JP2002172694A - Biaxially oriented polyester film and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film which shows a dramatically slight positional dependence of an orientation angle, as an indication of a conventional bowing phenomenon, in the center and at the end parts in the width direction of the film as well as a manufacturing method therefor. SOLUTION: During heating by a heat machine characteristics testing machine (hereafter called 'TMC') of the biaxially oriented film made of a polyester, the biaxially oriented polyester film has a time differential curwe of the expansion/contraction amount, in the width direction, of the film which shows a trace amount of a film expansion within the range of ±5 deg.C of expansion temperature (Te) in the width direction and 3% or less of a factor of heat shrinkage at 150 deg.C for 10 minutes in the longer and width directions of the film.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-bowing biaxially oriented polyester film having uniform physical properties in the film width direction and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, biaxially oriented polyester films have excellent mechanical properties, heat resistance, electrical properties, chemical resistance, and weather resistance, and have been used in various industrial fields. I have. As a typical film forming method for producing these characteristics, a sequential biaxial stretching method using a tenter is used. This is a film forming method in which the film is usually stretched in the longitudinal direction of the film, then stretched in the width direction in a tenter, and then heat-set.

In the process of heat-setting after stretching in the width direction in this tenter, a bowing phenomenon that does not have uniform physical properties in the film width direction has occurred. When the film is stretched and heat-set in a tenter,
In the longitudinal direction of the film, it is considered to occur due to shrinkage force and heat shrinkage stress based on Poisson's ratio.As a phenomenon, a straight line drawn with magic ink in the film width direction before the tenter, after the tenter, The behavior is such that the film comes out in a shape of being pulled back in a bow shape in the longitudinal direction of the film. Due to the bowing phenomenon, the orientation angle (here, the orientation angle is the smaller of the angles formed by the film width direction or the longitudinal direction and the main orientation axis) increases as the distance from the central portion in the film width direction increases. Position dependence of the orientation angle. Due to the position dependence of the orientation angle, there is a disadvantage that the strength and the dimensional stability are different in the width direction. For example, misalignment during printing,
The meandering, curling, and base film of a floppy (registered trademark) disk are caused by a decrease in recording characteristics due to warpage or the like in the apparatus. In particular, in the recent optoelectronics industry, the bowing phenomenon of a biaxially oriented polyester film is the most to be solved in order to be used as an optical member.

As a typical example, a biaxially oriented polyester film is currently used as a release film at the time of inspection of a polarizing plate. The polarizing plate usually includes a polarizing film 1, a surface protective film 2, an adhesive layer 3, and a release film 4, as shown in FIG. The polarizing film 1 is formed by coating a polarizer having a polarizing axis in which a polarizing element such as iodine or a dichroic dye or the like is adsorbed and oriented on a hydrophilic film such as a polyvinyl alcohol film with a cellulose film from above and below. Alternatively, it is provided by coating an acrylic resin. As the surface protective film 2, a transparent plastic film, such as a polyester film, having low moisture permeability and little deformation such as elongation is used. The surface protective film 2 and the polarizing film 1 are adhered with an adhesive (not shown).
Is strongly adhered to the polarizing film 1, but can be easily released from the polarizing film 1 even after a long time. The adhesive layer 3
The release film 4 is made of a biaxially oriented polyester film or the like, and the release film 4 is made of a pressure-sensitive adhesive for adhering the polarizing film 1 to a liquid crystal cell (not shown).

In manufacturing the polarizing plate, optical characteristics such as light transmittance, degree of polarization and haze of the polarizing film 1 as a raw material are inspected and used in advance, but in the manufacturing process for the polarizing plate. Mechanical stress on the polarizing film, foreign matter contamination,
Defects may occur due to adhesion or the like. For this reason, the cross Nicol method (2
A method of observing transmitted light in a state where the absorption axes of the polarizing plates are orthogonal to each other and a release film is sandwiched between the polarizing plates, is performed. However, the conventional polarizing plate has a lot of light leakage and low testability due to the optical anisotropy due to the bowing of the biaxially oriented polyester film used as the release film. In the low-boiling release film used,
Due to the large heat shrinkage when the solvent in the pressure-sensitive adhesive is removed by heating, the flatness is poor, and problems such as the inability to bond to the polarizing film arise.

