JP7836360B2 - Polyester film, release film, method for manufacturing polyester film - Google Patents

Description

This invention relates to a polyester film for manufacturing release films, a release film, and a method for manufacturing 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 onto the surface of a biaxially oriented polyester film are used in the production of ceramic sheets with dielectric layers for the manufacture of multilayer ceramic capacitors.

For example, Patent Document 1 discloses a release film for manufacturing ceramic sheets, which uses a biaxially oriented polyester film consisting of two or more layers as a base material. The base material has a surface layer A that substantially does not contain particles and a surface layer B that contains particles. A release coating layer is laminated on the surface of surface layer A, and a smoothing coating layer is laminated on the surface of surface layer B. The smoothing coating layer has a specific region average surface roughness (Sa) and maximum protrusion height (P).

Japanese Patent Publication No. 2016-060158

With the increasing capacitance and miniaturization of ceramic capacitors in recent years, there is a growing demand for even thinner ceramic sheets. However, as ceramic sheets become thinner, the influence of the surface shape of the release film on the performance of the ceramic sheet becomes greater. For example, if there are irregularities on the release surface of the release film, these irregularities may be transferred to the ceramic green sheet during its formation, causing variations in the thickness of the ceramic green sheet and the ceramic sheet obtained by firing, potentially degrading the performance of the ceramic capacitor product.
Furthermore, not only the release surface of the release film, but also the uneven shape of the transport surface on the opposite side of the release surface can potentially affect the performance of the ceramic sheet. The transport surface of the release film often has protrusions to suppress wrinkles during high-speed transport, but if these protrusions are too large, when the release film is rolled up for storage, the shape of the protrusions on the transport surface may be transferred to the release surface, forming transfer marks. The shape of the transfer marks transferred to the release surface is then transferred to the ceramic green sheet and the ceramic sheet, and is thought to affect the performance of the final product.

The present inventors, referencing the technology described in Patent Document 1, further investigated release films used in the manufacture of ceramic green sheets and found that, in addition to the issues of transfer marks on the surface of the release layer and transportability, 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 and/or defects caused by foreign matter during the formation of the release layer.

In view of the above circumstances, the present invention aims to provide a polyester film for manufacturing release films that 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, the present invention aims to provide a method for producing a release film and a polyester film.

The inventors of this invention, after diligently studying the above problems, have found that the above problems can be solved by the following configuration.

[1]
A polyester film comprising a polyester substrate substantially free of particles and a coating layer containing particles disposed on one surface of the polyester substrate, having a first main surface and a second main surface, and used for manufacturing a release film by forming a release layer on the first main surface, wherein the second main surface is the surface of the coating layer opposite to the polyester substrate side, the maximum protrusion 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 .
[2]
The polyester film according to [1], wherein the product (D × Sp) of the density D (unit: particles/ ^μm² ) of the particles constituting the protrusions of 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 above-mentioned coating layer further contains polyolefin.
[5]
The polyester film according to any one of [1] to [4], wherein the above-mentioned coating layer further contains a (meth)acrylate resin having an acid value of 30 mg KOH/g or less.
[6]
A polyester film according to any one of [1] to [5], wherein the average particle diameter of the above particles is 1 to 130 nm, the thickness of the above coating layer is 1 to 100 nm, and the average particle diameter of the above particles is greater than the thickness of the above coating layer.
[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 perfluoroalkyl groups having 1 to 4 carbon atoms.
[8]
A polyester film according to any one of [1] to [7], wherein the absolute value of the peel charge between the first main surface and the second main surface of a polyester film corresponding to a circle with 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 subjected to a heat treatment for 20 seconds under conditions that the temperature of the film surface becomes 90°C while being transported at a transport speed of 30 m/min and with a tension of 100 N/m in the transport direction, and the total area of the streaky defect regions observed on the polyester film is 40% or less of the total area of the observation region.
[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 expansion coefficient in the width direction of the polyester film at 90°C is -0.15 to 0.15% of the length in the width direction of the polyester film at 30°C.
[12]
A polyester film according to any one of [1] to [11], wherein the average surface roughness Sa of the second main surface is 1 to 10 nm.
[13]
A polyester film according to any one of [1] to [12], wherein the maximum projection height Sp of the first main surface is 1 to 60 nm.
[14]
A 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]
The polyester film according to any one of [1] to [14], wherein the variation in the thickness 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 above-mentioned release film is a release film for manufacturing ceramic green sheets.
[17]
A release film comprising a 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 protrusion height Sp on 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 to the polyester film side is 30 mJ/ ^m² or less.
[20]
A biaxial stretching process for biaxially stretching an unstretched polyester film having a polyester substrate,
The process includes a coating layer formation step in which a coating layer is formed in-line using a coating layer formation composition containing particles,
A method for manufacturing a polyester film as described in any of [1] to [16].
[21]
A method for manufacturing a polyester film according to [20], comprising: a heat setting step of heating the polyester film that has been biaxially stretched by the above-mentioned biaxial stretching step at a temperature of less than 240°C to heat-set it; a heat relaxation step of heating the polyester film that has been heat-set by the above-mentioned heat setting step at a temperature lower than that of the above-mentioned heat setting step to heat-relax it; a cooling step of cooling the polyester film that has been heat-relaxed by the above-mentioned heat relaxation step; and an expansion step of expanding the polyester film that has been heat-relaxed in the width direction during the cooling step, wherein the cooling rate of the polyester film during the cooling step is greater than 2000°C/min and less than 4000°C/min.

According to the present invention, it is possible to provide a polyester film for manufacturing release films that 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, it is possible to provide a method for producing a release film and a polyester film.

This is a cross-sectional view showing an example of the structure of the polyester film relating to this disclosure. This is an observation image of a polyester film in which streaky defect regions have occurred. This is a plan view showing an example of a stretching machine used in the manufacture of polyester film.

The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the following embodiments and can be implemented with appropriate modifications within the scope of the object of the present invention.

In this disclosure, a numerical range expressed using "~" means a range that includes the numbers before and after "~" as the lower and upper limits. In numerical ranges described in stages in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Also, in numerical ranges described in this disclosure, the upper or lower limit stated in one numerical range may be replaced with the values shown in the examples.
In this disclosure, the amount of each component in a composition means the total amount of multiple substances present in the composition, unless otherwise specified, if there are multiple substances corresponding to each component in the composition.
In this disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as their intended purpose is achieved.
In this disclosure, "mass%" and "weight%" are synonymous, and "parts by mass" and "parts by weight" are synonymous.
In this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

In this disclosure, the term "polyester film" encompasses both the polyester substrate alone and the laminate of the polyester substrate and the coating layer.
In this disclosure, "longitudinal direction" means the longitudinal direction of the polyester film during the manufacturing of the polyester film, and is synonymous with "conveying direction" and "machine direction".
In this disclosure, “width direction” means the direction perpendicular to the longitudinal direction. In this disclosure, “orthogonal” is not limited to strictly orthogonal, but includes approximately orthogonal. “Approximately orthogonal” means that the directions intersect at 90° ± 5°, preferably at 90° ± 3°, and more preferably at 90° ± 1°.
Furthermore, in this disclosure, "film width" means the distance between the two ends of the polyester film in the width direction.

[Polyester film]
The polyester film relating to this disclosure (hereinafter also referred to as "the film") comprises a polyester substrate and a coating layer containing particles (hereinafter also referred to as "specific coating layer") disposed on one surface of the polyester substrate, and has a first main surface and a second main surface, and is used to manufacture a release film by forming a release layer on the first main surface.
Furthermore, in this film, the second main surface is the surface opposite to the polyester substrate side of the specific coating layer, the maximum protrusion 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 this film will be explained with reference to the diagram.
Figure 1 is a cross-sectional view showing an example of the structure of the 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), while the polyester substrate 2 substantially does not contain particles.

The first main surface 1a of the polyester film 1 is the surface for forming the release layer. That is, after the polyester film 1 is manufactured, a release layer is laminated onto 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 that faces the polyester substrate 2. In other words, the specific coating layer 3 is the outermost layer of the polyester film 1.
The second main surface 1b of this polyester film 1 has the above-mentioned specific maximum protrusion height Sp and a specific surface free energy.

By having the above-described structure, this film can suppress the formation of transfer marks on the surface of the release layer, and also exhibits excellent transportability and excellent coating properties for the release layer (hereinafter, at least one of these effects is also referred to as "the effect of the present invention").
The reason why this film produces the effects of the present invention described above is not clear, but it can be inferred as follows.
When a release layer is formed on the first main surface of this film to produce a release film, the second main surface of this film corresponds to the transport surface, which is the surface opposite to the release layer. As described above, while providing a protruding shape on the transport surface (second main surface) improves transportability, if the protrusion is too large, there is a problem in that transfer marks are formed on the release layer when the release film is stored in a roll.
In contrast, the polyester film according to this disclosure suppresses the formation of transfer marks by keeping the protrusions (maximum protrusion height Sp) of the second main surface of the polyester film, which serves as the transport surface for the release film, small, and improves the transportability that has been reduced by keeping the surface free energy of the second main surface low.
On the other hand, if the maximum protrusion height Sp of the second main surface is made too small, the transportability will be significantly reduced, wrinkles may form during transport, and uneven thickness may occur in the release layer formed on the first main surface. Also, if the surface free energy of the second main surface is made too small, the polyester film will become easily charged, foreign matter may adhere to the first and second main surfaces, and coating defects may occur in the coating film of the release layer forming composition. In contrast, by setting both the maximum protrusion height Sp and the surface free energy of the second main surface to above a specific lower limit, it is presumed that uneven thickness and/or coating defects in the release layer are reduced, and the coatability of the release layer is improved.
Furthermore, this film has a structure that includes a polyester substrate that is substantially free of particles and a coating layer that contains particles. This improves the smoothness of the film, resulting in a well-balanced and excellent combination of suppression of transfer mark formation on the surface of the release layer and excellent transportability.

The film has the above-mentioned polyester substrate and a specific coating layer, and the maximum protrusion height Sp of the second main surface and the surface free energy are specified within the above-mentioned ranges. However, the specific embodiment is not particularly limited, and it may have an embodiment other than the one shown in Figure 1.
For example, in the configuration shown in Figure 1, the first main surface 1a of the polyester film 1 is the surface opposite to the specific coating layer 3 of the polyester substrate 2, but other layers may be arranged on the surface opposite to the specific coating layer of the polyester substrate, with one side of the surface being the first main surface 1a.
Furthermore, in the configuration shown in Figure 1, the specific coating layer 3 is placed 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.

The following provides a detailed explanation of each layer of this film.

<Polyester base material>
A polyester substrate is a film-like object containing polyester as its main polymer component. Here, "main polymer component" refers to the polymer that is present in the largest quantity (by mass) of all polymers contained in the film.
The polyester base material may contain one type of polyester or two or more types of polyester.

(polyester)
Polyester is a polymer having ester bonds in its main chain. Polyester is usually formed by polycondensation of dicarboxylic acid compounds and diol compounds, as described later.
The polyester is not particularly limited, and known polyesters can be used. Examples of polyesters include polyethylene terephthalate (PET), polyethylene-2,6-naphthalate (PEN), polypropylene terephthalate (PPT), polybutylene terephthalate (PBT), and copolymers thereof. Among these, 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 polyester is preferably 220 to 270°C, and more preferably 245 to 265°C.
The glass transition temperature (Tg) of polyester is preferably 65 to 90°C, and more preferably 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 polycondensation of 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 polyester is not particularly limited, and any known catalyst usable for the synthesis of polyester can be used.
Examples of catalysts 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 viewpoint 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 more preferably to use a titanium compound and a phosphorus compound in combination.

As the titanium compound, organic chelate titanium complexes are preferred. Organic chelate titanium complexes are titanium compounds that have an organic acid as a ligand.
Examples of organic acids include citric acid, lactic acid, trimellitic acid, and malic acid.
As titanium compounds, 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.

-Dicarboxylic acid compounds-
Examples of dicarboxylic acid compounds include 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 methyl aromatic dicarboxylic acids are preferred.

Examples of aliphatic dicarboxylic acid compounds include malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedionic acid, dimer acid, eicosanedionic acid, pimelic acid, azelaic acid, methylmalonic acid, and ethylmalonic acid.
Examples of alicyclic dicarboxylic acid compounds 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-sodium sulfisoisophthalic acid, phenylindanedicarboxylic acid, anthracenedicarboxylic acid, phenantradicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorenic acid, and their methyl esters.
Among these, terephthalic acid or 2,6-naphthalenedicarboxylic acid is preferred, and terephthalic acid is more preferred.

