JP5356764B2 - Optical base film - Google Patents

Description

  The present invention relates to an optical substrate film used as a constituent member of various displays such as a liquid crystal display device.

  Polyester films are used as constituent members for various displays such as liquid crystal display devices. In the liquid crystal display device, when the light beam emitted from the light source passes through the optical film, there is a problem that the amount of transmitted light decreases due to reflection on the surface and the luminance decreases.

  As a method for preventing a decrease in the amount of transmitted light, it is conceivable to increase the amount of light of the backlight, but there is a problem that the energy consumption is increased (Patent Document 1). A method of improving the amount of light transmission by preventing reflection on the film surface has also been proposed, and it is proposed to directly provide a layer for suppressing reflection on the film by a dry coating method such as sputtering, vapor deposition or CVD. Has been. However, the dry coating method requires vacuum processing equipment, and the production cost is expensive. Therefore, recently, a technique for forming a layer for suppressing reflection on a film at a low cost by a wet coating method has been developed (Patent Document 2).

  In wet coating, it is necessary to use a material having a refractive index sufficiently lower than that of a film for the antireflection layer. However, since such a low refractive index agent is difficult to obtain, it is generally reflected by laminating a plurality of layers. A surface having a low light reflectivity is obtained by causing light to interfere. In order to reduce the reflectance of the surface by laminating a plurality of layers as described above, it is necessary to strictly control the thickness of each layer, and it is necessary to provide a plurality of processes, which is complicated and expensive.

JP 2007-044954 A Japanese Patent Laid-Open No. 2003-075603

  It is an object of the present invention to provide an optical base film having a sufficiently low surface reflectance and excellent adhesion to a polyester film as a base material without using a high-cost multilayer antireflection layer. And

That is, the present invention comprises a biaxially stretched polyester film, and a coating layer provided directly on at least one surface of the biaxially stretched polyester film,
The coating layer is composed of 50 to 90% by weight of an acrylic copolymer polymerized from 65 to 99 parts by weight of trifluoroethyl (meth) acrylate and 1 to 35 parts by weight of ethylene glycol di (meth) acrylate, and 10 to 50 surfactants. And consists of
The refractive index N of the coating layer is 1.20 to 1.45, and the following formula is satisfied for the thickness d (nm) and the refractive index N of the coating layer,
(300/4) / N ≦ d (nm) ≦ (800/4) / N
It is an optical base film characterized in that the minimum value of the reflectance on the surface of the coating layer in the wavelength region of 300 to 800 nm is 0.5 to 4.0 %.

  According to the present invention, the present invention provides an optical base film having a sufficiently low surface reflectance and excellent adhesion to a polyester film as a base material, even without using a high-cost multilayer antireflection layer. Can be provided.

Hereinafter, the present invention will be described in detail.
The optical base film of the present invention comprises a biaxially stretched polyester film and a coating layer provided directly on at least one surface of the biaxially stretched polyester film. Although this coating layer is a single coating layer, it acts to suppress surface reflection of the film. In addition, the application layer may be provided on both surfaces of the biaxially stretched polyester film.

[Polyester film]
In the present invention, the polyester constituting the biaxially stretched polyester film is a linear saturated polyester synthesized from an aromatic dibasic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof. Specific examples of such polyester include polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, poly (1,4-cyclohexylenedimethylene terephthalate), and polyethylene-2,6-naphthalate.

  The polyester used for the biaxially stretched polyester film may be a copolymer of the above polyester. In the case of a copolymer, the amount of the copolymer component is, for example, 20 mol% or less based on the total repeating units of the polyester.

As the polyester, polyethylene terephthalate or polyethylene-2,6-naphthalate is particularly preferable because of a good balance between mechanical properties and optical properties.
The biaxially stretched polyester film may contain a colorant, an antistatic agent, an antioxidant and a catalyst, but preferably contains no filler from the viewpoint of obtaining high transparency.

  The thickness of the biaxially stretched polyester film is necessary when used as a substrate film (support) for an optical substrate film such as a hard coat, a touch panel, an antiglare treatment, an electromagnetic wave shielding film for PDP, or an organic EL. In order to obtain strength, the thickness is preferably 10 to 500 μm, more preferably 50 to 450 μm.

[Coating layer]
In the present invention, the coating layer is an acrylic copolymer (hereinafter referred to as “fluorine-containing”) having a fluoroalkyl (meth) acrylate represented by the following formula as a main polymerization component and a fluorine-free (meth) acrylate as a polymerization component. It may be referred to as “acrylic copolymer”).

