JP5607520B2 - Optical polyester film - Google Patents

Description

  The present invention relates to an optical polyester film. More specifically, the present invention has a coating layer having a high refractive index capable of reducing optical interference that has conventionally occurred when a hard coat layer is provided on a polyester film. It relates to a high optical polyester film.

  Polyester films, especially biaxially oriented films such as polyethylene terephthalate and polyethylene naphthalene dicarboxylate, have excellent mechanical properties, heat resistance, and chemical resistance, so magnetic tape, ferromagnetic thin film tape, photographic film, packaging film It is widely used as a material for films for electronic parts, electrical insulating films, films for metal laminates, films to be attached to the surface of glass displays, and protective films for various members.

  Conventionally, a glass substrate has been used for an image display device represented by a liquid crystal display. In recent years, however, image display devices have been required to be thinner, lighter, larger in screen, more flexible in shape, curved display, and the like. Therefore, instead of a heavy and easily broken glass substrate, a highly transparent polymer film substrate has been studied.

  In general, electroluminescent (EL) display elements, especially self-luminous elements such as organic EL elements, and displays with a self-printing function using electronic ink such as electronic paper are inferior in durability to humidity, so a flexible display is created. In this case, formation of a gas barrier layer is indispensable. When such a gas barrier layer is formed, it may be laminated directly on the polymer film. However, in many cases, a hard coat layer is formed on the polymer film because the gas barrier performance deteriorates due to minute defects. A method of laminating a gas barrier layer after laminating is used.

  On the other hand, when a hard coat layer is laminated on a polymer film, if the difference in refractive index between the hard coat layer and the polymer film is large, interference spots are generated due to interface reflection, which affects the display. Reduction is required. Moreover, in order to improve the adhesiveness of a polymer film and a hard-coat layer, laminating | stacking through an easily bonding layer is examined.

  As a method for reducing the interface reflection generated between the hard coat layer and the polymer film, for example, in Patent Document 1, as the easy adhesion layer provided on the polyethylene terephthalate film, both the refractive index of the polymer binder and the fine particles are 1.50 to 1. A method of reducing the haze value of the film and suppressing interface reflection by using a film having a range of .60 is proposed.

  For example, Patent Document 2 proposes to provide a coating layer using fine particles having a high refractive index on a polyethylene terephthalate film in order to suppress interface reflection with a hard coat having a high refractive index. However, in the method of adding fine particles having a high refractive index, a large amount of fine particles must be added, resulting in an increase in film haze and a decrease in transparency.

  In the case of a biaxially oriented polyester film using polyethylene naphthalene dicarboxylate having excellent heat resistance as a resin material, the refractive index of the polyester film becomes a high refractive index of 1.7 or more. The difference in refractive index is large, and interference spots are more likely to occur than in the case of polyethylene terephthalate. In order to reduce interference spots generated when a hard coat layer is provided on such a high refractive index polyester film, for example, in Patent Document 3, a low melting point polyester is formed on a polyethylene naphthalene dicarboxylate layer by coextrusion. A method has been proposed in which a layer is provided, the refractive index is controlled by heat treatment, and the average refractive index in the in-plane direction of the entire film is 1.70 or less.

  On the other hand, Patent Document 4 proposes an easy-adhesion layer using a copolyester having bis (4- (2-hydroxyethoxy) phenyl) fluorene or naphthalenedicarboxylic acid as a copolymerization component as an easy-adhesion layer having excellent heat resistance. ing. In addition, an easily adhesive layer containing such a copolymerized polyester has a high refractive index and can be used for an optical film. On the other hand, since water dispersibility is not sufficient, it contains a certain amount of a dicarboxylic acid component having a metal sulfonate group as a copolymerization component. This is described in Patent Document 5.

  Thus, when the polyester film is used as the polymer base film, the polyester base film has a relatively high refractive index compared with the refractive index of the hard coat layer. In order to eliminate the optical interference by suppressing the interface reflection of the easy-adhesive layer, a copolyester as described in Patent Documents 4 and 5 is considered as an easy-adhesive layer having a refractive index close to that of the polyester film. Has been.

  However, copolyester resins as described in Patent Documents 4 and 5 are brittle and have poor film-forming properties in the stretching process after applying the easy-adhesion layer in the film-forming process, so the surface due to cracks in the coating layer Under the present circumstances, haze reduction occurs and improvement is demanded.

JP 2004-284331 A JP 2004-54161 A JP 2004-98324 A Japanese Patent Laid-Open No. 10-110091 JP 2009-242461 A

  The purpose of the present invention is to have a coating layer with a high refractive index that can reduce the optical interference that has conventionally occurred when a hard coat layer is provided on a polyester film, and to eliminate coating layer cracking due to film-forming properties in the stretching process. By doing, it is providing the optical polyester film excellent in the surface haze characteristic resulting from an application layer.

  As a result of intensive studies to solve the above problems, the present inventors use a polyester binder having a low glass transition temperature together with a high refractive index polyester binder component having a fluorene structure as a binder component constituting the coating layer. As a result, the film-forming property in the stretching process is improved, and the cracking of the coating layer in the stretching process is eliminated. As a result, the surface haze characteristics resulting from the coating layer are improved, and the present invention has been completed.

（Ｒ_１は炭素数２から４のアルキレン基、Ｒ_２、Ｒ_３、Ｒ_４、及びＲ_５は水素、炭素数１から４のアルキル基、アリール基またはアラルキル基であり、それぞれ同じであっても異なっていてもよい）
ガラス転移温度が２５℃以上８０℃以下であるポリエステルＢとを含有し、ポリエステルＢがポリエステルＢの全ジカルボン酸成分に対して９０モル％以上のテレフタル酸に由来する成分を含む光学用ポリエステルフィルムによって達成される。 That is, an object of the present invention is an optical polyester film in which a coating layer is provided on at least one surface of a polyester film, and the coating layer is a diol component that is 20 mol% or more and 80 mol% or less based on the total diol component. A polyester A having a glass transition temperature of 90 ° C. or more and 135 ° C. or less containing a component having a fluorene structure represented by the formula (I);

(R ₁ is an alkylene group having 2 to 4 carbon atoms, R ₂ , R ₃ , R ₄ , and R ₅ are hydrogen, an alkyl group having 1 to 4 carbon atoms, an aryl group, or an aralkyl group, and each is the same. May be different)
An optical polyester film containing a polyester B having a glass transition temperature of 25 ° C. or more and 80 ° C. or less, wherein the polyester B contains a component derived from terephthalic acid of 90 mol% or more with respect to all dicarboxylic acid components of the polyester B Achieved.

