JP7532004B2 - Polyester Film - Google Patents

Description

The present invention relates to a polyester film that is excellent in thickness unevenness and transparency, and has excellent transparency and thermal deformation suppression after high-temperature heat treatment, and is particularly suitable for optical applications and surface protection.

Polyester film has excellent transparency, dimensional stability, mechanical properties, heat resistance, and electrical properties, and is used in a wide range of fields.

In recent years, in particular, they have been used as the base material for transparent conductive laminates used in touch panels, electronic paper, etc., and as protective films in the manufacturing process for their conductive electrodes.

The transparent conductive laminate is made of a polyester film as a substrate, on which various conductive layers are formed directly or via a resin anchor layer. The conductive layer is formed, for example, by sputtering, which deposits and adheres metals such as indium tin oxide or copper to the substrate. In order to minimize the thermal dimensional stability of the substrate due to the high temperature during deposition, the substrate film is generally heat-treated and processed in advance. In addition, as a protective film, in the manufacturing process of transparent electrodes for touch panels, a transparent conductive film formed with a metal transparent conductive film is manufactured through many heating and chemical treatment processes such as annealing, metal crystallization, resist printing, and edging. In order to prevent the transparent electrode from being soiled or damaged during the manufacturing process, a polyester film is attached as a surface protective film to the side of the transparent conductive film opposite to the side on which the transparent conductive film is formed. In addition, generally, foreign matter inspection of the transparent conductive film is performed with the surface protective film attached.

In the manufacturing process of the transparent electrode, for example, heat treatment is performed at 130°C to reduce the thermal shrinkage rate (Patent Document 1), or heat treatment is performed at 150°C to crystallize the metal of the conductive film (Patent Document 2), so the polyester film used in the substrate film for the transparent conductive film and the surface protective film for the transparent conductive film is required to have heat deformation resistance. In addition, low molecular weight substances (oligomers) that remain in the polyester film or are generated by thermal decomposition or the like during the heat treatment process are precipitated on the surface, causing whitening of the polyester film appearance, reduced yield due to transfer to the metal layer surface, contamination of the process, and an increase in extra processes such as cleaning, which significantly reduces the productivity of the product. In particular, in recent years, the temperature during the heat treatment of the ITO has been increased and the time has been increased in order to further reduce the electrical resistance of the ITO, and a higher level of heat deformation resistance is required. In addition, there is an increasingly high demand for the densification and appearance quality of the conductive circuits of smartphones and tablets equipped with capacitive touch panels, and further suppression of oligomer precipitation on the film surface is required.

However, when polyester film is exposed to such high-temperature treatment, the film deforms due to heat, resulting in a decrease in electrical conductivity and a decrease in visibility due to deformation, and therefore the properties of transparent conductive laminates using polyester film as a substrate cannot be said to be fully satisfactory in terms of heat deformation resistance.

As a measure to prevent the above-mentioned thermal deformation, for example, a method has been proposed in which a crystal nucleating agent is added to the polyester resin constituting the polyester film to increase the crystallinity and thereby improve the resistance to thermal deformation (Patent Document 3).

JP 2007-42473 A JP 2007-200823 A JP 2011-213770 A JP 2014-46503 A

However, the method described in Patent Document 3 tends to deteriorate transparency and cause the film to whiten, reducing visibility.

In addition, a method for preventing oligomer precipitation has been proposed in the past, in which a laminated film is formed on the surface by coating or the like, and the laminated film is endowed with the function of preventing oligomer precipitation (Patent Document 4), but this is insufficient due to the harsh heat treatment conditions mentioned above.

For this reason, polyester films are not fully satisfactory for use as surface protection films for conductive films and transparent conductive substrate films, and there is a demand for films that are both transparent and easy to process.

In view of the problems with the conventional technology described above, the object of the present invention is to provide a polyester film that is resistant to thermal deformation even when used in heat treatment at high temperatures and that suppresses the deterioration of transparency associated with the precipitation of oligomers.

As a result of intensive research into the above-mentioned problems, the inventors discovered that the above-mentioned problems could be solved by using a film having a specific configuration, and thus completed the present invention.

That is, the present invention which achieves the above object is obtained as follows.
(1) A film made of a polyester resin composition containing a polyester as a main component, the polyester resin composition having a content of an alkali metal element therein being M1 (mol/t), a content of a divalent metal element therein being M2 (mol/t), and a content of a trivalent metal element therein being M3 (mol/t), the molar ratio (M/P) of M (mol/ton) calculated by M = 0.5 × (M1) + M2 + 1.5 × (M3) to the content of a phosphorus element P (mol/ton) being 2.0 to 5.4, the film having an intrinsic viscosity (IV) of 0.55 to 0.75, a cold crystallization temperature (Tcc) of 150°C or higher and lower than 165°C, and a cyclic trimer content of 0.5% by weight or less.
(2) The polyester film according to (1), wherein the haze value of the polyester film is 2.0% or less.
(3) The polyester film according to (1) or (2), which has a haze value of 2.0% or less after heating at 170° C. for 3 hours.
(4) The polyester film according to any one of (1) to (3), wherein the difference in haze value ΔHz between before and after heating at 170° C. for 3 hours is 0.5% or less.
(5) The polyester film according to any one of (1) to (4), which contains polyethylene terephthalate as a main component.
(6) The polyester film according to any one of (1) to (5), which is used for optical purposes.
(7) The polyester film according to any one of (1) to (5), which is used for surface protection purposes.

According to the present invention, it is possible to provide a polyester film that is excellent in thickness unevenness and transparency, and has excellent film transparency and suppression of thermal deformation after high-temperature heat treatment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The polyester film of the present invention will be described below with reference to the accompanying drawings.
The polyester film of the present invention is a film made of a polyester resin containing polyester as the main component. The polyester resin is a general term for a polymer having an ester bond as the main bond in the main chain, and contains at least one component selected from ethylene terephthalate, ethylene-2,6-naphthalate, butylene terephthalate, ethylene-α,β-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylate, etc. as the main component. These components may be used alone or in combination of two or more, but among them, polyethylene terephthalate is preferred as the main component when considering the overall quality, economy, etc. In addition, these polyester resins may further contain other dicarboxylic acid components or diol components partially copolymerized, preferably at 20 mol% or less. However, since the flatness of the film after high-temperature heat treatment may decrease, the copolymerization amount is preferably 10 mol% or less, more preferably 5 mol% or less, and more preferably a homopolyester that is not copolymerized. When polyethylene terephthalate is used as the main component, the glass transition temperature Tg (°C) of polyethylene terephthalate is preferably 75 to 80°C, more preferably 79 to 80°C.