The case where the above-mentioned film having a large influence of the bowing phenomenon is applied to a release film of a polarizing plate will be described in more detail. In a polarizing plate maker, the absorption axis and the polarization axis of the polarizer are laminated and attached in the longitudinal direction and the width direction of the film, respectively, so that the positional relationship between the main alignment axis and the polarization axis is uniquely determined. The inspectability of the polarizing plate is determined by the degree of the orientation angle, which is an index of the positional relationship. Specifically, when a visual inspection is performed by the crossed Nicols method, there is no problem in application to the application because the orientation angle is small at the center in the film width direction, but the hue is large at the edges because the orientation angle is large. A change and a light transmission phenomenon occur, and there is a disadvantage that it is difficult to accurately inspect a polarizing film for inclusion of foreign matter or a defect.

A countermeasure for the bowing phenomenon that causes this problem has been studied conventionally, and will be described below.

[0008] For example, Japanese Patent Publication No. 39-29214 proposes a heat treatment method using a heating roll.
In addition, Japanese Patent Publication No. 42-9273 and Japanese Patent Application Laid-Open No. 7-3
No. 14552 discloses a method of performing heat treatment while giving a temperature gradient in the film width direction, and JP-A Nos. 62-18327 and 62-183328 disclose a method of forcibly heating both ends of a film. Heat treatment methods have been proposed.

Also, Japanese Patent Application Laid-Open No. 50-73978 proposes a method of heat-treating a film after width stretching by a nip roll between a stretching step and a heat treatment step in a width stretching machine (tenter). Japanese Patent Publication No. 63-24459 proposes a method of forcibly advancing a central portion of a film by a nip roll.

[0010] Japanese Patent No. 2936688 and Japanese Patent Application Laid-Open No. 6-262675 propose an apparatus in which a cooling step is provided between the width stretching step and the heat treatment step.

Further, Japanese Patent Application Laid-Open No. Sho 62-43856 discloses that after stretching in the transverse direction, it is cooled to a temperature below the glass point and then heat-treated at T1 (200 ° C. to 240 ° C.) in the first heat treatment section.
1 to 20% in T2 (T1 or lower temperature) in the second heat treatment section
T3 (T2
(A temperature less than). Also,
As a similar example, JP-A-1-165423 discloses that
After the film is transversely stretched with a tenter, the film is cooled and held at a temperature equal to or lower than the transverse stretching temperature, and subsequently heated in a temperature region divided into two or more while stretching in the width direction by 2 to 20%, and then heat-fixed. A method has been proposed. It is considered as a method having a bowing reduction effect described above, a cooling step after transverse stretching, a stepwise temperature increase in a heat treatment step,
Japanese Patent No. 2825727 and Patent No. 2825727 all incorporate all manufacturing methods such as transverse redrawing in a heat treatment step.
No. 25728. However, the effects of these manufacturing methods are still insufficient to be used as optoelectronic components in the future.

[0012]

As described above, it is desired to reduce bowing.
No film satisfying low bowing and thermal dimensional stability required for a release film for a polarizing plate was obtained. In view of such circumstances, the present invention provides a biaxially oriented polyester film and a method for producing the biaxially oriented polyester film, in which the position dependence of the orientation angle at the center and the end in the film width direction, which is an indicator of the bowing phenomenon, is dramatically less than before. To provide.

[0013]

That is, the object of the present invention is to provide a biaxially oriented film made of polyester which expands and contracts in the film width direction when the temperature is raised by a thermomechanical property tester (hereinafter referred to as TMA). The derivative curve of the amount of time has a minimum value which is in the range of ± 5 ° C. of the stretching temperature (Te) in the width direction and the heat in the longitudinal direction and the width direction of the film at 150 ° C. for 10 minutes. This is achieved by a biaxially oriented polyester film, wherein the shrinkage is 3% or less.

