JP7513718B2 - Polyester film, release film, and method for producing polyester film - Google Patents

Description

The present invention relates to a polyester film for producing a release film, a release film, and a method for producing a polyester film.

Biaxially oriented polyester films are used in a wide range of applications due to their processability, mechanical properties, electrical properties, dimensional stability, transparency, and chemical resistance. For example, in the field of multilayer ceramic capacitors, release films formed by laminating a release layer on the surface of a biaxially oriented polyester film are used to produce ceramic sheets having a dielectric layer for manufacturing multilayer ceramic capacitors.

For example, Patent Document 1 discloses a release film for producing ceramic sheets, which uses a biaxially oriented polyester film consisting of two or more layers as a substrate, the substrate having a surface layer A that is substantially free of particles and a surface layer B that contains particles, a release coating layer laminated on the surface of surface layer A and a smoothing coating layer laminated on the surface of surface layer B, and the smoothing coating layer has a specific area average surface roughness (Sa) and maximum projection height (P).

JP 2016-060158 A

In recent years, with the increase in capacity and miniaturization of ceramic capacitors, there is a demand for ceramic sheets to be thinner. On the other hand, it is considered that the thinner the ceramic sheet, the greater the influence of the surface shape of the release film on the performance of the ceramic sheet. For example, if there is an uneven shape on the release surface of the release film, the uneven shape is transferred to the ceramic green sheet when the ceramic green sheet is formed, causing the thickness of the ceramic green sheet and the ceramic sheet obtained by firing to vary, which may result in a decrease in the performance of the ceramic capacitor product.
In addition, not only the release surface of the release film, but also the uneven shape of the conveying surface, which is the surface opposite to the release surface, may affect the performance of the ceramic sheet. The conveying surface of the release film is often provided with protrusions for the purpose of suppressing wrinkles during high-speed conveyance, but if the protrusions are too large, the protrusion shape of the conveying surface may be transferred to the release surface when the release film is wound into a roll and stored, forming a transfer mark. It is believed that the shape of the transfer mark transferred to the release surface is transferred to the ceramic green sheet and ceramic sheet, affecting the performance of the final product.

The inventors further studied the release film used in the production of ceramic green sheets with reference to the technology described in Patent Document 1 and found that in addition to the issues of the formation of transfer marks on the release layer surface and transportability mentioned above, depending on the properties of the base film of the release film, the coatability of the release layer may be reduced due to uneven coating during formation of the release layer and/or defects due to foreign matter.

In view of the above-mentioned circumstances, an object of the present invention is to provide a polyester film for producing a release film, which can suppress the formation of transfer marks on the surface of the release layer and has excellent transportability and coatability of the release layer.
Another object of the present invention is to provide a release film and a method for producing a polyester film.

After extensive investigation into the above-mentioned problems, the inventors have discovered that the above-mentioned problems can be solved by the following configuration.

[1]
A polyester film comprising a polyester substrate substantially free of particles and a particle-containing coating layer disposed on one surface of the polyester substrate, the polyester film having a first main surface and a second main surface, and used to produce a release film by forming a release layer on the first main surface, wherein the second main surface is a surface of the coating layer opposite to the polyester substrate side, the second main surface has a maximum projection height Sp of 1 nm or more and less than 60 nm, and the second main surface has a surface free energy of 25 to 60 mJ/ ^m2 .
[2]
The polyester film according to [1], wherein the product (D×Sp) of the density D (unit: particles/μm ² ) of the particles constituting the protrusions on the second main surface and the maximum protrusion height Sp (unit: nm) is 20 or more.
[3]
The polyester film according to [1] or [2], wherein the thickness of the polyester film is 40 μm or less.
[4]
The polyester film according to any one of [1] to [3], wherein the coating layer further contains a polyolefin.
[5]
The polyester film according to any one of [1] to [4], wherein the coating layer further contains a (meth)acrylate resin having an acid value of 30 mgKOH/g or less.
[6]
The polyester film according to any one of [1] to [5], wherein the average particle size of the particles is 1 to 130 nm, the thickness of the coating layer is 1 to 100 nm, and the average particle size of the particles is larger than the thickness of the coating layer.
[7]
[7] The polyester film according to any one of [1] to [6], wherein the coating layer contains at least one surfactant selected from the group consisting of hydrocarbon surfactants and fluorine-based surfactants containing a perfluoroalkyl group having 1 to 4 carbon atoms.
[8]
The polyester film according to any one of [1] to [7], wherein the absolute value of the peel charge between the first and second main surfaces of the polyester film corresponding to a circle having a diameter of 1.5 cmφ is 0.12 nC or less.
[9]
The polyester film according to any one of [1] to [8], wherein the polyester film is transported at a transport speed of 30 m/min and a tension in the transport direction of 100 N/m, and then heat-treated for 20 seconds under conditions in which the film surface temperature is 90°C, and the total area of line-like defect regions observed in the polyester film is 40% or less of the total area of the observed regions.
[10]
The polyester film according to any one of [1] to [9], wherein the density of the polyester film is 1.39 to 1.41 g/ ^cm3 .
[11]
The polyester film according to any one of [1] to [10], wherein the polyester film has a width-direction expansion coefficient at 90° C. of −0.15 to 0.15% relative to the width of the polyester film at 30° C.
[12]
[12] The polyester film according to any one of [1] to [11], wherein the second main surface has a surface average roughness Sa of 1 to 10 nm.
[13]
[13] The polyester film according to any one of [1] to [12], wherein the first main surface has a maximum projection height Sp of 1 to 60 nm.
[14]
The polyester film according to any one of [1] to [13], wherein the surface free energy of the first main surface is 50 to 70 mJ / ^m2 .
[15]
[15] The polyester film according to any one of [1] to [14], wherein the thickness variation of the polyester film is 5% or less of the average thickness of the polyester film.
[16]
The polyester film according to any one of [1] to [15], wherein the release film is a release film for producing a ceramic green sheet.
[17]
A release film comprising the polyester film according to any one of [1] to [16] and a release layer disposed on the first main surface of the polyester film.
[18]
The release film according to [17], wherein the maximum projection height Sp of the surface of the release layer opposite to the polyester film side is 1 to 60 nm.
[19]
The release film according to [17] or [18], wherein the surface free energy of the surface of the release layer opposite the polyester film side is 30 mJ/ ^m2 or less.
[20]
A biaxial stretching step of biaxially stretching an unstretched polyester film having a polyester substrate;
and a coating layer forming step of in-line coating using a coating layer forming composition containing particles.
[17] A method for producing a polyester film according to any one of [1] to [16].
[21]
The method for producing a polyester film according to [20], further comprising: a heat setting step of heating the polyester film biaxially stretched by the biaxial stretching step at a temperature lower than 240°C to heat set the polyester film; a heat relaxing step of heating the polyester film heat-set by the heat setting step at a temperature lower than that of the heat setting step to heat relax the polyester film; a cooling step of cooling the polyester film heat-relaxed by the heat relaxing step; and an expansion step of expanding the heat-relaxed polyester film in the width direction in the cooling step, wherein the cooling rate of the polyester film in the cooling step is more than 2000°C/min and less than 4000°C/min.

According to the present invention, it is possible to provide a polyester film for producing a release film, which can suppress the formation of transfer marks on the surface of the release layer and has excellent transportability and coatability of the release layer.
Furthermore, according to the present invention, a release film and a method for producing a polyester film can be provided.

FIG. 1 is a cross-sectional view showing an example of a configuration of a polyester film according to the present disclosure. 1 is an observed image of a polyester film in which a streak-like defective region occurs. FIG. 2 is a plan view showing an example of a stretching machine used in the production of a polyester film.

The following describes in detail the embodiments of the present invention. Note that the present invention is not limited to the following embodiments, and can be modified as appropriate within the scope of the present invention.

In the present disclosure, a numerical range expressed using "to" means a range including the numerical values described before and after "to" as the lower and upper limits. In the numerical ranges described in stages in the present disclosure, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples.
In the present disclosure, when a plurality of substances corresponding to each component are present in the composition, the amount of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified.
In the present disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.
In the present disclosure, "mass %" and "weight %" are synonymous, and "parts by mass" and "parts by weight" are synonymous.
In the present disclosure, a combination of two or more preferred aspects is a more preferred aspect.

In the present disclosure, the mere description of "polyester film" includes both a polyester substrate alone and a laminate of a polyester substrate and a coating layer.
In the present disclosure, the term "longitudinal direction" refers to the longitudinal direction of a polyester film during production of the polyester film, and is synonymous with the terms "conveyance direction" and "machine direction."
In the present disclosure, the "width direction" refers to a direction perpendicular to the longitudinal direction. In the present disclosure, "perpendicular" is not limited to strictly perpendicular, but includes approximately perpendicular. "Approximately perpendicular" means intersecting at 90°±5°, preferably intersecting at 90°±3°, and more preferably intersecting at 90°±1°.
In the present disclosure, the term "film width" means the distance between both ends of the polyester film in the width direction.

[Polyester film]
The polyester film according to the present disclosure (hereinafter also referred to as "the film") is a polyester film comprising a polyester substrate and a particle-containing coating layer (hereinafter also referred to as "specific coating layer") disposed on one surface of the polyester substrate, having a first main surface and a second main surface, and used to produce a release film by forming a release layer on the first main surface.
In addition, in this film, the second main surface is the surface opposite the polyester substrate side of the specific coating layer, the maximum projection height Sp of the second main surface is 1 nm or more and less than 60 nm, and the surface free energy of the second main surface is 25 to 60 mJ/ ^m2 .

〔composition〕
The structure of the present film will be described with reference to the drawings.
Fig. 1 is a cross-sectional view showing an example of the configuration of the present film. The polyester film 1 comprises a polyester substrate 2 and a specific coating layer 3 disposed on one surface of the polyester substrate 2, and has a first main surface 1a and a second main surface 1b.
The specific coating layer 3 contains particles (not shown), whereas the polyester base material 2 does not substantially contain particles.

The first main surface 1a of the polyester film 1 is a surface for forming a release layer. That is, after the polyester film 1 is produced, a release layer is laminated on the first main surface 1a to produce a release film having the polyester film 1 and the release layer.
The second main surface 1b of the polyester film 1 is the surface opposite to the surface of the specific coating layer 3 facing the polyester base material 2. In other words, the specific coating layer 3 is the outermost layer of the polyester film 1.
The second main surface 1b of the polyester film 1 has the above-mentioned specific maximum projection height Sp and specific surface free energy.

By having the above-mentioned configuration, the present film can suppress the formation of transfer marks on the surface of the release layer, and also has the effects of excellent transportability and excellent coatability of the release layer (hereinafter, at least one of these effects will be referred to as the ``effect of the present invention'').
The reason why the present film exhibits the above-mentioned effects of the present invention is not clear, but is presumed to be as follows.
When a release layer is formed on the first main surface of the film to produce a release film, the second main surface of the film corresponds to the conveying surface, which is the surface opposite to the release layer. As described above, while the conveying property is improved by providing a protrusion shape on the conveying surface (second main surface), if the protrusion shape is too large, there is a problem that transfer marks are formed on the release layer during storage of the release film in a roll, etc.
In contrast, in the polyester film according to the present disclosure, the formation of transfer marks can be suppressed by keeping the protrusions (maximum protrusion height Sp) on the second main surface of the polyester film, which serves as the transport surface of the release film, small, and the surface free energy of the second main surface can be kept low, thereby improving the transportability that was reduced by making the protrusions smaller.
On the other hand, if the maximum projection height Sp of the second main surface is too small, the conveyability is significantly reduced, wrinkles are generated during conveyance, and thickness unevenness may occur in the release layer formed on the first main surface. Also, if the surface free energy of the second main surface is too small, the polyester film becomes easily charged, and foreign matter may adhere to the first and second main surfaces, causing coating defects in the coating film of the composition for forming the release layer. On the other hand, by setting each of the maximum projection height Sp and surface free energy of the second main surface to a specific lower limit value or more, it is presumed that the thickness unevenness and/or coating defects of the release layer are reduced and the coating property of the release layer is improved.
In addition, by configuring the present film to have a polyester base material that is substantially free of particles and a coating layer that contains particles, the smoothness of the film is improved, and it is believed that the film will have a good balance between preventing the formation of transfer marks on the release layer surface and transportability.

The present film has the above-mentioned polyester base material and specific coating layer, and the maximum projection height Sp and surface free energy of the second main surface are each specified within the above ranges, so long as the specific embodiment is not particularly limited, and the film may have an embodiment other than the configuration shown in FIG. 1.
For example, in the configuration shown in Figure 1, the first main surface 1a of the polyester film 1 is the surface opposite the specific coating layer 3 of the polyester substrate 2, but another layer, one of whose surfaces is the first main surface 1a, may be disposed on the surface of the polyester substrate opposite the specific coating layer.
In the configuration shown in FIG. 1, the specific coating layer 3 is disposed in contact with the surface of the polyester substrate 2, but a primer layer or the like may be provided between the specific coating layer and the polyester substrate.

Below, we will explain in detail each layer of this film.

<Polyester base material>
The polyester substrate is a film-like object that contains polyester as the main polymer component, where "main polymer component" means the polymer that is contained in the film in the greatest amount (by mass).
The polyester substrate may contain one type of polyester alone or two or more types of polyester.

(polyester)
Polyester is a polymer having an ester bond in the main chain, and is usually formed by polycondensation of a dicarboxylic acid compound and a diol compound, which will be described later.
The polyester is not particularly limited, and known polyesters can be used. Examples of the polyester include polyethylene terephthalate (PET), polyethylene-2,6-naphthalate (PEN), polypropylene terephthalate (PPT), polybutylene terephthalate (PBT), and copolymers thereof. Among them, polyesters selected from the group consisting of polyethylene terephthalate (PET), polyethylene-2,6-naphthalate (PEN), and copolymers thereof are preferred, and PET is more preferred.

The intrinsic viscosity of the polyester is preferably 0.50 dl/g or more and less than 0.80 dl/g, and more preferably 0.55 dl/g or more and less than 0.70 dl/g.
The melting point (Tm) of the polyester is preferably from 220 to 270°C, more preferably from 245 to 265°C.
The glass transition temperature (Tg) of the polyester is preferably from 65 to 90°C, and more preferably from 70 to 85°C.

The method for producing polyester is not particularly limited, and known methods can be used. For example, polyester can be produced by polycondensing at least one dicarboxylic acid compound and at least one diol compound in the presence of a catalyst.

-catalyst-
The catalyst used in the production of the polyester is not particularly limited, and any known catalyst that can be used in the synthesis of polyesters can be used.
Examples of the catalyst include alkali metal compounds (e.g., potassium compounds, sodium compounds), alkaline earth metal compounds (e.g., calcium compounds, magnesium compounds), zinc compounds, lead compounds, manganese compounds, cobalt compounds, aluminum compounds, antimony compounds, titanium compounds, germanium compounds, and phosphorus compounds. Among these, titanium compounds are preferred from the viewpoints of catalytic activity and cost.
The catalyst may be used alone or in combination of two or more. It is preferable to use at least one metal catalyst selected from potassium compounds, sodium compounds, calcium compounds, magnesium compounds, zinc compounds, lead compounds, manganese compounds, cobalt compounds, aluminum compounds, antimony compounds, titanium compounds, and germanium compounds in combination with a phosphorus compound, and it is more preferable to use a titanium compound in combination with a phosphorus compound.

The titanium compound is preferably an organic chelate titanium complex, which is a titanium compound having an organic acid as a ligand.
Organic acids include, for example, citric acid, lactic acid, trimellitic acid, and malic acid.
As the titanium compound, the titanium compounds described in paragraphs 0049 to 0053 of Japanese Patent No. 5575671 can also be used, and the contents of the above publication are incorporated herein by reference.

-Dicarboxylic acid compound-
Examples of the dicarboxylic acid compound include dicarboxylic acids such as aliphatic dicarboxylic acid compounds, alicyclic dicarboxylic acid compounds, and aromatic dicarboxylic acid compounds, as well as dicarboxylic acid esters such as methyl ester compounds and ethyl ester compounds of these dicarboxylic acids. Among these, aromatic dicarboxylic acids or aromatic methyl dicarboxylates are preferred.

