.

[0015] Hereinafter, embodiments of the present invention will be described in detail. The biaxially stretched syndiotactic polystyrene film according to the embodiment of the present invention may have excellent film surface smoothness and excellent handleability. One example of the application of the biaxially stretched syndiotactic polystyrene film according to the embodiment of the present invention is a circuit board application. When the biaxially stretched syndiotactic polystyrene film according to the embodiment of the present invention is used in this application, the dielectric loss can be reduced.

[0016] The film of the present invention includes a first side and a second side opposite to the first side, that is, both sides of the film of the present invention are composed of the first side and the second side.

[0017] The film of the present invention contains a polystyrene resin having a syndiotactic structure, i.e., a syndiotactic polystyrene resin. The polystyrene polymer constituting the syndiotactic polystyrene resin preferably has a syndiotactic structure in which the tacticity of the side chain phenyl groups or substituted phenyl groups is 85% or more for diads (two structural units) and 50% or more for pentads (five structural units), as determined by nuclear magnetic resonance spectroscopy.

[0018] Examples of the polystyrene polymer include polystyrene, poly(alkylstyrenes) such as poly(p-, m-, or o-methylstyrene), poly(2,4-, 2,5-, 3,4-, or 3,5-dimethylstyrene), and poly(p-tertiarybutylstyrene), poly(halogenated styrenes) such as poly(p-, m-, or o-chlorostyrene), poly(p-, m-, or o-bromostyrene), poly(p-, m-, or o-fluorostyrene), and poly(o-methyl-p-fluorostyrene), and poly(p-, m-, or o-chloromethylstyrene). and the like), poly(alkoxystyrenes) such as poly(p-, m-, or o-methoxystyrene) and poly(p-, m-, or o-ethoxystyrene), poly(carboxyalkylstyrenes) such as poly(p-, m-, or o-carboxymethylstyrene), poly(alkyl ether styrenes) such as poly(p-vinylbenzyl propyl ether), poly(alkylsilylstyrenes) such as poly(p-trimethylsilylstyrene), and even poly(vinylbenzyl dimethoxyphosphide).

[0019] In the present invention, among the polystyrene-based polymers, polystyrene is particularly suitable. The polystyrene-based polymer does not necessarily have to be a single compound, and may be a mixture with a polystyrene-based polymer having an atactic structure or an isotactic structure, a copolymer, or a mixture thereof, as long as the syndiotacticity is within the above range.

[0020] The weight-average molecular weight of the syndiotactic polystyrene resin is preferably 10,000 or more, more preferably 50,000 or more. If the weight-average molecular weight is less than 10,000, it is impossible to obtain a biaxially stretched film with excellent strength and elongation properties and heat resistance. Although there is no particular upper limit for the weight-average molecular weight, it is preferably 1,500,000. If it exceeds 1,500,000, breakage may occur due to increased stretching tension.

[0021] The film of the present invention can have heat resistance, chemical resistance, low surface tension, etc. The film of the present invention may contain other resins. For example, the content of syndiotactic polystyrene resin is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 75% by mass or more, relative to 100% by mass of the film. The content of syndiotactic polystyrene resin may be, for example, 90% by mass or more, 95% by mass or more, or 98% by mass or more.

[0022] For example, the film of the present invention may contain a styrene-based thermoplastic elastomer (TPS). TPS is a thermoplastic elastomer (TPE) whose hard segment is made of polystyrene. By containing TPS, the film of the present invention can improve the roughness, flexibility, and matte finish of the film.

[0023] The TPS is preferably hydrogenated, which improves the heat resistance of the TPS and prevents side reactions from occurring during the high-temperature melt-extrusion process of the syndiotactic polystyrene resin.

[0024] Examples of hydrogenated TPS include polystyrene-poly(ethylene / butylene)-polystyrene (TPS-SEBS), polystyrene-poly(ethylene / propylene)-polystyrene (TPS-SEPS), and polystyrene-poly(ethylene-ethylene / propylene)-polystyrene (TPS-SEEPS).

[0025] The film of the present invention may contain two or more different TPSs, and in this case, the soft segments of all or part of the TPSs may be poly(ethylene / propylene) blocks or poly(ethylene-ethylene / propylene) random copolymer blocks.

[0026] Regarding the amount of TPS blended, the mass ratio of syndiotactic polystyrene resin (a) to TPS (b) is preferably (a) / (b) = 100 / 0 to 60 / 40, more preferably 100 / 0 to 80 / 20. The greater the amount of TPS blended, the rougher the film becomes and the more flexible the film becomes. On the other hand, if the amount of TPS blended is too high, the heat resistance, chemical resistance, and low surface tension, which are characteristics of syndiotactic polystyrene film, decrease.

[0027] The film of the present invention is a film consisting of at least one layer, and may be a film consisting of a single layer or a film consisting of multiple layers.

[0028] The film of the present invention may contain additives such as antioxidants, ultraviolet absorbers, light stabilizers, lubricant particles, antistatic agents, colorants, crystal nucleating agents, and flame retardants.

