HK1196021A - Biaxially stretched polyester film and method for producing same - Google Patents

Description

Biaxially stretched polyester film and method for producing same

Technical Field

The present invention relates to a polyester film having tear linearity suitable as a film for use in packaging materials, electrical insulating materials, general industrial materials, and the like.

Background

Packaging bags using various plastic films are widely used for packaging foods, pharmaceuticals, and miscellaneous goods, and packaging bags obtained by laminating 2 or more layers of heat-sealable non-oriented plastic films are widely used for biaxially stretched plastic films.

Among plastic films, biaxially stretched polyester films are excellent in durability, moisture resistance, mechanical strength, heat resistance, and oil resistance, are produced by a tube method, a planar simultaneous biaxial stretching method, a planar sequential biaxial stretching method, and the like, and are widely used in the field of food packaging and the like.

However, the packaging bag using the biaxially stretched polyester film has a problem of poor tear openability. Although there is a method of providing a notch to improve the unsealability, the notch is often torn out without straight lines, and the contents are scattered and wasted, and the following troubles may occur: soft snacks such as cakes are broken when opened, or clothes are stained when the contents are liquid.

As an easy-to-open material having excellent tearing linearity in which a film is torn straight when torn, there is a material in which a uniaxially stretched polyolefin film is laminated as an intermediate layer. As such a material, for example, there is a 3-layer laminate of a biaxially stretched polyester film/a uniaxially stretched polyolefin film/a non-stretched polyolefin film, but an intermediate layer must be specially provided, which has a problem of cost and also has limited applications.

The present applicant has found that, as a method for imparting tear linearity to a biaxially stretched polyester film itself in advance, a method in which polybutylene terephthalate (modified PBT) containing polytetramethylene glycol (PTMG) having a molecular weight of 600 to 4000 in a proportion of 5 to 20% and polyethylene terephthalate (PET) are blended in a proportion of modified PBT/PET of 30/70 to 5/95 (patent documents 1 to 6).

Further, the present applicant has proposed a biaxially stretched polyester film and a laminate having high barrier properties against oxygen and water vapor and tearing linearity by vapor-depositing a metal compound such as alumina or silica on at least one surface of a biaxially stretched polyester film having tearing linearity (patent documents 7 to 8).

However, the biaxially stretched polyester film provided with the tear linearity has a larger amount of outgas remaining in the film than a normal polyester film. Therefore, the film cannot be used as a film constituting a container or a package for storing electronic materials such as semiconductors requiring high cleanability. Further, when the film is subjected to vapor deposition processing in a vacuum atmosphere, there is a problem that the quality of the vapor deposition layer tends to become unstable as the roll length increases.

Documents of the prior art

Patent document

Patent document 1 Japanese patent application laid-open No. H10-168293

Patent document 2 Japanese patent application laid-open No. 11-227135

Patent document 3: japanese patent laid-open publication No. 11-300916

Patent document 4 Japanese patent application laid-open No. 11-302405

Patent document 5 Japanese laid-open patent publication No. 2000-318035

Patent document 6 Japanese laid-open patent application No. 2002-

Patent document 7 Japanese patent laid-open No. 2001-162752

Patent document 8, Japanese patent laid-open No. 2006-150617

Disclosure of Invention

The invention provides a biaxially stretched polyester film having a linear tear and reduced residual gas emission in the film.

As a result of intensive studies to solve such problems, the inventors of the present invention have found that the outgas generated from the polyester film having a tear linearity particularly contains much Tetrahydrofuran (THF) and polytetramethylene glycol (PTMG) -containing PBT (modified PBT) compared with the outgas generated from a general PET film. The present inventors have focused on the amount of THF remaining in the modified PBT, and found that a biaxially stretched polyester film having a function of imparting linearity in tearing to a polyester and a small amount of residual outgassing can be obtained by forming a film by blending with a PET resin after removing the THF significantly at the stage of slicing, and reached the present invention.

That is, the gist of the present invention is as follows.

