JPWO2020175067A1 - Biaxially oriented polyester film and method for producing biaxially oriented polyester film - Google Patents

Abstract

A biaxial orientation characterized by containing a polyester resin composition containing 60 to 100% by mass of polybutylene terephthalate resin (A), having 7 or less layers, and satisfying the following requirements (1) to (4). Polyester film. (1) The intrinsic viscosity of the biaxially oriented polyester film is 0.7 dl / g or more. (2) The plane orientation ΔP of the biaxially oriented polyester film is 0.145 to 0.160. (3) The piercing strength of the biaxially oriented polyester film measured in the piercing test according to JIS-Z1707 is 0.40 N / μm or more. (4) The thickness unevenness of the biaxially oriented polyester film is 0.7% or less.

Description

The present invention relates to a biaxially oriented polyester film and a method for producing a biaxially oriented polyester film.

Polybutylene terephthalate (hereinafter, polybutylene terephthalate may be abbreviated as "PBT") resin is superior to polyethylene terephthalate (hereinafter, polyethylene terephthalate is abbreviated as PET) resin in moldability. Applications are being studied in fields such as draw-forming films, can inner bag films, and mold release films.

For example, in Patent Document 1, the PBT resin is 60% by weight or more, the yield stress in the longitudinal direction and the width direction is 70 MPa or more, the breaking strength is 160 MPa or more, and the breaking elongation is 100% or more. It is disclosed that it can be suitably used for flexible film applications.

In Patent Document 2, a release film having a release layer containing an acid-modified olefin resin and a cross-linking agent on one side of a polyester film containing substantially no particles and an easy-slip layer on the other outermost layer is provided. It is disclosed. Further, the arithmetic average roughness of the surface of the release layer is 10 nm or less, the maximum height is 100 nm or less, the easy-slip layer contains particles, the easy-slip layer contains particles, and the arithmetic mean of the surface of the easy-slip layer. It is disclosed that it can be preferably used when the roughness is less than 10 nm.

In Patent Document 3, the surface roughness is 0.005 to 0.050 μm, the number of fish eyes of 30 μm or more ^{is 5/1000 cm 2} or less, and the difference in heat shrinkage rate in the longitudinal direction at both ends of the film is 0 to 0. It is disclosed that it can be suitably used as a release film when it is 3% and the anisotropy difference is 0 to 0.20.

Japanese Patent No. 5994864 Japanese Unexamined Patent Publication No. 2016-221737 Japanese Patent No. 3031520

Since the PBT resin has a high crystallization rate, crystallization proceeds even during casting. In particular, the edges of the cast film tend to be thicker than the center for manufacturing reasons. Therefore, the crystallization becomes more remarkable at the edge of the film obtained by casting.
In the above-mentioned Patent Document 1, a static mixer is used to super-multilayer the molten resin before extruding it from the T-die. Therefore, crystals do not grow between layers, and as a result, crystallization during casting can be suppressed. On the other hand, by introducing a static mixer to the melt line, the dead part (retained part) increases, gel is generated in that part, and the fish eyes (small particulate defects) in the resulting film increase. Therefore, there is a possibility that the quality will be insufficient for industrial use where high quality is required.
In the above-mentioned Patent Documents 2 and 3, polyethylene terephthalate is suitably used as a release film by using it as a base film. On the other hand, in mold release film applications that involve molding, it may not be possible to handle deep draw molding based on the characteristics of polyethylene terephthalate, which is the base material.

The present invention has been made in view of the above-mentioned problems. That is, an object of the present invention is to provide a biaxially oriented polyester film having good film quality suitable for industrial use and also suitable for a release film accompanied by deep drawing molding.

The present inventors have diligently studied a biaxially oriented polyester film. As a result, they have found that a biaxially oriented polyester film having good deep drawing moldability and excellent quality can be obtained by adopting the following configuration, and have completed the present invention.

That is, the biaxially oriented polyester film according to the present invention is
A polyester resin composition containing 60 to 100% by mass of polybutylene terephthalate resin (A) is contained.
The number of layers is 7 or less,
It is characterized by satisfying the following requirements (1) to (4).
(1) The intrinsic viscosity of the biaxially oriented polyester film is 0.7 dl / g or more.
(2) The plane orientation ΔP of the biaxially oriented polyester film is 0.145 to 0.160.
(3) The piercing strength of the biaxially oriented polyester film measured in the piercing test according to JIS-Z1707 is 0.40 N / μm or more.
(4) The thickness unevenness of the biaxially oriented polyester film is 0.70% or less.

According to the above configuration, since the polyester resin composition containing 60% by mass or more of the polybutylene terephthalate resin (A) is contained, the puncture strength can be improved. Further, since the polyester resin composition containing 60% by mass or more of the polybutylene terephthalate resin (A) is contained, the drawability can be improved.
Further, since the number of layers is 7 or less, it can be manufactured without introducing a static mixer into the melt line. Therefore, it is possible to prevent the occurrence of fish eyes (small particulate defects) in the film caused by the introduction of the static mixer. As a result, a film having high quality can be obtained.
Further, since the intrinsic viscosity of the film is 0.70 dl / g or more, crystallization during casting is suppressed and the yield stress of the unstretched sheet is lowered, and as a result, fracture is less likely to occur during stretching. It becomes.
Further, since the plane orientation ΔP is 0.145 or more, the plane orientation is preferably high, the piercing strength is sufficient, and the deep drawing moldability is excellent. Further, since the plane orientation ΔP is 0.160 or less, the heat shrinkage rate can be maintained low, and the thermal stability can be improved.
Further, since the piercing strength is 0.40 N / μm or more, the deep drawing moldability can be improved.
Further, since the thickness unevenness is 0.70% or less, uniform molding can be performed when deep drawing molding is performed, and tearing or pinholes of the film are unlikely to occur.

In the above configuration, the polyester resin composition preferably contains a polyester resin (B) other than the polybutylene terephthalate resin (A).

When the polyester resin composition contains a polyester resin (B) other than the polybutylene terephthalate resin (A), the film-forming property at the time of biaxial stretching and the mechanical properties of the obtained film can be adjusted.

In the above configuration, it is preferable that the maximum diameter of the biaxially oriented polyester film is 0.3 mm or more and the number of fish eyes is 5 / m ^{2 or less.}

When the number of fish eyes is 5 / m ² or less, even if one surface of the biaxially oriented polyester film has a release layer, the surface opposite to the release layer during winding and storage. On top of that, it is possible to suppress uneven transfer to the release layer due to fish eyes. As a result, the film quality can be made less likely to be impaired. Further, an elaborate printing layer can be provided on the film.

In the above configuration, the difference (absolute value) between the three-dimensional average roughness SRa on one surface of the biaxially oriented polyester film and the three-dimensional average roughness SRa on the other surface is 0.01 μm or less. It is preferable to have.

When the difference is 0.010 μm or less, it is possible to prevent the difference between the lamination strength of the one surface and the lamination strength of the other surface from becoming too large. That is, it is possible to suppress the concentration of stress on the surface having low lamination strength and to make the bag breaking resistance more sufficient.

In the above configuration, the heat shrinkage of the biaxially oriented polyester film after heating at 150 ° C. for 15 minutes is 0 to 5% in the vertical direction (MD direction) and -1 to 5% in the horizontal direction (TD direction). Is preferable.

When the heat shrinkage rate of the biaxially oriented polyester film after heating at 150 ° C. for 15 minutes in the MD direction is 5% or less, it is possible to prevent the film from shrinking significantly when heat is applied in the post-processing, and the film can be processed. Will be easier.
When the heat shrinkage rate of the biaxially oriented polyester film after heating at 150 ° C. for 15 minutes in the MD direction is 0% or more, the piercing strength can be increased and the bag breaking resistance can be maintained high.
When the heat shrinkage rate of the biaxially oriented polyester film after heating at 150 ° C. for 15 minutes in the TD direction is 5% or less, it is possible to prevent the film from shrinking significantly when heat is applied in the post-processing, and the film is processed. Will be easier.
When the heat shrinkage rate of the biaxially oriented polyester film after heating at 150 ° C. for 15 minutes in the TD direction is -1% or more, the piercing strength can be increased and the bag breaking resistance can be maintained high.

Further, the present invention is a method for producing the biaxially oriented polyester film.
Step A of casting a resin composition for producing a biaxially oriented polyester film onto a cooling roll to form an unstretched sheet, and
It is characterized by having a step B of blowing a wind of 5 ° C. or lower on the unstretched sheet on the cooling roll.

According to the above configuration, the F surface (the surface opposite to the surface in contact with the cooling roll) of the unstretched sheet is rapidly cooled by blowing wind of 5 ° C. or lower on the unstretched sheet on the cooling roll. The difference in crystallinity from the surface in contact with the cooling roll) becomes small. In addition, the crystallinity of the entire unstretched sheet is low, and the crystallinity unevenness is reduced. As a result, biaxial stretching can be easily performed, and a film having high surface orientation and puncture strength and less thickness unevenness can be obtained.

According to the present invention, it is possible to provide a biaxially oriented polyester film having good deep drawing moldability and excellent quality.

It is a front schematic diagram for demonstrating the method of blowing the cooling air from a multi-duct to the B surface of the unstretched sheet on a cooling roll. It is a side view of FIG. It is a cross-sectional view of the mold used for the evaluation of the deep drawing formability. It is a top view of the mold shown in FIG.