  In a preferred embodiment of the optical polyester film of the present invention, the total content of polyester A and polyester B is 50% by weight to 100% by weight based on the weight of the coating layer, and polyester A and polyester B. The content of polyester A is 50 wt% or more and 80 wt% or less, the content of polyester B is 20 wt% or more and 50 wt% or less, and polyester A is the total dicarboxylic acid component of polyester A. In contrast, the coating layer contains a component derived from 2,6-naphthalenedicarboxylic acid in an amount of 40 mol% or more and 99 mol% or less, and the coating layer further has a range of 0.1 wt% or more and 10 wt% or less based on the weight of the coating layer. A surfactant, and the surfactant is an ethylene oxide of a linear unsaturated fatty acid ester having 16 to 20 carbon atoms. That the polyester film is composed of polyethylene naphthalene dicarboxylate, the coating layer has a refractive index of 1.60 or more and 1.65 or less, and the surface haze value of the film is 0.00. It also includes those having at least one of 1% or more and 1.5% or less, a further hard coat layer provided on the coating layer, and a flexible display substrate.

  The optical polyester film of the present invention has an application layer of a specific component, can suppress the occurrence of interference spots when the hard coat layer is laminated, and is excellent in the film forming property of the application layer. It has excellent surface haze characteristics, and is excellent in improving visibility when used as an optical film such as a flexible display substrate such as an organic EL display or electronic paper.

The present invention will be described in detail below.
<Polyester film>
The optical polyester film of the present invention has a layer structure in which a coating layer is provided on at least one surface of a polyester film (hereinafter sometimes referred to as a base material layer).

Hereinafter, the component which comprises a polyester film is demonstrated.
(polyester)
The main component of the polyester film of the present invention is a polyester obtained by polycondensation of an aromatic dicarboxylic acid or an ester-forming derivative thereof and a diol or an ester-forming derivative thereof. Specific examples of the polyester include polyethylene terephthalate, polyethylene isophthalate, polybutylene terephthalate, poly (1,4-cyclohexylenedimethylene terephthalate), and polyethylene naphthalene dicarboxylate. Here, “mainly” means 90% by weight or more, preferably 95% by weight or more, particularly preferably 99% by weight or more based on the weight of the polyester film.

  Among such polyesters, polyethylene terephthalate and polyethylene naphthalene dicarboxylate are particularly preferable. When used for optics such as a flexible display, polyethylene naphthalene dicarboxylate is preferable from the viewpoint of heat resistance and gas barrier properties. Among the polyethylene naphthalene dicarboxylates, polyethylene-2,6-naphthalene dicarboxylate is more preferable.

  These polyesters may be homopolymers or copolymers. In the case of a copolymer, the copolymer component other than the main component is preferably 10 mol% or less, more preferably 5 mol% or less, particularly preferably 1 mol% or less of all repeating units constituting the polyester. is there.

  As copolymerization components other than the main components, diol components such as trimethylene diol, tetramethylene diol, hexamethylene diol, neopentyl diol, cyclohexane dimethanol, diethylene glycol, polyethylene oxide diol, ethylene oxide adduct of bisphenol sulfone; adipic acid, Sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, 4,4′-diphenyl Examples include dicarboxylic acid components such as dicarboxylic acid and 5-sodium sulfoisophthalic acid. These compounds may be used alone or in combination of two or more.

  Among these copolymer components, more preferable dicarboxylic acid components include isophthalic acid, terephthalic acid, 4,4′-diphenyldicarboxylic acid, 2,6-naphthalenedicarboxylic acid, and 2,7-naphthalenedicarboxylic acid other than the main component. Of the ingredients. Preferred diol components include diethylene glycol, trimethylene diol, hexamethylene diol, neopentyl diol, and ethylene oxide adducts of bisphenol sulfone.

  Polyester can be produced by applying a known method. For example, it can be produced by a method in which a diol and a dicarboxylic acid are esterified, and then a reaction product obtained is subjected to a polycondensation reaction to obtain a polyester. Alternatively, these raw material monomer derivatives may be transesterified, and then the resulting reaction product may be subjected to a polycondensation reaction to obtain a polyester.

  The intrinsic viscosity of the polyester is preferably 0.40 dl / g or more, more preferably 0.40 dl / g or more and 0.90 dl / g or less at 35 ° C. in o-chlorophenol. If the intrinsic viscosity of the polyester is less than the lower limit, cutting may occur frequently during film formation, or the strength of the product after molding may be insufficient. On the other hand, when the intrinsic viscosity of the polyester exceeds the upper limit, melt extrusion is difficult because the melt viscosity is high, and polymerization may take a long time and productivity may deteriorate.

(Other ingredients)
In addition to the polyester component, the polyester film of the present invention may contain a small amount of a resin component within a range not impairing the problems of the present invention, and the content thereof is 10% by weight or less based on the weight of the base material layer. More preferred is 5% by weight or less, and particularly preferred is 1% by weight or less.

  The polyester film of the present invention may contain particles as necessary, but in order to improve transparency, the amount of particles contained in the film should be small and substantially free of particles. It is preferable. When the particles are used, the particles can be contained preferably in the range of 0.0005 to 0.01% by weight based on the weight of the base material layer. If the particle content of the base material layer is within such a range, the total light transmittance of the film exceeds 85%, and the haze value of the film becomes 1.0% or less, which is excellent in transparency. The particles can be added at the time of film formation, or may be particles contained in the recovered film.

  The type of particles is not particularly limited, but for example, calcium carbonate, calcium oxide, aluminum oxide, kaolin, silicon oxide, zinc oxide, carbon black, silicon carbide, tin oxide, crosslinked acrylic resin particles, crosslinked polystyrene resin particles, melamine resin Particles and crosslinked silicone resin particles are exemplified.

<Method for forming polyester film>
The polyester film of the present invention is obtained by melt-extruding a polyester resin composition into a film shape, cooling and solidifying it with a casting drum to form an unstretched film, and this unstretched film at a temperature of Tg to (Tg + 60) ° C. in the longitudinal direction (hereinafter, continuous). Biaxial stretching may be performed at a magnification of 3.1 to 4.5 times in the film forming direction, the longitudinal direction, and the MD direction) and in the width direction (hereinafter sometimes referred to as the transverse direction and the TD direction). preferable. The difference between the draw ratio in the longitudinal direction and the draw ratio in the width direction is preferably 0.5 or less. Here, Tg serving as an index of the film stretching temperature represents the glass transition temperature of the polyester constituting the base material layer.
Stretching can be performed by a generally used method, for example, a method using a roll or a method using a stenter. The stretching may be performed simultaneously in the longitudinal direction and the width direction, or may be sequentially performed in the longitudinal direction and the width direction.