(1) A biaxially stretched polyester film comprising a modified polybutylene terephthalate (modified PBT) and a polyethylene terephthalate (PET), characterized in that the mass ratio of the modified PBT to the PET (modified PBT/PET) is 20/80-5/95, the modified PBT is a PBT containing 5-20 mass% of a polybutylene glycol unit having a molecular weight of 600-4000, the biaxially stretched film has a Tetrahydrofuran (THF) content of 50 [ mu ] g/g or less, which is generated by heat treatment at 180 ℃ for 30 minutes in a helium atmosphere, and has a tear linearity in the longitudinal direction.

(2) A biaxially stretched polyester film, which is obtained by vapor-depositing at least one member selected from the group consisting of aluminum, aluminum oxide and silicon oxide on at least one surface of the biaxially stretched polyester film described in the above (1), and which has gas barrier properties and tear linearity.

(3) A method for producing a biaxially stretched polyester film, which comprises blending a modified polybutylene terephthalate (modified PBT) containing 5 to 20 mass% of a polybutylene glycol unit having a molecular weight of 600 to 4000 with a polyethylene terephthalate (PET) in a mass ratio of the modified PBT to the PET (modified PBT/PET) of 20/80 to 5/95, and stretching the blend to produce a film, wherein the amount of Tetrahydrofuran (THF) produced by heat treatment at 180 ℃ for 30 minutes in a helium atmosphere is 800 [ mu ] g/g or less, for producing the biaxially stretched polyester film of (1).

The present invention provides a biaxially stretched polyester film which has durability, moisture resistance, mechanical properties, heat resistance and oil resistance, has a tear linearity in the longitudinal direction, and has a small amount of residual gas emission. Further, the film of the present invention is particularly suitable for a packaging bag in a field where cleanliness is required because the amount of generation of outgas is small, and further, a vapor-deposited layer can be stably formed even in a vacuum state.

Drawings

Fig. 1 shows the shape of a test piece used for evaluation of the tear linearity of a film.

Fig. 2 is a diagram showing the shape of a test piece after tearing in a tearing test, where (a) shows an example of a test piece after tearing a sample having good tearing linearity, and (b) shows an example of a test piece after tearing a sample having poor tearing linearity.

Detailed Description

The biaxially stretched polyester film of the present invention contains a modified polybutylene terephthalate (modified PBT) and a polyethylene terephthalate (PET).

In the present invention, PET contains terephthalic acid and ethylene glycol as polymerization components, and other components may be copolymerized within a range not impairing the effects of the present invention.

Examples of the other copolymerizable component include dicarboxylic acids such as isophthalic acid, phthalic acid, 2, 6-naphthalenedicarboxylic acid, 5-sodium sulfoisophthalate, succinic acid, adipic acid, sebacic acid, dodecanedioic acid, dimer acid, maleic anhydride, maleic acid, fumaric acid, itaconic acid, citraconic acid, mesaconic acid, and cyclohexanedicarboxylic acid; hydroxycarboxylic acids such as 4-hydroxybenzoic acid, epsilon-caprolactone and lactic acid; diols such as 1, 3-propanediol, 1, 6-hexanediol, and cyclohexanedimethanol; and polyfunctional compounds such as trimellitic acid, trimesic acid, pyromellitic acid, trimethylolpropane, glycerol, pentaerythritol and the like.

PET can be obtained by a known production method, that is, by obtaining an oligomer by a transesterification method using dimethyl terephthalate and ethylene glycol or by a direct esterification method using terephthalic acid and ethylene glycol, and then melt-polymerizing or further solid-phase polymerizing the oligomer.

In the present invention, the modified polybutylene terephthalate (modified PBT) is polybutylene terephthalate (PBT) containing 5 to 20 mass% of polytetramethylene glycol (PTMG) units having a molecular weight of 600 to 4000.