Examples of the aliphatic dicarboxylic acid compound include malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, dimer acid, eicosanedioic acid, pimelic acid, azelaic acid, methylmalonic acid, and ethylmalonic acid.
Examples of the alicyclic dicarboxylic acid compound include adamantanedicarboxylic acid, norbornenedicarboxylic acid, cyclohexanedicarboxylic acid, and decalindicarboxylic acid.

Examples of aromatic dicarboxylic acid compounds include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodiumsulfoisophthalic acid, phenylindanedicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorene acid, and methyl esters thereof.
Among these, terephthalic acid or 2,6-naphthalenedicarboxylic acid is preferred, and terephthalic acid is more preferred.

The dicarboxylic acid compound may be used alone or in combination of two or more. When terephthalic acid is used as the dicarboxylic acid compound, terephthalic acid may be used alone or may be copolymerized with other aromatic dicarboxylic acids such as isophthalic acid or aliphatic dicarboxylic acids.

-Diol compound-
Examples of the diol compound include an aliphatic diol compound, an alicyclic diol compound, and an aromatic diol compound, and an aliphatic diol compound is preferred.

Examples of the aliphatic diol compound include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, and neopentyl glycol, with ethylene glycol being preferred.
Examples of alicyclic diol compounds include cyclohexanedimethanol, spiroglycol, and isosorbide.
Examples of aromatic diol compounds include bisphenol A, 1,3-benzenedimethanol, 1,4-benzenedimethanol, and 9,9'-bis(4-hydroxyphenyl)fluorene.
The diol compound may be used alone or in combination of two or more kinds.

-End-capping agent-
In the production of the polyester, a terminal capping agent may be used as necessary. By using the terminal capping agent, a structure derived from the terminal capping agent is introduced at the terminal of the polyester.
The terminal blocking agent is not limited, and any known terminal blocking agent can be used, including, for example, an oxazoline compound, a carbodiimide compound, and an epoxy compound.
For the terminal blocking agent, reference can also be made to the contents described in paragraphs 0055 to 0064 of JP 2014-189002 A, the contents of which are incorporated herein by reference.

- Manufacturing conditions -
The reaction temperature is not limited and may be appropriately set depending on the raw materials. The reaction temperature is preferably 260 to 300° C., more preferably 275 to 285° C.
The pressure is not limited and may be appropriately set depending on the raw materials. The pressure is preferably 1.33×10 ⁻³ to 1.33×10 ⁻⁵ MPa, and more preferably 6.67×10 ⁻⁴ to 6.67×10 ⁻⁵ MPa.

As a method for synthesizing polyester, the method described in paragraphs 0033 to 0070 of Japanese Patent No. 5,575,671 can also be used, and the contents of the above publication are incorporated herein by reference.

The content of the polyester in the polyester base material is preferably 85% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 98% by mass or more, based on the total mass of the polymer in the polyester base material.
The upper limit of the polyester content is not limited, and can be appropriately set within the range of 100% by mass or less based on the total mass of the polymer in the polyester base material.

When the polyester base material contains polyethylene terephthalate, the content of polyethylene terephthalate is preferably 90 to 100 mass%, more preferably 95 to 100 mass%, even more preferably 98 to 100 mass%, and particularly preferably 100 mass%, based on the total mass of polyester in the polyester base material.

The polyester substrate may contain components other than polyester (for example, catalysts, unreacted raw material components, particles, water, etc.).
The polyester substrate does not substantially contain particles. Examples of the particles include particles contained in the specific coating layer described later. The term "substantially does not contain particles" is defined as the content of particles being 50 ppm by mass or less relative to the total mass of the polyester substrate when the polyester substrate is quantitatively analyzed for elements derived from particles by fluorescent X-ray analysis, preferably 10 ppm by mass or less, and more preferably below the detection limit. This is because even if particles are not actively added to the polyester substrate, contaminants derived from foreign matter, raw material resin, or dirt attached to the line or equipment in the manufacturing process of the polyester substrate may peel off and be mixed into the polyester substrate.

The thickness of the polyester substrate is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 40 μm or less, in terms of being able to control the releasability. The lower limit of the thickness is not particularly limited, but is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 10 μm or more, in terms of improving the strength and improving the processability.
The thickness of the polyester substrate is measured according to the method for measuring the thickness of a polyester film described below.

<Specific Coating Layer>
The specific coating layer is a layer containing particles and is formed on one surface of the polyester substrate. The surface of the specific coating layer opposite to the surface facing the polyester substrate constitutes the second main surface.
The present film has a specific coating layer, which can improve the transportability of the polyester film and the release film. More specifically, it can improve the winding quality (suppress blocking), suppress the occurrence of scratches and defects during transport, and reduce transport wrinkles during high-speed transport.

The specific coating layer may be provided directly on the surface of the polyester substrate, or may be provided on the surface of the polyester substrate via another layer, but it is preferable to provide it directly on the surface of the polyester substrate in terms of better adhesion. In other words, it is preferable that the surface of the first main surface side of the specific coating layer is in contact with the polyester substrate.

The specific coating layer is not particularly limited as long as it contains particles and the second main surface has a specific maximum projection height Sp and surface free energy, but it is preferable that it contains a binder in addition to the particles. The specific coating layer may also contain additives other than the particles and binder.

(particle)
The average particle size of the particles contained in the specific coating layer is not particularly limited, and is preferably 1 to 250 nm, more preferably 10 to 200 nm, and even more preferably 30 to 130 nm, in terms of better transportability and suppression of transfer marks.
In addition, in terms of better transportability and suppression of transfer marks, it is preferable that the average particle size of the particles contained in the specific coating layer is 10 to 250 nm (more preferably 30 to 130 nm), the thickness of the specific coating layer is 1 to 200 nm (more preferably 10 to 100 nm), and the average particle size of the particles is larger than the thickness of the specific coating layer.

The specific coating layer may contain one type of particles alone or two or more types of particles.
When the specific coating layer contains two or more types of particles having different particle sizes, it is preferable that the specific coating layer contains at least one type of particle having an average particle size within the above range, and it is more preferable that all of the two or more types of particles having different particle sizes have average particle sizes within the above range.

The average particle diameter of the particles contained in the specific coating layer is determined by the following method using a scanning electron microscope (SEM). That is, the second main surface of the polyester film is observed at a magnification of 20,000 times using an SEM. Observation is performed for 10 arbitrarily selected visual fields, and for particles that can be identified as protrusions in each visual field (particles that can be visually recognized as protrusions protruding from the base surface), the area of each particle is measured using image software, and the diameter of a circle having the same area (diameter of a circle equivalent to the area) is calculated. The arithmetic average value of the diameter of the circle equivalent to the area obtained is the average particle diameter of the particles. At this time, even if dust and/or coarse aggregated particles of 1 μm or more are present, the dust and coarse aggregated particles are not counted when calculating the average particle diameter.
In the measurement of the average particle size, for aggregated particles, the particle size of secondary particles in an aggregated state (secondary particle size) is measured.
In addition, when the specific coating layer contains two or more kinds of particles with different particle diameters, the distribution of the equivalent circle diameter measured by the above-mentioned measuring method has two or more peaks with different particle diameters. In this way, when the distribution of the equivalent circle diameter measured by the above-mentioned measuring method has two or more peaks with different particle diameters, the average value of the equivalent circle diameter for each peak is calculated, and the average particle diameter is calculated for each particle with different particle diameters.

Examples of the particles contained in the specific coating layer include organic particles and inorganic particles. Among them, inorganic particles are preferred from the viewpoint of further improving the film winding quality, haze, and durability (for example, thermal stability).
The organic particles are preferably resin particles. Examples of the resin constituting the resin particles include acrylic resins such as polymethyl methacrylate resin (PMMA), polyester resins, silicone resins, and styrene-acrylic resins. The resin particles preferably have a crosslinked structure. Examples of the resin particles having a crosslinked structure include divinylbenzene crosslinked particles.
Examples of inorganic particles include silica particles (silicon dioxide particles, colloidal silica), titania particles (titanium oxide particles), calcium carbonate, barium sulfate, and alumina particles (aluminum oxide particles). Among the above, the inorganic particles are preferably silica particles from the viewpoint of further improving haze and durability.

The shape of the particles is not particularly limited, and examples thereof include rice grain, spherical, cubic, spindle, scaly, aggregated, and irregular shapes. "Aggregated" refers to a state in which primary particles are aggregated. The shape of the particles in the aggregated state is not limited, but spherical or irregular shapes are preferred.

The agglomerated particles are preferably fumed silica particles. Examples of commercially available products include the Aerosil series manufactured by Nippon Aerosil Co., Ltd.
As the non-aggregated particles, colloidal silica particles are preferably used, and examples of commercially available products include the Snowtex series manufactured by Nissan Chemical Industries, Ltd.

From the viewpoints of transportability and coating properties of the release layer, the content of particles in the specific coating layer is preferably 0.1 to 30 mass %, more preferably 1 to 25 mass %, and even more preferably 1 to 15 mass %, relative to the total mass of the specific coating layer.
The content of the particles is preferably from 0.0001 to 0.01% by mass, and more preferably from 0.0005 to 0.005% by mass, based on the total mass of the polyester film.

(binder)
The specific coating layer preferably contains a binder. The binder is preferably a resin binder. Examples of the resin binder include polyacrylic, polyurethane, polyester, and polyolefin.

The specific coating layer is preferably formed by coating an aqueous dispersion containing a binder. In this respect, the binder is preferably an acid-modified resin. The acid-modified resin includes a copolymer of (meth)acrylate and (meth)acrylic acid, a polyolefin having a carboxyl group, and an acid-modified polyurethane, and the copolymer of (meth)acrylate and (meth)acrylic acid or a polyolefin having a carboxyl group is preferable, and the polyolefin having a carboxyl group is more preferable.
In addition, the binder is preferably a (meth)acrylate resin, polyolefin, or polyurethane, more preferably a (meth)acrylate resin or polyolefin, and even more preferably a polyolefin, in that the surface free energy of the specific coating layer can be easily adjusted to the above-mentioned specific range. The (meth)acrylate resin, polyolefin, and polyurethane are not particularly limited, and known resins can be used.

The polyolefin may contain a structural unit derived from an olefin in the main chain, and preferably contains a structural unit derived from an olefin as the main component. By having an olefin structure in the main chain, the compatibility with the polyester substrate becomes insufficient, and as a result, the transfer mark after long-term storage can be further improved. The olefin is not particularly limited, but is preferably an alkene having 2 to 6 carbon atoms, more preferably ethylene, propylene, or hexene, and even more preferably ethylene.
In this specification, when a polymer "has as a main component" a structural unit derived from a certain monomer, it is intended that the structural unit accounts for 50 mol % or more of all structural units of the polymer.
The proportion of olefin-derived structural units in the polyolefin is preferably 50 to 99 mol %, more preferably 60 to 98 mol %, based on all structural units in the polyolefin.

The polyolefin is preferably an acid-modified polyolefin, since it can prevent charging when applying the release layer.The acid-modified polyolefin may be, for example, a copolymer obtained by modifying the above polyolefin with an acid-modifying component such as an unsaturated carboxylic acid or its anhydride.The polymerization form of this copolymer is not particularly limited, and may be random copolymerization, block copolymerization, graft copolymerization, or the like.
Examples of the acid-modified component include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, itaconic acid, itaconic anhydride, fumaric acid, and crotonic acid, as well as half esters and half amides of unsaturated dicarboxylic acids. From the viewpoint of dispersion stability of the resin, acrylic acid, methacrylic acid, maleic acid, and maleic anhydride are preferred.

The acidic group contained in the acid-modified polyolefin includes a carboxyl group, a sulfo group, and a phosphoric acid group, which are acidic groups corresponding to the above-mentioned acid-modified component, and a carboxyl group is preferred.The acidic group may form an acid anhydride, or may be neutralized with at least one selected from an alkali metal, an organic amine, and ammonia.In terms of suppressing charging and improving the coating property of the release layer, it is preferred that the acid-modified component is neutralized with an alkali metal.
The acid-modified polyolefin may contain only one type of structural unit having an acidic group, or may contain two or more types. Examples of the structural unit having an acidic group include a structural unit derived from the monomer of the acid-modified component described above, and a structural unit in which the acid-modified component is grafted onto a structural unit derived from the olefin monomer described above. The content of the structural unit having an acidic group is not particularly limited, but is preferably 0.1 to 30 mol% based on all structural units of the acid-modified polyolefin.

Commercially available acid-modified polyolefins include, for example, the ZAIXXEN (registered trademark) series, such as ZAIXXEN AC, A, L, NC, and N (manufactured by Sumitomo Seika Chemicals Co., Ltd.), the CHEMIPEARL (registered trademark) series, such as CHEMIPEARL S100, S120, S200, S300, S650, and SA100 (manufactured by Mitsui Chemicals, Inc.), and the HI-TECH (registered trademark) series, such as HI-TECH S3121 and S3148K (manufactured by Mitsui Chemicals, Inc.). Examples of such an adhesive include the Arrowbase (registered trademark) series (manufactured by Unitika Ltd.), Arrowbase SE-1013, SE-1010, SB-1200, SD-1200, SD-1200, DA-1010, and DB-4010 (manufactured by Toho Chemical Industries, Ltd.), Hardlen AP-2, NZ-1004, and NZ-1005 (manufactured by Toyobo Co., Ltd.), Sepolsion G315, and VA407 (manufactured by Sumitomo Seika Chemicals Co., Ltd.).
In addition, the acid-modified polyolefins described in paragraphs 0022 to 0034 of JP-A-2014-076632 can also be preferably used.

The (meth)acrylate resin is a resin containing a structural unit derived from a (meth)acrylate, and may be copolymerized with a vinyl monomer such as styrene. The (meth)acrylate resin is not particularly limited, but preferably contains a structural unit derived from a (meth)acrylate having an alkyl group with 1 to 12 carbon atoms, and more preferably contains a structural unit derived from a (meth)acrylate having an alkyl group with 1 to 6 carbon atoms.
In terms of preventing charging when applying the release layer, the (meth)acrylate resin preferably has an acid-modified component. The (meth)acrylate resin preferably contains a structural unit derived from (meth)acrylic acid as the acid-modified component. The (meth)acrylic acid may form an acid anhydride, or may be neutralized with at least one selected from an alkali metal, an organic amine, and ammonia. In terms of suppressing charging and improving the coatability of the release layer, the (meth)acrylic acid is preferably neutralized with an alkali metal.
In terms of suppressing electrostatic charge and improving the coatability of the release layer, the content of the structural unit having an acid-modified group is preferably 0.1 to 10% by mass relative to all structural units of the (meth)acrylate resin, and more preferably the structural unit consisting of (meth)acrylic acid is 0.1 to 10% by mass. By setting the content of the structural unit having an acid-modified group within the above range, the acid value can be lowered and the surface free energy can be adjusted to a desired range. In addition, as the aqueous dispersion of the (meth)acrylate resin, an aqueous dispersion containing a (meth)acrylate resin and a dispersant is also preferred.

The acid value of the (meth)acrylate resin is preferably 30 mgKOH/g or less, more preferably 20 mgKOH/g or less. The lower limit of the acid value is not particularly limited and is, for example, 0 mgKOH/g, but from the viewpoint of coating as an aqueous dispersion, 2 mgKOH/g or more is preferable. By adjusting the acid value of the (meth)acrylate resin to the above range and containing a structural unit derived from a (meth)acrylate having an alkyl group having 1 to 12 carbon atoms, the surface free energy can be adjusted to a desired range. When the polyester film of the present invention is stored for a long period of time and used as a release film for producing a ceramic green sheet, it is preferable to satisfy both of the acid value of the acrylic resin being in the above range and containing a structural unit derived from a (meth)acrylate having an alkyl group having 1 to 12 carbon atoms, in order to suppress defects.