In the polyester resin constituting the present invention, the molar ratio (M/P) of M (mol/ton) calculated by M = 0.5 x (M1) + M2 + 1.5 x (M3), where M1 (mol/t) is the content of alkali metal elements, M2 (mol/t) is the content of divalent metal elements, and M3 (mol/t) is the content of trivalent metal elements contained in the polyester resin composition, to the phosphorus content P (mol/ton), must be in the range of 2.0 to 5.4 (wherein, M represents the total number of moles of alkali metal elements, alkaline earth metal elements, and P represents the total number of moles of phosphorus per 10 ⁶ g of polyester).
In the present invention, M (mol/t) is calculated by M = 0.5 x (M1) + M2 + 1.5 x (M3), where M1 (mol/t) is the content of alkali metal elements, M2 (mol/t) is the content of divalent metal elements, and M3 (mol/t) is the content of trivalent metal elements. In general, anions derived from phosphorus elements such as phosphoric acid have a trivalent negative charge, and therefore are considered to interact with cations of monovalent metal elements in a ratio of 1:3, with cations of divalent metal elements in a ratio of 2:3, and with cations of trivalent metal elements in a ratio of 1:1. However, as a result of the intensive research conducted by the present inventors, based on (M/P) calculated by assuming that phosphorus element derived from phosphorus compound such as phosphoric acid interacts as divalent anion with cation of metal element (i.e., the formula that anion due to phosphorus element interacts with monovalent metal element cation 1:2, divalent metal element cation 1:1, and trivalent metal element cation 3:2), when M/P is 2.0 or more, the volume resistivity of the polyethylene terephthalate resin composition when melted is low, and the electrostatic casting method can be suitably used during film formation, and the occurrence of uneven thickness of the film can be suppressed. Furthermore, when M/P is 5.4 or less, it is easy to suppress the regeneration of cyclic trimer during melting and the catalyst residue remaining as foreign matter in the film, so that the initial haze value after film formation and the haze value after heating can be suppressed low. It is more preferable that M/P is in the range of 2.5 to 4.5. When M/P is high, the amount of alkali metal element and alkaline earth metal element is high, the crystallization of the polymer is easily promoted, the cold crystallization temperature (Tcc) of the polyester film described later is low, and the ΔHz value after heating may be high. When M/P is low, the amount of alkali metal element and alkaline earth metal element is low, the polymer is difficult to crystallize, the cold crystallization temperature (Tcc) of the polyester film described later is high, and the flatness after high-temperature heating may be affected.

The intrinsic viscosity (IV) of the polyester film of the present invention must be 0.55 dl/g or more and 0.75 dl/g or less. When the intrinsic viscosity (IV) of the film is within this range, the thickness unevenness of the film is suppressed, and the film can be stably produced. If the intrinsic viscosity (IV) is below 0.55 dl/g, the intrinsic viscosity (IV) of the polyester raw material used for the film is low, the melt viscosity is reduced, and local thickness unevenness is likely to occur from the extrusion port to the landing of the resin on the drum for melting and cooling the resin. If the intrinsic viscosity (IV) exceeds 0.75 dl/g, the intrinsic viscosity (IV) of the polyester raw material used for the film is high, so pressure is applied to the extrusion site during the extrusion process during film production, and thickness unevenness is likely to occur. More preferably, the intrinsic viscosity (IV) of the film is 0.60 dl/g or more and less than 0.70 dl/g. In addition, the less the thickness unevenness of the film, the more the variation in refraction can be suppressed in the longitudinal refractive index nMD, the widthwise refractive index nTD, and the perpendicular refractive index nZD of the polyester film, making it easier to suppress the occurrence of interference unevenness. The thickness unevenness of the polyester film is preferably 5.0% or less in both the longitudinal direction (film-forming direction) and width direction (direction perpendicular to the film-forming direction) of the film, and more preferably 3.0% or less.

The polyester film of the present invention must have a cold crystallization temperature (Tcc) of 150°C or more and less than 165°C. The cold crystallization temperature refers to the 2nd run crystallization peak determined by differential scanning calorimetry, which will be described later. If the cold crystallization temperature (Tcc) is less than 150°C, the difference between the haze value of the film before heating (hereinafter sometimes referred to as the initial haze value) and the haze value of the film after heating at 170°C for 3 hours (hereinafter sometimes simply referred to as the haze value after heating) tends to become large, and transparency is impaired when used as a transparent conductive substrate film or a surface protection film for a conductive film. Specifically, it may be difficult to make the ΔHz, which is the difference in haze value before and after heating the film at 170°C for 3 hours, 0.5% or less. This is thought to be because thermal crystallization progresses when the film is heated, and the cyclic oligomers contained in the film exist as multiple polymers, and the cyclic oligomers precipitate on the surface of the film, causing the haze value of the film to deteriorate. In addition, since the majority of cyclic oligomers are cyclic trimers, the present invention mainly describes cyclic trimers. On the other hand, if the cold crystallization temperature (Tcc) is 165°C or higher, the flatness of the film after heating may be affected. More preferably, the cold crystallization temperature (Tcc) is less than 160°C.

In the present invention, the cold crystallization temperature (Tcc) is a value obtained by measuring 5 mg of a polyester film sample on an electronic balance, sandwiching it between aluminum packing, and measuring it using a robot DSC-RDC220 thermal differential scanner manufactured by Seiko Instruments Inc., and analyzing the data using a disk session SSC/5200 manufactured by the same company in accordance with JIS-K-7121 (1987). Specific measurement conditions include heating from 25°C to 300°C at 20°C/min, then rapidly cooling to 25°C, and heating again to 300°C at 20°C/min (2nd run), and determining the crystallization peak temperature as the apex temperature of the crystallization peak. If multiple crystallization peaks are observed in the 2nd run, the crystallization peak apex temperature showing the maximum peak area as determined by JIS-K-7122 is measured. This is repeated three times, and the average value is determined as the cold crystallization peak temperature (Tcc).

There are no particular limitations on the means for setting the cold crystallization temperature (Tcc) to 150°C or higher and lower than 165°C, but in the present invention, in addition to the above-mentioned method of controlling M/P, particularly, methods of controlling the intrinsic viscosity (IV), particle content, and cyclic trimer content of the polyester resin are included.

The inventors believe that the cold crystallization temperature (Tcc) can be controlled by the above method for the following reasons. Regarding the method for controlling the cyclic trimer content, in the stretching, preheating, heat setting, and other processes in the film manufacturing process, if the cyclic trimer content in the polyester resin is low, there will be less of the components that inhibit the oriented crystallization of the polymer molecular chains, making crystallization easier and the cold crystallization temperature (Tcc) tend to be lower, whereas if the cyclic trimer content is high, there will be more components that inhibit the oriented crystallization of the polymer molecular chains, making crystallization more difficult and the cold crystallization temperature (Tcc) tend to be higher.