In the present invention, the molecular weight of the PTMG constituting the modified PBT must be 600 to 4000, preferably 1000 to 3000, and more preferably 1000 to 2000. When the molecular weight of the PTMG is less than 600, the obtained film cannot obtain tear straightness, and when it exceeds 4000, the obtained film is deteriorated in properties such as mechanical strength, dimensional stability, haze and the like, and further, does not exhibit stable tear straightness.

The content of the PTMG unit in the modified PBT must be 5 to 20 mass%, preferably 10 to 20 mass%, and more preferably 10 to 15 mass%. When the content of PTMG is less than 5% by mass, the resulting film does not exhibit tear linearity, and when it exceeds 20% by mass, the properties such as mechanical strength, dimensional stability, haze and the like of the resulting film are degraded, and it is difficult to obtain stable tear linearity of the film. When the content of PTMG is more than 20 mass%, particularly in mass production, a phenomenon (so-called "baras phenomenon") in which the film pulsates during extrusion may occur, and the thickness unevenness of the film may become large.

The modified PBT can be obtained by adding PTMG and polycondensing in the polymerization step of PBT. For example, the polyester can be obtained by the polycondensation reaction of an ester exchange reactant of dimethyl terephthalate and 1, 4-butanediol and PTMG with the molecular weight of 600-4000.

In the biaxially stretched polyester film of the present invention, the mass ratio of the modified PBT to the PET (modified PBT/PET) is required to be 20/80 to 5/95, preferably 15/85 to 10/90. When the mass ratio of the modified PBT is less than 5 mass%, it becomes difficult to obtain the tear linearity of the film. If the amount is more than 20% by mass, the variation in the thickness of the film may be large, the linearity of the tear of the film obtained may be low, and the amount of outgas generated may exceed the amount specified in the present invention. That is, in order to impart the tearing linearity to the film and reduce the amount of outgas generation, it is necessary to set the mixing ratio of the modified PBT and the PET within the above range.

The biaxially stretched polyester film of the present invention may contain other polymers such as polyethylene naphthalate and polycyclohexylenedimethylene terephthalate as long as the effects of the present invention are not impaired.

The biaxially stretched polyester film of the present invention is required to have an amount of Tetrahydrofuran (THF) generated when heat-treated at 180 ℃ for 30 minutes in a helium atmosphere of 50. mu.g/g or less, preferably 30. mu.g/g or less, and more preferably 20. mu.g/g or less. In the present invention, the reason why the amount of THF produced is particularly specified among the produced components is because THF is a volatile (low boiling point) component.

When the amount of THF produced exceeds 50. mu.g/g, the content of the package may be contaminated with the produced THF. Further, even when vapor deposition processing is performed on a biaxially stretched polyester film for the purpose of improving the gas barrier property and the water vapor barrier property, THF may be generated from the film during the vapor deposition processing, which may adversely affect the formation of a vapor deposited film, and the gas barrier property and the water vapor barrier property of the film cannot be stably improved.

In order to make the amount of THF generated from the biaxially stretched polyester film of the present invention 50. mu.g/g or less as defined in the present invention, the amount of THF generated from the modified PBT chip used in the production of the film is preferably 800. mu.g/g or less, more preferably 500. mu.g/g or less, and most preferably 300. mu.g/g or less. When the amount of THF generated from the modified PBT is more than 800. mu.g/g, the amount of THF generated from the film may be more than the value specified in the present invention depending on the blending ratio of the modified PBT to PET.

The amount of THF generated from the modified PBT chip is 800. mu.g/g or less, and examples thereof include a method of drying the polymerized modified PBT chip under a reduced pressure atmosphere, a method of drying under a nitrogen atmosphere, and a method of drying by a hot air dryer.

When the drying is carried out by these methods, the temperature is preferably 80 to 180 ℃ and the time is preferably 12 hours or longer, more preferably 24 hours or longer. In addition, when drying under a reduced pressure atmosphere, the degree of reduced pressure is preferably 10Pa or less, in addition to the above temperature and time.

