TW201706329A - Biaxially oriented polyester film - Google Patents

Abstract

Provided is a film excellent in terms of mechanical property and processability, the film being a biaxially oriented polyester film in which the values of dimensional stability in the film-width direction (TD) and in the direction (MD) perpendicular thereto, as measured through temperature declining from 150 DEG C to 50 DEG C, are 50-130 ppm/ DEG C each and the coefficient of planar orientation (fn) is 0.111-0.145.

Description

Biaxially oriented polyester film

The present invention relates to a biaxially oriented polyester film which is excellent in mechanical properties and workability.

a polyester resin, in particular, polyethylene terephthalate (hereinafter sometimes abbreviated as PET) or polyethylene 2,6-naphthalenedicarboxylate (hereinafter sometimes abbreviated as PEN), It is excellent in thermal properties, chemical resistance, electrical properties, and formability, and is used in various applications. The polyester film obtained by thinning the polyester, wherein the biaxially oriented polyester film is used as a step film for protecting the transparent electrode substrate from being damaged during the processing step, in terms of excellent mechanical properties and processability. .

In general, in a film-forming substrate (ITO (Indium Tin Oxide) vapor-deposited substrate, etc.) of a transparent conductive film used for a display or the like, in order to improve the conductivity of the ITO film, it is necessary to harden the substrate at a certain temperature. step. In this step, heat is applied to the substrate and the protective film simultaneously. Therefore, if the thermal conductivity of the film-forming substrate of the transparent conductive film and the protective film, in particular, the dimensional change rate (linear expansion coefficient (CTE)) when the temperature is lowered from 150 ° C to 50 ° C, there is a transparent conductive film. The planarity of the film substrate is deteriorated, or the protective film is peeled off and the protective function is lowered. Therefore, it is preferable that the dimensional change rate of the film-forming substrate and the protective film of the transparent conductive film is close to each other. Therefore, conventionally, a PET film which is biaxially aligned is used for a film-forming substrate of a transparent conductive film (Patent Document 1). Therefore, a PET film is often used as the protective film.

However, in recent years, from the viewpoint of improving the performance of the display and thinning, a film containing an amorphous resin such as a cycloolefin polymer (COP) is used for the film formation substrate of the transparent conductive film (Patent Document 2) 3).

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open Publication No. 2013-093310

Patent Document 2: Japanese Patent Laid-Open Publication No. 2013-114344

Patent Document 3: Japanese Patent Laid-Open Publication No. 2014-112510

When a biaxially oriented PET film is used for the film formation substrate of the transparent conductive film, the biaxially oriented PET can be preferably used in the protective film of the substrate. However, when a sheet containing COP is used for a film-form substrate of a transparent conductive film, the COP is usually an amorphous resin, and the dimensional change ratio is about 2 to 3 times larger than that of the biaxial alignment PET, so if the two axes are used, When the alignment PET film is used as a protective film, the planarity of the film-forming substrate on which the transparent conductive film is produced is deteriorated, or the film is peeled off.

On the other hand, it is considered that when a difference in dimensional change ratio between the film-forming substrate and the protective film of the transparent conductive film is reduced, a film containing COP is used as the protective film. However, since the COP is an amorphous resin when the film containing COP is used as a protective film, the toughness is low and the flexibility is poor compared with the biaxially oriented PET film, so that cracking occurs in the processing step. The problem.

The subject of the present invention is to provide a background in view of the background of the prior art. It can be preferably used as a biaxially oriented polyester film for a protective film of COP for use in a film-forming substrate or the like of a transparent conductive film.

In order to solve the above problems, the present invention adopts the following configuration. which is,

[I] A biaxially oriented polyester film having a dimensional change ratio of 50 ppm/in the film width direction (TD direction) and a direction perpendicular thereto (MD direction) from 150 ° C to 50 ° C, respectively. °C or more and 130 ppm/°C or less, and the surface alignment coefficient (fn) is 0.111 or more and 0.145 or less.

[II] A biaxially oriented polyester film having a dimensional change rate of 50 ppm/in the film width direction (TD direction) and a direction perpendicular thereto (MD direction) from 150 ° C to 50 ° C, respectively. The heat shrinkage ratio at a temperature of 130 ° C or 30 minutes in a film width direction (TD direction) and a direction (MD direction) at a right angle of ° C or more and 130 ppm / ° C or less is 1.0% or less.

[III] The biaxially oriented polyester film according to [I] or [II], wherein the surface alignment coefficient (fn) is 0.120 or more and 0.140 or less.

[IV] The biaxially oriented polyester film according to any one of [1] to [III] wherein the film width direction (TD direction), the direction perpendicular thereto (MD direction), and the film width When the direction of formation of the direction of 45° is 130° C. and the heat shrinkage ratio at 30 minutes is compared in each direction, the absolute value of the difference is 0% or more and 0.5% or less, and the average value thereof is 0.5. % or less; and the dimensional change rate in the film width direction (TD direction), the direction perpendicular thereto (MD direction), and the direction in which the film width direction forms 45° from 150 ° C to 50 ° C When the direction is compared, the absolute value of the difference is 0 ppm/° C. or more and 10 ppm/° C. or less.

[V] The biaxially oriented polyester film according to any one of [I] to [IV], wherein The polyester resin of the polyester film has a heat of crystal melting of 30 J/g or less.

[VI] The biaxially oriented polyester film according to any one of [1] to [V] wherein the polyester film is a laminated polyester film comprising at least three layers, and constitutes a surface layer on both sides of the film. The heat of crystal melting (ΔHmA) of the polyester resin is 30 J/g or more, and the heat of melting (?HmB) of the polyester resin constituting the layer other than the surface layer on both sides of the film is 30 J/g or less.

[VII] The biaxially oriented polyester film according to [VI], wherein the polyester resin constituting the layer other than the surface layer on both sides of the film is a resin containing terephthalic acid and ethylene glycol as main components. The constituent unit other than the above includes only one or only two kinds of isophthalic acid and cyclohexyl dimethanol.

The biaxially oriented polyester film according to any one of [1], wherein the polyester film is a laminated polyester film comprising at least three layers, and constitutes a surface layer on both sides of the film. The melting point TmA of the polyester resin is 250 ° C or more and 280 ° C or less.

[IX] The biaxially oriented polyester film according to [IV] to [VIII], wherein the ratio of the sum of the thicknesses of the surface layers on both sides of the polyester film to the sum of the thicknesses of the layers other than the surface layer (both sides) The sum of the thicknesses of the surface layers/the thickness of the layers other than the surface layer is 1/9 to 1/2.

