JP2007130997A - Biaxially-stretched resin film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially-stretched resin film in which the generation of a rainbow pattern is inhibited in the inspection of a crossed Nichol method. <P>SOLUTION: The biaxially-stretched resin film comprises a layer containing a polyethylene terephthalate-based resin (A) and a layer containing a syndiotactic polystyrene-based resin (B) alternately overlying into five or more layers, wherein the birefringence ΔP in each direction of 0°, 45°, 90° and 135° is 0.05 or less. Further, the difference between the maximum value and the minimum value in the birefringence ΔP in each direction of 0°, 45°, 90° and 135° is preferably 0.01 or less. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a biaxially stretched resin film suitable for various uses such as various food packaging, general industrial use, optical use, electric material use, and molding processing.

  Biaxially stretched films based on polyethylene terephthalate (PET) are used in various fields because of their excellent mechanical properties, heat resistance, flatness, and the like.

  One of them is a release polyester film used for inspection of a polarizing plate by the crossed Nicols method (see, for example, Patent Document 1). The crossed Nicols method inspects two polarizing plates so that their orientation principal axes are orthogonal to each other and extinguish them, and if there are foreign objects or defects in the polarizing plates, they will appear as bright spots, so visual defect inspection can be performed. It is carried out with a release polyester film sandwiched between two polarizing plates. In general, when a PET film is used for this, since the birefringence of the film is large, a rainbow pattern (color stripes appearing with a rainbow-like color) occurs, which becomes an obstacle to the inspection of the crossed Nicols method, In some cases, there is a problem that foreign matters in the polarizing plate or defects are easily overlooked.

Moreover, in the inspection of the crossed Nicols method, foreign matter and defects in the film sandwiched between the polarizing plates are similarly detected, and a method for inspecting the foreign matter and defects in the film by the crossed Nicols method has been proposed ( For example, see Patent Document 2). Even when the PET film is inspected for foreign matter in the film, the above-mentioned rainbow pattern is an obstacle to the inspection.
JP-A-2005-15726 JP 2005-49158 A

  An object of the present invention is to provide a biaxially stretched resin film in which the generation of a rainbow pattern is suppressed in the inspection by the crossed Nicols method.

  As a result of intensive studies by the present inventors, syndiotactic polystyrene resin is used in addition to polyethylene terephthalate resin as a raw material, and a multilayer structure in which polyethylene terephthalate resin layers and syndiotactic polystyrene resin layers are alternately laminated As described above, it was found that the above problem can be solved by setting the birefringence ΔP of the film to a specific value or less.

  That is, in the present invention, the layer containing the polyethylene terephthalate resin (A) and the layer containing the syndiotactic polystyrene resin (B) are alternately laminated to form 5 layers or more, and 0 °, 45 °, 90 ° The biaxially stretched resin film is characterized in that the birefringence ΔP in each direction of 135 ° is 0.05 or less.

  Regarding the birefringence index ΔP of the present invention, among the birefringence index ΔP values in the directions of 0 °, 45 °, 90 ° and 135 °, the difference between the maximum value and the minimum value is 0.01 or less. In some cases, the effect of reducing the occurrence of rainbow patterns is particularly great.

  The film of the present invention preferably has a haze of 5% or less in order to exert the effects of the present invention. From the viewpoint of productivity, the film in the vertical direction and the horizontal direction when heat-treated at 150 ° C. for 30 minutes is preferable. The heat shrinkage rate is preferably 3% or less. Moreover, it is a preferable aspect of the present invention that the mass ratio of the polyethylene terephthalate resin (A) and the syndiotactic polystyrene resin (B) is 10/90 to 90/10.

  According to the present invention, since there is almost no rainbow pattern in the cross-Nicol method, when the film of the present invention is used as a release PET film for the cross-Nicol method of the polarizing plate, It is easy to find contamination and defects. In addition, when detecting foreign matters and defects in the film of the present invention by the inspection of the crossed Nicols method, the detection is easy, so the inspection can be surely performed. It can be supplied with high reliability as a film product for various uses such as industrial use, general industrial use, optical use, electric material use, and molding use.

  In the present invention, a layer containing a polyethylene terephthalate resin (A) and a layer containing a polystyrene resin (B) are alternately laminated to form 5 or more layers, each of 0 °, 45 °, 90 °, and 135 °. A biaxially stretched resin film having a birefringence ΔP in a direction of 0.05 or less.

