JP2024014238A - Film using polyester resin composition and polystyrene resin composition - Google Patents

Abstract

To provide a film which is low in birefringence and good in transparency and toughness and contains a polyester resin composition and a polystyrene resin composition.SOLUTION: The film contains a polyester resin composition A and a polystyrene resin composition B represented by the following formula. (R1 represents H or a CH3 group; and R2 represents a substituent containing a hydroxy group, an ester group or an ether group.)SELECTED DRAWING: None

Description

The present invention relates to a film using a polyester resin composition and a polystyrene resin composition.

Polyester resins, represented by polyethylene terephthalate, have excellent mechanical properties, thermal properties, chemical resistance, electrical properties, and moldability, and are used for a variety of purposes. Among these polyester resins, especially in recent years, demand has increased for various optical films such as polarizing plate protective films (polarizer protection members), circularly polarizing plate retardation films (circularly polarizing plate members), and transparent conductive films in the fields of flat panel displays and touch panels. is increasing. Among these, in applications for polarizing plate protective films, replacement of conventional TAC (triacetyl cellulose) films with biaxially oriented polyester films is being actively studied for the purpose of cost reduction. However, birefringence occurs in biaxially oriented polyester films due to the orientation of the polyester during stretching, and interference colors that occur when assembled into a liquid crystal display cannot be sufficiently controlled, resulting in poor screen display quality. descend. Therefore, it is preferable to reduce the birefringence of the biaxially oriented polyester film and the polyester resin used therein from the viewpoint of quality when displayed on a screen. One way to reduce birefringence is to reduce crystallinity and orientation by not stretching or by slightly stretching, but polyester whose crystallinity has been lowered by such a method will not undergo thermal crystallization during the heating process. Transparency is insufficient due to whitening, and if the orientation is reduced, film deformation may occur during the heating process, making it difficult to apply in applications that require low birefringence, such as polarizer protective films. Ta.

On the other hand, by combining resins with birefringence with opposite signs, that is, by combining resins with negative birefringence with polyester resins with positive birefringence, the molecular Even if the orientation remains frozen, birefringence can be reduced in principle, and the following proposals have been made.

Patent Document 1 discloses that a transparent molded product with low birefringence can be obtained by block copolymerizing an aromatic polyester and a styrene polymer.

Patent Document 2 discloses that a transparent film with low birefringence can be obtained by alternately laminating polyethylene terephthalate resin and syndiotactic polystyrene resin.

Japanese Unexamined Patent Publication No. 178119/1983 Japanese Patent Application Publication No. 2008-137304

In Patent Document 1, since the block copolymer forms a phase-separated structure, the transparency is insufficient and it is difficult to apply it to optical applications.

In Patent Document 2, since the compatibility between the polyethylene terephthalate resin and the syndiotactic polystyrene resin is insufficient, peeling occurs at the interface between the polyethylene terephthalate resin and the syndiotactic polystyrene resin due to stress during stretching of the film. Due to the occurrence of delamination and the formation of voids, transparency was insufficient, making it difficult to apply for optical purposes.

As described above, with the conventional techniques, it has been difficult to obtain an optical resin material made of polyester resin and polystyrene resin that has transparency, low birefringence, and mechanical properties.

An object of the present invention is to overcome the problems of the prior art and provide a film that is made of a resin composition including a polyester resin composition and a polystyrene resin composition and has a low birefringence, transparency, and toughness. That's true.

As a result of studies to solve the above problems, a film containing a polyester resin composition and a polystyrene resin composition with a low birefringence and good transparency and toughness was discovered according to the present invention.

That is, the object of the present invention is achieved by the following means.
(1) A film containing a polyester resin composition A and a polystyrene resin composition B represented by (Chemical formula 1).

(R ₁ is H or CH ₃ group, R ₂ is a substituent containing a hydroxy group, ester group, or ether group)
(2) The film according to (1), wherein the polystyrene resin composition B shown by (Chemical formula 1) is the polystyrene resin composition B shown by (Chemical formula 2).

(R ₃ is H or CH ₃ group, R ₄ is a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or an ether group including polyether)
(3) The film according to (1), which has a weight loss rate (under N ₂ flow (200 mL/min), 280° C., 60 minutes) measured after vacuum drying at 80° C. for 15 hours is 5 wt% or less.
(4) The film according to (1), which satisfies the following formula (I).

0.8<(Ma/2+Mb)/P<2 (I)
(Ma: total content of alkali metal elements (mol/t), Mb: total content of magnesium elements, manganese elements, and calcium elements (mol/t), P: phosphorus element content (mol/t))
(5) The film according to (1), wherein the copolymerization rate n of the polystyrene resin composition B is 0.1≦n≦0.9.
(6) The film according to (1), wherein the polystyrene resin composition B has a number average molecular weight (Mn) of 1.0×10 ⁴ ≦Mn≦1.0×10 ⁶ .
(7) In the differential scanning calorimetry measurement, the glass transition temperature was 30 when the sample was melted at 300°C for 5 minutes, cooled to room temperature at 40°C/min, and then heated at 20°C/min (2nd Run). The film according to (1), wherein the difference between the maximum value and the minimum value is within 20°C when one or more than one temperature is observed at a temperature of 20°C or higher.
(8) The film according to (1), which has an internal haze of 30% or less. Note that the internal haze is a value obtained by converting a value measured with a haze meter using water as a solvent to 100 μm.
(9) The film according to (1), which is a biaxially stretched film.
(10) In differential scanning calorimeter measurement, a glass transition temperature of 30°C or higher is not observed when the sample is heated at 20°C/min (1st Run), or when the sample is heated at 20°C/min (1st Run). ), a glass transition temperature Tg ^A of 30°C or higher was observed, and after melting at 300°C for 5 minutes, the glass transition temperature was lowered to room temperature at a rate of 40°C/min, and then heated at a rate of 20°C/min (2nd Run). The biaxially stretched film according to (9), wherein the difference between Tg ^A and Tg ^B is 10° C. or more when the temperature is Tg ^B.
(11) The biaxially stretched film according to (9), wherein the absolute value of the birefringence index in the thickness direction is 0.1 or less.

According to the present invention, a film containing a polyester resin composition and a polystyrene resin composition with a low birefringence and good transparency and toughness can be provided.

The present invention will be explained in detail below.