Hereinafter, embodiments of the present invention will be described.

The biaxially oriented polyester film according to the present embodiment contains a polyester resin composition containing 60 to 100% by mass of PBT resin (A).
The number of layers is 7 or less,
A biaxially oriented polyester film characterized by satisfying the following requirements (1) to (4).
(1) The intrinsic viscosity of the biaxially oriented polyester film is 0.7 dl / g or more.
(2) The plane orientation ΔP of the biaxially oriented polyester film is 0.145 to 0.160.
(3) The piercing strength of the biaxially oriented polyester film measured in the piercing test according to JIS-Z1707 is 0.40 N / μm or more.
(4) The thickness unevenness of the biaxially oriented polyester film is 0.70% or less.

[Composition of biaxially oriented polyester film]
The biaxially oriented polyester film according to the present embodiment contains a polyester resin composition containing 60 to 100% by mass of PBT resin (A). The content of the PBT resin (A) is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more. Since the polyester resin composition containing 60% by mass or more of the PBT resin (A) is contained, the piercing strength can be improved. Further, since the polyester resin composition containing 60% by mass or more of the PBT resin (A) is contained, the drawability can be improved.
The PBT resin (A) used as the main constituent component preferably contains terephthalic acid in an amount of 90 mol% or more, more preferably 95 mol% or more, and further preferably 98 mol% or more as a dicarboxylic acid component. It is preferably 100 mol%. The glycol component of 1,4-butanediol is preferably 90 mol% or more, more preferably 95 mol% or more.

The lower limit of the intrinsic viscosity of the PBT resin (A) is preferably 0.8 dl / g, more preferably 0.95 dl / g, and further preferably 1.0 dl / g. By setting the intrinsic viscosity of the PBT resin (A) to 0.8 dl / g or more, the intrinsic viscosity of the film obtained by forming a film can be increased, and the bag breaking resistance and the film forming property can be improved. it can.
The upper limit of the intrinsic viscosity of the PBT resin (A) is preferably 1.3 dl / g. By setting the value to 1.3 dl / g or less, it is possible to prevent the stress at the time of stretching the film from becoming too high, and to improve the film forming property. Furthermore, when a PBT resin having a high intrinsic viscosity is used, the melt viscosity of the resin becomes high, so that the extrusion temperature needs to be raised. However, a PBT resin (A) having an intrinsic viscosity of 1.3 dl / g or less is used. As a result, it is not necessary to extrude at a high temperature, and the generation of decomposition products can be suppressed.

The polyester resin composition preferably contains a polyester resin (B) other than the PBT resin (A) for the purpose of adjusting the film-forming property at the time of biaxial stretching and the mechanical properties of the obtained film.
Examples of the polyester resin (B) include polyester resins such as PET, polyethylene naphthalate, polybutylene naphthalate, and polypropylene terephthalate, or isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid, cyclohexanedicarboxylic acid, adipic acid, and azeline. PBT resin copolymerized with at least one dicarboxylic acid selected from the group consisting of acid and sebacic acid, ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, neopentyl glycol, 1,5- PBT resin, isophthalic acid, orthophthalic acid copolymerized with at least one diol component selected from the group consisting of pentanediol, 1,6-hexanediol, diethylene glycol, cyclohexanediol, polyethylene glycol, polytetramethylene glycol and polycarbonate. , A PBT resin copolymerized with at least one dicarboxylic acid selected from the group consisting of naphthalenedicarboxylic acid, biphenyldicarboxylic acid, cyclohexanedicarboxylic acid, adipic acid, azelaic acid and sebacic acid, or 1,3-butanediol. From the group consisting of 1,3-propylene glycol, 1,2-propylene glycol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, cyclohexanediol, polyethylene glycol, polytetramethylene glycol and polycarbonate. Examples thereof include at least one resin selected from PET resins in which at least one selected diol component is copolymerized.

Among them, a resin containing PET as a main component has a high melting point, excellent heat resistance, good compatibility with PBT resin, and excellent transparency. Therefore, PET resin or copolymerized PET resin is preferable, and PET resin is particularly preferable.

The amount of the polyester resin (B) added is preferably 30% by mass or less, more preferably 20% by mass or less, still more preferably 10% by mass or less, based on the entire polyester resin composition. When the amount of the polyester resin (B) added is 30% by mass or less, the piercing strength can be further increased and the bag breaking resistance is more excellent. Moreover, the transparency can be kept high.
The amount of the polyester resin (B) added can be 0% by mass or more, 5% by mass or more, 10% by mass or more, or the like with respect to the entire polyester resin composition.

The lower limit of the intrinsic viscosity of the polyester resin (B) is preferably 0.5 dl / g, more preferably 0.6 dl / g, and further preferably 0.8 dl / g. The upper limit of the intrinsic viscosity of the polyester resin (B) is preferably 1.3 dl / g. By setting the intrinsic viscosity of the polyester resin (B) within the above numerical range, it becomes possible to better mix with the PBT resin (A).

In addition to the polyester resin composition, the biaxially oriented polyester film contains conventionally known additives such as lubricants, stabilizers, colorants, antioxidants, antistatic agents, and ultraviolet absorbers. You may.

When the entire biaxially oriented polyester film is 100% by mass, the content of the polyester resin composition is preferably 99.5% by mass or more, more preferably 99.6% by mass or more, and 99.7% by mass or more. Is even more preferable.
The content of the polyester resin composition is preferably 100% by mass or less, more preferably 99.9% by mass or less, and further preferably 99.8% by mass or less, when the entire biaxially oriented polyester film is taken as 100% by mass. preferable.

The lubricant can adjust the dynamic friction coefficient of the film, and examples thereof include inorganic lubricants such as silica, calcium carbonate, and alumina, and organic lubricants. Silica and calcium carbonate are more preferable, and silica is particularly preferable in that haze is reduced. These can be expressed as transparent and slippery.

The lower limit of the lubricant content in the biaxially oriented polyester film is preferably 100 mass ppm, more preferably 800 mass ppm. The slipperiness can be improved by setting the content to 100 mass ppm or more.
The upper limit of the content of the lubricant is preferably 20000 mass ppm, more preferably 1000 mass ppm, and particularly preferably 1800 mass ppm. Transparency can be maintained by setting the mass to 20000 mass ppm or less.

[Layer structure of biaxially oriented polyester film]
The biaxially oriented polyester film has 7 or less layers. The number of layers is more preferably 3 or less. From the viewpoint of reducing fish eyes, a single layer is most preferable. When it is desired to improve the surface characteristics of the film, a layer structure of 2 types 2 layers, 2 types 3 layers, or 3 types 3 layers in which a plurality of types having different compositions are laminated is preferable. Properties to be improved include slipperiness, antistatic property, ultraviolet ray blocking property, and easy adhesiveness. Improvement of these surface properties can be achieved by adding a lubricant, an antistatic agent, an ultraviolet absorber, a copolymer polymer, and an auxiliary agent thereof to the surface layer of the plurality of layers.

[Characteristics of biaxially oriented polyester film]
(1) Intrinsic Viscosity of Biaxially Oriented Polyester Film The intrinsic viscosity of the biaxially oriented polyester film is 0.7 dl / g or more. The intrinsic viscosity is more preferably 0.75 dl / g, further preferably 0.80 dl / g, and particularly preferably 0.90 dl / g. Since it is 0.70 dl / g or more, crystallization during casting is suppressed and the yield stress of the unstretched sheet is lowered, and as a result, fracture is less likely to occur during stretching.
The upper limit of the intrinsic viscosity of the obtained unstretched sheet is preferably 1.2 dl / g, and more preferably 1.1 dl / g. When it is 1.2 dl / g or less, the stress at the time of stretching does not become too high, and the film forming property becomes better.
The intrinsic viscosity of the unstretched sheet is substantially the same as the intrinsic viscosity of the biaxially oriented polyester film, and the lower limit is preferably 0.7 dl / g, more preferably 0.75 dl / g, still more preferable. Is 0.80 dl / g, particularly preferably 0.90 dl / g.
In order to keep the intrinsic viscosity of the biaxially oriented polyester film in a preferable range, it is preferable to set the extrusion temperature as low as possible. Although the temperature changes depending on the intrinsic viscosity of the polyester resin used, the shape of the extruder, etc., it is possible to suppress the decrease in the intrinsic viscosity of the biaxially oriented polyester film by extrusion at a low temperature, and as a result, breakage occurs during stretching. It tends to be less likely to occur.

(2) Surface orientation of biaxially oriented polyester film ΔP
The plane orientation ΔP of the biaxially oriented polyester film is 0.145 to 0.160. The plane orientation ΔP is preferably 0.148 or more, and more preferably 0.151 or more. The plane orientation ΔP is preferably 0.158 or less, more preferably 0.156 or less, and further preferably 0.154 or less. Since the plane orientation ΔP is 0.145 or more, the plane orientation is preferably high, the piercing strength is sufficient, and the deep drawing moldability is excellent. Further, since the plane orientation ΔP is 0.160 or less, the heat shrinkage rate can be maintained low, and the thermal stability can be improved.