[X] The biaxially oriented polyester film according to any one of [I] to [IX], which is used for bonding to a film containing an amorphous resin.

[XI] The biaxially oriented polyester film according to any one of [I] to [IX], which is used for bonding to a film comprising a cyclic olefin polymer (COP).

[XII] The biaxially oriented polyester film according to any one of [I] to [XI], which is used for the purpose of protecting a film comprising a cyclic olefin polymer (COP).

According to the present invention, it is possible to obtain a near-containing COP or PC (polycarbonate) A biaxially oriented polyester film having a dimensional change ratio of a film of an amorphous resin such as an ester, and having excellent mechanical properties and good workability.

Hereinafter, the present invention will be described in detail by way of specific examples.

The polyester film of the present invention must be a biaxially oriented polyester film from the viewpoint of mechanical properties. The term "polyester" as used herein means a component having a dicarboxylic acid component and a diol component. In the present specification, the constituent component means a minimum unit which can be obtained by hydrolyzing a polyester. The polyester film of the present invention preferably comprises a copolymer of polyethylene terephthalate or polyethylene terephthalate from the viewpoint of mechanical properties.

One aspect of the present invention is a film dimensional change in the width direction (TD direction) of the film and the direction of the right angle (MD direction) from 150 ° C to 50 ° C, respectively, 50 ppm / ° C and 130 ppm / ° C Hereinafter, the biaxial alignment polyester film having a plane alignment coefficient (fn) of 0.111 or more and 0.145 or less is used.

In general, a transparent conductive film is formed by forming a film on a substrate in a state where the temperature is higher than room temperature, and then performing a step of hardening in a state where the temperature is higher than room temperature, and cooling down to room temperature. process. That is, after the film formation of the transparent conductive film, in order to prevent the transparent conductive film from being damaged and impairing the conductivity, it is important to maintain the planarity of the substrate. The protective film of the substrate also passes through this step. In other words, in order to maintain the planarity of the substrate, it is important that the dimensional change rate at the time of temperature reduction of the protective film for the substrate is close to the value of the substrate.

In recent years, as a film-forming substrate for a transparent conductive film, it is widely used. A film of COP as an amorphous resin. The dimensional change rate of the film containing COP from the temperature of 150 ° C to 50 ° C is also dependent on the molecular skeleton of the COP, and is 50 ppm / ° C or more and 150 ppm / ° C or less. By changing the dimensional change rate of the polyester film of the present invention from 150 ° C to 50 ° C to a range close to the dimensional change rate of the film containing COP, it can be prevented from being damaged after the film formation of the conductive film. The planarity of the film-forming substrate of the transparent conductive film maintains the conductivity to be good. It is particularly preferably 60 ppm/° C. or more and 110 ppm/° C. or less, and more preferably 80 ppm/° C. or more and 100 ppm/° C. or less.

Further, the dimensional change rate of the biaxially oriented polyester film is determined by the alignment of the molecular chains of the polyester constituting the film. That is, in the case where the molecular chain is aligned, the molecular chain cannot move freely by heat, and as a result, the value of the dimensional change rate is lowered. That is, in the case of the usual biaxial alignment of the polyester film, when the orientation of the molecular chain such as fn exceeds 0.145 is high, the value of the dimensional change rate is lowered (less than 50 ppm/° C.). On the other hand, when the alignment of the film having a fn of less than 0.111 is insufficient, the mechanical properties, particularly the elongation at break, are inferior, and as a result, the workability is inferior. Further, since the alignment of the film is insufficient and the molecular link is nearly amorphous, the random coarse crystals are generated due to the application of heat to the film, and the transparency of the film is not only impaired, but also exists by random coarse crystals. The surrounding molecular chains are also fixed, and as a result, the value of the dimensional change rate is also lowered. Therefore, the biaxially oriented polyester film of the present invention can be formed into a film in the width direction (TD direction) and a direction in which it is in a right angle (MD direction) from 150 ° C to 50 ° C, respectively. The biaxially oriented polyester film having a surface alignment coefficient (fn) of 0.111 or more and 0.145 or less is 50 ppm/° C. or more and 130 ppm/° C. or less, and is excellent in mechanical properties and workability, and is maintained even when heated. High transparency film. More preferably, fn is 0.120 or more and 0.140 or less.

Another aspect of the present invention is a film width direction (TD direction), and The dimensional change rate in the direction of the right angle (MD direction) from 150 ° C to 50 ° C is 50 ppm / ° C or more and 130 ppm / ° C or less, and the film width direction (TD direction), and the right angle thereof The biaxially oriented polyester film having a heat shrinkage rate of 130% or less in the direction (MD direction) at 30 minutes and 1.0% or less, respectively. As described above, in the film forming step of the transparent conductive film, the step of forming a transparent conductive film on the substrate in a state where the temperature is higher than the room temperature is included. Therefore, when the heat shrinkage rate of the protective film of the substrate is large, the planarity of the substrate is impaired. When the polyester film having a dimensional change rate at a temperature of from 150 ° C to 50 ° C at a temperature lower than the above range and a heat shrinkage ratio at 130 ° C for 30 minutes is used as a protective film for a COP film, The smoothness of the COP film became good, and the adhesion to the COP became good. More preferably, the heat shrinkage ratio in the MD direction, the TD direction at 130 ° C, and the 30 minutes is 0.5% or less. The lower limit of the heat shrinkage ratio is preferably -0.2%. The so-called "-" means expansion. When the film expands more than -0.2%, there is a case where the substrate and the protective film are peeled off.

The dimensional change rate and the heat shrinkage ratio of the polyester film of the present invention are preferably such as to satisfy the following requirements. That is, it is preferable to have a heat shrinkage ratio in the film width direction (TD direction), a direction perpendicular thereto (MD direction), and a direction of 45° in the film width direction at 130 ° C for 30 minutes. When comparing, the absolute value of the difference is the absolute value of the difference between the thermal contraction rate of the TD direction and the MD direction, and the difference between the TD direction and the thermal contraction rate in the direction from the TD direction of 45°. The absolute value of the difference between the MD direction and the heat shrinkage rate in the direction from the TD direction of 45° is 0% or more and 0.5% or less, and the average value thereof is 0.5% or less, and the temperature is lowered from 150 ° C to 50 ° C. When the dimensional change rate is compared in all directions, the absolute value of the difference is the absolute value of the difference between the dimensional change rates of the TD direction and the MD direction, and the TD direction forms a 45° direction with the TD direction. Absolute value of the difference in dimensional change rate, MD direction and TD side The absolute value of the difference in dimensional change rate in the direction in which 45° is formed is 0 ppm/° C. or more and 10 ppm/° C. or less.