Regarding the intrinsic viscosity (IV) of the polyester resin raw material, when the intrinsic viscosity (IV) of the polyester resin raw material is high, the molecular chain of the polyester becomes longer on average, so the polymer is less likely to crystallize, and the cold crystallization temperature (Tcc) tends to be higher. On the other hand, when the intrinsic viscosity (IV) of the polyester resin raw material is low, the molecular chain of the polyester becomes shorter on average, so the polymer is more likely to crystallize, and the cold crystallization temperature (Tcc) is thought to be lower. It is thought that the cold crystallization temperature (Tcc) is also affected by the time of solid-phase polymerization and the intrinsic viscosity at the end of liquid-phase polymerization (melt polymerization) when producing the polyester resin raw material. Specifically, the longer the time of solid-phase polymerization and the higher the intrinsic viscosity at the end of liquid-phase polymerization (melt polymerization), the higher the cold crystallization temperature (Tcc) tends to be. Under these manufacturing conditions for raw materials, if the intrinsic viscosity of the polyester resin is high when liquid-phase polymerization (melt polymerization) is completed, extending the solid-phase polymerization time may cause the film to have a greater uneven thickness due to the increase in intrinsic viscosity (IV).

Next, regarding the particle content, if the particle content in the polyester resin is high, the particles will act as nuclei to promote the crystallization of the polymer and the crystallization of the film will also proceed, so the cold crystallization temperature (Tcc) will be lower than the cold crystallization temperature (Tcc) of the polyester resin that will be the matrix of the polyester layer, and the haze value of the film after heating will tend to deteriorate. Also, if the particle content is high, the initial haze value of the film will tend to deteriorate. If the particle content in the polyester resin is low, there will be fewer nuclei to promote crystallization, so the crystallization of the film will be suppressed, and the cold crystallization temperature (Tcc) will tend to be higher than the cold crystallization temperature (Tcc) of the polyester resin that will be the matrix of the polyester layer, and it is believed that the flatness of the film after heating will tend to deteriorate.

It is preferable to incorporate particles into polyester films to improve slipperiness during the film-making process and for post-processing. However, the cold crystallization temperature (Tcc) varies depending on the particle content, so it is important to adjust the cold crystallization temperature (Tcc) to prevent an increase in haze value after heating and to maintain the flatness of the film after high-temperature heating.

By specifying the cold crystallization temperature (Tcc) of the film, the present invention is able to simultaneously achieve transparency of the film after high-temperature heat treatment and suppression of thermal deformation.

The polyester film of the present invention preferably has a haze value of 2.0% or less. The haze value of the film may be the haze value after the film is formed, or the haze value of the film after heating or the like under certain conditions after the film is formed. By setting the haze value to 2.0% or less, a film with excellent transparency can be obtained. The haze value is preferably 1.5% or less, and more preferably 1.0% or less.

In addition, the polyester film of the present invention preferably has a haze value of 2.0% or less after heating under heating conditions of 170°C for 3 hours. In this way, when used as a transparent conductive substrate film, the transparency of the film of the conductive electrode itself, which is the final product, can be maintained even if it is subjected to heat effects in the processing step. Also, when used as a surface protection film for a conductive film, it is difficult to impair visibility during foreign matter and defect inspection. The haze value of the film after heating is preferably 1.5% or less, more preferably 1.0% or less. Furthermore, it is preferable that the difference ΔHz in the haze value of the polyester film of the present invention before heating and after heating at 170°C for 3 hours (hereinafter sometimes simply referred to as before and after heating) is 0.5% or less. The smaller the ΔHz value, the more excellent the transparency of the film can be even when subjected to heat effects, and it is preferably 0.4% or less, more preferably 0.2% or less, and more preferably 0.1% or less. If ΔHz exceeds 0.5%, the haze value of the film after heating will be high, which may reduce visibility when used as a transparent electrode substrate and inspection visibility when used as a protective film for a transparent conductive laminate.

In addition, ΔHz exceeds 0.5% when the polyester film contains more than 0.5% by weight of cyclic trimer or when the cold crystallization temperature (Tcc) of the film is less than 150°C. When the polyester resin raw material contains a large amount of cyclic trimer, the content of cyclic trimer when the film is made is also large, and it is believed that the cyclic trimer present in the film is more likely to precipitate on the film surface after heating, which increases the difference in haze value of the film before and after heating. In addition, when the cold crystallization temperature (Tcc) of the film is less than 150°C, even if the cyclic trimer present in the film is less than 0.5% by weight, the polymer is easily crystallized, so the polymer crystallization progresses from the time of melting, and when the film is stretched to orient the molecular chains, a distorted portion of orientation is created between the locally crystallized portion and the oriented molecular chains, and the cyclic trimer is more likely to precipitate on the film surface from the distorted portion, which is believed to increase the difference in haze value of the film before and after heating. When the polyester film of the present invention is laminated to another component to form a surface protection film for a conductive film, it is peeled off after processing, but the cyclic trimer precipitated on the polyester film surface may be transferred to the surface of the mating component, deteriorating the quality of the product.

The polyester film of the present invention is preferably configured to have an easy-adhesion layer to which particles are added without substantially containing particles in the polyester film, or, if an easy-adhesion layer is not provided, to contain particles in the polyester film in order to impart easy slippage during the film manufacturing process or processing. When particles are added, the particle content is 0.001% by weight or more and 0.015% by weight or less with respect to the polyester film. By setting the particle content to 0.001% by weight or more and 0.015% by weight or less, it is possible to impart easy slippage while maintaining transparency, and further, it is easy to make the initial haze value, which is an index of the transparency of the film, 2.0% or less, and further, the transparency after heating of the film (after heating at 170°C for 3 hours) 2.0% or less. In addition, by imparting easy slippage, the occurrence of scratches on the film can be suppressed. When the particle content is less than 0.001% by weight, the slipperiness of the film decreases, and when the particle content exceeds 0.015% by weight, the slipperiness of the film improves, but the initial haze value of the film may exceed 2.0%, and the haze after heating also tends to be high. The reasons for the deterioration of the initial transparency are the regeneration speed of the cyclic trimer during melting, catalyst residue, and the decrease in transparency due to particles, as described above, but the reason for the deterioration of the transparency after heating is believed to be that when the polymer crystallizes during melting, and the film is stretched to orient the molecular chains, a distorted portion of orientation is created between the locally crystallized portion and the oriented molecular chain, and the cyclic trimer is easily precipitated on the film surface from the distorted portion, which increases the difference in the haze value of the film before and after heating. In addition, since the particles are not compatible with the polymer, the interface between the polyester resin and the particles is noticeable near the film surface, and the cyclic trimer contained in the film precipitates on the surface from the interface, which is believed to hinder the transparency of the film.