[0029] Syndiotactic polystyrene resins tend to crystallize when forming unstretched films, making it difficult to biaxially stretch unstretched films. Therefore, the thickness of the film after biaxial stretching is preferably 80 μm or less, more preferably 70 μm or less, and even more preferably 60 μm or less. That is, the thickness of the film of the present invention is preferably 80 μm or less, more preferably 70 μm or less, and even more preferably 60 μm or less. On the other hand, the thickness of the film of the present invention may be, for example, 10 μm or more, 20 μm or more, or 25 μm or more.

[0030] The film of the present invention preferably contains an antioxidant. Examples of antioxidants include amine-based antioxidants, sulfur-based antioxidants, phosphorus-based antioxidants, and phenol-based antioxidants. The antioxidant is preferably a phosphorus-based antioxidant or a hindered phenol-based antioxidant. It is more preferable to use a combination of a phosphorus-based antioxidant and a phenol-based antioxidant. Phenol-based antioxidants have low volatility and little antioxidant-induced coloration. Phosphorus-based antioxidants have a phosphite structure and therefore have high reducing properties. The content of the antioxidant is preferably 10% by mass or less, more preferably 7% by mass or less, even more preferably 3% by mass or less, and particularly preferably 1% by mass or less, relative to 100% by mass of the film. The content of the antioxidant is preferably 0.01% by mass or more, relative to 100% by mass of the film. The content of the antioxidant may be, for example, 0.05% by mass or more, 0.10% by mass or more, or 0.12% by mass or more, relative to 100% by mass of the film. These upper and lower limits can be appropriately combined, and the antioxidant content is, for example, 0.01% by mass to 10% by mass, or 0.01% by mass to 1% by mass. Furthermore, when a phosphorus-based antioxidant and a phenol-based antioxidant are used in combination, they are preferably mixed in a mass ratio of, for example, 1:10 to 10:1. A mass ratio of 1:5 to 5:1 is more preferred, a mass ratio of 1:3 to 3:1 is even more preferred, and a mass ratio of 1:1 is particularly preferred. By incorporating an antioxidant into the film of the present invention, crystallization during film formation of the unstretched film can be suppressed. Therefore, by incorporating an antioxidant, it becomes easier to biaxially stretch the unstretched film even if the film thickness is 10 μm or more. Furthermore, by incorporating an antioxidant, the film of the present invention can have extremely good transparency.

[0031] Examples of phosphorus-based antioxidants include tris(2,4-di-tert-butylphenyl)phosphite, bis(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite, tetrakis(2,4-di-tert-butylphenyl)-4,4′-biphenylene diphosphonite, distearylpentaerythritol diphosphite, bis(2,4-dicumylphenyl)pentaerythritol diphosphite, cyclic neopentanetetraylbis(2,6-di-tert-butyl-4-methylphenyl phosphite), and bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite.

[0032] Examples of hindered phenol antioxidants include octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, pentaerythritol-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], triethylene glycol-bis[3-(3-tert-butyl-5-methyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,3,5-trimethyl-2,4,6-tris(3,5-di

[0033] Examples of suitable hydroxybenzyl compounds include N,N-hexamethylenebis(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, N,N-hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamide), 3,5-di-tert-butyl-4-hydroxybenzylphosphonate-diethyl ester, tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, and 3,9-bis{1,1-dimethyl-2-[β-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]ethyl}-2,4,8,10-tetraoxaspiro(5,5)undecane.

[0033] When used as a release film, the film of the present invention preferably contains an antistatic agent or is coated on one side with an antistatic agent such as a surfactant. The amount of the antistatic agent is preferably 10% by mass or less relative to 100% by mass of the film. If the amount exceeds 10% by mass, the film is likely to break during stretching, resulting in poor production stability.

[0034] The refractive index Nx in the MD direction of the biaxially stretched syndiotactic polystyrene film of the present invention is 1.580 or more. More preferably, it is 1.585 or more, and particularly preferably, it is 1.589 or more. Furthermore, the refractive index Nx is preferably 1.650 or less. More preferably, it is 1.600 or less, even more preferably, it is 1.595 or less, and particularly preferably, it is 1.591 or less. These upper and lower limits can be appropriately combined; for example, the refractive index Nx is 1.580 or more and 1.650 or less.

[0035] The refractive index Ny in the TD direction of the biaxially stretched syndiotactic polystyrene film of the present invention is preferably 1.500 or more. More preferably, it is 1.510 or more, even more preferably, it is 1.520 or more, and particularly preferably, it is 1.530 or more. The refractive index Ny is preferably 1.554 or less, more preferably, it is 1.550 or less, even more preferably, it is 1.548 or less, and particularly preferably, it is 1.546 or less. Furthermore, these upper and lower limits can be appropriately combined; for example, the refractive index Ny is 1.500 or more and 1.554 or less.