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

-Diol compounds-
Examples of diol compounds include aliphatic diol compounds, alicyclic diol compounds, and aromatic diol compounds, with aliphatic diol compounds being preferred.

Examples of aliphatic diol compounds 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.
Diol compounds may be used individually or in combination of two or more.

- End-capsulation agent -
In the manufacture of polyester, end-capping agents may be used as needed. By using end-capping agents, structures derived from the end-capping agent are introduced to the ends of the polyester.
The end-captive agent is not limited, and known end-captive agents can be used. Examples of end-captive agents include oxazoline compounds, carbodiimide compounds, and epoxy compounds.
As end-capturing agents, the contents described in paragraphs 0055 to 0064 of Japanese Patent Publication No. 2014-189002 can also be referenced, and the contents of the above publication are incorporated herein.

-Manufacturing conditions-
The reaction temperature is not limited and can be set appropriately depending on the raw materials. The reaction temperature is preferably 260 to 300°C, and more preferably 275 to 285°C.
The pressure is not limited and can be set appropriately according to 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 Publication No. 5575671 can also be used, and the contents of the above publication are incorporated herein.

The polyester content in the polyester substrate 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 substrate.
There is no upper limit to the polyester content, and it can be appropriately set within a range of 100% by mass or less relative to the total mass of the polymer in the polyester substrate.

When the polyester substrate contains polyethylene terephthalate, the polyethylene terephthalate content is preferably 90 to 100% by mass, more preferably 95 to 100% by mass, even more preferably 98 to 100% by mass, and particularly preferably 100% by mass, relative to the total mass of polyester in the polyester substrate.

The polyester substrate may contain components other than polyester (for example, catalysts, unreacted raw material components, particles, and water).
The polyester substrate is substantially free of particles. Examples of particles include those contained in the specific coating layer described later. "Substantially free of particles" is defined as a polyester substrate in which, when elements derived from particles are quantitatively analyzed by fluorescent X-ray analysis, the particle content is 50 ppm by mass or less relative to the total mass of the polyester substrate, preferably 10 ppm by mass or less, and more preferably below the detection limit. This is because even without actively adding particles to the polyester substrate, contaminants derived from foreign substances, raw resins, or dirt adhering to the lines or equipment in the manufacturing process of the polyester substrate may detach and become 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 order to control the peelability. There is no particular lower limit to the thickness, but in order to improve strength and processability, it is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 10 μm or more.
The thickness of the polyester substrate is measured according to the method for measuring the thickness of the polyester film described later.

<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.
This film, by having a specific coating layer, can improve the transportability of polyester film and release film. More specifically, it can improve 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 applied directly to the surface of the polyester substrate or to the surface of the polyester substrate via another layer. However, it is preferable to apply it directly to the surface of the polyester substrate for superior adhesion. That is, it is preferable that the surface of the first main surface 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 protrusion height Sp and surface free energy; however, it is preferable that it contains a binder in addition to the particles. Furthermore, the specific coating layer may also contain additives other than particles and binders.

(particle)
The average particle size of the particles contained in the specific coating layer is not particularly limited, but is preferably 1 to 250 nm, more preferably 10 to 200 nm, and even more preferably 30 to 130 nm, in terms of having better transportability and being able to suppress transfer marks.
Furthermore, in terms of superior transportability and suppression of transfer marks, it is preferable that the average particle diameter 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 diameter of the particles is greater than the thickness of the specific coating layer.

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

The average particle size of particles contained in a specific coating layer is determined using a scanning electron microscope (SEM) by the following method: The second main surface of the polyester film is observed using an SEM at a magnification of 20,000x. Ten arbitrarily selected fields of view are observed, and for particles that can be identified as protrusions in each field of view (particles that can be seen as protrusions protruding from the base surface), the area of each particle is measured using image software, and the diameter of a circle with the same area (area circle equivalent diameter) is calculated. The arithmetic mean of the obtained area circle equivalent diameters is taken as the average particle size of the particles. At this time, even if dust and/or coarse aggregated particles of 1 μm or larger are present, dust and coarse aggregated particles are not counted when calculating the average particle size.
Furthermore, in measuring the average particle diameter, for aggregated particles, the particle diameter of the secondary particles in the aggregated state (secondary particle diameter) shall be measured.
Furthermore, if a specific coating layer contains two or more particles of different particle sizes, the distribution of area-equivalent diameter measured by the above measurement method will show two or more peaks of different particle sizes. In such cases, where the distribution of area-equivalent diameter measured by the above measurement method has two or more peaks of different particle sizes, the average value of the area-equivalent diameter shall be calculated for each peak, and the average particle size shall be calculated for each particle of different particle sizes.

Examples of particles contained in a specific coating layer include organic particles and inorganic particles. Among these, inorganic particles are preferred from the viewpoint of further improving film winding quality, haze, and durability (e.g., thermal stability).
As organic particles, resin particles are preferred. Examples of resins constituting the resin particles include acrylic resins such as polymethyl methacrylate (PMMA), polyester resins, silicone resins, and styrene-acrylic resins. The resin particles preferably have a crosslinked structure. Examples of 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 these, silica particles are preferred as the inorganic particles from the viewpoint of improving haze and durability.

The particle shape is not particularly limited and can be, for example, rice-grain-shaped, spherical, cubic, spindle-shaped, flaky, aggregated, or irregularly shaped. Aggregated refers to a state in which primary particles are aggregated. While the shape of aggregated particles is not limited, spherical or irregularly shaped particles are preferred.

As aggregated particles, fumed silica particles are preferred. Examples of commercially available products include the Aerosil series from Nippon Aerosil Co., Ltd.
Colloidal silica particles are preferred as non-aggregated particles. Examples of commercially available products include the Snowtex series manufactured by Nissan Chemical Corporation.

From the viewpoint of transportability and the applicability of the release layer, the particle content in the specific coating layer is preferably 0.1 to 30% by mass, more preferably 1 to 25% by mass, and even more preferably 1 to 15% by mass, relative to the total mass of the specific coating layer.
Furthermore, the particle content is preferably 0.0001 to 0.01% by mass, and more preferably 0.0005 to 0.005% by mass, relative to the total mass of the polyester film.

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

The specific coating layer is preferably formed by applying an aqueous dispersion containing a binder. In this regard, an acid-modified resin is preferred as the binder. Examples of acid-modified resins include copolymers of (meth)acrylate and (meth)acrylic acid, polyolefins having carboxyl groups, and acid-modified polyurethanes, with copolymers of (meth)acrylate and (meth)acrylic acid or polyolefins having carboxyl groups being preferred, and polyolefins having carboxyl groups being more preferred.
Furthermore, 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, as it is easy to adjust the surface free energy of a particular coating layer to the above-mentioned specific range. The (meth)acrylate resin, polyolefin, and polyurethane are not particularly limited, and known resins can be used.

Polyolefins only need to contain olefin-derived structural units in their main chain, and it is preferable that they contain olefin-derived structural units as the main component. Having an olefin structure in the main chain results in insufficient compatibility with the polyester substrate, which in turn improves the transfer marks after long-term storage. There are no particular limitations on the olefin, but alkenes having 2 to 6 carbon atoms are preferred, ethylene, propylene, or hexene are more preferred, and ethylene is even more preferred.
In this specification, when a polymer is said to "have as a main component" a constituent unit derived from a certain monomer, it means that the constituent unit makes up 50 mol% or more of the total constituent units of the polymer.
The olefin-derived structural units in the polyolefin are preferably 50 to 99 mol%, and more preferably 60 to 98%, relative to the total structural units of the polyolefin.

As for the polyolefin, acid-modified polyolefins are preferred because they can prevent static charge buildup when applying the release layer. Examples of acid-modified polyolefins include copolymers obtained by modifying the above-mentioned 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 includes random copolymerization, block copolymerization, and graft copolymerization.
Examples of acid-modified components 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 resin dispersion stability, acrylic acid, methacrylic acid, maleic acid, or maleic anhydride are preferred.

Acidic groups contained in acid-modified polyolefins include carboxyl groups, sulfol groups, and phosphoric acid groups, which are acidic groups corresponding to the above-mentioned acid-modifying components, with carboxyl groups being preferred. The acidic groups may form acid anhydrides or be neutralized with at least one selected from alkali metals, organic amines, and ammonia. It is preferable that the acid-modifying components are neutralized with alkali metals in order to suppress static charge and improve the coatability of the release layer.
Acid-modified polyolefins may contain only one or more constituent units having acidic groups. Examples of constituent units having acidic groups include constituent units derived from the monomer of the acid-modified component, and constituent units in which the acid-modified component is grafted onto constituent units derived from the olefin monomer. The content of constituent units having acidic groups is not particularly limited, but it is preferably 0.1 to 30 mol% relative to all constituent units of the acid-modified polyolefin.

Examples of commercially available acid-modified polyolefins include the Zaixen® series (manufactured by Sumitomo Seika Co., Ltd.), such as Zaixen AC, A, L, NC, N, etc.; the Chemipearl® series (manufactured by Mitsui Chemicals, Inc.), such as Chemipearl S100, S120, S200, S300, S650, SA100, etc.; and the Hi-Tec® series (manufactured by Mitsui Chemicals, Inc.), such as Hi-Tec S3121, S3148K, etc. Examples include the Arrowbase (registered trademark) series (manufactured by Unitika Ltd.), such as Arrowbase SE-1013, SE-1010, SB-1200, SD-1200, SD-1200, DA-1010, DB-4010, etc., Hardlen AP-2, NZ-1004, NZ-1005 (manufactured by Toyobo Co., Ltd.), Sepolion G315, VA407 (manufactured by Sumitomo Seika Co., Ltd.), etc.
Furthermore, acid-modified polyolefins described in paragraphs 0022 to 0034 of Japanese Patent Publication No. 2014-076632 can also be preferably used.

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

The acid value of the (meth)acrylate resin is preferably 30 mg KOH/g or less, and more preferably 20 mg KOH/g or less. The lower limit of the acid value is not particularly limited; for example, 0 mg KOH/g is acceptable, but from the perspective of application as an aqueous dispersion, 2 mg KOH/g or more is preferred. By adjusting the acid value of the (meth)acrylate resin to be within the above range, and by including structural units derived from (meth)acrylate having an alkyl group with 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 and used as a release film for ceramic green sheet production, it is preferable to satisfy both of the above conditions: the acid value of the acrylic resin being within the above range, and the inclusion of structural units derived from (meth)acrylate having an alkyl group with 1 to 12 carbon atoms.

The polyurethane is not limited as long as it is a polymer having urethane bonds in its main chain; known polyurethanes such as reaction products of isocyanate compounds and polyol compounds can be used.
As mentioned above, acid-modified polyurethane is preferred because it is easy to prepare an aqueous dispersion. Acid-modified polyurethane refers to polyurethane having acidic groups. Examples of acidic groups include those listed above as acidic groups contained in acid-modified polyolefins. Furthermore, an aqueous dispersion of polyurethane containing polyurethane and a dispersant is also preferred.
The polyurethane contained in a specific coating layer can have its surface free energy controlled within a specific range by adjusting, for example, the structure and hydrophobicity (hydrophilicity) of the polyol compound and/or isocyanate compound used as raw materials.
Examples of commercially available polyurethanes include Hydran® AP-20, AP-40N, and AP-201 (all manufactured by DIC Corporation); Takelac® W-605, W-5030, and W-5920 (all manufactured by Mitsui Chemicals, Inc.); and Superflex® 210 and 130, and Elastron® H-3-DF, E-37, and H-15 (all manufactured by Daiichi Kogyo Seiyaku Co., Ltd.).

The specific coating layer may contain one type of binder or two or more types of binders. When using the above-mentioned polyolefin or (meth)acrylate resin in combination with a resin other than polyolefin and (meth)acrylate resin in order to control the surface free energy, the above-mentioned polyurethane is preferred as the resin used in combination.
From the viewpoint of adjusting Sp to a desired range, the binder content is preferably 30 to 99.8% by mass, and more preferably 50 to 99.5% by mass, relative to the total mass of the specific coating layer.