There are no limitations on the polyurethane as long as it is a polymer having a urethane bond in the main chain, and known polyurethanes such as reaction products of isocyanate compounds and polyol compounds can be used.
As mentioned above, acid-modified polyurethane is preferred in terms of ease of preparation of the aqueous dispersion. Acid-modified polyurethane means polyurethane having an acidic group. The acidic group may be the group mentioned as the acidic group contained in the acid-modified polyolefin. In addition, the aqueous dispersion of polyurethane is also preferably an aqueous dispersion containing polyurethane and a dispersant.
The surface free energy of the polyurethane contained in the specific coating layer can be controlled within a specific range, for example, by adjusting the structure and hydrophobicity (hydrophilicity) of the respective raw materials, the polyol compound and/or the isocyanate compound.
Commercially available polyurethane products include, for example, HYDRAN (registered trademark) AP-20, AP-40N, and AP-201 (all manufactured by DIC Corporation); TAKELAC (registered trademark) W-605, W-5030, and W-5920 (all manufactured by Mitsui Chemicals, Inc.); and SUPERFLEX (registered trademark) 210 and 130, ELASTRON (registered trademark) H-3-DF, E-37, and H-15 (all manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.).

The specific coating layer may contain one kind of binder alone or may contain two or more kinds of binders. In order to control the surface free energy, when the above-mentioned polyolefin or (meth)acrylate resin is used in combination with a resin other than the polyolefin and the (meth)acrylate resin, the resin to be used in combination is preferably the above-mentioned polyurethane.
From the viewpoint of adjusting Sp within a desired range, the content of the binder is preferably from 30 to 99.8% by mass, and more preferably from 50 to 99.5% by mass, based on the total mass of the specific coating layer.

(Additive)
The specific coating layer may contain additives other than the above-mentioned particles and binder.
Examples of additives contained in the specific coating layer include surfactants, waxes, crosslinking agents, antioxidants, ultraviolet absorbers, colorants, reinforcing agents, plasticizers, antistatic agents, flame retardants, rust inhibitors, and fungicides.

It is preferable that the specific coating layer contains a surfactant, since this improves the smoothness of the areas of the second principal surface other than the areas where the protrusions formed by the particles are present. By improving the smoothness of the above-mentioned areas of the second principal surface and reducing the surface roughness of the second principal surface due to factors other than the particles, it is possible to control Sp within a desired range and improve the effects of the present invention.

The surfactant is not particularly limited, and examples thereof include silicone-based surfactants, fluorine-based surfactants, and hydrocarbon-based surfactants. Hydrocarbon-based surfactants are preferred because they can improve the coatability of the release layer by suppressing charging on the first main surface and suppressing the occurrence of defects due to foreign matter when applying the release layer.

The silicone surfactant is not particularly limited as long as it has a silicon-containing group as a hydrophobic group, and examples thereof include polydimethylsiloxane, polyether-modified polydimethylsiloxane, and polymethylalkylsiloxane.
Commercially available silicone surfactants include, for example, BYK (registered trademark)-306, BYK-307, BYK-333, BYK-341, BYK-345, BYK-346, BYK-347, BYK-348, and BYK-349 (all manufactured by BYK Corporation), as well as KF-351A, KF-352A, KF-353, KF-354L, KF-355A, KF-615A, KF-945, KF-640, KF-642, KF-643, KF-6020, X-22-4515, KF-6011, KF-6012, KF-6015, and KF-6017 (all manufactured by Shin-Etsu Chemical Co., Ltd.).

The fluorosurfactant is not particularly limited as long as it has a fluorine-containing group as a hydrophobic group, and examples thereof include perfluorooctanesulfonic acid and perfluorocarboxylic acid.
Commercially available fluorine-based surfactants include, for example, Megafac (registered trademark) F-114, F-410, F-440, F-447, F-553, and F-556 (all manufactured by DIC Corporation), and Surflon (registered trademark) S-211, S-221, S-231, S-233, S-241, S-242, S-243, S-420, S-661, S-651, and S-386 (manufactured by AGC Seimi Chemical Co., Ltd.).

Furthermore, from the viewpoint of improving environmental compatibility, it is preferable to use, as the fluorine-based surfactant, a surfactant derived from an alternative material to a compound having a linear perfluoroalkyl group having 7 or more carbon atoms, such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), and it is more preferable to use a surfactant derived from an alternative material to a compound having a linear perfluoroalkyl group having 6 or more carbon atoms.
More specifically, it is preferable to use a surfactant having a perfluoroalkyl group having 1 to 4 carbon atoms, since the coating property of the release layer can be improved compared to a surfactant having a linear perfluoroalkyl group having 6 or more carbon atoms. The perfluoroalkyl group having 1 to 4 carbon atoms may be linear or branched. Among them, a surfactant having a linear perfluoroalkyl group having 1 to 4 carbon atoms or a branched perfluoroalkyl group having 3 carbon atoms is more preferable, and a surfactant having a perfluoroalkyl group having 1 or 2 carbon atoms or a branched perfluoroalkyl group having 3 carbon atoms is even more preferable.
Examples of the perfluoroalkyl group include CF ₃ -*, C ₂ F ₅ -*, C ₃ F ₇ -*, n-C ₄ F ₉ -*, and (CF ₃ ) ₂ CF-*. Here, * indicates the bonding position to a carbon atom other than the carbon atom substituted with a fluorine atom. The carbon atom bonding to these perfluoroalkyl groups preferably has a hydrogen atom or is bonded only to carbon atoms.

Commercially available fluorosurfactants having a perfluoroalkyl group with 1 to 4 carbon atoms include, for example, Futergent (registered trademark) 100, 100C, 110, 150, 150H, 212M, 215M, 250, 251, 222F, 245F, 208G, FTX-218, DFX-18, 300, 310, 320, 400SW, 710FL, 683, 601AD, 602A, and 681 (all manufactured by Neos Co., Ltd.), as well as PF-136A, PF-156A, PF-151N, PF-636, PF-6320, PF-656, PF-6520, PF-652-NF (all manufactured by OMNOVA).

Although the reason why the use of a surfactant having a perfluoroalkyl group with 1 to 4 carbon atoms improves the coatability of the release layer compared to the use of a surfactant having a linear perfluoroalkyl group with 6 or more carbon atoms is unclear, a surfactant having a perfluoroalkyl group with 1 to 4 carbon atoms contains many _CF3 groups per weight, which have low surface tension, and therefore can reduce the surface free energy of the second main surface with a small amount added. This improves the transportability of the polyester film, and since the composition of the specific coating layer does not change significantly, it is presumed that the peel electrification described below can be suppressed and the coatability of the release layer can be improved.

Examples of the hydrocarbon surfactant include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants.
Examples of anionic surfactants include alkyl sulfates, alkyl benzene sulfonates, alkyl phosphates, and fatty acid salts.
Examples of the nonionic surfactant include polyalkylene glycol mono- or dialkyl ethers, polyalkylene glycol mono- or dialkyl esters, and polyalkylene glycol monoalkyl esters and monoalkyl ethers.
Examples of cationic surfactants include primary to tertiary alkylamine salts and quaternary ammonium compounds.
Amphoteric surfactants include surfactants having both anionic and cationic moieties in the molecule.

Commercially available anionic surfactants include, for example, Rapisol (registered trademark) A-90, A-80, BW-30, B-90, and C-70 (all manufactured by NOF Corp.), NIKKOL (registered trademark) OTP-100 (all manufactured by Nikko Chemical Co., Ltd.), Kohacool (registered trademark) ON, L-40, and Phosphanol (registered trademark) 702 (all manufactured by Toho Chemical Industry Co., Ltd.), and Viewlite (registered trademark) A-5000 and SSS (all manufactured by Sanyo Chemical Industries, Ltd.).
Examples of commercially available nonionic surfactants include Naroacty (registered trademark) CL-95 and HN-100 (product names: manufactured by Sanyo Chemical Industries, Ltd.), Lysolex BW400 (product name: manufactured by Kokyu Alcohol Kogyo Co., Ltd.), EMALEX (registered trademark) ET-2020 (all manufactured by Nippon Emulsion Co., Ltd.), and Surfynol (registered trademark) 104E, 420, 440, 465, and Dynol (registered trademark) 604, 607 (all manufactured by Nissin Chemical Industry Co., Ltd.).

When used in combination with an acid-modified resin, anionic surfactants and/or nonionic surfactants are preferred, with anionic surfactants being more preferred, in that they can form a coating layer with a smooth surface without impeding the dispersion of the resin. In other words, anionic hydrocarbon surfactants are more preferred as surfactants in terms of improving surface smoothness and improving the coatability of the release layer.

The anionic hydrocarbon surfactant preferably has a plurality of hydrophobic end groups in terms of further improving smoothness.The hydrophobic end group may be a part of the hydrocarbon group of the hydrocarbon surfactant.For example, a hydrocarbon surfactant having a branched chain structure at its end will have a plurality of hydrophobic end groups.
Examples of anionic hydrocarbon surfactants having multiple hydrophobic end groups include sodium di-2-ethylhexyl sulfosuccinate (having four hydrophobic end groups), sodium di-2-ethyloctyl sulfosuccinate (having four hydrophobic end groups), and branched alkylbenzene sulfonate (having two hydrophobic end groups).

The surfactant may be used alone or in combination of two or more kinds.
The content of the surfactant is preferably 0.1 to 10 mass % based on the total mass of the specific coating layer, and from the viewpoint of superior antistatic properties during formation of the release layer and superior surface smoothness, it is more preferably 0.1 to 5 mass %, and even more preferably 0.5 to 2 mass %.

The wax is not particularly limited, and may be either natural or synthetic wax. Examples of natural waxes include carnauba wax, candelilla wax, beeswax, montan wax, paraffin wax, and petroleum wax. In addition, the slip agents described in [0087] of International Publication No. 2017/169844 can also be used.
The content of the wax is preferably 0 to 10% by mass based on the total mass of the specific coating layer.

The crosslinking agent is not particularly limited, and any known crosslinking agent can be used.
Examples of the crosslinking agent include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate compounds, and carbodiimide compounds, with oxazoline compounds and carbodiimide compounds being particularly preferred. Commercially available products include, for example, Carbodilite V-02-L2 (manufactured by Nisshinbo Co., Ltd.) and Epocross K-2020E (manufactured by Nippon Shokubai Co., Ltd.). For details of the epoxy compounds, isocyanate compounds, and melamine compounds, reference can be made to the descriptions in [0081] to [0083] of JP 2015-163457 A. The crosslinking agents described in [0082] to [0084] of WO 2017/169844 A can also be preferably used. For the carbodiimide compounds, reference can be made to the descriptions in [0038] to [0040] of JP 2017-087421 A.
For the oxazoline compound, carbodiimide compound, and isocyanate compound, the crosslinking agents described in paragraphs [0074] to [0075] of WO 2018/034294 can also be preferably used.
The content of the crosslinking agent is preferably 0 to 50% by mass based on the total mass of the specific coating layer.

(thickness)
The specific coating layer is formed by coating one surface of the polyester substrate with a composition containing particles, and the thickness of the specific coating layer is often 1 μm or less.
The thickness of the specific coating layer is preferably from 1 to 200 nm, more preferably from 10 to 100 nm, and even more preferably from 20 to 100 nm, from the viewpoints of manufacturability of the specific coating layer and reduction of haze.
The thickness of the specific coating layer is defined as the arithmetic average value of the thicknesses of five points on a slice prepared by preparing a slice having a cross section perpendicular to the main surface of the polyester film and measuring the thickness at the five points on the slice using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
When the specific coating layer is soft and it is difficult to stably prepare a cross-sectional slice, the measurement may be performed using a refractometer. Specifically, the thickness of the specific coating layer can be determined by fitting the measured reflectance spectrum to the thickness and refractive index of the specific coating layer and the polyester substrate.

The method for forming the specific coating layer will be explained in detail in the "Specific coating layer formation process" described below.

The present film may have layers other than the polyester substrate and the specific coating layer described above, but it is more preferable that the film consists of a polyester substrate and a specific coating layer.

[Physical properties, etc.]
Next, the physical properties of the present film will be described.

(Surface Free Energy of Second Principal Surface)
In the present film, the surface free energy of the second main surface is 25 to 60 mJ/ ^m2 . By having the surface free energy of the second main surface in the above range, even if the maximum projection height Sp of the second main surface is in the above range, a polyester film having excellent transportability and excellent coatability of the release layer can be obtained.
The surface free energy of the second main surface is preferably 25 to 50 mJ/ ^m2 in order to further suppress the formation of transfer marks on the surface of the release layer of the release film after long-term storage. Furthermore, from the viewpoints of the transportability and the coatability of the release layer, the surface free energy of the second main surface is more preferably 30 to 50 mJ/ ^m2 , even more preferably 30 to 45 mJ/ ^m2 , and particularly preferably 40 to 45 mJ/ ^m2 .
The surface free energy of the second main surface (the surface of the specific coating layer) can be adjusted, for example, by selecting the particles constituting the specific coating layer, the binder and additives described above, and the like.

The surface free energy of the second main surface of the polyester film is determined by dropping droplets of purified water, methylene iodide, and ethylene glycol onto the second main surface (the surface on the specific coating layer side) at 25°C using a contact angle meter (e.g., "DROPMASTER-501" manufactured by Kyowa Interface Science Co., Ltd.), measuring the contact angle 1 second after the droplets adhere to the surface, and calculating the surface free energy from each of the obtained contact angles according to the Kitazaki and Hata method.
The "surface free energy" obtained by the above method is the sum of the polar component and the hydrogen bond component of the surface free energy.

(Surface Free Energy of First Principal Surface)
From the viewpoint of preventing static electricity when the present film is wound up, the surface free energy of the first main surface is preferably 50 to 70 mJ/ ^m2 .
In addition, the larger the difference between the surface free energy of the first main surface and the surface free energy of the second main surface, the less likely the film is to be charged, which is preferable. The difference between the surface free energy of the first main surface and the surface free energy of the second main surface is preferably 1 to 35 mJ/ ^m2 , more preferably 5 to 35 mJ/ ^m2 , and even more preferably 10 to 30 mJ/ ^m2 .
The surface free energy of the first main surface can be adjusted by the type of resin and additives forming the layer having the first main surface. For example, when the first main surface is the surface of the polyester substrate opposite to the specific coating layer, the surface free energy of the first main surface can be adjusted by the type of resin and additives forming the polyester substrate.

(Maximum protrusion height Sp, average surface roughness Sa of the second main surface)
In the present film, the second main surface has a maximum projection height Sp of 1 nm or more and less than 60 nm. By having the second main surface have a maximum projection height Sp in the above range, a release film having a good balance between suppression of transfer marks on the release layer surface and transportability can be produced.
From the above viewpoints, the maximum projection height Sp of the second main surface is preferably from 10 to 50 nm, and more preferably from 20 to 50 nm.

In addition, in this film, in order to provide better stability in suppressing transfer marks, the surface average roughness Sa of the second main surface is preferably 1 to 10 nm, more preferably 1 to 9 nm, and even more preferably 1 to 8 nm.

The maximum projection height Sp and average surface roughness Sa of the second principal surface (specific coating layer surface) can be adjusted, for example, by the average particle size and content of the particles contained in the specific coating layer, and the thickness of the specific coating layer. When the specific coating layer is formed by in-line coating, the above adjustments can be made more easily.

The maximum protrusion height Sp and surface average roughness Sa of the second main surface of the polyester film are determined by measuring the surface of the polyester film on the side of a specific coating layer using an optical interferometer (e.g., "Vertscan 3300G Lite" manufactured by Hitachi High-Tech Corporation) under the following conditions, and then analyzing the results using the built-in data analysis software.
In measuring the maximum projection height Sp, the measurement position is changed and measurements are taken five times, and the maximum value of the obtained measurements is regarded as the measured value of the maximum projection height Sp (denoted as P in the built-in data analysis software). In measuring the average surface roughness Sa, the measurement position is changed and measurements are taken five times, and the average value of the obtained measurements is regarded as the measured value of the average surface roughness Sa.
(Measurement condition)
Measurement mode: WAVE mode Objective lens: 50x Measurement area: 186μm x 155μm

(Maximum protrusion height Sp, average surface roughness Sa of the first main surface)
In order to smooth the release layer, it is preferable that the first main surface is as smooth as possible. Specifically, the maximum projection height Sp of the first main surface is preferably 1 to 60 nm, and more preferably 5 to 30 nm. The surface average roughness Sa of the first main surface is preferably 0 to 10 nm, and more preferably 0 to 5 nm.
The maximum projection height Sp and average surface roughness Sa of the first main surface can be adjusted by techniques such as selecting the type of polyester and the type of additives constituting the polyester base material so as to form a smooth film while substantially not including particles in the polyester base material.
The maximum projection height Sp and surface average roughness Sa of the first main surface can be measured according to the above-mentioned method for measuring the maximum projection height Sp and surface average roughness Sa of the second main surface.

