JP7644602B2 - Biaxially oriented polyester film - Google Patents

Description

[CROSS REFERENCE TO RELATED APPLICATIONS]
This application claims the benefit of priority based on Korean Patent Application No. 10-2017-0156747 filed on November 22, 2017, the entire contents of which are incorporated herein by reference.

The present invention relates to a polyester resin composition and a biaxially oriented polyester film containing the same.

Recently, metal materials have been replaced with plastic materials to reduce the weight of automobiles, but the plastic materials used in such automobile parts are required to have high heat resistance and be slim to reduce volume. Among the electrical equipment (electrical devices) parts of such automobile parts, flexible flat cables (FFCs) are a type of connecting cable used to connect PCBs (Printed Circuit Boards) or PBAs (Printed Board Assemblies), and are characterized by being relatively small and thin compared to general connectors. In addition, such FFCs are flexible and can be folded, and are widely used in connecting connectors in electronic devices such as mobile phones.

Normally, FFCs are manufactured using PET (Polyethylene terephthalate) stretched film, but since PET stretched film has a heat resistance of approximately 130°C, FFCs using PET stretched film cannot be used for key parts such as automobile powertrains and engine control parts, which require high heat resistance. Therefore, attempts have been made to improve heat resistance by using technology to coat/laminate PET stretched film, but the reality is that it is difficult to give PET stretched film high heat resistance to withstand high temperatures of 150°C or more.

Polyimide films, which have high heat resistance, are also used for the above-mentioned FFC applications, but because polyimide films are made of thermosetting resins, they have the disadvantage that they are difficult to reuse.

The present invention provides a polyester resin composition having excellent heat resistance and moisture resistance, and a biaxially oriented polyester film containing the same.

According to one embodiment of the present invention, a polyester resin composition can be provided that includes a polyester resin containing a polycondensate of a dicarboxylic acid component containing terephthalic acid and isophthalic acid and a diol component containing cyclohexanedimethanol, the polyester resin having a melting temperature of 250°C or higher, a difference (Tm-Tmc) between the melting temperature (Tm) and the melt crystallization temperature (Tmc) of 45°C or higher, and a difference (Tcc-Tg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) of 40°C or higher.

According to another embodiment of the present invention, a biaxially oriented polyester film formed using the polyester resin composition can be provided.

According to the present invention, a polyester resin composition having excellent heat resistance and moisture resistance and a polyester film containing the same can be provided.

1 is a graph showing the results of measuring the moisture absorption rates of the films of Example 1 (PCT) and Comparative Example 1 (PET). 1 is a graph showing the measurement results of hydrolysis resistance (intrinsic viscosity retention rate) of the films of Example 1 (PCT) and Comparative Example 1 (PET). 1 is a photograph of the film surface during evaluation of the hydrolysis resistance rate (intrinsic viscosity retention rate) of the films of Example 1 (PCT) and Comparative Example 1 (PET). Photographs of the film surfaces of Example 1 (PCT) and Comparative Example 1 (PET) before and after oligomer evaluation.

Hereinafter, the polyester resin composition according to a specific embodiment of the present invention and the biaxially oriented polyester film for flexible flat cables containing the same will be described in more detail.

The inventors have experimentally confirmed that when a polyester resin containing a polycondensate of a dicarboxylic acid component containing terephthalic acid and isophthalic acid and a diol component containing cyclohexanedimethanol has a melting temperature of 250°C or higher, a difference (Tm-Tmc) between the melting temperature (Tm) and the melt crystallization temperature (Tmc) of 45°C or higher, and a difference (Tcc-Tg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) of 40°C or higher, it is possible to produce an unstretched sheet having a thickness of 1.0 mm or more, and that this can be used to increase the elongation rate during the production of a stretched film and improve heat resistance and moisture absorption, thereby completing the invention.

Specifically, the biaxially stretched polyester film of the embodiment includes a polyester resin including a polycondensate of a dicarboxylic acid component including terephthalic acid and isophthalic acid and a diol component including cyclohexanedimethanol. The melting temperature of the polyester resin may be 250°C or higher, 250 to 350°C, or 253 to 340°C. If the melting temperature of the polyester resin is less than 250°C, there is a problem that the heat resistance is low and the film cannot be applied to parts that require high heat resistance.