  The polyethylene terephthalate resin (A) (hereinafter also referred to as resin (A)) used in the present invention is composed mainly of polyethylene terephthalate (PET), and may be a homopolymer of polyethylene terephthalate. The copolymer may be a copolymer in which other copolymerizable components are copolymerized. Examples of copolymerizable acid components include aromatic dicarboxylic acids such as isophthalic acid, naphthalenedicarboxylic acid, diphenic acid, and phthalic acid, alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, adipic acid, dimer acid, and sebacin. Examples include acids, aliphatic dicarboxylic acids such as dodecanedioic acid, eicoic acid, polyfunctional acids such as trimellitic acid and pyromellitic acid, polylactic acid, polyglycolic acid, poly (2-oxybutyric acid), and derivatives thereof. . Examples of the copolymerizable alcohol component include propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, 1,4-cyclohexanedimethanol, and their Derivatives. When the copolymerizable component is copolymerized, the copolymerizable acid component is preferably 30 mol% or less and preferably 10 mol% or less in 100 mol% of the total acid component of the copolymer. More preferred. Further, the copolymerizable alcohol component is preferably 30 mol% or less, more preferably 10 mol% or less, in 100 mol% of all alcohol components of the copolymer.

  The reduced viscosity of the resin (A) is preferably 0.55 to 1.10, more preferably 0.58 to 1.00. When the reduced viscosity of the resin (A) is less than 0.55, it becomes difficult to obtain a film having mechanical strength that can be practically used. On the other hand, when the reduced viscosity of the resin (A) exceeds 1.10, The thermocompression bonding property of the film to the metal plate tends to be impaired.

In the present invention, the syndiotactic polystyrene resin (B) is mainly composed of a polystyrene polymer having a syndiotactic structure. Examples of the polystyrene-based polymer include polystyrene, poly (p-, m- or o-methylstyrene), poly (2,4-, 2,5-, 3,4- or 3,5-dimethylstyrene), poly Poly (alkylstyrene) such as (p-tertiarybutylstyrene); poly (p-, m- or o-chlorostyrene), poly (p-, m- or o-bromostyrene), poly (p-, m -Or o-fluorostyrene), poly (halogenated styrene) such as poly (o-methyl-p-fluorostyrene); poly (halogen-substituted alkylstyrene) such as poly (p-, m- or o-chloromethylstyrene) ); Poly (alkoxystyrene) such as poly (p-, m- or o-methoxystyrene), poly (p-, m- or o-ethoxystyrene); poly ( poly (carboxyalkyl styrene) such as p-, m- or o-carboxymethyl styrene); poly (alkyl ether styrene) such as poly (p-vinylbenzylpropyl ether); poly (alkyl ether styrene) such as poly (p-trimethylsilyl styrene) Alkylsilyl styrene); and poly (vinylbenzyldimethoxyphosphide). Of these, polystyrene is preferred.

In the present invention, a polystyrene-based polymer having a syndiotactic structure refers to phenyl groups (in some cases, substituted phenyl groups) that are side chains with respect to the main chain formed from carbon-carbon bonds, which are alternately positioned in opposite directions. It means a polystyrene-based polymer mainly having a three-dimensional structure. In the present invention, a polystyrene-based polymer having a syndiotactic structure is a value determined by a nuclear magnetic resonance method ( ¹³ C-NMR method), which is a diat (two constituent units) and an r (rasemo) of 85%. As described above, it is preferable to have stereoregularity with pentad (5 structural units) and rrrr of 50% or more.

  In the present invention, the polystyrene-based polymer having a syndiotactic structure may be mixed with a polymer having an isotactic structure or a polymer having an atactic structure as long as the tacticity of all polymers is within the above range. In addition, the polystyrene polymer having a syndiotactic structure may be a resin copolymerized with other types of monomers within a range not impairing the effects of the present invention as long as the tacticity is within the above range.

  In the film of the present invention, the mass ratio of the resin (A) to the resin (B) is preferably 10/90 to 90/10 as resin (A) / resin (B), and 20/80 to 80 / 20 is more preferable, 30/70 to 70/30 is still more preferable, and 40/60 to 60/40 is most preferable.