When a sheet containing an amorphous resin is used for a film-forming substrate of a transparent conductive film, it is usually used without being stretched, so that the sheet containing the amorphous resin is responsive to heat (dimension change rate, heat) Shrinkage) is isotropic. On the other hand, the biaxially oriented polyester film is in the film width direction (TD direction) as the extending direction, the direction in which it is a right angle (MD direction), and the direction in which the film width direction is 45 degrees in the middle direction. There is a difference in heat responsiveness (dimension change rate, heat shrinkage rate). When the polyester film having a large difference is bonded to a sheet and a film containing an amorphous resin to be used as a laminate, the laminate may be curled. By forming a biaxially oriented polyester film which satisfies the above range, even if it is bonded to a sheet containing an amorphous resin to form a laminate, the responsiveness to heat is close, and the curl of the laminate is suppressed. Therefore, it is better.

Further, the heat shrinkage ratio at 150 ° C for 30 minutes of the biaxially oriented polyester film of the present invention is preferably 1.5% or less in both MD and TD directions. Further, it is preferable that the heat shrinkage ratio at 150 ° C for 30 minutes is -0.2% or more and 0.5% or less in both the MD and TD directions.

In order to set the dimensional change ratio, the surface alignment coefficient (fn), and the heat shrinkage ratio of the biaxially oriented polyester film to the above range, for example, the following method (a) can be employed.

(a) A method in which the heat of crystal melting of the polyester resin constituting the biaxially oriented polyester film of the present invention is 30 J/g or less, and biaxial stretching is carried out by the following method.

The method of biaxial stretching can adopt the following method.

First, the polyester resin was heated and melted in an extruder, and then discharged from a nozzle to obtain an unstretched sheet.

(1) When the melted polyester is discharged from a nozzle to form an unstretched sheet, the sheet is cooled to a temperature of 10 ° C or more and 40 ° C or less on the drum having a surface temperature, and is subjected to an adhesion cooling hardening by static electricity to prepare an unstretched sheet.

(2) The unstretched sheet obtained in (1) is biaxially oriented in the longitudinal direction (MD) of the film and the width direction (TD) of the film by satisfying the temperature T1n (°C) of the following formula (i) The area is extended by 8.5 times or more and 16.0 times or less.

(i) Tg (°C) ≦T1n (°C) ≦Tg+40 (°C)

Tg: glass transition temperature (°C) of the resin constituting the polyester film

(3) The heat-fixing treatment of the biaxially stretched film obtained in (2) is performed at a temperature (Th0 (° C)) satisfying the following formula (ii) for 1 second or more and 30 seconds or less, uniformly After slow cooling, it was cooled to room temperature to obtain a polyester film.

(ii) Tm-60 (°C) ≦ Th0 (°C) ≦ Tm-20 (°C)

Tm: melting point of the resin constituting the film (°C)

By obtaining the unstretched sheet according to the condition of (1), a substantially amorphous polyester film can be obtained, and the film can be easily aligned in the subsequent step (2), and a film having good mechanical properties can be easily obtained.

By obtaining a biaxially stretched film according to the condition of satisfying (2), it is possible to impart a proper alignment to the film to form a film having good mechanical properties.

By ending the crystal alignment according to the condition of satisfying (3), it is possible to form a film in which the structure of the aligned polyester molecular chain is stable, and the mechanical properties and heat shrinkage ratio are good.

Further, in (2), as a method of biaxial stretching, a film may be used in the longitudinal direction (MD) and the film in the width direction (direction perpendicular to the longitudinal direction of the film, TD). The sequential two-axis stretching method by extension and separation, and the simultaneous biaxial stretching method in which the extension in the longitudinal direction and the width direction are simultaneously performed Any one. Further, when the extension temperature (T1n) (°C) is less than Tg (°C), stretching is difficult. When T1n (°C) exceeds Tg+40 (°C), there is a case where film rupture frequently occurs, and a film cannot be obtained by stretching. More preferably, it is Tg+10 (°C) ≦T1n (°C) ≦Tg+30 (°C).

In the step (3), when Th0 exceeds Tm-20 ° C, there is a case where the thermal shrinkage rate is increased by the alignment failure of the film imparted by the stretching. When Th0 is lower than Tm-60 ° C, the structure of the molecular chain is unstable, and the planarity is deteriorated or the film forming property is deteriorated. Further, by setting the value of Th0 to Tm-30 (°C) ≦Th0 (°C) ≦Tm-20 (°C), the stretching of the molecular chain during film extension can be relaxed, and the heat shrinkage ratio can be set to be The range of good. In the step (3), a relaxation treatment for reducing the distance in the film width direction from 2% to 10% may also be performed.

The heat of crystal melting of the polyester resin constituting the biaxially oriented polyester film of the present invention is preferably 30 J/g or less. When the heat of crystal melting exceeds 30 J/g, the crystallinity of the resin is high, and even when the biaxial stretching is carried out by the above method, the alignment of the molecular chain is enhanced, fn is large, and the dimensional change rate is lowered. tendency. The lower limit of the heat of crystal melting is preferably 2 J/g or more. In the case of less than 2 J/g, there is a case where the film forming property is poor, or fn is lower than the lower limit of the preferred range. In the case where the polyester resin is PET, the method of copolymerizing the dicarboxylic acid component is exemplified as a method of copolymerizing phthalic acid as a dicarboxylic acid component, and A method in which a diol component is subjected to copolymerization of cyclohexanedimethanol. These may be copolymerized separately or in several copolymerizations. It is preferred that the control of the crystallinity of the individual copolymerizers is relatively easy. In particular, when isophthalic acid is used as a copolymerization component, it is particularly preferable because it is close to the structure of terephthalic acid, and it is easy to extend the orientation and to set fn to a preferable range. As the amount of copolymerization, the total of the copolymerized components is The total amount of the constituent components of the polyester is preferably 7 mol% or more and 20 mol% or less.

As a method of a further preferred embodiment, a method in which the film is configured as the following (b) is exemplified.

(b) The polyester film is formed into a laminated polyester film comprising at least three layers, and the heat of crystal melting (ΔHmA) of the polyester resin constituting the surface layers on both sides of the film is set to 30 J/g or more, and will constitute The heat of crystal melting (?HmB) of the polyester resin of the layer other than the surface layer on both sides of the film is set to 30 J/g or less.