（上記式において、Ｒ^１基は、水素原子、または炭素原子数１〜５のアルキル基であり、Ｒ^２基は、炭素数１〜４かつフッ素原子数１〜７のアルキル基である。）

(In the above formula, the R ¹ group is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, and the R ² group is an alkyl group having 1 to 4 carbon atoms and 1 to 7 fluorine atoms.)

  The “main polymerization component” in the present invention refers to a polymerization component occupying, for example, 65 to 99% by weight, preferably 70 to 90% by weight, based on the total weight of all monomer components constituting the fluorine-containing acrylic copolymer. "Subordinate polymerization component" refers to a polymerization component occupying, for example, 1 to 35% by weight, preferably 10 to 30% by weight, based on the total weight of all monomer components constituting the fluorine-containing acrylic copolymer.

  By using a fluorine-containing acrylic copolymer composed of a polymer component of this ratio for the coating layer, it exhibits good adhesion with the light diffusion film, and a coating layer having an appropriate refractive index is formed on the biaxially stretched polyester film. Can be provided.

  Examples of the fluoroalkyl (meth) acrylate as the main polymerization component include trifluoropropyl (meth) acrylate, trifluoroethyl (meth) acrylate, tetrafluoropropyl (meth) acrylate, and hexafluorobutyl (meth) acrylate. it can. Water-soluble or dispersible ones are preferred, but water-soluble ones containing some organic solvents are also preferred, and trifluoroethyl (meth) acrylate is particularly preferred.

  For example, the following copolymerization component that does not contain fluorine (meth) acrylate may be exemplified by those exemplified below. That is, ethylene glycol diacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, 1,4-butane Diol diacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, pentaerythritol diacrylate, pentaerythritol dimethacrylate, pentaerythritol triacrylate, penta Erythritol trimethacrylate, pentaerythritol tetraa Relate, pentaerythritol tetramethacrylate, glycerol diacrylate, glycerol dimethacrylate, glycerol acoxydimethacrylate, 1,1,1-trishydroxymethylethane diacrylate, 1,1,1-trishydroxymethylethanedimethacrylate, 1,1 1-trishydroxymethylethane triacrylate, 1,1,1-trishydroxymethylethane trimethacrylate, 1,1,1-trishydroxymethylpropane diacrylate, 1,1,1-trishydroxymethylpropane dimethacrylate Among the (meth) acrylates, ethylene glycol diacrylate, ethylene glycol dimethacrylate, triethylene glycol dimethacrylate, La ethylene glycol dimethacrylate are preferred, especially ethylene glycol di (meth) acrylate.

  A preferable fluorine-containing acrylic copolymer for the coating layer is a fluorine-containing acrylic copolymer composed of trifluoroethyl (meth) acrylate and ethylene glycol di (meth) acrylate.

  The glass transition point of the fluorine-containing acrylic copolymer in the coating layer is preferably 40 to 100 ° C, more preferably 60 to 80 ° C. If it is this range, the outstanding adhesiveness and the outstanding film forming property can be obtained. On the other hand, when the glass transition temperature is less than 40 ° C., blocking tends to occur between the films, and when it exceeds 100 ° C., the film forming property is deteriorated.

  The fluorine-containing acrylic copolymer in the coating layer can be polymerized, for example, by emulsion polymerization. Examples of the polymerization initiator that can be used for emulsion polymerization include hydrogen peroxide, ammonium persulfate, potassium persulfate, t-butyl hydroperoxide, azobisisobutyronitrile, and 2,2′-azobis (2-methylpropionamide). ) Dihydroxychloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydroxychloride, 2,2′-azobis (2-amidinopropane) dihydroxychloride. In addition, a known redox initiator, preferably ammonium persulfate may be used.

  The emulsion polymerization reaction can be carried out at a temperature and reaction time depending on the type of monomer and radical polymerization initiator used and other conditions. As the polymerization conditions, for example, the polymerization reaction temperature can be 50 to 90 ° C., and the polymerization reaction time can be 3 to 24 hours. Emulsion polymerization can also be performed in inert gas atmosphere, such as nitrogen gas or argon gas, for example.

  The fluorine-containing acrylic copolymer used for forming the coating layer may be in the form of an emulsion or a powder. As a method of making the fluorine-containing acrylic copolymer into a powder form, for example, a method of drying the fluorine-containing acrylic copolymer can be used. Since the coating layer can be easily produced in a large area, the fluorine-containing acrylic resin is preferably used in the form of an emulsion.