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

The specific coating layer preferably contains a surfactant, as this improves the smoothness of the areas on the second main surface other than those where protrusions formed by particles exist. By improving the smoothness of the aforementioned areas on the second main surface and reducing the surface roughness of the second main surface due to factors other than particles, Sp can be controlled within a desired range, thereby improving the effects of the present invention.

The surfactant is not particularly limited, and examples include silicone-based surfactants, fluorine-based surfactants, and hydrocarbon-based surfactants. Hydrocarbon-based surfactants are preferred because they suppress static charge on the first main surface and reduce the occurrence of defects caused by foreign matter during application of the release layer, thereby improving the coatability of the release layer.

The silicone-based surfactant is not particularly limited as long as it is a surfactant having a silicon-containing group as a hydrophobic group, and examples include polydimethylsiloxane, polyether-modified polydimethylsiloxane, and polymethylalkylsiloxane.
Examples of commercially available silicone-based surfactants include BYK®-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 fluorinated surfactant is not particularly limited as long as it is a surfactant having a fluorine-containing group as a hydrophobic group, and examples include perfluorooctanesulfonic acid and perfluorocarboxylic acid.
Examples of commercially available fluorine-based surfactants include Megafac® F-114, F-410, F-440, F-447, F-553, and F-556 (all manufactured by DIC Corporation), and Surflon® 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, as a fluorine-based surfactant, from the viewpoint of improving environmental suitability, it is preferable to use a surfactant derived from a substitute material for compounds having a linear perfluoroalkyl group with 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 a substitute material for compounds having a linear perfluoroalkyl group with 6 or more carbon atoms.
More specifically, compared to surfactants having linear perfluoroalkyl groups with 6 or more carbon atoms, it is preferable to use surfactants having perfluoroalkyl groups with 1 to 4 carbon atoms in order to improve the applicability of the release layer. The perfluoroalkyl groups with 1 to 4 carbon atoms may be linear or branched. Among these, surfactants having linear perfluoroalkyl groups with 1 to 4 carbon atoms or branched perfluoroalkyl groups with 3 carbon atoms are more preferable, and surfactants having perfluoroalkyl groups with 1 or 2 carbon atoms or branched perfluoroalkyl groups with 3 carbon atoms are even more preferable.
Examples _{of the} above-mentioned perfluoroalkyl groups include _CF3- *, _C2F5- *, _C3F7- *, n- _C4F9- *, and ( _CF3 ) _2CF- *. Here, * indicates _the bonding position with carbon atoms other than those substituted with fluorine atoms. It is preferable that the carbon atoms bonded to these perfluoroalkyl groups either have hydrogen atoms or are bonded only to carbon atoms.

Examples of commercially available fluorinated surfactants having a perfluoroalkyl group with 1 to 4 carbon atoms include, for example, Futergent® 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.), and PF-136A, PF-156A, PF-151N, PF-636, PF-6320, PF-656, PF-6520, and PF-652-NF (all manufactured by OMNOVA).

The reason why using surfactants having perfluoroalkyl groups with 1 to 4 carbon atoms improves the coating properties of the release layer compared to using surfactants having linear perfluoroalkyl groups with 6 or more carbon atoms is unclear. However, surfactants having perfluoroalkyl groups with 1 to 4 carbon atoms contain a large number of _CF3 groups per unit weight, which have low surface tension. Therefore, a small amount of additive can reduce the surface free energy of the second main surface. This improves the transportability of the polyester film, and because the composition of the specific coating layer does not change significantly, it is presumed that this suppresses the release charging described later and improves the coating properties of the release layer.

Examples of hydrocarbon-based surfactants include anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants.
Examples of anionic surfactants include alkyl sulfates, alkylbenzene sulfons, alkyl phosphates, and fatty acid salts.
Examples of nonionic surfactants include polyalkylene glycol mono- or dialkyl ethers, polyalkylene glycol mono- or dialkyl esters, and polyalkylene glycol monoalkyl esters/monoalkyl ethers.
Examples of cationic surfactants include primary to tertiary alkylamine salts and quaternary ammonium compounds.
Examples of amphoteric surfactants include surfactants that have both anionic and cationic parts within their molecule.

Examples of commercially available anionic surfactants include Rapisol® A-90, A-80, BW-30, B-90, and C-70 (all manufactured by NOF Corporation), NIKKOL® OTP-100 (all manufactured by Nikko Chemical Co., Ltd.), Kohacool® ON, L-40, and Phosphanol® 702 (all manufactured by Toho Chemical Industry Co., Ltd.), as well as Viewlight® A-5000 and SSS (all manufactured by Sanyo Chemical Industries, Ltd.).
Examples of commercially available nonionic surfactants include Naroacty® CL-95 and HN-100 (product name: manufactured by Sanyo Chemical Industries, Ltd.), Risolex BW400 (product name: manufactured by Higher Alcohol Industry Co., Ltd.), EMALEX® ET-2020 (all manufactured by Nippon Emulsion Co., Ltd.), and Surfinol® 104E, 420, 440, 465, and Dynol® 604, 607 (all manufactured by Nisshin Chemical Industry Co., Ltd.).

When used in combination with acid-modified resins, anionic and/or nonionic surfactants are preferred, and anionic surfactants are more preferred, in that they can form a smooth coating layer without hindering the dispersion of the resin. Specifically, anionic hydrocarbon surfactants are more preferred as surfactants in terms of improving surface smoothness and the applicability of the release layer.

Anionic hydrocarbon surfactants are preferable to have multiple hydrophobic end groups in that they have improved smoothness. The hydrophobic end groups may be some of the hydrocarbon groups that the hydrocarbon surfactant has. For example, a hydrocarbon surfactant having a branched-chain hydrocarbon group at its end will have multiple 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-chain alkylbenzene sulfonates (having two hydrophobic end groups).

One type of surfactant may be used, or two or more types may be used in combination.
The surfactant content is preferably 0.1 to 10% by mass relative to the total mass of the specific coating layer, more preferably 0.1 to 5% by mass, and even more preferably 0.5 to 2% by mass, in terms of superior antistatic properties and surface smoothness during the formation of the peeling layer.

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

There are no particular restrictions on the crosslinking agent; known agents can be used.
Examples of crosslinking agents include melamine compounds, oxazoline compounds, epoxy compounds, isocyanate compounds, and carbodiimide compounds, with oxazoline compounds and carbodiimide compounds being particularly preferred. Examples of commercially available products include Carbodilite V-02-L2 (manufactured by Nisshinbo Inc.) and Epocross K-2020E (manufactured by Nippon Shokubai Inc.). For details on epoxy compounds, isocyanate compounds, and melamine compounds, refer to sections [0081] to [0083] of Japanese Patent Application Publication No. 2015-163457. Crosslinking agents described in sections [0082] to [0084] of International Publication No. 2017/169844 can also be preferably used. For carbodiimide compounds, refer to sections [0038] to [0040] of Japanese Patent Application Publication No. 2017-087421.
For oxazoline compounds, carbodiimide compounds, and isocyanate compounds, the crosslinking agents described in sections [0074] to [0075] of International Publication No. 2018/034294 can also be preferably used.
The crosslinking agent content is preferably 0 to 50% by mass relative to the total mass of the specific coating layer.

(Thickness)
The specific coating layer is often formed by coating a particle-containing composition onto one surface of a polyester substrate, resulting in a thickness of 1 μm or less.
From the viewpoint of the suitability of the specific coating layer for manufacturing and haze reduction, the thickness of the specific coating layer is preferably 1 to 200 nm, more preferably 10 to 100 nm, and even more preferably 20 to 100 nm.
The thickness of a specific coating layer is determined by preparing a section of the polyester film with a cross-section perpendicular to the main surface, and measuring the thickness at five points on the section using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The arithmetic mean of these thicknesses is then used.
If the specific coating layer is soft and it is difficult to reliably prepare cross-sectional sections, a refractometer may be used for measurement. Specifically, the thickness of the specific coating layer can be determined by fitting the measured reflectance spectrum to the film 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" section below.

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

[Physical properties, etc.]
Next, we will explain the physical properties of this film.

(Surface free energy of the second principal surface)
In this film, the surface free energy of the second main surface is 25 to 60 mJ/ ^m² . Because the surface free energy of the second main surface is within this range, a polyester film with excellent transportability and excellent coating properties for the release layer can be obtained, even when the maximum protrusion height Sp of the second main surface is within this range.
The surface free energy of the second main surface is preferably 25 to 50 mJ/ ^m² , in that it can 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 viewpoint of the above-mentioned transportability and coating properties 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 mentioned above, etc.

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

(Surface free energy of the first principal surface)
From the viewpoint of preventing static electricity when winding this film, the surface free energy of the first main surface is preferably 50 to 70 mJ/ ^m² .
Furthermore, a larger difference between the surface free energy of the first main surface and the surface free energy of the second main surface is preferable because it makes the film less prone to becoming charged. 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, if the first main surface is the surface of the polyester substrate opposite to the side with a 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 projection height Sp of the second main surface, average surface roughness Sa)
This film has a maximum protrusion height Sp of 1 nm or more and less than 60 nm on its second main surface. Having the maximum protrusion height Sp of the second main surface within this range allows for the production of a release film with a good balance between suppressing transfer marks on the release layer surface and excellent transportability.
From the above viewpoint, the maximum projection height Sp of the second main surface is preferably 10 to 50 nm, and more preferably 20 to 50 nm.

Furthermore, in this film, the average surface 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, in terms of superior stability in suppressing transfer marks.

The maximum protrusion height Sp and average surface roughness Sa of the second main 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, as well as the thickness of the specific coating layer. When forming the specific coating layer by in-line coating, the above adjustments can be performed more easily.

The maximum protrusion height Sp and average surface roughness Sa of the second main surface of the polyester film are determined by measuring the surface of the polyester film on the side with a specific coating layer using an optical interferometer (for example, "Vertscan 3300G Lite" manufactured by Hitachi High-Tech Corporation) under the following conditions, and then analyzing the data using the built-in data analysis software.
In measuring the maximum protrusion height Sp, five measurements are taken at different measurement positions, and the maximum value obtained is taken as the maximum protrusion height Sp (indicated as P in the built-in data analysis software). Similarly, in measuring the average surface roughness Sa, five measurements are taken at different measurement positions, and the average value obtained is taken as the average surface roughness Sa.
(Measurement conditions)
• Measurement mode: WAVE mode • Objective lens: 50x • Measurement area: 186 μm x 155 μm

(Maximum projection height Sp of the first main surface, average surface roughness Sa)
In terms of smoothing the delamination layer, the first main surface is preferably as smooth as possible. Specifically, the maximum protrusion height Sp of the first main surface is preferably 1 to 60 nm, and more preferably 5 to 30 nm. Furthermore, the average surface roughness Sa of the first main surface is preferably 0 to 10 nm, and more preferably 0 to 5 nm.
The maximum protrusion height Sp and the average surface roughness Sa of the first main surface can be adjusted by methods such as selecting the type of polyester and the type of additives that constitute the polyester substrate so that the polyester substrate is substantially free of particles and a smooth film is formed.
The maximum protrusion height Sp and average surface roughness Sa of the first main surface can be measured according to the measurement method for the maximum protrusion height Sp and average surface roughness Sa of the second main surface described above.

(Product of the maximum projection height Sp of the second principal surface and the particle density D)
In this film, for superior transportability, the product (D × Sp) of the density D of the particles constituting the protrusions on the second main surface (unit: particles/ ^μm² , also called "particle density D") 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 for superior suppression of transfer marks, it is preferably 400 or less, and more preferably 300 or less.
The above product (D × Sp) is an indicator of the size and density of protrusions present on the second main surface. When the above product (D × Sp) is within the above range, a suitable amount of protrusions, which are desirable from the viewpoint of suppressing transfer marks and transportability, are present on the second main surface, and the effects of suppressing transfer marks and transportability are further improved, which is preferable.

The particle density D, as described above, can be adjusted, similar to the maximum protrusion height Sp and average surface roughness Sa, by, for example, the average particle diameter and content of particles in a specific coating layer, as well as the thickness of the specific coating layer. When forming a specific coating layer using in-line coating, the above adjustments can be performed more easily.

Furthermore, the particle density D of the particles constituting the protrusions on the second main surface of the polyester film is determined using a SEM in the same manner as the method for measuring the average particle diameter. Specifically, the surface of the polyester film on the side with a specific coating layer is observed at a magnification of 20,000 times. Observations are performed on 10 arbitrarily selected fields of view, and the number of individual particles that can be identified as protrusions (particles that can be seen as protrusions protruding from the base surface) in each field of view is measured using image software. The calculated value obtained by dividing the total number of particles measured in all fields of view by the total area of all fields of view is defined as the particle density D (unit: particles/ ^μm² ).