(Product of maximum projection height Sp of second main surface and particle density D)
In the present film, in terms of better transportability, the product (D×Sp) of the density D (unit: particles/ ^μm2 , also referred to as "particle density D") of the particles constituting the protrusions on the second main surface and the maximum protrusion height Sp (unit: nm) on the second main surface is preferably 1 or more, more preferably 20 or more, and even more preferably 50 or more. There is no particular upper limit, but in terms of better suppression of transfer marks, it is preferably 400 or less, and more preferably 300 or less.
The above product (D × Sp) is an index that represents the size of the protrusions and the density at which they are present on the second main surface, and when the product (D × Sp) is within the above range, protrusions that are preferable from the standpoint of suppressing transfer marks and conveyability will be present in an appropriate amount on the second main surface, which is preferable because the effects of suppressing transfer marks and conveyability will be even better.

The above particle density D, like the above maximum projection height Sp and average surface roughness Sa, can be adjusted, for example, by the average particle size and content of the particles contained in the specific coating layer, and the thickness of the specific coating layer. When the specific coating layer is formed by in-line coating, the above adjustments can be made more easily.

In addition, the particle density D of the particles constituting the protrusions on the second main surface of the polyester film is obtained by the same method as the measurement method of the average particle diameter of the particles using SEM. That is, the surface of the specific coating layer side of the polyester film is observed at a magnification of 20,000 times. Observation is performed for 10 arbitrarily selected visual fields, and the number of individual particles that can be identified as protrusions in each visual field (particles that can be visually recognized as protrusions protruding from the base surface) is counted using image software. The calculated value obtained by dividing the total number of particles measured in all visual fields by the total area of all visual fields is the particle density D (unit: particles/μm ² ).

(Orientation)
The present film is a biaxially oriented polyester film. In the present disclosure, "biaxially oriented" refers to the property of having molecular orientation in two axial directions.
The molecular orientation is measured using a microwave transmission type molecular orientation meter (e.g., MOA-6004, manufactured by Oji Scientific Instruments Co., Ltd.). The angle between the two axial directions is preferably 90°±5°, more preferably 90°±3°, and even more preferably 90°±1°. The present film preferably has molecular orientation in the longitudinal direction and the width direction.

(Striped defect area)
In the present disclosure, a "line defect" refers to a wrinkle that extends in a line shape along the longitudinal direction of the film and appears as an unevenness in the width direction of the film. As described below, line defects occur in the film after production, and are therefore often wrinkles that occur irreversibly. Line defects are not generated during the heat treatment during film production, but originate from wavy wrinkles that occur during the heat treatment of the film after production, and the wavy wrinkles are solidified by cooling after the heat treatment. Furthermore, a "line defect region" refers to a portion in the film plane where a line defect occurs.
The occurrence of a line-shaped defect region (i.e., the occurrence of line-shaped defects partially within the film surface) may cause thickness unevenness in the release layer, reducing the coatability of the release layer, and as a result, affecting the performance of the ceramic capacitor. Furthermore, the line-shaped defect region tends to occur significantly when the film is heated while a tensile load is applied in the longitudinal direction.

The ratio of the total area of line-shaped defect regions that occur when heated at 90°C to the total area of the observation region of the polyester film (hereinafter also referred to as the "area ratio of line-shaped defect regions") is preferably 40% or less, more preferably 30% or less, and even more preferably 20% or less, in order to further improve the coatability of the release layer.
Although there is no particular lower limit for the area ratio of the line defect regions, it is preferable that the area ratio of the line defect regions generated when heated at 90° C. is small, and it is more preferable that there are no line defect regions, i.e., 0%.

The area ratio of the line defect region is measured by the following method.
(1) Using a heating and transporting device, a polyester film is transported at a transport speed of 30 m/min and a tension in the transport direction of 100 N/m, while being heat-treated for 20 seconds under conditions that the surface temperature of the film is 90° C. The heating time in the heat treatment is calculated from the point when the surface temperature of the film reaches the target temperature (90° C.), and the film is heated for 20 consecutive seconds from that point. The method for measuring the surface temperature of the film will be described later.
(2) The heat-treated polyester film is placed on a black flat plate, and the polyester film is visually observed from an oblique angle while changing the viewpoint so that the light of a fluorescent lamp installed on the ceiling in the room (e.g., Lupica Ace (color temperature: 5000K, average color rendering index (Ra): 84) manufactured by Mitsubishi Electric Corporation) is reflected. The region where the reflected image of the fluorescent lamp reflected on the surface of the polyester film is wavy as observed by visual observation is defined as a stripe defect region.
(3) The number of observed line defect areas is counted, and the periphery of each line defect area present in the observation area (area of 1 ^m2 ) of the polyester film observed visually is marked. Next, of the two parallel tangents circumscribing the periphery of each line defect area, the distance between the two parallel tangents selected so that the distance between the tangents is the maximum is measured as the length L of the major axis, and the distance between the two parallel tangents that are perpendicular to the two parallel tangents that give the length L and circumscribing the periphery of the line defect area is measured as the length S of the minor axis. From the obtained lengths L and S, the area of each line defect area is calculated by the following formula. From these values, the ratio of the total area of the line defect areas to the total area of the polyester film is calculated.
Length of the long axis of the streak defect area L × length of the short axis of the streak defect area S × π = area of the streak defect area Since the streak defect area is often elliptical or circular as described above, the area of the streak defect area can be calculated by the calculation method (3) above.

Figure 2 shows an image (photograph) of a polyester film in which a line defect region generated by the heat treatment of (1) above is observed. The region surrounded by a solid line in Figure 2 is the line defect region. In the line defect region shown in Figure 2, an uneven shape extending in the machine direction (MD) is observed. Note that the image (photograph) shown in Figure 2 shows only a portion of the observation region.
Thus, the line defect region is often elliptical or circular. When a line defect region occurs, at least one line defect region having an elliptical shape with its major axis aligned with the conveyance direction often appears.

A biaxially oriented polyester film in which the area ratio of the streak defect regions is within the above range can be produced by adjusting the heat setting temperature in the heat setting process, the cooling rate of the polyester film in the cooling process, and the widthwise expansion rate of the polyester film in the expansion process in the polyester film manufacturing method described below.

(Expansion rate)
The polyester film preferably has a widthwise expansion coefficient at 90°C of -0.15 to 0.15% relative to the film width at 30°C, more preferably -0.10 to 0.10%, even more preferably 0 to 0.10%, and particularly preferably 0 to 0.05%.
By adjusting the expansion coefficient of the polyester film in the width direction at 90° C. to the above range, it is possible to suppress not only the expansion of the film in the width direction during heating but also to reduce unevenness in the expansion coefficient from place to place on the film surface. As a result, it is presumed that the occurrence of streak defect regions due to heating can be suppressed.

The expansion coefficient in the width direction at 90° C. is measured by the following method using a thermomechanical analyzer.
(1) Prepare a sample having a length of at least 20 mm in a direction parallel to the width direction of the biaxially oriented film and 4 mm in a direction perpendicular to the width direction of the biaxially oriented film.
(2) Using a thermomechanical analyzer (for example, TMA-60, manufactured by Shimadzu Corporation), a tensile load of 0.1 g is applied to a sample having a width of 4 mm and a length (chuck distance) of 20 mm.
(3) The sample is heated from a temperature of 20° C. or more but less than 30° C. (preferably 25° C.) to 150° C. at a heating rate of 5° C./min, and the length of the sample at each temperature (° C.) is obtained.
(4) The expansion coefficient in the width direction at 90°C is calculated using the following formula from the length of the sample at 30°C (L30) and the length at 90°C (L90). In this disclosure, the expansion coefficient in the width direction is the arithmetic average value of the expansion coefficients obtained using five samples. Note that a positive expansion coefficient means expansion, and a negative expansion coefficient means contraction.
Formula: Expansion rate (%) = (L90 - L30) / L30 x 100

The expansion rate of the polyester film in the width direction can be adjusted, for example, by appropriately setting the stretching ratio, heat treatment temperature, and film width during cooling during the manufacturing process of the biaxially oriented film.

(Film Density)
The density of the polyester film is preferably from 1.39 to 1.41 g/cm ³ , more preferably from 1.395 to 1.405 g/cm ³ , and even more preferably from 1.398 to 1.400 g/cm ³ , in terms of achieving the effects of the present invention.
The density of the polyester film can be measured using an electronic specific gravity meter (product name "SD-200L", manufactured by Alpha Mirage Co., Ltd.).

(thickness)
The thickness of the polyester film is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 40 μm or less, from the viewpoint of excellent releasability. The lower limit of the thickness is not particularly limited, but is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more, from the viewpoint of excellent handleability.
The thickness of the polyester film is defined as the arithmetic average value of thicknesses measured at five points by a continuous stylus film thickness meter.

The thickness variation of the polyester film is preferably 7% or less, more preferably 5% or less, of the average thickness of the polyester film, in order to provide a more excellent surface smoothness of the first main surface on which the release layer is formed. The lower limit of the thickness variation is not particularly limited, and may be 0% or more of the average thickness of the polyester film.
The thickness variation is measured by the following method. The thickness of the polyester film is measured over 10 m along the longitudinal direction using a continuous stylus film thickness meter. This measurement is performed at five different positions in the width direction. The difference between the maximum and minimum values obtained is divided by the arithmetic mean value of all the measured values to obtain the thickness variation ((maximum thickness - minimum thickness) / average thickness).

(Amount of peeling charge)
In terms of improving the coatability of the release layer, the polyester film has an absolute value of peel charge between the first and second main surfaces of the polyester film equivalent to a circle having a diameter of 1.5 cmφ, measured by the following measurement method under an environment of a temperature of 23° C. and a relative humidity (RH) of 20%, of preferably 0.12 nc (nanocoulomb) or less, more preferably 0.11 nc or less, and even more preferably 0.1 nc or less, where the unit nc (nanocoulomb) is 10 ⁻⁹ coulomb.

The peel charge amount of the polyester film was measured as follows.
The measurement device used is equipped with a stage on which a reference sample of polyester film (with the first main surface facing up) is placed, a head that can hold a measurement sample (with the second main surface facing down) and move up and down vertically to repeatedly press and peel the second main surface of the measurement sample against the first main surface of the reference sample, and an electrometer connected to this head that can measure the amount of charge on the measurement sample.
The polyester film is cut into a circle having a diameter of 1.5 cm to prepare a sample for measuring the peeling charge amount, and cut into a rectangle having a size of 13 cm x 4 cm to prepare a reference sample for measuring the peeling charge amount. Next, the obtained polyester film sample is left in advance under the above-mentioned measurement temperature and humidity environment for 2 hours or more. After that, the reference sample is placed on the table of the measuring device, and the measurement sample is attached to the head. At this time, the first main surface of the reference sample placed on the table is placed on the upper side, and the second main surface of the measurement sample attached to the head is placed on the lower side, so that the first main surface of the reference sample and the second main surface of the measurement sample face each other.
After the measurement sample is neutralized, the head is raised or lowered to repeatedly press and peel the reference film against the measurement sample. The same measurement sample is used to measure the charge of the measurement sample after each of the first to fifth peels, and the average of the measured values is calculated. The measurement sample is changed, and the position where the measurement sample contacts the reference sample is changed for each measurement sample, and measurements are performed on a total of four samples, and the average of all the measurements is taken as the peel charge amount.
For the method of measuring the peeling charge of a polyester film, reference can also be made to the contents of JP-A-2003-194865 (particularly [0053] to [0067]), the contents of which are incorporated herein by reference.

The peel charge amount can be adjusted by selecting the type and amount of components such as the polyester base material and the binder and surfactant contained in the specific coating layer. More specifically, the peel charge amount can be adjusted to the above range by selecting two of the components selected from the group consisting of the polyester base material, the binder, and the surfactant that are closer in the triboelectric series.

〔Production method〕
The method for producing the present film may, for example, include a biaxial stretching step of biaxially stretching an unstretched polyester film having a polyester substrate, and a specific coating layer forming step of forming a specific coating layer containing particles.

The biaxial stretching may be simultaneous biaxial stretching in which longitudinal stretching and transverse stretching are performed simultaneously, or sequential biaxial stretching in which longitudinal stretching and transverse stretching are performed in two or more stages. Examples of sequential biaxial stretching include longitudinal stretching → transverse stretching, longitudinal stretching → transverse stretching → longitudinal stretching, longitudinal stretching → longitudinal stretching → transverse stretching, and transverse stretching → longitudinal stretching, with longitudinal stretching → transverse stretching being preferred.

<Stretching machine>
The apparatus used for biaxial stretching is not particularly limited, and a known stretching machine can be used. An example of the stretching machine will be described below with reference to the drawings.

FIG. 3 is a plan view showing an example of a stretching machine used in the production of a polyester film.
The stretching machine 100 shown in Fig. 3 includes a pair of annular rails 60a and 60b, and gripping members 2a to 2l attached to each annular rail and movable along the rails. The annular rails 60a and 60b are disposed symmetrically with respect to each other with the film 200 in between. The stretching machine 100 grips the film 200 with the gripping members 2a to 2l and moves the gripping members 2a to 2l along the rails, thereby stretching the film 200 in the width direction.

The stretching machine 100 has areas including, in order from the upstream side in the conveying direction, a preheating section 10, a stretching section 20, a heat setting section 30, a heat relaxing section 40, and a cooling section 50.
The above-mentioned zones in the stretching machine 100 are divided by windshield curtains, and the temperature in each zone can be adjusted by hot air or the like.

The preheating section 10 is an area where the film 200 is preheated.

The stretching section 20 is a region where the preheated film 200 is stretched by applying tension in the direction of the arrow TD (width direction), which is perpendicular to the direction of the arrow MD (longitudinal direction). As shown in Figure 3, in the stretching section 20, the film 200 is stretched from width L0 to width L1.

The heat fixing section 30 is an area in which the tensioned film 200 is heated and heat fixed while still under tension.

The heat-relaxing section 40 is a region in which the tension of the heat-set film 200 is thermally relaxed by heating the heat-set film 200 .
As shown in FIG. 3, in the heat-relaxing section 40, the film 200 is reduced (relaxed) from width L1 to width L2.

The cooling section 50 is an area for cooling the thermally relaxed film 200. By cooling the film 200, the shape of the film 200 can be fixed.
FIG. 3 shows that the width of film 200 carried into cooling section 50 is L2, and the width of film 200 carried out from cooling section 50 is L3.

Gripping members 2a, 2b, 2e, 2f, 2i, and 2j are attached to the annular rail 60a and are movable along the annular rail 60a. Gripping members 2c, 2d, 2g, 2h, 2k, and 2l are attached to the annular rail 60b and are movable along the annular rail 60b.
Gripping members 2a, 2b, 2e, 2f, 2i, and 2j grip one end of film 200 in the direction of arrow TD. Gripping members 2c, 2d, 2g, 2h, 2k, and 2l grip the other end of film 200 in the direction of arrow TD. Gripping members 2a to 2l are generally referred to as chucks, clips, or the like.
Gripping members 2a, 2b, 2e, 2f, 2i, and 2j move counterclockwise along circular rail 60a, while gripping members 2c, 2d, 2g, 2h, 2k, and 2l move clockwise along circular rail 60b.

The gripping members 2a to 2d move along the annular rail 60a or 60b while gripping the end of the film 200 in the preheating section 10, and proceed through the stretching section 20, the heat fixing section 30, and the heat relaxing section 40 to the cooling section 50. Next, the gripping members 2a and 2b and the gripping members 2c and 2d release the end of the film 200 at the end downstream of the direction of the arrow MD of the cooling section 50 (for example, the grip release point P and the grip release point Q in FIG. 3) in the order of the conveying direction, and then move further along the annular rail 60a or 60b and return to the preheating section 10. In the above process, the film 200 moves in the direction of the arrow MD, and is preheated in the preheating section 10, stretched in the stretching section 20, heat fixed in the heat fixing section 30, heat relaxed in the heat relaxing section 40, and cooled in the cooling section 50, and is transversely stretched.