  The coating layer preferably contains organic particles and / or inorganic particles in addition to the fluorine-containing acrylic copolymer and the surfactant. The average particle diameter of the particles is preferably 0.02 to 1.0 μm, more preferably 0.05 to 0.5 μm, and particularly preferably 0.05 to 0.2 μm. If the average particle size exceeds 1.0 μm, whiteness is generated after coating and the surface may be roughened, which is not preferable. If the average particle size is less than 0.02 μm, the contact area between the particles increases and aggregation occurs. This is not preferable because there are problems such as difficulty in manufacturing in the current technology.

  The coating layer is preferably composed of a fluorine-containing acrylic copolymer and a surfactant, and the content of the fluorine-containing acrylic copolymer constituting the coating layer in the coating layer is preferably 5 to 95% by weight, More preferably, it is 50 to 90% by weight, particularly preferably 70 to 90% by weight, and the content of the surfactant in the coating layer is 5 to 95% by weight, preferably 10 to 50% by weight, and more preferably. Is 10 to 30% by weight. When the coating layer is composed of this composition, it is possible to obtain a coating film with high cohesive force and to obtain excellent adhesive force with the biaxially stretched polyester film as the substrate.

  Examples of the surfactant include polyoxyethylene-fatty acid esters, sorbitan fatty acid esters, glycerin fatty acid esters, fatty acid metal soaps, alkyl sulfates, alkyl sulfonates, alkyl sulfosuccinates, and other anionic and nonionic surfactants. Can be used.

[Refractive index and thickness of coating layer]
The optical base film of the present invention has a configuration in which the coating layer is directly provided on at least one surface of a biaxially stretched polyester film. In the present invention, the reflectance of the surface can be lowered by providing the coating layer directly on at least one surface of the biaxially stretched polyester film.

  In the optical base film of the present invention, the minimum value of the reflectance of the coating layer surface in the wavelength region of 300 to 800 nm is 0.5 to 8.0%, preferably 0.5 to 7.0%, more preferably. Is 0.5 to 3.0%. By satisfying these conditions, it is possible to obtain a film with suppressed surface reflection, which is preferable as an optical substrate film.

The refractive index N of the coating layer is preferably 1.20 to 1.45, more preferably 1.35 to 1.45. And it is preferable that the thickness d (nm) and refractive index N of an application layer satisfy the following formula.
(300/4) / N ≦ d (nm) ≦ (800/4) / N

By satisfying these conditions, the reflectance of the film surface can be made sufficiently low.
For example, when the refractive index N of the coating layer is 1.35 to 1.45, the thickness d of the coating layer is preferably 80 to 120 nm. By setting it as the thickness of this range, the reflectance in wavelength 300-800 nm can be restrained low.

In the optical base film of the present invention, the surface haze value of the coating layer is preferably 0.5% or less, and the average light transmittance in the wavelength region of 300 to 800 nm is preferably 90 to 99%. .
If it is said range, reflection of the surface can fully be suppressed and it is preferable because it has transparency suitable as an optical substrate film.

[Production method]
Hereinafter, the melting point is expressed as Tm, and the glass transition temperature is expressed as Tg.
The biaxially stretched polyester film is obtained by, for example, melt-extruding polyester into a film and cooling and solidifying it with a casting drum to form an unstretched film. The unstretched film is once or twice or more in the longitudinal direction at Tg to (Tg + 60) ° C. The film is stretched so that the total magnification is 3 to 6 times, and then stretched so that the magnification is 3 to 5 times in the width direction at Tg to (Tg + 60) ° C., and further at 180 to 240 ° C. as necessary. It can be obtained by performing heat treatment for 1 to 60 seconds and performing reheat treatment while shrinking 0 to 20% in the width direction at a temperature lower by 10 to 20 ° C. than the heat treatment temperature.

  In the present invention, the coating layer is provided on at least one surface of the biaxially stretched polyester film, but it is not always necessary to provide the coating layer on the biaxially stretched polyester film, and the unstretched or uniaxially stretched polyester. You may manufacture by providing an application layer in a film and extending | stretching after that to make a biaxially stretched film of a film.