(Orientation)
This film is a biaxially oriented polyester film. In this disclosure, "biaxial orientation" means having molecular orientation in two axial directions.
Molecular orientation is measured using a microwave transmission molecular orientation meter (e.g., MOA-6004, manufactured by Oji Instruments Co., Ltd.). The angle between the two axes is preferably 90°±5°, more preferably 90°±3°, and even more preferably 90°±1°. The film preferably has molecular orientation in the longitudinal and width directions.

(Striated defect area)
In this disclosure, "striated defects" refer to wrinkles that extend in a striated manner along the longitudinal direction of the film and appear as irregularities in the width direction of the film. As will be described later, since striated defects occur in the film after manufacturing, they are often irreversible wrinkles. Striated defects do not occur during the heat treatment during the manufacturing of the film, but rather originate from wavy wrinkles that occur during the heat treatment of the film after manufacturing, and these wavy wrinkles solidify upon cooling after the heat treatment. Furthermore, "striated defect region" refers to the portion of the film surface in which striated defects occur.
When streaky defect regions occur (i.e., streaky defects occur partially within the film surface), they can cause thickness variations in the release layer, reducing the coatability of the release layer and potentially affecting the performance of the ceramic capacitor. Furthermore, streaky defect regions tend to occur more significantly when the film is heated while a tensile load is applied in the longitudinal direction.

The ratio of the total area of streaky 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 streaky 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 applicability of the release layer.
There is no particular lower limit to the area ratio of the streaky defect region, but it is preferable that the area ratio of the streaky defect region that occurs when heated at 90°C is small, and it is even more preferable that there is no streaky defect region, i.e., 0%.

The area ratio of the above-mentioned striated defect region is measured by the following method.
(1) Using a heating and conveying device, the polyester film is conveyed at a conveying speed of 30 m/min and with a tension of 100 N/m in the conveying direction, and a heat treatment is performed for 20 seconds under conditions that the surface temperature of the film reaches 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 heating is performed continuously for 20 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 then the polyester film is observed visually from an oblique angle, changing the viewpoint so that the light from a fluorescent lamp installed on the ceiling of the room [for example, a Lupica Ace manufactured by Mitsubishi Electric Corporation (color temperature: 5000K, average color rendering index (Ra): 84)] is reflected off it. The areas where the reflected image of the fluorescent lamp projected onto the surface of the polyester film appears wavy are defined as streaky defect areas.
(3) The number of observed streaky defect regions is counted, and the outer perimeter of each streaky defect region present in the observation area (area of 1 ^m² ) of the polyester film is marked. Next, the distance between two parallel tangents selected from two parallel tangents that circumscribe the outer perimeter of each streaky defect region, such that the distance between the tangents is maximized, is measured as the major axis length L, and the distance between two parallel tangents perpendicular to the two parallel tangents that give length L, and that circumscribe the outer perimeter of the streaky defect region, is measured as the minor axis length S. From the obtained lengths L and S, the area of each streaky defect region is calculated using the following formula. From these values, the ratio of the total area of the streaky defect regions to the total area of the polyester film is calculated.
The area of the striated defect region is calculated as follows: Length of the major axis L × Length of the minor axis S × π = Area of the striated defect region. Since the striated defect region is often elliptical or circular as described above, the area of the striated defect region can be calculated using the calculation method in (3) above.

Figure 2 shows an image (photograph) of a polyester film in which streaky defect regions generated by the heat treatment described in (1) above are observed. The area enclosed by the solid line in Figure 2 is the streaky defect region. In the streaky defect region shown in Figure 2, an uneven shape extending in the transport (MD) direction is observed. Note that the image (photograph) in Figure 2 shows only a portion of the observed area.
Thus, striated defect regions are often elliptical or circular in shape. Furthermore, when striated defect regions occur, at least one elliptical striated defect region appears whose major axis is aligned with the transport direction.

A biaxially oriented polyester film with a streaky defect area ratio within the above range can be manufactured by adjusting the heat setting temperature in the heat setting step, the cooling rate of the polyester film in the cooling step, and the widthwise expansion ratio of the polyester film in the expansion step, as described later in the polyester film manufacturing method.

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

The coefficient of expansion in the width direction at 90°C is measured using a thermomechanical analyzer by the following method.
(1) Prepare a sample that is adjusted to 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 with 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 higher but less than 30°C (preferably 25°C) to 150°C at a heating rate of 5°C/min to obtain the length of the sample at each temperature (°C).
(4) The expansion coefficient in the width direction at 90°C is determined from the length of the sample at 30°C (L30) and the length at 90°C (L90) using the following formula. In this disclosure, the expansion coefficient in the width direction is the arithmetic mean of the expansion coefficients obtained using five samples. A positive expansion coefficient means expansion, and a negative expansion coefficient means contraction.
Formula: Expansion rate (%) = (L90 - L30) / L30 × 100

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

(Film density)
The density of the polyester film is preferably 1.39 to 1.41 g/ ^cm³ , more preferably 1.395 to 1.405 g/ ^cm³ , and even more preferably 1.398 to 1.400 g/ ^cm³ , in terms of superior effects of the present invention.
The density of polyester film can be measured using an electronic hydrometer (product name "SD-200L," manufactured by Alpha Mirage).

(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, in terms of superior release properties. There is no particular lower limit to the thickness, but in terms of superior handling properties, it is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more.
The thickness of the polyester film shall be the arithmetic mean of the thicknesses measured at five locations using a continuous stylus-type film thickness gauge.

Furthermore, the variation in the thickness of the polyester film is preferably 7% or less of the average thickness of the polyester film, and more preferably 5% or less, in that it provides better surface smoothness of the first main surface that forms the release layer. 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.
Thickness variation is obtained by the following measurement method: Using a continuous stylus-type film thickness gauge, the thickness of the polyester film is measured along the longitudinal direction over a length of 10 m. This measurement is performed at five different locations in the width direction. From the obtained measurements, the difference between the maximum and minimum values is divided by the arithmetic mean of all measurements to obtain the value ((maximum thickness - minimum thickness) / average thickness), which is defined as the thickness variation.

(Amount of electrostatic charge removed)
In terms of improving the applicability of the release layer, the polyester film preferably has an absolute value of the release charge between the first and second main surfaces of a polyester film corresponding to a circle with a diameter of 1.5 cmφ, measured at a temperature of 23°C and a relative humidity (RH) of 20% using the measurement method described below, which is 0.12 nc (nanocoulombs) or less, more preferably 0.11 nc or less, and even more preferably 0.1 nc or less. Here, the unit nc (nanocoulomb) is ^10⁻⁹ coulombs.

The method for measuring the amount of electrostatic charge on a polyester film after peeling is as follows:
The measuring device used comprises a stand on which a reference sample of polyester film (with the first main surface facing upwards) is placed, a head that can repeatedly press and peel the second main surface of the measuring sample against the first main surface of the reference sample by moving up and down along the vertical direction while holding the measuring sample (with the second main surface facing downwards), and an electrometer connected to this head that can measure the amount of charge of the measuring sample.
A sample for measuring the amount of charge on the peel is prepared by cutting a polyester film into a circle with a diameter of 1.5 cm, and a standard sample for measuring the amount of charge on the peel is prepared by cutting it into a rectangle measuring 13 cm x 4 cm. Next, the obtained polyester film samples are left for at least two hours in the environment of the measurement temperature and humidity described above. After that, the standard sample is placed on the stand of the measuring device, and the measurement sample is attached to the head. At this time, the first main surface of the standard sample and the second main surface of the measurement sample are positioned so that they face each other, with the first main surface of the standard sample on the stand facing upwards and the second main surface of the measurement sample attached to the head facing downwards.
After discharging the measurement sample, the head is raised or lowered to repeatedly press and peel the reference film against the measurement sample. Using the same measurement sample, the amount of charge on the measurement sample is measured after each of the first to fifth peels, and the average of the measured values is calculated. The measurement sample is changed, and the contact position of the measurement sample with the reference film is also changed for each measurement sample, and measurements are taken with a total of four samples. The average of all measurements is taken as the peel charge amount.
Regarding the method for measuring the amount of electrostatic charge generated when peeling a polyester film, refer to the contents described in Japanese Patent Application Publication No. 2003-194865 (especially [0053] to [0067]), and the contents described in the above publication are incorporated herein by reference.

The amount of electrostatic charge generated during peeling can be adjusted by selecting the type and amount of components such as binders and surfactants contained in the polyester substrate and the specific coating layer. More specifically, by selecting two materials that are closer in the triboelectric series from the group consisting of the polyester substrate, binder, and surfactant, the amount of electrostatic charge generated during peeling can be adjusted to the above range.

[Manufacturing method]
One example of a method for manufacturing this film is a method comprising a biaxial stretching step of biaxially stretching an unstretched polyester film having a polyester substrate, and a specific coating layer formation step of forming a specific coating layer containing particles.

Biaxial stretching may be simultaneous biaxial stretching, where longitudinal and transverse stretching are performed at the same time, or it may be sequential biaxial stretching, where longitudinal 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 equipment used for biaxial stretching is not particularly limited, and any known stretching machine can be used. An example of a stretching machine will be described below with reference to the drawings.

Figure 3 is a plan view showing an example of a stretching machine used in the manufacture of polyester film.
The stretching machine 100 shown in Figure 3 comprises 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 arranged symmetrically with respect to the film 200. The stretching machine 100 can stretch the film 200 in the width direction by gripping the film 200 with the gripping members 2a to 2l and moving the gripping members 2a to 2l along the rails.

The stretching machine 100 has regions consisting of, 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 relaxation section 40, and a cooling section 50.
The above-mentioned area of the stretcher 100 is separated by a windbreak curtain, and the temperature within each area can be individually adjusted by hot air or the like.

The preheating section 10 is the region for preheating the film 200.

The stretching section 20 is the region where the preheated film 200 is stretched by applying tension in the direction of arrow TD (width direction), which is perpendicular to the direction of 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-setting section 30 is a region where the film 200, which has been subjected to tension, is heated and heat-set while maintaining that tension.

The heat relaxation section 40 is a region that relieves the tension of the heat-fixed film 200 by heating the heat-fixed film 200.
As shown in Figure 3, in the heat relaxation section 40, the film 200 is reduced (relaxed) from a width L1 to a width L2.

The cooling section 50 is a region for cooling the heat-relaxed film 200. By cooling the film 200, the shape of the film 200 can be fixed.
Figure 3 shows that the width of the film 200 being fed into the cooling unit 50 is L2, and the width of the film 200 being discharged from the cooling unit 50 is L3.

The annular rail 60a is fitted with gripping members 2a, 2b, 2e, 2f, 2i, and 2j that are movable along the annular rail 60a. The annular rail 60b is fitted with gripping members 2c, 2d, 2g, 2h, 2k, and 2l that are movable along the annular rail 60b.
Gripping members 2a, 2b, 2e, 2f, 2i, and 2j grip one end of the film 200 in the direction of arrow TD. Gripping members 2c, 2d, 2g, 2h, 2k, and 2l grip the other end of the film 200 in the direction of arrow TD. Gripping members 2a to 2l are generally referred to as chucks, clips, etc.
The gripping members 2a, 2b, 2e, 2f, 2i, and 2j move counterclockwise along the annular rail 60a. The gripping members 2c, 2d, 2g, 2h, 2k, and 2l move clockwise along the annular 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, proceeding through the stretching section 20, the heat-setting section 30, and the heat-relaxing section 40 to the cooling section 50. Next, the gripping members 2a and 2b, and gripping members 2c and 2d, release the end of the film 200 at the downstream end of the cooling section 50 in the direction of arrow MD (for example, grip release points P and Q in Figure 3), then move further along the annular rail 60a or 60b and return to the preheating section 10. During this process, the film 200 undergoes preheating in the preheating section 10, stretching in the stretching section 20, heat-setting in the heat-setting section 30, heat-relaxing in the heat-relaxing section 40, and cooling in the cooling section 50, resulting in lateral stretching.

The transport speed of the film 200 can be adjusted by adjusting the movement speed of the gripping members 2a to 2l. Furthermore, the movement speed of each gripping member 2a to 2l can be changed independently.