In the case of the constitution, the surface layers on both sides of the film have higher crystallinity than the layers other than the layers, and the alignment is more easily imparted. Therefore, the alignment of the layer other than the layer is also increased, and the alignment of the entire film is improved. As a result, the mechanical properties are improved and the workability is improved, which is preferable. Further, as a result of the improvement in the alignment property, whitening in the case where heat is applied to the film is suppressed, which is preferable. ΔHmA is preferably 31 J/g or more and 60 J/g or less, and ΔHmB is preferably 2 J/g or more and less than 30 J/g.

When the constitution is adopted, the alignment of the resin constituting the surface layers on both sides can be improved. Therefore, it is preferred that the elongation temperature satisfy the following formula (iii).

(iii) TgA (°C) ≦T1n (°C) ≦TgA+40 (°C)

The TgA system shows the glass transition temperature of the polyester resin constituting the surface layers on both sides of the film. Further, in the case of a film comprising a polyester resin having a different surface layer on both sides of the polyester film of the present invention (for example, A/B/C), it is preferably a polyester resin constituting the surface layers on both sides. In the Tg, the higher temperature satisfies the formula (iii).

Further, in the case where the polyester film of the present invention contains at least three layers of the polyester film, the melting point TmA of the polyester resin constituting the surface layers on both sides of the film is preferably 250 ° C or more and 280 ° C or less. Implementation form.

When the value of TmA is below 250 ° C, the heat in the film formation When heat is received in the treatment or the like, the planarity is deteriorated, or the film forming property is deteriorated. When the polyester resin having the surface layers constituting both sides of the film has a melting point TmA of 250 ° C or more and 280 ° C or less, the planarity and the film forming property are good, and therefore it is particularly preferable.

In the case where the film of the present invention is a laminated polyester film of three or more layers, it is preferably a ratio of a sum of thicknesses of surface layers on both sides of the polyester film to a sum of thicknesses of layers other than the surface layer (surface layers on both sides) The sum of the thicknesses/the thickness of the layers other than the surface layer is 1/9 to 1/2.

When the thickness of the surface layer is thin, and the ratio of the sum of the thicknesses of the surface layers on both sides to the sum of the thicknesses of the layers other than the surface layer is less than 1/9, the film forming property by the laminate is not improved, and the mechanical properties are improved. The effect of the situation. On the other hand, when the thickness of the surface layer is thicker and the ratio of the sum of the thicknesses of the surface layers on both sides to the sum of the thicknesses of the layers other than the surface layer exceeds 1/2, it is strongly affected by the alignment of the surface layer, and the inner layer is excessive. When the ground is stretched, the film formability is deteriorated.

Since the heat shrinkage ratio is set to a more preferable range, the step (c) is also a preferred embodiment.

(c) The film obtained by the method of (a) or (b) is annealed at a heat treatment temperature Th1 (° C.) satisfying the following formula (iv) at a time of from 70 seconds to 600 seconds. As a method of performing the annealing treatment, a method of performing heat treatment (off-annealing) by using an oven provided between the film take-up roll and the film take-up roll is exemplified.

(iv) 120 ° C ≦ Th1 (°C) ≦ Th0 (thermal fixed temperature) (°C)

When Th1 (°C) exceeds Th0 (heat setting temperature) (°C), in the step (4), the structure of the molecular chain in the immobilized film is destroyed in the step (3), and the knot is broken. As a result, there is a case where the film becomes largely shrunk and the planarity is deteriorated. On the other hand, when Th1 (°C) is less than 120 ° C, there is a case where the heat shrinkage ratio at 130 ° C cannot be set to a preferable range.

By the step (c), the stress remaining in the molecular chain constituting the film can be removed in the steps (a) and (b), and the decrease in the heat shrinkage rate and the isotropic property and the dimensional change rate can be obtained. It is an equivalent embodiment and is a preferred embodiment of the present invention. In particular, in the case of the film obtained by the method of (b), since the resin having a TmA of 250 ° C or more and 280 ° C or less is disposed on both surface layers, when heat is applied in the step (c), It is preferred that the alignment of the film is not damaged and only the heat shrinkage rate is lowered.

The thickness of the biaxially oriented polyester film of the present invention is preferably 30 μm or more and 150 μm or less. If it is less than 30 μm, it may be easily broken when it is used as a protective film, and if it exceeds 150 μm, it may be inferior in handleability. More preferably, it is 50 μm or more and 125 μm or less.

The biaxially oriented polyester film of the present invention preferably has a haze change amount (Δ haze) of 2% or less before and after treatment at 100 ° C for 12 hr. By setting the Δ haze to the above range, even when the film of the present invention is bonded to a sheet containing an amorphous resin, the sheet containing the amorphous resin can be visually recognized by the film of the present invention. Therefore, it is better. As a factor for increasing the Δ haze, it is conceivable that the amorphous portion of the film is coarsely crystallized by heating or the oligomer is deposited on the surface of the film by heating. By using the former, the film is given an alignment, fn is set to 0.111 or more, or in the latter, the film is formed into a laminate, and the oligomer is formed by solid phase polymerization of the resin constituting the outermost layer. The resin having a reduced content lowers the precipitation of the oligomer, whereby the Δ haze can be reduced.

The biaxial alignment polyester film of the present invention obtained in the above manner is In terms of mechanical properties and workability, since the value of the dimensional change rate at a temperature lowering from 150 ° C to 50 ° C is close to that of a film containing an amorphous resin, it can be preferably used for bonding to a film containing an amorphous resin. use. In particular, it can be preferably used as a protective film for a film comprising COP. Further, since it is excellent in transparency even when heated, it can be preferably used as a protective film for a COP film for film formation of a transparent conductive film.

[Method of evaluation of characteristics] A. Film, melting point of the resin constituting each layer (Tm, TmA, TmB) (°C)

The sample was scanned using the differential scanning calorimeter "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd. according to the method of JIS K 7121 (1999), and the Disk Session "SSC/5200" was used for data analysis. The measurement was carried out in the following manner.

The sample was weighed 5 mg each in the sample pan, and the sample was heated from 25 ° C to 300 ° C (1stRUN) at a temperature increase rate of 20 ° C / min, and kept in this state for 5 minutes, and then became 25 ° C or less. Quenching. Immediately thereafter, the temperature was further raised from 25 ° C to 300 ° C at a temperature increase rate of 20 ° C /min, and a differential scanning calorimetry chart of 2nd RUN was obtained (the vertical axis was set as heat energy and the horizontal axis was set as temperature). In the 2ndRUN differential scanning calorimetry chart, the temperature at the peak top of the crystal melting peak which is the endothermic peak was determined, and this was defined as the melting point (° C.). When two or more crystal melting peaks are observed, the temperature at the peak top where the peak area is the largest is taken as the melting point.