The biaxially stretched film is then preferably heat-set for 1 to 100 seconds at a temperature of (Tm-100) to (Tm-5) ° C, more preferably 200 to 250 ° C, particularly preferably heat. The fixing temperature is 220-240 ° C. By carrying out heat relaxation treatment at a relaxation rate of 0.5 to 3% on the biaxially stretched film subjected to such heat setting treatment, the heat shrinkage rate at 200 ° C. can be reduced to 0.1% or less. it can. Here, Tm serving as an index of the heat setting temperature represents the melting point of the polyester constituting the base material layer.
When the polyester film base layer thus obtained has a refractive index of, for example, a polyethylene naphthalene dicarboxylate film, both the longitudinal direction and the width direction exceed 1.70 in the film in-plane direction.

<Coating layer>
In the present invention, a coating layer (hereinafter sometimes referred to as a coating film or an easy adhesion layer) is provided on at least one surface of the polyester film, and the coating layer contains a certain amount of a diol component having a specific fluorene structure. It contains polyester A having a glass transition temperature of 90 ° C. or more and 135 ° C. or less and polyester B having a glass transition temperature of 25 ° C. or more and 80 ° C. or less as a polymer binder.

(Polyester A)
Polyester A constituting the coating layer contains a component having a fluorene structure represented by the following formula (I) as a diol component in an amount of 20 mol% to 80 mol% with respect to the total diol component of polyester A, and has a glass transition temperature of 90. It is a polyester having a temperature of from ℃ to 135 ℃.

（Ｒ₁は炭素数２から４のアルキレン基、Ｒ₂、Ｒ₃、Ｒ₄、及びＲ₅は水素、炭素数１から４のアルキル基、アリール基またはアラルキル基であり、それぞれ同じであっても異なっていてもよい）

(R ₁ is an alkylene group having 2 to 4 carbon atoms, R ₂ , R ₃ , R ₄ , and R ₅ are hydrogen, an alkyl group having 1 to 4 carbon atoms, an aryl group, or an aralkyl group, and each is the same. May be different)

  By using the polyester A obtained by using the component having the fluorene structure as one of the diol components as the polymer binder component of the coating layer, the refractive index of the coating layer is increased, and the refractive index of the adjacent polyester film and the refractive index is increased. As a result, the interface reflection is reduced and the optical interference is reduced. Further, by using the component having the fluorene structure as one of the diol components, the solvent resistance to the solvent used for forming the hard coat layer provided on the coating layer, for example, the solvent resistance to the methyl ethyl ketone solvent is improved.

  Specific examples of the diol component having a fluorene structure include 9,9-bis [4- (2-hydroxyethoxy) phenyl] fluorene and 9,9-bis [4- (2-hydroxyethoxy) -3-methylphenyl]. Fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-dimethylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3-ethylphenyl] fluorene, 9, 9-bis [4- (2-hydroxyethoxy) -3,5-diethylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3-propylphenyl] fluorene, 9,9-bis [ 4- (2-hydroxyethoxy) -3,5-dipropylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3-isopropyl Pyrphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-diisopropylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3-n-butylphenyl] Fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-di-n-butylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3-isobutylphenyl] Fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-diisobutylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3- (1-methylpropyl) phenyl Fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-bis (1-methylpropyl) phenyl] fluorene, 9,9-bi [4- (2-hydroxyethoxy) -3-phenylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-diphenylphenyl] fluorene, 9,9-bis [4- ( 2-hydroxyethoxy) -3-benzylphenyl] fluorene, 9,9-bis [4- (2-hydroxyethoxy) -3,5-dibenzylphenyl] fluorene, 9,9-bis [4- (3-hydroxy) And propoxy) phenyl] fluorene 9,9-bis [4- (4-hydroxybutoxy) phenyl] fluorene.

  Among these, 9,9-bis [4- (2-hydroxyethoxy) phenyl] fluorene is particularly preferable. 9,9-bis [4- (2-hydroxyethoxy) phenyl] fluorene is obtained, for example, by adding ethylene oxide to 9,9-bis (4-hydroxyphenyl) fluorene.

Content of the diol component which has this fluorene structure is 20 mol% or more and 80 mol% or less with respect to all the diol components of the polyester A. The diol component in the present invention indicates a monomer component compound constituting the polyester, and is present in the polyester in an esterified state with a dicarboxylic acid component.
The lower limit of the content of the component having such a fluorene structure is preferably 25 mol%, and more preferably 30 mol%. On the other hand, the upper limit of the content of the component having a fluorene structure is preferably 75 mol%, and more preferably 70 mol%.
If the content of the component having the fluorene structure is less than the lower limit value, the effect of improving the refractive index is small, and in addition, the amorphous property is insufficient, making it difficult to disperse in water. This effect increases as the content of the component having the fluorene structure increases. However, if the content exceeds the upper limit value, the adhesion to the hard coat layer decreases or the surface becomes rough. On the other hand, if the upper limit is exceeded, the non-crystalline property becomes too large and the blocking resistance of the film is lowered, and the polymerization rate in the production of polyester A is lowered.

  Examples of the other diol component constituting the polyester A include ethylene glycol, and it is preferably contained in the range of 20 mol% or more and 80 mol% or less with respect to the total diol component of the polyester A. The lower limit of the ethylene glycol content is preferably 35 mol%, more preferably 40 mol%. The upper limit of the ethylene glycol content is preferably 75 mol%, more preferably 65 mol%. If the content of ethylene glycol is less than the lower limit, the blocking resistance of the film may be lowered. On the other hand, when the content of ethylene glycol exceeds the upper limit, the content of the diol component having a fluorene structure is relatively reduced, which leads to a decrease in the refractive index of the coating layer.

  Examples of the diol component other than ethylene glycol and the component having the fluorene structure constituting the polyester A include 1,4-butanediol, 1,4-cyclohexanedimethanol, bisphenol A, and the like. Further, although not added as a monomer component, it may contain diethylene glycol generated in the polymerization process. The lower limit of the content of these other diol components is preferably 5 mol%. On the other hand, the upper limit of the content of other diol components is preferably 40 mol%, more preferably 20 mol%, and even more preferably 10 mol%.

  As a dicarboxylic acid component used for polyester A, it is preferable to contain a component derived from naphthalenedicarboxylic acid in an amount of 40 to 99 mol% based on the total dicarboxylic acid component of polyester A, and further derived from 2,6-naphthalenedicarboxylic acid. Preferably it is a component. By using naphthalenedicarboxylic acid as the dicarboxylic acid component, the refractive index of the coating layer is further increased, and the refractive index is close to that of the adjacent polyester film. As a result, interface reflection is reduced and optical interference is reduced. In particular, when the constituent component of the polyester film is polyethylene naphthalene dicarboxylate, naphthalene dicarboxylic acid is preferably used as the dicarboxylic acid component of the polyester A from the viewpoint of reducing the difference in refractive index from the polyester film.