(3) Puncture strength The piercing strength of the biaxially oriented polyester film measured in a piercing test according to JIS-Z1707 is 0.40 N / μm or more. The piercing strength is preferably 0.5 N / μm, more preferably 0.6 N / μm. Since the piercing strength is 0.40 N / μm or more, the deep drawing moldability can be improved.
The upper limit of the piercing strength is preferably 0.9 N / μm. It is more preferably 0.8 N / μm, and even more preferably 0.7 N / μm. When it is 0.9 N / μm or less, the heat shrinkage rate is suppressed and the heat stability is more excellent.

(4) Thickness and Thickness Spot of Biaxially Oriented Polyester Film The lower limit of the thickness of the biaxially oriented polyester film is preferably 5 μm. It is more preferably 7 μm and even more preferably 9 μm. When it is 5 μm or more, the strength as a film can be made sufficient.
The upper limit of the thickness of the biaxially oriented polyester film is preferably 40 μm. It is more preferably 30 μm, still more preferably 20 μm.
From the viewpoint of flexibility and resource saving, it is preferable that the thickness is thin.

The thickness unevenness of the biaxially oriented polyester film is 0.7% or less. The thickness unevenness is preferably 0.6% or less, more preferably 0.5% or less. Since the thickness unevenness is 0.007% or less, uniform molding can be performed when deep drawing molding is performed, and tearing or pinholes of the film are unlikely to occur. The method for determining the thickness unevenness is the method described in the examples.

^{(5) Fish Eye The biaxially oriented polyester film preferably has 5 fish eyes / m 2} or less having a maximum diameter of 0.3 mm or more on the biaxially oriented polyester film. The fish eyes are more preferably 4 pieces / m ² or less, and further preferably 3 pieces / m ² or less. When the number of fish eyes is 5 / m ² or less, even if one surface of the biaxially oriented polyester film has a release layer, the surface opposite to the release layer during winding and storage. On top of that, it is possible to suppress uneven transfer to the release layer due to fish eyes. As a result, the film quality can be made less likely to be impaired.

(6) Difference in Three-Dimensional Average Roughness SRa Difference between the three-dimensional average roughness SRa on one surface of the biaxially oriented polyester film surface and the three-dimensional average roughness SRa on the other surface (absolute). The value) is preferably 0.01 μm or less. The difference is more preferably 0.008 μm or less, still more preferably 0.006 μm or less. When the difference is 0.010 μm or less, it is possible to prevent the difference between the lamination strength of the one surface and the lamination strength of the other surface from becoming too large. That is, it is possible to suppress the concentration of stress on the surface having low lamination strength and to make the bag breaking resistance sufficient.

The lower limit of the three-dimensional average roughness SRa of the one surface of the biaxially oriented polyester film and the three-dimensional average roughness SRa of the other surface is preferably 0.01 μm. It is more preferably 0.02 μm, and even more preferably 0.03 μm. When the three-dimensional average roughness SRa is 0.01 μm or more, the slipperiness of the film can be made more sufficient.
The upper limit of the three-dimensional average roughness SRa of the one surface of the biaxially oriented polyester film and the three-dimensional average roughness SRa of the other surface is preferably 0.10 μm. It is more preferably 0.08 μm, and even more preferably 0.06 μm. When the three-dimensional average roughness SRa is 0.10 μm or less, a sufficient contact area at the time of film bonding can be sufficiently secured, the lamination strength can be further increased, and the bag breaking resistance can be made more sufficient. it can.

(7) Heat Shrinkage Rate The upper limit of the heat shrinkage rate after heating the biaxially oriented polyester film at 150 ° C. for 15 minutes in the MD direction is preferably 5%. It is more preferably 3.0%, still more preferably 2.5%. When it is 5% or less, it is possible to prevent the film from shrinking significantly when heat is applied in the post-processing, and the processing becomes easier.
The lower limit of the heat shrinkage rate after heating the biaxially oriented polyester film at 150 ° C. for 15 minutes in the MD direction is preferably 0%. It is more preferably 0.8%, still more preferably 1.2%. When it is 0%% or more, the piercing strength can be increased and the bag breaking resistance can be maintained high.
The upper limit of the heat shrinkage rate after heating the biaxially oriented polyester film at 150 ° C. for 15 minutes in the TD direction is preferably 5%. It is more preferably 3.0%, still more preferably 2.5%. When it is 5% or less, it is possible to prevent the film from shrinking significantly when heat is applied in the post-processing, and the processing becomes easier.
The lower limit of the heat shrinkage rate after heating the biaxially oriented polyester film at 150 ° C. for 15 minutes in the TD direction is preferably -1%. It is more preferably 0.8%, still more preferably 1.2%. When it is -1%% or more, the piercing strength can be increased and the bag breaking resistance can be maintained high.

The lower limit of the impact strength of the biaxially oriented polyester film is preferably 0.65 J / 15 μm. It is more preferably 0.70 J / 15 μm, and even more preferably 0.75 J / 15 μm.
When it is 0.65 J / 15 μm or more, the deep drawing moldability can be improved.
The upper limit of the impact strength of the biaxially oriented polyester film is preferably 1.20 J / 15 μm. It is more preferably 1.10 J / 15 μm, and even more preferably 1.00 J / 15 μm. When it is 1.20 J / 15 μm or less, the heat shrinkage rate is suppressed and the heat stability is more excellent.

The pinhole resistance of the biaxially oriented polyester film is such that the number of pinholes generated after a bending test using a gelboflex tester (repeating 2000 cycles continuously at a speed of 40 cycles per minute at 5 ° C.) is 15 or less. Is preferable. It is more preferably 10 or less, and further preferably 5 or less.

The biaxially oriented polyester film preferably satisfies the following requirement (X).
(X) Fourier Transform Infrared Spectroscopy The ratio A / B of the absorption intensity A of the peak of ^{1450 ± 10 cm -1} and the absorption intensity B of the peak of ^{1410 ± 10 cm -1} obtained by using the total reflection method in the spectral intensity. The difference (absolute value) between the ratio A / B of one surface and the ratio A / B of the other surface is 0.1 or less.
The difference is more preferably 0.08 or less, still more preferably 0.06 or less.
The absorption intensity A of the peak of 1450 ± 10 cm ^-1 is the absorption derived from the methylene group of the α-type crystal of PBT, and ^{the absorption intensity B of the peak of 1410 ± 10 cm -1} is the absorption derived from the carbon bond of the benzene ring. Is. The value of the absorption intensity ratio A / B indicates the orientation coefficient. The smaller the orientation coefficient, the lower the crystallinity, and the higher the orientation coefficient, the higher the crystallinity.
When the difference is 0.1 or less, the biaxially oriented polyester film has the same crystallinity, so that the difference in lamination strength between one surface and the other surface becomes small. As a result, it is possible to suppress the concentration of stress on the surface having low lamination strength, and it is possible to make the bag breaking resistance sufficient.
In this specification, the difference and the absorption intensity ratio A / B are values measured with the direction in which the measurement sample is set as the MD direction. This is because the measured value in the MD direction more clearly reflects the crystallinity. A more detailed measurement method is based on the method described in Examples.
When the biaxially oriented polyester film to be measured is a single-wafer film, the MD direction may be unknown. In this case, the absorption intensity ratios A / B are obtained for a plurality of directions, and the direction in which the difference is maximum is defined as the MD direction.

In the biaxially oriented polyester film, the lower limit of the value of the ratio A / B of the absorption intensity A and the absorption intensity B is preferably 0.2 on either one surface or the other surface. It is more preferably 0.3 and even more preferably 0.4. When the ratio A / B is 0.2 or more, the orientation becomes higher, the crystallinity becomes higher, and the mechanical strength can be made more sufficient.
The upper limit of the value of the ratio A / B is preferably 0.9. It is more preferably 0.8, and even more preferably 0.7. When the ratio A / B is 0.9 or less, the orientation does not become too high and the crystallinity can be made appropriate. As a result, the affinity with the adhesive can be enhanced, and the lamination strength can be made more sufficient. In addition, the bag tear resistance can be made sufficient.

The lower limit of the value of the refractive index Nz in the thickness direction of the one surface of the biaxially oriented polyester film and the value of the refractive index Nz in the thickness direction of the other surface is preferably 1.480. It is more preferably 1.485 and even more preferably 1.490. When it is 1.480 or more, the heat shrinkage rate is suppressed and the heat stability is more excellent.
The upper limit of the value of the refractive index Nz in the thickness direction of the one surface of the biaxially oriented polyester film and the value of the refractive index Nz in the thickness direction of the other surface is preferably 1.510. It is more preferably 1.505 and even more preferably 1.500. When it is 1.510 or less, the piercing strength can be further increased, and as a result, the bag breaking resistance can be made more sufficient.

It is preferable that the lamination strength of both the one surface and the other surface of the biaxially oriented polyester film is 4.0 N / 15 mm or more. It is more preferably 5.0 N / 15 mm, and more preferably 6.0 N / 15 mm.
When it is 4.0 N / 15 mm or more, it is possible to prevent the bag from breaking from the laminated portion when the bag is produced and dropped parallel to the floor, and the bag break resistance is more excellent.

A print layer may be laminated on the biaxially oriented polyester film. As the printing ink forming the printing layer, water-based and solvent-based resin-containing printing inks can be preferably used. Examples of the resin used for the printing ink include an acrylic resin, a urethane resin, a polyester resin, a vinyl chloride resin, a vinyl acetate copolymer resin, and a mixture thereof. Known printing inks include antistatic agents, light blocking agents, UV absorbers, plasticizers, lubricants, fillers, colorants, stabilizers, lubricants, defoamers, cross-linking agents, blocking agents, antioxidants and the like. Additives may be included.