    In general, a rainbow pattern is generated in a biaxially stretched film because the directions of benzene rings in the film material are aligned and light is easily bent in a specific direction. In this film, polyester with a benzene ring in the main chain in the molecule and syndiotactic polystyrene in the side chain are alternately present, making it difficult for the benzene ring to be oriented in a specific direction when viewed as a whole film. Therefore, the occurrence of rainbow patterns is considered to be suppressed. Therefore, it is considered that the direction in which the numbers of the benzene rings of the resins A and B are aligned is preferable.

  Resin (A) and resin (B) may contain particles. By adding the particles, the film of the present invention can be provided with handling properties when the film is produced or when it is wound into a roll. Examples of particles that can be used include inorganic particles such as silica, kaolin, clay, calcium carbonate, aluminum oxide, barium sulfate, zinc oxide, and zinc sulfide; organic particles such as crosslinked acrylic particles, crosslinked styrene particles, and silicone particles. In addition, particles having effects equivalent to those exemplified above can also be used. Of these, silica is preferred.

  Resin (A) and resin (B) contain conventionally known additives, for example, lubricants, stabilizers, colorants, antioxidants, antifoaming agents, antistatic agents, ultraviolet absorbers, etc., as necessary. It may be.

  In the present invention, the layer containing the resin (A) and the layer containing the resin (B) are alternately laminated in the thickness direction by 5 or more layers, preferably 11 layers or more, more preferably 30 layers or more, most Preferably, 100 or more layers are laminated. If the number of layers is 4 or less, it becomes difficult to make ΔP 0.05 or less, and the effect of reducing the rainbow pattern may be reduced.

  In the biaxially stretched film of the present invention, the birefringence ΔP in each direction of 0 °, 45 °, 90 ° and 135 ° of the film is 0.05 or less, preferably 0.01 or less, more preferably 0. 0.005 or less. If it exceeds 0.05, a rainbow pattern will be observed remarkably. Since the birefringence ΔP is preferably as close to 0 as possible, the lower limit is not particularly limited, and the lower limit is determined by the type of resin constituting the film and the manufacturing method. The minimum value (that is, the lower limit) of the birefringence ΔP of the film of the present invention is considered to be about 0.0001, about 0.0002, or about 0.0005 at the present time.

  In the biaxially stretched film of the present invention, the difference between the maximum value and the minimum value among the birefringence ΔP values in the directions of 0 °, 45 °, 90 ° and 135 ° is preferably 0.01 or less, More preferably it is 0.008 or less, and still more preferably 0.005 or less. If the difference between the maximum value and the minimum value exceeds 0.01, a rainbow pattern may appear prominently.

  The film of the present invention preferably has a haze of 5% or less, more preferably 2% or less. If the haze exceeds 5%, it may be difficult to carry out the cross Nicol inspection itself.

  The film of the present invention preferably has a thermal shrinkage rate of 3% or less in the longitudinal and lateral directions when heat-treated at 150 ° C. for 30 minutes, more preferably 2% or less, particularly preferably 1.5%. It is as follows. If it exceeds 3%, productivity during post-processing such as mold release processing will decrease. The heat treatment for evaluating the heat shrinkage rate is performed in a state where the biaxially stretched resin film is under no tension.

  The film of the present invention has F-5 values in the vertical and horizontal directions of 50 MPa to 1000 MPa, preferably 100 MPa to 400 MPa. If it is less than 50 MPa, the film is likely to be broken at the time of mold release processing or continuous defect inspection. In addition, if it exceeds 1000 MPa, the film productivity is poor and the production is difficult.

  The film of the present invention has a thickness of 1 μm to 1000 μm, preferably 3 μm to 300 μm, more preferably 10 μm to 50 μm. A film exceeding this range is not preferable because the productivity as a biaxially stretched resin film is poor.

  Hereinafter, the production method of the film of the present invention will be specifically described. However, the production method is not limited to the following production method, and the film of the present invention was produced by another production method. May be.

  The polyethylene terephthalate resin (A) chip and the polystyrene resin (B) chip dried by a known method are respectively supplied to separate melt-extrusion apparatuses and heated to a temperature equal to or higher than the melting point of each polymer to be melted. 260-300 degreeC is suitable for resin (A), and 270-310 degreeC is suitable for resin (B).