[0036] The refractive index Nz in the thickness direction of the biaxially stretched syndiotactic polystyrene film of the present invention is preferably 1.605 or more. More preferably, it is 1.610 or more, even more preferably, it is 1.620 or more, and particularly preferably, it is 1.624 or more. The refractive index Nz is preferably 1.700 or less. More preferably, it is 1.680 or less, even more preferably, it is 1.660 or less, and particularly preferably, it is 1.630 or less. Furthermore, when a reduction in the dielectric loss tangent is desired, the refractive index Nz is preferably 1.628 or less. More preferably, it is 1.627 or less. These upper and lower limits can be appropriately combined; for example, the refractive index Nz is 1.605 or more and 1.700 or less. Furthermore, when a reduction in the dielectric loss tangent is desired, the refractive index Nz is, for example, 1.605 or more and 1.628 or less.

[0037] With respect to the biaxially stretched syndiotactic polystyrene film of the present invention, from the viewpoint of reducing dielectric loss, the dielectric dissipation factor at 10 GHz is preferably 0.0003 or less. More preferably, it is 0.00028 or less, even more preferably, it is 0.00026 or less, and particularly preferably, it is 0.00025 or less. Furthermore, from the viewpoint of production costs, the dielectric dissipation factor at 10 GHz is preferably 0.0001 or more. More preferably, it is 0.00012 or more, even more preferably, it is 0.00015 or more, and particularly preferably, it is 0.00017 or more. These upper and lower limits can be appropriately combined; for example, the dielectric dissipation factor at 10 GHz is 0.0001 or more and 0.0003 or less.

[0038] In the film of the present invention, the difference between the refractive index Nx and the refractive index Ny (i.e., Nx-Ny) is preferably 0.04 or more. More preferably, it is 0.05 or more. Furthermore, Nx-Ny is preferably 0.10 or less. More preferably, it is 0.08 or less, even more preferably, it is 0.07 or less, and particularly preferably, it is 0.06 or less. These upper and lower limits can be appropriately combined, and for example, Nx-Ny is 0.04 or more and 0.10 or less.

[0039] Here, since Nx is 1.580 or more, the film of the present invention has strong anisotropy and is excellent in ease of cutting in the direction of high anisotropy. Therefore, the present invention has excellent ease of cutting in the TD direction. Therefore, the film of the present invention is easy to process. In other words, the film of the present invention has excellent processability. If it were a biaxially stretched polyethylene terephthalate film, Ny could be used as an index of ease of cutting in the TD direction. In contrast, in a biaxially stretched syndiotactic polystyrene film, the orientation of the benzene rings in the side chains changes during TD stretching, and Nx tends to increase and Ny tends to decrease. Therefore, in the film of the present invention, Nx, rather than Ny, is more suitable as an index of ease of cutting in the TD direction.

[0040] The TDMD breaking strength ratio of the biaxially stretched syndiotactic polystyrene film of the present invention is preferably 1.25 or more from the viewpoint of improving anisotropy. Here, the TDMD breaking strength ratio refers to the ratio of the breaking strength in the TD direction to the breaking strength in the MD direction (i.e., TD breaking strength / MD breaking strength). The TDMD breaking strength ratio is more preferably 1.30 or more, and even more preferably 1.35 or more. From the viewpoint of production costs, the TDMD breaking strength ratio is preferably 1.60 or less, and more preferably 1.50 or less. These upper and lower limits can be appropriately combined, and for example, a ratio of 1.25 to 1.60 is preferred.

[0041] The TD breaking strength ratio of the biaxially stretched syndiotactic polystyrene film of the present invention is preferably 1.30 or more from the viewpoint of improving the anisotropy of the film. Here, the TD breaking strength ratio refers to the ratio of the breaking strength in the TD direction to the TD 50% tensile strength (i.e., TD breaking strength / TD 50% tensile strength). The TD 50% tensile strength is the tensile strength at a tensile strain equivalent to half the tensile strain corresponding to the breaking strength in the TD direction. In other words, the TD 50% tensile strength is the tensile strength at 50% of the TD breaking elongation. The TD breaking strength ratio is more preferably 1.40 or more, even more preferably 1.45 or more, and particularly preferably 1.50 or more. From the viewpoint of production costs, the TD breaking strength ratio is preferably 1.70 or less, more preferably 1.60 or less. These upper and lower limits can be appropriately combined; for example, a range of 1.3 to 1.7 is preferred.

[0042] The breaking strength in the MD direction of the biaxially stretched syndiotactic polystyrene film of the present invention (hereinafter sometimes referred to as "MD breaking strength") is preferably 90 MPa or more in order to maintain strength during film processing and use. It is more preferably 95 MPa or more, even more preferably 100 MPa or more, and particularly preferably 105 MPa or more. From the viewpoint of production costs, the MD breaking strength is preferably 160 MPa or less, more preferably 110 MPa or less. These upper and lower limits can be appropriately combined, and for example, it is preferably 90 MPa or more and 160 MPa or less.