The printing method for providing the print layer is not particularly limited, and known printing methods such as an offset printing method, a gravure printing method, and a screen printing method can be used. For drying the solvent after printing, known drying methods such as hot air drying, hot roll drying, and infrared drying can be used.

Further, the biaxially oriented polyester film may be subjected to corona discharge treatment, glow discharge treatment, flame treatment, surface roughening treatment, as long as the object of the present invention is not impaired, and a known anchor coat. It may be processed, printed, decorated, etc.

A gas barrier layer such as an inorganic thin film layer or a metal foil such as an aluminum foil can be provided on at least one surface of the biaxially oriented polyester film.

When an inorganic thin film layer is used as the gas barrier layer, the inorganic thin film layer is a thin film made of a metal or an inorganic oxide. The material for forming the inorganic thin film layer is not particularly limited as long as it can be formed into a thin film, but from the viewpoint of gas barrier properties, inorganic oxidation such as silicon oxide (silica), aluminum oxide (alumina), and a mixture of silicon oxide and aluminum oxide Things are preferred. In particular, a composite oxide of silicon oxide and aluminum oxide is preferable from the viewpoint of achieving both flexibility and denseness of the thin film layer.

In this composite oxide, the mixing ratio of silicon oxide and aluminum oxide is preferably in the range of 20 to 70% of Al in terms of mass ratio of metal content. When the Al concentration is 20% or more, the water vapor gas barrier property can be further enhanced. On the other hand, when it is 70% or less, the inorganic thin film layer can be softened, and it is possible to prevent the film from being destroyed during secondary processing such as printing or laminating to reduce the gas barrier property. The silicon oxide referred to here is _{various silicon oxides such as SiO and SiO 2} or a mixture thereof, and aluminum oxide is various aluminum oxides such as _{AlO and Al 2} O _{3 or a mixture thereof.}

The film thickness of the inorganic thin film layer is usually 1 to 100 nm, preferably 5 to 50 nm. When the film thickness of the inorganic thin film layer is 1 nm or more, more satisfactory gas barrier properties can be easily obtained. On the other hand, if it is 100 nm or less, it is advantageous in terms of bending resistance and manufacturing cost.

The method for forming the inorganic thin film layer is not particularly limited, and is known, for example, a physical vapor deposition method (PVD method) such as a vacuum vapor deposition method, a sputtering method, or an ion plating method, or a chemical vapor deposition method (CVD method). The method may be adopted as appropriate. Hereinafter, a typical method for forming the inorganic thin film layer will be described by taking a silicon oxide / aluminum oxide thin film as an example. For example, when the vacuum vapor deposition method is adopted, a _{mixture of SiO 2} and Al ₂ O ₃ or a mixture of SiO ₂ and Al is preferably used as the vapor deposition raw material. Particles are usually used as these vapor deposition raw materials, but at that time, it is desirable that the size of each particle is such that the pressure at the time of vapor deposition does not change, and the particle size is preferably 1 mm to 5 mm. For heating, methods such as resistance heating, high frequency induction heating, electron beam heating, and laser heating can be adopted. Further, it is also possible to introduce oxygen, nitrogen, hydrogen, argon, carbon dioxide gas, water vapor or the like as the reaction gas, or to adopt reactive vapor deposition using means such as ozone addition and ion assist. Further, the film forming conditions can be arbitrarily changed, such as applying a bias to the film to be vapor-deposited (laminated film to be subjected to vapor deposition) or heating or cooling the film to be vapor-deposited. Such a vapor-deposited material, a reaction gas, a bias of the material to be vapor-deposited, heating / cooling, and the like can be similarly changed when the sputtering method or the CVD method is adopted. Further, a printing layer may be laminated on the inorganic thin film layer.

In the present embodiment, it is preferable to provide a protective layer on the gas barrier layer. The gas barrier layer made of metal oxide is not a completely dense film, but is dotted with minute defects. By applying a specific protective layer resin composition described later on the metal oxide layer to form the protective layer, the resin in the protective layer resin composition permeates the defective portion of the metal oxide layer. As a result, the effect of stabilizing the gas barrier property can be obtained. In addition, by using a material having a gas barrier property for the protective layer itself, the gas barrier performance of the laminated film is greatly improved.

Examples of the protective layer include urethane-based, polyester-based, acrylic-based, titanium-based, isocyanate-based, imine-based, and polybutadiene-based resins to which epoxy-based, isocyanate-based, and melamine-based curing agents are added. .. Examples of the solvent (solvent) used for forming the protective layer include aromatic solvents such as benzene and toluene; alcohol solvents such as methanol and ethanol; ketone solvents such as acetone and methyl ethyl ketone; ethyl acetate and acetate. Esther-based solvents such as butyl; polyhydric alcohol derivatives such as ethylene glycol monomethyl ether and the like can be mentioned.

In the urethane resin, the polar group of the urethane bond interacts with the inorganic thin film layer and also has flexibility due to the presence of the amorphous portion, so that damage to the inorganic thin film layer is suppressed even when a bending load is applied. It is preferable because it can be used.
The acid value of the urethane resin is preferably in the range of 10 to 60 mgKOH / g. It is more preferably in the range of 15 to 55 mgKOH / g, and even more preferably in the range of 20 to 50 mgKOH / g. When the acid value of the urethane resin is within the above range, the liquid stability is improved when it is made into an aqueous dispersion, and the protective layer can be uniformly deposited on the highly polar inorganic thin film, so that the coat appearance is good. It becomes.

The urethane resin has a glass transition temperature (Tg) of preferably 80 ° C. or higher, more preferably 90 ° C. or higher. By setting the Tg to 80 ° C. or higher, the swelling of the protective layer due to molecular motion in the wet heat treatment process (heating-retaining-lowering) can be reduced.

From the viewpoint of improving gas barrier properties, it is more preferable to use a urethane resin containing an aromatic or aromatic aliphatic diisocyanate component as a main component.
Among them, it is particularly preferable to contain a metaxylylene diisocyanate component. By using the above resin, the cohesive force of the urethane bond can be further enhanced by the stacking effect of the aromatic rings, and as a result, a good gas barrier property can be obtained.

In the present embodiment, the proportion of aromatic or aromatic aliphatic diisocyanate in the urethane resin is preferably in the range of 50 mol% or more (50 to 100 mol%) in 100 mol% of the polyisocyanate component (F). .. The ratio of the total amount of the aromatic or aromatic aliphatic diisocyanate is preferably 60 to 100 mol%, more preferably 70 to 100 mol%, still more preferably 80 to 100 mol%. As such a resin, the "Takelac (registered trademark) WPB" series commercially available from Mitsui Chemicals, Inc. can be preferably used. When the ratio of the total amount of aromatic or aromatic aliphatic diisocyanate is 50 mol% or more, better gas barrier property can be obtained.

The urethane resin preferably has a carboxylic acid group (carboxyl group) from the viewpoint of improving the affinity with the inorganic thin film layer. In order to introduce a carboxylic acid (salt) group into the urethane resin, for example, a polyol compound having a carboxylic acid group such as dimethylolpropionic acid or dimethylolbutanoic acid may be introduced as a copolymerization component as a polyol component. Further, if the carboxylic acid group-containing urethane resin is synthesized and then neutralized with a salt-forming agent, an aqueous dispersion urethane resin can be obtained. Specific examples of the salt forming agent include trialkylamines such as ammonia, trimethylamine, triethylamine, triisopropylamine, tri-n-propylamine and tri-n-butylamine, and N such as N-methylmorpholin and N-ethylmorpholin. Examples thereof include N-dialkylalkanolamines such as −alkylmorpholins, N-dimethylethanolamine and N-diethylethanolamine. These may be used alone or in combination of two or more.

A layer of another material may be laminated on the biaxially oriented polyester film, and as a method thereof, the biaxially oriented polyester film can be bonded after production or during film formation.

The biaxially oriented polyester film can be used as a packaging material, for example, by providing an inorganic vapor-deposited layer on the biaxially oriented polyester film and further forming a thermosetting resin layer (also referred to as a sealant layer) called a sealant. .. The formation of the thermosetting resin layer is usually performed by an extrusion laminating method or a dry laminating method. The thermoplastic polymer forming the heat-sealable resin layer may be any as long as it can sufficiently exhibit sealant adhesiveness, and polyethylene resins such as HDPE, LDPE, and LLDPE, and polypropylene resins. Ethylene-vinyl acetate copolymer, ethylene-α-olefin random copolymer, ionomer resin and the like can be used.

The sealant layer may be a single-layer film or a multilayer film, and may be selected according to the required function. For example, in terms of imparting moisture resistance, a multilayer film in which a resin such as an ethylene-cyclic olefin copolymer or polymethylpentene is interposed can be used.
Further, the sealant layer may contain various additives such as a flame retardant, a slip agent, an antiblocking agent, an antioxidant, a light stabilizer, and a tackifier.
The thickness of the sealant layer is preferably 10 to 100 μm, more preferably 20 to 60 μm.