  Both resins are introduced into a feed block in a molten state and merged. Next, in order to form a laminated structure in which the layer containing the resin (A) and the layer containing the resin (B) are alternately combined to form 5 layers or more, the resin is added to a laminating apparatus (eg, feed block, multi-manifold die, It is used for a static mixer or the like, preferably a static mixer.

A static mixer, for example, manufactured by Noritake Co., is an element in which rectangular plates are twisted 180 degrees in a resin flow path, and the number of layers is doubled each time this element is passed. become. Therefore, logically, when passing through n elements, it becomes 2 ⁿ layers, but in reality, it may change depending on the relationship between the flow path diameter, the discharge amount, the viscosity and surface tension of each resin, and the like. The number of elements in the static mixer is preferably 2 or more and 10 or less, more preferably 4 or more and 8 or less. By using such a static mixer having the number of elements, an appropriate multilayer structure can be obtained, and an effect of reducing the rainbow pattern can be seen.

  Further, it is possible to adopt a method of multilayering by a feed block method without using a static mixer.

  Subsequently, the resin is guided from the laminating apparatus to the slit die, and extruded from the die onto a rotary cooling drum having a surface temperature of about 20 to 60 ° C. in a molten state. The extruded resin is rapidly cooled and solidified to a temperature not higher than the glass transition temperature on a rotary cooling drum to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed.

  The film of the present invention can be obtained by stretching the sheet thus obtained in the biaxial direction. Specifically describing the stretching conditions, the unstretched sheet is preferably 80 to 130 ° C in the machine direction, more preferably 100 ° C to 120 ° C, preferably 1.3 to 6 times, more preferably 3.0 to After stretching 4.0 times to make a longitudinally uniaxially stretched film, it is stretched in the transverse direction at 90 to 160 ° C, more preferably at 105 to 140 ° C and 1.3 to 6 times, more preferably 3.2 to 4.2. Do it twice. After biaxial stretching, heat treatment is preferably performed at 150 to 240 ° C. for 1 to 600 seconds. Since the glass transition points of polyethylene terephthalate and syndiotactic polyester are about 69 ° C. (Tg1) and about 90 ° C. (Tg2), longitudinal stretching and longitudinal stretching at Tg2 + 10 ° C. or higher are preferable in order to enable stretching. Stretching at a stretching temperature of + 5 ° C. or higher is required. Further, in order to eliminate the rainbow pattern, it is necessary to control thickness unevenness. For that purpose, a longitudinal stretching temperature of Tg1 + 50 ° C. or less and a lateral stretching of longitudinal stretching temperature + 20 ° C. or less are necessary.

  The film of the present invention can be obtained by stretching the sheet thus obtained in the biaxial direction. Specifically describing the stretching conditions, the unstretched sheet is preferably stretched 1.3 to 6 times at 80 to 130 ° C. in the longitudinal direction to form a longitudinal uniaxially stretched film, and then at 90 to 160 ° C. in the lateral direction. Stretching 1.3 to 6 times. After biaxial stretching, heat treatment is preferably performed at 150 to 240 ° C. for 1 to 600 seconds. Further, at this time, it is preferable to relax in the longitudinal direction and / or the transverse direction by 0.1 to 20% in the maximum temperature zone of the heat treatment and / or the cooling zone at the heat treatment outlet.

  The film of the present invention can be formed into a release polyester film by providing a release layer according to a known method. The material constituting the release layer is not particularly limited as long as it has releasability, and may be a type mainly composed of a curable silicone resin, or an organic resin such as a urethane resin, an epoxy resin, or an alkyd resin. A modified silicone type by graft polymerization with a resin or the like may be used. Among them, those having a curable silicone resin as a main component are good in terms of good releasability. The said release polyester film can be conveniently used for the test | inspection by the cross Nicol method of a polarizing plate by joining with a polarizing plate through an adhesive bond layer.

  Further, the film of the present invention has excellent mechanical properties, heat resistance and flatness as a PET film, and further has unique optical properties. And when using the film of this invention, a coating layer can be provided in the surface of the film of this invention according to a well-known method. Further, the film surface can be vapor-deposited with a metal or metal oxide according to a known method. Therefore, the film of the present invention can be suitably used for various uses such as various food packaging, general industrial use, optical use, electric material use, and molding processing.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited by the following examples, and may be appropriately changed within a range that can meet the purpose described above and below. It is also possible to implement them, and they are all included in the technical scope of the present invention.
First, in this example, the film characteristics were evaluated as follows.