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

The stretching machine 100 may have additional gripping members (not shown) in addition to the gripping members 2a to 2l to support the film 200.

Next, we will explain this manufacturing method in detail.
This manufacturing method includes, for example, an extrusion molding step of extruding a molten resin containing raw polyester into a film to form an unstretched polyester film having a polyester substrate; a biaxial stretching step consisting of a longitudinal stretching step of stretching the unstretched polyester film in the transport direction to form a uniaxially oriented polyester film and a transverse stretching step of stretching the uniaxially oriented polyester film in the width direction to form a biaxially oriented polyester film; a heat setting step of heating and heat-fixing the biaxially oriented polyester film; a heat relaxation step of heating the polyester film heat-fixed in the heat setting step at a lower temperature than the heat setting step to heat-relax it; a cooling step of cooling the polyester film heat-relaxed in the heat relaxation step; an expansion step of expanding the heat-relaxed polyester film in the width direction during the cooling step; and a specific coating layer forming step of providing a specific coating layer 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 molten resin containing polyester raw material is extruded into a film shape using the extrusion molding method to form an unstretched polyester film. The polyester raw material is the same as the polyester described in the (Polyester) section above.

Extrusion molding is a method of molding raw material resin into a desired shape by, for example, using an extruder to push out a molten raw material resin.
A molten resin containing polyester is formed, for example, by using an extruder equipped with one or more screws to heat the polyester to a temperature above its melting point, and then rotating the screws to melt and knead the polyester. The polyester melts in the extruder due to the heating and kneading with the screws, becoming a molten material.

The molten material is extruded from the extrusion die through a gear pump and filter, etc. The extrusion die is also simply referred to as "die" (see JIS B8650:2006, a, extrusion molding machine, number 134). For example, the extrusion die described in Japanese Patent Publication No. 2005-297266, the extrusion die described in Japanese Patent Publication No. Hei 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 purge the extruder with nitrogen to suppress thermal decomposition (e.g., hydrolysis of polyester) within the extruder. Furthermore, a twin-screw extruder is preferred because it allows for a lower mixing temperature.

The molten material extruded from the extrusion die is formed into a film by cooling. For example, the molten material can be formed into a film by bringing it into contact with a casting roll and cooling and solidifying it on the casting roll. Furthermore, it is preferable to apply air (preferably cold air) to the molten material during cooling.

The temperature of the casting roll is preferably above (Tg-10)°C and below (Tg+30)°C, more preferably between (Tg-7) and (Tg+20)°C, and even more preferably between (Tg-5) and (Tg+10)°C. The above "Tg" refers to the glass transition temperature of the polyester constituting the film.
In this manufacturing method, the temperature of the polyester film and each component can be measured using a non-contact thermometer (e.g., an infrared thermometer). The surface temperature of the film is determined by measuring the temperature of the center of the film in the width direction five times and calculating the average of the obtained measurements.

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

The molded product (unstretched polyester film), cooled using a casting roll or similar device, is peeled off from the cooling device using a peeling roll or similar device.

<Biaxial stretching process>
The biaxial stretching process includes a longitudinal stretching process in which an unstretched polyester film is stretched in the transport 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 process, it is preferable to preheat the unstretched polyester film before longitudinal stretching. Preheating the unstretched polyester film makes it easier to stretch the polyester film longitudinally.
The preheating temperature for 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.
One method for preheating an unstretched polyester film is to place a preheating roll, which has the function of preheating the film, upstream of the stretching roll that stretches the film longitudinally, and preheat the unstretched polyester film while it is being transported.

Furthermore, the stretching roll may also have a function to preheat the film. The preferred range of preheating temperature for the film using the stretching roll is the same as the preferred range of preheating temperature for the preheating roll described above.

Longitudinal stretching can be performed, for example, by conveying an unstretched polyester film longitudinally while applying tension between two or more stretching rolls installed in the conveying direction. For instance, if one pair of stretching rolls A are installed upstream in the conveying direction and one pair of stretching rolls B are installed downstream, the unstretched polyester film will be stretched longitudinally by increasing the rotation speed of stretching rolls B to be faster than that of stretching rolls A during conveying.

In the longitudinal stretching process, the film transport speed (peripheral speed) by a pair of stretching rolls A located upstream in the transport direction and a pair of stretching rolls B located downstream in the transport direction is not particularly limited, as long as the film transport speed by stretching rolls A is slower than the film transport speed by stretching rolls B.
The film transport speed by 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 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 process is preferably 800 to 1500%/second, more preferably 1000 to 1400%/second, and even more preferably 1200 to 1400%/second. Here, "stretching speed" refers to the value obtained by dividing the length Δd of the stretched polyester film in the transport direction per second in the longitudinal stretching process by the length d0 of the polyester film in the transport direction before stretching, expressed as a percentage.

In the longitudinal stretching process, it is preferable to heat the unstretched polyester film. This is because heating facilitates longitudinal stretching.
The heating temperature in the longitudinal stretching process 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 process 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 the stretching rolls or other rolls that come into contact with the unstretched polyester film. Methods for heating the rolls include, for example, installing a heater inside the roll, and installing piping inside the roll and flowing heated fluid through the piping. In addition to the above, other methods include, for example, applying hot air to the unstretched polyester film, and heating the unstretched polyester film by bringing it into contact with a heat source such as a heater, or by passing it near a heat source.

The longitudinal stretching process for an unstretched polyester film is not limited to the method described above.
In the above longitudinal stretching process, the unstretched polyester film is longitudinally stretched by utilizing the difference in conveying speeds between two pairs of stretching rolls. However, a uniaxially oriented polyester film may also be produced by longitudinally stretching the unstretched polyester film using one or more high-speed stretching rolls positioned between the two stretching rolls and conveying the film at a faster speed than those stretching rolls.
Furthermore, although the above-described longitudinal stretching process involves transporting the film by sandwiching it between two opposing rolls (a pair of rolls), the stretching rolls used in the longitudinal stretching process may also consist of only one roll that is in contact with one side of the polyester film, without having opposing rolls.

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

In the transverse stretching process, it is preferable to preheat the polyester film before transverse stretching. Preheating the polyester film makes it easier to transversely stretch.
The preheating temperature is preferably (Tg-10) to (Tg+60)°C, and more preferably (Tg) to (Tg+50)°C. Specifically, the preheating temperature is preferably 80 to 120°C, and more preferably 90 to 110°C.

The stretching ratio in the width direction of the uniaxially oriented polyester film in the transverse stretching process (transverse stretching ratio) is not particularly limited, but it is preferably greater than the stretching ratio in the longitudinal stretching process. The stretching ratio in the transverse stretching process 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 determined from the ratio (L1/L0) of the film width L1 when the film is unloaded from the transverse stretching section 20 to the film width L0 when the film is loaded into the transverse stretching section 20.

The area ratio, expressed as the product of the stretching ratio in the longitudinal stretching process and the stretching 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 above the lower limit, molecular orientation in the film width direction is good. Furthermore, when the area ratio is below the upper limit, it is easier to maintain a state where 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%/second, more preferably 10 to 30%/second, and even more preferably 15 to 20%/second.

<Heat setting process>
In this manufacturing method, it is preferable to perform a heat setting step and a heat relaxation step as heat treatment on the polyester film that has been horizontally stretched by the horizontal stretching step.
In the heat-setting process, the biaxially oriented polyester film obtained in the transverse stretching process can be heated and heat-set. By crystallizing the polyester through heat-setting, shrinkage of the polyester film can be suppressed.
The heat setting process is carried out, for example, in the heat setting section 30 of the stretching machine 100.

The surface temperature of the polyester film in the heat-setting process (heat-setting temperature) 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 higher, more preferably 200°C or higher, and even more preferably 210°C or higher.
In the heat setting process, the heat treatment is performed while controlling the temperature so that the highest temperature reached on the surface of the polyester film is the heat setting temperature.

In the heat-setting process, the variation in surface temperature in the film width direction 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 film width direction within the above range, variations in crystallinity in the width direction can be suppressed.

Heating methods include, for example, applying hot air to the film and radiant heating of the film. An example of a device used in radiant heating is an infrared heater.

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.

<Thermal relaxation process>
In the thermal relaxation process, it is preferable to thermally relax the polyester film, which has been thermally fixed in the thermal fixing process, by heating it at a lower temperature than that of the thermal fixing process. Thermal relaxation can alleviate residual strain in the polyester film.
The thermal relaxation process is carried out, for example, in the thermal relaxation section 40 of the stretching machine 100.

In the heat relaxation process, the surface temperature of the polyester film (heat relaxation temperature) is preferably 5°C or more lower than the heat fixing temperature, more preferably 15°C or more lower, even more preferably 25°C or more lower, and particularly preferably 30°C or more lower. That is, the heat relaxation 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 thermal 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 process, the heat treatment is performed while controlling the temperature so that the highest temperature reached on the surface of the polyester film is the heat relaxation temperature mentioned above.

Heating methods include, for example, applying hot air to the film or radiant heating the film. An example of equipment used in radiant heating is an infrared heater.

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

Methods for cooling the polyester film during the cooling process include, for example, blowing air (preferably cold air) onto the film, and bringing the film into contact with a temperature-controllable component (e.g., a temperature-controlled roll).

The cooling rate of the polyester film in the cooling process is not particularly limited, but a rate of more than 2000°C/min and less than 4000°C/min is preferred, 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, as this reduces thickness unevenness of the release layer laminated on the biaxially oriented film and improves the applicability of the release layer.

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

In this manufacturing method, it is preferable to perform the heat setting, heat relaxation, and cooling steps described above in this order and in succession. This reduces the load (thermal history) on the polyester film caused by repeated heating and cooling, thereby reducing inherent strain in the film and suppressing the occurrence of streak-like defects.

<Extended Process>
In the above cooling process, it is also preferable to include a step of expanding the heat-relaxed polyester film in the width direction.
"Expanding the polyester film in the width direction" during the cooling process means applying tension to the polyester film in the width direction during the cooling process so that the film width at the end of the cooling process (L3 in Figure 3) is greater than the film width at the start of the cooling process (L2 in Figure 3).

In the expansion process, the method of expanding the polyester film in the width direction is not particularly limited. For example, when manufacturing a biaxially oriented polyester film using the stretching machine 100 described above, the film 200 held by each gripping member can be expanded in the width direction by increasing the distance between the annular rails 60a and 60b at the end point of the cooling section 50 (grip release point P and grip release point Q) compared to the distance between the annular rails 60a and 60b at the start point of the cooling section 50.
The expansion process may be carried out continuously or intermittently from the start to the end of the cooling process, or only at a certain point during the cooling process, as long as the film width is expanded before and after the cooling process.

The expansion rate in the width direction of the polyester film due to the expansion process, that is, the ratio of the film width at the end of the cooling process to the film width before the start of the cooling process, is preferably 0% or more, more preferably 0.001% or more, and even more preferably 0.01% or more.
There is no particular upper limit to the expansion ratio, but it is preferably 1.3% or less, more preferably 1.2% or less, and even more preferably 1.0% or less. By setting the expansion ratio of the film width to be below the above upper limit, even when strong tension is applied in the transport direction to transport the film at high speed during film manufacturing (for example, when the tension in the transport direction is 100 N/m or more), it is possible to suppress irregularities in the cut surface during the trimming process described later, and furthermore, to prevent the film from breaking due to such irregularities in the cut surface.

<Specific coating layer formation process>
The present manufacturing method preferably includes a specific coating layer formation step in which a specific coating layer formation composition containing particles (hereinafter also referred to as "Composition A") is used for in-line coating. The specific coating layer formed on one surface of the polyester substrate by the specific coating layer formation step is the same as the layer described in detail in the <Specific Coating Layer> section above.
The formation of the specific coating layer may be carried out at any stage of this manufacturing 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, we will explain a method for forming a specific coating layer using composition A.
Composition A can be prepared by mixing particles contained in a specific coating layer, binders and additives added as needed, and a solvent.
Examples of solvents 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 viewpoint of environmental impact, safety, and economic efficiency.

Composition A may contain one solvent or two or more solvents.
The solvent content is preferably 80 to 99% by mass, and more preferably 90 to 98% by mass, relative to the total mass of composition A.
In other words, in composition A, the total content of components other than the solvent (solids) is preferably 0.5 to 20% by mass, and more preferably 1 to 10% by mass, relative to the total mass of composition A.