The film of the present invention needs to contain polyester resin composition A and polystyrene resin composition B. In the film of the present invention, by making polyester resin composition A having a positive birefringence and polystyrene resin composition B having a negative birefringence compatibilizing, the birefringence is reduced and transparency is improved. It becomes possible to do so.

The polyester resin composition in the present invention refers to a polyester resin composition obtained by polycondensing a dicarboxylic acid component and a glycol component as main raw materials. The main raw material indicates that the total amount of structural units obtained from the dicarboxylic acid component and the glycol component in the polyester resin is 70 mol% or more. More preferably, it is 80 mol% or more, and still more preferably 90 mol% or more.

Examples of the dicarboxylic acid component in the present invention include aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, aromatic dicarboxylic acids, or ester derivatives thereof. Among these, from the viewpoint of heat resistance and processability of the polyester resin composition, an aromatic dicarboxylic acid component is preferable, and it is preferable that 90 mol% or more of the dicarboxylic acid component is an aromatic dicarboxylic acid component. Among them, terephthalic acid, isophthalic acid, 2,6-naphthalene dicarboxylic acid, 5-sodium sulfoisophthalic acid, and ester derivatives thereof are preferable from the viewpoint of both transparency and processability.Terephthalic acid is preferable from the viewpoint of heat resistance. Particularly preferred are acids, 2,6-naphthalene dicarboxylic acid, and ester derivatives thereof.

Examples of the glycol component include aliphatic diols, alicyclic diols, and aromatic cyclic diols. In addition to diols, polyfunctional alcohols such as trimethylolpropane and pentaerythritol can also be used within a range that does not impair processability. In particular, it is preferred that 90 mol% or more of the glycol component consists of aliphatic diols and/or alicyclic diols. Among these, ethylene glycol and 1,4-cyclohexanedimethanol are particularly preferred from the viewpoint of mechanical properties such as elongation and flexibility when the resin composition is made into a film.

Note that other dicarboxylic acid components, hydroxycarboxylic acid derivatives, and glycols may be copolymerized to the extent that the scope of the effects of the present invention is not impaired.

The intrinsic viscosity (measured in o-chlorophenol at 25°C) of the polyester resin composition A of the present invention is preferably 0.4 to 1.2 dL/g, more preferably 0.4 to 1.2 dL/g, from the viewpoint of mechanical properties. .5 to 0.8 dL/g.

The polyester resin composition A of the present invention may contain a manganese element, a magnesium element, and a calcium element, and the total content M _p thereof is preferably 0.5 to 10 (mol/t) per ton of film. preferable. More preferably 0.7 to 3.0 (mol/t). By satisfying the above range, thermal decomposition during melt mixing and melt molding can be suppressed. Note that elements are used synonymously with atoms.

Moreover, the polyester resin composition A of the present invention needs to contain phosphorus element. The content P _p is preferably 0.5 to 5 (mol/t) per ton of film. More preferably, it is 0.5 to 3.0 (mol/t). By satisfying the above range, thermal decomposition during melt mixing and melt molding can be suppressed without impairing the polymerization reactivity of the polyester.

The polyester resin composition A of the present invention preferably has a ratio of M _p to P _p , M _p /P _p, of 0.5 to 10.0. More preferably 0.5 to 5.0, still more preferably 0.8 to 2.0. By satisfying the above range, thermal decomposition during melt mixing and melt molding can be suppressed.

Polystyrene resin composition B in the present invention is a polystyrene copolymer and has the structure (Chemical formula 1 = Chemical formula 3 below). Further, a random copolymer or a block copolymer may be used.

(R ₁ is H or CH ₃ group, R ₂ is a substituent containing a hydroxy group, ester group, or ether group)
Examples of the substituent containing a hydroxy group include a hydroxymethyl group, a hydroxyethyl group, a hydroxybutyl group, a hydroxyoctyl group, a hydroxymethyl ester group, a hydroxyethyl ester group, a hydroxybutyl ester group, and a hydroxyoctyl ester group. From the viewpoint of compatibility with polyester resin composition A, hydroxymethyl group and hydroxyethyl group are preferable.

Examples of the substituent containing an ester group include a methyl ester group, an ethyl ester group, a butyl ester group, an octyl ester group, and the like. From the viewpoint of compatibility with polyester resin composition A, methyl ester groups and ethyl ester groups are preferred.

Examples of the substituent containing an ether group include a methyl ether group, an ethyl ether group, a butyl ether group, an octyl ether group, and a polyether group. From the viewpoint of compatibility with polyester resin composition A, methyl ether group, ethyl ether group, and polyether group are preferable.

Moreover, it is preferable that the polystyrene resin composition B in the present invention has a structure of (Chemical formula 2=Chemical formula 4 below).

(R ₃ is H or CH ₃ group, R ₄ is a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or an ether group including polyether)
By having the structure of (Chemical formula 2), the compatibility with the polyester resin composition A becomes good, and the transparency of the film becomes good.

Examples of the linear or branched hydrocarbon group having 1 to 8 carbon atoms include methylene group, ethylene group, propylene group, butylene group, isopropylene group, isobutylene group, hexylene group, and octylene group. From the viewpoint of compatibility with polyester resin composition A, methylene groups and ethylene groups are preferred.

Examples of the ether group containing polyether include a polyethylene glycol group, a polypropylene glycol group, a polytetramethylene glycol group, and the like. From the viewpoint of compatibility with polyester resin composition A, polyethylene glycol groups are preferred.

It is necessary that m, which is the copolymerization amount of polystyrene resin composition B in the present invention, satisfies 0<m<1. From the viewpoint of compatibility with polyester resin composition A, the lower limit is preferably 0.1 or more, more preferably 0.2 or more. Further, the upper limit is preferably 0.9 or less, more preferably 0.85 or less.

It is necessary that n, which is the copolymerization rate of polystyrene resin composition B in the present invention, satisfies 0<n<1. From the viewpoint of compatibility with polyester resin composition A, the lower limit is preferably 0.1 or more, more preferably 0.15 or more. Further, the upper limit is preferably 0.9 or less, more preferably 0.8 or less.

The copolymerization amounts m and n of the polystyrene resin composition B in the present invention need to satisfy m+n≦1. As long as the above range is satisfied, other copolymer components may be included to the extent that the range of effects of the present invention is not impaired.