Further, a method for producing a biaxially oriented polyester film for achieving the above object of the present invention is a method for producing a biaxially oriented polyester film in which an unstretched film is sequentially stretched in a longitudinal direction and a width direction and then heat-set. A glass transition temperature (T) between stretching in the width direction and heat setting.
g) Cooling is performed at the following temperature, and then, at the highest temperature of the heat setting temperature, the redrawing ratio defined by the following formula (1) is 5 to 3 in the width direction.
A method for producing a biaxially oriented polyester film, characterized in that restretching is given at 0%. Redrawing ratio = (maximum rail width−rail width after cooling) / rail width after cooling × 100 Equation (1)

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

The polyester used in the polyester film of the present invention is a polyester containing an aromatic dicarboxylic acid or an aliphatic dicarboxylic acid and a diol as main components. Here, as the aromatic dicarboxylic acid, for example, terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyl Dicarboxylic acid, 4,4'-diphenyl ether dicarboxylic acid,
4,4'-diphenylsulfonedicarboxylic acid and the like can be mentioned. Examples of the aliphatic dicarboxylic acid include adipic acid, suberic acid, sebacic acid, dodecanedioic acid and the like. Among them, terephthalic acid and isophthalic acid are preferred.

These acid components may be used alone.
Two or more kinds may be used in combination, and further, an oxyacid such as hydroxybenzoic acid may be partially copolymerized. As the diol component, for example, ethylene glycol, 1,2-
Propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,
Examples include 2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2-bis (4-hydroxyethoxyphenyl) propane, and the like. it can. Among them, ethylene glycol is preferably used. These diol components may be used alone or in combination of two or more.

The polyester used in the polyester film of the present invention is preferably polyethylene terephthalate, a copolymer of ethylene terephthalate and ethylene isophthalate, polybutylene terephthalate and its copolymer, polybutylene naphthalate and its copolymer, Furthermore, polyhexamethylene terephthalate and its copolymer, polyhexamethylene naphthalate and its copolymer, etc. can be mentioned, and polyethylene terephthalate is particularly preferred.

The polyester in the present invention can be produced by a conventionally known method. For example, a method in which an acid component is directly esterified with a diol component, and then the product of this reaction is heated under reduced pressure to remove the excess diol component and polycondensate to produce a dialkyl acid. There is a method of using an ester, performing a transesterification reaction between the ester and a diol component, and then performing polycondensation in the same manner as described above. At this time, if necessary, a conventionally known alkali metal, alkaline earth metal, manganese, cobalt, zinc, antimony, germanium or titanium compound can be used as a reaction catalyst.

The biaxially oriented polyester film of the present invention is obtained by feeding the above molten polymer to an extruder, melt extruding it into a sheet using a T-type die, and then solidifying it by cooling on a casting drum. Can be obtained by stretching at a temperature equal to or higher than the glass transition temperature of the resin composition. The stretching method may be any method, a method of stretching in the width direction after stretching in the longitudinal direction,
A method of stretching in the width direction and then stretching in the longitudinal direction, a method of stretching in the longitudinal direction and the width direction simultaneously, or a combination of stretching in the longitudinal direction and stretching in the width direction may be performed plural times. The lower limit of the stretching ratio is preferably at least 2.5 times in the longitudinal direction from the viewpoint of suppressing thickness unevenness and bowing, and the upper limit is preferably 4 times or less from the viewpoint of suppressing bowing. In the width direction, the release film for a polarizing plate is preferably 3.5 times or more so that the main orientation axis is directed in the film width direction. The stretching temperature is preferably 95 ° C. or higher in both the longitudinal direction and the width direction from the viewpoint of suppressing bowing.

The biaxially oriented polyester film of the present invention has an amount of expansion and contraction (L) in the film width direction when the temperature is raised by a thermomechanical property tester (hereinafter referred to as TMA).
Of the time (t) (hereinafter, referred to as ΔL / Δt)
Has a minimum value of expansion within the range of ± 5 ° C. of the stretching temperature (Te) in the width direction, and has a heat shrinkage of 3% or less at 150 ° C. for 10 minutes in the longitudinal direction and the width direction of the film, respectively. There is a feature that there is.

By having such characteristics, it has excellent thermal dimensional stability and uniform physical properties in the film width direction, ie, the position dependence of the orientation angle at the center and the edge in the film width direction is dramatically increased. A low biaxially oriented polyester film is achieved. Here, the TMA is a device capable of measuring the amount of expansion and contraction of a film when a film set in an electric furnace is load-controlled with respect to time and the temperature in the furnace is temperature-controlled.