In the case of measuring the melting point of the resin constituting each layer of the laminated polyester film, only the resin constituting each layer was cut off from the laminated polyester film using a microtome for measurement.

B. Thin film, crystal melting heat of the resin constituting each layer (ΔHm, ΔHmA, ΔHmB) (J/g)

The sample was subjected to the Disk Scan "SSC/5200" for data analysis by using the differential scanning calorimeter "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd. based on JIS K 7121 (1999). The method is to carry out the measurement.

The sample was weighed 5 mg each in the sample pan, and the sample was heated from 25 ° C to 300 ° C (1stRUN) at a temperature increase rate of 20 ° C / min, and kept in this state for 5 minutes, and then became 25 ° C or less. The way to quench. Immediately thereafter, the temperature was further raised from 25 ° C to 300 ° C at a temperature increase rate of 20 ° C /min, and a differential scanning calorimetry chart of 2nd RUN was obtained (the vertical axis was set as heat energy and the horizontal axis was set as temperature). In the 2ndRUN differential scanning calorimetry chart, the peak area of the endothermic peak was determined and the crystal melting heat was determined. When two or more crystal melting peaks are observed, the area of the peak having the highest temperature is the heat of crystal melting, and when two or more peaks cannot be separated, the two peaks are combined to obtain a peak area.

In the case of measuring the heat of crystal melting of the resin constituting each layer of the laminated polyester film, only the resin constituting each layer was shaved by a slicer self-laminated polyester film for measurement.

C. Thin film, glass transition temperature (Tg, TgA) of the resin forming the outermost layer ((°C)

Based on JIS K 7121 (1999), the differential scanning calorimeter "Robot DSC-RDC220" manufactured by Seiko Electronics Co., Ltd. was used, and Disk Session "SSC/5200" was used for data analysis, and the measurement was carried out according to the following method. .

The sample was weighed 5 mg each in the sample pan, and the sample was heated from 25 ° C to 300 ° C (1stRUN) at a temperature increase rate of 20 ° C / min, and kept in this state for 5 minutes, and then became 25 ° C or less. The way to quench. Immediately thereafter, the temperature was again raised from 25 ° C to 300 ° C at a temperature increase rate of 20 ° C / minute, and 2nd RUN was obtained. The differential scanning calorimetry map (the vertical axis is set to thermal energy and the horizontal axis is set to temperature). In the 2ndRUN differential scanning calorimetry diagram, in the stepwise change of the glass transition, the curve of the straight line extending from each baseline in the longitudinal direction and the stepwise change of the glass transition in the longitudinal direction Find the point of intersection. In the case where a stepwise change of two or more glass transitions is observed, the glass transition temperature is determined for the above, and the average value of the glass transition temperature is determined as the glass transition temperature (Tg) of the sample. ) (°C).

In the case of measuring the glass transition temperature of the resin constituting the outermost layer of the laminated polyester film, only the resin constituting the outermost layer was peeled off from the laminated polyester film using a microtome for measurement.

D. Film face alignment coefficient (fn)

The refractive index at 20 ° C was determined based on JIS K 7105 (1999) using an Abbe refractometer manufactured by Atago. The refractive index (Nmd) in the longitudinal direction, the refractive index (Nd) in the width direction, and the refractive index (Nz) in the thickness direction of the surface of the film were measured, and the surface alignment coefficient (fn) was calculated. The measurement was carried out at n=5, and the average value was calculated.

(v)fn=(Nmd ten Ntd)/2-Nz

E. Thermal shrinkage of film (%)

The heat shrinkage rate of the film was measured based on JIS C 2318 (1997). The film was cut into a strip having a width of 10 mm and a length of 150 mm. The measurement was carried out by attaching a reticle to the film and measuring the length of the reticle at 23 ° C in such a manner that the length measurement portion became approximately 100 mm, and set it as L0. Thereafter, a 2 g weight was placed in a hot air oven heated to a specific temperature (200 ° C or 220 ° C) to lift the film, and left for 30 minutes. After the film was taken out from the oven and cooled to 23 ° C, the length of the mark was measured and set to L1. By the following formula (vi) The shrinkage rate of the film. The measurement was carried out by randomly cutting out five places so that the longitudinal direction of the film or the film width direction became 150 mm. The average value was calculated for each of the longitudinal direction and the width direction, and the thermal contraction rate of the film was set.

(vi) (film heat shrinkage rate) = (L0-L1) / L0 × 100

F. Thickness of film (μm)

The thickness of the film was measured using a dial gauge according to JIS K7130 (1992) A-2, and the thickness was measured at any five places in a state in which the film was overlapped by 10 sheets. The average value was divided by 10 to set the film thickness.

G. Thickness of each layer of laminated polyester film (μm)

In the case where the film is a laminated film, the thickness of each layer is determined by the following method. The cross section of the film was cut out by a microtome in a direction parallel to the width direction of the film. This section was observed by a scanning electron microscope at a magnification of 5000 times, and the thickness ratio of each layer of the laminate was determined. The thickness of each layer was calculated from the obtained laminate ratio and the film thickness.

H. Film formation

The number of times the film was broken in 1 hour in the film formation was counted, and the number of times when the film was less than once was set to A, and the number of times of one time or less was set to B, and the time of 5 or more times was set to C. A is the best film forming property, and C is the worst.

I. Dimensional change rate (ppm/°C) from 150°C to 50°C

Based on JIS K7197 (1991), using a thermomechanical measuring device TMA/SS6000 (manufactured by Seiko Instruments Co., Ltd.), the sample width was set to 4 mm, and the length of the sample was used. (Distance between chucks) A sample of 20 mm with a load of 3 g. The temperature was raised from room temperature to 160 ° C at a temperature increase rate of 10 ° C /min for 10 minutes, and thereafter, the temperature was lowered to 20 ° C at 10 ° C / min to obtain the value of the sample at each temperature (° C.). The size L (150 ° C) (mm) of the sample at 150 ° C and the size L (50 ° C) (mm) of the sample at 50 ° C were calculated according to the following formula (vii). Further, the dimensional change rate was carried out in the film width direction (TD) and the direction orthogonal thereto (MD) at n = 5, and the average value was calculated.