The conveying speed of the film 200 can be adjusted by adjusting the moving speed of the gripping members 2a to 2l. In addition, the moving speed of each of the gripping members 2a to 2l can be changed independently.

As described above, the stretching machine 100 enables transverse stretching in which the film 200 is stretched in the direction of the arrow TD in the stretching section 20. On the other hand, the stretching machine 100 can also stretch the film 200 in the direction of the arrow MD by changing the movement speed of the gripping members 2a to 2l. In other words, it is also possible to perform simultaneous biaxial stretching using the stretching machine 100.

In addition to the gripping members 2a to 2l, the stretching machine 100 may further have other gripping members (not shown) to support the film 200.

Next, the present manufacturing method will be specifically described.
The present manufacturing method includes, for example, an extrusion molding step in which a molten resin containing a raw material polyester is extruded into a film to form an unstretched polyester film having a polyester substrate, a longitudinal stretching step in which the unstretched polyester film is stretched in the conveying direction to form a uniaxially oriented polyester film, and a transverse stretching step in which the uniaxially oriented polyester film is stretched in the width direction to form a biaxially oriented polyester film, a heat setting step in which the biaxially oriented polyester film is heated and heat set, a heat relaxation step in which the polyester film heat-set in the heat setting step is heated at a temperature lower than that in the heat setting step to heat relax, a cooling step in which the polyester film heat-relaxed in the heat relaxation step is cooled, an expansion step in which the heat-relaxed polyester film is expanded in the width direction in the cooling step, and a specific coating layer forming step in which a specific coating layer is provided on one side of the polyester substrate by an in-line coating method using a specific coating layer forming composition containing particles.

<Extrusion molding process>
The extrusion molding process is a process in which a molten resin containing a raw material polyester is extruded into a film shape by an extrusion molding method to form an unstretched polyester film. The raw material polyester has the same meaning as the polyester described in the above (Polyester) section.

The extrusion molding method is a method in which a molten raw material resin is extruded using an extruder, for example, to mold the raw material resin into a desired shape.
The polyester-containing molten resin is formed, for example, by heating the polyester to a temperature equal to or higher than the melting point using an extruder equipped with one or more screws, and then melt-kneading the polyester by rotating the screws. The polyester melts in the extruder by heating and kneading with the screws to become a molten material (melt).

The molten material is extruded from the extrusion die through a gear pump and a filter. The extrusion die is also simply called a "die" (see JIS B8650:2006, a, Extrusion Molding Machine, No. 134). For example, the extrusion die described in JP-A-2005-297266, the extrusion die described in JP-A-1-154720, and combinations thereof can also be used. The molten material may be extruded in a single layer or in multiple layers.

In melt extrusion, it is preferable to replace the atmosphere in the extruder with nitrogen in order to suppress thermal decomposition (e.g., hydrolysis of polyester) in the extruder. In addition, a twin-screw extruder is preferable because it can keep the kneading temperature low.

The molten material extruded from the extrusion die is cooled and formed into a film. For example, the molten material can be formed into a film by contacting the molten material with a casting roll and cooling and solidifying the molten material on the casting roll. When cooling the molten material, it is preferable to further blow air (preferably cold air) on the molten material.

The temperature of the casting roll is preferably more than (Tg-10)° C. and not more than (Tg+30)° C., more preferably from (Tg-7) to (Tg+20)° C., and even more preferably from (Tg-5) to (Tg+10)° C. The above "Tg" means the glass transition temperature of the polyester constituting the film.
Here, the temperatures of the polyester film and each member in this production method can be measured using a non-contact thermometer (e.g., a radiation thermometer). The surface temperature of the film is determined by measuring the temperature of the center part in the width direction of the film five times and calculating the average value of the obtained measurement values.

When using a casting roll in the extrusion molding process, it is preferable to increase the adhesion between the casting roll and the molten material. Methods for increasing adhesion include, for example, the electrostatic application method, the air knife method, the air chamber method, the vacuum nozzle method, and the touch roll method.

The molded body (unstretched polyester film) cooled using a casting roll or the like is peeled off from the cooling member such as the casting roll using a peeling member such as a peeling roll.

<Biaxial stretching process>
The biaxial stretching process includes a longitudinal stretching process in which an unstretched polyester film is stretched in the conveying direction (hereinafter also referred to as "longitudinal stretching") to form a uniaxially oriented polyester film, and a transverse stretching process in which the uniaxially oriented polyester film is stretched in the width direction (hereinafter also referred to as "transverse stretching") to form a biaxially oriented polyester film.

(Longitudinal stretching process)
In the longitudinal stretching step, it is preferable to preheat the unstretched polyester film before the longitudinal stretching. By preheating the unstretched polyester film, the polyester film can be easily longitudinally stretched.
The preheating temperature of the unstretched polyester film is preferably (Tg-30) to (Tg+40)° C., and more preferably (Tg-20) to (Tg+30)° C. Specifically, the preheating temperature is preferably 60 to 100° C., and more preferably 65 to 80° C.
As a method for preheating the unstretched polyester film, for example, a method in which a preheating roll having a function of preheating the film is disposed upstream of the stretching rolls for longitudinal stretching, and the unstretched polyester film is preheated while being transported, can be mentioned.

The stretching roll may also have the function of preheating the film. The preferred range of the preheating temperature of the film by the stretching roll is the same as the preferred range of the preheating temperature of the preheating roll described above.

Longitudinal stretching can be performed, for example, by applying tension between two or more pairs of stretching rolls installed in the transport direction while transporting the unstretched polyester film in the longitudinal direction. For example, if a pair of stretching rolls A is installed on the upstream side of the transport direction and a pair of stretching rolls B is installed on the downstream side of the transport direction, the unstretched polyester film is stretched in the longitudinal direction by making the rotation speed of stretching roll B faster than the rotation speed of stretching roll A when transporting the unstretched polyester film.

In the longitudinal stretching process, the film transport speed (circumferential speed) by a pair of stretching rolls A provided upstream in the transport direction and a pair of stretching rolls B provided downstream in the transport direction is not particularly limited as long as the film transport speed by the stretching rolls A is slower than the film transport speed by the stretching rolls B.
The film transport speed by the stretching roll A is, for example, 5 to 60 m/min, preferably 10 to 50 m/min, and more preferably 15 to 45 m/min. The film transport speed by the stretching roll B is, for example, 40 to 160 m/min, preferably 50 to 150 m/min, and more preferably 60 to 140 m/min.

The stretching ratio in the longitudinal stretching process is set appropriately depending on the application, but is preferably 2.0 to 5.0 times, more preferably 2.5 to 4.0 times, and even more preferably 2.8 to 4.0 times.

The stretching speed in the longitudinal stretching step is preferably 800 to 1500%/sec, more preferably 1000 to 1400%/sec, and even more preferably 1200 to 1400%/sec. Here, the "stretching speed" refers to the value obtained by dividing the length Δd in the conveying direction of the polyester film stretched in 1 second in the longitudinal stretching step by the length d0 in the conveying direction of the polyester film before stretching, expressed as a percentage.

In the longitudinal stretching step, it is preferable to heat the unstretched polyester film, since heating facilitates longitudinal stretching.
The heating temperature in the longitudinal stretching step is preferably (Tg-20) to (Tg+50)° C., more preferably (Tg-10) to (Tg+40)° C., and even more preferably (Tg) to (Tg+30)° C. Specifically, the heating temperature in the longitudinal stretching step is preferably 70 to 120° C., more preferably 80 to 110° C., and even more preferably 85 to 100° C.

Methods for heating the unstretched polyester film in the longitudinal stretching process include heating a roll, such as a stretching roll, that comes into contact with the unstretched polyester film. Methods for heating the roll include, for example, providing a heater inside the roll, and providing a pipe inside the roll and running a heated fluid through the pipe. In addition to the above, for example, there is a method of blowing hot air onto the unstretched polyester film, and a method of heating the unstretched polyester film by bringing the unstretched polyester film into contact with a heat source such as a heater or passing it near a heat source.

The longitudinal stretching step of longitudinally stretching the unstretched polyester film is not limited to the above-mentioned method.
In the above-mentioned longitudinal stretching process, the unstretched polyester film is longitudinally stretched by utilizing the difference in the transport speed between two pairs of stretching rolls. However, the unstretched polyester film may also be longitudinally stretched using one or more high-speed stretching rolls that are arranged between the two stretching rolls and transport the film at a faster transport speed than the stretching rolls to produce a uniaxially oriented polyester film.
In addition, in the above-mentioned longitudinal stretching process, the film is sandwiched between two opposing rolls (a pair of rolls) and transported, but the stretching roll used in the longitudinal stretching process may not have opposing rolls and may be composed of only one roll that contacts one side of the polyester film.

(Transverse stretching process)
The transverse stretching step is a step of transversely stretching the uniaxially oriented polyester film. The transverse stretching step is carried out, for example, in the transverse stretching section 20 of the stretching machine 100.

In the transverse stretching step, it is preferable to preheat the polyester film before the transverse stretching. By preheating the polyester film, the polyester film can be easily transversely stretched.
The preheating temperature is preferably from (Tg-10) to (Tg+60)° C., and more preferably from (Tg) to (Tg+50)° C. Specifically, the preheating temperature is preferably from 80 to 120° C., and more preferably from 90 to 110° C.

The stretching ratio in the width direction of the uniaxially oriented polyester film in the transverse stretching step (transverse stretching ratio) is not particularly limited, but is preferably larger than the stretching ratio in the longitudinal stretching step. The stretching ratio in the transverse stretching step is preferably 3.0 to 6.0 times, more preferably 3.5 to 5.0 times, and even more preferably 3.5 to 4.5 times.
When the transverse stretching process is carried out in the transverse stretching section 20 of the stretching machine 100, the transverse stretching ratio is calculated from the ratio (L1/L0) of the film width L1 at the time of discharge from the transverse stretching section 20 to the film width L0 at the time of entry into the transverse stretching section 20.

The area ratio, which is expressed as the product of the stretch ratio in the longitudinal stretching process and the stretch ratio in the transverse stretching process, is preferably 12.8 to 15.5 times, more preferably 13.5 to 15.2 times, and even more preferably 14.0 to 15.0 times. When the area ratio is equal to or greater than the above lower limit, the molecular orientation in the width direction of the film is good. When the area ratio is equal to or less than the above upper limit, the molecular orientation is less likely to be relaxed when subjected to heat treatment.

The heating temperature in the transverse stretching process is preferably (Tg-10) to (Tg+80)°C, more preferably (Tg) to (Tg+70)°C, and even more preferably (Tg) to (Tg+60)°C. Specifically, the heating temperature in the transverse stretching process is preferably 100 to 140°C, more preferably 110 to 135°C, and even more preferably 115 to 130°C.

The stretching speed in the transverse stretching process is preferably 8 to 45%/sec, more preferably 10 to 30%/sec, and even more preferably 15 to 20%/sec.

<Heat setting process>
In the present production method, it is preferable to carry out a heat setting step and a heat relaxing step as the heat treatment of the polyester film transversely stretched in the transverse stretching step.
In the heat setting step, the biaxially oriented polyester film obtained in the transverse stretching step can be heat set by heating. By crystallizing the polyester through heat setting, it is possible to suppress shrinkage of the polyester film.
The heat setting step is carried out, for example, in the heat setting section 30 of the stretching machine 100 .

The surface temperature (heat setting temperature) of the polyester film in the heat setting step is not particularly limited, but is preferably less than 240° C., more preferably 235° C. or less, and even more preferably 230° C. or less. The lower limit is not particularly limited, but is preferably 190° C. or more, more preferably 200° C. or more, and even more preferably 210° C. or more.
In the heat setting step, the heat treatment is carried out while controlling the maximum temperature reached on the surface of the polyester film to the above heat setting temperature.

In the heat setting process, the variation in surface temperature in the width direction of the film is preferably 0.5 to 10.0°C, more preferably 0.5 to 7.0°C, even more preferably 0.5 to 5.0°C, and particularly preferably 0.5 to 4.0°C. By controlling the variation in surface temperature in the width direction of the film within the above range, the variation in crystallinity in the width direction can be suppressed.

Examples of heating methods include applying hot air to the film and radiant heating of the film. Examples of devices used in radiant heating methods include infrared heaters.

The heating time in the heat setting process is preferably 5 to 50 seconds, more preferably 5 to 30 seconds, and even more preferably 5 to 10 seconds.

<Heat relaxation step>
In the heat-relaxing step, the polyester film heat-set in the heat-setting step is preferably heat-relaxed by heating at a temperature lower than that in the heat-setting step, which can relax the residual strain of the polyester film.
The heat-relaxing step is carried out, for example, in the heat-relaxing section 40 of the stretching machine 100 .

The surface temperature of the polyester film in the heat-relaxing step (heat-relaxing temperature) is preferably at least 5° C. lower than the heat setting temperature, more preferably at least 15° C. lower, even more preferably at least 25° C. lower, and particularly preferably at least 30° C. lower. That is, the heat-relaxing temperature is preferably 235° C. or lower, more preferably 225° C. or lower, even more preferably 210° C. or lower, and particularly preferably 200° C. or lower.
The lower limit of the heat relaxation temperature is preferably 100° C. or higher, more preferably 110° C. or higher, and even more preferably 120° C. or higher.
In the heat relaxation step, the heat treatment is carried out while controlling the maximum temperature of the surface of the polyester film to be the heat relaxation temperature.

Examples of heating methods include applying hot air to the film and radiant heating of the film. Examples of devices used in radiant heating methods include infrared heaters.

<Cooling process>
The present production method preferably includes a cooling step of cooling the heat-relaxed polyester film. The cooling step and the stretching step described below are carried out, for example, in the cooling section 50 of the stretching machine 100.

Methods for cooling the polyester film in the cooling process include, for example, blowing air (preferably cold air) onto the film and contacting the film with a temperature-controllable member (e.g., a temperature-control roll).

The cooling rate of the polyester film in the cooling process is not particularly limited, but is preferably more than 2000°C/min and less than 4000°C/min, more preferably 2000 to 3500°C/min, even more preferably more than 2200°C/min and less than 3000°C/min, and particularly preferably 2300 to 2800°C/min, in that it reduces unevenness in the thickness of the release layer laminated to the biaxially oriented film and provides better coatability of the release layer.

The cooling rate of the polyester film in the cooling step can be measured using a non-contact thermometer. For example, when the cooling step is performed in the cooling section 50 of the stretching machine 100, the surface temperature of the film 200 fed from the heat relaxation section 40 to the cooling section 50 and the surface temperature of the film 200 fed from the cooling section 50 are measured to obtain the temperature difference ΔT (°C) between them. The obtained temperature difference ΔT (°C) is divided by the residence time ta of the film 200 in the cooling section 50 to obtain the cooling rate.
The cooling rate of the polyester film can be adjusted by the operating conditions of the cooling device and the transport speed of the film.

In the present manufacturing method, it is preferable to carry out the above-mentioned heat setting step, heat relaxation step, and cooling step consecutively in this order. This is because it is possible to reduce the load (thermal history) caused by repeated heating and cooling on the polyester film, reduce the distortion and other defects inherent in the film, and suppress the occurrence of streak defects.

<Expansion process>
It is also preferable that the cooling step includes a step of expanding the heat-relaxed polyester film in the width direction.
"Expanding the polyester film in the width direction" in the cooling process means applying tension in the width direction to the polyester film during the cooling process so that the film width (L3 in Figure 3) at the end of the cooling process is wider than the film width (L2 in Figure 3) at the start of the cooling process.

In the stretching step, the method of stretching the polyester film in the width direction is not particularly limited. For example, when a biaxially oriented polyester film is produced using the stretching machine 100, the distance between the annular rails 60a and 60b at the end point (gripping release point P and gripping release point Q) of the cooling section 50 is made wider than the distance between the annular rails 60a and 60b at the start point of the cooling section 50, so that the film 200 gripped by each gripping member can be stretched in the width direction.
The expanding step may be carried out continuously or intermittently from the start to the end of the cooling step, or may be carried out only for a certain period during the cooling step, so long as the film width is expanded before and after the cooling step.