In the production of the biaxially stretched polyester film of the present invention, for example, a mixture of the modified PBT and PET is fed into an extruder, heated and melted, and then extruded from the die hole of a T-die into a sheet form to produce an unstretched sheet. A sheet extruded from a die hole of a T-die is wound in a close contact manner on a cooling drum by an external electrostatic casting method or the like to be cooled, then stretched at a temperature of 90 to 140 ℃ at a magnification of 3.0 to 5.0 times in each of the longitudinal direction and the transverse direction, and further heat-treated at a temperature of 210 to 245 ℃ to form a biaxially stretched film.

When the stretching temperature is lower than 90 ℃, a homogeneous stretched film may not be obtained, and when it exceeds 140 ℃, crystallization of PET may be promoted, and transparency may be deteriorated. When the stretch ratio is less than 3.0 times, the strength of the obtained stretched film is low, pinholes tend to be generated when the film is formed into a bag, and when the stretch ratio exceeds 5.0 times, stretching may be difficult. When the heat treatment temperature is lower than 210 ℃, the heat shrinkage of the obtained stretched film increases, and the bag after bag making may be deformed, and when the heat treatment temperature is higher than 245 ℃, the film may be fused.

As the biaxial stretching method, a tenter simultaneous biaxial stretching method, or a sequential biaxial stretching method using a roll and a tenter may be used. Further, a biaxially stretched film can also be produced by a tubular method.

The biaxially stretched polyester film of the present invention obtained as described above can improve the oxygen barrier property and the water vapor barrier property of the film by performing vapor deposition treatment on at least one surface of the film with aluminum, silicon oxide, aluminum oxide, or a combination of two or more of these components, i.e., silicon oxide and aluminum oxide. As a method for producing a vapor-deposited film, a vacuum vapor deposition method, an EB vapor deposition method, a sputtering method, an ion plating method, or the like can be used, but from the viewpoint of productivity and cost, the vacuum vapor deposition method is most preferable.

The vacuum deposition method is a method in which a film is formed in a vacuum state in a chamber, and then a component to be deposited is melted and evaporated to adhere to the film, thereby forming the film. In this method, if a large amount of exhaust gas is present in the film, there is a problem that the degree of vacuum in the chamber becomes unstable due to diffusion of the exhaust gas, and the formation of the vapor deposition film becomes unstable, and it is difficult to obtain the desired oxygen barrier property and water vapor barrier property. In particular, when a roll film is vacuum-deposited, the roll surface is continuously renewed, and therefore, even if a stable vapor-deposited film can be formed at the start of vapor deposition, if exhaust gas remains in the film, the formation of the vapor-deposited film may become unstable as the vapor deposition process proceeds. However, the biaxially stretched polyester film of the present invention has a small residual outgassing amount, and therefore, a vapor-deposited layer can be stably formed even in a roll film.

In order to improve the adhesion to a vapor deposited film of aluminum or the like, the surface of the biaxially stretched polyester film of the present invention is preferably pretreated in advance by a method such as corona discharge treatment or application of a tackifier.

The biaxially stretched polyester film of the present invention may be surface-treated by corona discharge treatment, surface curing treatment, plating treatment, coloring treatment, or various coating treatments depending on the use thereof.

Examples

The present invention will be described in detail with reference to examples. The evaluation methods of examples and comparative examples are as follows.

(1) Evaluation of tear straightness

A long film sheet having a length of 205mm in the longitudinal direction (MD) and 20mm in the width direction (TD) was obtained from a biaxially stretched polyester film, 10 samples were prepared by cutting a notch (notch) having a length of 5mm into the center of one TD side of the film sheet, and then the film sheet was manually torn in the MD direction from the notch, and the number of samples reaching the TD side opposite to the side of the notch where the tear propagation end was cut was evaluated.

The evaluation criteria are as follows.

Evaluation excellent: 9-10 sample pieces arrive

Evaluation ∘: 7-8 arrived sample pieces

Evaluation Δ: 5-6 sample pieces arrive

Evaluation x: the number of the reached sample pieces is 4 or less

In the present invention, the evaluation value is not less than O.