[0043] The breaking strength in the TD direction of the biaxially stretched syndiotactic polystyrene film of the present invention (hereinafter sometimes referred to as "TD breaking strength") is preferably 100 MPa or more in order to maintain strength during film processing and use. It is more preferably 110 MPa or more, even more preferably 120 MPa or more, and particularly preferably 130 MPa or more. From the viewpoint of production costs, the TD breaking strength is preferably 200 MPa or less, more preferably 160 MPa or less. These upper and lower limits can be appropriately combined, and for example, a range of 100 MPa or more and 200 MPa or less is preferred.

[0044] The TD 50% tensile strength (i.e., tensile strength at 50% of the TD breaking elongation) of the biaxially stretched syndiotactic polystyrene film of the present invention is preferably 75 MPa or more in order to maintain strength during film processing and use. It is more preferably 80 MPa or more, even more preferably 85 MPa or more, and particularly preferably 90 MPa or more. From the viewpoint of production costs, the tensile strength at 50% of the TD breaking elongation is preferably 130 MPa or less, more preferably 120 MPa or less. These upper and lower limits can be appropriately combined, and for example, a range of 75 MPa or more and 130 MPa or less is preferred.

[0045] The haze of the biaxially stretched syndiotactic polystyrene film of the present invention must be 25% or less from the viewpoint of transparency. It is preferably 20% or less, and more preferably 12% or less. In particular, for photoengraving applications, it is undesirable for the haze to exceed 12%. Furthermore, from the viewpoint of production costs, the haze is preferably 0.1% or more. For example, the haze may be 0.5% or more. These upper and lower limits can be appropriately combined, and for example, a range of 0.1% to 25% is preferred.

[0046] Furthermore, the arithmetic mean roughness SRa of the first surface of the biaxially stretched syndiotactic polystyrene film of the present invention is preferably 40.0 nm or less, and the maximum projection height SRp is preferably 400.0 nm or less. Here, the arithmetic mean roughness SRa refers to the three-dimensional arithmetic surface roughness. The maximum projection height SRp refers to the three-dimensional maximum projection height. SRa is preferably 30.0 nm or less, and SRp is more preferably 350.0 nm or less. SRa is even more preferably 20.0 nm or less, and SRp is 300.0 nm or less. Furthermore, SRa is preferably 4.0 nm or more, and SRp is preferably 80.0 nm or more.

[0047] These upper and lower limits can be combined as appropriate. For example, SRa is 4.0 nm or more and 40.0 nm or less, and SRp is 80.0 nm or more and 400.0 nm or less. Preferably, SRa is 4.0 nm or more and 20.0 nm or less, and SRp is 80.0 nm or more and 300.0 nm or less. More preferably, SRa is 5.0 nm or more and 19.0 nm or less, and SRp is 90.0 nm or more and 290.0 nm or less. Even more preferably, SRa is 6.0 nm or more and 18.0 nm or less, and SRp is 100.0 nm or more and 280.0 nm or less.

[0048] When SRa is 4.0 nm or more and SRp is 80.0 nm or more, excessive adhesion between films can be reduced or prevented when the film is wound into a roll.

[0049] The smaller the SRa and SRp, the higher the transparency, i.e., the smaller the haze. Furthermore, when the SRa is 40.0 nm or less and the SRp is 400.0 nm or less, excessive air entrapment between the film and the winding roll can be prevented when the film is wound into a roll at high speed, thereby reducing excessive reduction in friction between the film and the winding roll due to air. Therefore, unevenness of the roll end surface, i.e., winding slippage, can be reduced or prevented. In addition, since excessive air entrapment can be prevented when the film is wound into a roll at high speed, the occurrence of wrinkles and valves can be reduced or prevented. Therefore, it is preferable that the SRa is 40.0 nm or less and the SRp is 400.0 nm or less. Furthermore, when the SRa is 40.0 nm or less and the SRp is 400.0 nm or less, for example, the electromagnetic conversion characteristics are improved in magnetic tape applications, the transparency is improved in photographic and platemaking applications, and the voltage resistance characteristics are improved in film capacitor applications.

[0050] The explanation of the arithmetic mean roughness SRa on the second surface is omitted here because it overlaps with the explanation of the arithmetic mean roughness SRa on the first surface. The explanation of the arithmetic mean roughness SRa on the first surface can also be used as an explanation of the arithmetic mean roughness SRa on the second surface.

[0051] The explanation of the maximum protrusion height SRp on the second surface is omitted here because it overlaps with the explanation of the maximum protrusion height SRp on the first surface. The explanation of the maximum protrusion height SRp on the first surface can also be used as an explanation of the maximum protrusion height SRp on the second surface.

[0052] The method for producing the film of the present invention will be described below.

[0053] The film of the present invention can be produced by mixing syndiotactic polystyrene resin with, if necessary, lubricant particles, an antioxidant, and other materials, melting and kneading the mixture, and extruding the mixture to produce an unstretched film, and then biaxially stretching the unstretched film obtained.

[0054] The unstretched film may be produced, for example, by melting and kneading a mixture of desired components in an extruder, extruding the kneaded mixture through a T-die, and then cooling it.