The biaxially oriented polyester film can be used as a base film (base material layer) of a packaging laminate. As for the layer structure of the packaging laminate, when the boundary between the layers is represented by /, for example, a base material layer / gas barrier layer / protective layer, a base material layer / gas barrier layer / protective layer / adhesive layer / sealant layer, a base material. Layer / Gas barrier layer / Protective layer / Adhesive layer / Resin layer / Adhesive layer / Sealant layer, Base material layer / Adhesive layer / Resin layer / Gas barrier layer / Protective layer / Adhesive layer / Sealant layer, Base material layer / Gas barrier layer / protective layer / printed layer / adhesive layer / sealant layer, base material layer / printed layer / gas barrier layer / protective layer / adhesive layer / sealant layer, base material layer / gas barrier layer / protective layer / adhesive layer / Resin layer / printing layer / adhesive layer / sealant layer, base material layer / adhesive layer / resin layer / printing layer / gas barrier layer / protective layer / adhesive layer / sealant layer, base material layer / printing layer / gas barrier layer / Protective layer / adhesive layer / resin layer / adhesive layer / sealant layer, base material layer / printing layer / adhesive layer / resin layer / gas barrier layer / protective layer / adhesive layer / sealant layer, base material layer / adhesive Examples include a layer / resin layer / gas barrier layer / protective layer / printing layer / adhesive layer / sealant layer.

The laminate using the biaxially oriented polyester film can be suitably used for applications such as packaging products, various label materials, lid materials, sheet molded products, and laminated tubes. In particular, it is used for packaging bags (for example, pillow bags, pouches such as standing pouches and 4-way pouches). The thickness of the laminate can be appropriately determined according to the application. For example, it is used in the form of a film or sheet having a thickness of about 5 to 500 μm, preferably about 10 to 300 μm.

[Manufacturing method of biaxially oriented polyester film]
As a suitable method for obtaining the biaxially oriented polyester film, the T-die method is preferable from the viewpoint of thickness accuracy in the width direction. In the inflation method, the draw ratio is difficult to increase due to the manufacturing method, and thickness defects in the width direction may occur.
Further, as a suitable method for obtaining the biaxially oriented polyester film, when the molten polyester resin composition is cast on a cooling roll, conventionally, a static mixer or the like is used to crystallize the resin having the same composition in eight or more layers. A method of obtaining a uniform unstretched sheet by suppressing crystallization has been proposed, but from the viewpoint of obtaining a film having high quality, and from the viewpoint of convenience of equipment and maintainability, the homogeneous unstretched sheet described below is used. In the casting method obtained, it is preferable to form a stack of 7 layers or less when melt-extruding. The number of layers is more preferably 3 layers or less. In terms of equipment maintenance, a single layer is most preferable. When it is desired to improve the surface characteristics of the film, a layer structure of 2 types, 2 layers, 2 types, 3 layers, or 3 types, 3 layers is preferable.
Since the PBT resin has a high crystallization rate, the lower limit of the intrinsic viscosity of the obtained unstretched sheet is preferably 0.7 dl / g, more preferably 0.75 dl / g, and further preferably 0.80 dl / g. It is particularly preferably 0.90 dl / g. When it is 0.70 dl / g or more, crystallization during casting is suppressed and the yield stress of the unstretched sheet becomes low, and as a result, fracture tends to be less likely to occur during stretching.
The upper limit of the intrinsic viscosity of the obtained unstretched sheet is preferably 1.2 dl / g, and more preferably 1.1 dl / g. When it is 1.2 dl / g or less, the stress at the time of stretching does not become too high, and the film forming property becomes better.

The lower limit of the die temperature is preferably 240 ° C., more preferably 245 ° C., and particularly preferably 250 ° C. When the temperature is 240 ° C. or higher, the discharge can be more stable and the thickness can be made more uniform.
Further, when the temperature is 240 ° C. or higher, it is possible to prevent the PET resin retained in the melt extrusion process of the resin from becoming an unmelted substance and being mixed into the film, thereby impairing the quality of the film. The upper limit of the resin melting temperature is preferably 275 ° C, more preferably 270 ° C, and most preferably 265 ° C. When the temperature is 275 ° C. or lower, decomposition of the resin can be suppressed and the film can be prevented from becoming brittle. In addition, the intrinsic viscosity of the obtained film can be maintained high, and the film-forming property can be improved.
The upper limit of the die temperature is preferably 300 ° C., more preferably 290 ° C. or lower, and further preferably 280 ° C. or lower. When the temperature is 300 ° C. or lower, it is possible to prevent the thickness from becoming non-uniform. In addition, it is possible to prevent the resin from being deteriorated and having a poor appearance due to stains on the die lip or the like.

The lower limit of the rotation speed of the screw in the melt extrusion step of the resin is preferably 70 rpm, more preferably 80 rpm, and particularly preferably 90 rpm. When it is 70 rpm or more, the discharge becomes more stable and the thickness becomes more uniform. In addition, the resin is more sufficiently mixed, and poor appearance can be further suppressed.
The upper limit of the rotation speed of the screw in the process of melt-extruding the resin is preferably 150 rpm, more preferably 130 rpm, and particularly preferably 110 rpm. When it is 150 rpm or less, it is possible to suppress the decomposition of the molten resin due to the heat generated by shearing, the decrease in the intrinsic viscosity of the obtained film can be suppressed, and the film forming property can be improved.

The upper limit of the cooling roll temperature is preferably 40 ° C., more preferably 10 ° C. or lower. When the temperature is 40 ° C. or lower, the crystallinity of the molten polyester resin composition when it is cooled and solidified does not become too high, and stretching becomes easier. In addition, the laminate strength can be increased, and the bag breaking resistance can be improved.
The lower limit of the cooling roll temperature is preferably 0 ° C. When the temperature is 0 ° C. or higher, the effect of suppressing crystallization when the molten polyester resin composition is cooled and solidified can be sufficiently exhibited. When the temperature of the cooling roll is within the above range, it is preferable to lower the humidity of the environment near the cooling roll in order to prevent dew condensation.

When the molten polyester resin composition is cast on the surface of the cooling roll, the temperature of the surface of the cooling roll rises because the high-temperature resin comes into contact with the surface. Normally, the chill roll is cooled by flowing cooling water through a pipe inside, but the surface of the chill roll is cooled by ensuring a sufficient amount of cooling water, devising the arrangement of the pipe, and performing maintenance to prevent sludge from adhering to the pipe. It is necessary to reduce the temperature difference in the width direction. In particular, if a method for obtaining a uniform unstretched sheet by forming multiple layers into eight or more layers is not used, crystallization of the unstretched sheet tends to proceed, so caution is required.
At this time, the thickness of the unstretched sheet is preferably in the range of 15 to 2500 μm. It is more preferably 500 μm or less, still more preferably 300 μm or less.

For manufacturing biaxially oriented polyester films from the viewpoint of reducing the difference in crystallinity between the F surface (the surface in contact with the cooling roll) and the B surface (the surface opposite to the surface in contact with the cooling roll) of the unstretched sheet. After casting the resin composition onto the cooling roll (after step A), it is preferable that the B surface of the unstretched sheet on the cooling roll is also rapidly cooled. By quenching the B-plane, the difference in crystallinity from the F-plane becomes small. In addition, the crystallinity of the entire unstretched sheet is low, and the crystallinity unevenness is reduced. As a result, biaxial stretching can be easily performed, and a film having high surface orientation and puncture strength and less thickness unevenness can be obtained. The method of quenching is not particularly limited, but a method of blowing cooling air by a multi-duct is preferable from the viewpoint of convenience and maintainability of the equipment.
The upper limit of the temperature of the cooling air is preferably 20 ° C., more preferably 5 ° C. or lower. When the temperature is 20 ° C. or lower, the crystallinity of the B surface of the unstretched sheet does not become too high, the laminate strength can be maintained high, and the bag breaking resistance can be improved.
The lower limit of the temperature of the cooling air is -5 ° C. When the temperature is −5 ° C. or higher, the effect of suppressing crystallization of the B surface of the unstretched sheet can be sufficiently obtained.

FIG. 1 is a front schematic view for explaining a method of blowing cooling air from a multi-duct onto the B surface of an unstretched sheet on a cooling roll, and FIG. 2 is a side view thereof.
As shown in FIGS. 1 and 2, the die 10 is arranged so that the discharge port 12 faces the surface of the cooling roll 20. The distance (shortest distance) between the discharge port 12 and the surface of the cooling roll 20 is not particularly limited, but is generally about 2 cm to 10 cm.
Further, a multi-duct 30 is arranged on the outer peripheral surface of the cooling roll 20.
The position of the multi-duct 30 is the rotation direction of the cooling roll 20 (clockwise rotation direction in FIG. 2) when the position of the die 10 is 0 ° with respect to the center of the cooling roll 20 in the side view (FIG. 2). On the other hand, it is preferably installed in the range of 0 to 45 °, and more preferably installed in the range of 10 to 35 °. When the multi-duct 30 is arranged within the above range, the B surface of the unstretched sheet 40 cast on the cooling roll 20 can be cooled immediately after casting.
Further, the time from the time when the resin composition for producing the biaxially oriented polyester film is cast to the cooling roll (the time when it is touched) until the wind is blown to the portion is preferably 2.0 seconds or less. Within 0 seconds is more preferable, and within 0.5 seconds is even more preferable. From the time when the resin composition for producing the biaxially oriented polyester film is cast on the cooling roll (when it is touched) to the time when the wind is blown on the portion (until the portion moves directly under the multi-duct 30). When the time) is within 2.0 seconds, the B surface of the unstretched sheet 40 can be cooled immediately after casting.