The particles, binder, and additives contained in composition A, including their preferred embodiments, are as described in detail in the section on <Specific Coating Layer> above.
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 solution so that the content of each component relative to the total mass of solids in composition A is the same as the preferred content of each component relative to the total mass of the specific coating layer described above.

The method of applying composition A is not particularly limited, and known methods 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 formation process, an in-line coating method is applied, in which the coating solution is applied to one surface of the 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 no thermal history is applied, the area of streaky defects can be reduced, and as a result, the applicability of the release layer can be further improved.
In the in-line coating method, the polyester substrate to which composition A is applied may be an unstretched polyester substrate or a uniaxially oriented polyester substrate, but a uniaxially oriented polyester substrate is preferred. 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 simultaneously transversely stretching the uniaxially oriented polyester substrate and the specific coating layer.

This manufacturing method may include a winding step to obtain a roll-shaped biaxially oriented polyester film by winding up the biaxially oriented polyester film obtained through the above steps.
Furthermore, this manufacturing method may further include a trimming step in which the polyester film is continuously cut along the conveying direction before the winding step is performed, thereby cutting off at least one end of the polyester film in the width direction.

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

Furthermore, while there are no particular restrictions on the tension applied to the polyester film in the transport direction during each process other than the longitudinal stretching process, when the transverse stretching process, heat setting process, heat relaxation process, cooling process, and expansion process are performed using the stretching machine 100, the tension applied to the polyester film in the transport direction can be adjusted by the stretching conditions.
Furthermore, the tension applied to the polyester film in the transport direction after the cooling process and before it is wound up in the winding process described above is preferably 3 to 30 N/m, and more preferably 5 to 20 N/m.

In the method for manufacturing the film described above, a combination of two or more preferred embodiments is a more preferred embodiment.
In particular, a manufacturing method having two or more combinations selected from the group consisting of the following manufacturing conditions is a preferred embodiment because it serves as an indicator for adjusting the area ratio of the streaky defect region and the expansion rate in the width direction of the manufactured biaxially oriented film.
- The stretching ratio in the transverse stretching process 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 process is less than 240°C, preferably 190°C or higher and less than 240°C, more preferably 200 to 230°C, and even more preferably 210 to 230°C.
- The thermal relaxation temperature in the thermal relaxation process is lower than the thermal fixing temperature, preferably 5°C or more lower, more preferably 15°C or more lower, even more preferably 25°C or more lower, and particularly preferably 30°C or more lower.
- The cooling rate of the polyester film in the cooling process 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 in the width direction of the polyester film during the expansion process is 0 to 1.3%, preferably 0.001 to 1.2%, and more preferably 0.01 to 1.0%.

Furthermore, the area ratio of the streaky defect region 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, as well as other manufacturing conditions. Therefore, a manufacturing method having two or more combinations selected from the group of manufacturing conditions described above does not necessarily guarantee the production of a polyester film with the desired streaky defect region area ratio and/or expansion rate in the width direction. Moreover, the method for manufacturing a polyester film with the desired streaky defect region area ratio and/or expansion rate in the width direction is not limited to a method having two or more of the above manufacturing conditions.

[Release film]
The polyester film described above can be used in the manufacture of release films. More specifically, by providing a release layer on the first main surface of the polyester film, a release film can be manufactured having a polyester film and a release layer disposed 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; examples include silicone resins, fluororesins, alkyd resins, acrylic resins, various waxes, and aliphatic olefins, with silicone resins being preferred.

Silicone resin refers to a resin that has a silicone structure within its molecule. Examples of silicone resins include curable silicone resins, silicone graft resins, and modified silicone resins such as alkyl-modified resins, with reactive curable silicone resins being preferred.
Examples of reactive curable silicone resins include addition reaction-type silicone resins, condensation reaction-type silicone resins, and ultraviolet or electron beam curable silicone resins. Among these, addition reaction-type silicone resins or ultraviolet or electron beam curable silicone resins are preferred because they have low-temperature curing properties, allowing for the formation of a release layer at low temperatures.

Examples of silicone resins used in addition reactions include resins obtained by reacting polydimethylsiloxane, which has vinyl groups introduced to its terminals or side chains, with hydrodienesiloxane using a platinum catalyst and then curing the reaction.
Examples of silicone resins used in condensation reactions include resins having a three-dimensional crosslinked structure, formed by condensing polydimethylsiloxane having OH groups at its ends with polydimethylsiloxane having H groups at its ends using an organotin catalyst.
Examples of UV-curable silicone resins include those that utilize the same radical reaction as silicone rubber crosslinking, those that are photocured by introducing unsaturated groups, those that decompose onium salts with ultraviolet light or electron beams to generate strong acids, which then cleave epoxy groups and cause crosslinking, and those that are crosslinked by the addition reaction of thiols to vinylsiloxane. More specifically, examples include acrylate-modified polydimethylsiloxane and glycidoxy-modified polydimethylsiloxane.

The release layer may contain additives other than the resin mentioned above. These additives may include light and heavy release additives for adjusting the release force, adhesion enhancers, and antistatic agents.
The resin contained in the release layer may be used alone or in combination of two or more types.
The resin content in the release layer is preferably 50 to 99% by mass, and more preferably 60 to 98% by mass, relative to the total mass of the release layer. The remainder of the release layer other than the resin may be the additives and/or residues of solvents and catalysts contained in the coating liquid used to form the release layer.

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

(Surface free energy of the delamination surface)
From the viewpoint of preventing static electricity when winding the release film, the surface free energy of the surface of the release layer opposite to the polyester substrate side (also called 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 .
Furthermore, a wider difference between the surface free energy of the release surface of the release layer and the surface free energy of the second main surface of the polyester film is preferable because it makes the film less prone to static charge. The difference between the surface free energy of the release surface 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 delamination surface of the delamination layer can be adjusted by the type of resin and additives used to form the delamination layer.

(Maximum protrusion height Sp on the peeled surface, average surface roughness Sa)
In order to smooth the functional layer, such as the ceramic green sheet, formed on the release layer, it is preferable that the release surface be 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. Furthermore, the average surface roughness Sa of the release surface is preferably 0 to 10 nm, and more preferably 0 to 5 nm.
The maximum protrusion height Sp and average surface roughness Sa of the release surface can be adjusted by not incorporating particles into the release layer when forming it, and by selecting the resin and additives that form the release layer.
The maximum protrusion height Sp and average surface roughness Sa of the peeled surface can be measured in accordance with the measurement method for the maximum protrusion height Sp and average surface roughness Sa of the second main surface described above.

The method for providing a release layer on the first main surface of this film is not particularly limited, but one method involves applying a release layer-forming coating solution, which is obtained by dissolving or dispersing a release agent in a solvent, to the first main surface of the film, removing the solvent by drying, and forming a cured product by heating or irradiating with light as necessary.

The method of applying the coating solution for forming the release layer is not particularly limited, and known methods can be used. Examples of application methods include spray coating, slit coating, roll coating, blade coating, spin coating, bar coating, and dip coating.
The heating temperature for forming the release layer is preferably 180°C or lower, more preferably 150°C or lower, and even more preferably 120°C or lower. The lower limit is not particularly limited and may be 60°C or higher.

The coating solution for forming the release layer contains the above-mentioned resin and solvent, and may optionally contain the above-mentioned additives and/or the above-mentioned catalyst used for curing the resin. The coating solution for forming the release layer can be prepared by mixing these components.
Examples of solvents include water, as well as 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 organic solvents being preferred.

The coating solution for forming the release layer may contain one solvent or two or more solvents.
The solvent content is preferably 80 to 99.5% by mass, and more preferably 90 to 99% by mass, relative to the total mass of the coating solution for forming the release layer.
In other words, in the coating solution for forming a release layer, the total content of components other than the solvent (solids) is preferably 0.5 to 20% by mass, and more preferably 1 to 10% by mass, relative to the total mass of the coating solution for forming a release layer.

Furthermore, to improve the adhesion between the film and the release layer, pretreatment such as anchor coating, corona treatment, and plasma treatment may be applied to the first main surface of the film before applying the release layer.

<Application>
The release film equipped with this film has excellent transportability, can suppress the formation of transfer marks during roll storage, and can suppress thickness unevenness and/or coating defects in the release layer, making it preferable to use it as a release film (carrier film) for the manufacture of ceramic green sheets. The ceramic green sheet manufactured using the above release film can be suitably used in the manufacture of ceramic capacitors, where multilayering of internal electrodes is required due to miniaturization and increased capacitance.

The method for producing a ceramic green sheet using the above-mentioned release film is not particularly limited and can be carried out by known methods. For example, a method for producing a ceramic green sheet may involve applying a prepared ceramic slurry to the surface of the release layer of the release film and drying off the solvent contained in the ceramic slurry.
The method for applying the ceramic slurry is not particularly limited. For example, known methods such as applying a ceramic slurry, which is prepared by dispersing ceramic powder and a binder agent in a solvent, using a reverse roll method and removing the solvent by heating and drying can be applied. The binder agent is not particularly limited and examples include polyvinyl butyral. The solvent is also not particularly limited and examples include ethanol and toluene.

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

The present disclosure will be further explained with reference to the following examples. The materials, quantities used, proportions, processing details, and processing procedures shown in the following examples may be modified as appropriate, provided that they do not deviate from the spirit of this disclosure. Therefore, the scope of this disclosure is not limited to the specific examples shown below. Unless otherwise specified, "parts" and "%" refer to mass.

In this embodiment, the term "film" encompasses both the polyester substrate alone and the embodiment having a polyester substrate and a specific coating layer, as well as all forms of unstretched film, uniaxially oriented film, and biaxially oriented film.
Furthermore, in each step of this embodiment, a non-contact thermometer (AD-5616 (product name), manufactured by A&D, emissivity 0.95) was used to measure the temperature of the center of the film in the width direction five times, and the arithmetic mean of the obtained measurements was taken as the measured surface temperature of the film.

[Example 1]
<Extrusion molding process>
A titanium compound (titanium citrate chelate complex, VERTEC AC-420, manufactured by Johnson Matthey) described in Japanese Patent No. 5575671 was used as a polymerization catalyst to produce polyethylene terephthalate pellets. The obtained pellets were dried until the moisture content was 50 ppm or less, then placed into the hopper of a 30 mm diameter single-screw kneading extruder, and subsequently melted and extruded at 280°C. The molten material was passed through a filter (pore size 3 μm) and then extruded from the die into a cooling drum at 25°C to obtain an unstretched film made of polyethylene terephthalate. The extruded molten material was then brought into close contact with the cooling drum by electrostatic application.
The melting point (Tm) of polyethylene terephthalate, which constitutes the unstretched film, was 258°C, and the glass transition temperature (Tg) was 80°C.

<Longitudinal stretching process>
The above unstretched film was subjected to a longitudinal stretching process using the following method.
A uniaxially oriented film was produced by passing a preheated, unstretched film between two pairs of rolls with different peripheral speeds under the following conditions and stretching it in the longitudinal direction (conveying direction).
(Conditions for longitudinal extension)
Preheating temperature: 75°C
Stretching temperature: 90℃
Stretching ratio: 3.4 times Stretching speed: 1300%/sec

<Specific coating layer formation process>
A specific coating layer was formed by applying composition A (composition for forming a specific coating layer) to one side of a longitudinally stretched uniaxially oriented film (polyester substrate) using a bar coater, and drying the formed coating film with hot air at 100°C. At this time, the amount of composition A applied was adjusted so that the thickness of the formed specific coating layer was 60 nm.