The expansion rate in the width direction of the polyester film in the expansion step, i.e., the ratio of the film width at the end of the cooling step to the film width before the start of the cooling step, is preferably 0% or more, more preferably 0.001% or more, and even more preferably 0.01% or more.
The upper limit of the expansion rate is not particularly limited, but is preferably 1.3% or less, more preferably 1.2% or less, and even more preferably 1.0% or less. By setting the expansion rate of the film width to the above upper limit or less, even when a strong tension is applied in the conveying direction to convey the film at high speed during film production (for example, when the tension in the conveying direction is 100 N/m or more), it is possible to suppress the disturbance of the cut surface in the trimming process described below, and further the breakage of the film due to the disturbance of the cut.

<Specific Coating Layer Forming Step>
The present production method preferably includes a specific coating layer forming step of in-line coating with a particle-containing composition for forming a specific coating layer (hereinafter also referred to as "composition A"). The specific coating layer formed on one surface of the polyester substrate by the specific coating layer forming step is the same as the layer described in detail in the above section <Specific coating layer>.
The specific coating layer may be formed at any stage of the present production method. For example, a coating film may be formed on one surface of an unstretched or stretched polyester substrate, and then dried as necessary.

First, the method for forming the specific coating layer using composition A will be described.
Composition A can be prepared by mixing the particles contained in the specific coating layer, a binder and additives that are added as necessary, and a solvent.
Examples of the solvent include water, ethanol, toluene, ethylene glycol monoethyl ether, ethylene glycol dimethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether. Among these, water is preferred from the viewpoints of the environment, safety, and economy.

Composition A may contain one kind of solvent alone, or may contain two or more kinds of solvents.
The content of the solvent is preferably from 80 to 99 mass %, more preferably from 90 to 98 mass %, based on the total mass of composition A.
That is, in composition A, the total content of components (solid contents) other than the solvent is preferably 0.5 to 20 mass % relative to the total mass of composition A, and more preferably 1 to 10 mass %.

The particles, binder and additives contained in composition A, including preferred embodiments thereof, are as described in detail above in the section <Specific Coating Layer>.
With respect to each component other than the solvent in composition A, it is preferable to adjust the content of each component in the coating liquid so that the content of each component relative to the total mass of the solid content of composition A is the same as the preferred content of each component relative to the total mass of the specific coating layer described above.

There are no particular limitations on the method of applying composition A, and any known method can be used. Examples of application methods include spray coating, slit coating, roll coating, blade coating, spin coating, bar coating, and dip coating.

In the specific coating layer forming step, an in-line coating method is applied in which a coating liquid is applied to one surface of a polyester substrate while the polyester substrate is being transported. By applying the in-line coating method, the heating time of the polyester substrate in the manufacturing process is shortened, and since the substrate is not subjected to a heat history, the streak defect area can be reduced, and as a result, the coating property of the release layer can be further improved.
In the in-line coating method, the polyester substrate to which the composition A is applied may be an unstretched polyester substrate or a uniaxially oriented polyester substrate, but is preferably a uniaxially oriented polyester substrate. That is, it is preferable to perform the specific coating layer formation step by the in-line coating method between the longitudinal stretching step and the transverse stretching step. This is because the adhesion between the polyester substrate and the specific coating layer can be improved by transversely stretching the uniaxially oriented polyester substrate and the specific coating layer at the same time.

The present production method may include a winding step of winding the biaxially oriented polyester film obtained through the above steps to obtain a roll of biaxially oriented polyester film.
The present production method may further include a trimming step of continuously cutting the polyester film along the transport direction to cut off at least one end portion in the width direction of the polyester film before carrying out the winding step.

The conveying speed of the polyester film in each step other than the longitudinal stretching step in this manufacturing method is not particularly limited, but when the transverse stretching step, heat setting step, heat relaxation step, cooling step and expansion step are performed using the above-mentioned stretching machine 100, from the viewpoints of productivity and quality, it is preferably 50 to 200 m/min, and more preferably 80 to 150 m/min. The conveying speed of the polyester film in the longitudinal stretching step is as described above.

In each process other than the longitudinal stretching process, the tension in the transport direction applied to the polyester film is not particularly limited. However, when the transverse stretching process, heat setting process, heat relaxation process, cooling process and expansion process are performed using the above-mentioned stretching machine 100, the tension in the transport direction applied to the polyester film can be adjusted by the stretching conditions.
The tension in the conveying direction applied to the polyester film after the cooling step and before being wound up in the winding step is preferably 3 to 30 N/m, more preferably 5 to 20 N/m.

In the method for producing the present film specifically described above, a combination of two or more preferred embodiments is a more preferred embodiment.
In particular, a manufacturing method having a combination of two or more selected from the group consisting of the manufacturing conditions below can be said to be a preferred embodiment in that it serves as an indicator for adjusting the area ratio of the streak defect regions and the expansion coefficient in the width direction of the produced biaxially oriented film.
The stretching ratio in the transverse stretching step is 3.0 to 6.0 times, preferably 3.5 to 5.0 times, and more preferably 3.5 to 4.5 times.
The heat setting temperature in the heat setting step is less than 240°C, preferably 190°C or more and less than 240°C, more preferably 200 to 230°C, and even more preferably 210 to 230°C.
The heat-relaxing temperature in the heat-relaxing step is lower than the heat setting temperature, preferably by 5° C. or more, more preferably by 15° C. or more, even more preferably by 25° C. or more, and particularly preferably by 30° C. or more.
The cooling rate of the polyester film in the cooling step is more than 2000° C./min and less than 4000° C./min, preferably 2000 to 3500° C./min, more preferably more than 2200° C./min and less than 3000° C./min, and even more preferably 2300 to 2800° C./min.
The expansion rate of the polyester film in the width direction in the expansion step is 0 to 1.3%, preferably 0.001 to 1.2%, and more preferably 0.01 to 1.0%.

Note that the area ratio of the line defect regions and the expansion rate in the width direction of a biaxially oriented film vary depending on the materials constituting the polyester substrate and the specific coating layer, and on the other manufacturing conditions, etc., and therefore a manufacturing method having a combination of two or more selected from the group consisting of the manufacturing conditions listed above does not necessarily produce a polyester film having the desired area ratio of the line defect regions and/or expansion rate in the width direction. Furthermore, the manufacturing method for a polyester film having the desired area ratio of the line defect regions and/or expansion rate in the width direction is not limited to a method having two or more of the manufacturing conditions listed above.

[Release film]
The polyester film can be used to produce a release film. More specifically, a release film having a polyester film and a release layer disposed on the first main surface of the polyester film can be produced by providing a release layer on the first main surface of the polyester film.

The release layer contains at least a resin as a release agent. The resin contained in the release layer is not particularly limited, and examples thereof include silicone resin, fluororesin, alkyd resin, acrylic resin, various waxes, and aliphatic olefins, with silicone resin being preferred.

The silicone resin means a resin having a silicone structure in the molecule. Examples of the silicone resin include curable silicone resin, silicone graft resin, and modified silicone resin such as alkyl-modified silicone resin, and reactive curable silicone resin is preferred.
Examples of reactive curable silicone resins include addition reaction silicone resins, condensation reaction silicone resins, and ultraviolet or electron beam curable silicone resins. Among them, addition reaction silicone resins having low temperature curing properties and ultraviolet or electron beam curable silicone resins are preferred because the release layer can be formed at low temperature.

An example of an addition reaction silicone resin is a resin obtained by reacting and curing polydimethylsiloxane having vinyl groups introduced at the terminals or side chains with hydrogen siloxane using a platinum catalyst.
An example of a condensation reaction silicone resin is a resin having a three-dimensional crosslinked structure formed by condensing polydimethylsiloxane having an OH group at its terminal with polydimethylsiloxane having an H group at its terminal using an organotin catalyst.
Examples of UV-curable silicone resins include those that utilize the same radical reaction as silicone rubber crosslinking, those that introduce unsaturated groups to cause photocuring, those that use UV or electron beams to decompose onium salts to generate strong acids and cleave epoxy groups to cause crosslinking, and those that crosslink by addition reaction of thiols to vinyl siloxanes, etc. More specifically, examples include acrylate-modified polydimethylsiloxanes and glycidoxy-modified polydimethylsiloxanes.

The release layer may contain additives other than the above resins, such as a light release additive and a heavy release additive for adjusting the release force, an adhesion improver, and an antistatic agent.
The release layer may contain one type of resin alone or two or more types of resins.
The content of the resin in the release layer is preferably 50 to 99% by mass, more preferably 60 to 98% by mass, based on the total mass of the release layer. The remainder other than the resin in the release layer may be the above-mentioned additives and/or residues of the solvent, catalyst, and the like contained in the release layer-forming coating liquid used to form the release layer.

The thickness of the release layer may be set according to the intended use and is not particularly limited, but a thickness of 0.005 to 2.0 μm is preferred, and 0.05 to 1.0 μm is more preferred, in that it provides a good balance between release performance and smoothness of the release layer surface.

(Surface free energy of peeled surface)
From the viewpoint of preventing static electricity when the release film is wound up, the surface free energy of the surface of the release layer opposite the polyester substrate (also referred to as the release surface) is preferably 30 mJ/ ^m2 or less, more preferably 1 to 30 mJ/ ^m2 , and even more preferably 10 to 30 mJ/ ^m2 .
In addition, the larger the difference between the surface free energy of the release layer and the surface free energy of the second main surface of the polyester film, the less likely the film is to be charged, and therefore it is preferable. The difference between the surface free energy of the release layer and the surface free energy of the second main surface of the polyester film is preferably 1 to 50 mJ/ ^m2 , more preferably 1 to 40 mJ/ ^m2 , even more preferably 1 to 35 mJ/ ^m2 , particularly preferably 5 to 30 mJ/ ^m2 , and most preferably 10 to 25 mJ/ ^m2 .
The surface free energy of the release surface of the release layer can be adjusted by the type of resin forming the release layer and additives.

(Maximum protrusion height Sp, average surface roughness Sa of peeled surface)
In order to smooth the functional layer, such as a ceramic green sheet, formed on the release layer, it is preferable that the release surface is as smooth as possible. Specifically, the maximum protrusion height Sp of the release surface is preferably 1 to 60 nm, and more preferably 1 to 40 nm. The surface average roughness Sa of the release surface is preferably 0 to 10 nm, and more preferably 0 to 5 nm.
The maximum projection height Sp and average surface roughness Sa of the release surface can be adjusted by not including particles in the release layer when providing the release layer and by selecting the resin and additives that form the release layer.
The maximum projection height Sp and surface average roughness Sa of the peeled surface can be measured in accordance with the above-mentioned method for measuring the maximum projection height Sp and surface average roughness Sa of the second main surface.

The method for providing a release layer on the first main surface of the film is not particularly limited, but examples include a method in which a release layer-forming coating liquid, which is prepared by dissolving or dispersing a release agent in a solvent, is applied to the first main surface of the film, the solvent is removed by drying, and, if necessary, the film is heated or irradiated with light to form a cured product.

The coating method of the release layer forming coating liquid is not particularly limited, and any known method can be used, such as spray coating, slit coating, roll coating, blade coating, spin coating, bar coating, and dip coating.
The heating temperature in forming the release layer is preferably 180° C. or less, more preferably 150° C. or less, and even more preferably 120° C. or less. There is no particular lower limit, and the heating temperature may be 60° C. or more.

The coating liquid for forming the release layer contains the above-mentioned resin and solvent, and may contain the above-mentioned additives and/or the above-mentioned catalyst used for curing the resin, if necessary. The coating liquid for forming the release layer can be prepared by mixing these components.
Examples of the solvent include water and organic solvents such as toluene, methyl ethyl ketone, ethanol, ethylene glycol monoethyl ether, ethylene glycol dimethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether, with the organic solvents being preferred.

The coating liquid for forming the release layer may contain one type of solvent alone, or may contain two or more types of solvents.
The content of the solvent is preferably from 80 to 99.5% by mass, more preferably from 90 to 99% by mass, based on the total mass of the coating liquid for forming the release layer.
That is, in the coating liquid for forming a release layer, the total content of components (solid content) other than the solvent is preferably 0.5 to 20% by mass, more preferably 1 to 10% by mass, based on the total mass of the coating liquid for forming a release layer.

In addition, in order to improve adhesion between the film and the release layer, the first main surface of the film may be subjected to pretreatment such as anchor coating, corona treatment, and plasma treatment before the release layer is provided.

<Applications>
The release film having the present film has excellent transportability, can suppress the formation of transfer marks during storage in a roll, and can suppress uneven thickness of the release layer and/or coating defects, so that it is preferably used as a release film (carrier film) for producing a ceramic green sheet. The ceramic green sheet produced using the release film can be suitably used for producing a ceramic capacitor, which is required to have a multi-layered internal electrode due to miniaturization and large capacity.

The method for producing a ceramic green sheet using the release film is not particularly limited and can be carried out by a known method. For example, the method for producing a ceramic green sheet includes a method of applying a prepared ceramic slurry to the release layer surface of the release film and drying and removing the solvent contained in the ceramic slurry.
The method of applying the ceramic slurry is not particularly limited, and for example, a known method can be applied, such as a method in which a ceramic slurry in which a ceramic powder and a binder are dispersed in a solvent is applied by a reverse roll method, and the solvent is removed by heating and drying. The binder is not particularly limited, and examples thereof include polyvinyl butyral. The solvent is also not particularly limited, and examples thereof include ethanol and toluene.

Release films comprising this film can be used as protective films for dry film resists, films for sheet molding such as decorative layers and resin sheets, release films for semiconductor manufacturing processes, release films for polarizing plate manufacturing processes, and separators for adhesive films for labels, medical and office supplies, etc.

The present disclosure will be explained in more detail below with reference to examples. The materials, amounts used, ratios, processing contents, and processing procedures shown in the following examples can be changed as appropriate without departing from the spirit of this disclosure. Therefore, the scope of the present disclosure is not limited to the specific examples shown below. Note that, unless otherwise specified, "parts" and "%" are based on mass.

Hereinafter, in the present examples, the mere notation "film" includes both a polyester substrate alone and an embodiment having a polyester substrate and a specific coating layer, and also includes all of unstretched films, uniaxially oriented films, and biaxially oriented films.
In each step of this example, the temperature at the center of the width direction of the film was measured five times using a non-contact thermometer (AD-5616 (product name), manufactured by A&D Corporation, emissivity 0.95), and the arithmetic mean value of the obtained measurement values was regarded as the measured value of the film surface temperature.

Example 1
<Extrusion molding process>
Pellets of polyethylene terephthalate were produced using a titanium compound (citric acid chelate titanium complex, VERTEC AC-420, Johnson Matthey) described in Japanese Patent No. 5575671 as a polymerization catalyst. The obtained pellets were dried until the moisture content was 50 ppm or less, and then charged into a hopper of a single-screw kneading extruder having a diameter of 30 mm, and then melted and extruded at 280°C. The melt was passed through a filter (hole diameter 3 μm) and then extruded from a die onto a cooling drum at 25°C to obtain an unstretched film made of polyethylene terephthalate. The extruded melt was brought into close contact with the cooling drum by an electrostatic application method.
The polyethylene terephthalate constituting the unstretched film had a melting point (Tm) of 258°C and a glass transition temperature (Tg) of 80°C.

<Longitudinal Stretching Process>
The unstretched film was subjected to a longitudinal stretching process by the following method.
The preheated unstretched film was passed between two pairs of rolls having different peripheral speeds under the conditions described below to be stretched in the machine direction (transport direction), thereby producing a uniaxially oriented film.
(Longitudinal stretching conditions)
Preheat temperature: 75°C
Stretching temperature: 90°C
Stretching ratio: 3.4 times Stretching speed: 1300%/sec

<Specific Coating Layer Forming Step>
The following composition A (composition for forming a specific coating layer) was applied to one side of a longitudinally stretched uniaxially oriented film (polyester substrate) using a bar coater, and the formed coating film was dried with hot air at 100° C. to form a specific coating layer. At this time, the coating amount of composition A was adjusted so that the formed specific coating layer had a thickness of 60 nm.