[0055] Biaxial stretching involves stretching the film in two axial directions, followed by optional heat setting. This biaxial stretching crystallizes the syndiotactic polystyrene resin, raising the film's glass transition temperature and improving its heat resistance and mechanical strength. Biaxial stretching can also reduce the dielectric loss tangent.

[0056] The stretching method for the biaxially stretched syndiotactic polystyrene film of the present invention includes, for example, a sequential biaxial stretching method in which longitudinal stretching and transverse stretching are performed in sequence, and a simultaneous biaxial stretching method in which longitudinal stretching and transverse stretching are performed simultaneously. The simultaneous biaxial stretching method is suitable for producing a film with a well-balanced molecular orientation in the in-plane direction, but the stretching behavior is complex, and stretching unevenness due to stress relaxation makes it difficult to further improve transparency and film surface smoothness. Furthermore, the simultaneous biaxial stretching method limits the range of stretching condition changes, making it difficult to flexibly adjust film production conditions. Therefore, from the perspective of productivity, the sequential biaxial stretching method is preferred, as it allows for a variety of condition settings.

[0057] In the sequential biaxial stretching method, longitudinal stretching is preferably 3.6 times or less, and transverse stretching is preferably 3.5 times or more. The temperature during longitudinal stretching is preferably 100°C or more and 130°C or less, more preferably 105°C or more and 125°C or less. The temperature during transverse stretching is preferably 100°C or more and 130°C or less, more preferably 105°C or more and 125°C or less. After sequential biaxial stretching, the film is preferably heat-set at 220°C or more and 260°C or less. After heat-setting, it is preferable to perform a relaxation treatment of 1.0% or more in the longitudinal or transverse direction.

[0058] The biaxially stretched syndiotactic polystyrene film of the present invention is sometimes required to have excellent heat resistance at high temperatures. The heat resistance at high temperatures is greatly affected by the stretching conditions as well as the selection of the stretching method. To achieve excellent heat resistance, particularly a heat shrinkage rate of 5% or less, or even 2% or less at 150°C, it is preferable to perform a heat setting treatment, a transverse relaxation treatment, or the like.

[0059] The haze, arithmetic mean roughness SRa, maximum protrusion height SRp, and film handleability can be adjusted by film-forming conditions and lubricant particles. The type, presence, and content of lubricant particles can be appropriately set.

[0060] The lubricant particles may be particles made of, for example, metal oxides such as silica, titanium dioxide, talc, or kaolinite, metal salts such as calcium carbonate, calcium phosphate, or barium sulfate, or organic polymers. The lubricant particles are preferably inactive with the syndiotactic polystyrene resin. These lubricant particles may be used alone or in combination of two or more. The average particle size of the lubricant particles is preferably, for example, 0.01 μm to 2.0 μm, particularly 0.05 μm to 1.5 μm. These upper and lower limits can be appropriately combined. The particle size variation of the lubricant particles (the ratio of the standard deviation to the average particle size) is preferably 25% or less.

[0061] From the viewpoint of the transparency of the film of the present invention, the content of the lubricant particles is preferably 2.0% by mass or less, more preferably 1.0% by mass or less, based on 100% by mass of the film, while from the viewpoint of the handleability of the film, the content of the lubricant particles is preferably 0.005% by mass or more, more preferably 0.01% by mass or more, based on 100% by mass of the film.

[0062] The film of the present invention may have some other layer, such as an antistatic layer. The thickness of the antistatic layer may be, for example, 0.1 μm or less. The antistatic layer may be formed, for example, by applying a conductive resin to another film and then drying or crosslinking the resin.

[0063] The film of the present invention is suitably used, for example, as a material for electric and electronic devices (particularly, circuit boards, resin plates for millimeter-wave radomes, resin plates with good radio wave transmission, for magnetic tapes, and for capacitors), in-vehicle and electrical components, home appliances, various machine parts, industrial materials, photographic and plate-making materials, and packaging. In particular, it is suitably used as an optical film to be mounted on a display.

[0064] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The film evaluation methods are as follows.

[0065] (1) Breaking strength Breaking strength was evaluated by a tensile test in accordance with JIS K 7127:1999. A rectangular test piece measuring 180 mm x 10 mm, with the long side in the measurement direction, was cut out from the film. 10 mm long benchmark lines parallel to the short sides were drawn 40 mm inward from both short sides of the test piece, and the thickness (mm) of the test piece was measured at five points in a 100 mm long section between the benchmark lines, and the average value was calculated. The product of these values ​​and the width (10 mm) of the test piece was calculated as the cross-sectional area (mm) of the test piece. 2 ) The film thickness was measured using an electric micrometer (Militron 1245D, manufactured by Fineruf Co., Ltd.). The tensile test was performed by gripping the area from the benchmark line to the short side with a chuck so that the long side of the test piece was the tensile direction. A precision universal testing machine (Autograph AGX-V, manufactured by Shimadzu Corporation) was used for the tensile test, with a chuck distance of 100 mm and a tensile speed of 100 mm / min. The load (N) at break was calculated based on the cross-sectional area (mm 2 ) to obtain the breaking strength (MPa). By this procedure, the breaking strength in the MD direction (MPa) and the breaking strength in the TD direction (MPa) were obtained. The breaking strength in the TD direction was divided by the breaking strength in the MD direction to obtain the TDMD breaking strength ratio. In other words, the ratio of the breaking strength in the TD direction to the breaking strength in the MD direction was calculated. Furthermore, the tensile strength at 50% of the elongation at the breaking strength in the TD direction (i.e., TD50% tensile strength) was read. In other words, for the tensile test in the TD direction, the tensile strength at the tensile strain equivalent to half of the tensile strain corresponding to the breaking strength was read as the TD50% tensile strength. The TD breaking strength ratio was obtained by dividing the breaking strength in the TD direction (MPa) by the TD50% tensile strength (MPa).