The polyester film of the present invention must have a cyclic trimer content of 0.5% by weight or less. More preferably, it is 0.45% by weight or less. When the cyclic trimer content is 0.50% by weight or less, it is easy to make the initial haze value, which is an index of the transparency of the film, 2.0% or less, and further, the haze value after heating the film (after heating at 170°C for 3 hours) 2.0% or less. The lower the cyclic trimer content, the less likely the cyclic trimer will precipitate on the film surface during film molding or film processing, and the easier it is to maintain the transparency of the film after heating.
The lower limit of the content of the cyclic trimer is not particularly limited, but in the present invention, it is 0.01% by weight or more. If it is less than 0.01% by weight, the time required for solid-phase polymerization to reduce the cyclic trimer becomes long, which increases the intrinsic viscosity (IV) of the polyester resin raw material, and the intrinsic viscosity (IV) of the polyester film of the present invention also increases, which increases the load during melt extrusion and may cause uneven thickness of the film. If the cyclic trimer exceeds 0.5% by weight, the initial haze value of the film and the haze value after heating may exceed 2.0%, and similar to the amount of added particles, when the film of the present invention is used as a substrate for a transparent electrode, the visibility of the final transparent electrode may decrease, making it unsuitable for applications requiring high visibility such as touch panels. The haze value of the polyester film of the present invention after heating (after heating at 170°C for 3 hours) is preferably 1.5% or less. More preferably, it is 1.0% or less.

The means for reducing the cyclic trimer content in the polyester film to 0.5% by weight or less is not particularly limited, but it is particularly preferred to produce the film using a polyester resin with a low cyclic trimer content as the starting material. Various known methods can be used to produce polyester resins with a low cyclic trimer content, and as listed in the above-mentioned polyester resin production conditions, examples of such methods include a method of solid-phase polymerization after the polyester resin is produced, a method of adjusting the intrinsic viscosity of the polyester resin when liquid-phase polymerization (melt polymerization) before solid-phase polymerization is completed, and adjustment of the amount of alkali metal elements and alkaline earth metal elements.

The amount of terminal carboxyl groups in the polyester resin raw material used in the polyester film of the present invention is preferably 10 equivalents/t or more and 60 equivalents/t or less, more preferably 15 equivalents/t or more and 50 equivalents/t or less, and particularly preferably 20 equivalents/t or more and 45 equivalents/t or less. When the amount of terminal carboxyl groups is 10 equivalents/t or more, it is easier to suppress scratches caused by the stretching rolls and nip rolls in the longitudinal stretching section and the conveying section in the film manufacturing process. When the amount of terminal carboxyl groups is 60 equivalents/t or less, the amount of cyclic oligomers precipitated in the film manufacturing process is small, making it easier to suppress scratches caused by the precipitated cyclic oligomers. Generally, adding particles to a film makes the film surface rough, which helps to prevent scratches, so adding particles is effective when the amount of terminal carboxyl groups is low. However, adding particles in an amount exceeding 0.015% by weight can impair the transparency of the film, lower the cold crystallization temperature (Tcc) of the film, and tend to worsen ΔHz before and after heating at 170°C for 30 minutes. On the other hand, when the amount of terminal carboxyl groups is high, the cause of scratches is cyclic oligomers, so adding particles to the film tends not to prevent scratches.

The polyester film of the present invention preferably has a thickness of 10 μm or more and 500 μm or less. If the thickness of the film is less than 10 μm or more than 500 μm, it may be difficult to stably manufacture the film, and in particular, if the thickness exceeds 500 μm, it may be difficult to achieve both transparency and thickness. The thickness of the polyester film of the present invention is more preferably 16 μm or more and 300 μm or less, and even more preferably 18 μm or more and 250 μm or less.

The polyester film constituting the present invention can have a single-layer structure, a composite structure such as a two-layer structure or a three-layer structure, and a coating layer can be provided on a single-layer or composite film, as long as the effect of the present invention is not impaired. The coating layer can be provided during the film manufacturing process, or while rewinding the film after it has been manufactured and wound up. The composite film can have a two-layer or three-layer structure in which the polyester film of the present invention is the outermost layer and is laminated on at least one of the surface layers. It is also possible to have a pseudo-composite single-layer structure (such as a layer A/layer A or a layer A/layer A/layer A) in which the film of the present invention is used as one layer and multiple layers of the same type are laminated.

Biaxially oriented films are generally obtained by biaxial stretching, that is, stretching an unstretched sheet by about 2.5 to 5.0 times in each of the longitudinal and transverse directions, and then heat treating the sheet to complete the crystal orientation. The biaxial stretching method may be a sequential biaxial stretching method or a simultaneous biaxial stretching method. Furthermore, a restretching method may be used in which the film is stretched again in the longitudinal or transverse direction after biaxial stretching.

The polyester film constituting the present invention may also contain various additives, such as antioxidants, heat stabilizers, weather stabilizers, UV absorbers, organic lubricants, pigments, dyes, organic or inorganic fine particles, fillers, antistatic agents, nucleating agents, crosslinking agents, etc., as long as they do not impair the effects of the present invention.

When the polyester film of the present invention is used as a composite film of multiple layers, the lamination method is not limited. For example, when obtaining an unstretched sheet, lamination by co-extrusion, lamination by lamination, or a combination of these methods can be used. From the viewpoint of transparency and manufacturing stability, it is preferable to use the co-extrusion method. It is also possible to use a configuration in which the present invention is the outermost layer, or to use the present invention as a pseudo-composite film in which all layers are applied.

Next, a method for producing the polyester film of the present invention will be described as an example. However, the polyester film of the present invention is not limited to this method.

First, the polyethylene terephthalate resin composition can be produced by a process in which terephthalic acid and ethylene glycol are used as raw materials, a low polymer is obtained by a direct esterification reaction, and a high molecular weight polymer is obtained by a subsequent polycondensation reaction. Alternatively, the polyethylene terephthalate resin composition can be produced by a process in which dimethyl terephthalate and ethylene glycol are used as raw materials, a low polymer is obtained by an ester exchange reaction, and a high molecular weight polymer is obtained by a subsequent polymerization reaction. Either method can be adopted in the present invention. Furthermore, if necessary, a heat stabilizer, an electrostatic agent, an antifoaming agent, an antioxidant, and the like can be added before or during the reaction. Examples of antimony compounds include antimony trioxide, antimony pentoxide, antimony acetate, and antimony salts of aliphatic carboxylic acids. Examples of methods for adding antimony compounds include powder, ethylene glycol slurry, and ethylene glycol solution, but the method of adding the compound as an ethylene glycol solution is preferred because it can prevent coarsening due to aggregation of antimony, resulting in good transparency (haze value). Examples of phosphorus element compounds include phosphoric acid, phosphorous acid, phosphonic acid, and ester compounds thereof, and phosphoric acid and ethyl phosphate are particularly preferred.

The intrinsic viscosity of the polyethylene terephthalate resin composition can be determined by the polymer stirring torque at the end of the melt polymerization, and the end point determination torque of the melt polymerization device can be set to obtain the desired intrinsic viscosity (IV).

The resulting molten polyethylene terephthalate is then extruded from a nozzle in the form of strands, cooled, and pelletized using a cutter to produce a liquid phase polymerization (melt polymerization) polyethylene terephthalate resin composition.