The number average molecular weight (Mn) of the polystyrene resin composition B in the present invention is preferably 1.0×10 ⁴ or more and 1.0×10 ⁶ or less. By using a polystyrene resin having Mn within the above range, it is possible to achieve both mechanical properties and workability, and it is suitable for molding. Further, the ratio Mw/Mn of weight average molecular weight (Mw) to number average molecular weight (Mn) is not particularly limited, but is approximately 1.0 or more and 5.0 or less. From the viewpoint of stretchability of the film, it is preferable that Mw/Mn is 2.0 or more because mechanical properties are good.

The film of the present invention has a weight ratio (W _A /W _B ) of 10/90, where W _A is the content of polyester resin composition A and W _B is the content of polystyrene resin composition B in the film. The ratio is preferably 90/10, more preferably 30/70 to 70/30. When W _A /W _B is within the above range, the birefringence can be more effectively reduced and mechanical properties are improved. When W _A /W _B increases, that is, when the ratio of polyester resin increases, the mechanical properties become better. On the other hand, when W _A /W _B decreases, that is, when the ratio of polystyrene resin increases, the effect of reducing birefringence increases. Become.

The film of the present invention may contain an antioxidant. Various antioxidants can be used as the antioxidant, and may be used alone or in combination.

Specifically, phosphite-based antioxidants and hindered phenol-based antioxidants are preferred. As the phosphite antioxidant, 3,9-Bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro [5.5] undecane, 3,9-Bis(Octadecyloxy)-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5] undecane, 2,2'-Methylene-bis(4,6-di-tert- Examples include, but are not limited to, tris(nonylphenyl) phosphite, tris(nonylphenyl) phosphite, and trisodecyl phosphite. Further, as hindered phenolic antioxidants, Pentaerythritol tetrakis [3-(3,5-di-tert-butyl-4-hydroxyphenyl) propinate], 1,3,5-tris(3,5-di-tert -butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 4,4',4''-(1-methylpropanyl-3-ylidene)tris( Examples include, but are not limited to, 6-tert-butyl-m-cresol), etc. Among them, the effect of suppressing the decomposition of the resin composition during melt processing and the embrittlement of the obtained film is high, and the toughness is good. Among them, phosphite-based antioxidants are particularly preferred.

The content of the antioxidant is preferably 0.01 to 5 wt%, more preferably 0.1 to 2 wt%, based on the resin composition. By satisfying the above range, it becomes possible to improve toughness without impairing transparency.

The film of the present invention preferably has a weight loss rate (under N ₂ flow (200 mL/min), 280° C., 60 minutes) measured after vacuum drying at 80° C. for 15 hours of 5 wt% or less. More preferably, it is 3 wt% or less, and still more preferably 1.5 wt% or less. By satisfying the above range, embrittlement can be suppressed, resulting in good toughness.

The film of the present invention may contain alkali metal elements such as lithium, sodium, potassium, magnesium element, manganese element, calcium element, and the total content of alkali metal elements is Ma (mol/t), magnesium element, When the total content of manganese element and calcium element is Mb (mol/t), and the phosphorus element content is P (mol/t), it is preferable that the following formula (I) is satisfied.

0.8<(Ma/2+Mb)/P<2 (I)
The upper limit is more preferably 1.5 or less, still more preferably 1 or less. By satisfying the above range, it becomes possible to improve the toughness.

When the film of the present invention is melted at 300°C for 5 minutes, cooled to room temperature at a rate of 40°C/min, and subsequently heated at a rate of 20°C/min (2nd Run), one glass transition temperature is observed. Or, when a plurality of values are observed, it is preferable that the difference between the maximum value and the minimum value is within 20°C. The glass transition temperature is based on JIS K 7121 (1999), and uses the value of the midpoint between the extrapolated glass transition start temperature and the extrapolated glass transition end temperature measured using a differential scanning calorimeter. If one glass transition temperature is observed, or if multiple values are observed, the difference between the maximum and minimum values is within 20°C, which results in less cracking and chipping during film forming, and good mechanical properties. . More preferably, the difference between the maximum and minimum glass transition temperatures is within 10°C, even more preferably within 5°C, and particularly preferably only one glass transition temperature is observed.

The film of the present invention preferably has an internal haze of 30% or less. More preferably it is 20% or less, and still more preferably 10% or less. Note that this internal haze is a value measured in the film thickness direction with a haze meter using water as a solvent and converted to a film thickness of 100 μm. By satisfying the above range, the film becomes a highly transparent film that is preferable for polarizer protective films and the like.

In addition, the biaxially stretched film of the present invention has a glass transition of 30°C or more when the sample is heated at 20°C/min in the first measurement (1st run) using a differential scanning calorimeter (DSC). No temperature was observed, or a glass transition temperature ^TgA of 30°C or higher was observed when the temperature was raised at 20°C/min (1st Run), and after melting at 300°C for 5 minutes, it was cooled to room temperature at 40°C/min. In the subsequent second measurement (2nd Run), the difference between Tg ^A and Tg ^B is preferably 10° C. or more when the glass transition temperature is Tg ^B when the temperature is increased at 20° C./min. The difference between Tg ^A and Tg ^B of 10°C or more indicates that the polymer is oriented, which means that cracks, chips, and dimensional changes during the forming process of the biaxially stretched film are small, and mechanical properties are also improved. Becomes good. More preferably, the difference is 15° C. or more, and particularly preferably, no glass transition temperature is observed in the 1st Run.

In addition, when multiple Tg's of 30 degreeC or more are observed in 1st Run, the lowest glass transition temperature is set as Tg ^A. Similarly, in the 2nd Run, if a plurality of Tg values of 30° C. or higher are observed, the lowest glass transition temperature is determined as Tg ^B.

The absolute value of the birefringence index in the thickness direction of the biaxially stretched film of the present invention is preferably 0.1 or less. More preferably it is 0.05 or less, and still more preferably 0.02 or less. By setting it below the above upper limit, a display with excellent image quality can be obtained without disturbing polarized light when used, for example, as a polarizer protective film, which is preferable.

Next, a method for manufacturing the film of the present invention will be described. The film of the present invention includes a polyester resin composition A and a polystyrene resin composition B, and can be obtained by mixing these and then forming a film.