  The application layer can be applied to the polyester film at any stage of the film production process. The biaxially stretched polyester film is preferably manufactured during the production process, and more preferably applied to the film before the orientation crystallization is completed. Here, the film before completion of crystal orientation is an unstretched film, a uniaxially oriented film in which the unstretched film is oriented in either the longitudinal direction or the transverse direction, and further in two directions, the longitudinal direction and the transverse direction. And a biaxially stretched film before it is re-stretched in the machine direction or the transverse direction to complete orientation crystallization. Especially, it is preferable to apply a coating liquid to an unstretched film or a uniaxially stretched film oriented in one direction, and to perform longitudinal stretching and / or lateral stretching and heat setting as it is.

  The coating liquid used for forming the coating layer is preferably used in the form of an aqueous coating liquid such as an aqueous solution, an aqueous dispersion, or an emulsion. When an aqueous coating liquid is used as the coating liquid, the solid content concentration of the aqueous coating liquid is usually 20% by weight or less, and particularly preferably 1 to 10% by weight. If this ratio is less than 1% by weight, the applicability to the polyester film may be insufficient. On the other hand, if it exceeds 20% by weight, the stability of the coating liquid and the appearance of the coating layer are deteriorated.

  When the coating liquid is an aqueous dispersion or emulsion, the aqueous solution dispersed so that the solid content has an average particle size of about 0.1 μm is diluted with ion-exchanged water. Similarly, the wetting agent is diluted with ion-exchanged water. It is preferable to prepare by mixing.

  As the coating method, any known coating method can be applied. For example, a roll coating method, a gravure coating method, a roll brush method, a spray coating method, an air knife coating method, an impregnation method, and a curtain coating method can be used alone or in combination. In addition, a coating film may be formed only in the single side | surface of the light-diffusion film of a film as needed, and may be formed in both surfaces.

  When applying an aqueous coating liquid to a film, as a pretreatment for improving the coating property, the film surface is subjected to physical treatment such as corona surface treatment, flame treatment, plasma treatment, etc., or chemically combined with the composition. It is preferable to use an inert surfactant in combination.

  Hereinafter, the present invention will be described in detail with reference to examples. The physical properties were measured and evaluated by the following methods. “Parts” means parts by weight.

(1) Total light transmittance According to JIS K7361, the total light transmittance of the film was measured using the Nippon Denshoku Industries Co., Ltd. haze measuring device (NDH-2000). The total light transmittance of the film was evaluated according to the following criteria.
◎: 92.5% ≦ total light transmittance value: very good ○: 90.0% ≦ total light transmittance value <92.5% ... good ×: total light transmittance value <90.0% ... bad

(2) Surface haze According to JIS K7136, the surface haze of the film was measured using the Nippon Denshoku Industries Co., Ltd. haze measuring device (NDH-2000). The surface haze of the film was evaluated according to the following criteria.
◎: Surface haze value ≦ 0.5% ・ ・ ・ Extremely good ○: 0.5% <Surface haze value ≦ 1.5% ・ ・ ・ Good ×: 1.5% <Surface haze value ・ ・ ・ Poor

(3) Minimum value of reflectance Using a spectrophotometer MPC3100 manufactured by Shimadzu Corporation, the reflectance at a wavelength of 300 nm to 2100 nm was measured using the coating layer surface of the film as a measurement target. The minimum reflectance value at a wavelength of 300 to 800 nm was read from the chart, and the minimum reflectance value was evaluated according to the following criteria.
◎: Minimum reflectance value ≦ 2.0%... Very good ○: 2.0% <Minimum reflectance value ≦ 4.0%... Good ×: 4.0% <Minimum reflectance value. Bad

(4) Refractive index of coating layer The refractive index of the dried coating material for coating was measured using a laser refractometer prism coupler, model 2010, manufactured by Metricon, using a wavelength of 633 nm. The refractive index was evaluated according to the following criteria.
◎: Refractive index value ≦ 1.400 ... very good ○: 1.400 <Refractive index value ≤ 1.450 ... Good ×: 1.450 <Refractive index value ... Defective

(5) Coating layer thickness Using the bottom wavelength (nm) obtained from the minimum reflectance and the refractive index of the coating liquid dried product, the thickness was calculated from the following formula. (Thickness d (nm) is rounded to one decimal place.)
d = λ / (4 × n)
d: coating layer thickness (nm)
λ: Bottom wavelength (nm)
n: Refractive index of the dried coating liquid

(6) Film thickness A small piece of film for an optical substrate is embedded in an epoxy resin (Epomount manufactured by Refinetech Co., Ltd.), and sliced to 50 nm thickness with the embedded resin using a Microtome 2050 manufactured by Reichert-Jung. The film was observed with a transmission electron microscope (LEM-2000) at an acceleration voltage of 100 KV and a magnification of 100,000, and the thickness of the coating layer was measured.