The width of the multi-duct 30 (the length in the left-right direction in FIG. 1) is preferably equal to or larger than the width of the unstretched sheet 40 to be cast. By setting the width of the multi-duct 30 to be equal to or larger than the width of the cast unstretched sheet 40, it becomes easy to sufficiently obtain the effect of suppressing crystallization up to the end of the B surface of the unstretched sheet 40. As a result, it is possible to increase the laminating strength at the end portion and the laminating strength at the central portion equally.

The vertical width (length in the vertical direction in FIG. 1) of the multi-duct 30 is preferably 30 cm or more and 80 cm or less, and more preferably 50 cm or more and 60 cm or less.
The speed at which the unstretched sheet 40 moves on the cooling roll 20 is preferably 20 m / min to 100 m / min, and more preferably 40 m / min to 80 m / min.
By setting the vertical width of the multi-duct 30 within the numerical range and the speed of the unstretched sheet 40 within the numerical range, a suitable amount of cooling air can be blown onto the B surface of the unstretched sheet 40.

The upper limit of the height of the multi-duct 30 from the cooling roll 20 is 20 cm or less, more preferably 10 cm or less. When it is 20 cm or less, the cooling efficiency is improved, the effect of suppressing crystallization of the B surface of the unstretched sheet 40 is sufficiently obtained, and the lamination strength can be increased.
The lower limit of the height of the multi-duct 30 from the cooling roll 20 is not particularly limited, but a range that does not come into contact with the unstretched sheet 40 is desirable.

The upper limit of the wind speed of the cooling air from the multi-duct 30 is preferably 200 m / min, more preferably 180 m / min or less. When it is 200 m / min or less, it is possible to prevent the ground contact point when casting the molten sheet-forming resin composition from being shaken by the cooling air.
The lower limit of the wind speed of the cooling air is preferably 50 m / min. When it is 50 m / min or more, the effect of suppressing crystallization of the B surface of the unstretched sheet 40 can be sufficiently obtained, and the lamination strength can be increased.

Among the above, the method for producing a biaxially oriented polyester film according to the present embodiment includes a step A of casting a resin composition for producing a biaxially oriented polyester film onto a cooling roll to form an unstretched sheet, and the cooling roll. It is preferable to have a step B of blowing a wind of 5 ° C. or lower on the unstretched sheet.

Next, the stretching method will be described. The stretching method can be simultaneous biaxial stretching or sequential biaxial stretching, and is not particularly limited.

The lower limit of the stretching temperature in the longitudinal direction (hereinafter, also referred to as the MD direction) is preferably 55 ° C., more preferably 60 ° C. When the temperature is 55 ° C. or higher, breakage can be further suppressed. In addition, it is possible to prevent the orientation in the vertical direction from becoming too strong and suppress the increase in the heat shrinkage rate in the MD direction. The upper limit of the stretching temperature in the MD direction is preferably 100 ° C., more preferably 95 ° C. When the temperature is 100 ° C. or lower, the orientation can be sufficiently maintained and the mechanical properties can be further enhanced.

The lower limit of the draw ratio in the MD direction is preferably 2.5 times, particularly preferably 2.7 times. When it is 2.5 times or more, the orientation can be sufficiently provided and the mechanical properties can be further enhanced. Further, when it is 2.5 times or more, unevenness in thickness can be suppressed and loosening of the film roll can be prevented.
The upper limit of the draw ratio in the MD direction is preferably 3.8 times, more preferably 3.6 times, and particularly preferably 3.4 times. When it is 3.8 times or less, the effect of improving the mechanical strength and the uneven thickness can be sufficiently obtained.

The lower limit of the stretching temperature in the width direction (hereinafter, also referred to as the TD direction) is preferably 55 ° C., more preferably 60 ° C. When the temperature is 55 ° C. or higher, breakage can be made less likely to occur. In addition, it is possible to prevent the orientation in the lateral direction from becoming too strong and suppress the increase in the heat shrinkage rate in the TD direction.
The upper limit of the stretching temperature in the TD direction is preferably 100 ° C., more preferably 95 ° C. When the temperature is 100 ° C. or lower, the orientation can be sufficiently maintained and the mechanical properties can be further enhanced.

The lower limit of the draw ratio in the TD direction is preferably 3.7 times, more preferably 3.8 times, and particularly preferably 3.9 times. When it is 3.7 times or more, the degree of orientation in the width direction can be increased, and the mechanical strength can be increased.
The upper limit of the draw ratio in the TD direction is preferably 5.0 times, more preferably 4.6 times, and particularly preferably 4.3 times. When it is 5.0 times or less, the film is less likely to break, and the effect of improving the mechanical strength and thickness unevenness can be sufficiently obtained.

The lower limit of the heat fixing temperature is preferably 185 ° C, more preferably 190 ° C. When the temperature is 185 ° C. or higher, the heat shrinkage rate can be further reduced.
The upper limit of the heat fixing temperature is preferably 220 ° C. When the temperature is 220 ° C. or lower, it is possible to prevent the film from melting and becoming extremely brittle.
The heat fixing temperature may be different between the B surface and the F surface. By making the heat fixing temperature different between the B surface and the F surface, the crystallinity can be adjusted and the lamination strength can be further increased. As a result, the bag tear resistance can be further improved. When the heat fixing temperature is different between the B surface and the F surface, the temperature difference is preferably 10 ° C. or higher and 30 ° C. or lower, and more preferably 10 ° C. or higher and 20 ° C. or lower.

The lower limit of the relaxation rate is preferably 0.5%. When it is 0.5% or more, the heat shrinkage rate in the TD direction can be kept low.
The upper limit of the relaxation rate is preferably 10%. If it is 10% or less, it is possible to prevent sagging and the like, and it is possible to improve the flatness.

The lower limit of the temperature of the relaxing step is preferably 130 ° C., more preferably 150 ° C. When the temperature is 130 ° C. or higher, the film shrinks sufficiently when relaxed, and the effect of reducing the heat shrinkage rate can be sufficiently obtained.
The upper limit of the relaxing step temperature is preferably 190 ° C., more preferably 170 ° C. When the temperature is 190 ° C. or lower, it is possible to prevent deterioration of the flatness of the film due to wrinkles and the like.

The method for producing the biaxially oriented polyester film according to the present embodiment has been described above.

Next, the present invention will be described in more detail by way of examples, but the present invention is not limited to the following examples. The film was evaluated by the following measurement method.

[Intrinsic viscosity of film]
From the films produced in Examples and Comparative Examples, samples having a length of 5 mm and a width of 5 mm centered on the center position in the film width direction were cut out.
The intrinsic viscosity of the sample was measured using an automatic viscosity measuring device "SS-600-L1" manufactured by Shiyama Scientific Instruments Mfg. Co., Ltd. As the solvent solution, a mixed solution of p-chlorophenol / 1,1,2,2-tetrachloroethane = 6/4 (mass ratio) was used.

[Film surface orientation ΔP]
From the films produced in Examples and Comparative Examples, samples having a length of 5 mm and a width of 5 mm centered on the center position in the film width direction were cut out.
Samples According to JIS K 7142-1996 A method, using sodium D line as a light source and diiodomethane as a contact liquid, using an Abbe refractive index meter, the refractive index (Nx) in the longitudinal direction of the film, the refractive index (Ny) in the width direction, and the thickness direction The refractive index (Nz) of was measured, and ΔP was calculated by the following formula.
Planar orientation (ΔP) = [(Nx + Ny) / 2] -Nz

[Film piercing strength]
From the films produced in Examples and Comparative Examples, samples having a length of 5 mm and a width of 5 mm centered on the center position in the film width direction were cut out.
For the sample, the value measured by the test method described in JIS-Z1707 was calculated by the following formula in terms of 1 μm.
Puncture strength (N / μm) = Measured puncture strength / Film thickness

[Film thickness and thickness unevenness]
The film was sampled in a long strip shape having a length of 1 m and a width of 40 mm in the longitudinal direction and the width direction, and measured at a speed of 5 m / min using a continuous contact type thickness gauge manufactured by Micron Measuring Instruments Co., Ltd.
From the standard deviation of the measured thickness and the average value of the thickness, the thickness unevenness (%) in the longitudinal direction and the thickness unevenness (%) in the width direction are calculated by the following formula 1, and further, the thickness unevenness (%) and the width in the longitudinal direction are calculated. The average value of the thickness spots (%) in the direction was defined as the thickness spots (%).
Thickness unevenness = {(standard deviation of thickness) / (average value of thickness)} x 100 (%) ... Equation 1

[Heat shrinkage of film after heating at 150 ° C for 15 minutes]
The heat shrinkage of the polyester film was measured by the dimensional change test method described in JIS-C-2318, except that the test temperature was 150 ° C. and the heating time was 15 minutes.

[Impact strength of film]
Using a film impact tester manufactured by Toyo Seiki Seisakusho Co., Ltd., measurements were taken 10 times in an environment of a temperature of 23 ° C. and a relative humidity of 65%, and the average value was evaluated. The impact spherical surface used was one with a diameter of 1/2 inch. The unit used was J / 15 μm.