(Composition A)
Composition A was prepared by mixing the components listed below. The prepared composition A was subjected to filtration using a filter with a pore size of 6 μm (F20, manufactured by MAHLE FILES SYSTEMS, Inc.) and membrane degassing (2x6 radial flow superphobic, manufactured by POLIPORE, Inc.), and then the obtained composition A was applied to the surface of a uniaxially oriented film.
- Acid-modified polyolefin (Zyxene® NC, manufactured by Sumitomo Seika Co., Ltd., aqueous dispersion prepared by adding water to 25% solids): 157 parts - Anionic hydrocarbon surfactant (Rapizol® A-90, sodium di-2-ethylhexyl sulfosuccinate, manufactured by NOF Corporation, aqueous dilution with 1% solids): 56 parts - Particles (Snowtex® ZL, manufactured by Nissan Chemical Corporation, colloidal silica, aqueous dispersion with 40% solids): 11 parts - Water: 776 parts

<Horizontal stretching process>
A biaxially oriented film was produced by stretching a film that had undergone a longitudinal stretching process and a specific coating layer formation process in the width direction using a tenter under the following conditions.
(lateral stretching conditions)
Preheating temperature: 100℃
Stretching temperature: 120℃
Stretching ratio: 4.2 times Stretching speed: 50%/sec

<Heat setting process>
The biaxially oriented film subjected to the above transverse stretching process was then subjected to a heat setting process by heating it with a tenter under the following conditions to heat-set the film.
(Heat fixation conditions)
Heat fixing temperature: 227℃
Thermal setting time: 6 seconds

<Thermal relaxation process>
Next, a thermal relaxation process was performed on the heat-fixed film to relieve tension by heating it under the following conditions. In addition, during the thermal relaxation process, the film width was reduced compared to the end of the thermal fixing process by narrowing the distance between the gripping members of the tenter that grips both ends of the film (tenter width). The thermal relaxation rate Lr shown below was calculated from the film width L2 at the end of the thermal relaxation process relative to the film width L1 at the start of the thermal relaxation process using the formula Lr = (L1 - L2) / L1 × 100.
(Thermal relaxation conditions)
Thermal relaxation temperature: 190℃
Thermal relaxation rate Lr: 4%

<Cooling process and expansion process>
A cooling process was performed on the heat-relaxed film under the following conditions. In addition, an expansion process was carried out during the cooling process to widen the tenter width, thereby expanding the film width compared to the end of the heat relaxation process.
The cooling rate shown below was determined by dividing the temperature difference ΔT (°C) between the film surface temperature measured when the film was loaded into the cooling section 50 of the stretcher 100 and the film surface temperature measured when the film was removed from the cooling section 50 by the cooling time ta, with the cooling time ta being the time the film stayed in the cooling section 50 from the time the film was loaded into the cooling section 50 until it was removed from the cooling section 50.
Furthermore, the expansion ratio ΔL shown below was calculated using the formula ΔL = (L3 - L2) / L2 × 100, where L3 is the film width of the polyester film at the end of the cooling process and L2 is the film width of the polyester film at the start of the cooling process.
(Cooling conditions)
Cooling rate: 2500°C/min (extended conditions)
Expansion rate ΔL: 0.6%

<Winding process>
After the film had been cooled in the cooling process, a trimming device was used to continuously cut the film along the transport direction at a position 20 cm from both ends in the width direction, thereby trimming both ends of the film. Next, an extrusion process (knurling) was performed on the area from both ends of the film up to 10 mm in the width direction, and then the film was wound up with a tension of 40 kg/m.
A biaxially oriented film was fabricated using the method described above. The obtained biaxially oriented film had a thickness of 31 μm, a width of 1.5 m, and a roll length of 7000 m. Furthermore, the thickness of a specific coating layer on the obtained biaxially oriented film was measured using a SEM, and the thickness of the specific coating layer was found to be 60 nm. Specifically, a sample with a cross-section aligned with the thickness direction of the biaxially oriented film was prepared by cutting the biaxially oriented film using a microtome. The obtained sample was subjected to etching with Ar ions and deposition with Pt, and then the cross-section of the sample was observed using an SEM (Hitachi High-Tech Corporation, "S-4800"). From the obtained observation images, the thickness of the specific coating layer was measured at five locations, and the thickness of the specific coating layer was determined by arithmetic mean of the measured values.

[Example 2]
In the preparation of composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Snowtex MP2040" (manufactured by Nissan Chemical Corporation, colloidal silica, 40% by mass aqueous dispersion) was used instead of "Snowtex ZL" as 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 a specific coating layer was adjusted to 50 nm.

[Example 4]
A biaxially oriented film was prepared in accordance with the method described in Example 1, except that the amount of particles ("Snowtex ZL") added to Composition A was changed from 11 parts to 22 parts.

[Example 5]
A biaxially oriented film was prepared in accordance with the method described in Example 1, except that the amount of particles ("Snowtex ZL") added to Composition A was changed from 11 parts to 1.1 parts.

[Example 6]
In the preparation of composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Snowtex XL" (manufactured by Nissan Chemical Corporation, colloidal silica, 40% by mass aqueous dispersion) was used instead of "Snowtex ZL" as 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 according to the method described in Example 1, except that 40 parts of the following additive (crosslinking agent) were added to composition A.
- Crosslinking agent (an isocyanate compound, "Duranate® WM44L70" manufactured by Asahi Kasei Chemicals Corporation (70% solids by mass), a dispersion prepared by adding water to adjust the solids concentration to 10% by mass)

[Example 8]
A biaxially oriented film was prepared in accordance with the method described in Example 1, except that the amount of anionic hydrocarbon surfactant added to Composition A was changed from 56 parts to 112 parts.

[Example 9]
In the preparation of composition A, a silicone-based surfactant (BYK-346, solid content 52% by mass, manufactured by BYK) was diluted with water to a solid content of 1% by mass, and 56 parts of the resulting diluted solution were used. Otherwise, a biaxially oriented film was prepared according to the method described in Example 1.

[Example 10]
In the preparation of Composition A, a fluorine-based surfactant (Surflon S-211, 50% solids by mass, manufactured by AGC Seimi Chemical Co., Ltd.) was diluted with water to a solids content of 1% by mass, and 56 parts of the resulting diluted solution were used. Otherwise, a biaxially oriented film was prepared according to the method described in Example 1.

[Examples 11-13 and 23]
In the preparation of Composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Zyxene L" (manufactured by Sumitomo Seika Co., Ltd., acid-modified polyolefin, 25% by mass solids aqueous dispersion) (Example 11), "Zyxene A" (manufactured by Sumitomo Seika Co., Ltd., acid-modified polyolefin, 25% by mass solids aqueous dispersion) (Example 12), and "Chemipearl S120" (manufactured by Mitsui Chemicals, Inc., acid-modified polyolefin, 25% by mass solids 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, solids concentration 25% by mass, acid value 16 mg KOH/g) (Example 23) were used instead of "Zyxene NC" as the resin.

[Examples 14-22]
A biaxially oriented film was prepared according to the method of 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]
In the preparation of composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Takelac W605" (manufactured by Mitsui Chemicals, Inc., polyurethane, 30% by mass aqueous dispersion) was used as the resin instead of "Zychsen NC" in an amount such that the binder content relative to the total mass of composition A was the same.

[Example 25]
In the preparation of composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Hydran AP-40N" (manufactured by DIC Corporation, polyurethane, 35% by mass aqueous dispersion) was used as the resin instead of "Zychsen NC" in an amount such that the binder content relative to the total mass of composition A was the same.

[Example 26]
A biaxially oriented film was prepared according to the method described in Example 25, except that five parts of a fluorine-based surfactant (Futergent 215M, manufactured by Neos Co., Ltd., 1% by mass solids diluted with water) were added to the preparation of composition A.

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

[Comparative Example 1]
In the preparation of composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Snowtex MP2040" was used instead of "Snowtex ZL" as 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 described later.

[Comparative Example 2]
In the preparation of Composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that an aqueous dispersion of acrylic resin (an aqueous dispersion obtained by neutralizing a copolymer obtained by polymerizing methyl methacrylate, 2-hydroxyethyl methacrylate, and methacrylic acid in a mass ratio of 28:48:24) (solid content 25% by mass, acid value 157 mg KOH/g) was used instead of "Zyxene NC" as the resin, 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 described later.

[Comparative Example 3]
In the preparation of composition A, a biaxially oriented film was prepared according to the method described in Example 1, except that "Snowtex MP2040" was used instead of "Snowtex ZL" as particles, "Cymac US-480" (manufactured by Toagosei Co., Ltd., silicone-modified acrylic resin, solid content 25% by mass) was used instead of "Zychsen 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 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 described later.

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

<Maximum protrusion height Sp, average surface roughness Sa>
The maximum protrusion height Sp and average surface roughness Sa of the second main surface of a specific coating layer on a biaxially oriented film were measured by the following method.
The surface of the specific coating layer on the manufactured biaxially oriented film was measured using an optical interferometer (Vertscan 3300G Lite, manufactured by Hitachi High-Tech Corporation) under the following conditions. Subsequently, the maximum protrusion height Sp and average surface roughness Sa of the second main surface of the specific coating layer were determined by analyzing the data using the built-in data analysis software (VS-Measure 5).
For measuring the maximum protrusion height Sp, the maximum value obtained from five measurements taken at different measurement positions was adopted. For measuring the average surface roughness Sa, the average value obtained from five measurements taken at different measurement positions was adopted.
(Measurement conditions)
• Measurement mode: WAVE mode • Objective lens: 50x • Measurement area: 186 μm x 155 μm
The maximum protrusion height Sp and average surface roughness Sa of the first main surface of the biaxially oriented film were measured using a similar method. In all examples, Sp was 20 nm and Sa was 3 nm.

<Surface Free Energy>
The surface free energy of the second principal surface of a specific coating layer on a biaxially oriented film was measured by the following method.
Using a contact angle meter (DROPMASTER-501, manufactured by Kyowa Interface Chemical Co., Ltd.), droplets were dropped onto the surface of a specific coated layer of a manufactured biaxially oriented film at 25°C, and the contact angle was measured 1 second after the droplet adhered to the surface. Purified water (2 μL), methylene iodide (1 μL), and ethylene glycol (1 μL) were used as droplets, and the surface free energy was calculated from the measured contact angles using the Kitazaki-Hata method.
The surface free energy of the first principal surface of the biaxially oriented film was measured using a similar method. In all examples, the value was 59.7 mJ/ ^m² .

<Thickness variation>
A 10 m long sample was taken from the manufactured biaxially oriented film in the longitudinal direction. The thickness of this sample was measured along the longitudinal direction for 10 m using a continuous stylus-type film thickness gauge (TOF-6R001, manufactured by Yamabun Co., Ltd.). This measurement was performed at five different locations in the width direction. From the obtained measurements, the thickness variation was calculated by dividing the difference between the maximum and minimum values by the arithmetic mean of all measurements ((maximum thickness - minimum thickness) / average thickness).

<Film density>
The density (g/ ^cm³ ) of the biaxially oriented film was measured using an electronic hydrometer (product name "SD-200L", manufactured by Alpha Mirage).

<Area of muscle-like defects>
Using a heated conveying device, a biaxially oriented film was heated at 90°C for 20 seconds while being conveyed at a conveying speed of 30 m/min and with 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). After the heat treatment, the biaxially oriented film was placed on a black flat plate, and then the biaxially oriented film was visually observed from an oblique angle, changing the viewpoint so that the light from a fluorescent lamp (Lupica Ace manufactured by Mitsubishi Electric Corporation (color temperature: 5000K, average color rendering index (Ra): 84)) installed on the ceiling of the room reflected off it. A 1m x 1m area was visually observed, and areas on the surface of the biaxially oriented film where the reflected image of the fluorescent lamp was wavy were defined as streaky defect areas. Next, the ratio (area ratio) of the total area of the observed streaky defect regions to the total area of the observation region of the biaxially oriented film was calculated using the method described above (see the "streaky defect regions" section).

<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 described above (see the "Expansion Coefficient" section).

<Average particle diameter, particle density D>
The average particle size and particle density D of the particles contained in a specific coating layer were measured using the following method.
Using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, S4700), the surface of a specific coated layer of a biaxially oriented film was observed at a magnification of 20,000x, and 10 fields of view were obtained. 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 to the diameter of a circle with the same area (area circle equivalent diameter). After that, the arithmetic mean of all particles was calculated.
Furthermore, the particle density D (unit: particles/ ^μm² ) was calculated by dividing the number of particles identifiable from the image data of each field of view by the field of view area.

<Measurement of electrostatic charge due to peeling>
The amount of charge removed from each manufactured biaxially oriented film was measured using a measuring device comprising a platform on which a reference sample (with the first main surface facing upwards) is placed, a head that can repeatedly press and peel the measurement sample against the upper surface of the reference sample by moving up and down vertically while holding the measurement sample (with the second main surface facing downwards), and an electrometer connected to this head that can measure the amount of charge of the measurement sample.
First, each manufactured biaxially oriented film was cut into a circle with a diameter of 1.5 cm to prepare a sample for measuring the amount of charge removed from the film, and a rectangle measuring 13 cm x 4 cm was cut to prepare a reference sample. Each sample was then left for more than two hours in an environment with a temperature of 23°C and a relative humidity (RH) of 20%.
The obtained reference sample was placed on the measuring device's stand, and the measurement sample was mounted on the head. At this time, the first main surface of the reference sample and the second main surface of the measurement sample were positioned so that they faced each other, with the first main surface of the reference sample on the stand facing upwards and the second main surface of the measurement sample mounted on the head facing downwards. After the measurement sample was discharged, the head was raised or lowered, and the pressing and detaching of the measurement sample from the reference sample was repeated five times (contact pressure of 566 g/ ^cm² , contact time of 2 seconds). The amount of charge on the measurement sample was measured after each detachment from the first to the fifth time, and the average of the measured values was calculated. The measurement sample was changed, and the position in which the measurement sample contacted the reference sample was also changed for each measurement sample, and measurements were taken with a total of four samples. The average of all measurements was obtained as the amount of charge after detachment.