(1) Birefringence index ΔP
Samples obtained by cutting out the films obtained in Examples and Comparative Examples so as to have a width of 2 cm and a length of 3 cm were used as samples. At this time, a sample cut out so that the length direction of the sample is parallel to the length direction of the film (0 ° sample), and the relationship between the length direction of the sample and the length direction of the film is 45 °, Samples cut out to be 90 ° and 135 ° (45 ° sample, 90 ° sample, and 135 ° sample, respectively) were prepared.
About each said measurement sample, the refractive index of the length direction (longitudinal stretch direction) of the film, the width direction (lateral stretch direction), and the thickness direction was measured using the Abbe refractometer 4T by an Atago optical company. The solvent used for the measurement was diiodomethane, and the measurement conditions were 23 ° C. and 60 RH%. Measurement was performed at n = 5, and the average value (ΔP) of the refractive indexes of each sample was determined based on the following formula.
ΔP = | 0.5 (Nx + Ny) −Nz |
However, Nx, Ny, and Nz show the refractive index of the length direction of a film, the width direction, and the thickness direction, respectively.

(2) Heat Shrinkage Ten films were cut into a length of 10 mm and a length of 150 mm, respectively, in the length direction and the width direction as samples, and display lines were provided at intervals of 100 mm in the length direction of the samples.
It heat-processed, holding one side of the length direction of a sample so that it might be in a tension | tensile_strength state for 30 minutes in 150 degreeC oven (The product made by Tabai Espec, model PHH-101).
Thereafter, the film was taken out, and the thermal shrinkage rate was determined according to the following formula.
Thermal contraction rate = (1−interval between display lines after heating / interval between display lines before heating) × 100%

(3) F-5 value Ten films were cut into a length of 10 mm and a length of 150 mm, respectively, in the length direction and the width direction as samples, and display lines were provided at intervals of 100 mm in the length direction of the samples. These samples were subjected to a tensile test according to JIS-K7127 (1999) at a chuck distance of 100 mm, a temperature of 23 ° C., and a tensile speed of 100 mm / min. The test piece was type 2, and the tensile stress at 5% elongation was F-5.

(4) Rainbow pattern The film is placed on a light source (two 10W fluorescent lamps (Toshiba FL100-ELD-56K)) and viewed through a polarizing plate. At that time, when the film was rotated, the color change was evaluated as x, the acceptable range was evaluated as ◯, and the good condition was evaluated as ◎.

(5) Haze Based on JIS-K7136, it was determined using NDH2000 manufactured by Nippon Denshoku Industries Co., Ltd.

(6) Measurement of the number of layers inside the film The number of layers inside the film was determined by observation using a transmission electron microscope.
1. Preparation of measurement sample First, the film was embedded in an epoxy resin. As the epoxy resin, Luabeck 812, Luavec NMA (manufactured by Nacalai Tesque), and DMP30 (manufactured by TAAB) were mixed well at a ratio of 100: 89: 3 (mass), respectively. After embedding the film in the above-mentioned epoxy resin, the film was left in an oven adjusted to a temperature of 60 ° C. for 16 hours to cure the epoxy resin and obtain an embedding block.
The obtained embedding block was attached to Nissan Sangyo Ultracut N, and trimming was performed using a glass knife, and the cross section of the portion to be subjected to film observation was exposed on the resin surface. Subsequently, using a diamond knife (Sumitomo Electric Sumiknife SK2045), an ultrathin section was cut out from the resin surface. The cut out ultrathin sections were collected on a mesh and then stained in a ruthenium tetroxide vapor at room temperature (25 ° C.) for 30 minutes, and then thinly deposited with carbon.

2. Observation with an electron microscope The electron microscope observation was performed using JEM-2010 manufactured by JEOL under the conditions of an acceleration voltage of 200 kV and a magnification of 500 to 20000. The obtained image was recorded on an imaging plate (FDL UR-V manufactured by Fuji Photo Film), and the signal recorded on the imaging plate was read out using digital luminography (Pixsys TEM manufactured by JEOL Ltd.). Windows (registered trademark) It recorded as digital image information on the personal computer, and the number of layers confirmed from the difference in the dyeing degree of the layer containing resin (A) and the layer containing resin (B) was counted.