(2) Determination of exhaust gas composition and measurement of amount of production

The determination of the outgassing component was carried out by precisely weighing about 15mg of a biaxially stretched polyester film or a modified PBT chip, placing it in a sample cup, heating it in a thermal cracker (PY-2020 iD) at 180 ℃ for 30 minutes under a helium atmosphere, and measuring the generated volatile components by GC/MS (GC: Agilent6890N, MS: Agilent 5975C). In this case, the generated vent gas was Tetrahydrofuran (THF), 1, 4-Butanediol (BD), and water, and Ethylene Glycol (EG) was also generated from the biaxially stretched polyester film.

The amount of each exhaust gas generated was measured using the following standard sample and internal standard. Specifically, solutions of 100ppm each of hexadecane, THF, BD and EG dissolved in hexane were prepared as standard samples, and GC/MS measurement was performed under the same conditions as the samples to calculate peak area ratios of the respective components with respect to hexadecane. Next, a hexane solution having a hexadecane concentration of 100ppm was prepared as an internal standard, and about 15mg of a biaxially stretched polyester film or a modified PBT cut sample was precisely weighed, and 5. mu.l of the internal standard was added thereto, and GC/MS measurement was performed, whereby the amount of each outgas component generated was determined.

(3) Evaluation of Performance of vapor deposition film

A polyester resin (ELITEL UE-3200, manufactured by YOU GmbH., Tg: 65 ℃) and an isocyanate curing agent (CAT-10, manufactured by TOYOBO MORTON) were mixed at a mixing ratio of 10/1 (mass ratio) to prepare a coating material. The obtained coating was applied as a tackifier to a biaxially stretched polyester film having a roll length of 8000m at a thickness of 0.1 μm, and then an aluminum oxide layer was vapor-deposited at a thickness of 40 to 50nm using a continuous vacuum vapor deposition apparatus. The surface layer of the vapor deposited film (core side of the charged biaxially stretched polyester film) was sampled at 100m, 2000m, 4000m, and 7500m, and the oxygen permeability and the water vapor permeability were measured.

(3-a) oxygen permeability (ml/m)²·day·MPa）

According to JIS K-7129, the measurement was carried out at 20 ℃ and 100% RH humidity using OX-TRAN100 manufactured by Modern Control.

The evaluation criteria are as follows.

Evaluation excellent: oxygen permeability of less than 20 ml/(m)²·day·MPa）

Evaluation ∘: oxygen permeability of 20 or more and less than 30 ml/(m)²·day·MPa）

Evaluation Δ: oxygen permeability of 30 to less than 50 ml/(m)²·day·MPa）

Evaluation x: the oxygen permeability is 50 ml/(m)²day.MPa) or more

In the present invention, the evaluation value is not less than O.

(3-b) Water vapor Transmission Rate (g/m)²·day）

The measurement was carried out according to JIS K-7129 using PARMATRANW3/31 manufactured by Modern Control under the conditions of a temperature of 40 ℃ and a humidity of 90% RH.

The evaluation criteria are as follows.

Evaluation excellent: water vapor permeability less than 2 g/(m)²·day）

Evaluation ∘: a water vapor permeability of 2 or more and less than 5 g/(m)²·day）

Evaluation Δ: a water vapor permeability of 5 or more and less than 10 g/(m)²·day）

Evaluation x: the water vapor permeability is 10 g/(m)²Day) or more

In the present invention, the evaluation value is not less than O.

Example 1

< production of modified PBT >

194 parts by mass of dimethyl terephthalate, 108 parts by mass of 1, 4-butanediol and 80ppm of tetrabutyl titanate (a numerical value in terms of the mass of titanium metal relative to the polymer) were subjected to a transesterification reaction for 2.5 hours while being heated from 150 ℃ to 210 ℃. 85 parts by mass of the obtained transesterification reaction product was transferred to a polymerization pot, 40ppm of tetrabutyltitanate was added, 15 parts by mass of PTMG having a molecular weight of 1100 was added, the pressure was reduced, finally, the temperature was raised from 210 ℃ under a reduced pressure of 1hPa, and finally, melt polymerization was carried out at a temperature of 245 ℃ for 2 hours to produce a modified PBT having a relative viscosity of 1.62.