The polyester resin may have a difference (Tm-Tmc) between the melting temperature (Tm) and the melt crystallization temperature (Tmc) of 45°C or more, 45 to 120°C, or 50 to 115°C. If the difference between the melting temperature and the melt crystallization temperature of the polyester resin is less than 45°C, the polyester resin will rapidly crystallize after the T-die as it is cooled from the molten state (near Tm) to the melt crystallized state (near Tmc), resulting in a problem that it is difficult to form an unstretched sheet having a thickness of 1.0 mm or more and in a transparent state that can be stretched at the same time.

In addition, the polyester resin may have a difference (Tcc-Tg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) of 40°C or more, 40 to 100°C, or 43 to 90°C. If the difference between the cold crystallization temperature and the glass transition temperature of the polyester resin is less than 40°C, when the temperature is raised (heated) to perform the process of stretching the unstretched sheet, the section from the glass transition state (Tg) to the cooling crystallization state (Tcc) becomes shorter, so that the resin crystallizes rapidly, making it difficult to set the stretching processing conditions, and therefore stretching may be difficult.

In other words, since the melting temperature of the polyester resin is 250°C or higher, it exhibits high heat resistance, and since the difference between the melting temperature and the melt crystallization temperature is 45°C or higher, and the difference between the cold crystallization temperature and the glass transition temperature is 40°C or higher, it has excellent moldability and stretchability.

The content of isophthalic acid contained in the polyester resin may be 3 to 20 mol%, 5 to 20 mol%, 6 to 18 mol%, 8 to 16 mol%, or 10 to 14 mol% based on the total dicarboxylic acid components. If the content of isophthalic acid exceeds 20 mol%, the thermal shrinkage rate is high at high temperatures of 150°C or higher, and heat resistance is reduced. If the content is less than 3 mol%, the thickness of the transparent unstretched sheet is very limited to about 0.1 mm, and the stretching ratio is also limited, so that the elongation rate is reduced when producing a stretched film, and the desired mechanical properties and heat resistance cannot be obtained.

The dicarboxylic acid component and the diol component contained in the polyester resin may have a molar ratio of 1:1 to 2, or 1:1.2 to 1.8. If the molar ratio of the dicarboxylic acid component and the diol component is less than 1:1, the melting temperature is low and heat resistance is reduced, and if it exceeds 1:2, moldability is reduced, and the desired mechanical properties cannot be obtained.

The dicarboxylic acid component may further include, in addition to the terephthalic acid and isophthalic acid, one or more other dicarboxylic acid components selected from the group consisting of 2,6-naphthalenedicarboxylic acid, dimethylisophthalic acid, and dimethyl 2,6-naphthalenedicarboxylic acid, but is not limited thereto. The content of the other dicarboxylic acid components may be 20 mol % or less, 0.1 to 20 mol %, or 5 to 15 mol % based on the total dicarboxylic acid components.

The diol component may further include an aliphatic diol having 2 to 20 carbon atoms, for example, one or more other diol components selected from the group consisting of ethylene glycol, diethylene glycol, 1,4-butanediol, 1,3-propanediol, and neopentyl glycol, but is not limited thereto. The content of the other diol components may be 20 mol% or less, 0.1 to 20 mol%, or 5 to 15 mol% based on the total diol components.

The polyester resin may have an intrinsic viscosity (IV) of 0.4 to 1.2 dl/g, 0.5 to 1.0 dl/g, or 0.7 to 0.9 dl/g. If the intrinsic viscosity is less than 0.4 dl/g, the melting temperature will be low and heat resistance will be reduced, and if it exceeds 1.2 dl/g, moldability will be reduced and desired mechanical properties will not be obtained.

The polyester resin may have a number average molecular weight of 15,000 to 50,000 g/mol, 20,000 to 45,000 g/mol, or 25,000 to 40,000 g/mol. If the number average molecular weight is less than 15,000 g/mol, the melting temperature is low and heat resistance is reduced, and if it exceeds 50,000 g/mol, moldability is reduced and desired mechanical properties cannot be obtained.

In addition, the polyester resin may have a weight average molecular weight of 50,000 to 150,000 g/mol, 60,000 to 140,000 g/mol, or 70,000 to 130,000 g/mol. If the weight average molecular weight is less than 50,000 g/mol, the melting temperature is low and heat resistance is reduced, and if it exceeds 150,000 g/mol, moldability is reduced and desired mechanical properties cannot be obtained.