The lower limit of the content of the component derived from naphthalenedicarboxylic acid is more preferably 50 mol%, still more preferably 60 mol%, particularly preferably 70 mol%. Further, the upper limit of the proportion of the component derived from naphthalenedicarboxylic acid is more preferably 95 mol%.
When the content of the component derived from naphthalenedicarboxylic acid is less than the lower limit, depending on the type of polyester film, the difference in refractive index from the coating layer may not be sufficient, and optical interference may occur. On the other hand, when the content of the component derived from naphthalenedicarboxylic acid exceeds the upper limit, it may be difficult to dissolve the polyester A in the hydrophilic organic solvent, and it may be difficult to disperse in water.

  As other dicarboxylic acid components used for polyester A, 0.1 to 5 mol% of the components derived from aromatic dicarboxylic acid having a sulfonate group with respect to the total dicarboxylic acid components of polyester A, and other fragrances It is preferable to contain 0-60 mol% of group dicarboxylic acid. By including an aromatic dicarboxylic acid having a sulfonate group as a copolymerization component, the polyester A can be easily dispersed in water.

  The lower limit of the content of the aromatic dicarboxylic acid having a sulfonate group is more preferably 1 mol%, still more preferably 2 mol%, and particularly preferably 3 mol%. When the content of the aromatic dicarboxylic acid having a sulfonate group is less than the lower limit, the hydrophilicity of the polyester A may be lowered and water dispersion may not be sufficient. On the other hand, when the content of the aromatic dicarboxylic acid having a sulfonate group exceeds the upper limit value, the hydrophilicity of the film increases, and as a result, blocking resistance may decrease.

  Preferred examples of the aromatic dicarboxylic acid having a sulfonate group include 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, 5-lithium sulfoisophthalic acid, and 5-phosphonium sulfoisophthalic acid. In order to improve water dispersibility, 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, and 5-lithium sulfoisophthalic acid are more preferable, and 5-sodium sulfoisophthalic acid is most preferable.

  As a dicarboxylic acid component of polyester A, it contains a component derived from naphthalenedicarboxylic acid from the viewpoint of refractive index characteristics, and further derived from an aromatic dicarboxylic acid having a sulfonate group from the viewpoint of easily dispersing polyester A in water. While it is preferred to use the components, other aromatic dicarboxylic acids may be used with them. Examples of other aromatic dicarboxylic acids include isophthalic acid, terephthalic acid, and biphenyl dicarboxylic acid. Of these, isophthalic acid is particularly preferred.

  The lower limit of the proportion of other aromatic dicarboxylic acids is preferably 3 mol%, more preferably 5 mol%. Moreover, the upper limit of the ratio of the other aromatic dicarboxylic acid is preferably 50 mol%, more preferably 40 mol%, still more preferably 30 mol%.

  The glass transition temperature of polyester A is 90 ° C. or higher and 135 ° C. or lower, preferably 100 ° C. or higher and 130 ° C. or lower. Such a glass transition temperature is determined by the constituents of polyester A and the content thereof being in the above-mentioned range.

The refractive index of polyester A is preferably 1.60 or more and 1.65 or less. The refractive index of polyester A is preferably 1.63 or more and 1.65 or less. Such a refractive index characteristic is achieved by having a diol component having a fluorene structure as a diol component of polyester A and further including a naphthalenedicarboxylic acid component as a dicarboxylic acid component.
The intrinsic viscosity of polyester A is preferably 0.2 to 0.8 dl / g. Such intrinsic viscosity is a value measured at 35 ° C. using orthochlorophenol.

  Polyester A can be produced by a known polyester production technique. For example, it can be produced by a method of forming a monomer or oligomer by reacting the above-mentioned dicarboxylic acid component or an ester-forming derivative thereof with the above-mentioned diol component and then polycondensing under vacuum to obtain a polyester having a predetermined intrinsic viscosity. . At that time, a catalyst for promoting the reaction, for example, an esterification or transesterification catalyst or a polycondensation catalyst can be used. Various additives such as stabilizers can be added as necessary.

(Polyester B)
The coating layer of the present invention is characterized in that polyester B having a certain glass transition temperature is included in addition to polyester A. Polyester B constituting the coating layer is a polyester having a glass transition temperature of 25 ° C. or higher and 80 ° C. or lower.
By using polyester B having a glass transition temperature in such a temperature range together with polyester A, the coating layer is sufficiently softened at the process temperature at which the coating layer is coated on the polyester film and then the film is subjected to stretching treatment. The brittleness of the coating layer can be improved by improving the brittleness of the coating layer. And by improving the film forming property of the coating layer, the coating layer surface becomes smooth and surface scattering is reduced, so that the surface haze caused by the coating layer can be reduced.

  The lower limit of the glass transition temperature of polyester B is preferably 50 ° C, more preferably 60 ° C. The upper limit of the glass transition temperature of polyester B is preferably 75 ° C. When the glass transition temperature of the polyester B is less than the lower limit, the solvent resistance derived from the polyester A may be lowered. On the other hand, when the glass transition temperature of polyester B exceeds the upper limit value, the effect of improving the film forming property of the coating layer is not exhibited.

Examples of the dicarboxylic acid component used in the polyester B include terephthalic acid, isophthalic acid, phthalic acid, adipic acid, sebacic acid, 5-Na sulfoisophthalic acid, and among them, terephthalic acid and isophthalic acid are preferable.
Examples of the diol component used for polyester B include ethylene glycol, diethylene glycol, neopentyl glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, and alkylene oxide adducts of bisphenol A. In addition, polyhydroxy compounds such as glycerin and trimethylolpropane may be used in a small amount. In addition, among all the dicarboxylic acid components of polyester B, a naphthalenedicarboxylic acid component or the like may be used in a small amount, for example, within a range of 20 mol% or less.

  Among these, particularly preferable polyester B includes a copolymer polyester containing terephthalic acid in an amount of 90 mol% or more based on the total dicarboxylic acid component of polyester B, and using ethylene glycol and bisphenol A as the diol component. By adjusting the ratio of bisphenol A, the glass transition temperature of polyester B can be adjusted.

The refractive index of polyester B is preferably 1.50 or more and 1.59 or less, and more preferably 1.55 or more and 1.58 or less.
Similarly to polyester A, polyester B can be produced by a known polyester production technique.

(Contents of polyester A and polyester B)
The total content of polyester A and polyester B is preferably 50% by weight or more and 100% by weight or less, more preferably 60% by weight or more and 97% by weight or less, and still more preferably 60% by weight, based on the weight of the coating layer. % To 95% by weight. When the content of polyester A and polyester B in the coating layer is in such a range, both the refractive index characteristics and the film forming property of the coating layer can be improved.