The amount of off-gassing from the modified PBT produced was 3000. mu.g/g of THF and 10. mu.g/g of 1, 4-Butanediol (BD).

The modified PBT produced by the above-described method was dried in a vacuum dryer at 120 ℃ and 5Pa for 72 hours. The amount of outgas generated from the dried modified PBT was 280. mu.g/g for THF and 5. mu.g/g for BD.

< production of biaxially stretched polyester film >

The mixture obtained by simply slicing and mixing the modified PBT and PET (relative viscosity: 1.38) at a mass ratio of 15/85 was melt-extruded at a resin temperature of 280 ℃ by using a 200 mm-phi extruder equipped with a coat-hanger T-die, and then a pinning wire was applied with an applied voltage of 7kV to closely cool the wire on a casting roll adjusted to 20 ℃ to obtain an undrawn sheet having a thickness of about 190. mu.m.

The obtained unstretched sheet was stretched 3.5 times at 90 ℃ by a roll longitudinal stretcher and 4.5 times at 120 ℃ by a tenter type transverse stretcher, and then the transverse relaxation rate was adjusted to 3%, heat treatment was performed at 235 ℃ and slow cooling was performed to room temperature to obtain a biaxially stretched polyester film having a thickness of 12 μm.

The amounts of outgas generated from the produced biaxially stretched polyester were 8. mu.g/g of THF, 1. mu.g/g of BD and 70. mu.g/g of Ethylene Glycol (EG).

Examples 2 to 3 and comparative example 1

A biaxially stretched polyester film was obtained in the same manner as in example 1, except that the drying time of the modified PBT pellet was changed to that shown in table 1.

Example 4

Putting the modified PBT slice into a container with the diameter of 5m³In a dryer at a temperature of 120 ℃ for 5m³The drying was carried out for 24 hours while flowing nitrogen at a flow rate of/s. The amount of outgas generated from the dried modified PBT was 480. mu.g/g in THF and 8. mu.g/g in BD. Using the modified PBT pellet, a biaxially stretched polyester film was obtained in the same manner as in example 1.

Example 5 and comparative example 2

A biaxially stretched polyester film was obtained in the same manner as in example 4, except that the drying time of the modified PBT pellet was changed to that shown in table 1.

Example 6

The modified PBT chips were dried in a hot air dryer at 120 ℃ for 72 hours. The amount of outgas generated from the dried modified PBT was 1200. mu.g/g in THF and 9. mu.g/g in BD. Using the modified PBT pellet, a biaxially stretched polyester film was obtained in the same manner as in example 1.

Comparative example 3

A biaxially stretched polyester film was obtained in the same manner as in example 6, except that the drying time of the modified PBT pellet was changed to that shown in table 1.

Examples 7 to 9 and comparative examples 4 to 5

A biaxially stretched polyester film was obtained in the same manner as in example 1, except that the ratio of PTMG in the modified PBT pellet was changed as shown in table 1.

Examples 10 to 12 and comparative examples 6 to 7

A biaxially stretched polyester film was obtained in the same manner as in example 1, except that the blending ratio of the modified PBT and PET was changed as shown in table 1.

Examples 13 to 15 and comparative examples 8 to 9

A biaxially stretched polyester film was obtained in the same manner as in example 1, except that the molecular weight of the PTMG used for the modified PBT was changed as shown in table 1.

Example 16

A biaxially stretched polyester film was obtained in the same manner as in example 6, except that the blending ratio of the modified PBT and PET was changed as shown in table 1.

Comparative example 10

A biaxially stretched polyester film was obtained in the same manner as in comparative example 1, except that the blending ratio of the modified PBT and PET was changed as shown in table 1.