(Composition A)
Composition A was prepared by mixing the components shown below. The prepared composition A was filtered using a filter with a pore size of 6 μm (F20, manufactured by Mahle Filter Systems Co., Ltd.) and subjected to membrane degassing (2x6 Radial Flow Superphobic, manufactured by Polypore Co., Ltd.), and the obtained composition A was then applied to the surface of a uniaxially oriented film.
Acid-modified polyolefin (Zaixen (registered trademark) NC, manufactured by Sumitomo Seika Chemicals Co., Ltd., aqueous dispersion prepared by adding water to a solid content of 25% by mass): 157 parts Anionic hydrocarbon surfactant (Lapisol (registered trademark) A-90, di-2-ethylhexyl sodium sulfosuccinate, manufactured by NOF Corporation, aqueous dilution with a solid content of 1% by mass): 56 parts Particles (Snowtex (registered trademark) ZL, manufactured by Nissan Chemical Industries, Ltd., colloidal silica, aqueous dispersion with a solid content of 40% by mass): 11 parts Water: 776 parts

<Transverse stretching process>
The film that had been subjected to the longitudinal stretching step and the specific coating layer forming step was stretched in the width direction using a tenter under the following conditions to prepare a biaxially oriented film.
(Transverse stretching conditions)
Preheat temperature: 100°C
Stretching temperature: 120°C
Stretching ratio: 4.2 times Stretching speed: 50%/sec

<Heat setting process>
The biaxially oriented film that had been subjected to the transverse stretching step was subjected to a heat setting step in which the film was heat set by heating it using a tenter under the following conditions.
(Heat Fixation Conditions)
Heat setting temperature: 227°C
Heat fixing time: 6 seconds

<Heat relaxation step>
Next, the heat-set film was subjected to a heat-relaxing step in which tension in the film was relaxed by heating under the following conditions. In the heat-relaxing step, the distance between the gripping members of the tenter gripping both ends of the film (tenter width) was narrowed to reduce the film width compared to that at the end of the heat-setting step. The heat-relaxing rate Lr below was calculated from the film width L2 at the end of the heat-relaxing step relative to the film width L1 at the start of the heat-relaxing step, using the formula Lr = (L1 - L2) / L1 x 100.
(Thermal relaxation conditions)
Heat relaxation temperature: 190°C
Thermal relaxation rate Lr: 4%

<Cooling process and expansion process>
The heat-relaxed film was subjected to a cooling step in which the film was cooled under the following conditions. In addition, in the cooling step, an expansion step was carried out in which the tenter width was expanded to expand the film width compared to the width at the end of the heat-relaxation step.
The cooling rate described below was calculated by dividing the temperature difference ΔT (°C) between the film surface temperature measured when the film was brought into the cooling section 50 and the film surface temperature measured when the film was taken out of the cooling section 50 by the cooling time ta, where ta is the residence time of the film from when it was brought into the cooling section 50 of the stretching machine 100 until it was taken out.
The expansion rate ΔL was calculated from the film width L3 at the end of the cooling step relative to the film width L2 at the start of the cooling step by the formula ΔL=(L3-L2)/L2×100.
(Cooling conditions)
Cooling rate: 2500°C/min (expansion condition)
Expansion rate ΔL: 0.6%

<Winding process>
The film cooled in the cooling step was cut continuously along the conveying direction at positions 20 cm from both ends in the width direction of the film using a trimming device, and both ends of the film were trimmed. Next, extrusion processing (knurling) was performed on the region up to 10 mm in the width direction from both ends of the film, and the film was then wound up under a tension of 40 kg/m.
A biaxially oriented film was produced by the above method. The thickness of the obtained biaxially oriented film was 31 μm, the width was 1.5 m, and the roll length was 7000 m. In addition, when the thickness of the specific coating layer of the obtained biaxially oriented film was measured using an SEM, the thickness of the specific coating layer was 60 nm. Specifically, the biaxially oriented film was cut using a microtome to prepare a sample having a cross section along the thickness direction of the biaxially oriented film, and the obtained sample was subjected to an etching treatment with Ar ions and a vapor deposition treatment with Pt, and then the cross section of the sample was observed using an SEM ("S-4800" manufactured by Hitachi High-Technologies Corporation). From the obtained observation image, the thickness of five points of the specific coating layer was measured, and the measured values were arithmetically averaged to determine the thickness of the specific coating layer.

Example 2
A biaxially oriented film was produced according to the method described in Example 1, except that in the preparation of composition A, "Snowtex MP2040" (Nissan Chemical Industries, Ltd., colloidal silica, 40% by mass solids aqueous dispersion) was used instead of "Snowtex ZL" as the particles, and the thickness of the specific coating layer was adjusted to 160 nm.

Example 3
A biaxially oriented film was prepared according to the method described in Example 1, except that the thickness of the specific coating layer was adjusted to 50 nm.

Example 4
A biaxially oriented film was produced according to the method described in Example 1, except that in the preparation of composition A, the amount of particles ("Snowtex ZL") added was changed from 11 parts to 22 parts.

Example 5
A biaxially oriented film was produced according to the method described in Example 1, except that in the preparation of composition A, the amount of particles ("Snowtex ZL") added was changed from 11 parts to 1.1 parts.

Example 6
A biaxially oriented film was produced according to the method described in Example 1, except that in the preparation of composition A, "Snowtex XL" (Nissan Chemical Industries, Ltd., colloidal silica, aqueous dispersion with solid content of 40% by mass) was used instead of "Snowtex ZL" as the particles, the amount of particles added was changed from 11 parts to 3 parts, and the thickness of the specific coating layer was adjusted to 40 nm.

Example 7
A biaxially oriented film was prepared in the same manner as in Example 1, except that 40 parts of the following additive (crosslinking agent) was further added to composition A.
Crosslinking agent (a dispersion of an isocyanate compound, "Duranate (registered trademark) WM44L70" manufactured by Asahi Kasei Chemicals Corporation (solid content 70% by mass) adjusted by adding water to the dispersion to a solid content concentration of 10% by mass)

Example 8
A biaxially oriented film was produced in the same manner as in Example 1, except that in the preparation of composition A, the amount of the anionic hydrocarbon surfactant added was changed from 56 parts to 112 parts.

Example 9
A biaxially oriented film was produced in the same manner as in Example 1, except that in preparation of composition A, a silicone-based surfactant (BYK-346, solid content 52% by mass, manufactured by BYK Corporation) was diluted with water to a solid content of 1% by mass instead of the anionic hydrocarbon-based surfactant, and 56 parts of the resulting diluted solution was used.

Example 10
A biaxially oriented film was produced in the same manner as in Example 1, except that in preparation of composition A, a fluorosurfactant (Surflon S-211, solid content 50 mass%, manufactured by AGC Seimi Chemical Co., Ltd.) was diluted with water to a solid content of 1 mass% instead of the anionic hydrocarbon surfactant, and 56 parts of the resulting diluted solution was used.

[Examples 11 to 13 and 23]
In the preparation of composition A, instead of "Zaixen NC", "Zaixen L" (manufactured by Sumitomo Seika Chemicals Co., Ltd., acid-modified polyolefin, solid content 25% by mass aqueous dispersion) (Example 11), "Zaixen A" (manufactured by Sumitomo Seika Chemicals Co., Ltd., acid-modified polyolefin, solid content 25% by mass aqueous dispersion) (Example 12), and "Chemipearl S120" (manufactured by Mitsui Chemicals, Inc., acid-modified polyolefin, solid content 25% by mass aqueous dispersion) (Example 13), and acrylic resin (aqueous dispersion of a copolymer obtained by polymerizing methyl methacrylate, styrene, 2-ethylhexyl acrylate, 2-hydroxyethyl methacrylate and acrylic acid in a mass ratio of 59:8:26:5:2, solid content concentration 25% by mass, acid value 16 mgKOH/g) (Example 23) were used as the resin. A biaxially oriented film was produced according to the method described in Example 1.

[Examples 14 to 22]
A biaxially oriented film was produced according to the method described in Example 1, except that the heat setting temperature in the heat setting process, the cooling rate in the cooling process, and the expansion rate in the expansion process were controlled to the values shown in Table 1 below.

Example 24
A biaxially oriented film was produced according to the method described in Example 1, except that in the preparation of composition A, "Takelac W605" (manufactured by Mitsui Chemicals, Inc., polyurethane, 30% by weight solids aqueous dispersion) was used instead of "Zaixen NC" as the resin, in an amount such that the binder content relative to the total weight of composition A was the same.

Example 25
A biaxially oriented film was produced according to the method described in Example 1, except that in the preparation of composition A, "HYDRAN AP-40N" (manufactured by DIC Corporation, polyurethane, 35% by weight solids aqueous dispersion) was used instead of "ZAIKXEN NC" as the resin, in an amount such that the binder content relative to the total weight of composition A was the same.

Example 26
A biaxially oriented film was produced according to the method described in Example 25, except that in the preparation of composition A, 5 parts of a fluorine-based surfactant (Ftergent 215M, manufactured by Neos Co., Ltd., 1% by weight solids diluted with water) was further added.

Example 27
A biaxially oriented film was produced according to the method described in Example 25, except that in the preparation of composition A, 20 parts of a diluted solution (solid content 1% by mass) of a fluorine-based surfactant (Surflon S-211, solid content 50% by mass, manufactured by AGC Seimi Chemical Co., Ltd.) with water was added.

Comparative Example 1
In preparing composition A, a biaxially oriented film was produced according to the method described in Example 1, except that "Snowtex MP2040" was used instead of "Snowtex ZL" as the particles, the thickness of the specific coating layer was adjusted to 100 nm, and the heat setting temperature in the heat setting process, the cooling rate in the cooling process, and the expansion rate in the expansion process were controlled to the values shown in Table 1 below.

Comparative Example 2
In preparing composition A, a biaxially oriented film was produced according to the method described in Example 1, except that in place of "Zaixen NC" as the resin, an aqueous dispersion of acrylic resin (a neutralized aqueous dispersion of a copolymer obtained by polymerizing methyl methacrylate, 2-hydroxyethyl methacrylate, and methacrylic acid in a mass ratio of 28:48:24) (solid content 25 mass%, acid value 157 mgKOH/g) was used, and the heat setting temperature in the heat setting step and the cooling rate in the cooling step were controlled to the values shown in Table 1 below.

Comparative Example 3
In preparing composition A, "Snowtex MP2040" was used instead of "Snowtex ZL" as the particles, and "Simac US-480" (manufactured by Toagosei Co., Ltd., silicone modified acrylic resin, solid content 25% by mass) was used instead of "Zaixen NC" as the resin, the thickness of the specific coating layer was adjusted to 100 nm, and the heat setting temperature in the heat setting step, the cooling rate in the cooling step, and the expansion rate in the expansion step were controlled to the values shown in Table 1 below. A biaxially oriented film was produced according to the method described in Example 1.

[Physical property measurements]
The following physical properties were measured for each of the biaxially oriented films of Examples 1 to 27 and Comparative Examples 1 to 3. The measurement results are shown in Table 1.

<Maximum projection height Sp, average surface roughness Sa>
The maximum projection height Sp and average surface roughness Sa of the second main surface of the specific coating layer of the biaxially oriented film were measured by the following method.
The surface of the specific coating layer side of the produced biaxially oriented film was measured under the following conditions using an optical interferometer (Vertscan 3300G Lite, manufactured by Hitachi High-Tech Corporation), and then analyzed using the built-in data analysis software (VS-Measure5) to determine the maximum protrusion height Sp and average surface roughness Sa of the second main surface of the specific coating layer.
In measuring the maximum protrusion height Sp, the maximum value of the measured values obtained from five measurements taken at different measurement positions was used, and in measuring the average surface roughness Sa, the average value of the measured values obtained from five measurements taken at different measurement positions was used.
(Measurement condition)
Measurement mode: WAVE mode Objective lens: 50x Measurement area: 186μm x 155μm
The maximum projection height Sp and the average surface roughness Sa of the first main surface of the biaxially oriented film were measured in the same manner. In all Examples, Sp was 20 nm and Sa was 3 nm.

<Surface free energy>
The surface free energy of the second main surface of the specific coating layer of the biaxially oriented film was measured by the following method.
Using a contact angle meter (DROPMASTER-501, manufactured by Kyowa Interface Science Co., Ltd.), droplets were dropped onto the surface of the specific coating layer side of the produced biaxially oriented film at 25° C., and the contact angle was measured 1 second after the droplets attached to the surface. 2 μL of purified water, 1 μL of methylene iodide, and 1 μL of ethylene glycol were used as droplets, and the surface free energy was calculated from the measured contact angles by the Kitazaki-Hata method.
The surface free energy of the first main surface of the biaxially oriented film was measured in the same manner, and was 59.7 mJ/ ^m2 in all examples.

<Thickness variation>
A sample having a length of 10 m in the longitudinal direction was taken from the produced biaxially oriented film. The thickness of this sample was measured over 10 m along the longitudinal direction using a continuous stylus film thickness meter (TOF-6R001, manufactured by Yamabun Co., Ltd.). This measurement was performed at five different positions in the width direction. From the obtained measurements, the difference between the maximum and minimum values was divided by the arithmetic mean value of all the measurements to calculate the thickness variation ((maximum thickness - minimum thickness) / average thickness).

<Film density>
The density (g/cm ³ ) of the biaxially oriented film was measured using an electronic specific gravity meter (product name "SD-200L", manufactured by Alpha Mirage).

<Line-like defect area>
Using a heating and conveying device, the biaxially oriented film was heat-treated at 90°C for 20 seconds while being conveyed at a conveying speed of 30 m/min and a tension of 100 N/m in the conveying direction. The heating temperature in the heat treatment refers to the surface temperature of the film. The heating time in the heat treatment was calculated from the point when the surface temperature of the film reached the target temperature (90°C). The biaxially oriented film after the heat treatment was placed on a black flat plate, and then the biaxially oriented film was visually observed from an oblique angle while changing the viewpoint so that the light of a fluorescent lamp [Lupica Ace (color temperature: 5000K, average color rendering index (Ra): 84) manufactured by Mitsubishi Electric Corporation] installed on the ceiling in the room was reflected. A 1m x 1m area was visually observed, and the area where the reflected image of the fluorescent lamp was wavy on the surface of the biaxially oriented film was defined as a stripe defect area. Next, the ratio (area ratio) of the total area of the observed line defect regions to the total area of the observed regions of the biaxially oriented film was calculated by the method already described (see the above item "Line Defect Region").

<Expansion rate>
The expansion coefficient in the width direction of the biaxially oriented film at 90° C. was measured using a thermomechanical analyzer (TMA-60, manufactured by Shimadzu Corporation) according to the method previously described (see the above item "Expansion coefficient").

<Average particle size, particle density D>
The average particle size and particle density D of the particles contained in the specific coating layer were measured by the following method.
Using a scanning electron microscope (SEM, Hitachi High-Technologies Corporation, S4700), the surface of the specific coating layer side of the biaxially oriented film was observed at a magnification of 20,000 times to obtain observation images of 10 fields of view. For particles that could be identified as protrusions from the obtained image data, the area of each particle was measured using image software, and the particle diameter of each particle was obtained by converting it into the diameter of a circle having the same area (area-equivalent circle diameter), and then the arithmetic average value of all particles was calculated.
Furthermore, the number of particles identifiable from the image data of each visual field was divided by the visual field area to calculate the particle density D (unit: particles/μm ² ).