The polyester resin preferably has a melt crystallization temperature of 150 to 230°C. If the melt crystallization temperature of the polyester resin is less than 150°C, the heat setting effect decreases during post-processing or actual use of the stretched film, making it difficult to impart dimensional stability to the final stretched film, specifically, high shrinkage may occur. If the melt crystallization temperature exceeds 230°C, it is difficult to produce a transparent unstretched sheet during film extrusion.

According to another embodiment of the present invention, a biaxially oriented polyester film formed using the polyester resin composition can be provided. The biaxially oriented polyester film exhibits heat resistance at high temperatures of 150 to 200°C, and can therefore be applied to safety parts such as automotive powertrains, engine controls, and steering devices, and can be used as a material for flexible flat cables (FFCs), which require excellent heat resistance, moisture resistance, electrical insulation properties, etc.

Typically, PET film is used as a flexible flat cable material, which can control the thickness of the film up to 1.2 mm when unstretched. However, PET film has the disadvantage of being vulnerable to heat, as it has a heat resistance of approximately 130°C. Therefore, there have been attempts to use polycyclohexylene dimethylene terephthalate (PCT) film for parts that require heat resistance, but conventional PCT films have high glass transition temperatures (Tg) and melting temperatures (Tm).

However, conventional PCT films have a high melt crystallization temperature (Tmc) and crystallization occurs rapidly immediately after extrusion using a T-die, etc., resulting in a problem that the thickness of the transparent unstretched sheet is very limited to approximately 0.1 mm. If the thickness of the transparent unstretched sheet is as thin as approximately 0.1 mm, the elongation rate is low during the stretching process. Meanwhile, conventional PCT films have a low cold crystallization temperature (Tcc), so when the temperature is raised (heated) to perform the stretching process of the unstretched sheet, the section from the glass transition state (Tg) to the cooling crystallization state (Tcc) is short, causing rapid crystallization and making it difficult to set the stretching processing conditions, making stretching difficult.

However, the polyester resin according to the embodiment has a difference (Tm-Tmc) between the melting temperature (Tm) and the melt crystallization temperature (Tmc) of 45°C or more, and the difference (Tcc-Tg) between the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) of 40°C or more, so that the crystallization rate can be delayed immediately after extrusion, and the maximum thickness of the transparent unstretched sheet can be controlled to approximately 1.2 mm, which is a similar level to the thickness of the transparent unstretched sheet of a PET film. In addition, because the crystallization rate can be delayed in the stretching process, the unstretched sheet can be easily stretched, and the heat resistance of the polyester film can be improved.

The biaxially oriented polyester film according to the other embodiment may be a multilayer film having two or more layers in which the polyester resin composition is co-extruded, or a laminate (thermally bonded) film.

Specifically, two or more of the polyester resin compositions are melted in different extruders, and the molten resin is fed into a die to laminate two or more layers, and then a non-stretched multilayer film (or laminate film) can be provided by a method such as blown or casting. Each layer contained in the non-stretched multilayer film can be transparent and have a thickness of about 1.2 mm or more. In this case, the two or more polyester resin compositions can be the same composition or compositions with different components or contents. In addition, the non-stretched multilayer film can be biaxially stretched in the longitudinal and transverse directions to obtain a biaxially stretched polyester film with excellent heat resistance, etc.

The biaxially stretched polyester film may have a heat shrinkage rate of less than 0.5%, 0.01 to 0.4%, or 0.01 to 0.3% at 150°C for 30 minutes, and a heat shrinkage rate of less than 1.0%, 0.01 to 0.9%, or 0.01 to 0.8% at 200°C for 30 minutes. Such biaxially stretched polyester films have a significantly low heat shrinkage rate at temperatures of 150 to 200°C, and therefore can be confirmed to have excellent heat resistance.

The biaxially oriented polyester film also has excellent moisture resistance; specifically, when the temperature is 85°C and the relative humidity is 85%, the moisture absorption resistance can be 1% or less, 0.01 to 0.9%, or 0.01 to 0.8%.

On the other hand, the method for producing the biaxially stretched polyester film is not limited thereto. For example, the polyester resin composition is vacuum dried to thoroughly remove moisture, and then fed to an extruder, melt extruded at a temperature of 200 to 300°C, and molded into a sheet shape from a T-shaped die. The sheet-shaped product thus obtained is cooled and solidified on a mirror-finished cooling drum to obtain an unstretched sheet. At this time, it is preferable to use an electrostatic application method in order to increase adhesion to the cast drum. The unstretched sheet obtained is then stretched in the longitudinal direction (length direction or machine direction, MD; mechanical direction). It is desirable that the longitudinal stretching lowers the crystal orientation and also promotes thermal crystallization. After the longitudinal stretching, the film is stretched in the transverse direction (width direction, TD; trans machine direction) and heat-treated to obtain a biaxially stretched film.