[0066] (2) Haze: The haze of the film in each example and comparative example was measured using a haze meter (NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.). The transparency of the film was then evaluated based on the measured haze value as follows: Good: 12.0% or less; Fair: More than 12.0% and 25.0% or less; Bad: More than 25.0%.

[0067] (3) Arithmetic Mean Roughness (SRa) and Maximum Protrusion Height (SRp) The film surface was measured using a stylus-type three-dimensional roughness meter (ET4000A, manufactured by Kosaka Laboratory Co., Ltd.). Measurements were taken over a 1 mm measurement length in the longitudinal direction of the film with a stylus radius of 2 μm and a load of 30 mg, with a cutoff value of 0.25 mm and a stylus feed rate of 0.1 mm / s. The measurement was divided into 500 points at 2 μm intervals, and the height of each point was recorded on a three-dimensional roughness analyzer (TDA-31). The same procedure was repeated 150 times across the width of the film at 2 μm intervals, i.e., across a 0.3 mm width of the film, and the data was then recorded on the analyzer. The arithmetic mean roughness (SRa) and maximum protrusion height (SRp) were then calculated using the analyzer.

[0068] (4) Film handling ability When a wide slit roll is slit at high speed and rewound into a narrow roll, whether or not a problem-free roll can be obtained without miswinding, wrinkles, valves, etc. at the end of the roll was evaluated in four stages, and the evaluation was made using the following ranking system: Grade 1: It is extremely difficult to obtain a problem-free slit roll Grade 2: A problem-free slit roll can be obtained at low speed Grade 3: A problem-free slit roll can be obtained at medium speed Grade 4: A problem-free slit roll can be obtained at high speed

[0069] (5) Average particle size: The lubricant particles were observed with a Hitachi S-510 scanning electron microscope, and a photograph was taken, which was then enlarged and copied, and the outlines of the lubricant particles were traced, and 200 particles were arbitrarily selected and painted black. The Feret's diameter in the horizontal direction of each particle was measured using a Luzex 500 image analyzer manufactured by Nicolet Co., Ltd., and the average value was taken as the average particle size.

[0070] (6) Ease of Cutting (Tearing) Sensory evaluation was carried out to evaluate ease of cutting. When the film was cut by hand in the TD direction, those that could be easily torn by hand were rated as ◯, those that could not be easily torn by hand but could still be torn by hand were rated as △, and those that could not be torn by hand were rated as ×.

[0071] (7) Molecular Orientation and Refractive Index The in-plane slow axis direction of the film, i.e., the orientation axis direction, was determined using a molecular orientation meter (MOA-6004 molecular orientation meter, manufactured by Oji Scientific Instruments Co., Ltd.). The refractive indexes Nx in the MD direction, Ny in the TD direction, and Nz in the thickness direction were determined using an Abbe refractometer (NAR-4T, manufactured by Atago Co., Ltd., measurement wavelength 589 nm).

[0072] (8) Dielectric Loss Tangent (Sample Preparation) The film was cut into 8 cm x 3 mm strips to obtain measurement samples. (Sample Measurement) The dielectric loss tangent of the measurement samples was measured using a network analyzer (manufactured by Anritsu Corporation) by the cavity resonator perturbation method at a temperature of 23°C and a frequency of 10 GHz. The dielectric loss tangent values ​​were ranked as follows: ○: 0.00025 or less △: More than 0.00025 and 0.00030 or less ×: More than 0.00030

[0073] (Masterbatch A with antioxidant) Masterbatch A with antioxidant was prepared by adding 10.0 mass% of antioxidant to 90.0 mass% of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.). This antioxidant consists of a phosphorus-based antioxidant and a hindered phenol-based antioxidant. The phosphorus-based antioxidant and the hindered phenol-based antioxidant were mixed in equal amounts by mass.

[0074] (Masterbatch B containing lubricant particles) A ​​masterbatch B containing lubricant particles was prepared by adding 2.0 mass% of lubricant particles to 98.0 mass% of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.). The lubricant particles were calcium carbonate particles whose surfaces were coated with silica. The lubricant particles had an average particle size of 0.15 μm.

[0075] (Masterbatch C containing lubricant particles) A ​​masterbatch C containing lubricant particles was prepared by adding 2.0 mass% of lubricant particles to 98.0 mass% of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.). The lubricant particles were calcium carbonate particles whose surfaces were coated with silica. The lubricant particles had an average particle diameter of 0.30 μm.