The polystyrene resin composition B may be mixed with the polyester resin composition A before the polymerization of the polyester resin composition A, for example, before the esterification reaction, or after the polymerization. There are also methods of mixing using an extruder, methods of mixing after pulverizing the mixed components into powder with a pulverizer, methods of mixing by dissolving both in a solvent and co-precipitation, and methods of mixing by dissolving one in a solvent and forming a solution. Examples include, but are not limited to, a method in which one component is mixed with the other after the first component is mixed with the other component. Among these, kneading using a kneading machine is preferable because it allows direct molding into a film after melt extrusion, short heat history, and suppresses discoloration.

When mixing polyester resin composition A and polystyrene resin composition B, a compound having an antioxidant effect may be added. Various antioxidants can be used as the antioxidant, and may be used alone or in combination. The specific type and content of the antioxidant are as shown above.

The method for producing polyester resin composition A of the present invention uses a dicarboxylic acid component or its ester and a glycol component as main raw materials, and consists of a first step consisting of an esterification reaction or a transesterification reaction, followed by a polycondensation reaction. It consists of a second stage process. Further, as a manufacturing method, batch polymerization, semi-continuous polymerization, and continuous polymerization can be applied.

As raw materials for producing the polyester resin composition A of the present invention, dicarboxylic acids or dicarboxylic acid esters and glycols can be used, and each of them can be used alone or in combination of two or more types. Preferred dicarboxylic acid components and glycol components are as shown above.

In the method for producing polyester resin composition A of the present invention, the catalyst used in the esterification reaction may be a compound such as manganese, cobalt, zinc, titanium, calcium, etc.; From the viewpoint of suppressing the generation of foreign matter and the like, it is preferable to carry out the esterification reaction without a catalyst. Even in the absence of a catalyst, the esterification reaction proceeds satisfactorily due to the autocatalytic action of the carboxylic acid. Further, as a catalyst used for the transesterification reaction, a known transesterification catalyst can be used. Examples of transesterification catalysts include organic manganese compounds, organic magnesium compounds, organic calcium compounds, organic cobalt compounds, organic lithium compounds, etc. Specifically, carbonates, acetates, benzoates, oxides, hydroxides, etc. There are things, but it is not limited to these.

Further, as the catalyst used for the polycondensation reaction, a known polycondensation catalyst can be used. Examples include compounds such as antimony, titanium, aluminum, tin, and germanium. Specifically, oxides, hydroxides, organic acid salts, alcoholates containing the above metal elements, glycol solutions of these compounds, and the like can be used.

In the method for producing polyester resin composition A, solid phase polymerization may be performed to obtain a high molecular weight material. Solid phase polymerization is carried out by heat-treating the polyester resin composition in an inert gas atmosphere or under reduced pressure, although the apparatus and method are not particularly limited.

The method for producing polystyrene resin composition B in the present invention is not particularly limited as long as it is a polystyrene resin composition that satisfies (Chemical formula 1 = Chemical formula 5 below).

(R ₁ is H or CH ₃ group, R ₂ is a substituent containing a hydroxy group, ester group, or ether group)
Examples of the substituent containing a hydroxy group include a hydroxymethyl group, a hydroxyethyl group, a hydroxybutyl group, a hydroxyoctyl group, a hydroxymethyl ester group, a hydroxyethyl ester group, a hydroxybutyl ester group, and a hydroxyoctyl ester group.

Examples of the substituent containing an ester group include a methyl ester group, an ethyl ester group, a butyl ester group, an octyl ester group, and the like.

Examples of the substituent containing an ether group include a methyl ether group, an ethyl ether group, a butyl ether group, an octyl ether group, and a polyether group.

Examples of the method for producing polystyrene resin composition B include a method in which an initiator is added to a styrene monomer and a copolymer monomer, and a copolymer is obtained by reacting.

A method for obtaining a copolymer by adding an initiator to a styrene monomer and a copolymer monomer and reacting the same will be exemplified below, but the present invention is not limited to the following method.

A method for producing poly(styrene-co-hydroxyethyl methacrylate) includes a method of adding an initiator to styrene and hydroxyethyl methacrylate and causing a polymerization reaction to obtain a copolymer.

The method for producing poly(styrene-co-polyethylene glycol acrylate) involves adding an initiator to styrene and alkyl acrylate, causing a polymerization reaction, and then transesterifying the alkyl ester group with polyethylene glycol to form a copolymer. There are several ways to obtain it.

The polymerization method may be radical polymerization, anionic polymerization, or cationic polymerization. Living polymerization may be used to narrow the molecular weight distribution. A copolymer obtained by radical polymerization with a relatively wide molecular weight distribution is preferable because it has relatively good mechanical properties. Radical polymerization may be bulk polymerization or polymerization in an organic solvent.

Organic solvents include toluene, benzene, dioxane, tetrahydrofuran, 1-butanol, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and mixtures thereof.

Examples of the initiator include potassium persulfate, hydrogen peroxide, peroxides such as benzoyl peroxide, and azo compounds such as azobisisobutyronitrile.

The monomer concentration in the reaction solution can be appropriately determined depending on the type of monomer and the type of organic solvent, and can be in the range of 0.01 to 30% by weight, for example. The concentration of the initiator in the reaction solution can be appropriately determined depending on the type and concentration of the monomer, and can be in the range of, for example, 0.01 to 10 mol% in molar ratio per mol of monomer.

The reaction temperature is appropriately determined depending on the reactivity of the monomers and initiator used. Heating for the polymerization reaction is carried out under conditions such that the radical polymerization initiator contained in the reaction solution is simultaneously cleaved in order to obtain a polymer having a relatively uniform molecular weight. If the temperature is too low, the polymerization reaction will not be completed in a short time and the molecular weight will be broad; if the temperature is too high, the resulting polymer will depolymerize into monomers. From this point of view, the reaction temperature is determined by considering the type of radical polymerization initiator. For example, if the initiation temperature of the radical polymerization initiator is T°C, the reaction temperature is in the range of T°C to T+100°C. be able to.