The obtained polyethylene terephthalate resin composition is preferably pre-crystallized before being subjected to solid-state polymerization. Pre-crystallization can be performed by applying a mechanical impact to the polyethylene terephthalate resin composition to perform a shear treatment, or by performing a heat treatment under a hot air flow.

After preliminary crystallization, the polyethylene terephthalate resin composition is subsequently subjected to solid-state polymerization. The polyethylene terephthalate resin composition is charged into a solid-state polymerization apparatus, and solid-state polymerization is carried out at a predetermined temperature while an inert gas is circulated. After solid-state polymerization is completed, the polyethylene terephthalate resin composition is cooled and removed from the apparatus.

The intrinsic viscosity (IV) of the polyethylene terephthalate resin composition to be subjected to solid-phase polymerization is preferably in the range of 0.40 dl/g to 0.52 dl/g, and more preferably in the range of 0.45 dl/g to 0.50 dl/g. If it is 0.40 dl/g or more, the amount of cyclic trimer is reduced at a specified temperature or time, which is preferable because it improves productivity. If it is 0.52 dl/g or less, crystallization proceeds in a short time, and the amount of cyclic trimer is reduced before the intrinsic viscosity of the polyethylene terephthalate resin composition increases, which is preferable, and the heat history is small, which is preferable because coloring of the polyethylene terephthalate resin composition is suppressed.

Examples of inert gases include rare gases such as helium, neon, and argon, as well as nitrogen gas and carbon dioxide gas.

The polyethylene terephthalate resin composition raw material thus obtained (cyclic trimer content 0.01 wt % to 0.5 wt %, intrinsic viscosity 0.60 dl/g to 0.80 dl/g, terminal carboxyl group amount 10 to 60 equivalents/t) is vacuum dried, fed to an extruder and melted at 260 to 300°C, extruded from a T-shaped die into a sheet, wound around a mirror-finished casting drum having a surface temperature of 10 to 60°C by an electrostatic casting method, and cooled and solidified to produce an unstretched PET film.
This unstretched film is stretched 2.5 to 5.0 times in the machine direction, which is the running direction of the film, between rolls heated to 70 to 100° C. Then, this film is held with clips and introduced into a preheating zone, where it is heated to a temperature of 75 to 95° C., and then continuously stretched 3.0 to 5.0 times in the transverse direction perpendicular to the machine direction in a heating zone at 90 to 115° C., and then heat-treated for 5 to 60 seconds in a heating zone at 200 to 240° C., and passed through a cooling zone at 100 to 200° C. to obtain a polyester film in which crystal orientation is completed.
In order to improve the thermal dimensional stability during the heat treatment, a relaxation treatment of 3 to 12% may be performed in the machine direction or width direction. The biaxial stretching method may be the above-mentioned sequential stretching method or a simultaneous biaxial stretching method in which the machine direction and the width direction are stretched simultaneously. After the machine direction and width direction stretching, the film may be re-stretched in either the machine direction or the width direction.
The ends of the obtained polyester film are cut and wound into an intermediate product, and then the wound film is rewound and cut into a desired width using a slitter, and then wound around a cylindrical core to obtain a film roll of a desired length. Note that, in order to prevent the occurrence of scratches on the film after winding and blocking, in which films stick to each other, knurling may be applied to both lateral ends of the film, and the film may be embossed.

The polyester film of the present invention may have a coating layer on one or both sides of the film, as long as the object of the present invention is not impaired.

The coating layer provided on the surface of the polyester film of the present invention may be a known coating layer, such as a particle-containing layer for imparting slipperiness to the film, a hard coat layer for complementing the surface hardness and abrasion resistance of the film, an easy-adhesion layer for complementing the adhesion to a functional layer such as a separately provided hard coat layer, a refractive index adjustment layer for suppressing uneven light interference caused by the difference in refractive index between the hard coat layer and the base polyester film, or an adhesive layer for laminating with other films.

The polyester film of the present invention is suitable for optical applications. In particular, it can be used as a transparent conductive substrate film and a surface protection film for conductive films.

The configuration and effects of the present invention will be explained in more detail below using examples. Note that the present invention is not limited to the following examples. Before describing each example, the methods for measuring various physical properties will be described.

(1) Thickness Unevenness The polyester film was measured using a digital micrometer manufactured by Sony Corporation in accordance with JIS-C-2151. The polyester film was measured in the longitudinal direction and the width direction as described below to determine the thickness unevenness, and the thickness unevenness was evaluated by taking the average.
[Measurement of thickness unevenness]
The polyester film was measured at 10 points in the longitudinal or transverse direction, and the average value was taken as the center value A. The value obtained by subtracting the minimum value from the maximum value was taken as the thickness unevenness B, and the ratio of the thickness unevenness to the center value was calculated. That is, the thickness unevenness of the film (%) = 100 x B/A, and the average in the longitudinal and transverse directions was evaluated as follows.
[Evaluation of thickness unevenness]
×: Thickness unevenness exceeds 5.0%. △: Thickness unevenness exceeds 3.0% and is 5.0% or less. ○: Thickness unevenness is 3.0% or less.

(2) Haze Value Using a 5 cm square film as a sample, the haze before heating and after heating at 170°C for 3 hours was measured with a haze meter ("HZ-V3" manufactured by Suga Test Instruments) based on JIS-K-7105 (1985). Three samples were prepared for each measurement, and the average of the haze before heating was taken as the initial haze value. ΔHz was calculated from the difference between the average haze value before heating and the average haze value after heating, and ΔHz was evaluated as follows.
[Evaluation of ΔHz]
×: ΔHz exceeds 1.0%. △: ΔHz exceeds 0.5% and is 1.0% or less. ○: ΔHz exceeds 0.1% and is 0.5% or less. ◎: ΔHz is 0.1% or less.

(3) Cold crystallization temperature (Tcc)
The film was weighed out at 5 mg on an electronic balance, sandwiched between aluminum packing, and measured using a Seiko Instruments Inc. Robot DSC-RDC220 differential thermal scanner. Data was analyzed using a Seiko Instruments Inc. Disk Session SSC/5200. The test was performed according to JIS-K-7121 (1987). The temperature was raised from 25°C to 300°C at 20°C/min. Then, the sample was rapidly cooled to 25°C, and the temperature was raised again to 300°C at 20°C/min (2nd Run). The midpoint glass transition temperature was determined as the glass transition temperature (Tg), and the apex temperature of the crystallization peak was determined as the crystallization temperature.
When a plurality of second-run crystallization peaks are observed, the crystallization peak apex temperature showing the maximum peak area determined according to JIS-K-7122 is used.
This was repeated three times, and the average value was taken as the cold crystallization temperature (Tcc).