The content of polyester A is preferably 50% by weight or more and 80% by weight or less, more preferably 60% by weight or more and 75% by weight or less, based on the total amount of polyester A and polyester B.
The content of polyester B is preferably 20% by weight or more and 50% by weight or less, more preferably 25% by weight or more and 40% by weight or less, based on the total amount of polyester A and polyester B.

  When the content of the polyester A is in such a range, the film forming property in the film stretching step can be improved at the same time while improving the refractive index characteristics of the coating layer. On the other hand, when the content of polyester A is less than the lower limit or when the content of polyester B exceeds the upper limit, the refractive index of the coating layer may not be sufficiently increased. On the other hand, when the content of polyester A exceeds the upper limit value, or when the content of polyester B is less than the lower limit value, the film-forming property in the film stretching step with polyester B is not sufficient, and the coating layer In addition to cracking and surface scattering, surface haze due to the coating layer may be reduced, and adhesion to the hard coat layer may be poor.

(Other polymer binder components)
As other polymer binder constituting the coating layer, an acrylic resin, a urethane resin, a modified product thereof, and the like can be further added as necessary. In addition to the polyesters A and B, by using a small amount of these resins, it is possible to enhance the adhesion with the hard coat layer in addition to the refractive index characteristics. When a polymer binder component other than polyesters A and B is used, its content is preferably 40% by weight or less, more preferably 30% by weight or less, further preferably 1 to 20% by weight based on the weight of the coating layer. %.

(Surfactant)
The coating layer of the present invention preferably further contains a surfactant. When a surfactant is used, the surfactant is added in the range of 0.1 wt% to 10 wt% based on the weight of the coating layer. It is preferable to contain. Moreover, when it contains surfactant, it is more preferable that content of surfactant is 0.5 weight% or more and 7 weight% or less. By containing the surfactant in such a range, the water dispersibility of the polyester A can be increased, the particle size of the polyester A in the coating liquid is reduced, and the flatness of the coating layer surface is further increased, so that the surface haze The characteristics can be further improved.
As such a surfactant, an alkylene oxide adduct of a fatty acid ester is preferable, and among these surfactants, an ethylene oxide adduct of a linear unsaturated fatty acid ester having 16 to 20 carbon atoms is particularly preferable.

(particle)
In order to increase the visibility when the coating layer of the present invention is used as a substrate film for optical applications such as an organic EL display and an electronic paper display, the particle content in the coating layer is based on the weight of the coating layer. It is preferably 1% by weight or less. The particle content is more preferably 0.001 to 0.8% by weight, still more preferably 0.1 to 0.5% by weight. Particles in the coating layer are added in a very small amount for the purpose of imparting slipperiness, and by keeping the particle content in such a range, the surface haze value resulting from the coating layer can be reduced, and further the entire film The haze value can be reduced.

The present invention is characterized in that interference spots at the time of hard coat lamination can be reduced by using polyester A as a main polymer binder component, and it is not necessary to increase the refractive index of the coating layer by particles. The particle content can be reduced within a range that does not impair the slipperiness.
Such particles preferably have an average primary particle size of 5 to 200 nm. If the average primary particle diameter of the particles exceeds 200 nm, optical scattering may occur, and the transparency of the coating layer may be reduced. On the other hand, if the average primary particle size of the particles is less than 5 nm, the particles are aggregated more and the secondary particle size is increased, so that optical scattering may occur and the transparency of the coating layer may be lowered.

  The type of particle is not particularly limited. For example, calcium carbonate, magnesium carbonate, calcium oxide, zinc oxide, magnesium oxide, silicon oxide, sodium silicate, aluminum hydroxide, iron oxide, zirconium oxide, barium sulfate, titanium oxide, tin oxide Inorganic particles such as antimony trioxide, carbon black, molybdenum disulfide, etc., organic crosslinked polymers such as acrylic crosslinked polymers, styrene crosslinked polymers, silicone resins, fluororesins, benzoguanamine resins, phenolic resins, nylon resins, etc. it can.

<Formation of coating layer>
The coating layer of the present invention is formed by mixing polyesters A and B with other coating layer components in the form of an aqueous dispersion and coating the polyester film on at least one side of the polyester film in the form of a coating solution.

The polyester aqueous dispersion is preferably produced by the following method.
Polyesters A and B are preferably dissolved in a hydrophilic organic solvent having a solubility in water of 1 liter at 20 ° C. and 20 g or more and a boiling point of 100 ° C. or less, or azeotropic with water at 100 ° C. or less. By using a polyester having such solubility and a solvent, the polyester can be made into an aqueous dispersion. Examples of the organic solvent include dioxane, acetone, tetrahydrofuran, and methyl ethyl ketone. Smaller amounts of surfactant can also be added to such solutions.

Next, water is added to the organic solvent in which the polyesters A and B are dissolved, with stirring, preferably with heating and high-speed stirring, to obtain a blue white to milky white dispersion. A blue-white to milky white dispersion can also be obtained by a method of adding the organic solution to water under stirring.
The target polyester aqueous dispersion is obtained by separating and removing the organic solvent from the obtained dispersion. For example, the desired polyester aqueous dispersion can be obtained from the obtained dispersion by a method of evaporating a hydrophilic organic solvent under normal pressure or reduced pressure. In addition, when polyesters A and B are dissolved in a hydrophilic organic solvent azeotroped with water, the water azeotropes when the organic solvent is distilled off. It is desirable to keep it dispersed.

  In addition, if the solid content concentration after removing the organic solvent exceeds 40% by weight, re-aggregation of the polyester dispersed in water is likely to occur, and the stability of the aqueous dispersion is lowered. The partial concentration is preferably 40% by weight or less. On the other hand, the lower limit of the solid content concentration is preferably 0.1% by weight or more because the time required for drying becomes longer if the concentration is too small. Even within such a range, the solid content concentration of the aqueous coating liquid used in the present invention is preferably 20% by weight or less, more preferably 1 to 10% by weight, based on the weight of the coating liquid. If this ratio is less than 1% by weight, the coatability 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 may be deteriorated.

Application of the aqueous coating liquid to the polyester film can be carried out at any stage, but it is preferably carried out during the production process of the polyester film, and further applied to the polyester film before completion of orientation crystallization. Is preferred.
Here, the polyester film before the crystal orientation is completed 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 the like that have been oriented at a low magnification (biaxially stretched film before being finally re-stretched in the machine direction or the transverse direction to complete orientation crystallization). In particular, it is preferable to apply the aqueous coating liquid of the above composition to an unstretched film or a uniaxially stretched film oriented in one direction, and perform longitudinal stretching and / or lateral stretching and heat setting as it is.