After the polymerization reaction, the product may be taken out by a known method. For example, in the case of bulk polymerization, it may be heated and melted as it is, discharged in the form of a strand, and then cut into pellets with a pelletizer or the like for use. In addition, when using an organic solvent, add a poor solvent such as alcohol to the reaction solution to precipitate the desired polymer, and then separate the solid and liquid by filtration according to a conventional method, and if necessary, wash with water, dry, etc. It can be taken out by repeating the process. Alternatively, the reaction solution and water may be mixed and the reaction solvent may be separated and removed by separation or distillation to obtain an aqueous solution or dispersion of the polymer, and the resulting aqueous solution or dispersion may be used in that state, or the water may be removed as necessary. It can be used in solid state.

The film containing the polyester resin composition A and the polystyrene resin composition B obtained above can be formed into a film by a known forming method.

In addition, when forming the film, various additives may be added to the extent that the effects of the present invention are not impaired, such as colorants including pigments and dyes, lubricants, flame retardants, ultraviolet absorbers, antioxidants, antibacterial agents, nucleating agents, One or more additives such as plasticizers and mold release agents may also be added.

The film manufacturing method is not particularly limited in terms of equipment or method, but for example, the polyester resin composition A and polystyrene resin composition B obtained above are melt-extruded using an ordinary extruder or T-die to form a film. Then, a desired stretched film can be obtained by biaxially stretching. Also, two or more layers can be provided during melt extrusion. In addition, in the case of the above-mentioned laminated film, evaluation of each physical property and effect is carried out by cutting out the resin composition of the corresponding layer.

The film produced according to the present invention has excellent heat resistance, low birefringence, and transparency, so it can be used for agricultural materials, gardening materials, fishing materials, civil engineering and construction materials, stationery, medical supplies, automobile parts, etc. It is useful for optical parts, electrical/electronic parts, and other uses, and is particularly suitable as a polarizing plate protective film.

The present invention will be explained in more detail with reference to Examples below. The physical properties were measured and the effects evaluated according to the following methods.

(1) Intrinsic viscosity of polyester resin composition A (unit: dL/g)
0.1 g of polyester resin composition A was weighed with an accuracy of within 0.001 g, and dissolved in 10 mL of o-chlorophenol by heating at 100° C. for 30 minutes. The solution was cooled to room temperature, 8 mL of the solution was placed in an Ostwald viscometer placed in a 25° C. water tank, and the number of seconds it passed through the marked line was measured (A seconds).

Further, using 8 mL of o-chlorophenol alone, the number of seconds it took to pass the marked line was measured using an Ostwald viscometer placed in a water tank at 25° C. (B seconds) in the same manner as above.

The intrinsic viscosity was calculated using the following formula.

IV=-1+[1+4×K×{(A/B)-1}]＾0.5/(2×K×C)
Here, K is 0.343, and C is the concentration of the sample solution (g/100 mL).

(2) Content of polyester resin composition A and polystyrene resin composition B in the film (unit: weight %)
The film sample was dissolved in a mixed solvent of heavy hexafluoroisopropanol/deuterated chloroform = 1/1 (volume ratio), and 1H-NMR was observed using an H-NMR analyzer. Each peak was assigned, and based on the integral ratio, it was determined that the polyester resin The contents (wt%) of composition A and polystyrene resin composition B were determined.
・Device: “GSX-400” manufactured by JEOL Ltd.
(3) Copolymerization rate n of polystyrene resin composition B
Polystyrene resin composition B was dissolved in a deuterated chloroform solvent, 1H-NMR was observed using an H-NMR measuring device, each peak was assigned, and the copolymerization rate n relative to the number of moles of styrene repeating units was determined from the integral ratio. Ta.
・Device: “GSX-400” manufactured by JEOL Ltd.
(4) Molecular weight measurement of polystyrene resin composition B After dissolving 30 mg of polystyrene resin composition B in 20 mL of tetrahydrofuran (THF) (however, if insoluble matter in THF was present, the insoluble matter was removed by filtration. After that, measurement was performed by GPC method under the analysis conditions shown below, and the peak start position due to polystyrene resin in the chart obtained by this measurement (in the present invention, for convenience, the molecular weight 1.9 x 10 ⁷ position is adopted) A baseline is drawn horizontally (parallel to the horizontal axis) as a reference, and each molecular weight is calculated using a standard calibration curve created using standard polystyrene.

Equipment used: GPC specification high-performance liquid chromatograph manufactured by GL Sciences Co., Ltd. Column: Showa Denko Co., Ltd. Column, product name Shodex GPC KF-806, Shodex KF-805, and Shodex KF-803 connected in series in this order Column Temperature: 40℃
Solvent: THF
Flow rate: 1.0mL/min Concentration: 0.15w/v%
Injection volume: 0.2mL
Detector: UV-visible detector manufactured by GL Science Co., Ltd., trade name UV702 type (measurement wavelength 254 nm)
Molecular weight range of calibration curve used for calculation of molecular weight distribution: 5.4×10 ³ to 1.9×10 ⁷
(5) Glass transition temperature Tg of polyester resin composition A, polystyrene resin composition B, and film (unit: °C)
According to JIS 7122 (1987), using a differential scanning calorimeter, 3 mg of resin was heated from 30° C. to 300° C. at a rate of 20° C./min in a nitrogen atmosphere (1st Run). The glass transition temperature obtained during this temperature increase was calculated as the glass transition temperature of the 1st Run using the following formula. Next, after holding at 300°C for 5 minutes, the temperature was lowered to 30°C at a rate of 40°C/min. Furthermore, after holding at 30°C for 5 minutes, the temperature was raised from 30°C to 300°C at a rate of 20°C/min (2nd Run). The glass transition temperature obtained during this temperature increase was calculated as the glass transition temperature of the 2nd Run using the following formula.
Glass transition temperature = (extrapolated glass transition start temperature + extrapolated glass transition end temperature)/2
Here, the extrapolated glass transition start temperature is the intersection of the straight line extending the baseline on the low temperature side to the high temperature side and the tangent line drawn at the point where the slope of the curve of the step-like change part of the glass transition is maximum. Temperature. The extrapolated glass transition end temperature is the temperature at the intersection of a straight line extending the baseline on the high temperature side to the low temperature side and a tangent line drawn at the point where the slope of the curve of the step-like change part of the glass transition is maximum. did. The following equipment was used.
・Device: Seiko Instruments “EXSTAR DSC6220”
(6) Internal haze of film (unit: %)
The thickness of the film is measured using a film thickness meter, and the film thickness is defined as t (μm). This film was measured using a haze meter (“HGM-2DP” manufactured by Suga Test Instruments Co., Ltd.) using water as a solvent. The internal haze in terms of 100 μm was calculated by converting the measured haze value Ht using the following formula.