[0076] Example 1: 2.0% by mass of antioxidant-containing masterbatch A and 98.0% by mass of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.) were mixed. The mixture was then dried, melted at 300°C in an extruder, extruded through a T-die, and adhered to a cooling roll at 30°C by electrostatic loading and cooled to solidify, yielding an unstretched sheet of 380 μm. The unstretched sheet was first heated by an infrared heater using a roll, and then stretched 3.2 times in the longitudinal direction at a roll temperature of 100°C using the speed difference between the rolls. The sheet was then introduced into a tenter and stretched 3.8 times in the transverse direction at 120°C. The sheet was then heat-treated at 240°C in a heat-setting zone. The sheet was then subjected to a 5.0% relaxation treatment in the transverse direction to obtain a 30 μm-thick sequentially biaxially stretched syndiotactic polystyrene film.

[0077] Example 2: 5.0% by mass of masterbatch B containing lubricant particles, 2.0% by mass of masterbatch A containing antioxidant, and 93.0% by mass of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.) were mixed. The mixture was then dried, melted at 300°C in an extruder, extruded through a T-die, and adhered to a cooling roll at 30°C by electrostatic loading and cooled to solidify, yielding a 450 μm unstretched sheet. The unstretched sheet was first heated by an infrared heater using a roll, and then stretched 3.2 times in the longitudinal direction at a roll temperature of 100°C using a speed differential between the rolls. The sheet was then introduced into a tenter and stretched 4.0 times in the transverse direction at 120°C. The sheet was then heat-treated at 240°C in a heat-setting zone. It was then relaxed 5.0% in the transverse direction to yield a 34 μm thick sequentially biaxially stretched syndiotactic polystyrene film.

[0078] Example 3: 5.0% by mass of masterbatch C containing lubricant particles, 2.0% by mass of masterbatch A containing antioxidant, and 93.0% by mass of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.) were mixed. The mixture was then dried, melted at 300°C in an extruder, extruded through a T-die, and adhered to a cooling roll at 30°C by electrostatic loading and cooled to solidify, yielding a 450 μm unstretched sheet. The unstretched sheet was first heated by an infrared heater using a roll, and then stretched 3.2 times in the longitudinal direction at a roll temperature of 100°C using a speed differential between the rolls. The sheet was then introduced into a tenter and stretched 4.0 times in the transverse direction at 120°C. The sheet was then heat-treated at 240°C in a heat-setting zone. It was then relaxed 5.0% in the transverse direction to yield a 34 μm thick sequentially biaxially stretched syndiotactic polystyrene film.

[0079] Examples 4, 5, and 6 Sequentially biaxially stretched syndiotactic polystyrene films were obtained in the same manner as in Example 3, except that the film thickness was changed as shown in Table 1 by changing the discharge amount during extrusion.

[0080] Example 7 A sequentially biaxially stretched syndiotactic polystyrene film was obtained in the same manner as in Example 2, except that the film thickness was changed as shown in Table 1 by changing the discharge amount during extrusion.

[0081] (Example 8) A sequentially biaxially stretched syndiotactic polystyrene film was obtained in the same manner as in Example 2, except that the film thickness was changed as shown in Table 1 by changing the discharge amount during extrusion and the TD stretching ratio was changed to 4.2 times.

[0082] (Example 9) A sequentially biaxially stretched syndiotactic polystyrene film was obtained in the same manner as in Example 1, except that 5.0% by mass of the masterbatch B containing lubricant particles, 2.0% by mass of the masterbatch A containing an antioxidant, and 93.0% by mass of a syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.) were mixed.

[0083] Comparative Example 1: 2.0% by mass of antioxidant-containing masterbatch A and 98.0% by mass of syndiotactic polystyrene resin (Xarek 90ZC, manufactured by Idemitsu Kosan Co., Ltd.) were mixed. The mixture was then dried, melted at 290°C in an extruder, extruded through a T-die, and adhered to a cooling roll at 30°C by electrostatic loading and cooled to solidify, yielding a 630 μm unstretched sheet. This unstretched sheet was simultaneously biaxially stretched at 110°C and a stretching rate of approximately 500% / min to a longitudinal stretch ratio of 3.5 times and a stretching ratio of 3.5 times. After stretching, the sheet was heat-set at 210°C with a relaxation ratio of 0.95 times in the machine direction (MD) and 0.95 times in the transverse direction (TD) to obtain a simultaneously biaxially stretched syndiotactic polystyrene film with a thickness of approximately 50 μm.

[0084] Comparative Example 2 A simultaneously biaxially stretched syndiotactic polystyrene film was obtained in the same manner as in Comparative Example 1, except that the film thickness was changed as shown in Table 1 by changing the discharge amount during extrusion.

[0085] Comparative Example 3 A simultaneously biaxially stretched syndiotactic polystyrene film was obtained in the same manner as in Comparative Example 1, except that the TD stretching ratio was changed to 3.7 times.