When the aqueous coating liquid is applied to the film, physical treatment such as corona surface treatment, flame treatment, plasma treatment or the like may be applied to the film surface as a preliminary treatment for improving the coatability.
The coating amount of the coating liquid is adjusted and used so that the thickness of the coating film falls within the range described later. As a 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, a curtain coating method and the like can be used alone or in combination. Of these, the gravure coating method is more preferable.
In addition, it is preferable to provide the coating layer of this invention in the side which provides a hard-coat layer, and it may form on only one side or both surfaces of a film.

<Applied layer thickness>
The thickness of the coating layer of the present invention is preferably 50 nm or more and 200 nm or less. The thickness of the coating layer is more preferably from 80 nm to 150 μm, particularly preferably from 80 nm to 100 nm. When the coating layer thickness is in such a range, interference spots during hard coat lamination can be reduced, and when the coating layer thickness deviates from such a range, a coating layer composition containing polyester A and polyester B may be used. The effect of reducing interference spots may not be fully manifested. Further, when the coating layer thickness exceeds the upper limit value, the surface haze may be increased.

<Film characteristics>
(Refractive index of coating layer)
From the viewpoint of suppressing interference spots, the refractive index of the coating layer is preferably in the range of 1.60 to 1.65, more preferably 1.62 to 1.65, particularly 1.63 to 1. .64 or less is preferable.
As a specific method for setting the refractive index of the coating layer in such a range, a glycol component having a fluorene structure is used as the diol component of polyester A having a high refractive index, and 2,6-naphthalenedicarboxylic acid is further used as the dicarboxylic acid component of polyester A. The ratio of polyester A when using polyester A together with polyester B is 50% by weight or more.

(Surface haze)
The surface haze value of the film of the present invention is preferably from 0.1% to 3.7%, more preferably from 0.1% to 2.5%, and even more preferably from 0.1% to 1. 5% or less, particularly preferably 0.1% or more and 1.0% or less. The surface haze of the film of the present invention is an index representing the transparency of the coating layer, and when the surface haze value resulting from the coating layer is in such a range, the visibility when used as an optical film such as a flexible display substrate, for example. Will be good. Due to the characteristics of the components constituting the coating layer, it is technically difficult to make the surface haze value attributable to the coating layer smaller than the lower limit value. On the other hand, when the haze value exceeds the upper limit value, the light transmittance decreases. , Visibility is reduced.

  In order to make the surface haze value in such a range, it is possible to use polyester B having a low glass transition temperature as an essential component in addition to polyester A as a polymer binder constituting the coating layer. Furthermore, surface haze can be made favorable by containing the surfactant of the present invention in the coating layer.

(Heat shrinkage)
The optical polyester film of the present invention preferably has a heat shrinkage of 0.1% or less in the longitudinal direction and the width direction when heat-treated at 200 ° C. for 10 minutes. Moreover, it is preferable that the minimum of the thermal contraction rate of each direction is -0.1%.
The lower limit of the thermal shrinkage is preferably −0.05%, more preferably −0.03%, particularly preferably 0.00% in both the longitudinal direction and the width direction of the film. The upper limit of the thermal shrinkage is preferably 0.08%, more preferably 0.05%, and particularly preferably 0.03% in both the longitudinal direction and the width direction of the film.

If the thermal shrinkage exceeds this range, for example, if the electronic paper substrate is heat treated at a temperature of about 200 ° C., which is the heat treatment temperature of the TFT layer laminated on the substrate film, the pattern will be misaligned, leading to a decrease in resolution. There is.
As a specific method for bringing the thermal shrinkage rate into such a range, a method using the draw ratio, heat setting treatment and heat relaxation treatment described in the method for forming a biaxially oriented polyester film can be mentioned.

(Film thickness)
The thickness of the optical polyester film of the present invention is preferably a thickness suitable for a thin flexible display, and is preferably in the range of 50 to 300 μm. The film thickness is more preferably 100 to 200 μm, particularly preferably 100 to 150 μm. The film thickness is preferably thinner in terms of portability and ease of bending. However, if the lower limit is not reached, it may be difficult to maintain sufficient strength as a substrate film for a flexible display.

<Hard coat layer>
In the optical polyester film of the present invention, it is preferable that a hard coat layer is further provided on the coating layer.
As the hard coat layer, a radiation curable resin or a silane resin can be used, and in particular, a hard coat layer using a radiation curable resin is preferable. Among them, an ultraviolet (UV) curable resin is used. A hard coat layer is preferred. Examples of the UV curable composition used for forming the hard coat layer include UV curable compositions such as urethane-acrylate, epoxy-acrylate, and polyester-acrylate.

  In order to laminate the hard coat layer on the coating layer of the optical polyester film of the present invention, the composition constituting the hard coat layer is coated on the coating layer, and the composition is formed by heating, irradiation with radiation (for example, ultraviolet rays) or the like. What is necessary is just to harden a thing. Although the thickness of a hard-coat layer is not specifically limited, The range of 1-15 micrometers is preferable.

<Optical use>
The optical polyester film of the present invention is suitably used as a flexible display substrate such as an organic EL display or electronic paper.
When used in such optical applications, a configuration in which a hard coat layer is provided on the coating layer of the optical polyester film of the present invention and a gas barrier layer, a transparent conductive layer such as ITO, etc. is further laminated thereon is exemplified.

  By using the optical polyester film of the present invention as such a display substrate, the interface reflection between the polyester film and the coating layer is reduced, and the visibility is improved due to less interference spots at the time of hard coat lamination. Moreover, since the surface haze value of a film is small and it is excellent in transparency, the visibility when it is set as a display can be improved.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited only to these Examples. Each characteristic value was measured by the following method. Moreover, unless otherwise indicated, the part and% in an Example mean a weight part and weight%, respectively.

(1) Identification and quantification of constituent components of coating layer Identification of constituent components of coating layer by ¹ H, ¹³ C-NMR measurement and GC / MS measurement from coating layer provided on polymer binder composition or film And quantified.

(2) Glass transition temperature of polyester About 10 mg of a polyester sample is sealed in an aluminum pan for measurement and attached to a differential calorimeter (DuPont V4.OB2000 type DSC), and the rate is 25 ° C. to 20 ° C./min. The glass transition temperature was measured by raising the temperature to 200 ° C.

(3) Coating layer thickness The film was fixed with an epoxy embedding resin, and the cross section was cut into a thickness of 50 nm with a microtome. This was stained with 2% osmic acid at 60 ° C. for 2 hours, and the thickness of the coating layer was measured using a transmission electron microscope (JEM2010, manufactured by JEOL).

(4) Film thickness Film thickness was measured at 30 g of needle pressure using an electronic micrometer (trade name “K-312A type” manufactured by Anritsu Co., Ltd.).