(7) Weight reduction rate of film (mass ratio wt% to film)
After vacuum drying the film at 80° C. for 15 hours, 10 mg of the film was weighed into an aluminum pan. The weight loss rate was measured using TG-DTA when the temperature was raised from 25° C. at a rate of 10° C./min under N ₂ flow (200 mL/min) and held at 280° C. for 60 minutes. The following equipment was used.
・Device: HITACHI STA7200RV
(8) Metal element content Ma, Mb and phosphorus content P of the film (unit: mol/t)
7g of film was formed into a cylindrical shape using a melt press machine, and measured using a fluorescent X-ray analyzer (model number: 3270) manufactured by Rigaku Denki Co., Ltd., and calculated using a calibration curve prepared in advance with samples whose content was known. did.

(9) Preparation of unstretched film and biaxially stretched film A polyester resin composition and a polystyrene resin composition were blended at a specified blending ratio and vacuum-dried at 80° C. for 24 hours. Thereafter, the mixture was supplied to a twin-screw extruder with a cylinder temperature of 280° C. for melt-kneading. The gut discharged from the die was rapidly cooled by passing it through a cooling bath filled with water whose temperature was controlled at 10° C. for 10 seconds, and then pelletized using a strand cutter to obtain a chip-shaped resin composition.

The chip-shaped resin composition was vacuum dried at 80° C. for 24 hours, put into an extruder, melt-extruded at 280° C., and transferred to a die through a filter. Next, the sheet-like melt extruded from the die was cooled and solidified on a cooling drum with a surface temperature of 25° C. by electrostatic application to obtain a sheet-like unstretched film.

Furthermore, the obtained unstretched film was stretched 3.3 times in the longitudinal direction using a stretching roll at a glass transition temperature of +5°C (the higher temperature was adopted among the glass transition temperatures of polyester resin and polystyrene resin), and then uniaxially stretched. A stretched film was obtained. Further, this uniaxially stretched film was stretched 3.3 times in the width direction in a hot air atmosphere with a glass transition temperature of +5°C (the higher temperature was adopted among the glass transition temperatures of polyester resin and polystyrene resin) to form a biaxially stretched film. I got it. Note that the thickness of the biaxially stretched film was 40 μm.

(10) Film toughness The unstretched film prepared in (9) above was bent and judged as follows, and embrittlement was considered to be good when it was ``〇'' or ``△''.

〇: No chipping △: Some chipping ×: Cracking and breakage.

(11) Transparency of film The appearance of the biaxially stretched film prepared in (9) above was observed and judged as follows, and transparency was determined to be good when it was rated as “〇” or “△”.

○: Transparent without turbidity △: Slightly turbid but transparent ×: Opaque.

(12) Birefringence of biaxially stretched film For the biaxially stretched film prepared in (9) above, the in-plane refractive index nMD in the longitudinal direction, the in-plane refractive index nTD in the width direction, and the in-plane refractive index nTD in the thickness direction are The refractive index is measured as nZD, the birefringence in the thickness direction is calculated using the formula below, and the judgment is made as follows. When the absolute value of the birefringence is 0.1 or less, the birefringence is low and good. I suppose there is.

○: 0.00≦｜Birefringence｜≦0.05
△: 0.05<|Birefringence|≦0.1
×: 0.1 < | Birefringence |
Birefringence=|(nMD-nTD)/2-nZD|
・Measuring device: SAIRON TECHNOLOGY, INC. Manufactured by PRISM COUPLER & LOSS MEASUREMENT SPA-4000
-Measurement wavelength: 632.8nm.

(Reference Example 1) Synthesis of polyester resin composition A-1 Into an esterification reactor charged with 105 parts by weight of bishydroxyethyl terephthalate dissolved at 255°C, 86 parts by weight of terephthalic acid and ethylene glycol were added. A slurry consisting of 37 parts by weight (1.15 times the mole of terephthalic acid) was gradually added to proceed with the esterification reaction. The temperature in the reaction system was controlled to be 245 to 255°C, and the esterification reaction was terminated when the reaction rate reached 95%.

105 parts by weight of the esterification reaction product (equivalent to 100 parts by weight of PET) at 255°C thus obtained was transferred to a polymerization apparatus, and phosphoric acid was added until the phosphorus element content in polyester resin composition A-1 was 0.9 mol/t. Then, diantimony trioxide was added to the polyester resin composition A-1 so that the antimony element content was 0.7 mol/t, and manganese acetate tetrahydrate was added to the polyester resin composition A-1. The content of manganese element in 1 was 0.9 mol/t.

Thereafter, while the temperature inside the polymerization apparatus was gradually raised to 290° C., the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill out ethylene glycol. When a melt viscosity equivalent to an intrinsic viscosity of 0.6 is reached, the reaction is terminated, the reaction system is brought to normal pressure with nitrogen gas, and the polyester resin composition is discharged into cold water from the bottom of the polymerization apparatus in the form of a strand and cut. A-1 was obtained. Table 1 shows the physical properties of the obtained polyester resin composition A-1.

(Reference example 2) Synthesis of polyester resin composition A-2 Reference example except that manganese acetate tetrahydrate was added so that the manganese element content in polyester resin composition A-2 was 1.8 mol/t. Polyester resin composition A-2 was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained polyester resin composition A-2.

(Reference example 3) Synthesis of polyester resin composition A-3 Reference example except that manganese acetate tetrahydrate was added so that the manganese element content in polyester resin composition A-3 was 9.0 mol/t. Polyester resin composition A-3 was obtained in the same manner as in Example 1. Table 1 shows the physical properties of the obtained polyester resin composition A-3.

(Reference Example 4) Synthesis of polyester resin composition A-4 101 parts by weight of dimethyl 2,6-naphthalene dicarboxylate, 51 parts by weight of ethylene glycol (2.0 times the mole of the dicarboxylic acid component), and diantimony trioxide. Manganese acetate tetrahydrate was added so that the antimony element content in the polyester resin composition A-4 was 0.7 mol/t, and the manganese element content in the polyester resin composition A-4 was 2.5 mol/t. t, and the contents were dissolved at 180°C. Thereafter, methanol was distilled off while raising the temperature to 240°C. After an amount of methanol equivalent to a reaction rate of 95% was distilled off, phosphoric acid was added so that the phosphorus element content in polyester resin composition A-4 was 2.5 mol/t, and the transesterification reaction was completed. did.