Table 1 shows the linearity of tear of the biaxially stretched polyester films obtained in examples and comparative examples, the amount of THF in the generated exhaust gas, and the evaluation results of oxygen permeability and water vapor permeability of the films after vapor deposition treatment.

As is clear from table 1, in the examples, by making the amount of PTMG contained in the modified PBT and the molecular weight thereof, the blending ratio of the modified PBT and PET, and the amount of THF generated from the biaxially stretched polyester film satisfy the ranges specified in the present invention, a film having excellent linear tearability, a small amount of outgas generation, and stable gas barrier properties of the film after the vapor deposition treatment can be produced.

In contrast, the comparative example had the following problems.

In comparative examples 1 to 3, the amount of PTMG contained in the modified PBT and the molecular weight thereof, and the blending ratio of the modified PBT and PET satisfy the ranges specified in the present invention, and thus a film having linear tearability can be obtained, but the oxygen permeability and water vapor permeability of the vapor deposited film become unstable because the THF generation amount exceeds the ranges specified in the present invention.

In comparative example 4, the ratio of PTMG contained in the modified PBT was more than the range defined in the present invention, and therefore film formation was difficult, and a biaxially stretched polyester film could not be obtained. In comparative example 5, the oxygen permeability and the water vapor permeability of the deposited film were stable because the THF generation amount specified in the present invention was satisfied, but the ratio of PTMG contained in the modified PBT was less than the range specified in the present invention, and thus a film having good linearity of tearing could not be obtained.

In comparative examples 6 to 7, the oxygen permeability and the water vapor permeability of the vapor deposited film were stable because the THF generation amount specified in the present invention was satisfied, but the film having good linearity in tearing could not be obtained because the ratio of the modified PBT and the PET was outside the range specified in the present invention.

In comparative examples 8 to 9, the oxygen permeability and the water vapor permeability of the vapor deposited film were stable because the THF generation amount specified in the present invention was satisfied, but a film having good linearity of tearing could not be obtained because the molecular weight of PTMG contained in the modified PBT chips was outside the range specified in the present invention.

In comparative example 10, the THF content generated from the biaxially stretched polyester film exceeded the range specified in the present invention, and therefore the oxygen permeability and water vapor permeability of the evaporated film became unstable. Furthermore, since the ratio of the modified PBT to the PET is outside the range specified in the present invention, a film having good tear linearity cannot be obtained.

Claims (3)

1. A biaxially stretched polyester film comprising a modified PBT (polybutylene terephthalate) which is a modified PBT/PET, and a PET (polyethylene terephthalate) which is a modified PBT/PET, wherein the mass ratio of the modified PBT to the PET is 20/80-5/95, the modified PBT is a PBT containing 5-20 mass% of a polytetramethylene glycol unit having a molecular weight of 600-4000, the amount of THF (tetrahydrofuran) produced by heat-treating a biaxially stretched film at 180 ℃ for 30 minutes in a helium atmosphere is 50 [ mu ] g/g or less, and the biaxially stretched film has a tear linearity in the longitudinal direction.

2. A biaxially stretched polyester film, which is characterized in that at least one surface of the biaxially stretched polyester film according to claim 1 is vapor-deposited with at least one of aluminum, aluminum oxide and silicon oxide, and which has gas barrier properties and tear straightness.

3. A process for producing a biaxially stretched polyester film according to claim 1, wherein a modified PBT (polybutylene terephthalate), which contains 5 to 20 mass% of polytetramethylene glycol units having a molecular weight of 600 to 4000, and THF (tetrahydrofuran), which is generated by heat treatment at 180 ℃ for 30 minutes in a helium atmosphere, is blended with PET (polyethylene terephthalate), which is a modified PBT, so that the mass ratio of the modified PBT to the PET is 20/80 to 5/95, and the resulting film is stretched to form a film, wherein the THF is in an amount of 800 μ g/g or less.