(5) Surface haze value of film In accordance with JIS K7136, using a haze measuring device (NDH-2000) manufactured by Nippon Denshoku Industries Co., Ltd., a sample of one film, a sample obtained by stacking two films, and three films The film haze of each sample was measured. In measuring the film haze, the place where the films were overlapped was measured by canceling the coating layer haze with a cedder oil in between.
The haze increment accompanying the increase in the number of films corresponds to the internal haze of the film, and the surface haze due to the coating layer was determined from the value obtained by subtracting the internal haze from the film haze value of one film.

(6) Refractive index of the obtained biaxially oriented polyester film, on which the coating layer is not coated, Abbe refracts the refractive indexes in the longitudinal direction (MD direction), the width direction (TD direction), and the film thickness direction (Z direction). Measured with a rate meter. In addition, since the coating layer of the present invention is thin, when the coating layer is formed on both sides, it is not affected by the refractive index of the coating layer even if measured using an Abbe refractometer from above the coating layer. The refractive index of the biaxially oriented polyester film can be determined.

(7) Refractive index of coating layer Coating at a wavelength of 550 nm from the phase difference (delta) and amplitude (psi) measured on the coating layer coating surface of the obtained film with an ellipsometer (M-200) manufactured by JASCO The refractive index of the layer was determined. The refractive index obtained by the method (6) was used as the substrate refractive index.

(8) Solvent resistance Ten drops of methyl ethyl ketone (MEK) are dropped on a film sample with a dropper and allowed to dry naturally at room temperature. When the surface was sufficiently dried, the surface was observed with an optical microscope. When the texture of the coating layer was almost unchanged, “O” was indicated, and when swelling marks or irregularities due to the solvent were observed, “X” was assigned.

(9) Reflectance amplitude (evaluation on interference spots)
The opposite surface of the polyester film on which the hard coat layer was formed was painted with black magic, the reflected light on the opposite surface was eliminated, and the spectral reflectance was measured using a spectrophotometer (Shimadzu Corporation UV-3101PC). The reflectance at a wavelength of 500 nm to 600 nm was measured, and the amplitude of the reflectance was evaluated according to the following criteria. As the amplitude of the measured reflectance is larger, interference spots are generated, and the visibility as a display is lowered.
◎: Reflectance amplitude ≦ 0.5% ・ ・ ・ Very good ○: 0.5% <Reflectance amplitude ≦ 1.0% ・ ・ ・ Good × ： 1.0% <Reflectance amplitude ・ ・ ・ Poor

[Example 1]
Establish 100 parts of dimethyl naphthalene-2,6-dicarboxylate and 60 parts of ethylene glycol with 0.03 part of manganese acetate tetrahydrate as a transesterification catalyst and gradually raise the temperature from 150 ° C. to 238 ° C. for 120 minutes. An exchange reaction was performed. In the middle, when the reaction temperature reached 170 ° C., trimethyl phosphate (135 ° C. in ethylene glycol, heat-treated at 0.11 to 0.16 MPa for 5 hours: 0.023 part in terms of trimethyl phosphate) ) And 0.024 parts of antimony trioxide was added after the transesterification reaction.
Thereafter, the reaction product is transferred to a polymerization reactor, heated to 290 ° C., and subjected to a polycondensation reaction under a high vacuum of 27 Pa or less, and has an intrinsic viscosity of 0.61 dl / g and substantially no particles. Polyethylene-2,6-naphthalenedicarboxylate was obtained.
The polyethylene-2,6-naphthalenedicarboxylate pellets were dried at 170 ° C. for 6 hours, then fed to an extruder hopper, melted at a melting temperature of 305 ° C., and filtered through a stainless steel fine wire filter having an average opening of 17 μm. The film was extruded through a 2 mm slit die on a rotary cooling drum having a surface temperature of 60 ° C. and rapidly cooled to obtain an unstretched film.

The unstretched film thus obtained is preheated at 120 ° C., and further heated by a 900 ° C. IR heater from above 15 mm between low-speed and high-speed rolls and stretched 3.3 times in the longitudinal direction. An aqueous coating solution having a coating concentration of 6% shown in Table 1 was uniformly applied to both surfaces with a roll coater. Subsequently, it was supplied to a tenter and stretched 3.5 times in the transverse direction at 145 ° C.
The obtained biaxially oriented polyester film was heat-fixed at a temperature of 250 ° C. for 40 seconds, and a polyester film having a thickness of 125 μm was wound up, and then a treatment temperature of 200 ° C. and a relaxation rate of 1.0 using an IR heater heating zone. %, A relaxation treatment was performed in the width direction to obtain a polyester film. The thickness of the coating film was 0.1 μm. The properties of the obtained polyester film are shown in Table 1.
Furthermore, using a roll coater, a UV curable composition having the following composition was uniformly applied on the coating layer so that the film thickness after curing was 5 μm, and then a high pressure having a strength of 80 W / cm. A hard coat layer was formed by irradiating with a mercury lamp for 30 seconds to cure. Table 1 shows the characteristics of the obtained laminated film.

[Coating layer composition]
(Polyester A) The acid component is 2,6-naphthalenedicarboxylic acid 90 mol% / isophthalic acid 6 mol% / 5-sodium sulfoisophthalic acid 4 mol%, the diol component is ethylene glycol 25 mol% / 9,9-bis [4 -(2-hydroxyethoxy) phenyl] fluorene 60 mol% / diethylene glycol 15 mol% (Tg = 115 ° C.).
Such polyester A comprises 100 parts dimethyl 2,6-naphthalenedicarboxylate, 5.3 parts dimethyl isophthalate, 5.4 parts dimethyl 5-sodium sulfoisophthalate, 45 parts ethylene glycol and 9,9-bis (4- ( 28.3 parts of 2-hydroxyethoxy) phenyl) fluorene is charged into a transesterification reactor, 0.1 parts of tetrabutoxytitanium is added thereto, and the temperature is controlled at 230 ° C. in a nitrogen atmosphere to produce by heating. Methanol was distilled off to conduct transesterification.
Next, 0.5 part of Irganox 1010 (manufactured by Ciba Geigy) was added to the reaction system, and then the temperature was gradually raised to 255 ° C., and the pressure in the system was reduced to 1 mmHg to remove excess ethylene glycol while removing heavy ethylene glycol. A condensation reaction was performed to obtain a copolyester A having an intrinsic viscosity of 0.48 dl / g. 20 parts of this copolyester A was dissolved in 80 parts of tetrahydrofuran, and 180 parts of water was added dropwise to the resulting solution under high speed stirring at 10,000 rpm to obtain a bluish milky white dispersion. Subsequently, this dispersion was distilled under a reduced pressure of 20 mmHg to distill off the tetrahydrofuran. Thus, a polyester aqueous dispersion having a solid content concentration of 10 wt% was obtained.