In this case, the stretching ratio may be 2 to 5 times, preferably 2.5 to 5 times, and more preferably 2.5 to 4.0 times in the longitudinal direction, and 2.5 to 5 times, preferably 3 to 4.5 times, and more preferably 3.2 to 4.2 times in the transverse direction.

The stretching temperature can be in the range of the glass transition temperature (Tg) of the polyester resin +5°C to Tg +50°C, or in the range of Tg +10°C to Tg +40°C. In this case, the lower the Tg, the better the stretchability, but breakage may occur. When the stretching temperature is particularly in the range of Tg +10°C to Tg +40°C, the brittleness of the produced film can be improved.

The longitudinal stretching speed may be 22 to 500 m/min, 25 to 400 m/min, or 25 to 200 m/min. In this case, a longitudinal stretching speed of 22 m/min or more is advantageous for maintaining the orientation aimed at in the present invention, and since crystallinity is imparted by the longitudinal stretching speed and stretch ratio, the transverse stretching speed varies depending on the longitudinal stretching conditions.

The present invention will be described in more detail in the following examples. However, the following examples are merely illustrative of the present invention, and the contents of the present invention are not limited to the following examples.

Example 1
1) Preparation of polyester resin composition 2.0 kg of 1,4-cyclohexanedimethanol, 1.8 kg of dicarboxylic acid containing terephthalic acid and isophthalic acid in a molar ratio of 95 mol%:5 mol%, 0.4 g of triethyl phosphate, 0.2 g of a titanium oxide catalyst (Hombifast PC, manufactured by Sachtleben, Ti atom content in catalyst: 15 wt%), and 0.2 g of antimony trioxide (antimony atom content in catalyst: 83.5 wt%) were charged into a reactor, and an esterification reaction was carried out by increasing the temperature to 280°C at normal pressure for 3 hours. Next, the esterification reaction product was polycondensed by heating at a temperature of 295°C under a pressure of 0.5 to 1 torr for 150 minutes to prepare a polyester resin, which was then processed into chips.

2) Manufacturing of biaxially stretched polyester film The polyester resin chips were melt-extruded by an extruder, formed into a sheet, and cooled. The obtained sheet was stretched 3.6 times in the machine direction (MD), and then stretched 3.8 times in the transverse direction (TD). The stretched film was heat-set at 238°C to obtain a biaxially stretched polyester film.

Examples 2 and 3
A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that the content of the isophthalic acid was the molar ratio shown in Table 1 below.

Comparative Example 1
1.2 kg of terephthalic acid and 1.2 kg of ethylene glycol were charged into a reactor and a normal polymerization reaction was carried out at 258° C. to produce a polyethylene terephthalate (PET) polymer, which was then processed in the form of chips. The PET polymer was melt-extruded in an extruder, formed into a sheet, cooled, and stretched 3.7 times in the machine direction (MD), and then stretched 4.0 times in the transverse direction (TD) to produce a polyethylene terephthalate film.

Comparative Examples 2 to 5
A biaxially stretched polyester film was obtained in the same manner as in Example 1, except that the content of the isophthalic acid was the molar ratio shown in Table 1 below.

Evaluation 1. Measurement of glass transition temperature, etc. by DSC analysis The polyester resins of Examples 1 to 3 and Comparative Examples 1 to 5 were analyzed by DSC (Differential Scanning Calorimeter) to measure the glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm), and melt crystallization temperature (Tmc). Specifically, the polyester resins were heated from 30° C. to 320° C. at a heating rate of 10° C./min using DSC, maintained at 320° C. for 5 minutes, and then quenched to 30° C. and maintained at 320° C. for 5 minutes. Thereafter, the glass transition temperature (Tg), cold crystallization temperature (Tcc) and melting temperature (Tm) were measured while increasing the temperature from 30° C. to 320° C. at a heating rate of 10° C./min. After maintaining at 320° C. for 5 minutes, the melt crystallization temperature (Tmc) was measured while cooling to 30° C. at a cooling rate of 10° C./min. The results are shown in Table 1 below.