(7) Wavelength at which reflectivity is minimized (bottom wavelength)
Using a spectrophotometer MPC3100 manufactured by Shimadzu Corporation, the reflectance at a wavelength of 300 nm to 2100 nm was measured using the coating layer surface of the film as a measurement target. The minimum reflectance value at a wavelength of 300 to 800 nm was read from the chart.

(8) Coating defects The coating layer of the film for optical substrate thus formed was visually observed with a halogen lamp for a certain range (width 20 cm, length 20 cm), and the occurrence degree of longitudinal stripe defects and repellency defects was as follows. Evaluation based on the criteria.
◎: Neither streak-like defect nor repelling defect ・ ・ ・ very good ○: Total of stripe-like defect, repellent defect is less than 10 ・ ・ ・ Good △: Total of flaw-like defect, repellent defect is 10 Exceeding 30 or less ... Somewhat good ×: Total of streaky defects and repelling defects exceeds 30 ... Defect

(9) Adhesion Referring to JIS-K5600-8 coating film deterioration evaluation, 10 g of gauze was loaded on the surface of the coating layer while being loaded with a load of 500 g / cm ² , and then observed on the surface. The coating film width and scratches from which the coating layer was peeled off were observed, and the adhesion was evaluated. The width was such that it could be visually confirmed, and the following criteria were used.
◎: No peeling and scratches ・ ・ ・ very good ○: No peeling, scratches ・ ・ ・ Good △: Slightly peeled, scratched ・ ・ ・ Slightly good ×: Peeled, scratched ・ ・ ・ Poor

(10) Glass transition temperature About 10 mg of sample is sealed in an aluminum pan for measurement and attached to a differential calorimeter (DuPont V4.OB2000 type DSC), 300 to 300 at a rate of 25 ° C. to 20 ° C./min. The temperature was raised to 0 ° C., held at 300 ° C. for 5 minutes, then taken out, immediately transferred onto ice and rapidly cooled. The pan was again mounted on the differential calorimeter, and the glass transition temperature (Tg: ° C.) was measured by increasing the temperature from 25 ° C. to 20 ° C./min.

(11) Intrinsic viscosity Intrinsic viscosity ([η] dl / g) was measured with an o-chlorophenol solution at 25 ° C.

(12) Component The components used in the examples are as follows.
Fluorine-containing acrylic copolymer A:
It is composed of trifluoroethyl methacrylate, ethylene glycol dimethacrylate, and alkyl diphenyl ether disulfonate as an emulsifier (Tg = 76 ° C.). The acrylic was produced as follows according to the method described in Production Examples 1 to 3 of JP-A-63-37167. That is, 1050 parts of ion-exchanged water and 22.6 parts of alkyldiphenyl ether disulfonate as an emulsifier were charged in a four-necked flask and heated to 80 ° C. in a nitrogen stream, and then ammonium persulfate 6.4 as a polymerization initiator. Further, 110.0 parts of trifluoroethyl methacrylate and 6.9 parts of ethylene glycol dimethacrylate, which are monomers, were added dropwise while adjusting the liquid temperature to 70 to 80 ° C. over 3 hours. While maintaining the temperature for 5 hours, the reaction was continued with stirring, then cooled, dried in a vacuum dryer, and the fluorine-containing acrylic copolymer A having an average particle size of 0.1 μm and a solid content concentration of 25 wt% An aqueous dispersion was obtained. This fluorine-containing acrylic resin A contains 90 mol% of trifluoroethyl (meth) acrylate as a repeating unit and further 10 mol% of ethylene glycol dimethacrylate as a repeating unit.