[Pinhole resistance]
The films of Examples and Comparative Examples were cut to a size of 20.3 cm (8 inches) × 27.9 cm (11 inches), and the cut rectangular test film was cut at a temperature of 23 ° C. and a relative humidity of 50%. Under the conditions, it was left for 24 hours or more for conditioning. After that, the rectangular test film was wound around it into a cylindrical shape with a length of 20.32 cm (8 inches). Then, one end of the cylindrical film is fixed to the outer circumference of the disk-shaped fixing head of a gelboflex tester (manufactured by Rigaku Kogyo Co., Ltd., NO.901 type) (based on the MIL-B-131C standard), and the cylindrical film is fixed. The other end of the tester was fixed to the outer circumference of the disk-shaped movable head of the tester facing the fixed head at a distance of 17.8 cm (7 inches). Then, the movable head is rotated by 440 ° in the direction of the fixed head while approaching 7.6 cm (3.5 inches) along the axes of both heads facing each other in parallel, and then 6.4 cm (6.4 cm) without rotation. After going straight (2.5 inches), a one-cycle bending test in which those movements are executed in the opposite directions to return the movable head to the initial position is performed continuously at a speed of 40 cycles per minute for 2000 cycles. Repeated. The implementation was carried out at 5 ° C. After that, the number of pinholes generated in the portion within 17.8 cm (7 inches) × 27.9 cm (11 inches) excluding the fixed head and the portion fixed to the outer circumference of the movable head of the tested film was measured (that is,). The number of pinholes per 497 cm ² (77 square inches) was measured).

[Fourier transform infrared spectroscopy (total reflection method)]
Using Varian3100FT-IR manufactured by Varian, the medium crystal was brought into close contact with the film surface layer as diamond, and the spectral intensity was measured by the total reflection method while injecting light parallel to the MD direction. The resolution of the spectroscope was 4 cm ^-1 , and the number of spectrum integrations was 64. The spectral intensity is the absorbance at each wave number. It was calculated by the following formula.
Absorbance ratio (A / B) = Absorbance A (spectral intensity of peak of ^{1450 ± 10 cm -1} ) / Absorbance B (spectral intensity of peak of ^{1410 ± 10 cm -1)}
Further, while incident light parallel to the TD direction, the spectral intensity was measured by the same method as the measurement in the MD direction.

[Three-dimensional average roughness SRa of film]
Using a contact type three-dimensional surface roughness meter (model ET-4000A) manufactured by Kosaka Laboratory Co., Ltd., the average roughness (SRa) of the surface of the film was measured by the stylus method under the following conditions. The conditions are as follows, and the average value of the three measurements was used as the value.
Radius of stylus tip: 0.5 μm
Needle pressure: 50 μN
Cutoff value: 800 μm
Measurement length: 500 μm
Measurement speed: 0.1 μm / sec Measurement interval: 5 μm

[Refractive index Nz in the thickness direction of the film]
The sample was measured by the JIS K 7142-1996 A method with an Abbe refractive index meter using sodium D line as a light source and diiodomethane as a contact liquid.

[Preparation of evaluation laminated film (laminated body)]
Urethane-based two-component effect adhesive (Mitsui Chemicals, Inc. "Takelac (registered trademark) A525S" and "Takenate (registered trademark) A50" at a ratio of 13.5: 1 (mass ratio) on the B side of the polyester film A 70 μm-thick unstretched polypropylene film (“P1147” manufactured by Toyo Boseki Co., Ltd.) is laminated as a thermosetting resin layer by the dry laminating method, and aged at 40 ° C. for 4 days. A laminated film (laminated body) was obtained. The thickness of the adhesive layer formed of the urethane-based two-component curable adhesive after drying was about 4 μm.

[Manufacturing of bag-made products with retort treatment for evaluation]
Cut the above laminated film into a size of 15 cm square, stack two sheets so that the sealant is on the inside, and heat-seal the three sides at a seal temperature of 160 ° C and a seal width of 1.0 cm to achieve internal dimensions. A 13 cm 3-way seal bag was obtained.
After filling 250 mL of water in the obtained three-way seal bag, the fourth mouth was closed by heat sealing to prepare a four-way seal bag filled with water.
The four-way seal bag filled with the obtained water was immersed in hot water at 130 ° C. for 30 minutes to obtain a retort-treated bag. In this embodiment, a sealant (heat-sealable resin layer) is formed on the B surface side of the polyester film, and when the B surfaces of the polyester film are bonded to each other via the sealant, the bag resistance is B. It depends only on the laminating strength on the surface side, not on the laminating strength on the F surface side.
In the polyester film, the laminating strength on the B side is lower than the laminating strength on the F side. Therefore, if the bag in which the B sides of the polyester film are bonded to each other via a sealant has good tear resistance. As a matter of course, the rupture resistance of a bag in which the F surfaces of the polyester film are bonded to each other via a sealant is good. Therefore, in the following, the rupture resistance was evaluated only for the bag in which the B surfaces of the polyester film were bonded to each other via the sealant.

[Evaluation of rupture resistance]
The bag filled with the retort-treated water is dropped 10 times on a concrete plate from a height of 100 cm in an environment of room temperature of 5 ° C. and relative humidity of 35% in the horizontal direction. The percentage of bags that did not tear when dropped in was calculated. Similarly, the bag was dropped 10 times with the surface of the bag vertical, and the percentage of bags that did not tear due to the vertical drop was calculated. The number of bags used in the test was 20 in each of the horizontal and vertical directions.

[Laminate strength]
A test piece having a width of 15 mm and a length of 200 mm was cut out from the above-mentioned laminated film, and under the conditions of a temperature of 23 ° C. and a relative humidity of 65%, a Tencilon universal material tester (Tencilon UMT-II-500 type manufactured by Toyo Boldwin Co., Ltd.) ”) Was used to measure the laminate strength. As for the lamination strength, the tensile strength was set to 200 mm / min, and the strength when the polyester film and the thermosetting resin layer were peeled off at a peeling angle of 90 degrees was measured.

[Evaluation of film forming property]
When the films of each Example and Comparative Example were produced, the case where continuous film formation was possible without breaking for 30 minutes or more was evaluated as 〇, and the case where the film was broken at least once within 30 minutes was evaluated as ×.

[Evaluation of fish eyes]
From the films produced in Examples and Comparative Examples, a sample of 210 mm in the width direction and 300 m in the longitudinal direction was cut out, and a loupe (L-10X) manufactured by Tokai Sangyo Co., Ltd. (observation magnification 10 times) was used to make a fish eye under deflection. The number of was counted. The same operation was repeated 10 times, and the average value of the counts of 10 times was calculated as the number of fish eyes per ^{1 m 2 by the following formula.}
[Fish eyes (pieces / m ² )] = [Average count of fish eyes (pieces)] / [Measurement range (63 m ² )]

[Evaluation of transfer to release layer]
The following coating liquid for forming a release layer was applied to one surface of the base material so that the coating amount after drying was 0.05 g / m ^2. Then, the coated film was dried with hot air at 130 ° C. and 5 m / sec for 10 seconds, hot air at 170 ° C. and 20 m / sec for 10 seconds, and further dried with hot air at 130 ° C. and 20 m / sec for 10 seconds. As described above, the release layer was laminated on one surface of the base material. After that, it was wound up as a roll.
The number of transfer marks was visually counted using bromolite in the range of 210 mm in the width direction and 300 m in the longitudinal direction of the release layer side of the obtained roll. The same operation was repeated 10 times, and the average value of the counts of 10 times ^{was calculated as the number of transfer marks per 1 m 2} by the following formula, and the determination was made based on the number. The part to be counted was the winding core side of the center of the roll in the longitudinal direction.
[Transfer marks (pieces / m ² )] = [Count number of average transfer marks (pieces)] / [Measurement range (63 m ² )]
Judgment 3 pieces / m ² or less: 〇, 4 pieces / m ² or more: ×

[Preparation of coating liquid for mold release layer formation]
Each component was mixed with the composition shown below to obtain a coating liquid for forming a release layer.
Toluene: 56.05% by mass
Isopropanol: 14.01% by mass
Acid-modified polyolefin resin solution: 29.4% by mass
Hexamethylene diisocyanate-based blocked isocyanate compound: 0.54% by mass
(Duranate (registered trademark) MF-K60B solid content concentration 60% NCO% = 6.5% manufactured by Asahi Kasei Chemicals Co., Ltd.)
The method for preparing the acid-modified polyolefin resin solution is shown below. 2 g of oxidized wax (NPS-9125 acid value 32 mgKOH / g manufactured by Nippon Wax Co., Ltd.) was added to 98 g of toluene heated to 80 ° C., and the mixture was stirred for 30 minutes to dissolve it. The dissolved acid-modified polyolefin resin solution was cooled to 25 ° C. and then filtered through a 300 mesh stainless steel filter (wire diameter 0.035 mm, plain weave) to obtain an acid-modified polyolefin resin solution.

[Evaluation of deep drawing moldability]
From the films prepared in Examples and Comparative Examples, a sample of 15 cm in length (longitudinal direction) × 10 cm in width (width direction) was cut out. This sample was set in the dies shown in FIGS. 3 and 4, and pressed from above to perform drawing molding. FIG. 3 is a cross-sectional view of the mold used for evaluating the deep drawing moldability, and FIG. 4 is a plan view of the mold shown in FIG. Specifically, the film F (films of Examples and Comparative Examples) is placed on a mold 50 having a length of 89 mm, a width of 54 mm, and an R size of 3 mm, and the mold 50 is held in a state where the film F is held by the film holder 52. It was pressed with a punch 54 having a shape corresponding to the above. The aperture speed was 6 mm / s.
It was carried out at N = 10 for each drawing depth, and the maximum drawing depth when no film tearing or pinholes occurred at N = 10 was defined as the deep drawing molding value of the sample.