Example 1
As the resin (A), a copolymerized polyester obtained by copolymerizing 100 mol% of terephthalic acid as the acid component and 85 mol% of ethylene glycol and 15 mol% of neopentyl glycol as the glycol component (inherent viscosity 0.62) Was put into a 60 mmφ single screw extruder and melted at 285 ° C. Syndiotactic polystyrene (300 zc made by Idemitsu Petrochemical Co., Ltd.) was charged as a resin (B) into a twin screw extruder (22.5 mmφ × 2) and melted at 300 ° C. A static mixer (6-element, manufactured by Noritake Co., Ltd.) was introduced to the feed block so that each resin was in a molten state to form a two-layer laminated structure of resin (A) / resin (B) and then heated to 295 ° C. I let it pass. The ratio of the resin at this time was resin (A) / resin (B) = 40/60 (mass ratio).

  The resin after passing through the static mixer was extruded from a T-die and cooled with a cooling roll at 30 ° C. to obtain an unstretched sheet. This sheet was stretched 3.3 times in the longitudinal direction on a roll whose surface was heated to 110 ° C., and stretched 3.5 times in the transverse direction with a tenter having a preheating temperature of 110 ° C. and a stretching temperature of 120 ° C. A biaxially stretched resin film having a thickness of 50 μm was obtained by heat treatment at 220 ° C. for 4 seconds. The number of layers of the obtained film was 60 layers.

Example 2
A biaxially stretched resin film having a thickness of 50 μm was obtained in the same manner as in Example 1 except that resin (A) / resin (B) = 50/50 (mass ratio). The number of layers of the obtained film was 60 layers.

Example 3
Resins (A) and (B) were the same as those used in Example 1, and resin A was charged into a 60 mmφ single screw extruder and melted at 285 ° C. Resin B was put into a twin screw extruder (22.5 mmφ × 2) and melted at 300 ° C. Except for the resin A and B being in a molten state, the resin A and B are guided to a feed block so as to have an 8-layer laminated structure of A / B / A / B / A / B / A / B, and extruded from a T-die. A biaxially stretched resin film was obtained in the same manner as in Example 1.

Comparative Example 1
In this comparative example, only the resin (A) was used. The resin (A) used in Example 1 was melted at 285 ° C., extruded as it was from a T-die, and cooled with a cooling roll at 25 ° C. to obtain an unstretched sheet. This sheet was stretched 3.3 times in the longitudinal direction on a roll whose surface was heated to 90 ° C., and stretched 3.5 times in the transverse direction with a tenter having a preheating temperature of 110 ° C. and a stretching temperature of 120 ° C. A biaxially stretched resin film having a thickness of 50 μm was obtained by heat treatment at 220 ° C. for 4 seconds.

  The evaluation results of the biaxially stretched resin films obtained in Examples 1 and 2 and Comparative Example 1 are shown in Tables 1 to 4.

  The film of this invention is suitable for the release polyester film used for the test | inspection by the cross Nicol method of a polarizing plate. Moreover, the film of this invention can be utilized for various uses, such as various food packaging use, general industrial use, optical use, an electrical material use, and a shaping | molding process.

Claims (5)

  Layers containing polyethylene terephthalate resin (A) and layers containing syndiotactic polystyrene resin (B) are alternately laminated to form 5 or more layers in each direction of 0 °, 45 °, 90 ° and 135 ° A biaxially stretched resin film having a birefringence ΔP of 0.05 or less.   The biaxially stretched resin according to claim 1, wherein the difference between the maximum value and the minimum value of the birefringence ΔP in each direction of 0 °, 45 °, 90 ° and 135 ° is 0.01 or less. the film.   The biaxially stretched resin film according to claim 1 or 2, wherein the haze is 5% or less.   The biaxially stretched resin film according to any one of claims 1 to 3, which has a thermal shrinkage rate of 3% or less when heat-treated at 150 ° C for 30 minutes.   The biaxially stretched resin film according to any one of claims 1 to 4, wherein a mass ratio of the polyethylene terephthalate resin (A) and the syndiotactic polystyrene resin (B) is 10/90 to 90/10.