Thereafter, while the temperature inside the polymerization apparatus was gradually raised to 290° C., the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill out ethylene glycol. When a melt viscosity equivalent to an intrinsic viscosity of 0.6 is reached, the reaction is terminated, the reaction system is brought to normal pressure with nitrogen gas, and the polyester resin composition is discharged into cold water from the bottom of the polymerization apparatus in the form of a strand and cut. A-4 was obtained. Table 1 shows the physical properties of the obtained polyester resin composition A-4.

(Reference Example 5) Synthesis of polyester resin composition A-5 83 parts by weight of dimethyl terephthalate, 18 parts by weight of dimethyl 2,6-naphthalene dicarboxylate, 62 parts by weight of ethylene glycol (2.0 times the mole of the dicarboxylic acid component) ), diantimony trioxide was added so that the antimony element content in the polyester resin composition A-5 was 0.7 mol/t, and manganese acetate tetrahydrate was added to the manganese element content in the polyester resin composition A-5. It was added so that the content was 2.5 mol/t, and the contents were dissolved at 150°C. Thereafter, methanol was distilled off while raising the temperature to 240°C. After an amount of methanol corresponding to a reaction rate of 95% was distilled off, phosphoric acid was added so that the phosphorus element content in polyester resin composition A-5 was 2.5 mol/t, and the transesterification reaction was completed. did.

Thereafter, while the temperature inside the polymerization apparatus was gradually raised to 290° C., the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill out ethylene glycol. When a melt viscosity equivalent to an intrinsic viscosity of 0.6 is reached, the reaction is terminated, the reaction system is brought to normal pressure with nitrogen gas, and the polyester resin composition is discharged into cold water from the bottom of the polymerization apparatus in the form of a strand and cut. A-5 was obtained. Table 1 shows the physical properties of the obtained polyester resin composition A-5.

(Reference Example 6) Synthesis of polyester resin composition A-6 86 parts by weight of dimethyl terephthalate, 15 parts by weight of dimethyl isophthalate, 65 parts by weight of ethylene glycol (2.0 times the mole of dicarboxylic acid component), dimethyl trioxide Antimony was added so that the antimony element content in polyester resin composition A-6 was 0.7 mol/t, and manganese acetate tetrahydrate was added so that the manganese element content in polyester resin composition A-6 was 2. It was added at a concentration of 5 mol/t, and the contents were dissolved at 150°C. Thereafter, methanol was distilled off while raising the temperature to 240°C. After an amount of methanol equivalent to a reaction rate of 95% was distilled off, phosphoric acid was added so that the phosphorus element content in polyester resin composition A-6 was 2.5 mol/t, and the transesterification reaction was completed. did.

Thereafter, while the temperature inside the polymerization apparatus was gradually raised to 290° C., the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill out ethylene glycol. When a melt viscosity equivalent to an intrinsic viscosity of 0.6 is reached, the reaction is terminated, the reaction system is brought to normal pressure with nitrogen gas, and the polyester resin composition is discharged into cold water from the bottom of the polymerization apparatus in the form of a strand and cut. A-6 was obtained. Table 1 shows the physical properties of the obtained polyester resin composition A-6.

(Reference Example 7) Synthesis of polyester resin composition A-7 80 parts by weight of dimethyl terephthalate, 21 parts by weight of dimethyl 5-sodium sulfoisophthalate, 60 parts by weight of ethylene glycol (twice the mole of the dicarboxylic acid component), Diantimony oxide was added so that the antimony element content in polyester resin composition A-7 was 0.7 mol/t, and manganese acetate tetrahydrate was added so that the manganese element content in polyester resin composition A-7 became 0.7 mol/t. It was added at a concentration of 2.5 mol/t, and the contents were dissolved at 150°C. Thereafter, methanol was distilled off while raising the temperature to 240°C. After an amount of methanol equivalent to a reaction rate of 95% was distilled off, phosphoric acid was added so that the phosphorus element content in polyester resin composition A-7 was 2.5 mol/t, and the transesterification reaction was completed. did.

Thereafter, while the temperature inside the polymerization apparatus was gradually raised to 290° C., the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill out ethylene glycol. When a melt viscosity equivalent to an intrinsic viscosity of 0.6 is reached, the reaction is terminated, the reaction system is brought to normal pressure with nitrogen gas, and the polyester resin composition is discharged into cold water from the bottom of the polymerization apparatus in the form of a strand and cut. A-7 was obtained. Table 1 shows the physical properties of the obtained polyester resin composition A-7.

(Reference Example 8) Synthesis of polyester resin composition A-8 86 parts by weight of dimethyl terephthalate, 49 parts by weight of ethylene glycol, 13 parts by weight of 1,4-cyclohexanedimethanol), diantimony trioxide and polyester resin Manganese acetate tetrahydrate was added so that the antimony element content in composition A-8 was 0.7 mol/t, and manganese acetate tetrahydrate was added so that the manganese element content in polyester resin composition A-8 was 2.5 mol/t. The contents were dissolved at 150°C. Thereafter, methanol was distilled off while raising the temperature to 240°C. After an amount of methanol equivalent to a reaction rate of 95% was distilled off, phosphoric acid was added so that the phosphorus element content in polyester resin composition A-8 was 2.5 mol/t, and the transesterification reaction was completed. did.

Thereafter, while the temperature inside the polymerization apparatus was gradually raised to 290° C., the pressure inside the polymerization apparatus was gradually reduced from normal pressure to 133 Pa or less to distill out ethylene glycol. When a melt viscosity equivalent to an intrinsic viscosity of 0.6 is reached, the reaction is terminated, the reaction system is brought to normal pressure with nitrogen gas, and the polyester resin composition is discharged into cold water from the bottom of the polymerization apparatus in the form of a strand and cut. A-8 was obtained. Table 1 shows the physical properties of the obtained polyester resin composition A-8.