[evaluation]
Release films were prepared using 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 films produced in each example and comparative example were fed out, and a 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 using a slot die method. The coated film was then dried using a hot air dryer at 120°C, wound up, and rolled up to produce a release film (biaxially oriented film with a release layer). The thickness of the release layer after drying was 0.5 μm.

[Transfer trace evaluation 1]
The obtained release film was cut into 3.5 cm squares, and 10 of these were stacked in a direction where the release layer and the specific coating layer were in contact to obtain a laminated sample. This sample was subjected to a load of 84 kg and kept in a 40°C oven for 3 days. After removing the sample from the oven, the release film was peeled off one sheet at a time. The surface of the release layer of the release film was observed at 20,000x magnification using a scanning electron microscope (SEM, Hitachi High-Tech Corporation, S4700), and the presence of indentations was evaluated according to the following criteria to determine the presence of transfer marks.

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

(Evaluation criteria)
A: No indentations were observed, and the surface was smooth.
B: Indentations were observed, and the surface of the delamination layer was roughened.

[Transfer trace evaluation 2: Evaluation after long-term storage]
A long-term storage test was conducted in which the release film prepared by the method described in [Preparation of Release Film] above was stored for three months in a normal temperature and humidity environment. 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 Transcription Trace Evaluation 2 are shown below.

(Evaluation criteria)
A: No indentations were observed, and the surface was smooth.
B: Of the 10 release films, only one showed an indentation; the remaining nine had smooth surfaces.
C: Indentations were observed in two or more of the ten release films, and the surface of the release layer was roughened.

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

(Evaluation criteria)
A: No wrinkles were observed during transport.
B: Wrinkles were observed only in the vicinity of the conveyor roll.
C: Wrinkles were consistently observed downstream of the conveyor rolls.

[Applicability of the release layer]
The release film prepared for the evaluation of the transfer marks described above was cut to a length of 30 m in the longitudinal direction. Under a three-wavelength fluorescent lamp, the surface of the release layer side of the obtained sample was visually observed, and the number of streaky coating irregularities and defects due to foreign matter observed by reflected light was measured. Based on the measurement results, the coatability of the release layer was evaluated according to the following criteria.

(Evaluation criteria)
A: No unevenness in coating or defects were observed.
B: Coating inconsistencies and/or defects were observed, but were within acceptable limits.
C: Coating irregularities and/or defects exceeding acceptable limits were observed.

Table 1 shows the evaluation results for each example and comparative example.
In Table 1, the following components were used for resins 1 to 9 in the "Resin" column, W-1 to W-4 in the "Surfactant" column, crosslinking agents in the "Additives" column, and particles 1 to 3 in the "Type" column of the "Particles" column.
(resin)
Resin 1: Zyxene NC (manufactured by Sumitomo Seika Co., Ltd., acid-modified polyolefin, aqueous dispersion)
Resin 2: Zyxene L (manufactured by Sumitomo Seika Co., Ltd., acid-modified polyolefin, aqueous dispersion)
Resin 3: Zyxene A (manufactured by Sumitomo Seika Co., Ltd., acid-modified polyolefin, aqueous dispersion)
Resin 4: Chemipearl S120 (manufactured by Mitsui Chemicals, Inc., acid-modified polyolefin, aqueous dispersion)
Resin 5: Acrylic resin (an aqueous dispersion obtained by neutralizing a copolymer produced by polymerizing methyl methacrylate, 2-hydroxyethyl methacrylate, and methacrylic acid in a mass ratio of 28:48:24)
Resin 6: Cymac US-480 (manufactured by Toagosei Co., Ltd., silicone-modified acrylic resin, aqueous 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 (manufactured by Mitsui Chemicals, Inc., polyurethane, aqueous dispersion)
Resin 9: Hydran AP-40N (manufactured by DIC Corporation, polyurethane, aqueous dispersion)
(Surfactants)
W-1: Anionic hydrocarbon surfactant (Rapizol A-90, manufactured by NOF Corporation)
W-2: Silicone-based 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 with 6 or more carbon atoms)
W-4: Fluorine-based surfactant (Futergent 215M, manufactured by Neos Co., Ltd., contains a perfluoroisopropyl group and does not contain a linear perfluoroalkyl group with 6 or more carbon atoms)
(Additives)
Crosslinking agent: Isocyanate compound, "DURANET® WM44L70," manufactured by Asahi Kasei Chemicals Corporation (particles)
Particle 1: Snowtex ZL (manufactured by Nissan Chemical Corporation, colloidal silica, aqueous dispersion)
Particle 2: Snowtex MP2040 (manufactured by Nissan Chemical Corporation, colloidal silica, aqueous dispersion)
Particle 3: Snowtex XL (manufactured by Nissan Chemical Corporation, colloidal silica, aqueous dispersion)

Table 1 shows that the biaxially oriented polyester films of Examples 1 to 27 according to the present invention exhibit superior effects compared to Comparative Examples 1 to 3.

[Comparative Example 4]
As the polyester film for Comparative Example 4, a polyethylene terephthalate film ("Cosmoshine® A-1517", manufactured by Toyobo Co., Ltd., 16 μm thick) was prepared by laminating a resin layer containing particles onto 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 the film) and a second surface with an uneven shape (corresponding to the second main surface of the film). The maximum protrusion height Sp of the uneven surface of the polyester film of Comparative Example 4 was 63 nm.
Using the polyester film of Comparative Example 4, a roll-shaped release film was prepared by providing a release layer on the first surface according to the method described in [Preparation of Release Film] above. When the transfer marks on the surface of the release layer were evaluated using the method described in [Transfer Mark Evaluation 1] above, depressions were observed, indicating that the release layer was roughened (Evaluation B).
Furthermore, when the transfer marks on the surface of the release layer were evaluated using the method described in [Transfer Mark Evaluation 2] above with the obtained release films, indentations were observed on the surface of two or more of the ten release films, indicating that the release layer was roughened (Evaluation C).

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

Furthermore, it was confirmed that the transportability is superior when the product of the particle density D (unit: particles/ ^μm² ) of the particles constituting the protrusions of the second main surface and the maximum protrusion height Sp (unit: nm) of the second main surface (D × Sp) is 20 or more (comparison between Examples 2, 5 and 6 and other examples).

It was confirmed that the application properties of the release layer were superior when the area ratio of the streaky defect region generated when heated to 90°C was 20% or less (comparison of Examples 1, 20-22 and Examples 14-19).

It was confirmed that when the cooling rate of the polyester film during the cooling process was greater than 2200°C/min but less than 3000°C/min, the coating properties of the release layer were superior (comparison of Examples 1 and 17-19).

It was confirmed that when a specific coating layer contains a hydrocarbon-based surfactant, the coatability of the release layer is superior (comparison of Examples 1 and 9-10).
It was confirmed that when a specific coating layer contains a fluorine-based surfactant having a perfluoroalkyl group with 1 to 4 carbon atoms, the coating properties of the release layer are superior compared to when it contains a fluorine-based surfactant having a linear perfluoroalkyl group with 6 or more carbon atoms (comparison of Examples 26 and 27).

Furthermore, it was confirmed that when the absolute value of the peel charge of the biaxially oriented film, measured using the above measurement method, was 0.12 nC or less, the coating properties of the peel layer were superior (comparison between Examples 9-10 and Examples 1-8, 11-13, 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 has been stored for a long period of time (comparison between Example 24 and other examples).

[Example 101: Manufacturing of Ceramic Green Sheets]
In each of Examples 1 to 26, a ceramic slurry having the following formulation K was applied to the release layer of the release film prepared in each of the above examples, so that the thickness after drying was 0.5 μm. The resulting slurry-coated film was then dried at 90°C. The ceramic slurry was prepared by mixing the raw materials described in the following formulation and dispersing them in a ball mill. Two release films with slurry coatings prepared by the above method were placed on top of each other so that the surface of the slurry coating and the surface of the specific coating were in contact, and a load of 1 kg/ ^cm² was applied for 10 minutes. After that, the release film was peeled off from the slurry-coated release film to obtain a ceramic green sheet.
The resulting ceramic green sheets were free of foreign matter and transfer failures, and possessed excellent properties.

<Formulation K: Ceramic Slurry>
5 parts of polyvinyl butyral (manufactured by Sekisui Chemical Co., Ltd., Esrec BX-5), 100 parts of barium titanate (manufactured by Fuji Titanium Industries Co., Ltd., HPBT), and 45 parts of a 6:4 mixed solvent of toluene and ethanol.

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

Claims (18)

A polyester substrate that is substantially free of particles,
The polyester substrate comprises a particle-containing layer disposed on one surface of the polyester substrate,
Having a first main surface and a second main surface,
A polyester film used to manufacture a roll-shaped release film by forming a release layer on the first main surface,
The thickness of the particle-containing layer is 1 to 200 nm.
The average particle diameter of the aforementioned particles is greater than the thickness of the particle-containing layer,
The second main surface is the surface of the particle-containing layer opposite to the polyester substrate side,
The maximum projection height Sp of the second main surface is 1 nm or more and less than 60 nm.
The surface free energy of the second main surface is 25 to 50 mJ/ ^m² .
A polyester film in which the density D of particles constituting the protrusions on the second main surface is 0.2 to 3.6 particles/ ^μm² . The polyester film according to claim 1, wherein the absolute value of the peel charge between the first and second main surfaces of a polyester film corresponding to a circle with a diameter of 1.5 cm is 0.12 nC or less. The polyester film according to claim 1 or 2, wherein the product (D × Sp) of the density D (unit: particles/ ^μm² ) of the particles constituting the protrusions of the second main surface and the maximum protrusion height Sp (unit: nm) is 20 or more. The polyester film according to any one of claims 1 to 3, wherein the thickness of the polyester film is 40 μm or less. The polyester film according to any one of claims 1 to 4, wherein the particle-containing layer further contains a polyolefin. The polyester film according to any one of claims 1 to 5, wherein the particle-containing layer further contains a (meth)acrylate resin having an acid value of 30 mg KOH/g or less. The polyester film according to any one of claims 1 to 6, wherein the particle-containing layer contains at least one surfactant selected from the group consisting of hydrocarbon surfactants and fluorine-based surfactants containing perfluoroalkyl groups having 1 to 4 carbon atoms. The polyester film according to any one of claims 1 to 7, wherein the polyester film is subjected to a heat treatment for 20 seconds under conditions of transporting at a transport speed of 30 m/min and a tension of 100 N/m in the transport direction, while the temperature of the film surface becomes 90°C, and the total area of the streaky defect regions observed on the polyester film is 40% or less of the total area of the observation region. The polyester film according to any one of claims 1 to 8 , wherein the density of the polyester film is 1.39 to 1.41 g/ ^cm³ . The polyester film according to any one of claims 1 to 9 , wherein the expansion coefficient in the width direction of the polyester film at 90°C is -0.15 to 0.15% of the length in the width direction of the polyester film at 30°C. The polyester film according to any one of claims 1 to 10 , 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 11 , 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 12 , wherein the surface free energy of the first main surface is 50 to 70 mJ/ ^m² . The polyester film according to any one of claims 1 to 13 , wherein the variation in the 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 14 , wherein the release film is a release film for manufacturing ceramic green sheets. A roll-shaped release film comprising a polyester film according to any one of claims 1 to 15 , and a release layer disposed on the first main surface of the polyester film. The roll-shaped release film according to claim 16 , wherein the maximum protrusion height Sp on the surface of the release layer opposite to the polyester film side is 1 to 60 nm. The roll-shaped release film according to claim 16 or 17 , wherein the surface free energy of the surface of the release layer opposite to the polyester film side is 30 mJ/ ^m² or less.