(Polyester B) The acid component is composed of 97 mol% terephthalic acid / 1 mol% isophthalic acid / 5 mol% 5-sodium sulfoisophthalic acid, and the diol component is 60 mol% ethylene glycol / 40 mol% bisphenol A (Tg = 70 ° C.).
Such polyester B is charged with 100 parts of dimethyl terephthalate, 3 parts of dimethyl isophthalate, 1 part of dimethyl 5-sodiumsulfoisophthalate, 26 parts of ethylene glycol and 14 parts of bisphenol A in a transesterification reactor. One part was added and heated under a nitrogen atmosphere while controlling the temperature at 230 ° C., and the produced methanol was distilled off to conduct a transesterification reaction.
Next, 0.5 part of Irganox 1010 (manufactured by Ciba Geigy) was added to this reaction system, the temperature was gradually raised to 255 ° C., and the pressure in the system was reduced to 1 mmHg to carry out a polycondensation reaction. Copolyester B of .48 dl / g was obtained. 20 parts of this copolyester B was dissolved in 80 parts of tetrahydrofuran, and 180 parts of water was added dropwise to the resulting solution under high-speed stirring at 10,000 rpm to obtain a bluish milky white dispersion. Subsequently, this dispersion was distilled under a reduced pressure of 20 mmHg to distill off the tetrahydrofuran. Thus, a polyester aqueous dispersion having a solid content concentration of 10 wt% was obtained.

(Surfactant) The trade name “Emulgen 420” (composition: polyoxyethylene oleyl ether) manufactured by Kao Corporation was used.
[UV curable composition]
Pentaerythritol acrylate: 45% by weight
N-methylolacrylamide: 40% by weight
N-vinylpyrrolidone: 10% by weight
1-hydroxycyclohexyl phenyl ketone: 5% by weight

[Example 2]
A polyester film and a laminated film were obtained in the same manner as in Example 1 except that the ratio of the diol component of polyester A constituting the coating layer was changed as shown in Table 1. The obtained characteristics are shown in Table 1.

[Example 3]
A polyester film and a laminated film were obtained in the same manner as in Example 1 except that the ratio of the diol component of polyester A constituting the coating layer was changed as shown in Table 1. The obtained characteristics are shown in Table 1.

[Example 4]
A polyester film and a laminated film were obtained in the same manner as in Example 1 except that the ratio of polyester A and polyester B constituting the coating layer was changed as shown in Table 1. The obtained characteristics are shown in Table 1.

[Example 5]
A polyester film and a laminated film were obtained in the same manner as in Example 1 except that the ratio of polyester A and polyester B constituting the coating layer was changed as shown in Table 1. The obtained characteristics are shown in Table 1.

[Example 6]
A polyester film and a polyester film were prepared in the same manner as in Example 1 except that the surfactant constituting the coating layer was changed to a C16 saturated alcohol having the molecular weight of about 1500 by adding ethylene oxide and propylene oxide in the same amount. A laminated film was obtained. The obtained characteristics are shown in Table 1.

[Example 7]
A polyester film and a laminated film were obtained in the same manner as in Example 1 except that the surfactant was not used as a constituent component of the coating layer. The obtained characteristics are shown in Table 1.

[Comparative Example 1]
A polyester film and a laminated film were obtained in the same manner as in Example 1 except that polyester B was not used as the polymer binder component constituting the coating layer. The obtained characteristics are shown in Table 1.

[Comparative Example 2]
The polyester film and laminated film are the same as in Example 1 except that the glass transition temperature of polyester B is changed by changing the type and ratio of the dicarboxylic acid component of polyester B constituting the coating layer as shown in Table 1. Got. The obtained characteristics are shown in Table 1.

[Comparative Example 3]
Polyester film in the same manner as in Example 1 except that the glass transition temperature of polyester B was changed by changing the type and ratio of the dicarboxylic acid component and diol component of polyester B constituting the coating layer as shown in Table 1. A laminated film was obtained. The obtained characteristics are shown in Table 1.

  The optical polyester film of the present invention has an application layer of a specific component, can suppress the occurrence of interference spots when the hard coat layer is laminated, and is excellent in the film forming property of the application layer. It has excellent surface haze characteristics, and is excellent in improving visibility when used as an optical film such as a flexible display substrate such as an organic EL display or electronic paper.

Claims (10)

An optical polyester film in which a coating layer is provided on at least one surface of a polyester film, and the coating layer is represented by the following formula (I) as a diol component, which is 20 mol% or more and 80 mol% or less with respect to all diol components. Polyester A having a glass transition temperature of 90 ° C. or more and 135 ° C. or less containing a component having a fluorene structure;

(R ₁ is an alkylene group having 2 to 4 carbon atoms, R ₂ , R ₃ , R ₄ , and R ₅ are hydrogen, an alkyl group having 1 to 4 carbon atoms, an aryl group, or an aralkyl group, and each is the same. May be different)
It contains polyester B having a glass transition temperature of 25 ° C. or more and 80 ° C. or less, and polyester B contains 90 mol% or more of a component derived from terephthalic acid with respect to all dicarboxylic acid components of polyester B. Optical polyester film.   The total content of polyester A and polyester B is 50% by weight to 100% by weight based on the weight of the coating layer, and the content of polyester A is 50% based on the total amount of polyester A and polyester B The optical polyester film according to claim 1, wherein the polyester B content is 20% by weight to 80% by weight and the polyester B content is 20% by weight to 50% by weight.   The optical polyester film according to claim 1 or 2, wherein the polyester A contains a component derived from 2,6-naphthalenedicarboxylic acid in an amount of 40 mol% or more and 99 mol% or less with respect to the total dicarboxylic acid component of the polyester A.   The optical polyester film according to any one of claims 1 to 3, wherein the coating layer further contains a surfactant in a range of 0.1 wt% to 10 wt% based on the weight of the coating layer.   The optical polyester film according to claim 4, wherein the surfactant is an ethylene oxide adduct of a linear unsaturated fatty acid ester having 16 to 20 carbon atoms.   The optical polyester film according to claim 1, wherein a constituent component of the polyester film is polyethylene naphthalene dicarboxylate.   The optical polyester film according to claim 1, wherein the coating layer has a refractive index of 1.60 or more and 1.65 or less.   The optical polyester film according to any one of claims 1 to 7, wherein the surface haze value of the film is from 0.1% to 1.5%.   The optical polyester film according to claim 1, wherein a hard coat layer is further provided on the coating layer.   The optical polyester film according to claim 1, which is used for a flexible display substrate.