(4) Content of cyclic trimer in film A 20 mg piece of film before heating was used as a sample, which was dissolved in OCP (o-chlorophenol) at 150°C for 30 minutes and cooled at room temperature. After that, 1,4-diphenylbenzene was added as an internal standard, and then 2 mL of methanol was added, and the polymer was separated using a high-speed centrifuge. The liquid layer was measured using a high-performance liquid chromatograph (Shimadzu Corporation "LC-10ADvp").

(5) Evaluation of flatness after heating (thermal deformation resistance)
A film cut into a 20 cm square was placed on a punched metal and held in an oven set at 150°C for 10 minutes without tension, after which the film sample was cooled at room temperature for 10 minutes and the state of the reflected image of the fluorescent light projected on the surface of the film by the reflected light of the fluorescent light was observed. The following two types of punched metal were used for the evaluation, and the heat deformation resistance was evaluated according to the following criteria.

・Punched metal A: Punched metal SUS304 manufactured by Okutani Wire Mesh Manufacturing Co., Ltd.
Embossed 1.5t x D4.5 x P7.5 60° zigzag Punching metal B: Punching metal made by Okutani Wire Mesh Manufacturing Co., Ltd. SUS304
Embossed 2t x D7/H1.3 x P10 60° zigzag [Evaluation criteria for heat deformation resistance]
◯: In both the case of the punched metal A and the punched metal B, no distortion was observed in the reflected image of the fluorescent lamp.
Δ: In both the cases of the punched metal A and the punched metal B, the reflected image of the fluorescent lamp was distorted at the embossing pitch of the punched metal in part of the film surface.
x: In both the cases of punching metal A and punching metal B, the reflected image of the fluorescent lamp was distorted over the entire surface of the film at the embossing pitch of the punching metal.

(6) Particle content in the film (% by weight)
The sample is thoroughly washed with methanol to remove any surface deposits, washed with water and dried, 2.7 kg of o-chlorophenol is added to the 300 g sample, which is then heated to 100°C with stirring, and left for another hour after heating to dissolve the polyester portion. However, if the polyester portion does not dissolve due to high crystallization, for example, the polyester portion is dissolved once and then rapidly cooled before the above dissolving operation is carried out.
Next, coarse insoluble matter such as dust contained in the polyester is filtered off using a G-1 glass filter and removed, and the weight of the filtered matter is subtracted from the weight of the sample.
A Hitachi preparative ultracentrifuge, Model 40p, is equipped with a rotor RP30, and 30 cc of the solution after filtration through the glass filter is poured into each cell. The rotor is then rotated at 4,500 rpm. After confirming that there are no rotational abnormalities, a vacuum is created in the rotor, and the rotation speed is increased to 30,000 rpm. The particles are centrifuged at this rotation speed.
The separation is completed after about 40 minutes, but this can be confirmed, if necessary, by checking that the light transmittance at 375 mμ of the separated liquid becomes a constant value higher than that before separation. After separation, the supernatant liquid is removed by decantation to obtain separated particles.
Since the separated particles may be contaminated with polyester due to insufficient separation, room temperature o-chlorophenol is added to the collected particles, and after suspending them almost uniformly, ultracentrifugation is performed again. This operation must be repeated until the melting peak corresponding to the polymer can no longer be detected when the particles are dried and subjected to differential scanning calorimetry as described below. Finally, the separated particles A obtained in this way are vacuum dried at 120°C for 16 hours and weighed.
This was taken as the particle content (wt %) in the film.

(7) Film Scratches The obtained film was unwound 1 m from the roll, and the film roll was observed in the width direction with transmitted light using a halogen light as a light source to evaluate the visibility of scratches on the film. Films with no visible scratches were rated as good (◎), those with faint scratches but no quality issues were rated as acceptable (◯), and those with one clearly visible scratch were rated as slightly poor (△).

(8) Amount of terminal carboxyl groups in polyester resin raw materials The polyester raw materials used in the film were evaluated, and the polyester was dissolved in o-cresol/chloroform (weight ratio 7/3) at 130-160°C and the potential difference was measured with an alkali to determine the amount of carboxyl groups.

[Polyester resin used]
(PET-A)
An esterification reaction product, which is a reaction product of terephthalic acid and ethylene glycol, was stored in advance in a molten state at 255°C, and further terephthalic acid and ethylene glycol were made into a slurry such that the molar ratio of ethylene glycol to terephthalic acid was 1.15, and was supplied in a fixed amount while maintaining the temperature of the esterification reaction tank, and an esterification reaction was carried out while distilling off water, to obtain an esterification reaction product. The obtained esterification reaction product was transferred to a polymerization reaction tank, and an ethylene glycol solution containing phosphoric acid, an ethylene glycol solution containing potassium hydroxide, an ethylene glycol solution containing magnesium acetate tetrahydrate, and an ethylene glycol solution containing antimony trioxide were added to the polyethylene terephthalate resin composition so that the alkali metal element (potassium element) was 2 ppm, and the M/P of potassium element, magnesium element, and phosphorus element was 2.8, and the antimony element was 60 ppm. Subsequently, the pressure in the polymerization reaction tank was gradually reduced to 0.13 kPa or less in 30 minutes, and at the same time, the temperature was gradually increased to 280°C, and polymerization reaction was carried out until the target intrinsic viscosity was reached. Thereafter, the polycondensation reaction tank was returned to normal pressure with nitrogen gas, the mixture was discharged from the die into cold water in the form of strands, pelletized into cylindrical form by an extrusion cutter, and pre-crystallized by a surface crystallizer to obtain a liquid-phase polymerized (melt-polymerized) polyester having an intrinsic viscosity (IV) of 0.50 dl/g. The liquid-phase polymerized (melt-polymerized) polyester thus obtained was used for solid-phase polymerization at a temperature of 215°C for 18 hours in a rotary vacuum dryer under a reduced pressure of 0.13 KPa and nitrogen gas with a dew point of 5°C to obtain a polyester resin (PET-A). The intrinsic viscosity of the obtained polyester resin (PET-A) was 0.65 dl/g, the cyclic trimer content was 0.42 wt%, the amount of terminal carboxyl groups was 24 equivalents/t, and the glass transition temperature Tg was 80°C.

(PET-B):
A polyester resin (PET-B) was obtained in the same manner as PET-A, except that solid-phase polymerization after obtaining the liquid-phase polymerized (melt-polymerized) polyester was carried out using a continuous vacuum drying apparatus. The intrinsic viscosity of the obtained polyester resin (PET-B) was 0.73 dl/g, the cyclic trimer content was 0.39% by weight, the amount of terminal carboxyl groups was 20 equivalents/t, and the glass transition temperature Tg was 80°C.

(PET-C):
The procedure was the same as for PET-A, except that the intrinsic viscosity before solid-state polymerization was 0.40 dl/g and the solid-state polymerization time was 26 hours. The resulting polyester resin (PET-C) had an intrinsic viscosity of 0.60 dl/g, a cyclic trimer content of 0.38% by weight, a terminal carboxyl group content of 30 equivalents/t, and a glass transition temperature Tg of 80°C.