2. Measurement of maximum thickness of transparent unstretched sheet The polyester resins of Examples 1 to 3 and Comparative Examples 1 to 5 were melt-extruded by an extruder, and the maximum thickness of the transparent unstretched sheet was measured before stretching in the machine direction (MD) and the transverse direction (TD). The results are shown in Table 1 below.

3. Measurement of Maximum Elongation The unstretched sheets of Examples 1 to 3 and Comparative Examples 1 to 5 were biaxially stretched in both the machine direction and the transverse direction, and the maximum elongation was measured. The results are shown in Table 1 below.

4. Measurement of areal heat shrinkage rate For the films of Examples 1 to 3 and Comparative Examples 1 to 5, the film was cut so that one end was parallel to the machine direction (longitudinal direction) and the other end was perpendicular to the machine direction (transverse direction) to prepare a square film sample of 10 cm x 10 cm. After the film was kept in an air circulating oven at 150°C for 30 minutes, the sample was taken out and the change in length in each of the longitudinal and transverse directions was measured at room temperature, and the longitudinal and transverse heat shrinkage rates were calculated according to the following formula 1. In the same manner, the film was kept in an oven at 200°C for 30 minutes, and the heat shrinkage rates were calculated, and the results are shown in Table 1 below.
[Equation 1]
Heat shrinkage rate (%)=[(L ₀ −L)/L ₀ ]×100
In the above formula, _L0 is the length before heat treatment and L is the length after heat treatment.

5. Evaluation of Tensile Elongation Retention Rate The films of Example 1 (PCT) and Comparative Example 1 (PET) were maintained at a temperature of 121° C., a relative humidity of 100%, and an atmospheric pressure of 2 atm (Pressure Cooker Test) for 48 hours and 60 hours, and then the tensile elongation retention rate was measured. The results are shown in Table 1 below.

6. Evaluation of Moisture Absorption Resistance The films of Example 1 (PCT) and Comparative Example 1 (PET) were maintained at a temperature of 85° C. and a relative humidity of 85% for 5,000 hours, and the weight increase was measured and shown in the graph in Figure 1. In addition, the moisture absorption rate after 5,000 hours was calculated, and the results are shown in Table 1 below.

7. Evaluation of Hydrolysis Resistance (1) Intrinsic Viscosity Measurement The films of Example 1 and Comparative Example 1 were dissolved in o-chlorophenol to a concentration of 1.2 g/dl, and the intrinsic viscosity was measured at 35° C. using an Ubbelodhe viscometer.

(2) Measurement of Hydrolysis Resistance (Intrinsic Viscosity Retention Rate) After the films of Example 1 and Comparative Example 1 were maintained at a temperature of 85° C. and a relative humidity of 85%, the extent to which the intrinsic viscosity was maintained was measured, and the results are shown in the graph of Figure 2. The hydrolysis resistance (intrinsic viscosity retention rate) after 5,000 hours is also shown in Table 1 below. The surfaces of Example 1 (PCT) and Comparative Example 1 (PET) after 5,000 hours were photographed and are shown in Figure 3.

8. Oligomer Evaluation In order to examine whether oligomers were eluted from the films of Example 1 and Comparative Example 1, the films were heat-treated in an oven at 150° C. for about 60 minutes, and then the changes were observed and measured using a haze meter. The results are shown in Table 1 below.

Haze Meter: NIPPON DENSOKU, model NDH-7000
9. Evaluation of Dielectric Breakdown Voltage Flexible flat cables were manufactured using the films of Example 1 and Comparative Example 1, and AC 1,000 V was applied to the adjacent conductors for 60 seconds to read the voltage at which the film broke down and short-circuited. The results are shown in Table 1 below.

-IPA ^* : Content of isophthalic acid (IPA) relative to 100 mol% of dicarboxylic acid component contained in polyester resin -P.C.T ^** : Pressure Cooker Test (P.C.T) or Autoclave test, a test to evaluate whether a product can withstand high temperature/high humidity environment. The test is carried out under conditions of 121°C temperature, 100% relative humidity, and 2 atm air pressure.

As shown in Table 1 above, the PCT films of Examples 1 to 3 were found to have lower thermal shrinkage rates and better heat resistance than the PET film of Comparative Example 1, as well as better moisture absorption resistance, hydrolysis resistance, and electrical properties.