FIG. 3 shows the amount of expansion and contraction in the film width direction when a biaxially oriented polyester film generally satisfying the thermal dimensional stability is measured by TMA. The behavior will be described below. When the temperature rises at a constant rate, the film shrinks because the strain due to stretching is released from around the glass transition temperature of the polymer (thermal motion due to freezing of the amorphous chains), although it depends on the higher-order structure of the polymer. Begin to. Thereafter, when the temperature is further increased, shrinkage occurs due to a change in the conformation of the molecule, generation of a folded crystal of the molecular chain, and finally shrinkage occurs due to the melting of the microcrystals provided by heat setting.

Next, a description will be given of a curve representing the amount of expansion and contraction in the film width direction, which is a series of behaviors as shown in FIG. ΔL / Δ
Since t is released in the order of weak intermolecular force as the temperature increases with increasing temperature, t differs depending on the distribution, but usually decreases monotonically to a maximum near the heat setting temperature. In the present invention, it has been found that ΔL / Δt has a minimum value of expansion within a range of ± 5 ° C. of the stretching temperature (Te) in the width direction before reaching the maximum value. Here, the maximum value is a value in which the amount of change in shrinkage with respect to the time that appears near the heat setting temperature when the temperature is raised from room temperature 30 ° C. to 240 ° C. at 10 ° C./min is the largest, and when the test length is 20 mm. Is preferably 30 μm / min or more and 600 μm / min or less. At 30 μm / min or less, it is predicted that almost no expansion or contraction occurs, and therefore, a noise-like minimum value due to poor installation of the sample or the like appears.
If it is 600 μm / min or more, the change in the amount of expansion and contraction is too large, and the heat shrinkage at 150 ° C. for 10 minutes is expected to exceed 3%. That is, in order to obtain a biaxially oriented polyester film suitable for general industrial use, the heat shrinkage in each of the film longitudinal direction and the width direction when exposed to a temperature of 150 ° C. for 10 minutes is 3%.
It must be:

In the present invention, in the method for producing a biaxially oriented polyester film in which an unstretched film is stretched sequentially in the longitudinal direction and the width direction and then heat-set, a glass transition temperature (between stretching in the width direction and heat setting) is set. Tg) cooling at a temperature not higher than Tg, and then giving re-stretching at the highest temperature of the heat setting temperature in the width direction at a re-stretching rate of 5 to 30% defined by the following formula (1). A method for producing a polyester film is preferred. Re-stretching ratio = (maximum rail width−rail width after cooling) / rail width after cooling × 100 Expression (1) Here, the maximum rail width means a film width direction in a heat fixing zone in a tenter. The rail width after cooling is the maximum value of the distance between the clips at both ends gripped, and the rail width after cooling is the distance between the clips at both ends just before entering the heat fixing zone that is equal to or higher than the stretching temperature in the width direction. Further, it is more preferable that the re-stretching rate is 10% or more from the viewpoint of returning the bowing to a straight line. The heat setting temperature is 150 ° C. for 10 minutes, and the heat shrinkage is 3% or less in both the longitudinal direction and the width direction of the film.
C. or higher is preferred.

When the biaxially oriented polyester film of the present invention is used as a release film for a polarizing plate, which is an optoelectronic member, it is preferable to perform a release treatment from the viewpoint of releasability. The release treatment is not particularly limited, but a silicone coating treatment is preferred. Among them,
A treatment for forming a cured silicone resin coating is preferred. This cured silicone resin coating film is coated with a coating liquid containing a curable silicone resin on at least one side of the film, dried,
It can be formed by curing. Further, the biaxially oriented polyester film is preferably polyethylene terephthalate.

Further, the thickness of the biaxially oriented polyester film of the present invention is preferably 10 μm or more and 60 μm or less from the viewpoint of ease of use as a release film.

[Method of Measuring Characteristics] In the following description,
The properties of the film were measured by the following methods.

(1) Change in the amount of expansion and contraction due to TMA with respect to time: The sample was cut out from the center in the film width direction to a size of 4 mm in the longitudinal direction × 20 mm in the width direction.
With a constant load of 5 g applied, the temperature control was changed from 30 ° C to 2
TMA was heated up to 40 ° C. at a rate of 10 ° C./min. TMA (Seiko Instruments Inc.)
Manufactured by: TMA / S using EXSTAR6000 system
S6000), the length (L) that changed for each time and temperature in the width direction of 20 mm was measured. This L is time (t)
And the amount of change with respect to time (ΔL / Δ
t) was determined. A positive value of ΔL / Δt corresponds to contraction, and a negative value corresponds to expansion.