(PET-D):
Polyester resin (PET-D) was obtained in the same manner as polyester resin (PET-A), except that an ethylene glycol solution containing antimony trioxide was added so that the antimony element was 300 ppm, solid-state polymerization was not performed, and the intrinsic viscosity was changed. The intrinsic viscosity of the obtained polyester resin (PET-D) was 0.70 dl/g, the cyclic trimer content was 1.07 wt%, the amount of terminal carboxyl groups was 35 equivalents/t, and the glass transition temperature Tg was 80° C.

(PET-E):
A melt polymerization reaction was carried out in the same manner as for PET-A, except that potassium hydroxide was not added and antimony trioxide was added so as to give 65 ppm of elemental antimony, to obtain a liquid-phase polymerized (melt-polymerized) polyethylene terephthalate resin composition having an intrinsic viscosity of 0.64, which was then charged into a dryer having a stirring blade and subjected to solid-phase polymerization at a temperature of 225°C for 8 hours in nitrogen gas, and the polyethylene terephthalate resin composition after solid-phase polymerization was further subjected to a wet heat treatment at 225°C to obtain a polyester resin (PET-E). The resulting polyester resin (PET-E) had an intrinsic viscosity of 0.60 dl/g, a cyclic trimer content of 0.40% by weight, a terminal carboxyl group amount of 45 equivalents/t, and a glass transition temperature Tg of 80°C.

(PET-F):
A polyester resin (PET-F) was obtained in the same manner as in PET-A, except that the solid-phase polymerization time was changed to 26 hours. The intrinsic viscosity of the obtained polyester resin (PET-F) was 0.70 dl/g, the cyclic trimer content was 0.38 wt%, the amount of terminal carboxyl groups was 20 equivalents/t, and the glass transition temperature Tg was 80° C.

(PET-G):
A polyester resin (PET-G) was obtained in the same manner as PET-A, except that the amounts of potassium hydroxide, magnesium acetate, and phosphoric acid added were changed so that M/P was 1.5. The polyester resin (PET-G) had an intrinsic viscosity of 0.63 dl/g, a cyclic trimer content of 0.43 wt%, an amount of terminal carboxyl groups of 23 equivalents/t, and a glass transition temperature Tg of 80°C.

(PET-H):
A polyester resin (PET-H) was obtained in the same manner as PET-A, except that the additives of magnesium acetate tetrahydrate and phosphoric acid were changed so that the M/P was 6.0. The polyester resin (PET-H) had an intrinsic viscosity of 0.67 dl/g, a cyclic trimer content of 0.41 wt%, an amount of terminal carboxyl groups of 21 equivalents/t, and a glass transition temperature Tg of 80°C.

(PET-I):
A polyester resin (PET-I) was obtained in the same manner as in the production of PET-A, except that the amounts of magnesium acetate tetrahydrate and phosphoric acid added were changed so that M/P was 2.0.
The polyester resin (PET-I) had an intrinsic viscosity of 0.64 dl/g, a cyclic trimer content of 0.42% by weight, a terminal carboxyl group amount of 28 equivalents/t, and a glass transition temperature Tg of 80°C.

(PET-J):
A polyester resin (PET-J) was obtained in the same manner as in the case of PET-A, except that the dew point of the nitrogen gas during the solid-state polymerization was −5° C. and the solid-state polymerization time was 15 hours.
The polyester resin (PET-J) had an intrinsic viscosity of 0.65 dl/g, a cyclic trimer content of 0.42% by weight, a terminal carboxyl group amount of 18 equivalents/t, and a glass transition temperature Tg of 80°C.

(PET-K):
A polyester resin (PET-K) was obtained in the same manner as PET-A, except that the dew point of the nitrogen gas during solid-state polymerization was changed to 15°C and the solid-state polymerization time was changed to 30 hours. The polyester resin (PET-K) had an intrinsic viscosity of 0.65 dl/g, a cyclic trimer content of 0.40 wt%, an amount of terminal carboxyl groups of 40 equivalents/t, and a glass transition temperature Tg of 80°C.

[Coating fluid used]
(Coating agent-1)
Under a nitrogen gas atmosphere, 40 mol parts of 2,6-naphthalenedicarboxylic acid, 50 mol parts of terephthalic acid, 5 mol parts of sodium 5-sulfoisophthalate as dicarboxylic acid components, 95 mol parts of ethylene glycol and 5 mol parts of diethylene glycol as glycol components were charged into an ester exchange reactor, 100 mol parts of tetrabutyl titanate (catalyst) was added to the mixture in an amount of 1,000,000 mol parts by weight based on the total dicarboxylic acid components, and an esterification reaction was carried out for 5 hours at 160 to 240° C., after which the distillate was removed. Thereafter, 5 mol parts of trimellitic acid and 100 mol parts of tetrabutyl titanate were further added to the mixture in an amount of 1,000,000 mol parts by weight based on the total dicarboxylic acids, and the distillate was removed at 240° C. until the reaction product became transparent, followed by a polycondensation reaction under reduced pressure at 220 to 280° C. to obtain polyester (A). Thereafter, 100 parts by weight of polyester (A), 50 parts by weight (active ingredient equivalent) of a melamine-based crosslinking agent ("Nicalac" (registered trademark) MW12LF manufactured by Sanwa Chemical Co., Ltd.: containing 70% by weight of active ingredient and 17% by weight of isopropyl alcohol), 5.0 parts by weight of an active ingredient obtained by mixing 1.5 parts by weight of colloidal silica (particle size 140 nm), 3.0 parts by weight of ethylene glycol mono-n-butyl ether, and 92.0 parts by weight of water were mixed to obtain coating liquid-1.

(Coating liquid-2)
Under a nitrogen gas atmosphere, 85 mol parts of terephthalic acid as a dicarboxylic acid component, 5 mol parts of sodium 5-sulfoisophthalate, and 100 mol parts of ethylene glycol as a glycol component were charged into an ester exchange reactor, and 100 parts by weight of tetrabutyl titanate (catalyst) was added to the mixture in an amount of 100 parts by weight per 1 million parts by weight of the total dicarboxylic acid components, and the esterification reaction was carried out for 5 hours at 160 to 240 ° C., after which the distillate was removed. Thereafter, 10 mol parts of 1,3,5-trimellitic acid and 100 parts by weight of tetrabutyl titanate were further added to the mixture in an amount of 100 parts by weight per 1 million parts by weight of the total dicarboxylic acids, and the distillate was removed at 240 ° C. until the reaction product became transparent, and then a polycondensation reaction was carried out under reduced pressure at 220 to 280 ° C. to obtain polyester (B). Thereafter, coating liquid-2 was obtained in the same manner as coating liquid-1, except that polyester (A) was changed to polyester (B).