It was also confirmed that the PCT films of Examples 1 to 3 had higher maximum elongation, lower thermal shrinkage, and better heat resistance than the PCT films of Comparative Examples 2 to 5. Furthermore, it was confirmed that the PCT films of Examples 1 to 3 could control the thickness of the unstretched sheet to be thicker than the PCT films of Comparative Examples 2 and 3.

Claims (10)

The polyester resin includes a polycondensate of a dicarboxylic acid component including terephthalic acid and isophthalic acid and a diol component including cyclohexanedimethanol,
The content of the isophthalic acid is 3 to 20 mol % based on the total dicarboxylic acid components, and the content of the terephthalic acid is 80 to 97 mol % based on the total dicarboxylic acid components.
The content of the cyclohexanedimethanol is 80 to 100 mol % based on the total diol components,
The diol component includes one or more other diol components selected from the group consisting of ethylene glycol, diethylene glycol, 1,4-butanediol, 1,3-propanediol, and neopentyl glycol, in an amount of 0 to 20 mol % based on the total diol components;
The polyester resin has a number average molecular weight of 15,000 to 50,000 g/mol;
The polyester resin has an intrinsic viscosity (IV) of 0.4 to 0.9 dl/g;
The polyester resin has a melting temperature of 250° C. or higher and 350° C. or lower,
The polyester resin has a difference (Tm-Tmc) between its melting temperature (Tm) and its melt crystallization temperature (Tmc) of 45°C or more and 120°C or less,
The polyester resin has a difference (Tcc-Tg) between its cold crystallization temperature (Tcc) and its glass transition temperature (Tg) of 40° C. or more and 100° C. or less,
The melting temperature (Tm), the cold crystallization temperature (Tcc) and the glass transition temperature (Tg) are measured by using a Differential Scanning Calorimeter (DSC) while heating the polyester resin from 30° C. to 320° C. at a heating rate of 10° C./min, maintaining the temperature for 5 minutes, rapidly cooling to 30° C., maintaining the temperature for 5 minutes, and then heating from 30° C. to 320° C. at a heating rate of 10° C./min. The melt crystallization temperature (Tmc) is measured by maintaining the temperature at 320° C. for 5 minutes after the heating, and then cooling to 30° C. at a cooling rate of 10° C./min.
It is formed using a polyester resin composition,
The polyester resin composition is a coextruded multilayer film or laminate film having two or more layers.
Biaxially oriented polyester film. The dicarboxylic acid component includes one or more other dicarboxylic acid components selected from the group consisting of 2,6-naphthalenedicarboxylic acid, dimethylisophthalic acid, and dimethyl 2,6-naphthalenedicarboxylic acid, in an amount of 0 to 17 mol % based on the total dicarboxylic acid components;
2. The biaxially oriented polyester film according to claim 1. The molar ratio of the dicarboxylic acid component and the diol component is 1:1 to 2.
2. The biaxially oriented polyester film according to claim 1. The polyester resin has a weight average molecular weight of 50,000 to 150,000 g/mol.
2. The biaxially oriented polyester film according to claim 1. The polyester resin has a melt crystallization temperature of 150 to 230°C.
2. The biaxially oriented polyester film according to claim 1. The biaxially oriented polyester film is for flexible flat cables or PCBs (Printed Circuit Boards).
2. The biaxially oriented polyester film according to claim 1. The biaxially oriented polyester film has a heat shrinkage rate of less than 0.5% at 150° C. for 30 minutes and a heat shrinkage rate of less than 1.0% at 200° C. for 30 minutes.
2. The biaxially oriented polyester film according to claim 1. The biaxially oriented polyester film is stretched 2 to 5 times in the machine direction (MD) and 2 to 5 times in the transverse direction (TD).
2. The biaxially oriented polyester film according to claim 1. The biaxially oriented polyester film has a tensile elongation retention rate of 50% to 75% after 48 hours and a tensile elongation retention rate of 30% to 60% after 60 hours in a high temperature and high humidity test (Pressure Cooker Test) under conditions of a temperature of 121° C., a relative humidity of 100%, and an atmospheric pressure of 2 atm.
2. The biaxially oriented polyester film according to claim 1. The biaxially oriented polyester film has a moisture absorption resistance of 1% or less when the temperature is 85° C. and the relative humidity is 85%.
2. The biaxially oriented polyester film according to claim 1.

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