(Reference Example 9) Polystyrene resin composition B
The polystyrene resin composition B used in the Examples is shown in Table 2. In Table 2, when the polystyrene resin composition B is represented by (Chemical formula 1), it is marked as ○, and when it is not represented, it is marked as ×. Moreover, when polystyrene resin composition B in Table 2 is represented by (Chemical formula 2), it is marked as ○, and when it is not represented, it is marked as ×.

Moreover, B-0 is a polystyrene resin composition without copolymerization.

(Example 1)
As shown in Table 3, polyester resin composition A-1 and polystyrene resin composition B-1 were blended at the stated compounding ratio (50/50 weight ratio), and unstretched film and A biaxially stretched film was obtained.

Table 3 shows the properties of the obtained film.

The resin composition obtained in Example 1 had good toughness, transparency, and birefringence.

(Examples 2 to 7)
A film was obtained in the same manner as in Example 1 except that the type of polystyrene resin composition was changed as shown in Table 3. Table 3 shows the properties of the obtained film.

The films obtained in Examples 2 and 3 had good toughness, transparency, and birefringence.

The film obtained in Example 4 had good transparency and birefringence, although some chipping was observed in the unstretched film.

The film obtained in Example 5 was slightly cloudy and tended to have high birefringence, but had good toughness.

The film obtained in Example 6 had good transparency and birefringence, although some chipping was observed in the unstretched film.

The film obtained in Example 7 was slightly cloudy and tended to have high birefringence, but had good toughness.

(Examples 8 to 11, Comparative Examples 1 and 2)
A film was obtained in the same manner as in Example 1, except that the type of polystyrene resin composition was changed as shown in Table 4. Table 4 shows the properties of the obtained film.

The films obtained in Examples 8 and 9 had good toughness, transparency, and birefringence.

The film obtained in Example 10 was slightly cloudy and tended to have high birefringence, but had good toughness.

The film obtained in Example 11 had chipping in the unstretched film and tended to have a high birefringence, but had good transparency.

In the film obtained in Comparative Example 1, chips were observed in the unstretched film and the film was cloudy, so that the birefringence could not be measured.

The film obtained in Comparative Example 2 was too brittle to be formed into a film and had a high birefringence, but had good transparency.

(Examples 12 to 15)
A film was obtained in the same manner as in Example 1, except that the type of polystyrene resin composition was changed as shown in Table 5. Table 5 shows the properties of the obtained film.

The film obtained in Example 12 had good toughness, transparency, and birefringence.

The film obtained in Example 13 was slightly cloudy and tended to have high birefringence, but had good toughness.

The film obtained in Example 14 had chipping in the unstretched film, was slightly cloudy, and had a tendency to have a high birefringence.

The film obtained in Example 15 had good toughness, transparency, and birefringence.

(Examples 16-22)
A film was obtained in the same manner as in Example 1, except that the type of polyester resin composition was changed as shown in Table 6. Table 6 shows the properties of the obtained film.

Although the films obtained in Examples 16 and 17 had some defects in the unstretched film, they had good transparency and birefringence.

The films obtained in Examples 18 and 19 tended to have high birefringence, but had good toughness and transparency.

The films obtained in Examples 20, 21, and 22 had good toughness, transparency, and birefringence.

(Examples 23 to 26, Comparative Examples 3 and 4)
A film was obtained in the same manner as in Example 1, except that the blending ratio of the polyester resin composition and the polystyrene resin composition was changed as shown in Table 7. Table 7 shows the properties of the obtained film.

The film obtained in Example 23 had chipping in the unstretched film, was slightly cloudy, and had a tendency to have a high birefringence.

The films obtained in Examples 24 and 25 had good toughness, transparency, and birefringence.

The film obtained in Example 26 tended to have a high birefringence, but had good toughness and transparency.

The film obtained in Comparative Example 3 had a high birefringence, but good toughness and transparency.

The film obtained in Comparative Example 4 was brittle and could not be formed into a film, and had a high birefringence, but had good transparency.

Claims (11)

A film comprising a polyester resin composition A and a polystyrene resin composition B represented by (Chemical formula 1).

(R ₁ is H or CH ₃ group, R ₂ is a substituent containing a hydroxy group, ester group, or ether group) The film according to claim 1, wherein the polystyrene resin composition B represented by (Chemical formula 1) is the polystyrene resin composition B represented by (Chemical formula 2).

(R ₃ is H or CH ₃ group, R ₄ is a linear or branched hydrocarbon group having 1 to 8 carbon atoms, or an ether group including polyether) The film according to claim 1, wherein the weight loss rate measured after vacuum drying at 80°C for 15 hours (under _N2 flow (200 mL/min), 280°C, 60 minutes) is 5 wt% or less. The film according to claim 1, which satisfies the following formula (I).
0.8<(Ma/2+Mb)/P<2 (I)
(Ma: total content of alkali metal elements (mol/t), Mb: total content of magnesium, manganese, and calcium elements (mol/t), P: phosphorus element content (mol/t)) The film according to claim 1, wherein the copolymerization rate n of the polystyrene resin composition B is 0.1≦n≦0.9. The film according to claim 1, wherein the number average molecular weight (Mn) of the polystyrene resin composition B is 1.0×10 ⁴ ≦Mn≦1.0×10 ⁶ . In differential scanning calorimeter measurement, the glass transition temperature is 30°C or higher when the sample is melted at 300°C for 5 minutes, cooled to room temperature at 40°C/min, and then heated at 20°C/min (2nd Run). 2. The film according to claim 1, wherein the difference between the maximum value and the minimum value is within 20°C. The film according to claim 1, having an internal haze of 30% or less. Note that the internal haze is a value measured with a haze meter using water as a solvent and converted to 100 μm. The film according to claim 1, which is a biaxially stretched film. In differential scanning calorimeter measurement, a glass transition temperature of 30°C or higher is not observed when the sample is heated at 20°C/min (1st Run), or 30°C or higher is observed when the sample is heated at 20°C/min (1st Run). A glass transition temperature Tg ^A of ℃ or higher was observed, and after melting at 300℃ for 5 minutes, the glass transition temperature Tg ^B was obtained when the temperature was cooled to room temperature at a rate of 40℃/min and then heated at a rate of 20℃/min (2nd Run). The biaxially stretched film according to claim 9, wherein the difference between Tg ^A and Tg ^B is 10° C. or more. The biaxially stretched film according to claim 9, wherein the absolute value of the birefringence index in the thickness direction is 0.1 or less.