<Measurement of peeling charge amount>
The peel charge of each of the produced biaxially oriented films was measured using a measuring device equipped with a stage on which a reference sample (first main surface up) was placed, a head that could hold a measurement sample (second main surface down) and raise and lower it vertically, thereby repeatedly pressing and peeling the measurement sample against the top surface of the reference sample, and an electrometer connected to this head that could measure the charge of the measurement sample.
First, each of the produced biaxially oriented films was cut into a circular shape having a diameter of 1.5 cm to prepare a sample for measuring the peel charge amount, and also cut into a rectangular shape having a size of 13 cm x 4 cm to prepare a reference sample. Each sample was left in an environment having a temperature of 23°C and a relative humidity (RH) of 20% for more than 2 hours.
The obtained reference sample was placed on the table of the measuring device, and the measurement sample was attached to the head. At this time, the first main surface of the reference sample was placed on the table on the upper side, and the second main surface of the measurement sample was placed on the lower side, so that the first main surface of the reference sample and the second main surface of the measurement sample were opposed to each other. After the measurement sample was neutralized, the head was raised or lowered, and the measurement sample was repeatedly pressed against the reference sample and peeled off five times (contact pressure: 566 g/cm ² , contact time: 2 seconds). The charge amount of the measurement sample was measured after each of the first to fifth peelings, and the average value of the measured values was calculated. The measurement sample was changed, and the position where the measurement sample contacted the reference sample was changed for each measurement sample, and measurements were performed on a total of four samples, and the average of all the measurements was obtained as the peeling charge amount.

[evaluation]
A release film was prepared using each of the biaxially oriented films of Examples 1 to 27 and Comparative Examples 1 to 3, and the obtained release films were evaluated as follows. The evaluation results are shown in Table 1.

[Preparation of Release Film]
The biaxially oriented film produced in each Example and Comparative Example was sent out, and the coating solution consisting of the following formulation A was applied to the surface of the biaxially oriented polyester film opposite to the particle-containing layer by the slot die method, and then the coating film was dried using a hot air dryer at 120°C and taken up to produce a roll-shaped release film (biaxially oriented film provided with a release layer). The thickness of the release layer after drying was 0.5 μm.

[Transfer mark evaluation 1]
The obtained release film was cut into 3.5 cm squares, and 10 sheets were stacked in the direction in which the release layer and the specific coating layer were in contact with each other to obtain a laminate sample. This sample was held in an oven at 40°C for 3 days with a load of 84 kg. The sample was removed from the oven, and the release film was peeled off one by one from each sample. The surface of the release layer of the release film was observed at 20,000 times using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, S4700), and the transfer marks were evaluated according to the following criteria based on the presence of dents.

(Formulation A: Coating solution for forming release layer)
Addition reaction type silicone (SRX-345, release agent, manufactured by Dow Corning Toray Co., Ltd.): 10 parts Mixed solvent of toluene and methyl ethyl ketone (mixing ratio = 7:3 (mass ratio)): 490 parts Platinum catalyst (SRX-212, manufactured by Dow Corning Toray Co., Ltd.): 0.1 part The coating liquid for forming the release layer was prepared by stirring and mixing the above components.

(Evaluation criteria)
A: No dents were observed and the surface was smooth.
B: Dimples were observed, and the surface of the release layer was roughened.

[Transfer mark evaluation 2: Evaluation after long-term storage]
A long-term storage test was carried out by storing the release film prepared by the method described in [Preparation of Release Film] above under normal temperature and humidity conditions for three months. Except for using the release film after storage, the transfer marks formed on the release film were evaluated according to the method described in [Transfer Mark Evaluation 1] above.
The evaluation criteria for transfer mark evaluation 2 are as follows.

(Evaluation criteria)
A: No dents were observed and the surface was smooth.
B: A dent was observed on only one of the ten release films, while the remaining nine films had smooth surfaces.
C: Dimples were observed in two or more of the ten release films, and the surface of the release layer was roughened.

[Transportability]
In each of the Examples and Comparative Examples, the biaxially oriented film that had been subjected to the above-mentioned cooling step was transported under the following transport conditions before being wound up according to the method described in the above-mentioned winding step. At this time, the state of wrinkles appearing on the biaxially oriented film transported by the transport roll was visually observed, and transportability was evaluated according to the following criteria.
(Transportation conditions)
Transport roll material: Stainless steel (SUS)
Transport roll wrap angle: 130° (coated surface)
Conveying speed: 100 m/min Conveying tension: 100 N/m

(Evaluation criteria)
A: No wrinkles were observed during transportation.
B: Wrinkles were observed only near the transport roll.
C: Wrinkles were constantly observed even downstream of the transport roll.

[Coating Properties of Release Layer]
The release film prepared in the evaluation of the transfer marks was cut into a length of 30 m in the longitudinal direction. The surface of the release layer side of the obtained sample was visually observed under a three-wavelength fluorescent lamp, and the number of streaky coating irregularities and defects due to foreign matter observed by reflected light was counted. From the measurement results, the coating property of the release layer was evaluated according to the following criteria.

(Evaluation criteria)
A: Neither coating unevenness nor defects were observed.
B: Coating unevenness and/or defects were observed, but were within the acceptable range.
C: Coating unevenness and/or defects beyond the acceptable range were observed.

Table 1 shows the evaluation results of each of the examples and comparative examples.
In Table 1, Resins 1 to 9 in the "Resin" column, W-1 to W-4 in the "Surfactant" column, Crosslinker in the "Additive" column, and Particles 1 to 3 in the "Type" column of "Particles" indicate the use of the following components, respectively.
(resin)
Resin 1: ZAIKXEN NC (Sumitomo Seika Chemicals Co., Ltd., acid-modified polyolefin, water dispersion)
Resin 2: ZAIKXEN L (Sumitomo Seika Chemicals Co., Ltd., acid-modified polyolefin, water dispersion)
Resin 3: ZAIKSENE A (Sumitomo Seika Chemicals Co., Ltd., acid-modified polyolefin, water dispersion)
Resin 4: Chemipearl S120 (Mitsui Chemicals, Inc., acid-modified polyolefin, aqueous dispersion)
Resin 5: Acrylic resin (a neutralized aqueous dispersion of a copolymer obtained by polymerizing methyl methacrylate, 2-hydroxyethyl methacrylate, and methacrylic acid in a mass ratio of 28:48:24)
Resin 6: SIMAC US-480 (manufactured by Toagosei Co., Ltd., silicone modified acrylic resin, water dispersion)
Resin 7: Acrylic resin (a copolymer obtained by polymerizing methyl methacrylate, styrene, 2-ethylhexyl acrylate, 2-hydroxyethyl methacrylate, and acrylic acid in a mass ratio of 59:8:26:5:2, aqueous dispersion)
Resin 8: Takelac W605 (Mitsui Chemicals, Inc., polyurethane, water dispersion)
Resin 9: HYDRAN AP-40N (DIC Corporation, polyurethane, water dispersion)
(Surfactant)
W-1: Anionic hydrocarbon surfactant (Rapisol A-90, manufactured by NOF Corporation)
W-2: Silicone surfactant (BYK-346, manufactured by BYK Corporation)
W-3: Fluorine-based surfactant (Surflon S-211, manufactured by AGC Seimi Chemical Co., Ltd., structure containing a linear perfluoroalkyl group having 6 or more carbon atoms)
W-4: Fluorine-based surfactant (Ftergent 215M, manufactured by Neos Co., Ltd., structure containing a perfluoroisopropyl group and not containing a linear perfluoroalkyl group having 6 or more carbon atoms)
(Additive)
Crosslinking agent: isocyanate compound, "Duranate (registered trademark) WM44L70" manufactured by Asahi Kasei Chemicals Corporation (particles)
Particle 1: Snowtex ZL (Nissan Chemical Industries, Ltd., colloidal silica, water dispersion)
Particle 2: Snowtex MP2040 (Nissan Chemical Industries, Ltd., colloidal silica, water dispersion)
Particle 3: Snowtex XL (Nissan Chemical Industries, Ltd., colloidal silica, water dispersion)

From Table 1, it was confirmed that the biaxially oriented polyester films of Examples 1 to 27 of the present invention have superior effects of the present invention compared to Comparative Examples 1 to 3.

Comparative Example 4
As the polyester film of Comparative Example 4, a polyethylene terephthalate film ("Cosmoshine (registered trademark) A-1517", manufactured by Toyobo Co., Ltd., thickness 16 μm) was prepared in which a particle-containing resin layer was laminated on one main surface of a polyethylene terephthalate substrate. The polyester film of Comparative Example 4 had a relatively flat first surface (corresponding to the first main surface of this film) and a second surface (corresponding to the second main surface of this film) having an uneven shape. The maximum projection height Sp of the uneven surface of the polyester film of Comparative Example 4 was 63 nm.
A roll-shaped release film was prepared by providing a release layer on the first surface of the polyester film of Comparative Example 4 according to the method described in [Preparation of Release Film] above. The obtained release film was used to evaluate the transfer marks on the surface of the release layer according to the method described in [Transfer Mark Evaluation 1] above. Indentations were observed, and the release layer was roughened (evaluation B).
In addition, the obtained release films were used to evaluate the transfer marks on the surface of the release layer using the method described above in [Transfer Mark Evaluation 2].Indentations were observed on the surface of two or more of the ten release films, and the release layer was roughened (Evaluation C).

It was confirmed that when the average particle diameter of the particles contained in the specific coating layer is 130 nm or less, the transportability is superior (comparison between Examples 1 and 2).

In addition, it was confirmed that the transportability was better when the product (D x Sp) of the particle density D (unit: pieces/ ^μm2 ) of the particles constituting the protrusions on the second main surface and the maximum protrusion height Sp (unit: nm) on the second main surface was 20 or more (comparison of Examples 2, 5 and 6 with other Examples).

It was confirmed that the coatability of the release layer is better when the area ratio of the streak defect areas that occurs when heated at 90°C is 20% or less (comparison of Examples 1, 20-22 with Examples 14-19).

It was confirmed that when the cooling rate of the polyester film in the cooling process is greater than 2,200°C/min and less than 3,000°C/min, the coatability of the release layer is superior (comparison of Examples 1 and 17 to 19).

It was confirmed that when the specific coating layer contained a hydrocarbon surfactant, the coating properties of the release layer were superior (comparison of Examples 1 and 9 to 10).
It was confirmed that when the specific coating layer contains a fluorosurfactant having a perfluoroalkyl group having 1 to 4 carbon atoms, the coatability of the release layer is superior compared to when the specific coating layer contains a fluorosurfactant having a linear perfluoroalkyl group having 6 or more carbon atoms (Comparison between Examples 26 and 27).

In addition, it was confirmed that when the absolute value of the peel charge of the biaxially oriented film measured by the above measurement method is 0.12 nC or less, the coatability of the release layer is superior (comparison of Examples 9 to 10 with Examples 1 to 8, 11 to 13, 23, 25, and 26).

It was confirmed that when the surface free energy of the second main surface is 50 mJ/ ^m2 or less, the formation of transfer marks on the surface of the release layer can be further suppressed even after the release film is stored for a long period of time (comparison of Example 24 with other examples).

[Example 101: Production of ceramic green sheet]
A ceramic slurry having the following formula K was applied to the release layer of each of the release films prepared in Examples 1 to 26 so that the thickness after drying was 0.5 μm, and the obtained slurry coating film was dried at 90 ° C. The ceramic slurry was prepared by mixing each raw material described in the formula below and dispersing it with a ball mill. Two release films with a slurry coating film prepared by the above method were overlapped so that the surface of the slurry coating film and the surface of the specific coating film were in contact with each other, and a load of 1 kg / cm ² was applied for 10 minutes. Thereafter, the release film was peeled off from the release film with the slurry coating film to obtain a ceramic green sheet.
The obtained ceramic green sheet had no foreign matter or transfer failure and had good properties.

<Formulation K: Ceramic Slurry>
Polyvinyl butyral (S-LEC BX-5, manufactured by Sekisui Chemical Co., Ltd.) 5 parts Barium titanate (HPBT, manufactured by Fuji Titanium Industry Co., Ltd.) 100 parts Mixture of toluene and ethanol in a ratio of 6:4 45 parts

1: Polyester film 1a: First main surface 1b: Second main surface 2: Polyester base material 2a-2l: Gripping member 3: Specific coating layer 10: Preheating section 20: Stretching section 30: Heat fixing section 40: Heat relaxation section 50: Cooling section 60a, 60b: Circular rail 100: Stretching machine 200: Film P, Q: Grip release point MD: Transport direction (longitudinal direction)
TD: Width direction L0, L1, L2, L3: Film width

Claims (21)

a polyester substrate substantially free of particles;
a coating layer containing particles disposed on one surface of the polyester substrate;
A first major surface and a second major surface,
A polyester film used for producing a roll-shaped release film by forming a release layer on the first main surface,
the second main surface is a surface of the coating layer opposite to the polyester substrate side,
The second main surface has a maximum projection height Sp of 1 nm or more and less than 60 nm;
A polyester film, wherein the second main surface has a surface free energy of 25 to 60 mJ/ ^m2 . 2 . The polyester film according to claim 1 , wherein the product (D×Sp) of a density D (unit: particles/μm ² ) of particles constituting the protrusions on the second main surface and the maximum protrusion height Sp (unit: nm) is 20 or more. The polyester film according to claim 1 or 2, wherein the thickness of the polyester film is 40 μm or less. The polyester film according to any one of claims 1 to 3, wherein the coating layer further contains a polyolefin. The polyester film according to any one of claims 1 to 4, wherein the coating layer further contains a (meth)acrylate resin having an acid value of 30 mgKOH/g or less. The particles have an average particle size of 1 to 130 nm;
The coating layer has a thickness of 1 to 100 nm, and
6. The polyester film according to claim 1, wherein the average particle size of the particles is greater than the thickness of the coating layer. The polyester film according to any one of claims 1 to 6, wherein the coating layer contains at least one surfactant selected from the group consisting of hydrocarbon surfactants and fluorine-based surfactants containing a perfluoroalkyl group having 1 to 4 carbon atoms. The polyester film according to any one of claims 1 to 7, in which the absolute value of the peel charge between the first and second main surfaces of the polyester film corresponding to a circle having a diameter of 1.5 cmφ is 0.12 nC or less. The polyester film according to any one of claims 1 to 8, wherein the polyester film is transported at a transport speed of 30 m/min and a tension in the transport direction of 100 N/m, and then heat-treated for 20 seconds under conditions that the temperature of the film surface is 90°C, and the total area of the stripe defect regions observed in the polyester film is 40% or less of the total area of the observed regions. The polyester film according to any one of claims 1 to 9, wherein the polyester film has a density of 1.39 to 1.41 g/ ^cm3 . The polyester film according to any one of claims 1 to 10, wherein the expansion rate of the polyester film in the width direction at 90°C is -0.15 to 0.15% relative to the width direction length of the polyester film at 30°C. The polyester film according to any one of claims 1 to 11, wherein the surface average roughness Sa of the second main surface is 1 to 10 nm. The polyester film according to any one of claims 1 to 12, wherein the maximum projection height Sp of the first main surface is 1 to 60 nm. The polyester film according to any one of claims 1 to 13, wherein the first main surface has a surface free energy of 50 to 70 mJ/ ^m2 . The polyester film according to any one of claims 1 to 14, wherein the variation in thickness of the polyester film is 5% or less of the average thickness of the polyester film. The polyester film according to any one of claims 1 to 15, wherein the release film is a release film for producing ceramic green sheets. A roll-shaped release film comprising the polyester film according to any one of claims 1 to 16 and a release layer disposed on the first main surface of the polyester film. The roll-shaped release film according to claim 17, wherein the maximum projection height Sp of the surface of the release layer opposite to the polyester film side is 1 to 60 nm. 19. The roll-shaped release film according to claim 17 or 18, wherein the surface of the release layer opposite to the polyester film has a surface free energy of 30 mJ/ ^m2 or less. A biaxial stretching step of biaxially stretching an unstretched polyester film having a polyester substrate;
and a coating layer forming step of in-line coating using a coating layer forming composition containing particles.
The method for producing a polyester film according to any one of claims 1 to 16. a heat setting step of heating the polyester film biaxially stretched by the biaxial stretching step at a temperature of less than 240° C. to heat set the film;
a heat relaxing step of heating the polyester film heat-fixed in the heat fixing step at a temperature lower than that in the heat fixing step to heat relax the polyester film;
a cooling step of cooling the polyester film heat-relaxed in the heat-relaxing step;
and an expanding step of expanding the heat-relaxed polyester film in a width direction in the cooling step,
The method for producing a polyester film according to claim 20, wherein the cooling rate of the polyester film in the cooling step is more than 2000° C./min and less than 4000° C./min.

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