Fluorine-containing acrylic copolymer B:
It is composed of trifluoroethyl methacrylate, ethylene glycol dimethacrylate, and alkyl diphenyl ether disulfonate as an emulsifier (Tg = 76 ° C.). The acrylic was produced as follows according to the method described in Production Examples 1 to 3 of JP-A-63-37167. That is, 1050 parts of ion-exchanged water and 22.6 parts of alkyldiphenyl ether disulfonate as an emulsifier were charged in a four-necked flask and heated to 80 ° C. in a nitrogen stream, and then ammonium persulfate 6.4 as a polymerization initiator. Further, 77.0 parts of trifluoroethyl methacrylate and 46.2 parts of ethylene glycol dimethacrylate, which are monomers, were added dropwise while adjusting the liquid temperature to 70 to 80 ° C. over 3 hours. While maintaining the temperature for 5 hours, the reaction was continued under stirring, then cooled, dried in a vacuum dryer, and the fluorine-containing acrylic copolymer B having an average particle size of 0.1 μm and having a solid content concentration of 25 wt% An aqueous dispersion was obtained. This fluorine-containing acrylic resin B contains 67 mol% of trifluoroethyl (meth) acrylate as a repeating unit and further contains 33 mol% of ethylene glycol dimethacrylate as a repeating unit.

Fluorine-containing acrylic monomer:
Trifluoroethyl methacrylate (Fluorostar manufactured by Tosoh F-Tech)
Acrylic monomer:
Ethylene glycol dimethacrylate (Sanester EG manufactured by Sanshin Chemical Industry Co., Ltd.)
emulsifier:
Alkyl diphenyl ether sulfonate (New Call 271S, manufactured by Nippon Emulsifier Co., Ltd.)
Polymerization initiator:
Ammonium persulfate (Alternatively, ammonium peroxodisulfate manufactured by Mitsubishi Gas Chemical Company)
Fluorine-containing olefin resin C:
Polytetrafluoroethylene (PTFE made by Daikin Industries)
acrylic resin:
Nicazole RX-906 made by Nippon Carbide
Surfactant:
Polyoxyethylene alkyl ether (Emalgen manufactured by Kao Corporation)
Additive:
Carnauba wax (Cerosol 524, manufactured by Chukyo Yushi Co., Ltd.)
Fine particles:
Colloidal silica (average particle size: 200 nm, refractive index: 1.46) (Snowtex manufactured by Nissan Chemical Industries, Ltd.)

[Examples 1-2, Comparative Examples 1-8]
Molten polyethylene terephthalate ([η] = 0.62 dl / g, Tg = 78 ° C.) was extruded from a die and cooled with a cooling drum by a conventional method to form an unstretched film. The coating liquid (solid content concentration 3.0%) was uniformly applied with a roll coater to obtain a coated film. The coated film was subsequently dried at 95 ° C., simultaneously biaxially stretched at 100 ° C. at a magnification of 3.4 times in the longitudinal direction and 3.6 times in the transverse direction, and at 225 ° C. in the longitudinal direction and the width direction. Each was heat-set while being shrunk by 2% to obtain an optical base film having a biaxially stretched polyester film thickness of 50 μm and a coating layer.
The evaluation results are shown in Table 1.

[Example 3]
A prism lens layer was formed on the surface opposite to the coating layer of the optical base film of Example 1 to produce a prism sheet as a brightness enhancement sheet.

  The optical base film of the present invention can be used as a base film for various optical members, particularly base materials such as prism lens sheets, diffusion sheets, touch panels, backlights, liquid crystal displays, plasma displays, and organic EL displays. It can be suitably used as a film.

Claims (5)

A biaxially stretched polyester film, and a coating layer provided directly on at least one surface of the biaxially stretched polyester film,
The coating layer is composed of 50 to 90% by weight of an acrylic copolymer polymerized from 65 to 99 parts by weight of trifluoroethyl (meth) acrylate and 1 to 35 parts by weight of ethylene glycol di (meth) acrylate, and 10 to 50 surfactants. And consists of
The refractive index N of the coating layer is 1.20 to 1.45, and the following formula is satisfied with respect to the thickness d (nm) and the refractive index N of the coating layer,
(300/4) / N ≦ d (nm) ≦ (800/4) / N
An optical base film, wherein the minimum value of the reflectance of the coating layer surface in the wavelength region of 300 to 800 nm is 0.5 to 4.0 % .   The optical base film according to claim 1, wherein the surface haze value of the coating layer is 0.5% or less, and the average light transmittance in a wavelength region of 300 to 800 nm is 90 to 99%. The optical base film according to claim 1 or 2, wherein the coating layer is applied during the production process of the biaxially stretched polyester film. A method for producing an optical base film, comprising: applying an application layer in the course of producing a biaxially stretched polyester film when producing the optical base film according to claim 1. The manufacturing method of the base film for optics of Claim 4 which coats an application layer to the film before orientation crystallization is completed.