[Example 1]
Unique consisting of PBT resin (1100-211XG (CANG CHUN PLASTICS CO., LTD., Intrinsic viscosity 1.28 dl / g)) and terephthalic acid // ethylene glycol = 100 // 100 (mol%) using a uniaxial extruder. A resin composition for producing a biaxially oriented polyester film, which is a mixture of a polyester resin composition containing a PET resin having a viscosity of 0.62 dl / g as shown in Table 1 and silica particles having an average particle size of 2.4 μm. Is melted at 290 ° C., cast from a T-die at 250 ° C., and adhered to a cooling roll at 10 ° C. by the electrostatic adhesion method so that cold air is applied to the entire width of the unstretched sheet as shown in FIG. A multiduct was installed at a height of 15 cm from the cooling roll, and cold air at 5 ° C. and 150 m / min was blown onto the B surface to obtain an unstretched sheet. At this time, the vertical width of the multi-duct was 50 cm, and the moving speed of the unstretched sheet was 60 m / min. The time from the time when the resin composition was cast to the cooling roll until the cold air was blown to the portion was 0.6 seconds.
The content of silica particles in the resin composition for producing a biaxially oriented polyester film is 0.16% by mass as a silica concentration.

Then, the obtained unstretched sheet was stretched 3.3 times in the longitudinal direction (MD) at a temperature of 70 ° C., and then passed through a tenter and stretched 4.0 times in the width direction (TD) at 80 ° C. A biaxially oriented polyester film having a thickness of 15 μm was obtained by performing a heat fixing treatment at 200 ° C. for 3 seconds and a relaxation treatment of 9% for 1 second. Table 1 shows the resin composition of the polyester resin composition and the film forming conditions. The physical characteristics of the obtained film and the evaluation results are shown in Tables 1 and 2.

[Examples 2, 3, 5]
In Example 1, a biaxially oriented polyester film having a thickness of 15 μm was obtained by forming a biaxially oriented film in the same manner as in Example 1 except that the resin composition of the polyester resin composition was changed as described in Table 1. It was. The physical characteristics and evaluation results of the obtained film are shown in Tables 1 and 2.

[Example 4]
In Example 1, the same as in Example 1 except that the temperature of the warm air on the F surface side and the B surface side was changed in the heat fixing treatment step, and the resin composition and film forming conditions of the polyester resin composition were described in Table 1. A biaxially oriented polyester film having a thickness of 15 μm was obtained. The physical characteristics and evaluation results of the obtained film are shown in Tables 1 and 2.

[Comparative Examples 1 to 4]
In Example 1, when the molten resin was brought into close contact with the cooling roll, cold air was not blown from the B surface side by the multi-duct, and the resin composition and film forming conditions of the polyester resin composition were described in Table 1, but the same as in Example 1. Similarly, a biaxially oriented film was formed to obtain a biaxially oriented polyester film having a thickness of 15 μm. Since the obtained film has a large difference in crystallinity between the F surface and the B surface of the film, the laminating strength is low and the bag breaking resistance for horizontal dropping is poor.

[Comparative Example 5]
In Example 1, a biaxially oriented polyester film having a thickness of 15 μm was obtained by forming a biaxially oriented film in the same manner as in Example 1 except that the resin composition of the polyester resin composition was changed as described in Table 1. It was. Since the obtained film has a large difference in crystallinity between the F surface and the B surface of the film, the laminating strength is low and the bag breaking resistance for horizontal dropping is poor.

[Comparative Example 6]
In Example 1, when the molten resin was brought into close contact with the cooling roll, cold air was not blown from the B surface side by the multi-duct, and the resin composition and film forming conditions of the polyester resin composition were described in Table 1, but the same as in Example 1. Similarly, a biaxially oriented film was formed to obtain a biaxially oriented polyester film having a thickness of 15 μm. Since the obtained film has a large difference in crystallinity between the F surface and the B surface of the film, not only the laminating strength is low and the bag resistance for horizontal dropping is poor, but also the PBT ratio is low and the piercing strength is low. Therefore, the tear resistance of the vertical drop was poor. The results are shown in Tables 1 and 2.

[Comparative Example 7]
In Example 1, a biaxially oriented polyester film having a thickness of 15 μm was obtained by forming a biaxially oriented film in the same manner as in Example 1 except that the resin composition of the polyester resin composition was shown in Table 1. Since the obtained film had a small proportion of PBT, the piercing strength was low, and the bag-breaking resistance of vertical dropping was poor. The results are shown in Tables 1 and 2.

[Reference example 1]
Using a uniaxial extruder, 80% by mass of PBT resin and 20% by mass of PET resin are mixed, and silica particles with an average particle size of 2.4 μm as inert particles have a silica concentration of 900 ppm with respect to the mixed resin. After melting the mixture in 290 ° C., a melt line was introduced into a 10-element static mixer. As a result, the polyester resin melt is divided and laminated to obtain a multilayer melt made of the same raw material, cast from a T-die at 250 ° C, and adhered to a cooling roll at 10 ° C by an electrostatic adhesion method. However, a biaxially oriented polyester film having a thickness of 15 μm was obtained by forming a biaxially oriented film in the same manner as in Example 2 except that an unstretched sheet was obtained. The physical characteristics and evaluation results of the obtained film are shown in Tables 1 and 2.

[Reference example 2]
A non-stretched sheet was obtained in the same manner as in Example 2 except that the resin composition for producing a biaxially oriented polyester film was melted at 290 ° C. and then cast from a T-die at 285 ° C., and then biaxially oriented. The alignment film was formed to obtain a biaxially oriented polyester film having a thickness of 15 μm. The physical characteristics and evaluation results of the obtained film are shown in Tables 1 and 2.

As shown in Tables 1 and 2, the biaxially stretched polyester films (Examples 1 to 5) obtained by the present invention have few thickness unevenness, have good deep drawing moldability, and are excellent in quality. A biaxially oriented polyester film was obtained.
On the other hand, in Comparative Examples 1 to 4, when the molten resin was brought into close contact with the cooling roll, cold air was not blown from the B surface side by the multi-duct, so that the obtained film had large thickness unevenness and poor deep drawing moldability. Was there. Further, since the difference in crystallinity between the F surface and the B surface of the film is large, the laminating strength is low and the bagging resistance for horizontal dropping is poor.
Further, in Comparative Examples 6 and 7, the piercing strength was poor because the ratio of PBT was low.
In Reference Example 1, a static mixer was introduced into the melt line to obtain a multilayer film having the same resin composition consisting of 1024 layers. The number of fish eyes in the obtained film was larger than that in the examples, and it was not suitable for applications requiring high quality.
In Reference Example 2, since the temperature at which the resin was melted and extruded was too high, the intrinsic viscosity of the film became low, and the film broke in the stretching step in the width direction, and a sample for evaluating the film could not be obtained.

The biaxially oriented polyester film of the present invention has good film quality suitable for industrial use and contains PBT as a main component, and is therefore suitable as a release film accompanied by deep drawing molding.

10 Dice 12 Discharge port 20 Cooling roll 30 Multi-duct 40 Unstretched sheet 50 Die for deep drawing molding 52 Film retainer 54 Punch F film

Claims (6)

A polyester resin composition containing 60 to 100% by mass of polybutylene terephthalate resin (A) is contained.
The number of layers is 7 or less,
A biaxially oriented polyester film characterized by satisfying the following requirements (1) to (4).
(1) The intrinsic viscosity of the biaxially oriented polyester film is 0.7 dl / g or more.
(2) The plane orientation ΔP of the biaxially oriented polyester film is 0.145 to 0.160.
(3) The piercing strength of the biaxially oriented polyester film measured in the piercing test according to JIS-Z1707 is 0.40 N / μm or more.
(4) The thickness unevenness of the biaxially oriented polyester film is 0.7% or less. The biaxially oriented polyester film according to claim 1, wherein the polyester resin composition contains a polyester resin (B) other than the polybutylene terephthalate resin (A). The biaxially oriented polyester film according to claim 1 or 2, wherein the number of fish eyes having a maximum diameter of 0.3 mm or more on the biaxially oriented polyester film is 5 pieces / m ^{2 or less.} The feature is that the difference (absolute value) between the three-dimensional average roughness SRa on one surface of the biaxially oriented polyester film and the three-dimensional average roughness SRa on the other surface is 0.01 μm or less. The biaxially oriented polyester film according to any one of claims 1 to 3. Any of claims 1 to 4, wherein the heat shrinkage rate of the biaxially oriented polyester film after heating at 150 ° C. for 15 minutes is 0 to 5% in the vertical direction and -1 to 5% in the horizontal direction. The biaxially oriented polyester film described. The method for producing a biaxially oriented polyester film according to any one of claims 1 to 5.
Step A of casting a resin composition for producing a biaxially oriented polyester film onto a cooling roll to form an unstretched sheet, and
A method for producing a biaxially oriented polyester film, which comprises a step B of blowing a wind of 5 ° C. or lower on the unstretched sheet on the cooling roll.

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