(2) Boeing situation: As a measure of the bowing, a sample cut out to a size of 4.0 × 3.5 cm long at a position of 2 m in the film width direction from the center portion was used as a sample. The measurement was performed using an automatic birefringence meter (KOBRA-21ADH manufactured by Shin-Oji Paper Co., Ltd.). In addition, the larger one of the left and right orientation angles was adopted.

(3) Thermal dimensional change rate: sample is 1 × 10
cm and cut in a gear oven (TAHP GHP)
In S-222), the length and width of the film before and after the heat treatment at 150 ° C. for 10 minutes were accurately measured with a universal projector (E04 manufactured by 77-7 Nikon Corporation).

(4) Visual inspection situation: As shown in FIG. 2, the film is arranged under crossed Nicols, and a white light source (a white light source has a spectral distribution almost entirely spread in a visible light region, and The effect of light leakage was evaluated according to the following criteria. That is, FIG.
Is a schematic explanatory view of the visual inspection of this experiment by the cross Nicol method, where 5 is a polarizer, 6 is a film, 7 is an analyzer, 8
Is a white light source, and 9 is an eye of a person to be inspected. Note that the polarizer 5 and the analyzer 7 are polarizing plates that do not use the release film and the surface protection film shown in FIG. In the actual visual inspection of the polarizing plate, the polarizing plate on the side to be inspected is obtained by laminating the polarizer 5 and the film 6 corresponding to the release film for the polarizing plate shown in FIG. 1 via an adhesive layer. . The evaluation was performed according to the following classification. Good: No light leakage. Acceptable: Light leakage but inspection possible Not possible: Light leakage and inspection impossible

[0033]

The present invention will be described in more detail with reference to the following examples.

Example 1 Polyethylene terephthalate was melted, extruded from a die, cooled and solidified by a casting drum at 25 ° C., and then first heated at a preheating temperature of 105 ° C.
A 045-fold pre-stretching was performed, and then 1.177 by a roll heated at 120 ° C. and a radiation heater.
The film is stretched 2.83 times at 116 ° C. while stretching slightly, and stretched 3.5 times in the longitudinal direction by stretching a plurality of times, followed by 4.37 times stretching at 105 ° C. in the width direction using a tenter. Then, intermediate cooling is performed at 70 ° C., and further, in the subsequent heat setting zone of the second tenter, heat setting is performed at 210 ° C. which is the highest temperature at a redrawing ratio of 7.5%, and relaxation heat setting of 4% is performed at 110 ° C. By applying, a polyester film having a thickness of 25 μm was obtained. Table 1 shows the evaluation results of the physical properties of Example 1. FIG. 4 shows the result of a differential curve of the amount of expansion and contraction by TMA over time.

Example 2 Polyethylene terephthalate was melted, extruded from a die, cooled and solidified by a casting drum at 25 ° C., and then first heated at a preheating temperature of 105 ° C.
Preliminary stretching of 045 times is performed, and then stretching is performed 2.6 times at 124 ° C while stretching slightly at 1.177 times by a roll and a radiation heater heated at 120 ° C. The film is stretched 3.2 times, then stretched 4.37 times in the width direction at 105 ° C. by a tenter, and then intermediately cooled at 70 ° C.
The film was heat-set at 210 ° C., which is the highest temperature, at a re-stretching rate of 11.7% in a heat-setting zone of a tenter, and subjected to 4% relaxation heat-setting at 110 ° C., thereby obtaining a polyester film having a thickness of 30 μm. Table 1 shows the physical property evaluation results of Example 2.
FIG. 4 shows the result of a differential curve of the amount of expansion and contraction by TMA over time.

[Comparative Example 1] Polyethylene terephthalate was melted, extruded from a die, cooled and solidified by a casting drum at 25 ° C, stretched 2.8 times in the longitudinal direction at 85 ° C, and then stretched in the width direction with a tenter. The film is stretched 3.74 times at 100 ° C. and further stepwise in the heat setting zone of the tenter.
The resultant was heat-set while sequentially increasing the temperature to 140 ° C., 180 ° C., and 230 ° C., and subjected to a 5.6% relaxation treatment to obtain a 25 μm-thick polyester film. Table 1 shows the physical property evaluation results of Comparative Example 1.
Shown in FIG. 4 shows the result of a differential curve of the amount of expansion and contraction by TMA over time.