(vii) Dimensional change rate (ppm/°C) = 10 ⁶ × (L (150 ° C) - L (50 ° C))) / {20 × (150-50)}

J. △ Haze (100 ° C, haze change before and after 12 hr treatment) (%)

The film was cut into a square shape of 10 cm on one side, and a haze meter NDH-5000 manufactured by Nippon Denshoku Co., Ltd. was used, and the haze at three places was randomly measured, and the average value was calculated, and the haze before the test was set to H0. (%). The sample was fixed in an oven made by Taba Espeke (stock) in a room maintained at 23 ° C, 65% RH, and fixed at 4 ° C and below 10 ° RH. The heat treatment was carried out for 12 hours under dry heat. The haze of the film after the heat treatment was measured in the same manner, and H1 (%) was determined. The Δ haze (ΔH) was determined according to the following formula (viii).

(viii) △ haze (%) = H1-H0

The judgment was made in the following manner by the value of Δ haze.

A: △ haze 1.5% or less

B: △ haze exceeds 1.5% and below 2.0%

C: △ haze exceeds 2.0%

A is the best, C is the worst.

K. Processability

The workability is determined by the value of the elongation at break (%) in the direction of the width direction (TD) of the film and the direction in which it is a right angle (MD) by n5, and is determined as follows based on the average value of the film.

A: elongation at break is 120% or more

B: elongation at break is 105% or more and less than 120%

C: elongation at break is more than 90% and less than 105%

D: elongation at break is 75% or more and less than 90%

E: elongation at break is less than 75%

A is the best, E is the worst.

The elongation at break retention rate was measured in the following manner. The film was cut into a size of 1 cm × 15 cm in such a manner that the long side of the film MD TD was parallel, and the film was measured at a length of 5 cm between the chucks and a tensile speed of 300 mm/min based on ASTM-D882 (1997). Elongation at break. In addition, the number of samples was set to n=5, and after measuring the longitudinal direction and the width direction of the film, the average value of the film was determined, and the elongation at break of the film was determined.

L. Evaluation of adhesion to COP film

The film of the present invention was cut into a size of 20 cm × 20 cm, and bonded to a COP film to form a laminate, which was placed in an oven at 120 ° C and allowed to stand for 1 hour. Thereafter, the temperature of the oven was cooled to room temperature at a rate of 20 ° C / minute. Then, the number of wrinkles having a length of 3 cm or more in the laminate obtained by laminating the film of the present invention and the COP film was measured in the following manner. The evaluation was carried out at n=5, and evaluation was performed based on the average value thereof.

Less than 4: S

4 or more and less than 10: A

10 or more and less than 16: B

More than 16: C

S is the best, C is the worst.

As the COP film, a "ZEONOR ZF14" manufactured by Zeon Corporation of Japan and a film having a thickness of 40 μm were used. At the time of bonding, a "Rheocoat" R5000 manufactured by Toray Coatex Co., Ltd., which is an adhesive, was used to adjust the amount of the adhesive to 15%, and Toray was added to 100 parts by mass of the toluene solution. 3 parts by mass of the cross-linking agent "Coronate L" manufactured by Coatex Co., Ltd. was applied in such a manner that the coating thickness after drying was 10 μm.

M. Curl between laminates of COP films

The laminate produced in the item L. was placed in an oven at 120 ° C and allowed to stand for 1 hour. Thereafter, the temperature of the oven was cooled to room temperature at a rate of 20 ° C / min, and allowed to stand for 1 hour. Thereafter, the film was placed on a horizontal surface, and the COP film was placed on the upper side, and the amount of floating from the surface of the four corners of the laminate was measured, and the average value was determined as the amount of curl (mm) as follows. The way to make a decision. In the case where the four corners of the horizontal layer body are not floated by the above method, the amount of curl is set to 0 mm.

0mm or more and less than 10mm: A

10mm or more and less than 25mm: B

25mm or more and less than 40mm: C

40mm or more and less than 55mm: D

55mm or more: E.

Further, in the above measurement, in the case of the longitudinal direction or the width direction of the film which is not measured, the direction having the largest refractive index in the film is regarded as the long side direction and will be parallel to the direction of the long side direction. Consider the width direction. Further, the direction of the largest refractive index in the film can be obtained by measuring the refractive index of all the directions of the film by a refractometer, and determining the direction of the slow axis by using a phase difference measuring device (birefringence measuring device) or the like. Out.

[Examples]

Hereinafter, the present invention will be described by way of examples, but the present invention is not necessarily limited thereto.

[Production of PET-1] From the terephthalic acid and ethylene glycol, the antimony trioxide was used as a catalyst, and polymerization was carried out by a usual method to obtain a melt-polymerized PET. The obtained melt-polymerized PET had a glass transition temperature of 80 ° C, a melting point of 255 ° C, and an intrinsic viscosity of 0.62.

[Production of PET-2] PET-1 was obtained by solid phase polymerization of PET-1 by a usual method. The obtained PET-A had a glass transition temperature of 82 ° C, a melting point of 255 ° C, and an intrinsic viscosity of 0.85.

[Production of PET-A] terephthalic acid, isophthalic acid, and ethylene glycol, using antimony trioxide as a catalyst, and the amount of isophthalic acid copolymerization is 7 mol% based on the total amount of the dicarboxylic acid component. The method was carried out by a usual method to obtain a copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 77 ° C, a melting point of 243 ° C, and an intrinsic viscosity of 0.62.

[Production of PET-B] terephthalic acid, isophthalic acid, and ethylene glycol, using antimony trioxide as a catalyst, and the amount of isophthalic acid copolymerization is 10 mol% based on the total amount of the dicarboxylic acid component. The method was carried out by a usual method to obtain a copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 76 ° C, a melting point of 235 ° C, and an intrinsic viscosity of 0.62.

[Manufacture of PET-C] from terephthalic acid, isophthalic acid and ethylene glycol to trioxane As a catalyst, the amount of the isophthalic acid copolymerization amount is 15 mol% with respect to the total amount of the dicarboxylic acid component, and polymerization is carried out by a usual method to obtain a copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 74 ° C, a melting point of 230 ° C, and an intrinsic viscosity of 0.62.

[Production of PET-D] terephthalic acid, isophthalic acid, and ethylene glycol, using antimony trioxide as a catalyst, and the amount of isophthalic acid copolymerization is 20 mol% based on the total amount of the dicarboxylic acid component. The method was carried out by a usual method to obtain a copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 73 ° C, a melting point of 220 ° C, and an intrinsic viscosity of 0.62.