[0086] (Comparative Examples 4 and 5) A transversely uniaxially stretched syndiotactic polystyrene film was obtained in the same manner as in Comparative Example 1, except that the MD stretch ratio was changed to 1.0 times, the TD stretch ratio was changed to 3.8 times, and the extrusion discharge amount was changed to change the film thickness as shown in Table 1.

[0087] In this table, SPS resin means syndiotactic polystyrene resin. For Comparative Examples 4 and 5, a "-" in the box for easy-to-cut property means that it was not measured. A "-" in the box for tensile properties also means that it was not measured. Nave is the arithmetic mean of Nx, Ny, and Nz. ΔNx is a value calculated using the following formula: ΔNx=Nx-Ny / 2-Nz / 2 ΔNy is a value calculated using the following formula: ΔNy=Ny-Nx / 2-Nz / 2 ΔP is a value calculated using the following formula: ΔP=Nx / 2+ Ny / 2- Nz

[0088] In the films according to Examples 1 to 9, the slow axis direction coincided with the MD direction. It is believed that stretching in the TD direction caused the main chains to align in the TD direction, and the planes of the benzene rings to face the TD direction, thereby increasing the refractive index in the MD direction. As such, it is believed that in the films according to Examples 1 to 9, the molecular chains of the syndiotactic polystyrene resin extend roughly in the TD direction.

[0089] The films of Examples 1 to 9, which had a high refractive index Nx, were excellent in ease of cutting. On the other hand, the films of Comparative Examples 1 to 3, which had a lower refractive index Nx than the films of Examples 1 to 9, were inferior in ease of cutting.

[0090] The films according to Comparative Examples 1 to 3, which were produced by simultaneous biaxial stretching, had excessively large maximum protrusion heights SRp and excessively large arithmetic mean roughness SRa. The transparency of the films according to Comparative Examples 1 to 3 was inferior to that of the films according to Examples 1 to 9, which were produced by sequential biaxial stretching. The films according to Comparative Examples 1 to 3 were more susceptible to wrinkles and valves than the films according to Examples 1 to 9. The films according to Comparative Examples 1 to 3 were also more susceptible to winding slippage than the films according to Examples 1 to 9. This is thought to be because the films according to Comparative Examples 1 to 3 were prone to excessive air intrusion when the films were wound into a roll, which tended to excessively reduce friction between the films.

[0091] The films of Comparative Examples 4 and 5, which were produced by transverse uniaxial stretching, had higher refractive indexes Nz and larger dielectric loss tangents than the films of Examples 1 to 9, which were produced by sequential biaxial stretching.

[0092] INDUSTRIAL APPLICABILITY The present invention can provide a biaxially stretched syndiotactic polystyrene film and is therefore industrially applicable.

Claims

1. A biaxially oriented syndiotactic polystyrene film containing a syndiotactic polystyrene resin, having a haze of 25.0% or less, and a refractive index Nx in the MD direction of 1.580 or more.

2. The biaxially stretched syndiotactic polystyrene film according to claim 1, wherein the difference between the refractive index Nx and the refractive index Ny in the TD direction is 0.04 or more.

3. The biaxially stretched syndiotactic polystyrene film according to claim 1, wherein at least one surface of the biaxially stretched syndiotactic polystyrene film has a maximum peak height SRp of 400.0 nm or less, and an arithmetic mean roughness SRa of the surface of 40.0 nm or less.

4. The biaxially stretched syndiotactic polystyrene film according to claim 3, wherein the maximum projection height SRp is 80.0 nm or more, and the arithmetic mean roughness SRa is 4.0 nm or more.

5. The biaxially oriented syndiotactic polystyrene film according to claim 1, having a thickness of 80 μm or less.

6. The biaxially oriented syndiotactic polystyrene film of claim 1, further comprising at least one of an antioxidant and lubricant particles.

7. The biaxially stretched syndiotactic polystyrene film according to claim 1, which contains at least one of a hindered phenol-based antioxidant and a phosphorus-based antioxidant.

8. The biaxially oriented syndiotactic polystyrene film according to claim 1, wherein the ratio of the breaking strength in the TD direction to the breaking strength in the MD direction is 1.25 or more.

9. The biaxially oriented syndiotactic polystyrene film according to claim 1, wherein the ratio of the breaking strength in the TD direction to the TD 50% tensile strength is 1.3 or more, and the TD 50% tensile strength is the tensile strength at a tensile strain equivalent to half the tensile strain corresponding to the breaking strength in the TD direction.

10. The biaxially stretched syndiotactic polystyrene film according to claim 1, having a breaking strength in the TD direction of 100 MPa or more.

11. The biaxially stretched syndiotactic polystyrene film according to claim 1, wherein the breaking strength in the MD direction is 90 MPa or more.

12. The biaxially stretched syndiotactic polystyrene film according to claim 1, having a refractive index Nz in the thickness direction of 1.628 or less.

13. The biaxially stretched syndiotactic polystyrene film according to claim 1, having a dielectric loss tangent at 10 GHz of 0.0003 or less.

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