Comparative Example 2 Polyethylene terephthalate was melted, extruded from a die, cooled and solidified by a casting drum at 25 ° C., stretched 2.8 times in the longitudinal direction at 85 ° C., and then stretched in the width direction with a tenter. The film is stretched 3.74 times at 90 ° C., and further heat-set at 140 ° C., 220 ° C. and 230 ° C. in a stepwise manner in the heat setting zone of the tenter, and subjected to a 5.6% relaxation treatment. A polyester film having a thickness of 23 μm was obtained. Table 1 shows the physical property evaluation results of Comparative Example 2. FIG. 4 shows the result of a differential curve of the amount of expansion and contraction by TMA over time.

[Comparative Example 3] Polyethylene terephthalate was melted, extruded from a die, cooled and solidified by a casting drum at 25 ° C, heated by a roll heated to 86 ° C and a radiation heater, and stretched by stretching. In the tenter, the film is stretched in the width direction in the width direction at 100 ° C. by 2.96 times, and then stretched by 110 ° C. by 1.25 times. 140, 180, 20 step by step
5, a heat treatment at 230 ° C. and finally a relaxation heat treatment of about 5% at 150 ° C. to obtain a 38 μm-thick polyester film. Table 1 shows the physical property evaluation results of Comparative Example 3. FIG. 4 shows the result of a differential curve of the amount of expansion and contraction by TMA over time.

As is clear from the results shown in Table 1 and FIG. 4, the method of the present invention has a thermal dimensional stability of less than 3% under a temperature condition of 150 ° C. under a temperature condition of 150 ° C. As a result, a low-bowing biaxially oriented polyester film could be provided.

[0040]

[Table 1]

[0041]

According to the present invention, thermal dimensional stability is excellent,
In addition, a biaxially oriented polyester film having uniform physical properties in the film width direction can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic model diagram showing a configuration of a polarizing plate.

FIG. 2 is a schematic diagram showing a visual inspection by a cross Nicol method.

FIG. 3 is a curve of the amount of expansion and contraction in the film width direction in TMA.

FIG. 4 is a diagram showing a differential curve of the amount of expansion and contraction with respect to time by TMA in the film width direction in Examples and Comparative Examples.

[Explanation of symbols]

1: Polarizing film 2: Surface protective film 3: Adhesive layer 4: Release film 5: Polarizer 6: Film 7: Analyzer 8: White light source 9: Eye to be inspected 10: In the width direction of Example 1 Differential curve of expansion and contraction amount by TMA in TMA 11: Differential curve of expansion and contraction amount by TMA in the width direction of Example 12 12: Differential curve of expansion and contraction amount by TMA in the width direction of Comparative Example 1 13: By TMA in the width direction of Comparative Example 2 Differential curve of expansion and contraction amount 14: Differential curve of expansion and contraction amount by TMA in the width direction of Comparative Example 3

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Claims (4)

[Claims] When a temperature of a biaxially oriented film made of polyester is increased by a thermomechanical property tester (hereinafter referred to as TMA), a differential curve of the amount of expansion and contraction in the film width direction is expressed by the drawing in the width direction. The film has a minimum value of expansion within a range of temperature (Te) of ± 5 ° C., and has a thermal shrinkage of 3% or less at 150 ° C. for 10 minutes in the longitudinal direction and width direction of the film. Biaxially oriented polyester film. 2. A method for producing a biaxially oriented polyester film in which an unstretched film is sequentially stretched in a longitudinal direction and a width direction and then heat-set, wherein a glass transition temperature (Tg) or lower is set between the width-direction stretching and the heat setting. A biaxially oriented polyester film characterized in that the film is re-stretched in the width direction at a maximum temperature of the heat setting temperature at a re-stretching rate of 5 to 30% as defined by the following formula (1). Production method. Redrawing ratio = (maximum rail width−rail width after cooling) / rail width after cooling × 100 Equation (1) 3. The biaxially oriented polyester film according to claim 1, which is a release film for a polarizing plate. 4. The biaxially oriented polyester film according to claim 1, wherein the polyester is polyethylene terephthalate.