[Production of PET-E] terephthalic acid, isophthalic acid, and ethylene glycol, using antimony trioxide as a catalyst, and the amount of isophthalic acid copolymerization is 25 mol% based on the total amount of the dicarboxylic acid component. The method was carried out by a usual method to obtain a copolymerized PET. The glass transition temperature of the obtained copolymerized PET was 70 ° C, and no melting point was observed. The intrinsic viscosity is 0.62.

[Manufacture of PET-F] from terephthalic acid, cyclohexanedimethanol (CHDM) and ethylene glycol, using antimony trioxide as a catalyst, and the amount of cyclohexane dimethanol copolymerization relative to the total amount of diol components The method of becoming 10 mol% was carried out by a usual method to obtain a copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 72 ° C, a melting point of 235 ° C, and an intrinsic viscosity of 0.62.

[Manufacture of PET-G] From terephthalic acid, cyclohexanedimethanol (CHDM) and ethylene glycol, with antimony trioxide as a catalyst, the amount of cyclohexane dimethanol copolymerization relative to the total amount of diol components In a manner of 20 mol%, polymerization was carried out by a usual method to obtain a copolymerized PET. The obtained copolymerized PET had a glass transition temperature of 70 ° C, a melting point of 221 ° C, and an intrinsic viscosity of 0.62. (Example 1)

The PET-A was vacuum dried at 160 ° C for 2 hours and then put into an extruder for extrusion. The inside of the machine was melted and extruded onto a casting drum having a surface temperature of 25 ° C to prepare an unstretched sheet. Then, the sheet was preheated by a heated roll group, and then extended at a temperature of 90 ° C in a direction perpendicular to the width direction (MD direction) by 3.1 times, and then subjected to a roll group at a temperature of 25 ° C. The uniaxially stretched film was obtained by cooling. The both ends of the obtained uniaxially stretched film were stretched by 3.6 times in the film width direction (TD direction) by the holding portion of the uniaxially stretched film which was gripped by the jig at a temperature of 100 ° C in the tenter. Further, heat treatment was carried out for 10 seconds at a temperature of 210 ° C in the heat treatment zone in the tenter. In the step of heat setting, a 2% relaxation treatment was carried out in the film width direction. Then, after uniformly cooling in the cooling zone, the coiling was carried out to obtain a biaxially oriented polyester film. The properties of the obtained biaxial alignment polyester film are shown in the table. The dimensional change rate is 50 ppm/° C. or more and 130 ppm/° C. or less in the MD direction and the TD direction, and is a film excellent in adhesion to COP. Further, it is a film which is excellent in workability and has a small change in haze due to heating.

(Examples 2-5, Comparative Examples 1, 2)

A biaxially oriented polyester film was obtained in the same manner as in Example 1 except that the resin constituting the film was changed as described above. The properties of the biaxially oriented polyester film are shown in the table. In Examples 2 to 5, the film was fn, the dimensional change rate was a preferable range, and the workability was excellent, and the haze change due to heating was also small. In Comparative Example 1, the crystallinity of the resin was high and ΔHm was large, and as a result, the film fn was increased and the dimensional change rate was small. In Comparative Example 2, the ΔHm was not observed, and the crystallinity of the resin was lower. Therefore, the film having a small fn and a small dimensional change ratio was obtained, and the film was inferior to the COP. Further, since fn is small, it is a film which is inferior in workability and which has a large change in haze due to heating.

(Example 6)

In the case of the resin constituting the surface layer, PET-2 was set to 100 parts by mass, and vacuum-dried at 160 ° C for 2 hours, and then it was put into the extruder 1. Further, 100 parts by mass of PET-A as a resin constituting the inner layer was vacuum-dried at 160 ° C for 2 hours, and then introduced into the extruder 2. The raw materials were melted in an extruder, and the mixture was introduced into the two surface layers of the film by a merging device, and the mixture was extruded to a casting drum having a surface temperature of 25 ° C to prepare A three-layer laminated sheet. Then, the sheet was preheated by a heated roll group, and then stretched 3.1 times in the longitudinal direction (MD direction) at a temperature of 90 ° C, and then cooled by a roll group having a temperature of 25 ° C to obtain a single sheet. Axial extension film. The both ends of the obtained uniaxially stretched film were stretched by a clamp at a temperature of 100 ° C in the tenter at a temperature of 100 ° C in the longitudinal direction to extend 3.6 times in the width direction (TD direction) of the right angle. Further, heat treatment was carried out for 10 seconds at a temperature of 210 ° C in the heat treatment zone in the tenter. Then, after uniformly cooling in the cooling zone, the coiling was carried out to obtain a laminated polyester film. The properties of the film are shown in the table. The dimensional change rate is 50 ppm/° C. or more and 130 ppm/° C. or less in the MD direction and the TD direction, and is a film excellent in adhesion to COP. Further, it is a film which is excellent in workability and has a small change in haze due to heating. It is understood that by using PET-2 as the surface layer, it is possible to obtain a film which is more excellent in workability and which has a small change in haze due to heating.

(Examples 7-21)

Film formation was carried out in the same manner as in Example 6 except that the composition of the resin and the film formation conditions were changed as in the table. The properties of the film are shown in the table. Dimensional change rate is MD Both the direction and the TD direction are 50 ppm/° C. or more and 130 ppm/° C. or less, and are excellent films for bonding with COP. Further, it is a film which is excellent in workability and has a small change in haze due to heating.

(Examples 22-24)

The film obtained in Example 6 was used in Example 22, the film obtained in Example 7 was used in Example 23, and the film obtained in Example 8 was used in Example 24, respectively. The film was subjected to an annealing treatment at a temperature of 140 ° C at a temperature of 140 ° C for 5 minutes to obtain a film having a thickness of 125 μm by a hot air oven disposed between the film take-up roll and the film take-up roll. The properties of the film are shown in the table. The dimensional change rate is a film having a small heat shrinkage rate of 130 ppm and 30 minutes in the MD direction and the TD direction of 50 ppm/° C. or more and 130 ppm/° C. or less, and is a film excellent in adhesion to COP. Further, it is a film which is excellent in workability and has a small change in haze due to heating. Further, the average value of the heat shrinkage ratio in the MD direction, the TD direction, and the 45° direction is 0.5% or less, and the absolute value of the difference in the heat shrinkage ratio is also 0.5% or less, and the difference in dimensional change ratio between the directions is The absolute value is also 10 or less, and the curling property of the laminate with COP is also good.