(Coating liquid-3)
Coating liquid-4 was obtained in the same manner as in coating liquid-1, except that the active ingredients were changed to a mixture of 100 parts by weight of polyester (A), 40 parts by weight (active ingredient equivalent) of a melamine-based crosslinking agent ("Nicalac" (registered trademark) MW12LF manufactured by Sanwa Chemical Co., Ltd.: containing 70% by weight of active ingredient and 17% by weight of isopropyl alcohol), 10 parts by weight (active ingredient equivalent) of an oxazoline-based crosslinking agent ("Epocross" (registered trademark) WS500 manufactured by Nippon Shokubai Co., Ltd.: containing 40% by weight of active ingredient and 38% by weight of 1-methoxy-2-propanol), and 1.5 parts by weight of colloidal silica (particle size 140 nm).

[Creating polyester film]
Example 1
After drying PET-A in a vacuum at 160°C for 4 hours, silica particles (average particle size 0.45 μm) were added to PET-A in an amount of 0.01% by weight relative to PET-A, and the mixture was fed to an extruder and melt-extruded at 285°C. The mixture was filtered through a filter with an average opening of 5 μm made of sintered and compressed stainless steel fibers, and then through a filter made of sintered stainless steel powder with an average opening of 14 μm. The mixture was extruded into a sheet from a T-shaped die, and was cooled and solidified by wrapping the sheet around a mirror-finished casting drum with a surface temperature of 20°C using an electrostatic casting method. At this time, cold air at a temperature of 10°C was blown onto the film from slit nozzles with a gap of 2 mm and eight stages installed in the longitudinal direction from the opposite side of the casting drum at a wind speed of 20 m/s, thereby cooling the film from both sides.

This unstretched film was preheated to 70°C using a preheat roll, and then heated from above and below using a radiation heater to 90°C while being stretched 3.1 times in the longitudinal direction using the difference in peripheral speed between the rolls. It was then cooled to 25°C using a cooling roll to produce a uniaxially oriented (uniaxially stretched) film.

The uniaxially stretched film was held with clips and introduced into an oven, where it was heated and dried with hot air at 120°C and a wind speed of 20 m/min. It was then continuously introduced into the stretching process, where it was stretched 3.7 times in the width direction while being heated with hot air at 100°C and a wind speed of 15 m/min. The obtained biaxially oriented film was then continuously heat-treated for 15 seconds with hot air at 230°C and a wind speed of 20 m/min, after which it was cooled from 230°C to 120°C while being relaxed by 5% in the width direction, and then cooled to 50°C. Both ends in the width direction were then removed and the film was wound up to obtain a polyester film with a thickness of 125 μm.

The polyester film obtained here has excellent transparency and heat deformation resistance, and can be suitably used as a surface protection film for conductive films and as a transparent conductive substrate film.

Example 2
After particles were not added to PET-A, and a uniaxially stretched film was obtained, both sides of the film were subjected to a corona discharge treatment in air to set the surface tension of the film to 55 mN/m, and the coating liquid shown in Table 1 was applied to both sides of the uniaxially stretched film using a bar coater so that the coating thickness after coating and drying would be 100 nm. A polyester film was obtained in the same manner as in Example 1, except that a metering wire bar with a diameter of 13 mm and a wire diameter of 0.1 mm (#4) was used.

Example 3
After obtaining the uniaxially stretched film, both sides of the film were subjected to corona discharge treatment in air to set the surface tension of the film to 55 mN/m, and the coating solution shown in Table 1 was applied to both sides of the uniaxially stretched film using a bar coater so that the coating thickness after coating and drying was 100 nm. The metering wire bar used had a diameter of 13 mm and a wire diameter of 0.1 mm (#4). A polyester film was obtained in the same manner as in Example 1, except that the coating was performed.

( Comparative Example 9 )
A polyester film was obtained in the same manner as in Example 3, except that PET-B was used instead of PET-A and no particles were added.

(Examples 6 to 7 , Comparative Example 10 )
A polyester film was obtained in the same manner as in Example 1, except that the polyester resin in Table 1 was used in place of PET-A.

(Examples 8 and 9)
A polyester film was obtained in the same manner as in Example 1, except that the particles added to PET-J were the particles shown in Table 1.

(Comparative Examples 1 and 2)
A polyester film was obtained in the same manner as in Example 1, except that the particles added to PET-A were the particles shown in Table 1.

(Comparative Example 3)
A polyester film was obtained in the same manner as in Comparative Example 9, except that the particles shown in Table 1 were added to PET-B.

(Comparative Example 4)
A polyester film was obtained in the same manner as in Example 2, except that PET-D was used, the coating material shown in Table 1 was used, and the extrusion thickness was adjusted so that the final film thickness was 50 μm.

(Comparative Example 5)
A polyester film was obtained in the same manner as in Example 3, except that PET-D was used, inorganic particles shown in Table 1 were added, and a coating material shown in Table 1 was used.

(Comparative Examples 6 and 7)
A polyester film was obtained in the same manner as in Example 1, except that the polyester resin in Table 1 was used instead of PET-A.

(Comparative Example 8)
A polyester film was obtained in the same manner as in Example 3, except that the polyester resin in Table 1 was used in place of PET-A.

Claims (9)

A biaxially stretched polyester film made of a polyester resin composition containing a polyester as a main component, wherein the content of an alkali metal element contained in the polyester resin composition is M1 (mol/t), the content of a divalent metal element is M2 (mol/t), and the content of a trivalent metal element is M3 (mol/t), the molar ratio (M/P) of M (mol/t) calculated by M = 0.5 × (M1) + M2 + 1.5 × (M3) to the content of phosphorus element P (mol/t) is 2.0 to 2.8, the alkali metal element is potassium element, and the divalent metal element is magnesium element, the intrinsic viscosity (IV) of the film is 0.60 dl/g or more and less than 0.70 dl/g, the cold crystallization temperature (Tcc) is 150°C or more and less than 160°C, the content of cyclic trimer is 0.5 wt% or less , and the content of the alkali metal element is 2 ppm or less . The biaxially oriented polyester film according to claim 1, having a thickness variation of 3.0% or less. The biaxially oriented polyester film according to claim 1 or 2, having a thickness of 10 μm or more and 250 μm or less. The biaxially oriented polyester film according to any one of claims 1 to 3, wherein the haze value of the biaxially oriented polyester film is 2.0% or less. The biaxially oriented polyester film according to any one of claims 1 to 4, wherein the haze value of the film after heating the biaxially oriented polyester film at 170°C for 3 hours is 2.0% or less. The biaxially stretched polyester film according to any one of claims 1 to 5, in which the difference in haze value ΔHz between before and after heating at 170°C for 3 hours is 0.5% or less. A biaxially oriented polyester film according to any one of claims 1 to 6, the main component of which is polyethylene terephthalate. The biaxially stretched polyester film according to any one of claims 1 to 7, which is used for optical purposes. The biaxially oriented polyester film according to any one of claims 1 to 7, which is used for surface protection purposes.