(Comparative Example 3-7)

Film formation was carried out in the same manner as in Example 6 except that the composition of the resin and the film formation conditions were changed as in the table. The properties of the film are shown in the table. In Comparative Examples 3 and 7, the crystallinity of the resin was high and ΔHmB was large, and as a result, the film had a large fn and a small dimensional change ratio. In Comparative Example 4, since ΔHm was not observed and the crystallinity was low, the film having a small fn and a small dimensional change ratio was obtained. Furthermore, due to fn It is small, so it is a film with poor processability and a large change in haze caused by heating. In Comparative Examples 5 and 6, the heat treatment temperature in the film formation was high, and the alignment of the film was disordered, and as a result, the fn was extremely lowered, the elongation at break was lowered, and not only the processability was poor, but also the film having a large ? .

(Example 26)

Film formation was carried out in the same manner as in Example 6 except that the composition of the resin and the film formation conditions were changed as in the table. The film properties are shown in the table. The dimensional change rate is 50 ppm/° C. or more and 130 ppm/° C. or less in the MD direction and the TD direction, and is a film excellent in adhesion to COP. Further, it is a film which is excellent in workability and has a small change in haze due to heating.

(Embodiment 25, 27-36)

Example 25 uses the film of Example 2, Example 27 uses the film of Example 26, Example 28 uses the film of Example 9, Example 29 uses the film of Example 11, and Example 30 is used. The film of Example 12 was used in Example 31, the film of Example 14 was used, the film of Example 15 was used in Example 32, the film of Example 16 was used in Example 33, and the film of Example 18 was used in Example 34. Example 35 uses the film of Example 19, and the film of Example 36 uses the film of Example 20, and the film obtained by respectively is placed in a hot air oven between the film take-up roll and the film take-up roll, at 140 The annealing treatment was carried out at a temperature of ° C so that the heat treatment time of the film was 5 minutes.

In Examples 29, 30, 31, and 34 to 36, the average heat shrinkage ratio in the MD direction, the TD direction, and the 45° direction was 0.5% or less, and the heat shrinkage ratios were the same. The absolute value of the difference is also 0.5% or less, and the absolute value of the difference in dimensional change ratio between the directions is also 10 or less, and the curling property of the laminate with COP is also good.

In Example 25, since it was a single film, the heat shrinkage rate could not be lowered, and although the curling property was slightly inferior, it was a level which was not problematic in practical use. In Example 27, since the copolymerization synthesis was divided into several types, the heat shrinkage ratio could not be lowered, and although the curling property was slightly inferior, it was a level which was not problematic in practical use. In Example 28, since fn was small and amorphous, the heat shrinkage ratio could not be lowered, and although the curling property was slightly inferior, it was a level at which there was no problem in practical use. In Examples 32 and 33, the difference in the dimensional change rate in each direction was large, and although the curling property was slightly inferior, it was a level at which there was no problem in practical use.

(industrial availability)

The polyester film of the present invention is excellent in mechanical properties and workability, and the dimensional change rate at the time of cooling from 150 ° C to 50 ° C is close to that of the film containing the amorphous resin, so that it can be preferably used for bonding to include amorphous. The use of a film of resin. Further, since it is excellent in transparency even when heated, it is particularly preferably used as a protective film for a COP film for forming a film of a transparent conductive film.

Claims (11)

A biaxially oriented polyester film having a dimensional change rate of 50 ppm/° C. or more in a film width direction (TD direction) and a direction from a right angle (MD direction) from 150° C. to 50° C. 130 ppm/° C. or less, and the surface alignment coefficient (fn) is 0.111 or more and 0.145 or less. A biaxially oriented polyester film having a dimensional change rate of 50 ppm/° C. or more in a film width direction (TD direction) and a direction from a right angle (MD direction) from 150° C. to 50° C. 130 ppm / ° C or less, and the heat shrinkage rate at 130 ° C and 30 minutes in the film width direction (TD direction) and the direction (MD direction) which is a right angle are respectively 1.0% or less. The two-axis alignment polyester film of claim 1 or 2, wherein the surface alignment coefficient (fn) is 0.120 or more and 0.140 or less. The two-axis alignment polyester film according to any one of claims 1 to 3, wherein the film width direction (TD direction), a direction perpendicular thereto (MD direction), and a film formation direction of 45° When the heat shrinkage ratio at 130 ° C for 30 minutes is compared in each direction, the absolute value of the difference is 0% or more and 0.5% or less, and the average value thereof is 0.5% or less; When the dimensional change ratio in the width direction (TD direction), the direction in which it is a right angle (MD direction), and the direction in which the film width direction is 45° from 150 ° C to 50 ° C is compared in each direction, The absolute value of the difference is 0 ppm/° C. or more and 10 ppm/° C. or less. The two-axis alignment polyester film according to any one of claims 1 to 4, wherein the polyester resin constituting the polyester film has a heat of crystal melting of 30 J/g or less. The two-axis alignment polyester film according to any one of claims 1 to 5, wherein the above-mentioned poly The ester film is a laminated polyester film comprising at least three layers, and the polyester resin constituting the surface layers on both sides of the film has a crystal heat of fusion (ΔHmA) of 30 J/g or more, and constitutes a layer other than the surface layer on both sides of the film. The crystal melting heat (?HmB) of the polyester resin is 30 J/g or less. The two-axis oriented polyester film of claim 6, wherein the polyester resin constituting the layer other than the surface layer on both sides of the film is a resin containing terephthalic acid and ethylene glycol as main constituents, and is other than The constituent unit contains only one or only two kinds of isophthalic acid and cyclohexyl dimethanol. The two-axis alignment polyester film according to any one of claims 1 to 5, wherein the polyester film is a laminated polyester film comprising at least 3 layers, and a melting point TmA of a polyester resin constituting a surface layer on both sides of the film Both are above 250 ° C and below 280 ° C. The biaxially oriented polyester film according to any one of claims 6 to 8, wherein the ratio of the sum of the thicknesses of the surface layers on both sides of the polyester film to the sum of the thicknesses of the layers other than the surface layer (the surface layer on both sides) The sum of the thicknesses/the thickness of the layers other than the surface layer is 1/9 to 1/2. The biaxially oriented polyester film according to any one of claims 1 to 9, which is used for bonding to a film comprising an amorphous resin. A biaxially oriented polyester film according to any one of claims 1 to 10 for use in bonding to a film comprising a cyclic olefin polymer (COP).